Construction of Organometallic Architectures from Aminoalkylidyne
Diiron Complexes
Fabio Marchetti *[a]
[a] University of Pisa, Dipartimento di Chimica e Chimica Industriale, Via Moruzzi 13, I-56124 Pisa. E-mail: [email protected]. Webpage: www.dcci.unipi.it/~fabmar/
Abstract.
The chemistry of diiron complexes has seen a recent renaissance, due to the prominent role played by the element iron in the development of sustainable synthetic processes and the cooperative effects provided by two metal centres working in concert, that is a concept amazingly exploited by Nature. Aminoalkylidyne derivatives of [Fe2Cp2(CO)4] are a convenient scaffold to grow organic fragments; in particular, the facile removal of one CO ligand opens the doors to the unconventional assembly of molecules/ions, affording unusual structural motifs. In particular, vinyliminium complexes result from carbyne-alkyne coupling and offers the opportunity for a great variety of selective derivatization pathways, including addition of nucleophiles, access to substituted ferrocenes, deprotonation and reduction processes, and trapping of small molecules.
1. Introduction
Following the ferrocene breakthrough, [Fe2Cp2(CO)4] was prepared for the first time over sixty years ago, and then established to be a milestone for classical organometallic chemistry.1 Related diiron complexes were largely investigated for pioneering studies on the reactivity of bridging hydrocarbyl ligands, with the aim of expanding basic knowledge, exploring new synthetic routes to CC bond formation and modelling surface processes such as the Fischer Tropsch reaction.2
I would say that this fruitful chemistry held some prophetical character, since it relied on key concepts that have been more clearly focused later on. In principle, a dinuclear complex is the most simple metal system offering the opportunity for cooperative effects and multisite coordination modes,3 thus giving rise to reactivity patterns not commonly observed with mononuclear species.4 In this sense, it is impressive that diiron units are present in nature in some enzymes,5 and in particular [FeFe]-hydrogenases are actually based on an organometallic core, comprising carbonyl and cyanide ligands.6 Furthermore, the increasing demand for sustainable and environmentally benign synthetic processes has designated iron as a multitasking candidate 7 in view of replacing precious metals with non toxic and earth abundant counterparts.8
In the light of these concepts, the chemistry of diiron complexes has seen a renaissance in the recent times. Thus, a huge number of dithiolate [FeFe]compounds has been designed to mimic biological systems, and investigated for various catalytic applications.Error: Reference source not foundb,9 It is worthy of notice that diiron complexes resembling the [Fe2Cp2(CO)4] structure, and lacking of dithiolate groups, may exhibit electrocatalytic properties which are comparable to those of closer [FeFe]-hydrogenase models.10 A wide range of diiron complexes containing CO and/or stabilizing Cp ligands have progressively emerged as versatile platforms for the building of uncommon organic fragments, exploiting unusual reaction pathways, assisted by the cooperative interaction of the two iron centers.Error: Reference source not founda,11,12
Diiron cationic complexes with one aminoalkylidyne ligand, 1, have been known since a long time,13 and are available from [Fe Cp(CO)] by a straightforward two step procedure (Scheme 1),14
consisting in carbon monoxide/isocyanide substitution followed by alkylation of the coordinated isocyanide.15 Analogous bis-aminoalkylidyne species were also reported in the past.16 On the other hand, stable polynuclear compounds with bridging aminoalkylidyne ligands are relatively rare in the literature, based on molybdenum,17 nickel,18 tungsten,19 platinum,20 iron 21 and heterodinuclear systems,22 and their chemistry has been sparingly developed.
In contrast to [Fe2Cp2(CO)4], existing in solution as cis and trans isomers (referred to the Cp ligands),Error: Reference source not founda 1 are exclusively cis: it is presumable that intramolecular ligand exchange between bridging and terminal sites (Adams-Cotton mechanism 23) is working during the formation of 1, and similar considerations could explain cis/trans interconversions affecting the dinuclear derivatives of 1 (vide infra).
The CN bond distance in the X-ray structures of 1 ranges from 1.280(8) 24 to 1.297(4) Å,Error: Reference source not found accounting for a partial iminium character due to competition of the nitrogen lone pair for the empty orbital on the carbyne carbon. Solution IR spectroscopy in the (CO) region is a practical tool to easily monitor the course of the transformations of 1 and also all the succeeding compounds. On the other hand, NMR spectroscopy is a diagnostic technique for the recognition of the aminoalkylidyne group, since the carbyne nucleus typically resonates at down fields (e.g. at 315.5 ppm in dmso-d6 for R = Me, and at 327.8 ppm in CDCl3 for R = Xyl = 2,6-dimethylphenyl Error: Reference source not found).
Herein, I will overview the coordination chemistry developed using 1 as a starting material,25,26 wherewith I have been involved since 2001. In this review, the terms “alkylidyne” and “alkylidene” will be preferentially (but not exclusively) used respect to the corresponding ones “carbyne” and “carbene”.27
2. Reactivity of aminoalkylidyne complexes. 2.1. Nucleophilic additions.
Cationic aminoalkylidyne (aminocarbyne) complexes 1 undergo the addition of various nucleophiles. These reactions are generally selective across three possible sites, i.e. the Cp ring,28 the CO ligand Error: Reference source not found and the aminocarbyne carbon,Error: Reference source not found,Error: Reference source not found depending on the nature of the nucleophile (Scheme 2). A bulky R substituent on the nitrogen atom may have a significant impact on the reactivity of the complex: for instance, the reaction of 1 (R = Xyl = 2,6-dimethylphenyl) with cyanide salts results in direct CO replacement (Scheme 2e) rather than attack to the carbyne (Scheme 2b).Error: Reference source not found
Scheme 2. C-C and C-H bond forming reactions from diiron -aminoalkylidyne complexes: a) NaBH4, R =
Me, Et, CH2Ph.Error: Reference source not found b) NaBH4, R = 2-naphthyl;Error: Reference source not
found NBu4CN, R = Me, Et, CH2Ph.Error: Reference source not found c) NaBH4, R = Xyl;Error: Reference
source not found LiR′ (R′ = Me, nBu, Ph), R = Me, CH
2Ph;Error: Reference source not found R′MgCl (R′ =
Me, CH2Ph, iPr), R = Me, CH2Ph.Error: Reference source not found d) Li2Cu(CN)R′2 (R′ = Me, nBu), R = Me, CH2Ph;Error:
Reference source not found LiC≡CR′ (R′ = Ph, 4-C6H4Me), R = Me;Error: Reference source not found
Xyl.Error: Reference source not found f) CF3SO3Me, R = Me or Xyl, R′ = Ph, 4-C6H4Me, nBu, SiMe3.Error:
Reference source not found,Error: Reference source not found
Cyclopentadienyl ligands often act as spectators in organometallic chemistry, therefore the formation of stable cyclopentadiene derivatives (Scheme 2c) evidences the relative inertness of the carbyne carbon in 1, reflecting the partial iminium character of the CN bond.29 Accordingly, mixed aminoalkylidyne-alkoxyalkylidene complexes 7, obtained by treatment of 5 with alkylating agents (Scheme 2f), undergo nucleophilic additions almost exclusively at the alkoxycarbene moiety.Error: Reference source not found,Error: Reference source not foundb,30 In particular, amination of 7 to aminoalkylidyne-aminoalkylidene derivatives is accomplished following a general reaction type.31
2.2. Activation of unsaturated nitrogen ligands.
Our original purpose was to modify the aminoalkylidyne unit: the replacement in 1 of one carbonyl ligand with the more labile acetonitrile, being a classical method to generate the equivalent of one vacant site at a metal centre,32 appeared as a possible strategy to by-pass the chemical inertness of the carbyne centre. In a range of similar diruthenium systems, the direct metal coordination of different species via acetonitrile removal permits the subsequent intramolecular coupling with bridging unsaturated hydrocarbyl ligands.Error: Reference source not foundd,33
From the aminoalkylidyne complexes 8 (R′ = CH3, Scheme 3), acetonitrile is readily displaced by a variety of anionic/neutral compounds (L), i.e. hydride,34 cyanide,Error: Reference source not found halides,Error: Reference source not found,35 pseudo-halides,36 diethyldithiocarbamate,Error: Reference source not foundb phosphines,Error: Reference source not foundb amines,37 imines,Error: Reference source not found and also a mild carbanion such as dicyanomethanide,Error: Reference source not foundb affording [Fe2Cp2(CO)(μ-CO){μ-CN(Me)(Xyl)}(L)]0/+ complexes in good to high yields. These reactions take place without affecting the aminocarbyne moiety. Two isomers (syn-anti) are usually generated in case the aminoalkylidyne nitrogen binds two different substituents (i.e., R Me). In addition,
(L)]0/+, and complete trans to cis conversion is normally achieved by heating in refluxing tetrahydrofuran.Error: Reference source not foundb,Error: Reference source not found
Somehow unexpectedly, when 8 (R′ = CH3, CH2Ph) are treated with strong bases (phenyl lithium, lithium alkyls, lithium acetylides), the nitrile ligand does not behave as a leaving one, otherwise providing unusual example of nitrile to cyano-alkyl (9) rearrangement via heterolytic CH activation (Scheme 3a).38 This reaction was subsequently reported on Rh, Ir and Ni systems,39 and also by means of (C2F5)3PF2 / NEt3 combination.40 On the other hand, pivalonitrile (tBuCN), lacking of acidic hydrogens, manifests its lability towards phenyl lithium (compound 10, Scheme 3b).Error: Reference source not foundb Notwithstanding, this substitution reaction remains an exception, since the same pivalonitrile and also arylnitriles do not allow, in general, carbanionic nucleophiles to reach the iron coordination sphere. Instead, unclean reactions often take place, unless xylyl containing complexes are treated with lithium (4-tolyl)acetylide: with such specific combination (e.g., phenylacetylide does not work), selective nucleophilic addition at the coordinated nitrile occurs to give the alkynyl-ketimide intermediates 11, followed by CN coupling with the aminocarbyne (Scheme 3c,d). Alkynyl-ketimide ligands have been alternatively obtained by nitrile insertion into metal-acetylide bonds,41 and are in general reactive species able to participate in insertion and cycloaddition processes.42 In the present situation, the final formation of the allenyl-diaminoalkylidene derivatives 12 43 or that of the polycyclic 13 via Fe-Fe bond cleavage 44 are completely different outcomes which are finely addressed by a compromise of electronic features associated to the nature of the various substituents and also the metal (note that the same organic fragments may assembly in different manners on homologous diruthenium complexes 45).
Scheme 3. Assembly of nitrile/acetylide/aminocarbyne units in diiron complexes (Tol = 4-C6H4Me).Error:
Reference source not foundb,Error: Reference source not founda,Error: Reference source not found,Error: Reference source not found
Intramolecular CN coupling between the aminocarbyne and a nitrogen ligand, triggered by nucleophilic addition, was observed also in the reaction of the benzophenone imine complex 14 with lithium 4-tolylacetylide, affording the ferra-dihydropyridinamine 15 (Scheme 4).Error: Reference source not found
Scheme 4. Assembly of aminocarbyne, imine and acetylide units in a diiron complex (Tol = 4-C6H4Me).Error:
In summary, unsaturated nitrogen compounds can be easily incorporated as monodentate, terminal ligands in cationic diiron aminoalkylidyne complexes, assuming the role of the most reactive site. Nucleophilic addition to the nitrogen ligand may prelude to CN bond formation with the peculiar and otherwise unreactive aminocarbyne ligand, resulting in the formation of unique organometallic structures.
2.3. Activation of isocyanides.
Carbonyl-isocyanide mono-substitution in 1 is performed under mild conditions in the presence of trimethylamine-N-oxide, giving the family of complexes [Fe2Cp2(CO)(μ-CO){μ-CN(Me)(R)}(CNR ′)]+, 16, in 72-90% yields.46
One complex of the series, namely 16Xyl,Xyl, undergoes selective hydride addition at the xylyl-isocyanide ligand affording 17, then unusual CC coupling between the so generated formimidoyl ligand and the bridging aminocarbyne is viable at high temperature (Scheme 5a).Error: Reference source not found The attack of lithium acetylides to 16Xyl,Xyl is directed to the carbonyl (19), then slow intramolecular acetylide migration to the isocyanide follows in refluxing dichloromethane. The subsequent CN bond formation by iminoacyl/aminocarbyne coupling is promoted by a further thermal treatment and provides an example of N-carbonyl-alkynyl-aminoalkylidene ligand in unusual coordination mode (Scheme 5b, compound 21).47
Scheme 5. Isocyanide activation in diiron aminoalkylidyne complex: a) CC coupling initiated by hydride addition;Error: Reference source not found b) CN coupling promoted by addition of LiC≡CR (R = Ph, 4-C6H4Me, SiMe3).Error: Reference source not found
In general, iminoacyl complexes 20 can be obtained by direct addition of lithium acetylides to a range of isocyanide complexes 16, without the intermediacy of acyl derivatives 19. According to the fact that the sequential nucleophile/electrophile addition to a coordinated isocyanide represents a classical strategy to access aminoalkylidene ligands,48 complexes 20 were converted into the alkynyl-alkylidene 22 by methylation (Scheme 6a).49 The reactivity with nucleophiles of one compound of the series 22 was explored, revealing to be substantially localized at the alkynyl function 50 (an example of structure growing is shown in Scheme 6b).
Scheme 6. Synthesis and functionalization of alkynyl-aminoalkylidene complexes (Tol = 4-C6H4Me).Error:
Up to two isocyanide ligands can be incorporated in the diiron frame upon replacement of carbonyl ligands. The bis-isocyanide complex 24 exhibits a distinct chemistry towards acetylides respect to the mono-isocyanide homologues 16; hence, acetylide attack promotes the coupling of the two xylyl-isocyanide units affording a uncommon example of aminoalkylidene-N-iminoacyl ligand (Scheme 7a).51 The use of trimethylsilylacetylide supplies the way to dimerization, presumably through the release of Me3SiC≡CSiMe3 by contact with the alumina used for chromatographic purification (Scheme 7b). It should be mentioned here that the metal mediated assembly of isocyanide units is a topic of current great interest, pointing to the definition of convenient strategies to access novel nitrogen containing organic structures.Error: Reference source not found,52
Scheme 7. Coupling of isocyanide ligands promoted by acetylide addition to aminoalkylidyne complexes. a)
R = Xyl, R′ = Ph; R = Xyl, R′ = 4-C6H4Me; R = Me, R′ = Ph. b) -Al2O3, R′ = SiMe3.Error: Reference source not
found
In summary, diiron complexes with a bridging aminoalkylidyne ligand can be modified by selective replacement of carbonyl ligands with either one or two isocyanides. Addition of nucleophiles to the resulting cationic complexes is preferentially addressed to the isocyanide ligand, then intramolecular coupling with either carbonyl or aminoalkylidyne may follow to afford unusual
organic fragments. When the aminoalkylidyne is not directly involved, it drives the reaction outcomes which are regulated by the N-substituents, as well as the nature of the isocyanide(s).
2.4. CC bond forming insertion reactions of unsaturated hydrocarbons into the Fe-carbyne bond.
Complexes 8 react with alkynes via removal of the nitrile ligand in mild conditions (vide infra), but they are generally unreactive towards alkenes and allenes. On the other hand, reactions of 1 with a variety of alkenes 53 and allenes 54 take place in the presence of two co-reactants, i.e. a base, facilitating the deprotonation of the organic substrate, and Me3NO, allowing the CO elimination (Scheme 8). These reactions lead to the formation of amino-allylidene 27 (from alkenes) and aminobutadienylidene 28 (from allenes) compounds, which are essentially the products of nucleophilic attack of alkenyl/allenyl anions to the carbyne centre, assisted by the generation of a metal site vacancy. A parallel chemistry is exhibited by 1-related diiron thio-alkylidyne Error: Reference source not found,55 and diruthenium aminoalkylidyne compounds.Error: Reference source not found The synthesis of complexes 27 proceeds with cis to trans rearrangement of the cyclopentadienyl ligands. Compounds 27 and 28 display an extended π-bond which is susceptible to electrophilic attack. Methylation (from methyl triflate) and protonation (from triflic acid) of 27 selectively occur at the nitrogen atom yielding ammonium moieties, whereas in the case of 28 the protonation is directed to the unsaturated chain, so to yield a stable vinyliminium structure (vide infra).
Scheme 8. Base-assisted coupling of aminoalkylidyne ligand (R = Me, Xyl or 4-C6H4OMe) with a) alkenes (R′
= H, R′′ = CN, CO2Me, Ph; R′ = R′′ = CO2Et) Error: Reference source not found and b) allenes (R′ = R′′ = Me;
R′ = Me, CO2Et, R′′ = H).Error: Reference source not found
The alkyne insertion into the metal--CO bond in [Fe2Cp2(CO)4] is an old-time reaction reported by Knox and co-workers, leading to dimetallacyclopentenone complexes with prior photolytic removal of one carbonyl ligand.56 Accordingly, diiron systems [Fe
2Cp2(CO)3(L)] undergo alkyne insertion into the bond between one iron and a bridging ligand (L) isolobal 57 to CO, i.e. thiocarbonyl 58 or isocyanide,59 the reaction being initiated by photolytic displacement of one carbonyl. These reactions may require long times, give low yields and not be regioselective. Similar alkyne insertion was reported to take place starting from [Fe2(CO)6(μ-CO)(μ-dppm)], dppm =
bis(diphenylphosphino)methane.60
The interaction of aminoalkylidyne complexes with alkynes takes advantage of the net cationic charge of the former, weakening electron backdonation and thus the strength of the terminal CO binding. This feature makes possible CO removal from 1 with a chemical method, i.e. the use of trimethylamine N-oxide, allowing the preliminary coordination of an acetonitrile ligand, rather than with the much less selective (and practical) photochemical route. In 8, the lability of acetonitrile towards the neutral alkyne molecule supplies a site available to the presumable, elusive initial coordination of the triple carbon-carbon bond. This is followed by fast alkyne insertion into the Fe-CNR(Me) linkage, usually in dichloromethane solution at ambient temperature during 24-48 h, to give air stable compounds 29 in good to high yields, even in gram-scale (Scheme 9).61 The insertion reaction works with a wide variety of terminal alkynes HC≡CR′, being tolerant of functional R′ groups such as carboxylic acid, carboxylates, alcohols 62 and ferrocenyl.63 Also disubstituted alkynes with limited steric hindrance (e.g., R′ = R′′ = Me, Et, Ph, not SiMe3, CH2Cl or CF3) have been employed to obtain complexes of type 29.Error: Reference source not found The use of the chloride precursor 30 in combination with a silver salt is a faster procedure that may be convenient with volatile alkynes, e.g. acetylene Error: Reference source not found,64 (Scheme 9, compound 31).
It is noteworthy that Ru complexes analogous to 29 have been reported, and exhibit approximately parallel reactivity.65
Scheme 9. Synthesis of vinyliminium complexes via alkyne insertion into Fe-μ-carbyne bond. Inset: possible
forms of isomerism.Error: Reference source not found-Error: Reference source not found
The -1:3–coordinated ligand in 29 consists of a π-delocalized system: its representation as a vinyliminium (Figure 1, structure A) seems satisfying and has been usually adopted, although vinyl-amino-alkylidene 66 (structure B) and keteniminium-alkylidene (structure C) contributions should be taken into account. As an example, in the X-ray structure of E-29 (R = Xyl, R' = SiMe3, R'' = H),
the C1N, C1C2 and C2C3 bond lengths are 1.320(7), 1.406(8) and 1.412(8) Å, respectively.Error: Reference source not found The double bond character of the C1N interaction is increased with respect to the parent aminoalkylidyne 1, as witnessed by the related IR wavenumber values in CH2Cl2 solution [e.g.,CÓN = 1604 cm-1 in 1, R = Me;Error: Reference source not found CÓN = 1676 cm-1 in 31, R = Me Error: Reference source not found]. The resonance forms B and C finds support
in the alkylidene nature of C1 and C3, which is manifested in the 13C NMR spectra [(C1) = 230-235 ppm; (C3) = 180-220 ppm]. The -1:3–coordination mode of the vinyliminium ligand in 29 is rather rare in the literature, being previously reported only for triruthenium and triosmium clusters.67 Otherwise, vinyliminium fragments can be found coordinated in a few polynuclear complexes
through the alkene carbons 68 or via -1:2–fashion.69 The reactivity of all of these cited species has been barely explored.
Figure 1. Resonance forms representing the structure of the bridging C1-C2-C3 ligand in 29.
Three kinds of isomerism may concern 29 (see inset in Scheme 9): 1) isomerism due to head-to-tail insertion of the alkyne; 2) cis-trans isomerism, with reference to the mutual orientation of the Cp ligands; 3) E-Z isomerism, with reference to the stereochemistry of the N-substituents. In general, the insertion reaction is highly regioselective, placing the more hindered R′ substituent far from the iminium moiety, therefore isomerism 1) is rarely observed. Complexes 29 generally exhibit a stable
cis configuration, however trans isomers may be obtained as kinetic products from internal alkynes,
and can be quantitatively converted into the respective cis structures upon thermal treatment.Error: Reference source not found The E-Z isomerism is more common, occurring when R ≠ Me, and the E to Z ratio essentially depends on steric factors. For instance, the encumbered xylyl group (R = Xyl) makes the E form prevalent when R′′ = H, while the Z form becomes almost exclusive when R ′′ ≠ H.
3. Reactivity of vinyliminium complexes. 3.1. Addition of nucleophiles.
The cationic vinyliminium complexes 29 are susceptible to nucleophilic attack due to their net positive charge. The additions of hydride (from NaBH4),Error: Reference source not foundb,70 cyanide (from NBu4CN),71 acetylides (from lithium salts) 72 and dimethylamide (from NHMe2) 73 to those complexes lacking sterically demanding N-bonded groups (R ≠ Xyl) are directed to the C1 carbon in a highly stereo-selective manner, affording 32-34 in good yields (Scheme 10). Hydride
addition is anti respect to the R′′ substituent, whereas other nucleophiles add in the opposite configuration. The bridging -1:3–ligand in 32-34 can be more conveniently described as vinylalkylidene (32, 34) or allylidene (33), depending on the cases. Compared to several analogous diiron and diruthenium complexes,Error: Reference source not foundb,d,Error: Reference source not foundb,74 32-34 are peculiar in that they comprise one or two C1-bound amino substituents. If on the one hand the reactions leading to 32-33 have a general scope, i.e. they are not sensitive to the nature of R′ and R′ ′, on the other hand the synthesis of 34 and the addition to 29 of primary amines, and also ammonia and hydrazine, is observable only in the presence of withdrawing groups on the vinyl moiety. Furthermore, primary amine addition is accompanied by proton loss and N to C2 hydrogen migration, resulting in the formation of the unusual amidine-alkylidene complexes 35 (Scheme 10e).Error: Reference source not found According to NMR experiments employing deuterated ethylamine, the hydrogen migration generating the sp3 C2 centre is nearly stereospecific. Subsequent H migration from C2 to C1 may be observed upon thermal treatment.
Scheme 10. Additions of nucleophiles (in blue; reactions performed in thf solutions) to diiron -vinyliminium complexes. a) R = Me, CH2Ph or 4-C6H4CF3, R′ = Me, CH2OH, CO2Me, SiMe3 or 4-C6H4Me, R′′ = H; R = Me
or CH2Ph, R′ = R′′ = Me, Et, Ph or CO2Me.Error: Reference source not foundb,Error: Reference source not found b) X =
N.Error: Reference source not found R = Me, CH2Ph or 4-C6H4CF3, R′ = CH2OH, CO2Me, SiMe3 or 4-C6H4Me,
R′′ = H; R′ = R′′ = Me, Et, Ph or CO2Me; R = Me or CH2Ph, R′ = R′′ = Me, Ph or CO2Me. c) X = CR′′′.Error:
Reference source not found R = Me, R′ = R′′ = Me or Et, R′′′ = H, Ph, 4-C6H4Me, SiMe3, nBu or C(Me)=CH2; R
= CH2Ph, R′ = R′′ = Me, R′′′ = Ph. d) R = Me, CH2Ph or 4-C6H4OMe, R′ = CO2Me, R′′ = H or CO2Me.Error:
As a further modification, the amine function in 32 (R ≠ Xyl) can be converted into ammonium upon methylation, accompanied by a slight change in the nature of the C1-C2-C3 chain (Scheme 11).75
Scheme 11. Synthesis of ammonium-allylidene compounds. R = Me or CH2Ph, R′ = Et, Ph, CO2Me, SiMe3 or
4-C6H4Me, R′′ = H; R = CH2Ph, R′ = R′′ = Me or CO2Me.Error: Reference source not found
The steric protection exerted towards the iminium moiety by the methyl arms, belonging to the 2,6-dimethylphenyl group (Xyl) in related complexes 29Xyl, has a profound impact on the chemistry.
Thus, additions of hydride Error: Reference source not foundb,Error: Reference source not found and, in some cases, cyanide Error: Reference source not found and acetylides Error: Reference source not found are directed to the C2 carbon to give the bis-alkylidene complexes 37 (Scheme 12a-c). In the 13C NMR spectrum (in CDCl3) of E-37 (R = Xyl, R' = SiMe3, R'' = X = H), the C1, C2 and C3 carbons resonate at 281.3, 67.5 and 160.1 ppm, respectively. The steric, rather than electronic, effect associated to Xyl is evident in that compound 29 with R = 4-C6H4CF3, R′ = 4-C6H4Me and R′′ = H reacts with NaBH4 and NBu4CN according to the pathways a) and b) in Scheme 10.
Nucleophilic attack at the iminium C1 carbon of 29
Xyl is forced when LiBHEt3 (superhydride) is employed in the place of NaBH4,Error: Reference source not found or with an appropriate choice of acetylide and vinyl substituents (Scheme 12d,e).
Dramatic effects on the reactivity of 29Xyl are determined not only by little changes in the electronic
and steric properties of the vinyliminium substituents, but also by the geometry of the Cp ligands. Thus, the trans isomer of 29 (R = Xyl, R' = R'' = Me) undergoes selective addition of acetylides at
C2, accompanied by trans to cis rearrangement (Scheme 12e).Error: Reference source not found Conversely, the parallel reactions with the cis isomer lead to complete cleavage of the carbon-oxygen bond of one CO ligand, yielding compounds 39 (Scheme 12f).76 The scission of carbon monoxide into C and O atoms is a key step of the Fischer–Tropsch process, taking place on a bulk metal catalyst; conversely, the same cleavage within molecular metal complexes has been rarely observed.77 In the present case, it occurs at ambient temperature and is promoted by the likely, initial nucleophilic addition of the acetylide at the carbonyl ligand.
Scheme 12. Additions of nucleophiles (in blue; reactions performed in thf solutions) to diiron -vinyliminium complexes containing xylyl as a N-substituent. a) X = H. R′ = H, Me, nBu, CH
2OH, CO2Me, SiMe3, Ph or
4-C6H4Me, R′′ = H; R′ = R′′ = Me or CO2Me.Error: Reference source not foundb,Error: Reference source not found b) X =
C≡CR′′′. R′ = R′′ = Me (trans-isomer); R′′′ = Ph, 4-C6H4Me or SiMe3.Error: Reference source not found c) X =
C≡N. R′ = R′′ = CO2Me.Error: Reference source not found d) X = H. R′ = Me or CO2Me, R′′ = H; R′ = R′′ = Me
or Et.Error: Reference source not found e) X = C≡CR′′′. R′ = R′′ = Et, R′′′ = H or SiMe3.Error: Reference
source not found f) R′ = R′′ = Me (cis-isomer), R′′′ = H, Ph, 4-C6H4Me, nBu, SiMe3 or C(Me)CH2.Error:
Reference source not found
Continuing the study of the electrophilic behaviour of 29, it became clear soon that the addition of nucleophiles being also good Brönsted bases and/or reducing agents was limited by the acidity of the C2-H function and the tendency of the complexes to undergo mono-electron reduction (vide infra). Notwithstanding, a systematic exploration of the chemistry of 29 allowed to find that, in
correspondence to particular R′ and R′′ substituents, methyl and phenyl anions (from the respective lithium salts) selectively add to one Cp ring presumably affording an intermediate cyclopentadiene complex. Hydrogen migration from the cyclopentadiene to C1, generating a stable vinyl-alkylidene ligand, is driven by the recovery of aromaticity of the C5 ring (Scheme 13).Error: Reference source not found,78 Methylation of 40 (R = Me, R' = R'' = Et, X = Me) by methyl triflate gave an ammonium derivative analogous to 36.
Scheme 13. Addition of lithium alkyls to diiron -vinyliminium complexes. E = Me, R = Me or Xyl, R′ = R′′ = Me, Et or Ph; E = Ph, R = Xyl, R′ = R′′ = Ph.Error: Reference source not found
3.2. Deprotonation reactions.
Vinyliminium complexes 29, resulting from terminal alkynes (R′′ = H), display a Brönsted acidic C2-H function. The deprotonated fragment may be envisaged as three complementary structures (Figure 2).
Figure 2. Resonance forms of the fragment derived from vinyliminium C2-H deprotonation.
A series of complexes 29 undergo deprotonation by sodium hydride at ambient temperature affording the tetranuclear 41 (Scheme 14a),79 which are essentially the result of dimerization of the carbene structure I in Figure 2. The presence of bulkier substituents on the vinyliminium frame disfavours the dimerization process, thus the acetylide compounds 42 are selectively afforded as deprotonation products (Scheme 14b). This is a C−C bond breaking de-insertion reaction which regenerates the aminoalkylidyne moiety (see Scheme 9), resembling the behaviour of other similar
C3-chain bridging structures on diiron complexes.80 It has to be remarked that the terminal acetylide ligand in 42 is rarely obtainable by direct reaction of aminocarbyne compounds with acetylides (see Section 2.2). The iron-trimethylsilylacetylide moiety is susceptible to hydrolysis promoted by alumina filtration, providing an alternative route to iron-acyl species (compare Scheme 14d with Scheme 2d).Error: Reference source not foundb Interestingly, methylation of the acetylide unit [Fe-C≡CR′] does not produce the expected iron vinylidene [Fe=C=CR′(Me)],81 but the vinyliminium complexes 44 via new insertion of MeC≡CR′.Error: Reference source not found
Scheme 14. Vinyliminium deprotonation. a) R = Me or CH2Ph, R' = Me or CO2Me. b) R = Me, CH2Ph or Xyl,
R' = Ph, 4-C6H4Me or SiMe3. c) R = CH2Ph, R' = 4-C6H4Me or SiMe3.Error: Reference source not found d) R
= Xyl, R' = SiMe3.Error: Reference source not foundb
3.3. Reduction.
A limited number of vinyliminium complexes 29 (R′′ = H) is involved in the deprotonation routes shown in Scheme 14. The remaining compounds, bearing different combinations of R and R′, react with sodium hydride (and also other bases such as amines) to give the mononuclear derivatives 46 that maintain the C1-C2-C3 chain (Scheme 15),Error: Reference source not found,Error: Reference source not found through the unusual fragmentation of the diiron skeleton.82 The same outcome is observed when complexes 29 (R′′ ≠ H) are treated with NaH.83 According to recent studies,84 the synthesis of 46 proceeds through the reversible formation of the ferraferrocene radical 45 and is initiated by
one-electron reduction of 29, being viable using a typical reducing agent such as cobaltocene (the reduced iron is formally eliminated as "FeCp").85 Electrochemical investigation on 29 (R = Me, R'
= CH2OH or 4-C6H4Me, R'' = H) revealed one reduction process occurring at ca. 1.3 V in acetonitrile (referred to ferrocene).Error: Reference source not foundb
Scheme 15. Fragmentation of diiron vinyliminium complexes initiated by monoelectron reduction. R = Me or
Xyl, R′ = Me, nBu, CO
2Me or C(Me)2OH, R'' = H;Error: Reference source not found R = Me or Xyl, R′ =
C5H4FeCp (Fc), R'' = H;Error: Reference source not found R = Xyl, R′ = CHMe2, R'' = H;Error: Reference
source not found R = Me or Xyl, R' = R'' = Me, Et or CO2Me.Error: Reference source not founda Intermediate 45: R = Xyl, R′ = Et, R′′ = H.Error: Reference source not found
The serendipitous isolation of 47 (Scheme 16, the allene does not apparently take part to the reaction) might result from the NaH-reduction of the vinyliminium precursor and interception by methanol of an intermediate like 45.86
Scheme 16. Methanol capture along the reductive pathway of a vinyliminium complex.Error: Reference
source not found
3.4. Functionalization of vinyliminium ligands.
The possible deprotonation and reduction (Schemes 14, 15) of 29 open the doors to several structural modifications. This story may start with the consideration that the transient deprotonated vinyliminium frame seems suitable to be captured by appropriate organic molecules, in view of its carbene character (Figure 2, structure I). Clear examples of carbene-isocyanide coupling affording ketenimines are available in the literature.87 Indeed, the reactions of a series of complexes 29 (R′′ =
H) with sodium hydride, in the presence of an isocyanide, afforded the first class of ketenimine-alkylidene complexes (48, Scheme 17a).Error: Reference source not found In the X-ray structure of 48 (R = X = Xyl, R' = Me), the bond lengths within the fragment C2=C=N are 1.339(4) Å (C2=C) and 1.210(4) Å (C=N), while the same group gives rise to a characteristic IR absorption at 1962 cm1 (in CH
2Cl2).Error: Reference source not foundb Two features deserve to be mentioned, having some general validity also for the other functionalization reactions described in the following. First, the vinyliminium precursors of 48 converts into 46-type compounds when allowed to react with NaH in the absence of the isocyanide, suggesting that the hypothesis of a reduction (rather than direct deprotonation) mechanism should not be ruled out trying to explain the formation of 48. The second feature is that the isocyanide-trapping reaction is strongly sensitive to electronic (not steric) factors. In particular, the absence of an aryl group on the vinyliminium nitrogen determines very different outcomes: both Fe-Fe bond cleavage pathways, comprising the incorporation of two isocyanide units, and alkyne de-insertion affording aminocarbyne-isocyanide derivative have been observed following C2-H deprotonation (Scheme 17b,c).Error: Reference source not found
Scheme 17. Activation reactions of isocyanides by deprotonation of vinyliminium complexes.Error:
Reference source not found a) R = Xyl or 4-C6H4OMe, R′ = Me or CO2Me, X = Xyl, tBu, 4-C6H4CN. b) R =
Me, R′ = CO2Me, X = tBu. c) R = Me, R′ = CO2Me, X = Xyl.
The formation of new C2-C bonds has been achieved also by reactions of 29 (R = Xyl, R′ = Me) with NaH in the presence of, respectively, Ph3PCH2, cyclopentadiene, cyclopentene or benzyl bromide.88
Similarly to isocyanides, diazo-compounds N2=C(X)(Y) (X = H, Y = CO2Et; X = Y = Ph) can be incorporated within a bis-alkylidene structure upon C2-H deprotonation, according to a general reaction working also with homologous diruthenium complexes (Scheme 18).89 It should be noted that, although diazoalkanes are in general potential carbene precursors,90 there is a number of reactions where N2=C(X)(Y) are entrapped in molecular structures without N2 release.91 The azine moiety in 51 undergoes selective electrophilic addition to afford the hydrazone-modified vinyliminium 52; the latter compounds undergo hydride addition at the C1 site, as a result of the competition between the steric hindrance at C1 and C2 (see also Scheme 12).
Scheme 18. Synthesis and reactivity of azine-bis-alkylidene complexes.Error: Reference source not found a)
R = Xyl, R′ = Me, nBu; 4-C
6H4Me, CO2Me, X = CO2Et, Y = H; R = R′ = Me, X = CO2Et, Y = H; R = Xyl, R′ =
The reactions of 29 (bearing suitable substituents) with NaH in the presence of carbon disulfide or elemental chalcogens (i.e., S8 or gray Se) yield functionalized zwitterionic vinyliminium complexes (formal attack to structure II in Figure 2). More precisely, the modification of the vinyliminium structure with S=C=S (or the isolobal species S=C=NPh) provides a dithiocarbamato moiety,92 which may work as an organometallic ligand towards various metal fragments (Scheme 19).93 Further ligand derivatization has been achieved by dimethyl acetylenedicarboxylate cycloaddition, followed by protonation to dithiacyclopentene (Scheme 19c) or enyne-like metathesis with a second alkyne affording a dithiafulvene group (Scheme 19d).94
Scheme 19. Synthesis and reactivity of dithiocarboxylate-vinyliminium complexes. a-b) R = Xyl, Me or
4-C6H4OMe, R′ = Me, nBu, 4-C6H4Me or CO2Me; Fp = [FeCp(CO)2]+.Error: Reference source not found,Error: Reference source not found c) R = Xyl or 4-C
6H4OMe, R′ = Me or 4-C6H4Me. d) R = R′ = Me.Error: Reference source not found
Although iminium sulphides can be generated only with encumbered R groups (Scheme 20),95 the synthesis of analogous iminium selenides 58 is quite general. The X-ray structures of Z-58 (R = Xyl, R′ = 4-C6H4Me, E = S; R = Xyl, R′ = Me, E = Se) display an almost pure single C2(sp2)E bond [E = S: 1.736(4) Å; E = Se: 1.899(3) Å].96
Scheme 20. Synthesis and reactivity of chalcogen-functionalized vinyliminium complexes (S from S8, Se
from gray Se). a) E = S: R = Xyl, R′ = Me, CH2OH, nBu, CO2Me or 4-C6H4Me; E = Se: R = Xyl or Me, R′ = Me,
CH2OH, nBu, CO2Me or 4-C6H4Me.Error: Reference source not found b) R = Xyl, R' = 4-C6H4Me, E = S, XY =
HSO3CF3, MeSO3CF3, SiMe3Cl, CH2PhBr, CH2CHCH2I or CH2Cl2; R = Xyl or Me, R' = Me, E = S or Se, XY =
CH2Cl2.Error: Reference source not found c) R = Xyl or Me, R' = Me, CH2OH or nBu, E = S or Se.Error:
Reference source not found d) R = Xyl or Me, R' = Me or 4-C6H4Me, E = S, X = CO2Me; E = Se, X = H or
CO2Me.Error: Reference source not found e) R = Xyl, R' = Me or 4-C6H4Me, E = S.Error: Reference source
not found
On the other hand, H+/O substitution at the C2 carbon starting from 29 has been achieved in one case only, using Me3NO as an oxygen source and affording 63 in 29% yield.Error: Reference source
not found On account of the intermediate single-double C2O bond order [1.254(3) Å], the structure of 63 may be viewed as a zwitterionic/bis-alkylidene hybrid (Figure 3, to be compared with Figure 2).
Figure 3. Resonance forms for the O-derivatized vinyliminium compound 63.Error: Reference source not
found
Complexes 58 comprise a uncommon 3-iminium-1-ene-2-thiolate(selenolate) fragment,97 and exhibit a marked reactivity associated with a substantially naked sulphide/selenide function, stabilized thanks to coordination of the C1-C2-C3 ligand to the diiron structure. Such reactivity includes electrophilic additions, reversible oxidative dimerization (Scheme 20b,c) 98 and coordination to metal fragments.99 It is noteworthy that even dichloromethane is attacked by 58.100 However, the most interesting aspect of the chemistry of 58 is offered by the involvement of the vinyliminium skeleton and eventually one carbonyl ligand in the reactions with alkynes.101 Thus, regio-selective cycloadditions occur in mild conditions to give the thio(seleno)phen-3-amino-2-alkylidene complexes 61 (Scheme 20d). Remarkably, the synthesis of amino-substituted selenophene rings is not a trivial task to organic chemists,102 and the procedure described here consists in the unusual assembly of isocyanide, two alkynes and atomic selenium, on a diiron site. On the other hand, the treatment of S-containing species with two equivalents of methyl propiolate proceeds with H-exchange between the two alkynes and cleavage of one CO, yielding a complex structure resembling 39 (Scheme 20e).
The ferrathiophene complexes 64 are obtained as side-products of the reactions leading to 58, and are originated from S-binding to the C3 carbon along a presumable reductive fragmentation process preserving the C2-hydrogen (see Schemes 21a and 15).Error: Reference source not found A similar pathway occurs when 29 (R = Me) are treated with S8/NaH; in this case, the C1 carbon comes sterically accessible to sulphur, and 65 are selectively formed (Scheme 21b).Error: Reference source not found
Complexes 29 lacking of the C2-H hydrogens undergo fragmentation by tert-butyllithium in the presence of chalcogens to give respectively the 2-ferra-3-amino-selenophenes 66 (analogues to 64) and the ferra-dithiopyran 67 (Scheme 21c,d).103
In summary, the vinyliminium complexes are able to incorporate chalcogen atoms within both dinuclear and mononuclear structures, respectively via de-hydrogenation and fragmentation routes; the different reaction outcomes are finely driven by steric factors associated to the iminium substituents and also the chalcogen size.
Scheme 21. Synthesis of S/Se heterocycles from diiron vinyliminium complexes. a) R = Xyl, R′ = Me or
CO2Me.Error: Reference source not found b) R = Me, R′ = Me, 4-C6H4Me or CO2Me.Error: Reference source
not found c) R = Me, CH2Ph or Xyl, R′ = R′′ = Me, Et, CO2Me or Ph.Error: Reference source not found d) R =
Me, R′ = R′′ = Me, CO2Me or Ph.Error: Reference source not found
Another way to introduce a sulphur group in the vinyliminium frame is by means of H/SPh replacement from 29 (Scheme 22).104,105 The resulting cationic thiophenolate-vinyliminium (68) can be modified by hydride addition into an efficient organometallic N,S-ligand (69), which makes possible the facile construction of heterometallic structures (70).Error: Reference source not found Alternatively to the synthesis of 68, as a result of peculiar electronic/steric effects exerted by R/R′ groups, vinyliminium complexes 29 may react with PhSSPh/NaH affording the aminoalkylidyne 71, via C2-C1 bond cleavage and release of the C2C3 unit (Scheme 22d).Error: Reference source not found,Error: Reference source not found
Scheme 22. Inclusion of the thiophenolate moiety in diiron complexes: synthesis of organometallic
N,S-ligand and alkyne de-insertion. a) R = Xyl, Me or 4-C6H4OMe, R′ = Me or CH2OH.Error: Reference source not
found b) R = Xyl or Me. c) R = Me, R' = Me or CH2OH.Error: Reference source not found d) R = 4-C6H4OMe,
R′ = Me, R′′ = H;Error: Reference source not found R = Xyl, R′ = R′′ = Me.Error: Reference source not found
Alkyl C3-substituents in the cationic complexes 68 are susceptible to sodium hydride deprotonation (Scheme 23).Error: Reference source not foundb The formation of the aminobutadienylidene 72 may be competitive with the reduction pathway leading to monoiron complexes 73, analogous to 46. The uses of potassium tert-butoxide (base) or sodium naphthalenide (reductant) instead of NaH (base and reductant) favour the selective synthesis of 72 and 73, respectively. On the other hand, deprotonation of the hydroxo-methyl group results in cyclizative C-O coupling involving one carbonyl ligand. This reaction affords the zwitterionic vinyliminium species 74 and is reversible upon treatment with triflic acid. It has to be mentioned that the deprotonation of a C2-ethyl substituent has been observed in the reaction of 29 (R = Me, R' = R'' = Et) with NaH.Error: Reference source not founda
Scheme 23. Deprotonation of C3-alkyl substituent and competitive reduction process.Error: Reference
source not foundb a) R = Xyl or 4-C
6H4OMe, R′ = Me. b) R = Xyl or Me, R′ = CH2OH.
A way to functionalize the vinyliminium skeleton with a phosphorous group consists in the capture of trimethylphosphite along the reductive pathway of a restricted series of complexes 29. This reaction is accompanied by PIII to PV oxidation giving a dimethylphosphonate moiety (75), which is finally found as bound to the sterically accessible C1 carbon (Scheme 24).Error: Reference source not founda
Scheme 24. Synthesis of amino-phosphonato-vinyl complexes (R′ = Me, R′′ = H; R′ = R′′ = CO2Me).Error:
Reference source not founda
4. Synthesis of functionalized sandwich complexes.
The bridging vinyliminium ligand in 29 (R = Me) and the vinylalkylidene in 32 are subjected to cyclization reactions with alkynes at high temperatures, affording ferrocenes, phenols and cyclopentadienyl/oxo-cyclohexadienyl sandwich compounds, which are separable by
chromatographic techniques.106 The formation of 76 is the result of [3+2+1] cycloaddition involving the bridging vinyliminium, the alkyne and one carbonyl ligand,Error: Reference source not founda and might be viewed as a dinuclear version of the Dötz reaction (Scheme 25a).107 On the other hand, amino-ferrocenes 77 108 contain a pendant alkynyl chain and are obtained in moderate yields using two equivalents of propargyl alcohol (Scheme 25b).109 Notably, this synthetic procedure allows the access to ferrocenes multi-functionalized exclusively at the (C1-C2-C3)-derived five-membered ring, including N- and S-substituted ferrocenes, which are quite rare.110 Furthermore, the vinyl-alkylidene complex 32 (R = Me, R' = Fc = ferrocenyl, R'' = H) Error: Reference source not found has been exploited as a starting material to prepare bis-ferrocenes (upon reaction with a series of terminal alkynes) and a ter-ferrocene by reaction with ethynylferrocene.111 This approach represents a valid alternative to more elaborate synthetic routes to 1,3-ter-ferrocenes.112
Scheme 25. Synthesis of sandwich iron complexes by cyclization reactions involving the vinyliminium ligand.
a) R' = SiMe3, R'' = H.Error: Reference source not found b) R' = SiMe3, 4-C6H4Me or Fc, R'' = H; R' = R'' =
Me; R' = Me, R'' = SPh.Error: Reference source not found
Conclusions
Aminoalkylidyne diiron complexes are accessible in two steps from the commercial [Fe2Cp2(CO)4] and exhibit a rich chemistry upon facile removal of one carbonyl ligand. Thus, the introduction of nitriles, isocyanides and alkynes permits the formation of uncommon structures by coupling with the otherwise unreactive aminocarbyne group. In particular, the carbonyl/alkyne replacement gives entry to cationic vinyliminium complexes, that are substantially the result of unusual isocyanide/alkyne assembly. These robust and versatile compounds (a large variety of substituents
can be incorporated) represent an ideal platform to build a multitude of unique organometallic architectures, by sequential nucleophilic/electrophilic additions and trapping strategies along reduction and deprotonation processes. Remarkably, this plethora of reactions often follow pathways hardly available to conventional organic synthesis and are generally regio- and stereo-selective, being finely regulated by electronic and steric factors. These features are the consequence of the great ability of the cooperating iron centres to assist the various reaction outcomes and to stabilize hydrocarbyl fragments by means of multisite coordination modes. The two iron atoms are like the two hands below a puppet stage that animate and move the bridging carbon(s) in the changing chemical scenarios.
Acknowledgements.
I am particularly grateful to my former PhD supervisor, Prof. Luigi Busetto, for introducing me into this chemistry, and to Prof. Valerio Zanotti and Prof. Stefano Zacchini (all from University of Bologna) for accompanying me along this scientific and life path. Prof. Guido Pampaloni (University of Pisa) is acknowledged for fruitful discussion.
Keywords.
1 a) J. A. Labinger, Inorg. Chim. Acta 2015, 424, 14–19. b) W. J. Kelly, e-EROS Encyclopedia of
Reagents for Organic Synthesis 2001, 1-3. c) T. E. Bitterwolf, Coord. Chem. Rev. 2000, 206-207, 419-450.
2 Selected references: (a) R. Mazzoni, M. Salmi, V. Zanotti, V. Chem. Eur. J. 2012, 18, 10174-10194.
(b) Y. Liu, R. Wang, J. Sun, J. Chen, Organometallics 2000, 19, 3498 - 3506. (c) M. Etienne, J. Talarin, L. Toupet, Organometallics 1992, 11, 2058-2068. (d) C. P. Casey, M. Crocker, P. C. Vosejpka, Organometallics 1989, 8, 278-282. (e) C. P. Casey, M. Crocker, G. P. Niccolai, P. J. Fagan, M. S. Konings, J. Am. Chem. Soc. 1988, 110, 6070-6076. (f) E. L. Heel, G. B. Ansell, S. Leta, Organometallics 1984, 3, 1633-1637.
3 (a) M. Esther García, Daniel García-Vivó, Alberto Ramos, Miguel A. Ruiz, Coord. Chem. Rev.
2017, 330, 1–36. (b) L. Busetto, P. M. Maitlis, V. Zanotti, Coord. Chem. Rev. 2010, 254, 470–486.
4 See for instance: (a) P. Lang, M. Schwalbe, Chem. Eur. J. 2017, 23, 17398-17412. (b) K. P. Chiang,
S. M. Bellows, W. W. Brennessel, P. L. Holland, Chem. Sci. 2014, 5, 267-274. (c) J. P. McInnis, M. Delferro, T. J. Marks, Acc. Chem. Res. 2014, 47, 2545-2557. (d) G.-H. Huang, M. Li, J. Huang, J.-D. Lin, G. J. Chuang, Chem. Eur. J. 2014, 20, 5240-5243. (e) S. Baehr, A. Simonneau, E. Irran, M. Oestreich, Organometallics 2016, 35, 925-928.
5 (a) D. M. Kurtz, E. Boice, J. D. Caranto, R. E. Frederick, C. A. Masitas, K. D. Miner, Encyclopedia
of Inorganic and Bioinorganic Chemistry 2013, 1-18. (b) V. C.-C. Wang, S. Maji, P.-Y. Chen, H. K. Lee, S.-F. Yu, S. I. Chan, Chem. Rev. 2017, 117, 8574-8621. (c) M. Senger, K. Laun, F. Wittkamp, J. Duan, M. Haumann, T. Happe, M. Winkler, U.-P. Apfel, S. T. Stripp, Angew. Chem., Int. Ed. 2017, 56, 16503-16506. (d) B. May, L. Young, A. L. Moore, Biochem. Soc. Trans. 2017, 45, 731-740.
6 [FeFe]-Hydrogenase, K. D Swanson, D. O. Ortillo, J. B. Broderick, J. W. Peters, Encyclopedia of
Inorganic and Bioinorganic Chemistry, 2011, Wiley, 2nd edition.
7 (a) I. Bauer, H. J. Knolker, Chem. Rev. 2015, 115, 3170-3387. (b) A. Fürstner, ACS Cent. Sci. 2016,
2, 778. (c) Topics in Organometallic Chemistry 50, Iron Catalysis II, ed. Springer, Ed. E. Bauer, 2016. (d) S. Enthaler, K. Junge, M. Beller, Angew. Chem. Int. Ed. 2008, 47, 3317-3321. (e) E. Bisz, M. Szostak, ChemSusChem 2017, 10, 3964-3981. (f) T. Bleith, H. Wadepohl, L. H. Gade, J. Am. Chem. Soc. 2015, 137, 2456−2459.
8 Selected recent references: (a) S. Fukuzumi, Y.-M. Lee, W. Nam, Coord. Chem. Rev. 2018, 355, 54–
73. (b) H. Takeda, C. Cometto, O. Ishitani, M. Robert, ACS Catal. 2017, 7, 70−88. (c) J. W. Comerford, I. D. V. Ingram, M. North, X. Wu, Green Chem. 2015, 17, 1966-1987. (d) J. R. Carney, B. R. Dillon, S. P. Thomas, Eur. J. Org. Chem. 2016, 3912–3929. (e) M. D. Kärkäs, B. Åkermark, Dalton Trans. 2016, 45, 14421–14461. (f) B. M. Hunter, H. B. Gray, A. M. Müller, Chem. Rev. 2016, 116, 14120 – 14136. (g) B. Su, Z.-C. Cao, Z.-J. Shi, Acc. Chem. Res. 2015, 48, 886−896.
9 Selected references: (a) A. J. Jasniewski, L. Que, Chem. Rev., 2018, 118, 2554–2592. (b) A.
Trehoux, J.-P. Mahy, F. Avenier, Coord. Chem. Rev. 2016, 322, 142-158. (c) V. Artero, G. Berggren, M. Atta, G. Caserta, S. Roy, L. Pecqueur, M. Fontecave, Acc. Chem. Res. 2015, 48, 2380-2387. (d) M. Y. Darensbourg, R. D. Bethel, Nat. Chem. 2012, 4, 11-13. (e) S. Friedle, E. Reisner, S. J. Lippard,
Chem. Soc. Rev. 2010, 39, 2768-2779.
10 (a) F. Arrigoni, L. Bertini, L. De Gioia, A. Cingolani, R. Mazzoni, V. Zanotti, G. Zampella, Inorg.
Chem. 2017, 56, 13852-13864. (b) R. Mazzoni, A. Gabiccini, C. Cesari, V. Zanotti, I. Gualandi, D. Tonelli, Organometallics 2015, 34, 3228−3235.
11 See for instance: (a) J. He, C.-L. Deng, Y. Li, Y.-L. Li, Y. Wu, L.-K. Zou, C. Mu, Q. Luo, B. Xie, J.
Wei, Organometallics 2017, 36, 1322-1330. (b) P. Tong, D. Yang, Y. Li, B. Wang, J. Qu, Organometallics 2015, 34, 3571−3576. c) Y. Chen, L. Liu, Y. Peng, P. Chen, Y. Luo, J. Qu, J. Am. Chem. Soc. 2011, 133, 1147-1149. (d) S. Doherty, M. R. J. Elsegood, W. Clegg, M. F. Ward, M. Waugh, Organometallics 1997, 16, 4251-4253. (e) A. Boni, T. Funaioli, F. Marchetti, G. Pampaloni, C. Pinzino, S. Zacchini, Organometallics 2011, 30, 4115-4122.
12 Phosphinidene-Bridged Complexes: (a) M. A. Alvarez, M. E. García, R. González, M. A. Ruiz,
Organometallics 2013, 32, 4601-4611. (b) M. A. Alvarez, M. E. García, R. González, M. A. Ruiz, Dalton Trans. 2012, 41, 14498-14513. (c) M. A. Alvarez, M. E. García, R. González, M. A. Ruiz, Organometallics 2010, 29, 5140–5153.
13 (a) S. Willis, A. R. Manning, J. Chem. Soc., Dalton Trans. 1981, 322-324. (b) S. Willis, A. R.
Manning, F. S. Stephens, J. Chem. Soc., Dalton Trans. 1979, 23-27.
14 G. Agonigi, M. Bortoluzzi, F. Marchetti, G. Pampaloni, S. Zacchini, V. Zanotti, Eur. J. Inorg. Chem.
2018, 960–971.
15 A. J. L. Pombeiro, F. C. Guedes da Silva, R. A. Michelin, Coord. Chem. Rev. 2001, 218, 43–74. 16 (a) K. Boss, C. Dowling, A. R. Manning, D. Cunningham, P. McArdle, J. Organomet. Chem. 1999,
579, 252. (b) G. Cox, C. Dowling, A. R. Manning, P. McArdle, D. Cunningham, J. Organomet. Chem. 1992, 438, 143-158.
17 (a) W.-S. Ojo, F. Y. Pétillon, P. Schollhammer, J. Talarmin, Organometallics 2008, 27, 4207–4222.
(b) N. Cabon, E. Paugam, F. Y. Petillon, P. Schollhammer, J. Talarmin, K. W. Muir, Organometallics 2003, 22, 4178-4180. (c) W. S. Ojo, E. Paugam, F. Y. Pétillon, P. Schollhammer, J. Talarmin, K. W. Muir, Organometallics, 2006, 25, 4009-4018.
18 D. L. DeLaet, P. E. Fanwick, C. P. Kubiak, Organometallics 1986, 5, 1807.
19 (a) M. A. Alvarez, M. E. Garca, M. A. Ruiz, M. F. Vega, Dalton Trans. 2011, 40, 8294-8297. (b) M.
A. Alvarez, M. E. Garcia, D. Garcia-Vivo, M. A. Ruiz, M. F. Vega, Organometallics 2013, 32, 4543-4555.
20 K. R. Grundy, K. N. Robertson, Organometallics 1983, 2, 1736-1742.
21 Y.-C. Shi, Q. Fu, Z. Anorg. Allg. Chem.
2013, 639, 1791–1794.
22 (a) M. Knorr, I. Jourdain, A. S. Mohamed, A. Khatyr, S. G. Koller, C. Strohmann, J. Organomet.
Chem. 2015, 780, 70-85. (b) J. H. Davis Jr., C. M. Lukehart, L. A. Sacksteder, Organometallics 1987, 6, 50-55. (c) M. Knorr, C. Strohmann, Eur. J. Inorg. Chem. 1998, 495-499.
24 V. G. Albano, L. Busetto, C. Castellari, M. Monari, A. Palazzi, V. Zanotti, J. Chem. Soc., Dalton
Trans. 1993, 3661-3666.
25 A different class of diiron amino-alkylidene complexes, [Fe
2Cp2(CO)3{C(NR2)X}] (R= phenyl or
alkyl group, X = H or CN), can be obtained by amination of thio-alkylidene precursors [Error: Reference source not found].
26 (a) V. Zanotti, S. Bordoni, L. Busetto, L. Carlucci, A. Palazzi, R. Serra, V. G. Albano, M. Monari, F.
Prestopino, F. Laschi, P. Zanello, Organometallics 1995, 14, 5232-5241. (b) V. G. Albano, S. Bordoni, D. Braga, L. Busetto, A. Palazzi, V. Zanotti, Angew. Chem. Int. Ed. 1991, 30, 847-849.
27 For a clear definition of the terms “carbene” and “alkylidene” see: R. Peloso, E. Carmona, Coord.
Chem. Rev. 2018, 355, 116–132.
28 (a) V. G. Albano, L. Busetto, C. Camiletti, C. Castellari, M. Monari, V. Zanotti, J. Chem. Soc.,
Dalton Trans. 1997, 4671–4676. (b) L. Busetto, F. Marchetti, S. Zacchini, V. Zanotti, Inorg. Chim. Acta 2005, 358, 1469–1484.
29 Selected references on the reactivity of diiron carbyne complexes: (a) C. P. Casey, M. W. Meszaros,
R. E. Colborn, D. M. Roddick, W. H. Miles, M. A. Gohdes, Organometallics 1986, 5, 1879-1886. (b) C. P. Casey, P. J. Fagan, W. H. Miles, J. Am. Chem. Soc. 1982, 104, 1134-1136. (c) C. P. Casey, L. K. Woo, J. Organomet. Chem. 1986, 328, 161-168. (d) C. P. Casey, E. A. Austin, J. Am. Chem. Soc. 1988, 11, 7106-7113.
30 L. Busetto, F. Marchetti, S. Zacchini, V. Zanotti, Eur. J. Inorg. Chem. 2005, 3250–3260
31 (a) M. Landman, R. Pretorius, B. E. Buitendach, P. H. van Rooyen, J. Conradie, Organometallics
2013, 32, 5491-5503. (b) J. L. M. Dillen, M. M. van Dyk, S. Lotz, J. Chem. Soc., Dalton Trans. 1989, 2199-2203. (c) P. D. Woodgate, H. S. Sutherland, C. E. F. Rickard, J. Organomet. Chem. 2001, 626, 199-220.
32 See for instance: (a) M. A. Alvarez, M. E. García, D. García-Vivó, E. Huergo, M. A. Ruiz, Inorg.
Chem. 2018, 57, 912−915. (b) X. Shen, H. Huo, C. Wang, B. Zhang, K. Harms, E. Meggers, Chem. Eur. J. 2015, 21, 9720-9726. (c) P. G. Edwards, B. M. Kariuki, P. D. Newman, W. Zhang, Organometallics 2014, 33, 5440-5447. (d) R. Wang, Z. Zheng, J. Am. Chem. Soc. 1999, 121, 3549-3550. (e) S. A. R. Knox, F. Marchetti, J. Organomet. Chem. 2007, 692, 4119–4128.
33 (a) M. Akita, R. Hua, S. A. R. Knox, Y. Moro-oka, S. Nakanishi, M. I. Yates, Chem. Commun. 1997,
51-52. (b) J. N. L. Dennett, S. A. R. Knox, J. P. H. Charmant, A. L. Gillon, A. G. Orpen, Inorg. Chim. Acta 2003, 354, 29-40. (c) S. A. R. Knox, J. Clu. Sci. 1992, 3, 385-396. (d) A. Boni, F. Marchetti, G. Pampaloni, S. Zacchini, Dalton Trans. 2010, 39, 10866–10875.
34 V. G. Albano, L. Busetto, M. Monari, V. Zanotti, J. Organomet. Chem. 2000, 606, 163–168
35 (a) L. Busetto, F. Marchetti, S. Zacchini, V. Zanotti, J. Braz. Chem. Soc. 2003, 14, 902-907. (b) V.
G. Albano, L. Busetto, F. Marchetti, M. Monari, S. Zacchini, V. Zanotti, Z. Naturforsch. 2007, 62b, 427-438.
36 L. Busetto, F. Marchetti, S. Zacchini, V. Zanotti, Inorg. Chim. Acta 2005, 358, 1204–1216.
38 (a) V. G. Albano, L Busetto, F. Marchetti, M. Monari, V. Zanotti, J. Organomet. Chem. 2002, 649,
64–69. (b) S. D. Ittel, C. A. Tolman, A. D. English, J. P. Jesson, J. Am. Chem. Soc. 1978, 100, 7577– 7585.
39 (a) M. E. Evans, T. Li, W. D. Jones, J. Am. Chem. Soc. 2010, 132, 16278-16284. (b) Y.-H. Chang,
K. Takeuchi, M. Wakioka, F. Ozawa, Organometallics 2015, 34, 1957−1962. (c) A. M. Oertel, V. Ritleng, M. J. Chetcuti, L. F. Veiros, J. Am. Chem. Soc. 2010, 132, 13588–13589.
40 J. Bader, B. Neumann, H.-G. Stammler, N. Ignat'ev, B. Hoge, Chem. Eur. J. 2018, 24, 6975-6982. 41 W. J. Evans, J. R. Walensky, J. W. Ziller, Organometallics 2010, 29, 945–950.
42 (a) L. Cuesta, D. Morales, J. Pérez, D. Miguel, New J. Chem. 2008, 32, 917–919, and references
therein. (b) H.-D. Vu, J. Renault, T. Roisnel, C. Robert, P. Jéhan, N. Gouault, P. Uriac, Eur. J. Org. Chem. 2015, 4868–4875.
43 V. G. Albano, S. Bordoni, L. Busetto, F. Marchetti, M. Monari, V. Zanotti, J. Organomet. Chem.
2003, 684, 37-43.
44 L. Busetto, F. Marchetti, S. Zacchini, V. Zanotti, E. Zoli, J. Organomet. Chem. 2005, 690, 1959–
1970.
45 L. Busetto, F. Marchetti, S. Zacchini, V. Zanotti, Eur. J. Inorg. Chem. 2004, 1494-1504. 46 F. Marchetti, S. Zacchini, V. Zanotti, Organometallics 2014, 33, 3990-3997.
47 F. Marchetti, S. Zacchini, V. Zanotti, Chem. Commun. 2015, 51, 8101-8104.
48 (a) V. P. Boyarskiy, N. A. Bokach, K. V. Luzyanin, V. Yu. Kukushkin, Chem. Rev. 2015, 115,
2698-2779. (b) R. A. Michelin, A. J. L. Pombeiro, M. F. C. Guedes da Silva, Coord. Chem. Rev. 2001, 218, 75–112.
49 F. Marchetti, S. Zacchini, V. Zanotti, Eur. J. Inorg. Chem. 2016, 4820–4828.
50 (a) H. G. Raubenheimer, Dalton Trans. 2014, 43, 16959–16973. (b) D. I. Bezuidenhout, S. Lotz, D.
C. Liles, B. van der Westhuizen, Coord. Chem. Rev. 2012, 256, 479-524.
51 F. Marchetti, S. Zacchini, V. Zanotti, Organometallics 2015, 34, 3658−3664.
52 Recent references include: (a) Z. Hu, J. Dong, Y. Men, Z. Lin, J. Cai, X. Xu, Angew. Chem. Int. Ed.
2017, 56, 1805-1809. (b) A. S. Gowda, A. Baur, C. A. Scaggs, J. L. Petersen, J. M. Hoover, Organometallics 2016, 35, 3720−3727. (c) K. Altenburger, P. Arndt, L. Becker, F. Reiß, V. V. Burlakov, A. Spannenberg, W. Baumann, U. Rosenthal, Chem. Eur. J. 2016, 22, 9169-9180.
53 L. Busetto, M. Salmi, S. Zacchini, V. Zanotti, J. Organomet. Chem. 2008, 693, 57–67.
54 L. Busetto, F. Marchetti, M. Salmi, S. Zacchini, V. Zanotti, Eur. J. Inorg. Chem. 2008, 2437–2447. 55 L. Busetto, F. Marchetti, M. Salmi, S. Zacchini, V. Zanotti, J. Organomet. Chem. 2007, 692, 2245–
2252.
56 (a) A. F. Dyke, S. A. R. Knox, P. J. Naish, G. E. Taylor, J. Chem. Soc., Dalton Trans. 1982,
1297-1307. (b) A. Boni, T. Funaioli, F. Marchetti, G. Pampaloni, C. Pinzino, S. Zacchini, J. Organomet. Chem. 2011, 696, 3551-3556.
57 M. Elian, M. M.-L. Chen, D. M. P. Mingos, R. Hoffmann, Inorg. Chem. 1976, 15, 1148–1155. 58 F. Marchetti, M. Monari, S. Zacchini, V. Zanotti, Organometallics 2016, 35, 2630−2637.
59 V. G. Albano, L. Busetto, F. Marchetti, M. Monari, S. Zacchini, V. Zanotti, Organometallics 2007,
26, 3448-3455.
60 S. A. R. Knox, B. R. Lloyd, D. A. V. Morton, A. G. Orpen, M. L. Turner, G. Hogarth, Polyhedron
1995, 14, 2723-2743.
61 V. G. Albano, L. Busetto, F. Marchetti, M. Monari, S. Zacchini, V. Zanotti, Organometallics 2003,
22, 1326-1331.
62 (a) V. G. Albano, L. Busetto, F. Marchetti, M. Monari, S. Zacchini, V. Zanotti, J. Organomet. Chem.
2004, 689, 528–538. (b) V. G. Albano, L. Busetto, F. Marchetti, M. Monari, S. Zacchini, V. Zanotti, J. Organomet. Chem. 2005, 690, 837–846
63 L. Busetto, R. Mazzoni, M. Salmi, S. Zacchini, V. Zanotti, J. Organomet. Chem. 2010, 695,
2519-2525.
64 L. Busetto, F. Marchetti, S. Zacchini, V. Zanotti, Organometallics 2008, 27, 5058–5066. 65 L. Busetto, F. Marchetti, S. Zacchini, V. Zanotti, J. Organomet. Chem. 2006, 691, 2424–2439. 66 R. D. Adams, G. Chen, L. Chen, J. Yin, Organometallics 1993, 12, 2644-2651.
67 (a) M. R. Churchill, C. H. Lake, R. A. Lashewycz-Rubycz, H. Yao, R. D. McCargar, J. B. Keister, J.
Organomet. Chem. 1993, 452, 151-160. (b) S. Aime, D. Osella, A. J. Deeming, A. J. Arce, M. B. Hursthouse, H. M. Dawes, J. Chem. Soc., Dalton Trans. 1986, 1459-1463. (c) S. Aime, D. Osella, A. J. Arce, A. J. Deeming, M. B. Hursthouse, A. M. R. Galas, J. Chem. Soc., Dalton Trans. 1984, 1981-1986.
68 (a) S. Zhang, Q. Xu, J. Sun, J. Chen, Chem. Eur. J. 2003, 9, 5111-5122. (b) S. Zhang, Q. Xu, J. Sun,
J. Chen, Organometallics 2002, 21, 4572–4574. (c) S. M. Breckenridge, A. J. Carty, M. A. Pellinghelli, A. Tiripicchio, E. Sappa, J. Organomet. Chem. 1994, 471, 211-219. (d) T. E. Snead, C. A. Mirkin, K.-L. Lu, S. T. Nguyen, W.-C. Feng, H. L. Beckman, G. L. Geoffroy, A. L. Rheingold, B. S. Haggerty, Organometallics 1992, 11, 2613–2622. (e) R. D. Adams, G. Chen, Y. Chi, Organometallics 1992, 11, 1473–1479.
69 C. Rosenberger, N. Steunou, S. Jeannin, Y. Jeannin, J. Organomet. Chem. 1995, 494, 17-35.
70 V. G. Albano, L. Busetto, F. Marchetti, M. Monari, S. Zacchini, V. Zanotti, Organometallics 2004,
23, 3348-3354.
71 V. G. Albano, L. Busetto, F. Marchetti, M. Monari, S. Zacchini, V. Zanotti, J. Organomet. Chem.
2006, 691, 4234–4243.
72 L. Busetto, F. Marchetti, S. Zacchini, V. Zanotti, Eur. J. Inorg. Chem. 2007, 1799–1807. 73 F. Marchetti, S. Zacchini, V. Zanotti, Organometallics 2018, 37, 107−115.
74 (a) M. Akita, R. Hua, S. Nakanishi, M. Tanaka, Y. Moro-oka, Organometallics 1997, 16, 5572–
5584. (b) J. A. K. Howard, S. A. R. Knox, N. J. Terrill, M. I. Yates, J. Chem. Soc., Chem. Commun. 1989, 640-642. (c) P. Q. Adams, D. L. Davies, A. F. Dyke, S. A. R. Knox, K. A. Mead, P. Woodward, J. Chem. Soc., Chem. Commun. 1983, 222-224. (d) R. E. Colborn, A. F. Dyke, B. P. Gracey, S. A. R. Knox, K. A. Macpherson, K. A. Mead, A. G. Orpen, J. Chem. Soc., Dalton Trans. 1990, 761-771.
75 V. G. Albano, L. Busetto, F. Marchetti, M. Monari, S. Zacchini, V. Zanotti, J. Organomet. Chem.
