Controlled Dissociation of Fe and Cp from a Diiron Complex with a
Bridging C
3-Ligand Triggered by One Electron Reduction
Gabriele Agonigi,a Gianluca Ciancaleoni,a Tiziana Funaioli,a Stefano Zacchini,b Francesco Pineider,a
Calogero Pinzino,c Guido Pampaloni,a Valerio Zanotti,b and Fabio Marchetti a,*
a University of Pisa, Dipartimento di Chimica e Chimica Industriale, Via G. Moruzzi 13, I-56124 Pisa,
Italy.
b University of Bologna, Dipartimento di Chimica Industriale “Toso Montanari”, Viale Risorgimento
4, I-40136 Bologna, Italy.
Abstract
The one electron reduction of a diiron cationic complex revealed unique features: cleavage of the diiron structure occurred despite a multidentate C3-bridging ligand, and was accompanied by the clean
dissociation of one 5-cyclopentadienyl ring and one iron as isolated units. Thus, the Fe(II)-Fe(II)
-vinyliminium complex [Fe2Cp2(CO)(-CO){-1:3-C3(Et)C2HC1N(Me)(Xyl)}][SO3CF3], [1a]SO3CF3,
reacted with cobaltocene in tetrahydrofuran affording the Fe(II) vinyl-aminoalkylidene [FeCp(CO)C1N(Me)(Xyl)C2HC3(Et)C(=O)], 2a, in 77% yield relative to the C
3 ligand. Analogously,
[FeCp(CO)C1N(Me)(Xyl)C2HC3(CH
2OH)C(=O)], 2b, was obtained in 64% yield from the
appropriate diiron precursor and CoCp2. The formation of 2a is initiated by the one electron reduction
of [1a]+, followed by a reversible intramolecular rearrangement terminating with the irreversible
release of CpH (NMR, GC-MS) and Fe (EPR, magnetometry). The key intermediate Fe(I) ferraferrocene 3 was detected by EPR and IR spectro-electrochemistry, while the related species 3-H-3 was isolated after addition of a hydrogen source, and then identified by X-ray diffraction. A plausible mechanism for the route from [1a]+ to 3 was ascertained by DFT calculations. The dication [1a]2+,
displaying both the carbonyl ligands in terminal positions, and the anion [3] were electrochemically
generated. The functionalized diiron compounds 4 (52% yield) and 5 (62%) were afforded through activation of O2 and S8 by a radical intermediate along the reductive pathway of [1a]+. The reaction of
[Fe2Cp2(CO)(-CO){-1:3-C(SiMe3)CHCN(Me)(Xyl)}][SO3CF3], [1c]SO3CF3, with CoCp2 in
tetrahydrofuran afforded [Fe2Cp2(CCSiMe3)(CO)(μ-CO)μ-CNMe(Xyl)], 6, in 65% yield.
Keywords: diiron complexes; metal-metal cooperativity; vinyliminium ligand; cyclopentadienyl
release; chalcogen trapping.
Dimetal complexes constitute the most simple framework able to provide multisite coordination for the ligands and cooperative effects between the metal centres, thus allowing reactivity patterns which are otherwise not viable on mononuclear species.1 In this setting, diiron complexes have conquered a
central position in sustainable synthetic chemistry, in view of replacing precious elements with non toxic and earth abundant, cost effective counterparts in metal-mediated processes.2 In particular, an
impressive research effort is currently done in the direction of designing suitable diiron complexes able to mimic the relevant biological systems,3 i.e. enzymes based on a fundamental [FeFe]-skeleton.4
Besides, a large variety of derivatives of [Fe2Cp2(CO)4], a milestone of organometallic chemistry,5 are
accessible through replacement of CO ligands and have progressively emerged as convenient and versatile scaffolds to explore unconventional reaction mechanisms and synthetic routes.6,7
Metal-centred electron exchange in dinuclear complexes based on 3d elements often result in the homolytic rupture of the single metal-metal bond in the absence of a tightly bound bridging ligand.8 In
other terms, the fragmentation generally takes place with a distribution of ligands and electrons between the two separating metal fragments.9 For instance, the one electron oxidation/reduction of the
Fe(I) prototype [Fe2Cp2(CO)4] gives rise to single mononuclear products [oxidation: Fe(II); reduction:
Fe(0)], favoured by the fluxionality of the carbonyl ligands.10 As a useful comparison, the 4d and 5d
homologues [M2Cp2(CO)4] (M = Ru,11 Os 12) maintain their dinuclear structure upon one electron
oxidation.
The introduction of a multidentate bridging ligand is often crucial to provide stability to the fundamental metal(3d)-metal(3d) single bond skeleton, when the complexes are involved in electron transfer.13 With specific reference to Fe-Fe compounds, the retention of the dinuclear structure across
different oxidation states is essential to the catalytic behaviour of bio-inspired species.14 Also
enhance the reactivity of the system.15 For instance, the -thiocarbyne cation [Fe
2Cp2(CO)3(μ-CSMe)]+
is subjected to one-electron reduction to form a fairly stable dinuclear radical, and the weakening of the Fe-CO bonds favours the replacement of one carbonyl with various organic groups.Error: Reference source not founda
After many years of investigations on the reactivity of diiron complexes by some of us,16 we became
interested in the electrochemical behaviour of complexes with a vinyliminium C3-ligand coordinated to
the frame [Fe2Cp2(CO)2] via unusual 1:3-fashion.17 Previous reactivity studies have evidenced that this
coordination type is robust but versatile, thus several derived organometallic structures supported on the Fe-Fe core have been obtained.18 We report herein that the vinyliminium three-carbon chain adapts across
different oxidation states and, following reduction, it is incorporated in a stable monoiron structure via unprecedented heterolytic metal-metal cleavage cleanly releasing isolated Fe and Cp units. These results will be discussed with reference to multiple techniques, moreover strategies for vinyliminium functionalization (including O2 activation) will be described.
Results and discussion
1. Synthesis and chemical reduction of diiron vinyliminium complex.
The new µ-vinyliminium compound [1a]SO3CF3 was obtained from Fe2Cp2(CO)4 using previously
reported synthetic protocols (Scheme 1).19,20 The 1H NMR spectrum of [1a]SO
3CF3 in CDCl3 solution
shows a mixture of two isomers, in ca. 3:1 ratio. Based on comparison of the NMR data with those related to a library of compounds,Error: Reference source not found the prevalent isomer displays E configuration of the iminium substituents and cis geometry of the Cp ligands, while the opposite stereochemistry (Z,trans) is adopted in the secondary isomer.21 The DFT calculated structure of
Scheme 1. Four-step synthesis of vinyliminium complex (Xyl = 2,6-C6H3Me2; TfO = CF3SO3). Inset: secondary
isomer.
According to the electrochemical outcomes (vide infra), CoCp2 appeared to be a suitable one electron
reductant for [1a]SO3CF3.22 The reaction was carried out in tetrahydrofuran, and allowed to isolate the
monoiron complex 2a, in 77% yield (relative to the C3 unit), see Scheme 2. We successfully extended
this result to a similar system containing a hydroxyl group (synthesis of 2b from [1b]SO3CF3,Error:
Reference source not foundb Scheme 2).
Scheme 2. Synthesis of monoiron compounds by reaction of diiron vinyliminium complexes with cobaltocene.
A few compounds analogous to 2a-b were obtained in the past, in an attempt to deprotonate the C2-H
two sets of signals attributed to E and Z isomers, while 2b was detected exclusively in the E configuration. The 13C NMR resonance of C1 occurs around 265 ppm, that is typical for an
aminoalkylidene carbon;24 on the other hand, C2 and C3 resonate at ca. 145 and 180 ppm, respectively,
these values indicating alkenyl character, which is manifested also by the low field 1H resonance of C2-H
(ca. 6.5 ppm).
The molecular structure of (E,cis)-2a was ascertained by a X-ray diffraction study, and a view of the structure is shown in Figure 1. The 1-metalla-2-aminocyclopenta-1,3-dien-5-one five-membered ring is essentially coplanar (mean deviation from the Fe(1)-C(1)-C(2)-C(3)-C(4) least-squares plane 0.0264 Å), and the bonding distances are in good agreement with the vinyl-aminoalkylidene nature evidenced by NMR spectroscopy.
Figure 1. Molecular structure of [FeCp(CO)CN(Me)(Xyl)CHC(R)C(=O)], 2a. Displacement ellipsoids are at the 30% probability level. H-atoms have been omitted for clarity. Main bond distances (Å) and angles (°): Fe(1)−C(1) 1.918(3), Fe(1)−C(11) 1.741(3), C(1)-C(2) 1.478(4), C(2)-C(3) 1.342(4), C(3)-C(4) 1.512(4), C(1)-N(1) 1.328(3), Fe(1)−C(4) 1.943(3), C(11)-O(11) 1.154(4), C(4)-O(1) 1.229(3), N(1)-C(8) 1.477(3), C(1)-Fe(1)-C(4) 82.86(11), C(1)-C(2)-C(3) 114.6(2), C(3)-C(4)-Fe(1) 113.68(18), Fe(1)-C(1)-C(2) 115.29(19), C(2)-C(3)-C(4) 113.2(2), Fe(1)-C(11)-O(11) 178.2(3).
The synthesis of 2a-b from [1a-b]+ deserves some comments. Noticeably, it implies the
accommodation of the vinyliminium C3 chain within a peculiar five-membered metallacycle which is
not obtainable starting from monoiron precursors. Conversely, basic aminoalkylidene cyclic structures of the type [FeCp(CO)CN(R)2(CH2)3] were previously prepared via sequential modification of a
cyclobutene ligand coordinated to [FeCp(CO)2]+.25 Moreover, no reduction apparently occurs on going
from [1a-b]+ to 2a-b, since the iron centre in 2a-b retains the +2 oxidation state. In order to shed light
on the mechanism of the unusual C3-ligand rearrangement and to clarify the destiny of the missing
"FeCp" unit (Scheme 2), the reaction of [1a]SO3CF3 with CoCp2 was studied by multiple techniques.
First, this reaction was conducted at 50 °C and monitored by IR and EPR spectroscopy. In principle, the initial CoCp2 to [1a]+ one electron transfer is expected to generate the elusive radical species 1a,
which actually was not experimentally observed (Scheme 3). A view of the DFT optimized structure of
1a and the spin density distribution are shown in Figures S2 and S3. According to the calculations, the
spin density is mainly localized on the C1-bound Fe atom (see SI for details).
The first species which could be readily detected along the pathway to 2a exhibited two IR bands at 1949 and 1554 cm1 (compound 3 in Scheme 3). Although this spectral pattern indicates a monoiron motif resembling that of 2a, the addition to the reaction mixture of an excess of I2, acting as oxidant,
resulted in almost quantitative recovery of [1a]+. This reversibility suggests that the "FeCp" fragment is
still incorporated in 3. On account of these observations and in the light of the X-ray determination of the related compound 3-H-3 (vide infra), 3 was identified to be the diiron radical [FeCp(CO)CN(Me) (Xyl)CHC(Et)COFeCp]. Figure 2 gives a view of the DFT calculated structure of 3.
Figure 2. DFT-optimized structure of [FeCp(CO)CN(Me)(Xyl)CHC(Et)COFeCp], 3. Hydrogens atoms are not
displayed for clarity. Selected bond distances (Å) and angles (°): Fe1C1 = 1.980; Fe1C4 = 2.014; C1N = 1.384; C1C2 = 1.449; C2C3 = 1.435; C3C4 = 1.476; Fe1Fe2 = 2.514; Fe2C1 = 2.122; Fe2C2 = 2.053; Fe2C3 = 2.076; Fe2C4 = 2.254; C4O1 = 1.265; C5O2 = 1.200; NC1C2 = 119.2; C1C2C3 = 116.5.
The computer simulated IR spectrum of 3 well matches the experimental spectrum in THF solution (bands at 1949 and 1554 cm1).26 Moreover, the EPR spectrum (Figure S4) and the calculated spin
Scheme 3. Oxidation and reduction pathways of a diiron μ-vinyliminium cationic complex, and functionalization
reactions. Fe atoms with formal oxidation state ≠ +2 are in green. Red steps: confirmed by spectro-electrochemistry. Blue steps: via electrochemistry only.
The intermediate 3 revealed to be strongly air-sensitive and unstable at ambient temperature, quantitatively converting into 2a during ca. 10 hours. In the course of the 3 to 2a transformation, slight variations in the position of the IR band related to the carbonyl ligand were recorded. However, we
respect to initial [1a]+) of a hydrogen atom source (i.e., H
2O, CH2Cl2 or butylated hydroxytoluene) led
to a clear change in the IR spectrum (formation of 3-H-3, Scheme 3), the carbonyl band shifting from 1949 to 1941 cm.27 The corresponding EPR spectrum matches that of 3, due to the unpaired electron
being substantially localized on iron (Figure S6). Likewise 3, 3-H-3 converts into 2a in tetrahydrofuran at ambient temperature (we propose that 3-H-3 is intermediate along the 3 2a pathway, vide infra). A small amount of 3-H-3 could successfully be isolated using a quick work-up procedure: the cold reaction solution was filtered through a short silica pad to remove impurities, and dried under vacuum. The residue was dissolved in dichloromethane, and few extremely air-sensitive crystals suitable to X-ray analysis were collected by slow diffusion of hexane into this solution at 30 °C. The crystals of
3-H-3 were found in admixture with 2a.
The molecular structure of 3-3 is composed of two symmetry related (by an inversion centre) and H-bonded monomers (3), bearing exactly the same structures. Each monomeric unit is composed of the same five membered ring as that of 2a, that is 5-coordinated to the Fe(2)Cp fragment. In other terms,
the overall structure might be viewed as a functionalized ferraferrocene.28 The O-bonded hydrogen
atom is disordered over the two symmetry related positions and it has been refined with 0.5 occupancy factor.
The monomeric unit of 3-H-3 is shown in Figure 3, while the hydrogen bonded dimer is represented in Figure 4, the related hydrogen bond parameters being: O(1)-H(1) 0.84(2) Å, H(1)···O(1)#1 1.87(4) Å, O(1)···O(1)#1 2.688(7) Å, <O(1)H(1)O(1)#1 166(12)° (symmetry transformation used to generate equivalent atoms: #1 -x,-y,-z+1). The main distances and angles within each monomer are reported in Table 1, where they are compared to the corresponding values for 2a. The Fe(1)-Fe(2) distance (2.5046(10) Å) is typical of a metal-metal bond, as previously reported for many other Fe-Fe bonded diiron carbonyl cyclopentadienide complexes.Error: Reference source not found,29 The Fe(1), C(1),
may be still considered coplanar (mean deviation from the Fe(1)-C(1)-C(2)-C(3)-C(4) least-squares plane 0.0906 Å), even if to a less extent respect to 2a.
Figure 3. Molecular structure of the monomeric unit [FeCp(CO)C1N(Me)(Xyl)C2HC3(Et)COFeCp] within the
H-bonded dimer of 3-H-3. Displacement ellipsoids are at the 30% probability level. H-atoms have been omitted for clarity.
Figure 4. View of the hydrogen bonded dimer 3-3. Displacement ellipsoids are at the 30% probability level.
H-atoms, except H(1) have been omitted for clarity. Symmetry transformation used to generate equivalent atoms: #1 -x,-y,-z+1.
Table 1. Comparative view of selected bond distances (Å) and angles (°) for 2a and 3-H-3.
2a 3-H-3 Fe(1)−C(1) 1.918(3) 2.032(5) Fe(1)−C(4) 1.943(3) 1.979(5) Fe(1)−C(11) 1.741(3) 1.751(6) Fe(1)-Cpav 2.123(7) 2.126(11) Fe(1)-Fe(2) - 2.5046(10) Fe(2)-C(1) - 2.041(5) Fe(2)-C(2) - 2.043(5) Fe(2)-C(3) - 2.033(5) Fe(2)-C(4) - 2.002(5) Fe(2)-Cpav - 2.084(11) C(1)-N(1) 1.328(3) 1.388(6) C(1)-C(2) 1.478(4) 1.437(7) C(2)-C(3) 1.342(4) 1.423(7) C(3)-C(4) 1.512(4) 1.443(7) C(4)-O(1) 1.229(3) 1.312(6) C(11)-O(11) 1.154(4) 1.158(6) C(1)-Fe(1)-C(4) 82.86(11) 82.8(2) Fe(1)-C(1)-C(2) 115.29(19) 110.4(3) C(1)-C(2)-C(3) 114.6(2) 116.1(4)
C(2)-C(3)-C(4) 113.2(2) 114.4(4)
C(3)-C(4)-Fe(1) 113.68(18) 113.0(3)
Fe(1)-C(11)-O(11) 178.2(3) 178.1(4)
A DFT study was carried out to elucidate the mechanism of the intramolecular rearrangement leading from 1a to 3 (Figure 5). In order to save computational resources, the Et and Xyl groups were replaced with H and Me, respectively (model compound: 1am); unless otherwise specified, the following energy values will be referred to 1am. The first likely molecular event is the isomerization of 1am to 1am* (G = +18.9 kcal∙mol). The salient aspect of this transformation is the exchange between C3 and C1 in
the occupancy of the nearly equidistant site between the two Fe atoms (this site is occupied by C3 in
1am as well as in [1a]+, and by C1 in 1am*). A similar “sliding” of bridging vinyliminium ligand over
a diiron frame was previously observed, although not resulting in disruption of the dinuclear structure.30
The transition state (TS1, ΔE╪ = 23.6 kcal∙mol) for the 1am to 1am* modification was located
through an extensive description of the potential energy surface (Figure S7); TS1 evidences a concerted mechanism whereby C1 approaches the distal Fe atom, while the distance between the latter and C3
increases.
Intramolecular attack of C3 to the bridging carbonyl ligand (CO insertion into Fe-C3 bond) is viable in
Figure 5. Calculated pathway for a model reaction, and optimized geometries of intermediate species.
The preliminary variation in the coordination mode of the vinyliminium ligand (from 1am to 1am*) seems necessary to access the final species 3m. Otherwise, direct CO insertion into the Fe-C3 bond of
1am is featured by a more prohibitive activation barrier (33.8 kcal∙mol), and leads to an isomer of 3m
(3mi). This alternative but unlikely pathway has been explored and is presented in details in the Supporting Information (Figure S8).
2. Characterization of fragmentation co-products: NMR, magnetometric and EPR analyses.
The calculated G variation on going from 1a to 3 is slightly positive (5.6 kcal∙mol), thus the
irreversible release of the “FeCp” moiety from 3 is reasonably the driving force allowing the quantitative formation of 2a (Scheme 3). In order to elucidate the destiny of “FeCp”, we carried out different experiments.
The reaction of [1a]SO3CF3 with CoCp2, in deuterated-THF, afforded a final solution whose NMR
analysis pointed out the presence of cyclopentadiene (CpH, but not CpD), in an equimolar amount with respect to 2a (no other Cp signals were recognized). In diluted solutions, cyclopentadiene is expected
to be relatively stable in its monomeric form.31 The formation of cyclopentadiene was confirmed by
GC-MS analysis on the stripped solution (see Figure S9 and Experimental for details).
The yellowish-red fine suspension produced in the reaction mixture [1a]SO3CF3/NaH 32 was dispersed
on a silica matrix, which was repeatedly washed with acetonitrile to remove organic/organoiron species (see Experimental for details). The resulting silica material underwent IR, magnetometric and EPR studies (Figure 6). The IR spectrum was perfectly superimposable on that of blank silica, confirming the absence of organoiron compounds. Although no quantitative information could be extracted from the magnetometric data,33 the magnetisation curves clearly indicate a temperature depending
paramagnetic behaviour, thus ruling out the presence of ferromagnetic Fe0 clusters or highly ordered
spin structures,34 and otherwise suggesting Fe(III)-based oxido-species. Accordingly, the EPR spectrum
(see also Figure S10 and Table S1 for details) evidenced a population of non interacting Fe(III) sites (lines at g ≈ 2.1 and g ≈ 2.4).
In conclusion, although the mechanism of the "FeCp" elimination accompanying the formation of 2a appears difficult to define in detail, it is evident that this process proceeds with the clean and separated release of the Fe and Cp units. According to Scheme 3, we tentatively propose that the radical 3 is prone to capture a hydrogen atom from the reaction medium converting into H-3 (the formation of
3-H-3 is fast adding a suitable H-source, see above); subsequent intramolecular hydrogen migration from
the oxygen atom to the leaving cyclopentadienyl ligand 35 would produce 2a, cyclopentadiene and
atomic iron. Iron is presumably oxidized to Fe(III) at the surface of silica, despite working under inert atmosphere. It has to be noted that the dissociation of a 5-coordinated cyclopentadienyl ligand from a
low valent late transition metal is an exceedingly rare reaction.36 To the best of our knowledge, the
present case is a unique example of straightforward Cp-release from an iron compound in mild conditions.37
Figure 6. Analyses of iron-loaded silica sample. Left: variable temperature magnetization versus applied
magnetic field plots; right: EPR spectrum (a: experimental, 298 K; b: calculated).
3. Electrochemical and IR-SEC studies.
The electrochemical investigation of [1a]SO3CF3 was carried out in THF/[NnBu4]PF6 0.2 M by cyclic
voltammetry at a platinum working electrode and by in situ infrared spectro-electrochemistry in an optically transparent thin-layer electrochemical (OTTLE) cell. The potential of the working electrode was swept between the selected potentials at the scan rate of 1.0 mV∙s1 and a sequence of vibrational
spectra in the 2100-1500 cm-1 region was collected at regular time intervals. By comparison between
the sequences of IR spectra recorded during the slow potential scan and the profile of the i/E curve, the spectra could be separated in groups, each of them attributable to an electron exchange.
The cyclovoltammetric profile shown in Figure 7 exhibits a chemically reversible oxidation at the formal electrode potential E°′ = +0.62 V (ΔEp= 92 mV), and two reductions at E°′ = 1.31 (ΔEp= 120
mV) and E°′ = 1.48 V (ΔEp= 95 mV), respectively.38 The latter are complicated by subsequent
chemical reactions, as shown by the appearance of new oxidation processes during the back scan toward positive potentials, in the second cycle of the voltammetric experiment (red line).
Figure 7. Differential pulse (a) and double cycle (b) voltammograms of [1a]SO3CF3 recorded at a platinum
electrode in 0.2 M [NnBu
4]PF6/THF solutions. Black line = first cycle; red line = second cycle.
The reversible oxidation marked with an asterisk has been attributed to 2a, by comparison with the electrochemical response of a sample of this compound in the same electrolyte (Figure S11).39
The IR spectro-electrochemical in situ experiment confirmed the electrochemical quasi-reversibility of the reduction process and major changes in the [1a]+ structure, in agreement with Scheme 3.
When the potential of the working electrode was progressively decreased from 0.0 to –1.4 V, the CO bands of [1a]+ (1988 and 1807 cm-1) were replaced by a single absorption at 1949 cm-1, due to clean
formation of 3; accordingly, the NC1C2 and aromatic C=C bands (at 1635 and 1587 cm-1, respectively)
were replaced by a N=C1C2=C3 stretching at 1553 cm1 (Figure 8). During the backward oxidation
step, [1a]+ was partially recovered, and new bands characteristic of 2a (1919, 1626 and 1606 cm-1)
Figure 8. IR spectral changes of a THF solution of [1a]SO3CF3 recorded in an OTTLE cell during the progressive
reduction of the potential from 0.0 to -1.4 V. [NnBu
4]PF6 (0.2 mol dm3) as the supporting electrolyte. The
absorptions of the solvent and the supporting electrolyte have been subtracted.
A further change in the IR pattern was observed when the working electrode potential was lowered until 1.9 V (Figure S13), indicating the probable formation of [3]. New bands appeared at 1895, 1671
and 1605 cm1 in correspondence to the second reduction step observed in CV (E°′ = 1.48 V, Figure
7), and were assigned respectively to the terminal carbonyl, the acyl group and the aminoalkylidene moiety of [3]. The DFT calculated structure of [3] is shown in Figure S14, resembling that of 3;
however, slight structural differences can be observed on going from 3 to [3], in particular the
lengthening of the C1-N (from 1.384 to 1.420), Fe1-Fe2 (from 2.514 to 2.578) and C5-O2 bonds (from 1.200 to 1.283), and the shortening of the Fe2-C1 (from 2.122 to 2.007) and Fe2-C4 bonds (2.254 to 2.123).
The anion [3] appears rather chemically stable on considering that, during the reverse oxidation step,
respective backward oxidation steps to 0.0 V following the first and the second reduction process, showed comparable relative intensities of the absorptions related to [1a]+ and 2a (Figure S12) .
Also the oxidation behaviour of [1a]+ was spectro-electrochemically investigated. When the potential
of the working electrode was swept from 0.0 and +0.8 V (Figure 9), during the chemically reversible one electron oxidation of [1a]+, the IR bands typical of [1a]+ were gradually replaced by higher
frequency absorptions [2069 (CO), 1989 (CO), 1656 (NC1C2), 1587 (aromatic C=C) cm-1], attributed to
the dicationic species [1a]2+. The new set of signals indicates a rearrangement of the carbonyl ligands,
both occupying terminal sites in [1a]2+. This stereochemical modification renders the oxidation
electrochemically quasi-reversible, in alignment with the peak to peak separation value provided by the CV experiment (ΔEp = 90 mV). The starting compound [1a]+ was almost completely recovered in the
backward potential scan from +0.8 to 0.0 V, suggesting a certain stability of [1a]2+ on the
spectro-electrochemistry time scale.
Figure 9. IR spectral changes of a THF solution of [1a]SO3CF3 recorded in an OTTLE cell during the progressive
increase of the potential from 0.0 to +0.8 V. [NnBu
The structure of [1a]2+ was optimized by DFT calculations, showing a significant accordance with the
electrochemical studies. In particular, the DFT-optimized geometry bears two terminal carbonyl ligands (Figure 10), and the computed IR wavenumbers are in excellent agreement with the experimental data.40 The movement of one carbonyl ligand from bridging ([1a]+) to terminal position
([1a]2+) allows to minimize electrostatic repulsions, the newly generated positive charge being localized
at the iron atom not directly connected to the iminium moiety (Scheme 3).
Figure 10. DFT-optimized geometry of [1a]2+. Hydrogen atoms omitted for clarity. Selected distances (Å) and
angles (°): Fe1Fe2 = 2.748; Fe1C1 = 1.920; Fe1C3 = 2.102; Fe2C3 = 2.073; C1N = 1.304; C1C2 = 1.424; C2C3 = 1.417; Fe1C4 = 2.501; F2C4 = 1.795; NC1C2 = 133.04; C2C3C4 = 119.41; Fe2C4O1 = 168.03.
4. Trapping of chalcogens by radical species: functionalization of the vinyliminium ligand.
In an attempt to study the reversibility of the [1a]+ ∏ 3 process using an oxidant different from I 2 (see
above), dry air was bubbled into a THF solution of freshly prepared 3, at 50 °C. The IR spectrum of the resulting mixture indicated the minor recovery of [1a]+, and, surprisingly, the generation of a new
4, was isolated in 52% yield (Scheme 3). Compound 4 was obtained in a lower yield when O2 was used
in the place of dry air. The product was purified by alumina chromatography and unambiguously characterized by mass spectrometry, elemental analysis, IR and NMR spectroscopy. The IR and NMR data of 4 closely resemble those of the X-ray characterized compound (Z,cis)-[Fe2{-1:3
-C(Me)C(O)CN(Me)(Xyl)}(-CO)(CO)(Cp)2], obtained in the past via a low yield route, and suggest a
hybrid bis-alkylidene/zwitterionic structure (Scheme 4).Error: Reference source not foundb
Scheme 4. Resonance forms of the O-derivatized vinyliminium compound 4.
The formation of 4 seems the result of 3 to 1a re-conversion, followed by oxygenation of the latter from O2 through C-H bond activation.41 The involvement of traces of water as a O-source should be
excluded since it has been demonstrated that the reaction of 3 with H2O has a different outcome (i.e.,
formation of 3-H-3, see above). The synthetic strategy, leading to functionalization of the vinyliminium ligand with an oxygen atom, was extended to the synthesis of analogous S-derivative.42 Thus, the
reaction of [1a]SO3CF3 with CoCp2 was carried out directly in the presence of elemental sulphur, and
afforded the zwitterionic complex [Fe2{-1:3-C(Et)=C(S)C=N(Me)(Xyl)}(-CO)(CO)(Cp)2], 5, in
about 60% yield (Scheme 3). A similar result (40% yield) was achieved by adding S8 to a solution of
freshly prepared 3. As for 4, the formation of 5 is accompanied by inversion of stereochemical configuration at the iminium moiety (from E to Z). Some complexes analogous to 5 were previously obtained from the reactions of the parent diiron vinyliminium precursors with S and sodium hydride,
syntheses of 4 and 5 are formally deprotonation reactions, proceeding via one electron reduction.Error: Reference source not found In order to deepen this point, we treated [Fe2Cp2(CO)(-CO){-1:3
-C(SiMe3)CHCN(Me)(Xyl)}][SO3CF3], [1c]SO3CF3, with CoCp2 in tetrahydrofuran. Interestingly, this
reaction afforded the aminocarbyne-acetylide complex [Fe2Cp2(CCSiMe3
)(CO)(μ-CO)μ-CNMe(Xyl)], 6, as the main product (Scheme 5). Compound 6 is the deprotonated derivative of [1c]+,
and can be straightforwardly obtained by reaction of [1c]SO3CF3 with NaH.Error: Reference source not
found In conclusion, the one electron reductive mechanism might be involved in all the previously reported NaH-deprotonation reactions of diiron vinyliminium complexes.Error: Reference source not foundb,Error: Reference source not found,43
Scheme 5. Cobaltocene-induced deprotonation of vinyliminium complex.
Conclusions
We have described the redox chemistry of a cationic diiron complex based on the [Fe2Cp2(CO)2] frame,
which is governed by the reactivity of the bridging vinyliminium ligand. In particular, the one electron reduction promotes a straightforward C-C coupling rearrangement retaining the C3 moiety, that is
finally incorporated in a stable five membered metallacycle not accessible from any Fe1 precursor. The
cleavage of the diiron core takes place with the clean release of the 5-Cp ligand and iron as isolated
units. These features do not find correspondence in the known reactivity of dinuclear transition metal complexes that, when involved in electron transfer processes, may be reluctant to undergo metal-metal bond cleavage or rupture with a distribution of ligands and electrons between the two separating metal
fragments. It is remarkable that the dissociation of a cyclopentadienyl ligand from low valent late transition metals is a very rare reaction, and was never observed to date from iron complexes unless under drastic and complicated conditions. The reductive pathway presented herein proceeds through intermediate radical complexes that can be exploited for functionalization purposes. In other words, it is possible to regulate the destiny of reactive radical iron species, some of them being relatively long-lived, turning off the Fe/Cp elimination and obtaining instead diiron complexes with a derivatized C3
-ligand.
Experimental
Materials and methods. All the reactions were routinely carried out under nitrogen atmosphere, using
standard Schlenk techniques. The reaction vessels were oven dried at 140°C prior to use, evacuated (10–2 mmHg) and then filled with nitrogen. Organic reactants (Sigma Aldrich) and CoCp
2 (Strem) were
commercial products of the highest purity available. Compounds [Fe2Cp2(CO)3-CN(Me)(Xyl)]
[SO3CF3] (Xyl = 2,6-C6H3Me2) Error: Reference source not found and [Fe2Cp2(CO)(-CO){-1:3
-C(R)CHCN(Me)(Xyl)}][SO3CF3] (R = CH2OH, 1b; R = SiMe3, 1c) Error: Reference source not foundb were prepared
according to the literature. Solvents were distilled before use under nitrogen from appropriate drying agents. Chromatography separations were carried out under nitrogen on columns of deactivated alumina (Sigma Aldrich, 4% w/w water) or silica (TCI Europe). Infrared spectra of solid samples were recorded on a Perkin Elmer Spectrum One FT-IR spectrometer, equipped with a UATR sampling accessory. Infrared spectra of solutions were recorded on a Perkin Elmer Spectrum 100 FT-IR spectrometer with a CaF2 liquid transmission cell (4000-1000 cm-1 range). NMR spectra were recorded
at 298 K on a Bruker Avance II DRX400 instrument equipped with a BBFO broadband probe. Chemical shifts (expressed in parts per million) are referenced to the residual solvent peaks (1H, 13C).44
experiments.45NMR signals due to a second isomeric form (where it has been possible to detect them)
are italicized. NOE measurements were recorded using the DPFGSE-NOE sequence.21 Magnetometric
experiments were carried out on a Quantum Design (Sacramento, CA, USA) MPMS SQUID magnetometer operating in the 1.8-350 K temperature range with applied field up to 5.0 T, with the powder sample pressed into a pellet under inert atmosphere. All data were corrected for the diamagnetic contribution of silica matrix. EPR spectra were recorded at 298 K on a Varian (Palo Alto, CA, USA) E112 spectrometer operating at X band, equipped with a Varian E257 temperature control unit and interfaced to IPC 610/P566C industrial grade Advantech computer, using acquisition board 46
and software package especially designed for EPR experiments.47 Experimental EPR spectra were
simulated by the WINSIM 32 program.48
Elemental analyses were performed on a Vario MICRO cube instrument (Elementar). GC-MS analyses were performed on a HP6890 instrument, interfaced with MSD-HP5973 detector and equipped with a Phenonex Zebron column. Mass spectrometry analysis was carried out in Flow Injection (FIA) on a triple quadrupole PE Sciex API 365 instrument, equipped with a Turbospray source and interfaced to a HPLC Agilent 1100 system with a binary pump, high pressure mixing and auto-sampling.
Synthesis of [Fe2Cp2(CO)(-CO){-1:3-C3(Et)C2HC1N(Me)(Xyl)}][SO3CF3], [1a]SO3CF3 (Chart 1).
A solution of [Fe2Cp2(CO)2(μ-CO)μ-CNMe(Xyl)]SO3CF3 (400 mg, 0.644 mmol) in acetonitrile (10
mL) was treated with Me3NO (58 mg, 0.772 mmol). The mixture was stirred for 30 minutes, during
which time progressive darkening was observed. The volatiles were removed under vacuum, then the residue was dissolved in dichloromethane (20 mL). An excess of 1-butyne was bubbled into the solution, which was stirred at ambient temperature for 48 h. The final solution was charged on an alumina column. The use of CH2Cl2 and CH2Cl2/THF mixtures were used to elute impurities. A major,
brown band was collected by using neat MeCN as eluent. The solvent was removed under vacuum, thus CH2Cl2 (5 mL) and petroleum ether (30 mL) were added in the order given to the residue. The
obtained dark brown powder was isolated and dried under vacuum. Yield 333 mg, 80%. Anal. Calcd. for C27H28F3Fe2NO5S: C, 50.10; H, 4.36; N, 2.16; S, 4.95. Found: C, 50.17; H, 4.21; N, 2.00; S, 4.83. IR (CH2Cl2): ν/cm = 1999vs (CO), 1813s (CO), 1634m (NC1C2), 1585w (C=C). IR (THF): ν/cm = 1988vs (CO), 1807s (CO), 1635m (NC1C2), 1587w (C=C). 1H NMR (CDCl 3): /ppm = 7.48-7.35, 7.20-7.12, 6.96-6.94 (3 H, C6H3Me2); 5.41, 5.20, 4.72, 4.58 (s, 10 H, Cp); 4.27, 3.88 (m, 2 H, CH2CH3); 4.20, 3.73 (s, 3 H, NMe); 3.95 (s, 1 H, CH); 2.53, 2.30, 2.02, 1.77 (s, 6 H, C6H3Me2); 1.52 (t, 3 H,
CH2CH3). (E,cis)/(Z,trans) ratio 3. 13C{1H} NMR (acetone-d6): /ppm = 254.9 (-CO); 233.8 (C1);
219.8 (C3); 210.7 (CO); 145.2 (ipso-C
6H3Me2); 132.1, 131.3, 129.5, 129.2 (C6H3Me2); 91.2, 87.9 (Cp);
50.7 (C2); 45.7 (NMe); 40.2 (CH
2); 19.3 (CH2CH3); 17.3, 16.3 (C6H3Me2).
Reactions of vinyliminium compounds with CoCp2: synthesis of [FeCp(CO)C1N(Me)
(Xyl)C2HC3(R)C(=O)] (R = Et, 2a, Chart 2; R = CH2OH, 2b, Chart 3). Chart 2. Structure of 2a
[FeCp(CO)C1N(Me)(Xyl)C2HC3(Et)C(=O)], 2a. A solution of [1a]SO
3CF3 (160 mg, 0.247 mmol)
in THF (15 mL) was treated with CoCp2 (68 mg, 0.36 mmol), and the mixture was stirred at ambient
temperature for 18 h. The mixture was then filtered through a short alumina pad. The filtered solution was dried under vacuum; the residue was dissolved in CH2Cl2/OEt2 (1:1 v/v) and charged on an
alumina column. A brown band was collected by using neat dichloromethane as eluent. The product was isolated as a brown solid upon removal of the solvent under vacuum. Yield 72 mg, 77% (relative to Et). Crystals suitable for X-ray analysis were obtained by slow diffusion of hexane into a CH2Cl2
solution of 2a, at 30 °C. Anal. Calcd. for C21H23FeNO2: C, 66.86; H, 6.15; N, 3.71. Found: C, 66.97;
H, 6.30; N, 3.62. IR (CH2Cl2): ν/cm = 1915vs (CO), 1622s (COacyl), 1598m (C1N). IR (THF): ν/cm =
1919vs (CO), 1626s (COacyl), 1606m (C1N). 1H NMR (CDCl3): /ppm = 7.29-7.19 (m, 3 H, C6H3Me2);
6.49 (s, 1 H, C2H); 4.64, 3.99 (s, 5 H, Cp); 3.85, 3.68 (s, 3 H, NMe); 2.40, 2.37, 2.22, 2.12 (s, 6 H,
C6H3Me2); 2.16 (q, 2 H, CH2); 1.13, 0.86 (t, 3 H, CH2CH3). E/Z ratio = 1.8. 13C{1H} NMR (CDCl3):
/ppm = 270.5 (C=O); 266.4 (C1); 221.4 (CO); 179.2, 176.9 (C3); 146.6 (C2); 145.0 (ipso-C
6H3Me2);
134.1, 133.3, 132.7, 132.1, 129.2, 128.9, 128.6, 128.0 (C6H3Me2); 85.2, 84.9 (Cp); 48.8, 44.3 (NMe);
20.8, 20.5 (CH2); 18.4, 17.7, 17.4 (C6H3Me2); 12.4, 12.2 (CH2CH3).
[FeCp(CO)C1N(Me)(Xyl)C2HC3(CH2OH)C(=O)], 2b. This compound was obtained by using a
procedure analogous to that described for 2a, from [Fe2Cp2(CO)(-CO){-1:3
-C3(CH
2OH)C2HC1N(Me)(Xyl)}]SO3CF3, [1b]SO3CF3 (230 mg, 0.354 mmol), and CoCp2 (135 mg,
0.712 mmol). Yield 86 mg, 64% (relative to CH2OH). Anal. Calcd. for C20H21FeNO3: C, 63.34; H, 5.58;
N, 3.69. Found: C, 63.03; H, 5.50; N, 3.78. IR (CH2Cl2): ν/cm = 1922vs (CO), 1602w-sh (C1N),
1579m (COacyl). IR (solid state): ν/cm = 3403br (OH), 1913s (CO), 1603w-m, 1574m. 1H NMR
(dmso-d6): /ppm = 7.27 (m, 3 H, C6H3Me2); 6.55 (s, 1 H, C2H); 4.82 (br, 1 H, OH); 4.63 (s, 5 H, Cp);
4.07, 3.92 (dd, 2J
HH = 20 Hz, 2 H, CH2); 3.79 (s, 3 H, NMe); 2.18, 2.05 (s, 6 H, C6H3Me2). 13C{1H}
NMR (dmso-d6): /ppm = 268.6 (COacyl); 262.5 (C1); 222.7 (CO); 176.8 (C3); 146.2 (C2); 145.5
(ipso-C6H3Me2); 132.7, 132.3, 129.5, 129.3, 129.0 (C6H3Me2); 85.3 (Cp); 58.3 (CH2); 49.5 (NMe); 17.7, 17.2
(C6H3Me2).
Study of the reaction of [1a]SO3CF3 with CoCp2.
A) IR and EPR detection of [FeCp(CO)C1N(Me)(Xyl)C2HC3(Et)COFeCp], 3 (Chart 4).
A solution of [1a]SO3CF3 (120 mg, 0.185 mmol) in THF (12 mL), cooled to ca. 50 °C, was treated
with CoCp2 (51 mg, 0.270 mmol). The mixture was stirred at ca. 50 °C. After 10 minutes, the IR
spectrum of the red solution clearly evidenced the consumption of the starting material and the presence of two new bands at 1949 (vs, CO) and 1554 (m, N=C1C2=C3) cm1, attributed to 3. An aliquot
of the same solution was analyzed by EPR spectroscopy (Figures S4-S5). Quantitative conversion of 3 into 2a was achieved after overnight stirring at ambient temperature. The addition of I2 (60 mg, 0.236
mmol) to the cooled solution of 3 resulted in the clean regeneration of the cation [1a]+ after 10 minutes,
according to IR spectroscopy.
B) Isolation and characterization of 3-H-3 (Chart 5). Chart 5. Structure of 3-H-3
A solution of compound 3 at 50 °C, obtained with the procedure described above from [1a]SO3CF3
(0.15 mmol), was added of an excess of H2O (ca. 2 mmol). The resulting mixture was stirred for
additional 20 minutes at 50 °C (an aliquot of the solution was analyzed by EPR spectroscopy, see Figure S6), then it was quickly passed through a short silica pad. Removal of the solvent under vacuum afforded an air sensitive, brown solid. Yield 28%. Crystals suitable for X-ray analysis were obtained by slow diffusion of hexane into a CH2Cl2 solution, at 30 °C. Anal. Calcd. for C52H57Fe4N2O4: C, 62.62;
H, 5.76; N, 2.81. Found: C, 61.70; H, 5.91; N, 2.60. IR (THF): ν/cm = 1941vs (CO), 1525w (NC1C2).
IR (CH2Cl2): ν/cm = 1953vs (CO), 1622w (COacyl), 1530m (NC1C2). When a THF solution of 3-H-3
was stirred at ambient temperature, complete disappearance of 3-H-3 was detected by IR in 3-4 hours, and 2a was the prevalent product.
C) NMR and GC-MS identification of cyclopentadiene (CpH).
A solution of [1a]SO3CF3 (78 mg, 0.12 mmol) in THF-d8 (5 mL) was treated with CoCp2 (18 mg, 0.095
mmol), then the mixture was stirred at ambient temperature for one week. NMR spectra were recorded on the final solution. 1H NMR (THF-d
8), 2a: /ppm = 7.27-7.21 (m, 3 H, C6H3Me2); 6.51 (s, 1 H, CH);
4.61 (s, 5 H, Cp), 2.23, 2.10 (s, 6 H, C6H3Me2); 2.06 (q, 2 H, CH2); 0.80 (t, 3 H, CH2CH3). N-Me signal
of 2a covered by non deuterated solvent peak. 1H NMR (THF-d
8), CpH: /ppm = 6.52, 6.44 (m, 4 H,
CH); 2.95 (m, 2H, CH2). 2a/CpH ratio = 1.0. 13C{1H} NMR (THF-d8), CpH: /ppm = 130.7, 130.0
(CH); 39.3 (CH2).Error: Reference source not founda
The volatiles were stripped under vacuum and thus condensed into a Schlenk immersed in liquid nitrogen. Subsequent GC-MS (Figure S9) and NMR analyses of the liquid confirmed the presence of CpH.
2 cm), and the filter was exhaustively washed with acetonitrile. The filtrated solution was dried under vacuum, then IR (in CH2Cl2) and 1H NMR (in CDCl3) analyses of the residue evidenced the presence of
2a as largely prevalent species. The orange-coloured silica material was dried under vacuum, stored in
a sealed glass tube under nitrogen and then analyzed by IR, EPR and magnetometry.
Reaction of [1a]SO3CF3 with CoCp2/air: synthesis of [Fe2{-1:3-C3(Et)C2(O)C1N(Me)(Xyl)}( -CO)(CO)(Cp)2], 4 (Chart 6).
Chart 6. Structure of 4
A solution of 3 in THF was obtained at 50 °C from [1a]SO3CF3 (0.250 mmol), according to the
procedure described above. Dry air was bubbled into the cooled THF solution for 1 minute. The resulting solution was stirred for additional 20 minutes during which time it was allowed to warm to ambient temperature. The IR spectrum recorded on an aliquot of the resulting mixture indicated the presence of 4 in admixture with a minor amount of [1a]SO3CF3. The solution was charged on an
alumina column. Elution with CH2Cl2/THF (3:1 v/v) allowed to collect a green band corresponding to
4. The title product was isolated as an air sensitive green solid upon removal of the solvent under
vacuum. Yield 68 mg, 52%. Anal. Calcd. for C26H27Fe2NO3: C, 60.85; H, 5.30; N, 2.73; O, 9.35. Found:
C, 60.68; H, 5.21; N, 2.78; O, 9.27. IR (CH2Cl2): ν/cm = 1946vs (CO), 1777s (CO), 1548w (C1N). 1H
NMR (CDCl3): /ppm = 7.34-7.24 (m, 3 H, C6H3Me2); 4.94, 4.28 (s, 10 H, Cp); 4.26, 3.66 (m, 2 H,
CH2); 3.44 (s, 3 H, NMe); 2.36, 2.13 (s, 6 H, C6H3Me2); 1.83 (t, 3 H, CH2CH3). 13C{1H} NMR (CDCl3):
/ppm = 270.3 (-CO); 258.0 (C1); 214.1 (CO); 194.2 (C3); 153.8 (C2); 143.9 (ipso-C
134.5, 129.3, 128.5, 128.2 (C6H3Me2); 88.3, 86.7 (Cp); 46.3 (NMe); 42.9 (CH2); 18.1, 17.4 (C6H3Me2);
17.4 (CH2CH3). ESI-MS(+): m/z found 514 [M+H]+, 486 [M+HCO]+, 458 [M+HCO]+.
Reactions of [1a]SO3CF3 with CoCp2/S8: synthesis of [Fe2{-1:3-C3(Et)=C2(S)C1=N(Me)(Xyl)} (-CO)(CO)(Cp)2], 5 (Chart 7).
Chart 7. Structure of 5
S8 (160 mg, 0.624 mmol) and then CoCp2 (116 mg, 0.613 mmol) were added to a solution of
[1a]SO3CF3 (220 mg, 0.340 mmol) in THF (20 mL), at ambient temperature. The resulting mixture was
allowed to stir for 24 h. The final mixture was filtered through a short alumina pad using MeCN as eluent. The filtered solution was dried under vacuum, then the residue was dissolved in CH2Cl2 and the
new solution was charged on an alumina column. A green band corresponding to 5 was separated by using neat THF as eluent. The product was isolated as an air sensitive, green solid upon removal of the solvent under vacuum. Yield 112 mg, 62%. Anal. Calcd. for C26H27Fe2NO2S: C, 59.00; H, 5.14; N,
2.65; S, 6.06. Found: C, 59.10; H, 4.98; N, 2.67; S, 6.21. IR (CH2Cl2): ν/cm = 1961vs (CO), 1790s (CO), 1600w (C1N), 1582w (Xyl). 1H NMR (CDCl 3): /ppm = 7.38-7.23 (3 H, C6H3Me2); 5.03, 3.90 (m, 2 H, CH2); 5.01, 4.34 (s, 10 H, Cp); 3.61 (s, 3 H, NMe); 2.65, 2.12 (s, 6 H, C6H3Me2); 1.85 (t, 3 H, CH2CH3). 13C{1H} NMR (CDCl3): /ppm = 264.6 (-CO); 233.9 (C1); 212.4 (CO); 208.3 (C3); 142.3 (ipso-C6H3Me2); 135.7, 134.8, 129.0, 128.9, 128.6 (C6H3Me2); 110.2 (C2); 89.9, 89.0 (Cp); 45.7 (NMe);
Reaction of [1c]SO3CF3 with CoCp2: formation of [Fe2Cp2(C2C3SiMe3)(CO)(μ-CO) μ-C1NMe(Xyl)], 6 (Chart 8).Error: Reference source not found
Chart 8. Structure of 6
A solution of [Fe2Cp2(CO)(-CO){-1:3-C(SiMe3)CHCN(Me)(Xyl)}]SO3CF3, [1c]SO3CF3 (120 mg,
0.174 mmol), in THF (12 mL) was cooled to 50 °C and then treated with CoCp2 (50 mg, 0.264
mmol). After 5 minutes, the IR spectrum of the solution evidenced the presence of several bands in the region 1700-2050 cm. The mixture was allowed to warm to ambient temperature and stirred for
additional 18 h. The final solution was filtered through a celite pad, then the volatiles were removed under reduced pressure. The residue was washed with diethyl ether (20 mL), and then dried under vacuum, thus affording 6 as an ochre-yellow solid in admixture with minor side products. Yield ca. 55%. IR (CH2Cl2): ν/cm = 2012 (C≡C), 1973vs (CO), 1792s (CO), 1506w (C1N). 1H NMR (CDCl3):
/ppm = 7.35-7.18 (3 H, C6H3Me2); 4.81, 4.30 (s, 10 H, Cp); 4.40 (s, 3 H, NMe); 2.65, 2.23 (s, 6 H,
C6H3Me2); 0.18 (s, 9 H, SiMe3).
Spectro-electrochemical studies. Electrochemical measurements were recorded on a PalmSens4
instrument interfaced to a computer employing PSTrace5 electrochemical software, and performed in THF solutions containing [NnBu
temperature (205 °C). HPLC grade THF (Sigma-Aldrich) was distilled from and stored under argon over 3-Å molecular sieves. Electrochemical grade [NnBu
4]PF6 and FeCp2 (Fluka) were used without
further purification. Cyclic voltammetry was performed in a home-made three-electrode cell, having a platinum-disc working electrode and a platinum-spiral counter electrode, both sealed in a glass tube. A quasi-reference platinum electrode was employed as reference. The cell was pre-dried by heating under vacuum and filled with argon. The Schlenk-type construction of the cell maintained anhydrous and anaerobic conditions. The solution of the supporting electrolyte , prepared under argon, was introduced into the cell and the CV of the solvent was recorded. The analyte was then introduced and the voltammograms were recorded; a small amount of ferrocene was added to the solution and a further voltammogram was repeated. Under the present experimental conditions, the one-electron oxidation of ferrocene occurs at E° = +0.39 V vs SCE.
Infrared (IR) spectroelectrochemical measurements were carried out using an optically transparent thin-layer electrochemical (OTTLE) cell equipped with CaF2 windows, platinum mini-grid working and
auxiliary electrodes and silver wire pseudo-reference electrode.49 During the microelectrolysis
proce-dures, the electrode potential was controlled by a PalmSens4 instrument interfaced to a computer em-ploying PSTrace5 electrochemical software. Argon-saturated THF solutions of the compound under study, containing [NnBu
4]PF6 0.2 M as the supporting electrolyte, were used. The in situ
spectroelectro-chemical experiments were performed by collecting IR spectra at constant time intervals during the ox-idation or reduction obtained by continuously increasing or lowering the initial working potential at a scan rate of 1.0 mV/sec. IR spectra were recorded on a Perkin-Elmer FT-IR 1725X spectrophotometer and UV-vis spectra on a Perkin-Elmer Lambda EZ201 spectrophotometer.
Bruker APEX II diffractometer equipped with a CCD detector using Mo–K radiation. Data were corrected for Lorentz polarization and absorption effects (empirical absorption correction SADABS).50
The structures were solved by direct methods and refined by full-matrix least-squares based on all data using F2.51 Hydrogen atoms were fixed at calculated positions and refined by a riding model. The
asymmetric unit of the unit cell of 3-H-3 contains one 3-H molecule which forms a H-bond with a symmetry related (by an inversion centre) molecule. The O-bonded hydrogen atom has been located in the Fourier difference map and refined isotropically with 0.5 occupancy factor, using the 1.5-fold Uiso
value of the parent O-atom with restrained O–H distance. The O(1)···O(1)#1 distance is 2.688(7) Å (symmetry transformation used to generate equivalent atoms: #1 -x,-y,-z+1). This indicates a H-bond between these two symmetry related molecules. Therefore, it must be assumed that the O-bonded hydrogen atom is disordered over two symmetry related positions and it has been refined with 0.5 occupancy factor. Overall, 3-H and 3 have exactly the same structures, since they are generated by an inversion centre.
Computational studies.
All the geometries were optimized with ORCA 4.0.1.2,52 using the B97 functional53 in conjunction with
a triple-ζ quality basis set (def2-TZVP). The dispersion corrections were taken into account using the Grimme D3-parametrized correction and the with Becke-Jonhson damping to the DFT energy.54All the
structures were confirmed to be local energy minima (no imaginary frequencies) for intermediate species and saddle points (one imaginary frequency) for transition states. In the case of [1a]2+, SCF
convergence was not obtained, therefore the geometry and vibrations were computed with the ADF package (version 2014.09),55 using the same functional, Slater-type triple-ζ quality basis set (TZ2P),
frozen-core approximation and ZORA Hamiltonian to account for scalar relativistic effects (numerical integration grid = 6.0).56
Corresponding Author
*E-mail address: [email protected]. Webpage: http://www.dcci.unipi.it/fabio-marchetti.html
Notes
The authors declare no competing financial interest.
Acknowledgement
We thank the Universities of Bologna and Pisa for financial support.
Supporting Information Available
DFT optimized geometries; details of DFT calculations; EPR spectra; GC-MS spectrum; spectro-electrochemical graphs; NMR spectra. CCDC reference numbers 1855386 (2a) and 1855387 (3-H-3) contain the supplementary crystallographic data for the X-ray studies reported in this paper. These data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: (internat.) +44-1223/336-033; e-mail: [email protected]).
1 Selected references: (a) Lang, P.; Schwalbe, M. Pacman Compounds: From Energy Transfer to
Cooperative Catalysis. Chem. Eur. J. 2017, 23, 17398-17412. (b) Chiang, K. P.; Bellows, S. M.; Brennessel, W. W.; Holland, P. L. Multimetallic cooperativity in activation of dinitrogen at iron– potassium sites. Chem. Sci. 2014, 5, 267-274. (c) McInnis, J. P.; Delferro, M.; Marks, T. J. Previous Article Next Article Table of Contents Multinuclear Group 4 Catalysis: Olefin Polymerization Pathways Modified by Strong Metal–Metal Cooperative Effects. Acc. Chem. Res. 2014, 47, 2545-2557. (d) Huang, G.-H.; Li, J.-M.; Huang, J.-J.; Lin, J.-D.; Chuang, G. J. Cooperative Effect of Two Metals:
CoPd(OAc)4 Catalyzed C-H Amination and Aziridination. Chem. Eur. J. 2014, 20, 5240-5243. (e)‐
Baehr, S.; Simonneau, A.; Irran, E.; Oestreich, M. An Air-Stable Dimeric Ru–S Complex with an NHC as Ancillary Ligand for Cooperative Si–H Bond Activation. Organometallics 2016, 35, 925-928.
2 (a) Bauer, I.; Knolker, H. J. Iron Catalysis in Organic Synthesis. Chem. Rev. 2015, 115, 3170-3387.
(b) Fürstner, A. Iron Catalysis in Organic Synthesis: A Critical Assessment of What It Takes To Make This Base Metal a Multitasking Champion. ACS Cent. Sci. 2016, 2, 778-789. (c) Topics in Organometallic Chemistry 50, Iron Catalysis II, ed. Springer, Ed. E. Bauer, 2016. (d) Enthaler, S.; Junge, K.; Beller, M. Angew. Chem. Int. Ed. 2008, 47, 3317-3321. (e) Bisz, E.; Szostak, M.; ChemSusChem 2017, 10, 3964-3981. (f) Bleith, T.; Wadepohl, H.; Gade, L. H. Iron Achieves Noble Metal Reactivity and Selectivity: Highly Reactive and Enantioselective Iron Complexes as Catalysts in the Hydrosilylation of Ketones. J. Am. Chem. Soc. 2015, 137, 2456−2459.
3 Selected references: (a) Jasniewski, A. J.; Que, L. Dioxygen Activation by Nonheme Diiron
Enzymes: Diverse Dioxygen Adducts, High-Valent Intermediates, and Related Model Complexes. Chem. Rev. 2018, 118, 2554–2592. (b) Trehoux, A.; Mahy, J.-P.; Avenier, F. A growing family of O2 activating dinuclear iron enzymes with key catalytic diiron(III)-peroxo intermediates: Biological systems and chemical models. Coord. Chem. Rev. 2016, 322, 142-158. (c) Artero, V.; Berggren, G.; Atta, M.; Caserta, G.; Roy, S.; Pecqueur, L.; Fontecave, M. From Enzyme Maturation to Synthetic Chemistry: The Case of Hydrogenases. Acc. Chem. Res. 2015, 48, 2380-2387. (d) Darensbourg, M. Y.; Bethel, R. D. Nat. Chem. 2012, 4, 11-13. (e) Friedle, S.; Reisner, E.; Lippard, S. J. Current challenges of modeling diiron enzyme active sites for dioxygen activation by biomimetic synthetic complexes. Chem. Soc. Rev. 2010, 39, 2768-2779.
4 (a) Kurtz, D. M.; Boice, E.; Caranto, J. D.; Frederick, R. E.; Masitas, C. A.; Miner, K. D.
Encyclopedia of Inorganic and Bioinorganic Chemistry 2013, 1-18. (b) Wang, V. C.-C.; Maji, S.; Chen, P.-Y.; Lee, H. K.; Yu, S.-F.; Chan, S. I. Alkane Oxidation: Methane Monooxygenases, Related Enzymes, and Their Biomimetics. Chem. Rev. 2017, 117, 8574-8621. (c) Senger, M.; Laun, K.; Wittkamp, F.; Duan, J.; Haumann, M.; Happe, T.; Winkler, M.; Apfel, U.-P.; Stripp, S. T. Proton-Coupled Reduction of the Catalytic [4Fe4S] Cluster in [FeFe]-Hydrogenases. Angew. Chem., Int. Ed. 2017, 56, 16503-16506. (d) May, B.; Young, L.; Moore, A. L. Structural insights into the alternative oxidases: are all oxidases made equal?. Biochem. Soc. Trans. 2017, 45, 731-740.
van de Weghe, P. Cyclopentadienyliron dicarbonyl dimer: A simple tool for the hydrosilylation of aldehydes and ketones under air. Catal. Commun. 2016, 78, 52–54. (c) Bitterwolf, T. E. Photochemistry and reaction intermediates of the bimetallic Group VIII cyclopentadienyl metal carbonyl compounds, (η5-C5H5)2M2(CO)4 and their derivatives. Coord. Chem. Rev. 2000, 206-207, 419-450.
6 Selected references: (a) He, J.; Deng, C.-L.; Li, Y.; Li, Y.-L.; Wu, Y.; Zou, L.-K.; Mu, C.; Luo, Q.;
Xie, B.; Wei, J. A New Route to the Synthesis of Phosphine-Substituted Diiron Aza- and Oxadithiolate Complexes. Organometallics 2017, 36, 1322-1330. (b) Tong, P.; Yang, D.; Li, Y.; Wang, B.; Qu, J. Hydration of Nitriles to Amides by Thiolate-Bridged Diiron Complexes. Organometallics 2015, 34, 3571−3576. (c) Chen, Y.; Liu, L.; Peng, Y.; Chen, P.; Luo, Y.; Qu, J. Unusual Thiolate-Bridged Diiron Clusters Bearing the cis-HN═NH Ligand and Their Reactivities with Terminal Alkynes. J. Am. Chem. Soc. 2011, 133, 1147-1149. (d) Busetto, L.; Maitlis, P. M.; Zanotti, V. Bridging vinylalkylidene transition metal complexes. Coord. Chem. Rev. 2010, 254, 470–486. (e) Marchetti, F.; Zacchini, S.; Zanotti, V. Carbon monoxide–isocyanide coupling promoted by acetylide addition to a diiron complex. Chem. Commun. 2015, 51, 8101-8104. (f) Doherty, S.; Elsegood, M. R. J.; Clegg, W.; Ward, M. F.; Waugh, M. New Reaction Pathways for μ-η1,η2-Allenyl Ligands: On−Off Allenyl Coordination and CO Insertion into the Hydrocarbyl Bridge in Ru2(CO)6(μ-PPh2){μ-η1,η2α,β-C(Ph)CCPh2}. Organometallics 1997, 16, 4251-4253. (g) Casey, C. P.; Crocker, M.; Vosejpka, P. C. Reactions of organocopper reagents with the cationic bridging acylium complex [C5H5(CO)Fe]2(.mu.-CO)(.mu.-CHCO)+. Organometallics 1989, 8, 278-282. (h) Casey, C. P.; Crocker, M.; Niccolai, G. P.; Fagan, P. J.; Konings, M. S. Formation of bridging acylium and nitrilium complexes by reaction of carbon monoxide and tert-butyl isocyanide with a bridging diiron methylidyne complex. Evidence for strong electron donation from the Fe2C core onto the .mu.-CHC.tplbond.O and .mu.-CHC.tplbond.NR ligands. J. Am. Chem. Soc. 1988, 110, 6070-6076.
7 (a) Alvarez, M. A.; García, M. E.; González, R.; Ruiz, M. A. P–C and C–C Coupling Processes in
the Reactions of the Phosphinidene-Bridged Complex [Fe2(η5-C5H5)2(μ-PCy)(μ-CO)(CO)2] with Alkynes. Organometallics 2013, 32, 4601-4611. (b) Alvarez, M. A.; García, M. E.; González, R.; Ruiz, M. A. Reactions of the phosphinidene-bridged complexes [Fe2(η5-C5H5)2(μ-PR)(μ-CO)(CO)2] (R = Cy, Ph) with electrophiles based on p-block elements. Dalton Trans. 2012, 41, 14498-14513. (c) Alvarez, M. A.; García, M. E.; García-Vivó, D.; Ramos, A.; Ruiz, M. A. Activation of H–H and H–O Bonds at Phosphorus with Diiron Complexes Bearing Pyramidal Phosphinidene Ligands. Inorg. Chem. 2012, 51, 3698-3706.
8 (a) Ellis, J. E.; Flom, E. A. The chemistry of metal carbonyl anions : III. Sodium-potassium alloy:
An efficient reagent for the production of metal carbonyl anions. J. Organomet. Chem. 1975, 99, 263-268. (b) Gladysz, A.; Williams, G. M.; Tam, W.; Lee Johnson, D.; Parker, D. W.; Selover, J. C. Synthesis of metal carbonyl monoanions by trialkylborohydride cleavage of metal carbonyl dimers: a convenient one-flask preparation of metal alkyls, metal acyls, and mixed-metal compounds. Inorg. Chem., 1979, 18, 553–558. (c) Fröhlich, H.-O.; Romhil, W.; Sulfonamidsubstituierte Thionoliganden
Allg. Chem. 1988, 556, 153-160.
9 Callan, B.; Manning, A. R. J. Chem. Soc., Silver(I) salts as one-electron or two-electron oxidants in
their reactions with [Fe2(η-C5H5)2(CO)4–n(CNMe)n] derivatives (n= 0–2). The effect of varying the reaction solvent. Chem. Commun. 1983, 6, 263-264.
10 (a) Duane, B.; Dombek, ; Choi, M.-G.; Angelici, R. J. Dicarbonyl(η5 Cyclopentadienyl)‐ ‐
(Thiocarbonyl)Iron(1+) Trifluoromethane Sulfonate(1–) and Dicarbonyl(η5 Cyclo Pentadienyl)‐ ‐ ‐
[(Methylthio)Thiocarbonyl]Iron. Inorg. Synth. 1990, 28, 186-189. (b) Williams, W. E.; Lalor, F. J. Synthetic applications of the reaction of silver(I) salts with the bis-[dicarbonyl(π-cyclopentadienyl)iron] complex. J. Chem. Soc., Dalton Trans. 1973, 13, 1329-1332. (c) Jiang, X.; Chen, L.; Wang, X.; Long,
L.; Xiao, Z.; Liu, X. Photoinduced Carbon Monoxide Release from Half Sandwich Iron(II) Carbonyl‐
Complexes by Visible Irradiation: Kinetic Analysis and Mechanistic Investigation. Chem. Eur. J. 2015, 21, 13065-13072.
11 Li, S.; Wang, X.; Zhang, Z.; Zhao, Y.; Wang, X. Isolation and structural characterization of a mainly
ligand-based dimetallic radical. Dalton Trans. 2015, 44, 19754–19757.
12 Laws, D. R.; Morris Bullock, R.; Lee, R.; Huang, K.-W.; Geiger, W. E. Comparison of the
One-Electron Oxidations of CO-Bridged vs Unbridged Bimetallic Complexes: One-Electron-Transfer Chemistry of Os2Cp2(CO)4 and Os2Cp*2(μ-CO)2(CO)2 (Cp = η5-C5H5, Cp* = η5-C5Me5). Organometallics 2014, 33, 4716-4728.
13 He, L.-P.; Yao, C.-L.; Naris, M.; Lee, J. C.; Korp, J. D.; Bear, J. L. Molecular structure and chemical
and electrochemical reactivity of Co2(dpb)4 and Rh2(dpb)4 (dpb=N,N'-Diphenylbenzamidinate). Inorg. Chem. 1992, 31, 620-625.
14 Selected references: (a) Senger, M.; Laun, K.; Wittkamp, F.; Duan, J.; Haumann, M.; Happe, T.;
Winkler, M.; Apfel, U.-P.; Stripp, S. T. Proton-Coupled Reduction of the Catalytic [4Fe-4S] Cluster in [FeFe]-Hydrogenases. Angew. Chem. Int. Ed. 2017, 56, 16503-16506. (b) Mebs, S.; Senger, M.; Duan, J.; Jifu, W.; Florian, A.; Ulf-Peter, H.; Happe, T.; Winkler, M.; Stripp, S. T.; Haumann, M. Bridging Hydride at Reduced H-Cluster Species in [FeFe]-Hydrogenases Revealed by Infrared Spectroscopy, Isotope Editing, and Quantum Chemistry. J. Am. Chem. Soc. 2017, 139, 12157-12160. (c) Li, Q.; Lalaoui, N.; Woods, T. J.; Rauchfuss, T. B. Electron-Rich, Diiron Bis(monothiolato) Carbonyls: C–S
Bond Homolysis in a Mixed Valence Diiron Dithiolate. Inorg. Chem. 2018, 57, 4409−4418. (d)
Abul-Futouh,H.; Almazahreh, L. R.; Kamal Harb, M.; Görls,H.; El-khateeb,M.; Weigand,W.
[FeFe]-Hydrogenase H-Cluster Mimics with Various −S(CH2)nS– Linker Lengths (n = 2–8): A Systematic Study. Inorg. Chem., 2017, 56, 10437–10451. (e) Arrigoni, F.; Mohamed Bouh, S.; De Gioia, L.; Elleouet, C.; Pétillon, F. Y.; Schollhammer, P.; Zampella, G. Influence of the Dithiolate Bridge on the Oxidative Processes of Diiron Models Related to the Active Site of [FeFe] Hydrogenases. Chem. Eur. J. 2017, 23, 4364-4372. (f) Boyke, C. A.; Rauchfuss, T. B.; Wilson, S. R.; Rohmer, M.-M.; Bérnard, M. [Fe2(SR)2(μ-CO)(CNMe)6]2+ and Analogues: A New Class of Diiron Dithiolates as Structural Models
D. L. Review of electrochemical studies of complexes containing the Fe2S2 core characteristic of [FeFe]-hydrogenases including catalysis by these complexes of the reduction of acids to form dihydrogen. J. Organomet. Chem. 2009, 694, 2681–2699.
15 (a) Schroeder, N. C.; Angelici, R. J. Synthesis and reactivity of the bridging thiocarbyne radical,
Cp2Fe2(CO)2(.mu.-CO)(.mu.-CSMe).cntdot.. J. Am. Chem. Soc. 1986, 108, 3688-3693. (b) Boni, A.; Funaioli, T.; Marchetti, F.; Pampaloni, G.; Pinzino, C.; Zacchini, S. Synthesis and characterization of (cyclopentadienyl)(2,4-dimethylpentadienyl)cobalt(III) fluoroborate and of the dimeric product resulting from its reduction. Organometallics 2011, 30, 4115-4122. (c) English, R. B.; Haines, R. J.; Nolte, C. R. Reactions of metal carbonyl derivatives. Part XVIII. Synthesis and redox properties of some binuclear derivatives of iron bridged by both carbonyl and alkylthio-groups. J. Chem. Soc., Dalton Trans. 1975, 11, 1030-1033. (d) Alvarez, M. A.; Garcìa, M. E.; Gonzalez, R.; Ramos, A.; Ruiz, M. A. Chemical and Structural Effects of Bulkness on Bent-Phosphinidene Bridges: Synthesis and Reactivity of the Diiron Complex [Fe2Cp2{μ-P(2,4,6-C6H2tBu3)}(μ-CO)(CO)2]. Organometallics 2010, 29, 1875–1878. (e) Bordoni, S.; Busetto, L.; Calderoni, F.; Carlucci, L.; Laschi, F.; Zanello, P.; Zanotti, V. Redox chemistry and substitution reactions of the μ-cyanoalkylidene complexes [Fe2(CO)2(cp)2(μ-CO) {μ-C(CN) (X)}]n+ (n = 0, X = CN, H, Me, SMe, OMe, OEt, OPh, OCH2CH = CH2, PEt2, or NC5H10; n = 1, X = PMe2Ph). J. Organomet. Chem. 1995, 496, 27-35.
16 Selected references: (a) Marchetti, F.; Zacchini, S.; Zanotti, V. Photochemical Alkyne Insertions into
the Iron–Thiocarbonyl Bond of [Fe2(CS)(CO)3(Cp)2]. Organometallics 2016, 35, 2630−2637. (b) Busetto, L.; Marchetti, F.; Mazzoni, R.; Salmi, M.; Zacchini, S.; Zanotti, V. [3+2+1] cycloaddition involving alkynes, CO and bridging vinyliminium ligands in diiron complexes: a dinuclear version of the Dötz reaction?. Chem. Commun., 2010, 46, 3327–3329. (c) Albano, V. G.; Busetto, L.; Marchetti, F.; Monari, M.; Zacchini, S.; Zanotti, V. Photochemical Alkyne Insertions into the Iron−Thiocarbonyl Bond of [Fe2(CS)(CO)3(Cp)2]. Organometallics 2007, 26, 3448-3455. (d) Boni, A.; Marchetti, F.; Pampaloni, G.; Zacchini, S. Cationic Diiron and Diruthenium μ-Allenyl Complexes: Synthesis, X-Ray Structures and Cyclization Reactions with Ethyldiazoacetate/Amine Affording Unprecedented Butenolide- and Furaniminium-Substituted Bridging Carbene Ligands. Dalton Trans., 2010, 39, 10866– 10875.
17 (a) Churchill, M. R.; Lake, C. H.; Lashewycz-Rubycz, R. A.; Yao, H.; McCargar, R. D.; Keister, J.
B.; Structural studies on ruthenium carbonyl hydrides: XVIII. Synthesis and characterization of XCCRCR′)(CO)9—n(PPh3)n complexes. Crystal structures of (μ-H)Ru3(μ3-η3-Et2NCCHCMe)(CO)8(PPh3) and (μ-H)Ru3(μ3-η3-MeOCCMeCMe)(CO)7(PPh3)2·2CH2Cl2. J. Organomet. Chem. 1993, 452, 151-160. (b) Aime, S.; Osella, D.; Deeming, A. J.; Arce, A. J.; Hursthouse, M. B.; Dawes, H. M. Molecular structures and dynamic behaviour of two isomers of [Ru3(µ-H)(µ3-Me2NC4H4)(CO)9] formed from 1-dimethylaminobut-2-yne and containing η2-alkene-1,3-diyl and η2-alkene-1,2-diyl ligands respectively. J. Chem. Soc., Dalton Trans. 1986, 1459-1463.
18 See for instance: (a) Marchetti, F.; Zacchini, S.; Zanotti, V. Amination of Bridging Vinyliminium
