Activation of C=N Bonds by High Valent Group 6 Metal Chlorides,
Including the Conversion of a α-Diimine into a Functionalized
Imidazolium
Niccolò Bartalucci,a,b Marco Bortoluzzi,b,c Stefano Zacchini,b,d Guido Pampaloni,a,b and Fabio
Marchetti* a,b
a Università di Pisa, Dipartimento di Chimica e Chimica Industriale, Via Moruzzi 13, I-56124 Pisa. b CIRCC, via Celso Ulpiani 27, I-70126 Bari, Italy.
c Università Ca’ Foscari Venezia, Dipartimento di Scienze Molecolari e Nanosistemi, Via Torino
155, I-30170 Mestre (VE), Italy.
d Università di Bologna, Dipartimento di Chimica Industriale “Toso Montanari”, Viale
Risorgimento 4, I-40136 Bologna, Italy.
* To whom correspondence should be addressed.
E-mail: [email protected]; Webpage: www.dcci.unipi.it/~fabmar/
Abstract. The α-diimine (Xyl)N=CHCH=N(Xyl) (DADMe; Xyl = 2,6-C
6H3Me2) was converted by
reaction with WCl6 into the iminomethyl-imidazolium compound
[(Xyl)NCH=CHN(Xyl)CCH=N(Xyl)][WCl6], 1, in 47% yield. Mono(N-protonated) α-diimine salts
were isolated from the reactions of WCl6 with (2,6-C6H3Et2)N=C(Me)C(Me)=N(2,6-C6H3Et2)
(Me-DADEt) and of MoCl
5 with (2,6-C6H3Et2)N=CHCH=N(2,6-C6H3Et2) (DADEt) and
(2,6-C6H3tBu2)N=CHCH=N(2,6-C6H3tBu2) (DADtBu), giving respectively
[(2,6-C6H3Et2)NH=CHCH=N(C6H2Et2Me)][WCl6], 2 (minor product), [DADEt(H)]2[Mo2Cl10], 3a, and
[DADtBu(H)]
affording in 70% yield MoCl4[(4-C6H4Me)NC(Cl)N(4-C6H4Me)], 4. All the products were
characterized by analytical and spectroscopic methods, and the X-ray structures of 1, 2 and 4 were elucidated by X-ray diffraction. DFT calculations were performed to shed light on mechanistic and structural aspects.
Introduction
α-Diimines, also called 1,4-diaza-1,3-dienes (DAD), represent an intriguing class of organic compounds, whose steric and electronic properties can be modulated by varying the substituents on the N=CC=N skeleton.1 Due to their variability and robustness, these compounds have been largely
employed as ancillary, chelating N,N-ligands in coordination chemistry,2 many of the resulting
complexes finding application in metal mediated organic synthesis.3 α-Diimines behave as redox
non-innocent ligands,Error: Reference source not foundb,4 since they may undergo a formal,
stepwise reduction (up to two electrons reduction) when coordinated to metal centres,Error: Reference source not foundc,5 thus determining a possible ambiguity in the assignment of the
oxidation states.6
In this framework, studies on the reactivity of α-diimines with compounds of metal elements in an oxidation state ≥ +4 are rare in the literature.Error: Reference source not found,7 In particular, the
reactions of α-diimines with high valent metal chlorides (HVMC), i.e. NbCl5,8 TaCl5,9 MoCl5,10
WCl6 11 and derivatives,12 have been reported, but usually performed in the presence of reductants
(e.g. Na/naphthalene,Error: Reference source not foundb Na,Error: Reference source not found Zn/Hg Error:
Reference source not foundb) or a chlorine abstracting agent
[1-methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexadiene Error: Reference source not found,Error: Reference source not founda,Error: Reference source not founda,Error: Reference source not found], serving to the preliminary reduction of the metal centre. This elegant strategy is
necessary to quench the Lewis acidity and the oxidative power typical of HVMC,13,14,15 thus
allowing the formation of coordination compounds containing intact α-diimine ligands. In other words, the full exploitation of the activation potential of HVMC towards α-diimines is a
substantially unexplored field of chemistry. In two recent communications, we have reported that allowing the direct interaction of a HVMC (WCl6) with a N-aryl substituted α-diimine,16 in a weakly
coordinating solvent (dichloromethane) and in the absence of further reactants, results in the activation of the organic reactant following a reaction pathway never observed before (Scheme 1).
Scheme 1. WCl6-mediated conversion of α-diimine to quinoxalinium
.
As an extension of the preliminary result, in order to further explore non conventional organic synthetic routes, we decided to investigate the reactions of Group 6 HVMC, i.e. WCl6 and MoCl5,
with a selection of α-diimines, see Chart 1. Herein, we will present the results of this study, which has been extended, in the case of MoCl5, to a compound containing two cumulated [C=N] bonds
(carbodiimide).
Results and discussion
The 1:1 molar reaction of WCl6 with DADMe was carried out in dichloromethane at room
temperature, and afforded the iminomethyl-imidazolium salt [(Xyl)NCH=CHN(Xyl)CCH=N(Xyl)] [WCl6], 1, in 47% yield, see Scheme 2 (Xyl = 2,6-C6H3Me2).
Scheme 2. WCl6-directed DADMe to imidazolium conversion.
Compound 1 was characterized by X-ray diffraction, IR, 1H and 13C NMR spectroscopy, and
elemental analysis. The solid state structure is shown in Figure 1, while relevant bond distances and angles are reported in Table 1.
Figure 1. Molecular structure of 1, with key atoms labeled. Displacement ellipsoids are at the 50% probability level.
Table 1. Selected bond distances (Å) and angles (°) for 1.
W(1)−Cl(1) 2.3420(8) W(1)−Cl(2) 2.3302(9) W(1)−Cl(3) 2.3149(9) W(1)−Cl(4) 2.3319(9) W(1)−Cl(5) 2.2921(10) W(1)−Cl(6) 2.3263(9) C(1)−N(1) 1.346(4) N(1)−C(2) 1.371(4) C(2)−C(3) 1.352(5) C(3)−N(2) 1.369(4) N(2)−C(1) 1.346(4) C(1)−C(4) 1.461(4) C(4)−N(3) 1.264(4) N(3)−C(21) 1.432(4) N(1)−C(5) 1.454(4) N(2)−C(13) 1.453(4) Cl(1)−W(1)−Cl(6) 179.16(4) Cl(2)−W(1)−Cl(4) 177.86(3) Cl(3)−W(1)−Cl(5) 178.89(4) C(1)−N(1)−C(2) 109.7(3) N(1)−C(2)−C(3) 106.3(3) C(2)−C(3)−N(2) 108.2(3) C(3)−N(2)−C(1) 108.7(3) N(2)−C(1)−N(1) 107.0(3) N(1)−C(1)−C(4) 123.8(3) N(2)−C(1)−C(4) 129.2(3) C(1)−C(4)−N(3) 120.4(3) C(4)−N(3)−C(21) 118.9(3)
The crystals of 1 consist of ionic packings of [WCl6]– anions and
[(Xyl)NCH=CHN(Xyl)CCH=N(Xyl)]+ cations. The structure of [WCl
6]– anion was previously
reported as miscellaneous salts,Error: Reference source not founda,c,Error: Reference source not found instead the
parameters within the imidazolium skeleton and the xylyl groups are as expected. The C(4)-N(3) bond [1.264(4) Å] holds substantial imine character, the C(4) atom exhibiting pure sp2 hybridization
[C(1)-C(4)-N(3) = 120.4(3) °]. The NMR spectra of 1 (in CD2Cl2) display one set of resonances, the
aldimine carbon resonating at 143.1 ppm.
The transformation of the diazadiene moiety into iminomethyl-imidazolium is a rare reaction, although not novel in the literature: it was formerly reported to be promoted by Lewis acidic species such as HCl and AlCl3.17
The formation of [(Xyl)NCH=CHN(Xyl)CCH=N(Xyl)]+ from DADMe is accompanied by the
elimination of one [N-Xyl] unit. In order to understand the destiny of this fragment, we monitored by NMR spectroscopy the reaction of WCl6 with DADMe in CD2Cl2. The NMR spectra of the
mixture evidenced the presence of 1 and of one additional xylyl containing species, whose methyl groups were detected significantly deshielded in the 1H NMR spectrum (3.43 ppm). We suggest the
co-formation of a N-chloro amide complex of W(V), WVCl
4(NClXyl), see the following DFT
discussion.
DFT calculations have pointed out that the synthetic pathway to 1 begins with the mono-electron oxidation of DADMe by means of WCl
6, affording the corresponding radical cation [DADMe]+• and
[WCl6] (see step A in Scheme 3, and Figure SI-1). The electron-poor [DADMe]+• species may
couple with still unreacted DADMe via CN bond formation (step B), thus generating the radical
cation [XylNCHCHN(Xyl)CH(NXyl)CHNXyl]+• (Figure SI-2). The unpaired electron in this latter
species is mainly localized on the nitrogen atom of the [N-Xyl] fragment. Further WCl6-oxidation
of [XylNCHCHN(Xyl)CH(NXyl)CHNXyl]+• to the corresponding dication appears to be slightly
endergonic (step C, Figure SI-3). Nevertheless, the possible loss of H+ as HCl by reaction with
[WCl6] renders thermodynamically favourable the formation of a WCl5 / imine monocationic
adduct, [WCl5{N(Xyl)C(CHNXyl)NXylCHCHNXyl}]+ (step D and Figure SI-4). The conversion
into the final product requires the formal loss of one [N-Xyl] nitrene fragment. Attempts to optimize the structure of cyclic products containing [N-Xyl] led in every case to the opened geometry,
therefore the release of the [N-Xyl] fragment appears essential to cyclization. The interaction between WCl5 and the cation [N(Xyl)C(CHNXyl)NXylCHCHNXyl]+ (step D) could be preliminary
to this release. Indeed, mono-electronic reduction of the organic frame by W(V), accompanied by Cl/[N-Xyl] exchange, should be thermodynamically favourable, affording the imido-complex W(VI)
(NXyl)Cl4 and the radical cationic chloro-species [XylNCHCHN(Xyl)C(Cl)CHNXyl]+• (step E and
Figure SI-6). The final ring closure would occur via intramolecular attack on the chlorine-bonded carbon by imine nitrogen (step F), accompanied by electron exchange between the organic intermediate and the imido complex. In particular, the calculations suggest formal migration of the chlorine atom from the organic cation to W(VI)Cl
4(NXyl) to give W(V)Cl4(NClXyl) (see above NMR
study and Figure SI-6). The charge of the resulting imidazolium is counterbalanced by [WCl6]
generated during the previous steps. The calculated Gibbs energy variation for the overall last step is strongly negative.
Scheme 3. Relative Gibbs free energies of proposed intermediates for the WCl6-mediated conversion of
DADMe to 1. C-PCM/B97X calculations, dichloromethane as continuous medium. Xyl = 2,6-C
6H3Me2. One
In summary, it seems that at least two distinct pathways may be viable in the reactions of WCl6 with
N-aryl α-diimines, i.e. intramolecular rearrangement to quinoxalinium (Scheme 1) or intermolecular coupling leading to imidazolium (Scheme 2). The reactions with other N-aryl α-diimines were not selective and generally afforded intractable solid materials which could not be characterized. Our attention focused in particular on the reactivity of DADEt and Me-DADEt, being easily prepared as
pure compounds and containing 2,6-alkyl-substituted aryl moieties (similarly to DADMe and
DADIpr). After many experiments, 2 was the only product which could be identified, as isolated in
very low amount from WCl6 and Me-DADEt, and characterized by X-ray diffraction (Figure 2 and
Table 2). The formation of the cation in 2 appears the result of a redistribution of the alkyl substituents on the diimine backbone, via breaking of CH and CN bonds and formation of a new CC bond. A similar reactivity was previously described for alkyl-arenes allowing to react with TaCl5.18
From another point of view, 2 contains a rare example of N-protonated α-diimine, only few examples being crystallographically characterized.19 The bonding parameters of the cation are
comparable to those previously reported for analogous species. The C=N bond of the protonated nitrogen is slightly elongated compared to the non-protonated bond, 1.276(9) versus 1.264(9) Å. The N=C−C=N skeleton of the cation adopts the typical E-s-cis-E conformation. A inter-molecular H-bond is present within the protonated imine group of the cation and one chloride ligand of the anion [N(1)−H(1) 0.88 Å, H(1)···Cl(5) 2.68 Å, N(1)···Cl(5) 3.327(7) Å, <N(1)H(1)Cl(5) 131.5°].
Figure 2. ORTEP drawing of 2. Displacement ellipsoids are at the 50% probability level. Only the main images of the disordered groups are represented.
Table 2. Selected bond distances (Å) and angles (°) for 2.
W(1)−Cl(1) 2.342(10) W(1)−Cl(2) 2320(11) W(1)−Cl(3) 2.320(7) W(1)−Cl(4) 2.323(8) W(1)−Cl(5) 2.319(4) W(1)−Cl(6) 2.289(3) C(1)−C(2) 1.509(10) C(1)−N(1) 1.276(9) C(2)−N(2) 1.264(9) C(5)−N(1) 1.441(9) C(15)−N(2) 1.438(9) Cl(1)−W(1)−Cl(4) 177.6(4) Cl(2)−W(1)−Cl(6) 179.4(3) Cl(3)−W(1)−Cl(5) 177.5(2) C(5)−N(1)−C(1) 127.2(7) N(1)−C(1)−C(2) 115.1(7) C(1)−C(2)−N(2) 114.2(6) C(2)−N(2)−C(15) 122.9(6)
As an extension of our work, we studied the reactions of MoCl5 with a series of α-diimines,
performed under conditions like those adopted for the synthesis of 1. The reactions with DADIpr and
DADMe afforded complicated mixtures of products. On the other hand, the reactions of MoCl
5 with
Compounds 3a-b were characterized by elemental analysis, IR spectroscopy and magnetic analysis. In agreement with DFT simulations, the IR spectrum of solid 3b displays absorptions due to the asymmetric and symmetric stretching vibrations of the imine groups, respectively at 1667 and 1599 cm. Only the band related to the symmetric vibration mode could be detected in the IR spectrum
of 3a (1586 cm). The NH moiety in 3a-b manifested itself with an absorption in the 3150-3250
cm region. The magnetic analyses gave magnetic susceptibility values fully consistent with those
available in the literature for MoIV chloride derivatives.20
The formation of 3a-b seems to be the result of electron transfer from the diazadiene to MoCl5,Error: Reference source not found,21 followed by hydrogen capture by the resulting radical
cation from the reaction mixture. This is a common pathway observed in the oxidation reactions of diverse organic substrates with high valent metal halides in chlorinated solvents, leading to halometalate salts of protonated species.22
Based on these facts, and considering that the anion MoCl5 was previously proved to hold a
dinuclear structure,23 we propose for 3a-b the formula given in Eq. 1.
We did many attempts to obtain crystals of 3a-b suitable to X-ray diffraction, but unsuccessfully. Hence, the DFT calculated structures of the ions in 3a-b are shown in Figure 3.
Figure 3. DFT-optimized structures (C-PCM/wB97X calculations) of the cations and the anion within 3a (A) and 3b (B) with spin density surface (isovalue = 0.01 a.u.). Hydrogen atoms on the nitrogen substituents are omitted for clarity. Colour map: Mo, cyan; N, blue; C, grey; H, white; spin density surface: yellow tones. Selected computed lengths (Å): [Mo2Cl10]2- Mo-Cl (terminal, average, trans--Cl) 2.342, Mo-Cl (terminal, average, cis--Cl) 2.342, Mo-(-Cl) 2.563; 3a-cation N1-C1 1.283, N2-C2 1.270, C1-C2 1.475, N1-H 1.032; 3b-cation N1-C1 1.275, N2-C2 1.263, C1-C2 1.485, N1-H 1.033.
We extended the present study to the 1:1 molar reaction of MoCl5 with (4-C6H4
Me)N=C=N(4-C6H4Me), containing two cumulated C=N bonds. The reaction proceeded selectively to give the
green compound MoCl4[(4-C6H4Me)NC(Cl)N(4-C6H4Me)], 4 (Scheme 4), which is clearly
originated by chlorine transfer from molybdenum to the carbodiimide function. Magnetic susceptibility measured for 4 (1.57 BM) indicates that molybdenum maintains the +5 oxidation state.Error: Reference source not found
Scheme 4. Reactions of MoCl5 with 1,3-di-p-tolylcarbodiimide.
The molecular structure of 4 was determined by X-ray diffraction: the ORTEP representation is shown in Figure 4 while relevant bond lengths and angles are reported in Table 3. The structure of 4 resembles that previously reported for a homologous compound.24 The molybdenum centre displays
a distorted octahedral geometry, with the Mo(1)−Cl(2) [2.3625(5) Å] and Mo(1)−Cl(3) [2.3377(6) Å] contacts in relative trans positions slightly longer than Mo(1)−Cl(4) [2.2789(5) Å] and Mo(1)−Cl(5) [2.2484(5) Å] trans to N-atoms. The C(1), N(1) and N(2) atoms display an almost perfect sp2 hybridization [sum angles 359.9(3), 359.7(6) and 357.8(6)°, respectively] and both
C(1)−N(1) [1.322(2) Å] and C(1)−N(2) [1.324(2) Å] display a considerable -character.25
Figure 4. Molecular structure of MoCl4[(4-C6H4Me)NC(Cl)N(4-C6H4Me)], 4, with key atoms labeled.
Displacement ellipsoids are at the 50% probability level. Table 3. Selected bond distances (Å) and angles (°) for 4.
Mo(1)−Cl(2) 2.3625(5) Mo(1)−Cl(3) 2.3377(6) Mo(1)−Cl(4) 2.2789(5) Mo(1)-Cl(5) 2.2484(5) Mo(1)−N(1) 2.0969(15) Mo(1)−N(2) 2.1130(15) N(1)−C(1) 1.322(2) N(2)−C(1) 1.324(2) C(1)−Cl(1) 1.7063(17) N(1)−C(2) 1.414(2) N(2)−C(9) 1.417(2) Cl(2)−Mo(1)−Cl(3) 177.929(17) Cl(4)−Mo(1)−N(2) 154.09(4) Cl(5)−Mo(1)−N(1) 150.47(4) Cl(4)−Mo(1)−Cl(5) 116.63(2) N(1)−Mo(1)−N(2) 61.44(6) N(1)−C(1)−N(2) 108.71(15) N(1)−C(1)−Cl(1) 125.43(13) N(2)−C(1)−Cl(1) 125.75(14) C(1)−N(1)−C(2) 127.53(15) C(1)−N(1)−Mo(1) 95.26(11) C(2)−N(1)−Mo(1) 136.90(11) C(1)−N(2)−C(9) 126.41(15) C(1)−N(1)−Mo(1) 94.48(11) C(9)−N(2)−Mo(1) 136.92(12)
Conclusions
α-Diimines are versatile and robust ligands that have been largely employed in coordination chemistry. Nevertheless, the interaction of α-diimines with easily available chlorides of metals in an oxidation state higher than +4 has been barely explored heretofore, unless in the presence of reducing agents so to quench the activation power of the metal centre. Following a recent report on the direct synthesis of quinoxalinium salts by reaction of WCl6 with two N-aryl α-diimines, herein
we have described a rare WCl6-directed transformation of another component of such class of
compounds. This represents a further example of the potentiality of high valent metal chlorides in promoting unusual activation pathways to organic species under mild conditions. In general, the modification of N-aryl α-diimines by means of WCl6 is finely affected by the nature of the
α-diimine substituents, and non selective reactions may be viable, including reactions leading to redistribution of alkyl groups. Also the chlorination of a carbodiimide by means of MoCl5 has been
discussed. All of the described reactions appear to be promoted by the dual nature of the high valent metal chloride, acting as both a mono-electron oxidative agent and/or a chlorine transferor.
Experimental section
Warning! The metal products reported in this paper are highly moisture-sensitive, thus rigorously anhydrous conditions were required for the reaction, crystallization and separation procedures. The reaction vessels were oven dried at 140 °C prior to use, evacuated (10–2 mmHg) and then filled
with argon. WCl6 (99.9% purity) and MoCl5 (99.9%) were purchased from Strem and stored under
argon atmosphere as received. α-Diimines were synthesized according to the literature.26,Error: Reference source not found Other organic reactants were commercial products (Sigma Aldrich or TCI Europe) of the
highest purity available, that were stored under nitrogen atmosphere as received. Solvents were distilled from P4O10 under argon before use. Infrared spectra were recorded at 298 K on a FT
(reported per Mo atom) were measured at 298 K on solid samples with a Magway MSB Mk1 magnetic susceptibility balance (Sherwood Scientific Ltd.). Diamagnetic corrections were introduced according to König.27 Carbon, hydrogen and nitrogen analyses were performed on a
Carlo Erba mod. 1106 instrument. The chloride content was determined by the Mohr method 28 on
solutions prepared by dissolution of the solid in aqueous KOH at boiling temperature, followed by cooling to room temperature and addition of HNO3 up to neutralization. NMR spectra were
recorded at 298 K on a Bruker Avance II DRX400 instrument equipped with a BBFO broadband probe. The chemical shifts for 1H and 13C were referenced to the non-deuterated aliquot of the
solvent. NMR assignments were assisted by DEPT experiments and 1H,13C correlation measured
using gs-HSQC and gs-HMBC experiments.
Reaction of WCl6 with DADMe: synthesis of [(Xyl)NCH=CHN(Xyl)CCH=N(Xyl)][WCl6], 1.
A solution of DADMe (235 mg, 0.889 mmol) in CH
2Cl2 (10 mL) was treated with WCl6 (350 mg,
0.883 mmol), and the mixture was stirred at room temperature for 24 h. The resulting red solution was concentrated to ca. 2 mL, layered with hexane and stored at 30 °C. Thus, 1 was recovered as a microcrystalline solid after one week. Yield 334 mg, 47%. Anal. Calcd. for C28H30Cl6N3W: C, 41.77;
H, 3.76; N, 5.22; Cl, 26.42. Found: C, 41.65; H, 3.84; N, 5.13; Cl, 26.27. IR (solid state): = 2919w-br, 2864w, 1634w-sh, 1605m, 1580m, 1558w-m, 1536w-m, 1497m-s, 1470s, 1453m-s, 1379m, 1319w, 1222m, 1189m, 1168m-s, 1089vs, 1031s, 953w-m, 902w-m-sh, 797s, 772vs, 732s, 701m, 661w-m cm. 1H NMR (CD 2Cl2): δ = 8.66 (s, 1 H, C13-H); 7.40 (s, 2 H, C2-H); 7.43, 7.13 (m, 3 H, C9-H + C10-H); 7.30, 6.92 (m, 6 H, C5-H + C6-H); 2.18 (s, 12 H, C11-H); 1.69 ppm (s, 6 H, C12-H). 13C{1H} NMR (CD 2Cl2): δ = 147.7 (C7); 143.1 (C13); 137.4 (C1); 134.3, 129.1 (C5 + C6); 132.5, 132.3 (C3 + C4); 131.4, 128.3 (C9 + C10); 126.6, 126.2 (C8); 125.3 (C2); 19.7, 17.4 ppm (C11 + C12), see Chart 2 for atom numbering. Analogous reaction of DADMe with WCl
6 in CD2Cl2 gave a red
solution which was analyzed by NMR. Additional signals, except those of 1, were clearly detected. 1H
Chart 2. Numbering of carbon atoms in the cation of 1.
Reaction of WCl6 with Me-DADEt: formation of [(2,6-C6H3Et2)NH=CHCH=N(C6H2Et2Me)]
[WCl6], 2. WCl6 (108 mg, 0.271 mmol) was added to a solution of Me-DADEt (94 mg, 0.268 mmol)
in CH2Cl2 (5 mL), and the resulting mixture was stirred at room temperature for 24 h. The final
solution was layered with pentane and stored at 30 °C. Few violet, X-ray quality crystals of 2 were collected after 72 h.
Reaction of MoCl5 with α-diimines.
A) Synthesis of [DADEt(H)]
2[Mo2Cl10], 3a. MoCl5 (332 mg, 1.22 mmol) and DADEt (391 mg, 1.22
mmol) were added of dichloromethane (15 mL), and the resulting mixture was stirred for 18 h at room temperature. The final mixture was dried under reduced pressure, and the residue was washed with hexane (30 mL). A dark green solid was isolated. Yield 492 mg, 68%. Anal. Calcd. for C22H29Cl5MoN2: C, 44.43; H, 4.92; N, 4.71; Cl, 29.81. Found: C, 44.27; H, 4.78; N, 4.58; Cl, 29.70.
IR (solid state): = 3251w (NH), 2936w, 2877w, 1586w-br (sym C=N), 1455w-m-sh, 1376w, 1165m-br, 1028s-br, 990s-br, 802m-sh cm. Magnetic measurement: Mcorr = 2.23103 cgsu, eff =
B) Synthesis of [DADtBu(H)]
2[Mo2Cl10], 3b. This compound was prepared using a procedure
analogous to that described for the synthesis of 3a, by allowing MoCl5 (300 mg, 1.10 mmol) to react
with DADtBu (187 mg, 1.12 mmol). Dark brown solid. Yield 380 mg, 78%. Anal. Calcd. for
C10H21Cl5MoN2: C, 27.14; H, 4.78; N, 6.33; Cl, 40.06. Found: C, 27.23; H, 4.65; N, 6.44; Cl, 39.85.
IR (solid state): = 3158w-br (NH), 3061w, 2979m, 2846w-m-br, 1667m (asym C=N), 1599w-br (sym C=N), 1477m-s, 1429m, 1386m-s-sh, 1373m-s, 1341w, 1315w-sh, 1263m-s, 1237s, 1191s, 1097m-s, 1031vs-sh, 981vs, 930w, 894w-m, 876w, 861w-m, 803s-sh, 758w-m, 733w-m cm.
Magnetic measurement: Mcorr = 1.87103 cgsu, eff = 2.13 BM.
Reaction of MoCl5 with (4-C6H4Me)N=C=N(4-C6H4Me): synthesis of MoCl4
[(4-C6H4Me)NC(Cl)N(4-C6H4Me)], 4. A suspension of MoCl5 (232 mg, 0.850 mmol) in CH2Cl2 (15
mL) was treated with (4-C6H4Me)N=C=N(4-C6H4Me) (189 mg, 0.850 mmol), then the mixture was
stirred at room temperature for 18 h. The resulting solution was dried in vacuo, and the residue was washed with hexane (30 mL). The product was obtained as a green-brown solid. Yield: 295 mg, 70%. Dark green crystals suitable for X-ray analysis were collected from a concentrated CH2Cl2
solution layered with hexane and stored at 30°C for one week. Anal. Calcd for C15H14Cl5MoN2: C,
36.36; H, 2.85; Cl, 35.78. Found: C, 36.11; H, 2.99; Cl, 35.29. IR (solid state): = 2963w, 1634w, 1592w, 1498w, 1260w, 1191m, 1107m-br, 1016vs-br, 986vs-sh, 962vs, 940vs, 912vs, 815s, 795s-br, 732m-s cm.29 Magnetic measurement:
Mcorr = 1.02103 cgsu, eff = 1.57 BM.
X-ray crystallography.
Crystal data and collection details for 1, 2 and 4 are listed in Table 4. The diffraction experiments were carried out on a Bruker APEX II diffractometer equipped with a CCD detector and using Mo-K radiation ( = 0.71073 Å). Data were corrected for Lorentz polarization and absorption effects (empirical absorption correction SADABS).30 The structures were solved by direct methods and
refined by full-matrix least-squares based on all data using F2.31 All non-hydrogen atoms were
refined with anisotropic displacement parameters. All hydrogen atoms were fixed at calculated positions and refined by a riding model. H-atoms were treated isotropically using the 1.2 fold Uiso
value of the parent atom except methyl protons, which were assigned the 1.5 fold Uiso value of the
parent C-atom. Crystals of 1 contain some voids that are likely to be occupied by one disordered CH2Cl2 molecule per formula unit. It has not been possible to refine the CH2Cl2 molecule associated
to 1 satisfactorily. Thus, the SQUEEZE routine of PLATON has been applied.32 The [WCl
6]– anion,
two ethyl groups and one methyl group on the cation of 2 are disordered. Disordered atomic positions were split and refined using one occupancy parameter per disordered group.
Table 4. Crystal data and measurement details for 1, 2 and 4.
1 2 4
Formula C28H30Cl6N3W C25H35Cl6N2W C15H14Cl5MoN2
Fw 805.10 760.10 495.47
, Å 0.71073 100(2) 0.71073
Temperature, K 100(2) 0.71073 100(2)
Crystal system Monoclinic Monoclinic Monoclinic
Space group P21/n P21/n P21/c a, Å 8.3116(4) 9.5610(5) 10.7612(17) b, Å 18.5141(9) 15.4452(8) 21.413(3) c, Å 22.4919(11) 19.6813(11) 8.0173(12) 90 97.532(3) 97.790(2) 99.973(2) 90 Cell volume, Å3 3431.2(3) 2879.5(3) 1819.5(5) Z 4 4 4 Dc, g cm3 1.559 1.753 1.809 , mm1 3.855 4.586 1.453 F(000) 1580 1500 980 Crystal size, mm 0.16×0.12×0.10 limits, 1.43–28.00 1.68–25.05 1.90-28.00 Reflections collected 60561 34346 20835
Independent reflections 8263 [Rint = 0.0676] 5107 [Rint = 0.0416] 4330 [Rint = 0.0275]
Data / restraints / parameters 8263 / 0 /349 5107 / 359 / 419 4330 / 0 / 210
Goodness of fit on F2 1.020 1.167 1.066
R1 (I> 2(I)) 0.0304 0.0498 0.0212
wR2 (all data) 0.0746 0.1061 0.0527
Largest diff. peak
Computational studies. The electronic structures of the compounds were optimized using the
range-separated ωB97X DFT functional 33 in combination with Ahlrichs’ split-valence polarized
basis set, with ECP on the Mo and W centres.34 The “unrestricted” formalism was applied to
compounds with unpaired electrons, and the lack of spin contamination was verified by comparing the computed <S2> values with the theoretical ones. 35 The C-PCM implicit solvation model (ε =
9.08) was added to B97X calculations. The stationary points were characterized by IR simulations (harmonic approximation), from which zero-point vibrational energies and thermal corrections (T= 298.15 K) were obtained. 36 The software used was Gaussian 09. 37
Supplementary Material. DFT optimized geometries of the reaction intermediates along the
formation of 1. Cartesian coordinates of all DFT-optimized compounds are collected in a separated .xyz file. CCDC reference numbers 1814757 (1), 1551420 (2), and 1814756 (4) 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, CambridgeCB2 1EZ, UK; fax: (internat.) +44-1223/336-033; e-mail: [email protected]).
Acknowledgements.
The authors thank the University of Pisa for financial support.
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