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The electrochemical behavior of complexes [FeMo(CO) 5 (κ 2 -dppe)(μ-pdt)] (1) and [FeMo(CO) 4 (MeCN)( κ 2 -dppe)(μ-pdt)] (2), in the absence and in the presence of acid, has been investigated. The reduction of 1 follows at slow scan rates, in CH 2 Cl 2 -[NBu 4 ][PF 6 ] and acid-free media, an EC rev E mechanism that is supported by cyclic voltammetry (CV) experiments and digital CV simulations. In MeCN-[NBu 4 ][PF 6 ], the electrochemical reduction of 1 is the same as in dichloromethane and follows an ECE mechanism at slow scan rates, but with a positive shift of the redox potentials. In contrast, the oxidation of 1 is strongly solvent-dependent. In dichloromethane, the oxidation of 1 is reversible and involves a single electron, while in acetonitrile, it is irreversible at moderate and slow scan rates (v ≤ ca. 1 V s -1 ), and some chemical reversibility is apparent at higher scan rates (v = 10 V s -1 ). Density functional theory calculations revealed that the chemical step in the EC rev E mechanism corresponds to the dissociation of one PPh 2 end of the diphosphine ligand and the transfer of the semibridging CO to the Fe atom, similarly to the mechanism observed in the FeFe analogue complex. However, in the case of 1, the subsequent coordination of the phosphine ligand to the other metal is an unfavorable process.

International audience; Dimolybdenum thiolate-bridged complexes [Mo2Cp2(μ-SMe)2(μ-SCH2CH2E)] (E = O (2) or NH (4)) with a proton-dependent protecting device have been synthesized by reaction of [Mo2Cp2(μ-SMe)2(μ-Cl)2] (1) with SCH2CH2EH. The reactivity of the resultant quadruply bridged complexes with acid was investigated in the absence and in the presence of a potential ligand (N2, MeCN, RNC). While the protonation of complexes 2 and 4 under N2 in dichloromethane produced only the oxidized derivatives instead of the desired diazenido compound, ligand binding was observed in MeCN or in the presence of RNC (R = t-Bu, Xyl). Whereas acetonitrile loss from [Mo2Cp2(μ-SMe)2(μ-SCH2CH2OH)(MeCN)2]+ (8+) prevented the isolation and characterization of this species, the t-BuNC analogue (6+) could be characterized by an X-ray crystal structure. The electrochemistry of 2 and 2+ was investigated in CH2Cl2 and in MeCN, both in the absence and in the presence of acid. While the addition of HBF4·Et2O to a dichloromethane solution of 2 only produced 2+ (and presumably H2), 8+ was the major product of the protonation in MeCN.

Mixed μ-phosphido or μ-thiolato μ-halo-dimolybdenum(III) compounds [Mo 2 Cp 2 (μ-SMe) 2 (μ-X)(μ-Y)] (X = PPh 2 , Y = Cl ( 1 ); X = SCH 3 , Y = Br ( 3 ), I( 4 )) have been studied. Syntheses and X-ray structures of the new bromo and iodo analogues of [Mo 2 Cp 2 (μ-SMe) 3 (μ-Cl)] ( 2 ) are reported. While preparing 3 a side-product 5 was obtained and structurally characterised as the Mo 5 system [{Mo 2 Cp 2 Br(μ-O)(μ-SMe) 2 } 2 (μ-MoO 4 )], containing a {Mo IV –Mo IV –O–(Mo VI O 2 )–O–Mo IV –Mo IV } unit. The influence of the bridging groups on the structures and electrochemical behaviour of the complexes [Mo 2 Cp 2 (μ-SMe) 2 (μ-X)(μ-Y)] 1 – 4 has been investigated. In MeCN–[NBu 4 ][PF 6 ] the first oxidation of [Mo 2 Cp 2 (μ-SMe) 2 (μ-PPh 2 )(μ-Cl)] ( 1 ) is quasi-reversible, contrasting with the essentially irreversible first oxidation of the thiolate analogue [Mo 2 Cp 2 (μ-SMe) 3 (μ-Cl)] ( 2 ) under similar conditions. The effects of lowering the temperature or increasing the scan rate on the oxidation of 1 were examined: the initial quasi-reversible oxidation at first became irreversible but at still higher scan rates or lower temperatures the oxidation was again quasi-reversible. This suggests that the oxidation of 1 in MeCN is followed by reversible coordination of acetonitrile to afford the species [Mo 2 Cp 2 (μ-SMe) 2 (μ-PPh 2 )(Cl)(MeCN)] + ( 7 + ). Cyclic voltammetry of [Mo 2 Cp 2 (μ-SMe) 3 (μ-Y)](Y = Br ( 3 ) or I ( 4 )) showed two quasi-reversible diffusion-controlled oxidation steps in CH 2 Cl 2 –[NBu 4 ][PF 6 ] or thf–[NBu 4 ][PF 6 ]. In acetonitrile, the fast loss of the halide ligands results in the formation of [Mo 2 Cp 2 (μ-SMe) 3 (MeCN) 2 ] + species.

cited By 10; International audience; A series of chemical and electrochemical transformations of systems in which a Mo2Cp2(μ-SMe)3 core is bridged by a μ-C2HnR ligand (n = 0-4) are described in this paper. The reaction of the alkylidene complex [Mo2Cp2(μ-SMe) 3(μ-η1:η2-CHCH2Tol)] (BF4) (1) with LiBun at 0°C produces the μ-σ,π-vinyl complex [Mo2Cp2(μ-SMe) 3(μ-η1:η2-CH=CHTol] (2) in good yield. The molecular structure of 2 has been confirmed by X-ray analysis. Upon treatment with NaBEL4 1 is readily converted into the semibridging alkyl species [Mo2Cp2(μ-SMe)3(μ-CH 2CH2Tol)] (3), which is also formed by electrochemical reduction of 1 in acidic medium. NMR and X-ray diffraction studies of 3 are consistent with, but do not definitively establish, the presence of a η1 α-agostic interaction. Density functional theory has been used to confirm the presence of agostic interactions in both 1 and 3 and also to explore the exchange pathways for these hydrocarbyl dimolybdenum systems. Electrochemical transformation of the μ-alkylidene complex 1 gives 3 as the major product when acid is present and a mixture of 2 and 3 when acid is absent, production of 2 being favored by low initial concentrations of 1. Theoretical, spectroscopic, and diffraction data are used to explain the formation and structures of closely related [Mo2Cp 2(μ-SMe)3(μ-C2HnR] z- complexes (n = 0-4 and z = 0, 1), including 1-3. © 2005 American Chemical Society.

Treatment of [Mo2(Cp)2(μ-SMe)3(MeCN)2]+ (1+) with NO3– or NO2– results in the conversion of one terminally bound acetonitrile ligand into an amidato bridge. The reaction produces [Mo2(Cp)2(μ-SMe)3{μ-η1,η1-OC(Me)NH}]0/+ (20/+) and involves the formation of an intermediate, which was detected by cyclic voltammetry but which could not be isolated, and which likely arises from the substitution of the NOx anion for one MeCN ligand. The electrochemical behaviour of 2+ was studied by cyclic voltammetry in THF and MeCN. The reduction of 2+ in the presence of acid (HBF4/H2O or HBF4/Et2O) in these solvents leads to the release of the amidate bridge. Controlled-potential electrolysis of 2+ in MeCN in the presence of acid produces 1+ quantitatively; the charge consumed (>1 F mol–1 of 1+) indicates that electrons are also used to reduce protons. This was confirmed by the formation of 2+ (in variable amounts depending on the conditions) on treating 2 with acid. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

The reduction of [Mo2(Cp)2(µ-SMe)3(µ-O)]+ (1+) has been investigated by cyclic voltammetry and controlled-potential electrolysis in THF- and MeCN/NBu4PF6 without added acid or in the presence of various acids HX (HX: HTsO, CF3CO2H, HBF4). Reduction in the presence of acid follows an ECrevE mechanism in which the intermediate chemical step is an acid-base equilibrium between 1 and [Mo2(Cp)2(µ-SMe)3(µ-OH)]+ (2+). This electrochemical process is followed by protonation of the neutral µ-hydroxo complex 2 to afford different products which depend both on the solvent (THF or MeCN) and on the nature of the acid. Controlled-potential electrolysis of 1+ in the presence of HX (2 equiv.) leads to the generation of binding sites and finally gives products identical to those obtained from protonation of 2 by HX. The complex [Mo2(Cp)2(µ-SMe)3(µ-η1,η1-OCOCF3)] (3) which can be obtained either by protonation of 2 by CF3CO2H or by reduction of 1+ in the presence of CF3CO2H, has been characterized crystallographically. In the presence of HBF4 protonation of 1+ gives 22+. The reactivity of 2+ and of the complexes [Mo2(Cp)2(µ-SMe)3(H2O)(TsO)] (4) and [Mo2(cp)2(µ-SMe)3(H2O)L]+ (L = H2O or THF) (6+), all of which contain a terminal aqua ligand, has also been investigated. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

International audience; The design of the new FeMo heterobimetallic species [FeMo(CO)(5)(kappa(2)-dppe)(mu-pdt)] is reported. Mossbauer spectroscopy and density functional theory calculations give deep insight into the electronic and structural properties of this compound.

International audience; The μ-alkylidyne complex [Mo2Cp2(μ-SMe)3(μ-CCH2Ph)] (1) reacts with HBF4 in acetonitrile to give the unstable bis-nitrile species [Mo2Cp2(μ-SMe)2(μ-CCH2Ph)](NCCH3)2](BF4) (2). Treatment with either borohydride or chloride converts 2 into [Mo2Cp2(μ-SMe)2(μ-CCH2Ph)(μ-κ1:κ1-BH4)] (3) or [Mo2Cp2(μ-SMe)2(μ-CCH2Ph)(μ-Cl)] (4), respectively. Clean evolution of 4 in non-degassed solvent affords the novel μ-thioalkylidene derivative [Mo2(O)(Cl)Cp2(μ-SMe)(μ-MeSCCH2Ph)](5).

International audience; The electrochemical reduction of isomeric complexes, [Mo2Cp2(μ-SMe)3(μ-η1:η1-HCCPh)]+ (1+) and [Mo2Cp2(μ-SMe)3(μ-η1:η2-C[double bond, length as m-dash]CHPh)]+ (3+), where the hydrocarbyl bridges in a η1:η1- or a η1:η2 mode, has been studied by cyclic voltammetry and controlled-potential electrolysis in thf–[NBu4][PF6] and CH2Cl2–[NBu4][PF6], in the absence and in the presence of acid. The binding mode of the CC fragment induces different electrochemical behaviour of the complexes in acid-free solutions since 1+ reduces in two diffusion-controlled one-electron steps while the first reduction of 3+ is characterized by slow electron transfer kinetics. Controlled-potential reduction of both 1+ and 3+ produces a mixture of the acetylide [Mo2Cp2(μ-SMe)3(μ-η1:η2-CCPh)] (2) and alkylidyne complexes [Mo2Cp2(μ-SMe)3(μ-η1-CCH2Ph)] (4). In the presence of acid, the electrochemical reduction of 1+ and of 3+ occurs according to ECE processes. The nature of the products formed by controlled-potential reduction of 1+ depends on the nature of the acid and of the solvent. The transient formation of a complex with a μ-alkenyl ligand, either [Mo2Cp2(μ-SMe)3(μ-η1:η2-CH[double bond, length as m-dash]CHPh)] (7) or an isomer, is suggested by the oxidative electrochemistry of 7 and by its reaction with acids. In thf–[NBu4][PF6] in the presence of an excess of acid (HBF4/Et2O) and of phenylacetylene, electrolysis of 1+ gives rise to catalytic reduction of phenylacetylene to styrene. However, unidentified reactions limit the efficiency of this process. The reduction of 3+ in acidic medium produces the alkyl complex [Mo2Cp2(μ-SMe)3(μ-CH2CH2Ph)] (6) through alkylidyne [Mo2Cp2(μ-SMe)3(μ-η1-CCH2Ph)] (4) and alkylidene [Mo2Cp2(μ-SMe)3(μ-η1-CHCH2Ph)]+ (5+) intermediates. Some ethylbenzene was formed after reduction of 5+ in the presence of acid. These results show an effect of the binding mode of the hydrocarbyl bridge on the mechanism and products of the reduction of the corresponding complexes.

The electrochemical oxidation of [Mo2(cp)2(μ-SMe)3(μ-N2Ph)] and [Mo2(cp)2(μ-SMe)3(μ-N2HPh)]+ complexes where the diazo bridge adopts either an η1 or an η1:η1 coordination mode has been studied by cyclic voltammetry and controlled-potential electrolysis in THF– and CH2Cl2–[NBu4][PF6]. The electrochemical oxidation of [Mo2(cp)2(μ-SMe)3(μ-η1-N2Ph)] 1 and of [Mo2(cp)2(μ-SMe)3(μ-η1-N2HPh)]+1-H+++ triggers the isomerization of the diazo bridge to the η1:η1 mode found in 2+++ and 2-H2+2+2+ respectively. The electrochemical oxidation of [Mo2(cp)2(μ-SMe)3(μ-η1:η1-N2Ph)] 3 and of [Mo2(cp)2(μ-SMe)3(μ-η1:η1-HN2Ph)]+3-H+++ with a syn (“up–up”) arrangement of the Me substituents of the equatorial sulfur bridges is also followed by an isomerization to 2+++ and 2-H2+2+2+, respectively, with an anti (“up–down”) configuration of the equatorial Me groups. The rates of the isomerization 1++ → 2+++, 1-H2+2+2+ → 2-H2+2+2+, and 3-H2+2+ → 2-H2+2+2+ were studied by cyclic voltammetry at different scan rates and at different temperatures. The isomerization of the protonated complexes with either a hydrazido(2−) or a diazene bridge (respectively 1-H2+2+2+ and 3-H2+2+) is faster than that of the diazenido precursors (respectively 1++ and 3++). The diazenido complex 2+++ protonates readily, affording 2-H2+2+2+, while 2-H3+3+ undergoes proton loss.