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The paramagnetic μ-amidato species [Mo2Cp2(μ-MeCONH)(μ-SMe)3]BF42a+ (Cp = η-C5H5) is formed exclusively when the compound [Mo2Cp2(MeCN)2(μ-SMe)3]BF41a in powder form is kept in air, whereas the same starting complex in solution gives a mixture of the μ-amidato species and the μ-oxo complex [Mo2Cp2(μ-O)(μ-SMe)3]BF43. The oxo complex is formed quantitatively on warming [Mo2Cp2(μ-Cl)(μ-SMe)3]BF4 in water. The reduction of 2a+ by NaBH4 in CH3CN affords the molecular product [Mo2Cp2(μ-MeCONH)(μ-SMe)3] 2a which has been characterised by X-ray analysis.

Monocubane clusters with exo-iron atoms were synthesized from the iron-bridged dicubane [NEt4]3[Mo2Fe7S8(SEt)12] by reactions with π-acid ligands. The structures of two derivatives, [NEt4][MoFe3S4X3(µ-SEt)3Fe(CNtBu)3], X = Cl or PhS, have been established by X-ray crystallography. Electrochemical and Mossbauer data showed that the cubane core retains the {MoFe3S4}3+ redox level of the parent material and that the exo-iron centre is in the low-spin iron(II) state. Infrared and electrochemical data showed that there is little electronic interaction between the cubane and exo-iron centres. The parent complex possesses a low-spin bridging iron(III) centre. This centre undergoes slow electron transfer which is ascribed to spin-state reorganisation to high-spin FeII. Reduction of the cubane units leads to reversible deco-ordination of a bridging thiolate group from a molybdenum centre.

The electrochemical reduction of molybdenum(IV) alkylimides trans-[MoX(NR)(dppe)2]+ (X = halide, R = alkyl, dppe = Ph2PCH2CH2PPh2) proceeded by two pathways. In the absence of a source of protons, moderately stable five-co-ordinate molybdenum(II) imides are formed via an initial single-electron transfer followed by rate-determining loss of the trans-halide ligand and an additional electron transfer. In the presence of a source of protons, molybdenum–halide bond cleavage is intercepted by protonation at N which gives an amide intermediate. Further electron-transfer chemistry liberates amines and yields a dinitrogen complex in an overall four-electron process. Molybdenum(IV) imides trans-[MoX(NH)(dppe)2]+ were reduced to amide intermediates with the parent imide cation providing the source of protons and water probably acts as a proton-transfer relay. The amide has two fates; it is either converted into a nitride by hydrogen loss or, at a potential which encompasses its further reduction, yields ammonia and a dinitrogen complex.

Reaction of trans-[Mo 2 (cp) 2 (CO) 2 (µ-SR) 2 ] (cp = η-C 5 H 5 ; R = Me, Pr i , Bu t , Ph or CF 3 ; syn and anti isomers) with cyanide ion gave the corresponding cyanide complexes trans-[Mo 2 (cp) 2 (CO)(CN)(µ-SR) 2 ] - , except with R = Bu t where no reaction was observed. For R = CF 3 , two isomers having a syn orientation of the sulfur substituents were obtained. The nature of the R groups is shown to have a crucial influence on the site of the reaction of the cyanide complexes with Me 3 O + . Complexes where R = Me, Pr i (syn and anti isomers) or Ph (anti isomer) were S-methylated, whereas N-methylation was observed for R = Ph (syn isomer) or CF 3 (anti and both syn isomers). This is ascribed to electronic effects of the R groups which control the site of methylation by switching the reaction from orbital control (S-methylation) to charge control (N-methylation). For R = CF 3 , the R groups also affect the reaction of the dicarbonyl precursor with a YZ substrate since the preferred site of attack is different for YZ = CN - and YZ = RNC.

International audience; The access to a co-ordination site in the quadruply bridged complex [Mo2(cp)2(μ-SMe)3(μ-Cl)] (cp = η5-C5H5) has been found to be redox controlled. Electrochemical one-electron oxidation of the complex unlocks the chloride bridge but the radical cation retains the quadruply bridged geometry of the neutral parent as indicated by cyclic voltammetry and EPR spectroscopy. The chloride bridge opens up in the presence of a substrate (Y≡Z = MeCN, ButNC, Me2C6H3NC or CO), leading to the formation of [Mo2(cp)2(μ-SMe)3Cl(Y≡Z)] •+ derivatives. The site is sensitive to the electronic properties of the substrate, and kinetic studies of the substrate-binding step demonstrated that ButNC reacts faster and is bound more tightly at [Mo2(cp)2(μ-SMe)3Cl]•+ than is MeCN. The reduction of [Mo2(cp)2(μ-SMe)3Cl(Y≡Z)] •+ is reversible for Y≡Z = CO and RNC (R = But or C6H3Me2) whereas MeCN is lost on reduction. In this case the fact that the chloride ligand is still present at the neighbouring molybdenum centre allows regeneration of the parent complex via bridge reclosure. The reactivity of [Mo2(cp)2(μ-SMe)3(μ-Cl)] in MeCN has also been investigated instead of the bridge-opening process of the radical cation, the neutral parent loses the chloride bridge in MeCN; the resulting bis(acetonitrile) cation, [Mo2(Cp)2(μ-SMe)3(MeCN)2] +, has been isolated and characterized.

The range of imides trans-[MoCl(NCHR1R2)(dppe)2]+ made from the nitride trans-[MoCl(N)(dppe)2](dppe = Ph2PCH2CH2PPh2, R1= H or organic group, R2= organic group) has been extended; deprotonation of the imide group at the α-carbon gives reactive alkenylamides which have nitrogen ylide character {MoN+–C–HR} and these are attacked by electrophiles. Stepwise C1 homologation of a methylimide to ethyl- and isopropyl-imides can be achieved by successive deprotonation and methylation steps. The crystal structure of the alkenylamide trans-[MoCl(NCHCO2Me)(dppe)2] has been determined, as has that of the imide produced directly from it by C-methylation, trans-[MoCl{NCH(Me)CO2Me}(dppe)2]+. Deprotonation at the α-carbon of an allylimide complex gives a species which is regioselectively and stereospecifically attacked by electrophiles at the γ-carbon; when the electrophile is the proton the overall reaction corresponds to a 1,3-prototropic rearrangement of the allylimide to the E-methylvinylimide. Electroreduction of imides in the presence of a source of protons releases the free amine; in this way esters of the amino acids glycine and DL-alanine have been synthesised. Two protons can be removed by base from imides with electron-withdrawing ester substituents and free cyanoformate esters are released from the metal centre by substitution with dinitrogen or CO.

International audience; Dissymetrically disubstituted di-iron azadithiolate complexes [Fe2(CO)4(κ2-LL){μ-SCH2N(iPr)CH2S}] (LL = dppe, phen) protonate exclusively at the N atom of the bridge, like the hexacarbonyl precursor but in contrast to symmetrically disubstituted analogues; substitution of dppe for two CO groups noticeably increases the kinetics of the electrocatalytic proton reduction process.

Treatment of [Fe(2)(micro-pdt)(CO)(6)] (pdt = S(CH(2))(3)S) with 1,10-phenanthroline (phen) in refluxing toluene affords the asymmetric complex [Fe(2)(micro-pdt)(CO)(4)(phen)] (1); the protonation of with HBF(4).OEt(2) in CD(2)Cl(2) at 203 K has been monitored by (1)H NMR.

Electrochemical dehydrodimerisation of an {Mo–NCHCHCH2} group gives an all-trans-{N+CHCHCHCHCHCHN+} ligand, bridging two MoIV centres; the {(MoIII)–bridge–(MoIV)} mixed-valence state is accessible by electrochemical reduction and exhibits very strong electronic coupling over the 11.7 A which separates the two metal centres; this accords with EHMO calculations which show that the SOMO has substantial (30%) bridging-ligand character; in its capacity to function as a molecular wire linking two metal centres, the eight-atom hexatriene di(imide) chain {N(CH)6N} compares favourably with C8 chains of acetylenic carbons bridging other metal centres.

The substituents of the bridging sulfur atoms exert an electronic control on the site of methylation of the cyanide complex [Mo2(η5-C5H5)2(CO)(CN)(µ-SR)2]–: the methylation can be diverted from a sulfur Ione pair to the cyanide ligand on changing the electronic properties of the R groups from electron-releasing to electron-withdrawing, respectively.

The anomalous redox properties of [MoX2(Me8[16]aneS4)], X - Cl or Br, reported previously are rationalised, the authentic reduction of the dibromide occuring at E1/2=–2.87 V versus ferrocinium–ferrocene to give an unstable 17-electron anion which reacts with carbon monoxide and dinitrogen; low temperature cyclic voltammetry shows the [MoBr2(Me8[16]aneS4)]+/0 system to be fluxional and this is explained by a metal ‘in- or out-of-plane’ deformation accompanying conformational rearrangement of the macrocyclic ligand, X-ray crystallography showing the neutral dibromide to have the ‘all-up’ arrangement of the macrocyclic tetrathioether ligand rather than the ‘up–up–down–down’ conformation of the one-electron oxidised species.

The electrochemical reduction of the 15-electron molybdenum(III) complex [MoCl3(dpepp)], dpepp = Ph2PCH2CH2P(Ph)CH2CH2PPh2, has been studied in the presence of PPh3 under argon and under molecular nitrogen. The primary reduction of the trichloride involves a reversible single-electron transfer to give the unstable 16-electron molybdenum(II) anion [MoCl3(dpepp)]–. Loss of chloride from this anion prior to or following further electron transfer provides two principal pathways to molybdenum(0) products, [Mo(η6-C6H5PPh2)(dpepp)] under Ar or cis-[Mo(N2)2(dpepp)(PPh3)] under molecular nitrogen, a relatively high potential pathway via molybdenum(I) solvated intermediates and a low potential pathway via trans-[MoCl2(dpepp)(PPh3)]. The cis-dinitrogen complex is unstable on the preparative time-scale, small amounts of trans-[Mo(N2)2(dpepp)(PPh3] are formed, presumably by isomerisation, but the major product is the η6-arene complex.

The compounds [(η5-C5H5)(CO)2[graphics omitted](CO)4L](1)(L = CO) and (2)[L = P(OMe)3] have been studied by electrochemical methods (cyclic voltammetry, controlled-potential electrolysis, and coulometry) in propylene carbonate. The substitution of P(OMe)3 for CO at the tungsten centre is shown to have various effects on the electrochemical behaviour. Both (1) and (2) are reduced according to an e.c.-disp. process (second electron transfer is homogeneous) which differs depending on the nature of L. The results reported are consistent with the presence of an acceptor–donor metal–metal bond in both compounds.

The compounds [(η5-C5H5)(CO)3M(µ-SR)W(CO)5] have been studied by usual electrochemical methods (cyclic voltammetry, controlled-potential electrolysis, and coulometry). They undergo a destructive two-electron reduction according to an electrochemical–chemical–electrochemical process with a diffusion-controlled intervening chemical step. The [W(CO)5(SMe)]– anion generated by the reduction of the complexes undergoes chemical reactions leading to the dinuclear species [W2(CO)10(µ-SMe)]– on the time-scale of controlled-potential electrolyses.

A radical cation, transiently produced in a single step two-electron transfer, is shown to be the reactive species in a substitution process which can be oxidatively-induced or catalysed via an electron transfer chain mechanism.

Protonation of trans-[W(N2)2(Ph2PCH2CH2PPh2)2] with toluene-p-sulphonic acid gives the hydrazido cation trans-[W(NNH2)(p-MeC6H4SO3)(Ph2PCH2CH2PPh2)2]+. Reduction of this cation at a Hg-pool electrode in a tetrahydrofuran electrolyte under N2(1 atm) affords free ammonia with regeneration of the parent dinitrogen complex in excellent yield. Protonation of the regenerated product allows the system to be cycled and this can be performed in one vessel, the cathode compartment of the electrolysis cell.