Molybdenum complexes monomeric

I) Simple Monomeric Molybdenum Complexes. The first report on the synthesis of simple Mo dithio complexes appeared in 1971 when Mo(IV)(R2Dtc)4 complexes were obtained by CS2 insertion into Mo(NR2)4 complexes (68). Shortly after this report, the synthesis of these complexes from MoC14 and NaR2Dtc in acetonitrile was reported (88). A variety of tetrakis R2Dtc complexes of Mo(IV) also were obtained by the oxidative decarbonyla-tion of Mo(CO)6 with tetraalkyl thiuram disulfide (481,482, 609). 

The reactions of dinuclear thioaldehyde and -ketone complexes with PR3 take a different course. When the dimeric complexes 112 are treated with PMe3 a Ti-S bond in 112 is cleaved and the monomeric complexes 113 are reversibly formed (see Scheme 26), 69 The reaction of 112 with benzene-thiol led to a Ti-C cleavage and formation of 1,3-propanedithiolato-(phenylthiolato) complex.69 The reaction of binuclear p-191 rf-thioketone molybdenum complexes related to 115 (M = Mo, E = S, R = Aryl) with P(OEt)3 also gave mononuclear complexes [Mo(CO)2(Et) i72-S = C(Aryl)2 (i75-C5H5)] by a rather complicated reaction sequence. When other phosphites were employed additionally binuclear complexes derived from addition of the phosphite to one molybdenum atom were isolated.225... 

In contrast to the molybdenum complex, [CpCr(CO)3]2 gives ions only up to m/e 202 and 201, corresponding to [CpCr(CO)3H]+ and [CpCr(CO)3]+, respectively (151). The ions CpCr(CO)+ ( = 0-2) were also produced, but no bimetallic ions have been observed, suggesting that the compound may be monomeric in the vapor phase and implying a very weak metal-metal bond. 

In order to avoid the unwanted dimerization, steric constraints were incorporated into the used ligands. This hinders the metal centres to approach each other, so either the equilibrium is shifted to the side of the monomeric molybdenum species or the dimerization completely prevented. One of the first bulky sulfur-containing ligands that was able to successfully promote oxygen atom transfer catalysis of its molybdenum complex was developed by Berg and Holm (Scheme 4.7 BuLNS = bis(4-tert-butylphenyl)-2-pyridylmethanthiolate). ... 

Hydride transfer reactions from [Cp2MoH2] were discussed above in studies by Ito et al. [38], where this molybdenum dihydride was used in conjunction with acids for stoichiometric ionic hydrogenations of ketones. Tyler and coworkers have extensively developed the chemistry of related molybdenocene complexes in aqueous solution [52-54]. The dimeric bis-hydroxide bridged dication dissolves in water to produce the monomeric complex shown in Eq. (32) [53]. In D20 solution at 80 °C, this bimetallic complex catalyzes the H/D exchange of the a-protons of alcohols such as benzyl alcohol and ethanol [52, 54]. 

The use of /r-hydroxo or ju-alkoxo bridged polynuclear complexes of chromium, molybdenum, tungsten, or rhenium in this route leads to the formation of monomeric bis(NHC) complexes, to the elimination of hydrogen, and to the partial oxidation of the metal [Eq.(ll)]. Chelating and nonchelating imidazolium salts as well as benzimidazolium and tetrazolium salts can be used. 

The formation of a monomeric amidino complex has been reported for the reaction of rhenium pentachloride with di-isopropylcarbodiimid. A composition of [ReCl4(prop 2N2CCI)] was derived from elemental analysis, spectroscopic data and the crystal structure of the molybdenum analogue. " ... 

All the monomeric molybdenum(III) complexes are paramagnetic, and in Table l5 the magnetic moments of some typical examples are given. For six-coordinate compounds the values lie between 3.53 and 3.86 BM, with most in the region 3.7 to 3.8 BM as predicted for octahedral d3 complexes.37 The value of 1.73 BM in ELt[Mo(CN)7] 2H20 is consistent with a spin-doublet ground state d3 system.3S... 

Molybdenum(V) chemistry is dominated by oxo complexes many of these exist as dimers, but monomeric species can be isolated from strongly addic solutions or under nonaqueous conditions. Non-oxo compounds are also known, both as monomers and as dimers or polymers with halide or sulfur bridges. ESR spectroscopy has been used extensively to investigate the properties of monomeric Mov systems, and has shown the participation of this oxidation state in the reactions of the oxomolybdoenzymes (see Section 36.6.7). 

Monomeric non-oxo molybdenum(V) complexes have been isolated with nitride (N3 ), imide (NR2-) and amide (NRR -) ligands, which may act as models for intermediates in the degradation of dinitrogen in nitrogenase. However, no molybdenum(V) compounds are yet known which correspond to the diazenide or hydrazide analogues of the molybdenum(IV) complexes described in Section 36.4.2.3. 

The molecular complexity of molybdenum (IV) alkoxides is determined by the size and ramification of the alkyl group — the polymeric (R = Me), dimeric (CN = 5, two bridging OR-groups, R = Pr ) with a double metal-metal bond, and even monomers (R = Bu ). The first representatives of W(OR)4 homologous series are tetramers with the [Ti4(OMe)16]-type structure. The derivatives of ramified or bulky alcohols are known only as mixed-ligand complexes (such as dimeric solvates of alkoxide halids with alcohols, various alkoxide hidrides and monomeric complexes with phenantroline, see Table 12.19). 

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