Osmium carbene complexes, preparation

Diazoalkanes are u.seful is precursors to ruthenium and osmium alkylidene porphyrin complexes, and have also been investigated in iron porphyrin chemistry. In an attempt to prepare iron porphyrin carbene complexes containing an oxygen atom on the /(-carbon atom of the carbene, the reaction of the diazoketone PhC(0)C(Ni)CH3 with Fe(TpCIPP) was undertaken. A low spin, diamagnetic carbene complex formulated as Fe(TpCIPP)(=C(CH3)C(0)Ph) was identified by U V-visible and fI NMR spectroscopy and elemental analysis. Addition of CF3CO2H to this rapidly produced the protonated N-alkyl porphyrin, and Bit oxidation in the presence of sodium dithionitc gave the iron(II) N-alkyl porphyrin, both reactions evidence for Fe-to-N migration processes. ... 

The most significant and widely studied reactivity of the ruthenium and osmium porphyrin carbene complexes is their role in catalyzing both the decomposition of diazoesters to produce alkenes and the cyclopropanation of alkenes by diazoesters. Ethyl diazoacetate is used to prepare the carbene complex 0s(TTP)(=CHC02Et)... 

Scheme 8.20 Preparation of the five-coordinate osmium carbynes 75a,b and their reactions with chalcogens, HCI and chlorine to afford the six-coordinate carbene osmium(II) complexes 76a,b, 77a,b, and 78a,b (a R = Me, b R = Ph E = S, Se, Te)...

A series of osmium meso-tetra-p-tolyl-porphyrin carbene complexes, (TTP) Os(CRR )(R,R = p-tolyl R = H, R = SiMe3 or C02Et) were first prepared by Woo and Smith [ 149] by treating [(TTP)20s] with the appropriate diazoalkanes (Eq. 19). As an indirect method, one of these carbene complexes can also be prepared by reaction of the silylene complex (TTP)Os(SiEt2)(THF) (150) with di-p-tolyl diazomethane. 

Che and coworkers [152] were able to isolate and characterize a pure bis-carbene (TPFPP)Os(CPh2)2 (Fig. 3). The bis-carbene species represents the first structurally characterized fra s-bis-carbene metal complex whose carbene groups are not stabilized by heteroatoms. The related pentacoor-dinated mono-carbene complex was also prepared and characterized by an X-ray structure. A comparison of the reactivity of these complexes with olefins suggests that the bis-carbene species acts as an intermediate in cy-clopropanation. Thus, the inertness of the mono-carbene complex towards stoichiometric styrene cyclopropanation and the observation of an efficient cyclopropanation of styrene in the presence of the bis-carbene complex as a catalyst support this suggestion [152]. A recent X-ray structure determination for (TPFPP)Os(CPh2)(MeIm) revealed an Os = C distance of 1.902(3) A (Table 3) [141]. Recently, Che and coworkers [153] and Miyamoto and coworkers [154] reported oxo-bridged carbene complexes of osmium porphyrins (see Table 3). These compounds are rare examples of oxo-binuclear carbene complexes. 

Dimetallacycles are commonly synthesized by substitution of a geminal dihalide (Equation (27)) <86JOM(3l4)C43, 89JOM(359)205>, (Equation (28)) <88JA7868>. The osmium compound can also be prepared by elimination of methane from an acyclic complex (Equation (29)) <82JA7325>. Many dimetallics have been synthesized by additions to metal carbenes <82AOC(20)159>. Transmetallation reactions are also known (Equation (30)) <90ICAll>. 

A series of benzylidene and low-valent alkylidene complexes have also been prepared from sulfur ylides. Deprotonation of benzyl or alkyl diphenylsulfonium salts with a strong base, such as KN(SiMe3)2, KNPr j or LiNPr generates the corresponding sulfur ylide. Reaction of this ylide with low-valent ruthenium, osmium, rhodium, or iridium complexes containing a labile ligand leads to transfer of the carbene unit from sulfur to... 

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