Ruthenium porphyrins hydride complexes

A ruthenium porphyrin hydride complex was lirst prepared by protonation of the dianion, [Ru(TTP) in THF using benzoic acid or water as the proton source. The diamagnetic complex, formulated as the anionic Ru(If) hydride Ru(TTP)(H )(THF)l , showed by H NMR spectroscopy that the two faces of the porphyrin were not equivalent, and the hydride resonance appeared dramatically shifted upheld to —57.04 ppm. The hydride ligand in the osmium analogue resonates at —66.06 ppm. Reaction of [Ru(TTP)(H)(THF)j with excess benzoic-acid led to loss of the hydride ligand and formation of Ru(TTP)(THF)2. 

The first HNO complex, Os(PPh3)2(CO)(HNO)Cl2, was reported in 1970 upon exposure of HC1 to Os(PPh3)2(CO)(NO)Cl (173), and the X-ray crystal structure was published in 1979 (174). Recent interest has resulted in isolation of additional examples of HNO complexes, and the structures of three similar complexes have been reported [(Ru(HNO)(2,6-bis(2-mercapto-3,5-di-fert-butyl-phenylthio)dimethylpyridine) (175), ReCl(CO)2(PR3)2(HNO) (176), and IrHCl2 (PPh3)2(FINO) (177)]. Preparative routes generally involve protonation, hydride addition, or reduction of a coordinated nitrosyl (175, 176, 178-186). Farmer and co-workers also described the first synthesis of an HNO complex directly as a result of exposure to a donor compound (187) while Lee and Richter-Addo recently observed the HNO adduct of a heme model complex [ruthenium porphyrin (188)]. 

Rhodium-based catalysis suffers from the high cost of the metal and quite often from a lack of stereoselectivity. This justifies the search for alternative catalysts. In this context, ruthenium-based catalysts look rather attractive nowadays, although still poorly documented. Recently, diruthenium(II,II) tetracarboxylates [42], polymeric and dimeric diruthenium(I,I) dicarboxylates [43], ruthenacarbor-ane clusters [44], and hydride and silyl ruthenium complexes [45 a] and Ru porphyrins [45 b] have been introduced as efficient cyclopropanation catalysts, superior to the Ru(II,III) complex Ru2(OAc)4Cl investigated earlier [7]. In terms of efficiency, electrophilicity, regio- and (partly) stereoselectivity, the most efficient ruthenium-based catalysts compare rather well with the rhodium(II) carboxylates. The ruthenium systems tested so far seem to display a slightly lower level of activity but are somewhat more discriminating in competitive reactions, which apparently could be due to the formation of less electrophilic carbenoid species. This point is probably related to the observation that some ruthenium complexes competitively catalyze both olefin cyclopropanation and olefin metathesis [46], which is at variance with what is observed with the rhodium catalysts. 

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