Osmium ligand deprotonation

Photochemical reactions involving the osmium(II) complex [Os(bipy)3] have been discussed in relation to electronic structural models for this and related complexes [Ru(bipy)3] and [Fe(bipy)3]. The mechanism of base hydrolysis of the osmium(III) equivalent [Os(bipy)3] has been discussed, including consideration of ligand deprotonation as an initial step. The dinuclear osmium(III) complex [(H3N)50s( -N2)-Os(NH3)5] aquates readily, whereas the Os(III)Os(II) mixed-valence cation is indefinitely stable in water. This contrasts with the ruthenium situation, where the mixed-valence [(H3N)5Ru(/i-N2)Ru(NH3)5] is relatively labile. ... 

A similar approach to the one described above was utilized for the formation of quinone methide derivatives of osmium.14 Reaction of OsCl2(PPh3)3 with a phenolic diphosphine ligand in the presence of Et3N resulted in phosphine exchange followed by C—H activation and deprotonation by the base to form the two isomeric QM... 

Intermolecular addition of carbon nucleophiles to the ri2-pyrrolium complexes has shown limited success because of the decreased reactivity of the iminium moiety coupled with the acidity (pKa 18-20) of the ammine ligands on the osmium, the latter of which prohibits the use of robust nucleophiles. Addition of cyanide ion to the l-methyl-2//-pyr-rolium complex 32 occurs to give the 2-cyano-substituted 3-pyrroline complex 75 as one diastereomer (Figure 15). In contrast, the 1-methyl-3//-pyrrolium species 28, which possesses an acidic C-3-proton in an anti orientation, results in a significant (-30%) amount of deprotonation in addition to the 2-pyrroline complex 78 under the same reaction conditions. Uncharacteristically, 78 is isolated as a 3 2 ratio of isomers, presumably via epimerization at C-2.17 Other potential nucleophiles such as the conjugate base of malononitrile, potassium acetoacetate, and the silyl ketene acetal 2-methoxy-l-methyl-2-(trimethylsiloxy)-l-propene either do not react or result in deprotonation under ambient conditions. 

A related process has been reported for the synthesis of osmium nitrido complexes with 1,2-benzenedithiolate ligands. Thus, the synthesis of 173 was effected by treatment of (Bu4N)[OsNC14] with deprotonated 1,2-benzenedithiol (Eq. (70)). Interestingly, when 173 is treated with Me3OBF4 alkylation is observed to occur on the sulfur this contrasts the reaction of 173 with Ph3CPF6, for which reaction at the terminal nitride is observed [93]. 

Several groups have completed computational studies on the relative stabilities of osmium carbyne, carbene, and vinylidene species. DFT calculations on the relative thermodynamic stability of the possible products from the reaction of OsH3Cl(PTr3)2 with a vinyl ether CH2=CH(OR) showed that the carbyne was favored. Ab initio calculations indicate that the vinylidene complex [CpOs(=C=CHR)L]+ is more stable than the acetylide, CpOs(-C=CR)L, or acetylene, [CpOs() -HC=CR)L]+, complexes but it doesn t form from these complexes spontaneously. The unsaturated osmium center in [CpOsL]+ oxidatively adds terminal alkynes to give [CpOsH(-C=CR)L]+. Deprotonation of the metal followed by protonation of the acetylide ligand gives the vinylidene product. 

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... 

Polymers containing benzimidazole units in their backbones have also been used in the synthesis of coordination metallopolymers (159-162). Osmium and ruthenium coordinated polymers with bipyridine ligands have been prepared (159,160). These polymers (72, 73) possessed metal-metal interactions through their conjugated backbones. Communication between the ruthenium centers of 72 increased by deprotonating the imidazole protons (160). The osmium coordinated polymer (73) showed two reduction waves separated by 0.32 V, indicative of strong cnmmimication between the Os centers (159). 

The formation of either mononuclear or binuclear ruthenium and osmium carbonyl complexes bearing bis(2-pyridyl)amine (Hdpa) or the deprotonated ligand (dpa) can be driven by choice of solvent and reaction temperature. For example, [Ru3(GO)i2] reacts with Hdpa in HGl solution at 200 °G to give mainly m,m-[Ru(hdpa) (GO)2Gl2], whereas in toluene solution in the absence of HGl at 200 °G, Ru(dpa)(GO)2 2 predominates. In related chemistry, [Ru(GO)2Gl2] reacts with bis(pyrazol-l-yl)methane (BPM), bis(3,5-dimethylpyrazol-l-yl)... 

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