Synthesis and reactivity of electron deficient benzoheterocycle triosmium complexes

A series of electron deficient benzoheterocycle complexes, Os3(CO) 9(mu3-eta2-L-H)(mu-H) (L = Phenanthridine, 29; 2-methylbenzimidazole, 32; 2,3-dimethylbenzimidazole, 71; 2-methylbenzotriazole, 33, benzoxazole, 37; 2-methylbenzoxazole, 38; benzothiazole, 39; 2-methylbenzothiazole, 40; quinoxaline, 43 and 2-methylquinoxaline, 66 ) of triosmium carbonyl were synthesized. These complexes possess a special mu3-eta2-bonding mode at the C(7) or C(8)-position of the carbocyclic ring and a protective coordination of the nitrogen atom to the metal core which changes the normal reactivity of these benzoheterocycles. Sequential reactions of these complexes with H -/H+ have been studied. Complexes of phenanthridine and other benzoheterocycles with an alkyl group at the C(2) or C(3)-positions of the heterocyclic ring such as, 2-methylbenzimidazole, 2,3-dimethylbenzimidazole, 2-methylbenzothiazole, 2-methylbenzothiazole, 2-methylquinoxaline undergo regioselective nucleophilic addition reactions at the carbocyclic ring to form sigma-pi vinyl complexes. For 2-methylbenzoxazole nucleophilic addition reaction occurs at the 2-position of the heterocyclic ring to give a ring opened sigma-pi vinyl complex. Electron deficient complexes without an alkyl at the heterocyclic ring undergo hydride attack at the C(2)-position of the heterocyclic ring followed by protonation at the metal core to form dihydride complexes.; The studies of these complexes with relatively bulky carbon based nucleophiles show that complexes with or without an alkyl group at the heterocyclic ring all go regioselective nucleophilic addition reactions at the carbocyclic ring to give sigma-pi vinyl complexes. These nucleophilic products have been rearomatized by facile oxidation of the intermediate anions without adding any deprotonating agent.; These complexes undergo ligand addition at the metal core with soft nucleophiles such as PPh3 but with harder nucleophiles, both small (hydride) and relatively bulky carbon based carbanion (isobutyronitrile), nucleophilic attack occur at the benzoheterocycle ligands. In light of this diverse reactivity, a study of the reactivity of these complexes with n-butyl amine and carboxylic acid which are intermediate in nucleophilicity relative to phosphine and amine was carried out to define the stereodynamics of their coordination modes. The structural features of these coordinated complexes and the mechanistic implications of these transformations have been discussed and compared with the previously reported complexes with quinoline.