Unusual transformation of n-arylbenzohydroxamic acids mediated by osmium. Formation of organometallic complexes of osmium(III)

Cyclometalated Osmium Compounds and beyond: Synthesis, Properties, Applications

The synthesis of cyclometalated osmium complexes is usually more complicated than of other transition metals such as Ni, Pd, Pt, Rh, where cyclometalation reactions readily occur via direct activation of C–H bonds. It differs also from their ruthenium analogs. Cyclometalation for osmium usually occurs under more severe conditions, in polar solvents, using specific precursors, stronger acids, or bases. Such requirements expand reaction mechanisms to electrophilic activation, transmetalation, and oxidative addition, often involving C–H bond activations. Osmacycles exhibit specific applications in homogeneous catalysis, photophysics, bioelectrocatalysis and are studied as anticancer agents. This review describes major synthetic pathways to osmacycles and related compounds and discusses their practical applications.

Easy access to bio-inspired osmium(II) complexes through electrophilic intramolecular C(sp2)-H bond cyclometalation

Mild electrophilic C(sp2)-H cyclometalation of 2-phenylpyridine and N,N-dimethylbenzylamine by the chloro-bridged osmium(II) dimer [OsCl(micro-Cl)(eta6-C6H6)]2 in acetonitrile affords cyclometalated pseudotetrahedral OsII complexes [Os(C approximately N)(eta6-C6H6)(NCMe)]PF6 (C approximately N=o-C6H4py-kappa C,N (2) and o-C6H4CH2NMe2-kappa C,N (5), respectively) in good to excellent yields. The cyclometalation reactions are super sensitive to the nature of an external base. Sodium hydroxide is essential for cyclometalation of 2-phenylpyridine, but NaOH retards metalation of N,N-dimethylbenzylamine, the tertiary amine being self-sufficient as a base. Further reactions of compounds 2 and 5 with 1,10-phenanthroline or 2,2'-bipyridine (N approximately N) lead to the substitution of the eta6-bound benzene to produce octahedral species [Os(C approximately N)(N approximately N)(NCMe)2]PF6 or [Os(C approximately N)(N approximately N)2]PF6 in MeCN or MeOH as solvent, respectively. The cis configuration of the MeCN ligands in [Os(C approximately N)(phen)(NCMe)2]PF6 has been confirmed by an X-ray crystallographic study. Electrochemical investigation of the octahedral osma(II)cycles by cyclic voltammetry showed a pseudoreversible MIII/II redox feature at (-50)-(+109) and 190-300 mV versus Ag/AgCl in water and MeCN, respectively. As a possible application of the compounds, a rapid electron exchange between the reduced active site of glucose oxidase enzyme from Aspergillus niger and the electrochemically generated OsIII species has been demonstrated. The corresponding second-order rate constants cover the range (0.7-4.8)x10(6) M(-1) s(-1) at 25 degrees C and pH 7.

Novel reactivity mode of hydroxamic acids: a metalla-pinner reaction

The reaction between the nitrile complex trans-[PtCl(4)(EtCN)(2)] and benzohydroxamic acids RC(6)H(4)C([double bond]O)NHOH (R = p-MeO, p-Me, H, p-Cl, o-HO) proceeds smoothly in CH(2)Cl(2) at approximately 45 degrees C for 2-3 h (sealed tube) or under focused 300 W microwave irradiation for approximately 15 min at 50 degrees C giving, after workup, good yields of the imino complexes [PtCl(4)[NH[double bond]C(Et)ON[double bond]C(OH)(C(6)H(4)R)](2)] which derived from a novel metalla-Pinner reaction. The complexes [PtCl(4)[NH[double bond]C(Et)ON[double bond]C(OH)(C(6)H(4)R)](2)] were characterized by elemental analyses (C, H, N), FAB mass spectrometry, and IR and (1)H and (13)C[(1)H] spectroscopies, and [PtCl(4)[NH[double bond]C(Et)ON[double bond]C(OH)(Ph)](2)] (as the bis-dimethyl sulfoxide solvate), by X-ray single-crystal diffraction. The latter disclosed its overall trans-configuration with the iminoacyl species in the hydroximic tautomeric form in E-configuration which is held by N[bond]H...N hydrogen bond between the imine [double bond]NH atom and the hydroximic N atom.