title: Osmium(VI) Nitrido and Osmium(IV) Phosphoraniminato Complexes Containing Schiff Base Ligands.
abstract: A series of osmium(VI) nitrido complexes containing Schiff base ligands, [Os(VI)(N)(L)Cl] (L = salophen or salen), have been synthesized by reaction of the ligand with [NBu(n)(4)][Os(VI)(N)Cl(4)] in the presence of 2,6-dimethylpyridine. The nu(Os&tbd1;N) for the salophen complexes occur at around 1070 cm(-)(1) and are insensitive to the nature of the substituents present on the Schiff base ligand. The structures of [Os(N)(salophen)(MeOH)]ClO(4) and [Os(N)(5,5-Cl(2)salophen)(MeOH)]ClO(4) have been determined by X-ray crystallography, and the Os&tbd1;N bond distances are 1.651 and 1.66 Å, respectively. The osmium(VI) nitrido complexes react rapidly with triphenylphosphine to produce the corresponding osmium(IV) phosphoraniminato complexes, [Os(IV)(NPPh(3))(L)Cl]. The osmium(IV) complexes exhibit reversible Os(V/IV) and Os(IV/III) couples in cyclic voltammetry. The E(1/2) values show linear correlations with the Hammett constants sigma(p) of the substituents on the Schiff base ligand. The structure of [Os(IV)(NPPh(3))(salophen)Cl] has been determined by X-ray crystallography. The rather long Os-N(P) bond length (1.92 Å) and acute Os-N-P bond angle (149.6 degrees ) suggest that there is no significant multiple-bond character in the Os-N bond. The kinetics of nitrogen atom transfer from a series of 5,5'-disubstituted salophen nitrido complexes to PPh(3) have been studied in CH(3)CN at 25.0 degrees C by stopped-flow spectrophotometric method. The following rate law was obtained: -d[Os(VI)]/dt = k(2)[Os(VI)][PPh(3)]. The reactivities of the complexes were found to follow a Hammett correlation of log(k(X)/k(H)) with sigma(p), with a rho value of 1.9 +/- 0.1. The positive rho value is consistent with a transition state involving electrophilic attack by the nitrido ligand on the phosphorus atom.
authors.affiliation: Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Hong Kong, People's Republic of China, and Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, People's Republic of China.
authors.forename: Tsz-Wing
authors.initials: TW
authors.lastname: Wong
authors.forename: Tai-Chu
authors.initials: TC
authors.lastname: Lau
authors.forename: Wing-Tak
authors.initials: WT
authors.lastname: Wong