Osmium(VI) nitrido and osmium(IV) phosphoraniminato complexes containing schiff base ligands

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 [NBuⁿ₄][Os^VI(N)Cl₄] in the presence of 2,6-dimethylpyridine. The ν(Os≡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)]ClO₄ and [Os(N)(5,5-Cl₂salophen)(MeOH)]ClO₄ have been determined by X-ray crystallography, and the Os≡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(NPPh₃)(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 σ_p of the substituents on the Schiff base ligand. The structure of [Os^IV(NPPh₃)(salophen)-Cl] has been determined by X-ray crystallography. The rather long Os-N(P) bond length (1.92 A) and acute Os-N-P bond angle (149.6°) 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 PPh₃ have been studied in CH₃CN at 25.0 °C by stopped-flow spectrophotometric method. The following rate law was obtained: -d[Os(VI)]/dt = k₂[Os(VI)][PPh₃]. The reactivities of the complexes were found to follow a Hammett correlation of log(k_X/k_H) with σ_p, with a ρ value of 1.9 ± 0.1. The positive ρ value is consistent with a transition state involving electrophilic attack by the nitrido ligand on the phosphorus atom. © 1999 American Chemical Society.