Nitrogen atom transfer and redox chemistry of terpyridyl phosphoraniminato complexes of osmium (IV)

Article abstract of DOI:10.1016/s0020-1693(97)06116-1

Rapid reactions occur between [Os^VI(tpy) (Cl)₂(N)] X (X = PF₆^-, Cl^-, tpy=2,2′:6′,2″-terpyridine) and aryl or alkyl phosphines (PPh₃, PPh₂Me, PPhMe₂, PMe₃ and PEt₃) in CH₂Cl₂ or CH₃CN to give [Os^IV(tpy)(Cl)₂(NPPh₃)]⁺ and its analogs. The reaction between trans[Os^VI(tpy) (Cl)₂(N)]⁺ and PPh₃ in CH₃CN occurs with a 1:1 stoichiometry and a rate law first order in both PPh₃ and Os^VI with k(CH₃CN, 25°C) = 1.36±0.08×10⁴ M^-1 s^-1. The products are best formulated as paramagnetic d⁴ phosphoraniminato complexes of Os^VI based on a room temperature magnetic moment of 1.8 μ_B for trans-[Os^IV(tpy)(Cl)₂(NPPh₃)] (PF₆), contact shifted ¹H NMR spectra and UV-Vis and near-IR spectra. In the crystal structures of trans-[Os^IV(tpy)(Cl)₂(NPPh₃)] (PF₆) · CH₃CN (monoclinic, P2₁/_n, with a=13.384(5) A?, b=15.222(7) A?, c=17.717(6) A?, β=103.10(3)°, V=3516(2) A?³, Z=4, R=3.40, R_w=3.50) and cis-[Os^IV(tpy)(Cl)₂(NPPh₂Me)]-(PF ₆)·CH₃CN (monoclinic, P2₁/c, with a=10.6348(2) A?, b=15.146(9) A?, c=20.876(6) A?, β=97.47(1)°, V=3334(2) A?³, Z=4, R=4.00, R_w=4.90), the long Os-N(P) bond lengths (2.093(5) and 2.061(6) A?), acute Os-N-P angles (132.4(3) and 132.2(4)°), and absence of a significant structural trans effect rule out significant Os-N multiple bonding. From cyclic voltammetric measurements, chemically reversible Os^V/IV and Os^IV/III couples occur for trans-[Os^IV(tpy)(Cl)₂(NPPh₃)](PF₆) in CH₃CN at +0.92 V (Os^V/IV) and -0.27 V (Os^IV/III) versus SSCE. Chemical or electrochemical reduction of trans-[Os^IV(tpy)(Cl)₂(NPPh₃)](PF₆) gives isolable trans-Os^III(tpy)(Cl)₂(NPPh₃). One-electron oxidation to Os^V followed by intermolecular disproportionation and PPh₃ group transfer gives [Os^VI(tpy)Cl₂(N)]⁺, [Os^III(tpy)(Cl)₂(CH₃CN)]⁺ and [Ph₃P=N=PPh₃]⁺ (PPN⁺). trans-[Os^IV(tpy)(Cl)₂(NPPh₃)](PF₆) undergoes reaction with a second phosphine under reflux to give PPN⁺ derivatives and Os^II(tpy) (Cl)₂(CH₃CN) in CH₃CN or Os^II(tpy) (Cl)₂(PR₃) in CH₂Cl₂. This demonstrates that the Os^VI nitrido complex can undergo a net four-electron change by a combination of atom and group transfers.