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

Rapid reactions occur between [Os-VI(tpy)(Cl)(2)(N)]X(X=PF6-, Cl-, tpy=2,2':6',2 degrees-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 degrees C)=1.36+/-0.08x10(4) M-1 s(-1). The products are best formulated as paramagnetic d(4) phosphoraniminato complexes of Os-VI based on a room temperature magnetic moment of 1.8 mu(B) for trans-[Os-IV(tpy)(Cl)(2)(NPPh3)](PF6), contact shifted H-1 NMR spectra and UV-Vis and near-IR spectra. In the crystal structures of trans-[Os-IV(tpy)(Cl)(2)(NPPh3)](PF6). CH3CN (monoclinic, P2(1)/n, with a=13.384(5) Angstrom, b=15.222(7) Angstrom, c=17.717(6) Angstrom, beta=103.10(3)degrees, V=3516(2) Angstrom(3), Z=4, R=3.40, R-w=3.50) and cis-[Os-IV(tpy)(Cl)(2)(NPPh2Me)](PF6). CH3CN (monoclinic, P2(1)/c, with a=10.6348(2) Angstrom, b=15.146(9) Angstrom, c=20.876(6) Angstrom, beta=97.47(1)degrees, V=3334(2) Angstrom(3), Z=4, R=4.00, R-w=4.90), the long Os-N(P) bond lengths (2.093(5) and 2.061(6) Angstrom), acute Os-N-P angles (132.4(3) and 132.2(4)degrees), 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)Cl-2(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. (C) 1998 Elsevier Science S.A.