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

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Rapid reactions occur between [Os VI(tpy)(Cl) 2(N)]X (X = PF 6 −, Cl −, tpy = 2,2′:6′,2″-terpyridine) and aryl or alkyl phosphi nes (PPh 3, PPh 2Me, PPhMe 2, PMe 3 and PEt 3) in CH 2Cl 2 or CH 3CN to give [Os IV(tpy)(Cl) 2(NPPh 3)] + and its analogs. The reaction between trans-[Os VI(tpy)(Cl) 2(N)] + and PPh 3 in CH 3CN occurs with a 1:1 stoichiometry and a rate law first order in both PPh 3 and Os VI with k(CH 3CN, 25°C) = 1.36 ± 0.08 × 10 4 M − s −1. The products are best formulated as paramagnetic d 4 phosphoraniminato complexes of Os IV based on a room temperature magnetic moment of 1.8 μ B for trans-[Os IV(tpy)(Cl) 2(NPPh 3)](PF 6), contact shifted 1H NMR spectra and UV-Vis and near-IR spectra. In the crystal structures of trans-[Os IV(tpy)(Cl) 2( NPPh 3)](PF 6)·CH 3CN (monoclinic, P2 1/ n with a = 13.384(5) Å, b = 15.222(7) Å, c = 17.717(6) Å, β = 103.10(3)°, V = 3516(2) Å 3, Z = 4, R w = 3.40, R w = 3.50) and cis-[Os IV(tpy)(Cl) 2(NPPh 2Me)]-(PF 6)·CH 3CN (monoclinic, P2 1/ c, with a = 10.6348(2) Å, b = 15.146(9) ÅA, c = 20.876(6) Å, β = 97.47(1)°, V = 3334(2) Å 3, Z = 4, R = 4.00, R w = 4.90), the long Os-N(P) bond lengths (2.093(5) and 2.061(6) Å), 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(NPPh 3)](PF 6) in CH 3CN 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(NPPh 3)](PF 6) gives isolable trans-Os III(tpy)(Cl) 2(NPPh 3). One-electron oxidation to Os V followed by intermolecular disproportionation and PPh 3 group transfer gives [Os VI(tpy)Cl 2(N)] +, [OS III(tpy)(Cl) 2(CH 3CN)] + and [Ph 3=N=PPh 3] + (PPN +). trans-[Os IV(tpy)(Cl) 2(NPPh 3)](PF 6) undergoes reaction with a second phosphine under reflux to give PPN + derivatives and Os II(tpy)(Cl) 2(CH 3CN) in CH 3CN or Os II(tpy)(Cl) 2(PR 3) in CH 2Cl 2. This demonstrates that the Os VI nitrido complex can undergo a net four-electron change by a combination of atom and group transfers.