Darryl S. Williams

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

Abstract 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 2 Me, PPhMe 2 , PMe 3 and PEt 3 ) in CH 2 Cl 2 or CH 3 CN 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 3 CN occurs with a 1:1 stoichiometry and a rate law first order in both PPh 3 and Os VI with k (CH 3 CN, 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 1 H 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 3 CN (monoclinic, P 2 1 / 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 w = 3.40, R w = 3.50) and cis -[Os IV (tpy)(Cl) 2 (NPPh 2 Me)]-(PF 6 )·CH 3 CN (monoclinic, P 2 1 / c , with a = 10.6348(2) A, b = 15.146(9) AA, 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 (NPPh 3 )](PF 6 ) in CH 3 CN 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 3 CN)] + 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 3 CN) in CH 3 CN or Os II (tpy)(Cl) 2 (PR 3 ) in CH 2 Cl 2 . This demonstrates that the Os VI nitrido complex can undergo a net four-electron change by a combination of atom and group transfers.