EPR-derived electronic characteristics of zerovalent manganese carbonyl di(tertiary phosphine) radicals

Abstract

radical~-'Mn(CO)~(P(n-Bu)~)~, 'Mn(CO)3(P(i-Bu)3)2, 'Mn(CO)3(P(i-Pr)3)2, and 'MII(CO),(P(O-~-P~)~)~. Anisotropic g shifts and metal hyperfine and phosphorus superhyperfine spin Hamiltonian parameters are reported and compared to values reported in the literature for 'Mn(CO)5. It is thus possible to examine some of the effects of varying ligand, geometry, and electronic parameters upon the characteristics of the EPR spectra obtained and upon the unpaired spin density localized in the manganese 3d9 and 4s metal orbitals and delocalized onto the phosphorus ligands. An LCAO-MO perturbation treatment of g and A values for these square-pyramidal d7 systems yields reasonable estimates of the distribution of unpaired spin density, upper and lower limits of core polarization, and the energy difference, AE, between the ground state and d?r excited state. of metal-centered organometallic species in hydride abstraction,8-lo electron transfers,11-13 ligand substituti~ns,~~~-~~~~l~-'~ atom transfer^?^^^'^ and oxidative additions/reductive elimination~.~J~ Because of rapid radical recombination it is difficult to prepare persistent zerovalent manganese-centered radicals: consequently, few examples are found in the literature?,l0 Persistent Mn(C0)3L2 metal-centered radicals have been generated, where the ligand, L, is either a tertiary phosphine or pho~phite.~J~J~ These species are isoelectronic with the pentacyanocobaltate radical anion, CO(CN)~~-, a well-characterized radical,6 and with the unsub- stituted manganese(0) pentacarbonyl radical.21v22 This paper presents a characterization via multifrequency EPR spectroscopy of a series of 'Mn(CO),L, radicals. We chose to study 'Mn- (CO)~(P(~-BU)~I~ , 'Mn(CO)dP(i-Bu)31,, 'Mn(CO)3(p(i-pr)312, and 'Mn(C0)3(P(O-i-Pr)3), in order to examine the effects of varying steric and electronic parameter~~~,~~ upon the charac- teristics of the EPR spectra obtained and upon the unpaired spin density localized in the manganese 3d,z and 4s metal orbitals and delocalized onto the phosphorus ligands. Anisotropic g shifts and metal hyperfine and phosphorus superhyperfine spin Hamiltonian parameters are reported and compared to values reported for 'Mn(C0)5.21 An LCAO-MO perturbation treatment of g and A values for these square-pyramidal d7 systems yields reasonable estimates of the distribution of unpaired spin density, upper and lower limits of core polarization, and the energy difference, AE, between the ground state and da excited state. A less detailed study of 'Mn(C0)3L2 radicals has previously appeared.25