High Spin Co(I): High-Frequency and -Field EPR Spectroscopy of CoX(PPh₃)₃ (X = Cl, Br)

The previously reported pseudotetrahedral Co(I) complexes, CoX(PR3)3, where R = Me, Ph, and chelating analogues, and X = Cl, Br, I exhibit a spin triplet ground state, which is uncommon for Co(I), although expected for this geometry. Described here are studies using electronic absorption and high-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy on two members of this class of complexes: CoX(PR3)3, where R = Ph and X = Cl and Br. In both cases, well-defined spectra corresponding to axial spin triplets were observed, with signals assignable to three distinct triplet species, and with perfectly axial zero-field splitting (zfs) given by the parameter D = +4.46, +5.52, +8.04 cm−1, respectively, for CoCl(PPh3)3. The crystal structure reported for CoCl(PPh3)3 shows crystallographic 3-fold symmetry, but with three structurally distinct molecules per unit cell. Both of these facts thus correlate with the HFEPR data. The investigated complexes, along with a number of structurally characterized Co(I) trisphosphine analogues, were analyzed by quantum chemistry calculations (both density functional theory (DFT) and unrestricted Hartree−Fock (UHF) methods). These methods, along with ligand-field theory (LFT) analysis of CoCl(PPh3)3, give reasonable agreement with the salient features of the electronic structure of these complexes. A spin triplet ground state is strongly favored over a singlet state and a positive, axial D value is predicted, in agreement with experiment. Quantitative agreement between theory and experiment is less than ideal with LFT overestimating the zfs, while DFT underestimates these effects. Despite these shortcomings, this study demonstrates the ability of advanced paramagnetic resonance techniques, in combination with other experimental techniques, and with theory, to shed light on the electronic structure of an unusual transition metal ion, paramagnetic Co(I). ■ INTRODUCTION Cobalt in the 1+ formal oxidation state (3d) is relatively uncommon compared to both Co(II) and Co(0). Cobalt(I) is found in one of redox forms relevant in the reaction pathway of cobalamin-dependent enzymes. In these systems, however, square planar or square pyramidal Co(I) is diamagnetic (b2 ea1 b1 ), as is usually the case for the far more numerous isoelectronic Ni(II) coordination complexes. Organometallic, monocyclopentadienyl Co(I) complexes, such as CpCo(CO)2 and its derivatives, which are of interest as catalysts for alkyne dimerization are diamagnetic, too. There is, however, a bisarene Co(I) complex which is paramagnetic, with an S = 1 ground state. Very recently, a remarkable two-coordinate Co(I) “NacNac” (β-diketiminate) complex, LCo (where L = 2,2,6,6-tetramethyl-3,5-bis(2,4,6-triisopropylphenylimido)hept-4-yl) with an S = 1 ground state has been reported by Holland and co-workers. Lastly, and of most relevance here, there exist Co(I) complexes with other ligands, such as isonitriles and phosphines. Most have trigonal bipyramidal geometry and are thus diamagnetic. Examples are trans-CoH(CO)(PPh3)3 6 and HCo[ROP(OR)2]4 (R 1,2 = Me, Et, etc.), which is of interest as a chemical vapor deposition (CVD) precursor. There are, however, a few which, by virtue of their bulky phosphine ligands, are tetrahedral, and should yield a T1(F) (orbital and spin triplet; e t2 ) ground state. The best known among these is the series CoX(PPh3)3, where X = Cl, Br, I, reported by Aresta et al. in 1969. The tetrahedral structure was proposed mainly on the basis of the S = 1 ground state found for these complexes by magnetometry and was confirmed for the chlorido complex only more than 20 years later by Cassidy and Whitmire; no structures have been reported for the bromido and iodido analogues. Not long after the report on the PPh3 complexes, analogous S = 1 complexes, CoX(PMe3)3, where X = Cl, Br, I, were reported by Klein and Karsch, and the structures of the chlorido and iodido, but not bromido, complexes were shown to be tetrahedral. Two Co(I) complexes with tripodal ligands, in one case triphos (1,1,1tris((diphenylphosphino)methyl)ethane) and in the other Received: October 9, 2011 Published: April 6, 2012 Article