S. A. Zvyagin

Cobalt(II) Scorpionate Complexes as Models for Cobalt-Substituted Zinc Enzymes: Electronic Structure Investigation by High-Frequency and -Field Electron Paramagnetic Resonance Spectroscopy

A series of complexes of formula Tp(R,R)CoL, where Tp(R,R-) = hydrotris(3-R,5-R-pyrazol-1-yl)borate (scorpionate) anion (R = tert-butyl, R = H, Me, 2-thienyl (Tn), L = Cl(-), NCS(-), NCO(-), N(3)(-)), has been characterized by electronic absorption spectroscopy in the visible and near-infrared (near-IR) region and by high-frequency and -field electron paramagnetic resonance (HFEPR). Reported here are also crystal structures of seven members of the series that have not been reported previously: R = H, L = NCO(-), N(3)(-); R = Me, L = Cl(-), NCS(-), NCO(-), N(3)(-); R = Tn, L = Cl(-), NCS(-). These include a structure for Tp(t-Bu,Me)CoCl different from that previously reported. All of the investigated complexes contain a four-coordinate cobalt(II) ion (3d(7)) with approximate C(3v) point group symmetry about the metal ion and exhibit an S = (3)/(2) high-spin ground state. The use of HFEPR allows extraction of the full set of intrinsic S = (3)/(2) spin Hamiltonian parameters (D, E, and g values). The axial zero-field splitting parameter, D, for all investigated Tp(R,R)CoL complexes is always positive, a fact not easily determined by other methods. However, the magnitude of this parameter varies widely: 2.4 cm(-1) <or= D <or= 12.7 cm(-1), indicating the extreme sensitivity of this parameter to environment. The spin Hamiltonian parameters are combined with estimates of 3d energy levels based on the visible-near-IR spectra to yield ligand-field parameters for these complexes following the angular overlap model (AOM). This description of electronic structure and bonding in pseudotetrahedral cobalt(II) complexes can enhance the understanding of similar sites in metalloproteins, specifically cobalt-substituted zinc enzymes.

Low-Spin Hexacoordinate Mn(III): Synthesis and Spectroscopic Investigation of Homoleptic Tris(pyrazolyl)borate and Tris(carbene)borate Complexes

Three complexes of Mn(III) with scorpionate type ligands have been investigated by a variety of physical techniques. The complexes are [Tp(2)Mn]SbF(6) (1), [Tp(2)*Mn]SbF(6) (2), and [{PhB(MeIm)(3)}(2)Mn](CF(3)SO(3)) (3a), where Tp(-) = hydrotris(pyrazolyl)borate anion, Tp*(-) = hydrotris(3,5-dimethylpyrazolyl)borate anion, and PhB(MeIm)(3)(-) = phenyltris(3-methylimidazol-2-yl)borate anion. The crystal structure of 3a is reported; the structures of 1 and 2 have been previously reported, but were reconfirmed in this work. The synthesis and characterization of [{PhB(MeIm)(3)}(2)Mn]Cl (3b) are also described. These complexes are of interest in that, in contrast to many hexacoordinate (pseudo-octahedral) complexes of Mn(III), they exhibit a low-spin (triplet) ground state, rather than the high-spin (quintet) ground state. Solid-state electronic absorption spectroscopy, SQUID magnetometry, and high-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy were applied. HFEPR, in particular, was useful in characterizing the S = 1 spin Hamiltonian parameters for complex 1, D = +19.97(1), E = 0.42(2) cm(-1), and for 2, D = +15.89(2), E = 0.04(1) cm(-1). In addition, frequency domain Fourier-transform THz-EPR spectroscopy, using coherent synchrotron radiation, was applied to 1 only and gave results in good agreement with HFEPR. Variable-temperature dc magnetic susceptibility measurements of 1 and 2 were also in good agreement with the HFEPR results. This magnitude of zero-field splitting (zfs) is over 4 times larger than that in comparable hexacoordinate Mn(III) systems with S = 2 ground states. Complexes 3a and 3b (i.e., regardless of counteranion) have a yet much larger magnitude zfs, which may be the result of unquenched orbital angular momentum so that the spin Hamiltonian model is not appropriate. The triplet ground state is rationalized in each complex by ligand-field theory (LFT) and by quantum chemistry theory, both density functional theory and unrestricted Hartree-Fock methods. This analysis also shows that spin-crossover behavior is not thermally accessible for these complexes as solids. The donor properties of the three different scorpionate ligands were further characterized using the LFT model that suggests that the tris(carbene)borate is a strong σ-donor with little or no π-bonding.

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(PR(3))(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(PR(3))(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(PPh(3))(3). The crystal structure reported for CoCl(PPh(3))(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(PPh(3))(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).

ESR study of (C5H12N)2CuBr4

Abstract ESR studies at 9.27, 95.4, and 289.7 GHz have been performed on (C5H12N)2CuBr4 down to 3.7 K . The 9.27 GHz data were acquired with a single crystal and do not indicate the presence of any structural transitions. The high frequency data were collected with a polycrystalline sample and resolved two absorbances, consistent with two crystallographic orientations of the magnetic sites and with earlier ESR studies performed at 300 K . Below B C1 =6.6 T , our data confirm the presence of a spin singlet ground state.