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 <i>D</i> = +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 <i>D</i> 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)

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 [Tp₂Mn]­SbF₆ (<b>1</b>), [Tp₂*Mn]­SbF₆ (<b>2</b>), and [{PhB­(MeIm)₃}₂Mn]­(CF₃SO₃) (<b>3a</b>), 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 <b>3a</b> is reported; the structures of <b>1</b> and <b>2</b> have been previously reported, but were reconfirmed in this work. The synthesis and characterization of [{PhB­(MeIm)₃}₂­Mn]­Cl (<b>3b</b>) 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 <i>S</i> = 1 spin Hamiltonian parameters for complex <b>1</b>, <i>D</i> = +19.97(1), <i>E</i> = 0.42(2) cm^–1, and for <b>2</b>, <i>D</i> = +15.89(2), <i>E</i> = 0.04(1) cm^–1. In addition, frequency domain Fourier-transform THz-EPR spectroscopy, using coherent synchrotron radiation, was applied to <b>1</b> only and gave results in good agreement with HFEPR. Variable-temperature dc magnetic susceptibility measurements of <b>1</b> and <b>2</b> 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 <i>S</i> = 2 ground states. Complexes <b>3a</b> and <b>3b</b> (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