Spin-State Tuning at Pseudotetrahedral d^7 Ions: Examining the Structural and Magnetic Phenomena of Four-Coordinate [BP_3]Co^(II)-X Systems

Electronic structure, spin-state, and geometrical relationships for a series of pseudotetrahedral Co(II) aryloxide, siloxide, arylthiolate, and silylthiolate complexes supported by the tris(phosphino)borate [BP_3] ligands [PhBP_3] and [PhBP^(iPr)_3] ([PhB(CH_2PPh_2)_3]^- and [PhB(CH_2PiPr_2)_3]^-, respectively) are described. Standard ^1H NMR, optical, electrochemical, and solution magnetic data, in addition to low-temperature EPR and variable temperature SQUID magnetization data, are presented for the new cobalt(II) complexes [PhBP_3]CoOSiPh_3 (2), [PhBP_3]CoO(4-^tBu-Ph) (3), [PhBP_3]CoO(C_6F_5) (4), [PhBP_3]CoSPh (5), [PhBP_3]CoS(2,6-Me_2-Ph) (6), [PhBP_3]CoS(2,4,6-iPr_3-Ph) (7), [PhBP_3]CoS(2,4,6-^tBu_3-Ph) (8), [PhBP_3]CoSSiPh_3 (9), [PhBP_3]CoOSi(4-NMe_2-Ph)_3 (10), [PhBP_3]CoOSi(4-CF_3-Ph)_3 (11), [PhBP_3]CoOCPh_3 (12), [PhBPiPr_3]CoOSiPh_3 (14), and [PhBPiPr_3]CoSSiPh_3 (15). The low-temperature solid-state crystal structures of 2, 3, 5−10, 12, and 15 are also described. These pseudotetrahedral cobalt(II) complexes are classified as featuring one of two limiting distortions, either umbrella or off-axis. Magnetic and spectroscopic data demonstrate that both S = 1/2 and S = 3/2 ground-state electronic configurations are accessible for the umbrella distorted structure type, depending on the nature of the X-type ligand, its denticity (η^1 versus η^3), and the tripodal phosphine ligand employed. Off-axis distorted complexes populate an S = 1/2 ground-state exclusively. For those four-coordinate complexes that populate S = 1/2 ground states, X-ray data show two Co−P bond distances that are invariably shorter than a third Co−P bond. The pseudotetrahedral siloxides 2, 10, and 11 are exceptional in that they display gradual spin crossover in the solid state. The diamagnetic cobalt(III) complex {[PhBP_3]CoOSiPh_3}{BAr_4} ({16}{BAr_4}) (Ar = Ph or 3,5-(CF_3)_2−C_6H_3) has also been prepared and structurally characterized. Accompanying electronic structure calculations (DFT) for complexes 2, 6, and {16}^+ support the notion of a close electronic structure relationship between these four-coordinate systems and octahedral, sandwich, and half-sandwich coordination complexes