Spin-State Tuning at Pseudotetrahedral d7Ions: Examining the Structural and Magnetic Phenomena of Four-Coordinate [BP3]CoII−X Systems

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Abstract
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 [BP3] ligands [PhBP3] and [PhBPiPr3] ([PhB(CH2PPh2)3]- and [PhB(CH2PiPr2)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 [PhBP3]CoOSiPh3 (2), [PhBP3]CoO(4-tBu-Ph) (3), [PhBP3]CoO(C6F5) (4), [PhBP3]CoSPh (5), [PhBP3]CoS(2,6-Me2-Ph) (6), [PhBP3]CoS(2,4,6-iPr3-Ph) (7), [PhBP3]CoS(2,4,6-tBu3-Ph) (8), [PhBP3]CoSSiPh3 (9), [PhBP3]CoOSi(4-NMe2-Ph)3 (10), [PhBP3]CoOSi(4-CF3-Ph)3 (11), [PhBP3]CoOCPh3 (12), [PhBPiPr3]CoOSiPh3 (14), and [PhBPiPr3]CoSSiPh3 (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 (η1 versus η3), 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 {[PhBP3]CoOSiPh3}{BAr4} ({16}{BAr4}) (Ar = Ph or 3,5-(CF3)2−C6H3) 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.