Spectroscopic and magnetic properties of an iodo Co^I tripodal phosphine complex

Abstract

Reaction of the tripodal phosphine ligand 1,1,1-tris((diphenylphosphino)phenyl)ethane (PhP3) with CoI_2 spontaneously generates a one-electron reduced complex, [(PhP3)Co^(I)(I)] (1). The crystal structure of 1 reveals a distorted tetrahedral environment, with an apical Co–I bond distance of ~2.52 Å. Co^(II/I) redox occurs at an unusually high potential (+0.38 V vs. SCE). The electronic absorption spectrum of 1 exhibits an MLCT peak at 320 nm (ε = 8790 M^(−1) cm^(−1)) and a d–d feature at 850 nm (ε = 840 M^(−1) cm^(−1)). Two more d–d bands are observed in the NIR region, 8650 (ε = 450) and 7950 cm−1 (ε = 430 M−1 cm^(−1)). Temperature dependent magnetic measurements (SQUID) on 1 (solid state, 20–300 K) give μ_eff = 2.99(6) μB, consistent with an S = 1 ground state. Magnetic susceptibilities below 20 K are consistent with a zero field splitting (zfs) |D| = 8 cm^(−1). DFT calculations also support a spin-triplet ground state for 1, as optimized (6-31G*/PW91) geometries (S = 1) closely match the X-ray structure. EPR measurements performed in parallel mode (X-band; 0–15 000 G, 15 K) on polycrystalline 1 or frozen solutions of 1 (THF/toluene) exhibit a feature at g ≈ 4 that arises from a (Δm = 2) transition within the MS = <+1,−1> manifold. Below 10 K, the EPR signal decreases significantly, consistent with a solution zfs parameter (|D| ≈ 8 cm^(−1)) similar to that obtained from SQUID measurements. Our work provides an EPR signature for high-spin Co^I in trigonal ligation.

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