Elucidation of a Low Spin Cobalt(II) System in a Distorted Tetrahedral Geometry

Abstract

We have prepared a series of divalent cobalt(II) complexes supported by the [PhBP3] ligand ([PhBP3] = [PhB(CH2PPh2)3]-) to probe certain structural and electronic phenomena that arise from this strong field, anionic tris(phosphine) donor ligand. The solid-state structure of the complex [PhBP3]CoI (1), accompanied by SQUID, EPR, and optical data, indicates that it is a pseudotetrahedral cobalt(II) species with a doublet ground statethe first of its type. To our knowledge, all previous examples of 4-coordinate cobalt(II) complexes with doublet ground states have adopted square planar structure types. Complex 1 provided a useful precursor to the corresponding bromide and chloride complexes, {[PhBP3]Co(μ-Br)}2, (2), and {[PhBP3]Co(μ-Cl)}2, (3). These complexes were similarly characterized and shown to be dimeric in the solid-state. In solution, however, the monomeric low spin form of 2 and 3 dominates at 25 °C. There is spectroscopic evidence for a temperature-dependent monomer/dimer equilibrium in solution for complex 3. Furthermore, the dimers 2 and 3 did not display appreciable antiferromagnetic coupling that is typical of halide and oxo-bridged copper(II) and cobalt(II) dimers. Rather, the EPR and SQUID data for solid samples of 2 and 3 suggest that they have triplet ground states. Complexes 1, 2, and 3 are extremely oxygen sensitive. Thus, stoichiometric oxidation of 1 by dioxygen produced the 4-coordinate, high spin complex [PhB(CH2P(O)Ph2)2(CH2PPh2)]CoI, (4), in which the [PhBP3] ligand had undergone a 4-electron oxidation. Reaction of 1 with TlOAr (Ar = 2,6-Me2Ph) afforded an example of a 4-coordinate, high spin complex, [PhBP3]Co(O-2,6-Me2Ph) (5), with an intact [PhBP3] ligand. The latter two complexes were spectroscopically and structurally characterized for comparison to complexes 1, 2, and 3. Our data for these complexes collectively suggest that the [PhBP3] ligand provides an unusually strong ligand-field to these divalent cobalt complexes that is chemically distinct from typical tris(phosphine) donor ligand sets, and distinct from tridentate borato ligands that have been previously studied. Coupling this strong ligand-field with a pronounced axial distortion away from tetrahedral symmetry, a geometric consequence that is enforced by the [PhBP3] ligand, provides access to monomeric [PhBP3]CoX complexes with doublet rather than quartet ground states.

Optical, electrochemical and thermal properties of Co2+-doped CdS nanoparticles using polyvinylpyrrolidone

“…nanorods. The average particle size was calculated using Scherrer formula (Jenkins and Snyder 1996): D XRD ¼ 0:9k= bcosh ð Þ where D XRD is the average crystalline size, k is the wavelength of CuKa, b is the full width at half maximum of the diffraction peak, and h is the Bragg's angle. The average particle size of uncapped CdS:Co 2?…”

“…complexes with S = 1/2 often appear for the square planar and in pseudo-tetrahedral geometry (Jenkins et al 2002), and they are easily distinguished from the high-spin complexes. The g-factors are in the range 1.5-3.3, and ground state strongly depends on the crystal field strength, geometry and mixing of configurations Daul et al 1979).…”

A ligand field chemistry of oxygen generation by the oxygen-evolving complex and synthetic active sites

“…The metal-based orbitals involved in the s and p M-O interactions are the highest in energy owing to their antibonding character. The ordering of the a 1 (d z 2 (s à )) and e(d xz , d yz (p à )) depends on the tridentate ligand's bite angle ( Jenkins et al 2002). The distinguishing trait of the application of this ligand field to the metal oxo is that a degenerate e(d x 2 Ky 2 , d xy ) level is lowest in energy and may accommodate up to four electrons in a low-spin configuration before the metal p à orbitals are populated.…”

Complexes of iron and cobalt with new tripodal amido-polyphosphine hybrid ligands

“…For example, complexes of iron and cobalt with tris(phosphino)borate ligands show a propensity to coordinate and stabilize multiply-bonded ligand moieties, 1 and cobalt complexes supported by these ligands have revealed a number of unexpected electronic properties due to the strong field and distorted tetrahedral geometry conferred by the ligand. 2,3 In this context, we were eager to consider analogues of the (phosphino)borates in which the negative charge would be carried by an X-type donor ligand rather than the borate backbone.Particularly interesting to us were the mixed amide-phosphine hybrid ligands first developed by Fryzuk and coworkers. 4,5 As Fryzuk has noted, these scaffolds combine ''hard'' amido and ''soft'' phosphine donors, allowing them to bind many transition metals, stabilize a variety of oxidation states, and induce interesting transformations.…”

“…For example, complexes of iron and cobalt with tris(phosphino)borate ligands show a propensity to coordinate and stabilize multiply-bonded ligand moieties, 1 and cobalt complexes supported by these ligands have revealed a number of unexpected electronic properties due to the strong field and distorted tetrahedral geometry conferred by the ligand. 2,3 In this context, we were eager to consider analogues of the (phosphino)borates in which the negative charge would be carried by an X-type donor ligand rather than the borate backbone.…”