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

Jenkins, David M. and Di Bilio, Angel J. and Allen, Matthew J. and Betley, Theodore A. and Peters, Jonas C. (2002) Elucidation of a Low Spin Cobalt(II) System in a Distorted Tetrahedral Geometry. Journal of the American Chemical Society, 124 (51). pp. 15336-15350. ISSN 0002-7863. doi:10.1021/ja026433e.

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We have prepared a series of divalent cobalt(II) complexes supported by the [PhBP_3] ligand ([PhBP_3] = [PhB(CH_2PPh_2)_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 [PhBP_3]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, {[PhBP_3]Co(μ-Br)}_2, (2), and {[PhBP_3]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(CH_2P(O)Ph_2)_2(CH_2PPh_2)]CoI, (4), in which the [PhBP_3] ligand had undergone a 4-electron oxidation. Reaction of 1 with TlOAr (Ar = 2,6-Me_2Ph) afforded an example of a 4-coordinate, high spin complex, [PhBP_3]Co(O-2,6-Me_2Ph) (5), with an intact [PhBP_3] 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 [PhBP_3] 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 [PhBP_3] ligand, provides access to monomeric [PhBP_3]CoX complexes with doublet rather than quartet ground states.

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Peters, Jonas C.0000-0002-6610-4414
Additional Information:© 2002 American Chemical Society. Received April 5, 2002. Revised Manuscript Received September 27, 2002. We are grateful to the Dreyfus Foundation, the ACS Petroleum Research Fund, and the National Science Foundation (Grant No. CHE-0132216) for financial support of this work. We thank the Beckman Institute (Caltech) for use of the SQUID magnetometer and the crystallographic facility. D.M.J. is grateful for a pre-doctoral fellowship from the National Science Foundation. M.J.A. is grateful for a NDSEG Fellowship. T.A.B. is grateful for a pre-doctoral fellowship from the DOD. J. C. Thomas is acknowledged for assistance with DFT. Finally, we acknowledge our inorganic colleagues at Caltech for many stimulating discussions, and the reviewers of this paper for helpful commentary.
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Camille and Henry Dreyfus FoundationUNSPECIFIED
American Chemical Society Petroleum Research FundUNSPECIFIED
NSF Predoctoral FellowshipUNSPECIFIED
Department of Defense (DOD) Predoctoral FellowshipUNSPECIFIED
Issue or Number:51
Record Number:CaltechAUTHORS:20140723-090945029
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Official Citation:Elucidation of a Low Spin Cobalt(II) System in a Distorted Tetrahedral Geometry David M. Jenkins,Angel J. Di Bilio,Matthew J. Allen,Theodore A. Betley, and, and Jonas C. Peters Journal of the American Chemical Society 2002 124 (51), 15336-15350
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:47416
Deposited By: Tony Diaz
Deposited On:23 Jul 2014 17:04
Last Modified:10 Nov 2021 17:39

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