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Molecular and Electronic Structures of Homoleptic Six-Coordinate Cobalt(I) Complexes of 2,2′:6′,2″-Terpyridine, 2,2′-Bipyridine, and 1,10-Phenanthroline. An Experimental and Computational Study

Jason England†, Eckhard Bill†, Thomas Weyhermüller†, Frank Neese†, Mihail Atanasov*†§, and Karl Wieghardt*†

† Max Planck Institute for Chemical Energy Conversion, Stiftstrasse 34−36, D-45470 Mülheim an der Ruhr,Germany

§ Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria

Inorg. Chem., Article ASAP

DOI: 10.1021/acs.inorgchem.5b02415

Publication Date (Web): December 4, 2015

Copyright © 2015 American Chemical Society

*E-mail: mihail.atanasov@cec.mpg.de., *E-mail: karl.wieghardt@cec.mpg.de.

http://pubs.acs.org/doi/10.1021/acs.inorgchem.5b02415

Synopsis

High-resolution single-crystal X-ray diffraction shows unambiguously that the homoleptic monocations [Co(tpy)2]+, [Co(tbpy)3]+, and [Co(phen)3]+, each possessing an S = 1 ground state, are genuine octahedral cobalt(I) complexes with a d8 electron configuration of the Co ion and neutral N-heterocyclic ligands. They are isostructural and isoelectronic with the corresponding nickel(II) complexes. Differences in the electronic structures and spectra of the cobalt(I) and nickel(II) complexes have been investigated by ab initio based ligand-field analysis. No structural indication of metal-to-ligand π-back-donation is detectable.

Abstract

The crystal structures of nine homoleptic, pseudooctahedral cobalt complexes, 1–9, containing either 2,2′:6′,2″-terpyridine (tpy), 4,4′-di-tert-butyl-2,2′-bipyridine (tbpy), or 1,10-phenanthroline (phen) ligands have been determined in three oxidation levels, namely, cobalt(III), cobalt(II), and, for the first time, the corresponding presumed cobalt(I) species. The intraligand bond distances in the complexes [CoI(tpy0)2]+, [CoI(tbpy0)3]+, and [CoI(phen0)3]+ are identical, within experimental error, not only with those in the corresponding trications and dications but also with the uncoordinated neutral ligands tpy0, bpy0, and phen0. On this basis, a cobalt(I) oxidation state assignment can be inferred for the monocationic complexes. The trications are clearly low-spin CoIII (S = 0) species, and the dicationic species [CoII(tpy0)2]2+, [CoII(tbpy0)3]2+, and [CoII(phen0)3]2+ contain high-spin (S = 3/2) CoII. Notably, the cobalt(I) complexes do not display any structural indication of significant metal-to-ligand (t2g → π*) π-back-donation effects. Consistent with this proposal, the temperature-dependent molar magnetic susceptibilities of the three cobalt(I) species have been recorded (3–300 K) and a common S = 1 ground state confirmed. In contrast to the corresponding electronic spectra of isoelectronic (and isostructural) [NiII(tpy0)2]2+, [NiII(bpy0)3]2+, and [NiII(phen0)3]2+, which display d → d bands with very small molar extinction coefficients (ε < 60 M–1 cm–1), the spectra of the cobalt(I) species exhibit intense bands (ε > 103 M–1 cm–1) in the visible and near-IR regions. Density functional theory (DFT) calculations using the B3LYP functional have validated the experimentally derived electronic structure assignments of the monocations as cobalt(I) complexes with minimal cobalt-to-ligand π-back-bonding. Similar calculations for the six-coordinate neutral complexes [CoII(tpy•)2]0 and [CoII(bpy•)2(bpy0)]0 point to a common S = 3/2 ground state, each possessing a central high-spin CoII ion and two π-radical anion ligands. In addition, the excited-states and ground state magnetic properties of [CoI(tpy0)2][CoI−(CO)4] have been explored by variable-temperature variable-magnetic-field magnetic circular dichroism (MCD) spectroscopy. A series of strong signals associated with the paramagnetic monocation exhibit pronounced C-term behavior indicative of the presence of metal-to-ligand charge-transfer bands [in contrast to d–d transitions of the nickel(II) analogue]. Time-dependent DFT calculations have allowed assignment of these transitions as Co(3d) → π*(tpy) excitations. Metal-to-ligand charge-transfer states intermixing with the Co(d8) multiplets explain the remarkably large (and negative) zero-field-splitting parameter D obtained from SQUID and MCD measurements. Ground-state electron- and spin-density distributions of [CoI(tpy0)2]+ have been investigated by multireference electronic structure methods: complete active-space self-consistent field (CASSCF) and N-electron perturbation theory to second order (NEVPT2). Both correlated CASSCF/NEVPT2 and spin-unrestricted B3LYP-based DFT calculations show a significant delocalization of the spin density from the CoI dxz,yz orbitals toward the empty π* orbitals located on the two central pyridine fragments in the trans position. This spin density is of an alternating α,β-spin polarization type (McConnel mechanism I) and is definitely not due to magnetic metal-to-radical coupling. A comparison of these results with those for [NiII(tpy0)2]2+ (S = 1) is presented.