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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

England, J., Bill, E., Weyhermüller, T., Neese, F., Atanasov, M., & Wieghardt, K. (2015). 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. Inorganic Chemistry, 54(24), 12002-12018. doi:10.1021/acs.inorgchem.5b02415.

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England, Jason1, Author           
Bill, Eckhard2, Author           
Weyhermüller, Thomas2, Author           
Neese, Frank2, Author           
Atanasov, Mihail2, 3, Author           
Wieghardt, Karl1, Author
Affiliations:
1Research Department Wieghardt, Max Planck Institute for Chemical Energy Conversion, Max Planck Society, ou_3023875              
2Research Department Neese, Max Planck Institute for Chemical Energy Conversion, Max Planck Society, ou_3023886              
3Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria, ou_persistent22              

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 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.

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Language(s): eng - English
 Dates: 2015-10-192015-12-042015-12-21
 Publication Status: Published in print
 Pages: 17
 Publishing info: -
 Table of Contents: -
 Rev. Type: Peer
 Identifiers: DOI: 10.1021/acs.inorgchem.5b02415
 Degree: -

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Title: Inorganic Chemistry
  Abbreviation : Inorg. Chem.
Source Genre: Journal
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Publ. Info: Washington, DC : American Chemical Society
Pages: - Volume / Issue: 54 (24) Sequence Number: - Start / End Page: 12002 - 12018 Identifier: ISSN: 0020-1669
CoNE: https://pure.mpg.de/cone/journals/resource/0020-1669