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

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• 作者：Jason England ; Eckhard Bill ; Thomas Weyherm眉ller ; Frank Neese ; Mihail Atanasov ; Karl Wieghardt
• 刊名：Inorganic Chemistry
• 出版年：2015
• 出版时间：December 21, 2015
• 年：2015
• 卷：54
• 期：24
• 页码：12002-12018
• 全文大小：867K
• ISSN：1520-510X

文摘

The crystal structures of nine homoleptic, pseudooctahedral cobalt complexes, 1鈥?b>9, containing either 2,2鈥?6鈥?2鈥?terpyridine (tpy), 4,4鈥?di-tert-butyl-2,2鈥?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 [Co^I(tpy⁰)₂]⁺, [Co^I(^tbpy⁰)₃]⁺, and [Co^I(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 Co^III (S = 0) species, and the dicationic species [Co^II(tpy⁰)₂]²⁺, [Co^II(^tbpy⁰)₃]²⁺, and [Co^II(phen⁰)₃]²⁺ contain high-spin (S = ³/₂) Co^II. Notably, the cobalt(I) complexes do not display any structural indication of significant metal-to-ligand (t_2g 鈫?蟺*) 蟺-back-donation effects. Consistent with this proposal, the temperature-dependent molar magnetic susceptibilities of the three cobalt(I) species have been recorded (3鈥?00 K) and a common S = 1 ground state confirmed. In contrast to the corresponding electronic spectra of isoelectronic (and isostructural) [Ni^II(tpy⁰)₂]²⁺, [Ni^II(bpy⁰)₃]²⁺, and [Ni^II(phen⁰)₃]²⁺, which display d 鈫?d bands with very small molar extinction coefficients (蔚 < 60 M^鈥? cm^鈥?), the spectra of the cobalt(I) species exhibit intense bands (蔚 > 10³ M^鈥? cm^鈥?) 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 蟺-back-bonding. Similar calculations for the six-coordinate neutral complexes [Co^II(tpy^{鈥?/sup>)₂]0 and [CoII(bpy鈥?/sup>)₂(bpy0)]0 point to a common S = 3/₂ ground state, each possessing a central high-spin CoII ion and two 蟺-radical anion ligands. In addition, the excited-states and ground state magnetic properties of [CoI(tpy0)₂][CoI鈭?/sup>(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鈥揹 transitions of the nickel(II) analogue]. Time-dependent DFT calculations have allowed assignment of these transitions as Co(3d) 鈫?蟺*(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 d_xz,yz orbitals toward the empty 蟺* orbitals located on the two central pyridine fragments in the trans position. This spin density is of an alternating 伪,尾-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.}