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

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.