Spin Isomers and Ligand Isomerization in a Three-Coordinate Cobalt(I) Carbonyl Complex

Hemilabile ligands,
which have one donor that can reversibly bind
to a metal, are widely used in transition-metal catalysts to create
open coordination sites. This change in coordination at the metal
can also cause spin-state changes. Here, we explore a cobalt­(I) system
that is poised on the brink of hemilability and of a spin-state change
and can rapidly interconvert between different spin states with different
structures (“spin isomers”). The new cobalt­(I) monocarbonyl
complex LtBuCo­(CO) (2) is a singlet (12) in the solid state, with an unprecedented
diketiminate binding mode where one of the CC double bonds
of an aromatic ring completes a pseudo-square-planar coordination.
Dissolving the compound gives a substantial population of the triplet
(32), which has exceptionally
large uniaxial zero-field splitting due to strong spin–orbit
coupling with a low-lying excited state. The interconversion of the
two spin isomers is rapid, even at low temperature, and temperature-dependent
NMR and electronic absorption spectroscopy studies show the energy
differences quantitatively. Spectroscopically validated computations
corroborate the presence of a low minimum-energy crossing point (MECP)
between the two potential energy surfaces and elucidate the detailed
pathway through which the β-diketiminate ligand “slips”
between bidentate and arene-bound forms: rather than dissociation,
the cobalt slides along the aromatic system in a pathway that balances
strain energy and cobalt–ligand bonding. These results show
that multiple spin states are easily accessible in this hemilabile
system and map the thermodynamics and mechanism of the transition.