Syntheses, Structures, and Properties of New Manganese Carbonyls as Photoactive CO-Releasing Molecules: Design Strategies That Lead to CO Photolability in the Visible Region

The unusual role of CO as a signaling molecule in several
physiological pathways has spurred research in the area of synthesizing
new CO-releasing molecules (CORMs) as exogenous CO donors. Auxiliary
control on CO delivery can be achieved if CO can be released under
the control of light. To synthesize such photoactive CORMs (photoCORMs)
with the aid of smart design, a series of manganese carbonyls have
been synthesized with ligands that contain extended conjugation and
electron-rich donors on their frames. Five such photoCORMs, namely,
[Mn­(pimq)­(CO)3(MeCN)]­ClO4 (1, where
pimq = (2-phenyliminomethyl)­quinoline), [Mn­(qmtpm)­(CO)3(MeCN)]­ClO4 (2, where qmtpm = 2-quinoline-N-(2′-methylthiophenyl) methyleneimine), [Mn­(qmtpm)­(CO)3Br] (3) [Mn­(pmtpm)­(CO)3(MeCN)]­ClO4 (4, where pmtpm = 2-pyridyl-N-(2′-methylthiophenyl)­methyleneimine), and [Mn­(pmtpm)­(CO)3Br] (5), have been synthesized and structurally
characterized. These designed carbonyls readily release CO upon exposure
to light (400–550 nm). The apparent CO release rates and quantum
yield values at 509 nm (ϕ509) of the photoCORMs increase
steadily with rise in conjugation in the ligand frame and inclusion
of a −SMe group. Addition of Br– as an ancillary
ligand also improves the CO-donating capacity. Results of density
functional theory (DFT) and time dependent DFT (TDDFT) studies indicate
that low energy metal-to-ligand charge transfer (MLCT) transitions
from Mn-CO bonding into ligand-π orbitals lead to reduction
of M-CO­(π*) back-bonding and loss of CO from these photoCORMs.
Inclusion of −SMe and Br– in the coordination
sphere attenuates the energies of the HOMO and LUMO levels and causes
further enhancement of CO photorelease. Collectively, the results
of this work demonstrate that new photoCORMs with excellent sensitivity
to visible light can be synthesized on the basis of smart design principles.