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) methyleneirnine), [Mn(qmtpm)(CO)(3)Br] (3) [Mn(pmtpm)(CO)(3)(MeCN)]ClO4 (4, where pmtpm = 2-pyridyl-N-(2'-methylthiophenyl)methyleneimine), and [Mn(pmtpm)(CO)(3)Br] (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 (phi(509)) 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 MnCO bonding into ligand-pi orbitals lead to reduction of M-CO(pi(*)) 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.