Syntheses, Structures, and Photochemistry of Manganese Nitrosyls Derived from Designed Schiff Base Ligands: Potential NO Donors That Can Be Activated by Near-Infrared Light

Abstract

Two manganese nitrosyls, namely [Mn(SBPy3)(NO)](ClO4)2 (1) and [Mn(SBPy2Q)(NO)](ClO4)2 (2) have been synthesized by using designed pentadentate Schiff base ligands N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-aldimine (SBPy3) and N,N-bis(2-pyridyl methyl)amine-N-ethyl-2-quinoline-2-aldimine (SBPy2Q). Reaction of NO(g) with [Mn(SBPy3)(MeOH)](ClO4)2 and [Mn(SBPy2Q)(EtOH)](ClO4)2 in MeCN affords 1 and 2 respectively in good yields. Narrow-width peaks in the 1H NMR spectra and strong νNO at 1773 cm-1 (of 1) and 1759 cm-1 (of 2) confirm a strongly-coupled {low-spin Mn(II)-NO• }formulation for both these {Mn-NO}6 nitrosyls. In MeCN, 1 exhibits two strong absorption bands with λmax at 500 and 720 nm. These bands red shifts to 550 and 785 nm in case of 2 due to substitution of the pyridyl-imine moiety of SBPy3 with quinolyl-imine moiety in the SBPy2Q ligand frame. Exposure of solutions 1 and 2 to near-infrared (NIR) light (780 nm, 5 mW) results in rapid bleaching of the orange and fuchsia solutions and free NO is detected in the solutions by an NO-sensitive electrode. The high quantum yield values (Φ) of 1 (0.580 ± 0.010, λirr = 550 nm, MeCN) and 2 (0.434 ± 0.010, λirr = 550 nm, MeCN) and in particular their sensitivity to NIR light of 800-950 nm range strongly suggest that these designed manganese nitrosyls could be used as NIR light-triggered NO donors.

Photo-Controlled Release of NO and CO with Inorganic and Organometallic Complexes

“…When bound to a metal center, nitric oxide can either accept or donate electron density from or to the metal ( Figure 4 properties that indicate the bond order for NO to be greater than that of N=O. [61][62][63][64][65][66][67][68].…”

“…As a result, various strategies have been designed to make such species more susceptible to longer wavelength excitation [45,48,51,52,54,76,80,85,90,[116][117][118][119]. With certain platforms, it has proved possible to achieve the NO release at longer wavelength by extending the conjugation of the ligand frame and by adding key substituent groups at strategic positions [65,85,90]. However, another approach is to develop antenna-photoNORM conjugates such as illustrated in Scheme 2, where the strongly absorbing antenna harvests one or more photons in order to form excited states from which energy transfer to the photoNORM occurs.…”

Multi-photon excitation in uncaging the small molecule bioregulator nitric oxide

“…For example, we and others have worked to design molecular species and larger constructs that are photoactive for NO release at longer excitation wavelengths in the visible [10,[18][19][20] and even in the NIR ranges [21]. However, doing so requires a compound that has appropriate absorption properties and reactivity when excited by longer wavelength light.…”

Visible Light‐Induced Release of Nitrogen Monoxide from a Nitrosylrhodium Complex

“…[4,5,[12][13][14][15][16][17] For medicinal purposes, photochemical sources should deliver small, steady concentrations of NO by irradiation in the visible/near IR region of the spectrum where tissue transparency is the greatest. Promising results have been reported for nitrosyl complexes of ruthenium, [18,19] iron, [4,20,21] and manganese, [14,22] but compounds with a good combination of chemical stability and high quantum yield for release of NO in the visible range in aqueous solutions are quite rare. Even the most recently reported complexes that are photoactive in the desired spectral region require complex pyridine-based ligands and/or coordinated organic dyes, [13,20] and, with one exception, [14] require organic solvents.…”