A new photoactivable NO‐releasing {Ru−NO}⁶ ruthenium nitrosyl complex with a tetradentate ligand containing aniline and pyridine moieties

Abstract

A new type of photoactivable NO-releasing ruthenium nitrosyl complex, [Ru(EPBP)Cl(NO)], with a tetradentate ligand, N,N'-(ethane-1,2-diyldi-o-phenylene)-bis(pyridine-2carboxamide) (= H 2 EPBP) was synthesized. Single crystal Xray crystallography revealed that the complex has a distorted octahedral coordination geometry and NO is positioned at cis to Cl À ion. NO-photolysis was observed under a white room light. The photodissociation of RuÀ NO bond was identified by various techniques including X-ray crystallography, IR, UV/ Vis absorption, electron paramagnetic resonance (EPR), and NMR spectroscopies. Quantum yields for the NO-photolysis of the complex in CH 3 OH, CHCl 3 , DMSO, CH 3 CN, and CH 3 NO 2 were measured to be 0.19-0.36 with 400 (� 5) nm excitation. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations were performed to understand the details of the photodissociation of the complex. The calculations suggest that the NO photolysis is most likely initiated by the electronic transition from the aniline moiety π MOs (π (aniline)) of the EPBP 2À chelating ligand to the π-antibonding MO of RuÀ NO (π*(RuÀ NO)). Experimental and theoretical investigations indicate that the EPBP 2À ligand provides an effective platform forming ruthenium nitrosyl complexes useful for NO-photoreleasing.

Visible-light NO photolysis of ruthenium nitrosyl complexes with N2O2 ligands bearing π-extended rings and their photorelease dynamics

“…Previous experimental and theoretical investigations on the NO photodissociations of the {Ru–NO} 6 complexes have revealed that the electronic transitions initializing the dissociations changed from direct metal-to-ligand charge transfers (MLCT), dπ(Ru) → π*(Ru–NO), to indirect LLCT characters when {Ru–NO} 6 complexes contained the ligands of multidentate π-extended systems. 23,27,32,39,73–75 Our results show the effect of the size of the ligand π-systems and electron-donating abilities on the photoactive bands and give some insights into the future direction of the development of {Ru–NO} 6 complexes for PDT applications.…”

“…The observed 14 N nuclear hyperfine coupling constant ( A N ) of 12.65 G and the isotropic g -value ( g iso ) of 2.039 are in agreement with the previously reported g -values of the NO–Fe( ii )(MGD) 2 adducts. 32,38,66–69 UV/Vis, IR, X-ray crystallography, EPR, NMR, and spin-trap EPR investigations on 1 and 2 unambiguously demonstrate the photodissociation of the Ru( ii )(L)–NO + (L = salophen 2− or naphophen 2− ) bond to release the NO radical and solvent–bound Ru( iii )(L) (eqn (1)). …”

“…The g -tensors of g x ≥ g y > 2.0 > g z are the typical characteristics of low-spin octahedral Ru( iii ) complexes. 32,59–65 In addition to the mentioned EPR signals, Fig. 4 displays some other EPR features.…”

“…N -Methyl-glucamine-dithiocarbamato-iron( ii ), Fe( ii )(MGD) 2 , has been known to trap NO to become an EPR-detectable spin adduct both in vivo and in vitro . 32,38,66–69 Fig. 5 shows the EPR signals of such adducts produced in the mixture solutions of 1 + Fe( ii )(MGD) 2 and 2 + Fe( ii )(MGD) 2 before and after photo-illumination.…”

“…Among the metal nitrosyl complexes, ruthenium nitrosyls are one of the most promising NO donors to biological targets due to their stability under physiological conditions, low cytotoxicity, predictable NO-releasing behaviors, and easy structural modification. 20,25–29 Ruthenium nitrosyl complexes, with various types of ligands including macrocycles, 30,31 carboxamide-containing types, 23,32–35 and polypyridines, 36–42 have been intensively investigated for potential application in PDT requiring a high quantum yield of NO photolysis by long wavelength excitation. Salen (= N , N ′-bis(salicylidene)ethylenediamine dianion) type ligands, which form octahedral metal complexes with square planar N 2 O 2 ligands at equatorial positions, have also been successfully applied to ruthenium nitrosyls.…”