Chromium(III) Complexes for Photochemical Nitric Oxide Generation from Coordinated Nitrite:  Synthesis and Photochemistry of Macrocyclic Complexes with Pendant Chromophores, trans-[Cr(L)(ONO)₂]BF₄

Abstract

Several new dinitritochromium(III) complexes of the type trans-[Cr(L)(ONO)(2)]BF(4), where L is a derivative of the macrocyclic ligand cyclam having pendant aromatic chromophores attached (L = 5,7-dimethyl-6-(substituted)-1,4,8,11-tetraazacyclotetradecane), have been prepared and characterized. Photoexcitation of aqueous solutions containing these complexes at wavelengths corresponding to the pendant chromophore absorption bands led to the generation of NO as detected by an electrochemical sensor. Photophysical data show that the expected fluorescence of the pendant chromophores is largely quenched when the macrocyclic ligand is coordinated to these Cr(III) centers, and this is interpreted in terms of fast energy transfer processes from the ligand-centered pipi states to the Cr(III)-centered ligand field states leading to subsequent cleavage of the Cr(III)-coordinated nitrito ligand. Thus, the chromophores tethered to the coordinated cyclam serve as light-gathering antennae for the intramolecular sensitization of the NO-generating photoreactions at the metal center.

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

“…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].…”

“…This chromium nitrito complex was designed considering that Cr is an oxophilic metal, and therefore, when nitrite is a ligand, the β-cleavage of the CrO-NO bond may be more favorable than the cleavage of the Cr-ONO bond [54,55]. Changes in the absorbance spectrum when a deaerated solution of CrONO was subjected to long-term photolysis at 436 nm, did indicate the formation of the corresponding 2+ ), the product of NO 2 -aquation; however, the quantum yield was relatively small (0.0092) [17,18,21].…”

Nitrogen Monoxide and Nitrous Oxide Binding and Reduction

“…The nitrite to NO conversion can be effected by either a heme or copper enzyme (vide infra) and the NO to N 2 O transformation also involves hemes (see below). Further, relevant syn- thetic/chemical research includes the study of the catalytic reduction of NO (or other nitrogen oxides) (mentioned below) and as mentioned above the development of (metallo) reagents as drugs designed to thermally or photochemically release NO [28][29][30][31][32][33][34][35][36][37][38].…”

“…The use of metal ion chemistry for such purposes is a very active area of research, i.e. to sense biological NO [23][24][25][26][27], deliver it as drug [28][29][30][31][32][33][34][35][36][37][38] or modulate NO concentrations (as in septic shock therapy [39,40]). Another form of cellular signaling is thought to also involve NO, but with biological thiols [41], consisting of nitrosylation-denitrosylation chemistry, i.e.…”

Photoinduced Release of Nitroxyl and Nitric Oxide from Diazeniumdiolates

“…This approach is analogous to photodynamic therapy, and there is a significant ongoing effort toward the development of efficient and selective photochemical NO · donors. 11,14,[16][17][18][19][20][21] In each case, it is important to know the distribution of primary products resulting from the donor excited states. This is particularly so for diazeniumdiolates, when both NO · and HNO/NO -can be generated and these species have been reported to produce distinctly different physiological effects.…”