A critical evaluation of [ML(ONO)]n+ (M = Fe, Ru, Os) as nitric oxide precursor influenced by spin multiplicity and geometrical parameters (M-O-NO and MO-N-O) for the NO release: A theoretical study

Abstract

In cell biology, the physiological functions control blood pressure by the production of nitric oxide, and the exploitation of metal complexes such as [ML-ONO] (M = Fe2+, Ru2+, Os2+) has been motivated to study the photolytic liability of NO. The structural and electronic properties of the complexes of 1,4,8,11-tetrakis-(1-methyl-pyridyl)-1,4,8,11-tetraazacyclotetradecane were analyzed theoretically. The results show that β-cleavage of nitrito group in the complex [ML-ONO]n+ (M = Fe, Ru, Os; n = 1 or 2) favors the NO release, and observed the role of spin multiplicity. The M(II)-O-NO angle is associated with the degree of activation of the M(II)O—NO bond, relying on the excitations (S = 1 or S = 2), and it is corelated to the reduction of the bond angle ONO (~133° at S = 0) to ~ 105° at S = 1 or 2, favoring the nitrito β-cleavage for the complexes. This is consistent with orbital energy (dx2-y2) stabilizing the axial M(III)-O bond greater than that in M(II)-O [Fe < Ru ~ Os] as the excitation of singlet to triplet or to quintet prompts the NO delivery according to Gibbs free energy, agreeing with the transition state energy. However, at high spin state like 3/2 or 5/2, irrespective of bond angle M(III)-O-NO or ONO, the bond O-NO was broken spontaneously, resulting relatively a greater bond distance. Natural Transition Orbitals (NTOs), Molecular orbital (MO) Electron density contour on molecular orbital surface were also studied to understand the M-ONO bonding nature, and observed its performance in the TD-DFT electronic spectra.