Mechanism of NO Photodissociation in Photolabile Manganese–NO Complexes with Pentadentate N5 Ligands

Abstract

The Mn-nitrosyl complexes [Mn(PaPy(3))(NO)](ClO(4)) (1; PaPy(3)(-) = N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-carboxamide) and [Mn(PaPy(2)Q)(NO)](ClO(4)) (2, PaPy(2)Q(-) = N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-quinoline-2-carboxamide) show a remarkable photolability of the NO ligand upon irradiation of the complexes with UV-vis-NIR light [Eroy-Reveles, A. A.; Leung, Y.; Beavers, C. M.; Olmstead, M. M.; Mascharak, P. K. J. Am. Chem. Soc. 2008, 130, 4447]. Here we report detailed spectroscopic and theoretical studies on complexes 1 and 2 that provide key insight into the mechanism of NO photolabilization in these compounds. IR- and FT-Raman spectroscopy show N-O and Mn-NO stretching frequencies in the 1720-1750 and 630-650 cm(-1) range, respectively, for these Mn-nitrosyls. The latter value for ν(Mn-NO) is one of the highest transition-metal-NO stretching frequencies reported to this date, indicating that the Mn-NO bond is very strong in these complexes. The electronic structure of 1 and 2 is best described as Mn(I)-NO(+), where the Mn(I) center is in the diamagnetic low-spin state and the NO(+) ligand forms two very strong π backbonds with the d(xz) and d(yz) orbitals of the metal. This explains the very strong Mn-NO bonds observed in these complexes, which even supersede the strengths of the Fe- and Ru-NO bonds in analogous (isoelectronic) Fe/Ru(II)-NO(+) complexes. Using time-dependent density functional theory (TD-DFT) calculations, we were able to assign the electronic spectra of 1 and 2, and to gain key insight into the mechanism of NO photorelease in these complexes. Upon irradiation in the UV region, NO is released because of the direct excitation of d(π)_π* → π*_d(π) charge transfer (CT) states (direct mechanism), which is similar to analogous NO adducts of Ru(III) and Fe(III) complexes. These are transitions from the Mn-NO bonding (d(π)_π*) into the Mn-NO antibonding (π*_d(π)) orbitals within the Mn-NO π backbond. Since these transitions lead to the population of Mn-NO antibonding orbitals, they promote the photorelease of NO. In the case of 1 and 2, further transitions with distinct d(π)_π* → π*_d(π) CT character are observed in the 450-500 nm spectral range, again promoting photorelease of NO. This is confirmed by resonance Raman spectroscopy, showing strong resonance enhancement of the Mn-NO stretch at 450-500 nm excitation. The extraordinary photolability of the Mn-nitrosyls upon irradiation in the vis-NIR region is due to the presence of low-lying d(xy) → π*_d(π) singlet and triplet excited states. These have zero oscillator strengths, but can be populated by initial excitation into d(xy) → L(Py/Q_π*) CT transitions between Mn and the coligand, followed by interconversion into the d(xy) → π*_d(π) singlet excited states. These show strong spin-orbit coupling with the analogous d(xy) → π*_d(π) triplet excited states, which promotes intersystem crossing. TD-DFT shows that the d(xy) → π*_d(π) triplet excited states are indeed found at very low energy. These states are strongly Mn-NO antibondi...

Three Redox States of Metallonitrosyls in Aqueous Solution

“…Diamagnetic nonheme complexes show quasilinear low-spin MnNO fragments, multiple Mn-NO and N-O bonding, and υ NO $1700-1800 cm À1 , in agreement with a stronger π-donor character of Mn in the {MnNO} 6 fragments, formally described as Mn I NO + . DFT calculations and RR results support this electronic distribution in [Mn(Papy 3 )(NO)]ClO 4 (62), modifying the previous assignment as Mn II NO • (63). A high-spin trigonal bipyramidal (with NO in the equatorial plane) Mn III NO À (S ¼ 2) state has been proposed for the 5C [Mn(TC-5,5) (NO)] complex on the basis of IR spectroscopy (ν NO ¼ 1662 cm À1 ) and SQUID susceptometry (64).…”

NO binding to Mn-substituted homoprotocatechuate 2,3-dioxygenase: relationship to O2 reactivity

“…Thus, the absence of an EPR signal suggests an Mn III -NO − formulation. A Mn III -NO − complex could be diamagnetic [25] or have a ground S = 1 spin state from an antiferromagnetic exchange coupling between the S = 2 Mn III and S = 1 NO - ligand. The diamagnetic complex would of course be EPR-silent, and while EPR detection of S = 1 complexes is possible [26], it is unlikely due to the greater zero-field energies of Mn III ions.…”

Light-triggered nitric oxide delivery to malignant sites and infection

“…Results of DFT calculations on the [M(PaPy 3 )(NO)] n+ (M=Fe, Ru, Mn) nitrosyls indicate that, for Fe and Ru analogues, electronic transition(s) from the predominantly d π (M)-NO(π *) bonding orbital with partial carboxamide character to orbitals with d π (M)-NO(π *) antibonding character leads to NO photolability [25]. In the case of the Mn analogue 3, additional transitions between the Mn dorbitals and the Py π -frame in the visible region contribute to sensitivity to low-energy light (500-800 nm) through an indirect mechanism involving intersystem crossing [25,34,35]. This kind of transition allows one to alter the ligand frame judiciously to isolate metal nitrosyls that are sensitive to NIR light.…”