Electronic Structure and FeNO Conformation of Nonheme Iron-Thiolate-NO Complexes: An Experimental and DFT Study

摘要

Reactions of NO and CO with Fe(Ⅱ) complexes of the tripodal trithiolate ligands NS3 and PS3~* yield trigonal-bipyramidal (TBP) complexes with varying redox states and reactivity patterns with respect to dissociation of the diatomic ligand. The previously reported four-coordinate [Fe~Ⅱ(NS3)]~ complex reacts irreversibly with NO gas to yield the S = 3/2 {FeNO}~7 [Fe(NS3)(NO)]~- anion, isolated as the Me_4N~+ salt. In contrast, the reaction of NO with the species generated by the reaction of FeCl_2 with Li_3PS3~* gives a high yield of the neutral, TBP, S = 1 complex, [Fe(PS3~*)(NO)], the first example of a paramagnetic {FeNO}~6 complex. X-ray crystallographic analyses show that both [Fe(NS3)(NO)]~- and [Fe(PS3~*)(NO)] feature short Fe-N(NO) distances, 1.756(6) and 1.676(3) A, respectively. However, whereas [Fe(NS3)(NO)]~- exhibits a distinctly bent FeNO angle and a chiral pinwheel conformation of the NS3 ligand, [Fe(PS3~*)(NO)] has nearly C_(3v) local symmetry and a linear FeNO unit. The S = 1 [Fe~Ⅱ(PS3)L] complexes, where L = 1-Melm, CN~-, CO, and NO~+, exhibit a pronounced lengthening of the Fe-P distances along the series, the values being 2.101(2), 2.142(1), 2.165(7), and 2.240(1) A, respectively. This order correlates with the π-backbonding ability of the fifth ligand L. The cyclic voltammogram of the [Fe(NS3)(NO)]~- anion shows an irreversible oxidation at +0.394 V (vs SCE), apparently with loss of NO, when scanned anodically in DMF. In contrast, [Fe(PS3~*)(NO)] exhibits a reversible {FeNO}~6/{FeNO}~7 couple at a low potential of -0.127 V. Qualitatively consistent with these electrochemical findings, DFT (PW91/STO-TZP) calculations predict a substantially lower gas-phase adiabatic ionization potential for the [Fe(PS3)(NO)]~- anion (2.06 eV) than for [Fe(NS3)(NO)]~ (2.55 eV). The greater instability of the {FeNO}~7 state with the PS3~* ligand results from a stronger antibonding interaction involving the metal d_(z~2) orbital and the phosphine lone pair than the analogous orbital interaction in the NS3 case. The antibonding interaction involving the NS3 amine lone pair affords a relatively "stereochemically active" d_(z~2) electron, the z direction being roughly along the Fe-N(NO) vector. As a result, the {FeNO}~7 unit is substantially bent. By contrast, the lack of a trans ligand in [Fe(S~tBu)_3(NO)]~-, a rare example of a tetrahedral {FeNO}~7 complex, results in a "stereochemically inactive" d_(z~2) orbital and an essentially linear FeNO unit.