| 17685516 |
Conradie J, Quarless DA Jr, Hsu HF, Harrop TC, Lippard SJ, Koch SA, Ghosh A: Electronic structure and FeNO conformation of nonheme iron-thiolate-NO complexes: an experimental and DFT study. J Am Chem Soc. 2007 Aug 29;129(34):10446-56. Epub 2007 Aug 8.
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 (4) N (+) salt. In contrast, the reaction of NO with the species generated by the reaction of FeCl (2) with Li (3) PS3* 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-MeIm, 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 pi-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" dz2 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 (t) Bu) 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. |
83(1,1,1,3) |