Effect of the Sixth Axial Ligand in CS-Ligated Iron(II)octaethylporphyrinates: Structural and Mossbauer Studies

We present here the synthesis, characterization, and flash photolysis study of [(F8TPP)Fe^II(CO)(THF)] (1) {F8TPP = tetrakis(2,6difluorophenyl)porphyrinate(2-)}. Complex 1 crystallizes from THF/heptane solvent system as a tris-THF solvate, [(F8TPP)Fe^II(CO) (THF)]·3THF (1·3THF), with ferrous ion in the porphyrin plane (C61H52F8FeN4O5; a = 11.7908(2) (angstrom), b = 20.4453(2) (angstrom), c = 39.9423(3),(alpha)= 90,(beta)= 90,(gamma)= 90; orthorhombic, P212121, Z = 8; Fe-N4(av) = 2.00 (angstrom); N-Fe-N (all) = 90.0). This complex (as 1·THF) has also been characterized by 1H NMR {six-coordinate, low-spin heme; CD3CN, RT, 8.82 (s, pyrrole-H, 8H), 7.89 (s, para-phenyl-H, 8H), 7.46 (s, meta-phenyl-H, 4H), 3.58 (s, THF, 8H), 1.73 (s, THF, 8H)}, 2H NMR (pyrroledeuterated analogue) [(F8TPP-d8)FeII(CO)(THF)] {THF, RT, 8.78 ppm (s, pyrrole-D)}, 13C NMR (on 13CO-enriched adduct) {THF-d8, RT, 206.5 ppm; CD2Cl2, RT, 206.1 ppm}, UV-vis {THF, RT, max, 411 (Soret), 525 nm}, and IR {293 K, solution, CO 1979 cm^-1 (THF), 1976 cm^-1 (acetone), 1982 cm^-1 (CH3CN)} spectroscopies. In order to more fully understand the intricacies of solvent-ligand binding (as compared to CO rebinding to the photolyzed heme), we have also synthesized the bis-THF adduct [(F8TPP)Fe^II(THF)2]. Complex 2 also crystallizes from THF/heptane solvent system as a bis-THF solvate, [(F8TPP)Fe^II(THF)2]·2THF (2·2THF), with ferrous iron in the porphyrin plane (C60H52F8FeN4O4; a = 21.3216(3) (angstrom), b = 12.1191(2) (angstrom), c = 21.0125(2) (angstrom), (alpha)= 90,(beta)= 105.3658(5), (gamma) = 90; monoclinic, C2/c, Z = 4; Fe-N4(av) = 2.07 (angstrom); N-Fe-N (all) = 90.0). Further characterization of 2 includes UV-vis {THF, max, 421 (Soret), 542 nm} and 1H NMR {six-coordinate, high spin heme; THF-d8, RT, 56.7 (s, pyrrole-H, 8H), 8.38 (s, para-phenyl-H, 8H), 7.15 (s, meta-phenyl-H, 4H)} spectroscopies. Flash photolysis studies employing 1 were able to resolve the CO rebinding kinetics in both THF and cyclohexane solvents. In CO saturated THF {[CO] ~ 5 mM} and at [1] 5 M, the conversion of [(F8TPP)Fe^II(THF)2] (produced after photolytic displacement of CO) to [(F8TPP)Fe^II(CO)(THF)] was monoexponential, with kobs = 1.6 ((plus-minus)0.2) × 104 s^-1. Reduction in [CO] by vigorous Ar purging gave kobs 103 s^-1 in cyclohexane. The study presented in this report lays the foundation for applying fast-time scale studies based on CO flash photolysis to the more complicated heterobimetallic heme/Cu systems.