A reversible NO complex of Fe-II(TIM): An S=1/2(FeNO)(7) nitrosyl

[Fe(TIM)(CH3CN)(2)](PF6)(2) (1) (TIM=2,3,9,10-tetramethyl-1,4,8,11-tetraazacyclodeca-1,3,8,10-tetraene) forms a complex with NO reversibly in CH3CN (53+/-1% converted to the NO complex) or 60% CH3OH/40% CH3CN (81+/-1% conversion). Quantitative NO complexation occurs in H2O or CH3OH solvents. The EPR spectrum of [Fe(TIM)(solvent)NO](2+) in frozen 60/40 CH3OH/CH3CN at 77 K shows a three line feature at g=2.01, 1.99 and 1.97 of an S=1/2{FeNO}(7) ground state. The middle line exhibits a three-line N-shf coupling of 24 G indicating a six-coordinate complex with either CH3OH or CH3CN as a ligand trans to NO. In H2O [Fe(TIM)(H2O)(2)](2+) undergoes a slow decomposition, liberating 2,3-butanedione, as detected by H-1 NMR in D2O, unless a pi-acceptor axial ligand, L=CO, CH3CN or NO is present. An equilibrium of 1 in water containing CH3CN forms [Fe(TIM)(CH3CN)(H2O)](2+) which has a formation constant K-CH3N=320 M-1. In water K-NO>K-CH3CN since NO completely displaces CH3CN. [Fe(TIM)(CH3CN)(2)](2+) binds either CO or NO in CH3CN with K-NO/K-CO=0.46, significantly lower than the ratio for [Fe-II(hemes)] of similar to 1100 in various media. A steric influence due to bumping of beta-CH2 protons of the TIM macrocycle with a bent S=1/2 nitrosyl as opposed to much lessened steric factors for the linear Fe-CO unit is proposed to explain the lower K-NO/K-CO ratio for the [Fe(TIM)(CH3CN)](2+) adducts of NO or CO. Estimates for formation constants with [Fe(TIM)](2+) in CH3CN of K-NO=80.1 M-1 and K-CO=173 M-1 are much lower than to hemoglobin (where K-NO=2.5X10(10) M-1 and K-CO=2.3X10(7)) due to a reversal of steric factors and stronger pi-backdonation from [Fe-II(heme)] than from [Fe-II(TIM)(CH3CN)](2+).