A reversible NO complex of Fe II(TIM): an [formula omitted] nitrosyl

Abstract

[Fe(TIM)(CH 3CN) 2](PF 6) 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 CH 3CN (53±1% converted to the NO complex) or 60% CH 3OH/40% CH 3CN (81±1% conversion). Quantitative NO complexation occurs in H 2O or CH 3OH solvents. The EPR spectrum of [Fe(TIM)(solvent)NO] 2+ in frozen 60/40 CH 3OH/CH 3CN 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 CH 3OH or CH 3CN as a ligand trans to NO. In H 2O [Fe(TIM)(H 2O) 2] 2+ undergoes a slow decomposition, liberating 2,3-butanedione, as detected by 1H NMR in D 2O, unless a π-acceptor axial ligand, L=CO, CH 3CN or NO is present. An equilibrium of 1 in water containing CH 3CN forms [Fe(TIM)(CH 3CN)(H 2O)] 2+ which has a formation constant K CH 3 CN=320 M −1. In water K NO K CH 3CN since NO completely displaces CH 3CN. [Fe(TIM)(CH 3CN) 2] 2+ binds either CO or NO in CH 3CN with K NO/ K CO=0.46, sigificantly lower than the ratio for [Fe II(hemes)] of ∼1100 in various media. A steric influence due to bumping of β-CH 2 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)(CH 3CN)] 2+ adducts of NO or CO. Estimates for formation constants with [Fe(TIM)] 2+ in CH 3CN of K NO=80.1 M −1 and K CO=173 M − are much lower than to hemoglobin (where K NO=2.5×10 10 M −1 and K CO=2.3×10 7) due to a reversal of steric factors and stronger π-backdonation from [Fe II(heme)] than from [Fe II(TIM)(CH 3CN)] 2+.