A reversible NO complex of Fe(TIM): an S = 1/2FeNO nitrosyl |
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Authors: | Ya Chen Michael A. Sweetland Rex E. Shepherd |
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Affiliation: | Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, USA |
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Abstract: | [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=1/2FeNO7 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 1H NMR in D2O, unless a π-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 KCH3CN=320 M−1. In water KNOKCH3CN since NO completely displaces CH3CN. [Fe(TIM)(CH3CN)2]2+ binds either CO or NO in CH3CN with KNO/KCO=0.46, sigificantly lower than the ratio for [FeII(hemes)] of 1100 in various media. A steric influence due to bumping of β-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 KNO/KCO ratio for the [Fe(TIM)(CH3CN)]2+ adducts of NO or CO. Estimates for formation constants with [Fe(TIM)]2+ in CH3CN of KNO=80.1 M−1 and KCO=173 M− are much lower than to hemoglobin (where KNO=2.5×1010 M−1 and KCO=2.3×107) due to a reversal of steric factors and stronger π-backdonation from [FeII(heme)] than from [FeII(TIM)(CH3CN)]2+. |
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Keywords: | Iron complexes Azamacrocyle complexes Nitrosyl complexes |
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