Iron dinitrosyl complexes of TCNE: a synthetic, X-ray crystallographic, high field NMR and electrochemical study

Abstract

Treatment of Fe(NO) 2[PR 3](CO) with tetracyanoethylene (TCNE) in diethyl ether leads to the formation of Fe(NO) 2[PR 3]( η 2-TCNE), where PR 3=P(OCH 3) 3, 1, P( n-Bu) 3, 2, PMe 2Ph, 3, and PEt 2Ph, 4. An X-ray crystallographic study of 1 shows the iron to be situated in a nearly tetrahedral environment with a π-bonded tetracyanoethylene and two linearly bound nitrosyl groups. From the ambient-temperature NMR spectral data, it is evident that there exist two non-equivalent cyanocarbon environments, indicating that the rotation about the Fe-TCNE π-bond is slowed at room temperature; variable-temperature NMR studies on Fe(NO) 2[P(OMe) 3]( η 2-TCNE), 1, yielded an activation energy barrier of approximately 18.1±0.5 kcal mol −1 for this rotational process. Electrochemical studies revealed that the neutral Fe(NO) 2[PR 3]( η 2-TCNE) complexes undergo irreversible reductions at positively shifted potentials, relative to the related Fe(NO) 2[PR 3](CO) complexes. Moreover, a trend toward cathodic shift of the reduction potentials with increasing phosphine p K a has been observed. The high energy barrier for alkene rotation and the shift towards positive reduction potentials are rationalized in terms of a strong π-interaction between the iron center and TCNE.