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

Treatment of Fe(NO)(2)[PR3](CO) with tetracyanoethylene (TCNE) in diethyl ether leads to the formation of Fe(NO)(2)[PR3](eta(2)-TCNE), where PR3 = P(OCH3)(3), 1, P(n-Bu)(3), 2, PMe2Ph, 3, and PEt2Ph, 4. An X-ray crystallographic study of 1 shows the iron to be situated in a nearly tetrahedral environment with a pi-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;pi-bond is slowed at room temperature; variable-temperature NMR studies on Fe(NO)(2)[P(OMe)(3)](eta(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)[PR3](eta(2)-TCNE) complexes undergo irreversible reductions at positively shifted potentials, relative to the related Fe(NO)(2)[PR3](CO) complexes. Moreover, a trend toward cathodic shift of the reduction potentials with increasing phosphine pK(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 pi-interaction between the iron center and TCNE. (C) 1998 Elsevier Science S.A. All rights reserved.