Structural and Reactivity Studies of "Pincer" Pyridine Dicarbene Complexes of Fe0: Experimental and Computational Comparison of the Phosphine and NHC Donors

Reduction of [Fe(C-N-C)(Br)(2)] or [Fe(C-N-Me-C(I)(2)] with Na/Hg under N-2 gave the Fe-0 complexes [Fe(C-N-C)(N-2)(2)] and [Fe(C-N-Me-C)(N-2)(2)] for which C-N-C=2.6-bis(arylimidazol-2-ylidene)-pyridine, C-N-Me-C=2,6-bis(arylimidazol-2-ylidene)-3,5-dimethyl-pyridine and aryl= 2,6-iPr(2)C(6)H(3). Substitution of the coordinated N-2 by CO or CN(2,6-xyl), xyl= 2,6-dimethylphenyl, took place readily to afford [Fe(C-N-C)(L)(2)] and [Fe(C-N-Me-C(L)(2)]. L=CO or CN(2,6-xyl). The electronic characteristics of the N-heterocyclic carbene and phosphine donors in the complexes [Fe(C-N-C)(CO)(2)] [Fe(C-N-Me-C)(CO)(2)] and [Fe(P-N-P)(CO)(2)], P-N-P=2,6-bis(di-tert-butylphosphinomethyl)-pyridine, have been evaluated by spectroscopic. structural and computational methods. The unexpected reduced Fe-0 to CO backbonding in [Fe(C-N-C)(CO)(2)] and [Fe(C-N-Me-C)(CO)(2)] compared to [Fe-(P-N-P)(CO)(2)] is accounted for by backbonding from the Fe-N-pyridine bond in the imidazol-2-ylidene complexes. Reduction of [Fe(C-N-C)(Br)(2)] under Ar gave mixtures from Which a Complex with it metalated pyridine and a dangling imidazolium group was isolated. Photolysis of [Fe(C-N-C)(N-2)(2)] in the presence of benzaldehyde phenylimine gave all ortho-metalated benzaldehyde phenylimine. The Fe-II complex [Fe(C-N-C)(CO)(Br)(2)] was prepared by the reaction of [Fe(C-N-C)(Br)(2)] with CO or reduction of CO2 with [Fe(C-N-C)N-2)(2)].