Synthesis and solvent dependent reactivity of chelating bis-N-heterocyclic carbene complexes of Fe(II) hydrides

Abstract

The synthesis and isolation of low coordinate methylenebis-(N-DIPP-imidazole-2-ylidene)iron(II)hydrides, ((DIPPC)2CH2)FeH2-yIy ((DIPP = 2,6-di-isopropylphenyl, y = 1 or 0), was complicated by competitive reactions with solvent, rapid reductive elimination of H2 and/or dissociation of the bis-N-heterocyclic carbene ligand. Addition of KH to ((DIPPC)2CH2)FeI2 in THF/haloalkane mixtures enabled a short lived mono-hydride to be trapped by reaction with CH2Cl2 or cyclo-heptylbromide to form ((DIPPC)2CH2)FeI(X) (X = Cl or Br, respectively). Toluene coordination stabilises iron-mono hydride complexes as ((DIPPC)2CH2)FeIIH{η6-(toluene)} species, which can be isolated in low yield from combination of borohydride salts and ((DIPPC)2CH2)FeI2 in toluene, including an imidazole C4 deprotonated carbene-borane, methylene(N-DIPP-imidazole-2-ylidene)(N-DIPP-4-triethyl-borane-imidazole-2-ylidene)](hydrido)(η6-toluene)iron. In the absence of toluene, or at short reaction times compounds with empirical formula ((DIPPC)2CH2)Fe(H)(HB(R)3)·LiI (R = Et or sec-Bu) that function as a masked Fe(II)-dihydride are isolated. Whilst ((DIPPC)2CH2)Fe(H)(HB(R)3)·LiI was stable for days in Et2O, more polar solvents (MeCN, THF) led to formation of the carbene borane adducts ((DIPPC)2CH2)(BR3)2. The addition of CO or cyclo-heptylbromide to ((DIPPC)2CH2)Fe(H)(HB(R)3)·LiI formed ((DIPPC)2CH2)Fe(CO)3 and ((DIPPC)2CH2)FeBr2, respectively with BR3 evolved from both reactions as a by-product.