Square planar iron methyl complexes containing bis(imino)pyridine (PDI) ligands have been prepared by reductive alkylation of the corresponding ferrous dichloride; dialkylation is observed upon treatment with a larger alkyl lithium.

Synthesis and characterization of new, four-coordinate, high-spin iron(II) and manganese(II) complexes of the general form L2MR2 (L2 = neutral chelating ligand, R = alkyl) are described. Alkylation of the α-diimine complex, [ArNC(Me)−C(Me)NAr]FeCl2 (Ar = 2,6-diisopropylphenyl), as well as the enantiopure iron dichloride compounds, (−)-(sparteine)FeCl2 and (S)-(tBuBox)FeCl2 ((S)-(tBuBox) = 2,2-bis[2-[4(S)-(R‘)-1,3-oxazolinyl]propane), with LiCH2SiMe3 afforded the corresponding dialkyl derivatives. Solution magnetic susceptibility measurements and X-ray diffraction studies reveal each of the new iron(II) bis-trimethylsilylmethyl complexes to be high-spin, S = 2, tetrahedral molecules. In addition (−)-(sparteine)Fe(CH2CMe3)2, (−)-(sparteine)Fe(CH2C6H5)2, and (S)-(tBuBox)Fe(CH2C6H5)2 were also prepared and characterized by NMR spectroscopy and elemental analysis. An enantiopure, high-spin, tetrahedral manganese(II) dialkyl complex, (−)-(sparteine)Mn(CH2SiMe3)2, has also been synthesized. The catalytic activity of the new iron complexes in carbon−carbon bond forming processes has been evaluated, and stoichiometric reactions of the dialkyls with olefins, carbon monoxide, and the Lewis acid B(C6F5)3 have been examined.

The geometric preferences of a family of four coordinate, iron(II) d6 complexes of the general form L2FeX2 have been systematically evaluated. Treatment of Fe2(Mes)4 (Mes = 2,4,6-Me3C6H2) with monodentate phosphine and phosphite ligands furnished square planar trans-P2Fe(Mes)2 derivatives. Identification of the geometry has been accomplished by a combination of solution and solid-state magnetometry and, in two cases (P = PMe3, PEt2Ph), X-ray diffraction. In contrast, both tetrahedral and square planar coordination has been observed upon complexation of chelating phosphine ligands. A combination of crystallographic and magnetic susceptibility data for (depe)Fe(Mes)2 (depe = 1,2-bis(diethylphosphino)ethane) established a tetrahedral molecular geometry whereas SQUID magnetometry and Mössbauer spectroscopy on samples of (dppe)Fe(Mes)2 (dppe = 1,2-bis(diphenylphosphino)ethane) indicated a planar molecule. When dissolved in chlorinated solvents, the latter compound promotes chlorine atom abstraction, forming tetrahedral (dppe)Fe(Mes)Cl and (dppe)FeCl2. Ligand substitution reactions have been studied for both structural types and are rapid on the NMR time scale at ambient temperature.

A family of low-valent R-diimine iron complexes has been synthesized and their utility in catalytic olefin hydrogenation reactions evaluated. Reduction of the ferrous dichloride complex [ArNdC(Me)C(Me)dNAr]FeCl 2 (Ar ) 2,6-(CHMe 2 ) 2 -C 6 H 3 ) with sodium amalgam in benzene or toluene furnished the iron arene complexes, [ArNdC(Me)C(Me)dNAr]Fe(η 6 -C 6 H 5 R) (R ) H, Me). The solid-state structure of the toluene adduct revealed a contracted carbon-carbon backbone, short iron-imine bonds, and elongated imine nitrogen-carbon distances, suggesting significant reduction of the R-diimine ligand. The analogous reduction in alkane solvents afforded the bis(R-diimine) complex [ArNdC(Me)C(Me)dNAr] 2 Fe, which has also been crystallographically characterized. The arene complexes and the bis(R-diimine) complexes are inactive for catalytic olefin hydrogenation. Performing the reduction in the presence of internal alkynes such as diphenylacetylene and bis(trimethylsilyl)acetylene furnished the alkyne adducts [ArNdC(Me)C(Me)dNAr]Fe(η 2 -RCtCR) (R ) Ph, SiMe 3 ). Analogous olefin complexes with 1,5-cyclooctadiene and cycloctene have also been isolated using similar reduction procedures. The olefin adducts provide more active precatalysts than the alkyne compounds for the hydrogenation of 1-hexene. In each case, formation of η 6arene adducts serves as a major catalyst deactivation pathway.