The bis(arylimidazol-2-ylidene)pyridine cobalt methyl complex, (iPrCNC)CoCH3, was evaluated for the catalytic hydrogenation of alkenes. At 22 °C and 4 atm of H2 pressure, (iPrCNC)CoCH3 is an effective pre-catalyst for the hydrogenation of sterically hindered, unactivated alkenes such as trans-methylstilbene, 1-methyl-1-cyclohexene and 2,3-dimethyl-2-butene, representing one of the most active cobalt hydrogenation catalysts reported to date. Preparation of the cobalt hydride complex, (iPrCNC)CoH was accomplished by hydrogenation of (iPrCNC)CoCH3. Over the course of 3 hours at 22 °C, migration of the metal-hydride to the 4-position of the pyridine ring yielded (4-H2-iPrCNC)CoN2. Similar alkyl migration was observed upon treatment of (iPrCNC)CoH with 1,1-diphenylethylene. This reactivity raised the question as to whether this class of chelate is redoxactive, engaging in radical chemistry with the cobalt center. A combination of structural, spectroscopic and computational studies was conducted and provided definitive evidence for bis(arylimidazol-2-ylidene)pyridine radicals in reduced cobalt chemistry. Spin density calculations established that the radicals were localized on the pyridine ring, accounting for the observed reactivity and suggest a wide family of pyridine-based pincers may also be redox active.

The activity of aryl-substituted bis(imino)pyridine and bis(arylimidazol-2-ylidene)pyridine iron dinitrogen complexes has been evaluated in a series of catalytic olefin hydrogenation reactions. In general, more electron donating chelates with smaller 2,6-aryl substituents produce more active iron hydrogenation catalysts. Establishment of this structure-activity relationship has produced base metal catalysts that exhibit high turnover frequencies for the hydrogenation of unfunctionalized, tri- and tetrasubstituted alkenes, one of the most challenging substrate classes for homogenous hydrogenation catalysts.

The synthesis and characterization of dimeric, aryl-substituted bis(imino)pyridine iron dinitrogen complexes is described. In contrast to reduction with sodium amalgam where bis(chelate) iron compounds were isolated, stirring ((Ar)PDI)FeBr(2) or ((Me)BPDI)FeBr(2) (PDI = 2,6-(ArN=CMe)(2)C(5)H(3)N; Ar = 2,6-Et(2)-C(6)H(3)N ((Et)PDI), 2,6-Me(2)-C(6)H(3)N ((Me)PDI), 2-(i)Pr,6-Me-C(6)H(3)N ((Me,iPr)PDI); (Me)BPDI = 2,6-(2,6-Me(2)-C(6)H(3)N=CPh)(2)C(5)H(3)N) with sodium naphthalenide resulted in isolation of the desired iron dinitrogen compounds as diamagnetic solids. Two examples, [((Et)PDI)Fe(N(2))](2)(mu(2)-N(2)) and [((Me)BPDI)Fe(N(2))](2)(mu(2)-N(2)), were characterized by X-ray diffraction. The solid state metrical parameters, in combination with infrared and Mossbauer spectroscopic data, establish ferrous compounds with doubly reduced chelates. Each new bis(imino)pyridine iron dinitrogen compound was screened for the catalytic hydrogenation of ethyl-3-methylbut-2-enoate, and the compound bearing the smallest aryl substituent, [((Me)PDI)Fe(N(2))](2)(mu(2)-N(2)), offers significant improvement over the original ((iPr)PDI)Fe(N(2))(2) pre-catalyst and is one of the most active iron pre-catalysts known.

Addition of biphenylene to the bis(imino)pyridine iron dinitrogen complexes, (iPrPDI)Fe(N2)2 and [(MePDI)Fe(N2)]2(μ2-N2) (RPDI = 2,6-(2,6-R2—C6H3— N=CMe)2C5H3N; R = Me, iPr), resulted in oxidative addition of a C—C bond at ambient temperature to yield the corresponding iron biphenyl compounds, (RPDI)Fe-(biphenyl). The molecular structures of the resulting bis-(imino)pyridine iron metallacycles were established by X-ray diffraction and revealed idealized square pyramidal geometries. The electronic structures of the compounds were studied by Mössbauer spectroscopy, NMR spectroscopy, magnetochemistry, and X-ray absorption and X-ray emission spectroscopies. The experimental data, in combination with broken-symmetry density functional theory calculations, established spin crossover (low to intermediate spin) ferric compounds antiferromagnetically coupled to bis(imino)pyridine radical anions. Thus, the overall oxidation reaction involves cooperative electron loss from both the iron center and the redox-active bis(imino)pyridine ligand.