The catalytic hydrogenation of carbon-carbon and carbon-nitrogen multiple bonds by a cobalt(I) complex

Abstract

The widespread use of homogenous late second- and third-row transition metal complexes for the catalytic hydrogenation of carbon-carbon multiple bonds can be attributed to the well-defined reactivity, stability and overall high activity of such catalytic systems. The development of sustainable, more abundant and relatively less toxic late first-row transition metal complexes to parallel such catalysis is an attractive alternative and has gained much interest. However, due to the inherent electronic structure of these metals, common first-row transition metal complexes have a propensity to undergo one-electron or radical reactivity, which can be detrimental to the desired reactivity. Toward this end, an electron-rich monoanionic bis(carbene) aryl pincer ligand, MesCCC, was chosen study two-election chemistry with cobalt. With the MesCCC pincer ligand platform well-suited to stabilize a range of oxidation states of cobalt (Co(III), Co(II), and Co(I)), studies into the room temperature metalation of the ligand precursor salt, [H3(MesCCC)]Cl2, resulted in the formation (MesCCC)CoCl2py. Furthermore, the reduction of (MesCCC)CoCl2py with one or two reducing electron equivalents yielded the corresponding Co(II) and Co(I) complexes, respectively. Characterization of these complexes by 1H and 13C NMR, EPR, and FT-IR spectroscopies, as well as single crystal X-ray crystallography is described. The interconversion between a series of low-spin cobalt complexes in the +3, +2 and +1 oxidation states provides a platform for which to study two-electron chemistry with cobalt. The reactivity of the low-valent (MesCCC)Co(N2)(PPh3) towards dihydrogen resulted in a reversible H2 coordination to the cobalt center and based on multinuclear NMR studies, a nonclassical binding mode of H2 was established, (MesCCC)Co(H2)(PPh3). The scrambling of H2 and D2 by the Co(I)-(H2) species suggests a Co(III)-(H2)(H)2 intermediate may be involved. Furthermore, the competency of the Co-(H2) complexes towards the catalytic hydrogenation of alkenes was also investigated. The hydrogenation of terminal olefins proceeded selectivity at room temperature, while more sterically demanding 1,2-disubstitued olefins proceeded at elevated temperatures. Mechanistic studies based on multinuclear and parahydrogen (p-H2) induced polarization (PHIP) transfer NMR reveled a Co(I)/Co(III) redox process was operative. Based on the mechanistic studies of the olefin hydrogenation studies, the hydrogenation utility of (MesCCC)Co(N2)(PPh3) was extended toward the semi-hydrogenation of a broad scope of alkynes with excellent selectivity toward E-alkenes. Mechanistic studies of the alkyne hydrogenation process using 1H, 2H and PHIP transfer NMR spectroscopy enabled the identification of key reaction intermediates and established that cis-hydrogenation occurs first, followed by trans-isomerization under a H2 atmosphere. The catalytic utility of the (MesCCC)Co complexes was extended to the selective hydrogenation nitriles to primary amines. The active catalyst was generated by the in situ reduction of a bench-stable Co(III) precatalyst, (MesCCC)CoCl2py, using NaHBEt3. The resulting BEt3 was found to be vital to the observed catalysis, acting as Lewis acid. Based on the PHIP transfer NMR studies, the role of BEt3 was proposed to facilitate a side-on coordination of the nitrile to cobalt center, thus permitting a pairwise transfer of H2 through a Co(I/III) redox process to proceed. Given the robust functional group tolerance and enhancement of the 1H NMR resonances in the hydrogenation products using PHIP, the low-valent (MesCCC)Co complex were shown to effectively hyperpolarize the 13C NMR signals using p-H2. Comparisons of the carboxylate 13C signal enhancement of ethyl propionate using [(dppb)Rh(COD)]BF4 and (MesCCC)Co-py, showed that the cobalt complex hyperpolarization efficiency is comparable with the rhodium system, demonstrating that the monoanionic bis(carbene) pincer ligand supports noble-metal reactivity with cobalt. Lastly, stoichiometric oxidative addition and reductive elimination chemistry with the MesCCC ligand platform demonstrate the viability of a Co(I)/Co(III) redox couple in the proposed catalytic hydrogenation reactions. The preparation of cationic cobalt(III)-hydride dinitrogen phosphine complexes served as a platform to study migratory insertion of alkenes, as well as, the scrambling of H2 and D2. These studies also demonstrate that the MesCCC ligand system not only supports the hydrogenation activity but has the proper spectroscopic features to elucidate key mechanistic steps involved.LimitedAuthor requested closed access (OA after 2yrs) in Vireo ETD syste