3 years ago

Synthesis and Catalytic Activity of PNP-Supported Iron Complexes with Ancillary Isonitrile Ligands

Synthesis and Catalytic Activity of PNP-Supported Iron Complexes with Ancillary Isonitrile Ligands
Brandon Q. Mercado, Nilay Hazari, Nicholas E. Smith, Wesley H. Bernskoetter
Pincer-supported iron complexes of the form [(iPrPNHP)Fe(H)(HBH3)(CO)] (iPrPNHP = HN(CH2CH2PiPr2)2), [(iPrPNHP)Fe(H){OC(O)H}(CO)], [(iPrPNP)Fe(H)(CO)]; iPrPNP = N(CH2CH2PiPr2)2) have proven to be a privileged class of catalysts for hydrogenation and dehydrogenation reactions. Here, the synthesis, characterization, and reactivity of a family of complexes related to these species, in which the ancillary carbonyl ligand has been replaced with an aryl isonitrile ligand, is described. The complexes [(iPrPNHP)Fe(Cl)2(C≡NR)] (R = 2,6-dimethylphenyl (1a), 4-methoxyphenyl (1b)) were prepared in good yield through the dropwise addition of the aryl isonitrile to an in situ generated solution of [(iPrPNHP)Fe(Cl)2]. If the aryl isonitrile was added too quickly, significant amounts of the cationic bis(isonitrile) complexes [(iPrPNHP)Fe(Cl)(C≡NR)2][Cl] (R = 2,6-dimethylphenyl (2a); 4-methoxyphenyl (2b)) were also formed. Treatment of 1a with excess NaBH4 generated [(iPrPNHP)Fe(H)(HBH3)(C≡NR)] (3a), but the corresponding reaction with 1b was unsuccessful. In contrast, the reaction of 1a or 1b with 1 equiv of nBu4NBH4 generated the hydrido chloride complexes [(iPrPNHP)Fe(H)(Cl)(C≡NR)] (R = 2,6-dimethylphenyl (4a), 4-methoxyphenyl (4b)). The reaction of 4a or 4b with KOtBu resulted in deprotonation of the pincer ligand and the formation of the five-coordinate amido complexes [(iPrPNP)Fe(H)(C≡NR)] (R = 2,6-dimethylphenyl (5a), 4-methoxyphenyl (5b)). The ability of the pincer ligand to participate in bifunctional reactivity was demonstrated through the reactions of 5a,b with H2 and a H2/CO2 mixture, which generated the unstable complexes [(iPrPNHP)Fe(H)2(C≡NR)] (R = 2,6-dimethylphenyl (6a), 4-methoxyphenyl (6b)) and [(iPrPNHP)Fe(H){OC(O)H}(C≡NR)] (R = 2,6-dimethylphenyl (7a), 4-methoxyphenyl (7b)), respectively. The five-coordinate amido complexes 5a,b were shown to be catalysts for the hydrogenation of CO2 to formate. Complexes 1a,b, 2a, 4b, 5a, and 7a were characterized by X-ray crystallography.

Publisher URL: http://dx.doi.org/10.1021/acs.organomet.7b00602

DOI: 10.1021/acs.organomet.7b00602

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