Dinitrogen Cleavage and Multicoupling with Isocyanides in a Dititanium Dihydride Framework

Dinitrogen (N-2) activation and functionalization through N-N bond cleavage and N-C bond formation are of great interest and importance but remain highly challenging. We report here for the first time N-2 cleavage and selective multicoupling with isocyanides in a dititanium dihydride framework. The reaction of a dinitrogen dititanium dihydride complex [{((PNP)-P-acri)Ti}(2)(mu-eta(1):eta(2)-N-2)(mu-H)(2)] (1) with an excess (four or more equivalents) of p-methoxyphenyl isocyanide at room temperature gave a novel amidoamidinatoguanidinate complex [((PNP)-P-acri)Ti{NC(& boxH;NR)NC(& boxH;NR)CH2NR}Ti((PNP)-P-acri)(CNR)] (2, (PNP)-P-acri = 4,5-bis(diisopropylphosphino)-2,7,9,9-tetramethyl-9H-acridin-10-ide; R = p-MeOC6H4) through N-2 splitting and coupling with three isocyanide molecules. When 1 equiv of p-methoxyphenyl isocyanide was used to react with 1 at -30 degrees C, the hydrogenation of the isocyanide unit by the two hydride ligands in 1 took place, affording an amidomethylene-bridged dititanium dinitrogen complex [{((PNP)-P-acri)Ti}(2)(mu-eta(1):eta(2)-N-2){mu-eta(1):eta(2)-CH2N(p-MeOC6H4)}] (3), which upon reaction with another equivalent of p-methoxyphenyl isocyanide at room temperature gave an amidomethylene/nitrido/carbodiimido complex [((PNP)-P-acri)Ti(N & boxH;C & boxH;NR)(mu-N)(mu-eta(1):eta(2)-CH2NR)Ti((PNP)-P-acri)] (4) through N-2 cleavage and N & boxH;C bond formation. Further reaction of 4 with 1 equiv of p-methoxyphenyl isocyanide led to an unprecedented four-component (carbodiimido, nitrido, isocyanide, and amidomethylene) coupling, yielding an amidoamidinatoguanidinate complex [{((PNP)-P-acri)Ti}(2){NC(& boxH;NR)NC(& boxH;NR)CH2NR}] (5), which on reaction with another equivalent of p-methoxyphenyl isocyanide afforded the isocyanide-coordinated analogue 2. The reaction of 1 with 2-naphthyl isocyanide also took place in a similar multicoupling fashion. Moreover, the cross-coupling reactions of the p-methoxyphenyl isocyanide-derived amidomethylene/nitrido/carbodiimido complex 4 with 2-naphthyl isocyanide, cyclohexyl isocyanide, and tert-butyl isocyanide were also achieved, which afforded the corresponding amidoamidinatoguanidinate products consisting of two different isocyanides. Density functional theory (DFT) calculations further elucidated the mechanistic details.