Dinitrogen Cleavage and Multicoupling with Isocyanides in a Dititanium Dihydride Framework

Dinitrogen
(N2) 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 N2 cleavage and selective multicoupling
with isocyanides in a dititanium dihydride framework. The reaction
of a dinitrogen dititanium dihydride complex [{(acriPNP)Ti}2(μ–η1:η2-N2)(μ-H)2] (1) with an excess
(four or more equivalents) of p-methoxyphenyl isocyanide
at room temperature gave a novel amidoamidinatoguanidinate complex
[(acriPNP)Ti{NC(NR)NC(NR)CH2NR}Ti(acriPNP)(CNR)] (2, acriPNP
= 4,5-bis(diisopropylphosphino)-2,7,9,9-tetramethyl-9H-acridin-10-ide; R = p-MeOC6H4) through N2 splitting and coupling with three isocyanide
molecules. When 1 equiv of p-methoxyphenyl isocyanide
was used to react with 1 at −30 °C, the hydrogenation
of the isocyanide unit by the two hydride ligands in 1 took place, affording an amidomethylene-bridged dititanium dinitrogen
complex [{(acriPNP)Ti}2(μ–η1:η2-N2){μ–η1:η2-CH2N(p-MeOC6H4)}] (3), which upon reaction with
another equivalent of p-methoxyphenyl isocyanide
at room temperature gave an amidomethylene/nitrido/carbodiimido complex
[(acriPNP)Ti(NCNR)(μ-N)(μ–η1:η2-CH2NR)Ti(acriPNP)]
(4) through N2 cleavage and N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 [{(acriPNP)Ti}2{NC(NR)NC(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.