DFT Study of C–C and C–N Coupling on a Quintuple-Bonded Cr₂ Template: MECP (Minimum Energy Crossing Point) Barriers Control Product Distribution

A detailed DFT study involving 50 intermediates, 26 transition states, a range of spin multiplicities and 3 MECPs explains the products (and their distribution) of the reaction of cyclohexyl isocyanide with the Cr–Cr quintuple-bonded complex [LCr]₂ (L = N,N′-bis­(2,6-diisopropylphenyl-1,4-diazadiene). Using methyl isocyanide as the prototypical isocyanide and [LCr]₂ (L = N,N′-bis­(2,6-dimethyl-1,4-diazadiene) as the quintuple-bonded system, mechanistic pathways for the formation of diamagnetic [LCr­(MeNC)₄] (2b) and paramagnetic [L₂Cr₂[C₄(NMe)₄]­(MeNC)₂] (3b) and [L₂Cr₂[C₆(NMe)₆]] (4b) with an excess of isocyanide and of a paramagnetic product of C–H activation (5b) with 4 equiv of isocyanide are obtained. The ground-state electronic configurations of different products are discussed using an electron-counting scheme. Barriers for minimum energy crossing points (MECPs) and for different transition states control the experimental product distributions.