β-Diketiminato Calcium Acetylides: Synthesis, Solution Dimerization, and Catalytic Carbon−Carbon Bond Formation

The β-diketiminate-stabilized calcium amide [{ArNC(Me)CHC(Me)NAr}Ca{N(SiMe3)2}(THF)] (1) reacts with terminal acetylenes in hydrocarbon solvents to yield the corresponding calcium acetylide complexes [{ArNC(Me)CHCN(Me)Ar}Ca{CCR1}]2 (R1 = n-Bu, t-Bu, Ph, 4-MeC6H4, ferrocenyl, Ar = 2,6-di-isopropylphenyl, 2a−e). Although in all instances solid and solution state data were consistent with the reaction products existing as dimeric species with aggregation occurring via three-center−two-electron bridging acetylide units, a further reaction of 1 with HCCSi(iPr)3 demonstrated that both monomeric solvated [{ArNC(Me)CHC(Me)NAr}Ca{CCSi(iPr)3}(THF)2] (3b) or dimeric acetylide [{ArNC(Me)CHC(Me)NAr}Ca{CCSi(iPr)3}]2 (3a) species could be isolated from the reaction depending upon the exact conditions of the crystallization of the reaction product from solution. Further solution studies demonstrated the presence of a monomer−dimer equilibrium in solution. A van’t Hoff analysis allowed ΔG°(298 K) for the dimerization reaction to be calculated as +27.0 kJ mol−1. The reaction of these hydrocarbon-soluble kinetically stabilized calcium acetylides with 1,3-dialkylcarbodiimides gave the corresponding heteroleptic calcium C-propargyl amidinate complexes [{ArNC(Me)CHCN(Me)Ar}Ca{(R2N)2CCCR1}(THF)n] (R1 = 4-MeC6H4, n = 0, 4a; 4-MeC6H4, n = 1, 4a·THF; R2 = iPr; R1 = Si(iPr3), R2 = Cy, n = 1, 4b·THF) via insertion of the carbodiimide into the calcium−carbon σ-bond. The latter complexes have been characterized in both solution and the solid state including single-crystal X-ray analysis of 4a·THF. Extension of this reactivity to catalytic systems has allowed the application of amide 1 (5 mol %) to the catalytic hydroacetylenation of 1,3-di-isopropylcarbodiimide with phenylacetylene, yielding the corresponding propargyl amidine in 59% yield following crystallization from hexane solution.