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

The beta-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) (R-1 = 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(Pr-i)(3) demonstrated that both monomeric solvated [{ArNC(Me)CHC(Me)NAr}Ca{C CSi(Pr-i)(3)}(THF)(2)] (3b) or dimeric acetylide [{ArNC(Me)CHC(Me)NAr}Ca{C CSi(Pr-i)(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 Delta G degrees(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{((RN)-N-2)(2)CC CR1}(THF)(n)] (R-1 = 4-MeC6H4, n = 0, 4a; 4-MeC6H4, n = 1, 4a center dot THF; R-2 = Pr-i; R-1 = Si(Pr-i(3)), R-2 = Cy, n = 1, 4b center dot THF) via insertion of the carbodiimide into the calcium-carbon sigma-bond. The latter complexes have been characterized in both solution and the solid state including single-crystal X-ray analysis of 4a center dot 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.