Synthesis and Solution Behavior of (Tetraisopropylcyclopentadienyl)calcium Acetylide Complexes. Molecular Structure of {[(C₃H₇)₄C₅H]Ca(μ-C⋮CPh)(thf)}₂

The reaction of (Cp 4i )Ca[N(SiMe 3 ) 2 ](thf) (Cp 4i = ( i -Pr) 4 C 5 H) ( 1 ) with several terminal alkynes HC⋮CR in either toluene or hexanes produces the corresponding calcium acetylide complexes (Cp 4i )Ca[C⋮CR](thf) ( 2 ) (R = Ph ( 2a ), Fc ( 2b ), SiMe 3 ( 2c ), Si( i -Pr) 3 ( 2d ), SiPh 3 ( 2e )) in good yield. The (Cp 4i )Ca[C⋮CR](thf) complexes do not react with excess alkyne. (Cp 4i )Ca[C⋮CPh](thf) crystallizes from toluene as an acetylide-bridged dimer, [(Cp 4i )Ca[μ-C⋮CPh)(thf)] 2 ·C 7 H 8, with a pentahapto [Cp 4i ] - ligand and a terminal THF ligand on each calcium. The calcium−carbon bond distances for the bridging [C⋮CPh] - ligands are 2.551(8) and 2.521(7) Å; the average Ca−C(ring) bond distance is 2.713(15) Å. As with other (Cp 4i )CaX(thf) n complexes, the (Cp 4i )Ca[C⋮CR](thf) compounds are kinetically stabilized against disproportionation in the solid state and in THF solution; (Cp 4i )Ca[C⋮CSi( i -Pr) 3 ](thf) is also indefinitely stable in aromatic solutions. Slow but substantial dissociation of the THF ligand in (Cp 4i )Ca[C⋮CR](thf) (R = Ph, Fc, SiMe 3 ) occurs over several days in aromatic solution, leading to partial disproportionation of the complexes into (Cp 4i ) 2 Ca and {Ca[C⋮CR]} n . THF dissociation is even more pronounced in aromatic solutions of (Cp 4i )Ca[C⋮CSiPh 3 ](thf), leading to rapid disproportionation in minutes. Attempts to isolate the unsolvated complex {(Cp 4i )Ca[C⋮CPh]} n ( 3a ) are frustrated by its facile disproportionation at elevated temperatures.