Synthesis and solution behavior of (tetraisopropylcyclopentadienyl)calcium acetylide complexes. Molecular structure of {[(C3H7)(4)C5H]Ca(mu-C CPh)(thf)}(2)

The reaction of (Cp(4i))Ca[N(SiMe(3))(2)](thf) (Cp(4i)=(i-Pr)(4)C5H) (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), Pc (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[mu-C=CPh)(thf)](2) . C7H8, 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) Angstrom; the average Ca-C(ring) bond distance is 2.713(15) Angstrom. 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.