SYNTHESIS AND SOLUTION BEHAVIOR OF (TETRAISOPROPYLCYCLOPENTADIENYL)CALCIUM ACETYLIDE COMPLEXES - MOLECULAR-STRUCTURE OF ([(C3H7)(4)C5H]CA(MU-CCPH)(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 pr oduces 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 a s an acetylide-bridged dimer, [(Cp(4i))Ca[mu-C=CPh)(thf)](2) . C7H8, w ith 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 stabil ized against disproportionation in the solid state and in THF solution ; (Cp(4i))Ca[C=CSi(i-Pr)(3)](thf) is also indefinitely stable in aroma tic 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 comple xes into (Cp(4i))(2)Ca and {Ca[C=CR]}(n). THF dissociation is even mor e pronounced in aromatic solutions of (Cp(4i))Ca[C=CSiPh(3)](thf), lea ding to rapid disproportionation in minutes. Attempts to isolate the u nsolvated complex {(Cp(4i))Ca[C=CPh]}(n) (3a) are frustrated by its fa cile disproportionation at elevated temperatures.