ENCAPSULATED ALKALINE-EARTH METALLOCENES .5. KINETIC STABILIZATION OFMONO[TETRAISOPROPYLCYCLOPENTADIENYL]CALCIUM COMPLEXES

In the solid state, 1,2,3,4-tetraisopropylcyclopentadiene (HCp4i) form s stacked parallel rings in which the isopropyl groups display the sam e semigeared orientation as is found for the [Cp4i]-anion in its metal complexes. The [tetraisopropylcyclopentadienyl]calcium halide (Cp4i)C aI-(THF)2 (1) is isolated in high yield (>90%) from the 1:1 reaction o f KCp4i and CaI2 in THF or by the conproportionation of (Cp4i)2Ca and CaI2 in THF. One THF ligand in (Cp4i)CaI(THF)2 is easily removed by re crystallization from toluene to generate the monosolvated derivative ( Cp4i)CaI(THF) (2); allowing KCp4i and CaI2 (1:1) to react in a toluene /THF solvent mixture produces (Cp4i)CaI(THF) directly. (Cp4i)CaI(THF) crystallizes from toluene as an iodide-bridged dimer, [(Cp4i)Ca(mu-I)( THF)]2.C7H8, with a pentahapto [Cp4i]- ligand and a terminal THF on ea ch calcium atom. The Ca-I and Ca-I' distances are nearly equal at 3.10 1(4) angstrom and 3.110(4) angstrom. No disproportionation of (Cp4i)Ca I(THF)(1,2) into (Cp4i)2Ca and CaI2(THF)n is observed in either THF or aromatic solvents at room temperature. This stability arises from the inability of THF to dissociate completely from the oxophilic calcium center in the mono-(cyclopentadienyl) complexes, which consequently bl ocks the formation of the necessarily base-free (Cp4i)2Ca. Refluxing a toluene solution of (Cp4i)CaI(THF)(1,2) for 4 h, however, does lead t o its near quantitative (93%) conversion into (Cp4i)2Ca and CaI2(THF)n . Disproportionation is also observed after adding 1,4-dioxane to a TH F solution of (Cp4i)CaI(THF)(1,2). Heating (Cp4i)CaI(THF)(1,2) at 110- degrees-C and 10(-6) Torr for 4 h removes all coordinated THF without causing disproportionation, leaving unsolvated [(Cp4i)CaI]n (3). In ar omatic solution, [(Cp4i)CaI]n slowly disproportionates into (CP4i)2Ca and CaI2; this underscores the importance of coordinated THF to the st ability of (Cp4i)CaI(THF)(1,2). Heating solid [(Cp4i)CaI], at 215-220- degrees-C and 10(-6) Torr produces(CP4i)2Ca as a whitesublimateingoody ield(ca.65%). Attempts to synthesize complexes analogous to (Cp4i)CaI( THF)(1,2) using pyridine, diethyl ether, or 1,2-dimethoxyethane were n ot as successful; only (Cp4i)CaI(DME)(4) could be obtained in a mixtur e with CaI2(DME)n. (Cp4i)-CaI(THF)(1,2) can be derivatized by metathet ical reactions with K[N(SiMe3)2] and K[BHT] (HBHT= HOC6H2-t-Bu2-2,6-Me -4) to yield (Cp4i)Ca[N(SiMe3)2](THF) (5) and(Cp4i)Ca[BHT]-(THF) (6), respectively, in high yield. (Cp4i)Ca[BHT](THF) also can be cleanly pr epared by the reaction of (Cp4i)Ca[N(SiMe3)2](THF) with HBHT in toluen e. Unlike the reaction that occurs between (CP4i)2Ca and CaI2, (Cp4i)2 Ca and Ca[N(SiMe3)2]2 do not conproportionate in THF to form a mono(ri ng) complex. (Cp4i)Ca[N(SiMe3)2)](THF) sublimes readily at 120-degrees -C and 10(-6) Torr in ca. 50-60% yield to give a waxy material contain ing (Cp4i)Ca[N(SiMe3)2] (THF), (CP4i)2Ca, and Ca[N(SiMe3)2]2(THF)n. In the solid state, (Cp4i)Ca[N(SiMe3)2](THF) is monomeric with a pseudot rigonal planar arrangement of the ligands around the calcium. Two crys tallographically independent enantiomers are present in the asymmetric unit, with Ca-N bond distances of 2.29(1) and 2.30(1) angstrom. Both molecules of (Cp4i)Ca[N(SiMe3)2] (THF) possess structural features tha t suggest an agostic interaction exists between the calcium and one of the trimethylsilyl groups of the amido ligand, with Ca...C(Me) contac ts at 2.99(2) and 2.95(2) angstrom. These results illustrate the high level of kinetic control possible over the reactions of organocalcium species containing encapsulating ligands.