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

In the solid state, 1,2,3,4-tetraisopropylcyclopentadiene (HCp4i) forms stacked parallel rings in which the isopropyl groups display the same semigeared orientation as is found for the [Cp4i]-anion in its metal complexes. The [tetraisopropylcyclopentadienyl]calcium halide (Cp4i)CaI-(THF)2 (1) is isolated in high yield (>90%) from the 1:1 reaction of 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 recrystallization 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 each calcium atom. The Ca-I and Ca-I' distances are nearly equal at 3.101(4) angstrom and 3.110(4) angstrom. No disproportionation of (Cp4i)CaI(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 blocks 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 to its near quantitative (93%) conversion into (Cp4i)2Ca and CaI2(THF)n. Disproportionation is also observed after adding 1,4-dioxane to a THF 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 aromatic solution, [(Cp4i)CaI]n slowly disproportionates into (CP4i)2Ca and CaI2; this underscores the importance of coordinated THF to the stability of (Cp4i)CaI(THF)(1,2). Heating solid [(Cp4i)CaI], at 215-220-degrees-C and 10(-6) Torr produces(CP4i)2Ca as a whitesublimateingoodyield(ca.65%). Attempts to synthesize complexes analogous to (Cp4i)CaI(THF)(1,2) using pyridine, diethyl ether, or 1,2-dimethoxyethane were not as successful; only (Cp4i)CaI(DME)(4) could be obtained in a mixture with CaI2(DME)n. (Cp4i)-CaI(THF)(1,2) can be derivatized by metathetical 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 prepared by the reaction of (Cp4i)Ca[N(SiMe3)2](THF) with HBHT in toluene. Unlike the reaction that occurs between (CP4i)2Ca and CaI2, (Cp4i)2Ca and Ca[N(SiMe3)2]2 do not conproportionate in THF to form a mono(ring) 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 containing (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 pseudotrigonal planar arrangement of the ligands around the calcium. Two crystallographically 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 that suggest an agostic interaction exists between the calcium and one of the trimethylsilyl groups of the amido ligand, with Ca...C(Me) contacts 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.