PENTAMETHYLCYCLOPENTADIENYL AND CYCLOPENTADIENYL TANTALUM AND NIOBIUM CALIXARENE COMPOUNDS AND THEIR WATER AND ACETONITRILE INCLUSION COMPLEXES

The syntheses, spectroscopic properties, crystal and molecular structures, and bonding of several cyclopentadienyl or pentamethylcyclopentadienyl niobium and tantalum calixarene complexes are described. Included are the mononuclear complexes [Cp*Ta(calix[4]arene)] (1), [Cp*Ta(p-tert-butylcalix[4]arene)] (2a), and [CpNb(p-tert-butylcalix[4]arene)] (3), in which the electron-deficient metal centers are protected in the exo-calix position by sterically bulky ligands. These complexes are well suited to bind small-molecule guests within their macrocyclic pockets, as demonstrated in the solid state and in solution for the complexes [Cp*Ta(p-tert-butylcalix[4]arene)]. toluene (2a . toluene), [Cp*Ta(OH2)(p-tert-butylcalix[4]arene)] (2b), and [Cp*Ta(NCCH3) (p-tert-butylcalix[4]arene)] (2c). In 1, the calixarene cavity is elliptically shaped as a result of two sets of trans-Ta-O-Ph interbond angles, one wide and one narrow, a geometry that nicely accommodates the toluene guest molecule in 2a . toluene. In 2b and 2c, however, the cavity is cylindrical. Insights into these differences and bonding in these compounds are provided by Fenske-Hall molecular orbital calculations on [Cp*Ta](4+)[calix[4]arene](4-), (1), a model of 1 with C-4V symmetry, [Cp*Ta(OH2)(calix[4]arene)], and [Cp*Ta(NCCH3)(calix[4]arene)]. Crystal data: 1, monoclinic, P2(1)/c, a = 11.270(3) Angstrom, b = 18.168(4) Angstrom, c = 15.655(2) Angstrom, beta = 110.86(1)degrees, Z = 4, R = 0.028, R(w) = 0.034; 2a . 2toluene . pentane, monoclinic, P2(1)/c, a = 13.678(2) Angstrom, b = 19.404(2) Angstrom, c = 23.230(2) Angstrom, beta = 94.790(9)degrees, Z = 4, R = 0.030, R(w) = 0.038; 2b, monoclinic, P2(1)/n, a = 11.168(3) A, b = 22.211(6) Angstrom, 20.957(5) A, beta = 91.17 (1)degrees, Z = 4, R = 0.031, R(w) = 0.037; 2 . 2.5CH(2)Cl(2), triclinic, P ($) over bar 1 a = 20.21(1) Angstrom, b = 22.70(1) Angstrom, c = 13.346(4) Angstrom, alpha = 95.10(2)degrees, beta = 97.62(3)degrees, gamma = 108.67(4)degrees, Z = 4, R = 0.036, R(w) = 0.040.