Tantalum-Carbon Functionalities Bonded to a Calix[4]arene-Oxo Matrix: The Chemistry of Mono-, Dialkyl, and Butadiene Derivatives of Tantalum(V)

The synthesis of the starting compounds [Ta-2{mu-p-tBu-calix[4]-(O)(4)}(2)Cl-2] (3) and [Ta{p-tBu-calix[4]-(OMe)(O)(3)}Cl-2] (4) for our investigations of calix[4] arene - tantalum organometallic chemistry has been achieved by the reaction of TaCl5 with p-tBu-calix[4](OH)(2) (1) and p-tBu-calix[4]-(OMe)(2)(OH)(2) (2), whereby the latter compound underwent an acid-assisted demethylation. The alkylation of 3 by standard procedures led to the dimeric [Ta-2{mu-p-tBu-calix[4]-(O)(4)}(2)R-2] derivatives (R = alkyl or aryl), which are in equilibrium with the corresponding monomeric forms in solution. An alternative route to these complexes is the pyridine-assisted demethylation of [Ta{p-tBu-calix[4]-(OMe)(O)(3)}R-2], which is obtained from the alkylation of 4. The latter complexes, unlike [Ta-2{mu-p-tBu-calix[4]-(O)(4)}(2)R-2], undergo migratory insertion reactions with CO and tBuNC to give the corresponding eta(2)-ketones and eta(2)-imines. Both classes of compounds undergo a pyridine-induced demethylation, The bis(allyl) derivative [Ta-{p-tBu-calix[4]-(OMe)(O)(2)}(eta(3)-allyl)(2)] undergoes a double migratory insertion reaction of the allyl groups to give the corresponding eta(2)-ketone and eta(2)-imine, respectively, in the reactions with CO and tBuNC. Although butadiene is pi(2),eta(4)-bonded to the metal, the tantalum-butadiene fragment in [Ta{p-tBu-calix[4]-(OMe)(O)(3)}(eta(4)-C4H6)] (25) behaves as if it were eta(4),sigma(2)-pi bonded in the migratory insertion reaction with tBuNC and Me2CO. In the reaction with PhN3, complex 25 behaves as a source of d(2)-tantalum(III) and leads to the nitrene derivative 28.