Organometallic reactivity on a calix[4]arene oxo surface. The stepwise migratory insertion of carbon monoxide and isocyanides into zirconium-carbon bonds anchored to a calix[4]arene moiety

The present report deals with the multistep migratory insertion of carbon monoxide and isocyanides into the metal-carbon bonds of the ZrR2 fragment anchored to a tetroxo matrix, defined by the dimethoxycalix[4]arene dianion [p-Bu-t-calix[4]-(OMe)(2)(O)(2)](2-). The relevant differences between the two fragments [p-Bu-t-calix[4]-(OMe)(2)(O)(2)Zr](2+) and [Cp2Zr](2+) are experimentally proven and theoretically interpreted. Unlike those for [Cp2Zr](2+) derivatives, the reaction of CO with [p-Bu-t-calix[4]-(OMe)(2)(O)(2)ZrR2] [R = Me, 1; CH2Ph, 2; p-MeC6H4, 3] proceeds via a two-step migration directly to the corresponding eta(2)-metal-bonded-ketones [p-Bu-t-calix[4]-(OMe)(2)(O)(2)Zr{eta(2)-C(R-2)O}] (R = Me, 4; CH2Ph, 5; p-MeC6H4, 6). The Zr-C functionality in 4-6 maintains its inserting capability in the reaction with ketones, i.e., Ph2CO, leading to the diolate derivative [p-Bu-t-calix[4]-(OMe)(2)(O)(2)Zr{OC(R-2)C(Ph-2)O}] (8, R = p-MeC6H4), or with (BuNC)-N-t, forming [p-Bu-t-calix[4]-(OMe)(2)(O)(2)Zr{OC(Me-2)C(CNBut)N(Bu-t)}] (7). The reaction of 1-3 with (BuNC)-N-t proceeds through two different pathways, depending on the temperature and the R substituent at the metal. The first one leads to the eta(2)-metal-bonded imine [p-Bu-t-calix[4]-(OMe)(2)(O)(2)Zr{eta(2)-N(Bu-t)C(R-2)}] [R = Me, 12; CH2Ph, 13], which inserts two additional molecules of (BuNC)-N-t to give a metalladiazacyclopentane, [p-Bu-t-calix[4]-(OMe)(2)(O)(2)Zr{N(Bu-t)C(Me-2)C(CNBut)N(Bu-t)}] (14). The second pathway led, at low temperature, to the bis-eta(2)-iminoacyls [p-Bu-t-calix[4]-(OMe)(2)(O)(2)Zr{eta(2)-N(Bu-t)=C(R)}(2)] [R = Me, 15; p-MeC6H4, 16], which at 60 degrees C couple to the corresponding enediamido ligand in [p-Bu-t-calix[4]-(OMe)(2)(O)(2)Zr-{N(Bu-t)C(R)=C(R)N(Bu-t)}] [R = Me, 17; p-MeC6H4, 18]. The coupling of 15 and 16 showed first order kinetics in both cases and allowed the calculation of the corresponding activation parameters. The most significant differences between [p-Bu-t-calix[4]-(OMe)(2)(O)(2)Zr](2+) and [Cp2Zr](2+) have been interpreted on the basis of the frontier orbital sets of the two fragments.