A synthetic methodology to niobium alkylidenes: Reactivity of a Nb=Nb double bond anchored to a calix[4]arene oxo surface with ketones, aldehydes, imines, and isocyanides

The active compound leading to the synthesis of niobium alkylidenes and niobium alkylidynes from ketones and aldehydes is a [Nb-III = Nb-III] dimer, [{p-Bu-t-calix[4]-(O)(4)}(2)Nb2Na2] (3), supported by the [p-Bu-t-calix[4]-(O)(4)] tetraanion. It was obtained from a stepwise reduction of [{p-Bu-t-calix[4]-O-4}(2)Nb-2(Cl)(2)] (1) through the intermediate formation of a [Nb-Nb] dimer, [{mu-p-Bu-t-calix[4]-(O)(4)}(2)Nb-2] (2). Complex 3 reacted with ketones and aldehydes, RR'CO, via the metathesis of the Nb = Nb bond with the carbonyl functionality, forming an equimolar amount of the niobium alkylidene [{p-Bu-t-calix[4]-(O)(4)}Nb = CRR']Na [R = R' = Ph, 4; R = Ph, R' = Me, 5; R = Cp2Fe, R' = Me, 6; R = Ph, R' = CH2Ph, 7; RR' = (CH2)(4), 8; R = Pr-n, R' = H, 10; R = Ph, R' = H, 11] and of the oxoniobium(V) complex [{p-Bu-t-calix[4]-(O)(4)}Nb = ONa] (9). The easy separation of 9 from the niobium alkylidenes makes the reaction of 3 with ketones a useful synthetic methodology to obtain metal alkylidene derivatives. The niobium alkylidene 11 underwent a reversible protonation and deprotonation reaction, leading to the corresponding benzyl derivative [{p-Bu-t-calix[4]-(O)(4)}Nb-CH2Ph] (12) and to the bridging alkylidyne [{p-Bu-t-calix[4]-(O)(4)}(2)Nb-2(mu-PhC)(2)Na-4] (13) A proton transfer from 12 to 13, assisted by a basic solvent such as pyridine, led to 11. The latter compound has been obtained as a magnesium salt from a direct alkylation of 1 using 2 equiv of [Mg(CH2Ph)(2)]. The significant difference in the reaction rate of 3 with aldehydes or ketones, and the reaction of the niobium alkylidenes with an excess of aldehydes or ketones, leading to the coupling between the alkylidene and the carbonyl functionality, allowed us to carry out the McMurry synthesis of nonsymmetric olefins in a stepwise manner. The four-electron reduction of RNC by complex 3 led to the formation of a dimetalla-imino-alkylidyne, [{p-Bu-t-calix [4]-(O)(4)}Nb(mu-RNC)Nb{mu-p-Bu-t-calix[4]-(O)(4)}Na-2] [R = Bu-t, 14; R = 2,6-Me2C6H3, 15], while the reaction with the imine PhCH = NPh led to the eta(2)-imino complex [{p-Bu-t-calix[4]-(O)(4)}Nb(eta(2)-PhCH-NPh)Na] (16). Both reactions shed some light on the niobium alkylidene formation from ketones and aldehydes. An X-ray structural analysis is given for one of each class of compounds, namely 6, 9, 11, 13, 15, and 16.