Neutral and Cationic Palladium(II) Complexes of a Diazapyridinophane. Structure, Fluxionality, and Reactivity toward Ethylene

The palladium complexes (t-Bu-N4)PdCl2 (1), (t-Bu-N4)PdMeCl (2), (t-Bu-N4)PdMe(NCMe)+BAr4 - (3), and (t-Bu-N4)PdMe(C2H4)+BAr4 - (4) [t-Bu-N4 = N,N‘-ditertiobutyl-2,11-diaza[3.3](2,6)pyridinophane, Ar = 3,5-(CF3)2-C6H3] have been synthesized and characterized by VT 1H NMR and X-ray (for 1 and 3) analysis. The macrocyclic ligand is only bi- or tridentate, both coordination modes coexisting in solution for 1 and 3. Cationic methyl-ethylene complex 4 decomposes readily through insertion of ethylene into the Pd−Me bond, and ethylene polymerization with use of 1 (in the presence of MAO) or 3 as catalysts is therefore precluded. Complexes 1, 3, and 4 are fluxional, the t-Bu-N4 ligand undergoing an η2−η3 oscillatory process (ΔG ⧧ = 14.1−14.9 kcal mol-1) as well as hindered rotation of the t-Bu substituent of the coordinated amine function around the corresponding N−C bond (ΔG ⧧ = 8.8−10.1 kcal mol-1).