Facile immobilization of PNNNP-Pd pincer complexes in MFU-4l-OH and the effects of guest loading on Lewis acid catalytic activity

Abstract

A palladium diphosphine pincer complex H₃(P^NN^NP-PdI) has been encapsulated in the benzotriazolate metal-organic framework MFU-4l-OH ([Zn₅(OH)₄(btdd)₃], btdd²⁻ = bis(1,2,3-triazolo)dibenzodioxin), and the resulting materials were investigated as Lewis acid catalysts for cyclization of citronellal to isopulegol. Rapid catalyst immobilization is facilitated by a Brønsted acid–base reaction between the H₃(P^NN^NP-PdI) benzoic acid substituents and Zn–OH groups at the framework nodes. Catalyst loading can be controlled up to a maximum of 0.5 pincer complexes per formula unit [PdI-x, Zn₅(OH)_4−nx(btdd)₃(H_3−nP^NN^NP-PdI)_xx = 0.06–0.5, n ≈ 2.75]. Oxidative ligand exchange was used to replace I⁻ with weakly coordinating BF₄⁻ anions at the Pd–I sites, generating the activated PdBF₄-x catalysts (x = 0.06, 0.10, 0.18, 0.40). The Lewis acid catalytic activity of the PdBF₄-x series decreases with increasing catalyst density as a result of the appearance of mass transport limitations. Initial catalytic rates show that the activity of PdBF₄-0.06 approaches the intrinsic activity of a homogeneous P^NN^NP-PdBF₄ catalyst analogue. In addition, PdBF₄-0.06 exhibits better catalytic activity than the metallolinker-based MOF Zr-PdBF₄ and was not subject to leaching or catalyst degradation processes observed for the homogeneous analogue.