Synthesis and Characterization of Imine-Palladacycles Containing Imidate “Pseudohalide” Ligands: Efficient Suzuki–Miyaura Cross-Coupling Precatalysts and Their Activation To Give Pd0Ln Species (L = Phosphine)

Dinuclear palladacyclic complexes [{Pd(C∧N)(μ-NCO)}2] (C∧N = N-phenylbenzaldimine, Phbz) containing asymmetric imidato units (−NCO– = succinimidate (succ; 1), phthalimidate (phthal; 2), maleimidate (mal; 3), 2,3-dibromomaleimidate (2,3-diBrmal; 4), glutarimidate (glut; 5)) have been readily prepared by reaction between the di-μ-acetate precursor and cyclic imide ligands in a 1:2 molar ratio. Base treatment of the less acidic ligands 2-oxazolidone and δ-valerolactame with KOH/MeOH was required to give analogous −NCO– bridged complexes (6 and 7). Reactions of the dinuclear complexes with tertiary phosphines provide novel mononuclear N-bonded imidate derivatives of the general formula [Pd(Phbz)(imidate)(PR3)] (R = Ph (a), 4-F-C6H4 (b), 4-MeO-C6H4 (c), CH2CH2CN (d)). The application of these novel palladacyclic complexes as precatalysts for the Suzuki–Miyaura cross-coupling reactions of both aryl and benzyl bromides with phenylboronic acid has been examined. The acetate adducts [Pd(Phbz)(CH3COO)(PR3)] (8a,c) were prepared to assess the role of imidate ligands in catalyst performance. The mononuclear imidate derivatives possess greater activity than the parent dinuclear complexes, exhibiting comparable performance in the cross-coupling of benzyl bromide with arylboronic acids to the best examples reported in the literature. The mononuclear imidate derivatives give a common Pd0Ln intermediate, as inferred by the release of the organic fragment (first reductive elimination product). Catalyst activation occurs by reaction of phenylboronic acid with the palladacycle in the absence of exogenous base (as shown by GC-MS and ESI-MS), with implications for the reliable comparison of catalyst performance across a series of related precatalysts (e.g., how catalyst/reagents are mixed and what is their order of addition). The single-crystal X-ray structures of compounds 4, 7, 1d, 3c, and 8a have been determined.