Bis-Chelated Palladium(II) Complexes with Nitrogen-Donor Chelating Ligands Are Efficient Catalyst Precursors for the CO/Styrene Copolymerization Reaction

Abstract

A series of dicationic bis-chelated palladium(II) complexes [Pd(N-N)2][X]2 (N-N = 2,2‘-bipyridine (bipy), 1,10-phenanthroline (phen), and their substituted derivatives; X = PF6 -, BF4 -, OTf-, OTs-) has been synthesized and completely characterized both in the solid state and in solution. The synthetic procedure involves a simple one-pot reaction between Pd(MeCOO)2 and [(N-N)H][X]. These compounds are very active precatalysts for the CO/styrene copolymerization yielding perfectly alternating polyketones. The crystal structures of some complexes of the series provide evidence that a distorsion from the ideal square planar geometry toward a twist conformation occurs. In DMSO solution, one of the two nitrogen-donor ligands is involved in a dissociative equilibrium yielding a monochelated complex with two cis coordination sites available for the copolymerization catalytic process. The catalytically active species is very stable in 2,2,2-trifluoroethanol, where its activity was found unaltered for at least 48 h of reaction without apparent decomposition to palladium metal. The addition of 1,4-benzoquinone (BQ) to the catalytic system has a strong influence on the yield and, above all, on the molecular weight of polyketones. The zerovalent palladium complexes [Pd(N-N)(BQ)], which might be formed during the copolymerization process, have been synthesized and characterized. The crystal structure of [Pd(bipy)(BQ)] shows that benzoquinone acts as a mono-olefinic ligand to Pd. In the presence of protons, the Pd(0) complexes are readily oxidized to Pd(II) with the reduction of benzoquinone to hydroquinone. When [(N-N)H][X] is used as the source of protons, the resulting Pd(II) species is the precatalyst and can immediately re-enter the catalytic cycle.

Synthesis, stability and reactivity of palladium(0) olefin complexes bearing labile or hemi-labile ancillary ligands and electron-poor olefins

“…X-ray diffractometric studies 10.1. Complexes bearing mono-bi-and ter-dentate nitrogen ligands A significant number of palladium(0) olefin compounds bearing the title ligands was synthesized by several authors [30,31,35,37,75,[78][79][80][81][82][83][84][85][86]. All the structures display the typical trigonal arrangement around the palladium centre with the Pd-N bond lengths being scarcely influenced by the olefin nature, spanning within 2.10 and 2.18 Å.…”

Copolymerization of styrene with CO catalysed by palladium‐substituted 1,10‐phenanthroline bis(methoxycarbonyl) complexes [Pd(L—L)(COOMe) 2 ]

“…Actually, preformed palladium catalytic precursors, such as Pd(L1L) 2 (X) 2 (L1L = nitrogen or phosphorus bischelating ligands and X = PF 6 -, CF 3 COO -), allow one to reach good yields in ethylenecarbon monoxide copolymerization 4,5) . Many efforts have been also devoted to copolymerize CO with a-olefins, such as propene [6][7][8][9][10] and styrene [11][12][13][14][15][16][17][18][19][20][21] . Since in the latter case, the initation step is hypothesized to be a 2,1-insertion of styrene in a palladium carbomethoxy bond 22) , followed by CO insertion and successive 2,1-insertion of styrene in Pd-acyl bonds formed 23) , a new class of palladium bis(methoxycarbonyl) complexes, Pd(L1L)(COOMe) 2 , bearing substituted phenanthrolines as ligands, have been prepared 24) to prove their involvement in styrene-CO copolymerization.…”

Investigation of the reactivity of palladium(0) complexes with nitroso compounds: relevance to the palladiumphenanthroline-catalysed carbonylation reactions of nitroarenes

“…First we reacted Pd(phen)(pbq) (pbq= p-benzoquinone) [6], a source of 'Pd(0)(phen)', with different amounts of nitrosobenzene and in different solvents (THF, diglyme, methanol). Invariably an insoluble green paramagnetic product 1, was obtained.…”