Tetradentate selenium ligand as a building block for homodinuclear complexes of Pd(ii) and Ru(ii) having seven membered rings or bis-pincer coordination mode: high catalytic activity of Pd-complexes for Heck reaction

Abstract

1,2,4,5-Tetrakis(phenyselenomethyl)benzene (L) has been synthesized by reaction of in situ generated PhSe⁻ with 1,2,4,5-tetrakis(bromomethyl)benzene in N₂ atmosphere. Its first bimetallic complexes and a bis-pincer complex having compositions [(η³-C₃H₅)₂Pd₂(L)][ClO₄]₂ (1) [Pd₂(C₅H₅N)₂(L)][BF₄]₂ (2) and [(η⁶-C₆H₆)₂Ru₂(L)Cl₂][PF₆]₂ (3) have been synthesized by reacting L with [Pd(η³-C₃H₅)Cl]₂, [Pd(CH₃CN)₄][BF₄]₂ and [(η⁶-C₆H₆)₂RuCl₂]₂ respectively. The structures of ligand L and its all three complexes have been determined by X-ray crystallography. In 1 and 3, ligand L forms with two organometallic species seven membered chelate rings whereas in 2 it ligates in a bis-pincer coordination mode. The geometry around Pd in 1 or 2 is close to square planar whereas in 3, Ru has pseudo-octahedral half sandwich “Piano–Stool” geometry. The Pd–Se bond distances are in the ranges 2.4004(9)–2.4627(14) Å and follow the order 1 > 2, whereas Ru–Se bond lengths are between 2.4945(16) and 2.5157(17) Å. The 1 and 2 have been found efficient catalysts for Heck reaction of aryl halides with styrene and methyl acrylate. The 2 is superior to 1. The TON and TOF values (per Pd) are up to ∼ 47500 and ∼2639 h⁻¹ respectively.