Palladium-catalysed cross-coupling reactions of ruthenium bis-terpyridyl complexes: strategies for the incorporation and exploitation of boronic acid functionality

Abstract

The palladium catalysed Miyaura cross-coupling reactions of 4′-(4-bromophenyl)-2,2′:6′,2″-terpyridine (tpy-ϕ-Br) and 4′-bromo-2,2′:6′,2″-terpyridine (tpy-Br) with bis(neopentyl glycolato)diboron (B₂neo₂) lead to the first reported examples of boronate ester-substituted terpyridine ligands, L¹ and L². Ligand L¹, which incorporates a benzene ring between the terpyridine group and the boron, reacts with transition metals such as iron and ruthenium to generate complexes containing the analogous boronic acid-substituted terpyridine L³. The heteroleptic complex [Ru(ttpy)L³]²⁺ has also been prepared by an analogous cross-coupling reaction of the bromo complex [Ru(ttpy)(tpy-ϕ-Br)]²⁺ with B₂neo₂ (ttpy = 4′-tolyl-2,2′:6′,2″-terpyridine). The structurally related complex [Ru(ttpy)L⁴]²⁺ (L⁴  = terpyridine-4′-boronic acid) could not be prepared, either directly from L² or from [Ru(ttpy)(tpy-Br)]²⁺, apparently due to competitive hydrodeboration and solvolysis. The complex [Ru(ttpy)L³]²⁺ reacts with aryl halides under standard palladium-catalysed Suzuki–Miyaura cross-coupling conditions to generate more elaborate 4′-aryl-substituted terpyridyl complexes. Cross-coupling has also been achieved by reaction of [Ru(ttpy)(tpy-Br)]²⁺ with an arylboronic acid. The photophysical properties of [Ru(ttpy)L³]²⁺ are shown to be largely typical of ruthenium bis-terpyridyl complexes.