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-phi -Br) and 4'- bromo-2,2':6',2"-terpyridine (tpy-Br) with bis(neopentyl glycolato)diboron (B(2)neo(2)) lead to the first reported examples of boronate ester-substituted terpyridine ligands, L-1 and L-2. Ligand L-1, 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 L-3. The heteroleptic complex [Ru(ttpy)L-3](2+) has also been prepared by an analogous cross-coupling reaction of the bromo complex [Ru(ttpy)(tpy-phi -Br)](2+) with B(2)neo(2) (ttpy = 4'- tolyl-2,2': 6',2"-terpyridine). The structurally related complex [Ru(ttpy)L-4](2+) (L-4 = terpyridine-4'-boronic acid) could not be prepared, either directly from L-2 or from [Ru(ttpy)(tpy-Br)](2+), apparently due to competitive hydrodeboration and solvolysis, The complex [Ru(ttpy)L-3](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)L-3](2+) are shown to be largely typical of ruthenium bis-terpyridyl complexes.