Selective para-halogenation and dimerization of N,C,N′- arylruthenium(II) and -(III) 2,2′:6′,2″-terpyridine cations

The N,C,N′-bonded arylruthenium 2,2′:6′,2′′-terpyridine (tpy) complex salts [Ru(NCN)(tpy)](Cl) ([1a](Cl), NCN ) 2,6-bis[(dimethylamino)methyl]phenyl) and [Ru(N∧C∧N)(tpy)](PF6), ([2a](PF6), N∧C∧N ) 2,6-bis(2-pyridyl)phenyl) can be halogenated under very mild conditions by oxidation with copper(II) halogen salts. Halogenation occurs exclusively para to the site of metalation and yields the cations [Ru(4-RNCN)( tpy)]+ (R ) Cl, [1b]+ and R ) Br [1c]+) and [Ru(4-R-N∧C∧N)(tpy)]+ (R ) Cl, [2b]+ and R ) Br [2c]+). In the presence of an excess of oxidant relative to [1a]+, the halogenation reaction follows first order kinetics in the oxidized ruthenium complex. However, by using a small excess of copper(II) compared to [1a]+, dimerization of the complex cation to [{Ru(tpy)}2(μ-NCN-NCN)]4+ ([3]4+) is observed, which obeys second order kinetics. Both halogenation (C-X coupling) and dimerization (C-C coupling) are a result of the unique properties of the ruthenium(III) complexes compared to their parent ruthenium(II) species. According to the nature of the highest occupied spin orbital (HOSO) in DFT calculations the unpaired electron in [1a]2+ and [2a]2+ is partially localized on the para position. The involvement of the cyclometalated ligand in the HOSO is supported by redox data and electronic absorption spectroscopy. The ruthenium(III) species can best be considered a persistent organometallic radical.