Macrocyclic complexes of [Ru(N-N)2]2+ units [N-N=1,10 phenanthroline or 4-(p-anisyl)-1,10-phenanthroline]:: Synthesis and photochemical expulsion studies

Two ruthenium complexes [Ru(phen)(2)(m-36)](2+) and [Ru(aphen)2(m-36) 121, indicated as 1 and 2 respectively, which contain a 36-membered macrocycle incorporating a sterically hindered bipyridine (m-36), and two 1,10-phenanthroline (phen) or 4-(P-anisyl)-1,10-phenanthroline (aphen) ligands, have been synthesised. Under light irradiation, as seen from the metal-to-ligand-charge-transfer (MLCT) bands of the visible spectral region, the [Ru(phen)12 2+ core of I and 2 is selectively expelled from the macrocycle to give the corresponding bis-acetonitrile derivative. This photochemical process was found to be notably less efficient than that observed in a simpler related complex [Ru(phen)(2)dmbp](2+) (dmbp = 6,6'dimethyl-2,2'-bipyridine), that lacks the macrocyclic motif. In the case of I thermal back reaction takes place quantitatively affording a completely reversible system. The isolation, by chromatographic techniques, of the axial-axial cis-[(a,a)Ru(dphen)(2)Cl-2] isomer, where the two aphen ligands bear two p-anisyl groups disposed trans to one another enabled the preparation of 2. This type of complex undergoes a more complicated photochemical process, in fact, following the expected photolabilisation of the [Ru(aphen)(2)](2+) unit, photoisomerisation of the primary photo-product occurs and gives a statistical mixture of the three geometrical isomers: cis-[(a, a)Ru(aphen)(2)(CH3CN)(2)] (PF6)(2), cis-[ (a, e)Ru(aphen)(2)(CH3CN)(2)](PF6)(2) and cis-[ (e,e)Ru(aphen)(2)(CH3CN)(2)](PF6)(2) This side-reaction precludes its use as rotaxane precursor suggesting that, for this purpose, a bis-phenanthroline ligand stabilising the ruthenium core is needed. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003).