Probing ruthenium-acetylide bonding interactions: Synthesis, electrochemistry, and spectroscopic studies of acetylide-ruthenium complexes supported by tetradentate macrocyclic amine and diphosphine ligands

The synthesis and spectroscopic properties of trans-[RuL 4(C≡CAr)2] (L4 = two 1,2-bis- (dimethylphosphino)ethane, (dmpe)2; 1,5,9,13-tetramethyl-1,5,9,13- tetraazacyclohexadecane, 16-TMC; 1,-12-dimethyl-3,4:9,10-dibenzo-1,12-diaza-5,8- dioxacyclopentadecane, N2O2) are described. Investigations into the effects of varying the [RuL4] core, acetylide ligands, and acetylide chain length for the [-C≡C(C6H 4C=C)n-1Ph] and [-C≡C(C6H 4)n-1Ph] (n = 1-3) series upon the electronic and electrochemical characteristics of trans-[RuL4(C≡CAr) 2]0/+ are presented. DFT and TD-DFT calculations have been performed on frans-[Ru(L′)4(C≡CAr)2] 0/+ (L′ = PH3 and NH3) to examine the metal-acetylide π-interaction and the nature of the associated electronic transition(s). It was observed that (1) the relationship between the transition energy and 1/n for trans-[Ru(dmpe)2{C≡C(C6H 4C≡C)n-1Ph}2] (n = 1-3) is linear, and (2) the sum of the dπRuII) → π*(C≡CAr) MLCT energy for trans-[Ru(16-TMC or N2O2)(C≡CAr) 2] and the π-(C≡CAr) → dπ(RuIII) LMCT energy for trans-[Ru(16-TMC or N2O2)(C≡CAr) 2]+ corresponds to the intraligand ππ* absorption energy for trans-[Ru(16-TMC or N2O2) (C≡CAr)2]. The crystal structure of trans-[Ru(dmpe) 2-{C≡C(C6H4C≡C)2Ph} 2] shows that the two edges of the molecule are separated by 41.7 Å. The electrochemical and spectroscopic properties of these complexes can be systematically tuned by modifying L4 and Ar to give E 1/2 values for oxidation of trans-[RuL4(C≡CAr) 2] that span over 870 mV and λmax values of trans-[RuL4(C≡CAr)2] that range from 19 230 to 31 750 cm-1. The overall experimental findings suggest that the π-back-bonding interaction in trans-[RuL4(C≡CAr) 2] is weak and the [RuL4] moiety in these molecules may be considered to be playing a "dopant" role in a linear rigid π-conjugated rod.