Diruthenium-DMBA compounds bearing extended cross-conjugated ligands

Abstract

A series of bis(alkynyl) Ru2(DMBA)4 (DMBA = N,N′-dimethylbenzamidinate) compounds bearing extended, acyclic, cross-conjugated frameworks (number of acetylene units ≥ 3), namely Ru2(DMBA)4(L1SiiPr3)2 (1a, see Scheme 1 for the definition of L1 and L2) and Ru2(DMBA)4(L2SiiPr3)2 (2a), were prepared under aerobic, weak base conditions in good yields. Compounds 1a and 2a showed moderate stability and underwent protiodesilylation aided with the addition of (n-Bu)4NF to the free ethynyl compounds Ru2(DMBA)4(L1H)2 (1b) and Ru2(DMBA)4(L2H)2 (2b), respectively. An X-ray structural study of 1a revealed a significant deviation from the idealized D4h geometry in the coordination sphere of the Ru2 core, accompanied by one of the shortest Ru-Cα bonds reported thus far. Electronic absorption spectral measurements imply that compounds 1a and 1b possess a smaller HOMO-LUMO gap (ca. 1.38 eV), while the gap of 2a and 2b (ca. 1.40 eV) is closer to the range of typical Ru2(DMBA)4(C2R)2-type compounds (ca. 1.42 eV). Cyclic voltammetry measurements indicate a subtle interplay between structural and electronic properties, dependent only on the placement of the intervening olefin across the conjugation pathway.