Diruthenium-DMBA compounds bearing extended cross-conjugated ligandsd

A series of bis(alkynyl) Ru-2(DMBA) 4 (DMBA = N, N'-dimethylbenzamidinate) compounds bearing extended, acyclic, cross-conjugated frameworks (number of acetylene units >= 3), namely Ru2(DMBA) (4)(L1Si(i)Pr(3))(2) (1a, see Scheme 1 for the definition of L1 and L2) and Ru-2(DMBA)(4)(L2Si(i)Pr(3))(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)(4)NF to the free ethynyl compounds Ru-2(DMBA)(4)(L1H)(2) (1b) and Ru-2(DMBA)(4)(L2H)(2) (2b), respectively. An X-ray structural study of 1a revealed a significant deviation from the idealized D-4h geometry in the coordination sphere of the Ru-2 core, accompanied by one of the shortest Ru-C-alpha 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 Ru-2(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. (C) 2017 Elsevier B.V. All rights reserved.