Role of the bridging arylethynyl ligand in bi- and trinuclear ruthenium and iron complexes

A series of binuclear homometallic complexes [M-(mu-L)-M] containing ruthenium (M = [-Ru(dppe)(2)Cl], dppe = 1,2-bis(diphenylphosphino)ethane) and iron (M = [-C5H4-Fe-C5H5] = ferrocenyl) and trinuclear heterobimetallic complexes [M-(mu-L)-M'-(mu-L)-M] (M' = [Pd(p(n)Bu(3))(2)], [Ru(dppe)(2)]) in which the metals are bridged by arylethynyl ligands (mu-L =-C equivalent to C{(p-C6H4)C=C}(n)-) of various lengths (n = 1-3) was prepared and investigated, focusing on their electrochemical behavior. Depending on the length and nature of the bridge the coupling of the fully reversible electrochemical oxidations varies from strong to zero. The comproportionation constant K-C and hence the stability of the intermediate mixed-valent species is discussed in view of the nature of the bridge and compared to related systems with other types of unsaturated carbon ligands. The character of the oxidized states was examined using spectroelectrochemical techniques (UV/vis/near-IR, IR, or EPR) with special focus on the intervalence charge-transfer band (IVCT) of the mixed-valent monocations and the EPR behavior. Since the IVCT bands could not be assigned unequivocally and the EPR reveals marked alkynyl ligand contribution to the oxidized state for the ruthenium complexes, an alternative assignment (intraligand transitions) for the long-wavelength bands is discussed.