2,2'-Azobis(pyridine) (abpy) as a multiply reducible tetradentate ligand. EPR evidence for the configurational dependence of intramolecular electron transfer in the stereoisomeric tris-chelate ruthenium complexes [Ru(abpy)n(bpy)3-n]m (n = 2, 3; m = 2+ to 3-)

Abstract

The mononuclear title complexes can exist as stereoisomers due to the unsymmetrically chelating a-azimino functionality of the abpy ligand. Both fac and mer isomers of [Ru(abpy)3]2+ and two out of the three possible diastereoisomers of [Ru(abpy)2(bpy)]2+ were isolated via HPLC and identified by virtue of their NMR spectra. Electrochemical potentials for several one-electron reduction steps and UV/vis spectroelectrochemical features vary to a small but detectable degree for the respective pairs of isomers. The absence of a room-temperature EPR signal for one of the two isolated isomers of [RuII(abpy)(abpy)(bpy)]*+ demonstrates the structural requirements for intramolecular electron transfer. A 99'101Ru EPR hyperfine coupling of about 0.8 mT was established for the previously reported radical ion [Ru(abpy)(bpy)2],+. The free -azimino chelate sites in mononuclear complexes of abpy render these systems suitable for the construction of polynuclear compounds. The exceptional «--acceptor capability of abpy is illustrated by the reversible acceptance of at least eight electrons by the dinuclear complex {[Ru(bpy)2] 2(M,7?4-abpy)}".

Synthesis, crystal structure and spectrothermal characterization of zinc(II) salicylato complex with 2,2′-azobispyridine, [Zn(Hsal)2(H2O)(abpy)]·H2O

“…Both p / p* transition and charge-transfer absorption bands of 2 are more broad and intense than 1. The similar MLCT charge-transfer bands were observed in the various abpy complexes [6,22,23].…”

EPR Insensitivity of the Metal‐Nitrosyl Spin‐Bearing Moiety in Complexes [LnRuII‐NO·]k

“…The maximum calculated isotropic hyperfine splitting of 0.94 mT according to Table 2 is only slightly larger than the A iso ( 101 Ru) parameters measured for radical complexes containing cyanoruthenium [37,38] (Յ0.52 mT) or other ruthenium() complex fragments (Յ0.86 mT). [39] Species with metal-centered spin containing ruthenium() [40] are expected to have isotropic metal coupling constants of several milliteslas.…”

Characterization by NMR Spectroscopy, X‐ray Analysis and Cytotoxic Activity of the Ruthenium(II) Compounds [RuL3](PF6)2(L = 2‐Phenylazopyridine or o‐Tolylazopyridine) and [RuL'2L''](PF6)2(L', L'' = 2‐Phenylazopyridine, 2,2'‐Bipyridine)

“…In general, for tris(ligand) complexes with 2-phenylazopyridine-or 2-phenylazoimidazole-like ligands only 1 H NMR chemical shift data are mentioned, [19][20][21]28] without performing 2D NMR experiments and no broadening is mentioned. In our opinion the 1 H NMR spectroscopic data of these very complicated spectra [19][20][21]28] are questionable. Therefore, an extensive description of the assignment of the tris(ligand) complexes mentioned in this paper using several 2D NMR techniques seems to be useful.…”

“…This is in agreement with the fact that the complexes [Ru(azpy) 2+ [(abpy = 2,2-azobis(pyridine)] 1 H NMR spectra data are reported, but no broadening has been mentioned. [28] One mechanism that may be responsible for signal broadening in (azpy)ruthenium complexes is sterically hindered rotations of one or more phenyl rings. [30] In theory this implies that upon cooling the sample during the NMR experiment, the phenyl ring rotation becomes slower on the NMR time scale and at the lowest temperatures it is possible to stop the phenyl ring rotation (on the NMR time scale).…”