Syntheses and electronic, electrochemical, and theoretical studies of a series of μ-oxo-triruthenium carboxylates bearing orthometalated phenazines

Abstract

This work reports a series of five-acetate triruthenium clusters [Ru₃O(OAc)₅(L)(py)₂]PF₆, where L = dppn (benzo[i]dipyrido[3,2-a:2′,3′-c]phenazine, 1); dppz (dipyrido[3,2-a:2′,3′-c]phenazine, 2); CH₃-dppz (7-methyldipyrido [3,2-a:2′,3′-c] phenazine, 3); Cl-dppz (7-chlorodipyrido [3,2-a:2′,3′-c] phenazine, 4); and phen (1,10-phenanthroline, 5). The EPR spectra collected at 10 K displayed one isotropic signal without a hyperfine structure and with g values of ∼2.0, which showed that the five-acetate triruthenium clusters are paramagnetic, and that their electronic delocalization resembled the electronic delocalization of the parent hexa-acetate complexes. ¹H NMR analysis showed that the orthometalated phenazines lowered the symmetry of the compounds significantly. Inductive effects from the carbanion and ring current effects outweighed the effect of paramagnetic anisotropy and dominated the spectra. This resulted in a lack of typical correlations with ligand parameters such as pK_a that are observed for the parent hexa-acetate compounds. DFT calculations allowed for a discussion of those parameters in terms of the optimized geometry of compound 2. Natural bond orbital (NBO) results, in turn, aided the rationalization of the orthometalation reaction. The intra-cluster transitions (IC) at ∼690 nm consistently shifted to higher energies, and the redox pair [Ru₃O]^0/+1 also shifted to more positive E_1/2 values. Again, the shifts were small and produced poor correlations with phenazine basicity. Overall, the substitution of one acetate bridge caused poor π-interactions between the delocalized [Ru₃O] unit and the phenazine electron cloud. fsTA experiments, performed for the first time for such systems, showed that an ²IC excited state decayed very fast on the picosecond timescale.