¹H NMR, Electron Paramagnetic Resonance, and Density Functional Theory Study of Dinuclear Pentaammineruthenium Dicyanamidobenzene Complexes

Abstract

Paramagnetic (1)H NMR and electron paramagnetic resonance (EPR) spectroscopies and density functional theory (DFT) spin density calculations were selectively performed on the [{(NH(3))(5)Ru}(2)(μ-L)](3+, 4+, 5+) complexes, where L is 2,3,5,6-tetrachloro-, 2,5-dichloro-, 2,5-dimethyl-, and unsubstituted 1,4-dicyanamidobenzene dianion, to characterize the electronic structure of these complexes. EPR spectra of the [{(NH(3))(5)Ru}(2)(μ-L)](3+) complexes in N,N'-dimethylformamide at 4 K showed a ruthenium axial signal, and thus the complexes are [Ru(II),L(2-), Ru(III)] mixed-valence systems. DFT spin density calculations of [{(NH(3))(5)Ru}(2)(μ-L)](3+) where L = 1,4-dicyanamidobenzene dianion gave mostly bridging-ligand centered spin distribution for both vacuum and implicit solvent calculations, in poor agreement with EPR, but more realistic results were obtained when explicit electrostatic interactions between solute and solvent were included in modeling. For the [{(NH(3))(5)Ru}(2)(μ-L)](4+) complexes, EPR spectroscopy showed no signal down to 4 K. Nevertheless, solvent-dependent (1)H NMR data and analysis support a [Ru(III),L(2-), Ru(III)] state. Hyperfine coupling constants (A(c)/h) of trans- and cis-ammine and phenyl hydrogens were determined to be 17.2, 3.8, and -1.5 MHz respectively. EPR studies of the [{(NH(3))(5)Ru}(2)(μ-L)](5+) complexes showed a metal-radical axial signal and based on previously published (1)H NMR data, a [Ru(IV),L(2-), Ru(III)] state is favored over a [Ru(III),L(-), Ru(III)] state.

Tuning the Electronic Properties in Ruthenium–Quinone Complexes through Metal Coordination and Substitution at the Bridge

“…Our results here show the absolute necessity of incorporating solvent effects into DFT calculations when dealing with systems that are capable of undergoing weak intermolecular interactions or those that may possess many energetically close electronic states. Such a strong solvent dependence of the DFT‐calculated spin density has also been reported recently for ruthenium–amine complexes containing cyanamide bridges 15. The irreversible nature of the first oxidation step of 1 + precluded the characterization of 1 2+ through EPR spectroscopy.…”

Speciation in diethanolamine-moderated TiO2 precursor sols and their use in film formation

“…following the species present via their 1 H and 13 C NMR chemical shifts [30]. Jung et al [31,32] followed the preparation of modified Herein we describe the effect of the DEA moderator on the anatase/rutile phase behaviour of films and powders produced from TiO2 precursor sols, and examine the speciation in those sols in an attempt to link speciation to phase behaviour.…”

Mixed Valency in Dinuclear Ruthenium Complexes Bridged by Tetrazine- and Azo-Cyanamide Ligands

“…According to these calculation spin density resides mostly in p atomic orbitals of the bridging ligand with slight d orbital contributions from the ruthenium ions. Based on this calculation alone, a radical bridging ligand state is predicted however as has been stated before solvent interactions can shift spin density onto the metal 29 and because of this interpretation of Figure 60 should be qualified. Gas phase DFT calculations on both the mixed-valence dinuclear complexes (11) and (12) showed spin-density distribution to be localized mostly on the bridging ligand with only a small contribution from the ruthenium centres in disagreement with IR spectroelectrochemical studies.…”

“…The HOMO of tdpc 2-(5) at 0.7 eV is 0.7 eV more stable than that of adpc 2-(8) and this extra stability is expected to decrease metal-metal coupling via hole-transfer superexchange when tdpc 2bridges two metal ions. 29 The DFT calculation of the spin density distribution in the mixed-valence complex [{Ru(tpy)(bpy)}2(µ-tdpc)] 3+ (6) is shown in Figure 28 in which spin density resides mostly in p atomic orbitals of the bridging ligand with minor d orbital contributions from the ruthenium ions. Based on this calculation alone, a radical bridging ligand state is predicted however solvent interactions can shift spin density onto the metal ions as has been demonstrated for [{Ru(NH3)6)}2(µ-dicyd)] 3+ where dicyd 2is 1,4dicyanamide benzene dianion 29…”

“…For instance NMR and EPR studies on [Ru(NH3)5(pcyd)] 2+ and [{Ru(NH3)5}2(-dicyd)] 3+ complexes showed Ru(III) metal-centered spin density while the DFT calculation suggested spin density mostly on cyanamide ligands. Introducing solvents properties perturbed the spin density slightly(Figure 8) while simulating the polar solvent-solute interactions by using an explicit electrostatic model showed the most effect on the calculated spin density 29.…”