The homolytic P-P bond fission in a series of sterically congested tetraaminodiphosphanes (R2N)2P-P(NR2)2 ({4}2-{9}2, two of which were newly synthesized and fully characterized) into diaminophosphanyl radicals (R2N)2P˙ (4-9) was monitored by VT EPR spectroscopy. Determination of the radical concentration from the EPR spectra permitted to calculate free dissociation energies ΔGDiss(295) as well as dissociation enthalpies ΔHDiss and entropies ΔSDiss, respectively. Large positive values of ΔGDiss(295) indicate that the degree of dissociation is in most cases low, and the concentration of persistent radicals--even if they are spectroscopically observable at ambient temperature--remains small. Appreciable dissociation was established only for the sterically highly congested acyclic derivative {9}2. Analysis of the trends in experimental data in connection with DFT studies indicate that radical formation is favoured by large entropy contributions and the energetic effect of structural relaxation (geometrical distortions and conformational changes in acyclic derivatives) in the radicals, and disfavoured by attractive dispersion forces. Comparison of the energetics of formation for CC-saturated N-heterocyclic diphosphanes and the 7π-radical 3c indicates that the effect of energetic stabilization by π-electron delocalization in the latter is visible, but stands back behind those of steric and entropic contributions. Evaluation of spectroscopic and computational data indicates that diaminophosphanyl radicals exhibit, in contrast to aminophosphenium cations, no strong energetic preference for a planar arrangement of the (R2N)2P unit.

The compounds Ru(acac)2(Q) (1), [Ru(bpy)2(Q)](ClO4)2 ([2](ClO4)2), and [Ru(pap)2(Q)]PF6 ([3]PF6), containing Q = N,N'-diphenyl-o-benzoquinonediimine and donating 2,4-pentanedionate ligands (acac(-)), π-accepting 2,2(/)-bipyridine (bpy), or strongly π-accepting 2-phenylazopyridine (pap) were prepared and structurally identified. The electronic structures of the complexes and several accessible oxidized and reduced forms were studied experimentally (electrochemistry, magnetic resonance, ultraviolet-visible-near-infrared (UV-vis-NIR) spectroelectrochemistry) and computationally (DFT/TD-DFT) to reveal significantly variable electron transfer behavior and charge distribution. While the redox system 1(+)-1(-) prefers trivalent ruthenium with corresponding oxidation states Q(0)-Q(2-) of the noninnocent ligand, the series 2(2+)-2(0) and 3(2+)-3(-) retain Ru(II). The bpy and pap co-ligands are not only spectators but can also be reduced prior to a second reduction of Q. The present study with new experimental and computational evidence on the influence of co-ligands on the metal is complementary to a report on the substituent effects in o-quinonediimine ligands [Kalinina et al., Inorg. Chem. 2008, 47, 10110] and to the discussion of the most appropriate oxidation state formulation Ru(II)(Q(0)) or Ru(III)(Q(• -)).

Activating chemical bonds through external triggers and understanding the underlying mechanism are at the heart of developing molecules with catalytic and switchable functions. Thermal, photochemical, and electrochemical bond activation pathways are useful for many chemical reactions. In this Article, a series of Ru(II) complexes containing a bidentate and a tripodal ligand were synthesized. Starting from all-pyridine complex 1(2+), the pyridines were stepwise substituted with "click" triazoles (2(2+)-7(2+)). Whereas the thermo- and photoreactivity of 1(2+) are due to steric repulsion within the equatorial plane of the complex, 3(2+)-6(2+) are reactive because of triazoles in axial positions, and 4(2+) shows unprecedented photoreactivity. Complexes that feature neither steric interactions nor axial triazoles (2(2+) and 7(2+)) do not show any reactivity. Furthermore, a redox-triggered conversion mechanism was discovered in 1(2+), 3(2+), and 4(2+). We show here ligand design principles required to convert a completely inert molecule to a reactive one and vice versa, and provide mechanistic insights into their functioning. The results presented here will likely have consequences for developing a future generation of catalysts, sensors, and molecular switches.

Electrochemical and photochemical bond-activation steps are important for a variety of chemical transformations. We present here four new complexes, [Ru(L(n) )(dmso)(Cl)]PF6 (1-4), where L(n) is a tripodal amine ligand with 4-n pyridylmethyl arms and n-1 triazolylmethyl arms. Structural comparisons show that the triazoles bind closer to the Ru center than the pyridines. For L(2) , two isomers (with respect to the position of the triazole arm, equatorial or axial), trans-2sym and trans-2un , could be separated and compared. The increase in the number of the triazole arms in the ligand has almost no effect on the Ru(II) /Ru(III) oxidation potentials, but it increases the stability of the RuSdmso bond. Hence, the oxidation waves become more reversible from trans-1 to trans-4, and whereas the dmso ligand readily dissociates from trans-1 upon heating or irradiation with UV light, the RuS bond of trans-4 remains perfectly stable under the same conditions. The strength of the RuS bond is not only influenced by the number of triazole arms but also by their position, as evidenced by the difference in redox behavior and reactivity of the two isomers, trans-2sym and trans-2un . A mechanistic picture for the electrochemical, thermal, and photochemical bond activation is discussed with data from NMR spectroscopy, cyclic voltammetry, and spectroelectrochemistry.