On the energetics of P-P bond dissociation of sterically strained tetraamino-diphosphanes

The homolytic P-P bond fission in a series of sterically congested tetraaminodiphosphanes (R2N)(2)P-P-(NR2)(2) ({4}(2)-{9}(2), two of which were newly synthesized and fully characterized) into diaminophosphanyl radicals (R2N)(2)P-center dot (4-9) was monitored by VT EPR spectroscopy. Determination of the radical concentration from the EPR spectra permitted to calculate free dissociation energies Delta G(Diss)(295) as well as dissociation enthalpies Delta H-Diss and entropies Delta S-Diss, respectively. Large positive values of Delta G(Diss)(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 7 pi-radical 3c indicates that the effect of energetic stabilization by pi-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)(2)P unit.Peer reviewe

Uncommon cis Configuration of a Metal-Metal Bridging Noninnocent Nindigo Ligand

In contrast to several reported coordination compounds of trans-Nindigo ligands [Nindigo = indigo-bis(N-arylimine) = LH2] with one or two six-membered chelate rings involving one indole N and one extracyclic N for metal binding, the new diruthenium complex ion [(acac)(2)Ru(mu,eta(2):eta(2)-L)Ru(bpy)(2)](2+) = 2(2+) exhibits edge-sharing five- and seven-membered chelate rings in the first documented case of asymmetric bridging by a Nindigo ligand in the cis configuration [L2- = indigo-bis(N-phenylimine)dianion]. The dication in compound [2](ClO4)(2) displays one Ru(alpha-diimine)(3) site and one ruthenium center with three negatively charged chelate ligands. Compound [2](ClO4)(2) is obtained from the [Ru(bpy)(2)](2+)-containing cis precursor [(LH)Ru(bpy)(2)]ClO4 = [1]ClO4, which exhibits intramolecular H-bonding in the cation. Four accessible oxidation states each were characterized for the 1(n) and 2(n) redox series with respect to metal- or ligand-centered electron transfer, based on X-ray structures, electron paramagnetic resonance, and ultraviolet-visible-near-infrared spectroelectrochemistry in conjunction with density functional theory calculation results. The structural asymmetry in the Ru-III/Ru-II system 2(2+) is reflected by the electronic asymmetry (class I mixed-valence situation), leaving the noninnocent Nindigo bridge as the main redox-active site

Sensitivity of a Strained C-C Single Bond to Charge Transfer: Redox Activity in Mononuclear and Dinuclear Ruthenium Complexes of Bis(arylimino)acenaphthene (BIAN) Ligands

The new compounds [Ru(acac)(2)(BIAN)], BIAN = bis(arylimino)acenaphthene (aryl = Ph (1a), 4-MeC6H4 (2a), 4-OMeC6H4 (3a), 4-ClC6H4 (4a), 4-NO2C6H4 (5a)), were synthesized and structurally, electrochemically, spectroscopically, and computationally characterized. The alpha-dine sections of the compounds exhibit intrachelate ring bond lengths 1.304 angstrom < d(cN) < 1.334 and 1.425 angstrom < d(cc) < 1.449 angstrom, which indicate considerable metal-to-ligand charge transfer in the ground state, approaching a Ru-III(BIAN(center dot-)) oxidation state formulation. The particular structural sensitivity of the strained pen-connecting C-C bond in the BIAN ligands toward metal-to-ligand charge transfer is discussed. Oxidation of [Ru(acac)(2)(BIAN)] produces electron paramagnetic resonance (EPR) and UV-vis-NIR (NIR = near infrared) spectroelectrochemically detectable Ru-III species, while the reduction yields predominantly BIAN-based spin, in agreement with density functional theory (DFT) spin-density calculations. Variation of the substituents from CH3 to NO2 has little effect on the spin distribution but affects the absorption spectra. The dinuclear compounds {(mu-tppz)[Ru(Cl)(BIAN)](2)}(ClO4)(2), tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine; aryl (BIAN) = Ph ([1b](ClO4)(2)), 4-MeC6H4 ([2b](ClO4)(2)), 4-OMeC6H4 ([3b](ClO4)(2)), 4-ClC6H4 ([4b](ClO4)(2)), were also obtained and investigated. The structure determination of [2b](ClO4)(2) and [3b](ClO4)(2) reveals trans configuration of the chloride ligands and unreduced BIAN ligands. The DFT and spectroelectrochemical results (UV-vis-NIR, EPR) indicate oxidation to a weakly coupled (RuRuII)-Ru-III mixed-valent species but reduction to a tppz-centered radical state. The effect of the pi electron-accepting BIAN ancillary ligands is to diminish the metal-metal interaction due to competition with the acceptor bridge tppz

Sensitivity of the Valence Structure in Diruthenium Complexes As a Function of Terminal and Bridging Ligands

The compounds [(acac)(2)Ru-III(mu-H2L2-)Ru-III(acac)(2)] (rac, 1, and meso, 1') and [(bpy)(2)Ru-III(mu-H2L center dot-)Ru-III(bpy)(2)](C1O(4))(3) (meso, [2](ClO4)(3)) have been structurally, magnetically, spectroelectrochemically, and computationally characterized (acac(-) = acetylacetonate, bpy = 2,2'-bipyridine, and H4L = 1,4-diamino-9,10-anthraquinone). The N,O;N',O'-coordinated mu-H2Ln- forms two beta-ketiminato-type chelate rings, and 1 or 1' are connected via NH center dot center dot center dot O hydrogen bridges'in the crystals. 1 exhibits a complex magnetic behavior, while [2](ClO4)(3) is a radical species with mixed ligand/metal-based spin. The combination of redox noninnocent bridge (H2L0 -> -> -> -> H2L4-) and {(acac)(2)Ru-II} -> ->{(acac)(2)Ru-IV} or {(bpy)(2)Ru-II} -> {(bpy)(2)Ru-III} in 1/1' or 2 generates alternatives regarding the oxidation state formulations for the accessible redox states (1(n) and 2(n)), which have been assessed by UV-vis-NIR, EPR, and DFT/TD-DFT calculations. The experimental and theoretical studies suggest variable mixing of the frontier orbitals of the metals and the bridge, leading to the following most appropriate oxidation state combinations: [(acac)(2)Ru-III(mu-H2L center dot-)Ru-III(acac)(2)](+) (1(+)) -> [(acac)(2)Ru-III(mu-H2L2-)Ru-III(acac)(2)] (1) -> [(acac)(2)Ru-III mu-H2L center dot 3-)Ru-III(acac)(2)](-)/[(acac)(2)](-) Ru-III(mu-H2L2-)Ru-III(acac)(2)](-) (1(-)) -> [(acac)(2)Ru-III(mu-H2L4-)Ru-III(acac)(2)](2-)/[(acac)(2)Ru-III(mu-H2L2-)Ru-III(acac)(2)](2-) (1(2-)) and {(bpy)(2)Ru-III(mu-H2L2-)Ru-III(bpy)(2)](4+) (2(4+)) -> [(bpy)(2)Ru-III(mu-H2L center dot-)Ru-II(bpy)(2)](3+)/[(bpy)(2)Ru-III(mu-H2L2-)Ru-III(bpy)(2))(3+) (2(3+)) -> [(bpy)(2)Ru-II(mu-H2L2-)Ru-II(bpy)(2)](2+) (2(2+)). The favoring of Ru-III by sigma-donating acac(-) and of Ru-II by the pi-accepting bpy coligands shifts the conceivable valence alternatives accordingly. Similarly, the introduction of the NH donor function in H2Ln as compared to 0 causes a cathodic shift of redox potentials with corresponding consequences for the valence structure