the very different redox behaviour of isoelectronic complexes with [PtCl2] and [AuCl2]+

The new, potentially ambidentate heterocyclic ligand 2,3-bis(1-methylimidazol-2-yl)quinoxaline (bmiq) was obtained from 2,3-bis(1-methylimidazol-2-yl)glyoxal and 1,2-diaminobenzene. Its coordination to PtCl2 and to the isoelectronic [AuCl2]+ in [AuCl2(bmiq)](AuCl4) occurs via the imine N donors of the imidazolyl groups, leading to the formation of seven-membered chelate rings with boat conformation. According to the spectroelectrochemistry (UV-vis-NIR, EPR), the reversible electron addition to the [PtCl2(bmiq)] and the free ligand takes place in the (non-coordinated) quinoxaline part of the molecule, similarly as for related complexes of dipyrido[3,2-a:2′,3′-c]phenazines (dppz), 2,3-bis(2-pyridyl)quinoxalines (bpq) and 2,3-bis(dialkylphosphino)quinoxalines (QuinoxP). DFT calculations confirm the experimental results (structures, spectroscopy) and also point to the coordination potential of the quinoxaline N atoms. The electron addition to [AuCl2(bmiq)]+ takes place not at the ligand but at the metal site, according to experimental and DFT results

polaron pair vs. bipolaron

The molecular structure of a cyclic oligothiophene, C10T, has been determined by single-crystal X-ray structure analysis. The exclusive syn-conformation of all thiophene units as confirmed in the solid state and the ring strain in this macrocycle result in its unusual and optoelectronic properties. This does not only apply to neutral C10T but also to its oxidized states, as demonstrated by absorption and ESR spectroscopy, supporting the formation of a polaron-pair structure upon oxidation of C10T to C10T2(·+) as has been discussed for linear oligothiophenes. To the best of our knowledge, C10T2(·+) represents an unambiguous example comprising a two-polaron structure (polaron- pair) of a thiophene-based conjugated macrocycle

Homodinuclear complexes of [Cu(dppf)]+ or [Ru(bpy)2]2+ with 1,4‐Bis(camphorquinoneimino)benzene (bcqb) as a redox‐active bridging ligand

The product μ‐bcqb from the reaction between p‐phenylenediamine and two equivalents of camphorquinone has been used as a potentially conjugated molecular bridge between two complex fragments [Cu(dppf)]+, dppf=1,1‘‐bis(diphenylphosphino)ferrocene, to yield 12+, and between two [Ru(bpy)2]2+ moieties, resulting in 24+. The molecular structure of 1(BF4)2 shows an intramolecular Cu-Cu distance of 8.04 Å and a torsionally twisted conformation of the bridge, while cyclic voltammetry, EPR, IR and UV‐vis‐NIR spectroelectrochemistry reveal two closely spaced one‐electron reductions and a ferrocene‐based two‐electron oxidation. Compound 2(PF6)4 exhibits at least three one‐electron reduction waves and one 2e‐oxidation, the former attributed to μ‐bcqb and the latter assigned to metal‐based processes showing no evidence for a Ru-Ru mixed‐valent intermediate. However, intra‐ligand based mixed valency with intense LLIVCT absorptions in the near infrared was observed for the metal coordinated μ‐bcqb.- and μ‐bcqb.3- bridges. The two examples 1n+ and 2n+ illustrate that a combination of physical methods is advantageous to analyze and define the correct electronic structures in complexes involving several metal centers and noninnocent ligand components.State of Baden-WürttembergProjekt DEA

UV-vis-NIR and EPR characterisation of the redox series [MQ3]2+,+,0,−,2−, M = Ru or Os, and Q = o-quinone derivative

The neutral title compounds with Q = 3,5-di-tert-butyl-o-quinone or 4,6-di- tert-butyl-N-phenyl-o-iminobenzoquinone (Qx) were studied by UV-vis-NIR spectroelectrochemistry and by EPR spectroscopy in the case of the odd- electron monocation and monoanion intermediates. Supported by DFT and TD-DFT calculations, the results indicate stepwise electron removal from predominantly ligand-based delocalised MOs on oxidation whereas the stepwise electron uptake on reduction involves unoccupied MOs with considerably metal–ligand mixed character. In both cases, the strong near-infrared absorption of the neutral precursors diminishes. In comparison to the ruthenium series, the osmium analogues exhibit larger transition energies from enhanced MO splitting and a different EPR response due to the higher spin–orbit coupling. The main difference between the quinone (1nnn, 2nnn) and corresponding monoiminoquinone systems (3nnn, 4nnn) is the shift of about 0.6 V to lower potentials for the monoimino analogues. While the absorption features do not differ markedly, the EPR data reflect a higher degree of covalent bonding for the complexes with monoimino ligands

Intramolecular charge transfer in ruthenium complexes [Ru(acac)2(ciq)] with ambidentate camphoriminoquinone (ciq) ligands

Reaction of [Ru(acac)2(MeCN)2], acac−=acetylacetonate, with N‐phenylcamphoriminoquinone (pciq) or the new N‐(2‐thiomethylphenyl)‐camphoriminoquinone (tciq) produces complexes [Ru(acac)2(pciq)] and [Ru(acac)2(tciq)] with N,O or N,S coordination and E or Z configuration at the C=N bond, respectively. Oxidation state assignments in comparison with classical iminoquinone ligands are based on structural data in connection with DFT calculations. Reversible oxidation yields complex ions [RuIII(acac)2(pciq)]+ and [RuIII(acac)2(tciq)]+ as characterized by EPR, IR and UV‐vis‐NIR spectroelectrochemistry.state of Baden-WürttembergbwHPCDF

Mixed valency vs radical bridge formulation in symmetrically and asymmetrically ligated diruthenium complexes

The asymmetrical dinuclear [{(trpy*)Ru}2(μ‐adc‐Salph)Cl](PF6) 1(PF6), trpy*=4,4’,4”‐tri‐tert‐butyl‐2,6,2’,6”‐terpyridine, adc‐Salph=1‐benzoyl‐2‐salicyloylhydrazido(3‐), and the related symmetrical dinuclear [{Cl(trpy*)Ru}2(μ,η2 : η2‐adc‐Ph)](PF6) 2(PF6), adc‐Ph=1,2‐bis(benzoyl)hydrazido(2‐), were synthesized and structurally characterized. Both paramagnetic compounds were compared with the previously reported symmetrical [{(trpy*)Ru}2(μ,η3 : η3‐adc‐Sal)](PF6) 3(PF6) containing the bis‐tridentate bridge 1,2‐bis(salicyloyl)hydrazido(4‐). Molecular structures and magnetic resonance features (1H NMR, EPR) indicate spin density distribution over the metal(s) and the bridging ligand. Reversible one‐electron reduction and oxidation were possible in all instances yielding comproportionation constants Kc of about 109 for the paramagnetic intermediates 1+-3+. Structural results, spin density distribution and UV‐Vis‐NIR spectroelectrochemistry were analyzed for 1+ with the help of TD‐DFT calculations for a model compound (tert‐Bu→Me). Intense absorptions around λmax=1450-1650 nm for the cations were assigned to mixed metal/ligand transitions with significant inter‐valence charge transfer (IVCT) character. For both the symmetrical and asymmetrical arrangements the cationic intermediates can be described as considerably mixed metal/ligand systems.State of Baden-WürttembergCzech Science FoundationProjekt DEA

Rhenium tricarbonyl complexes of azodicarboxylate ligands

The excellent π-accepting azodicarboxylic esters adcOR (R = Et, iPr, tBu, Bn (CH2-C6H5) and Ph) and the piperidinyl amide derivative adcpip were used as bridging chelate ligands in dinuclear Re(CO)3 complexes [{Re(CO)3Cl}2(µ-adcOR)] and [{Re(CO)3Cl}2(µ-adcpip)]. From the adcpip ligand the mononuclear derivatives [Re(CO)3Cl(adcpip)] and [Re(CO)3(PPh3)(µ-adcpip)]Cl were also obtained. Optimised geometries from density functional theory (DFT) calculations show syn and anti isomers for the dinuclear fac-Re(CO)3 complexes at slightly different energies but they were not distinguishable from experimental IR or UV–Vis absorption spectroscopy. The electrochemistry of the adc complexes showed reduction potentials slightly below 0.0 V vs. the ferrocene/ferrocenium couple. Attempts to generate the radicals [{Re(CO)3Cl}2(µ-adcOR)]•− failed as they are inherently unstable, losing very probably first the Cl− coligand and then rapidly cleaving one [Re(CO)3] fragment. Consequently, we found signals in EPR very probably due to mononuclear radical complexes [Re(CO)3(solv)(adc)]•. The underlying Cl−→solvent exchange was modelled for the mononuclear [Re(CO)3Cl(adcpip)] using DFT calculations and showed a markedly enhanced Re-Cl labilisation for the reduced compared with the neutral complex. Both the easy reduction with potentials ranging roughly from −0.2 to −0.1 V for the adc ligands and the low-energy NIR absorptions in the 700 to 850 nm range place the adc ligands with their lowest-lying π* orbital being localised on the azo function, amongst comparable bridging chelate N^N coordinating ligands with low-lying π* orbitals of central azo, tetrazine or pyrazine functions. Comparative (TD)DFT-calculations on the Re(CO)3Cl complexes of the adcpip ligand using the quite established basis set and functionals M06-2X/def2TZVP/LANL2DZ/CPCM(THF) and the more advanced TPSSh/def2-TZVP(+def2-ECP for Re)/CPCMC(THF) for single-point calculations with BP86/def2-TZVP(+def2-ECP for Re)/CPCMC(THF) optimised geometries showed a markedly better agreement of the latter with the experimental XRD, IR and UV–Vis absorption data.University of Cologne, Faculty of Mathematics and Natural Science

The indigo isomer epindolidione as a redox‐active bridging ligand for diruthenium complexes

Epindolidione (H2L), a heteroatom‐modified analogue of tetracene and a structural isomer of indigo, forms dinuclear complexes with [RuX2]2+, X=bpy (2,2′‐bipyridine, [1]2+) or pap (2‐phenylazopyridine, [2]2+), in its doubly deprotonated bridging form μ‐L2-. The dications in compounds meso‐[1](ClO4)2 and meso‐[2](ClO4)2, [X2Ru(μ‐L)RuX2](ClO4)2, contain five‐membered chelate rings N‐C-C‐O‐RuII with π bridged metals at an intramolecular distance of 7.19 Å. Stepwise reversible oxidation and reduction is mainly ligand centered (oxidation: L2-; reduction: X), as deduced from EPR of one‐electron oxidized and reduced intermediates and from UV/Vis/NIR spectroelectrochemistry, supported by TD‐DFT calculation results. The results for [1](ClO4)2 and [2](ClO4)2 are qualitatively similar to the ones observed with the deprotonated indigo‐bridged isomers with their six‐membered chelate ring structures, confirming the suitability of both π systems for molecular electronics applications, low‐energy absorptions, and multiple electron transfers.Science and Engineering Research Board (SERB, Department of Science and Technology),J. C. Bose Fellowship (G.K.L., SERB)Land Baden-WürttembergCouncil of Scientific and Industrial Research (fellowship to M.K.)University Grant Commission (fellowship to S.K.B)-New Delhi (India

Strong metal–metal coupling in mixed-valent intermediates [Cl(L)Ru(μ-tppz)Ru(L)Cl]+, L = β-diketonato ligands, tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine

Five diruthenium(II) complexes [Cl(L)Ru(μ-tppz)Ru(L)Cl] (1–5) containing differently substituted β-diketonato derivatives (1: L = 2,4-pentanedionato; 2: L = 3,5-heptanedionato; 3: L = 2,2,6,6-tetramethyl-3,5-heptanedionato; 4: L = 3-methyl-2,4-pentanedionato; 5: L = 3-ethyl-2,4-pentanedionato) as ancillary ligands (L) were synthesized and studied by spectroelectrochemistry (UV-Vis- NIR, electron paramagnetic resonance (EPR)). X-ray structural characterisation revealed anti (1, 2, 5) or syn (3) configuration as well as non-planarity of the bis-tridentate tppz bridge and strong dπ(RuII) → π*(pyrazine, tppz) back- bonding. The widely separated one-electron oxidation steps, RuIIRuII/RuIIRuIII and RuIIRuIII/RuIIIRuIII, result in large comproportionation constants (Kc) of ≥1010 for the mixed-valent intermediates. The syn-configurated 3n exhibits a particularly high Kc of 1012 for n = 1+, accompanied by density functional theory (DFT)-calculated minimum Ru–N bond lengths for this RuIIRuIII intermediate. The electrogenerated mixed-valent states 1+–5+ exhibit anisotropic EPR spectra at 110 K with average values of 2.304–2.234 and g anisotropies Δg = g1–g3 of 0.82–0.99. Metal-to-metal charge transfer (MMCT) absorptions occur for 1+–5+ in the NIR region at 1660 nm–1750 nm (ε ≈ 2700 dm3 mol−1 cm−1, Δν1/2 ≈ 1800 cm−1). DFT calculations of 1+ and 3+ yield comparable Mulliken spin densities of about 0.60 for the metal ions, corresponding to valence-delocalised situations (Ru2.5)2. Rather large spin densities of about −0.4 were calculated for the tppz bridges in 1+ and 3+. The calculated electronic interaction values (VAB) for 1+–5+ are about 3000 cm−1, comparable to that for the Creutz–Taube ion at 3185 cm−1. The DFT calculations predict that the RuIIIRuIII forms in 12+–52+ prefer a triplet (S = 1) ground state with ΔE (S = 0 − S = 1) [similar]5000 cm−1. One-electron reduction takes place at the tppz bridge which results in species [Cl(L)RuII(μ-tppz˙−)RuII(L)Cl]− (1˙−–3˙−, 5˙−) which exhibit free radical-type EPR signals and NIR transitions typical of the tppz radical anion. The system 4n is distinguished by lability of the Ru–Cl bonds

Structural and oxidation state alternatives in platinum and palladium complexes of a redox‐active amidinato ligand

Reaction of [Pt(DMSO)2Cl2] or [Pd(MeCN)2Cl2] with the electron‐rich LH=N,N’‐bis(4‐dimethylaminophenyl)ethanimidamide yielded mononuclear [PtL2] (1) but dinuclear [Pd2L4] (2), a paddle‐wheel complex. The neutral compounds were characterized through experiments (crystal structures, electrochemistry, UV‐vis‐NIR spectroscopy, magnetic resonance) and TD‐DFT calculations as metal(II) species with noninnocent ligands L-. The reversibly accessible cations [PtL2]+ and [Pd2L4]+ were also studied, the latter as [Pd2L4][B{3,5‐(CF3)2C6H3}4] single crystals. Experimental and computational investigations were directed at the elucidation of the electronic structures, establishing the correct oxidation states within the alternatives [PtII(L-)2] or [Pt.(L )2], [PtII(L0.5-)2]+ or [PtIII(L-)2]+, [(PdII)2(μ‐L−)4] or [(Pd1.5)2(μ‐L0.75-)4], and [(Pd2.5)2(μ‐L-)4]+ or [(PdII)2(μ‐L0.75-)4]+. In each case, the first alternative was shown to be most appropriate. Remarkable results include the preference of platinum for mononuclear planar [PtL2] with an N‐Pt‐N bite angle of 62.8(2)° in contrast to [Pd2L4], and the dimetal (Pd24+→Pd25+) instead of ligand (L-→L ) oxidation of the dinuclear palladium compound.Ministerium für Wissenschaft, Forschung und Kunst Baden-WürttembergDeutsche ForschungsgemeinschaftProjekt DEA