A Neutral Borylene and its Conversion to a Radical by Selective Hydrogen Transfer.

A successful selective reduction of X2B-Tip (Tip = 1,3,5-iPr3-C6H2, X = I, Br) with KC8 and Mg metal, respectively, in the presence of a hybrid ligand (C6H4(PPh2)LSi) leads to a stable low-valent five-membered ring as a boryl radical [C6H4(PPh2)LSiBTip][Br] (1) and neutral borylene [C6H4(PPh2)LSiBTip] (2). Compound 2 reacts with 1,4-cyclohexadiene, resulting in hydrogen abstraction to afford the radical [C6H4(PPh2)LSiB(H)Tip] (3). Quantum chemical studies reveal that compound 1 is a B-centered radical, and compound 2 is a phosphane and silylene stabilized neutral borylene in a trigonal planar environment, whereas compound 3 is an amidinate-centered radical. Although compounds 1 and 2 are stabilized by hyperconjugation and π-conjugation, they display high H-abstraction energy and basicity, respectively.

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.

The Indigo Isomer Epindolidione as a Redox-Active Bridging Ligand for Diruthenium Complexes.

Epindolidione (H2 L), 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 , [X2 Ru(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.

Structural and Oxidation State Alternatives in Platinum and Palladium Complexes of a Redox-Active Amidinato Ligand.

Reaction of [Pt(DMSO)2 Cl2 ] or [Pd(MeCN)2 Cl2 ] with the electron-rich LH=N,N'-bis(4-dimethylaminophenyl)ethanimidamide yielded mononuclear [PtL2 ] (1) but dinuclear [Pd2 L4 ] (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 [Pd2 L4 ]+ were also studied, the latter as [Pd2 L4 ][B{3,5-(CF3 )2 C6 H3 }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 [Pd2 L4 ], and the dimetal (Pd2 4+ →Pd2 5+ ) instead of ligand (L- →L ) oxidation of the dinuclear palladium compound.

Selective Route to Stable Silicon-Boron Radicals and Their Corresponding Cations.

Herein, we report on a facile and selective one-pot synthetic route to silicon-boron radicals. Reduction of Br2BTip (Tip = 2,4,6-iPrC6H2) with KC8 in the presence of LSi-R affords LSi(tBu)-B(Br)Tip (1) and LSi(N(TMS)2)-B(Br)Tip (2) [L = PhC(NtBu)2]. These first examples of silicon-boron isolated radical species feature spin density on the silicon and boron atoms. 1 and 2 exhibit extraordinary stability to high temperatures under inert conditions in solution and air stability in the solid state. Both radicals have been isolated and fully characterized by electron paramagnetic resonance spectroscopy, SQUID magnetometry, mass spectrometry, cyclic voltammetry, single-crystal X-ray structure analysis, and density functional theory calculations. Moreover, compound 1 exhibits one-electron transfer when treated with 1 equiv of AgSO3CF3 and [Ph3C]+[B(C6F5)4]-, respectively, resulting in the corresponding cations [LSi(tBu)-B(Br)Tip]+[CF3SO3]- (3) and [LSi(tBu)-B(Br)Tip]+[B(C6F5)4]- (4). Compounds 3 and 4 have been characterized with multinuclear NMR and mass spectrometry.

Fused N-Heterocyclic-Bridged Isomeric Diruthenium Complexes [(acac)2Ru(µ-DIPQD)Ru(acac)2]n, n = +2, + 1, 0, -1, -2.

π-Conjugated bridged isomeric diruthenium(II) complexes [(acac)2RuII(µ-DIPQD)RuII(acac)2], 1 (trans) and 2 (cis) (acac- = acetylacetonate, (8E,16E)-N8,N16-diphenylindeno[1,2-b]indeno[2',1':5,6]pyrazino[2,3-g]quinoxaline-8,16-diimine (trans-DIPQD), and (12E,16E)-N12,N16-diphenylindeno[1,2-b]indeno[1',2':5,6]pyrazino[2,3-g]quinoxaline-12,16-diimine (cis-DIPQD) were separated and structurally characterized. The structures of the rac (ΔΔ/ΛΛ) forms of 1 and 2 exhibit two units of {Ru(acac)2}, linked to adjacent pyrazine and imine nitrogen donors of the bridge (DIPQD) in trans and cis modes, with metal-metal separations of 9.050 and 6.330 Å, respectively. The packing diagrams of 1 and 2 revealed an intermolecular π···π stacking interaction (3.202-3.398 Å) involving the face-to-face arrangement of the aromatic rings of DIPQD in adjacent molecules and varying solid-state packing modes, slipped stacking in the former versus brick-layer stacking in the latter. The electronic forms associated with multiple reversible one-electron redox steps of 1 and 2 were addressed by DFT (MO composition, Mulliken spin density distribution), supported by EPR of intermediate paramagnetic states and by UV-vis-NIR spectroelectrochemistry in all redox states. The results reveal similar electronic forms along the redox series irrespective of their isomeric identities in 1 and 2, viz., primarily metal-based oxidations ([(acac)2RuII(µ-DIPQD)RuIII(acac)2]+, 1+/2+, S = 1/2; [(acac)2RuIII(µ-DIPQD)RuIII(acac)2]2+, 12+/22+, S = 1) and bridge-based reductions ([(acac)2RuII(µ-DIPQD•-)RuII(acac)2]-, 1•-/2•-, S = 1/2; [(acac)2RuII(µ-DIPQD2-)RuII(acac)2]2-, 12-/22-, S = 1). TD-DFT analysis of the electronic transitions in the complexes suggests bridge-targeted mixed metal/ligand-based multiple charge transfer transitions over the visible to NIR region in all redox states, while a weak band involving the radical bridge appeared in the long-wavelength region (∼2000 nm) in 1•-/2•-.

Isolation of Transient Acyclic Germanium(I) Radicals Stabilized by Cyclic Alkyl(amino) Carbenes.

Despite the notable progress in the stabilization of main group radicals by NHCs and cAACs, no germanium radicals have been isolated so far due to synthetic challenges. Stabilization of neutral [:EIR]• (E = Si, Ge) radicals is an uphill task, as these reactive transient species are highly susceptible to dimerization. Herein, we report the synthesis of acyclic neutral germanium(I) radicals Cy-cAAC:GeN(SiMe3)Dip (1) and Me-cAAC:GeN(SiPh3)Mes (2) obtained by the reduction of [Ar(SiR3)NGeCl3] with KC8 in the presence of cAAC. Compounds 1 and 2 are well characterized by single crystal X-ray structural analysis, cyclic voltammetry, and EPR spectroscopy. Furthermore, the structure and bonding of compounds 1 and 2 have been investigated by theoretical methods.

A Route to Base Coordinate Silicon Difluoride and the Silicon Trifluoride Radical.

Silicon difluoride (SiF2 ) is highly unstable at room temperature and condenses at this temperature rapidly to a polymeric material of unknown structure. Therefore, the isolation of a stable monomeric silicon difluoride species is a challenging task. The cyclic alkyl(amino) carbene (cAAC) coordinated silicon difluoride was isolated as (cAAC)2 SiF2 (2), synthesized from the reduction of cAAC-SiF4 (1) by using two equivalents of KC8 in the presence of one equivalent of cAAC. In the solid state, compound 2 is stable at room temperature for a long time under inert conditions. The reduction of compound 1 in the presence of one equivalent KC8 resulted in the first stable silicon trifluoride monoradical (cAAC)SiF3 (3).

Mononuclear and Dinuclear Ruthenium Complexes of cis- and trans-Thioindigo: Geometrical and Electronic Structure Analyses.

The compounds [Ru(acac)2(L)] (1) and [Ru2(acac)4(µ-L)] (2) with acac- = acetylacetonato and L = thioindigo were characterized crystallographically with a cis-configurated L and O,O'-coordinated metal in 1 and with trans-configurated L and two S,O-coordinated bridged ruthenium centers for 2. The electronic structures of 1 and 2 were confirmed by spectroscopy and density functional theory calculations, suggesting considerable metal-to-ligand electron transfer resulting in the formation of the thioindigo radical anion and oxidized metal(s). UV-Vis-Near-IR and IR (spectro)electrochemistry was used to investigate charged forms 1 n ( n = 1+, 1-) and 2 n ( n = 1+, 1-, 2-), which reveal electron transfer activity of both the metal and the thioindigo ligand as well as sizable orbital mixing. An intense near-IR absorption is observed for 2- at 2180 nm. The remarkable ligand properties of thioindigo are being discussed in connection to related coordination compounds of indigo derivatives such as dehydroindigo and corresponding ("Nindigo") diimines.

At the Borderline between Metal-Metal Mixed Valency and a Radical Bridge Situation: Four Charge States of a Diruthenium Complex with a Redox-Active Bis( mer-tridentate) Ligand.

The complex ions [L3Ru(µ,η3:η3-BL)RuL3] n+ (1 n+, L3 = 4,4',4″-tri- tert-butyl-2,6,2',6″-terpyridine and H2BL2- = 1,2-bis(salicyloyl)hydrazide(2-)) were isolated with PF6- or ClO4- counterions ( n = 1) and as bis(hexafluorophosphate) ( n = 2). Structural, electrochemical, and spectroscopic characterization reveals the monocation as intermediate ( Kc = 108.2) in the three-step reversible redox system 10/+/2+/3+. The 1+ ion has the molecule-bridged (Ru- - -Ru 4.727 Å) ruthenium centers involved in five- and six-membered chelate rings, and it exhibits long-wavelength absorptions at λmax 2240, 1660, and 1530 nm (εmax = 1000, 3000, and 8000 M-1 cm-1, respectively), which would be compatible with a RuIIIRuII mixed-valent situation or with a coordinated radical ion bridge. In fact, EPR and DFT analysis of 1+ reveals that the spin is equally distributed over the ligand bridge and over both metals. The oxidized paramagnetic ions 12+ and 13+ have been studied by 1H NMR and EPR and by TD-DFT supported UV-vis-NIR and MIR (mid-IR) spectroelectrochemistry. The capacity of various kinds of bis( mer-tridentate) bridging ligands (π donors or π acceptors, cyclometalated or noncyclometalated) for mediating metal-metal interactions is discussed.

Organosilicon Radicals with Si-H and Si-Me Bonds from Commodity Precursors.

The cyclic alkyl(amino) carbene (cAAC) stabilized biradicals of composition (cAAC)2SiH2 (1), (cAAC)SiMe2-SiMe2(cAAC) (2), and (cAAC)SiMeCl-SiMeCl(cAAC) (3) have been isolated as molecular species. All the compounds are stable at room temperature for more than 6 months under inert conditions in the solid state. All radical species were fully characterized by single-crystal X-ray structure analysis and EPR spectroscopy. Furthermore, the structure and bonding of compounds 1-3 have been investigated by theoretical methods. Compound 1 contains the SiH2 moiety and this is the first instance, where we have isolated 1 without an acceptor molecule.

Near-Infrared-Absorbing Organometallic Diruthenium Complex Intermediates: Evidence for Bridging Anthrasemiquinone Formation and against Mixed Valency.

The new redox-active complexes [RuH(CO)(EPh3 )2 (µ-Q2- )RuH(CO)(EPh3 )2 ], E=P (1) and E=As (2) with the bis-chelate bridging ligand Q2- =1,4-dioxido-9,10-anthraquinone were prepared and characterised. The related compound [RuCl(CO)(PPh3 )2 (µ-Qx2- )RuCl(CO)(PPh3 )2 ] (4) with E=P and Qx2- =5,8-dioxido-1,4-naphthoquinone 4 revealed trans-positioned PPh3 groups. The electrogenerated one-electron oxidised states 1+ and 2+ were examined using spectroelectrochemical techniques (EPR, IR and UV/Vis/NIR). In situ EPR studies gave spectra with 31 P or 75 As hyperfine splitting of about 16 Gauss, small 99, 101 Ru coupling and small g-anisotropy in the frozen solution state. The 31 P and 75 As hyperfine values reflect axial positioning of the four Ru-E bonds relative to the plane of an anthrasemiquinone bridge. Single CO stretching bands around 1910 cm-1 of the precursors 1 and 2 shift by about 25 cm-1 to higher energies on oxidation. The direction, uniformity and the extent of the shifts confirm ligand bridge-based oxidation. Absorbance by the cations in the near infrared region is thus assigned to intra-ligand transitions of ruthenium(II)-bonded anthrasemiquinones and not to intervalence charge transfer of mixed-valent species. Ruthenium(II) stabilisation by CO and EPh3 is made responsible for the anthrasemiquinone formation instead of metal-centered oxidation.

Organometallic Fe-Fe Interactions: Beyond Common Metal-Metal Bonds and Inverse Mixed-Valent Charge Transfer.

The compounds [Fe(CO)3 (dRpf)]n+ , n=0, 1, 2 and dRpf=1,1'-bis(dicyclohexylphosphino)ferrocene ([1]n+ ) or 1,1'-bis(diisopropylphosphino)ferrocene ([2]n+ ), were obtained as two-step reversible redox systems by photolytic and redox reactions. The iron-iron distance decreases from about 4 Što about 3 Šon oxidation, which takes place primarily at the tricarbonyliron moiety. Whereas ferrocene oxidation is calculated to occur only in excited states, the near infrared absorptions of the mixed-valent monocations are due to an unprecedented "inverse" inter-valence charge transfer from the electron-rich iron(II) in the ferrocene backbone to the electron-deficient tricarbonyliron(I). Protonation of complex 1 results in the formation of the structurally characterized hydride [1H]BF4 , which reacts with acetone to form the dication, 12+ , and isopropanol. While the hydride [2H]BF4 was found to be unstable, protonation of 2 in acetone resulted in the clean formation of 22+, formally a hydrogen transfer.

Redox Induced Configurational Isomerization of Bisphosphine-Tricarbonyliron(I) Complexes and the Difference a Ferrocene Makes.

The tricarbonyliron (TCFe) complexes Fe(CO)3(dppf) and Fe(CO)3(dppp), where dppf = 1,1'-bis(diphenylphosphino)ferrocene and dppp = 1,3-bis(diphenylphosphino)propane, exhibit redox activity that induces configurational isomerization. The presence of the ferrocenyl (Fc) group stabilizes higher oxidized forms of TCFe. Using spectroelectrochemistry (IR, UV-vis, Mössbauer, and EPR) and computational analysis, we can show that the Fc in the backbone of the dppf ligand tends to form a weak dative bond to the electrophilic TCFeI and TCFeII species. The open shell TCFeI intermediate was stabilized by the distribution of spin between the two Fe centers (Fc and TCFe), whereas lacking the Fc moiety resulted in highly reactive TCFeI species. The [Fe(CO)3(dppf)]+ cation adopts two possible configurations, square-pyramidal (without an Fe-Fe interaction) and trigonal-bipyramidal (containing an Fe-Fe interaction). The two configurations are in equilibrium with the trigonal-bipyramidal configuration being enthalpically favored (ΔH = -7 kJ mol-1). There is an entropic penalty (ΔS = -20 J mol-1) due to tilting of the Cp (cyclopentadienide) rings of the dppf moieties by ∼8°. Additionally, the terminal iron hydride [FeH(CO)3(dppf)]BF4 was formed by protonation with a strong acid (HBF4·Et2O).

Metal-Metal Bridging Using the DPPP Dye System: Electronic Configurations within Multiple Redox Series.

Redox series [LnRu(µ-DPPP)RuLn]k, H2DPPP = 2,5-dihydro-3,6-di-2-pyridylpyrrolo(3,4-c)pyrrole-1,4-dione and L = 2,4-pentanedionato (acac-), 2,2'-bipyridine (bpy), and 2-phenylazopyridine (pap), have been studied by voltammetry (CV, DPV), EPR, and UV-vis-NIR spectroelectrochemistry, supported by TD-DFT calculations. Crystal structure analysis and 1H NMR revealed oxidation states [(acac)2RuIII(µ-DPPP2-)RuIII(acac)2] and [(bpy)2RuII(µ-DPPP2-)RuII(bpy)2]2+ for the corresponding precursors, isolated as rac diastereomers. Oxidation was observed to occur mainly at the bridging ligand (DPPP2- → DPPP•-), whereas the site of reduction (DPPP, Ru, or L) depends on effects from the ancillary ligands L. The metal coordination of a derivative of the pigment forming 2,5-dihydro-pyrrolo(3,4-c)pyrrole-1,4-dione (DPP) dyes and the analysis of corresponding multistep redox series add to the previously recognized coordinative and electron transfer potential of dye molecules of the azo, indigo, anthraquinone, and formazanate type.

A Stable Neutral Radical in the Coordination Sphere of Aluminum.

The neutral radical (Me2 -cAAC)2 AlCl2 (2) is stabilized by cyclic (alkyl)(amino)carbenes (cAACs). Complex 2 was synthesized by reduction of the Me2 -cAAC:→AlCl3 (1) adduct with KC8 in the presence of another equivalent of Me2 -cAAC. The crystal structure of 2 shows that the Al-C bond lengths of the two carbenes bound to the Al center are considerably different, which is likely the result of intermolecular interactions. Quantum-chemical calculations from the gas phase give an equilibrium structure with identical Al-C bond lengths. Compound 2 exhibits monoradical character, which was confirmed by EPR measurements. A bonding analysis indicates that the unpaired electron resides mainly at the carbene carbon atoms. Compound 2 is an example for an unusual neutral Al radical.

Noninnocence of Indigo: Dehydroindigo Anions as Bridging Electron-Donor Ligands in Diruthenium Complexes.

Complexes of singly or doubly deprotonated indigo (H2Ind) with one or two [Ru(pap)2](2+) fragments (pap = 2-phenylazopyridine) have been characterized experimentally [molecular structure, voltammetry, electron paramagnetic resonance (EPR), and UV-vis-near-IR spectroelectrochemistry] and by time-dependent density functional theory calculations. The compound [Ru(pap)2(HInd(-))]ClO4 ([1]ClO4) was found to contain an intramolecular NH---O hydrogen bond, whereas [{Ru(pap)2}2(µ-Ind(2-))](ClO4)2 ([2](ClO4)2), isolated as the meso diastereoisomer with near-IR absorptions at 1162 and 991 nm, contains two metals bridged at 6.354 Å distance by the bischelating indigo dianion. The spectroelectrochemical study of multiple reversible reduction and oxidation processes of 2(n) (n = 4+, 3+, 2+, 1+, 0, 1-, 2-, 3-, 4-) reveals the stepwise addition of electrons to the terminal π-accepting pap ligands, whereas the oxidations occur predominantly at the anionic indigo ligand, producing an EPR-identified indigo radical intermediate and revealing the suitability of deprotonated indigo as a σ- and π-donating bischelating bridge.

Analysis of Redox Series of Unsymmetrical 1,4-Diamido-9,10-anthraquinone-Bridged Diruthenium Compounds.

The unsymmetrical diruthenium complexes [(bpy)2Ru(II)(µ-H2L(2-))Ru(III)(acac)2]ClO4 ([3]ClO4), [(pap)2RuII(µ-H2L(2-))Ru(III)(acac)2]ClO4 ([4]ClO4), and [(bpy)2Ru(II)(µ-H2L(2-))Ru(II)(pap)2](ClO4)2 ([5](ClO4)2) have been obtained by way of the mononuclear precursors [(bpy)2Ru(II)(H3L(-))]ClO4 ([1]ClO4) and [(pap)2Ru(II)(H3L(-))]ClO4 ([2]ClO4) (where bpy = 2,2'-bipyridine, pap = 2-phenylazopyridine, acac(-) = 2,4-pentanedionate, and H4L = 1,4-diamino-9,10-anthraquinone). Structural characterization by single-crystal X-ray diffraction and magnetic resonance (nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR)) were used to establish the oxidation state situation in each of the isolated materials. Cyclic voltammetry, EPR, and ultraviolet-visible-near-infrared (UV-vis-NIR) spectroelectrochemistry were used to analyze the multielectron transfer series of the potentially class I mixed-valent dinuclear compounds, considering the redox activities of differently coordinated metals, of the noninnocent bridge and of the terminal ligands. Comparison with symmetrical analogues [L2'Ru(µ-H2L)RuL2'](n) (where L' = bpy, pap, or acac(-)) shows that the redox processes in the unsymmetrical dinuclear compounds are not averaged, with respect to the corresponding symmetrical systems, because of intramolecular charge rearrangements involving the metals, the noninnocent bridge, and the ancillary ligands.

1,5-Diamido-9,10-anthraquinone, a Centrosymmetric Redox-Active Bridge with Two Coupled ß-Ketiminato Chelate Functions: Symmetric and Asymmetric Diruthenium Complexes.

The dinuclear complexes {(µ-H2L)[Ru(bpy)2]2}(ClO4)2 ([3](ClO4)2), {(µ-H2L)[Ru(pap)2]2}(ClO4)2 ([4](ClO4)2), and the asymmetric [(bpy)2Ru(µ-H2L)Ru(pap)2](ClO4)2 ([5](ClO4)2) were synthesized via the mononuclear species [Ru(H3L)(bpy)2]ClO4 ([1]ClO4) and [Ru(H3L)(pap)2]ClO4 ([2]ClO4), where H4L is the centrosymmetric 1,5-diamino-9,10-anthraquinone, bpy is 2,2'-bipyridine, and pap is 2-phenylazopyridine. Electrochemistry of the structurally characterized [1]ClO4, [2]ClO4, [3](ClO4)2, [4](ClO4)2, and [5](ClO4)2 reveals multistep oxidation and reduction processes, which were analyzed by electron paramagnetic resonance (EPR) of paramagnetic intermediates and by UV-vis-NIR spectro-electrochemistry. With support by time-dependent density functional theory (DFT) calculations the redox processes could be assigned. Significant results include the dimetal/bridging ligand mixed spin distribution in 3(3+) versus largely bridge-centered spin in 4(3+)-a result of the presence of Ru(II)-stabilizig pap coligands. In addition to the metal/ligand alternative for electron transfer and spin location, the dinuclear systems allow for the observation of ligand/ligand and metal/metal site differentiation within the multistep redox series. DFT-supported EPR and NIR absorption spectroscopy of the latter case revealed class II mixed-valence behavior of the oxidized asymmetric system 5(3+) with about equal contributions from a radical bridge formulation. In comparison to the analogues with the deprotonated 1,4-diaminoanthraquinone isomer the centrosymmetric H2L(2-) bridge shows anodically shifted redox potentials and weaker electronic coupling between the chelate sites.

Isomeric Diruthenium Complexes of a Heterocyclic and Quinonoid Bridging Ligand: Valence and Spin Alternatives for the Metal/Ligand/Metal Arrangement.

5,7,12,14-Tetraazapentacene-6,13-quinone (L) reacts with 2 equiv of [Ru(acac)2(CH3CN)2] to form two linkage isomeric bis(chelate) compounds, [{RuII(acac)2}2(µ-L)], blue 1, with 5,6;12,13 coordination and violet 2 with 5,6;13,14 coordination. The linkage isomers could be separated, structurally characterized in crystals as rac diastereomers (ΔΔ/ΛΛ), and studied by voltammetry (CV, DPV), EPR, and UV-vis-NIR spectroelectrochemistry (meso-1, rac-2). DFT and TD-DFT calculations support the structural and spectroscopic results and suggest a slight energy preference (ΔE = 263 cm-1) for the rac-isomer 1 as compared to 2. Starting from the RuII-(µ-L0)-RuII configurations of 1 and 2 with low-lying metal-to-ligand charge transfer (MLCT) absorptions, the compounds undergo two reversible one-electron oxidation steps with open-shell intermediates 1+ (Kc = 4 × 104) and 2+ (Kc = 6 × 105). Both monocations display metal-centered spin according to EPR, but the DFT-calculated spin densities suggest a RuIII(µ-L•-)RuIII three-spin situation with opposite spin density at the bridging ligand for the meso form of 1+, estimated to lie 1887 cm-1 lower in energy than rac-1+, which is calculated with a Class II mixed-valent situation RuIII-(µ-L0)-RuII. A three-spin arrangement RuIII-(µ-L•-)-RuIII with negative spin density at one metal site is suggested by DFT for rac-2+ which is more stable by ΔE = 890 cm-1 than rac-1+. Reduction of 1 or 2 (Kc = 107-108) occurs mainly at the central bridging ligand with notable contributions (30%) from the metals in 1- and 2-. The mixed-valent RuIII(µ-L)RuII versus radical-bridged RuIII(µ-L•-)RuIII alternative is discussed comprehensively in comparison with related valence-ambiguous cases.