Comproportionation Constants Research Articles

3,6‐Divinyl‐N‐arylcarbazoles‐bridged diruthenium complexes bearing two Ru(CO)Cl(PiPr3)2 moieties: Polyelectrochromism and electronic coupling

A series of five 3,6‐divinyl‐N‐arylcarbazoles‐bridged diruthenium complexes with a general formula of [{Ru(CO)Cl(PiPr3)2}2(μ‐(CH=CH)2‐(N‐arylcarbazole)‐3,6); aryl = 4‐anisyl; [1a], 4‐tolyl; [1b], phenyl; [1c], 4‐(trifluoromethyl)phenyl; [1d], and 4‐nitrophenyl; [1e]] were successfully synthesized. These new square‐pyramidal bis(ruthenium‐vinyl)‐type complexes were characterized in their neutral state via classical NMR, UV/Vis, and IR spectroscopic techniques and in their three various accessible oxidized states via electrochemical techniques and ESR spectroscopy along with UV/Vis/NIR and IR spectroelectrochemistry as well as with DFT calculations. Our obtained results were compared with the closely related bis(ruthenium‐vinyl)‐appended N‐(4‐anisyl)diphenylamine TAARu2 counterpart of [1a]. Electrochemical studies on complexes [1a]–[1e] revealed three consecutive, chemically and electrochemically, well‐separated, reversible one‐electron oxidation processes at low and well‐accessible potentials. Their half‐wave potential separations ΔE½ between the individual redox waves and the derived comproportionation constants Kc1–Kc3 attribute to noteworthy stability of every oxidized intermediate state. The concert electrochemical and UV/Vis/NIR/IR spectro(electro)scopic results on these rotationally restricted N‐arylcarbazoles‐bridged bis(ruthenium‐alkenyl)‐type complexes [1a]–[1e] confirmed the relative weakness influence of altering the para‐substituent on the peripheral aryl ring. This is most probably due to the sizable torsion with nearly 55° between the co‐planarity of the carbazole heterocycle core and the peripheral N‐bound aryl ring. In their mixed‐valent (MV) radical states [1a]+–[1e]+, the observed pattern of two well‐separated Ru(C≡O) bands mirrors the uneven distribution of the uni‐positive charge over the two chemically equivalent bis(ruthenium‐vinyl) moieties as indicated by classical class II of Robin–Day classification of MV states.

Manipulating Ligand Density at the Surface of Polyoxovanadate-Alkoxide Clusters.

We present the post-synthetic modification of a polyoxovanadate-alkoxide (POV-alkoxide) cluster via the reactivity of its cationic form, [V6O7(OCH3)12]1+, with water. This result indicates that cluster oxidation increases the lability of bridging methoxide ligands, affording a ligand exchange reaction that serves to compensate for the increased charge of the cluster core. This synthetic advance affords the isolation of a series of POV-alkoxide clusters with varying degrees of μ2-O2- ligands incorporated at the surface, namely, [V6O8(OCH3)11], [V6O9(OCH3)10], and [V6O10(OCH3)9]. Characterization of the POV-alkoxide clusters is described; changes in the infrared and electronic absorption spectra are consistent with the oxidation of the cluster core. We also examine the consequences of ligand substitution on the redox properties of the series of POV-alkoxide clusters via cyclic voltammetry; decreased alkoxide ligand density translates to a cathodic shift of analogous redox events. Ligand substitution also increases comproportionation constants of the Lindqvist core, indicating electron exchange between vanadium centers is promoted in structures with greater numbers of μ2-O2- ligands.

Near‐IR Electrochromic Dihydroindolo(2,3a)carbazole‐Based Macrocycles and Their [b]‐Annulated BODIPY Complexes

AbstractA new class of redox‐active dihydroindolo(2,3a)carbazole(InC)‐based porphyrin‐like macrocycles (thiophene embedded: InCT and p–phenylene embedded: InCB) and the corresponding [b]‐annulated BODIPY complexes (InCTBF2 and InCBBF2) are reported. A facile synthetic methodology is adopted to accomplish the new expanded‐porphyrin‐like macrocyclic systems using the key precursor 11 based on 11,12‐dihydroindolo(2,3a)carbazole building block 4. In these macrocyclic systems, the subunit 4 is deemed to be a donor and an azafulvene‐extended with π‐conjugated fragment (thiophene or p‐phenylene) acts as an acceptor. Single crystal X‐ray structures are obtained for both InCT and InCB. Spectroelectrochemical studies on these macrocycles and their BF2 complexes display intriguing near‐IR electrochromic properties with reversible multi‐color switching upon cathodic scans at room temperature. InCTBF2 shows high stability with a radical comproportionation constant (Kc) of 1.22 × 106. Thiophene and p‐phenylene subunits in InCT and InCB exhibit ring rotation at room temperature as evident from NMR studies. Such ring rotation is restricted upon BF2 complexation, and they show unusual stability in acidic conditions.

Mixed Valency vs Radical Bridge Formulation in Symmetrically and Asymmetrically Ligated Diruthenium Complexes

Abstract: The asymmetrical dinuclear [{(trpy*)Ru} 2 (µ-adc-Salph)Cl](PF 6 ) 1 (PF 6 ), 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,η 2 :η 2 -adc-Ph)](PF 6 ) 2 (PF 6 ), 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,η 3 :η 3 -adc-Sal)](PF 6 ) 3 (PF 6 ) containing the bis-tridentate bridge 1,2-bis(salicyloyl)hydrazido(4-). Molecular structures and magnetic resonance features ( 1 H 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 K c of about 10 9 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 λ max = 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.

Tetrazine Chelate Ligands Bridging Two [Ru(acac)2] Fragments: Mixed Valency and Radical Complex Formation

AbstractUsing bis(3‐methyl‐2‐pyridyl)‐1,2,4,5‐tetrazine 1, 3‐(2‐pyrimidyl)‐6‐methyl‐1,2,4,5‐tetrazine 2 and bis(2‐pyrimidyl)‐1,2,4,5‐tetrazine=bmtz as ligands, the complexes 3=[Ru(acac)2(1)], 4={[Ru(acac)2]2(1)], 5={[Ru(acac)2]2(bmtz)], and 6={[Ru(acac)2]2(2)] were prepared and identified by structure analysis of crystallized material. The one‐electron oxidized form 6(PF6) was also studied structurally, suggesting a Class II mixed‐valent situation. The neutral dinuclear systems exhibit two reversible oxidation processes with comproportionation constants 109.2<Kc<1014.1 and one reduction which were analyzed UV/vis/NIR and EPR spectroscopically. Oxidation produces largely metal‐based mixed‐valent cations with very weak intervalence absorptions in the near IR whereas the electron uptake occurs at the tetrazine acceptor.

Development and Characterisation of Highly Conjugated Functionalised Ferrocenylene Macrocycles

AbstractMultiple synthetic pathways were explored for the preparation of ethynylarene‐containing ferrocenylene macrocyles. Accordingly, the synthesis of novel bis‐ and tris‐ferrocenylene macrocycles, containing a diethynylpyridine bridging group was achieved, and evidence is also provided for a number of linear‐analogues formed as by‐products. The electrochemical properties of the macrocycles were examined, and it was shown that the number of observable redox processes does not necessarily correlate with the number of redox active centres. Additionally, examination of the comproportionation constants of the tris‐ferrocenylene macrocycle demonstrated that the monocationic and dicationic species could be assigned to class II and I/II borderline of the Robin‐Day classification, respectively. The synthesis of these molecules presents an opportunity towards the development of conjugated ferrocenylene macrocycles for electronic applications.

Zinc-[7]helicenocyanine and Its Discrete π-Stacked Homochiral Dimer.

In this communication, we demonstrate a novel approach to prepare a discrete dimer of chiral phthalocyanine (Pc) by exploiting the flexible molecular geometry of helicenes, which enables structural interlocking and strong aggregation tendency of Pcs. Synthesized [7]helicene‐Pc hybrid molecular structure, zinc‐[7]helicenocyanine (Zn‐7HPc), exclusively forms a stable dimeric pair consisting of two homochiral molecules. The dimerization constants were estimated to be as high as 8.96×106 M−1 and 3.42×107 M−1 in THF and DMSO, respectively, indicating remarkable stability of dimer. In addition, Zn‐7HPc exhibited chiral self‐sorting behavior, which resulted in preferential formation of a homochiral dimer also in the racemic sample. Two phthalocyanine subunits in the dimeric form strongly communicate with each other as revealed by a large comproportionation constant and observation of an IV‐CT band for the thermodynamically stable mixed‐valence state.

Supramolecular Ladder Assemblies as a Model for Probing Electronic Interactions between Multiple Stacked π-Conjugated Systems.

A series of mono-, di-, and tri-topic receptors in which H-bonding sites, complementary to those of barbituric acid (BA), are fused is used to induce the supramolecular assembly of n×m ladders containing 1, 2, or 3 triphenylenevinylene units appended with BA. The topological constraint enforced by the architectures induces through-space interactions between the electroactive moieties that are reflected in the electronic absorption and emission spectrum. The n=2, m=2 or m=3 architectures undergo two single electron oxidation events, indicative of the formation of the corresponding mono- and di-radical cation species with comproportionation constants of 340 and 70, respectively. Comparison of the electrochemical potentials suggests that the charges are delocalized over the electroactive units in the assembly.

Synthesis and Characterization of Diferrocenyl Conjugates: Varying π‐Conjugated Bridging Ligands and its Consequence on Electrochemical Communication

Organometallic wire‐like complexes with ferrocenyl termini, conjugated with one and two units of thienylethynyl (M1 and M2) and thienyl (M3 and M4) groups, have been synthesized with a general formula of [Fc‐C≡C‐(Th‐C≡C)1‐2‐Fc] and [Fc‐(Th)1‐2‐Fc] (Fc = ferrocenyl, Th = thienyl) respectively. The diferrocenyl organometallic complexes have been characterized by various spectroscopic tools such as multinuclear NMR, FTIR, elemental analysis, and mass spectrometry. The electrochemical properties of these compounds have been investigated by cyclic (CV) and differential pulse voltammetry (DPV). The single reversible oxidation wave in diferrocenyl complexes with thienylethynyl spacers (M1 and M2) indicates the absence of intramolecular electrochemical communication between the two ferrocenyl termini. Interestingly, the diferrocenyl complexes with one and two thienyl spacers (M3 and M4) show two successive reversible one‐electron oxidation waves, indicating electronic coupling between the two ferrocenyl (Fc) termini. The potential difference (ΔE1/2) between the two redox centers is 160 mV and 130 mV, respectively, with the corresponding comproportionation constants (Kc) of 6.2 × 102 and 1.5 × 102. Upon mono‐oxidation of M3 by [Cp2Fe][BF4], a broad and weak intervalence charge‐transfer (IVCT) transition is observed in the NIR region of 1200–2200 nm.

Synthesis, Crystal Structures, and Magnetic and Electrochemical Properties of Highly Phenyl Substituted Trinuclear 5,6,11,12,17,18-Hexaazatrinaphthylene (HATNPh6)-Bridged Titanium Complexes.

Trinuclear titanium complexes coordinated by one ligand with three coordination sites have shown properties of mixed valency and a high number of reversible redox steps. Herein we report on the hexaphenyl-substituted derivative (Cp2Ti)3(μ3-HATNPh6) (2). On reaction of 2 with the ferrocenium salt [Cp2Fe]BF4, the cationic complexes [(Cp2Ti)3(μ3-HATNPh6)] n+ ( n = 1-3; 3-5) become available in a selective way. Cyclic voltammograms show 10 reversible redox states of the trinuclear species 2 without decomposition. In order to classify the degree of electronic communication between the titanium centers, comproportionation constants Kc, IVCT bands in NIR spectra, and magnetic measurements were analyzed. These parameters show strong coupling effects between the titanium centers but no full delocalization. In addition, single-crystal X-ray analysis of the neutral complex 2 and its oxidation products (1+ (3), 2+ (4), and 3+ (5)) revealed the geometric structure of the molecule in the solid state. For the cationic species anion-π interactions between the electron-deficient central ring of the HATNPh6 ligand and BF4- counterions were found.

Mixed Donor–Acceptor‐Derived N,N′‐Diarylpyrazine‐2,5‐dicarboxamido‐Bridged Diruthenium Systems: Structures, Magnetic Properties, and Electronic Forms in Multiredox States

Herein, we report diruthenium(III) complexes bridged by the deprotonated N,N′‐diphenylpyrazine‐2,5‐dicarboxamide (H2L1) and N,N′‐dimesitylpyrazine‐2,5‐dicarboxamide (H2L2) ligands in meso diastereomeric form, namely, (acac)2RuIII(µ‐L1)RuIII(acac)2 (1, acac = acetylacetonato) and (acac)2RuIII(µ‐L2)RuIII(acac)2 (2), as well as the monodeprotonated HL1‐derived mononuclear counterpart (acac)2RuIII(HL1) (3). Variable‐temperature magnetic studies of 1 revealed a weak bridge‐mediated intramolecular antiferromagnetic interaction with J = –7.58 cm–1. The dinuclear complexes exhibited anisotropic electron paramagnetic resonance (EPR) signals at g ≈ 2 along with half‐field signals at g1/2 ≈ 4, whereas the mononuclear counterpart 3 displayed an EPR signal with rhombic symmetry. Complexes 1 and 2 exhibited two oxidation and reduction steps with comproportionation constants (Kc) of 102 and 106 or 107. Complex 3 displayed one oxidation and two reductions steps. The electrogenerated compounds 1– and 2– exhibited metal‐based EPR signals and a very weak and broad near‐infrared (NIR) transition at λ ≈ 1200 nm. The C=O bands of the bridge were monitored through IR spectroelectrochemistry, which revealed a shift of these bands to lower energies on reduction. However, the native and reduced states have the same number of bands, which suggests a similar local symmetry for both redox forms and a delocalized electronic situation in the reduced compounds. A combination of electrochemistry and spectroelectrochemistry was used to probe the electronic structures of the various redox states in these complexes.

Ruthenium‐Carbon(Aryl) Bond Cleavage and Change in the Ligand Coordination Mode in a Four‐Membered Ortho‐Metalated Ruthenium(II) Organometallics Promoted by Thiolato Ligands

AbstractThe reaction of Ru(η2‐RL)(PPh3)2(CO)Cl [η2‐RL is C6H2O‐2‐CHNHC6H4R(p)‐3‐Me‐5 and R=Me, OMe, Cl] with excess sodium p‐chlorothiophenolate (p‐ClC6H4SNa) in dichloromethane–tetrahydrofuran medium afforded the binuclear complexes of the type [Ru(PPh3)(η2‐RL)(CO)(p‐ClC6H4S)]2 [η2‐RL is C6H3O‐2‐CHNC6H4R(p)‐3‐Me‐5] in excellent yield. The binding of the thiolato ligands is attended with the cleavage of the Ru–C(aryl) and Ru–Cl bonds in Ru(η2‐RL)(PPh3)2(CO)Cl and the RL ligands are now coordinated with the metals in [Ru(PPh3)(η2‐RL)(CO)(p‐ClC6H4S)]2 via the imine nitrogen and the phenolato oxygen atoms. The CO ligand in [Ru(PPh3)(η2‐RL)(CO)(p‐ClC6H4S)]2 remains trans to the phenolato function as in Ru(η2‐RL)(PPh3)2(CO)Cl. The spectral (UV‐vis, IR, NMR, mass) and electrochemical data of the complexes are included. In dichloromethane solution the complexes display two successive one‐electron oxidation waves. The comproportionation constants (KC) as calculated by the ▵E1/2 values of the complexes are in the order of 105 indicating that the two metal centers in [Ru(PPh3)(η2‐RL)(CO)(p‐ClC6H4S)]2 are only weakly coupled by the bridging thiolato liagnds. Structure determinations of [Ru(PPh3)(η2‐RL)(CO)(p‐ClC6H4S)]2 (R=Me, OMe) have revealed a distorted octahedral RuCONPS2 coordination sphere with the pairs (P, S), (C, O), and (N, S) defining the three trans directions. The Ru⋅⋅⋅⋅Ru distances in the complexes are clearly outside of the range for a Ru‐Ru single bond. The electronic structures and the absorption spectra of the complexes are also scrutinized by the density functional theory (DFT) and Time‐dependent DFT analysis.

Polyelectrochromism and electronic coupling in vinylruthenium-modified carbazoles

Six carbazole-derived mono-, di- and trinuclear ruthenium complexes featuring one to three square pyramidal, five-coordinated {Ru(CO)Cl(PiPr3)2(CHCH-)} entities were prepared by the regio- and stereospecific insertion of the hydride precursor HRu(CO)Cl(PiPr3)2 into the CC bond of the corresponding ethynylated N-substituted carbazoles. They were characterized by IR, UV/Vis, and NMR spectroscopy, electrochemistry, and, in up to four different oxidation states, by UV/Vis/NIR and IR spectroelectrochemistry. Their radical cations were additionally investigated by EPR spectroscopy. Experimental studies are complemented by (TD-)DFT calculations. Electrochemical studies on these complexes showed two to four reversible, consecutive one-electron oxidation processes. Half-wave potential separations ΔE½ between individual redox-waves and the derived comproportionation constants Kc indicate that intermediate oxidation states are sufficiently stable with respect to disproportionation to be generated and investigated in solution. Spectroscopic data indicate strong carbazole ligand contributions to every oxidation and notable involvement of the vinylruthenium moieties to the lower oxidation steps. UV/Vis/NIR spectroelectrochemistry reveals a polyelectrochomic behavior of all complexes with, however, less pronounced absorptivities when compared to the simple triarylamine counterparts. The pattern of two separate, moderately spaced Ru(CO) bands for the MV radical cations IIa+, IIb+ and III+ indicates that the unipositive charge is unevenly distributed over the two chemically equivalent vinylruthenium subunits.

Effects of electron transfer on the stability of hydrogen bonds.

The measurement of the dimerization constants of hydrogen-bonded ruthenium complexes (12, 22, 32) linked by a self-complementary pair of 4-pyridylcarboxylic acid ligands in different redox states is reported. Using a combination of FTIR and UV/vis/NIR spectroscopies, the dimerization constants (KD) of the isovalent, neutral states, 12, 22, 32, were found to range from 75 to 130 M-1 (ΔG0 = -2.56 to -2.88 kcal mol-1), while the dimerization constants (K2-) of the isovalent, doubly-reduced states, (12)2-, (22)2-, (32)2-, were found to range from 2000 to 2500 M-1 (ΔG0 = -4.5 to -4.63 kcal mol-1). From the aforementioned values and the comproportionation constant for the mixed-valent dimers, the dimerization constants (KMV) of the mixed-valent, hydrogen-bonded dimers, (12)-, (22)-, (32)-, were found to range from 0.5 × 106 to 1.2 × 106 M-1 (ΔG0 = -7.78 to -8.31 kcal mol-1). On average, the hydrogen-bonded, mixed-valent states are stabilized by -5.27 (0.04) kcal mol-1 relative to the isovalent, neutral, hydrogen-bonded dimers and -3.47 (0.06) kcal mol-1 relative to the isovalent, dianionic hydrogen bonded dimers. Electron exchange in the mixed valence states imparts significant stability to hydrogen bonding. This is the first quantitative measurement of the strength of hydrogen bonds in the presence and absence of electronic exchange.

Inter-ligand electronic coupling mediated through a dimetal bridge: dependence on metal ions and ancillary ligands.

A series of Mo2, Ru2, Rh2 and Cu2 complexes with redox-active NP-R [2-(2-R)-1,8-naphthyridine; R = pyrazinyl (NP-pz, L1) and thiazolyl (NP-tz, L2)] ligands have been synthesized and characterized by X-ray crystallography and spectroscopic methods. Two NP-R ligands wrap the dimetal core by occupying four equatorial positions and two axial sites. The remaining four equatorial sites are engaged by bridging acetates in quadruply bonded cis-[Mo2(L1)2(OAc)2][BF4]2 (1), cis-[Mo2(L2)2(OAc)2][BF4]2 (1A), doubly bonded cis-[Ru2(L1)2(OAc)2][ClO4]2 (3), cis-[Ru2(L2)2(OAc)2][ClO4]2 (3A) and singly bonded trans-[Rh2(L1)2(OAc)2][BF4]2 (5) and trans-[Rh2(L2)2(OAc)2][BF4]2 (5A). Compounds cis-[Mo2(L1)2(CH3CN)4][BF4]4 (2), cis-[Mo2(L2)2(CH3CN)4][BF4]4 (2A), cis-[Ru2(L1)2(CO)4][OTf]2 (4) and cis-[Ru2(L2)2(CO)4][ClO4]2 (4A) contain acetonitriles or carbonyls as the ancillary ligands. The dicopper complexes trans-[Cu2(CH3CN)(L1)2][ClO4]2 (6) and trans-[Cu2(L2)2(ClO4)2] (6A) involve no bonding interaction between two Cu(i) units. Cyclic voltammogram studies reveal that two one-electron processes corresponding to each of the two ligands bound to the metal-metal bonded dimetal core result in four reversible one-electron reductions, with the exception of dirhodium(ii,ii) compounds 5 and 5A which show two one-electron reductions. The highest comproportionation constant (Kc) values are obtained for inter-valence complexes originating from the diruthenium(ii,ii) compounds 3 and 3A, whereas no electron delocalization is observed for dicopper(i,i) complexes 6 and 6A. The dimetal bridge and the ancillary ligands tune the degree of inter-ligand electronic coupling in these complexes. DFT calculations reveal a π*(NP)-δ*(M2)-π*(NP) orbital conduit for electron delocalization. For diruthenium(ii,ii) compounds 3 and 3A, an additional π*(NP)-π*(M2)-π*(NP) pathway is accessible contributing to high Kc values. The ancillary π-ligands (acetates and carbonyls) reduce the extent of the electron flow through π*(NP)-δ*(M2)-π*(NP) and thus lower the Kc values. The absence of metal-metal bond orbitals and the reduced metal-ligand covalency in dicopper(i,i) compounds are responsible for the lack of electron delocalization in these systems.

A Dicobalt Complex with an Unsymmetrical Quinonoid Bridge Isolated in Three Units of Charge: A Combined Structural, (Spectro)electrochemical, Magnetic and Spectroscopic Study.

Quinonoid ligands are excellent bridges for generating redox-rich dinuclear assemblies. A large majority of these bridges are symmetrically substituted, with examples of unsymmetrically substituted quinonoid bridges being extremely rare. We present here a dicobalt complex in its various redox states with an unsymmetrically substituted quinonoid bridging ligand. Two homovalent forms and one mixed-valent form have been isolated and characterized by single crystal X-ray diffraction. The complex displays a large comproportionation constant for the mixed-valent state which is three orders of magnitude higher than that observed for the analogous complex with a symmetrically substituted bridge. Results from electrochemistry, UV/Vis/NIR spectroelectrochemistry, SQUID magnetometry, multi-frequency EPR spectroscopy and FIR spectroscopy are used to probe the electronic structures of these complexes. FIR provides direct evidence of exchange coupling. The results presented here display the advantages of using an unsymmetrically substituted bridge: site specific redox chemistry, high thermodynamic stabilization of the mixed-valent form, isolation and crystallization of various redox forms of the complex. This work represents an important step on the way to generating heterodinuclear complexes for use in cooperative catalysis.

Unsymmetric (μ-oxido)/(μ-pyrazolato) and Symmetric (μ-pyrazolato)2 Bridged Diosmium Frameworks: Electronic Structure and Magnetic Properties.

The present article deals with the structurally characterized unsymmetric oxido/pyrazolato-bridged [(bpy)2Os(III)(μ-oxido)(μ-pz)Os(III)(bpy)2](ClO4)3 ([1](ClO4)3) and symmetric dipyrazolato-bridged [(bpy)2Os(II)(μ-pz)2Os(II)(bpy)2](ClO4)2 ([2](ClO4)2) (pz = pyrazolato, bpy = 2,2'-bipyridine) complexes with the Os···Os separations of 3.484 and 4.172 Å, respectively. The anti-ferromagnetically coupled Os(III) centers [E(S = 1)-E(BS(1,1) S = 0) = 322.504 cm(-1)] in 1(3+) and diamagnetic (S = 0) 2(2+) exhibit well-resolved (1)H NMR resonances. [1](ClO4)3 shows temperature- and magnetic field-dependent paramagnetism at low magnetic field and diamagnetism at high magnetic field. 1(3+) and 2(2+) display successive metal-based oxidation processes involving the intermediate mixed-valent states and isovalent congeners: Os(IV)Os(IV) (1(5+))→Os(III)Os(IV) (1(4+))⇌Os(III)Os(III) (1(3+))⇌Os(III)Os(II) (1(2+)) and Os(III)Os(III) (2(4+))→Os(II)Os(III) (2(3+))⇌Os(II)Os(II) (2(2+)) as well as bpy-centered reductions. The effect of π donor O(2-) and σ/π-donating pz(-) in 1(3+) and 2(2+), respectively, leads to varying oxidation state of the metal ions in the isolated complexes: Os(III)Os(III) versus Os(II)Os(II). UV-visible-near-IR-electron paramagnetic resonance spectro-electrochemistry and density functional theory (DFT)/time-dependent DFT calculations collectively reveal overlapping of the metal- and ligand (pz, O, bpy)-based frontier orbitals in the delocalized mixed-valent states in 1(4+) and 1(2+) with comproportionation constant (Kc) value > 1 × 10(14) as well as in isovalent 1(3+), resulting in mixed metal/ligand to metal/ligand near-IR transitions in all the three states. The mixed-valent Os(II)Os(III) state in 2(3+) exhibits high Kc value of 1 × 10(22) corresponding to a strong electrochemical coupling situation. However, closeness of the bandwidth (Δν1/2, 4861 cm(-1)) of broad and weak intervalence charge transfer transition of 2(3+) at 1360 nm (ε/M(-1) cm(-1): 490) with the calculated Δν1/2 of 4121 cm(-1) based on the Hush formula as well as spin-density distributions of Os1: 0.811/0.799, Os2: 0.045/0042, and pz: 0.162/0.173 in meso and rac diastereomeric forms, respectively, attribute its localized class II state.

The Electron-Rich {Ru(acac)2} Directed Varying Configuration of the Deprotonated Indigo and Evidence for Its Bidirectional Noninnocence.

This article highlights the hitherto unexplored varying binding modes of the deprotonated natural dye indigo (H2L) and its bidirectional noninnocent potential. The reaction of H2L with the selective metal precursor Ru(II)(acac)2(CH3CN)2 (acac(-) = acetylacetonate) leads to the simultaneous formation of paramagnetic Ru(III)(acac)2(HL(-)) (1; blue solid) and diamagnetic Ru(II)(acac)2(L) (2; red solid), which have been characterized by standard analytical, spectroscopic, and structural analysis. Crystal structures establish that the usual trans configurated and twisted monodeprotonated HL(-) and unprecedented cis configurated nearly planar dehydroindigo (L) bind to the {Ru(acac)2} metal fragment via the N(-),O and N,N donors, forming six- and five-membered chelates, respectively. It also reveals the existence of intramolecular N-H···O hydrogen-bonding interaction between the NH proton and C═O group at the back face of the coordinated HL(-), in addition to an intermolecular N-H···O hydrogen bonding between the NH proton of HL(-) of Molecule B and oxygen atom of the nearby acac of the second molecule (Molecule A) in the asymmetric unit of 1. The specific role of the electron-rich {Ru(acac)2} metal fragment in stabilizing the cis-configuration of the electron-deficient L in 2 has been pointed out. Both 1 and 2 exhibit reversible one-electron oxidation and successive three reductions with varying Kc (comproportionation constant) values in the range of 10(18)-10(6). The potentials for the redox processes of 2 are positively shifted with respect to those of 1. The involvement of the metal or HL(-)/L or mixed metal-HL(-)/L-based orbitals in the accessible redox processes of 1(n) and 2(n) has been analyzed by spectroelectrochemistry, EPR at the paramagnetic states, and DFT calculated MO compositions/spin density distributions. The collective consideration of the experimental results and DFT/TD-DFT data has ascertained the participation of both the metal fragment {Ru(acac)2} and the HL(-)/L in the redox processes, which in effect result in mixed electronic structural forms of 1(n) and 2(n) (n = +1, 0, -1, -2, -3).

Fullerene–Pyrrolidine Biferrocene Donor–Acceptor Triads: Synthesis and Effects of Fullerene[60] on the Electronic Communication of the Two Ferrocene Units in Biferrocene

A novel nanoscale molecule, fullerene[60]-based biferrocene (C60-ph-Fc-Fc), was prepared via covalent grafting of biferrocene onto C60 using 1,3-dipolar cycloaddition and was fully characterized by mass spectrometry, nuclear magnetic resonance spectroscopy, infrared spectroscopy, ultraviolet-visible spectroscopy, and fluorescence spectroscopy. Cyclic voltammetry studies showed that C60-ph-Fc-Fc undergoes four successive quasi-reversible processes in the potential window of –2.0~0.40 V (versus Fc+/Fc). The biferrocene (moiety) exhibited significant electronic communication between the two ferrocenyl groups both before and after grafting onto C60. Grafting of fullerene led to weakening in the electronic interaction, and the comproportionation constant decreased by nearly five orders of magnitude from 1.56 × 106 for Fc-Fc to 88 for C60-ph-Fc-Fc, indicating that the electronic communication between halves of the biferrocene could be effectively modified by grafting of C60. The temperature-dependent magnetic susceptibility (2–300 K) study showed that there was a strong antiferromagnetic interaction between the ferrocene FeII centres.

Ligand Field Strength Mediates Electron Delocalization in Octahedral [((H)L)2Fe6(L')m](n+) Clusters.

To assess the impact of terminal ligand binding on a variety of cluster properties (redox delocalization, ground-state stabilization, and breadth of redox state accessibility), we prepared three electron-transfer series based on the hexanuclear iron cluster [((H)L)2Fe6(L')m](n+) in which the terminal ligand field strength was modulated from weak to strong (L' = DMF, MeCN, CN). The extent of intracore M-M interactions is gauged by M-M distances, spin ground state persistence, and preference for mixed-valence states as determined by electrochemical comproportionation constants. Coordination of DMF to the [((H)L)2Fe6] core leads to weaker Fe-Fe interactions, as manifested by the observation of ground states populated only at lower temperatures (<100 K) and by the greater evidence of valence trapping within the mixed-valence states. Comproportionation constants determined electrochemically (Kc = 10(4)-10(8)) indicate that the redox series exhibits electronic delocalization (class II-III), yet no intervalence charge transfer (IVCT) bands are observable in the near-IR spectra. Ligation of the stronger σ donor acetonitrile results in stabilization of spin ground states to higher temperatures (∼300 K) and a high degree of valence delocalization (Kc = 10(2)-10(8)) with observable IVCT bands. Finally, the anionic cyanide-bound series reveals the highest degree of valence delocalization with the most intense IVCT bands (Kc = 10(12)-10(20)) and spin ground state population beyond room temperature. Across the series, at a given formal oxidation level, the capping ligand on the hexairon cluster dictates the overall properties of the aggregate, modulating the redox delocalization and the persistence of the intracore coupling of the metal sites.