Sensitive Valence Structures of [(pap)2Ru(Q)]n (n = +2, +1, 0, −1, −2) with Two Different Redox Noninnocent Ligands, Q = 3,5-Di-tert-butyl-N-aryl-1,2-benzoquinonemonoimine and pap = 2-Phenylazopyridine

Bidirectional noninnocence of hinge-like deprotonated bis-lawsone on selective ruthenium platform: a function of varying ancillary ligands

The present work aimed to obtain discrete heavier metal complexes of unperturbed deprotonated bis-lawsone (hinge-like H₂L = 2,2'-bis(3-hydroxy-1,4-napthoquinone). This is primarily due to its limited examples with lighter metal ions (Co, Zn, and Ga) and the fact that our earlier approach with the osmium ion facilitated its functionalisation. Herein, we demonstrated the successful synthesis and structural characterisation of L^2--derived diruthenium [(bpy)₂Ru^II(μ-L^2-)Ru^II(bpy)₂](ClO₄)₂ [1](ClO₄)₂ (S = 0), (acac)₂Ru^III(μ-L^2-)Ru^III(acac)₂2 (S = 1) and monoruthenium (pap)₂Ru(L^2-) 3 (S = 0) derivatives (bpy = 2,2'-bipyridine, acac = acetylacetonate, and pap = 2-phenylazopyridine). The crystal structures established that (i) O,O^-/O,O^- donating five-membered bis-bidentate and O^-,O^- donating seven-membered bidentate chelating modes of deprotonated L^2- in rac (ΔΔ/ΛΛ) diastereomeric [1](ClO₄)₂, 2 and 3, respectively. (ii) The L^2- bridging unit in [1](ClO₄)₂, 2 and 3 underwent twisting its two naphthoquinone rings with respect to the ring connecting C-C bond by 73.01°, 62.15° and 59.12°, respectively. (iii) Intermolecular π-π interactions (∼3.5 Å) between the neighbouring molecules. The paramagnetic complex 2 (S = 1) with two non-interacting Ru(III) (S = 1/2) ions exhibited weak antiferromagnetic coupling only at very low temperatures. In agreement with the magnetic results, 2 displayed typical Ru^III-based anisotropic EPR in CH₃CN (<g>/Δg: 2.314/0.564) but without any forbidden g_1/2 signal at 120 K. The complexes exhibited multiple redox processes in CH₃CN in the experimental potential window of ± 2.0 V versus SCE. The analysis of the redox steps via a combined experimental and theoretical (DFT/TD-DFT) approach revealed the involvement of L^2- to varying extents in both the oxidative and reductive processes as a consequence of its bidirectional redox non-innocent feature. The mixing of the frontier orbitals of the metal ion and L^2- due to their closeness in energy indeed led to the resonating electronic form in certain redox states instead of any precise electronic structural state.

Radical versus Nonradical States of Azobis(benzothiazole) as a Function of Ancillary Ligands on Selective Ruthenium Platforms

The paper deals with the electronic impact of ancillary ligands on the varying redox features of azobis(benzothiazole) (abbt) in the newly introduced mononuclear ruthenium complexes [Ru(pap)₂(abbt)]ⁿ (1ⁿ) and [Ru(bpy)₂(abbt)]ⁿ (2ⁿ), where pap = 2-phenylazopyridine and bpy = 2,2'-bipyridine. In this regard, the complexes [Ru^II(pap)₂(abbt^•-)]ClO₄ ([1]ClO₄), [Ru^II(pap)₂(abbt⁰)](ClO₄)₂ ([1](ClO₄)₂), [Ru^II(bpy)₂(abbt⁰)](ClO₄)₂ ([2](ClO₄)₂), and [Ru^II(bpy)₂(abbt^•-)]ClO₄ ([2]ClO₄) were structurally and spectroscopically characterized. Unambiguous assignments of the aforestated radical and nonradical forms of abbt in 1⁺/2⁺ and 1²⁺/2²⁺, respectively, were made primarily based on their redox-sensitive azo (N═N) bond distances as well as by their characteristic electron paramagnetic resonance (EPR)/NMR signatures. Although the radical form of abbt^•- was isolated as an exclusive product in the case of strongly π-acidic pap-derived 1⁺, the corresponding moderately π-acidic bpy ancillary ligand primarily delivered an oxidized form of abbt⁰ in 2²⁺, along with the radical form in 2⁺ as a minor (<10%) component. The oxidized abbt⁰-derived [1](ClO₄)₂ was, however, obtained via the chemical oxidation of [1]ClO₄. Both 1⁺ and 2²⁺ displayed multiple closed by reversible redox processes (one oxidation O1 and four successive reductions R1-R4) within the potential window of ±2.0 V versus saturated calomel electrode. The involvement of metal-, ligand-, or metal/ligand-based frontier molecular orbitals along the redox chain was assigned based on the combined experimental (structure, EPR, and spectroelectrochemisry) and theoretical [density functional theory (DFT): molecular orbitals, Mulliken spin densities/time-dependent DFT] investigations. It revealed primarily ligand (abbt/pap or bpy)-based redox activities, keeping the metal ion as a simple spectator. Moreover, frontier molecular orbital analysis corroborated the initial isolation of the radical and nonradical species for the pap-derived 1⁺ and bpy-derived 2²⁺ as well as facile reduction of pap and abbt in 1⁺ and 2⁺, respectively.

Variable electronic structure and spin distribution in bis(2,2'-bipyridine)-metal complexes (M = Ru or Os) of 4,5-dioxido- and 4,5-diimido-pyrene

The odd-electron compounds [M(bpy)₂(L¹)](ClO₄) M = Ru ([1](ClO₄)) or Os ([2](ClO₄)), and the even-electron species [M(bpy)₂(H₂L²)](ClO₄)₂, M = Ru ([3](ClO₄)₂) or Os ([4](ClO₄)₂) were obtained from pyrene-4,5-dione, L¹, or 4,5-diaminopyrene, H₄L², and were characterised structurally, electrochemically and spectroscopically. Experimental and computational analysis (TD-DFT) revealed rather different electronic structures and spin distributions of the paramagnetic monocations 1⁺-4⁺. EPR investigations and electronic absorption studies exhibit increasing metal contributions to the singly occupied MO along the series 1⁺ < 3⁺ < 4⁺ < 2⁺, illustrated by g value and long-wavelength absorbance. In addition to variations of the metal (Ru,Os) and the donor atoms (O,NH) the extension of the π system of the semiquinone-type ligand has a large effect on the electronic structure of the paramagnetic cations.

Cis-Bis(2,2′-Azopyridinido)dicarbonylruthenium(II)

An [Ru(apy)2Cl2] precursor (apy = 2,2′-azopyridine) in 2-methoxyethanol was heated under a pressurized CO atmosphere to afford a diradical complex, [Ru(apy·−)2(CO)2], containing one-electron-reduced azo anion radical ligands. The electronic states of the complex were characterized by spectroscopic techniques and computational studies. Magnetic measurements revealed the existence of antiferromagnetic interactions in the diradical complex.

Probing the electronic structure of [Ru(L1)2]Z (z = 0, 1+ and 2+) (H2L1: a tridentate 2-aminophenol derivative) complexes in three ligand redox levels

Aerobic reaction between [RuII(DMSO)4Cl2], a redox-active 2-aminophenol-based ligand (H2L1: 2-[2-(benzylthio)phenylamino]-4,6-di-tert-butylphenol) and Et3N in MeOH under refluxing conditions afforded a purple complex [Ru(L1)2] (S = 0). Structural analysis reveals that the tridentate ligand coordinates in a mer conformation providing a distorted octahedral RuN2O2S2 coordination. Cyclic voltammetry on 1 in CH2Cl2 reveals the accessability of the monocation, dication and monoanion forms. Reddish purple monocation [Ru(L1)2](PF6)·CH2Cl2 ([1OX1](PF6)·CH2Cl2; S = 1/2) and green dication [Ru(L1)2](BF4)2·H2O ([1OX2](BF4)2·H2O; S = 0) have been isolated through the chemical oxidation of 1 in CH2Cl2 by [FeIII(η5-C5H5)2](PF6) and AgBF4, respectively. A structural analysis of the single crystals of the monocation and the dication with the compositions [1OX1](PF6)·CH2Cl2·H2O (2) and [1OX2](BF4)2·1.7H2O (3), respectively, has been done. Metrical (metal-ligand and ligand backbone) parameters, values of metrical oxidation states of coordinated ligands, 1H NMR spectra of 1 and [1OX2](BF4)2·H2O, EPR spectra of [1OX1](PF6)·CH2Cl2, X-ray photoelectron and UV-VIS-NIR spectra of 1-3, spin population analysis from broken-symmetry (BS) density functional theory (DFT) calculations and quasi-restricted orbital (QRO) analysis have allowed us to assign the electronic structure of the complexes. The complexes exhibit highly covalent metal-ligand interactions. The electronic states of 1, [1OX1]1+ and [1OX2]2+ are best described as [RuII{(LISQ)˙-}2] ↔ [RuIII{(LAP)2-}{(LISQ)˙-}] (S = 0), [RuIII{(LISQ)˙-}2]1+ (S = 1/2) and [RuII{(LIBQ)0}2]2+ ↔ [RuIII{(LISQ)˙-}{(LIBQ)0}]2+ (S = 0), respectively. Notably, all redox processes are ligand-centred. Absorption spectral properties have been rationalized based on time-dependent (TD)-DFT calculations. For 1, the appearance of an IVCT band at 1100 nm supports its Class II-III (borderline) ligand-based mixed-valence character.

Assigning Ligand Redox Levels in Complexes of 2-Aminophenolates: Structural Signatures

The purpose of this Viewpoint is to provide a broad-ranging update of advances in the coordination chemistry of redox-active (noninnocent) 2-aminophenolates, with emphasis on two ligand backbone structural parameters, the average of C-O and C-N (C-O/N) bond distances and Δa values, signifying the degree of bond-length alternation in the six-membered ring, in order to identify the redox level of the coordinated ligands. In the absence of magnetic, spectroscopic, and redox results, it has been established that it is possible to assign the electronic ground state of a coordination complex of 2-aminophenolates with consideration of charge, metal-ligand bond distances, and two very promising ligand backbone structural parameters. From a closer look at the sensitive ligand backbone metrical parameters of a diversified group of about 120 transition-metal complexes, a few very useful generalizations have been made.

Polyaromatic hydrocarbon derivatized azo-oximes of cobalt(iii) for the ligand-redox controlled electrocatalytic oxygen reduction reaction

Two new polyaromatic hydrocarbon (PAH) derivatized cobalt(iii) azo-oxime complexes were synthesized and their activity in electrocatalytic oxygen reduction reaction (ORR) were explored.

Effects of Ancillary Ligands on Redox and Chemical Properties of Ruthenium Coordinated Azoaromatic Pincer

In this work, the effect of the electronically different ancillary ligands on the overall properties of the Ru^IIL moiety (L = 2,6-bis(phenylazo)pyridine) in heteroleptic complexes of general formula [RuLQCl]^0/+ was investigated. Four different ancillary ligands (Q) with different electronic effects were used to prepare the heteroleptic compounds from the precursor complex, [RuL(CH₃CN)Cl₂] (1); Q = pcp: 2-(4-chloro-phenylazo)pyridine (strong π-acceptor), [2]⁺; bpy: 2,2'-bipyridyl (moderate π-acceptor), [3]⁺; acac^-: acetylacetonate (strong σ-donor), 4; and DTBCat^2-: 3,5-di- tert-butyl catecholate (strong π-donor), 5. The complexes [2]⁺, [3]⁺, 4, and 5 were fully characterized and structurally identified. The electronic structures of these complexes along with their redox partners were elucidated by using a host of physical measurements: nuclear magnetic resonance, cyclic voltammetry, electronic paramagnetic resonance, UV-vis-NIR spectroscopy, and density functional theory. The studies revealed significant effects of the coligands on azo bond lengths of the RuL moiety and their redox behavior. Aerobic alcohol oxidation reactions using these Ru complexes as catalysts were scrutinized. It was found that the catalytic efficiency is primarily controlled by the electronic effect of the coligand. Accordingly, the complex [2]⁺ (containing a strong π-acceptor coligand, pcp) brings about oxidation efficiently, producing 86% of benzaldehyde. In comparison, however, the complexes 4 and 5 (containing electron donating coligand) furnished only 15-20% of benzaldehyde under identical reaction conditions. Investigations of the reaction mechanism suggest that an unstable Ru-H species is formed, which is transformed to a Ru-hydrazo intermediate by H-walking as reported by Hall et al. ( J. Am. Chem. Soc., 2015, 137, 12330). Aerial O₂ regenerates the catalyst via oxidation of the hydrazo intermediate.

Ambient-Stable Bis-Azoaromatic-Centered Diradical [(L•)M(L•)] Complexes of Rh(III): Synthesis, Structure, Redox, and Spin-Spin Interaction

Bis-azoaromatic electron traps, viz. 2-(2-pyridylazo)azoarene 1, have been synthesized by colligating electron-deficient pyridine and azoarene moieties, and they act as apposite proradical templates for the formation of stable open-shell diradical complexes [(1^•-)Rh^III(1^•-)]⁺ ([2]⁺), starting from the low-valent electron reservoir [Rh^I]. The less stable monoradical [Rh^III(1^•-)Cl₂(PPh₃)₃] (3) has also been isolated as a minor product. These π-radical complexes are multiredox systems, and the electron transfer processes occur exclusively within the pincer-type NNN ligand backbone 1. Molecular and electronic structures of the diradicals and monoradicals have been ascertained with the aid of X-ray diffraction, electrochemical, spectroelectrochemical, and spectral (electronic, IR, NMR, and EPR) studies. In the diradicals [2]⁺, the orthogonal disposition of two ligand π orbitals linked via a closed-shell metal center (t₂⁶) impedes significant coupling between the radicals. Indeed, the observed magnetic moment of [2a]⁺ lies near ∼2.3 μ_B over the temperature range 50-300 K. A very weak antiferromagnetic (AF) intramolecular spin-spin interaction between two ligand π arrays in [(1^•-)Rh^III(1^•-)]⁺ have been found experimentally (J ≈ -5 cm^-1), and this is further substantiated by density functional theory (DFT) calculations at the (U)B3LYP/6-31G(d,p) level.

Luminescent closed shell nickel(ii) pyridyl-azo-oximates and the open shell anion radical congener: molecular and electronic structure, ligand redox behaviour and biological activity

Luminescent nickel(ii) complexes have been synthesized using redox-active azo-oximes. The superior π-acidity of the organic backbone is a key to the isolation of Ni(ii) anion radicals.

Insight into luminescent bisazoaromatic CNN pincer palladacycle: synthesis, structure, electrochemistry and some catalytic applications in C–C coupling

The 2-(phenylazo)azobenzene furnished novel palladacycles in excellent yield, which showed luminescence at rt and catalytic activity. The optoelectronic and electrochemical responses were substantiated with DFT and TDDFT.

Bidirectional non-innocence of the β-diketonato ligand 9-oxidophenalenone (L-) in [Ru([9]aneS3)(L)(dmso)]n, [9]aneS3 = 1,4,7-trithiacyclononane

The new compound [Ru(II)([9]aneS3)(L)(dmso)]ClO4 ([]ClO4) ([9]aneS3 = 1,4,7-trithiacyclononane, HL = 9-hydroxyphenalenone, dmso = dimethylsulfoxide) has been structurally characterised to reveal almost equal C-O bond distances of coordinated L(-), suggesting a delocalised bonding situation of the β-diketonato ligand. The dmso ligand is coordinated via the sulfur atom in the native (1+) and reduced states (1 and 1-) as has been revealed by X-ray crystallography and by DFT calculations. Cyclic voltammetry of 1+ exhibits two close-lying one-electron oxidation waves at 0.77 V and 0.94 V, and two similarly close one-electron reduction processes at -1.43 V and -1.56 V versus SCE in CH2Cl2. The electronic structures of 1n in the accessible redox states have been analysed via experiments (EPR and UV-vis-NIR spectroelectrochemistry) and by DFT/TD-DFT calculations, revealing the potential for bidirectional non-innocent behaviour of coordinated L(˙/-/˙2-). Specifically, the studies establish significant involvement of L based frontier orbitals in both the oxidation and reduction processes: [([9]aneS3)(dmso)Ru(III)-L˙](3+) (1(3+)) ⇌ [([9]aneS3)(dmso)Ru(III)-L(-)](2+)/[([9]aneS3)(dmso)Ru(II)-L˙](2+) (1(2+)) ⇌ [([9]aneS3)(dmso)Ru(II)-L(-)](+) (1(+)) ⇌ [([9]aneS3)(dmso)Ru(II)-L(˙2-)] (1) ⇌ [([9]aneS3)(dmso)Ru(II)-L(3-)](-)/[([9]aneS3)(dmso)Ru(I)-L(˙2-)](-) (1-).

Sensitivity of the valence structure in diruthenium complexes as a function of terminal and bridging ligands

The compounds [(acac)2Ru(III)(μ-H2L(2-))Ru(III)(acac)2] (rac, 1, and meso, 1') and [(bpy)2Ru(II)(μ-H2L(•-))Ru(II)(bpy)2](ClO4)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 μ-H2L(n-) forms two β-ketiminato-type chelate rings, and 1 or 1' are connected via NH···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 (H2L(0) → → → →H2L(4-)) and {(acac)2Ru(II)} → →{(acac)2Ru(IV)} or {(bpy)2Ru(II)} → {(bpy)2Ru(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)2Ru(III)(μ-H2L(•-))Ru(III)(acac)2](+) (1(+)) → [(acac)2Ru(III)(μ-H2L(2-))Ru(III)(acac)2] (1) → [(acac)2Ru(III)(μ-H2L(•3-))Ru(III)(acac)2](-)/[(acac)2Ru(III)(μ-H2L(2-))Ru(II)(acac)2](-) (1(-)) → [(acac)2Ru(III)(μ-H2L(4-))Ru(III)(acac)2](2-)/[(acac)2Ru(II)(μ-H2L(2-))Ru(II)(acac)2](2-) (1(2-)) and [(bpy)2Ru(III)(μ-H2L(•-))Ru(II)(bpy)2](4+) (2(4+)) → [(bpy)2Ru(II)(μ-H2L(•-))Ru(II)(bpy)2](3+)/[(bpy)2Ru(II)(μ-H2L(2-))Ru(III)(bpy)2](3+) (2(3+)) → [(bpy)2Ru(II)(μ-H2L(2-))Ru(II)(bpy)2](2+) (2(2+)). The favoring of Ru(III) by σ-donating acac(-) and of Ru(II) by the π-accepting bpy coligands shifts the conceivable valence alternatives accordingly. Similarly, the introduction of the NH donor function in H2L(n) as compared to O causes a cathodic shift of redox potentials with corresponding consequences for the valence structure.

Experimental and DFT evidence for the fractional non-innocence of a β-diketonate ligand

New compounds [Ru(pap)(2)(L)](ClO(4)), [Ru(pap)(L)(2)], and [Ru(acac)(2)(L)] (pap = 2-phenylazopyridine, L(-) = 9-oxidophenalenone, acac(-) = 2,4-pentanedionate) have been prepared and studied regarding their electron-transfer behavior, both experimentally and by using DFT calculations. [Ru(pap)(2)(L)](ClO(4)) and [Ru(acac)(2)(L)] were characterized by crystal-structure analysis. Spectroelectrochemistry (EPR, UV/Vis/NIR), in conjunction with cyclic voltammetry, showed a wide range of about 2 V for the potential of the Ru(III/II) couple, which was in agreement with the very different characteristics of the strongly π-accepting pap ligand and the σ-donating acac(-) ligand. At the rather high potential of +1.35 V versus SCE, the oxidation of L(-) into L(⋅) could be deduced from the near-IR absorption of [Ru(III)(pap)(L(⋅))(L(-))](2+). Other intense long-wavelength transitions, including LMCT (L(-) → Ru(III)) and LL/CT (pap(⋅-) → L(-)) processes, were confirmed by TD-DFT results. DFT calculations and EPR data for the paramagnetic intermediates allowed us to assess the spin densities, which revealed two cases with considerable contributions from L-radical-involving forms, that is, [Ru(III)(pap(0))(2)(L(-))](2+) ↔ [Ru(II)(pap(0))(2)(L(⋅))](2+) and [Ru(III)(pap(0))(L(-))(2)](+) ↔ [Ru(II)(pap(0))(L(⋅))(L(-))](+). Calculations of electrogenerated complex [Ru(II)(pap(⋅-))(pap(0))(L(-))] displayed considerable negative spin density (-0.188) at the bridging metal.

Recent advances on the chemistry of transition metal complexes of 2-(arylazo)pyridines and its arylamino derivatives

Recent advancement on the redox properties of a selection of transition metal complexes of the azoaromatic ligands: bidentate L(1) [2-(arylazo)pyridine] and tridentate HL(2) [2-(aminoarylphenylazo)pyridine] are described and compared. Due to the presence of a low lying azo-centered π*-orbital, these azoaromatic ligands may exist in multiple valent states. The coordination chemistry of the L(1) ligands was thoroughly studied during the 1980s. These complexes undergo facile reduction in solution at low accessible potentials. One electron reduced azo-complexes, though known for a long time to occur in solution, have only recently been isolated in a crystalline state. New synthetic protocols for the synthesis of metal-bound azo-radical complexes have been developed. Low-valent metal complexes such as metal carbonyls have been found to be excellent starting materials for this purpose. In a few selected cases, syntheses of these complexes were also achieved from very high valent metal oxides using triphenylphosphine as both a reducing and oxo-abstracting agent. Issues related to the ambiguities of the electronic structures in the azo-metal complexes have been discussed considering bond parameters, redox and spectral properties. Unusual redox events such as RIET (Redox-Induced Electron Transfer) phenomena in a few systems have been elaborated and compared with the known example. Novel examples of N=N bond cleavage reactions via four-electron reduction and subsequent C-N bond formation in metal-bound coordinated ligands have been noted.

o-Iminobenzosemiquinonate and o-imino-p-methylbenzosemiquinonate anion radicals coupled VO2+ stabilization

The diamagnetic VO(2+)-iminobenzosemiquinonate anion radical (L(R)(IS)(•-), R = H, Me) complexes, (L(-))(VO(2+))(L(R)(IS)(•-)): (L(1)(-))(VO(2+))(L(H)(IS)(•-))•3/2MeOH (1•3/2MeOH), (L(2)(-))(VO(2+))(L(H)(IS)(•-)) (2), and (L(2)(-))(VO(2+))(L(Me)(IS)(•-))•1/2 L(Me)(AP) (3•1/2 L(Me)(AP)), incorporating tridentate monoanionic NNO-donor ligands {L = L(1)(-) or L(2)(-), L(1)H = (2-[(phenylpyridin-2-yl-methylene)amino]phenol; L(2)H = 1-(2-pyridylazo)-2-naphthol; L(H)(IS)(•-) = o-iminobenzosemiquinonate anion radical; L(Me)(IS)(•-) = o-imino-p-methylbenzosemiquinonate anion radical; and L(Me)(AP) = o-amino-p-methylphenol} have been isolated and characterized by elemental analyses, IR, mass, NMR, and UV-vis spectra, including the single-crystal X-ray structure determinations of 1•3/2MeOH and 3•1/2 L(Me)(AP). Complexes 1•3/2MeOH, 2, and 3•1/2 L(Me)(AP) absorb strongly in the visible region because of intraligand (IL) and ligand-to-metal charge transfers (LMCT). 1•3/2MeOH is luminescent (λ(ext), 333 nm; λ(em), 522, 553 nm) in frozen dichloromethane-toluene glass at 77 K due to π(diimine→)π(diimine)* transition. The V-O(phenolato) (cis to the V═O) lengths, 1.940(2) and 1.984(2) Å, respectively, in 1•3/2MeOH and 3•1/2 L(Me)(AP) are consistent with the VO(2+) description. The V-O(iminosemiquinonate) (trans to the V═O) lengths, 2.1324(19) in 1•3/2MeOH and 2.083(2) Å in 3•1/2 L(Me)(AP), are expectedly ∼0.20 Å longer due to the trans influence of the V═O bond. Because of the stronger affinity of the paramagnetic VO(2+) ion to the L(H)(IS)(•-) or L(Me)(IS)(•-), the V-N(iminosemiquinonate) lengths, 1.908(2) and 1.921(2) Å, respectively, in 1•3/2MeOH and 3•1/2 L(Me)(AP), are unexpectedly shorter. Density functional theory (DFT) calculations using B3LYP, B3PW91, and PBE1PBE functionals on 1 and 2 have established that the closed shell singlet (CSS) solutions (VO(3+)-amidophenolato (L(R)(AP)(2-)) coordination) of these complexes are unstable with respect to triplet perturbations. But BS (1,1) M(s) = 0 (VO(2+)-iminobenzosemiquinonate anion radical (L(R)(IS)(•-)) coordination) solutions of these species are stable and reproduce the experimental bond parameters well. Spin density distributions of one electron oxidized cations are consistent with the [(L(-))(VO(2+))(L(R)(IQ))](+) descriptions [VO(2+)-o-iminobenzoquinone (L(R)(IQ)) coordination], and one electron reduced anions are consistent with the [(L(•2-))(VO(3+))(L(R)(AP)(2-))](-) descriptions [VO(3+)-amidophenolato (L(R)(AP)(2-)) coordination], incorporating the diimine anion radical (L(1)(•2-)) or azo anion radical (L(2)(3-)). Although, cations and anions are not isolable, but electro-and spectro-electrochemical experiments have shown that 3(+) and 3(-) ions are more stable than 1(+), 2(+) and 1(-), 2(-) ions. In all cases, the reductions occur with simultaneous two electron transfer, may be due to formation of coupled diimine/azo anion radical-VO(2+) species as in [(L(•2-))(VO(2+))(L(R)(AP)(2-))](2-).

Structure and ultrafast dynamics of the charge-transfer excited state and redox activity of the ground state of mono- and binuclear platinum(II) diimine catecholate and bis-catecholate complexes: a transient absorption, TRIR, DFT, and electrochemical study

A series of mononuclear complexes of the type [Pt(Bu(2)cat)(4,4'-R(2)-bipy)] [where Bu(2)cat is the dianion of 3,5-(t)Bu(2)-catechol and R = H, (t)Bu, or C(O)NEt(2)] and analogous dinuclear complexes based on the "back-to-back" bis-catechol ligand 3,3',4,4'-tetrahydroxybiphenyl have been studied in detail in both their ground and excited states by a range of physical methods including electrochemistry, UV/vis/near-IR, IR, and electron paramagnetic resonance spectroelectrochemistry, and time-resolved IR (TRIR) and transient absorption (TA) spectroscopy. Density functional theory calculations have been performed to support these studies, which provide a detailed picture of the ground- and excited-state electronic structures, and excited-state dynamics, of these complexes. Notable observations include the following: (i) for the first time, the lowest-energy catecholate → bipyridine (bpy) ligand-to-ligand charge-transfer (LL'CT) excited states of these chromophores have been studied by TRIR spectroscopy, showing a range of transient bands associated with the bpy radical anion and semiquinone species, and back-electron-transfer occurring in hundreds of picoseconds; (ii) strong electronic coupling between the two catecholate units in the bridging ligand of the dinuclear complexes results in a delocalized, planar (class 3) "mixed-valence" catecholate(2-)/semiquinone(•-) state formed by one-electron oxidation of the bridging ligand; (iii) in the LL'CT excited state of the dinuclear complexes, the bridging ligand is symmetrical and delocalized, whereas the bpy radical anion is localized at one terminus of the complex. This study is the first example of an investigation of excited-state behavior in platinum(II) catecholate complexes, performed with the use of picosecond TRIR and femtosecond TA spectroscopy.