Mixed-valent Intermediates Research Articles

Electrostatic vs. electronic interactions within oxidized multinuclear Pt(bipyridine)(dithiolene) complexes.

Bi- and trimetallic platinum(bipyridine)(dithiolene) complexes involving different organic linkers between the redox active platinum(dithiolene) moieties have been synthesized and studied by electrochemistry and spectroelectrochemistry. Cyclic voltammetry experiments carried out on these multinuclear complexes in dichloromethane using [NBu4][PF6] as the supporting salt show only one oxidation process involving the metallacycles. Nevertheless, spectro-electrochemical studies, carried out under the same conditions, show a different evolution of the spectra during oxidation depending on the position of the metallacycles on the phenyl ring. For instance, only the 1,3-disubtituted (Pt21,3-P) and 1,3,5-trisubstituted (Pt31,3,5-P) complexes show the growth of an absorption band in the NIR region upon oxidation while it was not observed for the other complexes. In contrast, electrochemical studies carried out using the poorly coordinating supporting electrolyte, [Na][B(C6H4(CF3)2)4], indicate sequential oxidation of the redox centers with ΔE values varying according to the nature of the bridge and the distance between metal centers. For all the investigated multinuclear complexes, spectro-electrochemical experiments performed in the presence of [Na][B(C6H4(CF3)2)4] show an absorption band in the NIR region consistent with appreciable electrostatic effects and charge delocalization in the mixed valent intermediates.

Stepwise Construction of the CrCr Quintuple Bond and Its Destruction upon Axial Coordination

Give me five! Terdentate 2,6-diamidopyridyl ligands were used to stabilize the Cr-Cr quintuple bond and have made it possible to isolate and characterize not only the Cr-Cr quintuple-bonded complex, but also the mixed-valent intermediates (Cr(I) and Cr(II)), which are important species in the formation of type I quintuple-bonded complexes.

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(Ru(II)) → π*(pyrazine, tppz) back-bonding. The widely separated one-electron oxidation steps, Ru(II)Ru(II)/Ru(II)Ru(III) and Ru(II)Ru(III)/Ru(III)Ru(III), result in large comproportionation constants (K(c)) of ≥10(10) for the mixed-valent intermediates. The syn-configurated (n) exhibits a particularly high K(c) of 10(12) for n = 1+, accompanied by density functional theory (DFT)-calculated minimum Ru-N bond lengths for this Ru(II)Ru(III) intermediate. The electrogenerated mixed-valent states 1(+)-5(+) exhibit anisotropic EPR spectra at 110 K with average values <g> of 2.304-2.234 and g anisotropies Δg = g(1)-g(3) 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 dm(3) mol(-1) cm(-1), Δν(1/2) ≈ 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 (Ru(2.5))(2). Rather large spin densities of about -0.4 were calculated for the tppz bridges in 1(+) and 3(+). The calculated electronic interaction values (V(AB)) 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 Ru(III)Ru(III) forms in 1(2+)-5(2+) prefer a triplet (S = 1) ground state with ΔE (S = 0 - S = 1) ∼5000 cm(-1). One-electron reduction takes place at the tppz bridge which results in species [Cl(L)Ru(II)(μ-tppz˙(-))Ru(II)(L)Cl](-) (1˙(-)-3˙(-), 5˙(-)) which exhibit free radical-type EPR signals and NIR transitions typical of the tppz radical anion. The system 4(n) is distinguished by lability of the Ru-Cl bonds.

Formation and mixed-valent behaviour of a substituted tetraferrocenylstannane

A tetrasubstituted tetraferrocenylstannane is formed from 1-bromoferrocene-1'-carboxylic acid methyl ester and copper bronze. The molecular structure is almost perfectly tetrahedral with FeFe distances of around 6 A. In solution two sequential one-electron processes and one two-electron process are indicative of mixed-valent intermediates. Intermetallic interactions have been probed by preparative oxidation, paramagnetic NMR spectroscopy, Mössbauer spectroscopy, UV/Vis/NIR spectroscopy and DFT calculations.

Ground and Excited Electronic States of Quininone-Containing Re(I)-Based Rectangles: a Comprehensive Study of Their Preparation, Electrochemistry, and Photophysics

The self-assembly of two rectangular compounds [{(CO)(3)Re(mu-QL)Re(CO)(3)}(2)(mu-bpy)(2)] (1, QL = 6,7-dimethyl 1,4-dioxido-9,10-anthraquinone (QL(1)); 2, QL = 1,4-dioxido-9,10-anthraquinone (QL(2)), bpy = 4,4'-bipyridine) via an orthogonal-bonding approach was achieved in high yields. Their structures were characterized by single-crystal X-ray diffraction analysis. The rectangles exhibited multielectron-redox properties. The introduction of a bridging quininone moiety made notable changes in two well-separated single-electron reductions of the bpy moiety, as compared with other 2,2'-bisbenzimidazolate (BiBzlm) or thiolate- or alkoxide-bridged rectangles, followed by quasi-reversible reduction of the quininone moiety to allow the existence of different redox states. Electrochemical assessment using cyclic voltammetry and UV-vis-NIR spectroelectrochemistry revealed reversibly accessible 0, 1-, and 2- redox states. The comproportionation constant of the successive reduction processes was K(c) = 4.18 x 10(8) for complex 1 and 4.08 x 10(8) for 2. In spite of the high K(c) values, no obvious intervalence charge transfer bands were detected in either the vis, NIR, or IR regions, suggesting very weak electronic coupling between the ligand centers in the mixed-valent intermediates. In the mixed-valent intermediate, the overlap between donor and acceptor orbitals of the two bpy ligands engendered weak electronic coupling associated with distance that exceeded van der Waals ligand/ligand distances and created a class I fully isolated, non-interacting, valence-localized situation. Furthermore, unusual ligand-to-metal-to-ligand charge-transfer (LMLCT) transitions of complexes 1 and 2 at 298 K were observed in the visible region. Molecule 2 exhibited multiple emissions from the triplet-centered pi-pi* intraligand ((3)IL), metal-to-ligand charge-transfer ((3)MLCT) and triplet ligand-ligand charge transfer ((3)LLCT) levels and showed biexponential decay. By contrast, in complex 1, (3)IL emission was absent and only single-exponential decay was observed. These results reveal the different nature of the electronically excited states between 1 and 2. The mechanisms of the photophysical deactivation processes in these systems can be explained in terms of the electronic characteristics of the quininone molecules and possible geometrical differences of the excited states involved. In addition, the energies, characteristics, and molecular structures of the ground and lowest triplet excited state were calculated using the density functional theory method.

Pseudo-base formation in the attempted synthesis of a conjugatively coupled bis(nitrosylruthenium) complex and spectroelectrochemistry of bipyrimidine-bridged dinuclear Ru(terpy)X precursor compounds (X = Cl, NO2)

Reaction of {(mu-bpym)[RuCl(terpy)]2}(PF6)2, bpym = 2,2'-bipyrimidine and terpy = 2,2':6',2''-terpyridine, with NaNO2 yields {(mu-bpym)[Ru(NO2)(terpy)]2}(PF6)2. In CH3CN/0.1 M Bu4NPF6 both dinuclear complexes can undergo two reversible bpym-centered one-electron reduction processes and two metal-centered one-electron oxidation steps, the latter involving mixed-valent intermediates with weak intermetallic coupling. Acidification of {(mu-bpym)[Ru(NO2)(terpy)]2}(PF6)2 does not lead to the expected {(mu-bpym)[Ru(NO)(terpy)]2}6+ but, probably because of the high charge, to the insoluble but structurally and IR-spectroscopically characterised pseudo-base product syn-{(mu-bpym-(4-OH))[Ru(NO)(terpy)]2}(PF6)5. The addition of one hydroxide to one of the 4-positions of bis-chelating bpym interrupts the aromatic pi conjugation and is accompanied by corresponding intra-pyrimidine bond length variations, however, the effect on the electronic interaction of the two different syn positioned {RuNO}6 moieties remains small, possibly due to their situation within the central molecular pi plane.

Multiple one-electron oxidation and reduction of trinuclear bis(2,4-pentanedionato)ruthenium complexes with substituted diquinoxalino[2,3- a:2′,3′- c]phenazine ligands

The complexes (μ 3-L 1/L 2)[Ru(acac) 2] 3, acac − = 2,4-pentanedionato, L 1 = 2,3,8,9,14,15-hexachlorodiquinoxalino[2,3- a:2′,3′- c]phenazine and L 2 = 2,3,8,9,14,15- hexamethyldiquinoxalino[2,3- a:2′,3′- c]phenazine, undergo stepwise one-electron oxidation involving a total of three electrons and stepwise one-electron reduction with three (L 2) or four electrons (L 1). All reversibly accessible states were characterized by UV–Vis–NIR spectroelectrochemistry. Oxidation leads to mixed-valent intermediates {(μ 3-L)[Ru(acac) 2] 3} + and {(μ 3-L)[Ru(acac) 2] 3} 2+ of which the Ru IIIRu IIRu II combinations exhibit higher comproportionation constants K c than the Ru IIIRu IIIRu II states – in contrast to a previous report for the unsubstituted parent systems {(μ 3-L 3)[Ru(acac) 2] 3} +/2+, L 3 = diquinoxalino[2,3- a:2′,3′- c]phenazine. No conspicuous inter-valence charge transfer absorptions were observed for the mixed-valent intermediates in the visible to near-infrared regions. The monocations and monoanions were characterized by EPR spectroscopy, revealing rhombic ruthenium(III) type signals for the former. Electron addition produces ruthenium(II) complexes of the reduced forms of the ligands L, a high resolution EPR spectrum with 14N and 35,37Cl hyperfine coupling and negligible g anisotropy was found for {(μ 3-L 1)[Ru(acac) 2] 3} −. DFT calculations of (μ 3-L 1)[Ru(acac) 2] 3 confirm several ligand-centered low-lying unoccupied MOs for reduction and several metal-based high-lying occupied MOs for electron withdrawal, resulting in low-energy metal-to-ligand charge transfer (MLCT) transitions.

Mixed valence aspects of diruthenium complexes [((L)ClRu)2(mu-tppz)]n+ incorporating 2-(2-pyridyl)azoles (L) as ancillary functions and 2,3,5,6-tetrakis(2-pyridyl)pyrazine (Tppz) as bis-tridentate bridging ligand.

Tppz [2,3,5,6-tetrakis(2-pyridyl)pyrazine]-bridged complexes [((L)ClRu)(2)(mu-tppz)]n+ with structurally similar but electronically different ancillary ligands, 2-(2-pyridyl)azoles (L), were synthesized as diruthenium(II) species. Cyclic voltammetry, EPR of paramagnetic states, and UV-vis-NIR spectroelectrochemistry show that the first two reduction processes occur at the tppz bridge and that oxidation involves mainly the metal centers. The mixed valent intermediates from one-electron oxidation exhibit moderate comproportionation constants 10(4) < K(c) < 10(5) but appear to be valence-averaged according to the Hush criterion. Redox potentials, EPR, and UV-vis-NIR results show the effect of increasing donor strength of the ancillary ligands along the sequence L(1) < L(2) < L(4) << L(3), L(1) = 2-(2-pyridyl)benzoxazole, L(2) = 2-(2-pyridyl)benzthiazole, L(3) = 2-(2-pyridyl)benzimidazolate, L(4) = 1-methyl-2-(2-pyridyl)-1H-benzimidazole. Whereas the mixed valent complexes with L(1) and L(2) remain EPR silent at 4 K, the analogues with L(4) and L(3) exhibit typical ruthenium(III) EPR signals, albeit with some noticeable ligand contribution in the case of the L(3)-containing complex. Intervalence charge transfer (IVCT) bands were found in the visible spectrum for the complex with L(3) but in the near-infrared range (at ca. 1500 nm) for the other systems.

The triruthenium complex [[(acac)2Ru(II)]3(L)] containing a conjugated diquinoxaline[2,3-a:2',3'-c]phenazine (L) bridge and acetylacetonate (acac) as ancillary ligands. Synthesis, spectroelectrochemical and EPR investigation.

The compound [[(acac)2Ru]3(L)] (1) undergoes three well-separated one-electron oxidation and reduction processes. The EPR results indicate electron removal from the ruthenium(II) centres on oxidation and the occupation of a largely L-based molecular orbital on reduction. In spite of well-separated (DeltaE > or = 340 mV) oxidation no obvious intervalence charge transfer bands were detected in the Vis, NIR or IR regions, suggesting very weak electronic coupling between the metal centres in the mixed-valent intermediates 1+ and 1(2+). The separated (DeltaE > or = 540 mV) stepwise reduction produces weak near-infrared features associated with partially occupied pi* orbitals of L, the unusually high g anisotropy in the EPR spectrum of 1- is attributed to the occupation of a degenerate MO by the unpaired electron.