Catecholate and semiquinone complexes of vanadium. Factors that direct charge distribution in metal-quinone complexes

Cass, Marion E.; Gordon, Nancy; Pierpont, Cortlandt G.

doi:10.1021/ic00242a027

Cited by 89 publications

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“…The structures of [V IV (Cat) 3 ] 2-, formally isoelectronic with the chromium monoanion, and d 0 [V V (3,5-DBCat) 3 ] -have appeared and clearly show the expected catecholate features for the ligands. 17,18 Similarly, the [M VI (Cl 4 -Cat) 3 ] 2 (M ) Mo, W) dimers have been reported. Lengths between the metals and oxygen atoms of bridging ligands are quite short, due to strong π donation, and yet the ligands retain their catecholate structure.…”

mentioning

confidence: 69%

Redox Isomerism for Quinone Complexes of Chromium and Chromium Oxidation State Assignment from X-ray Absorption Spectroscopy

Pierpont

2001

Inorg. Chem.

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Complexes consisting of redox active metal ions coordinated by one or more ligands that may exist in different charged states offer the potential for redox isomerism. Redox isomers differ in the distribution of charge between electronic levels that remain localized on either metal or ligand in the electronic structure of the complex. Complexes prepared from the orthoquinone ligands are of recent interest as examples of ambiguous charge distribution. 1 Three isomeric structures may be envisioned for the metal-quinone chelate ring; these are related by successive metal-ligand electron-transfer steps.Charge distribution within the chelate ring may be directed by the bonding properties of ancillary ligands also coordinated to the metal and the influence of quinone ligand substituents. Under appropriate thermodynamic conditions, equilibria between redox isomers may be observed, and electrochemical redox processes may take place selectively at either the ligand or metal ion. 2 In cases where orbital mixing is significant, charge delocalization within the chelate ring obscures assignment of the metal and ligand charge. The more highly oxidized complexes of the 1,2-dithiolate/1,2-dithiolene ligands illustrate this feature of electronic structure. 3 A number of probes are available for assessing charge distribution, localized or delocalized, and for making localized charge assignments. Magnetic properties have been particularly helpful in providing insight on charge distribution. Observations on magnetic exchange between radical semiquinonate ligands and paramagnetic metal ions may point to a unique metal spin state. 4 EPR spectra provide information on spin localization, metal or ligand, and on the possibility of spin delocalization over both ligand and metal. For complexes containing localized mixed-charge semiquinonate (SQ) and catecholate (Cat) ligands, intense LL'CT electronic transitions appear in the near infrared.The reaction between an o-benzoquinone and Cr(CO) 6 has been reported to give tris(quinone) complexes of general form CrQ 3 . 5,6 The view of these complexes as consisting of radical SQ ligands coordinated to Cr(III), Cr III (SQ • ) 3 , is based on observations of weak residual paramagnetism and structural features. Radical ligand spins couple antiferromagnetically with metal dπ (t 2g 3 ) electrons to give temperature-dependent magnetic moments for the molecules that are typically less than 1.0 µ B . 5,7,8 Mössbauer spectra recorded on the related complexes of iron, Fe III (SQ • ) 3 , verify that the metal ion is in the form of high-spin Fe(III) and that temperature-dependent magnetic properties result from similar metal-radical magnetic exchange. 9 Electrochemical characterization of members of the Cr III (SQ) 3 series have shown three-membered oxidation and reduction series. The range of redox potentials is dependent upon quinone substituent effects. This, with other structural, magnetic, and spectral properties obtained for various members of the redox series, has led to the view that the redox cou...

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confidence: 69%

Redox Isomerism for Quinone Complexes of Chromium and Chromium Oxidation State Assignment from X-ray Absorption Spectroscopy

Pierpont

2001

Inorg. Chem.

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“…No evidence was obtained for further reduction to divalent vanadium complexes. When catecholate was substituted with four electron-withdrawing chlorine atoms, either the nonoxido V(V) [V(Cl 4 cat) 3 ] − or the V(III) V(Cl 4 bsq •− ) 3 were obtained [101], for both of which a complex electrochemical behavior is reported.…”

Section: Non-oxido Vanadium(iv) Speciesmentioning

confidence: 99%

“…The anionic [V(Cl 4 cat) 3 ] − complex underwent one-electron oxidation and reduction reactions reversibly [101]. However, the latter occurred at a potential considerably more negative than that of the corresponding catechol complex and therefore both electrochemical processes were attributed to the ligand.…”

Section: Non-oxido Vanadium(v) Speciesmentioning

confidence: 99%

“…The same compound was re-investigated few years later (Table 9, entry 1b [193]) with results in excellent agreement, at least, according to the authors. 3 in acetonitrile were studied by voltammetry and constant-potential coulometry [101]. The anionic complex underwent one-electron oxidation and reduction reactions reversibly (Table 7, entry 3).…”

Section: Electrochemistry Of Vanadium(iii) Compoundsmentioning

confidence: 99%

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A journey into the electrochemistry of vanadium compounds

Galloni

Conte

2015

Coordination Chemistry Reviews

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“…1,7–11 The major method for characterization of metal complexes with redox active ligands in the solid state is X-ray crystallography. 9,10,12–15 Solution characterization is often done using electrochemistry, UV- vis spectroscopy and EPR spectroscopy. 8–10,15 Another characterization method used less frequently for such complexes in solution is multinuclear NMR spectroscopy.…”

Section: Introductionmentioning

confidence: 99%

Characterization of Noninnocent Metal Complexes Using Solid-State NMR Spectroscopy: o-Dioxolene Vanadium Complexes

et al. 2011

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51V solid-state NMR (SSNMR) studies of a series of non-innocent vanadium(V) catechol complexes have been conducted to evaluate the possibility that 51V NMR observables, quadrupolar and chemical shift anisotropies, and electronic structures of such compounds can be used to characterize these compounds. The vanadium(V) catechol complexes described in these studies have relatively small quadrupolar coupling constants, which cover a surprisingly small range from 3.4 to 4.2 MHz. On the other hand, isotropic 51V NMR chemical shifts cover a wide range from −200 ppm to 400 ppm in solution and from −219 to 530 ppm in the solid state. A linear correlation of 51V NMR isotropic solution and solid-state chemical shifts of complexes containing non-innocent ligands is observed. These experimental results provide the information needed for the application of 51V SSNMR spectroscopy in characterizing the electronic properties of a wide variety of vanadium-containing systems, and in particular those containing non-innocent ligands and that have chemical shifts outside the populated range of −300 ppm to −700 ppm. The studies presented in this report demonstrate that the small quadrupolar couplings covering a narrow range of values reflect the symmetric electronic charge distribution, which is also similar across these complexes. These quadrupolar interaction parameters alone are not sufficient to capture the rich electronic structure of these complexes. In contrast, the chemical shift anisotropy tensor elements accessible from 51V SSNMR experiments are a highly sensitive probe of subtle differences in electronic distribution and orbital occupancy in these compounds. Quantum chemical (DFT) calculations of NMR parameters for [VO(hshed)(Cat)] yield 51V CSA tensor in reasonable agreement with the experimental results, but surprisingly, the calculated quadrupolar coupling constant is significantly greater than the experimental value. The studies demonstrate that substitution of the catechol ligand with electron donating groups results in an increase in the HOMO-LUMO gap and can be directly followed by an upfield shift for the vanadium catechol complex. In contrast, substitution of the catechol ligand with electron withdrawing groups results in a decrease in the HOMO-LUMO gap and can directly be followed by a downfield shift for the complex. The vanadium catechol complexes were used in this work because the 51V is a half-integer quadrupolar nucleus whose NMR observables are highly sensitive to the local environment. However, the results are general and could be extended to other redox active complexes that exhibit similar coordination chemistry as the vanadium catechol complexes.

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Catecholate and semiquinone complexes of vanadium. Factors that direct charge distribution in metal-quinone complexes

Cited by 89 publications

References 1 publication

Redox Isomerism for Quinone Complexes of Chromium and Chromium Oxidation State Assignment from X-ray Absorption Spectroscopy

Redox Isomerism for Quinone Complexes of Chromium and Chromium Oxidation State Assignment from X-ray Absorption Spectroscopy

A journey into the electrochemistry of vanadium compounds

Characterization of Noninnocent Metal Complexes Using Solid-State NMR Spectroscopy: o-Dioxolene Vanadium Complexes

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