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![Resonance Raman spectra of (A) 1 (black), (B) [1˙] + (blue), and (C) [1˙˙] 2+ (red). Conditions: 1 mM complex in CH 2 Cl 2 , 213 K, λ ex = 413.1 nm. Asterisk denotes solvent CH 2 Cl 2 .](profile/Tim-Dunn-2/publication/234701778/figure/fig1/AS:396134997610498@1471457351437/Resonance-Raman-spectra-of-A-1-black-B-1-blue-and-C-1-2-red.png)
Resonance Raman spectra of (A) 1 (black), (B) [1˙] + (blue), and (C) [1˙˙] 2+ (red). Conditions: 1 mM complex in CH 2 Cl 2 , 213 K, λ ex = 413.1 nm. Asterisk denotes solvent CH 2 Cl 2 .
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![Fig. 2 Resonance Raman spectra of (A) 1 (black), (B) [1˙] + (blue), and...](profile/Tim-Dunn-2/publication/234701778/figure/fig1/AS:396134997610498@1471457351437/Resonance-Raman-spectra-of-A-1-black-B-1-blue-and-C-1-2-red_Q320.jpg)
![Fig. 4 Concentration matched EPR spectra of [1˙] + and [1˙˙] 2+ ....](profile/Tim-Dunn-2/publication/234701778/figure/fig3/AS:396134997610501@1471457351866/Concentration-matched-EPR-spectra-of-1-and-1-2-X-Band-EPR-spectrum-of-1_Q320.jpg)
The electronic structure of a doubly oxidized Ni salen complex NiSal(tBu) (Sal(tBu) = N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexane-(1R,2R)-diamine) has been investigated by both experimental and theoretical methods. The doubly oxidized product was probed by resonance Raman spectroscopy, UV-vis-NIR, and EPR to determine the locus of oxida...
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... Raman (rR) Spectroscopy provides strong evidence for the presence of phenoxyl radicals in both the oxidized forms (Fig. 2). ...
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... journal is © The Royal Society of Chemistry 2013Dalton Trans., 2013 As previously described [1˙] + exhibits bands at 1504 and 1605 cm −1 (Fig. 2B) attributed to characteristic phenoxyl radical C-O stretching, ν 7a , and C ortho -C meta stretching, ν 8a , modes respectively. 5,37 The rR spectrum for [1] 2+ (Fig. 2C) shows a significant increase in the intensity of ν 7a at 1504 cm −1 and a shift of ν 8a by 26 cm −1 to 1579 cm −1 ; the energy and intensity of the two bands match ...
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... journal is © The Royal Society of Chemistry 2013Dalton Trans., 2013 As previously described [1˙] + exhibits bands at 1504 and 1605 cm −1 (Fig. 2B) attributed to characteristic phenoxyl radical C-O stretching, ν 7a , and C ortho -C meta stretching, ν 8a , modes respectively. 5,37 The rR spectrum for [1] 2+ (Fig. 2C) shows a significant increase in the intensity of ν 7a at 1504 cm −1 and a shift of ν 8a by 26 cm −1 to 1579 cm −1 ; the energy and intensity of the two bands match those reported for localized Ni II -phenoxyl radical complexes with the same tert-butyl substitution pattern. 8, 38 The 2+ . The lack of shift in ν 7a in comparison to ν 8a ...
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... spin-spin interaction between the two localized radicals in [1˙˙] 2+ was investigated by EPR spectroscopy. The X-band EPR spectrum of a frozen solution of [1˙˙] 2+ exhibits a weak feature at g = 2 and lacks a half-field feature at g = 4 indicative of an accessible triplet spin state (Fig. 4, Fig. S2 †). The signal intensity integrates to ca. 8% compared to the g = 2 feature in a concentration-matched sample of [1˙] + . The small residual signal at g = 2 in [1˙˙] 2+ is likely excess chemical oxidant in the sample (Fig. S3 †). The EPR data are consistent with either anti- ferromagnetic coupling of the radical spins in [1˙˙] 2+ , or ...
Citations
... [14] Years later, Storr and coworkers oxidized 1 via 2 equiv. aminium radical cation [N(C 6 H 3 Br 2 ) 3 * ] + to form [1 II -L ** ]. [15] Very recently, we reported the action of excess of mCPBA on 1 leading to the formation of Ni(III) bisphenoxyl diradical species, [1 III -L ** ] (Scheme 1a) both in CH 3 CN and CH 2 Cl 2 at 233 K. [16] It has been demonstrated in the literature that the site of oxidation from metal to the ligand (i. e., [1 II -L * ] and [1 III -L]) can be shuffled depending upon the conditions employed. ...
... The EPR in case of 2Py at both the temperatures displayed a similar rhombic signal as MI with super hyperfine splitting originating from the axial binding of two pyridine ligands ( Figure S8 and Scheme 1b) making it a [1 III -Py 2 -L ** ] species. [15] Surprisingly, at 120 K, the EPR spectrum for 2Q in CH 3 CN revealed exclusively an isotropic signal with a g iso value of 1.98. The appearance of an isotropic signal combined with the generation of 2.5 equivalent Fc + is an exhibitive of three antiferromagnetically coupled species with the effective spin localized on one of the phenolate rings ( Figure 4). ...
... The bands around 1500-1700 cm À 1 are due to the phenoxyl radical species. [14][15][16] The resonance Raman signals observed for 2Q were found to be similar to [1 III -L ** ]. The identical spectrum in the case of Q as an exogenous ligand corresponds to its weaker binding to the Ni center, as suggested in UV/Vis absorption at 233 K (Figure 2 and Scheme 1b). ...
The participation of both ligand and the metal center in the redox events has been recognized as one of the ways to attain the formal high valent complexes for the late 3d metals, such as Ni and Cu. Such an approach has been employed successfully to stabilize a Ni(III) bisphenoxyl diradical species in which there exist an equilibrium between the ligand and the Ni localized resultant spin. The present work, however, broadens the scope of the previously reported three oxidized equivalent species by conveying the approaches that tend to affect the reported equilibrium in CH3CN at 233 K. Various spectroscopic characterization revealed that employing exogenous N‐donor ligands like 1‐methyl imidazole and pyridine favors the formation of the Ni centered localized spin though axial binding. In contrast, due to its steric hinderance, quinoline favors an exclusive ligand localized radical species. DFT studies shed light on the novel intermediates′ complex electronic structure. Further, the three oxidized equivalent species with the Ni centered spin was examined for its hydrogen atom abstraction ability stressing their key role in alike reactions.
... aminium radical cation [N(C 6 H 3 Br 2 ) 3 * ] + in CH 2 Cl 2 ( Figure 1c). [22] Since the precursor ([1 II -L], or simply 1 as in Figure 1c) is a neutral complex, the charge of [1 II -L * ] and [1 II -L ** ] is + 1 and + 2, respectively. Please note that in our compact notation, charges are omitted for brevity. ...
... Shimazaki and co-workers report that the two redox reversible waves can be assigned to Ni *,II/II and Ni **,II/*,II redox events, respectively. [22] However, in addition to two reversible redox waves, we also observed an irreversible oxidation wave at E p,a = 1.52 V vs. Ag/AgCl in the cyclic voltammogram and differential pulse voltammetry (DPV) studies (Figure 2b). Since both the phenolate rings and the metal center in 1 are susceptible to oxidation, this preliminary information points towards the formation of a Ni III -diradical species. ...
... As indicated in previous studies, the use of Ce IV or AgSbF 6 , and [N(C 6 H 3 Br 2 ) 3 * ] + (magic blue), as one-electron oxidants with 1, made the one and two-electron oxidized complexes accessible, respectively. [17,22] However, the additional redox event we observed in the cyclic voltammetry ( Figure 2a) steered us towards employing a different approach. Specifically, we followed the oxidation of 1 by mCPBA in CH 3 CN and CH 2 Cl 2 at À 40°C via UV/Vis absorption spectroscopy. ...
Cytochrome P450s and Galactose Oxidases exploit redox active ligands to form reactive high valent intermediates for oxidation reactions. This strategy works well for the late 3d metals where accessing high valent states is rather challenging. Herein, we report the oxidation of NiII(salen) (salen=N,N′‐bis(3,5‐di‐tert‐butyl‐salicylidene)‐1,2‐cyclohexane‐(1R,2R)‐diamine) with mCPBA (meta‐chloroperoxybenzoic acid) to form a fleeting NiIII bisphenoxyl diradical species, in CH3CN and CH2Cl2 at −40 °C. Electrochemical and spectroscopic analyses using UV/Vis, EPR, and resonance Raman spectroscopies revealed oxidation events both on the ligand and the metal centre to yield a NiIII bisphenoxyl diradical species. DFT calculations found the electronic structure of the ligand and the d‐configuration of the metal center to be consistent with a NiIII bisphenoxyl diradical species. This three electron oxidized species can perform hydrogen atom abstraction and oxygen atom transfer reactions.
... The oxidation locus in pristine nickel(II) complexes depends on the coordinating ability of the solvent, the nature of the supporting electrolyte and the presence of electron-donating or electron-withdrawing substituents in the phenyl moieties. Nickel(II) complexes with Salen-type ligands exhibit ligand-centered oxidation to phenoxyl radicals (one electron oxidation) [25] and bis-phenoxyl radical species (two electron oxidation) [26] in weakly coordinating media. ...
Most non-metalized Salen-type ligands form passivation thin films on electrode surfaces upon electrochemical oxidation. In contrast, the H2(3-MeOSalen) forms electroactive polymer films similarly to the corresponding nickel complex. There are no details of electrochemistry, doping mechanism and charge transfer pathways in the polymers of pristine Salen-type ligands. We studied a previously uncharacterized electrochemically active polymer of a Salen-type ligand H2(3-MeOSalen) by a combination of cyclic voltammetry, in situ ultraviolet–visible (UV–VIS) spectroelectrochemistry, in situ electrochemical quartz crystal microbalance and Fourier Transform infrared spectroscopy (FTIR) spectroscopy. By directly comparing it with the polymer of a Salen-type nickel complex poly-Ni(3-MeOSalen) we elucidate the effect of the central metal atom on the structure and charge transport properties of the electrochemically doped polymer films. We have shown that the mechanism of charge transfer in the polymeric ligand poly-H2(3-MeOSalen) are markedly different from the corresponding polymeric nickel complex. Due to deviation from planarity of N2O2 sphere for the ligand H2(3-MeOSalen), the main pathway of electron transfer in the polymer film poly-H2(3-MeOSalen) is between π-stacked structures (the π-electronic systems of phenyl rings are packed face-to-face) and C-C bonded phenyl rings. The main way of electron transfer in the polymer film poly-Ni(3-MeOSalen) is along the polymer chain, while redox processes are ligand-based.
... [22,30] The electronic structure of monometallic bis-ligand radical complexes has been studied by a number of groups, and the electronic structure is dependent on the interaction between the open-shell ligands, and in some cases additional interactions with the paramagnetic transition metal ion. [31][32][33] In this work we detail the synthesis and characterization of the neutral and oxidized forms of a monometallic Ni salen complex 1 and its bimetallic analogue 2 (Scheme 1). Although extensively investigated for catalysis, the ligand radical chemistry of multimetallic salen complexes has received little attention. ...
... The two oxidation waves can be attributed to ligand-based oxidation processes, forming the mono-and bis-phenoxyl radical species. [12,33] The electronic coupling between the redox-active phenolates in [1] + , and degree of electronic delocalization, was assessed by considering the difference between the first and second oxidation potentials (DE ox ) and the comproportionation constant, K c [Eqs. (1)-(3)]. ...
... [52] J ¼ (Table 5). The complete disappearance of the low-energy band upon two electron oxidation of a similar monomer to 1, [33] precludes double oxidation on one of the salen units of 2. www.chemeurj.org TD-DFT calculations on both the triplet and BS solutions for [2CC] 2 + were used to gain further insight into the nature of the low-energy transitions, and in particular the presence of the two intense low-energy bands in the NIR. ...
The geometric and electronic structure of an oxidized bimetallic Ni complex incorporating two redox-active Schiff-base ligands connected via a 1,2-phenylene linker has been investigated and compared to a monomeric analogue. Information from UV/Vis/NIR spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, electrochemistry, and density functional theory (DFT) calculations provides important information on the locus of oxidation for the bimetallic complex. The neutral bimetallic complex is conformationally dynamic at room temperature, which complicates characterization of the oxidized forms. Comparison to an oxidized monomer analogue 1 provides critical insight into the electronic structure of the oxidized bimetallic complex 2. Oxidation of 1 provides [1.]+, which is characterized as a fully delocalized ligand radical complex; the spectroscopic signature of this derivative includes an intense NIR band at 4500 cm−1. Oxidation of 2 to the bis-oxidized form affords a bis-ligand radical species [2..]2+. Variable temperature EPR spectroscopy of [2..]2+ shows no evidence of coupling, and the triplet and broken symmetry solutions afforded by theoretical calculations are essentially isoenergetic. [2..]2+ is thus best described as incorporating two non-interacting ligand radicals. Interestingly, the intense NIR intervalence charge transfer band observed for the delocalized ligand-radical [1.]+ exhibits exciton splitting in [2..]2+, due to coupling of the monomer transition dipoles in the enforced oblique dimer geometry. Evaluating the splitting of the intense intervalence charge transfer band can thus provide significant geometric and electronic information in less rigid bis-ligand radical systems. Addition of excess pyridine to [2..]2+ results in a shift in the oxidation locus from a bis-ligand radical species to the NiIII/NiIII derivative [2(py)4]2+, demonstrating that the ligand system can incorporate significant bulk in the axial positions.
The phenoxyl radical binding copper complexes have been widely developed and their detailed geometric and electronic structures have been clarified. While many one-electron oxidized CuII-phenolate complexes have been reported previously, recent studies of the Cu-phenolate complexes proceed toward elucidation of the complexes with other oxidation states, such as the phenoxyl radical binding CuI complexes and CuIV-phenolate complexes in the formal oxidation state. This Perspective focuses on new aspects of the properties and reactivities of various Cu-phenolate and Cu-phenoxyl radical complexes with emphasis on the relationship between geometric and electronic structures.
Cytochrome P450s and Galactose Oxidases exploit redox active ligands to form reactive high valent intermediates for oxidation reactions. This strategy works well for the late 3d metals where accessing high valent states is rather challenging. Herein, we report the oxidation of NiII(salen) (salen = N,N’‐bis(3,5‐di‐tert‐butyl‐salicylidene)‐1,2‐cyclohexane‐(1R,2R)‐diamine) with mCPBA (meta‐chloroperoxybenzoic acid) to form a fleeting Ni(III) bisphenoxyl diradical species, in CH3CN and CH2Cl2 at ‐40 °C. Electrochemical and spectroscopic analyses using UV‐Vis, EPR, and resonance Raman spectroscopies revealed oxidation events both on the ligand and the metal centre to yield a Ni(III) bisphenoxyl diradical species. DFT calculations found the electronic structure of the ligand and the d‐configuration of the metal center to be consistent with a Ni(III) bisphenoxyl diradical species. This three electron oxidized species can perform hydrogen atom abstraction and oxygen atom transfer reactions.
A combined theoretical/experimental study of the new thiophene-based NiSalen complex with unconjugated bridging fragment. This complex demonstrates unusual stability of the oxidized form, which is not typical for this class of compounds.
Metal- salen polymers are electrochemically active metallopolymers functionalized with multiple redox centers, with a potential for high performance in various fields such as heterogeneous catalysis, chemical sensors, energy conversion, saving, and storage. In light of the growing world demand for the development of superior energy storage systems, the prospects of employing these polymers for advancing the performance of supercapacitors and lithium-ion batteries are particularly interesting. This article provides a general overview of the results of investigating key structure-property relationships of metal- salen polymers and using them to design polymer-modified electrodes with improved energy storage characteristics. The results of independent and collaborative studies conducted by the members of two research groups currently affiliated to the Saint–Petersburg State University and the Ioffe Institute, respectively, along with the related data from other studies are presented in this review.
