Rhenium Complex with Noninnocent Dioxolene Ligand: Combined Experimental and ab Initio Study of [(3,5-di-<i>tert</i>-Bu₂C₆H₂O₂)ReCl₃(OPPh₃)]

Reaction of [ReOCl₃(PPh₃)₂] with 3,5-di-<i>tert</i>-butyl-1,2-benzoquinone (3,5-DTBQ) in hot toluene produces a new complex [(3,5-di-<i>tert</i>-Bu₂C₆H₂O₂)­Re­(OPPh₃)­Cl₃] (<b>1</b>), which was isolated and characterized by elemental analysis, IR, UV–vis spectroscopy, and cyclic voltammetry. In order to clarify the charge state of rhenium and the coordinated dioxolene ligand, X-ray experiments at 150 and 290 K were carried out. The C–O, C–C, and Re–O bond distances at both 150 and 290 K fall between those for semiquinolate (3,5-DTBSQ) and catecholate (3,5-DTBCat) forms; an empirical “metrical oxidation state” of the dioxolene ligand was estimated to be −1.5. High-level <i>ab initio</i> calculations (SOC-CASSCF/NEVPT2) revealed a mixed valence nature of the triplet ground state of complex <b>1</b> corresponding to a superposition of the <i>Re</i>^<i>IV</i>–<i>SQ</i> and <i>Re</i>^<i>V</i>–<i>cat</i> forms. In agreement with the high-level <i>ab initio</i> and DFT calculations, the temperature dependence of the magnetic susceptibility (5–300 K) is well described in the assumption of the triplet ground state, with the anomalously large zero-field splitting (ZFS) arising from the spin–orbit coupling. According to the <i>ab initio</i> calculations, all absorption bands in the visible region of the electronic absorptions spectrum are assigned to the LMCT bands, with significant contribution of the intraligand transition in the most intense band at 555 nm

Mixed Phenyl and Thiophene Oligomers for Bridging Nitronyl Nitroxides

The synthesis of four nitronyl nitroxide (NN) biradicals is described which are conjugatively linked through <i>p-ter</i>-phenyl (PPP), <i>ter</i>-thiophene (TTT) and alternating phenylene (P) and thiophene (T) units as PTP and TPT. We first utilized Suzuki and Stille coupling reactions through protection and deprotection protocols to synthesize these (NN) biradicals. Single crystals were efficiently grown for radical precursors of <b>3</b>, <b>5</b>, <b>6</b>, <b>PPP-NNSi, PTP-NNSi</b>, and final biradicals of <b>TTT-NN</b>, <b>TPT-NN</b>, and <b>PPP-NN</b>, whose structures and molecular packing were examined by X-ray diffraction studies. As a result, much smaller torsions between the NN and thiophene units (∼10°) in <b>TTT-NN</b> and <b>TPT-NN</b> than for NN and phenyl units (∼29°) in <b>PPP-NN</b> were observed due to smaller hindrance for a five vs a six membered ring. All four biradicals <b>TTT-NN</b>, <b>TPT-NN</b>, <b>PTP-NN</b>, and <b>PPP-NN</b> were investigated by EPR and optical spectroscopy combined with DFT calculations. The magnetic susceptibility was studied by SQUID measurements for <b>TTT-NN</b> and <b>TPT-NN</b>. The intramolecular exchange interactions for <b>TPT-NN</b> and <b>TTT-NN</b> were found in good agreement with the ones calculated by broken symmetry DFT calculations

Mixed Phenyl and Thiophene Oligomers for Bridging Nitronyl Nitroxides

The synthesis of four nitronyl nitroxide (NN) biradicals is described which are conjugatively linked through <i>p-ter</i>-phenyl (PPP), <i>ter</i>-thiophene (TTT) and alternating phenylene (P) and thiophene (T) units as PTP and TPT. We first utilized Suzuki and Stille coupling reactions through protection and deprotection protocols to synthesize these (NN) biradicals. Single crystals were efficiently grown for radical precursors of <b>3</b>, <b>5</b>, <b>6</b>, <b>PPP-NNSi, PTP-NNSi</b>, and final biradicals of <b>TTT-NN</b>, <b>TPT-NN</b>, and <b>PPP-NN</b>, whose structures and molecular packing were examined by X-ray diffraction studies. As a result, much smaller torsions between the NN and thiophene units (∼10°) in <b>TTT-NN</b> and <b>TPT-NN</b> than for NN and phenyl units (∼29°) in <b>PPP-NN</b> were observed due to smaller hindrance for a five vs a six membered ring. All four biradicals <b>TTT-NN</b>, <b>TPT-NN</b>, <b>PTP-NN</b>, and <b>PPP-NN</b> were investigated by EPR and optical spectroscopy combined with DFT calculations. The magnetic susceptibility was studied by SQUID measurements for <b>TTT-NN</b> and <b>TPT-NN</b>. The intramolecular exchange interactions for <b>TPT-NN</b> and <b>TTT-NN</b> were found in good agreement with the ones calculated by broken symmetry DFT calculations

Rhenium Complex with Noninnocent Dioxolene Ligand: Combined Experimental and ab Initio Study of [(3,5-di-<i>tert</i>-Bu₂C₆H₂O₂)ReCl₃(OPPh₃)]

Reaction of [ReOCl₃(PPh₃)₂] with 3,5-di-<i>tert</i>-butyl-1,2-benzoquinone (3,5-DTBQ) in hot toluene produces a new complex [(3,5-di-<i>tert</i>-Bu₂C₆H₂O₂)­Re­(OPPh₃)­Cl₃] (<b>1</b>), which was isolated and characterized by elemental analysis, IR, UV–vis spectroscopy, and cyclic voltammetry. In order to clarify the charge state of rhenium and the coordinated dioxolene ligand, X-ray experiments at 150 and 290 K were carried out. The C–O, C–C, and Re–O bond distances at both 150 and 290 K fall between those for semiquinolate (3,5-DTBSQ) and catecholate (3,5-DTBCat) forms; an empirical “metrical oxidation state” of the dioxolene ligand was estimated to be −1.5. High-level <i>ab initio</i> calculations (SOC-CASSCF/NEVPT2) revealed a mixed valence nature of the triplet ground state of complex <b>1</b> corresponding to a superposition of the <i>Re</i>^<i>IV</i>–<i>SQ</i> and <i>Re</i>^<i>V</i>–<i>cat</i> forms. In agreement with the high-level <i>ab initio</i> and DFT calculations, the temperature dependence of the magnetic susceptibility (5–300 K) is well described in the assumption of the triplet ground state, with the anomalously large zero-field splitting (ZFS) arising from the spin–orbit coupling. According to the <i>ab initio</i> calculations, all absorption bands in the visible region of the electronic absorptions spectrum are assigned to the LMCT bands, with significant contribution of the intraligand transition in the most intense band at 555 nm

Heteroligand o‑Semiquinonato-Formazanato Cobalt Complexes

Two novel heteroligand o-semiquinonato-formazanato cobalt complexes [Co­(3,6-SQ)­(Form)] (<b>1</b>) and [Co­(3,6-SQ)₂(Form)] (<b>2</b>) (3,6-SQ is 3,6-di-<i>tert</i>-butyl-o-benzosemiquinonate radical anion; Form is 1,3,5-triphenylformazanate anion) were synthesized and characterized in detail. The molecular structures of [Co­(3,6-SQ)­(Form)] and [Co­(3,6-SQ)₂(Form)] were determined by X-ray analysis. Magnetic susceptibility measurements and spectroscopic studies have shown that square-planar complex <b>1</b> is diamagnetic with a residual paramagnetism due to antiferromagnetic exchange metal–ligand. Complex <b>2</b> includes cobalt­(III) in the low-spin state and two anionic-radical o-semiquinonato and one formazanate ligand

Ferrocene‑<i>o</i>‑Benzosemiquinonato Tin(IV) Electron-Transfer Complexes

The interaction of ferrocene with tin­(IV) <i>o</i>-benzosemiquinonato complexes in acetonitrile results in a reversible electron transfer (ET) from ferrocene to the redox-active ligand with the formation of electron-transfer complexes [(3,6-Cat)­SnBr₃]⁻[Cp₂Fe]⁺ (<b>1</b>) and [(3,6-Cat)­(3,6-SQ)­SnCl₂]⁻[Cp₂Fe]⁺ (<b>2</b>), where 3,6-Cat is the 3,6-di-<i>tert</i>-butyl-catecholate dianion and 3,6-SQ is the 3,6-di-<i>tert</i>-butyl-<i>o</i>-benzosemiquinonato radical anion. The ET process and the solvent effect in the system ≪ferrocene–<i>o</i>-benzosemiquinonato tin­(IV) complexes≫ were investigated on the basis of a combination of spectroscopic and X-ray diffraction methods. The molecular structures of <b>1</b> and <b>2</b> were confirmed by X-ray analysis. Complex <b>2</b> demonstrates the ferromagnetic coupling in the linear chain alternating ···D^+•A^–•D^+•A^–•··· motif

Ferrocene‑<i>o</i>‑Benzosemiquinonato Tin(IV) Electron-Transfer Complexes

The interaction of ferrocene with tin­(IV) <i>o</i>-benzosemiquinonato complexes in acetonitrile results in a reversible electron transfer (ET) from ferrocene to the redox-active ligand with the formation of electron-transfer complexes [(3,6-Cat)­SnBr₃]⁻[Cp₂Fe]⁺ (<b>1</b>) and [(3,6-Cat)­(3,6-SQ)­SnCl₂]⁻[Cp₂Fe]⁺ (<b>2</b>), where 3,6-Cat is the 3,6-di-<i>tert</i>-butyl-catecholate dianion and 3,6-SQ is the 3,6-di-<i>tert</i>-butyl-<i>o</i>-benzosemiquinonato radical anion. The ET process and the solvent effect in the system ≪ferrocene–<i>o</i>-benzosemiquinonato tin­(IV) complexes≫ were investigated on the basis of a combination of spectroscopic and X-ray diffraction methods. The molecular structures of <b>1</b> and <b>2</b> were confirmed by X-ray analysis. Complex <b>2</b> demonstrates the ferromagnetic coupling in the linear chain alternating ···D^+•A^–•D^+•A^–•··· motif

Indirect Magnetic Exchange between <i>o</i>‑Iminosemiquinonate Ligands Controlled by Apical Substituent in Pentacoordinated Gallium(III) Complexes

A number of pentacoordinated gallium complexes iSQ₂GaR (<b>1</b>–<b>7</b>) (R = Et (<b>1</b>), Me (<b>2</b>), N₃ (<b>3</b>), Cl (<b>4</b>), Br (<b>5</b>), I (<b>6</b>), NCS (<b>7</b>)) where iSQ is a radical anion of 4,6-di-<i>tert</i>-butyl-<i>N</i>-(2,6-diisopropylphenyl)-<i>o</i>-iminobenzoquinone were synthesized, and crystalline samples of <b>1</b>–<b>7</b> were characterized using magnetic susceptibility measurements. The character of magnetic exchange interaction between spins of <i>o</i>-iminosemiquinonate radicals was found to be strongly influenced by the nature of the apical substituent. The antiferromagnetic coupling is predominant when the apical position is occupied by halogens or other tested inorganic anions, and the value of exchange interaction parameter varies from −99 to −176 K for R = I and NCS, respectively. In the case of alkyl groups the ferromagnetic exchange prevails and, as the result, the triplet ground state for pentacoordianted biradical compounds was observed. Compounds <b>1</b>–<b>7</b> demonstrate a biradical X-band EPR spectrum in frozen toluene matrix. The molecular structures of <b>4</b>, <b>6</b>, and <b>7</b> have been established by single-crystal X-ray analysis. A computational DFT UB3LYP/6-31G­(d,p) study was performed on complexes <b>1–7</b> in order to understand the reason for changes in the magnetic behavior of the related diradical gallium compounds. The calculations showed that the magnetic behavior of the complexes with inorganic anions is conditioned by the presence of antiferromagnetic exchange channel formed as a consequence of overlapping between donor atomic orbitals of iminoquinone with π-orbitals of halogen atoms (<b>4</b>–<b>6</b>) or nitrogen atom (<b>3</b>, <b>7</b>)

First Example of a Reversible Single-Crystal-to-Single-Crystal Polymerization–Depolymerization Accompanied by a Magnetic Anomaly for a Transition-Metal Complex with an Organic Radical

The reaction of copper­(II) hexafluoroacetylacetonate [Cu­(hfac)₂] with the stable nitronyl nitroxide 2-(1-ethyl-3-methyl-1<i>H</i>-pyrazol-4-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1<i>H</i>-imidazole-3-oxide-1-oxyl (L^a) resulted in a paired heterospin complex [[Cu­(hfac)₂]₃(μ-<i>O</i>,<i>N</i>-L^a)₂]­[Cu­(hfac)₂(<i>O</i>-L^a)₂]. The crystals of the compound were found to be capable of a reversible single-crystal-to-single-crystal (SC–SC) transformation initiated by the variation of temperature. At room temperature, the molecular structure of [[Cu­(hfac)₂]₃(μ-<i>O</i>,<i>N</i>-L^a)₂]­[Cu­(hfac)₂(<i>O</i>-L^a)₂] is formed by the alternating fragments of the pair complex. Cooling the crystals of the complex below 225 K caused considerable mutual displacements of adjacent molecules, which ended in a transformation of the molecular structure into a polymer chain structure. A reversible topotactic polymerization–depolymerization coordination reaction actually takes place in the solid during repeated cooling–heating cycles: [[Cu­(hfac)₂]₃(μ-<i>O</i>,<i>N</i>-L^a)₂]­[Cu­(hfac)₂(<i>O</i>-L^a)₂] ⇌ Cu­(hfac)₂(μ-<i>O</i>,<i>N</i>-L^a)]_∞. Polymerization during cooling is the result of the anomalously great shortening of intermolecular distances (from 4.403 Å at 295 K to 2.460 Å at 150 K; Δ<i>d</i> = 1.943 Å) between the terminal Cu atoms of the trinuclear fragments {[[Cu­(hfac)₂]₃(μ-<i>O</i>,<i>N</i>-L^a)₂]} and the noncoordinated N atoms of the pyrazole rings of the mononuclear {[Cu­(hfac)₂(<i>O</i>-L^a)₂]} fragments. When the low-temperature phase was heated above 270 K, the polymer chain structure was destroyed and the compound was again converted to the pair molecular complex. The specifics of the given SC–SC transformation lies in the fact that the process is accompanied by a magnetic anomaly, because the intracrystalline displacements of molecules lead to a considerable change in the mutual orientation of the paramagnetic centers, which, in turn, causes modulation of the exchange interaction between the odd electrons of the Cu²⁺ ion and nitroxide. On the temperature curve of χ<i>T</i>, this shows itself as a hysteresis loop. The nontrivial character of the recorded spin transition during the cooling of the sample below 225 K lies in the fact that the magnetic moment abruptly increased. In contrast, heating the sample above 270 K led to a drastic decrease in χ<i>T</i>. This behavior of χ<i>T</i> is caused by a stepwise change in the character of the exchange interaction in the {>N–^•O–Cu²⁺–O^•–N<} fragments. The lengthening of distances between the paramagnetic centers on cooling below 225 K led to a transition from antiferromagnetic to ferromagnetic exchange and, vice versa, the shortening of distances between the paramagnetic centers during the heating of the heterospin polymer above 270 K led to a transition from ferromagnetic exchange to antiferromagnetic exchange

Ferrocene‑<i>o</i>‑Benzosemiquinonato Tin(IV) Electron-Transfer Complexes

The interaction of ferrocene with tin­(IV) <i>o</i>-benzosemiquinonato complexes in acetonitrile results in a reversible electron transfer (ET) from ferrocene to the redox-active ligand with the formation of electron-transfer complexes [(3,6-Cat)­SnBr₃]⁻[Cp₂Fe]⁺ (<b>1</b>) and [(3,6-Cat)­(3,6-SQ)­SnCl₂]⁻[Cp₂Fe]⁺ (<b>2</b>), where 3,6-Cat is the 3,6-di-<i>tert</i>-butyl-catecholate dianion and 3,6-SQ is the 3,6-di-<i>tert</i>-butyl-<i>o</i>-benzosemiquinonato radical anion. The ET process and the solvent effect in the system ≪ferrocene–<i>o</i>-benzosemiquinonato tin­(IV) complexes≫ were investigated on the basis of a combination of spectroscopic and X-ray diffraction methods. The molecular structures of <b>1</b> and <b>2</b> were confirmed by X-ray analysis. Complex <b>2</b> demonstrates the ferromagnetic coupling in the linear chain alternating ···D^+•A^–•D^+•A^–•··· motif