Rhodium Complexes of a New Structurally Adaptive PNN-Pincer Type Ligand

A new PNN-pincer type ligand with pyrazolyl and diphenylphosphine flanking donors on a diarylamido anchor has been prepared. Its bis­(<i>tert</i>-butyl isocyanide)­rhodium­(I) complex exhibits hemilabile behavior in solution, and its solid-state structure verified the elusive κ²<i>P</i>,<i>N</i> coordination mode for this type of ligand. Reactions between (PNN)­Rh­(CN^tBu)₂ and iodomethane afford both <i>fac</i>- and <i></i><i>cis</i>,<i>mer</i>-[(PNN)­Rh­(CN^tBu)₂(Me)]­(I), which further showcases the structural versatility of the ligand

Syntheses and Electronic Properties of Rhodium(III) Complexes Bearing a Redox-Active Ligand

A series of rhodium­(III) complexes of the redox-active ligand, H­(<b>L</b> = bis­(4-methyl-2-(1<i>H</i>-pyrazol-1-yl)­phenyl)­amido), was prepared, and the electronic properties were studied. Thus, heating an ethanol solution of commercial RhCl₃·3H₂O with H­(<b>L</b>) results in the precipitation of insoluble [H­(<b>L</b>)]­RhCl₃, <b>1</b>. The reaction of a methanol suspension of [H­(<b>L</b>)]­RhCl₃ with NEt₄OH causes ligand deprotonation and affords nearly quantitative yields of the soluble, deep-green, title compound (NEt₄)­[(<b>L</b>)­RhCl₃]·H₂O, <b>2</b>·H₂O. Complex <b>2</b>·H₂O reacts readily with excess pyridine, triethylphosphine, or pyrazine (pyz) to eliminate NEt₄Cl and give charge-neutral complexes <i>trans</i>-(<b>L</b>)­RhCl₂(py), <i>trans</i>-<b>3</b>, <i>trans</i>-(<b>L</b>)­RhCl₂(PEt₃), <i>trans</i>-<b>4</b>, or <i>trans</i>-(<b>L</b>)­RhCl₂(pyz), <i>trans</i>-<b>5</b>, where the incoming Lewis base is <i>trans</i>- to the amido nitrogen of the meridionally coordinating ligand. Heating solutions of complexes <i>trans</i>-<b>3</b> or <i>trans</i>-<b>4</b> above about 100 °C causes isomerization to the appropriate <i>cis</i>-<b>3</b> or <i>cis</i>-<b>4</b>. Isomerization of <i>trans</i>-<b>5</b> occurs at a much lower temperature due to pyrazine dissociation. <i>Cis</i>-<b>3</b> and <i>cis</i>-<b>5</b> could be reconverted to their respective <i>trans</i>- isomers in solution at 35 °C by visible light irradiation. Complexes [(<b>L</b>)­Rh­(py)₂Cl]­(PF₆), <b>6</b>, [(<b>L</b>)­Rh­(PPh₃)­(py)­Cl]­(PF₆), <b>7</b>, [(<b>L</b>)­Rh­(PEt₃)₂Cl]­(PF₆), <b>8</b>, and [(<b>L</b>)­RhCl­(bipy)]­(OTf = triflate), <b>9</b>, were prepared from <b>2</b>·H₂O by using thallium­(I) salts as halide abstraction agents and excess Lewis base. It was not possible to prepare dicationic complexes with three unidentate pyridyl or triethylphosphine ligands; however, the reaction between <b>2</b>, thallium­(I) triflate, and the tridentate 4′-(4-methylphenyl)-2,2′:6′,2″-terpyridine (ttpy) afforded a high yield of [(<b>L</b>)­Rh­(ttpy)]­(OTf)₂, <b>10</b>. The solid state structures of nine new complexes were obtained. The electrochemistry of the various derivatives in CH₂Cl₂ showed a ligand-based oxidation wave whose potential depended mainly on the charge of the complex, and to a lesser extent on the nature and the geometry of the other supporting ligands. Thus, the oxidation wave for <b>2</b> with an anionic complex was found at +0.27 V versus Ag/AgCl in CH₂Cl₂, while those waves for the charge-neutral complexes <b>3</b>–<b>5</b> were found between +0.38 to +0.59 V, where the <i>cis</i>- isomers were about 100 mV more stable toward oxidation than the <i>trans</i>- isomers. The oxidation waves for <b>6</b>–<b>9</b> with monocationic complexes occurred in the range +0.74 to 0.81 V while that for <b>10</b> with a dicationic complex occurred at +0.91 V. Chemical oxidation of <i>trans</i>-<b>3</b>, <i>cis</i>-<b>3</b>, and <b>8</b> afforded crystals of the singly oxidized complexes, [<i>trans</i>-(<b>L</b>)­RhCl₂(py)]­(SbCl₆), <i>cis</i>-[(<b>L</b>)­RhCl₂(py)]­(SbCl₄)·2CH₂Cl₂, and [(<b>L</b>)­Rh­(PEt₃)₂Cl]­(SbCl₆)₂, respectively. Comparisons of structural and spectroscopic features combined with the results of density functional theory (DFT) calculations between nonoxidized and oxidized forms of the complexes are indicative of the ligand-centered radicals in the oxidized derivatives

Syntheses and Electronic Properties of Rhodium(III) Complexes Bearing a Redox-Active Ligand

A series of rhodium­(III) complexes of the redox-active ligand, H­(<b>L</b> = bis­(4-methyl-2-(1<i>H</i>-pyrazol-1-yl)­phenyl)­amido), was prepared, and the electronic properties were studied. Thus, heating an ethanol solution of commercial RhCl₃·3H₂O with H­(<b>L</b>) results in the precipitation of insoluble [H­(<b>L</b>)]­RhCl₃, <b>1</b>. The reaction of a methanol suspension of [H­(<b>L</b>)]­RhCl₃ with NEt₄OH causes ligand deprotonation and affords nearly quantitative yields of the soluble, deep-green, title compound (NEt₄)­[(<b>L</b>)­RhCl₃]·H₂O, <b>2</b>·H₂O. Complex <b>2</b>·H₂O reacts readily with excess pyridine, triethylphosphine, or pyrazine (pyz) to eliminate NEt₄Cl and give charge-neutral complexes <i>trans</i>-(<b>L</b>)­RhCl₂(py), <i>trans</i>-<b>3</b>, <i>trans</i>-(<b>L</b>)­RhCl₂(PEt₃), <i>trans</i>-<b>4</b>, or <i>trans</i>-(<b>L</b>)­RhCl₂(pyz), <i>trans</i>-<b>5</b>, where the incoming Lewis base is <i>trans</i>- to the amido nitrogen of the meridionally coordinating ligand. Heating solutions of complexes <i>trans</i>-<b>3</b> or <i>trans</i>-<b>4</b> above about 100 °C causes isomerization to the appropriate <i>cis</i>-<b>3</b> or <i>cis</i>-<b>4</b>. Isomerization of <i>trans</i>-<b>5</b> occurs at a much lower temperature due to pyrazine dissociation. <i>Cis</i>-<b>3</b> and <i>cis</i>-<b>5</b> could be reconverted to their respective <i>trans</i>- isomers in solution at 35 °C by visible light irradiation. Complexes [(<b>L</b>)­Rh­(py)₂Cl]­(PF₆), <b>6</b>, [(<b>L</b>)­Rh­(PPh₃)­(py)­Cl]­(PF₆), <b>7</b>, [(<b>L</b>)­Rh­(PEt₃)₂Cl]­(PF₆), <b>8</b>, and [(<b>L</b>)­RhCl­(bipy)]­(OTf = triflate), <b>9</b>, were prepared from <b>2</b>·H₂O by using thallium­(I) salts as halide abstraction agents and excess Lewis base. It was not possible to prepare dicationic complexes with three unidentate pyridyl or triethylphosphine ligands; however, the reaction between <b>2</b>, thallium­(I) triflate, and the tridentate 4′-(4-methylphenyl)-2,2′:6′,2″-terpyridine (ttpy) afforded a high yield of [(<b>L</b>)­Rh­(ttpy)]­(OTf)₂, <b>10</b>. The solid state structures of nine new complexes were obtained. The electrochemistry of the various derivatives in CH₂Cl₂ showed a ligand-based oxidation wave whose potential depended mainly on the charge of the complex, and to a lesser extent on the nature and the geometry of the other supporting ligands. Thus, the oxidation wave for <b>2</b> with an anionic complex was found at +0.27 V versus Ag/AgCl in CH₂Cl₂, while those waves for the charge-neutral complexes <b>3</b>–<b>5</b> were found between +0.38 to +0.59 V, where the <i>cis</i>- isomers were about 100 mV more stable toward oxidation than the <i>trans</i>- isomers. The oxidation waves for <b>6</b>–<b>9</b> with monocationic complexes occurred in the range +0.74 to 0.81 V while that for <b>10</b> with a dicationic complex occurred at +0.91 V. Chemical oxidation of <i>trans</i>-<b>3</b>, <i>cis</i>-<b>3</b>, and <b>8</b> afforded crystals of the singly oxidized complexes, [<i>trans</i>-(<b>L</b>)­RhCl₂(py)]­(SbCl₆), <i>cis</i>-[(<b>L</b>)­RhCl₂(py)]­(SbCl₄)·2CH₂Cl₂, and [(<b>L</b>)­Rh­(PEt₃)₂Cl]­(SbCl₆)₂, respectively. Comparisons of structural and spectroscopic features combined with the results of density functional theory (DFT) calculations between nonoxidized and oxidized forms of the complexes are indicative of the ligand-centered radicals in the oxidized derivatives

Electronic Communication Across Diamagnetic Metal Bridges: A Homoleptic Gallium(III) Complex of a Redox-Active Diarylamido-Based Ligand and Its Oxidized Derivatives

Complexes with cations of the type [Ga­(L)₂]^<i>n</i>+ where L = bis­(4-methyl-2-(1H-pyrazol-1-yl)­phenyl)­amido and <i>n</i> = 1, 2, 3 have been prepared and structurally characterized. The electronic properties of each were probed by electrochemical and spectroscopic means and were interpreted with the aid of density functional theory (DFT) calculations. The dication, best described as [Ga­(L^–)­(L⁰)]²⁺, is a Robin-Day class II mixed-valence species. As such, a broad, weak, solvent-dependent intervalence charge transfer (IVCT) band was found in the NIR spectrum in the range 6390–6925 cm^–1, depending on the solvent. Band shape analyses and the use of Hush and Marcus relations revealed a modest electronic coupling, <i>H</i>_ab of about 200 cm^–1, and a large rate constant for electron transfer, <i>k</i>_et, on the order of 10¹⁰ s^–1 between redox active ligands. The dioxidized complex [Ga­(L⁰)₂]³⁺ shows a half-field Δ<i>M</i>_s = 2 transition in its solid-state X-band electron paramagnetic resonance (EPR) spectrum at 5 K, which indicates that the triplet state is thermally populated. DFT calculations (M06/Def2-SV­(P)) suggest that the singlet state is 21.7 cm^–1 lower in energy than the triplet state

Electronic Communication Across Diamagnetic Metal Bridges: A Homoleptic Gallium(III) Complex of a Redox-Active Diarylamido-Based Ligand and Its Oxidized Derivatives

Complexes with cations of the type [Ga­(L)₂]^<i>n</i>+ where L = bis­(4-methyl-2-(1H-pyrazol-1-yl)­phenyl)­amido and <i>n</i> = 1, 2, 3 have been prepared and structurally characterized. The electronic properties of each were probed by electrochemical and spectroscopic means and were interpreted with the aid of density functional theory (DFT) calculations. The dication, best described as [Ga­(L^–)­(L⁰)]²⁺, is a Robin-Day class II mixed-valence species. As such, a broad, weak, solvent-dependent intervalence charge transfer (IVCT) band was found in the NIR spectrum in the range 6390–6925 cm^–1, depending on the solvent. Band shape analyses and the use of Hush and Marcus relations revealed a modest electronic coupling, <i>H</i>_ab of about 200 cm^–1, and a large rate constant for electron transfer, <i>k</i>_et, on the order of 10¹⁰ s^–1 between redox active ligands. The dioxidized complex [Ga­(L⁰)₂]³⁺ shows a half-field Δ<i>M</i>_s = 2 transition in its solid-state X-band electron paramagnetic resonance (EPR) spectrum at 5 K, which indicates that the triplet state is thermally populated. DFT calculations (M06/Def2-SV­(P)) suggest that the singlet state is 21.7 cm^–1 lower in energy than the triplet state

Syntheses and Electronic Properties of Rhodium(III) Complexes Bearing a Redox-Active Ligand

A series of rhodium­(III) complexes of the redox-active ligand, H­(<b>L</b> = bis­(4-methyl-2-(1<i>H</i>-pyrazol-1-yl)­phenyl)­amido), was prepared, and the electronic properties were studied. Thus, heating an ethanol solution of commercial RhCl₃·3H₂O with H­(<b>L</b>) results in the precipitation of insoluble [H­(<b>L</b>)]­RhCl₃, <b>1</b>. The reaction of a methanol suspension of [H­(<b>L</b>)]­RhCl₃ with NEt₄OH causes ligand deprotonation and affords nearly quantitative yields of the soluble, deep-green, title compound (NEt₄)­[(<b>L</b>)­RhCl₃]·H₂O, <b>2</b>·H₂O. Complex <b>2</b>·H₂O reacts readily with excess pyridine, triethylphosphine, or pyrazine (pyz) to eliminate NEt₄Cl and give charge-neutral complexes <i>trans</i>-(<b>L</b>)­RhCl₂(py), <i>trans</i>-<b>3</b>, <i>trans</i>-(<b>L</b>)­RhCl₂(PEt₃), <i>trans</i>-<b>4</b>, or <i>trans</i>-(<b>L</b>)­RhCl₂(pyz), <i>trans</i>-<b>5</b>, where the incoming Lewis base is <i>trans</i>- to the amido nitrogen of the meridionally coordinating ligand. Heating solutions of complexes <i>trans</i>-<b>3</b> or <i>trans</i>-<b>4</b> above about 100 °C causes isomerization to the appropriate <i>cis</i>-<b>3</b> or <i>cis</i>-<b>4</b>. Isomerization of <i>trans</i>-<b>5</b> occurs at a much lower temperature due to pyrazine dissociation. <i>Cis</i>-<b>3</b> and <i>cis</i>-<b>5</b> could be reconverted to their respective <i>trans</i>- isomers in solution at 35 °C by visible light irradiation. Complexes [(<b>L</b>)­Rh­(py)₂Cl]­(PF₆), <b>6</b>, [(<b>L</b>)­Rh­(PPh₃)­(py)­Cl]­(PF₆), <b>7</b>, [(<b>L</b>)­Rh­(PEt₃)₂Cl]­(PF₆), <b>8</b>, and [(<b>L</b>)­RhCl­(bipy)]­(OTf = triflate), <b>9</b>, were prepared from <b>2</b>·H₂O by using thallium­(I) salts as halide abstraction agents and excess Lewis base. It was not possible to prepare dicationic complexes with three unidentate pyridyl or triethylphosphine ligands; however, the reaction between <b>2</b>, thallium­(I) triflate, and the tridentate 4′-(4-methylphenyl)-2,2′:6′,2″-terpyridine (ttpy) afforded a high yield of [(<b>L</b>)­Rh­(ttpy)]­(OTf)₂, <b>10</b>. The solid state structures of nine new complexes were obtained. The electrochemistry of the various derivatives in CH₂Cl₂ showed a ligand-based oxidation wave whose potential depended mainly on the charge of the complex, and to a lesser extent on the nature and the geometry of the other supporting ligands. Thus, the oxidation wave for <b>2</b> with an anionic complex was found at +0.27 V versus Ag/AgCl in CH₂Cl₂, while those waves for the charge-neutral complexes <b>3</b>–<b>5</b> were found between +0.38 to +0.59 V, where the <i>cis</i>- isomers were about 100 mV more stable toward oxidation than the <i>trans</i>- isomers. The oxidation waves for <b>6</b>–<b>9</b> with monocationic complexes occurred in the range +0.74 to 0.81 V while that for <b>10</b> with a dicationic complex occurred at +0.91 V. Chemical oxidation of <i>trans</i>-<b>3</b>, <i>cis</i>-<b>3</b>, and <b>8</b> afforded crystals of the singly oxidized complexes, [<i>trans</i>-(<b>L</b>)­RhCl₂(py)]­(SbCl₆), <i>cis</i>-[(<b>L</b>)­RhCl₂(py)]­(SbCl₄)·2CH₂Cl₂, and [(<b>L</b>)­Rh­(PEt₃)₂Cl]­(SbCl₆)₂, respectively. Comparisons of structural and spectroscopic features combined with the results of density functional theory (DFT) calculations between nonoxidized and oxidized forms of the complexes are indicative of the ligand-centered radicals in the oxidized derivatives

Homoleptic Nickel(II) Complexes of Redox-Tunable Pincer-type Ligands

Different synthetic methods have been developed to prepare eight new redox-active pincer-type ligands, H­(X,Y), that have pyrazol-1-yl flanking donors attached to an <i>ortho</i>-position of each ring of a diarylamine anchor and that have different groups, X and Y, at the <i>para</i>-aryl positions. Together with four previously known H­(X,Y) ligands, a series of 12 Ni­(X,Y)₂ complexes were prepared in high yields by a simple one-pot reaction. Six of the 12 derivatives were characterized by single-crystal X-ray diffraction, which showed tetragonally distorted hexacoordinate nickel­(II) centers. The nickel­(II) complexes exhibit two quasi-reversible one-electron oxidation waves in their cyclic voltammograms, with half-wave potentials that varied over a remarkable 700 mV range with the average of the Hammett σ_p parameters of the <i>para</i>-aryl X, Y groups. The one- and two-electron oxidized derivatives [Ni­(Me,Me)₂]­(BF₄)_<i>n</i> (<i>n</i> = 1, 2) were prepared synthetically, were characterized by X-band EPR, electronic spectroscopy, and single-crystal X-ray diffraction (for <i>n</i> = 2), and were studied computationally by DFT methods. The dioxidized complex, [Ni­(Me,Me)₂]­(BF₄)₂, is an <i>S</i> = 2 species, with nickel­(II) bound to two ligand radicals. The mono-oxidized complex [Ni­(Me,Me)₂]­(BF₄), prepared by comproportionation, is best described as nickel­(II) with one ligand centered radical. Neither the mono- nor the dioxidized derivative shows any substantial electronic coupling between the metal and their bound ligand radicals because of the orthogonal nature of their magnetic orbitals. On the other hand, weak electronic communication occurs between ligands in the mono-oxidized complex as evident from the intervalence charge transfer (IVCT) transition found in the near-IR absorption spectrum. Band shape analysis of the IVCT transition allowed comparisons of the strength of the electronic interaction with that in the related, previously known, Robin–Day class II mixed valence complex, [Ga­(Me,Me)₂]²⁺