7 research outputs found
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
κ<sup>2</sup><i>P</i>,<i>N</i> coordination
mode for this type of
ligand. Reactions between (PNN)Rh(CN<sup>t</sup>Bu)<sub>2</sub> and
iodomethane afford both <i>fac</i>- and <i></i><i>cis</i>,<i>mer</i>-[(PNN)Rh(CN<sup>t</sup>Bu)<sub>2</sub>(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 RhCl<sub>3</sub>·3H<sub>2</sub>O with H(<b>L</b>) results in the precipitation of insoluble
[H(<b>L</b>)]RhCl<sub>3</sub>, <b>1</b>. The reaction
of a methanol suspension of [H(<b>L</b>)]RhCl<sub>3</sub> with
NEt<sub>4</sub>OH causes ligand deprotonation and affords nearly quantitative
yields of the soluble, deep-green, title compound (NEt<sub>4</sub>)[(<b>L</b>)RhCl<sub>3</sub>]·H<sub>2</sub>O, <b>2</b>·H<sub>2</sub>O. Complex <b>2</b>·H<sub>2</sub>O
reacts readily with excess pyridine, triethylphosphine, or pyrazine
(pyz) to eliminate NEt<sub>4</sub>Cl and give charge-neutral complexes <i>trans</i>-(<b>L</b>)RhCl<sub>2</sub>(py), <i>trans</i>-<b>3</b>, <i>trans</i>-(<b>L</b>)RhCl<sub>2</sub>(PEt<sub>3</sub>), <i>trans</i>-<b>4</b>,
or <i>trans</i>-(<b>L</b>)RhCl<sub>2</sub>(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)<sub>2</sub>Cl](PF<sub>6</sub>), <b>6</b>, [(<b>L</b>)Rh(PPh<sub>3</sub>)(py)Cl](PF<sub>6</sub>), <b>7</b>, [(<b>L</b>)Rh(PEt<sub>3</sub>)<sub>2</sub>Cl](PF<sub>6</sub>), <b>8</b>, and [(<b>L</b>)RhCl(bipy)](OTf
= triflate), <b>9</b>, were prepared from <b>2</b>·H<sub>2</sub>O 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)<sub>2</sub>, <b>10</b>. The solid state structures of nine new
complexes were obtained. The electrochemistry of the various derivatives
in CH<sub>2</sub>Cl<sub>2</sub> 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 CH<sub>2</sub>Cl<sub>2</sub>, 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>)RhCl<sub>2</sub>(py)](SbCl<sub>6</sub>), <i>cis</i>-[(<b>L</b>)RhCl<sub>2</sub>(py)](SbCl<sub>4</sub>)·2CH<sub>2</sub>Cl<sub>2</sub>, and [(<b>L</b>)Rh(PEt<sub>3</sub>)<sub>2</sub>Cl](SbCl<sub>6</sub>)<sub>2</sub>, 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)<sub>2</sub>]<sup><i>n</i>+</sup> 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<sup>–</sup>)(L<sup>0</sup>)]<sup>2+</sup>, 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<sup>–1</sup>, depending on
the solvent. Band shape analyses and the use of Hush and Marcus relations
revealed a modest electronic coupling, <i>H</i><sub>ab</sub> of about 200 cm<sup>–1</sup>, and a large rate constant for
electron transfer, <i>k</i><sub>et</sub>, on the order of
10<sup>10</sup> s<sup>–1</sup> between redox active ligands.
The dioxidized complex [Ga(L<sup>0</sup>)<sub>2</sub>]<sup>3+</sup> shows a half-field Δ<i>M</i><sub>s</sub> = 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<sup>–1</sup> 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 RhCl<sub>3</sub>·3H<sub>2</sub>O with H(<b>L</b>) results in the precipitation of insoluble
[H(<b>L</b>)]RhCl<sub>3</sub>, <b>1</b>. The reaction
of a methanol suspension of [H(<b>L</b>)]RhCl<sub>3</sub> with
NEt<sub>4</sub>OH causes ligand deprotonation and affords nearly quantitative
yields of the soluble, deep-green, title compound (NEt<sub>4</sub>)[(<b>L</b>)RhCl<sub>3</sub>]·H<sub>2</sub>O, <b>2</b>·H<sub>2</sub>O. Complex <b>2</b>·H<sub>2</sub>O
reacts readily with excess pyridine, triethylphosphine, or pyrazine
(pyz) to eliminate NEt<sub>4</sub>Cl and give charge-neutral complexes <i>trans</i>-(<b>L</b>)RhCl<sub>2</sub>(py), <i>trans</i>-<b>3</b>, <i>trans</i>-(<b>L</b>)RhCl<sub>2</sub>(PEt<sub>3</sub>), <i>trans</i>-<b>4</b>,
or <i>trans</i>-(<b>L</b>)RhCl<sub>2</sub>(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)<sub>2</sub>Cl](PF<sub>6</sub>), <b>6</b>, [(<b>L</b>)Rh(PPh<sub>3</sub>)(py)Cl](PF<sub>6</sub>), <b>7</b>, [(<b>L</b>)Rh(PEt<sub>3</sub>)<sub>2</sub>Cl](PF<sub>6</sub>), <b>8</b>, and [(<b>L</b>)RhCl(bipy)](OTf
= triflate), <b>9</b>, were prepared from <b>2</b>·H<sub>2</sub>O 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)<sub>2</sub>, <b>10</b>. The solid state structures of nine new
complexes were obtained. The electrochemistry of the various derivatives
in CH<sub>2</sub>Cl<sub>2</sub> 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 CH<sub>2</sub>Cl<sub>2</sub>, 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>)RhCl<sub>2</sub>(py)](SbCl<sub>6</sub>), <i>cis</i>-[(<b>L</b>)RhCl<sub>2</sub>(py)](SbCl<sub>4</sub>)·2CH<sub>2</sub>Cl<sub>2</sub>, and [(<b>L</b>)Rh(PEt<sub>3</sub>)<sub>2</sub>Cl](SbCl<sub>6</sub>)<sub>2</sub>, 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)<sub>2</sub>]<sup><i>n</i>+</sup> 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<sup>–</sup>)(L<sup>0</sup>)]<sup>2+</sup>, 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<sup>–1</sup>, depending on
the solvent. Band shape analyses and the use of Hush and Marcus relations
revealed a modest electronic coupling, <i>H</i><sub>ab</sub> of about 200 cm<sup>–1</sup>, and a large rate constant for
electron transfer, <i>k</i><sub>et</sub>, on the order of
10<sup>10</sup> s<sup>–1</sup> between redox active ligands.
The dioxidized complex [Ga(L<sup>0</sup>)<sub>2</sub>]<sup>3+</sup> shows a half-field Δ<i>M</i><sub>s</sub> = 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<sup>–1</sup> 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 RhCl<sub>3</sub>·3H<sub>2</sub>O with H(<b>L</b>) results in the precipitation of insoluble
[H(<b>L</b>)]RhCl<sub>3</sub>, <b>1</b>. The reaction
of a methanol suspension of [H(<b>L</b>)]RhCl<sub>3</sub> with
NEt<sub>4</sub>OH causes ligand deprotonation and affords nearly quantitative
yields of the soluble, deep-green, title compound (NEt<sub>4</sub>)[(<b>L</b>)RhCl<sub>3</sub>]·H<sub>2</sub>O, <b>2</b>·H<sub>2</sub>O. Complex <b>2</b>·H<sub>2</sub>O
reacts readily with excess pyridine, triethylphosphine, or pyrazine
(pyz) to eliminate NEt<sub>4</sub>Cl and give charge-neutral complexes <i>trans</i>-(<b>L</b>)RhCl<sub>2</sub>(py), <i>trans</i>-<b>3</b>, <i>trans</i>-(<b>L</b>)RhCl<sub>2</sub>(PEt<sub>3</sub>), <i>trans</i>-<b>4</b>,
or <i>trans</i>-(<b>L</b>)RhCl<sub>2</sub>(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)<sub>2</sub>Cl](PF<sub>6</sub>), <b>6</b>, [(<b>L</b>)Rh(PPh<sub>3</sub>)(py)Cl](PF<sub>6</sub>), <b>7</b>, [(<b>L</b>)Rh(PEt<sub>3</sub>)<sub>2</sub>Cl](PF<sub>6</sub>), <b>8</b>, and [(<b>L</b>)RhCl(bipy)](OTf
= triflate), <b>9</b>, were prepared from <b>2</b>·H<sub>2</sub>O 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)<sub>2</sub>, <b>10</b>. The solid state structures of nine new
complexes were obtained. The electrochemistry of the various derivatives
in CH<sub>2</sub>Cl<sub>2</sub> 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 CH<sub>2</sub>Cl<sub>2</sub>, 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>)RhCl<sub>2</sub>(py)](SbCl<sub>6</sub>), <i>cis</i>-[(<b>L</b>)RhCl<sub>2</sub>(py)](SbCl<sub>4</sub>)·2CH<sub>2</sub>Cl<sub>2</sub>, and [(<b>L</b>)Rh(PEt<sub>3</sub>)<sub>2</sub>Cl](SbCl<sub>6</sub>)<sub>2</sub>, 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)<sub>2</sub> 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<sub>p</sub> parameters
of the <i>para</i>-aryl X, Y groups. The one- and two-electron
oxidized derivatives [Ni(Me,Me)<sub>2</sub>](BF<sub>4</sub>)<sub><i>n</i></sub> (<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)<sub>2</sub>](BF<sub>4</sub>)<sub>2</sub>, is an <i>S</i> = 2 species, with
nickel(II) bound to two ligand radicals. The mono-oxidized complex
[Ni(Me,Me)<sub>2</sub>](BF<sub>4</sub>), 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)<sub>2</sub>]<sup>2+</sup>