14 research outputs found
Isomer Dependence in the Assembly and Lability of Silver(I) Trifluoromethanesulfonate Complexes of the Heteroditopic Ligands, 2‑, 3‑, and 4‑[Di(1<i>H</i>‑pyrazolyl)methyl]phenyl(di‑<i>p</i>‑tolyl)phosphine
Three isomers of a new heteroditopic
ligand that contains a diÂ(1<i>H</i>-pyrazolyl)Âmethyl (−CHpz<sub>2</sub>) moiety connected to a diÂ(<i>p</i>-tolyl)Âphosphine
group via a <i>para</i>-, <i>meta</i>-, or <i>ortho</i>-phenylene spacer (<i><b>pL</b></i>, <i><b>mL</b></i>, and <i><b>oL</b></i>, respectively) have been synthesized by using a palladium(0)-catalyzed
coupling reaction between HPÂ(<i>p</i>-tolyl)<sub>2</sub> and the appropriate isomer of (IC<sub>6</sub>H<sub>4</sub>)ÂCHpz<sub>2</sub>. The 1:1 complexes of silverÂ(I) trifluoromethanesulfonate,
AgÂ(OTf), were prepared to examine the nature of ligand coordination
and the type of supramolecular isomer (monomeric, cyclic oligomeric,
or polymeric) that would be obtained. The single crystal X-ray diffraction
studies showed that [AgÂ(<i><b>pL</b></i>)]Â(OTf), <b>1</b>, and [AgÂ(<i><b>mL</b></i>)]Â(OTf), <b>2</b>, possessed cyclic dimeric dications, whereas [AgÂ(<i><b>oL</b></i>)]Â(OTf), <b>3</b>, was a coordination
polymer. The polymeric chain in <b>3</b> could be disrupted
by reaction with triphenylphosphine, and the resulting complex, [AgÂ(<i><b>oL</b></i>)Â(PPh<sub>3</sub>)]Â(OTf), <b>4</b>, possessed a monometallic cation where the ligand was bound to silver
in a chelating κ<sup>2</sup><i>P,N</i>- coordination
mode. The solution structures of <b>1</b>–<b>4</b> were probed via a combination of IR, variable-temperature multinuclear
(<sup>1</sup>H, <sup>13</sup>C, <sup>31</sup>P) NMR spectroscopy,
as well as by electron spray ionization (ESI)Â(+) mass spectrometry.
A related complex [AgÂ(<i>m</i>-IC<sub>6</sub>H<sub>4</sub>CHpz<sub>2</sub>)<sub>2</sub>]Â(OTf), <b>5</b>, was also
prepared, and its solid-state and solution spectroscopic properties
were studied for comparison purposes. These studies suggest that the
cyclic structures of <b>1</b> and <b>2</b> are likely
preserved but are dynamic in solution at room temperature. Moreover,
both <b>3</b> and <b>4</b> have dynamic solution structures
where <b>3</b> is likely extensively dissociated in CH<sub>3</sub>CN or acetone rather than being polymeric as in the solid state
Isomer Dependence in the Assembly and Lability of Silver(I) Trifluoromethanesulfonate Complexes of the Heteroditopic Ligands, 2‑, 3‑, and 4‑[Di(1<i>H</i>‑pyrazolyl)methyl]phenyl(di‑<i>p</i>‑tolyl)phosphine
Three isomers of a new heteroditopic
ligand that contains a diÂ(1<i>H</i>-pyrazolyl)Âmethyl (−CHpz<sub>2</sub>) moiety connected to a diÂ(<i>p</i>-tolyl)Âphosphine
group via a <i>para</i>-, <i>meta</i>-, or <i>ortho</i>-phenylene spacer (<i><b>pL</b></i>, <i><b>mL</b></i>, and <i><b>oL</b></i>, respectively) have been synthesized by using a palladium(0)-catalyzed
coupling reaction between HPÂ(<i>p</i>-tolyl)<sub>2</sub> and the appropriate isomer of (IC<sub>6</sub>H<sub>4</sub>)ÂCHpz<sub>2</sub>. The 1:1 complexes of silverÂ(I) trifluoromethanesulfonate,
AgÂ(OTf), were prepared to examine the nature of ligand coordination
and the type of supramolecular isomer (monomeric, cyclic oligomeric,
or polymeric) that would be obtained. The single crystal X-ray diffraction
studies showed that [AgÂ(<i><b>pL</b></i>)]Â(OTf), <b>1</b>, and [AgÂ(<i><b>mL</b></i>)]Â(OTf), <b>2</b>, possessed cyclic dimeric dications, whereas [AgÂ(<i><b>oL</b></i>)]Â(OTf), <b>3</b>, was a coordination
polymer. The polymeric chain in <b>3</b> could be disrupted
by reaction with triphenylphosphine, and the resulting complex, [AgÂ(<i><b>oL</b></i>)Â(PPh<sub>3</sub>)]Â(OTf), <b>4</b>, possessed a monometallic cation where the ligand was bound to silver
in a chelating κ<sup>2</sup><i>P,N</i>- coordination
mode. The solution structures of <b>1</b>–<b>4</b> were probed via a combination of IR, variable-temperature multinuclear
(<sup>1</sup>H, <sup>13</sup>C, <sup>31</sup>P) NMR spectroscopy,
as well as by electron spray ionization (ESI)Â(+) mass spectrometry.
A related complex [AgÂ(<i>m</i>-IC<sub>6</sub>H<sub>4</sub>CHpz<sub>2</sub>)<sub>2</sub>]Â(OTf), <b>5</b>, was also
prepared, and its solid-state and solution spectroscopic properties
were studied for comparison purposes. These studies suggest that the
cyclic structures of <b>1</b> and <b>2</b> are likely
preserved but are dynamic in solution at room temperature. Moreover,
both <b>3</b> and <b>4</b> have dynamic solution structures
where <b>3</b> is likely extensively dissociated in CH<sub>3</sub>CN or acetone rather than being polymeric as in the solid state
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 bisÂ(<i>tert</i>-butyl isocyanide)ÂrhodiumÂ(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)ÂRhÂ(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)ÂRhÂ(CN<sup>t</sup>Bu)<sub>2</sub>(Me)]Â(I), which further showcases the structural versatility
of the ligand
Reaction Chemistry of Silver(I) Trifluoromethanesulfonate Complexes of Nitrogen-Confused <i>C</i>‑Scorpionates
Two new <i>C</i>-scorpionate
ligands with a bisÂ(3,5-dimethylpyrazol-1-yl)Âmethyl group bound to
the 3 position of either an <i>N</i>-tosyl (<sup>Ts</sup>L*) or an N–H pyrazole (<sup>H</sup>L*) ring have been prepared.
The silverÂ(I) complexes of these new ligands and the two previously
reported analogous ligands with unsubstituted bisÂ(pyrazol-1-yl)Âmethyl
groups (<sup>Ts</sup>L and <sup>H</sup>L) in both 1:1 and 2:1 ligand/metal
ratios were investigated to explore the effects of ligand sterics
on their physical and chemical properties. The structurally characterized
derivatives of the type [AgÂ(L)<sub>2</sub>]Â(OTf) are four-coordinate,
where the confused pyrazolyl is not bound to the metal. On the other
hand, three 1:1 complexes [AgÂ(L)]Â(OTf) had all pyrazolyls bound, while
the μ–κ<sup>1</sup>,κ<sup>1</sup>-<sup>Ts</sup>L derivative had an unbound confused pyrazolyl. The molecularity
of the latter four ranged from polymeric to dimeric to monomeric in
the solid with increasing steric bulk of the ligand. The utility of
these complexes in stoichiometric ligand-transfer reactions and in
styrene aziridination was demonstrated. Thus, tricarbonylmanganeseÂ(I)
complexes were prepared as kinetically inert models for comparative
solution diffusion NMR studies. Also, [FeÂ(<sup>H</sup>L)<sub>2</sub>]Â(OTf)<sub>2</sub> was prepared for similar reasons and to compare
the effects of anion on spin-crossover properties
Cyclic versus Polymeric Supramolecular Architectures in Metal Complexes of Dinucleating Ligands: Silver(I) Trifluoromethanesulfonate Complexes of the Isomers of Bis(di(1H-pyrazolyl)methyl)-1,1′-biphenyl.
In the search for new examples of
systems that self-assemble into
cyclic metal–organic architectures, the six isomers of <i>X</i>,<i>Y</i>′-bisÂ(diÂ(1<i>H</i>-pyrazolyl)Âmethane)-1,1′-biphenyl, <b>L</b><sub><b>XY</b></sub>, and their silverÂ(I) trifluoromethanesulfonate
complexes were prepared. Five of the six silver complexes gave crystals
suitable for single crystal X-ray diffraction, with only the microcrystalline
derivative of 2,3′-bisÂ(diÂ(1<i>H</i>-pyrazolyl)Âmethane)-1,1′-biphenyl, <b>L</b><sub><b>23</b></sub>, proving to be unsuitable for
this analysis. Of the structurally characterized silverÂ(I) complexes,
that with <b>L</b><sub><b>22</b></sub> showed an unusual
trans-spanning chelating coordination mode to silver. At the same
time the ligand was also bound to a second silver center giving rise
to a cyclic supramolecular isomer with a 22-membered metallacycle.
The complex of <b>L</b><sub><b>34</b></sub> also gave
a cyclic dication but with a remarkable 28-membered metallacycle ring.
The remaining three derivatives were polymeric. The results of this
study underscore that a 120° angle between dipyrazolylmethyl
moieties across aromatic spacers will give rise to a cyclic dication
but this is not an exclusive requirement for the formation of cyclic
architectures. Also, the supramolecular structures of complexes are
assembled via a variety of noncovalent interactions involving the
diÂ(pyrazolyl)Âmethyl cation most notably by weak hydrogen bonding interactions
involving the methine hydrogen and an oxygen atom of the triflate
anion
Cyclic versus Polymeric Supramolecular Architectures in Metal Complexes of Dinucleating Ligands: Silver(I) Trifluoromethanesulfonate Complexes of the Isomers of Bis(di(1H-pyrazolyl)methyl)-1,1′-biphenyl.
In the search for new examples of
systems that self-assemble into
cyclic metal–organic architectures, the six isomers of <i>X</i>,<i>Y</i>′-bisÂ(diÂ(1<i>H</i>-pyrazolyl)Âmethane)-1,1′-biphenyl, <b>L</b><sub><b>XY</b></sub>, and their silverÂ(I) trifluoromethanesulfonate
complexes were prepared. Five of the six silver complexes gave crystals
suitable for single crystal X-ray diffraction, with only the microcrystalline
derivative of 2,3′-bisÂ(diÂ(1<i>H</i>-pyrazolyl)Âmethane)-1,1′-biphenyl, <b>L</b><sub><b>23</b></sub>, proving to be unsuitable for
this analysis. Of the structurally characterized silverÂ(I) complexes,
that with <b>L</b><sub><b>22</b></sub> showed an unusual
trans-spanning chelating coordination mode to silver. At the same
time the ligand was also bound to a second silver center giving rise
to a cyclic supramolecular isomer with a 22-membered metallacycle.
The complex of <b>L</b><sub><b>34</b></sub> also gave
a cyclic dication but with a remarkable 28-membered metallacycle ring.
The remaining three derivatives were polymeric. The results of this
study underscore that a 120° angle between dipyrazolylmethyl
moieties across aromatic spacers will give rise to a cyclic dication
but this is not an exclusive requirement for the formation of cyclic
architectures. Also, the supramolecular structures of complexes are
assembled via a variety of noncovalent interactions involving the
diÂ(pyrazolyl)Âmethyl cation most notably by weak hydrogen bonding interactions
involving the methine hydrogen and an oxygen atom of the triflate
anion
Tricarbonylrhenium(I) Complexes of Dinucleating Redox-Active Pincer Ligands
Two homobimetallic tricarbonylrheniumÂ(I)
complexes of new redox-active
dinucleating pincer ligands have been prepared to assess the impact
of spacer size on the first oxidation potentials with respect to mononucleating
analogues and on intramolecular electronic communication. The new
pincer ligands feature two tridentate <i>NNN</i>- sites
each composed of two pyrazolyl flanking donors and a diarylamido anchor
that are either directly linked (to form a central benzidene core,
H<sub>2</sub>(L1)) or linked via a <i>para</i>-phenylene
group (to form a <i>para</i>-terphenyldiamine core, H<sub>2</sub>(L2)). The bimetallic complexes of the deprotonated ligands,
[<i>fac</i>-ReÂ(CO)<sub>3</sub>]<sub>2</sub>(μ-L1), <b>1</b>, and [<i>fac</i>-ReÂ(CO)<sub>3</sub>]<sub>2</sub>(μ-L2), <b>2</b>, were fully characterized in solution
and the solid state including by single-crystal X-ray diffraction
for <b>1</b>. The electrochemical properties of each depended
strongly on solvent and electrolyte. Complex <b>1</b> exhibits
two one-electron oxidations in all electrolyte-containing solutions
but with separations between first and second oxidation potentials,
Δ<i>E</i><sub>1/2</sub>, between 119 and 316 mV depending
on conditions. On the other hand, cyclic voltammetry of <b>2</b> showed one two-electron oxidation in DMF with NBu<sub>4</sub>PF<sub>6</sub> as an electrolyte but two one-electron oxidations with a
maximal separation in Δ<i>E</i><sub>1/2</sub> of 96
mV in CH<sub>2</sub>Cl<sub>2</sub> with NBu<sub>4</sub>BÂ(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub> as an electrolyte. The oxidized complexes <b>1</b><sup><b>n+</b></sup> and <b>2</b><sup><b>n+</b></sup> (<i>n</i> = 1, 2) were
prepared by chemical oxidation and were studied spectroscopically
(UV–vis/NIR, EPR). The mono-oxidized complex <b>1</b><sup><b>+</b></sup> behaves as a Robin–Day Class III
species, while <b>2</b><sup><b>+</b></sup> is a Robin–Day
Class II species that shows thermal valence trapping at 77 K by EPR
spectroscopy. As suggested from theoretical studies using DFT methods,
the oxidized complexes maintain considerable ligand radical character,
so their electronic structures can be formulated as (CO)<sub>3</sub>Re<sup>I</sup>(μ-L<sup><i>n</i>+</sup>)ÂRe<sup>I</sup>(CO)<sub>3</sub> (<i>n</i> = 1 or 2)
Tricarbonylrhenium(I) Complexes of Dinucleating Redox-Active Pincer Ligands
Two homobimetallic tricarbonylrheniumÂ(I)
complexes of new redox-active
dinucleating pincer ligands have been prepared to assess the impact
of spacer size on the first oxidation potentials with respect to mononucleating
analogues and on intramolecular electronic communication. The new
pincer ligands feature two tridentate <i>NNN</i>- sites
each composed of two pyrazolyl flanking donors and a diarylamido anchor
that are either directly linked (to form a central benzidene core,
H<sub>2</sub>(L1)) or linked via a <i>para</i>-phenylene
group (to form a <i>para</i>-terphenyldiamine core, H<sub>2</sub>(L2)). The bimetallic complexes of the deprotonated ligands,
[<i>fac</i>-ReÂ(CO)<sub>3</sub>]<sub>2</sub>(μ-L1), <b>1</b>, and [<i>fac</i>-ReÂ(CO)<sub>3</sub>]<sub>2</sub>(μ-L2), <b>2</b>, were fully characterized in solution
and the solid state including by single-crystal X-ray diffraction
for <b>1</b>. The electrochemical properties of each depended
strongly on solvent and electrolyte. Complex <b>1</b> exhibits
two one-electron oxidations in all electrolyte-containing solutions
but with separations between first and second oxidation potentials,
Δ<i>E</i><sub>1/2</sub>, between 119 and 316 mV depending
on conditions. On the other hand, cyclic voltammetry of <b>2</b> showed one two-electron oxidation in DMF with NBu<sub>4</sub>PF<sub>6</sub> as an electrolyte but two one-electron oxidations with a
maximal separation in Δ<i>E</i><sub>1/2</sub> of 96
mV in CH<sub>2</sub>Cl<sub>2</sub> with NBu<sub>4</sub>BÂ(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub> as an electrolyte. The oxidized complexes <b>1</b><sup><b>n+</b></sup> and <b>2</b><sup><b>n+</b></sup> (<i>n</i> = 1, 2) were
prepared by chemical oxidation and were studied spectroscopically
(UV–vis/NIR, EPR). The mono-oxidized complex <b>1</b><sup><b>+</b></sup> behaves as a Robin–Day Class III
species, while <b>2</b><sup><b>+</b></sup> is a Robin–Day
Class II species that shows thermal valence trapping at 77 K by EPR
spectroscopy. As suggested from theoretical studies using DFT methods,
the oxidized complexes maintain considerable ligand radical character,
so their electronic structures can be formulated as (CO)<sub>3</sub>Re<sup>I</sup>(μ-L<sup><i>n</i>+</sup>)ÂRe<sup>I</sup>(CO)<sub>3</sub> (<i>n</i> = 1 or 2)
Syntheses and Electronic Properties of Rhodium(III) Complexes Bearing a Redox-Active Ligand
A series of rhodiumÂ(III) complexes of the redox-active
ligand,
HÂ(<b>L</b> = bisÂ(4-methyl-2-(1<i>H</i>-pyrazol-1-yl)Âphenyl)Âamido),
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 HÂ(<b>L</b>) results in the precipitation of insoluble
[HÂ(<b>L</b>)]ÂRhCl<sub>3</sub>, <b>1</b>. The reaction
of a methanol suspension of [HÂ(<b>L</b>)]ÂRhCl<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>)Â[(<b>L</b>)ÂRhCl<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>)ÂRhCl<sub>2</sub>(py), <i>trans</i>-<b>3</b>, <i>trans</i>-(<b>L</b>)ÂRhCl<sub>2</sub>(PEt<sub>3</sub>), <i>trans</i>-<b>4</b>,
or <i>trans</i>-(<b>L</b>)ÂRhCl<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>)ÂRhÂ(py)<sub>2</sub>Cl]Â(PF<sub>6</sub>), <b>6</b>, [(<b>L</b>)ÂRhÂ(PPh<sub>3</sub>)Â(py)ÂCl]Â(PF<sub>6</sub>), <b>7</b>, [(<b>L</b>)ÂRhÂ(PEt<sub>3</sub>)<sub>2</sub>Cl]Â(PF<sub>6</sub>), <b>8</b>, and [(<b>L</b>)ÂRhClÂ(bipy)]Â(OTf
= triflate), <b>9</b>, were prepared from <b>2</b>·H<sub>2</sub>O by using thalliumÂ(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>, thalliumÂ(I) triflate, and the
tridentate 4′-(4-methylphenyl)-2,2′:6′,2″-terpyridine
(ttpy) afforded a high yield of [(<b>L</b>)ÂRhÂ(ttpy)]Â(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>)ÂRhCl<sub>2</sub>(py)]Â(SbCl<sub>6</sub>), <i>cis</i>-[(<b>L</b>)ÂRhCl<sub>2</sub>(py)]Â(SbCl<sub>4</sub>)·2CH<sub>2</sub>Cl<sub>2</sub>, and [(<b>L</b>)ÂRhÂ(PEt<sub>3</sub>)<sub>2</sub>Cl]Â(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
Syntheses and Electronic Properties of Rhodium(III) Complexes Bearing a Redox-Active Ligand
A series of rhodiumÂ(III) complexes of the redox-active
ligand,
HÂ(<b>L</b> = bisÂ(4-methyl-2-(1<i>H</i>-pyrazol-1-yl)Âphenyl)Âamido),
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 HÂ(<b>L</b>) results in the precipitation of insoluble
[HÂ(<b>L</b>)]ÂRhCl<sub>3</sub>, <b>1</b>. The reaction
of a methanol suspension of [HÂ(<b>L</b>)]ÂRhCl<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>)Â[(<b>L</b>)ÂRhCl<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>)ÂRhCl<sub>2</sub>(py), <i>trans</i>-<b>3</b>, <i>trans</i>-(<b>L</b>)ÂRhCl<sub>2</sub>(PEt<sub>3</sub>), <i>trans</i>-<b>4</b>,
or <i>trans</i>-(<b>L</b>)ÂRhCl<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>)ÂRhÂ(py)<sub>2</sub>Cl]Â(PF<sub>6</sub>), <b>6</b>, [(<b>L</b>)ÂRhÂ(PPh<sub>3</sub>)Â(py)ÂCl]Â(PF<sub>6</sub>), <b>7</b>, [(<b>L</b>)ÂRhÂ(PEt<sub>3</sub>)<sub>2</sub>Cl]Â(PF<sub>6</sub>), <b>8</b>, and [(<b>L</b>)ÂRhClÂ(bipy)]Â(OTf
= triflate), <b>9</b>, were prepared from <b>2</b>·H<sub>2</sub>O by using thalliumÂ(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>, thalliumÂ(I) triflate, and the
tridentate 4′-(4-methylphenyl)-2,2′:6′,2″-terpyridine
(ttpy) afforded a high yield of [(<b>L</b>)ÂRhÂ(ttpy)]Â(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>)ÂRhCl<sub>2</sub>(py)]Â(SbCl<sub>6</sub>), <i>cis</i>-[(<b>L</b>)ÂRhCl<sub>2</sub>(py)]Â(SbCl<sub>4</sub>)·2CH<sub>2</sub>Cl<sub>2</sub>, and [(<b>L</b>)ÂRhÂ(PEt<sub>3</sub>)<sub>2</sub>Cl]Â(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