16 research outputs found
Photoinduced Ligand Exchange and Covalent DNA Binding by Two New Dirhodium Bis-Amidato Complexes
Two new dirhodium complexes, the head-to-tail (<i>H,T</i>) and head-to-head (<i>H,H</i>) isomers of <i>cis</i>-[Rh<sub>2</sub>(HNOCCH<sub>3</sub>)<sub>2</sub>(CH<sub>3</sub>CN)<sub>6</sub>]<sup>2+</sup>, were synthesized, separated,
and characterized
following the reaction of Rh<sub>2</sub>(HNOCCH<sub>3</sub>)<sub>4</sub> with trimethyloxonium tetrafluoroborate in CH<sub>3</sub>CN. The
products were characterized by <sup>1</sup>H NMR spectroscopy, mass
spectrometry, elemental analysis, and single crystal X-ray diffraction.
Each bis-amidato isomer has a total of six CH<sub>3</sub>CN ligands,
two along the internuclear Rh–Rh axis, CH<sub>3</sub>CN<sub><i>ax</i></sub>, two in equatorial positions <i>trans</i> to the oxygen atoms of the bridging amidato groups, CH<sub>3</sub>CN<sub><i>eq</i></sub><sup><i>O</i></sup>, and
two in equatorial positions <i>trans</i> to the amidato
nitrogen atoms, CH<sub>3</sub>CN<sub><i>eq</i></sub><sup><i>N</i></sup>. When aqueous solutions of the complexes
are irradiated with low energy light (λ<sub>irr</sub> ≥
495 nm, 60 min), both types of CH<sub>3</sub>CN<sub><i>eq</i></sub> ligands undergo efficient ligand exchange with solvent H<sub>2</sub>O molecules to form monoaqua, followed by bis-aqua, adducts,
releasing two CH<sub>3</sub>CN<sub><i>eq</i></sub> ligands
in the process. The quantum yields, Φ<sub>400nm</sub>, for the <i>H,T</i> and <i>H,H</i> isomers to form monoaqua adducts
are 0.43 and 0.38, respectively, which are substantially greater than
the 0.13 yield observed for <i>cis</i>-[Rh<sub>2</sub>(O<sub>2</sub>CCH<sub>3</sub>)<sub>2</sub>(CH<sub>3</sub>CN)<sub>6</sub>]<sup>2+</sup>; importantly, no ligand exchange is observed when
the complexes are kept in the dark. Finally, low energy excitation
(λ<sub>irr</sub> ≥ 610 nm, 30 min) of the <i>H,T</i> isomer was shown to generate photoproducts that covalently bind
to linearized DNA, making <b>2</b> a potential agent for photochemotherapy
that does not require the formation of <sup>1</sup>O<sub>2</sub>,
as is typical of organic photodynamic therapy (PDT) agents
THF Exchange and Molecular Dynamics in the Series (BDI)MgX(THF), Where X = Bu<sup><i>n</i></sup>, NEt<sub>2</sub>, and OBu<sup><i>t</i></sup> and BDI = 2‑[(2,6-Diisopropylphenyl)amino]-4-[(2,6-diisopropylphenyl)imino]pent-2-ene
The complexes (BDI)ÂMgXÂ(THF), where
X = Bu<sup><i>n</i></sup>, NEt<sub>2</sub>, and OBu<sup><i>t</i></sup>, are
shown to undergo THF exchange at low added concentrations of THF by
a dissociative mechanism: X = Bu<sup><i>n</i></sup>, Δ<i>H</i><sup>#</sup> (kcal mol<sup>–1</sup>) = 13.4 ±
0.4 and Δ<i>S</i><sup>#</sup> (cal mol<sup>–1</sup> K<sup>–1</sup>) = 6.3 ± 1.6; X = NEt<sub>2</sub>, Δ<i>H</i><sup>#</sup> (kcal mol<sup>–1</sup>) = 15.2 ±
0.5 and Δ<i>S</i><sup>#</sup> (cal mol<sup>–1</sup> K<sup>–1</sup>) = 11.4 ± 2.3; X = OBu<sup><i>t</i></sup>, Δ<i>H</i><sup>#</sup> (kcal mol<sup>–1</sup>) = 16.4 ± 0.3 and Δ<i>S</i><sup>#</sup> (cal
mol<sup>–1</sup> K<sup>–1</sup>) = 9.5 ± 1.3. The
apparent aryl group rotations involving the BDI ligands have, within
experimental error, the same activation parameters as the THF dissociation,
which suggests that the two are correlated involving a three coordinate
reactive intermediate akin to what is well-known for related (BDI)ÂZnR
compounds involving three-coordinate trigonal planar Zn<sup>2+</sup>. At higher concentrations of THF for X = Bu<sup><i>n</i></sup> and OBu<sup><i>t</i></sup>, but not for X = NEt<sub>2</sub>, the coalescence temperatures for apparent aryl group rotation
are depressed from those of the pure compounds in toluene-<i>d</i><sub>8</sub>, and evidence is presented that this correlates
with an associative interchange process which becomes dominant in
neat THF. We estimate the I<sub>a</sub> mechanism to have activation
parameters: Δ<i>H</i><sup>#</sup> (kcal mol<sup>–1</sup>) = 5.4 ± 0.1 and Δ<i>S</i><sup>#</sup> (cal
mol<sup>–1</sup> K<sup>–1</sup>) = −20.9 ±
0.3 for X = Bu<sup><i>n</i></sup> and Δ<i>H</i><sup>#</sup> (kcal mol<sup>–1</sup>) = 8.3 ± 0.8 and
Δ<i>S</i><sup>#</sup> (cal mol<sup>–1</sup> K<sup>–1</sup>) = −19.8 ± 3.0 for X = OBu<sup><i>t</i></sup>. For the complex (BDI)ÂMgBu<sup><i>n</i></sup>(2-MeTHF), the dissociative exchange with added 2-MeTHF occurs
more readily than for its THF analogue, as expected for the more sterically
demanding Lewis base O-donor: Δ<i>H</i><sup>#</sup> (kcal mol<sup>–1</sup>) = 12.8 ± 0.5 and Δ<i>S</i><sup>#</sup> (cal mol<sup>–1</sup> K<sup>–1</sup>) = 8.6 ± 1.8. At very low temperatures in toluene-<i>d</i><sub>8</sub>, restricted rotation about the Mg–OÂ(THF) bond
is observed for the complexes where X = NEt<sub>2</sub> and OBu<sup><i>t</i></sup> but not for the complex where X = Bu<sup><i>n</i></sup>. These observations, which have been determined
from dynamic NMR studies, are correlated with the reactivities of
these complexes in solution
THF Exchange and Molecular Dynamics in the Series (BDI)MgX(THF), Where X = Bu<sup><i>n</i></sup>, NEt<sub>2</sub>, and OBu<sup><i>t</i></sup> and BDI = 2‑[(2,6-Diisopropylphenyl)amino]-4-[(2,6-diisopropylphenyl)imino]pent-2-ene
The complexes (BDI)ÂMgXÂ(THF), where
X = Bu<sup><i>n</i></sup>, NEt<sub>2</sub>, and OBu<sup><i>t</i></sup>, are
shown to undergo THF exchange at low added concentrations of THF by
a dissociative mechanism: X = Bu<sup><i>n</i></sup>, Δ<i>H</i><sup>#</sup> (kcal mol<sup>–1</sup>) = 13.4 ±
0.4 and Δ<i>S</i><sup>#</sup> (cal mol<sup>–1</sup> K<sup>–1</sup>) = 6.3 ± 1.6; X = NEt<sub>2</sub>, Δ<i>H</i><sup>#</sup> (kcal mol<sup>–1</sup>) = 15.2 ±
0.5 and Δ<i>S</i><sup>#</sup> (cal mol<sup>–1</sup> K<sup>–1</sup>) = 11.4 ± 2.3; X = OBu<sup><i>t</i></sup>, Δ<i>H</i><sup>#</sup> (kcal mol<sup>–1</sup>) = 16.4 ± 0.3 and Δ<i>S</i><sup>#</sup> (cal
mol<sup>–1</sup> K<sup>–1</sup>) = 9.5 ± 1.3. The
apparent aryl group rotations involving the BDI ligands have, within
experimental error, the same activation parameters as the THF dissociation,
which suggests that the two are correlated involving a three coordinate
reactive intermediate akin to what is well-known for related (BDI)ÂZnR
compounds involving three-coordinate trigonal planar Zn<sup>2+</sup>. At higher concentrations of THF for X = Bu<sup><i>n</i></sup> and OBu<sup><i>t</i></sup>, but not for X = NEt<sub>2</sub>, the coalescence temperatures for apparent aryl group rotation
are depressed from those of the pure compounds in toluene-<i>d</i><sub>8</sub>, and evidence is presented that this correlates
with an associative interchange process which becomes dominant in
neat THF. We estimate the I<sub>a</sub> mechanism to have activation
parameters: Δ<i>H</i><sup>#</sup> (kcal mol<sup>–1</sup>) = 5.4 ± 0.1 and Δ<i>S</i><sup>#</sup> (cal
mol<sup>–1</sup> K<sup>–1</sup>) = −20.9 ±
0.3 for X = Bu<sup><i>n</i></sup> and Δ<i>H</i><sup>#</sup> (kcal mol<sup>–1</sup>) = 8.3 ± 0.8 and
Δ<i>S</i><sup>#</sup> (cal mol<sup>–1</sup> K<sup>–1</sup>) = −19.8 ± 3.0 for X = OBu<sup><i>t</i></sup>. For the complex (BDI)ÂMgBu<sup><i>n</i></sup>(2-MeTHF), the dissociative exchange with added 2-MeTHF occurs
more readily than for its THF analogue, as expected for the more sterically
demanding Lewis base O-donor: Δ<i>H</i><sup>#</sup> (kcal mol<sup>–1</sup>) = 12.8 ± 0.5 and Δ<i>S</i><sup>#</sup> (cal mol<sup>–1</sup> K<sup>–1</sup>) = 8.6 ± 1.8. At very low temperatures in toluene-<i>d</i><sub>8</sub>, restricted rotation about the Mg–OÂ(THF) bond
is observed for the complexes where X = NEt<sub>2</sub> and OBu<sup><i>t</i></sup> but not for the complex where X = Bu<sup><i>n</i></sup>. These observations, which have been determined
from dynamic NMR studies, are correlated with the reactivities of
these complexes in solution
New Syntheses and Structural Characterization of NH<sub>3</sub>BH<sub>2</sub>Cl and (BH<sub>2</sub>NH<sub>2</sub>)<sub>3</sub> and Thermal Decomposition Behavior of NH<sub>3</sub>BH<sub>2</sub>Cl
New convenient procedures for the preparation of ammonia
monochloroborane (NH<sub>3</sub>BH<sub>2</sub>Cl) and cyclotriborazane
[(BH<sub>2</sub>NH<sub>2</sub>)<sub>3</sub>] are described. Crystal
structures have been determined by single-crystal X-ray diffraction.
Strong H···Cl···H bifurcated hydrogen
bonding and weak N–H···H dihydrogen bonding
are observed in the crystal structure of ammonia monochloroborane.
When heated at 50 °C or under vacuum, ammonia monochloroborane
decomposes to (NH<sub>2</sub>BHCl)<sub><i>x</i></sub>, which
was characterized by NMR, elemental analysis, and powder X-ray diffraction.
Redetermination of the crystal structure of cyclotriborazane at low
temperature by single-crystal X-ray diffraction analysis provides
accurate hydrogen positions. Similar to ammonia borane, cyclotriborazane
shows extensive dihydrogen bonding of N–H···H
and B–H···H bonds with H<sup>δ+</sup>···H<sup>δ−</sup> interactions in the range of 2.00–2.34
Ã…
THF Exchange and Molecular Dynamics in the Series (BDI)MgX(THF), Where X = Bu<sup><i>n</i></sup>, NEt<sub>2</sub>, and OBu<sup><i>t</i></sup> and BDI = 2‑[(2,6-Diisopropylphenyl)amino]-4-[(2,6-diisopropylphenyl)imino]pent-2-ene
The complexes (BDI)ÂMgXÂ(THF), where
X = Bu<sup><i>n</i></sup>, NEt<sub>2</sub>, and OBu<sup><i>t</i></sup>, are
shown to undergo THF exchange at low added concentrations of THF by
a dissociative mechanism: X = Bu<sup><i>n</i></sup>, Δ<i>H</i><sup>#</sup> (kcal mol<sup>–1</sup>) = 13.4 ±
0.4 and Δ<i>S</i><sup>#</sup> (cal mol<sup>–1</sup> K<sup>–1</sup>) = 6.3 ± 1.6; X = NEt<sub>2</sub>, Δ<i>H</i><sup>#</sup> (kcal mol<sup>–1</sup>) = 15.2 ±
0.5 and Δ<i>S</i><sup>#</sup> (cal mol<sup>–1</sup> K<sup>–1</sup>) = 11.4 ± 2.3; X = OBu<sup><i>t</i></sup>, Δ<i>H</i><sup>#</sup> (kcal mol<sup>–1</sup>) = 16.4 ± 0.3 and Δ<i>S</i><sup>#</sup> (cal
mol<sup>–1</sup> K<sup>–1</sup>) = 9.5 ± 1.3. The
apparent aryl group rotations involving the BDI ligands have, within
experimental error, the same activation parameters as the THF dissociation,
which suggests that the two are correlated involving a three coordinate
reactive intermediate akin to what is well-known for related (BDI)ÂZnR
compounds involving three-coordinate trigonal planar Zn<sup>2+</sup>. At higher concentrations of THF for X = Bu<sup><i>n</i></sup> and OBu<sup><i>t</i></sup>, but not for X = NEt<sub>2</sub>, the coalescence temperatures for apparent aryl group rotation
are depressed from those of the pure compounds in toluene-<i>d</i><sub>8</sub>, and evidence is presented that this correlates
with an associative interchange process which becomes dominant in
neat THF. We estimate the I<sub>a</sub> mechanism to have activation
parameters: Δ<i>H</i><sup>#</sup> (kcal mol<sup>–1</sup>) = 5.4 ± 0.1 and Δ<i>S</i><sup>#</sup> (cal
mol<sup>–1</sup> K<sup>–1</sup>) = −20.9 ±
0.3 for X = Bu<sup><i>n</i></sup> and Δ<i>H</i><sup>#</sup> (kcal mol<sup>–1</sup>) = 8.3 ± 0.8 and
Δ<i>S</i><sup>#</sup> (cal mol<sup>–1</sup> K<sup>–1</sup>) = −19.8 ± 3.0 for X = OBu<sup><i>t</i></sup>. For the complex (BDI)ÂMgBu<sup><i>n</i></sup>(2-MeTHF), the dissociative exchange with added 2-MeTHF occurs
more readily than for its THF analogue, as expected for the more sterically
demanding Lewis base O-donor: Δ<i>H</i><sup>#</sup> (kcal mol<sup>–1</sup>) = 12.8 ± 0.5 and Δ<i>S</i><sup>#</sup> (cal mol<sup>–1</sup> K<sup>–1</sup>) = 8.6 ± 1.8. At very low temperatures in toluene-<i>d</i><sub>8</sub>, restricted rotation about the Mg–OÂ(THF) bond
is observed for the complexes where X = NEt<sub>2</sub> and OBu<sup><i>t</i></sup> but not for the complex where X = Bu<sup><i>n</i></sup>. These observations, which have been determined
from dynamic NMR studies, are correlated with the reactivities of
these complexes in solution
Catalytic EnantioÂselective Hetero-dimerization of Acrylates and 1,3-Dienes
1,3-Dienes are ubiquitous and easily
synthesized starting materials
for organic synthesis, and alkyl acrylates are among the most abundant
and cheapest feedstock carbon sources. A practical, highly enantioÂselective
union of these two readily available precursors giving valuable, enantio-pure
skipped 1,4-diene esters (with two configurationally defined double
bonds) is reported. The process uses commercially available cobalt
salts and chiral ligands. As illustrated by the use of 20 different
substrates, including 17 prochiral 1,3-dienes and 3 acrylates, this
hetero-dimerization reaction is tolerant of a number of common organic
functional groups (e.g., aromatic substituents, halides, isolated
mono- and di-substituted double bonds, esters, silyl ethers, and silyl
enol ethers). The novel results including ligand, counterion, and
solvent effects uncovered during the course of these investigations
show a unique role of a possible cationic CoÂ(I) intermediate in these
reactions. The rational evolution of a mechanism-based strategy that
led to the eventual successful outcome and the attendant support studies
may have further implications for the expanding use of low-valent
group 9 metal complexes in organic synthesis
Femtosecond Study of Dimolybdenum Paddlewheel Compounds with Amide/Thioamide Ligands: Symmetry, Electronic Structure, and Charge Distribution in the <sup>1</sup>MLCT S<sub>1</sub> State
Four
photophysically interesting dimolybdenum paddlewheel compounds are
synthesized and characterized: <b>I</b> and <b>II</b> contain
amide ligand (<i>N</i>,3-diphenyl-2-propynamide), and <b>III</b> and <b>IV</b> contain thioamide ligand (<i>N</i>,3-diphenyl-2-propynethioamide). <b>I</b> and <b>III</b> are <i>trans</i>-Mo<sub>2</sub>L<sub>2</sub>(O<sub>2</sub>C-T<sup><i>i</i></sup>PB)<sub>2</sub>-type
compounds, and <b>II</b> and <b>IV</b> are Mo<sub>2</sub>L<sub>4</sub>-type compounds, where O<sub>2</sub>C-T<sup><i>i</i></sup>PB is 2,4,6-triisopropylbenzoate. <b>I</b>–<b>IV</b> display strong light absorption due to metal to ligand
charge transfer (MLCT) transitions from molybdenum to the amide/thioamide
ligands. Charge transfer dynamics in the MLCT excited states of <b>I</b>–<b>IV</b> have been examined using femtosecond
transient absorption (fs-TA) spectroscopy and femtosecond time-resolved
infrared (fs-TRIR) spectroscopy. The asymmetric amide/thioamide ligands
show two forms of regioarrangements in the paddlewheel compounds.
Analyses of the νÂ(Cî—¼C) bands in the fs-TRIR spectra of <b>I</b> and <b>II</b> show similar electron density distribution
over ligands in their <sup>1</sup>MLCT S<sub>1</sub> states where
only two amide ligands are involved and the transferred electron is
mainly localized on one of them. The fs-TRIR spectra of <b>III</b> and <b>IV</b>, however, show different charge distribution
patterns where the transferred electron is fully delocalized over
two thioamide ligands in <b>III</b> and partially delocalized
in <b>IV</b>. Fast interligand electron transfer (ILET) was
recognized as the explanation for the various charge distribution
patterns, and ILET was shown to be influenced by both the ligands
and the ligand arrangements
Femtosecond Study of Dimolybdenum Paddlewheel Compounds with Amide/Thioamide Ligands: Symmetry, Electronic Structure, and Charge Distribution in the <sup>1</sup>MLCT S<sub>1</sub> State
Four
photophysically interesting dimolybdenum paddlewheel compounds are
synthesized and characterized: <b>I</b> and <b>II</b> contain
amide ligand (<i>N</i>,3-diphenyl-2-propynamide), and <b>III</b> and <b>IV</b> contain thioamide ligand (<i>N</i>,3-diphenyl-2-propynethioamide). <b>I</b> and <b>III</b> are <i>trans</i>-Mo<sub>2</sub>L<sub>2</sub>(O<sub>2</sub>C-T<sup><i>i</i></sup>PB)<sub>2</sub>-type
compounds, and <b>II</b> and <b>IV</b> are Mo<sub>2</sub>L<sub>4</sub>-type compounds, where O<sub>2</sub>C-T<sup><i>i</i></sup>PB is 2,4,6-triisopropylbenzoate. <b>I</b>–<b>IV</b> display strong light absorption due to metal to ligand
charge transfer (MLCT) transitions from molybdenum to the amide/thioamide
ligands. Charge transfer dynamics in the MLCT excited states of <b>I</b>–<b>IV</b> have been examined using femtosecond
transient absorption (fs-TA) spectroscopy and femtosecond time-resolved
infrared (fs-TRIR) spectroscopy. The asymmetric amide/thioamide ligands
show two forms of regioarrangements in the paddlewheel compounds.
Analyses of the νÂ(Cî—¼C) bands in the fs-TRIR spectra of <b>I</b> and <b>II</b> show similar electron density distribution
over ligands in their <sup>1</sup>MLCT S<sub>1</sub> states where
only two amide ligands are involved and the transferred electron is
mainly localized on one of them. The fs-TRIR spectra of <b>III</b> and <b>IV</b>, however, show different charge distribution
patterns where the transferred electron is fully delocalized over
two thioamide ligands in <b>III</b> and partially delocalized
in <b>IV</b>. Fast interligand electron transfer (ILET) was
recognized as the explanation for the various charge distribution
patterns, and ILET was shown to be influenced by both the ligands
and the ligand arrangements
Femtosecond Study of Dimolybdenum Paddlewheel Compounds with Amide/Thioamide Ligands: Symmetry, Electronic Structure, and Charge Distribution in the <sup>1</sup>MLCT S<sub>1</sub> State
Four
photophysically interesting dimolybdenum paddlewheel compounds are
synthesized and characterized: <b>I</b> and <b>II</b> contain
amide ligand (<i>N</i>,3-diphenyl-2-propynamide), and <b>III</b> and <b>IV</b> contain thioamide ligand (<i>N</i>,3-diphenyl-2-propynethioamide). <b>I</b> and <b>III</b> are <i>trans</i>-Mo<sub>2</sub>L<sub>2</sub>(O<sub>2</sub>C-T<sup><i>i</i></sup>PB)<sub>2</sub>-type
compounds, and <b>II</b> and <b>IV</b> are Mo<sub>2</sub>L<sub>4</sub>-type compounds, where O<sub>2</sub>C-T<sup><i>i</i></sup>PB is 2,4,6-triisopropylbenzoate. <b>I</b>–<b>IV</b> display strong light absorption due to metal to ligand
charge transfer (MLCT) transitions from molybdenum to the amide/thioamide
ligands. Charge transfer dynamics in the MLCT excited states of <b>I</b>–<b>IV</b> have been examined using femtosecond
transient absorption (fs-TA) spectroscopy and femtosecond time-resolved
infrared (fs-TRIR) spectroscopy. The asymmetric amide/thioamide ligands
show two forms of regioarrangements in the paddlewheel compounds.
Analyses of the νÂ(Cî—¼C) bands in the fs-TRIR spectra of <b>I</b> and <b>II</b> show similar electron density distribution
over ligands in their <sup>1</sup>MLCT S<sub>1</sub> states where
only two amide ligands are involved and the transferred electron is
mainly localized on one of them. The fs-TRIR spectra of <b>III</b> and <b>IV</b>, however, show different charge distribution
patterns where the transferred electron is fully delocalized over
two thioamide ligands in <b>III</b> and partially delocalized
in <b>IV</b>. Fast interligand electron transfer (ILET) was
recognized as the explanation for the various charge distribution
patterns, and ILET was shown to be influenced by both the ligands
and the ligand arrangements
Crystal Structures and Human Leukemia Cell Apoptosis Inducible Activities of Parthenolide Analogues Isolated from <i>Piptocoma rufescens</i>
The molecular structures of three
parthenolide analogues, (−)-goyazensolide
(<b>1</b>), (−)-15-deoxygoyazensolide (<b>2</b>), and (−)-ereglomerulide (<b>3</b>), isolated from
the leaves of <i>Piptocoma rufescens</i> in a previous study
were determined by X-ray analysis, and the absolute configuration
of (−)-goyazensolide (<b>1</b>) was confirmed crystallographically
using Cu Kα radiation at low temperature. Compounds <b>1</b>–<b>3</b>, (+)-rufesolide A (<b>4</b>), and commercial
parthenolide were found to be growth inhibitory toward MOLM-13 and
EOL-1 human acute myeloid leukemia cells using PKC412 (midostaurin)
as the positive control, with <b>1</b>–<b>3</b> being more active than parthenolide. Also, compounds <b>1</b>–<b>4</b> exhibited synergistic effects when tested
with PKC412, but parthenolide did not show this type of activity.
At a concentration lower than 2.0 μM, both <b>1</b> and <b>2</b> induced approximately 50% of the cells to become apoptotic
at a late stage of the cell cycle, but no similar apoptotic effects
were observed for <b>3</b>, <b>4</b>, or parthenolide.
Leukemia cell apoptosis was induced by these compounds through the
activation of caspase-3 and the inhibition of NF-κB, as indicated
by immunoblotting analysis, and compounds <b>1</b> and <b>2</b> seem to be promising leads for development as potential
antileukemic agents