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>-[Rh₂(HNOCCH₃)₂(CH₃CN)₆]²⁺, were synthesized, separated, and characterized following the reaction of Rh₂(HNOCCH₃)₄ with trimethyloxonium tetrafluoroborate in CH₃CN. The products were characterized by ¹H NMR spectroscopy, mass spectrometry, elemental analysis, and single crystal X-ray diffraction. Each bis-amidato isomer has a total of six CH₃CN ligands, two along the internuclear Rh–Rh axis, CH₃CN_<i>ax</i>, two in equatorial positions <i>trans</i> to the oxygen atoms of the bridging amidato groups, CH₃CN_<i>eq</i>^<i>O</i>, and two in equatorial positions <i>trans</i> to the amidato nitrogen atoms, CH₃CN_<i>eq</i>^<i>N</i>. When aqueous solutions of the complexes are irradiated with low energy light (λ_irr ≥ 495 nm, 60 min), both types of CH₃CN_<i>eq</i> ligands undergo efficient ligand exchange with solvent H₂O molecules to form monoaqua, followed by bis-aqua, adducts, releasing two CH₃CN_<i>eq</i> ligands in the process. The quantum yields, Φ_400nm, 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>-[Rh₂(O₂CCH₃)₂(CH₃CN)₆]²⁺; importantly, no ligand exchange is observed when the complexes are kept in the dark. Finally, low energy excitation (λ_irr ≥ 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 ¹O₂, as is typical of organic photodynamic therapy (PDT) agents

THF Exchange and Molecular Dynamics in the Series (BDI)MgX(THF), Where X = Bu^<i>n</i>, NEt₂, and OBu^<i>t</i> and BDI = 2‑[(2,6-Diisopropylphenyl)amino]-4-[(2,6-diisopropylphenyl)imino]pent-2-ene

The complexes (BDI)­MgX­(THF), where X = Bu^<i>n</i>, NEt₂, and OBu^<i>t</i>, are shown to undergo THF exchange at low added concentrations of THF by a dissociative mechanism: X = Bu^<i>n</i>, Δ<i>H</i>^# (kcal mol^–1) = 13.4 ± 0.4 and Δ<i>S</i>^# (cal mol^–1 K^–1) = 6.3 ± 1.6; X = NEt₂, Δ<i>H</i>^# (kcal mol^–1) = 15.2 ± 0.5 and Δ<i>S</i>^# (cal mol^–1 K^–1) = 11.4 ± 2.3; X = OBu^<i>t</i>, Δ<i>H</i>^# (kcal mol^–1) = 16.4 ± 0.3 and Δ<i>S</i>^# (cal mol^–1 K^–1) = 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)­ZnR compounds involving three-coordinate trigonal planar Zn²⁺. At higher concentrations of THF for X = Bu^<i>n</i> and OBu^<i>t</i>, but not for X = NEt₂, the coalescence temperatures for apparent aryl group rotation are depressed from those of the pure compounds in toluene-<i>d</i>₈, and evidence is presented that this correlates with an associative interchange process which becomes dominant in neat THF. We estimate the I_a mechanism to have activation parameters: Δ<i>H</i>^# (kcal mol^–1) = 5.4 ± 0.1 and Δ<i>S</i>^# (cal mol^–1 K^–1) = −20.9 ± 0.3 for X = Bu^<i>n</i> and Δ<i>H</i>^# (kcal mol^–1) = 8.3 ± 0.8 and Δ<i>S</i>^# (cal mol^–1 K^–1) = −19.8 ± 3.0 for X = OBu^<i>t</i>. For the complex (BDI)­MgBu^<i>n</i>(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>^# (kcal mol^–1) = 12.8 ± 0.5 and Δ<i>S</i>^# (cal mol^–1 K^–1) = 8.6 ± 1.8. At very low temperatures in toluene-<i>d</i>₈, restricted rotation about the Mg–O­(THF) bond is observed for the complexes where X = NEt₂ and OBu^<i>t</i> but not for the complex where X = Bu^<i>n</i>. 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^<i>n</i>, NEt₂, and OBu^<i>t</i> and BDI = 2‑[(2,6-Diisopropylphenyl)amino]-4-[(2,6-diisopropylphenyl)imino]pent-2-ene

The complexes (BDI)­MgX­(THF), where X = Bu^<i>n</i>, NEt₂, and OBu^<i>t</i>, are shown to undergo THF exchange at low added concentrations of THF by a dissociative mechanism: X = Bu^<i>n</i>, Δ<i>H</i>^# (kcal mol^–1) = 13.4 ± 0.4 and Δ<i>S</i>^# (cal mol^–1 K^–1) = 6.3 ± 1.6; X = NEt₂, Δ<i>H</i>^# (kcal mol^–1) = 15.2 ± 0.5 and Δ<i>S</i>^# (cal mol^–1 K^–1) = 11.4 ± 2.3; X = OBu^<i>t</i>, Δ<i>H</i>^# (kcal mol^–1) = 16.4 ± 0.3 and Δ<i>S</i>^# (cal mol^–1 K^–1) = 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)­ZnR compounds involving three-coordinate trigonal planar Zn²⁺. At higher concentrations of THF for X = Bu^<i>n</i> and OBu^<i>t</i>, but not for X = NEt₂, the coalescence temperatures for apparent aryl group rotation are depressed from those of the pure compounds in toluene-<i>d</i>₈, and evidence is presented that this correlates with an associative interchange process which becomes dominant in neat THF. We estimate the I_a mechanism to have activation parameters: Δ<i>H</i>^# (kcal mol^–1) = 5.4 ± 0.1 and Δ<i>S</i>^# (cal mol^–1 K^–1) = −20.9 ± 0.3 for X = Bu^<i>n</i> and Δ<i>H</i>^# (kcal mol^–1) = 8.3 ± 0.8 and Δ<i>S</i>^# (cal mol^–1 K^–1) = −19.8 ± 3.0 for X = OBu^<i>t</i>. For the complex (BDI)­MgBu^<i>n</i>(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>^# (kcal mol^–1) = 12.8 ± 0.5 and Δ<i>S</i>^# (cal mol^–1 K^–1) = 8.6 ± 1.8. At very low temperatures in toluene-<i>d</i>₈, restricted rotation about the Mg–O­(THF) bond is observed for the complexes where X = NEt₂ and OBu^<i>t</i> but not for the complex where X = Bu^<i>n</i>. 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 NH₃BH₂Cl and (BH₂NH₂)₃ and Thermal Decomposition Behavior of NH₃BH₂Cl

New convenient procedures for the preparation of ammonia monochloroborane (NH₃BH₂Cl) and cyclotriborazane [(BH₂NH₂)₃] 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 (NH₂BHCl)_<i>x</i>, 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^δ+···H^δ− interactions in the range of 2.00–2.34 Å

THF Exchange and Molecular Dynamics in the Series (BDI)MgX(THF), Where X = Bu^<i>n</i>, NEt₂, and OBu^<i>t</i> and BDI = 2‑[(2,6-Diisopropylphenyl)amino]-4-[(2,6-diisopropylphenyl)imino]pent-2-ene

The complexes (BDI)­MgX­(THF), where X = Bu^<i>n</i>, NEt₂, and OBu^<i>t</i>, are shown to undergo THF exchange at low added concentrations of THF by a dissociative mechanism: X = Bu^<i>n</i>, Δ<i>H</i>^# (kcal mol^–1) = 13.4 ± 0.4 and Δ<i>S</i>^# (cal mol^–1 K^–1) = 6.3 ± 1.6; X = NEt₂, Δ<i>H</i>^# (kcal mol^–1) = 15.2 ± 0.5 and Δ<i>S</i>^# (cal mol^–1 K^–1) = 11.4 ± 2.3; X = OBu^<i>t</i>, Δ<i>H</i>^# (kcal mol^–1) = 16.4 ± 0.3 and Δ<i>S</i>^# (cal mol^–1 K^–1) = 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)­ZnR compounds involving three-coordinate trigonal planar Zn²⁺. At higher concentrations of THF for X = Bu^<i>n</i> and OBu^<i>t</i>, but not for X = NEt₂, the coalescence temperatures for apparent aryl group rotation are depressed from those of the pure compounds in toluene-<i>d</i>₈, and evidence is presented that this correlates with an associative interchange process which becomes dominant in neat THF. We estimate the I_a mechanism to have activation parameters: Δ<i>H</i>^# (kcal mol^–1) = 5.4 ± 0.1 and Δ<i>S</i>^# (cal mol^–1 K^–1) = −20.9 ± 0.3 for X = Bu^<i>n</i> and Δ<i>H</i>^# (kcal mol^–1) = 8.3 ± 0.8 and Δ<i>S</i>^# (cal mol^–1 K^–1) = −19.8 ± 3.0 for X = OBu^<i>t</i>. For the complex (BDI)­MgBu^<i>n</i>(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>^# (kcal mol^–1) = 12.8 ± 0.5 and Δ<i>S</i>^# (cal mol^–1 K^–1) = 8.6 ± 1.8. At very low temperatures in toluene-<i>d</i>₈, restricted rotation about the Mg–O­(THF) bond is observed for the complexes where X = NEt₂ and OBu^<i>t</i> but not for the complex where X = Bu^<i>n</i>. These observations, which have been determined from dynamic NMR studies, are correlated with the reactivities of these complexes in solution

Catalytic Enantio­selective 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 enantio­selective 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 Co­(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 ¹MLCT S₁ 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>-Mo₂L₂(O₂C-T^<i>i</i>PB)₂-type compounds, and <b>II</b> and <b>IV</b> are Mo₂L₄-type compounds, where O₂C-T^<i>i</i>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 ν­(CC) bands in the fs-TRIR spectra of <b>I</b> and <b>II</b> show similar electron density distribution over ligands in their ¹MLCT S₁ 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 ¹MLCT S₁ 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>-Mo₂L₂(O₂C-T^<i>i</i>PB)₂-type compounds, and <b>II</b> and <b>IV</b> are Mo₂L₄-type compounds, where O₂C-T^<i>i</i>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 ν­(CC) bands in the fs-TRIR spectra of <b>I</b> and <b>II</b> show similar electron density distribution over ligands in their ¹MLCT S₁ 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 ¹MLCT S₁ 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>-Mo₂L₂(O₂C-T^<i>i</i>PB)₂-type compounds, and <b>II</b> and <b>IV</b> are Mo₂L₄-type compounds, where O₂C-T^<i>i</i>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 ν­(CC) bands in the fs-TRIR spectra of <b>I</b> and <b>II</b> show similar electron density distribution over ligands in their ¹MLCT S₁ 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