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1. Expansion of the (BB)Ru metallacycle with coinage metal cations: formation of B–M–Ru–B (M = Cu, Ag, Au) dimetalacyclodiboryls

   https://doi.org/10.1039/C8SC00190A  

   Eleazer, Bennett J. ; Smith, Mark D. ; Popov, Alexey A. ; Peryshkov, Dmitry V. ( January 2018 , Chemical Science)

   In this work, we introduce a novel approach for the selective assembly of heterometallic complexes by unprecedented coordination of coinage metal cations to strained single ruthenium–boron bonds on a surface of icosahedral boron clusters. M( i ) cations (M = Cu, Ag, and Au) insert into B–Ru bonds of the (BB)–carboryne complex of ruthenium with the formation of four-membered B–M–Ru–B metalacycles. Results of theoretical calculations suggest that bonding within these metalacycles can be best described as unusual three-center-two-electron B–M⋯Ru interactions that are isolobal to B–H⋯Ru borane coordination for M = Cu and Ag, or the pairs of two-center-two electron B–Au and Au–Ru interactions for M = Au. These transformations comprise the first synthetic route to exohedral coinage metal boryl complexes of icosahedral closo -{C 2 B 10 } clusters, which feature short Cu–B (2.029(2) Å) and Ag–B (2.182(3) Å) bonds and the shortest Au–B bond (2.027(2) Å) reported to date. The reported heterometallic complexes contain Cu( i ) and Au( i ) centers in uncharacteristic square-planar coordination environments. These findings pave the way to rational construction of a broader class of multimetallic architectures featuring M–B bonds. 

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2. Cooperative C–H activation of pyridine by PBP complexes of Rh and Ir can lead to bridging 2-pyridyls with different connectivity to the B–M unit

   https://doi.org/10.1039/D1SC01850G  

   Cao, Yihan ; Shih, Wei-Chun ; Bhuvanesh, Nattamai ; Zhou, Jia ; Ozerov, Oleg V. ( November 2021 , Chemical Science)

   Pyridine and quinoline undergo selective C–H activation in the 2-position with Rh and Ir complexes of a boryl/bis(phosphine) PBP pincer ligand, resulting in a 2-pyridyl bridging the transition metal and the boron center. Examination of this reactivity with Rh and Ir complexes carrying different non-pincer ligands on the transition metal led to the realization of the possible isomerism derived from the 2-pyridyl fragment connecting either via B–N/C–M bonds or via B–C/N–M bonds. This M–C/M–N isomerism was systematically examined for four structural types. Each of these types has a defined set of ligands on Rh/Ir besides 2-pyridyl and PBP. A pair of M–C/M–N isomers for each type was computationally examined for Rh and for Ir, totaling 16 compounds. Several of these compounds were isolated or observed in solution by experimental methods, in addition to a few 2-quinolyl variants. The DFT predictions concerning the thermodynamic preference within each M–C/M–N isomeric match the experimental findings very well. In two cases where DFT predicts <2 kcal mol −1 difference in free energy, both isomers were experimentally observed in solution. Analysis of the structural data, of the relevant Wiberg bond indices, and of the ETS-NOCV partitioning of the interaction of the 2-pyridyl fragment with the rest of the molecule points to the strength of the M–C(pyridyl) bond as the dominant parameter determining the relative M–C/M–N isomer favorability. This M–C bond is always stronger for the analogous Ir vs. Rh compounds, but the nature of the ligand trans to it has a significant influence, as well. DFT calculations were used to evaluate the mechanism of isomerization for one of the molecule types. 

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3. Trimerization and cyclization of reactive P-functionalities confined within OCO pincers

   https://doi.org/10.1039/d1ra05926b  

   Chinen, Beatrice L. ; Hyvl, Jakub ; Brayton, Daniel F. ; Riek, Matthew M. ; Yoshida, Wesley Y. ; Chapp, Timothy W. ; Rheingold, Arnold L. ; Cain, Matthew F. ( August 2021 , RSC Advances)

   null (Ed.)

   In order to stabilize a 10–P–3 species with C 2v symmetry and two lone pairs on the central phosphorus atom, a specialized ligand is required. Using an NCN pincer, previous efforts to enforce this planarized geometry at P resulted in the formation of a C s -symmetric, 10π-electron benzazaphosphole that existed as a dynamic “bell-clapper” in solution. Here, OCO pincers 1 and 2 were synthesized, operating under the hypothesis that the more electron-withdrawing oxygen donors would better stabilize the 3-center, 4-electron O–P–O bond of the 10–P–3 target and the sp 3 -hybridized benzylic carbon atoms would prevent the formation of aromatic P-heterocycles. However, subjecting 1 to a metalation/phosphination/reduction sequence afforded cyclotriphosphane 3, resulting from trimerization of the P( i ) center unbound by its oxygen donors. Pincer 2 featuring four benzylic CF 3 groups was expected to strengthen the O–P–O bond of the target, but after metal–halogen exchange and quenching with PCl 3 , unexpected cyclization with loss of CH 3 Cl was observed to give monochlorinated 5. Treatment of 5 with ( p -CH 3 )C 6 H 4 MgBr generated crystalline P-( p -Tol) derivative 6, which was characterized by NMR spectroscopy, elemental analysis, and X-ray crystallography. The complex 19 F NMR spectra of 5 and 6 observed experimentally, were reproduced by simulations with MestreNova. 

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4. Aluminum‐Ligand Cooperative O−H Bond Activation Initiates Catalytic Transfer Hydrogenation

   https://doi.org/10.1002/cctc.202101869  

   Carr, Cody R. ; Vesto, James I. ; Xing, Xiujing ; Fettinger, James C. ; Berben, Louise A. ( April 2022 , ChemCatChem)

   Molecular design ultimately furnishes improvements in performance over time, and this has been the case for Rh‐ and Ir‐based molecular catalysts currently used in transfer hydrogenation (TH) reactions for fine chemical synthesis. In this report, we describe a molecular pincer ligand Al catalyst for TH, (I₂P²⁻)Al(THF)Cl (I₂P=diiminopyridine; THF=tetrahydrofuran). The mechanism for TH is initiated by two successive Al‐ligand cooperative bond activations of the O−H bonds in two molecules of isopropanol (ⁱPrOH) to afford six‐coordinate (H₂I₂P)Al(OⁱPr)₂Cl. Stoichiometric chemical reactions and kinetic experiments suggest an ordered transition state, supported by polar solvents, for concerted hydride transfer fromⁱPrO⁻to substrate. Metal‐ligand cooperative hydrogen bonding in a cyclic transition state is a likely support for the concerted hydride transfer event. The available data does not support involvement of an intermediate Al‐hydride in the TH. Proof‐of‐principle reactions including the conversion of isopropanol and benzophenone to acetone and diphenylmethanol with 90 % conversion in 1 h are described. The analogous hydride compound, (I₂P²⁻)Al(THF)H, also cleaves the O−H bond inⁱPrOH to afford (HI₂P⁻)Al(OⁱPr)H and (HI₂P⁻)Al(OⁱPr)₂, but no activity for catalytic TH was observed.

    

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5. Geometrical variations of two manganese(II) complexes with closely related quinoline-based tripodal ligands

   https://doi.org/10.1107/S2056989021009786  

   Frey, Steven T. ; Ballot, Jasper G. ; Hands, Allison ; Cirka, Haley A. ; Rinaolo, Katheryn C. ; Phalkun, Nich N. ; Kaur, Manpreet ; Jasinski, Jerry P. ( October 2021 , Acta Crystallographica Section E Crystallographic Communications)

   Structural analyses of the compounds di-μ-acetato-κ 4 O : O ′-bis{[2-methoxy- N , N -bis(quinolin-2-ylmethyl)ethanamine-κ 4 N , N ′, N ′′, O ]manganese(II)} bis(tetraphenylborate) dichloromethane 1.45-solvate, [Mn 2 (C 23 O 2 ) 2 (C 23 H 23 N 3 O) 2 ](C 24 H 20 B)·1.45CH 2 Cl 2 or [Mn(DQMEA)(μ-OAc) 2 Mn(DQMEA)](BPh 4 ) 2 ·1.45CH 2 Cl 2 or [1] (BPh 4 ) 2 ·1.45CH 2 Cl 2 , and (acetato-κ O )[2-hydroxy- N , N -bis(quinolin-2-ylmethyl)ethanamine-κ 4 N , N ′, N ′′, O ](methanol-κ O )manganese(II) tetraphenylborate methanol monosolvate, [Mn(CH 3 COO)(C 22 H 21 N 3 O)(CH 3 OH)](C 24 H 20 B)·CH 3 OH or [Mn(DQEA)(OAc)(CH 3 OH)]BPh 4 ·CH 3 OH or [2] BPh 4 ·CH 3 OH, by single-crystal X-ray diffraction reveal distinct differences in the geometry of coordination of the tripodal DQEA and DQMEA ligands to Mn II ions. In the asymmetric unit, compound [1] (BPh 4 ) 2 ·(CH 2 Cl 2 ) 1.45 crystallizes as a dimer in which each manganese(II) center is coordinated by the central amine nitrogen, the nitrogen atom of each quinoline group, and the methoxy-oxygen of the tetradentate DQMEA ligand, and two bridging-acetate oxygen atoms. The symmetric Mn II centers have a distorted, octahedral geometry in which the quinoline nitrogen atoms are trans to each other resulting in co-planarity of the quinoline rings. For each Mn II center, a coordinated acetate oxygen participates in C—H...O hydrogen-bonding interactions with the two quinolyl moieties, further stabilizing the trans structure. Within the crystal, weak π – π stacking interactions and intermolecular cation–anion interactions stabilize the crystal packing. In the asymmetric unit, compound [2] BPh 4 ·CH 3 OH crystallizes as a monomer in which the manganese(II) ion is coordinated to the central nitrogen, the nitrogen atom of each quinoline group, and the alcohol oxygen of the tetradentate DQEA ligand, an oxygen atom of OAc, and the oxygen atom of a methanol ligand. The geometry of the Mn II center in [2] BPh 4 ·CH 3 OH is also a distorted octahedron, but the quinoline nitrogen atoms are cis to each other in this structure. Hydrogen bonding between the acetate oxygen atoms and hydroxyl (O—H...O) and quinolyl (C—H...O and N—H...O) moieties of the DQEA ligand stabilize the complex in this cis configuration. Within the crystal, dimerization of complexes occurs by the formation of a pair of intermolecular O3—H3...O2 hydrogen bonds between the coordinated hydroxyl oxygen of the DQEA ligand of one complex and an acetate oxygen of another. Additional hydrogen-bonding and intermolecular cation–anion interactions contribute to the crystal packing. 

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