Icosahedral Meta-Carboranes Containing Exopolyhedral B-Se and B-Te Bonds

Chalcogen-containing carboranes have been known for several decades and possess stable exopolyhedral B(9)-Se and B(9)-Te σ bonds despite the electron-donating ability of the B(9) vertex. While these molecules are known, little has been done to thoroughly evaluate their electrophilic and nucleophilic behavior. Herein, we report an assessment of the electrophilic reactivity of meta-carboranyl selenyl (II), tellurenyl (II), and tellurenyl (IV) chlorides and establish their reactivity pattern with Grignard reagents, alkenes, alkynes, enolates, and electron-rich arenes. These electrophilic reactions afford unique electron-rich B-Y-C (Y = Se, Te) bonding motifs not commonly found before. Furthermore, we show that meta-carboranyl selenolate, and even meta-carboranyl tellurolate, can be competent nucleophiles and participate in nucleophilic aromatic substitution reactions. Arene substitution chemistry is shown to be further extended to electron-rich species via the palladium mediated cross-coupling chemistry.

Efficient Direct Nitrosylation of α-Diimine Rhenium Tricarbonyl Complexes to Structurally Nearly Identical Higher Charge Congeners Activable towards Photo-CO Release

The reaction of rhenium α-diimine (N-N) tricarbonyl complexes with nitrosonium tetrafluoroborate yields the corresponding dicarbonyl-nitrosyl [Re(CO)2(NO)(N-N)X]+ species (where X = halide). The complexes, accessible in a single step in good yield, are structurally nearly identical higher charge congeners of the tricarbonyl molecules. Substitution chemistry aimed at the realization of equivalent dicationic species (intended for applications as potential antimicrobial agents), revealed that the reactivity of metal ion in [Re(CO)2(NO)(N-N)X]+ is that of a hard Re acid, probably due to the stronger π-acceptor properties of NO+ as compared to those of CO. The metal ion thus shows great affinity for π-basic ligands, which are consequently difficult to replace by, e.g., σ-donor or weak π-acids like pyridine. Attempts of direct nitrosylation of α-diimine fac-[Re(CO)3]+ complexes bearing π-basic OR-type ligands gave the [Re(CO)2(NO)(N-N)(BF4)][BF4] salt as the only product in good yield, featuring a stable Re-FBF3 bond. The solid state crystal structure of nearly all molecules presented could be elucidated. A fundamental consequence of the chemistry of [Re(CO)2(NO)(N-N)X]+ complexes, it that the same can be photo-activated towards CO release and represent an entirely new class of photoCORMs.

Beyond the Simple Copper(II) Coordination Chemistry with Quinaldinate and Secondary Amines

Copper(II) acetate has reacted in methanol with quinaldinic acid (quinoline-2-carboxylic acid) to form [Cu(quin)2(CH3OH)]∙CH3OH (1) (quin− = an anionic form of the acid) with quinaldinates bound in a bidentate chelating manner. In the air, complex 1 gives off methanol and binds water. The conversion was monitored by IR spectroscopy. The aqua complex has shown a facile substitution chemistry with alicyclic secondary amines, pyrrolidine (pyro), and morpholine (morph). trans-[Cu(quin)2(pyro)2] (2) and trans-[Cu(quin)2(morph)2] (4) were obtained in good yields. The morpholine system has produced a by-product, trans-[Cu(en)2(H2O)2](morphCOO)2 (5) (morphCOO− = morphylcarbamate), a result of the copper(II) quinaldinate reaction with ethylenediamine (en), an inherent impurity in morpholine, and the amine reaction with carbon dioxide. (pyroH)[Cu(quin)2Cl] (3) forms on the recrystallization of [Cu(quin)2(pyro)2] from dichloromethane, confirming a reaction between amine and the solvent. Similarly, a homologous amine, piperidine (pipe), and dichloromethane produced (pipeH)[Cu(quin)2Cl] (11). The piperidine system has afforded both mono- and bis-amine complexes, [Cu(quin)2(pipe)] (6) and trans-[Cu(quin)2(pipe)2] (7). The latter also exists in solvated forms, [Cu(quin)2(pipe)2]∙CH3CN (8) and [Cu(quin)2(pipe)2]∙CH3CH2CN (9). Interestingly, only the piperidine system has experienced a reduction of copper(II). The involvement of amine in the reduction was undoubtedly confirmed by identification of a polycyclic piperidine compound 10, 6,13-di(piperidin-1-yl)dodecahydro-2H,6H-7,14-methanodipyrido[1,2-a:1′,2′-e][1,5]diazocine.

Oxidative Generation of Boron-Centered Radicals in Carboranes

<div><div><div><p>We report the first indirect observation and use of boron vertex-centered carboranyl radicals generated by the oxidation of modified carboranyl precursors. These radical intermediates are formed by the direct oxidation of a B−B bond between a boron cluster cage and an exopolyhedral boron-based substituent (e.g., −BF3K, −B(OH)2). The in situ generated radical species are shown to be competent substrates in reactions with oxygen-based radicals, dichalcogenides, and N-heterocycles, yielding the corresponding substituted carboranes containing B−O, B−S, B−Se, B−Te, and B−C bonds. Remarkably, this chemistry tolerates various electronic environments, providing access to facile substitution chemistry at both electron-rich and electron-poor B−H vertices in carboranes.</p></div></div></div>

Oxidative Generation of Boron-Centered Radicals in Carboranes

<div><div><div><p>We report the first indirect observation and use of boron vertex-centered carboranyl radicals generated by the oxidation of modified carboranyl precursors. These radical intermediates are formed by the direct oxidation of a B−B bond between a boron cluster cage and an exopolyhedral boron-based substituent (e.g., −BF3K, −B(OH)2). The in situ generated radical species are shown to be competent substrates in reactions with oxygen-based radicals, dichalcogenides, and N-heterocycles, yielding the corresponding substituted carboranes containing B−O, B−S, B−Se, B−Te, and B−C bonds. Remarkably, this chemistry tolerates various electronic environments, providing access to facile substitution chemistry at both electron-rich and electron-poor B−H vertices in carboranes.</p></div></div></div>

Competitive Ligand Exchange and Dissociation in Ru Indenyl Complexes

Kinetic profiles obtained from monitoring the solution phase substitution chemistry of [Ru(η⁵-indenyl)(NCPh)(PPh₃)₂]⁺ (<b>1</b>) by both ESI-MS and ³¹P{¹H} NMR are essentially identical, despite an enormous difference in sample concentrations for these complementary techniques. These studies demonstrate dissociative substitution of the NCPh ligand in <b>1</b>. Competition experiments using different secondary phosphine reagents provide a ranking of phosphine donor abilities at this relatively crowded half-sandwich complex: PEt₂H > PPh₂H >> PCy₂H. The impact of steric congestion at Ru is evident also in reactions of <b>1</b> with tertiary phosphines; initial substitution products [Ru(η⁵-indenyl)(PR₃)(PPh₃)₂]⁺ rapidly lose PPh₃, enabling competitive recoordination of NCPh. Further solution experiments, relevant to the use of <b>1</b> in catalytic hydrophosphination, show that PPh₂H out-competes PPh₂CH₂CH₂CO₂Bu<i>^t</i> (the product of hydrophosphination of <i>tert</i>-butyl acrylate by PPh₂H) for coordination to Ru, even in the presence of a ten-fold excess of the tertiary phosphine. Additional information on relative phosphine binding strengths was obtained from gas-phase MS/MS experiments, including collision-induced dissociation (CID) experiments on the mixed phosphine complexes [Ru(η⁵-indenyl)PP’P’’]⁺, which ultimately appear in solution during the secondary phosphine competition experiments. Unexpectedly, unsaturated complexes [Ru(η⁵-indenyl)(PR₂H)(PPh₃)]⁺, generated in the gas-phase, undergo preferential loss of PR₂H. We propose competing orthometallation of PPh₃ is responsible for the surprising stability of the [Ru(η⁵-indenyl)(PPh₃)]⁺ fragment under these conditions.

Competitive Ligand Exchange and Dissociation in Ru Indenyl Complexes

Kinetic profiles obtained from monitoring the solution phase substitution chemistry of [Ru(η⁵-indenyl)(NCPh)(PPh₃)₂]⁺ (<b>1</b>) by both ESI-MS and ³¹P{¹H} NMR are essentially identical, despite an enormous difference in sample concentrations for these complementary techniques. These studies demonstrate dissociative substitution of the NCPh ligand in <b>1</b>. Competition experiments using different secondary phosphine reagents provide a ranking of phosphine donor abilities at this relatively crowded half-sandwich complex: PEt₂H > PPh₂H >> PCy₂H. The impact of steric congestion at Ru is evident also in reactions of <b>1</b> with tertiary phosphines; initial substitution products [Ru(η⁵-indenyl)(PR₃)(PPh₃)₂]⁺ rapidly lose PPh₃, enabling competitive recoordination of NCPh. Further solution experiments, relevant to the use of <b>1</b> in catalytic hydrophosphination, show that PPh₂H out-competes PPh₂CH₂CH₂CO₂Bu<i>^t</i> (the product of hydrophosphination of <i>tert</i>-butyl acrylate by PPh₂H) for coordination to Ru, even in the presence of a ten-fold excess of the tertiary phosphine. Additional information on relative phosphine binding strengths was obtained from gas-phase MS/MS experiments, including collision-induced dissociation (CID) experiments on the mixed phosphine complexes [Ru(η⁵-indenyl)PP’P’’]⁺, which ultimately appear in solution during the secondary phosphine competition experiments. Unexpectedly, unsaturated complexes [Ru(η⁵-indenyl)(PR₂H)(PPh₃)]⁺, generated in the gas-phase, undergo preferential loss of PR₂H. We propose competing orthometallation of PPh₃ is responsible for the surprising stability of the [Ru(η⁵-indenyl)(PPh₃)]⁺ fragment under these conditions.