Role of Low-Valent Rhenium Species in Catalytic Hydrosilylation Reactions with Oxorhenium Catalysts

The catalytic competency of a Re­(III) complex has been demonstrated. In the presence of silane, oxorhenium­(V) catalysts are deoxygenated to produce species that are significantly more active than the metal oxo precursors in hydrosilylation reactions. The results presented suggest that, in evaluating mechanisms for catalytic hydrosilylation reactions that involve high-valent metal oxo complexes, the activity of species that may be generated by deoxygenation of the metal with silane should also be systematically investigated as potential catalysts

Organoplatinum Chemistry with a Dicarboxamide–Diphosphine Ligand: Hydrogen Bonding, Cyclometalation, and a Complex with Two Metal–Metal Donor–Acceptor Bonds

The chemistry of the ligand bis­(2-diphenylphosphinoethyl)­phthalamide, dpppa, with platinum­(II) is described. The reaction of dpppa with [Pt₂Me₄(μ-SMe₂)₂], <b>1</b>, in a 2:1 ratio gave a mixture of [PtMe₂(dpppa)] and [Pt₂Me₄(μ-dpppa)₂], both of which contain Pt···H–N hydrogen bonds. However, reaction in a 1:1 ratio gave a remarkable tetraplatinum complex, [Pt₄Me₆(μ-dpppa-H)₂], which is shown to contain two Pt–Pt donor–acceptor bonds and in which one arm of the dpppa ligand has been cyclometalated. The reaction of [PtCl₂(dpppa)] with silver trifluoroacetate, to abstract chloride, and triethylamine as base has given the bis­(cyclometalated) complex [Pt­(dpppa-2H)], and this has been crystallized in three different forms, in which one or both of the carbonyl groups act as donors to a proton or to silver­(I). The complex [Pt­(dpppa-2H)]·AgO₂CCF₃·dmso forms a dimer and [Pt­(dpppa-2H)]·(AgO₂CCF₃)₂ forms a coordination polymer in the solid state

Synthesis of Oxorhenium Acetyl and Benzoyl Complexes Incorporating Diamidopyridine Ligands: Implications for the Mechanism of CO Insertion

A series of oxorhenium alkyl, phenyl, and vinyl complexes of the form [(DAP)­Re­(O)­(R)] (R = aryl, vinyl, alkyl) was reported, and their reactivity with CO was examined. The methyl complex <b>5a</b> reacts with CO at a significantly faster rate (2.5 h) than the phenyl complex <b>7a</b> (24 h). Computational (B3PW91) studies reveal that although the acyl complex is the least stable (Δ<i>G</i>₃₅₃ = −11.2 kcal/mol) with respect to CO insertion compared to the benzoyl complex (Δ<i>G</i>₃₅₃ = −14.5 kcal/mol), the activation energy for CO insertion is lower for the methyl complex (Δ<i>G</i>^⧧₃₅₃ = 14.6 kcal/mol) than for the phenyl complex (Δ<i>G</i>^⧧₃₅₃ = 17.4 kcal/mol). This is consistent with the previously proposed mechanism, where CO inserts directly into the Re–R bond without prior formation of a CO adduct. The X-ray crystal structures of complexes <b>6</b>, <b>7a</b>, <b>8a</b>, and <b>9a</b> are reported

Carbon–Hydrogen versus Nitrogen–Oxygen Bond Activation in Reactions of N‑Oxide Derivatives of 2,2′-Bipyridine and 1,10-Phenanthroline with a Dimethylplatinum(II) Complex

The reactions of the potential oxygen atom donor ligands 1,10-phenanthroline <i>N</i>-oxide (phenO) and 2,2′-bipyridine <i>N</i>-oxide (bipyO) with the dimethylplatinum­(II) complex [Pt₂Me₄(μ-SMe₂)₂] are reported. The reaction with the more rigid ligand phenO gave [PtMe₂(κ²<i>N</i>,<i>O</i>-phenO)], which underwent oxidative addition with 4-<i>t</i>-Bu-C₆H₄CH₂Br to give the platinum­(IV) complex [PtBrMe₂(CH₂C₆H₄-4-<i>t</i>-Bu)­(phenO)]. The complex [PtMe₂(phenO)] reacted with methanol in air to give [Pt­(OH)­(OMe)­Me₂(phenO)], but under an inert atmosphere it gave [Pt­(OH)­(OMe)­Me₂(phen)], in a reaction involving N–O bond activation. In contrast, the reaction of [Pt₂Me₄(μ-SMe₂)₂] with bipyO occurred by C–H bond activation to give methane and [PtMe­(κ²<i>N</i>,<i>C</i>-C₅H₄N-C₅H₃NO)­(SMe₂)], which underwent ligand substitution with pyridine, triphenylphosphine, or bis­(diphenylphosphino)­methane (dppm) to give [PtMe­(κ²<i>N</i>,<i>C</i>-C₅H₄N-C₅H₃NO)­(NC₅H₅)], [PtMe­(κ²<i>N</i>,<i>C</i>-C₅H₄N-C₅H₃NO)­(PPh₃)], or the binuclear [{PtMe­(κ²<i>N</i>,<i>C</i>-C₅H₄N-C₅H₃NO)}₂(μ-dppm)], respectively. With bis­(diphenylphosphino)­ethane (dppe), ligand substitution gave [PtMe­(κ¹<i>C</i>-C₅H₄N-C₅H₃NO)­(dppe)], which contains a monodentate metalated bipyO ligand. The mechanisms of the key reactions are discussed

A Versatile Diphosphine Ligand: cis and trans Chelation or Bridging, with Self Association through Hydrogen Bonding

The diphosphine ligand, <i>N</i>,<i>N</i>′-bis­(2-diphenylphosphinoethyl)­isophthalamide, dpipa, contains two amide groups and can form <i>cis</i> or <i>trans</i> chelate complexes or <i>cis</i>,<i>cis</i> or <i>trans</i>,<i>trans</i> bridged complexes. The amide groups are likely to be involved in intramolecular or intermolecular hydrogen bonding. This combination of properties of the ligand dpipa leads to very unusual structural properties of its complexes, which often exist as mixtures of monomers and dimers in solution. In the complex [Au₂(μ-dpipa)₂]­Cl₂, the ligands adopt the <i>trans,trans</i> bridging mode, with linear gold­(I) centers, and the amide groups hydrogen bond to the chloride anions. In [Pt₂Cl₄(μ-dpipa)₂], the ligands adopt the <i>cis,cis</i> bridging mode, with square planar platinum­(II) centers, and the amide groups form intermolecular hydrogen bonds to the chloride ligands to form a supramolecular one-dimensional polymer. Both the monomeric and dimeric complexes [PtMe₂(dpipa)] and [Pt₂Me₄(μ-dpipa)₂] have <i>cis</i>-PtMe₂ units with <i>cis</i> chelating or <i>cis,cis</i> bridging dpipa ligands respectively; each forms a supramolecular dimer through hydrogen bonding between amide groups and each contains an unusual NH···Pt interaction. An attempted oxidative addition reaction with methyl iodide gave the complex [PtIMe­(dpipa)], which contains <i>trans</i> chelating dpipa, while a reaction with bromine gave a disordered complex with approximate composition [Pt₂Me₃Br₅(μ-dpipa)₂], which contains <i>trans</i>,<i>trans</i> bridging dpipa ligands

Understanding The Fascinating Origins of CO₂ Adsorption and Dynamics in MOFs

Metal–organic frameworks (MOFs) have shown great promise for the adsorption and separation of gases, including the greenhouse gas CO₂. In order to improve performance and realize practical applications for MOFs as CO₂ adsorbents, a deeper understanding of the number and type of CO₂ adsorption mechanisms must be unlocked, along with fine details of CO₂ motion within MOFs. Using several complementary characterization methods is a promising protocol for comprehensively investigating the various host–guest interactions between MOFs and CO₂. In this work, a combination of solid state NMR (SSNMR) and single crystal X-ray diffraction (SCXRD) has been utilized to reveal both the location and dynamics of adsorbed CO₂ within the related PbSDB and CdSDB MOFs, as well as to probe the role of metal centers in CO₂ adsorption. ¹³C SSNMR experiments targeting CO₂ reveal the number of unique adsorption sites and the types of CO₂ dynamics present, as well as their associated motional rates and angles. ¹¹¹Cd and ²⁰⁷Pb SSNMR methods are used to probe the influence of CO₂ adsorption on the MOF metal centers, and also to investigate the possibility of any metal–guest interactions. SCXRD experiments yield the exact locations and occupancies of adsorbed CO₂ in both MOFs; by pairing this information with SSNMR data, a comprehensive model of CO₂ adsorption and dynamics in PbSDB and CdSDB has been established. Both MOFs share a common adsorption site in the V-shaped “π-pocket” formed by the phenyl rings of an individual V-shaped organic linker, while CdSDB also features an additional π-pocket adsorption site arising from the phenyl rings of two linkers joined by Cd. SCXRD and SSNMR data indicate that CO₂ adsorbed at the SDB-based π pocket in both MOFs exhibits a local rotation or “wobbling” at an individual adsorption site, as well as a nonlocalized jumping or “hopping” between symmetry-equivalent adsorption sites. The combined analysis of SCXRD and SSNMR data has the potential to yield rich information regarding guest dynamics, adsorption locations, and host–guest interactions in many MOFs

The Electronic Nature of Terminal Oxo Ligands in Transition-Metal Complexes: Ambiphilic Reactivity of Oxorhenium Species

The synthesis of the Lewis acid–base adducts of B­(C₆F₅)₃ and BF₃ with [DAAmRe­(O)­(X)] DAAm = <i>N</i>,<i>N</i>-bis­(2-arylaminoethyl)­methylamine; aryl = C₆F₅ (X = Me, <b>1</b>, COCH₃, <b>2</b>, Cl, <b>3</b>) as well as their diamidopyridine (DAP) (DAP=(2,6-bis­((mesitylamino)­methyl)­pyridine) analogues, [DAPRe­(O)­(X)] (X = Me, <b>4</b>, Cl, <b>5</b>, I, <b>6</b>, and COCH_3, <b>7</b>), are described. In these complexes the terminal oxo ligands act as nucleophiles. In addition we also show that stoichiometric reactions between <b>3</b> and triarylphosphine (PAr₃) result in the formation of triarylphosphine oxide (OPAr₃). The electronic dependence of this reaction was studied by comparing the rates of oxygen atom transfer for various para-substituted triaryl phosphines in the presence of CO. From these experiments a reaction constant ρ = −0.29 was obtained from the Hammett plot. This suggests that the oxygen atom transfer reaction is consistent with nucleophilic attack of phosphorus on an electrophilic metal oxo. To the best of our knowledge, these are the first examples of mono-oxo d² metal complexes in which the oxo ligand exhibits ambiphilic reactivity

Oxyfunctionalization with Cp*Ir^III(NHC)(Me)L Complexes

A series of monomethyl Cp*Ir^III complexes were synthesized and studied for the formation of methanol in water. Methanol yields of 75(4)% in the presence of O₂ were obtained. From isotope labeling studies, it was determined that O₂ is the source of the oxygen atom in the product. From kinetic studies, oxyfunctionalization appears to proceed by dissociation of an L-type ligand followed by O₂ binding and insertion

Cp*Ir^III-Catalyzed Oxidative Coupling of Benzoic Acids with Alkynes

Cp*Ir­(III) complexes have been shown to catalyze the oxidative coupling of benzoic acids with alkynes in methanol at 60 °C to form a variety of isocoumarins. The use of AgOAc as an oxidant was required to facilitate significant product formation. Alkyl alkynes were shown to be more reactive substrates than aryl alkynes, and a number of functional groups were tolerated on benzoic acid. Combined mechanistic and computational studies (BP86) revealed that (1) C–H activation occurs via an acetate-assisted mechanism; (2) C–H activation is not turnover limiting; and (3) the oxidant oxidizes the reduced form of the catalyst via an Ir­(I)–Ir­(II)–Ir­(III) sequence