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1.
Dalton Trans ; 49(1): 95-101, 2020 Jan 07.
Artigo em Inglês | MEDLINE | ID: mdl-31782454

RESUMO

A scandium hydridoborate complex supported by the dianionic pentadentate ligand B2Pz4Py is prepared via hydride abstraction from the previously reported scandium hydride complex with tris-pentafluorophenyl borane. Exposure of [(B2Pz4Py)Sc][HB(C6F5)3] to CO2 immediately forms [(B2Pz4Py)Sc][HCOOB(C6F5)3] at room temperature. The formatoborate complex can also be synthesized directly from the starting material (B2Pz4Py)ScCl with Et3SiH and B(C6F5)3 while in the presence of an atmosphere of CO2 in 81% yield. This compound was evaluated as the transition metal component of a tandem deoxgenative CO2 hydrosilation catalyst. At 5% loadings, complete consumption of Et3SiH was observed along with CO2 reduction products, but conversion to an inactive scandium complex identified as (B2Pz4Py)ScOSiEt3 was observed.

2.
Chem Commun (Camb) ; 56(6): 841-853, 2020 Jan 21.
Artigo em Inglês | MEDLINE | ID: mdl-31872836

RESUMO

In 1995, the synthesis, properties and remarkable hydroboration activity of bis-pentafluorophenyl borane was first reported. Its reactivity stems from the ready accessibility of the monomeric borane and its high Lewis acidity. In the intervening twenty five years, this reagent has been widely exploited as a means of incorporating Lewis acidic -B(C6F5)2 groups into complex structures for a range of applications. In this "25th Anniversary" Feature article, we highlight the synthetic methods to the borane, its fundamental properties and chemistry as well as the diverse array of uses of this borane. These include self-activating olefin polymerization catalysts, frustrated Lewis pair generation, small molecule activation, bond cleavage reactions, Lewis acid catalysis and modification of organic materials.

3.
Chem Commun (Camb) ; 55(74): 11095-11098, 2019 Sep 21.
Artigo em Inglês | MEDLINE | ID: mdl-31460525

RESUMO

The electrophilic borylation of 2,5-diarylpyrazines results in the formation of boron-nitrogen doped dihydroindeno[1,2-b]fluorene which can be synthesized using standard Schlenk techniques and worked up and handled readily under atmospheric conditions. Through transmetallation via diarylzinc reagents a series of derivatives were synthesized which show broad visible to near-IR light absorption profiles that highlight the versatility of this BN substituted core for use in optoelectronic devices. The synthesis is efficient, scalable and allows for tuning through changes in substituents on the planar heterocyclic core and at boron. Exploratory evaluation in organic solar cell devices as non-fullerene acceptors gave power conversion efficiencies of 2%.

4.
Chemistry ; 25(17): 4305-4308, 2019 Mar 21.
Artigo em Inglês | MEDLINE | ID: mdl-30723967

RESUMO

Isolable cationic PtII and PtIV alkylidenes, proposed intermediates in catalytic organic transformations, are reported. The bonding in these species was probed by experimental, structural, spectroscopic, electrochemical and computational methods, providing direct evidence for π-bonding, the often-theorized relativistic stabilization of these species, and the influence of oxidation state.

5.
J Am Chem Soc ; 140(47): 16094-16105, 2018 11 28.
Artigo em Inglês | MEDLINE | ID: mdl-30398331

RESUMO

In reactions of significance to alternative energy schemes, metal catalysts are needed to overcome kinetically and thermodynamically difficult processes. Often, high-oxidation-state, high-energy metal oxo intermediates are proposed as mediators in elementary steps involving O-O bond cleavage and formation, but the mechanisms of these steps are difficult to study because of the fleeting nature of these species. Here we utilized a novel dianionic pentadentate ligand system that enabled a detailed mechanistic investigation of the protonation of a cobalt(III)-cobalt(III) peroxo dimer, a known intermediate in oxygen reduction catalysis to hydrogen peroxide. It was shown that double protonation occurs rapidly and leads to a low-energy O-O bond cleavage step that generates a Co(III) aquo complex and a highly reactive Co(IV) oxyl cation. The latter was probed computationally and experimentally implicated through chemical interception and isotope labeling experiments. In the absence of competing chemical reagents, it dimerizes and eliminates dioxygen in a step highly relevant to O-O bond formation in the oxygen evolution step in water oxidation. Thus, the study demonstrates both facile O-O bond cleavage and formation in the stoichiometric reduction of O2 to H2O with 2 equiv of Co(II) and suggests a new pathway for selective reduction of O2 to water via Co(III)-O-O-Co(III) peroxo intermediates.

6.
Dalton Trans ; 47(46): 16789-16797, 2018 Dec 14.
Artigo em Inglês | MEDLINE | ID: mdl-30427346

RESUMO

A rigidified PCalkylP ligand allowed for the synthesis and characterization of cationic and radical PCCarbeneP nickel complexes in which the carbene anchor of the pincer framework is electrophilic rather than nucleophilic. Alpha-hydride abstraction from a (PCalkylP)nickel halide complex readily leads to the cationic carbene complex, which furnishes the radical carbene complex by one electron reduction. The reactivity of these reverse polarity carbene complexes towards small molecules (H2, CO, CO2, R3SiH, NH3) reveals different modes of activation when compared to previously reported nucleophilic nickel carbene complexes, and a clear dependence on the redox state of the complex. For H2, CO and CO2, no reaction is observed, but silanes react via hydride transfer and formation of solvated silylium ions. Ammonia is activated in a novel way, wherein it coordinates the carbene carbon and is deprotonated to form a robust C-N bond. This is not only a rare example of ammonia activation by a first row transition metal but also evidence of the intermediacy of group 10 carbenes in direct C-N bond forming reactions.

7.
Dalton Trans ; 47(38): 13680-13688, 2018 Oct 02.
Artigo em Inglês | MEDLINE | ID: mdl-30209501

RESUMO

Alkyl and hydrido scandium complexes of the dianionic pentadentate ligand B2Pz4Py are reported. The key starting material (B2Pz4Py)ScCl is readily prepared and alkylated with organolithium reagents RLi (R = CH3, CH2SiMe3, CH2SiMe2Ph, CH2CH2CH3 and CH2CHMe2) to form alkyl derivatives in 61-93% yields. These compounds are very thermally stable and do not undergo sigma bond metathesis reactions with dihydrogen. The hydrido complex was prepared from (B2Pz4Py)ScCl and NaHBEt3 in 80% yield and was found to be more stable by 28 kcal mol-1 as a dimer, rather than a monomeric hydrido complex. However, the monomer is accessible through dissociation of the dimer at 80 °C. All of the compounds (B2Pz4Py)ScR react with water to form the bridging oxo dimer (B2Pz4Py)ScOSc(B2Pz4Py). The reactivity of the hydrido and methyl complexes towards carbon dioxide was explored; heating to 80 °C results in the formation of κ2 formato and acetate complexes, respectively. The mechanisms were studied via density functional theory and distinct transition states for insertion of CO2 into the Sc-R (R = H, CH3) were found, with the insertion into Sc-CH3 being more enthalpically difficult (by 18 kcal mol-1) than insertion into Sc-H. The slow rate of reaction between [(B2Pz4Py)ScH]2 and CO2 is attributed to the barrier associated with dimer dissociation. In both insertion reactions, the kinetic products are κ1 formato or acetate complexes that are only slightly less stable than the observed κ2 derivatives. The κ1 compounds can therefore be trapped by treating the κ2 isomers with tris-pentafluorophenyl borane.

8.
Dalton Trans ; 47(36): 12491-12492, 2018 Sep 18.
Artigo em Inglês | MEDLINE | ID: mdl-30191944
9.
Dalton Trans ; 47(3): 734-741, 2018 Jan 15.
Artigo em Inglês | MEDLINE | ID: mdl-29239418

RESUMO

The reactions of 1-bora-7a-azaindenide anions, prepared in moderate to excellent yields by reduction of the appropriate 1-bora-7a-azaindenyl chlorides with KC8 in THF, with alkyl halides and carbon dioxide were studied. With alkyl halides (CH2Cl2, CH3I and BrCH(D)CH(D)tBu), the anions behave as boron anions, alkylating the boron centre via a classic SN2 mechanism. This was established with DFT methods and via experiments utilizing the neo-hexyl stereoprobe BrCH(D)CH(D)tBu. These reactions were in part driven by a re-aromatization of the six membered pyridyl ring upon formation of the product. Conversely, in the reaction of the 1-bora-7a-azaindenide anions with CO2, a novel carboxylation of the C-2 carbon alpha to boron was observed. Computations indicated that while carboxylation of the boron centre was kinetically feasible, the products of B-carboxylation were not thermodynamically favored relative to the observed C-2 carboxylated species, which were formed preferably due to the generation of both C-C and B-O bonds. In these products, the pyridyl ring remains non-aromatic, in part accounting for the observed reversibility of carboxylation.

10.
Inorg Chem ; 57(1): 495-506, 2018 Jan 02.
Artigo em Inglês | MEDLINE | ID: mdl-29260872

RESUMO

Three PCsp3P pincer ligands differing in the aryl group linking the phosphine arms with the anchoring carbon donor were used to support square planar Ni(II) bromide complexes 1-3Br. Exchange of the coordinating bromide anion for the more weakly coordinating triflate (OTf) or hexafluoroantimonate (SbF6) anions was accomplished by treatment with AgX or TlX salts to give compounds 1-3X; compounds 1OTf, 1SbF6, 2Br, 2OTf, 3Br, and 3SbF6 were all characterized by X-ray crystallography. The reactions of these Ni(II) compounds with the amine-N-oxide oxygen atom transfer agents ONMe3 and ONMePh2 were explored. For ONMe3, reactions with 2 equiv gave products in which one arm of the pincer ligand was oxidized to a P═O unit, with the other amine-N-oxide ligated to the Ni(II) center, forming products 5-6X; compounds 4OTf, 5OTf, and 6SbF6 were characterized crystallographically. Transient amine-N-oxide adducts prior to ligand oxidation were observed in some reactions. For the more effective O atom donor ONMePh2, reactions were very rapid and a second oxidation of the remaining phosphine arm was observed, producing a Ni(II) species with an OCO pincer ligand (7SbF6). All compounds were fully characterized. Experiments aimed at trapping transient Ni(IV) oxo intermediates (with cyclohexadiene, KH, and various Lewis acids) indicated that such species were not involved in the reaction. This was supported by density functional theory (DFT) computations at the B3PW91 level, which indicated that direct O atom insertion into the Ni-P bonds without the intermediacy of a Ni oxo species was the low-energy pathway.

11.
Dalton Trans ; 46(13): 4346-4354, 2017 Mar 27.
Artigo em Inglês | MEDLINE | ID: mdl-28287666

RESUMO

Iridaepoxide dihydride complexes of a PCP ligand bearing benzo[b]thiophene linkers are synthesized through ligand coopertive N2O and H2 activations. These neutral complexes also eliminate water at elevated temperatures to form the corresponding PCcarbeneP complexes which results in the formal hydrogenation of N2O to water. The synthesis of cationic iridaepoxide dihydride complexes are reported herein where the room temperature elimination of water is observed when a donating solvent is used. This supports a previously proposed mechanism for this water elimination where hydrides cis to the epoxide are required. Ir(i) and Ir(iii) cationic PCcarbeneP complexes are also synthesized through protonation and through O-H oxidation additions of water and phenol.

12.
Inorg Chem ; 56(7): 4157-4168, 2017 Apr 03.
Artigo em Inglês | MEDLINE | ID: mdl-28296399

RESUMO

Neutral cobalt(II) complexes of the dianionic tetrapodal pentadentate ligand B2Pz4Py, in which borate linkers supply the anionic charges, are reported. Both the six-coordinate THF adduct 1-THF and the five-coordinate THF-free complex 1 are in a high-spin S = 3/2 configuration in the ground state and have been structurally characterized by X-ray crystallography. These two Co(II) starting materials react rapidly with aryl azides of moderate steric bulk. The thermodynamic products of these reactions are low-spin, diamagnetic, Co(III) amido complexes that are either monomeric, when an external hydrogen atom source such as 1,4-cyclohexadiene is present, or dimeric products formed via C-C coupling of the azide aryl group and internal transfer of H• to the nitrogen. These products are fully characterized and are rare examples of octahedral Co amido compounds; structural determinations reveal significant pyramidalization of the amido nitrogens due to π-π repulsion wherein the amido ligand is primarily a σ donor. The amido products arise from highly reactive Co(III) imido radical intermediates that are the kinetic products of the reactions of 1 or 1-THF with the azide reagents. The imido radicals can be detected by X-band EPR spectroscopy and have been probed by density functional theory computations, which indicate that this doublet species is characterized by a high degree of spin localization on the imido ligand, accounting for the reactivity with hydrogen atom sources and dimerization chemistry observed. The high coordination number and the electron-rich nature of the dianionic B2Pz4Py ligand framework render the imido ligand formed highly reactive.

13.
Dalton Trans ; 45(32): 12669-79, 2016 Aug 09.
Artigo em Inglês | MEDLINE | ID: mdl-27465584

RESUMO

The donor properties of five different PCcarbeneP ligands are assessed by evaluation of the CO stretching frequencies in iridium(i) and rhodium(i) carbonyl cations. The ligands feature dialkyl phosphine units (R = (i)Pr or (t)Bu) linked to the central benzylic carbon by either an ortho-phenylene bridge, or a 2,3-benzo[b]thiophene linker; in the former, substituent patterns on the phenyl linker are varied. The carbonyl complexes are synthesized from the (PCcarbeneP)M-Cl starting materials via abstraction of the chlorides in the presence of CO gas. In addition to the expected mono carbonyl cations, products with two carbonyl ligands are produced, and for the rhodium example, a novel product in which the second carbonyl ligand adds reversibly across the Rh[double bond, length as m-dash]C bond to give an η(2) ketene moiety was characterized. The IR data for the complexes shows the 2,3-benzo[b]thiophene linked system to be the poorest overall donor, while the phenyl bridged ligands incorporating electron donating dialkyl amino groups para to the anchoring carbene are very strongly donating pincer arrays.

14.
Chem Sci ; 7(9): 5939-5944, 2016 Sep 01.
Artigo em Inglês | MEDLINE | ID: mdl-30034736

RESUMO

A dianionic tetrapodal pentadentate diborate ligand is introduced. This ligand forms a high spin neutral iron(ii) complex that reacts with a variety of organoazides to yield transient Fe(iii) imido radicals that are extremely potent hydrogen atom abstractors. The nature of these species is supported by full characterization of the Fe(iii) amido products, kinetic studies, density functional computations and Mössbauer spectroscopy on the -C6H4-p- t Bu substituted derivative.

15.
Chem Sci ; 7(2): 921-931, 2016 Feb 01.
Artigo em Inglês | MEDLINE | ID: mdl-30155011

RESUMO

Iridium complexes of the PCsp2 P ligand in which the donors are linked by 2,3-benzo[b]thiophene groups engage in the cooperative activation of N2O and the resulting iridaepoxides can be treated with dihydrogen to effect elimination of water and regeneration of the starting iridium complex. The mechanism of the steps in this reaction have been investigated using low temperature NMR investigations that reveal H/D exchange processes that point to a highly reactive kinetic product of hydrogen addition to the iridaepoxide. This intermediate is also involved in the water elimination pathway, and model compounds have been synthesized to provide further evidence for the mechanistic proposals for water elimination. The adaptable donor properties of the PCsp2 P ligand framework, particularly the anchoring carbene donor, plays a significant role in the ability of these compounds to mediate the transformation of N2O in this way.

16.
Dalton Trans ; 45(14): 5920-4, 2016 Apr 14.
Artigo em Inglês | MEDLINE | ID: mdl-26583306

RESUMO

Polycyclic aromatic hydrocarbons in which one or more CC units have been replaced by isoelectronic BN units have attracted interest as potentially improved organic materials in various devices. This promise has been hampered by a lack of access to gram quantities of these materials. However, the exploitation of keystone reactions such as ring closing metathesis, borylative cyclization of amino styrenes and electrophilic borylation has lead to strategies for access to workable amounts of material. These strategies can be augmented by judicious postfunctionalization reactions to diversify the library of materials available. This Frontier article highlights some of the recent successes and shows that the long promised applications of BN-doped PAHs are beginning to be explored in a meaningful way.

17.
Chem Commun (Camb) ; 52(7): 1361-4, 2016 Jan 25.
Artigo em Inglês | MEDLINE | ID: mdl-26650257

RESUMO

Silicon-hydrogen bonds are shown to add to a nickel carbon double bond to yield nickel α-silylalkyl hydrido complexes. Kinetic and isotope labeling studies suggest that a concerted 4-centred addition across the Ni=C bond is operative rather than a mechanism involving Si-H oxidative addition. This constitutes an example of Si-H bond activation via ligand cooperativity.

18.
Dalton Trans ; 44(41): 18196-206, 2015 Nov 07.
Artigo em Inglês | MEDLINE | ID: mdl-26421949

RESUMO

A series of molecules capable of releasing of the strong organometallic Lewis acid B(C6F5)3 upon exposure to 254 nm light have been developed. These photo Lewis acid generators (PhLAGs) can now serve as photoinitiators for several important B(C6F5)3-catalyzed reactions. Herein is described the synthesis of the triphenylsulfonium and diphenyliodonium salts of carbamato- and hydridoborates, their establishment as PhLAGs, and studies aimed at defining the mechanism of borane release. Factors affecting these photolytic reactions and the application of this concept to photoinduced hydrosilylation reactions and construction of siloxane scaffolds are also discussed.

19.
Dalton Trans ; 44(27): 12027-8, 2015 Jul 21.
Artigo em Inglês | MEDLINE | ID: mdl-26090922
20.
J Am Chem Soc ; 137(6): 2187-90, 2015 Feb 18.
Artigo em Inglês | MEDLINE | ID: mdl-25642725

RESUMO

A new PC(carbene)P pincer ligand with 2,3-benzo[b]thiophene linkers connecting the flanking dialkyl phosphine donors to the central carbene can be attached to Ir(I). The chloro derivative reacts with N2O with loss of N2 to form an iridaepoxide species by addition of an oxygen atom to the Ir═C linkage. This compound reacts with H2 to afford the oxidative addition product, in which the hydride ligands are trans to the Ir-O bond. Heating this dihydride results in slow release of H2O; kinetic and spectroscopic studies show that conversion of the dihydride to its isomer, in which the hydrides are cis to the Ir-O bond, is required for H2O elimination to take place. Together, these reactions constitute the stoichiometric conversion of N2O and H2 to N2 and H2O; further mechanistic studies suggest ways to make the system catalytic.

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