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A gold(III) complex with the hitherto most electron poor mesoionic carbene ligand is presented. Aqua regia was the oxidizing agent of choice for the synthesis of this unusual organometallic compound. The AuIII complex is redox-rich, and also acts as a catalyst for oxazole formation, delivering selectively a completely different isomer compared to its AuI congener.

The bench-stable cationic bis(σ-B–H) aminoborane complex [Fe(PNPNMe-iPr)(H)(η2-H2B = NMe2)]+ (2) efficiently catalyzes the semihydrogenation of internal alkynes, 1,3-diynes and 1,3-enynes. Moreover, selective incorporation of deuterium was achieved in the case of 1,3-diynes and 1,3-enynes. The catalytic reaction takes place under mild conditions (25 °C, 4–5 bar H2 or D2) in 1 h, and alkenes were obtained with high Z-selectivity for a broad scope of substrates. Mechanistic insight into the catalytic reaction, explaining also the stereo- and chemoselectivity, is provided by means of DFT calculations. Intermediates featuring a bisdihydrogen moiety [Fe(PNPNMe-iPr)(η2-H2)2]+ are found to play a key role. Experimental support for such species was unequivocally provided by the fact that [Fe(PNPNMe-iPr)(H)(η2-H2)2]+ (3) exhibited the same catalytic activity as 2. The novel cationic bisdihydrogen complex 3 was obtained by protonolysis of [Fe(PNPNMe-iPr)(H)(η2-AlH4)]2 (1) with an excess of nonafluoro-tert-butyl alc...

Cyclometalated gold(III) complexes Au(L)(OAcF)2 (L = phenylpyridine dicarboxylic diester (ppyde) or phenylpyridine dicarboxylic acid (ppydc)) have been prepared reacting Au(OAc)3 with corresponding phenyl pyridines (ppyde or ppydc) in trifluoroacetic acid (HOAcF) under microwave heating. Further treatment of Au(L)(OAcF)2 with aqua regia resulted in dichloro complexes Au(L)Cl2. Au-functionalized UiO-67 MOF has been synthesized by exchanging linkers of UiO-67 with Au(ppydc)Cl2, furnishing the MOF with (N^C)-cyclometalated Au(III) centers. The catalytic activities of the molecular cyclometalated complexes and the Au-incorporated MOF were studied in gold-catalyzed propargyl ester cyclopropanations. Almost all complexes and the MOF showed catalytic activity to the cyclopropanation product (up to 97% conversion), with a preference for the trans diastereoisomer (up to 14:86 d.r.). The recyclability of the most active molecular complex has also been investigated. © 2020 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).

International audience; Amides functionalities are among the most widely found groups in biologically active molecules, and their selective catalytic reduction is an important target for synthetic methods. Recent advances in base metal catalysis have identified efficient systems for selective hydrogenolysis of the amide C–N linkage. This study examines in detail the mechanism for deaminative hydrogenation of formanilide and dimethylformamide (DMF) to the corresponding amines (aniline and dimethylamine, respectively) and methanol catalyzed by (iPrPNHP)Fe(H)2(CO) (iPrPNHP = HN{CH2CH2(PiPr2)}2) using density functional theory (DFT) calculations and microkinetic modeling. Following an initial hydrogenation of the amide carbonyl group, protonolysis of the C–N bond of the hemiaminal intermediate produces amine and formaldehyde, which is further hydrogenated to methanol. Remarkably, protonolysis of the C–N bond of the hemiaminal intermediate follows different pathways, depending on the nature of the substrate and the experimental conditions (presence or absence of cocatalyst) to yield the same products. In particular, cleavage of the C–N bond can be facilitated by either the metal catalyst or one of the organic species present (the amide substrate itself or an amide cocatalyst), with interesting consequences on the kinetics as evidenced by simulations using DFT calculated energy profiles for the entire pathway. This study reveals the subtle interplay between the nature of the substrate and the need for additives and changes the long-established principle that the hydrogenation of electron rich carbonyl substrates is governed by their carbonyl hydrogenation step and, therefore, only metal-hydride hydricity is relevant for catalyst design.

A (N,CAr,CAlk) Au(III) pincer complex has been synthesized from Au(OAc)3 (OAc = OCOCH3) and 2-(3,5-di-tert-butylphenyl)pyridine (L1) involving a Csp3–H bond activation by electrophilic substitution. In agreement with DFT calculations, the resulting complex significantly improves the performance of Au(tpy)(OAcF)2 (tpy = 2-(p-tolyl)pyridine, OAcF = OCOCF3) in the catalytic trifluoroacetylation of acetylene.

We describe here the application of the well-defined bench-stable cationic aminoborane complex as highly efficient pre-catalysts for the semi-hydrogenation of internal alkynes, 1,3-diynes and 1,3-enynes with molecular hydrogen under mild conditions.

A highly asymmetric AuIII η3‐allyl complex has been generated by treating Au(η1‐allyl)Br(tpy) (tpy=2‐(p‐tolyl)pyridine) with AgNTf2. The resulting η3‐allyl complex has been characterized by NMR spectroscopy and X‐ray crystallography. DFT calculations and variable temperature 1H NMR suggest that the allyl ligand is highly fluxional.

A new one-dimensional coordination polymer formed unexpectedly during the synthesis of a Pt-functionalized bi­pyridine linker for metal–organic frameworks. We report here the synthesis, structure determination and energy-dispersive X-ray spectroscopy analysis of this new coordination polymer., The title compound, {[MgPtCl2(C12H6N2O4)(C3H7NO)2(H2O)]·C3H7NO}n, is a one-dimensional coordination polymer. The structure consists of Pt-functionalized bi­pyridine ligands connected by MgII cations, as well as coordinating and non-coordinating solvent mol­ecules. The PtII cation is coordinated by the two N atoms of the bi­pyridine moiety and two Cl atoms in a square-planar fashion. This coordination induces an in-plane bend along the bi­pyridine backbone of approximately 10° from the linear ideal of a conjugated π-system. Likewise, the coordination to the MgII cation induces a significant bowing of the plane of the bi­pyridine of about 12°, giving it a distinct curved appearance. The carboxyl­ate groups of the bi­pyridine ligand exhibit moderate rotations relative to their parent pyridine rings. The MgII cation has a fairly regular octa­hedral coordination polyhedron, in which three vertices are occupied by O atoms from the carboxyl­ate groups of three different bi­pyridine ligands. The remaining three vertices are occupied by the O atoms of two di­methyl­formamide (DMF) mol­ecules and one water mol­ecule. The one-dimensional chains are oriented in the [01-1] direction, and non-coordinating DMF mol­ecules can be found in the space between the chains. The shortest inter­molecular O⋯H contacts are 2.844 (4) and 2.659 (4) Å, suggesting moderate hydrogen-bonding inter­actions. In addition, there is a short inter­molecular Pt⋯Pt contact of 3.491 (1) Å, indicating a Pt stacking inter­action. Some structure-directing contribution from the hydrogen bonding and Pt⋯Pt inter­action is probable. However, the crystal packing seems to be directed primarily by van der Waals inter­actions.

The Au(III) complex Au(tpy)(OAcF)2 (OAcF = OCOCF3; tpy = 2-(p-tolyl)pyridine) catalyzes the anti addition of trifluoroacetic acid (HOAcF) to acetylene to furnish vinyl trifluoroacetate. The Au(III) vinyl complex Au(tpy)(OAcF)(CH═CHOAcF) (vinyl group bonded trans to tpy-N) is formed during the early stage of the reaction. The vinyl complex, which has been isolated and structurally characterized, resists protolytic cleavage of the vinyl group, and therefore catalysis does not proceed by a simple formal insertion (i.e., coordination-nucleophilic attack-protolysis at the site trans to tpy-N) mechanism. Experimental evidence, including isotopic labeling, rather suggests that a double-insertion process is operative. The unobserved Au(III) divinyl complex Au(tpy)(CH═CHOAcF)2 is a crucial intermediate for which the true catalytic activity, comprising a coordination-nucleophilic attack-protolysis sequence, occurs at the site trans to tpy-C. The overall mechanism is in full agreement with DFT calculations and is a result of the considerable differences in the kinetic and thermodynamic trans effects of tpy-N versus tpy-C on each reaction step. The computational data provide a rationale for the catalytic functionalization of acetylene trans to tpy-C, whereas ethylene (previously reported) only undergoes a stoichiometric insertion, and then comes to a full stop, trans to tpy-N. © 2017 American Chemical Society

The title compound, {[MgPtCl2(C12H6N2O4)(C3H7NO)2(H2O)]·C3H7NO}n, is a one-dimensional coordination polymer. The structure consists of Pt-functionalized bipyridine ligands connected by MgII cations, as well as coordinating and non-coordinating solvent molecules. The PtII cation is coordinated by the two N atoms of the bipyridine moiety and two Cl atoms in a square-planar fashion. This coordination induces an in-plane bend along the bipyridine backbone of approximately 10° from the linear ideal of a conjugated π-system. Likewise, the coordination to the MgII cation induces a significant bowing of the plane of the bipyridine of about 12°, giving it a distinct curved appearance. The carboxylate groups of the bipyridine ligand exhibit moderate rotations relative to their parent pyridine rings. The MgII cation has a fairly regular octahedral coordination polyhedron, in which three vertices are occupied by O atoms from the carboxylate groups of three different bipyridine ligands. The remaining three vertices are occupied by the O atoms of two dimethylformamide (DMF) molecules and one water molecule. The one-dimensional chains are oriented in the [01-1] direction, and non-coordinating DMF molecules can be found in the space between the chains. The shortest intermolecular O...H contacts are 2.844 (4) and 2.659 (4) Å, suggesting moderate hydrogen-bonding interactions. In addition, there is a short intermolecular Pt...Pt contact of 3.491 (1) Å, indicating a Pt stacking interaction. Some structure-directing contribution from the hydrogen bonding and Pt...Pt interaction is probable. However, the crystal packing seems to be directed primarily by van der Waals interactions.

The Au(III) complex Au(OAc(F))2(tpy) (1, OAc(F) = OCOCF3; tpy = 2-p-tolylpyridine) undergoes reversible dissociation of the OAc(F) ligand trans to C, as seen by (19)F NMR. In dichloromethane or trifluoroacetic acid (TFA), the reaction between 1 and ethylene produces Au(OAc(F))(CH2CH2OAc(F))(tpy) (2). The reaction is a formal insertion of the olefin into the Au-O bond trans to N. In TFA this reaction occurs in less than 5 min at ambient temperature, while 1 day is required in dichloromethane. In trifluoroethanol (TFE), Au(OAc(F))(CH2CH2OCH2CF3)(tpy) (3) is formed as the major product. Both 2 and 3 have been characterized by X-ray crystallography. In TFA/TFE mixtures, 2 and 3 are in equilibrium with a slight thermodynamic preference for 2 over 3. Exposure of 2 to ethylene-d4 in TFA caused exchange of ethylene-d4 for ethylene at room temperature. The reaction of 1 with cis-1,2-dideuterioethylene furnished Au(OAc(F))(threo-CHDCHDOAc(F))(tpy), consistent with an overall anti addition to ethylene. DFT(PBE0-D3) calculations indicate that the first step of the formal insertion is an associative substitution of the OAc(F) trans to N by ethylene. Addition of free (-)OAc(F) to coordinated ethylene furnishes 2. While substitution of OAc(F) by ethylene trans to C has a lower barrier, the kinetic and thermodynamic preference of 2 over the isomer with CH2CH2OAc(F) trans to C accounts for the selective formation of 2. The DFT calculations suggest that the higher reaction rates observed in TFA and TFE compared with CH2Cl2 arise from stabilization of the (-)OAc(F) anion lost during the first reaction step.

Despite widespread use of complexes of the type Pd(L)(η(3)-allyl)Cl as precatalysts for cross-coupling, the chemistry of related Pd(I) dimers of the form (μ-allyl)(μ-Cl)Pd2(L)2 has been underexplored. Here, the relationship between the monomeric and the dimeric compounds is investigated using both experiment and theory. We report an efficient synthesis of the Pd(I) dimers (μ-allyl)(μ-Cl)Pd2(IPr)2 (allyl = allyl, crotyl, cinnamyl; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) through activation of Pd(IPr)(η(3)-allyl)Cl type monomers under mildly basic reaction conditions. The catalytic performance of the Pd(II) monomers and their Pd(I) μ-allyl dimer congeners for the Suzuki-Miyaura reaction is compared. We propose that the (μ-allyl)(μ-Cl)Pd2(IPr)2-type dimers are activated for catalysis through disproportionation to Pd(IPr)(η(3)-allyl)Cl and monoligated IPr-Pd(0). The microscopic reverse comproportionation reaction of monomers of the type Pd(IPr)(η(3)-allyl)Cl with IPr-Pd(0) to form Pd(I) dimers is also studied. It is demonstrated that this is a facile process, and Pd(I) dimers are directly observed during catalysis in reactions using Pd(II) precatalysts. In these catalytic reactions, Pd(I) μ-allyl dimer formation is a deleterious process which removes the IPr-Pd(0) active species from the reaction mixture. However, increased sterics at the 1-position of the allyl ligand in the Pd(IPr)(η(3)-crotyl)Cl and Pd(IPr)(η(3)-cinnamyl)Cl precatalysts results in a larger kinetic barrier to comproportionation, which allows more of the active IPr-Pd(0) catalyst to enter the catalytic cycle when these substituted precatalysts are used. Furthermore, we have developed reaction conditions for the Suzuki-Miyaura reaction using Pd(IPr)(η(3)-cinnamyl)Cl which are compatible with mild bases.

The contribution of Au(III) species to catalysis is still debated due to the limited number of characterized intermediates with this oxidation state. In particular, the coordination of alkenes and alkynes to Au(III) followed by insertion into Au(III)-X bonds has been suggested but rarely proven experimentally. Here, these reactions are explored by means of DFT and CCSD(T) calculations considering [AuX3(L)] and [AuX2(L)2](+) complexes. In these complexes, L = ethylene and acetylene have been chosen as substrates of high interest and representative of any unsaturated organic substrate, whereas X is Cl, Me or H, as found in metal salts and as model for intermediates involved in catalysis. Isoelectronic Pt(II) complexes are also considered for comparison. Ethylene coordination occurs preferentially perpendicular for all X except H, whereas for acetylene, coordination takes place in-plane for all X except Cl. These coordination isomers can represent either minima (intermediates) or saddle points (transition states) on the potential energy surface, depending on X. NBO analysis shows how this variety of structures results from the combination of electronic (M-L donation and back-donation) and steric (cis L-X repulsion) effects. With the sole exception of [AuMe2(ethylene)2](+), rotation of the unsaturated ligand and insertion into a cis Au-X bond involve low to moderate energy barriers, ΔG(‡) = 2.5 to 23.5 kcal mol(-1), and are thermodynamically feasible, ΔG = 4.3 to -47.2 kcal mol(-1). The paucity of experimental observations for such reactions should thus be caused by other factors, like the participation of the intermediates and products in competitive side reactions including the reductive elimination of XCHnCHnX (n = 1 or 2).

Everyone who works within the field of catalysis draws inspiration from the amazing functionality of nature’s catalysts, the enzymes. It is particularly the mild conditions that these catalysts are able to operate at and the selectivity that they demonstrate that make these materials dream targets for scientists involved in the art of synthesizing homogeneous and heterogeneous industrial catalysts. But enzymes also have their weak points; in particular their low thermal stability and their often too slow reaction rates for an economical industrial process are problems that have to be overcome. The obvious solution would be to copy the catalytic active center into a robust open framework. A key property of an enzyme is its selectivity; this property is partly regulated by steric constraints surrounding the catalytically active site. The microporous zeolite based catalysts in some cases show impressive selectivity based on the geometrical constraints imposed by the size and shape of the regular channels in these crystalline silicate and alumino-phosphate based structures, and enzyme-like properties have been claimed but the pure inorganic nature of the selective internal surface in these materials makes it impossible to mimic many important enzymatic properties. The new generation of microporous materials, Metal Organic Frameworks (MOFs) are hybrids of organic and inorganic structures. This dualistic nature offers an unprecedented flexibility in the possibility to incorporate both organic and metallic functional groups into the ordered crystalline lattice and thereby opening up for a much greater possibility to copy structural motifs known from enzymes into much simpler but also more stable open structures. Several groups are working on development of new catalysts by this approach. Here we will illustrate this approach with structures that mimic anhydrase and C–H activation.

Design of a selective homogeneous methane functionalizing catalyst based on [(ArN=CRCR=NAr)Pt(Me)(L)]+ requires knowledge of its stability in various reaction media, particularly water. Reaction of (diimine)PtMe2 (1) (diimine = ArN=CRCR=NAr, Ar = 2, 6-Me2C6H3, R = Me) with HOTf (OTf = OSO2CF3) gives the methane activating compound (diimine)Pt(Me)OTf (3). When varying amounts of H2O are added during the synthesis of 3, competing degradation pathways lead to two different characterizable products. With only trace amounts of water, two dimeric species, [(diimine)Pt(μ-Cl)(μ-OH)Pt(diimine)](OTf)2 (6) and [(diimine)Pt(μ-OH)2Pt(diimine)](OTf)2 (7), are isolated, in addition to an uncharacterized dark brown precipitate. When an excess of H2O is added, the aquo species [(diimine)Pt(Me)(H2O)][OTf] (5) is first observed, which then reacts further to give a dark brown precipitate and 7. The structures of 1, 6, and 7 are presented. Both 6 and 7 exhibit unusual conformations for their respective classes. Compound 6 has...

Two new Pd(II) N -heterocyclic iminocarbene complexes (C–N)PdCl 2 that contain 5-membered chelate rings have been prepared by carbene transfer from a silver iminocarbene precursor to (COD)PdCl 2 . The new Pd imonocarbene complexes, as well as two that have been previously reported (altogether three 5-membered and one 6-membered chelate ring complexes) have been evaluated as catalysts for the Suzuki–Miyaura coupling reaction. The complexes were found to be active in the reaction, but without exceptional catalytic performances. The 5-membered chelate ring complexes appeared to be more robust and remained active for a longer time than the 6-membered ring congener. The catalytic performance of the 5-membered chelate ring complexes appeared to be rather insensitive to the steric demands of the imine-N-aryl group. The X-ray structure of one of the Ag iminocarbene complexes reveals the κ 1 (C) bonding of the iminocarbene moiety in a nearly linear Ag(I) complex; two monomeric units are associated through a weak Ag–Ag interaction. The X-ray structures of two new Pd iminocarbene complexes (C–N)PdCl 2 confirm the chelating κ 2 (C,N) nature of the iminocarbene moiety; in both complexes, the Pd–Cl distances trans to carbene-C are slightly longer than those trans to imine-N.

Reactions between [Cp∗IrCl2]2 and diaryl-substituted diazabutadienes (dab) ArNCR′CR′NAr (R′=H: Ar=4-MeC6H4, 2,6-Me2C6H3, 2,6- i Pr 2 C 6 H 3 ; R′=Me: Ar=4-MeC6H4) proceed straightforwardly to give the dab complexes Cp∗Ir(dab)Cl+Cl−. Sterically more encumbered dab ligands (R′=Me: Ar=2,6-Me2C6H3, 2,6- i Pr 2 C 6 H 3 , 2,4,6-Me2C6H3) instead give products Cp ∗ IrClCH 2 C ( NHAr ) CMe  N ( Ar ) + Cl − in which the dab ligand has undergone CH activation at one of methyl groups of the dab bridge. The CH cleavage reaction is reversible in the sense that thermally, the cyclometalated Ir complex decomposes to regenerate the free ligand.

The C-H activation of toluene and p-xylene at cationic Pt(II) diimine complexes (N-N)Pt(CH(3))(H(2)O)(+)BF(4)(-) (N-N = Ar-N=CMe-CMe=N-Ar; 1(BF(4)(-)), N(f)-N(f), Ar = 3,5-(CF(3))(2)C(6)H(3)); 2(BF(4)(-)), N'-N', Ar = 2,6-(CH(3))(2)C(6)H(3)) has been investigated. The reactions were performed at ambient temperature in 2,2,2-trifluoroethanol (TFE), and after complete conversion of the starting material to mixtures of Pt-aryl/Pt-benzyl complexes and methane, acetonitrile was added to trap the products as more stable acetonitrile adducts. In the reactions with toluene, the relative amounts of products resulting from aromatic C-H activation were found to decrease in the order (N-N)Pt(m-tolyl)(NCMe)(+)(N-N)Pt(p-tolyl)(NCMe)(+)(N-N)Pt(o-tolyl)(NCMe)(+) for both 1 and 2. Unlike the reaction at 1, significant amounts of the benzylic activation product (N'-N')Pt(benzyl)(NCMe)(+) were concurrently formed in the C-H activation of toluene at 2. The C-H activation of p-xylene revealed an even more remarkable difference between 1 and 2. Here, the product ratios of (N-N)Pt(xylyl)(NCMe)(+) and (N-N)Pt(p-methylbenzyl)(NCMe)(+) were found to be 90:10 and 7:93 for reactions at 1 and 2, respectively. The elimination of toluene from (N(f)-N(f))Pt(Tol)(2) species (3a-c; a, Tol = o-tolyl; b, Tol = m-tolyl; c, Tol = p-tolyl) after protonolysis with 1 equiv of HBF(4) was investigated. Most notably, protonation in neat TFE followed by addition of acetonitrile gave a 77:23 mixture of (N(f)-N(f))Pt(m-tolyl)(NCMe)(+) (4b) and (N(f)-N(f))Pt(p-tolyl)(NCMe)(+) (4c) from all three isomeric bis(tolyl) complexes 3a-c. The presence of acetonitrile during the protonation reactions resulted in considerably less isomerization. This behavior is explained by an associative mechanism for the product-determining displacement of toluene by the solvent. For the C-H activation reactions, our findings suggest the existence of a dynamic equilibrium between the isomeric intermediates (N-N)Pt(aryl)(CH(4))(+) (aryl = tolyl/benzyl from 1; xylyl/p-methylbenzyl from 2). The observed selectivities might then be explained by steric and electronic effects in the pentacoordinate transition-state structures for the solvent-induced associative elimination of methane from these intermediates.

The platinum(II) methyl cation [(N−N)Pt(CH3)(solv)]+BF4- (N−N = ArNC(Me)C(Me)NAr, Ar = 2,6-(CH3)2C6H3, solv = H2O (2a) or TFE = CF3CH2OH (2b)) is prepared by treatment of (N−N)Pt(CH3)2 with 1 equiv of aqueous HBF4 in TFE. Reaction of a mixture of 2a and 2b with benzene in TFE/H2O solutions cleanly affords the platinum(II) phenyl cation [(N−N)Pt(C6H5)(solv)]+BF4- (3). Investigations of the kinetics and isotopic labeling experiments indicate that reaction of 2 with benzene proceeds via benzene coordination, reversible oxidative addition of benzene C−H bonds, reversible formation of a methane C,H-σ complex, and final dissociation of methane. Under conditions where [(N−N)Pt(CH3)(H2O)]+BF4- (2a) is the major starting complex, rate-determining benzene coordination to 2b is implicated by the observed kinetic rate law (inverse first order in [H2O] and first order in [C6H6] to 3.8 M) and the small kinetic deuterium isotope effect for C6H6 vs C6D6 (kH/kD = 1.06 ± 0.05 at 25 °C). When deuterated benzenes C6D6 and 1,...

Controlled protonolysis of (Nf-Nf)Pt(CH3)2 (1; Nf-Nf = ArNCMeCMeNAr, Ar = 3,5-(CF3)2C6H3) with HBF4·Et2O in dichloromethane in the presence of small quantities of water gives the BF4- salt of the aqua complex (Nf-Nf)Pt(CH3)(H2O)+ (6). When dissolved in trifluoroethanol (TFE), 6(BF4-) effects the activation of methane and benzene C−H bonds under very mild conditions. Thus, 6 reacted with benzene in TFE-d3 at ambient temperature to quantitatively yield (Nf-Nf)Pt(C6H5)(H2O)+ and methane after 2−3 h. The use of C6D6 led to multiple incorporation of deuterium into the methane produced and suggests the involvement of methane σ-complex and benzene σ- or π-complex intermediates. When the solution of 6(BF4-) was exposed to 13CH4, an exchange reaction produced ca. 50% of (Nf-Nf)Pt(13CH3)(H2O)+ and CH4 after ca. 48 h at 45 °C. The reaction was inhibited by added water, suggesting that water is reversibly lost from 6 before C−H activation takes place. The use of CD4 resulted in multiple deuterium incorporation into t...

International audience; A Missing Gold Link: An Au(III) alkene complex has been prepared and characterized by NMR spectroscopy and X-ray crystallography. Its bonding features have been analyzed by DFT calculations and natural bond orbital (NBO) analysis. In [(cod)AuMe(2) ](+) , the unequal AuC bond lengths result from the domination of the preference of 1,5-cyclooctadiene (cod) for nonparallel double bonds over back donation from the metal which favors parallel double bonds.

The performance of recently reported highly cis-diastereoselective Rh(I) cyclopropanation catalysts has been significantly improved by a systematic study of different reaction parameters (catalyst activation, solvent, temperature, stoichiometry). The catalyst efficiency and diastereoselectivity were enhanced by changing the activating agent from AgOTf to NaBArf. With this new system, the Rh(I) catalyst was shown to be a highly efficient and cis-diastereoselective cyclopropanation catalyst in reactions between α-diazoacetates and a range of different alkenes and substituted derivatives. Particularly noteworthy is the remarkable reactivity and cis-diastereoselectivity displayed in the reactions between ethyl diazoacetate and cyclopentene, 2,5-dihydrofuran, and benzofuran, with yields up to 99% and cis-selectivities greater than 99%.

The performance of recently reported highly cis-diastereoselective Rh(I) cyclopropanation catalysts has been significantly improved by a systematic study of different reaction parameters (catalyst activation, solvent, temperature, stoichiometry). The catalyst efficiency and diastereoselectivity were enhanced by changing the activating agent from AgOTf to NaBArf. With this new system, the Rh(I) catalyst was shown to be a highly efficient and cis-diastereoselective cyclopropanation catalyst in reactions between α-diazoacetates and a range of different alkenes and substituted derivatives. Particularly noteworthy is the remarkable reactivity and cis-diastereoselectivity displayed in the reactions between ethyl diazoacetate and cyclopentene, 2,5-dihydrofuran, and benzofuran, with yields up to 99% and cis-selectivities greater than 99%.

The energetics of the deprotonation and metal-hydrogen bond homolysis reactions of TpM(CO) 3 H and Tp'M(CO) 3 H (Tp=hydridotris(pyrazolyl)borate; Tp'=hydridotris(3,5-dimethylpyrazolyl)borate; M=Cr, Mo, W) and their cation radicals have been investigated, and the results are compared with available data for the analogous CpM(CO) 3 H complexes. Proton-transfer equilibrium measurements have established that for a given metal, acetonitrile pK a values of the metal hydrides decrease in the order CpM(CO) 3 H>TpM(CO) 3 H>Tp'M(CO) 3 H. This trend is the opposite of that expected on the basis of the relative electron-richness of the metal centers, as measured by infrared ν CO frequencies and oxidation potential data for the corresponding metal anions

Comparison of [Cat] [CpCr(CO) 2 PEt 3 ] (Cat=Et 4 N + and Ph 3 PNPPh 3 + (PPN + )) by NMR, visible/UV, and infrared spectroscopy in CH 3 CN and in THF shows that ion pairing for the PPN complex occurs in THF, and that such pairing involves a detectable amount of charge transfer from anion to cation. This cause a rise in the CO stretching frequencies, which can be compared to the values in the fully-oxidized species CpCr(CO) 2 (PEt 3 ) .

A structurally characterized Rh(I) iminocarbene complex (N,C)Rh(CO)Cl is activated with AgOTf to act as a highly cis-selective catalyst for the cyclopropanation of substituted styrenes and other alkenes with ethyl diazoacetate (11 examples, 10−99% yield, up to >99% cis-selectivity).

A structurally characterized Rh(I) iminocarbene complex (N,C)Rh(CO)Cl is activated with AgOTf to act as a highly cis-selective catalyst for the cyclopropanation of substituted styrenes and other alkenes with ethyl diazoacetate (11 examples, 10−99% yield, up to >99% cis-selectivity).

The oxidation chemistry of the organometallic hydride complexes ({eta}{sup 5}C{sub 5}H{sub 5})M(CO){sub 3}H (M = Cr, 1; Mo, 2; W, 3), ({eta}{sup 5}-C{sub 5}Me{sub 5})Mo(CO){sub 3}H (4), and ({eta}{sup 5}-C{sub 5}H{sub 5})W(CO){sub 2}(PMe{sub 3})H (5) has been investigated in acetonitrile solution by electrochemical and other methods. The thermodynamic acidities of the cation radicals of 1-5 have been estimated by the use of a thermochemical cycle based on the oxidation potentials of 1-5 and their conjugate bases (anions) as well as the solution pK{sub a} values of 1-5. The metal hydride cation radical pK{sub a} estimates fall in the range {minus}10 to +5, which makes these complexes the most acidic hydrides for which pK{sub a} values have been determined. Coulometric measurements show that 1-5 undergo overall two-electron oxidations. Chemical oxidation of 5 gives a 1:1 mixture of dihydride ({eta}{sup 5}-C{sub 5}H{sub 5})W(CO){sub 2}(PMe{sub 3})H{sub 2}{sup +} (13) and ({eta}{sup 5}-C{sub 5}H{sub 5})W(CO){sub 2}(PMe{sub 3})(NCMe){sup +} (10), the latter as a mixture of cis and trans isomers in the thermodynamic 95:5 ratio.

Protonation of (N-N)PtPh(2) (1; N-N = diimine ArN=CMe-CMe=NAr with Ar = 2,6-Me(2)C(6)H(3) (a), 2,4,6-Me(3)C(6)H(2) (b), 4-Br-2,6-Me(2)C(6)H(2) (c), 3,5-Me(2)C(6)H(3) (d), and 4-CF(3)C(6)H(4) (e)) in the presence of MeCN at ambient temperature generates (N-N)Pt(Ph)(NCMe)(+) (2). At -78 degrees C, protonation of 1a yielded (N-N)PtPh(2)(H)(NCMe)(+) (3a), which produced benzene and 2a at ca. -40 degrees C. Protonation of 1a-e in CD(2)Cl(2)/Et(2)O-d(10) furnished (N-N)Pt(C(6)H(5))(eta(2)-C(6)H(6))(+) (4a-e). The pi-benzene complexes 4a-c, sterically protected at Pt, eliminate benzene at ca. 0 degree C. The sterically less protected 4d-e lose benzene already at -30 degrees C. SST and 2D EXSY NMR demonstrate that phenyl and pi-benzene ligand protons undergo exchange with concomitant symmetrization of the diimine ligand, most likely via oxidative insertion of Pt into a C-H bond of coordinated benzene. The kinetics of the exchange processes for 4a-c were probed by quantitative EXSY spectroscopy, resulting in DeltaH() of 70-72 kJ mol(-1) and DeltaS of 37-48 J K(-1) mol(-1). A large, strongly temperature-dependent H/D kinetic isotope effect (9.7 at -34 degrees C; 6.9 at -19 degrees C) was measured for the dynamic behavior of 4a versus 4a-d(10), consistent with the proposed pi-benzene C-H bond cleavage. The fact that the pi-benzene complex 4a is thermally more robust in the absence of MeCN than is the Pt(IV) hydridodiphenyl complex 3a in the presence of MeCN agrees with the notion that arene elimination from Pt(IV) hydridoaryl complexes occurs via Pt(II) pi-arene intermediates that eliminate the hydrocarbon associatively, in this case, promoted by MeCN. Compounds 1a, 1b, 1d, 2a, and 2b have been crystallographically characterized.

Protonolysis of (diimine)PtMe2 (1) complexes in CD2Cl2 containing CD3CN at -78 degrees C yields (diimine)PtMe2(H)(NCCD3)+ (4), (diimine)PtMe(NCCD3)+ (5), and methane. The relative yields of 5 and methane decrease with increasing concentrations of CD3CN. This is consistent with protonation of 1 occurring directly at the metal, rather than at a methyl group. The principle of microscopic reversibility then implies that the deprotonation in "Shilov-type C-H activation" occurs from a Pt(IV) hydridomethyl intermediate, rather than from a Pt sigma-methane complex.

The compounds Cp*Fe(dppe)X ([Fe]X) and the corresponding cation radicals [Fe*]X*+ are available for the series X = F, Cl, Br, I, H, CH3. This has allowed for a detailed investigation of the dependence of the nature of Fe-X bonding on the identity of X and the oxidation state (charge) of the complex. Cyclic voltammetry demonstrates that the electrode potentials for the [Fe]X0/+ couples decrease in the order I > Br > Cl > H > F > CH3. An "inverse halide order" is seen, in which the most electronegative X leads to the most easily oxidized complex. This suggests that F is the best donor among the halides. The halide trend is also reflected in NMR spectroscopic data. Mossbauer spectroscopy data also suggest that the F ligand is a strong donor (relative to H and CH3) in [Fe*]X*+. DFT calculations on CpFe(dpe)X ([Fe]X) model complexes nicely reproduce the trend in the electrode potentials for the [Fe*]X0/+ couples. Analysis of the theoretical data within the halogen series indicates that the energy of the [Fe]X HOMO does not correlate with the extent of its Fe(d(pi))-X(p(pi)) antibonding character, which varies in the order I > Br > Cl > F, but rather depends on the destabilizing electrostatic effect caused by X. This effect varies in the order F > Cl > Br > I. A thermochemical cycle that incorporates the [Fe*]X0/+ and [Fe*]0/+ electrode potentials was used to investigate the effect of the oxidation state of the complex on the homolytic bond dissociation energy (BDEhom), defined for the processes Fe-X --> Fe* + X* and Fe-X*+ --> Fe*+ + X*. For all X, it was found that a one-electron oxidation leads to a weakening of the Fe-X bond. This trend was reproduced by the DFT calculations. On the other hand, IR nu(Fe-X) spectroscopy data showed an increase in the stretching frequencies for X = H and Cl upon oxidation. X-ray crystallographic data showed a shortening of the Fe-Cl bond upon oxidation. The trends in IR and Fe-Cl bond distances were reproduced in the DFT calculations. The combined data therefore suggest that oxidation leads to weaker, but shorter, Fe-X bonds. A second thermochemical cycle was applied to investigate the effect of the one-electron oxidation on the heterolytic bond dissociation energies (BDEhet), defined for the processes Fe-X --> Fe+ + X- and Fe-X*+ --> Fe2+ + X-. In this case, the oxidation led to bond strengthening in all cases. The computed BDE values have been analyzed within Ziegler's transition state methodology and decomposed into two components, one electrostatic and one covalent, describing the interaction between the unrelaxed fragments. In all the computed BDEhom and BDEhet values of the [Fe]X models the electrostatic component is important. This helps to understand their respective variations upon oxidation.

Detailed, quantitative knowledge about the energetics of metal-ligand bonding in organotransition-metal complexes is crucial to the understanding of stoichiometric and catalytic reactions and may eventually help in the design of new and improved processes [1]. While numerous methods are available to estimate the energetics of homolytic as well as heterolytic metal-ligand cleavage reactions, many of these can only be applied under equilibrium conditions. Thus, if the reactions involve transient species, the techniques may not be applicable.

Four new Rh(I) complexes bearing chelating imine-functionalized N-heterocyclic carbene ligands have been synthesized and characterized. The catalytic activity of these new Rh(I) complexes has been tested in the cyclopropanation reaction between ethyl diazoacetate and styrene. One of the new complexes, having ethyl groups on the ligand N-aryl ring, exhibited a reactivity and a cis-diastereoselectivity that were comparable to our previously reported Rh(I) cyclopropanation catalyst of this type, and a higher yield and cis-diastereoselectivity were obtained at lower catalyst loadings and higher temperatures. The other new Rh(I) complexes were found to be inferior to the previously reported Rh(I) cyclopropanation catalyst. The catalytic study gave important information about the effect that changing the steric requirements of the substituents at the ligand system has on the efficiency and cis-diastereoselectivity of the complexes as cyclopropanation catalysts.

The electrocatalytic ligand substitutions of Cp’Mn(CO)2(NCMe) and Cp’Mn(CO)2(pyr) with phosphines and phosphites have been investigated by derivative cyclic voltammetry (DCV) at sweep rates that are sufficiently fast to preclude high catalytic turnovers on the experimental time scale. Digital simulations have established that a single working curve may be used to analyze the DCV data, provided that the substrate is more readily oxidized than the product by 250 mV or more. Rate constants and activation parameters were obtained for the reactions between Cp’Mn(CO)2(NCMe)+ and PPh3 and P(OMe)3, and between Cp’Mn(CO)2(pyr)+ and PEt3 and PBu3. Excellent agreement is observed between the kinetic data obtained with this technique and previously reported values that were obtained under conditions that allowed for high catalytic turnover numbers.

The Fe–halide bonds in Cp*Fe(dppe)X (X = F, Cl, Br, I) complexes are weakened as a consequence of one and two-electron oxidations; the bond weakening decreases in the order F << Cl < Br < I and is much less pronounced for F than the other halides, indicating a pronounced effect of apparent fluoride-to-metal backbonding as a consequence of the removal of electrons.

L'etude est faite par voltammetrie cyclique et par voltammetrie a balayage lineaire, dans l'acetonitrile