Controlled monodefluorination and alkylation of C(sp3)-F bonds by lanthanide photocatalysts: importance of metal-ligand cooperativity.

The controlled functionalization of a single fluorine in a CF3 group is difficult and rare. Photochemical C-F bond functionalization of the sp3-C-H bond in trifluorotoluene, PhCF3, is achieved using catalysts made from earth-abundant lanthanides, (CpMe4)2Ln(2-O-3,5- t Bu2-C6H2)(1-C{N(CH)2N(iPr)}) (Ln = La, Ce, Nd and Sm, CpMe4 = C5Me4H). The Ce complex is the most effective at mediating hydrodefluorination and defluoroalkylative coupling of PhCF3 with alkenes; addition of magnesium dialkyls enables catalytic C-F bond cleavage and C-C bond formation by all the complexes. Mechanistic experiments confirm the essential role of the Lewis acidic metal and support an inner-sphere mechanism of C-F activation. Computational studies agree that coordination of the C-F substrate is essential for C-F bond cleavage. The unexpected catalytic activity for all members is made possible by the light-absorbing ability of the redox non-innocent ligands. The results described herein underscore the importance of metal-ligand cooperativity, specifically the synergy between the metal and ligand in both light absorption and redox reactivity, in organometallic photocatalysis.

A Dicopper Nitrenoid by Oxidation of a CuICuI Core: Synthesis, Electronic Structure, and Reactivity.

A dicopper nitrenoid complex was prepared by formal oxidative addition of the nitrenoid fragment to a dicopper(I) center by reaction with the iminoiodinane PhINTs (Ts = tosylate). This nitrenoid complex, (DPFN)Cu2(µ-NTs)[NTf2]2 (DPFN = 2,7-bis(fluorodi(2-pyridyl)methyl)-1,8-naphthyridine), is a powerful H atom abstractor that reacts with a range of strong C-H bonds to form a mixed-valence Cu(I)/Cu(II) µ-NHTs amido complex in the first example of a clean H atom transfer to a dicopper nitrenoid core. In line with this reactivity, DFT calculations reveal that the nitrenoid is best described as an iminyl (NR radical anion) complex. The nitrenoid was trapped by the addition of water to form a mixed-donor hydroxo/amido dicopper(II) complex, which was independently obtained by reaction of a Cu2(µ-OH)2 complex with an amine through a protonolysis pathway. This mixed-donor complex is an analogue for the proposed intermediate in copper-catalyzed Chan-Evans-Lam coupling, which proceeds via C-X (X = N or O) bond formation. Treatment of the dicopper(II) mixed donor complex with MgPh2(THF)2 resulted in generation of a mixture that includes both phenol and a previously reported dicopper(I) bridging phenyl complex, illustrating that both reduction of dicopper(II) to dicopper(I) and concomitant C-X bond formation are feasible.

Bimetallics in a Nutshell: Complexes Supported by Chelating Naphthyridine-Based Ligands.

Bimetallic motifs are a structural feature common to some of the most effective and synthetically useful catalysts known, including in the active sites of many metalloenzymes and on the surfaces of industrially relevant heterogeneous materials. However, the complexity of these systems often hampers detailed studies of their fundamental properties. To glean valuable mechanistic insight into how these catalysts function, this research group has prepared a family of dinucleating 1,8-naphthyridine ligands that bind two first-row transition metals in close proximity, originally designed to help mimic the proposed active site of metal oxide surfaces. Of the various bimetallic combinations examined, dicopper(I) is particularly versatile, as neutral bridging ligands adopt a variety of different binding modes depending on the configuration of frontier orbitals available to interact with the Cu centers. Organodicopper complexes are readily accessible, either through the traditional route of salt metathesis or via the activation of tetraarylborate anions through aryl group abstraction by a dicopper(I) unit. The resulting bridging aryl complexes engage in C-H bond activations, notably with terminal alkynes to afford bridging alkynyl species. The µ-hydrocarbyl complexes are surprisingly tolerant of water and elevated temperatures. This stability was leveraged to isolate a species that typically represents a fleeting intermediate in Cu-catalyzed azide-alkyne coupling (CuAAC); reaction of a bridging alkynyl complex with an organic azide afforded the first example of a well-defined, symmetrically bridged dicopper triazolide. This complex was shown to be an intermediate during CuAAC, providing support for a proposed bimetallic mechanism. These platforms are not limited to formally low oxidation states; chemical oxidation of the hydrocarbyl complexes cleanly results in formation of mixed valence CuICuII complexes with varying degrees of distortion in both the bridging moiety and the dicopper core. Higher oxidation states, e.g., dicopper(II), are easily accessed via oxidation of a dicopper(I) compound with air to give a CuII2(µ-OH)2 complex. Reduction of this compound with silanes resulted in the unexpected formation of pentametallic copper(I) dihydride clusters or trimetallic monohydride complexes, depending on the nature of the silane. Finally, development of an unsymmetrical naphthyridine ligand with mixed donor side-arms enables selective synthesis of an isostructural series of six heterobimetallic complexes, demonstrating the power of ligand design in the preparation of heterometallic assemblies.

A Dicopper Platform that Stabilizes the Formation of Pentanuclear Coinage Metal Hydride Complexes.

Reduction of a dicopper(II) bis(hydroxide) complex with silanes in the presence of external copper or silver cations results in the formation of multinuclear hydride clusters, which were characterized by a variety of NMR spectroscopic experiments and X-ray crystallography. In particular, the pentanuclear complexes adopt an unusual planar "bow tie" configuration. The copper hydride complexes are efficient catalysts for the dehydrogenation of formic acid to H2 and CO2 .

The Importance of Ligand-Induced Backdonation in the Stabilization of Square Planar d10 Nickel π-Complexes.

The electronic nature of Ni π-complexes is underexplored even though these complexes have been widely postulated as intermediates in organometallic chemistry. Herein, the geometric and electronic structure of a series of nickel π-complexes, Ni(dtbpe)(X) (dtbpe=1,2-bis(di-tert-butyl)phosphinoethane; X=alkene or carbonyl containing π-ligands), is probed using a combination of 31 P NMR, Ni K-edge XAS, Ni Kß XES, and DFT calculations. These complexes are best described as square planar d10 complexes with π-backbonding acting as the dominant contributor to M-L bonding to the π-ligand. The degree of backbonding correlates with 2 JPP from NMR and the energy of the Ni 1s→4pz pre-edge in the Ni K-edge XAS data, and is determined by the energy of the π*ip ligand acceptor orbital. Thus, unactivated olefinic ligands tend to be poor π-acids whereas ketones, aldehydes, and esters allow for greater backbonding. However, backbonding is still significant even in cases in which metal contributions are minor. In such cases, backbonding is dominated by charge donation from the diphosphine, which allows for strong backdonation, although the metal centre retains a formal d10 electronic configuration. This ligand-induced backbonding can be formally described as a 3-centre-4-electron (3c-4e) interaction, in which the nickel centre mediates charge transfer from the phosphine σ-donors to the π*ip ligand acceptor orbital. The implications of this bonding motif are described with respect to both structure and reactivity.

Diverse Reactivity of Diazatitanacyclohexenes: Coupling Reactions of 2H-Azirines Mediated by Titanium(II).

2H-Azirines are versatile coupling partners for the synthesis of N-heterocycles. Herein, we present our studies on the reactivity of Cp2Ti(BTMSA) (1; BTMSA = bis(trimethylsilyl)acetylene) with a variety of azirines. In all the cases examined, the initial organometallic products formed are diazatitanacyclohexenes, presumably formed via oxidative addition of Ti(II) into the C-N bond of the azirine to form an azatitanacyclobutene intermediate, followed by C═N insertion of a second equivalent of azirine into the Ti-C bond to form the observed products. Diazatitanacyclohexene 3, bearing phenyl substituents and derived from 2,3-diphenyl-2H-azirine, fragments to form an azabutadiene and nitrile, which is shown to be catalytic in the presence of excess 2,3-diphenyl-2H-azirine. H-substituted complex 8, derived from 3-phenyl-2H-azirine, decomposes via protonolysis of the Cp ligands. In contrast, the methyl-substituted diazatitanacyclohexene 10, derived from 2-methyl-3-phenyl-2H-azirine, is thermally robust. Attempts to trap the putative azatitanacyclobutene intermediate with an alkyne were unsuccessful, resulting instead in the formation of titanacyclopentadiene (12) from coupling of alkyne with BTMSA. Initial reactivity studies found that 10 could be protonolyzed with AcOH to form mixtures of pyrrole and aziridine products, whereas reacting 10 with MeOH results solely in the formation of 2,4-dimethyl-3,5-diphenyl-1H-pyrrole.

Oxaziridine cleavage with a low-valent nickel complex: competing C-O and C-N fragmentation from oxazanickela(ii)cyclobutanes.

Reacting the low-valent nickel complex [(dtbpe)Ni]2(µ-η2:η2-C6H6) with oxaziridines was found to form mixtures of imine, amide and aldehyde products. If the N-substituent of the oxaziridine is sufficiently bulky, a short-lived intermediate can be isolated and characterized by X-ray diffraction studies as an oxazanickela(ii)cyclobutane. This is the first well-defined example of N-O oxidative addition of an oxaziridine to a transition metal. Subsequent fragmentation of this oxazanickelacyclobutane forms a complex mixture of products, including a nickel(ii) imido complex, demonstrating that oxaziridines can serve as nitrene precursors. Preliminary mechanistic analysis is consistent with a bimetallic mechanism of fragmentation of the oxazanickelacyclobutane to form the nickel imido and η2-aldehyde complexes.

Catalytic Functionalization of Styrenyl Epoxides via 2-Nickela(II)oxetanes.

Low-valent nickel is shown to preferentially isomerize mono- or disubstituted epoxides into their corresponding aldehydes. Experiments with tetrasubstituted epoxides demonstrate that these reactions proceed via reactive 2-nickelaoxetane intermediates, and that the oxidative addition step likely occurs with retention of configuration. The monosubstituted aldehyde isomerization products were found to rapidly react with HBpin to form boronate esters. These hydroboration reactions could be performed catalytically.

Recent advances in well-defined, late transition metal complexes that make and/or break C-N, C-O and C-S bonds.

Chemical transformations that result in either the formation or cleavage of carbon-heteroatom bonds are among the most important processes in the chemical sciences. Herein, we present a review on the reactivity of well-defined, late-transition metal complexes that result in the making and breaking of C-N, C-O and C-S bonds via fundamental organometallic reactions, i.e. oxidative addition, reductive elimination, insertion and elimination reactions. When appropriate, emphasis is placed on structural and spectroscopic characterization techniques, as well as mechanistic data.

Exploring Regioselective Bond Cleavage and Cross-Coupling Reactions using a Low-Valent Nickel Complex.

Recently, esters have received much attention as transmetalation partners for cross-coupling reactions. Herein, we report a systematic study of the reactivity of a series of esters and thioesters with [{(dtbpe)Ni}2(µ-η(2):η(2)-C6H6)] (dtbpe=1,2-bis(di-tert-butyl)phosphinoethane), which is a source of (dtbpe)nickel(0). Trifluoromethylthioesters were found to form η(2)-carbonyl complexes. In contrast, acetylthioesters underwent rapid Cacyl-S bond cleavage followed by decarbonylation to generate methylnickel complexes. This decarbonylation could be pushed backwards by the addition of CO, allowing for regeneration of the thioester. Most of the thioester complexes were found to undergo stoichiometric cross-coupling with phenylboronic acid to yield sulfides. While ethyl trifluoroacetate was also found to form an η(2)-carbonyl complex, phenyl esters were found to predominantly undergo Caryl-O bond cleavage to yield arylnickel complexes. These could also undergo transmetalation to yield biaryls. Attempts to render the reactions catalytic were hindered by ligand scrambling to yield nickel bis(acetate) complexes, the formation of which was supported by independent syntheses. Finally, 2-naphthyl acetate was also found to undergo clean Caryl-O bond cleavage, and although stoichiometric cross-coupling with phenylboronic acid proceeded with good yield, catalytic turnover has so far proven elusive.

Reexamining Oxidation States during the Synthesis of 2-Rhodaoxetanes from Olefins.

Herein, we report experimental, spectroscopic, and computational data that indicate that a rhodium ethylene complex, formally described as rhodium(I) and which forms a 2-rhoda(III) oxetane following reaction with H2O2, is more accurately described as a rhodium(III) metallacyclopropane. X-ray absorption spectroscopy clearly demonstrates a change in the oxidation state at rhodium following ligand coordination with tris(2-pyridylmethyl)amine. Both NMR and density functional theory studies suggest a high energy barrier to rotation of the coordinated ethylene, which is attributed to large geometric and electronic reorganization resulting from the loss of π-back-bonding. These results imply that the role of H2O2 in the formation of 2-rhoda(III) oxetanes is to oxidize the C2H4 fragment rather than the metal center, as has been previously suggested.

Synthesis of 2-Nickela(II)oxetanes from Nickel(0) and Epoxides: Structure, Reactivity, and a New Mechanism of Formation.

2-Nickelaoxetanes have been frequently invoked as reactive intermediates in catalytic reactions of epoxides using nickel, but have never been isolated or experimentally observed in these transformations. Herein, we report the preparation of a series of well-defined nickelaoxetanes formed via the oxidative addition of nickel(0) with epoxides featuring ketones. The stereochemistry of the products is retained, which has not yet been reported for nickelaoxetanes. Theoretical calculations support a bimetallic ring-opening/closing pathway over a concerted oxidative addition. Initial reactivity studies of a nickelaoxetane demonstrated protonolysis, oxidatively induced reductive elimination, deoxygenation, and elimination reactions when treated with the appropriate reagents.

Ring expansion of a 2-rhodaoxetane: insertion chemistry with unsaturated molecules.

A 2-rhodaoxetane was found to react with unsaturated electrophiles, such as highly electron-deficient acetylene dicarboxylates, CS2 and various aldehydes, to form a series of six-membered metallacycles. These metallacycles were characterized by (1)H, (13)C and 2D NMR spectroscopic techniques, as well as HRMS and, for one complex, XRD. In some cases, the insertions were found to be reversible.

Direct synthesis of ligand-based radicals by the addition of bipyridine to chromium(II) compounds.

The reaction of 2,2'-bipyridine (bpy) with monomeric chromium(II) precursors was used to prepare the S = 1 complexes Cr(tBu-acac)2(bpy) (1) and (η(5)-Cp)(η(1)-Cp)Cr(bpy) (3), as well as the S = 2 compound Cr[N(SiMe3)2]2(bpy) (4). The crystallographically determined bond lengths indicate that the bpy ligands in 1 and 3 are best regarded as radical anions, while 4 shows no structural evidence for electron transfer from Cr(II) to the neutral bpy ligand.