James D. Mattock

Efficient NiII2LnIII2 Electrocyclization Catalysts for the Synthesis of trans-4,5-Diaminocyclopent-2-enones from 2-Furaldehyde and Primary or Secondary Amines

A series of heterometallic coordination clusters (CCs) [Ni(II)2Ln(III)2(L1)4Cl2(CH3CN)2] 2CH3CN [Ln = Y (1Y), Sm (1Sm), Eu (1Eu), Gd (1Gd), or Tb (1Tb)] were synthesized by the reaction of (E)-2-(2-hydroxy-3-methoxybenzylidene-amino)phenol) (H2L1) with NiCl2·6(H2O) and LnCl3·x(H2O) in the presence of Et3N at room temperature. These air-stable CCs can be obtained in very high yields from commercially available materials and are efficient catalysts for the room-temperature domino ring-opening electrocyclization synthesis of trans-4,5-diaminocyclopent-2-enones from 2-furaldehyde and primary or secondary amines under a non-inert atmosphere. Structural modification of the catalyst to achieve immobilization or photosensitivity is possible without deterioration in catalytic activity.

Organometallic probe for the electronics of base-stabilized group 11 metal cations

A number of trimetalloborides have been synthesized through the reactions of base-stabilized coinage metal chlorides with a dimanganaborylene lithium salt in the hope of using this organometallic platform to compare and evaluate the electronics of these popular coinage metal fragments. The adducts of Cu(I), Ag(I), and Au(I) ions, stabilized by tricyclohexylphosphine (PCy3), N-1,3-bis(4-methylphenyl)imidazol-2-ylidene (ITol), or 1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene (CAAC), with [{Cp(CO)2Mn}2B](-) were studied spectroscopically, structurally, and computationally. The geometries of the adducts fall into two classes, one symmetric and one asymmetric, each relying on the combined characteristics of both the metal and ligand. The energetic factors proposed as the causes of the structural differences were investigated by ETS-NOCV (extended transition state-natural orbitals for chemical valence) analysis, which showed the final geometry to be controlled by the competition between the tendency of the coinage metal to adopt a higher or lower coordination number and the willingness of the cationic fragment to participate in back-bonding interactions.

Strongly Phosphorescent Transition Metal π-Complexes of Boron–Boron Triple Bonds

Herein are reported the first π-complexes of compounds with boron-boron triple bonds with transition metals, in this case CuI. Three different compounds were isolated that differ in the number of copper atoms bound to the BB unit. Metalation of the B-B triple bonds causes lengthening of the B-B and B-CNHC bonds, as well as large upfield shifts of the 11B NMR signals, suggesting greater orbital interactions between the boron and transition metal atoms than those observed with recently published diboryne/alkali metal cation complexes. In contrast to previously reported fluorescent copper(I) π-complexes of boron-boron double bonds, the Cun-π-diboryne compounds (n = 2, 3) show intense phosphorescence in the red to near-IR region from their triplet excited states, according to their microsecond lifetimes, with quantum yields of up to 58%. While the Cu diborene bond is dominated by electrostatic interactions, giving rise to S1 and T1 states of pure IL(π-π*) nature, DFT studies show that the CuI π-complexes of diborynes reported herein exhibit enhanced metal d orbital contributions to HOMO and HOMO-1, which results in S1 and T1 having significant MLCT character, enabling strong spin-orbit coupling for highly efficient intersystem-crossing S1 → Tn and phosphorescence T1 → S0.

Engineering a Small HOMO-LUMO Gap and Intramolecular C-H Borylation by Diborene/Anthracene Orbital Intercalation

The diborene 1 was synthesized by reduction of a mixture of 1,2-di-9-anthryl-1,2-dibromodiborane(4) (6) and trimethylphosphine with potassium graphite. The X-ray structure of 1 shows the two anthryl rings to be parallel and their π(C14 ) systems perpendicular to the diborene π(B=B) system. This twisted conformation allows for intercalation of the relatively high-lying π(B=B) orbital and the low-lying π* orbital of the anthryl moiety with no significant conjugation, resulting in a small HOMO-LUMO gap (HLG) and ultimately a C-H borylation of the anthryl unit. The HLG of 1 was estimated to be 1.57 eV from the onset of the long wavelength band in its UV/Vis absorption spectrum (THF, λonset =788 nm). The oxidation of 1 with elemental selenium afforded diboraselenirane 8 in quantitative yield. By oxidative abstraction of one phosphine ligand by another equivalent of elemental selenium, the B-B and C1 -H bonds of 8 were cleaved to give the cyclic 1,9-diborylanthracene 9.

Monomeric 16-Electron π-Diborene Complexes of Zn(II) and Cd(II)

Despite the prevalence of stable π-complexes of most d10 metals, such as Cu(I) and Ni(0), with ethylene and other olefins, complexation of d10 Zn(II) to simple olefins is too weak to form isolable complexes due to the metal ions limited capacity for π-backdonation. By employing more strongly donating π-ligands, namely neutral diborenes with a high-lying π(B═B) orbital, monomeric 16-electron M(II)-diborene (M = Zn, Cd) π-complexes were synthesized in good yields. Metal-B2 π-interactions in both the solid and solution state were confirmed by single-crystal X-ray analyses and their solution NMR and UV-vis absorption spectroscopy, respectively. The M(II) centers adopt a trigonal planar geometry and interact almost symmetrically with both boron atoms. The MB2 planes significantly twist out of the MX2 planes about the M-centroid(B-B) vector, with angles ranging from 47.0° to 85.5°, depending on the steric interactions between the diborene ligand and the MX2 fragment.

Unsymmetrical, Cyclic Diborenes and Thermal Rearrangement to a Borylborylene

Cyclic diboranes(4) based on a chelating monoanionic benzylphosphine linker were prepared through boron-silicon exchange between arylsilanes and B2 Br4 . Coordination of Lewis bases to the remaining sp2 boron atom yielded unsymmetrical sp3 -sp3 diboranes, which were reduced with KC8 to their corresponding trans-diborenes. These compounds were studied with a combination of spectroscopic methods, X-ray diffraction, and DFT calculations. PMe3 -stabilized diborene 6 was found to undergo thermal rearrangement to gem-diborene 8. DFT calculations on 8 reveal a polar boron-boron bond, and indicate that the compound is best described as a borylborylene.

Isolierbare, neutrale Analoga des [B2H5]−-Ions als vielseitige und stark bindende η2-Donorliganden

Herein we report the discovery that two bottleable, neutral, base-stabilized diborane(5) compounds are able to bind strongly to a number of copper(I) complexes exclusively through their B–B bond. The resulting complexes represent the first known complexes containing unsupported, neutral σB-B diborane ligands. Single-crystal X-ray analyses of these complexes show that the X–Cu moiety (X = Cl, OTf, C6F5) lies opposite the bridging hydrogen of the diborane and is near perpendicular to the B–B bond, interacting almost equally with both boron atoms and causing a B–B bond elongation. DFT studies show that σ donation from and π backdonation to the pseudo-π-like B–B bond account for their formation. Astoundingly, these copper σB-B-complexes are inert to ligand exchange with pyridine under either heating or photoirradiation. In addition to their intriguing structural features, diboranes are also powerful borylating reagents in both metal-catalyzed and metal-free methodologies for the formation of B−C bonds and other processes. Generally, either oxidative addition or formal σ-bond metathesis reactions are proposed for the activation of B−B single bonds. As first proposed in the Dewar−Chatt−Duncanson model for metal-olefin complexes, the synergetic combination of σ donation from a bonding ligand orbital to a vacant metal orbital and π backdonation from a filled metal orbital to the ligand antibonding orbital are the basis of the metal-promoted activation of both multiple and single bonds. In stark contrast to the wide range of transition metal πcomplexes with unsaturated molecules, only a few unsupported σ-complexes with single bonds between non-hydrogen maingroup elements, such as C−C, B−C and Si−Si bonds, have been authenticated. Despite the extensive precedent for metal-mediated activation of B−B single bonds, the synthesis of unsupported metal-diborane complexes wherein the metal binds side-on to a B−B single bond has thus far not been achieved. In the last few decades, several interesting transition metal complexes bearing doubly base-stabilized diborane ligands bound through two metal−H−B bridges (A, Figure 1) have been characterized, in addition to a number of metal complexes of anionic diborane species bound to B−B σ-bonds supported by Coulombic interactions (B, Figure 1). However, unsupported transition metal complexes directly bound to the B−B single bond of neutral diboranes remain elusive. Herein we report that carbene-stabilized, hydrogen-bridged neutral diboranes(5) of the form VII (C, Figure 1), generated from an intramolecular borylborylene C−H bond insertion reaction, readily coordinate to transition metals, in this case Cu(I), in an η mode. This reactivity establishes neutral reagents VII as a highly unusual class of bottleable, neutral diboranes(5) that bind strongly to metals in a side-on, unsupported manner. These unsupported σcomplexes are surprisingly stable and inert towards ligand exchange with pyridine under either heating or photoirradiation. Figure 1. Coordination modes of diboranes with transition metals. Mes = 2,4,6-trimethylphenyl; α,2-Mes = methylene(3,5-dimethyl-1,2-phenylenediyl); An = 9-anthryl; 1,9-An = 1,9-anthracenediyl. In an attempted hydride abstraction reaction of diborane 1a with (CuOTf)2∙(toluene) in benzene at room temperature, [17] an intense red solution was obtained after removal of black solids by filtration, from which the unprecedented complex 2 was obtained as red crystals in 91% yield (Scheme 1). Notably, complexes 3 and 4 were also obtained in good yields using less electrophilic copper(I) reagents such as CuCl and CuPf (Pf: pentafluorophenyl). Furthermore, 1,2-diboraindane 1b also [*] Dr. S. R. Wang, D. Prieschl, Dr. M. Arrowsmith, Dr. C. Pranckevicius, Dr. T. E. Stennett, Dr. R. D. Dewhurst, Prof. Dr. H. Braunschweig Institute for Inorganic Chemistry Julius-Maximilians-Universität Würzburg Am Hubland, 97074 Würzburg (Germany) E-Mail: h.braunschweig@uni-wuerzburg.de and Institute for Sustainable Chemistry & Catalysis with Boron Julius-Maximilians-Universität Würzburg Am Hubland, 97074 Würzburg (Germany) J. D. Mattock, Dr. A. Vargas Department of Chemistry, School of Life Sciences University of Sussex, Brighton BN1 9QJ, Sussex, UK E-Mail: alfredo.vargas@sussex.ac.uk Supporting information for this article is given via a link at the end of the document.

Bottleable Neutral Analogues of [B2H5]− as Versatile and Strongly Binding η2 Donor Ligands

Herein we report the discovery that two bottleable, neutral, base-stabilized diborane(5) compounds are able to bind strongly to a number of copper(I) complexes exclusively through their B-B bond. The resulting complexes represent the first known complexes containing unsupported, neutral σB-B diborane ligands. Single-crystal X-ray analyses of these complexes show that the X-Cu moiety (X=Cl, OTf, C6 F5 ) lies opposite the bridging hydrogen atom of the diborane and is near perpendicular to the B-B bond, interacting almost equally with both boron atoms and causing a B-B bond elongation. DFT studies show that σ donation from and π backdonation to the pseudo-π-like B-B bond account for their formation. Astoundingly, these copper σB-B complexes are inert to ligand exchange with pyridine under either heating or photoirradiation.

Potassium Binding Adjacent to Cationic Transition-Metal Fragments: Unusual Heterobimetallic Adducts of a Calix[4]arene-Based Thione Ligand

The synthesis of cationic rhodium and iridium complexes of a bis(imidazole-2-thione)-functionalized calix[4]arene ligand and their surprising capacity for potassium binding are described. In both cases, uptake of the alkali metal into the calix[4]arene cavity occurs despite adverse electrostatic interactions associated with close proximity to the transition-metal fragment [Rh+···K+ = 3.715(1) Å; Ir+···K+ = 3.690(1) Å]. The formation and constituent bonding of these unusual heterobimetallic adducts have been interrogated through extensive solution and solid-state characterization, examination of the host-guest chemistry of the ligand and its upper-rim unfunctionalized calix[4]arene analogue, and use of density functional theory based energy decomposition analysis.

Identification of a Lead Candidate in the Search for Carbene‐Stabilised Homoaromatics

The effect of carbenes as Lewis donor groups on the homoaromaticity of mono- and bicyclic organic molecules is surveyed. The search for viable carbene-stabilised homoaromatics resulted in a large amount of rejected candidates as well as nine promising candidates that are further analysed for their homoaromaticity by using a number of metrics. Of these, five appeared to show modest homoaromaticity, whereas another compound showed a level of homoaromaticity comparable with the homotropylium cation benchmark compound. Isoelectronic analogues and constitutional isomers of the lead compound were investigated, however, none of these showed comparable homoaromaticity. The implications of these calculations on the design of donor-stabilised homoaromatics are discussed.