Alexander Higelin

Unusual Tilted Carbene Coordination in Carbene Complexes of Gallium(I) and Indium(I)

First attempts to isolate carbenes date back to the early 19th century, and interest increased throughout the 20th century, 3] but research and application of these subvalent carbon ligands did not really rise prior to the preparation of isolable and storable N-heterocyclic carbenes (NHCs). In the two decades since the publication of the first stable crystalline carbene by Arduengo et al., carbenes have evolved from mere reaction intermediates in biological or industrial processes to quintessential ligands in modern coordination chemistry and catalysis. 7] NHCs are most well-known for their ability to coordinate transition metals, 9] but a good number of main-group complexes have also been reported. Among those are a number of gallium(III) 13] and indium(III) 15] complexes, but no such compounds are known for gallium(I) and indium(I) to date. Previous attempts have led to the formation of dimerized M species. 17] This may be prevented by the use of suitable starting material. In this context we established a simple route to weak complexes of solvated [M(arene)2,3] [Al(OR)4] (M = Ga, In). Those are versatile precursors for coordination chemistry, for example, complexes with phosphanes and crown ethers. 21] NHCs are known to mimic phosphanes in their ability to form strong s-bonds but only weak p-bonds to metal centers. They have in fact outperformed and replaced phosphanes in many applications. Therefore, we now expand the coordination chemistry of gallium(I) and indium(I) to NHCs. A yellow solution formed when Ga[Al(OR)4] was dissolved in fluorobenzene together with two equivalents of 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr). The characteristic NMR signals of the [Al(OR)4] anion were observed in this solution (d(F) = 74.9 ppm; d(Al) = 33.8 ppm). The H NMR spectrum displayed signals that are reasonably close to reported shifts of the IPr ligand, but with a considerably upfield-shifted signal for the methine proton of the isopropyl group (2.19 vs. 2.96 ppm). This is in agreement with the spectra of similar complexes with transition metals. 26] Unlike in the bare starting material, no signal was observed in the Ga NMR spectrum, which is a similar situation to the gallium phosphane complexes. This is interpreted as an indication of the complexation of the Ga cation by the carbene. 19,27] Crystallization of the yellow Ga/IPr solution by concentration and storage at 40 8C yielded crystals that analyzed as [Ga(IPr)2] [Al(OC(CF3)3)4] ·PhF (1; Figure 1 a). Much like in the complexes with bulky phosphane ligands such as PtBu3, [18] the central gallium ion is coordinated in a bent fashion with an angle of 118.28 (dGa–C = 229 pm). The coordination is in fact not ideal, as the donation of electron density from carbon into the empty p orbitals of gallium would benefit from a much smaller angle (compare the gallium(I) phosphane complexes). 19] The gallium(I) ion is also not in plane with either ring. Figure 1b presents the

Structural Proof for a Higher Polybromide Monoanion: Investigation of [N(C3H7)4][Br9]

The chemistry of polyhalides, especially of polyiodides, has long been known. 2] The first systematic investigation of these anions goes back to Jçrgensen in 1870. Since these pioneering years, a great variety, mainly of polyiodides, have been investigated. The lighter and more reactive halogens, bromine, chlorine, and fluorine, have been less explored which is probably due to the relative ease of handling iodine. However, in the past year, the investigation of lighter polyhalides has once again come into the focus of the scientific community. Feldmann et al. have reported the preparation of a 3D polybromide network [C4MPyr]2[Br20] [7] in ionic liquids. A series of tetraethylammonium polybromides was also investigated by Raman spectroscopy. Moreover, the first free trifluoride monoanion was characterized by matrix-isolation spectroscopy under cryogenic conditions in argon and neon matrices. All these recent reports indicate that our knowledge of polyhalides is still relatively limited and provides room for new discoveries. The chemistry of polybromides is especially limited compared to the extensive chemistry of polyiodides. Among the polybromide monoanions, only the [Br3] anion was fully characterized, including single-crystal X-ray diffraction. All other known polybromide monoanions (penta, hepta, and nona) were only characterized by IR and/or Raman spectroscopy. Based on these data, their structures were only tentatively assigned. High level quantum-chemical calculations, which could support the structure assignment based on vibrational data for the nonabromide have not been performed. Only calculations at the HF level have been carried out but these do not provide definitive information because of the lack of electron correlation. By far the most prominent polybromides are dianions, such as [Br8] 2 , [Br10] 2 , and [Br20] 2 [7] or polybromide networks 14–16] [{Br3} ·=2 Br2], and [(Br )2·3 Br2]. These compounds are not only of academic interest, they can be used for many practical applications such as zinc–bromine batteries, 18] water treatment, or selective bromination reactions. Furthermore, an application as redox couple in dyesensitized solar cells (DSSC) is promising, a field which is a more and more important in energy generation. Herein, we report the first synthesis of the nonabromide salt [NPr4][Br9]. The reaction of tetrapropylammonium bromide and excess bromine leads to the formation of brownish red crystals. These crystals are relatively stable and can even be handled briefly in air. The single-crystal X-ray structure determination shows that the salt [NPr4][Br9], crystallizes in the tetragonal space group I 4, Figure 1. Similar to other known polyhalides, the [Br9] structure is based on a central bromide anion Br ,

Univalent Gallium and Indium Phosphane Complexes: From Pyramidal M(PPh3)3+ to Carbene-Analogous Bent M(PtBu3)2+ (M=Ga, In) Complexes

In a new oxidative route, Ag(+)[Al(OR(F))(4)](-) (R(F)=C(CF(3))(3)) and metallic indium were sonicated in aromatic solvents, such as fluorobenzene (PhF), to give a precipitate of silver metal and highly soluble [In(PhF)(n)](+) salts (n=2, 3) with the weakly coordinating [Al(OR(F))(4)](-) anion in quantitative yield. The In(+) salt and the known analogous Ga(+)[Al(OR(F))(4)](-) were used to synthesize a series of homoleptic PR(3) phosphane complexes [M(PR(3))(n)](+), that is, the weakly PPh(3)-bridged [(Ph(3)P)(3)In-(PPh(3))-In(PPh(3))(3)](2+) that essentially contains two independent [In(PPh(3))(3)](+) cations or, with increasing bulk of the phosphane, the carbene-analogous [M(PtBu(3))(2)](+) (M=Ga, In) cations. The M(I)-P distances are 27 to 29 pm longer for indium, and thus considerably longer than the difference between their tabulated radii (18 pm). The structure, formation, and frontier orbitals of these complexes were investigated by calculations at the BP86/SV(P), B3LYP/def2-TZVPP, MP2/def2-TZVPP, and SCS-MP2/def2-TZVPP levels.

Hexaazatrinaphthylenes with Different Twists

A synthetic strategy that allows the induction of twist angles of different sizes in 5,6,11,12,17,18-hexaazatrinaphthylene (HATNA) chromophores is reported. The different twist angles are accompanied by measurable changes in the emission and electrochemical characteristics of HATNA.

Isolated cationic crown ether complexes of gallium(I) and indium(I)

The recently reported homologous low-valent indium and gallium salts M(+)[Al(OR(F))(4)](-) (M = Ga, In; R(F) = C(CF(3))(3)) were used to extend the coordination chemistry of Ga(I) and In(I) to the isolated [18]crown-6 complexes [M([18]crown-6)(PhF)(2)](+)[Al(OR(F))(4)](-) in fluorobenzene solution (PhF = C(6)H(5)F). In contrast to known ion-paired compounds for M = In, our complexes are undisturbed and in the solid state free of contacts to the anion. A peculiar combination of very weak η(1)- and η(6)-coordination to the PhF-solvent was observed that allows speculation about the presence of a stereochemically active lone pair at M(I). Structure and energetics of these novel salts were rationalized on the basis of DFT calculations.

Synthesis, Characterisation and Reactions of Truly Cationic NiI-Phosphine Complexes

The recently published purely metallo-organic NiI salt [Ni(cod)2 ][Al(ORF )4 ] (1, cod=1,5-cyclooctadiene, RF =C(CF3 )3 ) provides a starting point for a new synthesis strategy leading to NiI phosphine complexes, replacing cod ligands by phosphines. Clearly visible colour changes indicate reactions within minutes, while quantum chemical calculations (PBE0-D3(BJ)/def2-TZVPP) approve exergonic reaction enthalpies in all performed ligand exchange reactions. Hence, [Ni(dppp)2 ][Al(ORF )4 ] (2, dppp=1,3-bis(diphenylphosphino)propane), [Ni(dppe)2 ][Al(ORF )4 ] (3, dppe=1,3-bis(diphenyl-phosphino)ethane), three-coordinate [Ni(PPh3 )3 ][Al(ORF )4 ] (4) and a remarkable two-coordinate NiI phosphine complex [Ni(PtBu3 )2 ][Al(ORF )4 ] (5) were characterised by single crystal X-ray structure analysis. EPR studies were performed, confirming a nickel d9 -configuration in complexes 2, 4 and 5. This result is supported by additional magnetization measurements of 4 and 5. Further investigations by cyclic voltammetry indicate relatively high oxidation potentials for these NiI compounds between 0.7 and 1.7 V versus Fc/Fc+ . Screening reactions with O2 and CO gave first insights on the reaction behaviour of the NiI phosphine complexes towards small molecules with formation of mixed phosphine-CO-NiI complexes and oxidation processes yielding new NiI and/or NiII derivatives. Moreover, 4 reacted with CH2 Cl2 at RT to give a dimeric NiII ylide complex (4 c). As CH2 Cl2 is a rather stable alkyl halide with relatively high C-Cl bond energies, 4 appears to be a suitable reagent for more general C-Cl bond activation reactions.