Cationic Niobium-Sandwich and Piano-Stool Complexes.

The syntheses of the homoleptic bis(arene) niobium cations [Nb(arene)2 ]+ (arene = C6 H3 Me3 , C6 H5 Me) with 16 valence electrons and heteroleptic arene-carbonyl cations [(CO)Nb(arene)2 ]+ (arene = C6 H3 Me3 , C6 H5 Me) and [(arene)M(CO)4 ]+ (arene = C6 H3 Me3 , C6 H6 ) obeying 18 valence electrons are described. Stabilization of these complexes was achieved by using the weakly coordinating anions [Al(ORF )4 ]- or [F{Al(ORF )3 }2 ]- (RF = C(CF3 )3 ). The limits of two synthesis routes starting from neutral Nb(arene)2 (arene = C6 H3 Me3 , C6 H5 Me) or [NEt4 ][M(CO)6 ] (M = Nb, Ta) were investigated. All compounds were analyzed by single crystal X-ray determination, vibrational and NMR spectroscopy. DFT calculations were executed to support the experimental data.

Altering Charges on Heterobimetallic Transition-Metal Carbonyl Clusters.

The homoleptic group 5 carbonylates [M(CO)6 ]- (M=Nb, Ta) serve as ligands in carbonyl-terminated heterobimetallic Agm Mn clusters containing 3 to 11 metal atoms. Based on our serendipitous [Ag6 {Nb(CO)6 }4 ]2+ (4 a2+ ) precedent, we established access to such Agm Mn clusters of the composition [Agm {M(CO)6 }n ]x (M=Nb, Ta; m=1, 2, 6; n=2, 3, 4, 5; x=1-, 1+, 2+). Salts of those molecular cluster ions were synthesized by the reaction of [NEt4 ][M(CO)6 ] and Ag[Al(ORF )4 ] (RF =C(CF3 )3 ) in the correct stoichiometry in 1,2,3,4-tetrafluorobenzene at -35 °C. The solid-state structures were determined by single-crystal X-ray diffraction methods and, owing to the thermal instability of the clusters, a limited scope of spectroscopic methods. In addition, DFT-based AIM calculations were performed to provide an understanding of the bonding within these clusters. Apparently, the clusters 3+ (m=6, n=5) and 42+ (m=6, n=4) are superatom complexes with trigonal-prismatic or octahedral Ag6 superatom cores. The [M(CO)6 ]- ions then bind through three CO units as tridentate chelate ligands to the superatom core, giving overall structures related to tetrahedral AX4 (42+ ) or trigonal bipyramidal AX5 molecules (3+ ).

Experimental and quantum chemical studies of anionic analogues of N-heterocyclic carbenes.

A combination of quantum chemical and synthetic/crystallographic methods have been employed to probe electronic structure in two series of anionic ligands related to the well known N-heterocyclic carbene (NHC) class of donor. Analyses of (i) the respective frontier orbital energies/compositions for the 'free' ligands and the results of ETS-NOCV studies of the bonding in model group 11 complexes; and (ii) the structural metrics for (new) linear gold(i) compounds, have been used to probe the bonding in complexes of NHC ligands which incorporate a backbone-appended weakly-coordinating anion component (WCA-NHCs) and in systems featuring the isoelectronic (formally anionic) diazaborolyl ligand family. Key findings are that WCA-NHC ligands - in which the anionic component is attached to the ligand heterocycle via a methylene (CH2) spacer - offer electronic (and steric) properties which are largely unperturbed from their 'simple' NHC counterparts, while diazaborolyl donors (in which the negative charge is formally located at the boron donor atom) offer significantly stronger σ-donation and a very high trans influence.

The Acidity of the HBr/AlBr3 System: Stabilization of Crystalline Protonated Arenes and Their Acidity in Bromoaluminate Ionic Liquids.

Bulk protonated mesitylene, toluene, and benzene bromoaluminate salts were stabilized and characterized in the superacidic system HBr/n AlBr3 with NMR spectroscopy and X-ray analysis of [HC6 H3 (CH3 )3 ](+) [AlBr4 ](-) (1), [HC6 H5 (CH3 )](+) [AlBr4 ](-) (2), and [C6 H7 ](+) [Al2 Br7 ](-) ⋅C6 H6 (3). Protonation attempts in bromoaluminate ILs led to a complete protonation of mesitylene, and a protonation degree of up to 15 % for toluene in the IL BMP(+) [Al2 Br7 ](-) . Benzene could only be protonated in the more acidic IL BMP(+) [Al3 Br10 ](-) , with a degree of 25 %. Protonation attempts on aromatics provide evidence that the bromoaluminate ILs tolerate superacidic environments. On the basis of the absolute Brønsted acidity scale, quantum chemical calculations confirmed the superacidic properties, and rank the acidities in ILs down to a pHabs value of 164 with an error of less than one pH unit compared with experimental findings. The neat AlBr3 /HBr system even may reach acidities down to pHabs 163.

The superacid HBr/AlBr(3) : protonation of benzene and ordered crystal structure of [C(6)H(7)](+)[Al(2)Br(7)](-).

Crystalline and properly ordered protonated benzene as the [C6 H7 ](+) [Al2 Br7 ](-) ⋅(C6 H6 ) salt 1 are obtained by the combination of solid AlBr3 , benzene, and HBr gas. Compound 1 was characterized and verified by NMR, Raman and X-Ray spectroscopy. This unexpected simple and straight forward access shows that HBr/AlBr3 is an underestimated superacid that should be used more frequently.