Jan A. P. Sprenger

The Hexacyanodiborane(6) Dianion [B2(CN)6]2−

Diborane(6) dianions with substituents that are bonded to boron via carbon are very reactive and therefore only a few examples are known. Diborane(6) derivatives are the simplest catenated boron compounds with an electron-precise B-B σ-bond that are of fundamental interest and of relevance for material applications. The homoleptic hexacyanodiborane(6) dianion [B2 (CN)6 ](2-) that is chemically very robust is reported. The dianion is air-stable and resistant against boiling water and anhydrous hydrogen fluoride. Its salts are thermally highly stable, for example, decomposition of (H3 O)2 [B2 (CN)6 ] starts at 200 °C. The [B2 (CN)6 ](2-) dianion is readily accessible starting from 1) B(CN)3 (2-) and an oxidant, 2) [BF(CN)3 ](-) and a reductant, or 3) by the reaction of B(CN)3 (2-) with [BHal(CN)3 ](-) (Hal=F, Br). The latter reaction was found to proceed via a triply negatively charged transition state according to an SN 2 mechanism.

Deprotonation of a Hydridoborate Anion

The first deprotonation of a borohydride anion was achieved by treatment of [BH(CN)3 ]- with strong non-nucleophilic bases, which resulted in the formation of alkali-metal salts of the tricyanoborate dianion B(CN)32- in up to 97 % yield and 99.5 % purity. [BH(CN)3 ]- is less acidic than (Me3 Si)2 NH but a stronger acid than iPr2 NH. Less sterically hindered, more nucleophilic bases such as PhLi and MeLi mostly attack a CN group under formation of imine dianions [RC(N)B(CN)3 ]2- , which can be hydrolyzed to ketones of the [RC(O)B(CN)3 ]- type. The boron-centered nucleophile B(CN)32- reacts with CO2 and CN+ reagents to give salts of the [B(CN)3 CO2 ]2- dianion and the tetracyanoborate anion [B(CN)4 ]- , respectively, in excellent yields.

The Role of [BF4]‐ and [B(CN)4]‐ Anions in the Ionothermal Synthesis of Chalcogenidometalates

The role of borate anions in ionothermal syntheses of crystalline chalcogenidometalates was shown by the directed ionothermal synthesis of seven new selenidostannates in ionic liquids with either [BF4 ]- or [B(CN)4 ]- anions. Single crystal structures are presented of four compounds, (C4 C1 C1 Im)5 [Sn9 Se20 ][BF4 ] (1, Im=imidazolium), (DMMPH)4 [Sn2 Se6 ] (2, DMMP=2,6-dimethylmorpholine), (C2 C1 Im)2 (DMMPH)2 [Sn6 Se14 ] (3), and K2 [Sn3 Se7 ] (4), and how the chosen ionic liquid anion, and the reaction temperature, influences the product spectrum is discussed: Notably, 1 contains an entire formula unit of the ionic liquid, (C4 C1 C1 Im)[BF4 ], while 4 contains neither the cation nor the anion of the ionic liquid-both observations being very uncommon for ionothermal syntheses. Three further compounds were obtained that exhibit strong disorder in the crystal structures (I-III), for which only their anionic substructures are discussed herein. Compound 4 and I-III are based on the same layered selenidostannate substructure, 2D-{[Sn3 Se7 ]2- }, yet with different inter-layer distances, which is clearly reflected in the crystal colors and quantitative optical absorption properties.

Perfluoroalkyltricyanoborate and Perfluoroalkylcyanofluoroborate Anions: Building Blocks for Low-Viscosity Ionic Liquids

The potassium perfluoroalkyltricyanoborates K[Cn F2 n+1 B(CN)3 ] [n=1 (1 d), 2 (2 d)] and the potassium mono(perfluoroalkyl)cyanofluoroborates K[Cn F2 n+1 BF(CN)2 ] [n=1 (1 c), 2 (2 c)] and [Cn F2 n+1 BF2 (CN)]- [n=1 (1 b), 2 (2 b), 3 (3 b), 4 (4 b)] are accessible with perfect selectivities on multi-gram scales starting from K[Cn F2 n+1 BF3 ] and Me3 SiCN. The K+ salts are starting materials for the preparation of salts with organic cations, for example, [EMIm]+ (EMIm=1-ethyl-3-methylimidazolium). These [EMIm]+ salts are hydrophobic room-temperature ionic liquids (RTILs) that are thermally, chemically and electrochemically very robust, offering electrochemical windows up to 5.8 V. The RTILs described herein, exhibit very low viscosities with a minimum of 14.0 mPa s at 20 °C for [EMIm]1 c, low melting points down to -57 °C for [EMIm]2 b and extraordinary high conductivities up to 17.6 mS cm-1 at 20 °C for [EMIm]1 c. The combination of these properties makes these ILs promising materials for electrochemical devices as exemplified by the application of selected RTILs as component of electrolytes in dye-sensitised solar cells (DSSCs, Grätzel cells). The efficiency of the DSSCs was found to increase with a decreasing viscosity of the neat ionic liquid. In addition to the spectroscopic characterisation, single crystals of the potassium salts of the anions 1 b-d, 2 d, 3 b and 4 c as well as of [nBu4 N]2 c have been studied by X-ray diffraction.

Formation of a Trifluorophosphane Platinum(II) Complex by P−F Bond Activation of Phosphorus Pentafluoride with a Pt0 Complex

The reaction of PF5 with [(Cy3 P)2 Pt] gave the PF3 complex trans-[(Cy3 P)2 PtF(PF3 )][PF6 ], which was characterized by single-crystal X-ray diffraction, multinuclear NMR spectroscopy, and elemental analysis. To the best of our knowledge, this reaction is the first example of the oxidative addition of a P-F bond to a transition metal and is a rare example of an activation of a main-group-element-fluorine bond by a metal. Relativistic DFT calculations showed that the formation of the Lewis pair [(Cy3 P)2 Pt→PF5 ], which was not observed even at low temperatures, represents the initial step of the reaction. From this key intermediate, the cation trans-[(Cy3 P)2 PtF(PF3 )]+ was furnished by a two-step mechanism involving, successively, a second and a third PF5 molecule.

Bis-Salicylatoborate as Versatile Sensitizer for Highly Luminescent Ln-oxoborates from UV to NIR with 4f- and 5d-Participation of the Lanthanides

Several lanthanide bis(salicylato)borates (BSB) have been synthesized from anhydrous Ln-chlorides together with pyridine. The manifold products start with the formation of small complexes, indicated by [ErCl2(py)4(BSB)], on to 1D-polymers 1∞[Ln(BSB)3(py)2], Ln = Y, La - Nd, Sm, with a reduced py content. Along the lanthanide contraction, the formation of 2D-networks of the constitution 2∞[Ln(BSB)3(py)], Ln = Sm, Eu, Tb, Dy, Er, is observed by further release of py. The coordination polymers exhibit intense photoluminescence from the UV to the NIR region by Ln-specific 4f-4f-emission for Nd3+, Sm3+, Eu3+, Tb3+, and Dy3+. Emission is remarkable for Dy3+ (yellow-white) and Nd3+ (NIR) based on sensitizer effects of the [BSB]- anion that shows energy transfer to most lanthanide ions. For Ce3+, participation of 5d-states is observed giving parity-allowed broad band 5d-4f-emission. The bis(salicylato)borate ligand shows fluorescence in the UV with a short lifetime of only 2 ns, which makes the energy transfer in the other Ln-compounds remarkable and marks it as versatile sensitizer for these metal ions.