Nickel(II) Cyanoborates and Cyanoborate-Ligated Nickel(II) Complexes.

Nickel(II) cyanoborates Ni[BH2(CN)2]2·H2O (1b·H2O), Ni[BH(CN)3]2·0.5H2O (1c·0.5H2O), and Ni[B(CN)4]2·0.5H2O (1d·0.5H2O) were synthesized, and their reactivity with respect to dppeO2 (=1,2-bis-(diphenylphosphinoethane dioxide)), pyNO (=pyridine-N-oxide), dppe (=1,2-bis-(diphenylphosphinoethane), and DMSO (=dimethyl sulfoxide) was examined. Using these ligands, either cyanoborate (CB) complex salts of [Ni(dppe)2]2+ (2b-d) and [Ni(pyNO)6]2+ (3c-d) were isolated or complexes [Ni(DMSO)4{NC-B(CN)3}2] (1dDMSO) and [Ni(dppeO2)2{NC-B(CN)3}2] (1ddppeO2) were formed. Salt metathesis of [Ni(dppe)Cl2] with alkali metal cyanoborates resulted in mono- and disubstituted coordination compounds [Ni(dppe){NC-BH(CN)2}Cl] (5c) and [Ni(dppe){NC-BH2CN)2}] (4b), which decomposed to salts 2b-d. The synthetical pathways explored offer convenient routes to nickel(II) cyanoborates, nickel(II) complexes ligated with cyanoborates, and nickel(II) complex salts of cyanoborates. Further, our studies demonstrate the diverse character of cyanoborates in coordination chemistry as noncoordinating counteranions and also as medium coordinating anions forming novel transition-metal complexes and salts.

Dismutation of Tricyanoboryllead Compounds: The Homoleptic Tetrakis(tricyanoboryl)plumbate Tetraanion.

A series of unprecedently air-stable (tricyanoboryl)plumbate anions was obtained by the reaction of the boron-centered nucleophile B(CN)3 2- with triorganyllead halides. Salts of the anions [R3 PbB(CN)3 ]- (R=Ph, Et) were isolated and found to be stable in air at room temperature. In the case of Me3 PbHal (Hal=Cl, Br), a mixture of the anions [Me4-n Pb{B(CN)3 }n ]n- (n=1, 2) was obtained. The [Et3 PbB(CN)3 ]- ion undergoes stepwise dismutation in aqueous solution to yield the plumbate anions [Et4-n Pb{B(CN)3 }n ]n- (n=1-4) and PbEt4 as by-product. The reaction rate increases with decreasing pH value of the aqueous solution or by bubbling O2 through the reaction mixture. Adjustment of the conditions allowed the selective formation and isolation of salts of all anions of the series [Et4-n Pb{B(CN)3 }n ]n- (n=2-4) including the homoleptic tetraanion [Pb{B(CN)3 }4 ]4- .

Stable and Storable N(CF3 )2 Transfer Reagents.

Fluorinated groups are essential for drug design, agrochemicals, and materials science. The bis(trifluoromethyl)amino group is an example of a stable group that has a high potential. While the number of molecules containing perfluoroalkyl, perfluoroalkoxy, and other fluorinated groups is steadily increasing, examples with the N(CF3 )2 group are rare. One reason is that transfer reagents are scarce and metal-based storable reagents are unknown. Herein, a set of CuI and AgI bis(trifluoromethyl)amido complexes stabilized by N- and P-donor ligands with unprecedented stability are presented. The complexes are stable solids that can even be manipulated in air for a short time. They are bis(trifluoromethyl)amination reagents as shown by nucleophilic substitution and Sandmeyer reactions. In addition to a series of benzylbis(trifluoromethyl)amines, 2-bis(trifluoromethyl)amino acetate was obtained, which, upon hydrolysis, gives the fluorinated amino acid N,N-bis(trifluoromethyl)glycine.

Hydroxytricyanoborate Anion: Synthetic Aspects and Structural, Chemical, and Spectroscopic Properties.

In recent years, salts of the hydridotricyanoborate anion [BH(CN)3]- (MHB) have become readily available. In spite of the unusually high stability of the MHB anion, it can be used as a valuable starting material for the preparation of selected tricyanoborates, for example, the boron-centered nucleophile B(CN)32-. A further unprecedented example is the hydroxytricyanoborate anion [B(OH)(CN)3]- that is accessible by oxidation of (H3O)MHB with elemental bromine in water. The Brønsted acid (H3O)[B(OH)(CN)3] was isolated as a crystalline solid. It serves as a versatile starting material for the synthesis of coordination compounds, metal salts, and ionic liquids. The [B(OH)(CN)3]- anion shows a rich coordination chemistry and a high tendency to form hydrogen-bonded motifs as demonstrated by a series of salts with different types of cations. Furthermore, the [B(OH)(CN)3]- anion itself serves as starting material for new tricyanoborates such as the unusual trianion [B{OB(CN)3}3]3- and the silylated anions [B(OSiR3)(CN)3]- (R = Me, Et, Ph). Some of these follow-up products have been characterized by single-crystal X-ray diffraction, e.g., [nBu4N]3[B{OB(CN)3}3] and [nBu4N][B(OSiPh3)(CN)3].

Cyanohydridoborate Anions: Synthesis, Salts, and Low-Viscosity Ionic Liquids.

High-yield syntheses up to molar scales for salts of [BH(CN)3 ]- (2) and [BH2 (CN)2 ]- (3) starting from commercially available Na[BH4 ] (Na5), Na[BH3 (CN)] (Na4), BCl3 , (CH3 )3 SiCN, and KCN were developed. Direct conversion of Na5 into K2 was accomplished with (CH3 )3 SiCN and (CH3 )3 SiCl as a catalyst in an autoclave. Alternatively, Na5 is converted into Na[BH{OC(O)R}3 ] (R=alkyl) that is more reactive towards (CH3 )3 SiCN and thus provides an easy access to salts of 2. Some reaction intermediates were identified, for example, Na[BH(CN){OC(O)Et}2 ] (Na7 b) and Na[BH(CN)2 {OC(O)Et}] (Na8 b). A third entry to 2 and 3 uses ether adducts of BHCl2 or BH2 Cl such as the commercial 1,4-dioxane adducts that react with KCN and (CH3 )3 SiCN. Alkali metal salts of 2 and 3 are convenient starting materials for organic salts, especially for low viscosity ionic liquids (ILs). [EMIm]3 has the lowest viscosity and highest conductivity with 10.2 mPa s and 32.6 mS cm-1 at 20 °C known for non-protic ILs. The ILs are thermally, chemically, and electrochemically robust. These properties are crucial for applications in electrochemical devices, for example, dye-sensitized solar cells (Grätzel cells).

Properties of perhalogenated {closo-B10} and {closo-B11} multiply charged anions and a critical comparison with {closo-B12} in the gas and the condensed phase.

closo-Borate anions [closo-BnXn]2- are part of the most famous textbook examples of polyhedral compounds. Substantial differences in their reactivity and interactions with other compounds depending on the substituent X and cluster size n have been recognized, which favor specific closo-borates for different applications in cancer treatment, chemical synthesis, and materials science. Surprisingly, a fundamental understanding of the molecular properties underlying these differences is lacking. Here, we report our study comparing the electronic structure and reactivity of closo-borate anions [closo-BnXn]2- (X = Cl, Br, I, n = 10, 11, 12 in all combinations) in the gas phase and in solution. We investigated the free dianions and the ion pairs [nBu4N]+[closo-BnXn]2- by gas phase anion photoelectron spectroscopy accompanied by theoretical investigations. Strong similarities in electronic structures for n = 10 and 11 were observed, while n = 12 clusters were different. A systematic picture of the development in electronic stability along the dimension X is derived. Collision induced dissociation shows that fragmentation of the free dianions is mainly dependent on the substituent X and gives access to a large variety of boron-rich molecular ions. Fragmentation of the ion pair depends strongly on n. The results reflect the high chemical stability of clusters with n = 10 and 12, while those with n = 11 are much more prone to dissociation. We bridge our study to the condensed phase by performing comparative electrochemistry and reactivity studies on closo-borates in solution. The trends found at the molecular level are also reflected in the condensed-phase properties. We discuss how the gas phase values allow evaluation of the influence of the condensed phase on the electronic stability of closo-borates. A synthetic method via an oxidation/chlorination reaction yielding [closo-B10Cl10]2- from highly chlorinated {closo-B11} clusters is introduced, which underlines the intrinsically high reactivity of the {closo-B11} cage.

Lanthanide Coordination Polymers and MOFs based on the Dicyanodihydridoborate Anion.

New lanthanide cyanoborates were synthesized from anhydrous lanthanide chlorides and the acid H[BH2 (CN)2 ] in either acetonitrile or pyridine. Reactions in acetonitrile lead to three-dimensional, anionic metal-organic frameworks (MOFs) 3 ∞ [Ln2 {BH2 (CN)2 }9 ]⋅[Ln(CH3 CN)9 ] (Ln=Ce, Eu, Tb) which incorporate complex cations [Ln(CH3 CN)9 ]3+ in the pores of the framework for charge compensation. In contrast, the reactions in pyridine result in the formation of one-dimensional coordination polymers 1 ∞ [H(py)2 ][LnCl2 {BH2 (CN)2 }2 (py)2 ]⋅0.5 py (Ln=Ce, Pr, py=pyridine) with [H(py)2 ]+ as counter ions for the anionic strand structure. The products show intense photoluminescence, for Ce3+ based on 5d-4f transitions in the blue spectral region, whereas the Eu3+ and Tb3+ compounds exhibit characteristic photoluminescence based on 4f-4f transitions of the respective lanthanide ions. The observed photoluminescence is mainly attributed to a direct excitation of the lanthanide ions and sensitization of the lanthanide ions by the [BH2 (CN)2 ]- anions. These results mark the utilized borate anions as versatile building block for new coordination compounds.

Stepwise Introduction of Cyano Groups into nido- and closo-Undecaborate Clusters.

Eleven-vertex closo and nido boron clusters with one or two exo-cyano groups were obtained by a series of consecutive cage-opening and cage-closure reactions starting from K2 [closo-B11 H11 ] (K2 1). In the first step, K2 1 reacts with KCN in water at elevated temperatures to yield [7-NC-nido-B11 H12 ]2- (5 a). Oxidation of 5 a with PbO2 gives [NC-closo-B11 H10 ]2- (2). In analogous subsequent reactions, dianion 2 was converted with KCN regioselectively to [7,9-(NC)2 -nido-B11 H11 ]2- (6 a), which was further oxidized to [(NC)2 -closo-B11 H9 ]2- (3). The {nido-B11 } dianions 5 a and 6 a were protonated to yield [7-NC-nido-B11 H13 ]- (5 b) and [7,9-(NC)2 -nido-B11 H12 ]- (6 b). All anions were studied by NMR spectroscopy and, except for 6 b, salts of all anions were characterized by IR and Raman spectroscopy and by elemental analysis. The position of the cyano group(s) in 5 a and 6 a were confirmed by NMR data and single-crystal X-ray diffraction studies on [Ph4 P]2 5 a⋅CH2 Cl2 and [Et4 N]2 6 a. The {closo-B11 } clusters are fluxional in solution. In the crystal of [EMIm]2 2 the BCN vertex is in the 2-position. Two isomers of dianion 3, [2,3-(NC)2 -closo-B11 H9 ]2- and [2,6-(NC)2 -closo-B11 H9 ]2- , were identified in the crystal of its [Et4 N]+ salt. The peak oxidation potentials Epa of anions 1-3, 5 a, 6 a, 5 b, and [nido-B11 H14 ]- (4 b), determined by cyclic voltammetry, increase with increasing number of cyano groups. Increasing Epa in the order 1<2<3 parallels the increasing reactivity toward cyanide anions.

Anhydrous, Homoleptic Lanthanide Frameworks with the Pentafluoroethyltricyanoborate Anion.

Pentafluoroethyltricyanoborate frameworks of rare-earth metal ions of the general formula [Ln{C2F5B(CN)3}3(OH2)n] (Ln = La, Eu, Ho; n = 0, 3; [Ln13(OH2)n]) were synthesized using the oxonium salt (H3O)[C2F5B(CN)3] ((H3O)1) and lanthanide chlorides LnCl3·nH2O as starting compounds. Single-crystals of ∞3[La{C2F5B(CN)3}3] (∞3[La13]) are obtained from the room temperature ionic liquid (RTIL) [EMIm]1 using either a ionothermal approach or by recrystallization of anhydrous microcrystalline [La13] that is obtained from reactions in aqueous media after drying in a vacuum. Removal of water from [Ln13(OH2)3] (Ln = Eu, Ho) to give microcrystalline ∞3[Ln13] is achieved in a vacuum at elevated temperatures. All compounds were characterized by vibrational and NMR spectroscopy, thermogravimetry, and elemental analysis. The structures of the three-dimensional coordination polymers ∞3[Eu13(OH2)3] and ∞3[La13] were elucidated by single-crystal X-ray-diffraction. According to powder diffraction studies on anhydrous ∞3[Ln13] (Ln = La, Eu, Ho), the three compounds are isotypic. A study of the photoluminescence properties reveals that both Eu3+ compounds, [Eu13] and [Eu13(OH2)3], are strongly luminescent, the emission of the anhydrous framework being significantly more intense than the one of the hydrate. The Eu-compounds benefit from a sensitizer effect of the anion. In contrast, the Ho-containing framework ∞3[Ho13] exhibits separate chromophores and a strong reabsorption of the fluorescence by the Ho3+ ions.

Protonation versus Oxonium Salt Formation: Basicity and Stability Tuning of Cyanoborate Anions.

Anhydrous H[BH2 (CN)2 ] crystallizes from acidic aqueous solutions of the dicyanodihydridoborate anion. The formation of H[BH2 (CN)2 ] is surprising as the protonation of nitriles requires strongly acidic and anhydrous conditions but it can be rationalized based on theoretical data. In contrast, [BX(CN)3 ]- (X=H, F) gives the expected oxonium salts (H3 O)[BX(CN)3 ] while (H3 O)[BF2 (CN)2 ]/H[BF2 (CN)2 ] is unstable. H[BH2 (CN)2 ] forms chains via N-H⋅⋅⋅N bonds in the solid state and melts at 54 °C. Solutions of H[BH2 (CN)2 ] in the room-temperature ionic liquid [EMIm][BH2 (CN)2 ] contain the [(NC)H2 BCN-H⋅⋅⋅NCBH2 (CN)]- anion and are unusually stable, which enabled the study of selected spectroscopic and physical properties. [(NC)H2 BCN-H⋅⋅⋅NCBH2 (CN)]- slowly gives H2 and [(NC)H2 BCN-BH(CN)2 ]- . The latter compound is a source of the free Lewis acid BH(CN)2 , as shown by the generation of [BHF(CN)2 ]- and BH(CN)2 ⋅py.