Foundations oF Coordination Chemistry – alFred Werner CHIMIA 2014, 68, Nr. 5 321 doi:10.2533/chimia.2014.321 Chimia 68 (2014) 321–324 © Schweizerische Chemische Gesellschaft Analyzing Ammonia Bridges – and more about Bonding in Boron-rich Solids Barbara Albert* and Kathrin Hofmann Abstract: Three types of boron-rich compounds in unusual bonding situations are described: First, salts that contain closo-hydroborate anions and exhibit hydrogen and dihydrogen bonds and a strong ammonia network; second, boron-rich metal borides with an unexpected metal–metal bond stabilized by Peierls distortion; and third, nanoscale metal borides that bind selectively to certain heptapeptides identified by the phage display technique. Keywords: Ammoniates · Borides · Crystal structures · Hydroborates · Properties 1. Introduction investigated and understood. Here, we reactions), and Roald Hoffmann (Nobel will discuss ammonia molecules in clo- Prize 1981, bonding theory). Nowadays, In 1893 Alfred Werner published his so-hydroborates with anions like B H 2–. hydroborates are being discussed in the 6 6 article ‘Beitrag zur Konstitution anor- Incidentally, such compounds are perfectly context of energy-related materials being ganischer Verbindungen’ and suggested an suited to discuss both features that excited potentially useful for hydrogen storage.[5] octahedral coordination of cobalt, for ex- Alfred Werner and his contemporaries: oc- We have synthesized, crystallized, and ample in [Co(NH ) ]Cl .[1] At that time it tahedrally and tetrahedrally arranged am- structurally analyzed many closo-hydrob- 3 6 3 seemed to be a revolutionary idea to place monia ligands – and chains of ammonia orates because they are fascinating as mod- the ammonia molecules at six corners of a molecules! Boranes and hydroborates are el compounds for comparison of molecular metal-centered octahedron (Fig. 1). molecular entities containing boron and solids to solids with extended frameworks. Werner’s idea caused a paradigm shift hydrogen. They are extremely interesting [6,7] Boron-rich borides like BaB for ex- 6 in coordination and inorganic chemistry in many ways. A very special bonding situ- ample show structural (B octahedra) and 6 that can easily be seen as significant as ation and a unique interplay between sym- bonding (electron deficiency) features that Kekulé’s dream of aromatic rings. Earlier, metry, molecular structure, electronic de- resemble the corresponding molecular salt Christian Wilhelm Blomstrand had postu- ficiency and properties have brought such with a B H 2– anion (Fig. 2). In our first 6 6 lated chains of ammonia molecules[2] and boron compounds even to the attention of section, we will present findings of topo- his picture was well accepted by important several Nobel-Prize winners like William logical analyses of selected hydroborates. scientists of that time, for example Sophus N. Lipscomb (Nobel Prize 1976, borane The second section presents a selection Mads Jørgensen, who apparently fought structures), Herbert C. Brown (Nobel of boron-rich borides in unusual bonding with Werner for the truth.[3] Prize 1979, boron compounds in organic situations. Typically, boron-rich borides Ammoniates and the role of ammonia as ligand or bridge remain an interesting research topic until today. Korber et al. in- Fig. 1 Werner’s sug- gestion for the am- vestigated the role of ammonia molecules monia arrangement in a huge variety of complex inorganic in [Co(NH ) ]Cl com- 3 6 3 solids that were synthesized in liquid am- pared to Jørgensen’s monia, and they found several different model, according to roles of ammonia and types of linkages.[4] ref. [1] (slightly modi- However, compared to hydrogen bridges fied). in hydrated salts or biomolecules, the in- teraction of ammonia with cations, anions and other molecules in solids is much less Fig. 2. closo-Hy- droborate B H 2–(left) 6 6 compared to barium hexaboride, BaB 6 (right). *Correspondence: Prof. Dr. B. Albert Eduard-Zintl-Institute of Inorganic and Physical Chemistry Technische Universität Darmstadt Alarich-Weiss-Str. 12 D-64287 Darmstadt, Germany Tel.: + 49 6151 162392 E-mail: [email protected] 322 CHIMIA 2014, 68, Nr. 5 Foundations oF Coordination Chemistry – alFred Werner exhibit crystal structures that are charac- terized by the motif of three-dimensionally interconnected polyhedra.[8] The example described here, MnB , takes us from poly- 4 hedral arrangements like the aforemen- tioned octahedron of boron atoms to very unusual diamond-like boron atom frame- works. BB tetrahedra were found that 4 resemble CC tetrahedra in londsdaleite, a 4 hexagonal diamond variant. An unexpect- ed metal atom arrangement with a Mn–Mn Fig. 3. Crystal growth in silica gel and structure determination: Unit cell of bond was also observed. [(C H ) ] B H ∙CH COOH, crystals of [(C H ) ] B H ∙CH COOH and [PPh ] B H ∙C H OH, unit 3 7 4 2 10 10 3 3 7 4 2 10 10 3 4 2 12 12 3 7 Finally, a third and closing section will cell of [PPh ] B H ∙C H OH (from left to right).[11] present new findings on nanoscale boride 4 2 12 12 3 7 particles. Here, the bonding situation that is Fig. 4 Both Werner- unique is that between a heptapeptide and like tetrahedral co- nickel boride, Ni B. To our knowledge, this 3 ordination of lithium is the first description of selective binding ions and Jørgensen- between a boride and a peptide. like chain formation between ammonia ligands, the latter 2. Ammonia Molecules in clo- emphasized by an ex- so-Hydroborates – Multipole perimentally derived Refinements Based on Electron bond critical point with the (3,-1) signa- Density Distributions ture.[10] Two of our synthetic procedures are novel for hydroborates and gave access to new compounds and their crystal struc- tures: i) the reaction in liquid ammonia at very low temperatures,[5,9,10] and ii) the gel crystallization (Fig. 3).[11] Formation and crystallization of hydro- borates in liquid ammonia has led to un- Fig. 5. Dihydrogen expected compounds like Cs[Na(NH ) ] 3 6 bond between [9] [10] B H ∗NH or[Li(NH ) ] B H ∗2NH . B H 2– cage (red) with 10 10 3 3 4 2 6 6 3 6 6 These compounds exhibit strong ammonia Hδ− and a [Li(NH ) ]+ 3 4 networks that allow for an accurate local- tetrahedron with Hδ+, ization of the atoms – including hydro- visualized by a bond gen – via X-ray data collected up to high critical point with the angles. Conventional structure determina- (3,-1) signature.[10] tion does not lead to deeper insights into the bonding situation of solids. But the experimental determination of the charge density followed by multipole refinements and topological analyses according to the atoms-in-molecules (AIM)-method by Bader et al.[12] permits a classification of the characteristics of interatomic areas of the electron density by means of critical points. Establishing bond critical point (BCP), ring critical points and cage criti- cal points helps to better understand the bonding situation. of a dihydrogen bond using bond critical 3. Unexpected Mn–Mn Bond in a As can be seen in Fig. 4, lithium ions points derived from experimentally de- Refractory Material, MnB 4 in [Li(NH ) ] B H ∗2NH are tetrahedral- termined electron density maps is rather 3 4 2 6 6 3 ly coordinated by ammonia molecules – unique in literature. Boron-rich borides are hard and re- Werner-like. In addition to that a hydrogen We obtained another hydroborate, fractory compounds that are very versatile bond was observed between the ligands of N(C H ) B H , by a different synthetic pro- concerning their structural arrangements 4 9 4 6 7 neighboring complex cations, thus leading cedure, and again performed a topological and their properties.[8] Semiconducting, to a chain-like arrangement of ammonia analysis of charge densities.[13] The anion superconducting, ferromagnetic and ther- molecules that reminds us of Jørgensen’s can be seen as a product of a Brønsted acid- moelectric properties have been described theory. For [Li(NH ) ] B H ∗2NH , charge base reaction between a B H 2– anion and for metal borides with varying boron con- 3 4 2 6 6 3 6 6 density analysis further revealed multi- a H+ cation. The detailed electron density tent. We recently found boron-rich scan- center bonds in the boron atom cluster of analysis of this compound allowed us to dium and europium borides with excel- the anion, and a dihydrogen bond between identify a rhomboid BBBH ring and a new lent thermoelectric properties at very high Hδ− and Hδ+ (Fig. 5). Such a visualization type of a 4c−2e bond (Fig. 6). temperatures. As mentioned above, typical Foundations oF Coordination Chemistry – alFred Werner CHIMIA 2014, 68, Nr. 5 323 Mn–Mn distances. One of them is very rather unusual method of preparation is to short with 269.61(7) pm, which is aston- precipitate nanoscale boride particles from ishing compared to other Mn-containing solvents. The method has been shown to be solids like MnP (Mn–Mn distances successful for metal-rich borides of iron, 4 are >292.1 pm[15]) or Mn (CO) (292.3 cobalt, and nickel: M B, M B, MB (M = Fe, 2 10 3 2 pm[15]). In MnP , an oxidation state of Co, Ni). Here, Ni B nanoparticles were ob- 4 3 Mn(ii), non-metallic properties and dia- tained by a precipitation route using nickel magnetism was observed.[16] For MnB we salts and sodium tetrahydroborate, NaBH , 4 4 measured a very small band-gap (0.04 eV), in water.[18] Earlier, we applied a similar and paramagnetism. According to theoreti- preparation technique using non-aqueous cal calculations the oxidation state Mn(i) solvents to obtain nanoscale powders of is very probable and the Mn–Mn bond in ferromagnetic iron boride, FeB, which is MnB may be described as a double bond.