Michael Hailmann

Metal-organic framework luminescence in the yellow gap by codoping of the homoleptic imidazolate ∞(3)[Ba(Im)2] with divalent europium.

The rare case of a metal-triggered broad-band yellow emitter among inorganic-organic hybrid materials was achieved by in situ codoping of the novel imidazolate metal-organic framework ∞(3)[Ba(Im)2] with divalent europium. The emission maximum of this dense framework is in the center of the yellow gap of primary light-emitting diode phosphors. Up to 20% Eu2+ can be added to replace Ba2+ as connectivity centers without causing observable phase segregation. High-resolution energy-dispersive X-ray spectroscopy showed that incorporation of even 30% Eu2+ is possible on an atomic level, with 2-10% Eu2+ giving the peak quantum efficiency (QE = 0.32). The yellow emission can be triggered by two processes: direct excitation of Eu2+ and an antenna effect of the imidazolate linkers. The emission is fully europium-centered, involving 5d → 4f transitions, and depends on the imidazolate surroundings of the metal ions. The framework can be obtained by a solvent-free in situ approach starting from barium metal, europium metal, and a melt of imidazole in a redox reaction. Better homogeneity for the distribution of the luminescence centers was achieved by utilizing the hydrides BaH2 and EuH2 instead of the metals.

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.

Generation of Dicoordinate Boron(I) Units by Fragmentation of a Tetra‐Boron(I) Molecular Square

Reduction of carbene-borane adduct [(cAAC)BBr2 (CN)] (cAAC=1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene) cleanly yielded the tetra(cyanoborylene) species [(cAAC)B(CN)]4 presenting a 12-membered (BCN)4 ring. The analysis of the Kohn-Sham molecular orbitals showed significant borylene character of the BI atoms. [(cAAC)B(CN)]4 was found to reduce two equivalents of AgCN per boron center to yield [(cAAC)B(CN)3 ] and fragmented into two-coordinate boron(I) units upon reaction with IMeMe (1,3,4,5-tetramethylimidazol-2-ylidene) to yield the corresponding tricoordinate mixed cAAC-NHC cyanoborylene. The analogous cAAC-phosphine cyanoborylene was obtained by reduction of [(cAAC)BBr2 (CN)] in the presence of excess phosphine.

Unprecedented Efficient Structure Controlled Phosphorescence of Silver(I) Clusters Stabilized by Carba-closo-dodecaboranylethynyl Ligands.

{Ag2 (12-C≡C-closo-1-CB11 H11 )}n and selected pyridine ligands have been used for the synthesis of photostable Ag(I) clusters that, with one exception, exhibit for Ag(I) compounds unusual room-temperature phosphorescence. Extraordinarily intense phosphorescence was observed for a distorted pentagonal bipyramidal Ag(I) 7 cluster that shows an unprecedented quantum yield of Φ=0.76 for Ag(I) clusters. The luminescence properties correlate with the structures of the central Ag(I) n motifs as shown by comparison of the emission properties of the clusters with different numbers of Ag(I) ions, different charges, and electronically different pyridine ligands.

Difunctionalized {closo‐1‐CB11} Clusters: 1‐ and 2‐Amino‐12‐ethynylcarba‐closo‐dodecaborates

Carba-closo-dodecaborate anions with two functional groups have been synthesized via a simple two-step procedure starting from monoamino-functionalized {closo-1-CB11 } clusters. Iodination at the antipodal boron atom provided access to [1-H2 N-12-I-closo-1-CB11 H10 ](-) (1 a) and [2-H2 N-12-I-closo-1-CB11 H10 ](-) (2 a), which have been transformed into the anions [1-H2 N-12-RCC-closo-1-CB11 H10 ](-) (R=H (1 b), Ph (1 c), Et3 Si (1 d)) and [2-H2 N-12-RCC-closo-1-CB11 H10 ](-) (R=H (2 b), Ph (2 c), Et3 Si (2 d)) by microwave-assisted Kumada-type cross-coupling reactions. The syntheses of the inner salts 1-Me3 N-12-RCC-closo-1-CB11 H10 (R=H (1 e), Et3 Si (1 f)) and 2-Me3 N-12-RCC-closo-1-CB11 H10 (R=H (2 e), Et3 Si (2 f)) are the first examples for a further derivatization of the new anions. All {closo-1-CB11 } clusters have been characterized by multinuclear NMR and vibrational spectroscopy as well as by mass spectrometry. The crystal structures of Cs1 a, [Et4 N]2 a, K1 b, [Et4 N]1 c, [Et4 N]2 c, 1 e, and [Et4 N][1-H2 N-2-F-12-I-closo-1-CB11 H9 ]⋅0.5 H2 O ([Et4 N]4 a⋅0.5 H2 O) have been determined. Experimental spectroscopic data and especially spectroscopic data and bond properties derived from DFT calculations provide some information on the importance of inductive and resonance-type effects for the transfer of electronic effects through the {closo-1-CB11 } cage.

Carba-closo-dodecaborate Anions with Two Functional Groups: [1-R-12-HC≡C-closo-1-CB11H10]− (R = CN, NC, CO2H, C(O)NH2, NHC(O)H)

Disubstituted carba-closo-dodecaborate anions with one functional group bonded to the cluster carbon atom and one ethynyl group bonded to the antipodal boron atom were synthesized from easily accessible {closo-1-CB11} clusters. [Et4N][1-NC-12-HC≡C-closo-1-CB11H10] ([Et4N]4b) was prepared starting from Cs[12-Et3SiC≡C-closo-1-CB11H11] (Cs1c) via salts of the anions [1-HO(O)C-12-HC≡C-closo-1-CB11H10](-) (2b) and [1-H2N(O)C-12-HC≡C-closo-1-CB11H10](-) (3b). In a similar reaction sequence [Et4N][1-CN-12-HC≡C-closo-1-CB11H10] ([Et4N]7b) was obtained from Cs[1-H2N-12-HC≡C-closo-1-CB11H10] (Cs5b) by formamidation to yield [Et4N][1-H(O)CHN-12-HC≡C-closo-1-CB11H10] ([Et4N]6b) and successive dehydration. In addition, the synthesis of the isonitrile [Et4N][1-CN-closo-1-CB11H11] ([Et4N]7a) is presented. The {closo-1-CB11} derivatives were characterized by multinuclear NMR as well as vibrational spectroscopy, mass spectrometry, and elemental analysis. The crystal structures of [Et4N][1-HO(O)C-12-HC≡C-closo-1-CB11H10] ([Et4N]2b), [Et4N][1-H2N(O)C-12-HC≡C-closo-1-CB11H10] ([Et4N]3b), [Et4N][1-NC-12-HC≡C-closo-1-CB11H10] ([Et4N]4b), [Et4N][1-H(O)CHN-12-HC≡C-closo-1-CB11H10] ([Et4N]6b), [Et4N][1-CN-12-HC≡C-closo-1-CB11H10] ([Et4N]7b), and K[1-H(O)CHN-closo-1-CB11H11] ([Et4N]6a) were determined. The transmission of electronic effects through the carba-closo-dodecaboron cage was studied based on (13)C NMR spectroscopic data, by results derived from density functional theory calculations, and by a comparison to the data of related benzene and bicyclo[2.2.2]octane derivatives.

Silver(I) Clusters with Carba-closo-dodecaboranylethynyl Ligands: Synthesis, Structure, and Phosphorescence

Salts of anionic silver(I) clusters with the carba-closo-dodecaboranylethynyl ligand were obtained from {Ag2 (12-C≡C-closo-1-CB11 H11 )}n , selected pyridines, and [Et4 N]Cl or [Ph4 P]Br. Salts of octahedral silver(I) clusters [Et4 N]2 [Ag6 (12-C≡C-closo-1-CB11 H11 )4 (4-X-C5 H5 N)x ] were formed with pyridine (X=H, x=8), 4-methylpyridine (X=Me, x=8), and 4-cyanopyridine (X=CN, x=10). In contrast, 3,5-lutidine (3,5-Me2 Py) did not result in salts of dianionic clusters, even in the presence of excess of [Et4 N]Cl or [Ph4 P]Br; instead salts of monoanionic AgI7 clusters, [Et4 N][Ag7 (12-C≡C-closo-1-CB11 H11 )4 (3,5-Me2 Py)9 ] and [Ph4 P][Ag7 (12-C≡C-closo-1-CB11 H11 )4 (3,5-Me2 Py)13 ] were obtained. The AgI7 cluster is pentagonal bipyramidal in the former, but is an edge-capped octahedron in the latter. The 4-methylpyridine and 3,5-lutidine complexes show green phosphorescence at room temperature. Although argentophilic interactions give rise to sufficient spin-orbit coupling for intersystem crossing S1 →Tn and moderate-to-high radiative rate constants, time-resolved measurements indicate that the quantum yields are greatly influenced by the pyridine ligands, which mainly determine the non-radiative rate constants. In addition, the crystal structures of [Ag16 (12-C≡C-closo-1-CB11 H11 )8 (Py)9.25 (CH3 CN)2 (CH2 Cl2 )0.75 ]⋅CH2 Cl2 , [Ag8 (12-C≡C-closo-1-CB11 H11 )4 (Py)12 ], [Ag10 (12-C≡C-closo-1-CB11 H11 )4 (4-MePy)10 Br2 ], [Ag7 (12-C≡C-closo-1-CB11 H11 )3 (4-tBuPy)11 Cl]⋅(4-tBuPy), and [Ag9 (12-C≡C-closo-1-CB11 H11 )4 (3,5-Me2 Py)11 Cl] were elucidated.