Bis(4-benzhydryl-benzoxazol-2-yl)methane - from a Bulky NacNac Alternative to a Trianion in Alkali Metal Complexes.

A novel sterically demanding bis(4-benzhydryl-benzoxazol-2-yl)methane ligand 6 (4-BzhH2 BoxCH2 ) was gained in a straightforward six-step synthesis. Starting from this ligand monomeric [M(4-BzhH2 BoxCH)] (M=Na (7), K (81 )) and dimeric [{M(4-BzhH2 BoxCH)}2 ] (M=K (82 ), Rb (9), Cs (10)) alkali metal complexes were synthesised by deprotonation. Abstraction of the potassium ion of 8 by reaction with 18-crown-6 resulted in the solvent separated ion pair [{(THF)2 K@(18-crown-6)}{bis(4-benzhydryl-benzoxazol-2-yl)methanide}] (11), including the energetically favoured monoanionic (E,E)-(4-BzhH2 BoxCH) ligand. Further reaction of 4-BzhH2 BoxCH2 with three equivalents KH and two equivalents 18-crown-6 yielded polymeric [{(THF)2 K@(18-crown-6)}{K@(18-crown-6)K(4-Bzh BoxCH)}]n (n→∞) (12) containing a trianionic ligand. The neutral ligand and herein reported alkali complexes were characterised by single X-ray analyses identifying the latter as a promising precursor for low-valent main group complexes.

Group 13 Heavier Carbene Analogues Stabilized by the Bulky Bis(4-benzhydryl-benzoxazol-2-yl)methanide Ligand.

On the basis of the bulky bis(4-benzhydryl-benzoxazyl-2-yl)methane ligand (4-BzhH2Box2CH2), neutral monovalent group 13 complexes [M13(4-BzhH2Box2CH)] [M13 = Tl (1), In (2), or Ga (3)] have been synthesized by salt metathesis reaction of the corresponding potassium or sodium precursor and TlOTf, InOTf, or "GaI". The diiodido gallium species [GaI2(4-BzhH2Box2CH)] (3a) was realized as a byproduct once the synthesis of 3 was carried out at higher temperatures. The synthesis of [AlI2(4-BzhH2Box2CH)] (6) as a potential precursor for an aluminum(I) congener was accomplished by two alternative synthetic routes. During one of those procedures, [AlMe2(4-BzhH2Box2CH)] (4) was synthesized in good yields by deprotonation with an AlMe3 solution (method A). Subsequently, 4 was converted to the monoiodinated species [AlMeI(4-BzhH2Box2CH2)] (5) using 1 equiv of I2 or to 6 by iodination with 2 equiv of I2 at 70 °C for 4 days. As an alternative, complex 6 could be prepared by iodination of 1 equiv of I2 and [AlH2(4-BzhH2Box2CH)] (7), which was previously obtained by facile reaction of 4-BzhH2Box2CH2 and AlH3NMe2Et. All main products 1-7 were completely characterized by nuclear magnetic resonance spectroscopy, mass spectrometry, elemental analysis, and single-crystal X-ray structure determination. Alane 7 was additionally analyzed by solid-state fluorescence spectroscopy. Density functional theory calculations on [M13(4-BzhH2Box2CH)] [M13 = Tl (1), In (2), Ga (3), or Al] revealed that the complexes consist of monovalent group 13 cations coordinated by an anionic (4-BzhH2Box2CH) ligand similar to metallacycles incorporating a NacNac ligand.

Analysis of Solid-State Luminescence Emission Amplification at Substituted Anthracenes by Host-Guest Complex Formation.

Invited for the cover of this issue are Dietmar Stalke and coworkers from the Georg-August Universität at Göttingen. The image symbolizes the organization of small arenes in the pocket of thiophosphoranyl substituted anthracene like the construction of a stable wall from a loose pile of bricks, shifting the emission to the green and amplifying it by a factor of five. Read the full text of the article at 10.1002/chem.202003017.

Mixed Low-Valent Alanes from the Bis(4-methyl-benzoxazol-2-yl)methanide Ligand.

Within this work, an aluminum dihydride complex ([(4-MeBox2CH)AlH2]) (1) based on the bis(4-methyl-benzoxazol-2-yl)methanide ligand was synthesized and characterized by spectroscopic methods (NMR, ATR-IR, and fluorescence), DSC (differential scanning calorimetry), mass spectrometry (LIFDI), and single crystal X-ray diffraction. The reactivity of alane 1 was investigated toward the reducing agents [DippNacNacAlI] and [(MesNacNacMgI)2], which gave the dialane compounds [(4-MeBox2CH)HAlII-AlIIH(DippNacNac)] (2) and [{(4-MeBox2CH)AlIIH}2] (4a), respectively. Furthermore, dialuminoxanes [{(4-MeBox2CH)AlH}2(µ-O)] (4b) and [({(MesNacNac)Mg}2(µ-H)){H3AlII-AlIIH(4-MeBox2CH)}] (4c) were isolated as byproducts, with 4b co-crystallizing with 4a. The hydricity of both hydrides in the mixed-ligated dialane 2 were examined by a reaction with 1 equiv of trityl borate ([Ph3C][B(C6F5)4]), which resulted in [(4-MeBox2CH)HAlII-AlII(DippNacNac)][B(C6F5)4] (3). Due to the formation of 4b, complex 1 was reacted with 0.5 equiv of water, which causes the likely synthesis of insoluble oligomeric alumoxanes. To prevent this reaction and support the formation of well-defined dialumoxanes, 1 was initially converted to [(4-MeBox2CH)(DippO)AlH] (5) by the deprotonation of 2,6-diisopropylphenol (propofol). This sterically encumbered compound 5 was subsequently reacted with 0.5 equiv of water, which resulted in defined molecules of [{(4-MeBox2CH)(DippO)Al}2(µ-O)] (6). All these compounds exemplify the versatility of the 4-MeBox2CH ligand in low-valent aluminum chemistry.

Analysis of Solid-State Luminescence Emission Amplification at Substituted Anthracenes by Host-Guest Complex Formation.

Small robust organic molecules showing solid-state luminescence are promising candidates for optoelectronic materials. Herein, we investigate a series of diphenylphosphanyl anthracenes [9-PPh2 -10-R-(C14 H8 )] and their sulfur oxidised analogues. The oxidation causes drastic changes in the molecular structure as the new orientation of the bulky (S)PPh2 substituent induces a strong butterfly bent structure of the anthracene core, which triggers a strong bathochromic shift resulting in a green solid-state fluorescence. As the emission properties change only slightly upon aggregation the origin of the emission is attributed to a typical monomer fluorescence. The host-guest complexes of [9-(S)PPh2 -10-Ethyl-(C14 H8 )] with four basic arenes reveal an emission enhancement up to five-times higher quantum yields compared to the pure host. Less interchromophoric interactions and a restriction of intramolecular motion within the host molecules due to fixation by weak C-H⋅⋅⋅π interactions with the co-crystallised arene are responsible for that emission enhancement.

Three colour solid-state luminescence from positional isomers of facilely modified thiophosphoranyl anthracenes.

Three positional isomers of thiophosphoranyl anthracene were synthesized and their photophysical properties were investigated. By varying the position of the substituents, blue, green and yellow solid-state fluorescence with differences in the emission wavelength of over 100 nm, assigned to the intra- and intermolecular effects, could be established.