Boron bis-(4-methylbenzoxazol-2-yl)-methanide complexes.

Boron compounds have attracted the attention of chemists because of their unique catalytic properties and potential wider material applications. Although group 13 metal compounds, which are based on the bis-(benzoxazol-2-yl)-methane system (Box, ({NCOC6H4}2CH2)), have been reported in the last several years, boron containing Box compounds were still missing. Now we report their successful syntheses and spectroscopic characterisation in this work. The borane compound [({NCOC6H3}2CH)BH2] (1) and haloboranes [({NCOC6H3}2CH)BF2] (2), [({NCOC6H3}2CH)BCl2] (3) and [({NCOC6H3}2CH)BBr2] (4) were characterised in the solid state by single crystal X-ray diffraction and in solution by NMR techniques. In addition, the fluorescence properties of compounds 1-4 are communicated.

Structure Elucidation of Lithium Compounds Using 7 Li Residual Quadrupolar Couplings.

A new NMR method for the structure elucidation of lithium compounds under solution-like conditions is presented. It is based on the measurement of 7 Li residual quadrupolar couplings (RQCs) in a stretched polystyrene (PS) gel, and comparison to RQCs predicted from crystal or DFT-derived model structures in combination with alignment tensors derived from one-bond 1 H,13 C residual dipolar couplings (RDCs). The method was applied to five lithium model complexes containing monoanionic, bidentate bis(benzoxazole-2-yl)methanide, bis(benzothiazole-2-yl)methanide and bis(pyridyl)methanide ligands, of which two are first introduced in this work. In agreement with the crystalline state, four complexes are monomeric with Li coordinated fourfold by two additional THF molecules, whereas in one complex bulky tBu groups only provide space for one additional THF molecule.

A Carbene-Stabilized Boryl-Phosphinidene.

Herein, the synthesis and characterization of the carbene-stabilized boryl phosphinidenes 1-3 are reported. Compounds 1-3 are obtained by reacting Me-cAAC=PK (Me2 -cAAC=dimethyl cyclic(alkyl)(amino)carbene) and dihaloaryl borane in toluene. All three compounds were characterized by X-ray crystallography. Quantum mechanical studies indicated that these compounds have two lone pairs on the P center viz., an σ-type lone pair and a "hidden" π-type lone pair. Hence, these compounds can act as double Lewis bases, and the basicity of the π-type lone pair is higher than the σ-type lone pair.

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.

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.

Cyclic (Alkyl)(Amino)Carbene-Stabilized Aluminum and Gallium Radicals Based on Amidinate Scaffolds.

Neutral, mononuclear aluminum and gallium radicals, stabilized by cyclic (alkyl)(amino)carbene (cAAC), were synthesized. LMCl2 upon reduction with KC8 in the presence of cAAC afforded the radicals LMCl(cAAC), where L = PhC(NtBu)2 and M = Al (1), Ga (2). The radicals were characterized by X-ray crystallography, electron paramagnetic resonance (EPR) spectroscopy, and mass spectrometry. EPR, SQUID measurement, and computational calculations confirmed paramagnetism of the radicals with unpaired spin mainly on cAAC.

Isolation of base stabilized fluoroborylene and its radical cation.

Herein, we report the synthesis and characterization of the metal free low valent fluoroborylene [(Me-cAAC)2BF] (1) stabilized by cyclic (alkyl)(amino) carbene (cAAC). The fluoroborylene 1 is obtained by the reductive defluorination of Me-cAAC:BF3 with 2.0 equivalents of KC8 in the presence of 1.0 equivalent of Me-cAAC. Due to its highly electron rich nature, 1 underwent one-electron oxidation with 1.0 equivalent of lithium tetrakis(pentafluorophenyl)borate [LiB(C6F5)4] to form the radical cation [(Me-cAAC)2BF]˙+[B(C6F5)4]- (2). DFT studies suggested that the lone pair of electrons is localized on the boron atom in 1, which explains its unprecedented reactivity. Both compounds 1 and 2 were characterized by X-ray crystallography. The radical cation 2 was studied by EPR spectroscopy.

Alkali metal complexes based on bisheterocyclomethanide ligands.

A series of new alkali metal complexes containing deprotonated bisheterocyclomethanide were synthesised and structurally as well as spectroscopically characterised. Starting from the neutral bis-(benzoxazol-2-yl)methane ({NCOC6H4}2CH2), the complexes were obtained by facile deprotonation of the methylene bridge with nbutyllithium (nBuLi) or potassium hydride. Each of the resulting complexes [({NCOC6H4}2CH)Li(OEt2)2] (1), [({NCOC6H4}2CH)K(thf)]n (2) and [({NCOC6H4}2CH)K(18-crown-6)]n (3) (n → ∞) was analysed in the solid state by single crystal X-ray diffraction and in solution by various NMR techniques. All the compounds show a nearly planar ligand with the alkali metal ion chelated by the two ring nitrogen atoms of the conjugated C3N2 moiety. Furthermore, water NMR titration experiments revealed that both compounds 2 and 3 are examples of water-stable reactive organo potassium compounds. Due to their hydrolysis stability, these complexes are under further investigations in deep eutectic solvents.

An Electrophilic Carbene-Anchored Silylene-Phosphinidene.

The cyclic alkyl(amino) carbene-anchored silylene-phosphinidene was isolated as L-Si-P(:cAAC-Me) (L=benzamidinate) at room temperature, synthesized from the reduction of L-Si(Cl2 )-P(:cAAC-Me) (1) using two equivalents of KC8 . Compound 1 was prepared by the oxidative addition of a chlorophosphinidene to the benzamidinate substituted silylene center. This is the first molecular example of a silylene-phosphinidene characterized by single-crystal X-ray structural analysis. Moreover, 1 H, 31 P, and also 29 Si NMR spectroscopic data supported the formulation of the products. The theoretical calculations of compound 2 are in good agreement with the experimental results.

An unprecedented 1,4-diphospha-2,3-disila butadiene (-P[double bond, length as m-dash]Si-Si[double bond, length as m-dash]P-) derivative and a 1,3-diphospha-2-silaallyl anion, each stabilized by the amidinate ligand.

The first acyclic 4π-electron -P[double bond, length as m-dash]Si-Si[double bond, length as m-dash]P- motif with two four coordinate silicon substituents supported by the amidinate ligand and two coordinate phosphorus has been synthesized from the reaction of heteroleptic chlorosilylene LSiCl (1), TripPCl2 (Trip = 2,4,6-iPr3C6H2) and KC8 in a 1 : 1 : 3 ratio. The same reaction in a 1 : 2 : 6 ratio in the presence of one equivalent of 18-crown-6 ether affords the 1,3-diphospha-2-silaallyl anion.