Group 13 trihalide complexes of 9-fluorenone: a comparison of methods for assigning relative Lewis acidity

The Field of Main Group Lewis Acids and Lewis Superacids: Important Basics and Recent Developments

New developments in the field of Lewis acidity are highlighted, with the focus of novel Lewis acids and Lewis superacids of group 2, 13, 14, and 15 elements. Several important basics, illustrated by modern examples (classification of Donor-Acceptor (DA) complexes, amphoteric nature of any compound in terms of DA interactions, reorganization energies of main group Lewis acids and the role of the energies of frontier orbitals) are presented and discussed. It is emphasized that the Lewis acidity phenomena are general and play vital role in different areas of chemistry: from weak "atomophilic" interactions to the complexes of Lewis superacids.

Evaluation of the Lewis acidity of metal complexes using ESI mass spectrometry

Metal complexes have extensive applications in catalysis, however, the efficient evaluation of Lewis acidity of metal complexes is still a challenge. Herein, we report a method by using electrospray ionization mass spectrometry (ESI-MS) to evaluate the Lewis acidity of metal complexes in the presence of a reference Lewis base, in which the value of the Lewis acidity can be quantized by the bond dissociation energy (BDE) of the resultant Lewis acid-base pairs. Using this method, the Lewis acidity of tetradentate Schiff-base metal complexes (designated as salenMX), a class of common metal complexes in the homogeneous catalysis, was studied in detail. For the salenM(III)X complexes (M = Al, Cr, Fe, Co), the Lewis acidity tendency is Al > Cr > Fe > Co due to a strong affinity between the Al complex and the reference Lewis base while a weak affinity concerning on the Co complex. Additionally, the effect of ligand steric and electronic nature on the Lewis acidity was studied by using Co complex. Furthermore, density functional theory (DFT) was employed to calculate the BDE, which consists with the results obtained from ESI-MS. The ESI-MS method provides a convenient and efficient method for evaluating the Lewis acidity of metal complexes.

Separation of GaCl3 from AlCl3 by Solid-Liquid Extraction and Stripping Using Anhydrous n-Dodecane and NaCl

Separation of GaCl3 from other associating chloride compounds (e.g. AlCl3, SbCl3, and InCl3) is generally achieved by hydrometallurgical processes. In this study, we explore the separation of GaCl3 from these compounds on the basis of the exceptionally high solubility of GaCl3 in hydrocarbon solvents. We found that GaCl3 can be efficiently extracted by anhydrous n-dodecane from a solid mixture of GaCl3 and AlCl3; on the contrary, SbCl3 and InCl3 significantly reduce the extraction of GaCl3. On the basis of Lewis acidity theory and study of the Raman spectra, it is shown that formation of the ionic compound [SbCl2][GaCl4] is responsible for the reduced GaCl3 extraction. Formation of [InCl2][GaCl4] is also likely, but further study is needed to support the existence of this compound. Further making use of the strong Lewis acidity of GaCl3, GaCl3 can be efficiently stripped from the loaded n-dodecane phase by solid NaCl through formation of NaGaCl4. The extraction of GaCl3 by n-dodecane, in combination with its stripping by NaCl, is a solvometallurgical process that is essentially different from the hydrometallurgical processes for the separation of GaCl3 and AlCl3.

Square-Planar Pt(II) and Ir(I) Complexes as the Lewis Bases: Donor-Acceptor Adducts with Group 13 Trihalides and Trihydrides

The stability and properties of donor-acceptor adducts of square-planar Pt(II) and Ir(I) complexes (designated as PtX, IrX, or generally MX complexes) with trihydrides and trihalides of group 13 elements of general formula YZ₃ (Y = B, Al, Ga; Z = H, F, Cl, Br) were studied theoretically using DFT methodology in the gas phase. MX complexes were represented by wide range of the ligand environment which included model complexes [Ir(NH₃)₃X]⁰ and cis-[Pt(NH₃)₂X₂]⁰ (X = H, CH₃, F, Cl, Br) and isoelectronic complexes [Ir(NNN)(CH₃)]⁰ and [Pt(NCN)(CH₃)]⁰ with tridentate NNN and NCN pincer ligands. MX complexes acted as the Lewis bases donating electron density from the doubly occupied 5d _z² atomic orbital of the metal M atom to the empty valence p _z orbital of Y whose evidence was clearly provided by the natural atomic orbital (NAO) analysis. This charge transfer led to the formation of pentacoordinated square pyramidal MX·(YZ₃) adducts with M·Y dative bond. Binding energies were -44.7 and -75.2 kcal/mol for interaction of GaF₃ as the strongest acid with PtNCN and IrNNN pincer ligands complexes. Only M·B bonds had covalent character although MX·BZ₃ adducts were the least stable due to large values of Pauli repulsion and deformation energies. The highest degree of covalent character was found for adducts of BH₃ in all series of structures studied. Al and Ga adducts showed remarkably similar behavior with respect to geometry and binding energies.

An Investigation of the Symmetric and Asymmetric Cleavage Products in the System Aluminum Trihalide/1-Butylimidazole

Mixtures of AlX₃ (X=Cl, Br) with 1-butylimidazole (BuIm) in various ratios were investigated. The mixtures were characterized by multinuclear (¹ H, ²⁷ Al, ¹³ C, and ¹⁵ N) NMR, IR, and Raman spectroscopy and in part by single-crystal X-ray diffraction. Depending on the molar fraction x(AlBr₃ ) of the AlBr₃ -based mixtures, the cationic aluminum complexes [Al(BuIm)₆ ]³⁺ and [AlBr₂ (BuIm)₄ ]⁺ , the neutral adduct [AlBr₃ (BuIm)], as well as the anions Br^- , [AlBr₄ ]^- , and [Al₂ Br₇ ]^- could be identified as the products of the symmetric and asymmetric cleavage of dimeric Al₂ Br₆ . Furthermore, there are hints at the formation of [AlBr₂ (BuIm)₂ ]⁺ or related cations. Comparison of the AlBr₃ /BuIm system with AlCl₃ -based mixtures revealed the influence of the halide: In contrast to AlBr₃ , the trication [Al(BuIm)₆ ]³⁺ could not be detected as main product in a 1:6 mixture of AlCl₃ and BuIm. Additionally, [AlCl₃ (BuIm)] crystallizes from a mixture with x(AlCl₃ )=0.60 at room temperature, whereas the corresponding AlBr₃ -based mixture remains liquid even at +6 °C. Three AlBr₃ -based mixtures are liquid at room temperature, whereas all other mixtures are solids with melting points between 46 and 184 °C. The three liquid mixtures exhibit medium to high viscosities (117 to >1440 mPa s), low conductivities (0.03-0.20 mS cm^-1 ), but high densities (1.80-2.21 g cm^-3 ). Aluminum could be successfully deposited from one of the neat Lewis acidic mixtures of the AlBr₃ -based system.

Tetrabutylammonium Salts of Aluminum(III) and Gallium(III) Phthalocyanine Radical Anions Bonded with Fluoren-9-olato- Anions and Indium(III) Phthalocyanine Bromide Radical Anions

Reduction of aluminum(III), gallium(III), and indium(III) phthalocyanine chlorides by sodium fluorenone ketyl in the presence of tetrabutylammonium cations yielded crystalline salts of the type (Bu₄ N⁺ )₂ [M^III (HFl-O^- )(Pc^.3- )]^.- (Br^- )⋅1.5 C₆ H₄ Cl₂ [M=Al (1), Ga (2); HFl-O^- =fluoren-9-olato^- anion; Pc=phthalocyanine] and (Bu₄ N⁺ ) [In^III Br(Pc^.3- )]^.- ⋅0.875 C₆ H₄ Cl₂ ⋅0.125 C₆ H₁₄ (3). The salts were found to contain Pc^.3- radical anions with negatively charged phthalocyanine macrocycles, as evidenced by the presence of intense bands of Pc^.3- in the near-IR region and a noticeable blueshift in both the Q and Soret bands of phthalocyanine. The metal(III) atoms coordinate HFl-O^- anions in 1 and 2 with short Al-O and Ga-O bond lengths of 1.749(2) and 1.836(6) Å, respectively. The C-O bonds [1.402(3) and 1.391(11) Å in 1 and 2, respectively] in the HFl-O^- anions are longer than the same bond in the fluorenone ketyl (1.27-1.31 Å). Salts 1-3 show effective magnetic moments of 1.72, 1.66, and 1.79 μ_B at 300 K, respectively, owing to the presence of unpaired S=1/2 spins on Pc^.3- . These spins are coupled antiferromagnetically with Weiss temperatures of -22, -14, and -30 K for 1-3, respectively. Coupling can occur in the corrugated two-dimensional phthalocyanine layers of 1 and 2 with an exchange interaction of J/k_B =-0.9 and -1.1 K, respectively, and in the π-stacking {[In^III Br(Pc^.3- )]^.- }₂ dimers of 3 with an exchange interaction of J/k_B =-10.8 K. The salts show intense electron paramagnetic resonance (EPR) signals attributed to Pc^.3- . It was found that increasing the size of the central metal atom strongly broadened these EPR signals.

A zwitterionic hydrocarbon-soluble borenium ion based on a β-diketiminate backbone

A hydrocarbon-soluble borenium/borate zwitterion has been developed which is more strongly Lewis acidic than B(C₆F₅)₃, despite featuring a pendant (non-fluorinated) aryl group and two flanking N-donors.

Metallo-Wittig chemistry of an alkylidene to form a terminal titanium oxo complex

The synthesis and characterization of a terminal titanium oxo complex generated by alkylidene benzophenone cross-metathesis is reported.

Masking of Lewis acidity trends in the solid-state structures of trichlorido- and tribromido(2,2':6',2''-terpyridine-κ(3)N,N',N'')gallium(III)

The molecular structures of trichlorido(2,2':6',2''-terpyridine-κ(3)N,N',N'')gallium(III), [GaCl3(C15H11N3)], and tribromido(2,2':6',2''-terpyridine-κ(3)N,N',N'')gallium(III), [GaBr3(C15H11N3)], are isostructural, with the Ga(III) atom displaying an octahedral geometry. It is shown that the Ga-N distances in the two complexes are the same within experimental error, in contrast to expected bond lengthening in the bromide complex due to the lower Lewis acidity of GaBr3. Thus, masking of the Lewis acidity trends in the solid state is observed not only for complexes of group 13 metal halides with monodentate ligands but for complexes with the polydentate 2,2':6',2''-terpyridine donor as well.

Tuning Lewis acidity using the reactivity of “frustrated Lewis pairs”: facile formation of phosphine-boranes and cationic phosphonium-boranes

The concept of "frustrated Lewis pairs" involves donor and acceptor sites in which steric congestion precludes Lewis acid-base adduct formation. In the case of sterically demanding phosphines and boranes, this lack of self-quenching prompts nucleophilic attack at a carbon para to B followed by fluoride transfer affording zwitterionic phosphonium borates [R(3)P(C(6)F(4))BF(C(6)F(5))(2)] and [R(2)PH(C(6)F(4))BF(C(6)F(5))(2)]. These can be easily transformed into the cationic phosphonium-boranes [R(3)P(C(6)F(4))B(C(6)F(5))(2)](+) and [R(2)PH(C(6)F(4))B(C(6)F(5))(2)](+) or into the neutral phosphino-boranes R(2)P(C(6)F(4))B(C(6)F(5))(2). This new reactivity provides a modular route to a family of boranes in which the steric features about the Lewis acidic center remains constant and yet the variation in substitution provides a facile avenue for the tuning of the Lewis acidity. Employing the Gutmann-Beckett and Childs methods for determining Lewis acid strength, it is demonstrated that the cationic boranes are much more Lewis acidic than B(C(6)F(5))(3), while the acidity of the phosphine-boranes is diminished.

B(C6F5)3- vs Al(C6F5)3-Derived Metallocenium Ion Pairs. Structural, Thermochemical, and Structural Dynamic Divergences

The thermodynamic and structural characteristics of Al(C6F(5)3-derived vs B(C6F5)3-derived group 4 metallocenium ion pairs are quantified. Reaction of 1.0 equiv of B(C6F5)3 or 1.0 or 2.0 equiv of Al(C6F5)3 with rac-C2H4(eta5-Ind)2Zr(CH3)2 (rac-(EBI)Zr(CH3)2) yields rac-(EBI)Zr(CH3)(+)H3CB(C6)F5)(3)(-) (1a), rac-(EBI)Zr(CH3)+H3CAl(C6F5)(3)(-) (1b), and rac-(EBI)Zr2+[H3CAl(C6F5)3](-)(2) (1c), respectively. X-ray crystallographic analysis of 1b indicates the H3CAl(C6F5)(3)(-) anion coordinates to the metal center via a bridging methyl in a manner similar to B(C6F5)3-derived metallocenium ion pairs. However, the Zr-(CH3)(bridging) and Al-(CH3)(bridging) bond lengths of 1b (2.505(4) A and 2.026(4) A, respectively) indicate the methyl group is less completely abstracted in 1b than in typical B(C6F5)3-derived ion pairs. Ion pair formation enthalpies (DeltaH(ipf)) determined by isoperibol solution calorimetry in toluene from the neutral precursors are -21.9(6) kcal mol(-1) (1a), -14.0(15) kcal mol(-1) (1b), and -2.1(1) kcal mol(-1) (1b-->1c), indicating Al(C6F5)3 to have significantly less methide affinity than B(C6F5)3. Analogous experiments with Me2Si(eta5-Me4C5)(t-BuN)Ti(CH3)2 indicate a similar trend. Furthermore, kinetic parameters for ion pair epimerization by cocatalyst exchange (ce) and anion exchange (ae), determined by line-broadening in VT NMR spectra over the range 25-75 degrees C, are DeltaH++(ce) = 22(1) kcal mol(-1), DeltaS++(ce) = 8.2(4) eu, DeltaH++(ae) = 14(2) kcal mol(-1), and DeltaS++(ae) = -15(2) eu for 1a. Line broadening for 1b is not detectable until just below the temperature where decomposition becomes significant ( approximately 75-80 degrees C), but estimation of the activation parameters at 72 degrees C gives DeltaH++(ce) approximately 22 kcal mol(-1)and DeltaH++(ae) approximately 16 kcal mol(-1), consistent with the bridging methide being more strongly bound to the zirconocenium center than in 1a.

An X-ray Absorption Study of Two VOCl3-Modified Silicas: Evidence for Chloride−Silica Interactions

The structures of the sites formed in the gas-solid reactions of VOCl3 with the surfaces of a fumed silica (Aerosil) and a silica gel (Sylopol) were investigated by using X-ray absorption spectroscopy. XANES and EXAFS analysis at the vanadium K-edge reveal that the sites have a uniform first coordination sphere regardless of the origin or the extent of hydroxylation of the silica support (controlled by thermal treatment in vacuo at 100 and 500 degrees C). Analysis of the second coordination sphere was limited by the lack of structural uniformity. EXAFS curve-fitting confirmed that the sites are [triple bond]SiOVOCl2, but revealed an unexpected asymmetry in the V-Cl bond distances. The latter is suggested to be a manifestation of silicon-chloride interactions.

Microwave-Assisted Allylation of Acetals with Allyltrimethylsilane in the Presence of CuBr

We describe the first synthesis of homoallyl ethers from acetals and allyltrimethylsilane using microwave heating and CuBr as a promoter. This method works best for aromatic acetals, giving the corresponding homoallyl ethers in good to quantitative yield.