Alkaline-earth complexes with macrocyclic-functionalised bis(phenolate)s and bis(fluoroalkoxide)s

The synthesis and structural features of several families of unsolvated molecular complexes of the heavy alkaline earths (Ae = calcium, strontium and barium) supported by bis(phenolate)s or bis(fluoroalkoxide)s are described. These dianionic, multidentate ligands are built around diaza-macrocycles that contain either five or six N- and O-heteroatoms. Several of these complexes have been characterised by X-ray diffraction crystallography. A list of comparative features was drawn upon close examination of the molecular structures of these complexes. It highlights the subtle influences of the identity of the central Ae metal, denticity and nature -fluoroalkoxide vs. phenolate- of the anionic tethers in the ligands. All complexes are seven- or eight-coordinate. It is observed in particular that a decrease of the number of heteroatoms in the macrocyclic backbone of the ligand will be compensated by the establishment of intramolecular AeF interactions (accounting for ca. 3.8-6.4% of the pertaining coordination spheres according to bond valence sum analysis), dimerisation of the complex, or, in one case, solvent (thf) retention. Attempts to gauge the Lewis acidity in these series of complexes were carried out by three independent methods (Childs, Gutmann-Beckett and global electrophilicity index). However, conflicting results were obtained and no clear trend can be delineated, even if on the whole, these measurements concur to suggest relatively low Lewis acidity.

Molecular Engineering of Metal Alkoxides for Solution Phase Synthesis of High-Tech Metal Oxide Nanomaterials

The 'bottom-up' synthesis of inorganic nanomaterials with precision at the atomic/molecular level offers many opportunities for the design and improvement of the nanomaterials for various applications. Molecular engineering during soft chemical processing for the synthesis of functional nanomaterials enables the desired chemical and physical properties of the precursors, such as solubility or volatility, clean decomposition, control of stoichiometry for multimetallic species to name a few, and leads to easy control of uniform particle size distribution, stoichiometry…. This Minireview illustrates some important aspects of the molecular engineering in light of some recent developments from the molecular synthesis of nanomaterials involving non-silicon metal alkoxide systems for high-tech applications.

Aminofluoroalkoxide amido and boryloxo lead(ii) complexes

We report here on the utilisation of a readily available bidentate aminofluoroalkoxide in lead(ii) chemistry. Stable heteroleptic three-coordinate complexes can be produced in high yields, including a convenient amido synthetic precursor and a rare case of Pb^II-boryloxide.

Copper(i)-based oxidation of polycyclic aromatic hydrocarbons and product elucidation using vacuum ultraviolet spectroscopy and theoretical spectral calculations

Fluorinated 1,3,5-triazapentadienyl complexes of copper catalyze the oxidation PAHs to quinones using H₂O₂as an oxidant under mild conditions.

Cadmium complexes bearing Me2N^E^O- (E = S, Se) organochalcogenoalkoxides and their zinc and mercury analogues

Heteroleptic zinc and cadmium complexes of the type [{^Me₂N^E^O^R₂}M-Nu]_n (M = Zn, Cd; E = S, Se; R = CH₃, CF₃; Nu = N(SiMe₃)₂, I, Cl; n = 1-2) were prepared by reacting the alcohol proteo-ligands {^Me₂N^E^O^R₂}H with [M(N(SiMe₃)₂)₂] (M = Zn, Cd) or [XMN(SiMe₃)₂] (M = Zn, X = Cl; M = Cd, X = I) in an equimolar ratio. These group 12 metal complexes were characterised in solution by multinuclear NMR spectroscopy and their solid-state structures were determined by X-ray diffractometry. The ligands {^Me₂N^E^O^(CH₃)₂}^- bearing CH₃ groups in α position to the alkoxide behave as κ²-O,E-bidentate moieties (E = S, Se) and form centro-symmetric dinuclear O-bridged heteroleptic alkoxo-amido complexes both with zinc and cadmium, with four-coordinate metal centres resting in tetrahedral environments. By contrast, complexes supported by the CF₃-substituted {^Me₂N^E^O^(CF₃)₂}^- crystallise as tetrahedral mononuclear species, with tridentate κ³-N,O,E-coordinated ligands. These structural differences resulting from changes in the ligand skeleton and in the electron-donating properties of the alkoxide were also observed in solution. Attempts to prepare congeneric heteroleptic mercury complexes from [Hg(N(SiMe₃)₂)₂] unexpectedly only afforded homoleptic bis(alkoxide)s such as [{^Me₂N^S^O^(CF₃)₂}₂Hg]. Owing to the strong Hg-C bond, treatment of [PhHgN(SiMe₃)₂] with {^Me₂N^S^O^(CF₃)₂}H afforded the heteroleptic, T-shaped [{^Me₂N^S^O^(CF₃)₂}HgPh] mercuric alkoxide upon elimination of hexamethyldisilazane. [{^Me₂N^S^O^(CF₃)₂}₂Hg] and [{^Me₂N^S^O^(CF₃)₂}HgPh] constitute very rare examples of structurally characterised mercuric alkoxides.

Dinuclear zinc(ii) pyrazolates with different degrees of ring-fluorination and their use in zinc(ii) mediated olefin aziridination

A highly fluorinated pyrazolate complex of zinc has been isolated and utilized as a competent catalyst for olefin aziridination.

Calcium, Strontium and Barium Homogeneous Catalysts for Fine Chemicals Synthesis

The large alkaline earths (Ae), calcium, strontium and barium, have in the past 15 years yielded a brand new generation of heteroleptic molecular catalysts for the production of fine chemicals. However, the integrity of these complexes is often plagued by ligand redistribution equilibria in solution. This personal account retraces the paths followed in our research group towards the design of stable heteroleptic alkalino-earth complexes, including the use of intramolecular noncovalent Ae···H-Si and Ae···F-C interactions. Their implementation as homogenous precatalysts for reactions such as the intramolecular and intermolecular hydroamination and hydrophosphination of activated alkenes, the hydrophosphonylation of ketones, and the dehydrogenative coupling of amines and hydrosilanes that enable the efficient and controlled formations of CP, CN, or SiN σ-bonds, is presented in a synthetic perspective that highlights their overall outstanding catalytic performance.

Aluminum alkyl complexes: synthesis, structure, and application in ROP of cyclic esters

Aluminum alkyl complexes have very useful applications as catalysts or reagents in small molecule transformations and as cocatalysts in olefin polymerization. This short review focuses on some recent developments in the design, synthesis and structure of aluminum(iii) alkyl complexes supported by various ligands bearing nitrogen, oxygen, sulfur or phosphorus atoms, and their catalytic applications in the ring-opening polymerization (ROP) of cyclic esters. The coordination chemistry of the Al metal centre and the catalytic activity changes of the complexes caused by ligand modifications are also discussed.

On the coordination chemistry of organochalcogenolates RNMe2^E− and RNMe2^E^O− (E = S, Se) onto lead(ii) and lighter divalent tetrel elements

The coordination chemistry of organochalcogenolato ligands containing hard (N, O) and soft (S, Se) atoms onto divalent Ge, Sn and Pb is explored.

(μ-1,2-dimethoxyethane-κ2O:O')bis[(1,2-dimethoxyethane-κ2O,O')tris(1,1,1,5,5,5-hexafluoro-4-oxopent-2-en-2-olato-κ2O,O')cerium(III)]

A previous analysis [Fatila et al. (2012). Dalton Trans. 41, 1352-1362] of the title complex, [Ce(2)(C(5)HF(6)O(2))(6)(C(4)H(10)O(2))(3)], had identified it as Ce(hfac)(3)(dme)(1.5) according to the (1)H NMR integration [hfac = 1,1,1,5,5,5-hexafluoroacetylacetonate (1,1,1,5,5,5-hexafluoro-4-oxopent-2-en-2-olate) and dme = 1,2-dimethoxyethane]; however, it was not possible to determine the coordination environment unambiguously. The structural data presented here reveal that the complex is a binuclear species located on a crystallographic inversion center. Each Ce(III) ion is coordinated to three hfac ligands, one bidentate dme ligand and one monodentate (bridging) dme ligand, thus giving a coordination number of nine (CN = 9) to each Ce(III) ion. The atoms of the bridging dme ligand are unequally disordered over two sets of sites. In addition, in two of the -CF(3) groups, the F atoms are rotationally disordered over two sets of sites. This is the first crystal structure of a binuclear lanthanide β-diketonate with a bridging dme ligand.

Thermochemistry of the initial steps of methylaluminoxane formation. Aluminoxanes and cycloaluminoxanes by methane elimination from dimethylaluminum hydroxide and its dimeric aggregates

Results are presented of ab initio studies at levels MP2(full)/6-31G* and MP2(full)/6-311G** of the hydrolysis of trimethylaluminum (TMA, 1) to dimethylaluminumhydroxide (DMAH, 2) and of the intramolecular 1,2-elimination of CH(4) from 2 itself to form methylaluminumoxide 3, from its dimeric aggregate 4 to form hydroxytrimethyldialuminoxane 5 and dimethylcyclodialuminoxane 6, and from its TMA aggregate 7 to form 8 and/or 9, the cyclic and open isomers of tetramethyldialuminoxane, respectively. Each methane elimination creates one new Lewis acid site, and dimethylether is used as a model oxygen-donor molecule to assess the most important effects of product stabilization by Lewis donor coordination. It is found that the irreversible formation of aggregate 4 (ΔG(298) = -29.2 kcal/mol) is about three times more exergonic than the reversible formation of aggregate 7 (ΔG(298) = -9.9 kcal/mol), that the reaction free enthalpies for the formations of 5 (ΔG(298) = -9.0 kcal/mol) and 6 (ΔG(298) = -18.8 kcal/mol) both are predicted to be quite clearly exergonic, and that there is a significant thermodynamic preference (ΔG(298) = -7.2 kcal/mol) for the formation of 6 over ring-opening of 5 to hydroxytrimethyldialuminoxane 10. The mechanism for oligomerization is discussed based on the bonding properties of dimeric aggregates and involves the homologation of HO-free aluminoxane with DMAH (i.e., 9 to 13), and any initially formed hydroxydialuminoxane 10 is easily capped to trialuminoxane 13. Our studies are consistent with and provide support for Sinn's proposal for the formation of oligoaluminoxanes, and in addition, the results point to the crucial role played by the kinetic stability of 5 and the possibility to form cyclodialuminoxane 6. Dialuminoxanes 9 and 10 are reversed-polarity heterocumulenes, and intramolecular O→Al dative bonding competes successfully with Al complexation by Lewis donors. Intramolecular O→Al dative bonding is impeded in cyclodialuminoxane 6, and the dicoordinate oxygen in 6 is a strong Lewis donor. Ethylene polymerization catalysts contain highly oxophilic transition metals, and our studies suggest that these transition metal catalysts should discriminate strongly in favor of cycloaluminoxane-O donors even if these are present only in small concentrations in the methylaluminoxane (MAO) cocatalyst.

Discrete, solvent-free alkaline-earth metal cations: metal···fluorine interactions and ROP catalytic activity

Efficient protocols for the syntheses of well-defined, solvent-free cations of the large alkaline-earth (Ae) metals (Ca, Sr, Ba) and their smaller Zn and Mg analogues have been designed. The reaction of 2,4-di-tert-butyl-6-(morpholinomethyl)phenol ({LO(1)}H), 2-{[bis(2-methoxyethyl)amino]methyl}-4,6-di-tert-butylphenol ({LO(2)}H), 2-[(1,4,7,10-tetraoxa-13-azacyclopentadecan-13-yl)methyl]-4,6-di-tert-butylphenol ({LO(3)}H), and 2-[(1,4,7,10-tetraoxa-13-azacyclo-pentadecan-13-yl)methyl]-1,1,1,3,3,3-hexafluoropropan-2-ol ({RO(3)}H) with [H(OEt(2))(2)](+)[H(2)N{B(C(6)F(5))(3)}(2)](-) readily afforded the doubly acidic pro-ligands [{LO(1)}HH](+)[X](-) (1), [{LO(2)}HH](+)[X](-) (2), [{LO(3)}HH](+)[X](-) (3), and [{RO(3)}HH](+)[X](-) (4) ([X](-) = [H(2)N{B(C(6)F(5))(3)}(2)](-)). The addition of 2 to Ca[N(SiMe(3))(2)](2)(THF)(2) and Sr[N(SiMe(3))(2)](2)(THF)(2) yielded [{LO(2)}Ca(THF)(0.5)](+)[X](-) (5) and [{LO(2)}Sr(THF)](+)[X](-) (6), respectively. Alternatively, 5 could also be prepared upon treatment of {LO(2)}CaN(SiMe(3))(2) (7) with [H(OEt(2))(2)](+)[X](-). Complexes [{LO(3)}M](+)[X](-) (M = Zn, 8; Mg, 9; Ca, 10; Sr, 11; Ba, 12) and [{RO(3)}M](+)[X](-) (M = Zn, 13; Mg, 14; Ca, 15; Sr, 16; Ba, 17) were synthesized in high yields (70-90%) by reaction of 3 or 4 with the neutral precursors M[N(SiMe(3))(2)](2)(THF)(x) (M = Zn, Mg, x = 0; M = Ca, Sr, Ba, x = 2). All compounds were fully characterized by spectroscopic methods, and the solid-sate structures of compounds 1, 3, 7, 8, 13, 14, {15}(4)·3CD(2)Cl(2), {16}(4)·3CD(2)Cl(2), and {{17}(4)·EtOH}·3CD(2)Cl(2) were determined by X-ray diffraction crystallography. Whereas the complexes are monomeric in the case of Zn and Mg, they form bimetallic cations in the case of Ca, Sr and Ba; there is no contact between the metal and the weakly coordinating anion. In all metal complexes, the multidentate ligand is κ(6)-coordinated to the metal. Strong intramolecular M···F secondary interactions between the metal and F atoms from the ancillary ligands are observed in the structures of {15}(4)·3CD(2)Cl(2), {16}(4)·3CD(2)Cl(2), and {{17}(4)·EtOH}·3CD(2)Cl(2). VT (19)F{(1)H} NMR provided no direct evidence that these interactions are maintained in solution; nevertheless, significant Ae···F energies of stabilization of 25-26 (Ca, Ba) and 40 kcal·mol(-1) (Sr) were calculated by NBO analysis on DFT-optimized structures. The identity and integrity of the cationic complexes are preserved in solution in the presence of an excess of alcohol (BnOH, (i)PrOH) or L-lactide (L-LA). Efficient binary catalytic systems for the immortal ring-opening polymerization of L-LA (up to 3,000 equiv) are produced upon addition of an excess (5-50 equiv) of external protic nucleophilic agents (BnOH, (i)PrOH) to 8-12 or 13-17. PLLAs with M(n) up to 35,000 g·mol(-1) were produced in a very controlled fashion (M(w)/M(n) ≈ 1.10-1.20) and without epimerization. In each series of catalysts, the following order of catalytic activity was established: Mg ≪ Zn < Ca < Sr ≈ Ba; also, Ae complexes supported by the aryloxide ligand are more active than their parents supported by the fluorinated alkoxide ancillary, possibly owing to the presence of Ae···F interactions in the latter case. The rate law -d[L-LA]/dt = k(p)·[L-LA](1.0)·[16](1.0)·[BnOH](1.0) was established by NMR kinetic investigations, with the corresponding activation parameters ΔH(++) = 14.8(5) kcal·mol(-1) and ΔS(++) = -7.6(2.0) cal·K(-1)·mol(-1). DFT calculations indicated that the observed order of catalytic activity matches an increase of the L-LA coordination energy onto the cationic metal centers with parallel decrease of the positive metal charge.