Geometries, Thermodynamic Properties and Reactions of Methylzinc Alkoxide Clusters Studied by Density Functional Theory Calculations

From Chain- to Graphene-like Hydroxyl-terminated (ZnO)n Clusters with n≤6 Obtained via Zinc Dimethoxide Hydrolysis and Condensation: Ab initio Structural, Electronic, Vibrational and Optical Properties Calculations

Recent reports are focusing on the structural evolution from the atomic-scale and also at the expenses of alkyl zinc alkoxide precursors towards (ZnO)_n clusters and nanostructures with different interesting motifs, but still not much is known about their electronic properties. In this manuscript, we present a theoretical study using DFT and TD-DFT methodologies on the hydrolysis and condensation of zinc dimethoxide precursor in its monomeric, dimeric and trimeric forms towards thermodynamically stable hydroxyl-terminated (ZnO)_n clusters with novel chain- and graphene-like fashions. For all cases, distinct vibrational and optical spectra features were assigned evidencing a global monotonic decrease in the opto-electronic gap with increasing oligomerization and cyclization stages. In addition, the electron-affinity of all clusters was also observed to be enhanced with increasing oligomerization and cyclization stages and the electronic charge localization in -e charged clusters was observed to be strongly related to the presence of zinc-oxo subunits and other particular structural features. Our calculations also indicate that the stabilization through hydroxyl termination of both chain- and graphene-like ZnO clusters not only could be a promising driving force to obtain larger atomic-scale 1D and 2D nanostructures but also envisage interesting properties, particularly as electronic acceptor materials for energy applications.

Synthesis of Trimeric Organozinc Compounds and their Subsequent Reaction with Oxygen

A conventional solution‐based route to a cyclic trimeric organozinc compound [{Zn(Et)(β‐diketonate)}₃] (β‐diketonate=OC(OMe)CHC(Me)O, 1) is described, with 1 structurally characterized for the first time. The ligand selection of bidentate β‐diketonates is shown to be key to isolating a cyclic trimer. Additional reaction of β‐diketonates with diethyl zinc were spectroscopically characterized as compounds of the type [{Zn(Et)(β‐diketonate)}_n] (β‐diketonate=OC(Me)CHC(Me)O, 2, OC(OtBu)CHC(Me)O, 3). Further studies have shown that selective oxidation of these species produces cubanes of the general formula [{Zn(OC(R)CHC(Me)O)₂Zn(Et)OEt}₂] (R=OMe, 4; Me, 5; OtBu, 6), allowing a high oxygen content whilst remaining structurally suitable for use as precursors. The successful deposition of thin films of zinc oxide through aerosol‐assisted chemical vapor deposition (AACVD), using a novel precursor, is described and fully characterized.

Development of Zn-ProPhenol-catalyzed asymmetric alkyne addition: synthesis of chiral propargylic alcohols

The development of a general and practical zinc-catalyzed enantioselective alkyne addition methodology is reported. The commercially available ProPhenol ligand (1) has facilitated the addition of a wide range of zinc alkynylides to aryl, aliphatic, and α,β-unsaturated aldehydes in high yield and enantioselectivity. New insights into the mechanism of this reaction have resulted in a significant reduction in reagent stoichiometry, enabling the use of precious alkynes and avoiding the use of excess dimethylzinc. The enantioenriched propargylic alcohols from this reaction serve as versatile synthetic intermediates and have enabled efficient syntheses of several complex natural products.

NMR-spectroscopic and solid-state investigations of cometal-free asymmetric conjugate addition: a dinuclear paracyclophaneimine zinc methyl complex

We present herein the first indications for dimeric structures in cometal-free asymmetric conjugate addition reactions of dialkylzinc reagents with aldehydes. These are revealed by nonlinear effect (NLE) studies. A monomer-dimer equilibrium can be assumed which explains the increase of the ee value in the product over time. Also, DOSY NMR spectroscopic measurements indicate the existence of the catalyst as [LZnEt](n) complexes in solution. Additionally, the first X-ray structure of a zinc complex with a [2.2]paracyclophane ligand was determined. The structures of the zinc complexes are supported by DFT calculations of monomeric and dimeric species.

Structural isomerism and thermodynamic properties of the methylzinc alkoxide molecules (MeZnOR)n (R = Me, tBu) and cations [(MeZnOMe)n]+ (n = 3, 4) studied by B3LYP and PCM calculations

Zinc oxide is a semiconductor as well as a catalyst in important industrial processes, and alkylzinc alkoxide species of the type (RZnOR)(n) are precursors for the preparation of nanoscaled zinc oxide particles. In this work, the structures, thermodynamic properties, dipole moments, and molecular vibrations of substituted zinc oxide molecules and cations have been studied by density functional calculations at the B3LYP/6-31+G(2df,p) level. The neutral tetramers (MeZnOMe)(4) and (MeZnO(t)Bu)(4) are cubane-like species of T(d) symmetry in the gas phase, whereas the cation [(MeZnOMe)(4)](+) is approximately of C(3v) symmetry due to a Jahn-Teller distortion. For the neutral trimers (MeZnOMe)(3) and (MeZnO(t)Bu)(3) two structural isomers of almost identical energy were located on the potential energy surfaces: A closo-cluster with eight ZnO bonds is most stable both as isolated molecule and in a polarizable continuum, but a roof-shaped structure with seven ZnO bonds differs in enthalpy by less than 8 kJ mol(-1). In the case of the analogous cation [(MeZnOMe)(3)](+), the isomer with the roof-shaped Zn(3)O(3) skeleton is 31.7 kJ mol(-1) more stable in the gas phase than the cluster isomer. A particularly long Zn-C bond in the most stable cationic tri- and tetramers explains the loss of a methyl group, observed mass spectrometrically after electron impact ionization. The dissociation of the tetramers (MeZnOMe)(4) and (MeZnO(t)Bu)(4) into 4/3 trimers is endothermic and endergonic, but less so for the tert-butyl derivative compared to the all-methyl cluster. This destabilizing effect of tert-butyl substituents on the tri- and tetrameric clusters follows also from the larger internuclear distances between the non-hydrogen atoms in the structures of (MeZnO(t)Bu)(n) (n = 3 and 4) compared to the analogous all-methyl derivatives. Remarkable differences of the atomic charges of the carbon atoms of methyl or tert-butyl groups attached to oxygen have been found.

Oxygenation of simple zinc alkyls: surprising dependence of product distributions on the alkyl substituents and the presence of water

Oxygenation reactions of dialkylzinc solutions have been investigated. Me2Zn reacts with oxygen in the absence of water to give the bis(heterocubane) [(MeZn)6Zn(OMe)8], whereas Et2Zn and iPr2Zn afford the mono(heterocubanes) [(RZn)4(OR)4]. In the presence of small amounts of water (added during or after the oxygenation) the product types are reversed for Me2Zn and Et2Zn giving [(MeZn)4(OMe)4] and [(EtZn)6Zn(OEt)8], while being retained for iPr2Zn (giving [(iPrZn)4(OiPr)4]). Full characterization of all products by NMR spectroscopy, mass spectrometry, and elemental analyses is provided, and crystal structures of [(EtZn)6Zn(OEt)8] and [(iPrZn)4(OiPr)4] are reported. A rationalization of the different reactivities is attempted on the basis of DFT-calculated energies of some key reactants.

Interaction of Zinc Oxide Clusters with Molecules Related to the Sulfur Vulcanization of Polyolefins (“Rubber”)

The vulcanization of rubber by sulfur is a large-scale industrial process that is only poorly understood, especially the role of zinc oxide, which is added as an activator. We used the highly symmetrical cluster Zn(4)O(4) (T(d)) as a model species to study the thermodynamics of the initial interaction of various vulcanization-related molecules with ZnO by DFT methods, mostly at the B3LYP/6-31+G* level. The interaction energy of Lewis bases with Zn(4)O(4) increases in the following order: CO<S(6)<C(2)H(4)<C(3)H(6)<Me(2)S(2)<1,4-C(5)H(8)<MeSH<Me(2)O<Me(2)S<Me(3)N<<CH(3)COO(-). The corresponding binding energies range from -57 to -262 kJ mol(-1). However, Brønsted acids react with the Zn(4)O(4) cluster with proton transfer from the ligand molecule to one of the oxygen atoms of Zn(4)O(4), and these reactions are all strongly exothermic [binding energies [kJ mol(-1)] in parentheses: H(2)O (-183), MeOH (-171), H(2)S (-245), MeSH (-230), C(3)H(6) (-121), and CH(3)COOH (-255)]. The important vulcanization accelerator mercaptobenzothiazole (C(7)H(5)NS(2), MBT) containing several donor sites reacts with the Zn(4)O(4) cluster with proton transfer from the NH group to one of the oxygen atoms of ZnO, and in addition the exocyclic thiono sulfur atom and the nitrogen atom coordinate to one and the same zinc atom, resulting in a binding energy of -247 kJ mol(-1). A second isomer of [(MBT)Zn(4)O(4)] with a strong O--HN hydrogen bond rather than a Zn--N bond is only slightly less stable (binding energy -243 kJ mol(-1)). The NH form of free MBT is 36 kJ mol(-1) more stable than the tautomeric SH form, while the sulfurized MBT derivative benzothiazolyl hydrodisulfide C(7)H(5)NS(3) (BtSSH) is most stable with the connectivity >CSSH.