Tetrahedral molecular geometry

About: Tetrahedral molecular geometry is a research topic. Over the lifetime, 1795 publications have been published within this topic receiving 30706 citations.

Phenoxyl radicals: H-bonded and coordinated to Cu(II) and Zn(II)

Abstract: Two pro-ligands (RLH) comprised of an o,p-di-tert-butyl-substituted phenol covalently bonded to a benzimidazole (BzLH) or a 4,5-di-p-methoxyphenyl substituted imidazole (PhOMeLH), have been structurally characterised. Each possesses an intramolecular O–H⋯N hydrogen bond between the phenolic O–H group and an imidazole nitrogen atom and 1H NMR studies show that this bond is retained in solution. Each RLH undergoes an electrochemically reversible, one-electron, oxidation to form the [RLH]˙+ radical cation that is considered to be stabilised by an intramolecular O⋯H–N hydrogen bond. The RLH pro-ligands react with M(BF4)2·H2O (M = Cu or Zn) in the presence of Et3N to form the corresponding [M(RL)2] compound. [Cu(BzL)2] (1), [Cu(PhOMeL)2] (2), [Zn(BzL)2] (3) and [Zn(PhOMeL)2] (4) have been isolated and the structures of 1·4MeCN, 2·2MeOH, 3·2MeCN and 4·2MeCN determined by X-ray crystallography. In each compound the metal possesses an N2O2-coordination sphere: in 1·4MeCN and 2·2MeOH the {CuN2O2} centre has a distorted square planar geometry; in 3·2MeCN and 4·2MeCN the {ZnN2O2} centre has a distorted tetrahedral geometry. The X-band EPR spectra of both 1 and 2, in CH2Cl2–DMF (9 : 1) solution at 77 K, are consistent with the presence of a Cu(II) complex having the structure identified by X-ray crystallography. Electrochemical studies have shown that 1, 2, 3 and 4 each undergo two, one-electron, oxidations; the potentials of these processes and the UV/vis and EPR properties of the products indicate that each oxidation is ligand-based. The first oxidation produces [M(II)(RL)(RL˙)]+, comprising a M(II) centre bound to a phenoxide (RL) and a phenoxyl radical (RL˙) ligand; these cations have been generated electrochemically and, for R = PhOMe, chemically by oxidation with Ag[BF4]. The second oxidation produces [M(II)(RL˙)2]2+. The information obtained from these investigations shows that a suitable pro-ligand design allows a relatively inert phenoxyl radical to be generated, stabilised by either a hydrogen bond, as in [RLH]˙+ (R = Bz or PhOMe), or by coordination to a metal, as in [M(II)(RL)(RL˙)]+ (M = Cu or Zn; R = Bz or PhOMe). Coordination to a metal is more effective than hydrogen bonding in stabilising a phenoxyl radical and Cu(II) is slightly more effective than Zn(II) in this respect.

Reorientational correlation functions for computer-simulated liquids of tetrahedral molecules

Abstract: Molecular dynamics simulations have been carried out on fluids of tetrahedral molecules for several sets of temperatures and pressures. The results were analysed to give reorientational and angular velocity autocorrelation functions. It was found that these could be related by a cumulant expansion in powers of the angular velocity and rotation operators. This type of analysis is likely to prove simple and useful for relating orientational correlation functions to the angular velocity correlation function in dense fluids where reorientation is hindered.

Characterization of Metal Binding by a Designed Protein: Single Ligand Substitutions at a Tetrahedral Cys2His2 Site

Abstract: The tetrahedral Cys2His2 Zn(I1)-binding site in the de novo designed protein ZQ (Regan, L., & Clarke, N. D. (1990) Biochemistry 29, 108781 has been studied by independently mutating each of the metal-binding ligands to alanine. The contribution of each ligand to the geometry and affinity of metal binding has been characterized using Co(II), Zn(II), and Cd(I1). The results indicate that all four ligands contribute to high-affinity metal binding in ZQ. Two of the four metal-site mutants retain the tetrahedral Zn(I1)-binding geometry of ZQ, with one water molecule presumed to bind in the vacant ligand position. These mutants provide the first examples of a demonstrated de novo tetrahedral three-coordinate site designed into a protein and as such are a first step toward the design of catalytic rather than structural Zn(I1) sites. One of the metal-site mutants binds Zn(I1) with either tetrahedral four-coordinate or five- coordinate geometry, while the last ligand-to-alanine substitution abolishes tetrahedral binding. The importance of ligand type for metal-binding in ZQ was investigated by characterizing two ligand-swap mutants in which a cysteine residue was replaced with a histidine. In both cases, tetrahedral metal binding was lost. Collectively, these results affirm the strategy used to design ZQ by showing that all designed liganding residues are participating in metal binding, and by suggesting that the tetrahedral geometry of the binding site is perturbed when the designed side chain ligands are replaced with alternate ligands. During the course of evolution, proteins have recruited ions and small molecules to increase the range of structures and activities accessible to them. In particular, the transition metal ion Zn(1I) has been used in a wide variety of structural and catalytic roles. A number of properties make this ion well-suited for these roles: Zn(I1) is a good Lewis acid, has

Induced chirality-at-metal and diastereoselectivity at Δ/Λ-configured distorted square-planar copper complexes by enantiopure Schiff base ligands: combined circular dichroism, DFT and X-ray structural studies.

Abstract: Bidentate enantiopure Schiff base ligands, (R or S)-N-1-(Ar)ethyl-2-oxo-1-naphthaldiminato-κ2N,O, diastereoselectively yield Δ/Λ-chiral four-coordinated, non-planar Cu(N^O)2 complexes [Ar = C6H5R/S-L1, m-C6H4OMe R-L2, p-C6H4OMe R/S-L3, and p-C6H4Br R/S-L4]. Two N,O-chelate ligands coordinate to the copper(II) atom in distorted square-planar mode, and induce metal-centered Δ/Λ-chirality at the copper atom in the C2-symmetric complexes. In the solid state, the R-L1 (or R-L4) ligand chirality diastereoselectively induces a Λ-Cu configuration in Λ-Cu-R-L1 (or Λ-Cu-R-L4), the S-L1 ligand a Δ-Cu configuration in Δ-Cu-S-L1, forming enantiopure crystals upon crystallization. Conversely, the R-L2 ligand combines both Λ/Δ-Cu-R-L2 as a diastereomeric pair in the crystals. In solution, electronic circular dichroism (CD) spectra show full or partial diastereoselectivity towards Λ-Cu for R ligands and towards Δ-Cu for S ligands. The electronic CD spectra measured on all complexes obtained from R ligands (or S ligands), e.g.Cu-R-L1, Cu-R-L2, Cu-R-L3, and Cu-R-L4 (or Cu-S-L1, Cu-S-L3, and Cu-S-L4), show consistent spectral features. TDDFT calculations of the electronic CD spectra for the diastereomers Λ-Cu-R-L1 and Δ-Cu-R-L1 suggest that the CD spectra are largely dominated by the configuration at the metal center (Λvs.Δ). The experimental CD spectrum of Cu-R-L1 agrees well with the one calculated for the Λ-Cu-R-L1 configuration. Cyclic voltammetry of Cu-R-L1 reveals a quasi-reversible redox wave corresponding to one-electron transfer for the [CuIIL2]0/[CuIL2]−1 couple in acetonitrile. DSC analyses for the complexes show an exothermic peak between 377 and 478 K (ΔH = −12 to −43 kJ mol−1), corresponding to a phase transformation from distorted square-planar/tetrahedral to regular tetrahedral geometry on heating.