Coordination Bond Angles Research Articles

Theoretical identification of key structural factors for strong magnetic anisotropy in Ni(II) complexes

Single-molecule magnets (SMMs) possess a crucial property called magnetic anisotropy (MA), which has an exceedingly delicate correlation with their structures. In recent years, the study on magneto-structural correlations has emerged as a challenging area in singlemolecule science. Understanding the fundamental physical mechanisms underlying the magneto-structural correlations is essential for building excellent high-temperature SMMs. In this work, we screened various four-coordinated nickel(II) SMMs and studied several key structural factors, such as the lengths and angles of the coordination bonds that may be closely associated with MA. Following that, we developed simple molecular models to deduce the evolution trends of MA with coordination bond angles and lengths. The findings on the magneto-structural correlations stimulated our interest to further explore the crystal structure database. We revealed that the magneto-structural correlation can be well described by a logarithmic function. Guided by such a relationship, we discovered a nickel(II) complex with the strongest MA to date among the tetrahedral-coordinated ones. Our work may be helpful for the empirical synthesis of exceptional high-temperature SMMs.

Seeking magneto-structural correlations in easily tailored pentagonal bipyramid Dy(III) single-ion magnets

Controlling molecular magnetic anisotropy via structural engineering is delicate and fascinating, especially for single-molecule magnets (SMMs). Herein a family of dysprosium single-ion magnets (SIMs) sitting in pentagonal bipyramid geometry have been synthesized with the variable-size terminal ligands and counter anions, through which the subtle coordination geometry of Dy(III) can be finely tuned based on the size effect. The effective energy barrier (Ueff) successfully increases from 439 to 632 K and the magnetic hysteresis temperature (under a 200 Oe/s sweep rate) raises from 11 to 24 K. Based on the crystal-field theory, a semi-quantitative magneto-structural correlation deduced experimentally for the first time is revealed that the Ueff is linearly proportional to the structural-related value S20 corresponding to the axial coordination bond lengths and the bond angles. Through the evaluation of the remanent magnetization from hysteresis, quantum tunneling of magnetization (QTM) is found to exhibit negative correlation with the structural-related value Stun corresponding to the axial coordination bond angles.

Synthesis, characterization and electrochemical behavior of new acyclic mono and binuclear copper (II) complexes: DNA binding and cleavage studies

Abstract A new series of acyclic mono and binuclear copper (II) complexes of the general formula [Cu(L1–5)](ClO4)(H2O)2 (1–5) and [Cu2(L1–5)(H2O)2](ClO4)2 (6–10), {L1–5 refers to [N-(salicylaldimine)-N′-[(2-formyl-4-methyl-6-(4-methylpiperazine-1-yl)methyl)phenol] diamines, where the diamines are L1 = 1,2-diamino ethane, L2 = 1,3-diamino propane, L3 = 1,2-diamino benzene, L4 = 2-aminobenzylamine and L5 = 1,8-diamino naphthalene} have been synthesized and characterized. From the single crystal X-ray diffraction method, the mononuclear complex [Cu(L1)](ClO4)(H2O)2 (1), in which the copper atom exhibits a distorted square planar geometry, coordinates with two phenolic oxygen atoms and two ethylenediamine nitrogen atoms. The coordination bond angles have values between 83.7(2)° [N1–Cu1–N2] and 94.08(19)° [O2–Cu1–N2]. Electrochemical studies were performed using cyclic voltammetry, in which the appearance of one quasi-reversible peak within the potential range −0.91 to −1.21 V (Epc) for mononuclear and two quasi-reversible peaks within the potential ranges −0.70 to −0.88 V (E1pc) and −0.96 to −1.15 V (E2pc) for binuclear complexes were observed respectively. The CT-DNA binding ability of the complexes was investigated using absorption and fluorescence spectroscopic studies, viscosity measurements and circular dichroic techniques. In the absorption spectra, the binding constant (Kb) values for all the complexes were found to be of the order of 0.4 × 104–5.5 × 104 and from fluorescence spectra the apparent binding constant (Kapp) values were of the order of 1.7 × 106–6.6 × 106. From the above Kb and Kapp values, the binding propensity of the complexes to CT-DNA is revealed to be through the intercalative mode. The complexes cleave supercoiled pBR322 DNA in the presence of mercaptoethanol as a reducing agent and with a mechanistic pathway involving the formation of singlet oxygen as a reactive oxygen species. Overall, the more aromatic containing binuclear Cu(II) complex (10) exhibits better DNA binding and cleavage activity than the substituted aliphatic complexes.

Crystal Structures and Solution Properties of Discrete Complexes Composed of Saddle-Distorted Molybdenum(V)-Dodecaphenylporphyrins and Keggin-Type Heteropolyoxometalates Linked by Direct Coordination

Reactions of a saddle-distorted Mo(V)-porphyrin complex, [Mo(DPP)(O)(H(2)O)]ClO(4) (1·ClO(4); DPP(2-) = dodecaphenylporphyrin dianion), with tetra-n-butylammonium (TBA) salts of Keggin-type heteropolyoxomatalates (POMs), α-[XW(12)O(40)](n-) (X = P, n = 3, 2; X = Si, n = 4, 3; X = B, n = 5; 4), in ethyl acetate/acetonitrile gave 2:1 complexes formulated as [{Mo(DPP)(O)}(2)(HPW(12)O(40))] (5), [{Mo(DPP)(O)}(2)(H(2)SiW(12)O(40))] (6), and [(n-butyl)(4)N](2)[{Mo(DPP)(O)}(2)(HBW(12)O(40))] (7) under mild reaction conditions. The crystal structures of the complexes were determined by X-ray crystallography. In these three complexes, named Porphyrin Hamburgers, the POM binds to two Mo(V) centers of porphyrin units directly via coordination of two terminal oxo groups. In spite of the similarity of those POM's structures, those Porphyrin Hamburgers exhibit different coordination bond angles between POM and the Mo(V) center in the porphyrin: 5 and 7 show two different coordination bond angles in one molecule in contrast to 6, which exhibits only one coordination bond angle. The Porphyrin Hamburgers involve protonation of the POM moieties to adjust the charge balance, as confirmed by spectroscopic titration with bases. In the crystals, the Porphyrin Hamburgers form two-dimensional (2D) sheets in the ac plane based on π-π interactions among peripheral phenyl substituents. Stacking of the 2D sheets toward the b axis constructs a 3D layered structure involving channels running into the crystallographic [1 0 0] and [0 0 1] directions in the crystal to include solvent molecules of crystallization for 5-7, and also counter cations for 7. Three complexes were revealed to be stable enough to maintain their structures even in solutions to show molecular ion peaks in the MALDI-TOF-MS measurements. They also exhibited different electron paramagnetic resonance (EPR) signals because of the Mo(V) (S = 1/2, I = 0) centers, reflecting the difference in the crystal structures. In addition, these complexes showed reversible multistep redox processes as observed in their cyclic voltammograms in benzonitrile to demonstrate high stability throughout the redox reactions in solution.

Synthesis and crystal structures of the flexible Schiff base complex bis( N-1,2-diphenylethyl-salicydenaminato-κ 2N,O)copper(II) (methanol): A rare case of solvent-induced distortion

A new mononuclear copper(II) complex incorporating a bulky Schiff base ligand, bis( N-1,2-diphenylethyl-salicydenaminato-κ 2N,O)copper(II) (green form) and its co-crystal containing methanol solvent molecules (brown form) have been synthesized and characterized by X-ray crystallography, XRD, IR and diffuse reflectance spectra in the solid state. This is a rare case in that both solvent free and containing crystals can be isolated and characterized. Both forms adopt a compressed tetrahedral trans-[CuN 2O 2] coordination geometry with coordination bond angles of trans-N–Cu–N = 157.0(4)° and trans-O–Cu–O = 151.1(3)° for the brown form and trans-N–Cu–N = 141.3(3)° and trans-O–Cu–O = 139.9(2)° for the green form. The methanol solvent molecules force the complex of the brown form to adopt a more planar coordination environment in the solid state. Semi-empirical calculations (ZINDO) revealed the magnitude of the dipole moment of both molecules and that the energy of the green form is 5.7 kJ mol −1 higher than that of the brown form. In addition, absorption spectra in various organic solvents were measured and a relationship between absorption spectra and polarity of the solvents is discussed.

Synthesis, crystal structures and electronic properties of Schiff base nickel (II) complexes: Towards solvatochromism induced by a photochromic solute

Two new mononuclear nickel (II) complexes incorporating Schiff base ligands, bis( N- R-1-phenylethyl- or N-2-propyl-3,5-dichlorosalicydenaminato)nickel (II), have been synthesized and characterized. The former chiral brown paramagnetic complex adopts a compressed tetrahedral [NiN 2O 2] coordination geometry of Λ(R,R) absolute configuration with the coordination bond angles of N–Ni–N = 116.3(2)° and O–Ni–O = 141.4(2)°. On the other hand, the latter green diamagnetic complex adopts a square planar trans-[NiN 2O 2] coordination geometry. It is revealed that flexible distortion of the [NiN 2O 2] coordination environment and the overall dipole moment of the complexes are affected by both electronic withdrawing Cl-substituents and steric factors of the ligands. A structural phase transition occurs by heating in the solid state for both complexes. Negative solvatochromism relating to d–d and π–π* transitions can be observed, depending on the polarity of the solvents in which these complexes are dissolved. Furthermore, we have attempted to design a supramolecular solvatochromism system induced by a photochromic solute, that is a CHCl 3 solution of the chiral Ni(II) complex and polar 4-hydroxyazobenzene solute. After UV light irradiation, we have successfully observed slight spectral changes of the π–π* bands in absorption and CD spectra due to conformational change of the R-1-phenylethylamine moieties of the chiral ligands.

Analysis of the coordination geometry in copper complexes

Geometrical parameters associated with a metal coordinated system have been analysed using data from 75 structures of copper complexes involving 370 coordination bonds. Coordination bond length and coordination bond angle, and deviation of the metal atom from the plane of equatorial ligand atoms have been analysed in detail for both oxygen and nitrogen ligand atoms. At the ligand end, the parameters defined closely follow that of the hydrogen bonded system and are related to the lone pair orbital directions. We find that (1) for oxygen ligands, the axial bond lengths are distinctly longer than the equatorial ones in octahedral (OCT) and square pyramidal (PY) geometries. The situation is reversed for trigonal bi-pyramidal (TBP) geometry with nitrogen ligands. (2) The coordinnation bond angle involving axial atoms varies between 130 and 180° for the OCT case and 170 and 180° for TBP geometry. Angles lying around 90° range from 75–105° with a few exceptions in OCT geometry. (3) The metal atom lies very nearly in the equatorial plane in TBP, while it deviates significantly in others. The extent of deviation can be explained qualitatively with the pull of the metal atom by the axial atoms. (4) The distribution of the ligand end parameters indicate that the bond direction strongly tends to cling to the associated orbital. When the ligand atom has two orbitals, the influence of the non-associated orbital is not significant and is far less as compared to hydrogen bonding.