Steric effect on construction of Cu(II) complexes with pyridine carboxamide ligands

The structures of three new Cu(II) complexes with pyridine carboxamide ligands (Me₂bpb, 6-Me₂-Mebpb, and 6-Me₂-Me₂bpb) have been determined. 6-Methyl-substituted pyridyl bpb ligands produced dimeric compounds with Cu(II) ions, and weak interactions between dimers can make even polymeric compounds, while bpb ligands without 6-methyl substitution produced monomeric Cu(II) complexes. The large distortion effects of 6-methyl-substitution are shown in Cu(II) complexes with 6-methyl-substituted pyridyl bpb ligands. This result suggests that the steric effect of 6-methyl-substitution plays important role for distortion of the structure, and 6-methyl-substitution can also influence to make polymeric compounds with interactions between Cu(II) ions and neighbor carbonyl oxygen atoms. In addition, the voltammetric behaviors of the Cu complexes were examined and classified into two groups, with/without 6-methyl group. The complexes without 6-methyl group show reversible redox waves at −1.6 V, and the complexes with 6-methyl group do irreversible redox ones at −1.3 V, indicating that the presence of the methyl group of 6-position of the complex makes the reduction of the complexes easier.     

Disorder and intermolecular interactions in a family of tetranuclear Ni(II) complexes probed by high-frequency electron paramagnetic resonance

High-frequency electron paramagnetic resonance (HFEPR) data are presented for four closely related tetranuclear Ni(II) complexes, [Ni(hmp)(MeOH)Cl]4.H2O (1a), [Ni(hmp)(MeOH)Br]4.H2O (1b), [Ni(hmp)(EtOH)Cl]4.H2O (2), and [Ni(hmp)(dmb)Cl]4 (3) (where hmp(-) is the anion of 2-hydroxymethylpyridine and dmb is 3,3'-dimethyl-1-butanol), which exhibit magnetic bistability (hysteresis) and fast magnetization tunneling at low temperatures, properties which suggest they are single-molecule magnets (SMMs). The HFEPR spectra confirm spin S = 4 ground states and dominant uniaxial anisotropy (DSz(2), D < 0) for all four complexes, which are the essential ingredients for a SMM. The individual fine structure peaks (due to zero-field splitting) for complexes 1a, 1b, and 2 are rather broad. They also exhibit further (significant) splitting, which can be explained by the fact that there exists two crystallographically distinct Ni 4 sites in the lattices for these complexes, with associated differences in metal-ligand bond lengths and different zero-field splitting (ZFS) parameters. The broad EPR lines, meanwhile, may be attributed to ligand and solvent disorder, which results in additional distributions of microenvironments. In the case of complex 3, there are no solvate molecules in the structure, and only one distinct Ni 4 molecule in the lattice. Consequently, the HFEPR data for complex 3 are extremely sharp. As the temperature of a crystal of complex 3 is decreased, the HFEPR spectrum splits abruptly at approximately 46 K into two patterns with very slightly different ZFS parameters. Heat capacity data suggest that this is caused by a structural transition at 46.6 K. A single-crystal X-ray structure at 12(2) K indicates large thermal parameters on the terminal methyl groups of the dmb (3,3-dimethyl-1-butanol) ligand. Most likely there exists dynamic disorder of parts of the dmb ligand above 46.6 K; an order-disorder structural phase transition at 46.6 K then removes some of the motion. A further decrease in temperature (<6 K) leads to further fine structure splittings for complex 3. This behavior is thought to be due to the onset of short-range magnetic correlations/coherences between molecules caused by weak intermolecular magnetic exchange interactions.     

Interplay of phenyl-perfluorophenyl stacking,C-H...F,C-F...pi and F...F interactions in some crystalline aromatic azines

Analysis of phenyl-perfluorophenyl stacking synthon, C-H...F, C-F...pi interactions, and F...F tetramer in three closely related azine crystal structures shows the dominance of Ar-ArF synthon while other interactions are turned on/off depending on the H/F stoichiometry in the molecule.     

Tuning N-heterocyclic carbenes in T-shaped Pt(II) complexes for intermolecular C-H bond activation of arenes

Small change matters: T-shaped Pt(II) complexes with less flexible substituents, than, for example, isopropyl or tert-butyl groups, on N-heterocyclic carbene (NHC) ligands allow for C-H bond activation reactions of aromatic compounds (see scheme; BAr(f)(4)(-) =tetrakis[(3,5-trifluoromethyl)phenyl]borate; F yellow, Pt red). NHC substituents that are not highly branched prevent agostic interactions and reduce the barriers to achieve the C-H bond cleavage.     

Mononuclear Ni(II)-thiolate complexes with pendant thiol and dinuclear Ni(III/II)-thiolate complexes with Ni...Ni interaction regulated by the oxidation levels of nickels and the coordinated ligands

Compared to [Ni(II)(SePh)(P(o-C(6)H(3)-3-SiMe(3)-2-S)(2)(o-C(6)H(3)-3-SiMe(3)-2-SH))]- (1a) and [Ni(II)(Cl)(P(o-C(6)H(3)-3-SiMe(3)-2-S)(2)(o-C(6)H(3)-3-SiMe(3)-2-SH))]- (3a) with a combination of the intramolecular [Ni...H-S] and [Ni-S...H-S] interactions, complexes [NiII(SePh)(P(o-C(6)H(3)-3-SiMe(3)-2-S)(2)(o-C(6)H(3)-3-SiMe(3)-2-SH))]- (1b) and [Ni(II)(Cl)(P (o-C(6)H(3)-3-SiMe(3)-2-S)(2)(o-C(6)H(3)-3-SiMe(3)-2-SH))]- (3b) with intramolecular [Ni...H-S] interaction exhibit lower nu(S-H) stretching frequencies (2137 and 2235 cm(-1) for 1b and 3b vs 2250 and 2287 cm(-1) for 1a and 3a, respectively) and smaller torsion angles (27.2 degrees for 3b vs 58.9 and 59.1 degrees for 1a and 3a, respectively). The pendant thiol interaction modes of 1a, 3a, and 3b in the solid state are controlled by the solvent pairs of crystallization. Oxygen oxidation of dinuclear [Ni(II)(P(o-C(6)H(3)-3-SiMe(3)-2-S)(2)(o-C(6)H(3)-3-SiMe(3)-2-SH))](2) (4) yielded thermally stable dinuclear [Ni(III)(P(o-C(6)H(3)-3-SiMe(3)-2-S)(2)(o-C(6)H(3)-3-SiMe(3)-2-mu-S))](2) (5). The two paramagnetic d(7) Ni(III) cores (S = 1/2) with antiferromagnetic coupling (J = -3.13 cm(-1)) rationalize the diamagnetic property of 5. The fully delocalized mixed-valence [Ni(II)-Ni(III)] complexes [Ni2(P(o-C(6)H(3)-3-SiMe(3)-2-S)(3))(2)]- (6) and [Ni(2)(P(o-C(6)H(3)-3-SiMe(3)-2-S)(3))(P(o-C(6)H(3)-3-SiMe(3)-2-S)(2)(o-C(6)H(3)-3-SiMe(3)-2-SCH(3)))] (7) were isolated upon the reduction of 5 and the methylation of 6, respectively. The electronic perturbation from the sulfur methylation of 6 triggers the stronger Ni...Ni interaction and the geometrical rearrangement from the diamond shape of the [NiS(2)Ni] core to the butterfly structure of [Ni(mu-S)(2)Ni] to yield 7 with Ni...Ni distances of 2.6088(1) A. The distinctly different Ni...Ni distances (2.6026(7) for 5 and 2.8289(15) A for 6) and the coordination number of the nickels indicate a balance of geometrical requirements for different oxidation levels of [PS(3)Ni-NiPS(3)] cores of 5 and 6.     

Theoretical studies for Lewis acid-base interactions and C-H...O weak hydrogen bonding in various CO2 complexes

Comprehension of the basic concepts for the design of CO2-philic molecules is important due to the possibility for "green" chemistry in supercritical CO2 of substitute solvent systems. Lewis acid-base interactions and C-H...O weak hydrogen bonding were suggested as two key factors in the solubility of CO2-philic molecules. To isolate the stabilization energy of weak hydrogen bonding from the overall binding energy, high-level quantum mechanical calculations were performed for the van der Waals complexes of CO2 with methane, methylacetate, dimethylether, acetaldehyde, and 1,2-dimethoxyethane. Structures and energies were calculated at the MP2 level of theory using the 6-31+G(d) and aug-cc-pVDZ basis sets with basis set superposition error corrections. In addition, the single-point energies were calculated using recently developed multilevel methods. This study shows that the Lewis acid-base interaction has a significant impact on the complex stability compared to the C-H...O weak hydrogen bond. The additional stabilization energy of the cooperative weak hydrogen bond with alpha-proton of the carbonyl group was negligible on the enhancement of supercritical CO2 solubility. However, the stabilization energy was larger for the ether group, such that it may have an important role in increasing the supercritical CO2 solubility. Additional formation of cooperative weak hydrogen bonds may not further increase the solubility due to the stability reduction by steric hindrance.     

Anion directed templated synthesis of mono- and di-Schiff base complexes of Ni(II)

Two sets of Schiff base ligands, set-1 and set-2 have been prepared by mixing the respective diamine (1,2-propanediamine or 1,3-propanediamine) and carbonyl compounds (2-acetylpyridine or pyridine-2-carboxaldehyde) in 1:1 and 1:2 ratios, respectively and employed for the synthesis of complexes with Ni(II) perchlorate and Ni(II) thiocyanate. Ni(II) perchlorate yields the complexes having general formula [NiL₂](ClO₄)₂ (L = L¹ [N¹-(1-pyridin-2-yl-ethylidine)-propane-1,3-diamine] for complex 1, L² [N¹-pyridine-2-ylmethylene-propane-1,3-diamine] for complex 2 or L³ [N¹-(1-pyridine-2-yl-ethylidine)-propane-1,2-diamine] for complex 3) in which the Schiff bases are mono-condensed terdentate whereas Ni(II) thiocyanate results in the formation of tetradentate Schiff base complexes, [NiL](SCN)₂ (L = L⁴ [N,N′-bis-(1-pyridine-2-yl-ethylidine)-propane-1,3-diamine] for complex 4, L⁵ [N,N′-bis(pyridine-2-ylmethyline)-propane-1,3-diamine] for complex 5 or L⁶ [N,N′-bis-(1-pyridine-2-yl-ethylidine)-propane-1,2-diamine] for complex 6) irrespective of the sets of ligands used. Formation of the complexes has been explained by anion modulation of cation templating effect. All the complexes have been characterized by elemental analyses, spectral and electrochemical results. Single crystal X-ray diffraction studies confirm the structures of four representative members, 1, 3, 4 and 5; all of them have distorted octahedral geometry around Ni(II). The bis-complexes of terdentate ligands, 1 and 3 are the mer isomers and the complexes of tetradentate ligands, 4 and 5 possess trans geometry.     

Geometric isotope effect of various intermolecular and intramolecular C-H...O hydrogen bonds, using the multicomponent molecular orbital method

The geometric isotope effect (GIE) of sp- (acetylene-water), sp(2)- (ethylene-water), and sp(3)- (methane-water) hybridized intermolecular C-H...O and C-D...O hydrogen bonds has been analyzed at the HF/6-31++G level by using the multicomponent molecular orbital method, which directly takes account of the quantum effect of proton/deuteron. In the acetylene-water case, the elongation of C-H length due to the formation of the hydrogen bond is found to be greater than that of C-D. In contrast to sp-type, the contraction of C-H length in methane-water is smaller than that of C-D. After the formation of hydrogen bonds, the C-H length itself in all complexes is longer than C-D and the H...O distance is shorter than D...O, similar to the GIE of conventional hydrogen bonds. Furthermore, the exponent (alpha) value is decreased with the formation of the hydrogen bond, which indicates the stabilization of intermolecular C-H...O hydrogen bonds as well as conventional hydrogen bonds. In addition, the geometric difference induced by the H/D isotope effect of the intramolecular C-H...O hydrogen bond shows the same tendency as that of intermolecular C-H...O. Our study clearly demonstrates that C-H...O hydrogen bonds can be categorized as typical hydrogen bonds from the viewpoint of GIE, irrespective of the hybridizing state of carbon and inter- or intramolecular hydrogen bond.     

Solvent directed templated synthesis of mono- and di(Schiff base) complexes of Ni(II)

The mono- and binuclear complexes Ni(Salen) (I) and Ni₂(Salen)₂ (II) (H₂Salen = N,N′-bis(salicylidene)ethane-1,2-diamine), have been synthesized and structurally characterized by single-crystal X-ray diffraction studies. The X-ray structural analyses show that the metal center of complex I is mononuclear and tetracoordinate with a distorted tetrahedron, whereas the metal-centered complex II is binuclear and pentacoordinate with rectangular pyramid geometries, respectively. The electrochemical studies evidenced for the mononuclear Ni(II) complex shows one quasireversible reduction potential at −0.80 V (E _pc ) and the binuclear Ni(II) complex shows a reduction potential at −0.90 V (E _pc ) in the cathodic region. The article is published in the original.     

Studies on the intermolecular interactions of metal chelate complexesVIII: On the interactions of dithiocarbamato-copper(II) and -copper(I) complexes with haloalkanes

The interaction of Cu(II)(dtc)₂ and Cu(I)(dtc) complexes with haloalkanes were studied by the EPR method. It was found that the Cu(II)(dtc)₂ complex reacted with haloalkanes only in the presence of weak Lewis bases which formed adducts with it. The intermediate reaction product is the mixed-ligand complex Cu(II)(dtc)X_n (X = Cl, Br, n = 1 or 2); the final products being CuX₂B_n (B = Lewis bases, n = 1 or 2) and unstable resin-like residue. Cu(I)(dtc) reacted with haloalkanes without any promoters giving the mixed-ligand complex Cu(II)(dtc)X_n as product. Free radicals were detected in the reaction of Cu(I)(dtc) using the method of “radical scavenger” and were not found in the reaction of Cu(II)(dtc)₂. The reported results confirmed one of the two reaction mechanisms proposed in the previous studies. The role of the solvent on the EPR parameters of the mixed-ligand Cu(II)(dtc)X complex is also discussed.     

Synthesis and properties of one-dimensional Ni(II) and Ni(II)Cu(II) complexes linked by hydrogen bond

Four dithiooxalato (Dto) bridged one-dimensional Ni(ll) and Ni(ll)Cu(ll) complexes (Me₆[14]dieneN₄)Ni₂(Dto)₂) (1), (Me₆[14]dieneN₄)CuNi(Dto)₂ (2), (Me₆[14]aneN₄)Ni₂(Dto)₂ (3), and (Me₆[14]aneN₄)CuNi(Dto)₂ (4), were synthesized. These complexes have been characterized by elemental analysis, IR, UV and ESR spectra. The crystal structure of complex3 was determined. It crystallizes in the monoclinic system, space group C2/c with a = 2. 2425(4) nm,b = 1.0088(2) nm,c= 1.4665(3) nm, β= 125.32(3)δ Z = 4;R = 0.076, R_w = 0.079. In the complex, Ni(1) coordinates four sulphur atoms of two Dto ligands in plane square environment. Ni(2) lies in the center of macrocyclic ligand. For Dto ligand, two sulphur atoms coordinate Ni(1), and O(1) coordinates Ni(2) and forms weak coordination bond. O(2) is linked to N(2) of macrocyclic ligand through hydrogen bond.     

Theoretical studies on the intra- and intermolecular C-H activation by T-shaped pincer complexes

Three coordinated, T-shaped (PNP)M^I (M = Co, Ru, Rh, Os and Ir) and [(PCP)Pt⁰]⁻, as well as their reactivities for intra- and intermolecular C-H activation have been studied by DFT methods. The experimental observed reactivities were well reproduced. The calculation also generated structural and energetic information which the experimental values were not yet available. We found that the intramolecular C-H activation is in general possible for the low spin (PNP)M^I. Intermolecular C-H activation is not preferred either thermodynamically or kinetically, but could be in competition if the intramolecular activation is reversible. Using model compounds, we found that the intramolecular C-H activation reactivity is not sensitive to steric effects of the bulky ligands. However, the strain of the four-membered ring in the product significantly reduces the reactivity, and the driving force increases by 4.51-12.95 kcal/mol if the strain was largely removed by changing from a four-membered ring to a five-membered ring. The C-H activation step is quite difficult for metals with a d¹⁰ configuration. Part of the reason is that one phosphine ligand dissociates during the reaction because the product has a d⁸ configuration and prefers a square planar structure.     

Studies on the electronic structure and intermolecular interactions of diacetyl-, and methylglyoxal- bis(thiosemicarbazone)copper(II) complexes

The electronic structure of the title complexes are studied by e.p.r. and electronic spectra in DMF solution (300 and 100 K) as well as in magnetically diluted powder sample. It is found that the electronic structure is determined mainly by the unit CuN₂S₂ of the complex. The interaction of the complexes with some Lewis acids is also studied. The formation of D-A complex with relatively weak acids is observed. The strong Lewis acids destroy the complexes.     

Variation in crystalline architectures through supramolecular interactions in copper(II) complexes with tridentate N2O donor Schiff bases

Three copper(II) complexes, [Cu(L¹)(H₂O)(ClO₄)]·0.5H₂O (1), [Cu(L²)(H₂O)(ClO₄)]·0.5H₂O (2), and [Cu(L²)(NCNC(OCH₃)NH₂)]ClO₄ (3), where HL¹ = 4-bromo-2-(-(quinolin-8-ylimino)methyl)phenol and HL² = 1-(-(quinolin-8-ylimino)methyl)naphthalen-2-ol, have been prepared and characterized by elemental analysis, IR, UV–vis and fluorescence spectroscopy and single-crystal X-ray diffraction studies. The copper(II) centers assume five-coordinate square-pyramidal geometries in 1 and 2, whereas square planar copper(II) is present in 3. A methanol molecule has been inserted in the pendant end of the ligated dicyanamide in 3. Various supramolecular architectures are formed by hydrogen bonding, π?π, C–H?π, and lp?π interactions.     

Ferro- and antiferromagnetic interactions in face-sharing trioctahedral Ni(II)Mn(II)Ni(II) and Ni(II)Fe(III)Ni(II) complexes with the same 1-5/2-1 spin system

Two heterotrinuclear complexes, [Mn(II)(Ni(II)L)2].2CH3OH (where H3L = 1,1,1-tris(N-salicylideneaminomethyl)ethane) and [Fe(III)(Ni(II)L)2]NO3.C2H5OH, consisting of three face-sharing octahedra have been prepared; although these complexes have closely related structures and have the same 1-5/2-1 spin system, they show completely different magnetic interactions between the adjacent metal ions: ferromagnetic (Ni(II)-Mn(II)) and antiferromagnetic (Ni(II)-Fe(III)).     

Supertetrahedral decametallic Ni(II) clusters directed by micro(6)-tris-alkoxides

We report the syntheses, structures and magnetic properties of two decametallic Ni(II) clusters with unprecedented supertetrahedral cores, stabilised by the (hitherto unobserved) micro(6)-coordination modes of the tris-alkoxides [MeC(CH(2)O)(3)](3-) and [C(6)H(9)O(3)](3-).     

Crystal structures and intermolecular interactions of two copper(II) complexes with an asymmetric diazine ligand

Reactions of asymmetric ligand N-phenylacetyl picoloylhydrazide (HL) and copper(II) acetate/chloride give two complexes CuL₂ (1) and Cu₂Cl₂L₂ (2). The coordination geometries of Cu(II) in 1 and 2 are a severely distorted octahedron and a distorted square pyramid, respectively. The binuclear copper complex 2 contains a centrosymmetric Cu₂(μ-Cl)₂ core. Individual molecules of 1 and 2 further self-assemble through non-covalent intermolecular bonds in the solid state to form extended 2-D polymers. The magnetic properties, IR, EA, and solid-state photoluminescence properties of the title complexes are presented.     

Studies on some Ni(II) and Zn(II) diaminomonocarboxylate complexes

pH-metric, calorimetric, NMR and spectro-photometric studies were carried out on the Ni(II) and Zn(II) complexes of 2,3-diaminopropionic acid (dapa), 2,4-diaminobutyric acid (daba), 2,5-diaminopentanoic acid or ornithine (Orn) and 2,6-diaminohexanoic acid or lysine (Lys).It is concluded that the maximum number of coordinated ligands (with the exception of totally deprotonated Orn) is three in the case of Ni(II), but only two in the Zn(II) complexes. The ω-amino groups of dapa, daba and Orn take part in the coordination in the Ni(II) complexes. Hence, with Ni(II) both “diamine type” and “glycine-like” complexes are formed. The tendency to “diamine-like” coordination increases from Orn to dapa, so much so that dapa behaves much rather as a C-substituted 1,2-diamino ethane than a substituted glycine. Only Lys is coordinated to the Ni(II) ion exclusively in a “glycine-like” manner.Zn(II) complexes were studied only to a pH value of 9 (because of precipitation). In this pH region the terminal NH₂ groups take part in the coordination only in the cases of dapa and daba. With Orn and Lys “glycine-like”parent and mixed hydroxo complexes are formed.