Tethering of nickel(II) Schiff-base complex onto mesoporous silica: An efficient heterogeneous catalyst for epoxidation of olefins

A new hybrid catalyst has been prepared by tethering a nickel(II) Schiff-base complex via post-synthesis modification of mesoporous silica, MCM-41. The Schiff-base has been derived from salicylaldehyde and 3-aminopropyltriethoxysilane (3-APTES) which is chemically anchored on MCM-41 via silicon alkoxide route. The anchored Schiff-bases imposed a stable planar coordination geometry around the central nickel ions. The catalyst has been characterized by elemental analysis, FT-IR, UV-Vis, small angle X-ray diffraction (SAX) and transmission electron microscopy (TEM) studies. The SAX and TEM measurement showed the mesoporosity of the catalyst. The activity of the catalyst has been assessed in the epoxidation of olefins using tert-butyl-hydroperoxide (tert-BuOOH) as oxidant in heterogeneous condition. Immobilized nickel catalyst was found to be catalytically more active and selective compared to the similar type of nickel(II) complex as well as Ni(NO₃)₂·6H₂O in homogeneous media. The catalyst can be recycled and reused several times without significant loss of activity.     

Heteromolecular grids of free-base and zinc-tetra(4-pyridyl)porphyrins with benzenetetracarboxylic acid

This study describes the formation of hetero-molecular networks involving the 1,2,4,5-benzenetetracarboxylic acid (BTCA) and either the free-base or zinc-metallated tetra(4-pyridyl)porphyrin (TPyP or Zn–TPyP, respectively), taking advantage of the complementary tetradentate H-atom donor and H-atom acceptor capacity of the component species. The reaction of BTCA with TPyP yields flat square-grid-type hydrogen bonded arrays, wherein every BTCA moiety interacts with four different porphyrin units and each one of the latter links laterally to four different tetraacid molecules. Replacement of TPyP by Zn–TPyP adds axial coordination capacity to the porphyrin unit and changes the intermolecular interaction pattern. In this case, the supramolecular self-assembly involves trans-axial coordination of BTCA to Zn–TPyP, into a 2:1 complex of the two species, as well as extended hydrogen bonding in four lateral directions between the (BTCA)₂(Zn–TPyP) units thus formed. The hydrogen-bond networking takes place between the four N(pyridyl)-sites of the porphyrin scaffold and the axial tetracid ligands of four neighboring complexes. In the two crystals, the open hydrogen bonded molecular networks stack in an offset manner, incorporating molecules of the 1,1,2,2-tetrachloroethane solvent within channel zones that penetrate through the layered structure. Application of the TPyP scaffold in the formation of hydrogen-bonded (rather than coordination-driven) assemblies has not been explored prior to our work on this subject.     

Analysis of host-assisted guest protonation exemplified for p-sulfonatocalix[4]arene--towards enzyme-mimetic pKa shifts

The pD dependence of the complexation of p-sulfonatocalix[4]arene (CX4) with the azoalkanes 2,3-diazabicyclo[2.2.1]hept-2-ene (1), 2,3-diazabicyclo[2.2.2]oct-2-ene (2), 2,3-diazabicyclo[2.2.3]non-2-ene (3), and 1-methyl-4-isopropyl-2,3-diazabicyclo[2.2.2]oct-2-ene (4) in D(2)O has been studied. The pD-dependent binding constants, determined by (1)H NMR spectroscopy, were analyzed according to a seven-state model, which included the CX4 tetra- and penta-anions, the protonated and unprotonated forms of the azoalkanes, the corresponding complexes, as well as the complex formed between CX4 and the deuteriated hydronium ion. The variation of the UV absorption spectra, namely the hypsochromic shift in the near-UV band of the azo chromophore upon protonation, was analyzed according to a four-state model. Measurements by independent methods demonstrated that complexation by CX4 shifts the pK(a) values of the guest molecules by around 2 units, thereby establishing a case of host-assisted guest protonation. The pK(a) shift can be translated into improved binding (factor of 100) of the protonated guest relative to its unprotonated form as a result of the cation-receptor properties of CX4. The results are discussed in the context of supramolecular catalytic activity and the pK(a) shifts induced by different types of macrocyclic hosts are compared.     

Cation-dependent nuclearity of the copper-azido moiety: synthesis, structure, and magnetic study

Mono-, di-, and trinuclear copper-azido moieties have been synthesized by varying the size of the countercations. [Bu4N]+ yielded a [Cu2(N3)6]2- copper-azido moiety in [Bu4N]2[Cu2(mu(1,1)-N3)2(N3)4], 1, and [Pr4N]+ yielded a [Cu3(N3)8]2- moiety in {[Pr4N]2[Cu3(mu(1,1)-N3)4(N3)4]}n, 2, in which symmetry-related [Cu3(N3)8]2- moieties are doubly mu(1,1)-azido bridged to form unprecedented infinite zigzag chains parallel to the crystallographic a-axis. In the case of [Et4N]+, the mononuclear species [Et4N]2[Cu(N3)4], 3, has been formed. All complexes have been characterized structurally by single-crystal X-ray analysis: 1, C32H72N20Cu2, triclinic, space group P, a = 10.671(9) A, b = 12.239(9) A, c = 10.591(5) A, alpha = 110.01(4) degrees , beta = 93.91(5) degrees , gamma = 113.28(5) degrees , V = 1160.0(1) A3; 2, C24H56N26Cu3, monoclinic, space group P2(1)/n, a = 8.811(2) A, b = 37.266(3) A, c = 13.796(1) A, beta = 107.05(1) degrees , V = 4330.8(10) A(3); 3, C16H40N14Cu, tetragonal, space group I4/m, a = b = 10.487(1) A, c = 12.084(2) A, V = 1328.9(3) A3. The variable-temperature magnetic susceptibility measurements showed that although the magnetic interaction in [Bu4N]2[Cu2(mu(1,1)-N3)2(N3)4], 1, is antiferromagnetic (J = -36 cm(-1)), it is ferromagnetic in {[Pr4N]2[Cu3(mu(1,1)-N3)4(N3)4]}n, 2 (J = 7 cm(-1)). As expected, the [Et4N]2[Cu(N3)4] complex, 3, is paramagnetic.     

Solid state thermal isomerisation of diethylenetriamine complexes of NiX2 (X=NO3, ClO4, BF4 and CF3SO3)

Summary [Ni(dien)₂]X₂·nH₂O (dien=diethylenetriamine; n=0, X=NO₃ or CF₃SO₃; n=0.5, X=ClO₄ or BF₄ and n=2, X=CF₃SO₃) complexes have been prepared and investigated thermally in the solid state. [Ni(dien)₂](NO₃)₂ (1) and [Ni(dien)₂](CF₃SO₃)₂ (2) undergo endothermic irreversible phase transitions (209–247°C and 184–205°C; H=5.6 kJ mol^–1 and 7.7 kJ mol^–1 for (1) and (2), respectively). [Ni(dien)₂](ClO₄)₂·0.5H₂O (3) shows an endothermic irreversible phase transition after deaquation (201–216°C; H=7.7 kJ mol^–1). [Ni(dien)₂](BF₄)₂·0.5H₂O also shows an endothermic irreversible phase transition after deaquation, accompanied by partial decomposition. All the complexes possess octahedral geometry with the ligands arranged meridionally. The phase transitions are explained in terms of conformational changes of the triamine chelate rings.Author to whom all correspondence should be directed. Supplementary data available: i.r. spectra (Table 4) and x-ray diffraction patterns (Table 5).     

Investigation of polar and stereoelectronic effects on pure excited-state hydrogen atom abstractions from phenols and alkylbenzenes

The fluorescence quenching of singlet-excited 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) by 22 phenols and 12 alkylbenzenes has been investigated. Quenching rate constants in acetonitrile are in the range of 10(8)-10(9) M(-1)s(-1) for phenols and 10(5)-10(6) M(-1)s(-1) for alkylbenzenes. In contrast to the quenching of triplet-excited benzophenone, no exciplexes are involved, so that a pure hydrogen atom transfer is proposed as quenching mechanism. This is supported by (1) pronounced deuterium isotope effects (kH/kD ca 4-6), which were observed for phenols and alkylbenzenes, and (2) a strongly endergonic thermodynamics for charge transfer processes (electron transfer, exciplex formation). In the case of phenols, linear free energy relationships applied, which led to a reaction constant of rho = -0.40, suggesting a lower electrophilicity of singlet-excited DBO than that of triplet-excited ketones and alkoxyl radicals. The reactivity of singlet-excited DBO exposes statistical, steric, polar and stereoelectronic effects on the hydrogen atom abstraction process in the absence of complications because of competitive exciplex formation.     

Entrapment of [Ru(bpy)3]2+ in the anionic metal-organic framework: novel photoluminescence behavior exhibiting dual emission at room temperature

[Ru(bpy)(3)](2+) (bpy = 2,2'-bipyridine) ions were entrapped into the cavities of two-dimensional anionic sheet-like coordination polymeric networks of [M(dca)(3)](-) (dca = dicyanamide; M = Mn(II) and Fe(II)). The prepared compounds, {[Ru(bpy)(3)][Mn(dca)(3)](2)}(n) (1) and {[Ru(bpy)(3)][Fe(dca)(3)](2)}(n) (2), were structurally characterized by X-ray single crystal analysis. The spectroscopic properties of the [Ru(bpy)(3)](2+) ion dramatically changed on its entrapment in [M(dca)(3)](-). The [Ru(bpy)(3)](2+) moiety present in 1 and 2 exhibits novel dual photo-emission at room temperature.     

Multicomponent One‐pot Reactions Towards the Synthesis of Stereoisomers of Dipicolylamine Complexes

Reported are multi‐component one‐pot syntheses of chiral complexes [M(L^ROR′)Cl₂] or [M(L^RSR′)Cl₂] from the mixture of an N‐substituted ethylenediamine, pyridine‐2‐carboxaldehyde, a primary alcohol or thiol and MCl₂ utilizing in‐situ formed cyclized Schiff bases where a C?O bond, two stereocenters, and three C?N bonds are formed (M=Zn, Cu, Ni, Cd; R=Et, Ph; R′=Me, Et, nPr, nBu). Tridentate ligands L^ROR′ and L^RSR′ comprise two chiral centers and a hemiaminal ether or hemiaminal thioether moiety on the dipicolylamine skeleton. Syn‐[Zn(L^PhOMe)Cl₂] precipitates out readily from the reaction mixture as a major product whereas anti‐[Zn(L^PhOMe)Cl₂] stays in solution as minor product. Both syn‐[Zn(L^PhOMe)Cl₂] and anti‐[Zn(L^PhOMe)Cl₂] were characterized using NMR spectroscopy and mass spectrometry. Solid‐state structures revealed that syn‐[Zn(L^PhOMe)Cl₂] adopted a square pyramidal geometry while anti‐[Zn(L^PhOMe)Cl₂] possesses a trigonal bipyramidal geometry around the Zn centers. The scope of this method was shown to be wide by varying the components of the dynamic coordination assembly, and the structures of the complexes isolated were confirmed by NMR spectroscopy, mass spectrometry, and X‐ray crystallography. Syn complexes were isolated as major products with Zn^II and Cu^II, and anti complexes were found to be major products with Ni^II and Cd^II. Hemiaminals and hemiaminal ethers are known to be unstable and are seldom observed as part of cyclic organic compounds or as coordinated ligands assembled around metals. It is now shown, with the support of experimental results, that linear hemiaminal ethers or thioethers can be assembled without the assistance of Lewis acidic metals in the multi‐component assembly, and a possible pathway of the formation of hemiaminal ethers has been proposed.