Jürgen Sander

Surface complexation of [Nb6O19]8− with NiII: solvothermal synthesis and X-ray structural characterization of two novel heterometallic NiNb-polyoxometalates ☆

Abstract Solvothermal reaction of NiCl2, Nb(OEt)5 and 1,3,5-triamino-1,3,5-trideoxy-cis-inositol (taci) in a 1:1:1 ratio under alkaline conditions (NaOH) resulted in the formation of two new NbNi heteropolyoxometalate complexes. [Ni(taci)2]2{trans-[Nb6O19][Ni(taci)]2}·26H2O (1) and [Na(H2O)6]2{cis-[H2Nb6O19][Ni(taci)]2}·18H2O (2), were characterized by single crystal X-ray analysis. In both compounds, two [Ni(taci)]2+ entities are each bonded to three NbONb bridges of a central Nb6O19 unit, resulting in the formation of an approximately C3v symmetric NiN3O3 coordination geometry. The mononuclear analogue [Ni(taci)(H2O)3]2+ was crystallized as the sulfate and structurally characterized for comparison. The two isolated anions {trans-[Nb6O19][Ni(taci)]2}4− and {cis-[H2Nb6O19][Ni(taci)]2}2− have approximate D3d and C2v symmetry, respectively. Compound 2 crystallized, however, in the cubic space group Pn 3 m (No. 224) with the two Ni centers equally distributed over four symmetry equivalent sites. In this structure, the [Na(H2O)6]+ cations and the {cis-[H2Nb6O19][Ni(taci)]2}2− anions represent linearly and tetrahedrally connecting building blocks in a hydrogen bonded supramolecular architecture which can be described in terms of two independent but interpenetrating diamond-type nets.

The Coordination Chemistry of cis-3, 5-Diaminopiperidine and Substituted Derivatives

An efficient and convenient method for the preparation of cis-3,5-diaminopiperidine (dapi) has been established and the coordination chemistry of this ligand with CoII, CoIII, NiII, CuII, ZnII, and CdII has been investigated in the solid state and in aqueous solution. Potentiometric measurements revealed a generally high stability for the bis complexes of the divalent cations with maximum stability for NiII (log beta2 = 21.2, beta2 = [M(dapi)2][M](-1)[dapi](-2), 25 degrees C, mu = 0.1 mol dm(-3)). Cyclic voltammetry established quasi-reversible formation of [Ni(dapi)2]3+ with a redox potential of 0.91 V (versus NHE) for the Ni(II/III) couple. [Co(dapi)2]3+ was prepared by aerial oxidation of the corresponding CoII precursor. The two isomers trans-[Co(dapi)2]3+ (1(3+), 26%) and cis-[Co(dapi)2]3+ (2(3+), 74%), have been separated and isolated as solid Cl- and CF3SO3- salts. In a non-aqueous medium 1(3+) and 2(3+) reacted with paraformaldehyde and NEt3 to give the methylidene-imino derivatives 3(3+) and 4(3+), in which the two piperidine rings are bridged by two or one N-CH2-O-CH2-N bridges, respectively. Crystal structure analyses were performed for H3dapi[ZnCl4]Cl, 1Cl3 x 2H2O, 2Cl3 x H2O, 3[ZnCl4]Cl, 4[ZnCl4]Cl, [Ni(dapi)2]Cl2 x H2O, [Cu(dapi)2](NO3)2, [Cu(dapi)Cl2], [(dapi)ClCd-(mu2-Cl)2-CdCl(dapi)], and [Co(dapi)(NO2)(CO3)]. The stability of [M(II)(dapi)]2+ and [M(II)(dapi)2]2+ complexes in aqueous solution, particularly the remarkably high tendency of [M(dapi)]2+ to undergo coordinative disproportionation is discussed in terms of the specific steric requirements of this ligand. Molecular mechanics calculations have been performed to analyze the different types of strain in these complexes. A variety of alkylated derivatives of dapi have been prepared by reductive alkylation with formaldehyde, benzaldehyde, salicylaldehyde, and pyridine-2-carbaldehyde. The NiII complexes of the pentadentate N3,N5-bis(2-pyridinylmethyl)-cis-3,5-diaminopiperidine (py2dapi) and the hexadentate N3,N5,1-tris(2-pyridinylmethyl)-cis-3,5-diaminopiperidine (py3dapi) have been isolated as crystalline ClO4- salts [Ni(py2dapi)Cl]ClO4 and [Ni(py3dapi)](ClO4)2 x H2O and characterized by crystal structure analyses.

The Coordination Chemistry ofcis-3,4-Diaminopyrrolidine and Related Polyamines

cis-3,4-Diaminopyrrolidine (cis-dap), trans-3,4-diaminopyrrolidine (trans-dap), cis-1,2-cyclopentanediamine (cis-cptn), and trans-1,2-cyclopentanediamine (trans-cptn) have been prepared in multigram quantities. The complexation of these ligands and of 3-aminopyrrolidine (ampy) with NiII, CuII, ZnII, and CdII has been studied in solution by means of potentiometric and spectrophotometric titrations. The complexes of the triamines cis-dap and trans-dap show a pronounced tendency to form protonated species such as [MII(HL)]3+, [MII(HL)2]4+, and [MII(HL)L]3+, indicative of a bidentate coordination mode of the ligand L. The UV/Vis spectra of the corresponding CuII complexes further confirmed bidentate coordination with trans-CuN4 geometry. The overall stabilities of the bis complexes [ML2]2+ decrease in the order cis-cptn > cis-dap > trans-cptn > ampy > trans-dap. The considerably lower stabilities of the ampy complexes as compared to the corresponding cis-dap complexes indicate metal binding to the two primary amino groups of the latter ligand. This was supported by molecular mechanics calculations (CuII and CoIII complexes) and confirmed by single-crystal X-ray diffraction studies of [Pt(Hcis-dap)Cl4]Cl·H2O, [Pd(Hcis-dap)2](ClO4)4·2H2O, and [Cu(Hcis-dap)2(OH2)2](SO4)2·3.5H2O − 2x H+ + x Cu2+ with 0.01 ⩽ x ⩽ 0.11. For the diamine ligands, coordination through the two exocyclic amino groups or through one exocyclic and one endocyclic amino group was established from the X-ray structure analyses of [Ni(cis-cptn)2](ClO4)2 and [Cu(3R-ampy)(3S-ampy)](ClO4)2, respectively. The crystal structure determination of [Co(cis-dap)(tach)][ZnCl4]Cl·C2H5OH (tach = cis-1,3,5-cyclohexanetriamine) revealed tridentate, facial coordination of cis-dap in this particular complex. However, the structural parameters of the [Co(cis-dap)(tach)]3+ moiety indicate significant strain for this coordination mode. The coordinating properties of the ligand cis-dap are compared with those of other aliphatic and alicyclic triamines.

T-symmetrical icosahedra: A new type of chirality in metal complexes

A homoleptic metal complex with four tripodal tridentate ligands can have chiral tetrahedral T symmetry. An example of such a complex has been prepared, and the proposed highly symmetric, but chiral structure (see schematic representation, MP=mirror plane, R=NHCH2 PH) has been confirmed.

Chemical sensors in water protection control

The application of several selective chemical sensors and biosensors for monitoring water quality and thus facilitating water pollution control is described. Any regulation in the field of pollution control and environmental chemistry has to be accompanied by an analysis of the toxic substances. Many hazardous substances require continuous surveillance to determine the current toxicological status and the necessary remedial actions. Chemical sensors designed to detect threshold levels of chemical compounds can perform this function. The sensors require a built-in calibration to produce results which are not biased by interfering compounds. Production of these reliable sensors in great quantities will lower unit costs and make their use more cost-effective.