Molecular Triangles Research Articles

Oxalate Bridged Triangles Incorporating Mo24+ Units. Electronic Structure and Bonding

The reactions between [Mo(2)L(2)(CH(3)CN)(6)][BF(4)](2) compounds and [Bu(n)(4)N](2)[O(2)CCO(2)] in CH(3)CN are shown to proceed under kinetic control to the formation of a mixture of molecular triangles and squares. The molecular triangles [L(2)Mo(2)(O(2)CCO(2))](3) I (L = DPhF, PhNCHNPh) and II (L = DAniF, p-MeO-C(6)H(4)NCHNC(6)H(4)-p-OMe) are the major products, and when 0.75 equivalents of [Bu(n)(4)N](2)[O(2)CCO(2)] is employed, they are formed to the exclusion of the square. The molecular structure of II is reported based on a single crystal X-ray determination. The molecular triangles do not enter into an equilibrium with their molecular square counterparts in CH(2)Cl(2), in contrast to their perfluoroterephthalate bridged counterparts. The compounds I and II are orange and have a strong electronic transition at lambda(max) approximately 460 nm assignable to metal-to-ligand charge transfer ((1)MLCT) involving the oxalate bridge. Electronic structure calculations employing density functional theory on model compounds [(HCO(2))(2)Mo(2)(O(2)CCO(2))](3) and [(HNCHNH)(2)Mo(2)(O(2)CCO(2))](3) have been carried out and indicate the frontier occupied molecular orbitals are Mo(2) delta combinations e(4)a(2), and the lowest unoccupied are bridge pi* for the formamidinates and delta* for formates as ancillary ligands. Compounds I and II show quasi-reversible oxidation waves in their cyclic voltammograms and oxidation of II in 2-methyl-THF by reaction with AgPF(6) (1 equivalent) leads to a metal centered EPR signal, g approximately 1.95. The electronic absorption spectrum shows a low-energy broad band centered at 6418 cm(-1), which is assigned to an intervalence charge transfer (IVCT) band of a class III mixed valence ion.

ChemInform Abstract: Ring-Closing Alkyne Metathesis with Simple Catalyst Systems: An Access to Molecular Triangles and Rhomboids.

Treatment of the siloxane monomer 2 with a mixture of molybdenum hexacarbonyl and 4-chlorophenol at 140 °C furnished the corresponding cyclotrimer 3 and the cyclotetramer 4.

Synthetic and Computational Studies on Factors Controlling Structures of Molecular Triangles and Squares and Their Equilibrium in Solutions

Reactions of (en*)Pd(NO(3))(2) (en* = N,N,N',N'-tetramethylethylenediamine)with a series of organic bridging ligands of pyrazine, 1,2-bis(4-pyridyl)ethylene, 1,2-bis(4-pyridyl)acetylene, and 1,4-bis(4-pyridyl)benzene are carried out to investigate factors controlling the supramolecular structures and the equilibrium between the molecular triangles and the squares in solutions. The molecular structures of solid triangular [(en*)Pd(L(n))](3)(NO(3))(6) with 1,2-bis(4-pyridyl)ethylene and 1,2-bis(4-pyridyl)acetylene bridging ligands are determined by X-ray crystallography: [(en*)Pd(1,2-bis(4-pyridyl)ethylene)](3)(NO(3))(6), monoclinic Pn (No. 7), a = 17.3242(3) A, b = 15.0804(3) A, c = 17.3223(3) A, beta = 103.5100(10) degrees , V = 4400.33(14) A(3), Z = 2; [(en*)Pd(1,2-bis(4-pyridyl)acetylene)](3)(NO(3))(6), orthorhombic Aba2 (No. 41), a = 14.6642(3) A, b = 27.8763(5) A, c = 21.4233(4) A, V = 8757.5(3) A(3), Z = 4. In contrast, an infinite chain structure of {[(en*)Pd(pyrazine)](NO(3))}(infinity) (monoclinic P2(1)/m (No. 11), a = 14.4740(7) A, b = 8.9209(3) A, c = 28.9705(13) A, beta = 89.974(2) degrees , V = 3740.7(3) A(3), Z = 2) is observed with the shortest pyrazine. The steric hindrance between the supporting and the bridging ligands or the neighboring supporting ligands would contribute to the formation of the infinite chain complex 1. The N(Py)-Pd-N(Py) angles in the solid molecular triangles monotonically increased closely to 90 degrees with the increase in the lengths of the bridging ligands, indicating the relaxation of the steric hindrance between the supporting and the bridging ligands. The structures of the molecular triangles and squares in solutions are optimized with density functional theory (DFT) calculations using the conductor-like polarizable continuum model (C-PCM), and the resulting structures are almost the same as those in the solid state. The (1)H NMR spectra indicate that (i) the equilibrium between the molecular triangles and the squares is attained upon the introduction of the methyl substituents into the en supporting ligands (en*), (ii) the DeltaG degrees values decreased with the increase in the lengths of the straight bridging ligands, and (iii) the equilibrium constants depend on the kinds of solvents. The (1)H NMR spectra estimated with the gauge invariant atomic orbital DFT (GIAO-DFT) calculations well reproduce the experimental data, and the single-point energy DFT calculations with C-PCM in the presence of the solvents approximately reproduce the facts (i)-(iii).

Assembling molecular triangles into discrete and infinite architectures

Having established that molecules with general formulae [MnIII6O2(R-sao)6(O2CR)2(L)4–6] ([Mn6]) and [MnIII3O(R-sao)3(X)(L)3] ([Mn3]) (saoH2 = salicylaldoxime; R = H, Me, Et etc; X = RCO2−, ClO4−; L = solvent), with the latter being the analogous “half” molecules of the former, exhibit the phenomenon of single-molecule magnetism, we have exploited them as building blocks to construct supramolecular architectures by means of host–guest interactions and coordination driven self-assembly. A number of discrete and infinite architectures, namely [MnIII3O(Ph-sao)3(4Cl-sbz)3(MeOH)3]2(OH)(ClO4)·2MeOH (1·2MeOH), [MnIII3O(Ph-sao)3(4Me-sbz)3(EtOH)3]2(OH)(NO3) (2), {[MnIII3O(Et-sao)3(4,4′-bpy)2(MeOH)] ClO4·1.5MeOH·Et2O}n (3·1.5MeOH·Et2O), {[MnIII3O(sao)3(4,4′-bpe)1.5]ClO4·3MeOH}n (4·3MeOH) and [{MnIII3O(Et-sao)3(O2CPh)(EtOH)}2{4,4′-bpe}2] (5), based on the molecular triangle [Mn3] and various pyridyl-type ligands (4Cl-sbz = 4-chlorostilbazole, 4Me-sbz = 4-methylstilbazole, 4,4′-bpy = 4,4′-bipyridine and 4,4′-bpe = trans-1,2-bis(4-pyridyl)ethylene) were obtained and structurally and magnetically characterized.

A unique single carboxylate-bridged spin-frustrated chiral Mn(ii) metallatriangle

The reaction between Hmbpymca ligand (prepared in situ from the hydrolysis of 5-methyl-4-cyano-bispyrimidine with NaOH and further neutralization with 2 M HCl) and Mn(ClO(4))(2)·4H(2)O in 1:1 molar ratio afforded the triangulo-trimanganese(II) complex [Mn(3)(bpymca)(3)(H(2)O)(6)]Cl(3)·6H(2)O 1. The chloride anions in this complex come from the HCl used in the neutralization process. The molecular structure of 1 consists of cationic molecular triangles [Mn(3)(μ-mbpymca)(3)(H(2)O)(6)](3+) with C(3) symmetry, chloride anions and crystallization water molecules, all of them involved in an extensive network of hydrogen bonds, leading to a chiral network. Within the [Mn(3)(μ-mbpymca)(3)(H(2)O)(6)](3+) cations, seven-coordinated Mn(II) ions are bridged by both oxygen atoms of the carboxylate groups and exhibit a MnO(5)N(2) compressed pentagonal bipyramidal coordination environment. The temperature dependence of the magnetic susceptibility shows the presence of weak antiferromagnetic interactions between Mn(II) ions mediated by the carboxylate group of the mbpymca ligand and the existence of a 3D antiferromagnetic ordering below 4 K, which has its origin in the AF inter-trimer exchange interactions mediated by the strong hydrogen bonds present in the crystal of 1. The experimental magnetic susceptibility data above 7 K could be satisfactorily fitted to the theoretical analytical expression derived from the spin Hamiltonian H = -J(S(1)S(2) + S(1)S(3) + S(2)S(3)) with J = -0.782(3) cm(-1) and g = 2.092(3). The model predicts a degenerate ground state with an S = 1/2, which is typical of triangular trimetallic spin frustrated systems containing metal with non-integer spins. DFT calculations were performed on the molecular structure as found in the solid state to support the experimental J value and the Mn-O(carb)-Mn as the primarily exchange pathway.

Synthesis and characterisation of heterometallic molecular triangles using ambidentate linker: self-selection of a single linkage isomer

The coordination driven self-assembly of discrete molecular triangles from a non-symmetric ambidentate linker 5-pyrimidinecarboxylate (5-pmc) and Pd(II)/Pt(II) based 90 degrees acceptors is presented. Despite the possibility of formation of a mixture of isomeric macrocycles (linkage isomers) due to different connectivity of the ambidentate linker, formation of a single and symmetrical linkage isomer in both the cases is an interesting observation. Moreover, the reported macrocycles represent the first example of discrete metallamacrocycles of bridging 5-pmc. While solution composition in both the cases was characterised by multinuclear NMR study and electrospray ionization mass spectrometry (ESI-MS), the identity of the assemblies in the solid state was established by X-ray single crystals structure analysis. Variable temperature NMR study clearly ruled out the formation of any other macrocycles by [4 + 4] or [2 + 2] self-assembly of the reacting components.

Molecular Pd(II) and Pt(II) triangles containing 4,5-dicyanoimidazolate and diimine ligands: Self-assembly, spectroscopic, and photophysical properties

Two isostructural molecular triangles, [M(d- t-bpy)(im-CN 2)] 3(ClO 4) 3 (M ≡ Pd 1, Pt 2; d- t-bpy ≡ 4,4′-di- tert-butyl-2,2′-bipyridine, im-CN 2 ≡ 4,5-dicyanoimidazolate), were synthesized and characterized by the X-ray diffraction study. Both 1 and 2 show molecular capsule-like structures in the solid state, which were built from two triangles in a head-to-tail manner and one ClO 4 - anion entrapped inside the cavity. It is noted that there are some anion⋯π and –C N⋯π interactions as an origin of electrostatic forces found in both 1 and 2, and these weak interactions with non-classical hydrogen bonding are most likely responsible for the formation of molecular capsules. In addition, 2 shows a vibronic-structured emission at ca. 457, 486, and 520 nm in solution, which is absent in 1. The observed vibronic spacing of 1200–1400 cm −1 is within the expected stretching frequencies of aromatic diimines in the excited state, and the vibronic emission is thus suggested due to an intraligand transition of d- t-bpy possibly mixing with a metal-to-ligand charge-transfer transition.

Self-Assembled Metallacycles with Pyrazine Edges: A New Example in Which the Unexpected Molecular Triangle Prevails over the Expected Molecular Square

The combination of cis-protected metal fragments with linear linkers is expected to yield molecular squares. We found instead that treatment of the 90 degrees angular precursor trans-[RuCl2(dmso-S)4] (1) with an equivalent amount of the linear and rigid pyrazine (pyz) linker unexpectedly yields, in a number of different experimental conditions, the molecular triangle [{trans,cis-RuCl2(dmso-S)2(mu-pyz)}3] (3), together with polymeric material. Very similar results were also obtained from the reaction between 1 and the preformed corner fragment trans,cis,cis-[RuCl2(dmso-S)2(pyz)2] (6). In both cases, the expected molecular square [{trans,cis-RuCl2(dmso-S)2(mu-pyz)}4] (4) was observed only as a transient species. These results suggest that 3, which is the first example of a neutral molecular triangle with octahedral metal corners and pyrazine edges, is both the thermodynamic and the kinetic product of the reactions described above. The X-ray structure of 3 shows that the main distortions from ideal coordination geometry concern the N-Ru-N angles, which are narrower than 90 degrees , and the coordination bonds of pyz. The pyrazine molecules, which are basically planar, are significantly tilted from linearity. Calculations performed on 6 indicated that the N-Ru-N angle is ca. six times more rigid than the tilt angle of pyrazine. The structural and theoretical findings on 3 and 6, together with the previous examples of molecular triangles and squares with cis-protected metal corners and linear pyz edges, suggest that the entropically favored molecular triangles might be preferred over the expected molecular squares with metal corner fragments that spontaneously favor Npyz-M-Npyz angles narrower than 90 degrees because of the presence of ancillary ligands with significant steric demand on the coordination plane. The rather-flexible coordination geometry of pyrazine can accommodate the moderate distortions from linearity required to close the small metallacycle with modest additional strain.

Self-Assembly Reactions between the Cis-Protected Metal Corners (N−N)MII(N−N = Ethylenediamine, 4,4‘-Substituted 2,2‘-Bipyridine; M = Pd, Pt) and the Fluorinated Edge 1,4-Bis(4-pyridyl)tetrafluorobenzene

The self-assembly reactions between the fluorinated ditopic ligand 1,4-bis(4-pyridyl)tetrafluorobenzene (A) and different nitrogen-protected palladium(II) and platinum(II) complexes have been investigated. While dynamic equilibria between molecular triangles and squares were observed when the diimine compounds 4,4'-R2bipy (bipy = 2,2'-bipyridine; R = H, Me, t-Bu) were employed as ancillary ligands, only square species were obtained from ethylenediamine (en) derivatives. Characterization of the obtained metallomacrocycles was accomplished by 1H and 19F NMR spectroscopy in combination with electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR). Molecular dynamics simulations (UFF) have been performed to interpret the influence of the fluorinated ring on the square/triangle relative stability. Density functional calculations using the GIAO method have been employed for the interpretation of the chemical shift assignments. The study of the ability of these compounds to act as hosts of electron-rich aromatic guests has shown that the palladium ethylenediamine square is capable of establishing this type of intermolecular interaction exclusively in aqueous media. The host-guest stoichiometry and association constants have been determinated by 1H NMR spectroscopy.

Substitution chemistry of MM quadruply bonded complexes (M = Mo or W) supported by the anion of 2-hydroxy-6-methylpyridine

The compounds M(2)(mhp)(4), where M = Mo or W and mhp is the anion formed from deprotonation of 2-hydroxy-6-methylpyridine, are shown to react with carboxylic acids RCOOH to give an equilibrium mixture of products M(2)(O(2)CR)(n)(mhp)(4-n) where R = 2-thienyl and phenyl. The equilibrium can be moved in favor of M(2)(O(2)CR)(4) by the addition of excess acid or by the favorable crystallization of these products. The latter provides a facile synthesis of the W(2)(O(2)CR)(4) compound where R = 9-anthracene. Reactions involving 2,4,6-triisopropyl benzoic acid, TiPBH, yield M(2)(TiPB)(2)(mhp)(2) compounds as thermodynamic products. Reactions involving Me(3)OBF(4) (1 and 2 equiv.) yield the complexes Mo(2)(mhp)(3)(CH(3)CN)(2)BF(4) and Mo(2)(mhp)(2)(CH(3)CN)(4)(BF(4))(2), respectively. The latter compound has been structurally characterized and shown to have mirror symmetry with two cis mhp ligands: MoMo = 2.1242(5) A, Mo-O = 2.035(2) A, Mo-N(mhp) = 2.161(2) A, and Mo-N(CH(3)CN) = 2.160(3) and 2.170(3) A. Reactions involving Mo(2)(mhp)(3)(CH(3)CN)(2)(2+) and Mo(2)(mhp)(2)(CH(3)CN)(4)(2+) with (n)Bu(4)NO(2)CMe (1 and 2 equiv.) yield the complexes Mo(2)(mhp)(3)(O(2)CMe) and Mo(2)(mhp)(2)(O(2)CMe)(2) which are shown to be kinetically labile to ligand scrambling. Reactions between Mo(2)(mhp)(3)(CH(3)CN)(2)(+)BF(4)(-) (2 equiv.) and [(n)Bu(4)N(+)](2)[O(2)C-X-CO(2)](2-) yielded dimers of dimers [Mo(2)(mhp)(3)](2)(micro-O(2)C-X-CO(2)] where X = nothing, 2,5- or 3,4-thienyl and 1,4-C(6)H(4). Reactions between Mo(2)(mhp)(2)(CH(3)CN)(4)(2+)(BF(4)(-))(2) and tetra-n-butylammonium oxalate and terephthalate yield compounds [Mo(mhp)(2)bridge](n) which by MALDI-TOF MS are proposed to be a mixture of molecular squares (n = 4) and triangles (n = 3) along with minor products of [Mo(2)(mhp)(3)](2)(bridge) and Mo(2)(mhp)(4) that arise from ligand scrambling.

An equilibrium between a loop and a triangle containing [formula omitted] species and perfluorodicarboxylate linkers

A new family of supramolecular loops with flexible perfluorodicarboxylate linkers, [ cis-Mo 2(DAniF) 2] 2(O 2C(CF 2) 2CO 2) 2 ( 1), [ cis-Mo 2(DAniF) 2] 2(O 2C(CF 2) 3CO 2) 2 ( 2a), and [ cis-Mo 2(DAniF) 2] 2(O 2C(CF 2) 4CO 2) 2 ( 3) have been prepared and characterized using X-ray structure analyses and NMR spectroscopy (DAniF = N, N′-di- p-anisylformamidinate). NMR spectra of 1 and 3 remain unchanged over long periods of time. However, the spectrum of 2a changes with time showing that there is a process that is consistent with a dynamic equilibrium between the molecular loop ( 2a) and molecular triangle [ cis-Mo 2(DAniF) 2] 3(O 2C(CF 2) 3CO 2) 3 ( 2b) in the solution of 2a in CDCl 3. A study of the concentration-dependent and temperature-dependent equilibrium using 1H and 19F NMR spectra of this hexafluoroglutarate-containing species suggested the existence of 2b and provided quantitative values for the thermodynamic equilibrium constant K, Δ H 0, and Δ S 0. Conversion of the molecular loops 2a to the molecular triangles 2b is enthalpically favored, but entropically disfavored with Δ H 0 = −41.9 kJ mol −1, and Δ S 0 = −82 J mol −1 K −1.

Dimolybdenum-Containing Molecular Triangles and Squares with Diamidate Linkers: Structural Diversity and Complexity

By employing cis-Mo2(DAniF)2(2+) (DAniF = N,N'-di(p-anisyl)formamidinate) as the vertex building block and terephthaloyldiamidate as the linker, four dimolybdenum-containing cyclic oligomers have been synthesized and structurally characterized. In these compounds, described by the general formula [cis-Mo2(DAniF)2((ArNOC)2C6H4)2]n, n = 3 and 4, the geometry and composition of the products are affected by the identity of the aromatic groups of the linker. When Ar = phenyl, n = 3 (1a and 1b); however, n = 4 for Ar = p-trifluoromethylphenyl (2) and when Ar = m-trifluoromethylphenyl (3). All these compounds have a central cavity, shaped by the diamidate linker, that is capable of serving as host to guest molecules in a selective manner. For compounds 2 and 3, self-assembly that takes place in the crystalline state entails intermolecular C-H...F-C interactions. Such interactions generate a one-dimensional network with a tunnel cross section of 10 x 10 A(2) in 2, whereas in 3, they result in a cage in which two THF molecules are encapsulated. The F...H distances vary in a broad range from 2.38 to 2.70 A.

Dynamic equilibrium between cyclic oligomers. Thermodynamic and structural characterization of a square and a triangle

A dynamic equilibrium has been found in CDCl3 between a neutral molecular square, [cis-Mo2(DAniF)2]4(O2CC6F4CO2)4 (1) and triangle, [cis-Mo2(DAniF)2]3(O2CC6F4CO2)3 (2) (DAniF = the anion of N,N'-di-p-anisylformamidine). The two components have been crystallographically characterized and solution studies by 1H and 19F NMR spectra of the concentration- and the temperature-dependence of the equilibrium have been performed. The conversion of three moles of molecular squares 1 to four moles of molecular triangles 2 has an equilibrium constant of 1.98(7) x 10(-4) at 23.7 degrees C. At this temperature, the DeltaG(0) for the conversion of three moles of squares to four moles of triangles is 21.0 kJ mol(-1). The conversion is enthalpically disfavored (DeltaH(0) = 23.5 kJ mol(-1)), but entropically favored (DeltaS(0) = 8.2 J K(-1) mol(-1)).

A highly electroluminescent molecular square

Chiral molecular triangles and squares containing the Pt(diimine) metallocorners were synthesized and characterized, and used as the triplet MLCT luminophore in highly efficient light-emitting devices.

Triangles and tetrahedra: metal directed self-assembly of metallo-supramolecular structures incorporating bis-beta-diketonato ligands.

The interaction of six aryl-linked bis-beta-diketones, including a new naphthylene linked species, with copper(II), iron(III) and, in one instance gallium(III), has been investigated with the aim of obtaining metallo-supramolecular assemblies exhibiting different geometries. New examples of two assembly types incorporating the above bis-beta-diketones (L) were generated. The first type is represented by a range of molecular triangles of formula [Cu(3)(L-H(2))(3)](solvent)(n) while the second is given by a corresponding selection of less-common neutral molecular tetrahedra of formula [Fe(4)(L-H(2))(6)](solvent)(n) as well as [Ga(4)(L-H(2))(6)].8.5THF.0.5H(2)O; an example of each type has been characterised by X-ray crystallography. A magnetochemical investigation of [Fe(4)(-H(2))(6)].6THF is reported. The susceptibility is Curie like and consistent with very weak coupling occurring between the iron(III) d(5)(high spin) centres. The X-ray structures of two trinuclear copper(II) as well as a tetranuclear iron(III) and a tetranuclear gallium(III) assembly confirm their discrete triangular and tetrahedral geometries, respectively. The structure of the gallium(III) species is closely related to that of the corresponding iron(III) species. The tetrahedral structures provide rare examples of such assemblies encapsulating guest solvent molecules--in each case tetrahydrofuran is incorporated in the central cavity.

Double-stranded helicates, triangles, and squares formed by the self-assembly of pyrrol-2-ylmethyleneamines and ZnII ions.

Neutral double-stranded helicates and molecular triangles and squares are formed through the self-assembly of spacer-bridged bis(pyrrol-2-ylmethyleneamine) ligands and ZnII ions (see scheme). The steric requirements of the Zn centers and the size and relative flexibility of the ligands dictate the degree of association, and hence, the geometry of the products.

A topology paradigm for metal-organic zeolites.

Coordination polymers that afford controllable cavities and incorporate different guest molecules may provide structural prototypes for the design of porous host lattices for applications such as adsorption,[1, 2] ion exchange[3] and heterogeneous catalysis.[4] The elaboration of such materials is a considerable challenge, as only a few of the previously synthesized solids are stable to the loss of the initially accumulated guests and capable of taking up small organic molecules in the resulting molecular holes.[1] New insights into the development of approaches for the engineering of metal–organic zeolites are possible on the basis of topological considerations.[5] Attractive metal–organic-zeolite models of metal–organic frameworks may be predicted for novel “2D building blocks” of semiregular topology[6] (Figure 1b and c) that are expanded in a third direction by pillaring.[7] In this case, initial plane tiling by a set of different polygons, instead of uniform square grids as in [Cu(bipy)2SiF6], generates closely packed molecular triangles (cf. molecular hexagons) with very open regions within the network. Thus, the free space appears to be concentrated, and the resulting framework of relatively low overall porosity, which is stable and robust, could maintain large cages for guest molecules. Realistic prototypes of such arrays are provided by the structures of purely inorganic materials—tungsten bronzes.[6b] Herein, we report how the combination of the inherent functional features of organic and inorganic counterparts allows an especially effective implementation of this assembly scenario, and the generation of a target coordination network with unprecedented hexagonal tungsten bronze topology. Despite the fact that the angles at the net vertex (2 6 60, 2 6 1208) do not match the demands of a typical coordination environment around octahedrally coordinated transitionmetal ions, the desired connectivity may be tuned by the conformational flexibility of the organic linker. First, the 3,3’,5,5’-tetramethylsubstituted 4,4’-bipyrazolyl ligand (4,4’bpz) acts as an angular linker.[8] Its frame includes two planar pyrazolyl fragments, which have an angle of rotation around 60–808 and the resulting noncollinear orientation of two N M vectors makes possible the assembly of the desired flat coordination net. Second, these coordination layers [M(4,4’bpz)2]n, formed by the octahedrally coordinated metal ions and two equivalents of the bridging 4,4’-bpz ligands, have a rich and versatile functionality for cross-linking into a 3D superstructure. Each four-coordinate point M(pyrazole)4 of the layer provides, in the two axial directions, six binding sites that include coordination positions at the metal atom, and four hydrogen bond donating NH groups of the coordinated pyrazole ligands. Thus, dense interconnection of the layers and generation of a rigid 3D network is feasible by the rational choice of an anionic counterpart to fit perfectly this set of binding sites. The 3D structure of a new family of framework solids [M(4,4’-bpz)2{S2O6}]n 1 (M=Co 1a, Zn 1b, Cd 1c) is supported by cooperation of bridging functions of organic and inorganic linkers. The 2D [M(4,4’-bpz)2]n linkage exists as a triangular so-called Kagom@ network[9] (Figure 2) and dithionate anions are used as pillars. The present 2D metalorganic subunit is a isomer of “square grids” and represents a mode of plane tiling by a combination of molecular triangles and hexagons in 2:1 ratio, instead of a 44 regular tiling topology. The fragments [M(4,4’-bpz)2]6 are distorted and adopt a roughly triangular shape that eliminates possible centers of inversion and the entire 3D structure is chiral. Dithionates are perfectly suited as pillars for the 1:4 metal– pyrazole coordination, as two oxygen atoms complete the octahedral environment of the metal atoms and the remaining four oxygen atoms form relatively strong NH···O hydrogen bonds with all available adjacent pyrazolyl groups (Scheme 1, Figure 3). This complementary set of donor/acceptor interactions, and effective shielding of the S2O6 linkers by eight methyl groups, presumably has an impact on the high thermal stability of the compounds, which remains unchanged in air up to 590 K.[10] Noteworthy, simple inorganic dithionates themselves are much less stable and readily loose SO2 upon heating. Successive planes of [M(4,4’-bpz)2]n are stacked with an interlayer separation of approximately 7.97 F, and the entire Figure 1. Four-connected plane nets of significance for chemical topology: a) regular 44 tiling, b),c) semiregular nets.

Self-assembly in palladium(II) and platinum(II) chemistry: the biomimetic approach.

The self-assembly of complex cationic structures by combination of cis-blocked square planar palladium(II) or platinum(II) units with bis(pyridyl) ligands having bridging amide units has been investigated. The reactions have yielded dimers, molecular triangles, and polymers depending primarily on the geometry of the bis(pyridyl) ligand. In many cases, the molecular units are further organized in the solid state through hydrogen bonding between amide units or between amide units and anions. The molecular triangle [Pt(3)(bu(2)bipy)(3)(mu-1)(3)](6+), M = Pd or Pt, bu(2)bipy = 4,4'-di-tert-butyl-2,2'-bipyridine, and 1 = N-(4-pyridinyl)isonicotinamide, stacks to give dimers by intertriangle NH.OC hydrogen bonding. The binuclear ring complexes [[Pd(LL)(mu-2)](2)](CF(3)SO(3))(4), LL = dppm = Ph(2)PCH(2)PPh(2) or dppp = Ph(2)P(CH(2))(3)PPh(2) and 2 = NC(5)H(4)-3-CH(2)NHCOCONHCH(2)-3-C(5)H(4)N, form transannular hydrogen bonds between the bridging ligands. The complexes [[Pd(LL)(mu-3)](2)](CF(3)SO(3))(4), LL = dppm or dppp, L = PPh(3), and 3 = N,N'-bis(pyridin-3-yl)-pyridine-2,6-dicarboxamide, and [[Pd(LL)(mu-4)](2)](CF(3)SO(3))(4), LL = dppm, dppp, or bu(2)bipy, L = PPh(3), and 4 = N,N'-bis(pyridin-4-yl)-pyridine-2,6-dicarboxamide, are suggested to exist as U-shaped or square dimers, respectively. The ligands N,N'-bis(pyridin-3-yl)isophthalamide, 5, or N,N'-bis(pyridin-4-yl)isophthalamide, 6, give the complexes [[Pd(LL)(mu-5)](2)](CF(3)SO(3))(4) or [[Pd(LL)(mu-6)](2)](CF(3)SO(3))(4), but when LL = dppm or dppp, the zigzag polymers [[Pd(LL)(mu-6)](x)](CF(3)SO(3))(2)(x) are formed. When LL = dppp, a structure determination shows formation of a laminated sheet structure by hydrogen bonding between amide NH groups and triflate anions of the type NH-OSO-HN.

Stacked molecular triangles: self-assembly using coordination chemistry and hydrogen bonding

The combination of the cis-blocked platinum(II) unit (bu2bipy)Pt2+ with the unsymmetrical bis(pyridine) ligand 4-NC5H4C(O)NH-4-C5H4N gives the molecular triangle complex [{Pt(bu2bipy)(μ-4-NC5H4C(O)NH-4-C5H4N)}3]6+, which forms stacked pairs in the solid state through intertriangle NH⋯OC hydrogen bonds and Pt⋯OC secondary bonds.

Polygons and Faceted Polyhedra and Nanoporous Networks

Small rhombihexahedra and small cubicuboctahedra (see picture), namely faceted polyhedra, form the basis of two novel low-density framework solids that are afforded by the self-assembly of molecular squares only or molecular squares and triangles, respectively.