Jon A. McCleverty

Anion-Templated Assembly of a Supramolecular Cage Complex

The templating effect of the tetrafluoroborate ion leads to assembly of four CoII ions and six bridging ligands around this anion to give a tetrahedral complex with a bridging ligand along each edge and the anion trapped in the central cavity (shown below). Surprisingly under identical conditions but with NiII a simpler dinuclear complex forms.

Coordination and supramolecular chemistry of multinucleating ligands containing two or more pyrazolyl-pyridine ‘arms’

Abstract A wide variety of new ligands has been prepared in which two or three bidentate or terdentate chelating units, based on pyrazolyl-pyridine (bidentate) or pyrazolyl-bipyridine (terdentate) fragments, have been attached to a central spacer. This may be a borohydride unit to give bis- or tris(pyrazolyl)borate derivatives, or an aromatic (phenyl or biphenyl) unit. A general characteristic of these ligands is that they can coordinate all of their binding ‘arms’ to a single metal ion, giving mononuclear complexes, or they can coordinate each separate ‘arm’ to a different metal ion to give assembly of polynuclear species of occasionally quite unexpected structural complexity. We have observed self-assembly of helicates, helical rings, and tetrahedral cages, in some cases with non-coordinated anions playing a crucial templating role in the assembly process. The tetrahedral cages in particular, which have been prepared with both M4(μ3-L)4 and M4(μ2-L)6 stoichiometries, provide elegant examples of how high-symmetry structures can assemble from simple components.

Switching of molecular second-order polarisability in solution

Second-order nonlinear optical (NLO) polarisabilities or first hyperpolarisabilities, β, of molecules may be manipulated by reversibly modifying the properties of specific parts of active molecules, usually by on/off switching. This may involve reducing the donor capacity of the electron-rich fragment of a typical donor–acceptor species, D–A, by oxidation or protonation. Conversely, the acceptor behaviour of A may be altered by reduction, or by deprotonation. Alteration of the first hyperpolarisability may also involve structural or chemical modification of the bridging group, thereby interfering with the communication between D and A. Much of the article focuses on redox behaviour of NLO-active metal-containing compounds, mainly describing dipolar species in which the donor fragment containing a metal is oxidised, but giving examples of switching behaviour in octopolar species and of protonation/deprotonation behaviour.

Anion-templated self-assembly of tetrahedral cage complexes of cobalt(II) with bridging ligands containing two bidentate pyrazolyl-pyridine binding sites

The bridging ligands L1 and L2 contain two N,N-bidentate pyrazolyl-pyridine units linked to a central aromatic spacer unit (1,2-phenyl or 2,3-naphthyl, respectively). Reaction with Ni(II) salts and treatment with the anions tetrafluoroborate or perchlorate result in formation of dinuclear complexes having a 2:3 metal:ligand ratio, with one bridging and two terminal tetradentate ligands. In contrast, reaction of L1 and L2 with Co(II) salts, followed by treatment with tetrafluoroborate or perchlorate, results in assembly of cage complexes having a 4:6 metal:ligand ratio; these complexes have a metal ion at each corner of an approximate tetrahedron, and a bis-bidentate bridging ligand spanning each edge. The central cavity is occupied by a tetrahedral counterion that forms multiple hydrogen-bonding interactions with the methylene protons of the bridging ligands. The anionic guest fits tightly into the central cavity of the cage to which it is ideally complementary in terms of shape, size, and charge. Solution NMR experiments show that the central anion acts as a template for cage formation, with a mixture of Co(II) and the appropriate bridging ligand alone giving no assembly into a cage until the tetrahedral anion is added, at which point cage assembly is fast and quantitative. The difference between the structures of the complexes with Ni(II) and Co(II) illustrate how the uncoordinated anions can exert a profound influence on the course of the assembly process.

In situ reversible electrochemical switching of the molecular first hyperpolarizability

In situ reversible electrochemical switching of the molecular second-order nonlinear optical (NLO) polarizability, or first hyperpolarizability, has been implemented in a specially designed cell.The redox-switchable NLO chromophore is based on the octamethylferrocene/octamethylferrocenium redox system as electron-donor (D) group, in conjunction with nitrothiophene as the electron-acceptor (A) group and ethenyl as the p-conjugation bridge.This D- p-A chro- mophore has been shown to exhibit reversible redox switching of its linear and nonlinear optical properties.The im- portance and potential of this electrochemical switching of the first hyperpolarizability is discussed in the context of current and future applications of second-order NLO effects. 2002 Elsevier Science B.V. All rights reserved.

Synthesis of the new tripodal ligand tris-[3-(2′-pyridyl)pyrazol-1-yl]hydroborate, and the crystal structure of its europium(III) complex

The new tripodal ligand tris-[3-(2′-pyridyl)pyrazol-1-yl]hydroborate (L–), comprising three N,N-bidentate chelating arms linked by the apical boron atom, has been synthesized; the crystal structure of [EuL(MeOH)2F][PF6] reveals the nine-coordinate metal lying within the hexadentate ligand cavity.

An assessment of second harmonic generation by donor acceptor molecules containing stilbenyl or diarylazo bridges between ferrocenyl donor and nitro acceptor groups

Abstract The compounds [Fe(η 5 -C 5 H 5 )(η 5 -C 5 H 4- -Ar) (Ar = 4,4′-C 6 H 4 N 2 C 6 H 4 NO 2 ( 1 ), 4, 4−C 6 H 4 N 2 C 6 H 3 (2′-Me)NO 2 ( 2 ), 4,4′-C 6 H 3 (3−Me)N 2 C 6 H 3 (2′-Me)NO 2 ( 3 ) and trans -4,4′-C 6 H 4 (CHCH)C 6 H 4 NO 2 ( 4 ) have been synthesized and their second order non-linear optical properties assessed by use of the Kurtz powder test for second hartnonic generation on irradiation at 1907 nm. The stilbenyl compound, 4 , gave a signal of similar intensity to that of a urea standard but the others gave no detectable signal.

Anionengesteuerter Aufbau eines supramolekularen Käfigkomplexes

Der Templateffekt eines Tetrafluoroborat-Ions fuhrt dazu, das sich vier CoII-Ionen und sechs zweifach zweizahnige Liganden um dieses Ion zusammenlagern, wobei ein tetraedrischer Vierkernkomplex mit einem verbruckenden Liganden entlang jeder Kante entsteht und das Anion im zentralen Hohlraum eingeschlossen ist (siehe unten). Uberraschenderweise wird mit NiII unter gleichen Bedingungen ein einfacherer Zweikernkomplex erhalten.

Non-innocent behaviour in mononuclear and polynuclear complexes: consequences for redox and electronic spectroscopic properties

This article reviews recent work in the area of non-innocent behaviour in polynuclear metal complexes. Non-innocence, which occurs when metal-based and ligand-based redox orbitals are similar in energy, has been known since the first dithiolene complexes of the Ni triad. Our recent work in this field is with complexes of two distinct types: polynuclear complexes of Ru(II) with dioxolene-type bridging ligands; and dinuclear complexes based on tris(pyrazolyl)borato-Mo(V) or -Mo(I) units linked by bis-phenolate or bis-pyridyl bridging ligands. Detailed redox and UV/Vis/NIR spectroelectrochemical studies on these complexes have been carried out. An important point which emerges is that non-innocent behaviour in dinuclear complexes is an essential prerequisite for strong metal–metal electronic coupling across extended bridging ligands. Many of the complexes studied show intense charge-transfer transitions in the near-IR region of the spectrum, and the use of these in prototypical optical devices is briefly discussed.

Transition metal complexes with terminal or bridging imidato(1–) ligands. X-Ray crystal structures of trans-[Ir(CO)(NCOC2F4CO)(PPh3)2] and [{Pd(o-C6H4CHNPh)(µ-NCOC2H4CO)}2]·CH2Cl2, spectroscopic studies of [Mn(CO)5(NCOC2H4CO)], and the nature of the metal–nitrogen bond

The reaction of [Mn(CO)5Br] with the thallium(I) salt of succinimide yields the novel succinimidato(1–) complex [Mn(CO)5([graphic omitted]O)]. The imide-derived ligand is found to behave as a pseudo-halogen in terms of its σ-acceptor and π-donor properties by ‘Graham’ analysis of i.r. spectroscopic data. Mononuclear arylpalladium complexes, trans-[Pd(Ph)(im)(PPh3)2][im =Imidato(1–) ligand], and related carbonyl–rhodium and –iridium complexes, trans-[M (CO)(im)(PPh3)2], may be synthesised by reaction of the corresponding chloro-complexes with succinimide, phthalimide, or tetrafluorosuccinimide. The structure of trans-[Ir(CO)([graphic omitted]O)(PPh3)2] has been established crystallographically (Ir–N 2.09 A). Binuclear arylpalladium complexes containing halide or acetate bridges react with imides in the presence of base to give complexes in which the Imidato(1–) ligand adopts a novel bridging co-ordination mode, via nitrogen and one of the carbonyl oxygens. The structure of one complex of this type, [{Pd(o-C6H4CHNPh)(µ-[graphic omitted]O)}2]·CH2Cl2, has been confirmed crystallographically.