David J. Bray

Self-assembled Metallo-supramolecular Systems Incorporating β-Diketone Motifs as Structural Elements

Publisher Summary This chapter discusses the synthesis and properties of metal-containing molecular architectures incorporating β-diketonate motifs. The chapter focuses on the systems in which the use of designed metal-ion-directed assembly is employed for constructing new supramolecular entities. The simplest β-diketone ligand is 2,4-pentanedione (acetylacetone, acacH). β-diketones such as acacH show a tendency to form neutral complexes—with the geometry adopted normally reflecting the preferred geometry of the metal ion involved. In the case of cobalt(II), tetrahedral complexes can also result if a bulky β-diketonate ion is used. The ability to form higher coordinate species can be tuned through variation of the terminal R groups on the β-diketone. Reflecting the available relative orientations of the β-diketonate groups in each ligand type, a range of both dinuclear (rectangular) and trinuclear (triangular) “platform-like” products are isolated and characterized. In the lanthanum-containing complex, the lanthanum(III) ion is eleven coordinate being bound by the six “internal” oxygens from the β-diketonate groups, the three pyridyl nitrogens, a tetrahydrofuran group, and a water molecule.

Synthetic, structural, electrochemical and solvent extraction studies of neutral trinuclear Co(II), Ni(II), Cu(II) and Zn(II) metallocycles and tetrahedral tetranuclear Fe(III) species incorporating 1,4-aryl-linked bis-beta-diketonato ligands.

Uncharged complexes, formulated as trimeric metallocycles of type [M3(L(1))3(Py)6] (where M = cobalt(II), nickel(II) and zinc(II) and L(1) is the doubly deprotonated form of a 1,4-phenylene linked bis-beta-diketone ligand of type 1,4-bis(RC(O)CH2C(O))C6H4 (R = t-Bu)) have been synthesised, adding to related, previously reported complexes of these metals with L(1) (R = Ph) and copper(ii) with L(1) (R = Me, Et, Pr, t-Bu, Ph). New lipophilic ligand derivatives with R = hexyl, octyl or nonyl were also prepared for use in solvent extraction experiments. The X-ray structures of H2L(1) (R = t-Bu) and of its trinuclear (triangular) nickel(II) complex [Ni3(L(1))3(Py)6].3.5Py (R = t-Bu) are also presented. Electrochemical studies of H2L(1), [Co3(L(1))3(Py)6], [Ni3(L(1))3(Py)6], [Cu3(L(1))3], [Zn3(L(1))3(Py)6] and [Fe4(L(1))6] (all with R = t-Bu) show that oxidative processes for the complexes are predominantly irreversible, but several examples of quasireversible behaviour also occur and support the assignment of an anodic process, seen between +1.0 and +1.6 V, as involving metal-centred oxidations. The reduction behaviour for the respective metal complexes is not simple, being irreversible in most cases. Solvent extraction studies (water/chloroform) involving the systematic variation of the metal, bis-beta-diketone and heterocyclic base concentrations have been performed for cobalt(II) and zinc(II) using a radiotracer technique in order to probe the stoichiometries of the respective extracted species. Significant extraction synergism was observed when 4-ethylpyridine was also present with the bis-beta-diketone ligand in the chloroform phase. Competitive extraction studies demonstrated a clear uptake preference for copper(II) over cobalt(II), nickel(II), zinc(II) and cadmium(II).

Assembly of a trinuclear metallo-capsule from a tripodal tris(β-diketone) derivative and copper(II)

A new metallo-capsule has been synthesised that consists of three copper(II) ions and two molecules of a tris-deprotonated tripodal ligand in which three 2,4-pentanedione groups are linked via their gamma-carbons through thioether spacers to the 1,3,5-positions of a triazine core.

New bis‐Pyrazole Derivatives Synthesized From Aryl‐ and Xylyl‐Linked bis(β‐Diketone) Precursors

Abstract The synthesis and characterization of six new aryl‐linked, and in one instance xylyl‐linked, bis‐pyrazoles are reported. The X‐ray structures of two of these derivatives are also presented.

Liquid-liquid extraction studies with bis-β-diketones

1,4- (H₂L¹) and 1,3-phenylene (H₂L²) linked bis-β-diketone ligands as well as corresponding metal complexes of composition M₃(L¹)₃B₆ and M₂(L²)₂B₂ with B = pyridine (Py) or 4-ethylpyridine (EtPy) have been synthesised. Extraction studies involving the systematic variation of the metal, bis-β-Bdiketone and 4-ethylpyridine concentrations have been performed for Co(Il) and Zn(II) in order to probe the solution stoichiometries of the extracted species. Significant synergistic effects were observed when EtPy was present along with the bis-β-diketone ligand in the organic (CHCI₃) phase. Competitive extraction studies (water/chloroform) demonstrated a clear uptake preference for Cu(II) over Co(II), Ni(II), Zn(Il) and Cd(II).

An unprecedented bridging [Ag2(NO3)6]4− anion as a component of an infinite silver(I) molecular ladder incorporating a dinuclear cationic silver complex of a bis-dipyridylamine ligand

The interaction of silver(I) nitrate with the polypyridyl ligand (L) formed from two dipyridylamine units joined at their secondary amines by a 2,6-lutidinyl bridge results in a molecular ladder motif that consists of pairs of cationic [Ag2(L)MeCN]2+ units bridged by an unprecedented [Ag2(NO3)6]4− anion.

Synthesis and co-crystallisation behaviour of copper(II) complexes of two isomeric p-tolyl-terpyridines

Copper(II) complexes incorporating the isomeric tolyl-derivatised terpyridine ligands, 4′-p-tolyl-2,2′:6′,2′′-terpyridine (L1) and 6′-p-tolyl-2,2′:2′′,4′-terpyridine (L2) have been prepared and characterised by X-ray diffraction. The first of these is a co-crystal of type [Cu(L1)(NO3)2]·[Cu(L1)(NO3)(EtOH)]NO3·MeOH while the second is a single complex of type [Cu(L2)2(NO3)]NO3·0.5MeOH·1.5H2O. Crystallisation of a mixture of both products from ethanol/methanol (1:1) yields an unusual co-crystalline product of stoichiometry [Cu(L2)2NO3]2[Cu(L1)(NO3)2](NO3)2 whose structure was also confirmed by an X-ray stucture determination. §Dedicated to Professor Peter Williams in honour of his contribution to chemistry over many years.

Assembly of a tri-silver metallo-capsule incorporating a tripodal tris-pyridyl ligand

A trinuclear metallo-capsule has been assembled from a new tripodal pyridyl ligand and three silver(I) ions; the X-ray structure shows the presence of a Ag-Ag interaction in the solid state giving rise to a non-symmetric capsule arrangement while NMR evidence indicates that the structure in solution is symmetrical.

Cobalt(II), iron(II), zinc(II) and palladium(II) complexes of di-topic 4′-{4-[bis(2-pyridyl)aminomethyl]phenyl}-2,2′:6′,2′′-terpyridine. Synthetic and X-ray structural studies

The di-topic terpy/dpa hybrid ligand, 4′-{4-[bis(2-pyridyl)aminomethyl]phenyl}-2,2′:6′,2′′-terpyridine (L), incorporating both 2,2′:6′,2′′-terpyridine (terpy) and 2,2′-dipyridylamino domains has been synthesised. The metal complexes [CoLCl2], [ZnLCl2], [ZnL(CH3CO2)2], [Fe(L)2(PF6)2] and [Fe(L)(HL)](PF6)0.5(ClO4)2.5 have been isolated. X-ray structures of all five structures show that the respective metal ions preferentially bind to the terpy site of L, with the dpa site in the first four complexes remaining empty. In the fifth complex, which was isolated following the attempted reaction of silver perchlorate with [Fe(L)2(PF6)2], one dpa site is protonated. The crystal packing of all these species is dominated by extensive π-interactions. The 5-coordinate zinc(II) and cobalt(II) complexes are isostructural and exhibit a 3D network of interactions involving π-stacking as well as CH⋯Cl and CH⋯N hydrogen bonding. Similar interactions occur in the packing of [ZnL(CH3CO2)2] and [Fe(L)(HL)](PF6)0.5(ClO4)2.5 giving rise to 3D arrays. Reaction of palladium(II) with L yields a product of stoichiometry Pd2LCl4 for which the evidence suggests that palladium binds at both the terpy and dpa domains. Based on this result, addition of [Pd(CH3CN)2Cl2] to [Fe(L)2](PF6) in acetonitrile yielded a heterometallic complex in which the “free” dpa sites of [Fe(L)2](PF6) are now occupied by palladium ions to form a heterometallic product of type [FePd2(L)2Cl4](PF6)2·1.5CH3CN.

Selective Solvent Extraction of Silver(I) by Tris-Pyridyl Tripodal Ligands and X-Ray Structure of a Silver(I) Coordination Polymer Incorporating One Such Ligand

Two tripodal ligands, each derived from 1,1,1-tris(hydroxymethyl)ethane and terminated respectively by 4-pyridyl (L1) and 2-pyridyl groups (L2), have been synthesised. Competitive seven-metal extraction studies (H2O/CHCl3) incorporating equal concentrations of cobalt(ii), nickel(ii), copper(ii), zinc(ii), silver(i), cadmium(ii), and lead(ii) in the aqueous phase and L1 or L2 in the organic phase showed selective extraction of silver(i) in each case. A parallel solvent extraction experiment involving a related tripodal tris-pyridyl ligand (L3) based on a 1,3,5-substituted aryl ring scaffold and incorporating thioether sulfurs in each tripod arm also showed extraction selectivity for silver(i); extraction efficiencies towards this metal ion fall in the order L3 > L1 > L2. Physical data are in accord with L1 forming a capsule-like complex of type [Ag3L12]3+ in which silver ions link pairs of pyridyl groups from different ligands. In contrast, L2 yields a complex of type [Ag2L2(NO)3]n whose X-ray structure showed it to be a two-dimensional coordination polymer in which the three pyridyl donors of each L2 coordinate to three silver(i) centres, two of which are crystallographically distinct, with the centres also bonded to bidentate and/or bridging bidentate nitrato groups.