Abstract

Abstract

Three new ligands have been prepared in which two terdentate chelating pyrazolyl-bipyridine units are connected by a central aromatic spacer via methylene “hinges”: the spacers are o-phenylene (L^Ph), 2,6-pyridine-diyl (L^Py) and 2,3-naphthalenediyl (L^naph). The ligands act as potentially hexadentate bridging ligands, with the central pyridyl N atom of L^Py not involved in coordination. The following complexes were prepared and structurally characterised: [M₂(L^Ph)₂][ClO₄]₄ (M = Ni, Cu), which are dinuclear double helicates; [Ag₂(L^Ph)(MeCN)₂][BF₄]₂, a dinuclear complex with an Ag···Ag bond in which the ligand adopts a helical twist around the pair of metal ions; [Ni₂(L^Py)₂][BF₄]₄, an achiral “mesocate” with a box-like structure and a face-to-face arrangement of ligands; [Ag₃(L^naph)₂](BF₄)₃, which contains a linear trinuclear array of Ag^I ions with the two ligands arranged in a shallow helical twist, each ligand spanning one terminal and the central metal ion; and [Cd₆(L^naph)₆](ClO₄)₁₂, a cyclic hexanuclear helicate with a perchlorate anion in the central cavity. Both [Cu₂(L^Ph)₂][ClO₄]₄ and [Cd₆(L^naph)₆](ClO₄)₁₂, which have architecturally similar bridging ligands, show evidence by electrospray mass spectrometry for formation of a range of cyclic oligomers in solution up to 11-mers for the Cd^II/L^naph system.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

The complexes [Pt(bipy){CC-(4-pyridyl)}₂] (1) and [Pt(tBu₂bipy){CC-(4-pyridyl)}₂] (2) and [Pt(tBu₂-bipy)(CC-phen)₂] (3) all contain a Pt(bipy)(diacetylide) core with pendant 4-pyridyl (1 and 2) or phenanthroline (3) units which can be coordinated to {Ln(diketonate)₃} fragments (Ln = a lanthanide) to make covalently-linked Pt^II/Ln^III polynuclear assemblies in which the Pt^II chromophore, absorbing in the visible region, can be used to sensitise near-infrared luminescence from the Ln^III centres. For 1 and 2 one-dimensional coordination polymers [1⋅Ln(tta)₃]_∞ and [2⋅Ln(hfac)₃]_∞ are formed, whereas 3 forms trinuclear adducts [3⋅{Ln(hfac)₃}₂] (tta=anion of thenoyl-trifluoroacetone; hfac=anion of hexafluoroacetylacetone). Complexes 1–3 show typical Pt^II-based ³MLCT luminescence in solution at ≈510 nm, but in the coordination polymers [1⋅Ln(tta)₃]_∞ and [2⋅Ln(hfac)₃]_∞ the presence of stacked pairs of Pt^II units with short Pt⋅⋅⋅Pt distances means that the chromophores have ³MMLCT character and emit at lower energy (≈630 nm). Photophysical studies in solution and in the solid state show that the ³MMLCT luminescence in [1⋅Ln(tta)₃]_∞ and [2⋅Ln(hfac)₃]_∞ in the solid state, and the ³MLCT emission of [3⋅{Ln(hfac)₃}₂] in solution and the solid state, is quenched by PtLn energy transfer when the lanthanide has low-energy f–f excited states which can act as energy acceptors (Ln=Yb, Nd, Er, Pr). This results in sensitised near-infrared luminescence from the Ln^III units. The extent of quenching of the Pt^II-based emission, and the PtLn energy-transfer rates, can vary over a wide range according to how effective each Ln^III ion is at acting as an energy acceptor, with Yb^III usually providing the least quenching (slowest PtLn energy transfer) and either Nd^III or Er^III providing the most (fastest PtLn energy transfer) according to which one has the best overlap of its f–f absorption manifold with the Pt^II-based luminescence.

We have prepared a series of five ligands with potentially N,S-bidentate chelating arms derived from 3-[2-(methylsulfanyl)phenyl]pyrazole linked to central aromatic spacers by methylene units. Complexes with a variety of architectures have been obtained, including simple mononuclear complexes and polynuclear chain complexes. The p-xylyl-spaced ligand L¹ forms one-dimensional helical coordination polymers with copper(I) and silver(I) ions. These polymers display interligand aromatic stacking interactions within each helical chain. The m-xylyl-spaced ligand L² forms a coordination polymer with copper(I) but a mononuclear complex with the larger silver(I) ion in which the central phenyl ring is involved in an η¹ π-type Ag···C interaction with the Ag^I. The 3,3′-biphenyl-spaced ligand L³ also forms one-dimensional polymers with silver(I) and copper(I) ions, but in this case the sequence of bridging ligands between one metal centre and the next follows a zig-zag path rather than being helical. The 1,8-naphthyl-spaced ligand L⁴ only forms mononuclear complexes with copper(I) and silver(I) ions showing that this spacer is not large enough to enforce a bridging coordination mode. The three-armed ligand L⁵, prepared from 2,4,6-tris(bromomethyl)mesitylene, also forms a mononuclear complex with silver(I) ions, where one of the three arms is pendant. However, when excess silver(I) ions are present two of these mononuclear complexes can be assembled into the trinuclear complex [Ag₃(L⁵)₂](ClO₄)₃. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

A series of dinuclear platinum(II)–lanthanide(iii) complexes has been prepared in which a square-planar Pt^II unit, either [(PPh₃)₂Pt(pdo)] (H₂pdo=5,6-dihydroxyphenanthroline) or [Cl₂Pt(dppz)] [dppz=2,3-bis(2-pyridyl)pyrazine], is connected to a {Ln(dik)₃} unit (“dik”=a 1,3-diketonate ligand). The mononuclear complexes [(PPh₃)₂Pt(pdo)] and [Cl₂Pt(dppz)] both have external, vacant N,N-donor diimine-type binding sites that react with various [Ln(dik)₃(H₂O)₂] units to give complexes [(PPh₃)₂Pt(μ-pdo)Ln(tta)₃] (series A; Htta=thenoyltrifluoroacetone), [Cl₂Pt(μ-dppz)Ln(tta)₃] (series B); and [Cl₂Pt(μ-dppz)Ln(btfa)₃] (series C; Hbtfa=benzoyltrifluoroacetone); in all of these the lanthanide centres are eight-coordinate. The lanthanides used exhibit near-infrared luminescence (Nd, Yb, Er). Crystal structures of members of each series are described. In all complexes, excitation into the Pt-centred absorption band (at 520 nm for series A complexes; 440 nm for series B and C complexes) results in characteristic near-IR luminescence from the Nd, Yb or Er centres in both the solid state and in CH₂Cl₂, following energy-transfer from the Pt antenna chromophore. This work demonstrates how d-block-derived chromophores, with their intense and tunable electronic transitions, can be used as sensitisers to achieve near-infrared luminescence from lanthanides in suitably designed heterodinuclear complexes based on simple bridging ligands.