Three-component coordination networks based on [Ru(phen)(CN)4]2? anions, near-infrared luminescent lanthanide(iii) cations, and ancillary oligopyridine ligands: structures and photophysical properties

Mononuclear Rare-Earth Metalloligands Exploiting a Divergent Ligand

Rare-earth tris-diketonato [RE(dike)3pyterpy] metalloligands can be prepared reacting at room temperature [RE(dike)3dme] (dme = 1,2-dimethoxyethane; dike = tta with Htta = 2-thenoyltrifluoroacetone and RE = La, 1; Y, 2; Eu, 3; Dy, 4; or dike = hfac with Hhfac hexafluoroacetylacetone, and RE = Eu, 5; Tb, 6; Yb 7) with 4'-(4‴-pyridil)-2,2':6',2″-terpyridine (pyterpy). The molecular structures of 1, 5, 6, and 7 have been studied through single-crystal X-ray diffraction showing mononuclear neutral complexes with the rare-earth ion in coordination number nine and with a muffin-like coordination geometry. [RE(tta)3pyterpy] promptly reacts with [M(tta)2dme] with formation of [Mpyterpy2][RE(tta)4]2 (M = Zn, RE = Y, 8; M = Co, RE = Dy, 9). Consistently, [Zn(hfac)2dme] reacts at room temperature with 2 equiv of pyterpy yielding [Znpyterpy2][hfac]2 10 that easily can be transformed by reaction with 2 equiv of [Eu(hfac)3] in [Znpyterpy2][Eu(hfac)4]2 11 that has been structurally characterized. Finally, 1, 2, 3, 5, and 7 metalloligands react at room temperature in few minutes with [PtCl(μ-Cl)PPh3]2 yielding the heterometallic molecular complexes [RE(dike)3pyterpyPtCl2PPh3] (dike = tta, RE = La, 12; Y, 13; Eu; 14; dike = hfac, RE = Eu, 15; Yb, 16).

Tunable magnetic anisotropy in luminescent cyanido-bridged {Dy2Pt3} molecules incorporating heteroligand PtIV linkers

The interest in the generation of photoluminescence in lanthanide(III) single-molecule magnets (SMMs) is driven by valuable magneto-optical correlations as well as perspectives toward magnetic switching of emission and opto-magnetic devices linking SMMs with optical thermometry. In the pursuit of enhanced magnetic anisotropy and optical features, the key role is played by suitable ligands attached to the 4f metal ion. In this context, cyanido complexes of d-block metal ions, serving as expanded metalloligands, are promising. We report two novel discrete coordination systems serving as emissive SMMs, {[Dy^III(H₂O)₃(tmpo)₃]₂[Pt^IVBr₂(CN)₄]₃}·2H₂O (1) and {[Dy^III(H₂O)(tmpo)₄]₂[Pt^IVBr₂(CN)₄]₃}·2CH₃CN (2) (tmpo = trimethylphosphine oxide), obtained by combining Dy^III complexes with uncommon dibromotetracyanidoplatinate(IV) ions, [Pt^IVBr₂(CN)₄]^2-. They are built of analogous Z-shaped cyanido-bridged {Dy₂Pt₃} molecules but differ in the coordination number of Dy^III (C.N. = 8 in 1, C.N. = 7 in 2) and the number of coordinated tmpo ligands (three in 1, four in 2) which is related to the applied solvents. As a result, both compounds reveal Dy^III-centred slow magnetic relaxation but only 1 shows SMM character at zero dc field, while 2 is a field-induced SMM. The relaxation dynamics in both systems is governed by the Raman relaxation mechanism. These effects were analysed using ac magnetic data and the results of the ab initio calculations with the support of magneto-optical correlations based on low-temperature high-resolution emission spectra. Our findings indicate that heteroligand halogeno-cyanido Pt^IV complexes are promising precursors for emissive SMMs with the further potential of sensitivity to external stimuli that may be related to the lability of the axially positioned halogeno ligands.

Octacyanidometallates for multifunctional molecule-based materials

Octacyanidometallates have been successfully employed in the design of heterometallic coordination systems offering a spectacular range of desired physical properties with great potential for technological applications. The [M(CN)8]n- ions comprise a series of complexes of heavy transition metals in high oxidation states, including NbIV, MoIV/V, WIV/V, and ReV. Since the discovery of the pioneering bimetallic {MnII4[MIV(CN)8]2} and {MnII9[MV(CN)8]6} (M = Mo, W) molecules in 2000, octacyanidometallates were fruitfully explored as precursors for the construction of diverse d-d or d-f coordination clusters and frameworks which could be obtained in the crystalline form under mild synthetic conditions. The primary interest in [M(CN)8]n--based networks was focused on their application as molecule-based magnets exhibiting long-range magnetic ordering resulting from the efficient intermetallic exchange coupling mediated by cyanido bridges. However, in the last few years, octacyanidometallate-based materials proved to offer varied and remarkable functionalities, becoming efficient building blocks for the construction of molecular nanomagnets, magnetic coolers, spin transition materials, photomagnets, solvato-magnetic materials, including molecular magnetic sponges, luminescent magnets, chiral magnets and photomagnets, SHG-active magnetic materials, pyro- and ferroelectrics, ionic conductors as well as electrochemical containers. Some of these materials can be processed into the nanoscale opening the route towards the development of magnetic, optical and electronic devices. In this review, we summarise all important achievements in the field of octacyanidometallate-based functional materials, with the particular attention to the most recent advances, and present a thorough discussion on non-trivial structural and electronic features of [M(CN)8]n- ions, which are purposefully explored to introduce desired physical properties and their combinations towards advanced multifunctional materials.

Wide-Range UV-to-Visible Excitation of Near-Infrared Emission and Slow Magnetic Relaxation in LnIII(4,4'-Azopyridine-1,1'-dioxide)[CoIII(CN)6]3- Layered Frameworks

Trivalent lanthanide ions combined with two molecular linkers, organic 4,4'-azopyridine-1,1'-dioxide (apdo), and inorganic hexacyanidocobaltate(III), gave a series of magnetoluminescent coordination polymers, [{Ln^III(apdo)(H₂O)₄}{Co^III(CN)₆}]·2H₂O (Ln = Nd, 1; Tb, 2; Dy, 3; Er, 4; Tm, 5; Yb, 6). They are hybrid organic-inorganic layered frameworks composed of cyanido-bridged {Ln₂(μ-NC)₄Co₂} squares linked by Ln-apdo-Ln bridges into a coordination network of a mixed 4- and 8-metal ring topology. Lanthanide(III) complexes, [Ln^III(μ-apdo)₂(H₂O)₄(μ-NC)₂]⁺, of a distorted dodecahedral geometry are isolated by diamagnetic [Co^III(CN)₆]^3- and apdo linkers. As a result, 1-6 reveal field-induced slow relaxation of magnetization, with typical temperature-dependent relaxation of a single-ion origin for Nd^III-containing 1, Dy^III-containing 3, and Yb^III-containing 6. The related alternate-current magnetic data were precisely analyzed, indicating the multiple magnetic relaxation pathways, including a direct process, strong quantum tunneling of magnetization, non-negligible Raman processes, and crucial two-phonon Orbach thermal relaxation. The thermal energy barriers of the Orbach process, Δ E/ k_B, are 15.1(9) K with τ₀ = 9.8(9) × 10^-6 s at H_dc = 4500 Oe, 16.1(8) K with τ₀ = 9.0(9) × 10^-5 s at H_dc = 1500 Oe, and 17.3(6) K with τ₀ = 3.2(7) × 10^-6 s at H_dc = 700 Oe, for 1, 3, and 6, respectively, proving the single-molecule magnet (SMM) behavior. Because of the presence of [Co(CN)₆]^3-, 1-6 show strong UV absorption, while the chromophoric apdo leads to the strong absorption in the visible range. As a result, the visible 4f/3d metal-centered emission is quenched, but the near-infrared luminescence from Nd^III and Yb^III is observed in 1 and 6, respectively. It is realized by Co-to-Ln metal-to-metal, and apdo-to-Ln ligand-to-metal energy transfers; thus, broad UV-to-visible excitation can be explored. Compounds 1-6 form a novel family of functional bimetallic assemblies, incorporating NIR-emissive SMMs as presented for NdCo (1) and YbCo (6) derivatives.

Sensitized near infrared emission through supramolecular d → f energy transfer within an ionic Ru(ii)-Er(iii) pair

The newly synthesized ionic triple salt Ru-Er, {[Ru^II(bpy)₂(dbim)][Er^III(hfac)₄][CF₃COO]·H₂O} (bpy = 2,2'-bipyridine; hfac^- = hexafluoroacetylacetonate; dbim = 2,2'-dibenzimidazole) exhibits near-infrared (NIR) emission at 1535 nm by intermolecular Ru → Er (d → f) energy transfer across supramolecular interactions when pumped within the Ru(ii) ³MLCT band. It is the first such observation for a transition metal-lanthanide ionic pair.

Hybrid organic–inorganic connectivity of NdIII(pyrazine-N,N′-dioxide)[CoIII(CN)6]3− coordination chains for creating near-infrared emissive Nd(iii) showing field-induced slow magnetic relaxation

Organic pzdo and inorganic hexacyanidocobaltate(iii) support the formation of Nd(iii) complexes showing NIR fluorescence and magnetic anisotropy.

Lanthanide Photoluminescence in Heterometallic Polycyanidometallate-Based Coordination Networks

Solid-state functional luminescent materials arouse an enormous scientific interest due to their diverse applications in lighting, display devices, photonics, optical communication, low energy scintillation, optical storage, light conversion, or photovoltaics. Among all types of solid luminophors, the emissive coordination polymers, especially those based on luminescent trivalent lanthanide ions, exhibit a particularly large scope of light-emitting functionalities, fruitfully investigated in the aspects of chemical sensing, display devices, and bioimaging. Here, we present the complete overview of one of the promising families of photoluminescent coordination compounds, that are heterometallic d–f cyanido-bridged networks composed of lanthanide(3+) ions connected through cyanide bridges with polycyanidometallates of d-block metal ions. We are showing that the combination of cationic lanthanide complexes of selected inorganic and organic ligands with anionic homoligand [M(CN)_x]ⁿ⁻ (x = 2, 4, 6 and 8) or heteroligand [M(L)(CN)₄]²⁻ (L = bidentate organic ligand, M = transition metal ions) anions is the efficient route towards the emissive coordination networks revealing important optical properties, including 4f-metal-centred visible and near-infrared emission sensitized through metal-to-metal and/or ligand-to-metal energy transfer processes, and multi-coloured photoluminescence switchable by external stimuli such as excitation wavelength, temperature, or pressure.

Development of Ion-Conductive and Vapoluminescent Porous Coordination Polymers Composed of Ruthenium(II) Metalloligand

We synthesized two new porous coordination polymers (PCPs) {Ln₇(OH)₅[Ru(dcbpy)₃]₄·4nH₂O} (Ln₇-Ru₄; Ln = Ce, Nd) composed of the luminescent ruthenium(II) metalloligand [Ru(4,4'-dcbpy)₃]^4- ([4Ru]; 4,4'-dcbpy = 4,4'-dicarboxy-2,2'-bipyridine) and lanthanide ions Ln³⁺ (Ln = Ce, Nd). These two PCPs Ln₇-Ru₄ are isomorphous with the previously reported PCP La₇-Ru₄, and the lattice constants (a, c, and unit cell volume V) changed systematically according to the lanthanide contraction. All three Ln₇-Ru₄ compounds have OH^- anion containing porous structures and a large number of hydrate water molecules within the pores, resulting in moderate ion conductivities (10^-6-10^-7 S cm^-1) at 90% relative humidity (RH) and 298 K. In contrast, the structural transformation of Ln₇-Ru₄ associated with water-vapor adsorption/desorption strongly depends on the lanthanide ion; the Ln₇-Ru₄ compounds with larger Ln³⁺ ions recover the original porous structure at lower relative humidities (RH). A similar trend was observed for the ion conduction activation energy, suggesting that the bridging Ln³⁺ ion plays an important role in the formation of the ion-conductive pathways. La₇-Ru₄ and Ce₇-Ru₄ exhibit vapochromic luminescence associated with water vapor adsorption/desorption, arising from the ³MLCT emission of [4Ru]. This vapochromic behavior is also affected by the replacement of the Ln³⁺ ion; the vapochromic shift of Ce₇-Ru₄ was observed at RH values (near 100% RH) higher than that of La₇-Ru₄. ³MLCT emissions of the [4Ru] metalloligand in Nd₇-Ru₄ were barely observable in the visible region, but sharp emission bands characteristic of 4f-4f transitions of the Nd³⁺ ion were observed in the near-infrared (NIR) region (arising from the ¹MLCT transition of [4Ru]), suggesting the transfer of energy from the [4Ru] ³MLCT excited state to the 4f-4f transition state of the Nd³⁺ ions.

Self-assembly of NIR luminescent 30-metal drum-like and 12-metal rectangular d-f nanoclusters with long-chain Schiff base ligands

Two classes of heterobimetallic d-f nanoclusters [Ln6Cd24(L(1))11(OAc)43(OH)] and [Ln4Zn8(L(2))2(OAc)20(OH)4] (Ln = Nd and Yb) were prepared using flexible long-chain Schiff base ligands which have (CH2)6 backbones. Their NIR luminescence properties were determined.

The Utility of 2,2′-Bipyrimidine in Lanthanide Chemistry: From Materials Synthesis to Structural and Physical Properties

This paper reviews the recent investigations undertaken on the use of 2,2′-bipyrimidine (bpm) as a ligand for designing molecular complexes as well as polymeric lanthanide materials. A special emphasis is put on the ability of this polydentate neutral ligand to yield compounds of various dimensionalities, to act as a connector between these large ions, and influence their emissive and magnetic properties. This ligand can adopt a terminal or a bridging coordination mode with lanthanide ions, thus generating a wealth of frameworks of various topologies with the 4f elements. The main focus of this review is to show the originality brought by bpm in lanthanide structural chemistry and solid-state photophysics and magnetism.

catena-poly[[[(2,2'-bipyridine-2κN,N')-μ-cyanido-1:2κN:C-cyanido-2κC-tris-(methanol-1κO)(nitrato-1κO,O')iron(II)yttrium(III)]-di-μ-cyanido-1:2'κN:C;2:1'κC:N] methanol solvate hemihydrate]

The title complex, {[Fe^IIY^III(CN)₄(NO₃)(C₁₀H₈N₂)(CH₃OH)₃]·CH₃OH·0.5H₂O}_n, is built up of ladder-like chains oriented along the c axis. Each ladder consists of two strands based on alternating Fe^II and Y^III ions connected by cyanide bridges. Two such parallel chains are connected by additional cyanide anions (the ‘rungs’ of the ladder), which likewise connect Fe^II and Y^III ions, such that each [Fe(bipy)(CN)₄]²⁻ (bipy is 2,2′-bipyridine) unit coordinates with three Y^III ions and each Y^III ion connects with three different [Fe(bipy)(CN)₄]²⁻ units. The Fe^II atom is six-coordinated in a distorted octa­hedral geometry and the Y^III atom cation is eight-coordinated in a distorted dodeca­hedral environment. The uncoordinated methanol solvent mol­ecules are involved in hydrogen-bonding inter­actions with the one terminal cyanide group and a coordinated methanol mol­ecule from another [Y^III(NO₃)(CH₃OH)₃]²⁺ unit. Adjacent ladder-like chains are also held together by hydrogen bonds between the terminal cyanide ligands of the [Fe(CN)₄(bipy)]²⁻ units in one chain and the OH donors of CH₃OH ligands from [Y^III(NO₃)(CH₃OH)³] units in neighboring chains. The water molecule exhibits half-occupation.

Syntheses, structures and photophysical properties of tetranuclear Cd-Ln coordination complexes

Four novel hetero-tetranuclear d-f coordination complexes, [Ln2Cd2(p-toluylate)10(phen)2] (Nd, 1; Pr, 2; Sm, 3) and [Ho2Cd2(p-chloro-benzoate)10(phen)2](4) (phen = 1, 10-phenanthroline), are synthesized by the hydrothermal method, and their structures are studied by single-crystal X-ray diffraction. Structures 1-3 are isomorphous and structure 4 has a similar molecular structure. In the near-infrared (NIR) region or in the visible region, complexes 1-4 show the characteristic emission bands of Ln(III) ions, which are attributed to the sensitization from the ligands and d-block (cadmium-ligand section) to f-block (lanthanide-ligand section). Moreover, in the Cd-Ln complexes, the CdLn separation is very close, so the d-orbital of the Cd(II) ion and the f-orbital of the Ln(III) ion may influence each other, which probably cause the intra-levels to be adjusted. This can have a significant effect on the emission bands in the NIR region, hence the corresponding emission bands of complexes 1, 2 and 4 exhibit shift or splitting compared with isolated Ln(III) ions. At the same time in the UV-Vis-NIR absorption spectra of the complexes, the corresponding absorption bands also show shift or splitting. The shift or splitting of bands in the NIR emission spectra and in the absorption spectra can evidence each other.

Near IR-emitting DNA-probes exploiting stepwise energy transfer processes

The synthesis and characterisation of two new cyclen-based near IR-emitting lanthanide complexes is reported; the lanthanides are sensitised by rhodamine, which in turn is excited by energy transfer from a coumarin 2 moiety. The three lumophores function as an energy transfer cascade spanning the UV-visible-near IR region of the spectrum, resulting in large Stokes shifts. Double stranded DNA selectively switches one of the two energy transfer processes off, enabling luminescent DNA-sensing in the near IR region. The regioselective di-alkylation of the cyclen scaffold is explained with the help of DFT calculations.