Homodinuclear Lanthanide Complexes with the Divergent Heterotopic 4,4′‐Bipyridine N ‐Oxide (bipyMO) Ligand

Sandwich d/f Heterometallic Complexes [(Ln(hfac)3)2M(acac)3] (Ln = La, Pr, Sm, Dy and M = Co; Ln = La and M = Ru)

Sandwich d/f heterometallic complexes [(Ln(hfac)3)2M(acac)3] (Ln = La, Pr, Sm, Dy and M = Co; Ln = La and M = Ru) were prepared in strictly anhydrous conditions reacting the formally unsaturated fragment [Ln(hfac)3] and [M(acac)3] in a 2-to-1 molar ratio. These heterometallic complexes are highly sensitive to moisture. Spectroscopic observation revealed that on hydrolysis, these compounds yield dinuclear heterometallic compounds [Ln(hfac)3M(acac)3], prepared here for comparison purposes only. Quantum mechanical calculations supported, on the one hand, the hypothesis on the geometrical arrangement obtained from ATR-IR and NMR spectra and, on the other hand, helped to rationalize the spontaneous hydrolysis reaction.

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).

Aluminium 8-Hydroxyquinolinate N-Oxide as a Precursor to Heterometallic Aluminium-Lanthanide Complexes

A reaction in anhydrous toluene between the formally unsaturated fragment [Ln(hfac)3] (Ln3+ = Eu3+, Gd3+ and Er3+; Hhfac = hexafluoroacetylacetone) and [Al(qNO)3] (HqNO = 8-hydroxyquinoline N-oxide), here prepared for the first time from [Al(OtBu)3] and HqNO, affords the dinuclear heterometallic compounds [Ln(hfac)3Al(qNO)3] (Ln3+ = Eu3+, Gd3+ and Er3+) in high yields. The molecular structures of these new compounds revealed a dinuclear species with three phenolic oxygen atoms bridging the two metal atoms. While the europium and gadolinium complexes show the coordination number (CN) 9 for the lanthanide centre, in the complex featuring the smaller erbium ion, only two oxygens bridge the two metal atoms for a resulting CN of 8. The reaction of [Eu(hfac)3] with [Alq3] (Hq = 8-hydroxyquinoline) in the same conditions yields a heterometallic product of composition [Eu(hfac)3Alq3]. A recrystallization attempt from hot heptane in air produced single crystals of two different morphologies and compositions: [Eu2(hfac)6Al2q4(OH)2] and [Eu2(hfac)6(µ-Hq)2]. The latter compound can be directly prepared from [Eu(hfac)3] and Hq at room temperature. Quantum mechanical calculations confirm (i) the higher stability of [Eu(hfac)3Al(qNO)3] vs. the corresponding [Eu(hfac)3Alq3] and (ii) the preference of the Er complexes for the CN 8, justifying the different behaviour in terms of the Lewis acidity of the metal centre.

Competing excitation paths in luminescent heterobimetallic Ln-Al complexes: Unraveling interactions via experimental and theoretical investigations

The interest for heterometallic lanthanide-d or-p metal (Ln-M) complexes is growing because of a potential cooperative or synergistic effect related to the proximity of two different metals in the same molecular architecture affording special tunable physical properties. To exploit the potentiality of Ln-M complexes, suitable synthetic approaches, and the in-depth understanding of the effect of each building block on their properties are mandatory. Here, we report the study on a family of heterometallic luminescent complexes [Ln(hfac)₃Al(L)₃], Ln= Eu³⁺ and Tb³⁺. Using different L ligands, we investigated the effect of the steric and electronic properties of the Al(L)₃ fragment, highlighting the general validity of the employed synthetic route. A marked difference in the light emission of [Eu(hfac)₃Al(L)₃] and [Tb(hfac)₃Al(L)₃] complexes has been observed. Thanks to photoluminescence experiments and Density Functional Theory calculations, Ln³⁺ emissions are explained with a model involving two non-interacting excitation paths through hfac or Al(L)₃ ligands.

Size Selectivity in Heterolanthanide Molecular Complexes with a Ditopic Ligand

The similar reactivity of lanthanides generally leads to statistically populated polynuclear complexes, making the rational design of ordered hetero-lanthanide compounds extremely challenging. Here we report on the site selectivity in hetero-lanthanide tetranuclear complexes afforded by the relatively simple ditopic pyterpyNO ligand (4'-(4-pyridil)-2,2':6',2"-terpyridine N-oxide). The sequential room temperature reaction of RE2 (tta)6 (pyterpyNO)2 (where RE=Y, (1); Eu, (2), Dy, (3) Htta=2-thenoyltrifluoroacetone) with La(tta)3 dme (dme=dimethoxyethane) yielded Y2 La2 (tta)12 (pyterpyNO)2 (4), Dy2 La2 (tta)12 (pyterpyNO)2 (5) and Eu2 La2 (tta)12 (pyterpyNO)2 (6). Single crystals X-ray diffraction studies showed that 4, 5 and 6 are isostructural, featuring a tetranuclear structure with two different metal coordination sites with coordination numbers 8 (CN8) and 9 (CN9). The two smaller cations are mainly bridged by the O-donor atoms of the NO groups of two pyterpyNO ligands (CN8), while the larger lanthanum centres are bound by a terpyridine unit (CN9). Size selectivity has been studied with structural and magnetic studies in the solid state and through 19 F NMR and photoluminescence studies in solution, showing a direct dependence on the difference of ionic radii of the ions and yielding a 91 % selectivity for 4. Furthermore, 19 F NMR, X-ray and PL studies pointed out that the nature of the product is independent from the synthetic route for compound Eu2 Y2 (tta)12 (pyterpyNO)2 (7), keeping the ion selectivity also for a self-assembly reaction. Unexpectedly, these studies have evidenced that selectivity is not exclusively governed by electrostatic interactions related to size dimensions.

Stoichiometrically Controlled Assembly of Lanthanide Molecular Complexes of the Heteroditopic Divergent Ligand 4'-(4-Pyridyl)-2,2':6',2″-terpyridine N-Oxide in Hypodentate or Bridging Coordination Modes. Structural, Magnetic, and Photoluminescence Studies

Mononuclear rare-earth tris-β-diketonato complexes RE(tta)₃dme [RE = Y (1), La (2), Dy (3), or Eu (4); Htta = 2-thenoylacetone; dme = 1,2-dimethoxyethane] react cleanly at room temperature in a 1:1 molar ratio with the heteroditopic divergent ligand 4'-(4-pyridyl)-2,2':6',2″-terpyridine N-oxide (pyterpyNO) to yield RE₂(tta)₆(pyterpyNO)_n, where n = 2 for RE = Y (5), Dy (6), or Eu (7) and n = 3 for RE = La (8). The crystal structure of 5 revealed a dinuclear compound with two pyterpyNO's bridging through the oxygen atom in a hypodentate mode leaving the terpyridine moieties uncoordinated. Using a metal:pyterpyNO molar ratio of 2 for RE = Y (9), Dy (10), or Eu (11), it was possible to isolate the molecular complexes RE₄(tta)₁₂(pyterpyNO)₂, while using a 5:3 molar ratio, the product La₅(tta)₁₂(pyterpyNO)₃ (12) can be obtained. ⁸⁹Y nuclear magnetic resonance spectroscopy revealed two different yttrium centers at room temperature for 9. An X-ray diffraction study of 10 showed a symmetrical tetranuclear structure resulting from the coordination of two Dy(tta)₃ fragments to the two hypodentate terpyridines of the dinuclear unit and presenting two different coordination sites for metals with coordination numbers of 8 and 9. Magnetic studies of 6 and 10 revealed the presence of an antiferromagnetic interaction between the two Dy(III) atoms bound by the NO bridges. These compounds displayed a slow relaxing magnetization through Orbach (6) and Raman (10) processes in the absence of an applied magnetic field; the rate increased upon application of a 1 kOe field. 7 and 11 showed a bright red emission typical of Eu³⁺. The two complexes have similar emission properties mainly determined by the employed β-diketonato ligands.

1D-Zigzag Eu3+/Tb3+ Coordination Chains as Luminescent Ratiometric Thermometers Endowed with Multicolor Emission

Two homometallic Coordination Polymers (CPs) with composition [Ln(hfac)₃bipy]_n (Ln³⁺ = Eu³⁺, 1, and Tb³⁺, 2; hfac = hexafluoroacetylacetonato, bipy = 4,4′-bipyridine) were used to develop a family of ratiometric luminescent thermometers containing Eu³⁺ and Tb³⁺ as red and green emitters, respectively. The thermometric properties of pure CPs and of their mixtures having an Eu³⁺/Tb³⁺ molar ratio of 1:1, 1:3, 1:5, and 1:10 (samples: Eu1Tb1, Eu1Tb3, Eu1Tb5, and Eu1Tb10) were studied in the 83–383 K temperature range. Irrespective of the chemical composition, we observed similar thermometric responses characterized by broad applicative temperature ranges (from 100 to 165 K wide), and high relative thermal sensitivity values (S_r), up to 2.40% K⁻¹, in the physiological temperature range (298–318 K). All samples showed emissions endowed with peculiar and continuous color variation from green (83 K) to red (383 K) that can be exploited to develop a colorimetric temperature indicator. At fixed temperature, the color of the emitted light can be tuned by varying composition and excitation wavelength.

Composition-Thermometric Properties Correlations in Homodinuclear Eu3+ Luminescent Complexes

A family of homodinuclear Ln³⁺ (Ln³⁺ = Gd³⁺, Eu³⁺) luminescent complexes with the general formula [Ln₂(β-diketonato)₆(N-oxide)_y] has been developed to study the effect of the β-diketonato and N-oxide ligands on their thermometric properties. The investigated complexes are [Ln₂(tta)₆(pyrzMO)₂] (Ln = Eu (1·C₇H₈), Gd (5)), [Ln₂(dbm)₆(pyrzMO)₂] (Ln = Eu (2), Gd (6)), [Ln₂(bta)₆(pyrzMO)₂] (Ln = Eu (3), Gd (7)), [Ln₂(hfac)₆(pyrzMO)₃] (Ln = Eu (4), Gd (8)) (pyrzMO = pyrazine N-oxide, Htta = thenoyltrifluoroacetone, Hdbm = dibenzoylmethane, Hbta = benzoyltrifluoroacetone, Hhfac = hexafluoroacetylacetone, C₇H₈ = toluene), and their 4,4′-bipyridine N-oxide (bipyMO) analogues. Europium complexes emit a bright red light under UV radiation at room temperature, whose intensity displays a strong temperature (T) dependence between 223 and 373 K. This remarkable variation is exploited to develop a series of luminescent thermometers by using the integrated intensity of the ⁵D₀ → ⁷F₂ europium transition as the thermometric parameter (Δ). The effect of different β-diketonato and N-oxide ligands is investigated with particular regard to the shape of thermometer calibration (Δ vs T) and relative thermal sensitivity curves: i.e.. the change in Δ per degree of temperature variation usually indicated as S_r (% K^–1). The thermometric properties are determined by the presence of two nonradiative deactivation channels, back energy transfer (BEnT) from Eu³⁺ to the ligand triplet levels and ligand to metal charge transfer (LMCT). In the complexes bearing tta and dbm ligands, whose triplet energy is ca. 20000 cm^–1, both deactivation channels are active in the same temperature range, and both contribute to determine the thermometric properties. Conversely, with bta and hfac ligands the response of the europium luminescence to temperature variation is ruled by LMCT channels since the high triplet energy (>21400 cm^–1) makes BEnT ineffective in the investigated temperature range.

A family of homodinuclear Eu³⁺ luminescent complexes with the general formula [Ln₂(β-diketonato)₆(N-oxide)_y] (y = 2, 3) was developed to study the effect of the β-diketonato and N-oxide ligands on the thermometric properties of the complexes. In this way, an effective tuning of the system’s thermometric properties can be achieved.

Structure and Stereochemistry of Adducts of Tris(dipivaloylmethanato)europium(III), Eu(dpm)3, with Some Dipolar Aprotic Unidentate O-Donors

Single crystal X-ray structural characterisations are reported for adducts of the form [(L-O)Eu(O,O′-dpm)3] obtained by the crystallisation of tris(dipivaloylmethanato)europium(iii) (dpm=[HC(C(tBu).CO)2]−) from an array of dipolar aprotic oxygen-donor solvents L (L=N-methylpyrrolidinone (nmp), trimethylphosphate, (MeO)3PO, (tmp), hexamethylphosphoramide (hmpa), dimethylacetamide (dma), dimethyl sulfoxide (dmso), and the bidentate octamethylpyrophosphoramide (ompa). In all adducts, the resulting arrays contain seven-coordinate metal atoms, which adopt two different isomeric forms of the mono-capped trigonal prismatic stereochemistry, the L=dma and dmso adducts corresponding to one type, nmp and tmp the other. The adduct formed with ompa behaves as a pair of discrete metal environments bridged by the O-ompa-O′ ligand, thus; [(dpm-O,O′)3Eu(O-ompa-O′)Eu(O,O′-dpm)3], and is found in two forms, one in which both Eu environments is of the tmp type, the other of the dmso/dma type. In the hmpa adduct, the asymmetric unit of the structure is a disordered composite of both types. In none of the adducts is there any further solvation beyond coordination of a single L.

Magnetic Behavior of Luminescent Dinuclear Dysprosium and Terbium Complexes Derived from Phenoxyacetic Acid and 2,2’-Bipyridine

Two dinuclear lanthanide complexes [Dy2(L1)6(L2)2]·2EtOH (1) and [Tb2(L1)6(L2)2]·2EtOH (2) (HL1 = phenoxyacetic acid and L2 = 2,2’-bipyridine) were synthesized and the crystal structures were determined. In both complexes, the lanthanide centers are nine-coordinated and have a muffin geometry. Detailed magnetic study reveals the presence of field-induced single molecule magnet (SMM) behavior for complex 1, whereas complex 2 is non-SMM in nature. Further magnetic study with 1’, yttrium doped magnetically diluted sample of 1, disclosed the presence of Orbach and Raman relaxation processes with effective energy barrier, ∆E = 16.26 cm−1 and relaxation time, τo = 2.42 × 10−8 s. Luminescence spectra for complexes 1 and 2 in acetonitrile were studied which show characteristic emission peaks for DyIII and TbIII ions, respectively.