Dinuclear lanthanide complexes supported by a hybrid salicylaldiminato/calix[4]arene-ligand: synthesis, structure, and magnetic and luminescence properties of (HNEt 3 )[Ln 2 (HL)(L)] (Ln = Sm III , Eu III , Gd III , Tb III ) †

The synthesis, structures, and properties of a new calix[4]arene ligand with an appended salicylaldimine unit (H 4 L = 25-[2-((2-methylphenol)imino)ethoxy]-26,27,28-trihydroxy-calix[4]arene) and four lanthanide complexes (HNEt 3 )[Ln 2 (HL)(L)] (Ln = Sm III ( 4 ), Eu III ( 5 ), Gd III ( 6 ), and Tb III ( 7 )) are reported. X-ray crystallographic analysis (for 4 and 6 ) reveals an isostructural series of dimeric complexes with a triply-bridged NO 3 Ln( μ -O) 2 (OH ⋯ O)LnO 3 N core and two seven coordinated lanthanide ions. According to UV-vis spectrometric titrations in MeCN and ESI-MS the dimeric nature is maintained in solution. The apparent stability constants range between log K = 5.8 and 6.3. The appended salicylaldimines sensitize Eu III and Tb III emission ( λ exc 311 nm) in the solid state or immersed in a polycarbonate glass at 77 K (for 5 , 7 ) and at 295 K (for 7 ).


Synthesis and characterization of the ligand
The salicylaldimine-appended calix [4]arene H 4 L was readily prepared according to a procedure reported by Zhang et al. for related bis(salicylaldimine)-p-tert-butylcalix [4]arenes (Scheme 1). 58 Alkylation of the parent calix [4]arene 1 with bromoacetonitrile followed by reduction of the nitrile 2 provided the amine 3, which was condensed with salicylaldehyde in the presence of MgSO 4 , to provide the title compound as a paleyellow solid in 21% overall yield. The IR spectrum of H 4 L reveals two sharp (3635 and 3500 cm −1 ) and one broad OH band (3320 cm −1 ) indicative of hydrogen bonding interactions. 59 The CN stretch appears at 1635 cm −1 , a typical value for salicylaldimines. 60 The calixarene adopts a cone conformation in CH 2 Cl 2 as evidenced by NMR (two characteristic AB systems for the Ar-CH 2 -Ar groups). [61][62][63] The free ligand displays intense absorption bands in the UV (Table 1), attributed to π → π* transitions of aromatic rings of the calix [4]arene (254, 286 nm) 64 and the salicylaldimine (311 nm). 65,66 A weak band around 403 nm (ε = 117 M −1 cm −1 ) can be assigned to the n → π* transition of the imine group.
The crystal structure of the free ligand ( Fig. 1) shows a cone conformation stabilized by three intramolecular OH⋯O hydrogen bonds (O1⋯O2, O2⋯O3, O3⋯O4). The pendant Schiffbase is almost perfectly planar forming an intramolecular Scheme 1 Synthesis of the ligand H 4 L. OH⋯N hydrogen bond, as in other o-hydroxyaryl Schiff bases. 67 An intramolecular CH⋯π interaction manifests itself by a short H8b⋯X1 centroid distance of 2.84 Å. Self-inclusion mediated by intermolecular CH 2 ⋯π interactions of length 2.90 Å occurs. This leads to one-dimensional chains as illustrated in Fig. 1.
The new ligand and all intermediates were characterized by IR, UV-vis, 1 H and 13 C NMR spectroscopy and electrospray ionization mass spectrometry (ESI-MS). 2D NMR experiments (NOESY, HSQC, HMBC) were used to correctly assign the chemical shifts of hydrogen and carbon atoms (ESI †).

Synthesis and characterization of complexes
The reaction of H 4 L with samarium(III) nitrate hexahydrate was performed with NEt 3 as a base ( pK a 18.82, MeCN) 68 to deprotonate the phenol functions. At a ∼1 : 1 : 4.5 molar ratio in a mixed CH 2 Cl 2 /MeOH solution at room temperature a paleyellow solution forms, from which a dinuclear compound of composition (NHEt 3 )[Sm 2 (HL)(L)] (4, where L and HL represent the fourfold and threefold deprotonated versions of H 4 L) could be reproducibly obtained in 82% yield (Scheme 2).
Analogous europium(III) (5), gadolinium(III) (6) and terbium(III) complexes (7) were also synthesized in this manner. According to ESI-MS, mononuclear complexes of composition [LnL] − are also present (Table 1), but all attempts to isolate these entities failed. The exclusive formation of the [Sm 2 (HL)(L)] − dimers may be due to a lower solubility although other factors such as packing or specific intermolecular interactions cannot be ruled out. All complexes are air-stable but hygroscopic, and exhibit good solubility in methylene chloride, chloroform and THF. They are moderately soluble in toluene, and only sparingly soluble in protic solvents.
The formulation of the complexes was ascertained in all cases by elemental analysis, mass spectrometry, IR and UV-vis spectroscopy, and in case of the Sm III and Gd III complexes also by X-ray crystallography. The negative ESI-MS spectra of dilute (10 −3 M) MeCN/CH 2 Cl 2 solutions exhibit molecular ion peaks at m/z = 1439.2 (4), 1441.3 (5), 1451.3 (6), and 1453.3 (7), respectively, with the correct isotopic peak pattern for dimeric [Ln 2 (HL)(L)] − anions (ESI). Under these conditions, signals at m/z = 719.1 (4), 720.1 (5), 725.1 (6) and 726.1 (7) for monomeric [LnL] − species are also observed. The IR spectra of all complexes reveal a band at 1635-1636 cm −1 for the CvN stretching frequency, a typical value for imine functions coordinated to Ln III ions. 66 O-H stretching bands were absent indicative of Ln III bound phenolate groups. The C-O stretching frequency observed for H 4 L at 1338 cm −1 is shifted to lower frequencies in the complexes (1327-1316 cm −1 ), indicative of the coordination of the phenol ether moiety as well. 69 Crystallographic characterization O atoms are not observed (N3⋯O7 4.33 Å), presumably due to the fact that the latter are buried by the organic residues of the supporting ligand. The complex has idealized C 2 symmetry comprising two mononuclear Sm III L units joined by two phenolato bridges to give a four-membered Sm 2 O 2 ring, a motif quite common in lanthanide calixarene structures but herein realized from phenol groups of the salicylidene moieties. [72][73][74] This assembly is reinforced by a hydrogen bond between O4 and O8 of the calixarene bowls (O4⋯O8 2.40 Å, O4-H4⋯O8 164°). [75][76][77] Each samarium atom is further bonded to four calix [4]arene O atoms and to the imine N atom of the Schiff base unit, giving rise to coordination number seven (Fig. 3).
The structure of the gadolinium compound (HNEt 3 ) [Gd 2 (HL)L(MeCN) 2 ]·MeCN (6·3MeCN) is isomorphous with 4·3MeCN, having slightly shorter Gd-O and Gd-N distances ( Table 2), in agreement with its smaller ionic radius. 79 The Ln⋯Ln distance is 3.9067(3) Å in 4 and 3.8965(4) in 6. In essence the NO 5 donor set of H 4 L cannot saturate the coordination sphere of the lanthanide ions and so dimerization occurs to share some of the O donors. 74 There are no significant intermolecular bonding interactions between the [Ln 2 (L) (HL)] − complexes. The shortest intermolecular Ln⋯Ln distances are 10.725 Å in 4 and 10.696 Å in 6.

Magnetic properties
The lanthanide complexes were further studied by temperature-dependent magnetic susceptibility measurements using a SQUID-Magnetometer (MPMS Quantum Design) in applied magnetic fields of 0.5 T over a temperature range 2-300 K. Plots of χ M T versus T for 4-7 are shown in Fig. 4.
For the Sm III 2 complex 4 the χ M T value is 0.72 cm 3 K mol −1 at room temperature, slightly larger than the expected value of 0.64 cm 3 K mol −1 for two non-interacting Sm III ions. 81 On lowering the temperature, χ M T decreases and tends to a value of ca. 0.01 cm 3 K mol −1 at 2 K. For Sm 3+ , with a 6 H 5/2 (g J = 2/7) ground state, the multiplet spacing is on the order of k B T and thermal population of excited 6 H J/2 states ( J = 7, 9,11,13,15) contributes significantly to the susceptibility. The crystal field, which partially lifts the degeneracy of the J states in zero field, may also affect the susceptibility.
We analyzed the temperature dependence of the magnetic susceptibility of the Sm III complex by utilizing the analytical expression given by Kahn (eqn (S1) †). 80 This model considers only the effect of spin-orbit coupling, which is appropriate given that magnetic exchange interactions are weak as suggested by the results for the analogous Gd complex (see below). Indeed, a reasonable fit was possible (excluding the    (4) low temperature data). The value of the spin-orbit coupling constant was determined to be λ = 254 cm −1 . This value is comparable to that of the free Sm 3+ ion (284 cm −1 ). 81 The χ M T value of the Gd III 2 complex 6 amounts to 16.16 cm 3 K mol −1 at 300 K, somewhat larger than the expected value 15.77 cm 3 K mol −1 of two uncoupled 8 S 7/2 centers. Upon cooling, χ M T slowly decreases to 15.5 cm 3 K mol −1 at 23.3 K and drops to 11.2 cm 3 K mol −1 at 3 K, indicative of a very weak antiferromagnetic exchange interaction as in other phenolatobridged Gd III 2 complexes. 82,83 For Gd III ions, first-order spinorbit coupling is absent (L = 0). Therefore, the exchange interaction can be analyzed by using the isotropic spin Hamiltonian H = −J·S Gd1 ·S Gd2 with S Gd1 = S Gd2 = 7 / 2 . The magnetic susceptibility for a dinuclear Gd III complex is given by eqn (1), where g is the Landé factor, μ B the Bohr magneton, N a the Avogadro number, k B the Boltzmann constant, and x = J/k B T. 84 A good fit of the experimental data is possible applying J = −0.065 cm −1 and g = 2.01. Such a weak antiferromagnetic coupling constant is a typical value for phenolato-bridged Gd III systems. 82,[84][85][86][87] The χ M T value of the Eu III 2 complex 5 is 2.80 cm 3 K mol −1 at 300 K, a value which is close to that expected for two non-interacting Eu III ions (2.65 cm 3 K mol −1 ), with non-negligible population of excited 7 F 1 -7 F 6 levels. The deviation from the Hund-Landé expectation value (0μ B ) is also attributable to contributions from the second order Zeeman effect in the ground 7 F 0 multiplet. 88 Upon cooling, the χ M T values decrease steadily, reaching 0.03 cm 3 K mol −1 at 2 K, as in other dinuclear Eu III complexes. 89 The magnetic susceptibility of the Eu III 2 complex can be fitted to the formula derived by Kahn (eqn (S2) †) by considering only λ (multiplet spacing) as a parameter as also done for the Sm III complex. Again, the magnetic interaction between the Eu III ions are assumed negligible.
Indeed, an excellent fit was possible over the whole temperature range to give λ = 324 cm −1 . The multiplet spacing is within the range of k B T, and significant population of the first excited state at 300 K explains the deviation from the Curie law. The λ parameter for 6 agrees with other dinuclear Eu III complexes. In [Eu 2 (L′) 2 ], for example, where L′ is derived from a calixarene ligand with two hydroxyquinolinolato arms, and the Eu III ions in an N 4 O 4 environment λ = 325 cm −1 . 90 The χ M T value of the dinuclear Tb III complex at 300 K with 23.87 cm 3 K mol −1 is slightly higher than the expected value of 23.60 cm 3 K mol −1 for a 7 F 6 ground state. Upon cooling the χ M T values decrease first slowly to 22.71 cm 3 K mol −1 at 100 K and then more rapidly to 15.87 cm 3 K mol −1 at 4 K. Tb III complexes are known to exhibit significant magnetic anisotropy, and fitting of susceptibility data is therefore difficult. 84 The field dependence of the magnetization for complex 7 was determined in order to see whether magnetic anisotropy is present in this complex. Indeed, at 2 K the magnetization values increase rapidly at low fields and then linearly but without a clear saturation, reflecting the presence of a significant magnetic anisotropy (Fig. 5). Moreover, a M vs. H/T plot ( Fig. 6) illustrates that the curves are not really superimposed on each other as expected for an isotropic system with a welldefined ground state. Nevertheless, hysteresis effects were not observed in M vs. H data at 2 K. Alternative current (ac) measurements were also undertaken to determine potential SMM behavior. However, neither maxima nor imaginary components of the ac susceptibility were observed in the χ″ M vs. T plots, ruling out an SMM behavior for 7. This may be attributed to the low local symmetry of the Tb III ions. 91

Spectrophotometric titrations
To study the complexation reactions of H 4 L with Sm III , Eu III , Gd III , and Tb III in solution UV-vis spectrophotometric batch titrations were carried out. The experiments were performed at   To determine the stoichiometry of the resulting species the mole ratio method was applied. 92 The inset of Fig. 7  Irrespective of the type of the lanthanide ion, only 1 : 1 compounds were systematically detected. Nonlinear leastsquares refinements of the titration data converged for a speciation model involving the ligand and its 1 : 1 complexes with apparent stability constants of 6.08(4) (Sm III ), 6.21(7) (Eu III ), 5.81(4) (Gd III ), and 6.34(6) (Tb III ). The stability constants show a strong affinity of L 4− towards lanthanides and decrease with decreasing ionic radii with the strongest interaction observed with Tb III and the weakest interaction observed for Sm III . There are only very few studies reporting thermodynamic data for f-element calixarene complexes in non-aqueous solvents. [93][94][95][96] Danil de Namor and Jafour have studied the complexation of trivalent cations by p-tert-butylcalix [4]arene tetraethanoate, p-tert-butylcalix [4]arene tetramethyl ketone, and p-tert-butylcalix [4]arene tetraacetamide in acetonitrile. 97 Borisova and co-workers have determined stability constants for lanthanide complexes supported by 2,2′-bipyridyl-6,6′-dicarboxylic acid diamide and 2,6-pyridinedicarboxylic acid diamide ligands in the same solvent. The binding constants of these complexes were found to lie in a similar range (log K ∼ 4-6). 98

Spectroscopic and photophysical properties
The new compounds were further characterized by UV-vis absorption and emission spectroscopy. The electronic absorption spectra were measured in acetonitrile (complexes) at room temperature. Table 1 lists the data. All complexes show three intense absorption bands around 220 nm, 300 nm, and 350 nm, respectively. The first two high-energy bands are associated with 1 (π-π)* transitions centered on the phenol ether and phenolate groups of the calix [4]arene backbone. The transition at 350 nm can be attributed to the phenyl ring of the salicylaldimine unit. Deprotonation and coordination of the lanthanides red-shifts these features by 15 and 40 nm relative to those of the free ligand. The change of the lanthanide ion appears to have little if any impact on the spectrophotometric properties. Hence, upon going from the Sm to the Tb complex a slight blue-shift of the lowest energy band of ca. 2 nm can be detected.
The luminescence properties of the Eu and Tb complexes were investigated in view of literature reports that calix [4] arenes can act as an antenna for the sensitization of lanthanide luminescence. 37,41 The free ligand shows a single emission band with a maximum at 455 nm when excited at 285 nm. The two complexes are not emissive in solution (CH 3 CN, CH 2 Cl 2 ). However, when embedded in a polymer Eu complex 5 displays four relatively broad and intense transitions (Fig. 8), attributed to 5 D 0 → 7 F J transitions ( J = 1-2) when excited at 311 nm at 77 K. 99 Both, the 5 D 0 → 7 F 1 (580 nm, 595 nm,) and the 5 D 0 → 7 F 2 transitions (620, 630 nm) appear as doublets. In view of the low local symmetry of the  coordination polyhedron (C 1 in this case), this may be related to crystal-field splitting of the 7 F 1 and 7 F 2 levels. Splitting of these levels is not unusual for Eu(III) complexes with such a low site symmetry. 100 The 5 D 0 → 7 F 0 transition (expected in the 570-585 nm range), is a strictly forbidden transition in site symmetries other than C nv , C n or C s . 100 It is also not observed for the present compound.
The intensity of the hypersensitive 5 D 0 → 7 F 2 transition (or the ratio R of the intensities I( 5 D 0 → 7 F 2 )/I( 5 D 0 → 7 F 1 ) is also often used as a measure for the asymmetry of the Eu 3+ site, since the 5 D 0 → 7 F 2 signal is strictly forbidden for a Eu 3+ at a site with inversion symmetry. In our case, there is no inversion symmetry about the Eu 3+ ion. The 5 D 0 → 7 F 2 is observed and is 1.6 times more intense than the 5 D 0 → 7 F 1 transition, in good agreement with the theoretical predictions. [101][102][103] The Tb complex gives rise to four transitions at 490, 545, 584 and 619 nm, assigned to the 5 D 4 → 7 F J ( J = 6, 5, 4, 3) transitions, again split by crystal-field effects. Of these, the "green" 5 D 4 → 7 F 5 transition at 545 nm has the highest intensity. Note that the intensity decreases with increasing temperature, which might be traced to quenching via enhanced vibrational relaxation (energy transfer to the O-H⋯O vibration modes). 104,105 The excited state luminescence decay of the immobilized Tb complex is biexponential, although the first exponential term is dominating (99% of the initializing luminescence intensity, I 0 ) with a lifetime of about τ 1 = 81 ± 2.5 µs. A small contribution of a second term with a time constant of τ 2 = 305 ± 3 µs was determined. The origin of the second term may be a different conformation of the complex due to the imbedding into the polymer matrix, as often observed for imbedded dyes. 106 This will be further investigated in a subsequent work. The features of the lifetimes are comparable to values reported for other luminescent Tb calixarene complexes ( Fig. 9 and 10). 37,107 The luminescence properties of the Gd compound 6 were examined in order to determine the triplet state energy of the Schiff base ligand. The emission spectrum of compound 6 embedded in polycarbonate at 77 K is strong (Fig. S38 †), with the shortest wavelength 0-0 transition of the ligand peaking at ca. 457 nm (21 882 cm −1 ). This triplet-state energy compares well with those of other Schiff base ligands 66 and lies well above the resonance energy levels of the Eu(III) and Tb(III) ions. These results suggest that a ligand triplet state is indeed involved in the energy transfer mechanism to the resonance state of the Ln(III) ions, from which emission occurs.

Conclusions
A new monofunctionalized calix [4]arene-Schiff base ligand has been synthesized and its coordination chemistry towards selected lanthanide ions (Sm, Eu, Gd, Tb) investigated in solution and solid state. The chemistry of this ligand system is distinct from that of the well-studied bis-and tetrakis-lower rim functionalized calix [4]arenes, which tend to support only monomeric structures. Dimerization occurs via the salicylidene's phenolate groups, not via bridging O atoms from the calix [4]arene, as seen for some heteroleptic complexes involving the parent calix [4]arenes to give coordination number 7 with an highly irregular coordination geometry. The assembly is further stabilized by an intramolecular OH⋯O-hydrogen bond established in second sphere of the calixarene bowls. The dimeric units are also present in MeCN solution as suggested by ESI MS. There are littleif anymagnetic exchange interactions in the dimers, and the absence of SMM behavior may be associated with the low local symmetry of the lanthanide ions. The present study enlarges the database, may contribute to current knowledge of structure-property relationship in Ln calixarene containing SMMs, luminescent materials, and chemosensors.

Materials and methods
The calix [4]arene 1 was prepared as described in the literature. 108 All reagents and solvents were commercial grade and used without further purification. Melting points were deter-  mined with an Electrothermal IA9000 series instrument using open glass capillaries and are uncorrected. Elemental analyses were carried out on a VARIO EL elemental analyzer (Elementar Analysensysteme GmbH, Hanau). NMR spectra were recorded on a Bruker FT 300 spectrometer or AVANCE DRX 400 spectrometer at 298 K. Chemical shifts refer to solvent signals. Mass spectra were obtained using the negative ion electrospray ionization modus (ESI) on a Bruker Daltronics ESQUIRE 3000 Plus ITMS or Impact II UHR Qq-TOF instrument. Infrared spectra (4000-400 cm −1 ) were recorded at 1 cm −1 resolution on a Bruker TENSOR 27 (equipped with a MIRacle ZnSe ATR accessory from PIKE Technologies) FT-IR spectrometer. Solution absorption spectra were collected on a Jasco V-670 UV-vis-NIR device. Steady state fluorescence absorption and emission spectra were recorded on a PerkinElmer LS 50B luminescence spectrometer using 1 cm quartz cells (Hellma). The magnetic susceptibility measurements were performed with the use of a MPMS 7XL SQUID magnetometer (Quantum Design) working between 1.8 and 330 K for applied dc fields ranging from −7 to 7 T. Measurements were performed on polycrystalline samples over the temperature range 2-330 K at applied magnetic field of 0.1, 0.5, 1.0 T. The observed susceptibility data were corrected for the underlying diamagnetism.

Spectrophotometric titrations/determination of stability constants
A series of UV-vis spectroscopic studies were performed in order to determine the composition and stability constants of the lanthanide complexes. The stoichiometry of the lanthanide complexes was determined by the mole ratio method. All titrations were performed at 298 K in Hellma 110-QS quartz cells of 1 cm optical path length containing solutions at constant ionic strength (N(nBu) 4 PF 6 0.01 M) and constant ligand concentrations (5 × 10 −5 M) in MeCN. For each experiment, 21 solutions were prepared by combining stock solutions of the ligand and the corresponding Ln(NO 3 ) 3 ·6H 2 O salts with an Eppendorf micropipette (volume range of 10-100 μL and 100-1000 µL; 0.71-0.10% error) and allowed to stir for 12 h. UV-vis absorption spectra were collected in the 190-650 nm range at uniform data point intervals of 1 nm with a doublebeam V-670 (Jasco) spectrophotometer. The multiwavelength data sets were analyzed by a nonlinear least-squares procedure implemented in the Hyperquad2008 v1.1.33 software.

Synthesis of the polycarbonate films
Polycarbonate Z200 (0.30 g) was dissolved in CH 2 Cl 2 (1.5 mL) and stirred for 10 min. A solution of the lanthanide complex (V = 1 mL, 8 × 10 −3 M) in CH 2 Cl 2 was added to the PC solution. The resulting mixture was spread on a Petri dish (d = 5 cm) and the solvent was allowed to evaporate in open air over night.

Luminescence lifetime measurements
The luminescence lifetime of the Tb complex 7 was measured applying a Fluoromax4 (HoribaScientific) equipped with a Fluorohub (Horiba Scientific) and the DataStation (Version 2.7) software package for TCSPC applications. The sample, imbedded in a thin polycarbonate matrix, was installed at an 35°angle towards the incident excitation light beam. Excitation and emission wavelengths of 310 and 545 nm were chosen. The time resolution was 1.33 µs per channel. In order to determine the instrument response function we used the identical setup with a highly reflective spectralon sample. The photon count rate was well below 1 percent of the excitation count rate ruling out pile up effects.

Conflicts of interest
There are no conflicts of interest to declare.