* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Sabina4_V10 - Universidade de Vigo
Survey
Document related concepts
Transcript
FULL PAPER Homo- and Heteronuclear Compounds with a Symmetrical Bis-Hydrazone Ligand: Synthesis, Structural Studies and Luminescent Properties Sabina Rodríguez-Hermida,[a],[b] Ana B. Lago,[a] Rosa Carballo,[a] Oscar Fabelo[c] and Ezequiel M. Vázquez-López*[a] Abstract: Nine new coordination compounds have been synthesized by the reaction of salts of bivalent metal ions (a = ZnII, b = CuII, c = NiII, d = CoII) with the bis(benzoylhydrazone) of 4,6diacetylresorcinol (H4L). Three kinds of complexes have been obtained: homodinuclear compounds [M2(H2L)2].nH2O (1a, 1b, 1c and 1d), homotetranuclear compounds [M4(L)2].n(solv) (2a and 2c) and heterotetranuclear compounds [Zn2M2(L)2].n(solv) (2ab, 2ac and 2ad). The structures of the free ligand H4L·2DMSO and its complexes [Zn2(H2L)2(DMSO)2] (1a*), [Zn4(L)2(DMSO)6] (2a*) and [Zn0.45Cu3.55(L)2(DMSO)6]·2DMSO (2ab*) were elucidated by singlecrystal X-ray diffraction. The ligand shows luminescence properties and its fluorimetric behavior towards MII metals (M = Zn, Cu, Ni and Co) has been studied. Furthermore, the solid-state luminescence properties of the ligand and compounds have been determined at room temperature. 1H-NMR monitoring of the reaction of H4L with ZnII showed the deprotonation sequence of the OH/NH groups upon metal coordination. Heteronuclear reactions have also been monitored by ESI-MS and spectrofluorimetric techniques. is often disturbed by intermolecular contacts. In this context, it is necessary to overcome this concentration quenching in order to obtain efficient solid-state emissive compounds.[4] Control of the coordination architecture is a crucial factor in determining the chemical and physical properties of the resulting materials. Appropriate synthetic routes are required to construct compounds with the desired topologies and these depend on the appropriate choice of metal ions, in terms of their coordination number and geometry, and bridging ligands with a suitable shape, size, flexibility and denticity. The metal ions play both a structural role (directing and sustaining the solid-state architecture) and a functional one (through magnetic, optical or redox properties).[5] Introduction The preparation of solids with useful functionalities is one of the main goals of crystal engineering. Fluorescent chemosensors are of interest in many research areas, such as environmental control,[1] medical diagnostics[2] and biological studies.[3] Nevertheless, many of the fluorescent materials prepared to date cannot fluoresce efficiently in the aggregated or solid state because the intrinsic radiative decay pathway of the fluorophore [a] [b] [c] Dr. S. Rodríguez-Hermida, Dr. A.B. Lago, Prof. R. Carballo, Prof. E.M. Vázquez-López Instituto de Investigación Biomédica - Universidade de Vigo Departamento de Química Inorgánica Facultade de Química, E-36310 Vigo, Galicia, Spain E-mail: [email protected] Current address: ICN2-Institut Catala de Nanociencia i Nanotecnologia, Campus UAB 08193 Bellaterra, Barcelona (Spain) Dr. O. Fabelo Institut Laue Langevin Diffraction Group 71 avenue des Martyrs Grenoble, France Supporting information for this article is given via a link at the end of the document. Scheme 1. Schematic representation of H4L showing the intramolecular O– H…N interactions and the two tridentate binding pockets. Schiff base ligands are commonly used as fluorescence probes[6] due to their ease of functionalization, convenient metalcoordination and, therefore, the possibility of modifying their optical properties. The use of tridentate Schiff bases to modulate the oxidation state of the metal ion has recently been reported[7a] but, although their properties in the formation of dynamic metallosupramolecular polymers[7b] and constitutional libraries were reported by the Lehn group[7c] almost a decade ago, tridentate poly-hydrazone ligands have been less widely studied.[7d,e] In the latter ligands, the presence of two or more reactive sites (which may provide additional properties to the molecule, such as fluorescence) provides an additional option to tune the molecular response towards various chemical species.[8] In this way, heterometallic transition metal complexes can be synthesized with polydentate ligands since one approach to isolate heteronuclear compounds is the use of metalloligands that incorporate different and separate binding sites in the organic skeleton or through a bridging-coordinative mode of the donor groups.[9] FULL PAPER In this study we selected H4L (Scheme 1) which is a multi-site coordinating ligand that contains six donor atoms separated by an aromatic ring to define two symmetrical coordination pockets. The intramolecular hydrogen bonds associated with the salicyl and the hydrazone groups preorganize the pockets towards a cisoid conformation (see for example the different conformation observed in the non-hydroxylated and in the mono-hydroxylated ligands[10]) while the methyl group at both C=N hydrazone groups leads the metal attack only by the same side of the ligand molecule. Preorganization of the hydrazone groups may play a relevant role in the stability[7c] and reactivity of the hydrazone complexes.[9d] An additional property associated with the presence of salicylaldimine groups is luminescence based on the existence of an excited-state intramolecular proton transfer mechanism (ESIPT).[11] In the work described here we explored the coordinative behavior and luminescent response of this ditopic hydrazone ligand[12] (H4L) towards different first-row transition metal ions. The presence of two clearly defined coordinative sites allowed us to explore the effect of the gradual coordination of different ions on the luminiscence properties. Simultaneously, the ability of the hydroxyl group to establish bridges[13] between two metal centers allowed us to isolate complexes with different nuclearity (Scheme 2). << Scheme 2 here >> Results and Discussion Synthesis and Characterization H4L was synthesized by a modification of the method previously published by Kotali[12] but using ethanol in an acidic medium as the solvent. The symmetrical, ditopic and potentially tridentate double-Schiff base ligand was obtained in good yield. The ligand is stable in air and is soluble in DMF and DMSO. The ligand was characterized by 1H-NMR, IR, EA and MS (ESI+), and its structure was confirmed by X-ray diffraction analysis. The optical properties (UV-Vis absorption and emission) were measured in solution and in the solid state. Three different types of complex based on the M/H4L (M = ZnII, CuII, NiII and CoII) system were obtained by adjusting the synthetic conditions (Scheme 2), although the complexes were isolated from reactions carried out with an equimolar metal acetate/ligand ratio (1:1:1 in heteronuclear compounds) using methanol as solvent. The different levels of deprotonation were achieved without additional base under conventional (complexes of H 2L2–, i.e., compounds 1a, 1b, 1c and 1d) or solvothermal (complexes of L4–, i.e., compounds 2a and 2c) conditions. Solvothermal reactions with CuII and CoII yielded unidentified products. All of the compounds were characterized by elemental analysis, mass spectrometry (ESI+), infrared, diffuse reflectance, and 1H-NMR spectroscopy in the case of the diamagnetic compounds. Attempts to obtain single crystals of all compounds from the mother liquor were unsuccessful, although single crystals of Zn II complexes 1a*, 2a* and the heteronuclear complex ZnII/CuII 2ab* were obtained from DMSO solutions. The results of spectroscopic studies (vide infra) suggest that the replacement of the water present in the as-synthesized compounds by DMSO in the single crystals did not lead to significant modifications in the coordinative characteristics of the metal-ligand core. The IR spectra of the complexes provide information about the involvement of one or two ligand pockets in the coordination to the corresponding metal ion. The structurally significant IR bands for the free ligand and its complexes are listed in Table S1. The infrared spectrum of the free ligand contains bands centered at 1641, 1513 and 1278 cm–1, which are attributed to ν (C=O), ν (C=N) and ν (C–N), respectively. In the IR spectrum of the homodinuclear compounds 1a–d, two bands corresponding to amide I (ν (C=O)) are observed. This behavior has been related to the co-existence of coordinated and uncoordinated carbonyl groups in other ditopic dihydrazones.[10b] By contrast, the IR spectra of 2a and 2c each contain a single red-shifted vibration attributed to amide I at 1627 and 1623 cm –1, respectively, indicating that both binding pockets are coordinated to metal centers. The diffuse reflectance spectra contain bands attributed to d-d transitions of metal atoms (Table S2) in 1b–d and 2c in addition to bands with CT (charge transfer) or IL (intra-ligand) character that are common to all of the complexes. The thermal stability of compounds 1a–d, 2a and 2c was studied by thermogravimetric analysis (Figures S6–S8). The compounds are stable up to the range 250–300 ºC, at which point decomposition takes place. The thermal behavior seems to be independent of the di-/tetra-nuclear metallic nature. The thermal analysis results helped us to assign the different number of trapped solvent molecules and the conclusions were consistent with the elemental analysis data. It was observed that all of the compounds seem to be stabilized with different numbers of water or methanol molecules and this is also evident in the isolated single crystals, in which DMSO molecules are present in all cases. However, the results do not allow the unequivocal determination of the true role of solvents in the structure. Consequently, the compounds were formulated as solvate complexes although the coordination of water, for example in compounds 1a and 2a, is very plausible. NMR studies (on the free ligand and its zinc complexes) and electrospray ionization (ESI) mass spectrometry were performed on DMSO-d6 solutions due to the low solubility of the compounds in other solvents. The 1H-NMR signals due to the – OH and –NH groups were observed at 13.99 and 11.30 ppm, respectively, for H4L (the atomic numbering schemes used in this paper are shown in Scheme 4, see experimental section). In 1a these signals were shifted to slightly higher field and the integration was half, which is consistent with the simultaneous deprotonation of the –NH and –OH of one binding pocket. In the spectrum of freshly prepared solutions of 2a, the absence of these signals in the set of main signals is consistent with the tetra-anionic character of the ligand and the coordination of two metal centers by the two pockets. Nevertheless, very weak signals due to traces of 1a could be detected and this suggests an equilibrium in solution between 2a and 1a (vide infra). Peaks in the ESI mass spectrum at 985, 983 and 974 (m/z) in 1a, 1b and 1d, respectively, can be assigned to the molecular ion FULL PAPER [M2(H2L)2 – H]+, thus suggesting the robustness of the dinuclear species even in DMSO solutions. Bearing in mind the frequent occurrence of the di-µ-hydroxybridge core motif (M2O2) in the complexes of Schiff base ligands derived from salicylaldehyde[13] (Scheme 2, vide infra), we believe that the same structural motif is present in all of the isolated structures and is a key factor in the stability of the compounds. As a consequence, the behavior of compounds 1 is very different to that of the asymmetric ligand recently reported by us (only an –OH group is present in the aromatic spacer) because coordination polymers were exclusively isolated for a 1:1 (MII/L) stoichiometry.[10b] On the other hand, the isolation of homotetranuclear compounds (2a, 2c) under solvothermal conditions and their reluctance to retain their discrete nature, encouraged us to study the {Zn2(H2L)2} system as metalloligand for the synthesis of new heterometallic complexes. This hypothesis was reinforced by the ESI-TOF characterization of the reaction solutions (DMSO) between 1a (formed in situ) and equimolar amounts of MII acetates (Table S13). The results of spectrofluorimetric studies (vide infra) on these reaction media are consistent with the spectrometry results. The mononuclear and homodinuclear species [M(H3L)2] and [M2(H2L)2] (M = Zn, Co, Ni) were detected in the mass spectra (Figure S5, Table S13) along with the heterodinuclear species [ZnCo(H2L)2] and [ZnNi(H2L)2], although the ZnII cation seems to be replaced by CuII and only mono- and di-copper species [Cu(H3L)2] and [Cu2(H2L)2] were observed. Finally, two heterotetranuclear compounds [Zn2M2(L)2].n(solv) (M = CuII, 2ab and NiII, 2ac) were isolated from the preformed dimeric compound [Zn2(H2L)2]. In addition, the resulting solids were characterized by ICP-OES (inductively coupled plasma optical emission spectrometry) and EDS (energy-dispersive Xray spectroscopy) analysis. Both techniques indicated an equimolar metal-to-metal ratio within the bulk materials. Interestingly, the resulting materials of the reaction with Co II (described as 2ad in the experimental section) are quite stable in solution since species such as |ZnCo(H2L)2+H|+ was detected in the ESI-MS. Nevertheless, the lack agreement in the ZnII:CoII molar ratio found by EDS (1:0.6) with ICP-OES (1:0.5) suggests a non-homogeneous nature of the materials. Moreover, a single shifted amide I band was observed in the IR spectra (Table S1) and this suggests a coordination mode involving both binding pockets. The diffuse reflectance spectra of the heterotetranuclear complexes confirm the presence of the two metal centers, with broad bands that are characteristic of dd transitions[14] of CuII (2ab), NiII (2ac) and CoII (2ad) (Table S2). The bimetallic nature of this kind of compound could also be studied by X-ray diffraction of the single crystals of 2ab*, obtained by slow evaporation of a DMSO solution of 2ab. This system can be modulated by exchange of the metal ion, as observed in the crystal structure of 2ab* where replacement of the zinc atom by copper was detected. This type of modularity is helpful in the design of new materials because it allows the physical properties to be tuned without changing the structure of the material.[15] Structural Studies Structure of the ligand H4L·2DMSO The molecular structure of H4L·2DMSO is represented in Figure 1 together with the atomic numbering scheme. Selected bond distances and angles are listed in Table S4. The ligand crystallizes with two crystallographically independent and distorted DMSO molecules in the monoclinic P21 space group. The molecule is almost planar: the angles between the planes defined by the three rings are 21.99(12) ° and 27.73(13) °, with the aromatic rings clearly located in planar segments with rms values ranging from 0.0039 to 0.0067. The C(6)–N(6) and C(6)– O(6) bond distances are typical of the amide resonance form (Table S4). Figure 1. Up: molecular structure of H4L·2DMSO showing the atomic numbering. Down: supramolecular arrangement of H4L·2DMSO showing details of the N–H…O, C–H…O interactions and the O–H…N intramolecular interaction (bottom). An interesting parameter that highlights differences between the free ligand and its complexes is the length of the intramolecular O–H···N hydrogen bond. The structural parameters in the interaction O(3)–H···N(5) are indicative of an interaction between a strong and moderate hydrogen bond[16] (Table S9) to generate six-membered rings [rms = 0.0338 (N5a–C5a–C4a– C3a–O3a–H3a) and 0.0498 (N5b–C5b–C4b–C3b–O3b–H3b)]. These intramolecular H-bonds are of particular interest for a number of reasons: (i) they are probably responsible for the planar arrangement of the molecular structure of the free ligand;[10] (ii) they are responsible for the luminescence of H4L FULL PAPER (vide infra) –a property that should be affected by the metal coordination. The molecular association in the crystal structure is achieved by means of weak H-bonds involving aromatic C–H groups and hydroxyl groups (Table S9, Figure 1) and the DMSO solvent molecules, which act as a ‘glue’ between the ligand molecules: as H-donors with the ligand oxygen atoms O(3) and O(6) and as acceptors with the N(6)–H groups. These interactions are reinforced by C(12)–H…O(s) and C(13)–H…O(s) interactions (Table S9) in a similar way to that observed in a previously published asymmetrical dihydrazone.[10b] In addition, C(11)– H…O(6) and C–H…π interactions are also observed in the supramolecular arrangement. Structures of complexes 1a*, 2a* and 2ab* Crystal data and selected bond distances and angles for 1a*, 2a* and 2ab* are listed in Tables S3 and S5–S7, respectively. Compound 2a* crystallizes in the triclinic P-1 space group and 1a* and 2ab* in the monoclinic P21/c and P21/n space groups, respectively. Compound 1a* is a homodinuclear dimer with an uncoordinated pocket. Compounds 2a* and 2ab*, both of which do not have uncoordinated pockets, are homotetra- and heterotetranuclear compounds, respectively. The structures correspond to neutral discrete compounds in which DMSO solvent molecules complete the coordination sphere and also contribute to the supramolecular association by establishing different interactions (vide infra) (Scheme 2). All MII cations are {NO4} pentacoordinated through O(3), N(5) and O(6) atoms of one ligand molecule to generate 5- and 6membered chelate rings and by an oxygen atom of a DMSO molecule. The fifth position is occupied by one O(3) atom belonging to a neighboring ligand (coordination sphere A, Figure 2) [in 1a* the Zn(1) atom of 2a* and in M(1) and M(3) in 2ab*] or by another DMSO molecule [Zn(2) in 2a* and Cu(2) and Cu(4) in 2ab*] (Coordination sphere B). The geometry around the metallic centers can be described as a square pyramid since the Addison parameter (τ) [17] values are within the range 0.008–0.355 (Table S8). The main distortion is found in the outer zinc metal atoms coordinated to two DMSO molecules in compound 2a*. The apical position is occupied by a DMSO molecule in the three structures with M–O distances in the range 2.016–2.461 Å for M = Zn and 2.045–2.236 Å for M = Cu. In the homodinuclear compound 1a* the ligand is bideprotonated to form a dianionic binding site in one pocket, which leads to a dimeric structure in which two metal-ligand units are linked by a µ-phenolate-bridged Zn2O2 core. In 2a*and 2ab* the ligand is tetradeprotonated and adopts the µ-κ3O,N,O´:κO´ and κ3O,N,O´ coordination modes. The bridging coordinative behavior of the O(3) atom allows the formation of M2O2 cores. This core is a common structural motif in all of the structures. The Zn···M (M = ZnII or CuII) distance is 3.102 Å, 3.184 Å and 3.058 Å in 1a*, 2a* and 2ab*, respectively.[13] In all cases this core is coplanar with the planar ligand binding pocket. This structural feature has been observed previously in aroylhydrazone complexes with M···M distances in a similar range (3.058–3.184 Å).[13b,18] This unit is always composed of zinc atoms except in 2ab*, where the structural disorder in the central core was interpreted as a partial replacement of zinc atoms (using EADP Shelxl instructions [19]) from the initial [Zn2(H2L)2] precursor by copper atoms to generate ((Cu/Zn)2-O2) units, whereas the two external pockets are exclusively occupied by copper atoms. Figure 2. Schematic representation and main structural differences in complexes 1a*, 2a* and 2ab*. The ligand moiety retains its planarity after coordination in 2ab* (29.18°, 5.04°, 6.38° and 30.34°), whereas in 1a* and 2a* the terminal phenyl rings are rotated and this planarity is broken (14.98°, 27.66°, 54.41° and 2.02° for 1a* and 14.45° and 57.59° for 2a*). In 1a* and 2a*, the ligand moieties are arranged in a parallel manner (the angles between the planes formed by the central aromatic ring of both ligand molecules is 9.09° and 0°, respectively). By contrast, in 2ab* the ligands form an angle of 50.35°, which is similar to the H-bond angle N(6b)–H–O(6b) established in 1a*, vide infra. Coordination prevents the formation of the O–H…N intramolecular H-bonds observed in the free ligand and in the uncoordinated pocket of 1a*. Moreover, the deprotonation of NH groups upon coordination implies the presence of the enol form in the carboxamide fragments in the coordinated pockets, as deduced from the bond distances (Tables S5–S7). The protonated nature of the –NH group in the uncoordinated pocket of 1a* allows the interaction with the oxygen atom of a DMSO molecule of a neighboring unit. The –NH and –CH groups of the ligand act as donors and the oxygen atom of the DMSO molecule acts as an acceptor in this interaction (Figure 3). This situation leads to the formation of the stable synthon observed in the free ligand and in a previous study with a related asymmetrical ligand (Table S10).[10b] Zig-zag chains are formed by C(10b)–H…O(6d) interactions along the crystallographic b axis. These chains are interconnected in a double strand by the supramolecular N(6b)–H…O(6b) synthon and this arrangement is reinforced by C–H…π interactions (Figure 3, Table S10). These double-strand chains are interconnected by means of N(6d)–H…O(2s) interactions with the DMSO molecules. FULL PAPER Figure 3. Supramolecular association in 1a* showing details of the supramolecular synthon. Similar hydrogen bonding interactions, based on a donor N–H group, are not possible due to the deprotonated character of the ligand in 2a* and 2ab*. Indeed, in these complexes nitrogen atoms act as acceptors and establish interactions with the C–H groups [C(11b)–H and C(43s)–H] in 2a*. Moreover, C(9b)– H…O(2s) interactions established with DMSO solvent molecules are observed (Figure S1, Table S11). In 2ab*, DMSO molecules (coordinated and uncoordinated) are responsible for the supramolecular association through DMSO···DMSO, DMSO···O(6) and C(9)–H···DMSO interactions (Figure S1, Table S12). Fluorescence studies The two intramolecular O–H…N hydrogen bonds associated with the ketimine groups of the ligand H4L encouraged us to explore the luminescence properties, since this intramolecular hydrogen bond allows triggered emission through an excited state intramolecular proton transfer mechanism (ESIPT) (Figure S2).[11,16,20] The ESIPT process involves the transfer of the hydroxyl proton to an imine nitrogen located less than 2 Å away through a pre-existing six-membered ring hydrogen bonding configuration.[21] ESIPT dyes generally have large Stokes shifts and are ideal candidates for use as fluorescence labels.[22] The photophysical properties of H4L were studied at room temperature in DMSO solution, due to the lack of solubility in other solvents, and in the solid state (Figure 4) in order to confirm the structural integrity in solution. Three absorption bands were observed in the spectrum of H4L in DMSO solution: two intense bands at 295 and 350 nm and a weak absorption at 420 nm. The solid diffuse reflectance spectrum of H4L exhibits intense absorptions at λ max 260 and 335 nm. The highest energy band is probably due to a π→π* transition of aromatic rings while the intense band above 300 nm is assigned to the n→π* transition of the hydrazone (azomethine) chromophore. The latter band (which is usually assigned to π-π* absorptions associated with the C=N azomethine bond[7a]) in the solid state has a tail at around 400 nm while there is a clear band in the solution spectrum with a maximum at 420 nm. This bathochromic shift is interesting because this band is associated with the emission: upon excitation at 430 nm (and 350 nm), the emission bands in solid state and in solution are centered at λ max 535 and 500 nm, respectively. The differences between the two states are probably due to different intermolecular interactions involving the solvent (DMSO) in the ground and excited states. However, it is not easy to establish correlations between the two states because the intrinsic radiative decay pathway is often disturbed by intermolecular contacts in the aggregate or the solid state.[23] Gradual addition of NaOH to the H4L solution led to an increase in the intensity of the absorption band around 430 nm (Figure S3A). More interestingly, although the intensity of the emission (along with splitting of the band) was initially observed, after reaching the maximum intensity at a 1:1 (H4L:NaOH) ratio the emission was finally quenched when a 1:2 molar ratio was reached (Figure S3B). Although the first process can be attributed to the improved protonation transfer in the H 3L– species as a result of resonance stabilization induced by the deprotonated pocket similarly to the effects were observed by Xu and Pang in bis(benzoxazole) derivatives,[24] the small value of the Stokes shift makes doubt of the true origin of this emission in H3L-. The final quenching at a 1:2 (H4L:NaOH) ratio suggests the formation of H2L2– by deprotonation of both phenolic OH groups – a situation that is consistent with the higher acidity observed for this group in the salicylaldimine hydrazones. The addition of M(AcO)2 (M = Zn, Co, Cu and Ni) to H4L in DMSO at r.t. also caused changes in the absorption spectra (Figures 5 and 6). The addition of the MII cation led to a progressive increase in the intensity of the band at 420 nm and this change was accompanied by a decrease in the absorption bands at 295 and 350 nm along with the emergence of a new band at around 320 nm. The presence of an isosbestic point in all experiments (at 375 nm) suggests the formation of new species during the additions. Figure 4. Absorption (left) and emission (right) spectrum at r.t. of H4L in the solid state (solid line) and in DMSO solution (dashed line; [H4L] = 1·10–5 M). λexc = 430 nm (black lines); λexc = 350 nm (grey lines). It was possible to distinguish two kinds of fluorescence response of H4L depending on the electronic nature of the M II cations. The addition of increasing amounts of CoII, NiII and CuII led to a quenching in the emission intensity, which was total when a 1:1 FULL PAPER molar ratio was reached (Figure 5). This feature can be related to the paramagnetic nature of the metal atoms, that plays a relevant role in the quenching process.[25] On the other hand, this quenching occurs at the equimolar point; therefore, the formation of a coordination polymer of the type [M(H2L)]n.[10b] cannot be ruled out. The behavior was completely different when ZnII was used. Fortunately, the diamagnetic nature of this cation allowed the reaction in DMSO-d6 to be monitored by 1H-NMR spectroscopy (Figure S4). In this experiment signals were observed for the O– H and N–H groups (13.49 and 11.12 ppm, respectively) at a 1:1 (H4L:ZnII) molar ratio and only when a 1:2 was reached were these signals absent. Spectrofluorometric monitoring of the reaction showed a marked increase in the emission intensity, i.e., around a 60-fold enhancement, upon addition of one equivalent of zinc acetate (Figure 6). The ESI-TOF mass spectra of the reaction mixtures at this molar ratio contained signals due to [Zn 2(H2L)2]+ and [Zn(H2L)]+, which suggests the predominance of species based on 1a. The solid state fluorescence spectrum of 1a displays a broad band centered at 475 nm and a shoulder at 535 nm (Figure 6) and this matches reasonably well with the spectra of the ZnII/H4L solutions in a 1:1 molar ratio. Once again, deprotonation, and probably coordination in the present case, of one of the pockets seems to increase the basic character of the nitrogen atom in the other pocket, thus improving the emission.[26] << Figure 5 here >> The addition of more zinc acetate caused changes in the absorption and emission spectra, which is consistent with the interaction of the second ZnII with the free pocket of the species 1a formed in solution. An interesting aspect of this reaction is that although the fluorescence intensity decreases when more ZnII is added, it is still higher compared with that of the free ligand [25 fold enhancement when a 1:2 (H4L:ZnII) molar ratio is reached]. Although the NMR spectrum of this medium showed only very small signals for the –OH proton, the better sensitivity of fluorescence spectroscopy provided evidence for the existence of a protonation-chelation equilibrium in solution on the second pocket (Scheme 3). The formation of heterotetranuclear [Zn2M2(L)2] complexes from [Zn2(H2L)2] was also monitored by spectrofluorimetric techniques. In these studies, zinc acetate was added to H4L until a 1:1 molar ratio was reached and increasing amounts of the other M II acetate (M = Cu, Ni, Co) were then added (Figure 7). The initial increase in the emission intensity is consistent with the formation of [Zn2(H2L)2] at a 1:1 molar ratio and this was followed by a gradual decrease in fluorescence on addition of M II until the molar ratio was 1:1:1 (L:Zn:M). When copper or cobalt acetates were added a plateau appeared in the spectrum and fluorescence quenching was observed. It is noteworthy that the point at which the fluorescence is switched off was only reached at a 1:1:3 ratio when nickel acetate was added. Figure 6. Spectrophotometric and fluorimetric monitoring of the reaction of H4L with ZnII (ldown) in DMSO at r.t. ([H4L] = 1·10–5 M, [ZnII] = 1·10–2 M). Fluorimetric measurement of H4L (black), 1a (gray) and 2a (gray dashed line) at r.t. in the solid state (up). λexc = 430 nm. FULL PAPER Scheme 3. Representation of the ESIPT mechanism of 1a and the equilibrium between 1a and 2a in solution. The switch-off in fluorescence emission can be attributed to the replacement of the Zn by Cu or Co (with behavior analogous to that observed in the direct reaction of H4L and CuII or CoII) or to the formation of 1:1:1 (L:Zn:M) complexes, where the paramagnetic nature of M may influences in final quenching. In agreement with the latter hypothesis, signals attributable to heterodinuclear species were observed in the ESI-TOF spectra of the H4L/Zn/Co reaction medium but they were absent in the H4L/Zn/Cu system. Consequently, a different behavior of [Zn2(H2L)2] in conjunction with CoII or CuII cannot be ruled out. Nevertheless, the case of NiII warrants further consideration because: (i) the ESI-TOF spectra of the reaction media display signals due to Ni/Zn species (Figure S5); (ii) in the absence of Zn, the reaction with the free ligand led to quenching of the fluorescence (vide supra); (iii) in the presence of [Zn 2(H2L)2] additional overconcentration is necessary to quench the fluorescence. In fact, the behavior in this case was similar to that observed in the reaction of [Zn2(H2L)2] with ZnII. << Figure 7 here >> Conclusions The nature of the symmetrical and multi-site coordinating hydrazone ligand H4L allowed the isolation, from different reactions with a variety of divalent metal atoms, of three different kinds of complexes: homodinuclear, homotetranuclear and heterotetranuclear compounds. The structural parameters of these three types of compounds in the solid state were analyzed by X-ray diffraction. It was demonstrated that the fluorescence behavior of H4L depends on the coordination of the {ONO} binding pockets because this emission is based on an excited state intramolecular proton transfer (ESIPT) mechanism. Deprotonation of one of the pockets on addition of a base or by coordination of ZnII gives rise to an extraordinary enhancement in the emission intensity and only if deprotonation of the second pocket is achieved is the emission finally quenched. The results of ESI-TOF mass spectrometry studies on the zinc-containing reaction media suggest the presence of dimeric species and the X-ray structures of [Zn2(H2L)2(DMSO)2] (1a*) and [Zn4(L)2(DMSO)6] (2a*) confirm this hypothesis. In both cases, the association of the two units is based on the diamond-shaped Zn2O2 core formed by the bridging phenolate groups. The fluorimetric response of the ligand H4L towards CoII, NiII and CuII is completely different because a gradual decrease in the emission intensity was observed on addition of the metal. Although the formation of polymeric compounds in which both binding pockets are used in the metal coordination cannot be ruled out, ESI-TOF studies on the reaction media suggest the formation of compounds based on dimeric [M2(H2L)2] units. The ability of the fragment {Zn2(H2L)2} to act as a metalloligand was also tested against MII and several heterotetranuclear compounds were isolated. The reactions were monitored by ESI-TOF MS, with peaks due to heterodinuclear species observed, and by analysis of the spectrophotometric and fluorimetric response. The fluorimetric responses of {Zn 2(H2L)2} (which is different to that of H4L) towards the different MII cations were not identical. The fluorescence was completely quenched at 1:1:1 (H4L:Zn:M) ratios when M = CoII and CuII, whereas an excess of NiII was necessary to achieve quenching. Although these results suggest that the reacting metal cations interact with the free pocket of the metalloligand, the isolation of single crystals of [Zn0.45Cu3.55(L)2(DMSO)6]·2DMSO (2ab*) proves that replacement of the metal in the original Zn2O2 diamond is also possible. The bimetallic nature of these materials could also be analyzed in the solid state by EDS and OIC-PC techniques. Experimental Section Materials and physical measurements Metallic salts and solvents were obtained commercially and were used as supplied. Elemental analyses (C, H, N) were carried out on a Fisons EA1108 microanalyser. IR spectra were recorded from KBr discs (4000–400 cm–1) on a Bruker Vector 22 spectrophotometer. 1H NMR spectra were obtained with a Bruker AMX 400 spectrometer. Mass spectra were recorded on a Hewlett-Packard 5989A spectrometer. TGA was performed on a SETSYS Evolution Setaram thermogravimetric analyser in a flow of N2 with a heating rate of 10 ºC min–1. UV–Vis absorption and emission spectra were obtained on HP 8453 and Horiba-Jobin–Yvon Fluoromax-3 TCSPC spectrophotometers in DMSO at r.t.; excitation and emission slits of 2.0 or 5.0 nm were used for the measurements. Spectrofluorimetric measurements in the solid state were collected on a Jasco-FP-8300 spectrofluorimeter with excitation and emission bandwidths of 5 nm. Determination of metals [Co(II), Ni(II), Cu(II), Zn(II)] in heterometallic compounds was carried out with a Perkin-Elmer Optima 4300 DV Optical Inductively Coupled Plasma Spectrometer (OIC-PC). The heterometallic nature of the compounds was also confirmed with a JEOL JSM6700F Field Emission Scanning Electron Microscope (FE- FULL PAPER SEM) equipped with an Oxford Inca Energy SEM 300 X-ray detector (EDS). Synthesis of the compounds Synthesis of H4L. A warm colorless solution of 4,6-diacetylresorcinol (0.1 g, 0.51 mmol) in EtOH (20 mL) was added dropwise to a solution of benzoic hydrazide (0.208 g, 1.53 mmol) in EtOH (20 mL). Immediately, 1 drop of HCl(c) was added. The colorless solution was heated under reflux for 2 h and the resulting white solid was filtered off and vacuum dried over CaCl2/NaOH. Suitable crystals of H4L·2DMSO were obtained after storing a DMSO solution at r.t. for 20 days. Data for H4L: Yield: 0.190 g (86%). 1H-NMR (400 MHz, [D6]DMSO, 25ºC): δ= 13.99 (s, 2H, O3-H), 11.30 (s, 2H, N6-H), 7.94 (d, J = 7.2 Hz, 4H, C8-H + C12-H), 7.83 (s, 1H, C2-H), 7.63 (t, J = 7.3 Hz, 2H, C10-H), 7.53 (t, J = 7.4 Hz, 4H, C9-H + C11-H), 6.37 (s, 1H, C1-H), 2.55 ppm (s, 6H, C13-H). IR (KBr): ν = 3434 + 3224m (OH-NH), 1641vs (C=O), 1513m (C=N), 1278s cm−1 (C–N). MS-ESI: m/z (%): 431 (100) [H5L+]. Elemental analysis calcd (%) for C24H22N4O4: C, 66.97; H, 5.15; N, 13.02; found: C, 66.65; H, 5.17; N, 12.97. Data for 1b, [Cu2(H2L)2].4(H2O) (Green): Yield: 0.109 g (45%). IR (KBr): ν = 3438vs (OH–NH), 1646m + 1618vs (C=O), 1517vs (C=N), 1267s cm−1 (C–N). MS-ESI: m/z (%): 431 (6) [H5L]+, 492 (7) [Cu(H3L)]+, 570 (100) [Cu(H3L)(DMSO)]+, 631 (27) [Cu2(HL)(DMSO)]+, 922 (27) [Cu(H3L)2 + H]+, 983 (11) [Cu2(H2L)2 + H]+. Elemental analysis calcd (%) for C48H40N8O8Cu2.4H2O: C, 54.59; H, 4.58; N, 10.61; found: C, 54.64; H, 4.23; N, 10.48. Data for 1c, [Ni2(H2L)2].5(H2O) (Dark orange): Yield: 0.108 g (42%). IR (KBr): ν= 3401m (OH–NH), 1622sh + 1592s (C=O), 1524vs (C=N), 1275s cm−1 (C–N). MS-ESI: m/z (%): 431 (29) [H5L]+, 565 (96) [Ni(H3L)(DMSO)]+, 917 (100) [Ni(H3L)2 + H]|+. Elemental analysis calcd (%) for C48H40N8O8Ni2.5H2O: C, 54.17; H, 4.74; N, 10.53; found: C, 54.55; H, 4.30; N, 10.33. Data for 1d, [Co2(H2L)2].8(H2O) (Dark brown): Yield: 0.110 g (43%). IR (KBr): ν= 3417m (OH–NH), 1625m + 1577vs (C=O), 1521vs (C=N), 1276m cm−1 (C–N). MS-ESI: m/z (%): 431 (7) [H5L]+, 487 (7) [Co(H3L)]+, 566 (27) [Co(H3L)(DMSO)]+, 916 (6) [Co(H3L)2 + H]+, 974 (11) [Co2(H2L)2 + H]+. Elemental analysis calcd (%) for C48H40N8O8Co2.8H2O requires: C, 51.53; H, 5.04; N, 10.01; C, 51.22; H, 4.46; N, 9.60. Synthesis of homotetranuclear complexes of general formula [M4(L)2].n(solv): 2a (ZnII) and 2c (NiII) The reaction in solvothermal conditions (heating at 110 ºC for 24 h followed by a slow cooling to r.t. at a rate of 0.60 ºC/h) of H4L (0.1 g, 0.23 mmol) and M(AcO)2·nH2O (0.22 mmol) in MeOH (40 mL) afforded homogeneous precipitates in both syntheses. The solids were filtered off and vacuum dried over CaCl2/NaOH. Scheme 4. Schematic representation of the ligand and its complexes showing the atomic labeling used. Synthesis of homodinuclear complexes of general [M2(H2L)2].nH2O: 1a (ZnII), 1b (CuII), 1c (NiII) and 1d (CoII) Data for 2a, [Zn4(L)2].2(H2O) (Yellow): Yield: 0.058 g (21%). 1H-NMR (400 MHz, [D6]DMSO, 25ºC): δ= 8.20–7.38 (m, 12H, C1-H + C2-H, C8-H + C9-H + C10-H + C11-H + C12-H), 2.79–2.67 ppm (m, 6H, C13-H). IR (KBr): ν= 3430s (OH), 1627s (C=O), 1539s (C=N), 1259m cm−1 (C–N). MS-ESI: m/z(%): 431 (9) [H5L]+, 493 (11) [Zn(H3L)]+, 571 (14) [Zn(H3L)(DMSO)]+, 730 (11) [Zn2(HL)(H2O)(DMSO)2]+, 923 (15) [Zn(H3L)2 + H]+. Elemental analysis calcd (%) for C48H36N8O8Zn4.2H2O: C, 50.11; H, 3.50; N, 9.74; found: C, 50.39; H, 3.87; N, 9.61. formula 2a*, [Zn4(L)2(DMSO)6] A solution of M(AcO)2·nH2O (0.22 mmol) in MeOH (10 mL) was added to a suspension of H4L (0.1 g, 0.23 mmol) in MeOH (20 mL). The suspension was heated under reflux for 2 h and the resulting solid was filtered off and vacuum dried over CaCl2/NaOH. 1H- Data for 1a, [Zn2(H2L)2].2(H2O) (Dark yellow): Yield: 0.102 g (43%). NMR (400 MHz, [D6]DMSO, 25ºC): δ= 13.43 (s, 1H, O3a-H), 11.09 (s, 1H, N6a-H), 8.09 (s, 2H, C8a-H + C12a-H), 7.94 (s, 2H, C8b-H + C12b-H), 7.71 (s, 1H, C2-H), 7.61–7.40 (m, 6H, C9a-H + C9b-H + C10a-H + C10bH + C11a-H + C911-H), 6.03 (s, 1H, C1-H), 2.73 ppm (s, 6H, C13a-H + C13b-H). IR (KBr): ν = 3442, 3226m (OH–NH), 1641vs + 1625vs (C=O), 1508vs (C=N), 1276s cm−1 (C–N). MS-ESI: m/z (%) = 431 (29) [H5L]+, 571 (10) [Zn(H3L)(DMSO)]+, 923 (30) [Zn(H3L)2 + H]+, 985 (17) [Zn2(H2L)2 + H]+. Elemental analysis calcd (%) for C48H40N8O8Zn2.2H2O requires: C, 56.32; H, 4.33; N, 10.95; found: C, 56.29; H, 4.68; N, 10.88. 1a*, [Zn2(H2L)2(DMSO)2] The DMSO solvate of this complex was obtained as single crystals by slow evaporation (1 month) at r.t. of a DMSO solution of 1a. The DMSO solvate of this complex was obtained as single crystals by slow evaporation (1 month) at r.t. of a DMSO solution of 2a. Data for 2c, [Ni4(L)2].2(MeOH).4(H2O) (Brown): Yield: 0.056 g (23%). IR (KBr): ν= 3413m (OH), 1623s (C=O), 1536m (C=N), 1260m cm−1 (C–N). MS-ESI: m/z(%): 431 (4) [H5L]+, 543 (44) [Ni2(HL)]+, 917 (4) [Ni(H3L)2 + H]+. Elemental analysis calcd (%) for C48H36N8O8Ni4.2MeOH.4H2O: C, 49.07; H, 4.28; N, 9.16; found: C, 49.02; H, 4.51; N, 9.51. Synthesis of heterotetranuclear complexes of general formula [Zn2M2(L)2].n(solv): 2ab (M = CuII), 2ac (M = NiII) and 2ad (M = CoII) A solution of Zn(AcO)2.2H2O (0.056 g, 0.26 mmol) in MeOH (10 mL) was added to a suspension of H4L (0.114 g, 0.26 mmol) in the same solvent (20 mL). The yellow suspension was heated under reflux for 2 h. The suspension was cooled down to r.t. and then a solution of 0.26 mmol of M(AcO)2.nH2O (0.057 g of Co(AcO)2.4H2O, 0.045 g of Cu(AcO)2.H2O or 0.064 g of Ni(AcO)2.4H2O) in MeOH (10 mL) was added. The mixture was heated under reflux for 2 h. The resulting solids were filtered off and dried under vacuum over CaCl2/NaOH. FULL PAPER Data for 2ab, [Zn2Cu2(L)2].2(H2O) (Pale green): Yield: 0.132 g (41%). IR (KBr): ν = 3446m (OH), 1590s (C=O), 1564s (C=N), 1266m cm−1 (C–N). MS-ESI: m/z(%): 431 (5) [H5L]+, 570 (69) [Cu(H3L)(DMSO)]+, 631 (9) [Cu2(HL)(DMSO)]+, 922 (100) [Cu(H3L)2 + H]+. Elemental analysis calcd (%) for C48H36N8O8Zn2Cu2·2H2O: C, 50.27; H, 3.52; N, 9.77; Cu, 10.47; Zn, 10.63; found: C, 50.16; H, 3.91; N, 9.65; Cu, 11.08; Zn, 11.40. 2ab*, [Zn0.45Cu3.55(L)2(DMSO)6]·2DMSO The DMSO solvate of this complex was obtained as single crystals by storing a DMSO solution of 2ab at r.t. for 1 month. Insufficient crystals were obtained for elemental analysis or a meaningful estimation of yield. Data for 2ac, [Zn2Ni2(L)2].4(H2O).2(MeOH) (Dark yellow): Yield: 0.122 g (38%). IR (KBr): ν= 3411m (OH), 1594s (C=O), 1565s (C=N), 1281m cm−1 (C–N). MS-ESI: m/z(%): 431 (24) [H4L]+, 565 (83) [Ni(H3L)(DMSO)]+, 571 (16) [Zn(H3L)(DMSO)]+, 917 (70) [Ni(H3L)2 + H]+, 923 (19) [Zn(H3L)2 + H]+. Elemental analysis calcd (%) for C48H36N8O8Zn2Ni2.4H2O.2MeOH: C, 48.54; H, 4.24; N, 9.06; Ni, 9.49; Zn, 10.57; found: C, 48.68; H, 4.00; N, 8.76; Ni, 9.41; Zn, 9.05. Data for 2ad, [Zn2Co2(L)2].(H2O).(MeOH) (Brown): Yield: 0.087 g (27%). IR (KBr): ν= 3422m (NH), 1585s (C=O), 1518s (C=N), 1273m cm−1 (C–N). MS-ESI: m/z(%): 431 (4) [H5L]+, 493 (11) [Zn(H3L)]+, 918 (92) [Co(H3L)2 + H]+, 923 (100) [Zn(H3L)2 + H]+, 980 (10) [ZnCo(H2L)2 + H]+, 985 (7) [Zn2(H2L)2 + H]+. Acknowledgements Financial support from the Spanish Ministry of Education and Science (project CTQ2010-19386/BQU) is gratefully acknowledged. We acknowledge the Spanish BM16 beamline (Dr. A. Labrador) facility at the ESRF (Grenoble, France) for provision of synchrotron radiation beam time under project 1601-747. A. B. L. thanks the Xunta de Galicia for a postdoctoral contract under the “ÁngelesAlvariño” program. Keywords: Hydrazone • luminescence • copper • zinc • nickel [1] [2] [3] [4] [5] [6] Crystallography Crystallographic data for H4L·2DMSO, 1a* and 2a* were collected at 100 K and with a λ value of 0.7513 Å in the ESRF synchrotron Spanish BM16-CRG beamline (Grenoble, France). Data were indexed, integrated and scaled using the HKL2000 program.[27] Absorption corrections were not applied. Data for 2ab* were collected at 100 K on a microfocal Bruker Smart 6000 CCD diffractometer using graphite monochromated Cu-Kα radiation (λ = 1.54178 Å) and were corrected for Lorentz and polarization effects. The frames were integrated with the Bruker SAINT[28] software package and absorption correction was applied using the program SADABS.[29]. The max/min transmission factors were 0.753/0.557. [7] [8] [9] The structures were solved by direct methods using the program SHELXS97.[30] All non-hydrogen atoms were refined with anisotropic thermal parameters by full-matrix least-squares calculations on F2 using the program SHELXL2013/2.[19] [10] Hydrogen atoms were inserted at calculated positions and constrained with isotropic thermal parameters, except for the hydrogen atoms of the – NH and –OH groups, which were located from difference Fourier maps in H4L·2DMSO and 1a*. In 2ab*, the AEDP command was used to refine the metal center positions, giving rise to 90% and 65% occupancy for Cu(1) and Cu(3), and 10% and 35% for Zn(1) and Zn(3), respectively. Drawings were produced with MERCURY[31] and special computations for the crystal structure discussions were carried out with PLATON.[32] The structural data have been deposited with the Cambridge Crystallographic Data Centre (CCDC) with the reference numbers included in Table S3. Selected bond lengths and angles and hydrogen bond distances for the crystal structures are listed in Tables S4–S12. [11] [12] [13] [14] [15] [16] a) J. Du, M. Liu, X. Lou, T. Zhao, Z. Wang, Y. Xue, J. Zhao, Y. Xu, Anal. Chem., 2012, 84, 8060-8066; b) Y. Yu, X. Cheng, H. Zhang, S. Hu, X. Li, A. Zhang, J. Polym.Sci., Part A: Polym. Chem., 2013, 51, 4592-4600. Y. Xiang, Y. Lu, Inorg.Chem., 2014, 53, 1925-1942. a) T. Mistri, M. Dolai, D. Chakraborty, A. R. Khuda-Bukhsh, K. K. Das, M. Ali, Org. Biomol. Chem., 2012, 10, 2380-2384; b) K. Kaur, M. Kaur, A. Kaur, J. Singh, N- Singh, S. K. Mittal, N. Kaur, Inorg. Chem. Front., 2014, 1, 99-108. K. Sakai, H. Kawamura, N. Kobayashi, T. Ishikawa, C. Ikeda, T. Kikuchib, T. Akutagawab, CrystEngComm, 2014, 16, 3180-3185. M. Andruh, Chem. Commun., 2007, 2565-2577. a) W. H. Hsieh, C-F. Wan, D-J.Liao, A-T. Wu, Tetrahedron Lett., 2012, 53, 5848-5851; b) X. Peng, X. Tang, W. Qin, W. Dou, Y. Guo, J. Zheng, W. Liu, D. Wang, Dalton Trans., 2011, 40, 5271-5277; c) L. Wang, W. Qin, X. Tang, W. Dou, W. Liu, J. Phys. Chem. A, 2011, 115, 16091616; d) D. Zhang, Z. Zang, X. Zhou, Y. Zhou, X. Tang, R. Wei, W. Liu, Inorg. Chem. Commun., 2009, 12, 1154-1156. a) M. S. Shongwe, K. S. Al-Barhi, M. Mikuriya, H. Adams, M. J. Morris, E. Bill, K. C. Molloy, Chem.–Eur. J., 2014, 20, 9693-9701; b) C.-F. Chow, S. Fujii, J.-M. Lehn, Angew. Chem. 2007, 119, 5095-5098; Angew. Chem. Int. Ed. Engl., 2007, 46, 5007-5010; c) G. Vantomme, S. Jiang, J. M. Lehn, J. Am. Chem. Soc., 2014, 136, 9509-9518; d) D. Plaul, A. Buchholz, H. Görls, W. Plass, Polyhedron, 2007, 26, 45814590; e) D. Plaul, W. Plass, Inorg. Chim.Acta, 2011, 374, 341-349. Q. Chu, D. A. Medvetz, Y. Pang, Chem. Mater.,2007, 19, 6421-6429. a) C. Mohapatra, V. Chandrasekhar, Cryst. Growth Des., 2014, 14, 406-409; b) M. C. Das, S. Xiang, Z. Zhang, B. Chen, Angew. Chem. 2011, 123, 10696-10707; Angew. Chem. Int. Ed., 2011, 50, 1051010520; c) A. Igashira-Kamiyama, T. Konno, Dalton Trans., 2011, 40, 7249-7263; d) D. J. Hutchinson, L. R. Hanton, S. C. Morattil, Inorg. Chem., 2010, 13, 5923-5934. a) S. Rodríguez-Hermida, C. Wende, A. B. Lago, R. Carballo, N. Kulak, E. M. Vázquez-López, Eur. J. Inorg. Chem., 2013, 5843-5853; b) S. Rodríguez-Hermida, A. B. Lago, L. Cañadillas-Delgado, R. Carballo, E. M. Vázquez-López, Cryst. Growth Des.,2013, 13,1193-1205. E. Hadjoudis, I. M. Mavridis, Chem. Soc. Rev., 2004, 33, 579-588. A. Kotali, Tetrahedron Lett.,1994, 35, 6753-6754. a) A. Roth, A. Buchholz, M. Gärtner, A. Malassa, H. Görls, G. Vaughan, W. Plass, Z. Anorg. Allg. Chem., 2007, 633, 2009-2018; b) M. Mishra, K. Tiwari, A. K. Singh, V. P. Singh, Polyhedron, 2014, 77, 57-65; c) F. Borbone, U. Caruso, M. Causà, S. Fusco, B. Panunzi, A. Roviello, R. Shikler, A. Tuzi, Eur. J. Inorg. Chem., 2014, 2695-2703. A. B. P. Lever, Inorganic Electronic Spectroscopy, Elsevier, Amsterdam, 1986. L.-M. Wu, H.-B. Teng, X.-C. Feng, X.-B. Ke, Q.-F. Zhu, J.-T. Su, W.-J. Xu, X.-M. Hu, Cryst. Growth Des.,2007, 7, 1337–1342 a) E. Hadjoudis, I. M. Mavridis, Chem. Soc. Rev., 2004, 33, 579-588; b) P. Barbazán, R. Carballo, B. Covelo, C. Lodeiro, J. C. Lima, E. M. Vázquez-López, Eur. J. Inorg. Chem., 2008, 2713–2720. FULL PAPER [17] [18] [19] [20] [21] [22] [23] [24] A. W. Addison, T. N. Rao, J. Reedijk, J. Van Rijn, G. C. Verschoor, J. Chem. Soc. Dalton Trans., 1984, 1349-1356. a) Z. Xu, X. Zhang, W. Zhang, Y. Gao, Z Zeng, Inorg. Chem. Commun., 2011, 14, 1569-1573; b) G. L. Parrilha, R. P. Vieira, A. P. Rebolledo, I. C. Mendes, L. M. Lima, E. J. Barreiro, O. E. Piro, E. E. Castellano, H. Beraldo, Polyhedron, 2011, 30, 1891-1898; c) J. J. Vittal, X. Yang, Cryst. Growth Des., 2002, 2, 259-262; d) A. K. Singh, M. Yadav, S. K. Singh, S. Sunkari, S. Pandey, Inorg. Chim. Acta, 2010, 363, 995-1000. G. M. Sheldrick, SHELXL-2013, Program for the Refinement of Crystal Structures from X-Ray Data, University of Göttingen: Germany,2013. Z. Zhou, W. Li, X. Hou, L. Chen, X. Hao, C. Redshaw, W. - H. Sun, Inorg.Chim.Acta, 2012, 392, 292-299. a) M. M. Henary, C. J. Fahrni, J. Phys. Chem. A, 2002, 106, 52105220; b) V. Luxami, S. Kumar, RSC Adv.,2012, 2, 8734-8740. J. Wu, W. Liu, J. Ge, H. Zhang, P. Wang, Chem. Soc. Rev., 2011, 40, 3483-3495. K. Sakai, H. Kawamura, N. Kobayashi, T. Ishikawa, C. Ikeda, T. Kikuchi, T. Akutagawa, CrystEngComm, 2014, 16, 3180–318. Y. Xu, Y. Pang, Dalton Trans., 2011, 40, 1503–1509. [25] [26] [27] [28] [29] [30] [31] [32] a) K. Rurack, Spectrochimica Acta Part A, 2001, 57, 2161-2195; b) J. H. Chang, Y. Jeong, Y-K. Shin, Bull. Korean Chem. Soc., 2003, 24, 119122; c) A. P. de Silva, H. Q. N. Gunaratne, T. Gunnlaugsson, A. J. M. Huxley, C. P. McCoy, T. Rademacher, T. E. Rice, Chem. Rev., 1997, 97, 1515-1566; d) D. Merrill, J. M. Harrington, H-S. Lee, R. D. Hancock, Inorg. Chem., 2011, 50, 8348-8385. Y. Xu, Y. Pang, Chem. Commun., 2010, 46, 4070-4072. Z. Otwinowski, W. Minor, Methods in Enzymology vol. 276: Macromolecular Crystallography, part A, (Eds.: C. W. Carter, Jr., R. M. Sweet) Academic Press, New York, 1997, pp. 307-326. Siemens SAINT, Version 4, Software Reference Manual, Siemens Analytical X-Ray Systems, Inc., Madison, WI, USA, 1996. G.M. Sheldrick, SADABS, Program for Empirical Absorption Correction of Area Detector Data, University of Göttingen, Germany, 1996. G.M. Sheldrick, Acta Crystallogr. A, 2008, 64, 112-122. I. J. Bruno, J. C. Cole, P. R. Edgington, M. K. Kessler, C. F. Macrae, P. McCabe, J. Pearson, R. Taylor, Acta Crystallogr. B, 2002, 58, 389-397. A. L. Spek, PLATON, A Multipurpose Crystallographic Tool, Utrecht University, Utrecht, The Netherlands, 2001. FULL PAPER Scheme 2. Schematic representation of the compounds isolated and those characterized by X-ray diffraction (bottom). Some of the DMSO molecules are omitted for clarity. FULL PAPER Figure 5. Spectrophotometric (top) and fluorimetric (bottom) monitoring of the reactions of H4L with CoII (left), NiII (middle) and CuII (right) cations in DMSO at r.t. [H4L] = 1·10–5 M, [MII] = 1·10–2 M. λexc= 430 nm. FULL PAPER Figure 7. Top: schematic representation of the coordination sequence. Bottom: spectrophotometric (top) and fluorometric (bottom) monitoring of the reactions of H4L (black lines) with ZnII (gray solid lines) to a 1:1 molar ratio and then with CoII (A), NiII (B), CuII (C) cations (gray dashed lines) in DMSO at r.t. ([H4L] = 1·10–5 M, [MII] = 1·10–2 M, λexc = 430 nm). FULL PAPER Entry for the Table of Contents (Please choose one layout) Layout 1: FULL PAPER Reactions of bis(benzoylhydrazone) of 4,6-diacetylresorcinol with M2+ (M = Zn, Cu, Ni, Co) yield three kinds of complexes: homodinuclear [M2(H2L)2], homotetranuclear [M4(L)2] and, heterotetranuclear compounds [Zn2M2(L)2]. The ligand luminescence properties and its fluorimetric behavior towards MII metals (M = Zn, Cu, Ni and Co) has been studied. Sabina Rodríguez-Hermida, Ana B. Lago, Rosa Carballo, Oscar Fabelo and Ezequiel M. Vázquez-López* Page No. – Page No. Homo- and Heteronuclear Compounds with a Symmetrical BisHydrazone Ligand: Synthesis, Structural Studies and Luminescent Properties Layout 2: FULL PAPER Author(s), Corresponding Author(s)* ((Insert TOC Graphic here)) Page No. – Page No. Title Text for Table of Contents