Different bridging preferences of zinc and palladium in dimeric complexes of a ligand based on 2-tosylaminomethylaniline

We report here the formation of dimeric palladium(II) and zinc(II) complexes of a ligand (H2L) derived from the condensation of 2-tosylaminomethylaniline and 2,3dihydroxybenzaldehyde. The structures in solution of the corresponding palladium(II) and zinc(II) complexes could be deduced from NMR studies. With the aim of establishing an energystructure relationship between the two possible linkage isomers (μ-O-bridged and μ-N-bridged), some theoretical energy calculations have been performed.


Introduction
Linkage isomerism attracts much interest, especially in connection to research on photochemical, biological and solid state reactions [1][2][3].A bibliographic study on dinuclear complexes of N-tosylated tridentate ligands has shown that the two metals ions are typically bonded through µ2-Ophenoxo [4,5] or µ2-Smercapto [6] bridges, but the structural characterisation of a dimeric copper complex displaying novel μ2-Nsulfonamido bridges [7] has demonstrated that this unfamiliar bridge was also possible, as well as that N-tosylated tridentate Schiff base ligands could be versatile linkers.Fig. 1 shows the molecular structures of four dinuclear complexes displaying µ2-Ophenoxo, µ2-Smercapto and μ2-Nsulfonamido bridges.
Since we have found that an N,N,N-donor Schiff base ligand based on 2-tosylaminomethylaniline was suitable for yielding dimeric Ni 2+ , Pd 2+ , Cu 2+ , Zn 2+ and Cd 2+ complexes through μ2-Nsulfonamido bridges [7], we have extended our investigations to H2L, which is a tridentate linear ligand that can simultaneously bond two metals through either µ2-Ophenoxo bridges or µ2-Nsulfonamido ones (Fig. 2).The easy bideprotonation of its OH and NH groups, as well as a suitable N,N,O binding domain have been considered during the design step of this N-tosylated tridentate Schiff base ligand, as adequate to forming dimeric complexes.

Results and discussion
Synthesis of the compounds.H2L has been synthesized in almost quantitative yield via nucleophilic addition of 2-tosylaminomethylaniline [8] to 2,3-dihydroxybenzaldehyde. Pd2(L)2 could be prepared by reaction of Pd(OAc)2 and H2L in low yield, with formation of acetic acid during the reaction.In view of this fact, we have used an electrochemical method for the synthesis of Zn2(L)2, in which a zinc metal anode was oxidised in the presence of a solution of the ligand.This method is particularly adequate to forming complexes of the type M2L2 because during the process H2 instead of H3O + is evolved to the reaction medium, and therefore, bideprotonation of the ligand is not hindered.Spectroscopic characterisation of the compounds.The structures of H2L and its complexes were elucidated on the basis of NMR, FT-IR, and mass spectroscopies.The dinuclear nature of the zinc(II) and palladium(II) complexes is evidenced by the corresponding MALDI-TOF mass spectra.Single crystal X-ray diffraction techniques have provided precise information about the molecular structure of H2L.NMR assignment of the signals was made by a combination of COSY and NOESY experiments.
The free Schiff base ligand was easily identified from the imino proton signal (8.7 ppm) and the magnetically equivalent methylene protons (4.1 ppm) [7].The absence of the signals corresponding to the most acidic protons, i.e. those corresponding to the OHphenol and NHsulphonamide groups, demonstrates the dianionic behavior of the ligand in Zn2(L)2 and Pd2(L)2.The infrared spectra of H2L displays five characteristic bands ring at about 3350, 3250, 1600, 1300 and 1150 cm −1 , which are attributed to ν

(O-H), ν(N-H), ν(C=N), νas(S-O) and νs(S-O)
vibrations, respectively.The absence of the two first bands in the spectra of the complexes supports the dianionic behaviour of the ligands after complexation.
Crystal structure of H2L.Single crystal X-ray diffraction techniques have confirmed that orange prismatic crystals obtained after room temperature evaporation from a methanol solution of H2L consist of crystallographically independent molecules of the enol tautomer of H2L (Fig. 3).The enol nature of the molecule is deduced from the bond distances of the phenol groups.Bond distances and angles, which are shown in Table 1, fall within the usual ranges for related ligands [4][5][6][7].Since the geminal protons at the methylene group (prochiral center) of Zn2(L)2 give rise to a single peak (isochronous NMR signal), they are enantiotopic protons (Fig. 4, left, δ = 3.90 ppm).
This result is consistent with the idea that Zn2(L)2 exhibits rapid rotational motion of methylene groups on the NMR time scale at room temperature [9].The two protons labelled as H7 can interconvert easily by fast rotation about a single carbon-carbon bond and therefore become equivalent.This spectral feature matches up with the observed in NMR spectra of free ligands based on 2-tosylaminomethylaniline and in its mononuclear complexes too [10].A dimeric structure displaying µ2-Ophenoxo bridges is consistent with a conformation that permits a rapid rotational motion of methylene groups on the NMR time scale [11].
In contrast, the 1

Conclusions
We have demonstrated that N-tosylated tridentate Schiff base ligand H2L derived from the condensation of 2-tosylaminomethylaniline and 2,3-dihydroxybenzaldehyde is adequate to obtain linkage isomers O-bridged and N-bridged.Experimental studies in solution have revealed the formation of syn-μ2-O-Zn2(L)2 and syn-μ2-N-Pd2(L)2.Theoretical calculations support the latter.

Experimental
Syntheses of the ligand H2L: A solution (absolute ethanol, 40 mL) of 2,3-dihydroxybenzaldehide (0,15 g, 0,73 mmol) and 2-tosylaminomethylaniline (0,20 g, 0,73 mmol) in was heated under reflux for 3 h.The resulting orange solution was concentrated to dryness under vacuum resulting in an oily fluid, which after stirring with diethyl ether (20 mL) for 8h leads to an orange solid that was filtered off and then dried under vacuum.Recrystallization of the brute in methanol leads to prismatic orange crystals, which were adequate to single X-ray diffraction techniques.Yield = 0,22 g (76%).Mp = 141 °C. 1 H NMR (500 MHz, DMSO-d6, δ in ppm): 12,56 (s, 1H, HO20), Although no problem has been encountered in this work, all perchlorate compounds are potentially explosive and should be handled in small quantities and with great care!).An acetonitrile solution (80 mL) of H2L (0,05 g, 0,13 mmol), containing tetraethylammonium perchlorate (ca.50 mg) that is used as supporting electrolyte, was electrolysed for 1h and 21 min at a current intensity of 5.2 mA and an initial voltaje of 5 V.The resulting pale yellow precipitate was filtered off, washed with acetonitrile (20 mL) and then dried under vacuum.

Fig. 3 .
Fig. 3. Top: Ellipsoids view for the molecular structure of H2L, showing an intramolecular H-bond.Bottom: Partial view of the crystal packing of H2L showing intra-and intermolecular H-bonds that associate two contiguous molecules.
H NMR spectrum of Pd2(L)2 revealed two very different chemical shifts, 4.50 and 6.25 ppm (anisochronous NMR signal) for the two geminal protons of the methylene group (Fig.4, right) despite being not placed in a chiral environment, and therefore are diastereotopic protons.The observed AB doublet with a chemical shift difference of ca.1.75 ppm persists from -80 °C to the high-temperature limit of 40 °C in acetone-d6.This result is consistent with the idea that Pd2(L)2 is conformationally locked and cannot undergo rapid rotational motion of methylene groups.A dimeric structure displaying μ2-Nsulfonamido bridges is consistent with a conformation that prevents/hinders fast rotation about a single carbon-carbon bond on the NMR time scale.