Chiral oxazolidine complexes derived from phenolic Schiff bases.

Schiff bases derived from pyridyl- or salicyl- aldehydes and aminoalcohols can evolve to heterocyclic oxazolidines which, in the presence of cations allow the formation of uncommon coordination compounds. In this work, we report new Ni II and mixed valence Mn II /Mn IV complexes derived from pyridyl oxazolidines and the unprecedented characterization of enantiomerically pure oxazolidines derived from the condensation of o -vanillin with phenylglycinol in the presence of Ni II cations. The different reactivity of the pyridinic or phenolic Schiff bases has been compared and the new systems have been structural, optical and magnetically characterized. 8.1/8.0. The green precipitate obtained immediately after the mixing of the reagents was not identified but its IR spectrum shows the presence of H 2 L2 ligands, benzoate and azido ligand. The cyclization of part of the ligands was probably produced during the


INTRODUCTION
Schiff bases are versatile ligands that, depending on the starting carbonyl and amino fragments used for the condensation, can exhibit an enormous variety of properties such as different kind and number of N,O-donors, different charge or different predefined shapes (linear, cyclic, compartmental, etc.). This versatility can be very useful in the definition of the topology and properties of the derived coordination complexes. 1,2 More concretely, the reaction of ketones or aldehydes with 1,2-aminoalcohols is a simple method to prepare a large variety of Schiff bases 3 but in some cases, a further nucleophilic attack of the alkoxo group on the iminic C-atom takes place. Thus, instead of the usual condensation it allows the formation of oxazolidines, which consist in one saturated heterocyclic five-membered NCOCC ring. Oxazolidines are less studied than the related unsaturated oxazoles or oxazolines despite that the organic molecules or their 3 coordination compounds offer promising possibilities in different applied fields like medicinal chemistry 4,5 or catalysis. [6][7][8][9][10] Reaction of pyridine-or quinolinealdehyde with tris(hydroxymethyl)aminomethane (TRIS) is one of the reactions that, usually in the presence of metallic cations, tends to produce oxazolidines instead of the corresponding Schiff bases. The number of coordination complexes derived from 2-pyridinecarboxaldehyde [11][12][13] or quinolinealdehydes, [14][15][16] often mononuclear systems, is limited but has been characterized for several 3d cations along the last years.
However, this catalysed formation of the oxazolidine has not been reported yet when using salicylaldehyde or the related o-vanillin as starting carbonyl source.
Recently, our group has developed a research line on coordination compounds derived from Schiff bases, mainly focused on chiral systems starting from enantiopure reactants. [17][18][19][20] During this period, the reactions of 2-pyridinecarboxaldehyde with TRIS or o-vanillin with enantiomerically pure (R) or (S)-phenylglycinol, designed for the synthesis of Schiff base/azide clusters, allowed us to the characterization in good yield of the derived oxazolidine complexes containing the ligands H3L3 and H2L4, instead of the expected H3L1 and H2L2 bases depicted in Chart 1. The later ligand, H2L4, becomes particularly interesting because, on one hand, the example presented here is the first case in which one oxazolidine has been obtained from this kind of Schiff bases and, on the other hand, its chiral character leads to selective characterization of two enantiomeric oxazolidines. Chart 1. Precursor Schiff bases and the derived oxazolidines employed in this work. Asterisk denotes the chiral C-atom of the H2L2 base.
The reported systems exhibit some remarkable features such the different reactivity of the expected Schiff bases in some specific conditions, the larger manganese-oxazolidine complex reported until today, the unusual Mn II /Mn IV coexistence of manganese cations in oxidation state 5 differing by two units and the unprecedented chiral oxazolidine derived from H2L2 family of ligands. These different products with different metals obtained in similar reaction conditions can be, in a near future, a new usual way to prepare oxazolidine-derived complexes and one more step in the study of the occurrence of this synthetic path in the catalytic presence of some transition metals.

Materials and Physical Measurements.
2-pyridinecarboxaldehyde, o-vanillin and the chiral (R) and (S)-phenylglycinol chemicals were purchased to TCI Chemicals and used as received. The syntheses were performed at open air in reagent grade solvents. IR spectra (4000-400 cm -1 ) were recorded on a Bruker IFS-125 FT-IR spectrometer with samples prepared as KBr pellets. Variable-temperature magnetic studies were performed using a MPMS5 Quantum Design magnetometer operating at 0.03 T in the 300-2.0 K range. Diamagnetic corrections were applied to the observed paramagnetic susceptibility using Pascal constants. Fits of the experimental magnetic measurements were performed with PHI program. 21 ECD spectra in methanolic solution were recorded on a Jasco-815 spectropolarimeter.
X-ray crystallography. Details of crystal data, data collection and refinement for complexes 1, 2, 3S, 4R and 4S are summarized in ESI Tables S1 and S2. Collection data were made on a Bruker D8 Venture system equipped with a multi-layer monochromator and a Mo microfocus (= 0.71073 Å). All structures were solved using the Brucker SHELXTL software package and refined with SHELXL computer program. 22 Data were corrected for absorption effects using the multi-scan method (SADABS). Plots for publication were generated with ORTEP3 for Windows 23 and plotted with Pov-Ray programs. 6 All data can be found in the supplementary crystallographic data for this paper in cif format with CCDC numbers 1993842 -1993846. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.  The synthesis of the manganese complex 2 has been described starting from manganese formate but the same complex can be obtained starting from other manganese salts (nitrate, perchlorate or triflate) and in these cases, the formate anions were formed by catalytic oxidation of methanol, meaning that the final complex is extremely stable.

Crystal Structure of [Ni(HL2) 2 (HL4)( 11 -N 3 )]ꞏ2MeOH (R)-HL2, (4R), (S)-HL2, (4S).
Shape and bond parameters for the two enantiomers are practically identical and thus a common description for both will be provided. The system consists of two Ni II cations, two monoanionic HL2ligands, one monoanionic HL4ligand and one end-on azido bridge, Figure 5. Main bond parameters are reported in Table 4. The HL2ligands act as tridentate donors and are mer-

Synthetic aspects
Schiff bases like H3L1, derived from the reactions among TRIS and pyridyl or quinolinealdehydes, linked to polyoxometallates or lanthanides with SMM response for the Dy III case, have been recently characterized. [26][27][28] However, 3d complexes, containing cycled oxazolidines like H3L3 instead the corresponding Schiff base, appears to be the most common reaction and some mononuclear Cu II or dinuclear Fe III compounds derived from the pyridyl ligands [11][12][13] and several mononuclear or low nuclearity 3d systems derived from the quinoline or from the reaction of salicylaldehyde with 3-aminopropane-1,2-diol, that yields either oxazinanes and oxazolidines. 30 One factor that becomes crucial in well studied ligands as could be the diol derivatives of dipyridyl ketone, 31 is the presence of pyridinic rings that helps to

Magnetic study.
The MT value at room temperature for the two dinuclear complexes 1 and 4 are 2.90 and 2.64 cm 3 ꞏmol -1 ꞏK respectively, larger than the expected value for two non-interacting S = 1 spins (g = 2.00). On cooling, the plots reach a maximum value of 3.39 cm 3 ꞏmol -1 ꞏK at 45 K (1) and 3.56 cm 3 ꞏmol -1 ꞏK at 17 K (4) and below the maxima strongly decreases down to 0.73 cm 3 ꞏmol -1 ꞏK at 2 K for 1 whereas the decrease is moderate for 4, reaching 2.71 cm 3 ꞏmol -1 ꞏK at 2 K, Figure 7, top. This response evidences intramolecular ferromagnetic response and a very different anisotropic behavior or intermolecular interactions at low temperature.
In light of the structural data for 1, that show strong intermolecular interactions, Figure 1, and that D and z'J' parameters are always strongly correlated  Crystal data, collection details, IR spectra for complexes 1-4 and BVS calculations for 2 can be found in Tables S1-S3 and Figures S1-S3.