Unusual Catalytic Process Involving OH and NH Exchange

Interaction of some transition metals species with a Schiff base, 1, derived from 4,6-diacetylresorcinol and 2-amino5-methylphenol (1:2 mole ratio) yielded the two compounds 4-ethanimidoyl-6-[(1E)-N-(2-hydroxy-4-methylphenyl) ethanimidoyl]benzene-1,3-diol, 2, and 4,6-diacetyl-1,3-diaminobenzene, 4 via unusual catalytic process involving OH and NH exchange. All the compounds are characterized by elemental analysis, IR, mass and H NMR. The structure of the two compounds 2 and 4 are also determined by X-ray analysis. The crystal structure analysis revealed that the two compounds crystallized in the monoclinic space group P21/c with a Z value of 4.


Introduction
Coordination chemistry of macrocyclic ligands has shown to be interesting subject of current research in the last two decades [1,2].Importance in designing new macrocyclic ligands arises mainly from their use as models for protein-metal binding sites in a substantial array of metalloproteins in biological systems, such as the synthetic ionophores, models for the magnetic exchange phenomena, therapeutic reagents in chelate therapy for treatment of metal intoxication and the cyclic antibiotics that retain their antibiotic actions to specific metal complexation [3][4][5][6].On the other hand, the synthesis of binuclear complexes in which a ligand structure accommodates two metal centers in close proximity but in a different compartments separated by an intervening group represents important criteria in the study of transitionmetal systems.In such molecularly designed ligands, the aromatic rings are expected to act as a bridge and a rigid separator between the two compartments.The interest in these complexes corresponds to their ability to serve as simple models for multi-metal-centered catalysts [7,8].From these types of ligands, the macrocyclic Schiff bases exhibited great importance in macrocyclic chemistry because they can selectively chelate certain metal ions depending on the number, type and position of their donor atoms, the ionic radii of the metal centers, and the coordinating property of the counter ions [9].In this report, we synthesized a molecularly designed macrocyclic Schiff base to serve as a host for two metal centers.However, unusual behavior was observed in the attempts of interactions of this ligand with different transition metal species.

Reagents
4,6-diacetylresorcinol, 2-amino-5-methylphenol, ferric chloride, nickel nitrate and palladium(II) chloride were purchased from Aldrich.All the solvents were of analytical reagent grade and were purified by standard methods.

Instruments
IR measurements (KBr discs) were carried out on a Unicam-Mattson 1000 FT-IR spectrometer.NMR measurements were performed on a Spectrospin-Bruker 300 MHz spectrometer.Samples were dissolved in (CD 3 ) 2 SO and TMS was used as an internal reference.Elemental analyses were performed on Perkin-Elmer 2400 CHN elemental analyzer.Mass spectrometry measurements of the solid complexes (70 eV, EI) were carried out on a Finnigan MAT SSQ 7000 spectrometer.

Preparation of 4,6-Diacetyl-1,3diaminobenzene (4)
Similar procedure was employed as that used for preparation of compound 2 but the mixture was heated to reflux for 3 h.The color of the solution was turned to dark brown.The mixture was left to stand overnight at room temperature where white needle crystals were isolated.[10] and refined by full matrix least-squares methods maXus [11].The displacement factors of non-hydrogen atoms of the two compounds were refined with anisotropic thermal parameters.The hydrogen atoms were refined isotropically.The function minimized was ( ) 2 .The final R and R w values are given in Table 1.

IR and NMR Studies
The Schiff base 4,6-di[(1E)-N-(2-hydroxy-4-methyl-phenyl)-ethanimidoyl]benzene-1,3-diol, 1, was prepared by the condensation reaction of 4,6-diacetylresorcinol and 2-amino-5-methylphenol in 1 to 2 molar ratio (Scheme 1).This Schiff base was molecularly designed to be used as a bicompartment ligand.The IR spectrum of 1 showed characteristic stretching frequencies due to the functional groups [12].Also, the presence of the OH groups was confirmed by 1 H NMR. Direct interaction of compound 1 with some transition metals species such as Fe(III), Ni(II) and Pd(II) in ethanol exhibited unusual catalytic behavior to give the two compounds 4-ethani-midoyl-6-[(1E)-N-(2-hydroxy-4-methylphenyl) ethanimidoyl] benzene-1, 3diol, 2, and 4,6-diacetyl-1,3-diaminobenzene, 4. Interestingly, this catalytic process was deactivated by carrying out the reactions in basic medium, i.e., by addition of sodium hydroxide.The two compounds 2, 4 were characterized by elemental analysis, IR, 1 H NMR as well as X-ray crystal structure.The IR spectra of both 2 and 4 showed characteristic bands corresponding to the functional groups in the compounds [12].Interestingly, the 1 H NMR spectrum of compound 4 showed broad signals at 12.72 and 6.41 ppm due to both OH and NH.Therefore, the structure of the compound in solution is different from the solid state structure which indicated by X-ray analysis.The presence of a signal at 12.72 ppm could be due to hydrogen bonding between one of the NH 2 protons and the carbonyl oxygen [13][14][15][16].
A plausible mechanism for the formation of the two compounds 2 and 4 is given in Scheme 2. It is suggested that the interaction of the metal ions with the bicompartment Schiff base, 1, caused fragmentation of the base to give compound 2 (Scheme 1).On the other hand, further reaction with the metal ions with 2 might lead to the formation of intermediate 3, also, via fragmentation of 2. The existence of this intermediate was suggested according to 1 H NMR evidence.Also, the presence of both NH and OH signals in the 1 H NMR spectrum of compound 4 is additional evidence.However, attempts to isolate intermediate 3 in the solid state were unsuccessful.Scheme 2 shows a speculated mechanism for the formation of 4,6-diacetyl-1,3-diaminobenzene, 4, through a radical mechanism.To our knowledge, this is the first example observed for such type of exchange on a phenyl ring.However, the proton NH/OH exchange using 1 H, 13 C and 15 N NMR was previously observed [17].Also, the NH/OH exchange as well as proton exchange is a familiar feature on metal centers [18][19][20].
The crystal structure of the compounds 2 and 4 were determined by X-ray analysis.The crystallographic data are presented in Table 1.The ORTEP representations of the two compounds are illustrated in Figures 1 and 2. Selected bond lengths and angles are given in Tables 2  and 3.The crystal analysis revealed that the two compounds crystallized in the monoclinic space group P2 1 /c with a Z value of 4. From the structural analysis of compound 2 (Figure 1), it can be noted that the compound is totally unsymmetrical having the point group C s .The fragments N1-C19-C18 and N1-C19-C9 are planar which revealed the sp 2 hybridization of the N1, C19 and C18 atoms.The bond angles between these atoms lie in the range of 120˚ (Table 2).Also, the bond angle C19-N1-H1 is linear (179.9˚).Furthermore, the bond length of C19-N1 is 1.212 Å, which is shorter than the normal single C-N bond (vide infra), and indicated the presence of double bond character [21,22].The bond angles in the azomethine part (C11-N5-C13) showed the presence of Scheme 1. Synthesis scheme of compounds 2 and 3.      .The content of one unit cell (Figure 3) showed that the dihydroxy phenyl moieties of every two molecules are located in opposite faces and are parallel.This could be due to a charge transfer between every two moieties with π-π* type of interaction [23].The X-ray crystal analysis of compound 4, Figure 2, revealed that it has a C 2 axis of rotation with two planes of symmetry perpendicular to the symmetry axis (C 2v symmetry).The bond angles between atoms are consistent with such symmetry (Table 3).However, the O-C-C angles located near the nitrogen atoms are slightly wider than the other angles.The bond length of C-N bond is typically single bond (N 1 -C 8 = 1.338Å and N 3 -C 7 = 1.348Å) [21,22].On the other hand, the C-O bond lengths are consistent with the presence of double bond character (O 2 -C 12 = 1.226Å and O 9 -C 11 = 1.224Å) [21,22].These bond lengths are shorter than the single C-O bonds observed for compound 2 (Scheme 1).The unit cell packing of 4 (Figure 4) exhibited that the  molecules are located in opposite faces and are in parallel stacks.Again, this could be due to a charge transfer between every two molecules with π-π* type of interaction [23].

Supplementary Data
Supplementary crystallographic data (Atomic positional parameters, all bond lengths and angles, anisotropic temperature factors and the calculated and observed structure factors) are available from the CCDC, 12 Union Road, Cambridge CB2 IEZ, UK on request, deposition number: CCDC for: (2) 821862; (4) 821863.

4 Scheme 2 .
Scheme 2. Suggested mechanism for the formation of the two compounds 2 and 4.
conjugation between the C-N bonds and the two phenyl groups attached to the group (1.431 and 1.324 Å).The X-ray analysis of compound 2 showed that it contains 3 OH groups attached to the phenyl rings.Interestingly, the C-O bond length of the OH group attached to the position para to the C-N-H group (C 16 -O 2 = 1.281Å) is shorter than the other C-O bonds (C 6 -O 3 = 1.355Å and C 20 -O 4 = 1.339Å)

Figure 3 .
Figure 3.The unit cell packing of 2. Hydrogen atoms are omitted for clearance.

Figure 4 .
Figure 4.The unit cell packing of 4. Hydrogen atoms are omitted for clearance.