ABSTRACT

The reaction of the cobalt(II) meso-tetraphenylporphyrin (TPP) starting material with an excess of 2- aminophenol (Hon) in organic solvents, yields the cobalt(II) porphyrin species [Co^II(TPP)(Hon)₂] (1). This compound has been characterized by UV-vis, IR, MSI MS and ¹H NMR spectroscopy. The UV-vis data and especially the proton NMR results, for the isolated product, indicated that complex 1 is a Co(II) mesoporphyrin derivative. The X-ray molecular structure of the title compound bis(2-aminophenol) (tetraphenyl-porphyrinato) cobalt(II) has been determined. This structure is the first one reported of a metalloporphyrin with a 2-aminophenol axial ligand species. The central metal is hexacoordinated by the four nitrogen atoms of the pyrrole rings and the nitrogen atoms of the two Hon trans axial ligands.

1. INTRODUCTION

Complexes with noble metal such as rhenium and technetium have been used for decades in medicine specially as potential radiotherapeutic agents for cancer [1] and many cobalt(II) phthalocyaninetetrasodiumsulfonate has been found to enhance the rate of oxidation of 2-aminophenol with dioxygen in water to 2-amino-phenoxazin- 3-one [2]. Thus, the main goal of the present work is to prepare a metalloporphyrin complex with much cheaper metal in particular transition metals such as cobalt, which is coordinated to a 2-aminophenol (Hon) axial ligand [3]. Such species can eventually replace the rhenium and technetium as potential radiotherapeutic agents and also can be used as catalysts in many organic reactions. It is noteworthy that no structures of 2-aminophenol Co(II) porphyrin derivative were reported up to day. The only reported cobalt(II) with a substituted 2-aminophenol is the polymer{[Co^II(m₂-AT)]}_n(where m₂-AT = m₂-3-amino- (S)-tyrosinato) [4]. In order to gain more insight into the physico-chemistry properties of this new Co(II)-porphyrin derivative. We report in this paper the results of the spectroscopy and the X-ray molecular structure investigation on the 2-aminophenol cobalt(II) meso-tetraphenylporphyrin complex [Co^II(TPP)(Hon)₂].

2. EXPERIMENTAL

2.1. Materials and Methods

Ultraviolet-visible (UV-vis) spectra were measured on a SHIMADZU UV-2401 spectrometer and IR spectra were recorded from pure products using a PerkinElmer Spectrum 100 FT-IR equipped with a single-bounce diamond attenuated total reflectance (ATR) sampling accessory. Proton magnetic resonance spectra were measured at room temperature on a Bruker 300 Ultrashield spectrometer. Mass spectra were recorded on spectrometers MS/ MS API-365 (Perkin Elmer Sciex) equipped with electrospray source (ESI). All chemicals were purchased from SIGMA-ALDRICH Co. LLC. They were used as received without further purification except the dichloromethane which was distilled under CaH₂ and freshly used.

Synthetic Methods

The free base 5,10,15,20-tetraphenylporphyrin and the corresponding cobalt(II) derivative [Co^II(TPP)] were synthesized by literature methods [5,6].

2.2. Synthesis of [Co^II(TPP)(Hon)₂]

[Co^II(TPP)] [6] (100 mg, 0.149 mmol) and (190 mg, 2.231 mmol) of 2-aminophenol in 25 mL of dichloromethane were stirred overnight at room temperature. The color of the solution turns from red-orange to dark-red and single crystals of the complex were prepared by slow diffusion of the hexanes into the CH₂Cl₂ solution. Typical yields were 55% - 65%. UV-vis [l_max (nm) in CH₂Cl₂, (loge)]: 434 (4.46), 521 (3.54), 555 (3.41). IR (KBr):n, cm^-1 3411, 3385, 3354, 3329, 3298, 3027, 2324, 1888, 1811, 1622, 1595, 1500, 1440, 1378 , 1362, 1350, 1271, 1229, 1203, 1175, 1156, 1147, 1071, 1000, 993, 954, 928,  834, 796, 747, 701. ¹H NMR (CDCl₃): d, ppm, 16.07 (s, 8 H), 13.20 (s, 8 H), 9.97 (s, 8 H), 9.76 (s, 4 H), 5.30 (s, H), 3.70 (s, 2H). ESI(+) MS m/z calc for (C₆H₇NO) [M]⁺ 109.05, found 109.20; m/z calc for (C₄₄H₂₈N₄Co) [M]⁺ 671.16, found 671.40; m/z calc for (C₅₀H₃₅CoN₅O) [M-H]⁺ 779.21 found 779.50.

2.3. X-Ray Diffraction

A dark purple crystal of the cobalt(II) 2-aminophenol  porphyrin derivative with approximate dimensions of  0.44 × 0.40 × 0.27 mm³ was mounted under inert perfluoropolyether at the tip of glass fiber and cooled in the cryostream of an Oxford Diffraction XCALIBUR.

The data were collected at 180 K using the monochromatic Mo Ka radiation (l = 0.71073). Lattice parameters were obtained by least-squares fit to the optimized setting angles of the 4447 collected unique reflections in the full theta range data collectioon 3.02˚ < q < 26.02˚. Intensity data were recorded using w scans. Data reduction was done using CrysAlisPro, Oxford Diffraction Ltd. [7]. Empirical absorption correction using sphereeeeeecal harmonics, implemented in SCALE3 ABSPACK scaling algorithm. The minimum and the maximum transmission factors values are 0.8592 and 1.000, respectively. The structure was solved by direct methods using SIR-2004-1.0 [8] and refined by full-matrix least squares on |F|² using the SHELXL - 97 program [9]. The asymmetric unit of the structure contains half [Co^II(TPP)(Hon)₂] molecule where the Co(II) is located on an inversion center. The two hydrogens of the NH₂ group and the H of the OH group of the axial ligand were found in the difference Fourier map and were included in the refinement using restraints (N-H = 0.92(1) Å; O-H = 0.85(1) Å) with U_iso(H) = 1.2U_eq(N₃, O1). In the last refinement cycles, the H of the NH₂ group were fixed and treated as riding on their parent N atom. The H attached to O1 was kept refining. The positions of H atoms attached to C atoms were calculated and they were treated as riding on their parent C atoms with C-H = 0.95Å and U_iso(H) = 1.2 U_eq(C). Refinement was then carried on to convergence with anisotropic thermal parameters for all non hydrogen atoms. Crystal data and experimental parameters used for the intensity data collection are summarized in Table 1. Complete crystallographic details are available from the CCDC (see Supporting information section).

3. RESULS AND DISCUSSION

3.1. UV-Visible, IR and Mass Spectrometry

The electronic spectra of complex 1 and the [Co^II(TPP)] starting compound are represented in Figure 1. The Soret bands of the related species [Co^II(TpivPP)(OAc)]⁻ [10], and other Co(II) meso-porphyrin derivatives (Table 2), are in the range [438 - 441 nm]. This data confirm complex creation of a new Co(II)-meso-porphyrin coordination compound by displaying red shifted Soret band of complex 1. The IR spectrum of [Co^II(TPP)(Hon)₂] (Figure 2) shows the existence of the coordinated 2-aminophenol ligand. Thus, the frequencies of the O-H(Hon) and the N-H(NH₂) stretching are 3386 and 3332 cm⁻¹ respectively.

The ESI MS experiments recorded in the positive ion mode show the presence of the three fragments: [Hon]⁺ with a m/z value of 190.20, the [Co^II(TPP)]⁺ and the [Co^II(TPP)(on)]⁺ with a m/z values of 671.40 and 779.50 respectively. For the latter fragment, it corresponds to [M-H]⁺ where the deprotonation of the OH group occurs on one 2-aminophenol axial ligand.

3.2. Proton NMR Spectroscopy

The paramagnetic starting material [Co^II(TPP)] species (with the ground state configuration 3d⁷) presents downfield chemical shifts of the b-pyrrole protons (Hb-pyrr) = 15.75 ppm) as shown in Figure 3. For the diamagnetic cobalt(III) porphyrin derivatives (with the ground state configuration 3d⁶), the b-pyrrole protons resonate in the normal regions of the free base TPP porphyrin (8.1 ppm < δ(Hb-pyrr) < 9.1 ppm) [11,12] (Table 3). Complex 1

presents a peak at 16.07 attributed to the b-pyrrole protons. This is an indication that our derivative is a paramagnetic cobalt(II) meso-porphyrin species [13]. The ¹H NMR peaks for the NH₂ and the OH protons of the two Hon ligands appear at δ ≈ 3.7 ppm and δ ≈ 5.3 ppm respectively.

3.3. Crystal Structure

The structure of [Co^II(TPP)(Hon)₂] compound was determined at 180 K and Figure 4 is an ORTEP diagram [15] of this complex. The Co(II) is coordinated to thefour nitrogens of the porphyrin ring and the nitrogen at oms of the NH₂ groups of the two 2-aminophenol trans axial ligands. Selected bond distance (Ǻ) angles (deg) for the [Co^II(TPP)(Hon)₂] complex are represented in Table 4.The preference for the amino nitrogen is understanddable since Co(II) would have a lower affinity for the more electronegative oxygen donor atom, which prefers

ionic interaction with the metal. In the Cambridge Structural Database (CSD, version 5.33 of November 2011[16]) few structures of transition-metal-Hon structures werereported among them only one structure of a Co(II) substituted 2-aminophenol ligand was mentioned [4]. For all these published structures the metal ion is either bonded only to the nitrogen of the amino group or to the NH₂ group and the deprotonated oxygen (Table 5). Figure 5 illustrates the geometry of the transition metal (M) andthe amino group of one coordinated Hon ligand. The Co^II-NH₂(Hon) bond length for 1 [ 2.391(3) Å] is slightly longer than the one of the related Co^II-bis[2-m₂-3amino-(S)-tyrosinato] polymer derivative [4] with values of 2.311(7) and 2.258(7) Å. The only reported structures of complexes with metal coordinating Hon ligand are those with the rhenium transition metal [17,18] (Table 6) where the M-NH₂ distance is in the range [2.048(4)- 2.270(4) Å]. The C_I–NH₂ and C_II-OH of the Hon ligand for complex 1 and the reported complexes are within the normal values: [1.40 - 1.45 Å] and [1.35 - 1.39 Å] respectively. The C_I-N-M angle is 120.9(2)˚ for our species and the related Co(II) derivatives are in the range [115.4˚ - 121.0˚]. The complex [Co^II(TPP)(Hon)₂] presents an intramolecular hydrogen-bond (N₃-H…..O_I) with a distance of 2.677(5) Å which falls in the range [2.639(5) - 2.788(3) Å] for related species (Table 5).It has been noticed [19] for cobalt-porphyrin complexes that ruffling of the porphyrin core always results in a shortening of the porphyrin equatorial cobalt pyrrole nitrogen M-N_p bonds. Thus, for the very ruffled structure [Co^II(TPP)] [6], the Co-N_p bond length value is 1.923(4) Å. while the practically planar porphyrin core of the dimer

{[Co^II(TPP)(µ-4,4’-bipy)].2bipy}_n [10] presents a Co-N_p distances of 1.985(5) and 1.993(1) Å. The Co-N_p value for our complex [Co^II(TPP)(Hon)₂] is 1.974(2) Å which is an indication that 1 presents a slightly ruffled porphyrin core. On the other hand, the average displacements of the meso carbons above and below the porphyrin mean plan are small [−0.01 and 0.034 Å] which indicate a moderate ruffling.

The crystal structure of 1 is stabilized by intermolecular interactions type O-H.....Cg and C-H.....Cg where Cg is the centroid of pyrrole five member ring or phenyl  six member ring. A selection of these intermolecular distances is summarized in Table 6. Figure 6 is a drawing showing the packing in lattice of [Co^II(TPP)(Hon)₂], viewed down the a axis.

4. SUPPORTING INFORMATION

Crystallographic data (excluding structure factors) for the [Co^II(TPP)(Hon)₂] compound have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication number CCDC-878829. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44 1223-336-033; or email: HUdeposit@ccdc.cam.ac.ukU

5. ACKNOWLEDGEMENTS

The authors gratefully acknowledge financial support from the Ministry of Higher Education and Scientific Research of  Tunisia.

REFERENCES