Synthesis, characterization and crystal structure of a new binuclear copper(II) complex with indole-3-propionic acid

Deng Xiujun, Bian Hedong, Yu Qing, Liang Hong
(College of Chemistry and Chemical Engineering, Guangxi Normal University, Guilin 541004, China)

Supported by Teaching and Research Program for Outstanding Young Teachers in Higher Education Institutions of Ministry of Education, the Nature Science Foundation of Guangxi Province and the Nature Science Foundation of Guangxi Normal University.

Abstract A binuclear copper(II) complex, Cu2(IPA)4(CH3OH)2 (1) (HIPA = indole-3-propionic acid), was synthesized and characterized by IR, elemental analysis and X-ray diffraction. The crystal of 1 belongs to Triclinic system, space group P. Two Cu(II) ions in 1 are bridged by four IPA anions to form a binuclear complex. Each central Cu(II) ion is coordinated by four carboxylate O atoms from four bridging IPA anions and one O atom from a methanol molecule in distorted square pyramidal geometry.

    Indole-3-propionic acid (HIPA), a deamination product of tryptophan is formed by symbiotic bacteria in the gastrointestinal tract of mammals and birds[1]. HIPA may hold particular promise as a therapeutic agent in human brain diseases[2]. It was found that anti-inflammatory and antibacterial activity of metal complexes was higher than in the parent carboxylic acids[3]. The carboxylate group displays a variety of binding geometries, such as monodentate, chelating, bidentate bridging and monodentate bridging so in coordination chemistry as well as in the active sites of metalloenzymes[4-8]. It should be noted that the crystal structures of HIPA have been studied[9,10], but there are seldom reports about the crystal structure of its metal complexes[11], as yet. In the present paper, Cu2(IPA)4(CH3OH)2 was synthesized and characterized by IR, elemental analysis, and X-ray diffraction.
    2.1 Instruments and chemicals
    IR spectrum was recorded on a Perkin-Elmer Spectrum one FT-IR Spectrometer in the 4000 -400 cm-1 region by using KBr pellets. Elemental analysis for C, H and N atoms were carried out on a Model 2400 II Perkin-Elmer elemental analyzer. All reagents were of analytical grade from commercial sources and were used without any further purification.
    2.2 Synthesis of Cu2(IPA)4(CH3OH)2 (1)
    HIPA was dissolved in 5 mL of methanol and the pH value was adjust to about 7 with 1 mol·L-1 NaOH solution. Then the solution of Cu(NO3)2·3H2O (0.5 mmol) in 10 mL of methanol was added. The mixture was stirred at 60ºC for 4 h. and then filtered. The filtrate was allowed to stand in air at room temperature for several days, yielding green platelike single crystals suitable for analysis.
    IR (KBr) n:. 3411.64(vs), 1606.18(s), 1423.69(vs), 1384.79(ms), 1358.64(w), 1226.90(w), 1094.85(m), 1010.66(m), 740.23(s), 613.77(m), 493.85(m)cm-1.
    Anal. calcd for C46H48Cu2N4O10: C, 58.48; H, 5.13; N, 5.93%. found: C, 57.62; H, 5.52; N, 5.86%.
    2.3 X-ray crystallography of 1

Table 1 Crystal data and structure refinement for complex 1

Empirical formula C46H48Cu2N4O10 Absorption coefficient£¨mm-1£© 1.020
Formula weight 943.96 F(000) 980
Crystal system Triclinic Crystal size (mm3) 0.21 ¡Á 0.17 ¡Á 0.07
space group P q (¡ã) 1.41 to 25.01
a (nm) 0.7521(2) Limiting indices -8¡Üh¡Ü8, -18¡Ük¡Ü13, -24¡Ül¡Ü21
b (nm) 1.5799 (3) Reflections collected 10895
c (nm) 2.0432(3) Independent reflections 7338
a (¡ã) 68.330(2) Data / restraints / parameters 7338 / 18/ 559
b (¡ã) 85.804(3) Goodness-of-fit on F2 0.993
g (¡ã) 79.622(2) R [I>2s(I)] 0.0995
Volume (nm3 ) 2.2192(8) wR2 0.1555
Z 2 Maximum diff. peak (e/nm3) 1.248
Dc (g/cm3) 1.413 Minimum diff. peak (e/nm3) -1.351

A single crystal of 1 with dimensions of 0.21 mm ¡Á 0.17 mm ¡Á 0.07 mm was selected and mounted on a Bruker Smart 1000 diffractometer with Mo Ka radiation (l = 0.071073 nm) by using an w-scan technique at 298(2) K. 10895 reflections were collected in the rang of 1.41 to 25.01, of which 7338 (Rint = 0.0863) unique reflections and 2858 observed ones with I >2s (I) were used in the succeeding refinements. The coordinates of the hydrogen atoms were obtained from difference Fourier maps and refined with a common isotropic thermal parameter. All calculations were carried out using SHELXS-97[12] and refined by full-matrix least-squares, based on F2, using SHELXL-97[13] programs. Details of data collection and processing are given in Table 1.


Fig. 1¡¡The molecular structure of Cu2(IPA)4(CH3OH)2 (H atoms are omitted for clarity)

The selected bond lengths and bond angles are listed in Table 2, and Figure 1 shows the molecular structure of 1. In the complex, four IPA anions adopt bidentate bridging coordination mode using two oxygen atoms of the carboxylate groups. The Cu···Cu separation is 0.26221(17) nm which is comparable well to that in the reported dinuclear caboxylate-bridged complexes[14-18]. Each Cu(II) atom is coordinated by five oxygen atoms from four bridging ligands and one methanol molecule. The Reejijk distortion index t value[19] is 0.00183 nm for Cu1 and 0.00267 nm for Cu2, respectively, indicating slightly distorted square pyramidal (sp) configuration for the copper(II) atoms. The equatorial plane of each copper(II) contains of four oxygen atoms from four ligands with the average Cu-O bond lengths of 0.19744 and 0.19817 nm for Cu1 and Cu2, respectively. Cu1 and Cu2 are displaced by 0.02006 and 0.01854 nm out of the basal plane towards the axial methanol ligand, respectively. The methanol O atom occupies the apical position of the coordination sphere. The molecular architecture is stabilized by an extensive 1-D band (Fig.2, Table 3) of hydrogen bonds (N-H···O) and (O-H···O) involving the IPA ions and methanol molecules.

Fig. 2 the 1D band diagram of the title compound along a-axis
(Some atoms have been omitted for carily)

Table 2¡¡Selected bond lengths (nm) and bond angles (o)

Bond length ¡¡ ¡¡ ¡¡
Cu(1)-O(5) 0.1963(7) Cu(2)-O(6) 0.1960(7)
Cu(1)-O(1) 0.1967(8) Cu(2)-O(4) 0.1976(7)
Cu(1)-O(7) 0.1974(7) Cu(2)-O(2) 0.1987(7)
Cu(1)-O(3) 0.1991(7) Cu(2)-O(8) 0.2001(7)
Cu(1)-O(9) 0.2156(7) Cu(2)-O(10) 0.2191(7)
Cu(1)-Cu(2) 0.26221(17)
Bond angle
O(5)-Cu(1)-O(1) 167.5(3) O(6)-Cu(2)-O(4) 89.6(3)
O(5)-Cu(1)-O(7) 91.0(3) O(6)-Cu(2)-O(2) 170.0(3)
O(1)-Cu(1)-O(7) 89.0(3) O(4)-Cu(2)-O(2) 89.4(3)
O(5)-Cu(1)-O(3) 90.9(3) O(6)-Cu(2)-O(8) 88.5(3)
O(1)-Cu(1)-O(3) 86.7(3) O(4)-Cu(2)-O(8) 168.4(3)
O(7)-Cu(1)-O(3) 168.6(3) O(2)-Cu(2)-O(8) 90.6(3)
O(5)-Cu(1)-O(9) 97.7(3) O(6)-Cu(2)-O(10) 93.2(3)
O(1)-Cu(1)-O(9) 94.7(3) O(4)-Cu(2)-O(10) 95.9(3)
O(7)-Cu(1)-O(9) 97.0(3) O(2)-Cu(2)-O(10) 96.8(3)
O(3)-Cu(1)-O(9) 93.8(3) O(8)-Cu(2)-O(10) 95.6(3)

Table 3 Hydrogen-bonding geometry (nm, ¡ã) for the title complex

D-H d(D-H) d(H..A) <DHA d(D..A) A
N4-H4 0.860 2.151 151.28 2.934 O1[-x+3, -y+3, -z+1]
O9-H9 0.820 2.452 118.53 2.934 O8[ x+1, y, z ]
O10-H10 0.820 2.484 133.86 3.109 O3[ x, y, z ]