Acta Chim. Slov. 2003, 50, 207-222. 207 NEW NICKEL(II) COMPLEXES INCORPORATING DISSYMMETRIC TETRADENTATE SCHIFF BASE LIGANDS DERIVED FROM AMINOTHIOETHER PYRIDINE WITH N2OS CHROMOPHORES: SYNTHESIS, SPECTROSCOPIC CHARACTERIZATION AND CRYSTAL STRUCTURES OF [Ni(pytIsal)]C104 and [Ni(pytAzosal)]C104.H20 Nezamaddin Daneshvar,*" Lotf A. Saghatforoush,* Ali A. Khandar/ and Ali A. Entezami "' 'c Department oflnorganic Chemistry, Faculty ofChemistry, Univrsity of Tabriz, Tabriz, Iran Department ofOrganic Chemistry, Faculty ofChemistry, University of Tabriz * Corresponding author. fax: +98-411-3340191, Tel: +98-411-3393146, e-mail: nezam_daneshvar@yahoo. com Received 03-01-2003 Abstract The synthesis of nickel (II) complexes with five dissymmetric Schiff base ligands (abbreviataed pytIsalH, pytBrsalH, pytN02salH, pytOMesalH, and pytAzosalH) which were prepared from aminothioether pyridine and a salicylaldehyde derivative, are described. The ligands were characterized by 'H,L,C NMR, FTIR and elemental analysis. The complexes of these ligands were synthesized by treating an ethanolic solution of appropriate ligand with equimolar amount of Ni(C104)2.6H20 and methanolic NaOH or alternatively by a more direct route in which the two reactants are added to a solution of the ligand immediately after formation of the ligand and prior to any isolation. The complexes were characterized by elemental analysis, FTIR, electronic spectra and molar conductivity. The complex [Ni(pytIsal)]C104 (6) crystallizes in triclinc space group Pl with a = 12.188(2) A, b = 13.312(3) A, c = 13.348(3) A, h = 115.48(3)°, i = 90.78(3)°, t = 90.90(3), Z = 4, Rx = 0.0617 for 4276 reflections with I>2o(I). The complex [Ni(pytAzosal)]C104.H20 (10) crystallizes in orthorhombic space group P212121 with a = 7.093(5) A, b = 9.277(6) A, c = 34.62(3) A, h = 90°, i= 90°, t = 90°, Z = 4, R: = 0.0635 for 2185 reflections with I>2o(I). The coordination geometry of nickel ions in the complexes with mixed N, S and O donor atoms is square planar with a slight tetrahedron distortion. Ali of the complexes were found to be 1:1 electrolyte systems in acetonitrile. Cyclic voltammetry studies indicates that the complexes [Ni(pytXsal)]C104 (X = I and OMe) show quasi-reversible reduction to Ni(I) while this process for X = Azo and N02 is reversible and irreversible, respectively. Introduction Low symmetry complexes are very important in biological systems as well as in industrial catalysis. From the theoritical point of view they are interesting, too.1"5 Transition metals occur in metalloenzymes bound to a macrocycle such as heme ring or to donor atoms of peptide chains in a distorted environment, as in hemerythrin (Fe2), N. Daneshvar, L. A. Saghatforoush, A. A. Khandar, A. A. Entezami: New Nickel(II) Complexes... 208 Acta Chim. Slov. 2003, 50, 207-222. hemocyanin (CU2) and [Ni-Fe] hydrogenase enzymes.6"7 A great deal of work has been reported on the synthesis, structural investigations, various crystallographic features, mesogenic characteristics, structure-redox relationships and catalytic properties of different types of Schiff bases and their complexes with transition and non-transition elements. Dissymmetrical Schiff base ligands can clearly offer many advantages over their symmetrical counterparts in the elucidation of the composition and geometry of metal ions binding sites in metalloproteins, and selectivity of natural systems with synthetic materials.9 Dissymmetric Schiff base complexes as chiral analogues become more effective and prevalent in asymmetric catalysis.6'9"14 The synthesis of transition metal complexes containing thiolate ligand is an important area of study with implication in bioinorganic chemistry, catalysis and medical chemistry.15 The structural characterization of active site of the [Ni-Fe] hydrogenase, by which the catalytic site was revealed to contain a heterodimetallic nickel-iron cluster in a sulfur rich environment, has renewed interest in the chemical modeling of [Ni-Fe] hydrogenases. This hyphothesis that the coordination environment of nickel in NiFe hydrogenase and CO-dehydrogenase contains sulfur, nitrogen and/or oxygen donors, resulted in a continuing 1^17 effort to obtain nickel complexes with this kind of coordination environment. Herein, we describe the synthesis and spectroscopic characterization of five nickel(II) complexes with N2OS co-ordination sphere as depicted in Scheme 1, and the crystal structures of 4-iodo-2- {[2-(2-pyridin-2-yl-ethylsulfanyl)-ethylimino]methyl}phenol-nickel(II) ([Ni(pytIsal)]C104) (6) and 4-phenylazo-2-{[2-(2-pyridin-2-yl-ethylsulfanyl)-ethylimino]methyl}phenol-nickel(II) ([Ni(pytAzosal)]C104.H20 (10) are reported. Results and discussion l-(2-Pyridyl)-3-thia-5-amino pentane (pyta) is a convenient starting material for the svnthesis of dissymmetric ligands containing NSON coordination sphere. Schiff base condensation of pyta with several substituted salicylaldehydes yields the desired ligand in a facile manner and with good purity. Nickel(II) complexes of these ligands depicted in Scheme 1 have been prepared using an ethanolic solution of the suitable ligand with equimolar amounts of Ni(C104)2-6H20 in the presence of methanolic NaOH or alternatively by a more direct route in which two reactants are added to a solution of the ligand immediately after formation of the ligand and prior to any isolation. The same N. Daneshvar, L. A. Saghatforoush, A. A. Khandar, A. A. Entezami: New Nickel(II) Complexes... Acta Chim. Slov. 2003, 50, 207-222. 209 products obtained in both routes but the latter was less tirne consuming and gave higher yields. ~<*SLxA «¦ X=I, Br, N02, OCH3, N2Ph Scheme 1. The formation of Schiff base ligands andNi(II) complexes. The Schiff base ligands and complexes were characterized by the usual methods: elemental analysis, absorption electronic spectroscopy, molar conductivity, FTIR, ]H and C NMR and also crystal structure detenninations of the two of the complexes. The •3 measunnent of molar conductivities at 10" M concentration carried out in acetonitrile, the obtained AM values for monocationic complexes, indicating a behavior attributable 77 to 1:1 electrolytes. These complexes display good stability in the air at room temperature and are soluble in acetonitrile, methanol, dimethyl formamide and dimethyl sulfoxide. The structures of the ligands confirmed by the H and C NMR spectra data. The signal for proton of the -NH group was not found in the 'H NMR spectra and it is 7'J >JA suggested that the Schiff base ligands do not undergo keto-enol tautomerism. " A signal at 59.33 ppm in the C NMR spectrum of pytOMesalH can be attributed to the carbon atom of the methoxy group. FTIR spectra of the Ni(II) complexes are ali fairly similar, and not very different from the free ligands spectra. The ligands show strong bands at 1634-1655 cm"1 characteristic of the v (C=N) stretching mode. The shift of these strong bands towards lower frequencies by 16-35 cm"1, compared to free ligands N. Daneshvar, L. A. Saghatforoush, A. A. Khandar, A. A. Entezami: New Nickel(II) Complexes... 210 Acta Chim. Slov. 2003, 50, 207-222. spectra, are consistent with the iminic nitrogen coordination. The disappearance of the OH bands of the free ligands in the complexes show that the OH group has been deprotonated and bonded to metal ion. The spectra of [Ni(pytN02sal)]C104 and its free ligand show two bands at 1325 and 1557 cm" attributable to the nitro group, v (N=0). 1 9S The complexes display very strong band for perchlorate anion at 1086-1094 cm" . A relatively medium broad absorption band with maximum at 3600 and 3531 cm"1 indicates the presence of crystal water or methanol as the elemental analyses of [Ni(pytAzosal)]C104.H20 and [Ni(pytOMesal)]ClO4.0.5CH3OH show the presence of one mole of water and half mole of methanol in the one mole of the complex, respectively. The electronic spectra of the Ni(II) complexes, which give light brown reddish solutions in acetonitrile, were obtained. The [Ni(PytXsal)]C104 (X=I, Br, NO2, Azo, OMe) complexes show a low energy band as a shoulder at 538, 535, 530 and 540 nm, respectively, with the exception of [Ni(PytOMesal)]C104 which shows a broad low energy band at 579 nm. The relatively intense bands at 314-417 nm may be assigned to ligand to metal charge transfer or metal to ligand charge transfer bands. ' ' ' ' The absorption spectra of ali the complexes are similar despite the changes of nature of the salicylaldehyde substituents. This is not surprising since the d-d bands occur as a shoulder on the intense UV band, which can be expected to obscure changes in the d-d band positions brought about by small changes in the environment of the nickel atom. The low energy band is typical of d-d transition in square planar Ni(II) complexes with mixed coordination sphere containing nitrogen, oxygen and sulfur atoms. ' ' ' ' Crystal Structures of [Ni(Pythal)]Cl04 (6) and[Ni(PytAzo sal)]Cl04.H20 (10) Compounds 6 and 10 are both cationic and have been crystallized as the perchlorate salt.The complex 6 does not include any solvent molecule, but the complex 10 includes one water molecule. Details of data collection procedures and structures are given in Table 1. The crystal structures of complexes 6 and 10 and their unit celi diagrams are shown in Figs. 1-4. The relevant distances and angles are listed in Table 2. The complex 6 crystallizes in the triclinic space group PT. The asymmetric unit contains two independent molecules (molecule I and II) of [Ni(PytIsal)]C104. The complex 10 crystallizes in the orthorhombic space group Y2{l{l\, with one asymmetric unit N. Daneshvar, L. A. Saghatforoush, A. A. Khandar, A. A. Entezami: New Nickel(II) Complexes... Acta Chim. Slov. 2003, 50, 207-222. 211 constituted by a molecule of complex and a molecule of water. The nickel ion in both complexes has N2OS coordination sphere, bound by deprotonated phenolic oxygen, imine and pyridine type nitrogens and the thioether sulfur atoms. Bond lengths of both independent molecules in complex 6 are similar, with only significant difference occuring in the Ni-S distance that is 0.01 A longer in molecule II compared to molecule I. While the 0-Ni-N(2), O-Ni-N(l), N(2)-Ni-S bond angles are similar in both molecules, the O- Ni- S and N(l)- Ni- N(2) bond angles are smaller and the N(l)-Ni-S bond angle is larger in molecule I than in molecule II (Table 2). In the complex 6 the Ni-0 (1.843 A) and Ni-N(l) (1.922 A) bond lengths are slightly longer while Ni-N(2) (1.866 A) and Ni-S (2.1775 A) are slightly shorter than related Schiff base complexes.15'17'21 In complex 10, Ni-0 (1.821 A), Ni-N(2) (1.884 A) andNi-N(l) (1.911 A) bond lengths are in the range found for similar Ni-0 (phenolate), Ni-N (imine) and Ni-N (pyridine) interactions, but Ni-S (2.158 A) distance is slightly shorter than related Schiff base complexes. ' ' Apparently, the double bonds in the chelate ring Ni(l)-N(2)-C(10)-C(ll)-C(16)-O(l) in both complexes are delocalized. In the complexes of 6 and 10 the C(10)-C(ll) distance of 1.428 A and 1.383 A, respectively, is much shorter than the normal C(sp2)-C(sp2) single bond (1.51 A),9 the C(10)-N(2) distance of 1.286 A and 1.293 A, respectively, are a little longer than normal imine C=N double bond (1.28 A)9 and the C(16)-0(l) bond lengths of 1.306 A and 1.297 A, respectively, are shorter than the normal C(sp2)-0 single bond (1.34 A).9 Ali these changes in bond lengths indicate delocalization of the donating ring of aromatic Schiff base portion, so this chelating ring tends to remain planar as a result of this delocalization to form a stable conjugation structure. Within the ligand-metal fragment, the four adjacent bond angles around the Ni center are unevenly distributed from the ideal 90°. The comparison of bond angles of complexes of 6 and 10 with similar tetradentate Schiff base complexes show that in both independent molecules of complex 6 the angles of N(l)-Ni(l)-S(l) and 0(1)-Ni(l)-N(2) are larger, but the angles of N(2)-Ni(l)-N(l), 0(1)-Ni(l)-N(l) and 71 0(1)-Ni(l)-S(l) are smaller. The main difference observed in molecule I is that the angles of N(2)-Ni(l)-N(l) and 0(1)-Ni(l)-S(l) are 6° and 4.35° smaller, respectively, and N(l)-Ni(l)-S(l) angle is 3.99° larger than the obtained values for earlier known 71 compounds. On the other hand, in complex 10 the N(l)-Ni(l)-S(l) and N(2)-Ni(l)-S(l) angles are larger, but the angle of 0(1)-Ni(l)-N(l) is smaller, and the only N. Daneshvar, L. A. Saghatforoush, A. A. Khandar, A. A. Entezami: New Nickel(II) Complexes... 212 Acta Chim. Slov. 2003, 50, 207-222. significant difference observed for N(l)-Ni(l)-S(l) angle which is 2.5° larger than reported values. It is emphasized that the bond angles of 0(1)-Ni(l)-N(2), N(2)-Ni(l)-S(l), N(l)-Ni(l)-S(l), 0(1)-Ni(l)-N(l), 0(1)-Ni(l)-S(l) and N(2)-Ni(l)-N(l) obtained for, already reported nickel Schiff base complexes are 94.8°, 87.5°, 87.6°, 87.6°, 176.5° and 176.5°, respectively.21 In complex 6 the weak interactions are observed between hydrogen atom of C(6) and iodine atom of another independent molecule with a distance of 2.98 A, also weak interaction between the nickel atom and iodine atom from the Molecule I Molecule II CC3J C(8') c(7i sin C(6) C<7) C(3' C(14') Fig. 1. The structure of [Ni(pytIsal)]C104. 0(4S) Fig. 2. The structure of [Ni(pytAzosal)]C104.H20. N. Daneshvar, L. A. Saghatforoush, A. A. Khandar, A. A. Entezami: New Nickel(II) Complexes... Acta Chim. Slov. 2003, 50, 207-222. 213 Fig. 3. Stereoview of the [Ni(pytIsal)]C104. Fig. 4. Stereoview of the [Ni(pytAzosal)]C104.H20. N. Daneshvar, L. A. Saghatforoush, A. A. Khandar, A. A. Entezami: New Nickel(II) Complexes... 214 Acta Chim. Slov. 2003, 50, 207-222. Table 1. Crystal and structure refinement data for the nickel complexes. [Ni(pytIsal)]C104 [Ni(pytAzosal)]C104.H20 Empirical formula Ci6H16ClIN2Ni05S C22H23CIN4Ni06S Formula weight 569.43 565.66 Temperature (K) 293(2) 110(2) Space group P-l P2,2i2, Crystal system Triclinic Orthorhombic Unit celi a (A) 12.188(2) 7.093(5) b(A) 13.312(3) 9.277(6) c (A) 13.348(3) 34.62(3) a(°) 115.48(3) 90.0 P(°) 90.78(3) 90.0 y(°) 90.90(3) 90.0 Volum(A3) 1945.2(7) 2278(3) Z 4 4 Density (Calc.) (Mgm"3) 1.935 1.649 Absorption Coefficient (mm"1) 2.846 1.110 F(000) 1120 1168 Crystal Size (mm) 0.2x0.08x0.02 0.08x0.08x0.4 0 Range for data collection (°) 2.40 to 27.06 2.27 to 26.02 Index ranges 0 2a (I)] R1 = 0.0617,wR2=0.1124 R! = 0.0635,wR2=0.1072 Rindices (ali data) Ri = 0.1364, wR2 =0.1282 Ri = 0.1330, wR2 =0.1215 Largest difference peak and hole (e A' 3) 0.649 and-0.801 0.834 and-0.568 same molecule with a distance of 3.565 A, and between one of the O atoms of CIO4" anion with iodine atom with a distance of 3.01 A, and between two O atoms from two separated perchlorate anions with a distance of 2.85 A are observed. The crvstal structure of complex 10 consists of discrete complex mononuclear cation and perchlorate anion with no significant intermolecular interactions. In regard to the bond angles of 0(1)-Ni(l)-S(l) (175.77°) and N(l)-Ni(l)-N(2) (176°) in complex 10 and in the molecule II which are 176 and 174.5° , respectivelv, we conclude that the coordination geometrv N. Daneshvar, L. A. Saghatforoush, A. A. Khandar, A. A. Entezami: New Nickel(II) Complexes... Acta Chim. Slov. 2003, 50, 207-222. 215 Table 2. Selected bond lengths (A ) and angles (°) for complexes. [Ni(pytIsal)]C104 [Ni(pytAzosal)]C104 Molecule I Molecule II Ni(l)-0(1) 1.843(4) 1.845(4) 1.821(6) Ni(l)-N(2) 1.866(5) 1.866(5) 1.884(7) Ni(l)-N(l) 1.922(5) 1.914(5) 1.911(6) Ni(l)-S(l) 2.1775(19) 2.1878(19) 2.158(3) N(2)-C(10) 1.286(7) 1.277(8) 1.293(9) N(2)-C(9) 1.485(8) 1.472(8) 1.478(9) 0(1)-C(16) 1.306(7) 1.303(7) 1.297(9) C(10)-C(ll) 1.428(8) 1.422(9) 1.383(11) 0(1)-Ni(l)-N(2) 95.35(19) 95.1(2) 94.60(3) 0(1)-Ni(l)-N(l) 86.641(18) 87.59(19) 86.60(3) N(2)-Ni(l)-N(l) 170.5(2) 174.5(2) 176.0(3) 0(1)-Ni(l)-S(l) 172.15(15) 176.10(14) 175.77(18) N(2)-Ni(l)-S(l) 87.65(15) 87.75(17) 88.80(2) N(l)-Ni(l)-S(l) 91.59(15) 89.80(16) 90.10(2) around nickel in complex 10 and molecule II is near to square planar in comparison to molecule I which has corresponding values of 172.15 and 170.5°, respectively. The nickel center resides on the basal ONSN plane, howerer in the independent molecule I the atoms of 0(1) and S(l) are, respectively, 0.131 and 0.153 A above the mean plane, and the N(l) and N(2) atoms are 0.141 and 0.143 A below the same plane, respectively. But this situation in molecule II is reversed and the atoms of 0(1) and S(l) are 0.068 and 0.060 A below this plane , respectively, and the atoms of N(l) and N(2) with a value of 0.065 and 0.063 A are placed above the same plane, respectively. Donor atoms situations in complex 10 are the same as molecule II, in which the atoms of 0(1) and S(l) are, respectively, 0.055 and 0.047 A below the plane and the atoms of N(l) and N(2) are respectively, 0.056 and 0.053 A above the same plane. Comparison of these data shows that the tetrahedral distortion for molecule I respect to ideal square planar arrangment is larger than complex 10 and molecule II. Cyclic voltammetry The electrochemistry data obtained for the Ni(II) complexes are given in Table 3. Moreover, one of the voltammograms of the complexes is shown in Fig. 5. On the basis N. Daneshvar, L. A. Saghatforoush, A. A. Khandar, A. A. Entezami: New Nickel(II) Complexes... 216 Acta Chim. Slov. 2003, 50, 207-222. Table 3. Cyclic voltammetry data for Ni(II) complexes. Copmlex Epc(I), Epa(I), AE(I), ipc(I)/ipa(I) Epc(II), Epa(II), AE(II), ipc(II)/ipa(II) mV mV mV mV mV mV [Ni(pytN02sal)]C104 -912 - - - - 1375 - - [Ni(pytAzosal)]C104 -994 -917 77 0.9 - 1025 - - [Ni(pytIsal)]C104 -1015 -886 129 0.75 627 1002 375 0.2 [Ni(pytOMesal)]C104 -1086 -962 124 0.85 707 964 257 0.57 -10B -i .2 -a.7 -0.2 e .3 a.a P OTENT I AL, U 1 .3 Fig. 5. The voltammogram of the [Ni(pytAzosal)]C104. of the voltammetric data, the complex of [Ni(pytN02sal)]C104 undergoes irreversible reduction process in potential range of -1200 - +1500 mV, as no anodic wave is observed. For other three complexes reduction waves are observed in the range of-1086 - -912 mV. The peak seperation AE (= Epc - Epa) for complexes [Ni(pytXsal)]C104 (X= OMe, I, Azo) are 124, 129, and 77 mV, respectively at scan rate of 100 mVs"1, thus the redox process for complexes of [Ni(pytOMesal)]C104 and [Ni(pytIsal)]C104 is quasi reversible and for complex of [Ni(pytAzosal)]C104 is reversible. The voltammetric responses are assignable to the Ni(II)/Ni(I) couple. For dissymmetric Schiff base complexes herein studied, Epc becomes less negative in the sequence of OCH3 < K Azo < NO2 with increasing electron-withdrawing effects of the substituent at para position respect to phenolic hydroxyl group, and this has been interpreted by assuming that the strong electron withdrawing effects stabilise the lower oxidation state. On the other hand, the complexes of [Ni(pytXsal)]C104 (X=N02, Azo) show similar irreversible oxidation waves while complexes of (X= OMe, I) show quasi reversible oxidation waves N. Daneshvar, L. A. Saghatforoush, A. A. Khandar, A. A. Entezami: New Nickelfll) Complexes... Acta Chim. Slov. 2003, 50, 207-222. 217 with the peak seperation (AE) 254 and 375 mV, respectively, implying that a stable Ni(III) is inaccessible, most probably due to oxidation of the ligands. Conclusions In this paper, we could synyhesize Ni(II) tetradentate dissymmetric Schiff base complexes containing sulfur, nitrogen and oxygen donor atoms. The structure determinations of two of these complexes were established by single crystal X-ray diffraction study. In both complexes the geometry around the nickel ions are square planar with a slight tetrahedron distortion. Crystallographic identification of molecules with two different geometries in the čase of the [Ni(pytIsal)]C104 (molecules I and II) is interesting from a structural point of view. Electrochemistry data show reversible and irreversible reductions for [Ni(pytXsal)]C104 ( X = Azo and NO2), respectively, while this process for [Ni(pytXsal)]C104 (X = OMe and I), respectively, is quasireversible. Experimental Materials Ali solvents used in the preparations were reagent grade. Chemicals for the preparations were reagent grade and commercially available, and were used without 1 o further purification. The solvents were purified by standard methods. 5-Phenylazo salicylaldehyde, 5-iodosalicylaldehyde, and l-(2-pyridyl)-3-thia-5-amino pentane (pyta) were synthesized according to the known procedures19"21 and 2-vinyl pyridine distilled in vacuum before using. Physical Measurements Elemental analyses (C, H, N) were performed using a Heraeus CHN-O-RAPID elemental analyzer. Infrared (FTIR) spectra were recorded using KBr discs on a FTIR Unicam 4600. H and C spectra were taken in CDCI3 on a Bruker spectrospin 400 MHz ultrashield spectrometer and chemical shifts were indicated in ppm relative to tetramethylsilane(TMS). The electronic spectra in 200-900 nm range were obtained in acetonitrile on a Shimadzu UV-265 FW spectrophotometer. The conductivity measurements were carried out in acetonitrile at room temperature using a Hanna conductometer HI 8828N instrument. Cyclic voltammograms were performed using an AMEL instrument Model 2053 as potentiostat connected with a function generator N. Daneshvar, L. A. Saghatforoush, A. A. Khandar, A. A. Entezami: New Nickel(II) Complexes... 218 Acta Chim. Slov. 2003, 50, 207-222. (AMEL Model 568). Ali solutions were deoxygenated by passing a stream of Ar into the solution for at least 10 min prior to recording the voltammogram. Ali potentials reported herein were measured at room temperature and referenced to the saturated calomel electrode (SCE) with ferrocene as an internal standard. A platinum wire was used as counter electrode and a glassy carbon disc with a diameter of 3 mm was used as working electrode. Before each experiment the working electrode was cleaned perfectly by polishing with alumina and rinsed thoroughly with distilled water and acetone. The electrolytic medium consisted of 0.1 M LiC104 in acetonitrile. Under these conditions the ferrocenium - ferrocene couple was located at 371 mV with a peak separation of 89 mV. X- ray Crystallography Crystallographic data for complexes of [Ni(PytIsal)]C104 (6) and [Ni(PytAzosal)]C104.H20 (10) are given in Table 1. Suitable single crystals for both complexes were acquired by slow evaporation from of the methanol solution of related complexes at room temperature and mounted in sealed glass capillaries. Diffraction data were collected on a Bruker Smart CCD diffractometer at 110 K for complex 10 and on a Siemens P3/PC diffractometer at 293 K for complex 6 and the co and k scan modes were used. Intensity data were obtained using Mo Ka radiation (0.7107A) monochromatized from graphite. The data were reduced and the structures were solved by direct methods using the program SHELXTL version 5.1, and refined by the full-matrix least-squares on F , using ali the unique data and the weighting scheme w=[o2(F0)2+(0.0554P)2+1.944P]"1 for complex 6 and w=[