DALTON J. Chem. Soc., Dalton Trans., 1997, Pages 273–276 273 Co-ordination ability of amino acid oximes. Potentiometric, spectroscopic and structural studies of complexes of 2-cyano-2- (hydroxyimino)acetamide Tatiana Yu. Sliva,*,a Anna M. Duda,b Tadeusz G�owiak,b Igor O. Fritsky,a Vladimir M. Amirkhanov, a Andrei A. Mokhir a and Henryk Koz�owski*,b a Department of Chemistry, Shevchenko University, 252017 Kiev, Ukraine b Faculty of Chemistry, University of Wroc�aw, F. Joliot-Curie 14, 50383 Wroc�aw, Poland X-Ray crystallographic and solution studies have revealed the first examples of complexes of Cu2+ and Ni2+ with an oxime derivative in which completely deprotonated ligand molecules bind in trans position because of the lack of a hydrogen bond between the hydroxyl oxygens which usually stabilises the cis positioning.Oximes of amino acids and peptides were shown to be specific and efficient ligands for Cu2+ and Ni2+ ions.1,2 The complexes formed are stable and soluble and extensive dimer formation for Cu2+-containing systems is observed at pH >5.The binuclear complex formation results from the two alternative donor centres at the oxime (]] N]OH) group (N and O) in the ligands which both have a high affinity for M2+ and cannot both coordinate to the same metal ion. The co-ordination properties of oxime analogues of amides or peptides are, to a large extent, governed by the presence of a planar RC(]] NOH)CONH framework which may allow the formation of different chelate rings with virtually no change in conformation.The formation of bis complexes of the MH21L2 type having cis ligand coordination stabilised by hydrogen bonding between the oxime oxygens is the most characteristic feature of complexes with amino acid or amide analogues. The deprotonation of the first proton of the bis complex of Ni2+ has a critical impact on binding ability of the neighbouring nitrogen. In the case of 2- (hydroxyimino)propanamide complexes the NiL2 species having both oxime hydroxyl groups protonated is octahedral, while its deprotonated product NiH21L2 is square-planar.There is no change of binding groups during this deprotonation process. The only change possible is the distinct shift of electron density from the deprotonated hydroxyl group of the oxime moiety towards the neighbouring oxime nitrogen. The exceptional stability of the NiH21L2 complex with the amide ligand is also seen in the log K value (7.64) of the deprotonation reaction NiL2 aA NiH21L2 when compared to that for deprotonation of free amide (9.87) or removal of the proton from the NiH21L2 species (10.54).The same behaviour is observed for the acidic form of the oxime ligand. In this work the binding ability of a compound having a strongly electron-withdrawing cyano group at the a-carbon of an amide oxime is studied. This modification may have a critical impact on the role of hydrogen bonding in coordination of oxime by affecting the basicity of the oxime hydroxyl group.Experimental Synthesis of 2-cyano-2-(hydroxyimino)acetamide (H2chia) The oxime was synthesized by treating cyanocetamide (Fluka) with sodium nitrite in aqueous solution in the presence of acetic acid; the sodium salt formed was then treated with hydrochloric acid in order to obtain the free oxime as described elsewhere,3 the purity being checked by elemental analysis and NMR spectroscopy, [Found (Calc.for C3H3N3O2): C, 31.55: (31.85); H, 2.9 (2.65); N, 37.1 (37.15)%]. Potentiometry also showed the oxime to be near 100% pure and a very good fit was obtained between calculated and experimental titration curves with alkali over the whole pH range. Potentiometric studies Titrations involved an ionic background of 0.1 mol dm23 KNO3, a pro-ligand concentration of 3 × 1023 mol dm23 and metal-to-pro-ligand ratios of 1 : 1, 1 : 2, 1 : 3 and 1 : 5. Stability constants for the complexes of H+, Cu2+ and Ni2+ were calculated from titrations carried out using total volumes of 2 cm3.Alkali was added from a 0.100 cm3 micrometer syringe which had been calibrated by weight titrations and the titration of standard materials. The pH-metric titrations were performed at 25 8C using a MOLSPIN automatic titration system with a microcombined glass–calomel electrode calibrated in hydrogenion concentration using HNO3.4 Titrations were performed in triplicate and the SUPERQUAD computer program was used for stability constant calculations (bpqr = [MpHrLq]/ [M]p[H]r[L]q).5 Standard deviations quoted refer to random errors only.They are, however, a good indication of the importance of a particular species in the equilibrium. Spectroscopic studies Absorption spectra were recorded on a Beckman DU 650 spectrophotometer. The metal-ion concentrations were 2.5 × 1023 mol dm23 and the metal-to-pro-ligand molar ratios were 1 : 1, 1 : 2 and 1 : 5.The EPR spectra were recorded on a Brüker ESP 300E spectrometer at X-band (9.3 GHz) at 120 K, in ethane- 1,2-diol–water (1 : 2) as a solvent. Concentrations used in the spectroscopic measurements were similar to those given for potentiometric titrations. Syntheses of complexes Nickel(II). The compound NiCl2?6H2O (0.238 g, 1 mmol) was dissolved in acetonitrile (20 cm3) and added to a solution of the acetamide (0.226 g, 2 mmol) in acetonitrile (10 cm3). A 25% aqueous solution (1.44 cm3) of tetramethylammonium hydroxide was added dropwise.The dark brown solution was set aside274 J. Chem. Soc., Dalton Trans., 1997, Pages 273–276 for 48 h at room temperature. The crystalline precipitate of the anionic complex was filtered off, washed with acetonitrile and dried in a vacuum desiccator over CaCl2. Red-brown needleshaped single crystals suitable for X-ray analysis were obtained by recrystallisation from acetonitrile–water (3 : 1) upon slow evaporation in the air at room temperature.Yield 65% [Found (Calc. for C14H30N8O6): C, 36.2 (36.15); H, 6.45 (6.5); N, 24.1 (24.1); Ni, 12.8 (12.6)%]. Copper(II). A 25% aqueous solution (0.72 cm3, 2 mmol) of tetramethylammonium hydroxide was added to a suspension of [Cu(Hchia)]?0.5H2O6 (0.297 g, 1 mmol) in water (5 cm3). In 30 min red crystals were filtered off, washed with water and acetone and dried in vacuum. Single crystals suitable for X-ray analysis were grown by slow diffusion of methanol vapour into a 15% solution of the complex in acetonitrile.Yield 70% [Found (Calc. for C14H30CuN8O6): C, 35.55 (35.8); H, 6.55 (6.45); Cu, 13.75 (13.5); N, 23.7 (23.85)%]. Crystallography Crystal data and parameters for data collection and structure refinement are presented in Table 3, while selected distances and angles are collected in Tables 4 and 5, respectively. For X-ray Table 1 Protonation constants and complex-formation constants of 2-cyano-2-(hydroxyimino)acetamide at 25 8C and I = 0.1 mol dm23 KNO3 Species log b log b* HL 5.12(1) 9.87 Nickel(II) complexes NiL NiH21L2 NiH22L2 3.38(1) 21.88(1) 27.72(3) 4.82 — 28.97 Copper(II) complexes CuL Cu2H22L2 CuH22L2 3.74(1) 21.06(2) 26.64(2) 7.87 5.66 24.74 * Data for 2-(hydroxyimino)propanamide.1 Table 2 Spectral parameters (visible and EPR) for metal(II)–2-cyano- 2-(hydroxyimino)acetamide systems at 25 8C and I = 0.1 mol dm23 (KNO3).Pro-ligand concentration 2.5 × 1023 mol dm23 and metal-topro- ligand molar ratio 1 : 5 (for Ni2+) and 1 : 2 (for Cu2+) UV/VIS Species (binding mode) l/nm e/dm3 mol21 cm21 NiL (N, N2) NiH21L (N, N2) NiH22L2 (2 × N, N2) 395.0a 517.0b 645.0b 974.0b 395.0a 535.0b,c 986.0b 398.0a 539.0c 79 2 0.8 1.2 355 17 3 708 100 CuL (N, N2) Cu2H22L2 (N, N2, O2) CuH22L2 (2 × N, N2) 710.0 662.0 607.0 47d 64 155e a Intraligand transition.b d–d Transition of Oh nickel(II) complex. c d–d Transition of square-planar nickel(II) complex. d A|| 156 G, g|| 2.260.e A|| 188, g|| 2.222. analysis a needle-shaped single crystal was chosen and sealed in a glass capillary to avoid crystal powdering. Data collection and processing. Intensity data were collected at 293(2) K with a KM4 computer-controlled four-circle diffractometer. 7 The structure was solved by direct methods and refined by full-matrix least sqsing SHELXL 93 8 with anisotropic thermal parameters for non-hydrogen atoms. All H atoms were located from the successive Fourier-difference maps and only their displacement parameters included in the refinement.Correction for extinction was taken into account in the course of refinement. Atomic coordinates, thermal parameters, and bond lengths and angles have been deposited at the Cambridge Crystallographic Data Centre (CCDC). See Instructions for Authors, J. Chem. Soc., Dalton Trans., 1997, Issue 1. Any request to CCDC for this material should quote the full literature citation and the reference number 186/279.Results and Discussion Solution studies 2-Cyano-2-(hydroxyimino)acetamide (H2chia) exhibits one measurable protonation constant corresponding to protonation of the hydroxyimino group C]] NO2. Its value, log K = 5.12, is more than six orders of magnitude lower than that of 2- (hydroxyimino)propanoic acid (log K = 11.6 1) and more than four and half orders of magnitude lower than that of 2- (hydroxyimino)propanamide (log K = 9.87 1). This very acidic behaviour of the oxime hydroxyl group derives from the strong electron-withdrawing effect of the 2-cyano substituent.Complexes with 2-cyano-2-(hydroxyimino)acetamide The complex species evaluated from the potentiometric calculations for Cu2+ and Ni2+ are collected in Table 1. Results Fig. 1 Species distribution curves for the complexes formed in the Cu2+–H2chia (a) and Ni2+–H2chia (b) systems as a function of pH. Pro-ligand concentration 0.003 mol dm23, and metal-to-ligand molar ratios 1 : 5 (for nickel) and 1 : 3 (for copper)J. Chem.Soc., Dalton Trans., 1997, Pages 273–276 275 Table 3 Crystal data and structure refinement for the 2-cyano-2-(hydroxyimino)acetamide complexes * Empirical formula M (Cu-Ka)/Å a/Å b/Å c/Å a/8 b/8 „/8 U/Å3 Dc/Mg m23 m/cm21 F(000) Crystal size/mm q Range for data collection/8 hhl Ranges Reflections collected Independent reflections Data, parameters Goodness of fit on F2 R1, wR2 Final indices [I > 3s(I)] Weighting scheme, w Extinction coefficient Largest difference peak and hole/e Å23 C14H30N8NiO6 465.18 1.541 80 7.905(2) 8.589(2) 9.017(2) 107.11(3) 97.69(3) 101.36(3) 561.5(2) 1.376 16.47 246 0.15 × 0.18 × 0.18 5.24–80.35 0–10, 210 to 10, 211 to 11 2302 2302 2143, 146 1.073 0.0356, 0.1092 1/[s2 (Fo 2) + (0.0722P)2 + 0.1537P] where P = (Fo 2 + 2Fc 2)/3 0.0058(14) 0.269, 20.263 C14H30CuN8O6 470.00 1.541 78 7.683(2) 8.754(2) 9.059(2) 108.42(3) 97.46(3) 98.44(3) 561.6(2) 1.390 1.773 247 0.4 × 0.4 × 0.2 5.24–58.05 21 to 8, 29 to 9, 29 to 9 1704 1553 [Rint = 0.0137] 1553, 137 0.892 0.0330, 0.0860 0.0126(13) 0.269, 20.263 * Details in common: triclinic, space group P1� ; Z = 1; full-matrix least-squares refinement on F2.obtained earlier for 2-(hydroxyimino)propanamide1 are also shown for comparison. Copper(II) forms three major complexes: two monomeric, CuL and CuH22L2, and the dinuclear species Cu2H22L2 (Fig. 1). The latter is EPR silent in the region pH 5.5–6.5.The same three species were also observed, among others, in the case of 2-(hydroxyimino)propanamide. The strongly acidic character of the present oxime donor results in much less stable complexes when compared to those of 2- (hydroxyimino)propanamide. The stability constant for the dimeric species differs by more than six orders of magnitude (Table 1). The EPR and absorption spectral data collected in Table 2 indicate the involvement of two or four nitrogen atoms.1,2 In the dinuclear species the involvement of deprotonated hydroxyimino oxygen is most likely, as was found earlier in similar oxime systems in the analogous pH regions.1 Unfortunately, the strong intraligand transitions observed in the range 400–200 nm do not allow one to observe O2 oximeÆCu2+ charge-transfer band to support this binding mode (see ref. 1). In the case of Ni2+-containing solutions the three major complexes formed are monomeric NiL, NiH21L and NiH22L2 species (Fig. 1).According to the absorption spectra (Table 2) the latter bis complex is square planar, while the first two are both octahedral. The binding mode is likely to be the same as that observed for Cu2+, i.e. involving the nitrogen donors of the ligand (see below). The interesting feature of the nickel system is the much lower difference in stabilities between the complexes of the present derivative and that of 2-(hydroxyimino) propanamide (Table 1). Table 4 Selected bond lengths (Å) and angles (8) for the nickel(II)–2- cyano-2-(hydroxyimino)acetamide complex Ni]N(2i) Ni]N(2) 1.858(2) 1.858(2) Ni]N(1) Ni]N(1i) 1.899(2) 1.899(2) N(2i)]Ni]N(2) N(2i)]Ni]N(1) N(2)]Ni]N(1) 180.0 96.38(7) 83.62(7) N(2i)]Ni]N(1i) N(2)]Ni]N(1i) N(1)]Ni]N(1i) 83.62(7) 96.38(7) 180.0 Symmetry transformation used to generate equivalent atoms: i 2x, 2y, 2z.Crystal structures of the complexes [NMe4]2[M{N(O)C(CN)C- (O)N}2]?2H2O (M = Ni or Cu) Both copper and nickel ions give the same molecular structure for the MH22L2 complex, consisting of tetramethylammonium cations, complex anions MH22L2, and water molecules.In the crystal packing (Fig. 2) the complex anions are linked by hydrogen bonds into chains spread along the z direction. The Fig. 2 Packing of molecules of the copper(II)–2-cyano-2-(hydroxyimino) acetamide complex Table 5 Selected bond lengths (Å) and angles (8) for the copper(II)–2- cyano-2-(hydroxyimino)acetamide complex Cu]N(2) 1.895(2) Cu]N(1) 2.056(2) N(2)]Cu]N(2i) N(2)]Cu]N(1) 180.0 81.60(8) N(2i)]Cu]N(1) N(1)]Cu]N(1i) 98.40(8) 180.0 Symmetry transformation as in Table 4.276 J.Chem. Soc., Dalton Trans., 1997, Pages 273–276 hydrogen bonds are formed between the amide and oxime oxygen atoms and the proton of the amide group and water. The NMe4 + cations form columns along the z axis of the crystal, occupying empty spaces between the translational chains of the complex anions. In complex anion (Fig. 3, for Ni2+) the metal atom occupies a partial position at the centre of symmetry.The distorted square-planar co-ordination sphere consists of four nitrogen atoms belonging to the deprotonated hydroxyimino and amide groups. The two dianions of 2-cyano-2- (hydroxyimino)acetamide are situated in trans position with respect to each other. The Cu]Noxime and Cu]Namide distances are 2.056 and 1.895 A respectively, noticeably different, while e.g. in the complex of Cu2+ with 2-(hydroxyimino)- propanamide9 these distances are in the ranges 1.91–1.94 and 1.92–1.95 A, respectively, i.e.close to each other. The elongation of the Cu–Noxime bond in the present complex clearly indicates the strong electron-withdrawing effect of the 2-cyano substituent (see above). In the nickel(II) complex with 2-cyano-2-(hydroxyimino)- acetamide the Ni]Noxime and Ni]Namide distances are 1.899 and 1.858 A, respectively, relatively close to each other, although the bond distance with oxime nitrogen is also the longer as also found in the copper complex.Both distances are very close to those observed for the nickel complex with 2-(hydroxyimino)- propanamide in which the Ni]Noxime and Ni]Namide bond lengths are 1.872 and 1.853 A, respectively.2 The geometrical parameters of the present ligand are similar to those observed for other ligands having deprotonated oxime and amide groups. The distances N(1)-O(1) of 1.289 and 1.281 A and N(1)-C(1) of 1.302 and 1.314 A for copper and nickel respectively, are close to those reported for the N-co-ordinated deprotonated oxime group.10 This indicates that the CNO2 moiety exists in the nitroso-form.11 Fig. 3 Crystal structure of the nickel(II)–2-cyano-2-(hydroxyimino)- acetamide complex Conclusion The high acidity of the oxime hydroxyl group in 2-cyano-2- (hydroxyimino)acetamide changes considerably the coordination mode of the oxime ligand with Cu2+ and Ni2+ ions. Although the donor atoms bound to the metal ions are the same as with 2-(hydroxyimino)propanamide, two molecules of the propanamide ligand are bound in cis position, while in the case of the 2-cyano derivative the ligands are trans to each other.The mor factor deciding the cis positions of the ligand molecules in the major complex, MH21L2, formed with oxime analogues of amino acids is the hydrogen bond between the two hydroxyl oxygens. This hydrogen bond is not present in the corresponding complex of the 2-cyano derivative, MH22L2, due to the very acidic oxime hydroxyl group.The MH21L2 species is not observed in the present case. Acknowledgements This work was supported by the Polish State Committee for Scientific Research (KBN 3T09A06908). T. Yu. Sliva thanks Kasa J. Mianowskiego for financial support. References 1 Ch. O. Onindo, T. Yu. Sliva, T. Kowalik-Jankowska, I. O. Fritsky, P. Buglyo, L. D. Pettit, H. Kozlowski and T. Kiss, J. Chem. Soc., Dalton Trans., 1995, 3911. 2 T. Yu. Sliva, T. Kowalik-Jankowska, V. M. Amirkhanov, T. Glowiak, L. D. Pettit, I. O. Fristky and H. Kozlowski, J. Inorg. Biochem., in the press. 3 C. A. Schultze, Berichte, 1909, 42, 735. 4 H. M. Irving, M. H. Miles and L. D. Pettit, Anal. Chim. Acta, 1967, 68, 475. 5 P. Gans, A. Sabatini and A. Vacca, J. Chem. Soc., Dalton Trans., 1985, 1195. 6 V. V. Skopenko, R. D. Lampeka and Yu. L. Zub, Zh. Neorg. Khim., 1981, 26, 142. 7 KUMA M4 Software User Guide, Version 3.1, Kuma Diffraction, Wroc�aw, 1986. 8 G. M. Sheldrick, SHELXL 93, Program for refinement of crystal structures, University of Göttingen, 1993. 9 Yu. A. Simonov, V. Kh. Kravtsov, I. O. Fritsky, E. E. Gubina, R. D. Lampeka, T. S. Iskenderov and A. Zh. Zhumabaev, Koord. Khim., 1995, 21, 407. 10 I. O. Fritsky, R. D. Lampeka, V. V. Skopenko, Yu. A. Simonov, A. A. Dvorkin and T. I. Malinovsky, Z. Naturforsch., Teil B., 1993, 48, 270. 11 K. V. Domasevich, A. N. Chernega, S. V. Lindeman and Yu. T. Struchkov, Zh. Neorg. Khim., 1995, 40, 426. Received 23rd July 1996; Paper 6/0512