Keto-amine Form Research Articles

Theoretical investigations on the structure and vibrational spectra of N-(2-hydroxy-1-naphthylidene)threonine

Hartree–Fock (HF) and hybrid density functional theory (DFT) have been used to investigate the electronic structure and energetics of enol-imine and keto-amine tautomers of N-(2-hydroxy-1-naphthylidene)threonine. Both these tautomers engender six-membered ring due to intramolecular hydrogen bonded interactions. The enol-imine tautomer turns out to be ∼9 kJ mol −1 lower in energy in the HF framework owing to strong O–H⋯N (1.832 Å) interactions compared to N–H⋯O interactions (H⋯O=1.906 Å) in keto-amine tautomer. Consequently the downshift of the C N stretching vibration to 1560 cm −1 in the enol-imine tautomer has been predicted. The intense C O stretching, insensitive to the tautomeric transformation, turns out to be at 1715 cm −1. The effect of solvents on the tautomeric stability has been investigated. Self-consistent reaction field (SCRF) calculations have shown that unlike in the gas phase structure protic polar solvents facilitate the keto-amine form to be of lower energy. Configuration interaction single excitation (CIS) calculations reveal relatively strong O–H⋯N (1.768 Å) interactions in the excited state of enol-imine tautomer.

Nitroaniline Derivatives of 2-Oxo-1-naphthylideneamines—Molecular Self-Assembling via C—H⋯O Intermolecular Hydrogen Bonds and Stabilization of O—H⋯N and N—H⋯O Tautomers in Solution and Solid State

Three Schiff-bases of the Ar—CH—N—Ar type were synthesized from 2-hydroxy-1-naphthaldehyde and o- (1), m- (2), and p-nitroaniline (3) in order to investigate the shift of the keto-amine/enol-imine tautomeric equilibrium that arises by the introduction of the electron-withdrawal nitro group. The compounds have been investigated experimentally, in the solid state and in the solution, by IR, NMR, UV spectroscopy, and X-ray single crystal analysis, and theoretically by using density functional theory (DFT). The tautomeric equilibrium is strongly shifted towards the keto-amine form despite of the nitro group position at N-phenyl ring in the solid state of molecular structures of 1, 2, and 3 (especially indicated by C—O and C—N bond distances). The keto-amine tautomeric form of the compounds is characterized by a strong, resonance assisted hydrogen bond (RAHB) of the N—H⋯ O type (the N⋅ ⋅ ⋅ O distances are 2.530(8), 2.554(2) and 2.555(5) A in 1, 2, and 3, respectively). The molecules are assembled in the crystalline state into a 3D HB network (1), centrosymmetrical dimers (2) or infinite chains (3) by the C—H⋯ O intermolecular hydrogen bonds. In contrast, UV/VIS spectra of the chloroform solution of the title compounds reveal predominance of the enol-imine form. The molecular geometries and relative stabilities of the compounds determined by using DFT find good agreement with the experimental data.

New Benzo-15-Crown-5 Ethers Featuring Salicylic Schiff Base Substitutions – Synthesis, Complexes and Structural Study

Condensation reactions between 4'-formyl-5'-hydroxybenzo-15-crown-5and 2-aminopyridine, 2-amino-6-methylpyridine, 2-amino-4-methylpyridine or2-(aminomethyl)furan yielded the new laterally functionalized crown ethers1–4. The crown compounds 1–3 form crystalline 1:1 (Na+:ligand) complexes 1a–3a with sodium perchlorate. Ligands and complexes have been characterized by elemental analyses, IR, UV-Vis, 1H-, 13C-NMR and mass spectra. The tautomeric equilibria (phenol-imine, O...H–N and keto-amine, O...H–N forms) have been systematically studied by using UV-Vis absorption spectra. The spectra of the ligands 1–4 and complexes 1a–3a were recorded in polar, non-polar, acidic, and basic media. In solutions of polar solvents, tautomeric interconversion of the Schiff base into the keto-amine form has been observed. A crystal structure [monoclinic, space group P21/c,a = 14.292(2), b = 9.449(6), c = 16.059(2) A, β = 114.20(1)°,V = 1978.4(13) A3, Z = 4 and Dx = 1.314 g cm-3] shows that compound 4 is in the form of phenol-imine in solid state. There is a strong intramolecular [O–H...N 1.78(6), O...N 2.581(7), O–H 0.89(6) A and N...H–O 148.4(5)°] hydrogen bond between the phenolic oxygen and imine nitrogen atoms. The C=N imine bond reveals a trans planar (1E) configuration. The molecules stack in columns parallel to the a/c plane of the unit cell.

SYNTHESIS AND SPECTROSCOPIC STUDIES IN SOME NEW SCHIFF BASES

Infrared, 1H-NMR and UV-visible spectra of a series of substituted 2-hydroxy-1-naphthaldehyde Schiff bases were used to investigate enol-imine and keto-amine tautomeric equilibrium. They were synthesised and studied by elemental analysis. From the IR spectra of the model compounds it was possible to assign the IR absorption for the C═O and the C═N groups in both the keto-amine and enol-imine forms. It was possible to assign other absorptions which were either specific to the keto-amine or the enol-imine forms. Specific pattern was observed for all the studied compounds. The UV-visible spectra of compounds have been investigated in different solvent, acidic and basic media. The compounds were in tautomeric equilibrium (enol-imine O─H ⃛N, keto-amine O ⃛H─N forms) in polar and non-polar solvents. The keto-amine form was observed in basic solutions of DMSO, ethanol, chloroform, benzene, cyclohexane and in acidic solutions of chloroform and benzene, but not in acidic solutions of DMSO and ethanol.

Intramolecular hydrogen bonding and tautomerism in N-(3-pyridil)-2-oxo-1-naphthylidenemethylamine

N-(3-pyridil)-2-oxo-1-naphthylidenemethylamine (C16H12N2O) was studied by elemental analysis, IR, 1H NMR, and UV–visible techniques and X-ray diffraction methods. The UV–visible spectrum of the compound was investigated in solutions effect polarity. The polarity of the some solvents was modifierly the additional (CF3COOH) and [(C2H5)3N]. The compound is in tautomeric equilibrium (phenol-imine O–H···N and keto-amine O···H–N forms) in polar and nonpolar solvents. The keto-amine form is observed in basic solutions of DMSO, ethanol, chloroform, benzene, cyclohexane, and in acidic solutions of chloroform and benzene, but not in acidic solutions of DMSO and ethanol. The compound crystallizes in the monoclinic, space group P21/a with a = 7.010(5) A, b = 13.669(4) A, c = 12.764(4) A, β = 101.23(4)°, V = 1199.6(10) A3, Z = 4, D c = 1.375 g/cm3, μ(Mo Kα) = 0.088 mm−1, R = 0.045 for 1658 reflections [I > 2σ(I)]. The title compound is not planar two Schiff base moieties A [C1–C11, O1] and B [N1, C12, C13, N2, C14, C15, C16] are inclined at an angle of 27.4(1)° reflecting mainly the twist about C12–N1 [C11–C12–N1–C13, 29.7(2)°]. There is a strong intramolecular hydrogen bond (O–H···N) of 2.529(2) A.

Synthesis, Complex Formation and Spectral Investigation of New Tritopic Bis(Crown Ether) Compounds Containing Recognition Sites for Sodium and Nickel Guest Cations*

Abstract New bis(crown ether) ligands (1 - 3) of Schiff base type containing recognition sites for sodium and nickel guest cations have been synthesized by the condensation of two equivalents of 4′-formyl-5′-hydroxy(benzo-15-crown-5)with 1,5-diamino-3-azapentane, 1,8-diamino-3,6- diazaoctane or 1,8-diamino-3,6-dioxaoctane. Homonuclear ditopic crystalline 2:1 (Na+:ligand) complexes (1a - 3a) of the ligands with NaClO4 have been prepared. Heteronuclear tritopic crystalline 2:1:1 (Na⊕ : Ni2⊕ : ligand) complexes (2b and 3b) have also been synthesized from the reactions of the ditopic complexes (2a and 3a) with Ni(CH3COO)2 · 6H2O. The UV-VIS spectra of 1 - 3, their sodium complexes 1a - 3a and sodium-nickel complexes 2b and 3b have been recorded in polar and non-polar solvents as well as in acidic and basic media. In polar solvents and basic solutions, tautomeric equilibria (phenol-imine and keto-amine forms, O-H···N⇌O···H-N ) are present, depending on the hydrogen bonding. The results indicate that the concentrations of the keto forms of the compounds generally increase as the polarity of the solvent increases

Intramolecular hydrogen bonding and tautomerism in Schiff bases. Structure of N-(2-pyridil)-2-oxo-1-naphthylidenemethylamine

N-(2-pyridil)-salicylidene (1) and N-(2-pyridil)-2-oxo-1-naphthylidene-methylamine (2) were studied by elemental analysis, IR, 1H NMR and UV–visible techniques and the structure of compound (2) was examined crystallographically. The UV–visible spectra of 2-hydroxy Schiff bases are investigated in different solvents, acidic and basic media. Compound (2) is in tautomeric equilibrium (phenol–imine, O–H⋯N⇌keto–amine, O⋯H–N forms) in polar and non-polar solvents. These tautomers are not observed in polar and non-polar solvents for (1) as also supported by 1H NMR and UV–visible data. The keto–amine form of compound (2) was observed in basic solutions of DMSO, ethanol, chloroform, benzene, cyclohexane and in acidic solutions of chloroform and benzene, but not in acidic solutions of DMSO and ethanol. On the contrary, this form for compound (1) was not observed in the same solutions. The asymmetric unit of compound (2) contains two independent molecules of (C16H12N2O) which constitute a tautomeric pair. The observed differences in the related CN (1.317(4) and 1.330(4)Å) and C–O (1.279(4) and 1.263(4)Å) bond lengths in the two crystallographically independent molecules indicate that the phenol–imine and the keto–amine forms coexist in the solid state. Intramolecular hydrogen bond lengths (O–H⋯N) are 2.586(4)Å and 2.518(4)Å for the two individual tautomers. It crystallizes in the monoclinic space group P21/n with a=5.837(2), b=17.476(2), c=24.295(3)Å,β=91.95(4)°, V=2476.9(6)Å3,Z=4 and Dx=1.3315gcm−3.

Electronic Structure of 2,6-Bis{.N-(2-hydroxyphenyl)immoinethyl}-4-methylphenol

The electronic and molecular structure of 2,6-bis{N-(2-hydroxyphenyl)iminomethyl}-4- methylphenol (hpimp) is clarified from the measurements of electronic absorption and 1H NMR spectra in various solvents and an X-ray diffraction analysis, together with MO calculations. Electronic absorption bands of hpimp are at 422, 397.9, 359, 341, 294.3, 265.8, and 224 nm in the non-polar solvent cyclohexane. In polar solvents, such as methanol, an additional band which is assigned to a partly formed keto-amine hpimp, is observed at 499 nm. From the 1H NMR spectra it is seen that hpimp exists in the enol-imine form in non-polar solvents, and as an equilibrium mixture of enol-imine and keto-amine forms in polar solvents. Each electronic absorption band of solid hpimp in a KBr disk is broadened compared with the solution state, and an additional band, again assigned to the keto-amine form, appears around 499 nm. An X-ray diffraction analysis shows that hpimp assumes a keto-amine structure in the solid state, and forms a column structure along the c-axis. MO calculations suggest that the enol-imine hpimp has a twist structure around the two C−C single bonds, the twist angle being 100° to 120°.

Intramolecular hydrogen bonding and tautomerism in Schiff bases. Part I. Structure of 1,8-di[N-2-oxyphenyl-salicylidene]-3,6-dioxaoctane

1,8-Di[N-2-oxyphenyl-salicylidene]-3,6-dioxaoctane (1) and 1,8-di[N-2-oxyphenyl-2-oxo-1-naphthylidenemethylamino]-3,6-dioxaoctane (2) are obtained from the reaction of triethylene glycol bis(2-aminophenyl ether) with salicylaldehyde and 2-hydroxy-1-naphthaldehyde. Compounds (1) and (2) have been characterized by elemental analysis, IR, 1H-NMR and UV-visible techniques and the structure of compound (1) has been examined crystallographically. The UV-visible spectra of 2-hydroxy Schiff bases are studied in different solvents and acidic media. Compound (2) is in tautomeric equilibria (phenolimine, OH…N ⇌ keto-amine, O…HN forms) in polar and non-polar solvents. These tautomers are not observed in polar and non-polar solvents for (1) as supported by 1H-NMR and UV-visible data. The keto-amine form is observed in acidic solutions of chloroform and benzene, but it is not observed in solutions DMSO and EtOH for compounds (1) and (2). Compound (1) is in the form of phenol-imine in solid state and a crystallographically centrosymmetric multidentate Schiff base ligand, containing two imine nitrogen, two hydroxy and four etheric O atoms. There is a strong intramolecular [ OH⋯ N = 1.488(3) A ̊ ] hydrogen bond. The C  N imine bond and CNC bond angle are 1.270(3)Å and 123.5(2)°. It crystallizes in the monoclinic space group P2 1 n with a = 6.301(1), b = 13.624(1), c = 16.189(1) A ̊ , β = 100.28(1)°, V = 1367.4(3) A ̊ 3 , Z = 2 and D x = 1.313 g cm −3.

Studies on the catabolism of glycated haemoglobin: Glycated globin is not a substrate for the glucosyl-galactosyl-hydroxylysine glucosidase but for a peptidase activity present in rat kidney and spleen

Rat kidney and spleen glucosyl-galactosyl-hydroxylysine glucosidase (EC.3.2.1.107) whose specificity for the hydroxylysine-linked disaccharide units present in collagens depends upon the substrate's free amino group was tested for its glycosidase activity on the ketoamine form of glycated [(14)C]Glc-Globin. The most stable preparations of the enzyme from normal and diabetic rat tissues, partially purified by ultracentrifugation and ammonium sulphate fractionation, were used. These glucosidase preparations did not release any significant amount of radioactive neutral hexose. But a radioactive glycopeptide of about 1,400 Da was released from [(14)C]Glc-Globin at a pH optimum of 4.0. It appears to be released by a peptidase activity present in the kidney and spleen of normal and diabetic rats.

Modification of sialic acid-MBTH assay for quantitative determination of nonenzymatically glucosylated proteins

A colorimetric assay for the quantitative determination of nonenzymatically glucosylated proteins is presented. The method employed is a modification of the method of Massamiri et al. (1978, Anal. Biochem., 91, 618–625) for quantitating sialic acid residues. The procedure involves periodate oxidation of the glucosylated protein releasing formaldehyde. The latter is then detected by complexing with the reagent, 3-methyl-2-benzothiazolinone hydrazone (MBTH). Fructose is used as a standard as it is structurally analogous to the ketoamine form on the protein. The MBTH assay can accurately detect fructose in concentrations as low as 1 μ M. The assay was utilized to quantitate in vitro nonenzymatically glucosylated calmodulin and bovine serum albumin.

Synthesis, structure, and action of some gossypol derivatives on the peroxidation of the lipids of biosubstrates

The synthesis has been effected of new gossypol derivatives using piperidino-and morpholinoethylamines. According to their PMR spectra, these substances exist in the keto-amine form. Their action on the peroxidation of lipids (POL) of various biosubstrates has been studied. Gossypol bis(piperinoethylimine) and bis(morpholinoethylimine) in concentrations of 1·10−7–5·10−6 mM exert a pronounced antioxidant action on human blood serum and rat brain synaptosomes. In the same concentrations, these substances suppressed the POL in enzymatic and nonenzymatic systems of the oxidation of rat liver microsomes.

Time-resolved e.s.r. study of triplet ketoamines generated by intramolecular proton transfer in free Schiff bases

Non-phosphorescent triplet states of the ketoamine forms of several free Schiff bases generated by intramolecular proton transfer have been observed using a time-resolved e.s.r. method.

Some New Fluorinated β-Ketoamines and Their Copper Complexes

Abstract Synthesis and characterization of some new fluorinated bidentate-monofunctional and tetradentate-bifunctional β-ketoamines and their copper complexes are reported. The spectral data of the free Schiff bases indicate that the site of amine condensation in the unsymmetrical β-diketones is the lesser enolized acyl-oxygen and the ketoamine form is the predominant tautomer. The copper complexes of these ligands have been characterized by elemental analysis and IR spectroscopy.

Cyclic and open-chain tautomerism and complex formation behaviour of the condensation product of 2-amino-3-aminomethyl-4- methoxymethyl-6-methylpyridine with salicylaldehyde

The crystalline product obtained by the condensation of 2-amino-3-aminomethyl-4-methoxymethyl-6-methylpyridine and salicylaldehyde was characterized by spectrometric methods (UV, IR, 1H NMR and MS). It was shown that the dissolution of the crystalline cyclic diamine afforded its tautomeric conversion to the enolimine form of the Schiff base [2-amino-4-methoxymethyl-6-methyl-3(salicylideneaminomethyl) pyridine] with a rate of the process and a ratio of both forms attained at equilibrium dependent upon the solvent. The presence of the ketoamine form was not evidenced. The behaviour of the condensation product as the ligand in formation of complexes with Cu(II), Ni(II), Co(II) and Cd(II) was investigated in methanolic solution. Coordination with Cu(II) and Ni(II) caused partial hydrolysis of the ligand and the formation of complexes of the bis Schiff base [4-methoxymethyl-6-methyl-2-(salicylideneamino)-3-(salicylideneaminomethyl)pyridine] characterized after their isolation by elemental analyses and spectrometric data.

Pre-HbA1c in children with insulin dependent diabetes mellitus.

Day-to-day fluctuations in blood concentration of the glycosylated hemoglobin, HbA1c or HbA1, are mainly due to variations in content of the intermediate aldimide form, preHbA1c. Using electrofocusing we have studied this component adjacent to the HbA1c band, which represents the stable ketoamine form. The fraction of preHbA1c present rapidly increases upon incubation of erythrocytes in 20 mM glucose at +37 degrees C, and the reaction is fully reversible if glucose is removed. In children with insulin-dependent diabetes mellitus with different degrees of metabolic control, levels of preHbA1c varied between 0.2 and 3.7% of total hemoglobin (median value 1.3%, N = 25). A clear correlation was found between preHbA1c and urinary glucose concentrations during the 12 hours preceding blood sampling (r = 0.75, p less than 0.001, df = 18). As expected, pre-HbA1c did not correlate to HbA1c or glycosylated albumin, which reflect long- and medium-term diabetic control.

The nonenzymatic glycosylation of collagen

Calf skin acid-soluble collagen, containing about 34 residues of lysine plus hydroxylysine per 100,000 dalton polypeptide chain, was treated with [ 14C]glucose in the presence or absence of NaCNBH 3. In 144 h, under the conditions employed, the presence of NaCNBH 3 increased the extent of glycosylation from 8 to 15% of the total residues of lysine plus hydroxylysine. The extent of glycosylation was estimated, using acid hydrolysates of the protein, by isolation and determination of reduced adducts (1-lysinohexitols) employing a system of paper chromatography followed by chromatography on an amino acid analyzer. By those means the difficulties of using specific color reactions such as that with thiobarbituric acid were obviated. Identification of the reduced adducts as forms of 1-lysinohexitol was made by comparison of that substance prepared by treatment of polylysine with [ 14C]glucose in the presence of NaCNBH 3. Of interest is the fact that treatment of the polymer with glucose for 144 h under conditions similar to those used for the collagen, resulted in an increase of extent of glycosylation from 3 to about 50% of the total lysine residues when NaCNBH 3 was present in the incubation medium. The greater degree of glycosylation of lysine residues in polylysine as compared with collagen (15 versus about 50%) may be ascribed to the different orders of macromolecular structure in the protein that could sequester certain of the residues from reaction with glucose. 1-Lysinohexitol was also identified in hydrolysates of neutral salt-soluble guinea pig skin collagen that had been reacted with glucose and then treated with NaB 3H 4. The glycosylated collagens were fragmented by treatment with CNBr, and modified lysine residues were found to occur along the entire length of the collagen chains. The use of NaCNBH 3 in the manner described above permits measurement of both aldimine and ketoamine forms of the adducts made with glucose. The possible physiological significance of the reversibility of the ketoamine form of adduct is discussed briefly.

1H, 13C, and 15N nuclear magnetic resonance studies on the tautomerism of the Schiff's bases of 3-acetyl-6-methyl-2H-pyran-2,4(3H)-dione and 3,5-diacetyltetrahydropyran-2,4,6-trione

The 1H and 13C n.m.r. spectra of 3-acetyl-6-methyl-2H-pyran-2,4(3H)-dione (I) and 3,5-diacetyltetrahydropyran-2,4,6-trione (III) and their Schiff's bases were studied in CDCl3 over a range of temperatures for elucidation of possible tautomerism. Whereas compound (I) appears to exist in only one form, at least two forms have been detected for compound (III). The Schiff's bases exist predominantly in keto-amine forms and the rotational barrier due to substantial double bond character of the exocyclic carbon to carbon bonds allows ready detection of rotamers. Structures of the predominant rotamers are deduced from the 1Hand 13C spectra and the assignments of NH proton signals are based on the relative strengths of the intramolecular hydrogen-bonds. The chemical shifts of NH protons correlate linearly with the pKa of the amine precursors. The 15N spectra of some 15N-enriched Schiff's bases were obtained in CDCl3 and in (CD3)2SO and the considerable downfield shifts of the 15N resonances, compared with those of typical amides, are indicative of extensive electron delocalisation.

Effects of acute changes in blood glucose on HbA1c.

SUMMARY Although hemoglobin A1c (HbA1c) is generally considered to be an accurate index of long-term blood glucose regulation, several recent studies suggest that HbA1c may be acutely responsive to changes in blood glucose. We have examined the effects of acute changes in glucose concentration in vivo and in vitro on HbA1c. HbA1c was measured by a high-performance liquid chromatography (HPLC) method. HbA1c and plasma glucose were measured in seven diabetic patients and five control subjects before and 2 h after a standard breakfast. Only diabetic patients showed increases in HbA1c and plasma glucose values 2 h after the test meal (Δ HbA1c = 0.87 ± 0.24% and Δ glucose = 210 ± 0.33 mg/dl). The increment in HbA1c correlated significantly with the increment in plasma glucose (r = 0.73, P < 0.05). to examine the lability of these postmeal increments in HbA1c, erythrocytes from pre- and postmeal blood samples were incubated in 0.9% NaCl for 5 h at 37°C and HbA1c was re-measured. After saline incubations %HbA1c in pre- and postmeal blood samples decreased in both diabetics and controls, and from each subject time 0 and 2-h HbA1c values were nearly identical. HbA1c was then measured before and after incubations of erythrocytes from a larger group of diabetic patients (N = 55) and control subjects (N = 7). In both diabetic and control cells HbAlc decreased after saline incubations. Pre- and post-saline HbA1c values in diabetics were (means ± SEM) 10.07 ± 0.30% and 9.15 ± 0.27%, respectively; values in controls were 5.87 ± 0.11% and 5.46 ± 0.09%, respectively. The mean decrement in HbA1c was significantly greater in cells from diabetics than from controls (P < 0.001). In diabetics the HbA1c decrement correlated with both plasma glucose (r = 0.58, P < 0.001) and the preincubation HbA1c (r = 0.55, P < 0.001). After dialysis of hemolysates for 5 h at 37°C or 48–72 h at 4°C, HbA1c values showed decreases comparable to those after saline incubations of intact erythrocytes. However, decreases in HbA1c after dialysis were accompanied by increases in HbA1a+b, findings that were not observed after saline incubation. The results suggest that HbA1c exists in two chromatographically indistinguishable forms: one that represents the major portion of HbA1c in normals and diabetics, is not altered by short-term changes in plasma glucose, and can be estimated by measuring HbA1c after saline incubation of erythrocytes; and a second form that is responsive to short-term fluctuations of the blood glucose level. These labile and stable fractions may be identical to the labile Schiff-base and stable ketoamine forms of HbA1c, which have been previously described. For HbA1c to be an accurate indicator of long-term glucose control, saline incubation of erythrocytes or perhaps dialysis of hemolysates before HbA1c assay may be necessary. Otherwise the assay results will reflect recent changes in blood glucose levels.

Unsymmetrical schiff base complexes of nickel(II) and palladium(II)

The unsemmetrical Schiff bases salicyladehyde-acetylacetoneethylenediimine (Hsal)(Hacac)en, o-hydroxyacetophenoneacetylacetoneethylenediimine(Hhap)(Hacac)en, o-hydroxybenzophenoneacetyl-acetoneethylenediimine(Hhbp)(Hacac)en, and their nickel(II) and palladium(II) complexes have been synthesized and studied by different techniques. Mass, pmr and double pmr spectra suggest that the free Schiff bases comprise two different hybridized nitrogen atoms. Diamagnetism and spectral data of Ni(II) and Pd(II) chelates are consistent with their square-planar stereochemistry. Pmr spectra indicate that a conjugation between aromatic rings in Ni(hbp)(acac)en and Pb(hbp)(acac)en favours the ketoamine form of the arromatic part of complexes rather than the normal Schiff base struture. Pmr spectrum of Ni(sa)(acac)en studied over the - 70°C, + 30°C range suggests a high flexibility of the five-membered chelate ring.