Some Complexes of N-Aryl Furfural Nitrones with Co(II), Ni(II), Cu(II), Zn(II) and Cd(II) Chlorides

Abstract

Some new metal(II) dichloride complexes with the ligands substituted nitrones of the general formula [Abbildung in dieser Leseprobe nicht enthalten] have been prepared and characterized by elemental analysis, IR,[1]H,[13]C NMR and Vis/Uv spectroscopy. The IR spectral data showed that the nitrone ligands coordinated with the metal ion through the most active atom of the N-oxide to give square planner coordinate (Cu,Ni,) complexes and (Zn,Cd,Co) tetrahedral complexes. No correlation was observed between the N-O vibrations stretching hing frequency ν(N-O) of the complexes and the Hammet (σ) constants.

Introduction

Preparation of nitrone compounds and it`s derivatives had much attention because of the biological importance, new methods of prepared nitrone compounds have much attention([1]-[2]). Some of these methods have been applied to the preparation of complexes molecules with useful biological activity such as antibiotics and glycosides inhibitors ([3]-[8]). Therefore, new methods of activation, such as microwave chemistry and coordination to a metal center, have been attempt. In fact, it was observed that the microwave field decreased the activation energy of various types of reactions in particular with organonitrones ([9]).Moreover, metals in coordination processes can dramatically increased the reactivity of organonitriles [11]) and metal-mediated processes can lead to the formation of heterocyclic species ([10]).

Nitrones have been recognized as having the following resonance structure

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When A,A` or B is a mesmeric substituent the nitrone group (-C=NO-) will, of course, interact mesomerrically with the substituent and thereby will probably exert an electron-attracting effect on the latter.

The [13]CNMR and [1]H1NMR study showed ([12]), when a strong electron-donating group is in the Para position of a phenyl ring the nitrone acted as an electron-withdrawing group. For strong electron-withdrawing substituent the nitrone group acted as an electron donor. Thus they concluded that the nitrone group behaved as an electron- withdrawing or donating

In the current work, we have presented the reaction between metal chloride Co(II), Ni(II), Cu(II),Zn(II),Cd(II) and N-arylfurfuralnitrones(fig1) in order to examine the type of interaction between metals and this type of ligands as they contain more than possible donor site. Also measure the effect of substituent on the mechanism formation of complexes using Hammet equation. To the best of our knowledge, this work is novel.

EXPERIMENTAL

The ligands p-x-phenyl-N-furfural nitrones (X=H, Cl, Br, F, OH, CH3, OCH3 and COCH3) were prepared as described previously with substituted X-C4H4NHOH in ethanol ([13]). The free nitrone ligands were purified by crystallization.

Preparation of complexes: The following standard method was used; molar quantities (usually 1:2 mmole) of metal salts and the nitrone ligand (L) were dissolved in absolute ethanol (25ml) at ambient temperature. The colored solution and the complex started to deposit. The formed precipitate were filtered, washed several times with small portions of ether and dried. The yield is almost quantitative.

Metal analysis for some of the complexes were determined, Nickel metal was determined as dimethylglyoxime complexes ([14]). Cobalt, zinc, copper and cadmium metals were determined by pyridine method ([14]).

[13]C and [1]H1NMR spectra were recorded at 25˚C on a Bruker DPX 300MHz spectrometer at the department of Chemistry, College of Science, Jordan.

IR spectra was recorded on an Infrared spectrophotometer BRUKER (TENSOR 27), conductivity measurements were done for 10-[3]M solutions of the complexes in ethanol and dimethylforamide at room temperature (25˚C), using a Jenway conductivities meter model 4070. Visible spectra were recorded Ultraviolet-Visible spectrophotometer (UV -1650 PC).

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Fig.1. The Nitrones Ligands used in the coordination with MCl2(M= Mn,Co,Ni,Cu,Zn and Cd )

Results and Discussion

The IR data in Table I confirmed the formation of the complexes. There are changes in the frequencies of the C=N band upon complexation, but especially significant is the appearance of a new band at ca. 340 cm-[1], attributed to υ(M-O), which served as a good indicator of coordination.( [15]). Moreover, the drastic shift in the υ(N-O) frequency is clear evidence for the interaction between the NO group of the nitrone ligand and metal. However, coordinatid lead to a shift to lower frequency and the values of ν(NO)complex- ν (NO)ligand showed a systematic variations from 15-to-50 cm-[1] (Table1), and this may be attributed to a decrease in the NO group upon coordination([16]). On the other hand the ν (C=N) frequency of the ligand show a great change to a higher values upon coordination and the values of ν (CN)complex- ν (CN)ligand show again systematic variations from 25 to 45cm-[1]. This may be due to the increase in the bond order between C and N upon coordination. In contrast, the ν (C-O) frequency of the ligand which appeared in the range ([17]) 1070- 1090cm-[1] remains almost constant upon coordination supported that the furfural oxygen remained unchenged upon coordination supported that the furfural oxygen is not involved in the coordination.

Molar conductivities

Molar conductivities for 10-[3]M solutions of the complexes in two different solvents, ethanol and DMF, at ambient temperature 25˚C were in the range 1.3-11.82 and 0.05- 10.2 ohm cm[2] mole-[1] respectively,(Table II) suggesting the present of non-conductive species([18])(i.e., non-ionic) complexes in the solvents used.

NMR Spectra

The magnetic resonance (Table III) showed shifted α-H of ligand to complex in d6DMSO solvent and this confirms coordination. The [13]C NMR spectral data were recorded to provide an additional indicator for the coordination number. In (Table IV), shows a clear change in the chemical shifts of the carbon atoms of the organic nucleus, especially significant are those of C-α, C-1, C2 `,C3`and C5`(Fig1). Carbon atoms were assigned by comparison with other related organic compounds as model compounds([19]). The carbon C-α showed a downfield shift on going from the free ligand to its complex (ca.1.5ppm) and this clearly suggested that the C=N bond order is increased due to complexiation. This is supported by the stretching frequency of the C=N bond which shifted to a higher value on complexation. Similarly, C-1 is also affected upon coordination and is shifted upfield by ca. 1ppm. Although the furfural nucleus had not been involved in the coordination, nevertheless, the peak for C-2` is shifted upfield whereas those of C-3` and C-5` are shifted downfield upon coordination. We believe that this may be due to the inductive effect caused by coordination. This in turn causes a great drainage of electron density from the furfural oxygen, through conjugation to C2`, then to C-α and so on.

From Fig1. It can be seen that only two active donor site, i.e., the furfural oxygen and the nitrone oxygen, participate in bonding. Whereas the ligands L3 and L4 contain further donor site, i.e., the methoxy oxygen and the acetoxy oxygen atoms, respectively. Nevertheless, the spectral data showed that both groups, the methoxy and the acetoxy, were not involved in the complexation with metal. Therefore, the only possible donor sites of all ligands are the furfural oxygen and the nitrone oxygen atoms, thus the nitrones can behave as bidentate ligands, since the furfural group can rotate freely around the C2-Cα bond, and both oxygens can be arranged in such a way that they can complex with metal in a bidentate fashion. However, this is not the case with complexes, in which the IR spectral data showd that all of these ligands coordinate with metal in a bidentate fashion via both oxygen atoms([16],[20]).

The electronic spectra of the copper(II) complexes showed a single broad and poorly defined asymmetric band around 17500 cm-[1] and the spectra of nickle(II) Complexes showed a band around 20000cm-[1]. These results were consistent with square-planar structures, since the four lower orbitals are often so close together in energy, that individual transitions there from to the upper d level cannot be distinguished - hence the single absorption band([21]).

The nickel (II) complexes showed a diamagnetic behavior consistent with square- planar environment around the metal ion. Magnetic susceptibilities of the copper (II)

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