Structural and thermal properties of three cyano-substituted azoderivatives of β-diketones
Introduction
It is well known that many azocompounds and β-diketones, under certain conditions, tend to perform tautomeric transformations. Thus, diazocompounds can exist as a mixture of azo- and hydrazone tautomeric forms [1], while β-diketones easily switch to the enol form [2], [3]. These tautomeric equilibria are important from theoretical and practical aspects: about 92% of azo dyes published in Colour Index possess this tautomery [4] which affects their tone, photostability, etc. On the other hand, tautomeriс equilibria of β-diketones also influence their spectral properties, reactivity, coordination ability, etc. [5], [6]. However, there is a lack of experimental data on the tautomerism of the pigments combining both of the those functionalities, i.e. azoderivatives of β-diketones (ADB).
Moreover, this tautomeric balance can play an important role for the application of ADB as bistate molecular switches [8], [9], [10] or regulation of the ionophore selectivity in analytical chemistry [11], [12]. Within this view, special attention should be paid to the nature of the strong intramolecular O⋯HN hydrogen bond and its influence on the enol-azo ⇌ hydrazo transformation [7]. Additionally, the synthetic potential of ADB for coordination chemistry was underestimated, notwithstanding their potentially rich chelating ability [7], [13], [14], [15], [16], [17], [18].
On the other hand, the cyano group has a well-recognized chemical versatility [19]. In particular, in organic chemistry the addition of nucleophiles or electrophiles and the asymmetric dipolar cycloaddition to the CN triple bond offer attractive routes for the creation of CC, CN, CO and CS bonds. Therefore, cyano-substituted ADB are expected to be good candidates towards the development of a rich organic chemistry.
Thus, taking in mind all the above mentioned considerations, the main aims of the current work are as follows: (i) to synthesize new ADB with cyano substituents in the aromatic part of the molecule (Scheme 1) and prove their coordination ability by synthesizing a new Pd(II)-ADB complex; (ii) to study the influence of the introduced substituents on the tautomeric balance and thermal properties of the synthesized materials.
Section snippets
Materials and instrumentation
All the chemicals were obtained from commercial sources (Aldrich) and used as received. Infrared spectra (4000–400 cm−1) were recorded on a BIO-RAD FTS 3000MX instrument in KBr pellets. 1D (1H, 13C{1H}) and 2D (1H, 1H-COSY, 1H, 13C-HMQC, 1H, 13C-HSQC and 1H, 13C-HMBC) NMR spectra were recorded on Bruker Avance II + 300 and 400 MHz (UltraShield™ Magnet) spectrometers at ambient temperature. Chemical shifts (δ) are relative to internal TMS. Carbon, hydrogen, and nitrogen elemental analyses were
Spectroscopic and crystallographic investigation of HL1–3
The new cyano-substituted azoderivatives of β-diketones HL1–3 were synthesized via the Japp–Klingemann reaction [20], [21], [22] between the respective cyano-substituted aromatic diazonium salts and pentane-2,4-dione or 1-ethoxybutane-1,3-dione in water solution containing sodium hydroxide. IR spectra of the isolated compounds show ν(NH) vibrations at 3437–3448 cm−1, while ν(CO), ν(CO⋯H), ν(CN) and ν(CN) are observed at 1672–1690, 1637–1660, 1601–1608 and 2220 cm−1, correspondingly, what is
Conclusions
Three new cyano-substituted ADB, HL1–3, were synthesized and characterized by IR and multi-nuclear NMR spectroscopies, ESI-MS, elemental and X-ray diffraction analysis (for HL1). The derivative of the unsymmetric β-diketone, HL3, exists in solution as a mixture of the enol-azo and hydrazo tautomeric forms, and a decrease of the solvent polarity shifts the tautomeric balance to the hydrazo form. The formation of the heterodienic system, HNNCCO, and the fairly weak heteronuclear
Acknowledgements
This work has been partially supported by the Foundation for Science and Technology (FCT), Portugal, and its PPCDT (FEDER funded) and “Science 2007” programs. M.N.K., K.T.M. K.V.L. and A.M. express gratitude to the FCT for a post-doc fellowship and working contracts. The authors gratefully acknowledge Dr. Conceição Oliveira for the ESI-MS analysis, and the Portuguese NMR Network (IST-UTL Centre) for the NMR facility.
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