An organometallic mercaptopyridine complex with unusual bond shift fluxionality: metal-mediated tautomerism of (pentamethylcyclopentadienyl)bis(pyridine-2-thiolato)rhodium(III)
Introduction
Tautomeric equilibria have often been investigated to deduce general structure–stability relationships [1], [2], [3], and the results have been extended beyond the initial studies to such diverse areas as quantum mechanical calculations [4], molecular switches [5], tautomeric catalysts [6], molecular recognitions [7], and theories of genetic mutation [1]. For instance, the tautomeric equilibria of purine and pyrimidine bases have been suggested to affect RNA transcription and DNA replication in biological systems [1], and behaviors of nucleic acids [8]. Thio-bases such as thiouracil and thiocytosine that can be tautomerized are normal constituents of some t-RNA species [9]. Among such tautomeric systems, the simplest example is pyridine-2-thiolate (PyS) and its analogs that are capable of bonding to metals as either pyridine-2-thiol (A, thiol) or pyridine-2-thione (B, thione) [10]. Their organometallic and inorganic complexes have both biological and chemical importance [10], but their unusual fluxional processes were not yet investigated fully. There are three important motivations for studying this system. First, it may be a model for elucidating factors that control tautomerism of purine or thio-bases. Second, it may possible to control the tautomer balance using substituent effects. Third, tautomer equilibria may control molecular recognition processes. In this paper, we have synthesized new complex of Cp*RhIII(PyS)2 and scrutinized its NMR behavior in solution. An unusual tautomeric non-rigidity established by the aid of a configurational- and conformational-rigid structure around room temperature has been described.
Section snippets
Materials and instrumentation
2-Mercaptopyridine (PyS), and pentamethylcyclopentadienyl-rhodium(III) chloride were purchased from Aldrich. Elemental analyses were performed by the Advanced Analysis Center at KIST. The infrared spectra in the 5000–400 cm−1 region were measured as KBr pellets on a Perkin–Elmer 16F PC model FT-IR spectrometer. 1H and 13C NMR spectra were recorded on a Varian Gemini-300 NMR spectrometer operating at 300.00 MHz (1H) and 75.48 MHz (13C), respectively, in pulse mode with Fourier transform.
Results and discussion
The synthetic procedure yielded single crystals of Cp*RhIII(PyS)2. 1H NMR spectrum of the PyS region consists of four broad peaks at 8.04, 7.28, 7.10, and 6.58 ppm in a 2:1:2:3 integral ratio at 30°C. So, X-ray characterization was performed to reveal its solid state structure depicted in Fig. 1. Selected bond lengths and angles were listed in Table 1. One PyS ligand bonds to the rhodium ion in an S-monodentate mode (RhS(2), 2.3799(9) Å) while the other chelates to the metal atom in an
Supplementary material
A listing of refinement details, atomic coordinates, thermal parameters, bond lengths and angles, and structure factors for Cp*RhIII(PyS)2 are available from the authors on request.
Acknowledgements
This research was supported financially by the Ministry of Science and Technology of Korea (OSJ), and Basic Science Research Institute Program of Ministry of Education of Korea (BSRI-97-3407) (HKC).
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