Elsevier

Inorganica Chimica Acta

Volume 299, Issue 1, 28 February 2000, Pages 100-103
Inorganica Chimica Acta

An organometallic mercaptopyridine complex with unusual bond shift fluxionality: metal-mediated tautomerism of (pentamethylcyclopentadienyl)bis(pyridine-2-thiolato)rhodium(III)

https://doi.org/10.1016/S0020-1693(99)00469-7Get rights and content

Abstract

Studies have been carried out on unique molecular non-rigidity of Cp*RhIII(PyS)2 (PyS=pyridine-2-thiolato). One PyS ligand bonds to the rhodium ion in an S-monodentate mode (RhS(2)=2.3799(9) Å) while the other ligand chelates to the metal in an N,S-bidentate mode (RhS(1)=2.4380(9); RhN(1)=2.089(2) Å). Both PyS ligands possess a significant contribution from the thiol tautomer in the solid state. Even though the conformation and configuration of the molecule are still retained in solution, an unusual metal-mediated tautomeric non-rigidity for the PyS region is observed in the solution around room temperature.

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 (RhS(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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