Synthesis, Characterization and Crystal Structures of Zwitterionic Triazolato Complexes by Reaction of a Ruthenium Azido Complex with Excess Ethyl Propiolate

The synthesis and structures of two novel zwitterionic ruthenium triazolato complexes are reported. The treatment of the ruthenium azido complex [Ru]-N 3 (1, [Ru] = (η 5 -C 5 H 5 )(dppe)Ru, dppe = Ph 2 PCH 2 CH 2 PPh 2 ) with an excess of ethyl propiolate in CHCl 3 or CH 2 Cl 2 under ambient conditions for 15 days results in the formation of a mixture of the Z- and E-forms of N(1)-bound ruthenium 3-ethylacryl-4-carboxylate-3H-1,2,3-triazolato complexes [Ru]N 3 (CH=CHCO 2 Et)C 2 H(CO 2 ) (Z-3) and (E-3) in a ratio of ca. 5:2. The structures of E-3 and Z-3 were confirmed by single-crystal X-ray diffraction analysis and fully characterized by 1 H, 31 P, 13 C NMR and IR spectroscopy, mass spectrometry, and elemental analysis. The negatively charged carboxylate moieties of the zwitterionic ruthenium triazolato complexes Z-3 and Z-3 are highly nucleophilic and reactive toward a variety of electrophiles, making Z-3 and Z-3 potential starting materials for the development of biologically active 1,2,3-triazole derivatives.


Introduction
Metal azido complexes are active starting materials in organometallic and material chemistry [1] because of their remarkable reactivity with various sub-strates. Coordinated azide undergoes cycloadditions with unsaturated organic substrates such as alkynes, alkenes, nitriles and heteroallenes, giving rise to multi-functional metal-bound five-membered heterocycles [2], have attracted intensive research activities. Over the past decade, many new examples for the metal-mediated cycloaddition to alkynes have emerged and many of them have been shown to give metal-coordinated 1,2,3-triazolato complexes [2]. The triazole ring, a bioisostere of the amide group exhibiting diverse biological activities and showing excellent hydrogen donating and accepting ability, has found widespread applications in medicinal and pharmaceutical chemistry [3] [4] [5] [6] [7]. Ethyl propiolate, an active α,β-unsaturated ester, is capable of participating in a variety of cyclization reactions [8] [9] [10], as well as Diels-Alder cycloaddition [11] [12] and conjugate addition reactions [13] [14] [15] [16]. In our previous study, we reported on reactions of a ruthenium azido complex with a variety of unsaturated organics to form ruthenium triazolato complexes [17] [18] [19] [20] [21]. Among the unsaturated alkynes that we had ever used, methyl propiolate and ethyl propiolate were found to be extraordinarily reactive and afforded several novel ruthenium triazolato products [17] [20] [21]. In our previous study [21], the treatment of [Ru]-N 3 (1, [Ru] = (η 5 -C 5 H 5 )(dppe)Ru, dppe = Ph 2 PCH 2 CH 2 PPh 2 ) with an excess of methyl propiolate afforded novel zwitterionic Zand E-form triazolato complexes in a ratio of ca. 4:1. As we mentioned above, the 1,3-dipolar cycloaddition of azido metal complexes with alkynes has attracted the interest of various research groups in recent years and hundreds of metal azides have been shown to give 1,2,3-triazolato complexes [1], but to the best of our knowledge, our previous work [21] represented the first and unique example of the further addition of methyl propiolate to the thus formed triazolato ring. Recently we treated the ruthenium azido compound 1 with excess ethyl propiolate in chloroform, the thus formed ruthenium triazolato complexes slowly transferred to some new complexes which were similar to the zwitterionic triazolato products we afforded before. To investigate the details of such interesting reactions, herein we report on the reaction of a ruthenium azide with an excess of ethyl propiolate, affording novel zwitterionic triazolato complexes. The structures of thus formed triazolato complexes were confirmed by single-crystal X-ray diffraction analysis.

General Procedures
All solvents and reagents were of reagent grade and were used without further purification. Elemental analyses were performed on a PerkinElmer 2400 CHN elemental analyzer. HR & LR-FAB mass spectra were recorded on a JMS-700 double focusing mass spectrometer (JEOL, Tokyo, Japan) with a resolution of 8000 (3000) (5% valley definition). IR spectra were collected on a PerkinElmer Paragon 1000 FT-IR spectrometer in the range of 4000 -400 cm −1 using KBr pellets. NMR spectra were recorded on Bruker AVA-300 (300 NMR) spectrometers (2) were prepared following methods reported in the literature [17]. Elemental analyses and X-ray diffraction studies were carried out at the Instrumentation Center located at the NTU.

Structure Analysis and Refinement
Single crystals f Z-3 suitable for X-ray diffraction study were afforded by slow evaporation of the CH 2 Cl 2 /n-hexane solution of 3 under ambient conditions for 7 days and single crystals of E-3 were afforded by the diffusion of n-pentane into CHCl 3 solution of 3 at −15˚C for 10 days. The chosen single crystal was glued to glass fiber and mounted on a Bruker SMART APEX diffractometer equipped with graphite monochromatic Mo-Kα radiation (λ = 0.71073 Å). Data collection was executed using the SMART program; cell refinement and data reduction were performed with the SAINT program. The structure was determined by the SHELXTL/PC [22] program and refined by the full-matrix least-squares me-Journal of Crystallization Process and Technology thods on F 2 . Hydrogen atoms were placed geometrically using the riding model with thermal parameters set to 1.2 times that for the atoms to which the hydrogen is attached and 1.5 times that for the methyl hydrogens. Crystallographic data of E-3 and Z-3 are summarized in Table 1.

Synthesis of Zwitterionic Triazolato Complexes Z-3 and E-3
The  ethanol, benzene and n-hexane, but sparingly soluble in n-pentane. The good solubility could be due to the zwitterionic structure and the excellent hydrogen donating and accepting ability of 3.

Single Crystal X-Ray Diffraction Analysis
Yellow crystals of Z-3 were afforded by slow evaporation of the CH 2 Cl 2 /n-hexane solution of 3 under ambient conditions for 7 days. Yellow crystals of E-3 were afforded by the diffusion of n-pentane into CHCl 3 solution of 3 at −15˚C for 10 days, a few yellow crystals of E-3 grew on the wall of the tube and were collected by hand. Structures of Z-3 and E-3 were determined by a single crystal X-ray diffraction analysis. ORTEP drawings are shown in Figure 1 and Figure 2, respectively. Selected bond distances and bond angles are given in Table 2.  (7) 87.86 (7) Ru−N1−N2 118.15 (19) 119

Conclusion
We successfully synthesized the zwitterionic 1,2,3-triazolato complexes Z-3 and E-3 in good yield by reaction of the ruthenium azido complex [Ru]-N 3 (1, [Ru] = (η 5 -C 5 H 5 )(dppe)Ru, dppe = Ph 2 PCH 2 CH 2 PPh 2 ) with an excess of ethyl propi-Journal of Crystallization Process and Technology olate. Results obtained from the elemental, spectral and X-ray crystallography had confirmed the proposed structures of the synthesized triazolates. Hopefully, these findings will enable a more complete understanding of such triazolato complexes that can be produced and the development of synthetic procedures that can be used to prepare novel metal-coordinated triazolato complexes. We are currently in the process of further exploring the reactivity of these zwitterionic 1,2,3-triazolato complexes. Studies of related reactions and applications of these complexes are currently underway.