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Reports: UR352779-UR3: Triazole-Containing Tridentate Anionic Chelators for Lanthanide and Group VIII Organometallic Complexes

James T. Fletcher, PhD, Creighton University

Ligand motifs containing 1,2,3-triazole subunits have become increasingly prevalent due to their ease of preparation as allowed by Sharpless-Meldal click methods.  A common example is the 2,6-bis(1-substituted-1,2,3-triazol-4-yl)pyridine ligand (1), which has been shown to coordinate a variety of transition metals in a neutral tridentate manner.  1,2,3-Triazole containing ligands able to coordinate metals as anionic ligands are much less prevalent.  Metal complexes comprised of anionic ligands can display properties unique to analogous neutral ligands, as well as preclude the requirement of outer sphere counter ions.  The overall goal of this study is to develop two previously unreported 1,2,3-triazole ligand motifs displaying the ability to coordinate transition metals in an anionic tridentate manner: 6-(1-substituted-1,2,3-triazol-4-yl)-2-carboxypyridine (2) and 2-(1H-tetrazol-5-yl)-6-(1-substituted-1,2,3-triazol-4-yl)pyridine (3).  Using a click chemistry approach to install the 1,2,3-triazole ring enables the target chelators to be prepared modularly and in high yield, while also allowing a diverse range of functionality to be incorporated onto the periphery of the chelator at the 1-triazolyl position.  Functional groups allowing the tuning of electronic properties and solubility are of particular interest, as is exploring the ability of such compounds to form stable 2:1 coordination compounds with Ru(II) and Fe(II) ions and stable 3:1 coordination compounds with trivalent lanthanide ions.

Figure 1.  Identity of tridentate chelators 1, 2 and 3.

Year one of this grant established an efficient synthesis for chelators 2 and 3 and identified their ability to form stable, charge-balanced 2:1 coordination compounds with Ru(II) ions.  This success was tempered by the fact that many of these Ru(2)2 and Ru(3)2 complexes suffered from poor solubility, limiting the ability to fully characterize these complexes by cyclic voltammetry. 

In year two of this grant, attempts to prepare highly soluble analogs of Ru(2)2 and Ru(3)2 complexes were unsuccessful, motivating the decision to invest more effort preparing new coordination complexes with other metal ions.  Stable, charge balanced 2:1 Fe(II) complexes were successfully prepared by reaction of 2 and 3 with FeCl2 using the conditions summarized in Figure 2.  While the success of these reactions was confirmed by mass spectrometry measurements, such compounds suffer from the same limited solubility as observed for analogous Ru(II) complexes.

Figure 2. Preparation of Fe(2)2 and Fe(3)2 complexes.

Representative charge balanced 3:1 lanthanide complexes were also successfully prepared using chelators 2 and 3.  Eu(III) and Tb(III) were selected due to their visible and long lived emission properties.  The conditions used to prepare these compounds are summarized in Figure 3.  X-ray structural analysis of the benzylated analog of Tb(3)3 supports the proposed 3:1 chelator:metal stoichiometry and anionic tridentate coordination environment at the metal center, as illustrated in Figure 4.

Figure 3. Preparation of Ln(2)2 and Ln(3)2 complexes (Ln = Eu, Tb).

Figure 4.  Structure of Tb(3)3 analog as measured by single crystal X-ray diffraction (courtesy of Eric Villa, Creighton University).

The observed solid-state fluorescence emission of these complexes was found to vary as chelator and peripheral substituents varied for Eu(III) and Tb(III) ions.  Analogs with small aryl units (benzene, naphthalene) peripherally attached showed the distinct sharp pattern of metal-centered emission bands for Eu(III) and Tb(III), respectively, while analogs with larger aryl units (anthracene) peripherally attached showed broad emission bands more indicative of ligand-centered emission.  Preparation of an expanded set of analogs is currently underway to further probe how variation of peripheral substituents impacts solid-state emission properties, including both emission wavelength and lifetime.

Also in year two of this grant, iterations of the tridentate triazolylpyridine ligand motif were explored.  It was found that replacement of the carboxylic acid component of chelator 2 with an aldehyde moiety enabled the efficient preparation of 2-imino-6-(1,2,3-triazol-4-yl)pyridine analogs (4) following the synthetic scheme summarized in Figure 5.  Starting with 6-bromo-2-formylpyridine, the target chelator can be prepared in three steps: Sonogashira coupling, tandem deprotection/click reaction and amine condensation.  This approach enables facile variation of functionality at both the triazole and imine positions of the chelator, attractive for structure-property relationship studies.

Figure 5.  Preparation of target compound 4.

A series of Ru(4)2Cl2 complexes were prepared using conditions analogous to preparing Ru(1)2Cl2 complexes, as summarized in Figure 6.  In contrast to the anionic chelators 2 and 3, analogs of 4 coordinate in a neutral tridentate manner and their Ru(II) complexes display greatly enhanced solubility in protic and polar aprotic organic solvents.  This has enabled preliminary structure property relationship studies to be completed as the electronic nature of the triazole and imine substituents are varied.  Relative to the chelators themselves, UV-visible absorbance measurements show the Ru(II) complexes display MLCT bands in the 500-650 nm range as peripheral substituents at the triazole and imine units are varied.  Additional studies are underway to expand the number of analogs, as well as characterize such compounds by cyclic voltammetry.

Figure 6.  Preparation of Ru(4)2Cl2 complexes.

With a better understanding of the chelating ability of compounds 2 and 3 with group VIII and lanthanide ions, work during the third year of the grant will focus on three major areas.  One goal is to complete a comprehensive study on the impact that peripheral substitution has on the solid state fluorescence emission properties of Eu(III) and Tb(III) complexes.  A second goal is to finish the ongoing studies on Ru(4)2Cl2 complexes in order to define how peripheral substitution impacts optoelectronic properties as measured by UV-Visible spectroscopy and cyclic voltammetry.  A third goal is to screen chelators 2, 3 and 4 with a panel of metal ions in order to identify attractive leads for future investigations involving coordination compounds outside the Fe(II), Ru(II) and trivalent lanthanide ions originally proposed by this study.

In year two, this project has directly supported two undergraduate research students devoting full time effort during the summer of 2015 and part time effort during the 2014-2015 academic year.  Presentations describing the results of these studies will be made at the 2015 ACS Midwest Regional Meeting and at Pacifichem 2015.  It is anticipated that multiple manuscripts including undergraduate coauthors describing the results summarized herein will be submitted for publication in the upcoming year.