Mononuclear transition metal complexes with sterically hindered carboxylate ligands: Synthesis, structural and spectral properties
Graphical abstract
Sterically hindered 2,6-di-(p-fluorophenyl)benzoate and 3,5-dimethylpyrazole ligand leads to uncommon coordination geometry in the mononuclear divalent transition metal complexes from tetrahedral to octahedral and square pyramidal.
Research highlights
► Mononuclear metal complexes are synthesized using carboxylates and pyrazoles. ► Sterically hindered ligands lead to uncommon coordination geometry. ► Coordination geometry spends from tetrahedral to octahedral and square pyramidal.
Introduction
The research on the metal carboxylates has always been intriguing in that they play important roles not only in synthetic chemistry with the essence of labile coordination modes of carboxylate group, such as architecture of open and porous framework [1], [2], but also in biologic activities [3], [4] and physiological effects [5], [6]. A versatile carboxylate anion can adopt a wide range of bonding modes, including monodentate, symmetric and asymmetric chelating, and bidentate and monodentate bridging [7]. It is generally known that the carboxylate complexes of the 3d elements in active sites of enzymes play an important role in redox chemistry. For example, metal active sites catalyze fundamental transformations such as the conversions of oxygen to water, water to oxygen, nitrogen to ammonia and methane to methanol. Because the behavior of metal ions in proteins cannot be divorced from the fundamental chemistry of the particular metal, the study of small molecule, active-site synthetic analogs is useful [8], [9]. Synthetic models with carboxylates, imines and thiolates as the ligands are exploited in order to sharpen or focus certain questions related to the role of metals in metalloenzymes [10], [11], [12]. The goal is to elucidate fundamental aspects of structure, spectroscopy, magnetic and electronic structure, reactivity and chemical mechanism. The biological functions of some selected metal ions like copper, iron, cobalt manganese and nickel have undergone an extensive research but the quest for more advancement still exists. We have focused on cobalt, nickel and copper mononuclear complexes of the carboxylate ligand with a potential to expand the understanding of the coordination chemistry and substrate binding in enzymes with these metal active sites.
A common structural feature found in most of the metalloproteins and enzymes is the presence of one or more carboxylate groups derived from aspartate or glutamate side chains of the protein. The carboxylate group of glutamate and aspartate works as supporting ligand for the metal center in various metalloproteins and behave as monodentate or bidentate depending upon the requirement of active site. Besides vitamin B12 which catalyses the trans-alkylation and isomerization reactions via Co(III)-alkyl intermediate [13], there are several other cobalt containing proteins and enzymes such as ribonucleotide reductase (RR), MAPs (methionine aminopeptidases), glucose isomerases, cobalt transporters, bromoperoxidase, etc. where cobalt plays directly or indirectly an important role [14]. Similarly, copper-containing oxidases and oxygenases comprise a large class of enzymes containing varying numbers of copper ions having diverse structures. The organic cofactors in copper-containing amine oxidases (AO) and lysyl oxidase (LO), dopamine β-monooxygenase (DβM) and peptidylglycine α-hydroxylating monooxygenase (PHM) all involves mononuclear copper active-oxygen species [15], [16], [17].
The literature also show that dinuclear nickel complexes with bridging carboxylates [18], [19] are interesting for their magnetic interactions, whereas the mononuclear complexes play an important role in modeling the active site of metalloproteins [2], [20], [21], [22]. With nickel(II) as its native metal ion, urease catalyses the hydrolysis of urea to ammonia and carbamate and in turn spontaneous decomposition of CO2 and a second molecule of ammonia [23]. However structurally characterized nickel carboxylate complexes are limited in literature [24], [25].
Ever since, the simultaneous recognition of Lippard and Tolman that sterically hindered m-terphenyl carboxylates were ideal ligands for the preparation of model compounds of the enzyme active site of non-heme diiron enzymes [26], [27], several reports on the use of this class of sterically hindered m-terphenyl ligands for diiron enzymes as well as copper and manganese enzymes have appeared in the literature which involves the dinuclear metal carboxylates [28], [29]. Lee et al. also reported the carboxylate bridged dinuclear cobalt and nickel complexes of sterically hindered 2,6-di(p-tolyl)benzoic acid as a model of metallohydrase active sites [30]. However, mononuclear copper(II), cobalt(II) and nickel(II) complexes of this class of ligands are rarely reported [31], [32]. Herein, we report the synthesis and characterization of mononuclear cobalt, nickel and copper complexes incorporating the sterically hindered terphenyl carboxylate (−O2CAr4-FPh) and 3,5-dimethylpyrazole (Hdmpz) as a co-ligand (Scheme 1).
Section snippets
General considerations
All chemicals purchased were of analytic grade and were used without further purification. All reactions were carried out under N2 atmosphere by using Schlenk techniques. FT-IR spectra were recorded using Varian 640-IR FT-IR spectrometer. UV–Vis spectra were recorded on a OPTIZEN 2120UV spectrophotometer. Electrochemical studies were performed using a Zahner Elektrik IM 6 model potentiostat with 0.5 M dichloromethane solutions of [(n-C4H9)4N]PF6 as supporting electrolyte under a nitrogen
Rationale for the synthetic approach
The use of pyrazole or pyrazole derivatives has drawn strong interest in modeling biological systems. Trofimenko has used pyrazole in polypyrazolyborates to stabilize a variety of organometallic and coordination complexes [39] which are valuable in biomimetic coordination chemistry of numerous metalloproteins since these monoanionic, facially coordinating ligands have histidine-like donors which can hold three cis sites fixed while leaving other coordination sites open. Thus, a variety of
Conclusion
In summary, use of the sterically-hindered 2,6-di-(p-fluorophenyl)benzoate and 3,5-dimethylpyrazole ligand afforded a series of mononuclear divalent transition metal complexes. Although compound 1, 2, 3 and [Fe(O2CAr4-FPh)2(Hdmpz)2] have identical ligand components, coordination geometry alters from tetrahedral to octahedral and square pyramidal depending on metal ion entity. It indicated that the geometry of coordination metal complexes is determined not only by the coordination environment
Acknowledgement
This work was supported partly by the Korea Science and Engineering Foundation (KOSEF) grant and the Converging Research Center Program through the National Research Foundation of Korea (NRF) (No. 2009-0082832) funded by the Korea government (MEST). This work was also supported in part by KIST (Korea Institute of Science & Technology) for ‘National Agenda Project program’, the faculty research program 2009 of Kookmin University in Korea and the Korea Science and Engineering Foundation (KOSEF)
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