Elsevier

Inorganica Chimica Acta

Volume 362, Issue 7, 15 May 2009, Pages 2396-2401
Inorganica Chimica Acta

Synthesis and characterization of a series of diphenyldipyrazolylmethane complexes with zinc(II)

https://doi.org/10.1016/j.ica.2008.10.024Get rights and content

Abstract

The zinc(II) coordination chemistry of a series of diphenyldipyrazolylmethane ligands was explored using 1H NMR and single crystal X-ray diffraction. Unsubstituted diphenyldipyrazolylmethane (dpdpm), diphenylbis(3-methylpyrazolyl)methane (dpdp′m), and diphenylbis(3,5-dimethylpyrazolyl)methane (dpdp″m) were reacted with Zn(NO3)2 to afford Zn(dpdpm)(NO3)2, Zn(dpdp′m)(NO3)2 and Zn(Pz″)2(NO3)2 where Pz″ = 3,5-dimethylpyrazole, respectively. All attempts to isolate Zn(dpdp″m)(NO3)2 with the intact dpdp″m ligand were unsuccessful due to decomposition of the ligand. These bidentate ligands support the formation of 1:1 ligand to metal complexes and structurally model the two histidine coordination mode common in zinc proteins.

Graphical abstract

Three zinc(II) complexes with diphenyldipyrazolylmethane ligands have been synthesized and characterized. Two of the three complexes have similar 1:1 metal to ligand structures in the solid state. Methylation of both the 3- and 5-positions of the pyrazoles prevents isolation of the target zinc complex due to ligand decomposition.

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Introduction

Diphenyldipyrazolylmethane is a bidentate, neutral ligand containing two pyrazole moieties available for coordination to metal centers. The sterics and electronics of diphenyldipyrazolymethane can be adjusted by employing substituted pyrazole moieties during ligand synthesis [1]. This control over ligand design can be used to enforce a desired geometry at a metal center and to tune the reactivity of the metal ligand complex.

In the literature, diphenyldipyrazolylmethane ligands have received relatively little attention compared to their polypyrazolylborate counterparts. In 1993, Shiu et al. isolated three molybdenum complexes with diphenyldipyrazolylmethanes [2], and in 1999 Jordan et al. synthesized cationic palladium(II) alkyl organometallic complexes for use in polymerization catalysis [3]. In addition, Reger et al. used these ligands in 2004 to gain a fundamental understanding of metal cation-π phenyl interactions in complexes with Ag(I) [4]. More recently, attention has shifted to generating diphenyldipyrazolylmethane complexes with the first row transition metals copper(II) and nickel(II). With regards to the copper(II) chemistry, these ligands have been used to mimic histidine coordination which is common in metalloproteins [1a], and have been used in oxalate bridged complexes to study electronic communication between d9 metal centers [1b]. Most recently, interesting solvato-, vapo-, and thermochromic properties of a series of nickel(II) diphenyldipyrazolylmethane complexes have been reported by Baho and Zargarian [5].

An investigation of the zinc(II) coordination chemistry of unsubstituted diphenyldipyrazolylmethane (dpdpm), diphenylbis(3-methylpyrazolyl)methane (dpdp′m), and diphenylbis(3,5-dimethylpyrazolyl)methane (dpdp″m) is presented here. Zinc is a biologically relevant metal that serves catalytic and structural roles in more than 300 enzymes [6]. A common amino acid for zinc ligation in these enzymes is histidine, and diphenyldipyrazolylmethane ligands may serve as accurate structural models for the 2 His coordination environment found in carboxypeptidase, thermolysin, and neutral protease [6c]. Here, the reactivity of these ligands and their coordination chemistry with zinc is explored using 1H NMR and single crystal X-ray diffraction in order to determine if diphenyldipyrazolylmethanes can be used to model the active site structures of zinc containing enzymes.

Section snippets

Experimental

All syntheses were carried out in air and the reagents and solvents were obtained commercially and used as received. Elemental analysis was performed by Atlantic Microlabs Inc. Fourier transform infrared spectroscopy (FTIR) was performed on powdered solids using a Perkin–Elmer SpectraOne spectrophotometer fitted with a diamond attenuated total reflectance stage. NMR spectra were recorded using a Bruker AVANCE 300 MHz instrument. Melting points (m.p.) were measured using a Mel-Temp® instrument.

Synthesis and spectroscopy

The room temperature 1H NMR spectra of Zn(dpdpm)(NO3)2 and Zn(dpdp′m)(NO3)2 are very similar so only Zn(dpdpm)(NO3)2 will be discussed here to avoid redundancy. In the spectrum of the free dpdpm ligand the phenyl protons appear as two sets of multiplets. One multiplet is centered at 7.1 ppm (4H) and the other is centered at 7.4 ppm (6H) [1]. Upon metallation with zinc(II), the phenyl protons of dpdpm separate into three distinct groups. The 1H NMR spectrum for Zn(dpdpm)(NO3)2 and the labeling

Conclusions

Three new zinc(II) complexes were synthesized using diphenyldipyrazolylmethane ligands. Zn(dpdpm)(NO3)Cl and Zn(dpdp′m)(NO3)2 were isolated as 1:1 metal to ligand complexes in the solid state. Attempts to isolate Zn(dpdp″m)(NO3)2 were unsuccessful due to decomposition of the dpdp″m ligand in the presence of metal. Instead, Zn(Pz″)2(NO3)2 was obtained and characterized. Future work will include a detailed NMR study of the conditions leading to dpdp″m ligand decomposition. We also plan to

Acknowledgements

J.L.S. would like to thank Dr. Chris Ziegler at the University of Akron for graciously collecting the single crystal X-ray diffraction data and the Center for Excellence in Teaching and Learning Incentive Grant at Kennesaw State University for financial support. We wish to also acknowledge NSF Grant CHE-0116041 for funds used to purchase the Bruker-Nonius diffractometer.

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