Elsevier

Polyhedron

Volume 114, 16 August 2016, Pages 172-178
Polyhedron

Structural characterization and electrochemical properties of nickel (II) complexes bearing sterically bulky hydrotris(3-phenyl)- and hydrotris(3-tert-butylpyrazol-1-yl)borato ligands

https://doi.org/10.1016/j.poly.2015.11.032Get rights and content

Abstract

Two series of pseudo-tetrahedral nickel (II) complexes bearing the bulky hydrotris(3-phenyl)- or hydrotris(3-tert-butylpyrazol-1-yl)borato ligands, TpPhNiL and TptBuNiL respectively, have been synthesized and characterized with the intent of investigating the affects of substituent size on the redox properties of the nickel center (L = Cl, Br, I, NCS, NO3). X-ray analysis reveals both Tp ligands bind in a κ3 fashion with the R groups pointing out towards the metal producing a pocket in which the 4th ligand, L, resides. The TpPh ligand results in a larger pocket and produces nickel complexes with distorted solid-state geometries, specifically bent B…Ni…L angles, unlike the corresponding TptBu complexes which show much less variation in structure. Cyclic voltammetry experiments indicate all complexes undergo a Ni2+/1+ reduction at large negative potentials vs. ferrocene with varying levels of reversibility. The NCS complexes are the most reversible and the easiest to reduce with an average reduction potential of −1.51 V due to the slightly electron withdrawing nature of the thiocyanate ligand. The strong donor properties of the bidentate nitrate ligand result in complexes that are the most difficult to reduce at an average of −1.80 V. Overall, the TpPhNiL complexes are easier to reduce than their corresponding TptBuNiL complexes resulting from the weakened donor ability of the TpPh ligand. While the two ligands do produce different structural motifs, the electrochemical trends are more in line with the electronic properties of the complexes than structural differences.

Graphical abstract

A series of nickel (II) complexes bearing sterically bulky TpR (R = tert-butyl or phenyl) ligands were synthesized and structurally characterized. The redox properties of the nickel center were investigated and results indicate that the redox trends fall more in line with the electronic properties of the complexes than the structural differences.

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Introduction

The hydrotris(pyrazolyl)borate, Tp, ligand has been a valuable tool for synthetic chemists ever since its development by Trofimenko in the late 1960s [1]. The ability to modify the electronic and steric properties by substitution around the pyrazole rings has lead to widespread use of Tp ligands in the fields of inorganic, organometallic and bioinorganic chemistry [2]. Of particular interest are derivatives with large groups at the 3 or 5 positions that discourage formation of the coordinatively saturated (TpR)2M sandwich complexes promoting monomeric metal complexes with available sites opposite the TpR ligand for additional ligand binding. Within the bulky TpR ligands, there is structural variation depending on the size and nature of the R group. Large aliphatic groups like tert-butyl produce “tetrahedral enforcer” ligands, which leave a small rigid pocket around the metal available for only one additional ligand [3], [4]. However, planar aromatic groups such as phenyl, produce larger, more flexible pockets with greater diversity in overall structure and ligand identity [5], [6]. Nickel complexes of the latter have been investigated for their biomimetic chemistry [7], [8], [9] and the their promise as homogeneous catalysts for organic transformations [10], [11], [12]. However, the basic redox properties of the nickel center in these types of complexes have not been thoroughly studied. Specifically, do the structural properties imposed by the bulky pyrazolyl substituent affect the redox properties of the metal center?

In this work, we investigate the role of substituent nature on the redox properties of Tp nickel complexes by generating two series of molecules bearing the TptBu and TpPh ligands. The solid-state structures of the novel complexes are investigated using single crystal X-ray diffraction and the redox properties of the nickel center are monitored by cyclic voltammetry. The TpPh ligand produces a series of complexes with far more structural diversity than the TptBu ligand. However, the differences in redox potential are more closely aligned with the electronic differences between the two sets of molecules.

Section snippets

Materials and general procedures

All solvents and chemicals (Aldrich Chemical Co.) were reagent grade and used as received unless otherwise indicated. Thallium hydrotris-(3-phenylpyrazol-1-yl)borate, Tl[TpPh], and sodium hydrotris-(3-t-butylpyrazol-1-yl)borate, Na[TptBu], were prepared from the melt method outlined previously for other bulky Tp ligands [5,6]. Caution: Thallium salts are known to be toxic, extra care should be taken when handling these complexes. FT-NMR spectra were recorded on a Varian Mercury 300 spectrometer

Synthesis of TpPhNiL complexes

Synthesis of the phenyl-substituted nickel complexes was achieved through the metathesis of Tl[TpPh] with the appropriate nickel (II) salt in a THF/MeOH mixture in the case of the Cl, Br and I complexes or through the metathesis of isolated chloride complex with 2-fold excess sodium iodide salt in methanol for the I complex. This is a modification of the previously procedure reported by Trofimenko [5], [6]. A 2-fold excess of the NaI gave the highest conversion to the desired complex as

Conclusions

We have synthesized two series of pseudo-tetrahedral nickel (II) complexes bearing bulky trispyrazolylborate ligands. The planar nature of the phenyl substituents on the TpPh ligand produces a large pocket around the metal center and results in a more structurally diverse set of complexes. The complexes all exhibit a pseudo-reversible Ni2+/1+ reduction at large negative potentials versus ferrocene. The iodide complexes in each series undergo an additional Ni1+/Ni0 reduction stabilized by the

Acknowledgements

We thank the Department of Chemistry at both Susquehanna University and Villanova University for financial support for this work. X-ray data was collected at Villanova University through support from the National Science Foundation (0521062).

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