A combined structural, spectroscopic, electrochemical, and magnetic study of Nickel (II) pyrazolates: dinuclear [Ni₂], linear [Ni₃], and triangular [Ni₃] incorporating five-/six-coordinate Ni²⁺ ions.

Zhi-Chun Shi ^a, Yiannis Sanakis ^b, Raphael G. Raptis ^{a, *}

a Department of Chemistry, Florida International University, 11200 SW 8th Street, Miami, FL 33199, USA
b Institute of Nanoscience and Nanotechnology, NCSR “Demokritos”, 15310 Ag. Paraskevi, Attiki, Greece

The single crystals of four nickel polynuclear clusters based on four differently substituted pyrazole ligands, including a dinuclear [Ni₂(4-Br-pz)₃]⁺[ClO₄]^- (1), two linear trinuclear [Ni₃(4-NO₂-pz)₆(py)₂] (2) and Ni₃(4-NO₂-pz)₆(4,4’-azp)₂] (3), and two triangular trinuclear clusters [Ni₃(µ₃-OH)(4-Cl-pz)₄]⁺[ClO₄]^- (4) and [Ni₃(µ₃-OH)(4-I-pz)₄]⁺[ClO₄]^- (5), have been prepared and studied. All five complexes have been fully characterized by SCXRD, FTIR, UV-Vis-NIR, NMR, Raman, Cyclic Voltammetry, and elemental analysis. Their magnetic properties are currently under investigation in order to probe their relationship to the topological structure. For instance, the linear [Ni₃] cluster is a paramagnetic species with an S = 1 ground state.

The dinickel cluster [Ni₂] is constructed by two Ni²⁺ metal ions, three bridging 4-bromo-pyrazolates, four pyridines and two water molecules as terminal ligands, and one perchlorate (ClO₄^-) counterion, Figure 1a.

Figure 1b and 1c show two neutral linear trinuclear nickel complexes [Ni₃], which are assembled Ni²⁺ metal centers in a linear fashion. These compounds incorporate pyridine and 4,4’-azopyridine as terminal ligands. The differences in electrochemical properties between these two [Ni₃] is due to alternation of the terminates. Such linear motifs consisting of a linear string have been well studied as extended metal atom chains (EMACs).

Two similar triangular trinuclear cationic nickel complexes ([Ni₃]), Figure 1c and 1d, are constructed by three Ni²⁺ centers, one µ₃-OH bridging group, six pyridines (each Ni²⁺ ion coordinating to two pyridines in the axial and equatorial direction), one perchlorate (ClO₄^-) anion as the counterion, and four bridging pyrazolates (4-choloro-pyrazolate or 4-iodine-pyrazolate). Two types of coordination modes of nickel ion are displayed in the crystal structure, i.e., two six-coordinate and one five-coordinate Ni²⁺ centers.

Figure 1. The crystal structure of the complex (a) [Ni₂(µ-4-Br-pz)₃](ClO₄), (b) [Ni₃(µ-4-NO₂-pz)₆(py)₂(H₂O)₄], (c) [Ni₃(µ-4-NO₂-pz)₆(4,4’-azp)₂(H₂O)₄] (d) [Ni₃(µ₃-OH)( µ-4-Cl-pz)₄](ClO₄) (side-view), and (e) [Ni₃(µ₃-OH)(µ-4-I-pz)₄](ClO₄) (top-view). H atoms and C atoms of pyridines (in 1c and 1d) are omitted for clarity.

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