Access to Formally Ni(I) States in a Heterobimetallic NiZn System

Abstract

Heterobimetallic NiZn complexes featuring metal centers in distinct coordination environments have been synthesized using diimine-dioxime ligands as binucleating scaffolds. A tetramethylfuran-containing ligand derivative enables a stable one-electron-reduced S = 1/2 species to be accessed using Cp₂Co as a chemical reductant. The resulting pseudo-square planar complex exhibits spectroscopic and crystallographic characteristics of a ligand-centered radical bound to a Ni(II) center. Upon coordination of a π-acidic ligand such as PPh₃, however, a five-coordinate Ni(I) metalloradical is formed. The electronic structures of these reduced species provide insight into the subtle effects of ligand structure on the potential and reversibility of the Ni^II/I couple for complexes of redox-active tetraazamacrocycles.

Fig. 1

Diimine-dioxime complexes of nickel containing either a bridging proton (1 and 2) or BF₂ (3 and 4) group.

Fig. 2

Solid-state structures of (a) 5 and (b) 6 excluding ClO₄^- counterions and non-coordinated solvent molecules (thermal ellipsoids at 50% probability). Selected bond distances (Å) and angles (°) for 5: Ni1–N1, 1.884(2); Ni1–N2, 1.835(3); Ni1–N3, 1.885(2); Ni1–N4, 1.836(2); N1–Ni–N3, 106.40(7); N2–Ni–N4, 86.3(1). Selected bond distances (Å) and angles (°) for 6: Ni1–N1, 1.923(2); Ni1–N2, 1.905(2); Ni1–N3, 1.911(2), Ni1–N4, 1.917(2)); N1–Ni–N3, 99.49(9); N2–Ni–N4, 96.5(1).

Fig. 3

Association of bridging anions to complex 5 and solid-state structures of (a) 7 and (b) 8 excluding ClO₄^- counterions (thermal ellipsoids at 50% probability). Only one of two crystallographically distinct molecules in the asymmetric unit is shown for 8. Selected bond distances (Å) for 7: Ni1–O3, 2.173(3); Zn1–O4, 2.060(2); O3–C20, 1.239(4); O4–C20, 1.260(4). Selected bond distances (Å) for 8: Ni1–N8, 2.209(3); Zn1–O3, 2.148(3); N8–O3, 1.266(5); N8–O4, 1.236(4).

Fig. 4

Solid-state structure of 10 excluding ClO₄^- and BPh₄^- counterions and non-coordinated solvent molecules (thermal ellipsoids at 50% probability).

Fig. 5

Cyclic voltammograms for 0.5 mM (a) 5 and (b) 10 in MeCN. Scans that extend past the second reduction wave are in black, and a scan that extends only past the first reduction wave is in red. (0.1 M [n-Bu₄N][ClO₄] supporting electrolyte; glassy carbon working electrode; N₂ atmosphere; 100 mV/s scan rate; internally referenced to the Fc/Fc⁺ redox couple at +0.38 V vs. SCE).

Fig. 6

Solid-state structures of (a) 11 and (b) 12 excluding BPh₄^- counterions and non-coordinated solvent molecules (thermal ellipsoids at 50% probability). Selected bond lengths (Å) and angles (°) for 11: Ni1–mean N₄-plane, 0.021; Σ N–Ni1–N, 360.0. Selected bond lengths (Å) and angles (°) for 12: Ni1–P1, 2.2263(5); Ni1–mean N₄-plane, 0.666; Σ N–Ni1–N, 334.4. Experimental (black, top) and simulated (red, bottom) X-band EPR spectra for (c) 11 (2-MeTHF/THF frozen solution, 77 K) and (d) 12 (2-MeTHF/THF frozen solution, 50 K). 11: g_iso = 2.020. Simulated parameters for 12: g_‖ = 2.208, g_⊥ = 2.044.

Fig. 7

Nickel K-edge spectra of 10 (top, red) and 11 (bottom, black).

Fig. 8

Comparison of selected bond lengths (Å) from solid-state structures (black) for (a) 10, (b) 11, and (c) 12. For 11, metrical parameters are shown for only one of the two crystallographically distinct molecules in the asymmetric unit. Calculated values from computationally-optimized structures (red) at the B3LYP/6-31G(d) level of DFT are shown for comparison.

Fig. 9

Calculated SOMO for (a) 11 and (b) 12, and spin density plots for (c) 11 and (d) 12. Geometries were optimized at the B3LYP/6-31G(d) level of DFT and verified by frequency analysis.

Fig. 10

Solid-state structure of 13 highlighting parts of the ligand relevant to the hydrogen-atom abstraction reactivity (thermal ellipsoids at 50% probability). The occupancy of C2, C2A, C11, and C11A refined to a value of 50%. Selected bond lengths (Å): Ni1–N1, 1.883(3); Ni1–N2, 1.820(3); Ni1–N3, 1.921(3); Ni1–N4, 1.881(4); N2–C2, 1.43(1); N2–C2A, 1.48(1); C17–C18, 1.357(6).

Scheme 1

Preparation of [Ni(^Medoen)Zn]²⁺ (5) and [Ni(^Medopn)Zn]²⁺ (6).

Scheme 2

Hydrogen-atom and proton abstraction reactions.

Scheme 3

Chemical reduction and coordination of PPh₃.

Scheme 4

Hydrogen-atom abstraction from the reduced complex.