The effects of distal ligand substitution on the copper(II)/bis-(2,6-dipyrazol-1-ylpyridine) centre

Abstract

The complex [Cu(L⁴)₂](BF₄)₂ (3; L⁴ = 2-[pyrazol-1-yl]-6-[3-{2,4,6-trimethylphenyl}pyrazol-1-yl]pyridine) has been synthesised. Complex 3 crystallises in two crystal forms from MeNO₂/Et₂O. The monoclinic α-form contains crystallographically ordered, pseudo-Jahn–Teller elongated {d_{y² − z²}}¹ Cu(II) ions that are structurally very similar to [Cu(L²)₂](BF₄)₂ (1; L² = 2,6-dipyrazol-1-ylpyridine). The Cu ion in the orthorhombic β-polymorph at 180 K exhibits Cu–N bond lengths that are rather different from the α-form, and which could correspond to a disordered {d_{y² − z²}}¹ Cu(II) centre; or, to a static, rhombically compressed ion with a {d_z²}¹ ground state. Variable temperature EPR data favour the former interpretation. A crystal structure at 31 K of [Cu(L³)₂](BF₄)₂ (2; L³ = 2,6-bis-[3-{2,4,6-trimethylphenyl}pyrazol-1-yl]pyridine), a genuine {d_z²}¹ ion, is also described. Comparison of the crystallographic data of 1–3 shows that the {d_{y² − z²}}¹-to-{d_z²}¹ ground state change occurs concomitantly with only a small z-axis compression.