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

The complex [Cu(L-4)(2)](BF4)(2) (3 L-4=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 alpha -form contains crystallographically ordered, pseudo-Jahn-Teller elongated {d(y)(2)-z(2)}(1) Cu(II) ions that are structurally very similar to [Cu(L-2)(2)](BF4)(2) (1; L-2=2,6-dipyrazol-1-ylpyridine). The Cu ion in the orthorhombic beta -polymorph at 180 K exhibits Cu-N bond lengths that are rather different from the alpha -form, and which could correspond to a disordered {d(y)(2)-z(2)}(1) Cu(II) centre; or, to a static, rhombically compressed ion with a {d(z)(2)}(1) ground state. Variable temperature EPR data favour the former interpretation. A crystal structure at 31 K of [Cu(L-3)(2)](BF4)(2) (2; L-3=2,6-bis-[3-{2,4,6-trimethylphenyl}pyrazol-1-yl]pyridine), a genuine {d(z)(2)}(1) ion, is also described. Comparison of the crystallographic data of 1-3 shows that the {d(y)(2)-z(2)}(1)-to-{d(z)(2)}(1) ground state change occurs concomitantly with only a small z-axis compression.