| Autor: |
Halcrow MA; School of Chemistry, University of Leeds, Woodhouse Lane, Leeds, LS2 9JT, UK. m.a.halcrow@leeds.ac.uk., Vasili HB; School of Physics and Astronomy, University of Leeds, W. H. Bragg Building, Leeds, LS2 9JT, UK., Pask CM; School of Chemistry, University of Leeds, Woodhouse Lane, Leeds, LS2 9JT, UK. m.a.halcrow@leeds.ac.uk., Kulak AN; School of Chemistry, University of Leeds, Woodhouse Lane, Leeds, LS2 9JT, UK. m.a.halcrow@leeds.ac.uk., Cespedes O; School of Physics and Astronomy, University of Leeds, W. H. Bragg Building, Leeds, LS2 9JT, UK. |
| Abstrakt: |
[Fe(bpp) 2 ][ClO 4 ] 2 (bpp = 2,6-bis{pyrazol-1-yl}pyridine; monoclinic, C 2/ c ) is high-spin between 5-300 K, and crystallises with a highly distorted molecular geometry that lies along the octahedral-trigonal prismatic distortion pathway. In contrast, [Ni(bpp) 2 ][ClO 4 ] 2 (monoclinic, P 2 1 ) adopts a more regular, near-octahedral coordination geometry. Gas phase DFT minimisations (ω-B97X-D/6-311G**) of [M(bpp) 2 ] 2+ complexes show the energy penalty associated with that coordination geometry distortion runs as M 2+ = Fe 2+ (HS) ≈ Mn 2+ (HS) < Zn 2+ ≈ Co 2+ (HS) ≲ Cu 2+ ≪ Ni 2+ ≪ Ru 2+ (LS; HS = high-spin, LS = low-spin). Slowly crystallised solid solutions [Fe x Ni 1- x (bpp) 2 ][ClO 4 ] 2 with x = 0.53 (1a) and 0.74 (2a) adopt the P 2 1 lattice, while x = 0.87 (3a) and 0.94 (4a) are mixed-phase materials with the high-spin C 2/ c phase as the major component. These materials exhibit thermal spin-transitions at T ½ = 250 ± 1 K which occurs gradually in 1a, and abruptly and with narrow thermal hysteresis in 2a-4a. The transition proceeds to 100% completeness in 1a and 2a; that is, the 26% Ni doping in 2a is enough to convert high-spin [Fe(bpp) 2 ][ClO 4 ] 2 into a cooperative, fully SCO-active material. These results were confirmed crystallographically for 1a and 2a, which revealed similarities and differences between these materials and the previously published [Fe x Ni 1- x (bpp) 2 ][BF 4 ] 2 series. Rapidly precipitated powders with the same compositions (1b-4b) mostly resemble 1a-4a, except that 2b is a mixed-phase material; 2b-4b also contain a fraction of amorphous solid in addition to the two crystal phases. The largest iron fraction that can be accommodated by the P 2 1 phase in this system is 0.7 ± 0.1. |
