Issue 16, 2024

Activating a high-spin iron(ii) complex to thermal spin-crossover with an inert non-isomorphous molecular dopant

Abstract

[Fe(bpp)2][ClO4]2 (bpp = 2,6-bis{pyrazol-1-yl}pyridine; monoclinic, C2/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][ClO4]2 (monoclinic, P21) 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 M2+ = Fe2+ (HS) ≈ Mn2+ (HS) < Zn2+ ≈ Co2+ (HS) ≲ Cu2+ ≪ Ni2+ ≪ Ru2+ (LS; HS = high-spin, LS = low-spin). Slowly crystallised solid solutions [FexNi1−x(bpp)2][ClO4]2 with x = 0.53 (1a) and 0.74 (2a) adopt the P21 lattice, while x = 0.87 (3a) and 0.94 (4a) are mixed-phase materials with the high-spin C2/c phase as the major component. These materials exhibit thermal spin-transitions at T½ = 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][ClO4]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 [FexNi1−x(bpp)2][BF4]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 P21 phase in this system is 0.7 ± 0.1.

Graphical abstract: Activating a high-spin iron(ii) complex to thermal spin-crossover with an inert non-isomorphous molecular dopant

Supplementary files

Article information

Article type
Paper
Submitted
15 feb 2024
Accepted
25 mrt 2024
First published
26 mrt 2024
This article is Open Access
Creative Commons BY license

Dalton Trans., 2024,53, 6983-6992

Activating a high-spin iron(II) complex to thermal spin-crossover with an inert non-isomorphous molecular dopant

M. A. Halcrow, H. B. Vasili, C. M. Pask, A. N. Kulak and O. Cespedes, Dalton Trans., 2024, 53, 6983 DOI: 10.1039/D4DT00443D

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