Highly Anisotropic Exchange Interactions in a Trigonal Bipyramidal Cyanide-Bridged Ni^II₃Os^III₂ Cluster

Abstract

This article is a part of our efforts to control the magnetic anisotropy in cyanide-based exchange-coupled systems with the eventual goal to obtain single-molecule magnets with higher blocking temperatures. We give the theoretical interpretation of the magnetic properties of the new pentanuclear complex {[Ni(II)(tmphen)(2)](3)[Os(III)(CN)(6)](2)} x 6 CH(3)CN (Ni(II)(3)Os(III)(2) cluster). Because the system contains the heavy Os(III) ions, spin-orbit coupling considerably exceeds the contributions from the low-symmetry crystal field and exchange coupling. The magnetic properties of the Ni(II)(3)Os(III)(2) cluster are described in the framework of a highly anisotropic pseudo-spin Hamiltonian that corresponds to the limit of strong spin-orbital coupling and takes into account the complex molecular structure. The model provides a good fit to the experimental data and allows the conclusion that the trigonal axis of the bipyramidal Ni(II)(3)Os(III)(2) cluster is a hard axis of magnetization. This explains the fact that in contrast with the isostructural trigonal bipyramidal Mn(III)(2)Mn(II)(3) cluster, the Ni(II)(3)Os(III)(2) system does not exhibit the single-molecule magnetic behavior.

Magnetic anisotropy in two- to eight-coordinated transition–metal complexes: Recent developments in molecular magnetism

“…An X-band EPR experiment using a polycrystalline material made of the complex (Ph 4 P)[Os III (CN) 6 ]•6H 2 O determined anisotropic g-factors of g ┴ = 1.79 and g ǁ = 1.21 [195]. Moreover, their potential use as building blocks for heterometallic magnetic complexes with strong anisotropic magnetic exchange interactions has also been demonstrated [114,138,190,191,193]. According to the hole formalism, the d orbital/spectroscopic term splitting pattern is the same for d 1 and d 6 ions [197], and thus the magnetic anisotropy of high-spin d 6 ions (Fe II or Co III ) is expected to be as interesting as that of d 1 ions (vide supra).…”

“…Examples of low-spin d 5 complexes that exhibit considerable magnetic anisotropy mainly comprise Re II [112,[177][178][179][180][181][182], Ru III [138,[183][184][185][186][187][188][189], and Os III [114,138,[190][191][192][193][194][195]. The magnitude of  for free Re II ranges between 300 and 2000 cm -1 .…”

Enhancing the Blocking Temperature in Single‐Molecule Magnets by Incorporating 3d–5d Exchange Interactions

“…Parallel to the work reported in this paper, the first cyanide-bridged materials based on the hexacyanoosmateA C H T U N G T R E N N U N G (III) anion were reported by Dunbars group in the form of the pentanuclear complexes [{Ni-A C H T U N G T R E N N U N G (tmphen) 2 } 3 {Os(CN) 6 } 2 ]·6 MeCN [29] (tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline) and [30] Although the former cluster was shown to exhibit magnetic anisotropy, the sign of this anisotropy is incompatible with SMM behaviour. [31] As to the use of other 3d ions instead of Ni, theoretical considerations suggest that the magnetisation reversal barriers for the hypothetical trigonal bipyramidal Os clusters are unlikely to be pronounced. [28] The different orientation of the local anisotropy axes of the exchange-coupled Os-CN-M pairs in trigonal bipyramidal structures does not take full advantage of the strong anisotropy of the orbitally dependent superexchange.…”

Analyzing Anisotropic Exchange in a Pentanuclear Os2Ni3 Complex

“…The experimental χT curve has again be presented in Figure 4 of Ref. [10] which has been used as the source of experimental data to compare with. To facilitate the analysis, we slightly modified the geometry such that it adopts axial C 3 symmetry.…”

“…) depend on the orientation of the molecule, so following Ref. [10], an orientation was chosen where Os 1,2 are on the positive and negative z axis, while Ni 3,4,5 are in the xy plane, with Ni 3 on the x axis. The cartesian coordinates are documented in the Supporting Information and used in all subsequent calculations.…”