Scheme
1: Proposed synthetic routes for the preparation of 1 .
The first synthetic route worked very well for some “traditional”
divalent Ln ions (Sm, Tm, Yb), but these methods have so far not proved
transferable to deliver the analogous Dy(II)
complex.[20] The second route (Scheme 1, Route B)
relies upon salt metathesis methodologies [i.e.MX3 + n M′L
M(L)3-n (X)n + n M′X;
where M = Ln, M′ = alkali metal, X = halide, L = ligand, n =
1-3] with Dy(III) precursors to afford
[Dy{N(SiiPr3)2}2(X)],
and have thus far been unsuccessful, presumably due to facile
deprotonation of alkyl silyl groups. However, whilst studying silylamide
systems, the Mills Group used an anion abstraction strategy commonly
employed elsewhere in the periodic table with the silylium reagent
[(Et3Si)2H][B(C6F5)4].[20]
In principle this strategy could be transferred to any
Ln(L)2X species, where X is a halide and L a sterically
demanding ligand, though very few examples like this
exist.[21] Fortunately, a family of heteroleptic
dysprosium metallocene complexes,
[Dy(Cpttt)2(X)]
(Cpttt =
C5H2-1,2,4-tBu3;
X = Br, I, BH4) reported by Nief and Jaroshick in 2007
were perfectly suited to the new synthetic
strategy;[22] additionally, the
[Dy(Cp*)2]+ (Cp* =
C5Me5) cation had recently been
postulated to be an excellent SMM.[23] Simply
substituting DyCl3 instead of DyI3 in
the Nief/Jaroschik protocol, and follow-up reaction with the silylium
reagent delivered the cationic Dy(III) complex
[Dy(Cpttt)2][B(C6F5)4]
(2 , Figure 1), which contained the first example of an isolated
dysprosocenium cation.[14a] The synthetic team
delivered the new molecule to the theoreticians, who suggested that
Cpttt did not fulfil the requirements of a
“point-charge like ligand” and “would likely be a poor SMM because
even the most axial SMMs lose magnetization at zero field”, but that
they would have a look anyway. On this occasion, the hunch of the
synthetic chemists was correct, and 2 was found to possess a
very high U eff (1760 K) and retained
magnetization at temperatures as high as 60 K
(T H, the maximum temperature at which hysteresis
loops remain open at zero applied field). This was a significant
improvement compared to the best previous SMMs, such asT H = 14 K in the radical bridged terbium complex
[K(18-crown-6)(THF)2][{Tb(N{SiMe3}2)2(THF)}2(μ-η2:η2-N2)].[24]