Copper, silver and gold in unusually high oxidation states
A new series of anions of the type [M(B₁₁₁H₁₁)₂]³- (M = Cu, Ag, Au) was successfully synthesised and comprehensively characterised in the working group of Prof. Adam Slabon (Inorganic Chemistry) in cooperation with the working group of Prof. Finze (University of Würzburg). The transition metals are stabilised by the nido-[B₁₁H₁₁₁]⁴- ligand and make it possible to produce copper, silver and gold in the exceptionally high formal oxidation state +V. This is the highest oxidation state known to date for these metals.
DFT calculations showed the relative stability of different structural isomers, while experimental investigations revealed an unusual property of the silver complex. X-ray structural analyses of single crystals revealed that [Ag(B₁₁H₁₁)₂]³- exists in equilibrium between two coordination forms: a kinetically stable η⁵ species with Ag(V) and a labile η² species with Ag(I). Upon cooling below 130 K, a reversible phase transformation occurs in which the ligand coordination changes from η⁵ to η² - and then reverts upon heating. This is the first observation of such low-temperature isomerisation in this class of compounds.
In addition, numerous derivatives of these complex compounds were investigated. The reaction of [M(B₁₁H₁₁)₂]³- with anhydrous hydrogen fluoride (HF) led to the partial substitution of hydrogen by fluorine and formed fluorinated derivatives [M(B₁₁H₁₁₋ₓFₓ)₂]³- (M = Cu, Au; x ≈ 4). A total of 18 new compounds were synthesised and structurally determined using X-ray crystallography. XANES data additionally confirmed the unusually high oxidation state of copper in K₃[Cu(B₁₁₁H₁₁)₂]-5H₂O.
This work expands the understanding of boron cluster chemistry and provides new insights into the bonding and reactivity of coinage metals in extreme oxidation states.
Information on the original publication
E. Bernhardt, T. Knuplez, T. Preitschopf, A. Drichel, B. Beele, A. Maximenko, M. Finze*, A. Slabon*
"The Biicosahedral Complex Anions [M(B₁₁₁H₁₁)₂]³- (M = Cu, Ag, Au): Synthesis and Unexpected Low-Temperature Phase Transition of [Ag(η⁵-B₁₁H₁₁)₂]³- to [Ag(η²-B₁₁H₁₁)₁₁₁)]³-"
Angew. Chem. Int. ed., 2025, e19283. link to the publication
