AuII: Rare or neglected? A critical literature review

Abstract

Dedicated kinetic studies of one of the first examples of electron transfer catalysis in the 1950s signalled the birth of Au^II chemistry [16]. Post-2000 developments in Au^II chemistry, which is the focus of this article, rest on mainly four pillars of discovery made in the previous century:

i      Isolation of the first mononuclear paramagnetic Au^II complex, using mnt as ligand (Figure 1) [17].
ii     Preparation and investigation of the structure and reactivity of an extended series of [Au^II–Au^II] complexes pioneered by Schmidbaur and others (Equation 2) using bipodal ligands [14], [22], [34–38].
iii    Determination of the first crystal structure of a Au^II complex, a macrocyclic, S-donor ([9]aneS3) product (II, Figure 2) [20].
iv    Characterisation of intermediate Au^II species formed during nanogold preparation by pulse radiolysis and photolytic reduction of Au^III [23, 24].
Since 2000, the results mentioned in i to iv have spawned separate areas of chemical activity, each one the focus of more in-depth investigations. The most important results obtained can be summarised as follows:

• Various new ligands to trap [Au^II–Au^II] units formed from AuI precursors within heterometallacyclic ring systems (compare ii above) are now known. The donor atoms vary from soft carbanionic carbon, C(carbene) (Equation 5) [48–50] and P [36, 37] to harder N (Equation 3 [39]; 12; Equation 4 [41–46]); and even O in the all-inorganic cyclic rings of AuSO4 (Figure 4) [51]. Rings are composed of 8 to 17 atoms. Stable, water-soluble variants are known (Equation 18) [91]. Yet most of the complexes in the family disproportionate in solution.
• Using dppn as a ligand – albeit under different oxidising conditions than under 1 – the AuI precursor is not converted into an aurocycle as expected but rather into an isomeric, unbridged [Au^II–Au^II] complex (Equation 6 [56, 57]; Figure 5). Other, related complexes can also be prepared by oxidation of carefully selected gold(I) complexes (Equations 7 and 9) [58, 60], comproportionation (Equation 8) [59] or, uniquely, by reductive condensation (Equation 10) [61]. Quantum chemical calculations [62] as well as electrochemical experiments [63] indicate a surprisingly strong Au–Au bond in such unbridged complexes.
• The crystal and molecular structure of the monomeric mnt complex of Au^II (I, Figure 1) isolated in the 1950s (see i above), is now known [70], whereas various other S-donor complexes (see iii) have been subjected to intensive scrutiny in solution and their structural details interpreted by means of DFT calculation [71–73]. Xe atoms also stabilise Au^II in mononuclear as well as F-bridged dinuclear form (Equations 11 and 12; 34) [67, 68]. More complex ligands in the form of porphyrin rings can be installed at Au^III and then reduced to Au^II complexes (38, Figure 7). Crystallographic as well as electrochemical parameters for such products are available [74–83].
• Nanogold formation from Au^III precursors effected by radiolysis and photolysis (see iv) is still receiving a great deal of attention. The role of the solvent remains a bone of contention [13, 84–89]. Of particular interest is the postulated Au^II–Au^II bond formation during g and pulse radiolysis [90].
• Recently, the involvement of a Au^II–Au^II complex has been postulated for a homogeneously catalysed oxidative heteroarylation reaction (Scheme 1) [92]. The integrity of the dinuclear gold unit as well as an intermediately formed Au^II–F bond seems to be of crucial importance in the conversion.

Considering future prospects, a number of challenging goals remain:

• Unexploited ligands and ligand combinations should be used to prepare more examples of both dinuclear and mononuclear Au^II complexes in order to increase the Au^II synthetic toolbox. Conspicuously absent carbenes, phosphites, isocyanides and cyanides offer soft donor atoms for coordination, whereas various imines could be considered as donor systems of intermediate hardness or softness. No hydrides of Au^II are yet known.
• Synthetic procedures could be refined. Ideal oxidising and reducing agents for converting AuI and Au^III into Au^II under specific conditions of ligation are required.
• Too often Au^II is identified in solution but suitable crystals not grown; crystallisation procedures need to be adapted.
• Too few reactivity studies have been undertaken; relevant reactions should be kinetically and quantum-chemically investigated.
• Applications in catalysis, biomedicine and the role of Au^II as promotor or intermediate in nanogold or material synthesis, require further study. Extremely promising in the context of heterogeneous catalysis is the observation that Au^II is stabilised on certain solid supports such as zeolites.

Keywords: Dinuclear Au^II complexes; mononuclear Au^II complexes; cyclic Au^II complexes; Au^II porphyrins; aurocycle; nanogold; homogeneous catalysis; comproportionation; Au^II complex isomers

Lees die volledige artikel in Afrikaans: Au^II: Skaars of afgeskeep? ’n Kritiese literatuuroorsig

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