Schorpp M, Krossing I. Soft interactions with hard Lewis acids: generation of mono- and dicationic alkaline-earth metal arene-complexes by direct oxidation.
Chem Sci 2020;
11:2068-2076. [PMID:
32180929 PMCID:
PMC7057851 DOI:
10.1039/c9sc06254h]
[Citation(s) in RCA: 28] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/10/2019] [Accepted: 01/12/2020] [Indexed: 12/26/2022] Open
Abstract
The synthesis of the first unsupported dicationic arene complexes of calcium and strontium [(η6-HMB)AE(oDFB)4]2+ is reported (HMB = hexamethylbenzene; AE = alkaline earth metal; oDFB = ortho-difluorobenzene). They were prepared by direct oxidation of the elemental metals employing the ligand-forming radical cation salt [HMB][WCA] as an oxidant (WCA = [Al(ORF)4] or [μF-{Al(ORF)3}2]; RF = C(CF3)3). In addition, monocationic η6-HMB complexes of calcium, strontium and barium supported by coordination of the monodentate anion [F-Al(ORF)3]- are reported. In all examples, almost undistorted η6-HMB coordination is observed with rather short M-arenecentroid distances approaching those observed with the isoelectronic but negatively charged pentamethylcyclopentadienyl ligand. The structure and bonding, thermodynamic stability and Lewis acidity (fluoride/hydride ion affinities, FIA/HIA) of the generated complexes were assessed by DFT methods. It followed that the gaseous dications [(η6-HMB)AE(oDFB)4]2+ are extremely hard Lewis acids that retain FIAs close to superacidity in solution.
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