The ligand-based quintuple bond-shortening concept and some of its limitations

Awal Noor, Tobias Bauer, Tanya K. Todorova, Birgit Weber, Laura Gagliardi, Rhett Kempe

Research output: Contribution to journalArticlepeer-review

56 Scopus citations

Abstract

Herein, the ligand-based concept of shortening quintuple bonds and some of its limitations are reported. In dichromium-diguanidinato complexes, the length of the quintuple bond can be influenced by the substituent at the central carbon atom of the used ligand. The guanidinato ligand with a 2,6-dimethylpiperidine backbone was found to be the optimal ligand. The reduction of its chromium(II) chloride-ate complex gave a quintuply bonded bimetallic complex with a Cr-Cr distance of 1.7056 (12) Å. Its metal-metal distance, the shortest observed in any stable compound yet, is of essentially the same length as that of the longest alkane C-C bond (1.704 (4) Å). Both molecules, the alkane and the Cr complex, are of remarkable stability. Furthermore, an unsupported Cr I dimer with an effective bond order (EBO) of 1.25 between the two metal atoms, indicated by CASSCF/CASPT2 calculations, was isolated as a by-product. The formation of this by-product indicates that with a certain bulk of the guanidinato ligand, other coordination isomers become relevant. Over-reduction takes place, and a chromium-arene sandwich complex structurally related to the classic dibenzene chromium complex was observed, even when bulkier substituents are introduced at the central carbon atom of the used guanidinato ligand. A sterically tailor-made guanidinate ligand is the key to the shortest metal-metal bond observed to date. The Cr-Cr quintuple bond is of the same length as the longest C-C bond in stable alkanes. The ligand-based metal-metal bond-shortening concept has a few limitations. Most importantly, the formation of coordination isomers with an unsupported Cr-Cr bond of significantly lower bond order was determined by CASSCF/CASPT2 calculations (see figure).

Original languageEnglish (US)
Pages (from-to)9825-9832
Number of pages8
JournalChemistry - A European Journal
Volume19
Issue number30
DOIs
StatePublished - Jul 22 2013

Keywords

  • N ligands
  • chemical bonds
  • chromium
  • electronic structure
  • multiple bonds

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