Chromium(II) amides: synthesis and structures

Abstract

A novel class of mono- and di-meric chromium(II) amides has been prepared and characterized. Reaction of [CrCl₂(thf)₂](thf = tetrahydrofuran) with 2 equivalents of M(NR₂)(R = C₆H₁₁, Prⁱ, Ph, or phenothiazinyl; M = Li or Na) allowed the formation of the homoleptic amides [{Cr(µ-NR₂)(NR₂)}₂](R = Ph 1a, C₆H₁₁1b or Prⁱ1c). The reaction is followed by co-ordination of a Lewis base which, although preserving the Cr₂(NR₂)₂ core, remarkably elongated the Cr ⋯ Cr distance forming [{Cr(µ-NR₂)(NR₂)L}₂](R = Ph, L = thf 2). Facile cleavage to give the square-planar monomeric species [Cr(NR₂)L₂](R = Ph, L = pyridine 3a, thf 3b; NR₂= phenothiazinyl, L = thf 3c) occurred upon treatment with an excess of co-ordinating solvent. Reaction with an excess of M(NR₂) led to the formation of anionic chromium(II) square-planar metalates [M₂Cr(NR₂)₄L₂](R = Ph, M = Na, L = thf 4a; R = Et, M = Li, L = thf 4b or pyridine 4c). A dimeric species with a very short Cr ⋯ Cr contact [{Cr[N(C₅H₄N-2)₂]₂}₂]·2dmf (dmf = dimethylformamide)5b has been obtained involving a unique three-centre chelating geometry of the bridging amide ligand. The crystal structures of compounds 1b, 2, 3a, 3c, 4c and 5b have been determined.