Chromium(II) amides: synthesis and structures

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.