Synthesis and characterization of 17-valence-electron [CpCr(NO)X-2](-) anions: Oxidatively induced loss of the nitrosyl ligand

Several anionic and paramagnetic [CpCr(NO)X-2](-) complexes (X = I [1], O3SCF3 (OTf) [2], Br [3], Cl [4]) have been prepared as potential precursors to neutral CpCr(NO)X-2 species. Reaction of [CpCr(NO)I](2) with [NBu4]I provides [NBu4][1], and halide abstraction from [NBu4][1] with 2 equiv of AgOTf affords [NBu4][2]. The weakly-bound OTf- ligands of [NBu4][2] are readily displaced by Br- to produce [NBu4][3]. The dichloro complexes [NEt4][4] and [PPN][4] are obtained by treating [CpCr(NO)Cl](2) with [NE4]Cl and [PPN]Cl, respectively. Use of acetonitrile as reaction solvent allows generation of the requisite [CpCr(NO)Cl](2) directly from CpCr(NO)(CO)(2) and PCl5, a marked improvement over previous synthetic routes to this dimer. Similar halogenations of Cp*Cr(NO)(CO)(2) in NCMe provide access to the previously unknown [Cp*Cr(NO)I](2) (5), and [Cp*Cr(NO)Cl](2) (6), halo-bridged, and dimers. The solid-state molecular structure of [PPN][4]. CH2Cl2 has been established by single-crystal X-ray crystallography to be a normal three-legged piano stool. The one-electron oxidation of [4] has been investigated both chemically (by reaction with [Cp2Fe](+)) and electrochemically (by cyclic voltammetry). These studies suggest that upon oxidation a high-spin CpCr(NO)Cl-2 complex is initially formed which then rapidly releases NO. Extended Huckel molecular-orbital calculations have been performed on [4], [CpCr(CO)(3)](-), and [CpCrCl3](-), three CpCr-containing anions with ligands of varying pi-bonding capabilities. Correlations between their orbital energies and electron occupancies and a rationale for the lability of the NO ligand in neutral CpCr(NO)Cl-2 are provided.