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Synthesis and Characterization of 17-Valence-Electron [CpCr(NO)X2]- Anions:  Oxidatively Induced Loss of the Nitrosyl Ligand1

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journal contribution
posted on 16.04.1997, 00:00 by Peter Legzdins, W. Stephen McNeil, Steven J. Rettig, Kevin M. Smith
Several anionic and paramagnetic [CpCr(NO)X2]- complexes (X = I [1], O3SCF3 (OTf) [2], Br [3], Cl [4]) have been prepared as potential precursors to neutral CpCr(NO)X2 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 [NEt4]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)Cl2 complex is initially formed which then rapidly releases NO. Extended Hückel molecular-orbital calculations have been performed on [4], [CpCr(CO)3]-, and [CpCrCl3]-, three CpCr-containing anions with ligands of varying π-bonding capabilities. Correlations between their orbital energies and electron occupancies and a rationale for the lability of the NO ligand in neutral CpCr(NO)Cl2 are provided.