Several anionic and paramagnetic [CpCr(NO)X 2 ] - complexes (X = I [ 1 ], O 3 SCF 3 (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 [NBu 4 ]I provides [NBu 4 ][ 1 ], and halide abstraction from [NBu 4 ][ 1 ] with 2 equiv of AgOTf affords [NBu 4 ][ 2 ]. The weakly-bound OTf - ligands of [NBu 4 ][ 2 ] are readily displaced by Br - to produce [NBu 4 ][ 3 ]. The dichloro complexes [NEt 4 ][ 4 ] and [PPN][ 4 ] are obtained by treating [CpCr(NO)Cl] 2 with [NEt 4 ]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 PCl 5, 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 ]·CH 2 Cl 2 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 [Cp 2 Fe] + ) 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 Hückel molecular-orbital calculations have been performed on [ 4 ], [CpCr(CO) 3 ] -, and [CpCrCl 3 ] -, 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)Cl 2 are provided.
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Legzdins et al. (1997) studied this question.