ca01 only detailed ToC 1..24

114 Science of Synthesis 2.6 Complexes of Cr, Mo, and W without CO Ligands
2.6.4.2 Method 2:
By Oxidative Addition of Alkyl Halides
This method has mostly been used for the synthesis of chromium(III) compounds, with
wide application in the Nozaki–Hiyama–Kishi reaction (Section 2.6.4.5). Other group 6 derivatives
have been prepared as well.
2.6.4.2.1 Variation 1:
One-Electron Oxidative Additions
Chromium(II) precursors react with alkyl halide reagents to afford a 1:1 mixture of chromium(III)
halide and alkylchromium(III) products. The mechanism involves single-electron-transfer
steps and radical intermediates (Scheme 24). Other radical sources may be
used instead, including hydroperoxides or alkane/hydrogen peroxide mixtures under
thermal, flash photolysis, or pulse radiolysis conditions. [109] This procedure has been
used to produce the alkylpentaaquachromium(III) ion 61. [110] The latter is long-lived but
cannot be isolated and must be generated in situ. Activated alkyl halides react thermally
with the chromium(III) ion, while other alkyls require photolytic or radiolytic conditions.
The addition of ligands such as ethylenediamine or saturated tetraaza macrocycles makes
the process more favorable for both thermodynamic and kinetic reasons.
Scheme 24 One-Electron Oxidative Addition of Alkyl Halides [110]
[Cr(OH2)6] 2+
− [CrX(OH2)5] 2+
R1X R1• [Cr(OH2)6] 2+
[CrR 1 (OH 2) 5] 2+
Reduction of Chloroform with Chromium(II) Perchlorate: [111]
H 2O (1 L) was freed of oxygen and then shaken with CHCl 3 (10 mL). A 0.2 M soln of chromium(II)
perchlorate (500 mL) was added in an atmosphere of N 2 and the soln allowed to
stand for 2 or 3 h at rt. The grayish-red soln gave no precipitate with AgNO 3 soln at rt. A
portion (100 mL) of this soln was placed on a column (15 cm ” 2 cm) of Dowex 50-X4 (200–
400 mesh) in the hydrogen form and the column was eluted with 1 M HClO 4 at the rate of
1–2 drops •s –1 . A green fraction (100 mL) was soon eluted, followed by a colorless soln
(150 mL), and then by a red fraction (110 mL). The green and red fractions were identified
as containing [Cr(H 2O) 5Cl] 2+ and [Cr(H 2O) 5(CHCl 2)] 2+ , respectively, by UV–vis spectroscopy
and quantitative analysis with AgClO 4 and KMnO 4.
2.6.4.2.2 Variation 2:
Two-Electron Oxidative Additions
Electron-rich metal complexes containing at least one electron pair in a metal-based orbital
may undergo a classical two-electron oxidative addition of alkyl halides, making a
new metal-alkyl bond (Scheme 25). This synthetic strategy is thus generally limited to
systems supported by electron-donating ancillary ligands. No synthetically useful examples
of this type of reactivity have been described for noncarbonyl-containing group 6 organometallic
species where the halide ion is incorporated in the coordination sphere of
the metal. However, there are examples of two-electron oxidative addition reactions of alkyl
halides where the halide remains as an outer sphere counterion, as in the preparation
of 62. [102]
61