Exploring Chromium(III)−Alkyl Bond Homolysis with CpCr[(ArNCMe)2CH](R) Complexes

摘要

A range of paramagnetic Cr(III) monohydrocarbyl complexes CpCr[(ArNCMe)₂CH](R) (Ar = ortho-disubstituted aryl; R = primary alkyl, trimethylsilylmethyl, benzyl, phenyl, alkenyl, or alkynyl) were synthesized to investigate how varying the steric and electronic properties of the R group affected their propensity for Cr−R bond homolysis. Most complexes were prepared by salt metathesis of known CpCr[(ArNCMe)₂CH](Cl) compounds in Et₂O with commercial RMgCl solutions, although more sterically demanding combinations of Ar and R groups necessitated the use of halide-free MgR₂ reagents and the Cr(III) tosylate or triflate derivatives. Alternative synthetic routes to Cr(III)−R species using the previously reported Cr(II) compounds CpCr[(ArNCMe)₂CH] and sources of R· radicals (e.g., BEt₃ and air) were also explored. The UV−vis spectra of the CpCr[(ArNCMe)₂CH](R) complexes possessed two strong bands with maximum absorbances in the ranges 395−436 nm and 535−582 nm, with the band in the latter range being particularly characteristic of the Cr(III)−R compounds. The Cr−CH₂R bond lengths as determined by single-crystal X-ray diffraction were longer than those in the corresponding Cr−CH₃ complexes, typically falling in the range 2.10 to 2.13 Å. The Cr(III) benzyl compounds displayed longer Cr−CH₂Ph distances, while the bond lengths for the alkenyl and alkynyl species were substantially shorter. The rate of Cr−R bond homolysis at room temperature was determined by monitoring the reaction of Cr(III) neopentyl, benzyl, and isobutyl complexes with excess PhSSPh using UV−vis spectroscopy. Although the other primary alkyl, phenyl, and alkenyl compounds did not undergo appreciable homolysis under these conditions, they were cleanly converted to CpCr[(ArNCMe)₂CH](SPh) by photolysis.