Phosphine Dissociation Mediates C−H Cleavage of Fluoroarenes by OsH(C6H5)(CO)(PtBu2Me)2

OsH(Ph)(CO)L 2 (L = P t Bu 2 Me, Ph = C 6 H 5 ) is synthesized and studied by spectroscopic and DFT (B3LYP) calculations. It forms 1:1 adducts with CO or MeCN, but D 2 causes reductive elimination of C 6 H 6 . Reaction with C 6 D 6 gives exclusively OsD(C 6 D 5 )(CO)L 2 . Lack of conversion of OsD(C 6 H 5 )(CO)L 2 to OsH(C 6 H 4 D)(CO)L 2 under the same conditions indicates that zerovalent Os(η 2 -C 6 H 5 D)(CO)L 2 is not kinetically relevant. DFT calculations confirm that an η 2 -arene complex is not accessible and also show that the dissociative reductive elimination of arene is a high energy process. Reaction of OsH(Ph)(CO)L 2 with fluorinated arenes, HAr F gives benzene and OsH(Ar F )(CO)(P t Bu 2 Me) 2 (i.e., attack on the HAr F C−H bond), with the fastest attack involving more heavily fluorinated arenes (but C 6 F 6 does not react) and the attack occurring preferentially at the C−H bond ortho to fluorine. DFT calculations show that the observed isomer is thermodynamically controlled. These results, isotope-labeling results, and rate suppression and exchange involving added free phosphine lead to a mechanism where reagent arene associates with OsH(Ph)(CO)L 2, but benzene loss demands preliminary dissociation of phosphine from such an adduct.