Mechanism of Hydrogenolysis of an Iridium–Methyl Bond: Evidence for a Methane Complex Intermediate

Abstract

Evidence for key σ-complex intermediates in the hydrogenolysis of the iridium-methyl bond of (PONOP)Ir(H)(Me)(+) (1) [PONOP = 2,6-bis(di-tert-butylphosphinito)pyridine] has been obtained. The initially formed η(2)-H(2) complex, 2, was directly observed upon treatment of 1 with H(2), and evidence for reversible formation of a σ-methane complex, 5, was obtained through deuterium scrambling from η(2)-D(2) in 2-d(2) into the methyl group of 2 prior to methane loss. This sequence of reactions was modeled by density functional theory calculations. The transition state for formation of 5 from 2 showed significant shortening of the Ir-H bond for the hydrogen being transferred; no true Ir(V) trihydride intermediate could be located. Barriers to methane loss from 2 were compared to those of 1 and the six-coordinate species (PONOP)Ir(H)(Me)(CO)(+) and (PONOP)Ir(H)(Me)(Cl).

The reaction of an iridium PNP complex with parahydrogen facilitates polarisation transfer without chemical change

“…Collectively, these data are consistent with the identity of 3 as the Ir( iii ) complex [(C 5 H 3 N(CH 2 P( t Bu) 2 ) 2 )Ir(H 2 )(H) 2 ]BF 4 which contains a set of rapidly interchanging hydride/dihydrogen ligands. 42 It is this complex that provides the indirect route to H 2 loss that is necessary for p -H 2 introduction into 2 . We note that the related 16-electron dihydride complex [(C 5 H 3 N(CH 2 P( t Bu) 2 ) 2 )Ir(H) 2 ]PF 6 has been observed previously in the absence of CH 3 CN and found as a minimum by DFT, where it contains inequivalent hydride ligands (see Scheme 2 ).…”

Methyl Complexes of the Transition Metals

“…The hydrogenolysis mechanism of a pincer Ir III −CH 3 complex was analyzed in detail by Brookhart and co‐workers (Scheme a). Spectroscopic studies and DFT calculations revealed that reductive elimination of methane proceeds through a σ‐metathesis or σ‐CAM mechanism, in which the oxidation state of iridium is unaltered over the course of the transformation and in which a σ‐methane complex 3.18 b is an instrumental intermediate . The metal–ligand cooperative hydrogenolysis of a Ni−CH 3 bond in 3.19 has recently been disclosed and the role of the boryl ligand in assisting such transformations is supported by computational studies (Scheme b) …”

Metathesis by Partner Interchange in σ‐Bond Ligands: Expanding Applications of the σ‐CAM Mechanism

“…Hydrogenolysis of an iridium methyl hydride complex with experimental energetics and activation barriers. [39] Scheme 14. Dissociation of methane from Pt(bpy)(CH 3 )] + by CID in gas phase.…”