Transition Metal Polyhydride Complexes. 11. Mechanistic Studies of the Cis to Trans Isomerization of the Iridium(III) Dihydride Ir(H)₂(CO)L (L = C₆H₃(CH₂P(H)₂)₂)

Abstract

Density functional theory (B3LYP) has been used to investigate the mechanism of the cis−trans isomerization in Ir(H)2(CO)L (L = C6H3(CH2P(H2))2), a model for Ir(H)2(CO)L‘ (L‘ = C6H3(CH2P(i-Pr2))2. This complex is initially formed as the cis isomer by bubbling H2 through a benzene solution of Ir(CO)L‘ at room temperature. Over period of 18 h at 90 °C, the cis isomer quantitatively converts to the trans isomer (Organometallics 1997, 16, 3786). Five possible mechanisms are examined in detail: (i) CO dissociation and reassociation; (ii) CO migratory insertion into the Ir−H bond and subsequent elimination; (iii) Ph−H reductive elimination to an arene intermediate and subsequent oxidative addition; (iv) phosphine dissociation, complex rearrangement, and phosphine reassociation; and (v) nondissociative trigonal twist. The preferred mechanism involves two consecutive trigonal twists in which the complex passes through a distorted octahedral intermediate.

Hydrogen Addition to (pincer)IrI(CO) Complexes: The Importance of Steric and Electronic Factors

“…23 No reaction was observed when 10 was pressurized with 8 atm of H 2 in C 6 D 6 and mixed at either room or at elevated temperature (100 °C) for an extended period of time. However, when THF-d 8 was used as the solvent, 7% of 10 was converted to trans-(tBu)4 (PCP)Ir(CO)(H) 2 (11) after 8 days at room temperature and 11 was observed in 41% yield after 4 days at 100 °C. A triplet hydride resonance at −9.74 ppm ( 2 J PH = 14 Hz) in the 1 H NMR spectrum is indicative of formation of a trans-dihydride Ir complex, where the two hydrides are chemically equivalent.…”

“…Comparable to the (iPr)4 (PCP)Ir(CO) system reported by Milstein and further computational studies by Hall and co-workers, the cis-dihydride may isomerize to the trans-dihydride via a trigonal twist mechanism where the meridionally coordinated pincer ligand would become facially coordinating to allow isomerization of the ligands at Ir. 11 Experimental evidence from work by Roddick and co-workers on hydrogen addition to (CF3)4 (PCP)-Ir(CO) demonstrates the ability of facial coordination of a pincer ligand to iridium. 34 Further investigations to understand the mechanism of the cis to trans isomerization in our system are currently underway in our group.…”

“…When the isomerization of cis-to trans-dihydrides in (PCP)Ir(CO) complexes was studied computationally by Hall and co-workers, steric effects were not considered; the phosphine arms were truncated to −PH 2 . 11 However, we have found that subtle changes in the substituent at phosphorus for (POCOP)Ir(CO) [POCOP = κ 3 -C 6 H 3 -2,6-(OPR 2 ) 2 for R = t Bu, i Pr, or OMe] complexes can have a profound effect on the chemistry at the metal center: Bulky t Bu-substituted phosphinites favor Ir(I) carbonyl species, while less bulky i Pr and OMe often afford Ir(III) carbonyl complexes under comparable metalation conditions. 12 In this work, we describe how a small change in the R substituent on the phosphine can affect the mechanism of hydrogen addition to (POCOP)-and (PCP)-supported Ir(CO) complexes.…”

Activation of Molecular Hydrogen and Oxygen by PSiP Complexes of Cobalt

“…[25c], , While such a process can involve dissociation of phosphine (either PMe 3 or a ligand arm), we have not observed direct 31 P NMR evidence for such an intermediate in the range of –80 to 80 °C. X‐ray crystallographic data has provided examples of the flexibility of κ 3 ‐(Cy‐PSiP) coordination, with P–Co–P angles ranging from 124–160° (vide supra), which suggests that both fac ‐ and mer ‐κ 3 ‐(Cy‐PSiP) coordination are feasible and non‐dissociative mechanisms for exchange, such as the “trigonal twist” that has been invoked in some circumstances,[27c], can be accommodated. Upon cooling a sample of 5 in [D 8 ]toluene solution under an H 2 atmosphere, decoalescence is observed, such that four relatively broad, but distinct resonances (ca.…”

26 Mechanisms of reactions in solution

“…1133 Theoretical work includes studies of the acidity of transition-metal dihydrogen complexes, 1134 M^H homolysis in [MH(CO) 3 (dab)] (M Mn or Re), 1135 oxidative addition of H 2 to [Ni II Cl 2 (PH 3 ) 2 ] (relevant to hydrodesulfurization catalysis), 1136 to trans-[MX 2 (PMe 3 ) 4 ] (M Mo or W), 1137 to [{Zr(PH 3 ) 2 (NH 2 ) 2 } 2 (m-Z 2 -N 2 )] 1138 and to nickel 1139 and palladium clusters. 1140,1141 Theoretical studies have also been made of reductive elimination in [NbH 3 Cp 2 ], 1142 of hydrogen site exchange in ansa-bridged [WH 3 {(Z-C 5 H 4 ) 2 SiH 2 }] 1143 and [OsH 3 (PH 3 ) 2 {kN,S-(NC 5 H 4 S-2)}] 1144 and of the interconversion of the model ``pincer'' complexes cis-and trans-[Ir(H) 2 (CO){C 6 H 3 (CH 2 PH 2 ) 2 -2,6}] 1145.…”