Computational study of the transition state for hydrogen addition to Vaska-type complexes (trans-Ir(L)2(CO)X): substituent effects on the energy barrier and the origin of the small hydrogen/deuterium kinetic isotope effect

Abstract

Ab initio molecular orbital methods have been used to study transition state properties for the concerted addition reaction of H2 to Vaska-type complexes, trans-Ir(L)z(CO)X, 1 (L = PH3 and X = F, C1, Br, I, CN, or H; L = NH3 and X = C1). Stationary points on the reaction path retaining the trans-12 arrangement were located at the Hartree-Fock level using relativistic effective core potentials and valence basis sets of double-b quality. The identities of the stationary points were confirmed by normal mode analysis. Activation energy barriers were calculated with electron correlation effects included via Moller-Plesset perturbation theory carried fully through fourth order, MP4(SDTQ). The more reactive complexes feature structurally earlier transition states and larger reaction exothermicities, in accord with the Hammond postulate. The experimentally observed increase in reactivity of Ir(PPh&(CO)X complexes toward H2 addition upon going from X = F to X = I is reproduced well by the calculations and is interpreted to be a consequence of diminished halide-to-Ir ?r-donation by the heavier halogens. Computed activation barriers (L = PH3) range from 6.1 kcal/mol (X = H ) to 21.4 kcal/mol (X = F). Replacing PH3 by NH3 when X = C1 increases the barrier from 14.1 to 19.9 kcal/mol.Using conventional transition state theory, the kinetic isotope effects for H2/D2 addition are computed to lie between 1.1 and 1.7 with largervalues corresponding to earlier transition states. Judging from the computational data presented here, tunneling appears to be unimportant for H2 addition to these iridium complexes.

Mechanistic insight into novel sulfoxide containing SABRE polarisation transfer catalysts

“…These values proved to be the same within error and hence, there is little or no kinetic isotope effect which is consistent with other reports. 56,57 We note that exchange between 3 and 3-d is not observed in the associated EXSY data which provides confirmation that the underlying deuterium exchange processes involving methanol-d 4 are slow.…”

Reductive Elimination of H2 Activates Nitrogenase to Reduce the N≡N Triple Bond: Characterization of the E4(4H) Janus Intermediate in Wild-Type Enzyme

“…As with photoinduced re, the progress curves during cryoannealing oa are the same for the two MoFe variants, and their joint fit yields KIE ∼ 5, rather larger than seen for closedshell monometallic complexes. 32,33 Combined with a strong temperature dependence of the time-constant (not shown), this KIE implies that oa of H 2 involves traversal of an energy barrier.…”

Heterolysis of Dihydrogen by Silver Alkoxides and Fluorides

“…Although these values are modest, KIE values for reactions in which HH bond‐breaking is likely rate‐limiting often fall in the range of k H / k D =1.2–1.6,41 and significantly higher values are rare 42. Several explanations, including early transition states43 and complex new vibrational modes as the transition state is approached,44 have been invoked for this behavior. The measured KIE for silver fluoride hydrogenolysis thus appears consistent with rate‐limiting HH bond cleavage.…”