Half‐Sandwich Metal‐Catalyzed Alkyne [2+2+2] Cycloadditions and the Slippage Span Model

Abstract

Half‐sandwich RhI compounds display good catalytic activity toward alkyne [2+2+2] cycloadditions. A peculiar structural feature of these catalysts is the coordination of the metal to an aromatic moiety, typically a cyclopentadienyl anion, and, in particular, the possibility to change the bonding mode easily by the metal slipping over this aromatic moiety. Upon modifying the ancillary ligands, or proceeding along the catalytic cycle, hapticity changes can be observed; it varies from η5, if the five metal–carbon distances are identical, through η3+η2, in the presence of allylic distortion, and η3, in the case of allylic coordination, to η1, if a σ metal–carbon bond forms. In this study, we present the slippage span model, derived with the aim of establishing a relationship between slippage variation during the catalytic cycle, quantified in a novel and rigorous way, and the performance of catalysts in terms of turnover frequency, computed with the energy span model. By collecting and comparing new data and data from the literature, we find that the highest performance is associated with the smallest slippage variation along the cycle.

In Silico Acetylene [2+2+2] Cycloadditions Catalyzed by Rh/Cr Indenyl Fragments

“…As recently reported [22], the slippage variations in intermediates and transition states along the catalytic cycle may be related to the chemical activity of the half-metallocene catalysts in this class of reactions. For this purpose, the slippage span ∆LISP * was calculated.…”

“…Along Path I (Scheme 3a), ∆LISP * values follow the trend anti-IndRh > IndRh > CpRh. Since TOFs' trend is reverse, i.e., anti-IndRh < IndRh < CpRh, this indicates that the higher the slippage span along the catalytic cycle is, the lower the performance of the catalyst is [22]. When following Path II, ∆LISP * values follow the trend anti-IndRh(CO) < IndRh(CO) < CpRh(CO), while TOFs follow the reverse trend anti-IndRh(CO) > IndRh(CO) > CpRh(CO), in agreement with the presence of extra indenyl and indenyl effect in the bimetallic and monometallic Ind catalysts, respectively.…”

“…The slippage variations occurring during the catalytic cycle can be quantified referring to the value of ∆, which changes from 0 Å (η 5 ) to nearly 0.3 Å (η 3 ) till 0.6 Å or even higher values (η 1 ). Some of us have recently pointed out that this definition suitably applies to symmetric systems [22], and thus have introduced another descriptor, i.e., the label independent slippage parameter (LISP) (Equation (2)).…”

“…Importantly, LISP is also suitable for describing non-symmetric displacements. In the same paper [22], a relationship between the catalytic activity of several half-sandwich group 9 metal complexes for alkyne [2+2+2] cycloadditions and the slippage span expressed in terms of LISP (computed as the difference between the maximum and the minimum value of LISP along the catalytic cycle) was established. The slippage span ∆LISP was then related to the turn-over frequency (TOF) values, calculated with the energy span model [23], and it emerged that the lower the ∆LISP is, the higher the TOF is.…”

“…The availability of a flexible parameter to quantify the metal slippage, which intuitively influences the catalytic activity, and of a relationship between the metal slippage and the turn-over frequency is a valuable tool. So far, the slippage span model has been applied to monometallic Co and Rh half-sandwich catalysts for alkyne [2+2+2] cycloadditions [22]; in this work, we apply it to bimetallic Rh/Cr indenyl catalysts to assess its general validity.…”