The [2+2] Addition of Ethylene to Metal−Ligand Multiple Bonds:  A Density Functional Study of Mo(E)OCl₂

Abstract

We have studied the [2+2] cycloaddition of ethylene to Mo(E)OCl2 for E = S, Se, O, NH, PH, SiH2, and CH2 based on gradient-corrected density functional theory. The reactant Mo(E)OCl2 has for E = NH, PH, SiH2, and CH2 a preferred conformation with all atoms of the E ligand in the plane bisecting the Cl−Mo−Cl angle. The [2+2] cycloaddition takes place with ethylene approaching Mo(E)OCl2 perpendicular to the plane bisecting the Cl−Mo−Cl angle. The reaction enthalpy follows the order (in kcal/mol) SiH2 (−24.1) < CH2 (−15.6) < PH (−7.4) < NH (−0.9) < S (5.9) < Se (6.2) < O (12.0) < Cl (13.0). The ligands with the less electronegative heteroatom give rise to the more exothermic reaction, as they have lone pairs of sufficiently high energy to donate charge to π* of the incoming olefin and form a strong C−E bond. The activation energy for the addition follows the order (in kcal/mol) SiH2 (2.3) < CH2 (4.7) < PH (5.0) < S(16.6) < Se (16.6) < Cl (19.0) < NH (21.1) < O (25.8). Again, ligands with the less electronegative heteroatoms afford the lowest barrier for the addition since they have lone pairs that at an early stage of the reaction can interact, stabilizing with π* of the incoming ethylene. The investigation affords a rationale for why [2+2] addition of olefin to ME bonds is observed experimentally to be facile for E = SiH2 and CH2 but not for E = O.

Density Functional Calculations of95Mo NMR Chemical Shifts: Applications to Model Catalysts for Imine Metathesis

“…A similar barrier, 88.2 kJ mol À1 , has recently been computed for the addition of ethylene to the imido group of [MoO(NH)Cl 2 ]. [46] These barriers are much higher than those involved in olefin metathesis with alkylidene complexes (see above), which is consistent with the more drastic reaction conditions necessary for imine metathesis. Under these reaction conditions, however, barriers in the order of 100 kJ mol À1 are not insurmountable and the mechanism illustrated in Figure 3 represents a viable pathway for imine metathesis, and supports the proposition that diazametallacyles and labile imine adducts are involved (A and B, respectively, in Scheme 2).…”

Metathesis activity of monomeric Mo-methylidene centres on (100) and (110)C surfaces of γ-Al2O3––a comparative DFT study

“…Several theoretical studies of molybdenum catalysts of olefin metathesis were reported [49][50][51][52][53][54][55][56][57]. They concerned homogeneous systems, however, and confirmed the commonly accepted carbene mechanism of the process [58], involving Moalkylidene complexes and molybdacyclobutane intermediates.…”

“…Becke three parameter hybrid functionals were successfully applied in calculations involving molybdenum compounds [55,57,88,89], including olefin metathesis catalysts [55,57]. Although the pure GGA functionals were used in such studies as well [53,56], other results reported in the present work are obtained with employing the B3LYP functional.…”

Synthesis and reactivity of 2-azametallacyclobutanes

“…37 Further computational studies of [2 + 2] additions of metalimido species with ethylene were performed among others in the Ziegler and Frenking labs. For molybdenum 38 and tungsten 39 complexes, the cycloadditions were essentially thermoneutral with fairly significant kinetic barriers. In the osmium systems, the [2 + 2] addition was kinetically and thermodynamically unfavourable by calculation.…”