Molybdacyclobutane complexes

Intermediate molybdacyclobutane complexes have also been detected in the reactions of 7 with 21-24115. Only in the case of 21 is the ultimate product a long-chain polymer, but in all cases one may observe, at 0-60 °C, a clean first-order rearrangement of the initial metallacyclobutane complex to the first metal carbene adduct, consisting of an equilibrium mixture of syn and anti rotamers in the ratio 9 1 (see below). Except in the case of 21, the metal carbene complexes do not survive for very long. For 21, however, ROMP is propagated, and distinct H NMR signals are seen for the longer-chain metal carbene complexes in both syn and anti forms. 

Intermediate molybdacyclobutane complexes have also been detected in the reactions of Mo(=CHCMe2R)(=NAr)(OCMe3)2 (Ar = C6H3-/-Pr2-2,6 R = Me, Ph) with 8-11 (Bazan 1991c). 

In the previous work [9] we have investigated ethene and propene metathesis reactions proceeding on molybdenaalkylidene centres. In the present work density functional study on ethene metathesis reaction proceeding on molybdenamethylidene centres of M0O3/AI2O3 catalyst is reported. Two variants of theoretical models of the active sites have been applied. In the first case, simple structures of the carbene and molybdacyclobutane complexes, in which hydroxyl groups replace the bonds between molybdenum and the carrier, are proposed. In the second case, molybdenum is attached to a small cluster of formula Al2(OH)6, which represents alumina. 

Formation of the Mo molybdacyclobutane is very exothermic and irreversible. It does not seem that the olefin metathesis active centres of M0O3/AI2O3 catalyst contain Mo, because the decomposition of the Mo molybdacyclobutane complex to ethene and Mo methylidene complex is very endothermic and endoergic. The energy barrier of this reaction appears much higher than experimental activation energies of olefin metathesis. 

Scheme 11.1 (a-c) Decomposition modes of different molybdacyclobutane complexes. 

There are a number of differences between the tungsten and molybdenum complexes. The main generality drawn from the data is that the tungsten complexes promote olefin metathesis quite a bit faster than the molybdenum complexes, but the tungsten complexes are less tolerant of functionality. The tungstacyclobutane is more stable than the molybdacyclobutane. Several tungstacyclobutanes have... 

Four-coordinate Mo methylidene complex (1) and both trigonal bipyramidal (5) and square pyramidal (6) structures of molybdacyclobutane have Cs symmetry. 

Investigation of the reaction pathway of ethene addition to Mo methylidene complex (1) lets us localise two transition structures 2 and 4. The first transition state is situated between the reactants and the intermediate complex (3). The second one leads to the trigonal bipyramidal molybdacyclobutane (5) and to the intermediate (3). 

Calculated reaction energies, enthalpies, Gibbs free energies and entropies at T=298.15 K are given in Table 1. The activation barriers of ethene addition to Mo methylidene complex (1) are low 21 kJmol for the first step (IV) and 13 Umof for the second one (VI). The activation energy of transformation of molybdacyclobutane... 

As results from Table 1, the reaction (VIII) of ethene addition to Mo methylidene complex (1) leading to square pyramidal molybdacyclobutane (6) is exothermic and not very exoergic. However, our efforts to find a square pyramidal transition state have failed. 

As the results indicate, the overall process of ethene addition to Mo methylidene complex (7) leading to molybdacyclobutane (10) is very exothermic and exoergic. The... 

The formation of square pyramidal structure of Mo molybdacyclobutane from methylidene complex and ethene is exothermic with small absolute value of the change of Gibbs free energy. The formation of trigonal bipyramidal structure of Mo molybdacyclobutane is clearly endoergic. However, we have only found a transition state leading to the trigonal bipyramidal structure. 

In mechanistic studies, the molybdacyclobutane of a MAP catalyst was found to break up to ethylene/methylidene intermediates 4500 times faster than the corresponding tungstacycle (at 40°C) [19]. Syn and anti proton exchange were also found to be significantly faster (up to lOOx s) for molybdacycles than for tungsta-cycles. Methylidene rotation about the M=C bond was determined to be comparatively slower for molybdenum complexes (<0.2 s ) than for tungsten complexes (3.6-230 s ). Schrock and coworkers proposed that many of these properties contribute to the superior efficiency of the tungsten MAP catalysts relative to their molybdenum counterparts. 

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