Kinetics and mechanism of pyridine substitution from [(η3-C3H5)Mo(CO)2(X)(py)2] (X = Cl, Br; py = pyridine) by 1,2-bis(diphenylphosphino)ethane

Autor: Mohammad El-khateeb, Wolfgang Weigand, Khalil J. Asali
Rok vydání: 2018
Předmět:
Zdroj: Transition Metal Chemistry. 43:549-554
ISSN: 1572-901X
0340-4285
DOI: 10.1007/s11243-018-0238-0
Popis: Reactions of [(η3-C3H5)Mo(CO)2(X)(py)2] (X = Cl (1), Br (2); py = pyridine) with 1,2-bis(diphenylphosphino)ethane (= dppe) in toluene solution resulted in replacement of the py ligands and the formation of [(η3-C3H5)Mo(CO)2(X)(κ2-P,Pʹ-dppe)]. Thermal kinetics of these substitution reactions, under pseudo-first-order reaction conditions, indicated that these reactions proceed by a rate law that is first order in the concentration of the metal complex and zero order in dppe concentration. The data suggest a two-step substitution process. The first step is slow and reversible, involving the dissociation of the first py coligand to form a coordinatively unsaturated [(η3-C3H5)Mo(CO)2(X)(py)] transient species. This is followed by an irreversible end-on coordination of the dppe ligand at the transient species to give [(η3-C3H5)Mo(CO)2(X)(py)(κ1-P-dppe)] as an intermediate. The latter complex undergoes a fast and entropy-driven ring closure, resulting in the substitution of the second py ligand to give [(η3-C3H5)Mo(CO)2(X)(κ2-P,Pʹ-dppe)]. Rate constants and activation parameters for these reactions have been determined and discussed. Competition studies for the [(η3-C3H5)Mo(CO)2(X)(py)] intermediates show that these transients are highly reactive and react almost indiscriminately with both py and dppe. Reactions of [(η3-C3H5)Mo(CO)2(X)(py)2] (X = Cl, Br; py = pyridine) with dppe give [(η3-C3H5)Mo(CO)2(X)(κ2-P,Pʹ-dppe)]. Thermal kinetics of these substitution reactions, under pseudo-first-order reaction conditions, indicate that these reactions proceed by a rate law that is first order in the concentration of the metal complex and zero order in dppe concentration. A mechanism is proposed and suggests initial replacement of py, followed by attack of one P-atom of dppe at [(η3-C3H5)Mo(CO)2(X)(py)] to give [(η3-C3H5)Mo(CO)2(X)(py)(κ1-P-dppe)] which losses another py to give [(η3-C3H5)Mo(CO)2(X)(κ2-P,Pʹ-dppe)]. Rate constants and activation parameters for these reactions have been determined and discussed. Competition studies for the [(η3-C3H5)Mo(CO)2(X)(py)] intermediates show that these transients are highly reactive and react almost indiscriminately with both py and dppe.
Databáze: OpenAIRE
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