Reactions of rhenium trialkylphosphite complexes (η 5-C 5Me 5)Re(CO) 2{P(OR) 3} (R  Me or Et) with halogens and SbCl 5: Reversible Arbuzov-like dealkylation to form rhenium dialkylphosphonate complexes (η 5-C 5Me 5)ReX(CO) 2{PO(OR) 2}

Abstract

The cationic complexes of general formula [(η 5-C 5Me 5)ReX(CO) 2{P(OR) 3}] +(R  Me; X  Cl ( 2a), Br ( 3a) or I ( 4a)) and (R  Et; X  Cl ( 2b), Br ( 3b) an I ( 4b)) have been synthesized by reactions of (η 5-C 5Me 5)Re(CO) 2{P(OR) 3} with either halogens X 2 or SbCl 5. Each is obtained as a mixture of cis and trans isomers of a typical four-legged piano-stool geometry and has been fully characterized in solution by a combination of IR and 1H, 13C and 31P nuclear magnetic resonance spectroscopy. It is observed that under appropriate conditions these cationic complexes undergo transformation to the corresponding neutral dialkylphosphonate complexes of general formula trans-(η 5-C 5Me 5)ReX(CO) 2{PO(OR) 2} ( 5a–7a and 5b–7b) where R  Me ( a) or Et ( b) and X  Cl ( 5), Br ( 6) and I ( 7). This is ascribed to a Michaelis—Arbuzov-type dealkylation reaction involving a nucleophilic attack at the alkyl group by the counter-anion, and it is possible to render the cation more (or less) stable to this transformation by suitable choice of the counter-anion and the phosphite. The cationic triethylphosphite complexes exhibit a decreased tendency to undergo the dealkylation reaction. Most interestingly, the product ( i.e. cationic phosphite or neutral dialkylphosphonate) in the reaction of (η 5-C 5Me 5)Re(CO) 5{P(OR) 3} with Br 2 at −78°C is dependent on the molar ratio of the reactants; when examined in detail, this is found to result from a counter-ion dependent reversible alkylation-dealkylation process. Thus the dialkylphosphonate is unchanged by treatment with alkylbromide alone but is completely transformed back to the cationic trialkylphosphite complex by alkyl bromide and bromine.