Hydrotris(pyrazolyl)borate metallacycles: conversion of a late-metal metallacyclopentene to a stable metallacyclopentadiene-alkene complex.

The bis(ethene) complex [(Tp)Ir(C(2)H(4))(2)] (3) undergoes reaction with dimethyl acetylenedicarboxylate (DMAD) in acetonitrile solvent at 60 degrees C to give the trispyrazolylborate metallacyclopent-2-ene complex [(Tp)Ir (CH(2)CH(2)C(CO(2)Me)=C(CO(2)Me))(NCMe)] (4). Spectroscopic analysis of a room-temperature reaction between 3 and DMAD in acetonitrile-d(3) provides evidence for the formation of an eta(2)-alkene/eta(2)-alkyne intermediate on the path to 4. The reaction of 3 with DMAD in THF solvent leads to the formation of the THF-ligated metallacyclopent-2-ene complex [(Tp)Ir(CH(2)CH(2)C(CO(2)Me)=C(CO(2)Me))(THF)] (5), which undergoes further reaction with DMAD at 60 degrees C in benzene to give [(Tp)Ir(C(CO(2)Me)=C(CO(2)Me)C(CO(2)Me)=C(CO(2)Me))(eta(2)-CH(2)=CH(2))] (6). Complex 4 was structurally characterized by X-ray crystallography.