Abstract
The reducing agent [Et3BH][Li] reacts with diag-CpRe(CO)2Br2 in THF at −78∘C by polyene ring attack to give the anionic rhenium(I) compound [(η4-C5H6)Re(CO)2Br2]−. Monitoring the reaction by low-temperature IR spectroscopy has revealed that the reaction between diag-CpRe(CO)2Br2 and hydride is instantaneous and that the known compounds CpRe(CO)2Br− and CpRe(CO)2Br(H) are not formed as the principal reduction products. The reduced compound [(η4-C5H6)Re(CO)2Br2]− is extremely temperature sensitive and decomposes upon warming to afford low yields of CpRe(CO)3. We have explored the reactivity of the hydride reduction product with added CO and have isolated and structurally characterized the anionic tetracarbonyl compound trans(Br)-[Re(CO)4Br2]− as the tetraphenylphosphonium salt. trans(Br)-[Re(CO)4Br2][PPh4] crystallizes in the monoclinic space group C2/c, a = 16.908(2) A, b = 7.1579(7) A, c = 23.543(2) A, β = 96.144(9)∘, V = 2832.9(5) A3, Z = 4, D cacl = 1.870 g/cm3; R = 0.0419, R w = 0.0450 for 1629 observed reflections with I > 3σ(I). The reaction between the product of hydride reduction and dppe is shown by IR and 31P NMR spectroscopies to produce mer-cis-BrRe(CO)2(η2-dppe)(η1-dppe).
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