Synthesis and reactivity of [(eta(6)-biphenylene)Mn(CO)(3)](+)

Biphenylene readily coordinates through one of the phenyl rings to form [(eta(6)-biphenylene)Mn(CO)(3)](+) (2(+)), the X-ray structure of which is reported. The nucleophiles H-, Me-, Me3CC(O)CH2-, and P(OEt)(3) add to 2(+) at a bridgehead carbon to afford high yields of cyclohexadienyl complexes (3); the X-ray structure of the product of hydride addition is reported. P(OMe)(3) adds to 2(+) to give 3, which then undergoes a spontaneous Arbuzov elimination to the dimethyl phosphonate adduct. Nucleophilic additions to 2(+) are unusual in that they occur at a substituted (bridgehead) carbon. A kinetic and thermodynamic study of the reaction with P(OEt)(3) shows that the bridgehead carbons in 2(+) are orders of magnitude more electrophilic than are the arene carbons in unstrained [(arene)Mn-(CO)(3)](+) systems.