Synthesis of Cyclopentadienyl-based Tricarbonyl Rhenium Complexes and some Unusual Reactivities of Cp-substituents

Cyclopentadienyl-based (Cp-based) tricarbonyl rhenium complexes [Cp′Re(CO)3] are convenient precursors for the corresponding Cp-based trioxorhenium complexes (Cp′ReO3), which are potential catalysts for the deoxydehydration (DODH) of diols to olefins. To evaluate the influence of different Cp substituents in Cp′ReO3 complexes in DODH, a series of alkyl-substituted Cp′Re(CO)3 complexes (1a–8a) were synthesized. High yields (86–98 %) were obtained from the reactions of Re2(CO)10 with the corresponding Cp′H ligands (1–8). The C–O infrared absorptions of 1a–8a indicate that the electron-donating character of the Cp ligand increases with the number of substituents attached directly to the Cp ring. Analogous aryl-substituted complexes 10a–12a containing bulky phenyl groups were accessed through the salt metathesis of ReBr(CO)5 with the lithium salt of the deprotonated ligand (Cp′Li), and the aryl groups decreased the electron donation. Furthermore, an unusual [6+4] cycloaddition reaction of (CpMe4H)Re(CO)3 (8a) with excess ligand resulted in the highly asymmetric Cp′Re(CO)3 complex 9a. Finally, the reaction of the tetraphenylcyclopentadienone ligand with Re2(CO)10 was investigated and led to the isolation of two unusual compounds, namely, Re(CO)3 complexes of the Shvo-type hydroxytetraphenylcyclopentadienyl ligand, [Ph4Cp(OH)]Re(CO)3 (13a), and a benzofuran-fused cyclopentadienyl ligand, [Ph3Cp(C6H4O)]Re(CO)3 (14a). X-ray crystal structures were obtained for the new Cp′Re(CO)3 complexes (CptBu2H3)Re(CO)3 (2a), [1,2,3-Me3(tetrahydroindenyl)]Re(CO)3 (7a), 9a, 13a, and 14a, which all have the typical three-legged “piano-stool” configuration.