Synthesis of Cyclopentadienyl‐Based Tricarbonyl Rhenium Complexes and Some Unusual Reactivities of Cyclopentadienyl Substituents

Cyclopentadienyl‐based (Cp‐based) tricarbonyl rhenium complexes [Cp′Re(CO) 3 ] are convenient precursors for the corresponding Cp‐based trioxorhenium complexes (Cp′ReO 3 ), which are potential catalysts for the deoxydehydration (DODH) of diols to olefins. To evaluate the influence of different Cp substituents in Cp′ReO 3 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 Re 2 (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 (CpMe 4 H)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 Re 2 (CO) 10 was investigated and led to the isolation of two unusual compounds, namely, Re(CO) 3 complexes of the Shvo‐type hydroxytetraphenylcyclopentadienyl ligand, [Ph 4 Cp(OH)]Re(CO) 3 ( 13a ), and a benzofuran‐fused cyclopentadienyl ligand, [Ph 3 Cp(C 6 H 4 O)]Re(CO) 3 ( 14a ). X‐ray crystal structures were obtained for the new Cp′Re(CO) 3 complexes (Cp t Bu 2 H 3 )Re(CO) 3 ( 2a ), [1,2,3‐Me 3 (tetrahydroindenyl)]Re(CO) 3 ( 7a ), 9a , 13a , and 14a , which all have the typical three‐legged “piano‐stool” configuration.