Haptotropic rearrangements in metal complexes of 4H-cyclopenta(def)phenanthrene

Abstract

<p>Transition-metal complexes of the tetracyclic system 4H-cyclopenta[def]phenanthrene (cppH) have been prepared for a series of metals, i.e. (η⁶-cppH)Cr(CO)₃, (η⁶-cppH)Mn(CO)₃⁺ and (η⁶-cppH)Fe(C₅H₅)⁺; in all cases the metal is bonded to a terminal six-membered ring. Upon deprotonation, the corresponding (η⁶-cpp) species can be trapped at low temperatures. Raising the temperature results in a η⁶- to η⁵-haptotropic shift, in the case of the Cr(CO)₃ complex at temperatures above -30℃ and +20℃ for the Mn(CO)₃ complex. However, heating of a sample of (η⁶-cpp)Fe(C₅H₅) did not result in a haptotropic rearrangement. The corresponding 8,9-dihydro-cpp derivatives (η⁶-H₂-cppH)Cr(CO)₃ and [(η⁶-H₂cppH)Mn(CO)₃] PF₆ have also been prepared. Deprotonation of the Mn(CO)₃ complex yielded the thermally stable compound (η⁶-H₂ccp)Mn(CO)₃ which only undergoes an η⁶- to η⁵-haptotropic shift upon heating to 60℃ for one hour. The relative ease of haptotropic shifts in the cpp complexes can be rationalized in terms of a transition state in which the migrating organometallic fragment is η³-bonded to the polycyclic ligand in which a 10π (naphthalene like) system is retained. EHMO calculations predict that the barriers to migration in case of the (cpp)FeCp system can be expected to be some 4.5 kcal mol⁻¹ lower than in the corresponding fluorenide system. Furthermore, calculations for the system (cpp)Mn(CO)₃ reveal that the barriers to migration can be expected to be much lower than in the analogous (cpp)Fe(C₅H₅) system. The barriers were calculated to be 17.5 kcal mol⁻¹ and 30.0 kcal mol⁻¹ respectively. Reactions have been carried out to prove the existence of the proposed η³-transition state. In the course of these studies, the complexes (η¹-cpp)Fe(C₅H₅) and (η¹-cpp)Mn(CO)₃(P(C₂H₅)₃)₂ have been synthesized, both of which were found to be unexpectedly stable.</p>