Alternative eta(5)- and eta(6)-bonding modes for bis(fluorenyl)lanthanide complexes by reactions with AlR3 and succesive addition of THF

Reversible and equilibrium isomerization of eta (5)-bis(Me3Si-fluorenyl)-ra re earth metal complexes to eta (6)-bis(Me3Si-fluorene-AlR3)-rare earth met al complexes, which is a novel type of metallotropic tautomerism, was reali zed by the addition of AlR3 to the former complexes. Resulting eta (6)-comp lexes also changed to the initial eta (5)-complexes reversibly by the addit ion of excess THF. eta (5)-Bis(Me3Si-fluorenyl)Sm(THF)(2) (1) was prepared by reaction of Me3Si-fluorenylpotassium with SmI2(THF)(2), and its structur e was analyzed by X-ray diffraction. eta (6)-Bis(Me3Si-fluorene-AlMe3)Sm (2 ) was synthesized by the reaction of 1 with excess AIMe(3). The correspondi ng reaction of excess AlEt3 with 1 gave eta (6)-bis(Me3Si-fluoren-AlEt3)Sm (3), The structures of 2 and 3 were determined by X-ray analyses. The react ion of bis(Me3Si-fluorenyl)-eta (6)-(THF)(2) (4) with an excess amount of A lMe3 gave eta (5)-(Me3Si-fluorenyl)-eta (6)-( Me3Si-fluorene-AlMe3)Yb (5), where one equimolar AlMe3 coordinates to the Yb atom through its two Me gro ups via an agostic interaction. One of the Me3Si-fluorenyl groups assumes e ta (5)-coordination, while the other Me3Si-fluorenyl group shows eta (6)-co ordination, The addition of excess THF to 5 produced (fluorenyl)Yb(THF)(4)/ AlMe4, (6) in low yield. To understand the primary factor for the alternati ve eta (5)- and eta (6)-bonding modes, we have synthesized ((i)Prindenyl)(2 )Yb(THF)(2) (7) without a Me3Si group and with a rather small indenyl ring and examined the reaction with AlR3.