Synthesis and Reactivity of Mono(pentamethylcyclopentadienyl) Tetraphenylborate Lanthanide Complexes of Ytterbium and Samarium:  Tris(ring) Precursors to (C₅Me₅)Ln Moieties

The synthesis of unsolvated (C 5 Me 5 )Ln(BPh 4 ) complexes (Ln = Sm, Yb) has been investigated to determine if the productive chemistry of the metallocene tetraphenylborate complexes, (C 5 Me 5 ) 2 Ln(μ-Ph) 2 BPh 2, can be expanded to mono(pentamethylcyclopentadienyl) systems. Precursors (C 5 Me 5 ) 2 Yb, 1, and (C 5 Me 5 ) 2 Sm, 2, were both prepared by desolvation of (C 5 Me 5 ) 2 Ln(THF) 2 under high vacuum in near quantitative yields. Compounds 1 and 2 react with [Et 3 NH][BPh 4 ] to form divalent (C 5 Me 5 )Ln(μ-η 6:η 1 -Ph) 2 BPh 2 ( 3, Yb; 4, Sm) complexes in which two of the phenyl rings of the tetraphenylborate counteranion coordinate η 6 to the lanthanide to generate a three-ring coordination geometry involving cyclopentadienyl and arene coordination. In contrast to the expected trigonal plane defined by the three-ring centroids in 3, the structure of 4 is pyramidal with Sm 0.41 Å out of the plane of the three-ring centroids. Complex 3 reacts with THF to make the polysolvated complex, [(C 5 Me 5 )Yb(THF) 4 ][BPh 4 ], 5, which can also be obtained from (C 5 Me 5 ) 2 Yb(THF) 2 and [Et 3 NH][BPh 4 ]. With 4, a monosolvated complex, (C 5 Me 5 )Sm[(μ-η 6:η 1 -Ph)(μ-η 2:η 1 -Ph)BPh 2 ](THF), 6, can be isolated that retains η 6 coordination by one aryl ring and displays η 2 coordination with the other aryl ring. Reaction of phenazine with 3 and 4 is accompanied by ligand redistribution to form the trivalent bis(pentamethylcyclopentadienyl) products, [(C 5 Me 5 ) 2 Ln] 2 [μ-η 3:η 3 -C 12 H 8 N 2 ], (Yb, 7; Sm, 8 ). Reduction of azobenzene by 4 generates a mono(pentamethylcyclopentadienyl)tetraphenylborate complex, (C 5 Me 5 )Sm[(μ-η 6:η 1 -Ph)BPh 3 ](N 2 Ph 2 ), 9, that contains an η 6 coordinated phenyl ring and an azobenzene radical anion.