Bulky Ytterbium Formamidinates Stabilise Complexes with Radical Ligands, and Related Samarium “Tetracyclone” Chemistry

Divalent [Yb(DippForm) 2 (thf) n ] ( n =2 ( 1 a ), or 1 ( 1 b ), DippForm= N , N ′‐bis(2,6‐diisopropylphenyl)formamidinate) complexes were treated with the ketones: 9‐fluorenone (fn), or 2,3,4,5‐tetraphenylcyclopentadienone (tpc, tetracyclone), giving ketyl complexes: [Yb(DippForm) 2 (fn . ‐ O )(thf)] ( 2 ), and [Yb(DippForm) 2 (tpc . ‐ O )] ( 3 ), respectively (ketyl=a radical anion containing a C . −O (−) group. By contrast, when perfluorobenzophenone (pfb) was treated with either 1 a or 1 b , transitory ketyl formation was followed by rapid decomposition through a C−F activation pathway, giving [YbF(DippForm) 2 (thf)] ( 4 a ) and a highly unusual fluoride/oxide‐bridged species: [Yb 5 F 6 O 2 (DippForm) 5 ] ( 4 b ). The reduction of diketones: 3,5‐di‐ tert ‐butyl‐1,2‐benzoquinone (tbbq), 9,10‐phenanthrenequinone (phen), or 1,2‐acenaphthenequinone (acen), was also examined giving ketyl complexes: [Yb(DippForm) 2 (tbbq . ‐ O 2 )] ( 5 ), [Yb(DippForm) 2 (phen . ‐ O 2 )] ( 6 ), and [Yb(DippForm) 2 (acen . ‐ O 2 )(thf)] ( 7 ). An unsolvated derivative of 7 , namely [Yb(DippForm) 2 (acen . ‐ O 2 )] ( 8 ), was obtained from PhMe. All ketyl complexes had suitably elongated C . −O bonds, were stable in both polar and non‐polar solvents—an uncommon trait for rare‐earth ketyl complexes—and, with the exception of 3 , showed radical signals in ESR spectra. To investigate the reactivity of the tpc . ‐ O ketyl complex, 3 was treated with oxidants (CS 2 , Se) and reducing agents (Mg 0 , KH, or [SmI 2 (thf) 2 ]). Thus 3 was oxidised to tpc by Se. Treatment of 3 with KH led to a ligand exchange process giving an unusual diketyl species [Yb(DippForm)(tpc . ‐ O ) 2 (thf) 2 ] ( 10 ), which has two cisoid tpc . ‐ O − ligands in very close proximity. When treated with [SmI 2 (thf) 2 ], the tpc . ‐ O ketyl was further reduced to a dianion (1‐oxido‐2,3,4,5‐tetraphenylcyclopentadianide 2− ), ({C 5 Ph 4 }‐ O ) 2− by [SmI 2 (thf) 2 ], giving dimeric [{SmI({C 5 Ph 4 }‐ O )(thf) 2 } 2 ] ( Sm11 ) and monomeric complexes [YbI(DippForm) 2 (thf)] ( 11 b ) and [YbI 2 (DippForm)(thf) 2 ] ( 11 c ). Activated Sm metal reduced neutral tetracyclone to the dianion, ({C 5 Ph 4 }‐ O ) 2− , in THF, giving tetranuclear [{Sm II 2 ({C 5 Ph 4 }‐ O ) 2 (thf) 3 } 2 ] ( Sm13 ). Treatment of Sm13 with iodine in situ provided access to [{SmI({C 5 Ph 4 }‐ O )(thf) 2 } 2 ] ( Sm11 ), in good yield.