Selective Cleavage of the Strong or Weak C–C Bonds in Biphenylene Enabled by Rare-Earth Metals

Abstract

Selective cleavage of C–C bonds within arene rings is of great interest but remains elusive, especially for the molecules possessing the active and inert C–C bonds. Here, we report that the active and inert C–C bonds of biphenylene could be controllably cleaved by the reaction of biphenylene, potassium graphite, and rare-earth complexes with different metal centers. For scandium, the bond activation occurs at the Caryl–Caryl single bond, yielding 9-scandafluorene. For Lu, the reaction goes through ring contraction of the aromatic ring in biphenylene to provide benzopentalene dianionic lutetium. The origin of the selectivity and the reaction mechanism were illustrated by the isolation of intermediates and DFT calculations.

Synthesis and reactivity of a uranium(iv) complex supported by a monoanionic nitrogen–phosphorus ligand

“…66 The bond angle of C1–U1–C2 is 69.1(7)°, which is close to the C–Ln–C angles in [Li(DME) 3 ][(C 5 Me 5 ) 2 Ln(biphen)] (Ln = Ce, 69.0(7)°; Ln = La, 68.55(8)°) 60 but is smaller than the C–M–C angles in other reported metallafluorenes (74°–130°). 56–61,67,68 The bond lengths of U–N amine (average of 2.735 Å) and U–N amido (average of 2.316 Å) are longer than those observed in complex 1 (2.695 and 2.249 Å, respectively), which is probably due to the steric hindrance of the biphenyl group. Complex 3 represents a new example of a heterofluorene containing an actinide element.…”

“…Heteroatoms in the heterofluorenes could be main group elements, 56,57 transition metals, 58,59 and rare-earth metals. 60,61 With complex 1 in hand, we attempted to synthesize a uranium-containing metallafluorene species via salt metathesis. Treatment of complex 1 with one equivalent of 2,2′-dilithiobiphenyl 62 in THF at RT overnight afforded a brown turbid solution, from which complex 3 was isolated in 76% yield as a yellow crystalline solid (Scheme 2).…”

“…The bond lengths of U1-C1 (2.50(2) Å) and U1-C2 (2.482(17) Å) are shorter than the U-C aryl bond lengths in [Li] ). [56][57][58][59][60][61]67,68 The bond lengths of U-N amine (average of 2.735 Å) and U-N amido (average of 2.316 Å) are longer than those observed in complex 1 (2.695 and 2.249 Å, respectively), which is probably due to the steric hindrance of the biphenyl group. Complex 3 represents a new example of a heterofluorene containing an actinide element.…”

“…Heteroatoms in the heterofluorenes could be main group elements, 56,57 transition metals, 58,59 and rare-earth metals. 60,61 With complex 1 in hand, we attempted to synthesize a uranium-containing metallafluorene species via salt Scheme 1 Synthesis of complexes 1 and 2. Fig.…”

Carbon–carbon bond activation by Mg, Al, and Zn complexes

“…The central C–C σ-bond of the four-membered has a low bond dissociation energy of 65.4 kcal mol −1 . 58,59 Both the anti-aromatic character and strain of the four-membered ring contribute to the weakening of this C–C σ-bond. In contrast, the C–C bond strength within the six-membered ring system has been estimated as 114.4 kcal mol −1 .…”