Metal⋅⋅⋅F-C Bonding in Low-Coordinate Alkaline Earth Fluoroarylamides.

A set of calcium and barium complexes containing the fluoroarylamide N(C F ) is presented. These compounds illustrate the key role of stabilising M⋅⋅⋅F-C secondary interactions in the construction of low-coordinate alkaline earth complexes. The nature of Ca⋅⋅⋅F-C bonding in calcium complexes is examined in the light of structural data, bond valence sum (BVS) analysis and DFT computations. The molecular structures of [Ca{N(C F ) } (Et O) ] (4'), [Ca{μ-N(SiMe ) }{N(C F ) }] (5 ), [Ba{μ-N(C F ) }{N(C F ) }⋅toluene] (6 ), [{BDI }CaN(C F ) ] (7 ), [{N^N }CaN(C F ) ] (8 ), and [Ca{μ-OB(CH(SiMe ) ) }{N(C F ) }] (9 ), where {BDI } and {N^N } are the bidentate ligands CH[C(CH )NDipp] and DippNC H CNDipp (Dipp=2,6-iPr -C H ), are detailed. Complex 6 displays strong Ba⋅⋅⋅F-C contacts at around 2.85 Å. The calcium complexes feature also very short intramolecular Ca-F interatomic distances at around 2.50 Å. In addition, the three-coordinate complexes 7 and 8 form dinuclear structures due to intermolecular Ca⋅⋅⋅F-C contacts. BVS analysis shows that Ca⋅⋅⋅F-C interactions contribute to 15-20 % of the bonding pattern around calcium. Computations demonstrate that Ca⋅⋅⋅F-C bonding is mostly electrostatic, but also contains a non-negligible covalent contribution. They also suggest that Ca⋅⋅⋅F-C are the strongest amongst the range of weak Ca⋅⋅⋅X (X=F, H, C ) secondary interactions, due to the high positive charge of Ca which favours electrostatic interactions.