Intermolecular hydrogen-fluorine interaction in dimolybdenum triply bonded complexes modified by fluorinated formamidine ligands for the construction of 2D- and 3D-networks

The formamidines ArNHC(H)=N-Ar [Ar = Ph (2a), 4-F-Ph (2b), 3,5-F ₂Ph (2c) and 2,6-F₂Ph (2d) and R = 2,3,5-F₃Ph (2e), 3,4,5-F₃Ph (2f), F₅Ph (2g) and 4-CF₃Ph (2h)] were synthesized and the influence of the introduction of a fluorine or a trifluoromethyl group into the aryl unit on the solid-state structures was investigated. On comparing the experimental data, only marginal differences in the geometrical and electronic features of the diverse substituted species were detected. DFT calculations and X-ray crystallography of 2d-2g revealed that the E-syn-configuration corresponded to the thermodynamically most stable motif of all of the examined formamidines. However, in their solid-state, these ligands showed a range of H⋯F interactions, which varied depending on the number and position of the fluorine atoms on the aryl group and thus led to interesting solid-state structures. Moreover, compounds 2 were used for the synthesis of the new heteroleptic dimolybdenum triply-bonded complexes, Mo ₂[(2a-2c; 2e-f)-H]₂(OtBu)₄ (3a-3c; 3e-3f). X-ray crystallography of complexes 3c and 3f revealed two different isomers in the solid state: in trans-3c the two formamidines are in one plane, while in cis-3c and cis-3f they are next to each other. The DFT calculations showed only a small distinction in energy between the configurations, which led us to assume that the different configurations were induced by the crystal packing. The specific H⋯F interactions provided by the different formamidines led to a two-dimensional arrangement for trans-3c and a three-dimensional network for cis-3c and cis-3f.