Intermolecular Hydrogen–Fluorine Interaction in Dimolybdenum Triply Bonded Complexes Modified by Fluorinated Formamidine Ligands for the Construction of 2D‐ and 3D‐Networks

Abstract

The formamidines ArNHC(H)=N–Ar [Ar = Ph (2a), 4‐F‐Ph (2b), 3,5‐F2Ph (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, Mo2[(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.

Potassium C–F Interactions and the Structural Consequences in N,N′-Bis(2,6-difluorophenyl)formamidinate Complexes

“…This tetradentate binding of the DFForm ligand contrasts that of the FForm ligand in [K(FForm)]∞ [19], where the ASU contains one FForm ligand bound η 4 (N,(Ar-C6,5),F) to potassium. As the K ion is bound by the DFForm NCHN bite in an almost symmetrical manner, and does not favour one nitrogen donor (as observed in [K(FForm)], the K···F-C bonding is weak, and thus the C-F bonds in 1 (of either C1/F1 or C9/F3, Figure 1) are almost unchanged from those of DFFormH (C-F: 1.3596(17)-1.3625(18)) [30]. By contrast, the asymmetrical NCHN binding of FForm to K in [K(FForm)]∞ (along with the coordination across the aromatic component), brings the fluorine atom into a closer proximity to the potassium atom (K-F: 3.029(4)), and weakens the C-F bond (C-F: 1.377(6) Å) [19].…”

“…Because of this additional coordination, the DFForm ligand is nearly planar across the K and K′′′ atoms, with the bond angle of K-N1/N2cent-K′′′ being 175.66(1)°. Although the auxiliary fluorine atoms are coordinated at a considerably shorter distance than the K-F1 and K-F3 analogues, there is still only a minor shortening of the C-F bonds from those of DFFormH (C-F: 1.3596(17)-1.3625(18)) [30]). As shown in Figure 2A, there is an apparent coordination gap in axial positions of the potassium ions, and it is in this position that the other two fluorine atoms (namely, F2 and F4) of the DFForm ligand become relevant, and expand the one-dimensional polymeric network into a two-dimensional polymer.…”

“…Because of this additional coordination, the DFForm ligand is nearly planar across the K and K atoms, with the bond angle of K-N1/N2 cent -K being 175.66(1) • . Although the auxiliary fluorine atoms are coordinated at a considerably shorter distance than the K-F1 and K-F3 analogues, there is still only a minor shortening of the C-F bonds from those of DFFormH (C-F: 1.3596(17)-1.3625(18)) [30]). [18].…”

Low‐Valent Molybdenum‐Based Dual Pre‐Catalysts for Highly Efficient Catalytic Epoxidation of Alkenes and Deoxygenation of Sulfoxides

“…Intriguingly, many reduction reactions are performed by molybdenum-based enzymes, which underline the high versatility of this metal. [11][12][13] This type of complex is well-known and established, which is a result of the seminal work by Cotton, Chisholm, and co-workers. [6] These enzymes efficiently catalyze the deoxygenation of sulfoxides and they have been intensively investigated for their structural properties and their catalytic applicability; even some model systems mimicking their active centre have been synthesized.…”

“…[10] Recently, our group have investigated the use of triply bonded dimolybdenum alkoxides as precursors for several applications. [11][12][13] This type of complex is well-known and established, which is a result of the seminal work by Cotton, Chisholm, and co-workers. Discovered in 1977, [14] various triplybonded analogues have been reported, [15,16] structurally characterized, and thoroughly investigated in terms of their coordination chemistry [17] and reactivity.…”