Structural Studies of N,N′‐Di(ortho‐fluorophenyl)formamidine Group 1 Metallation

Abstract

The treatment of N,N'-di(ortho-fluorophenyl)formamidine (HFPhF) in tetrahydrofuran with equimolar amounts of n-butyllithium, sodium bis(trimethylsilyl)amide or potassium bis(trimethylsilyl)amide affords the colourless crystalline formamidinate complexes [Li(FPhF)(thf)] (1), [Na(FPhF)(thf)] (2) and [K(FPhF)] (5). Low-temperature preparation of 2 in diethyl ether yields the Et(2)O adduct [Na(FPhF)(Et(2)O)] (3). At ambient temperature the sodium fluoride inclusion complex [Na(3)(FPhF)(3)(Et(2)O)(NaF)] (4) is also formed. Spectroscopic ((1)H, (13)C and (19)F((1)H) NMR) data for 1-5, microanalytical analyses for compounds 1, 2 and 5 and X-ray structure determinations for 1, 3-5 confirm the formulae of these species. In the solid-state, 1 and 3 possess a dimeric nature in which the formamidinate ligands coordinate through mu(2):eta(2):eta(1) (1) and mu(2):eta(2):eta(2) (3) binding modes. These are enabled by partial ortho-fluoro donation. Compound 4, which is also dimeric, contains two trisodium tris(formamidinate) units that comprise mu(2):eta(2):eta(2)-FPhF ligands, a bridging diethyl ether moiety and an unprecedented mu(3):eta(2):eta(2):eta(2)-formamidinate donor. Together, these trinuclear units encapsulate two sodium fluoride units by eta(2)-N,N-formamidinate chelation of the sodium cations (thereby creating further mu(3):eta(2):eta(2):eta(2)-bound formamidinates) and fluoride-sodium interactions. Compound 5 extends the coordinative versatility of FPhF to mu(2):eta(4):eta(3) coordination by the generation of K(2)(mu(2):eta(4):eta(3)-FPhF)(2) units that exhibit eta(2)-arene interactions. Macromolecularly, the overlaying of these units affords a polymeric solvent-free structure that incites coordination of the FPhF ligands to metal atoms above and below the K(2)(FPhF)(2) plane. Overall, this generates a remarkable mu(4):eta(4):eta(3):eta(2):eta(1)-amidinate binding mode that incorporates both bridging and terminal fluorine donors. Compounds 1-5 are the first non-chromium complexes of N,N'-di(ortho-fluorophenyl)formamidinate.

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

“…Complex 1 crystallised in the triclinic space group P-1, with only one potassium ion and one DFForm ligand in the ASU ( Figure 1A). The DFForm ligand of the ASU is bound (F,N,N ,F [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].…”

“…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]. Another example of such tetradentate (F,N,N ,F ) DFForm coordination was observed in the homoleptic cerium DFForm complex, [Ce(DFForm) 3 ] [17], where one of three DFForm ligands is tetradentate, with the other two being tri-dentate (F,N,N ).…”

“…Another means to engage the aromatic component in coordination is through the addition of donor functionalities (e.g., OMe, F), especially in the ortho-positions, thereby transforming the formamidinate ligand into a tri- [15][16][17], or tetra-dentate (e.g., N,N ,X or N,N ,X,X ) [17], chelate, with examples across a variety of different metal classes [18]. For s-block chemistry however, the use of such ligands has been restricted to very few examples, namely the use of N,N -bis(2-fluorophenyl)formamidine (FForm) [19].…”

“…The binding of the DFForm ligand in 1 is complex, and is discussed starting from the asymmetric unit (ASU), and then extending in both dimensions of the polymeric network. Nearly 15 years ago, FForm was complexed to the group one metals Li, Na, and K [19]. Akin to the transition metal complexes of Cotton and co-workers [20], the presence of the fluorine atom on the ortho-position of the aromatic rings permitted an additional coordinating site.…”

“…Akin to the transition metal complexes of Cotton and co-workers [20], the presence of the fluorine atom on the ortho-position of the aromatic rings permitted an additional coordinating site. This further led to partial, or complete, exclusion of bound donor molecules (e.g., Et2O, thf), by the formation of either binuclear, or for potassium, polymeric constructs (e.g., [Na(FForm)(Et2O)]2 or [K(FForm)]∞) [19]. This contrasts the group one complexes of the non-fluorinated N,N′-di(aryl)formamidinate ligands [21][22][23], which readily retain coordinating solvent.…”