Structure of [1.1]Ferrocenophanyllithium in the Solution and the Solid State. Absence of an Intramolecular [C−H−C]^- Hydrogen Bond

Abstract

[1.1]Ferrocenophanyllithium (1) is shown by dynamic NMR (DNMR) and isotopic perturbation to undergo a rapid intramolecular 1,12-proton transfer coupled with 1,12-lithium ion transfer. The first [C−H−C]- hydrogen bond previously reported to be present in 1 is shown by X-ray crystallography to be absent in the solid state. The 13C NMR spectrum of the bridge-labeled compound [1,12-13C2][1.1]ferrocenophanyl[1-6Li]lithium (6) in 2,5-dimethyltetrahydrofuran (DMTHF) displays a 1:1:1 triplet (1 J(13C,6Li) = 4.0 Hz) showing that 6Li is bonded to C-1 and that 1 is monomeric. Transient nuclear Overhauser effects (tNOE's) measured in DEE-d 10 in bridged monodeuterated 1 show that the bridges in 1 are syn and not anti to each other and that lithium is exo-coordinated to the anionic bridge carbon. The 1 J(13C,1H) coupling constants show that the hybridization of the carbanionic carbon is intermediate between sp2 and sp3. Thus the solution and solid state structures of 1 are similar. UV−vis spectroscopy shows that 1 exists mainly as contact ion pairs (CIPs) in the temperature range −100 to +25 °C in THF, DMTHF, and diethyl ether (DEE). In dimethoxyethane (DME), 1 is mainly CIPs above +25 °C but is mainly separated ion pairs (SIPs) below −30 °C.

Ab Initio Calculations on Conventional and Unconventional Hydrogen BondsStudy of the Hydrogen Bond Strength

“…The existence and strength of H-bond depend on the Lewis acidity of X−H bond and on the Lewis basicity of Y center. The understanding of hydrogen bonding has changed in last 2−3 decades since new types of H bonds have been investigated. − Such interactions known as nonconventional hydrogen bonds may be generalized in four ways: H bonds with C−H bonds as unconventional donors, − ,− those with unconventional acceptors (π electrons within aromatic systems or the other π-electronic moieties or simple C-atoms), hydrogen bonds with unconventional donors and unconventional acceptors such as C−H···C (or C−H···π) systems, and dihydrogen bonds. − The last case concerns X−H +δ ··· -δ H−M systems (designated as DHBs) for which X−H is usually the typical proton donating bond with an electronegative X atom and M−H is the metal−hydride σ bond behaving as an acceptor. In some cases of DHBs, the B−H is an accepting bond …”

“…The most important geometrical characteristic of X−H···Y hydrogen bond is that the distance between the proton and the acceptor atom (H···Y) is shorter than the sum of their van der Waals radii . For strong and medium conventional H bonds, H···Y distance is usually much less than the corresponding sum of van der Waals radii but for weaker C−H···Y (Y = O, N, or C) bonds it may be only slightly less than such sum or even equal to it. ,, However, the mentioned above geometrical characteristic should not be applied rigorously because it is known that a van der Waals cutoff is not the physical limit of the long-range electrostatic hydrogen bond interaction 16d. The use of energetic criteria is more problematic.…”

Intermolecular =C–H···:C hydrogen bond in a crystalline unsaturated Arduengo-type carbene

“…20 We have located only two experimentally well-authenticated examples, 21,22 both involving protonated species. 23 In addition, the results of an early IR and NMR study 24 were interpreted in terms of a C-H···C hydrogen bond in solution between phenylacetylene and benzyl isocyanide. Thus, this paper seems to report the first example of a C-H···C intermolecular hydrogen bond in a crystal involving neutral molecules.…”

Influence of substitution on the strength and nature of CH···N hydrogen bond in XCCH···NH3complexes

“…Such interactions are known as nonconventional hydrogen bonds. There are different types of nonconventional hydrogen bonds, for example, H‐bonds with CH bonds as unconventional donors 5–12, those with unconventional acceptors (π electrons within aromatic systems or the other π‐electronic moieties or simple C‐atoms) 13–17, hydrogen bonds with unconventional donors and unconventional acceptors such as CH···C (or CH···π) systems 18–24 and dihydrogen bonds 25–34.…”