Benchmarking of Halogen Bond Strength in Solution with Nickel Fluorides: Bromine versus Iodine and Perfluoroaryl versus Perfluoroalkyl Donors.

Sarah J Pike,Robin N Perutz,Christopher A Hunter,Lee Brammer

doi:10.1002/chem.201900924

Sarah J Pike, Robin N Perutz + Show 2 more

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https://doi.org/10.1002/chem.201900924

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Abstract

The energetics of halogen bond formation in solution have been investigated for a series of nickel fluoride halogen bond acceptors; trans‐[NiF(2‐C5NF4)(PEt3)2] (A1), trans‐[NiF{2‐C5NF3(4‐H)}(PEt3)2] (A2), trans‐[NiF{2‐C5NF3(4‐NMe2)}(PEt3)2] (A3) and trans‐[NiF{2‐C5NF2H(4‐CF3)}(PCy3)2] (A4) with neutral organic halogen bond donors, iodopentafluorobenzene (D1), 1‐iodononafluorobutane (D2) and bromopentafluorobenzene (D3), in order to establish the significance of changes from perfluoroaryl to perfluoroalkyl donors and from iodine to bromine donors. 19F NMR titration experiments have been employed to obtain the association constants, enthalpy, and entropy for the halogen bond formed between these donor‐acceptor partners in protiotoluene. For A2–A4, association constants of the halogen bonds formed with iodoperfluoroalkane (D2) are consistently larger than those obtained for analogous complexes with the iodoperfluoroarene (D1). For complexes formed with A2–A4, the strength of the halogen bond is significantly lowered upon modification of the halogen donor atom from I (in D1) to Br (in D3) (for D1: 5≤K 285≤12 m −1, for D3: 1.0≤K 193≤1.6 m −1). The presence of the electron donating NMe2 substituent on the pyridyl ring of acceptor A3 led to an increase in −ΔH, and the association constants of the halogen bond complexes formed with D1–D3, compared to those formed by A1, A2 and A4 with the same donors.

Highlights

We describe a systematic study of the influence of structural variations of the donor and acceptor species on the binding constants and energetics of halogen bond formation between a series of structurally related nickel fluorides A2-A4 and a range of organic iodo- and bromo-perfluorocarbon donors D1-D3 in protiotoluene (Chart 1)
Nickel fluorides were employed as halogen bond acceptors as they are soluble in toluene and do not display appreciable selfassociation.17a We reported the formation of the halogen-bonded adduct D1·A1 and D1·A4 earlier.[17]
A series of 19F NMR titration experiments were performed on metal fluorides (A2-A4) through the addition of increasing quantities of halogen bond donors (D1-D3) in protiotoluene

Summary

Introduction

Halogen bonding interactions are rapidly emerging as key constituents of the molecular recognition toolbox.[1,2,3] The utility and importance of halogen bonding interactions is evident through its widespread use in applications including crystal engineering,[4] materials chemistry,[5] supramolecular chemistry[6] and anion recognition[7] and through its emergent role in organocatalysis and reactivity.[8,9] Halogen bonding interactions are known to hold great significance in medicinal chemistry[10,11] and are recognized to Supporting information for this article is available on the WWW under http://www.chemeurj.org/ or from the author.be important in achieving function in biological systems.[12,13,14] The formation of halogen bonding interactions to species in the “ligand domain” has been revealed crystallographically[15,16] and in solution. (The ligand domain consists of those ligands directly bonded to the metal or with a strong electronic interaction with it.)[16] There remains, a distinct shortage of information about the energetics of these halogen bonding interactions in solution.[17,18,19] In contrast, the energetics of halogen-bonded systems involving organic acceptor and donor partners are better documented and include association constants for halogen bonds formed between haloalkynes,[20] haloarenes[21] and haloalkanes[22] as donors with neutral organic bases.

Results

Conclusion