Concerted halogen bonding and orthogonal metal-halogen interactions in dimers of lithium formamidinate and halogenated formamidines: an ab initio study.

Ruben Parra

doi:10.3390/molecules19011069

Abstract

Dimers of lithium formamidinate, CH(NH)2Li, and halogenated formamidines, HN=CHNHX, (X=Cl, Br, or I) are used as model systems to investigate simultaneous N-X···N and N-Li···N interactions, in tandem with orthogonal Li···X interactions. Geometry optimizations and energy calculations for the dimers are examined with the MP2 method and the M06-2X hybrid functional and the aug-cc-pVTZ basis set (the aug-cc-pVTZ-PP basis set is used for the iodine atom). Both methods predict the formation of a planar structure of C2v symmetry, regardless of the identity of the halogen atom. In this structure, the identities of the constituent monomers are essentially lost. Accordingly, the N-X···N interactions emerge as a rather symmetric quasi-linear N···X···N, where the covalent N-X bond in the halogenated formamidine is replaced by a partly covalent N···X interaction. Formation of the C2v structure is also driven by a fairly linear N···Li···N interaction parallel to the N···X···N interaction, and a Li···X interaction orthogonal to both the N···X···N and N···Li···N interactions. The strength of the interactions increases with the size of the halogen. The robustness of the interactions suggests that the dimers studied here or suitable analogues may find diverse applications including their use as novel polymeric synthons.

Highlights

A halogen bond (XB) interaction is commonly understood as an attractive and highly directional interaction Y···X-R between an electron rich species or Lewis base, Y, and the halogen atom, X, fromMolecules 2014, 19 a molecule or fragment R–X in which R is a group more electronegative than X, or is X itself [1,2,3,4,5].Very recently, the International Union of Pure and Applied Chemistry (IUPAC) released the following recommended definition for a halogen bond: a halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity [6]
The robustness of the interactions suggests that the dimers studied here or suitable analogues may find diverse applications including their use as novel polymeric synthons
Geometries obtained with both aug-cc-pVDZ and aug-cc-pVTZ basis sets are similar, and only the results with the larger basis set will be used for discussion

Summary

Introduction

A halogen bond (XB) interaction is commonly understood as an attractive and highly directional interaction Y···X-R between an electron rich species or Lewis base, Y, and the halogen atom, X, fromMolecules 2014, 19 a molecule or fragment R–X in which R is a group more electronegative than X, or is X itself [1,2,3,4,5].Very recently, the International Union of Pure and Applied Chemistry (IUPAC) released the following recommended definition for a halogen bond: a halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity [6]. The strength and directionality of the XB interaction have been rationalized in terms of the so called σ-hole, which is the positive electrostatic potential that a covalently bonded halogen atom may develop in its outer side, opposite to the covalent bond and pointing toward any potential electron donor [7,8]. Polarization results in regions of charge depletion and charge concentration that complement each other giving rise to the attractive interaction known as the XB interaction. Such bonding model is known as the lump-hole theory, and shows that a true positive permanent σ-hole is not mandatory for XB formation [9]. It has been demonstrated that halogen bonds should be studied with large basis sets and theoretical models that efficiently incorporate electrostatic, dispersion, polarization, and charge transfer interaction components

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Conclusion