Metal-Halogen Bonding Seen through the Eyes of Vibrational Spectroscopy.

Vytor P Oliveira,Elfi Kraka,Bruna L Marcial,Francisco B C Machado

doi:10.3390/ma13010055

Vytor P Oliveira, Elfi Kraka + Show 2 more

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https://doi.org/10.3390/ma13010055

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Abstract

Incorporation of a metal center into halogen-bonded materials can efficiently fine-tune the strength of the halogen bonds and introduce new electronic functionalities. The metal atom can adopt two possible roles: serving as halogen acceptor or polarizing the halogen donor and acceptor groups. We investigated both scenarios for 23 metal–halogen dimers trans-M(Y2)(NC5H4X-3)2 with M = Pd(II), Pt(II); Y = F, Cl, Br; X = Cl, Br, I; and NC5H4X-3 = 3-halopyridine. As a new tool for the quantitative assessment of metal–halogen bonding, we introduced our local vibrational mode analysis, complemented by energy and electron density analyses and electrostatic potential studies at the density functional theory (DFT) and coupled-cluster single, double, and perturbative triple excitations (CCSD(T)) levels of theory. We could for the first time quantify the various attractive contacts and their contribution to the dimer stability and clarify the special role of halogen bonding in these systems. The largest contribution to the stability of the dimers is either due to halogen bonding or nonspecific interactions. Hydrogen bonding plays only a secondary role. The metal can only act as halogen acceptor when the monomer adopts a (quasi-)planar geometry. The best strategy to accomplish this is to substitute the halo-pyridine ring with a halo-diazole ring, which considerably strengthens halogen bonding. Our findings based on the local mode analysis provide a solid platform for fine-tuning of existing and for design of new metal–halogen-bonded materials.

Highlights

Halogen bonding (XB) is a non-covalent interaction formed between the electrophilic region of a halogen atom (X) in a halogenated moiety, called halogen donor (DX), and the nucleophilic region of a partner molecule called halogen acceptor (ARn ), where A is usually a heteroatom with substituents Rn [1,2]
Similar to the hydrogen bond (HB), the XB tends to adopt a linear arrangement between DX and A [19,20,21]
We introduce in this work a new quantitative measure for the intrinsic bond strength of metal–halogen bonding based on the local vibrational mode analysis first introduced by Konkoli and Cremer [67,68,73,74]

Summary

Introduction

Halogen bonding (XB) is a non-covalent interaction formed between the electrophilic region of a halogen atom (X) in a halogenated moiety, called halogen donor (DX), and the nucleophilic region of a partner molecule (moiety) called halogen acceptor (ARn ), where A is usually a heteroatom with substituents Rn [1,2]. In the 1950s, the existence and characterization of XB was established by Mulliken’s groundbreaking theoretical work on charge-transfer interactions [4,5,6] and was supported by crystallographic studies of Hassel and coworkers [7] It took the scientific community until the 1980s to fully recognize the importance of XB across diverse fields of chemistry and biology [8,9,10], including chemical engineering and materials science [11,12], in particular discovering the potential of XB in guiding self-assembly [13,14], which goes far beyond the formation of hydrogen bond (HB) analogs. In contrast to a hydrogen atom, a monovalent halogen is amphoteric:

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