Factors Controlling Asymmetrization of the Simplest Linear I₃^–and I₄^2–Polyiodides with Implications for the Nature of Halogen Bonding

Abstract

This paper investigates geometric and electronic features of linear I 3 − and I 4 2− anions, as building blocks of larger polyiodides. Most experimental structures are quasi D ∞h , although one lateral linkage is occasionally elongated with I•••I separations approaching those of I•••I−R − species, typical of halogen bonding (HalB). Hirshfeld surfaces from crystal data highlight solid state effects depending on the distribution of the counterions around I 3 − or I 4 2− units. Corresponding experimental asymmetries have been mimicked with density functional theory calculations through different surroundings of positive point charges. The consequent deformations are interpreted in terms of the s/p rehybridizations occurring at the central I atom(s) of the populated frontier σ* wave functions. The origin is a charge-induced variation of the orbital energies at lateral iodides (electronegativity), hence by their the donor power in a nucleophilic attack. The calculations also provide energy information on I 2 + I − or I 2 + 2I − additions, and, in solvent, the intrinsic energy stability of I 4 2− is for the first time validated. In the absence of positive charge perturbations, the 1− charge of a remote iodide polarizes I 3 − and promotes incipient electrostatic attraction, which is quickly accompanied by electron transfer with a generalized σ delocalization throughout I 4 2− . Implicit orbital overlap supports a covalent picture, or better to say hypervalency, given the electron richness of the central atoms. Molecular electrostatic potential (MEP) surfaces are expected to show σ holes in support of the purely electrostatic HalB model, typically proposed for I•••I−R − systems. However, the computed surfaces show little evidence of σ holes in the equilibrium adducts I 3 − , I 4 2− and I•••I−R − suggesting that HalB cannot be purely electrostatic.

Bonding Analysis in Homo‐ and Hetero‐Trihalide Species: A Charge Displacement Study

“…On the other hand, the difference between the homo‐ and hetero‐trihalides is due to the electrostatic component, Δ E elstat , which increases significantly on going from Cl 2 ··· Cl – (–75.0 to –11.6 kcal mol –1 ) to ClI ··· Cl – ,(–98.0 to –17.4 kcal mol –1 ) and from Br 2 ··· Br – (–79.8 to –11.8 kcal mol –1 ) to BrI ··· Br – (–89.1 to –14.1 kcal mol –1 ). This is entirely in line with the presence of a pre‐formed dipole in the hetero‐dihalogen molecule, with the more positive atom facing the approaching anion, as already underlined in the literature , . The enhanced Δ E elstat is partially counterbalanced by a marked decrease in the orbital contribution, Δ E tot orb .…”

“…In particular, the paper by Hoffmann and co‐workers published in 1997 analyzed, on the basis of density functional calculations, the various contributions to the stabilization of trihalides by comparing the Rundle–Pimentel model for electron‐rich three‐centered four‐electron (3c, 4e) systems with that describing the interhalogenic bond as a donor/acceptor or charge‐transfer interaction between a halide and a dihalogen molecule. Later, Mealli and co‐workers extended the work to polyiodide systems . Although it is not easy to assign the contribution of each energy term to the bonding in these hypervalent anions, both models account for a total bond order of 1 in these systems, and, although the former fits better for symmetric systems, the latter describes better strongly asymmetric arrangements.…”

Halogen Bonding in Supramolecular Chemistry

“…Mealli and co-workers have developed a DFT-based model of I 3 − and I 4 2− anions with a focus on the σ-electron density delocalization rather than a simple electrostatic interpretation. 110 Esterhuysen and co-workers have performed a theoretical study of the triiodide ion and the I 3 − ···I 3 − dimer (both in the gas phase and in an implicit continuum solvent model using various levels of theory and basis sets) to understand the driving force behind the formation of [I 3 − ] ∞ chains and to study the effect of a uniform electric field (as found in solution) on the I 3 − ion and its interactions. 111 The MP2/cc-pVTZ-PPcalculated I 3 − bond length and I 3 − ···I 3 − intermolecular distances were consistent with those obtained from the Cambridge Structural Database.…”

Coherent ultrafast lattice-directed reaction dynamics of triiodide anion photodissociation

“…In our study, this scenario is more plausible for light species like I • than its diatomic co-product I 2 −• with an adjacent I 3 − , because the excess translation energy imparted to I 2 −• from bond-breaking will, according to momentum conservation, grant it only half of the recoil velocity as I • , and it will be further slowed down by the Coulombic repulsion from I 3 − . The alternative two-body dissociation channel (I 3 − ℎ � � I 2 + I − ) contains a distinct monatomic fragment, the iodide anion (I − ), but its reactivity is low and the formation of the dianion I 4 2− through an analogous bimolecular association pathway, I − + I 3 − ⇌ I 4 2− , is thermodynamically discouraged 34,35 . Furthermore, the other photofragment, I 2 , will generate the well-characterized absorptive signal in the region of 500-600 nm from its major absorption bands 36 , and will follow different kinetics with respect to the spectral in condensed phases [9][10][11][12]14,22,38 .…”