3.4 Second-order donor-acceptor charge transfer delocalization from NBO analyses
Consistent with the work of others,11 the results of the second-order perturbative estimates of donor-acceptor (bond-antibond) interactions obtained from an NBO analysis suggests that several complexes of Fig. 1 are the result of significant charge transfer (CT) delocalization between the σ* anti-bonding orbital of the chalcogen bond donor fragment and the lone-pair bonding orbitals associated with the bases A. For example, the complexes F2O2···A(A = F, Cl, Br) are predominately due to the result of the n (A) → σ*(O–O) CT delocalization, with E(2) of 112.9, 159.9 and 198.44 kcal mol-1 for F2O2···F, F2O2···Cl, and F2O2···Br, respectively, where n represents the lone-pair bonding orbital(s) of the anionic species. Similarly, the complexes F2O···F, F2O···Cl, and F2O···Br are the results of then (A) → σ*(O–F) CT, with E(2) of 184.4, 141.9 and 127.9 kcal mol-1, respectively.
The complex Cl2O···F is due to the combined effect of CT delocalizations such as n(2) (F) → σ*(O–Cl1)/σ*(O–Cl2) (E(2) = 8.3/1.0 kcal mol-1), n(3) (F) → σ*(O–Cl1)/ σ*(O–Cl2) (E(2) = 0.3/1.8 kcal mol-1) and n(4) (F) → σ*(O–Cl1)/ σ*(O–Cl2) (E(2) = 139.2/5.5 kcal mol-1). For the complex Cl2O···Cl, theE(2) for the CT interactions, viz .n(1) (Cl) → σ*(O–Cl1)/ σ*(O–Cl2) and n(3) (Cl) → σ*(O–Cl1)/σ*(O–Cl2) are 2.3/2.3 and 2.3/2.3 kcal mol-1, respectively. The corresponding CT delocalizations responsible for Cl2O···Br are n(1) (Br) → σ*(O–Cl1)/σ*(O–Cl2) and n(3) (Br) → σ*(O–Cl1)/σ*(O–Cl2), withE(2) of 2.2 and 2.1 kcal mol-1, respectively. These results indicate that several lone-pair bonding orbitals on the halide ion X facilitate CT interactions with the anti-bonding σ* orbitals of the C–X bonds, providing evidence of the formation of chalcogen bonding in these complexes. The many-fold interaction topologies between the donor and acceptor orbitals are apparently due to the non-linear nature of the F···O, Cl···O and Br···O interactions in these complexes, respectively (∠F···O–Cl = 149.9o in Cl2O···F, ∠Cl···O–Cl = 120.0o in Cl2O···Cl, ∠Br···O–Cl = 118.5o in Cl2O···Br).
The angle of interaction in the complexes Br2O···F, Br2O···Cl, and Br2O···Br is also significantly non-linear. For instance, ∠F···O–Br, ∠Cl···O–Br and ∠Br···O–Br are 124.0, 121.2 and 120.0o in the respective complexes, resulting in pseudo-windmill type geometries between the O and X atoms. There is, therefore, an expectation of orbital interaction in these complexes that could be associated with the two σ* anti-bonding orbitals of the two Br–O bonds in the OBr2 moiety and the lone-pair bonding orbitals of the halide anions. Indeed, the second order analysis suggests such a possibility; the Br2O···F complex is the result of (n(4) (F) → σ*(O–Br1)) and (n(4) (F) → σ*(O–Br2)) delocalizations, each with an E(2) of 61.2 kcal mol-1. The strongest orbital interaction occurs between n(4) on F and each of two σ*-orbitals of the O–Br bond;E(2) for analogous CT interactions involving the lone-pair bonding orbitals 2 and 3 on F and σ*(O–Br) were 5.4 and 2.0 kcal mol-1, respectively. Similarly, for the complex Br2O···Cl, the CT interactions were (n(1) (Cl) → σ*(O–Br1)/σ*(O–Br2) and (n(3) (Cl) → σ*(O–Br1)/σ*(O–Br2), each withE(2) of 2.4/1.9 kcal mol-1. For the complex Br2O···Br the corresponding CT delocalizations were (n(1) (Br) → σ*(O–Br1)/σ*(O–Br2) and (n(3) (Br) → σ*(O–Br1)/σ*(O–Br2), each with an E(2) of 2.4/1.8 kcal mol-1.
Along similar lines, the complexes of OX2 (X = Cl, Br) and the O end of OCN (22, 33) are described byn (O) → σ*(O–X) CT delocalizations. TheE(2) for these are 1.76 and 2.36 kcal mol-1, respectively. The trend inE(2) is consistent with the trend in ΔE (BSSE) of these complexes. The O···Br secondary interaction in Br2O···OCN is characterized by anE(2) of 0.38 kcal mol-1.
The complex, F2O2···NC, is a result of the π (C≡N) → σ*(O–O) and n (N) → σ*(O–O) CT delocalizations, with E(2) of 0.63 and 1.20 kcal mol-1, respectively. Although the former provides evidence of the involvement of a π···σ interaction, the latter supports the formation of lone-pairσ* type chalcogen bonding. In addition, and because the O atom in O2F2 is not entirely positive, the lone-pair orbital of O facilitates back-bonding CT interactions with the σ* orbital of the C≡N fragment (viz.n (O) → σ*(C≡N)), with E(2) of 1.42 kcal mol-1. Similarly, the complex (CN)2O···O(NO2)features CT delocalizations of n (O) (NO3) → σ*(O–C) ((CN)2O) (E(2) = 0.9 kcal mol-1) and n (O) (NO3) → π*(C≡N) (E(2) = 6.3 kcal mol-1), showing that in addition to the formation of the lone-pairσ* chalcogen bond, the σπ* interaction plays a crucial role in determining the equilibrium geometry of the complex. Most of the complexes discussed above do involve several other weak CT interactions; detailed discussion of these is beyond the scope of this study.
QTAIM’s bond path topologies discussed already above revealed that one of the two O···X interactions in the complexes of OX2with Br3 (14, 25 and 34 of Fig. 1) is marginally stronger than the other. The CT delocalizations corresponding to these two interactions in F2O···Br3 are described by n (terminal Br) → σ*(O–F2) and n (middle Br) → σ*(O–F3), , with E(2) of 1.06 and 0.86 kcal mol-1, respectively. The analogous delocalizations in Cl2O···Br3 weren (terminal Br) → σ*(O–Cl2) and n (middle Br) → σ*(O–Cl3), with E(2) of 0.32 and 1.12 kcal mol-1, respectively. Similarly, these aren (terminal Br) → σ*(O–Br2) and n (middle Br) → σ*(O–Br3) for Br2O···Br3, withE(2) of 0.73 and 1.58 kcal mol-1, respectively.