Charge transfer complexes

    A charge transfer complex is one in which a donor and acceptor interact weakly together with some transfer of electronic charge, usually facilitated by the acceptor.   The observed colours of the halogens arise from an electronic transition from the highest occupied π^* MO to the lowest unoccupied σ^* MO. The HOMO–LUMO energy gap decreases in the order F₂ > Cl₂ > Br₂ > I₂, leading to a progressive shift in the absorption maximum from the near-UV to the red region of the visible spectrum.  Dichlorine, dibromine and diiodine dissolve unchanged in many organic solvents (e.g. saturated hydrocarbons, CCl₄). However in, for example, ethers, ketones and pyridine, which contain donor atoms, Br₂ and I₂ (and Cl₂ to a smaller extent) form charge transfer complexes with the halogen σ^* MO acting as the acceptor orbital. In the extreme, complete transfer of charge could lead to heterolytic bond fission as in the formation of [I)py(2]‏⁺ (Figure 16.4 and equation 1.1).

                                                  (1.1)

   Solutions of I₂ in donor solvents, such as pyridine, ethers or ketones, are brown or yellow. Even benzene acts as a donor, forming charge transfer complexes with I₂ and Br₂; the colours of these solutions are noticeably different from those of I₂ or Br₂ in cyclohexane (a non-donor). Whereas amines, ketones and similar compounds donate electron density through a σ lone pair, benzene uses its π-electrons; this is apparent in the relative orientations of the donor (benzene) and acceptor (Br₂) molecules in Figure 1.1b. That solutions of the charge transfer complexes are coloured means that they absorb in the visible region of the spectrum (≈ 400–750 nm), but the electronic spectrum also contains an intense absorption in the UV region (≈ 230–330 nm) arising from an electronic transition from the solvent ^_X₂ occupied bonding MO to a vacant antibonding MO. This is the socalled charge transfer band. Many charge transfer complexes can be isolated in the solid state and examples are given in Figure 1.1.

Fig. 1.1 Some examples of charge transfer complexes involving Br₂; the crystal structure of each has been determined by X-ray diffraction: (a) 2MeCN.Br₂ [K.-M. Marstokk et al. (1968) Acta Crystallogr., Sect. B, vol. 24, p. 713]; (b) schematic representation of the chain structure of C₆H₆_Br₂; (c) 1,2,4,5-(EtS(₄C₆H_2. )Br₂(₂ in which Br₂ molecules are sandwiched between  layers of 1,2,4,5-(EtS)₄C₆H₂ molecules; interactions involving only one Br₂ molecule are shown and H atoms are omitted [H. Bock et al. (1996) J. Chem. Soc., Chem. Commun., p. 1529]; (d) Ph₃P.Br₂ [N. Bricklebank et al. (1992) J. Chem. Soc., Chem. Commun., p. 355]. Colour code: Br, brown; C, grey; N, blue; S, yellow; P, orange; H, white.

   In complexes in which the donor is weak, e.g. C₆H₆, the X^_X bond distance is unchanged (or nearly so) by complex formation. Elongation as in 1,2,4,5-(EtS)₄C₆H₂.(Br₂(₂ (compare the Br^_Br distance in Figure 1.1c with that for free Br₂, in Figure in below) is consistent with the involvement of a good donor; it has been estimated from theoretical calculations that ^_0.25 negative charges are transferred from 1,2,4,5-(EtS)₄C₆H₂ to Br₂.

Different degrees of charge transfer are also reflected in the relative magnitudes of Δ_rH given in equation 1.2. Further evidence for the weakening of the X^_X bond comes from vibrational spectroscopic data, e.g. a shift for ν(X^_X( from 215 cm^-1 in I₂ to 204 cm^-1 in C₆H₆.I₂.

         (1.2)

  Figure 1.1d shows the solid state structure of Ph₃P.Br₂; Ph₃P.I₂ has a similar structure (I^_I = 316 pm). In CH₂Cl₂ solution, Ph₃P.Br₂ ionizes to give [Ph₃PBr]⁺‏Br^- and, similarly, Ph₃PI₂ forms [Ph₃PI]⁺‏I^-  or, in the presence of excess I₂, [Ph₃PI]⁺‏[I₃]^-. The formation of complexes of this type is not easy to predict:

• the reaction of Ph₃Sb with Br₂ or I₂ is an oxidative addition yielding Ph₃SbX₂, 1.1;

• Ph₃AsBr₂ is an As(V) compound, whereas Ph₃As.I₂, Me₃As.I₂ and Me₃As.Br₂ are charge transfer complexes of the type shown in Figure 1.1d.

(1.1)

       The nature of the products from reaction 1.3 are dependent on the solvent and the R group in R₃P. Solid state structure determinations exemplify products of type [R₃PI]⁺‏[I₃]^- (e.g. R = nPr₂N, solvent = Et₂O) and [(R₃PI(₂I₃]‏⁺[I₃]^- (e.g. R = Ph, solvent = CH₂Cl₂; R = iPr, solvent = Et₂O). Structure 1.2 shows the [(iPr₃PI)₂I₃]⁺‏  cation in [(R₃PI)₂I₃][I₃].

                                                        (1.3)

(1.2)