Extreme environmental sensitivity of the rate of intramolecular electron transfer in mixed-valence 1',1'''-dibenzylbiferrocenium triiodide

Abstract

The extreme sensitivity of the rate of intramolecular electron transfer for the mixed-valence 1',1'-dibenzylbiferrocenium cation to different environments is explored. Two different crystalline morphologies of 1',1'-dibenzylbiferronium triiodide (1) could be identified by their different crystal habits (needle and plate-like crystals). They are structurally characterized by single-crystal and powder X-ray diffraction techniques. The single-crystal X-ray structure of a parallelepiped (needle) crystal of 1 at 298 K shows that this polymorph crystallizes in the triclinic space group Pi. The unit cell parameters are a = 9.900 (1) A, b = 9.962 (1) A, c = 10.193 (2) A, CY = 112.97 (2)O, @ = 114.50 (I)O, and y = 97.80 (1)O with Z = 1. The structure is also reported for the same crystal at 135 K. An analysis of the full sphere of data again indicates Pi as the appropriate space group with unit cell constants a = 9.782 (2) A, b = 9.930 (1) A, c = 10.108 (2) A, CY = 113.08 (2)O, @ = 114.08 and y = 98.06 (1)' with 2 = 1. Refinements were carried out with 2990 (6a) and 3315 (6a) observed reflections at 298 and 135 K, respectively, to give R = 0.026 and R, = 0.037 at 298 K and R = 0.025 and R, = 0.038 at 135 K. Both the mixed-valence l', 1'-dibenzylbiferrocenium cation and triiodide anion are centrosymmetric at both temperatures. The two crystallographically equivalent metallocene moieties of the cation have dimensions intermediate between those of Fe and Fell' metallocenes. The packing of 1 consists of weakly interacting 1',1'-dibenzylbiferronium cations forming chains which are surrounded by and separated from other similar chains by the triiodide anions. The 296 K single-crystal X-ray structure of the plate-like crystals of complex 1 show that this pol morph crystallizes in the monoclinic space group P2,/n. The unit cell parameters are a = 22.286 (9) A, b = 19.413 (10) 1, c = 22.840 (5) A, and @ = 98.55 (3)O with Z = 12. Refinement was carried out with 4486 (6a) observed reflections to give R = 0.089 and R, = 0.131. There are three different l',l'- dibenzylbiferrocenium cations and three different Ip- anions in the asymmetric unit. There is not crystallographically imposed symmetry for any of the cations or anions. The temperature dependencies of the S7Fe Mossbauer spectra of the Pi and P2,/n polymorphs of complex 1 differ widely. A sample of needle crystallites has a Mossbauer spectrum characteristic of a valence detrapped mixed-valence species down to a temperature of 25 K. On the other hand, a sample of the P2,/n plate-like crystals exhibits a Mhsbauer spectrum characteristic of a valence-trapped species which remains trapped even at 300 K. It was surprising to find that mild grinding of the needle crystallites produces a valence-trapped Mossbauer signal in addition to the valence detrapped signal. Appreciable amounts of the trapped species persist for the ground Pi needles even up to 300 K. At 7 K, the EPR spectrum of a recrystallized microcrystalline sample of 1 consists of the superposition of three signals: two axial powder patterns each with its own gH and g, signals and one isotropic g = 2.16 signal. The g tensor anisotropies of the two axial signals are Ag = 1.29 and 0.76. The axial pattern with Ag = 0.76 is assigned to the Pi form, whereas, the Ag = 1.29 pattern is assigned to the P2,/n form. The two axial EPR patterns broaden with increasing temperature and the isotropic g = 2 signal gains in relative intensity, so that by 150 K it is the only signal observed. This Lorentzian g = 2.16 signal persists to high temperatures. In contrast, the variable-temperature EPR spectrum of an ethanol glass of 1 shows a single axial signal (Ag = 1.82) which broadens with increasing temperature and becomes unobservable above -77 K. The EPR properties of mixed-valence l', 1'-dibenzylbiferrocenium cation are modified by the local environments. Not only does the EPR signal change from the Pi to the P2Jn crystal forms but also there is even a sample history dependence of the g values for the two crystal forms. The g = 2.16 signal is the result of intermolecular interactions.