Adducts of Ferrocenylboranes and Pyridine Bases: Generation of Charge-Transfer Complexes and Reversible Coordination Polymers

Abstract

Compared to parent ferrocene, the redox potential of the FeII/FeIII transition is shifted to much more cathodic values in B–N adducts 1·Do and 2·(Do)2 of borylated ferrocenes FcBMe2 (1) and 1,1′-fc(BMe2)2 (2) with pyridine bases Do [Fc: (C5H5)Fe(C5H4); fc: (C5H4)Fe(C5H4); Do: γ-picoline, 4-(dimethylamino)pyridine, N-(n-propyl)-4-(4′-pyridyl)pyridinium hexafluorophosphate]. Electron donation by one single BMe2·Do substituent at the cyclopentadienyl ligand is approximately equal to the positive inductive effect of the five methyl groups in a C5Me5 (cp*) moiety. Using the bidentate nitrogen ligand 4,4′-bipyridine (bipy), the dark-purple dimetallic complex 1·bipy·1 is obtained upon reaction with 2 equiv. of 1. 1·bipy·1 has been structurally characterized by X-ray crystallography. The compound tolerates loss of two electrons at the ferrocene groups, as well as two one-electron reductions at the bipy linker. Dark purple coloured coordination polymers [2·bipy]n are accessible from bipy and 1 equiv. of the diborylated derivative 2. The intense colours of 1·bipy·1 and [2·bipy]n are indicative of charge-transfer interactions between the electron-rich ferrocene fragments and the viologen-like R3B–bipy–BR3 acceptor. Neat, solid [2·bipy]n is thermally stable up to 240 °C, but in the presence of toluene, polymerization is fully reversible at about 85 °C.