Ferrocene-Based Bis(guanidines): Superbases for Tridentate N,Fe,N-Coordination

The
condensation of 1,1′-diaminoferrocene (1) with
a range of chloroformamidinium salts [(R′RN)2CCl]­X
(2) afforded the corresponding bis­(guanidines)
[Fe­{(η5-C5H4)–NC­(NRR′)2}2] (3; five examples). The two guanidine
units present in 3 are able to chelate a proton or a
water molecule. The structures
of 3 and of the corresponding hydrates were determined
by single-crystal X-ray diffraction. Cyclic voltammetry revealed that
the ferrocene moiety of 3 is exceptionally electron rich,
with E1/2 values down to −0.90
V vs the ferrocenium/ferrocene couple. According to the results of
DFT calculations, these bis­(guanidines) are superbases with proton
affinities considerably higher than that of the classic “Proton
Sponge” 1,8-bis­(dimethylamino)­naphthalene. Furthermore, they
are able to act as tridentate ligands, giving rise to Fe→M
dative bonds, as is shown by chelates of the type [PdCl­(3-κFe,κ2N)]+, which were obtained by reaction of 3 with [PdCl2(MeCN)2]. The unusual N,Fe,N-chelating
mode leads to exceptionally short Fe–Pd bonds (ca. 2.69 Å)
and causes an enormous coordination-induced anodic shift of the half-wave
potential of ca. 0.6 V.