Key Role of the Cation in the Crystallization of Chiral Tris(Anilato)Metalate Magnetic Anions

A complete study of the role played
by the usually considered “innocent”
cation in the synthesis of chiral tris­(anilato)­metalate magnetic complexes
is presented. This study is based on the rational synthesis of the
family of compounds formulated as A3[MIII(C6O4X2)3] with A+ = [PPh3Et]+, [PPh3Pr]+, [PBu4]+, [NHep4]+,
and [PPh4]+; MIII = CrIII, FeIII, and GaIII; and [C6O4X2]2– (X = Cl, Br, and NO2; [C6O4X2]2– = dianion of the 3,6-disubstituted derivatives of 2,5-dihydroxy-1,4-benzoquinone,
H4C6O4). We show and explain the
unexpected key role played by the cations in isolating chiral or achiral
crystals of these [MIII(C6O4X2)3]3– anions with D3 point group symmetry. Thus, among the 18 new compounds
prepared and characterized, the six obtained with the cation [PPh3Et]+, formulated as [PPh3Et]3[MIII(C6O4X2)3]3– [M/X = Cr/Cl (1), Fe/Cl (2), Ga/Cl (3), Cr/Br (4), Fe/Br
(5), and Ga/Br (6)] are chiral and crystallize
in the chiral and polar R3 space group. These mixtures of enantio-pure
chiral crystals are obtained using a racemic mixture of the anions
and cations. Interestingly, if we use the [MIII(C6O4X2)3]3– (X =
NO2) anions or if we change the cation (A), then we obtain
achiral salts presenting the same A3[MIII(C6O4X2)3] formula (X = Cl,
Br, and NO2) and the same monomeric D3 structure for the anion. In these last cases the anions crystallize
in the polar but not chiral P2cb space group (for X = NO2) or in the centrosymmetric (which
is neither chiral nor polar) P1̅ space group
for A+ = [PPh3Pr]+. These salts include
the following: [PPh3Et]3[Fe­(C6O4(NO2)2)3] (7), [PPh3Pr]3[MIII(C6O4X2)3] [M/X = Cr/Cl (8),
Fe/Cl (9), Ga/Cl (10), Cr/Br (11), Ga/Br (12), and Fe/NO2 (13)], [PBu4]3[MIII(C6O4X2)3] [M/X = Cr/NO2 (14) and Fe/NO2 (15)], [NHep4]3[MIII(C6O4X2)3] [M/X = Cr/Cl (16), Cr/Br (17)] and [PPh4]3[Fe­(C6O4(NO2)2)3] (18). Here
we report the synthesis of all these salts and the crystal structure
of compounds 2, 5, 9, 10, 14, and 15, as well as the spectroscopic
and powder X-ray characterization of all the members of this huge
family of compounds. Compound 10 contains the unknown
[Ga­(C6O4X2)3]3– anion, whereas compounds 14 and 15 are
the two first complexes of the type [M­(C6O4X2)3]3– with X = NO2. The magnetic properties of all the salts containing paramagnetic
Fe­(III) and Cr­(III) ions are also reported. All of them have been
satisfactorily reproduced with a simple monomeric model including
zero field splitting (ZFS) of the corresponding S = 5/2 Fe­(III) and S = 3/2 Cr­(III) metal ions. This
series constitutes a huge library of valuable precursors for the synthesis
of extended 1D, 2D, and 3D coordination polymers with tunable magnetic
ordering, chirality, and even porosity.