Influence of Bulky N-Substituents on the Formation of Lanthanide Triple Helical Complexes with a Ligand Derived from Bis(benzimidazole)pyridine: Structural and Thermodynamic Evidence

The planar aromatic tridentate ligand 2,6-bis(1-S-neopentylbenzimidazol-2-yl)pyridine (L11) reacts with Ln(III)
(Ln = La−Lu) in acetonitrile to give the successive complexes [Ln(L11)n]3+ (n = 1−3). However, stability constants
determined by spectrophotometry and NMR titrations show that formation of the tris complexes is not favored,
log K3 being around 1 for La(III) and Eu(III), while no such species could be evidenced for the smaller Lu(III)
ion. The X-ray structures of L11 (monoclinic, P21, a = 13.4850(12) Å, b = 12.0243(11) Å, c = 16.4239(14) Å,
β = 103.747(7)°), [La(ClO4)2(L11)2]3[La(ClO4)2(H2O)(L11)2](ClO4)4·15MeCN (1a, monoclinic, P21, a = 21.765(4)
Å, b = 30.769(6) Å, c = 21.541(5) Å, β = 116.01(3)°), and [Eu(L11)3](ClO4)3·4.28MeCN (5a, monoclinic, P1̄,
a = 14.166(3) Å, b = 19.212(4) Å, c = 21.099(4) Å, α = 108.91(3)°, β = 98.22(3)°, γ = 108.40(3)°) have been
solved. In 1a, two different types of complex cations are evidenced, both containing 10-coordinate La(III) ions.
In the first type, both perchlorate anions are bidentate, while in the second type, one perchlorate is monodentate,
the 10th coordination position being occupied by a water molecule. In 5a the three ligands are not equivalent.
Ligands A and B are wrapped in a helical way and are mirror images of each other, while ligand C lies almost
perpendicular to the two other ones. This stems from the steric hindrance generated by the bulky neopentyl groups
with the consecutive loss of any stabilizing interstrand π-stacking interactions. This explains the low stability of
the tris complexes and the difficulty of isolating them and points to the importance of the steric factors in the
design of self-assembled triple helical lanthanide-containing functional edifices [Ln(Li)3]3+.