Structural and Photophysical Properties of Mononuclear and Dinuclear Lanthanide(III) Complexes of Multidentate Podand Ligands Based on Poly(pyrazolyl)borates

Lanthanide(III) complexes have been prepared with [L1]- [the tetradentate chelating ligand bis{3-(2-pyridyl)pyrazolyl}dihydroborate], [L2]- [the tetradentate chelating ligand bis{3-(2-pyrazinyl)pyrazolyl}dihydroborate],
[L3]- [the hexadentate chelating ligand bis[3-{6‘-(2,2‘-bipyridyl)}pyrazol-1-yl]dihydroborate], and [L4]2- [the 12-dentate compartmental ligand hexakis{3-(2-pyridyl)pyrazol-1-yl}diboran(IV)ate, which has two hexadentate tris(pyrazolyl)borate-based cavities linked “back-to-back” by a B−B bond]. [Ln(L1)2(NO)3] are 10-coordinate with
two tetradentate N-donor ligands and one bidentate nitrate. [Ln(L2)2(NO)3] have 10-coordinate structures similar
to those of the [L1]- complexes except that the coordinated N1 of the pyrazine rings is not such a good donor as
the pyridine rings in the [L1]- complexes, leading to marked lengthening of these Ln−N bonds. [Ln(L3)(NO3)2]
are also 10-coordinate from one hexadentate chelating ligand which has a pseudoequatorial coordination mode
and two pseudoaxial bidentate nitrate ligands; the hexadentate ligand has a shallow helical twist to prevent steric
interference between its ends. Finally [{Ln(NO3)2(L4)] are dinuclear, with each metal center being 10-coordinate
from a tripodal hexadentate ligand cavity and two bidentate nitrates. Five complexes were structurally
characterized:  [Tb(L2)2(NO3)]·dmf is monoclinic (P21/c) with a = 14.881(3) Å, b = 15.5199(12) Å, c = 15.845(2)
Å, β = 92.387(12)°, and Z = 4. [Gd(L2)2(NO3)]·dmf is monoclinic (P21/c) with a = 14.926(2) Å, b = 15.465(2)
Å, c = 15.878(2) Å, β = 92.698(11)°, and Z = 4. [Eu(L3)(NO3)2]·dmf·0.5Et2O is triclinic (P1̄) with a = 10.020(3)
Å, b = 13.036(3) Å, c = 14.740(3) Å, α = 70.114(14)°, β = 71.55(2)°, γ = 79.66(2)°, and Z = 2. [{La(NO3)(dmf)2}2(L4)](NO3)2·dmf is orthorhombic (Pbca) with a = 18.813(2) Å, b = 15.241(2) Å, c = 27.322(2), and Z
= 4. [{Gd(NO3)2}2(L4)]·2.4dmf is tetragonal (P42/n) with a = 16.622(6), c = 24.19(5) Å, and Z = 4. Detailed
photophysical studies have been performed on the free ligands and their complexes with Gd(III), Eu(III), and
Tb(III) in several solvents. The results show a wide range in the emission properties of the complexes which can
be rationalized in terms of subtle variations in the steric and electronic properties of the ligands. In particular the
dinuclear Tb(III) complex of [L4]2- has an emission quantum yield of ca. 0.5 in D2O and MeOD.