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Three Conformational Polymorphs of Di-μ-chlorotetrakis(1-methylboratabenzene)diyttrium:  Synthesis, X-ray Structures, Quantum Chemical Calculations, and Lattice Energy Minimizations1

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journal contribution
posted on 19.05.2001, 00:00 by Xiaolai Zheng, Bing Wang, Ulli Englert, Gerhard E. Herberich
The reaction of yttrium trichloride with lithium 1-methylboratabenzene (1/2) in toluene (110 °C, 3 days) afforded the donor-free dinuclear sandwich complex [(C5H5BMe)2Y(μ-Cl)]2 (1) in 85% yield as pale-yellow crystals. By means of single crystal and powder diffraction methods, three conformational polymorphs, α-1 [P21/n (No. 14), monoclinic, a = 6.6124(8) Å, b = 14.352(9) Å, c = 14.120(1) Å, β = 95.57(1)°, V = 1333.7(9) Å3, Z = 2], β-1 [P21/a (No. 14), monoclinic, a = 8.542(2) Å, b = 13.712(6) Å, c = 11.76(1) Å, β = 102.60(4)°, V = 1344.5(13) Å3, Z = 2], and γ-1 [Pbca (No. 61), orthorhombic, a = 20.091(5) Å, b = 13.527(3) Å, c = 9.976(2) Å, V = 2711.2(11) Å3, Z = 4], were characterized in the solid state of 1. The molecules in the three phases vary remarkably in the rotational position of boratabenzene ligands with differences of 91.1, 133.1, and 24.9° between each pair. DFT calculations at the B3LYP/LanL2DZ level reveal that the three molecular structures observed in the solid state correspond closely to three minima on the gas-phase potential energy surface. The β conformation is 2.8 and 7.2 kJ/mol more stable than the α and γ conformations, respectively. Lattice energy minimizations predict that the α-1 phase is about 5.5 and 18.7 kJ mol-1 more stable than the β-1 and γ-1 modifications, in agreement with the packing coefficients and the molecular volumes of the three crystal structures. While the α-1 and β-1 modifications have comparable total energies, the γ-1 form is less stable. The total energy differences among the polymorphs are greater than generally expected.