Three conformational polymorphs of di-mu-chlorotetrakis(1-methylboratabenzene)diyttrium: synthesis, x-ray structures, quantum chemical calculations, and lattice energy minimizations

The reaction of yttrium trichloride with lithium 1-methylboratabenzene (1/2) in toluene (110 degrees C, 3 days) afforded the donor-free dinuclear sandwich complex (C(5)H(5)BMe)(2)Y(mu-Cl)](2) (1) in 85% yield as pale-yellow crystals. By means of single crystal and powder diffraction methods, three conformational polymorphs, alpha-1 P2(1)/n (No. 14), monoclinic, a = 6.6124(8) A, b = 14.352(9) A, c = 14.120(1) A, beta = 95.57(1) degrees, V = 1333.7(9) A(3), Z = 2], beta-1 P2(1)/a (No. 14), monoclinic, a = 8.542(2) A, b = 13.712(6) A, c = 11.76(1) A, beta = 102.60(4) degrees, V = 1344.5(13) A(3), Z = 2], and gamma-1 Pbca (No. 61), orthorhombic, a = 20.091(5) A, b = 13.527(3) A, c = 9.976(2) A, V = 2711.2(11) A(3), 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 degrees 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 beta conformation is 2.8 and 7.2 kJ/mol more stable than the alpha and gamma conformations, respectively. Lattice energy minimizations predict that the alpha-1 phase is about 5.5 and 18.7 kJ mol(-)(1) more stable than the beta-1 and gamma-1 modifications, in agreement with the packing coefficients and the molecular volumes of the three crystal structures. While the alpha-1 and beta-1 modifications have comparable total energies, the gamma-1 form is less stable. The total energy differences among the polymorphs are greater than generally expected.