Alkali Metal Induced Structural Changes in Complexes Containing Anionic Lanthanum Aryloxide Moieties. X-ray Crystal Structures of (THF)La(OAr)2(μ-OAr)2Li(THF), (THF)La(OAr)2(μ-OAr)2Na(THF)2, and CsLa(OAr)4 (Ar = 2,6-i-Pr2C6H3)
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Abstract
Reaction of La2(OAr)6 (1, Ar = 2,6-i-Pr2C6H3) with 2 equiv of LiOAr in THF produces the lanthanum tetrakis(aryloxide) salt (THF)La(OAr)2(μ-OAr)2Li(THF) (2). A similar reaction employing NaOAr leads to isolation of the analogous sodium salt (THF)La(OAr)2(μ-OAr)2Na(THF)2 (3). 1 reacts with 2 equiv of cesium aryloxide in THF to yield the base-free cesium salt CsLa(OAr)4 (4). Compounds 2, 3, and 4 have been characterized by 1H NMR and IR spectroscopy, microanalysis, and single-crystal X-ray diffraction studies. The overall molecular geometry of 2 comprises a five-coordinate La metal center coordinated by four aryloxide ligands and one THF ligand in a somewhat distorted trigonal bipyramidal geometry and a three-coordinate Li cation which is coordinated to the oxygen atoms of two aryloxide ligands as well as a single THF ligand. La−O bond lengths average 2.208(3) and 2.384(3) Å for terminal and bridging aryloxide ligands, respectively, while the average Li-O(aryloxide) bond distance is 1.866(10) Å. The molecular geometry of 3 is similar to that observed in 2, with a five-coordinate La metal center displaying a distorted trigonal bipyramidal geometry in which a THF ligand and an aryloxide ligand occupy axial positions. Two of the aryloxide ligands on the La metal center bridge to a sodium cation, which completes its coordination sphere by coordination of two additional molecules of THF. La−O bond lengths average 2.227(6) and 2.333(5) Å for terminal and bridging aryloxide ligands, respectively, while the average Na−OAr bond distance is 2.350(6) Å. The ipso carbon atom of one of the bridging aryloxide ligands is directed toward the fifth coordination site of the sodium cation at a distance of 3.085(9) Å. The solid state structure of CsLa(OAr)4 (4) features alternating tetrahedral [La(OAr)4]- anions (La−O = 2.241(6) Å (ave.)) and Cs+ cations held in an extended structure by means of cesium−π-arene interactions, resulting in the formation of a quasi one-dimensional infinite chain structure. The cesium environment in 4 consists exclusively of multihapto Cs−C interactions (Cs−C range 3.696(7)−3.847(7) Å), with no Cs−O contacts of less than 4.649 Å. Crystal data for 2 (at −100 °C): orthorhombic space group Pbca, a = 20.091(3) Å, b = 19.983(2) Å, c = 27.288(2) Å, V = 10956 Å3, Z = 8, Dcalc = 1.202 g cm-3. Crystal data for 3 (at −100 °C): orthorhombic space group Pbca, a = 26.035(3) Å, b = 20.771(4) Å, c = 26.441(3) Å, V = 14299 Å3, Z = 8, Dcalc = 1.174 g cm-3. Crystal data for 4 (at −100 °C): monoclinic space group C2/c, a = 21.261(2) Å, b = 22.616(3) Å, c = 20.740(2) Å, β = 108.126(7)°, V = 9478 Å3, Z = 8, Dcalc = 1.465 g cm-3.
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