Two Tris(imino)tin(II) and -lead(II) Cage Complexes. Syntheses and Structures of E[μ-N=C (^tBu) Ph]₃Li•THF (E = Sn, Pb)

The reactions of Cp₂Pb and Cp₂Sn with LiN=C(^tBu)Ph (1:1; 2 or 3 equiv) in THF produce the isostructural complexes Pb[μ-N=C(^tBu)Ph]₃Li•THF (1) and Sn[μ-N=C(^tBu) Ph]₃Li•THF (2), respectively. Complex 2 can also be prepared by the facile ligand-exchange reaction of 1 and Cp₂Sn in THE. The crystal structures of both complexes have been obtained at 153 K. Crystal data: 1, orthorhombic, space group P2₁2₁2, a = 16.912(2) Å, b = 19.214(2) Å, c = 11.212(3) Å, Z = 4; 2, monoclinic, space group P₂/n, a = 10.679(2) Å, b = 33.036(7) Å, c = 10.683(2) Å, β= 106.84(3)°, Z = 4. Both complexes have monomeric trigonal bipyramidal EN₃L1 cage structures (E = Pb, Sn) in which there are sharp (ca. 80°) angles at the bridging imino-N centers and close intermolecular E•••Li contacts which are slightly greater than the sum of the covalent radii of the metals [2.85(3) in 1 and 2.776(4) Å in 2]. Model semiempirical (MNDO) calculations suggest that there is no significant E-Li bonding in either complex and that the observed core geometries mainly reflect the optimal bridging angle of the imino ligands. Monomer molecules of 1 exhibit extreme angular distortions within the PbN₃Li core, resulting from two such molecules pairing up in the lattice. Their phenyl groups intermesh, and long-range (ca. 3.26 Å) (ortho)C—H•••Pb contacts are established. In contrast, molecules of 2 have a largely symmetrical core and do not associate in the manner of those of 1 in the solid state. This unexpected structural difference between 1 and 2 may suggest that the (ortho)C—H•••Pb contacts observed in 1 are more than simply van der Waals interactions.