Helmholtz-Zentrum Dresden-Rossendorf

The reactions of organosiliconpyridine-2-olates (pyridyl-2-oxysilanes) RSi(pyO)₃ (pyO = pyridine-2-olate, R = Me (1a), Ph (1b), Bn (1c) and Allyl (1d)) and [PdCl₂(NCMe)₂] in chloroform afforded the hexacoordinate silicon complexes RSi(μ²-pyO)₄PdCl (R = Me (2a), Ph (2b), Bn (2c) and Allyl (2d), respectively), which feature a Pd–Si bond, in which the Pd atom is the formal lone pair donor toward Si. The new compounds 2b, 2c, 2d were characterized with multi-nuclear NMR spectroscopy and elemental analysis. The effect of the Si-bound substituent R on the trans-disposed Pd–Si bond was studied by single-crystal X-ray diffraction and computational analyses (e.g., Natural Localized Molecular Orbitals, NLMO; topological analyses of the electron density at the bond critical point with Quantum Theory of Atoms-In-Molecules, QTAIM). A structurally related byproduct, (η³-allyl) ClPd(pyO)Si(μ²-pyO)₄PdCl 2d’, which formed along with target product 2d and features an Si–O bond trans to Pd–Si, was included in this systematic study. Another byproduct from the synthesis of 2d, the pentanuclear complex ClPd(μ²-pyO)₂Si(μ²-pyO)₂Pd(μ²-pyO)₂Si(μ²-pyO)₂PdCl (compound 3) was characterized crystallographically. This compound features pentacoordinate Si atoms within trigonal–bipyramidal Si(O₄Pd) coordination spheres with equatorial Pd–Si bonds to the terminal Pd atoms. The Pd–Si bond situation in this compound was elucidated with the aid of computational analyses. QTAIM analyses of 3 in conjunction with a model compound PdSi4, which features two silyl groups and two silylene ligands, indicate topological properties of the electron density at the Pd–Si bond critical point which are similar to Pd–Si bonds of silyl and silylene compounds. The latter exhibit greater similarity, which indicates features of a Pd←Si bond. In contrast, NLMO analyses of 3 identify a polar covalent Pd–Si bond with predominant Pd contribution (formal Pd→Si donation).