Carbene Complexes of Stannocenes

Abstract

Several stannocene carbene complexes, 3a-3g, were synthesized and examined in solution by NMR spectroscopy and in the solid state by single-crystal X-ray diffraction. In this new class of metallocene carbene complexes, coordination of the carbene to the tin atom was found to be comparably weak and mostly due to attractive dispersion forces, as indicated by density functional theory calculations. Furthermore, coordination of the N-heterocyclic carbenes results in a weakening of the Sn-Cp bonds, making these complexes very reactive and short-lived at room temperature.

On the Bonding Situation in Stannocene and Plumbocene N-Heterocyclic Carbene Complexes

“…As described earlier, the stannocene and plumbocene NHC complexes exist in a dissociative equilibrium in solution at room temperature, but can be isolated as crystalline solids at low temperatures. In addition to the already reported stannocene-and plumbocene-NHC complexes, 1a-e, 2b-d, 3b, 9,13 we were able to isolate and structurally characterize the 1,3,4,5-tetramethylimidazole-2-ylidene-stannocene adduct, 2a, and, more significantly, 1,3-dimesitylimidazole-2-ylidene-plumbocene complexes 3a,c (Figure 1). The latter represent rare examples of bis(cyclopentadienyl)lead(II) carbene complexes and allow for structural comparison to their previously reported tin analogues.…”

“…This goes in line with experimental observations that 1c is only marginally stable at room temperature and decomposes within minutes to hours in solution, while 2c is stable at ambient conditions for longer periods. 9 Lead complexes show stronger bonded NHC ligands than the tin complexes regardless the bond distances are longer. Thus, the bond length does not directly correlate with the bondstrength.…”

“…This has been rationalized by the shape of the frontier molecular orbitals. 9 While the HOMO and HOMO-1 in tetrelocenes correspond to the  system of the cyclopentadienyl ligands, the LUMO exhibits a large coefficient at the central atom in the shape of a p orbital, perpendicular to the Cp-E-Cp plane. In addition, the energetically low lying lone pair spatial demand also contributes to this coordination mode (see infra Figure 5).…”

“…Figure 3. Calculated geometries of complexes 1-5 with selected bond length (in pm) at B3LYP+D3/def2-TZVP (experimental values in parenthesis); 9,15,16 Hydrogen atoms omitted for clarity.…”

“…Among others, a well-recognized and particularly important class of such compounds are heavy tetrylene-Surprisingly, the reactivity of the longest known diorganotetrylene-type compounds, stannocene, and plumbocene, towards NHCs is almost unexplored. 9 This might be because these metallocenes are often not regarded as "true" tetrylenes, since they lack any strong  donor abilities, but rather exclusively exhibit Lewis acidic character. While complexes of plumbocene and stannocene with nitrogen bases are known since the 1990s and NHC complexes of group 2 and group 4 metallocenes have also been described, the first isolation and structural characterization of group 14 metallocene (tetrelocene) NHC complexes has only recently been reported by the Schäfer group (Chart 1).…”