Synthesis, Structure, and Bonding Analysis of Tin(II) Dihalide and Cyclopentadienyltin(II) Halide (Alkyl)(amino)carbene Complexes

Abstract

Tin­(II) dibromide, 2b, and cyclopentadienyltin­(II) chloride, 2c, and cyclopentadienyltin­(II) bromide, 2d, were reacted with cyclic (alkyl)­(amino)­carbene (cAAC), 1, to give the corresponding complexes cAAC·SnBr2, 3b, cAAC·SnCpCl, 3c, and cAAC·SnCpBr, 3d, which were studied in solution and in the solid state. Although isolatable as crystalline solids, 3c,d exist in a heteroleptic/homoleptic equilibrium with other tin­(II) species, in solution. In addition, the coordination chemistry of 3b was investigated and the corresponding iron tetracarbonyl complex, 5, could be isolated and structurally characterized. Furthermore, the mechanism of the equilibrium reaction observed for 3c,d in solution, as well as the chemical bonding nature of all cAAC complexes were investigated by quantum-chemical calculations within the DFT framework.

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.…”

“…We have recently observed such a behavior when low energy gain by π-backdonation is combined with steric hindrance. 10 We have taken compounds 4 and 5a,b to compare the coordination mode. NBO results in Table 1 reveal that the metallocene moieties bear less negative charge than in complexes 1-3.…”

“…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). [10][11][12] It was recognized early on that these complexes show much higher reactivity than their reactants, resulting in low thermal stability, requiring isolation and characterization at low temperatures in some cases. Furthermore, these complexes all exhibit unusually long metal-carbon bonds and the complexation energy of the NHC to the metallocene is rather small.…”

Mechanistic elucidation of monoalkyltin(iv)-catalyzed esterification

“…To complement our experimental findings under catalytically relevant conditions, we have further examined the reaction mechanism for the n-BuSn(OAc) 3 -catalyzed esterification by DFT calculations at the BP86-D3/def2-TZVP// M06-2X-D3/def2-TZVP/def2-QZVPP level of theory (Scheme 3). 31 The observed off-cycle resting states, monomer 3 and dimer 5A, are equal in free energy, and both can lead to the formation of intermediate D (ΔG = 4.5 kcal mol −1 ) which is the starting points of the catalytic cycle. The tin-bound ethoxy group can subsequently accommodate another ethanol molecule via a hydrogen bond (structure E).…”

Carbene‐Stabilized Dithiolene (L0) Zwitterions

“…Due to poor solubility in benzene (or toluene) at room temperature, the 13 C NMR spectrum of 3 (in C 6 D 6 ) was determined at an elevated temperature ( T =68 °C) [8] . The 13 C NMR resonance of the carbene carbon in 3 [C(28) in Figure 2] (in C 6 D 6 ) is observed at 208.53 ppm, which shifts upfield compared to that (313.6 ppm) of the free Me CAAC ligand (in C 6 D 6 ) [14a] …”