138312-84-2Relevant academic research and scientific papers
Structure and photochemistry of new base-stabilized silylene (silanediyl) complexes R2(HMPA)Si=M(CO)n of iron, chromium, and tungsten (R = t-BuO, t-BuS, MesO, 1-AdaO, 2-AdaO, NeopO, TritO, Me, Cl; M = Fe, n = 4; M = Cr, W, n = 5): Sila-Wittig reaction of the base-free reactive intermediate [Me2Si=Cr(CO)5] with dimethyl carbonate
Leis, Christian,Wilkinson, Dallas L.,Handwerker, Hermann,Zybill, Christian,Müller, Gerhard
, p. 514 - 529 (2008/10/08)
The reaction of the dichlorosilanes R2SiCl2 (R = t-BuO, t-BuS, MesO (Mes = mesityl), 1-AdaO (Ada = adamantyl), 2-AdaO, NeopO (Neop = neopentyl), TritO (Trit = triphenylmethyl), Me, Cl) with the carbonylate dianions M(CO)n2- (M = Fe, n = 4; M = Cr, W, n = 5) in the presence of HMPA (hexamethylphosphoric triamide) affords an effective access to the base-stabilized silylene complexes R2-(HMPA)Si=M(CO)n. Single-crystal X-ray structure analyses of the iron complexes Me2(HMPA)Si=Fe(CO)4 (4) and Cl2(HMPA)Si=Fe(CO)4 (5) and the chromium compounds Me2(HMPA)Si=Cr(CO)5 (7) and Cl2(HMPA)Si=Cr(CO)5 (8) have been performed and are compared with the previously determined structures of (t-BuO)2(HMPA)Si=Fe(CO)4 (1) and (t-BuO)2(HMPA)Si=Cr(CO)5 (6). The structures feature relatively short M-Si bond lengths with d(FeSi) = 2.280 (1)/2.294 (1) (4) and 2.214 (1)/2.221 (1) ? (5) and d(CrSi) = 2.410 (1) (7) and 2.342 (1) (8). In contrast, the Si-O (HMPA) bonds are fairly long (1.735 (3)/1.731 (3) (4), 1.683 (3)/1.676 (3) (5), 1.743 (2) (7), 1.690 (2) ? (8)). The coordination geometry at silicon is significantly distorted from tetrahedral and subject to strong substituent effects. Both metal-silicon bond lengths and the coordination geometry at silicon in the coordination compounds correlate to frontier orbital energies and electron population densities of the free silylenes. Data from cyclic voltammetric measurements are in agreement with this description. A M?ssbauer spectrum of 1 shows an unusual negative isomer shift of -0.477, which indicates particularly for the TBP d8 compounds 1-5 (axial silylene coordination) a high ionic contribution of the Fe-Si bond in the sense of a betaine structure. The relative orientation of the substituents at silicon, as found in the crystal, is reproduced by a force field calculation as the global energy minimum. VT NMR experiments show the fluxionality of the ligand framework of the TBP Fe d8 complexes at room temperature. The free energy of activation for the Berry pseudorotation process is ΔG250.0? = 40.3 (±5) kJ/mol. A reactivity study of the base adducts 1, 4, and 6 shows the photochemical activation of the silylenes: In the presence of triphenylphosphine, the trans phosphine complexes trans-(t-BuO)2Si(HMPA)=Fe(CO)3P(C6H 5)3 (23), trans-Me2Si(HMPA)=Fe(CO)3P(C6H 5)3 (24), and trans-(t-BuO)2Si(HMPA)=Cr(CO)4P(C6H 5)3 (26) are obtained by a photochemical CO displacement reaction. Subsequently, the silylenes are displaced by a further phosphine molecule to yield the trans-bis(phosphine) complexes trans-{P(C6H5)3}2Fe(CO)3 (25) and trans-{P(C6H5)3}2Cr(CO)4 (27). The liberated silylenes form polysilanes; di-tert-butoxysilylene can be trapped in the presence of 2,3-dimethylbutadiene by an 1,4-addition to form the silacyclopentene Si(t-BuO)2CH2C(CH3)=C(CH3)CH 2 (28) (photolysis of 1). As a secondary photolysis product, (2,3-dimethylbutadiene)iron tricarbonyl, (η4-C6H4)Fe(CO)3 (29), is isolated. The base-free complex [Me2Si=Cr(CO)5] is generated in situ from Me2SiCl2 and Na2Cr(CO)5 at -50°C as a reactive intermediate and yields a variety of interesting trapping reactions: in the presence of HMPA the stable HMPA adduct 7 is formed; the thermolabile THF adduct Me2Si(THF)=Cr(CO)5 (32) dimerizes above -40°C to give 21. With dimethyl carbonate as the trapping reagent, [Me2Si=Cr(CO)5] undergoes a sila-Wittig reaction to form hexamethyltrisiloxane, [Me2SiO-]3 (30), and (dimethoxycarbene)chromium pentacarbonyl, (MeO)2C=Cr(CO)5 (31).
