Krossing I
University of Karlsruhe, Germany.
Chemistry. 2001 Jan 19;7(2):490-502. doi: 10.1002/1521-3765(20010119)7:2<490::aid-chem490>3.0.co;2-i.
Purified LiAlH4 reacts with fluorinated alcohols HORF to give LiAl(ORF)4 (RF=-CH(CF3)2, 2a; -C(CH3)(CF3)2, 2b; -C(CF3)3, 2c) in 77 to 90% yield. The crude lithium aluminates LiAl(ORF)4 react metathetically with AgF to give the silver aluminates AgAl(ORF)4 (RF=-CH(CF3)2, 3a; -C(CH3)(CF3)2, 3b; -C(CF3)3, 3c) in almost quantitative yield. The solid-state structures of solvated 3a-c showed that the silver cation is only weakly coordinated (CN(Ag)=6-10; CN = coordination number) by the solvent and/or weak cation - anion contacts Ag-X (X=O, F, Cl, C). The strength of the Ag-X contacts of 3a-c was analysed by Brown's bond-valence method and then compared with other silver salts of weakly coordinating anions (WCAs), for example [CB11H6Cl6]- and [M(OTeF5)n]- (M=B, Sb, n=4, 6). Based on this quantitative picture we showed that the Al[OC(CF3)3]4 anion is one of the most weakly coordinating anions known. Moreover, the AgAl(ORF)4 species are certainly the easiest WCAs to access preparatively (20 g in two days), additionally at low cost. The Al-O bond length of Al(ORF)4- is shortest in the sterically congested Al[OC(CF3)3]4- anion-which is stable in H2O and aqueous HNO3 (35 weight%)--and indicates a strong and highly polar Al-O bond that is resistant towards heterolytic alkoxide ion abstraction. This observation was supported by a series of HF-DFT calculations of OR-, Al(OR)3 and Al(OR)4- at the MPW1PW91 and B3LYP levels (R= CH3, CF3, C(CF3)3). The alkoxide ion affinity (AIA) is highest for R=CF3 (AlA=384 +/- 9 kJ x mol(-1)) and R= C(CF3)3 (AIA=390 +/- 3 kJ x mol(-1)), but lowest for R=CH3 (AIA=363 +/- 7 kJ X mol(-1)). The gaseous AL(ORF)4-anions are stable against the action of the strong Lewis acid ALF3(g) by 88.5 +/- 2.5 (RF=CF3) and 63 +/- 12 kJ X mol(-1) (RF=C(CF3)3), while AL(OCH3)4- decomposes with -91 +/- 2 kJ X mol(-1). Therefore the presented fluorinated aluminates AL(ORF)4- appear to be ideal candidates when large and resistant WCAs are needed, for example, in cationic homogenous catalysis, for highly electrophilic cations or for weak cationic Lewis acid/base complexes.
纯化的氢化铝锂（LiAlH₄）与氟化醇HORF反应，以77%至90%的产率生成LiAl(ORF)₄（RF = -CH(CF₃)₂，2a；-C(CH₃)(CF₃)₂，2b；-C(CF₃)₃，2c）。粗制的铝酸锂LiAl(ORF)₄与AgF发生复分解反应，以几乎定量的产率生成铝酸银AgAl(ORF)₄（RF = -CH(CF₃)₂，3a；-C(CH₃)(CF₃)₂，3b；-C(CF₃)₃，3c）。溶剂化的3a - c的固态结构表明，银阳离子仅通过溶剂和/或弱的阳离子 - 阴离子接触Ag - X（X = O、F、Cl、C）进行弱配位（CN(Ag)=6 - 10；CN = 配位数）。通过布朗的键价法分析了3a - c中Ag - X接触的强度，然后与其他弱配位阴离子（WCA）的银盐进行比较，例如[CB₁₁H₆Cl₆]⁻和[M(OTeF₅)ₙ]⁻（M = B、Sb，n = 4、6）。基于这一定量描述，我们表明Al[OC(CF₃)₃]₄阴离子是已知的最弱配位阴离子之一。此外，AgAl(ORF)₄物种无疑是制备上最容易获得的WCA（两天内可制备20克），而且成本低廉。在空间位阻较大的Al[OC(CF₃)₃]₄⁻阴离子中，Al(ORF)₄⁻的Al - O键长最短，该阴离子在水和35%（重量）的硝酸水溶液中稳定，这表明存在一个强且高度极化的Al - O键，它能抵抗异裂醇盐离子的夺取。这一观察结果得到了在MPW1PW91和B3LYP水平上对OR⁻、Al(OR)₃和Al(OR)₄⁻（R = CH₃、CF₃、C(CF₃)₃）进行的一系列HF - DFT计算的支持。对于R = CF₃（AlA = 384 ± 9 kJ·mol⁻¹）和R = C(CF₃)₃（AlA = 390 ± 3 kJ·mol⁻¹），醇盐离子亲和力（AIA）最高，但对于R = CH₃（AIA = 363 ± 7 kJ·mol⁻¹）最低。气态的Al(ORF)₄⁻阴离子对强路易斯酸AlF₃(g)的作用具有稳定性，对于RF = CF₃时为88.5 ± 2.5 kJ·mol⁻¹，对于RF = C(CF₃)₃时为63 ± 12 kJ·mol⁻¹，而Al(OCH₃)₄⁻以 - 91 ± 2 kJ·mol⁻¹的能量分解。因此，当需要大的且稳定的WCA时，例如在阳离子均相催化中用于高亲电阳离子或弱阳离子路易斯酸/碱配合物时，所呈现的氟化铝酸盐Al(ORF)₄⁻似乎是理想的候选物。
