ABSTRACT
The overall neutral α-borylated phosphorus ylide Ph3PC(Me)BEt2 behaves like a polar borataalkene and can act as acyclic, ambiphilic π-type ligand with L- and Z-type functionalities side by side. In the complexes [MX{η2-Ph3PC(Me)BEt2}] (M = Cu, (Ag), Au; X = Cl, NTf2), the bonding is dominated by the highly nucleophilic ylidic carbon atom (L-type ligand). The Lewis acidic boron atom furnishes nonetheless a small but significant bonding contribution (Z-type ligand).
ABSTRACT
The synthesis and full characterization of α-silylated (α-SiCPs; 1-7) and α-germylated (α-GeCPs; 11-13) phosphorus ylides bearing one chloride substituent R3 PC(R1 )E(Cl)R2 2 (R=Ph; R1 =Me, Et, Ph; R2 =Me, Et, iPr, Mes; E=Si, Ge) is presented. The molecular structures were determined by X-ray diffraction studies. The title compounds were applied in halide abstraction studies in order to access cationic species. The reaction of Ph3 PC(Me)Si(Cl)Me2 (1) with Na[B(C6 F5 )4 ] furnished the dimeric phosphonium-like dication [Ph3 PC(Me)SiMe2 ]2 [B(C6 F5 )4 ]2 (8). The highly reactive, mesityl- or iPr-substituted cationic species [Ph3 PC(Me)SiMes2 ][B(C6 F5 )4 ] (9) and [Ph3 PC(Et)SiiPr2 ][B(C6 F5 )4 ] (10) could be characterized by NMR spectroscopy. Carrying out the halide abstraction reaction in the sterically demanding ether iPr2 O afforded the protonated α-SiCP [Ph3 PCH(Et)Si(Cl)iPr2 ][B(C6 F5 )4 ] (6 dec) by sodium-mediated basic ether decomposition, whereas successfully synthesized [Ph3 PC(Et)SiiPr2 ][B(C6 F5 )4 ] (10) readily cleaves the F-C bond in fluorobenzene. Thus, the ambiphilic character of α-SiCPs is clearly demonstrated. The less reactive germanium analogue [Ph3 PC(Me)GeMes2 ][B{3,5-(CF3 )2 C6 H3 }4 ] (14) was obtained by treating 11 with Na[B{3,5-(CF3 )2 C6 H3 }4 ] and fully characterized including by X-ray diffraction analysis. Structural parameters indicate a strong CYlide -Ge interaction with high double bond character, and consequently the C-E (E=Si, Ge) bonds in 9, 10 and 14 were analyzed with NBO and AIM methods.
ABSTRACT
The influence of substituents on α-borylated phosphorus ylides (α-BCPs) has been investigated in a combined experimental and quantum chemical approach. The synthesis and characterization of Me3 PC(H)B(iBu)2 (1), consisting of small Me substituents on phosphorous and iBu residues on boron, is reported. Compound 1 is accessible through a novel synthetic approach, which has been further elucidated through DFT studies. The reactivity of 1 towards various small molecules was probed and compared with that of a previously published derivative, Ph3 PC(Me)BEt2 (2). Both α-BCPs react with NH3 to undergo heterolytic N-H bond cleavage. Different di- and trimeric ring structures were observed in the reaction products of 1 with CO and CO2 . With PhNCO and PHNCS, the expected insertion products [Me3 PC(H)(PhNCO)B(iBu)2 ] and [Me3 PC(H)(PhNCS)B(iBu)2 ], respectively, were isolated.
ABSTRACT
The concept and the first example for electronic frustration within a C-B π-bond arising from the competition for a lone pair of electrons is reported. The α-borylated phosphorus ylide (α-BCP) Ph3 PC(CH3 )BEt2 (1) has been synthesized, characterized, and investigated with the aid of density functional theory calculations. These show the presence of a highly polarized C-B π-bond, induced by the electron-withdrawing Ph3 P+ substituent. This competition for the C-based lone pair of electrons leads to an FLP-type reactivity (FLP=frustrated Lewis pair), which has been demonstrated by reacting 1 with various small molecules: the dimeric insertion products [Ph3 PC(CH3 )COBEt2 ]2 (2) and [Ph3 PC(CH3 )(CO2 )BEt2 ]2 (5) result from the reaction of 1 with CO and CO2 , respectively, whereas monomeric products of the general formula [Ph3 PC(CH3 )(XYZ)BEt2 ] were obtained by treating 1 with COS (3), CS2 (4) or the heterocumulenes PhNCO (6) and PhNCS (7).
ABSTRACT
The synthesis and characterisation of two aluminium diphosphamethanide complexes, [Al(tBu)2 {κ(2) P,P'-Mes*PCHPMes*}] (3) and [Al(C6 F5 )2 {κ(2) P,P'-Mes*PCHPMes*}] (4), and the silylated analogue, Mes*PCHP(SiMe3 )Mes* (5), are reported. The aluminium complexes feature four-membered PCPAl core structures consisting of diphosphaallyl ligands. The silylated phosphine 5 was found to be a valuable precursor for the synthesis of 4 as it cleanly reacts with the diaryl aluminium chloride [(C6 F5 )2 AlCl]2 . The aluminium complex 3 reacts with molecular dihydrogen at room temperature under formation of the acyclic σ(2) λ(3) ,σ(3) λ(3) -diphosphine Mes*PCHP(H)Mes* and the corresponding dialkyl aluminium hydride [tBu2 AlH]3 . Thus, 3 belongs to the family of so-called hidden frustrated Lewis pairs.
