The synthesis of a new trisbenzylsilanephosphine P{(o-CHCH)SiMeH} (1) is shown to proceed with high yields from P(o-tolyl). Compound 1 coordinates to the Rh and Ir dimers [MCl(COD)] (M = Rh, Ir) in a tetradentate or tridentate fashion, depending on the strict exclusion of water. The dimeric compounds [ClM(SiMeCH-o-CH)P(o-CH-CHSiMeH)], 2Rh and 2Ir, feature a tetradentate coordination of the starting ligand with P and two Si atoms as well as a non-classical agostic Si-H group. The presence of adventitious water in the solvents leads to the formation of two new complexes [(μ-Cl)M(SiMeCH-o-CH)P(o-CH-CHSiMeOSiMeCH-o-CH-)P(SiMeCH-o-CH)], 3Rh and 3Ir, which feature a siloxane bridge through Si-H bond breaking in 2. Reaction of [RhCl(COD)] with the bisbenzylsilanephosphine PhP{(o-CHCH)SiMeH} leads to the formation of compound 4Rh which features also a dimeric structure with the SiPSi ligand coordinated through the two silicon atoms, one of which occupies the apical position of a square-pyramidal geometry in the solid state, while the second is disposed equatorially trans to π-donor Cl. Finally, bidentate coordination of a PSi ligand is achieved by reaction of [RhCl(COD)] with PhP{(o-CHCH)SiMeH} which leads to the monometallic species [RhCl(SiMeCH-o-CH-PPh)], 5Rh, incorporating two chelating PSi ligands and maintaining a Cl ligand.
Three new diboronic acid-substituted bisquinolinium salts were synthesized, structurally described by single-crystal X-ray diffraction, and studied in-depth as fluorescent receptors for six monosaccharides and two open-chain polyols in water at physiological pH. The dicationic pyridine-2,6-dicarboxamide-based receptors contain two N-quinolinium rings as the fluorescent units covalently linked to three different isomers of phenylboronic acid (ortho, 2; meta, 3; and para, 4) as chelating binding sites for polyols. Additions of glucose/fructose in the micromolar concentration range to receptors 2 and 3 induce significant fluorescence changes, but in the presence of arabinose, galactose, mannose, and xylose, only modest optical changes are observed. This optical change is attributed to a static photoinduced electron transfer mechanism. The meta-diboronic receptor 3 exhibited a high affinity/selectivity toward glucose (K = 3800 M −1 ) over other monosaccharides including common interfering species such as fructose and mannitol. Based on multiple spectroscopic tools, electrospray ionization high-resolution mass spectrometry, crystal structures, and density functional theory calculations, the binding mode between 3 and glucose is proposed as a 1:1 complex with the glucofuranose form involving a cooperative chelating diboronate binding. These results demonstrate the usefulness of a new set of cationic fluorescent diboronic acid receptors with a strong ability for optical recognition of glucose in the sub-millimolar concentration range.
Incorporating pendant silicon functionalities into phosphine ligands enables to profit from the strong σ‐electron donor and trans influence properties of Si while enhancing the coordination ability of the ligand. Herein, we show that the introduction of bulky sigma donor substituents on the Si atoms and modulation of the number of Si–H functional groups in a series of phosphinobenzylsilanes allow either the stabilization of rare highly unsaturated 14‐electron rhodium(III) and iridium(III) species devoid of agostic interactions or the access to mixed‐valence MI–MIII complexes. Our findings using isopropyl‐substituted silicon are markedly different from those obtained when employing the methyl‐substituted Si series.
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