Highly Lewis Acidic Arylboronate Esters Capable of Colorimetric Turn-On Response.
A series of boronate-π-acceptor compounds containing different types of π bridges (1,4-phenylen or thien-2,5-diyl or furan-2,5-diyl) that link the switchable boronate ester group with the efficient TCF acceptor group (TCF=2-dicyanomethylen-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran) has been synthesized. A TCF chromophore of this type undergoes transition to a donor-π-acceptor compound upon coordination of Lewis bases at the Lewis acidic boron center, which is accompanied by an enhanced intramolecular charge-transfer interaction. The Lewis acid character has been investigated by spectroscopic measurements (UV/Vis, NMR spectroscopies) as well as DFT and ab initio-based calculations. It is shown that the TCF acceptor group and thiophene or furan π-bridges directly bound to the boron atom cooperatively increase the Lewis acidity. UV/Vis titration experiments confirm fluoride binding constants in the range of up to 10(8)  M(-1) in CH2 Cl2 . In addition to the strong boron fluoride binding motif, Lewis interactions also occur with weaker Lewis bases, such as pyridine or aliphatic alcohols. The unique combination of chromophoric and Lewis acidic properties is responsible for the intense colorimetric turn-on response detectable after complex formation.
Department of Polymer Chemistry, Technische Universität Chemnitz, 09107 Chemnitz (Germany).
Alexander
A
Oehlke
Max-Planck-Institut für Chemische Energiekonversion, Stiftstrasse 34-36, 45470 Mülheim a. d. Ruhr (Germany).
Alexander A
AA
Auer
Department of Polymer Chemistry, Technische Universität Chemnitz, 09107 Chemnitz (Germany).
Katja
K
Schreiter
Department of Polymer Chemistry, Technische Universität Chemnitz, 09107 Chemnitz (Germany).
Nadine
N
Friebe
Department of Polymer Chemistry, Technische Universität Chemnitz, 09107 Chemnitz (Germany). stefan.spange@chemie.tu-chemnitz.de.
Stefan
S
Spange
Lewis acids; arylboronate esters; donor-acceptor systems; fluorides; solvent effects