Supramolecular Pt(II) and Ru(II) Trigonal Prismatic Cages Constructed with a Tris(pyridyl)borane Donor

We report a novel example of supramolecular cages containing a Lewis acidic trigonal boron center. Self-assembly of the tris(pyridyl)borane donor 1 with diruthenium (2) or platinum (3), as an electron acceptor, furnished boron-containing trigonal prismatic supramolecular cages 5 and 6, which were characterized by 1H NMR and electrospray ionization time-of-flight mass spectroscopy and X-ray crystallography. The molecular structure of cage 5 was confirmed as a trigonal prismatic cage with an inner dimension of about 400 Å3. The fluoride binding properties of borane ligand 1 and Pt cage 6 were studied. UV/vis absorption titration studies demonstrated that the boron center of cage 6 undergoes strong binding interaction with the fluoride ion, with an estimated binding constant of 1.3 × 1010 M-2 in acetone based on the 1:2 binding isotherm. The binding was also confirmed by 1H NMR titration. Photoluminescence titration studies showed that cage 6 emitted borane-centered fluorescence (τ = 2.21 ns), which was gradually quenched upon addition of fluoride. When excess fluoride was added to a solution of 6, however, dissociation of the pyridyl ligand from the Pt(II) center was observed.

Pyridalthiadiazole acceptor-functionalized triarylboranes with multi-responsive optoelectronic characteristics

A new class of Ar₂B-π-A dyads and A-π-B(Ar)-π-A triads that feature strong organic acceptor moieties (A = pyridalthiadiazole, PT) attached to a central triarylborane were synthesized via Stille cross-coupling of ArB(Th-SnMe₃)₂ (Th = thiophenediyl, Ar = 2,4,6-tri-tert-butylphenyl (Mes*) or 2,4,6-tris(trifluoromethylphenyl) (^FMes)) with one or two equivalents of dibromopyridalthiadiazole. Single-crystal X-ray crystallography data for the triad Mes*B(Th-PT-Br)₂ indicate a highly coplanar conformation, which is ideal for extended π-conjugation and favors intermolecular π-stacking. Despite the presence of Br substituents, these compounds exhibit strong photoluminescence in THF solution with quantum yields reaching up to 52%. Further extension of conjugation by coupling with 2-hexylthiophene leads to additional bathochromic shifts to give a highly soluble and strongly red-emissive material. All these compounds undergo facile reduction, first of the PT substituents and then at more negative potentials for the borane moiety. Upon chemical reduction with in THF, an intramolecular charge transfer (ICT) pathway from the reduced PT moieties to boron is enabled and this results in a change of the color to blue. Theoretical calculations reveal that, due to the electron-withdrawing effect of the PT moieties, not only the PT-centered LUMOs themselves but also the LUMO+1 or LUMO+2, which show contributions from the p orbital of boron, experience a significant decrease in energy; they are much lower in energy than those of typical conjugated triarylboranes. The relatively low energy of both the PT-centered LUMOs and boron-centered LUMO+1 or LUMO+2 opens up multiple pathways for reaction with highly nucleophilic fluoride anions. Evidence for very strong F^- binding to boron is obtained in the case of the more sterically accessible ^FMes derivatives. Fluoride anion binding leads to an electron-rich borate moiety and as such generates an ICT pathway to the electron-deficient PT moieties; the direction of this ICT is opposite to that observed upon chemical reduction. For the Mes* derivatives, F^- binding is hindered, resulting in competing reduction of the PT acceptors. Finally, the electron acceptor character of the hexylthiophene derivative is exploited in electron-only diodes that show an average electron mobility of 6.4 ± 1.6 × 10^-5 cm² V^-1 s^-1.

Lewis acidity enhancement of triarylborane by appended phosphine oxide groups

A series of mono-, di-, and tri-phosphine oxide substituted triarylboranes, Mes2BAr (1), MesBAr2 (2), and BAr3 (3) (Ar = 4-(Ph2PO)-2,6-Me2-C6H2) were prepared to investigate the effect of a phosphine oxide group (Ph2PO) on Lewis acidity enhancement of triarylboranes. The X-ray crystal structure of 3 revealed peripheral decoration of Ph2PO groups with a C3-axis perpendicular to the trigonal boron center. UV/Vis absorption and PL spectra indicated a significant contribution of π(Mes or phenylene) → pπ(B) charge transfer in the lower-energy electronic transition. The reduction potential measured by cyclic voltammetry showed apparent LUMO stabilization by introduction of phosphine oxide groups, the extent of which gradually increased with the increasing number of phosphine oxide groups. Lewis acidity enhancement was also supported by the gradual increase in fluoride ion affinity in the order 3 > 2 > 1. Theoretical calculations suggest that introduction of a Ph2PO group into a triarylborane significantly enhances the Lewis acidity of the boron center via an inductive electron-withdrawing effect and this effect is additive for multiple phosphine oxide groups.