Donor–Acceptor Boron-Ketoiminate Complexes with Pendent N-Heterocyclic Arms: Switched-on Luminescence through N-Heterocycle Methylation

Abstract

A series of intramolecular, donor-stabilized BF2 complexes supported by phenanthridinyl-decorated, β-ketoiminate chelating ligand scaffolds is described, along with their characterization by spectroscopy and X-ray diffraction. In solution, the relative orientation of the pendent phenanthridinyl arm is fixed despite not coordinating to the boron center, and a well-resolved through-space interaction between a phenanthridinyl C–H and a single fluorine atom can be observed by 19F–1H NOE NMR spectroscopy. The neutral compounds are nonetheless only weakly luminescent in fluid solution, ascribed to nonradiative decay pathways enabled by rotation of the N-heterocyclic unit. Methylation of the phenanthridinyl nitrogen restricts this rotation, “switching on” comparably strong emission in solution. Modeling by density functional theory (DFT) and time-dependent DFT (TDDFT) indicates that the character of the lowest energy excitation changes upon methylation, with shallow calculated potential energy surfaces of the neutral complexes consistent with their lack of significant radiative decay.

Yellow-Emitting, Pseudo-Octahedral Zinc Complexes of Benzannulated N^N^O Pincer-Type Ligands

“…The appearance of a higher-energy peak [Zn( Me L ) 2 1449 cm ‑1 ; Zn( t Bu L ) 2 1453 cm –1 ; Zn( CF3 L ) 2 1433 cm –1 ] is attributed to a combination of enolato–imine CC and phenanthridinyl CN stretches. In contrast, boron complexes of this ligand set show distinct signals for the ν C–O,stretch of the enolato–imine unit, which suggests that π bonds of the N^O fragment are more delocalized in the zinc complexes, extending over all five atoms of the chelating unit. Consistent with this description, Mayer bond order analysis of the enolato–imine π bonds O1–C CH3 –CH–C CH3 –N2 yields bond orders of 1.48, 1.33, 1.32, and 1.49, respectively.…”

“…The N^N­(H)^O proligands R LH (R = Me, t Bu, CF 3 ) were isolated following our previously reported protocol that utilizes the acid-catalyzed condensation of 4-aminophenanthridine with acetylacetone. , Zinc complexes of these ligands, Zn­( R L ) 2 , were prepared in a similar way to homoleptic Zn­(II) species supported by monoanionic, bidentate N -phenyl-4-aminophenanthridines . That is, via the dropwise addition of a solution of diethylzinc in hexanes into a solution of the proligands in 3:1 hexanes/toluene, both precooled to −38 °C, which is then warmed to ambient temperature (Scheme a).…”

“…Only one set of 1 H and 13 C NMR signals is observed in solution, suggesting that the two ligands have the same magnetic environment in solution which proffers C 2 symmetry to the complexes. In the 1 H NMR spectrum, coordination to Zn­(II) induces a ∼0.7 ppm shift to lower frequency of the NC 6 H resonance of the phenanthridinyl ligand arms [e.g., δ­(NC 6 H ): Me LH , 9.29 ppm; Zn­( Me L ) 2 , 8.58 ppm] which likely results from ring-current-induced shielding by the (N^O)Zn chelate …”

“…The Zn1–O1/2 (2.060–2.075 Å) and Zn1–N2/4 (2.112–2.131 Å) bond distances suggest that a metal–alkoxy descriptor is more appropriate here. In general, in Pt­(II) and boron complexes of R L , the enolato–imine tautomer predominates upon deprotonation or coordination to metal centers, − while structural and spectroscopic data for the proligands ( R LH ) are more consistent with the keto–enamine tautomer [C15–N2/C17–O1: t Bu LH , 1.363(3)/1.244(3) Å; Zn­( t Bu L ) 2 , 1.315(11)/1.255(10) Å; Zn­( CF3 L ) 2 , 1.328(3)/1.263(3) Å].…”

“…Diethylzinc (1 M in hexanes) was purchased from Sigma-Aldrich. R LH were synthesized according to previously published procedures. , 1D and 2D NMR spectra were recorded on a Bruker Avance 300 MHz spectrometer or a Bruker Avance 500 MHz spectrometer. 1 H and 13 C­{ 1 H} NMR spectra were referenced to residual solvent peaks.…”