Helicene Bis(pyrazol‐1‐yl)pyridine Ligands for Luminescent Transition‐Metal Complexes

Abstract

We describe herein the synthesis of 2,6-bis(pyrazol-1-yl)pyridine (1-bpp) ligands substituted by helicene units in the 4-position of the pyridine ring. The two-step procedure afforded the molecules bpp[n]helicene (n = 4, 5 and 6) and methyl-bpp[4]helicene, abbreviated as 4, 5, 6 and 4m respectively, with overall good yields. This series of ligands shows fluorescence in the violet region, with a red shift of the emission when increasing the number of aromatic rings in the helicene unit. The tridentate fluorophores have been reacted with Re(CO) 5 Cl and [Ru(terpy)Cl 3 ] to prepare a series of transition metal complexes formulated as [Re I (n)(CO) 3 Cl] (n = 4 and 5) and [Ru II (n)(terpy)](PF 6 ) 2 (n = 4, 4m, 5 and 6). The compounds [a] MOLTECH-4807 Scheme 1. Bpp[n]helicene ligands investigated in the present study.Full Paper 4, 4m, 5 and 6). The photophysical properties of the ligands and complexes have been investigated. Results and Discussion Synthesis and Crystal Structures of the LigandsThe synthetic procedure for the N-donor helicene-based ligands 4, 4m, 5 and 6 is described in Scheme 2 and Scheme 3. The preparation of bpp[n]helicene (n = 4, 5, 6) ligands has been achieved via a two steps procedure from racemic 2-bromo-[n]helicenes, synthesized according to literature reports (Scheme 2). [13] First, a Suzuki coupling between 2,6-dichloropyridine-4-boronic acid pinacol ester and bromo[n]helicene afforded molecules 1, 2 and 3 with excellent yields. These precursors were subsequently reacted with sodium pyrazolate to afford bpp[n]helicene ligands 4, 5 and 6 as racemic mixtures.Precursor 1 has also been reacted with 3-methyl-pyrazole to prepare the ligand methyl-bpp[4]helicene 4m (Scheme 3). This reaction provided two isomers as a consequence of the delocal-Scheme 2. Synthetic procedure for ligands 4, 5 and 6.Scheme 3. Synthetic procedure for ligand 4m.

Chiral Emissive Lanthanide Complexes from Enantiopure [6]Helicene‐bis(pyrazolyl)‐pyridine Ligands

“…[12] In the frame of our own interest in functional helicenes, [13,14] and their transition metal complexes, [9,15] we have recently reported the series of 2,6bis(pyrazol-1-yl)pyridine (1-bpp) ligands substituted by racemic [4]-, [5]-and [6]helicene units in the 4-position of the pyridine ring, together with the photophysical properties of their luminescent Re(I) and Ru(II) complexes. [16] While a large variety of functionalized 1-bpp and corresponding transition metal complexes have been reported, [17,18] chiral derivatives have received little attention so far, [19,20] especially driven by catalytic applications, [21,22] but not for emission or chiroptical properties.…”

[4]Helicene-based anions in electrocrystallization with tetrachalcogeno-fulvalene donors

“…19,20 Our team as well as others have dedicated much effort to develop chiral emissive or redox-active helicenes in order to observe the synergistic interaction between chirality and conductivity or luminescence. 21–23 In particular, TTF-helicenes have shown redox modulation of their chiroptical properties. 24 However, helicene-based anions have not been used so far as counter-ions in TTF-based radical cation salts.…”

Schiff-base [4]helicene Zn(ii) complexes as chiral emitters

“…23,24 Moreover, their combination with metal complexes has been explored to yield phosphorescent compounds. [25][26][27] Accordingly, we have designed electro- 28 and photo-active [29][30][31] helicene and heterohelicene compounds and investigated their properties. Herein, we describe the first enantiopure Zn(II)-salen-helicene complexes together with their photophysical and chiroptical properties, supported by TD-DFT calculations.…”