Exploring opportunities for tuning phenyltris(pyrazol-1-yl)borate donation by varying the extent of phenyl substituent fluorination

Abstract

Complexes of 3,4,5-trifluorophenyl- and 3,5-bis(trifluoromethyl)phenyltris(pyrazol-1-yl)borates have been prepared for comparison to pentafluorophenyl analogues to assess if donor modulation at κ3-N,N,N bound metals is feasible via this strategy.

Thallium(I) Complexes of Tris(pyridyl)borates and a Comparison to Their Pyrazolyl Analogues

“…This could be the cause for the slightly longer Tl–N distances of 4 compared to the corresponding bond lengths of 2 (Table ). These intermolecular interactions of 4 are similar to those reported for [(3,4,5-(F) 3 C 6 H 2 )­B­(Pz) 3 ]­Tl, but the latter features a bridging asymmetric κ 3 - N , N , N tris­(pyrazolyl)­borate coordination mode with much shorter intermolecular Tl–N bonds at 2.741 Å …”

“…19 F NMR (471 MHz, CDCl 3 ): δ (ppm) −67.81 (s). 11 Synthesis of [PhB(6-(CF 3 )Py) 3 ]K. To a suspension of KH (0.100 g, 2.49 mmol) in 20 mL of THF at 0 °C was slowly added a solution of [PhB(6-(CF 3 )Py) 3 ]H (1.20 g, 2.28 mmol) in 20 mL of THF via cannula transfer. After the complete cessation of hydrogen gas evolution, the reaction mixture was warmed to room temperature and was stirred overnight.…”

“…Due to the ease of ligand assembly from common starting materials, and readily modifiable ligand steric and electronic properties through pyrazolyl ring substituents, many tris­(pyrazolyl)­borate ligand varieties have been synthesized and utilized as supporting ligands in metal coordination chemistry, facilitating numerous applications ranging from enzyme active site modeling to catalysis. − Tris­(pyrazolyl)­borate ligands are commonly obtained as alkali-metal salts via the reactions between pyrazoles and alkali-metal borohydrides MBH 4 (M = Li, Na, K) under high-temperature conditions. Although tris­(pyrazolyl)­borato alkali-metal salts can serve as ligand transfer agents, conversion to their thallium­(I) derivatives (via metathesis using thallium salts such as TlNO 3 and TlOAc) is quite common to facilitate the separation and purification of tris­(pyrazolyl)­borate ligands from crude reaction mixtures, and because of their solubility in common organic solvents, ease of crystallization, and milder (less reducing) and efficient ligand transfer qualities. ,− Direct routes to tris­(pyrazolyl)­boratothallium complexes from TlBH 4 and pyrazoles are also available . Thallium complexes of tris­(pyrazolyl)­borates are also of interest in their own right since they display interesting NMR (Tl exists as two naturally occurring spin 1/2 isotopes; 203 Tl and 205 Tl) and structural (in certain cases, Tl···Tl contacts) features. ,− …”

“…1 H and 13 C NMR spectra were referenced internally to solvent signals (CDCl 3 , 7.26 ppm for 1 H NMR, 77.16 ppm for 13 C NMR; DMSO-d 6 , 2.50 ppm for 1 H NMR, 39.52 ppm for 13 C NMR) or externally to SiMe 4 (0 ppm). 1 H, 13 C, 19 F, and 11 B NMR chemical shifts are reported in ppm, and coupling constants (J) are reported in hertz (Hz). Abbreviations used for signal assignments: Py = pyridyl, Py n = noncoordinated pyridyl, Ph = phenyl, Me = methyl, s = singlet, d = doublet, dd = doublet of doublets, t = triplet, q = quartet, m = multiplet, brs = broad singlet.…”

Tris(pyridyl)borates: an emergent class of versatile and robust polydentate ligands for catalysis and materials applications

“…12 The replacement of the five-membered pyrazole (Pz) with six-membered pyridine (Py) rings also alters the binding geometry and bite angle of the ligand. This is evident, for example, when comparing the B–N (Tp, average 1.55 Å) and B–C (Tpyb, average 1.64 Å) bond lengths and the B–N–N (Tp, average 119.1) and B–C–N (Tpyb, average 116.9) bond angles for the octahedral Fe( ii ) complexes 12–15 of Ph-substituted Tp and Tpyb ligands. Furthermore, as a better σ-donor than pyrazole, pyridine is expected to more strongly donate to the metal centre (p K a (pyridine) = 12.5, p K a (pyrazole) = 9.1 for conjugate acid in CH 3 CN).…”