Syntheses, structures and electrochemistry of [CuL1(LR)]BF4 [L1 = 3-{2,5-dimethoxyphenyl)-1-(2-pyridyl)pyrazole; LR = tris(3-arylpyrazolyl)hydroborate] and [CuL12][BF4]2. Effects of graphitic interactions on the stability of an aryl radical cation†
Abstract
Reaction of potassium 3(5)-(2,5-dimethoxyphenyl)pyrazolate with 2-bromopyridine in diglyme (2,5,8-trioxanonane) at 130 °C for 3 d afforded 3-(2,5-dimethoxyphenyl)-1-(2-pyridyl)pyrazole (L1) in 69% yield after an aqueous quench and recrystallisation from hexanes. Complexation of hydrated Cu(BF4)2 with 1 molar equivalent of L1 and K[LR] in CH2Cl2 afforded [CuL1(LR)]BF4 [LR = tris(3-arylpyrazolyl)hydroborate: R = Ph 1, 2-furyl 2, 2-thienyl 3, 4-tolyl 4, 4-FC6H45 or 4-MeOC6H46]. The single-crystal structures of 1·0.5H2O and 2·2CH2Cl2 contain square-pyramidal complex cations with CuN5 donor sets. Both structures show the presence of intramolecular graphitic interactions between the pendant dimethoxyphenyl group of L1 and a R substituent from the LR ligand. Treatment of Cu(BF4)2·xH2O with 2 molar equivalents of L1 in MeCN afforded [CuL12][BF4]27, the single-crystal structure of which shows a tetrahedrally distorted planar geometry about the Cu2+ ion with additional 2.9 Å Cu · · · O contacts to the 2-methoxy groups of each ligand, which 1H NMR data suggest are retained in CD3CN solution. Complexes 1–7 contain a {dx2–y2}1 copper(II) centre according to solution UV/VIS and EPR spectroscopies, 1–6 remaining five-co-ordinate in CH2Cl2 solution. Voltammetric measurements in CH2Cl2–0.5 mol dm–3 NBun4PF6 at 293 K showed that E½ for the L1–[L1]˙+ oxidation associated with the dimethoxyphenyl group of L1 is unperturbed by the graphitic interaction involving this group in 1–6. However, for 1, 4 and 5 this couple is chemically reversible at scan rates between 10 and 1000 mV s–1; for 2, 3 and 6 this process is partially obscured by an irreversible LR ligand oxidation. For L1 and 7, this process is quasi-reversible or irreversible under the same conditions. Hence, the intramolecular graphitic interactions in complexes 1, 4 and 5 may kinetically stabilise the dimethoxyphenyl radical cation against decomposition.