SYNTHESES, STRUCTURES AND ELECTROCHEMISTRY OF [CUL1(L-R)]BF4 L-1=3-(2,5-DIMETHOXYPHENYL)-1-(2-PYRIDYL)PYRAZOLE, L-R=TRIS(3-ARYLPYRAZOLYL)-HYDROBORATE] AND [CUL1(2)][BF4](2) - EFFECTS OF GRAPHITIC INTERACTIONS ON THE STABILITY OF AN ARYL RADICAL-CATION

Reaction of potassium 3(5)-(2,5-dimethoxyphenyl)pyrazolate with 2-brom opyridine in diglyme (2,5,8-trioxanonane) at 130 degrees C for 3 d aff orded 3-(2,5-dimethoxyphenyl)-1-(2-pyridyl)pyrazole (L-1) in 69% yield after an aqueous quench and recrystallisation from hexanes. Complexat ion of hydrated Cu(BF4)(2) with 1 molar equivalent of L-1 and K[L-R] i n CH2Cl2 afforded [CuL1(L-R)]BF4[L-R = tris(3-arylpyrazolyl)hydroborat e: R = Ph 1, 2-furyl 2, 2-thienyl 3, 4-tolyl 4, 4-FC6H4 5 or 4-MeOC6H4 6]. The single-crystal structures of 1.0.5H(2)O and 2.2CH(2)Cl(2) con tain square-pyramidal complex cations with CuN5 donor sets. Both struc tures show the presence of intramolecular graphitic interactions betwe en the pendant dimethoxyphenyl group of L-1 and a R substituent from t he L-R ligand. Treatment of Cu(BF4)(2) . xH(2)O with 2 molar equivalen ts of L-1 in MeCN afforded [CuL21][BF4](2) 7, the single-crystal struc ture of which shows a, tetrahedrally distorted planar geometry about t he Cu2+ ion with additional 2.9 Angstrom Cu ... O contacts to the 2-me thoxy groups of each ligand, which H-1 NMR data suggest are retained i n CD3CN solution. Complexes 1-7 contain a {d(x2-y2)}(1) copper(II) cen tre according to solution UV/VIS and EPR spectroscopies, 1-6 remaining five-co-ordinate in CH2Cl2 solution. Voltammetric measurements in CH2 Cl2-0.5 mol dm(-3) (NBu4PF6)-P-n at 293 K showed that E-1/2 for the L- 1-[L-1](.+) oxidation associated with the dimethoxyphenyl group of L-1 is unperturbed by the graphitic interaction involving this group in 1 -6. However, for 1, 4 and 5 this couple is chemically reversible at sc an rates between 10 and 1000 mV s(-1); for 2, 3 and 6 this process is partially obscured by an irreversible L-R ligand oxidation. For L-1 an d 7, this process is quasi-reversible or irreversible under the same c onditions. Hence, the intramolecular graphitic interactions in complex es 1, 4 and 5 may kinetically stabilise the dimethoxyphenyl radical ca tion against decomposition.