SYNTHESIS, STRUCTURAL CHARACTERIZATION, AND ELECTROCHEMICAL-BEHAVIOR OF COPPER(I) COMPLEXES OF STERICALLY HINDERED TRIS(3-TERT-BUTYLPYRAZOLYL AND 3,5-DIPHENYLPYRAZOLYL)HYDROBORATE LIGANDS

A series of Cu(I) complexes of sterically hindered tris(pyrazolyl)hydr oborates [Tp(RR')Cu]2 (R and R' are the 3- and 5-pyrazolyl substituent s, respectively; R = t-Bu, R' = H or R = R' = Ph) and Tp(RR')CuL. (R = -Bu, R' = H, L = CH3CN; R = R' = Ph, L = CH3CN; R = R' = Ph, 1, = 3,5 -Ph2pz = 3,5-diphenylpyrazole) were isolated and characterized by elem ental analysis,variable-temperature H-1 NMR spectroscopy, osmometry, c yclicvoltammetry, and, in the case of [Tp(t-Bu)Cu]2,[Tp(Ph2)CU]2, and Tp(Ph2)CU(3,5-Ph2PZ), by X-ray crystallography. ([Tp(t-Bu)Cu]2.Et2O: C 46H78N12B2Cu2O, space group P2(1)/n (No. 14) at -101-degrees-C with a = 19.001(8) angstrom, b = 13.629(8) angstrom, c = 21.490(7) angstrom, beta = 113.01(3)-degrees, V = 522(8) angstrom3, Z = 4, R = 0.082 and R . = 0.075 for 3749 unique, observed reflections with I > 2sigma(I) and 541 variable parameters. [TpPh2Cu]2.2CH2Cl2: C47H38N6CuCl4B, space gr oup P2(1)/n (No. 14) at -101-degrees-C with a = 17.801(8) angstrom, b = 13.063(8) angstrom, c = 18.551(9) angstrom, beta = 99.42(4)-degrees, V = 4255(7) angstrom3, Z = 4, R 0.062 and R(w) = 0.061 for 5552 uniqu e, observed reflections with I > 2sigma(I) and 532 variable parameters . Tp(Ph2)Cu(3,5-Ph2PZ)-CH2Cl2: C61H48N8BCuCl2, space group P2(1)/n (No . 14) at -101-degrees-C with a = 10.308(7) angstrom, b = 29.105(9) ang strom, c = 17.158(8) angstrom, beta = 93.54(5)-degrees, Y = 5138(8) an gstrom3, Z = 4, R = 0.069 and R(w) = 0.061 for 5136 unique, observed r eflections with I > 2sigma(I) and 658 variable parameters). Solution m olecular weight data and H-1 NMR spectra of [Tp(t-Bu)CU]2, which conta ins linear 2-coordinate Cu(I) ions bridged by eta2-Tp(t-Bu) ligands in the solid state, indicated that it retains its dimeric structure in s olution, although the presence of a trace amount of a monomeric specie s could not be ruled out. A pyrazolyl ring exchange process occurs wit h DELTAH(double dagger) = 11.7(5) kcal mol-I and DELTAS(double dagger) = -9(2) eu, as determined by line shape analysis of variable-temperat ure H-1 NMR spectra. [Tp(Ph2)CU]2, in which eta3-Tp(Ph2) ligands bridg e between 3-coordinate Cu(I) ions in the solid state, apparently disso ciates in solution to ill-defined species of lower molecular weight. H -1 NMR spectroscopy indicated that the pyrazole ligand in the monomeri c, distorted tetrahedral Tp(Ph2)CU(3,5-Ph2PZ) is labile. Cyclic voltam mograms of the [Tp(RR')CU]2 and Tp(RR')Cu(CH3CN) complexes in 0.1 M (T BA)PF6/CH2Cl2 showed irreversible oxidations at high potentials (>+0.6 V vs SCE), but Tp(Ph2)Cu(3,5-Ph2PZ) (same conditions) and Tp(t-Bu)Cu( CH3CN) (in 10% CH3CN/CH2Cl2 with 0.1 M (TBA)OTf) exhibited chemically reversible redox processes attributed to Tp(Ph2)CU(3,5-Ph2PZ)/Tp(Ph2)C U(3,5-Ph2pz)+ and Tp(t-Bu)Cu(CH3CN)/Tp(t-Bu)CU(CH3CN)+ couples with E1 /2 = +0.69 and +0.93 V vs SCE, respectively.