AN ELECTRON SELF-EXCHANGE STUDY OF THE COORDINATION-NUMBER-INVARIANT PENTACOORDINATE COPPER(I/II) COUPLE [CUI,II((5-MEIMIDH)2DAP)]+/2+1

The electron self-exchange rate of the [Cu(I,II)((5-MeimidH)2DAP)]+/2+ cation couple has been determined in CD3CN, as a function of temperature, by dynamic NMR line-broadening techniques. Under anaerobic conditions with mu = 25 mM ((CH3)4NBF4), the rate constant ranged from 0.8 x 10(4) (243 K) to 3.5 x 10(4) M-1 s-1 (293 K). From the temperature dependence of the self-exchange rate, activation parameters of DELTA-H] = 16.2 +/- 3.3 kJ mol-1 and DELTA-S] = -103 +/- 12 J K-1 mol-1 have been obtained. An X-ray crystal structure of the [Cu(II)((5-MeimidH)2DAP)]2+ cation shows the same general pentacoordinate stucture as found earlier for the [Cu(II)((imidH)2DAP)]2+ parent compound, and it is assumed that [Cu(I)((5-MeimidH)2DAP)]+ is also pentacoordinate as is its Cu(I) parent compound. The present temperature-dependent electron self-exchange data are some of the first such data to be obtained for a synthetic Cu(I/II) couple that remains coordination-number-invariant (CN = 5) during electron transfer. The present electron self-exchange rate constant, together with those for two other, related coodination-number-invariant Cu(I/II) couples ([Cu(I,II)((imidH)2DAP)]+/2+ and [Cu(I,II)((py)2DAP)]+/2+) indicate a possible relationship between intramolecular conformational mobility in the Cu(I) partner complex and the electron self-exchange rate of the Cu(I/II) couple. Because of its coordination-number invariance, this small-molecule system resembles the situation at the active site in the blue copper protein plastocyanin (CN = 4) and possibly azurin. It is noteworthy that azurin has a much greater DELTA-H] for electron exchange than does the present synthetic system. Crystal data for [Cu(II)((5-MeimidH)2DAP)](BF4)2.1/2CH3OH, CuF8O0.5N7B2C21.5H29: a = 11.983 (4) angstrom, b = 10.360 (4) angstrom, c = 12.906 (6) angstrom, alpha = 68.51 (3)-degrees, beta = 73.63 (3)-degrees, gamma = 67.06 (3)-degrees, Z = 2, triclinic, space group P1BAR. A total of 7592 observed data were collected at -155 +/- 5-degrees-C and used in the solution. The pentacoordinate cation is disordered into two enantiometric forms around the copper(II) ion. The structures are closer to idealized trigonal bipyramidal than square pyramidal. Crystal data for [Cu(II)((imidH)2DAP)](BF4)2, CuF8N7B2C19H23: a = 12.431 (9) angstrom, b = 14.024 (6) angstrom, c = 14.296 (11) angstrom, beta = 104.45 (6)-degrees, Z = 4, monoclinic, space group P2(1)/c. A total of 1358 observed data were obtained. The structure of the cation (as its BF4- salt) is more square pyramidal than that of the perchlorate salt whose structure had been previously determined.