Reversible O-2 binding to a dinuclear copper(I) complex with linked tris(2-pyridylmethyl)amine units: Kinetic-thermodynamic comparisons with mononuclear analogues

fajThe kinetics and thermodynamics of reaction of O-2 with copper(I) complexes can provide fundamental information relevant to chemical and biological systems. Using diode-array variable-temperature (180-296 K) stopped-flow kinetic methods, we report detailed information on the O-2 reactivity (in EtCN) of dicopper(I) complex [(D-1)Cu-2(I)(RCN)(2)](2+) (2a) (R = Me or Et) [D-1 = dinucleating ligand with a -CH2CH2- group linking two tris(2-pyridylmethyl)amine (TMPA) units at a 5-pyridyl position of each tetradentate moiety]. A comparative study of mononuclear complex [(TMPAE)Cu(RCN)li (1a') [TMPAE has a -C(O)OCH3 ester substituent in the 5-position of one pyridyl group of TMPA] has been carried out. The results are compared with data from the previously investigated complex [(TMPA)Cu(RCN)](+) (1a). The syntheses of D-1 and 2a-(ClO4)(2) are described; an X-ray structure reveals two pentacoordinate Cu(I) ions (Cu ... Cu = 11.70 Angstrom), each bound by the N-4-tetradentate and an EtCN molecule. Cyclic voltammetric data for 1a' and 2a are reported. At 193 K in EtCN, 2a reacts with O-2 (Cu/O-2 = 2:1, manometry) to produce an intensely purple colored solution of adduct [(D-1)Cu-2(O-2)](2+) (2c), lambda(max) = 540 nm (epsilon 11 100 M(-1) cm(-1)). This peroxo-dicopper(II) species reacts with PPh(3), liberating O-2 and producing the isolatable bis-phosphine adduct [(D')Cu-2(PPh(3))(2)](2+). The kinetic investigation provides spectral characterization of transient Cu/O-2 1:1 adducts generated upon oxygenation of cold solutions of 1a' or 2a. [(TMPAE)Cu(O-2)](+) (1b') forms reversibly (lambda(max) = 415 nm) with k(1) (8.2 +/- 0.4) x 10(3) M(-1) s(-1) and K-1 = k(1)/k(-1) = (284 +/- 9) M(-1) at 183 K, with Delta H-1 degrees (-32 +/- 1) kJ mol(-1), Delta S-1 degrees = (-127 +/- 3) J K-1 mol(-1). Two types of Cu(II)-O-2(-) complexes form in the reaction of 2a: a 2:1 open form (i.e., [(D-1)Cu-2(O-2)(EtCN)](2+), 2b) and a bis-O-2 2:2 open adduct (i.e., [(D-1)Cu-2(O-2)(2)](2+), 2b'). For the formation of 2b, k(1) (1.63 +/- 0.01) x 10(4) M(-1) s(-1) and K-1 = (2.03 +/- 0.04) x 10(3) M(-1) at 183 K. Complexes 2b and 2b' have identical spectroscopic properties (lambda(max) = 416 nm, epsilon = 4500 M(-1) cm(-1)) per Cu-O-2 unit, and their rate constants are statistically related. Intermediates 1b' and 2b further convert into (mu-peroxo)dicopper(II) [(2 Cu):(1 O-2)] complexes. [((TMPAE)Cu)(2)(O-2)](2+) (1c') (lambda(max) = 532 nm, epsilon = 9380 M(-1) cm(-1)) forms in a second-order reaction of 1b' with 1a' with K1K2 = (2.1 +/- 0.4) X 10(11) M(-2) at 183 K (Delta H(12)degrees = -77 +/- 1 kJ mol(-1) and Delta S(12)degrees = -203 +/- 5 J K-1 mol(-1)), while [D-1)Cu-2(O-2)](2+) (2c) (lambda(max) 540 nn, epsilon = 11 100 M(-1) cm(-1)) is generated from 2b in an intramolecular reaction, with k(2) = (3.51 +/- 0. 05) x 10(1) s(-1) and k(on), = k(1)k(2)/k(-1) (7.1 +/- 0.2) x 10(4) M-l s(-1) (183 K). The overall formation of 2c is faster than for 1c' or [((TMPA)Cu)(2)(O-2)](2+) (1c) because of a more positive entropy of activation (Delta S-on(double dagger), = (-139 +/- 3) J K-1 mol(-1) for 2c vs Delta S-on(double dagger) = (-201 +/- 5) J K-1 mol(-1) for 1c). However, this significantly enhanced kinetic reactivity (for 2a --> 2c) is not reflected by an analogous increase in thermodynamic stability. [(D-1)Cu-2(O-2)](2+) (2c) is enthalpically less stable (Delta H(12)degrees = (-34.8 +/- 0.4) kJ mol(-1)) than Cu2O2 species 1c and 1c' (Delta H(12)degrees = -81 to -77 kJ mol(-1), respectively), which are formed from mononuclearprecursors. There is a substantially larger overall formation entropy for 2c [Delta S(12)degrees = (-89.3 +/- 1.5) J K-1 mol(-1) compared to -220 and -203 J K-1 mol(-1) for 1c and 1c', respectively] since Cu2O2 formation is an intramolecular, rather than intermolecular, process. Examination of other kinetic parameters and spectral differences provides complementary information that 2c has a strained structure. In fact, 2c is not the ultimate oxidation product: relief of steric constraints occurs at higher temperatures by a slow rearrangement (lambda(max) = 540 nm --> lambda(max) = 529 nm) producing (Cu2O2)(n) oligomers containing intermolecular Cu-O-2-Cu bonds. A particularly stable trimer species [((D-1)Cu-2(O-2))(3)](6+) (2d) was characterized, with Delta H-3 degrees (-153 kJ mol(-1))/3 = -51 KJ mol(-1) per Cu2O2 unit, intermediate between that seen for 2c, 1c, and 1c'. Thus, (peroxo)dicopper(II) complexes formed from mononuclear precursors are the most stable, while secondary rearrangements within intramolecularly formed Cu-2-O-2 complexes with dinucleating ligands can and do occur. Comparisons are made with relevant copper-dioxygen complexes, and the chemical and biological relevance of this chemistry is discussed.