REVERSIBLE O-2 BINDING TO A DINUCLEAR COPPER(I) COMPLEX WITH LINKED TRIS(2-PYRIDYLMETHYL)AMINE UNITS - KINETIC-THERMODYNAMIC COMPARISONS WITH MONONUCLEAR ANALOGS

fajThe kinetics and thermodynamics of reaction of O-2 with copper(I) c omplexes 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- gro up linking two tris(2-pyridylmethyl)amine (TMPA) units at a 5-pyridyl position of each tetradentate moiety]. A comparative study of mononucl ear complex [(TMPAE)Cu(RCN)li (1a') [TMPAE has a -C(O)OCH3 ester subst ituent in the 5-position of one pyridyl group of TMPA] has been carrie d out. The results are compared with data from the previously investig ated complex [(TMPA)Cu(RCN)](+) (1a). The syntheses of D-1 and 2a-(ClO 4)(2) are described; an X-ray structure reveals two pentacoordinate Cu (I) ions (Cu ... Cu = 11.70 Angstrom), each bound by the N-4-tetradent ate 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, manometr y) 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 an d producing the isolatable bis-phosphine adduct [(D')Cu-2(PPh(3))(2)]( 2+). The kinetic investigation provides spectral characterization of t ransient Cu/O-2 1:1 adducts generated upon oxygenation of cold solutio ns of 1a' or 2a. [(TMPAE)Cu(O-2)](+) (1b') forms reversibly (lambda(ma x) = 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 typ es of Cu(II)-O-2(-) complexes form in the reaction of 2a: a 2:1 open f orm (i.e., [(D-1)Cu-2(O-2)(EtCN)](2+), 2b) and a bis-O-2 2:2 open addu ct (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 pro perties (lambda(max) = 416 nm, epsilon = 4500 M(-1) cm(-1)) per Cu-O-2 unit, and their rate constants are statistically related. Intermediat es 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(1 2)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, eps ilon = 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 format ion of 2c is faster than for 1c' or [((TMPA)Cu)(2)(O-2)](2+) (1c) beca use 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 enhanc ed kinetic reactivity (for 2a --> 2c) is not reflected by an analogous increase in thermodynamic stability. [(D-1)Cu-2(O-2)](2+) (2c) is ent halpically 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 m ol(-1), respectively), which are formed from mononuclearprecursors. Th ere 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 complement ary information that 2c has a strained structure. In fact, 2c is not t he 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 interm olecular 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 (-15 3 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 fo rmed 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 relev ant copper-dioxygen complexes, and the chemical and biological relevan ce of this chemistry is discussed.