Reversible O2 binding to a dinuclear copper(I) complex with linked tris(2-pyridylmethyl)amine units: Kinetic-thermodynamic comparisons with mononuclear analogues

Article abstract of DOI:10.1021/ja00155a014

The kinetics and thermodynamics of reaction of O₂ 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 O₂ reactivity (in EtCN) of dicopper(I) complex [(D¹)Cu(I)₂(RCN)₂]²⁺ (2a) (R = Me or Et) [D¹ = 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)]⁺ (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 D¹ and 2a-(ClO₄)₂ are described; an X-ray structure reveals two pentacoordinate Cu(I) ions (Cu···Cu = 11.70 A?), each bound by the N₄-tetradentate and an EtCN molecule. Cyclic voltammetric data for 1a' and 2a are reported. At 193 K in EtCN, 2a reacts with O₂ (Cd/C₂ = 2:1, manometry) to produce an intensely purple colored solution of adduct [(D¹)Cu₂(O₂)]²⁺ (2c), λ(max) = 540 nm (ε = 11 100 M^-1 cm^-1). This peroxo-dicopper(II) species reacts with PPh₃, liberating O₂ and producing the isolatable bis-phosphine adduct [(D¹)Cu₂(PPh₃)₂]²⁺. The kinetic investigation provides spectral characterization of transient Cu/O₂ 1:1 adducts generated upon oxygenation of cold solutions of 1a' or 2a. [(TMPAE)Cu(O₂)]⁺ (1b') forms reversibly (λ(max) = 415 nm) with k₁ = (8.2 ± 0.4) x 10³ M^-1 s^-1 and K₁ = k₁/k_-1 = (284 ± 9) M^-1 at 183 K, with ΔH₁° = (-32 ± 1) kJ mol^-1, ΔS₁° = (-127 ± 3) J K^-1 mol^-1. Two types of Cu(II)-O₂^- complexes form in the reaction of 2a: a 2:1 open form (i.e., [(D¹)Cu₂(O₂)(EtCN)]²⁺, 2b) and a bis-O₂ 2:2 open adduct (i.e., [(D¹)Cu₂(O₂)₂]²⁺, 2b'). For the formation of 2b, k₁ = (1.63 ± 0.01) x 10⁴ M^-1 s^-1 and K₁ = (2.03 ± 0.04) x 10³ M^-1 at 183 K. Complexes 2b and 2b' have identical spectroscopic properties (λ(max) = 416 nm, ? = 4500 M^-1 cm^-1) per Cu-O₂ unit, and their rate constants are statistically related. Intermediates 1b' and 2b further convert into (μ-peroxo)dicopper(II) [(2 Cu):(1O₂)] complexes. [{(TMPAE)Cu}₂(O₂)]²⁺ (1c') (λ(max) = 532 nm, ? = 9380 M^-1 cm^-1) forms in a second-order reaction of 1b' with 1a' with K₁K₂ = (2.1 ± 0.4) x 10¹¹ M^-2 at 183 K (ΔH₁₂° = -77 ± 1 kJ mol^-1 and AS₁₂° = -203 ± 5 J K^-1 mol^-1), while [(D¹)Cu₂(O₂)]²⁺ (2c) (λ(max) = 540 nm, ? = 11 100 M^-1 cm^-1) is generated from 2b in an intramolecular reaction,with k₂ = (3.51 ± 0.05) x 10¹ s^-1 and k(on) = k₁k₂/k_-1 = (7.1 ± 0.2) x 10⁴ M^-1 s^-1 (183 K). The overall formation of 2c is faster than for 1c' or [{(TMPA)Cu}₂(O₂)]²⁺ (1c) because of a more positive entropy of activation (ΔS(on)paragraph = (-139 ± 3) J K^-1 mol^-1 for 2c vs ΔS(on)paragraph = (-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. [(D¹)Cu₂(O₂)]²⁺ (2c) is enthalpically less stable (ΔH₁₂° = (-34.8 ± 0.4) kJ < mol^-1) than Cu₂O₂ species 1c and 1c' (ΔH₁₂° = -81 to -77 kJ mol^-1, respectively), which are formed from mononuclear precursors. There is a substantially larger overall formation entropy for 2c [ΔS₁₂° = (-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 (λ(max) = 540 nm → λ(max) = 529 nm) producing {Cu₂O₂}(n) oligomers containing intermolecular Cu-O₂-Cu bonds. A particularly stable trimer species [{(D¹)Cu₂(O₂)}₃]⁶⁺ (2d) was characterized, with ΔH₃° = (-153 kJ mol^-1)/3 = -51 kJ mol^-1 per Cu₂O₂ unit, intermediate between that seen for 2c, 1c, and 1c'.