Reversible O₂ Binding to a Dinuclear Copper(I) Complex with Linked Tris(2-pyridylmethyl)amine Units: Kinetic—Thermodynamic Comparisons with Mononuclear Analogues

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)Cu¹₂(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-(C1O₄)₂ are described; an X-ray structure reveals two pentacoordinate Cu(I) ions (Cu•••Cu = 11.70 Å), 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 0₂ (Cu/O₂ = 2:1, manometry) to produce an intensely purple colored solution of adduct [(D¹)Cu₂(O₂)]²⁺ (2c), γ_max = 540 nm (∊ = 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 (γ_max = 415 nm) with k₁ = (8.2 ± 0.4) × 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) × 10⁴ M^–1 s^–1 and K₁ = (2.03 ± 0.04) × 10³ M^–1 at 183 K. Complexes 2b and 2b´ have identical spectroscopic properties (γ_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):(1 O₂)] 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) × 10¹¹ M^–2 at 183 K (ΔH₁₂° = –77 ± 1 kJ mol^–1 and ΔS₁₂° = –203 ± 5 J K^–1 mol^–1), while [(D¹)Cu₂(O₂)]²⁺ (2c) (γ_max = 540 nm, ∊ = 11 100 M^–1 cm^–1) is generated from 2b in an intramolecular reaction, with K₂ = (3.51 ± 0.05) × 10¹ s^–1 and k_on = k₁k₂/k_–1 = (7.1 ± 0.2) × 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^‡ = (—139 ± 3) J K^–1 mol^–1 for 2c vs ΔS_on^‡ = (–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 (γ_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´. Thus, (peroxo)dicopper(II) complexes formed from mononuclear precursors are the most stable, while secondary rearrangements within intramolecularly formed Cu₂—O₂ 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.