Copper(I)/Dioxygen Reactivity of Mononuclear Complexes with Pyridyl and Quinolyl Tripodal Tetradentate Ligands: Reversible Formation of Cu:O2 = 1:1 and 2:1 Adducts

Ning Wei, Narasimha N. Murthy, Qin Chen, Jon Zubieta, Kenneth D. Karlin

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Copper(I) complexes possessing a series of related tripodal tetradentate ligands with pyridyl- and/or quinolylcontaining groups have been investigated in reactions with dioxygen (O2). The ligand variations allow for the testing of effects of ligand donor ability and steric factors. Copper-dioxygen complex stabilities, preference for formation of 1:1 Cu-O2 and/or 2:1 Cu2-O2 adducts, Cun-O2 (n = 1,2), spectroscopic properties, and reactivity characteristics have been investigated. The ligands are L = tris(2-pyridylmethyl)amine (TMPA), bis(2-pyridylmethyl)(2- quinolylmethyl)amine (BPQA), bis(2-quinolylmethyl)(2-pyridylmethyl)amine (BQPA), and tris(2-quinolylmethyl)amine (TMQA). The 2:1 adduct [{(TMPA)Cu}2(O2)]2+ (1c), with a trans-{μ-1,2-peroxo)dicopper(II) structure, has previously been shown to form reversibly from the interaction of O2 with the copper(I) complex [(TMPA)Cu(RCN)]+ (1a) (R = Me, Et) (Tyklár, Z.; et al. J. Am. Chem. Soc. 1993, 115, 2677). The corresponding Cu(I) complexes [(BPQA)Cu]+ (2a), [(BQPA)Cu]+ (3a), and [(TMQA)Cu]+ (4a) have been synthesized as either PF6- or ClO4- salts. They lack an RCN nitrile ligand as isolated solids, but 2a-4a appear to coordinate nitriles in MeCN or EtCN solutions. The X-ray structure of 4a has been determined (triclinic space group [formula omitted]; a = 13.130 (2), b = 16.570 (2), c = 12.956 (2) Å, α = 111.44 (1) β = 98.49 (1), γ = 84.46 (1)°; V= 2592.1 (6) Å3 4567; Z = 2), and it exhibits a pseudotetrahedral coordination to the alkylamine and three quinolyl N-donors. Copper(I) complexes 2a and 3a react with triphenylphosphine to form adducts [LCu(PPh3)]+ (2d, 3d). The X-ray structure of 3d (monoclinic space group C2/c; a = 16.341 (4), b = 19.017 (6), c = 26.860 (7) Å; β = 105.48 (2)°, V = 8044 (4) Å3; Z = 8) indicates that one of the quinolyl arms of the BQPA ligand is uncoordinated and dangles away from the Cu(I) ion, leaving a distorted tetrahedral coordination, comprising Npyridyl, Nquinolyl, Nalkylamino, and P copper ligation. Cyclic voltammetric measurements carried out on 1a-4a plus [(TMPA′)Cu(CH3CN)]+(1a′; TMPA′ possesses a-C(O)Me ester group attached to the 5-position of one pyridyl ring of TMPA) show that the Cu(II)/Cu(I) redox potentials can be greatly influenced by these ligand variations. E1/2 values span a large range, from -0.61 to -0.24 V for 1a-4a, versus Ag/AgNO3 in dimethylformamide; the value for 1a′ is -0.56 V. The BPQA complex 2a reacts rapidly with O2 at -80 °C in EtCN solvent to give an intensely purple stable solution adduct [{(BPQA)Cu}2(O2)]2+ (2c), as evidenced by manometric O2-uptake measurements (Cu:O2 = 2:1) and the UV-vis spectrum, which closely resembles that of 1c, with λmax = 535 (ε = 10 500 M-1 cm-1), ca. 440 (sh, ε = 2000 M-1 cm-1), and ca. 600 nm (sh, ε = 7600 M-1 cm-1). The chemical reactivity parallels that of 1c; reaction with PPh3 gives 2d with evolution of O2, while exposure to H+ produces hydrogen peroxide in 94% yield, as determined by iodometric titration. The BQPA-containing complex 3a reacts to form a dark brown adduct [(BQPA)Cu(O2)]+ (3b), which is stable at -80 ° C in EtCN. Manometry (Cu:O2 =1:1) and the drastically different UV-vis characteristics with λmax = 378 nm (ε = 8200 M-1 cm-1) attest to its formulation as a 1:1 adduct, formally a Cu(II)-superoxide species. Reactions of 3b with PPh3 and H+ proceed with formation of products analogous to those found with 2c, and phosphine adduct 3d could be isolated. Reaction of 3b with the TMPA results in a ligand-exchange reaction, with production of peroxo species 1c, as followed by UV-vis spectroscopy at -80 °C. The binding of O2 to 3a to give 1:1 adduct 3b is reversible, as demonstrated by cycling experiments, i.e., the alternate and repetitive removal of O2 from 3b by application of a vacuum, followed by reoxygenation of 3a; this process was monitored spectrophotometrically. Steric factors are seen to be important in this unique stabilization of the Cu-O2 complex 3b. With the even more bulky TMQA ligand complex 4a, no reaction with O2 is observed. Possible structures of the Cu-bound O2 ligand in 1:1 adduct 3b are enumerated. Dioxygen binding characteristics and variations observed for 1a-4a and 1a′ are examined in terms of electronic (i.e. redox) characteristics and steric effects and in light of recent complete kinetic-thermodynamic characterization of the O2-binding in 1a-3a (Karlin, K. D.; et al. J. Am. Chem. Soc. 1993, 115, 9506). Steric and not electronic effects appear to predominate in determining the O2-reaction chemistry. The biological relevance of 2:1 and 1:1 Cu:O2 adduct chemistry is also mentioned.

Original languageEnglish (US)
Pages (from-to)1953-1965
Number of pages13
JournalInorganic Chemistry
Issue number9
StatePublished - Apr 1 1994

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Inorganic Chemistry

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