Theoretical modelling of tripodal CuN3 and CuN4 cuprous complexes interacting with O2, CO or CH3CN.

Dioxygen binding at copper enzymatic sites is a fundamental aspect of the catalytic activity observed in many biological systems such as the monooxygenases, especially peptidylglycine α-hydroxylating monooxygenase (PHM), in which two mononuclear Cu^I sites are involved. Biomimetic models have been developed: dipods, tripods, and, more recently, functionalized calixarenes. The modelling of calixarene systems, although not unreachable for theory yet, requires, however, a number of preliminary investigations to ensure proper calibrations if relevant description of the metal–ligand interaction at the hybrid quantum mechanical/molecular mechanics levels of theory is the aim. In this paper, we report quantum chemistry investigations on a coherent series of representative cuprous tripodal species characterized by (1) monodentate ligands [Cu(ImH)₃]⁺ (where ImH is imidazole), [Cu(MeNH₂)₃]⁺ and [Cu(MeNH₂)₄]⁺ , (2) neutral tripodal ligands [CuCH(ImH)₃]⁺, [Cu(tren)]⁺ [where tren is tris(2-aminoethyl)amine], and [Cu(trenMe₃)]⁺ [where trenMe₃ is tris(2-methylaminoethyl)amine] and (3) a hydrido-tris(pyrazolyl)borate [CuBH(Pyra)₃]. The structures of these complexes, the coordination mode ( η ² side-on or η ¹ end-on) of O₂ to Cu^I and the charge transfer from the metal to dioxygen have been computed. For some systems, the coordination by CH₃CN and CO is also reported. Beyond results relative to structural properties, an interesting feature is that it is possible to build from computational results only a set of abacuses linking the ν(¹⁶O–¹⁶O) vibrational frequency of the coordinated O₂ molecule to the O–O bond length or to the net charge of the O₂ moiety. Such abacuses may help experimentalists in distinguishing between the four possible ways of binding O₂ to CuN₃ and CuN₄ cuprous centres, namely (1) end-on triplet states, (2) side-on triplet states, (3) end-on singlet states and (4) side-on singlet states. These abacuses are extended to three tripods obtained by the substitution of one nitrogen atom by either a phosphorus or a sulphur atom. Moreover, it is shown that any factor favouring pyramidalization at copper favours charge transfer and thus coordination of the incoming O₂ moiety. All these allow insight into the coordination mode of O₂ and into the charge transfer from Cu^I in site Cu_M of PHM. [ABSTRACT FROM AUTHOR]