Accessing bimetallic complexes through a variable bridging ligand strategy

Abstract

In this contribution we are introducing a Variable Bridging Ligand (VBL) strategy for the preparation of heterometallic complexes. One synthetic challenge associated with other methods described in the existing literature stems from the fact that if there is a need to change the donor groups on a metalloligand, both the initial ligand and the resulting metalloligand must be synthesized all over again. To circumvent this problem, we reasoned that if a metal complex would have one labile ligand that can be easily replaced by various bridging polytopic species, several metalloligands can thus be generated starting from the same starting material. Experimentally, we describe here the synthesis and characterization of a new ligand, bis(pyrazol-1-yl)-N-(quinolin-8-yl)acetamide, HL^Q2Pz (Pz = pyrazolyl ring) and its reactions with copper(II) centers. In a first step, pro-metalloligand preparation was investigated using Cu(II) primary metallic centers and the HL^Q2Pz main ligand to produce (L^Q2Pz)Cu(OAc)·H₂O, (L^Q2Pz)Cu(BF₄), and (L^Q2Pz)Cu(ClO₄)·MeCN complexes. Using (L^Q2Pz)Cu(ClO₄)·MeCN as a pro-metalloligand, two metalloligands (L^Q2Pz)Cu[O₂CCH(pz)₂] and (L^Q2Pz)Cu[O₂CCH(CH₂pz)₂] were prepared. Employing the latter metalloligand, two mixed valence Cu^II/Cu^I complexes, {(L^Q2Pz)Cu^II[O₂CCH(CH₂pz)₂Cu^I(PPh₃)₂]}NO₃ and {(L^Q2Pz)Cu^II[O₂CCH(CH₂pz)₂]}₂Cu^I(PF₆) were synthesized. These compounds were characterized by NMR, UV-Vis, and IR spectroscopy, and in solid state by single crystal X-ray diffraction.