Synthesis and Characterization of Bimetallic Copper (I) Complexes Supported by a Hexadentate Naphthyridine-Based Macrocycle Ligand

Herein, we describe the synthesis and characterisation of N,N′–di–tert–butyl–3,7–diaza–1,5(2,7)–1,8–naphthyridinacyclooctaphane and its coordination chemistry with CuI centres. The macrocyclic naphtyridinaphane ligand (tBuN6) reacts selectively with 2 equivalents of tetrakis acetonitrile copper (I) precursor, yielding the bimetallic CuI–complex [Cu2(tBuN6)(MeCN)2][BF4]2, 1·BF4. Each CuI–atom has a pentacoordinate geometry, with elongated distances with the amine N–donors (2.25 Å), in which the fifth coordination position belongs to a cuprophilic interaction with its vicinal CuI–atom (2.74 Å). Complex 1·BF4 exists in equilibrium in acetonitrile solutions with complex 2, [Cu2(tBuN6)2][BF4]2, and free [Cu(MeCN)4[BF4] due to ligand/solvent exchange. Complex 2 can be formed in high yields when tBuN6 is combined with 1 equivalent of [Cu(MeCN)4[BF4]. Both CuI–atoms remain pentacoordinate with the same Cu···Cu distance but with increased distances with the amine N–donors (2.31 Å). Ligand tBuN6, however, exhibits two different conformations in the solid structure: syn(boat/boat) for the ligand with amine N–donors attached to the CuI–centre, and syn(chair/chair) for the ligand with free amine N–donors. Complex 1·BF4 reacts with 1 equivalent of [Bu4N][Cl] generating selectively complex 3, [Cu2(μ–Cl)(tBuN6)][BF4]. The presence of the bridging Cl–atom shortens the Cu···Cu distance (2.51 Å) and increases the stability of the dicopper complex in acetonitrile solutions and solid state. Counter ion exchange of 1·BF4 with sodium tetrakis 3,5–bis(trifluoromethyl)phenyl borate (NaB(ArF)4) generates 1·B(ArF)4, which behaves similarly to 1·BF4 in acetonitrile solutions. However, in tetrahydrofuran (THF) solutions, 1·B(ArF)4 exhibits higher stability, with no observed formation of complex 2. In THF, complex 1·B(ArF)4 contains one MeCN–bridging molecule, which is described as a three-centre two-electron bonding [Cu2(μ–MeCN)(tBuN6)][B(ArF)4]2. When complex 1·B(ArF)4 is placed under CO atmosphere in THF solutions, the syn,syn biscarbonyl complex 4·B(ArF)4 [Cu2(tBuN6)(CO)2][B(ArF)4]2 is generated. This complex is sufficiently stable in solution under CO and Ar atmosphere to be characterised by NMR and IR spectroscopy, with two stretching bands observed for the CO bound to the CuI–centres at 2102 and 2088 cm–1.