Binuclear Rhenium(I) Complexes with Bridging [2.2]Paracyclophane−Diimine Ligands:  Probing Electronic Coupling through π−π Interactions

Two pseudo-para substituted bis-diimino[2.2]paracyclophane ligands (4,16-bis(picolinaldimine)-[2.2]paracyclophane (BPPc) and 4,16-bis(methyl-picolinaldimine)-[2.2]paracyclophane (BmPPc)) were prepared by the condensation reaction of the appropriate picolinaldimine with 4,16-diamino-[2.2]paracyclophane (2). An improved synthesis of 2 from [2.2]paracyclophane also is reported. BPPc (3a): monoclinic, P21/c, a = 8.2238(11) Å, b = 15.336(2) Å, c = 8.4532(11) Å, β = 98.578(3)°, V = 1054.2(2) Å3, Z = 2. To investigate the binding properties of the bis-diimino[2.2]paracyclophane ligands, binuclear rhenium(I) tricarbonyl chloride complexes [Re(CO)3Cl]2(μ-BPPc) (5a) and [Re(CO)3Cl]2(μ-BmPPc) (5b) were prepared and fully characterized by infrared spectroscopy, 1H NMR spectroscopy, elemental analysis, UV−visible absorption spectroscopy, and cyclic voltammetry. Two model complexes, Re(tolyl-pyCa)(CO)3Cl (4) (tolyl-pyCa = N-(p-tolyl)-2-pyridinecarboxaldimine) and [Re(CO)3Cl]2(μ-PBP) (6) (PBP = p-phenylenebis(picolinaldimine)), also are reported. The dimeric compounds 5 and 6 each undergo two one-electron, predominantly diimine-centered reduction processes. Spectroscopic data and comproportionation constants (5a, 23 ± 9; 5b, 23 ± 9; 6, 2750 ± 540) are consistent with relatively weak interactions between the diimine groups mediated by the paracyclophane bridging group, and these results are consistent with steric and electronic factors.