Proof of innocence for the quintessential noninnocent ligand TCNQ in its tetranuclear complex with four [fac-Re(CO)3(bpy)]+ groups:: Unusually different reactivity of the TCNX ligands (TCNX = TCNE, TCNQ, TCNB)

The reactions of [fac-Re(CO)(3)(bpy)(MeOH)](PF6), bpy = 2.2'-bipyridine, with the TCNX ligands (TCNE = tetracyanoethene, TCNQ = 7,7,8,8-tetracyano-p-quinodimethane, and TCNB = 1,2,4,5-tetracyanobenzene) in CH2Cl2 gave very different results, No reaction was observed with TCNB whereas TCNE produced very labile intermediates which converted under mild conditions to structurally characterized {(mu-CN)[fac-Re(CO)(3)(bpy)](2)}-(PF6) with an eclipsed conformation relative to the almost linear Re-CN-Re axis (Re-N(NC) 2.134(8) Angstrom, Re-C(CN) 2.098(8) Angstrom). With TCNQ, a stable tetranuclear complex {(mu(4)-TCNQ)[Re(CO)(3)(bpy)](4)}(BF4)(4) was obtained. Its structural, electrochemical, and spectroscopic analysis indicates only negligible charge transfer from the rhenium-(1) centers to the extremely strong pi acceptor TCNQ. Evidence includes a calculated charge of only -0.09 for coordinated TCNQ according to the empirical structure/charge correlation of Kistenmacher, a high-energy nitrile stretching band nu(CN) = 2235 cm(-1), and unprecedented large anodic shifts >0.7 V of the reduction potentials. DFT calculations were used to confirm and explain the absence of electron delocalization from the electron-rich metals to the TCNQ acceptor bridge. Correspondingly, the X-band and high-frequency (285 GHz) EPR data (g = 2.007) as well as the IR and UV-vis-NIR spectroelectrochemical results (marginal nu(CO) shifts, TCNQ(.-) chromophore bands) support the almost exclusive confinement of the added electron in {(mu(4)-TCNQ)[Re(CO)(3)(bpy)](4)}(3+) to the TCNQ bridge,