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)

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Abstract
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 {(μ-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) Å, Re−C(CN) 2.098(8) Å). With TCNQ, a stable tetranuclear complex {(μ4-TCNQ)[Re(CO)3(bpy)]4}(BF4)4 was obtained. Its structural, electrochemical, and spectroscopic analysis indicates only negligible charge transfer from the rhenium(I) centers to the extremely strong π 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 νCN = 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 νCO shifts, TCNQ•- chromophore bands) support the almost exclusive confinement of the added electron in {(μ4-TCNQ)[Re(CO)3(bpy)]4}3+ to the TCNQ bridge.

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