(2-(Dimethylammonium)ethyl)cyclopentadienyltricarbonyl-metalates: Group VI metal zwitterions. Attenuation of the brønsted basicity and nucleophilicity of formally anionic metal centers

Protonation of (2-(dimethylamino)ethyl)cyclopentadienyl (Cp ^N) group VI metal carbonyl anions with acetic acid proceeds at the amine to provide zwitterionic (2-(dimethylammonium)- ethyl)cyclopentadienyltricarbonylmetalates, M(CO) ₃(η ⁵-Cp ^NH) (M = Cr (1), Mo (2), W (3)). Few zwitterionic organometalates with anionic metal centers and positive charges that cannot be delocalized have been reported to date. The impact of the intramolecular charge separation on the structures and reactivity of zwitterionic organometalates is of current interest. The M(CO) ₃ units of structurally characterized 1-3 are significantly perturbed in solution and the solid state due to ion-pairing between the pendant ammonium cation and anionic metal center. The concentration and solvent dependence of the ¹H NMR ammonium hydrogen chemical shifts of 1-3 and the intramolecular N⋯M separations of these zwitterions in the solid state are consistent with three-center-four-electron N - H⋯M hydrogen bonding. One other hydrogen bond with a zerovalent group VI metal acceptor has been characterized to date. The intramolecular charge separation attenuates the reactivity of the anionic metal centers of 1-3 relative to that of [M(CO) ₃(η ⁵-Cp)] ^-. Structurally characterized [WH(CO) ₃- (η ⁵-Cp ^NH)]Cl (4) was the only isolable salt obtained from protonation of 1-3; the Cr and Mo analogues are unstable relative to 1 and 2, respectively. Structurally characterized [M(AuPPh ₃)(CO) ₃(η ⁵-Cp ^NH)]Cl (M = Mo (5), W(6)) were synthesized via reactions of 2 and 3, respectively, with Ph ₃PAuCl and protonation of M(AuPPh ₃)(CO) ₃(η ⁵-Cp ^N) (M = Mo (8), W (9)) with HCl. Pure samples of the analogous chromium salt were inaccessible from either 1 or structurally characterized Cr(AuPPh ₃)(CO) ₃(η ⁵-Cp ^N) (7).