(2-(Dimethylammonium)ethyl)cyclopentadienyltricarbonylmetalates: Group VI Metal Zwitterions. Attenuation of the Brønsted Basicity and Nucleophilicity of Formally Anionic Metal Centers

Protonation of (2-(dimethylamino)ethyl)cyclopentadienyl (CpN) group VI metal carbonyl
anions with acetic acid proceeds at the amine to provide zwitterionic (2-(dimethylammonium)ethyl)cyclopentadienyltricarbonylmetalates, M(CO)3(η5-CpNH) (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(η5-Cp)]-. Structurally characterized [WH(CO)3(η5-CpNH)]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(AuPPh3)(CO)3(η5-CpNH)]Cl (M = Mo (5), W(6)) were synthesized via reactions of 2 and 3,
respectively, with Ph3PAuCl and protonation of M(AuPPh3)(CO)3(η5-CpN) (M = Mo (8), W
(9)) with HCl. Pure samples of the analogous chromium salt were inaccessible from either
1 or structurally characterized Cr(AuPPh3)(CO)3(η5-CpN) (7).