[(2-(Trimethylammonium)ethyl)cyclopentadienyl]tricarbonylmetalates: Group VI Metal Zwitterions

Zwitterionic [(2-(trimethylammonium)ethyl)cyclopentadienyl]tricarbonylmetalates, M(CO)3(η5-CpNMe) (M = Cr
(7), Mo (8), W (9)), provide useful models to assess intramolecular through-space electrostatic interactions on the reactivity
of formally negatively charged metals. The C3v M(CO)3 units
in these zwitterions appear largely unperturbed by the tethered
2-(trimethylammonium)ethyl group; neither significant intra- nor
intermolecular ion pairing occurs either in solution or in the
solid state. Although the donor ability of the Cp group in these
zwitterions is essentially identical with that in [M(CO)3(η5-Cp)]-
on the basis of spectroscopic and crystallographic data, the
metal-based reactivity of 7−9 appears to be decreased relative
to that of [M(CO)3(η5-Cp)]-. While the latter reacts with HCl,
AuPPh3Cl, CH3I, and I2 to provide robust M(II) complexes,
isolable products could only be obtained from reactions of 7−9
with I2 and CH3I. One important strategy in the design of
zwitterionic organometalates is that metal-based reactivity is
enhanced as the tethered counterion steric bulk increases to
decrease significant ion-pairing interactions. The relative
inertness of 7−9 compared to their protonated analogues
M(CO)3(η5-CpNH), despite the absence of structural perturbations in 7−9 due to ion pairing, indicates that ion-pairing
interactions are not the only important factors that determine
metal-based reactivity in organometallic zwitterions.