The
new, formally Mo(II)
complexes [Mo(η
3
-2-R-allyl)(6,6′-dmbipy)(CO)
2
Cl] (6,6′-dmbipy = 6,6′-dimethyl-2,2′-bipyridine;
2-R-allyl = allyl for R = H, 2-methallyl for R = CH
3
) and
[Mo(η
3
-2-methallyl)(pTol-bian)(CO)
2
Cl]
(pTol-bian = bis(p-tolylimino)acenaphthene) share, in this rare case,
the same structural type. The effect of the anionic π-donor
ligand X (Cl
–
vs NCS
–
) and the
2-R-allyl substituents on the cathodic behavior was explored. Both
ligands play a significant role at all stages of the reduction path.
While 2e
–
-reduced [Mo(η
3
-allyl)(6,6′-dmbipy)(CO)
2
]
−
is inert when it is ECE-generated from
[Mo(η
3
-allyl)(6,6′-dmbipy)(CO)
2
(NCS)], the Cl
–
ligand promotes Mo–Mo dimerization
by facilitating the nucleophilic attack of [Mo(η
3
-allyl)(6,6′-dmbipy)(CO)
2
]
−
at
the parent complex at ambient temperature. The replacement of the
allyl ligand by 2-methallyl has a similar effect. The Cl
–
/2-methallyl ligand assembly destabilizes even primary radical anions
of the complex containing the strongly π-accepting pTol-Bian
ligand. Under argon, the cathodic paths of [Mo(η
3
-2-R-allyl)(6,6′-dmbipy)(CO)
2
Cl] terminate at ambient
temperature with 5-coordinate [Mo(6,6′-dmbipy)(CO)
3
]
2–
instead of [Mo(η
3
-2-R-allyl)(6,6′-dmbipy)(CO)
2
]
−
, which is stabilized in chilled electrolyte.
[Mo(η
3
-allyl)(6,6′-dmbipy)(CO)
2
]
−
catalyzes CO
2
reduction only when
it is generated at the second cathodic wave of the parent complex,
while [Mo(η
3
-2-methallyl)(6,6′-dmbipy)(CO)
2
]
−
is already moderately active at the first
cathodic wave. This behavior is fully consistent with absent dimerization
under argon on the cyclic voltammetric time scale. The electrocatalytic
generation of CO and formate is hampered by the irreversible formation
of anionic tricarbonyl complexes replacing reactive [Mo(η
3
-2-methallyl)(6,6′-dmbipy)(CO)
2
]
2
along the cathodic route.