C O M M U N I C A T I O N S
mV/s and is assigned to the V4+/V3+ couple with E1/2 ) -1.56 V
vs Fc/Fc+. Complex 1-Nb(NCO) likewise displays two redox events
in its CV. One event, assigned as the 1e oxidation of 1-Nb(NCO)
(Nb5+/Nb4+ couple), has an E1/2 ) -1.1 V vs Fc/Fc+ and is
quasireversible. The other event is centered at -2.9 V vs Fc/Fc+
and is completely irreversible (see Supporting Information, SI).
Assigned to a metal-centered Nb4+/Nb3+ reduction, the irrevers-
ibility of this process is consistent with 1e reduction resulting in
rapid decarbonylation of 1-Nb(NCO) (Vide supra), the process of
interest.
set and the d0 electron count of vanadium, such a pathway cannot
be ruled out. Furthermore, this reaction is effectively the reverse
of the decarbonylation observed in the reduction of 1-Nb(NCO)
(Scheme 1A) and can be likened to an isoelectronic reverse reaction
of azide oxidation of 1-V, the method originally used to synthesize
1-VN-.11
N- abstraction by CO as documented herein is remarkable and
complements previously observed reactivity of metal-ligand
multiple bonds with CO. Both terminal imido and bridging nitrido
ligands have been shown to combine with CO while effecting 2e
reduction of the ligated metal.12–16 Similarly, discrete metal
complexes bearing oxo ligands have been shown to provide CO2
upon treatment with CO,17 and heterogeneous catalysis provides
many examples of CO acting as a terminal reducing agent on metal
oxide materials to produce CO2.18 In addition, the newly observed
reactivity may shed light on the process of isocyanate formation
observed in the reaction of CO with NH3 over transition metal
surfaces.19,20 Our observation of simple, complete transfer of N-
to CO to generate free (NCO)- significantly enlarges the precedent
for transformations of this type. Furthermore, the generation of an
open coordination site that has no observed affinity for CO leaves
the door open for the metal center to engage in further chemistry,
an attractive feature for catalytic schemes.
Given the observed propensity for cyanate dissociation from the
vanadium metal center upon reduction of 1-V(NCO), the question
arose as to whether CO could effect N- transfer from 1-VN- with
formal 2e reduction of vanadium. No change was observed upon
mixing when a solution of Na[1-VN] in THF was treated with 1
atm of CO at 23 °C. However, over the course of 24 h, a gradual
color change did ensue from bright yellow to dark forest green.
After removing the volatile components from the reaction mixture
under reduced pressure, the resulting dark green-brown residue was
extracted with n-hexane and filtered through a sintered glass frit.
Concentration and subsequent storage of the filtrate at -35 °C gave
dark green crystals of 1-V, as confirmed by spectroscopic methods,7
in 71% yield (eq 1). The IR spectrum of an off-white powder
collected in the initial filtration revealed it to be the reaction
coproduct sodium cyanate (νNCO ) 2228 cm-1). The yield of sodium
cyanate was measured to be 77% through gravimetric analysis.8
To further confirm the formation of sodium cyanate, the treatment
was repeated using 13CO. The 13C NMR spectrum of the off-white
powder dissolved in D2O contained a single resonance centered at
129 ppm (see SI), in agreement with the reported chemical shift of
Acknowledgment. The authors would like to thank BP for
financial support, the MIT DCIF staff for assistance with spectro-
scopic characterization, and Peter Mu¨ller for assistance with
crystallographic studies.
Supporting Information Available: Experimental details for the
synthesis of 1-V(NCO) and 1-V, crystallographic data on 1-V(NCO),
and electrochemical data on 1-V(NCO) and 1-Nb(NCO). This material
cyanate.9 Additionally, the reaction was monitored via 1H and 51
V
NMR by carrying it out in flame-sealed NMR tubes. Under a static
atmosphere of 1 atm of CO, the reaction proceeds to completion
in just over 24 h with complete consumption of Na[1-VN],
concomitant formation of 1-V, and no observed intermediates.
Furthermore, the addition of 2 equiv of 12-crown-4 as a sodium-
sequestering reagent to the reaction mixture had no qualitative effect
on the rate of reaction as assayed by NMR.
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