ACS Catalysis
Research Article
(CO)(H)]− generates (CO2-PNN)Ru(CO)(H) along with a
15% yield of KOCHO.18
value is more acidic compared to that of (PNP)Mn−H (pKip =
31.2).
Treatment of (CO2-PNP)Mn with 1 equiv of KOtBu
generates [(*CO2-PNP)Mn][K]. The 31P NMR spectrum has
doublets at 118.7 and 113.7 ppm (J = 104 Hz). Furthermore,
Reactivity of [(*CO2-PNP)Mn]− With H2. Addition of
0.85 atm of H2 to [(*CO2-PNP)Mn]− results in a color
change and precipitation of a colorless solid over 24 h. After
heating to 65 °C, NMR spectroscopy indicates that the major
Mn species in solution is (PNP)Mn−H. IR and NMR
spectroscopy was used to identify the solid as KOCHO.
Quantification of formate by ion chromatography gives 93%
yield.
1
the H NMR spectrum of (CO2-PNP)Mn shows that the
aromatic pyridine peaks of the (CO2-PNP)Mn shift to 6.21,
5.81, and 5.39 ppm, indicating dearomatization. Single-crystal
X-ray diffraction confirmed the dearomatization, with the bond
length of the pyridine carbon and the attached methylene
spacer decreasing from 1.499 (3) to 1.39 (1)Å (Figure 2). Of
To gain further insight, the reaction was monitored by 31P
and 1H NMR spectroscopy (Figure 3). After addition of H2 to
[(*CO2-PNP)Mn]−, the 31P NMR spectrum shows a pair of
doublets that correspond to the thermodynamic isomer of
[(CO2-PNP)Mn−H]− described above. Of note, this species
remains a minor component of the reaction mixture, with
[(*CO2-PNP)Mn]− present at all time points; higher H2
pressures are required to shift the equilibrium (see the SI).
Over time, signals associated with this species decay as those
for (PNP)Mn−H grow. The reaction requires heating and
days for completion. No other identifiable intermediates are
observed.
The conversion of [(*CO2-PNP)Mn]− to formate and
(PNP)Mn−H formally requires 2 equiv of H2. When only 1
equiv of H2 is added to [(*CO2-PNP)Mn]−, ∼30% converts
to (PNP)Mn−H.
Proposed Mechanism. We postulate that loss of formate
from [(CO2-PNP)Mn−H]− and conversion to (PNP)Mn−H
could occur by 3 different mechanisms. Bimolecular mecha-
nisms, whereby 2 equiv of anionic [(CO2-PNP)Mn−H]− must
react with one another, are not considered due to steric and
electrostatic arguments. Likewise, a mechanism that proceeds
from the neutral (PNP)Mn−OCHO (gray arrows) is not
thought to be operative, at least for low-temperature catalysis
(vide infra), given the thermodynamic and mechanistic studies
presented above. As catalysis ensues from [(CO2-PNP)Mn−
H]−, whereby the ligand is deprotonated, revised Noyori-type
mechanisms52,53 (whereby the protonated ligand does not
transfer a proton but rather serves as a hydrogen-bond donor)
are also not considered.
Figure 2. Thermal ellipsoid plots (50%) of (top): (CO2-PNP)Mn
and (bottom): [(*CO2-PNP)Mn][K]. Hydrogen atoms and solvent
molecules are removed for clarity, and only one Mn from the unit cell
is shown for the anionic species. Select bond distances (Å) for (CO2-
PNP)Mn: Mn1−O3: 2.074(2); O3−C3: 1.287(2); C3−O4:
1.227(3); C3−C4: 1.554(3); C4−C5: 1.500(3); and C9−C10:
1.499(3). Select bond distances (Å) for [(*CO2-PNP)Mn][K]:
Mn1−O3: 2.099(7); O3−C10: 1.273(2); C10−O4: 1.23(1); C9−
C10: 1.53(1); C4−C7: 1.51(1); C3−C4: 1.39(1). Similar bond
distances are obtained about Mn2 of the unit cell.
The first mechanism considered is unimolecular formate loss
from [(CO2-PNP)Mn−H]− to give (*PNP)Mn, which then
reacts with CO2 or H2 to generate (CO2-PNP)Mn or
(PNP)Mn−H, respectively (Scheme 10, tan arrows). Given
that the reaction of (*PNP)Mn with either gas is complete
within minutes and similar thermodynamics associated with
the two reactions, if this mechanism is correct then the rate of
formate loss should have no dependence on CO2 or H2. The
data presented are not consistent with this mechanism.
Alternatively, H2 addition across the C−C bond of [(CO2-
PNP)Mn−H]− generates (PNP)Mn−H and formate in a
single step (Scheme 10, blue arrows). If this mechanism is
correct, then addition of H2 to the reaction of [(CO2-
PNP)Mn−H]− and CO2 should be faster (compared to that
with no H2 added). That there is no rate enhancement, and
that the reaction of [(*CO2-PNP)Mn]− with excess H2 is
significantly slower (and requires heating), suggests that this
mechanism is also incorrect.
note in the crystal structure of [(*CO2-PNP)Mn][K] is the
∼0.02 Å increase in the Mn−O bond distance relative to
(CO2-PNP)Mn. Furthermore, the IR stretch associated with
the bound CO2 shifts from 1651 to 1605 cm−1 upon
deprotonation.
The anion, [(*CO2-PNP)Mn]−, exists as a potassium-
bridged dimer in the solid state. Treatment with 18-crown-6
yields the monomeric potassium adduct, confirmed through
single-crystal X-ray diffraction. Attempts to prepare analogous
sodium or lithium species were not successful and showed
limited stability (see the SI). This suggests that the potassium
is essential in stabilizing the anion.
Finally, the hydride of [(CO2-PNP)Mn−H]− can react with
free CO2 to generate formate and (CO2-PNP)Mn (Scheme
10, pink arrow). This is consistent with the observed rate
enhancement with more CO2 pressure. Moreover, the
Titration of (CO2-PNP)Mn with the Wittig base, Ph3P
CH(CH3)2 (pKip = 28.7),51 yielded a pKip = 28.7 0.2. This
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ACS Catal. 2021, 11, 8358−8369