Organometallics
Article
It is noteworthy that while both 4 and 5 are active the cis-
isomer (5) marginally outperforms trans-isomer 4 (compare
runs 7 and 8). A plausible explanation is that isomerization
occurs slowly over the catalyst run time scale and that each
isomer results in a distinct catalytic species or that there is a
longer induction time for the trans-isomer to form a
catalytically active species. To compare these results to a
literature catalyst, the pincer complex C was tested under a
similar protocol. C was more active than other catalysts after
shorter run times; for example, giving 10% isobutanol after 4 h
(entry 13) even if overall turnover numbers for C are similar to
other catalysts if these are allowed longer run times. The high
(78%) ethanol conversion over this 4 h run time for C
indicates a low overall selectivity for Guerbet products, with
significantly more of the mass balance being solid products. In
every case when catalytic activity is observed, these solids are
dissolved quickly upon addition of an excess of sodium
methoxide. Immediately after base addition, resonances
consistent with free ligand and monochelate 7 were observed,
along with minor resonances exhibiting the distinctive
broadening observed upon ligand manganese complexation.
This suggests a more dynamic system in which ligand
redistribution is occurring; similar observations are made
with ruthenium catalysts. Complex 7 was tested for isobutanol
formation to ascertain whether this was the most active species
(Table 1, Entry 9). Although a competent catalyst, it remains
inferior to the bis-chelate complexes of the same ligand
suggesting its formation is detrimental. It is not clear why
dppm remains the most effective ligand for isobutanol
production with both manganese and ruthenium. Given the
importance of ligand-assisted mechanisms in hydrogen transfer
catalysis, our working hypothesis is that involvement of the
acidic hydrogens in the methylene backbone of dppm may be
important; recent reports in related chemistry support this
hypothesis.25,36,37
A recent paper by Kireev et al. supports our hypothesis of
dppm acting as a noninnocent ligand on manganese.36 In this
paper, monochelate 7 is reacted with KHMDS to form
complex 7b, containing two highly strained 3-membered rings.
This can then be converted to hydride complex 7c under 50
atm of hydrogen (Scheme 4). Intriguingly, this paper shows
that substitution of the C-backbone atom with a phenyl group
makes conversion to the hydride significantly more favorable.
With this in mind, monochelate 8 was synthesized and tested
for isobutanol formation in our study (Table 1, entry 10).
Pleasingly, complex 8 outperforms both monochelate 7 and
the previously most effective catalyst, 2, by a significant margin,
with turnover numbers in excess of 200. This shows the
potential positive effects that substitution of the dppm
backbone can have toward catalytic activity for manganese
complexes. Again, a plausible explanation is a ligand-assisted
mechanism, that is, substitution of the backbone facilitating a
more favorable cycle of hydrogenation/dehydrogenation.
In conclusion, we show that simple dppm or dppea ligand
complexes of manganese are effective catalysts for the Guerbet
reaction leading to butanol biofuel molecules; pincer-type
complexes are not a prerequisite for competent performance.
Substitution of the dppm backbone also leads to a significant
increase in catalytic performance, giving isobutanol yields of
21%.
1
also isolated in the postreaction mixtures, and H and 13C
NMR analyses show this to be predominantly sodium formate
(peak in the 1H NMR spectrum at 8.45 ppm), which accounts
for most of the remainder of the mass balance in each case
between ethanol consumption and butanol yield. This formate
is presumably produced via a Cannizzaro-type reaction with
methanol. A small amount of sodium acetate is also seen by
conversion of ethanol with the same mechanism. Interestingly,
NMR spectra show an absence of any carbonate salt, in
contrast to isobutanol reactions using analogous ruthenium
catalysts.22
Catalysts 2 and 5 were subsequently tested for the
homocoupling of ethanol to form n-butanol (Scheme 3).
Scheme 3. Formation of n-Butanol via the Coupling of Two
Ethanol Molecules
Owing to the rate of the catalyst appearing to be one of the
limiting factors in the isobutanol chemistry, 90 h run times
were used once again. Pleasingly, it appears catalyst 5 is also
active for n-butanol formation with nearly 100 turnover
numbers over 90 h (see Table S2 for further details). Unlike in
isobutanol production where catalyst 2 shows little activity for
this reaction, this is attributed to a lack of catalyst stability
under these conditions; the postreaction mixture of 2 is dark
brown, implying catalyst decomposition. This is in contrast to
the postreaction mixture for catalyst 5 which is still a bright
yellow homogeneous solution if kept under anaerobic
conditions.
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To gain insight into the active form of the best catalyst, the
effect of adding base to catalyst 2 was investigated by 31P NMR
spectroscopy. Complex 2 is insoluble in methanol but
Full experimental procedures along with detailed
Scheme 4. Activation of Manganese Complexes Containing Substituted dppm Ligands and Their Reactivity with
aa 36
,
Hydrogen
a
Conditions: 7: R = H, (i) 50 atm H2, 50 °C, 16 h. 8: R = Ph, (ii) 1 atm H2, 25 °C, 5 min.
D
Organometallics XXXX, XXX, XXX−XXX