Reductive coupling of carbon monoxide in a rhenium carbonyl complex with pendant lewis acids

Article abstract of DOI:10.1021/ja805108z

Phosphinoborane ligands impart unique reactivity to a rhenium carbonyl cation relative to simple phosphine complexes. Addition of either triethylborohydride or a platinum hydride (that can be formed from H₂) forms a rhenium boroxycarbene. This carbene, which crystallizes as a dimer, disproportionates over a period of days to afford the starting cation and a structurally unprecedented boroxy(boroxymethyl)carbene, in which a new C-C bond has been formed between two reduced CO ligands. This product of C-C bond formation can be independently synthesized by addition of 2 equiv of hydride to the rhenium carbonyl cation. Copyright

Full text of DOI:10.1021/ja805108z

Published on Web 08/15/2008
Reductive Coupling of Carbon Monoxide in a Rhenium Carbonyl Complex
with Pendant Lewis Acids
Alexander J. M. Miller, Jay A. Labinger,* and John E. Bercaw*
Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology,
Pasadena, California 91125
Received July 2, 2008; E-mail: jal@caltech.edu; bercaw@caltech.edu
Scheme 1
Synthesis gas, a mix of CO and H2, could become a key part of
the global energy landscape as an intermediate in converting
methane and coal to fuels or chemical feedstocks.1 The heteroge-
neous Fischer-Tropsch (F-T) process, which converts syngas to
a complex mixture of hydrocarbons and oxygenates and operates
at 473-623 K, is becoming economically competitive as crude oil
prices rise.2,3 During the oil crisis of the 1970s, there was much
interest in developing homogeneous F-T chemistry, hoping to
achieve lower temperatures and greater selectivities.4 Since then,
however, there have been only scattered reports of homogeneous
catalytic syngas-to-C2 conversion, all under high temperature and/
or high pressure conditions.5,6
Scheme 2
The difficulty of forming a C-H bond from coordinated CO
appears to be a major obstacle to this approach. Stoichiometric
reductions of group 6-8 metal carbonyls have been achieved with
strong main group7,8 and early transition metal9 hydrides, with pro-
duction of C2 organics in the case of [CpFe(CO)3]+.10 Such
catalytically impractical reagents would need to be replaced with
hydrides that can be formed from H2; DuBois has shown that certain
group 10 hydrides may be promising candidates.11 Another
challenge is the facile formation of a C-C bond from a reduced
carbonyl species. Exogenous Lewis acids12 and amphoteric addi-
tives13 have previously been shown to accelerate alkyl migration
reactions, but reductive coupling of CO itself is unusual.14
We have sought to address both of these challenges with a single
design element: the incorporation of a Lewis acidic borane in the
secondary coordination sphere of a rhenium carbonyl complex. We
report here that this pendant Lewis acid facilitates the delivery of
multiple hydride equivalentssfrom both main group and late
transition metal hydridessto a CO ligand, followed by spontaneous
alkyl migration to form a C-C bond.
resonance at
δ .
13.95 and precipitation of [Pt(dmpe)2]2+
[(PPh3)2Re(CO)4]+ does not react with [HPt(dmpe)2]+, and even
addition of BEt3 (10 equiv) affords only ∼1% conversion,
highlighting the importance of the tethered borane.
Phosphinoborane15 complex 3 was constructed as shown in
Scheme 1.16 Commercially available diphenylvinylphosphine was
metallated with Re(CO)5Br at 120 °C in toluene in a sealed vessel,
yielding mer,trans-(Ph2PC2H3)2Re(CO)3Br (1) as a white powder
in good yield. Treatment of 1 with AgBF4 followed by 1 atm of
CO afforded cationic trans-[(Ph2PC2H3)2Re(CO)4][BF4] (2). Hy-
droboration with 9-BBN (9-borabicyclo[3.3.1]nonane) proceeded
over48hat70°Ctoaffordthedesiredproducttrans-[(Ph2P(CH2)2B(C8H14))2-
Re(CO)4][BF4] (3).
An X-ray diffraction (XRD) study confirmed the expected
structure of 3 (Figure S1). As strongly donating solvents were
avoided throughout the synthesis of 3, the boron centers in the
9-BBN groups remain three-coordinate. The IR spectrum of 3 shows
a single CO stretch at 1998 cm-1, similar to the PPh3 analogue
[(PPh3)2Re(CO)4][BF4] (νCO ) 2000 cm-1).17
We attribute the downfield NMR signal to a neutral boroxycarbene
such as 4 (Scheme 2), formed in 60-70% yield. In hopes of achieving
complete conversion, 1 equiv of NaHBEt3 (in toluene) was added to
a C6H5Cl solution of 3, affording a yellow solution which showed
quantitative formation of 4, as assessed by 31P{1H} NMR, IR, and
the unique 1H NMR resonance at δ 13.95. Decomposition with
formation of the starting material 3 and an unknown species took place
over several days. Performing the hydride addition in CD2Cl2 also gave
the NMR signal (along with additional products that may include
Re-Cl species), and colorless crystals grew overnight. An XRD study
verified that the boroxycarbene feature was present, but the structure
so obtained is bimetallic 4a, with the oxygen of the carbene on one
rhenium interacting with the boron from the other, generating a 14-
membered cycle (Figure 1).
The pendant borane makes cation 3 a markedly better hydride
acceptor despite the similar stretching frequencies.18 Addition of
1 equiv of [HPt(dmpe)2][PF6] (dmpe ) 1,2-bis(dimethylphos-
phino)ethane) in either C6D5Cl or 1,2-C6H4F2 leads to a new proton
Attempts to crystallize 4 by vapor diffusion of Et2O into a C6H5Cl
solution took days but eventually yielded colorless plates; XRD
revealed transformation to complex 5 (Scheme 2), a novel boroxy-
(boroxymethyl)carbene generated by formation of a new C-C bond.
9
11874 J. AM. CHEM. SOC. 2008, 130, 11874–11875
10.1021/ja805108z CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
It is noteworthy that the conversion of 3 to 5 can also be carried
out with 2 equiv of [HPt(dmpe)2]+, in slightly lower yield (∼70%).
Since the Pt hydride can be preformed externally by heterolytic
cleavage of H2 in the presence of a suitable base (KOPh or
tetramethylguanidine),21 this transformation amounts to the net
formation of a C2 species from intermediates directly obtainable
from CO and H2.
In summary, we have found that incorporation of a borane into
the secondary coordination sphere of a rhenium carbonyl complex
fundamentally alters reactivity by facilitating hydride transfer,
permitting a group 10 transition metal hydride generated from H2
to serve as hydride source, and promoting C-C bond formation
by alkyl migration, even in the absence of a strong donor. Ongoing
work is focused on further lowering the barrier to hydride transfer
as well as developing methods for liberation of the C2 organic
fragment and closing a catalytic cycle.
Acknowledgment. Larry Henling and Dr. Michael Day assisted
with crystallography. A.J.M.M. is grateful to Dr. Paul R. Elowe for
enlightening discussions. This research was generously funded by BP
through the Methane Conversion Cooperative (MC2) program.
Supporting Information Available: Full details on synthesis and
characterization for compounds 1-5, NMR experiments, and crystal-
lographic information are available. This information is available free
of charge via the Internet at http://pubs.acs.org.
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Figure 1. XRD structural representation (50% ellipsoids) of 4a·CH2Cl2
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C4-O4-B2 126.87(8). 5: Re-C4 2.0960(9), Re-O4 2.2322(7), C4-O3
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The oxygen of the boroxymethyl group coordinates to rhenium,
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We propose that 5 is the product of alkyl migration to CO in an
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transfer, to yield the boroxymethyl intermediate 6 and tetracarbonyl
cation 3 (Scheme 2). (The decanted supernatant after crystallization
of 5 indeed contained predominantly 3, along with some residual
5.) It is notable that no additional CO or other ligand is needed to
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(16) See Supporting Information for full experimental, crystallographic, and
spectroscopic details.
Addition of 2 equiv of NaHBEt3 to a C6H5Cl solution of 3
resulted in the immediate precipitation of 5 in 80-95% isolated
yield. (In contrast, [(PPh3)2Re(CO4)]+ yields a formyl with 1 equiv
of NaHBEt317 but does not react further with excess borohydride.)
This preparation allowed full characterization of 5. The asymmetry
shown in the crystal structure is evident by NMR as well, with
two doublets (12.0, 17.7 ppm) in the 31P{1H} NMR and complex
(17) Gibson, D. H.; Owens, K.; Mandal, S. K.; Sattich, W. E.; Franco, J. O.
Organometallics 1989, 8, 498–505.
(18) νCO can be roughly correlated with hydride acceptor ability: e.g.,
[Cp*Re(CO)2(NO)]+ (νCO ) 2092, 2036 cm-1) reacts with group 10
hydrides,11 while [(PPh3)2Re(CO)4]+ (νCO ) 2000 cm-1) does not.
(19) The structure shown in Figure 1 was obtained from isolated 5, which yielded
the solvate 5 · 3.5 THF · 0.5 Et2O from THF/Et2O vapor diffusion. The
structure is of higher quality than the one obtained from the solvate
5 · 3 Et2O, grown from the disproportionation reaction, Figure S2.
(20) Or intermolecular, as 4 may not be dimeric in solution. The XRD
structure of 4a shows the two carbene carbons in the 14-membered ring
separated by 5 Å, which could allow facile hydride transfer from one
to the other.
1
aromatic and aliphatic regions in the H NMR; the [CH2O] group
resonates as two doublets at 4.55 and 4.64 ppm. The infrared
spectrum of 5 exhibits two CO stretches at 1848 and 1933 cm-1
,
consistent with a relatively electron-rich species. The carbenoid
nature of 5 is apparent in the 13C{1H} NMR spectrum, with a
characteristic doublet of doublets at 303.4 ppm.
(21) Curtis, C. J.; Miedaner, A.; Ellis, W. W.; DuBois, D. L. J. Am. Chem. Soc.
2002, 124, 1918–1925.
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