Transformations of group 7 carbonyl complexes: Possible intermediates in a homogeneous syngas conversion scheme

Article abstract of DOI:10.1021/om900804j

A variety of C-H and C-C bond forming reactions of group 7 carbonyl complexes have been studied as potential steps in a homogeneously catalyzed conversion of syngas to C₂₊ compounds. The metal formyl complexes M(CO)₃(PPh₃)₂(CHO) (M = Mn, Re) are substantially stabilized by coordination of boranes BX₃ (X = F, C₆F₅) in the form of novel boroxycarbene complexes M(CO)_3- (PPh₃)₂(CHOBX₃), but these boron-stabilized carbenes do not react with hydride sources to undergo further reduction to metal alkyls. The related manganese methoxycarbene cations [Mn(CO)_5-x(PPh₃)_x(CHOMe)]⁺ (x = 1 or 2), obtained by methylation of the formyls, do react with hydrides to form methoxymethyl complexes, which undergo further migratory insertion under an atmosphere of CO. The resulting acyls, cis- and trans-Mn(PPh₃)(CO) ₄(C(O)CH₂OMe), can be alkylated to form the cationic carbene complex [Mn(PPh₃)(CO)₄(C(OR)CH₂OMe)] ⁺, which undergoes a 1,2 hydride shift to form 1,2-dialkoxyethylene, which is displaced from the metal, releasing triflate or diethyl ether adducts of [Mn(PPh₃)(CO)₄]⁺. The acyl can also be protonated with HOTf to form a hydroxycarbene complex, which rearranges to Mn(PPh₃)(CO)₄(CH₂COOMe) and is protonolyzed to yield methyl acetate and [Mn(PPh₃)(CO)₄]⁺; addition of L (L = PPh₃, CO) to the manganese cation regenerates [Mn(PPh₃)(CO)₄(L)]⁺. Since the original formyl complex can be obtained by the reaction of [Mn(PPh₃)(CO) ₅]⁺ with [PtH(dmpe)₂]⁺, which in turn can be generated from H₂, this set of transformations amounts to a stoichiometric cycle for selectively converting H₂ and CO into a C₂ compound under mild conditions.

Full text of DOI:10.1021/om900804j

6218 Organometallics 2009, 28, 6218–6227
DOI: 10.1021/om900804j
Transformations of Group 7 Carbonyl Complexes: Possible Intermediates
in a Homogeneous Syngas Conversion Scheme
Paul R. Elowe, Nathan M. West, Jay A. Labinger,* and John E. Bercaw*
Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology,
Pasadena, California 91125
Received September 14, 2009
A variety of C-H and C-C bond forming reactions of group 7 carbonyl complexes have been
studied as potential steps in a homogeneously catalyzed conversion of syngas to C2þ compounds. The
metal formyl complexes M(CO)3(PPh3)2(CHO) (M=Mn, Re) are substantially stabilized by coordi-
nation of boranes BX3 (X = F, C6F5) in the form of novel boroxycarbene complexes M(CO)3-
(PPh3)2(CHOBX3), but these boron-stabilized carbenes do not react with hydride sources to
undergo further reduction to metal alkyls. The related manganese methoxycarbene cations
[Mn(CO)5-x(PPh3)x(CHOMe)]þ (x=1 or 2), obtained by methylation of the formyls, do react with
hydrides to form methoxymethyl complexes, which undergo further migratory insertion under an
atmosphere of CO. The resulting acyls, cis- and trans-Mn(PPh3)(CO)4(C(O)CH2OMe), can be
alkylated to form the cationic carbene complex [Mn(PPh3)(CO)4(C(OR)CH2OMe)]þ, which under-
goes a 1,2 hydride shift to form 1,2-dialkoxyethylene, which is displaced from the metal, releasing
triflate or diethyl ether adducts of [Mn(PPh3)(CO)4]þ. The acyl can also be protonated with HOTf to
form a hydroxycarbene complex, which rearranges to Mn(PPh3)(CO)4(CH2COOMe) and is proto-
nolyzed to yield methyl acetate and [Mn(PPh3)(CO)4]þ; addition of L (L = PPh3, CO) to the
manganese cation regenerates [Mn(PPh3)(CO)4(L)]þ. Since the original formyl complex can be
obtained by the reaction of [Mn(PPh3)(CO)5]þ with [PtH(dmpe)2]þ, which in turn can be generated
from H2, this set of transformations amounts to a stoichiometric cycle for selectively converting
H2 and CO into a C2 compound under mild conditions.
Introduction
the context of organometallic chemistry.2,4-6 Coordination
of a Lewis acid to the oxygen of CO or an intermediate
derived therefrom has been found to facilitate one or both of
these processes in a number of cases;2,5,7-13 we recently
reported a rhenium carbonyl complex incorporating a pen-
dant Lewis acid that achieves both transfer of hydrogen
(from [(dmpe)2PtH]þ, a species obtainable directly from H2)
to CO and C-C bond formation.14 Generation of useful
products from syngas might be effected by a sequence of
reactions starting with those shown in Scheme 1.
With anticipated decreases in the availability of petro-
leum, there is great interest in new methods for utilizing coal
and natural gas as alternative feedstocks for production of
valuable chemicals. Many such approaches involve conver-
sion of synthesis gas (syngas), a mixture of CO and H2 readily
obtained from either resource. Syngas conversion is domi-
nated by the Fischer-Tropsch process and methanol synth-
esis, both heterogeneously catalyzed reactions. During the
oil crisis of the 1970s and 1980s, a good deal of attention
turned to homogeneous catalytic hydrogenation of CO as a
possible route to selective synthesis of more complex pro-
ducts, resulting in considerable progress toward the identi-
fication and preparation of potential intermediates in syngas
transformations.1-3 The recent volatility of oil supply and
prices has sparked renewed interest in this field.
Group 7 carbonyl complexes are attractive candidates
for exploring this approach; in particular, the cationic
species [M(CO)4(PPh3)2]þ (M =Mn, 1a; M=Re, 1b) and
[Mn(CO)5(PPh3)]þ (12) are easily synthesized and have been
(6) Crawford, E. J.; Bodnar, T. W.; Cutler, A. R. J. Am. Chem. Soc.
1986, 108, 6202.
(7) Butts, S. B.; Holt, E. M.; Strauss, S. H.; Alcock, N. W.; Stimson,
R. E.; Shriver, D. F. J. Am. Chem. Soc. 1979, 101, 5864.
(8) Labinger, J. A.; Miller, J. S. J. Am. Chem. Soc. 1982, 104, 6856.
(9) Grimmett, D. L.; Labinger, J. A.; Bonfiglio, J. N.; Masuo, S. T.;
Shearin, E.; Miller, J. S. J. Am. Chem. Soc. 1982, 104, 6858.
(10) Labinger, J. A.; Bonfiglio, J. N.; Grimmett, D. L.; Masuo, S. T.;
Shearin, E.; Miller, J. S. Organometallics 1983, 2, 733.
(11) Wayland, B.; Fu, X. Science 2006, 311, 790.
(12) Carnahan, E. M.; Protasiewicz, J. D.; Lippard, S. J. Acc. Chem.
Res. 1993, 26, 90.
Both C-H and C-C bond forming steps will obviously be
crucial components of catalytic transformation of syngas to
any C2þ product, and both have been shown to be difficult in
*Corresponding authors. E-mail: bercaw@caltech.edu; jal@its.
caltech.edu.
ꢀ
ꢀ
(1) Henrici-Olive, G.; Olive, S. The Chemistry of the Catalyzed
Hydrogenation of Carbon Monoxide; Springer-Verlag: New York, 1984.
(2) Gladysz, J. A. Adv. Organomet. Chem. 1982, 20, 1.
(13) Okazaki, M.; Ohtani, T.; Inomata, S.; Tagaki, N.; Ogino, H.
(3) Dombek, B. D. Adv. Catal. 1983, 32, 325.
(4) Cutler, A. R.; Hanna, P. K.; Vites, J. C. Chem. Rev. 1988, 88, 1363.
(5) Wolczanski, P. T.; Bercaw, J. E. Acc. Chem. Res. 1986, 13, 121.
J. Am. Chem. Soc. 1998, 120, 9135.
(14) Miller, A. J. M.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc.
2008, 130, 11874.
r
pubs.acs.org/Organometallics
Published on Web 10/12/2009
2009 American Chemical Society

Article
Organometallics, Vol. 28, No. 21, 2009 6219
Scheme 1
shown to react with strong hydride sources, such as LiH-
BEt3, to form neutral formyl complexes.15-17 We report here
on the stabilization of such formyl complexes by coordina-
tion to electron-deficient boranes, the insertion of CO into
manganese(methoxymethyl) bonds, and the reactions of
resulting acyl complexes with alkylating agents or Brønsted
acids to ultimately yield trans-dialkoxyethylenes or methyl
acetate, respectively. In addition, we have integrated indivi-
dual steps to produce a stoichiometric cycle for conversion of
CO to methyl acetate by reaction with [(dmpe)2PtH]þ,
methanol, and acid.
Figure 1. Structural drawing of 4b with thermal ellipsoids at the
˚
50% probability level. Selected bond lengths (A) and angles
Results and Discussion
(deg): Re-C4, 2.096(3); Re-CO (av), 1.980; C4-O4, 1.270(4);
O4-B, 1.544(4); Re-C4-O4, 125.8(2); C4-O4-B, 124.6(3).
Boroxycarbene Complexes. Reaction of the formyl com-
plexes M(PPh3)2(CO)3(CHO) (2)17 with methyl triflate has
been previously reported to afford the cationic Fischer
methoxycarbenes [M(CO)3(PPh3)2(CHOCH3)]þ[OTf]- (3);18
we anticipated that an analogous siloxycarbene might be more
reactive toward further transformation. Surprisingly, addition
of a CH2Cl2 solution of SiMe3OTf (OTf = CF3SO3) to 2b
(prepared in situ from [Re(PPh3)2(CO)4][BF4] and LiHBEt3)
led to the neutral borane-stabilized rhenium formyl Re-
(PPh3)2(CO)3(CHOBF3) (4b) in good yield. The unexpected
product probably results from abstraction of a fluoride from
[BF4]- by SiMe3OTf, releasing triflate and BF3, which coor-
dinates to the formyl oxygen. The direct reaction of 2b with
Scheme 2
BF3 OEt2 gives 4b nearly quantitatively; in the same way, 2b
3
reacts with B(C6F5)3 to form Re(PPh3)2(CO)3(CHOB(C6F5)3)
(5b). The Mn analogues 4a and 4b were obtained similarly
from 2a (Scheme 2).
20
˚
The crystal structure of 4b is shown in Figure 1; the Re-C4
(2.055 and 1.221 A, respectively). Typical Re-C single
21-24
˚
˚
˚
(2.096(3) A) and C4-O4 (1.270(4) A) bond lengths may be
compared to those of the isoelectronic cationic methoxy-
carbene complex [Re(CO)3(PPh3)2(CHOCH3)][OTf] (3b)
bond lengths fall in the range 2.24-2.32 A.
19
˚
(2.064(3) and 1.290(4) A, respectively), as well as the
neutral formyl complex Re(C5Me5)(NO)(PPh3)(CHO)
These values indicate that complex 4b can be best described as
somewhere between a borane-stabilized formyl (A) and a boroxy-
carbene complex (B) (eq 1); the C-O distances in particular
suggest a somewhat greater contribution of the latter form.
Structures of 5a and 5b were also determined (Figure 2)
and are completely analogous. The only such complex pre-
viously crystallographically characterized (to the best of our
knowledge) is the example with tethered boranes cited
above;14 a number of borane complexes of transition metal
acyls have been reported.25-27
(15) Weiler, G.; Huttner, G.; Zsolnai, L.; Berke, H. Z. Naturforsch.,
B. Anorg. Chem., Org. Chem. 1987, 42B, 203.
(16) Gibson, D. H.; Mandal, S. K.; Owens, K.; Richardson, J. F.
Organometallics 1987, 6, 2624.
(17) Gibson, D. H.; Owens, K.; Mandal, S. K.; Sattich, W. E.;
Franco, J. O. Organometallics 1989, 8, 498.
(18) Gibson, D. H.; Mandal, S. K.; Owens, K.; Richardson, J. F.
Organometallics 1990, 9, 1936.
(19) See Supporting Information for structure.
(20) Wong, W. K.; Tam, W.; Strouse, C. E.; Gladysz, J. A. J. Chem.
Soc., Chem. Commun. 1979, 530.
(21) Bleeke, J. R.; Rausher, D. J.; Moore, D. A. Organometallics
1987, 6, 2614.
(25) Anderson, G. D. W.; Boys, O. J.; Cowley, A. R.; Green, J. C.;
Green, M. L. H.; Llewellyn, S. A.; von Beckh, C. M.; Pascu, S. I.; Vei,
I. C. J. Organomet. Chem. 2004, 689, 4407.
(22) Bleeke, J. R.; Earl, P. L. Organometallics 1989, 8, 2735.
(23) Brown, D. A.; Mandal, S. K.; Ho, D. M.; Becker, T. M.
J. Organomet. Chem. 1999, 592, 61.
(26) Llewellyn, S. A.; Green, M. L. H.; Cowley, A. R. Dalton Trans.
(24) Bodner, G. S.; Patton, A. T.; Smith, D. E.; Georgiou, S.; Tam,
W.; Wong, W. K.; Strouse, C. E.; Gladysz, J. A. Organometallics 1987, 6,
1954.
2006, 1776.
(27) Butts, S. B.; Strauss, S. H.; Holt, E. M.; Stimson, R. E.; Alcock,
N. W.; Shriver, D. F. J. Am. Chem. Soc. 1980, 102, 5093.

6220 Organometallics, Vol. 28, No. 21, 2009
Elowe et al.
˚
Figure 2. Structural drawings of 5a and 5b with thermal ellipsoids at the 50% probability level. Selected bond lengths (A) and angles
(deg) for 5a: Mn-C4, 1.994(2); Mn-CO (av), 1.839; C4-O4, 1.275(2); O4-B, 1.593(3); Mn-C4-O4, 127.40(16); C4-O4-B,
˚
129.96(17). Selected bond lengths (A) and angles (deg) for 5b: Re-C4, 2.125(5); Re-CO (av), 1.980; C4-O4, 1.282(5); O4-B, 1.575(6);
Mn-C4-O4, 126.9(3); C4-O4-B, 130.6(4).
Scheme 3
In view of our recent findings that an intramolecularly
stabilized rhenium formyl can accept a second hydride to
yield a boroxymethyl species, which then undergoes CO
insertion to form a C-C bond,14 we treated 4a and 4b with
LiHBEt3 or NaHBEt3. However, these reactions led only to
the partial re-formation of the parent formyls (2a and 2b,
respectively) and [HBF3]-; no further reduction of the
borane-stabilized formyl to a boroxyalkyl species was ob-
served. It appears that the BX3 species used here exhibit
greater hydride affinity than the borane-stabilized formyls, a
trend also manifested in the decomposition of 5a.
Formyl complexes are typically quite labile in solution, often
decomposing irreversibly to the corresponding carbonyl hydride
species with concomitant loss of a ligand;1,28 in the case of 2,
preferential loss of a phosphine over a CO ligand is observed.17
Surprisingly, both 4a and 4b are quite stable, both in solution
and in the solid state. Virtually no reaction (less than 1% of free
PPh3 was detected) of 4b could be observed after several days,
even on treatment with PMe3 at 40 °C or exposure to 1-10 atm
CO, in attempts to induce C-C bond formation. 5b is likewise
stable, showing no sign of decomposition in solution at room
temperature after 24 h, but 5a decomposes at room temperature
to the borohydride salt of the parent cationic carbonyl,
[Mn(CO)4(PPh3)2][(C6F5)3BH], 6 (Scheme 3). Apparently the
manganese formyl is effectively a stronger hydride donor than
[(C6F5)3B-H]-. In the absence of light, the conversion of 5a to 6
Conversion of 4a to a traditional Fischer carbene by
reaction with CH3OTf proceeds slowly (15 h: 10%; 36 h:
17%) to give [Re(CO)3(PPh3)2(CHOMe)][OTf] (3b); the
reaction is only slightly accelerated when an excess of Et2O
is added to promote the formation of the BF3 OEt2 adduct.
3
The analogous reactions of 5a and 5b with CH3OTf gave
slow conversion to CH4 and [M(CO)4(PPh3)2][OTf] as well as
minor unidentified products; it is not clear at what stage CH4
is formed. The siloxycarbene complex [Re(PPh3)2(CO)3-
(CHOSiMe3)][BPh4] (8), the initial target of this part of the
project, can be obtained by generating Re(PPh3)2(CO)3-
(CHO) in situ from [Re(PPh3)2(CO)4][B(C6H5)4] and [BH-
Et3]-, followed by treatment with SiMe3OTf; use of the
1
-
(followed by H NMR) exhibits clean first-order kinetics. It
B(C6H5)4 counteranion eliminates the problems caused
seems reasonable to suggest that 5a reversibly dissociates to 2a
(although no evidence for this “naked” formyl was observed)
plus B(C6F5)3 followed by hydride transfer (Scheme 3). In
contrast, on exposure to light, 5a is converted to manganese
hydride 7 and the phosphine-borane adduct Ph3P-B(C6F5)3. 7
does not appear to arise via 6, since the latter is stable to
irradiation: exposing a 4:1 mixture of 6 and 5a to light results
in a 4:1 mixture of 6 and 7. The requirement of light to induce
formation of 7 suggests possible involvement of radical path-
ways, as has been shown for related systems.29,30
by reactive fluoride. However, somewhat surprisingly (consi-
dering the expected strong Si-O bond), 8 decomposes
rapidly at room temperature to the cationic tetracarbonyl
precursor 2a and trimethylsilane. This instability precluded
isolation of 8 in high purity; it was characterized by 1H
and 31P NMR only, with the former displaying a diagnostic
signal at δ 11.3.
Formation and Carbonylation of Manganese Methoxy-
methyl Complexes. Because of the resistance of the above
boroxycarbene complexes toward further reduction, we
turned to manganese methoxycarbene complexes, which
are known to accept a hydride to form manganese methox-
ymethyl compounds.18 Mn(PPh3)2(CO)3(CH2OCH3) (9)
can be synthesized via initial reduction of [Mn(PPh3)2(CO)4]-
[BF4] (1a) to the manganese formyl (2a) by reaction with a
(28) Byers, B. H.; Brown, T. L. J. Organomet. Chem. 1977, 127, 181.
(29) Ruszczyk, R. J.; Huang, B.-L.; Atwood, J. D. J. Organomet.
Chem. 1986, 299, 205.
(30) Kuchynka, D. J.; Amatore, C.; Kochi, J. K. Inorg. Chem. 1986,
25, 4087.

Article
Organometallics, Vol. 28, No. 21, 2009 6221
Scheme 4
Scheme 5
hydride source, such as LiHBEt3,17 followed by treatment
with methyl triflate to give methoxycarbene complex 3a,
which in turn is reduced with another equivalent of hydride
to the desired complex 9. All of these steps have been
demonstrated previously, and 9 has been characterized
crystallographically.31
A similar synthesis of monophosphine analog Mn(PPh3)-
(CO)4(CH2OCH3) (10) was thwarted by the instability of
intermediate Mn(PPh3)(CO)4(CHO) (11), which quickly
decomposes to cis-Mn(PPh3)(CO)4H. (The multiple decom-
position pathways of manganese and rhenium formyl com-
plexes have been studied previously.32,33) Attempts to slow
the decomposition by performing the reduction with LiH-
BEt3 in the dark at low temperature were only partially
successful. The best yield of the formyl was obtained using
[PtH(dmpe)2][PF6] as reductant, with <10% of the hydride
observed after 10 min at room temperature, but a stable
carbene complex could not be obtained under these condi-
tions.
However, 10 can be synthesized following a procedure
discovered by Gibson, which avoids isolation of an inter-
mediate carbene complex: protonation of Mn(PPh3)(CO)4-
(CHO) (11) in methanol results in formation of half an
equivalent each of 10 and [Mn(PPh3)(CO)5]þ (12), presum-
ably via initial formation of a hydroxycarbene, exchange
with methanol, and hydride transfer (Scheme 4).34 Indeed,
we find that addition of methanol to a flask containing
[(dmpe)2PtH][PF6] and [Mn(PPh3)(CO)5][BF4] (12) followed
by the addition of p-toluenesulfonic acid after 3 min at room
temperature affords Mn(PPh3)(CO)4(CH2OCH3) (10) and
[Mn(PPh3)(CO)5]þ (12) in good yields. 10 can also be pre-
pared via reaction of Na[Mn(PPh3)(CO)5] with ClCH2OCH3
in THF.35
comparison to an authentic sample),36 suggesting the possi-
ble formation of acyls. The products were definitively iden-
tified as the isomers cis-Mn(CO)4(PPh3)(COCH2OCH3) (cis-
13) and trans-Mn(CO)4(PPh3)(COCH2OCH3) (trans-13),
respectively, formed in a 6:1 ratio (Scheme 5), by comparison
to literature data as well as independent synthesis from
Na[Mn(CO)4(PPh3)] and ClC(O)CH2OCH3. A peak corre-
sponding to free PPh3 was observed in the 31P NMR
spectrum, consistent with phosphine loss from 9. Indepen-
dent synthesis of the acyl species yields an equilibrium
mixture of cis-13, trans-13, and Mn(PPh3)(CO)4(CH2OCH3)
(10), as previously reported.37,38 Addition of 2 equiv of free
PPh3 does not significantly change either the product dis-
tribution or the reaction rate. The reaction is accelerated in
coordinating solvents such as THF and even more so in
methanol (12 h for complete reaction, vs 7 days in benzene),
as has been observed in analogous migratory insertion
reactions.37
Carbonylation of 10 under the same conditions leads,
after several days, to an identical product distribution by
1H NMR; the 13C NMR spectrum shows the growth of the
diagnostic acyl peaks corresponding to cis-13 (272.8 ppm)
and trans-13 (263.4 ppm).39 The presence of Mn(CO)5-
(COCH2OCH3) can be ruled out, as its high-frequency IR
stretching band (2130 cm1)36 is absent from the spectrum of
the product mixture; moreover, 10 does not disproportionate
in solution to give 9 and Mn(CO)5(CH2OCH3). These results
suggest that 9 does not undergo direct carbonylation to an
acyl, but rather phosphine displacement by CO to give 10,
which subsequently undergoes migratory insertion
(Scheme 5).
Bisphosphine complex Mn(PPh3)2(CO)3(CH2OCH3) (9)
reacts slowly in C6D6 solution under 1 atm of CO at room
temperature to generate three new products as determined by
1H NMR. One minor product was identified as monophos-
phine complex 10 by comparison to an authentic sample.
Signals attributable to the Mn(CH2OMe) protons for both
the major and the other minor product show no coupling
to phosphorus (Mn(CO)5(CH2OCH3) was ruled out by
We also observed that 9 reacts with LiHBEt3 to give
dimethyl ether in moderate yield (ca. 60-75%, depending
on the conditions) along with minor unidentified products.
Presumably this transformation involves nucleophilic attack
of hydride at the methylene carbon (eq 2);40 the byproduct
[Mn(PPh3)2(CO)3]- likely undergoes further reaction with
reactive impurities or borane present in solution. The rhe-
nium analogue Re(PPh3)2(CO)3(CH2OCH3) showed no
(31) See Supporting Information for structural data.
(32) Narayanan, B. A.; Amatore, C.; Casey, C. P.; Kochi, J. K. J. Am.
Chem. Soc. 1983, 105, 6351.
(33) Narayanan, B. A.; Amatore, C. A.; Kochi, J. K. Organometallics
1984, 3, 802.
(34) Gibson, D. H.; Owens, K.; Mandal, S. K.; Sattich, W. E.;
Franco, J. O. Organometallics 1991, 10, 1203.
(35) Hevia, E.; D., M.; Perez, , J.; Riera, V. Organometallics 2006, 25,
4909.
(36) Cawse, J. N.; Fiato, R. A.; Pruett, R. L. J. Organomet. Chem.
1979, 172, 405.
(37) Kraihanzel, C. S.; Maples, P. K. Inorg. Chem. 1968, 7, 1806.
(38) Noack, K.; Ruch, M.; Calderazzo, F. Inorg. Chem. 1968, 7, 345.
(39) Crowther, D. J.; Tivakornpannarai, S.; Jones, W. M. Organo-
metallics 1990, 9, 739.
(40) Alternate pathways such as initial hydride transfer to CO or to
Mn followed by reductive elimination cannot be ruled out.

6222 Organometallics, Vol. 28, No. 21, 2009
Elowe et al.
Scheme 6
Scheme 7
such reactivity, possibly reflecting the stronger M-C
bond.
Liberation of C2 Species. The previous section demon-
strated formation of the C-C coupled species Mn(CO)4-
(PPh3)(COCH2OCH3) (13), whose organic group is deriv-
able entirely (in principle) from CO and H2. An attractive
approach to further progress would be addition of an electro-
phile to the acyl oxygen of 13 to form a new carbene complex,
which might add hydride to form an alkyl and grow the chain
via another CO insertion or could undergo protonolysis of the
Mn-C bond to give a C2 organic product. Addition of
MeOTf to acyl complex 13 gives no immediately detectable
reaction, but over a period of 24 h the MeOTf 1H NMR signal
rhenium carbenes.44-48 Apparently hydride migration is
stereoselective, as only trans olefins are produced, even
though the cis isomers are thermodynamically preferred; if
trace acid is present, some isomerization to the more stable
cis species is observed.41,49
Attempted reduction of the manganese carbene intermedi-
ate 15 with [(dpme)2PtH][PF6] resulted instead in deprotona-
tion to form the vinyl complex Mn(PPh3)(CO)4(C(OEt)d
C(H)(OMe)) (16); this transformation is also effected by
triethylamine (Scheme 7). Like the above hydride migration,
deprotonation is stereoselective, leading to the vinyl isomer
in which the alkoxy groups are trans to one another. Depro-
tonation of cationic rhenium carbenes has previously been
found to lead to rhenium vinyl complexes.46-48 In contrast,
Cutler found that the analogous iron carbene will accept a
hydride from LiHBEt3.4,50 Placing a methanol solution of 16
under 1000 psi of CO does not lead to observable carbonyla-
tion products.
Protonation gives a quite different outcome from alkyla-
tion: addition of HOTf to 13 results in rapid formation of
the hydroxycarbene [Mn(PPh3)(CO)4(dC(OH)CH2OMe)]-
[OTf] (17), whose 1H NMR spectrum displays signals for the
hydroxy proton (broad singlet at δ 14.76), the methylene
group (singlet at δ 4.01), and the methoxy group (singlet at δ
3.38). 17 decomposes over time to yield manganese triflate
14. Other strong acids may be used as well; the stability of the
product depends on the nature of the counterion. Behavior
1
disappears and a new product grows in, whose H NMR
spectrum corresponds to trans-1,2-dimethoxyethene,41 ac-
companied by growth of a 31P NMR signal at 42 ppm,
identified as belonging to Mn(PPh3)(CO)4(OTf) (14).42 The
yield of this reaction is greater than 80% by 1H NMR.
If EtOTf is used as the alkylating agent, the reaction
proceeds similarly to give the asymmetrically substituted
olefin trans-1-ethoxy-2-methoxyethene, whose 1H NMR
spectrum matches literature data.41 With triethyloxonium
tetrafluoroborate, in contrast, alkylation of the acyl oxygen
occurs over a period of several hours, forming the originally
expected cationic carbene complex [Mn(PPh3)(CO)4(dC-
(OEt)CH2OMe)][BF4] (15), identified by its 1H NMR signals
(ethyl triplet and quartet at δ 4.28 and 1.36, respectively;
methylene singlet at δ 4.12; methoxy singlet at δ 3.68). 15
decomposes over time to liberate trans-1-ethoxy-2-methoxy-
ethene.
The dialkoxyolefins probably arise via alkylation of the
acyl oxygen followed by a 1,2 hydride shift. The coordinated
olefin is then displaced by triflate anion, or diethyl ether in
the case of Et3Oþ (Scheme 6). The latter is apparently a
(kinetically) more effective alkylating agent, allowing the
intermediate carbene complex 15 to be observed. Similar
transformations have been reported for CpFe(CO)2-
(C(dO)CH2OMe)4,6,43 as well as unstabilized iron and
(44) Brookhart, M.; Tucker, J. R.; Husk, G. R. J. Am. Chem. Soc.
1983, 105, 258.
(45) Dazinger, G.; Kirchner, K. Organometallics 2004, 23, 6281.
(46) Hatton, W. G.; Gladysz, J. A. J. Am. Chem. Soc. 1983, 105, 6157.
(47) Bodner, G. S.; Smith, D. E.; Hatton, W. G.; Heah, P. C.;
Georgiou, S.; Rheingold, A. L.; Geib, S. J.; Hutchinson, J. P.; Gladysz,
J. A. J. Am. Chem. Soc. 1987, 109, 7688.
(41) Taskinen, E. Magn. Reson. Chem. 1998, 36, 573.
(42) Schwieger, M.; Beck, W. Z. Anorg. Allg. Chem. 1991, 595, 203.
(43) Crawford, E. J.; Lambert, C.; Menard, K. P.; Cutler, A. R.
J. Am. Chem. Soc. 1985, 107, 3130.
(48) Peng, T.-S.; Gladysz, J. A. Organometallics 1990, 9, 2884.
(49) Taskinen, E.; Bjorkqvist, H. Struct. Chem. 1994, 5, 321.
(50) Bodnar, T.; Coman, G.; LaCroce, S.; Lambert, C.; Menard, K.;
Cutler, A. J. Am. Chem. Soc. 1981, 103, 2471.

Article
Organometallics, Vol. 28, No. 21, 2009 6223
assigned to cation 19. The latter peaks are observed at
intermediate stages during NMR monitoring of the proton-
ation of 13, supporting the proposed mechanism via 19. The
analogous acid-catalyzed isomerization of CpFe(CO)2(C-
(dO)CH2OMe) to CpFe(CO)2(CH2C(dO)OMe), as well
as formation of methyl acetate with a stoichiometric amount
of acid, has been previously reported.4,6,43 Placing a metha-
nol solution of 18 under 1000 psi of CO does not lead to
products resulting from migratory insertion.
Prospects and Conclusions
Reactions of the sort shown in Scheme 1 have long been
studied as models for potential steps in promising homo-
geneous catalytic approaches to syngas conversion. The
more recent discovery of late transition metal complexes
that activate dihydrogen to generate metal hydrides suffi-
ciently nucleophilic to attack coordinated CO may provide a
way to realize that potential. However, a number of hurdles
still need to be surmounted; in particular, the nature of the
reagents and reaction conditions required to effect each of
the various steps will generally differ strongly. For example,
a Lewis acid added to facilitate hydride transfer or CO
insertion may interfere with generation of the metal hydride
reagent or the hydride transfer step, or it may bind so
strongly to an intermediate or product as to preclude cata-
lysis, as is the case with our earlier linked rhenium-boron
complex.14 More detailed understanding of the individual
steps and the range of reagents/conditions under which they
can be carried out is needed to determine whether and how
they can be assembled into a practical catalytic system.
In the present work we first attempted to overcome the
instability of group 7 formyls with the use of Lewis acidic
boranes. While this approach was successful in that it
allowed isolation and structural characterization of these
potential intermediates, coordination of boranes does not
facilitate further reduction or C-C coupling; indeed, in
several cases C-H bond formation was eventually reversed
by transfer of hydride to boron. We next turned to bis-
(phosphine)manganese complex [Mn(CO)4(PPh3)2]þ (1a),
which Gibson has previously used as a model system, where-
in it is possible to isolate all of the intermediates in the
reduction (using borohydrides) of CO to a methoxymethyl
group.34 Methoxymethyl complex Mn(PPh3)2(CO)3(CH2-
OCH3) (9) undergoes CO migratory insertion under very
mild conditions (RT, 1 atm CO), a result that seems surpris-
ing considering the strength of the Mn-CO bonds (as
indicated by the low average CO stretching frequency of
1938 cm-1). In fact, the reaction almost certainly begins with
displacement of one phosphine by CO, since carbonyla-
tion of the monophosphine complex Mn(PPh3)(CO)4-
(CH2OCH3) (10) gives the same product distribution.
Figure 3. X-ray crystal structure of (PPh3)Mn(CO)4(CH2CO2Me)
(18) with thermal ellipsoids at the 50% probability level. Selected
bond lengths (A) and angles (deg) for 18: Mn-C5, 2.1948(11);
C5-C6, 1.4775(14); C6-O5, 1.2231(12); C6-O6, 1.3555(13);
Mn-C5-C6, 108.98(6); O5-C6-O6, 121.39(10).
˚
Scheme 8
analogous to the above alkylation reactions would be ex-
pected to yield methoxyacetaldehyde by hydride migration
followed by tautomerization. We see none of this product;
rather methyl acetate is formed.51 Most probably this is the
result of protonolysis of the methoxy group to yield a ketene
intermediate, which undergoes rapid nucleophilic attack at
the central carbon to form carbomethoxymethyl complex 18,
followed by protonolysis (Scheme 8).6
Complex 18 was independently synthesized in good yield
by addition of TsOCH2C(O)OCH3 to a solution of [Na]-
[(PPh3)Mn(CO)4] in THF; the spectroscopic data match
those for the previously reported synthesis (in 0.2% yield),52
and the structure was further characterized by X-ray crystallo-
graphy (Figure 3). The 1H NMR spectrum of 18 in CD2Cl2
displays a doublet at δ 0.91 ppm (JPH=7 Hz) for the methy-
lene and a singlet at δ 3.51 for the methyl group; addition of
triflic acid causes downfield shifts to δ 1.42 and 4.00 ppm,
Unlike 1a, [Mn(PPh3)(CO)5]þ (12) reacts with the transi-
tion metal hydride donor [(dmpe)2PtH]þ. This represents
important progress toward a catalytic system, as Dubois has
shown that this hydride can be formed by cleavage of H2 with
[(dmpe)2Pt]2þ and base, potentially delivering H- and Hþ
from H2. By adding [(dmpe)2PtH][PF6] followed by p-tolu-
enesulfonic acid to 12 in methanol, we can carry out the
overall conversion to 10 (previously reported as a sequence
of separate steps) in a one-pot reaction. Since methanol
solutions of 10 undergo facile migratory insertion with CO,
this amounts to conversion of two CO molecules to the C2
species Mn(PPh3)(CO)4(C(dO)CH2OCH3) (13), using an
(51) This 1H NMR data matches that available for methyl acetate in
the Spectral Database for Organic Compounds. Vacuum transfer of the
volatiles from a completed NMR-scale reaction contained methyl
acetate, further confirming its release during the reaction.
(52) Engelbrecht, J.; Greiser, T.; Weiss, E. J. Organomet. Chem. 1981,
204, 79.

6224 Organometallics, Vol. 28, No. 21, 2009
Elowe et al.
Scheme 9
Scheme 10
approach to homogeneous syngas conversion, if a set of
compatible reagents and conditions can be found, appears
quite promising.
Experimental Section
General Considerations. All air- and moisture-sensitive com-
pounds were manipulated using standard vacuum line, Schlenk,
or cannula techniques or in a glovebox under a nitrogen atmo-
sphere. Solvents for air- and moisture-sensitive reactions were
dried over sodium benzophenone ketyl or calcium hydride or by
passage through activated alumina columns.53 Benzene-d6 was
purchased from Cambridge Isotopes and dried over sodium
benzophenone ketyl. Dichloromethane-d2 was purchased from
Cambridge Isotopes and distilled from calcium hydride. THF-d8
was purchased from Cambridge Isotopes and dried either over
sodium benzophenone ketyl or by passage through a column of
activated alumina. Other materials were used as received. Re-
(CO)5Br and Mn(CO)5Br were obtained from Strem, while
ClCH2OCH3 and ClC(O)CH2OCH3 were purchased from Al-
drich. Preparations of Re(CO)3(PPh3)2Br,54 [Re(CO)4(PPh3)2]-
[BF4],17 Mn(CO)4(PPh3)Br,55 [Mn(CO)5(PPh3)][BF4],17 Mn-
(CO)3(PPh3)2Br,54 [Mn(CO)4(PPh3)2][BF4],17 [Pt(dmpe)2][PF6]2,56
[Pt(dmpe)2H][PF6],56 [Re(CO)3(PPh3)2(CHOMe)][BPh4],18 [Mn-
(CO)3(PPh3)2(CHOCH3)][OTf],18 and cis-Mn(CO)4(PPh3)(CH2O-
CH3)35 were carried out using literature procedures.
H2-derived hydride reagent, in a one-pot reaction, in a
relatively short amount of time.
Alkylation of the acyl oxygen of 13 with alkyl triflates or
oxonium reagents is fairly slow. The alkoxycarbene inter-
mediate (not observed with triflates) undergoes hydride
migration to give free trans-dialkoxyethylene; no manganese
coordinated olefin is observed. The inability of this metal
center to support a bound olefin stands in contrast to the
[CpFe(CO)2]þ system, for which the olefin remains bound
after hydride migration; it is also interesting that while
hydride migration is stereoselective for both systems, they
give the opposite stereochemistry: cis olefin for Fe,4,6 trans
for Mn.
On the other hand, 13 reacts rapidly with HOTf, a strong
Brønsted acid; the initially formed hydroxycarbene
[Mn(PPh3)(CO)4(dC(OH)CH2OMe)][OTf] (17) rearranges
via coordinated ketene and ultimately yields free methyl
acetate. (Similar transformations have been reported by
Cutler for the release of C2 products from CpFe(CO)2(C-
(dO)CH2OR),4,6,43 but a key difference between the two
systems is that the iron methoxymethyl does not undergo
migratory insertion under CO, instead requiring phosphine
addition.) This completes (on paper) a cycle for the synthesis
of C2 organics from CO, starting with recyclable
[Mn(PPh3)(CO)5]þ (12), as shown in Scheme 9.
Of course, as written, this cycle cannot be carried out
catalytically, since (as noted above) several steps require
reagents and conditions that are not mutually compatible.
To cite just one example, the protonolytic release of methyl
acetate was achieved in CH2Cl2; it does not occur in metha-
nol, the solvent in which 13 is generated, probably because of
the leveling of the acid strength. Nonetheless, it suggests
intriguing possibilities, as can be seen if the reactions of
Scheme 9, along with the formation of the Pt hydride
reagent, are rewritten substituting a generic acid BHþ for
the various acids actually involved (Scheme 10). With the
addition of one more step, hydrolysis of methyl acetate, the
overall reaction amounts to direct catalytic conversion of
syngas to acetic acid, a process that currently is carried out
indirectly via methanol synthesis and carbonylation. This
Instrumentation. 1H and 31P NMR spectra were recorded on a
Varian Mercury 300 spectrometer at 299.868 and 121.389 MHz,
respectively, at room temperature. 13C NMR spectra were
recorded on a Varian INOVA-500 spectrometer at 125.903
1
MHz at room temperature. All H NMR chemical shifts are
1
reported relative to TMS, and H (residual) chemical shifts of
the solvent are used as secondary standard. 13C NMR chemical
shifts are reported relative to the internal solvent. 31P NMR
chemical shifts are reported relative to an external H3PO4 (85%)
standard. 19F NMR chemical shifts are reported relative to an
external CCl3F standard. Elemental analyses were performed by
Desert Analytics, Tuscon, AZ. X-ray crystallography was car-
ried out by Dr. Michael W. Day and Lawrence M. Henling using
an Enraf-Nonius CAD-4 diffractometer. IR spectra were re-
corded on a Nicolet 6700 FT-IR spectrometer. High-pressure
NMR experiments were carried out using similar equipment and
procedures to those described previously.57
(53) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518.
(54) Bond, A. M.; Colton, R.; McDonald, M. E. Inorg. Chem. 1978,
17, 2842.
(55) Angelici, R. J.; Basolo, F. J. Am. Chem. Soc. 1962, 84, 2495.
(56) Miedaner, A.; DuBois, D. L.; Curtis, C. J.; Haltiwanger, R. C.
Organometallics 1993, 12, 299.
(57) Owen, J. S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc.
2006, 128, 2005.

Article
NMR-Scale Preparation of Re(CO)3(PPh3)2(CHO) (2b).
[Re(CO)4(PPh3)2][BF4] (1b) (19 mg, 0.022 mmol) was placed in
a J-Young NMR tube and suspended in THF-d8 (0.7 mL).
LiHBEt3 (1 M in THF, 22 μL, 1 equiv) was syringed into the
tube. The sealed tube was shaken vigorously to give a yellow
solution. 1H NMR (RT, 300 MHz, THF-d8): δ 7.32-7.43 (18H,
m, ArH), 7.45-7.56 (12H, m, ArH), 13.86 (1H, s, CHO). 31P
NMR (RT, 121 MHz, THF-d8): 13.2 ppm (s).
Organometallics, Vol. 28, No. 21, 2009 6225
vial. LiHBEt3 (1 M in THF, 378 μL, 1 equiv) was syringed in to give
a yellow solution after filtration. THF was evaporated to give the
crude formyl species and LiBF4 byproduct as a yellow residue.
BF3 OEt2 (62 μL, 1.3 equiv) was dissolved in CH2Cl2 (5 mL) and
3
added to the residue, and the solution was stirred for 5 min, after
which the solvent was evaporated. The solid was recrystallized
from CH2Cl2/petroleum ether to give 292 mg (0.340 mmol, 90%)
of a white crystalline solid.
NMR-Scale Reduction of [Mn(CO)5(PPh3)][BF4] (12). In the
glovebox, [Mn(CO)5(PPh3)][BF4] (12) (11 mg, 0.019 mmol) was
placed in a J-Young NMR tube and suspended in THF-d8 (0.4
mL). The suspension was frozen in the cold well. Fresh THF-d8
was added to the top of the layer and frozen. Finally, LiHBEt3 (1
M in THF, 20 μL, 1 equiv) was syringed into the tube and frozen
as well. Outside the glovebox, the tube was placed in a -78 °C
bath until ready to collect data. The tube was thawed and shaken
vigorously immediately before placing it into the NMR probe,
giving a yellow solution that contained cis- and trans-Mn(CO)4-
(PPh3)(CHO) (11) and Mn(CO)4(PPh3)H in a 10.5:1:3 ratio,
Mn(CO)3(PPh3)2(CHOBF3) (4a). In the glovebox, a suspen-
sion of [Mn(CO)4(PPh3)2][BF4] (1a) (331 mg, 0.425 mmol) in
THF (3 mL) was stirred in a vial. LiHBEt3 (1 M in THF, 425 μL,
1 equiv) was syringed in to give a yellow solution after filtration.
BF3 OEt2 (70 μL, 0.56 mmol, 1.3 equiv) was syringed into the
3
THF solution and stirred for 5 min, after which the solvent was
evaporated. The solid was recrystallized from THF/petroleum
ether to give 204 mg (0.268 mmol, 63%) of a yellow crystalline
solid. Anal. Calcd for C40H31BF3MnO4P2: C, 62.93; H, 4.49.
Found: C, 62.86; H, 4.22. 1H NMR (RT, 300 MHz, CD2Cl2): δ
7.38-7.58 (30H, m, ArH), 12.80 (1H, s, CHOBF3). 13C NMR
(RT, 126 MHz, CD2Cl2) partial: 129.4 (t, JC-P=5.0 Hz, Ar),
131.2 (s, Ar), 133.4 (t, JC-P=5.2 Hz, Ar), 134.5 (d, JC-P=44.5
Hz, Ar), 218.2 (t, JC-P=18.2 Hz, cis CO’s), 220.9 (t, JC-P=16.5
Hz, trans CO); a signal for the carbene carbon could not be
1
respectively. H NMR (RT, 300 MHz, THF-d8): δ -7.26 (d,
2JH-P = 33.6 Hz, Mn-H), 7.23-7.74 (m, ArH), 13.59 (d,
3
3JH-P = 2.3 Hz, cis-CHO), 14.37 (d, JH-P = 9.0 Hz, trans-
CHO). 31P NMR (RT, 121 MHz, THF-d8): 57.2 (s, br).
When a tube containing a similar mixture was filled with 1
atm of CO on the Schlenk line before warming to room
temperature, the resulting yellow solution contained cis- and
trans-Mn(CO)4(PPh3)(CHO) and Mn(CO)4(PPh3)H in a 14:1:7
ratio, respectively. When the reagents were mixed at room
temperature, only cis-Mn(CO)4(PPh3)(CHO) and Mn(CO)4-
(PPh3)H were observed, in a 3:7 ratio.
observed. 31P NMR (RT, 121 MHz, CD2Cl2): 63.2 ppm (s). 19
F
NMR (RT, 471 MHz, CD2Cl2): -156.3 ppm (s).
Mn(CO)3(PPh3)2(CHOB(C6F5)3) (5a). In the glovebox, a
suspension of [Mn(CO)4(PPh3)2][BF4] (1a) (95 mg, 0.122 mmol)
in toluene (3 mL) was stirred in a vial. NaHBEt3 (1 M in toluene,
122 μL, 1 equiv) was syringed in to give a yellow solution after
filtration. B(C6F5)3 (62 mg, 0.122 mmol) was dissolved in
toluene (5 mL) and added to the reaction mixture, which was
stirred for 5 min before evaporating the solvent. The solid was
recrystallized from CH2Cl2/petroleum ether and dried in vacuo
to give 95 mg (0.079 mmol, 65%) of a yellow crystalline solid.
Anal. Calcd for C58H31BF15MnO4P2: C, 57.83; H, 2.59. Found:
C, 57.79; H, 3.04. 1H NMR (RT, 300 MHz, CD2Cl2): δ
7.29-7.48 (30H, m, ArH), 13.22 (1H, s, CHOB(C6F5)3). 13C
NMR (RT, 126 MHz, CD2Cl2) partial: 130.2 (t, JC-P=5.3 Hz,
Ar), 132.8 (s, Ar), 133.1 (t, JC-P=5.3 Hz, Ar), 134.1 (Ar), 137.1
(m, B(C6F5)), 148.6 (m, B(C6F5)), 212.4 (m, cis CO’s); a signal
for the carbene carbon could not be observed. 31P NMR (RT,
121 MHz, CD2Cl2): 64.8 ppm (s). 19F NMR (RT, 471 MHz,
CD2Cl2): -132.2, -159.2, -165.2 ppm.
Re(CO)3(PPh3)2(CHOB(C6F5)3) (5b). In the glovebox, a sus-
pension of [Re(CO)4(PPh3)2][BF4] (1b) (187 mg, 0.212 mmol) in
THF (3 mL) was stirred in a vial. LiHBEt3 (1 M in THF, 212 μL,
1 equiv) was syringed in to give a yellow solution after filtration.
THF was evaporated to give the crude formyl species and LiBF4
byproduct as a yellow residue. B(C6F5)3 (109 mg, 0.213 mmol)
was dissolved in CH2Cl2 (5 mL) and added to the residue; after
stirring for 5 min the solvent was evaporated. The solid was
recrystallized from THF/petroleum ether and dried in vacuo to
give125mg (0.0954 mmol, 45%) of a white crystalline solid. Anal.
Calcd for C58H31BF15O4P2Re: C, 52.15; H, 2.34. Found: C,
52.85; H, 2.34. 1H NMR (RT, 300 MHz, CD2Cl2): δ 7.30-7.47
(30H, m, ArH), 13.84 (1H, s, CHOB(C6F5)3). 13C NMR (RT, 126
MHz, CD2Cl2): 118.2 (m, B(C6F5)), 129.0 (t, JC-P=5.0 Hz, Ar),
131.2 (s, Ar), 133.4 (t, JC-P=5.5 Hz, Ar), 134.6 (t, JC-P=24.4 Hz,
Ar), 137.3 (dm, JC-F=250 Hz, B(C6F5)), 140.2 (dm, JC-F=252
Hz, B(C6F5)), 148.4 (dm, JC-F = 242 Hz, B(C6F5)), 192.9 (t,
NMR-Scale Reduction of [Mn(CO)5(PPh3)][BF4] (12) Using
[Pt(dmpe)2H][PF6] at Room Temperature. [Mn(CO)5(PPh3)]-
[BF4] (12) (13 mg, 0.024 mmol) and [Pt(dmpe)2H][PF6] (15
mg, 0.024 mmol) were placed in a J-Young NMR tube and
suspended in THF-d8 (0.7 mL). The tube was sealed and shaken
vigorously to give a yellow solution containing cis-Mn(CO)4-
(PPh3)(CHO) (11), trans-Mn(CO)4(PPh3)(CHO) (11), and Mn-
(CO)4(PPh3)H in a 11:1.5:1 ratio, respectively, as well as other
unidentified decomposition products. After 2.5 h, the 1H NMR
spectrum shows the same products in 7:1:3.5 ratio, demonstrat-
ing that the formyl decomposes to the hydride via loss of CO.
NMR-Scale Preparation of Mn(CO)3(PPh3)2(CHO) (2a).
[Mn(CO)4(PPh3)2][BF4] (1a) (5 mg, 0.006 mmol) was placed in
a J-Young NMR tube and suspended in THF-d8 (0.7 mL).
LiHBEt3 (1 M in THF, 6 μL, 1 equiv) was syringed into the tube.
The sealed tube was shaken vigorously to give a yellow solution.
1H NMR (RT, 300 MHz, THF-d8): δ 7.17-7.68 (30H, m, ArH),
13.55 (1H, t, 3JH-P=2.0 Hz, CHO).
Re(CO)3(PPh3)2(CHOBF3) (4b). Method A. In the glove-
box, a suspension of [Re(CO)4(PPh3)2][BF4] (1b) (125 mg, 0.142
mmol) in THF (3 mL) was stirred in a vial. LiHBEt3 (1 M in THF,
142 μL, 1 equiv) was syringed in to give a yellow solution after
filtration. THF was evaporated to give the crude formyl species
and LiBF4 byproduct as a yellow residue. TMSOTf (21 μL, ca. 1
equiv) was dissolved in CH2Cl2 (5 mL) and added to the residue,
stirring for 5 min, after which the solvent was evaporated. Color-
less crystals of the product were obtained upon recrystallization
with CH2Cl2/petroleum ether. Yield: 72 mg (0.085 mmol, 60%).
1H NMR (RT, 300 MHz, CD2Cl2): δ 7.32-7.64 (30H, m, ArH),
13.38 (1H, s, CHOBF3). 13C NMR (RT, 126 MHz, CD2Cl2):
129.3 (t, JC-P=5.1 Hz, Ar), 131.3 (s, Ar), 133.4 (t, JC-P=5.7 Hz,
Ar), 134.8 (t, JC-P =25.5 Hz, Ar), 192.9 (t, JC-P =8.3 Hz, cis
J
C-F=8.9 Hz, cis CO’s), 195.0 (t, JC-F=7.2 Hz, trans CO), 298.9
F
(s, CHOB). 31P NMR (RT, 121 MHz, CD2Cl2): 12.6 ppm (s). 19
CO’s), 196.1 (t, JC-P=8.1 Hz, trans CO), 300.7 (s, CHOBF3). 31
P
NMR (RT, 471 MHz, CD2Cl2): -132.1 (6F, m, ortho-C6F5),
-159.4 (3F, m, para-C6F5), -165.2 (6F, m, meta-C6F5). HRMS
(FABþ):m/z calcdfor C52H31BF10O4P2Re(M - C6F5) 1169.119,
found 1169.121; for C40H30O4P2Re (M- B(C6F5)3 - H) 823.1177,
found 823.1798.
NMR-Scale Preparation of [Re(CO)3(PPh3)2(CHOSiMe3)]-
[BPh4] (8). In the glovebox, a suspension of [Re(CO)4(PPh3)2]-
[BPh4] (1b) (11 mg, 0.010 mmol) in THF (3 mL) was stirred in a
NMR (RT, 121 MHz, CD2Cl2): 12.6 ppm (s). 19F NMR (RT, 471
MHz, CD2Cl2): -156.5 ppm (s). IR: νCO (cm-1, CH2Cl2) 2063,
2003, 1964. HRMS (FABþ) m/z calcd for C40H31BF2O4P2Re
(M- F) 873.1317, found 873.1331; for C40H30O4P2Re (M - BF3 -
H) 823.1177, found 823.1055.
Method B. In the glovebox, a suspension of [Re(CO)4(PPh3)2]-
[BF4] (1b) (333 mg, 0.378 mmol) in THF (3 mL) was stirred in a

6226 Organometallics, Vol. 28, No. 21, 2009
Elowe et al.
1
vial. LiHBEt3 (1 M in THF, 10 μL, 1 equiv) was syringed in to
give a yellow solution. Solvent was evaporated to give the crude
formyl species and LiBPh4 byproduct as a yellow residue. The
residue was dissolved in CD2Cl2 (0.4 mL) and transferred to a
J-Young NMR tube, and the solution frozen in the cold well.
SiMe3OTf (2 μL, 0.01 mmol, 1 equiv) was dissolved in CD2Cl2
(0.3 mL) and the resulting solution added to the J-Young tube
and frozen in the cold well. The contents of the tube were kept at
LN2 temperature until ready to be placed into the NMR probe,
where it was thawed and shaken vigorously to give a yellow
(C(O)CH2OCH3) (cis-13): H NMR (RT, 300 MHz, C6D6): δ
3.17 (3H, s, OCH3), 3.60 (2H, s, CH2), 6.98-7.07 (m, ArH),
7.35-7.44 (m, ArH), 7.59-7.67 (m, ArH). 13C NMR (RT, 126
MHz, C6D6): 58.9 (s, OCH3), 90.5 (d, JC-P = 3.0 Hz, CH2),
128.6, 130.8, 134.0, 135.1, 215.0 (CO), 215.6 (CO), 217.7 (CO),
272.3 (dt, JC-P=16.2 Hz, JC-C=3.5 Hz, C(O)CH2OCH3). 31
P
,
NMR (RT, 121 MHz, C6D6): 53.5 ppm (s, br). IR CO (cm-1
CH2Cl2): 2070, 1994, 1962, 1920, 1624. HRMS (FABþ): m/z for
C25H21MnO6P (M þ H) 503.0456, found 503.0465. trans-Mn-
(PPh3)(CO)4(C(O)CH2OCH3) (trans-13): 1H NMR (RT, 300
MHz, C6D6): 3.27 (3H, s, OCH3), 3.94 (2H, s, CH2), 6.98-7.07
(m, ArH), 7.71-7.80 (m, ArH), 7.80-7.88 (m, ArH). 13C NMR
(RT, 126 MHz, C6D6): 59.1 (s, OCH3), 91.6 (d, JC-P=4.6 Hz,
CH2), 128.8, 132.7, 135.6, 263.4 (br, C(O)CH2OCH3), The
carbonyl ligand signals could not be assigned, as they overlap
with other peaks. IR νCO (cm-1, CH2Cl2): 1636, other stretches
could not be assigned due to overlap.
1
solution. H NMR (RT, 300 MHz, CD2Cl2): δ -0.09 (9H, s,
OSi(CH3)3), 6.83-6.90 (4H, m, B(C6H5)4), 6.98-7.07 (8H, m,
B(C6H5)4), 7.27-7.36 (8H, m, B(C6H5)4), 7.37-7.47 (12H, m,
Ar-H), 7.48-7.56 (18H, m, ArH), 13.91 (1H, s, CHOSiMe3).
31P NMR (RT, 121 MHz, CD2Cl2): 11.3 ppm (s).
Mn(CO)3(PPh3)2(CH2OCH3) (9). In a vial, [Mn(CO)3(PPh3)2-
(CHOMe)][OTf] (40 mg, 0.047 mmol) was suspended in THF
(2 mL). LiHBEt3 (1 M in THF, 47 μL, 1 equiv) was syringed into
the vial. After 2 min of mixing followed by filtration, the resulting
yellow solution was placed into a small vial, which was in turn
placed in a larger vial containing petroleum ether (5 mL) for
crystallization by diffusion. After 15 h the resulting long yellow
needles were decanted, washed with petroleum ether, and dried
under vacuum to give 25 mg (0.036 mmol, 76% yield) of 9. Anal.
Calcd for C41H35MnO4P2: C, 69.20; H, 5.38. Found: C, 68.96; H,
5.17. 1H NMR (RT, 300 MHz, C6D6): δ 2.73 (3H, s, OCH3), 3.60
General Procedure for NMR-Scale Addition of Electrophiles to
Mn(PPh3)(CO)4(COCH2OCH3) (13). In the drybox 15 mg
(0.030 mmol) of Mn(PPh3)(CO)4(COCH2OCH3) (13) was
added to a vial. To the vial was added 0.6 mL of CD2Cl2
followed by 0.030 mmol of the desired electrophile. The solution
was pipetted into a J-Young NMR tube, and disappearance of
cis-13 and appearance of products monitored by H and 31P
1
NMR.
Methylation of 13 with MeOTf. MeOTf (3.4 uL, 4.9 mg, 0.030
mmol) was added, leading to formation of 14 and trans-1,2-
dimethoxyethene over time.
Ethylation of 13 with EtOTf. EtOTf (3.9 uL, 5.3 mg, 0.030
mmol) was added, leading to formation of 14 and trans-1-
ethoxy-2-methoxyethene over time.
3
(2H, t, JH-P = 7.6 Hz, CH2), 6.93-7.01 (6H, m, Ar-H),
7.02-7.11 (12H, m, Ar-H), 7.87-7.97 (12H, m, Ar-H). 13C
NMR (RT, 126 MHz, CD2Cl2): 63.8 (s, OCH3), 75.3 (t, JC-P=
12.9 Hz, CH2OMe), 128.6 (t, JC-P=4.6 Hz, Ar), 129.9 (s, Ar),
133.9 (t, JC-P=5.1 Hz, Ar) 136.7 (m, Ar), 222.8 (t, JC-P=17.8 Hz,
trans CO), 224.4 (t, JC-P=21.3 Hz, cis CO’s). 31P NMR (RT, 121
MHz, C6D6): 76.6 ppm (s). 19F NMR (RT, 471 MHz, C6D6): no
signal, confirming the absence of OTf-. IR: νCO (cm-1, CH2Cl2)
2009, 1921, 1885.
Ethylation of 13 with [Et3O][BF4]. [Et3O][BF4] (5.7 mg 0.030
mmol) was added, leading to formation of 15 and ultimately
trans-1-ethoxy-2-methoxyethene over time. Spectroscopic data
for [Mn(PPh3)(CO)4(C(OEt)CH2OCH3) (15): 1H NMR (RT,
300 MHz, CD2Cl2): δ 7.5 (15H, m, Ph); 4.28 (2H, q, 3JH-H=7
Hz, OCH2CH3); 4.12 (2H, s, CH2OCH3); 3.68 (3H, s, OCH3);
cis-Mn(CO)4(PPh3)(CH2OCH3) (10). In the glovebox, Na/
Hg (0.5 wt %, 4 equiv) was prepared in a flask. A THF (20 mL)
solution of Mn(PPh3)(CO)4Br (388 mg, 0.762 mmol) was slowly
added onto the amalgam. The mixture was allowed to stir in the
absence of light for 2 h. In another flask, ClCH2OCH3 (58 μL, 1
equiv) was dissolved in THF (10 mL) and placed in a Schlenk
tube. On the Schlenk line, the manganese solution was decanted
into the ClCH2OCH3 solution using a filter-tipped cannula. The
mixture was allowed to stir in the absence of light for 2 h, after
which all volatiles were removed. The residue was dissolved in
THF, filtered, recrystallized from THF/petroleum ether, and
dried under vacuum to give 319 mg (0.670 mmol, 88%) of 10 as a
yellow crystalline solid. 1H NMR (RT, 300 MHz, C6D6): δ 3.11
1.36 (3H, t, JH-H = 7 Hz, OCH2CH3). 31P NMR (RT, 121
3
MHz, C6D6): 52.2 (s, br).
Protonation of 13 with HOTf. HOTf (2.7 uL, 4.5 mg, 0.030
mmol) was added, leading to formation of 18 and ultimately 14
and methyl acetate over time.
Formation of Mn(PPh3)(CO)4(C(OEt)dCHOCH3) (16). In
the drybox 30 mg (0.060 mmol) of 13 and 11.4 mg (0.0600 mmol)
of [Et3O][BF4] were added to a vial. To the vial was added 0.6
mL of CD2Cl2. The solution was shaken, then pipetted into a
J-Young NMR tube, and the reaction was monitored by 1H and
31P NMR. When the alkoxycarbene 15 was at its maximum
concentration (∼ 6 h), triethylamine (11 uL, 7.9 mg, 7.8 mmol,
1.3 equiv) was added to the solution. 1H NMR (RT, 300 MHz,
3
(3H, s, OCH3), 3.90 (2H, d, JH-P=7.0 Hz, CH2), 6.93-7.02
(9H, m, Ar-H), 7.50-7.59 (6H, m, Ar-H). 13C NMR (RT,
126 MHz, CD2Cl2): 63.4 (s, OCH3), 71.1 (t, JC-P =11.7 Hz,
CH2OMe), 128.8 (d, JC-P =9.5 Hz, Ar), 130.6 (s, Ar), 133.4
(d, JC-P =10.4 Hz, Ar) 133.8 (d, JC-P =40.3 Hz, Ar), 215.7
(CO), 218.4 (d, JC-P = 21.8 Hz, CO), 218.8 (CO). 31P NMR
(RT, 121 MHz, C6D6): 61.2 ppm (s). HRMS (FABþ): m/z
calcd for C24H23MnO5P (M þ H - H2) 473.0351, found
473.0373.
4
CD2Cl2): δ 7.5 (15H, m, Ph); 6.35 (1H, d, JP-H = 2.7 Hz,
CdCH); 3.49 (3H, s, OMe); 3.17 (2H, q, JH-H = 7 Hz,
3
OCH2CH3); 0.75 (3H, t, 3JH-H=7 Hz, OCH2CH3). 31P NMR
(RT, 121 MHz, C6D6): 54.3 (s, br).
Synthesis of Mn(PPh3)(CO)4(CH2COOCH3) (18). In the dry-
box 500 mg (0.982 mmol) of Mn(PPh3)(CO)4Br was added to a
vial and dissolved in THF. This solution was pipetted into a
flask containing a Na/Hg amalgam (0.5%, 4 equiv), and the
mixture was stirred for 2 h. In a separate flask 287.5 mg (1.178
mmol, 1.2 equiv) of methyl 2-(tosyloxy)acetate was dissolved in
THF. The solution of [Na][Mn(PPh3)(CO)4] was filtered
through Celite into the flask containing methyl 2-(tosyloxy)ace-
tate. The mixture was stirred overnight before being filtered
through Celite, dried in vacuo, and triturated with pentane to
yield 420 mg (0.837 mmol, 85%) of 18. X-ray quality crystals
were obtained by layering a toluene solution of 18 with pentane
and placing in a -35 °C freezer overnight. 1H NMR (RT, 300
MHz, C6D6): δ 7.47 (6H, m, Ph); 6.93 (9H, m, Ph); 3.63 (3H, s,
OMe); 1.36 (2H, d, 3JP-H=7.2 Hz, CH2). 31P NMR (RT, 121
MHz, C6D6): 57.9 (s, br).
Carbonylation of Mn(CO)3(PPh3)2(CH2OCH3) (10) to Form
13. To a flask was added Mn(CO)3(PPh3)2(CH2OCH3) (150 mg,
0.212 mmol) followed by C6H6 (20 mL). The flask was degassed
on the Schlenk line and then filled with CO (1 atm). The flask
was sealed and allowed to stir for 7 days protected from light.
After removing all volatiles, the resulting yellow oil was tritu-
rated several times with hexanes and dried in vacuo to give 140
mg (0.197 mmol, 93%) of a yellow solid. The composition of the
solid is a mixture containing cis-Mn(PPh3)(CO)4(C(O)CH2-
OCH3) (cis-13) (80%), trans-Mn(PPh3)(CO)4(C(O)CH2OCH3)
(trans-13) (13%), and Mn(PPh3)(CO)4(CH2OCH3) (10) (7%). If
the same procedure is followed but CH3OH is used instead of
C6H6, the reaction is complete overnight. cis-Mn(PPh3)(CO)4-

Article
Organometallics, Vol. 28, No. 21, 2009 6227
Table 1. Crystal Data and Structure Refinement for 4b, 5a, 5b, and 18
4b
5a
5b
17
empirical formula
C
41H33BCl2F3O4P2Re
C58H31BF15O4P2Mn
1.37(CH2Cl2)58
1320.44
100(2)
0.71073
monoclinic
P21/c
12.514(1)
29.923(1)
14.991(1)
90
101.670(4)
90
5497.4(6)
4
1.595
C58H31BF15O4P2Re
0.51(CH2Cl2),0.49(C5H12
1414.34
100(2)
0.71073
monoclinic
P21/c
12.6862(8)
29.8277(19)
15.0080(9)
90
102.822(4)
90
5537.4(6)
4
1.697
C25H20O6PMn
3
3
59
)
mol wt
temperature, K
˚
wavelength, A
976.52
100(2)
0.71073
triclinic
P1
12.165(2)
12.4987(2)
14.550(3)
95.935(2)
97.794(2)
113.704(2)
1976.2(6)
2
1.641
3.346
964
0.30 ꢀ 0.20 ꢀ 0.06
1.81 to 28.23
-15 e h e 15
-16 e k e 16
-18 e l e 18
16 967
8763 [R(int)=0.0197]
8763/0/487
1.067
R1=0.0259
wR2=0.0647
R1=0.0275
wR2=0.0654
2.021 and -1.306
502.32
100(2)
0.71073
monoclinic
P21/n
9.9363(4)
14.5616(5)
15.7421(6)
90.000
92.966(2)
90.000
2274.65(15)
4
1.467
0.690
1032
0.26 ꢀ 0.23 ꢀ 0.22
1.91 to 38.37
-15 e h e 16
-20 e k e 25
-25 e l e 25
57 123
10 030 [R(int)=0.1014]
10 030/0/378
1.540
cryst syst
space group
˚
a, A
˚
b, A
˚
c, A
R, deg
β, deg
γ, deg
3
˚
vol, A
Z
density (calcd), Mg/m3
μ, mm-1
0.531
2653
2.401
2792
F(000)
cryst size, mm3
2θ range, deg
index ranges
0.31 ꢀ 0.24 ꢀ 0.14
2.37 to 29.67
-17 e h e 16
-41 e k e 41
-19 e l e 20
103 862
15 189 [R(int)=0.0536]
15 189/8/795
2.805
R1=0.0525
wR2=0.0843
R1=0.0766
wR2=0.0855
1.962 and -1.484
0.18 ꢀ 0.13 ꢀ 0.05
2.34 to 24.00
-19 e h e 19
-45 e k e 45
-23 e l e 23
177 247
21 028 [R(int)=0.1340]
21 028/16/778
1.739
R1=0.0639
wR2=0.0939
R1=0.1212
wR2=0.0975
7.047 and -4.595
reflns collected
indep reflns
data/restraints/params
goodness-of-fit on F2
final R indices [I > 2σ(I)]
R1=0.0326
wR2=0.0774
R1=0.0460
wR2=0.0793
0.679 and -0.702
R indices (all data)
-3
˚
largest diff peak and hole, e A
Formation of Mn(PPh3)(CO)4(CH2OCH3) (10) from
[Mn(PPh3)(CO)5][BF4] (12) with [(dmpe)2PtH][PF6] and HOTs.
To a Schlenk flask in the drybox was added [Mn(PPh3)(CO)5]-
[BF4] (12) (100 mg, 0.184 mmol) and [(dmpe)2PtH][PF6] (141.5
mg, 0.220 mmol, 1.2 equiv). The flask was removed from the
box, put on a Schlenk line, and cooled to -78 °C. Methanol was
cannulated into the flask and the mixture stirred for 15 min. A
solution of HOTs (77 mg, 0.41 mmol, 2.2 equiv) in methanol was
cannulated into the flask. The solution was allowed to warm to
RT and stirred for 1 h at RT. Solvent was removed in vacuo and
the residue dissolved in benzene, filtered through Celite, evapo-
rated to dryness, and triturated with pentane to yield 35 mg (0.74
mmol, 40% yield) of 10.
M. Henling and Dr. Michael W. Day for help with the
other X-ray structures, and Dr. Michael S. Malarek for
assistance in obtaining 13C NMR spectra. We thank
Alexander Miller for helpful discussions. We are grateful
to BP for generous financial support of this research
through the Methane Conversion Cooperative (MC2)
program.
Supporting Information Available: The crystal structures of 3b
and 9 are reported. The .cif files for all of the structures are
available free of charge via the Internet at http://pubs.acs.org.
Crystallographic data have been deposited at the CCDC, 12
Union Road, Cambridge CB2 1EZ, UK, and copies can be
obtained on request, free of charge, by quoting the publication
citation and the deposition numbers 685551 (3b), 686292 (5a),
685990 (5b), 644905 (9), and 742899 (17).
Acknowledgment. We thank Prof. Arnold L. Rheingold
for obtaining the X-ray structure of 4b, Lawrence