A bimetallic, coordinated-ketene complex formed from a bimetallic lithium-carbon spirocycle by lithium-mediated insertion of CO into a rhodium-carbon bond

Article abstract of DOI:10.1002/anie.200503814

(Chemical Equation Presented) Metals helping metals: The bimetallic spirocyclic bridged-carbene complex 1 reacts almost quantitatively with CO by formal insertion into the Rh-C(Li) bond to form the dimeric complex 2, the first example of a μ₂,η²-(O,C) lithium-rhodium ketene complex (see scheme). Complex 2 reacts with water to transform the ketene ligand into a methine group by CO elimination.

Full text of DOI:10.1002/anie.200503814

Angewandte
Chemie
Bimetallic Complexes
DOI: 10.1002/anie.200503814
A Bimetallic, Coordinated-Ketene Complex
Formed from a Bimetallic Lithium–Carbon
Spirocycle by Lithium-Mediated Insertion of CO
into a Rhodium–Carbon Bond**
Min Fang, Nathan D. Jones, Robert Lukowski,
Jim Tjathas, Michael J. Ferguson, and Ronald G. Cavell*
Bimetallic complexes play a large role in organometallic
chemistry in part because of the potential for cooperative
catalytic applications. Of particular interest in this context are
hetero-bimetallic systems constructed around a single-carbon
(methylene) bridge (sometimes also classified as a “bridging
carbene”). Most hetero-bimetallic systems are either “A-
frame” complexes that are bridged by bis(phosphine) ligands
[1]
À
or compounds containing a direct M M bond.
The use of lithium reagents as organic and organometallic
synthetic precursors[2–5] is also relevant to the system
described herein, and the importance of such reagents has
warranted extensive interest from synthetic, structural, and
theoretical perspectives.[6] There are, however, few structur-
ally characterized lithium systems which contain a carbon-
bridged Li–C–precious-metal assembly, and only three of
these cases involve Rh.[7,8] None of these bridged-carbene
Rh complexes carries a phosphorus substituent on the central
carbon atom.
A final facet of the system described herein is the
insertion of CO into a metal–carbon bond, which is an
important organometallic transformation.[9] In particular,
metal ketene complexes (which can formally be considered
as the derivatives of such insertion processes) are considered
to play important roles in industrial Fischer–Tropsch synthes-
es[1e,10] as well as in CO2 fixation and functionalization,[11] and
may be intermediates in the thermal and photochemical
reactions of carbene/CO complexes.[12] In most cases, the
existence of the intermediate ketene complex is only inferred
because the general instability of such complexes hampers
direct observation. Nevertheless, over the years, a number of
ketene complexes has been prepared and structurally char-
acterized. The varied chemistry of ketene complexes has been
reviewed;[10f] it was noted that up to nine coordination modes
[*] Dr. M. Fang, Dr. N. D. Jones, R. Lukowski, J. Tjathas,
Dr. M. J. Ferguson,[+] Prof. Dr. R. G. Cavell
Department of Chemistry
University of Alberta
Edmonton, Alberta T6G2G2 (Canada)
Fax: (+1)780-492-8231
E-mail: ron.cavell@ualberta.ca
[+] X-Ray Crystallography Laboratory
[**] We are indebted to the Natural Sciences and Engineering Research
Council of Canada and the Killam Trust (postdoctoral fellowship to
N.D.J.) for financial support.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2006, 45, 3097 –3101
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3097

Communications
have been identified. A few X-ray crystal structures of m2,h2-
insertion to yield the stable dimeric ketene complex 3
(Scheme 1).[23]
(O,C) M1–M2 ketene complexes have been reported;[10f,13]
however, none of these early examples contained Rh. The
isolation and characterization of stable h2-(C,O) and h2-(C,C)
ketene complexes of iridium(i) and rhodium(i)[14,15] have
renewed interest in ketene complexes in general.[15–18]
The molecular structure of 1[24] is shown in Figure 1. The
complex is monomeric, presumably as a result of the
Herein, we report a chemical sequence which combines
the above major themes and derives from the unusual
lithiated spirocyclic bridging-carbene complex 1 initially
P(1)
P(2)
formed by the 1:1 reaction[19] of [{Rh(cod)Cl}2] (cod = 1,5-
N(1)
N(2)
[20,21]
C(1)
Si(2)
=
cyclooctadiene) with [{Li2C(Ph2P NSiMe3)2}2] ([(Li2L)2],
Si(1)
Rh
Scheme 1). Although 1 is moisture sensitive, it can be isolated
C(12)
C(15)
Li
C(11)
O
C(16)
Figure 1. X-ray crystal structure of 1. Non-hydrogen atoms are repre-
sented by Gaussian ellipsoids set at 20% probability. Only the ipso-
carbon atom of each phenyl ring is shown, and hydrogen atoms are
not shown. Selected bond lengths [Š] and angles [8]: Rh–C(1) 2.105(6),
P(1)–C(1) 1.706(6), P(2)–C(1) 1.695(6), P(1)–N(1) 1.611(4), Rh–N(1)
2.160(4), P(2)–N(2) 1.579(5), Li–N(2) 2.02(1), Li–O 1.89(1), Li–C(1)
2.20(1); P(1)-C(1)-Rh 91.8(2), C(1)-P(1)-N(1) 101.5(2), P(1)-N(1)-Rh
92.6(2), N(1)-Rh-C(1) 74.1(2), P(2)-C(1)-Li 81.5(4), C(1)-Li-N(2)
77.7(4), Li-N(2)-P(2) 90.4(4), N(2)-P(2)-C(1) 108.1(3), P(1)-C(1)-P(2)
135.4(3); torsion angles [8]: Rh-C(1)-P(1)-N(1) 1.0(3), C(1)-Li-N(2)-P(2)
10.1(3).
=
Scheme 1. Synthesis of complexes 1, 3, and 8 (L=C(Ph2P NSiMe3)2).
substantial bulk of the substituents. The coordination geom-
etry around the Rh atom in 1 is nearly planar with a Rh-C(1)-
P(1)-N(1) ring torsion angle of 1.0(3)8. The Rh atom is
bonded to the cod ligand in the normal fashion with typical
distances from the metal to the centroids of the two olefin
bonds, and the metal lies 0.073 Š below the plane defined by
and stored for several weeks without apparent change at
À208C in a drybox freezer. Subsequent reaction of this
complex with other metallic precursors leads to a series of
bimetallic (or hetero-bimetallic) complexes of the form 2
(Scheme 2).[22] More significantly, 1 combines with CO by
=
the C C centroids, C(1), and N(1) (torsion angle: 2.2(3)8).
À
À
The Rh C(1) and Rh N(1) bond lengths are within the
normal ranges, but the former is shortened, which, along with
À
the relatively short P C(1) bonds, gives relatively large P-
C(1)-Rh and N-P-C(1) angles. The geometry around C(1) is
distorted tetrahedral, with angles ranging from 81.5(4)
(aP(2)-C(1)-Li) to 135.4(3)8 (aP(1)-C(1)-P(2)). This dis-
tortion could be a result of one or all of the following: the
À
short P C(1) bond lengths, the repulsion between the
positively charged Rh and Li atoms, and steric crowding.
The C(1)-Li-N(2)-P(2) ring is also nearly planar (torsion
angle: 10.1(3)8). The bond angles and lengths in 1 are
generally in good agreement with those found for other
four-membered CLiNP rings,[25] all of which are nearly planar
(C-Li-N-P torsion-angle range: À12.4(2)–6.74(8)8). The Rh–
Li separation (2.88(1) Š) is larger than the sum of covalent
radii of the individual atoms (1.35+1.34 = 2.69 Š), thus
À
indicating that there is no Rh–Li bonding. The Li C(1)
bond length (2.20(1) Š) in 1, in which the Li atom is
tricoordinate, is substantially shorter than those with tetra-
coordinate Li atoms (2.29–2.39 Š).[25a,e] The P C(1) bond
À
À
lengths (1.695(6), 1.706(6) Š) lie between typical P C single-
bond (1.80 Š) and P C double-bond (1.57 Š) lengths.[26] The
P N bond lengths (1.611(4), 1.579(5) Š) are closer to typical
À
Scheme 2. Spirocyclic bimetallic complexes 2 and the Li dimer 4. Bond
lengths are in Š.
À
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À
À
P N double-bond (1.57 Š) than to P N single-bond (1.77 Š)
lengths all lie between typical single- and double-bond lengths
À
À
= = =
À
À
values. The Si N bond lengths (1.693(5), 1.706(4) Š) are
(C O 1.43, C O 1.20, C C 1.48, C C 1.34, P C 1.80, P C
À
À
=
indicative of Si N single bonds (1.70 Š). Overall, this
structure is best formulated with an ylidic P-C-P backbone.
1.57, P N 1.77, P N 1.57 Š), thus indicating electron
delocalization in the O(1)-C(1)-C(10)-P(1)-N(1) and the
corresponding O(4)-C(4)-C(20)-P(3)-N(3) regions. The cen-
tral Li2O2 ring is nearly planar with a O(4)-Li(1)-O(1)-Li(2)
torsion angle of 8.9(4)8. The Li···Li separation of 2.58(1) Š
suggests some bonding interaction (covalent radius of Li:
Complex 1 reacts smoothly with CO by replacement of
À
the cod ligand and formal C C bond formation upon
“insertion” of CO into the Rh C bond to yield 3, which has
been fully characterized by elemental analysis, NMR spec-
troscopy (1H, 7Li, 13C, and 31P),[23] and X-ray structural
analysis.[24] The molecular structure of 3 (Figure 2) reveals a
À
À
À
1.34 Š). The Li N and Li O bond lengths of 3 are compa-
rable to those observed in complex 4 (Scheme 2).[29] The
Li(1)···O(5) contact (2.360(9) Š) is shorter than the
Li(2)···O(2) (2.715(10) Š) contact and is probably the
À
reason why the Li(1) O(4) bond is much longer than the
À
Li(2) O(1) bond. These contact separations are much smaller
than the sum of the van der Waals radii of Li and O
(3.32 Š).[30]
The metrical details for 3 suggest that resonance struc-
tures 3a–3d (see the Supporting Information) all contribute
to the overall bonding in this complex. The resonance
structure 3c (Scheme 4) suggests that the complex could
also be viewed as a rhodium-substituted enolate. Very few
metal enolate MC( O)CR2À complexes, except for a few iron
=
complexes,[13] have been structurally characterized. The
solution NMR spectroscopic chemical shifts are consistent
with the structure of complex 3. The 31P NMR spectrum
resonance signal at d = 9.1 ppm was assigned to the PNRh
phosphorus atom and showed a two-bond coupling (2JP, Rh
=
6 Hz) to Rh through mediation by the N atom. The RhCC
Figure 2. X-ray crystal structure of 3. Non-hydrogen atoms are repre-
sented by Gaussian ellipsoids set at 20% probability. Only the ipso-
carbon atom of each phenyl ring is shown, hydrogen atoms are not
shown, and only one part of the disordered carbonyl ligand
(C(3A),O(3A)) on Rh(1) is shown. Selected interatomic separations [Š]
and bond angles [8]: Si(1)–N(1) 1.742(4), N(1)–P(1) 1.611(4), P(1)–
C(10) 1.745(4), P(1)–C(11) 1.815(5), P(1)–C(21) 1.811(4), C(10)–C(1)
1.396(6), C(1)–O(1) 1.296(5), Rh(1)–C(1) 2.057(4), Rh(1)–C(2)
1.816(6), Rh(1)–C(3A) 1.922(10), C(3A)–O(3A) 1.152(13), C(2)–O(2)
1.159(7), C(10)–P(2) 1.785(4), P(2)–N(2) 1.575(4), N(2)–Li(1)
1.995(9), O(1)–Li(1) 1.933(9), Li(1)–O(4) 1.880(9), O(4)–Li(2)
1.924(9), O(1)–Li(2) 1.819(9), N(2)–Si(2) 1.699(4), Li(1)···Li(2)
2.582(13), Li(1)···O(5) 2.360(9), Li(2)···O(2) 2.715(10); Li(1)-O(1)-Li(2)
86.9(4), O(1)-Li(2)-O(4) 94.2(4), Li(2)-O(4)-Li(1) 85.5(4), O(4)-Li(1)-
O(1) 92.0(4), C(10)-C(1)-O(1) 118.0(4), C(10)-C(1)-Rh(1) 118.3(3),
Rh(1)-C(1)-O(1) 123.5(3); torsion angles [8]: O(4)-Li(1)-O(1)-Li(2)
8.9(4), Rh(2)-C(4)-C(20)-P(3) À0.5(4), P(2)-C(10)-C(1)-O(1) 3.4(5),
P(2)-N(2)-Li(1)-O(1) 6.5(4).
carbon atoms of 3 appeared at d = 238.8 ppm in the 13C NMR
1
spectrum with a JC,Rh coupling constant of 33 Hz.
We presume that 3 is formed from a Li-mediated insertion
reaction, which follows the normal substitutional replacement
of the cod ligand with CO on the Rh center to give 5
(Scheme 3). In the second step, a CO ligand on Rh inserts into
À
the Rh C bond to form 6, and the bridging CO ligand then
À
attacks at the Li atom and opens the Li C bond to form the
À
Li O bond, which is likely to be stronger due to the
oxophilicity of the Li atom, in 7. The coordinative unsatura-
tion at Rh which is created by this process is readily satisfied
by an additional equivalent of CO. The overall result is that
À
one CO molecule has “inserted” into the Rh Ccarbene bond.
central dilithium-core structure formed by two bridging
ketene moieties. The geometries around the OCRhC carbon
atoms C(1) and C(4) (sum of angles around each C atom (av):
359.9(4)8) as well as around the PCP carbon atoms C(10) and
C(20) (sum of angles around each C atom (av): 359.8(3)8) are
2
À
planar, thus suggesting sp hybridization. The Rh CC C bond
=
lengths in 3 (av: 2.058(5) Š) fall within the range of typical
Rh C(sp ) single-bond lengths (2.057(3)[27] to 2.123(5) Š[28]).
I
2
À
À
The Rh CCO bonds trans to the carbon atoms are much longer
(bond length (av): 1.923(5) Š) than the Rh CCO bonds trans
to the nitrogen atom (bond length (av): 1.817(6) Š) as a result
À
of the higher trans influence of carbon compared to nitrogen.
À
À
The O(1) C(1) (1.296(5) Š), C(1) C(10) (1.396(6) Š),
À
À
C(10) P(1) (1.745(4) Š), and P(1) N(1) (1.611(4) Š) bond
Scheme 3. A possible route to 3 from 1 and CO.
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Elimination of diethyl ether and dimerization yield 3, and this
“insertions” of free CO into Li-C-M bimetallic complexes
process probably enhances the stability of the final product.
There is no evidence for the formation of any lithium–
carbonyl intermediate.[10g]
of the type exemplified by 1.
Received: October 27, 2005
Revised: January 30, 2006
Published online: March 30, 2006
Notably, complexes analogous to 1, such as complexes 2a
À
and 2b (Scheme 2) which have weaker Rh Ccarbene bonds
(bond lengths: 2.148(2) (2a), 2.137(3) (2b), 2.105(6) Š (1)),
do not undergo insertion reactions with CO.[22] This situation
suggests that the Li atom plays a key role in promoting the
CO insertion reaction of 1, probably by inducing a higher
nucleophilicity of the proximal carbon through increased
polarity at the bridging carbene center. The nearly quantita-
tive formation of 3 also implies that a direct CO insertion into
Keywords: bimetallic compounds · bridging ligands · lithium ·
rhodium · spiro compounds
.
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Complex 3 reacts with H2O at room temperature accord-
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ligand. No intermediate was observed in the reaction.[22]
A
possible reaction pathway is illustrated in Scheme 4. Consid-
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the Supporting Information.
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negative charge, as suggested by one resonance structure 3c,
it is likely that the first step of the hydrolysis involves attack at
the Li atoms to form LiOH, and protonation of the carbene
center gives intermediate 9, which then rearranges to form
intermediate 10. Subsequent elimination of excess CO from
the Rh center affords 8.
In summary, the first m2,h2-(O,C) lithium–rhodium ketene
complex 3 was synthesized at room temperature in high yield
by insertion of CO into the spirocyclic carbene center of the
Li/Rh complex 1 by a reaction pathway in which CO first
coordinates then inserts. Hydrolysis of 3 gives 8 under
elimination of CO. The preparation of 3 suggests that a new
synthetic route to m2,h2-(O,C) lithium–metal ketene com-
plexes could be developed through the application of
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3100
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848.90, yellow plates, 0.15 ” 0.13 ” 0.05 mm3, monoclinic, space
group P21 (No. 4), a = 10.8558(7), b = 18.8081(12), c =
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10.9412(7) Š, b = 99.5810(10)8, V= 2202.8(2) Š3, Z = 2; 1calcd
=
1.280 gcmÀ3, m(MoKa) = 0.548 mmÀ1, T= 193 K, R1 (F) = 0.0540
[6633 reflections, F2o ꢀ 2s(F2ACHTUNGTRENNUNGo)], wR2 = 0.1046 for
all 8597 unique data; b) Crystallographic data for 3·Et2O:
C72H86Li2N4O7P4Rh2Si4, Mr = 1575.39, orange plates, 0.42 ”
0.38 ” 0.07 mm3, monoclinic, space group P21/n (an alternate
setting of P21/c [No. 14]), a = 15.6713(9), b = 17.2388(9), c =
31.3524(17) Š, b = 92.7650(10)8, V= 8460.1(8) Š3, Z = 4,
1calcd = 1.237 gcmÀ3, m(MoKa) = 0.570 mmÀ1, T= 193K, R1 (F) =
0.0504 [11650 reflections, F2o ꢀ 2s(Fo2)], wR2 = 0.1692 for all
17367 unique data. CCDC-277956 (1) and CCDC-285955
(3·Et2O) contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
[19] Synthesis of 1: [{RhCl(cod)}2] (0.253 g, 0.513 mmol) was added
as a solid in a single portion to a suspension of [(Li2L)2] (0.590,
0.517 mmol) in THF (15 mL) at room temperature. The resulting
suspension was stirred at room temperature for 3 h, over which
time the solution turned to a deep red color. The in situ yield
(31P NMR) of 1 was 89%. The solvent was removed under
dynamic vacuum, and the resultant solids were extracted with
diethyl ether (10 mL). The insoluble portion (LiCl) was removed
by centrifugation. The red solution was reduced in vacuo to
approximately 2 mL and kept at À208C overnight in the dry box
to yield 0.548 g of a crystalline orange solid. The supernatant
liquid was decanted, further reduced in volume, and cooled
again to obtain a second crop (0.095 g) of 1. The overall yield of 1
was 0.643 g (73.8%). Elemental analysis (%) calcd for
C43H60LiN2OP2RhSi2: C 60.8, H 7.1, N 3.3; found: C 60.2, H
7.2, N 3.2. Selected NMR spectral data (full data are given in the
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Supporting Information): 31P{1H} NMR (161.9 MHz, C6D6,
2
298K, 85% H3PO4): d = 42.4 (dd, 2JP, P = 80.4 Hz, JP, Rh
=
19.4 Hz; PNRh), 15.3 ppm (d, 2JP, P = 79.8 Hz, 2JP, Li = 5.7 Hz;
PNLi); Li{1H} NMR (155.4 MHz, C6D6, 298K, LiCl/D2O): d =
7
1.03 ppm (d, 2JP, Li = 5.7 Hz).
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1483 – 1484.
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2940.
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[22] M. Fang, N. D. Jones, K. Friesen, G. Lin, M. J. Ferguson, R.
McDonald, R. Lukowski, R. G. Cavell, unpublished results.
[23] Synthesis of 3: An orange benzene solution (7 mL) of complex 1
(0.247 g, 0.291 mmol) was exposed to CO (1 atm) in a closed
Pyrex glass vessel (50 mL) at room temperature with a gas-entry
stopcock. The solution was stirred for 2 h, and the orange
solution gradually turned green. The 31P NMR spectrum of the
solution which was extracted from the vessel showed complex 3
(89% P) and small amounts of two unidentified species: X1 at
d = 52.4 (dd, JP, P = 66 Hz, JP, R h = 20 Hz), 15.4 ppm (JP, P = 61 Hz,
JP, Rh = 0 Hz) (10% P) and X2 at d = 17.9 ppm (0.9% P). No
further reaction occurred according to 31P NMR monitoring
after the mixture was exposed to additional CO (1 atm) and
stirred for 2 h further. The CO and solvent were removed under
dynamic vacuum, and the residue was redissolved in diethyl
ether. The diethyl ether solution was concentrated to 1–2 mL
and then kept at À208C overnight in a drybox to afford 3 as a
yellow-green crystalline solid (0.127 g). The supernatant liquid
was decanted, further reduced in volume, and cooled again to
obtain a second crop (0.041 g) of 3. The overall yield of 3 was
0.168 g (73.3%). Elemental analysis (%) calcd for
C68H76Li2N4O6P4Rh2Si4·0.5Et2O:
C 54.89, H 5.50, N 3.56;
found: C 54.65, H 5.31, N 3.64. Selected NMR spectral data
(full data are given in the Supporting Information):
31P{1H} NMR (161.9 MHz, C6D6, 298 K, 85% H3PO4): d = 50.6
2
2
2
(d, JP, P = 90 Hz; PNLi), 9.1 ppm (dd, JP, P = 90 Hz, JP, Rh = 6 Hz;
PNRh); 7Li{1H} NMR (155.4 MHz, [D8]THF, 298 K, LiCl/D2O):
d = À0.25 ppm (s).
Angew. Chem. Int. Ed. 2006, 45, 3097 –3101
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3101