Structure and reactivity of a cobalt(I) phthalaldehyde complex with both σ- and π-bonded aldehyde groups

Article abstract of DOI:10.1002/(SICI)1521-3773(19990215)38:4<552::AID-ANIE552>3.0.CO;2-B

A bidentate phthalaldehyde ligand with both σ and π coordination of the aldehyde groups is found in [(C₅Me₅)Co{(C(O)H)₂C₆H₄}] (structure depicted). This complex is the 'resting state' of the catalyst in the ring closure of the dialdehyde to give the lactone. Interchange of coordination modes occurs with a barrier of 70 kJ mol^-1 at 35°C. Investigation of other Co¹ chelate complexes with a single aldehyde group shows that the coordination mode of the aldehyde is dictated by the nature of the bonding of the other ligating group.

Full text of DOI:10.1002/(SICI)1521-3773(19990215)38:4<552::AID-ANIE552>3.0.CO;2-B

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Keywords: antibiotics
Domino reactions ´ intermediates
´
autoxidation
´
cyclizations
´
Structure and Reactivity of a Cobalt(i)
Phthalaldehyde Complex with Both s- and
p-Bonded Aldehyde Groups**
Christian P. Lenges, Maurice Brookhart,* and
Peter S. White
[1] T. T. Dabrah, T. Kaneko, W. Massefski, Jr., E. B. Whipple, J. Am.
Chem. Soc. 1997, 119, 1594 ± 1598; T. T. Dabrah, H. J. Harwood, Jr.,
L. H. Huang, N. D. Jankovich, T. Kaneko, J.-C. Li, S. Lindsey, P. M.
Moshier, T. A. Subashi, M. Therrien, P. C. Watts, J. Antibiot. 1997, 50,
1 ± 7.
Dedicated to Professor Helmut Werner
on the occasion of his 65th birthday
[2] a) K. C. Nicolaou, M. W. Härter, L. Boulton, B. Jandeleit, Angew.
Chem. 1997, 109, 1243 ± 1245; Angew. Chem. Int. Ed. Engl. 1997, 36,
1194 ± 1196; b) K. C. Nicolaou, M. H. D. Postema, N. D. Miller, G.
Yang, Angew. Chem. 1997, 109, 2922 ± 2925; Angew. Chem. Int. Ed.
Engl. 1997, 36, 2821 ± 2823; c) H. M. L. Davies, R. Calvo, G. Ahmed,
Tetrahedron Lett. 1997, 38, 1737 ± 1740; d) P. W. M. Sgarbi, D. L. J.
Clive, Chem. Commun. 1997, 2157 ± 2158; e) A. Armstrong, T. J.
Critchley, A. A. Mortlook, Synlett 1998, 552 ± 553; f) O. Kwon, D.-S.
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Chem. 1998, 110, 1978 ± 1981; Angew. Chem. Int. Ed. 1998, 37, 1877 ±
1880; g) O. Kwon, D.-S. Su, D. Meng, W. Deng, D. C. DꢁAmico, S. J.
Danishefsky, Angew. Chem. 1998, 110, 1981 ± 1983; Angew. Chem. Int.
Ed. 1998, 37, 1880 ± 1882; h) N. Waizumi, T. Itoh, T. Fukuyama,
Tetrahedron Lett. 1998, 39, 6015 ± 6018.
The coordination of the carbonyl group of a ketone or
aldehyde to a transition metal is of importance in metal-
mediated catalysis and has been investigated in some detail.
Aldehydes coordinate in a s or p fashion, depending on the
metal center, the oxidation state, the formal charge of the
complex, and the steric requirements of the substrate or
ancillary ligands (Scheme 1). In some cases an equilibrium
M
O
O
O
M
M
[3] The synthesis of 4 by a route superior to that presented in ref. [2a] will
be reported in due course.
H
R
H
R
H
R
σ
π
Scheme 1. Coordination modes of dialdehydes to transition metal com-
plexes.
[4] Strategies involving alkylation a to the ketone in 4 followed by several
attempts to append various substituents onto the ketone failed
presumably due to excessive steric screening; details will be provided
in a full account of this work.
[5] K. Ito, K. Yakushijin, Heterocycles 1978, 9, 1603 ± 1606; K. Yakushijin,
M. Kozuka, H. Furukawa Chem. Pharm. Bull. 1980, 28, 2178 ± 2184;
D. J. Lythgoe, I. McCleanaghan, C. A. Ramsden, J. Heterocycl. Chem.
1993, 30, 113 ± 117.
[6] N. Bodor, M. J. S. Dewar, A. J. Harget, J. Am. Chem. Soc. 1970, 92,
2929 ± 2936.
[7] K. B. Sharpless, R. C. Michaelson, J. Am. Chem. Soc. 1973, 95, 6136 ±
6137.
[8] a) W. Nagata, M. Yoshioka, T. Okumura, Tetrahedron Lett. 1966, 847 ±
852; b) G. Liu, T. C. Smith, H. Pfander, ibid. 1995, 36, 4979 ± 4982. The
configuration was confirmed by ROESY and COSY spectra.
[9] HMQC and COSY spectra were also in accordance with the proposed
structure 5.
[10] F. Farina, M. V. Martin, M. C. Paredes, Synthesis 1973, 167 ± 168.
[11] The configuration and connectivities of compounds 11a, 11b, and 12
were determined using ROESY, COSY and HMQC NMR spectros-
copy.
[12] C. Geletneky, S. Berger, Eur. J. Org. Chem. 1998, 1625 ± 1627.
[13] R. A. Jones in Comprehensive Heterocyclic Chemistry, Vol. 4 (Eds.:
A. R. Katritzky, C. W. Rees) Pergamon Press, Oxford, 1984, pp. 57 ±
255.
between the coordination modes is established.[1±16] Here we
describe the unique structure and reactivity of a CoI
phthalaldehyde complex in which both coordination modes
are realized.
Cobalt aldehyde[17, 18] complexes are quite rare, and tran-
sition metal complexes of dialdehydes such as phthalaldehyde
are also uncommon.[19, 20] Bosnich et al. reported[19] the
spectroscopic characterization of the cationic RhI phthalalde-
‡
hyde complex [Rh(dcpe){h1,h2-(C(O)H)2C6H4)}] .[21] This
rhodium complex is stable only at low temperatures and
undergoes catalytic formation of a five-membered lactone.
The resting state of the catalyst in this process is unknown.
The cobalt complex reported here is to our knowledge the
first structurally characterized transition metal complex in
which a dialdehyde exhibits both coordination modes in the
same molecule. The complex is also the resting state in a
catalytic intramolecular aldehyde condensation reaction
(Tishchenko reaction)[22±31] that generates a five-membered
lactone.
Treatment of the labile CoI bis-olefin complex 1[32] with
phthalaldehyde according to equation (1) ([D6]acetone,
108C, 10 equiv) resulted in a rapid color change from
orange to dark green; NMR analysis showed complete
substitution of coordinated olefin to generate a single new
Co species after 10 min. One new C5Me5 signal was observed
at d ˆ 1.70 (s, 15H) along with four resonances in the aromatic
[*] Prof. M. Brookhart, Dipl.-Chem. C. P. Lenges, Dr. P. S. White
Department of Chemistry
University of North Carolina at Chapel Hill
Chapel Hill, NC 27599-3290 (USA)
Fax: (‡1)919-962-2476
E-mail: brook@net.chem.unc.edu
[**] We thank the National Institutes of Health (Grant GM 28938) for
financial support. C.P.L. thanks the Fonds der Chemischen Industrie,
Â
Germany, for a Kekule fellowship.
552
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of the cobalt(i) center, thus reducing the C O bond order. The
structurally characterized nickel(0) benzaldehyde complex
O
O
Me5
Me5
O
H
H
2
[33±35]
ˆ
ˆ
[Ni(PCy3)2(h -C6H5CH O)]
has a C O bond length of
Co
Co
1.325(7) Š (1H NMR: d ˆ 5.58), which is similar to that of the
(1)
SiMe3
Me3Si
H
ˆ
2
O
p-coordinated aldehyde in 2. The shorter C O distance of the
9.58 (1H)
188.9 (13C)
other aldehyde group is indicative of the greater double-bond
character typical of a s-bound aldehyde group. This bond
length lies in the range reported for cationic and neutral Lewis
acidic transition metal aldehyde adducts which bind alde-
hydes in a s fashion.[1±3, 6, 36±38] The two Co O distances are
quite similar (Co O1 1.8900(1), Co O2 1.8932(10) Š). The
Co C1 distance is well outside bonding range (2.85 Š), and the
long Co C8 bond (2.00 Š) suggests that a change in coordina-
tion mode, which may be required for further reactions of this
complex, might be initiated along this trajectory.
SiMe3
H
1
10
10 oC
2
6.67 (1H)
91.3 (13C)
region for an unsymmetrically bound phthalaldehyde mole-
cule at d ˆ 7.29, 7.62, 7.66, and 8.09. Most significant are two
singlets for the aldehyde protons at d ˆ 6.67 and 9.58. The
13C NMR spectrum ( 108C, [D6]acetone) showed signals at
d ˆ 91.3 (d, JC,H ˆ 173 Hz) and 188.9 (d, JC,H ˆ 188 Hz) for the
two differently coordinated carbonyl groups. Free phthalal-
dehyde has NMR signals for the equivalent aldehyde groups
at d ˆ 10.52 (1H) and 192 (13C) in [D6]acetone. On the basis of
this NMR evidence, 2 was formulated as a Co phthalaldehyde
complex in which one aldehyde group is coordinated in a s
fashion and the other in a p fashion.
Attempts to isolate 2 by chromatography were unsuccess-
ful, but 2 could be crystallized from acetone at 788C after
3 d in moderate yield (32%). Black-green crystals were
obtained which are air sensitive and decompose under
ambient conditions in an inert atmosphere.
Coordination of phthalaldehyde to the Co center also clearly
influences the bond lengths in the aromatic ring, which range
from 1.42 Š (C2 C7) to 1.37 Š (C3 C4, C5 C6). The most
significant influence of the transition metal on the conforma-
tion of phthalaldehyde is the C1-C2-C7-C8 torsion angle of 238,
which reflects the deviation from planarity of the aldehyde
units upon coordination to the (C5Me5)Co moiety. This leads
to a C5-C6-C7-C2 torsion angle of 6.68 in the aromatic ring.
The bond angles of the carbonyl groups to the aromatic ring
are within the expected range (ca. 1208); however, the O1-Co-
O2 angle is only 918, which seems required to generate a
favorable bonding interaction with both aldehyde groups. The
O1-Co-C8-O2 torsion angle of 868 indicates that the s-
coordinated aldehyde group (C1 O1) is nearly parallel to the
Co C8 axis, and the angle between the coordination axes of
the p- and s-coordinated aldehyde groups is nearly 908. This
arrangement facilitates backbonding between d orbitals of
cobalt and the s-coordinated aldehyde group.
Complex 2 was characterized by single-crystal X-ray struc-
ture determination (Figure 1), which confirmed the proposed
structure. The two aldehyde groups of a phthalaldehyde mole-
cule are coordinated to the (C5Me5)CoI moiety to generate an
1
At 208C the two aldehyde H NMR signals are sharp
singlets (n1/2 ˆ 1.5 Hz). Warming the acetone solution results
in line broadening of both peaks; this indicates a dynamic
process which exchanges the sites of the p- and s-coordinated
aldehyde groups. Line-shape analysis of spectra obtained at
different temperatures gave activation energies for this
1
1
process of DG=ˆ 69 kJmol 1 (k ˆ 45 s ) at 518C, 70 kJmol
1
1
1
(11 s ) at 358C, and 68 kJmol (4.2 s ) at 218C. Since the
presence of excess phthalaldehyde had no influence on the
line broadening, the process does not involve intermolecular
exchange via free phthalaldehyde. Hence, this site exchange
on the NMR time scale is attributed to a reversible
interchange of s and p coordination modes of the aldehyde
groups. As indicated above, the long distance Co C8 suggests
the release of the p-coordinated aldehyde group and rotation
of the (C5Me5)Co moiety to generate a s-bound aldehyde
group and vice versa [Eq. (2)].
Figure 1. Crystal structure of 2 (ORTEP plot, 50% probability ellipsoids).
Selected bond lengths [Š] and angles [8]: Co O1 1.8900(1), Co O2
1.8932(10), Co C8 2.0036(15), O1 C1 1.2590(19), O2 C8 1.3306(20),
C1 C2 1.4305(22), C7 C8 1.4523(22), C2 C7 1.4192(22), C2 C3
1.4161(22), C3 C4 1.371(3), C4 C5 1.385(3), C5 C6 1.377(3), C6 C7
1.4034(23); O1-Co-O2 90.92(4), Co-O1-C1 128.66(10), O1-Co-C8 93.82(6),
Co-O2-C8 39.80(6), O2-Co-C8 39.80(6), Co-C8-O2 65.62(8), O2-Co-O1-C1
39.64(14), C8-Co-O1-C1 0.10(14), C1-C2-C7-C8 23.61(16), O1-Co-C8-O2
86.85(17), C2-C3-C4-C5 1.39(17), C5-C6-C7-C2 6.64(18).
Me5
O
Me5
asymmetric, 18-electron cobalt(i) bis-aldehyde complex (only
two oxygen atoms and one carbon atom of the aldehyde are
∆G = 70 kJ mol–1
35 o
Co
Co
ˆ
within bonding distance to cobalt). The C O bond lengths
O
(2)
differ considerably: 1.26 Š for the s-coordinated aldehyde
(C1 O1) and 1.33 Š for the p-coordinated aldehyde (C8 O2)
H
H
O
H
O
C
H
ˆ
group. The elongated C O bond of the p-coordinated alde-
hyde indicates considerable backbonding from a filled d orbital
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In the presence of excess phthalaldehyde, catalytic forma-
tion of the five-membered lactone occurs at 58C and above.
Turnover numbers (TON) on the order of 100 were observed
in the reaction of 1 mol% of 1 with phthalaldehyde. To obtain
more detailed information, the reaction of 20 equivalents of
phthalaldehyde with 1 at 208C was monitored by NMR
spectroscopy. Complex 2 is generated rapidly in the initial
phase of catalysis. After 9 h 50% conversion to the lactone is
observed in toluene, and after 24 h conversion of the aldehyde
is complete [Eq. (3)]. In the initial stages of catalysis, 2 is the
density from the CoI center and thus favors s binding of the
aldehyde by interaction with the lone pair of electrons of the
carbonyl group. In complex 4, the PPh2 group binds predom-
inately by a s-donor interaction, and the higher electron
density at the CoI center favors binding of the aldehyde in a p
fashion with significant backbonding from cobalt to the p*
orbital of the carbonyl group. On the basis of the behavior of
complexes 3 and 4, it is easy to rationalize why the dialdehyde
complex 2 prefers to bind through one s and one p
interaction.
Me5
Experimental Section
O
O
O
Co
All operations were carried out under an argon atmosphere. All solvents
were degassed and purified by standard methods.
O
20oC
H
H
O
(3)
H
O
H
1
2: Five equivalents of phthalaldehyde were added to a solution of 1 (0.15 g,
0.38 mmol) in acetone (5 mL) at 08C. The mixture was stirred for 1 h at this
temperature during which time the color changed to dark green. Cooling to
788C resulted in the deposition of black-green crystals, which were
isolated by filtration and stored at 208C in a dry-box freezer (yield:
0.04 g, 0.12 mmol, 32%). The material obtained in this way was suitable for
X-ray structure analysis. 1H NMR (300 MHz, [D6]acetone, 108C): d ˆ
1.70 (s, 15H; C5Me5), 6.67 (s, 1H; p-C(O)H), 7.29, 7.62 ± 7.66, 8.09 (m, 4H;
ArH), 9.58 (s, 1H; s-C(O)H); 13C{1H} NMR: d ˆ 8.6 (C5Me5), 88.6
(C5Me5), 91.3 (d, J ˆ 173 Hz; p-C(O)H), 188.9 (d, J ˆ 188.2 Hz; s-
C(O)H), 123.4, 125.6, 126.7, 131.2, 133.5, 143.6 (Ar).
2
TON: 10 - 100
only cobalt-containing species present; after 3 h other organo-
metallic cobalt complexes had been formed, and after
complete conversion a variety of unidentified cobalt com-
plexes were observed, which primarily arise from decarbon-
ylation reactions.[39] This suggests that the rate of lactonization
is independent of aldehyde concentration and that the
intramolecular cyclization is the slow step in this process.
With simple aldehydes 1 undergoes oxidative addition
reactions to generate cobalt(iii) acyl hydride complexes, which
are intermediates in catalytic hydroacylation.[40] In the pres-
ence of an excess of various alkyl aldehydes, catalytic
dimerization to give the Tishchenko esters was observed.
This suggests that the intramolecular Tishchenko reaction
also follows a pathway that is initiated by oxidative addition of
the aldehyde. This mechanistic scenario was also proposed by
Bosnich et al. for cationic rhodium bis-phosphane complexes
as catalysts.[19]
Crystal structure data for 2: C18H21O2Co, Mr ˆ 328.29, monoclinic, space
group P21/n, Z ˆ 4, a ˆ 10.1111(5), b ˆ 8.7370(4), c ˆ 17.4463(8) Š, b ˆ
96.1400(10)8, Vˆ 1532.38(12) Š3, 1calcd ˆ 1.423 gcm
,
Tˆ 1108C,
1
2qmax ˆ 608, MoKa radiation (l ˆ 0.71073 Š); 20178 reflections were
measured, 4435 unique reflections were obtained, and 3914 of these with
I > 2.5s(I) were used in the refinement; data were collected on a Siemens
SMART diffractometer by the omega scan method. For significant
reflections R(merg) ˆ 0.022; residuals: RF ˆ 0.032, Rw ˆ 0.037 (significant
reflections), GOF ˆ 3.11.[41]
4: To a stirred solution of 1 (0.15 g, 0.38 mmol) in acetone was added one
equivalent of o-diphenylphosphanylbenzaldehyde at 208C. An immediate
color change from red to brown was observed, and stirring was continued
for 30 min. Removal of all volatile materials left 4 as a brown oil (0.18 g,
0.38 mmol). 1H NMR (300 MHz, [D6]acetone, 108C): d ˆ 1.49 (s, 15H;
C5Me5), 4.67 (s, 1H; C(O)H), 7.05 ± 7.19 (m, 3H; ArH), 7.36 ± 7.45 (m, 6H;
ArH), 7.52 (m, 4H; ArH), 8.33 (m, 1H; ArH); 13C{1H} NMR: d ˆ 9.61
(C5Me5), 88.01 (d, 168.6 Hz, C(O)H), 90.80 (C5Me5), 124.1, 126.6, 128.8,
129.9, 130.1, 133.3, 135.0, 133.2, 137.2, 159.5 (Ar); 31P{1H} NMR: d ˆ 58.1
(br).
It is informative to compare compound 2 with other CoI
aldehyde complexes. We previously reported the synthesis of
the chelated aldehyde olefin complex 3[32] [Eq. (4)] by
Me5
Me5
Me5
H
H
O
O
Received: October 12, 1998 [Z12520IE]
German version: Angew. Chem. 1999, 111, 535 ± 538
Co
Co
Co
O
PPh2
PPh2
(4)
Me3Si
H
O
H
10
Me2CO
10 oC
Keywords: aldehydes ´ C H activation ´ cobalt ´ coordina-
tion modes ´ structure elucidation
10
10 o
Me2CO
C
SiMe3
1
4
3
4.67 (1H)
88.0 (13C)
9.35 (1H)
178.8 (13C)
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phosphanylbenzaldehyde to a solution of 1 in acetone, the
aldehyde coordinates in a p fashion to the cobalt center, as
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ambivalent nature of the carbonyl group allows s or p
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554
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COMMUNICATIONS
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Precursors to Nonshrinking Sol ± Gel Systems
and Their Use as Encapsulants
Douglas A. Loy,* Kamyar Rahimian, and
Michael Samara
Sol ± gel polymerization has been the focus of much
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has been limited by the shrinkage associated with the
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6.94 in the 1H NMR spectrum, and at d ˆ 200.5 and 62.8 in the
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Me
Me
Me
Me
Me
Me
Me
O
O
Si
Si
Me
Me
Me
Si
Si
Me
Me Si
O
Si
2
3
4
[29] S. Onozawa, T. Sakakura, M. Tanaka, M. Shiro, Tetrahedron 1996, 52,
4291.
[30] K.-I. Morita, Y. Nishiyama, Y. Ishii, Organometallics 1993, 12, 3748.
[31] T. Ito, H. Horino, Y. Koshiro, A. Yamamoto, Bull. Chem. Soc. Jpn.
1982, 55, 504.
Me Me
Si
Me
O
Me Me
Me
Si
Si
O
Si
O
Si
Me
Si
Me
Me
Me
Me
Me
[32] C. P. Lenges, M. Brookhart, J. Am. Chem. Soc. 1997, 119, 3165.
[33] D. Walther, J. Organomet. Chem. 1980, 190, 393 ± 401.
[34] J. Kaiser, J. Sieler, D. Walther, E. Dinjus, L. Golic, Acta Crystallogr.
Sect. B 1982, 38, 1584 ± 1586.
5
6
Me Me
Si
Me
Me
Me
Me
[35] A structurally characterized Ni0 ethylene formaldehyde complex has a
O
Si
Si
[16]
ˆ
C O bond length of 1.311 Š.
O
Si
Me Me
[36] P. Courtot, R. Pichon, J. Y. Salaun, J. Organomet. Chem. 1985, 286,
cat.
n
Me Me
Si
C17.
[37] J. Auffret, P. Courtot, R. Pichon, J. Y. Salaun, J. Chem. Soc. Dalton
O
Trans. 1987, 1687.
Me
Me
Si
Si
Si
Me
[38] The
C O
bond
length
of
[TpMo(MeC2Me)(CO)(h1-p-
Me
ˆ
Me
O
Me
1
O CHC6H4OMe)] is 1.237 Š.[6]
‡
n
ˆ
[39] This assignment is supported by the formation of benzene and
additional aldehyde, presumably benzaldehyde, in the deactivation
process. NMR signals in the C5Me5 region also indicate the formation
of species such as [{(C5Me5)Co(CO)}2].
Scheme 1. Ring-opening polymerization of 1 to the corresponding poly-
mer.
Unlike the sol ± gel chemistry of alkoxysilanes, which
requires stoichiometric quantities of water, ROP of disila-
oxacyclopentane groups only requires catalytic quantities of
an anionic base, such as tetrabutylammonium hydroxide
[40] M. Brookhart, C. P. Lenges, P. S. White, J. Am. Chem. Soc. 1998, 120,
6965 ± 6979.
[41] Crystallographic data (excluding structure factors) for the structure
reported in this paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication
no. CCDC-113115. Copies of the data can be obtained free of charge
on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK
(fax: (‡44) 1223-336-033; e-mail: deposit@ccdc.cam.ac.uk.
[*] Dr. D. A. Loy, K. Rahimian, M. Samara
Sandia National Laboratories, Encapsulants and Foams Department
P.O. Box 5800, MS 1407, Albuquerque, NM 87185 (USA)
Fax: (‡)505-844-9624
E-mail: daloy@sandia.gov
Supporting information for this article is available on the WWW
under http://www.wiley-vch.de/home/angewandte/ or from the author.
Angew. Chem. Int. Ed. 1999, 38, No. 4
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