Synthesis and first X-ray structures of cobalt(II) and cobalt(III) complexes bearing 2,4-dioxo-alkanoic acid dialkylamide ligands

Article abstract of DOI:10.1139/V08-151

The aerobic oxidation of 5-hydroxy pentenes to trans-tetrahydrofurans (THFs), the Mukaiyama oxidation, has in recent years emerged as a powerful tool in synthetic chemistry. In this report we describe the first crystal structures of 2,4-dioxo-alkanoic acid dialkylamide complexes for both Co(II) and Co(III) salts. The Co(II) structures show an unprecedented arrangement of atoms, with a central Co(II)(H₂O)₆ flanked by two Co(II) atoms each coordinated with three anionic chelating ligands. The dimeric Co(III) structures show two anionic chelating ligands and bridging hydroxyl groups.

Full text of DOI:10.1139/V08-151

328
Synthesis and first X-ray structures of cobalt(II)
and cobalt(III) complexes bearing 2,4-dioxo-
alkanoic acid dialkylamide ligands1
Jian Wang, Nicholas A. Morra, Hongda Zhao, Jeffrey S.T. Gorman, Vincent Lynch,
Robert McDonald, John F. Reichwein, and Brian L. Pagenkopf
Abstract: The aerobic oxidation of 5-hydroxy pentenes to trans-tetrahydrofurans (THFs), the Mukaiyama oxidation,
has in recent years emerged as a powerful tool in synthetic chemistry. In this report we describe the first crystal struc-
tures of 2,4-dioxo-alkanoic acid dialkylamide complexes for both Co(II) and Co(III) salts. The Co(II) structures show
an unprecedented arrangement of atoms, with a central Co(II)(H2O)6 flanked by two Co(II) atoms each coordinated
with three anionic chelating ligands. The dimeric Co(III) structures show two anionic chelating ligands and bridging
hydroxyl groups.
Key words: aerobic oxidation, trans-tetrahydrofuran (THF), cobalt, X-ray crystallography, oxidation, coordination.
Résumé : L’oxydation aérobique de 5-hydroxypentènes en trans-tétrahydrofuranes (THF), l’oxydation de Mukaiyama,
est devenue au cours des années récentes un outil puissant. Dans ce travail, on décrit les structures cristallines de sels
complexes de Co(II) et de Co(III) de dialkyamides de l’acide 2,4-dioxoalcanoïque. Les structures des sels du Co(II)
présentent un arrangement d’atomes, inconnu jusqu’à maintenant, comportant un groupe central Co(II)(H2O)6 flanqué
de deux atomes de Co(II) qui sont chacun liés à trois ligands chélatants anioniques. Les structures dimères du Co(III)
comportent deux ligands chélatants anioniques et des groupes hydroxyles servant de ponts.
Mots-clés : oxydation aérobique, THF, cobalt, diffraction des rayons X, oxydation, coordination.
[Traduit par la Rédaction] Wang et al. 334
Introduction
Scheme 1. Representative Mukaiyama oxidation.
Co(III)
complexes
of
2,4-dioxo-alkanoic
acid
dialkylamides are useful catalysts for the aerobic oxidation
of olefins to peroxides or alcohols (1–5) and for the oxida-
tive cyclization of 5-hydroxy pentenes to tetrahydrofurans
(THFs) (6). In recent years, the stereoselective aerobic oxi-
dation of 5-hydroxy pentenes to trans-THFs (henceforth the
Mukiayama oxidation, Scheme 1) has emerged as a powerful
tool in organic synthesis and has been strategically em-
ployed in the total synthesis of natural products by the
groups of Wang et al. (7), Takahashi et al. (8), Evans et al.
(9), and Pagenkopf and co-workers (10). Surprisingly, de-
spite the obvious synthetic importance of these catalysts,
there is little structural information available on transition
metal 2,4-dioxo amide complexes, and to the best of our
knowledge no X-ray structures of these species have been
reported. In this paper we describe crystal structures of two
Co(II) complexes and one Co(III) complex.
Experimental
General procedure for ligands 2
To a cooled (0 °C) solution of dialkylamine (10 mmol)
and Et3N (10 mmol) in CH2Cl2 (20 mL) was added ethyl
oxalyl chloride (10 mmol). After stirring at room tempera-
ture (rt) for 16 h, the reaction mixture was washed with
Received 11 March 2008. Accepted 10 September 2008. Published on the NRC Research Press Web site at canjchem.nrc.ca on
15 November 2008.
J. Wang, N.A. Morra, and B.L. Pagenkopf.2 Department of Chemistry, The University of Western Ontario, London, ON N6A
5B7, Canada.
H. Zhao, J.S.T. Gorman, V. Lynch,3 and J.F. Reichwein. Department of Chemistry, The University of Texas at Austin, Austin,
TX 78712, USA.
R. McDonald.4 Chemistry Department, University of Alberta, Edmonton, AB T6G 2G2, Canada.
1This article is part of a Special Issue dedicated to Professor R. Puddephatt.
2Corresponding author (e-mail: bpagenko@uwo.ca).
3Corresponding author for questions concerning structures 3e and 7 (e-mail: vmlynch@mail.utexas.edu).
4Corresponding author for questions concerning structure 9 (e-mail: Bob.McDonald@ualberta.ca).
Can. J. Chem. 87: 328–334 (2009)
doi:10.1139/V08-151
© 2008 NRC Canada

Wang et al.
329
Table 1. Crystallographic data collection parameters for 7, 3e, and 9.
Complex
3e
7
9
Chemical formula
C213H218Co3N6O22
C141H162Co3N6O25
C98.5H110Co2N4O14
Formula mass
Crystal dimensions (mm)
Crystal system
Space group
Z
3390.72
2517.56
0.50 × 0.14 × 0.11
Monoclinic
P21/n
1691.77
0.53 × 0.27 × 0.15
Monoclinic
Ia
0.32 × 0.26 × 0.15
Triclinic
P1
2
2
4
a (Å)
b (Å)
c (Å)
17.9663(1)
23.9160(2)
24.1629(2)
67.360(1)
85.866(1)
84.983(1)
9537.27(12)
1.181
16.5500(5)
20.8953(6)
19.1655(6)
90
98.326(2)
90
24.6233(16)
13.1161(9)
29.4833(19)
90
98.3545(10)
90
α (°)
β (<)
γ (<)
Volume (Å3)
Dcalcd. (mg cm–3)
6557.9(3)
1.275
9420.9 (11)
1.193
T (K)
153(2)
153(2)
193(2)
Radiation
MoKα (λ = 0.710 73)
MoKα (λ = 0.710 73)
MoKα (λ = 0.710 73)
µ (mm–1)
0.324
0.448
0.414
Total reflections collected
Total unique reflections
Observed reflections [I > 2σ(I)]
2θmax (α)
74 396
43 644 [R(int) = 0.0439]
16 105
54.98
16 105
8 560 [R(int) = 0.0606]
74 396
45
36 466
19 200 [R(int) = 0.0365]
15 217
52.78
Goodness-of-fit on F2
R1 [I > 2σ(I)]
2.423
0.0847
1.165
0.0602
0.987
0.0504
wR2 (all data)
Largest diff. peak and hole (e Å–3)
0.1576
0.1373
0.129
1.103 and –0.503
0.486 and –0.320
0.557 and –0.316
1 mol/L HCl (2×), satd. NaHCO3, and brine and then dried
(MgSO4). Concentration in vacuo gave 1 as a white solid or
colorless oil in quantitative yield, which was used without
further purification.
3a
Pink amorphous solids. MS (CI) m/z: 540 [M + H]+, 242
[ligand + H]+.
3b
A solution of t-BuOK (23 g, 205 mmol, 2.1 equiv.) in
THF (180 mL) was added via cannula to a solution of
pinacolone (12 mL, 96 mmol, 1.0 equiv.) and amide 1a
(18 g, 96 mmol, 1.0 equiv.) in THF (40 mL) over 40 min at
rt. After 3 h, AcOH (20 mL, 350 mmol, 3.6 equiv.) was
added over 5 min and the resulting heterogeneous mixture
was filtered and the solids were washed with CH2Cl2.
The filtrate was washed with NaHCO3 and brine, dried,
filtered, and concentrated. Purification of the resulting oil
by flash chromatography (SiO2, 50% EtOAc/hexanes) pro-
vided the title compound as a slightly yellow solid (18.1 g,
78%).
To a solution of 2 (7.0 mmol) in hexanes or benzene (3a)
was added Co(II)(2-ethyl-hexanoate)2 (3.5 mmol) solution
in mineral spirits. The reaction mixture was stirred at rt for
8 h and hexane (ca. 70 mL) was added to induce precipita-
tion. The solids were separated by centrifugation or filtration
and washed with hexane.
Full characterization of Co(II) complexes by NMR was
not possible because the salts are paramagnetic. Combustion
analysis is significantly out of range because the number of
water molecules within the powder varies and drying gener-
ally lead to decomposition. Attempts to record discrete melt-
ing points also lead to decomposition. Several of these
complexes are known compounds, including 3a, 3b, and 3c
(3a, 11) (Table 1).
Pink amorphous solids. MS (CI) m/z: 536 [M + H]+, 240
[ligand + H]+.
3c
Pink amorphous solids. MS (CI) m/z: 512 [M + H]+, 228
[ligand + H]+.
3d
Brownish peach colored amorphous solids. MS (CI) m/z:
760 [M + H]+.
3e
Red crystals. MS (CI) m/z: 1064 [M + H]+.
4
Pink amorphous solids. MS (CI) m/z: 676 [M + H]+.
5
Reddish brown brittle foam. MS (CI) m/z: 732 [M + H]+.
7
Red crystals. MS (CI) m/z: 732 [M + H]+.
Synthesis and characterization of 9
To a mixture of 3d (0.76 g, 1 mmol, 1.0 equiv.) in acetone
(20 mL) was added 30% w/w H2O2 (1.2 mL, 10 mmol,
© 2008 NRC Canada

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Can. J. Chem. Vol. 87, 2009
Table 2. Preparation of Co(II) complexes.
Scheme 2. Synthesis of Co(II) complexes.
10 equiv.) at rt. After 15 h, the solution was concentrated
under reduced pressure. TLC showed that the residue
showed numerous spots. Flash chromatography over silica
gel (25% EtOAc/hexanes) allowed separation of at least six
distinct bands, one of which (Rf = 0.25, 40% EtOAc/hex-
anes) provided material that eventually crystallized. The
crystal for X-ray analysis was grown from a vapor diffusion
1
chamber containing EtOAc, toluene, and hexanes. H NMR
(600 MHz, C6D6) δ: 7.43 (d, J = 7.3 Hz, 4H), 7.24–7.20 (m,
8H), 7.15–7.08 (m, 8H), 7.00 (d, J = 7.1 Hz, 4H), 6.88–6.81
(m, 10H), 6.75 (t, J = 7.50, 4H), 6.53 (s, 2H), 6.34 (s, 2H),
6.26 (t, J = 7.3 Hz, 2H), 5.58 (d, J = 14.5 Hz, 2H), 5.29 (d,
J = 15.4 Hz, 2H), 5.20 (d, J = 14.1 Hz, 2H), 4.28 (d, J =
16.8 Hz, 2H), 3.44–3.38 (m, 6H), 3.02 (d, J = 16.5 Hz, 2H),
1.26 (s, 18H), 1.01 (s, 18H), –3.39 (s, 1H), –3.72 (s, 1H).
59Co NMR (144 MHz, C6D6) δ: 13364 (brs). Combustion
calcd.: C 67.28, H 6.31, N 3.62; found: C 68.03, H 6.36, N
3.62.
Note: The compounds shown are representative of the metal to ligand
stoichiometry only, and are not meant to imply structure.
aUnoptimized yields of complexation.
bPoor solubility.
Results and discussion
This modified procedure proved effective for the prepara-
tion of a variety of 2,4-dioxo amide complexes and these
results are summarized in Table 2. Replacement of the
morpholine with piperidine (3b), diethylamine (3c), or
dibenzyl amine (3d) gave complexes as microcrystalline
pink powders unsuitable for single crystal X-ray analysis. Of
these, the dibenzyl amide 3d displayed the most promising
crystalline character, and in entries 5–7 of Table 2 the
dibenzyl amide portion of the ligand was conserved while
substitutions were made in place of the tert-butyl group. The
methyl (4), isopropyl (5), and phenyl (6) analogs of 3d were
each prepared successfully, but they too failed to give crys-
tals suitable for X-ray analysis.
Cobalt(II) structures
Our initial efforts to employ the Mukaiyama oxidation in
synthesis were complicated by fickle oxidation results as a
consequence of inconsistent purity of the Co(II) complex.
The first reports on Co(II) 2,4-dioxo-alkanoic acid amide
complexes utilized CoCl2 under aqueous alkaline conditions,
but we found that such conditions caused decomposition of
the morpholine amide 2a (Scheme 2). The result was that
the preparation of catalyst 3a under the aqueous conditions
provided a highly contaminated amorphous brown solid and
attempts at purification proved fruitless. However, these dif-
ficulties were overcome by complexation under neutral con-
ditions in organic solvent. When a solution of the free ligand
2a in benzene was treated with 0.5 equiv. of Co(II)(2-ethyl
hexanoate)2, the initial dark blue color quickly dissipated
and the resulting red solution was treated with hexane to
precipitate the complex 3a.
A more substantial change was made by installing a
methyl group at C(3) and, gratifyingly, the complex 7 was
obtained as red crystals and the unusual X-ray structure of 7
is shown in Fig. 1.5 Selected bond lengths and angles are re-
ported in Table 3. There are three cobalt atoms present in the
5 Supplementary data for this article are available on the journal Web site (canjchem.nrc.ca) or may be purchased from the Depository of Un-
published Data, Document Delivery, CISTI, National Research Council Canada, Ottawa, ON K1A 0R6, Canada. DUD 3859. For more in-
formation on obtaining material refer to cisti-icist.nrc-cnrc.gc.ca/cms/unpub_e.shtml. CCDC 203707 (7), 679940 (3e), and 203708 (9)
contain the crystallographic data for this manuscript. These data can be obtained, free of charge, via www.ccdc.cam.ac.uk/conts/retriev-
ing.html (Or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax +44 1223 336033; or de-
posit@ccdc.cam.ac.uk).
© 2008 NRC Canada

Wang et al.
331
Fig. 1. (a) View of the complex 7, [(C21H22NO3)3Co]2-Co(H2O)6. The complex resides on a crystallographic inversion center at ½, ½,
½ with Co(H2O)6 at the center, which bridges a pair of (C21H22NO3)3Co complexes via O-HLO hydrogen bonds (12). (b) View of 7
showing a single Co-ligand assembly and the central Co(H2O)6 with selected atoms labeled. Displacement ellipsoids are scaled to the
30% probability level. Hydrogen atoms and aromatic groups have been removed for clarity.
unit cell, each separated by 4.870 Å. The two outer cobalt
atoms are each surrounded by three dioxoamide ligands and
their negative charge is balanced by a central Co(II)(H2O)6.
The dihedral angle between the amide carbonyl and the
acac-like plane is 118(1)°, which might be expected to mini-
mize A(1,3) strain with the C(3) methyl group. The unfavor-
able A(1,3) interaction was also evident in the 1H NMR
spectra of the metal-free ligand 8, which in CDCl3 existed as
a 1:1 ratio of both the keto and enol forms (Scheme 3). In
contrast, the preceding ligands existed primarily in the enol
form. The distinctly different structure and behavior of
ligand 8 compared with the others caused concern that the
structure of 7 may be unique and not representative of the
series.
To address concerns that the C(3) methyl may have lead
to a structural anomaly, additional complexes more closely
resembling 3d were prepared. Of these, the biphenyl analog
3e (Table 2, entry 9) formed crystals suitable for X-ray anal-
ysis (Fig. 2). Importantly, the major features of the two
structures are conserved, including the arrangement of the
three cobalt atoms and the angle of the amide. A minor dif-
ference is that the central cobalt in 3e is surrounded by only
© 2008 NRC Canada

332
Can. J. Chem. Vol. 87, 2009
Fig. 2. View of the H-bound complex 3e, [(C34H32NO3)3Co]2-Co(H2O)3, showing the labeling scheme for selected atoms (12). Dis-
placement ellipsoids are scaled to the 30% probability level. Hydrogen atoms and the aromatic groups have been removed for clarity.
The three water molecules of the bis Co complex are H-bound to carbonyl oxygen atoms of the adjacent mono Co complex.
Table 3. Selected bond lengths and angles for 7.
Bond lengths (Å)
Table 4. Selected bond lengths and angles for
3e.
Bond lengths (Å)
C(45)–C(46)
C(45)–O(7)
C(46)–C(48)
C(48)–C(49)
C(48)–O(8)
C(49)–O(9)
C(49)–N(2)
Co(1)–O(7)
Co(1)–O(8)
N(2)–C(57)
1.450(7)
1.246(6)
1.367(7)
1.535(7)
1.287(5)
1.236(6)
1.339(6)
2.021(4)
2.075(3)
1.471(6)
Co(3)–O(31)
Co(3)–O(34)
C(32d)–C(33)
C(30d)–C(32)
C(30d)–O(31)
C(29)–O(30)
C(29)–C(30)
N(1)–C(2)
2.058(2)
2.091(2)
1.407(4)
1.379(4)
1.285(3)
1.236(4)
1.515(4)
1.468(4)
1.349(4)
1.250(4)
N(1)–C(29)
C(33)–O(34)
Bond angles (°)
Bond angles (°)
O(7)-C(45)-C(46)
C(48)-C(46)-C(45)
O(8)-C(48)-C(46)
C(45)-O(7)-Co(1)
C(48)-O(8)-Co(1)
O(7)-Co(1)-O(8)
124.3(5)
121.4(5)
129.2(4)
132.2(3)
125.9(3)
85.73(13)
C(33)-O(34)-Co(3)
C(30)-O(31)-Co(3)
O(31)-Co(3)-O(34)
O(34)-C(33)-C(32)
O(31)-C(30)-C(32)
C(30)-C(32)-C(33)
125.1(2)
120.78(9)
86.88(8)
123.4(3)
127.4(3)
125.8(3)
Scheme 3. Equilibrium disfavors the enol form.
In both X-ray structures there are neutral donors, includ-
ing water, surrounding the central cobalt atom. The reactions
were run under an argon atmosphere using undried bulk sol-
vents and this was presumably the origin of the water. To de-
termine if additional water would facilitate crystallization,
several complexation reactions were simply performed open
to air. Remarkably, the preparation of 3d under these condi-
tions gave larger crystals that were easily obtained by filtra-
tion (instead of centrifugation). Importantly, no oxidation to
Co(III) was observed under these conditions (TLC), whereas
the analogous Co(acac)2 species oxidize more readily.
three water molecules, with the coordination sphere being
fulfilled by dative bonding from a neighboring ligand. The
distance between Co(1) and Co(2) was found to be 4.980 Å,
Co(2) and Co(3) are separated by 2.926 Å. Bond lengths and
angles of significance are reported in Table 4.
© 2008 NRC Canada

Wang et al.
333
Fig. 3. View of the complex 9, [(C34H32NO3)3Co]2(µ-OH)2, showing the labeling scheme for selected atoms (12). Nonhydrogen atoms
are represented by Gaussian ellipsoids at the 30% probability level. Hydroxyl group hydrogen atoms are shown with arbitrarily small
thermal parameters; all other hydrogens and aromatic groups have been removed for clarity.
Cobalt(III) structure
Obtaining structural information about the Co(III) com-
Table 5. Selected bond lengths and angles for 9.
Bond lengths (Å)
plexes (which are generated in situ for the Mukaiyama oxi-
dation) might be more helpful in speculating about the
nature of the active catalytic species. However, the Co(III)
complexes are generally obtained as green oils or glasses.
Efforts to grow crystals at –20 °C were promising, but the
resultant crystals were not of suitable quality for X-ray anal-
ysis. The Co(III) complexes are stable to flash chromatogra-
phy on silica gel and the complexes elute as numerous
bands, some of which are readily separable while others are
overlapping. We speculated one origin of these different
bands to be diastereomeric complexation, and when sepa-
rated the individual diastereomers might crystallize more
easily. At rt individually eluted bands isomerize within a day
or two to multiple species, as monitored by TLC. However,
at –20 °C the isomerization is slowed dramatically and crys-
tals suitable for X-ray crystallography were obtained from
one of the more polar bands (Rf = 0.25, 40% EtOAc/hex-
anes) of compound 9 (the oxidized version of 3d) (Fig. 3).
In this binuclear cluster, the two central cobalt atoms are
bridged by two hydroxyl groups and each cobalt is sur-
rounded by two chelating ligands. Significant bond lengths
and angles have been summarized in Table 5.
Co(1)–O(1)
Co(1)–O(2)
Co(1)–O(11)
Co(1)–O(12)
Co(2)–O(1)
Co(2)–O(2)
O(12)–C(13)
O(11)–C(11)
O(10)–C(10)
C(12)–C(13)
C(11)–C(12)
C(10)–C(11)
N(10)–C(10)
N(10)–C(18)
1.903(2)
1.904(2)
1.888(2)
1.887(2)
1.903(2)
1.918(2)
1.254(4)
1.284(4)
1.234(4)
1.415(4)
1.351(5)
1.541(4)
1.321(4)
1.486(4)
Bond angles (°)
O(1)-Co(1)-O(2)
O(1)-Co(1)-O(11)
O(1)-Co(1)-O(12)
Co(1)-O(1)-Co(2)
Co(1)-O(2)-Co(2)
Co(1)-O(11)-C(11)
Co(1)-O(12)-C(13)
O(12)-C(13)-C(12)
O(11)-C(11)-C(12)
C(11)-C(12)-C(13)
81.73(9)
174.77(10)
89.52(9)
98.74(10)
98.17(10)
120.3(2)
124.1(2)
124.6(3)
128.9(3)
122.4(3)
Use of crystals in oxidative cyclizations
All of the new complexes reported in Table 2 possess
some catalytic activity for the oxidation of γ-hydroxyl
olefins to tetrahydrofurans. The complexes 3a and 3d have
emerged as particularly useful catalysts and the decision to
employ one versus the other is influenced by product polar-
ity and chromatographic characteristics. Crystals of 9 were
dissolved in iPrOH and appeared as a single spot by TLC.
These crystals successfully catalyzed the oxidation shown in
Scheme 1 (1 atm O2, iPrOH, 50 °C, 16 h) in yields typically
ranging from 70%–90%. Some spots isolated from the
Co(III) species via chromatography were found to be inac-
© 2008 NRC Canada

334
Can. J. Chem. Vol. 87, 2009
tive under standard Mukaiyama oxidation conditions. Inves-
tigations are currently underway to determine the structural
differences between the active and inactive Co(III) com-
plexes.
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We thank the Natural Sciences and Engineering Research
Council of Canada (NSERC), donors of the Petroleum Re-
search Fund, and the DOD Prostate Cancer Research Pro-
gram DAMD17-01-1-0109 for financial support. We also
thank Professor Paul J. Ragogna and Jason Dutton for help-
ful discussions.
© 2008 NRC Canada