A masked two-coordinate cobalt(I) complex that activates C-F bonds

Article abstract of DOI:10.1021/ja2052914

We report the isolation, characterization, and reactions of the unsaturated complex L^tBuCo (L^tBu = bulky β-diketiminate ligand). The unusual slipped κN,η⁶-arene binding mode in L ^tBuCo interconverts rapidly and

Full text of DOI:10.1021/ja2052914

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A Masked Two-Coordinate Cobalt(I) Complex That Activates
CꢀF Bonds
Thomas R. Dugan,† Xianru Sun,‡ Elena V. Rybak-Akimova,‡ Olayinka Olatunji-Ojo,§ Thomas R. Cundari,§
and Patrick L. Holland*,†
†Department of Chemistry, University of Rochester, Rochester, New York 14627, United States
‡Department of Chemistry, Tufts University, 62 Talbot Avenue, Medford, Massachusetts 02155, United States
§CASCaM, Department of Chemistry, University of North Texas, Denton, Texas 76203, United States
S Supporting Information
b
ABSTRACT: We report the isolation, characterization, and
reactions of the unsaturated complex LtBuCo (LtBu = bulky
β-diketiminate ligand). The unusual slipped kN,η6-arene
binding mode in LtBuCo interconverts rapidly and reversibly
with the traditional k2N,N0 ligation mode upon binding of
Lewis bases, making it a “masked” two-coordinate complex.
The mechanism of this isomerization is demonstrated using
kinetic studies. LtBuCo is a stable yet reactive synthon for
low-coordinate cobalt(I) complexes and is capable of cleav-
ing the CꢀF bond in fluorobenzene.
Figure 1. Molecular structures of LtBuCo(THF) (1) and LtBuCo (2)
using 50% probability thermal ellipsoids. H atoms have been omitted for
clarity. Selected bond distances (Å) and bond angles (deg) for 1:
Co1ꢀO1, 2.002(2); Co1ꢀN11, 1.902(3); Co1ꢀN21, 1.881(3); N11ꢀ
Co1ꢀO1, 125.0(1); N21ꢀCo1ꢀO1, 135.2(1); N11ꢀCo1ꢀN21,
99.5(1). For 2: Co1ꢀN21, 1.9037(8); Co1ꢀcentroid, 1.61;
N11ꢀC21, 1.291(1); C21ꢀC31, 1.464(1); C31ꢀC41, 1.378(1);
C41ꢀN21, 1.382(1); N21ꢀCo1ꢀcentroid, 148.
emilabile ligands, which contain a Lewis basic moiety that
H
can reversibly dissociate from a metal, create transient
coordinative unsaturation that can be used for bond activation
and catalysis.1 Notable examples include catalysts for olefin
metathesis and cross-coupling reactions.2 In this communication,
we report that a bulky β-diketiminate ligand on cobalt(I) under-
goes a novel isomerization that allows it to behave as a hemilabile
ligand. Though β-diketiminates are represented in thousands of
metal complexes,3 this is the first example of this new binding
mode. We show that the ligand isomerization is rapidly reversible
and that it provides a “masked two-coordinate” cobalt(I) center
for ligand binding and for activation of a strong CꢀF bond.
The addition of tetrahydrofuran (THF) to a solution of
LtBuCoNNCoLtBu [LtBu = 2,2,6,6-tetramethyl-3,5-bis(2,4,6-tri-
isopropylphenylimido)hept-4-yl]4 in C6D6 gives an immediate
color change from brown to dark-green. This color change
THF-d8 at 23 °C, corresponding to an S = 1 ground state; 1 is a
high-spin d8 cobalt(I) complex.
Dissolution of 1 in C6D6 under Ar gives a dark-orange solution
whose 1H NMR spectrum shows peaks from 1 plus a number of
additional resonances. Evaporation of the volatile materials and
redissolution in fresh C6D6 leads to an 1H NMR spectrum that
contains very little 1. The displacement of THF with aromatic
solvents can be used for the synthesis of the unknown species on
a gram scale. Thus, 1 was dissolved in toluene, and two cycles of
solvent removal and addition of more toluene removed all of the
THF. Crystallization from a concentrated pentane solution at
ꢀ45 °C under Ar gave LtBuCo (2) in 72% yield as brown crystals.
The molecular structure of 2 (Figure 1 right) shows a cobalt
atom and a single β-diketiminate ligand with no additional
1
corresponds to the appearance of a set of signals in the H
NMR spectrum for a new species, LtBuCo(THF) (1), in quanti-
tative yield. On a preparative scale, 1 can be produced from the
reaction of LtBuCoCl with KC8 in THF under Ar, which gives 1 as
a dark-green crystalline solid in 64% yield. Complex 1 (Figure 1
left) has pseudo-C2 symmetry in the solid state. The three-
coordinate cobalt atom is planar, as the sum of the NꢀCoꢀN
and NꢀCoꢀO bond angles is 359.7(2)°. The THF ligand is bent
slightly toward one aryl arm of the β-diketiminate ligand, with
NꢀCoꢀO angles of 135.2(1) and 125.0(1)°.
donors. In contrast to the typical k2N,N0 binding mode for LtBu
,
the β-diketiminate ligand in 2 is bound to cobalt in a kN,η6-arene
mode in which the cobalt is bound to the arene ring of an uncoordi-
nated nitrogen atom. The bonds in the NCCCN ring are
localized (see the center of Scheme 1 for a valence-bond picture),
as evidenced by the alternating bond lengths of 1.291(1),
1.464(1), 1.378(1), and 1.382(1) Å along the β-diketiminate
backbone. These may be compared with the analogous bond
A solution of 1 in 5:1 C6D6/THF gives an 1H NMR spectrum
with nine paramagnetically shifted resonances, indicating C2v
symmetry in solution. Two resonances at δꢀ2.2 and ꢀ19.6 ppm are
absent in THF-d8, consistent with rapid exchange of bound THF
with solvent. The solution magnetic moment is 3.23(4) μB in
Received: June 8, 2011
Published: July 19, 2011
r
2011 American Chemical Society
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The 1H NMR spectrum of 2 in cyclohexane-d12 indicates Cs
symmetry in solution, with a mirror plane containing the
NCCCN of the β-diketiminate ligand. The number of peaks in
the 1H NMR spectrum of 2 in cyclohexane-d12 is invariant between
25 and 80 °C, with no sign of coalescence from trading of coordinated
arene rings through a C2v-symmetric intermediate. Therefore, the
barrier for isomerization from the kN,η6-arene binding mode to the
k2N,N0 mode in LtBuCo is greater than 15 kcal/mol.9
The relationship between THF complex 1 and rearranged 2 is
interesting in that dissociation of THF from a 14-electron complex
(1) produces a 16-electron complex (2). Intramolecular coordina-
tion of ligand aryl groups has been seen in other complexes of
bulky ligands and may be reversible10 or irreversible.11 “Masked”
coordination has been observed recently in related cobalt(I) and
nickel(I) species.12 In view of the extensive reorganization of the
binding modes of LtBu in conversions between 1and 2, it is surprising
that the conversion of 2 to 1 takes place in a few seconds upon
addition of THF. However, 2 and 1 are in slow exchange on the
NMR time scale, even at 80 °C. Therefore, values of Keq for
Figure 2. Overlay of the two SOMOs in 2on the DFT-optimized geometry.
H atoms have been omitted for clarity. C, yellow; N, magenta.
Scheme 1. Ligand-Binding Reactions of LtBuCo (2)
1
binding of THF to 2 to form 1 can be derived from H NMR
integrations for the two species as a function of [THF].
The equilibrium constants measured at temperatures from 25
to 85 °C in cyclohexane-d12 lead to the thermodynamic para-
meters ΔH° = ꢀ14.7 ( 0.5 kcal/mol and ΔS° = ꢀ40.9 ( 1.7 eu.
The equilibrium shifts significantly toward 1 at low temperature,
and the large negative entropy of reaction is consistent with
association of an additional ligand. The binding affinity at room
temperature is dependent on the size of the ligand: for THF, Keq =
19 ( 5 Mꢀ1 at 25 °C, whereas for a mixture of cis- and trans-2,5-
dimethyltetrahydrofuran, Keq = 0.55 ( 0.18 Mꢀ1 at 25 °C.
Compound 2 reacts with N2 to give the previously char-
acterized end-on/end-on bridging dinitrogen complex
L
tBuCoNNCoLtBu.4 Binding of N2 is relatively weak, as only
95% conversion is evident in cyclohexane-d12 with 1 atm N2. The
conversion to LtBuCoNNCoLtBu is >95% when the pressure of
N2 is increased to 8 atm. Arenes also bind to 2, although weakly.
Addition of benzene to a solution of 2 in cyclohexane-d12 results
distances of 1.345(4), 1.403(4), 1.402(4), and 1.337(4) Å in 1.
The β-diketiminate backbone deviates only slightly from planar-
ity. The CoꢀN21 distance of 1.9037(8) Å is similar to the
CoꢀN distance in 1. The CoꢀC(arene) bond distances, which
range from 2.092(1) to 2.158(1) Å, are normal.5 The CꢀC bond
distances in the bound arene are ∼0.02 Å longer than in the
unbound arene, indicating back-bonding from cobalt(I) into the
aromatic ring. The N21ꢀCoꢀaryl-centroid angle is 148°, mak-
ing this the first example of an isolated compound with a bent
one-legged piano-stool geometry.6
The magnetic moment of 2 in cyclohexane-d12 solution is
3.07(7) μB at 27 °C, indicating a high-spin (S = 1) electronic
configuration. Density functional theory (DFT) calculations
(M06 functional and augmented triple-ζ, all-electron basis sets)
indicate that the two singly occupied molecular orbitals
(SOMOs) are almost entirely metal-based, with small contribu-
tions from the ligands (Figure 2). These half-filled orbitals
extend lobes toward the open side of the complex. Lower in
energy are three doubly occupied Co-based MOs. The electro-
nic structure of 2 is thus consistent with a high-spin d8
configuration that is isolobal to triplet CH2.7 Though it has
the same geometry and number of d electrons as the highly
reactive but unobserved Cp*Ir(L) and Cp*Rh(L) (L = phos-
phine, CO) intermediates in CꢀH activation reactions, the
heavier analogues are singlets while 2 is a triplet, reflecting the
small ligand-field splitting of the 3d metal.8
1
in the appearance of seven new resonances in the H NMR
spectrum, and the dependence of the integrations on [benzene]
gives Keq = 0.81 ( 0.26 Mꢀ1 at 25 °C. The formation of
L
tBuCo(benzene) also accounts for the extra resonances that
are observed in the 1H NMR spectrum of 2 in C6D6 (see above).
The temperature dependence of the equilibrium constant gives
ΔH° = ꢀ7.4 ( 0.3 kcal/mol and ΔS° = ꢀ28.9 ( 1.0 eu.
Similarly, Keq = 0.65 ( 0.10 Mꢀ1 for toluene at 25 °C. The
symmetric H NMR spectra of LtBuCo(arene) are consistent
1
with η6 binding, as observed in a close analogue with a smaller
diketiminate ligand.13 In our system, arenes bind so weakly that
the equilibrium concentration of the arene complex is low, and as
a result, we cannot rule out lower-hapticity binding modes that
exchange rapidly on the NMR time scale.
Compound 2 coordinates stronger donors and π-acidic ligands
irreversibly to give new cobalt(I) complexes (Scheme 2). The
addition of 1 equiv of PPh3 to a solution of 2 in C6D6 gives high-
spin LtBuCo(PPh3) (3) (Figure 3 left), which was isolated in 70%
yield. Sparging a solution of 2 in hexane with CO gas for 5 min at
20 °C gives cherry-red LtBuCo(CO)2 (4) (Figure 3 right) in 87%
yield. Compound 4 is diamagnetic, as expected for a 16-electron
square-planar CoI complex. The IR spectrum contains two CꢀO
stretching bands at 2014 and 1949 cmꢀ1 that shift to 1970 and
1909 cmꢀ1 when 13CO is used. The 13C{1H} NMR spectrum of
this 13CO-labeled sample of 4 shows a singlet for bound CO at
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Scheme 2. Possible Mechanisms for Ligand Binding to
L
tBuCo
Figure 3. Molecular structures of LtBuCo(PPh3) (3) and LtBuCo(CO)2
(4) using 50% probability thermal ellipsoids. H atoms and cocrystallized
solvent have been omitted for clarity. Selected bond distances (Å) and
bond angles (deg) for 3: Co1ꢀP1, 2.2176(6); Co1ꢀN11, 1.998(2);
Co1ꢀN21, 1.949(2); N11ꢀCo1ꢀP1, 119.52(5); N21ꢀCo1ꢀP1,
142.81(6); N11ꢀCo1ꢀN21, 97.41(7). For 4: Co1ꢀC1, 1.759(1);
Co1ꢀC2, 1.753(1); C1ꢀO1, 1.146(1); C2ꢀO2, 1.149(1); Co1ꢀN11,
1.9250(8); Co1ꢀN21, 1.9209(8); Co1ꢀC1ꢀO1, 172.50(9); Co1ꢀ
C2ꢀO2, 172.14(9); C1ꢀCo1ꢀC2, 81.37(4); N11ꢀCo1ꢀC2,
92.55(4); N21ꢀCo1ꢀC1, 93.62(4); N11ꢀCo1ꢀN21, 94.63(3).
with k = 1.48(3) ꢂ 103 Mꢀ1 sꢀ1 at ꢀ40 °C. The rate constant for
binding of DMAP is 52 times larger than that for pyridine under
identical conditions. The reaction of 2 with THF is at least 105-
fold slower at ꢀ40 °C. Thus, the rate markedly depends on the
concentration and identity of the incoming ligand.
Two possible mechanisms for the β-diketiminate isomeriza-
tion process are shown in Scheme 2. Mechanism A starts with
step i, in which the η6-arene ligand dissociates from the metal and
the imine nitrogen flips to give 2a, a two-coordinate complex
wherein the β-diketiminate binds in the traditional k2N,N0
geometry. In step ii, coordination of the added ligand gives the
three-coordinate product. In contrast, mechanism B starts with
coordination of the second ligand to 2, forming 2b (step iii).15 In
step iv, 2b then undergoes the arene slip/imine flip isomerization
of the β-diketiminate ligand to form the product.
When the steady-state approximation is used with the reason-
able assumption that kikii . kꢀikꢀii because of the large overall
equilibrium constant, mechanism A predicts a zeroth-order
dependence on ligand concentration (see the Supporting In-
formation for the derivation), which does not agree with the rate
law obtained using stopped-flow kinetics. Additionally, the rate
constant for the reaction of 2 and pyridine at ꢀ40 °C corre-
sponds to ΔGq = 10.1 kcal/mol, while ΔGq for the isomerization
of 2 to 2a in step i is at least 15 kcal/mol (on the basis of the 1H
NMR data described above). Therefore, mechanism A is incon-
sistent with the experimental data. The rate law derived from
mechanism B using analogous simplifying assumptions predicts a
first-order dependence on both the cobalt and pyridine concen-
trations. The observed kinetic data are consistent with this
prediction and with the rate of ligand binding to 2 being highly
dependent on the nucleophilicity and size of the Lewis base.
Thus, the evidence most strongly indicates an associative mecha-
nism where coordination of the added ligand precedes β-
diketiminate isomerization.
Figure 4. Spectral changes during the reaction of 2 (0.3 mM) and pyridine
(3.75 mM) in hexane at ꢀ40 °C. Inset: time course at λ = 715 nm.
191.24 ppm with a width of 60 Hz; the broadening is attributed to
relaxation by quadrupolar 59Co.
The addition of 1 equiv of pyridine to a solution of 2 in C6D6
1
gives a color change from brown to dark-green. The H NMR
spectrum shows that 2 is quantitatively converted to the three-
coordinate pyridine complex LtBuCo(pyridine).14 Addition of 1
equiv of 4-tert-butylpyridine or 4-dimethylaminopyridine
(DMAP) to 2 yields LtBuCo(tert-butylpyridine) or LtBuCo-
(DMAP), respectively. Because of the strong binding of pyridine
ligands to 2 and the distinct visible bands in the products, we used
these reactions to investigate the mechanism of ligand binding.
The rates of binding of pyridine were measured by monitoring
the exponential growth of the characteristic metal-to-ligand
charge-transfer bands for LtBuCo(pyridine) at ꢀ40 °C
(Figure 4). The spectrum was identical throughout the time
course, indicating that no intermediates were present in signifi-
cant concentrations. The first-order rate constants for pyridine
binding increased linearly with pyridine concentration. Thus, the
experimental rate law is
d½LtBuCoꢁ
ꢀ
¼ k½LtBuCoꢁ½pyridineꢁ
dt
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Scheme 3. CꢀF Activation of Fluorobenzene by 2
DE-FG02-09ER16089 to P.L.H. and DE-FG02-03ER15387 to
T.R.C.) and by the NSF (CHE-0750140, CRIF-0639138, and
MRI-0821508 to E.V.R.-A.).
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We also explored oxidative addition reactions of 2. A solution
of 2 in cyclohexane-d12 does not react with H2 over 1 day,
whether 1 atm H2 60 °C or 10 atm H2 at 25 °C is used. The
resistance of 2 to oxidative addition reactions is reminiscent of
the behavior of other unsaturated cobalt complexes that have a
triplet ground state.8,16 The addition of fluorobenzene (5 equiv)
to a solution of 2 in cyclohexane-d12 results in the appearance of
new set of signals in the 1H NMR spectrum consistent with two
new species. Heating to 60 °C for 18 h results in complete
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identified as LtBuCoPh on the basis of a comparison to fully
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characterized LtBuCoPh (synthesized independently using LtBu
-
CoCl and PhMgCl). LtBuCoPh accounts for 50% of the cobalt in
the fluorobenzene reaction as determined by integrations. The
other product was identified crystallographically as [LtBuCo(μ-
F)]2.17 Accurate integration of its broad, overlapping resonances
in the crude 1H NMR spectrum was not possible, so its yield was
determined by crystallization, which showed that it accounts for
roughly 50% of the cobalt. These yields point toward the
stoichiometry shown in Scheme 3. This is a binuclear oxidative
addition, which has been observed for other aryl halides but is
rare for the strong CꢀF bond in aryl fluorides.18 Therefore, the
hemilabile β-diketiminate ligand in 2 can rearrange to the bidentate
mode during reductive cleavage of CꢀF bonds in a fluoroarene. The
mechanism and scope of CꢀF activation by 2 is under study.
In conclusion, the cobalt(I) complex LtBuCo has a β-diketi-
minate ligand coordinated in an unprecedented kN,η6-arene
binding mode. Even though our kinetic studies show that a two-
coordinate complex is never present, LtBuCo may be termed a
“masked two-coordinate complex” because the addition of a
variety of different Lewis bases gives three-coordinate complexes
in which the β-diketiminate has undergone rearrangement to the
traditional k2N,N0 binding mode. The use of the masked two-
coordinate complex for small-molecule activation has been
demonstrated by cleavage of the CꢀF bond in fluorobenzene.
Overall, LtBuCo is a versatile compound that shows promise for
the development of new cobalt chemistry.
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’ ASSOCIATED CONTENT
S
Supporting Information. Details on experiments, com-
b
putations, and crystallography (CIF). This material is available
(13) Dai, X.; Kapoor, P.; Warren, T. H. J. Am. Chem. Soc. 2004,
126, 4798.
free of charge via the Internet at http://pubs.acs.org.
(14) This compound was characterized previously. See: Ding, K.;
Dugan, T. R.; Brennessel, W. W.; Bill, E.; Holland, P. L. Organometallics
2009, 28, 6650.
’ AUTHOR INFORMATION
Corresponding Author
holland@chem.rochester.edu
(15) Though intermediate 2b is drawn as η6, the hapticity of the
bound arene is only a speculation.
(16) Ingleson, M.; Fan, H.; Pink, M.; Tomaszewski, J.; Caulton,
K. G. J. Am. Chem. Soc. 2006, 128, 1804.
(17) This compound has also been synthesized independently. See:
Dugan, T. R.; Holland, P. L. Manuscript in preparation.
(18) Zhu, D.; Budzelaar, P. H. M. Organometallics 2010, 29, 5759.
’ ACKNOWLEDGMENT
We thank Nicole Wedgwood and William Brennessel for
crystallographic assistance. Funding was provided by the U.S.
Department of Energy, Office of Basic Energy Sciences (Grants
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