Kinetic studies of cyclooctadiene substitution reactions
The rate and mechanism of substitution of the cyclooctadiene ligands in both cobalt(0) and cobalt(I) was of fundamental interest and may provide insights into precatalyst activation and substrate coordination events. Our studies commenced with a self-exchange experiment between free and coordinated 1,5-cyclooctadiene with neutral
P2CoCOD. Because of the paramagnetically shifted
1H NMR resonances of the formally cobalt(0) compound, 1,5-cyclooctadiene-
d12 would provide additional spectroscopic handles for these experiments. The iron-catalyzed [4 + 4] cycloaddition of dienes, pioneered by tom Dieck
13a and later optimized by Ritter
13b and expanded by our laboratory,
13c was well suited for the preparation of cyclooctadiene-
d12 from commercially available 1,3-butadiene-
d6. The targeted cobalt(0) diene complex, (
R,
R)-(
iPrDuPhos)Co(COD-
d12) (
P2CoCOD-d12), was successfully synthesized following the previously reported procedure
5c in 92% isolated yield. The natural abundance compound,
P2CoCOD, exhibits paramagnetically shifted resonances in the benzene-
d6 1H NMR spectrum at ambient temperatures that range from −80.23 to 34.35 ppm. Comparing these data with the analogous spectrum of
P2CoCOD-d12 identified the six resonances corresponding to the coordinated cyclooctadiene (Supplementary Fig. S2).
With the
P2CoCOD-d12 in hand, exchange with natural abundance 1,5-cyclooctadiene was explored (
Fig. 2A). Addition of 20 equivalents of free 1,5-cyclooctadiene to a benzene-
d6 solution of
P2CoCOD-d12 and monitoring the progress of the reaction by
1H NMR spectroscopy established appearance of coordinated COD resonances over the course of 8 h at ambient temperature (Supplementary Fig. S7). Notably, the isotopic composition of the cyclooctadiene ligand also impacted the
1H NMR chemical shifts of the
iPrDuPhos protons (
Fig. 2B). The resolution of the
1H NMR resonances of the
iPrDuPhos ligand in
P2CoCOD and
P2CoCOD-d12 provided a convenient measure for determining the degree of conversion for diene exchange. The reaction reached 10% and 50% conversions in approximately 4 and 27 h, respectively, and equilibrium was established after 96 h.
To determine the reaction order in both cobalt and 1,5-cyclooctadiene, initial rate (<10% conversion) measurements were performed. Three separate reactions: (A)
P2CoCOD-d12 (32 mmol/L) and 1,5-cyclooctadiene (640 mmol/L); (B)
P2CoCOD-d12 (32 mmol/L) and 1,5-cyclooctadiene (1280 mmol/L) and (C)
P2CoCOD-d12 (64 mmol/L), and 1,5-cyclooctadiene (640 mmol/L) in benzene-
d6 solutions were monitored by
1H NMR spectroscopy, and the formation of [
P2CoCOD] product as a function of time is shown in
Fig. 3. Doubling the concentration of 1,5-cycloctadiene did not result in a discernable acceleration in initial rate (A vs B), whereas an approximate two-fold enhancement in rate was observed when the initial
P2CoCOD-d12 concentration was doubled (A vs C), establishing zeroth order behavior in [COD-
d12] and 1st order in [
P2CoCOD-d12], respectively (
Fig. 3B). These findings support a dissociative substitution pathway where ligand coordination is fast compared with rate-determining cobalt–carbon bond breaking. Although 17-electron complexes have been reported to undergo associative substitutions through 19-electron intermediates,
14 the steric properties of the ancillary ligands are known to dramatically influence the preferred substitution mechanism.
15 The solid-state structure of
P2CoCOD demonstrates that the
iso-propyl groups on the rigid DuPhos ligand backbone impose steric hindrance above and below the P–Co–P plane, likely inhibiting formation a five-coordinate, 19-electron intermediate with two COD ligands coordinated in (η
2, η
2) and η
2 arrangement. Substitution of
P2CoCOD with 1,5-cyclooctadiene-
d12 was also monitored and exhibited the same kinetic profile, suggesting there was no secondary kinetic isotope effect. An overall first order rate constant
k = 6.6 (±0.5) × 10
−6 s
−1 was derived from the linear fits of the initial rates, supporting slow diene substitution. The entire reaction time courses for trials A–C were followed over 103 h until [
P2CoCOD] plateaued and the equilibria were established (
Fig. 3C). The increase in [
P2CoCOD] slowed significantly at higher conversions, as the reverse reaction rate of the equilibrium became significant. Plotting conversions versus time for trials A–C showed similar traces but slightly different conversions (positions of final equilibrium) relevant to the initial [
P2CoCOD-d12]:[COD] ratio (
Fig. 3D).
The observation of a dissociative substitution mechanism for the 17-electron, neutral cobalt(0) complex prompted related studies on cyclooctadiene exchange with the 16-electron, cationic cobalt(I) complex,
[P2CoCOD]+. The propensity of
[P2CoCOD]+ to form the more stable 18-electron, η
6-arene complexes
[P2Co(arene)]+ precluded the use of benzene-
d6 as solvent.
12 Although
[P2CoCOD]+ was also unstable upon prolonged standing at ambient temperature in THF-
d8 due to solvent substitution and formation of high-spin, paramagnetic cobalt species,
12 adding 20 equivalents of 1,5-cyclooctadiene-
d12 to a THF-
d8 solution of
[P2CoCOD]+ and monitoring the reaction by
1H NMR spectroscopy after 10 min demonstrated that the reaction had reached equilibrium (Supplementary Fig. S8). No additional spectroscopic changes were observed until competing THF-
d8 substitution occurred. Although no quantitative kinetic data could be obtained due to the fast rate of substitution, these observations clearly establish the substantial increase in the substitutional lability of
[P2CoCOD]+ compared with its one-electron reduced analogue,
P2CoCOD.
Cyclooctadiene substitution reactions with carbon monoxide
Substitution of the diene with stronger field ligands such as CO was also explored. Replacement of the 1,5-cyclooctadiene ligand by carbon monoxide with
P2CoCOD was previously studied and afforded the coordinatively saturated, dimeric cobalt(0) dicarbonyl complex with bridging, terminal CO ligands and a cobalt–cobalt bond, [(
R,
R)-
iPrDuPhosCo(η
1-CO)(μ
2-CO)]
2 (
Scheme 2A).
11 The carbonylation of
[P2CoCOD]+ was studied for comparison. Treatment of a diethyl ether solution of
[P2CoCOD]+ with 1 atm CO resulted in an instant colour change from blue to orange. The product was isolated in 86% yield and X-ray diffraction established formation of the 18-electron cobalt carbonyl complex, [(
R,
R)-
iPrDuPhosCo(η
1-CO)
3][BAr
F4] (
Scheme 2B;
Fig. 4). An idealized trigonal bipyramidal geometry at cobalt was observed. The
1H,
31P{
1H}, and
13C{
1H} NMR spectra (THF-
d8, 298 K) of [(
R,
R)-(
iPrDuPhos)Co(η
1-CO)
3][BAr
F4] suggest chemically equivalent phosphine atoms (singlet, 98.15 ppm) and carbon atoms of CO ligands (199.5 ppm) at ambient temperature (Supplementary Figs. S3–S5). The
1H and
13C NMR signals are consistent with an overall C
2-symmetric environment for the ligand, despite the overall C
1-symmetry of the molecule. This observation is consistent with those previously reported for η
6-Ph(BPh
3) and η
2, κ
1-methyl 2-acetamidoacrylate coordination.
12 Solution infrared spectra (Et
2O, 298 K) of [(
R,
R)-
iPrDuPhosCo(η
1-CO)
3][BAr
F4] showed terminal CO stretching frequencies at 2079, 2035, and 2011 cm
−1, which are higher than those of the neutral [(
R,
R)-
iPrDuPhosCo(η
1-CO)(μ
2-CO)]
2 (
Scheme 2), consistent with weaker back-bonding to CO from Co(I)
+ than the neutral, formal Co(0) center. The observation that both the neutral
P2CoCOD and cationic
[P2CoCOD]+ underwent fast and complete displacement of COD ligand by CO to afford coordinatively saturated carbonyl complexes highlight their substitutional lability to the stronger field ligand.
Electronic structure studies of [P2CoCOD]+ and P2CoCOD
The electronic structures of the neutral and cationic cobalt complexes were investigated by full molecule density functional theory (DFT) using the B3LYP functional. The qualitative d-orbital splitting diagrams for
[P2CoCOD]+ are shown in
Fig. 5A. The HOMO is principally d
z2 in character as expected for a square-planar d
8 metal complex with σ-donating phosphines and π-accepting alkene (diene) ligands.
16 The qualitative d-orbital splitting diagram and the Mulliken spin density plot for
P2CoCOD are likewise presented in
Figs. 5B and
6. The spin density distribution on cobalt approximates the shape of the d
z2 orbital, suggesting d
z2 as the singly occupied molecular orbital (SOMO) of the
S = 1/2 complex. This contrasts expectations for a cobalt(0),
d9 complex where Jahn-Teller distortion resulting in a D
2d geometry would be expected and the d
xy, d
xz, and d
yz orbitals would have higher energies than d
x2-
y2 and d
z2 orbitals and d
z2 would be filled.
A closer examination of the metrical parameters from the solid-state structure of
P2CoCOD5c revealed that one alkene double bond of COD was longer than the other (1.417(3) Å vs. 1.396(3) Å), whereas the average Co–C bond length was shorter than the other (2.044(3) Å vs. 2.128(2) Å), indicating stronger back-bonding interaction and metallacyclopropane character
17 of one “Co–C=C” bonds (
Fig. 7A). The elongated C=C bond length is comparable with that (1.423 Å) of a structurally characterized cobalt ethylene complex, (PMe
3)
3Co(Ph)(η
2-C
2H
4).
18 By comparison, both alkene double bond distances of COD of the cationic
[P2CoCOD]+ are 1.39(1) Å (
Fig. 7A), suggesting weaker back-bonding from Co(I)
+ and a lack of metallacyclopropane character. The solid-state structure of
P2CoCOD showed distortion from an idealized D
2d geometry towards a more square-pyramidal configuration, where the phosphines, cobalt, and one C=C bond of the cyclooctadiene define the basal plane and the other alkene is apical. A similar distortion towards square-pyramidal geometry and metallacyclopropane character have also been identified in the solid-state structures of (dppe)Co(COD)
5b (dppe = 1,2-Bis(diphenylphosphino)ethane) and (
R,
R)-(BenzP*)Co(COD)
5d ((
R,
R)-BenzP* = (
R,
R)−1,2-Bis(t-butylmethylphosphino)benzene) (
Fig. 7A). As such, these formally P
2Co(0)COD complexes are best described as five-coordinate,
d7, cobalt(II) compounds with the cyclooctadiene being viewed as an LX
2-type ligand. The four strong-field donors consisting of phosphines and alkyls occupy the
xy plane and largely determine the relative
d-orbital energies similar to those in a square-planar field with perturbation from the weaker field alkene donor coordinating through the z-axis and slightly raising the energies of the d
z2, d
xz, and d
yz parentage orbitals. Accordingly, the spin density analysis and qualitative
d-orbital splitting diagram suggest that the SOMO of the low spin, d
7 cobalt complex has primarily
dz2 character with contributions from the cyclooctadiene carbon
p orbitals, whereas the d
xz, d
yz, and d
xy orbitals are filled and d
x2-
y2 orbital is empty (
Figs. 5B and
6).
In addition, the X-band EPR spectra of
P2CoCOD, (dppe)CoCOD, (
R,
R)-(BenzP*)CoCOD and (
R,
R)-(
PhBPE)CoCOD ((
R,
R)-
PhBPE = 1,2-bis[(2 R,5 R)−2,5-diphenylphospholano]-ethane) all exhibit characteristic hyperfine coupling of one
g tensor to the
59Co nucleus (I = 7/2) but no coupling to the
31P nuclei,
5b–5d indicating a lack of
dx2-
y2 character (phosphines are on the xy plane) of the SOMO and further supporting a d
7 instead of d
9 electronic configuration. Nevertheless, the relatively small
g anisotropy for all P
2CoCOD complexes
5b–5d compared with the large
g anisotropy characteristic of square-planar, low spin, d
7 cobalt(II) complexes, including (dppe)Co(CH
2SiMe
3)
25b and (
iPrmPNP)Co(X) (X = CH
3, Cl and H,
iPrmPNP is an L
2X-type ligand),
19 also suggest that the P
2CoCOD compounds are electronically differentiated from low spin, planar L
2CoX
2 complexes. For a related structurally characterized P
2Co(0)(η
6-arene) precatalyst, (
R,
R)-(
PhBPE)Co(η
6-C
6H
6),
5c DFT-computed Mulliken spin density distribution on cobalt approximate the shape of the d
x2-
y2 orbital (Supplementary Fig. S10), supporting a Co(0), d
9 assignment for the η
6-arene complex.
Taken together, the slow substitution rate of the 17-electron
P2CoCOD may be attributed to the strong coordination of one cyclooctadiene C=C bond to the cobalt through strong π-back donation, resulting in a rate-determining alkene dissociation step. In addition, the shift of electron density from cobalt to the alkene fragment likely makes the metal less reducing and consequently less reactive towards protic solvents such as methanol. A stepwise substitution mechanism accounting for the overall dissociative process established from kinetic measurements is proposed in
Scheme 3, where the transition states have primarily cobalt–carbon bond-breaking character. Dissociation of the apical alkene arm provided space for an incoming cyclooctadiene molecule affording a 17-electron bis(η
2-cyclooctadiene) intermediate, whereas formation of a 15-electron mono(η
2-cyclooctadiene) intermediate is less likely. Subsequent tautomerization between the apical and equatorial COD on the metallacyclopropane component and alkene dissociation generated the product. In comparison, oxidation by one electron to furnish the corresponding cation, [
P2CoCOD]+, resulted in markedly higher substitutional lability and faster ligand self-exchange rate, likely originating from the electrophilicity of the cobalt(I) center and a stabilization effect from coordination of another alkene. An associative substitution for [
P2CoCOD]+ is likely operative and occurs through an 18-electron intermediate. The planar geometry observed with
[P2CoCOD]+ may also provide greater access of an incoming ligand from the apical direction, resulting in an increased rate of substitution.