A Masked Two-Coordinate Cobalt (I) Complex That Activates C-F Bonds Page: 12,420
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Journal of the American Chemical Society
Scheme 2. Possible Mechanisms for Ligand Binding to
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 (A) and
bond angles (deg) for 3: Col-P1, 2.2176(6); Col-Nil, 1.998(2);
Col-N21, 1.949(2); N11-Col-P1, 119.52(5); N21-Col-P1,
142.81(6); NI1-Col-N21, 97.41(7). For 4: Col-C1, 1.759(1);
Col-C2, 1.753(1); C1-01, 1.146(1); C2-O2, 1.149(1); Col-N1i,
1.9250(8); Col-N21, 1.9209(8); Col-C1-01, 172.50(9); Col-
C2-O2, 172.14(9); C1-Col-C2, 81.37(4); N11-Col-C2,
92.55(4); N21-Col-C1, 93.62(4); N11-Col-N21, 94.63(3).
500 600 700
Figure 4. Spectral changes during the reaction of 2 (0.3
(3.75 mM) in hexane at -40 OC. Inset: time course at
191.24 ppm with a width of 60 Hz; the broadening
relaxation by quadrupolar 59Co.
The addition of 1 equiv of pyridine to a solut
gives a color change from brown to dark-green
spectrum shows that 2 is quantitatively convert
coordinate pyridine complex LtBUCo(pyridine).
equiv of 4-tert-butylpyridine or 4-dimeth
(DMAP) to 2 yields LtBuCo(tert-butylpyridi
(DMAP), respectively. Because of the strong bir
ligands to 2 and the distinct visible bands in the p
these reactions to investigate the mechanism o
The rates of binding of pyridine were measure
the exponential growth of the characteristic
charge-transfer bands for LtBUCo(pyridine)
(Figure 4). The spectrum was identical throat
course, indicating that no intermediates were p
cant concentrations. The first-order rate const2
binding increased linearly with pyridine concent
experimental rate law is
d[LtBu = k[LtBuCo][pyridine]
t(s) with k = 1.48(3) x 10 M-1 s 1 at -40 OC. 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 OC. Thus, the rate markedly depends on the
concentration and identity of the incoming ligand.
Two possible mechanisms for the P-diketiminate isomeriza-
tion process are shown in Scheme 2. Mechanism A starts with
800 900 step i, in which the yr6-arene ligand dissociates from the metal and
the imine nitrogen flips to give 2a, a two-coordinate complex
mM) and pyridine wherein the j3-diketiminate binds in the traditional K2N,N'
S= 715 nm. geometry. In step ii, coordination of the added ligand gives the
three-coordinate product. In contrast, mechanism B starts with
ig is attributed to coordination of the second ligand to 2, forming 2b (step iii).is5In
step iv, 2b then undergoes the arene slip/imine flip isomerization
ion of 2 in C6D6 of the j3-diketiminate ligand to form the product.
i. The 1H NMR When the steady-state approximation is used with the reason-
ted to the three- able assumption that kiki > k _k_,, because of the large overall
.14 Addition of 1 equilibrium constant, mechanism A predicts a zeroth-order
ylaminopyridine dependence on ligand concentration (see the Supporting In-
ne) or LtBuCo- formation for the derivation), which does not agree with the rate
hiding of pyridine law obtained using stopped-flow kinetics. Additionally, the rate
roducts, we used constant for the reaction of 2 and pyridine at -40 OC corre-
f ligand binding. sponds to AG* = 10.1 kcal/mol, while AG* for the isomerization
d by monitoring of 2 to 2a in step i is at least 15 kcal/mol (on the basis of the 1H
metal-to-ligand NMR data described above). Therefore, mechanism A is incon-
at -40 OC sistent with the experimental data. The rate law derived from
ighout the time mechanism B using analogous simplifying assumptions predicts a
resent in signifi- first-order dependence on both the cobalt and pyridine concen-
ants for pyridine trations. The observed kinetic data are consistent with this
ration. Thus, the 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 3-
dx.doi.org/10.1021/ja2052914 IJ. Am. Chem. Soc. 2011, 133, 12418-12421
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Dugan, Thomas R.; Sun, Xianru; Rybak-Akimova, Elena V.; Olatunji-Ojo, Olayinka; Cundari, Thomas R., 1964- & Holland, Patrick L. A Masked Two-Coordinate Cobalt (I) Complex That Activates C-F Bonds, article, July 19, 2011; [Washington, D.C.]. (https://digital.library.unt.edu/ark:/67531/metadc107787/m1/3/: accessed June 19, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT College of Arts and Sciences.