10.1021/ja970697x.s001
Richard R. Schrock
Richard R.
Schrock
Scott W. Seidel
Scott W.
Seidel
Nadia C. Mösch-Zanetti
Nadia C.
Mösch-Zanetti
Keng-Yu Shih
Keng-Yu
Shih
Myra B. O'Donoghue
Myra B.
O'Donoghue
William M. Davis
William M.
Davis
William M. Reiff
William M.
Reiff
Synthesis and Decomposition of Alkyl Complexes of
Molybdenum(IV) That Contain a [(Me<sub>3</sub>SiNCH<sub>2</sub>CH<sub>2</sub>)<sub>3</sub>N]<sup>3-</sup>
Ligand. Direct Detection of α-Elimination Processes That Are
More than Six Orders of Magnitude Faster than β-Elimination
Processes
American Chemical Society
1997
2.9 μ Β
CH 2 Ph
cyclopentyl
room temperature
3 N
field splitting effects
3 SiNCH 2 CH 2
CH 2 CMe 3
TMS
2 NCH 2 CH 2 SiMe 2 CH 2
elimination
NMR
alkyl complexes
CH 2 SiMe 3
cyclohexyl complexes show
1997-12-10 00:00:00
Dataset
https://acs.figshare.com/articles/dataset/Synthesis_and_Decomposition_of_Alkyl_Complexes_of_Molybdenum_IV_That_Contain_a_Me_sub_3_sub_SiNCH_sub_2_sub_CH_sub_2_sub_sub_3_sub_N_sup_3-_sup_Ligand_Direct_Detection_of_-Elimination_Processes_That_Are_More_than_Six_Orders_of_Magnitude_Faster_than_-Elimi/3665184
A variety of paramagnetic molybdenum complexes,
[N<sub>3</sub>N]MoR ([N<sub>3</sub>N]<sup>3-</sup>
=
[(Me<sub>3</sub>SiNCH<sub>2</sub>CH<sub>2</sub>)<sub>3</sub>N]<sup>3-</sup>;
R
= Me, Et, Bu, CH<sub>2</sub>Ph,
CH<sub>2</sub>SiMe<sub>3</sub>,
CH<sub>2</sub>CMe<sub>3</sub>, cyclopropyl, cyclobutyl,
cyclopentyl, cyclohexyl, cyclopentenyl, phenyl),
have been prepared from [N<sub>3</sub>N]MoCl. The several
that have been examined all follow Curie−Weiss <i>S</i> = 1
behavior
with a magnetic moment in the solid state between 2.4 and 2.9
μ<sub>Β</sub> down to 50 K. Below ∼50 K the
effective
moments undergo a sharp decrease as a consequence of what are proposed
to be a combination of spin−orbit coupling
and zero field splitting effects. NMR spectra are temperature
dependent as a consequence of “locking” of the
backbone
into one <i>C</i><sub>3</sub>-symmetric conformation and as a
consequence of Curie−Weiss behavior. The cyclopentyl and
cyclohexyl
complexes show another type of temperature-dependent fluxional behavior
that can be ascribed to a rapid and reversible
α-elimination process. For the cyclopentyl complex the rate
constant for α-elimination is ∼10<sup>3</sup>
s<sup>-1</sup> at room
temperature, while the rate constant for α-elimination for the
cyclohexyl complex is estimated to be ∼200
s<sup>-1</sup> at
room temperature. An isotope effect for α-elimination for the
cyclohexyl complex was found to be ∼3 at 337 K.
Several of the alkyl complexes decompose between 50 and 120 °C.
Of the complexes that contain linear alkyls,
only [N<sub>3</sub>N]Mo(CH<sub>2</sub>CMe<sub>3</sub>)
decomposes cleanly (but slowly) by α,α-dehydrogenation to give
[N<sub>3</sub>N]Mo⋮CCMe<sub>3</sub>.
[N<sub>3</sub>N]MoMe is by far the most stable of the
alkyl complexes; no [N<sub>3</sub>N]Mo⋮CH can be detected
upon attempted
thermolysis at 120 °C. Other decompositions of linear alkyl
complexes are complicated by competing reactions,
including β-hydride elimination. β-Hydride elimination (to
give [N<sub>3</sub>N]MoH) is the sole mode of decomposition
of
the cyclopentyl and cyclohexyl complexes; the former decomposes at a
rate calculated to be approximately 10× that
of the latter at 298 K. β-Hydride elimination in
[N<sub>3</sub>N]Mo(cyclopentyl) to give (unobservable)
[N<sub>3</sub>N]Mo(cyclopentene)(H) has been shown to be 6−7 orders of magnitude slower than
α-hydride elimination to give (unobservable)
[N<sub>3</sub>N]Mo(cyclopentylidene)(H).
[N<sub>3</sub>N]Mo(cyclopropyl) evolves ethylene in a
first-order process upon being heated to
give [N<sub>3</sub>N]Mo⋮CH, while
[N<sub>3</sub>N]Mo(cyclobutyl) is converted into
[N<sub>3</sub>N]Mo⋮CCH<sub>2</sub>CH<sub>2</sub>CH<sub>3</sub>.
[N<sub>3</sub>N]MoH decomposes
slowly and reversibly at 100 °C to yield molecular hydrogen and
[(Me<sub>3</sub>SiNCH<sub>2</sub>CH<sub>2</sub>)<sub>2</sub>NCH<sub>2</sub>CH<sub>2</sub>SiMe<sub>2</sub>CH<sub>2</sub>]Mo
([bitN<sub>3</sub>N]Mo). X-ray structures of
[N<sub>3</sub>N]Mo(triflate),
[N<sub>3</sub>N]MoMe,
[N<sub>3</sub>N]Mo(cyclohexyl), and
[bitN<sub>3</sub>N]Mo show
that the degree of twist of the TMS groups away from an “upright”
position correlates with the size of the ligand in
the apical pocket and that steric congestion in the cyclohexyl complex
is significantly greater than in the methyl
complex. Relief of steric strain in the ground state in molecules
of this general type to give a less crowded alkylidene
hydride intermediate is proposed to be an important feature of the high
rate of α-elimination relative to β-elimination
in several circumstances.