3 research outputs found
Tertiary and Quaternary Phosphonium Borane Bifunctional Catalysts for CO<sub>2</sub>/Epoxide Copolymerization: A Mechanistic Investigation Using In Situ Raman Spectroscopy
Tertiary
and quaternary phosphonium borane catalysts are employed
as catalysts for CO2/epoxide copolymerization. Catalyst
structures are strategically modified to gain insights into the intricate
structure–activity relationship. To quantitatively and rigorously
compare these catalysts, the copolymerization reactions were monitored
by in situ Raman spectroscopy, allowing the determination of polymerization
rate constants. The polymerization rates are very sensitive to perturbations
in phosphonium/borane substituents as well as the tether length. To
further evaluate catalysts, a nonisothermal kinetic technique has
been developed, enabling direct mapping of polymerization rate constant
(kp) as a function of polymerization temperatures.
By applying this method, key intrinsic attributes governing catalyst
performance, such as activation enthalpy (ΔH‡), entropy (ΔS‡), and optimal polymerization temperature (Topt), can be extracted in a single continuous temperature sweep
experiment. In-depth analyses reveal intricate trends between ΔH‡, ΔS‡, and Lewis acidity (as determined using the Gutmann–Beckett
method) with respect to structural variations. Collectively, these
results are more consistent with the mechanistic proposal in which
the resting state is a carbonate species, and the rate-determining
step is the ring-opening of epoxide. In agreement with the experimental
results, DFT calculations indicate the important contributions of
noncovalent stabilizations exerted by the phosphonium moieties. Excitingly,
these efforts identify tertiary phosphonium borane analogues, featuring
an acidic phosphonium proton, as leading catalysts on the basis of kp and Topt. Mediated
by phosphonium borane catalysts, epoxides such as butylene oxide (BO), n-butyl glycidyl ether (BGE), 4-vinyl cyclohexene oxide
(VCHO), and cyclohexene oxide (CHO) were copolymerized with CO2 to form polyalkylene carbonate with >95% chemo-selectivity.
The tertiary phosphonium catalysts maintain their high activity in
the presence of large excess of di-alcohols as chain-transferring
agents, affording well-defined telechelic polyols. The results presented
herein shed light on the cooperative catalysis between phosphonium
and borane
Tertiary and Quaternary Phosphonium Borane Bifunctional Catalysts for CO<sub>2</sub>/Epoxide Copolymerization: A Mechanistic Investigation Using In Situ Raman Spectroscopy
Tertiary
and quaternary phosphonium borane catalysts are employed
as catalysts for CO2/epoxide copolymerization. Catalyst
structures are strategically modified to gain insights into the intricate
structure–activity relationship. To quantitatively and rigorously
compare these catalysts, the copolymerization reactions were monitored
by in situ Raman spectroscopy, allowing the determination of polymerization
rate constants. The polymerization rates are very sensitive to perturbations
in phosphonium/borane substituents as well as the tether length. To
further evaluate catalysts, a nonisothermal kinetic technique has
been developed, enabling direct mapping of polymerization rate constant
(kp) as a function of polymerization temperatures.
By applying this method, key intrinsic attributes governing catalyst
performance, such as activation enthalpy (ΔH‡), entropy (ΔS‡), and optimal polymerization temperature (Topt), can be extracted in a single continuous temperature sweep
experiment. In-depth analyses reveal intricate trends between ΔH‡, ΔS‡, and Lewis acidity (as determined using the Gutmann–Beckett
method) with respect to structural variations. Collectively, these
results are more consistent with the mechanistic proposal in which
the resting state is a carbonate species, and the rate-determining
step is the ring-opening of epoxide. In agreement with the experimental
results, DFT calculations indicate the important contributions of
noncovalent stabilizations exerted by the phosphonium moieties. Excitingly,
these efforts identify tertiary phosphonium borane analogues, featuring
an acidic phosphonium proton, as leading catalysts on the basis of kp and Topt. Mediated
by phosphonium borane catalysts, epoxides such as butylene oxide (BO), n-butyl glycidyl ether (BGE), 4-vinyl cyclohexene oxide
(VCHO), and cyclohexene oxide (CHO) were copolymerized with CO2 to form polyalkylene carbonate with >95% chemo-selectivity.
The tertiary phosphonium catalysts maintain their high activity in
the presence of large excess of di-alcohols as chain-transferring
agents, affording well-defined telechelic polyols. The results presented
herein shed light on the cooperative catalysis between phosphonium
and borane
Heterobimetallic Complexes Comprised of Nb and Fe: Isolation of a Coordinatively Unsaturated Nb<sup>III</sup>/Fe<sup>0</sup> Bimetallic Complex Featuring a Nbî—¼Fe Triple Bond
Heterometallic multiple
bonds between niobium and other transition
metals have not been reported to date, likely owing to the highly
reactive nature of low-valent niobium centers. Herein, a <i>C</i><sub>3</sub>-symmetric trisÂ(phosphinoamide) ligand framework is used
to construct a Nb/Fe heterobimetallic complex Cl–NbÂ(<sup><i>i</i></sup>PrNPPh<sub>2</sub>)<sub>3</sub>Fe−Br (<b>2</b>), which features a Fe→Nb dative bond with a metal–metal
distance of 2.4269(4) Ã…. Reduction of <b>2</b> in the presence
of PMe<sub>3</sub> affords NbÂ(<sup><i>i</i></sup>PrNPPh<sub>2</sub>)<sub>3</sub>Fe–PMe<sub>3</sub> (<b>6</b>), a
compound with an unusual trigonal pyramidal geometry at a Nb<sup>III</sup> center, a Nbî—¼Fe triple bond, and the shortest bond distance
(2.1446(8) Ã…) ever reported between Nb and any other transition
metal. Complex <b>6</b> is thermally unstable and degrades via
P–N bond cleavage to form a Nb<sup>V</sup>NR imide
complex, <sup><i>i</i></sup>PrNî—»NbÂ(<sup><i>i</i></sup>PrNPPh<sub>2</sub>)<sub>3</sub>Fe−PMe<sub>3</sub> (<b>9</b>). The heterobimetallic complexes <sup><i>i</i></sup>PrNî—»NbÂ(<sup><i>i</i></sup>PrNPPh<sub>2</sub>)<sub>3</sub>Fe−Br (<b>8</b>) and <b>9</b> are
independently synthesized, revealing that the strongly π-bonding
imido functionality prevents significant metal–metal interactions.
The <sup>57</sup>Fe Mössbauer spectra of <b>2</b>, <b>6</b>, <b>8</b>, and <b>9</b> show a clear trend in
isomer shift (δ), with a decrease in δ as metal–metal
interactions become stronger and the Fe center is reduced. The electronic
structure and metal–metal bonding of <b>2</b>, <b>6</b>, <b>8</b>, and <b>9</b> are explored through
computational studies, and cyclic voltammetry is used to better understand
the effect of metal–metal interaction in early/late heterobimetallic
complexes on the redox properties of the two metals involved