posted on 2017-08-07, 00:00authored byRomain
J.-C. Dubey, Robert J. Comito, Zhenwei Wu, Guanghui Zhang, Adam J. Rieth, Christopher H. Hendon, Jeffrey T. Miller, Mircea Dincă
Molecular
catalysts offer tremendous advantages for stereoselective
polymerization because their activity and selectivity can be optimized
and understood mechanistically using the familiar tools of organometallic
chemistry. Yet, this exquisite control over selectivity comes at an
operational price that is generally not justifiable for the large-scale
manufacture of polyfolefins. In this report, we identify Co-MFU-4l, prepared by cation exchange in a metal–organic
framework, as a solid catalyst for the polymerization of 1,3-butadiene
with high stereoselectivity (>99% 1,4-cis). To
our
knowledge, this is the highest stereoselectivity achieved with a heterogeneous
catalyst for this transformation. The polymer’s low polydispersity
(PDI ≈ 2) and the catalyst’s ready recovery and low
leaching indicate that our material is a structurally resilient single-site
heterogeneous catalyst. Further characterization of Co-MFU-4l by X-ray absorption spectroscopy provided evidence for
discrete, tris-pyrazolylborate-like coordination of Co(II). With this
information, we identify a soluble cobalt complex that mimics the
structure and reactivity of Co-MFU-4l, thus providing
a well-defined platform for studying the catalytic mechanism in the
solution phase. This work underscores the capacity for small molecule-like
tunability and mechanistic tractability available to transition metal
catalysis in metal–organic frameworks.