Application of ultrahigh-field 59Co solid-state NMR spectroscopy in the investigation of the 1,2-polybutadiene catalyst [Co(C8H13)(C4H6)].

Syndiotactic 1,2-polybutadiene (sPBD) is an important industrial polymer with a wide variety of uses; however, there is only a very limited number of catalysts currently capable of producing this polymer with a high degree of syndiotacticity. Only one such catalyst has been structurally characterized, when in 1967 Natta et al. published a low-resolution crystal structure of the cobalt complex [Co(h:h-C8H13)(h -C4H6)] (1). Since then, no spectroscopic data have been published on this or any other sPBD catalyst, a fact which frustrates discussions on the mechanism of the reaction, though the mechanism of syndiotactic formation of the larger 1,3-dienes is quite well understood. This is especially unfortunate in the case of 1, which requires the addition of CS2 to increase the formation of syndiotactic polymer, as there are many possibilities as to how this molecule may be altering the catalyst behavior. In sharp contrast to the sPBD systems, the formation of 1,4polybutadiene has been fully explored by Taube and Tobisch, and recent work has experimentally confirmed the mechanism. Thus, the application of spectroscopic tools to 1 will greatly aid in our understanding of this and other catalysts. Solution-state NMR studies would provide the best insights into the mechanism of the homogeneous catalyst 1; however, information garnered from solid-state studies may also provide crucial insights into the activity of this unique system. Co solid-state NMR (SSNMR) spectroscopy has been used in the past to investigate various small molecules, biological model systems, and homogeneous catalysts; only recently has the potential of ultrahigh-field Co SSNMR spectroscopy of powdered samples been demonstrated in cases where the cobalt nucleus experiences a very large electric field gradient (EFG). Most of these previous SSNMR studies, however, focussed on the + III oxidation state of Co. It is clear that this technique has the potential to become a valuable new tool for the investigation of catalytically important Co complexes with diamagnetic electron configurations. Following a modified literature procedure, single crystals of 1 were isolated and an X-ray structure determination was carried out. The structure of 1 is in agreement with the original Natta formulation, in which the Co center is located in a distorted square-pyramidal coordination environment (Figure 1). The basal plane comprises an h-bound butadiene molecule [C(1) C(2) and C(3) C(4)] and the allylic portion