Solution and Solid-State Structures of Phosphine Adducts of Monomeric Zinc Bisphenoxide Complexes. Importance of These Derivatives in CO₂/Epoxide Copolymerization Processes

Abstract

Phosphine derivatives of the monomeric zinc phenoxide complexes, (phenoxide)2ZnLn, where phenoxide equals 2,6-di-tert-butylphenoxide, 2,4,6-tri-tert-butylphenoxide, and 2,6-diphenylphenoxide and n = 1 or 2, have been synthesized from the reaction of Zn[N(SiMe3)2]2 and the corresponding phenol followed by the addition of phosphine. The complexes have been characterized in solution by 31P NMR spectroscopy and in selected instances in the solid-state by X-ray crystallography. The small, basic phosphine, PMe3, provided the only case of an isolated complex possessing two phosphine ligands (i.e., n = 2). For all other larger phosphines only the monophosphine adducts were obtained. Furthermore, only fairly basic phosphines were found to bind to zinc, e.g., whereas PPh3 (pKa = 2.73) was ineffective, PPh2Me (pKa = 4.57) did form a strong bond to zinc. The solid-state structures of the monophosphine adducts consist of a near-trigonal planar geometry about the zinc center, where the average P-Zn-O angles are larger than the O-Zn-O angles. On the other hand, the bisphosphine adduct, Zn(O-2,4,6-tBu3C6H2)(2).2PMe3, is a distorted tetrahedral structure with O-Zn-O and P-Zn-P bond angles of 108.8(2) degrees and 107.1(9) degrees, respectively. Competitive phosphine binding studies monitored by 31P NMR spectroscopy provided a relative binding order of PPh3 approximately PtBu3 << PPh2Me < PCy3 < PMe2Ph < PnBu3 < PEt3 < PMe3. Hence, the relative binding of basic phosphine ligands at these congested zinc sites is largely determined by their steric requirements. All phosphine adducts, with the exception of PMe2Ph and PMe3, were found to undergo slow self-exchange (< 600 s-1) with free phosphine by 31P NMR spectroscopy. However, the two small phosphines, PMe2Ph (cone angle = 122 degrees) and PMe3 (cone angle = 118 degrees), were shown to undergo rapid exchange presumably via an associative mechanism. Although there was no kinetic preferences for PCy3 binding to cadmium vs zinc, cadmium was thermodynamically favored by about a factor of 2.5. The addition of up to 3 equiv of PCy3 to the Zn(O-2,6-tBu2C6H3)2 or Zn(O-2,4,6-tBu3C6H2)2 derivatives did not significantly alter the reactivity of these catalysts for the copolymerization of cyclohexene oxide (CHO) and CO2 to high-molecular weight poly(cyclohexene carbonate). However, the presence of PCy3 greatly retarded their ability to homopolymerize CHO to polyether or to afford polyether linkages during the copolymerization of CHO/CO2.

Coordination chemistry of the main group elements with phosphine, arsine and stibine ligands

“…Although a few complexes of phosphine substituted bi-or poly-dentates have found specific catalytic applications, e.g. zinc complexes in olefin oxide polymerisations [48][49][50], it seems unlikely that many catalytic applications will emerge. The major area of application lies in the chemistry related to III-V semi-conductor materials, which has been described in detail in excellent reviews.…”

Discrete Metal‐Based Catalysts for the Copolymerization of CO2 and Epoxides: Discovery, Reactivity, Optimization, and Mechanism

“…[125] Addition of 2,6-dihalophenols to Zn[N(SiMe 3 ) 2 ] 2 gave four-coordinate, dimeric zinc phenoxides with coordinated THF solvent molecules. [125,127] To probe this theory, phosphane (PCy 3 , PMe 3 , etc.) Analysis of the PCHC revealed PDIs of 6.0, molecular weights of 42 000 g mol À1 , a T g of 115 8C, and > 99 % carbonate linkages.…”

Aspects of carbon dioxide utilization

“…Recently, many active zinc catalysts such as zinc phenoxides [167], bulky fluorophenoxides (C1) [168,169], zinc diimines (C2) [170][171][172] and zinc bis-Schiff bases (C3) [173] have been investigated and the production of a copolymer having a narrow molecular weight distribution (M w /M n = 1.07-1.17) and a high molecular weight (M w = about 420,000) at a high TON (1441 g/1 g cat) were reported. …”