259188-20-0Relevant academic research and scientific papers
Solution and solid-state structures of phosphine adducts of monomeric zinc bisphenoxide complexes. Importance of these derivatives in CO2/epoxide copolymerization processes
Darensbourg, Donald J.,Zimmer, Marc S.,Rainey, Patrick,Larkins, David L.
, p. 1578 - 1585 (2008/10/08)
Phosphine derivatives of the monomeric zinc phenoxide complexes, (phenoxide)2ZnL(n), 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 (pK(a) = 2.73) was ineffective, PPh2Me (pK(a) = 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-(t)Bu3C6H2)2·2PMe3, is a distorted tetrahedral structure with O-Zn-O and P-Zn-P bond angles of 108.8(2)°and 107.1(9)°, respectively. Competitive phosphine binding studies monitored by 31P NMR spectroscopy provided a relative binding order of PPh3 ? P(t)Bu3 2Me 3 2Ph 3 3 3. 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 (-1) with free phosphine by 31P NMR spectroscopy. However, the two small phosphines, PMe2Ph (cone angle = 122°) and PMe3 (cone angle = 118°), 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-(t)Bu2C6H3)2 or Zn(O-2,4,6-(t)Bu3C6H2)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.
Solution 31P and 113Cd NMR studies of phosphine adducts of monomeric cadmium (bisphenoxide) complexes and the solid-state structures of (2,6-di-tert-butylphenoxide)2Cd(PCy3) and (2,6-di-tert-butylphenoxide)2Cd(PMe3)2
Darensbourg, Donald J.,Rainey, Patrick,Larkins, David L.,Reibenspies, Joseph H.
, p. 473 - 479 (2008/10/08)
Monomeric phosphine derivatives of cadmium phenoxides, (phenoxide)2CdL(n), where the phenoxide ligands contain sterically demanding substituents in the 2,6-positions (2,6-di-tert-butylphenoxide and 2,6-diphenylphenoxide) are described. These derivatives were synthesized from the reaction of Cd[N(SiMe3)2]2 and the corresponding phenol followed by the addition of the phosphine. For the large PCy3 ligand (Tolman's cone angle (θ) = 170°) n = 1, whereas for the smaller Me3P (θ = 118°) and n-Bu3P (θ = 132°) ligands both mono- and bis(phosphine) derivatives (n = 1 and 2) were prepared. The (2,6-di-tert-butylphenoxide)2Cd(PCy3) and (2,6-di-tert-butylphenoxide)2Cd-(PMe3)2 were characterized in the solid-state by X-ray crystallography. The structure of the monophosphine adduct of PCy3 consists of a near trigonal planar geometry about the cadmium center, where the average P-Cd-O angle of 131.4°is larger than the O-Cd-O angle of 96.72°with a Cd-P bond length of 2.5247(12) A. On the other hand, the bis(phosphine) adduct, Cd(O-2,6-(t)Bu2C6H3)2(PMe3)2 is a distorted tetrahedral structure with O-Cd-O and P-Cd-P bond angles of 116.7(6)°and 104.3(2)°, respectively. The average Cd-P bond length in this derivative was determined to be 2.737[5] A. The monotricyclohexylphosphine derivatives of these cadmium bisphenoxides were shown by 31P NMR spectroscopy not to be undergoing facile exchange with free phosphine in solution at ambient temperature. On the contrary, the corresponding Me3P and n-Bu3P analogues readily undergo self-exchange with free phosphine in solution via a rapid equilibrium between monophosphine adduct plus free phosphine and the bis(phosphine) adduct. The 113Cd chemical shifts in the CdO2P moieties shift downfield and the 113Cd-31P coupling constants decrease by about 900 Hz upon binding an additional phosphine ligand.
