Solation and solid-state structures of phosphine adducts of monomeric zincbisphenoxide complexes. Importance of these derivatives in CO2/epoxide copolymerization processes

Phosphine derivatives of the monomeric zinc phenoxide complexes, (phenoxide )(2)ZnLn, 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 synthe sized from the reaction of Zn[N(SiMe3)(2)](2) and the corresponding phenol followed by the addition of phosphine. The complexes have been characterize d in solution by P-31 NMR spectroscopy and in selected instances in the sol id-state by X-ray crystallography. The small, basic phosphine, PMe3, provid ed 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 m onophosphine adducts consist of a near-trigonal planar geometry about the z inc center, where the average P-Zn-O angles are larger than the O-Zn-O angl es, On the other hand, the bisphosphine adduct, Zn(O-2,4,6-(Bu3C6H2)-Bu-t)( 2). 2PMe(3), is a distorted tetrahedral structure with O-Zn-O) and P-Zn-P b ond angles of 108.8(2)degrees and 107.1(9)degrees, repectively. Competitive phosphine binding studies monitored by (31) NMR spectroscopy provided a re lative, binding order of PPh3 approximate to (PBU3)-B-t much less than PPh2 Me < PCy3 < PMe2Ph < PnBu(3) < 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)) wi th free phosphine by P-31 NMR spectroscopy. However, the two small phosphin es, PMe2Ph (cone angle = 122 degrees) and PMe3 (cone angle = 118 degrees), were shown to undergo rapid exchange presumably via an associative mechanis m. 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-(Bu2CsH3)-Bu-t)(2) or Zn( O-2,4,6-(Bu3C6H2)-Bu-t)(2) derivatives did not significantly alter the reac tivity of these catalysts for the copolymerization of cyclohexene oxide (CH O) and CO2 to high-molecular weight poly(cyclohexene carbonate). However, t he presence of PCy3 greatly retarded their ability to homopolymerize CHO to polyether or to afford polyether linkages during the copolymerization of C HO/CO2.