Formation of large chelate rings and cyclometallated products from diphosphines of type But2P(CH2)nPBut2(n= 5–8) and Ph2P(CH2)5PPh2with palladium and platinum chlorides: factors affecting the stability and conformation of large chelate rings

Publisher Website
Google Scholar
Abstract
The new diphosphines But 2P(CH2)nPBut 2(n= 5–7) are described and the previously known Ph2P(CH2)5PPh2 characterized more fully. The compounds, But 2P(CH2)nPBut 2(n= 5–8) react with [PdCl2(NCPh)2] to give crystalline complexes of the type [{PdCl2[But 2P(CH2)nPBut 2]}x], where x= 2 for n= 5, 7, or 8 but unknown for n= 6. Also for n= 5 a volatile cyclometallated complex [[graphic omitted]But 2)] is formed. With [PtCl2(NCPh)2] or [PtCl2(NCBut)2], But 2P(CH2)5Pbut 2 gives trans-[{PtCl2[But 2P(CH2)5PBut 2]}x](x is unknown) and a cyclometallated product [[graphic omitted]But 2)](X = Cl) contaminated by a very similar species, possibly [[graphic omitted]But 2)] from which it could not be separated. The complex [{PtCl2[But 2P(CH2)5PBut 2]}x] with CF3CO2H, however, gives pure [[graphic omitted]But 2)](X = O2CCF3) in almost quantitative yield and other pure complexes of this type with X = Cl, Br, I, or H are readily prepared by metathesis. The compound But 2P(CH2)6PBut 2 with [PtCl2(NCPh)2] gives trans-[{PtCl2[But 2P(CH2)6PBut 2]}x](x is unknown) and the cyclometallated [[graphic omitted]But 2)]. A remarkable difference in extent of deuteriation is observed on treating [[graphic omitted]But 2)](X = Cl or Br) with CF3CO2D. The chloro-complex is not deuteriated but the bromo-complex is deuteriated in both t-butyl and methylene groups. Using 31P n.m.r. spectroscopy the large-chelate mixed compounds described in this and previous papers, containing PBut 2 groups in ‘corner’ positions, are shown to be stable in solution relative to open-chain structures. The complexes trans-[Pd2Cl4{Ph2P(CH2)5PPh2}2] and cis-[Pt2Cl4{Ph2P(CH2)5PPh2}2] are described. The literature on large-chelate-ring compounds is surveyed. Factors affecting the stability and conformational homogeneity of carbocyclic and hetero-substituted rings, cyclic peptides, and depsipeptides, and cyclic monomer–polymer equilibria are discussed. Since the effects of sterically demanding substituents on the stability of purely organic rings (large and otherwise) has been explained purely in terms of bond lengths, angles, torsion angles, etc., sterically demanding substituents must have similar effects on chelate rings. Enthalpy and entropy factors affecting the stability of large chelate rings and their conformational homogeneity in solution are discussed from this point of view. Infrared, 1H, 13C, and 31P n.m.r., and mass spectral data are given and discussed.