Formation of large chelate rings and cyclometallated products from diphosphines of type But2P(CH2)nPBut2(n= 5–8) and Ph2P(CH2)5PPh2 with palladium and platinum chlorides: factors affecting the stability and conformation of large chelate rings

Abstract

The new diphosphines Bu^t₂P(CH₂)_nPBu^t₂(n= 5–7) are described and the previously known Ph₂P(CH₂)₅PPh₂ characterized more fully. The compounds, Bu^t₂P(CH₂)_nPBu^t₂(n= 5–8) react with [PdCl₂(NCPh)₂] to give crystalline complexes of the type [{PdCl₂[Bu^t₂P(CH₂)_nPBu^t₂]}_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]Bu^t₂)] is formed. With [PtCl₂(NCPh)₂] or [PtCl₂(NCBu^t)₂], Bu^t₂P(CH₂)₅Pbu^t₂ gives trans-[{PtCl₂[Bu^t₂P(CH₂)₅PBu^t₂]}_x](x is unknown) and a cyclometallated product [[graphic omitted]Bu^t₂)](X = Cl) contaminated by a very similar species, possibly [[graphic omitted]Bu^t₂)] from which it could not be separated. The complex [{PtCl₂[Bu^t₂P(CH₂)₅PBu^t₂]}_x] with CF₃CO₂H, however, gives pure [[graphic omitted]Bu^t₂)](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 Bu^t₂P(CH₂)₆PBu^t₂ with [PtCl₂(NCPh)₂] gives trans-[{PtCl₂[Bu^t₂P(CH₂)₆PBu^t₂]}_x](x is unknown) and the cyclometallated [[graphic omitted]Bu^t₂)]. A remarkable difference in extent of deuteriation is observed on treating [[graphic omitted]Bu^t₂)](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 PBu^t₂ 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.