Synthesis of a chelating hexadentate ligand with a P3N3 donor set. Crystal and molecular structure of [OC-6-22]-[Co{(RP*,RP*,RP*)-CH3C(CH2PPhC6H4NH2-2)3}](PF6)3

Abstract

The first structurally authenticated example of a hexadentate chelating tertiary phosphine in which all six donors are bound to a single metal centre is described. The multidentate ligand (R_P*,R_P*,R_P*)- and (R_P*,R_P*,S_P*)-CH₃C(CH₂PPhC₆H₄NH₂-2)₃ has been prepared in 80% yield via the reaction of five equivalents of sodium (2-aminophenyl)phenylphosphide (generated in situ from (2-aminophenyl)phenylphosphine and sodium in thf) with 1,1,1-tri(bromomethyl)ethane in thf. The diastereomeric mixture has been complexed to cobalt(III) and the resulting pair of complexes, viz. [Co{(R_P*,R_P*,R_P*)-CH₃C(CH₂PPhC₆H₄NH₂-2)₃}]Cl₃ and [CoCl{(R_P*,R_P*,S_P*)-CH₃C(CH₂PPhC₆H₄NH₂-2)₃}]Cl₂, separated by ion exchange chromatography. The structure of the former (as the corresponding hexafluorophosphate salt) has been confirmed by X-ray crystallography and clearly shows all six donors of the P₃N₃ ligand coordinated to a single cobalt(III) centre. The related hexadentate ligand with internal N donors and terminal diphenylphosphino groups, viz. CH₃C(CH₂NHC₆H₄PPh₂-2)₃, has also been synthesised, albeit in low yield, via the reaction of [Li(tmeda)][2-NHC₆H₄PPh₂] (generated in situ from (2-aminophenyl)diphenylphosphine, n-butyllithium and tmeda in diethyl ether) with 1,1,1-tri(iodomethyl)ethane in thf. No formation of a P₃N₃ ligand has been observed when either Na[2-PPhC₆H₄NH₂] or [Li(tmeda)][2-NHC₆H₄PPh₂] is reacted with the related tripodal substrate 1,1,1-tris(tolyl-4-sulfonyloxymethyl)ethane in thf. Rather the P-methyloxetane (±)-[3-{(2-aminophenyl)phenylphosphinomethyl}]-3-methyloxetane and the sulfonamide 2-(4-CH₃C₆H₄SO₂)NHC₆H₄PPh₂ and the corresponding N-methyloxetane [3-{(2-diphenylphosphinophenyl)aminomethyl}]-3-methyloxetane have been isolated from the respective reactions. The structure of the sulfonamide has been confirmed by an X-ray analysis of the platinum(II) complex trans-[PtCl(CH₃){2-PPh₂C₆H₄NH(SO₂C₆H₄CH₃-4)}₂].