3H-1,3-Azaphospholo[4,5-b]pyridines – novel heterocyclic P,N-bridging or hybrid ligands: synthesis and first d8-transition metal complexes

Abstract

The first 3H-1,3-azaphospholo-pyridines 2a–c were synthesized as racemic mixtures in modest to medium yield by the reaction of N-(2-chloropyrid-3-yl)-trimethylacetimidoyl chloride 1 with RPLi₂ (R = Ph, n-Bu, i-Bu), generated from RPH₂ and BuLi in THF at −70 °C, and studied with respect to their suitability as ligands (L) in transition metal complexes. Reactions of 2a with group 6 metal(0) pentacarbonyls led to P-coordinated LM(CO)₅ complexes 3a–5a (Cr, Mo, W) and the reaction of 2c with (norbornadiene)Mo(CO)₄ surprisingly to 4c. [Rh(1,5-COD)Cl]₂ and 2a,b, in metal/ligand ratio 1 : 1, furnished LRh(1,5-COD)Cl complexes 6a,b with P-coordination, 6b accompanied by a minor contamination by the bis-coordinated L[Rh(COD)Cl]₂ complex 7b. Reactions of 2a,b with [(allyl)PdCl]₂ proceeded in THF with dismutation of N-coordinated (allyl)PdCl and formed with 2a a labile crude product [(2a){(allyl)PdCl}_1.2(PdCl₂)_0.8]·C₄H₈O, with the composition close to L[Pd(allyl)Cl]PdCl₂ THF (8a·THF), which converted during crystallization to 9a, whereas 2b directly formed the N,N′-PdCl₂-bridged bis[LPd(allyl)chloride] complex 9b. Conversion of 2b with equimolar amounts of Pd(CH₃CN)₂Cl₂ in THF, or Na₂PdCl₄ in methanol, gave rise to the dimeric P,N-bridging complex 10b. Crystal structure analyses of 6a (rac), 9b·2CDCl₃ (meso), 10b·4.5THF and 10b·2D₆-acetone (rac) provided detailed structural information. 10b, but more efficiently complexes formed in situ from 2a,b and Pd₂(DBA)₃ or Pd(OAc)₂, catalysed the arylamination of 2-bromopyridine with 2,4,6-trimethylaniline.