Comparing the reactivity of isomeric phosphinoferrocene nitrile and isocyanide in Pd(ii) complexes: synthesis of simple coordination compounds vs. preparation of P-chelated insertion products and Fischer-type carbenes

Abstract

Isomeric phosphinoferrocene ligands, viz. 1′-(diphenylphosphino)-1-cyanoferrocene (1) and 1′-(diphenylphosphino)-1-isocyanoferrocene (2), show markedly different coordination behaviours. For instance, the reactions of 1 with [PdCl₂(MeCN)₂] and [(L^NC)Pd(μ-Cl)]₂ (L^NC = [2-(dimethylamino-κN)methyl]phenyl-κC¹) produced the “phosphine” complexes [PdCl₂(1-κP)₂] (7) and [(L^NC)PdCl(1-κP)] (8), and the latter was converted into the coordination polymer [(L^NC)Pd(μ(P,N)-1)][SbF₆] (9). Conversely, the reaction of 2 with [(L^NC)Pd(μ-Cl)]₂ involved coordination of the phosphine moiety and simultaneous insertion of the isocyanide group into the Pd–C bond, giving rise to the P,η¹-imidoyl complex [PdCl(Ph₂PfcNCC₆H₄CH₂NMe₂-κ³C,N,P)] (10; fc = ferrocene-1,1′-diyl). Compound 10 was further transformed into the Fischer carbene [PdCl(Ph₂PfcN(Me)CC₆H₄CH₂NMe₂-κ³P,C,N)][BF₄] (11) by methylation with [Me₃O][BF₄]. The reactions of 2 with Pd–Me and Pd(η³-allyl) precursors also led to imidoyl complexes [Pd(μ-Cl)(Ph₂PfcNCR-κ²C,P)]₂ (R = Me: 12, R = allyl: 15), which were cleaved with PPh₃ into the corresponding monopalladium complexes [PdCl(PPh₃)(Ph₂PfcNCR-κ²C,P)] (R = Me: 13, R = allyl: 16). The treatment of 12 and 15 with thallium(I) acetylacetonate (acac) produced [Pd(acac-O,O′)(Ph₂PfcNCR-κ²C,P)] (R = Me: 17, R = allyl: 18). Through proton transfer, these complexes reacted with Ph₂PCH₂CO₂H, ultimately producing bis-chelate complexes [Pd(Ph₂PCH₂CO₂-κ²O,P)(Ph₂PfcNCR)] (R = Me: 19, R = prop-1-enyl (sic!): 20). In addition, compound 13 was converted into the P-chelated carbene [PdCl(PPh₃)(Ph₂PfcN(Me)CMe-κ²C,P)][BF₄] (14). Compounds 10, 11, 13 and 14 were studied by cyclic voltammetry and by DFT computations.