Direct Phosphination of Ferrocenium Ion with Tertiary Phosphines by the Mechanism of Oxidative Nucleophilic Substitution

Abstract

Ferrocenium salts [(C5H5)2Fe]X (X = BF4, PF6) (I+·) react with tertiary phosphines PR3 (PMe3, PEt3, PnBu3, PMe2Ph, and PMePh2) in dichloromethane at room temperature to form a mixture of the corresponding ferrocenylphosphonium salts [(C5H5)FeC5H4PR3]X (III), ferrocene (I), and phosphonium salts [HPR3]X. The same reaction was carried out in an electrochemical cell as the electrolysis of a solution of ferrocene and phosphine in dichloromethane at the oxidation potential of ferrocene. Possible reaction pathways for phosphination of ferrocenium ion were studied by DFT at the M06‐L/6‐311++G(d,p) theory level. The experimental and theoretical studies of this reaction suggest it to proceed according to the Scheme of oxidative nucleophilic substitution of hydrogen in I+· including the following steps: a) nucleophilic exo addition of a phosphine to the cyclopentadienyl ring of I+· to form the radical cation adduct [(η5‐C5H5)Fe·(η4‐C5H5–+PR3)]X (II+·); b) redox reaction of the initial adduct II+· with the starting ferrocenium salt I+· to result in ferrocene (C5H5)2Fe (I) and iron phosphoniocyclopentadiene dication [(η5‐C5H5)Fe+(η4‐C5H5–+PR3)](X)2 (II2+) where the Csp3–H bond is involved in the agostic interaction with the metal atom; and c) deprotonation of the Csp3–H bond in II2+ by the starting phosphine to form III. The relation between the data obtained and current concepts of C–H functionalization in transition metal arene and cyclopentadienyl complexes is discussed.

DFT, DLPNO‐CCSD(T), and NEVPT2 benchmark study of the reaction between ferrocenium and trimethylphosphine

“…We also studied two additional side reactions, which were described in the original paper. [5] The first side reaction is the endoaddition of PR 3 to 1 (endo). The second is the deprotonation of 5 (deprrad).…”

“…However, frequency calculation (as well as geometry optimization) for the condensed phase is often disputed because the SMD model has been derived to reproduce the solvation Gibbs free energies only. Table 1 shows the thermochemical parameters for the reaction steps (see Figure 1) taken from the original paper ("SMD") [5] and computed in the present work ("gas"). "SMD" values were obtained at the M06-L/6-311++G(d,p) level directly within the SMD model.…”

“…[54] The results obtained in the present study show that M06-L was definitely a good choice for the theoretical analysis of the mechanism of phosphination of ferrocenium carried out in the original papers. [5][6] Because the accurate values of E gas are readily available at the DLPNO-CCSD(T) level, it is possible to give more accurate estimations for ΔG SMD values as well. In this way, the Gibbs free energy values for the compounds are constructed as G SMD = E gas + G corr + E solv , where E gas is obtained at the DLPNO-CCSD(T) level and G corr and E solv are taken from the corresponding M06-L/6-311++G(d,p) calculation (see References [55][56] for more examples of such an approach).…”

“…It was shown recently that ferrocenium reacts with nucleophilic phosphorus compounds PR 3 (namely, tertiary phosphines and aminophosphines) to yield the corresponding ferrocenylphosphonium salts [CpFe(C 5 H 4 PR 3 )] + , that is, this reaction represents a novel approach for C H functionalization of ferrocene. [5][6] At the same time, other nucleophilic reactions of ferrocenium leading to functionalization of the cyclopentadienyl (Cp) ring are extremely rare and their mechanisms remain unclear.…”

“…At this point, a quantum-mechanical calculation is the method of choice. A possible reaction pathway for the phosphination of ferrocenium was proposed in the original paper and was studied using DFT (M06-L/6-311++G(d,p) level of theory with the continuum solvation model [SMD]) for PMe 3 and PMePh 2 [5] and later for PhP(NMe 2 ) 2 , PhP(NEt 2 ) 2 , Ph 2 P(NMe 2 ), and Ph 2 P(NEt 2 ). [6] However, DFT calculations for organometallic compounds are challenging and some doubts about the reliability of the original study arise.…”