Experimental and DFT study of the tautomeric behavior of cobalt-containing secondary phosphine oxides

摘要

New cobalt-containing secondary phosphine oxides [(VLPPh(2)CH(2)pPh(2))CO2(CO)(4){mu,eta-PhC C- P(=O)(H)(R))] (8a: R=tBu; 8b: R= Ph) were prepared by reaction of secondary phosphine oxides PhC=CP(=O)(H)(R) (6a: R=tBu; 6b: R=Ph) with dppm-bridged dicobalt complex [(mu-PPh2CH2PPh2)Co-2(CO)(6)] (2). The molecular structures of 8a and 8b were determined by single-crystal X-ray diffraction. Although palladium-catalyzed Heck reactions employing 8b as ligand gave satisfying results, 8a performed poorly in the same reaction. Judging from these results, a tautomeric equilibrium between 8b and its isomeric form [(mu-PPh(2)CH(2)pPh(2))Co-2(CO)(4){mu,eta-PhC CP(OH)(Ph)}] 8b' indeed takes place, but it is unlikely between 8a and [(mu-PPh2CH2PPh2)Co-2(CO)(4){mu,eta-PhC CP(OH)(tBu)}] (8a'). The DFT studies demonstrated that reasonable activation energies for the tautomeric conversions can be achieved only via a bimolecular pathway. Since a tBu group is much larger than a Ph group, the conversion is presumably only feasible in the case of 8b reversible arrow 8b', but not in the case of 8a reversible arrow 8a'. Another cobalt-containing phosphine, namely, [(mu-PhC CP(OH)(Ph))] 8b' indeed takes place, but it is unlikely between 8a and [(mu-PPh2CH2PPh2)CO2(CO)(4){mu,eta-PhC CP(OH)(tBu)}] (8a'). The DFT studies demonstrated that reasonable activation energies for the tautomeric conversions can be achieved only via a bimolecular pathway. Since a tBu group is much larger than a Ph group, the conversion is presumably only feasible in the case of 8b reversible arrow 8b', but not in the case of 8a reversible arrow 8a'. Another cobalt-containing phosphine, namely, [(mu-PPh2CH2PPh2)Co-2(CO)(4){mu,eta-PhC CP- (NEt2(tBu)}], and its oxidation product [(mu-PPh2CH2PPh2)Co-2(CO)(4)- {mu,eta-PhC CP(=O)(NEt2)(tBu)}] 7a' were prepared from the reaction of PhC CP(NEt2)(tBu) (5a) with 2. The molecular structures of 7a and 7a' were determined by single-crystal Xray diffraction. The phosphorus atom is surrounded by substituents in a tetrahedral environment. A P-N single bond (1.676(3) angstrom) is observed in the molecular structure of 7a. Heck reactions employing 7a/Pd(OAC)(2) as catalyst system exhibited efficiency comparable to that of 8a/Pd(OAC)(2).