Experimental and DFT Study of the Tautomeric Behavior of Cobalt-Containing Secondary Phosphine Oxides

New cobalt‐containing secondary phosphine oxides [(μ‐PPh2CH2PPh2)Co2(CO)4{μ,η‐PhCCP(O)(H)(R)}] (8 a: R=tBu; 8 b: R=Ph) were prepared by reaction of secondary phosphine oxides PhCCP‐ (O)(H)(R) (6 a: R=tBu; 6 b: R=Ph) with dppm‐bridged dicobalt complex [(μ‐PPh2CH2PPh2)Co2(CO)6] (2). The molecular structures of 8 a and 8 b were determined by single‐crystal X‐ray diffraction. Although palladium‐catalyzed Heck reactions employing 8 b as ligand gave satisfying results, 8 a performed poorly in the same reaction. Judging from these results, a tautomeric equilibrium between 8 b and its isomeric form [(μ‐PPh2CH2PPh2)Co2(CO)4{μ,η‐PhCCP(OH)(Ph)}] 8 b′ indeed takes place, but it is unlikely between 8 a and [(μ‐PPh2CH2PPh2)Co2(CO)4{μ,η‐PhCCP(OH)(tBu)}] (8 a′). 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 8 b⇌8 b′, but not in the case of 8 a⇌8 a′. Another cobalt‐containing phosphine, namely, [(μ‐PPh2CH2PPh2)Co2(CO)4{μ,η‐PhCCP(NEt2)(tBu)}] (7 a), and its oxidation product [(μ‐PPh2CH2PPh2)Co2(CO)4{μ,η‐PhCCP(O)(NEt2)(tBu)}] 7 a′ were prepared from the reaction of PhCCP(NEt2)(tBu) (5 a) with 2. The molecular structures of 7 a and 7 a′ were determined by single‐crystal X‐ray diffraction. The phosphorus atom is surrounded by substituents in a tetrahedral environment. A PN single bond (1.676(3) Å) is observed in the molecular structure of 7 a. Heck reactions employing 7 a/Pd(OAc)2 as catalyst system exhibited efficiency comparable to that of 8 a/Pd(OAc)2. Evidence for tautomerism between phosphine oxide 1 and its isomeric phosphinous acid form 1′ in solution has been obtained. Thus, Heck reactions employing 1/Pd(OAc)2 as the catalyst system, in which 1′ but not 1 is expected to be active, gave satisfying results. DFT studies demonstrated that the tautomeric conversion 1⇌1′ might be achieved with reasonably low activation energy through a bimolecular pathway.