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

New cobalt‐containing secondary phosphine oxides [(μ‐PPh 2 CH 2 PPh 2 )Co 2 (CO) 4 {μ,η‐PhCCP(O)(H)(R)}] ( 8 a : R= t Bu; 8 b : R=Ph) were prepared by reaction of secondary phosphine oxides PhCCP‐ (O)(H)(R) ( 6 a : R= t Bu; 6 b : R=Ph) with dppm‐bridged dicobalt complex [(μ‐PPh 2 CH 2 PPh 2 )Co 2 (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‐PPh 2 CH 2 PPh 2 )Co 2 (CO) 4 {μ,η‐PhCCP(OH)(Ph)}] 8 b′ indeed takes place, but it is unlikely between 8 a and [(μ‐PPh 2 CH 2 PPh 2 )Co 2 (CO) 4 {μ,η‐PhCCP(OH)( t Bu)}] ( 8 a′ ). The DFT studies demonstrated that reasonable activation energies for the tautomeric conversions can be achieved only via a bimolecular pathway. Since a t Bu 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‐PPh 2 CH 2 PPh 2 )Co 2 (CO) 4 {μ,η‐PhCCP(NEt 2 )( t Bu)}] ( 7 a ), and its oxidation product [(μ‐PPh 2 CH 2 PPh 2 )Co 2 (CO) 4 {μ,η‐PhCCP(O)(NEt 2 )( t Bu)}] 7 a′ were prepared from the reaction of PhCCP(NEt 2 )( t Bu) ( 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 PN 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 .