C(spⁿ )-X (n=1-3) Bond Activation by Palladium

We have studied the palladium-mediated activation of C(spⁿ )-X bonds (n = 1-3 and X = H, CH₃ , Cl) in archetypal model substrates H₃ C-CH₂ -X, H₂ C=CH-X and HC≡C-X by catalysts PdL_n with L_n = no ligand, Cl^- , and (PH₃ )₂ , using relativistic density functional theory at ZORA-BLYP/TZ2P. The oxidative addition barrier decreases along this series, even though the strength of the bonds increases going from C(sp³ )-X, to C(sp² )-X, to C(sp)-X. Activation strain and matching energy decomposition analyses reveal that the decreased oxidative addition barrier going from sp³ , to sp² , to sp, originates from a reduction in the destabilizing steric (Pauli) repulsion between catalyst and substrate. This is the direct consequence of the decreasing coordination number of the carbon atom in C(spⁿ )-X, which goes from four, to three, to two along this series. The associated net stabilization of the catalyst-substrate interaction dominates the trend in strain energy which indeed becomes more destabilizing along this same series as the bond becomes stronger from C(sp³ )-X to C(sp)-X.