Probing the Limits of Alkaline Earth-Transition Metal Bonding: An Experimental and Computational Study

摘要

Reduction of Fp_2 (Fp = CpFe(CO)_2) or [Co(CO)_3(PCy_3)]_2 (15) with Mg-mercury amalgam gave [Mg{TM(L)}_2(THF)]_2 (TM(L) = Fp or Co(CO)_3(PCy_3) (19)) in which the TM is bonded to two Mg atoms. Reduction of 15 with Ca~-, Sr~-, Ba~-, Yb~-, Eu~- and Sm-mercury amalgam gave a series of compounds "M{Co(CO)_3(PCy_3)}_2(THF)_n" (M = Ae or Ln) in which the M-Co bonding varies with the charge-to-size ratio of M. For M = Ca or Yb (24), each metal forms one M-Co bond and one M(μ-OC)Co η~1-isocarbonyl linkage. With M = Sr (21) or Eu (25), a switch from M-Co bonding to side-on (η~2) CO ligand coordination is found. Sm~Ⅱ{Co(CO)_3(PCy_3)}_2(THF)_3 disproportionates in pentane to form Sm ~Ⅲ{Co(CO)_3(PCy_3)}_3(THF)_3 containing two Sm~Ⅲ- Co bonds, in contrast with 25, showing the importance of the Ln charge on Ln-TM bonding. Diffusion NMR spectroscopy found that in solution, 21 and 24 are dimeric compounds [M{Co(CO)_3(PCy_3)}_2(THF)_3]_2 that, according to DFT calculations, contain either one (Ae = Ca) or two (Ae = Sr) Ae-Co bonds per Co atom. DFT calculations in combination with Ziegler Rauk energy decomposition and atoms in molecules analysis were used to assess the nature and energy of Ae-Co bonding in a series of model compounds. The Ae-Co interaction energies decrease from Be to Sr, and toward the bottom of the group, side-on (η~2) CO ligand coordination competes with Ae-Co bonding. The PCy_3 ligand plays a pivotal role by increasing solubility in nondonor solvents and the Ae-Co interaction energy.