Synthetic access to half-sandwich manganese C₄ cumulenic complexes

The vinylalkynyl complexes Mn(C₅H₄R′) (R″₂PCH₂CH₂PR″₂)(=C= CSnPh₃C≡CSnPh₃) (5, 6) were obtained in good yields by treatment of Mn(C₅H₄R′)(η⁶- cycloheptatriene) (R′ = H (1a), Me (1b)) with 1 equiv of Ph ₃SnC≡CC≡CSnPh₃ and R″₂PCH₂CH₂PR″₂ (R″ = CH₃ (dmpe), C₂H₅ (depe)). The theoretically characterized (DFT) C₄ cumulenic species Mn(C₅H ₄R′)(R″₂PCH₂CH ₂PR″₂){=C=C=C=C(SnPh₃)₂} (7, 8) were obtained by photolysis of the tinsubstituted vinylalkynyl complexes 5 and 6. Subsequently 7 and 8 could be converted into the parent cumulenic species Mn(C₅H₄R′)(R″₂PCH ₂CH₂PR″₂){=C=C=C=C(H)₂} (9, 10) by reacting complexes 7 and 8 with TBAF (5% H₂O) at -40 °C. The difference in the thermodynamic stabilities between the complexes 7 and 8 and complexes 9 and 10 was traced by DFT calculations using Mn(C ₅H₅)(dHpe){=C=C=C=C(SnMe₃)₂} (4-H) and Mn(C₅H₅)(dHpe){=C=C=C=CH₂} (9-H) as model complexes. Treatment of the vinylalkynyl complex with an excess of MeOH led to the formation of a dinuclear complex with a C₈ chain between two manganese centers. The cumulenic complexes and the vinylalkynyl complexes were characterized by NMR and vibrational spectroscopy and elemental analyses. An X-ray diffraction study has been performed on complex 4b.