Facile Axial Ligand Substitution in Linear Mo≣Mo-Ni Complexes.

Clean axial ligand substitution reactions of heterometallic extended metal atom chains (HEMACs) supported by the dpa ligand (dpa = 2,2'-dipyridylamine) have been synthetically challenging due to side reactions that alter the trimetallic core. Following the hypothesis that a heterometallic core containing second-row transition metals would be more robust toward ligand substitution, we report the synthesis of three new heterotrimetallic compounds, Mo ₂ Ni(dpa) ₄ (OTf) ₂ (1), Mo ₂ Ni(dpa) ₄ (NCS) ₂ (2), and Mo ₂ Ni(dpa) ₄ (NCSe) ₂ (3) that are obtained cleanly and in good yield. Compound 1 may be synthesized either directly by reaction of Ni(OTf) ₂ with Mo ₂ (dpa) ₄ (4) or indirectly, by reaction of Mo ₂ Ni(dpa) ₄ Cl ₂ (5) with 2 equiv of TlOTf. Axial ligand substitution on 1 via solutions containing NaNCS or KNCSe afford 2 or 3, respectively. X-ray crystal structures of 1, 2, and 3 present short Mo-Ni distances of 2.458(8)Å /2.47(1) Å, 2.548(1), and 2.546(1), respectively. Density functional theory (DFT) calculations indicate a 3-center 3-electron σ bonding interaction between the Mo ₂ quadruply bonded core and the Ni in both 1 and 2. These complexes were analyzed by SQUID magnetometry, supporting the presence of a high spin Ni ²⁺ center with S = 1.