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, Mo2Ni(dpa)4(OTf)2 (1), Mo2Ni(dpa)4(NCS)2 (2), and Mo2Ni(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 Mo2(dpa)4 (4) or indirectly, by reaction of Mo2Ni(dpa)4Cl2 (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 Mo2 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 Ni2+ center with S = 1.