Model Ziegler-Type Hydrogenation Catalyst Precursors, [(1,5-COD)M(μ-O₂C₈H₁₅)]₂ (M = Ir and Rh): Synthesis, Characterization, and Demonstration of Catalytic Activity En Route to Identifying the True Industrial Hydrogenation Catalysts

The compounds [(1,5-COD)M(mu-O2C8H15)]2 (COD = cyclooctadiene, M = Ir (1) or Rh (2), O2C8H15 = 2-ethylhexanoate) were synthesized by addition of Bu3NH(2-ethylhexanoate) or Na(2-ethylhexanoate) to acetone suspensions of [(1,5-COD)Ir(mu-Cl)]2 or [(1,5-COD)Rh(mu-Cl)]2, respectively. The synthesis of such well-defined second and third row model precursors is key to determining the true nature of commercial Ziegler-type hydrogenation catalysts (i.e., catalysts made from the combination of a non-zerovalent, group 8-10 transition metal precatalyst and a trialkylaluminum cocatalyst), an unsolved, approximately 40 year old problem. The characterizations of 1 and 2 were accomplished by elemental analysis, melting point, FAB-MS, FT-IR, UV-vis, NMR spectroscopy, and single crystal X-ray diffraction. The complexes, C32H54Ir2O4 and C32H54O4Rh2, are isostructural: monoclinic, P2(1)/n, Z = 4. The lattice constants for 1 are a = 15.7748(5) A, b = 9.8962(3) A, c = 20.8847(7) A, beta = 108.408(2) degrees. The lattice constants for 2 are a = 15.7608(4) A, b = 9.9032(3) A, c = 20.8259(5) A, beta = 108.527(1) degrees. Complexes 1 and 2 are dimeric, bridged by the 2-ethylhexanoates, and with one 1,5-COD ligand bound to each metal. The formally 16 electron metal atoms are in square ligand planes with dihedral angles between the planes of 56.5 degrees for 1 and 58.1 degrees for 2. The M-M distances of 3.2776(2) and 3.3390(4) A for 1 and 2, respectively, fall in the range of similar structures thought to have some M-M interaction despite the lack of a formal M-M bond. Demonstration that active Ziegler-type hydrogenation catalysts are made when 1 or 2 combine with AlEt3 is provided, results that open the door to the use of 1 and 2 as well-defined third and second row congeners, respectively, of Ziegler-type hydrogenation catalysts. These compounds have proven important in addressing the previously unsolved problem of the true nature of the catalyst in industrial Ziegler-type hydrogenation catalyst systems; their high yield synthesis and unequivocal characterization reported herein are the necessary first steps of that work.