Mechanistic insight into cooperative catalysis with pentanuclear nickel clusters: catalytic alkene dimerization and...

This work gives structural insights into potential intermediates relevant to the mechanism of stereoselective catalysis by pentanuclear Ni complexes, such as the previously reported stereoselective dimerization of norbornene to the C₂ symmetric (Z) anti-(bis-2,2'-norbornylidene) by [(ⁱPr₃P)Ni]₅H₆ (1). Attempts to generate a polymer by reaction of norbornadiene with catalytic 1 instead selectivity gave the exo-trans-exo 2 + 2 cyclodimerized product, 1,4,4a,4b,5,8,8a,8b-octahydro-1,4:5,8-dimethanobiphenylene. The reaction of cyclopentene with catalytic [(ⁱPr₃P)Ni]₅H₆ (1) gave a mixture of 1,1'-bi(cyclopentylidene) and 1-cyclopentylcyclopentene as organic products. Catalysis terminated when 1 was fully converted to the twisted trapezoidal pentanuclear cluster (ⁱPr₃P)₄Ni₅(C₁₀H₁₃)H₅ (2). Complex 2 reacted with H₂ to give back 1. Attempts to functionalize the organic fragment in 2 with diphenylsilane instead gave (ⁱPr₃P)₄Ni₅(SiPh₂)(SiPh₂H)H₅ (3), which retains the twisted trapezoidal geometry. Complex 3 was also prepared by direct reaction of Ph₂SiH₂ with 1. Triethylsilane reacts with 1 to give a new distorted pentagonal cluster [(ⁱPr₃P)Ni]₅(μ₅-SiEt)H₇ (4) from Si-H and multiple Si-C bond cleavages. The new structures demonstrate the remarkable flexibility in the geometry of the persistent Ni₅ core and provide insight into the structures of intermediates in the reactions with [(ⁱPr₃P)Ni]₅H₆.