Assembling Heterometals through Oxygen: An Efficient Way To Design Homogeneous Catalysts

摘要

Assembling a molecule containing two metal centers with entirely different chemical properties remains a synthetic chal-nlenge. One of the major motivations for this chemistry is its ability to catalyze various organic transformations. Thenproximity between two different metals in a heterometallic complex allows more pronounced chemical communicationnbetween the metals and often leads to the modification of the fundamental properties of the individual metal atoms throughnthe well-known cooperative interaction. Although various types of heterometallic systems are known, the M O M′ frame-nwork is particularly important because it brings the metals into close proximity with each other.nIn this Account, we describe several suitable synthetic routes for the assembly of heterometals of entirely different chem-nical properties through an oxygen atom. The new synthetic strategies for the construction of heterobimetallic complexes takenadvantage of unprecedented syntheses of a number of hydroxide precursors of the type LMR(OH) [L CH{N(Ar)(CMe)}2,nAr 2,6-iPr2C6H3;M Al, Ga, or Ge; R alkyl, aryl, or lone pair of electrons], [LSr(µ-OH)]2 · (THF)3, and Cp*2ZrMe(OH).nWe used the Brønsted acidic character of the proton in the M(O H), Sr(O H), or Zr(O H) moiety, to build a new class ofnheterobimetallic complexes based on M O M′ motif. This synthetic strategy assembles a main group element with anothernmain group element, a transition metal, or a lanthanide metal.nThis synthetic development provides access to a new class of heterobimetallic complexes through oxygen bridging. In manyncases these complexes prove to be excellent candidates for polymerization of monomers including ε-caprolactone, ethylene, andnstyrene. Some of these catalysts bear a chemically grafted methylalumoxane (MAO) unit in the backbone of an active metal cen-nter, which led to efficient ethylene polymerizations at an unusually low MAO concentration. We attribute this reactivity both tonthe presence of a chemically grafted (Me)Al O backbone in the active catalysts (a part of externally added cocatalyst, MAO) andnto the enhanced Lewis acidity from the bridging oxygen at the active metal center.nIn addition, we have demonstrated the development of heterometallic systems having two catalytically active centers. Such struc-ntures could aid in the development of a catalytic system bearing two active centers with different chemistries..