Mesoporous Silica-Supported Amidozirconium-Catalyzed Carbonyl Hydroboration

The hydroboration of aldehydes and ketones using a silica-supported zirconium catalyst is reported. Reaction of Zr­(NMe₂)₄ and mesoporous silica nanoparticles (MSN) provides the catalytic material Zr­(NMe₂)_<i>n</i>@MSN. Exhaustive characterization of Zr­(NMe₂)_<i>n</i>@MSN with solid-state (SS)­NMR and infrared spectroscopy, as well as through reactivity studies, suggests its surface structure is primarily SiOZr­(NMe₂)₃. The presence of these nitrogen-containing zirconium sites is supported by ¹⁵N NMR spectroscopy, including natural abundance ¹⁵N NMR measurements using dynamic nuclear polarization (DNP) SSNMR. The Zr­(NMe₂)_<i>n</i>@MSN material reacts with pinacolborane (HBpin) to provide Me₂NBpin and the material ZrH/Bpin@MSN that is composed of interacting surface-bonded zirconium hydride and surface-bonded borane SiOBpin moieties in an approximately 1:1 ratio, as well as zirconium sites coordinated by dimethylamine. The ZrH/Bpin@MSN is characterized by ¹H/²H and ¹¹B SSNMR and infrared spectroscopy and through its reactivity with D₂. The zirconium hydride material or the zirconium amide precursor Zr­(NMe₂)_<i>n</i>@MSN catalyzes the selective hydroboration of aldehydes and ketones with HBpin in the presence of functional groups that are often reduced under hydroboration conditions or are sensitive to metal hydrides, including olefins, alkynes, nitro groups, halides, and ethers. Remarkably, this catalytic material may be recycled without loss of activity at least eight times, and air-exposed materials are catalytically active. Thus, these supported zirconium centers are robust catalytic sites for carbonyl reduction and that surface-supported, catalytically reactive zirconium hydride may be generated from zirconium-amide or zirconium alkoxide sites