Trinuclear Zirconium Polyhydride ({Cp*Zr(BH₃CH₃)}­(μ-H)₂­{Cp*Zr(BH₃CH₃)}­(μ-H)­{Cp*Zr(BH₃CH₃)})­(μ‑κ²_C,H:κ¹_C:κ²_C,H-CHBH₃) and Its Derivatives: Compounds Containing a Pentacoordinated Carbon Atom

The reaction of Cp*Zr­(BH₃CH₃)₃ with an excess amount of trimethylamine in a toluene solution yields the hypercarbon-containing complex ({Cp*Zr­(BH₃CH₃)}­(μ-H)₂­{Cp*Zr­(BH₃CH₃)}­(μ-H)­{Cp*Zr­(BH₃CH₃)})­(μ-κ²_C,H:κ¹_C:κ²_C,H-CHBH₃), <b>1</b>. To our knowledge, this is the first example in which a hypercoordinated carbon-containing complex was prepared from the reaction of a hydroborate complex with a Lewis base. The reaction of <b>1</b>, NaH, and [N­(CH₃)₄]Cl produces the anionic product [N­(CH₃)₄]­[({Cp*ZrCl}­{(μ-H)₂­{Cp*Zr­(BH₃CH₃)}}₂)­(μ-κ²_C,H:κ¹_C:κ²_C,H-CHBH₃)], <b>2</b>, whereas the reaction of <b>1</b> with B­(C₆F₅)₃ produces the hydride abstraction cationic product [({Cp*Zr­(BH₃CH₃)}­{(μ-H)­{Cp*Zr­(BH₃CH₃)}}₂)­(μ-κ²_C,H:κ¹_C:κ²_C,H-CHBH₃)]­[HB­(C₆F₅)₃], <b>3</b>. The further reaction of <b>3</b> with [N­(CH₃)₄]Cl and NaOH produces the neutral complex ({Cp*Zr­(BH₃CH₃)}­((μ-H)­{Cp*Zr­(BH₃CH₃)})₂)­(μ-κ²_C,H:κ¹_C:κ²_C,H-CHBH₃)­(μ₃-X) (X = Cl (<b>4</b>), OH (<b>5</b>)). Single-crystal X-ray structures of <b>1</b>, <b>2</b>, <b>3</b>, <b>4</b>, and <b>5</b> reveal a pentacoordinated carbon atom, which coordinates to a boron atom, a hydrogen atom, and three Zr atoms in each complex. The geometry around the hypercarbon in each complex can be best described as either distorted trigonal bipyramidal or distorted square pyramidal. The μ₃-bridging Cl^– ligand in <b>4</b> and OH^– ligand in <b>5</b> bond to three Zr atoms on the opposite side of the hypercarbon. These hypercarbon-containing complexes were further characterized by elemental analysis, infrared spectroscopy, and NMR spectroscopy. Formations of <b>2</b>–<b>5</b> confirm the robust framework of the hypercarbon when undergoing reactions