Chemistry of Reduced Monomeric and Dimeric Cobalt Complexes Supported by a PNP Pincer Ligand

The reduction chemistry of cobalt complexes with HPNP (HPNP = HN­(CH₂CH₂P^<i>i</i>Pr₂)₂) as a supporting ligand is described. Reaction of [(HPNP)­CoCl₂] (<b>1</b>) with <i>n</i>-BuLi generated both the deprotonated Co­(II) species [(PNP)­CoCl] (<b>2</b>) along with the Co­(I) complex [(HPNP)­CoCl] (<b>3</b>). Products resulting from reduction of <b>2</b> with KC₈ vary depending upon the atmosphere under which the reduction is performed. Monomeric square planar [(PNP)­CoN₂] (<b>4</b>) is obtained under dinitrogen, whereas dimeric [(PNP)­Co]₂ (<b>5</b>) is formed under argon. Over time, <b>5</b> activates a C–H bond in the PNP ligand to form the species [Co­(H)­(μ-PNP)­(μ-^<i>i</i>Pr₂PCH₂CH₂NCHCH₂P^<i>i</i>Pr₂)­Co] (<b>6</b>). We also observed the oxidative addition of H–Si bond to complex <b>3</b> to form [(HPNP)­CoCl­(H)­SiH₂Ph] (<b>7</b>). ¹H NMR studies showed that species <b>7</b> is in equilibrium with <b>3</b> and silane in solution. Complex <b>3</b> can be oxidized with AgBPh₄ to generate {(HPNP)­CoCl}­BPh₄ (<b>8</b>), a square planar species with a formal electron count of 15 electrons

Chemistry of Reduced Monomeric and Dimeric Cobalt Complexes Supported by a PNP Pincer Ligand

The reduction chemistry of cobalt complexes with HPNP (HPNP = HN­(CH₂CH₂P^<i>i</i>Pr₂)₂) as a supporting ligand is described. Reaction of [(HPNP)­CoCl₂] (<b>1</b>) with <i>n</i>-BuLi generated both the deprotonated Co­(II) species [(PNP)­CoCl] (<b>2</b>) along with the Co­(I) complex [(HPNP)­CoCl] (<b>3</b>). Products resulting from reduction of <b>2</b> with KC₈ vary depending upon the atmosphere under which the reduction is performed. Monomeric square planar [(PNP)­CoN₂] (<b>4</b>) is obtained under dinitrogen, whereas dimeric [(PNP)­Co]₂ (<b>5</b>) is formed under argon. Over time, <b>5</b> activates a C–H bond in the PNP ligand to form the species [Co­(H)­(μ-PNP)­(μ-^<i>i</i>Pr₂PCH₂CH₂NCHCH₂P^<i>i</i>Pr₂)­Co] (<b>6</b>). We also observed the oxidative addition of H–Si bond to complex <b>3</b> to form [(HPNP)­CoCl­(H)­SiH₂Ph] (<b>7</b>). ¹H NMR studies showed that species <b>7</b> is in equilibrium with <b>3</b> and silane in solution. Complex <b>3</b> can be oxidized with AgBPh₄ to generate {(HPNP)­CoCl}­BPh₄ (<b>8</b>), a square planar species with a formal electron count of 15 electrons

Reversible Sigma C–C Bond Formation Between Phenanthroline Ligands Activated by (C₅Me₅)₂Yb

The electronic structure and associated magnetic properties of the 1,10-phenanthroline adducts of Cp*₂Yb are dramatically different from those of the 2,2′-bipyridine adducts. The monomeric phenanthroline adducts are ground state triplets that are based upon trivalent Yb­(III), f¹³, and (phen^•– ) that are only weakly exchange coupled, which is in contrast to the bipyridine adducts whose ground states are multiconfigurational, open-shell singlets in which ytterbium is intermediate valent (J. Am. Chem. Soc 2009, 131, 6480; J. Am. Chem. Soc 2010, 132, 17537). The origin of these different physical properties is traced to the number and symmetry of the LUMO and LUMO+1 of the heterocyclic diimine ligands. The bipy^•– has only one π*₁ orbital of b₁ symmetry of accessible energy, but phen^•– has two π* orbitals of b₁ and a₂ symmetry that are energetically accessible. The carbon p_π-orbitals have different nodal properties and coefficients and their energies, and therefore their populations change depending on the position and number of methyl substitutions on the ring. A chemical ramification of the change in electronic structure is that Cp*₂Yb­(phen) is a dimer when crystallized from toluene solution, but a monomer when sublimed at 180–190 °C. When 3,8-Me₂phenanthroline is used, the adduct Cp*₂Yb­(3,8-Me₂phen) exists in the solution in a dimer–monomer equilibrium in which Δ<i>G</i> is near zero. The adducts with 3-Me, 4-Me, 5-Me, 3,8-Me₂, and 5,6-Me₂-phenanthroline are isolated and characterized by solid state X-ray crystallography, magnetic susceptibility and L_III-edge XANES spectroscopy as a function of temperature and variable-temperature ¹H NMR spectroscopy