Alison R. Fout

Evidence for the Existence of a Terminal Imidoscandium Compound: Intermolecular CH Activation and Complexation Reactions with the Transient ScNAr Species

Terminal imide ligands in early-transition-metal chemistry have recently undergone a dramatic renaissance, given their potential applications in processes such as group transfer and catalysis. Absent from this extensive list are Group 3 transition-metal imides, an antithesis given the inherent affinity of the highly electropositive metal ions for a hard donor such as nitrogen. To date, complexes of Group 3 transition-metal elements (including the lanthanides) with terminal imido ligands have been neither directly detected nor isolated; their existence during the formation of a narrow list of dinuclear or polynuclear bridging imides has only been speculated. The inability to isolate a terminal imide linkage may be due to the discrepancy in energy between the lanthanide and imide-nitrogen orbitals, rendering this type of bond highly polarized and thus prohibiting the formal M=NR or M NR bond that is prototypical among most early transition metals. As a result, such a mismatch in orbital energies should bestow unprecedented nucleophilic behavior to the imido nitrogen atom when coordinated to an ion such as a lanthanide. Herein, we present credible evidence for the existence of a terminal scandium imido complex by applying a combination of isotopic labeling and reactivity studies of a transient Sc=NR complex, evidenced by the intermolecular C H activation of pyridine and benzene as well as complexation with Al(CH3)3. The fact that we can generate transient, reactive titanium alkylidynes of the type [(PNP)Ti CR] (PNP = N[2-P(CHMe2)2-4-methylphenyl]2, R = Ph, SiMe3, and tBu, among other groups) encouraged the search for an isolobal {(PNP)Sc NR} fragment, owing to the comparable atomic radii between titanium(IV) and scandium(III) when weighed against the other Group 3 congeners. Likewise, the PNP ligand type has been recently demonstrated to be an ideal ancillary support in the preparation of an unprecedented bridging phosphinidene moiety on lutetium(III). For us, assembling the PNP ancillary ligand and Sc to form [(PNP)ScCl2] (1) in 95% yield proved straightforward by treatment of Li(PNP) with [ScCl3(thf)3] in toluene at 70 8C over 48 h. Bright yellow 1 can be readily transmetalated with LiNHAr (Ar = 2,6-iPr2C6H3) to afford [(PNP)Sc(NHAr)Cl] (2) in 76% yield (Scheme 1). To incorporate a

Lewis acid stabilized methylidene and oxoscandium complexes.

The methylidene scandium complex (PNP)Sc(mu3-CH2)(mu2-CH3)2[Al(CH3)2]2 (PNP = N[2-P(CHMe2)2-4-methylphenyl]2-) can be prepared from the reaction of (PNP)Sc(CH3)2 and 2 equiv of Al(CH3)3. The Lewis acid stabilized methylidenes candium complex has been crystallographically characterized, and its bonding scheme analyzed by DFT. In addition, we report preliminary reactivity studies of the Sc-CH2 ligand with substrates such as H2NAr and OCPh2. While the former results in an Brønsted acid-base reaction, the latter reagent produces the olefin H2C CPh2 along with the novel oxoscandium complex (PNP)Sc(mu3-O)(mu2-CH3)2[Al(CH3)2]2, quantitatively.

Facile Nitrite Reduction in a Non-heme Iron System: Formation of an Iron(III)-Oxo

Reaction of tetrabutylammonium nitrite with [N(afa(Cy))3Fe(OTf)](OTf) cleanly resulted in the formation of an iron(III)-oxo species, [N(afa(Cy))3Fe(O)](OTf), and NO(g). Formation of NO(g) as a byproduct was confirmed by reaction of the iron(II) starting material with half an equivalent of nitrite, resulting in a mixture of two products, the iron-oxo and an iron-NO species, [N(afa(Cy))3Fe(NO)](OTf)2. Formation of the latter was confirmed through independent synthesis. The results of this study provide insight into the role of hydrogen bonding in the mechanism of nitrite reduction and the binding mode of nitrite in biological heme systems.

Well-Defined Cobalt(I) Dihydrogen Catalyst: Experimental Evidence for a Co(I)/Co(III) Redox Process in Olefin Hydrogenation

The synthesis of a cobalt dihydrogen Co(I)-(H2) complex prepared from a Co(I)-(N2) precursor supported by a monoanionic pincer bis(carbene) ligand, (Mes)CCC ((Mes)CCC = bis(mesityl-benzimidazol-2-ylidene)phenyl), is described. This species is capable of H2/D2 scrambling and hydrogenating alkenes at room temperature. Stoichiometric addition of HCl to the Co(I)-(N2) cleanly affords the Co(III) hydridochloride complex, which, upon the addition of Cp2ZrHCl, evolves hydrogen gas and regenerates the Co(I)-(N2) complex. Furthermore, the catalytic olefin hydrogenation activity of the Co(I) species was studied by using multinuclear and parahydrogen (p-H2) induced polarization (PHIP) transfer NMR studies to elucidate catalytically relevant intermediates, as well as to establish the role of the Co(I)-(H2) in the Co(I)/Co(III) redox cycle.

Alkyne Semihydrogenation with a Well-Defined Nonclassical Co–H2 Catalyst: A H2 Spin on Isomerization and E-Selectivity

The reactivity of a CoI-H2 complex was extended toward the semihydrogenation of internal alkynes. Under ambient temperatures and moderate pressures of H2, a broad scope of alkynes were semihydrogenated using a CoI-N2 precatalyst, resulting in the formation of trans-alkene products. Furthermore, mechanistic studies using 1H, 2H, and para-hydrogen induced polarization (PHIP) transfer NMR spectroscopy revealed cis-hydrogenation of the alkyne occurs first. The Co-mediated alkene isomerization afforded the E-selective products from a broad group of alkynes with good yields and E/Z selectivity.

Accessing Pincer Bis(carbene) Ni(IV) Complexes from Ni(II) via Halogen and Halogen Surrogates

This communication describes the two-electron oxidation of ((DIPP)CCC)NiX ((DIPP)CCC = bis(diisopropylphenyl-benzimidazol-2-ylidene)phenyl); X = Cl or Br) with halogen and halogen surrogates to form ((DIPP)CCC)NiX3. These complexes represent a rare oxidation state of nickel, as well as an unprecedented reaction pathway to access these species through Br2 and halogen surrogate (benzyltrimethylammonium tribromide). The Ni(IV) complexes have been characterized by a suite of spectroscopic techniques and can readily reduce to the Ni(II) counterpart, allowing for cycling between the Ni(II)/Ni(IV) oxidation states.

Isolation of Iron(II) aqua and hydroxyl complexes featuring a tripodal H-bond donor and acceptor ligand

A tripodal ligand platform, tris(5-cycloiminopyrrol-2-ylmethyl)amine (H3[N(pi(Cy))3]), that features a hydrogen bond-accepting secondary coordination sphere when bound anionically to an iron center is reported. Neutral coordination to iron affords ligand tautomerization, resulting in a hydrogen bond-donating secondary coordination sphere, and formation of the tris(5-cyclohexyl-amineazafulvene-2-methyl)amine, H3[N(afa(Cy))3], scaffold. Both binding motifs result in formation of stable, high-spin iron(II) complexes featuring ancillary water, triflate, or hydroxo ligands. Structural analysis reveals that these complexes exhibit distorted trigonal-bipyramidal geometries with coordination of the apical nitrogen to iron as well as three equatorial amine or imine nitrogens, depending on the axial ancillary ligand. Formation of the aqua complex K[(N(pi(Cy))3)Fe(OH2)] (3) illustrated the propensity of the ligand to be hydrogen bond-accepting, whereas the iron triflate species [N(afa(Cy))3Fe](OTf)2 (4) features a hydrogen bond-donating secondary coordination sphere. The ability of each of the three arms of the ligand to tautomerize independently was observed during the formation of the iron-hydroxyl species [N(afa(Cy))2(pi(Cy))]FeOH (5) and characterized by X-ray crystallography and IR spectroscopy. The combined data for the iron complexes established that each arm of the tripodal ligand can tautomerize independently and is likely dependent on the electronic needs of the iron center when binding various substrates.

A bioinspired iron catalyst for nitrate and perchlorate reduction

Biological inspiration for reduction Microorganisms have evolved sophisticated enzymatic machinery to reduce perchlorate and nitrate ions. Although the energetics of the pathways are different, the heme-containing active sites of the corresponding reductase enzymes are remarkably similar. Ford et al. constructed an inorganic catalyst to mediate these reactions based on these active sites, using a nonheme iron complex. A secondary coordination sphere near the iron center aligned the nitrate or perchlorate oxyanions and formed an iron-oxo complex. Regenerating the catalyst in the presence of protons and electrons released water—a potentially much more sustainable process than reduction strategies that require the use of harsh reagents. Science, this issue p. 741 An iron-based biological mimic catalyzes the reduction of nitrate and chlorine oxyanions. Nitrate and perchlorate have considerable use in technology, synthetic materials, and agriculture; as a result, they have become pervasive water pollutants. Industrial strategies to chemically reduce these oxyanions often require the use of harsh conditions, but microorganisms can efficiently reduce them enzymatically. We developed an iron catalyst inspired by the active sites of nitrate reductase and (per)chlorate reductase enzymes. The catalyst features a secondary coordination sphere that aids in oxyanion deoxygenation. Upon reduction of the oxyanions, an iron(III)-oxo is formed, which in the presence of protons and electrons regenerates the catalyst and releases water.

Oxidative atom-transfer to a trimanganese complex to form Mn6(μ6-E) (E = O, N) clusters featuring interstitial oxide and nitride functionalities.

Utilizing a hexadentate ligand platform, a trinuclear manganese complex of the type ((H)L)Mn(3)(thf)(3) was synthesized and characterized ([(H)L](6-) = [MeC(CH(2)N(C(6)H(4)-o-NH))(3)](6-)). The pale-orange, formally divalent trimanganese complex rapidly reacts with O-atom transfer reagents to afford the μ(6)-oxo complex ((H)L)(2)Mn(6)(μ(6)-O)(NCMe)(4), where two trinuclear subunits bind the central O-atom and the ((H)L) ligands cooperatively bind both trinuclear subunits. The trimanganese complex ((H)L)Mn(3)(thf)(3) rapidly consumes inorganic azide ([N(3)]NBu(4)) to afford a dianionic hexanuclear nitride complex [((H)L)(2)Mn(6)(μ(6)-N)](NBu(4))(2), which subsequently can be oxidized with elemental iodine to ((H)L)(2)Mn(6)(μ(6)-N)(NCMe)(4). EPR and alkylation of the interstitial light atom substituent were used to distinguish the nitride from the oxo complex. The oxo and oxidized nitride complexes give rise to well-defined Mn(II) and Mn(III) sites, determined by bond valence summation, while the dianionic nitride shows a more symmetric complex, giving rise to indistinguishable ion oxidation states based on crystal structure bond metrics.

Synthesis of Open-Shell, Bimetallic Mn/Fe Trinuclear Clusters

Concomitant deprotonation and metalation of hexadentate ligand platform (tbs)LH6 ((tbs)LH6 = 1,3,5-C6H9(NHC6H4-o-NHSiMe2(t)Bu)3) with divalent transition metal starting materials Fe2(Mes)4 (Mes = mesityl) or Mn3(Mes)6 in the presence of tetrahydrofuran (THF) resulted in isolation of homotrinuclear complexes ((tbs)L)Fe3(THF) and ((tbs)L)Mn3(THF), respectively. In the absence of coordinating solvent (THF), the deprotonation and metalation exclusively afforded dinuclear complexes of the type ((tbs)LH2)M2 (M = Fe or Mn). The resulting dinuclear species were utilized as synthons to prepare bimetallic trinuclear clusters. Treatment of ((tbs)LH2)Fe2 complex with divalent Mn source (Mn2(N(SiMe3)2)4) afforded the bimetallic complex ((tbs)L)Fe2Mn(THF), which established the ability of hexamine ligand (tbs)LH6 to support mixed metal clusters. The substitutional homogeneity of ((tbs)L)Fe2Mn(THF) was determined by (1)H NMR, (57)Fe Mössbauer, and X-ray fluorescence. Anomalous scattering measurements were critical for the unambiguous assignment of the trinuclear core composition. Heating a solution of ((tbs)LH2)Mn2 with a stoichiometric amount of Fe2(Mes)4 (0.5 mol equiv) affords a mixture of both ((tbs)L)Mn2Fe(THF) and ((tbs)L)Fe2Mn(THF) as a result of the thermodynamic preference for heavier metal substitution within the hexa-anilido ligand framework. These results demonstrate for the first time the assembly of mixed metal cluster synthesis in an unbiased ligand platform.