Grant W. Margulieux

Catalytic Hydrogenation Activity and Electronic Structure Determination of Bis(arylimidazol-2-ylidene)pyridine Cobalt Alkyl and Hydride Complexes

The bis(arylimidazol-2-ylidene)pyridine cobalt methyl complex, ((iPr)CNC)CoCH3, was evaluated for the catalytic hydrogenation of alkenes. At 22 °C and 4 atm of H2 pressure, ((iPr)CNC)CoCH3 is an effective precatalyst for the hydrogenation of sterically hindered, unactivated alkenes such as trans-methylstilbene, 1-methyl-1-cyclohexene, and 2,3-dimethyl-2-butene, representing one of the most active cobalt hydrogenation catalysts reported to date. Preparation of the cobalt hydride complex, ((iPr)CNC)CoH, was accomplished by hydrogenation of ((iPr)CNC)CoCH3. Over the course of 3 h at 22 °C, migration of the metal hydride to the 4-position of the pyridine ring yielded (4-H2-(iPr)CNC)CoN2. Similar alkyl migration was observed upon treatment of ((iPr)CNC)CoH with 1,1-diphenylethylene. This reactivity raised the question as to whether this class of chelate is redox-active, engaging in radical chemistry with the cobalt center. A combination of structural, spectroscopic, and computational studies was conducted and provided definitive evidence for bis(arylimidazol-2-ylidene)pyridine radicals in reduced cobalt chemistry. Spin density calculations established that the radicals were localized on the pyridine ring, accounting for the observed reactivity, and suggest that a wide family of pyridine-based pincers may also be redox-active.

Isocyano Analogues of [Co(CO)4]n: A Tetraisocyanide of Cobalt Isolated in Three States of Charge

The encumbering m-terphenyl isocyanide ligand, CNAr(Mes2) (Mes = 2,4,6-Me(3)C(6)H(2)), is used to stabilize homoleptic tetraisocyanide complexes of cobalt in the 1-, 0, and 1+ charge state. Most importantly, these complexes serve as isolable analogues of the binary carbonyl complexes [Co(CO)(4)](-), Co(CO)(4), and [Co(CO)(4)](+). Sodium amalgam reduction of CoCl(2) in the presence of CNAr(Mes2) provides the salt Na[Co(CNAr(Mes2))(4)], which can be oxidized with 1 equiv of ferrocenium triflate (FcOTf) to the neutral complex, Co(CNAr(Mes2))(4). X-ray diffraction, FTIR spectroscopy, and low-temperature EPR spectroscopy reveal that Co(CNAr(Mes2))(4) modulates between D(2d)- and C(2v)-symmetric forms. DFT calculations are used to rationalize this structural modulation in terms of thermal access to low-energy b(2)-symmetric C-Co-C bending modes. Treatment of Na[Co(CNAr(Mes2))(4)] with 2 equiv of FcOTf, followed by addition of Na[BAr(F)(4)], provides the salt [Co(CNAr(Mes2))(4)]BAr(F)(4), which contains a diamagnetic, square planar monovalent cobalt center. The molecular and electronic structures of [Co(CNAr(Mes2))(4)]BAr(F)(4) are compared and contrasted to the reported properties of the carbonyl cation, [Co(CO)(4)](+).

Bis(phosphine)cobalt Dialkyl Complexes for Directed Catalytic Alkene Hydrogenation

Planar, low-spin cobalt(II) dialkyl complexes bearing bidentate phosphine ligands, (P-P)Co(CH2SiMe3)2, are active for the hydrogenation of geminal and 1,2-disubstituted alkenes. Hydrogenation of more hindered internal and endocyclic trisubstituted alkenes was achieved through hydroxyl group activation, an approach that also enables directed hydrogenations to yield contrasteric isomers of cyclic alkanes.

Cobalt-Catalyzed Enantioselective Hydrogenation of Minimally Functionalized Alkenes: Isotopic Labeling Provides Insight into the Origin of Stereoselectivity and Alkene Insertion Preferences

The asymmetric hydrogenation of cyclic alkenes lacking coordinating functionality with a C1-symmetric bis(imino)pyridine cobalt catalyst is described and has been applied to the synthesis of important substructures found in natural products and biologically active compounds. High activities and enantioselectivities were observed with substituted benzo-fused five-, six-, and seven-membered alkenes. The stereochemical outcome was dependent on both the ring size and exo/endo disposition. Deuterium labeling experiments support rapid and reversible 2,1-insertion that is unproductive for generating alkane product but accounts for the unusual isotopic distribution in deuterated alkanes. Analysis of the stereochemical outcome of the hydrogenated products coupled with isotopic labeling, stoichiometric, and kinetic studies established 1,2-alkene insertion as both turnover limiting and enantiodetermining with no evidence for erosion of cobalt alkyl stereochemistry by competing β-hydrogen elimination processes. A stereochemical model accounting for the preferred antipodes of the alkanes is proposed and relies on the subtle influence of the achiral aryl imine substituent on the cobalt catalyst.

Zwitterionic Stabilization of a Reactive Cobalt Tris‐Isocyanide Monoanion by Cation Coordination

As a result of their metal-based nucleophilicity and ability to mediate a wide range of transformations involving maingroup and carbon-based electrophiles, organometallic metallates have received considerable attention. The homoleptic carbonyl metallates [Fe(CO)4] 2 and [Co(CO)4] serve as the prototypical examples of this molecular class and are stabilized in part by strong p-back-bonding interactions between the highly reduced metal centers and the CO ligands. In addition, several metallates containing other pacidic ligands, including olefins, arenes, isocyanides, and PF3, have been reported. h] However, in the vast majority of these cases the reduced metal centers possess coordinative and/or electronic saturation (i.e. 18e configurations), which adds to their stability. Accordingly, the chemistry of metallates featuring coordinatively unsaturated metal centers is significantly underdeveloped, despite the promise of coupling metal-based nucleophilic character with enhanced reactivity toward Lewis basic substrates. In an effort to stabilize organometallic complexes featuring both highly reduced metal centers and coordinative unsaturation, we have surveyed the ligation properties of the encumbering and p-acidic m-terphenyl isocyanide ligands CNAr and CNAr (Ar = 2,6-(2,4,6-Me3C6H2)C6H3); Ar = 2,6-(2,6-(iPr)2C6H3)C6H3). [4] Herein, we report the use of CNAr for the isolation of a zwitterionic complex that functions as a highly reactive source of the coordinatively unsaturated, tris-isocyanide cobalt monoanion [Co(CNAr)3] toward electrophilic reagents. In this respect, [Co(CNAr)3] serves as an isocyano mimic of the unstable tricarbonyl monoanion, [Co(CO)3] , which has been observed exclusively in the gas phase. Furthermore, we detail a unique method for the stabilization of the [Co(CNAr)3] anion that relies on h-arene coordination of the well-known and traditionally non-interacting bis-(triphenylphosphine)iminium cation, [PhP3=N=PPh3] + ([PPN]). Previously we reported the [PPN] salt, [PPN][Co(CNAr)4], [7] as a unique example of a discrete cobalt tetra-isocyanometallate unperturbed by anion–cation interactions.Prolonged stirring (30 h) of this salt in n-hexane solution results in CNAr ligand dissociation and the precipitation of the black, zwitterionic tris-isocyanide complex [(h-PPN)Co(CNAr)3, 1; Figures 1 and 2]. Crystallographic characterization of (h-PPN)Co(CNAr)3 (1)

Cobalt-Catalyzed [2π + 2π] Cycloadditions of Alkenes: Scope, Mechanism, and Elucidation of Electronic Structure of Catalytic Intermediates

Aryl-substituted bis(imino)pyridine cobalt dinitrogen compounds, (RPDI)CoN2, are effective precatalysts for the intramolecular [2π + 2π] cycloaddition of α,ω-dienes to yield the corresponding bicyclo[3.2.0]heptane derivatives. The reactions proceed under mild thermal conditions with unactivated alkenes, tolerating both amine and ether functional groups. The overall second order rate law for the reaction, first order with respect to both the cobalt precatalyst and the substrate, in combination with electron paramagnetic resonance (EPR) spectroscopic studies established the catalyst resting state as dependent on the identity of the precatalyst and diene substrate. Planar S =1/2 κ3-bis(imino)pyridine cobalt alkene and tetrahedral κ2-bis(imino)pyridine cobalt diene complexes were observed by EPR spectroscopy and in the latter case structurally characterized. The hemilabile chelate facilitates conversion of a principally ligand-based singly occupied molecular orbital (SOMO) in the cobalt dinitrogen and alkene compounds to a metal-based SOMO in the diene intermediates, promoting C–C bond-forming oxidative cyclization. Structure–activity relationships on bis(imino)pyridine substitution were also established with 2,4,6-tricyclopentyl-substituted aryl groups, resulting in optimized catalytic [2π + 2π] cycloaddition. The cyclopentyl groups provide a sufficiently open metal coordination sphere that encourages substrate coordination while remaining large enough to promote a challenging, turnover-limiting C(sp3)–C(sp3) reductive elimination.

Synthesis and ligand modification chemistry of a molybdenum dinitrogen complex: redox and chemical activity of a bis(imino)pyridine ligand.

The bis(imino)pyridine 2,6-(2,6-iPr2-C6H3N=CPh)2-C5H3N ((iPr)BPDI) molybdenum dinitrogen complex, [{((iPr)BPDI)Mo(N2)}2(μ2,η(1),η(1)-N2)] has been prepared and contains both weakly (terminal) and modestly (bridging) activated N2 ligands. Addition of ammonia resulted in sequential N-H bond activations, thus forming bridging parent imido (μ-NH) ligands with concomitant reduction of one of the imines of the supporting chelate. Using primary and secondary amines, model intermediates have been isolated that highlight the role of metal-ligand cooperativity in NH3 oxidation.

Synthesis and Protonation of an Encumbered Iron Tetraisocyanide Dianion

Reported here are synthetic studies probing highly reduced iron centers in an encumbering tetraisocyano ligand environment. Treatment of FeCl2 with sodium amalgam in the presence of 2 equiv of the m-terphenyl isocyanide CNAr(Mes2) (Ar(Mes2) = 2,6-(2,4,6-Me3C6H2)2C6H3) produces the disodium tetraisocyanoferrate Na2[Fe(CNAr(Mes2))4]. Structural characterization of Na2[Fe(CNAr(Mes2))4] revealed a tight ion pair, with the Fe center adopting a tetrahedral coordination geometry consistent with a d(10) metal center. Attempts to disrupt the cation-anion contacts in Na2[Fe(CNAr(Mes2))4] with cation-sequestration reagents lead to decomposition, except for the case of 18-crown-6, where a mononuclear complex featuring a dianionic 1-azabenz[b]azulene ligand was isolated in low yield. Formation of this 1-azabenz[b]azulene is rationalized to proceed by an aza-Büchner ring expansion of a CNAr(Mes2) ligand mediated by a coordinatively unsaturated Fe center. Disodium tetraisocyanoferrate Na2[Fe(CNAr(Mes2))4] is readily protonated by trimethylsilanol (HOSiMe3) to produce the monohydride ferrate salt, Na[HFe(CNAr(Mes2))4], the anionic portion of which serves as an isocyano analogue of the hydrido-tetracarbonyl metalate [HFe(CO)4](-). Treatment of Na[HFe(CNAr(Mes2))4] with methyl triflate (MeOTf; OTf = [O3SCF3](-)) at low temperature in the presence of dinitrogen yields the five-coordinate Fe(0) complex Fe(N2)(CNAr(Mes2))4. The formation of Fe(N2)(CNAr(Mes2))4 in this reaction is consistent with the intermediacy of the neutral tetraisocyanide Fe(CNAr(Mes2))4. The decomposition of Fe(N2)(CNAr(Mes2))4 to the dimeric complex [Fe(η(6)-(Mes)-μ(2)-C-CNAr(Mes))]2 and a seven-membered cyclic imine derived from a CNAr(Mes2) ligand is presented and provides insight into the ability of CNAr(Mes2) and related m-terphenyl isocyanides to stabilize zerovalent four-coordinate iron complexes in a strongly π-acidic ligand field.

Ammonia Activation, H2 Evolution and Nitride Formation from a Molybdenum Complex with a Chemically and Redox Noninnocent Ligand

Treatment of the bis(imino)pyridine molybdenum η6-benzene complex (iPrPDI)Mo(η6-C6H6) (iPrPDI, 2,6-(2,6-iPr2C6H3N═CMe)2C5H3N) with NH3 resulted in coordination induced haptotropic rearrangement of the arene to form (iPrPDI)Mo(NH3)2(η2-C6H6). Analogous η2-ethylene and η2-cyclohexene complexes were also synthesized, and the latter was crystallographically characterized. All three compounds undergo loss of the η2-coordinated ligand followed by N-H bond activation, bis(imino)pyridine modification, and H2 loss. A dual ammonia activation approach has been discovered whereby reversible M-L cooperativity and coordination induced bond weakening likely contribute to dihydrogen formation. Significantly, the weakened N-H bonds in (iPrPDI)Mo(NH3)2(η2-C2H4) enabled hydrogen atom abstraction and synthesis of a terminal nitride from coordinated ammonia, a key step in NH3 oxidation.

Photochemically Induced Reductive Elimination as a Route to a Zirconocene Complex with a Strongly Activated N2 Ligand

The zirconocene dinitrogen complex [{(η(5)-C5Me4H)2Zr}2(μ2,η(2),η(2)-N2)] was synthesized by photochemical reductive elimination from the corresponding zirconium bis(aryl) or aryl hydride complexes, providing a high-yielding, alkali metal-free route to strongly activated early-metal N2 complexes. Mechanistic studies support the intermediacy of zirconocene arene complexes that in the absence of sufficient dinitrogen promote C-H activation or undergo comproportion to formally Zr(III) complexes. When N2 is in excess arene displacement gives rise to strong dinitrogen activation.