Determination of the N-H Bond Dissociation Free Energy in a Pyridine(diimine)molybdenum Complex Prepared by Proton-Coupled Electron Transfer.

The pyridine(diimine)molybdenum bis(imido) complex (iPrPDI)Mo(═NTol)2 (Tol = 4-methylphenyl) was synthesized by the addition of 2 equiv of 4-methylphenylazide to the corresponding molybdenum benzene derivative, (iPrPDI)Mo(η6-C6H6) [iPrPDI = 2,6-(2,6-iPr2C6H3N═CMe)2C5H3N]. Protonation of (iPrPDI)Mo(═NTol)2 with 2,6-lutinidum triflate yielded a cationic molybdenum amido complex, [(iPrPDI)Mo(NHTol)(═NTol)][OTf], which was further transformed into the neutral molybdenum amido (iPrPDI)Mo(NHTol)(═NTol) by reduction with zinc powder. A series of spectroscopic, synthetic, and pKa determination studies along with electrochemical measurements by the protonation-reduction pathway were used to establish an N-H bond dissociation free energy (BDFE) between 65 and 69 kcal/mol for the molybdenum imido-amido compound, (iPrPDI)Mo(NHTol)(═NTol). Full-molecule density functional theory studies provided a computed value of 61 kcal/mol. By contrast, reduction of (iPrPDI)Mo(═NTol)2 with KC8 afforded the corresponding anionic molybdenum complex K[(iPrPDI)Mo(═NTol)2], which has a potassium cation intercalated with the pyridine and tolyl groups. Protonation of K[(iPrPDI)Mo(═NTol)2] with the weak amidinium acid [TBD(H)][BArF24] (TBD = triazabicyclodecene; BArF24 = B[3,5-(CF3)2C6H3]4) also produced the neutral molybdenum amido complex (iPrPDI)Mo(NHTol)(═NTol). Measurement of the pKa and oxidation potential of K[(iPrPDI)Mo(═NTol)2] provided a range of 69-73 kcal/mol for the N-H BDFE of (iPrPDI)Mo(NHTol)(═NTol), in good agreement with the protonation-reduction route and completing the square scheme. The similar pKa and redox potentials obtained from each pathway demonstrate that both sequences are energetically feasible for proton-coupled electron-transfer (PCET) events. This study on the determination of N-H BDFE of the molybdenum amido complex renders fundamental insight into the N2 reduction cycle by PCET.

Synthesis, Structure, and Hydrogenolysis of Pyridine Dicarbene Iron Dialkyl Complexes.

Two methods for the synthesis of bis(imidazol-2-ylidene)pyridine iron dialkyl complexes, (CNC)Fe(CH2SiMe3)2, have been developed. The first route consists of addition of two equivalents of LiCH2SiMe3 to the iron dihalide complex, (CNC)FeBr2, while the second relies on addition of the free CNC ligand to readily-prepared (py)2Fe(CH2SiMe3)2 (py = pyridine). With aryl-substituted CNC ligands, octahedral complexes of the type ( Ar CNC)Fe(CH2SiMe3)2(N2) ( Ar CNC = bis(arylimidazol-2-ylidene)pyridine) were isolated, where the dinitrogen ligand occupies the site trans to the pyridine of the CNC-chelate. In contrast, the alkyl-substituted variant, (tBuACNC)Fe(CH2SiMe3)2 (tBuACNC = 2,6-(tBu-imidazol-2-ylidene)2pyridine) was isolated as the five-coordinate compound lacking dinitrogen. Exposure of the ( Ar CNC)Fe(CH2SiMe3)2(N2) derivatives to an H2 atmosphere resulted in formation of the corresponding iron hydride complexes ( Ar CNC)FeH4. These compounds catalyzed hydrogen isotope exchange between the deuterated benzene solvent and H2, generating isotopologues and isotopomers of ( Ar CNC)Fe(H n )(D4-n ) (n = 0-4). When (3,5-Me2 MesCNC)Fe(CH2SiMe3)2(N2) (3,5-Me2 MesCNC = 2,6-(2,4,6-Me3-C6H2-imidazol-2-ylidene)2-3,5-Me2-pyridine) was treated successively with H2 and then N2, the corresponding reduced dinitrogen complex (3,5-Me2 MesCNC)Fe(N2)2 was isolated. The same product was also obtained following addition of pinacolborane to (3,5-Me2 MesCNC)Fe(CH2SiMe3)2(N2).

Synthesis and Electronic Structure Diversity of Pyridine(diimine)iron Tetrazene Complexes.

A series of pyridine(diimine)iron tetrazene compounds, (iPrPDI)Fe[(NR)NN(NR)] [iPrPDI = 2,6-(ArN = CMe)2C5H3N; Ar = 2,6-iPr2C6H3] has been prepared either by the addition of 2 equiv of an organic azide, RN3, to the corresponding iron bis(dinitrogen) compound, (iPrPDI)Fe(N2)2 or by the addition of azide to the iron imide derivatives, (iPrPDI)FeNR. The electronic structures of these compounds were determined using a combination of metrical parameters from X-ray diffraction, solution and solid-state magnetic measurements, zero-field 57Fe Mössbauer and 1H NMR spectroscopies, and density functional theory calculations. The overall electronic structure of the iron tetrazene compounds is sensitive to the nature of the tetrazene nitrogen substituent with three distinct classes of compounds identified: (i) overall diamagnetic ( S = 0) compounds arising from intermediate-spin iron(II) centers ( SFe = 1) engaged in antiferromagnetic coupling with both pyridine(diimine) and tetrazene radical anions ( SPDI = -1/2 and Stetrazene = -1/2; R = 2-adamantyl, cyclooctyl, benzyl); (ii) overall S = 1 compounds best described as intermediate-spin iron(III) ( SFe = 3/2) derivatives engaged in antiferromagnetic coupling with a pyridine(diimine) radical anion ( SPDI = -1/2; R = 3,5-Me2C6H3, 4-MeC6H4); (iii) overall S = 2 compounds best described as high-spin iron(III) centers ( SFe = 5/2) engaged in antiferromagnetic coupling to a pyridine(diimine) radical anion ( SPDI = -1/2; R = 1-adamantyl). For both the intermediate- and high-spin ferric cases, the tetrazene ligand adopts the closed-shell, dianionic form, [N4R2]2-. For the case where R = SiMe3, spin-crossover behavior is observed, arising from a spin-state change from intermediate- to high-spin iron(III).

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.

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 ß-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.

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, ((R)PDI)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 = ½ κ(3)-bis(imino)pyridine cobalt alkene and tetrahedral κ(2)-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(sp(3))-C(sp(3)) reductive elimination.

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.

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.

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.

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.

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.

High-Activity Iron Catalysts for the Hydrogenation of Hindered, Unfunctionalized Alkenes.

The activity of aryl-substituted bis(imino)pyridine and bis(arylimidazol-2-ylidene)pyridine iron dinitrogen complexes has been evaluated in a series of catalytic olefin hydrogenation reactions. In general, more electron donating chelates with smaller 2,6-aryl substituents produce more active iron hydrogenation catalysts. Establishment of this structure-activity relationship has produced base metal catalysts that exhibit high turnover frequencies for the hydrogenation of unfunctionalized, tri- and tetrasubstituted alkenes, one of the most challenging substrate classes for homogenous hydrogenation catalysts.

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)](+).