Samuel A. Johnson

The continuing story of dinitrogen activation

Abstract This review attempts to survey the recent literature concerning the coordination chemistry and the reactivity patterns of metal–dinitrogen complexes. In order that this work may stand alone, a certain amount of background information is included; however, the emphasis is on synthesis and reactivity patterns of recently discovered dinitrogen complexes. In addition, some effort is made to discuss new trends in dinitrogen chemistry as well as to point out underdeveloped topics in this area.

Unexpected intermediates and products in the C-F bond activation of tetrafluorobenzenes with a bis(triethylphosphine)nickel synthon: direct evidence of a rapid and reversible C-H bond activation by Ni(0).

The reaction of (PEt(3))(2)Ni(eta(2)-C(14)H(10)), a source of the reactive Ni(PEt(3))(2) moiety, with 1,2,4,5-F(4)C(6)H(2) yields a mixture of three C-F bond activation products that include the unexpected products (PEt(3))(2)NiF-2,3,5,6-F(4)C(6)H and (PEt(3))(2)NiF-2,3,5-F(3)C(6)H(2). Monitoring the reaction mixture via (19)F and (1)H NMR also reveals the presence of the C-H bond activation product, (PEt(3))(2)NiH-2,3,5,6-F(4)C(6)H which is produced in a rapid equilibrium reaction. This observation provides insight into the steps necessary to modify nickel complexes for selective C-F bond activation in a variety of polyfluorinated aromatic substrates, but also expands the potential of simple nickel compounds for C-H bond activation and functionalization reactions.

Catalytic C−H Bond Stannylation: A New Regioselective Pathway to C−Sn Bonds via C−H Bond Functionalization

The ubiquitous Stille coupling reaction utilizes Sn-C bonds and is of great utility to organic chemists. Unlike the B-C bonds used in the Miyaura-Suzuki coupling reaction, which are readily obtained via direct borylation of C-H bonds, routes to organotin compounds via direct C-H bond functionalization are lacking. Here we report that the nickel-catalyzed reaction of fluorinated arenes and pyridines with vinyl stannanes does not provide the expected vinyl compounds via C-F activation but rather provides new Sn-C bonds via C-H functionalization with the loss of ethylene. This mechanism provides a new unanticipated methodology for the direct conversion of C-H bonds to carbon-heteroatom bonds.

Selective CF Bond Activation of Tetrafluorobenzenes by Nickel(0) with a Nitrogen Donor Analogous to N‐Heterocyclic Carbenes

N, not NHC: A neutral, basic, strong sigma-donor nitrogen ancillary ligand with properties analogous to those of N-heterocyclic carbenes (NHCs) was developed to aid in the oxidative additions of challenging substrates to late transition metals. Selective, room-temperature C-F bond activation was observed with hexa-, penta-, and all three isomers of tetrafluorobenzene using a nickel(0) source in the presence of this donor.

Mechanistic implications of an asymmetric intermediate in catalytic C–C coupling by a dinuclear nickel complex

An asymmetric nickel-nickel bonded intermediate was isolated in the reaction of biphenylene with bis(1,5-cyclooctadiene)nickel and i-Pr(3)P, where three of the four carbons are σ-bonded to one nickel. Mechanistic investigations support reactivity as a formal Ni(III)-Ni(I) complex; reductive elimination of cis-disposed Ni-C bonds from a single nickel centre directly provides a dinuclear Ni(I)-Ni(I) complex, a reaction relevant to dinuclear catalysis.

Solid-state 91Zr NMR spectroscopy studies of zirconocene olefin polymerization catalyst precursors.

(91)Zr (I = 5/2) solid-state NMR (SSNMR) spectra of the zirconocene compounds, Cp(2)ZrCl(2), Cp*(2)ZrCl(2) (1), Cp(2)ZrBr(2) (2), (Me(3)SiC(5)H(4))(2)ZrBr(2) (3), O(Me(2)SiC(5)H(4))(2)ZrBr(2) (4), (1,3-C(5)H(3))(SiMe(2)OSiMe(2))(2)(1,3-C(5)H(3))ZrBr(2) (5), Ind(2)ZrCl(2) (6), Cp(2)ZrMeCl (7), Cp(2)ZrMe(2) (8), and [Cp(2)ZrMe][MeB(C(6)F(5))(3)] (9) have been acquired. Static (91)Zr SSNMR spectra have been acquired for all complexes at magnetic fields of 9.4 and 21.1 T. Cp(2)ZrCl(2) and complexes 1 to 5 possess relatively narrow central transition powder patterns which allows for magic-angle spinning (MAS) (91)Zr solid-state NMR spectra to be acquired at a moderate field strength of 9.4 T. Complexes 6 to 9 possess ultrawideline central transition SSNMR spectra necessitating piece-wise acquisition techniques. From the static and MAS (91)Zr SSNMR spectra, it is possible to measure (91)Zr electric field gradient (EFG) and chemical shift (CS) tensor parameters, as well as the Euler angles which describe their relative orientation. Basis sets and methods for the accurate quantum chemical calculation of (91)Zr EFG and CS tensors have been identified. The origin of the observed EFG and CS tensor parameters are further investigated by visualization of the EFG and CS tensor orientations within the molecular frames. Correlations between the observed and calculated NMR tensor parameters and molecular symmetry and structure are made. All of these observations suggest that (91)Zr SSNMR spectroscopy can be utilized to probe the molecular structure of a variety of homogeneous and heterogeneous olefin polymerization catalysts.

Mesityl Alkyne Substituents for Control of Regiochemistry and Reversibility in Zirconocene Couplings: New Synthetic Strategies for Unsymmetrical Zirconacyclopentadienes and Conjugated Polymers

Reaction of 2 equivs of MesC[triple bond]CPh with Cp(2)Zr(eta(2)-Me(3)SiC[triple bond]CSiMe(3))(pyr) afforded the zirconacyclopentadiene Cp(2)Zr[2,5-Ph(2)-3,4-Mes(2)C(4)]. The regiochemistry of this isomer (betabeta with respect to the mesityl substituents) was determined through single-crystal X-ray analysis and 2D (NOESY, HSQC, HMBC) NMR experiments. This selectivity is attributed largely to a steric-based directing effect of the o-methyl ring substituents since coupling of 1,3-dimethyl-2-(phenylethynyl)benzene with zirconocene gave a single regioisomer (o-xylyl groups in both beta-positions) while coupling of 1,3-dimethyl-5-(phenylethynl)benzene gave a statistical distribution of zirconacyclopentadiene regioisomers. The coupling reaction of 2 equivs of MeC[triple bond]CMes or PrC[triple bond]CMes with Cp(2)Zr(eta(2)-Me(3)SiC[triple bond]CSiMe(3))(pyr) at ambient temperature gave the betabeta regioisomers, Cp(2)Zr[2,5-Me(2)-3,4-Mes(2)C(4)] and Cp(2)Zr[2,5-Pr(2)-3,4-Mes(2)C(4)], respectively, as the major products. Heating solutions of these zirconacycles at 80 degrees C for several hours resulted in an increase in the amount of the unsymmetrical product. For reaction mixtures of PrC[triple bond]CMes and Cp(2)Zr(eta(2)-Me(3)SiC[triple bond]CSiMe(3))(pyr) the major (and apparently thermodynamic) product under these reaction conditions was Cp(2)Zr[2,4-Mes(2)-3,5-Pr(2)C(4)]. The steric strain in the mesityl-substituted zirconacycles allowed for facile substitution reactions of MesC[triple bond]CPh or PrC[triple bond]CMes by less bulky alkynes (i.e., tolan and 3-hexyne) to give the unsymmetrical ziconacyclopentadienes Cp(2)Zr[2,4,5-Ph(3)-3-MesC(4)], Cp(2)Zr[2-Ph-3-Mes-4,5-Et(2)C(4)], and Cp(2)Zr[2-Pr-3-Mes-4,5-Ph(2)C(4)]. Reaction of a mesityl-terminated diyne containing a rigid dihexylfluorenylene spacer with zirconocene afforded poly(p-fluorenylenedienylene) after demetalation with benzoic acid.

Facile Deep and Ultradeep Hydrodesulfurization by the [(iPr3P)Ni]5H6 Cluster Compared to Mononuclear Ni Sources

The pentanuclear nickel cluster [((i)Pr3P)Ni]5H6 facilitates the room-temperature hydrodesulfurization of dibenzothiophene, 4-methyldibenzothiophene, and 4,6-dimethydibenzothiophene. These reactions provide the new tetranuclear nickel hydride sulfide [((i)Pr3P)Ni]4(μ-H)4(μ4-S) (1). In comparison, the dinuclear dinitrogen nickel complex [((i)Pr3P)2Ni]2(μ-N2) undergoes oxidative addition of the C-S bonds of dibenzothiophene and 4-methyl dibenzothiophene to provide the metallacycles Ni3(P(i)Pr3)3C12H8S (2) and Ni3(P(i)Pr3)3C13H10S (3), respectively, but 4,6-dimethydibenzothiophene is unreactive, even with heating to 70 °C for a week. The reaction of [((i)Pr3P)Ni]5H6 with SP(i)Pr3 in toluene provided [((i)Pr3P)Ni]5H6(S) (4), which was observed and characterized by NMR spectroscopy. The addition of vinyltrimethylsilane to 4 provided the best synthetic route to 1, with ((i)Pr3P)Ni(η(2)-CH2═CHSiMe3)2 (5) generated as a byproduct.

A mechanistic investigation of carbon-hydrogen bond stannylation: synthesis and characterization of nickel catalysts.

The complex ((i)Pr(3)P)Ni(η(2)-Bu(3)SnCH=CH(2))(2) (1a) was characterized by NMR spectroscopy and was identified as the active species for catalytic C-H bond stannylation of partially fluorinated aromatics, for example in the reaction between pentafluorobenzene and Bu(3)SnCH=CH(2), which generates C(6)F(5)SnBu(3) and ethylene. The crystalline complex ((i)Pr(3)P)Ni(η(2)-Ph(3)SnCH=CH(2))(2) (1b) provides a more easily handled analogue, and is also capable of catalytic stannylation with added Ph(3)SnCH=CH(2) and C(6)F(5)H. Mechanistic studies on 1b show that the catalytically active species remains mononuclear. The rate of catalytic stannylation is proportional to [C(6)F(5)H] and inversely proportional to [Ph(3)SnCH=CH(2)]. This is consistent with a mechanism where reversible Ph(3)SnCH=CH(2) dissociation provides ((i)Pr(3)P)Ni(η(2)-Ph(3)SnCH=CH(2)), followed by a rate-determining reaction with C(6)F(5)H to generate the stannylation products. Kinetic competition reactions between the fluorinated aromatics pentafluorobenzene, 1,2,4,5-tetrafluorobenzene, 1,2,3,5-tetrafluorobenzene, 1,2,4-trifluorobenzene, 1,3,5-trifluorobenzene and 1,3-difluorobenzene all suggest significant Ni-aryl bond formation in the rate-determining step under catalytic conditions. Labelling studies are consistent with an insertion of the hydrogen of the arene into the vinyl group, followed by β-elimination or β-abstraction of the SnPh(3) moiety.

Chelating amides of lithium. Synthesis, structure and coordination chemistry

Abstract A limited review of the use of mixed-donor ligands that contain amides is presented. By combining amides with phosphines and amines in a chelating array, a variety of tridentate and macrocyclic ligands have been prepared. The synthesis and structures of a variety of lithium derivatives are presented since these are the typical starting materials in the preparation of transition metal, main group and lanthanide complexes that incorporate these ligand systems. It is found that the structures of these lithium derivatives depend on the nature of the other donors present in the chelating array.