In this Account, we describe the transition metal-mediated cleavage of C−F and C−H bonds in fluoroaromatic and fluoroheteroaromatic molecules. The simplest reactions of perfluoroarenes result in C−F ...oxida tive addition, but C−H activation competes with C−F activation for partially fluorinated molecules. We first consider the reactivity of the fluoroaromatics toward nickel and platinum complexes, but extend to rhenium and rhodium where they give special insight. Sections on spectroscopy and molecular structure are followed by discussions of energetics and mechanism that incorporate experimental and computational results. We highlight special characteristics of the metal−fluorine bond and the influence of the fluorine substituents on energetics and mechanism. Fluoroaromatics reacting at an ML2 center initially yield η2-arene complexes, followed usually by oxidative addition to generate MF(ArF)(L)2 or MH(ArF)(L)2 (M is Ni, Pd, or Pt; L is trialkylphosphine). The outcome of competition between C−F and C−H bond activation is strongly metal dependent and regioselective. When C−H bonds of fluoroaromatics are activated, there is a preference for the remaining C−F bonds to lie ortho to the metal. An unusual feature of metal−fluorine bonds is their response to replacement of nickel by platinum. The Pt−F bonds are weaker than their nickel counterparts; the opposite is true for M−H bonds. Metal−fluorine bonds are sufficiently polar to form M−F···H−X hydrogen bonds and M−F···I−C6F5 halogen bonds. In the competition between C−F and C−H activation, the thermodynamic product is always the metal fluoride, but marked differences emerge between metals in the energetics of C−H activation. In metal−fluoroaryl bonds, ortho-fluorine substituents generally control regioselectivity and make C−H activation more energetically favorable. The role of fluorine substituents in directing C−H activation is traced to their effect on bond energies. Correlations between M−C and H−C bond energies demonstrate that M−C bond energies increase far more on ortho-fluorine substitution than do H−C bonds. Conventional oxidative addition reactions involve a three-center triangular transition state between the carbon, metal, and X, where X is hydrogen or fluorine, but M(d)−F(2p) repulsion raises the activation energies when X is fluorine. Platinum complexes exhibit an alternative set of reactions involving rearrangement of the phosphine and the fluoroaromatics to a metal(alkyl)(fluorophosphine), M(R)(ArF)(PR3)(PR2F). In these phosphine-assisted C−F activation reactions, the phosphine is no spectator but rather is intimately involved as a fluorine acceptor. Addition of the C−F bond across the M−PR3 bond leads to a metallophosphorane four-center transition state; subsequent transfer of the R group to the metal generates the fluorophosphine product. We find evidence that a phosphine-assisted pathway may even be significant in some apparently simple oxidative addition reactions. While transition metal catalysis has revolutionized hydrocarbon chemistry, its impact on fluorocarbon chemistry has been more limited. Recent developments have changed the outlook as catalytic reactions involving C−F or C−H bond activation of fluorocarbons have emerged. The principles established here have several implications for catalysis, including the regioselectivity of C−H activation and the unfavorable energetics of C−F reductive elimination. Palladium-catalyzed C−H arylation is analyzed to illustrate how ortho-fluorine substituents influence thermodynamics, kinetics, and regioselectivity.
Three-coordinate bipyridyl complexes of gold, (κ2-bipy)Au(η2-C2H4)NTf2, are readily accessed by direct reaction of 2,2′-bipyridine (bipy), or its derivatives, with the homoleptic gold ethylene ...complex Au(C2H4)3NTf2. The cheap and readily available bipyridyl ligands facilitate oxidative addition of aryl iodides to the Au(I) center to give (κ2-bipy)Au(Ar)INTf2, which undergo first aryl-zinc transmetalation and second C–C reductive elimination to produce biaryl products. The products of each distinct step have been characterized. Computational techniques are used to probe the mechanism of the oxidative addition step, offering insight into both the origin of the reversibility of this process and the observation that electron-rich aryl iodides add faster than electron-poor substrates. Thus, for the first time, all steps that are characteristic of a conventional intermolecular Pd(0)-catalyzed biaryl synthesis are demonstrated from a common monometallic Au complex and in the absence of directing groups.
Conspectus Group 14 Zintl anions E x q− (E = Si–Pb, x = 4, 5, 9, 10) are synthetically accessible, and their diverse chemical reactivity makes them valuable synthons in the construction of larger ...nanoclusters with remarkable structures, intriguing patterns of chemical bonding, and tunable physical and chemical properties. A plethora of novel cluster anions have now been isolated from the reactions of polyanionic E x q− precursors with low-valent d-/f-block metal complexes, main-group organometallics, or organics in polar aprotic solvents. The range of products includes intermetalloid clusters with transition metal atom(s) embedded in main-group element cages, organometallic Zintl anions in which E x q− acts as a ligand, intermetallic Zintl anions where E x q− is bridged by ligand-free transition metal atom(s), organo-Zintl anions where E x q− is functionalized with organic-group(s), and oligomers formed through oxidative coupling reactions. The synthesis and characterization of these unconventional complexes, where important contributions to stability come from ionic, covalent, and metal–metal bonds as well as weaker aurophilic and van der Waals interactions, extend the boundaries of coordination chemistry and solid-state chemistry. Substantial progress has been made in this field over the past two decades, but there are still many mysteries to unravel related to the cluster growth mechanism and the controllable synthesis of targeted clusters, along with the remarkable and diverse patterns of chemical bonding that present a substantial challenge to theory. In this Account, we hope to shed some light on the relationship between structure, electronic properties, and cluster growth by highlighting selected examples from our recent work on homoatomic deltahedral E x q− anions, including (1) germanium-based Zintl clusters, such as the supertetrahedral intermetallic clusters M6Ge164– (M = Zn, Cd) and the sandwich cluster {(Ge9)2η6-Ge(PdPPh3)3}4– with a heterometallic Ge@Pd3 interlayer; (2) tin-based intermetalloid clusters M x @Sn y q− and the application of Co@Sn94– in bottom-up synthesis; and (3) lead clusters with precious metal cores, including the largest Zintl anion Au12Pb448–. In addition to their intrinsic appeal from a structural and electronic perspective, these new cluster anions also show promise as precursors for the development of new materials with applications in heterogeneous catalysis, where we have recently reported the selective reduction of CO2.
Abstract
Transition-metal oxyhydrides are of considerable current interest due to the unique features of the hydride anion, most notably the absence of valence
p
orbitals. This feature distinguishes ...hydrides from all other anions, and gives rise to unprecedented properties in this new class of materials. Here we show via a high-pressure study of anion-ordered strontium vanadium oxyhydride SrVO
2
H that H
−
is extraordinarily compressible, and that pressure drives a transition from a Mott insulator to a metal at ~ 50 GPa. Density functional theory suggests that the band gap in the insulating state is reduced by pressure as a result of increased dispersion in the
ab
-plane due to enhanced V
dπ
-O
pπ
-V
dπ
overlap. Remarkably, dispersion along
c
is limited by the orthogonal V
dπ
-H
1s
-V
dπ
arrangement despite the greater
c
-axis compressibility, suggesting that the hydride anions act as π-blockers. The wider family of oxyhydrides may therefore give access to dimensionally reduced structures with novel electronic properties.
The synthesis and characterization of an (arsino)phosphaketene, As(PCO){N(Dipp)(CH2)}2 (Dipp=2,6‐diisopropylphenyl) is reported along with its subsequent reactivity with B(C6F5)3. When reacted in a ...stoichiometric ratio, B(C6F5)3 drove the insertion of the P=C bond of the phosphaketene into one of the As−N bonds of the arsino functionality, affording an acid‐stabilized, seven‐membered, cyclic arsaphosphene. In contrast, when catalytic amounts of B(C6F5)3 were employed, dimeric species, which formed through a formal 2+2 cycloaddition of the cyclic arsaphosphene, were generated. The cyclic arsaphosphene product represents the first example of such a compound in which the two substituents are arranged in a cis‐configuration.
Reagent and promoter: Stoichiometric and catalytic reactions between B(C6F5)3 and an (arsino)phosphaketene, As(PCO){N(Dipp)(CH2)}2 (Dipp=2,6‐diisopropylphenyl) are reported. In a stoichiometric ratio, an acid‐stabilized, cyclic cis‐arsaphosphene was isolated. In contrast, when catalytic amounts of B(C6F5)3 were employed, dimeric species, which form through a formal 2+2 cycloaddition of the cyclic arsaphosphene, were generated.
In the field of molecular electronics, an intimate link between the delocalization of molecular orbitals and their ability to support current flow is often assumed. Delocalization, in turn, is ...generally regarded as being synonymous with structural symmetry, for example, in the lengths of the bonds along a molecular wire. In this work, we use density functional theory in combination with nonequilibrium Green’s functions to show that precisely the opposite is true in the extended metal atom chain Cr3(dpa)4(NCS)2 where the delocalized π framework has previously been proposed to be the dominant conduction pathway. Low-symmetry distortions of the Cr3 core do indeed reduce the effectiveness of these π channels, but this is largely irrelevant to electron transport at low bias simply because they lie far below the Fermi level. Instead, the dominant pathway is through higher-lying orbitals of σ symmetry, which remain essentially unperturbed by even quite substantial distortions. In fact, the conductance is actually increased marginally because the σnb channel is displaced upward toward the Fermi level. These calculations indicate a subtle and counterintuitive relationship between structure and function in these metal chains that has important implications for the interpretation of data emerging from scanning tunnelling and atomic force microscopy experiments.
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