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Expanded helicenes are an emerging class of helical nanocarbons composed of alternating linear and angularly fused rings, which give rise to an internal cavity and a large diameter. The latter is expected to impart exceptional chiroptical properties, but low enantiomerization free energy barriers (Δ

Despite the widespread use of copper catalysis for the formation of C-C bonds, debate about the mechanism persists. Reductive elimination from Cu(III) is often invoked as a key step, yet examples of its direct observation from isolable complexes remain limited to only a few examples. Here, we demonstrate that incorporation of bulky mesityl (Mes) groups into the α-positions of a phenanthrene-appended zirconacyclopentadiene, Cp

High-valent multimetallic-oxo/oxyl species have been implicated as intermediates in oxidative catalysis involving proton-coupled electron transfer (PCET) reactions, but the reactive nature of these oxo species has hindered the development of an in-depth understanding of their mechanisms and multimetallic character. The mechanism of C-H oxidation by previously reported RuCo3O4 cubane complexes bearing a terminal RuV-oxo ligand, with significant oxyl radical character, was investigated. The rate-determining step involves H atom abstraction (HAA) from an organic substrate to generate a Ru-OH species and a carbon-centered radical. Radical intermediates are subsequently trapped by another equivalent of the terminal oxo to afford isolable radical-trapped cubane complexes. Density functional theory (DFT) reveals a barrierless radical combination step that is more favorable than an oxygen-rebound mechanism by 12.3 kcal mol-1. This HAA reactivity to generate organic products is influenced by steric congestion and the C-H bond dissociation energy of the substrate. Tuning the electronic properties of the cubane (i.e., spin density localized on terminal oxo, basicity, and redox potential) by varying the donor ability of ligands at the Co sites modulates C-H activations by the RuV-oxo fragment and enables construction of structure-activity relationships. These results reveal a mechanistic pathway for C-H activation by high-valent metal-oxo species with oxyl radical character and provide insights into cooperative effects of multimetallic centers in tuning PCET reactivity.

Carbon nanobelts are molecules of high fundamental and technological interest due to their structural similarity to carbon nanotubes, of which they are molecular cutouts. Despite this attention, synthetic accessibility is a major obstacle, such that the few known strategies offer limited structural diversity, functionality, and scalability. To address this bottleneck, we have developed a new strategy that utilizes highly fused monomer units constructed via a site-selective [2 + 2 + 2] cycloaddition and a high-yielding zirconocene-mediated macrocyclization to achieve the synthesis of a new carbon nanobelt on large scale with the introduction of functional handles in the penultimate step. This nanobelt represents a diagonal cross section of an armchair carbon nanotube and consequently has a longitudinally extended structure with an aspect ratio of 1.6, the highest of any reported nanobelt. This elongated structure promotes solid-state packing into aligned columns that mimic the parent carbon nanotube and facilitates unprecedented host-guest chemistry with oligo-arylene guests in nonpolar solvents.

The synthesis of polycyclic aromatic hydrocarbons (PAHs) and related nanographenes requires the selective and efficient fusion of multiple aromatic rings. For this purpose, the Diels-Alder cycloaddition has proven especially useful; however, this approach currently faces significant limitations, including the lack of versatile strategies to access annulated dienes, the instability of the most commonly used dienes, and difficulties with aromatization of the [4 + 2] adduct. In this report we address these limitations via the marriage of two powerful cycloaddition strategies. First, a formal Cp2Zr-mediated [2 + 2 + 1] cycloaddition is used to generate a stannole-annulated PAH. Secondly, the stannoles are employed as diene components in a [4 + 2] cycloaddition/aromatization cascade with an aryne, enabling π-extension to afford a larger PAH. This discovery of stannoles as highly reactive - yet stable for handling - diene equivalents, and the development of a modular strategy for their synthesis, should significantly extend the structural scope of PAHs accessible by a [4 + 2] cycloaddition approach.

Expanded helicenes are large, structurally flexible π-frameworks that can be viewed as building blocks for more complex chiral nanocarbons. Here we report a gram-scale synthesis of an alkyne-functionalized expanded [11]helicene and its single-step transformation into two structurally and functionally distinct types of macrocyclic derivatives: (1) a figure-eight dimer via alkyne metathesis (also gram scale) and (2) two arylene-bridged expanded helicenes via Zr-mediated, formal [2+2+n] cycloadditions. The phenylene-bridged helicene displays a substantially higher enantiomerization barrier (22.1 kcal/mol) than its helicene precursor (

A tetrameric cluster composed entirely of (aryl)Sn units, [DMPSn]4 (DMP=2,6-dimesitylphenyl), has been prepared by reduction of [DMPSnCl]2 with a variety of reductants. This cluster was characterized in solution by multinuclear NMR spectroscopies, as well as in the solid-state by single crystal X-ray diffraction analysis. This species is stereochemically nonrigid in solution and possesses a cyclo-Sn4 core whose DMP substituents are equivalent at higher temperatures. The solid-state molecular structure is remarkably unsymmetrical and possesses a nearly planar cyclo-Sn4 core. The DMP substituents are arranged such that three are approximately coplanar, while one is nearly perpendicular to the cyclo-Sn4 core. Density functional theory calculations for a [PhSn]4 model system show that this distorted geometry about the cyclo-Sn4 core maximizes σ-bonding between the Sn centers in a manner reminiscent of trans-bent bonding in the heavy group 14 analogues of alkenes and alkynes.

Copper boryl species have been widely invoked as reactive intermediates in Cu-catalysed C−H borylation reactions, but their isolation and study have been challenging. Use of the robust dinucleating ligand DPFN (2,7-bis(fluoro-di(2-pyridyl)methyl)-1,8-naphthyridine) allowed for the isolation of two very thermally stable dicopper(I) boryl complexes, [(DPFN)Cu2(µ-Bpin)][NTf2] (2) and [(DPFN)Cu2(µ-Bcat)][NTf2] (4) (pin = 2,3-dimethylbutane-2,3-diol; cat = benzene-1,2-diol). These complexes were prepared by cleavage of the corresponding diborane via reaction with the alkoxide [(DPFN)Cu2(µ-OtBu)][NTf2] (3). Reactivity studies illustrated the exceptional stability of these boryl complexes (thermal stability in solution up to 100 °C) and their role in the activation of C(sp)−H bonds. X-ray diffraction and computational studies provide a detailed description of the bonding and electronic structures in these species, and suggest that the dinucleating character of the naphthyridine-based ligand is largely responsible for their remarkable stability.

High-valent Ru(V)–oxo intermediates have long been proposed in catalytic oxidation chemistry, but investigations into their electronic and chemical properties have been limited due to their reactive nature and rarity. The incorporation of Ru into the [Co(3)O(4)] subcluster via the singlestep assembly reaction of Co(II)(OAc)(2)(H(2)O)(4) (OAc = acetate), perruthenate (RuO(4)(−)), and pyridine (py) yielded an unprecedented Ru(O)Co(3)(μ(3)-O)(4)(OAc)(4)(py)(3) cubane featuring an isolable, yet reactive, Ru(V)–oxo moiety. EPR, ENDOR, and DFT studies reveal a valence-localized [Ru(V)(S = 1/2)Co(III)(3)(S = 0)O(4)] configuration and non-negligible covalency in the cubane core. Significant oxyl radical character in the Ru(V)–oxo unit is experimentally demonstrated by radical coupling reactions between the oxo cubane and both 2,4,6-tri-tert-butylphenoxyl and trityl radicals. The oxo cubane oxidizes organic substrates and, notably, reacts with water to form an isolable μ-oxo bis-cubane complex [(py)(3)(OAc)(4)Co(3)(μ(3)-O)(4)Ru]–O–[RuCo(3)(μ(3)-O)(4)(OAc)(4)(py)(3)]. Redox activity of the Ru(V)–oxo fragment is easily tuned by the electron-donating ability of the distal pyridyl ligand set at the Co sites demonstrating strong electronic communication throughout the entire cubane cluster. Natural bond orbital calculations reveal cooperative orbital interactions of the [Co(3)O(4)] unit in supporting the Ru(V)–oxo moiety via a strong π-electron donation.

Reactions of [(dippe)PtMe(Et2O)][BArf4] (4[BArf4], dippe = 1,2-bis(diisopropylphosphino)ethane; Arf = 3,5-(CF3)2C6H3) with Mes2EH2 (Mes = mesityl) liberate methane and produce silylene, germylene, or stannylene products [(dippe)Pt(H)═EMes2][BArf4] (E = Si, 1[BArf4]; Ge, 8; Sn, 9). In contrast, treatment of 4[BArf4] with tertiary silanes HSiR3 (R = Ph, Et, OEt) failed to give the expected cationic silyl complexes but instead produced the bridging hydride [(dippe)Pt(μ-H)]2[BArf4]2 (10) along with the corresponding disilane Si2R6. Complex 10 reacts with primary stibines RSbH2 (R = Mes, dmp) to afford dimeric stibido complexes [(dippe)Pt(μ-SbHR)]2[BArf4]2 (R = Mes, 11; dmp, 12) via Sb–H bond activation.

High-valent multimetallic-oxo/oxyl species have been implicated as intermediates in oxidative catalysis involving proton-coupled electron transfer (PCET) reactions, but the reactive nature of these oxo species has hindered the development of an in-depth understanding of their mechanisms and multimetallic character. The mechanism of C-H oxidation by previously reported RuCo

A novel dihydridoruthenate, [(solv)Na][Cp*(iPr2MeP)RuH2] (2; solv = THF or OEt2), was synthesized from Cp*(iPr2MeP)RuCl (1) and sodium triethylborohydride. Compound 2 was used to generate “Cp*(iPr2MeP)RuH” equivalents by salt metathesis with 1, which resonance Raman spectroscopy indicates is a mixture of the terminal dinitrogen complex, Cp*(iPr2MeP)RuH(N2) (4), and diastereomers of the bridging dinitrogen complex [Cp*(iPr2MeP)RuH]2(μ-N2) (meso, 5, and a pair of enantiomers, 6). Compound 2 also reacted with the late transition metal chloride complexes [(COD)IrCl]2 and (IPr)CuCl [IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene] to form hydride-bridged heterobimetallic complexes Cp*(iPr2MeP)Ru(μ-H)2Ir(COD) (7) and Cp*(iPr2MeP)Ru(μ-H)2Cu(IPr) (8), respectively, that feature weakened Ru–H interactions relative to those of 2.

A tetrameric cluster composed entirely of (aryl)Sn units, [DMPSn]

The [BP3iPr]Co(I) synthon Na(THF)6{[BP3iPr]CoI} (1, [BP3iPr] = κ3-PhB(CH2PiPr2)3–) reacts with PhSiH3 or SiH4 to form unusual {[BP2iPr](SiH2R)CoH2}═Si═{H2Co[BP3iPr]} species (R = Ph, 2a; R = H, 2b;...

This article describes the differing chemical properties imparted by the two ligands, hemilabile 2-[(diisopropylphosphino)methyl]-3-methylpyridine (iPr2PN) and the related 1,2-bis(dimethylphosphino)ethane (dmpe), when attached to the 14e Cp*M(NO) scaffolds (Cp* = η5-C5Me5; M = W, Mo). For instance, the treatment of [Cp*W(NO)Cl2]2 with 2 or 1 equiv of dmpe in C6H6 affords excellent yields of [Cp*W(NO)(κ2-dmpe)Cl]Cl (1) or [Cp*W(NO)Cl2]2[μ-dmpe] (2). In contrast, the treatment of [Cp*W(NO)Cl2]2 with 1 equiv of iPr2PN in C6H6 does not produce the complex analogous to 1 but rather affords orange [Cp*W(NO)(κ2-P-N-iPr2PN)Cl][Cp*W(NO)Cl3] (3) in 90% yield. Furthermore, subsequent reduction of 1 or 2 with 2 or 4 equiv of Cp2Co in tetrahydrofuran (THF), respectively, results in the production of orange Cp*W(NO)(κ2-dmpe) (4) in good yields. However, a similar treatment of 3 with 1 equiv of Cp2Co in THF does not result in the production of Cp*W(NO)(κ2-P,N-iPr2PN), the analogue of 4, but rather generates a 1:1 mixtur...

Treatment of CH

Treatment of CH2Cl2 solutions of Cp*M(NO)Cl2 (Cp* = η5-C5(CH3)5; M = Mo, W) first with 2 equiv of AgSbF6 in the presence of PhCN and then with 1 equiv of Ph2PCH2CH2PPh2 affords the yellow–orange salts [Cp*M(NO)(PhCN)(κ2-Ph2PCH2CH2PPh2)](SbF6)2 in good yields (M = Mo, W). Reduction of [Cp*M(NO)(PhCN)(κ2-Ph2PCH2CH2PPh2)](SbF6)2 with 2 equiv of Cp2Co in C6H6 at 80 °C produces the corresponding 18e neutral compounds, Cp*M(NO)(κ2-Ph2PCH2CH2PPh2) which have been isolated as analytically pure orange–red solids. The addition of 1 equiv of the Lewis acid, Sc(OTf)3, to solutions of Cp*M(NO)(κ2-Ph2PCH2CH2PPh2) at room temperature results in the immediate formation of thermally stable Cp*M(NO→Sc(OTf)3)(H)(κ3-(C6H4)PhPCH2CH2PPh2) complexes in which one of the phenyl substituents of the Ph2PCH2CH2PPh2 ligands has undergone intramolecular orthometalation. In a similar manner, addition of BF3 produces the analogous Cp*M(NO→BF3)(H)(κ3-(C6H4)PhPCH2CH2PPh2) complexes. In contrast, B(C6F5)3 forms the 1:1 Lewis acid–base addu...

The complexes trans-Cp*W(NO)(CH2CMe3)(H)(L) (Cp* = η5-C5Me5) result from the treatment of Cp*W(NO)(CH2CMe3)2 in n-pentane with H2 (∼1 atm) in the presence of a Lewis base, L. The designation of a particular geometrical isomer as cis or trans indicates the relative positions of the alkyl and hydrido ligands in the base of a four-legged piano-stool molecular structure. The thermal behavior of these complexes is markedly dependent on the nature of L. Some of them can be isolated at ambient temperatures [e.g., L = P(OMe)3, P(OPh)3, or P(OCH2)3CMe]. Others undergo reductive elimination of CMe4 via trans to cis isomerization to generate the 16e reactive intermediates Cp*W(NO)(L). These intermediates can intramolecularly activate a C–H bond of L to form 18e cis complexes that may convert to the thermodynamically more stable trans isomers [e.g., Cp*W(NO)(PPh3) initially forms cis-Cp*W(NO)(H)(κ2-PPh2C6H4) that upon being warmed in n-pentane at 80 °C isomerizes to trans-Cp*W(NO)(H)(κ2-PPh2C6H4)]. Alternatively, the...

The complexes trans-Cp*W(NO)(CH

Copper boryl species have been widely invoked as reactive intermediates in Cu-catalysed C-H borylation reactions, but their isolation and study have been challenging. Use of the robust dinucleating ligand DPFN (2,7-bis(fluoro-di(2-pyridyl)methyl)-1,8-naphthyridine) allowed for the isolation of two very thermally stable dicopper(i) boryl complexes, [(DPFN)Cu2(μ-Bpin)][NTf2] (2) and [(DPFN)Cu2(μ-Bcat)][NTf2] (4) (pin = 2,3-dimethylbutane-2,3-diol; cat = benzene-1,2-diol). These complexes were prepared by cleavage of the corresponding diborane via reaction with the alkoxide [(DPFN)Cu2(μ-O t Bu)][NTf2] (3). Reactivity studies illustrated the exceptional stability of these boryl complexes (thermal stability in solution up to 100 °C) and their role in the activation of C(sp)-H bonds. X-ray diffraction and computational studies provide a detailed description of the bonding and electronic structures in these complexes, and suggest that the dinucleating character of the naphthyridine-based ligand is largely responsible for their remarkable stability.