Cluster dynamics of heterometallic trinuclear clusters during ligand substitution, redox chemistry, and group transfer processes.

Stepwise metalation of the hexadentate ligand tbsLH6 (tbsLH6 = 1,3,5-C6H9(NHC6H4-o-NHSiMe2tBu)3) affords bimetallic trinuclear clusters (tbsL)Fe2Zn(thf) and (tbsL)Fe2Zn(py). Reactivity studies were pursued to understand metal atom lability as the clusters undergo ligand substitution, redox chemistry, and group transfer processes. Chloride addition to (tbsL)Fe2Zn(thf) resulted in a mixture of species including both all-zinc and all-iron products. Addition of ArN3 (Ar = Ph, 3,5-(CF3)2C6H3) to (tbsL)Fe2Zn(py) yielded a mixture of two trinuclear products: (tbsL)Fe3(µ3-NAr) and (tbsL)Fe2Zn(µ3-NAr)(py). The two imido species were separated via crystallization, and outer sphere reduction of (tbsL)Fe2Zn(µ3-NAr)(py) resulted in the formation of a single product, [2,2,2-crypt(K)][(tbsL)Fe2Zn(µ3-NAr)]. These results provide insight into the relationship between heterometallic cluster structure and substitutional lability and could help inform both future catalyst design and our understanding of metal atom lability in bioinorganic systems.

A [CoSiH2] Silylene Synthon Provides Modular Access to Homo- and Heterobimetallic [Co═Si═M] (M = Co, Fe) Silicide Complexes.

Base-stabilized [BP3iPr](H)2CoSiH2(DMAP) (1, [BP3iPr] = PhB(CH2PiPr2)3-; DMAP = 4-dimethylaminopyridine) is a rare instance of a synthon for the simplest "parent" silylene complex (LM═SiH2). Complex 1 was accessed in high yields via double Si-H bond activation in SiH4 by [BP3iPr]Co(DMAP), and in solution, it undergoes rapid exchange between bound and free DMAP by an associative mechanism (as determined by variable-temperature 1H NMR dynamic studies). The DMAP ligand of 1 is readily displaced by metal-based fragments that bind silicon and cleave the Si-H bonds of the SiH2 moiety to produce bimetallic [Co═Si═M] (M = Co, Fe) molecular silicides. Thus, treatment of 1 with 0.5 equiv of (LCoI)2(µ-N2) (L = a tripodal ligand) resulted in the spontaneous formation of [BP3iPr](H)2Co═Si═Co(H)2L (L = [BP2tBuPz], PhB(CH2PtBu2)2(pyrazolyl)- (3); Tp″, HB(3,5-diisopropylpyrazolyl)3- (4)) with the concomitant release of DMAP. The symmetrical silicide [BP3iPr](H)2Co═Si═Co(H)2[BP3iPr] (5) was prepared by treatment of a mixture of 1 and [BP3iPr]Co(DMAP) with 2 equiv of Ph3B, which in this case is required to sequester DMAP as the elimination product Ph3B-DMAP. A heterobimetallic silicide, [BP3iPr](H)2Co═Si═Fe(H)2[SiP3iPr] (7; [SiP3iPr] = PhSi(CH2PiPr2)3), was obtained via in situ KC8 reduction of [SiP3iPr]FeCl and subsequent addition of 1 and Ph3B. These transformations involving a metal-SiH2 derivative demonstrate a fundamentally new type of reactivity for silylene complexes and provide a unique synthetic method for construction of molecular silicide complexes.

Tetracopper σ-Bound µ-Acetylide and -Diyne Units Stabilized by a Naphthyridine-based Dinucleating Ligand.

Reactions of a dicopper(I) tert-butoxide complex with alkynes possessing boryl or silyl capping groups resulted in formation of unprecedented tetracopper(I) µ-acetylide/diyne complexes that were characterized by NMR and UV/Vis spectroscopy, mass spectrometry and single-crystal X-ray diffraction. These compounds possess an unusual µ4 -η1 :η1 :η1 :η1 coordination mode for the bridging organic fragment, enforced by the rigid and dinucleating nature of the ligand utilized. Thus, the central π system remains unperturbed and accessible for subsequent reactivity and modification. This has been corroborated by addition of a fifth copper atom, giving rise to a pentacopper acetylide complex. This work may provide a new approach by which metal-metal cooperativity can be exploited in the transformation of acetylide and diyne groups to a variety of substrates, or as a starting point for the controlled synthesis of copper(I) alkyne-containing clusters.

Dimetalloylene (M-E-M) Complexes of Heavier Main Group Elements Ge, Sn, Pb, Bi via Cleavage of E-X Bonds (X=N(SiMe3 )2 , OtBu) with an Iridium Hydride.

Reactions of the IrV hydride [Me BDIDipp ]IrH4 {BDI=(Dipp)NC(Me)CH(Me)CN(Dipp); Dipp=2,6-iPr2 C6 H3 } with E[N(SiMe3 )2 ]2 (E=Sn, Pb) afforded the unusual dimeric dimetallotetrylenes ([Me BDIDipp ]IrH)2 (µ2 -E)2 in good yields. Moreover, ([Me BDIDipp ]IrH)2 (µ2 -Ge)2 was formed in situ from thermal decomposition of [Me BDIDipp ]Ir(H)2 Ge[N(SiMe3 )2 ]2 . These reactions are accompanied by liberation of HN(SiMe3 )2 and H2 through the apparent cleavage of an E-N(SiMe3 )2 bond by Ir-H. In a reversal of this process, ([Me BDIDipp ]IrH)2 (µ2 -E)2 reacted with excess H2 to regenerate [Me BDIDipp ]IrH4 . Varying the concentrations of reactants led to formation of the trimeric ([Me BDIDipp ]IrH2 )3 (µ2 -E)3 . The further scope of this synthetic route was investigated with group 15 amides, and ([Me BDIDipp ]IrH)2 (µ2 -Bi)2 was prepared by the reaction of [Me BDIDipp ]IrH4 with Bi(NMe2 )3 or Bi(OtBu)3 to afford the first example of a "naked" two-coordinate Bi atom bound exclusively to transition metals. A viable mechanism that accounts for the formation of these products is proposed. Computational investigations of the Ir2 E2 (E=Sn, Pb) compounds characterized them as open-shell singlets with confined nonbonding lone pairs at the E centers. In contrast, Ir2 Bi2 is characterized as having a closed-shell singlet ground state.

Direct Transformation of SiH4 to a Molecular L(H)2Co═Si═Co(H)2L Silicide Complex.

The synthesis of bimetallic molecular silicide complexes is reported, based on the use of multiple Si-H bond activations in SiH4 at the metal centers of 14-electron LCoI fragments (L = Tp″, HB(3,5-diisopropylpyrazolyl)3-; [BP2tBuPz], PhB(CH2PtBu2)2(pyrazolyl)). Upon exposure of (Tp″Co)2(µ-N2) (1) to SiH4, a mixture of (Tp″Co)2(µ-H) (2) and (Tp″Co)2(µ-H)2 (3) was formed and no evidence for Si-H oxidative addition products was observed. In contrast, [BP2tBuPz]-supported Co complexes led to Si-H oxidative additions with the generation of silylene and silicide complexes as products. Notably, the reaction of ([BP2tBuPz]Co)2(µ-N2) (5) with SiH4 gave the dicobalt silicide complex [BP2tBuPz](H)2Co═Si═Co(H)2[BP2tBuPz] (8) in high yield, representing the first direct route to a symmetrical bimetallic silicide. The effect of the [BP2tBuPz] ligand on Co-Si bonding in 7 and 8 was explored by analysis of solid-state molecular structures and density functional theory (DFT) investigations. Upon exposure to CO or DMAP (DMAP = 4-dimethylaminopyridine), 8 converted to the corresponding [BP2tBuPz]Co(L)x adducts (L = CO, x = 2; L = DMAP, x = 1) with concomitant loss of SiH4, despite the lack of significant Si-H interactions in the starting complex. On heating to 60 °C, 8 underwent reaction with MeCl to produce small quantities of MexSiH4-x (x = 1-3), demonstrating functionalization of the µ-silicon atom in a molecular silicide to form organosilanes.

Expanded [23]-Helicene with Exceptional Chiroptical Properties via an Iterative Ring-Fusion Strategy.

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 (ΔG‡e) have largely precluded experimental interrogation of this prediction. Here, we report the syntheses of expanded helicenes containing 15, 19, and 23 rings on the inner helical circuit, using two iterations of an Ir-catalyzed, site-selective [2 + 2 + 2] reaction. This series of compounds displays a linear relationship between the number of rings and ΔG‡e. The expanded [23]-helicene, which is 7 rings longer than any known single carbohelicene and among the longest known all-carbon ladder oligomers, exhibits a ΔG‡e that is high enough (29.2 ± 0.1 kcal/mol at 100 °C in o-DCB) to halt enantiomerization at ambient temperature. This enabled the isolation of enantiopure samples displaying circular dichroism dissymmetry factors of ±0.056 at 428 nm, which are ≥1.7× larger than values for previously reported classical and expanded helicenes. Computational investigations suggest that this improved performance is the result of both the increased diameter and length of the [23]-helicene, providing guiding design principles for high dissymmetry molecular materials.

A sequential cyclization/π-extension strategy for modular construction of nanographenes enabled by stannole cycloadditions.

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.

Robust dicopper(i) µ-boryl complexes supported by a dinucleating naphthyridine-based ligand.

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.

Copper(III) Metallacyclopentadienes via Zirconocene Transfer and Reductive Elimination to an Isolable Phenanthrocyclobutadiene.

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, Cp2Zr(2,5-Mes2-phenanthro[9,10]C4), enables efficient oxidative transmetalation to the corresponding, formal Cu(III) metallacyclopentadiene dimer. The dimer was quantitatively converted to a structurally analogous anionic monomer [nBu4N]{Cl2Cu(2,5-Mes2-phenanthro[9,10]C4)} upon treatment with [nBu4N][Cl]. Both metallacycles undergo quantitative reductive elimination upon heating to generate phenanthrocyclobutadiene and a Cu(I) species. Due to the steric protection provided by the mesityl groups, this cyclobutadiene was isolated and thoroughly characterized to reveal antiaromaticity comparable to that of free cyclobutadiene, which imbues it with a small highest occupied molecular orbital-lowest unoccupied molecular orbital energy gap of 1.85 eV and accessible reduced and oxidized electronic states.

Versatile Fe-Sn Bonding Interactions in a Metallostannylene System: Multiple Bonding and C-H Bond Activation.

The metallostannylene Cp*(iPr2MeP)(H)2Fe-SnDMP (1; Cp* = η5-C5Me5; DMP = 2,6-dimesitylphenyl), formed by hydrogen migration in a putative Cp*(iPr2MeP)HFe[Sn(H)DMP] intermediate, serves as a robust platform for exploration of transition-metal main-group element bonding and reactivity. Upon one-electron oxidation, 1 expels H2 to generate the coordinatively unsaturated [Cp*(iPr2MeP)Fe═SnDMP][B(C6F5)4] (3), which possesses a highly polarized Fe-Sn multiple bond that involves interaction of the tin lone pair with iron. Evidence from EPR and 57Fe Mössbauer spectroscopy, along with DFT studies, shows that 3 is primarily an iron-based radical with charge localization at tin. Upon reduction of 3, C-H bond activation of the phosphine ligand was observed to produce Cp*HFe(κ2-(P,Sn)═Sn(DMP)CH2CHMePMeiPr) (5). Complex 5 was also accessed via thermolysis of 1, and kinetics studies of this thermolytic pathway indicate that the reductive elimination of H2 from 1 to produce a stannylyne intermediate, Cp*(iPr2MeP)Fe[SnDMP] (A), is likely rate-determining. Evidence indicates that the production of 5 proceeds through a concerted C-H bond activation. DFT investigations suggest that the transition state for this transformation involves C-H cleavage across the Fe-Sn bond and that a related transition state where C-H bond activation occurs exclusively at the tin center is disfavored, illustrating an effect of iron-tin cooperativity in this system.

C-H Activation by RuCo3O4 Oxo Cubanes: Effects of Oxyl Radical Character and Metal-Metal Cooperativity.

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.

Scalable, Divergent Synthesis of a High Aspect Ratio Carbon Nanobelt.

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.

Expanded Helicenes as Synthons for Chiral Macrocyclic Nanocarbons.

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 (<11.9 kcal/mol), which makes this a promising strategy to access configurationally stable expanded helicenes. In contrast, the topologically distinct figure-eight retains the configurational lability of the helicene precursor. Despite its lability in solution, this compound forms homochiral single crystals. Here, the configuration is stabilized by an intricate network of two distinct yet interconnected helical superstructures. The enantiomerization mechanisms for all new compounds were probed using density functional theory, providing insight into the flexibility of the figure-eight and guidance for future synthetic modifications in pursuit of non-racemic macrocycles.

Structure and Bonding in a Diamond-Shaped Tin Cluster Possessing a cyclo-Sn4 Core.

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.

Isolation and Study of Ruthenium-Cobalt Oxo Cubanes Bearing a High-Valent, Terminal RuV-Oxo with Significant Oxyl Radical Character.

High-valent RuV-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 [Co3O4] subcluster via the single-step assembly reaction of CoII(OAc)2(H2O)4 (OAc = acetate), perruthenate (RuO4-), and pyridine (py) yielded an unprecedented Ru(O)Co3(µ3-O)4(OAc)4(py)3 cubane featuring an isolable, yet reactive, RuV-oxo moiety. EPR, ENDOR, and DFT studies reveal a valence-localized [RuV(S = 1/2)CoIII3(S = 0)O4] configuration and non-negligible covalency in the cubane core. Significant oxyl radical character in the RuV-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)4Co3(µ3-O)4Ru]-O-[RuCo3(µ3-O)4(OAc)4(py)3]. Redox activity of the RuV-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 [Co3O4] unit in supporting the RuV-oxo moiety via a strong π-electron donation.

Activations of all Bonds to Silicon (Si-H, Si-C) in a Silane with Extrusion of [CoSiCo] Silicide Cores.

The [BP3 iPr]Co(I) synthon Na(THF)6{[BP3 iPr]CoI} (1, [BP3 iPr] = κ3-PhB(CH2P iPr2)3-) reacts with PhSiH3 or SiH4 to form unusual {[BP2 iPr](SiH2R)CoH2}═Si═{H2Co[BP3 iPr]} species (R = Ph, 2a; R = H, 2b; [BP2 iPr] = κ2-PhB(CH2P iPr2)2) that result from activation of all Si-H and Si-C bonds in the starting silanes. Solution-spectroscopic data (multinuclear NMR, IR) for 2a,b, and the solid-state structure of 2a, indicate substantial Co═Si═Co multiple bonding and minimal interaction of the core Si atom with nearby hydride ligands. In the presence of 4-dimethylaminopyridine (DMAP), 1 reacts with PhSiH3 to give [BP3 iPr](H)2CoSiHPh(DMAP) (3). Complexes 2a,b eliminate RSiH3 upon thermolysis in the presence of DMAP to generate {[BP2 iPr]Co(NC5H3NMe2)}═Si═{H2Co[BP3 iPr]} (4).

Effects of Coordinating a Hemilabile Ligand to 14e Cp*M(NO) Scaffolds (M = Mo, W).

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 mixture of the novel complexes Cp*W(NO)(H)(κ1-P-iPr2PN)Cl (5) and Cp*W(NO)(κ2-P,N-iPr2PCH-2-(3-Me-C5H3N))Cl (6), which are separable by crystallization from pentane and diethyl ether solutions, respectively. The divergent reactivity imparted by the dmpe and iPr2PN proligands is a unique demonstration of the unusual properties of a mixed-donor ligand. In the case of molybdenum, the reaction of [Cp*Mo(NO)Cl2]2 with 2 equiv of iPr2PN in C6H6 first forms Cp*Mo(NO)(κ1-P-iPr2PN)Cl2, which then converts to [Cp*Mo(NO)(κ2-P,N-iPr2PN)Cl][Cp*Mo(NO)Cl3], the analogue of 3. Reduction of the Cp*Mo(NO)(κ1-P-iPr2PN)Cl2 intermediate complex with 2 equiv of Cp2Co affords dark-green Cp*Mo(NO)(κ2-P,N-iPr2PN) (7). All new complexes have been characterized by conventional spectroscopic and analytical methods, and the solid-state molecular structures of most of them have been established by single-crystal X-ray crystallographic analyses.

Cationic and Neutral Cp*M(NO)(κ2-Ph2PCH2CH2PPh2) Complexes of Molybdenum and Tungsten: Lewis-Acid-Induced Intramolecular C-H Activation.

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 adducts, Cp*M(NO→B(C6F5)3)(κ2-Ph2PCH2CH2PPh2) in CH2Cl2 at room temperature. Upon warming to 80 °C, Cp*Mo(NO→B(C6F5)3)(κ2-Ph2PCH2CH2PPh2) converts cleanly to the orthometalated product Cp*Mo(NO→B(C6F5)3)(H)(κ3-(C6H4)PhPCH2CH2PPh2), but Cp*W(NO→B(C6F5)3)(κ2-Ph2PCH2CH2PPh2) generates a mixture of products whose identities remain to be ascertained. Attempts to extend this chemistry to include related Ph2PCH2PPh2 compounds have had only limited success. All new complexes have been characterized by conventional spectroscopic and analytical methods, and the solid-state molecular structures of most of them have been established by single-crystal X-ray crystallographic analyses.

Thermal Chemistry of Cp*W(NO)(CH2CMe3)(H)(L) Complexes (L = Lewis Base).

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 Cp*W(NO)(L) intermediates can effect the intermolecular activation of a substrate R-H to form trans-Cp*W(NO)(R)(H)(L) complexes [e.g., L = P(OMe)3 or P(OCH2)3CMe; R-H = C6H6 or Me4Si] probably via their cis isomers. These latter activations are also accompanied by the formation of some Cp*W(NO)(L)2 disproportionation products. An added complication in the L = P(OMe)3 system is that thermolysis of trans-Cp*W(NO)(CH2CMe3)(H)(P(OMe)3) results in it undergoing an Arbuzov-like rearrangement and being converted mainly into [Cp*W(NO)(Me)(PO(OMe)2)]2, which exists as a mixture of two isomers. All new complexes have been characterized by conventional and spectroscopic methods, and the solid-state molecular structures of most of them have been established by single-crystal X-ray crystallographic analyses.