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Transition-metal arene complexes are important species in catalysis and arene functionalization. Certain electron-rich metal fragments are capable of binding arenes across two adjacent carbon atoms, inducing significant dearomatization through strong back-bonding interactions. Such complexes generally adopt low-spin, closed-shell configurations with electronic and coordinative saturation. Herein, we report an asymmetric tris­(pyrazolyl)­hydroborate (Tp) ligand and its Fe­(I) fragment, which forms η2 complexes with a range of arenes and heteroarenes, including benzene, trifluoromethylbenzene, naphthalene, anthracene, and furan. X-band EPR and solution magnetometry definitively establish these complexes as high spin (S = 3/2), owing to the relatively weak ligand field provided by Tp. These compounds expand a small but growing family of complexes that feature strongly back-bonding metal fragments in high-spin configurations.

The exceptionally π-basic metal fragments {MoTp-(NO)(DMAP)} and {WTp(NO)(PMe(3))} (Tp = tris(pyrazolyl)borate; DMAP = 4-(N,N-dimethylamino)pyridine) form thermally stable η(2)-coordinated complexes with a variety of electron-deficient arenes. The tolerance of substituted arenes with fluorine-containing electron withdrawing groups (EWG; −F, −CF(3), −SF(5)) is examined for both the molybdenum and tungsten systems. When the EWG contains a π bond (nitriles, aldehydes, ketones, ester), η(2) coordination occurs predominantly on the nonaromatic functional group. However, complexation of the tungsten complex with trimethyl orthobenzoate (PhC(OMe)(3)) followed by hydrolysis allows access to an η(2)-coordinated arene with an ester substituent. In general, the tungsten system tolerates sulfur-based withdrawing groups well (e.g., PhSO(2)Ph, MeSO(2)Ph), and the integration of multiple electron-withdrawing groups on a benzene ring further enhances the π-back-bonding interaction between the metal and aromatic ligand. While the molybdenum system did not form stable η(2)-arene complexes with the sulfones or ortho esters, it was capable of forming rare examples of stable η(2)-coordinated arene complexes with a range of fluorinated benzenes (e.g., fluorobenzene, difluorobenzenes). In contrast to what has been observed for the tungsten system, these complexes formed without interference of C–H or C–F insertion.

A method for the resolution of η(2)-alkene-complex isomers of the type MoTp(NO)(DMAP)(η(2)-alkene) and WTp(NO)(PMe(3))(η(2)-alkene) (where Tp = hydridotris(pyrazolyl)-borate and DMAP = 4-(dimethylamino)pyridine) has been explored. Alkene and polyene compounds form as a mixture of kinetically trapped isomers. For both types of complexes, it was found that addition of either a fluorinated alcohol or one-electron oxidant reduces the number of isomers in solution. Accelerated ligand exchange was also observed, although these reactions were accompanied by significant decomposition.

A second-row transition metal complex {MoTp(NO)(DMAP)} (DMAP = 4-(dimethylamino)pyridine; Tp = tris(pyrazolyl)borate) is shown to form dihapto-coordinate complexes with a range of substituted pyrid...

The complex MoTp(NO)(DMAP)(η2-naphthalene) (1; DMAP = 4-(dimethylamino)pyridine; Tp = tris(pyrazolyl)borate) is demonstrated to undergo Michael–Michael ring-closure (MIMIRC) reactions promoted by t...

The preparation and properties of the complex (RMo,R)-MoTp(NO)(DMAP)(η2-α-pinene) are described (∼10 g scale; DMAP = 4-(dimethylamino)pyridine; Tp = hydridotris(pyrazolyl)borate). This complex undergoes exchange of the pinene with a wide range of other π ligands including acetone, ethyl acetate, N,N-dimethylformamide, acetonitrile, and naphthalene. Treatment of the α-pinene complex with iodine results in the complex (S)-MoTp(NO)(DMAP)(I), which is recovered in enantioenriched form (er = 99:1; yield >90%; scale 4.6 g). Reduction of this molybdenum(I) precursor results in enantioenriched molybdenum(0) complexes, including (R)-MoTp(NO)(DMAP)(η2-trifluorotoluene). Sequential treatment of this arene complex with acid, a masked enolate, and iodine regenerates MoTp(NO)(DMAP)(I) along with an alkylated 1-(trifluoromethyl)cyclohexa-1,3-diene with an er value as high as 99:1. This process demonstrates the efficient transfer of asymmetry from α-pinene to the diene product. Accompanying studies with (1R)-myrtenal rev...

The dearomatization of naphthalene and anthracene is explored by their η2 coordination to {TpMo(NO)(MeIm)} and {TpMo(NO)(DMAP)} (where Tp = hydridotris(pyrazolyl)borate, MeIm = 1-methylimidazole, and DMAP = 4-(dimethylamino)pyridine). The DMAP and MeIm complexes have nearly identical redox properties and abilities to bind these polycyclic aromatic hydrocarbons (PAHs), but unlike MeIm, the DMAP ligand can be protonated at N while remaining bound to the metal. This action enhances the π-acidic properties of DMAP, resulting in greater stability of the molybdenum toward oxidation by acid. Utilizing this feature of the DMAP ligand, several new 1,2-dihydronaphthalenes and 1,2-dihydroanthracenes were prepared. Furthermore, it was found that acetals and Michael acceptors could function as electrophiles for the PAHs using the DMAP system, resulting in several new mono- and 1,4-dialkylated products.

The transiently stable indoline complex TpW(NO)(PMe3)(η2-N-ethyl-6,7-indoline) is shown to be a synthon for a broad range of novel hexahydroindoles. The tungsten dearomatizes the indoline, allowing for its sequential protonation, electrophilic addition (e.g., protonation, fluorination, hydroxylation, acylation), nucleophilic addition (e.g., amination, arylation, etheration, alkylation), and reduction, all with well-defined and predictable stereochemistry. The tungsten stereocenter directly governs the stereoselective formation of four adjacent asymmetric carbons originating from the aromatic ring of the indoline.

Polycyclic aromatic hydrocarbons (PAHs; e.g., naphthalene and anthracene) form stable η2-bound complexes with the dearomatizing fragment {TpMo(NO)(MeIm)} (where Tp = hydridotris(pyrazolyl)borate; MeIm = 1-methylimidazole). These complexes undergo protonation at the α carbon followed by regioselective nucleophilic addition at the adjacent β carbon. The nucleophile (a pyrrole or an enolate) adds stereoselectively, anti to the face of metal coordination. The resulting 1,2-dihydroarene ligand may be isolated via metal oxidation by iodine to provide the free 1,2-dihydroarene in moderate yield (∼60%), as well as TpMo(NO)(MeIm)(I), the precursor of the original PAH complex (∼80–90%). Thus, a formal catalytic cycle for the dearomatization of naphthalene and anthracene has been demonstrated.

Indoline and tetrahydroquinoline derivatives form η2-bound complexes with the dearomatization agent {TpW(NO)(PMe3)} that can be isolated as triflic acid salts. Protonation occurs selectively in the aromatic ring, either ortho or para to the nitrogen. With the tetrahydroquinoline complexes, para protonation dominates. For the indoline ligands, alkylation of the nitrogen dramatically affects the ortho/para isomer ratio present after protonation. In the case of the N-ethylindoline and N-isopropylindoline ligands, the bridgehead-protonated isomer (ortho) is formed with >10/1 selectivity. This indolinium isomer is found to undergo acid-catalyzed hydroarylation or hydroamination with various heterocycles. Oxidative decomplexation is demonstrated for the pyrazole and 2-methylfuran derivatives using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). The resulting tetrahydroindolines feature three new stereocenters determined by the configuration of the tungsten complex.

2-(Dimethylamino)pyridine (2-DMAP) and 2-(dimethylamino)pyrimidine derivatives form η2-bound complexes with the dearomatization agent {TpW(NO)(PMe3)} that are each capable of undergoing a double protonation. In the case of 2-DMAP, the resulting π-allyl species reacts with the α-carbon of thiophene or 2-methylfuran, thereby coupling the heterocyclic rings. In the case of the thiophene-derived product, subsequent oxidative decomplexation using ceric ammonium nitrate affords a novel organic amidine derivative. Examples of tungsten-promoted acetylation and fluorination of the aminopyridine ring are also described.

The complexes TpW(NO)(PMe3)(L), where L = 2H-phenol, 2H-p-cresol, 2H-5,6,7,8-tetrahydro-2-naphthol, 2H-N,N-dimethylanilinium were cyclopropanated using Simmons–Smith conditions. Cyclopropanated derivatives of 2H-N,N-dimethylanilinium were selectively ring-opened with HOTf/MeCN to form allylic species, which could be coupled with various nucleophiles. The nucleophilic addition occurs anti to the metal fragment, as determined by X-ray crystallography. Moreover, the cyclopropane ring opening occurs regioselectively, owing to the stabilization of the allylic cation by the metal fragment. The resulting ligands can, in some cases, be removed from the metal by oxidative decomplexation using ceric ammonium nitrate (CAN).

Naphthalene, when bound to the complex {TpW(NO)(PMe3)}, is shown to be activated toward Diels–Alder cycloaddition in the bound ring with N-methylmaleimide (NMM). In its most stable form, the tungsten in this complex binds C1 and C2. However, the cyclization reaction is thought to proceed through an intermediate in which the metal binds C2 and C3, rendering the uncoordinated portion of the hydrocarbon similar to a quinodimethane. The reaction rates and regio- and stereochemistry are investigated for a series of substituted naphthalene ligands, and these observations support the proposed mechanism. An X-ray structure of the complex TpW(NO)(PMe3)(η2-L), where L is the cycloadduct of NMM and 1,4-dimethylnaphthalene, confirms the endo stereochemistry of the cycloaddition.

Several π-allyl complexes of {TpW(NO)(PMe3)} are investigated as possible sources of η2-diene complexes. In order to prepare the diene complex as a single diastereomer, the allyl complex must undergo deprotonation stereoselectively. Allyl complexes of this tungsten system are highly distorted, with the difference between the W−C bond lengths for the two allyl termini being as much as 0.69 A. DFT calculations and several crystal structures are presented that collectively suggest that one terminus, C1, distal to the PMe3 group, tends toward an sp2 carbocation. Consistent with this interpretation, deprotonation preferentially occurs at a carbon adjacent to this allyl terminus for six-membered rings. However, in the presence of base the five-membered cyclic analogue [TpW(NO)(PMe3)(C5H7)]+ fails to form either isomer of the corresponding η2-diene complex.

Phenols, when coordinated to tungsten, undergo conversion to their 2H tautomers. In this form, these phenols undergo a variety of reactions with electrophiles including cyclopropanation, epoxidation, and electrophilic addition to the meta carbon, eventually leading to highly functionalized cyclohexenones.

Nucleophiles capable of four-electron donation (e.g., anilines, enolates) add to C2 of the N-acetylpyridinium (NAPy) ligand of TpW(NO)(PMe3)(NAPy)(OTf) and induce a Zincke−Konig-like ring scission to generate 3,4-coordinated acetamidotrienes or 1-azatrienes with 3Z,5E stereochemistry. This ring opening is driven by the enhanced π-acidity of the cyanine, relative to its purported dihydropyridine predecessor, which stabilizes the π-base {TpW(NO)(PMe3)}. X-ray data of the indoline-derived analogue (2) indicates that metal coordination disrupts the donor−acceptor (i.e., amide−iminium) conjugation in the cyanine ligand. Photolysis of 2 in the presence of indoline liberates a further derivatized cyanine.

Several pyrrole complexes of {TpW(NO)(PMe3)} are evaluated as potential synthons to the indole core. The 2,5-dimethylpyrrole analogue undergoes Michael addition with several enones and enals to form 3H-pyrrole complexes, which subsequently tautomerize and undergo an aldol ring closure to form tetrahydroindole complexes (including one confirmed by crystallographic data). In some cases an organic tetrahydroindole could be recovered, albeit in low yield. Pyrrole N−H insertion was observed for 2-methylpyrrole and 2,4-dimethylpyrrole, and for the latter, a crystal structure is reported for the corresponding seven-coordinate pyrrolyl hydride derived from 2,4-dimethylpyrrole.

A dihapto-coordinated pyridine ligand of a tungsten complex is readily alkylated or acylated at nitrogen, and the resulting pyridinium species undergoes regio- and stereoselective addition at C2 wi...

The addition of singly and doubly activated dienophiles to TpW(NO)(PMe3)(5,6-η2-anisole) and TpW(NO)(PMe3)(5,6-η2-1,3-dimethoxybenzene) to generate dihapto-coordinated [2.2.2]bicyclooctadienes and -trienes is explored. The highly functionalized bicyclic scaffolds are isolated from the metal fragment intact. Although moderate yields hinder their synthetic application, a unique stereoelectronic effect is uncovered. Coordination diastereomers of anisoles with the methoxy substituent distal to the PMe3 ligand have faster reaction rates relative to the proximal diastereomer. DFT calculations reveal the metal fragment possesses a dipole that directs positive charge away from the PMe3 ligand, and a polarized cycloaddition transition state is invoked to explain this observation.

The complex TpW(NO)(PMe3)(η2-benzene) cleanly inserts into the C−F bond of fluorobenzene to form the seven-coordinate complex TpW(NO)(PMe3)(F)(Ph) (confirmed by X-ray diffraction), while no C−H insertion is detected. Treatment of this product with triethyl- or phenyldimethylsilane results in liberation of benzene and the corresponding silyl fluoride. DFT calculations suggest a low-energy reaction pathway for C−F activation involving a fluorine-bound σ-complex that does not require the presence of an η2-arene intermediate. As the fluorine content of the benzene increases, or the benzene is replaced by fluoronaphthalene, C−F addition is no longer observed, and either C−H addition or η2-coordination dominates. Two species thought to be η2-silanes are also reported.

Complexes of the form TpRe(CO)(MeIm)(η2-pyr) and TpW(NO)(PMe3)(η2-pyr) (where pyr = 1-methylpyrrole, 2-methylpyrrole, and 2,5-dimethylpyrrole) were prepared and combined with a range of potential dipolarophiles. In several cases a dipolar cycloaddition resulted in the formation of 7-azanorbornene complexes (one crystal structure for each metal is presented), but attempts to develop a general route to the organic bicyclic system were unsuccessful. Using the complex [Os(NH3)5(η2-pyrrole)]2+ an organic cycloadduct was successfully prepared with dimethyl fumarate, and this complex was elaborated into advanced synthons to several naturally occurring pyrrolizidines.

A series of complexes of the form TpM(NO)(L‘)(η2-L) are prepared (where L is a nitrile, ketone, or alkene, M = W or Mo, L‘ = PMe3, MeIm, Tp = hydridotris(pyrazolyl)borate). These complexes are subjected to various electrophiles (e.g., alkyl halides, Bronsted acids) to systematically probe the ability of the π-basic transition metal to affect the reactivity of the dihapto-coordinated ligand. Alkylation is observed at the heteroatom of the ketone and the nitrile, but depending on the reagent, alkylation of the nitrosyl ligand or addition to the complexed π-bond also occurs. The structures of several nitrilium complexes and a rare example of an η2-acetonium complex are also reported.

A series of complexes having the general formulas [TpM(NO)(L)Br] and [TpmM(NO)(L)Br]BF4 (M = Mo or W and L = a moderate σ-donor) have been prepared. Variations in L provide access to M(I) complexes having a broad range of electronic and steric properties. Reduction of a limited number of these complexes in the presence of aromatic molecules produces novel complexes with the general formula [TpM(NO)(L)(η2-aromatic)]n+ (n = 0, 1).

The complex TpW(NO)(PMe3)(η2-benzene) was treated with an excess of phenol to generate TpW(NO)(PMe3)(η2-2H-phenol) as a mixture of two stereoisomers. This complex in the presence of base undergoes reactions with several common classes of electrophiles, including benzaldehyde, alkyl iodides, and Michael acceptors, to form new C−C bonds. Methyl and ethyl iodide react at C2 to form 2-alkyl-2H-phenol complexes, whereas the Michael acceptors react at C4 to give 4-alkyl-4H-phenol complexes. In both cases, the electrophile adds to the complexed phenol stereoselectively, anti to the metal. In the case of benzaldehyde, an aldol condensation reaction occurs at C2 to form a rare example of a thermally stable o-quinone methide complex. Crystal structures of the 2-ethyl-2H-phenol and the phenyl o-quinone methide complexes are included.

Reactions are reported for the TpW(NO)(PMe3) complex of either 2-methylfuran or 2,5-dimethylfuran with various dipolarophiles (N-methylmaleimide, N-phenylmaleimide, acrylonitrile). The resulting 7-oxanorbornene complexes are thought to be formed from a two-step reaction sequence. The first step involves the metal shifting the coordination site from C4−C5 to C3−C4, an action that renders the furan similar to a carbonyl ylide. This is followed by a 1,3-dipolar cycloaddition. In the case of the maleimides, a high preference (>10:1) for exo stereochemistry is observed. For acrylonitrile, the preference for exo is only modest (2:1). Attempts to liberate the intact 7-oxanorbornene were unsuccessful. Two crystal structures are presented that support the stereochemical assignments of the complexed cycloadducts.

Complexes of the form TpRe(CO)(MeIm)(η2-L) and TpW(NO)(PMe3)(η2-L) have been synthesized, where L = 1-methylpyrrole, 2-methylpyrrole, and 2,5-dimethylpyrrole. Depending on the nature of both the pyrrole and the metal, either a η2-1H- or an η2-3H-pyrrole complex can be isolated. In contrast, for the parent pyrrole, oxidative addition occurs across the N−H bond, generating either a N-pyrrolyl or N-pyrrolyl hydride. The protonation or methylation of the η2-pyrrole complexes and the reduction of the resulting pyrrolium species have also been investigated.

Complexes of the form TpW(NO)(PMe3)(η2-thiophene) have been synthesized in 31−40% yield by reduction of TpW(NO)(PMe3)(Br) in the presence of the thiophene ligand. The dynamics of protonation and the subsequent deprotonation for the corresponding 2H-thiophenium complexes have been investigated. Hydrogenation of the uncoordinated double bond was accomplished, and sulfur−carbon, sulfur−oxygen, and carbon−carbon bond-forming reactions were explored with these complexes.

The π-basic metal fragment {TpW(NO)(PMe3)} forms thermally stable η2 complexes with acetaldehyde, acetone, ethyl acetate, methyl formate, and N,N-dimethylformamide (where Tp = hydridotris(pyrazolyl)borate). The related system {TpMo(NO)(MeIm)} (MeIm = 1-methylimidazole) also forms stable η2 complexes with acetone, methyl formate, and N,N-dimethylformamide. When the tungsten fragment is combined with a carboxylic acid, oxidative addition yields a seven-coordinate hydride complex. These results are compared to those observed for analogous Re and Os systems. TpMo(NO)(MeIm)(η2-DMF) and TpW(NO)(PMe3)(η2-DMF) are rare examples of η2 amide complexes, demonstrating the exceptional strength of these group 6 π bases.

A series of complexes of the form TpMo(NO)(L)(η2-Lπ) were synthesized, where Tp = hydridotris(pyrazolyl)borate, L = 1-methylimidazole or ammonia, and Lπ is an alkene, alkyne, ketone, polyaromatic hydrocarbon, or aromatic heterocycle. The thermal stability of the dihapto-coordinated aromatic complexes is indicative of a strong π back-bonding interaction similar to that found in the heavy metal analogues [Os(NH3)5(η2-arene)]2+, TpRe(CO)(L)(η2-arene), and TpW(NO)(L)(η2-arene). Several synthetic routes to these molybdenum complexes are outlined.

Dihapto-coordinated furan complexes of the form TpRe(CO)(t-BuNC)(2,3-η2-furan) (Tp = hydridotris(pyrazolyl)borate) react with aldehydes in the presence of BF3·OEt2 to yield dihydrofuran complexes. Depending on the reaction conditions, these products take one of three forms: a dihapto-coordinated trans-2-alkyl-3-acyl-2,3-dihydrofuran, trans-2,3-dialkyl-2,3-dihydrofuran, or 4a,7a-dihydro-4H-furo[2,3-d][1,3]dioxine. The first two are formed exclusively as trans-disubstituted products, whereas the last exhibits cis stereochemistry at the ring fusion. These are isolated as mixtures of coordination diastereomers, which are separable by chromatography. Of the two diastereomers of the trans-2-alkyl-3-acyl-2,3-dihydrofuran product obtained in each case, only the major isomer undergoes an interfacial isomerization under ambient conditions. Attempts to decomplex the dihydrofurans were unsuccessful.

A computational investigation of metal-based C−H activation with complexes that possess scorpionate (i.e., tris(pyrazolyl)borate) and cyclopentadienyl ligands was reported. The model tris(azo)borate ligand (Tab; [HB(NNH)3]-) was created as an alternative to the parent tris(pyrazolyl)borate (Tp). The Tab ligand was found to closely reproduce the structure and energetics of Tp in Re-mediated C−H activation of methane. Relative to separated reactants oxidative addition of methane to CpRe(CO)2 was exothermic, while the same transformations with TpRe(CO)2 and (Tab)Re(CO)2 were endothermic. The differences between the Tp and Cp systems were attributed primarily to steric effects. The reactivity differences between Tp and Cp drew attention to how each system would respond, sterically and electronically, to ring substitutions. TpRe(CO)3 complexes consistently had a broader νCO (a measure of electronic properties) and cone angle (a measure of steric bulk) range in comparison to CpRe(CO)3 congeners. While Cp was mo...

A series of asymmetric N-substituted pyrroles (1−7) has been synthesized from amino acid derivatives and complexed to the pentaammineosmium(II) fragment. Many of these pyrrole complexes (8−14) show a thermodynamic preference for one coordination diastereomer according to their NMR spectra (−5 to −20 °C). This stereoselective coordination results in stereoselective electrophilic addition at the uncoordinated β carbon (C3) when the complexes are treated with trifluoromethanesulfonic acid (HOTf), methyl triflate, or dimethoxymethane. These stereoselective reactions at C3 are a direct result of differentiation of the pyrrole enantiofaces.

The metal fragment {TpRe(CO)(1-methylimidazole)} (Tp = hydridotris(pyrazolyl)borate) forms stable complexes with a variety of arenes and aromatic heterocycles via dihapto coordination. In this study, a series of olefins, ketones, and aldehydes are bound to ascertain the relative steric environment near the stereogenic metal center. Utilizing this information, predictions of the diastereomeric selectivity of η2-aromatic systems can be made.

A variety of rhenium asymmetric π-basic dearomatization agents of the general formula TpRe(CO)(L)(Lπ) (Tp = hydridotris(pyrazolyl)borate; L = tBuNC, pyridine, PMe3, 1-methylimidazole, or NH3; Lπ = dihapto-coordinated ligand) have been synthesized via three different routes. By varying the ligand, L, the steric and electronic properties of these complexes can be tuned, and thus the stability and selectivity of the η2-aromatic systems can be adjusted. Comparisons among the various rhenium complexes are presented as well as comparisons to the established pentaammineosmium(II) system.

A series of complexes of the type [TpRe(CO)(LD)(η2-ethylene)] has been synthesized, where LD is tBuNC, PMe3, pyridine, 1-methylimidazole, or NH3 (increasing in electron-donating ability of LD). The rates of the propeller-like rotation of ethylene about the ethylene−rhenium bond have been determined using spin saturation transfer experiments at low temperatures (−85 to −60 °C). It was found that the ΔG⧧ values correlate with both the C⋮O stretching frequencies and Re(II/I) reduction potentials for these complexes, indicating that the barrier to rotation is primarily electronic in nature.

An aniline coordinated to tungsten undergoes alkylation at C4 or protonation at C2. In the latter case, a second electrophilic addition is possible at C3, followed by nucleophilic addition at C4.

A comparison of the electron-donating abilities of the hydridotris(pyrazolyl)borate ligand and cyclopentadienyl ligand toward transition metals is presented. These data demonstrate that there is no definitive trend in donor ability across the periodic table, and consequently generalizations about the relative donating abilities of the two ligands should be discouraged.