Facile Double C−H Activation of Tetrahydrofuran by an Iridium PNP Pincer Complex

C-H Bond Activation by Iridium(III) and Iridium(IV) Oxo Complexes

Oxidation of an iridium(III) oxo precursor enabled the structural, spectroscopic, and quantum-chemical characterization of the first well-defined iridium(IV) oxo complex. Side-by-side examination of the proton-coupled electron transfer thermochemistry revealed similar driving forces for the isostructural oxo complexes in two redox states due to compensating contributions from H+ and e- transfer. However, C-H activation of dihydroanthracene revealed significant hydrogen tunneling for the distinctly more basic iridium(III) oxo complex. Our findings complement the growing body of data that relate tunneling to ground state properties as predictors for the selectivity of C-H bond activation.

Ruthenium Complexes of a Triphosphorus-Coordinating Pincer Ligand: Ru-P Ligand-Substituent Exchange Reactions Driven by Large Variations of Bond Energies

The reaction of [(p-cymene)RuCl₂]₂ with the triphosphine ligand bis(2-di-tert-butylphosphinophenyl)phosphine (^tBuP^HPP) results in an unusual exchange reaction in which a chloride ligand and a phosphorus-bound H atom are exchanged ("H-P/Ru-Cl exchange") to give the (chlorophosphine)ruthenium hydride complex (^tBuP^ClPP)RuHCl [1^Cl-HCl; ^tBuP^ClPP = bis(2-di-tert-butylphosphinophenyl)chlorophosphine]. Density functional theory calculations indicate that the presumed initial product of metalation, (^tBuP^HPP)RuCl₂ (1^H-Cl2), undergoes an H-P/Ru-Cl exchange via sequential P-to-Ru α-H migration to give the intermediate (^tBuPPP)RuHCl₂, followed by Ru-to-P α-Cl migration to give the observed product 1^Cl-HCl (crystallographically characterized). Dehydrochlorination of 1^Cl-HCl under a H₂ atmosphere gives (^tBuP^ClPP)RuH₄ (1^Cl-H4), which then can undergo a second dehydrochlorination and addition of H₂ to give (^tBuP^HPP)RuH₄ (1^H-H4). This reaction may proceed via the reverse of the intramolecular exchange by 1^H-Cl2, i.e., loss of H₂ from 1^Cl-H4 to give 1^Cl-H2, which could undergo Cl-P/Ru-H exchange to give (^tBuP^HPP)RuHCl (1^H-HCl). Accordingly, the thermodynamics of Cl-P/Ru-H exchange are found to be highly dependent on the nature of the ancillary anionic ligand (H or Cl), which is not directly involved in the exchange. The origin of this thermodynamic dependence can be explained in terms of the high stability of complexes (^RP^XPP)RuHCl (X = H, Cl; R = Me, ^tBu), in which the hydride is approximately trans to a vacant coordination site and the central phosphine group is approximately trans to the weak-trans-influence chloride ligand. This conclusion has general implications for five-coordinate d⁶ complexes, both pincer- and nonpincer-ligated.

The Backbone of Success of P,N-Hybrid Ligands: Some Recent Developments

Organophosphorus ligands are an invaluable family of compounds that continue to underpin important roles in disciplines such as coordination chemistry and catalysis. Their success can routinely be traced back to facile tuneability thus enabling a high degree of control over, for example, electronic and steric properties. Diphosphines, phosphorus compounds bearing two separated P^III donor atoms, are also highly valued and impart their own unique features, for example excellent chelating properties upon metal complexation. In many classical ligands of this type, the backbone connectivity has been based on all carbon spacers only but there is growing interest in embedding other donor atoms such as additional nitrogen (–NH–, –NR–) sites. This review will collate some important examples of ligands in this field, illustrate their role as ligands in coordination chemistry and highlight some of their reactivities and applications. It will be shown that incorporation of a nitrogen-based group can impart unusual reactivities and important catalytic applications.

Ir-Catalyzed Cycloaddition of Tribenzocyclyne with Biphenylenes

We demonstrate that Ir-catalyzed C-C bond activation in biphenylenes followed by a reaction with tribenzocyclyne is a suitable method for synthesizing strained and unknown monoadducts with the tetradehydrotetrabenzo[a,c,e,i]cyclododecene scaffold ([12]annulenes). Modification of reaction conditions also furnished [12]annulene products with cis and/or trans double bonds formed by hydrogen transfer. The [9]annulene side product was formed upon the reaction of the benzyl radical with tribenzocyclyne during the Bergman cyclization. All isolated compounds were fully characterized by HRMS, NMR, and X-ray diffraction analysis.

Rare-earth-metallocene alkylaluminates trigger distinct tetrahydrofuran activation

Thermal treatment of Cp*2YMe(thf) (Cp* = C5Me5), obtained from Cp*2Y(AlMe4) via donor-induced AlMe3 cleavage, in THF resulted in the concomitant formation of vinyl oxide Cp*2Y(OC2H3)(thf) and 2-ethylene-tetrahydrofuranyl complex Cp*2Y(2-C2H4-OC4H7) via the release of methane. In stark contrast, dissolving Cp*2La(AlMe4) in THF/n-hexane led to the quantitative formation of AlMe3-stabilized 2-tetrahydrofuranyl complex Cp*2La(2-AlMe3-OC4H7), with methane elimination.

Decarbonylative ether dissection by iridium pincer complexes

A unique chain-rupturing transformation that converts an ether functionality into two hydrocarbyl units and carbon monoxide is reported, mediated by iridium(i) complexes supported by aminophenylphosphinite (NCOP) pincer ligands. The decarbonylation, which involves the cleavage of one C-C bond, one C-O bond, and two C-H bonds, along with formation of two new C-H bonds, was serendipitously discovered upon dehydrochlorination of an iridium(iii) complex containing an aza-18-crown-6 ether macrocycle. Intramolecular cleavage of macrocyclic and acyclic ethers was also found in analogous complexes featuring aza-15-crown-5 ether or bis(2-methoxyethyl)amino groups. Intermolecular decarbonylation of cyclic and linear ethers was observed when diethylaminophenylphosphinite iridium(i) dinitrogen or norbornene complexes were employed. Mechanistic studies reveal the nature of key intermediates along a pathway involving initial iridium(i)-mediated double C-H bond activation.

Group-Transfer Reactions of a Cationic Iridium Alkoxycarbene Generated by Ether Dehydrogenation

Despite broad interest in metal carbene complexes, there remain few examples of catalytic transformations of ethers that proceed via alkoxycarbene intermediates generated by α,α-dehydrogenation. We demonstrate that both neutral and cationic alkoxycarbene derivatives are accessible via ether dehydrogenation at a PNP(ⁱPr)₄ pincer-supported iridium complex (PNP(ⁱPr)₄ = 2,6-bis((diisopropylphosphino)methyl)pyridine). Both cationic and neutral alkoxycarbene complexes undergo group transfer imination with azides, with the cationic derivative serving as a more efficient catalyst for cyclopentyl ether imination. Mechanistic studies support an iridium(I)dinitrogen complex as the resting state in the dark and a role for light-promoted N₂ dissociation. Isoamyl nitrite and phenyl ethyl ketene are also found to engage with the cationic alkoxycarbene complex in formal alkoxide and O atom transfer reactions, respectively. In the former case an isolable dialkoxyalkyliridium complex is obtained, representing only the second example of a structurally characterized dialkoxyalkyl complex of a transition metal.

Reactivity of a Sterically Unencumbered α-Borylated Phosphorus Ylide towards Small Molecules

The influence of substituents on α‐borylated phosphorus ylides (α‐BCPs) has been investigated in a combined experimental and quantum chemical approach. The synthesis and characterization of Me₃PC(H)B(iBu)₂ (1), consisting of small Me substituents on phosphorous and iBu residues on boron, is reported. Compound 1 is accessible through a novel synthetic approach, which has been further elucidated through DFT studies. The reactivity of 1 towards various small molecules was probed and compared with that of a previously published derivative, Ph₃PC(Me)BEt₂ (2). Both α‐BCPs react with NH₃ to undergo heterolytic N−H bond cleavage. Different di‐ and trimeric ring structures were observed in the reaction products of 1 with CO and CO₂. With PhNCO and PHNCS, the expected insertion products [Me₃PC(H)(PhNCO)B(iBu)₂] and [Me₃PC(H)(PhNCS)B(iBu)₂], respectively, were isolated.

Rhodium-Catalyzed Alkylation of C-H Bonds in Aromatic Amides with Non-activated 1-Alkenes: The Possible Generation of Carbene Intermediates from Alkenes

The alkylation of C-H bonds (hydroarylation) in aromatic amides with non-activated 1-alkenes using a rhodium catalyst and assisted by an 8-aminoquinoline directing group is reported. The addition of a carboxylic acid is crucial for the success of this reaction. The results of deuterium-labeling experiments indicate that one of deuterium atoms in the alkene is missing, suggesting that the reaction does not proceed through the commonly accepted mechanism for C-H alkylation reactions. Instead the reaction is proposed to proceed through a carbene mechanism. The carbene mechanism is also supported by preliminary DFT calculations.

Reactions of GeCl2 with the Thiolate LiSC(SiMe3 )3 : From thf Activation to Insertion of GeCl2 Molecules into C-S Bonds

The reaction system GeCl₂ ⋅dioxane/LiSTsi (Tsi=C(SiMe₃ )₃ ) opens a fruitful area in germanium chemistry, depending on the stoichiometry and solvent used during the reaction. For example, the reaction of GeCl₂ ⋅dioxane in toluene with two equivalents of the thiolate gives the expected germylene Ge(STsi)₂ in excellent yield. This germylene readily reacts with hydrogen and acetylene, however, in a non-selective way. By using an excess amount of the thiolate and toluene as the solvent, the germanide [Ge(STsi)₃ ][Li(thf)] is obtained. Performing the same reaction in thf leads to a C-H activation of thf to give (H₇ C₄ O)Ge[STsi](μ² -S)₂ Ge[STsi]₂ , in which the thf molecule is still intact. Using a sub-stoichiometric amount of the thiolate leads to the heteroleptic compound [ClGe(STsi)]₂ and to the insertion product (thf)Ge[S-GeCl₂ -Tsi]₂ , in which additional GeCl₂ molecules insert into the C-S bonds of Ge(STsi)₂ . The synthesis and the experimentally determined structures of all compounds are presented together with first reactivity studies of Ge(STsi)₂ .

Reversible alkoxycarbene formation by C-H activation of ethers via discrete, isolable intermediates

A cationic iridium complex bearing a pendant ether ligand is shown to be in equilibrium with the corresponding dihydridoalkoxycarbene resulting from two consecutive C-H activations. Control of the conditions allows for the crystallographic characterization of all three complexes involved in the interconversion of ether and alkoxycarbene functionalities via sequential oxidative addition and α elimination.

Symmetrical and unsymmetrical diphosphanes with diversified alkyl, aryl, and amino substituents

Synthesis, classification, and analysis of the structural, electronic and spectroscopic properties of a series of novel diphosphanes with diversified substituents.

Csp3 -H Activation without Chelation Assistance in an Iridium Pincer Complex Forming Cyclometallated Products

Cyclometallation of 8-methylquinoline and 2-(dimethylamino)-pyridine in an iridium-based pincer complex is described. The C-H activation of 2-(dimethylamino)pyridine is not chelation assisted, which has not been described before for Csp³ -H bonds in cyclometallation reactions. The mechanism of the cyclometallation of 2-(dimethylamino)pyridine was studied by DFT calculations and kinetic measurements.

First-row transition metal complexes of ENENES ligands: the ability of the thioether donor to impact the coordination chemistry

The reactions of two variants of ENENES ligands, E(CH₂)₂NH(CH)₂SR, where E = 4-morpholinyl, R = Ph (a), Bn (b) with MCl₂ (M = Mn, Fe, Co, Ni and Cu) in coordinating solvents (MeCN, EtOH) affords isolable complexes, whose magnetic susceptibility measurements suggest paramagnetism and a high-spin formulation.

C–H activation of ethers by pyridine tethered PCsp3P-type iridium complexes

An iridium complex supported by a PC^PyP ligand featuring an internal pyridine tether showed selective mono-C–H activation of ethers, which represent intermediates in the synthesis of the corresponding Fischer carbenes.

Rhodium-catalyzed regioselective addition of the ortho C-H bond in aromatic amides to the C-C double bond in α,β-unsaturated γ-lactones and dihydrofurans

An unprecedented C-H alkylation using α,β-unsaturated γ-lactones (butenolides) and dihydrofurans was achieved by the Rh-catalyzed reaction of benzamides. C-C bond formation occurs between the ortho-position of the benzamide derivative and the γ-position of the butenolide or the α-position of the dihydrofuran. The presence of an 8-aminoquinoline directing group is crucial for the success of the reaction. The results of deuterium labeling experiments indicate that the cleavage of the C-H bond is reversible and suggest that a migratory carbene insertion is involved as the key step.

Oxygen Reduction with a Bifunctional Iridium Dihydride Complex

The iridium dihydride [Ir(H)2 (HPNP)](+) (PNP=N(CH2 CH2 PtBu2 )2 ) reacts with O2 to give the unusual, square-planar iridium(III) hydroxide [Ir(OH)(PNP)](+) and water. Regeneration of the dihydride with H2 closes a quasi-catalytic synthetic oxygen-reduction reaction (ORR) cycle that can be run several times. Experimental and computational examinations are in agreement with an oxygenation mechanism via rate-limiting O2 coordination followed by H-transfer at a single metal site, facilitated by the cooperating pincer ligand. Hence, the four electrons required for the ORR are stored within the two covalent MH bonds of a mononuclear metal complex.

Iridium(i) PNP complexes in the sp3C–H bond activation of methyl propanoate and related esters

The utilisation of the PNP iridium pincer complex [Ir(PNP)(COE)][BF₄] [PNP = 2,6-bis{(di-tert-butylphosphino)methyl}pyridine; COE = cyclooctene] in the sp³C–H activation of methyl propanoate and other related esters was explored.

A phosphomide based PNP ligand, 2,6-{Ph2PC(O)}2(C5H3N), showing PP, PNP and PNO coordination modes

Coordination chemistry of a versatile phosphomide ligand, 2,6-{Ph₂PC(O)}₂(C₅H₃N), is described.

[IrCl{N(CHCHPtBu2)2}]−: a versatile source of the IrI(PNP) pincer platform

The iridium(ii) complex [IrCl{N(CHCHPtBu₂)₂}] is reduced by KC₈ to give the anionic iridium(i) pincer complex [IrCl{N(CHCHPtBu₂)₂}]⁻ which was isolated and fully characterized upon stabilization of the counter cation with crown ether as [K(15-cr-5)₂][IrCl{N(CHCHPtBu₂)₂}].

Coordination chemistry of Co complexes containing tridentate SNS ligands and their application as catalysts for the oxidation of n-octane

Fully characterized cobalt complexes of tridentate SNS ligands were selective (up to 90%) for the oxidation of n-octane to octanones.

Fischer carbene complexes remain favourite targets, and vehicles for new discoveries

Fifty years after their introduction, Fischer-type carbene complexes still enthral synthetic and theoretical chemists interested in their preparation and characterization.

Reversible double C-H bond activation of linear and cyclic ethers to form iridium carbenes

The double C-H bond activation of a series of linear and cyclic ethers by the iridium complex [Tp(tol')Ir(C(6)H(5))(N(2))] (2⋅N(2)), which features a cyclometalated hydrotris(3-p-tolylpyrazol-1-yl)borate ligand (Tp(tol')) coordinated in a κ(4)-N,N',N'',C manner, has been studied. Two methyl ethers, namely, Me(2)O and MeOtBu, along with diethyl ether and the cyclic ethers tetrahydrofuran, tetrahydropyran (THP), and 1,4-dioxane have been investigated with formation in every case of the corresponding hydride carbene complexes 3-8, which are stabilized by κ(4)-coordination of the ancillary Tp(tol') ligand. Five of the compounds have been structurally authenticated by X-ray crystallography. A remarkable feature of these rearrangements is the reversibility of the double C-H bond activation of Me(2)O, MeOtBu, Et(2)O, and THP. This has permitted catalytic deuterium incorporation into the methyl groups of the two methyl ethers, although in a rather inefficient manner (for synthetic purposes). Although possible in all cases, C-C coupling by migratory insertion of the carbene into the Ir-C σ bond of the metalated linkage has only been observed for complex 8 that contains a cyclic carbene that results from α,α-C-H activation of 1,4-dioxane. Computational studies on the formation of iridium carbenes are also reported, which show a role for metalated Tp ligands in the double C-H activation and account for the reversibility of the reaction in terms of the relative stability of the reagents and the products of the reaction.