Cyclometalated Iridium Complexes of Bis(Aryl) Phosphine Ligands: Catalytic C–H/C–D Exchanges and C–C Coupling Reactions

Bringing Selectivity in H/D Exchange Reactions Catalyzed by Metal Nanoparticles through Modulation of the Metal and the Ligand Shell

Ru and Rh nanoparticles catalyze the selective H/D exchange in phosphines using D₂ as the deuterium source. The position of the deuterium incorporation is determined by the structure of the P-based substrates, while activity depends on the nature of the metal, the properties of the stabilizing agents, and the type of the substituent on phosphorus. The appropriate catalyst can thus be selected either for the exclusive H/D exchange in aromatic rings or also for alkyl substituents. The selectivity observed in each case provides relevant information on the coordination mode of the ligand. Density functional theory calculations provide insights into the H/D exchange mechanism and reveal a strong influence of the phosphine structure on the selectivity. The isotope exchange proceeds via C-H bond activation at nanoparticle edges. Phosphines with strong coordination through the phosphorus atom such as PPh₃ or PPh₂Me show preferred deuteration at ortho positions of aromatic rings and at the methyl substituents. This selectivity is observed because the corresponding C-H moieties can interact with the nanoparticle surface while the phosphine is P-coordinated, and the C-H activation results in stable metallacyclic intermediates. For weakly coordinating phosphines such as P(o-tolyl)₃, the interaction with the nanoparticle can occur directly through phosphine substituents, and then, other deuteration patterns are observed.

Preparation of Deuterium Labeled Compounds by Pd/C-Al-D2O Facilitated Selective H-D Exchange Reactions

The chemo/regioselective H-D exchange of amino acids and synthetic building blocks by an environmentally benign Pd/C-Al-D₂O catalytic system is described. Due to the importance of isotope labeled compounds in medicinal chemistry and structural biology, notably their use as improved drug candidates and biological probes, the efficient and selective deuteration methods are of great interest. The approach is based on selective H-D exchange reactions where the deuterium source is simple D₂O. D₂ gas is generated in situ from the reaction of aluminum and D₂O, while the commercially available palladium catalyst assists the H-D exchange reaction. The high selectivity and efficiency, as well as the simplicity and safe nature of the procedure make this method an environmentally benign alternative to current alternatives.

Bulky 1,1'-bisphosphanoferrocenes and their coordination behaviour towards Cu(i)

Two bulky mesityl substituted dppf-analogues Fe(C₅H₄PMes₂)₂ (Mes = 2,4,6-Me₃C₆H₂, 1) and Fe(C₅H₄PMes₂)(C₅H₄PPh₂) (Mes = 2,4,6-Me₃C₆H₂, Ph = C₆H₅, 3) have been prepared and their properties as donor ligands have been explored using heteronuclear NMR spectroscopy and in particular via¹J_P-Se coupling, cyclic voltammetry and DFT calculations. Based on the results obtained, a series of mono- and dinuclear Cu(i) complexes have been prepared with these new diphosphane ligands using Br^-, I^-, and BF₄^- as counter anions. For the very bulky ligand 1 rare and unprecedented double bridging complexation modes have been observed containing two non-planar Cu₂Br₂ units, while for the other dinuclear complexes planar Cu₂Br₂ units have been found. The Cu(i) complexes of 1 and 3 were then used as catalysts for CO₂-fixation reaction with terminal alkynes, and complexes with ligand 3 were found to be more efficient than those with 1. DFT calculations performed on compounds 1, 3 and their Cu(i) complexes were able to verify the trend of these catalytic reactions.

A Versatile Approach to Access Trimetallic Complexes Based on Trisphosphinite Ligands

A straightforward method for the preparation of trisphosphinite ligands in one step, using only commercially available reagents (1,1,1-tris(4-hydroxyphenyl)ethane and chlorophosphines) is described. We have made use of this approach to prepare a small family of four trisphosphinite ligands of formula [CH3C{(C6H4OR2)3], where R stands for Ph (1a), Xyl (1b, Xyl = 2,6-Me2-C6H3), iPr (1c), and Cy (1d). These polyfunctional phosphinites allowed us to investigate their coordination chemistry towards a range of late transition metal precursors. As such, we report here the isolation and full characterization of a number of Au(I), Ag(I), Cu(I), Ir(III), Rh(III) and Ru(II) homotrimetallic complexes, including the structural characterization by X-ray diffraction studies of six of these compounds. We have observed that the flexibility of these trisphosphinites enables a variety of conformations for the different trimetallic species.

Group 9 and 10 Metal Complexes of an Ylide-Substituted Phosphine: Coordination versus Cyclometalation and Oxidative Addition

Ylide-substituted phosphines (YPhos) have been shown to be excellent ligands for several transition metal catalyzed reactions. Investigations of the coordination behavior of the YPhos ligand YSPPh2 (1) [with YS = (Ph3P)(SO2Tol)C] toward group 9 and 10 metals revealed a surprisingly diverse coordination chemistry of the ligand. With Ni(CO)4, the formation of a di- as well as tricarbonyl complex is observed depending on the reaction conditions. In [( κP,η 2 -benzene-1)Ni(CO)2] the phosphine ligand also coordinates via a phosphonium bound phenyl group to the metal leading to a unique nickel η 2 -arene interaction, which can be viewed as an intermediate state toward P-C bond activation. Full cleavage of the P-C bond takes place with [Rh(COD)Cl]2 leading to a complex salt with [( κP,κO-1)Rh(COD)]+ as cation and a dirhodium trichloride complex anion. Here, YSPPh2 underwent P-C bond cleavage to thus act as an anionic diphosphine ligand. In contrast, in [( κP,κO-1)Rh(COD)]+ as well as [( κP,κO-1)Rh(CO)Cl], formed from the reaction of 1 with [Rh(CO)2Cl]2, the YPhos ligand acts as bidentate ligand complexing the metal via the phosphine and sulfonyl moiety with an intact PPh3 unit. A further type of coordination is observed with [Ir(COD)Cl]2. Here, phosphine coordination is accompanied by C-H activation at one of the phosphonium bound phenyl groups leading to a cyclometalated complex.

Reactivity of a gold(i)/platinum(0) frustrated Lewis pair with germanium and tin dihalides

The rich reactivity of tetrylene dihalides towards a metallic FLP: phosphine exchange reactions and formation of homo and heterobimetallic compounds.

Synthesis, Structures, and Reactivity of Single and Double Cyclometalated Complexes Formed by Reactions of [Cp*MCl2]2 (M = Ir and Rh) with Dinaphthyl Phosphines

Reactions of two dinaphthyl phosphines with [Cp*IrCl₂]₂ have been carried out. In the case of di(α-naphthyl)phenylphosphine (1a), a simple P-coordinated neutral adduct 2a is obtained. However, tert-butyldi(α-naphthyl)phenylphosphine (1b) is cyclometalated to form [Cp*IrCl(P^C)] (3b). Complexes 2a and 3a undergo further cyclometalation to give the corresponding double cyclometalated complexes [Cp*Ir(C^P^C)] (4a,b) upon heating. In the presence of sodium acetate, reactions of 1a,b with [Cp*IrCl₂]₂ directly afford the final double cyclometalated complexes (4a,b). In the absence of acetate, [Cp*RhCl₂]₂ shows no reaction with 1a,b, whereas with acetate the reactions form the corresponding single cyclometalated complexes [Cp*RhCl(P^C)] (5a,b), which react with ^t BuOK to form the corresponding rhodium hydride complexes (6a,b). Treatment of 4a with CuCl₂ or I₂ leads to opening of two Ir-C σ bonds to yield the corresponding P-coordinated iridium dihalide (7 or 8) by means of an intramolecular C-C coupling reaction. A new chiral phosphine (11) is formed by the ligand-exchange reaction of 8 with PMe₃. Reactions of the single cycloiridated complex 3b with terminal aromatic alkynes result in the corresponding five- and six-membered doubly cycloiridated complex 12 and/or η²-alkene coordinated complexes 13-15; the latter discloses that the electronic effect of terminal alkynes affects the regioselectivity. While the single cyclorhodated complex 5b reacts with terminal aromatic alkynes to form the corresponding six-membered cyclometalated complexes 16a-c by vinylidene rearrangement/1,1-insertion. Plausible pathways for formation of insertion products 13-16 were proposed. Molecular structures of twelve new complexes were determined by X-ray diffraction.

C-H Functionalisation for Hydrogen Isotope Exchange

The various applications of hydrogen isotopes (deuterium, D, and tritium, T) in the physical and life sciences demand a range of methods for their installation in an array of molecular architectures. In this Review, we describe recent advances in synthetic C-H functionalisation for hydrogen isotope exchange.

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.

Synthesis and Reactivity toward H2 of (η(5)-C5Me5)Rh(III) Complexes with Bulky Aminopyridinate Ligands

Electrophilic, cationic Rh(III) complexes of composition [(η(5)-C5Me5)Rh(Ap)](+), (1(+)), were prepared by reaction of [(η(5)-C5Me5)RhCl2]2 and LiAp (Ap = aminopyridinate ligand) followed by chloride abstraction with NaBArF (BArF = B[3,5-(CF3)2C6H3]4). Reactions of cations 1(+) with different Lewis bases (e.g., NH3, 4-dimethylaminopyridine, or CNXyl) led in general to monoadducts 1·L(+) (L = Lewis base; Xyl = 2,6-Me2C6H3), but carbon monoxide provided carbonyl-carbamoyl complexes 1·(CO)2(+) as a result of metal coordination and formal insertion of CO into the Rh-Namido bond of complexes 1(+). Arguably, the most relevant observation reported in this study stemmed from the reactions of complexes 1(+) with H2. (1)H NMR analyses of the reactions demonstrated a H2-catalyzed isomerization of the aminopyridinate ligand in cations 1(+) from the ordinary κ(2)-N,N' coordination to a very uncommon, formally tridentate κ-N,η(3) pseudoallyl bonding mode (complexes 3(+)) following benzylic C-H activation within the xylyl substituent of the pyridinic ring of the aminopyridinate ligand. The isomerization entailed in addition H-H and N-H bond activation and mimicked previous findings with the analogous iridium complexes. However, in dissimilarity with iridium, rhodium complexes 1(+) reacted stoichiometrically at 20 °C with excess H2. The transformations resulted in the hydrogenation of the C5Me5 and Ap ligands with concurrent reduction to Rh(I) and yielded complexes [(η(4)-C5Me5H)Rh(η(6)-ApH)](+), (2(+)), in which the pyridinic xylyl substituent is η(6)-bonded to the rhodium(I) center. New compounds reported were characterized by microanalysis and NMR spectroscopy. Representative complexes were additionally investigated by X-ray crystallography.

Electronic effects of substituents on the stability of the iridanaphthalene compound [Ir[upper bond 1 start]Cp*{=C(OMe)CH=C(o-C6[upper bond 1 end]H4)(Ph)}(PMe3)]PF6

Iridanaphthalene complexes are synthesized from the corresponding methoxy(alkenyl)carbeneiridium compounds. The electronic character of the substituents on the 6-position of the metallanaphthalene ring is crucial from the point of view of the stability of the iridanaphthalene, [Ir[upper bond 1 start]Cp*{=C(OMe)CH=C(o-C[upper bond 1 end]6H4)(Ph)}(PMe3)]PF6, vs. its transformation to the corresponding indanone derivatives. Stability studies of the iridanaphthalene compounds revealed that strong electron donor substituents (-OMe) stabilize the iridanaphthalene, while weak electron donor (-Me) and electron withdrawing (-NO2) groups favor the formation of indanone derivatives. Two possible indanone isomers can be obtained in the conversion of the unstable iridanaphthalene complexes and a mechanism for the formation of these isomers is proposed.

Formation of a C-C double bond from two aliphatic carbons. Multiple C-H activations in an iridium pincer complex

The search for novel, atom-economic methods for the formation of C-C bonds is of crucial importance in synthetic chemistry. Especially attractive are reactions where C-C bonds are formed through C-H activation, but the coupling of unactivated, alkane-type C_sp³ -H bonds remains an unsolved challenge. Here, we report iridium-mediated intramolecular coupling reactions involving up to four unactivated C_sp³ -H bonds to give carbon-carbon double bonds under the extrusion of dihydrogen. The reaction described herein is completely reversible and the direction can be controlled by altering the reaction conditions. With a hydrogen acceptor present a C-C double bond is formed, while reacting under dihydrogen pressure leads to the reverse process, with some of the steps representing net C_sp³ -C_sp³ bond cleavage. Mechanistic investigations revealed a conceptually-novel overall reactivity pattern where insertion or deinsertion of an Ir carbene moiety, formed via double C-H activation, into an Ir-C bond is responsible for the key C-C bond formation and cleavage steps.