The scope of ambiphilic acetate-assisted cyclometallation with half-sandwich complexes of iridium, rhodium and ruthenium

C? H activation by acetate‐assisted cyclometallation of a phenyl group with half‐sandwich complexes [{MCl₂Cp*}₂] (M=Ir, Rh) and [{RuCl₂(p‐cymene)}₂] can be directed by a wide range of nitrogen donor ligands including pyrazole, oxazoline, oxime, imidazole and triazole, and X‐ray structures of a number of complexes are reported. All the ligands tested cyclometallated at iridium, however ruthenium and rhodium fail to cause cyclometallation in some cases. As a result, the nitrogen donors have been categorised based on their reactivity with the three metals used. The relevance of these cyclometallation reactions to catalytic synthesis of carbocycles and heterocycles is discussed.     

Reactions of [Ru(CO)3Cl2]2 with aromatic nitrogen donor ligands in alcoholic media

The reactions of mono‐ and bidentate aromatic nitrogen‐containing ligands with [Ru(CO)₃Cl₂]₂ in alcohols have been studied. In alcoholic media the nitrogen ligands act as bases promoting acidic behaviour of alcohols and the formation of alkoxy carbonyls [Ru(N–N)(CO)₂Cl(COOR)] and [Ru(N)₂(CO)₂Cl(COOR)]. Other products are monomers of type [Ru(N)(CO)₃Cl₂], bridged complexes such as [Ru(CO)₃Cl₂]₂(N), and ion pairs of the type [Ru(CO)₃Cl₃]^? [Ru(N–N)(CO)₃Cl]⁺ (N–N = chelating aromatic nitrogen ligand, N = non‐chelating or bridging ligand). The reaction and the product distribution can be controlled by adjusting the reaction stoichiometry. The reactivity of the new ruthenium complexes was tested in 1‐hexene hydroformylation. The activity can be associated with the degree of stability of the complexes and the ruthenium–ligand interaction. Chelating or bridging nitrogen ligands suppresses the activity strongly compared with the bare ruthenium carbonyl chloride, while the decrease in activity is less pronounced with monodentate ligands. A plausible catalytic cycle is proposed and discussed in terms of ligand–ruthenium interactions. The reactivity of the ligands as well as the catalytic cycle was studied in detail using the computational DFT methods. Copyright © 2005 John Wiley & Sons, Ltd.     

Copper Versus Thioether‐Centered Oxidation: Mechanistic Insights into the Non‐Innocent Redox Behavior of Tripodal Benzimidazolylaminothioether Ligands

A series of Cu⁺ complexes with ligands that feature varying numbers of benzimidazole/thioether donors and methylene or ethylene linkers between the central nitrogen atom and the thioether sulfur atoms have been spectroscopically and electrochemically characterized. Cyclic voltammetry measurements indicated that the highest Cu²⁺/Cu⁺ redox potentials correspond to sulfur‐rich coordination environments, with values decreasing as the thioether donors are replaced by nitrogen‐donating benzimidazoles. Both Cu²⁺ and Cu⁺ complexes were studied by DFT. Their electronic properties were determined by analyzing their frontier orbitals, relative energies, and the contributions to the orbitals involved in redox processes, which revealed that the HOMOs of the more sulfur‐rich copper complexes, particularly those with methylene linkers (? N? CH₂? S? ), show significant aromatic thioether character. Thus, the theoretically predicted initial oxidation at the sulfur atom of the methylene‐bridged ligands agrees with the experimentally determined oxidation waves in the voltammograms of the NS₃‐ and N₂S₂‐type ligands as being ligand‐based, as opposed to the copper‐based processes of the ethylene‐bridged Cu⁺ complexes. The electrochemical and theoretical results are consistent with our previously reported mechanistic proposal for Cu²⁺‐promoted oxidative C? S bond cleavage, which in this work resulted in the isolation and complete characterization (including by X‐ray crystallography) of the decomposition products of two ligands employed, further supporting the novel reactivity pathway invoked. The combined results raise the possibility that the reactions of copper–thioether complexes in chemical and biochemical systems occur with redox participation of the sulfur atom.     

Ligand Effects on the Electronic Structure of Heteroleptic Antimony‐Centered Radicals

We report on the structures of three unprecedented heteroleptic Sb‐centered radicals [L(Cl)Ga](R)Sb^. ( 2‐R , R=B[N(Dip)CH]₂ 2‐B , 2,6‐Mes₂C₆H₃ 2‐C , N(SiMe₃)Dip 2‐N ) stabilized by one electropositive metal fragment [L(Cl)Ga] (L=HC[C(Me)N(Dip)]₂, Dip=2,6‐i‐Pr₂C₆H₃) and one bulky B‐ ( 2‐B ), C‐ ( 2‐C ), or N‐based ( 2‐N ) substituent. Compounds 2‐R are predominantly metal‐centered radicals. Their electronic properties are largely influenced by the electronic nature of the ligands R, and significant delocalization of unpaired‐spin density onto the ligands was observed in 2‐B and 2‐N . Cyclic voltammetry (CV) studies showed that 2‐B undergoes a quasi‐reversible one‐electron reduction, which was confirmed by the synthesis of [K([2.2.2]crypt)][L(Cl)GaSbB[N(Dip)CH]₂] ([K([2.2.2]crypt)][ 2‐B ]) containing the stibanyl anion [ 2‐B ]^?, which was shown to possess significant Sb?B multiple‐bonding character.     

Ligand π‐Radical Interaction with f‐Shell Unpaired Electrons in Phthalocyaninato–Lanthanoid Single‐Molecule Magnets: A Solution NMR Spectroscopic and DFT Study

The phthalocyaninato double‐decker complexes [M(obPc)₂]⁰ (M= Y^III, Tb^III, Dy^III; obPc=2,3,9,10,16,17,23,24‐octabutoxyphthalocyaninato), along with their reduced ([M(obPc)₂]^?[P(Ph)₄]⁺; M=Tb^III, Dy^III) and oxidized ([M(obPc)₂]⁺[SbCl₆]^? (M=Y^III, Tb^III) counterparts were studied with ¹H, ¹³C and 2D NMR. From the NMR data of the neutral (i.e., with one unpaired electron in the ligands) and anionic Tb^III complexes, along with the use of dispersion corrected DFT methods, it was possible to separate the metal‐centered and ligand‐centered contributions to the hyperfine NMR shift. These contributions to the ¹H and ¹³C hyperfine NMR shifts were further analyzed in terms of pseudocontact and Fermi contact shifts. Furthermore, from a combination of NMR data and DFT calculations, we have determined the spin multiplicity of the neutral complexes [M(obPc)₂]⁰ (M=Tb^III and Dy^III) at room temperature. From the NMR data of the cationic Tb^III complex, for which actually no experimental structure determination is available, we have analyzed the structural changes induced by oxidation from its neutral/anionic species and shown that the interligand distance decreases upon oxidation. The fast electron exchange process between the neutral and anionic Tb^III double‐decker complexes was also studied.     

Porphyrins Fused to N‐Heterocyclic Carbenes (NHCs): Modulation of the Electronic and Catalytic Properties of NHCs by the Central Metal of the Porphyrin

We report herein a detailed study of the use of porphyrins fused to imidazolium salts as precursors of N‐heterocyclic carbene ligands 1 M . Rhodium(I) complexes 6 M – 9 M were prepared by using 1 M ligands with different metal cations in the inner core of the porphyrin (M=Ni^II, Zn^II, Mn^III, Al^III, 2H). The electronic properties of the corresponding N‐heterocyclic carbene ligands were investigated by monitoring the spectroscopic changes occurring in the cod and CO ancillary ligands of [( 1 M )Rh(cod)Cl] and [( 1 M )Rh(CO)₂Cl] complexes (cod=1,5‐cyclooctadiene). Porphyrin–NHC ligands 1 M with a trivalent metal cation such as Mn^III and Al^III are overall poorer electron donors than porphyrin–NHC ligands with no metal cation or incorporating a divalent metal cation such as Ni^II and Zn^II. Imidazolium salts 3 M (M=Ni, Zn, Mn, 2H) have also been used as NHC precursors to catalyze the ring‐opening polymerization of L ‐lactide. The results clearly show that the inner metal of the porphyrin has an important effect on the reactivity of the outer carbene.     

Synthesis,Structure, and Bonding of d3 Molybdenum–Oxo Complexes

Reduction of d² metal–oxo ions of the form [MO(PP)₂Cl]⁺ (M=Mo, W; PP=chelating diphosphine) produces d³ MO(PP)₂Cl complexes, which include the first isolated examples in group 6. The stability and reactivity of the MO(PP)₂Cl compounds are found to depend upon the steric bulk of the phosphine ligands: derivatives with bulky phosphines that shield the oxo ligand are stable enough to be isolated, whereas those with phosphines that leave the oxo ligand exposed are more reactive and observed transiently. Magnetic measurements and DFT calculations on MoO(dppe)₂Cl indicate the d³ compounds are low spin with a ²[(d_xy)²(π*(MoO))¹] configuration. X‐ray crystallographic and vibrational‐spectroscopic studies on d² and d³ [MoO(dppe)₂Cl]^0/+ establish that the d³ compound possesses a reduced M?O bond order and significantly longer Mo?O bond, accounting for its greater reactivity. These results indicate that the oxo‐centered reactivity of d³ complexes may be controlled through ligand variation.     

Correlating Structure and KA2 Catalytic Activity of ZnII Hydrazone Complexes

Two new Zn(II) complexes bearing tridentate hydrazone-based ligands with NNO or NNS donor atoms were synthesised and characterised by elemental analysis, infrared (IR) and nuclear magnetic resonance (NMR) spectroscopies, and single crystal X-ray diffraction methods. These complexes, together with four previously synthesised analogues, having hydrazone ligands with a NNO donor set of atoms, were successfully employed as catalysts in the ketone-amine-alkyne (KA²) coupling reaction, furnishing tetrasubstituted propargylamines, compounds with unique applications in organic chemistry. DFT calculations at the CAM-B3LYP/TZP level of theory were performed to elucidate the electronic structure of the investigated Zn(II) complexes, excellently correlating the structure of the complexes to their catalytic reactivity.     

Stabilized Carbenium Ions as Latent,Z‐type Ligands

Controlling the reactivity of transition metals using secondary, σ‐accepting ligands is an active area of investigation that is impacting molecular catalysis. Herein we describe the phosphine gold complexes [(o‐Ph₂P(C₆H₄)Acr)AuCl]⁺ ([ 3 ]⁺; Acr=9‐N‐methylacridinium) and [(o‐Ph₂P(C₆H₄)Xan)AuCl]⁺ ([ 4 ]⁺; Xan=9‐xanthylium) where the electrophilic carbenium moiety is juxtaposed with the metal atom. While only weak interactions occur between the gold atom and the carbenium moiety of these complexes, the more Lewis acidic complex [ 4 ]⁺ readily reacts with chloride to afford a trivalent phosphine gold dichloride derivative ( 7 ) in which the metal atom is covalently bound to the former carbocationic center. This anion‐induced Au^I/Au^III oxidation is accompanied by a conversion of the Lewis acidic carbocationic center in [ 4 ]⁺ into an X‐type ligand in 7 . We conclude that the carbenium moiety of this complex acts as a latent Z‐type ligand poised to increase the Lewis acidity of the gold center, a notion supported by the carbophilic reactivity of these complexes.     

Stabilized Carbenium Ions as Latent,Z‐type Ligands

Controlling the reactivity of transition metals using secondary, σ‐accepting ligands is an active area of investigation that is impacting molecular catalysis. Herein we describe the phosphine gold complexes [(o‐Ph₂P(C₆H₄)Acr)AuCl]⁺ ([ 3 ]⁺; Acr=9‐N‐methylacridinium) and [(o‐Ph₂P(C₆H₄)Xan)AuCl]⁺ ([ 4 ]⁺; Xan=9‐xanthylium) where the electrophilic carbenium moiety is juxtaposed with the metal atom. While only weak interactions occur between the gold atom and the carbenium moiety of these complexes, the more Lewis acidic complex [ 4 ]⁺ readily reacts with chloride to afford a trivalent phosphine gold dichloride derivative ( 7 ) in which the metal atom is covalently bound to the former carbocationic center. This anion‐induced Au^I/Au^III oxidation is accompanied by a conversion of the Lewis acidic carbocationic center in [ 4 ]⁺ into an X‐type ligand in 7 . We conclude that the carbenium moiety of this complex acts as a latent Z‐type ligand poised to increase the Lewis acidity of the gold center, a notion supported by the carbophilic reactivity of these complexes.     

Pyridylalkylamine ligands and their palladium complexes: structure and reactivity revisited by NMR

Pyridylmethylamines or pma are versatile platforms for different catalytic transformations. Five pma‐ligands and their respective Pd complexes have been studied by liquid state NMR. By comparing ¹H, ¹³C and ¹⁵N chemical shifts for each pma/pma–Pd couple, a general trend for the metallacycle atoms concerns variations of the electronic distribution at the pendant arm, especially at the nitrogen atom of the ligand. Moreover, the increase of the chemical shift of the pendant arm nitrogen atom from primary to tertiary amine is also related to the increase of crowding within the complex. This statement is in good agreement with X‐ray data collected for several complexes. Catalytic results for the Suzuki–Miyaura reaction involving the pma–Pd complexes showed within this series that a sterically crowded and electron‐rich ligand in the metallacycle was essential to reach the coupling product with a good selectivity. In this context, NMR study of chemical shifts of all active nuclei especially in the metallacycle could give a trend of reactivity in the studied family of pma–Pd complexes. Copyright © 2014 John Wiley & Sons, Ltd.     

Catalytic Activity of Cationic and Neutral Silver(I)–XPhos Complexes with Nitrogen Ligands or Tolylsulfonate for Mannich and Aza‐Diels–Alder Coupling Reactions

Cationic and neutral silver(I)–L complexes (L=Buchwald‐type biaryl phosphanes) with nitrogen co‐ligands or organosulfonate counter ions have been synthesised and characterised through their structural and spectroscopic properties. At room temperature, both cationic and neutral silver(I)–L complexes are extremely active catalysts in the promotion of the single and double A³ coupling of terminal (di)alkynes, pyrrolidine and formaldehyde. In addition, the aza‐Diels–Alder two‐ and three‐component coupling reactions of Danishefsky’s diene with an imine or amine and aldehyde are efficiently catalysed by these cationic or neutral silver(I)–L complexes. The solvent influences the catalytic performance due to limited complex solubility or solvent decomposition and reactivity. The isolation of new silver(I)–L complexes with reagents as ligands lends support to mechanistic proposals for such catalytic processes. The activity, stability and metal–distal arene interaction of these silver(I)–L catalysts have been compared with those of analogous cationic gold(I) and copper(I) complexes.     

RuII,OsII, and IrIII Complexes with Chelating Pyridyl–Mesoionic Carbene Ligands: Structural Characterization and Applications in Transfer Hydrogenation Catalysis

Chelating ligands with one pyridine donor and one mesoionic carbene donor are fast establishing themselves as privileged ligands in homogeneous catalysis. The synthesis of several new Ir^III–Cp*‐ and Os^II–Cym complexes (Cp*=pentamethylcyclopentadienyl, Cym=p‐cymene=4‐isopropyl‐toluene) derived from chelating pyridyltriazolylidenes where the additional pyridine donor was incorporated via the azide part of the triazole is presented. Furthermore, different 4‐substituted phenylacetylene building blocks have been used to introduce electronic fine‐tuning in the ligands. The ligands thus can be generally described as 4‐(4‐R‐phenyl)‐3‐methyl‐1‐(pyridin‐2‐yl)‐1H‐1,2,3‐triazol‐5‐ylidene (with R being H (L¹), Me (L²), OMe (L³), CN (L⁴), CF₃ (L⁵), Br (L⁶) or NO₂ (L⁷)). The corresponding complexes (Ir‐ 1 to Ir‐ 7 and Os‐ 1 to Os‐ 7 ) were characterized by standard spectroscopic methods, and the expected three‐legged, piano‐stool type coordination was unambiguously confirmed by X‐ray diffraction analysis of selected compounds. Together with Ru^II analogues previously reported by us, a total of 21 complexes were tested as (pre)catalysts for the transfer hydrogenation of carbonyl groups, showing a remarkable reactivity even at very low catalyst loadings. The electronic effects of the ligands as well as different substrates were investigated. Some mechanistic elucidations are also presented.     

Next‐Generation D2‐Symmetric Chiral Porphyrins for Cobalt(II)‐Based Metalloradical Catalysis: Catalyst Engineering by Distal Bridging

Novel D₂‐symmetric chiral amidoporphyrins with alkyl bridges across two chiral amide units on both sides of the porphyrin plane (designated “HuPhyrin”) have been effectively constructed in a modular fashion to permit variation of the bridge length. The Co^II complexes of HuPhyrin, [Co(HuPhyrin)], represent new‐generation metalloradical catalysts where the metal‐centered d‐radical is situated inside a cavity‐like ligand with a more rigid chiral environment and enhanced hydrogen‐bonding capability. As demonstrated with cyclopropanation and aziridination as model reactions, the bridged [Co(HuPhyrin)] functions notably different from the open catalysts, exhibiting significant enhancement in both reactivity and stereoselectivity. Furthermore, the length of the distal alkyl bridge can have a remarkable influence on the catalytic properties.     

Multi‐faceted Reactivity of Alkyltellurophenols Towards Peroxyl Radicals: Catalytic Antioxidant Versus Thiol‐Depletion Effect

Hydroxyaryl alkyl tellurides are effective antioxidants both in organic solution and aqueous biphasic systems. They react by an unconventional mechanism with ROO^. radicals with rate constants as high as 10⁷ M ^?1 s^?1 at 303 K, outperforming common phenols. The reactions proceed by oxygen atom transfer to tellurium followed by hydrogen atom transfer to the resulting RO^. radical from the phenolic OH. The reaction rates do not reflect the electronic properties of the ring substituents and, because the reactions occur in a solvent cage, quenching is more efficient when the OH and TeR groups have an ortho arrangement. In the presence of thiols, hydroxyaryl alkyl tellurides act as catalytic antioxidants towards both hydroperoxides (mimicking the glutathione peroxidases) and peroxyl radicals. The high efficiency of the quenching of the peroxyl radicals and hydroperoxides could be advantageous under normal cellular conditions, but pro‐oxidative (thiol depletion) when thiol concentrations are low.     

The Effect of Substituents on Catalytic Performance of <Emphasis Type="Italic">bis</Emphasis>-Thiosemicarbazone Mo(VI) Complexes: Synthesis and Spectroscopic,Electrochemical, and Functional Properties

The preparation, characterization and electrochemical properties are reported for three new types of molybdenum(VI) complexes with bis-thiosemicarbazone ligands. All compounds were characterized by elemental analysis, electronic spectra, IR and ¹H NMR spectroscopies, thermogravimetric analysis, and cyclic voltammetry. The bis-thiosemicarbazone Mo(VI) complexes were tested as a catalyst for the homogeneous oxidation of olefins using tert-butyl hydrogen peroxide as an oxidant. The catalysts showed efficient reactivity in the olefins epoxidation reactions giving high yield and selectivity of the products, in most cases. Results showed that the bis-thiosemicarbazone ligands introduced both electronic and steric effects on catalytic performance of the prepared Mo(VI) complexes.     

Transition‐Metal Chemistry of Alkaline‐Earth Elements: The Trisbenzene Complexes M(Bz)3 (M=Sr,Ba)

We report the synthesis and spectroscopic identification of the trisbenzene complexes of strontium and barium M(Bz)₃ (M=Sr, Ba) in low‐temperature Ne matrix. Both complexes are characterized by a D₃ symmetric structure involving three equivalent η⁶‐bound benzene ligands and a closed‐shell singlet electronic ground state. The analysis of the electronic structure shows that the complexes exhibit metal–ligand bonds that are typical for transition metal compounds. The chemical bonds can be explained in terms of weak donation from the π MOs of benzene ligands into the vacant (n?1)d AOs of M and strong backdonation from the occupied (n?1)d AO of M into vacant π* MOs of benzene ligands. The metals in these 20‐electron complexes have 18 effective valence electrons, and, thus, fulfill the 18‐electron rule if only the metal–ligand bonding electrons are counted. The results suggest that the heavier alkaline earth atoms exhibit the full bonding scenario of transition metals.     

Transition‐Metal Chemistry of Alkaline‐Earth Elements: The Trisbenzene Complexes M(Bz)3 (M=Sr,Ba)

We report the synthesis and spectroscopic identification of the trisbenzene complexes of strontium and barium M(Bz)₃ (M=Sr, Ba) in low‐temperature Ne matrix. Both complexes are characterized by a D₃ symmetric structure involving three equivalent η⁶‐bound benzene ligands and a closed‐shell singlet electronic ground state. The analysis of the electronic structure shows that the complexes exhibit metal–ligand bonds that are typical for transition metal compounds. The chemical bonds can be explained in terms of weak donation from the π MOs of benzene ligands into the vacant (n?1)d AOs of M and strong backdonation from the occupied (n?1)d AO of M into vacant π* MOs of benzene ligands. The metals in these 20‐electron complexes have 18 effective valence electrons, and, thus, fulfill the 18‐electron rule if only the metal–ligand bonding electrons are counted. The results suggest that the heavier alkaline earth atoms exhibit the full bonding scenario of transition metals.     

A mass spectrometry and DFT study of pyrithione complexes with transition metals in the gas phase

2‐Mercaptopyridine N ‐oxide (pyrithione, PTOH) along with several transition metal ions forms coordination compounds displaying notable biological activities. Gas‐phase complexes formed between pyrithione and manganese (II), cobalt (II), nickel (II), copper (II), and zinc (II) were investigated by infusion in the electrospray source of a quadrupole‐time of flight mass spectrometer. Remarkably, positive ion mode spectra displayed the singly charged metal adduct ion [C₁₀H₈MN₂O₂S₂]²⁺ ([M(PTO)₂]^+• or [M(DPTO)]^+•), where DPTO is dipyrithione, 2,2′‐dithiobis(pyridine N ‐oxide), among the most abundant peaks, implying a change in the oxidation state of whether the metal ion or the ligands. In addition, doubly charged ions were recognized as metal adduct ions containing DPTO ligands, [M(DPTO)_n]²⁺. Generation of [M(PTO)₂]^+• / [M(DPTO)]^+• could be traced by CID of [M(DPTO)₂]²⁺, by observation of the sequential losses of a charged (PTO⁺) and a radical (PTO^•) deprotonated pyrithione ligand. The fragmentation pathways of [M(PTO)₂]^+• / [M(DPTO)]^+• were compared among the different metal ions, and some common features were noticed. Density functional theory (DFT) calculations were employed to study the structures of the observed adduct ions, and especially, to decide in the adduct ion [M(PTO)₂]^+• / [M(DPTO)]^+• whether the ligands are 2 deprotonated pyrithiones or a single dipyrithione as well as the oxidation state of the metal ion in the complex. Characterization of gas‐phase pyrithione metal ion complexes becomes important, especially taking into account the presence of a redox‐active ligand in the complexes, because redox state changes that produce new species can have a marked effect on the overall toxicological/biological response elicited by the metal system.