Dinuclear Palladium Complexes of Pyrazole-Bridged Bis(NHC) Ligands: A Delicate Balance between Normal and Abnormal Carbene Coordination

Pyrazole-bridged bis­(imidazolium) salts [H₃<b>L</b>^<b>R</b>]­(PF₆)₂ (R = Et, ^<i>n</i>Bu, C₆H₂Me₃-2,4,6, and ^<i>t</i>Bu), which are precursors to the corresponding pyrazole-bridged bis­(N-heterocyclic carbene) ligands, were found to give, upon reaction with Pd­(OAc)₂, dinuclear palladium complexes with either normal or abnormal binding modes of the two NHC groups: via C2 in [<b>L</b>^<b>R</b>₂Pd₂]­(PF₆)₂ (except R = ^<i>t</i>Bu) and via C4/5 in [^a<b>L</b>^<b>R</b>₂Pd₂]­(PF₆)₂. It has been shown that the course of the reaction crucially depends on the amount of NH₄OAc added, suggesting an acetate-assisted pathway leading to [<b>L</b>^<b>R</b>₂Pd₂]­(PF₆)₂. Further reaction of [<b>L</b>^<b>R</b>₂Pd₂]­(PF₆)₂ and [^a<b>L</b>^<b>R</b>₂Pd₂]­(PF₆)₂ with PdCl₂ and NEt₄Cl gave the corresponding neutral dinuclear complexes <b>L</b>^<b>Et</b>Pd₂Cl₃ and ^a<b>L</b>^<b>Et</b>Pd₂Cl₃ selectively, without any normal/abnormal rearrangement occurring during transmetalation. Only ^a<b>L</b>^<b>tBu</b>Pd₂Cl₃ is accessible directly from [H₄<b>L</b>^{<i><b>t</b></i><b>Bu</b>}]­Cl₃ and Pd­(OAc)₂. All complexes have been characterized by NMR spectroscopy and elemental analysis, and several of them have also been characterized by ESI mass spectrometry and single-crystal X-ray diffraction. The observed binding modes and structural features have been rationalized by density functional theory calculations, which evidence that for a given complex the thermodynamically favored conformer is found in the solid state

Dinuclear Palladium Complexes of Pyrazole-Bridged Bis(NHC) Ligands: A Delicate Balance between Normal and Abnormal Carbene Coordination

Pyrazole-bridged bis­(imidazolium) salts [H₃<b>L</b>^<b>R</b>]­(PF₆)₂ (R = Et, ^<i>n</i>Bu, C₆H₂Me₃-2,4,6, and ^<i>t</i>Bu), which are precursors to the corresponding pyrazole-bridged bis­(N-heterocyclic carbene) ligands, were found to give, upon reaction with Pd­(OAc)₂, dinuclear palladium complexes with either normal or abnormal binding modes of the two NHC groups: via C2 in [<b>L</b>^<b>R</b>₂Pd₂]­(PF₆)₂ (except R = ^<i>t</i>Bu) and via C4/5 in [^a<b>L</b>^<b>R</b>₂Pd₂]­(PF₆)₂. It has been shown that the course of the reaction crucially depends on the amount of NH₄OAc added, suggesting an acetate-assisted pathway leading to [<b>L</b>^<b>R</b>₂Pd₂]­(PF₆)₂. Further reaction of [<b>L</b>^<b>R</b>₂Pd₂]­(PF₆)₂ and [^a<b>L</b>^<b>R</b>₂Pd₂]­(PF₆)₂ with PdCl₂ and NEt₄Cl gave the corresponding neutral dinuclear complexes <b>L</b>^<b>Et</b>Pd₂Cl₃ and ^a<b>L</b>^<b>Et</b>Pd₂Cl₃ selectively, without any normal/abnormal rearrangement occurring during transmetalation. Only ^a<b>L</b>^<b>tBu</b>Pd₂Cl₃ is accessible directly from [H₄<b>L</b>^{<i><b>t</b></i><b>Bu</b>}]­Cl₃ and Pd­(OAc)₂. All complexes have been characterized by NMR spectroscopy and elemental analysis, and several of them have also been characterized by ESI mass spectrometry and single-crystal X-ray diffraction. The observed binding modes and structural features have been rationalized by density functional theory calculations, which evidence that for a given complex the thermodynamically favored conformer is found in the solid state

Convenient Synthetic Route to Palladium Complexes of Unconventional N‑Heterocyclic Carbenes Derived from Pyridazine and Phthalazine

Several Pd­(II) complexes with unconventional pyridazine- and phthalazine-derived carbene ligands were synthesized via direct oxidative addition of Cl derivatives of the alkylated diazine heterocycles to Pd(0) species. The alkylated ligand precursors are readily prepared from commercial starting materials, and oxidative addition is regioselective. DFT calculations confirm that the thermodynamically favored products are formed. Four complexes (<b>1</b>–<b>4</b>) have been fully characterized, including by X-ray crystallography. Attractive intramolecular π–π stacking between the electron-poor N-alkylated diazine heterocycles and adjacent phenyl groups of the PPh₃ coligands is revealed by the solid-state structures

Gold(I), Gold(III), and Heterometallic Gold(I)–Silver(I) and Gold(I)–Copper(I) Complexes of a Pyridazine-Bridged NHC/Pyrazole Hybrid Ligand and Their Initial Application in Catalysis

The pyridazine-bridged NHC/pyrazole ligand L (HL = 3-[3-(2,4,6-trimethylphenyl)-3<i>H</i>-imidazolium-1-yl]-6-(3,5-dimethylpyrazol-1-yl)-pyridazine) that provides an organometallic and a classical N-donor compartment is shown to serve as a versatile scaffold for a variety of homo- and heterometallic gold­(I) carbene complexes. Complexes [LAuX] (<b>1</b>^<b>Cl</b>, X = Cl; <b>1</b>^<b>Br</b>, X = Br), [L₂Au]­(PF₆) (<b>2</b>), [L₂AuAg]­(BF₄)­(PF₆) (<b>3</b>), [L₂AuAg₃(MeCN)₆]­(BF₄)₄ (<b>5</b>), and [L₂AuCu]­(OTf)_0.75(PF₆)_1.25 (<b>6</b>) have been characterized by X-ray crystallography. In all cases Au­(I) binds to the NHC site while the additional Ag­(I) in <b>3</b> or Cu­(I) in <b>6</b> is accommodated in the pyrazole-derived site. Both <b>3</b> and <b>6</b> form two-stranded helical structures; racemization of the <i>P</i> and <i>M</i> enantiomers is much more facile in the Ag­(I) case <b>3</b> but has a barrier of around 65 kJ/mol in the Cu­(I) case <b>6</b>, which is rationalized on the basis of the different coordination chemistry preferences of these two metal ions. <b>3</b> may bind two further Ag­(I) ions to the central pyridazine N, giving <b>5</b>. Treatment of <b>1</b>^<b>Br</b> with Br₂ leads to bromination at the pyrazole C⁴ of the ligand backbone, yielding [L^BrAuBr] (<b>8</b>). In contrast, <b>1</b>^<b>Cl</b> could be successfully oxidized to the Au­(III) complex [LAuCl₃] (<b>7</b>) using PhICl₂; both <b>7</b> and the gold­(I) complex <b>8</b> have been characterized crystallographically. Preliminary screening shows that <b>7</b>, in combination with AgBF₄, is a good catalyst for the etherification of 1-indanol with a variety of alcohol substrates and shows significantly higher activity than the gold­(I) catalyst <b>1</b>^<b>Cl</b>

Gold(I), Gold(III), and Heterometallic Gold(I)–Silver(I) and Gold(I)–Copper(I) Complexes of a Pyridazine-Bridged NHC/Pyrazole Hybrid Ligand and Their Initial Application in Catalysis

The pyridazine-bridged NHC/pyrazole ligand L (HL = 3-[3-(2,4,6-trimethylphenyl)-3<i>H</i>-imidazolium-1-yl]-6-(3,5-dimethylpyrazol-1-yl)-pyridazine) that provides an organometallic and a classical N-donor compartment is shown to serve as a versatile scaffold for a variety of homo- and heterometallic gold­(I) carbene complexes. Complexes [LAuX] (<b>1</b>^<b>Cl</b>, X = Cl; <b>1</b>^<b>Br</b>, X = Br), [L₂Au]­(PF₆) (<b>2</b>), [L₂AuAg]­(BF₄)­(PF₆) (<b>3</b>), [L₂AuAg₃(MeCN)₆]­(BF₄)₄ (<b>5</b>), and [L₂AuCu]­(OTf)_0.75(PF₆)_1.25 (<b>6</b>) have been characterized by X-ray crystallography. In all cases Au­(I) binds to the NHC site while the additional Ag­(I) in <b>3</b> or Cu­(I) in <b>6</b> is accommodated in the pyrazole-derived site. Both <b>3</b> and <b>6</b> form two-stranded helical structures; racemization of the <i>P</i> and <i>M</i> enantiomers is much more facile in the Ag­(I) case <b>3</b> but has a barrier of around 65 kJ/mol in the Cu­(I) case <b>6</b>, which is rationalized on the basis of the different coordination chemistry preferences of these two metal ions. <b>3</b> may bind two further Ag­(I) ions to the central pyridazine N, giving <b>5</b>. Treatment of <b>1</b>^<b>Br</b> with Br₂ leads to bromination at the pyrazole C⁴ of the ligand backbone, yielding [L^BrAuBr] (<b>8</b>). In contrast, <b>1</b>^<b>Cl</b> could be successfully oxidized to the Au­(III) complex [LAuCl₃] (<b>7</b>) using PhICl₂; both <b>7</b> and the gold­(I) complex <b>8</b> have been characterized crystallographically. Preliminary screening shows that <b>7</b>, in combination with AgBF₄, is a good catalyst for the etherification of 1-indanol with a variety of alcohol substrates and shows significantly higher activity than the gold­(I) catalyst <b>1</b>^<b>Cl</b>

Oxidation States, Stability, and Reactivity of Organoferrate Complexes

We have applied a combination of electrospray-ionization mass spectrometry, electrical conductivity measurements, and Mössbauer spectroscopy to identify and characterize the organoferrate species R_<i>n</i>Fe_<i>m</i>^– formed upon the transmetalation of iron precursors (Fe­(acac)₃, FeCl₃, FeCl₂, Fe­(OAc)₂) with Grignard reagents RMgX (R = Me, Et, Bu, Hex, Oct, Dec, Me₃SiCH₂, Bn, Ph, Mes, 3,5-(CF₃)₂-C₆H₃; X = Cl, Br) in tetrahydrofuran. The observed organoferrates show a large variety in their aggregation (1 ≤ <i>m</i> ≤ 8) and oxidation states (I to IV), which are chiefly determined by the nature of their organyl groups R. In numerous cases, the addition of a bidentate amine or phosphine changes the distributions of organoferrates and affects their stability. Besides undergoing efficient intermolecular exchange processes, several of the probed organoferrates react with organyl (pseudo)­halides R′X (R′ = Et, ^<i>i</i>Pr, Bu, Ph, <i>p</i>-Tol; X = Cl, Br, I, OTf) to afford heteroleptic complexes of the type R₃FeR′^–. Gas-phase fragmentation of most of these complexes results in reductive eliminations of the coupling products RR′ (or, alternatively, of R₂). This finding indicates that iron-catalyzed cross-coupling reactions may proceed via such heteroleptic organoferrates R₃FeR′^– as intermediates. Gas-phase fragmentation of other organoferrate complexes leads to β-hydrogen eliminations, the loss of arenes, and the expulsion of organyl radicals. The operation of both one- and two-electron processes is consistent with previous observations and contributes to the formidable complexity of organoiron chemistry

Proton-Induced, Reversible Interconversion of a μ‑1,2-Peroxo and a μ‑1,1-Hydroperoxo Dicopper(II) Complex

The μ-1,2-peroxo dicopper­(II) complex (<b>2</b>) of a compartmental bis­(tetradentate) pyrazolate-based ligand is shown to convert, upon protonation, to the corresponding μ-1,1-hydro­peroxo dicopper­(II) complex (<b>3</b>). The transformation is cleanly reversed with base, and an apparent p<i>K</i>_a = 22.2 ± 0.3 for the Cu₂OOH unit in MeCN has been determined. The unprecedented stability of <b>3</b> (<i>t</i>_1/2 = 9 h in nitrile solvents at room temperature, giving the hydroxo-bridged dicopper complex) has allowed for its structural characterization by X-ray diffraction. While the O–O bond length (1.462(3) Å) barely changes upon protonation from <b>2</b> to <b>3</b>, the O–O stretching frequency is much higher in the hydro­peroxo complex <b>3</b> (860 cm^–1). <b>3</b> mediates 2e^– oxo transfer to the nucleophilic substrate PPh₃ but is not activated for H-atom abstraction

1,1′-Bis(pyrazol-3-yl)ferrocene: A Clip Ligand That Forms Supramolecular Aggregates and Prismatic Hexanuclear Coinage Metal Complexes

Two ferrocene derivatives with appended pyrazole substituents, namely, 1,1′-bis­(5-methyl-1<i>H</i>-pyrazol-3-yl)­ferrocene (H₂L^H) and 1,1′-bis­(5-trifluoromethyl-1<i>H</i>-pyrazol-3-yl)­ferrocene (H₂L^F), were synthesized. In solid state they form distinct H-bonded dimers with orthogonal (H₂L^H, <i>C</i>₂ symmetry) or antiparallel (H₂L^F, <i>C</i>_2<i>h</i> symmetry) arrangement of the two ferrocene/pyrazole hybrid molecules. Supramolecular dimerization was also detected in solution at low temperatures, though diffusion-ordered spectroscopy and variable-temperature NMR spectroscopy revealed several dynamic processes. Redox potentials of the ferrocene derivatives are affected by the nature of the pyrazole substituent (Me, CF₃). In their deprotonated form [L^R]^2–, both ferrocene/pyrazole hybrids serve as ligands and form oligonuclear Cu^I, Ag^I, and Au^I complexes that were identified by matrix-assisted laser desorption ionization mass spectrometry. X-ray crystallography revealed the structures of Cu₆L₃^H and Ag₆L₃^F, which both contain two parallel and eclipsed [M­(μ-pz)]₃ metallamacrocycles (M = Cu, Ag) linked by three ferrocene units. M^I···M^I distances between the two triangular M₃N₆ decks are shorter in Ag₆L₃^F (3.28–3.30 vs 3.44–3.51 Å in the case of Cu₆L₃^H), indicating substantial <i>intra</i>molecular closed-shell Ag­(d¹⁰)–Ag­(d¹⁰) interactions. However, Cu₆L₃^H features close <i>inter</i>molecular Cu···Cu contacts as short as 3.37 Å. Mössbauer data for both the ligands and complexes were collected, and electrochemical properties were measured; preliminary luminescence data are reported

Crowning of Coinage Metal Pyrazolates: Double-Decker Homo- and Heteronuclear Complexes with Synergic Emissive Properties

A new pyrazole ligand with flexible thioether chelate arms was synthesized and was used to obtain an unprecedented class of hexanuclear coinage metal complexes of general formula [MM′L]₃Y₃ (M, M′ = Cu, Ag, Au; Y = OTf, BF₄). Three of them were characterized by X-ray crystallography, namely, homometallic [Ag₂L]₃(OTf)₃ and [Ag₂L]₃(BF₄)₃ as well as heterometallic [CuAgL]₃(OTf)₃, revealing that the classical [M­(μ-pz)]₃ core is crowned by a second deck of S-bound M′ ions. Depending on the solvent, these oligonuclear systems undergo rapid dynamics and show cation–anion aggregation in solution, which has been investigated by DOSY and temperature dependent NMR spectroscopy. Preliminary luminescence data for selected hexametallic [MM′L]₃Y₃ complexes show that the combination of ligand-directed intramolecular and supramolecular d¹⁰ metal ion interactions in the solid state gives rise to synergic emissive properties that allow for a selective addressing of different emission wavelengths

Reaching across the Divide: How Monometalation of One Binding Pocket Affects the Empty Binding Pocket in a Siamese-Twin Porphyrin Palladium Complex

Siamese-twin porphyrin is a pyrazole-containing expanded porphyrin incorporating two porphyrin-like binding pockets. The macrocycle, however, does not possess an aromatic π system but rather two separated conjugation pathways that are isolated by the pyrazole junctions. Mono- and bimetallic complexes of the Siamese-twin porphyrin are known. This work addresses in detail the electronic consequences that monometalation (with Pd^II) has on the electronic properties of the nonmetalated binding pocket by studying the solid-state structure, acid/base, and electrochemical properties of the monopalladium twin-porphyrin complex. Specifically, metalation leads to a switch of the protonation sites of the free-base pocket. The unusual location of the protons at adjacent pyrrolic nitrogen atoms was revealed using X-ray diffraction and 1D/2D NMR spectroscopy. The one-electron oxidation and reduction events are both ligand-centered, as derived by spectroelectrochemical and electron paramagnetic resonance measurements, but are located on different halves of the molecule. Single-electron oxidation (−0.32 V vs Fc/Fc⁺) generated an organic radical centered on the metal-coordinating side of the ligand, while single-electron reduction (−1.59 V vs Fc/Fc⁺) led to the formation of an organic radical on the free-base side of the macrocycle. Density functional theory calculations corroborated the redox chemistry observed. The possibility of selectively preparing the monometallic complexes carrying two distinct redox sitesa metal-containing oxidation site and a metal-free reduction sitefurther expands the potential of Siamese-twin porphyrins to serve as an adjustable platform for multielectron redox processes in chemical catalysis or molecular electronics applications