Structurally Similar, Thermally Stable Copper(I), Silver(I), and Gold(I) Ethylene Complexes Supported by a Fluorinated Scorpionate

Going for gold - the chemistry of structurally authenticated gold(I)-ethylene complexes

Gold coordination chemistry and catalysis involving unsaturated hydrocarbons such as olefins have experienced a remarkable growth during the last few decades. Despite the importance, isolable and well-characterized molecules with ethylene, the simplest and the most widely produced olefin, on gold are still limited. This review aims to cover features of, and strategies utilized to stabilize, gold-ethylene complexes and their diverse use in chemical transformations and homogeneous catalytic processes. Isolable and well-authenticated gold-ethylene complexes are important not only for structural, spectroscopic, and bonding studies but also as models for likely intermediates in gold mediated reactions of alkenes and gold-alkene species observed in the gas phase. There has also been development on AuI/III catalytic cycles. Nitrogen based ligands have been the most widely utilized ligand supports thus far for the successful stabilization of gold-ethylene adducts. Gold has a bright future in olefin chemistry and with ethylene.

In situ studies of reversible solid-gas reactions of ethylene responsive silver pyrazolates

Solid-gas reactions and in situ powder X-ray diffraction investigations of trinuclear silver complexes {[3,4,5-(CF3)3Pz]Ag}3 and {[4-Br-3,5-(CF3)2Pz]Ag}3 supported by highly fluorinated pyrazolates reveal that they undergo intricate ethylene-triggered structural transformations in the solid-state producing dinuclear silver-ethylene adducts. Despite the complexity, the chemistry is reversible producing precursor trimers with the loss of ethylene. Less reactive {[3,5-(CF3)2Pz]Ag}3 under ethylene pressure and low-temperature conditions stops at an unusual silver-ethylene complex in the trinuclear state, which could serve as a model for intermediates likely present in more common trimer-dimer reorganizations described above. Complete structural data of three novel silver-ethylene complexes are presented together with a thorough computational analysis of the mechanism.

Thallium(I) Complexes of Tris(pyridyl)borates and a Comparison to Their Pyrazolyl Analogues

Thallium(I) complexes of B-methylated and B-phenylated tris(pyridyl)borates featuring trifluoromethyl groups at the pyridyl ring 6-positions have been synthesized by metathesis using the corresponding potassium salts [MeB(6-(CF3)Py)3]K and [PhB(6-(CF3)Py)3]K with thallium(I) acetate. The closely related tris(pyrazolyl)borate analogue [PhB(3-(CF3)Pz)3]Tl has also been prepared, and comparisons of structural and spectroscopic features between the two scorpionate families are presented. [MeB(6-(CF3)Py)3]Tl displays κ3-coordination of the tris(pyridyl)borate similar to that of tris(pyrazolyl)borate in [MeB(3-(CF3)Pz)3]Tl, while [PhB(6-(CF3)Py)3]Tl and [PhB(3-(CF3)Pz)3]Tl feature κ2-N,N ligand coordination modes with the B-phenyl groups flanking the thallium sites. 19F NMR spectroscopy of [MeB(6-(CF3)Py)3]Tl reveals the presence of a remarkably large 1208 Hz four-bond thallium-fluorine coupling constant in chloroform at room temperature, which is considerably larger than 878 Hz observed for the pyrazolyl borate analogue [MeB(3-(CF3)Pz)3]Tl. Although [PhB(6-(CF3)Py)3]Tl is structurally nonrigid at room temperature in chloroform, at lower temperatures, the ligand arm exchange slows down, revealing 4JTl-F = 1110 Hz. Steric demands of these ligands have been quantified using the buried volume concept. In addition, ligand transfer chemistry from [MeB(6-(CF3)Py)3]Tl and [PhB(6-(CF3)Py)3]Tl to copper(I) under ethylene and computational analyses of the various coordination modes of tris(pyrazolyl)borates and tris(pyridyl)borates are reported.

Structural diversity of copper(I)-ethylene complexes with 2,4-bis(2-pyridyl)pyrimidine directed by anions

The 3 : 1 reaction of [Cu(C2H4)n]ClO4 with 2,4-bis(2-pyridyl)pyrimidine (bpprd) in Me2CO under C2H4 afforded yellow prism crystals of the dinuclear Cu(I)-C2H4 complex [Cu2(bpprd)(η2-C2H4)2(ClO4)2] (1). The 3 : 1 reaction of [Cu(C2H4)n]NO3 with bpprd in Me2CO under C2H4 afforded yellow plate crystals of the tetranuclear Cu(I)-C2H4 complex [Cu4(bpprd)2(η2-C2H4)4(μ-NO3)2](NO3)2 (2). The 10 : 1 reaction of [Cu(C2H4)n]BF4 with bpprd in Me2CO under C2H4 afforded yellow plate crystals of the dinuclear Cu(I)-C2H4 complex [Cu2(bpprd)(η2-C2H4)2(BF4)]BF4 (3). The 3 : 1 reaction of [Cu(C2H4)n]BF4 with bpprd in Me2CO under C2H4 afforded red prism crystals of the polymeric Cu(I)-C2H4 complex {[Cu6(bpprd)4(η2-C2H4)2(μ-η2:η2-C2H4)(μ-BF4)2](BF4)4}n (4). The X-ray crystal structures of complexes 1-4 have been determined. The structural diversity of Cu(I)-C2H4 complexes bridged by bpprd with different anions was demonstrated. The 1D Cu(I)-bpprd/C2H4 coordination polymer 4 bridged by unusual μ-η2:η2-C2H4 and the μ-BF4- anion is of particular significance. Complex 1 exhibited relatively well-resolved 1H NMR signals of bpprd and C2H4 (δ = 4.97 ppm) in (CD3)2CO at 23 °C.

Copper(I), Silver(I), and Gold(I) Ethylene Complexes of Fluorinated and Boron-Methylated Bis- and Tris(pyridyl)borate Chelators

Bis- and tris-pyridyl borate ligands containing pyridyl donor arms, a methylated boron cap, and a fluorine-lined coordination pocket have been prepared and utilized in coinage metal chemistry. The tris(pyridyl)borate ligand has been synthesized using a convenient boron source, [NBu₄][MeBF₃]. These N-based ligands permitted the isolation of group 11 metal-ethylene complexes [MeB(6-(CF₃)Py)₃]M(C₂H₄) and [Me₂B(6-(CF₃)Py)₂]M(C₂H₄) (M = Cu, Ag, Au). The gold complexes display the largest coordination-induced upfield shifts of the ethylene ¹³C resonance relative to that of the free ethylene in their NMR spectra, while the silver complexes show the smallest shift. Solid-state structures of five of these metal-ethylene complexes as well as the related free ligands were established by X-ray crystallography. Surprisingly, all three [MeB(6-(CF₃)Py)₃]M(C₂H₄) adopt the rare κ² coordination mode rather than the typical κ³ coordination mode of facial capping tridentate ligands. Computational analyses indicate that κ² coordination mode is favored over the κ³-mode in these coinage metal-ethylene complexes and point to the effects CF₃-substituents have on κ²/κ³-energy difference. The M-C and M-N bond distances of [MeB(6-(CF₃)Py)₃]M(C₂H₄) follow the trend expected based on covalent radii of M(I) ions. The calculated ethylene-M interaction energy of κ²-[MeB(6-(CF₃)Py)₃]M(C₂H₄) indicated that the gold(I) forms the strongest interaction with ethylene. A comparison to the related poly(pyrazolyl)borates is also presented.

Ethylene binding in mono- and binuclear CuI complexes with tetradentate pyridinophane ligands

Herein we report a series of Cu^I complexes supported by tetradentate ^RN4 pyridinophane ligands that coordinate to ethylene forming either mononuclear complexes with ethylene coordinated in an η²-mode or a binuclear complex where ethylene binds to two Cu atoms in a μ-η²-η²-mode, depending on the steric effects of the ^RN4 ligand and the reaction conditions. In the binuclear complex with bridging ethylene, the CC bond is significantly elongated, with a bond length of 1.444(8) Å according to X-ray diffraction analysis. This complex represents the only examination a μ-η²-η²-coordinated Cu-olefin complex reported to date, featuring one of the longest reported CC bonds. The spectroscopic characterization, structure, electrochemical properties and solution behavior are analyzed in this study. Coordination of ethylene was found to be reversible in these complexes and more favored in less sterically hindered ^RN4 ligands, so that ethylene binding is observed in a coordinating solvent (MeCN) environment. In the case of the ^MeN4 ligand, the ethylene complex is photoluminescent in the solid state. The ethylene binding modes in mono- and binuclear complexes are elucidated through Natural Bond Orbital and QTAIM analyses.

Dicoordinate Au(I)-Ethylene Complexes as Hydroamination Catalysts

A series of gold(I)–ethylene π-complexes containing a family of bulky phosphine ligands has been prepared. The use of these sterically congested ligands is crucial to stabilize the gold(I)–ethylene bond and prevent decomposition, boosting up their catalytic performance in the highly underexplored hydroamination of ethylene. The precatalysts bearing the most sterically demanding phosphines showed the best results reaching full conversion to the hydroaminated products under notably mild conditions (1 bar of ethylene pressure at 60 °C). Kinetic analysis together with density functional theory calculations revealed that the assistance of a second molecule of the nucleophile as a proton shuttle is preferred even when using an extremely congested cavity-shaped Au(I) complex. In addition, we have measured a strong primary kinetic isotopic effect that is consistent with the involvement of X–H bond-breaking events in the protodeauration turnover-limiting step.

Binding Interactions in Copper, Silver and Gold π-Complexes

The copper(I), silver(I), and gold(I) metals bind π‐ligands by σ‐bonding and π‐back bonding interactions. These interactions were investigated using bidentate ancillary ligands with electron donating and withdrawing substituents. The π‐ligands span from ethylene to larger terminal and internal alkenes and alkynes. Results of X‐ray crystallography, NMR, and IR spectroscopy and gas phase experiments show that the binding energies increase in the order Ag<Cu<Au and the binding energies are slightly higher for alkynes than for alkenes. Modulation of the electron density at the metal using substituents on the ancillary ligands shows that the π‐back bonding interaction plays a dominant role for the binding in the copper and gold complexes.

Fluorinated tris(pyridyl)borate ligand support on coinage metals

A useful ligand involving three pyridyl donor arms and fluorocarbon substituents surrounding the coordination pocket has been assembled and utilized in coinage metal chemistry. This tris(pyridyl)borate serves as an excellent ligand support for the stabilization of ethylene complexes of copper, silver and gold.

An Unsymmetrical, Cyclic Diborene Based on a Chelating CAAC Ligand and its Small-Molecule Activation and Rearrangement Chemistry

A one‐pot synthesis of a CAAC‐stabilized, unsymmetrical, cyclic diborene was achieved via consecutive two‐electron reduction steps from an adduct of CAAC and B₂Br₄(SMe₂)₂. Theoretical studies revealed that this diborene has a considerably smaller HOMO–LUMO gap than those of reported NHC‐ and phosphine‐supported diborenes. Complexation of the diborene with [AuCl(PCy₃)] afforded two diborene–Au^I π complexes, while reaction with DurBH₂, P₄ and a terminal acetylene led to the cleavage of B−H, P−P, and C−C π bonds, respectively. Thermal rearrangement of the diborene gave an electron‐rich cyclic alkylideneborane, which readily coordinated to Ag^I via its B=C double bond.

Copper(I) and silver(I) chemistry of vinyltrifluoroborate supported by a bis(pyrazolyl)methane ligand

Although unsaturated organotrifluoroborates are common synthons in metal-organic chemistry, their transition metal complexes have received little attention. [CH2(3,5-(CH3)2Pz)2]Cu(CH2[double bond, length as m-dash]CHBF3), (SIPr)Cu(MeCN)(CH2[double bond, length as m-dash]CHBF3) and [CH2(3,5-(CH3)2Pz)2]Ag(CH2[double bond, length as m-dash]CHBF3) represent rare, isolable molecules featuring a vinyltrifluoroborate ligand on coinage metals. The X-ray crystal structures show the presence of three-coordinate metal sites in these complexes. The vinyltrifluoroborate group binds asymmetrically to the metal site in [CH2(3,5-(CH3)2Pz)2]M(CH2[double bond, length as m-dash]CHBF3) (M = Cu, Ag) with relatively closer M-C(H)2 distances. The computed structures of [CH2(3,5-(CH3)2Pz)2]M(CH2[double bond, length as m-dash]CHBF3) and M(CH2[double bond, length as m-dash]CHBF3), however, have shorter M-C(H)BF3 distances than M-C(H)2. These molecules feature various inter- or intra-molecular contacts involving fluorine of the BF3 group, possibly affecting these M-C distances. The binding energies of [CH2[double bond, length as m-dash]CHBF3]- to Cu+, Ag+ and Au+ have been calculated at the wB97XD/def2-TZVP level of theory, in the presence and absence of the supporting ligand CH2(3,5-(CH3)2Pz)2. The calculation shows that Au+ has the strongest binding to the [CH2[double bond, length as m-dash]CHBF3]- ligand, followed by Cu+ and Ag+, irrespective of the presence of the supporting ligand. However, in all three metals, the supporting ligand weakens the binding of olefin to the metal. The same trends were also found from the analysis of the σ-donation and π-backbonding interactions between the metal fragment and the π and π* orbitals of [CH2[double bond, length as m-dash]CHBF3]-.

Gold(I) ethylene complexes supported by electron-rich scorpionates

Ethylene complexes of gold(i) have been stabilized by electron-rich, κ2-bound tris(pyrazolyl)borate ligands. Large up-field shifts of olefinic carbon NMR resonances and relatively long C[double bond, length as m-dash]C distances of gold bound ethylene are indicative of significant Au(i) → ethylene π-backbonding relative to the analog supported by a weakly donating ligand, consistent with the computational data.

Isolable 1-Butene Copper(I) Complexes and 1-Butene/Butane Separation Using Structurally Adaptable Copper Pyrazolates

Non-porous small molecule adsorbents such as {[3,5-(CF₃ )₂ Pz]Cu}₃ (where Pz=pyrazolate) are an emerging class of materials that display attractive features for ethene-ethane separation. This work examines the chemistry of fluorinated copper(I) pyrazolates {[3,5-(CF₃ )₂ Pz]Cu}₃ and {[4-Br-3,5-(CF₃ )₂ Pz]Cu}₃ with much larger 1-butene in both solution and solid state, and reports the isolation of rare 1-butene complexes of copper(I), {[3,5-(CF₃ )₂ Pz]Cu(H₂ C=CHC₂ H₅ )}₂ and {[4-Br-3,5-(CF₃ )₂ Pz]Cu(H₂ C=CHC₂ H₅ )}₂ and their structural, spectroscopic, and computational data. The copper-butene adduct formation in solution involves olefin-induced structural transformation of trinuclear copper(I) pyrazolates to dinuclear mixed-ligand systems. Remarkably, larger 1-butene is able to penetrate the dense solid material and to coordinate with copper(I) ions at high molar occupancy. A comparison to analogous ethene and propene complexes of copper(I) is also provided.

Versatility and adaptative behaviour of the P^N chelating ligand MeDalphos within gold(i) π complexes

The hemilabile P^N ligand MeDalphos enables access to a wide range of stable gold(i) π-complexes with unbiased alkenes and alkynes, as well as electron-rich alkenes and for the first time electron-poor ones. All complexes have been characterized by multi-nuclear NMR spectroscopy and whenever possible, by X-ray diffraction analyses. They all adopt a rare tricoordinate environment around gold(i), with chelation of the P^N ligand and side-on coordination of the alkene, including the electron-rich one, 3,4-dihydro-2H-pyrane. The strength of the N → Au coordination varies significantly in the series. It is the way the P^N ligand accommodates the electronic demand at gold, depending on the alkene. Comparatively, when the chelating P^P ligand (ortho-carboranyl)(PPh₂)₂ is used, gold(i) π-complexes are only isolable with unbiased alkenes. The bonding situation within the gold(i) P^N π-complexes has been thoroughly analyzed by DFT calculations supplemented by Charge Decomposition Analyses (CDA), Natural Bond Orbital (NBO) and Atoms-in-Molecules (AIM) analyses. Noticeable variations in the donation/back-donation ratio, C[double bond, length as m-dash]C weakening, alkene to gold charge transfer and magnitude of the N → Au coordination were observed. Detailed examination of the descriptors for the Au/alkene interaction and the N → Au coordination actually revealed intimate correlation between the two, with linear response of the MeDalphos ligand to the alkene electronics. The P^N ligand displays non-innocent and adaptative character. The isolated P^N gold(i) π-complexes are reactive and catalytically relevant, as substantiated by the chemo and regio-selective alkylation of indoles.

π Complexes of P^P and P^N chelated gold(i)

Tricoordinate gold(i) π-complexes containing P-based chelating ligands (P^P and P^N) were prepared. The structure of the gold(i) styrene complexes has been analysed in detail based on NMR and XRD data. The P^N complex is a competent catalyst for indole alkylation. The reaction proceeds with complete C3 and Markovnikov selectivity.

Monoanionic, Bis(pyrazolyl)methylborate [(Ph3B)CH(3,5-(CH3)2Pz)2)]- as a Supporting Ligand for Copper(I)-ethylene, cis-2-Butene, and Carbonyl Complexes

The monoanionic bidentate ligand [(Ph₃B)CH(3,5-(CH₃)₂Pz)₂)]^- has been prepared from lithium bis(pyrazolyl)methanide and triphenylborane. This useful new ligand is closely related to the well-established bis(pyrazolyl)borate and bis(pyrazolyl)methane ligands but has key differences to both analogues as well. The ethylene, cis-2-butene, and carbon monoxide adducts [(Ph₃B)CH(3,5-(CH₃)₂Pz)₂]Cu(L) (where L = C₂H₄, cis-CH₃HC═CHCH₃, and CO) have been prepared from [(Ph₃B)CH(3,5-(CH₃)₂Pz)₂)]Li(THF), copper(I) triflate, and the corresponding coligand. These complexes have been characterized by NMR spectroscopy and X-ray crystallography. In all cases the bis(pyrazolyl) moiety is bound in κ²N fashion with the BPh₃ group rotated to sit over the metal center, sometimes coordinating to the metal via phenyl carbons as in [(Ph₃B)CH(3,5-(CH₃)₂Pz)₂)]Li(THF) and [(Ph₃B)CH(3,5-(CH₃)₂Pz)₂]Cu(CO) or simply hovering above the metal site as in [(Ph₃B)CH(3,5-(CH₃)₂Pz)₂)]Cu(C₂H₄) and [(Ph₃B)CH(3,5-(CH₃)₂Pz)₂)]Cu(cis-CH₃HC═CHCH₃). The ¹³C and ¹H resonances of the ethylene carbon and protons of [(Ph₃B)CH(3,5-(CH₃)₂Pz)₂)]Cu(C₂H₄) appear at δ 81.0 and 3.71 ppm in CD₂Cl₂, respectively. The characteristic CO frequency for [(Ph₃B)CH(3,5-(CH₃)₂Pz)₂]Cu(CO) has been observed at υ̅ 2092 cm^-1 by infrared spectroscopy and is lower than that of free CO suggesting moderate M → CO π-back-donation. A detailed analysis of these complexes has been presented herein.

Synthesis and Properties of "Sandwich" Diimine-Coinage Metal Ethylene Complexes

Synthesis and full characterization of cationic isostructural "sandwich" diimine-coinage metal ethylene complexes are reported. Ethylene self-exchange kinetics proceeds by an associative exchange mechanism for Cu and Au complexes. The fastest ligand exchange was observed for Ag complex 8a. The upper limit of ΔG‡, assuming associative ligand exchange, was found to be ca. 5.0 kcal/mol. Ethylene self-exchange in Cu complex 7b proceeds with ΔG298‡ = 12.9 ± 0.1 kcal/mol, while the exchange is the slowest in Au complex 9, with ΔG298‡ = 16.7 ± 0.1 kcal/mol. Copper complex 7b is unusually stable and can survive in air for years.

An element through the looking glass: exploring the Au-C, Au-H and Au-O energy landscape

Gold, the archetypal "noble metal", used to be considered of little interest in catalysis. It is now clear that this was a misconception, and a multitude of gold-catalysed transformations has been reported. However, one consequence of the long-held view of gold as inert metal is that its organometallic chemistry contains many "unknowns", and catalytic cycles devised to explain gold's reactivity draw largely on analogies with other transition metals. How realistic are such mechanistic assumptions? In the last few years a number of key compound classes have been discovered that can provide some answers. This Perspective attempts to summarise these developments, with particular emphasis on recently discovered gold(iii) complexes with bonds to hydrogen, oxygen, alkenes and CO ligands.

Copper hydrotris(3,5-diphenylpyrazolyl)borate dithiocarbamates: mimicking green copper proteins

Three new copper hydrotris(pyrazolyl)borate dithiocarbamate complexes [Tp^Ph2Cu(dtc)] which mimic green copper proteins are reported.

Copper(i), silver(i) and gold(i) complexes of N-heterocyclic carbene-phosphinidene

IMes·PPh serves as a good bridging ligand for group 11 metal ions affording molecules with diverse solid state structures.

Thallium(I) complexes of fluorinated bis- and tris(pyrazolyl)borate ligands: [H2B{3,5-(CF3)2pz}2]Tl and [HB{3,5-(CF3)2pz}3]Tl

X-ray crystal structural data at 100 (2) K for the thallium(I) complex of a fluorinated bis(pyrazolyl)borate, {bis[3,5-bis(trifluoromethyl)-1H-pyrazol-1-yl-κN2]borato}thallium(I), [Tl(C10H4BF12N4)], abbreviated as [H2B{3,5-(CF3)2pz}2]Tl, and the related tris(pyrazolyl)borate, {tris[3,5-bis(trifluoromethyl)-1H-pyrazol-1-yl-κN2]borato}thallium(I), [Tl(C15H4BF18N6)], abbreviated as [HB{3,5-(CF3)2pz}3]Tl, are reported. [H2B{3,5-(CF3)2pz}2]Tl features a two-coordinate TlIatom with a bent geometry and a boat-shaped bis(pyrazolyl)borate ligand. It also has several additional inter- and intramolecular Tl...H and Tl...F contacts in the solid state. The tris(pyrazolyl)borate analogue [HB{3,5-(CF3)2pz}3]Tl has a trigonal–pyramidal thallium centre and features inter- and intramolecular Tl...F contacts.