Selectivity Effects in Bimetallic Catalysis: Role of the Metal Sites in the Decomposition of Formic Acid into H 2 and CO 2 by the Coinage Metal Binuclear Complexes [dppmMM′(H)] +

Homo and heteromultimetallic complexes containing a group 8 transition metal and μ-diphosphine bridging ligands involved in anticancer research: A review

Herein, we present a comprehensive review focusing on synthetic strategies, detailed structural analysis, and anticancer activity investigations of complexes following the general formula [LnM(μ-diphosphine)M'Lm] where M = group 8 metal; M' = any transition metal; μ-diphosphine = bridging ligand; Ln and Lm = ligand spheres). Both homo- and heteromultimetallic complexes will be discussed in detail. We review in vitro, in vivo and in silico anticancer activity investigations, in an attempt to draw comparisons between the various complexes and derive structure-activity relationships (SAR). This review solely focuses on complexes falling under the general formula stated above that have been studied for their anticancer activities, other complexes falling into that scheme but which have not undergone anticancer testing are not included in this review. We compare the anticancer activities of these complexes to their mononuclear counterparts, and a positive control (cisplatin) when possible and present a summary of all existing data to date and attempt to draw some conclusions on the future development of these complexes.

Formate dehydrogenase activity by a Cu(II)-based molecular catalyst and deciphering the mechanism using DFT studies

Due to the requirement to establish renewable energy sources, formic acid (FA), one of the most probable liquid organic hydrogen carriers (LOHCs), has received great attention. Catalytic formic acid dehydrogenation in an effective and environmentally friendly manner is still a challenge. The N3Q3 ligand (N3Q3 = N,N-bis(quinolin-8-ylmethyl)quinolin-8-amine) and the square pyramidal [Cu(N3Q3)Cl]Cl complex have been synthesised in this work and characterised using several techniques, such as NMR spectroscopy, mass spectrometry, EPR spectroscopy, cyclic voltammetry, X-ray diffraction and DFT calculations. This work investigates the dehydrogenation of formic acid using a molecular and homogeneous catalyst [Cu(N3Q3)Cl]Cl in the presence of HCOONa. The mononuclear copper complex exhibits catalytic activity towards the dehydrogenation of formic acid in H2O with the evolution of a 1 : 1 CO2 and H2 mixture. The activation energy of formic acid dehydrogenation was calculated to be Ea = 86 kJ mol-1, based on experiments carried out at various temperatures. The Gibbs free energy was found to be 82 kJ at 298 K for the decomposition of HCOOH. The DFT studies reveal that [Cu(N3Q3)(HCOO-)]+ undergoes an uphill process of rearrangement followed by decarboxylation to generate [Cu(N3Q3)(H-)]+. The initial uphill step for forming a transition state is the rate-determining step. The [Cu(N3Q3)(H-)]+ follows an activated state in the presence of HCOOH to liberate H2 and generate the [Cu(N3Q3)(OH2)]2+.

Liberation of carbon monoxide from formic acid mediated by molybdenum oxyanions

Multistage mass spectrometry experiments, isotope labelling and DFT calculations were used to explore whether selective decarbonylation of formic acid could be mediated by molybdate anions [(MoO3)x(OH)]- (x = 1 and 2) via a formal catalytic cycle involving two steps. In step 1, both molybdate anions undergo gas-phase ion-molecule reactions (IMR) with formic acid to produce the coordinated formates [(MoO3)x(O2CH)]- and H2O. In step 2, both coordinated formates [(MoO3)x(O2CH)]- undergo decarbonylation under collision-induced dissociation (CID) conditions to reform the molybdate anions [(MoO3)x(OH)]- (x = 1 and 2), thus closing a formal catalytic cycle. In the case of [MoO3(O2CH)]- an additional decarboxylation channel also occurs to yield [MoO3(H)]-, which is unreactive towards formic acid. The reaction between [Mo18O3(18OH)]- and formic acid gives rise to [Mo18O3(O2CH)]- highlighting that ligand substitution occurs without 18O/16O exchange between the coordinated 18OH ligand and HC16O2H. The reaction between [(MoO3)x(OD)]- (x = 1 and 2) and DCO2H initially produces [(MoO3)x(OH)]- (x = 1 and 2), indicating that D/H exchange occurs. DFT calculations were carried out to investigate the reaction mechanisms and energetics associated with both steps of the formal catalytic cycle and to better understand the competition between decarbonylation and decarboxylation, which is crucial in developing a selective catalyst. The CO and CO2 loss channels from the monomolybdate anion [MoO3(O2CH)]- have similar barrier heights which is in agreement with experimental results where both fragmentation channels are observed. In contrast, the dimolybdate anion is more selective, since the decarbonylation pathway of [(MoO3)2(O2CH)]- is both kinetically and thermodynamically favoured, which agrees with experimental observations where the CO loss channel is solely observed.

Electronic spectroscopy of homo- and heterometallic binuclear coinage metal phosphine complexes in isolation

Binuclear coinage metal phosphine complexes are examined under ion trap isolation in order to elucidate their noncovalent binding, structural properties and intrinsic electronic spectra. Our survey shows an intriguing order of electronic transitions obtained by in situ synthesis and mass-spectrometrically supported UV photodissociation spectroscopy on a series of six isolated homo- and heterobinuclear complexes of type [MM'(dcpm)2]2+ (M, M' = CuI, AgI, AuI; dcpm = bis(dicyclohexyl-phosphino)methane). This approach provides the unique opportunity to study all possible coinage metal interactions within a fixed ligand framework. A successive blue-shift (33 700-38 500 cm-1; 297-260 nm) of the lowest-energy bright electronic transition energy in gas phase was observed in the order of Cu2 < CuAu < CuAg < Au2 < AgAu < Ag2. This order was reproduced by quantum chemical calculations using a scalar-relativistic GW-Bethe-Salpeter-equation (GW-BSE) approach. Theory ascribes the electronic bands of all complexes to metal-centered 1MC(dσ*-pσ) transitions revealing a strengthening of metal-metal' (M-M') binding upon excitation, in agreement to mass spetrometric results. A test of the correlation of transition energies with M-M' distance by quantum chemical calculations of single point energies as a function of intermetallic distance indicates qualitative agreement with experimental results. However, the experimentally observed high sensitivity of spectroscopic shifts towards metal composition cannot be accounted for solely by M-M' distance variation. The differences in electronic transitions are qualitatively rationalized by the varying (n + 1)s (n = 3, 4, 5) orbital contributions (increase from Cu2via CuAu/CuAg to Au2/AgAu/Ag2) within the nd(n + 1)s/p-hybridization for the ground electronic state of the different complexes, whereas the excited state (of (n + 1)p orbital character) shows significantly less variation in energy. In particular, the observed spectroscopic and mass spectrometric sequence for the Ag/Au complexes is traced back to the interplay of Pauli repulsion and variation in metal-ligand bond strength within the orbital hybridization model.

Near thermal, selective liberation of hydrogen from formic acid catalysed by copper hydride ate complexes

A near thermal two-step catalytic cycle for the selective release of hydrogen from formic acid by mononuclear cuprate anions was revealed using multistage mass spectrometry experiments, deuterium labelling and DFT calculations. In gas-phase ion-molecule reactions, mononuclear copper hydride anions [(L)Cu(H)]^- (where L = H^-, O₂CH^-, BH₄^- and CN^-) were found to react with formic acid (HCO₂H) to yield [(L)Cu(O₂CH)]^- and H₂. The copper formate anions [(L)Cu(O₂CH)]^- can decarboxylate via collision-induced dissociation (CID) to reform the copper hydride [(L)Cu(H)]^-, thereby closing the two-step catalytic cycle. Analogous labelling experiments with d₁-formic acid (DCO₂H) reveal that the decarboxylation process also occurs spontaneously. A kinetic study was carried out to provide further insights into the species involved in this reaction. Energetics from density functional theory (DFT) calculations show that the key decarboxylation step can occur without CID, thus in support of experimental observations.

Photochemical hydrogen production based on the HCOOH/CO2 cycle promoted by a pentanuclear cobalt complex

The first catalytic cycle for hydrogen production based on the photochemical two-electron reduction of carbon dioxide (CO₂) and the dehydrogenation of formic acid at ambient temperature was demonstrated using a pentanuclear cobalt complex (Co5). A series of mechanistic studies were performed to elucidate the mechanism responsible for the promotion of the photocatalytic cycle by Co5.

On the Hydrogen Oxalate Binding Motifs onto Dinuclear Cu and Ag Metal Phosphine Complexes

We report the binding geometries of the isomers that are formed when the hydrogen oxalate ((CO₂)₂H=HOx) anion attaches to dinuclear coinage metal phosphine complexes of the form [M₁M₂dcpm₂(HOx)]⁺ with M=Cu, Ag and dcpm=bis(dicyclohexylphosphino)methane, abbreviated [MM]⁺. These structures are established by comparison of isomer‐selective experimental vibrational band patterns displayed by the cryogenically cooled and N₂‐tagged cations with DFT calculations of the predicted spectra for various local minima. Two isomeric classes are identified that feature either attachment of the carboxylate oxygen atoms to the two metal centers (end‐on docking) or attachment of oxygen atoms on different carbon atoms asymmetrically to the metal ions (side‐on docking). Within each class, there are additional isomeric variations according to the orientation of the OH group. This behavior indicates that HOx undergoes strong and directional coordination to [CuCu]⁺ but adopts a more flexible coordination to [AgAg]⁺. Infrared spectra of the bare ions, fragmentation thresholds and ion mobility measurements are reported to explore the behaviors of the complexes at ambient temperature.

ORGANOMETALLIC GAS-PHASE ION CHEMISTRY AND CATALYSIS: INSIGHTS INTO THE USE OF METAL CATALYSTS TO PROMOTE SELECTIVITY IN THE REACTIONS OF CARBOXYLIC ACIDS AND THEIR DERIVATIVES

Carboxylic acids are valuable organic substrates as they are widely available, easy to handle, and exhibit structural and functional variety. While they are used in many standard synthetic protocols, over the past two decades numerous studies have explored new modes of metal-mediated reactivity of carboxylic acids and their derivatives. Mass spectrometry-based studies can provide fundamental mechanistic insights into these new modes of reactivity. Here gas-phase models for the following catalytic transformations of carboxylic acids and their derivatives are reviewed: protodecarboxylation; dehydration; decarbonylation; reaction as coordinated bases in C-H bond activation; remote functionalization and decarboxylative C-C bond coupling. In each case the catalytic problem is defined, insights from gas-phase studies are highlighted, comparisons with condensed-phase systems are made and perspectives are reached. Finally, the potential role for mechanistic studies that integrate both gas- and condensed-phase studies is highlighted by recent studies on the discovery of new catalysts for the selective decomposition of formic acid and the invention of the new extrusion-insertion class of reactions for the synthesis of amides, thioamides, and amidines. © 2020 John Wiley & Sons Ltd. Mass Spec Rev.

Spectroscopy and photochemistry of copper nitrate clusters

The investigation of copper nitrate cluster anions Cu(ii)n(NO3)2n+1-, n ≤ 4, in the gas phase using ultraviolet/visible/near-infrared (UV/vis/NIR) spectroscopy provides detailed insight into the electronic structure of the copper salt and its intriguing photochemistry. In the experimentally studied region up to 5.5 eV, the spectra of copper(ii) nitrate exhibit a 3d-3d band in the vis/NIR and well-separated bands in the UV. The latter bands originate from Ligand-to-Metal Charge Transfer (LMCT) as well as n-π* transitions in the nitrate ligands. The clusters predominantly decompose by loss of neutral copper nitrate in the electronic ground state after internal conversion or via the photochemical loss of a neutral NO3 ligand after a LMCT. These two decomposition channels are in direct competition on the ground state potential energy surface for the smallest copper nitrate cluster, Cu(ii)(NO3)3-. Here, copper nitrate evaporation is thermochemically less favorable. Population of π* orbitals in the nitrate ligands may lead to N-O bond photolysis. This is observed in the UV region with a small quantum efficiency, with photochemical loss of either nitrogen dioxide or an oxygen atom.

Quantification of Cooperativity between Metal Sites in Dinuclear Transition Metal Complexes Containing the (2-Dimethylamino)-4-(2-pyrimidinyl)pyrimidine Ligand

A concept for the quantification of cooperative effects in transition-metal complexes is presented. It is demonstrated for a series of novel N,N- (mononuclear) and C,N-coordinated homo- and heterometallic binuclear complexes based on the (2-dimethylamino)-4-(2-pyrimidinyl)pyrimidine ligand, which are accessible by applying roll-over cyclometallation. These iridium-, platinum-, and palladium-containing compounds are investigated with respect to their absorption and fluorescence spectra. The cooperative effects in the electronic absorptions, i. e., the energetic shifts between mononuclear and dinuclear complexes, and free ligands are analyzed on the basis of the lowest energy π-π* transitions and compared to calculated data, obtained from TD-DFT calculations. Furthermore the corresponding fluorescence spectra are presented and analyzed with respect to the concept of cooperativity.

Rivalry between Regium and Hydrogen Bonds Established within Diatomic Coinage Molecules and Lewis Acids/Bases

A theoretical study of the complexes formed by Ag₂ and Cu₂ with different molecules, XH (FH, ClH, OH₂ , SH₂ , HCN, HNC, HCCH, NH₃ and PH₃ ) that can act as hydrogen-bond donors (Lewis acids) or regium-bond acceptors (Lewis bases) was carried out at the CCSD(T)/CBS computational level. The heteronuclear diatomic coinage molecules (AuAg, AuCu, and AgCu) have also been considered. With the exception of some of the hydrogen-bonded complexes with FH, the regium-bonded binary complexes are more stable. The AuAg and AuCu molecules show large dipole moments that weaken the regium bond (RB) with Au and favour those through the Ag and Cu atoms, respectively.

Phenyl-Group Exchange in Triphenylphosphine Mediated by Cationic Gold-Platinum Complexes-A Gas-Phase Mimetic Approach

The PPh₃ ligands in the heterodinuclear AuPt complex [(Ph₃P)AuPt(PPh₃)₃][BAr₄^F] (BAr₄^F = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) exhibit a high fluxionality on the AuPt core. Fast intramolecular and slow intermolecular processes for the reversible exchange of the PPh₃ ligands have been identified. When [(Ph₃P)AuPt(PPh₃)₃][BAr₄^F] is heated in solution, the formation of benzene is observed, and a trinuclear, cationic AuPt₂ complex is generated. This process is preceded by reversible phenyl-group exchange between the PPh₃ ligands present in the reaction mixture as elucidated by deuterium-labeling studies. Both the elimination of benzene and the preceding reversible phenyl-group exchange have originally been observed in mass-spectrometry-based CID experiments (CID = Collision-Induced Dissociation). While CID of mass-selected [Au,Pt,(PPh₃)₄]⁺ results exclusively in the loss of PPh₃, the resulting cation [Au,Pt,(PPh₃)₃]⁺ selectively eliminates C₆H₆. Thus, the dissociation of a PPh₃ ligand from [Au,Pt,(PPh₃)₃]⁺ is energetically not able to compete with processes which result in C-H- and C-P-bond cleavage. In both media, the heterobimetallic nature of the employed complexes is the key for the observed reactivity. Only the intimate interplay of the gas-phase investigations, studies in solution, and thorough DFT computations allowed for the elucidation of the mechanistic details of the reactivity of [(Ph₃P)AuPt(PPh₃)₃][BAr₄^F].

Using electrospray ionization-tandem mass spectrometry to explore formation and gas-phase chemistry of silver nanoclusters generated from the reaction of silver salts with NaBH4 in the presence of bis(diphenylarsino)methane

Electrospray ionization-mass spectrometry (ESI-MS) of mixtures of AgBF4 or AgNO3 with the capping ligand bis(diphenylarsino)methane ((Ph2 As)2 CH2 = dpam) in a solution of acetonitrile revealed the formation of the following cations: [Ag(CH3 CN)(dpam)]+ , [Ag(dpam)2 ]+ , [Ag2 (Cl)(dpam)2 ]+ , and [Ag3 (Cl)2 (dpam)3 ]+ . Addition of NaBH4 to these solutions results in the formation of the cluster cations [Ag2 (BH4 )(dpam)2 ]+ , [Ag2 (BH4 )(dpam)3 ]+ , [Ag3 (H)(BH4 )(dpam)3 ]+ , [Ag3 (BH4 )2 (dpam)3 ]+ , [Ag3 (H)(Cl)(dpam)3 ]+ , and [Ag3 (I)(BH4 )(dpam)3 ]+ , as established by ESI-MS. Use of NaBD4 confirmed that borohydride is the source of the hydride in these clusters. An Orbitrap Fusion LUMOS mass spectrometer was used to explore the gas-phase unimolecular chemistry of selected clusters via multistage mass spectrometry (MSn ) experiments employing low-energy collision-induced dissociation (CID) and high-energy collision-induced dissociation (HCD) experiments. The borohydride containing clusters fragment via two competing pathways: (i) ligand loss and (ii) B-H bond activation involving BH3 loss. Density functional theory (DFT) calculations were used to calculate the energetics of the optimized structures for all precursor ions, fragment ions, and neutrals and to estimate the reaction endothermicities. Generally, there is reasonable agreement between the most abundant product ion formed and the predicted endothermicity of the associated reaction channel. The DFT calculations predicted that the novel dimer [Ag2 (BH4 )(dpam)2 ]+ has a paddlewheel structure in which the dpam and BH4 - ligands bridge both silver centers.

Photoinitiated Charge Transfer in a Triangular Silver(I) Hydride Complex and Its Oxophilicity

The photoexcitation of a triangular silver(I) hydride complex, [Ag₃ (μ₃ -H)(μ₂ -dcpm)₃ ](PF₆ )₂ ([P](PF₆ )₂ , dcpm=bis(dicyclohexylphosphino)methane), designed with "UV-silent" bis-phosphine ligands, provokes hydride-to-Ag₃ single and double electron transfer. The nature of the electronic transitions has been authenticated by absorption and photodissociation spectroscopy in parallel with high-level quantum-chemical computations utilizing the GW method and Bethe-Salpeter equation (GW-BSE). Specific photofragments of mass-selected [P]²⁺ ions testify to charge transfer and competing pathways resulting from the unique [Ag₃ (μ₃ -H)]²⁺ scaffold. This structural motif of [P](PF₆ )₂ has been unequivocally verified by ¹ H NMR spectroscopy in concert with DFT and X-ray diffraction structural analysis, which revealed short equilateral Ag-Ag distances (d_AgAg =3.08 Å) within the range of argentophilic interactions. The reduced radical cation [P]^{. +} exhibits strong oxophilicity, forming [P+O₂ ]^.+ ,which is a model intermediate for silver oxidation catalysis.

Role of Ligand in the Selective Production of Hydrogen from Formic Acid Catalysed by the Mononuclear Cationic Zinc Complexes [(L)Zn(H)]+ (L=tpy, phen, and bpy)

A series of zinc-based catalysts was evaluated for their efficiency in decomposing formic acid into molecular hydrogen and carbon dioxide in the gas phase using quadrupole ion trap mass spectrometry experiments. The effectiveness of the catalysts in the series [(L)Zn(H)]⁺ , where L=2,2':6',2''-terpyridine (tpy), 1,10-phenanthroline (phen) or 2,2'-bipyrydine (bpy), was found to depend on the ligand used, which turned out to be fundamental in tuning the catalytic properties of the zinc complex. Specifically, [(tpy)Zn(H)]⁺ displayed the fastest reaction with formic acid proceeding by dehydrogenation to produce the zinc formate complex [(tpy)Zn(O₂ CH)]⁺ and H₂ . The catalysts [(L)Zn(H)]⁺ are reformed by decarboxylating the zinc formate complexes [(L)Zn(O₂ CH)]⁺ by collision-induced dissociation, which is the only reaction channel for each of the ligands used. The decarboxylation reaction was found to be reversible, since the zinc hydride complexes [(L)Zn(H)]⁺ react with carbon dioxide yielding the zinc formate complex. This reaction was again substantially faster for L=tpy than L=phen or bpy. The energetics and mechanisms of these processes were modelled using several levels of density functional theory (DFT) calculations. Experimental results are fully supported by the computational predictions.

Selective C-O Coupling Hidden in the Thermal Reaction of [Al2 CuO5 ]+ with Methane

The thermal gas-phase reaction of [Al₂ CuO₅ ]⁺ with methane has been explored by using FT-ICR mass spectrometry complemented by high-level quantum chemical calculations. The generation of atomic [Cu]⁺ from the [Al₂ CuO₅ ]⁺ /CH₄ couple corresponds to the only reaction channel. Labeling experiments and computational studies strongly suggest that methane activation is indeed involved in the production of [Cu]⁺ , and generation of CH₂ O prevails. Mechanistic aspects and the associated doping effects are discussed.

Synthesis, structure, and condensed-phase reactivity of [Ag3(μ3-H)(μ3-BH4)LPh3](BF4) (LPh = bis(diphenylphosphino)amine) with CS2

Electrospray ionisation mass spectrometry (ESI-MS) was used to monitor the reaction of AgBF₄, bis(diphenylphosphino)amine (dppa = (Ph₂P)₂NH = L^Ph) and NaBH₄ in acetonitrile.