Activation of Anisole by Organoplatinum(II) Complexes: Evidence for Rate-Determining C–H Activation

Theoretical study on the mechanism of the hydrogen evolution reaction catalyzed by platinum subnanoclusters

The smallest subnanocluster models of platinum colloid (Pt_n) are supposed to diffuse in aqueous media in order to examine their behaviors when they are subjected to the electrocatalytic hydrogen evolution reaction under zero overpotential conditions at pH 0. The DFT approach allows us to clarify the nature of individual proton transfer (PT) and electron transfer (ET) processes together with the importance of relying on concerted proton-electron transfer (CPET) pathways to promote the majority of H* adsorption processes by Pt_n subnanoclusters. Although the CPET processes are closely correlated with the Volmer steps (Pt + H⁺ + e^- → Pt-H*) described so far in electrochemistry, our study for the first time points out the essential capability of the Pt_n clusters to promote the multiple PT steps without the need to transfer any electrons, revealing the fundamentally high basicity of the naked Pt_n clusters (pK_a = 27-28 for Pt₄, Pt₅, and Pt₆). The discrete cluster models adopted herein avoid the structural constraints forced by the standard slab models and enable us to discuss the drastic alterations in the geometric and electronic structures of the intermediates given by the consecutive promotion of multiple CPET steps. The weakening of the Pt-H* bond strength with the increasing number of CPET steps is well rationalized by carefully examining the changes in the ν(Pt-H*) vibrational frequencies, the hydricity, and the H₂ desorption energy. The behaviors are also correlated with the underpotential and overpotential deposited hydrogen atoms (H_UPD and H_OPD) discussed in electrochemical studies for many years.

Multiple Reaction Pathways of Eight-Membered Rhodacycles in Rh-Catalyzed Annulations of 2-Alkenyl Phenols/Anilides with Alkynes

Density functional theory calculations were performed to study the competing pathways of rhodacycle intermediates generated in Rh(III)-catalyzed annulations of 2-alkenyl phenols and 2-alkenyl anilides with alkynes. The results show that the multiple pathways of eight-membered rhodacycles can be subtly tuned to give specific cyclic products. The seven-membered oxacyclic and spirocyclic products from 2-alkenyl phenols are formed by favoring the pathway of dissociating the Rh-O bond of O-contained rhodacycles, which are followed by antarafacial nucleophilic attack. The indoline product from 2-alkenyl anilides is generated through the pathway of intramolecular olefin migratory insertion of the N-contained rhodacycle.

Ligand-Controlled Csp2-H versus Csp3-H Bond Formation in Cycloplatinated Complexes: A Joint Experimental and Theoretical Mechanistic Investigation

The cyclometalated platinum(II) complexes [PtMe(C^∧N)(L)] [1_PS: C^∧N = 2-phenylpyridinate (ppy), L = SMe₂; 1_BS: C^∧N = benzo[h]quinolate (bhq), L = SMe₂; 1_PP: C^∧N = ppy, L = PPh₃; and 1_BP: C^∧N = bhq, L = PPh₃] containing two different cyclometalated ligands and two different ancillary ligands have been investigated in the reaction with CX₃CO₂H (X = F or H). When L = SMe₂, the Pt-Me bond rather than the Pt-C bond of the cycloplatinated complex is cleaved to give the complexes [Pt(C^∧N)(CX₃CO₂)(SMe₂)]. When L = PPh₃, the selectivity of the reaction is reversed. In the reaction of [PtMe(C^∧N)(PPh₃)] with CF₃CO₂H, the Pt-C^∧N bond is cleaved rather than the Pt-Me bond. The latter reaction gave [PtMe(κ¹N-Hppy)(PPh₃)(CF₃CO₂)] as an equilibrium mixture of two isomers. For L = PPh₃, no reaction was observed with CH₃CO₂H. The reasons for this difference in selectivity for complexes 1 are computationally discussed based on the energy barrier needed for the protonolysis of the Pt-C_sp³ bond versus the Pt-C_sp² bond. Two pathways including the direct one-step acid attack at the Pt-C bond (S_E2) and stepwise oxidative-addition on the Pt(II) center followed by reductive elimination [S_E(ox)] are proposed. A detailed density functional theory (DFT) study of these protonations along with experimental UV-vis kinetics suggests that a one-step electrophilic attack (S_E2) at the Pt-C bond is the most likely mechanism for complexes 1, and changing the nature of the ancillary ligand can influence the selectivity in the Pt-C bond cleavage. The effect of the nature of the acid and cyclometalated ligand (C^∧N) is also discussed.

Reversible PtII-CH3 deuteration without methane loss: metal-ligand cooperation vs. ligand-assisted PtII-protonation

Di(2-pyridyl)ketone dimethylplatinum(ii), (dpk)Pt^II(CH₃)₂, reacts with CD₃OD at 25 °C to undergo complete deuteration of Pt-CH₃ fragments in ∼5 h without loss of methane to form (dpk)Pt^II(CD₃)₂ in virtually quantitative yield. The deuteration can be reversed by dissolution in CH₃OH or CD₃OH. Kinetic analysis and isotope effects, together with support from density functional theory calculations indicate a metal-ligand cooperative mechanism wherein DPK enables Pt-CH₃ deuteration by allowing non-rate-limiting protonation of Pt^II by CD₃OD. In contrast, other model di(2-pyridyl) ligands enable rate-limiting protonation of Pt^II, resulting in non-rate-limiting C-H(D) reductive coupling. Owing to its electron-poor nature, following complete deuteration, DPK can be dissociated from the Pt^II-centre, furnishing [(CD₃)₂Pt^II(μ-SMe₂)]₂ as the perdeutero analogue of [(CH₃)₂Pt^II(μ-SMe₂)]₂, a commonly used Pt^II-precursor.

Reactions of organoplatinum complexes with dimethylamine-borane

Reactions of organoplatinum complexes with dimethylamineborane are reported.

Experiments and Direct Dynamics Simulations That Probe η2-Arene/Aryl Hydride Equilibria of Tungsten Benzene Complexes

Key steps in the functionalization of an unactivated arene often involve its dihaptocoordination by a transition metal followed by insertion into the C-H bond. However, rarely are the η²-arene and aryl hydride species in measurable equilibrium. In this study, the benzene/phenyl hydride equilibrium is explored for the {WTp(NO)(PBu₃)} (Bu = n-butyl; Tp = trispyrazoylborate) system as a function of temperature, solvent, ancillary ligand, and arene substituent. Both face-flip and ring-walk isomerizations are identified through spin-saturation exchange measurements, which both appear to operate through scission of a C-H bond. The effect of either an electron-donating or electron-withdrawing substituent is to increase the stability of both arene and aryl hydride isomers. Crystal structures, electrochemical measurements, and extensive NMR data further support these findings. Static density functional theory calculations of the benzene-to-phenyl hydride landscape suggest a single linear sequence for this transformation involving a sigma complex and oxidative cleavage transition state. Static DFT calculations also identified an η²-coordinated benzene complex in which the arene is held more loosely than in the ground state, primarily through dispersion forces. Although a single reaction pathway was identified by static calculations, quasiclassical direct dynamics simulations identified a network of several reaction pathways connecting the η²-benzene and phenyl hydride isomers, due to the relatively flat energy landscape.

Photoswitchable and pH responsive organoplatinum(ii) complexes with azopyridine ligands

Several platinum(ii) complexes with ligands containing azo groups have been prepared and structurally characterised, and their photoswitching between trans and cis azo group isomers has been studied. The azo groups in the cationic complexes [PtMe(bipy)(4-NC₅H₄-N[double bond, length as m-dash]N-4-C₆H₄X)][PF₆], X = H, OH or NMe₂, and in the dicationic complex [Pt(bipy)(4-H₂NC₆H₄-N[double bond, length as m-dash]N-C₆H₅)₂][OTf]₂ undergo trans to cis photoswitching on irradiation at 365 nm. The complex [PtMe(bipy)(4-NC₅H₄-N[double bond, length as m-dash]N-4-C₆H₄NMe)₂][PF₆] also exhibits a reversible halochromic effect on protonation to give the dicationic complex [PtMe(bipy)(4-NC₅H₄-NH[double bond, length as m-dash]N-4-C₆H₄NMe₂]²⁺. The nature of the frontier orbitals in the platinum(ii) complexes depends on the charge on the complex and on the degree of metal-ligand π-bonding.

The use of Resonant X-ray Emission Spectroscopy (RXES) for the electronic analysis of metal complexes and their interactions with biomolecules

This review presents a new application of Resonant X-ray Emission Spectroscopy (RXES) to study the mechanism of action of metal containing anticancer derivatives and in particular platinum in situ and in vivo. The technique is an example of a photon-in photon-out X-ray spectroscopic approach, which enables chemical speciation of drugs to be determined and therefore to derive action mechanisms, and to determine drug binding rates under physiological conditions and therapeutic concentrations. This is made feasible due to the atomic specificity and high penetration depth of RXES. The review presents examples of the three main types of information that can be obtained by RXES and establishes an experimental protocol to perfect the measurements within cells.

Novel in situ methodology to observe the interactions of chemotherapeutical Pt drugs with DNA under physiological conditions

The binding of the antitumor drug cisplatin with DNA was determined by means of in situ resonant inelastic X-ray scattering (RIXS) spectroscopy.

Rollover cyclometalation with 2-(2'-pyridyl)quinoline

Rollover cyclometalation of 2-(2'-pyridyl)quinoline, L, allowed the synthesis of the family of complexes [Pt(L-H)(X)(L')] and [Pt(L*)(X)(L')][BF4] (X = Me, Cl; L' = neutral ligand), the former being the first examples of Pt(II) rollover complexes derived from the ligand L. The ligand L* is a C,N cyclometalated, N-protonated isomer of L, and can also be described as an abnormal-remote pyridylene. The corresponding [Pt(L-H)(Me)(L')]/[Pt(L*)(Me)(L')](+) complexes constitute an uncommon Brønsted-Lowry acid-base conjugated couple. The species obtained were investigated in depth through NMR and UV-vis spectroscopy, cyclic voltammetry, and density functional theory (DFT) methods to correlate different chemico-physical properties with the nature of the cyclometalated ligand (e.g., L vs bipy or L* vs L) and of the neutral ligand (DMSO, CO, PPh3). The crystal structures of [Pt(L-H)(Me)(PPh3)], [Pt(L-H)(Me)(CO)] and [Pt(L*)(Me)(CO)][BF4] were determined by X-ray powder diffraction methods, the latter being the first structure of a Pt(II)-based, protonated, rollover complex to be unraveled. The isomerization of [Pt(L*)(Me)(PPh3)](+) in solution proceeds through a retro-rollover process to give the corresponding adduct [Pt(L)(Me)(PPh3)](+), where L acts as a classical N,N chelating ligand. Notably, the retro-rollover reaction is the first process, among the plethora of Pt-C bond protonolysis reactions reported in the literature, where a Pt-C(heteroaryl) bond is cleaved rather than a Pt-C(alkyl) one.

Self-assembly of isomeric clamshell dimers of platinum(II)

The controlled synthesis of isomeric organoplatinum clamshell dimers [Pt(2)Me(2)(μ(2)-κ(3)-6-dppd)(2)](2+), 6-dppd = 1,4-di-2-pyridyl-5,6,7,8,9,10-hexahydrocycloocta[d]pyridazine, is reported. The new complexes are formed selectively by self-assembly from mononuclear precursors, taking advantage of the slow cis-trans isomerization at platinum(II). Thus reaction of endo-[PtClMe(κ(2)-6-dppd)] with AgOTf gave endo,endo-[Pt(2)Me(2)(μ(2)-κ(3)-6-dppd)(2)](2+), while the reaction of [PtMe(2)(κ(2)-6-dppd)] with HOTf in solvent S = Me(2)C=O or MeCN gave first a mixture of exo- and endo-[PtMe(S)(κ(2)-6-dppd)](+) and then, by loss of solvent, a mixture of exo,exo- and endo,endo-[Pt(2)Me(2)(μ(2)-κ(3)-6-dppd)(2)](2+). The endo,endo isomer slowly isomerized to the more stable exo,exo isomer in solution. Reaction of PPh(3) with endo-[PtClMe(κ(2)-6-dppd)] gave a mixture of endo- and exo-[PtMe(PPh(3))(κ(2)-6-dppd)](+) but reaction with exo,exo-[Pt(2)Me(2)(μ(2)-κ(3)-6-dppd)(2)](2+) gave exo-[PtMe(PPh(3))(κ(2)-6-dppd)](+) selectively, with retention of stereochemistry. The structures of the clamshell dimers and of key precursors are reported and equilibria are studied both experimentally and by DFT calculations.