Semiquinone Radical Anion Research Articles

Catalysis of first triplet p-benzoquinone for electron ejection from sulfite: Significance of single electron transfer

In this study, the combination of p-benzoquinone (p-BQ) and sulfite under the ultraviolet (UV) irradiation demonstrated markedly superior efficiency in the ejection of hydrated electrons (eaq−) compared to the individual UV/p-BQ and UV/sulfite processes. Theoretical calculations reveal that the 1,2- and 1,4-addition of bisulfite (HSO3−) to p-BQ was unlikely to occur kinetically, and the 1,4-addition of sulfite (SO32−) might be the major pathway for the reaction with p-BQ. Via single electron transfer from the highest occupied molecular orbitals (HOMOs) of HSO3− and SO32− to the HOMO–1 of 3p-BQ*, the oxidation of HSO3− and SO32− by the first triplet state of p-BQ (3p-BQ*, the major excited species during the excitation) could result in the generation of HSO3•, SO3•− and semiquinone radical anion. This mechanism could elucidate the synergism of p-BQ and sulfite for ejecting eaq− in the water under the UV irradiation. Given the abundance of quinones in natural waters, these findings may be helpful for the utilization of sulfite to efficiently release eaq− in the water treatment applications.

Utilizing coordination chemistry through formation of a CuII-quinalizarin complex to manipulate cell biology: An in vitro, in silico approach

Quinalizarin, an analogue of anthracycline anticancer agents, is an anticancer agent itself. A CuII complex was prepared and characterized by elemental analysis, UV-Vis & IR spectroscopy, mass spectrometry, EPR and DFT. The intention behind the preparation of the complex was to increase cellular uptake, compare its binding with DNA against that of quinalizarin, modulation of semiquinone formation, realization of human DNA topoisomerase I & human DNA topoisomerase II inhibition and observation of anticancer activity. While the first two attributes of complex formation lead to increased efficacy, decrease in semiquinone generation could results in a compromise with efficacy. Inhibition of human DNA topoisomerase makes up this envisaged compromise in free radical activity since the complex shows remarkable ability to disrupt activities of human DNA topoisomerase I and II. The complex unlike quinalizarin, does not catalyze flow of electrons from NADH to O2 to the extent known for quinalizarin. Hence, decrease in semiquinone or superoxide radical anion could make modified quinalizarin [as CuII complex] less efficient in free radical pathway. However, it would be less cardiotoxic and that would be advantageous to qualify it as a better anticancer agent. Although binding to calf thymus DNA was comparable to quinalizarin, it was weaker than anthracyclines. Low cost of quinalizarin could justify consideration as a substitute for anthracyclines but the study revealed IC50 of quinalizarin/CuII-quinalizarin was much higher than anthracyclines or their complexes. Even then, there is a possibility that CuII-quinalizarin could be an improved and less costly form of quinalizarin as anticancer agent.

Electrochemical behaviour of anthraquinone dyes in non-aqueous solvent solution. Part II. Controlled potential electrolysis of alizarin

This article is the second part of our already published research results (Caram, Banera, Martínez Suárez, Mirífico, 2017). The present work aims to characterise/identify reaction intermediates and final products of the cathodic electrolysis of alizarin (AH2) in solution of DMF and ACN with TBAP and NaClO4 as supporting electrolytes to corroborate the reaction mechanism previously published by us. Intermediates and reaction products are accumulated by controlled potential electrolysis (CPE) of AH2 performed at the potentials of the first, second, and third charge transfers. Progress of the electrolysis is followed by CV. The techniques employed for the characterisation/identification of the involved chemical species are CV, UV–Vis, FTIR, 1H and 13C NMR, ESR, elemental analysis, and quantum chemical computations based on density functional theory (DFT). Some aspects of the mechanism are corroborated, some details are modified, and new insights are obtained. Intermediates are detected by CV, and the structure of some of them (AH2● ─, AH3●, and AH─) is confirmed by ESR and by the identity of the isolated reaction products (1-hydroxy-2-methoxyanthracene-9,10-dione (AH-CH3) by adding an excess of CH3I at the end of the CPE in DMF/TBAP and NaClO4 as the electrolyte systems, and sodium 1-hydroxy-9,10-dioxo-9,10-dihydroanthracene-2-olate (AH-Na) in ACN/NaClO4). Ion pair formation between AH2● ─ semiquinone radical anion with Na+ is significant. The homogeneous dissociation equilibrium AH3─ ⇄AH22– + H+ does not occur, and AH2• – + AH2 ⇄ AH – + AH3• is a slow process more shifted to the right in DMF/TBAP electrolyte medium. AH-CH3 and AH-Na are clear evidence of the self-protonation reaction of AH2• –. Ca. 20 % of the initial moles of AH2 are converted into AH-CH3 in DMF/TBAP and NaClO4 as the electrolyte systems. CPE of AH2 in ACN/NaClO4 results in reaction products that are insoluble and are coating the electrode, ca. 80 % of the initial moles of AH2 are recuperated as pure AH-Na. AH-CH3 and AH-Na contain the carbonyl-quinone groups in their structures. The applied potential does not modify these results. AH-CH3 and AH-Na are electrochemically (CV) characterised. The acidity of the quinone dye, the homogeneous protonation/deprotonation equilibriums, the intramolecular hydrogen bonding, and the easy air oxidation and protonation of some of the intermediates have led to the obtained results.

Square Wave Voltcoulommetry of two-electron molecular electrocatalytic processes with adsorbed species. Application to the surface O2 reduction in acetonitrile at anthraquinone-modified glassy carbon electrodes

An adequate analysis of the performance and efficiency of a given multi-redox molecular electrocatalyst versus a particular substrate of interest requires the knowledge of the overall reaction pathway and the determination of the rate constants of the catalytic steps taking place. For this purpose, a theoretical model is presented for the Square Wave Voltcoulommetry response of surface-confined two-electron catalysts, that is, molecular species with two different redox states that can act in different electrocatalytic forms as a function of the applied potential. The rigorous theoretical model incorporates convective mass transport effects and non-idealities in the electrochemical behaviour of the immobilized redox probes. The results obtained have been applied to the study of the surface oxygen reduction in an aprotic medium (acetonitrile) at an anthraquinone-modified glassy carbon electrode. The charge-potential responses indicate that the reaction pathway involves two different catalytic routes due to the two reduction products of the anthraquinone moiety, that is, the semiquinone anion radical and the dianion. A Koutecky-Levich approach, combined with the fittings of the experimental responses to the theoretical expressions deduced here, has allowed us to obtain the values of the different thermodynamic (formal potential, interaction parameters, surface coverage) and kinetic (rate constant) parameters of the process.

A CobaltII/CobaltIII complex of alizarin that was analyzed from the stand point of binding with DNA, for ROS generation and anticancer drug prospecting was identified as an analogue of anthracyclines

A coordination compound of CoII with alizarin was prepared to see if it resembles various activities that are reported for anthracyclines as anticancer agents. Alizarin, which has a comparatively simpler structure than anthracyclines, contain the same hydroxy-9,10-anthraquinone as found in anthracyclines. Hence, if tried in cancer chemotherapy, it is expected to show some advantages related to cost and for all those reasons that become applicable in using a relatively simpler compound. However, to be acceptable, it must show comparable efficacy as known for anthracyclines. Like anthracyclines, alizarin forms the semiquinone radical anion that generates superoxide radical anion, responsible for cardiotoxicity. Complex formation decreases generation of superoxide, that was verified by an enzyme assay that might help to decrease toxic side effects. Although hydroxy-9,10-anthraquinones (here alizarin), resemble anthracyclines, biophysical and/or biochemical interactions of the former at the cellular level are less efficient than the latter. Besides, under physiological conditions alizarin is anionic. Hence, increase in pH affects DNA binding adversely. The prepared complex, which is anionic, when it has CoII and neutral when CoIII, binds DNA better than alizarin under varying conditions of ionic strength and pH. Since reactive oxygen species are present in cells, there is a high probability that the CoII complex could either completely or partially convert to CoIII. The CoIII species would then manifest in several biological responses affecting the growth of cells. Experiments pertaining to ROS were performed on three human cell lines (SIHA, HepG2, WI 38) that suggest a shift in mechanism for the complex/complexes from that known for anthracyclines or hydroxy-9,10-anthraquinones. An expected loss in efficacy due to decreased semiquinone formation is compensated by the CoII/CoIII couple owing to the metal ion being present in two oxidation states in cellular medium thus promoting cytotoxicity. The CoII/CoIII complex tried on cells reveal several attributes that enable it/them to overcome some of the shortcomings of alizarin to become comparable to anthracyclines.

Rules for Magnetic Exchange in Azulene-Bridged Biradicals: Quo Vadis?

Electronic coupling through organic bridges facilitates magnetic exchange interactions and controls electron transfer and single-molecule device electron transport. Electronic coupling through alternant π-systems (e.g., benzene) is better understood than the corresponding coupling through nonalternant π-systems (e.g., azulene). Herein, we examine the structure, spectroscopy, and magnetic exchange coupling in two biradicals (1,3-SQ2Az and 1,3-SQ-Az-NN; SQ = the zinc(II) complex of spin-1/2 semiquinone radical anion, NN = spin-1/2 nitronylnitroxide; Az = azulene) that possess nonalternant azulene π-system bridges. The SQ radical spin density in both molecules is delocalized into the Az π-system, while the NN spin is effectively localized onto the five-atom ONCNO π-system of NN radical. The spin distributions and interactions are probed by EPR spectroscopy and magnetic susceptibility measurements. We find that J = +38 cm-1 for 1,3-SQ2Az and J = +9 cm-1 for 1,3-SQ-Az-NN (). Our results highlight the differences in exchange coupling mediated by azulene compared to exchange coupling mediated by alternant π-systems.

O-Semiquinone radical anion isolated as an amorphous porous solid.

The use of metal cations is a commonly applied strategy to create S > 1/2 stable molecular systems containing semiquinone radicals. Persistent mono-semiquinonato complexes of diamagnetic metal ions (S = 1/2) have been hitherto less common and mostly limited to the complexes of heavy metal ions. In this work, a mono-semiquinonato complex of aluminum, derived from 1,2-dihydroxybenzene, is obtained using a surprisingly short and uncomplicated procedure. The isolated product is an amorphous and porous solid that exhibits very good stability under ambient conditions. To characterise its molecular and electronic structure, 9.7, 34 and 406 GHz EPR spectroscopy was used in concert with computational techniques (DFT and DLPNO-CCSD). It was revealed that the radical complex is composed of two chemically equivalent aluminum cations and two catechol-like ligands with the unpaired electron uniformly distributed between the two organic molecules. The good stability and porous structure make this complex applicable in heterogeneous aerobic reactions.

EPR detection of key radicals during coking process of lignin monomer pyrolysis

Lignin pyrolysis is a promising method for the sustainable production of phenolic compounds from biomass. However, detailed knowledge about the radicals involved in this process and their influence during the coking is missing. Herein, we report on the pyrolysis of guaiacol, a lignin monomer at 600 °C under inert and hydrogen gas conditions. During the pyrolysis process, the formed radicals were detected by two improved electron paramagnetic resonance spectroscopy methods. Successfully observed o-phenyl semiquinone radical anion, cyclopentadienyl radical, methyl radical, hydroxyl radical, methoxy radical, phenyl radical, phenoxy radical, etc. during the pyrolysis of guaiacol, as well as the changes of related radicals in different pyrolysis stages. It was found that o-semiquinone radical anions and cyclopentadienyl radicals were the key free radicals in the generation of coke and tar during the pyrolysis process. And through hydrogenation pyrolysis experiments, it was verified that the relative content of o-semiquinone radical and the solid coke were reduced with a hydrogen supply source. Our results showed that by controlling the key free radicals in the coking process, the formation of coke can be suppressed and the conversion efficiency can be improved.

Semiquinone radical anions derived from 2,3-dimethylchrysazin, 7-deoxyaklavinone, and aclacinomycin T. Computational studies of the influence of aprotic and protic solvents on the electron spin resonance spectra

The influence of the solvent on the molecular structures and the ESR (EPR) spectra of the three title semiquinones is investigated by theoretical calculations, using density functional theory (DFT) and polarizable continuum models (PCM). The reordering of the relative magnitudes of the isotropic hyperfine constants observed when passing from an aprotic to a protic solvent is well reproduced by calculations using the integral equation formalism PCM (IEFPCM), provided discrete solute-solvent hydrogen-bonding interactions in the protic solvent is considered explicitly. A mixed discrete-continuum procedure with a simplified super-molecular model leads to excellent prediction of all proton hyperfine constants measured in methanol solution, including methine as well as alkyl protons. The dependence of the alkyl proton hyperfine constants on the conformation of the A rings in the semiquinones of 7-deoxyaklavinone and aclacinomycin T is discussed.

Stimuli‐Responsive Resorcin[4]arene Cavitands: Toward Visible‐Light‐Activated Molecular Grippers

Resorcin[4]arene cavitands, equipped with diverse quinone (Q) and [Ru(bpy)2 dppz]2+ (bpy=2,2'-bipyridine, dppz=dipyrido[3,2-a:2',3'-c]phenazine) photosensitizing walls in different configurations, were synthesized. Upon visible-light irradiation at 420 nm, electron transfer from the [Ru(bpy)2 dppz]2+ to the Q generates the semiquinone (SQ) radical anion, triggering a large conformational switching from a flat kite to a vase with a cavity for the encapsulation of small guests, such as cyclohexane and heteroalicyclic derivatives, in CD3 CN. Depending on the molecular design, the SQ radical anion can live for several minutes (≈10 min) and the vase can be generated in a secondary process without need for addition of a sacrificial electron donor to accumulate the SQ state. Switching can also be triggered by other stimuli, such as changes in solvent, host-guest complexation, and chemical and electrochemical processes. This comprehensive investigation benefits the development of stimuli-responsive nanodevices, such as light-activated molecular grippers.

Chrysazin semiquinone radical anion. A theoretical study of the influence of the solvent on the electron spin resonance spectrum

The strong influence of the solvent on the hyperfine coupling constants of the semiquinone radical anion derived from chrysazin (1,8-dihydroxy-9,10-anthraquinone) was investigated by theoretical calculations, using the polarizable continuum models IEFPCM and IPCM. The current implementation of the popular IEFPCM model does not describe the influence of specific interactions between solute and solvent molecules. This model thus requires incorporation of hydrogen-bonded solute–solvent clusters to approximate the situation in alcoholic and aqueous solvents. The results using the IPCM model suggest that it may be used as a tool to simulate the effect of rapidly changing formation and breaking of solute–solvent complexes by means of an average electrostatic reaction field at the position of the solute species, thus approximating the influence of specific effects within the polarizable continuum picture.

Stopped-flow-optical Absorption and -electron Spin Resonance Studies on Short-lived Quercetin Semiquinone Radical Produced by Redox Reactions with O2−• Radical in DMSO

The stopped-flow-optical absorption and -ESR spectra of a quercetin (QueH2) derived semiquinone anion radical (Que−•, half-life time ∼31 ms) were detected by mixing the O2−• radical and QueH2 in di...

Uno Ferro, a de novo Designed Protein, Binds Transition Metals with High Affinity and Stabilizes Semiquinone Radical Anion.

Metalloenzymes often utilize radicals in order to facilitate chemical reactions. Recently, DeGrado and co-workers have discovered that model proteins can efficiently stabilize semiquinone radical anion produced by oxidation of 3,5-di-tert-butylcatechol (DTBC) in the presence of two zinc ions. Here, we show that the number and the nature of metal ions have relatively minor effect on semiquinone stabilization in model proteins, with a single metal ion being sufficient for radical stabilization. The radical is stabilized by both metal ion, hydrophobic sequestration, and interactions with the hydrophilic residues in the protein interior resulting in a remarkable, nearly 500 mV change in the redox potential of the SQ. - /catechol couple compared to bulk aqueous solution. Moreover, we have created 4G-UFsc, a single metal ion-binding protein with pm affinity for zinc that is higher than any other reported model systems and is on par with many natural zinc-containing proteins. We expect that the robust and easy-to-modify DFsc/UFsc family of proteins will become a versatile tool for mechanistic model studies of metalloenzymes.

In situ reactivity of electrochemically generated semiquinone on Emodin and its CuII/MnII complexes with pyrimidine based nucleic acid bases and calf thymus DNA: Insight into free radical induced cytotoxicity of anthracyclines

There is enough proof to believe that free-radical intermediates of anthracycline based anticancer agents are involved in different stages of drug action. Subtle therapeutic differences observed in the actions of different anthracyclines largely influence the mechanism of action of the drugs that distinguish one member from another. Redox properties control biological responses related either to the one-electron quinone/semiquinone couple or the two-electron quinone/quinone-dianion couple. Comproportionation also leads to generation of semiquinone. Hence, whatever the form of reduction of the quinone moiety, a substantial amount of semiquinone is eventually formed in the system. Immediately after formation, there is competition between natural radical-decay pathways and one-electron transfer reactions that generate the superoxide-radical anion. Prototropic properties control rate of radical decay while redox properties control rate of electron transfer to molecular oxygen. In aerated medium, semiquinone-radical anion and superoxide-radical anion co-exist while in de-aerated medium semiquinone-radical anion predominates. All the radicals are damaging to the biological system. Through this study, attempt was made to detect changes induced by the radicals on pyrimidine based nucleic acid bases and calf thymus DNA in aerated and de-aerated (Ar saturated) medium to know the mechanism by which Emodin, its CuII/MnII complexes might affect DNA. Semiquinone-radical anion was generated electrochemically maintaining a glassy carbon electrode at the first reduction potential of each compound. Since the chosen compound (Emodin), its complexes are analogues of anthracyclines, findings on them can be extrapolated to understand the differences in anticancer activity or of adverse drug reactions reported in an innumerable number of clinical studies related to anthracyclines where the difference in structure of different members is due to differences in the relative positioning of hydroxy groups on the hydroxy-9, 10-anthraquinone moiety of anthracyclines. The study helps to realize action of compounds of this class as anticancer agents.

Tracking electrochemical reactions inside organic electrodes by operando IR spectroscopy

IR spectroscopy can be a non-destructive and straightforward probing tool in the battery research. However, its application has been limited due to the difficulties related to the handling and interpretation of ex situ samples along with the lack of widely applicable in situ and operando cells. Herein, we show a simple, operando ATR-IR two electrode pouch cell with an IR-transmissive silicon window and discuss its advantages and limitations. This setup is applied to the study of the polyanthraquinone (PAQ) reaction mechanism in Li- and Mg-organic batteries. During the reduction/oxidation process of the PAQ, not only the conversion of both CO groups into C–O– species is observed, but also the formation of an intermediate semiquinone radical anion as an intermediate product. Furthermore, continuous measurement of IR spectra allows visualization of the gradual solid-electrolyte interphase (SEI) buildup on the cathode during cycling.

The Quest for Molecular Grippers: Photo-Electric Control of Molecular Gripping Machinery.

The quest for nanoscale molecular machines has inspired the search for their close relatives, molecular grippers. This path was paved by the development of resorcin[4]arene cavitands and their quinone-based redox-active congeners. In this Concept article, the efforts to design and establish the control of quinone-functionalized resorcin[4]arenes by electronic and electromagnetic stimuli is described. This was achieved by relying on paramagnetic semiquinone radical anions formed electrochemically or by photoredox catalysis. The gripper-like motion of such species could not be studied by conventional NMR spectroscopy. Instead, an entirely different approach had to be developed that included various electroanalytical and spectroelectrochemical methods, including UV/Vis/NIR spectroelectrochemistry, pulsed EPR and Davies 1 H ENDOR spectroscopy, transient absorption spectroscopy, and time-resolved luminescence measurements, besides density functional theory calculations and X-ray crystallography. The conceptual breakthroughs are reviewed as well as the current state and future perspectives of photoredox-switchable molecular grippers.

Influence of organic cations on the stacking of semiquinone radical anions

A series of salts of tetrachloro- and tetrabromosemiquinone radical anions reveal four types of stacks: 1) pancake bonded dimers, 2) pancake-bonded trimers, 3) equidistant radicals and 4) a novel type of equidistant stacks of partially charged radicals.

Oxidation of nucleic acids: Chemistry of pyrene quinone (PQ 1) and development of dihydrodioxins (DHD 2) as DNA photooxidizing agents

Pyrene-4,5-dione (pyrene quinone, PQ 1) is a polycyclic aromatic hydrocarbon (PAH), a planar group and DNA-cutting functionality that can cause cytotoxic, mutagenic, and carcinogenic damage to DNA and can encourage cancers in humans. It is extensively accepted that PAHs require metabolic activation in order to exert their biological activities, including carcinogenicity. Upon entering the cell PAHs are either metabolized into diol epoxides, quinones, or free-radical intermediates, all of which can react with cellular DNA to produce PAH-DNA covalent adducts and cause other forms of DNA damage. We have studied the binding and photo-reaction of PQ 1 with herring sperm (HS) DNA in order to see if it binds to the DNA. PQ 1 alone does not bind to herring sperm DNA, even though it might form hydrogen bonds with bases. In addition, PQ 1 alone does not linearized super-coiled ΦX174 plasmid DNA. Therefore, PQ 1 alone does not have the capability to photochemically cleave and/or damages DNA duplexes directly. We attribute this lack of reactivity to the low solubility of PQ 1 in water, since when it is masked with a water solubilizing group (dipyridinium dihydrodioxin), DHD 2, it is very effective in causing DNA damage. Electrochemical results indicate that the reduction potential of the first electron donated to PQ 1 in acetonitrile has E0red = −0.052 V vs. NHE, and converts it to the semiquinone radical anion. Thus, the Gibbs free energy (ΔG0) of the electron transfer to the excited state of PQ 1 in DNA is roughly estimated using the Rehm–Weller equation to have ΔG0 = −33.536 kcal/mol, which indicates the electron transfer can happen spontaneously from adenine to the photoexcited PQ 1. These results strongly suggest, and are consistent with the hypothesis, that PQ 1 cleaves and/or damages DNA duplexes at Gs or As when it is intercalated between base pairs. DNA–DHD 2 interactions have previously been investigated by transient absorption spectroscopy which indicates that PQ 1 is photochemically released into the base pair stacks where it can photochemically triggers the further release of PQ 1 and cause extensive DNA damage. Thus the photochemical release of PQ 1 from DHD 2 is autocatalytic, which makes this system a very effective DNA oxidizing system. The nature of this DNA damage has been investigated by mass spectroscopy using electrospray ionization. In this work, experiments using DNA with known sequences of nucleotides such as: A7 + DHD 2, A7 + PQ 1, ds (T7A7) + DHD 2, and ds (T7A7) + PQ 1, were investigated. DHD+22-2BF4− photochemically damages the DNA and the generated PQ 1 and dipyridinium stilbene become attached to the DNA bases at different positions. Furthermore, atomic force microscopy of the plasmid ΦX174 photochemically damaged with DHD 2 indicates that the DNA strands become extensively crosslinked together forming aggregated DNA that would not be subject to repair.

Choreography of the Reductase and P450BM3 Domains Toward Electron Transfer Is Instigated by the Substrate.

The driving force for the electron transfer (ET) step in the catalytic cycle of cytochrome P450BM3 is investigated using three sets of 1 μs molecular dynamic simulations for the resting state of P450 in complex with the flavin (FMN) in the reductase domain. These sets involve the following: (i) substrate-free (SF), (ii) substrate (N-palmitoyl glycine, i.e., NPG)-bound (SB), and (iii) SB with the semiquinone radical anion (SQ-) of FMN. Starting from the X-ray structure of the SF heme domain, we observe that the α1-helix (of the reductase) and the C-helix (of the heme) undergo reorientation to a parallel orientation, which is the thermochemically stable form. The reorientation of the helices pushes away the FMN to a distance of 18.4 Å from the heme's center. When the substrate binds it causes the I-helix of the heme domain to kink and push the C-helix toward the α1-helix, thereby locking the latter two into a stabilized perpendicular conformation, wherein the FMN-heme distance is 12 Å. The distance drops further in the SQ- form, and upon QM/MM geometry optimization the two moieties approach 8.8 Å, which enhances the ET rate (by 104-106 fold) to the heme's Fe3+ ion. These motions are driven by hydrogen bond strengthening between the C- and the α1-helices. Finally, substrate binding leads to formation of an organized water chain connecting the FMN and heme moieties. The water channel assists the ET and couples it to the proton transfer steps that should activate O2 and create the oxo-iron active species.

The semiquinone radical anion of 1,10-phenanthroline-5,6-dione: synthesis and rare earth coordination chemistry

Reduction of 1,10-phenanthroline-5,6-dione (pd) with CoCpR2 resulted in the first molecular compounds of the pd˙- semi-quinone radical anion, [CoCpR2]+[pd]˙- (R = H, (1); R = Me4, (2)). Furthermore compounds 1 and 2 were reacted with [Y(hfac)3(thf)2] (hfac = 1,1,1-5,5,5-hexafluoroacetylacetonate) to synthesise the rare earth-transition metal heterometallic compounds, [CoCpR2]+[Y(hfac)3(N,N'-pd)]˙- (R = H, (3); R = Me4, (4)).