Two‐Coordinate Fe0 and Co0 Complexes Supported by Cyclic (alkyl)(amino)carbenes

The CAAC [CAAC=cyclic (alkyl)(amino)carbene] family of carbene ligands have shown promise in stabilizing unusually low‐coordination number transition‐metal complexes in low formal oxidation states. Here we extend this narrative by demonstrating their utility in affording access to the first examples of two‐coordinate formal Fe⁰ and Co⁰ [(CAAC)₂M] complexes, prepared by reduction of their corresponding two‐coordinate cationic Fe^I and Co^I precursors. The stability of these species arises from the strong σ‐donating and π‐accepting properties of the supporting CAAC ligands, in addition to steric protection.     

Iodination of cyclo‐E5‐Complexes (E=P,As)

In a high‐yield one‐pot synthesis, the reactions of [Cp*M(η⁵‐P₅)] (M=Fe ( 1 ), Ru ( 2 )) with I₂ resulted in the selective formation of [Cp*MP₆I₆]⁺ salts ( 3 , 4 ). The products comprise unprecedented all‐cis tripodal triphosphino‐cyclotriphosphine ligands. The iodination of [Cp*Fe(η⁵‐As₅)] ( 6 ) gave, in addition to [Fe(CH₃CN)₆]²⁺ salts of the rare [As₆I₈]^2? (in 7 ) and [As₄I₁₄]^2? (in 8 ) anions, the first di‐cationic Fe‐As triple decker complex [(Cp*Fe)₂(μ,η^5:5‐As₅)][As₆I₈] ( 9 ). In contrast, the iodination of [Cp*Ru(η⁵‐As₅)] ( 10 ) did not result in the full cleavage of the M?As bonds. Instead, a number of dinuclear complexes were obtained: [(Cp*Ru)₂(μ,η^5:5‐As₅)][As₆I₈]_0.5 ( 11 ) represents the first Ru‐As₅ triple decker complex, thus completing the series of monocationic complexes [(Cp^RM)₂(μ,η^5:5‐E₅)]⁺ (M=Fe, Ru; E=P, As). [(Cp*Ru)₂As₈I₆] ( 12 ) crystallizes as a racemic mixture of both enantiomers, while [(Cp*Ru)₂As₄I₄] ( 13 ) crystallizes as a symmetric and an asymmetric isomer and features a unique tetramer of {AsI} arsinidene units as a middle deck.     

Stability of arsenic peptides in plant extracts: off-line versus on-line parallel elemental and molecular mass spectrometric detection for liquid chromatographic separation

The instability of metal and metalloid complexes during analytical processes has always been an issue of an uncertainty regarding their speciation in plant extracts. Two different speciation protocols were compared regarding the analysis of arsenic phytochelatin (As^IIIPC) complexes in fresh plant material. As the final step for separation/detection both methods used RP-HPLC simultaneously coupled to ICP-MS and ES-MS. However, one method was the often used off-line approach using two-dimensional separation, i.e. a pre-cleaning step using size-exclusion chromatography with subsequent fraction collection and freeze-drying prior to the analysis using RP-HPLC–ICP-MS and/or ES-MS. This approach revealed that less than 2% of the total arsenic was bound to peptides such as phytochelatins in the root extract of an arsenate exposed Thunbergia alata, whereas the direct on-line method showed that 83% of arsenic was bound to peptides, mainly as As^IIIPC₃ and (GS)As^IIIPC₂. Key analytical factors were identified which destabilise the As^IIIPCs. The low pH of the mobile phase (0.1% formic acid) using RP-HPLC–ICP-MS/ES-MS stabilises the arsenic peptide complexes in the plant extract as well as the free peptide concentration, as shown by the kinetic disintegration study of the model compound As^III(GS)₃ at pH 2.2 and 3.8. But only short half-lives of only a few hours were determined for the arsenic glutathione complex. Although As^IIIPC₃ showed a ten times higher half-life (23 h) in a plant extract, the pre-cleaning step with subsequent fractionation in a mobile phase of pH 5.6 contributes to the destabilisation of the arsenic peptides in the off-line method. Furthermore, it was found that during a freeze-drying process more than 90% of an As^IIIPC₃ complex and smaller free peptides such as PC₂ and PC₃ can be lost. Although the two-dimensional off-line method has been used successfully for other metal complexes, it is concluded here that the fractionation and the subsequent freeze-drying were responsible for the loss of arsenic phytochelatin complexes during the analysis. Hence, the on-line HPLC–ICP-MS/ES-MS is the preferred method for such unstable peptide complexes. Since freeze-drying has been found to be undesirable for sample storage other methods for sample handling needed to be investigated. Hence, the storage of the fresh plant at low temperature was tested. We can report for the first time a storage method which successfully conserves the integrity of the labile arsenic phytochelatin complexes: quantitative recovery of As^IIIPC₃ in a formic acid extract of a Thunbergia alata exposed for 24 h to 1 mg As^v L⁻¹ was found when the fresh plant was stored for 21 days at 193 K. Figure On-line HPLC–ICP-MS/ES-MS (bottom) is the preferred method for MS determination of unstable arsenic peptide complexes in plant extracts, since this avoids fractionation and subsequent freeze-drying that are responsible for loss of arsenic phytochelatin complexes in the 2D off-line method (top) Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Solution Chemistry of Arsenic Anions in the Presence of Metal Cations

The interaction of arsenic(V) and arsenic(III) oxyanions with metal cations was investigated by potentiometry under temperature and ionic strength conditions approaching those prevailing in natural waters. The selection includes the major metal cations and some other ions of high environmental relevance. Ionic pairs [M(As^VO₄)]^?, [M(HAs^VO₄)] and [M(H₂As^IIIO₃)]⁺ formation is suggested for all +2 metal cations, based on the potentiometric results. These ion-pairs between arsenic anions and other metal cations are hardly ever mentioned or taken into account when arsenic speciation in natural waters is considered. These results provide the basis for studying arsenic speciation in natural aquatic systems, on which environmental fate, bioavailability and toxicity of the element depend. Some extrapolations to the conditions of the natural waters are presented as well as some insights into the adsorption process onto hydrous oxides.     

Bis{[phthalocyaninato(2–)]arsenic(III)} octa­iodo­diarsenic(III)

Crystals of the novel title arsenic(III)–phthalocyanine complex, [As(C₃₂H₁₆N₈)]₂[As₂I₈] or [AsPc]₂[As₂I₈], where Pc is the phthalocyaninate(2−) macrocycle, have been obtained from the reaction of pure powdered arsenic with phthalonitrile under oxidizing conditions (iodine vapour) at 463 K. The crystals are formed by separate but inter­acting [AsPc]⁺ cations and centrosymmetric [As₂I₈]²⁻ anions. The As atom of the [AsPc]⁺ ion is bonded to the four isoindole N atoms of the Pc macrocycle and lies 0.762 (1) Å out of their plane. The anionic part of the complex consists of two [AsI₄]⁻ units joined together into a centrosymmetric [As₂I₈]²⁻ counter‐ion. The arrangement of oppositely charged moieties, viz. [AsPc]⁺ and [As₂I₈]²⁻, in the crystal structure is determined mainly by their ionic attractions and by π–π inter­actions between the aromatic phthalocyaninate(2−) macrocycles.     

Samarium Polyarsenides Derived from Nanoscale Arsenic

Zintl phases of arsenic and molecular compounds containing Zintl‐type polyarsenide ions are of fundamental interest in basic and applied sciences. Unfortunately, the most obvious and reactive arsenic source for the preparation of defined molecular polyarsenide compounds, yellow arsenic As₄, is very inconvenient to prepare and neither storable in pure form nor easy to handle. Herein, we present the synthesis and reactivity of elemental As⁰ nanoparticles (As⁰_Nano, d=7.2±1.8 nm), which were successfully utilized as a reactive arsenic source in reductive f‐element chemistry. Starting from [Cp*₂Sm] (Cp*=η⁵‐C₅Me₅), the samarium polyarsenide complexes [(Cp*₂Sm)₂(μ‐η²:η²‐As₂)] and [(Cp*₂Sm)₄As₈] were obtained from As⁰_nano, thereby generating the largest molecular polyarsenide of the f‐elements and circumventing the use of As₄ in preparative chemistry.     

Homoleptic Two‐Coordinate Silylamido Complexes of Chromium(I), Manganese(I), and Cobalt(I)

Anionic two‐coordinate complexes of first‐row transition‐metal(I) centres are rare molecules that are expected to reveal new magnetic properties and reactivity. Recently, we demonstrated that a N(SiMe₃)₂^? ligand set, which is unable to prevent dimerisation or extraneous ligand coordination at the +2 oxidation state of iron, was nonetheless able to stabilise anionic two‐coordinate Fe^I complexes even in the presence of a Lewis base. We now report analogous Cr^I and Co^I complexes with exclusively this amido ligand and the isolation of a [Mn^I{N(SiMe₃)₂}₂]₂^2? dimer that features a Mn?Mn bond. Additionally, by increasing the steric hindrance of the ligand set, the two‐coordinate complex [Mn^I{N(Dipp)(SiMe₃)}₂]^? was isolated (Dipp=2,6‐iPr₂‐C₆H₃). Characterisation of these compounds by using X‐ray crystallography, NMR spectroscopy, and magnetic susceptibility measurements is provided along with ligand‐field analysis based on CASSCF/NEVPT2 ab initio calculations.     

A Mixed‐Valence Tri‐Zinc Complex, [LZnZnZnL] (L=Bulky Amide), Bearing a Linear Chain of Two‐Coordinate Zinc Atoms

Reduction of a variety of extremely bulky amido Group 12 metal halide complexes, [LMX(THF)_0,1] (L=amide; M=Zn, Cd, or Hg; X=halide) with a magnesium(I) dimer gave a homologous series of two‐coordinate metal(I) dimers, [L′MML′] (L′=N(Ar^?)(SiMe₃), Ar^?=C₆H₂{C(H)Ph₂}₂Prⁱ‐2,6,4); and the formally zinc(0) complex, [L*ZnMg(^MesNacnac)] (L*=N(Ar*)(SiPrⁱ₃); Ar*=C₆H₂{C(H)Ph₂}₂Me‐2,6,4; ^MesNacnac=[(MesNCMe)₂CH]^?, Mes=mesityl), which contains the first unsupported Zn? Mg bond. Two equivalents of [L*ZnMg(^MesNacnac)] react with ZnBr₂ or ZnBr₂(tmeda) to give the mixed valence, two‐coordinate, linear tri‐zinc complex, [L*Zn^IZn⁰Zn^IL*], and the first zinc(I) halide complex, [L*ZnZnBr(tmeda)], respectively. The analogues [L*ZnMZnL*] (M=Cd or Hg), were also prepared, the Cd species contains the first Zn? Cd bond in a molecular compound. Metal–metal bonding was studied by DFT calculations.     

Low‐Valent Iron(I) Amido Olefin Complexes as Promotors for Dehydrogenation Reactions

Fe^I compounds including hydrogenases show remarkable properties and reactivities. Several iron(I) complexes have been established in stoichiometric reactions as model compounds for N₂ or CO₂ activation. The development of well‐defined iron(I) complexes for catalytic transformations remains a challenge. The few examples include cross‐coupling reactions, hydrogenations of terminal olefins, and azide functionalizations. Here the syntheses and properties of bimetallic complexes [MFe^I(trop₂dae)(solv)] (M=Na, solv=3 thf; M=Li, solv=2 Et₂O; trop=5H‐dibenzo[a,d]cyclo‐hepten‐5‐yl, dae=(N‐CH₂‐CH₂‐N) with a d⁷ Fe low‐spin valence‐electron configuration are reported. Both compounds promote the dehydrogenation of N,N‐dimethylaminoborane, and the former is a precatalyst for the dehydrogenative alcoholysis of silanes. No indications for heterogeneous catalyses were found. High activities and complete conversions were observed particularly with [NaFe^I(trop₂dae)(thf)₃].     

Combining Topological and Steric Constraints for the Preparation of Heteroleptic Copper(I) Complexes

Heteroleptic copper(I) complexes have been prepared from a macrocyclic ligand incorporating a 2,9‐diphenyl‐1,10‐phenanthroline subunit ( M30 ) and two bis‐phosphines, namely bis[(2‐diphenylphosphino)phenyl] ether (POP) and 1,3‐bis(diphenylphosphino)propane (dppp). In both cases, the diphenylphosphino moieties of the PP ligand are too bulky to pass through the 30‐membered ring of M30 during the coordination process, hence the formation of C_2v‐symmetrical pseudo‐rotaxanes is prevented. When POP is used, X‐ray crystal structure analysis shows the formation of a highly distorted [Cu( M30 )(POP)]⁺ complex in which the POP ligand is only partially threaded through the M30 unit. This compound is poorly stable as the Cu^I cation is not in a favorable coordination environment due to steric constraints. By contrast, in the case of dppp, the bis‐phosphine ligand undergoes both steric and topological constraints and adopts a nonchelating coordination mode to generate [Cu₂( M30 )₂(μ‐dppp)](BF₄)₂. This compound exhibits metal‐to‐ligand charge transfer (MLCT) emission characterized by a very large Stokes’ shift (≈200 nm) that is not attributed to a dramatic structural distortion between the ground and the emitting states but to very weak MLCT absorption transitions at longer wavelengths. Accordingly, [Cu₂( M30 )₂(μ‐dppp)](BF₄)₂ shows unusually high luminescence quantum yields for Cu^I complexes, both in solution and in the solid state (0.5 and 7 %, respectively).     

Low‐Spin Pseudotetrahedral Iron(I) Sites in Fe2(μ‐S) Complexes

Fe^I centers in iron–sulfide complexes have little precedent in synthetic chemistry despite a growing interest in the possible role of unusually low valent iron in metalloenzymes that feature iron–sulfur clusters. A series of three diiron [(L₃Fe)₂(μ‐S)] complexes that were isolated and characterized in the low‐valent oxidation states Fe^II? S? Fe^II, Fe^II? S? Fe^I, and Fe^I? S? Fe^I is described. This family of iron sulfides constitutes a unique redox series comprising three nearly isostructural but electronically distinct Fe₂(μ‐S) species. Combined structural, magnetic, and spectroscopic studies provided strong evidence that the pseudotetrahedral iron centers undergo a transition to low‐spin S=1/2 states upon reduction from Fe^II to Fe^I. The possibility of accessing low‐spin, pseudotetrahedral Fe^I sites compatible with S^2? as a ligand was previously unknown.     

Bimetallic Au2Cu6 Nanoclusters: Strong Luminescence Induced by the Aggregation of Copper(I) Complexes with Gold(0) Species

The concept of aggregation‐induced emission (AIE) has been exploited to render non‐luminescent Cu^ISR complexes strongly luminescent. The Cu^ISR complexes underwent controlled aggregation with Au⁰. Unlike previous AIE methods, our strategy does not require insoluble solutions or cations. X‐ray crystallography validated the structure of this highly fluorescent nanocluster: Six thiolated Cu atoms are aggregated by two Au atoms (Au₂Cu₆ nanoclusters). The quantum yield of this nanocluster is 11.7 %. DFT calculations imply that the fluorescence originates from ligand (aryl groups on the phosphine) to metal (Cu^I) charge transfer (LMCT). Furthermore, the aggregation is affected by the restriction of intramolecular rotation (RIR), and the high rigidity of the outer ligands enhances the fluorescence of the Au₂Cu₆ nanoclusters. This study thus presents a novel strategy for enhancing the luminescence of metal nanoclusters (by the aggregation of active metal complexes with inert metal atoms), and also provides fundamental insights into the controllable synthesis of highly luminescent metal nanoclusters.     

Synthesis and Structure of the Novel Pentaselenidohexaarsenate(I,II) Cluster Anion [As6Se5]2−

Methanolothermal reaction of [MnCl₃(9‐ane‐N₃)] with As₂Se₃ at 150 °C in the presence of Cs₂CO₃ affords violet‐coloured [Mn(9‐ane‐N₃)₂]As₆Se₅ ( 1 ). Its novel tricyclic selenidoarsenate(I,II) anion [As₆Se₅]^2? contains two five‐membered [As₃As(Se)Se] rings that are symmetry‐related by a crystallographic C₂ axis passing through the common As^I‐As^I bond between their respective first two ring members. The adjacent As^I atoms in the individual rings are bridged by the Se atom of the third [As₄Se] ring.     

Bonding in Diborane–Metal Complexes: A Quantum‐Chemical and Experimental Study of Complexes Featuring Early and Late Transition Metals

The coordination chemistry of the doubly base‐stabilised diborane(4), [HB(hpp)]₂ (hpp=1,3,4,6,7,8‐hexahydro‐2H‐pyrimido‐[1,2‐a]pyrimidinate), was extended by the synthesis of new late transition‐metal complexes containing Cu^I and Rh^I fragments. A detailed experimental study was conducted and quantum‐chemical calculations on the metal–ligand bonding interactions for [HB(hpp)]₂ complexes of Group 6, 9, 11 and 12 metals revealed the dominant B? H? M interactions in the case of early transition‐metal fragments, whereas the B? B? M bonding prevails in the case of the late d‐block compounds. These findings support the experimental results as reflected by the IR and NMR spectroscopic parameters of the investigated compounds. DFT calculations on [MeB(hpp)]₂ and model reactions between [B₂H₄ ? 2NMe₃] and [Rh(μ‐Cl)(C₂H₄)₂] showed that the bicyclic guanidinate allows in principle for an oxidative addition of the B? B bond. However, the formation of σ‐complexes is thermodynamically favoured. The results point to the selective B? H or B? B bond‐activation of diborane compounds by complexation, depending on the chosen transition‐metal fragment.     

Bis{[phthalocyaninato(2–)]arsenic(III)} tetradecaiodotetraarsenic(III)

Crystals of the novel title arsenic(III) phthalocyanine complex, [As(C₃₂H₁₆N₈)]₂[As₄I₁₄] or [(AsPc)⁺]₂·[As₄I₁₄]²⁻, where Pc is phthalocyaninate(2−), have been obtained by the reaction of pure powdered As with phthalo­nitrile under a stream of iodine vapour at 493 K. The crystals are built up of separate but interacting [AsPc]⁺ cations and [As₄I₁₂]²⁻ anions. The As atom of the [AsPc]⁺ unit is bonded to the four iso­indole N atoms of the Pc macrocycle and lies 0.743 (2) Å out of the plane defined by these four N atoms. The anionic part of the complex consists of AsI₃ and [AsI₄]⁻ units joined together into an [As₄I₁₄]²⁻ anion. The arrangement of the oppositely charged moieties, [AsPc]⁺ and [As₄I₁₄]²⁻, in the crystal is determined mainly by ionic attraction and by donor–acceptor interactions between the [AsPc]⁺ and [As₄I₁₄]²⁻ ions.     

Redox Chemistry of Heterobimetallic Polypnictogen Triple-Decker Complexes – Rearrangement,Fragmentation and Transfer

The redox chemistry of the heterobimetallic triple-decker complexes [(Cp*Fe)(Cp′′′Co)(μ,η⁵:η⁴-E₅)] (E=P ( 1 ), As ( 2 ), Cp*=1,2,3,4,5-pentamethyl-cyclopentadienyl, Cp′′′=1,2,4-tri-tertbutyl-cyclopentadienyl) and [(Cp′′′Co)(Cp′′′Ni)(μ,η³:η³-E₃)] (E=P ( 10 ), As ( 11 )) was investigated. Compound 1 and 2 could be oxidized to the monocations 3 and 4 and further to the dications 5 and 6 , while the initially folded cyclo-E₅ ligand planarizes upon oxidation. The reduction leads to an opposite change in the geometry of the middle deck, which is now folded stronger into the direction of the other metal fragment (formation of monoanions 7 and 8 ). For the arsenic compound 8 , a different behavior is found since a fragmentation into an As₆ ( 9 ) and As₃ ligand complex occurs. The Co and Ni triple-decker complexes 10 and 11 can be oxidized initially to the heterometallic monocations 12 and 13 , which are not stable in solution and convert selectively into the homometallic nickel complexes 14 and 15 and the cobalt complexes 16 and 17 . This behavior was further proven by the oxidation of [(Cp′′′Co)(Cp′′Ni)(μ,η³:η²-P₃)] ( 19 , Cp′′=1,3-di-tertbutyl-cyclopentadienyl) comprising two different Cp ligands. The transfer of {Cp^RM} fragments can be suppressed when a {W(CO)₅} unit is coordinated to the P₃ ligand ( 20 ) prior to the oxidation and the mixed cobalt and nickel cation 21 can be isolated. The reduction of 10 and 11 yields the heterometallic monoanions 22 and 23 , where no transfer of the {Cp^RM} fragments is observed.     

CpPEt2As4—An Organic‐Substituted As4 Butterfly Compound

Cp^PEt₂As₄ (Cp^PEt=C₅(4‐EtC₆H₄)₅) ( 1 ) is synthesized by the reaction of Cp^PEt. radicals with yellow arsenic (As₄). In solution an equilibrium of the starting materials and the product is found. However, 1 can be isolated and stored in the solid state without decomposition. As₄ can be easily released from 1 under thermal or photochemical conditions. From powder samples of Cp^PEt₂As₄, yellow arsenic can be sublimed under rather mild conditions (T=125 °C). A similar behavior for the P₄‐releasing agent was determined for the related phosphorus compound Cp^BIG₂P₄ ( 2 ; Cp^BIG=C₅(4‐nBuC₆H₄)₅). DFT calculations show the importance of dispersion forces for the stability of the products.     

A Stable Crystalline Copper(I)–N2O Complex Stabilized as the Salt of a Weakly Coordinating Anion

Nitrous oxide is considered a poor ligand, and therefore only a handful of well‐defined metal–N₂O complexes are known. Oxidation of copper powder with an extreme oxidant, [Ag₂I₂][ An ]₂ ([ An ]^?=[Al(OC(CF₃)₃)₄]^?) in perfluorinated hexane leads to Cu^I[ An ], the first auxiliary ligand‐free Cu^I salt of the perfluorinated alkoxyaluminate anion. The compound is capable of forming a stable and crystalline complex with nitrous oxide, Cu(N₂O)[ An ], where the Cu?N₂O bond is by far the strongest among all other molecular metal–N₂O complexes known. Thorough characterization of the compounds together with the crystal structure of Cu(N₂O)[ An ] complex supported with DFT calculations are presented. These give insight into the bonding in the Cu⁺–N₂O system and confirm N‐end coordination of the ligand.     

Scorpionate‐Type Coordination in MFU‐4l Metal–Organic Frameworks: Small‐Molecule Binding and Activation upon the Thermally Activated Formation of Open Metal Sites

Postsynthetic metal and ligand exchange is a versatile approach towards functionalized MFU‐4l frameworks. Upon thermal treatment of MFU‐4l formates, coordinatively strongly unsaturated metal centers, such as zinc(II) hydride or copper(I) species, are generated selectively. Cu^I‐MFU‐4l prepared in this way was stable under ambient conditions and showed fully reversible chemisorption of small molecules, such as O₂, N₂, and H₂, with corresponding isosteric heats of adsorption of 53, 42, and 32 kJ mol^?1, respectively, as determined by gas‐sorption measurements and confirmed by DFT calculations. Moreover, Cu^I‐MFU‐4l formed stable complexes with C₂H₄ and CO. These complexes were characterized by FTIR spectroscopy. The demonstrated hydride transfer to electrophiles and strong binding of small gas molecules suggests these novel, yet robust, metal–organic frameworks with open metal sites as promising catalytic materials comprising earth‐abundant metal elements.     

Synthesis,Characterization, and Reactivity of a Series of Homo- and Hetero-dinuclear Complexes based on an Asymmetric FloH Ligand System

In a previous communication we reported the site-directed generation of a heterodinuclear Fe^IIICu^II complex ( 1 ) by using an asymmetric dinucleating ligand FloH. The iron(III) ion was introduced first on the preferential metal-binding site of the ligand that led to the formation of the thermodynamically favored five-membered chelate ring upon metal-binding. Copper(II) was introduced in the next step. The stepwise metalation strategy reported previously has now been extended to synthesize a series of heterodinuclear Fe^IIIM^II [M = Mn ( 2 ), Fe ( 3 ), Co ( 4 ), and Ni ( 5 )] and Fe^IICu^I ( 1a ) as well as the homodinuclear Cu^ICu^I ( 6 ) complexes. The complexes were characterized by X-ray crystallography (except for 1a and 6 ), and by a limited number of spectroscopic methods. Complex 1 with a labile solvent binding site at Fe^III reacted with H₂O₂ to form a transient intermediate that showed reactivity typical of metal peroxide complexes. The metal centers in the complexes 2 – 5 are coordinatively saturated, and hence they showed no reactivity with H₂O₂. Complex 1a reacted with O₂ via an intermolecular pathway to form a μ-oxo bridged tetrameric complex 1b , which was structurally characterized. This is in contrast to the homodinuclear Cu^ICu^I and heme Fe^IICu^I cores, which prefer an intramolecular pathway for O₂ activation.