Heterobimetallic and Heterotrimetallic Clusters Containing a Redox‐Active Metalloligand

Selective Manipulation of Well-Defined Trinuclear Pd(II)-Complexes

Several strategies are available to design well-defined multimetallic molecular entities bearing functional ligands. Substoichiometric exchange reactions in the coordination sphere of pre-existing multinuclear precursors are relatively underexploited in this context. Palladium(II) acetate is not a mononuclear compound in the solid state but rather exists as a trimer, i. e. [Pd3(OAc)6]. Although this material is ubiquitously used to synthesize mononuclear Pd species, it may principally also lend itself to selective exchange of some of the edge-sharing acetate units in its triangular motif, whilst keeping the overall multinuclear architecture intact. Strikingly, little is known about the controlled manipulation and substoichiometric substitution chemistry of this well-defined conglomerate. We herein conclusively demonstrate that, for the first time, the targeted exchange of two or four acetate units from the Pd3(acetate)6 platform is possible, thereby installing either one or two new tridentate ligands onto this trinuclear architecture. Follow-up exchange and substitution chemistry is available without disrupting the multimetallic nature of the core structure. New complexes 2-7 are all conclusively characterized using multinuclear NMR spectroscopy, UV-vis and IR spectroscopy as well as X-ray diffraction analysis.

Three-Coordinate Nickel and Metal-Metal Interactions in a Heterometallic Iron-Sulfur Cluster

Biological multielectron reactions often are performed by metalloenzymes with heterometallic sites, such as anaerobic carbon monoxide dehydrogenase (CODH), which has a nickel-iron-sulfide cubane with a possible three-coordinate nickel site. Here, we isolate the first synthetic iron-sulfur clusters having a nickel atom with only three donors, showing that this structural feature is feasible. These have a core with two tetrahedral irons, one octahedral tungsten, and a three-coordinate nickel connected by sulfide and thiolate bridges. Electron paramagnetic resonance (EPR), Mössbauer, and superconducting quantum interference device (SQUID) data are combined with density functional theory (DFT) computations to show how the electronic structure of the cluster arises from strong magnetic coupling between the Ni, Fe, and W sites. X-ray absorption spectroscopy, together with spectroscopically validated DFT analysis, suggests that the electronic structure can be described with a formal Ni1+ atom participating in a nonpolar Ni-W σ-bond. This metal-metal bond, which minimizes spin density at Ni1+, is conserved in two cluster oxidation states. Fe-W bonding is found in all clusters, in one case stabilizing a local non-Hund state at tungsten. Based on these results, we compare different M-M interactions and speculate that other heterometallic clusters, including metalloenzyme active sites, could likewise store redox equivalents and stabilize low-valent metal centers through metal-metal bonding.

Expanding the Utility of β-Diketiminate Ligands in Heavy Group VI Chemistry of Molybdenum and Tungsten

We report the synthesis of 17 molybdenum and tungsten complexes supported by the ubiquitous BDI ligand framework (BDI = β-diketiminate). The focal entry point is the synthesis of four molybdenum and tungsten(V) BDI complexes of the general formula [MO(BDI^R)Cl₂] [M = Mo, R = Dipp (1); M = W, R = Dipp (2); M = Mo, R = Mes (3); M = W, R = Mes (4)] synthesized by the reaction between MoOCl₃(THF)₂ or WOCl₃(THF)₂ and LiBDI^R. Reactivity studies show that the BDI^Dipp complexes are excellent precursors toward adduct formation, reacting smoothly with dimethylaminopyridine (DMAP) and triethylphosphine oxide (OPEt₃). No reaction with small phosphines has been observed, strongly contrasting the chemistry of previously reported rhenium(V) complexes. Additionally, the complexes 1 and 2 are good precursors for salt metathesis reactions. While 1 can be chemically reduced to the first stable example of a Mo(IV) BDI complex 15, reduction of 2 resulted in degradation of the BDI ligand via a nitrene transfer reaction, leading to MAD (4-((2,6-diisopropylphenyl)imino)pent-2-enide) supported tungsten(V) and tungsten(VI) complexes 16 and 17. All reported complexes have been thoroughly studied by VT-NMR and (heteronuclear) NMR spectroscopy, as well as UV-vis and EPR spectroscopy, IR spectroscopy, and X-ray diffraction analysis.

Switching of the redox centers of a tris-2-mercaptophenolato chromium(III) metalloligand by a guest metal ion

This work reports that the redox-active metalloligand (ML) fac-[Cr^III(mp)₃]^3- (mp: 2-mercaptophenolato) coordinates with a Co(III) ion to afford the trianionic complex [Co^III{fac-Cr^III(mp)₃}₂]^3-. The free ML shows ligand-centered redox processes, whereas the guest-metal-bound trinuclear structure exhibited a guest-metal-centered Co(II)/Co(III) redox couple, demonstrating redox switching through guest-metal binding to the MLs.

Two-Electron Mixed Valency in a Heterotrimetallic Nickel-Vanadium-Nickel Complex

Mixed-valence complexes represent an enticing class of coordination compounds to interrogate electron transfer confined within a molecular framework. The diamagnetic heterotrimetallic anion, [V(SNS)₂{Ni(dppe)}₂]^-, was prepared by reducing (dppe)NiCl₂ in the presence of the chelating metalloligand [V(SNS)₂]^- [dppe = bis(diphenylphosphino)ethane; (SNS)^3- = bis(2-thiolato-4-methylphenyl)amide]. Vanadium-nickel bonds span the heterotrimetallic core in the structure of [V(SNS)₂{Ni(dppe)}₂]^-, with V-Ni bond lengths of 2.78 and 2.79 Å. One-electron oxidation of monoanionic [V(SNS)₂{Ni(dppe)}₂]^- yielded neutral, paramagnetic V(SNS)₂{Ni(dppe)}₂. The solid-state structure of V(SNS)₂{Ni(dppe)}₂ revealed that the two nickel ions occupy unique coordination environments: one nickel is in a square-planar S₂P₂ coordination environment (τ₄ = 0.19), with a long Ni···V distance of 3.45 Å; the other nickel is in a tetrahedral S₂P₂ coordination environment (τ₄ = 0.84) with a short Ni-V distance of 2.60 Å, consistent with a formal metal-metal bond. Continuous-wave X-band electron paramagnetic resonance spectroscopy, electrochemical investigations, and density functional theory computations indicated that the unpaired electron in the neutral V(SNS)₂{Ni(dppe)}₂ cluster is localized on the bridging [V(SNS)₂] metalloligand, and as a result, V(SNS)₂{Ni(dppe)}₂ is best described as a two-electron mixed-valence complex. These results demonstrate the important role that metal-metal interactions and flexible coordination geometries play in enabling multiple, reversible electron transfer processes in small cluster complexes.

Transition Metal Chain Complexes Supported by Soft Donor Assembling Ligands

The chemistry of discrete molecular chains constituted by metals in low oxidation states, displaying metal-metal proximity and stabilized by suitable metal-bridging, assembling ligands comprising at least one soft donor atom is comprehensively reviewed; complexes with a single (hard or soft) bridging atom (e.g., μ-halide, μ-sulfide, or μ-PR₂ etc.) as well as "closed" metal arrays (that fall in the realm of cluster chemistry) are excluded. The focus is on transition metal-based systems, with few excursions to cases combining transition and post-transition elements. Most relevant supporting ligands have neutral C, P, O, or S donor (mainly, N-heterocyclic carbene, phosphine, ether, thioether) or anionic donor (mainly phenyl, ylide, silyl, phosphide, thiolate) groups. A supporting-ligand-based classification of the metal chains is introduced, using as the classifying parameter the number of "bites" (i.e., ligand bridges) subtending each intermetallic separation. The ligands are further grouped according to the number of donor atoms interacting with the metal chain (called denticity in the following) and the column of the Periodic Table to which the set of donor atoms belongs (in ascending order). A complementary metal-based compilation of the complexes discussed is also provided in a concise tabular form.

Stepwise assembly of heterobimetallic complexes: synthesis, structure, and physical properties

Bimetallic active sites are ubiquitous in metalloenzymes and have sparked investigations of synthetic models to aid in the establishment of structure-function relationship. We previously reported a series of discrete bimetallic complexes with [FeIII-(μ-OH)-MII] cores in which the ligand framework provides distinct binding sites for two metal centers. The formation of these complexes relied on a stepwise synthetic approach in which an FeIII-OH complex containing a sulfonamido tripodal ligand served as a synthon that promoted assembly. We have utilized this approach in the present study to produce a new series of bimetallic complexes with [FeIII-(μ-OH)-MII] cores (M = Ni, Cu, Zn) by using an ancillary ligand to the FeIII center that contains phosphinic amido groups. Assembly began with formation of an FeIII-OH that was subsequently used to bind the MII fragment that contained a triazacyclononane ligand. The series of bimetallic complexes were charactered structurally by X-ray diffraction methods, spectroscopically by absorption, vibrational, electron paramagnetic resonance spectroscopies, and electrochemically by cyclic voltammetry. A notable finding is that these new [FeIII-(μ-OH)-MII] complexes displayed significantly lower reduction potentials than their sulfonamido counterparts, which paves way for future studies on high valent bimetallic complexes in this scaffold.

Dimensional Structures and Electrocatalytic Activities of Platinum(II)-Palladium(II)-Manganese(II) Coordination Polymers Controlled by Chloride versus Bromide

Here we report the synthesis and structural characterization of heterotrimetallic (Pt^II₂Pd^II₂Mn^II₂)_n coordination polymers that show different dimensionalities and electrocatalytic activities depending on chloride and bromide employed in the system. The reaction of the Pt^II₂Pd^II₂ tetranuclear complex [Pd₂Pt₂(NH₃)₄(d-pen)₄] (1), bearing free carboxylate groups, with Mn²⁺ in the presence of chloride produced the (Pt^II₂Pd^II₂Mn^II₂)_n coordination polymer [Mn₂Cl₂(H₂O)₆(1)]Cl₂ (2^Cl), in which the Pt^II₂Pd^II₂ units of 1 are linked by [MnCl(H₂O)₃]⁺ moieties in a 1:2 ratio to form a 2D sheetlike structure. The corresponding reaction in the presence of bromide also gave the (Pt^II₂Pd^II₂Mn^II₂)_n coordination polymer [Mn₂(H₂O)₆(1)]Br₄ (3^Br), but 3^Br adopted a 3D network structure via a 1:1 linkage of the Pt^II₂Pd^II₂ units with [Mn₂(H₂O)₆]⁴⁺ moieties. These complexes showed appreciably different electrocatalytic activities for H₂ evolution.

Molybdenum imidazole citrate and bipyridine homocitrate in different oxidation states – balance between coordinated α-hydroxy and α-alkoxy groups

Oxo and thiomolybdenum(iv/vi) imidazole hydrocitrates K₂{Mo^IV₃O₄(im)₃[Mo^VIO₃(Hcit)]₂}·3im·4H₂O (1), (Him)₂{Mo^IV₃SO₃(im)₃[Mo^VIO₃(Hcit)]₂}·im·6H₂O (2), molybdenum(v) bipyridine homocitrate trans-[(Mo^VO)₂O(H₂homocit)₂(bpy)₂]·4H₂O (3) and molybdenum(vi) citrate (Et₄N)[Mo^VIO₂Cl(H₂cit)]·H₂O (4) (H₄cit = citric acid, H₄homocit = homocitric acid, im = imidazole and bpy = 2,2′-bipyridine) with different oxidation states were prepared. 1 and 2 are the coupling products of [Mo^VIO₃(Hcit)]³⁻ anions and incomplete cubane units [Mo^IV₃O₄]⁴⁺ ([Mo^IV₃SO₃]⁴⁺) with monodentate imidazoles, respectively, where tridentate citrates coordinate with α-hydroxy, α-carboxy and β-carboxy groups, forming pentanuclear skeleton structures. The molybdenum atoms in 1 and 2 show unusual +4 and +6 valences based on charge balances, theoretical bond valence calculations and Mo XPS spectrum. The coordinated citrates in 1 and 2 are protonated with α-hydroxy groups, while 3 and 4 with higher oxidation states of +5 and +6 are deprotonated with α-alkoxy group even under strong acidic condition, respectively. This shows the relationship between the oxidation state and protonation of the α-alkoxy group in citrate or homocitrate, which is related to the protonation state of homocitrate in FeMo-cofactor of nitrogenase. The homocitrate in 3 chelates to molybdenum(v) with bidentate α-alkoxy and monodentate α-carboxy groups. Molybdenum(vi) citrate 4 is only protonated with coordinated and uncoordinated β-carboxy groups. The solution behaviours of 1 and 2 are discussed based on ¹H and ¹³C NMR spectroscopies and cyclic voltammograms, showing no decomposition of the species.

Oxo and thiomolybdenum(iv/vi) citrates, molybdenum(v) homocitrate and molybdenum(vi) citrate were obtained, showing the influence of coordinated α-hydroxy and α-alkoxy groups with different oxidation states.

Cyanoferrocenes as redox-active metalloligands for coordination-driven self-assembly

Ferrocene-based Lewis bases have found utility as metalloligands in a wide variety of applications. The coordination chemistry of cyanoferrocenes however, is underexplored. Herein, we describe a new synthetic protocol for the generation of cyanoferrocenes. The coordination chemistry of these metalloligands to [Cu(NCMe)4][PF6], [(PPh3)2Cu(NCMe)2][PF6] and [(dppf)Cu(NCMe)2][PF6] salts has been explored, providing crystallographic evidence of cluster and polymeric forms of 1,1'- and 1,2-dicyanoferrocene complexes. The stability of the complexes and ligand dissociation were found to be strongly solvent-dependent.

Ligand-Based Storage of Protons and Electrons in Dihydrazonopyrrole Complexes of Nickel

A newly developed dihydrazonopyrrole ligand and corresponding Ni complexes have been synthesized and thoroughly characterized. Electrochemical studies and chemical reactivity tests show that these complexes can reversibly store both electrons and protons, or equivalently H-atoms, via ligand-based events. The stored H-atom equivalent can be transferred to small molecules such as acetonitrile or oxygen. Furthermore, this series of complexes can adopt a variety of different coordination modes. In addition to one e^- reactivity, the two e^- electrophilic oxidation of phosphines is also demonstrated. Taken together, these results show that dihydrazonopyrrole complexes represent a geometrically and electronically flexible scaffold for controlling the flow of both electrons and protons.