David M. Ermert

The title compound, (C₈H₂₂N₂O)₂[Mo₈O₂₆]·H₂O, (cis-H₂ L)₂[β-Mo₈O₂₆]·H₂O, where L = (bis[2-N,N-dimethylamino)ethyl] ether), was synthesized from bis[2-(dimethylamino)ethyl] ether and MoO₃ under solvothermal conditions and characterized by multinuclear NMR and single-crystal X-ray diffraction techniques. The structure displays two [oxybis(ethane-1,2-diyl)]bis(dimethylammonium), or [cis-H₂ L]²⁺, cations, a central [β-Mo₈O₂₆]⁴⁻ anionic cluster consisting of eight distorted MoO₆ octahedra, and two water molecules in their deuterated form. The central anion lies across an inversion center. The [cis-H₂ L]²⁺ cations are hydrogen bonded to the central [β-Mo₈O₂₆]⁴⁻ cluster via bridging water molecules. In the crystal, O—H...O hydrogen bonds link the components into chains along [010]. Weak C—H...O hydrogen bonds link these chains into a three-dimensional network.

Polynuclear manganese compounds have garnered interest as mimics and models of the water oxidizing complex (WOC) in photosystem II and as single molecule magnets. Molecular systems in which composition can be correlated to physical phenomena, such as magnetic exchange interactions, remain few primarily because of synthetic limitations. Here, we report the synthesis of a family of trimanganese(II) complexes of the type Mn₃X₃L (X = Cl^–, H^–, and MeO^–) where L^3– is a tris(β-diketiminate) cyclophane. The tri(chloride) complex (2) is structurally similar to the reported tri(bromide) complex (1) with the Mn₃X₃ core having a ladder-like arrangement of alternating M–X rungs, whereas the tri(μ-hydride) (3) and tri(μ-methoxide) (4) complexes contain planar hexagonal cores. The hydride and methoxide complexes are synthesized in good yield (48% and 56%) starting with the bromide complex employing a metathesis-like strategy. Compounds 2–4 were characterized by combustion analysis, X-ray crystallography, X-band EPR spectroscopy, SQUID magnetometry, and infrared and UV–visible spectroscopy. Magnetic susceptibility measurements indicate that the Mn₃ clusters in 2–4 are antiferromagnetically coupled, and the spin ground state of the compounds (S = 3/2 (1, 2) or S = 1/2 (3, 4)) is correlated to the identity of the bridging ligand and structural arrangement of the Mn₃X₃ core (X = Br, Cl, H, OCH₃). Electrochemical experiments on isobutyronitrile solutions of 3 and 4 display broad irreversible oxidations centered at 0.30 V.

The rational design of trimetallic transition metal clusters supported by a trinucleating cyclophane ligand, L³⁻, and the reactivities of these complexes with dinitrogen and carbon dioxide are discussed.

The designed [3M–3(μ-H)] clusters (M = Fe^II, Co^II) Fe₃H₃L (1-H) and Co₃H₃L (2-H) [where L^3– is a tris(β-diketiminate) cyclophane] were synthesized by treating the corresponding M₃Br₃L complexes with KBEt₃H. From single-crystal X-ray analysis, the hydride ligands are sterically protected by the cyclophane ligand, and these complexes selectively react with CO₂ over other unsaturated substrates (e.g., CS₂, Me₃SiCCH, C₂H₂, and CH₃CN). The reaction of 1-H or 2-H with CO₂ at room temperature yielded Fe₃(OCHO)(H)₂L (1-CO₂) or Co₃(OCHO)(H)₂L (2-CO₂), respectively, which evidence the differential reactivity of the hydride ligands within these complexes. The analogous reactions at elevated temperatures revealed a distinct difference in the reactivity pattern for 2-H as compared to 1-H; Fe₃(OCHO)₃L (1-3CO₂) was generated from 1-H, while 2-H afforded only 2-CO₂.

Using a simple metathesis approach, the triiron(II) tribromide complex Fe₃Br₃L (1) reacts with tetrabutylammonium azide to afford the monoazide dibromide analogue Fe₃(Br)₂(N₃)L (2) in high yield. The inclusion of azide was confirmed by IR spectroscopy with a ν(N₃) = 2082 cm^–1 as well as combustion analysis and X-ray crystallography. Heating 2 in the solid state results in the complete loss of the azide vibration in the IR spectra and the isolation of the olive-green mononitride complex Fe₃(Br)₂(N)L (3). Solution magnetic susceptibility measurements support that the trimetallic core within 2 is oxidized upon generation of 3 (5.07 vs 3.09 μ_B). Absorption maxima in the UV–visible–near-IR (NIR) spectra of 2 and 3 support the azide-to-nitride conversion, and a broad NIR absorption centered at 1117 nm is similar to that previously reported for the intervalence charge-transfer band for a mixed-valent nitridodiiron cluster. The cyclic voltammograms recorded for 3 are comparable to those of 1 with no reductive waves observed between ∼0 and −2.5 V (vs Fc/Fc⁺), whereas a reversible one-electron redox process is observed for Fe₃(NH₂)₃L (4). These results suggest that intercluster cooperativity is unlikely to predominate the dinitrogen reduction mechanism when 1 is treated with KC₈ under N₂.

The reaction of [Zn₃Cl₃L], in which L³⁻ is a tris(β‐diketiminate) cyclophane, with K(sBu)₃BH afforded [Zn₃(μ‐H)₃L] (2), as confirmed by NMR spectroscopy, NOESY, and X‐ray crystallography. The complex 2 was air‐stable and unreactive towards water, methanol, and other substrates (e.g., nitriles) at room temperature over 24 h but reacted with CO₂ (ca. 1 atm) to generate [Zn₃(μ‐H)₂(μ‐1,1‐O₂CH)] (3). In contrast, [Zn₃(OH)₃L] (4) was found to be unreactive toward CO₂ over the course of several days at 90 °C.

A trinucleating cyclophane bearing three β-diketimine arms, and the corresponding iron(II) and manganese(II) complexes have been synthesized and characterized. The three metal ions are oriented towards the internal void space, and are coordinated by a N₂Br₂ donor set, in which a unique combination of μ³, μ, and terminal coordination modes are observed for the halide donors.