Correlating Bridging Ligand with Properties of Ligand-Templated [MnII3X3]3+ Clusters (X = Br-, Cl-, H-, MeO-).

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 L3- is a tris(ß-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.

Insights into small molecule activation by multinuclear first-row transition metal cyclophanates.

The rational design of trimetallic transition metal clusters supported by a trinucleating cyclophane ligand, L(3-), and the reactivities of these complexes with dinitrogen and carbon dioxide are discussed. Emphasis is placed on the differences in the observed reactivity between these trimetallic cyclophane complexes and that of the mono- and dinuclear transition metal compounds.

Reactivity of Hydride Bridges in High-Spin [3M-3(µ-H)] Clusters (M = FeII, CoII).

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(ß-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.

Nitride-bridged triiron complex and its relevance to dinitrogen activation.

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 µB). 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.

An Air- and Water-Tolerant Zinc Hydride Cluster That Reacts Selectively With CO2.

The reaction of [Zn3Cl3L], in which L(3-) is a tris(ß-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.

Preorganized assembly of three iron(II) or manganese(II) ß-diketiminate complexes using a cyclophane ligand.

A trinucleating cyclophane bearing three ß-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 µ(3), µ, and terminal coordination modes are observed for the halide donors.