The low-temperature (<-35 °C) reduction of the trivalent uranium monoarene complex [{((Ad,Me) ArO)3 mes}U] (1), with potassium spheres in the presence of a slight excess of 2.2.2-cryptand, affords the quantitative conversion of 1 into the uranium(II) monoarene complex [K(2.2.2-crypt)][(((Ad,Me) ArO)3 mes)U] (1-K). The molecular and electronic structure of 1-K was established experimentally by single-crystal X-ray diffraction, variable-temperature (1) H NMR and X-band EPR spectroscopy, solution-state and solid-state magnetism studies, and optical absorption spectroscopy. The electronic structure of the complex was further investigated by DFT calculations. The complete body of evidence confirms that 1-K is a uranium(II) monoarene complex with a 5f (4) electronic configuration supported by δ backbonding and that the nearly reversible, room-temperature reduction observed for 1 at -2.495 V vs. Fc/Fc(+) is principally metal-centered.

Supramolecular coordination compounds bear exceptional advantages over their organic counterparts. They are available in one-pot reactions and in high yields and display physical properties that are generally inaccessible with organic species. Moreover, their weak, reversible, noncovalent bonding interactions facilitate error checking and self-correction. This Review emphasizes the achievements in supramolecular coordination chemistry initiated by serendipity and their materialization based on rational design. The recognition of similarities in the synthesis of different supramolecular assemblies allows prediction of potential results in related cases. Supramolecular synthesis obeys guidelines comparable to the "lead sheet" used by small jazz ensembles for improvisation and therefore more often leads to unpredicted results. The combination of detailed symmetry considerations with the basic rules of coordination chemistry has only recently allowed for the design of rational strategies for the construction of a variety of nanosized systems with specified size and shape.

The new neopentyl (Neop)-substituted tris(aryloxide) U(III) complex [(((Neop,Me)ArO)(3)tacn)U(III)] reacts with CO(2) to form CO and the bridging carbonate complex [{(((Neop,Me)ArO)(3)tacn)U(IV)}(2)(μ-CO(3))]. The uranium(IV) bridging oxo [{(((Neop,Me)ArO)(3)tacn)U(IV)}(2)(μ-O)] has been determined to be the intermediate in this reaction. For the first time, both U(iv) complexes can be reduced back to the U(III) starting material. Thus, with KC(8) as reductant, [(((Neop,Me)ArO)(3)tacn)U(III)] engages in a synthetic cycle, in which CO(2) is converted to CO and CO(3)(2-).

The reaction of the cobalt(I) complex [(TIMMN mes )Co I ](BPh 4 )(2)(TIMMN mes = tris-[2-(3-mesitylimidazolin-2-ylidene)methyl]amine) with 1-adamantylazide yields the cobalt(III) imido complex [(TIMMN mes )Co III -(NAd)](BPh 4 )( 3)w ith concomitant release of dinitrogen. The N-anchor in diamagnetic 3 features an unusual, planar tertiary amine,w hich results from repulsive electrostatic interaction with the filled d(z 2 )-orbital of the cobalt ion and negative hyperconjugation with the neighboring methylene groups.O ne-electron oxidation of 3 with [FeCp 2 ](OTf) provides access to the rare,h igh-valent cobalt(IV) imido complex [(TIMMN mes )Co IV (NAd)](OTf) 2 (4). Despite ah alflife of less than 1hat room temperature, 4 could be isolated at low temperatures in analytically pure form. Single-crystal Xray diffractometry and EPR spectroscopyc orroborate the molecular structure and the d 5 low-spin, S = 1 = 2 ,e lectron configuration. Ac omputational analysis of 4 suggests high covalency within the Co IV =NAdbond with non-negligible spin density located at the imido moiety,w hicht ranslates into substantial triplet nitrene character.

Star-shaped complex [Fe(III)[Fe(III)(L1)2]3] (3) was synthesized starting from N-methyldiethanolamine H2L1 (1) and ferric chloride in the presence of sodium hydride. For 3, two different high-spin iron(III) ion sites were confirmed by Mössbauer spectroscopy at 77 K. Single-crystal X-ray structure determination revealed that 3 crystallizes with four molecules of chloroform, but, with only three molecules of dichloromethane. The unit cell of 3.4CHCl3 contains the enantiomers (delta)-[(S,S)(R,R)(R,R)] and (lambda)-[(R,R)(S,S)(S,S)], whereas in case of 3.3CH2Cl2 four independent molecules, forming pairs of the enantiomers [lambda-(R,R)(R,R)(R,R)]-3 and [lambda-(S,S)(S,S)(S,S)]-3, were observed in the unit cell. According to SQUID measurements, the antiferromagnetic intramolecular coupling of the iron(III) ions in 3 results in a S = 10/2 ground state multiplet. The anisotropy is of the easy-axis type. EPR measurements enabled an accurate determination of the ligand-field splitting parameters. The ferric star 3 is a single-molecule magnet (SMM) and shows hysteretic magnetization characteristics below a blocking temperature of about 1.2 K. However, weak intermolecular couplings, mediated in a chainlike fashion via solvent molecules, have a strong influence on the magnetic properties. Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) were used to determine the structural and electronic properties of star-type tetranuclear iron(III) complex 3. The molecules were deposited onto highly ordered pyrolytic graphite (HOPG). Small, regular molecule clusters, two-dimensional monolayers as well as separated single molecules were observed. In our STS measurements we found a rather large contrast at the expected locations of the metal centers of the molecules. This direct addressing of the metal centers was confirmed by DFT calculations.

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Iron–nitrosyls have fascinated chemists for a long time due to the noninnocent nature of the NO ligand that can exist in up to five different oxidation and spin states. Coordination to an open-shell iron center leads to complex electronic structures, which is the reason Enemark−Feltham introduced the {Fe–NO} n notation. In this work, we succeeded in characterizing a series of {Fe–NO}6–9 complexes, including a reactive {Fe–NO}10 intermediate. All complexes were synthesized with the tris-N-heterocyclic carbene ligand tris­[2-(3-mesitylimidazol-2-ylidene)­ethyl]­amine (TIMENMes), which is known to support iron in high and low oxidation states. Reaction of NOBF4 with [(TIMENMes)­Fe]2+ resulted in formation of the {Fe–NO}6 compound [(TIMENMes)­Fe­(NO)­(CH3CN)]­(BF4)3 (1). Stepwise chemical reduction with Zn, Mg, and Na/Hg leads to the isostructural series of high-spin iron nitrosyl complexes {Fe–NO}7,8,9 (2–4). Reduction of {Fe–NO}9 with Cs electride finally yields the highly reduced {Fe–NO}10 intermediate, key to formation of [Cs­(crypt-222)]­[(TIMENMes)­Fe­(NO)], (5) featuring a metalacyclic [Fe−(NO−NHC)3−] nitrosoalkane unit. All complexes were characterized by single-crystal XRD analyses, temperature and field-dependent SQUID magnetization methods, as well as 57Fe Mössbauer, IR, UV/vis, multinuclear NMR, and dual-mode EPR spectroscopy. Spectroscopy-based DFT analyses provide insight into the electronic structures of all compounds and allowed assignments of oxidation states to iron and NO ligands. An alternative synthesis to the {Fe–NO}8 complex was found via oxygenation of the nitride complex [(TIMENMes)­Fe­(N)]­(BF4). Surprisingly, the resulting {Fe–NO}8 species is electronically and structural similar to the [(TIMENMes)­Fe­(N)]+ precursor. Based on the structural and electronic similarities between this nitrosyl/nitride complex couple, we adopted the strategy, developed by Wieghardt et al., of extending the Enemark−Feltham nomenclature to nitrido complexes, rendering [(TIMENMes)­Fe­(N)]+ as a {Fe–N}8 species.

Keywords:Manganese clusters / Self-assembly / Mixed valency / X-ray diffraction / Cyclic voltammetry