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Understanding host-guest interactions is one of the key requirements for adjusting properties in metal- organic frameworks (MOFs). In particular, systems with coordinatively unsaturated Lewis acidic metal sites feature highly selective adsorption processes. This is attributed to strong interactions with Lewis basic guest molecules. Here we show that a combination of 13C MAS NMR spectroscopy with state-of-the-art density functional theory (DFT) calculations allows one to unravel the interactions of water, 2-aminopyridine, 3-aminopyridine, and diethylamine with the open metal sites in Cr-MIL-101. The 13C MAS NMR spectra, obtained with ultrafast magic-angle spinning, are well resolved, with resonances distributed over 1000 ppm. They present a clear signature for each guest at the open metal sites. Based on competition experiments this leads to the following binding preference: water < diethylamine ≈ 2-aminopyridine < 3-aminopyridine. Assignments were done by exploiting distance sum relations derived from spin-lattice relaxation data and 13C{¹H} REDOR spectral editing. The experimental data were used to validate NMR shifts computed for the Cr-MIL-101 derivatives, which contain Cr3O clusters with magnetically coupled metal centers. While both approaches provide an unequivocal assignment and the arrangement of the guests at the open metal sites, the NMR data offer additional information about the guest and framework dynamics. We expect that our strategy has the potential for probing the binding situation of adsorbate mixtures at the open metal sites of MOFs in general and thus accesses the microscopic interaction mechanisms for this important material class, which is essential for deriving structure-property relationships. [ABSTRACT FROM AUTHOR]

Ruthenium-based compounds are potential candidates for use as anticancer metallodrugs. The central ruthenium atom can be in the oxidation state +2 (e.g., RAPTA, RAED) or +3 (e.g., NAMI, KP). In this study we focus on paramagnetic NAMI analogs of a general structure [4-R-pyH]+trans-[RuIIICl4(DMSO)(4-R-py)]-, where 4-R-py stands for a 4-substituted pyridine. As paramagnetic systems are generally considered difficult to characterize in detail by NMR spectroscopy, we performed a systematic structural and methodological NMR study of complexes containing variously substituted pyridines. The effect of the paramagnetic nature of these complexes on the 1H and 13C NMR chemical shifts was systematically investigated by temperature-dependent NMR experiments and density-functional theory (DFT) calculations. To understand the electronic factors influencing the orbital (δorb, temperature-independent) and paramagnetic (δpara, temperature-dependent) contributions to the total NMR chemical shifts, a relativistic two-component DFT approach was used. The paramagnetic contributions to the 13C NMR chemical shifts are correlated with the distribution of spin density in the ligand moiety and the 13C isotropic hyperfine coupling constants, Aiso(13C), for the individual carbon atoms. To analyze the mechanism of spin distribution in the ligand, the contributions of molecular spin-orbitals (MSOs) to the hyperfine coupling constants and the spatial distribution of the z-component of the spin density in the MSOs calculated at the relativistic four-component DFT level are discussed and rationalized. The significant effects of the substituent and the solvent on δpara, particularly the contact contribution, are demonstrated. This work should contribute to further understanding of the link between the electronic structure and the NMR chemical shifts in open-shell systems, including the ruthenium-based metallodrugs investigated in this account. [ABSTRACT FROM AUTHOR]

The oxidation state assignment of the manganese ions present in the superoxidized manganese (III/IV) catalase active site is determined by comparing experimental and broken symmetry density functional theory calculated 14N, 17O, and 1H hyperfine couplings. Experimental results have been interpreted to indicate that the substrate water is coordinated to the Mn(III) ion. However, by calculating hyperfine couplings for both scenarios we show that water is coordinated to the Mn(IV) ion and that the assigned oxidation states of the two manganese ions present in the site are the opposite of that previously proposed based on experimental measurements alone. [ABSTRACT FROM AUTHOR]

The active site of [FeFe] hydrogenase contains a catalytic binuclear iron subsite coordinated by CN- and CO ligands as well as a unique azadithiolate (adt2-) bridging ligand. It has been established that this binuclear cofactor is synthesized and assembled by three maturation proteins HydE, -F, and -G. By means of in vitro maturation in the presence of 15N- and 13C-labeled tyrosine it has been shown that the CN- and CO ligands originate from tyrosine. The source of the bridging adt2- ligand, however, remains unknown. In order to identify the nitrogen of the bridging amine using HYSCORE spectroscopy and distinguish its spectroscopic signature from that of the CN- nitrogens, we studied three isotope-labeled variants of the H-cluster (15N-adt2-/C14N-, 15N-adt2-/C15N-, and 14N-adt2-/C15N-) and extracted accurate values of the hyperfine and quadrupole couplings of both CN- and adt2- nitrogens. This will allow an evaluation of isotopologues of the H-cluster generated by in vitro bioassembly in the presence of various 15N-labeled potential precursors as possible sources of the bridging ligand. [ABSTRACT FROM AUTHOR]

The photoexcited triplet states of a series of linear and cyclic butadiyne-linked porphyrin oligomers were investigated by transient Electron Paramagnetic Resonance (EPR) and Electron Nuclear DOuble Resonance (ENDOR). The spatial delocalization of the triplet state wave function in systems with different numbers of porphyrin units and different geometries was analyzed in terms of zero-field splitting parameters and proton hyperfine couplings. Even though no significant change in the zero-field splitting parameters (D and E) is observed for linear oligomers with two to six porphyrin units, the spin polarization of the transient EPR spectra is particularly sensitive to the number of porphyrin units, implying a change of the mechanism of intersystem crossing. Analysis of the proton hyperfine couplings in linear oligomers with more than two porphyrin units, in combination with density functional theory calculations, indicates that the spin density is localized mainly on two to three porphyrin units rather than being distributed evenly over the whole π-system. The sensitivity of the zero-field splitting parameters to changes in geometry was investigated by comparing free linear oligomers with oligomers bound to a hexapyridyl template. Significant changes in the zero-field splitting parameter D were observed, while the proton hyperfine couplings show no change in the extent of triplet state delocalization. The triplet state of the cyclic porphyrin hexamer has a much decreased zero-field splitting parameter D and much smaller proton hyperfine couplings with respect to the monomeric unit, indicating complete delocalization over six porphyrin units in this symmetric system. This surprising result provides the first evidence for extensive triplet state delocalization in an artificial supramolecular assembly of porphyrins. [ABSTRACT FROM AUTHOR]

Reduction of the bis(9-borafluorenyl)methane 1 with excess lithium furnishes the red dianion salt Li2[1]. The corresponding dark green monoanion radical Li[1] is accessible through the comproportionation reaction between 1 and Li2[1]. EPR spectroscopy on Li[1] reveals hyperfine coupling of the unpaired electron to two magnetically equivalent boron nuclei (a(11B) = 5.1 ± 0.1 G, a (10B) = 1.7 ± 0.2 G). Further coupling is observed to the unique B-CH-B bridgehead proton (a(¹H) = 7.2 ± 0.2 G) and to eight aromatic protons (a(¹H) = 1.4 ± 0.1 G). According to X-ray crystallography, the B⋯B distances continuously decrease along the sequence 1 → [1]•- → [1]2- with values of 2.534(2), 2.166(4), and 1.906(3) Å, respectively. Protonation of Li2[1] leads to the cyclic borohydride species Li[1H] featuring a B-H-B two-electron-three-center bond. This result strongly indicates a nucleophilic character of the boron atoms; the reaction can also be viewed as rare example of the protonation of an element-element σ bond. According to NMR spectroscopy, EPR spectroscopy, and quantum-chemical calculations, [1]2- represents a closed-shell singlet without any spin contamination. Detailed wave function analyses of [1]•- and [1]2- reveal strongly localized interactions of the two boron pz-type orbitals, with small delocalized contributions of the 9-borafluorenyl π systems. Overall, our results provide evidence for a direct B-B one-electron and two-electron bonding interaction in [1]•- and [1]2-, respectively. [ABSTRACT FROM AUTHOR]

The mainline feature in metal Kβ X-ray emission spectroscopy (XES) has long been recognized as an experimental marker for the spin state of the metal center. However, even within a series of metal compounds with the same nominal oxidation and spin state, significant changes are observed that cannot be explained on the basis of overall spin. In this work, the origin of these effects is explored, both experimentally and theoretically, in order to develop the chemical information content of Kβ mainline XES. Ligand field expressions are derived that describe the behavior of Kβ mainlines for first row transition metals with any d" count, allowing for a detailed analysis of the factors governing mainline shape. Further, due to limitations associated with existing computational approaches, we have developed a new methodology for calculating Kβ mainlines using restricted active space configuration interaction (RAS-CI) calculations. This approach eliminates the need for empirical parameters and provides a powerful tool for investigating the effects that chemical environment exerts on the mainline spectra. On the basis of a detailed analysis of the intermediate and final states involved in these transitions, we confirm the known sensitivity of I