A Domino Fusion of an Organic Ligand Depended on Metal-Induced and Oxygen Insertion, Unraveled by Crystallography, Mass Spectrometry, and DFT Calculations.

Herein, the reaction of (1-methyl-1 H-benzo[d]imidazol-2-yl)methanamine (L1) with Co(H2 O)6 Cl2 , in CH3 CN at 120 °C, leading to the 2,3,5,6-tetrakis(1-methyl-1 H-benzo[d]imidazol-2-yl)pyrazine (3), isolated as a dimeric cluster {[CoII 2 (3)Cl4 ]⋅2 CH3 CN} (2), is reported. When O2 and H2 O are present, (1-methyl-1 H-benzo[d]imidazole-2-carbonyl)amide (HL1') is first formed and crystallized as [CoIII (L1)2 (L1')]Cl2 ⋅2 H2 O (4) before fusion of HL1' with L1, giving 1-methyl-N-(1-methyl-1 H-benzo[d]imidazol-2-carbonyl)-1 H-benzo[d]imidazol-2-carboxamide (HL2'') forming a one-dimensional (1D) chain of [CoII 3 (L2'')2 Cl4 ]n (5). The combination of crystallography and mass spectrometry (ESI-MS) of isolated crystals and the solutions taken from the reaction as a function time reveal seven intermediate steps leading to 2, but six steps for 5, for which a different sequence takes place. Control and isotope labeling experiments confirm the two carbonyl oxygen atoms in 5 originate from both air and water. The dependence on the metals, compared with FeCl3 ⋅6 H2 O leading to a stable triheteroarylmethyl radical, is quite astounding, which could be attributed to the different oxidation states of the metals and coordination modes confirmed by DFT calculations. This metal and valence dependent process is a very useful way for selectively obtaining these large molecules, which are unachievable by common organic synthesis.

Tracking the formation of a polynuclear Co16 complex and its elimination and substitution reactions by mass spectroscopy and crystallography.

We present the syntheses and structures of the biggest chiral cobalt coordination cluster, [Co16(L)4(H3L)8(N3)6](NO3)2·16H2O·2CH3OH (1, where H4L = S,S-1,2-bis(1H-benzimidazol-2-yl)-1,2-ethanediol). 1 consists of two Co4O4 cubes (Co4(L)2(H3L)2) alternating with Co2(EO-N3)2Co2 (Co4(L)2(H3L)2(N3)2), bridged by the benzimidazole and azide nitrogen atoms to form a twisted ring. The ligand adopts both cis and trans forms, and all the rings have the same chiralilty. ESI-MS of 1 from a methanol solution of crystals reveals the fragment [Co16(L)4(H3L)8(N3)6+2H](4+), suggesting the polynuclear core is stable in solution. ESI-MS measurements from the reaction solution found smaller fragments, [Co4(H3L)4-H](3+), [Co4(H3L)4-2H](2+), [Co4(H3L)4(N3)2](2+), and [Co2(H3L)2](2+), and ESI-MS from a methanol solution of the solid deposit found in addition the Co16 core. These results and the dependence on the synthesis time allow us to propose the process for the formation of 1, which opens up a new way for the direct observation of the ligand-controlled assembly of clusters. In addition, the isolation of [Co4(H3L)4](NO3)4·4H2O (2) consisting of separate Co4O4 cubes with the ligands being only cis in crystalline form supports the proposal. Interestingly, N3(-) is replaced by either CH3O(-) or OH(-), and this is the first time that high-resolution ESI-MS is successfully utilized to examine both the step-by-step elimination and substitution of inner bridging ligands in such a high nuclear complex. Increasing the voltage results in stepwise elimination of azide from the parent cluster. The preliminary magnetic susceptibility of 1 indicates ferromagnetic cubes antiferromagnetically coupled to the squares within the cluster, though in a field of 2.5 kOe, weak and slow relaxation is observed below 4 K.

[Excitation-wavelength dependent photoluminescence from porous silicon].

With the technique of fluorescence spectral analysis, the dependence of the fluorescence from porous silicon on the excitation wavelength was investigated. It was found when the excitation wavelength decreases from 650 to 340 nm, the fluorescence spectrum of porous silicon blue shifts continuously from 780 to 490 nm. Using scanning electron microscopy (SEM) and computer simulation, the cross-sectional structures of porous silicon were studied. The authors' results showed that the microstructures of porous silicon exhibit fractal characteristics. With the additional information extracted from the excitation spectra of porous silicon, the authors' results can be interpreted in terms of the quantum size effect and the fractal structures of porous silicon.