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The synthesis and structures of mono- and bis(amidinate) aluminum complexes are described. The reaction of AlMe3 and 1 equiv of carbodiimide, R‘NCNR‘, affords {MeC(NR‘)2}AlMe2 (1a, R‘ = i Pr; 1b, R‘ = Cy = cyclohexyl). The reaction of R‘NCNR‘ with MeLi or t BuLi generates Li[RC(NR‘)2] (2a, R = Me, R‘ = i Pr; 3a, R = t Bu, R‘ = i Pr; 3b, R = t Bu, R‘ = Cy; 3c, R = t Bu, R‘ = SiMe3). 2a, 3a, and 3b may be isolated or reacted in situ, while attempted isolation of 3c gave [Li( t BuCN){μ-N(SiMe3)2}]2 (3d). The reaction of 1 equiv of AlCl3 with 2a or 3a − c affords {RC(NR‘)2}AlCl2 (4a, R = Me; 5a − c, R = t Bu), and the reaction of AlMe2Cl with 3a − c affords { t BuC(NR‘)2}AlMe2 (6a − c). Alkylation of 5a,b with 2 equiv of PhCH2MgCl or Me3CCH2Li yields { t BuC(NR‘)2}Al(CH2Ph)2 (7a,b) or { t BuC(NR‘)2}Al(CH2CMe3)2 (8a,b). The reaction of 0.5 equiv of AlCl3 with 2a or 3a,b yields {RC(NR‘)2}2AlCl (9a, R = Me; 10a,b, R = t Bu). Complexes 1b, 3d, 4a, 5a,b, 6b, 8b, 9a, and 10a,b have been characterized by X-ray crystallography. The crystallographic results establish that steric interactions between the R and R‘ groups influence the R‘−N−Al angle and, hence, the steric environment at aluminum.

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Derivatized s-triazine (C3N3) precursors have seen significant recent use in the production of carbon nitride materials. Larger polycyclic molecular precursors, such as those containing an s-heptazine core (C6N7 or tri-s-triazine), may improve stability and order in carbon nitride products. In this Communication, we describe the synthesis and crystal structure of 2,5,8-triazido-s-heptazine (2). Synthesis of 2 was achieved from melon, an oligomeric s-heptazine synthesized by the pyrolysis of NH4SCN. Melon was converted to molecular 2,5,8-trichloro-s-heptazine, which was then transformed to the triazide upon reaction with (CH3)3SiN3. The crystal structure of 2 verifies that the s-heptazine is planar and the azides adopt a pinwheel-like C3h arrangement around the periphery. The s-heptazine core shows pi delocalization in the C-N bonds around the periphery (av. 1.33 A), while the internal planar C-N bonds are longer (1.40 A). The heptazine units pack into parallel, but offset, layered sheets in the crystal. The triazide 2 exhibits photoluminescence at 430 nm and rapidly and exothermically decomposes upon heating at 185 degrees C to produce a tan thermally stable carbon nitride powder with a formula near C3N4.

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Three new caryophyllene-type sesquiterpene alcohols, 6-hydroxypunctaporonin E (1), 6-hydroxypunctaporonin B (2), and 6-hydroxypunctaporonin A (3), have been isolated from cultures of the fungicolous fungus Pestalotiopsis disseminata. The structures of 1-3 were determined mainly by analysis of 1D and 2D NMR data. The structure and absolute configuration of 6-hydroxypunctaporonin E (1) was confirmed through X-ray crystallographic analysis of its mono-bromobenzoate derivative. Compounds 1 and 2 showed activity against Gram-positive bacteria.

X-ray studies on human insulins prepared by semisynthetic and biosynthetic methods have recently been undertaken. Human insulin differs from porcine insulin only at the COOH terminus of the B-chain. The present study reports the crystal structure of 4-zinc human insulin, which is used clinically as a slow-acting preparation. The structure has been refined, using 1.85-A resolution data, to a residual of 0.173. The unit cell is rhombohedral, space group R3, with hexagonal cell constants a = 80.953 and c = 37.636 A, and it is nearly isomorphous with that of 4-zinc porcine insulin. As a result of a conformational change of the first eight residues of the Bchain of molecule 1 from an extended conformation observed in the 2-zinc structure to an a-helical one, the coordination around one of the zinc ions on the 3-fold axis has changed, an additional zinc ion in a general position is bound by the hexamer, and additional hydrogen-bonded interactions help stabilize dimer and hexamer formation. Unlike the surface of the 2-zinc insulin hexamer, which possesses a shallow depression containing a zinc ion and its coordinating water molecules, the 4-zinc human insulin hexamer contains a zinc and chloride ion at the bottom of an 8-A tunnel produced by three parallel ahelices. These a-helices shield the zinc ion from the environment, decreasing the rate of dissociation of the hexamer, and provide an explanation for the slow-acting aspect of the 4-zinc crystalline form.In mammals, insulin is synthesized in the B cells of the pancreas and stored as a zinc-containing hexameric aggregate. However, this hormone, consisting of an A and a B chain, is a monomer as it circulates in the blood stream and when it interacts with its receptor. Binding and activity data on several insulin analogues suggest that the insulin molecule must undergo a substantial conformational change when it binds to its receptor (1, 2).Crystallographic investigations have been carried out on insulin derived from various species (3), chemically modified insulins (2, 4), and different crystalline modifications (5-7). The most extensive studies have been carried out on porcine insulin, which differs from human insulin by the change of alanine to threonine at B30. As a result of these studies, the nature of the interactions that hold the hexamer together, the interactions between monomers to produce dimers, and finally, the identity and spatial relationship of the residues that are thought to bind to the receptor are known.Comparison between the human and porcine 2-zinc insulin crystal structures revealed alterations in the hormone's conformation only at B29 and B30 (8). While other studies have shown that the insulin molecule is flexible, the largest changes in conformation have been observed in the 4-zinc insulin structure. This is of particular interest, since a 4-zinc crystalline preparation is used clinically as a "slow-acting" insulin because it is slow to dissolve and enter the bloodstream. The present study was undertaken to determine the effect of the...

Reactions in crystals: Interactions between AgI ions are used to preorganize olefins for a regiocontrolled single‐crystal‐to‐single‐crystal (SCSC) [2+2] photodimerization that results in a quantitative conversion of a finite dinuclear complex into a one‐dimensional coordination network (see scheme).