X-ray quality crystals

The synthesis of the first quadruply bonded tungsten(II) car-boxylate, W2(02CCF3)4 (hereafter referred to as V TFA) ), was reported by us last year (1). The structure of its diglyme adduct, W2(TFA)4 2/3 diglyme is shown in Figure 1. This particular view shows the partial contents of two unit cells and it emphasizes the tridentate nature of the axially coordinated polyether which "stiches" W2(TFA) units together in the solid state. We now have in hand several other adducts of W2(TFA) and one of these, W2(TFA)4 2PPh3 (an axial adduct), has been structurally characterized (2). Despite many attempts, X-ray quality crystals of unsolvated W2(TFA) have not been obtained. 

Dendrimers are structurally defined branched polymeric molecules and can form good X-ray quality crystals. This is particularly useful in the correlation of structure and properties. Two main synthetic approaches have been used divergent, which is appropriate for smaller dendrimers, and convergent, which is better for larger dendrimers. Branched polysilanes with a structurally defined branched core were described in 1994, but the growth from the core was not controlled and non-uniform structures were obtained.357... 

An acetonitrile solution of CpFe(CO)(/r-CN)2Cu(CH3CN)2 2, prepared as described in Section 37.B, is added to a 50-mL Schlenk flask containing 0.4 mmol of tricyclohexylphosphine (PCya, 0.112 g). A yellow solid precipitates from solution. On removal of the solvent by cannula filtration and drying the residue under vacuum, the crude product is isolated in 91% yield. X-ray quality crystals are obtained on slow diffusion of acetonitrile into a dichloromethane solution of CpFe(CO)(//-CN)2Cu(PCy3) 2 maintained at 10°C for several days. 

The reaction of 1-chlorosiloles with alkali metals leads to unstable species that react with methyl iodide, ethyl bromide or chlorotrimethylsilane in the manner expected for the 1-silacyclopentadienide anion. Interestingly, X-ray quality crystals were isolated from the reaction with Li in THF and the molecular structure was shown to be that of a [2 + 2] head-to-tail dimer, which is formed by the 1,5-rearrangement of the anion in the silole ring185 (equation 70). 

The ability of the boron atom of 59 to engage in a donor-acceptor interaction was illustrated with DMAP and DABCO (DABCO = diazabi-cyclo-[2.2.2]-octane) that readily formed the corresponding Lewis adducts. Interestingly, a similar behavior was retained after coordination of the phosphorus atom to palladium. The formation of the Lewis base adducts 66a and 66b of complex 65 (Scheme 38) was supported by solid-state 31P and nB CP/MAS-NMR spectroscopy (<5 1 B = 5-6 ppm), although the occurrence of decomposition and/or dissociation processes impeded spectroscopic characterization in solution and recrystallization to obtain X-ray quality crystals. Compounds 66a and 66b substantiate the ability of ambiphilic compounds to engage concomitantly into the coordination of donor and acceptor moieties. Such a dual behavior opens interesting perspectives for the preparation of metallo-polymers and multimetallic complexes. 

By cooling an NO-saturated solution of 6 in aromatic solvent, X-ray-quality crystals of Tp "BuCu(NO), 8, were obtained and the resulting... 

Multinuclear NMR has been used extensively to investigate the structures of rhodium clusters in solution. Where X-ray quality crystals arc not available, as is the case for [Rh6(CO)i5H] , the study of H, H- Rh, C- Rh, and ° Rh NMR has allowed determination of the cluster structure (Scheme 13) (397). A combination of this technique with X-ray diffraction studies in the hydrido species CRhi3(CO)24H,5 (n = 2, 3)... 

Once X-ray quality crystals have been grown data collection using several wavelengths or derivatives is required in order to obtain the protein structure. X-ray data collection has been revolutionized in the last decade by both better X-ray sources and detectors. Third generation synchrotrons are now available across the... 

The 1,4-dicyanamidebenzene dianion (dicyd2 ) and its substituted derivatives can also be precipitated from acetone solution as yellow thallium salts.18 Tetraphenylarsonium salts have greater solubility in non-aqueous solvents than the Tl1 salts and have been used to grow X-ray-quality crystals.19... 

Obtaiinng X-ray quality crystals is usually the most difficult and time-consuming step of a new structure determination project, notably in the case of a novel, poorly characterized gene product. Recent advances in crystallization robotics and the provision of numerous commercial crystallization screens have considerably simplified the process and shortened the time needed to set up extensive crystallization screeiung experiments, while, at the same time, dramatically reducing the amount of material needed. Nowadays, 5-lOmg of a homogeneous protein sample are... 

Attempts at producing X-ray quality crystals of the spiroketals were unsuccessful. The compounds were treated with fluoride to effect C-3 deprotection, and nOe effects used to assign C-12 stereochemistry. Oxidation of the diols gave ketones 79 and 80 (Scheme 34), whose structures were determined by 2D-NMR experiments. 

Obtaining X-ray quality crystals is often the most difficult step of a structure determination. Strong and dedicated support in molecular biology and biochemistry is an absolute must for the production of sufficient quantities of crystalhzation grade protein. When the crystallization conditions are not known, one often needs as much as 100 mg of protein to find the right crystallization conditions. In most cases, several protein constructs or protein variants need to be generated until one is found which is amenable to crystallization. AVhen crystallization conditions are already known and one is lucky, 1 mg of protein may be enough to produce a series of crystals with different inhibitors. One should bear in mind, however, that published crystallization protocols are often difficult to reproduce In such cases, particular attention should be paid to small differences in the protein construct or in the purification protocol, which can have a dramatic influence on crystallization behaviour. 

The availability of X-ray quality crystals and suitable spectroscopic samples has enabled detailed investigations to be carried out into the active site structure of these iron-based proteins. Concurrently, several functional model systems have been developed that have aided in the elucidation of the mechanistic pathways involved in this family of enzymes. 

Despite the air and water sensitivity of extracted FeMoco, it is thermally quite stable compared to many synthetic iron-sulfur clusters. A coordination chemist might wonder why, then, solutions of extracted FeMoco have not yielded X-ray quality crystals. Complicating issues include (i) extracted FeMoco solutions always contain some salts and impurities, (ii) the solutions are extremely air sensitive, and sensitive to excess water, (iii) roughly 1 kg of bacteria is required to make 6-7 mg of extracted FeMoco, and (iv) the reversible aggregation phenomena described above. 

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