Steven F. Watkins

V. Molecular structures of acetate-bridged dimers of a 2-arylpyridine and a 2,6-diarylpyridine cyclometallated by palladium(II)

Abstract The crystal and molecular structures of dimeric cyclopalladated acetato(OAc)-bridged complexes of a 2-arylpyridine ( 1 ) and a 2,6-diarylpyridine ( 2 ), [Pd( 1 )OAc] 2 ( 3 ) and [Pd( 2 )OAc] 2 ( 4 ) have been determined by X-ray diffraction. In each. two planar acetates, nearly perpendicular to each other, cis -bridge two Pd II atoms, while the other two square planar sites on each metal are occupied by the cyclometallated aryl-substituted pyridine. The two metal coordination planes are in near parallel registry with one another, giving a “boat” molecule shape of exact ( 3 or near ( 4 ) two-fold symmetry. The non-bonding Pd⋯Pd distance (δ) strongly correlates with the angle (α) subtended by the metal coordination planes, which in turn seems related to the steric requirements of the non-bridging ligands. In 3 , with the planar nitrophenylpyridine ligand, α 25.7° and δ 2.822(6) A, the shortest Pd⋯Pd distance yet observed in dimers of this type. The PdC lengths, 1.94 A in 3 and 1.944 A in 4 , are significantly shorter than the calculated length, 2.05 A, suggesting partial multiple bond character in these shortest PdC (aryl) bonds so far observed. PdN bond lengths are near expected single bond lengths. Failure of trans -dimetallation to occur in 4 , analogous to that recently observed with certain 2,6-dialkylpyridines or to the tridentate behavior of terpyridine in [Pd terpy X] + compounds, appears due to a combination of the shortened PdC bond in the first-formed metallated ring and the larger constraining angles (near 120° for sp 2 carbons) in the remaining (unmetallated) ring.

THE PHENOMENON OF KRYPTORACEMIC CRYSTALLIZATION. PART 1. COUNTERION CONTROL OF CRYSTALLIZATION PATHWAY SELECTION. PART 4. THE CRYSTALLIZATION BEHAVIOR OF (+/−)-[Co(tren)(NO2)2]Br(I), (+/−)-[Co(tren)(NO2)2]2Br(ClO4) · H2O(II), (+/(−)-[Co(tren)(NO2)2]ClO4(III) AND ATTEMPTS TO SOLVE THE STRUCTURE OF (+/−)-[Co(tren)(NO2)2]NO3(IV)

Abstract Racemic aqueous solutions of (+/−)-[Co(tren)(NO2)2]Br (I), (+/−)-[Co(tren)(NO2)2]ClO4 (III) and [Co(tren)(NO2)2]NO3(IV) crystallize as racemates. By contrast, the double salt, (+/−)-[Co(tren)(NO2)2]2Br(ClO4) · H2O(II), produces kryptoracemic crystals belonging to the enantiomorphic space group P212121 (No. 19). The former three species crystallize with one molecule in the asymmetric unit; in the latter, a racemic pair is the asymmetric unit, a fact which is hidden by the enantiomorphic nature of its space group – thus the name of the crystallization phenomenon reported. In (II) pairs of cations are related by an approximate, non-crystallographic, inversion center. The crystal structure and polarity of (I) and the absolute configuration of (II) were determined by refinement. The crystalline contents of (I) to (III) consist of infinite strings of hydrogen bonded cations, the counter ions and (where relevant) waters of crystallization acting as a hydrogen-bonding glue linking the spiral strings In (...

A list of organometallic kryptoracemates

The vast majority of racemic solutions of chiral compounds apparently crystallize at room temperature in non-Sohncke space groups as racemic crystals. However, kryptoracemic crystals composed of nearly enantiomeric pairs occasionally crystallize at room temperature, or appear as low-temperature phases, in Sohncke space groups. As a complement to the previously published catalog of organic kryptoracemates [Fábián & Brock (2010). Acta Cryst. B66, 94-103], 1412 chiral organometallic crystal structures have now been extracted from the Cambridge Structural Database and analyzed. 26 are listed herein as credible kryptoracemates. The possible influence of temperature is discussed, together with some problems in characterizing and classifying these structures.

The crystal structure of a macrocycle containing pyridine and piperazine subunits, and of a Cu(I) complex of its diprotonated cation

The 2:2 macrocycle C22H30N6, containing pyridine and piperazine subunits, crystallizes in monoclinic space group C2c with a = 17.614(3), b = 9.696(2), c = 12.662(2) A, β = 111.65(2)°, V = 2010(1) A3, Z = 4. The structure was refined to R = 0.036 for 1863 observed reflections. The molecule lies on an inversion center, and the piperazine subunits have the chair conformation. Surprisingly, this macrocycle becomes diprotonated, and the Cu(II) is reduced to Cu(I) upon reaction with CuCl2 in methanol. The resulting Cu(I) complex of the diprotonated macrocyclic dication, [CUCl(C22H32N6)]Cl2·2H2O, crystallizes in monoclinic space group P21n with a = 10.745(1), b = 13.728(2), c = 17.774(2) A , β = 98.08(1)°, V = 2596(1) A3, Z = 4. The structure was refined to R = 0.058 for 3989 observed reflections. The Cu atom coordinates approximately linearly to one of the pyridine N atoms and a chloro ligand with CuN distance 1.947(3) A, CuCl distance 2.133(1) A, and NCuCl angle 164.5(1)°. While the piperazine subunits maintain their chair conformations, one N of each is protonated and turned outward. The unprotonated piperazine N atoms form long interactions of 2.488(4) and 2.508(4) A, distance to the Cu atom.

The crystal and molecular structure of 3,3,6,6-tetramethyl-s-tetrathiane, ((CH3)2CS2]2,

Abstract The X-ray crystal structure of 3,3,6,6-tetramethyl-s-tetrathiane (“duplodithioacetone”) confirms earlier nmr studies predicting the molecule to adopt a twist-boat conformation.

Meridinol, a lignan from Zanthoxylum fagara

Abstract The structure of a novel lignan, meridinol, isolated from Zanthoxylum fagara was determined by the evidence of its spectroscopic data and X-ray single crystal structure as (1 S, 2R)-1,2-bis (3,4-methylenedioxy benzyl)- 1-hydroxy butyrolactone.

Direct Phase Determination from Neutron Diffraction Data of the Structure of Melampodin

The structure of a large complex molecule, C21H24O9, has been solved directly from neutron diffraction data by using sigma-2 and tangent refinement methods. As a consequence, the neutron diffraction technique must be reevaluated as a possible primary tool for crystal structure determination.

Symmetric hydrogen bonding in dimethylammonium hydrogen diphenyldiphosphonate

In the title compound, C2H8N+.C12H11O5P2-, pairs of hydrogen diphenyldiphosphonate anions form dimers across a twofold axis, with two symmetric O...H...O hydrogen bonds [O...O = 2.406 (3) and 2.418 (3) A]. The 12-membered ring thus formed has crystallographic 2 and quasi-222 symmetry. Cations on either side of the ring form N-H...O hydrogen bonds to the four extraannular O atoms, with N...O distances of 2.765 (2) and 2.748 (3) A.

The x-ray crystal structure of 2,3:4,5-di-O-isopropylidene-1-O-methyl-β-d-fructopyranose

2,3:4,5-Di-O-isopropylidene-1-O-methyl-beta-D-fructopyranose, C13H22O6, Mr = 274.3, orthorhombic, P212121, a = 12.388 (2), b = 13.307 (5), c = 8.660 (1) A, V = 1427.4 (9) A3, Z = 4, Dm = 1.24 g.cm-3, Dx = 1.276 g.cm-3, CuKalpha, lambda = 1.54184 A, mu = 8.0 cm-1, F(000) = 592, T = 295 (1) K, R = 0.032 for 1586 observations (of 1693 unique data). The molecule is a derivative of the naturally occurring carbohydrate D-fructose. The data reported here indicate that the ketose six-membered ring is constrained by the presence of two fused five-membered rings into the 3SO conformation. These findings agree with the n.m.r.-spectroscopic results for 2,3:4,5-di-O-benzylidene-beta-D-fructopyranose. As a result of crystal packing forces, the exocyclic side-chain has a C-C-O-C torsion angle of -102 degrees, quite different from the expected value of 180 degrees.

Synthesis, structure, thermal, magnetic properties and quantum mechanical calculations of bridged [bis(di(2-methylpyridyl)amine)-(μ2-1,2-bis(4-pyridyl)ethane)-tetraperchlorato-dicopper(II)] dihydrate complex

The title compound [Cu2(DPA)2(μ-bpe)(ClO4)4]·2H2O, I where DPA = di(2-methylpyridyl)amine and bpe = 1,2-bis(4-pyridyl)ethane was studied by single crystal X-ray crystallography at different temperatures, spectroscopic methods, differential scanning calorimetry (DSC) and by magnetic susceptibility methods. DSC studies were recorded between 125 K and 647 K and magnetic studies between 2 K and 300 K. The X-ray diffraction studies were carried out at 293, 200, 150 and 100 K. Cooling the crystal from 293 K to 100 K produces a significant change in cell constants but no change in space group. X-ray and DSC studies revealed that significant phase transitions occur at temperatures between 200 K and 100 K that are described herein. The X-ray studies revealed a thermally-induced case of polymorphism whereby the cell volume triples on going from 293 to 100 K. This transition was reversible even though there is a marked hysteresis in the process. The major structural change between 200 K and 100 K is due to the displacement of the central bpe pyridyl rings relative to the plane defining CuN1N2N3 and the bending angle Cu–N1–C3. This transitional phase change was indirectly supported by quantum mechanical calculations which indicated the absence of significant π–π stacking interaction between the central bpe pyridyl rings forming the two parallel dinuclear species. This was attributed to the long 8.28 A distances between the two indicated moieties. Lack of energetic and spatial constraints justified the pyridyl ring displacement along the N–C3 axis. The temperature dependence of the magnetic behavior of I was that of a simple paramagnet down to 2 K. The complex revealed a very weak to non-existent magnetic coupling between the Cu(II) centers (J = −0.34 cm−1).