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A Meetsma

Publications and source records attributed to A Meetsma.

10 recordsLinked to original sources
PubMed2001

Polymorphism in pentacene.

Pentacene, C(22)H(14), crystallizes in different morphologies characterized by their d(001)-spacings of 14.1, 14.5, 15.0 and 15.4 A. We have studied the crystal structure of the 14.1 and 14.5 A d-spacing morphologies grown by vapour transport and from solution. We find a close correspondence between the 14.1 A structure reported by Holmes, Kumaraswamy, Matzeger & Vollhardt [Chem. Eur. J. (1999), 5, 3399-3412] and the 14.5 A structure reported by Campbell, Monteath Robertson & Trotter [Acta Cryst. (1961), 14, 705-711]. Single crystals commonly adopt the 14.1 A d-spacing morphology with an inversion centre on both molecules in the unit cell. Thin films grown on SiO(2) substrates above 350 K preferentially adopt the 14.5 A d-spacing morphology, with a slightly smaller unit-cell volume.

Journal Article↗
PubMed2001

Synthesis, structural characterization, and transmetalation reactions of a tetranuclear magnesium silsesquioxane complex.

The reaction of a silsesquioxane trisilanol with methylmagnesium chloride leads to an unprecedented tetranuclear magnesium silsesquioxane complex 1 in high yield. The crystal structure shows an unusually short Mg-Cl bond, indicative of an electron-deficient magnesium atom; 1 has been used as transmetalation agent for the synthesis of metal silsesquioxane complexes. Transmetalation activity was low, but can easily be followed by multinuclear NMR. Crystal data for 1: C(78)H(142)Cl(2)Mg(4)O(26)Si(14).6(C(4)H(8)O), a = 15.744(1) A, b = 26.526(2) A, c = 16.917(1) A, beta = 113.229(2) degrees, monoclinic, P2(1)/n, Z = 2.

Journal Article↗
PubMed2001

Hexagonal YMnO(3).

The crystal structure of hexagonal yttrium trioxomanganate has been determined at room temperature and at 180 K. It is isomorphous with LuMnO(3). The Mn displacement vector has a frustrated component in the ab plane and a ferroelectric part along the c axis. The net ferroelectricity is zero due to inversion twinning, with 1:1 twin fractions.

Journal Article↗
PubMed2000

Cationic and neutral diphenyldiazomethanerhodium(I) complexes as catalytically active species in the C-C coupling reaction of olefins and diphenyldiazomethane

Cationic rhodium(I) complexes cis-[Rh(acetone)2(L)(L')]+ (2: L = L'=C8H14; 3: L=C8H14; L'=PiPr3; 4: L=L'=PiPr3), prepared from [RhCl(C8H14)2]2] and isolated as PF6 salts, catalyze the C-C coupling reaction of diphenyldiazomethane with ethene, propene, and styrene. In most cases, a mixture of isomeric olefins and cyclopropanes were obtained which are formally built up by one equivalent of RCH=CH2 (R = H, Me, Ph) and one equivalent of CPh2. The efficiency and selectivity of the catalyst depends significantly on the coordination sphere around the rhodium(I) center. Treatment of 4 with Ph2CN2 in the molar ratio of 1:1 and 1:2 gave the complexes trans-[Rh(PiPr3)2(acetone)(eta1-N2CPh2)]PF6 (8) and trans-[Rh(PiPr3)2(eta1-N2CPh2)2]PF6 (9), of which 8 was characterized by X-ray crystallography. Since 8 and 9 not only react with ethene but also catalyze the reaction of C2H4 and free Ph2CN2, they can be regarded as intermediates (possibly resting states) in the C-C coupling process. The lability of 8 and 9 is illustrated by the reactions with pyridine and NaX (X=Cl, Br, I, N3) which afford the mono(diphenyldiazomethane)rhodium(I) compounds trans-[Rh(PiPr3)2(py)(eta1-N2CPh2)]PF6 (10) and trans-[RhX(eta1-N2CPh2)(PiPr3)2] (11-14), respectively. The catalytic activity of the neutral complexes 11 - 14 is somewhat less than that of the cationic species 8, 9 and decreases in the order Cl > Br> I > N3.

Journal Article↗
PubMed2000

Spin-singlet clusters in the ladder compound NaV2O5

The space group of alpha(')-NaV2O5 turns below T(c) = 34 K from Pmmn with all V sites equivalent, into Fmm2 with three independent vanadium sites per layer. This is incompatible with models of charge ordering into V4+ and V5+. Our structure determination indicates that the phase transition consists of a charge ordering with three distinct valence states, formally V4+, V4.5+, and V5+. The singlet formation is not associated with dimerization on the spin ladder, but with the formation of spin clusters. Finally, we ascribe the quadrupling of the c axis to the large polarizability of the V2O5 skeleton.

Journal Article↗
PubMed2000

Chiral overcrowded alkenes; asymmetric synthesis of (3S,3'S)-(M, M)-(E)-(+)-1,1',2,2',3,3',4,4'-octahydro-3,3',7,7'-tetramethyl-4, 4'-biphenanthrylidenes.

An asymmetric synthesis route towards (3S,3'S)-(M,M)-(E)-(+)-1,1',2, 2',3,3',4,4'-octahydro-3,3',7,7'-tetramethyl-4,4'-biphenanthrylidene was developed using the Evans procedure as a key step. The absolute configurations of the title compound and of its parent ketone were determined by CD spectroscopy and could be correlated with the stereochemical results of the asymmetric alkylation. Furthermore, a comparison was made with the known (3R,3'R)-(P,P)-(E)-(-)-1,1',2,2', 3,3',4,4'-octahydro-3,3',7,7'-dimethyl-4,4'-biphenanthrylidene. Finally, the X-ray crystallographic analysis of (3S,3'S)-(M, M)-(E)-(+)-1,1',2,2',3,3',4,4'-octahydro-3,3',7,7'-tetramethyl-4, 4'-biphenanthrylidene is presented.

Alkenes↗
PubMed1999

Asymmetric Synthesis of Overcrowded Alkenes by Transfer of Axial Single Bond Chirality to Axial Double Bond Chirality.

Optically active overcrowded alkenes were synthesized by employing bis-beta-naphthol as a chiral template during an intramolecular coupling reaction. The major isomer 2 has a unique helical structure with twisted and folded structural moieties. Removal of the chiral template afforded overcrowded thioxanthylidene 3 with 96 % ee, which indicates that no racemization or isomerization of the enantiomers took place.

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