Structural studies of two tetracyclines: 4-dedimethylaminotetracycline hydrate and 6-methylene-5-oxytetracycline hydrochloride

The crystal structures of two tetracycline derivatives, 4-dedimethylaminotetracycline hydrate and 6-methylene-5-oxytetracycline hydrochloride (methacycline·HCl), were determined using X-ray intensity data measured on a diffractometer. The first compound crystallizes with two chemical formula and one water molecule per asymmetric unit. The monoclinic crystal [a=7.174(5),b=12.029(9),c=21.117(15) Å,=93.16(6)°] shows the space groupP2₁. Crystals of methacycline hydrochloride are orthorhombic, space groupP2₁2₁2₁,a=9.472(1),b=11.921(3),c=18.945(4)Å, with one molecule in the asymmetric unit.     

Bond valence sums in coordination chemistry. Calculation of the oxidation state of chromium in complexes containing only Cr-O bonds and a redetermination of the crystal structure of potassium tetra(peroxo)chromate(V)

A simple method for calculating the oxidation state of Cr in complexes containing only Cr-O bonds is presented. A total of 242 CrOn fragments with n = 3-6 were retrieved from the Cambridge Structural Database (CSD) and, together with the data for K3CrO8, were analyzed using the bond valence sum method. New R0 values for Cr(II) of 1.739(21) A, Cr(III) of 1.708(7) A, Cr(V) of 1.762(14) A, and Cr(VI) of 1.793(7) A were derived. An average R0 value of 1.724 A for Cr-O reproduces the oxidation state of 96 of the 110 Cr(II), Cr(III), and Cr(IV) CrOn complexes (n = 3-6) and that of K3CrO8 within 0.30 valence units. The crystal structure of K3CrO8 was redetermined at 173 K to provide accurate data for a Cr complex with both high oxidation state and coordination number. Potassium tetraperoxochromate(V), K3CrO8, is tetragonal, Space group I42m, a = b = 6.6940(3) A, c = 7.7536(5) A, Z = 2. The difficulties with fitting the observed valence for Cr(V) and Cr(VI) complexes with coordination numbers 4 and 5 are discussed. The use of bond valence sums in gaining chemical insight into Cr complexes with noninnocent ligands and in establishing oxidation states in Cr clusters is presented. An analysis of the Cr-O bond distances used in the calculations shows a large range of values that can be understood in terms of the bond valence sum calculation.     

The synthesis and molecular structure of π-cyclopentadienyl-π-tetraphenylcyclobutadienerhodium(I)

The reaction of π-cyclopentadienyl-π-1,5-cyclooctadienerhodium(I) and diphenylacetylene gives hexaphenylbenzene as the principal product with a low yield of π-cyclopentadienyl-π-tetraphenylcyclobutadienerhodium(I). The crystal structure of π-cyclopentadienyl-π-tetraphenylcyclobutadienerhodium(I) was determined by X-ray diffraction techniques using diffractometer data. The crystals are monoclinic with unit cell dimensions of a=13.416(3), b=19.534(6), c=13.411(2) Å and β=135.01 (1)°. The space group is P2₁/c and, with four molecules per unit cell, no molecular symmetry is required (D_m=1.40 g/cm³ and D_c=1.411 g/cm³). The structure was solved by the heavy atom method and refined by least-squares methods to a final R of 0.043 for the 3675 observed reflections used in the analysis. The rhodium atom is coordinated to both the cyclopentadienyl group (Rh-ring distance is 1.868 Å) and the tetraphenylcyclobutadiene group (Rh-ring distance is 1.828 Å). The phenyl groups are twisted relative to the C₄ ring and bent away from the rhodium atom.     

Structural studies of antihistamines. The crystal structure ofH 4aH 9a -cis-H 9,H 9a-trans-2-methyl-9-phenyl-2, 3,4,4a,9,9a-hexahydro-1H-indeno[2,1-c]pyridine hydrochloride

The crystal and molecular structure ofH _4a ,H _9a -cis-H ₉,H _9a -trans-2-methyl-9-phenyl-2, 3,4,4 _a ,9,9 _a -hexahydro-1H-indeno[2,1-c]pyridine hydrochloride was determined. The compound crystallizes in the orthorhombic space groupPbca, with eight molecules in the unit cell of dimensionsa = 10.461(2),b = 20.141(10), andc = 15.469(3) Å. Intensity data were measured using Mo radiation and a – scan method. The structure was solved by direct methods and refined by least-squares techniques to a finalR of 4.6%. Thecis-trans isomer has higher antihistamine activity compared to thecis-cis isomer but is less active than phenindamine, the 2,3,4,9-tetrahydro derivative. The bond distances in thecis-trans andcis-cis derivatives are identical so that the differences in activity must be related to differences in conformation. The difference in the conformation is discussed from a receptor-site view of the two molecules.     

Bond Valence Sum Analysis of Tl（Ⅰ） and Tl（Ⅲ） Complexes with O and N Donors

The influence of the lone pair of electrons in thallium complexes is analyzed using the bond valence sum method.Bond length data for metal-organic Tl complexes were obtained from the Cambridge Structural Database(CSD),and problems with searching the CSD file for Tl complexes are discussed.The recommended R0 values for Tl(Ⅰ)-O of 2.162 ,Tl(Ⅲ)-O of 2.016 ,Tl(Ⅰ)-N of 2.286 ?,and for Tl(Ⅲ)-N of 2.014  used with b = 0.37 were derived from analyses of homoleptic Tl-O,Tl-N,and heteroleptic Tl-O and-N metal organic complexes.These R0 values can be used to assign correctly the oxidation state of Tl in complexes containing any combination of Tl-O or Tl-N bonds.Examples of questionable oxidation states for Tl complexes are given.The R0 value for Tl(Ⅲ)-Cl of 2.300  was also determined.     

Derivatization for electrospray ionization-mass spectrometry. 4. Alkenes and alkynes

A survey of derivatization strategies and prospective derivatization reactions for conversion of simple alkenes and alkynes to 'electrospray-active' species is presented. General synthetic strategies are discussed and illustrative examples of prospective derivatives prepared from model compounds are presented along with their electrospray ionization (ES) mass spectra. The identified derivatives of these neutral, nonpolar analytes are either ionic or are ionizable in solution through Bronsted acid/base chemistry, by Lewis acid/base chemistry, or by chemical or electrochemical electron-transfer chemistry. Once ionized, the derivatives are expected to be amenable to detection by electrospray ionization-mass spectrometry. Derivatives are identified for positive and negative ion analysis of both alkenes and alkynes. Copyright 2000 John Wiley & Sons, Ltd.     

Electrochemical processes in electrospray ionization mass spectrometry

Editorial Comment Last month we presented, as a Special Feature, a set of five articles that constituted a Commentary on the fundamentals and mechanism of electrospray ionization (ESI). These articles produced some lively discussion among the authors on the role of electrochemistry in ESI. Six authors participated in a detailed exchange of views on this topic, the final results of which constitute this month's Special Feature. We particularly hope that younger scientists will find value in this month's Special Feature, not only for the science that it teaches but also what it reveals about the processes by which scientific conclusions are drawn. To a degree, the contributions part the curtains on these processes and show science in action. We sincerely thank the contributors to this discussion. The give and take of intellectual debate is not always easy, and to a remarkable extent this set of authors has maintained good humor and friendships, even when disagreeing strongly on substance. Graham Cooks and Richard Caprioli Copyright 2000 John Wiley & Sons, Ltd.