The effect of tin(II) chloride on the liquid-liquid extraction of tetrachloroplatinate(II) ions by triphenylphosphine in dichloromethane

The effect of tin(II) chloride on the extraction of tetrachloroplatinate(II) in 1.0–1.5 M HCl into dichloromethane with triphenylphosphine (TPP) is described. Tin(II) chloride dramatically increases the rate and efficiency of platinum extraction. The percentage of platinum extracted depends in a complicated way on the time allowed for extraction, the Pt:Sn(II) ratio, the Pt:TPP ratio, and to a lesser extent to the hydrochloric acid concentration. Tin is initially extracted into the organic phase, probably as [Pt(SnCl₃)Cl(PPh₃)₂], but is subsequently back-extracted into the aqueous phase, as a result of the relatively slow disproportionation reaction: [Pt(SnCl₃)Cl(PPh₃)₂]_org + cl^? ? [Pt(PPh₃)₂Cl₂]_org + SnCl^?₃.     

Synthesis of the 3-methyl and 4-methyl derivatives of 3-amino-3,4-dihydro-1-hydroxycarbostyril and related compounds

The 3-methyl and 4-methyl derivatives of 3-amino-3,4-dihydro-1-hydroxycarbostyril were synthesized by the reductive cyclization of α-methyl-β-(o-nitrophenyl)alanine and α-amino-β-(o-nitrophenyl)butyric acid hydrohalides, respectively, under conditions of catalytic hydrogenation in acidic solution. The free bases of the latter two o-nitroaromatic amino acids were also catalytically hydrogenated under neutral conditions to yield the respective α-methyl-β-(o-aminophenyl)alanine and α-amino-β-(o-aminophenyl)butyric acid which were converted to the corresponding lactams, 3-methyl- and 4-methyl-3-amino-3,4-dihydrocarbostyrils. α-Methyl-β-(o-nitrophenyl)alanine was obtained by acid hydrolysis of 5-methy)-5-(o-nitrobenzyl)hydantoin which was prepared by treatment of o-nitrophenylacetone with potassium cyanide and ammonium carbonate. α-Amino-β-(o-nitrophenyl)butyric acid was synthesized by condensation of α-bromo-o-nitroethylbenzene with diethyl acetamidomalonate, followed by acid hydrolysis of the condensation product. The 4-methylated compounds were obtained as synthetic mixtures of two diasteromeric racemates in nearly the same amounts as shown by nmr spectral analysis. Unlike the demethylated parent compound, 3-amino-3,4-dihydro-1-hydroxycarbostyril, neither the 3-methyl nor 4-methyl analog was found to possess any antibacterial activity.     

Preparation and characterization of rhenium(I) compounds with amino ester derivatized diimine ligands. Investigations of luminescence. Crystal structures of Re(CO)3Cl(pyca-β-Ala-OEt) and Re(CO)3Cl(pyca-l-Asp(OMe)-OMe)

The title compounds were prepared in good yield by heating ester protected amino acids (H-l-Ala-OEt, H-β-Ala-OEt, H-L-Val-OMe, H-GABA-OMe, H-L-Asp(OMe)-OMe, H-L-Met-OMe) with Re(CO)₅Cl in the presence of pyridine-2-carboxaldehyde. The resulting novel complexes contain a bound, conjugated diimine ligand with a pendant ester group. All compounds were characterized by ¹H and ¹³C NMR, IR and UV-Vis spectroscopy. Compounds prepared from chiral amino esters give diastereomers because of the presence of a stereogenic metal center. Syntheses using Re(CO)₅Br as starting material were discontinued because of metathesis involving the chloride in the amino ester · HCl salts and the metal bromide. Photophysical studies on Re(CO)₃Cl(pyca-β-Ala-OEt), 2, show that it is luminescent in organic solvents in air at room temperature. The crystal structures of 2 and Re(CO)₃Cl(pyca-l-Asp(OMe)-OMe), 5, were determined.     

A comparative study of H+ and Li+ interactions with oxygen bases

Ab initio SCF calculations have been performed to investigate the structural and electronic features of the interactions of H⁺ and Li⁺ with the oxygen bases H₂CO and OH^?. The data indicate that the OH interactions are primarily covalent while the LiO interactions are primarily electrostatic, although the LiO interaction in LiOH has considerable covalent character.     

Ab initio study of complexes with two cations as N-H donors to F-: structures and spin-spin coupling constants across N-H-F hydrogen bonds

A systematic ab initio study has been carried out to determine the MP2/6-31+G(d,p) structures and EOM-CCSD coupling constants across N-H-F-H-N hydrogen bonds for a series of complexes F(H(3)NH)(2)(+), F(HNNH(2))(2)(+), F(H(2)CNH(2))(2)(+), F(HCNH)(2)(+), and F(FCNH)(2)(+). These complexes have hydrogen bonds with two equivalent N-H donors to F(-). As the basicity of the nitrogen donor decreases, the N-H distance increases and the N-H-F-H-N arrangement changes from linear to bent. As these changes occur and the hydrogen bonds between the ion pairs acquire increased proton-shared character, (2h)J(F)(-)(N) increases in absolute value and (1h)J(H)(-)(F) changes sign. F(H(3)NH)(2)(+) complexes were also optimized as a function of the N-H distance. As this distance increases and the N-H...F hydrogen bonds change from ion-pair to proton-shared to traditional F-H...N hydrogen bonds, (2h)J(F)(-)(N) initially increases and then decreases in absolute value, (1)J(N)(-)(H) decreases in absolute value, and (1h)J(H)(-)(F) changes sign. The signs and magnitudes of these coupling constants computed for F(H(3)NH)(2)(+) at short N-H distances are in agreement with the experimental signs and magnitudes determined for the F(collidineH)(2)(+) complex in solution. However, even when the N-H and F-H distances are taken from the optimized structure of F(collidineH)(2)(+), (2h)J(F)(-)(N) and (1h)J(H)(-)(F) are still too large relative to experiment. When the distances extracted from the experimental NMR data are used, there is excellent agreement between computed and experimental coupling constants. This suggests that the N-H-F hydrogen bonds in the isolated gas-phase F(collidineH)(2)(+) complex have too much proton-shared character relative to those that exist in solution.     

Tetrakis(μ‐guanidino­acetic acid‐κ2O:O′)­bis­[(nitrato‐κO)copper(II)]

The title compound, [Cu₂(NO₃)₂(C₃H₇N₃O₂)₄], forms a centrosymmetric dimer, with the two Cu²⁺ ions separated by 2.6525 (6) Å. The asymmetric unit contains a Cu atom coordinated to two guanidino­acetic acid ligands (via one carboxyl­ate O atom from each ligand) and to a nitrate group. The inversion centre in P generates the entire mol­ecule, in which each Cu atom is coordinated to four carboxyl­ate and to one nitrate O atom; ignoring the Cu—Cu separation, the geometry about each Cu atom is square pyramidal. The amino acid ligand is in the zwitterionic form. Strong N—H?O hydrogen bonds lead to a three‐dimensional supramolecular structure, in which the N?O distances are in the range 2.931 (4)–3.278 (3) Å, with N—H?O angles ranging from 128 to 170°.     

One-bond spin-spin coupling constants of X-1H proton donors in complexes with X-H-Y hydrogen bonds, for X = 13C, 15N, 17O, and 19F: predictions, comparisons, and relationships among 1J X-H, 1K X-H, and X-H distances

Ab initio calculations at the equation-of-motion coupled cluster (EOM-CCSD) level of theory have been carried out to investigate one-bond (13)C-(1)H, (15)N-(1)H, (17)O-(1)H, and (19)F-(1)H coupling constants in a systematic study of monomers and hydrogen-bonded complexes. Computed coupling constants ((1)J(X-H)) for monomers are in good agreement with available experimental data. All reduced Fermi-contact terms and reduced coupling constants ((1)K(X-H)) for monomers and complexes are positive. Plots of (1)K(X-H) versus the X-H distance for the 16 monomers and the 64 complexes in which these monomers are proton donors exhibit significant scatter. However, a linear relationship has been demonstrated for the first time between coupling constants and X-H distances for different X atoms by plotting the ratios of the coupling constants for complexes and corresponding monomers versus the ratios of distances for complexes and corresponding monomers times the square of the Pauling electronegativity. Since the ratio removes the dependence of coupling constants on the magnetogyric ratios of X, this relationship holds for both (1)K(X-H) and (1)J(X-H). The decrease in reduced coupling constants ((1)K(X-H)) as the X-H distance increases is due primarily to the increased proton-shared character of the hydrogen bond.     

Molecular orbital theory of the hydrogen bond. 25. Water–uracil complexes in excited n → π states

Hydrogen bonding of uracil with water in excited n → π* states has been investigated by means of ab initio SCF -CI calculations on uracil and water–uracil complexes. Two low-energy excited states arise from n → π* transitions in uracil. The first is due to excitation of the C₄? O group, while the second is associated with excitation of the C₂? O group. In the first n → π* state, hydrogen bonds at O₄ are broken, so that the open water–uracil dimer at O₄ dissociates. The “wobble” dimer, in which a water molecule is essentially free to move between its position in an open structure at N₃? H and a cyclic structure at N₃? H and O₄ in the ground state, collapses to a different “wobble” dimer at N₃? H and O₂ in the excited state. The third dimer, a “wobble” dimer at N₁? H and O₂, remains intact, but is destabilized relative to the ground state. Although hydrogen bonds at O₂ are broken in the second n → π* state, the three water–uracil dimers remain bound. The “wobble” dimer at N₁? H and O₂ changes to an excited open dimer at N₁? H. The “wobble” dimer at N₃? H and O₄ remains intact, and the open dimer at O₄ is further stabilized upon excitation. Dimer blue shifts of n → π* bands are nearly additive in 2:1 and 3:1 water:uracil structures. The fates of the three 2:1 water:uracil trimers and the 3:1 water:uracil tetramer in the first and second n → π* states are determined by the fates of the corresponding excited dimers in these states.     

Ab initio study of the influence of trimer formation on one- and two-bond spin-spin coupling constants across an X-H-Y hydrogen bond: AH:XH:YH3 complexes for A, X = 19F, 35Cl and Y = 15N, 31P

Ab initio equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) calculations have been carried out to investigate the effect of a third polar near-neighbor on one-bond ((1)J(X)(-)(H) and (1h)J(H)(-)(Y)) and two-bond ((2h)J(X)(-)(Y)) spin-spin coupling constants in AH:XH:YH(3) complexes, where A and X are (19)F and (35)Cl and Y is either (15)N or (31)P. The changes in both one- and two-bond spin-spin coupling constants upon trimer formation indicate that the presence of a third molecule promotes proton transfer across the X-H-Y hydrogen bond. The proton-shared character of the X-H-Y hydrogen bond increases in the order XH:YH(3) < ClH:XH:YH(3) < FH:XH:YH(3). This order is also the order of decreasing shielding of the hydrogen-bonded proton and decreasing X-Y distance, and is consistent with the greater hydrogen-bonding ability of HF compared to HCl as the third molecule. For all complexes, the reduced X-H and X-Y spin-spin coupling constants ((1)K(X)(-)(H) and (2h)K(X)(-)(Y)) are positive, consistent with previous studies of complexes in which X and Y are second-period elements in hydrogen-bonded dimers. (1h)K(H)(-)(Y) is, as expected, negative in these complexes which have traditional hydrogen bonds, except for ClH:FH:NH(3) and FH:FH:NH(3). In these two complexes, the F-H-N hydrogen bond has sufficient proton-shared character to induce a change of sign in (1h)K(H)(-)(Y). The effects of trimer formation on spin-spin coupling constants are markedly greater in complexes in which NH(3) rather than PH(3) is the proton acceptor.     

Photochemistry of A1E, a retinoid with a conjugated pyridinium moiety: competition between pericyclic photooxygenation and pericyclization

The photochemistry of the retinoid analogue A1E shows an oxygen and solvent dependence. Irradiation of A1E with visible light (lambda(irr) = 425 nm) in methanol solutions resulted in pericyclization to form pyridinium terpenoids. Although the quantum yield for this cyclization is low (approximately 10(-4)), nevertheless the photochemical transformation occurs with quantitative chemical yield with remarkable chemoselectivity and diastereoselectivity. Conversely, irradiation of A1E under the same irradiation conditions in air-saturated carbon tetrachloride or deuterated chloroform produced a cyclic 5,8-peroxide as the major product. Deuterium solvent effects, experiments utilizing endoperoxide, phosphorescence, and chemiluminescence quenching studies strongly support the involvement of singlet oxygen in the endoperoxide formation. It is proposed that, upon irradiation, in the presence of oxygen, A1E acts as a sensitizer for generation of singlet oxygen from triplet oxygen present in the solution; the singlet oxygen produced reacts with A1E to produce cyclic peroxide. Thus, the photochemistry of A1E is characterized by two competing reactions, cyclization and peroxide formation. The dominant reaction is determined by the concentration of oxygen, the concentration of A1E, and the lifetime of singlet oxygen in the solvent employed. If the lifetime of singlet oxygen in a given solvent is long enough, then oxidation (peroxide formation) is the major reaction. If the singlet oxygen produced is quenched by the protonated solvent molecules faster than singlet oxygen reacts with A1E, then cyclization dominates.