Robert E. Tapscott

Tropodegradable fluorocarbon replacements for ozone-depleting and global-warming chemicals

Abstract Incorporation of certain molecular features into fluorocarbons can decrease the tropospheric lifetime, providing commercially applicable chemicals with low global warming and stratospheric ozone impacts.

ISOMER STABILITIES OF TARTRATE-BRIDGED BINUCLEAR COMPLEXES

Abstract The seven possible isomers for a tetragonal coordination geometry and the twenty-four possible isomers for trigonal-bipyramidal and octahedral geometries are enumerated and described for binuclear tartrate-bridged complexes. Two geometrical parameters corresponding to the strain in the binuclear structure and to the ligand conformation energy have been evaluated as a function of the coordination geometry. Predictions of the relative energies of tartrate-bridged isomers made from these parameters agree with known stability differences when protonation of the tartrate ligand is also considered. An entropy factor of Rln2 contributes to the stabilities of the mixed-ligand isomers. This work indicates that tartrate-bridging may be used to induce known absolute configurations about metal ions for CD studies and for stereochemical correlations and to lock tartrate groups into known conformations for thermochemical determinations of conformational energies.

EQUILIBRIA OF VANADYL(IV) TARTRATES IN AQUEOUS SOLUTION ABOVE pH 7

Abstract In aqueous solution at pH ∼8, the reaction of binuclear vanadyl(IV) tartrate(4–), (VO)2 (C4 H2 O6)2 4−, with excess tartrate(2–) ligand gives the mononuclear complex, VO(C4 H3 O6)2 4−. Spectral evidence indicates that the product has a trans coordination by two ionized hydroxyl oxygen atoms and structures are proposed. At ionic strengths of 1.00 M (3.00 M) at 298°K, stoichiometric equilibrium constants of 0.34 ± 0.02 M−1 (0.8 ± 0.2 M−1) and 0.054 ± 0.002 M−1 (0.07 ± 0.01 M−1) have been determined for the binuclear-mononuclear reactions in, respectively, the active system, (VO)2 (d-C4 H2 O6)2 4− + 2d-C4 H4 O6 2− ⇆ 2VO(d-C4 H3 O6)2 4−, and the racemic system, (VO)2 (d-C4 H2 O6)(l-C4 H2 O6)4− + 2dl-C3 H4 O6 2− ⇆ 2VO(dl-C4 H3 O6)2 4−. From these experimentally determined equilibrium constants, the equilibrium constant for the interconversion of the active and racemic binuclear complexes has been derived for two different assumptions of the monomeric isomer distribution in the racemic system. Assuming...

ISOMERS OF TRIS(2-METHYL-1,3-PROPANEDIAMINE)COBALT(III)‐A COMPLEX OF A PROCHIRAL SINAMBIC LIGAND

Abstract The complex ion [Co(metn)3]3+ (metn=2-methyl-1,3-propanediamine) has been synthesized and the facial and meridional isomers have been separated by column chromatography and identified by 13C and 1H NMR spectroscopy. This is the first reported separation of isomers of a tris complex where the isomerism is due to the presence of prochiral ligands with enantiotopic donor atoms (prochiral “sinambic” ligands). The 13C and visible spectra indicate a large contribution from conformers containing skew-boat chelate rings in solution. An incomplete (owing to disorder) crystal structure shows a tris-(skew-boat) conformer for the cationic complex present in fac-[Co(metn)3] Cl3.

Electronic and molecular structure of DL vanadyl(IV) tartrate(4−) and methyl-substituted tartrate(4−) binuclear complexes

Abstract Several Fenske-Hall type molecular orbital calculations have been carried out in order to elucidate electronic and structural changes that occur upon methyl substitution of the DL isomer of binuclear vanadyl(IV) tartrate(4−). In addition, in order to confirm those structural changes that accompany methyl substitution, as observed in a recent crystal structure determination on tetrasodium [μ-(+)-dimethyltartrato(4−)]·[μ(−)dimethyltartrato(4−)]-bis(oxovanadate(IV)] dodecahydrate, an X-ray structure determination on a second crystal form, a hexahydrate, has been completed. The salt Na4[VO)2-((+)-dmt((−)-dmt]·6H2O, ‘dmt’ = dimethyltartrate-(4−), OOCC(CH3)(O)C(CH3)(O)COO4−, crystallizes in the monoclinic space group P21/c with a = 10.624(2), b = 11.621(2), c = 11.719(3) », β = 124.07(2)°, Z = 2. The structure was refined to R = 0.041, Rw = 0.041 for 2248 independent, observed reflections. Like the blue dodecahydrate studied earlier, the pink hexahydrate exhibits a decreased VV distance, a dropping of the vanadium atom toward the plane of the four equatorial oxygen ligators, an increased vanadium to tartrate hydroxyl oxygen atom distance (all relative to the nonmethyl-substituted complex), and sixth-site coordination by an ionized tartrate hydroxyl oxygen atom in the other half of the binuclear complex. The complex present in the hexahydrate salt is even more severely distorted from the idealized D2h geometry than is the dodecahydrate structure (though a crystallographic center of symmetry is maintained in both). The molecular orbital calculation confirm an energy level ordering for the HOMO and the lowest four LUMOs of a′(dx2-y2), a′(dxz), a′′(dyz), a′′(dxy), and a′′(dz2) in C2 localized symmetry with the A′(dxz), a′′(dyz) pair having nearly the same energy.

Synthesis, characterization and structure of cis and trans cobalt(III) bis chelates of 2-methyl-1,3-propanediamine, a prochiral sinambic ligand

Abstract Cis and trans isomers of [Co(metn)2Cl2]+ (metn = 2-methyl-1,3-propanediamine) have been prepared and characterized. Owing to the presence of rotationally nonequivalent donor atoms in the prochiral ligand, three diastereomers are possible for the cis isomer (C2(anti), C2(syn), and C1) and two diastereomers are possible for the trans complex (C2h and C2ν). A crystal structure of the purple compound [Co(metn)2Cl2]Cl· 1 4 CH3OH shows the presence of both the cis C2(anti) and trans C2h isomers. An uncompleted structure determination on green [Co(metn)2Cl2](H3O)Cl2 ·H2O reveals only the trans C2h isomer. X-Ray crystallography also shows a flattening of the 6-membered metn chelate ring compared with that of 1,3-propanediamine. Though 13C NMR spectra of trans-[Co(metn)2Cl2]+ show no evidence for more than one isomer in solution, this may be due to undetectably small chemical shift differences between the two diastereomers possible. IR spectra indicate a solid state cis to trans rearrangement of [Co(metn)2Cl2]+ in the presence of KBr.

An ESR study of vanadyl(IV) tartrate and methyl-substituted tartrate complexes

Abstract ESR spectra have been determined for nine binuclear, exchange-coupled complexes of vanadyl(IV) with tartrate(4−), monomethyltartrate(4−) and dimethyltartrate(4−) ligands in frozen glasses at 77 K. Spin-Hamiltonian parameters have been obtained from the spectra collected for these triplet state species using an axial symmetry approximation. The fact that the zero-field splitting parameter, D, increases with increasing methyl substitution indicates a corresponding decrease in the metal-metal distance.

Magnetic studies of vanadyl(IV) tartrate dimers

Abstract Temperature-dependent magnetic susceptibility data have been collected for solid sodium salts of the binuclearvanadyl(IV) complexes [(VO) 2 ( d -tart) 2 ] 4− , [(VO) 2 ( dl -tart) 2 ] 4− , [(VO) 2 ( dl -mmt) 2 ] 4− ,and [(VO) 2 ( dl -dmt) 2 ] 4− . (“tart” = tartrate(4−), “mmt” = monomethyl-tartrate(4−), “dmt” = dimethyltartrate(4−).) Ferromagnetic intradimer- and antiferromagnetic interdimer exchange is found; however, both interactions are small and similar in magnitude and reliable exchange constants cannot be extracted. The intradimer interaction probably occurs by a superexchange mechanism.

The crystal and molecular structure of 1,9-diamino-4-methyl-3,7-diazanonane-3,7-diacetato(2−)cobalt(III) nitrate monohydrate—a complex exhibiting large 13C NMR steric shifts

Abstract Past studies have shown major 13 C NMR chemical shift differences between methylene carbon atoms proximal (In a γ position) and distal to a ligand methyl substituent in [Co(mddda)] + (mddda = 1,9-diamino-4-methyl-3,7-diazanonane-3,7-diacetate(2−), NH 2 CH 2 CH 2 N(CH 2 COO − )CH 2 CH 2 CH(CH 3 )N(CH 2 COO − )CH 2 CH 2 NH 2 ). Since the chemical shift differences have been attributed to γ steric shifts associated with the methyl group on the hexadentate ligand, X-ray crystal structure determination has been carried out on [Co(mddda)]NO 3 ·H 2 O to examine the steric features. The compound crystallizes in the space group P2 1 /c with a = 9.756(2) A, b = 14.912(4) A, c = 11.783(2)°, A, β = 95.70(2)°, and Z = 4. Intensities were collected on an automated diffractometer and the structure was refined to a conventional R factor of 0.058. The crystal contains a racemic mixture of ΔΛΔ(R) and ΛΔΛ(S) enantiometric cationic complexes, where the R and S labels designate the absolute configurations of the chiral methyl-substituted carbon atom of the ligands. These diastereomers are those required to maintain an equatorial methyl group on the central six-membered chelate ring, whose conformations are λ skew-boat for the ΔΛΔ isomer and δ skew-boat for the ΛΔΛ isomer. The respective conformations of the two skewed five-membered chelate rings are δδ and λλ. Though most corresponding bond lengths and bond angles in the two portions of the molecule are nearly the same, those differences which are observed indicate nonbonded interactions between the methyl substituent and adjacent methylene groups. Consideration of these interactions and geometrical variations indicates that bond angle distortion may play an important role in the γ shifts observed.

Kinetic studies of stereoselective vanadyl(IV) tartrate reactions

Abstract The stereoselective ligand substitution reaction, (VO)2(l-C4H2O6)4−2 + d-C4H4O2−6 ⇄ (VO)2(d-C4H2O6)(l-C4H2O6)4− + l-C4H4O2−6, and ligand exchange reaction, 1 2 (VO)2(l-C4H2O6)4−2 + 1 2 (VO)2(d-C4H2O6)4−2 ⇄ (VO)2(d-C4H2O6(l-C4H2O6)4−, both occur by parallel path processes. The major paths for the two reactions have reaction rate laws of forms d[(VO)2(d-C4H2O6)(l-C4H2O6)4−]/dt = k1[(VO)2(l-C4H2O6)4−2] for ligand substitution and d[(VO)2(d-C4H2O6)(l-C4H2O6)4−]/dt = k3{[(VO1)2(l-C4H2O6)4−2] + [(VO)2(d-C4H2O6)4−2]} (for equimolar reactants) for ligand exchange. The respective kinetic parameters at 25.0 °C and μ = 3.00 mol l−1 (LiClO4) are k1 = 0.0112 ± 0.0013 sec−1, ΔH≠ = 25 ± 2 kcal mol−1, ΔS≠ = 17 ± eu and k3 = 0.00693 ± 0.00027 sec−1, ΔH≠ = 23.5 ± 1.0 kcal mol−1, ΔS≠ = 10 ± 3 eu. That corresponding activation parameters are equal indicates that the first-order pathways of the two reactions may have identical dissociative rate determining steps. The secondary, autocatalytic paths, which are also present in these reactions, apparently involve product-assisted ligand dissociations.