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D B Rorabacher

Publications and source records attributed to D B Rorabacher.

11 recordsLinked to original sources
PubMed2006

Comparative study of donor atom effects on the thermodynamic and electron-transfer kinetic properties of copper(II/I) complexes with sexadentate macrocyclic ligands. [CuII/I([18]aneS4N2)] and [CuII/I([18]aneS4O2)].

Studies have been conducted on the copper complexes formed with two sexadentate macrocyclic ligands containing four thioether sulfur donor atoms plus either two nitrogen or two oxygen donor atoms on opposing sides of the ring. The resulting two ligands, L, designated as [18]aneS(4)N(2) and [18]aneS(4)O(2), respectively, represent homologues of the previously studied Cu(ii/i) system with a macrocycle having six sulfur donor atoms, [18]aneS(6). Crystal structures of [Cu(II)([18]aneS(4)O(2))](ClO(4))(2) and [Cu(I)([18]aneS(4)O(2))]ClO(4) have been determined in this work. Comparison of the structures of all three systems reveals that the oxidized complexes are six coordinate with two coordinate bonds undergoing rupture upon reduction. However, the geometric changes accompanying electron transfer appear to differ for the three systems. The stability constants and electrochemical properties of both of the heteromacrocyclic complexes have been determined in acetonitrile and the Cu(II/I)L electron-transfer kinetics have been studied in the same solvent using six different counter reagents for each system. The electron self-exchange rate constants have then been calculated using the Marcus cross relationship. The results are compared to other Cu(II/I)L systems in terms of the effect of ligand geometric changes upon the overall kinetic behavior.

Journal Article↗
PubMed2006

Effect of constrained donor atom orientations on the stabilities, complexation kinetics, redox potentials, and structures of macrocyclic polythiaether Complexes. Copper(II) complexes with cyclopentanediyl derivatives of [14]aneS4 in 80% methanol.

The two ethylene bridges in the macrocyclic tetrathiaether 1,4,8,11-tetrathiacyclotetradecane ([14]aneS(4)) have been systematically replaced by cis- or trans-1,2-cyclopentane to generate a series of new ligands that exhibit differing preferences for the orientation of the sulfur donor atoms while maintaining constant inductive effects. The resulting five dicyclopentanediyl derivatives, along with two previously synthesized monocyclopentanediyl analogues, have been complexed with Cu(II), and their stability constants, formation and dissociation rate constants, and redox potentials have been determined in 80% methanol/20% water (by weight). The crystal structures of the Cu(II) complexes with the five dicyclopentanediyl-[14]aneS(4) diastereomers as well as the structures for a representative Cu(I) complex and one of the free ligands have also been determined. The properties of these complexes are compared to previous data obtained for the corresponding cyclohexanediyl derivatives in an attempt to shed additional light on the influence of sterically constraining substituents upon the properties of macrocyclic ligand complexes.

Journal Article↗
PubMed2005

Physical parameters and electron-transfer kinetics of the copper(II/I) complex with the macrocyclic sexadentate ligand [18]aneS6.

The electron-transfer kinetics of the complex formed by copper(II/I) with the sexadentate macrocyclic ligand 1,4,7,10,13,16-hexathiacyclooctadecane ([18]aneS6) have been measured in acetonitrile with a series of three oxidizing agents and three reducing agents. These studies have been supplemented by determinations of the redox potential and the stability constants of the Cu(I)- and Cu(II)([18]aneS6) complexes in both acetonitrile and aqueous solution. The Marcus cross relationship has been applied to the cross-reaction rate constants for the six reactions studied to resolve the electron self-exchange rate constant for the Cu(II/I)([18]aneS6) complex. An average value of k11 = 3 x 10(3) M(-1) s(-1) was obtained at 25 degrees C, mu = 0.10 M in acetonitrile. This value is approximately 2 orders of magnitude smaller than the values reported previously for the corresponding Cu(II/I) complexes with the quadridentate and quinquedentate homoleptic homologues having all ethylene bridges, namely, 1,4,7,10-tetrathiacyclododecane ([12]aneS4) and 1,4,7,10,13-pentathiacyclopentadecane ([15]aneS5). This significant difference in reactivity is attributed to the greater rearrangement in the geometry of the inner-coordination sphere that accompanies electron transfer in the Cu(II/I)([18]aneS6) system, wherein two Cu-S bonds are ruptured upon reduction. In contrast to other Cu(II/I) complexes with macrocyclic polythiaethers that have self-exchange rate constants within the same range, no evidence for conformationally gated electron transfer was observed, even in the case of the most rapid oxidation reaction studied.

Journal Article↗
PubMed2003

Electron-transfer kinetics of copper(II/I) tripodal ligand complexes.

The electron-transfer kinetics for each of three copper(II/I) tripodal ligand complexes reacting with multiple reducing and oxidizing counter reagents have been examined in aqueous solution at 25 degrees C, mu = 0.10 M. For all of the ligands studied, an amine nitrogen serves as the bridgehead atom. Two of the ligands (PMMEA and PEMEA) contain two thioether sulfurs and one pyridyl nitrogen as donor atoms on the appended legs while the third ligand (BPEMEA) has two pyridyl nitrogens and one thioether sulfur. Very limited kinetic studies were also conducted on two additional closely related tripodal ligand complexes. The results are compared to our previous kinetic study on a Cu(II/I) system involving a tripodal ligand (TMMEA) with thioether sulfur donor atoms on all three legs. In all systems, the Cu(II/I) electron self-exchange rate constants (k(11)) are surprisingly small, ranging approximately 0.03-50 M(-)(1) s(-)(1). The results are consistent with earlier studies reported by Yandell involving the reduction of Cu(II) complexes with four similar tripodal ligand systems, and it is concluded that the dominant reaction pathway involves a metastable Cu(II)L intermediate species (designated as pathway B). Since crystal structures suggest that the ligand reorganization accompanying electron transfer is relatively small compared to our earlier studies on macrocyclic ligand complexes of Cu(II/I), it is unclear why the k(11) values for the tripodal ligand systems are of such small magnitude.

Journal Article↗
PubMed2001

Electron-transfer kinetics of tris(2-(methylthioethyl))aminecopper(II/I). A tripodal ligand complex exhibiting virtual C3v symmetry.

The tripodal ligand TMMEA (tris(2-methylthioethyl)amine) forms a trigonal bipyramidal complex with copper(II) in which the bridgehead nitrogen occupies one axial site, a solvent molecule (or anion) occupies the opposite axial site, and the three thioether sulfurs occupy the three planar sites. Upon reduction to copper(I), the axial solvent molecule (or anion) dissociates to leave a trigonal pyramidal complex with shortened Cu-S bonds and an elongated Cu-N bond. Therefore, both oxidation states maintain virtual C3v symmetry similar to that found in the type 1 blue copper protein sites. The electron-transfer cross-reaction rate constants have been determined for the Cu(II/I)(TMMEA) system reacting with three reductants and three oxidants. The Marcus cross relation was then utilized to generate apparent values for the Cu(II/I) electron self-exchange rate constant (k(11)) from the kinetic data for each of the six reactions. The median value obtained from the three reduction reactions is log k(11(Red)) = -1.5 while the median from the three oxidation reactions is log k(11(Ox)) = +0.9. This difference of 2.4 orders of magnitude is consistent with the dual-pathway square scheme mechanism which we have previously proposed for electron transfer in Cu(II/I) complexes. For this tripodal ligand system, however, the pathway involving a metastable Cu(II)L intermediate (pathway B) appears to be preferred over the pathway involving a metastable Cu(I)L intermediate (pathway A), which is opposite to the trend we have previously observed for a number of systems involving macrocyclic and acyclic tetrathiaethers. Both pathways exhibit relatively sluggish electron-transfer kinetics which is attributed to the rupture/formation of the strongly bound inner-sphere water molecule and the accompanying solvent reorganization.

Amines↗
PubMed2001

Direct evidence for a geometrically constrained "entatic state" effect on copper(II/I) electron-transfer kinetics as manifested in metastable intermediates.

The absolute magnitude of an "entatic" (constrained) state effect has never been quantitatively demonstrated. In the current study, we have examined the electron-transfer kinetics for five closely related copper(II/I) complexes formed with all possible diastereomers of [14]aneS(4) (1,4,8,11-tetrathiacyclotetradecane) in which both ethylene bridges have been replaced by cis- or trans-1,2-cyclohexane. The crystal structures of all five Cu(II) complexes and a representative Cu(I) complex have been established by X-ray diffraction. For each complex, the cross-reaction rate constants have been determined with six different oxidants and reductants in aqueous solution at 25 degrees C, mu = 0.10 M. The value of the electron self-exchange rate constant (k(11)) has then been calculated from each cross reaction rate constant using the Marcus cross relation. All five Cu(II/I) systems show evidence of a dual-pathway square scheme mechanism for which the two individual k(11) values have been evaluated. In combination with similar values previously determined for the parent complex, Cu(II/I)([14]aneS(4)), and corresponding complexes with the two related monocyclohexanediyl derivatives, we now have evaluated a total of 16 self-exchange rate constants which span nearly 6 orders of magnitude for these 8 closely related Cu(II/I) systems. Application of the stability constants for the formation of the corresponding 16 metastable intermediates--as previously determined by rapid-scan cyclic voltammetry--makes it possible to calculate the specific electron self-exchange rate constants representing the reaction of each of the strained intermediate species exchanging electrons with their stable redox partners--the first time that calculations of this type have been possible. All but three of these 16 specific self-exchange rate constants fall within--or very close to--the range of 10(5)-10(6) M(-1) s(-1), values which are characteristic of the most labile Cu(II/I) systems previously reported, including the blue copper proteins. The results of the current investigation provide the first unequivocal demonstration of the efficacy of the entatic state concept as applied to Cu(II/I) systems.

Copper↗
PubMed2000

Effect of conformational constraints on gated electron-transfer kinetics. 3. Copper(II/I) complexes with cis- and trans-cyclopentanediyl-1,4,8,11-tetrathiacyclotetradecane.

Previous kinetic and electrochemical studies of copper complexes with macrocyclic tetrathiaethers-such as 1,4,8,11-tetrathiacyclotetradecane ([14]aneS4)-have indicated that electron transfer and the accompanying conformational change occur sequentially to give rise to a dual-pathway mechanism. Under appropriate conditions, the conformational change itself may become rate-limiting, a condition known as "gated" electron transfer. We have recently hypothesized that the controlling conformational change involves inversion of two donor atoms, which suggests that "gated" behavior should be affected by appropriate steric constraints. In the current work, two derivatives of [14]aneS4 have been synthesized in which one of the ethylene bridges has been replaced by either cis- or trans-1,2-cyclopentane. The resulting copper systems have been characterized in terms of their Cu(II/I)L potentials, the stabilities of their oxidized and reduced complexes, and their crystal structures. The electron self-exchange rate constants have been determined both by NMR line-broadening and by kinetic measurements of their rates of reduction and oxidation with six or seven counter reagents. All studies have been carried out at 25 degrees C, mu = 0.10 M (NaClO4 and/or Cu(ClO4)2), in aqueous solution. Both Cu(II/I) systems show evidence of a dual-pathway mechanism, and the electron self-exchange rate constants representative of both mechanistic pathways have been determined. The first-order rate constant for gated behavior has also been resolved for the Cu(I)(trans-cyclopentane-[14]aneS4) complex, but only a limiting value can be established for the corresponding cis-cyclopentane system. The rate constants for both systems investigated in this work are compared to values previously determined for the Cu(II/I) systems with the parent [14]aneS4 macrocycle and its derivatives involving phenylene and cis- or trans-cyclohexane substituents. The results are discussed in terms of the influence of the fused rings on the probable conformational changes accompanying the electron-transfer process.

Journal Article↗
PubMed2000

Formation and dissociation kinetics and crystal structures of nickel(II)-macrocyclic tetrathiaether complexes in acetonitrile. Comparison to nickel(II)-macrocyclic tetramines.

Complex formation and dissociation rate constants have been independently determined for solvated nickel(II) ion reacting with eight macrocyclic tetrathiaether ligands and one acyclic analogue in acetonitrile at 25 degrees C, mu = 0.15 M. The macrocyclic ligands include 1,4,8,11-tetrathiacyclotetradecane ([14]aneS4) and seven derivatives in which one or both ethylene bridges have been substituted by cis- or trans-1,2-cyclohexane, while the acyclic ligand is 2,5,9,12-tetrathiatridecane (Me2-2,3,2-S4). In contrast to similar complex formation kinetic studies on Ni(II) reacting with corresponding macrocyclic tetramines in acetonitrile and N,N-dimethylformamide (DMF), the kinetics of complex formation with the macrocyclic tetrathiaethers show no evidence of slow conformational changes following the initial coordination process. The differing behavior is ascribed to the fact that such conformational changes require donor atom inversion, which is readily accommodated by thiaether sulfurs but requires abstraction of a hydrogen from a nitrogen (to form a temporary amide). The latter process is not facilitated in solvents of low protophilicity. The rate-determining step in the formation reactions appears to be at the point of first-bond formation for the acyclic tetrathiaether but shifts to the point of chelate ring closure (i.e., second-bond formation) for the macrocyclic tetrathiaether complexes. The formation rate constants for Ni(II) with the macrocyclic tetrathiaethers parallel those previously obtained for Cu(II) reacting with the same ligands in 80% methanol-20% water (w/w). By contrast, the Ni(II) dissociation rate constants show significant variations from the trends in the Cu(II) behavior. Crystal structures are reported for the Ni(II) complexes formed with all five dicyclohexanediyl-substituted macrocyclic tetrathiaethers. All but one are low-spin species.

Journal Article↗
PubMed2000

Kinetics and mechanism of copper(II) complex formation with tripodal aminopolythiaether and aminopolypyridyl ligands in aqueous solution.

The complex formation kinetics of aquated copper(II) ion reacting with 12 related tripodal ligands have been studied in aqueous solution at 25 degrees C, mu = 0.10 M (NaClO4). For most of the ligands studied, specific formation rate constants have been resolved for both the unprotonated and monoprotonated ligand species. All of the tripodal ligands included in this study contain a bridgehead amine nitrogen with the three legs consisting of 2-methylthioethyl or 2-ethylthioethyl and/or 2-pyridylethyl or 2-pyridylmethyl. Since the bridgehead nitrogen is too sterically hindered to participate in initial coordinate bond formation, the first bond must involve a thiaether sulfur or a pyridine nitrogen on one of the pendant legs followed by coordination to the bridgehead nitrogen to complete the first chelate ring. All kinetic data are interpreted in terms of this presumed sequence in the bond formation steps. For the two ligands in which all three pendant legs contain thiaether sulfur donor atoms, the rate-determining step appears to be at the point of second bond formation (chelate ring closure), although the distinction is not well defined. For all other unprotonated ligands, the kinetic behavior is consistent with the first-bond formation being rate-determining. Upon protonation, the rate-determining step appears to shift to the point of proton loss associated with second-bond formation in several cases. A particularly interesting observation is that the tripodal ligand tris(ethylthioethyl)amine (TEMEA) exhibits specific Cu(II) complex formation rate constants that are virtually identical to those for a closely related macrocyclic ligand, 1,4,8-trithia-11-azacyclotetradecane ([14]aneNS3), but the calculated CuIIL dissociation rate constants differ by a factor of 1000. A further comparison of the calculated dissociation rate constants for Cu(II)-tripodal ligand complexes indicates that a Cu(II)-N(pyridine) bond is approximately 10(4) times stronger than a Cu(II)-SR2 bond. This leads to the conclusion that a 1:1 Cu(II)-SR2 complex would have a predicted stability constant of about 0.04 M-1 in aqueous solution--the first estimate obtained for the strength of a single Cu(II)-S(thiaether) bond.

Journal Article↗
PubMed1997

Avoiding interferences from Good's buffers: A contiguous series of noncomplexing tertiary amine buffers covering the entire range of pH 3-11.

Of the 20 well-known buffers proposed by Good, all but 3 form metal ion complexes which can result in serious interferences, particularly in protein analyses. The structural features responsible for such complex formation have been identified. Based on a mechanistic analysis of the metal complexation process, it is proposed that tertiary amine compounds, having N-substituents which are ethyl or larger, are sterically inaccessible for initial bond formation with solvated metal ions in aqueous solution. Thus, in the absence of donor atoms on the alpha-, beta-, or gamma-carbons, metal complexation cannot proceed. The proposed noncomplexing compounds include Good's 3 noncomplexing buffers (Mes, Mops, Pipes) plus six related species. Mixed-mode acid dissociation constants (hydrogen ion in terms of activity, conjugate acid-base species in molar concentrations) have been determined for all compounds in their protonated form at 25 degrees C, mu = 0.10 M (NaNO3). The values for four compounds in this series are reported here for the first time: viz., N,N'-diethylpiperazine for which log Ka1m = 4.67 +/- 0. 03 and log Ka2m = 8.83 +/- 0.02; N,N,N',N'-tetraethylmethylenediamine for which log Ka1m < 1 and log Ka2m = 11.01 +/- 0.03; N,N'-diethyl-N,N'-bis(3-sulfopropyl)ethylenediamine for which log Ka1m = 5.75 +/- 0.03 and log Ka2m = 9.37 +/- 0.02; and piperazine-N,N'-bis(2-ethanesulfonic acid) (Pipes) for which log Ka1m = 2.81 +/- 0.01 (the value of log Ka2m having been previously reported).

Amines↗