Liberation Of Ligand Research Articles

Zinc(II)-mediated nitrile-amidoxime coupling gives new insights into H⁺-assisted generation of 1,2,4-oxadiazoles.

The cyanamides Me2NCN (1a), OC4H8NCN (1b), and PhC(═O)N(H)CN (1c) and the conventional nitriles PhCN (1d) and EtCN (1e) react with 1 equiv of each of the amidoximes R'C(═NOH)NH2 (R' = Me (2a), Ph (2b)) in the presence of 1 equiv of ZnCl2 producing the complexes [ZnCl2{HN═C(R)ON═C(R')NH2}] (R/R' = NMe2/Me (3a), NMe2/Ph (3b), NC4H8O/Me (3c), NC4H8O/Ph (3d), N(H)C(═O)Ph/Me (3e), N(H)C(═O)Ph/Ph (3f), Ph/Me (3g), Ph/Ph (3h), Et/Ph (3j)) with the chelate ligands originating from the previously unreported zinc(II)-mediated nitrile-amidoxime coupling. Addition of 1 equiv of p-TolSO3H to any of one 3a-h, 3j results in the ligand liberation and formation of the iminium salts [H2N═C(R)ON═C(R')NH2](p-TolSO3) ([4a-j](p-TolSO3)), which then at 20-65 °C spontaneously transform to 1,2,4-oxadiazoles (5a-j). As a side reaction, cyanamide derived species [4a-f](p-TolSO3) undergo Tiemann rearrangement to produce the substituted ureas R'NHC(═O)NH2 (R' = Me (6a), Ph (6b)) and RC(═O)NH2 (R = NMe2 (6c), NC4H8O (6d), N(H)C(═O)Ph (6e)), whereas phenyl and ethyl cyanide derivatives besides their transformation to the oxadiazoles undergo hydrolysis to the parent amidoxime R'C(═NOH)NH2 (R' = Me (2a), Ph (2b)) and the carboxamides RC(═O)NH2 (R = Ph (6f), Et (6g)). All new obtained compounds were characterized by HRESI-MS, IR, ATR-FTIR, (1)H NMR, CP-MAS TOSS (13)C NMR, elemental analyses (C, H, N), and single crystal X-ray diffraction for seven species (3a-e, [4b](p-TolSO3), and [4d](p-TolSO3)). Two previously unknown heterocycles 5c and 5e were isolated and characterized by elemental analyses (C, H, N), HRESI-MS, IR, (1)H and (13)C{(1)H} NMR. The observed conversion of [4a-j](p-TolSO3) to the 1,2,4-oxadiazoles uncovers the mechanism of the previously reported H(+)-assisted generation of these heterocycles (Augustine; et al. J. Org. Chem. 2009, 74, 5640).

Template Syntheses of Copper(II) Complexes from Arylhydrazones of Malononitrile and their Catalytic Activity towards Alcohol Oxidations and the Nitroaldol Reaction: Hydrogen Bond‐Assisted Ligand Liberation and E/Z Isomerisation

A one-pot template condensation of 2-(2-(dicyanomethylene)hydrazinyl)benzenesulfonic acid (H(2)L(1), 1) or 2-(2-(dicyanomethylene)hydrazinyl)benzoic acid (H(2)L(2), 2) with methanol (a), ethylenediamine (b), ethanol (c) or water (d) on copper(II), led to a variety of metal complexes, that is, mononuclear [Cu(H(2)O)(2)(κO(1),κN(2)L(1a)] (3) and [Cu(H(2)O)(κO(1),κN(3)L(1b))] (4), tetranuclear [Cu(4)(1κO(1),κN(2):2κO(1)L(2a))(3)-(1κO(1), κN(2):2κO(2)L(2a))] (5), [Cu(2)(H(2)O)(1κO(1), κN(2):2κO(1)L(2c))-(1κO(1),1κN(2):2κO(1),2 κN(1)- L(2c))](2) (6) and [Cu(2)(H(2)O)(2)(κO(1),κN(2)- L(1dd))-(1κO(1),κN(2):2κO(1)L(1dd))(μ-H(2)O)](2·) 2H(2)O (7·2H(2)O), as well as polymer- ic [Cu(H(2)O)(κO(1),1κN(2):2κN(1)L(1c))](n) (8) and [Cu(NH(2)C(2)H(5))(κO(1),1κN(2):2κN(1)L(2a))](n) (9). The ligands 2-SO(3)H-C(6)H(4)-(NH)N=C{(CN)[C(NH(2))-(=NCH(2)CH(2)NH(2))]} (H(2)L(1b), 10), 2-CO(2)H-C(6)H(4)-(NH)N={C(CN)[C(OCH(3))-(=NH)]} (H(2)L(2a), 11) and 2-SO(3)H-C(6)H(4)-(NH)N=C{C(=O)-(NH(2))}(2) (H(2)L(1dd), 12) were easily liberated upon respective treatment of 4, 5 and 7 with HCl, whereas the formation of cyclic zwitterionic amidine 2-(SO(3)(−))-C(6)H(4)-N=NC(-C=(NH(+))CH(2)CH(2)NH)(=CNHCH(2)CH(2)NH) (13) was observed when 1 was treated with ethylenediamine. The hydrogen bond-induced E/Z isomerization of the (HL(1d))(−) ligand occurs upon conversion of [{Na(H(2)O)(2)(μ-H(2)O)(2)}(HL(1d))](n) (14) to [Cu(H(2)O)(6)][HL(1d)](2)·2H(2)O (15) and [{CuNa(H(2)O)-(κN(1),1κO(2):2κO(1)L(1d))(2)}K(0.5)(μ-O)(2)]n·H(2)O (16). The synthesized complexes 3–9 are catalyst precursors for both the selective oxidation of primary and secondary alcohols (to the corresponding carbonyl compounds) and the following diastereoselective nitroaldol (Henry) reaction, with typical yields of 80–99%.

The Growing Promiscuity of G Protein Coupled Receptors

The Growing Promiscuity of G Protein Coupled Receptors

The generation of palladium silicide nanoalloy particles in a SiCN matrix and their catalytic applications

The synthesis, characterization and catalytic studies of single phase palladium silicide nanoalloy particles supported by a polymer derived, non-oxide SiCN matrix (Pd2Si@SiCN) are reported. Simultaneous chemical modification of a polyorganosilazane as well as its cross-linking was achieved by the use of an aminopyridinato palladium complex at room temperature. Cross-linking takes place with an evolution of hydrogen and increases the ceramic yield by the retention of carbon and nitrogen atoms. Liberation of ligand, as confirmed by 1H NMR spectroscopy, provides indirect evidence of the transfer of palladium to the nitrogen functions producing metal modified polyorganosilazane whose pyrolysis at 1100 °C under nitrogen atmosphere provides Pd2Si@SiCN. Powder X-ray diffraction (PXRD) studies confirmed the presence of the hexagonal Pd2Si phase in the amorphous SiCN matrix. The size of the particles formed seems to depend upon the nature of the solvent used in the cross-linking step. The amount of palladium complex added seems not to affect the size of particles formed but does increase their population density. Pd2Si@SiCN catalysts were found active for the hydrogenation of ketones. The selectivity of the reaction towards the alcoholic product remains very high. The conversion of the reaction however decreases both with increase in alkyl chain length as well as its branching at alpha carbon. The recyclable Pd2Si@SiCN could be a suitable choice for catalytic transformations under a harsh chemical environment and at higher temperatures.

Potential biochromium sources; kinetic studies on acid- and base-catalyzed aquation of [Cr(ox)2(2-(aminomethyl)pyridine

Acid- and base-catalyzed hydrolysis of [Cr(ampy)(ox)2]−, where ampy = 2-(aminomethyl)pyridine, leads to successive dissociation of the ligands via concurrent reaction paths, whereas at pH 1–9 only ampy is liberated as a result of spontaneous processes. The first ligand dissociation proceeds via aqua intermediates with one-end bonded ampy (1) or ox ligands (2), respectively, which in alkaline media undergo rapid deprotonation to give the appropriate hydroxo-forms. The kinetics of two reaction stages, namely the chelate ring opening and the ligand liberation, were studied spectrophotometrically. In acidic media, the first stage is much faster than the second, whereas in alkaline solutions, both the stages are characterized by similar rate constants. The dependences of kobs on [H+] are as follows: kobs1,H = a1 + b1/[H+], kobs2,H = a2 + b2[H+]. At pH > 13, rate constants kobs1,OH and kobs2,OH are [OH−] independent. The effect of pH on the complex reactivity was rationalized based on proposed mechanisms.

Photocatalytic Coalescence of Functionalized Gold Nanoparticles

A novel strategy for the synthesis of chromophore-functionalized AuNPs with a narrow size distribution is reported. It consists of increasing the size of preprepared NPs by means of a fast (second scale) and clean (light and an organic photocatalyst) method. The results agree with thiolate ligand liberation from the NP surface promoted by photogenerated radicals. This lets gold cores come together and finally coalesce.

Surface reactions during atomic layer deposition of Pt derived from gas phase infrared spectroscopy

Infrared spectroscopy was used to obtain absolute number information on the reaction products during atomic layer deposition of Pt from (methylcyclopentadienyl)trimethylplatinum [(MeCp)PtMe3] and O2. From the detection of CO2 and H2O it was established that the precursor ligands are oxidatively decomposed during the O2 pulse mainly. Oxygen atoms chemisorbed at the Pt lead to likewise ligand oxidation during the (MeCp)PtMe3 pulse however the detection of a virtually equivalent density of CO2 and CH4 also reveals a concurrent ligand liberation reaction. The surface coverage of chemisorbed oxygen atoms found is consistent with the saturation coverage reported in surface science studies.

Chromium(III) complexes with lutidinic acid: kinetic studies in HClO4 and NaOH solutions

Chromium(III)-lutidinato complexes of general formula [Cr(lutH)n(H2O)6−2n]3−n (where lutH− is N,O-bonded lutidinic acid anion) were obtained and characterized in solution. Acid-catalysed aquation of [Cr(lutH)3]0 leads to only one ligand dissociation, whereas base hydrolysis produces chromates(III) as a result of subsequent ligand liberation steps. The kinetics of the first ligand dissociation were studied spectrophotometrically, within the 0.1–1.0 M HClO4 and 0.4–1.0 M NaOH range. In acidic media, two reaction stages, the chelate-ring opening and the ligand dissociation, were characterized. The dependencies of pseudo-first-order rate constants on [H+] are as follows: kobs1 = k1 + k−1/K1[H+] and kobs2 = k2K2[H+]/(1 + K2[H+]), where k1 and k2 are the rate constants for the chelate-ring opening and the ligand dissociation, respectively, k−1 is the rate constant for the chelate-ring closure, and K1 and K2 are the protonation constants of the pyridine nitrogen atom and coordinated 2-carboxylate group in the one-end bonded intermediate, respectively. In alkaline media, the rate constant for the first ligand dissociation depends on [OH−]: kobs1 = kOH(1) + kO[OH−], where kOH(1) and kO are rate constants of the first ligand liberation from the hydroxo- and oxo-forms of the intermediate, respectively, and K2 is an equilibrium constant between these two protolytic forms. Kinetic parameters were determined and a mechanism for the first ligand dissociation is proposed. The kinetics of the ligand liberation from [Cr(lut)(OH)4]3− were also studied and the values of the pseudo-first-order rate constants are [OH−] independent.

Chromium(III)-isocinchomeronato and quinolinato complexes: kinetic studies in NaOH solutions and effect on 3T3 fibroblast proliferation

The aquation of chromium(III)-isocinchomeronato and quinolinato complexes, mer-[Cr(icaH)3]0 and mer-[Cr(quinH)3]0 (where icaH− and quinH− are N,O-bonded isocinchomeronic and quinolinic acid anion, respectively) was studied in NaOH solutions. The process leads to successive ligand liberation in the fully deprotonated species. The kinetics of the first ligand liberation were studied spectrophotometrically in the visible region. A mechanism is proposed in which the rate of the chelate-ring opening at the Cr–N bond is much faster than the rate of the Cr–O bond breaking. The rate-determining step is described by the rate law: kobs1 = kOH(1) + kOQ2 [OH−], where kOH(1) and kO are rate constants of the first ligand liberation from the hydroxo- and oxo-forms of the intermediate, respectively, and Q2 is an equilibrium constant between these two protolytic forms. The first pseudo-first-order rate constants (kobs1) were calculated using SPECFIT software for an A → B → C reaction pattern. The results are compared with those determined in acidic medium. Kinetics of the second and third ligand liberation were also studied and values of successive pseudo-first-order rate constants (kobs2, kobs3) are [OH−] independent. Effect of chromium(III)-quinolinato and isocinchomeronato complexes on 3T3 fibroblast proliferation was evaluated. Cytotoxicity of these complexes is low, suggesting they may be promising candidates as novel dietary supplements.

Synthesis and kinetic studies in aqueous solution on chromium(III) complexes with isocinchomeronic acid—potential new biochromium sources

The chromium(III) complexes with a new potential chromium transporting ligand—2,5-pyridinedicarboxylic acid (isocinchomeronic acid, icaH2):[Cr(icaH)3]0, [Cr(icaH)2 (H2O)2]+ and [Cr(icaH)(H2O)4]2+ (where icaH = N,O-bonded isocinchomeronic acid anion), have been obtained and characterized in solution. The [Cr(icaH)3]0 complex undergoes aquation in acidic media to the diaqua-product. Kinetics of this process was studied spectrophotometrically in the 0.1–1.0 M HClO4 range, at I = 1.0 M. The first aquation stage, the chelate-ring opening at the Cr–N bond, is a much faster than the second one. The rate laws are of the form: k obs = k 1 + k −1/Q 1[H+] and k obs = k 2 Q 2[H+]/(1 + Q 2[H+]), where k 1 and k 2 are the rate constants for the chelate-ring opening and the ligand liberation, respectively, k −1 is the rate constant of the chelate-ring closure, Q 1 and Q 2 are the protonation constants of the pyridine nitrogen and 5-carboxylate group in the one-end bonded intermediate, respectively. The results are discussed in terms of potential pharmaceutical application of the complex.

Synthesis of P-chiral phosphines via chiral metal template promoted asymmetric furan Diels–Alder reaction

The organoplatinum complex containing ortho-metalated ( S)-(1-(dimethylamino)ethyl)-naphthalene as the chiral auxiliary has been used to promote the asymmetric [4+2] Diels–Alder reaction between phenyldivinylphosphine and 2-diphenylphosphinofuran. The reaction was complete in 6 days at room temperature, with the formation of four isomeric diphosphino-substituted oxanorbornene metal complexes in the ratio of 4:2:2:1. Only the exo-cycloaddition products were formed. The formation of stereogenic carbon centers within the oxanorbornene skeleton are highly stereoselective, with all four cycloadducts adopting the same absolute configurations. However, the stereocontrol at the external phosphorus stereogenic center is less efficient ( S p: R p = 2:1 for the template cycloadducts). The chiral naphthylamine auxiliary could be removed chemoselectively by treatment with concentrated hydrochloric acid, and further ligand liberation of the dichloro complexes with aqueous cyanide gave the diphosphino-substituted oxanorbornene ligands. Hydrogenation of the double bonds in the cycloadduct stabilizes the phosphorus stereogenic center of the free diphosphine ligand which otherwise undergoes inversion of absolute configuration.

Kinetics and mechanism of tris-quinolinatochromium(III) aquation in HClO4 media

The chromium(III)-quinolinato complexes, [Cr(quinH)3]0, [Cr(quinH)2(H2O)2]+ and [Cr(quinH)(H2O)4]2+ (where quinH = N,O-bonded quinolinic acid anion), were obtained and characterized in solution. The tris-quinolinato complex undergoes acid-catalyzed aquation to give the diaqua-product, whereas subsequent ligand liberation processes are exceptionally slow. Kinetics of the aquation were studied spectrophotometrically over the 0.1–1.0 M HClO4 range, at I = 1.0 M. The first aquation stage, the chelate-ring opening at the Cr-N bond, is much faster than the second one. The following rate laws were established: kobs = k1 + k−1/Q1[H+] and kobs = k2Q2[H+]/(1 + Q2[H+]), where k1 and k2 are the rate constants for the chelate-ring opening and the ligand liberation, respectively, k−1 is the rate constant of the chelate-ring closure, Q1 and Q2 are the protonation constants of the pyridine nitrogen and 3-carboxylate group in the one-end bonded intermediate, respectively. Kinetic parameters have been determined and the mechanism has been discussed.

Kinetics and Mechanism of Chromium(III)-oxalates-2-hydroxynicotinate and Chromium(III)-oxalates-3-hydroxypicolinate Aquation in HClO4 Solutions

New chromium(III) complexes, [Cr(C2O4)2(2-hnic)]2− and [Cr(C2O4)2(3-hpic)]2− (where 2-hnic = O,O′-bonded 2-hydroxynicotinic acid and 3-hpic = N,O-bonded 3-hydroxypicolinic acid), were obtained and characterized in solution. The acid-catalyzed aquation of the both complexes leads to liberation of the appropriate pyridinecarboxylic acid and formation of cis-[Cr(C2O4)2(H2O)2]−. Kinetics of these reactions were studied spectrophotometrically in the 0.1–1.0 M HClO4 range, at I = 1.0 M. In the case of [Cr(C2O4)2(2-hnic)]2−, a slow chelate-ring opening at the Cr–O (phenolate) bond is followed by a fast Cr–O (carboxylate) bond breaking. The rate law: kobs = kHQH[H+] was established, where kH is the acid-catalyzed rate constant and QH is the protonation constant of the coordinated phenolate oxygen atom. In the case of [Cr(C2O4)2(3-hpic)]2−, the reversible chelate-ring opening at Cr–N bond is followed by the rate determining step – the one-end bonded ligand liberation. The rate law for the first step was determined: kobs = k1+k−1/Q1[H+], where k1 and k−1 are the rate constants of the chelate-ring opening and closure and Q1 is the protonation constant of the pyridine nitrogen atom. The aquation mechanisms are proposed and the effect of ligand coordination mode on complex reactivity is discussed.

Chromium(III)-isonicotinate Complexes. Kinetics and Mechanism of Isonicotinate Ligand Liberation in HClO4 Solutions

Chromium(III)-isonicotinate complexes, cis-[Cr(C2O4)2(N-inic)(H2O)]- and [Cr(C2O4)(H2O)3-OH-Cr(C2O4)2(O-inic)]-(N-inic)(H2 (N-inic = N-bonded and O-inic = O-bonded isonicotinic acid) were obtained and characterized in solution. Kinetics of acid-catalyzed isonicotinate ligand liberation were studied spectrophotometrically in the 0.1–1.0 m HClO4 range, at I=1.0 m. The dependencies of the pseudo-first order rate constant on [H+] were established: kobs = k0+kHQH[H+] and kobs = kHQH[H+] for the N-inic and O-inic complex, respectively, where k0 and kH are the rate constants of the spontaneous and the acid-catalyzed reaction paths, and QH is the protonation constant of the carboxylic group in isonicotinic ligand. The obtained results indicate that N-bonded isonicotinic acid liberation occurs mainly via a spontaneous reaction path and is much slower than O-bonded inic liberation. The mechanisms for these processes are proposed.

Oxidation States and CO Ligand Exchange Kinetics in a Self‐Assembled Monolayer of a Triruthenium Cluster Studied by In Situ Infrared Spectroscopy

Oxidation states and CO ligand exchange kinetics in a self-assembled monolayer (SAM) of an oxo-centered triruthenium cluster [Ru(3)(mu3-O)(mu-CH3COO)6(CO)(L1)(L2)] (L1 = [(NC5H4)CH2NHC(O)(CH2)10S-]2, L2 = 4-methylpyridine) have been extensively investigated on the surface of a gold electrode in aqueous and nonaqueous solutions. The SAM exhibits three consecutive one-electron transfers and four oxidation states, which have been characterized by electrochemistry, in situ infrared spectroscopy, and in situ sum frequency generation (SFG) vibrational spectroscopy measurements. The original electron-localized state of the Ru cluster center was changed to electron delocalization states by oxidation or reduction of the central Ru ions. These changes are revealed by the IR absorptions of the CO ligand and the bridging acetate ligands of the triruthenium cluster in the SAM. The IR absorptions of the two kinds of ligands are strongly dependent on the oxidation state of the Ru cluster center. One-electron oxidation of the central Ru ion in the SAM triggers a CO ligand liberation process. Solvent molecules may then occupy the CO site to result in a CO-free SAM. One-electron reduction of this CO-free SAM in a CO-saturated solution leads to re-coordination of the CO ligand into the SAM. Both processes can be precisely controlled by tuning the electrode potential. The kinetics of the CO exchange cycle in the SAM, including liberation and coordination, has been investigated by in situ IR and SFG measurements for the first time. The CO exchange cycle is significantly dependent on the temperature. The reaction rate greatly decreases with decreasing solution temperature, which is an important factor in the CO ligand exchange process. The activation energies of both CO liberation and coordination have been evaluated from the reaction rate constants obtained at various temperatures.

New chromium(III)–nicotinate complexes. Kinetics and mechanism of nicotinate ligand liberation in acidic media

Two new chromium(III)–nicotinate complexes, cis-[Cr(C2O4)2(O-nic)(H2O)]− and cis-[Cr(C2O4)2(N-nic)(H2O)]−, were obtained and characterized in solution (where O-nic=O-bonded and N-nic=N-bonded nicotinic acid). The kinetics of nicotinate ligand liberation were studied spectrophotometrically in the 0.1–1.0 m HClO4 range, at I=1.0 m. The rate equations were determined and a mechanism is proposed. The rate of Cr–O bond breaking is [H+] dependent: kobs=kHQH[H+], where kH is the acid-catalyzed rate constant and QH is the protonation constant of the nonbonded oxygen atom in the O-coordinated ligand. The Cr–N bond breaking proceeds via two paths: spontaneous and acid-catalyzed; kobs=k0 + kHQH[H+], where k0 and kH are the spontaneous and acid catalyzed rate constants and QH is the protonation constant of the carboxylic group in the N-bonded nicotinic acid. The results demonstrate by comparison that Cr–N bond breaking is a much slower process than Cr–O bond fission.

Kinetics and mechanism of aquation of chromium(III) complexes with 3-hydroxypicolinic and 2-hydroxynicotinic acids in HClO{4{ solutions

Two complexes, [Cr(3-hpic){3}]{0} and [Cr(2-hnic){3}]{0} (where 3-hpic = hydroxypicolinic acid and 2-hnic = 2-hydroxynicotinic acid anions), were prepared and characterized in solution. The 3-hpic ligand forms a 5-membered chelate ring via pyridine nitrogen and carboxlate oxygen atoms, whereas the 2-hnic ligand forms a 6-membered chelate ring via carboxylate and phenolate oxygen atoms. The kinetics of the acid-catalyzed aquation were studied spectrophotometrically in the 0.1–1.0 HClO{4} range, at I = 1.0 . The rate equations for the first aquation step – the chelate-ring opening – was determined and a mechanism was proposed. In the case of [Cr(3-hpic){3}]{0}, the reversible chelate-ring opening at the Cr—N bond precedes much slower than the second aquation step – a one-end bonded ligand liberation. The equation rate is of the form: k{obs} = k{1} + k{-1} /Q{1}[H{+}], where k{1} and k{-1} are the rate constants for the forward and the reverse processes in the unprotonated substrate and Q{1} is the protonation constant of the non-bonded pyridine nitrogen atom. In the case of [Cr(2-hnic)3]{0}, the chelate-ring opening at the Cr—O (phenolate) bond is the rate-determining step. The observed pseudo-first order rate constant increases as [H{+}] increases: k{obs} = k{0} + k{H}Q{H}[H{+}], where k{0} and k{H} are the rate constants of the spontaneous and acid catalyzed processes and Q{H} is the protonation constant of the coordinated phenolate oxygen atom. The results lead to the conclusion that an aquation mechanism depends on the coordination mode of the ligand.

Kinetics and Mechanism of Chromium(III)-Picolinato and Chromium(III)-Dipicolinato Complexes Aquation in HNO3Solutions

The acid-catalyzed aquation of [Cr(pic)(H2O)4]22+ and [Cr(dpic)(H2O)3]+(pic = picolinic acid anion, dpic = dipicolinic acid dianion) in nitrate(V) media was studied. The reaction is reversible in the case of the pic-complex and practically irreversible in the case of the dpic-complex. It is assumed that the reactive form of the substrate undergoes fast chelate ring-opening followed by protolytic equilibria, followed by the rate of the Cr—O bond breaking of the monodentate bonded ligand which is the rate-determining step. The kinetics of pic/dpic ligand liberation were followed spectrophotometrically in the 0.4–2.0 M HNO3 range at I= 2.0 M. The following dependences of the pseudo-first order rate constants on [H+] have been established:kobs=a+b[H+](where b and a are apparent rate constants for the forward and the reverse reaction of the pic-complex) and kobs=b[H+]+c[H+]2(where b and c are apparent rate constants for the dpic liberation). Fast protolytic pre-equilibria, leading to protonation of the carboxylic oxygen atom on the monodentate bonded ligand, preceeds ligand liberation.

Kinetics and mechanism of the first aquation stage for the [Cr(pic)3]0 and [Cr(pic)2(OH)]20 complexes in HClO4 solutions

Aquation of [Cr(pic)3]0 and [Cr(pic)2(OH)]20 in aqueous HClO4 solutions leads to formation of the common product – [Cr(pic)2(H2O)2]+. The first, reversible stage, the ring opening via Cr—N bond breaking in [Cr(pic)3]0 is followed by the second, rate-determining step – one-end bonded pic ligand liberation. In the case of the [Cr(pic)2(OH)]20 complex, the first faster stage produces the singly bridged dimer, which undergoes cleavage into the parent monomers in the second, much slower step. The subsequent aquation of [Cr(pic)2(H2O)2]+ is extremely slow and leads to [Cr(pic)(H2O)4]2+ formation, which practically does not undergo further ligand substitution under the conditions applied. Kinetics of the first aquation stage for [Cr(pic)3]0 and of the second step for [Cr(pic)2(OH)]20 were studied spectrophotometrically in the 0.1–1.0 M HClO4 range at I = 1.0 M. The observed pseudo-first order rate constant for [Cr(pic)3]0 decreases with [H+] increase according to the rate law: kobs = k1 + k−1Q1/[H+], where k1 and k−1 are the rate constants of the forward and the reverse processes in the unprotonated substrate and Q1 is the protonation constant of the pyridine nitrogen atom. In the case of the [Cr(pic)2(OH)]20 complex, the rate for the singly bridged dimer cleavage does not depend on [H+]. The activation parameters for the chelate-ring opening in [Cr(pic)3]0 and for the singly bridged dimer cleavage have been determined and discussed. Some kinetic data of the slow, second aquation stage for the [Cr(pic)3]0 complex and of the fast, first aquation stage for the doubly bridged dimer have been studied; for both reactions the rate increases linearly with the increase in [H+].

Kinetics and mechanism of one dipicolinato ligand substitution in the chromium(III)-bis(dipicolinato) complex: an unusual rate dependence on acid concentration

The Na[Cr(PDA)2] · 2H2O complex (PDA1 = dipicolinic acid anion) and its aquation product, [Cr(PDA)(H2O)3]+, were prepared and characterized. The electronic spectra demonstrate that the bis(dipicolinato) complex undergoes very fast partial dechelation during dissolution. In acidic media, pH controlled, rapid protolytic and ring opening processes lead to coexistence of complexes with one tridentate (PDA) and the other bi- or mono-dentate (PDA′). The kinetics of PDA ligand liberation were followed spectrophotometrically within the 0.1–2.0 M HClO4 range at I = 2.0 M. The observed first-order rate constant depends on [H+] according to the equation: kobs = A[H+]/(1 + B[H+] + C[H+]2). A reaction course via the uncharged [Cr(PDA)(HPDA′)(H2O)2]0 complex is proposed. The observed rate increase, followed by rate retardation with [H+] increase, is attributed to the unreactive [Cr(PDA)(H2PDA′)(H2O)2]+ complex. In terms of the proposed mechanism, A, B, C parameters have been defined as: A = k1Q1, B = Q1, C = Q1Q2 where k1 is the rate constant of the CrIII-carboxylato oxygen bond-breaking in the monodentate HPDA ligand, Q1 is a composite value describing protolytic and dechelation processes and Q2 is the protonation constant of the uncharged [Cr(PDA)(HPDA′)(H2O)2]0 complex.