Chelate Ring Opening Research Articles

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.

Intramolecular C−H Activation Directed Self-Assembly of an Organoplatinum(II) Molecular Square

A Pt(II) complex that contains a N,N-chelate ligand with the formula of Pt(NPA)(CH3)2 (1), NPA = N-(2‘-pyridyl)-7-azaindole, has been found to undergo a rapid chelate ring opening and “roll-over” process that leads to intramolecular C−H activation and the subsequent formation of a rare organoplatinum Pt4 macrocycle, Pt4(CH3)4(N,C,N-NPA)4 (2), without extra linkers. Complex 2 has been fully characterized by NMR, elemental, and X-ray diffraction analyses. Kinetic study using NMR spectroscopy has established that, in the absence of strong coordinating solvent molecules, the rate-determining step for the conversion process of 1 to 2 is the C−H cleavage step, and 2 is formed exclusively. The presence of strong coordinating solvent molecules, such as CH3CN, terminates the reaction at the Pt(CH3)(N,C-NPA)(L) formation step where L = the solvent molecule. The facile roll-over intramolecular C−H activation and the facile and selective formation of the cyclic Pt4 complex by compound 1 are attributed to the poor sta...

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.

New chromium(III)–picolinamide complexes. Kinetics and mechanism of picolinamide liberation in HClO4 solutions

Two new chromium(III) complexes with picolinamide (pica) and oxalates, [Cr(C2O4)2(N,N′-pica)]2− and [Cr(C2O4)2(N,O-pica)]−, were obtained and the kinetics of their aquation in HClO4 solutions were studied. The aquation leads to pica liberation and proceeds in two stages: (i) the chelate-ring opening at the Cr–amide bond and (ii) the Cr–N-pyridine bond breaking, which gives free pica and cis-[Cr(C2O4)2(H2O2)2]−. In the case of N,N′-bonded pica the kinetics of both stages was determined and in the case of the N,O-bonded pica only the second stage was investigated. The following rate laws were established: (kobs)1 = k0 + k1Q1[H+] and (kobs)2 = k2Q2[H+], where k0 and k1 are the rate constants of the chelate-ring opening in the unprotonated and protonated starting complex, and k2 is the rate constant of the pica liberation from the protonated intermediate. Kinetic parameters are calculated and the aquation mechanism is discussed.

Acid-catalyzed Hydrolysis of the cis-[Cr(1,10-phenanthroline)2(O2CO)]+ Ion Studied by the u.v.–vis Stopped Flow Method

The cis-[Cr(phen)2(O2CO)]+ ion was prepared through the displacement of two molecules of water from the cis-[Cr(phen)2(OH2)2]3+ by the bidentate carbonate anion. It underwent two-phase hydrolysis reactions under acidic conditions (0.1 < [H+] < 2.7 m) at 5, 10, 15, 20 and 25 °C. Via slow carbonato chelate ring opening (first step k1slow) and a second fast decarboxylation(k2fast value). The first step was preceded by protonation of the coordinate bidentate carbonate ligand. The second step exhibited no pH dependence, while k1slow values increased with acid concentration that suggested the presence of both protonated and deprotonated reactant species. Based on these observations we have proposed a hydrolysis mechanism featuring H2O-induced ring-opening of the coordinate CO32− group in the first step k1slow followed by loss of CO32− from two intermediates, [Cr(phen)2(O2COH)]2+ (k1slow) and [Cr(phen)2(OH2)(O2COH)]2+ (k2fast).

Evidence for a dynamic role for homocitrate during nitrogen fixation: the effect of substitution at the α-Lys426 position in MoFe-protein of Azotobacter vinelandii

Although it is generally accepted that the active site of nitrogenase is located on the FeMo-cofactor, the exact site(s) of N2 binding and reduction remain the subject of continuing debate, with both molybdenum and iron atoms being suggested as key players. The current consensus favours binding of acetylene and some other non-biologically relevant substrates to the central iron atoms of the FeMo-cofactor [Dos Santos, Igarashi, Lee, Hoffman, Seefeldt and Dean (2005) Acc. Chem. Res. 38, 208-214]. The reduction of N2 is, however, a more demanding process than reduction of these alternative substrates because it has a much higher activation energy and does not bind until three electrons have been accumulated on the enzyme. The possible conversion of bidentate into monodentate homocitrate on this three electron-reduced species has been proposed to free up a binding site for N2 on the molybdenum atom. One of the features of this hypothesis is that alpha-Lys426 facilitates chelate ring opening and subsequent orientation of the monodentate homocitrate by forming a specific hydrogen bond to the homocitrate -CH2CH2CO2- carboxylate group. In support of this concept, we show that mutation of alpha-Lys426 can selectively perturb N2 reduction without affecting acetylene reduction. We interpret our experimental observations in the light of a detailed molecular mechanics modelling study of the wild-type and altered MoFe-nitrogenases.

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.

Protonation of metal‐bound α‐hydroxycarboxylate ligand and implication for the role of homocitrate in nitrogenase: Computational study of the oxy‐bidentate chelate ring opening

AbstractProtonation of the metal‐bound oxy‐bidentate ligand in the model complexes of [(HS)3(NH3)M(OCH2COO)]q (M = Mo, Fe, V, Co; q = −2, −1) in the gas phase and in solutions of water and acetonitrile has been explored by the density functional approach. Calculations show that protonation of the carboxyl oxygen can open the α‐hydroxycarboxylate chelate ring ligated to a transition‐metal center under specific oxidation and spin states. The feasibility of the chelate ring opening by protonation depends on the electronic nature of the metal site in tune with conversion of a six‐coordinate with a five‐coordinate metal atom. Such selective dissociation of the metal‐bound chelate ligand manipulates the availability of an empty site at the metal center and significantly affects reactivity of the metal‐mediated chemical processes. Protonation changes the stability of species with different spin multiplicities and impels spin transition at the metal center in dissociation of the oxy‐bidentate ligand. Solvent environments of water and acetonitrile play an important role in stabilizing the negatively charged species. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006

A new route to the asymmetric Schiff base ligands. The ligand exchange in [Mo(CN)3O(salhy)]2? ion. Kinetics and mechanism

The kinetics of the ligand exchange in (PPh4)2[Mo(CN)3O(salhy)]. 6H2O (Hsalhy = salicylaldehyde hydrazone) by a solvent molecule and by 2,2′-bipyridine (bpy) have been studied in EtOH. For the ligand exchange by a solvent molecule the pseudo-first order rate constant equals kobs = 3.2 (±0.2) × 10−3 s−1 (t=25 °C), ΔH‡=67 (± 7) kJ mol−1, ΔS‡=−75 (±23) J mol−1 K−1, while for the exchange by a bpy molecule kobs=3.5 (±0.2) × 10−3 s−1 (t=25 °C), ΔH‡=56 (±7) KJ mol−1, ΔS‡ = −104 (±8) J mol−1 K−1. It was found, that all reactions proceed via the same mechanism which involves the chelate ring opening cis to the Mo=O bond. The mechanism of the reaction was proposed and was proved by the synthesis of (PPh4)2[Mo(CN)3O(N-pic)]. 2.5H2O (N-pic denotes that the nitrogen of picolinic acid is trans to Mo=O) by ligand exchange in EtOH, while in aqueous solution the O-pic analogue is formed exclusively.

Antisymbiosis. Preferential coordination of anionic oxygen versus neutral sulfur donor atoms of methylsulfanyl- or methylsulfinyl-acetato, 2-benzoato and 2-phenolato to the cis-Pt II(PPh 3) 2 and Pt II(dppe) residues

The interaction of an excess of the title ligands L − with the cis-Pt(phos) 2 moieties gives compounds a– b cis-[Pt(L- O) 2(phos) 2] ( a, phos = P(Ph) 3; b, phos = 1/2 dppe), in which O- is preferred to S-coordination. Such preference is confirmed by the fact that the same products are obtained by reaction of excess of L − with the previously reported a– d complexes [Pt(L- O,S)(phos) 2] +, ( c, phos = PPh 3, d, phos = 1/2 dppe), for which chelate ring opening occurs with rupture of Pt–S rather than Pt–O bonds. Compound a can be obtained also by oxidative addition of HL to [Pt(PPh 3) 3]. The Pt–O bonds in compounds a– d are stable towards substitution by Me 2SO, pyridine and tetramethylthiourea. Substitution of L’s occurs with N, N′-diethyldithiocarbamate, which forms a very stable chelate with Pt(II). Thiourea and N, N′-dimethylthiourea also react, because they give rise to cyclometallated products [Pt(phos) 2( NRC( S)NHR)] + (R = H, CH 3), with one ionised thioamido group, as revealed by an X-ray investigation of [Pt(PPh 3) 2( NHC( S)NH 2)] +. The preference of O versus S coordination, as well as the stability of the Pt–O bonds, are discussed in terms of antisymbiosis.

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.

Enantioselective Catalysis. Part 156 [1]. Ruthenium Procatalysts and 2-Pyridinealdehyde/(S)-NOBIN-Derived Cocatalysts in the Transfer Hydrogenation of Acetophenone with 2-Propanol

Several ruthenium procatalysts were tested in the transfer hydrogenation of acetophenone with 2-propanol using the chiral imine ligand (S)-2-(2-pyridinylmethyleneamino)-2′-hydroxy-1,1′-binaphthyl and the corresponding amine (S)-2-(2-pyridinylmethylamino)-2′-hydroxy-1,1′-binaphthyl. Ru(PPh3)3Cl2 was the best procatalyst. Its triphenylphosphane ligands were crucial for the catalytic activity and take part in the chirality transfer. Triphenylphosphane removing reagents such as copper(I) chloride, TEMPO, or TMAO improved the catalytic performance to enantioselectivities up to 99% ee. The findings led to a mechanistic proposal including dissociation equilibria of triphenylphosphane and chelate ring opening of the tridentate chiral binaphthyl ligand. New ligands with an additional chiral center were synthesized and tested as cocatalysts. The nature of catalytically active intermediates was examined by MS and NMR spectroscopy.

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 the acid-catalyzed hydrolysis of [carbonatoethylenediamine(L)2cobalt(III)]+ ions

The kinetics of acid-catalyzed hydrolysis of the [Co(en)(L)2(O2CO)]+ ion (L = imidazole, 1-methylimidazole, 2-methylimidazole) follows the rate law −d[complex]/dt = {k1K[H+]/(1 + K[H+])}[complex] (15–30 or 25–40 °C, [H+] = 0.1–1.0 M and I = 1.0 M (NaClO4)). The reaction course consists of a rapid pre-equilibrium protonation, followed by a rate determining chelate ring opening process and subsequent fast release of the one-end bound carbonato ligand. Kinetic parameters, k1 and K, at 25 °C are 5.5 × 10−2 s−1, 0.44 M−1 (ImH), 5.1 × 10−2 s−1, 0.54 M−1 (1-Meim) and 3.8 × 10−3 s−1, 0.74 M−1 (2-MeimH) respectively, and activation parameters for k1 are ΔH1≠ = 43.7 ± 8.9 kJ mol−1, ΔS1≠ = −123 ± 30 J mol−1 deg−1 (ImH), ΔH1≠ = 43.1 ± 0.3 kJ mol−1, ΔS1≠ = −125 ± 1 J mol−1 deg−1 (1-Meim) and ΔH1≠ = 64.2 ± 4.3 kJ mol−1, ΔS1≠ = −77 ± 14 J mol−1 deg−1 (2-MeimH). The results are compared with those for similar cobalt(III) complexes.

Structural evidence for the facile chelate-ring opening reactions of novel platinum(ii)–pyridine carboxamide complexes

The design of Pt(II) complexes with novel structural features is of great relevance in the search for new anticancer agents with innovative chemical and biological properties. In this work we have synthesised and structurally characterised three novel Pt(II) complexes with a pyridine carboxamide ligand (HL). The X-ray structural analysis shows that complex 1, [Pt(L-N, N')2] crystallises in space group C2, while complex 2, [Pt(L-N,N')(L-N)Cl] crystallises in space group P21. ESMS and 1H NMR data show that one of the two ligands in complex 1 undergoes a fast ring-opening reaction to give complex 2 in the presence of Cl−. Complex 2 demonstrated notable activity against human and murine leukemia cells (HL-60 and P388), but rather weak potency against lung adenocarcinoma A-549. However, complex 1 was found to be basically inactive against the above cell lines. This work provides new examples in the design of platinum compounds that can be activated in vivo by biological anions.

Kinetics and mechanism of the chromium(III)–picolinato chelate ring opening in some chromium(III)–oxalato–picolinato complexes in acidic aqueous solutions

Two CrIII–picolinato complexes were obtained and characterized in solution. The [Cr(C2O4)(pyac)2]− and [Cr(C2O4)2(pyac)]2− ions (pyac = picolinic acid anion) in acidic solutions undergo a reversible one-end CrIII–picolinato chelate ring opening via CrIII—N bond breaking. The reaction rate was determined spectrophotometrically in the 0.1–1.0 M HClO4 range at I = 1.0 M. The observed pseudo-first order rate constant depends on [H+] according to the equation: k obs = a + b[H+] + c/[H+]. A reaction mechanism, which assumes participation of the protonated and unprotonated forms of the reactants, has been proposed. The kinetic parameters a, b, c have been defined as a = k 1, b = k 2 Q 1, c = k −1/Q 2, where k 1, k −1,k 2 are rate constants for the forward and reverse processes and Q 1, Q 2 are the protolytic equilibrium constants in the term of the proposed mechanism. The activation parameters have been determined and discussed.

Potentiometric and multinuclear NMR study of the binary and ternary uranium(VI)-L-fluoride systems, where L is alpha-hydroxycarboxylate or glycine.

Equilibria, structures, and ligand-exchange dynamics in binary and ternary U(VI)-L-F- systems, where L is glycolate, alpha-hydroxyisobutyrate, or glycine, have been investigated in 1.0 M NaClO4 by potentiometry and 1H, 17O, and 19F NMR spectroscopy. L may be bonded in two ways: either through the carboxylate end or by the formation of a chelate. In the glycolate system, the chelate is formed by proton dissociation from the alpha-hydroxy group at around pH 3, indicating a dramatic increase, a factor of at least 10(13), of its dissociation constant on coordination to uranium(VI). The L exchange in carboxylate-coordinated UO2LF3(2-) follows an Eigen-Wilkins mechanism, as previously found for acetate. The water exchange rate, k(aq) = 4.2 x 10(5) s(-1), is in excellent agreement with the value determined earlier for UO2(2+)(aq). The ligand-exchange dynamics of UO2(O-CH2-COO)2F3- and the activation parameters for the fluoride exchange in D2O (k(obs) = 12 s(-1), deltaH(double dagger) = 45.8 +/- 2.2 kJ mo(-1), and deltaS(double dagger) = -55.8 +/- 3.6 J K(-1) mol(-1)) are very similar to those in the corresponding oxalate complex, with two parallel pathways, one for fluoride and one for the alpha-oxocarboxylate. The same is true for the L exchange in UO2(O-CH2-COO)2(2-) and UO2(oxalate)2(2-). The exchange of alpha-oxocarboxylate takes place by a proton-assisted chelate ring opening followed by dissociation. Because we cannot decide if there is also a parallel H+-independent pathway, only an upper limit for the rate constant, k1 < 1.2 s(-1), can be given. This value is smaller than those in previously studied ternary systems. Equilibria and dynamics in the ternary uranium(VI)-glycine-fluoride system, investigated by 19F NMR spectroscopy, indicate the formation of one major ternary complex, UO2LF3(2-), and one binary complex, UO2L2 (L = H2N-CH2COO-), with chelate-bonded glycine; log beta(9) = 13.80 +/- 0.05 for the equilibrium UO2(2+) + H2N-CH2COO- + 3F- = UO2(H2N-CH2COO)F3(2-) and log beta(11) = 13.0 +/- 0.05 for the reaction UO2(2+) + 2H2N-CH2COO- = UO2(H2N-CH2COO)2. The glycinate exchange consists of a ring opening followed by proton-assisted steps. The rate of ring opening, 139 +/- 9 s(-1), is independent of both the concentration of H+ and the solvent, H2O or D2O.

Synthesis, characterisation and coordination chemistry of novel chiral N,N-dialkyl-N ′-menthyloxycarbonylthioureas. Crystal and molecular structures of N,N-diethyl-N ′-(−)-(3R)-menthyloxycarbonylthiourea and cis-(S,S)-[Pt(L)Cl(DMSO)] [where HL = N-(+)-(3R)-menthyloxycarbonyl-N′-morpholinothiourea or N-benzoyl-N′,N′-diethylthiourea

The novel ligands N,N-diethyl-N′-(−)-(3R)-menthyloxycarbonylthiourea (HL3), N,N-diethyl-N′-(+)-(3R)-menthyloxycarbonylthiourea (HL4), N-(−)-(3R)-menthyloxycarbonyl-N′-morpholinothiourea (HL5) and N-(+)-(3R)-menthyloxycarbonyl-N′-morpholinothiourea (HL6) have been prepared and characterised. The molecular structure of HL3 has been confirmed by X-ray crystallography. The reaction of these N,N-disubstituted-N′-menthyloxycarbonylthiourea ligands with cis-[PtCl2(DMSO)2] in the presence of sodium acetate yields geometric isomers of the resultant [Pt(L)Cl(DMSO)] complexes, that is the DMSO is sulfur bonded to the platinum in either a cis-(S,S) or trans-(S,S) arrangement with respect to the sulfur donor atom of the chelated ligand. This is in contrast to the complexation reaction of cis-[PtCl2(DMSO)2] with N-benzoyl-N′,N′-diethylthiourea (HL1) or N-benzoyl-N′-morpholinothiourea (HL2) which yields only one [Pt(L)Cl(DMSO)] complex in which the DMSO is in a cis-(S,S) arrangement with respect to the sulfur donor atom of the chelated ligand. The molecular structures of cis-(S,S)-[Pt(L)Cl(DMSO)], where L = (L1)− or (L6)−, have been determined by X-ray crystallography. The difference in the coordination chemistry of the acylthiourea and alkoxycarbonylthiourea ligands has been examined further by treating the [Pt(L)Cl(DMSO)] complexes with PPh3 to give the corresponding mono- and bis-(phosphine) complexes, [Pt(L)Cl(PPh3)] and [Pt(L)(PPh3)2]+. The 31P NMR studies of these complexes reveal that the alkoxycarbonylthiourea ligands bind less strongly than the acylthiourea ligands, which is consistent with the crystallographic studies. The weaker binding properties of the alkoxycarbonylthiourea ligands might be a possible explanation for the observed geometric isomerisation of the complexes and that the mechanism could involve a chelate ring opening step.

Reaction of [Ni(Ph2PCH2CH2PPh2)2] with DCl: controlling the formation of HD and D2 *

The reaction between anhydrous DCl and [Ni(Ph2PCH2CH2PPh2)2] in dichloromethane produced mixtures of HD and D2. The relative amounts of the dihydrogen isotopomers produced depends on the concentration of acid. Mechanistic investigations showed that the reaction involves initial formation of [NiD(Ph2PCH2CH2PPh2)2]+. This deuteriation labilises the nickel to phosphine dissociation. At low concentrations of acid, phosphine chelate ring opening produces [NiCl2(Ph2PCH2CH2PPh2)] (confirmed by X-ray crystallography) and free Ph2PCH2CH2PPh2 together with HD (65 ± 5) and D2 (35 ± 5%). The hydrogen atom of HD originates from a phosphine ligand. At high concentrations of acid the rate of attack of DCl at [NiD(Ph2PCH2CH2PPh2)2]+ becomes faster than phosphine chelate ring opening and results in the formation of [Ni(Ph2PCH2CH2PPh2)2]Cl2 and predominantly D2. Experimentally, the highest concentration of DCl that can be used without appreciable decomposition of the complex is 0.2 mol dm–3. At this concentration of acid the dihydrogen isotopomer distribution is HD (35 ± 5) and D2 (65 ± 5%); however, analysis of the product distribution indicates that at much higher acid concentrations D2 could be the exclusive isotopomer.

Kinetics and mechanism of the interaction of pentaammine(pyridine-2-carboxylato)cobalt(III) ions with hexaaquanickel(II) and nickel(II) complexes: the role of chelating ligands

The reversible complexation of the pentaammine(pyridine-2-carboxylato)cobalt(III) ion [N5Co{O2C-(2)-C5H4 N}]2+ [N5=5HN3 and tetraethylenepentaammine (tetren)] with NiIIL(OH2)6-n [L=H2O (N5=tetren); L=bipy, ida2- (iminodiacetate) and nta3- (nitrilotriacetate), N5=5NH3 and tetren] has been investigated by the stopped-flow technique at 20-40 degC, and I= 0.3mol dm-3. At 25degC, the rate constants, kf(dm3 mol-1s-1), DeltaH(kJmol-1) and DeltaS(JK-1mol-1) for the formation of the ternary complexes [(tetren)-CoIII{O2C-(2)-C5H4N} NiIIL(OH2)6-n] are as follows: L=H2O, 530+9, 53+2, -15+7, respectively; L=bipy, 640+30, 37+3, -65+9; L=ida2-, 3900+100, 47+3, -18+11; L=nta3-, 10200+400, 49+1, −2+2. Nickel(II), in the ternary complexes, is chelated by the free pyridyl-N and the carboxylato moiety of the pyridine-2-carboxylate bound to the cobalt centre. The formation rate constant (kf) and the associated activation parameters are relatively insensitive to the N5 moieties for a given ligand L; kf increased in the order: Ni(OH2)62+Ni(bipy)(OH2)42+ Ni(ida)(OH2)3 (nta)(OH2)2-. Data analysis indicated that the mechanism shifted from the dissociative interchange (Id) to the chelation-controlled one, with the decrease of the available sites for coordination in NiIIL(OH2)6−n. The rate constants (kr) for the dissociation of [N5CoIII{O2C-(2)-C5 H4N}NiIIL(OH2) 6-(n+2)] to the parent reactants indicated steric acceleration [krL(5NH3) kr Ni(ida) >krNi(bipy)2+ for both pentaammine substrates. The chelate ring opening rate constants for the ternary complexes were estimated, from which it was apparent that the tetren envelope of cobalt(III) exerted relatively greater steric pressure as compared with 5NH3 in favouring opening up of the chelate ring.