Ronald M. Milburn

Cobalt(III)-promoted hydrolysis of 4-nitrophenyl phosphate: the role of dinuclear species

Abstract The reactions which follow on mixing aqueous solutions of Co III (tn) 2 (aq) and 4-nitrophenyl phosphate (NPP) have been reexamined using spectrophotometry and 31 P NMR (here ‘(aq)’ refers to (H 2 O) 2 , (H 2 O)(OH), or (OH) 2 depending on pH; charges omitted). When solutions of Co III (tn) 2 (aq) and NPP are mixed with pH maintained in the neutral region, equilibrium formation of the mononuclear monodentate complex Co III (tn) 2 (OH 2 )(NPP) ( cis and trans ) is established in a few minutes. This 1:1 complex (or its deprotonated hydroxo form) undergoes NPP hydrolysis by two parallel paths: (a) direct rate determining hydrolysis to produce 4-nitrophenol(ate) (NP) and Co(tn) 2 PO 4 ; and (b) reaction with additional Co III (tn) 2 (aq) to form a dinuclear complex of NPP, which never builds up to directly detectable levels and which undergoes hydrolysis to produce NP and the dinuclear complex [(tn) 2 Co] 2 (μ 4 -PO 4 ) 3+ . Path (b) is favored by high reactant concentrations and by high cobalt to NPP ratios. Additional dinuclear complex of orthophosphate is produced by reaction of Co(tn) 2 PO 4 with Co III (tn) 2 (aq). At pH 7.3 and at 25°C: for path (a) k =1.75×10 −4 s −1 (a rate enhancement of ∼3×10 4 over the unpromoted NPP hydrolysis rate); for path (b), the first order rate constant for hydrolysis of the dinuclear complex of NPP is estimated to be >5.8×10 −3 s −1 (a rate enhancement of at least 1.0×10 6 over the unpromoted NPP hydrolysis rate).

Depyrophosphorylation of adenosine 5′-triphosphate (ATP)

Abstract The slow hydrolysis of free adenosine 5′-triphosphate (ATP) in both weakly and strongly basic aqueous solution (NaOH, 4 °C) is found to favor depyrophosphorylation over dephosphorylation. In the middle pH region (25 °C) the addition of N4Co(H2O)23+ (N4=tn2, trpn; N4Co(H2O)(OH)2+ pre-dominant species) to preformed N4CoATP− complexes (N4 = tn2, trpn, tren) results in increased rates in production of Pi, but not of PPi. However, addition of Cu2+ or Ca2+ to preformed N4CoATP− complexes (N4 = tn2, trpn, tren) leads to increased rates in the production of PPi as well as of Pi. Mechanistic implications are discussed. The production of Pi from ATP (dephosphorylation) has been monitored by quenching aliquots with Eu2+ (H+) and rapidly replacing multivalent cations with Na+ (ion exchange) before developing phosphomolybdate for analysis. The production of PPi from ATP (depyrophosphorylation ) has been monitored, following similar quenching with Eu2+ (H+) and replacement of multivalent cations, by assaying for free PPi using a coupled enzyme system.

Hydrolysis of 2,4-dinitrophenylphosphate promoted by hydroxoaquatetraaminecobalt(III) ions

Abstract The hydrolysis of 2,4-dinitrophenylphosphate (DNPP) to orthophosphate and 2,4-dinitrophenolate (DNP) is accelerated in the presence of excess tn 2 Co(H 2 O) 2 3+ or trpnCo(H 2 O) 2 3+ at rates which maximize at pHs close to those at which the hydroxoaquatetraaminecobalt(III) complex concentrations peak (tn 2 , pH ∼6.4; trpn, pH ∼6.0; tn = trimethylenediamine; trpn = 3,3′,3″-triaminotripropylamine). For dilute DNPP solutions (∼10 −4 M) the hydrolysis rates (25°C, 0.50 M NaClO 4 ) increase with increasing Co/DNPP ratio in ways that are qualitatively as well as quantitatively different for the two systems (trpn: steady increase moving toward rate saturation, higher rates; tn 2 : ‘S’-shaped curve with very low rates at low ratios, lower rates compared to trpn for comparable ratios). For the trpn system the results are interpreted on the basis of pre-equilibrium formation of the 1:1 monodentate-DNPP cobalt complex by substitution of the labile water on cobalt, and rate-determining attack by the cis -coordinated hydroxide on the phosphorus center to affect hydrolysis. For the tn 2 system the main path to hydrolysis is through a 2:1 cobalt to DNPP complex in which attack by a cis -coordinated hydroxide is again involved. The more complex rate behavior and the slower hydrolysis rates observed for tn 2 system result from the formation of cis and trans isomers in which trans arrangements of coordinated DNPP and hydroxide leave the latter unavailable to participate in intramolecular hydrolysis. Computer fitting of the observed rate data provides values of equilibrium and rate constants for the two systems. Detailed mechanistic schemes are proposed. For the trpn system at pH 6.0 and a 25:1 cobalt to DNPP ratio (5 × 10 −5 M DNPP) the observed acceleration over hydrolysis in the absence of the cobalt complex is ∼3 × 10 3 ; the calculated specific rate constant for hydrolysis in the reactive 1:1 complex ( k ∼0.2 s −1 ) represents an acceleration over the unpromoted rate of ∼3 × 10 4 .

The preparation of [Cotn2PO4] and [CotrpnPO4] and kinetics of phosphato complex formation from the highly reactive [Cotn2(OH)(H2O)]2+ and [Cotrpn(OH)(H2O)]2+ cations

Abstract The complexes [Cotn 2 PO4]·2H 2 0 and [CotrpnPO 4 ]·5H 2 O have been prepared and characterized. The kinetics of the anation reactions of [Co(N 4 )(OH)(H 2 0)] 2+ by phosphate was studied for the two systems (N 4 )=tn 2 and trpn, at pH 6.5, 25 °C and μ=1.0 M. The results are consistent with S N l IP mechanism involving rapid preequilibrium ion-pair formation, followed by a slow substitution step. The reactivity of [Co(N 4 )(OH)H 2 0)] 2+ for phosphate complex formation decreases in the order trpn>tn 2 >en 2 .

Kinetics and mechanism of aquation reactions of dihalo-, diacetonitrile- and bromoacetonitrile-(3,3',3″-triaminotripropylamine)cobalt(III) perchlorate

Abstract The octahedral cobalt(III) complexes with the tripodal quadridentate amine 3,3′,3″-triaminotripropylamine [trpn = N(CH2CH2CH2NH3)3], [Co(trpn)Cl2]ClO4, [Co(trpn)Br2ClO4, [Co(trpn)F2]CO4·3/2H2O, [Co(trpn)(CH3CN)2](ClO4)3 and [Co(trpn)(CH3CN)Br]BrClO4 have been synthesized. The acid hydrolysis of [Co(trpn)Cl2]+ and the rate of bromide substitution in [Co(trpn)(CH3CN)Br]2+, clearly demonstrate the extreme lability of these complexes. The aquation of [Co(trpn)F2]+ ion was examined in both neutral and acidic medium. In neutral aqueous medium, the primary aquation determined at 25°C and μ = 0.5 M, gave k1 = 1.28 × 10−3 s −1, ΔH†1 = 12.8 kcal mol−1 and ΔS†1 − 28.9 cal K−1 mol−1, while for the secondary aquation step (25°C and μ = 1.0 M) the acid-independent pathway gave k2 = 1.4 × 10−4 s −1, ΔH†2 = 21.3 kcal mol−1 and ΔS = −4.7 cal K−1 mol−1. The acid hydrolysis of [Co(trpn)(CH3CN)2]3+ was studied over the pH range 1–4 and the temperature range 8–20°C. The hydrolysis proceeds in two consecutive steps : [Co(trpn)(CH3CN)2]3+ k1→ [Co(trpn)(CH3CN)(H2O)]3+ 2→ [Co(trpn)(H2O)2]3+. Analysis of the kinetic data gave k1 = 1.2 × 10−2 s −1(20°C), ΔH†1 = 16.0 kcal mol−1 and ΔS1‡ = 12.7 cal K−1 mol−1, and k2 = 2.38 × 10−3 s −1 (20°C), ΔH2‡ = 14.9 kcal mol−1 and ΔS‡2 = − 19.4 cal K−1 mol−1. The results of the kinetics of these substitution reactions are discussed in relation to data for the other tetraamine cobalt(III) complexes.

Formation and dephosphorylation of dptCo(ATP)−. Comparisons for triamine- and tetraamine-cobalt(III) complexes

Abstract The neutral complex [dptCo(α,β,γ-HATP)] has been prepared and characterized (dpt = NH(CH2CH2CH2NH2)2) and the kinetics of dptCo(ATP)− formation from dptCo(OH)(H2O)22+ and HATP3− have been measured spectrophotometrically (25°C, I = 0.5 M). The results are consistent with a mechanism involving rapid formation of the monodentate complex dptCo(H2O)2(γ-ATP)− by water replacement with proton transfer (t 1 2 ∼ 20 sec), and rate-determining completion of the chelate rings with replacement of the two remaining waters (t 1 2 ∼ 5 min). Also, initial dephosphorylation rates for the preformed dptCo(ATP)− chelate have been determined in the presence of the added aqua complexes dptCoIII(aq), trpnCoIII(aq) and tn2CoIII(aq) over the pH range 4–7 (25°C, I = 0.1 M, dptCoATP−, and added aqua complexes each 1 mM). The effectiveness of these aqua complexes in promoting ATP dephosphorylation decreases in the order trpn > dpt > tn2, when the rates at the pH of maximum promotion are compared. The enhancement in the hydrolysis rate of ATP (10−3 M) for the added dptCoIII(aq) (10−3 M) at pH 4.9, where dptCo(OH)(H2O)22+ predominates, is ∼ 105. In comparison to dephosphorylation rates for systems of the type N4Co(ATP)− + N4CoIII(aq) (where the rates depend very little on the N4 identity in N4Co(ATP)−), dephosphorylation rates for corresponding dptCo(ATP)− + N4CoIII(aq) systems are about 1 3 to 1 2 as large. The change in ATP coordination, from bidentate in complexes of type cis-N4Co(β,γ-ATP)− to tridentate in a complex of type fac-N3Co(α,β,γ-ATP)−, thus reduces the reactivity of the 1:1 complex towards ATP hydrolysis in the presence of added aqua-metal ions.

Kinetics of solvolysis of dihalo-(3,3′,3″-triaminotripropylamine)cobalt(III) perchlorate

The kinetics of the primary solvolysis reaction of [Co(trpn)X 2]+ ions (trpn = 3,3′,3″-triaminotripropylamine = N(CH2CH2CH2NH2)3; X = F, Cl, Br) in acetonitrile were studied. At 25°C, the rate constants for the release of one halide are 7.8 × 10−4, 1.66 × 10−3 and 1.87 × 10−2 s−1 for X = F, Cl and Br, respectively. The corresponding activation parameters ΔH‡ (kcal mol−1) and ΔS‡ (cal K−1 mol−1) are as follows: 24.7(±0.6), 10.8(±1.8); 13.6(±0.6), −25.8(±1.8); 5.05(±0.8), −49.5(±2.4) for the same series. The rate constants were found to be independent of the initial complex concentration. The solvolysis product [Co(trpn)(CH3CN)Br]BrClO4, of the reaction of the complex [Co(trpn)Br2]ClO4 with acetonitrile, was isolated and characterized. No evidence was found for the second solvolysis step. The results are discussed in terms of the dissociative Id mechanism.

Synthesis and characterization of some octahedral cobalt(III) complexes of 3,3',3″-triaminotripropylamine

Abstract The tripod quadridentate ligand 3,3′,3″-triaminotripropylamine (trpn) has been used to synthesize a series of cis-octahedral cobalt(III)-trpn complexes, including the type [CotrpnX2] (ClO4)n (X = F, Cl, Br, NO3 and n = 1; X = CH3CN and n = 3) as well as [Cotrpn (CH3CN)Br]BrClO4, where the coordinating groups are monodentate ligands. With chelating oxy-anions, preparation of the complexes [Cotrpn (CO3)] (CF3SO3), [Cotrpn(SO4)] ClO4, [Cotrpn(HSO4)](ClO4)2, [Cotrpn(C2O4)] ClO4 and [Cotrpn(H2P2O7)] ClO4 are described. The visible, IR and 13C NMR spectra have been used to characterize the isolated complexes.

Preparation and 13C NMR study of carbonato-, bicarbonato- and oxalato-tetraaminecobalt(III) complexes

Abstract A series of diamagnetic carbonato-, bicarbonato- and oxalato-tetraaminecobalt(III) complexes have been prepared, where the amines are: ammonia, 1,2-diaminoethane (en), 1,3-diaminopropane (tn), 2,2′,2″-triaminotriethylamine (tren), and 3,3′,3″- triaminotripropylamine (trpn), and the 13 C NMR spectra for these complexes have been examined. 13 C NMR provides a valuable diagnostic probe for the complexes, and assignments of chemical shifts to specific carbons are in most cases readily made. Results for [Co(trpn)(H 2 O)(OCO 2 H)] 2+ indicate the presence of both geometric isomers. The studies have relevance to the use of 13 C NMR in examining metal-ligand interactions in systems of biological interest.

Synthesis and characterization of triaminecobalt(III) complexes and acid hydrolysis kinetics of fac-[Codpt(H2O)nX3−n]n+ (X = Cl, Br; n = 0, 1, 2)

Abstract The complexes CodptX 3 and [Codpt(H 2 O)X 2 ]ClO 4 (X = Cl, Br; dpt = dipropylenetriamine = NH(CH 2 CH 2 CH 2 NH 2 ) 2 ) have been prepared and characterized. Rate constants (s −1 ) for aqueous solution at 25 °C and μ = 0.5 M (NaClO 4 ), for the acid-independent sequential ractions. have been measured spectrophotometrically. For X = Cl: k 1 ⋍ 2 × 10 −2 , k 2 = 1.7 × 10 −4 and k 3 = 4.8 × 10 −6 , and for X = Br: k 1 ⋍ 2 × 10 −2 , k 2 = 5.25 × 10 −4 and k 3 = 2.5 × 10 −5 The primary equation was found to be acid independent, while the secondary and tertiary aquations were acid-inhibited reactions. For the second step, the rate of the reaction was given by the rate equation where C t is the complex concentration in the aqua-and hydroxodihalo species, k ′ 2 is the rate constant for the acid-dependent pathway and K a is the equilibrium constant between the hydroxo and aqua complex ions. The activation parameters were evaluated, for X = Cl: Δ H ≠ 2 = 106.3 ± 0.4 kJ mol −1 and Δ S ≠ 2 = 40.2 ± 1.7 J K −1 mol − , and for X = Br: Δ H ≠ 2 = 91.6 ± 0.4 kJ mol −1 and Δ S ≠ 2 = 0.4 ± 1.7 J K −1 mol −1 . The results are discussed and detailed comparisons of the reactivities of these complexes with other haloaminecobalt(III) species are presented.