The unsymmetrical N,O-dibridging NO2– group in the threo (‘rac’) or erythro (‘meso’) isomers of [(en)2Co(μ-NH2,μ-NO2)Co(en)2]4+ is cleaved in hot triflic acid to yield a complex that is predominantly meso. This reacts in water to yield the new meso-[(en)2Co(μ-NH2,μ-OH)Co(en)2]4+ ion. The reactions of the threo- and erythro-μ-NO2– isomers in basic (0.1 M) aqueous solution are each retentive, giving the rac- and meso-[(en)2Co(μ-NH2,μ-OH)Co(en)2]4+ ions as products, respectively. The optically resolved erythro complex gives the achiral meso isomer, which completes the threo/rac and erythro/meso chemical correlations between the two series of dinuclear complexes; the meso synthesis completes the pair – the meso- and rac-[(en)2Co(μ-NH2,μ-OH)Co(en)2]4+ ions. In HCl, stable μ-Cl, μ-NH2 dinuclear ions are formed from these μ-NH2, μ-OH complexes, with retention. The peroxo or superoxo ions rac-[Co(en)2(μ-NH2,μ-O2)Co(en)2]n+ react with SnII to produce symmetrically bridged intermediates that decay to the common and new meso-[(en)2Co(μ-NH2,μ-OH)Co(en)2]4+ isomer on reaction in Me2SO. Both the rac and meso isomers of the μ-hydroxo-μ-peroxo complexes are also reduced by SnII to give intermediates that in Me2SO decay to the respective diols with retention. The new rac-[Co(en)2(μ-OH)2Co(en)2]4+ and meso-[(en)2Co(μ-NH2,μ-OH)Co(en)2]4+ isomers complete the characterization of rac/meso pairs in both systems. The corresponding μ-sulfate pairs are also reported.

The rates of base catalysed hydrolysis of six asym-[Co(dmptacn)X](n+) and six asym-[Co(dmpmetacn)X](n+) complexes (X = Br-, NO3-, MeSO3- CF3,SO3-, Me2SO, SO42-) have been studied to explore the role of the leaving group and the formal charge on these complex ions in determining the effective site of deprotonation, at one of the four distinct alpha-CH2 pyridyl sites or at an NH centre. The work corroborates the new pseudo-aminate mechanism established for the chloro derivatives. Deuterium exchange and proton NMR experiments have shown that the site of deprotonation is leaving group dependent, the first time this long-standing speculative idea has been proven. This work has also shown that the effect of improving the leaving group can override rate limiting deprotonation to the point where NH deprotonation, via the normal S(N)1CB mechanism, operates rather than the rate limiting pseudo-aminate mechanism. The steric course of substitution has been determined and the reaction via NH deprotonation, enforced by employing an exceptionally good leaving group such as the triflate ion, has uncovered a non-retentive pathway leading to the previously unknown (and unstable) sym isomeric product. The steric course of substitution is shown to be mechanism dependent. For reaction via alpha-CH2 deprotonation, which of the two arms is utilised proved quite leaving group dependent, although the specific proton on a particular 'arm' was always the same, consistent with the requirements of this new mechanism. (c) 2006 Elsevier Ltd. All rights reserved.

The kinetics and stereochemistry of acid hydrolysis of the classic complexes cis- and trans-[Co(NH3)(4)Cl-2](+) have been determined. They hydrolyse at virtually the same rate, 2.4 x 10(-3) s(-1) and 2.2 x 10(-3) s(-1) for the cis and trans tons. respectively, (0.02 M HClO4, 25 degrees C). The 'immeasurably fast' hydrolysis of the cis isomer has been dismissed as a myth. The trans-(Co(NH3)(4)(OH2)Cl](2+) isomer has been isolated for the first time, and the trans/cis isomerisation equilibration rate measured. again remarkably at virtually the same rate, 2.3 x 10(-1) s(-1). The cis isomer is the more stable at equilibrium. 88%). The steric course of substitution has been determined as 55% cis product for the cis-dichloro reactant and 35% cis for the trans reactant. The kinetic and equilibrium data are compared to that for the well-studied [Co(en)(2)Cl-2](+) system, and the synthetic chemistry for the simpler tetraammine systems is elaborated. (c) 2006 Elsevier Ltd. All rights reserved.

The mixed diamine complexes trans-[Co(tmen)(diamine)Cl-2](+) have been synthesised (tmen = NH2C(Me)(2)C(Me)(2)NH2; diamine = en = NH2(CH2)(2)NH2, and ibn = NH2C(Me)(2)CH2NH2). Replacement of one en ligand in trans-[Co(en)(2)Cl-2](+) by one linen ligand engenders an enormous rate enhancement (2000-fold) for acid hydrolysis. Solvolysis rates have been measured in Me2SO and DMF for these complexes and also trans-[Co(tmen)(2)Cl-2](+) which is more reactive again (10(4)-fold). The measured reactivities in DMF at 2 degrees C establish that the kinetic effect of replacing each en by tmen is incremental, and the extreme base catalysed racemisation rate for (+)-[Co-(tmen)(3)](3+) can now be explained on this basis. (c) 2006 Elsevier B.V. All rights reserved.

The hexaaminecobalt(III) complex [Co(tmptacn)]3+ (tmptacn = 1,4,7-tris(2'-pyridylmethyl)-1,4,7-triazacyclononane) undergoes a novel base-catalyzed N- to C-bonded rearrangement in which a tacn nitrogen is displaced by the alpha-carbon which deprotonates and binds to the metal ion as a carbanion. The X-ray structure establishes the configuration for the regio- and stereoselectively (100%) formed product. The reaction involves both ring expansions and ring contraction. The carbanion is part of a strained four-membered ring. The kinetics are reported for the N- to C-rearrangement, shown to be retentive for the optically resolved (+)-[Co(tmptacn)]3+ reactant, and also the kinetics for a competitive and somewhat faster base-catalyzed racemization reaction of this complex. The reaction is completely but very slowly reversed in acid, also with retention, and in D2O/D+ there is 1:1 D-incorporation into the two sets of inequivalent tacn carbons. Extensive 1D and 2D NMR studies establish mechanistic details, and alternative mechanisms are proposed for the forward and reverse reactions. In neutral solution, there is a competitive oxidation reaction for the reverse C- to N-bonded process, involving the regio- and stereoselective (100%) incorporation of an alpha-OH substituent into the tacn ring.

The kinetics and stereochemistry for the base catalysed substitution reactions of all seven isomers (4 mer and 3 fac) of both [Co(dien)(ibn)Cl]2+ and [Co(dien)(ampy)Cl]2+ have been studied in detail, for water and azide ion as entering groups. The stereochemistry for the azide ion anation of some of the [Co(dien)(diamine)OH]2+ species have also been investigated. The mer isomers are of comparable reactivity and amongst the fastest reacting pentaaminechlorocobalt(III) complexes known. They are also much faster to hydrolyse than the fac species. In both the ibn and ampy systems, a common product stereochemistry is observed for the four reactant mer isomers (the product is a mixture of all four mer configurations), for both azide ion and water as nucleophiles, but not for the three fac reactants (H2O as nucleophile). The kinetic and equilibrium distributions are quite different. For the mer isomer reactions, a common trigonal bipyramidal five-coordinate intermediate deprotonated at the sec-NH of the dien is overwhelmingly implicated. The substitution mechanisms are argued in detail. Other data reported include isomerisation rates and equilibrium distributions for some mer-hydroxo and a mer-aqua complex of exceptional reactivity, equilibrium distributions for the mer-phosphato complexes in the ampy system under different pH conditions, the crystal structure for the isolated m1-[Co(dien)(ampy)OP(OH)3]Cl3 · 2H2O species, and a rationale for its predominance at neutral pH based on internal H-bonding.

The synthesis and characterisation of Co(III) complexes derived from a condensation reaction with a central or terminal nitrogen of a dien ligand and the α-carbon of a range of substituted bis(pyridin-2-yl)methane ligands are described. Aerial oxidation of bpm {bis(pyridin-2-yl)methane with Co(II)/dien or direct reaction with Co(dien)Cl3 provided in low yield a single C–N condensation product 1 (at the primary terminal NH2) after the pyridyl –CH2– is formally oxidised to –CH+–. The methyl substituted ligand bpe {1,1-bis(pyridin-2-yl)ethane} behaves likewise, except both terminal (prim) and central (sec) amines condense to yield isomeric products 2 and 3. Two of these three materials have been characterised by single crystal X-ray crystallography. The corresponding reactions for the bis(pyridyl) ligand bpk {bis(pyridin-2-yl)ketone} provided C–N condensation products without the requirement for oxidation at the α-C center; two carbinolamine complexes in different geometrical configurations resulted, mer-anti-[Co(dienbpc)Cl]ZnCl4, 5, and unsym-fac-[Co(dienbpc)Cl]ZnCl4, 6, {dienbpc=[2-(2-aminoethylamino)-ethylamino]-di-pyridin-2-yl-methanol}. In addition, a novel complex, [Co(bpk)(bpd-OH)Cl]ZnCl4, 4, in which one bidentate N, N-bonded bpk ligand and one tridentate N, O, N-bonded bpd (the diol from bpk+OH−) were coordinated, was obtained via the Co(II)/O2 synthetic route. When the bpc ligand (bpc=bis(pyridin-2-yl)methanol) was employed directly as a reagent along with dien, no condensation reactions were observed, but rather a single isomeric complex [Co(dien)(bpc)]Cl.ZnCl4, 7, in which the ligand bpc acted as a N,N,O-bonded tridentate ligand rather than as a N,N-bidentate ligand was isolated. 13C, 1D and 2D 1H NMR studies are reported for all the complexes; they establish the structures unambiguously.

AbstractFor Abstract see ChemInform Abstract in Full Text.

The 1,4-bis(2-pyridylmethyl)-1,4-diazacyclononane (dmpdacn) ligand with a N4C donor set deprotonates at a CH2 gamma to an amine under extraordinarily mild conditions (pH 7) and binds as a pentadentate ligand to Co(III) as the [Co(dmpdacn-C)(OH2)](2+) complex. This complex was characterized by 1D and 2D NMR techniques, and a single-crystal X-ray structure is reported. In an alternative synthesis from Co(II), dmpdacn, and air, the same C-bonded complex is obtained along with a novel hydroxylated Co(III) complex [Co(dmpdacnOH-O)Cl](2+) which has been similarly characterized. Here the carbanion has been oxidized, a C- to O-bonded rearrangement has taken place, and the bound aqua group is replaced by Cl-. The base hydrolysis kinetics of the hydroxylated Co(III) complex are reported, and mechanisms for this and the unusually facile C-H cleavage and CH2 oxidation reactions are discussed.

High-resolution (1)H and (13)C NMR studies on the linkage isomers [(NH(3))(5)CoOC(S)NHCH(3)](2+) and [(NH(3))(5)CoSC(O)NHCH(3)](2+) reveal that the O-bonded form exists as a 5:1 mixture of Z and E isomers arising from restricted rotation about the C-N bond. Similarly, restricted rotation is observed (at 20 degrees C) for the S-bonded isomer (Z/E ca. 18:1), but not for the isoelectronic carbamate ion [(NH(3))(5)CoOC(O)NHCH(3)](2+), nor for the unsubstituted carbamato complex [(NH(3))(5)CoOC(O)NH(2)](2+). An analysis of the variable-temperature NMR data for the O-bonded carbamato and urea complexes has provided quantitative data on the rotational barriers, and these ions involve much faster C-N bond rotations than the thiocarbamato complexes. The acid-catalyzed reaction of [(NH(3))(5)CoOC(S)NHCH(3)](2+) is confirmed, but there is much less parallel hydrolysis (ca. 2%) than previously reported (40 +/- 10%) for 0.1 M HClO(4). In 1 M HClO(4), [(NH(3))(5)CoSC(O)NHCH(3)](2+) and [(NH(3))(5)CoOH(2)](3+) are formed in parallel as an 83:17 mixture. The kinetic data suggest that the protonated form is at least 20-fold more reactive than the free ion and that the linkage isomerization and hydrolysis pathways are both acid-catalyzed, the latter clearly more so than the rearrangement.

Both isomers of [(en)(2)Co(micro-NO(2))(micro-NH(2))Co(en)(2)](4+) synthesized by Alfred Werner in 1913 have been optically resolved and are therefore the classic inorganic analogues of the sugars threose and erythrose, rather than the rac- and meso-tartaric acids, as Werner believed. The nitro bridge is unsymmetrically N-O bonded, and each dinuclear ion is therefore asymmetric, a fact also clearly apparent in the (1)H and (13)C NMR spectra.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

The two closely spaced NH signals in the (1)H NMR spectrum of trans-[Co(en)(2)(OSMe(2))(N(3))](2+) have been reassigned using 2D NMR and other techniques. Thus, the unusual syn to anti (to Co-N(3)) NH rearrangement on base catalyzed substitution of the selectively deuterated complex in ND(3)(l) has been reinterpreted as "normal", with inversion of the effective deprotonation site accompanying the act of substitution. The re-examination of this system required a repeat study of the secondary isotope effect for the acid hydrolysis reaction, previously used to assign syn and anti amine sites, and this has been extended to other solvents (Me(2)SO, MeCN). The relative NH proton exchange rates are also reconsidered. A systematic rate reduction for Me(2)SO substitution is observed for deuterium incorporation into the cis-NH centers, irrespective of whether these are syn or anti, and the effect is much greater in Me(2)SO than in water. The results are interpreted in terms of zero point energy effects and coupled vibrations.

A compound originally prepared by Alfred Werner but structurally misassigned has been shown by three independent X-ray structural analyses to be a hexanuclear species which can now be regarded as Werner's second hexol, his first being that famous for being the first non-carbon containing species to be optically resolved.

The structures of five of the seven possible isomeric forms in the [Co(dien)(ampy)Cl]2+ system (three mer- and two fac-) have been determined by single crystal X-ray diffraction: (1) m1·Cl2·H2O, triclinic P1̄, a=7.3757(12), b=9.9250(16), c=11.5002(19) Å, α=100.242(3)°, β=100.832(3)°, γ=94.615(3)°, Z=2; (2) m3·ZnCl4, monoclinic P21/c, a=11.057(3), b=9.581(2), c=17.568(4) Å, β=95.560(5)°, Z=4; (3) m4·ZnCl4 monoclinic P21/n, a=8.4464(14), b=24.398(4), c=10.0645(17) Å, β=112.510(3)°, Z=4; (4) f1·ZnCl4, triclinic P1̄, a=10.318(1), b=12.706(3), c=7.578(1) Å, α=107.01(2)°, β=94.90(1)°, γ=99.07(1)°, Z=2; (5) f2·(ZnCl4)0.5·Cl·H2O, triclinic P1, a=7.7225(18), b=8.359(2), c=15.898(4) Å, α=96.025(4)°, β=93.298(5)°, γ=116.191(4)°, Z=1. The last structure provides an example of spontaneous resolution. Two additional and independently determined structures are reported, in excellent agreement with these results. The structural work confirms the results of the detailed 2D NMR study. The agreement between the single crystal data and the calculated ab initio molecular parameters is excellent (within ±3%). The calculated total energies have been computed for the gas phase and aqueous solution, and are been used to explain the relative abundances of the seven isomers observed experimentally. The modes of isomer rearrangement are discussed.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Cobalt(II) salts react with H2O2 in the presence of 2 equiv of tetramethylethylenediamine (tmen) to produce a stable sideways bonded mononuclear peroxo complex [Co(tmen)2O2]ClO4 which has been characterized chemically and by a single crystal X-ray determination. This is the first such tetraaminecobalt(III) complex, indeed, the first for any tetraaminemetal ion complex. The mononuclear peroxo complex can also be synthesized by O2 2- anation of cis-[Co(tmen)2(OH2)2]3+. This reaction is reversed in acid, and this offers the potential to develop Co(III) catalyzed oxidation reactions.

The bis(pentadentate) ligand tmpdtne binds two Co(II) centers, and the entity is readily oxidized to the dicobalt(III) derivative [Co(2)(tmpdtne)Cl(2)](4+) which has been separated into two isomeric forms. NMR studies establish these as meso and rac isomers arising from the different or same absolute configurations for the asym configuration about each Co(III) center. Each dinuclear ion base hydrolyses to the dihydroxo derivative [Co(2)(tmpdtne)(OH)(2)](4+) with retained asym configurations about each metal ion and also retained rac or meso configurations. The kinetics for the stepwise loss of the two Cl(-) ligands is uniphasic, and data are presented to show that the loss of the first chloride is rate determining and is followed by very rapid intramolecular and loss of the second Cl(-) via a hydroxo-bridged species to yield the observed dihydroxo derivative. Meso and rac forms of the latter have been crystallized. The X-ray crystal structure of the rac-dihydroxo complex is reported, and it establishes the configurations of all the complexes reported. The (1)H NMR spectra for the hydroxo ions show very high field Co-OH resonances (ca. delta-0.5 ppm) not observed previously for such ions, and this result is discussed in the context of published (1)H NMR data for bridged Co-OH-Co species. The base hydrolysis kinetics for the dichloro ions are first order in [OH(-)], and deprotonation at an alpha-CH(2) center (alpha to a pyridyl) is identified as the source of the catalysis, since there is no NH center available for deprotonation on the ligand. These data further support the new pseudoaminate base hydrolysis mechanism first reported in 2003. The values of k(OH) for the second-order base-catalyzed reaction are ca. 4.0 M(-1) s(-1) for both the rac and meso isomers, and these results are discussed in terms of the increased acidities of these 4+ cations compared to their 2+ ion counterparts.

The [Co(dmptacn)Cl](2+) (dmptacn = 1,4-bis(pyridylmethyl)-1,4,7-triazacyclononane) complex has been shown to be the asym isomer through 1D and 2D NMR studies, its optical resolution, and the single-crystal X-ray structure of its perchlorate salt. The kinetics of base-catalyzed hydrolysis establishes the usual [OH(-)] dependence (k(OH) = 0.040 M(-1) s(-1), 25 degrees C, I = 1.0 M, NaCl), but D-exchange experiments reveal that substantial if not complete reaction proceeds via the new pseudoaminate mechanism, i.e., via deprotonation at an alpha-CH(2) center rather than the NH. The significant kinetic isotope effect (k(H)/k(D) = 2.1) is interpreted in terms of rate-limiting deprotonation followed by reprotonation of the conjugate base at a rate competitive with loss of Cl(-). NMR and polarimetric studies establish geometric and optical retention for the hydrolysis reaction and exclude even the transient formation of a sym isomer intermediate.