Small molecule activation at uranium coordination complexes: control of reactivity via molecular architecture

Electron-rich uranium coordination complexes display a pronounced reactivity toward small molecules. In this Feature article, the exciting chemistry of trivalent uranium ions coordinated to classic Werner-type ligand environments is reviewed. Three fundamentally important reactions of the [(((R)ArO)3tacn)U]-system are presented that result in alkane coordination, CO/CO2 activation, and nitrogen atom-transfer chemistry.     

Synthesis and characterization of N-heterocyclic carbene complexes of uranium(III)

Reaction of [(((Ad)ArO)(3)tacn)U(III)] (1) or [((Me(3)Si)(2)N)(3)U(III)] (3) with tetramethylimidazol-2-ylidene (Me(4)IMC:) yields novel N-heterocyclic carbene complexes [(((Ad)ArO)(3)tacn)U(III)(Me(4)IMC:)] (2) and [((Me(3)Si)(2)N)(3)U(III)(Me(4)IMC:)] (4). Uranium complexes 2 and 4 represent the first examples of compounds with an N-heterocyclic carbene ligand coordinated to a low-valent uranium center. The paramagnetic complexes 1, 2, and 4 were characterized by (1)H NMR, UV-vis-NIR, and EPR spectroscopy as well as SQUID magnetization measurements and X-ray diffraction analyses. DFT studies indicate a significant degree of pi-bonding in the U(III)-carbene entity.     

Uranium-mediated reductive conversion of CO(2) to CO and carbonate in a single-vessel, closed synthetic cycle

The new neopentyl (Neop)-substituted tris(aryloxide) U(iii) complex [(((Neop,Me)ArO)(3)tacn)U(III)] reacts with CO(2) to form CO and the bridging carbonate complex [{(((Neop,Me)ArO)(3)tacn)U(IV)}(2)(μ-CO(3))]. The uranium(iv) bridging oxo [{(((Neop,Me)ArO)(3)tacn)U(IV)}(2)(μ-O)] has been determined to be the intermediate in this reaction. For the first time, both U(iv) complexes can be reduced back to the U(iii) starting material. Thus, with KC(8) as reductant, [(((Neop,Me)ArO)(3)tacn)U(III)] engages in a synthetic cycle, in which CO(2) is converted to CO and CO(3)(2-).     

Structural and spectroscopic characterization of a charge-separated uranium benzophenone ketyl radical complex

The reaction of [((t-Bu)ArO) 3tacn)U (III)] ( 1) with 4,4'-di- tert-butylbenzophenone affords a unique isolable U(IV) ketyl radical species [((t-Bu)ArO) 3tacn)U (IV)(OC* (t-Bu)Ph 2)] (2) supported by XRD data, magnetization measurements, and DFT calculations. Isolation and full characterization of the corresponding diphenyl methoxide complex [((t-Bu)ArO) 3tacn)U (IV)(OCH ( t-Bu )Ph 2)] (3) is also presented. The one-electron reduction of benzophenone by [((Ad)ArO) 3tacn)U (III)] (4) leads to a purple U(IV) ketyl radical intermediate [((Ad)ArO) 3tacn)U (IV)(OC*Ph 2)] (5). This species is highly reactive, and attempts at isolation were unsuccessful and resulted in methoxide complex [((Ad)ArO) 3tacn)U (IV)(OCHPh 2)] (6) from H abstraction and dinuclear para-coupled complex [((Ad)ArO) 3tacn)U (IV)(OCPhPhCPh 2O)U (IV)((Ad)ArO) 3tacn)] (7).     

Synthesis of uranium(VI) terminal oxo complexes: molecular geometry driven by the inverse trans-influence

Oxidation of our previously reported uranium(V) oxo complexes, supported by the chelating ((R)ArO)(3)tacn(3-) ligand system (R = tert-butyl (t-Bu), 1-t-Bu; R = 1-adamantyl (Ad), 1-Ad), yields terminal uranium(VI) oxo complexes [(((R)ArO)(3)tacn)U(VI)(O)]SbF(6) (R = t-Bu, 2-t-Bu; R = Ad, 2-Ad). These complexes differ in their molecular geometry in that 2-t-Bu possesses pseudo-C(s) symmetry in solution and solid state as the terminal oxo ligand lies in the equatorial plane (as defined by the three aryloxide arms of the ligand) in order to accommodate the thermodynamic preference of high-valent uranium oxo complexes to have a σ- and π-donating ligand trans to the oxo (vis-à-vis the ubiquity of the linear UO(2)(2+) moiety). The distortion of the ligand--which stands in contrast to all other complexes of uranium supported by the ((R)ArO)(3)tacn(3-) ligand, including 2-Ad--is most clearly seen in the structures of 2-t-Bu, [(((t-Bu)ArO)(3)tacn)U(VI)(O)(eq)]SbF(6), and 3-t-Bu, [(((t-Bu)ArO)(3)tacn)U(VI)(O)(eq)(OC(O)CF(3))(ax)]. The solid-state structure of 3-t-Bu reveals that the trans U-O(ArO) bond length is shortened by 0.1 ? in comparison to the cis U-O(ArO) bonds and the trans U-O-C(ipso) angle is linearized (157.67° versus 147.85° and 130.03°). Remarkably, the minor modification of the ligand to have Ad groups at the ortho positions of the aryloxide arms is sufficient to stabilize a C(3v)-symmetric terminal uranium(VI) oxo complex (2-Ad) without a ligand trans to the oxo. These experimental results were reproduced in DFT calculations and allow the qualitative bracketing of the relative thermodynamic stabilization afforded by the inverse trans-influence as ～6 kcal mol(-1).     

Uranium tris-aryloxide derivatives supported by triazacyclononane: engendering a reactive uranium(III) center with a single pocket for reactivity

The synthesis and spectroscopic characterization of the mononuclear uranium complex [((ArO)(3)tacn)U(III)(NCCH(3))] is reported. The uranium(III) complex reacts with organic azides to yield uranium(IV) azido as well as uranium(V) imido complexes, [((ArO)(3)tacn)U(IV)(N(3))] and [((ArO)(3)tacn)U(V)(NSi(CH(3))(3))]. Single-crystal X-ray diffraction, spectroscopic, and computational studies of this analogous series of uranium tris-aryloxide complexes supported by triazacyclononane are described. The hexadentate, tris-anionic ligand coordinates to the large uranium ion in unprecedented fashion, engendering coordinatively unsaturated and highly reactive uranium centers. The macrocyclic triazacyclononane tris-aryloxide derivative occupies six coordination sites, with the three aryloxide pendant arms forming a trigonal plane at the metal center. DFT quantum mechanic methods were applied to rationalize the reactivity and to elucidate the electronic structure of the newly synthesized compounds. It is shown that the deeply colored uranium(III) and uranium(V) species are stabilized via pi-bonding interaction, involving uranium f-orbitals and the axial acetonitrile and imido ligand, respectively. In contrast, the bonding in the colorless uranium(IV) azido complex is purely ionic in nature. The magnetism of the series of complexes with an [N3O3-N(ax)] core structure and oxidation states +III, +IV, and +V is discussed in context of the electronic structures.     

A new diamantane functionalized tris(aryloxide) ligand system for small molecule activation chemistry at reactive uranium complexes

The diamantane functionalized triazacyclononane ligand (^DiaArOH)₃tacn (L₃) has been synthesized and the reactivity of its U(III) metallated product [((^DiaArO)₃tacn)U] (1) has been explored. Complex 1 promotes dichloromethane and azidotrimethylsilane activation to generate U(IV) complex [((^DiaArO)₃tacn)U(Cl)] (2) and U(V) complex [((^DiaArO)₃tacn)U(NTMS)] (3), respectively. Spectroscopic investigations of complexes 1, 2, and 3 will be discussed, along with molecular structures where possible.     

Carbon dioxide activation with sterically pressured mid- and high-valent uranium complexes

Sterically pressured mid- to high-valent uranium complexes with an aryloxide substituted triazacyclononane ligand scaffold, [(((R)ArO)3tacn)(3-)], were studied for carbon dioxide activation and transformation chemistry. The high valent uranium(V) imido species [(((R)ArO)3tacn)U(NR)] (R = (t)Bu, R' = 2,4,6-trimethylphenyl (2-(t)Bu); R = Ad, R' = 2,4,6-trimethylphenyl (2-Ad); R = (t)Bu, R' = phenyl (3-(t)Bu)) were synthesized and spectroscopically characterized. X-ray crystallography of the tert-butyl mesityl imido derivative, 2-(t)Bu , reveals coordination of a bent imido fragment with a relatively long U-N bond distance of 2.05 A. The mesityl imido complexes reacted with carbon dioxide, readily extruding free isocyanate to produce uranium(V) terminal oxo species, [(((R)ArO)3tacn)U(O)] (R = (t)Bu (4-(t)Bu), Ad (4-Ad)), presumably through multiple bond metathesis via a uranium(V) carbimate intermediate. Using the smaller phenyl imido fragment in 3-(t) Bu slowed isocyanate loss, allowing the uranium(V) carbimate intermediate to undergo a second metathesis reaction, ultimately producing the diphenyl ureate derivative, [(((tBu)ArO)3tacn)U(NPh2)CO] (5-(t)Bu). Single crystal X-ray diffraction studies were carried out on both uranium(V) terminal oxo complexes and revealed short U-O bonds (1.85 A) indicative of a formal UO triple bond. The electronic structure of the oxo U(V) complexes was investigated by electronic absorption and EPR spectroscopies as well as SQUID magnetization and DFT studies, which indicated that their electronic properties are highly unusual. To obtain insight into the reactivity of CO2 with U-N bonds, the reaction of the uranium(IV) amide species, [(((R)ArO)3tacn)U(NHMes)] (R = (t)Bu (6-(t)Bu), Ad (6-Ad) with carbon dioxide was investigated. These reactions produced the uranium(IV) carbamate complexes, [(((R)ArO)3tacn)U(CO2NHMes)] (R = (t)Bu (7-(t)Bu), Ad (7-Ad)), resulting from insertion of carbon dioxide into U-N(amide) bonds. The molecular structures of the synthesized uranium carbamate complexes highlight the different reactivities due to the steric pressure introduced by the alkyl derivatized tris(aryloxide) triazacyclononane ligand. The sterically open tert-butyl derivative creates a monodentate eta(1)-O bound carbamate species, while the sterically more bulky adamantyl-substituted compound forces a bidentate kappa(2)-O,O coordination mode of the carbamate ligand.     

Modifying the reactivity in the homologation of propane by introducing aryloxide ligands on a silica supported zirconium alkyl system

Grafting the well-defined molecular complexes [(ArO)Zr(CH2tBu)3], , and [(ArO)2Zr(CH2tBu)2], , on SiO2-(700) (ArO=2,6-Ph2C6H3O) gives the corresponding monosiloxy surface complexes [([TRIPLE BOND]SiO)Zr(CH2tBu)2(OAr)] and [([TRIPLE BOND]SiO)Zr(CH2tBu)(OAr)2] as major surface species as evidenced by mass balance analysis, IR and NMR spectroscopies. In both cases, minor cyclometallated species (ca. 20%) are also probably formed during the grafting process. While /SiO2-(700) catalytically transforms propane into its lower and higher homologues, /SiO2-(700) remains inactive. Moreover, the formation of butane as the major higher homologues is consistent with the formation of metallocarbene intermediates in this system in contrast to what was observed for the corresponding homologation reaction on silica supported zirconium hydrides.     

Observation of the inverse trans influence (ITI) in a uranium(V) imide coordination complex: an experimental study and theoretical evaluation

An inverse trans influence has been observed in a high-valent U(V) imide complex, [(((Ad)ArO)(3)N)U(NMes)]. A thorough theoretical evaluation has been employed in order to corroborate the ITI in this unusual complex. Computations on the target complex, [(((Ad)ArO)(3)N)U(NMes)], and the model complexes [(((Me)ArO)(3)N)U(NMes)] and [(NMe(3))(OMe(2))(OMe)(3)U(NPh)] are discussed along with synthetic details and supporting spectroscopic data. Additionally, the syntheses and full characterization data of the related U(V) trimethylsilylimide complex [(((Ad)ArO)(3)N)U(NTMS)] and U(IV) azide complex [(((Ad)ArO)(3)N)U(N(3))] are also presented for comparison.     

Uranium(III)-mediated C-C-coupling of terminal alkynes: formation of dinuclear uranium(IV) vinyl complexes

The previously reported uranium(III) complex [(((Ad)ArO)(3)N)U(III)(DME)] (1; Ad = adamantane, DME = 1,2-dimethoxyethane) reacts with the terminal bis-alkynes 1,7-octadiyne or 1,6-heptadiyne in C-C-coupling reactions to form the uranium(IV) vinyl complexes [{(((Ad)ArO)(3)N)U(IV)}(2)(μ-η(2):η(1)-1,2-(CH)(2)-cyclohexane)] (2) and [{(((Ad)ArO)(3)N)U(IV)}(2)(μ-η(2):η(2)-1,2-(CH)(2)-cyclopentane)] (3). With the monoalkynes 1-hexyne or 4-(t)butyl-phenylacetylene, the complexes [{(((Ad)ArO)(3)N)U(IV)}(2)(μ-η(2)(C1):η(1)(C4)-2-(n)Bu-1,3-octadiene)] (4) and [{(((Ad)ArO)(3)N)U(IV)}(2)(μ-η(2)(C4):η(1)(C1)-1,3-di-(p-(t)Bu-phenyl)butadiene))] (5), are formed. These are the first four examples of uranium vinyl complexes that are reported and crystallographically characterized. In addition, detailed DFT calculations are presented to establish a possible mechanism for their formation and explain the differences found for the coordination of the hydrocarbon fragments. In contrast to a previously proposed monometallic pathway for catalytic hydroamination of alkynes and alkyne dimerization involving a uranium vinyl intermediate at uranium(III) complexes, the calculations clearly support a bimetallic mechanism, since its transition states are energetically the most favored.     

Cyano-bridged FeIII2CuII3 and FeIII4NiII4 complexes: syntheses, structures, and magnetic properties

Two tricyanometallate precursors, (Bu4N)[(Tp4Bo)Fe(CN)3].H2O.2MeCN (1) and (Bu4N)[(pzTp)Fe(CN)3] (2) [Bu4N+ = tetrabutylammonium cation; Tp4Bo = tris(indazolyl)hydroborate; pzTp = tetrakis(pyrazolyl)borate], with a low-spin FeIII center have been synthesized and characterized. The reactions of 1 or 2 with [Cu(Me3tacn)(H2O)2](ClO4)2 (Me3tacn = N,N',N' '-trimethyl-1,4,7-triazacyclononane) afford two pentanuclear cyano-bridged clusters, [(Tp4Bo)2(Me3tacn)3Cu3Fe2(CN)6](ClO4)4.5H2O (3) and [(pzTp)2(Me3tacn)3Cu3Fe2(CN)6](ClO4)4.4H2O (4), respectively. Assembly reactions between 2 and [Ni(phen)(CH3OH)4](ClO4)2 (phen = 1,10-phenanthroline) or Zn(OAc)2.2H2O afford a molecular box [(pzTp)4(phen)4Ni4Fe4(CH3OH)4(CN)12](ClO4)4.4H2O (5) and a rectangular cluster [(pzTp)2Zn2Fe2(OAc)2(H2O)2(CN)6] (6). Their molecular structures were determined by single-crystal X-ray diffraction. In complexes 1 and 2, the central FeIII ions are coordinated by three cyanide carbon atoms and three nitrogen atoms of Tp4Bo- or pzTp-. Both complexes 3 and 4 show a trigonal-bipyramidal geometry, in which [(L)Fe(CN)3]- units occupy the apical positions and are linked through cyanide to [Cu(Me3tacn)]2+ units situated in the equatorial plane. Complex 5 possesses a cubic arrangement of eight metal irons linked through edge-spanning cyanide bridges, while complex 6 shows Zn2Fe2(CN)4 rectangular structure, in which FeIII and ZnII ions are alternately bridged by the cyanide groups. Intramolecular ferromagnetic couplings are observed for complexes 3-5, and they have S = 5/2, 5/2, and 6 ground states and appreciable magnetic anisotropies with negative D values equal to -0.49, -2.39, and -0.39 cm-1, respectively.     

Reactions of Ph3Sb═S with copper(I) complexes supported by N-donor ligands: formation of stable adducts and S-transfer reactivity

In the exploration of sulfur-delivery reagents useful for synthesizing models of the tetracopper-sulfide cluster of nitrous oxide reductase, reactions of Ph(3)Sb═S with Cu(I) complexes of N,N,N',N'-tetramethyl-2R,3R-cyclohexanediamine (TMCHD) and 1,4,7-trialkyltriazacyclononanes (R(3)tacn; R = Me, Et, iPr) were studied. Treatment of [(R(3)tacn)Cu(NCCH(3))]SbF(6) (R = Me, Et, or iPr) with 1 equiv of S═SbPh(3) in CH(2)Cl(2) yielded adducts [(R(3)tacn)Cu(S═SbPh(3))]SbF(6) (1-3), which were fully characterized, including by X-ray crystallography. The adducts slowly decayed to [(R(3)tacn)(2)Cu(2)(μ-η(2):η(2)-S(2))](2+) species (4-6) and SbPh(3), or more quickly in the presence of additional [(R(3)tacn)Cu(NCCH(3))]SbF(6) to 4-6 and [(R(3)tacn)Cu(SbPh(3))]SbF(6) (7-9). The results of mechanistic studies of the latter process were consistent with rapid intermolecular exchange of S═SbPh(3) between 1-3 and added [(R(3)tacn)Cu(NCCH(3))]SbF(6), followed by conversion to product via a dicopper intermediate formed in a rapid pre-equilibrium step. Key evidence supporting this step came from the observation of saturation behavior in a plot of the initial rate of loss of 1 versus the initial concentration of [(Me(3)tacn)Cu(NCCH(3))]SbF(6). Also, treatment of [(TMCHD)Cu(CH(3)CN)]PF(6) with S═SbPh(3) led to the known tricopper cluster [(TMCHD)(3)Cu(3)(μ(3)-S)(2)](PF(6))(3) in good yield (79%), a synthetic procedure superior to that previously reported (Brown, E. C.; York, J. T.; Antholine, W. E.; Ruiz, E.; Alvarez, S.; Tolman, W. B. J. Am. Chem. Soc. 2005, 127, 13752-13753).     

fac-[TcO3(tacn)]+: a versatile precursor for the labelling of pharmacophores, amino acids and carbohydrates through a new ligand-centred labelling strategy

Herein, we report a protocol for the synthesis of [(99m)TcO(3)(tacn)](+) ([1](+)) (tacn = 1,4,7-triazacyclononane) that is suitable for clinical translation. Bioconjugates containing pharmacophores ([TcO(NO(2)-Imi)(tacn)](+); [3](+)), artificial amino acids ([TcO(Fmoc-allyl-His)(tacn)](+); [5](+)), and glucose derivatives ([TcO(allyl-tetraacetylglucose)(tacn)](+); [7](+)) were synthesized by cycloaddition strategies and fully characterized ((99)Tc and (99m)Tc). These new technetium complexes are stable at neutral pH and demonstrate the potential and flexibility of the [3+2] cycloaddition labelling concept. In addition to the synthetic work, the first biodistribution studies of [1](+) and the small [3+2] cycloadduct [(99m)TcO(NO(2)-Imi)(tacn)](+) ([3](+)) were completed. The biodistribution studies suggest the stability of these complexes in vivo. Furthermore, it was demonstrated that the high hydrophilicity of the [(99m)TcO(3)(tacn)](+) building block is a complement to the complexes of the fac-{Tc(CO)(3)}(+) core.     

Synthesis and alkali metal ion-binding properties of a chromium(III) triacetylide complex

A simple triacetylide complex of chromium(III) is synthesized for use as a potential precursor to metal-dicarbide clusters. Reaction of Me(3)SiCCLi with [(Me(3)tacn)Cr(CF(3)SO(3))(3)] (Me(3)tacn = N,N',N"-trimethyl-1,4,7-triazacyclononane) in THF generates [(Me(3)tacn)Cr(CCSiMe(3))(3)], which subsequently reacts with Bu(4)NF to supply [(Me(3)tacn)Cr(CCH)(3)] as an air-stable orange solid. The crystal structure of this unprecedented triacetylide complex reveals octahedral coordination of the chromium center, with linear Cr-C(triple bond)C bond angles and C(triple bond)C bond distances essentially identical to the corresponding distance in acetylene. Crystallization of the complex from a DMF solution containing K(CF(3)SO(3)) leads to the sandwich complex ([(Me(3)tacn)Cr(CCH)(3)](2)K)(+), in which the K(+) ion is coordinated in a side-on fashion by each of the six C(triple bond)C units. With the larger Cs(+) cation, a triangular ([(Me(3)tacn)Cr(CCH)(3)](3)Cs)(+) complex is instead observed. The magnetic properties of these alkali metal complexes are indicative of weak antiferromagnetic exchange between Cr(III) centers, with J = -0.8 and -0.3 cm(-1), respectively.     

Crystal structures and solution behavior of paramagnetic divalent transition metal complexes (Fe, Co) of the sterically encumbered tridentate macrocycles 1,4,7-R3-1,4,7-triazacyclononane: coordination numbers 5 (R = i-Pr) and 6 (R = i-Bu)

The coordination chemistry of the sterically hindered macrocyclic triamines, 1,4,7-R3-1,4,7-triazacyclononane (R = i-Pr, i-Pr3tacn, and R = i-Bu, i-Bu3tacn) with divalent transition metals has been investigated. These ligands form a series of stable novel complexes with the triflate salts MII(CF3SO3)2 (M = Fe, Co, or Zn) under anaerobic conditions. The complexes Fe(i-Pr3tacn)(CF3SO3)2 (2), [Co(i-Pr3tacn)(SO3CF3)(H2O)](CF3SO3) (3), [Co(i-Pr3tacn)(CH3CN)2](BPh4)2 (4), Zn(i-Pr3tacn)(CF3SO3)2 (5), [Fe(i-Bu3tacn)(CH3CN)2(CF3SO3)](CF3SO3) (6), Fe(i-Bu3tacn)-(H2O)(CF3SO3)2 (7), and Co(i-Bu3tacn)(CF3SO3)2 (8) have been isolated. The behavior of these paramagnetic complexes in solution is explored by their 1H NMR spectra. The solid-state structures of four complexes have been determined by X-ray single-crystal crystallography. Crystallographic parameters are as follows. 2: C17H33F6FeN3O6S2, monoclinic, P2(1)/n, a = 10.895(1) A, b = 14.669(1) A, c = 16.617(1) A, beta = 101.37(1) degrees, Z = 4. 3: C17H35CoF6N3O7S2, monoclinic, P2(1)/c, a = 8.669(2) A, b = 25.538(3) A, c = 12.4349(12) A, beta = 103.132(13) degrees, Z = 4. 6: C24H45F6FeN5O6S2, monoclinic, P2(1)/c, a = 12.953(6) A, b = 16.780(6) A, c = 15.790(5) A, beta = 96.32(2) degrees, Z = 4. 7: C20H41F6FeN3O7S2, monoclinic, C2/c, a = 22.990(2) A, b = 15.768(2) A, c = 17.564(2) A, beta = 107.65(1) degrees, Z = 8. The ligand i-Pr3tacn leads to complexes in which the metal ions are five-coordinate, while it's isobutyl homologue affords six-coordinate complexes. This difference in the stereochemistries around the metal center is attributed to steric interactions involving the bulky alkyl appendages of the macrocycles.     

Osmium Complexes of 1,4,7-Triazacyclononane (tacn) and 1,4,7-Trimethyl-1,4,7-triazacyclononane (Me(3)tacn) and the X-ray Crystal Structure of [(Me(3)tacn)Os(eta(6)-C(6)H(5)BPh(3))]BPh(4).CH(3)CN

The complexes of osmium with tacn (1,4,7-triazacyclononane) and Me(3)tacn (1,4,7-trimethyl-1,4,7-triazacyclononane), [LOs (eta(6)-C(6)H(6))](PF(6))(2) (L = tacn) and LOsCl(3) (L = tacn, Me(3)tacn), have been prepared by substitution of L on [Os(eta(6)-C(6)H(6))Cl(2)](2) or [Os(2)Cl(8)](2)(-), respectively. Reaction of LOsCl(3) with neat triflic acid leads to partial replacement of chloride and formation of the binuclear Os(III)-Os(III) complexes [LOs(&mgr;-Cl(3))OsL](PF(6))(3) (L = tacn, Me(3)tacn). The binuclear nature was established by NMR spectroscopy and elemental analysis and, for L = tacn, a partially refined X-ray crystal structure which shows the Os-Os separation to be 2.667 ?, indicative of significant metal-metal bonding. Reduction of [LOs(&mgr;-Cl(3))OsL](3+) over zinc amalgam in either aqueous or non-aqueous solution yields the intensely colored Os(II)-Os(III) mixed-valence ions [LOs(&mgr;-Cl(3))OsL](2+). Electrochemical measurements on [LOs(&mgr;-Cl(3))OsL](3+) in CH(3)CN reveal the reversible formation of the mixed valence ions. These are further reduced at lower potential to the Os(II)-Os(II) binuclear species, reversibly for L = Me(3)tacn. (Me(3)tacn)OsCl(3) is oxidized by persulfate ion to give [(Me(3)tacn)OsCl(3)](+); zinc amalgam reduction in an aqueous solution at high concentration produces the binuclear complex [(Me(3)tacn)Os(&mgr;-Cl(3))Os(Me(3)tacn)](3+) or, at low concentration, a solution containing an air sensitive osmium(II) species. Addition of BPh(4)(-) results in the eta(6)-arene zwitterion [(Me(3)tacn)Os(eta(6)-C(6)H(5)BPh(3))](+), which was characterized by X-ray diffraction on the BPh(4)(-) salt. The compound crystallizes in the triclinic space group P1 with a = 11.829(2) ?, b = 12.480(3) ?, c = 17.155(4) ?, alpha = 84.42(2) degrees, beta = 83.52(2) degrees, gamma = 71.45(2) degrees, V = 2380(2) ?(3), Z = 2, and R = 7.62%, and R(w) = 7.39%.     

Insight into binding of alkanes to transition metals from NMR spectroscopy of isomeric pentane and isotopically labeled alkane complexes

Alkane complexes of the type Cp'Re(CO)2(alkane) (Cp' = cyclopentadienyl or (isopropyl)cyclopentadienyl; alkane = isotopomers of n-pentane and cyclopentane) have been characterized using NMR spectroscopy following photolysis of Cp'Re(CO)3 in the appropriate alkane at 163-193 K. In the case of n-pentane, three different complexes are observed corresponding to binding of the three different types of carbon in this alkane. ROESY NMR experiments indicate that these isomeric complexes are slowly interconverting intramolecularly at 173 K. The order of the energetically preferred site of coordination is methylene (C2) approximately central methylene (C3) > methyl (C1) but with a spread of <0.2 kcal mol-1. Isotopic perturbation of resonance (IPR) experiments, conducted on several isotopomers of (i-PrCp)Re(CO)2(1-pentane), showed a large shielding of the 1H NMR chemical shift of the proton in a bound CHD2 moiety (delta -3.62) and CH2D (delta -2.64) compared with that of a bound CH3 moiety (delta -1.99). Likewise, the value of 1JCH for the coordinated methyl group of isotopomers of (i-PrCp)Re(CO)2(1-pentane) reduces in the order CH3 > CH2D > CHD2. This suggests that the alkane coordinates in an eta2-C,H fashion with a rapid exchange of bound hydrogen or deuterium within a methyl or methylene group, and that binding of a hydrogen atom is preferred over a deuterium by an amount of 0.23 +/- 0.03 kcal mol-1.     

Dinuclear 1,4,7-triazacyclononane (tacn) complexes of cobalt(III) with amido and tacn bridges. Synthesis, characterization and reversible acid-accelerated bridge cleavage

Amido-bridged dinuclear cobalt(III) complexes with 1,4,7-triazacyclononane (tacn) were synthesized from [Co(tacn)(O3SCF3)3] by treatment with potassium amide in liquid ammonia at 100 degrees C. Two isomeric triply bridged complexes, [(tacn)Co(mu-NH2)3Co(tacn)]3+ and [(tacn)Co(mu-NH2)2[mu-tacn(-H)]Co(NH3)]3+, were isolated as perchlorates, and the crystal structure of the perrhenate of the latter complex was determined by X-ray diffraction. In this compound a nitrogen atom (deprotonated) from one of the tacn ligands forms a third bridge together with two amido bridges. In 1.0 M (Na,H)ClO4 ([H+] 0.1-1.0 M) the two isomers undergo acid-accelerated amido bridge cleavage, as earlier found for chromium(III) analogues, in spite of the fact that such bridges are co-ordinatively saturated. The triamido-bridged isomer is in this acid medium in equilibrium with [(H2O)(tacn)Co(mu-NH2)2Co(tacn)(NH3)]4+. An isolated perchlorate of this complex appeared to be the salt of the trans-ammineaqua isomer as determined by X-ray diffraction. Equilibration from both sides fits the first-order rate constant dependence k(obs)=6.2(3) x 10(-5)[H+] + 2.1(2) x 10(-5)(s(-1)) at 40 degrees C. Prolonged treatment of the two triply bridged isomers in 1.0 M HClO4 at elevated temperature produces primarily triply bridged dinuclear species where one or two amido bridges have been replaced by hydroxo bridges.     

First row divalent transition metal complexes of aryl-appended tris((pyridyl)methyl)amine ligands: syntheses, structures, electrochemistry, and hydroxamate binding properties

Divalent manganese, cobalt, nickel, and zinc complexes of 6-Ph(2)TPA (N,N-bis((6-phenyl-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine; [(6-Ph(2)TPA)Mn(CH(3)OH)(3)](ClO(4))(2) (1), [(6-Ph(2)TPA)Co(CH(3)CN)](ClO(4))(2) (2), [(6-Ph(2)TPA)Ni(CH(3)CN)(CH(3)OH)](ClO(4))(2) (3), [(6-Ph(2)TPA)Zn(CH(3)CN)](ClO(4))(2) (4)) and 6-(Me(2)Ph)(2)TPA (N,N-bis((6-(3,5-dimethyl)phenyl-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine; [(6-(Me(2)Ph)(2)TPA)Ni(CH(3)CN)(2)](ClO(4))(2) (5) and [(6-(Me(2)Ph)(2)TPA)Zn(CH(3)CN)](ClO(4))(2) (6)) have been prepared and characterized. X-ray crystallographic characterization of 1A.CH(3)()OH and 1B.2CH(3)()OH (differing solvates of 1), 2.2CH(3)()CN, 3.CH(3)()OH, 4.2CH(3)()CN, and 6.2.5CH(3)()CN revealed mononuclear cations with one to three coordinated solvent molecules. In 1A.CH(3)()OH and 1B.2CH(3)()OH, one phenyl-substituted pyridyl arm is not coordinated and forms a secondary hydrogen-bonding interaction with a manganese bound methanol molecule. In 2.2CH(3)()CN, 3.CH(3)()OH, 4.2CH(3)()CN, and 6.2.5CH(3)()CN, all pyridyl donors of the 6-Ph(2)TPA and 6-(Me(2)Ph)(2)TPA ligands are coordinated to the divalent metal center. In the cobalt, nickel, and zinc derivatives, CH/pi interactions are found between a bound acetonitrile molecule and the aryl appendages of the 6-Ph(2)TPA and 6-(Me(2)Ph)(2)TPA ligands. (1)H NMR spectra of 4 and 6 in CD(3)NO(2) solution indicate the presence of CH/pi interactions, as an upfield-shifted methyl resonance for a bound acetonitrile molecule is present. Examination of the cyclic voltammetry of 1-3 and 5 revealed no oxidative (M(II)/M(III)) couples. Admixture of equimolar amounts of 6-Ph(2)TPA, M(ClO(4))(2).6H(2)O, and Me(4)NOH.5H(2)O, followed by the addition of an equimolar amount of acetohydroxamic acid, yielded the acetohydroxamate complexes [((6-Ph(2)TPA)Mn)(2)(micro-ONHC(O)CH(3))(2)](ClO(4))(2) (8), [(6-Ph(2)TPA)Co(ONHC(O)CH(3))](ClO(4))(2) (9), [(6-Ph(2)TPA)Ni(ONHC(O)CH(3))](ClO(4))(2) (10), and [(6-Ph(2)TPA)Zn(ONHC(O)CH(3))](ClO(4))(2) (11), all of which were characterized by X-ray crystallography. The Mn(II) complex 8.0.75CH(3)()CN.0.75Et(2)()O exhibits a dinuclear structure with bridging hydroxamate ligands, whereas the Co(II), Ni(II), and Zn(II) derivatives all exhibit mononuclear six-coordinate structures with a chelating hydroxamate ligand.