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%.     

Giant metal-cyanide coordination clusters: tetracapped edge-bridged cubic Cr(12)Ni(12)(CN)(48) and double face-centered cubic Cr(14)Ni(13)(CN)(48) species

The crystal structures of two new metal-cyanide clusters with record high nuclearities are reported. A direct assembly reaction involving [(Me(3)tacn)Cr(CN)(3)] (Me(3)tacn = N,N',N"-trimethyl-1,4,7-triazacyclononane), NiI(2), and KCN in aqueous solution affords [(Me(3)tacn)(12)Cr(12)Ni(12)(CN)(48)](12+). The structure of this 24-metal cluster features a cube of eight Cr(III) centers linked along the edges by 12 trans-coordinated [Ni(CN)(4)](2)(-) units, and capped on four faces by [(Me(3)tacn)Cr](3+) moieties. Its metal-cyanide cage encloses a 900 A(3) cavity that is accessible through the two noncapped cube faces. A still larger cluster, [(Me(3)tacn)(14)Cr(14)Ni(13)(CN)(48)](20+), was obtained from a related reaction excluding the addition of KCN. This 27-metal species possesses a highly anisotropic geometry in which two face-centered cubic units are fused through a common Ni(II) vertex.     

Tetracyanide-bridged divanadium complexes: redox switching between strong antiferromagnetic and strong ferromagnetic coupling

Reaction of [(Me3tacn)V(CF3SO3)3] (Me3tacn = N,N',N'-trimethyl-1,4,7-triazacyclononane) with LiCN in DMF results in oligomerization of cyanide to form [(Me3tacn)2V2(CN)4(mu-C4N4)]. The structure of this binuclear complex features a planar tetracyanide unit bridging two VIV centers via imido type linkages. The conjugated pathway provided by the bridging ligand leads to strong antiferromagnetic coupling (J = -112 cm-1) and an S = 0 ground state. Reduction of the complex with cobaltocene generates the Class III mixed-valence anion [(Me3tacn)2V2(CN)4(mu-C4N4)]1-, wherein resonance exchange induces strong ferromagnetic coupling to give a well-isolated S = 3/2 ground state.     

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.     

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-).     

Nickel(II)-molybdenum(III)-cyanide clusters: synthesis and magnetic behavior of species incorporating [(Me(3)tacn)Mo(CN)(3)

The substitution of Mo(III) for Cr(III) in metal-cyanide clusters is demonstrated as an effective means of increasing the strength of the magnetic exchange coupling and introducing magnetic anisotropy. Synthesis of the octahedral complex [(Me(3)tacn)Mo(CN)(3)] (Me(3)tacn = N,N',N"-trimethyl-1,4,7-triazacyclononane) is accomplished with the addition of precisely 3 equiv of LiCN to a solution of [(Me(3)tacn)Mo(CF(3)SO(3))(3)] in DMF. An excess of LiCN prompts formation of a seven-coordinate complex, [(Me(3)tacn)Mo(CN)(4)](1)(-), whereas less LiCN produces multinuclear species such as [(Me(3)tacn)(2)Mo(2)(CN)(5)](1+). In close parallel to reactions previously performed with [(Me(3)tacn)Cr(CN)(3)], assembly reactions between [(Me(3)tacn)Mo(CN)(3)] and [Ni(H(2)O)(6)](2+) or [(cyclam)Ni(H(2)O)(2)](2+) (cyclam = 1,4,8,11-tetraazacyclotetradecane) afford face-centered cubic [(Me(3)tacn)(8)Mo(8)Ni(6)(CN)(24)](12+) and linear [(Me(3)tacn)(2)(cyclam)NiMo(2)(CN)(6)](2+) clusters, respectively. Generation of the former involves a thermally induced cyanide linkage isomerization, which rapidly leads to a low-spin form of the cluster containing diamagnetic Ni(II) centers. The cyclic voltammagram of this species in DMF reveals a sequence of six successive reduction waves spaced approximately 130 mV apart, suggesting class II mixed-valence behavior upon reduction. The magnetic properties of the aforementioned linear cluster are consistent with the expected ferromagnetic coupling and an S = 4 ground state, but otherwise vary slightly with the specific conformation adopted (as influenced by the packing of associated counteranions and solvate molecules in the crystal). Magnetization data indicate an axial zero-field splitting parameter with a magnitude falling in the range [D] = 0.44-0.72 cm(-1), and fits to the magnetic susceptibility data yield exchange coupling constants in the range J = 17.0-17.6 cm(-1). These values represent significant increases over those displayed by the analogous Cr(III)-containing cluster. When perchlorate is used as a counteranion, [(Me(3)tacn)(2)(cyclam)NiMo(2)(CN)(6)](2+) crystallizes from water in a dimeric form with pairs of the linear clusters directly linked via hydrogen bonding. In this case, fitting the magnetic susceptibility data requires use of two coupling constants: one intramolecular with J = 14.9 cm(-1) and another intermolecular with J' = -1.9 cm(-1). Reacting [(Me(3)tacn)Mo(CN)(3)] with a large excess of [(cyclam)Ni(H(2)O)(2)](2+) produces a [(Me(3)tacn)(2)(cyclam)(3)(H(2)O)(2)Ni(3)Mo(2)(CN)(6)](6+) cluster possessing a zigzag structure that is a simple extension of the linear cluster geometry. Its magnetic behavior is consistent with weaker ferromagnetic coupling and an S = 6 ground state. Similar reactions employing an equimolar ratio of reactants afford related one-dimensional chains of formula [(Me(3)tacn)(cyclam)NiMo(CN)(3)](2+). Once again, the ensuing structure depends on the associated counteranions, and the magnetic behavior indicates ferromagnetic coupling. It is hoped that substitutions of the type exemplified here will be of utility in the design of new single-molecule magnets.     

Synthesis and characterization of a trigonal bipyramidal supramolecular cage based upon rhodium and platinum metal centers

The reaction of 4-ethynyl-pyridine with tert-butyl lithium followed by its addition to (Me3tacn)RhCl3 affords the facial octahedral complex (Me3tacn)Rh(CCPy)3, condensation of which with the square planar complex cis-(DCPE)Pt(NO3)2 results in a self-assembled trigonal bipyramidal cage with Rh(III) and Pt(II) atoms occupying the vertices.     

Kinetics and mechanism of hydrolysis of a model phosphate diester by [Cu(Me3tacn)(OH2)2]2+ (Me3tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane)

The kinetics of hydrolysis of bis(p-nitrophenyl)phosphate (BNPP) by [Cu(Me3tacn)(OH2)2]2+ has been studied by spectrophotometrical monitoring of the release of the p-nitrophenylate ion from BNPP. The reaction was followed for up to 8000 min at constant BNPP concentration (15 microM) and ionic strength (0.15 M) and variable concentration of complex (1.0-7.5 mM) and temperature (42.5-65.0 degrees C). Biphasic kinetic traces were observed, indicating that the complex promotes the cleavage of BNPP to NPP [(p-nitrophenyl)phosphate] and then cleavage of the latter to phosphate, the two processes differing in rate by 50-100-fold. Analysis of the more amenable cleavage of BNPP revealed that the rate of BNPP cleavage is among the highest measured for mononuclear copper(II) complexes and is slightly higher than that reported for the close analogue [Cu(iPr3tacn)(OH2)2]2+. Detailed analysis required the determination of the pKa for [Cu(Me3tacn)(OH2)2]2+ and the constant for the dimerization of the conjugate base to [(Me3tacn)Cu(OH)2Cu(Me3tacn)]2+ (Kdim). Thermodynamic parameters derived from spectrophotometric pH titration and the analysis of the kinetic data were in reasonable agreement. Second-order rate constants for cleavage of BNPP by [Cu(Me3tacn)(OH2)(OH)]+ and associated activation parameters were obtained from initial rate analysis (k = 0.065 M(-1) s(-1) at 50.0 degrees C, deltaH = 56+/-6 kJ mol(-1), deltaS = -95+/-18 J K(-1) mol(-1)) and biphasic kinetic analysis (k = 0.14 M(-1) s(-1) at 50.0 degrees C, deltaH = 55+/-6 kJ mol(-1), deltaS = -92+/-20 J K(-1) mol(-1)). The negative entropy of activation is consistent with a concerted mechanism with considerable associative character. The complex was found to catalyze the cleavage of BNPP with turnover rates of up to 1 per day. Although these turnover rates can be considered low from an application point of view, the ability of the complexes to catalyze phosphate ester cleavage is clearly demonstrated.     

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.     

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).     

Macrocyclic copper(II) and zinc(II) complexes incorporating phosphate esters

Slow evaporation of solutions prepared by adding either Cu(ClO(4))(2).6H(2)O or Zn(ClO(4))(2).6H(2)O to solutions containing appropriate proportions of Me(3)tacn (1,4,7-trimethyl-1,4,7-triazacyclononane) and sodium phenyl phosphate (Na(2)PhOPO(3)) gave dark blue crystals of [Cu(3)(Me(3)tacn)(3)(PhOPO(3))(2)](ClO(4))(2).(1)/(2)H(2)O (1) and colorless crystals of [Zn(2)(Me(3)tacn)(2)(H(2)O)(4)(PhOPO(3))](ClO(4))(2).H(2)O (2), respectively. Blue crystals of [Cu(tacn)(2)](BNPP)(2) (3) formed in an aqueous solution of [Cu(tacn)Cl(2)], bis(p-nitrophenyl phosphate) (BNPP), and HEPES buffer (pH 7.4). Compound 1 crystallizes in the triclinic space group P1 (No. 2) with a = 9.8053(2) A, b = 12.9068(2) A, c = 22.1132(2) A, alpha = 98.636(1) degrees, beta = 99.546(1) degrees, gamma = 101.1733(8) degrees, and Z = 2 and exhibits trinuclear Cu(II) clusters in which square pyramidal metal centers are capped by two phosphate esters located above and below the plane of the metal centers. The trinuclear cluster is asymmetric having Cu...Cu distances of 4.14, 4.55, and 5.04 A. Compound 2 crystallizes in the monoclinic space group P2(1)/c (No. 14) with a = 13.6248(2) A, b = 11.6002(2) A, c = 25.9681(4) A, beta = 102.0072(9) degrees, and Z = 4 and contains a dinuclear Zn(II) complex formed by linking two units of [Zn(Me(3)tacn)(OH(2))(2)](2+) by a single phosphate ester. Compound 3 crystallizes in the monoclinic space group C2/c (No. 15) with a = 24.7105(5) A, b = 12.8627(3) A, c = 14.0079(3) A, beta = 106.600(1) degrees, and Z = 4 and consists of mononuclear [Cu(tacn)(2)](2+) cations whose charge is balanced by the BNPP(-) anions.     

A cyano-bridged single-molecule magnet: slow magnetic relaxation in a trigonal prismatic MnMo(6)(CN)(18) cluster

We report the synthesis of the first well-documented example of a cyano-bridged single-molecule magnet. An assembly reaction parallel to that employed in producing the trigonal prismatic [(Me(3)tacn)(6)MnCr(6)(CN)(18)](2+) (Me(3)tacn = N,N',N"-trimethyl-1,4,7-triazacyclononane) cluster affords K[(Me(3)tacn)(6)MnMo(6)(CN)(18)](ClO(4))(3) (1), containing an analogous molybdenum(III)-substituted cluster. Fits to the DC magnetic susceptibility and magnetization data for 1 show that the MnMo(6) cluster possesses weak antiferromagnetic coupling (J = -6.7 cm(-1)), leading to an S = (13)/(2) ground state with significantly enhanced magnetic anisotropy (D = -0.33 cm(-1) and E = -0.018 cm(-1)). Consistent with these results, AC magnetic susceptibility measurements show the molecule to exhibit slow magnetic relaxation indicative of a single-molecule magnet with an energy barrier of 10 cm(-1) for spin reversal.     

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.     

Symmetry-based magnetic anisotropy in the trigonal bipyramidal cluster [Tp2(Me3tacn)3Cu3Fe2(CN)6]4+

Reaction of [(Me3tacn)Cu(H2O)2]2+ (Me3tacn = N,N',N' '-trimethyl-1,4,7-triazacyclononane) with [TpFe(CN)3]- (Tp- = hydrotris(pyrazolyl)borate) in a mixture of ethanol and acetonitrile affords the pentanuclear cluster [Tp2(Me3tacn)3Cu3Fe2(CN)6]4+. Single-crystal X-ray analysis reveals a trigonal bipyramidal structure featuring a D3h-symmetry core in which two opposing FeIII (S = 1/2) centers are linked through cyanide bridges to an equatorial triangle of three CuII (S = 1/2) centers. Fits to variable-temperature dc magnetic susceptibility data are consistent with ferromagnetic coupling to give an S = 5/2 ground state, while fits to low-temperature magnetization data indicate the presence of a large axial zero-field splitting (D = -5.7 cm-1). Frequency dependence observed in the ac magnetic susceptibility data confirms single-molecule magnet behavior, with an effective spin reversal barrier of Ueff = 16 cm-1. When compared with the much lower anisotropy barrier previously observed for the face-centered cubic cluster [Tp8(H2O)6Cu6Fe8(CN)6]4+, the results demonstrate the enormous influence of the geometry in which a given set of metal ions are arranged.     

Nucleophilic reactivity and oxo/sulfido substitution reactions of MVIO3 Groups (M = Mo, W)

The nucleophilic reactivity of oxo ligands in the groups M(VI)O(3) in the trigonal complexes [(Me(3)tacn)MO(3)] (M = Mo (1), W (10)) and [(Bu(t)(3)tach)MO(3)] (M = Mo (5), W (14)) has been investigated. Complexes 1/10 can be alkylated with MeOTf to give [(Me(3)tacn)MO(2)(OMe)](1+) (2/11), silylated with Pr(i)(3)SiOTf to form [(Me(3)tacn)MO(2)(OSiPr(i)(3))](+) (3/12), and protonated with HOTf to yield [(Me(3)tacn)MoO(2)(OH)](+) (4). Similarly, complexes 5/14 can be silylated to [(Bu(t)(3)tach)MO(2)(OSiPr(i)(3))](+) (6/15) and protonated to [(Bu(t)(3)tach)MO(2)(OH)](+) (7/16). Products were isolated as triflate salts in yields exceeding 70%. When excess acid was used, the dinuclear mu-oxo species [(Bu(t)(3)tach)(2)M(2)O(5)](2+) (8/17) were obtained. X-ray structures are reported for 2-4, 6-8, 12, and 15-17. All mononuclear complexes have dominant trigonal symmetry with a rhombic distortion owing to a M[bond]OR bond (R = Me, SiPr(i)(3), H), which is longer than M[double bond]O oxo interactions; the latter exert a substantial trans influence on M[bond]N bond lengths. Oxo ligands in 5/14 undergo replacement with sulfide. Lawesson's reagent effects formation of [(Bu(t)(3)tach)MS(3)] (9/18), 14 with excess B(2)S(3) yields incompletely substituted [(Bu(t)(3)tach)WOS(2)] (20), and 5 with excess B(2)S(3) yields [(Bu(t)(3)tach)Mo(IV)O(S(4))] (19). The structures of 9, 19, and 20 are reported. Precedents for M(VI)S(3) groups in five- and six-coordinate molecules are limited. This investigation is the first detailed study of the behavior of M(VI)O(3) groups in nucleophilic and oxo/sulfido substitution reactions and should be useful in synthetic approaches to the active sites of the xanthine oxidase enzyme family and of certain tungstoenzymes. (Bu(t)(3)tach = 1,3,5-tri-tert-butyl-1,3,5-triazacyclohexane, Me(3)tacn = 1,4,7-trimethyl-1,4,7-triazacyclonane; OTf = triflate).     

Synthesis, X-ray crystal structures, magnetism, and phosphate ester cleavage properties of copper(II) complexes of N-substituted derivatives of 1,4,7-triazacyclononane

Two new N-substituted derivatives of the 1,4,7-triazacyclononane (tacn) macrocycle, 1-benzyl-4,7-dimethyl-1,4,7-triazacyclononane (L2) and 1,4,7-tris(3-cyanobenzyl)-1,4,7-triazacyclononane (L3), have been prepared and, together with 1,4-dimethyl-1,4,7-triazacyclononane (L1), have been used to synthesize the corresponding hydroxo-bridged binuclear copper (II) complexes, [Cu2(mu-OH)2L2](ClO4)2.xH2O (1 L = L1, x = 0; 2 L = L2, x = 1; 3 L = L3, x = 2). The X-ray crystal structures of all three complexes reveal the presence of [Cu2(mu-OH)2]2+ cores capped by pairs of facially coordinating tacn ligands so that the Cu(II) centers reside in distorted square pyramidal coordination environments. Variable-temperature magnetic susceptibility measurements indicate weak antiferromagnetic coupling (J = -36.4 cm(-1)) between the Cu(II) centers in 1, while the centers in 2 and 3 have been shown to interact ferromagnetically (J = 11.2 and 49.3 cm(-1), respectively). The variation in the strength and sign of these interactions has been rationalized in terms of the differing geometries of the [Cu2(mu-OH)2]2+ cores. The ability of the Cu(II) complexes to cleave phosphate ester bonds has been probed using the model phosphate ester bis(4-nitrophenyl)phosphate (BNPP) at pH 7.4 and a temperature of 50 degrees C. The measured rate constant for 3 (3 x 10(-4) s(-1)) is significantly greater than those previously reported for the Cu(II) complexes of the fully alkylated tacn ligands, Me3tacn and iPr3tacn, which until now have been rated as the most effective tacn-based phosphate ester cleavage agents.     

Amidation of unfunctionalized hydrocarbons catalyzed by ruthenium cyclic amine or bipyridine complexes

Selective amidation of simple hydrocarbons with pre-isolated and in-situ formed iminoiodanes catalyzed by ruthenium complexes [Ru(III)(Me(3)tacn)(CF(3)CO(2))(3).H(2)O] (2b, Me(3)tacn = N,N', N"-trimethyl-1,4,7-triazacyclononane) and cis-[Ru(II)(6, 6'-Cl(2)bpy)(2)Cl(2)] (3, 6,6'-Cl(2)bpy = 6,6'-dichloro-2, 2'-bipyridine) was investigated. With PhI=NTs as nitrogen source, both catalysts efficiently promote the amidation of adamantane, cyclohexene, ethylbenzene, cumene, indan, tetralin, and diphenylmethane to afford N-substituted sulfonamides in 80-93% yields with high selectivity. Competitive amidations of para-substituted ethylbenzenes and kinetic isotope effect for the amidation of cyclohexene/cyclohexene-d(10) suggest that the amidation processes probably proceed via the hydrogen abstraction by a reactive Ru=NTs species to form a carboradical intermediate. The amidation with PhI(OAc)(2)/TsNH(2) gave results comparable to those obtained with PhI=NTs. Extension of the "PhI(OAc)(2)/TsNH(2) + catalyst 2b or 3" protocol to MeSO(2)NH(2) and PhCONH(2) with ethylbenzene as substrate produced the corresponding N-substituted amides in up to 89% yield.     

Syntheses, structures, and magnetic properties of low-dimensional heterometallic complexes based on the versatile building block [(Tp)Cr(CN)3]-

Using the tricyano precursor (Bu(4)N)[(Tp)Cr(CN)(3)] (Bu(4)N(+) = tetrabutylammonium cation; Tp = tris(pyrazolyl)hydroborate), a pentanuclear heterometallic cluster [(Tp)(2)Cr(2)(CN)(6)Cu(3)(Me(3)tacn)(3)][(Tp)Cr(CN)(3)](ClO(4))(3)·5H(2)O (1, Me(3)tacn = N,N',N'-trimethyl-1,4,7-triazacyclononane), three tetranuclear heterometallic clusters [(Tp)(2)Cr(2)(CN)(6)Cu(2)(L(OEt))(2)]·2.5CH(3)CN (2, L(OEt) = [(Cp)Co(P(O)(OEt)(2))(3)], Cp = cyclopentadiene), [(Tp)(2)Cr(2)(CN)(6)Mn(2)(L(OEt))(2)]·4H(2)O (3), and [(Tp)(2)Cr(2)(CN)(6)Mn(2)(phen)(4)](ClO(4))(2) (4, phen = phenanthroline), and a one-dimensional (1D) chain polymer [(Tp)(2)Cr(2)(CN)(6)Mn(bpy)](n) (5, bpy = 2,2'-bipyridine) have been synthesized and structurally characterized. Complex 1 shows a trigonal bipyramidal geometry in which [(Tp)Cr(CN)(3)](-) units occupy the apical positions and are linked through cyanide to [Cu(Me(3)tacn)](2+) units situated in the equatorial plane. Complexes 2-4 show similar square structures, where Cr(III) and M(II) (M = Cu(II) or Mn(II)) ions are alternatively located on the rectangle corners. Complex 5 consists of a 4,2-ribbon-like bimetallic chain. Ferromagnetic interactions between Cr(III) and Cu(II) ions bridged by cyanides are observed in complexes 1 and 2. Antiferromagnetic interactions are presented between Cr(III) and Mn(II) ions bridged by cyanides in complexes 3-5. Complex 5 shows metamagnetic behavior with a critical field of about 22.5 kOe at 1.8 K.     

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).     

Alkene cis-dihydroxylation by [(Me3tacn)(CF3CO2)RuVI O2)]ClO4(Me(3)tacn = 1,4,7-Trimethyl-1,4,7-triazacyclononane): structural characterization of [3 + 2] cycloadducts and kinetic studies

cis-Dioxoruthenium(VI) complex [(Me(3)tacn)(CF(3)CO(2))Ru(VI)O(2)]ClO(4) (1, Me(3)tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane) reacted with alkenes in aqueous tert-butyl alcohol to afford cis-1,2-diols in excellent yields under ambient conditions. When the reactions of 1 with alkenes were conducted in acetonitrile, oxidative C=C cleavage reaction prevailed giving carbonyl products in >90% yields without any cis-diol formation. The alkene cis-dihydroxylation and C=C cleavage reactions proceed via the formation of a [3 + 2] cycloadduct between 1 and alkenes, analogous to the related reactions with alkynes [Che et al. J. Am. Chem. Soc. 2000, 122, 11380]. With cyclooctene and trans-beta-methylstyrene as substrates, the Ru(III) cycloadducts (4a) and (4b) [formula; see text] were isolated and structurally characterized by X-ray crystal analyses. The kinetics of the reactions of 1 with a series of p-substituted styrenes has been studied in acetonitrile by stopped-flow spectrophotometry. The second-order rate constants varied by 14-fold despite an overall span of 1.3 V for the one-electron oxidation potentials of alkenes. Secondary kinetic isotope effect (KIE) was observed for the oxidation of beta-d(2)-styrene (k(H)/k(D) = 0.83 +/- 0.04) and alpha-deuteriostyrene (k(H)/k(D) = 0.96 +/- 0.03), which, together with the stereoselectivity of cis-alkene oxidation by 1, is in favor of a concerted mechanism.