Preparation and Protonation Studies of trans-Dioxorhenium(V) Complexes with Imidazoles

Two rhenium compounds with benzimidazole ligands: synthesis and DNA interactions

Two rhenium compounds: cis-tetrachlorotetrabenzimidazoldirhenium(iii) chloride - I and tetrabenzimidazoldioxorhenium(v) - II have been synthesized and characterized. X-ray data are presented for the new complex II. I and II show strong emission that has been used to investigate their interaction with several non-canonical DNA structures. Both compounds have a quenching effect on the fluorescence intensity upon addition of the investigated oligonucleotides; I was more selective for binding G4-than II. Association constant values obtained for I and II with G-quadruplexes reached 106 M-1, which suggests a strong interaction between both complexes and these sequences. FRET-melting assays show that I and II have a rather high level of stabilization of ckit1 and ckit2 quadruplexes. I is toxic against macrophages RAW267.7 only in high concentrations, while complex II shows no toxicity against these cells. I and II accumulate inside cells in different degrees. Molecular dynamic simulation studies have provided insights into the binding modes of II with ckit1 and ckit2 G-quadruplexes. The results obtained show the DNA binding activity of the rhenium complexes and their ability to be players in the anti-cancer fight since they can bind to non-canonical DNA forms in oncogene promoters, accumulate in some cancer cells, and influence the cancer cells microenvironment.

Reactivity umpolung (reversal) of ligands in transition metal complexes

The success and power of homogeneous catalysis derives in large part from the wide choice of transition metal ions and their ligands. This tutorial review introduces examples where the reactivity of a ligand is completely reversed (umpolung) from Lewis basic/nucleophilic to acidic/electrophilic or vice versa on changing the metal and co-ligands. Understanding this phenomenon will assist in the rational design of catalysts and the understanding of metalloenzyme mechanisms. Labelling a metal and ligand with Seebach donor and acceptor labels helps to identify whether a reaction involving the intermolecular attack on the ligand is displaying native reactivity or reactivity umpolung. This has been done for complexes of nitriles, carbonyls, isonitriles, dinitrogen, Fischer carbenes, alkenes, alkynes, hydrides, methyls, methylidenes and alkylidenes, silylenes, oxides, imides/nitrenes, alkylidynes, methylidynes, and nitrides. The electronic influence of the metal and co-ligands is discussed in terms of the energy of (HOMO) d electrons. The energy can be related to the pKLACa (LAC is ligand acidity constant) of the theoretical hydride complexes [H-[M]-L]+ formed by the protonation of pair of valence d electrons on the metal in the [M-L] complex. Preliminary findings indicate that a negative pKLACa indicates that nucleophilic attack by a carbanion or amine on the ligand will likely occur while a positive pKLACa indicates that electrophilic attack by strong acids on the ligand will usually occur when the ligand is nitrile, carbonyl, isonitrile, alkene and η6-arene.

trans-Tetrakis(4-methylpyridine-κN)dioxidorhenium(V) hexafluoridophosphate

The title compound, [ReO₂(C₆H₇N)₄]PF₆, contains octa­hedral [ReO₂(4-Mepy)₄]⁺ cations (4-Mepy is 4-methyl­pyridine) and PF₆⁻ anions. Both the cation and the anion reside on special positions, the Re atom on a crystallographic center of inversion and the P atom on a C₂ axis parallel to the b axis. The Re^V atom in the cation exhibits an octa­hedral coordination geometry with two O atoms in the apical positions and four N atoms of the 4-Mepy ligands in the equatorial plane. The Re=O and Re—N bond lengths fall in the typical ranges of trans-dioxidorhenium(V) complexes.

Binding of the Oxo−Rhenium(V) Core to Methionine and to N-Terminal Histidine Dipeptides

The ReOX(2)(met) compounds (X = Cl, Br) adopt a distorted octahedral structure in which a carboxylato oxygen lies trans to the Re=O bond, whereas the equatorial plane is occupied by two cis halides, an NH(2), and an SCH(3) group. Coordination of the SCH(3) unit creates an asymmetric center, leading to two diastereoisomers. X-ray diffraction studies reveal that the crystals of ReOBr(2)(d,l-met).1/2H(2)O and ReOBr(2)(d,l-met).1/2CH(3)OH contain only the syn isomer (S-CH(3) bond on the side of the Re=O bond), whereas ReOCl(2)(d-met) and ReOCl(2)(d,l-met) consist of the pure anti isomer. (1)H NMR spectroscopy shows that both isomers coexist in equilibrium in acetone (anti/syn ratio = 1:1 for X = Br, 3:1 for X = Cl). Exchange between these two isomers is fast above room temperature, but it slows down below 0 degrees C, and the sharp second-order spectra of both isomers at -20 degrees C were fully assigned. The coupling constants are consistent with the solid-state conformations being retained in solution. Complexes of the type [ReOX(2)(His-aa)]X (X = Cl, Br) are isolated with the dipeptides His-aa (aa = Gly, Ala, Leu, and Phe). X-ray diffraction work on [ReOBr(2)(His-Ala)]Br reveals the presence of distorted octahedral cations containing the Re=O(3+) core and a dipeptide coordinated through the histidine residue via the imidazole nitrogen, the terminal amino group, and the amide oxygen, the site trans to the Re=O bond being occupied by the oxygen. The alanine residue is ended by a protonated carboxylic group that does not participate in the coordination. The constant pattern of the(1)H NMR signals for the protons in the histidine residue confirms that the various dipeptides adopt a similar binding mode, consistent with the solid-state structure being retained in CD(3)OD solution.

Preparation and electronic properties of rhenium(V) complexes with bis(diphenyl phosphino)ethane

Complexes of bis(diphenylphosphino)ethane (dppe) of the types ReOX3(dppe) and ReO(OR)X2(dppe) (with X= Cl or Br, and R = Me, Et, Pr, Ph, cyclohexyl (Cy), or -CH2CH2OH) were prepared to evaluate the influence of ligand changes on the low-energy d–d transitions in these Re(V) low-spin d2 systems. Arylimido compounds Re(NR)Cl3(dppe) (with R = Ph and p-ClC6H4) were also obtained. X-ray diffraction studies on ReO(OPr)Cl2(dppe), ReO(OPh)Br2(dppe), and ReO(OCy)Cl2(dppe) confirmed the presence of the trans O=Re-OR unit and showed that the structural characteristics of the Re-O-R segment are not affected by the presence of a bulky cyclohexyl substituent or an aromatic phenyl group. The structure of the arylimido complex Re(p-ClC6H4N)Cl3(dppe) was also determined. The electronic absorption spectra of the ReOX3(dppe) compounds include two low-energy components at ~11 500 and ~16 000 cm–1, assigned to the two spin-allowed d–d transitions expected for these low-symmetry systems. Substitution of the oxo ligand by an arylimido group has little effect on the lower-energy component, but moves the two components closer together. Replacing the halogen trans to the Re=O bond by an alkoxo group shifts the whole system to higher energies. These variations were found to correlate well with the energies of the frontier orbitals determined from DFT calculations. While attempting to prepare ReOCl3(dppe) from ReOCl3(PPh3)2, the Re(III) compound fac-ReCl3(PPh3){Ph2PC2H4PPh2(O)} was obtained, in which one end of dppe had become a phosphine oxide. Upon standing in DMSO, this compound gave the octahedral compound ReCl4{Ph2PC2H4PPh2(O)}, in which the formation of a chelate ring involving a phosphine and a phosphine oxide was ascertained by X-ray diffraction.Key words: rhenium, crystal structure, DFT calculations, d–d electron transitions.

Oxorhenium(V) complexes with the non-sulfur-containing amino acid histidine

Equivalent amounts of ReOX(3)(OPPh(3))(Me(2)S) (where X = Cl, Br) and L-histidine (L-hisH) in acetonitrile yield ReOX(2)(L-his), in which the amino acid monoanion is N,N,O-tridentate. X-ray diffraction work on both compounds shows that the three donors occupy a face in a distorted octahedron and the carboxylate oxygen is coordinated trans to the Re=O bond. The 2:1 complex [ReO(L-his)(2)]I is obtained by reacting 2 equiv of L-histidine with ReO(2)I(PPh(3))(2) in methanol in the presence of NaOCH(3). (1)H NMR spectroscopy indicates that these complexes contain one N,N,O-tridentate histidine anion coordinated as above and one N,N-bidentate histidine anion, whose carboxylate group is free. By refluxing ReOX(2)(L-his) in methanol, the carboxylic groups esterify and two octahedral units condense into an oxo-bridged dinuclear complex [ReOX(2)(L-hisMe)](2)O containing N,N-bidentate histidine methyl ester. The O=Re-O-Re=O backbone is approximately linear, and the two ReOX(2)(L-hisMe) units are related by a 2-fold axis through the central oxygen. Crystals of [ReOBr(2)(L-hisMe)](2)O consist of an ordered phase containing two of the possible diastereoisomers in a 1:1 ratio. (1)H NMR spectra of these crystals include two sets of signals, consistent with the presence of two isomers with C(2) symmetry, and the spectra of the nonrecrystallized material confirm that these are the only two isomers formed.

Steric control of substituted phenoxide ligands on product structures of uranyl aryloxide complexes

A series of uranyl aryloxide complexes has been prepared via metathesis reactions between [UO(2)Cl(2)(THF)(2)](2) and di-ortho-substituted phenoxides. Reaction of 4 equiv of KO-2,6-(t)()Bu(2)C(6)H(3) with [UO(2)Cl(2)(THF)(2)](2) in THF produces the dark red uranyl compound, UO(2)(O-2,6-(t)()Bu(2)C(6)H(3))(2)(THF)(2).THF, 1. Single-crystal X-ray diffraction analysis of 1 reveals a monomer in which the uranium is coordinated in a pseudooctahedral fashion by two apical oxo groups, two cis-aryloxides, and two THF ligands. A similar product is prepared by reaction of KO-2,6-Ph(2)C(6)H(3) with [UO(2)Cl(2)(THF)(2)](2) in THF. Single-crystal X-ray diffraction analysis of this compound reveals it to be the trans-monomer UO(2)(O-2,6-Ph(2)C(6)H(3))(2)(THF)(2), 2. Dimeric structures result from the reactions of [UO(2)Cl(2)(THF)(2)](2) with less sterically imposing aryloxide salts, KO-2,6-Cl(2)C(6)H(3) or KO-2,6-Me(2)C(6)H(3). Single-crystal X-ray diffraction analyses of [UO(2)(O-2,6-Cl(2)C(6)H(3))(2)(THF)(2)](2), 3, and [UO(2)Cl(O-2,6-Me(2)C(6)H(3))(THF)(2)](2), 4, reveal similar structures in which each U atom is coordinated by seven ligands in a pseudopentagonal bipyramidal fashion. Coordinated to each uranium are two apical oxo groups and five equatorial ligands (3, one terminal phenoxide, two bridging phenoxides, and two nonadjacent terminal THF ligands; 4, one terminal chloride, two bridging phenoxides, and two nonadjacent terminal THF ligands). Apparently, the phenoxide ligand steric features exert a greater influence on the solid-state structures than the electronic properties of the substituents. Emission spectroscopy has been utilized to investigate the molecularity and electronic structure of these compounds. For example, luminescence spectra taken at liquid nitrogen temperature allow for a determination of the dependence of the molecular aggregation of 3 on the molecular concentration. Electronic and vibrational spectroscopic measurements have been analyzed to examine trends in emission energies and stretching frequencies. However, comparison of the data for compounds 1-4 reveals that the innate electron-donating capacity of phenoxide ligands is only subtly manifest in either the electronic or vibrational energy distributions within these molecules.

Rhenium oxo complexes of a chelating diyne ligand. Synthesis and study of the kinetics of protonation

A series of oxo complexes, Re(O)X(diyne) (X = I, Me, Et), have been prepared from 2,7-nonadiyne and Re(O)I(3)(PPh(3))(2). Addition of B(C(6)F(5))(3) to Re(O)I(2,7-nonadiyne) (5) results in coordination of the oxo ligand to the boron. The protonation of Re(O)(X)(2-butyne)(2) and Re(O)(X)(2,7-nonadiyne)(2) with a variety of acids has been examined. With 5 and HBF(4)/Et(2)O, the ultimate product was [Re(CH(3)CN)(3)(I)(2,7-nonadiyne)](2+) (7). The conversion of 5 to 7 changes the conformation of the diyne ligand from a "chair" to a "boat" and shifts its propargylic protons considerably downfield in the (1)H NMR. The kinetics of the protonation of Re(O)I(2,7-nonadiyne) (5) by CF(3)SO(3)H in CH(3)CN have been monitored by visible spectroscopy, in a stopped-flow apparatus, and by low temperature (1)H NMR. Two second-order rate constants, presumably successive protonations, were observed in the stopped-flow, k(1) = 11.9 M(-)(1) s(-)(1) and k(2) = 3.8 M(-)(1) s(-)(1). Low temperature (1)H NMR spectroscopy indicated that the resulting solution contained a mixture of two doubly protonated intermediates X and Y, each of which slowly formed the product 7 via an acid-independent process.

Preparations, characterizations, and structures of (biimidazole)dihalobis(triphenylphosphine)rhenium(III) salts: strong ion-pairing and acid-base properties

Two-electron reduction occurs when the Re(V) precursors ReOX3(PPh3)2 and ReO(OEt)X2(PPh3)2 are reacted with biimidazole (biimH2) in boiling chloroform, affording rhenium(III) cationic complexes of the type cis,trans-[ReX2(PPh3)2(biimH2)]X with X = Cl, Br, and I. Crystal structures are determined for the compounds with the three halogens, as well as for the [ReCl2(PPh3)2(biimH2)](benzoate) salt. In all cases, the counterion is attached to the complex cation via hydrogen bonding with the N-H groups of coordinated biimidazole. Variable-temperature 1H NMR spectroscopy shows that a mixture of [ReCl2(PPh3)2(biimH2)](benzoate) and [ReCl2(PPh3)2(biimH2)]Cl is in slow exchange below -50 degrees C in CD2Cl2, indicating that ion pairing is retained in solution. Both N-H groups can be deprotonated with sodium methoxide, and their acidities are evaluated from UV-visible spectra. Competition between monodeprotonated [ReCl2(PPh3)2(biimH)] and various carboxylic acids reveals that the acidity of the first N-H proton corresponds to that of acetic acid (pKa(aq) approximately 4.8). By a similar competitive reaction between bis-deprotonated [ReCl2(PPh3)2(biim)]- and phenols, the second acidity is estimated to be close to that of phenol (pKa(aq) approximately 9.8).

Re(V) complexes with an open-chain quadridentate ligand containing two amine and two amido donors. Synthesis, characterization, and solution equilibria of Re2O3(dioxo-tetH4)2 and [ReO(H2O)(dioxo-tetH4)]Cl (dioxo-tetH6 = 1,4,8,11-tetraazaundecane-5,7-dione)

We are interested in identifying mononuclear cationic [M(V)=O]3+ (M = Tc, Re) complexes for radiopharmaceutical applications. The open-chain ligand, 1,4,8,11-tetraazaundecane-5,7-dione-(dioxo-tetH6) with two amine and two amide donors, was selected for investigation since the literature led us to expect that a five-coordinate [Re(V)=O(dioxo-tetH4)]+ cation would dominate. Instead, the neutral mu-oxo bridged dinuclear complex, Re2O3(dioxo-tetH4)2 (1), and a salt of the six-coordinate mononuclear cation, [ReO(H2O)(dioxo-tetH4)]+ (2), were isolated; the structure of each was determined by X-ray crystallography. The cation (2) is unusual because it has a trans-oxo/aqua core. Such aqua compounds are rarely isolated, and the Re-OH2 distance is relatively short (2.185 A). The cation has two pKa values, 4.1 and 8.7, determined with visible spectroscopy. Since the Re-OH2 bond is short, the coordinated water is likely to be acidic. Thus the two pKa's are assigned to the stepwise deprotonation of the water ligand to give a trans-oxo/hydroxo neutral form and a trans-dioxo anion. Although 1 was the first product isolated following ligand exchange of ReOCl3(Me2S)(OPPh3) with dioxo-tetH6 under neutral conditions, it probably formed from the hydroxo mononuclear complex. Under concentrated conditions (approximately 300 mM) the dinuclear complex deposited from solution, but the 1H NMR spectra of 2 (approximately 20 mM) were consistent with the presence of only monomeric forms in D2O, pH 3-12. 1H NMR experiments demonstrated that in DMSO-d6 2 converts to 1 upon addition of base, consistent with the proposal that two units of the hydroxo monomer condense to give the dinuclear form. In addition, all spectra of pure 1 dissolved in DMSO-d6 included extra low intensity signals that were characteristic of the monomer. Thus, although 1 is favored over the neutral monomer in DMSO-d6, the two complexes exist as a mixture of equilibrating forms. Our results do not support the previous findings for the Re(V) complex with a macrocyclic diamine-diamide ligand related to dioxo-tetH6. The data indicate that the ability of an amido group to donate electron density to a Re(V) center is moderately greater than the donating ability of a neutral amine group.

Cationic Re(V) oxo complexes with poly(pyrazolyl)borates: synthesis, characterization, and stability

Cationic Re(V) oxo compounds of the type [ReO(OSiMe3)(eta 2-B(pz)4)(L)2]X [X = Cl, L = 4-(NMe2)C5H4N (1), 1-Meimz (1-methylimidazole; 2), 1/2 dmpe (1,2-bis(dimethylphosphino)ethane; 3), py (4a); X = I, L = py (4b)] can be prepared by reacting trans-[ReO2(eta 2-B(pz)4)(L)2] with XSiMe3. In solution, cations 1-4 are reactive species, and those with unidentate nitrogen donor ligands (1, 2, and 4) rearrange into the neutral derivatives [ReO(Cl)(OSiMe3)(eta 2-B(pz)4)(L)] [L = py (5), 4-(NMe2)C5H4N (6), 1-Meimz (7)], which are also reported herein. Compounds 1-3 and 5-7 have been fully characterized by the usual spectroscopic techniques, which in some cases includes X-ray crystallographic analysis (3, 6, and 7). Compound 3 crystallizes from CH2Cl2/n-hexane as yellow crystals with one molecule of CH2Cl2 solvent, and compounds 6 and 7 crystallize from THF/n-hexane as violet and red crystals, respectively, with one molecule of THF solvent in the case of 6. Crystallographic data: 3, orthorhombic space group Pn2(1)a, a = 11.311(2) A, b = 19.135(2) A, c = 15.443(2) A, V = 3342.4(8) A3, Z = 4; 6, triclinic space group P1, a = 8.7179(11) A, b = 12.5724(8) A, c = 17.750(2) A, alpha = 70.454(7) degrees, beta = 77.935(9) degrees, gamma = 77.129(8) degrees, V = 1768.1(3) A3, Z = 2; 7, monoclinic space group P2(1)/c, a = 16.356(2) A, b = 20.384(3) A, c = 17.360(3) A, beta = 106.971(12) degrees, V = 5535.8(14) A3, Z = 8.

Basicity of uranyl oxo ligands upon coordination of alkoxides

Uranium(VI) alkoxide complexes are prepared via metathesis reactions of [UO2Cl2(THF)2]2 with potassium alkoxides in nonaqueous media. The dark red compound U[OCH2C(CH3)3]6, 1, results from redistributive exchange of oxo and neopentoxide ligands between more than one uranium species. Single-crystal X-ray diffraction analysis of 1 reveals a monomer in which the uranium is coordinated in a pseudooctahedral fashion by six neopentoxide ligands. Imposition of steric congestion at the metal center prevents oxo-alkoxide ligand exchange in the reactions using more sterically demanding alkoxides. Simple metathesis between uranyl chloride and alkoxide ligands occurs in the synthesis of golden yellow-orange UO2(OCHPh2)2(THF)2, 2, and yellow UO2[OCH(tBu)Ph]2(THF)2, 3. Single-crystal X-ray diffraction analysis of 2 reveals a monomer in which the uranium is coordinated in a pseudooctahedral fashion by two apical oxo ligands, two diphenylmethoxide ligands occupying trans positions, and two tetrahydrofuran ligands. Coordination of diisopropylmethoxide allows for synthesis of a more complex binary alkoxide system. Single-crystal X-ray diffraction analysis of watermelon red [UO2(OCH(iPr)2)2]4, 4, reveals a tetramer in which each uranium is coordinated in a pseudooctahedral fashion by two apical oxo ligands, one terminal alkoxide, two bridging alkoxide ligands, and one bridging oxo ligand from a neighboring uranyl group. These compounds are characterized by elemental analysis, 1H NMR, infrared spectroscopy, and, for 1, 2, and 4, single-crystal X-ray diffraction analysis. Luminescence spectroscopy is employed to evaluate the extent of aggregation of compounds 2-4 in various solvents. Vibrational spectroscopic measurements of 2-4 imply that, in contrast to the case of uranyl complexes prepared in aqueous environments, coordination of relatively strongly donating alkoxide ligands allows for enhancement of electron density on the uranyl groups such that the uranyl U=O bonds are weakened. Crystal data are as follows. 1: monoclinic space group C2/m, a = 10.6192(8) A, b = 18.36(1) A, c = 10.6151(8) A, beta = 109.637(1) degrees, V = 1949.1(3) A3, Z = 2, dcalc = 1.297 g cm-3. Refinement of 2065 reflections gave R1 = 0.045. 2: monoclinic space group P2(1)/c, a = 6.1796(4) A, b = 15.669(1) A, c = 16.169(1) A, beta = 95.380(1) degrees, V = 1558.7(2) A3, Z = 2, dcalc = 1.664 g cm-3. Refinement of 3048 reflections gave R1 = 0.036. 4: tetragonal space group I4, a = 17.8570(6) A, b = 17.8570(6) A, c = 11.4489(6) A, V = 3650.7(3) A3, Z = 2, dcalc = 1.821 g cm-3. Refinement of 1981 reflections gave R1 = 0.020.

Preparation and characterization of oxorhenium(V) complexes with 2,2'-biimidazole: the strong affinity of coordinated biimidazole for chloride ions via N-H...Cl- hydrogen bonding

N,N'-Dimethylbiimidazole and bipyridine (N-N) react with ReOCl3(OPPh3)(Me2S) to give mer-ReOCl3(N-N) compounds. Nonmethylated biimidazole forms a trans-O,O [ReOCl2(OPPh3)(biimH2)]+ cation, which is tightly associated with the Cl- counterion via N-H...Cl- hydrogen bonding. Hydrolysis of ReOCl3(biimMe2) in wet acetone (5% water) leads to the linear oxo-bridged dinuclear species [(OReCl2(biimMe2)2(mu-O)] containing chelated biimMe2. Acetone solutions containing only 1% water yield the bent oxo-bridged dinuclear species [(OReCl2)2(mu-O)(mu-biimMe2)2], where each Re center retains the ReO2Cl2N2 coordination but the biimMe2 ligands are bridging. The linear oxo-bridged [(OReCl2(biimH2)2(mu-O)] complex obtained with nonmethylated biimidazole includes two Cl- ions held via N-H...Cl- hydrogen bonds, leading to a dianionic [(OReCl2(biimH2...Cl)2(mu-O)]2- unit in the crystals of the PPh4+ salt. The compounds are characterized by IR and NMR spectroscopies, and the structures of [ReOCl2(OPPh3)(biimH2)]Cl, [(OReCl2(biimH2)2(mu-O)](PPh4Cl)(2).2H2O, and [(OReCl2)2(mu-O)(mu-biimMe2)2].acetone are determined by X-ray diffraction.

Neutral trans-Dioxorhenium(V) Complexes with the Anionic Tetrakis(pyrazolyl)borate Ligand

Reduction of [ReO(3){eta(3)-B(pz)(4)}] (1) with PPh(3) in the presence of mono- or bidentate sigma-donor ligands (pyridines, imidazoles, or diphosphines) is a very convenient method for the synthesis of the neutral dioxorhenium(V) complexes: trans-[ReO(2){eta(2)-B(pz)(4)}(L)(2)] (L = py (3), 4-Mepy (4), 4-NMe(2)py (5), 1-MeImz (6)) and trans-[ReO(2){eta(2)-B(pz)(4)}(P&arcraise;P)] (P&arcraise;P = dmpe (7), dppe (8)). In the presence of pyridine or dimethylphosphinoethane, the analogous [ReO(3){eta(3)-HB(pz)(3)}] is also reduced by PPh(3) to trans-[ReO(2){eta(2)-HB(pz)(3)}(py)(2)] (9) and trans-[ReO(2){eta(2)-HB(pz)(3)}(dmpe)] (10), respectively. In contrast, the reaction of [ReO(2)(py)(4)]Cl with K[B(pz)(4)] leads to a mixture of species from which were identified the neutral mono-oxo complexes [ReO(eta(2)-N,O)(&mgr;-O)B(pz)(3)}(pz)(pzH)(2)] (11) and [ReO{(eta(2)-N,O)(&mgr;-O)B(pz)(3)}Cl(py)(2)] (12). Complexes 3-12 were characterized by different techniques, namely, IR, (1)H/(31)P{(1)H} NMR spectroscopies and X-ray crystallographic analysis (5, 10, and 11). Compound 5 crystallizes from dichloromethane/n-hexane as orange crystals containing 3 molecules of solvated CH(2)Cl(2) (crystal data: triclinic space group P&onemacr;, with cell parameters a = 10.907(2) Å, b = 11.113(1) Å, c = 16.922(2) Å, alpha = 97.91(1) degrees, beta = 102.37(1) degrees, gamma = 94.21(1) degrees, V = 1973(1) Å(3), Z = 2). Compound 10 crystallizes from dichloromethane/n-hexane as yellowish crystals containing one molecule of pzH (crystal data: orthorhombic space group Pnma, with cell parameters a = 18.422(2) Å, b = 11.850(1) Å, c = 11.434(1) Å, alpha = beta = gamma = 90 degrees, V = 2496.1(4) Å(3), Z = 4). Compound 11 crystallizes from dichloromethane/n-hexane in the monoclinic space group P2(1)/n, with cell parameters a = 10.890(1) Å, b = 15.162(1) Å, c = 14.137(2) Å, beta = 102.07(1) degrees, V = 2282.6(4) Å(3), Z = 4.

Oxo Ligand Reactivity in the [ReO(2)L(4)](+) Complex of 1-Methylimidazole. Preparation and Crystal Structures of Salts Containing the ReOL(4)(3+) Core and Apical CH(3)O(-), BF(3)O(2)(-), and (CH(3)O)(2)PO(2)(-) Groups

The oxo group of [ReO(2)(1-MeIm)(4)](+) can be methylated with excess methyl trifluoromethanesulfonate in CH(2)Cl(2) under mild conditions, leading to [ReO(OCH(3))(1-MeIm)(4)](CF(3)SO(3))(2), which is converted to the B(C(6)H(5))(4)(-) and PF(6)(-) salts. When the reaction is carried out in methanol in the presence of excess PF(6)(-), the phosphate ester complex [ReO{OP(O)(OCH(3))(2)}(1-MeIm)(4)](PF(6))(2) is isolated. [ReO(2)(1-MeIm)(4)](+) in the presence of acid, 2,2-dimethoxypropane, and excess BF(4)(-) transforms to the oxo-boron adduct [ReO(OBF(3))(1-MeIm)(4)](+). The presence of the trans-O=Re-OR core was demonstrated by crystallographic studies on one compound of each type: [ReO(OCH(3))(1-MeIm)(4)](PF(6))(2), tetragonal, P4/ncc, Z = 4, a = 13.022(3) Å, c = 17.218(5) Å, R = 0.0271; [ReO{OP(O)(OCH(3))(2)}(1-MeIm)(4)](PF(6))(2).toluene, monoclinic, P2(1)/c, Z = 4, a = 14.165(4) Å, b = 13.052(6) Å, c = 20.931(6) Å, beta = 95.83(2) degrees, R = 0.0373; [ReO(OBF(3))(1-MeIm)(4)](I(3)), monoclinic, P2(1)/c, Z = 4, a = 13.373(4) Å, b = 13.237(3) Å, c = 16.689(6) Å, beta = 103.67(3) degrees, R = 0.0445. The IR spectra show nu(Re=O) vibrations near 960 cm(-)(1) in all cases. The solution UV-vis spectra of the CH(3)O(-) and BF(3)O(2)(-) compounds are similar to that of the parent oxo-hydroxo [ReO(OH)(1-MeIm)(4)](2+) species, whereas the visible spectrum of the blue phosphate ester compound resembles that of the oxo-aquo [ReO(OH(2))(1-MeIm)(4)](3+) ion. The compounds were also characterized by solution (1)H, (13)C, and (31)P NMR spectroscopy. This work confirms earlier conclusions that ReO(2)L(4)(+) units with good sigma-donor imidazole ligands are more resistant to ligand loss than the pyridine analogues.