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.
The syntheses, crystal structures, and electronic absorption spectra of the copper(I) and copper(II) complexes of 2,9-diphenyl-1,10-phenanthroline (dpp) are reported. The complex [Cu(dpp)(2)](PF(6)) (1) crystallizes in space group P2(1)/c with a = 11.081(4) Å, b = 25.491(8) Å, c = 14.263(5) Å, beta = 92.84(3) degrees, Z = 4, and V = 4024(2) Å(3). For 4813 unique data with F > 4.0sigma(F), R = 5.41% and R(w) = 6.43%. The coordination geometry about the copper(I) center in [Cu(dpp)(2)](+) is best described as distorted tetrahedral with approximate C(2) symmetry. The structure of [Cu(dpp)(2)](+) is largely determined by interligand pi-stacking interactions that occur between the phenyl groups of one ligand and the phenanthroline moiety of the other ligand. Solution-state absorption and (1)H NMR spectra indicate that the [Cu(dpp)(2)](+) complex is fluxional in solution, rocking between two enantiomeric structures of C(2) molecular symmetry through an intermediate of C(s)() symmetry. The complex [Cu(dpp)(2)](ClO(4))(2) (2) crystallizes in space group P&onemacr; with a = 7.809(3) Å, b = 13.027(6) Å, c = 20.344(10) Å, alpha = 87.68(4) degrees, beta = 89.16(4) degrees, gamma = 79.26(4) degrees, Z = 2, and V = 2032(1) Å(3). For 4943 unique data with F > 4.0sigma(F), R = 5.22% and R(w) = 5.37%. The coordination geometry about the copper(II) center in [Cu(dpp)(2)](2+) is best described as flattened tetrahedral with approximate D(2) symmetry. There are no interligand pi-stacking interactions in the structure of [Cu(dpp)(2)](2+). The four-coordinate geometry in [Cu(dpp)(2)](2+) persists in solution on the basis of solution-state and solid-state absorption spectroscopy. Structural distortion in the metal-to-ligand charge-transfer excited state of [Cu(dpp)(2)](+) is discussed on the basis of the structures of 1 and 2.
A novel N-heterocyclic tridentate polycarbene ligand has been synthesized and stabilized as its corresponding silver complex. An electronic structure investigation reveals significant π-bonding interactions within the Ag−carbene unit of the [(TIMEMe)2Ag3]3+ cation.
Metal-alkane complexes are believed to be key intermediates in C-H activation processes. The C-H σ bond of saturated hydrocarbons is strong and notoriously unreactive, and thus, selective intermolecular carbon-hydrogen bond activation has been identified as a fundamental and practical challenge to synthetic chemists. 1 Although theoretical chemists have made significant progress to elucidate the fundamental nature of metal-alkane interactions, detailed structural information for metal-alkane adducts is exceedingly rare. Most known examples of transition metal-alkane complexes to date have been detected in gas phases, matrices, and solutions in situ. 2 In virtually all reported cases, the metal-alkane adducts were identified spectroscopically as fleeting intermediates at cryogenic temperatures.Noteworthy exceptions were recently reported by George et al. 3 and Geftakis and Ball. 4 The latter group generated a cyclopentane adduct, [(Cp)Re(CO) 2 (C 5 H 10 )], via photolysis of [(Cp)Re(CO) 3 ] that was detected NMR-spectroscopically as an intermediate in neat cyclopentane solution at -93°C. On the basis of a comparison of the experimentally determined 13 C and 1 H coupling constants and chemical shifts with those of structurally closely related, -agostic bonded C-H moieties, an η 2 -H,C metal-alkane interaction was proposed (see below).In 1997, Reed et al. reported the only example of an X-ray diffraction analysis of a simple alkane in the coordination sphere of a metal complex. 5 In this iron porphyrin complex, (dap)Fe‚(nheptane), the hydrophobic pocket of a double A-framed porphyrin supported the heptane-iron adduct through a host/guest effect.We report here the X-ray diffraction analysis of a series of alkane adducts of the low-valent, coordinatively unsaturated, tris-aryl oxide uranium(III) complex [((ArO) 3 tacn)U] (1, Scheme 1). 6,7 These species exhibit evidence for bonding interactions between the uranium ion as well as the macrocyclic ligand and the axial alkane and, thus, raise the question whether the axial alkane is held in place through metal-alkane coordination, a host-guest effect, or a combination of both.Recrystallization of highly reactive 1 from neat n-pentane, n-hexane, benzene, and/or toluene, or mixtures thereof, did not yield single crystals suitable for X-ray diffraction analysis. We found, however, that cube-shaped, red-brown crystals could be obtained from an n-pentane solution if trace amounts of cyclohexane were present in the glovebox atmosphere. If a solution of 1 in n-pentane is treated with 50 equiv of cyclohexane or cyclopentane, cubeshaped crystals of [((ArO) 3 tacn)U(cy-C6)]‚(cy-C6) (1a) and [((ArO) 3 tacn)U(cy-C5)]‚(cy-C5) (1b) can be isolated reproducibly.The X-ray diffraction analysis of both complexes clearly revealed atom positions and connectivities of one molecule of cycloalkane in the coordination sphere of the uranium(III) center and a second molecule of cycloalkane cocrystallized in the lattice. The quality of the X-ray data, however, did not allow for discussion of metri...
The syntheses, crystal structures, electronic absorption spectra, electrochemical properties, and photophysical properties of a series of copper(I) bis(phenanthroline) complexes are reported. The phenanthroline ligands that have been prepared and investigated are the following: dop (2,9-di-(2-methylphenyl)-1,10-phenanthroline), xop (2-(2-methylphenyl)-9-(2,6-dimethylphenyl)-1,10-phenanthroline), dpep (2,9-diphenylethynyl-1,10-phenanthroline), and dmesp (2,9-dimesityl-1,10-phenanthroline). The complex [Cu(dop)(2)](PF(6)).Et(2)O crystallizes in space group P&onemacr;with a = 11.854(3) Å, b = 14.705(3) Å, c = 15.866(4) Å, alpha = 107.81(2) degrees, beta = 106.72(2) degrees, gamma = 97.56(2) degrees, V = 2447.6(10) Å(3), and Z = 2. For 5739 unique data with F > 4.0sigma(F), R = 7.52%. The complex [Cu(xop)(2)](PF(6)).(3)/(2)CH(3)OH crystallizes in space group C2/c with a = 23.096(6) Å, b = 23.387(6) Å, c = 17.873(7) Å, beta = 100.08(3) degrees, V = 9505(5) Å(3), and Z = 8. For 5631 unique data with F > 4.0sigma(F), R = 6.02%. The complex [Cu(dpep)(2)](PF(6)) crystallizes in space group P&onemacr; with a = 13.327(7) Å, b = 14.114(7) Å, c = 15.175(5) Å, alpha = 87.23(4) degrees, beta = 66.48(3) degrees, gamma = 61.84(4) degrees, V = 2273(2) Å(3), and Z = 2. For 4851 unique data with F > 4.0sigma(F), R = 5.47%. The complex [Cu(dmesp)(dpep)](PF(6)) crystallizes in space group Pbca with a = 14.547(6) Å, b = 22.868(6) Å, c = 30.659(10) Å, V = 10199(6) Å(3), and Z = 8. For 2281 unique data with F > 4.0sigma(F), R = 9.43%. The electrochemical, spectral, and structural properties of [Cu(dop)(2)](+) and [Cu(xop)(2)](+) demonstrate that the copper coordination environment is more sterically encumbered and more rigid in these two complexes than the coordination environment in the comparison molecule [Cu(dpp)(2)](+) (dpp = 2,9-diphenyl-1,10-phenanthroline). A larger energy gap is predicted for [Cu(dop)(2)](+) and [Cu(xop)(2)](+) based on these data, and consequently, a blue-shifted emission is observed relative to [Cu(dpp)(2)](+). The room-temperature excited-state lifetimes in dichloromethane and methanol of the dop and xop complexes are shown to be shorter than the dpp complex, and these results are interpreted as due to a reduction in ligand pi-electron delocalization in the former two complexes. The complexes [Cu(dpep)(2)](+) and [Cu(dmesp)(dpep)](+) are shown to have increased ligand pi-electron delocalization relative to [Cu(dpp)(2)](+); however, neither complex displays room-temperature steady-state emission in dichloromethane.
Several single-molecule magnets with the composition [Mn12O12(O2CR)16(H2O)x] (x = 3 or 4) exhibit two out-of-phase ac magnetic susceptibility signals, one in the 4-7 K region and the other in the 2-3 K region. New Mn12 complexes were prepared and structurally characterized, and the origin of the two magnetization relaxation processes was systematically examined. Different crystallographic forms of a Mn12 complex with a given R substituent exist where the two forms have different compositions of solvent molecules of crystallization and this results in two different arrangements of bound H2O and carboxylate ligands for the two crystallographically different forms with the same R substituent. The X-ray structure of cubic crystals of [Mn12O12(O2CEt)16(H2O)3]. 4H2O (space group P1) (complex 2a) has been reported previously. The more prevalent needle-form of [Mn12O12(O2CEt)16(H2O)3] (complex 2b) crystallizes in the monoclinic space group P2(1)/c, which at -170 degrees C has a = 16.462(7) A, b = 22.401(9) A, c = 20.766(9) A, beta = 103.85(2) degrees, and Z = 4. The arrangements of H2O and carboxylate ligands on the Mn12 molecule are different in the two crystal forms. The complex [Mn12O12-(O2)CC6H4-p-Cl)16(H2O)4].8CH2Cl2 (5) crystallizes in the monoclinic space group C2/c, which at -172 degrees C has a = 29.697(9) A, b = 17.708(4) A, c = 30.204(8) A, beta = 102.12(2) degrees, and Z = 4. The ac susceptibility data for complex 5 show that it has out-of-phase signals in both the 2-3 K and the 4-7 K ranges. X-ray structures are also reported for two isomeric forms of the p-methylbenzoate complex. [Mn12O12(O2CC6H4-p-Me)16(H2O)4]. (HO2CC6H4-p-Me) (6) crystallizes in the monoclinic space group C2/c, which at 193 K has a = 40.4589(5) A, b = 18.2288(2) A, c = 26.5882(4) A, beta = 125.8359(2) degrees, and Z = 4. [Mn12O12(O2CC6H4-p-Me)16(H2O)4].3(H2O) (7) crystallizes in the monoclinic space group I2/a, which at 223 K has a = 29.2794(4) A, b = 32.2371(4) A, c = 29.8738(6) A, beta = 99.2650(10) degrees, and Z = 8. The Mn12 molecules in complexes 6 and 7 differ in their arrangements of the four bound H2O ligands. Complex 6 exhibits an out-of-phase ac peak (chi(M)' ') in the 2-3 K region, whereas the hydrate complex 7 has a chi(M)' ' signal in the 4-7 K region. In addition, however, in complex 6, one Mn(III) ion has an abnormal Jahn-Teller distortion axis oriented at an oxide ion, and thus 6 and 7 are Jahn-Teller isomers. This reduces the symmetry of the core of complex 6 compared with complex 7. Thus, complex 6 likely has a larger tunneling matrix element and this explains why this complex shows a chi(M)' ' signal in the 2-3 K region, whereas complex 7 has its chi(M)' ' peak in the 4-7 K region, i.e., the rate of tunneling of magnetization is greater in complex 6 than complex 7. Detailed 1H NMR experiments (2-D COSY and TOCSY) lead to the assignment of all proton resonances for the benzoate and p-methyl-benzoate Mn12 complexes and confirm the structural integrity of the (Mn12O12) complexes upon dissolution. In solution there is rapi...
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