- The extraordinary ability of guanidinate derivatives to stabilize higher oxidation numbers in dimetal units by modification of redox potentials: Structures of Mo25+ and Mo26+ compounds
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Full characterization of the first homologous series of dimolybdenum paddlewheel compounds having electronic configurations of the types σ2π4σx, x = 2, 1, 0, and Mo-Mo bond orders of 4, 3.5, and 3, respectively, has been accomplished with the guanidinate-type ligand hpp (hpp = the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine). Essentially quantitative oxidation of Mo2(hpp)4, 1, by CH2Cl2 gives Mo2(hpp)4Cl, 2. The halide in 2 can be replaced by reaction with TlBF4 to produce Mo2(hpp)4(BF4); 3. Further oxidation of 2 by AgBF4 produces Mo2(hpp)4ClBF4, 4. The change from bond order 4 (in 1) to 3.5 in Mo2(hpp)4Cl is accompanied by an increase in the Mo-Mo bond length of 0.061 to 2.1280(4) A. A further increase of 0.044 A in the Mo-Mo distance to 2.172(1) A is observed as the bond order decreases to 3 in 4. At the same time, the Mo-N distances decrease smoothly as the oxidation state of the Mo atoms increases. Electrochemical studies have shown two chemically reversible processes at very negative potentials, E1/21 = -0.444 V and E1/21 = -1.271 V versus Ag/AgCl. These correspond to the processes Mo26+/5+ and Mo25+/4+, respectively. The latter potential is displaced by over 1.5 V relative to those of the Mo2(formamidinate)4 compounds and the first one has never been observed in such complexes. Thus, in surprising contrast to previously observed behavior of the dimolybdenum unit, when it is surrounded by the very basic guanidinate ligand hpp, there is an extraordinary stabilization of the higher oxidation numbers of the molybdenum atoms.
- Cotton, F. Albert,Daniels, Lee M.,Murillo, Carlos A.,Timmons, Daren J.,Wilkinson, Chad C.
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- Molecular and electronic structures by design: Tuning symmetrical and unsymmetrical linear trichromium chains
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The preparation, properties, and crystal structures of 12 trichromium extended metal atom chain (EMAC) compounds of the type Cr3(L) 4X2 (L = equatorial ligands dipyridylamide (dpa) or di-4,4′-ethyl-2,2′-pyridylamide (depa), and X = axial ligands, e.g., halide or pseudohalide ions) with large variations in metal-metal distances are reported here. These complexes, which belong to a broad class of fundamentally interesting trinuclear molecules over which the electrons may or may not be delocalized, pose significant theoretical and experimental challenges which are dealt with in this report. Complexes with strongly donating axial or equatorial ligands tend to favor a symmetrical (D4) molecular structure, while more weakly donating ligands give rise to unsymmetrical (C4) structures; the physical properties of these two classes of compounds are discussed fully, and important comparisons with a reported DFT model of the electronic structures of the compounds are made.
- Berry, John F.,Cotton, F. Albert,Lu, Tongbu,Murillo, Carlos A.,Roberts, Brian K.,Wang, Xiaoping
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- Synthesis and catalytic properties of molybdenum(VI) complexes with tris(3,5-dimethyl-1-pyrazolyl)methane
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The complex [MoO2Cl{HC(3,5-Me2pz)3}] BF4 (1) (HC(3,5-Me2pz)3 = tris(3,5-dimethyl-1- pyrazolyl)methane) has been prepared and examined as a catalyst for epoxidation of olefins at 55 °C using tert-butyl hydroperoxide (TBHP) as the oxidant. For reaction of cis-cyclooctene, epoxycyclooctane is obtained quantitatively within 5 h when water is rigorously excluded from the reaction mixture. Increasing amounts of water in the reaction mixture lead to lower activities (without affecting product selectivity) and transformation of 1 into the trioxidomolybdenum(VI) complex [{HC(3,5-Me2pz)3}MoO 3] (4). Complex 4 was isolated as a microcrystalline solid by refluxing a suspension of 1 in water. The powder X-ray diffraction pattern of 4 can be indexed in the orthorhombic Pnma system, with a = 16.7349(5) A, b = 13.6380(4) A, and c = 7.8513(3) A. Treatment of 1 in dichloromethane with excess TBHP led to isolation of the symmetrical [Mo 2O4(μ2-O){HC(3,5-Me2pz) 3}2](BF4)2 (2) and unsymmetrical [Mo2O3(O2)2(μ2- O)(H 2O){HC(3,5-Me2pz)3}] (3) oxido-bridged dimers, which were characterized by single-crystal X-ray diffraction. Complex 2 displays the well-known (Mo2O5)2+ bridging structure where each dioxidomolybdenum(VI) center is coordinated to three N atoms of the organic ligand and one μ2-bridging O atom. The unusual complex 3 comprises dioxido and oxidodiperoxo molybdenum(VI) centers linked by a μ2-bridgingO atom, with the former center being coordinated to the tridentate N-ligand. The dinuclear complexes exhibit a similar catalytic performance to that found for mononuclear 1. For complexes 1 and 2 use of the ionic liquids (ILs) 1-butyl-3-methylimidazolium tetrafluoroborate and N-butyl-3-methylpyridinium tetrafluoroborate as solvents allowed the complexes to be completely dissolved, and in each case the catalyst and IL could be recycled and reused without loss of activity.
- Neves, Patricia,Gago, Sandra,Balula, Salete S.,Lopes, Andre D.,Valente, Anabela A.,Cunha-Silva, Luis,Paz, Filipe A. Almeida,Pillinger, Martyn,Rocha, Joao,Silva, Carlos M.,Gonc-Alves, Isabel S.
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p. 3490 - 3500
(2011/07/07)
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- Synthesis and studies of chemical properties of arylthallium(III) bis(tetrafluoroborates)
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Synthesis of arylthallium(III)bis(tetrafluoroborates) was carried out and their pyrolysis was studied. Synthetic procedure for preparation of fluoroaromatic compounds is developed.
- Gun'kin
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p. 455 - 457
(2010/08/04)
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- Organic thallium compounds. Synthesis and chemical properties of phenylthallium(III) bis(tetrafluoroborate)
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Phenylthallium(III) bis(tetrafluoroborate) was obtained by the reaction of phenylthallium dihydroxide with tetrafluoroboric acid in ether.
- Gun'kin,Pankst'yanov,Popova,Kleimenova
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- Chemistry of [Cp*Rh(μ-Cl)]2 and its dioxygen and nitrosobenzene insertion products
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Treating [Cp*Rh(μ-Cl)Cl]2 (Cp* = C5Me5 = pentamethylcyclopentadienyl) with excess Na(Hg) (0.4%) in CH2Cl2 gives deep blue [Cp*Rh(μ-Cl)]2 (1). Crystals of 1 from toluene/petroleum ether are monoclinic (C2/c) with a = 33.068(8) A?, b = 13.173(3) A?, c = 16.810(4) A?, β = 117.77(2)°, V = 6479.4 A?3, and Z = 12. The structure consists of two independent, nearly identical molecules (one on a 2-fold axis and the other in a general position) with short Rh-Rh bonds (2.623 A? average), bridging chlorides, and η5-Cp* ligands. Methylene chloride solutions of 1 react rapidly with dioxygen at temperatures slightly above -78°C to give an unstable (above ca. -15°C in CD2Cl2) unsymmetrical, dioxygen adduct [(Cp*Rh)2(μ-Cl)(Cl)(μ,η1:η 2-O2)] (2). Complex 2 is fluxional on the NMR time scale. The addition of PPh3 to cold solutions of 2 gives Cp*RhCl2(PPh3) and the unstable (above ca. 0°C in CD2Cl2) complex Cp*Rh(O2)(PPh3) (3). With excess PMe3, 2 gives the fulvene complex [(η4-C5Me4CH2)Rh(PMe 3)3]Cl (4). Crystals of 4 from CH2Cl2/petroleum ether are monoclinic (C2/c) with a = 16.618(5) A?, b = 12.074(6) A?, c = 27.319(7) A?, β = 99.04(2)°, V = 5413 A?3, and Z = 8. Treating 1 with PhNO at 22°C gives [(Cp*Rh)2(μ-Cl)(Cl)(μ,η1:η 2-PhNO)] (5) which, by NMR and a crystal structure determination, has a structure similar to that proposed for 2. Crystals of 5 from toluene/hexane are triclinic (P1) with a = 9.290(3) A?, b = 17.830(3) A?, c = 8 941(3) A?, α = 92.60(2)°, β = 110.87(3)°, γ = 104.36(2)°, V = 1325.9 A?3, and Z = 2. The structure consists of an η1:η2-bound PhNO ligand (bridging the two η5-Cp*Rh centers via the nitrogen atom), a bridging Cl, and a terminal Cl bound to the η1-Rh center. A chloride ion is removed from 5 with TlBF4 to give the first μ,η2:η2-nitrosobenzene complex, [(Cp*Rh)2(μ-Cl)(μ,η2:η 2-PhNO)]BF4 (6). Crystals of 6 from CH2Cl2/petroleum ether are monoclinic (P21/c) with a = 10.933(4) A?, b = 15.166(6) A?, c = 18.136(10) A?, β = 95.33(4)°, V = 2994 A?3, and Z = 4. The Na/Hg reduction of 5 in the presence of PhNO and TlBF4 gives [Cp*Rh(μ,η1:η2-PhNO)]2 (7). Treating the nitrosobenzene complexes with CO gives up to 35% yields of PhNCO.
- Hoard, David W.,Sharp, Paul R.
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p. 612 - 620
(2008/10/08)
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- Electrochemical Generation of Soluble and Reactive Cadmium, Lead, and Thallium Cations in Noncoordinating Solvents: Relative Strengths of Perchlorate, Tetrafluoroborate, and Hexafluorophosphate Ligation in Dichloromethane and Benzene
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Electrochemical oxidation of metal amalgam electrodes in noncoordinating solvents generates soluble forms of highly active metal ions at the electrode surface that can form very strong complexes with the tetrafluoroborate and perchlorate anions.Oxidation at Cd, Pb, and Tl dropping mercury amalgam electrodes in a solution containing dichloromethane and either tetrabutylammonium hexafluorophosphate, , tetrabutylammonium tetrafluoroborate, , or tetrabutylammonium perchlorate, , occurs reversibly, thereby allowing thermodynamic data to be obtained on the nature of the complex formed.The - ligation is considerably weaker than that of - or -.Data obtained in dichloromethane with hexafluorophosphate, as a reference electrolyte allowed the following complexes with their equilibrium constants to be identified: 2-, log β4=9.1; -, log β3=7.3; -, log β3=8.3; Pb(BF4)2, log β2=7.5; Tl(ClO4), log β1=3.3; Tl(BF4), log β1=2.9.The equilibrium constants for these complexes are larger then those obtained in aqueous media for many classical ligands.In benzene, which is of lower dielectric constant and is less polar than dichoromethane, the half-wave potential for the oxidation of the cadmium amalgam electrode is approximately 700 mV more negative with pechlorate than with hexafluorophosphate as the electrolyte anion.In contrast, the difference is only 200 mV in dichloromethane.This unprecendented difference may be attributed to the weaker coordination of benzene and the consequent amplification of the differences in strength of perchlorate and hexafluorophosphate ligation.Consequently, a method of preparing highly activated and previously unknown forms of soluble metal ions is available in solvents such as chlorinated and aromatic hydrocarbons. This feature is further illustrated by the large negative shift in half-wave potential for the metal oxidations observed after the coordinating solvent dimethyl sulfoxide is added to dichloromethane (0.2 M ) solutions.Controlled-potential electrolysis experiments at a mercury amalgam pool in dichloromethane lead to the formation of the expected nonsolvated insoluble salts, demonstrating that the kinetics of precipitation are slower then the polarographic time scale.Concepts developed on this work provide prospects for new forms of mechanistic, thermodynamic and synthetic metal ion chemistry.
- Bond, Alan M.,Ellis, S. Richard,Hollenkamp, Anthony F.
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p. 5293 - 5297
(2007/10/02)
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- Matrix-isolation study of oxide-transfer reactions to halogen-containing Lewis acids: Synthesis of the CO2F22- anion and attempted synthesis of the BF3O2- anion
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The oxide-transfer technique has been coupled with matrix isolation in an attempt to synthesize and characterize the BF3O2- and CO2F22- anions, through the reaction of Tl2O with either BF3 or COF2. In the latter experiments, two sets of product bands were observed; one agreed well with the absorptions of the known CO2F- anion. The second set has been assigned to the CO2F22- anion in the matrix-isolated Tl+2CO2F22- triple ion, which rapidly eliminated TlF upon warming to form the Tl+CO2F- anion. Attempts to synthesize the BF3O2- anion in a similar fashion were not successful, suggesting that previous reports of the room-temperature synthesis of this anion are incorrect.
- David, Shelle J.,Ault, Bruce S.
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p. 1238 - 1241
(2008/10/08)
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