The photoreaction of Fe(CO) 5 and cyanide salts in MeCN solution affords the dianion [Fe (CN) 2 (... more The photoreaction of Fe(CO) 5 and cyanide salts in MeCN solution affords the dianion [Fe (CN) 2 (CO) 3 ] 2− , conveniently isolated as [K(18-crown-6)] 2 [Fe(CN) 2 (CO) 3 ]. Solutions of [Fe (CN) 2 (CO) 3 ] 2− oxidize irreversibly at −600 mV (vs. Ag/AgCl) to give primarily [Fe(CN) 3 (CO) 3 ] −. Protonation of the dianion affords the hydride [K(18-crown-6)][HFe(CN) 2 (CO) 3 ] with a pK a ≈ 17 (MeCN). The ferrous hydride exhibits enhanced electrophilicity vs. its dianionic precursor, which resists substitution. Treatment of [K(18-crown-6)][Fe(CN) 2 (CO) 3 ] with tertiary phosphines and phosphites gives isomeric mixtures of [HFe(CN) 2 (CO) 2 L] − (L = P(OPh) 3 and PPh 3). Carbonyl substitution on [1H(CO) 2 ] − by P(OPh) 3 is first-order in both the phosphite and iron (k = 0.18 M −1 s −1 at 22 °C) with ΔH ‡ = 51.6 kJ mol −1 and ΔS ‡ = −83.0 J K −1 mol −1. These ligands are displaced under an atmosphere of CO. With cis-Ph 2 PCH=CHPPh 2 (dppv), we obtained the monocarbonyl, [HFe(CN) 2 (CO)(dppv)] − , a highly basic hydride (pK a > 23.3) that rearranges in solution to a single isomer. Treatment of [K(18-crown-6)][HFe(CN) 2 (CO) 3 ] with Et 4 NCN resulted in rapid proton transfer to give [Fe(CN) 2 (CO) 3 ] 2− and HCN. The tricyano hydride [HFe(CN) 3 (CO) 2 ] 2− is prepared by the reaction of [HFe(CN) 2 (CO) 2 (PPh 3)] − and [K(18-crown-6)]CN. Similar to the phosphine and phosphite derivatives, [HFe(CN) 3 (CO) 2 ] 2− exists as a mixture of all three possible isomers. Protonation of the hydrides [HFe(CN) 2 (CO)(dppv)] − and [HFe(CN) 3 (CO) 2 ] − in acetonitrile solutions releases H 2 and gives the corresponding acetonitrile complexes [K(18-crown-6)][Fe (CN) 3 (NCMe)(CO) 2 ] and Fe(CN) 2 (NCMe)(CO)(dppv). Alkylation of [K(18-crown-6)] 2 [Fe (CN) 2 (CO) 3 ] with MeOTf gives the thermally-unstable [MeFe(CN) 2 (CO) 3 ] − , which was characterized spectroscopically at −40 °C. Reaction of dppv with [MeFe(CN) 2 (CO) 3 ] − gives the acetyl complex, [Fe(CN) 2 (COMe)(CO)(dppv)] −. Whereas [Fe(CN) 2 (CO) 3 ] 2− undergoes protonation and methylation at Fe, acid chlorides give the iron(0) N-acylisocyanides [Fe(CN) (CO) 3 (CNCOR)] − (R = Ph, CH 3). The solid state structures of [K(18-crown-6)][HFe(CN) 2 (CO) (dppv)], Fe(CN) 2 (NCMe)(CO)(dppv), and [K(18-crown-6)] 2 [HFe(CN) 3 (CO) 2 ] were confirmed crystallographically. In all three cases, the cyanide ligands are cis to the hydride or acetonitrile ligands.
The reactivity of metal olefin complexes with non-innocent ligands (NILs) was examined. Treatment... more The reactivity of metal olefin complexes with non-innocent ligands (NILs) was examined. Treatment of PtCl 2 (diene) with the deprotonated catechol or aminophenol ligands afforded the corresponding Pt(NIL)(diene) complexes. The Pt(t BA F Ph)(COD), Pt(t BA F Ph)(nbd), and Pt(O 2 C 6 H 2 t Bu 2)(COD) (H 2 t BA F Ph) 2-(2-trifluoromethyl)anilino-4,6-di-tertbutylphenol, H 2 O 2 C 6 H 2 t Bu 2) 3,5-di-tert-butylcatechol) complexes were examined by cyclic voltammetry. Treatment of Pt(t BA F Ph)(COD) or Pt(t BA F Ph)(nbd) with AgPF 6 afforded the imino-semiquinones [Pt(t BA F Ph)(COD)]PF 6 or [Pt(t BA F Ph)(nbd)]PF 6 , respectively. The [Pt(t BA F Ph)(COD)] complex was unreactive toward nucleophiles, whereas the oxidized derivative, [Pt(t BA F Ph)(COD)]PF 6 , rapidly and stereospecifically added alkoxides at the carbon trans to the phenolate. The Pt(t BA F Ph)(COD), [Pt(t BA F Ph)(COD)]PF 6 , Pt(t BA F Ph)(C 8 H 12 OMe), and [Cp 2 Co][Pt-(t BA F Ph)(C 8 H 12 OMe)] complexes were characterized crystallographically.
Condensation of Fe 2 (SH) 2 (CO) 6 , acetaldehyde, and (NH 4) 2 CO 3 affords the methyl-substitut... more Condensation of Fe 2 (SH) 2 (CO) 6 , acetaldehyde, and (NH 4) 2 CO 3 affords the methyl-substituted azadithiolate Fe 2 [(SCHMe) 2 NH](CO) 6 (1). The complex exists mainly (~95%) as the meso diastereomer, but the d,l diastereoisomers could be detected. DFT calculations predict that the meso isomer would be 2.5 kcal/mol more stable than the d,l isomer due to conventional nonbonding interactions between the methyl groups and the ring hydrogen atoms. Crystallographic analysis of meso-1 confirms that the two methyl groups are equatorial, constraining the diferraazadithiolate bicycle to a conformation that desymmetrizes the diiron center. The lowered symmetry is confirmed by the observation of two 13 C NMR signals in the FeCO region under conditions of fast turnstile rotation at the Fe(CO) 3 groups. The pK a value of the amine in 1 is 7.89 (all pK a 's determined in MeCN solution), which is similar to a redetermined value for Fe 2 [(SCH 2) 2 NH](CO) 6 (2, pK a = 7.98) and only slightly less basic than the tertiary amine Fe 2 [(SCH 2) 2 NMe](CO) 6 (pK a = 8.14). Substitution of 1 with PMe 3 proceeded via the intermediacy of two isomers of Fe 2 [(SCHMe) 2 NH] (CO) 5 (PMe 3), affording Fe 2 [(SCHMe) 2 NH](CO) 4 (PMe 3) 2 (3). 31 P NMR spectra confirm that the two PMe 3 ligands in 3 are nonequivalent, consistent with the desymmetrizing effect of the dithiolate. The pK a value of the amine in 3 was found to be 11.3. Using triphenylphosphine, we prepared Fe 2 [(SCHMe) 2 NH](CO) 5 (PPh 3) as a single regioisomer.
Journal of the American Chemical Society, Aug 14, 2008
This study probes the impact of electronic asymmetry of diiron(I) dithiolato carbonyls. Treatment... more This study probes the impact of electronic asymmetry of diiron(I) dithiolato carbonyls. Treatment of Fe 2 (S 2 C n H 2n)(CO) 6-x (PMe 3) x compounds (n = 2, 3) with NOBF 4 gave the derivatives [Fe 2 (S 2 C n H 2n)(CO) 5-x (PMe 3) x (NO)]BF 4 (x = 1, 2, 3) which are electronically unsymmetrical due to the presence of a single NO + ligand. Whereas the mono phosphine derivative is largely undistorted, the bis PMe 3 derivatives are distorted such that the CO ligand on the Fe(CO)(PMe 3)(NO) + subunit is semibridging. Two isomers of [Fe 2 (S 2 C 3 H 6)(CO) 3 (PMe 3) 2 (NO)]BF 4 were characterized spectroscopically and crystallographically. Each isomer features electron-rich [Fe(CO) 2 PMe 3 ] and electrophilic [Fe(CO)(PMe 3)(NO)] + subunits. These species are in equilibrium with an unobserved isomer that reversibly binds CO (ΔH = −35 kJ/mol, ΔS = −139 J/mol•K) to give the symmetrical adduct [Fe 2 (S 2 C 3 H 6)(μ-NO)(CO) 4 (PMe 3) 2 ]BF 4. In contrast to Fe 2 (S 2 C 3 H 6)(CO) 4 (PMe 3) 2 , the bis (PMe 3) nitrosyls readily undergo CO-substitution to give the (PMe 3) 3 derivatives. The nitrosyl complexes reduce at potentials that are ~1 V milder than their carbonyl counterparts. DFT calculations, specifically NBO values, reinforce the electronic resemblance of the nitrosyl complexes with the corresponding mixed-valence diiron complexes. Unlike other diiron dithiolato carbonyls, these species undergo reversible reductions at mild conditions. The results show that the novel structural and chemical features associated with mixed valence diiron dithiolates-the so-called H ox models-can be replicated in the absence of mixed-valency by introducing electronic asymmetry.
Journal of the American Chemical Society, Nov 29, 2010
The report summarizes studies on the redox behavior of synthetic models for the [FeFe]hydrogenase... more The report summarizes studies on the redox behavior of synthetic models for the [FeFe]hydrogenases, consisting of diiron dithiolato carbonyl complexes bearing the amine cofactor and its N-benzyl derivative. Of specific interest are the causes of the low reactivity of oxidized models toward H 2 , which contrasts with the high activity of these enzymes for H 2 oxidation. The redox and acid-base properties of the model complexes [Fe 2 [(SCH 2) 2 NR](CO) 3 (dppv)(PMe 3)] + ([2] + for R = H and [2′] + for R = CH 2 C 6 H 5 , dppv = cis-1,2-bis(diphenylphosphino)ethylene)) indicate that addition of H 2 and followed by deprotonation are (i) endothermic for the mixed valence (Fe II Fe I) state and (ii) exothermic for the diferrous (Fe II Fe II) state. The diferrous state is shown to be unstable with respect to coordination of the amine to Fe, a derivative of which was characterized crystallographically. The redox and acid-base properties for the mixed valence models differ strongly for those containing the amine cofactor versus those derived from propanedithiolate. Protonation of [2′] + induces disproportionation to a 1:1 mixture of the ammonium-Fe I Fe I and the dication [2′] 2+ (Fe II Fe II). This effect is consistent with substantial enhancement of the basicity of the amine in the Fe I Fe I state vs the Fe II Fe I state. The Fe I Fe I ammonium compounds are rapid and efficient H-atom donors toward the nitroxyl compound TEMPO. The atom transfer is proposed to proceed via the hydride, as indicated by the reaction of [HFe 2 [(SCH 2) 2 NH](CO) 2 (dppv) 2 ] + with TEMPO. Collectively, the results suggest that proton-coupled electron-transfer pathways should be considered for H 2 activation by the [FeFe]-hydrogenases.
The one-electron oxidations of a series of diiron(I) dithiolato carbonyls were examined to evalua... more The one-electron oxidations of a series of diiron(I) dithiolato carbonyls were examined to evaluate the factors that affect the oxidation state assignments, structures, and reactivity of these lowmolecular weight models for the H ox state of the [FeFe]-hydrogenases. The propanedithiolates Fe 2 (S 2 C 3 H 6)(CO) 3 (L)(dppv) (L = CO, PMe 3 , Pi-Pr 3) oxidize at potentials ~180 mV milder than the related ethanedithiolates (Angew. Chem. Int. Ed. 2007, 46, 6152). The steric clash between the central methylene of the propanedithiolate and the phosphine favors the rotated structure, which forms upon oxidation. EPR spectra for the mixed-valence cations indicate that the unpaired electron is localized on the Fe(CO)(dppv) center in both [Fe 2 (S 2 C 3 H 6)(CO) 4 (dppv)]BF 4 and [Fe 2 (S 2 C 3 H 6) (CO) 3 (PMe 3)(dppv)]BF 4 , as seen previously for the ethanedithiolate [Fe 2 (S 2 C 2 H 4)(CO) 3 (PMe 3) (dppv)]BF 4. For [Fe 2 (S 2 C n H 2n)(CO) 3 (Pi-Pr 3)(dppv)]BF 4 , however, the spin is localized on the Fe (CO) 2 (Pi-Pr 3) center, although the Fe(CO)(dppv) site is rotated in the crystalline state. IR and EPR spectra, as well as redox potentials and DFT-calculations, suggest, however, that the Fe(CO) 2 (Pi-Pr 3) site is rotated in solution, driven by steric factors. Analysis of the DFT-computed partial atomic charges for the mixed-valence species shows that the Fe atom featuring a vacant apical coordination position is an electrophilic Fe(I) center. One-electron oxidation of [Fe 2 (S 2 C 2 H 4)(CN) (CO) 3 (dppv)] − resulted in 2e oxidation of 0.5 equiv to give the μ-cyano derivative [Fe I 2 (S 2 C 2 H 4) (CO) 3 (dppv)](μ-CN)[Fe II 2 (S 2 C 2 H 4)(μ-CO)(CO) 2 (CN)(dppv)], which was characterized spectroscopically. Correspondence to: Luca De Gioia; Thomas B. Rauchfuss. Supporting Information Available: Crystallographic data for [Fe 2 (S 2 C 3 H 6)(CO) 3 (PMe 3)(dppv)]BF 4 and [Fe 2 (S 2 C 3 H 6)(CO) 3 (Pi-Pr 3)(dppv)]BF 4 , voltammograms, as well as IR, EPR, and NMR spectra. For DFT calculations: atomic coordinates, energy values, spin populations and partial atomic charges for the DFT models [Fe 2 (S 2 C n H 2n)(CO) 3 (PH 3) 3 ] + , [Fe 2 (S 2 C n H 2n)(CO) 3 (PMe 3)(dppv)] + , and [Fe 2 (S 2 C n H 2n)(CO) 3 (Pi-Pr 3)(dppv)] +. This material is available free of charge via the Internet at …
The [FeFe] hydrogenase enzymes are the most efficient catalysts known for the reduction of proton... more The [FeFe] hydrogenase enzymes are the most efficient catalysts known for the reduction of protons to H 2. [1] The active site exists in two functional states (Scheme 1), H red , which is S =0, and H ox , which is S = 1/2. [2] Research in this area is aimed at elucidating the mechanism of the enzymatic catalysis and at using this information to develop protein-free bioinspired synthetic catalysts. [3] A specific research goal is the preparation of molecules that resemble the functional states of the active site with the expectation that function will follow form. Most studies on diiron dithiolato carbonyl complexes rely on organic ligands (e.g. phosphanes) in place of the naturally occurring cyanide and μ-SR[Fe 4 S 4 ] ligands, [4] which have complicated acid-base behavior that is often difficult to control outside of the protein. Another barrier to modeling has been the rarity of mixed-valence diiron dithiolate compounds with the appropriate structures, stability, and reactivity. ** This research was supported by the US National Institutes of Health. We thank Teresa Prussak-Wieckowska for assistance on X-ray crystallography.
Two isostructural metalloporphyrin framework solids have been synthesized. Both frameworks contai... more Two isostructural metalloporphyrin framework solids have been synthesized. Both frameworks contains manganese(III) metal complexes of trans-dicarboxylateporphyrins whose peripheralcarboxylates coordinate the edges of tetrahedral Zn4O[Formula: see text] clusters; the two metalloporphyrins explored are Mn(III) and Co(II). The cubic interpenetrated frameworks have 72% free volume and 4 × 7 Å averaged size pores. The evacuated frameworks are robust and retain a structure open to the sorption of substrates with medium polarity. The manganese porphyrin framework catalyzes the hydroxylation of cyclic and linear alkanes with iodosylbenzene as oxidant in a size- and polarity-selective manner. In addition, the catalysis was found to occur within the pores, making this a rare case of porphyrin framework solid with interior catalysis.
The report summarizes studies on the redox behavior of synthetic models for the [FeFe]hydrogenase... more The report summarizes studies on the redox behavior of synthetic models for the [FeFe]hydrogenases, consisting of diiron dithiolato carbonyl complexes bearing the amine cofactor and its N-benzyl derivative. Of specific interest are the causes of the low reactivity of oxidized models toward H 2 , which contrasts with the high activity of these enzymes for H 2 oxidation. The redox and acid-base properties of the model complexes [Fe 2 [(SCH 2) 2 NR](CO) 3 (dppv)(PMe 3)] + ([2] + for R = H and [2′] + for R = CH 2 C 6 H 5 , dppv = cis-1,2-bis(diphenylphosphino)ethylene)) indicate that addition of H 2 and followed by deprotonation are (i) endothermic for the mixed valence (Fe II Fe I) state and (ii) exothermic for the diferrous (Fe II Fe II) state. The diferrous state is shown to be unstable with respect to coordination of the amine to Fe, a derivative of which was characterized crystallographically. The redox and acid-base properties for the mixed valence models differ strongly for those containing the amine cofactor versus those derived from propanedithiolate. Protonation of [2′] + induces disproportionation to a 1:1 mixture of the ammonium-Fe I Fe I and the dication [2′] 2+ (Fe II Fe II). This effect is consistent with substantial enhancement of the basicity of the amine in the Fe I Fe I state vs the Fe II Fe I state. The Fe I Fe I ammonium compounds are rapid and efficient H-atom donors toward the nitroxyl compound TEMPO. The atom transfer is proposed to proceed via the hydride, as indicated by the reaction of [HFe 2 [(SCH 2) 2 NH](CO) 2 (dppv) 2 ] + with TEMPO. Collectively, the results suggest that proton-coupled electron-transfer pathways should be considered for H 2 activation by the [FeFe]-hydrogenases.
Nitrosyl derivatives of diiron dithiolato carbonyls have been prepared starting from the versatil... more Nitrosyl derivatives of diiron dithiolato carbonyls have been prepared starting from the versatile precursor Fe 2 (S 2 C n H 2n)(dppv)(CO) 4 (dppv = cis-1,2-bis(diphenylphosphinoethylene). These studies expand the range of substituted diiron(I) dithiolato carbonyl complexes. From [Fe 2 (S 2 C 2 H 4)(CO) 3 (dppv)(NO)]BF 4 ([1(CO) 3 ]BF 4), the following compounds were prepared: [1 (CO) 2 (PMe 3)]BF 4 , [1(CO)(dppv)]BF 4 , NEt 4 [1(CO)(CN) 2 ], and 1(CO)(CN)(PMe 3). Some if not all of these substitution reactions occur via the addition of two equiv of the nucleophile followed by dissociation of one nucleophile and decarbonylation. Such a double adduct was characterized crystallographically in the case of [Fe 2 (S 2 C 2 H 4)(CO) 3 (dppv)(NO)(PMe 3) 2 ]BF 4. This result shows that the addition of two ligands causes scission of the Fe-Fe bond and one Fe-S bond. When cyanide is the nucleophile, nitrosyl migrates away from the Fe(dppv) site, yielding a Fe(CN)(NO)(CO) derivative. Compounds [1(CO) 3 ]BF 4 , [1(CO) 2 (PMe 3)]BF 4 , and [1(CO)(dppv)]BF 4 were also prepared by the addition of NO + to the di-, tri-and tetrasubstituted precursors. In these cases, the NO + appears to form an initial 36e-adduct containing terminal Fe-NO, followed by decarbonylation. Several complexes were prepared by the addition of NO to the mixed-valence Fe(I)Fe(II) derivatives. The diiron nitrosyl complexes reduce at mild potentials and in certain cases form weak adducts with CO.
The accuracy of the Content should not be relied upon and should be independently verified with p... more The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden.
The titanium(II) alkyl trans-TiMe 2 (dmpe) 2 , where dmpe is 1,2-bis(dimethylphosphino)ethane, re... more The titanium(II) alkyl trans-TiMe 2 (dmpe) 2 , where dmpe is 1,2-bis(dimethylphosphino)ethane, reacts with 1,3-butadiene and trans,trans-1,4-diphenyl-1,3-butadiene at-20°C to produce the titanium(II) butadiene complexes TiMe 2 (η 4-C 4 H 4 R 2)(dmpe), where R is H or Ph. NMR spectra are consistent with structures in which the methyl groups are mutually cis, and this has been verified crystallographically for the 1,4-diphenylbutadiene complex. These molecules are fluxional on the NMR time scale, and the activation parameters for exchange are ∆H q) 9.1 (0.2 kcal mol-1 and ∆S q) 3 (1 eu for the 1,4-diphenylbutadiene complex. The process that exchanges the two TiMe groups, the two ends of the dmpe ligand, and the two ends of the butadiene ligand is proposed to be a trigonal twist, although we cannot entirely rule out the possibility that the exchange involves five-coordinate intermediates generated by dissociation of one "arm" of a chelating ligand. If the reaction of TiMe 2 (dmpe) 2 and 1,3-butadiene is allowed to proceed at-20°C for prolonged periods (>12 h), a second titanium "butadiene" complex is formed, which has been identified as the titanium(IV) η 3 ,η 1octa-1,6-diene-1,8-diyl complex TiMe 2 (η 3 ,η 1-C 8 H 12)(dmpe). Warming a solution of TiMe 2-(dmpe) 2 and 1,3-butadiene to 25°C results in the catalytic dimerization of butadiene to the Diels-Alder dimer 4-vinylcyclohexene at rates of 5 turnovers/h. A mechanism for the catalytic dimerization is proposed, which involves coupling of two butadiene ligands to form a divinyltitanacyclopentane species, allylic rearrangement to a vinyltitanacycloheptene intermediate, and reductive elimination to form the cyclic product. Treatment of TiMe 2-(dmpe) 2 with 1,3-butadiene in the presence of AlEt 3 results in reduction to the titanium(0) complex Ti(η 4-C 4 H 6) 2 (dmpe), which has also been crystallographically characterized. Unlike the behavior seen for certain other early transition metal butadiene complexes, in both Ti-(η 4-C 4 H 6) 2 (dmpe) and TiMe 2 (η 4-C 4 H 4 Ph 2)(dmpe) the butadiene ligands are bound like true dienes. We propose that the preferred bonding mode for butadiene complexes of the lower valent early transition metals is the π,η 4 mode and that increasing σ 2 ,π character is introduced only when there are significant steric repulsions between the ancillary ligands and the meso butadiene substituents.
All manipulations were conducted using standard Schlenk techniques. All solvents were dried using... more All manipulations were conducted using standard Schlenk techniques. All solvents were dried using the Grubbs' solvent purification process. Elemental analyses were conducted by the University of Illinois Microanalytical Laboratory. 1 H NMR spectra were acquired on a Unity Varian 400 or a Unity Varian 500 spectrometer. IR spectra were collected on a Mattson Infinity Gold FTIR spectrometer. ESI-MS were collected on a Quattro quadrupole-hexapole-quadrapole (QHQ) mass spectrometer. (Et 4 N) 2 [Fe(SPh) 2 (CO) 2 (CN) 2 ] (1a). A 100 mL Schlenk flask was charged with 0.127 g (1.00 mmol) of FeCl 2 and 10 mL of MeCN under CO. NaSPh was generated from 0.080 g (2.00 mmol) of NaH and 205 µL (2.00 mmol) of PhSH in 5 mL of MeOH followed by evaporation. A solution of the NaSPh in 15 mL of MeCN was transferred to the FeCl 2 solution under CO. After 2 h, a solution of 0.300 g (1.92 mmol) of NEt 4 CN in 2 mL MeCN was added, and the color turned orange in ∼10 min. After 1 h, the orange solution was filtered through Celite. The filtrate was reduced to ∼10 mL and 80 mL of THF was added followed by cooling to-20 °C for 48 h to yield orange microcrystals. These crystals were washed with 5 mL of THF to remove oily impurites.
The box-like cages {M[Cp*Rh(CN) 3 ] 4 [Mo(CO) 3 ] 4 } 3form as the sole metal-containing products... more The box-like cages {M[Cp*Rh(CN) 3 ] 4 [Mo(CO) 3 ] 4 } 3form as the sole metal-containing products of the reaction of [Cp*Rh(CN) 3 ]and (η 6-C 6 H 3 Me 3)Mo(CO) 3 in the presence of K + and Cs +. Well-defined species could not be identified in solutions of Cp*Rh(CN) 3and (η 6-C 6 H 3 Me 3)Mo(CO) 3 in the absence of alkali metal cations. The new cages were isolated as their Et 4 N + salts, M) K + (1), Cs + (2). Crystallographic characterization of 1 and 2 reveals box-like M 8 (µ-CN) 12 cages containing alkali metal cations. The cages feature 12 external CO and 4 external C 5 Me 5 ligands. In 1, the K + is disordered over two off-center positions, whereas in the case of 2, the Cs + is centered in the cage with a formal coordination number of 24. Otherwise, the structures of the two compounds are virtually indistinguishable. The persistence of the solid-state structures in solution was established through 13 C NMR spectroscopy and electrospray mass spectrometric measurements. 133 Cs NMR spectroscopy, which readily distinguishes free from included Cs + , shows that the boxes preferentially bind Cs + relative to K + .
We have investigated the charge density, F(r), its curvature, ∂ 2 F/∂r ij , the dipole moment, µ,... more We have investigated the charge density, F(r), its curvature, ∂ 2 F/∂r ij , the dipole moment, µ, and the electrostatic potential, Φ(r), in L-asparagine monohydrate by using high-resolution single-crystal X-ray crystallography and quantum chemistry. In addition, we have compared electric field gradient, ∇E, results obtained from crystallography and quantum chemistry with those obtained from single-crystal 14 N nuclear magnetic resonance spectroscopy. A multipole model of the X-ray F(r) is compared to Hartree-Fock and density functional theory predictions, using two different large basis sets. The quality of the calculated charge densities is evaluated from a simultaneous comparison of eight Hessian-of-F(r) tensors at bond critical points between non-hydrogen atoms. These tensors are expressed in an icosahedral representation, which includes information on both tensor magnitude and orientation. The best theory-versus-experiment correlation is found at the B3LYP/6-311++G(2d,2p) level, which yields a slope of 1.09 and an R 2 value of 0.96. Both DFT and HF results give molecular dipole moments in good accord with the value extracted from the X-ray diffraction data, 14.3(3) D, and both sets of calculations are found to correctly reproduce the experimental molecular electrostatic potential, Φ(r). The intermolecular hydrogen bond F(r) is also subjected to a detailed theoretical and experimental topological analysis, and again good agreement is found between theory and experiment. For the comparison of the ∇E tensors, the icosahedral representation is again used. There is found to be moderate accord between theory and experiment when using results obtained from diffraction data, but much better accord when using results obtained from NMR data (slope) 1.14, R 2) 0.94, for the 12 icosahedral tensor elements for N1 and N2). Overall, these results strongly support the idea that both HF and DFT methods give excellent representations of the electrostatic properties F(r), ∂ 2 F/∂r ij , µ, Φ(r), and ∇E, for crystalline L-asparagine monohydrate, encouraging their future use in situations where experimental results are lacking, such as in peptides and in enzyme active sites.
The temperature and kinetic data in Table 2 for cis-hydrindanyl and trans-bicyclo[3.3.0]octanyl p... more The temperature and kinetic data in Table 2 for cis-hydrindanyl and trans-bicyclo[3.3.0]octanyl p-nitrobenzoates should be reversedscis-hydrindanyl pNB: t) 80°C, k) 2.11 × 10-5 s-1 and t) 50°C, k) 0.054 × 10-5 s-1 , k rel) 19; trans-bicyclo[3.3.0]octyl pNB: t) 50°C, k) 6.18 × 10-5 s-1 , k rel) 2200.
Thermolysis of a xylene solution of Cp 2 Fe 2 (CO) 4 and PPh 3 yields primarily Cp 4 Fe 4 (CO) 4 ... more Thermolysis of a xylene solution of Cp 2 Fe 2 (CO) 4 and PPh 3 yields primarily Cp 4 Fe 4 (CO) 4 (1) together with smaller amounts of (C 5 H 4 Ph)Cp 3 Fe 4 (CO) 4 and Cp 3 Fe 3 (CO) 3 (PPh 2). Cluster 1 can be alkylated and arylated by using organolithium reagents to give the derivatives (C 5 H 4 R)Cp 3 Fe 4 (CO) 4. This reaction is competitive with reduction of 1 by the organolithium reagent. A more versatile method for functionalizing 1 involves its deprotonation with lithium diisopropylamide (LDA) followed by treatment with electrophiles to give (C 5 H 4 X)Cp 3 Fe 4 (CO) 4 (X) C(OH)HCH 3 , CO 2 H, CHO, SPh, PPh 2). An excess of LDA gave increased amounts of the di-and even trifunctionalized derivatives (C 5 H 4 X) x Cp 4-x Fe 4 (CO) 4 (x) 2, 3). Treatment of (C 5 H 4-CHO)Cp 3 Fe 4 (CO) 4 with the lithiated cluster gave the double cluster [(C 5 H 4)Cp 3 Fe 4 (CO) 4 ] 2 CHOH. The use of the cluster as a ligand was demonstrated by the synthesis of the adducts (C 5 H 4 PPh 2 ML n)Cp 3 Fe 4 (CO) 4 , where ML n) RuCl 2 (cymene), IrCl(1,5-C 8 H 12). Single-crystal X-ray diffraction was employed to characterize [(C 5 H 4)Cp 3 Fe 4 (CO) 4 ] 2 CHOH and (C 3 H 4 PPh 2)Cp 3 Fe 4 (CO) 4 RuCl 2 (cymene).
The photoreaction of Fe(CO) 5 and cyanide salts in MeCN solution affords the dianion [Fe (CN) 2 (... more The photoreaction of Fe(CO) 5 and cyanide salts in MeCN solution affords the dianion [Fe (CN) 2 (CO) 3 ] 2− , conveniently isolated as [K(18-crown-6)] 2 [Fe(CN) 2 (CO) 3 ]. Solutions of [Fe (CN) 2 (CO) 3 ] 2− oxidize irreversibly at −600 mV (vs. Ag/AgCl) to give primarily [Fe(CN) 3 (CO) 3 ] −. Protonation of the dianion affords the hydride [K(18-crown-6)][HFe(CN) 2 (CO) 3 ] with a pK a ≈ 17 (MeCN). The ferrous hydride exhibits enhanced electrophilicity vs. its dianionic precursor, which resists substitution. Treatment of [K(18-crown-6)][Fe(CN) 2 (CO) 3 ] with tertiary phosphines and phosphites gives isomeric mixtures of [HFe(CN) 2 (CO) 2 L] − (L = P(OPh) 3 and PPh 3). Carbonyl substitution on [1H(CO) 2 ] − by P(OPh) 3 is first-order in both the phosphite and iron (k = 0.18 M −1 s −1 at 22 °C) with ΔH ‡ = 51.6 kJ mol −1 and ΔS ‡ = −83.0 J K −1 mol −1. These ligands are displaced under an atmosphere of CO. With cis-Ph 2 PCH=CHPPh 2 (dppv), we obtained the monocarbonyl, [HFe(CN) 2 (CO)(dppv)] − , a highly basic hydride (pK a > 23.3) that rearranges in solution to a single isomer. Treatment of [K(18-crown-6)][HFe(CN) 2 (CO) 3 ] with Et 4 NCN resulted in rapid proton transfer to give [Fe(CN) 2 (CO) 3 ] 2− and HCN. The tricyano hydride [HFe(CN) 3 (CO) 2 ] 2− is prepared by the reaction of [HFe(CN) 2 (CO) 2 (PPh 3)] − and [K(18-crown-6)]CN. Similar to the phosphine and phosphite derivatives, [HFe(CN) 3 (CO) 2 ] 2− exists as a mixture of all three possible isomers. Protonation of the hydrides [HFe(CN) 2 (CO)(dppv)] − and [HFe(CN) 3 (CO) 2 ] − in acetonitrile solutions releases H 2 and gives the corresponding acetonitrile complexes [K(18-crown-6)][Fe (CN) 3 (NCMe)(CO) 2 ] and Fe(CN) 2 (NCMe)(CO)(dppv). Alkylation of [K(18-crown-6)] 2 [Fe (CN) 2 (CO) 3 ] with MeOTf gives the thermally-unstable [MeFe(CN) 2 (CO) 3 ] − , which was characterized spectroscopically at −40 °C. Reaction of dppv with [MeFe(CN) 2 (CO) 3 ] − gives the acetyl complex, [Fe(CN) 2 (COMe)(CO)(dppv)] −. Whereas [Fe(CN) 2 (CO) 3 ] 2− undergoes protonation and methylation at Fe, acid chlorides give the iron(0) N-acylisocyanides [Fe(CN) (CO) 3 (CNCOR)] − (R = Ph, CH 3). The solid state structures of [K(18-crown-6)][HFe(CN) 2 (CO) (dppv)], Fe(CN) 2 (NCMe)(CO)(dppv), and [K(18-crown-6)] 2 [HFe(CN) 3 (CO) 2 ] were confirmed crystallographically. In all three cases, the cyanide ligands are cis to the hydride or acetonitrile ligands.
The reactivity of metal olefin complexes with non-innocent ligands (NILs) was examined. Treatment... more The reactivity of metal olefin complexes with non-innocent ligands (NILs) was examined. Treatment of PtCl 2 (diene) with the deprotonated catechol or aminophenol ligands afforded the corresponding Pt(NIL)(diene) complexes. The Pt(t BA F Ph)(COD), Pt(t BA F Ph)(nbd), and Pt(O 2 C 6 H 2 t Bu 2)(COD) (H 2 t BA F Ph) 2-(2-trifluoromethyl)anilino-4,6-di-tertbutylphenol, H 2 O 2 C 6 H 2 t Bu 2) 3,5-di-tert-butylcatechol) complexes were examined by cyclic voltammetry. Treatment of Pt(t BA F Ph)(COD) or Pt(t BA F Ph)(nbd) with AgPF 6 afforded the imino-semiquinones [Pt(t BA F Ph)(COD)]PF 6 or [Pt(t BA F Ph)(nbd)]PF 6 , respectively. The [Pt(t BA F Ph)(COD)] complex was unreactive toward nucleophiles, whereas the oxidized derivative, [Pt(t BA F Ph)(COD)]PF 6 , rapidly and stereospecifically added alkoxides at the carbon trans to the phenolate. The Pt(t BA F Ph)(COD), [Pt(t BA F Ph)(COD)]PF 6 , Pt(t BA F Ph)(C 8 H 12 OMe), and [Cp 2 Co][Pt-(t BA F Ph)(C 8 H 12 OMe)] complexes were characterized crystallographically.
Condensation of Fe 2 (SH) 2 (CO) 6 , acetaldehyde, and (NH 4) 2 CO 3 affords the methyl-substitut... more Condensation of Fe 2 (SH) 2 (CO) 6 , acetaldehyde, and (NH 4) 2 CO 3 affords the methyl-substituted azadithiolate Fe 2 [(SCHMe) 2 NH](CO) 6 (1). The complex exists mainly (~95%) as the meso diastereomer, but the d,l diastereoisomers could be detected. DFT calculations predict that the meso isomer would be 2.5 kcal/mol more stable than the d,l isomer due to conventional nonbonding interactions between the methyl groups and the ring hydrogen atoms. Crystallographic analysis of meso-1 confirms that the two methyl groups are equatorial, constraining the diferraazadithiolate bicycle to a conformation that desymmetrizes the diiron center. The lowered symmetry is confirmed by the observation of two 13 C NMR signals in the FeCO region under conditions of fast turnstile rotation at the Fe(CO) 3 groups. The pK a value of the amine in 1 is 7.89 (all pK a 's determined in MeCN solution), which is similar to a redetermined value for Fe 2 [(SCH 2) 2 NH](CO) 6 (2, pK a = 7.98) and only slightly less basic than the tertiary amine Fe 2 [(SCH 2) 2 NMe](CO) 6 (pK a = 8.14). Substitution of 1 with PMe 3 proceeded via the intermediacy of two isomers of Fe 2 [(SCHMe) 2 NH] (CO) 5 (PMe 3), affording Fe 2 [(SCHMe) 2 NH](CO) 4 (PMe 3) 2 (3). 31 P NMR spectra confirm that the two PMe 3 ligands in 3 are nonequivalent, consistent with the desymmetrizing effect of the dithiolate. The pK a value of the amine in 3 was found to be 11.3. Using triphenylphosphine, we prepared Fe 2 [(SCHMe) 2 NH](CO) 5 (PPh 3) as a single regioisomer.
Journal of the American Chemical Society, Aug 14, 2008
This study probes the impact of electronic asymmetry of diiron(I) dithiolato carbonyls. Treatment... more This study probes the impact of electronic asymmetry of diiron(I) dithiolato carbonyls. Treatment of Fe 2 (S 2 C n H 2n)(CO) 6-x (PMe 3) x compounds (n = 2, 3) with NOBF 4 gave the derivatives [Fe 2 (S 2 C n H 2n)(CO) 5-x (PMe 3) x (NO)]BF 4 (x = 1, 2, 3) which are electronically unsymmetrical due to the presence of a single NO + ligand. Whereas the mono phosphine derivative is largely undistorted, the bis PMe 3 derivatives are distorted such that the CO ligand on the Fe(CO)(PMe 3)(NO) + subunit is semibridging. Two isomers of [Fe 2 (S 2 C 3 H 6)(CO) 3 (PMe 3) 2 (NO)]BF 4 were characterized spectroscopically and crystallographically. Each isomer features electron-rich [Fe(CO) 2 PMe 3 ] and electrophilic [Fe(CO)(PMe 3)(NO)] + subunits. These species are in equilibrium with an unobserved isomer that reversibly binds CO (ΔH = −35 kJ/mol, ΔS = −139 J/mol•K) to give the symmetrical adduct [Fe 2 (S 2 C 3 H 6)(μ-NO)(CO) 4 (PMe 3) 2 ]BF 4. In contrast to Fe 2 (S 2 C 3 H 6)(CO) 4 (PMe 3) 2 , the bis (PMe 3) nitrosyls readily undergo CO-substitution to give the (PMe 3) 3 derivatives. The nitrosyl complexes reduce at potentials that are ~1 V milder than their carbonyl counterparts. DFT calculations, specifically NBO values, reinforce the electronic resemblance of the nitrosyl complexes with the corresponding mixed-valence diiron complexes. Unlike other diiron dithiolato carbonyls, these species undergo reversible reductions at mild conditions. The results show that the novel structural and chemical features associated with mixed valence diiron dithiolates-the so-called H ox models-can be replicated in the absence of mixed-valency by introducing electronic asymmetry.
Journal of the American Chemical Society, Nov 29, 2010
The report summarizes studies on the redox behavior of synthetic models for the [FeFe]hydrogenase... more The report summarizes studies on the redox behavior of synthetic models for the [FeFe]hydrogenases, consisting of diiron dithiolato carbonyl complexes bearing the amine cofactor and its N-benzyl derivative. Of specific interest are the causes of the low reactivity of oxidized models toward H 2 , which contrasts with the high activity of these enzymes for H 2 oxidation. The redox and acid-base properties of the model complexes [Fe 2 [(SCH 2) 2 NR](CO) 3 (dppv)(PMe 3)] + ([2] + for R = H and [2′] + for R = CH 2 C 6 H 5 , dppv = cis-1,2-bis(diphenylphosphino)ethylene)) indicate that addition of H 2 and followed by deprotonation are (i) endothermic for the mixed valence (Fe II Fe I) state and (ii) exothermic for the diferrous (Fe II Fe II) state. The diferrous state is shown to be unstable with respect to coordination of the amine to Fe, a derivative of which was characterized crystallographically. The redox and acid-base properties for the mixed valence models differ strongly for those containing the amine cofactor versus those derived from propanedithiolate. Protonation of [2′] + induces disproportionation to a 1:1 mixture of the ammonium-Fe I Fe I and the dication [2′] 2+ (Fe II Fe II). This effect is consistent with substantial enhancement of the basicity of the amine in the Fe I Fe I state vs the Fe II Fe I state. The Fe I Fe I ammonium compounds are rapid and efficient H-atom donors toward the nitroxyl compound TEMPO. The atom transfer is proposed to proceed via the hydride, as indicated by the reaction of [HFe 2 [(SCH 2) 2 NH](CO) 2 (dppv) 2 ] + with TEMPO. Collectively, the results suggest that proton-coupled electron-transfer pathways should be considered for H 2 activation by the [FeFe]-hydrogenases.
The one-electron oxidations of a series of diiron(I) dithiolato carbonyls were examined to evalua... more The one-electron oxidations of a series of diiron(I) dithiolato carbonyls were examined to evaluate the factors that affect the oxidation state assignments, structures, and reactivity of these lowmolecular weight models for the H ox state of the [FeFe]-hydrogenases. The propanedithiolates Fe 2 (S 2 C 3 H 6)(CO) 3 (L)(dppv) (L = CO, PMe 3 , Pi-Pr 3) oxidize at potentials ~180 mV milder than the related ethanedithiolates (Angew. Chem. Int. Ed. 2007, 46, 6152). The steric clash between the central methylene of the propanedithiolate and the phosphine favors the rotated structure, which forms upon oxidation. EPR spectra for the mixed-valence cations indicate that the unpaired electron is localized on the Fe(CO)(dppv) center in both [Fe 2 (S 2 C 3 H 6)(CO) 4 (dppv)]BF 4 and [Fe 2 (S 2 C 3 H 6) (CO) 3 (PMe 3)(dppv)]BF 4 , as seen previously for the ethanedithiolate [Fe 2 (S 2 C 2 H 4)(CO) 3 (PMe 3) (dppv)]BF 4. For [Fe 2 (S 2 C n H 2n)(CO) 3 (Pi-Pr 3)(dppv)]BF 4 , however, the spin is localized on the Fe (CO) 2 (Pi-Pr 3) center, although the Fe(CO)(dppv) site is rotated in the crystalline state. IR and EPR spectra, as well as redox potentials and DFT-calculations, suggest, however, that the Fe(CO) 2 (Pi-Pr 3) site is rotated in solution, driven by steric factors. Analysis of the DFT-computed partial atomic charges for the mixed-valence species shows that the Fe atom featuring a vacant apical coordination position is an electrophilic Fe(I) center. One-electron oxidation of [Fe 2 (S 2 C 2 H 4)(CN) (CO) 3 (dppv)] − resulted in 2e oxidation of 0.5 equiv to give the μ-cyano derivative [Fe I 2 (S 2 C 2 H 4) (CO) 3 (dppv)](μ-CN)[Fe II 2 (S 2 C 2 H 4)(μ-CO)(CO) 2 (CN)(dppv)], which was characterized spectroscopically. Correspondence to: Luca De Gioia; Thomas B. Rauchfuss. Supporting Information Available: Crystallographic data for [Fe 2 (S 2 C 3 H 6)(CO) 3 (PMe 3)(dppv)]BF 4 and [Fe 2 (S 2 C 3 H 6)(CO) 3 (Pi-Pr 3)(dppv)]BF 4 , voltammograms, as well as IR, EPR, and NMR spectra. For DFT calculations: atomic coordinates, energy values, spin populations and partial atomic charges for the DFT models [Fe 2 (S 2 C n H 2n)(CO) 3 (PH 3) 3 ] + , [Fe 2 (S 2 C n H 2n)(CO) 3 (PMe 3)(dppv)] + , and [Fe 2 (S 2 C n H 2n)(CO) 3 (Pi-Pr 3)(dppv)] +. This material is available free of charge via the Internet at …
The [FeFe] hydrogenase enzymes are the most efficient catalysts known for the reduction of proton... more The [FeFe] hydrogenase enzymes are the most efficient catalysts known for the reduction of protons to H 2. [1] The active site exists in two functional states (Scheme 1), H red , which is S =0, and H ox , which is S = 1/2. [2] Research in this area is aimed at elucidating the mechanism of the enzymatic catalysis and at using this information to develop protein-free bioinspired synthetic catalysts. [3] A specific research goal is the preparation of molecules that resemble the functional states of the active site with the expectation that function will follow form. Most studies on diiron dithiolato carbonyl complexes rely on organic ligands (e.g. phosphanes) in place of the naturally occurring cyanide and μ-SR[Fe 4 S 4 ] ligands, [4] which have complicated acid-base behavior that is often difficult to control outside of the protein. Another barrier to modeling has been the rarity of mixed-valence diiron dithiolate compounds with the appropriate structures, stability, and reactivity. ** This research was supported by the US National Institutes of Health. We thank Teresa Prussak-Wieckowska for assistance on X-ray crystallography.
Two isostructural metalloporphyrin framework solids have been synthesized. Both frameworks contai... more Two isostructural metalloporphyrin framework solids have been synthesized. Both frameworks contains manganese(III) metal complexes of trans-dicarboxylateporphyrins whose peripheralcarboxylates coordinate the edges of tetrahedral Zn4O[Formula: see text] clusters; the two metalloporphyrins explored are Mn(III) and Co(II). The cubic interpenetrated frameworks have 72% free volume and 4 × 7 Å averaged size pores. The evacuated frameworks are robust and retain a structure open to the sorption of substrates with medium polarity. The manganese porphyrin framework catalyzes the hydroxylation of cyclic and linear alkanes with iodosylbenzene as oxidant in a size- and polarity-selective manner. In addition, the catalysis was found to occur within the pores, making this a rare case of porphyrin framework solid with interior catalysis.
The report summarizes studies on the redox behavior of synthetic models for the [FeFe]hydrogenase... more The report summarizes studies on the redox behavior of synthetic models for the [FeFe]hydrogenases, consisting of diiron dithiolato carbonyl complexes bearing the amine cofactor and its N-benzyl derivative. Of specific interest are the causes of the low reactivity of oxidized models toward H 2 , which contrasts with the high activity of these enzymes for H 2 oxidation. The redox and acid-base properties of the model complexes [Fe 2 [(SCH 2) 2 NR](CO) 3 (dppv)(PMe 3)] + ([2] + for R = H and [2′] + for R = CH 2 C 6 H 5 , dppv = cis-1,2-bis(diphenylphosphino)ethylene)) indicate that addition of H 2 and followed by deprotonation are (i) endothermic for the mixed valence (Fe II Fe I) state and (ii) exothermic for the diferrous (Fe II Fe II) state. The diferrous state is shown to be unstable with respect to coordination of the amine to Fe, a derivative of which was characterized crystallographically. The redox and acid-base properties for the mixed valence models differ strongly for those containing the amine cofactor versus those derived from propanedithiolate. Protonation of [2′] + induces disproportionation to a 1:1 mixture of the ammonium-Fe I Fe I and the dication [2′] 2+ (Fe II Fe II). This effect is consistent with substantial enhancement of the basicity of the amine in the Fe I Fe I state vs the Fe II Fe I state. The Fe I Fe I ammonium compounds are rapid and efficient H-atom donors toward the nitroxyl compound TEMPO. The atom transfer is proposed to proceed via the hydride, as indicated by the reaction of [HFe 2 [(SCH 2) 2 NH](CO) 2 (dppv) 2 ] + with TEMPO. Collectively, the results suggest that proton-coupled electron-transfer pathways should be considered for H 2 activation by the [FeFe]-hydrogenases.
Nitrosyl derivatives of diiron dithiolato carbonyls have been prepared starting from the versatil... more Nitrosyl derivatives of diiron dithiolato carbonyls have been prepared starting from the versatile precursor Fe 2 (S 2 C n H 2n)(dppv)(CO) 4 (dppv = cis-1,2-bis(diphenylphosphinoethylene). These studies expand the range of substituted diiron(I) dithiolato carbonyl complexes. From [Fe 2 (S 2 C 2 H 4)(CO) 3 (dppv)(NO)]BF 4 ([1(CO) 3 ]BF 4), the following compounds were prepared: [1 (CO) 2 (PMe 3)]BF 4 , [1(CO)(dppv)]BF 4 , NEt 4 [1(CO)(CN) 2 ], and 1(CO)(CN)(PMe 3). Some if not all of these substitution reactions occur via the addition of two equiv of the nucleophile followed by dissociation of one nucleophile and decarbonylation. Such a double adduct was characterized crystallographically in the case of [Fe 2 (S 2 C 2 H 4)(CO) 3 (dppv)(NO)(PMe 3) 2 ]BF 4. This result shows that the addition of two ligands causes scission of the Fe-Fe bond and one Fe-S bond. When cyanide is the nucleophile, nitrosyl migrates away from the Fe(dppv) site, yielding a Fe(CN)(NO)(CO) derivative. Compounds [1(CO) 3 ]BF 4 , [1(CO) 2 (PMe 3)]BF 4 , and [1(CO)(dppv)]BF 4 were also prepared by the addition of NO + to the di-, tri-and tetrasubstituted precursors. In these cases, the NO + appears to form an initial 36e-adduct containing terminal Fe-NO, followed by decarbonylation. Several complexes were prepared by the addition of NO to the mixed-valence Fe(I)Fe(II) derivatives. The diiron nitrosyl complexes reduce at mild potentials and in certain cases form weak adducts with CO.
The accuracy of the Content should not be relied upon and should be independently verified with p... more The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden.
The titanium(II) alkyl trans-TiMe 2 (dmpe) 2 , where dmpe is 1,2-bis(dimethylphosphino)ethane, re... more The titanium(II) alkyl trans-TiMe 2 (dmpe) 2 , where dmpe is 1,2-bis(dimethylphosphino)ethane, reacts with 1,3-butadiene and trans,trans-1,4-diphenyl-1,3-butadiene at-20°C to produce the titanium(II) butadiene complexes TiMe 2 (η 4-C 4 H 4 R 2)(dmpe), where R is H or Ph. NMR spectra are consistent with structures in which the methyl groups are mutually cis, and this has been verified crystallographically for the 1,4-diphenylbutadiene complex. These molecules are fluxional on the NMR time scale, and the activation parameters for exchange are ∆H q) 9.1 (0.2 kcal mol-1 and ∆S q) 3 (1 eu for the 1,4-diphenylbutadiene complex. The process that exchanges the two TiMe groups, the two ends of the dmpe ligand, and the two ends of the butadiene ligand is proposed to be a trigonal twist, although we cannot entirely rule out the possibility that the exchange involves five-coordinate intermediates generated by dissociation of one "arm" of a chelating ligand. If the reaction of TiMe 2 (dmpe) 2 and 1,3-butadiene is allowed to proceed at-20°C for prolonged periods (>12 h), a second titanium "butadiene" complex is formed, which has been identified as the titanium(IV) η 3 ,η 1octa-1,6-diene-1,8-diyl complex TiMe 2 (η 3 ,η 1-C 8 H 12)(dmpe). Warming a solution of TiMe 2-(dmpe) 2 and 1,3-butadiene to 25°C results in the catalytic dimerization of butadiene to the Diels-Alder dimer 4-vinylcyclohexene at rates of 5 turnovers/h. A mechanism for the catalytic dimerization is proposed, which involves coupling of two butadiene ligands to form a divinyltitanacyclopentane species, allylic rearrangement to a vinyltitanacycloheptene intermediate, and reductive elimination to form the cyclic product. Treatment of TiMe 2-(dmpe) 2 with 1,3-butadiene in the presence of AlEt 3 results in reduction to the titanium(0) complex Ti(η 4-C 4 H 6) 2 (dmpe), which has also been crystallographically characterized. Unlike the behavior seen for certain other early transition metal butadiene complexes, in both Ti-(η 4-C 4 H 6) 2 (dmpe) and TiMe 2 (η 4-C 4 H 4 Ph 2)(dmpe) the butadiene ligands are bound like true dienes. We propose that the preferred bonding mode for butadiene complexes of the lower valent early transition metals is the π,η 4 mode and that increasing σ 2 ,π character is introduced only when there are significant steric repulsions between the ancillary ligands and the meso butadiene substituents.
All manipulations were conducted using standard Schlenk techniques. All solvents were dried using... more All manipulations were conducted using standard Schlenk techniques. All solvents were dried using the Grubbs' solvent purification process. Elemental analyses were conducted by the University of Illinois Microanalytical Laboratory. 1 H NMR spectra were acquired on a Unity Varian 400 or a Unity Varian 500 spectrometer. IR spectra were collected on a Mattson Infinity Gold FTIR spectrometer. ESI-MS were collected on a Quattro quadrupole-hexapole-quadrapole (QHQ) mass spectrometer. (Et 4 N) 2 [Fe(SPh) 2 (CO) 2 (CN) 2 ] (1a). A 100 mL Schlenk flask was charged with 0.127 g (1.00 mmol) of FeCl 2 and 10 mL of MeCN under CO. NaSPh was generated from 0.080 g (2.00 mmol) of NaH and 205 µL (2.00 mmol) of PhSH in 5 mL of MeOH followed by evaporation. A solution of the NaSPh in 15 mL of MeCN was transferred to the FeCl 2 solution under CO. After 2 h, a solution of 0.300 g (1.92 mmol) of NEt 4 CN in 2 mL MeCN was added, and the color turned orange in ∼10 min. After 1 h, the orange solution was filtered through Celite. The filtrate was reduced to ∼10 mL and 80 mL of THF was added followed by cooling to-20 °C for 48 h to yield orange microcrystals. These crystals were washed with 5 mL of THF to remove oily impurites.
The box-like cages {M[Cp*Rh(CN) 3 ] 4 [Mo(CO) 3 ] 4 } 3form as the sole metal-containing products... more The box-like cages {M[Cp*Rh(CN) 3 ] 4 [Mo(CO) 3 ] 4 } 3form as the sole metal-containing products of the reaction of [Cp*Rh(CN) 3 ]and (η 6-C 6 H 3 Me 3)Mo(CO) 3 in the presence of K + and Cs +. Well-defined species could not be identified in solutions of Cp*Rh(CN) 3and (η 6-C 6 H 3 Me 3)Mo(CO) 3 in the absence of alkali metal cations. The new cages were isolated as their Et 4 N + salts, M) K + (1), Cs + (2). Crystallographic characterization of 1 and 2 reveals box-like M 8 (µ-CN) 12 cages containing alkali metal cations. The cages feature 12 external CO and 4 external C 5 Me 5 ligands. In 1, the K + is disordered over two off-center positions, whereas in the case of 2, the Cs + is centered in the cage with a formal coordination number of 24. Otherwise, the structures of the two compounds are virtually indistinguishable. The persistence of the solid-state structures in solution was established through 13 C NMR spectroscopy and electrospray mass spectrometric measurements. 133 Cs NMR spectroscopy, which readily distinguishes free from included Cs + , shows that the boxes preferentially bind Cs + relative to K + .
We have investigated the charge density, F(r), its curvature, ∂ 2 F/∂r ij , the dipole moment, µ,... more We have investigated the charge density, F(r), its curvature, ∂ 2 F/∂r ij , the dipole moment, µ, and the electrostatic potential, Φ(r), in L-asparagine monohydrate by using high-resolution single-crystal X-ray crystallography and quantum chemistry. In addition, we have compared electric field gradient, ∇E, results obtained from crystallography and quantum chemistry with those obtained from single-crystal 14 N nuclear magnetic resonance spectroscopy. A multipole model of the X-ray F(r) is compared to Hartree-Fock and density functional theory predictions, using two different large basis sets. The quality of the calculated charge densities is evaluated from a simultaneous comparison of eight Hessian-of-F(r) tensors at bond critical points between non-hydrogen atoms. These tensors are expressed in an icosahedral representation, which includes information on both tensor magnitude and orientation. The best theory-versus-experiment correlation is found at the B3LYP/6-311++G(2d,2p) level, which yields a slope of 1.09 and an R 2 value of 0.96. Both DFT and HF results give molecular dipole moments in good accord with the value extracted from the X-ray diffraction data, 14.3(3) D, and both sets of calculations are found to correctly reproduce the experimental molecular electrostatic potential, Φ(r). The intermolecular hydrogen bond F(r) is also subjected to a detailed theoretical and experimental topological analysis, and again good agreement is found between theory and experiment. For the comparison of the ∇E tensors, the icosahedral representation is again used. There is found to be moderate accord between theory and experiment when using results obtained from diffraction data, but much better accord when using results obtained from NMR data (slope) 1.14, R 2) 0.94, for the 12 icosahedral tensor elements for N1 and N2). Overall, these results strongly support the idea that both HF and DFT methods give excellent representations of the electrostatic properties F(r), ∂ 2 F/∂r ij , µ, Φ(r), and ∇E, for crystalline L-asparagine monohydrate, encouraging their future use in situations where experimental results are lacking, such as in peptides and in enzyme active sites.
The temperature and kinetic data in Table 2 for cis-hydrindanyl and trans-bicyclo[3.3.0]octanyl p... more The temperature and kinetic data in Table 2 for cis-hydrindanyl and trans-bicyclo[3.3.0]octanyl p-nitrobenzoates should be reversedscis-hydrindanyl pNB: t) 80°C, k) 2.11 × 10-5 s-1 and t) 50°C, k) 0.054 × 10-5 s-1 , k rel) 19; trans-bicyclo[3.3.0]octyl pNB: t) 50°C, k) 6.18 × 10-5 s-1 , k rel) 2200.
Thermolysis of a xylene solution of Cp 2 Fe 2 (CO) 4 and PPh 3 yields primarily Cp 4 Fe 4 (CO) 4 ... more Thermolysis of a xylene solution of Cp 2 Fe 2 (CO) 4 and PPh 3 yields primarily Cp 4 Fe 4 (CO) 4 (1) together with smaller amounts of (C 5 H 4 Ph)Cp 3 Fe 4 (CO) 4 and Cp 3 Fe 3 (CO) 3 (PPh 2). Cluster 1 can be alkylated and arylated by using organolithium reagents to give the derivatives (C 5 H 4 R)Cp 3 Fe 4 (CO) 4. This reaction is competitive with reduction of 1 by the organolithium reagent. A more versatile method for functionalizing 1 involves its deprotonation with lithium diisopropylamide (LDA) followed by treatment with electrophiles to give (C 5 H 4 X)Cp 3 Fe 4 (CO) 4 (X) C(OH)HCH 3 , CO 2 H, CHO, SPh, PPh 2). An excess of LDA gave increased amounts of the di-and even trifunctionalized derivatives (C 5 H 4 X) x Cp 4-x Fe 4 (CO) 4 (x) 2, 3). Treatment of (C 5 H 4-CHO)Cp 3 Fe 4 (CO) 4 with the lithiated cluster gave the double cluster [(C 5 H 4)Cp 3 Fe 4 (CO) 4 ] 2 CHOH. The use of the cluster as a ligand was demonstrated by the synthesis of the adducts (C 5 H 4 PPh 2 ML n)Cp 3 Fe 4 (CO) 4 , where ML n) RuCl 2 (cymene), IrCl(1,5-C 8 H 12). Single-crystal X-ray diffraction was employed to characterize [(C 5 H 4)Cp 3 Fe 4 (CO) 4 ] 2 CHOH and (C 3 H 4 PPh 2)Cp 3 Fe 4 (CO) 4 RuCl 2 (cymene).
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