Isomerization of an Enantiomerically Pure Phosphorus-Bridged [1]Ferrocenophane

The enantiomerically pure phospha[1]­ferrocenophane <b>4-</b><i><b>C</b></i>_<b>1</b> was prepared through a salt-metathesis reaction between <i>t</i>BuPCl₂ and a chiral dilithioferrocene derivative (Li₂fc^3‑Pen), which was equipped with two 3-pentyl groups in α positions with respect to lithium on the Cp rings (<i>S</i>_p<i>,S</i>_p isomer; <i>C</i>₂ symmetry). The chiral <b>4-</b><i><b>C</b></i>_<b>1</b> isomerizes in reaction mixtures to give the <i>C</i>_<i>s</i>-symmetrical phospha[1]­ferrocenophane <b>4-</b><i><b>C</b></i>_{<b><i>s</i></b>}. This thermal isomerization involves haptotropic η⁵ to η¹ shifts of Cp rings and is catalyzed by the chelating ligand 1,1′-(<i>t</i>BuClP)₂fc^3‑Pen, which is a byproduct of the salt-metathesis reaction. Both phospha[1]­ferrocenophanes, <b>4-</b><i><b>C</b></i>_<b>1</b> and <b>4-</b><i><b>C</b></i>_{<b><i>s</i></b>}, were isolated and characterized as pure compounds; the molecular structure of <b>4-</b><i><b>C</b></i>_{<b><i>s</i></b>} was determined by a single-crystal X-ray analysis. According to DFT calculations, the equilibrium constant <i>K</i>° for <b>4-</b><i><b>C</b></i>_<b>1</b> ⇌ <b>4-</b><i><b>C</b></i>_{<b><i>s</i></b>} is 4.43 (Δ<i>E</i>^SCF = −3.74 kcal/mol; Δ<i>H</i>° = −3.81 kcal/mol; Δ<i>G</i>° = −3.68 kcal/mol). As deduced from calculated molecular geometries, the thermodynamic difference between both isomers is mainly caused by a steric repulsion between the <i>t</i>Bu group on phosphorus and one of the 3-pentyl groups on a Cp ring in the isomer <b>4-</b><i><b>C</b></i>_<b>1</b>

Electronic Structure of Re₂(O₂CR)₄Cl₂ Complexes (R = H, CMe₃) and Reassignment of the Electronic Absorption Spectrum of Re₂(O₂CCMe₃)₄Cl₂

Electronic structure calculations on two dinuclear rhenium(III) carboxylate complexes, Re2(O2CH)4Cl2 and Re2(O2CCMe3)4Cl2, are presented and discussed. Allowed electronic transitions for both molecules were calculated using time-dependent density functional theory (TDDFT). The results for the pivalate dimer, Re2(O2CCMe3)4Cl2, are compared with previously reported single-crystal polarized absorption spectra obtained by Martin and co-workers (Inorg. Chem.1984, 23, 699−701). Several revisions to the previous spectral assignments are proposed

Bonding in Complexes of Bis(pentalene)dititanium, Ti₂(C₈H₆)₂

Bonding in the bis­(pentalene)­dititanium “double-sandwich” species Ti₂Pn₂ (Pn = C₈H₆) and its interaction with other fragments have been investigated by density functional calculations and fragment analysis. Ti₂Pn₂ with <i>C</i>_2<i>v</i> symmetry has two metal–metal bonds and a low-lying metal-based empty orbital, all three frontier orbitals having a₁ symmetry. The latter may be regarded as being derived by symmetric combinations of the classic three frontier orbitals of two bent bis­(cyclopentadienyl) metal fragments. Electrochemical studies on Ti₂Pn^†₂ (Pn^† = 1,4-{SiⁱPr₃}₂C₈H₄) revealed a one-electron oxidation, and the formally mixed-valence Ti­(II)–Ti­(III) cationic complex [Ti₂Pn^†₂]­[B­(C₆F₅)₄] has been structurally characterized. Theory indicates an <i>S</i> = ¹/₂ ground-state electronic configuration for the latter, which was confirmed by EPR spectroscopy and SQUID magnetometry. Carbon dioxide binds symmetrically to Ti₂Pn₂, preserving the <i>C</i>_2<i>v</i> symmetry, as does carbon disulfide. The dominant interaction in Ti₂Pn₂CO₂ is σ donation into the LUMO of bent CO₂, and donation from the O atoms to Ti₂Pn₂ is minimal, whereas in Ti₂Pn₂CS₂ there is significant interaction with the S atoms. The bridging O atom in the mono­(oxo) species Ti₂Pn₂O, however, employs all three O 2p orbitals in binding and competes strongly with Pn, leading to weaker binding of the carbocyclic ligand, and the sulfur analogue Ti₂Pn₂S behaves similarly. Ti₂Pn₂ is also capable of binding one, two, or three molecules of carbon monoxide. The bonding demands of a single CO molecule are incompatible with symmetric binding, and an asymmetric structure is found. The dicarbonyl adduct Ti₂Pn₂(CO)₂ has <i>C_s</i> symmetry with the Ti₂Pn₂ unit acting as two MCp₂ fragments. Synthetic studies showed that in the presence of excess CO the tricarbonyl complex Ti₂Pn^†₂(CO)₃ is formed, which optimizes to an asymmetric structure with one semibridging and two terminal CO ligands. Low-temperature ¹³C NMR spectroscopy revealed a rapid dynamic exchange between the two bound CO sites and free CO

The Reductive Activation of CO₂ Across a TiTi Double Bond: Synthetic, Structural, and Mechanistic Studies

The reactivity of the bis­(pentalene)­dititanium double-sandwich compound Ti₂Pn^†₂ (<b>1</b>) (Pn^† = 1,4-{SiⁱPr₃}₂C₈H₄) with CO₂ is investigated in detail using spectroscopic, X-ray crystallographic, and computational studies. When the CO₂ reaction is performed at −78 °C, the 1:1 adduct <b>4</b> is formed, and low-temperature spectroscopic measurements are consistent with a CO₂ molecule bound symmetrically to the two Ti centers in a μ:η²,η² binding mode, a structure also indicated by theory. Upon warming to room temperature the coordinated CO₂ is quantitatively reduced over a period of minutes to give the bis­(oxo)-bridged dimer <b>2</b> and the dicarbonyl complex <b>3</b>. In situ NMR studies indicated that this decomposition proceeds in a stepwise process via monooxo (<b>5</b>) and monocarbonyl (<b>7</b>) double-sandwich complexes, which have been independently synthesized and structurally characterized. <b>5</b> is thermally unstable with respect to a μ-O dimer in which the Ti–Ti bond has been cleaved and one pentalene ligand binds in an η⁸ fashion to each of the formally Ti^III centers. The molecular structure of <b>7</b> shows a “side-on” bound carbonyl ligand. Bonding of the double-sandwich species Ti₂Pn₂ (Pn = C₈H₆) to other fragments has been investigated by density functional theory calculations and fragment analysis, providing insight into the CO₂ reaction pathway consistent with the experimentally observed intermediates. A key step in the proposed mechanism is disproportionation of a mono­(oxo) di-Ti^III species to yield di-Ti^II and di-Ti^IV products. <b>1</b> forms a structurally characterized, thermally stable CS₂ adduct <b>8</b> that shows symmetrical binding to the Ti₂ unit and supports the formulation of <b>4</b>. The reaction of <b>1</b> with COS forms a thermally unstable complex <b>9</b> that undergoes scission to give mono­(μ-S) mono­(CO) species <b>10</b>. Ph₃PS is an effective sulfur transfer agent for <b>1</b>, enabling the synthesis of mono­(μ-S) complex <b>11</b> with a double-sandwich structure and bis­(μ-S) dimer <b>12</b> in which the Ti–Ti bond has been cleaved

Mechanistic Studies of the Insertion of CO₂ into Palladium(I) Bridging Allyl Dimers

In contrast to the chemistry of momomeric η¹-Pd allyls, which act as nucleophiles, and monomeric η³-Pd allyls, which act as electrophiles, relatively little is known about the reactivity of Pd complexes with bridging allyl ligands. Recently we demonstrated that Pd^I dimers containing two bridging allyl ligands react with one equivalent of CO₂ to form species with one bridging allyl and one bridging carboxylate ligand. In this work we have prepared complexes from three different classes of Pd^I bridging allyl dimers: (i) dimers containing two bridging allyl ligands, (ii) dimers with one bridging allyl and one bridging chloride ligand, and (iii) dimers with one bridging allyl and one bridging carboxylate ligand. Complexes from all three groups have been characterized by X-ray crystallography, and their structures compared. Complexes with two bridging allyl ligands have the longest Pd bridging allyl bond lengths due to the high <i>trans</i> influence of the opposing bridging allyl ligand. For these species the HOMO is located almost entirely on the bridging allyl ligands, whereas for chloride- and carboxylate-bridged species the HOMO is primarily Pd based. A combined experimental and theoretical study has been performed to investigate the reactivity of the three different types of bridging allyl dimers with CO₂. Complexes with one bridging allyl and one bridging chloride ligand and complexes with one bridging allyl and one bridging carboxylate ligand do not insert CO₂ because the reaction is thermodynamically unfavorable. In contrast, in most cases the reaction of CO₂ with species containing two bridging allyl ligands is facile and involves nucleophilic attack of the bridging allyl ligand on electrophilic CO₂. An alternative pathway for CO₂ insertion, which involves a monomer/dimer equilibrium, can occur in the presence of a weakly coordinating ligand. Overall, our results suggest that although the bridging allyl ligand is likely to be unreactive in carboxylate- and chloride-bridged species, complexes with two bridging allyl ligands can act as nucleophiles like monomeric η¹-Pd allyls

Mechanistic Studies of the Insertion of CO₂ into Palladium(I) Bridging Allyl Dimers

In contrast to the chemistry of momomeric η¹-Pd allyls, which act as nucleophiles, and monomeric η³-Pd allyls, which act as electrophiles, relatively little is known about the reactivity of Pd complexes with bridging allyl ligands. Recently we demonstrated that Pd^I dimers containing two bridging allyl ligands react with one equivalent of CO₂ to form species with one bridging allyl and one bridging carboxylate ligand. In this work we have prepared complexes from three different classes of Pd^I bridging allyl dimers: (i) dimers containing two bridging allyl ligands, (ii) dimers with one bridging allyl and one bridging chloride ligand, and (iii) dimers with one bridging allyl and one bridging carboxylate ligand. Complexes from all three groups have been characterized by X-ray crystallography, and their structures compared. Complexes with two bridging allyl ligands have the longest Pd bridging allyl bond lengths due to the high <i>trans</i> influence of the opposing bridging allyl ligand. For these species the HOMO is located almost entirely on the bridging allyl ligands, whereas for chloride- and carboxylate-bridged species the HOMO is primarily Pd based. A combined experimental and theoretical study has been performed to investigate the reactivity of the three different types of bridging allyl dimers with CO₂. Complexes with one bridging allyl and one bridging chloride ligand and complexes with one bridging allyl and one bridging carboxylate ligand do not insert CO₂ because the reaction is thermodynamically unfavorable. In contrast, in most cases the reaction of CO₂ with species containing two bridging allyl ligands is facile and involves nucleophilic attack of the bridging allyl ligand on electrophilic CO₂. An alternative pathway for CO₂ insertion, which involves a monomer/dimer equilibrium, can occur in the presence of a weakly coordinating ligand. Overall, our results suggest that although the bridging allyl ligand is likely to be unreactive in carboxylate- and chloride-bridged species, complexes with two bridging allyl ligands can act as nucleophiles like monomeric η¹-Pd allyls

140 H/D Isotopomers Identified by Long-Range NMR Hyperfine Shifts in Ruthenium(III) Ammine Complexes. Hyperconjugation in Ru–NH₃ Bonding

¹H NMR spectra of the paramagnetic cyanide-bridged mixed-valence compound [(η⁵-C₅H₅)­Fe­(CO)₂(μ-CN)­Ru­(NH₃)₅]­(CF₃SO₃)₃ (<b>I</b>) have been obtained in several solvents. When traces of partially deuterated water are present, instead of a single cyclopentadienyl (Cp) resonance shifted by the hyperfine interaction, numerous well-resolved resonances are observed. The spectra were simulated satisfactorily by giving the appropriate statistical weight to 140 possible H/D isotopomers formed by deuteration in the five ruthenium­(III) ammine ligands. The proliferation of distinct resonances occurs because (a) the hyperfine shifts (HSs) due to each sequential deuteration in a single ammine are different and (b) while deuteration in an ammine cis to the cyanide bridge causes a downfield shift, in the trans ammine it causes an upfield shift that is nearly twice as large. All of these shifts exhibit a 1/<i>T</i> dependence, but temperature-independent components, due to large second-order Zeeman effects at the Ru^III center, are also present. Combining the results of density functional theory calculations with data from metal–metal charge-transfer optical transitions and with the effect of solvent-induced NMR HSs, it is argued that Fermi contact shifts at the Cp protons are insignificant compared to those due to the dipolar (pseudocontact) mechanism. Analytical expressions are presented for the dependence of the HS on the tetragonal component of the ligand field at the Ru^III ion. The tetragonal field parameter, defined as the energy by which the 4d_<i>xy</i> orbital exceeds the mean t_2g orbital energy, was found to be 147, 52, and 76 cm^–1, in dimethylformamide, acetone, and nitromethane, respectively. The effects of deuteration show that there is a significant component of hyperconjugation in the Ru–ammine interaction and that ND₃ is a weaker π donor than NH₃. A single deuteration in an axial ammine increases the tetragonal field parameter (ν) by +2.8 cm^–1, resulting in a HS of −37 ppb in the Cp proton resonance, whereas a single deuteration in an equatorial ammine decreases the field by −1.5 cm^–1 with a HS of +20 ppb, despite a nominal separation of seven chemical bonds. We analyze the origin of this remarkable sensitivity, which relies on the favorable characteristics of the Ru^III low-spin t_2g⁵ configuration, having a spin–orbit coupling constant ζ ≈ 950 cm^–1

140 H/D Isotopomers Identified by Long-Range NMR Hyperfine Shifts in Ruthenium(III) Ammine Complexes. Hyperconjugation in Ru–NH₃ Bonding

¹H NMR spectra of the paramagnetic cyanide-bridged mixed-valence compound [(η⁵-C₅H₅)­Fe­(CO)₂(μ-CN)­Ru­(NH₃)₅]­(CF₃SO₃)₃ (<b>I</b>) have been obtained in several solvents. When traces of partially deuterated water are present, instead of a single cyclopentadienyl (Cp) resonance shifted by the hyperfine interaction, numerous well-resolved resonances are observed. The spectra were simulated satisfactorily by giving the appropriate statistical weight to 140 possible H/D isotopomers formed by deuteration in the five ruthenium­(III) ammine ligands. The proliferation of distinct resonances occurs because (a) the hyperfine shifts (HSs) due to each sequential deuteration in a single ammine are different and (b) while deuteration in an ammine cis to the cyanide bridge causes a downfield shift, in the trans ammine it causes an upfield shift that is nearly twice as large. All of these shifts exhibit a 1/<i>T</i> dependence, but temperature-independent components, due to large second-order Zeeman effects at the Ru^III center, are also present. Combining the results of density functional theory calculations with data from metal–metal charge-transfer optical transitions and with the effect of solvent-induced NMR HSs, it is argued that Fermi contact shifts at the Cp protons are insignificant compared to those due to the dipolar (pseudocontact) mechanism. Analytical expressions are presented for the dependence of the HS on the tetragonal component of the ligand field at the Ru^III ion. The tetragonal field parameter, defined as the energy by which the 4d_<i>xy</i> orbital exceeds the mean t_2g orbital energy, was found to be 147, 52, and 76 cm^–1, in dimethylformamide, acetone, and nitromethane, respectively. The effects of deuteration show that there is a significant component of hyperconjugation in the Ru–ammine interaction and that ND₃ is a weaker π donor than NH₃. A single deuteration in an axial ammine increases the tetragonal field parameter (ν) by +2.8 cm^–1, resulting in a HS of −37 ppb in the Cp proton resonance, whereas a single deuteration in an equatorial ammine decreases the field by −1.5 cm^–1 with a HS of +20 ppb, despite a nominal separation of seven chemical bonds. We analyze the origin of this remarkable sensitivity, which relies on the favorable characteristics of the Ru^III low-spin t_2g⁵ configuration, having a spin–orbit coupling constant ζ ≈ 950 cm^–1

Double-Sandwich Pentalene Complexes M₂(pent^†)₂ (M = Rh, Pd; pent^† = 1,4-Bis(triisopropylsilyl)pentalene): Synthesis, Structure, and Bonding

The bis­(pentalene) complexes M₂(pent^†)₂ (M = Rh (<b>1</b>), Pd (<b>2</b>); pent^† = 1,4-bis­(triisopropylsilyl)­pentalene) have been synthesized and structurally characterized. In both <b>1</b> and <b>2</b> the metals have a formal electron count in excess of 18 per metal center, and DFT calculations indicate antibonding metal–metal interactions are present in <b>1</b>, whereas <b>2</b> involves antibonding metal–ligand interactions

Photoelectron Spectroscopy of Palladium(I) Dimers with Bridging Allyl Ligands

The dianionic Pd^I dimers [TBA]₂[(TPPMS)₂Pd₂(μ-C₃H₅)₂] (<b>1</b>) [TBA = tetrabutylammonium, TPPMS = PPh₂(3-C₆H₄SO₃)⁻] and [TBA]₂[(TPPMS)₂Pd₂(μ-C₃H₅)­(μ-Cl)] (<b>2</b>), containing two bridging allyl ligands and one bridging allyl ligand and one bridging chloride ligand, respectively, were synthesized. The electronic structures of these complexes were investigated by combining electrospray mass spectrometry with gas phase photodetachment photoelectron spectroscopy. The major difference between the photoelectron spectra of the anions of <b>1</b> and <b>2</b> is the presence of a low-energy detachment band with an adiabatic electron detachment energy of 2.44(6) eV in <b>1</b>, which is not present in <b>2</b>. The latter has a much higher adiabatic electron detachment energy of 3.24(6) eV. Density functional theory calculations suggest that this band is present in <b>1</b> due to electron detachment from the out-of-phase combination of the π₂ orbitals, which are localized on the terminal carbon atoms of the bridging allyl ligands. In <b>2</b>, the Pd centers stabilize the single π₂ orbital of the bridging allyl ligand, and it is lowered in energy. The presence of the high-energy out-of-phase combination of the π₂ allyl orbitals makes <b>1</b> a better nucleophile, which explains why species with two bridging allyl ligands react with CO₂ in an analogous fashion to momoneric Pd η¹-allyls, whereas species with one bridging allyl and one bridging chloride ligand are unreactive