Laser-Induced Fluorescence of Decamethylrhenocene in Low-Temperature Matrices

The emission and excitation spectra of decamethylrhenocene, Re(η5-C5Me5)2 have been recorded in argon and in nitrogen matrices with a tunable pulsed laser as exciting source. In argon matrices two sets of spectra may be excited selectively corresponding to different trapping sites or conformers of the guest molecules. Each set is dominated by a progression in ν8, a vibration corresponding to the in-phase combination of ring−metal−ring and ring−methyl bending modes (ν8‘ = 392 (2) cm-1 ν8‘‘ = 383 (1) cm-1). Much weaker features can be detected corresponding to progressions involving the three other totally symmetric modes (νx + nν8, x = 6, 7, 9, n = 0, 1, 2, ...). The emission and excitation bands in the principal site are symmetrical in shape with full width at half-maximum of only ca. 6 cm-1. The spectra of Re(η5-C5Me5)2 in nitrogen matrices are similar to those in argon matrices, but differ in site/conformer structure. The excited state lifetime in nitrogen matrices has been determined by single photon counting methods to be 3.69 (7) ns. The quantum yield for emission in nitrogen matrices is estimated as 0.010

Structure and Dynamic Exchange in Rhodium η²-Naphthalene and Rhodium η²-Phenanthrene Complexes: Quantitative NOESY and EXSY Studies

Complexes of naphthalene and phenanthrene with rhodium(η5-cyclopentadienyl)(trimethylphosphine) have been studied by quantitative two-dimensional nuclear Overhauser (NOESY) and exchange spectroscopy (EXSY). Naphthalene coordinates in the η2-1,2-mode as (η5-C5H5)Rh(PMe3)(η2-C10H8). At 260 K, NOESY peaks establish that the solution conformer has the hydrogen atoms on the coordinated double bond bent out of the arene plane toward the PMe3 ligand. The effective average distance, reff, of these hydrogen atoms from those in the PMe3 ligand is calculated as 3.52 Å by matrix analysis of the NOESY spectrum. At room temperature, (η5-C5H5)Rh(PMe3)(η2-C10H8) undergoes an intramolecular [1,3]-metallotropic shift within the coordinated ring with ΔG⧧300 of 74.4 kJ mol-1 detected by EXSY spectroscopy. This species is in equilibrium (a) with the C−H activated isomer (η5-C5H5)Rh(PMe3)(C10H7)H and (b) with the dinuclear complex [(η5-C5H5)Rh(PMe3)]2(μ-1,2-η2-3,4-η2-C10H8) and free naphthalene. The free energy change at 300 K for conversion of (η5-C5H5)Rh(PMe3)(η2-C10H8) to (η5-C5H5)Rh(PMe3)(C10H7)H is +11.5 kJ mol-1 compared to +2.2 kJ mol-1 for the (η5-C5Me5) analogue. The crystal structure of the dinuclear complex, [(η5-C5H5)Rh(PMe3)]2(μ-1,2-η2-3,4-η2-C10H8), shows that this molecule adopts the structure with the two rhodium centers coordinated antifacially to the same ring of the naphthalene ligand. The C−C bond lengths of the coordinated ring show conspicuous alternation, while those of the uncoordinated ring differ less than those in free naphthalene. The mean separation of the hydrogen atoms attached to the coordinated CC bond from the PMe3 hydrogen atoms, averaged from the crystal structure as 1/〈r-3〉1/3 for each independent rhodium center, is 3.56 Å, compared to 3.48 Å for reff measured in solution by NOESY spectroscopy. The phenanthrene complex (η5-C5H5)Rh(PMe3)(η2-9,10-C14H10) adopts a conformation similar to the naphthalene complex; the value of reff is estimated to be reduced to 3.43 Å

A Comparison of C−F and C−H Bond Activation by Zerovalent Ni and Pt: A Density Functional Study

Density functional theory indicates that oxidative addition of the C−F and C−H bonds in C6F6 and C6H6 at zerovalent nickel and platinum fragments, M(H2PCH2CH2PH2), proceeds via initial exothermic formation of an η2-coordinated arene complex. Two distinct transition states have been located on the potential energy surface between the η2-coordinated arene and the oxidative addition product. The first, at relatively low energy, features an η3-coordinated arene and connects two identical η2-arene minima, while the second leads to cleavage of the C−X bond. The absence of intermediate C−F or C−H σ complexes observed in other systems is traced to the ability of the 14-electron metal fragment to accommodate the η3-coordination mode in the first transition state. Oxidative addition of the C−F bond is exothermic at both nickel and platinum, but the barrier is significantly higher for the heavier element as a result of strong 5dπ−pπ repulsions in the transition state. Similar repulsive interactions lead to a relatively long Pt−F bond with a lower stretching frequency in the oxidative addition product. Activation of the C−H bond is, in contrast, exothermic only for the platinum complex. We conclude that the nickel system is better suited to selective C−F bond activation than its platinum analogue for two reasons:  the strong thermodynamic preference for C−F over C−H bond activation and the relatively low kinetic barrier

Light-Controlled Ion Switching: Direct Observation of the Complete Nanosecond Release and Microsecond Recapture Cycle of an Azacrown-Substituted [(bpy)Re(CO)₃L]⁺ Complex

A [(bpy)Re(CO)3L]+ complex (bpy = 2,2‘-bipyridine) in which L contains an azacrown ether (MacQueen, D. B.; Schanze, K. S. J. Am. Chem. Soc. 1991, 113, 6108) acts as a reversible light-controlled switch of alkali and alkaline earth metal cations bound to the azacrown, as observed directly by time-resolved UV−vis spectroscopy. Excitation to the metal-to-ligand charge-transfer (MLCT) state of the metal-complexed form, [(bpy)Re(CO)3L]+-Mn+, results in cation release on the nanosecond time scale for Mn+ = Li+, Na+, Ca2+, and Ba2+, with Li+ and Na+ being released more rapidly than Ca2+ and Ba2+; by contrast, Mg2+ is not released. After decay to the ground state, [(bpy)Re(CO)3L]+ recaptures metal cations on the microsecond time scale to restore the starting thermal equilibrium. A multistep rebinding mechanism is observed for Li+ and Na+, in which the cation attaches initially to the azacrown nitrogen atom before binding to the equilibrium position within the azacrown ring. The excited states and other intermediates in the cation release-and-recapture cycle have been observed directly in real time, and their decay rate constants have been determined as a function of cation identity, enabling a generalized light-controlled cation-switching mechanism to be developed for this generic molecular design

Structure and Dynamic Exchange in Rhodium η²-Naphthalene and Rhodium η²-Phenanthrene Complexes: Quantitative NOESY and EXSY Studies

Complexes of naphthalene and phenanthrene with rhodium(η5-cyclopentadienyl)(trimethylphosphine) have been studied by quantitative two-dimensional nuclear Overhauser (NOESY) and exchange spectroscopy (EXSY). Naphthalene coordinates in the η2-1,2-mode as (η5-C5H5)Rh(PMe3)(η2-C10H8). At 260 K, NOESY peaks establish that the solution conformer has the hydrogen atoms on the coordinated double bond bent out of the arene plane toward the PMe3 ligand. The effective average distance, reff, of these hydrogen atoms from those in the PMe3 ligand is calculated as 3.52 Å by matrix analysis of the NOESY spectrum. At room temperature, (η5-C5H5)Rh(PMe3)(η2-C10H8) undergoes an intramolecular [1,3]-metallotropic shift within the coordinated ring with ΔG⧧300 of 74.4 kJ mol-1 detected by EXSY spectroscopy. This species is in equilibrium (a) with the C−H activated isomer (η5-C5H5)Rh(PMe3)(C10H7)H and (b) with the dinuclear complex [(η5-C5H5)Rh(PMe3)]2(μ-1,2-η2-3,4-η2-C10H8) and free naphthalene. The free energy change at 300 K for conversion of (η5-C5H5)Rh(PMe3)(η2-C10H8) to (η5-C5H5)Rh(PMe3)(C10H7)H is +11.5 kJ mol-1 compared to +2.2 kJ mol-1 for the (η5-C5Me5) analogue. The crystal structure of the dinuclear complex, [(η5-C5H5)Rh(PMe3)]2(μ-1,2-η2-3,4-η2-C10H8), shows that this molecule adopts the structure with the two rhodium centers coordinated antifacially to the same ring of the naphthalene ligand. The C−C bond lengths of the coordinated ring show conspicuous alternation, while those of the uncoordinated ring differ less than those in free naphthalene. The mean separation of the hydrogen atoms attached to the coordinated CC bond from the PMe3 hydrogen atoms, averaged from the crystal structure as 1/〈r-3〉1/3 for each independent rhodium center, is 3.56 Å, compared to 3.48 Å for reff measured in solution by NOESY spectroscopy. The phenanthrene complex (η5-C5H5)Rh(PMe3)(η2-9,10-C14H10) adopts a conformation similar to the naphthalene complex; the value of reff is estimated to be reduced to 3.43 Å

Transient Photochemistry, Matrix Isolation, and Molecular Structure of <i>cis</i>-Ru(dmpm)₂H₂ (dmpm = Me₂PCH₂PMe₂)

Photolysis of cis-Ru(dmpm)2H2 (dmpm = Me2PCH2PMe2) generates the four-coordinate species Ru(dmpm)2, which has been studied by laser flash photolysis and matrix isolation techniques. Ru(dmpm)2 displays weak bands in the visible region of the spectrum. Comparison with analogues containing diphosphines with a CH2CH2 bridge demonstrates the sensitivity of the spectrum to variation of the bite angle, P−Ru−P, of the ring formed by the metal and the diphosphine ligands. The rate constants for the reaction of Ru(dmpm)2 with H2, CO, C2H4, and Et3SiH have been measured. All of the rate constants lie in the range from 2.8 × 108 to 4.9 × 108 dm3 mol-1 s-1 at 295 K, showing that Ru(dmpm)2 is very unselective. Ru(dmpm)2 reacts with benzene with complex kinetics, which can be interpreted in terms of a rapid equilibrium between Ru(dmpm)2 and the benzene complex Ru(dmpm)2(η2-C6H6). The latter forms the phenyl hydride complex Ru(dmpm)2(Ph)H relatively slowly (rate constant 3.5 × 103 s-1, kinetic isotope effect k(C6H6)/k(C6D6) = 1.8). A single-crystal X-ray structure of cis-Ru(dmpm)2H2 shows that the Ru−P bonds trans to hydrogen (mean length 2.304(2) Å) are longer than the remaining two Ru−P bonds (mean length 2.282(2) Å). The mean bite angle P−Ru−P of the bridged phosphorus atoms is 72.0°. The unbridged, P−Ru−P angles are 108.0° and 179.2°

Reactivity of a Nickel Fluoride Complex: Preparation of New Tetrafluoropyridyl Derivatives^†

Treatment of trans-[NiF(2-C5F4N)(PEt3)2] (1), obtained by reaction of Ni(COD)2 with PEt3 and pentafluoropyridine, with Me3SiOTf effects the formation of the air-stable triflate complex trans-[Ni(OTf)(2-C5F4N)(PEt3)2] (2). The X-ray crystal structure reveals a molecular complex with approximately square-planar coordination at nickel, a Ni−O distance of 1.957(2) Å, and a Ni−C distance of 1.851(3) Å. The reaction of 2 with NaOPh yields the phenoxy complex trans-[Ni(OPh)(2-C5F4N)(PEt3)2] (3). The crystal structure of 3 was determined. The Ni−O and Ni−C distances are 1.894(4) and 1.861(6) Å, respectively. The complexes trans-[NiPh(2-C5F4N)(PEt3)2] (5) and trans-[NiMe(2-C5F4N)(PEt3)2] (6) were obtained on treatment of 1 with PhLi and Me2Zn, respectively. Treatment of 6 with CO yielded 2-acetyltetrafluoropyridine, while reaction with air yielded 2-methyltetrafluoropyridine. The studies reported in this paper demonstrate the synthesis of nickel derivatives of tetrafluoropyridine with the metal in a 2-position as well as the preparation of new 2-substituted tetrafluoropyridines by C−C coupling reactions

Facile Insertion of CO₂ into the Ru−H Bonds of Ru(dmpe)₂H₂ (dmpe = Me₂PCH₂CH₂PMe₂): Identification of Three Ruthenium Formate Complexes

The reaction of cis-Ru(dmpe)2H2 (dmpe = Me2PCH2CH2PMe2), 1, with carbon dioxide has been investigated. Addition of CO2 at 293 K results in the formation of two formate complexes:  the major product is the trans formate hydride, trans-Ru(dmpe)2(OCHO)H, 2, while the minor species is the bis(formate) complex, cis-Ru(dmpe)2(OCHO)2, 3. When the addition of carbon dioxide is performed at 195 K, both the bis(formate) and the cis-formate hydride complex, cis-Ru(dmpe)2(OCHO)H, 4, are observed in larger concentrations. All three complexes have been characterized by multinuclear NMR while 2 has been isolated and characterized by X-ray crystallography. The ruthenium formate group is planar with Ru−O(1) = 2.243(4) Å. The Ru−O(1) vector is tilted away from the normal to the P4 plane with resultant O(1)−Ru−P angles of 84.4(2), 88.4(1), 96.7(1), and 101.5(2)°. The C−O(2) vector of the formate group points toward the RuP4 plane. The symmetric and antisymmetric OCO stretching modes in the IR spectrum of 2 were identified by reaction with 13CO2

Exceptional Sensitivity of Metal−Aryl Bond Energies to <i>ortho</i>-Fluorine Substituents: Influence of the Metal, the Coordination Sphere, and the Spectator Ligands on M−C/H−C Bond Energy Correlations

DFT calculations are reported of the energetics of C−H oxidative addition of benzene and fluorinated benzenes, ArFH (ArF = C6FnH5−n, n = 0−5) at ZrCp2 (Cp = η5-C5H5), TaCp2H, TaCp2Cl, WCp2, ReCp(CO)2, ReCp(CO)(PH3), ReCp(PH3)2, RhCp(PH3), RhCp(CO), IrCp(PH3), IrCp(CO), Ni(H2PCH2CH2PH2), Pt(H2PCH2CH2PH2). The change in M−C bond energy of the products fits a linear function of the number of fluorine substituents, with different coefficients corresponding to ortho-, meta-, and para-fluorine. The values of the ortho-coefficient range from 20 to 32 kJ mol−1, greatly exceeding the values for the meta- and para-coefficients (2.0−4.5 kJ mol−1). Similarly, the H−C bond energies of ArFH yield ortho- and para-coefficients of 10.4 and 3.4 kJ mol−1, respectively, and a negligible meta-coefficient. These results indicate a large increase in the M−C bond energy with ortho-fluorine substitution on the aryl ring. Plots of D(M−C) vs D(H−C) yield slopes RM−C/H−C that vary from 1.93 to 3.05 with metal fragment, all in excess of values of 1.1−1.3 reported with other hydrocarbyl groups. Replacement of PH3 by CO decreases RM−C/H−C significantly. For a given ligand set and metals in the same group of the periodic table, the value of RM−C/H−C does not increase with the strength of the M−C bond. Calculations of the charge on the aryl ring show that variations in ionicity of the M−C bonds correlate with variations in M−C bond energy. This strengthening of metal−aryl bonds accounts for numerous experimental results that indicate a preference for ortho-fluorine substituents

Hydrofluoroarylation of Alkynes with Ni Catalysts. C–H Activation via Ligand-to-Ligand Hydrogen Transfer, an Alternative to Oxidative Addition

The mechanism of the hydrofluoroarylation of alkynes, RCCR, by nickel phosphine complexes, described by Nakao et al. (Dalton Trans. 2010, 39, 10483), was studied by density functional theory (DFT) calculations. The oxidative addition of a C–H bond of partially fluorinated benzenes, C₆F_<i>n</i>H_6–<i>n</i> (<i>n</i> = 0–5) to a Ni(0) phosphine complex is reversible, but the oxidative addition of a C–F bond yields a stable product via a high-energy barrier. A pathway via the Ni­(II) hydride complex is eliminated on the basis of a calculated H/D kinetic isotope effect (KIE) that does not agree with the measured value. An alternate pathway was determined, using as reactant a Ni­(phosphine)­(alkyne) complex that is shown to be the major species in the reactive media under the catalytic conditions. This pathway is initiated by arene coordination to the Ni alkyne complex followed by proton transfer from the σ-C–H bond of the coordinated arene to the alkyne as the C–H activation step. Analysis of the charge distribution shows that the alkyne is strongly negatively charged when coordinated to the Ni­(phosphine) species, which favors a C–H activation as a proton transfer, similar to that in CMD and AMLA but not previously seen between hydrocarbyl ligands for electron rich metals. The C–H activation step thus represents an example of a general class of mechanism that we term ligand-to-ligand hydrogen transfer (LLHT). The product of this reaction is a nickel­(vinyl)­(aryl) complex, which rearranges to place the aryl and vinyl groups cis to one another before undergoing reductive elimination of the arylalkene. An analysis of the calculated turnover frequencies shows that the rate-determining states that control the energy span are the alkyne complex + free arene and the transition state for the vinyl-aryl complex trans-to-cis rearrangement. The calculated KIE agrees with the observed lack of isotope effect. Analysis of the effects of fluorine substituents shows that the Ni–C­(aryl) bond energies control the energy barriers for the arene C–H activation step and the energy spans. A correlation between bond dissociation energies for the Ni–C­(aryl) bond and the arene C–H bond follows the behavior presented previously (J. Am. Chem Soc. 2009, 131, 7817), in which the effects of ortho fluorine substituents are dominant. Consequently, fluorine substitution of the arene, especially at the ortho positions, strengthens the Ni–C bond and increases the TOF. The LLHT mechanism described here may also apply to nickel-catalyzed C–H activation reactions with other substrates