Article pubs.acs.org/JPCA
Coordination and Bond Activation in Complexes of Regioisomeric Phenylpyridines with the Nickel(II) Chloride Cation in the Gas Phase Alexandra Tsybizova, Lubomír Rulíšek, Detlef Schröder,* and Tibor András Rokob*,† Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Flemingovo nám. 2, 166 10 Prague 6, Czech Republic S Supporting Information *
ABSTRACT: Electrospray ionization of dilute solutions of phenylpyridines (phpy) in the presence of nickel(II) chloride leads to gaseous ions of the type [Ni(phpy)m]2+ with m = 3−5 and [NiCl(phpy)n]+ with n = 1−3, which are characterized by various gas-phase experiments in combination with calculations using density functional theory. Of the regioisomeric phpy's, 2-phpy behaves drastically different compared to 3- and 4-phpy. Ion mobility mass spectrometry allows a differentiation of the gaseous ions and an elucidation of characteristic properties of the metal complexes. For 2-phpy, C−H bond activation in the [NiCl(phpy)2]+ complex is significant, whereas this route is almost suppressed for the corresponding complexes of 3- and 4-phpy and only occurs at elevated energies.
1. INTRODUCTION C−H bond activation by transition-metal compounds is a key theme in catalysis, which has been explored for decades but still remains one of the major challenges in contemporary chemistry. Specifically, large amounts of methane are still burnt in flares today, thereby not only wasting the energy content of the feedstock itself but also contributing to possible climate effects by the CO2 released in the course of combustion. Likewise, the industrial functionalization of arenes is often still based on chlorine chemistry as a first step, which has experienced large progress but is inherently associated with the production of problematic waste. A major problem in C−H bond activation is in fact not the activation itself (this is realized in every flame) but to achieve a sufficient selectivity in the overall process. In the case of methane, for example, the C−H bond activation is a challenge, but the real problem is to prevent the subsequent activation of the initial products.1 In this respect, gas-phase ion chemistry has been established as a useful complement in catalysis research because it can unravel the elementary steps in bond-activation processes in general and oxidation reactions in particular.2 Pyridine (py) and its derivatives are common ligands in coordination chemistry. One of the salient questions in this respect is whether coordination of a metal cation M+ occurs to the aromatic π-system (π-py/M+) or the lone pair electrons of the basic nitrogen atom (σ-py/M+), which is decisive for the actual coordination geometry (Chart 1). For the parent compound py, Rodgers and co-workers have demonstrated that σ-coordination is generally preferred over the π-type geometry, and the same type of σ-coordination is preferred for related ligands such a bipyridyl and 1,10-phenanthroline.3,4 2-Phenylpyridine (2-phpy) is frequently used as a substrate in model studies for the activation of C−H bonds by organometallic complexes because the nitrogen atom incorpo© 2012 American Chemical Society
Chart 1. Two of the Possible Coordination Modes of py with a Metal Cation Mn+
rated in one of the aromatic rings facilitates the activation of the 2′-C−H bond of the other one.5 In this respect, it is instructive to consider the concept of “activated C−H bonds”. This term is typically used for compounds in which neighbor effects significantly reduce the strength of one or several C−H bonds in a molecule such that its activation becomes more facile; reference values are D(C−H) = 439 kJ mol−1 in methane and D(C−H) = 472 kJ mol−1 in benzene.6 Scheme 1 lists the computed strengths of the various C−H bonds in 2-phpy, which turn out to be all very close to each other. Thus, in 2phpy itself, the 2′-C−H bond is only very slightly weaker and Scheme 1. Computed C−H Bond Dissociation Energies (M06/def2-TZVPP//B3LYP-D3/def2-SVP Values in kJ mol−1 at 0 K) of the Phenyl Substituent in 2-phpy and the Regioisomers 3-phpy and 4-phpy
Special Issue: Peter B. Armentrout Festschrift Received: May 30, 2012 Revised: August 14, 2012 Published: August 14, 2012 1171
dx.doi.org/10.1021/jp3052455 | J. Phys. Chem. A 2013, 117, 1171−1180
The Journal of Physical Chemistry A
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hence not “activated” appreciably compared to the other C−H bonds. Yet, after insertion of a metal in the 2′-C−H bond, the metal can profit from coordination to the nitrogen atom in the other ring. Activation in this sense is thus operative in the complexes of 2-phpy's with the reactive species if the latter prefers coordination to nitrogen, despite its absence in the isolated substrate itself. Here, we report a comparative study of the coordination properties of the regioisomeric phpy's (2-, 3-, and 4-phpy) and their bond activation by gaseous nickel(II) chloride cations NiCl+, eventually leading to cyclometalation.7,8 Chlorine is chosen as a simple, noninterfering ligand, whose binding to the released hydrogen atom provides a sufficient driving force to assist the bond-activation reaction from the side of the products (D(H−Cl) = 432 kJ mol−1).6 Nickel is chosen as a 3d metal with a high potential for C−H bond activation in isolated model systems in the gas phase;9−11 4d- and 5d-elements are often more reactive but much less attractive from an economical point of view.12 Furthermore, the combination of nickel and chlorine in NiCl+ mimics the most common oxidation state of nickel(II) of the metal.13 In comparison, the often very reactive bare metal monocations (e.g., Fe+)12b bear unrealistic oxidation states with respect to catalytic processes, and the chemistry of metal dications (e.g., Y2+ and La2+)14 is often dominated by mere electron-transfer processes driven by the enormous electrophilicity of bare multiply charged ions, which is, however, of rather limited relevance for catalysis in more dense media, where complete solvation (ligation) can be assumed. Here, we address the coordination chemistry and bond activations of phpy/NiCl+ complexes by a combination of gas-phase experiments and quantum chemical calculations.
SYNAPT G2 mass spectrometer (WATERS, Manchester, U.K.), which has a standard mass resolution of m/Δm ≈ 20000. The SYNAPT G2 was also used for ion mobility experiments.23,24 In brief, the instrument has an ESI source, from which the ions are transferred to the vacuum manifold using a traveling waveguide, followed by mass selection of the ions of interest using a quadrupole analyzer (Q). In the ion mobility mode, the mass-selected ions are collected in a linear ion trap filled with argon, from which they are admitted as a single pulse at ta = 0 via a helium cooling cell to the ion mobility section filled with ∼2 mbar of nitrogen. After extraction from the drift tube, the ions pass a transfer cell and enter the source region of a reflectron time-of-flight mass spectrometer. It is important to note that the absolute values of the arrival times (ta) in the SYNAPT G2 very much depend on the adjustments of the pressures and the voltage settings. Any comparison can therefore only be made relative to each other under identical settings.25 In the instrument, several options exist to energize the generated ions in order to induce isomerization and/or fragmentation.24 At first, the ionization process can be modified from soft to increasingly harsher conditions by the increase of the cone voltage (Uc) in the ion source. At larger cone voltages, the ions undergo multiple collisions with the nebulizing nitrogen gas (1 bar), which increase their internal energy. Hence, variation of the cone voltage provides a simple method for modification of the ions’ internal energy and induction of fragmentation and, in some cases, even atomization.17,26 Another option exists in variation of the voltage settings of the trap (Utr) in front of the ion mobility section. The larger the Utr applied, the more the ions are heated in collisions with lowpressure argon (10−2 mbar) present in the trap. Finally, energy can be provided in the transfer cell behind the ion mobility section by the increase of the applied voltage in the transfer cell (Utf).
2. EXPERIMENTAL METHODS Initial experiments for the characterization of the ions of interest were performed using a Finnigan LCQ Classic ion trap mass spectrometer (IT-MS).15 The ions of interest were generated by electrospray ionization (ESI) of dilute solutions of nickel(II) chloride in water/methanol (1:1) with traces of the respective phpy. In brief, the LCQ bears a conventional ESI source consisting of the spray unit (typical flow rates between 5 and 30 μL/min; typical spray voltage of 5 kV) with nitrogen as a sheath gas, followed by a heated transfer capillary (kept at 200 °C), a set of lenses that determines the soft- or hardness of ionization by variation of the degree of collisional activation in the medium pressure regime,16,17 two transfer octopoles, and a Paul ion trap with ∼10−3 mbar of helium for ion storage and manipulation, including a variety of MSn experiments.18 For detection, the ions are ejected from the trap to an electron multiplier. Low-energy CID was performed by application of an excitation AC voltage to the end caps of the trap to induce collisions of the isolated ions with the helium buffer gas.19 For a CID excitation period of 20 ms and a trapping parameter of qz = 0.25, we have recently introduced an empirical calibration scheme that allows conversion of the experimental appearance energies (AEs) of the fragmentations to an absolute energy scale.4b,20 In brief, this calibration scheme involves measurements of ions with known dissociation thresholds under the above-mentioned conditions, followed by correlation of the measured appearance energies in the laboratory frame with the reference values.21,22 Further, the observed isotope patterns confirm all ion assignments made in the following. The assignments find additional support by experiments on a
3. COMPUTATIONAL METHODS In the quantum chemical calculations, geometry optimizations were done at the B3LYP-D3/def2-SVP level,27,28 which includes Grimme’s empirical dispersion correction in its version 3.29 For all optimized structures, frequency analysis was performed at the same level of theory in order to assign them as genuine minima or transition structures on the potential energy surface (PES), as well as to calculate zeropoint vibrational energies (ZPVEs). Subsequently, single-point energy calculations were performed using the M06 functional30 and the def2-TZVPP basis set,28 where no empirical dispersion is added because the functional itself accounts for dispersion in the vicinity of equilibrium geometries. Selected energies were also computed at the dispersion-corrected BP86-D3/def2TZVPP27c,31 and B3LYP-D3/def2-TZVPP levels of theory; these are reported in the Supporting Information. The energies in the paper refer to a temperature of 0 K in the gaseous state; singlet−triplet state splittings are adiabatic and include ZPVE. Singlet states were calculated with broken-symmetry32 openshell wave functions that were lower in energy than the closedshell solutions. No spin-projection corrections in the energy were applied. The ⟨S2⟩ values are reported in the Supporting Information. The calculations were done using the Turbomole 6.333 and Gaussian0934 program packages. Molecular graphics were drawn using XYZ Viewer.35 Experimental ion mobilities, and thereby arrival times, can be related to the collision cross section of the ion with the buffer gas.36 For the comparative purposes of interest here, we used a 1172
dx.doi.org/10.1021/jp3052455 | J. Phys. Chem. A 2013, 117, 1171−1180
The Journal of Physical Chemistry A
Article
Table 1. Relative Intensitiesa of the Major Nickel Complexes in the ESI Mass Spectra of Solutions of NiCl2 (∼1 × 10−4 M) with the Isomeric phpy's (∼3 × 10−4 M) in Water/Methanol (1:1) under Soft Ionization Conditions in the ESI Source 2-phpy 3-phpy 4-phpy
[Ni(phpy)3]2+,b
[Ni(phpy)4]2+
[Ni (phpy)2−H]+
[NiCl(phpy)2]+
[NiCl(phpy)3]+
2 2 12
