Aryl Group in the Hydrolysis

Article pubs.acs.org/Organometallics

Role of the Metal, Ligand, and Alkyl/Aryl Group in the Hydrolysis Reactions of Group 10 Organometallic Cations [(L)M(R)]+ Matthew J. Woolley,†,‡,∥ George N. Khairallah,†,‡,∥ Gabriel da Silva,§ Paul S. Donnelly,†,∥ Brian F. Yates,⊥ and Richard A. J. O’Hair*,†,‡,∥ †

School of Chemistry, The University of Melbourne, Victoria 3010, Australia ARC Centre of Excellence for Free Radical Chemistry and Biotechnology, The University of Melbourne, Victoria 3010, Australia § Department of Chemical and Biomolecular Engineering, The University of Melbourne, Victoria 3010, Australia ∥ Bio21 Institute of Molecular Science and Biotechnology, The University of Melbourne, Victoria 3010, Australia ⊥ School of Chemistry, University of Tasmania, Private Bag 75 Hobart, Tasmania 7001, Australia ‡

S Supporting Information *

ABSTRACT: The reactions of water with the coordinatively unsaturated group 10 organometallic cations [(L)M(R)]+ (4; where L = 1,10-phenanthroline (phen), neocuproine (neo); M = nickel, palladium, platinum; R = CH3, C6H5, CH2C6H5), formed via decarboxylation of the carboxylate complexes [(L)M(O2CR)]+, were examined in the gas phase using a combination of multistage mass spectrometry experiments and DFT calculations at the M06/SDD6-31+G(d) level of theory. Two main types of primary product ions were observed: the aqua adduct [(L)M(R)(H2O)]+ (5) and the hydroxide [(L)M(OH)]+ (7), formed via a hydrolysis reaction. A secondary product ion, arising from formation of the adduct [(L)M(OH)(H2O)]+, was also observed when L = phen, R = CH3, and M = Pt. The rates of reaction of 4 and the product branching ratios for 5 and 7 were dependent upon the nature of M, L, and R. When L = phen and R = CH3, the hydroxide 7 dominates for Ni, with the adduct 5 as the major product for both Pd and Pt. For R = C6H5 the rate of the reaction is slower, while for R = CH2C6H5 no reaction occurs. Replacing the phen auxiliary ligand with neo dramatically slows down the rate of reaction with water. DFT calculations reveal that an acid−base hydrolysis mechanism is favored over an oxidative addition/reductive elimination mechanism proceeding via the M(IV) intermediate [(L)M(CH3)(H)(OH)]+. Furthermore, the relative energies calculated for the barriers of these hydrolysis reactions are consistent with the experimentally observed reactivity trends. This mechanism is also supported by RRKM theory/master equation simulations, which demonstrate that formation of the aqua adduct and hydroxide can be explained by competition between unimolecular dissociation and collisional deactivation of the chemically activated reaction adduct within the ion trap. The lack of reactivity of the benzyl systems appears to arise from η3 binding of the benzyl group, which blocks access to the incoming water. Finally, links are made to group 10 three-coordinate organometallic complexes in the condensed phase.

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INTRODUCTION Solvents play a significant role in the structure and reactivity of main-group and transition-metal organometallic compounds.1 Beyond their role as a medium, solvents can be active participants in reactions. They can coordinate to the metal center or can directly react with the organometallic reagent via Brønsted−Lowry and Lewis acid−base interactions or via redox reactions. Solvents can also influence the oligomeric state of organometallic reagents and perturb their equilbria.2 There is intense interest in developing “greener” organometallic processes, and water-based catalytic reactions have gone beyond mere curiosities to become industrially important processes.3 While the general roles of water as solvent, ligand, and active participant in organometallic reactions are well-known,4 © 2013 American Chemical Society

remarkably little is known about reactivity trends as a function of metal center and coordination environment. An intriguing question is: Can alkyl/aryl and aqua ligands coexist in the coordination sphere of organometallic complexes? While the answer appears to be yes for certain transition metals, relatively few such complexes have been isolated and structurally characterized. A search of the Cambridge Structural Database (CSD)5 uncovered a number of such structures, including organometallic complexes of Ta,6 Ru,7 Co,8 Rh,9 Re,10 Ir,11 Pd,12 Pt,13 and Au.14 A selection of these isolated complexes, which highlights the diversity of the coordination environment around the metal centers, is shown in Scheme 1. Received: April 25, 2013 Published: November 25, 2013 6931

dx.doi.org/10.1021/om400358q | Organometallics 2013, 32, 6931−6944

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Scheme 1. Examples of Organometallic Complexes with both Alkyl (or Aryl) and Aqua Ligands That Have Been Structurally Characterized via X-ray Crystallography6,10,13e

(eq 3, where R = CH3) which are collisionally stabilized by the helium bath gas.20f These three-coordinate complexes are very reactive, due to a vacant coordination site allowing direct attack on the metal, and the water adducts formed in eq 3 are related to Chen’s gas-phase complexes [(L)Pt(R)(OH2)]+ 17 as well as complexes such as 2 that have been isolated in the condensed phase (Scheme 1). While these three-coordinate Ni(II), Pd(II), and Pt(II) complexes may seem exotic, several are involved in ligand exchange reactions in solution (e.g., nickel, 21 palladium, 22 and platinum23). Stable three-coordinate complexes have been isolated and structurally characterized in the condensed phase with the use of bulky phosphine ligands with a large bite angle24 or another sterically hindering functional group such as a cyclopentane.25 In contrast, the diimine heterocyclic ligands used in the gas-phase experiments presented here are less sterically demanding than bulky phosphine ligands or cyclopentane.

Although it seems counterintuitive, gas-phase studies using mass-spectrometry-based methods can be used to gain insights into the solvation of metal ions and metal complexes.15 For example, the hydration and dehydration of metal ions and inorganic complexes has been well studied in the gas phase.15a−g With the advent of electrospray ionization, massspectrometry-based techniques have been used to examine microhydration of ligated inorganic complexes.15h,i These MSbased methods range from IR multiple-photon dissociation (IRMPD) spectroscopy15h,16 to thermochemical measurements. Examples of the latter include a determination of the gas-phase Pt−OH2 binding energies in the cationic Pt(II) complexes [(L)Pt(R)(OH2)]+ (where L = a diimine ligand and R = CH3, C6H5) via a combination of energy-resolved CID experiments and DFT calculations.17 We have been using the Pesci decarboxylation reaction (eq 1)18 to “synthesize” organometallic anions19 and cations20 [(L)M(O2 CR)]+ / − → [(L)M(R)]+ / − + CO2

(1)

[(L)M(R)]+ + H 2O → [(L)M(R)(H 2O)]+

of the type [(L)M(R)]+/− in order to study their unimolecular and bimolecular reactivity as a function of the nature of the metal M, the R group, and the type of coordinating ligands L. A reaction we have studied in some detail is the hydrolysis reaction of organoalkali and organoalkaline organometallic anions19a−d and cations.20a−c These reactions all proceed via Lewis acid−base chemistry, to yield the metal hydroxide and the alkane (eq 2). [(L)M(R)]+ / − + H 2O → [(L)M(OH)]+ / − + RH

(3)

In this paper we describe the use of ion trap mass spectrometry experiments26 to extend our qualitative experimental observations20f to include detailed kinetic experiments on the larger range of complexes shown in Scheme 2. Finally, DFT calculations are carried out for the first time to investigate the mechanisms of these hydrolysis reactions. These calculations are also extended to RRKM theory/master equation based reaction rate simulations. Of particular interest is to establish whether these reactions proceed via a Lewis acid−base mechanism akin to those observed for organomagnesium ions19a−d,20b,c or whether they involve oxidative addition/reductive elimination (OA/RE) mechanisms, a reaction of considerable interest with regard to water activation.27

(2)

Recently we have noted that the three-coordinate Ni(II), Pd(II), and Pt(II) organometallic cations [(L)M(CH3)]+ (where L = 1,10-phenanthroline (phen)) bind water to the vacant coordination site, thereby undergoing a competition between hydrolysis (eq 2) and the formation of water adducts

Scheme 2. Three-Coordinate Complexes Examined in This Study, Where the Auxiliary Ligand Is either 1,10-Phenanthroline (phen, Where R2 = H) or Neocuproine (neo, Where R2 = CH3)

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Germany) LTQ FT hybrid mass spectrometer consisting of a linear ion trap (LTQ) coupled to a Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometer. The metal acetate complex cations [(L)M(O2CCH3)]+ were formed via direct electrospray ionization of methanolic solutions of the acetates [(L)M(O2CCH3)2] (∼0.5 mM). All other metal carboxylate complex cations [(L)M(O2CR)]+ were formed via electrospray ionization of methanolic solutions of the acetates [(L)M(O2CCH3)2] (∼0.5 mM) which had undergone carboxylate exchange with the appropriate carboxylic acid RCO2H (R = C6H5, CH2C6H5; ∼5 mM). In either case, the resultant solution was introduced into the ESI source of the mass spectrometer at a flow rate of 5 μL/min. ESI conditions used were as follows: spray voltage 4.0 kV; capillary temperature 300 °C; nitrogen sheath gas flow rate ca. 10 arbitrary units. The three-coordinate organometallic cations [(L)M(R)]+ were “synthesized” in the gas phase in an MS2 experiment via low-energy collision-induced dissociation (CID) of the massselected metal carboxylate complex cations [(L)M(O2CR)]+ in the linear ion trap. The helium bath gas was used as the collision gas, and the CID conditions used included the following: a Q value of 0.25 and an excitation time of 30 ms, with the normalized collisional energy varied from 20 to 30 (arbitrary units) to ensure that 15−25% of the precursor ion remained. ion−molecule reactions between the massselected organometallic cations [(L)M(R)]+ and background water were carried out in a series of MS3 experiments with reaction times varying between 30 and 3,000 ms. Reactions with water introduced via an ion−molecule reaction line, in addition to the background water inherently present in the ion trap, were used to test the reactivity of the synthesized organometallic ions at longer reaction times (10000 ms) and higher concentrations of water (>5 × 1010 molecules/cm3) as well as to determine the background concentration of water. This latter was achieved by adding known amounts of water into the ion trap via the ion−molecule reaction line and assuming a linear relationship between water concentration and the rate of the reaction. Single isotopes were selected (58Ni, 106Pd, and 195Pt) to allow for distinction between complexes with similar mass to charge ratios such as [(phen)60Ni(CH3)]+ and [(phen)58Ni(OH)]+. Measurement of the Rates of Reaction between the Organometallic Cations and Water. The reaction kinetics for the hydrolysis of the organometallic ions [(L)M(R)]+ were examined using an LTQ FT hybrid mass spectrometer.31 Ion−molecule reaction rate measurements were conducted by isolating the reactant ion, [(L)M(R)]+ in an MS3 experiment and then allowing it to react with water as previously reported.20c A relevant study has demonstrated that ions in ion−molecule reactions in this instrument are quasithermalized to the bath gas temperature (∼298 K).31c Least-squares regression of plots of ln(relative reactant ion intensity) vs time delay over 30−3000 ms confirmed pseudo-first-order kinetics. Single isotope peaks were used in several independent measurements taken over several days. The mass selection windows and scan mass range were kept constant throughout. Theoretical rates for the reaction were calculated with the program COLRATE32 using the average dipole orientation (ADO) theory of Su and Bowers.33 Extraction of Kinetic Data for the Hydrolysis Reaction via Modeling of the Kinetics. The potential reaction pathways associated with the reaction of [(L)M(R)]+ in an ion trap mass spectrometer are shown in Scheme 3. Rate measurements typically involve monitoring the decay of the organometallic complex [(L)M(R)]+ at different reaction times (ranging from 30 to 3000 ms) under pseudo-firstorder conditions where the concentration of neutral water was kept constant and was in a large excess. The reactions were carried out in a helium bath gas (2 mTorr), which provides quasi-thermal conditions in which product ions are thermalized to the temperature of the bath gas (∼298 K). The overall rate of decay of [(L)M(R)]+ is related to rates of formation of the energized adduct 5*, its back-reaction, collisional stabilization (k1), and decomposition to give the hydrolysis product (k2). The rate constants k1 and k2 can be determined via a kinetic analysis of the temporal profile of 4, 5, and 7 at different concentrations of water in addition to background water. To extract the individual rate constants shown in Scheme 3, the temporal profiles of [(L)M(R)]+, [(L)M(R)(H2O)]+, [(L)M(OH)]+, [(L)M(R)(N2)]+, and [(L)M(OH)(H2O)]+

EXPERIMENTAL SECTION

Materials. Pd(O2CCH3)2, Ni(O2CCH3)2, K2PtCl4, 1,10-phenanthroline (phen), and neocuproine (neo) were obtained from Sigma Aldrich (reagent grade). All solvents were HPLC grade. All purchased materials were used without further purification. Synthesis of the [(L)M(O2CCH3)]+ Complexes (Where L = phen, neo; M = NiII, PdII, PtII). All metal acetate complexes were prepared by literature procedures and were characterized by exact mass measurements and examination of the isotopic pattern of the metal from high-resolution mass spectrometry experiments on the monoacetate cations [(L)M(O2CCH3)]+, formed via electrospray ionization in the positive ion mode (ion assignments available in the Supporting Information, Table S1). Brief details of each synthesis are given as well as references to more detailed synthetic procedures. [(phen)Ni(O2CCH3)2].28 Phenanthroline (0.180 g, 1 mmol) in methanol (5 mL) was added to Ni(O2CCH3)2·4H2O (0.249 g, 1 mmol) in methanol (10 mL). The mixture was stirred at room temperature for 3 h and then filtered. The green product was obtained by diffusion of diethyl ether into the filtrate, collected by filtration, washed with acetone and diethyl ether, and dried overnight in vacuo to give green crystals of [(phen)Ni(O2CCH3)2] (0.28 g, 0.79 mmol, 79%). [(phen)Pd(O2CCH3)2].29 Phenanthroline (0.081 g, 0.45 mmol) was added to a solution of Pd(O2CCH3)2 (0.100 g, 0.45 mmol) in acetone (10 mL). The solution was left to sit for 30 min and then filtered, washed with cold acetone, and vacuum-dried to give a yellow powder of [(phen)Pd(O2CCH3)2] (0.13 g, 0.33 mmol, 74%). [(phen)Pt(O2CCH3)2].30 Dimethyl sulfoxide (DMSO, 0.30 g, 4.5 mmol) was added to a solution of K2[PtCl4] (0.63 g, 1.5 mmol) in water (10 mL), and the mixture was left at ambient temperature for 3 h. The yellow solid was then collected by filtration, washed with water, ethanol, and diethyl ether, and then dried in vacuo for 4 h to give [(DMSO)Pt(Cl)2] (0.55 g, 1.3 mmol, 89%). A mixture of phenanthroline (0.18 g, 1 mmol) dissolved in methanol (5 mL) was added dropwise to a stirred suspension of [(DMSO)2Pt(Cl)2] (0.45 g, 1 mmol) in methanol (40 mL). The mixture was stirred overnight at ambient temperature, and the resulting yellow solid was collected by filtration, washed with methanol and diethyl ether, and dried in air to give [(phen)Pt(Cl)2] (0.39 g, 0.9 mmol, 88%). Ag(O2CCH3) (0.049 g, 0.29 mmol) was added to [(phen)Pt(Cl)2] (0.072 g, 0.16 mmol, 10/18 ratio) dissolved in tetrahydrofuran/dichloromethane (5/1 v/v, 10 mL) and stirred overnight with exclusion of ambient light; then the solution was filtered and the filtrate was collected and evaporated to dryness under reduced pressure to give the yellow powder [(phen)Pt(O2CCH3)2] (0.058 g, 0.12 mmol, 72%). [(neo)Ni(O2CCH3)2].28 Neocuproine (0.21 g, 1 mmol) in methanol (5 mL) was added to Ni(O2CCH3)2·4H2O (0.25 g, 1 mmol) in methanol (10 mL). The mixture was stirred at room temperature for 3 h and filtered. A green precipitate was obtained by the addition of diethyl ether to the filtrate, which was collected by filtration, washed with acetone and diethyl ether, and dried overnight in vacuo to give green crystals of [(neo)Ni(O2CCH3)2] (0.28 g, 0.72 mmol, 72%). [(neo)Pd(O2CCH3)2].29 Neocuproine (0.094 g, 0.45 mmol) was added to a solution of Pd(O2CCH3)2 (0.10 g, 0.45 mmol) in acetone (10 mL). The solution was left to sit for 30 min and then filtered, washed with cold acetone, and vacuum-dried to give the yellow powder [(phen)Pd(O2CCH3)2] (0.16 g, 0.36 mmol, 80%). [(neo)Pt(O2CCH3)2].30 Neocuproine (0.21 g, 1 mmol) in methanol (5 mL) was added dropwise to a stirred suspension of [(DMSO)2Pt(Cl)2] (0.45 g, 1 mmol) in methanol (40 mL). The solution was stirred overnight, and the resulting yellow solid was filtered, washed with methanol and diethyl ether, and dried in air to give [(neo)Pt(Cl)2] (0.43 g, 0.91 mmol, 91%). Ag(OAc) (0.050 g, 0.29 mmol) was added to the complex (0.077 g, 0.16 mmol, 10/18 ratio) dissolved in tetrahydrofuran/dichloromethane (5/1 v/v, 10 mL) and stirred overnight in the dark; then the solution was filtered and the filtrate was collected and evaporated to dryness to give a yellow powder (0.060 g, 0.12 mmol, 70%). Mass Spectrometry Experiments. All mass spectrometric experiments were conducted on a Thermo Scientific (Bremen, 6933


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Scheme 3. Kinetic Model for the Competing Reactions of [(L)M(R)]+ Used to Extract Kinetic Data for the Formation of the Collisionally Stabilized Adducts [(L)M(R)(H2O)]+ and the Hydrolysis Product [(L)M(OH)]+

from the experimental data were modeled using the program DynaFit.34 This program performs a least-squares regression to simultaneously fit both reactant and product concentration vs time data. Theoretical Methods. Geometry optimizations and electronic energy calculations were performed using the Gaussian 09 package35 to provide insights into the reactions of the organometallic cations, [(L)M(R)]+, with water (Supporting Information, Figures S5−S21) and nitrogen (Supporting Information, Figure S22). Structures of minima and transition states were optimized using the M06 hybrid meta-GGA functional.36 The Stuttgart−Dresden (SDD) basis set and effective core potential were used for the nickel, palladium, and platinum atoms, while the 6-31+G(d) all-electron basis set was used for carbon, nitrogen, and hydrogen.37 All optimized structures were subjected to vibrational frequency analysis to ensure they corresponded to true minima (no imaginary frequencies) or transition states (1 imaginary frequency). The final energies used to calculate the reaction energies were corrected with the M06/6-31+G(d) zero-point vibrational energies (Ereported = Eelectronic + Ezpve). IRC calculations were performed on transition states to confirm that they connect the stated reactants and products. Reaction Rate Theory. For the bimolecular [(L)M(R)]+ + H2O reactions where M = Ni, rate constants were simulated via energygrained master equation simulations, using Rice−Ramsperger−Kassel-Marcus (RRKM) theory for microcanonical rate constants k(E). The general procedure is similar to that used in recent studies to model ion−molecule reaction kinetics.20c,31b,38 Structures and vibrational frequencies at the M06 level of theory were used to determine densities and sums of states and moments of inertia. Barrierless association of the cations with water and the reverse of the RH loss processes are assumed to proceed at the ion−molecule collision rate and are modeled using restricted Gorin transition states.39 Master equation simulations were performed with an energy grain of 10 cm−1 under temperature and pressure conditions representative of the ion trap (298 K and 2.0 mTorr). The value of ΔEdown was adjusted so as to achieve agreement between theory and experiment, as described in the text. Collisions between adduct ions and He are treated as proceeding at the Langevin collision rate. Reported results represent 1 × 107 trials, with a steady state being achieved within around 100 collisions. All rate constant calculations were performed using a modification of MultiWell201340 that allows for Langevin ion−molecule collision rates. The PPM code was used to facilitate data postprocessing and to confirm steady-state product distributions.41

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to low-energy CID, resulting in the formation of each of the organometallic ions [(L)M(R)]+ shown in Scheme 2 via gasphase decarboxylation reactions (cf. eq 1; data not shown). Each of these coordinatively unsaturated organometallic complexes [(L)M(R)]+ were mass-selected in a series of MS3 experiments and allowed to undergo ion−molecule reactions with water. We first describe general reactivity trends and then quantify the kinetics of these reactions. The complexes that reacted with background water with a time delay of 300 ms are shown in Figure 1. 1. General Reactivity Trends. The major product formed via reaction between [(phen)Ni(CH3)]+ (m/z 253, Figure 1a) and water is the hydrolysis product [(phen)Ni(OH)]+ (m/z 255, eq 2), as confirmed by high-resolution mass spectrometry (Supporting Information, Table S1). Addition of water formed [(phen)Ni(CH3)(H2O)]+ (m/z 271, eq 3), which is a minor product. Thus, [(phen)Ni(CH3)]+ mainly appears to react with water via a hydrolysis reaction (eq 2). [(phen)Pd(CH3)]+ (m/z 301, Figure 1c) solely reacts with water to generate the adduct [(phen)Pd(CH3)(H2O)]+ (m/z 319, eq 3). A range of product ions are observed in the MS3 spectrum of [(phen)Pt(CH3)]+ (m/z 390, Figure 1e), including the following: addition of water, [(phen)Pt(CH3)(H2O)]+ (m/z 408, eq 3); hydrolysis, [(phen)Pt(OH)]+ (m/z 392, eq 2); hydrolysis followed by water adduction, [(phen)Pt(OH)(H2O)]+ (m/z 410, eq 7, Scheme 3). A major product is due to the addition of background N2 present in the mass spectrometer to yield [(phen)Pt(CH3)(N2)]+ (m/z 418), which was confirmed by high-resolution mass spectrometry (Supporting Information, Table S1) as reported previously.20f As the background nitrogen is likely to originate from the ambient air as well as the nebulizing gas used in the ESI process, and background water is likely to originate from the ambient air only, the concentration of nitrogen in the mass spectrometer is believed to be much greater than that of water. Despite the smaller binding energy of nitrogen to platinum (−1.22 eV; Supporting Information, Figure S22) in comparison to that of water (−1.45 eV; 0.23 eV difference), this higher concentration leads to a more competitive addition of nitrogen. In contrast, the higher difference in the binding energy in the nickel (−1.21 eV for water vs −0.86 eV for nitrogen; 0.35 eV difference) and palladium cases (−1.15 eV for water vs −0.82 eV for nitrogen; 0.33 eV difference) helps explain why reactions with nitrogen are not observed

RESULTS AND DISCUSSION

Electrospray ionization of the metal carboxylates [(L)M(O2CR)2] generated the desired carboxylate precursor ions [(L)M(O2CR)]+ (L = phen, neo; R = CH3, C6H5, CH2C6H5; M = Ni, Pd, Pt), which were then mass-selected and subjected 6934


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Figure 1. LTQ-FT-ICR MS3 spectra of ion−molecule reactions of mass-selected organometallic ions with background water (∼5 × 109 molecules/ cm3) for a period of 300 ms in the linear ion trap: (a) [(phen)58Ni(CH3)]+; (b) [(phen)58Ni(C6H5)]+; (c) [(phen)106Pd(CH3)]+; (d) [(phen)106Pd(C6H5)]+; (e) [(phen)195Pt(CH3)]+; (f) [(phen)195Pt(C6H5)]+. The mass-selected organometallic ions are denoted by an asterisk (*). The hash mark (#) represents the product of hydrolysis, the ampersand (&) represents addition of water to the product of hydrolysis, and the caret (∧) represents the product formed via addition of N2. The insets in parts c and d show the peak fronting for the aqua complexes [(phen)Pd(R)(H2O)]+.42

A further examination of Figure 1 reveals that, in the case of R = CH3 and L = phen, the hydrolysis reaction (eq 2) follows the order nickel > platinum, with palladium being unreactive. For platinum the hydrolysis reaction seems slower, with the reaction with nitrogen to form the adduct [(phen)Pt(CH3)(N2)]+ being competitive. As the R group becomes larger, with phenyl replacing the methyl group, the hydrolysis reaction is only observed for nickel. Furthermore, when the R group was changed to benzyl no hydrolysis reaction occurred (Supporting Information, Figures S1 and S2). Switching the phenanthroline ligand with neocuproine (neo, 2,9-dimethylphenanthroline), where the methyl groups replace the hydrogen atoms at the 2and 9- positions of the diimine, slowed this reaction considerably (Supporting Information, Figures S3 and S4). This effect is likely due to the methyl groups on the ligand hindering the access of water to the metal center. In order to quantify the trends discussed above, kinetic measurements for the reactions of each of the organometallic ions with water were carried out; the results of these experiments are described in section 2. Since no reaction with R = benzyl were observed, these cases are not discussed in this section. 1.1. Reversibility of Water Addition: H2O Loss from the [(L)M(R)(H2O)]+ Complexes. The insets to Figures 1c and 1d show evidence for peak fronting for the water adducts [(phen)Pd(R)(H2O)]+ during the mass analysis step, consistent with them being “fragile” ions which become metastable as they are ejected from the quadrupole ion trap.42 More direct evidence that the aqua ligand is weakly bound to the metal center comes from the observation that when each of the water adducts [(phen)M(R)(H2O)]+ are mass-selected without

experimentally. Removing water from the ion trap would allow insight into the binding of nitrogen with [(phen)Ni(CH3)]+ and [(phen)Pd(CH3)]+. When the R group is changed from methyl to phenyl, the order of reactivity of the metals stays the same but the overall reactivity slows down. This change in reactivity is quantified and discussed further in section 3. [(phen)Ni(C6H5)]+ (m/z 315, Figure 1b) undergoes hydrolysis to form [(phen)Ni(OH)]+ (m/z 255, eq 2) with the loss of C6H6, in competition with the addition of water to form [(phen)Ni(C6H5)(H2O)]+ (m/z 333, eq 3). In the case of palladium, [(phen)Pd(C6H5)]+ (m/z 363, Figure 1d) reacts with background water to form the adduct [(phen)Pd(C6H5)(H2O)]+ (m/z 381, eq 3) and does not undergo hydrolysis, similarly to the case for R = CH3. The spectrum of [(phen)Pt(C6H5)]+ (m/z 452, Figure 1f) shows addition of background N2 to yield [(phen)Pt(C6H5)(N2)]+ (m/z 480) and addition of water to give [(phen)Pt(C6H5)(H2O)]+ (m/z 470, eq 3). Interestingly, the addition of N2 is minimal in this case in comparison to that for R = CH3 and the hydrolysis reaction is absent. A possible explanation for the lack of reactivity for [(phen)M(C6H5)]+ with water is that the more basic methyl group undergoes hydrolysis more readily than the less basic phenyl group. Indeed, previous gas-phase studies on the reaction of [(L)M(CH3)]+ (L = phen, bipy) with benzene have shown that the metathesis reaction (eq 8) readily occurs for the group 10 metals,43,44 indicating that the hydrolysis reaction with methyl is more thermodynamically favorable than with phenyl, which supports our findings. [(L)M(CH3)]+ + C6H6 → [(L)M(C6H5)]+ + CH4

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Table 1. Overall Reaction Rate Constants (kexpt),a Collision Rate (kADO),b Reaction Efficiencies (φ),c and Individual Rate Constants for Reactions k1, k2, k3, k5, and k6 (Scheme 3)d for the Reaction of [(phen)M(R)]+ with Watere kinetic param a,d

kexpt kADOb reaction efficiency (φ)c k1d k2d k3d,h k5d,h k6d

[(phen)Ni(CH3)]+ 2.35 1.72 13.6 5.16 1.83 2.13 i 3.48 −1 −1

−10

× 10 × 10−9

× 10−11 × 10−10

× 10−12

[(phen)Pd(CH3)]+ 1.20 × 1.71 × 7.0 1.20 × 5.15 × 0.105 i 3.93 ×

−10

10 10−9

10−10 10−13

10−10

[(phen)Pt(CH3)]+ 5.84 × 1.71 × 34.0 5.54 × 2.98 × 0.121 1.24 3.71 ×

−10

10 10−9

10−10 10−11

10−10

[(phen)Ni(C6H5)]+ −11

5.9 × 10 1.71 × 10−9 3.5 1.36 × 10−11 4.58 × 10−11 0.195 i 1.34 × 10−12

[(phen)Pd(C6H5)]+ −10

1.28 × 10 1.70 × 10−9 7.6 1.28 × 10−10 i 0.0685 i i

[(phen)Pt(C6H5)]+ 4.25 × 10−10 1.70 × 10−9 25.0 4.18 × 10−10 i i 0.0752 i

In units of cm molecule s . As in our previous study, errors are conservatively estimated as ±25%. The numbers listed represent the average of three runs. bThe collision rate was calculated using the theory of Chesnavich et al.33 The calculation was performed using the program COLRATE.32 The background concentration of water in the ion trap was found to be ∼5 × 109 molecules/cm3. cReaction efficiency (φ) = kexpt/ kADO × 100. dRates determined using the program DynaFit.34 eComplexes which undergo hydrolysis or water adduction (see footnotes f and g) are considered. k4 is negligible in all cases for this reaction. fAll reactions with [(neo)M(R)]+ were too slow to allow rate measurements, even with increased concentrations of water. gAll complexes containing the R group benzyl were found to be unreactive toward water, with an upper bound rate of 0 eV), suggesting that the reaction is not expected to occur under the near-thermal conditions of the ion trap, which is entirely consistent with the experimental results. 6939


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More importantly, the geometries of the reactant complex highlight that the methyl groups at the 2- and 9-positions of the phenanthroline ligand do indeed block the vacant coordination site on the metal center, forcing the complex to twist and adopt a distorted-square-planar geometry, as shown in Figure 4. Thus, by

Figure 4. Results of DFT calculations at the M06/SDD6-31+G(d) level of theory: Deviation from planarity for the water adduct 5 induced by the neocuproine auxiliary ligand: (a) planar [(phen)Ni(CH3)(H2O)]+; (b) distorted [(neo)Ni(CH3)(H2O)]+. The Cartesian coordinates and energies for these calculated structures are available in the Supporting Information, Figures S5 and S16.

comparing the structures of [(phen)Ni(CH3)]+ (Figure 4a) and [(neo)Ni(CH3)]+ (Figure 4b), we observe that the dihedral angle between the metal atom, the two nitrogen atoms, and the rings, in the case of L = neo, is 168°. Hence, the nickel atom is 12° out of the plane of the neocuproine. When L = phen, the same dihedral angle for nickel is 180°, with the nickel lying in the plane of the phenanthroline. In addition, for L = neocuproine, the methyl groups at positions 2 and 9 are also also observed to be out of the plane of the nitrogen atoms and the rings, in the opposite direction to nickel with a dihedral angle of 189°. 3.5. Role of the R Group in the Acid−Base Hydrolysis of [(L)M(R)]+. While addition of water to [(L)M(R)]+ occurs readily for all three metals when L = phenanthroline and R = CH3), changes in the R group cause changes in the reactivity. When the methyl group was replaced by a phenyl group, the reaction with water slows down for both nickel and palladium and increases slightly for palladium. In order to explain this observation, DFT calculations were performed on [(phen)M(C6H5)]+ systems. The results of these calculations (Figure 5) suggest that although the transition states appear to be lower in energy than the entrance channel in all cases, the energies of the final products, 7, are relatively higher than those in the case of R = CH3. For instance, in the case of nickel, the hydrolysis product was at 0.55 eV lower in energy than the entrance channel in the case of R = CH3, whereas it is only 0.01 eV lower in the case where R = C6H5. Interestingly, these calculations show that the intermediates formed between water and the metal complex should be stable. However, as the product energies are higher than the corresponding transition state, this might lead to a substantial reversible reaction to regenerate the reactants. These might serve as an explanation to the slower rates observed. The slower rate of hydrolysis for [(phen)Ni(C6H5)]+ in comparison to [(phen)Ni(CH3)]+ can be explained from the potential energy diagrams in Figures 3 and 5 and the energies from Table 2. The reverse reaction, whereby the products react to re-form the reactants, is energetically favorable when the energy of the separated products is higher than that of the appropriate transition state (i.e. TS5‑6). This appears to occur when R = C6H5. For example, in the case of Ni (Figures 3 and 5) the relative energies of the transition states for R = CH3 and R = C6H5 are similar at −0.38 eV. Therefore, the rates should be the same on the basis of kinetics. Thermodynamically, however, the relative energy of the product (7) for R = CH3 is

Figure 5. Results of DFT calculations at the M06/SDD6-31+G(d) level of theory. (a) Plot of the energy required to undergo hydrolysis for each of the complexes [(phen)M(C6H5)]+, where M = Ni, Pd, Pt. The barriers for hydrolysis are Ni (−0.38 eV), Pt (−0.37 eV), and Pd (−0.08 eV). (b) DFT calculated structures of the addition of water to the unsaturated organometallic species [(phen)Ni(C6H5)]+ and further hydrolysis via the acid−base mechanism (path A, Scheme 4; Supporting Information, Figure 9). The Cartesian coordinates and energies for all calculated structures are available in the Supporting Information, Figures S9−S11.

at −0.55 eV and that for R = C6H5 is at −0.01 eV. Hence, in the case of R = CH3 the separated products are lower in energy than the transition state, suggesting that the reverse reaction is not favorable. However, for R = C6H5 the separated products are higher in energy than the transition state, opening up the possibility for a reverse reaction to re-form the reactants. Interestingly, on the basis of the energies relative to the separated reactants, the product [(L)M(OH)(C6H6)]+ complexes appear to be more stable than [(L)M(OH)(CH4)]+ complexes. Indeed, an examination of the structures of the [(L)M(OH)(C6H6)]+ complexes reveal that the benzene is bound in a η2 fashion (Figure 5, structure 6), consistent with previous DFT calculations on related [(L)M(C6H5)(C6H6)]+ complexes.17a This is in contrast to the agostic interaction of CH4 with the metal in [(L)M(OH)(CH4)]+. The experiments reveal that changing the methyl group to a benzyl group completely shuts down the hydrolysis reaction, since neither the hydroxide product (eq 2) nor the water adduct (eq 3) is observed (Supporting Information, Figure S1). The DFT calculations on the monodentate benzyl group suggest that, while hydrolysis is endothermic, coordination of water would occur and thus the [(phen)M(CH2C6H5)(OH2)]+ 6940


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adduct and He in deactivating collisions. Although little is known about the collisional energy transfer properties of large cations such as these, this value is of reasonable magnitude, although it is somewhat larger than values suggested from recent studies of organic cation−He collisions (∼200 cm−1).31b,38c Using ΔEd = 500 cm−1, the master equation modeling predicts a total hydrolysis rate constant of 5.37 × 10−10 cm3 molecule−1 s−1, in comparison to an experimental value of 2.35 × 10−10 cm3 molecule−1 s−1. The theoretical branching ratio for hydrolysis is 89%, in agreement with the experimental value (90%; Supporting Information, Table S2). For the [(phen)Ni(C6H5)]+ + H2O reaction the theoretically determined rate constant is 1.04 × 10−10 cm3 molecule−1 s−1, whereas the experimental value is 5.94 × 10−11 cm3 molecule−1 s−1. In this case the hydrolysis branching ratio could be matched relatively well (90% vs 92%; Supporting Information, Table S2) using ΔEd = 150 cm−1. Note that, because this latter reaction features a product set with energy just below that of the reactants, the computed rate constant is very sensitive to this barrier height; increasing it within a reasonable range slows the predicted rate constant dramatically. It is apparent from these reaction rate simulations that the experimental observations can be reconciled by a mechanism in which chemically activated aqua adduct ions formed within the ion trap undergo competitive collisional deactivation and unimolecular dissociation to yield the observed products. The DFT calculated barrier heights also result in calculated rate constants that are accurate to within around a factor of 2, which validates the suggested acid−base mechanism. 4. Links to Condensed-Phase Three-Coordinate NiII, PdII, and PtII Organometallic Complexes. The reactivity of d8 NiII, PdII, and PtII organometallic complexes has led to them being of interest as catalysts. It is likely that partially solvated metal complexes play important roles in their reactions in the solution. While there are numerous reports of protonolysis reactions of square -planar NiII, PdII and PtII organometallic complexes in solution, most mechanistic discussions do not invoke three-coordinate complexes as key intermediates in these reactions. Directly related three-coordinate complexes have, however, been isolated as the product of oxidative addition to M(0) complexes22d and have been invoked previously as intermediates in C−H activation reactions23a and ligand exchange reactions.22b The gas-phase studies and theoretical calculations presented here highlight that solvent association to the three-coordinate complexes [(phen)M(R)]+ is reversible. For the “forward” reaction, there is a potential competition between hydrolysis to form the new three-coordinate complex [(phen)M(OH)]+ (eq 2) and solvation to form the fourcoordinate complexes [(phen)M(R)(H2O)]+ (eq 3). Both reactions have precedent in solution. In the complexes studied here, the smaller nickel ion favors hydrolysis, due to the polarizing effect of the Ni2+ ion as evident in the Ni−O bond length (1.95 Å) in comparison to Pd2+ (Pd−O bond length 2.15 Å) or Pt2+ (Pt−O bond length 2.14 Å), which weakens the O−H bond, allowing hydrolysis to occur.54 In the case of palladium and platinum the three-coordinate complexes preferentially undergo addition of water to give the complexes [(phen)M(R)(H2O)]+ (eq 3), which are directly related to four-coordinate palladium13 and platinum14 complexes containing both a C-bonded alkyl (or an aryl) ligand and a coordinated water that have been isolated and characterized by X-ray crystallography. In contrast, Ni(II) complexes containing coordinated alkyl (or aryl) and aqua ligands have not been structurally characterized by X-ray crystallography.

adduct should be experimentally observed (Supporting Information, Figure S2). An explanation for this discrepancy is that for [(phen)M(CH2C6H5)]+ DFT results suggest that the binding of the benzyl in a η1 fashion is less favorable than an η3 binding mode, therefore blocking a coordination site and retarding the formation of the aqua adduct (Figure 6).

Figure 6. Binding modes of the benzyl group to metal centers in [(phen)M(CH2C6H5)]+ complexes. Examples of DFT calculated complexes: (a) [(phen)Pd(η1-CH2C6H5)]+ (+1.52 eV relative to η3 complex); (b) [(phen)Pd(η3-CH2C6H5)]+.

Examination of the literature reveals several examples of η2 and η3 binding of benzyl groups to metal complexes,49 including X-ray crystal structures of nickel,50 palladium,51 and platinum52 complexes. In a recent study on the protonolysis of cis-[Pt(CH2Ar)2(PEt3)2] in acetonitrile DFT calculations have been used to invoke a [Pt(η2-CH2Ar)(PEt3)2]+ complex to rationalize the experimental observation of cis−trans isomerization of the product complex [Pt(CH2Ar)(NCCH3)(PEt3)2]+.53 While DFT calculations suggest that the most favorable binding mode is [(phen)M(η3-CH2C6H5)]+ (Figure 6), calculations also suggest that, starting from the stable structures, the complexes can undergo isomerization to the η1 structures, which opens up a coordination site that allows a reaction with H2O. This isomerization barrier, however, is 1.02 eV in energy and, thus, the stable [(phen)M(η3-CH2C6H5)]+ complex is unreactive in the LTQ mass spectrometer used in these experiments. When these more stable isomeric structures are considered, hydrolysis of water is predicted to be endothermic by 1.02 eV for [(phen)Ni(CH2C6H5)]+, 1.37 eV for [(phen)Pt(CH2C6H5)]+, and 1.52 eV for [(phen)Pd(CH2C6H5)]+ (Table 2). IRMPD spectroscopy16 could be used in future studies as a structural probe to establish the nature of the coordination of the benzyl group to the metal center, since the DFT calculations predict different IR spectra for the different isomers (Supporting Information, Figures S23−S28). 3.6. Reaction Rate Simulations for the [(phen)Ni(R)]+ + H2O Reactions. To help confirm the proposed acid−base mechanism, theoretical reaction rate simulations were performed for the [(phen)Ni(R)]+ + H2O reactions, where R = CH3, C6H5. From Table 1 both reactions were experimentally found to involve collisional deactivation of the aqua adduct as well as chemically activated hydrolysis to yield [(phen)Ni(OH)]+ cations, but with a marked difference in the total reaction rate with changing R group. For the [(phen)Ni(CH3)]+ + H2O reaction excellent agreement can be found between experiment and theory when using a value of 500 cm−1 for ΔEd, which is the average energy transferred between the reaction 6941


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material is available free of charge via the Internet at http:// pubs.acs.org.

The reversibility of water adduction has been demonstrated in this work. Thus, the complexes [(phen)M(R)(H2O)]+ can lose the aqua ligand (eq 5, Scheme 3), as discussed in section 1.1. The small amount of energy required for these complexes to lose the aqua ligand suggests that similar ligand dissociation reactions may play a role in solution-phase ligand exchange reactions.55

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Corresponding Author

*R.A.J.O.: fax, +613 9347-5180; tel, +61 3 8344-2452; e-mail, [email protected].

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Notes

CONCLUSIONS Decarboxylation of group 10 metal carboxylate complexes [(L)M(O2CR)]+ yields the three-coordinate organometallic complexes [(L)M(R)]+, which react with water via hydrolysis reactions (eq 2) and/or water adduction (eq 3). Both experiment and DFT calculations reveal that the reactivity of [(L)M(R)]+ depends on the nature of the metal (M), the identity of R, and the ligand L (L = 1,10-phenanthroline (phen), neocuproine (neo); M = nickel, palladium, platinum; R = CH3, C6H5, CH2C6H5). Hydrolysis occurs the fastest for L = phen, R = CH3, and M = Ni. When M = Pd, Pt the addition of water to form [(phen)M(CH3)(H2O)]+ was the major product, with the alkyl group and aqua ligand coexisting in the coordination sphere of the metal complex. Changing the neutral ligand to neocuproine instead of phenanthroline causes the addition of water to cease. The lack of reactivity for L = neo was postulated to be due to the methyl groups in the 2- and 9positions on the ligand, blocking access of the water molecule to the metal center. DFT calculations reveal that [(neo)Ni(CH3)(H2O)]+ adopts a distorted-square-planar geometry. Increasing the size of the R group also alters the reactivity. When R = C6H5, all three metals showcase the same reactivity as in the case of CH3, the difference being that the reaction is slowed down in all three cases, suggesting an associative mechanism. Interestingly, when R = CH2C6H5, no reaction was observed in all cases. DFT calculations suggested that the most stable binding mode for the benzyl complexes was an η3 binding mode that blocked water from complexing with the metal center. Theory was found to agree well with experiments, with barriers for hydrolysis calculated for [(phen)M(CH3)]+ as −0.38 eV for Ni, −0.14 eV for Pt, and +0.15 eV for Pd, which follow the experimental reactivity trend. The calculated higher stability of the complexes [(L)M(OH)(C6H6)]+ in comparison to [(L)M(OH)(CH4)]+ is believed to be due to the η2 binding mode of C6H6 in comparison to the agostic interaction between the metal and CH4. Both the work reported here and previous work43 on metathesis reactions of [(L)M(CH3)]+ (eq 8) suggests that three-coordinate M(II) complexes exhibit interesting reactivity trends with substrates. Given that palladium complexes catalyze protodecarboxylation reactions (eq 9)56 and decarboxylative allylation57 (eq 10, R1 = allyl) and benzylation reactions (eq 10, R1 = benzyl,)58 current work is underway to establish whether [(phen)M(R)]+ complexes catalyze such reactions in the gas phase.

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RCO2 H → RH + CO2

(9)

RCO2 R1 → RR1 + CO2

(10)

AUTHOR INFORMATION

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the ARC for financial support via grant DP110103844 (to R.A.J.O. and G.N.K.) and DP1096134 (to G.N.K.) and through the ARC CoE program. M.J.W. thanks the Faculty of Science for a Melbourne Research Scholarship. The authors gratefully acknowledge the generous allocation of computing time from the Victorian Partnership for Advanced Computing (VPAC) Facility.

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ASSOCIATED CONTENT

S Supporting Information *

Text, figures, and tables giving Cartesian coordinates of all species examined, a full citation of ref 35, mass spectra, potential energy diagrams, and DFT predicted IR spectra. This 6942


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Organometallics

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Aryl Group in the Hydrolysis

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