Article pubs.acs.org/Organometallics
Metal−Carbonyl Bond Energies in Phosphine Analogue Complexes of Co(CO)3NO by Photoelectron Photoion Coincidence Spectroscopy Csaba István Pongor,†,‡ László Szepes,† Rosemarie Basi,§ Andras Bodi,∥ and Bálint Sztáray*,§ †
Institute of Chemistry, Eötvös Loránd University, Budapest, Hungary 1117 Institute of Biophysics and Radiation Biology, Semmelweis University, Budapest, Hungary 1094 § Department of Chemistry, University of the Pacific, Stockton, California 95211, United States ∥ Paul Scherrer Institut, Villigen 5232, Switzerland ‡
ABSTRACT: Threshold photoelectron photoion coincidence spectroscopy was used to study a series of cobalt−organic complexes with phosphine and phosphine analogue ligands: PMe3Co(CO)2NO, PEt3Co(CO)2NO, AsMe3Co(CO)2NO, and SbMe3Co(CO)2NO. The two lowest energy dissociative photoionization channels were sequential carbonyl losses in all four cases. Nitrosyl loss was also observed as a minor channel from the molecular ion and as a major competitive dissociation from the first (carbonyl loss) daughter ion. Further sequential CO and NO losses lead to the LCo+ (L = PMe3, PEt3, AsMe3, SbMe3) ions, which, similarly to an earlier threshold collision-induced dissociation (TCID) mass spectrometry study on the phosphine complexes,1 exhibited parallel ethene loss and methane loss dissociation reactions, although the bare metal ion was not observed. Unimolecular statistical rate theory (RRKM) calculations were performed to model the first two carbonyl loss channels and relate cobalt−carbonyl bond energy trends to the electron donor and acceptor properties of the phosphine analogue ligands. Co−CO bond energies of 0.90 ± 0.09, 0.84 ± 0.08, 1.13 ± 0.08, and 1.15 ± 0.09 eV were obtained in LCo(CO)2NO+ (L = PMe3, PEt3, AsMe3, SbMe3, respectively) and 0.82 ± 0.11, 0.74 ± 0.11, 0.95 ± 0.10, and 0.94 ± 0.09 in the first daughter ions, respectively.
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INTRODUCTION Transition-metal complexes are used as catalysts in numerous reactions of academic and industrial importance. The ligands influence the electronic structure, catalytic activity, and physical and chemical properties of a complex, determining its possible applications. Therefore, systematic studies of the physical and chemical properties of transition-metal compounds and their ligands can be useful for catalyst design and in predicting the effect of a certain ligand upon substitution. Transition-metal carbonyls are one of the most extensively studied groups of organometallic compounds. Their relative stability and physical properties make them ideal candidates for both theoretical and experimental studies. Modern textbooks of inorganic and organometallic chemistry2,3 all discuss the electronic structure of transition-metal carbonyls in the context of molecular orbital (MO) bond theories involving donor− acceptor interactions to explain trends in bonding.4 Today, one of the most prominent bonding models in transition-metal coordination complexes is the Dewar−Chatt−Duncanson model (DCD),4,5 in which the structure is discussed in terms of synergetic donor and acceptor interactions between the ligand and the transition-metal center. In carbonyl complexes, the highest occupied orbital in CO is of C−O bonding σ character and can interact with unoccupied d orbitals of the transition metal. Occupied d orbitals of the transition metal can also interact with the lowest unoccupied π* orbital of CO, resulting in π back-donation. According to quantum chemical © 2012 American Chemical Society
calculations, the back-donation appears to be more important than σ donation in the quantitative partitioning of the metal− carbonyl bond energy.6 In this study, we will confirm this finding using experimental data. The popularity of the DCD model can be ascribed to the fact that it is a very versatile framework to discuss the electronic structure of the complexes in a qualitative way. However, quantitatively, this advantage can also be a drawback, since it results in oversimplifying otherwise complicated phenomena. When Dewar,5 Chatt, and Duncanson4 originally proposed the model, the synergy of donor−acceptor interactions was strongly emphasized. The DCD model underlines the role of d orbitals of the transition metal, which can lead to underestimating the contributions of s or p orbitals.7−9 The terminology used also implies that changes in charge distribution are the sole driving force in ligand binding, which is not necessarily true.6 It is widely accepted that π backdonation is energetically more significant than σ donation,6 even though changes in charge density show a larger contribution of σ donation.6,10−12 The σ donor orbitals have significant electron density between the carbon and the transition metal. When coordinated, the ligand encounters significant electron repulsion on interacting with occupied orbitals of the transition metal, making σ donation the Received: February 16, 2012 Published: April 13, 2012 3620
dx.doi.org/10.1021/om300132g | Organometallics 2012, 31, 3620−3627
Organometallics
Article
tions. We concluded that the phosphine analogue ligands’ σ donor properties are similar, with a slight decrease from P to Sb. The other difference between these ligands was found to be their π acceptor quality, which decreases in the order CO > PMe3 > AsMe3 > SbMe3. The photoelectron photoion coincidence studies on the bond dissociation energies, presented in this paper, were carried out to shed some more light on how π acceptor or σ donor qualities influence metal− carbonyl binding.
energetically less favorable interaction.12 Furthermore, theoretical analyses of electronic properties of nonclassical carbonyls suggested that electrostatic interactions may also play a significant role.9,13 The carbonyl ligand is often used as a reporter group within organometallic compounds.2,14 Changes caused by ligand substitution in the C−O stretching frequencies,7 bond lengths,15,16 ionization energies,17−20 and electron distributions21−24 have been used extensively to study changes in local electron density, electronic structure, and bonding in organometallics. Cotton et al. were among the first to study force constants and C−O stretching frequencies systematically in organometallic compounds and ranked ligands based on their π acceptor strength.25−28 Angelici et al. suggested that σ donor strength alone could perhaps account for changes in C−O stretching frequencies in a study on a group of amine- and phosphine-substituted compounds.29 Soon after, Graham developed a method to separate σ and π interactions.30,31 Tolman suggested a set of steric and electronic parameters to describe phosphine and related ligands.32 On the basis of the experiments of Strohmeier et al., Tolman suggested an electronic parameter, ν, to be the A1 carbonyl mode in the substituted derivatives of Ni(CO)4, which can be used to rank ligands on the basis of their donor−acceptor properties.21 Ni(CO)4, which has the ominous nickname “liquid death” and was, therefore, not studied in our iPEPICO experiments, is advantageous due to its high reactivity toward phosphine compounds in stochiometric amounts, almost regardless of the ligand size. Advances in quantum chemical methods have led to the possibility of using calculated descriptors for predictive models.33 Thermochemical data may also be used to understand the effects of different ligands better. Bond dissociation energies (BDE) and heats of formation can be calculated and used to explain substituent effects;10,34−38 however, measured dissociation energies are needed to test the validity of these results. Our groups have aimed to determine such BDE data systematically using photoelectron photoion coincidence spectroscopy (PEPICO)1,39−41 and to explain the trends based on ultraviolet photoelectron spectroscopy (UPS).42−44 In this work, we present an imaging photoelectron photoion coincidence (iPEPICO) study on a series of trialkylpnictogensubstituted cobalt complexes, PMe3Co(CO)2NO, PEt3Co(CO)2NO, AsMe3Co(CO)2NO, and SbMe3Co(CO)2NO. Previously, we have published two preludes to this study: a threshold PEPICO (TPEPICO) paper on the two phosphine complexes1 and a more recent photoelectron spectroscopic study on all four title compounds.43 In the first paper, we also published the results of TCID experiments,1 carried out in the Armentrout lab, on the dissociation of the PR3Co+ ions, as the TPEPICO experiments were limited by the available photon energy on the laboratory-based instrument. More importantly, the mass resolution was much lower and possible competitive NO-loss processes could not be resolved. The iPEPICO instrument at the vacuum ultraviolet (VUV) beamline of the Swiss Light Source offers higher mass resolution, which was proven to be crucial in distinguishing the carbonyl and nitrosyl loss steps, and we could now extend our studies to complexes with the heavier phosphine analogue ligands (AsMe3 and SbMe3), as well. In the photoelectron spectroscopy paper,43 we studied the substitution effect of these ligands on the electron structure of Co(CO)3NO and CpMn(CO)3 with the aid of photoelectron spectroscopy and quantum chemical calcula-
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EXPERIMENTAL SECTION
Synthesis. All manipulations were carried out under a nitrogen atmosphere using standard Schlenk techniques. The solvents (hexane, tetrahydrofuran, dibutyl ether, and diethyl ether) were purchased from Sigma-Aldrich and were freshly distilled from sodium and benzophenone before use. Co(CO)3NO was obtained from Strem Chemicals, and no further purification was needed. Trimethylarsine and trimethylstibine were synthesized according to the literature.43−48 Briefly, the appropriate trichloride was methylated using methylmagnesium iodide, which was prepared in an ether type solvent, to which the trihalide solution was added dropwise. Dibutyl ether was used in the case of AsMe3 and diethyl ether in the case of SbMe3. The resulting trimethylpnictogens were purified by distillation and used in the preparation of the substituted organometallic complexes. Trimethylphosphine was purchased from Sigma-Aldrich and was used without further purification. A method described in an earlier paper was followed for the synthesis of the trimethylphosphine (PMe3), trimethylarsine (AsMe3), and trimethylstibine (SbMe3) monosubstituted derivatives of Co(CO)3NO.43 Under a nitrogen atmosphere, 2.5 g (14.5 mmol) of Co(CO)3NO was dissolved in 20 mL of tetrahydrofuran (THF). After the addition of a small excess of trimethylphosphine (
