Note pubs.acs.org/Organometallics
Structure and Bonding of Palladium Oxos as Possible Intermediates in Metal−Carbon Oxy Insertion Reactions Travis M. Figg,† George Schoendorff,‡ Bhaskar Chilukuri,§ and Thomas R. Cundari*,‡ †
Cherry L. Emerson Center for Scientific Computation, Emory University, 1515 Dickey Drive, Atlanta, Georgia 30322, United States Department of Chemistry and Center for Advanced Scientific Computing and Modeling (CASCaM), University of North Texas, Denton, Texas 76203, United States § Department of Chemistry, Washington State University, P.O. Box 644630, Pullman, Washington 99163-4630, United States t@unt. edu ‡
S Supporting Information *
ABSTRACT: Analysis of the electronic structure of PdO complexes is reported utilizing multiconfigurational self-consistent field (MCSCF) theory. Models include hard (N) and soft (Cl) donors to mimic proposed Pd oxo intermediates. Calculations argue against formulation of a PdIV oxo intermediate as posited in earlier experimental studies of PdII-mediated oxy insertion: i.e., a square-pyramidal complex with a basal oxo ligand. However, low-energy structures with other coordination geometries (trigonal bipyramidal) and isomerism (basal O) were identified. The supporting ligand plays a role in stabilization of the PdO bond as indicated by calculations on the PdOCl2 fragment and LPdOCl2 (L = model diimine ligand).
P
Pellarin et al. reported an interesting analysis of the challenges in isolation of PtO complexes, including thwarting H+ addition to yield Pt−OH species.12 The foregoing research indicates that for late transition metals in low to medium formal oxidation states an oxo ligand can display redox noninnocence, thus altering the reactivity of the resulting complexes.13 For example, an oxyl may facilitate the M(O)R → MOR transformation7 in comparison to an oxo, due to a less negative formal O charge in the former, making it more compatible with a nucleophilic alkyl (R) migrating group. Thus, experimental and computational data hint at group 10 oxos/oxyls as possessing the requisite balance of reactivity and kinetic/thermodynamic accessibility to participate in important catalysis such as partial hydrocarbon oxidation. Previous computational studies have focused on diatomic PdO−, PdO, and PdO+,14,15 and their descriptions of the O ligand lie closer to that of an oxo (O2−). To our knowledge, the bonding of PdO complexes similar to those studied by the Van Koten and Bandyopadhyay groups3,4 (Scheme 1) has not been explored. In this note, a computational study is reported on model Pd oxo/oxyl complexes to investigate their electronic structure in a more realistic ligand environment. Given the challenges involved in modeling late transition metal oxo complexes, we opted for small chemical models (Scheme 2) to make the utilization of more expensive, correlated wave
alladium oxo complexes have been proposed as key intermediates for several important transformations.1,2 For example, oxy insertion into Pd−carbon bonds to form Pd−OR moieties was proposed to go via PdIV oxo intermediates (Scheme 1).3,4 Oxy insertion is a key step in catalytic Scheme 1. Proposed PdIV Oxo Intermediates in Pd-Mediated Oxy Insertiona
a
Adapted, with permission, from refs 3 and 4. Copyright 1993 and 1999 American Chemical Society.
hydrocarbon oxidation, and little precedent exists for selective MR + YO → MOR + Y (YO = oxidant) transformations. Mechanisms of Pd-mediated oxy insertions were never fully elucidated, and proposed PdIV oxo intermediates were not isolated. Earlier proposals of Pd oxo entities5 were revisited in light of newer experimental data.6 A recent DFT study of the reaction of NiII alkyls with N2O producing NiII alkoxides suggested the intermediacy of a NiIII oxyl (i.e., O−), likely to avoid an unfavorable NiIV oxidation state.7 Computations by Pierpont et al. describe C−H bond activation mediated by nickel oxyls;8 recent experiments also implicate LnNiO ↔ LnNi−O• intermediates in oxidation.9 Milstein et al. reported the structure and bonding of a Pt oxo complex.10,11 Finally, © 2013 American Chemical Society
Received: April 12, 2013 Published: August 19, 2013 4993
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function techniques, viz. multiconfigurational self-consistent field (MCSCF)16 theory, feasible. Scheme 2. Model Pd Oxo Complexes in a Square-Pyramidal Geometry with Apical (Left) or Basal (Center) O Ligand or a Trigonal-Bipyramidal Geometry (Right) with an Oxo Ligand in the Equatorial Position
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Figure 1. CAS(4,4)-optimized low-energy singlet (left) and triplets (middel and right) PdOCl2.
space natural orbitals indicated correlation among σPdO, πPdO, π*PdO, and σ*PdO orbitals. C. Computations on the (NH(CH) 2 NH)Pd(O)(Cl) 2 Model. With the various complex components assessed, attention was turned to the full complex LPdOCl2. MCSCF calculations were performed using a CAS(8,8) active space for each isomer and spin state indicated to be reasonable by initial MP2 and DFT simulations. As discussed more fully in the Supporting Information, preliminary ORMAS calculations with larger active spaces indicated minimal correlation between L π/ π* and PdO-based frontier orbitals, and hence the CAS(8,8) active space was deemed suitable for geometry optimizations. Note that the combination of hard (N) and soft (Cl) donors mimics the proposed Pd oxo intermediates of the Van Koten and Bandyopadhyay groups3,4 (Scheme 1). Before the discussion of CAS(8,8) results, a brief survey of experimental Pd−ligand bond lengths is warranted.26 Constraining the search to high-quality (R < 5%) crystal structures of neutral complexes with neither identified errors nor crystallographic disorder yields typical Pd−Cl(terminal) and Pd−N(imine) bond lengths of 2.33(4) Å (n = 4454) and 2.05(5) Å (n = 1320) with sample (n) standard deviations indicated in parentheses. The CAS(8,8)-optimized Pd−Cl bond lengths are in reasonable accord with experiment, while computed Pd−N bond lengths seem long. The average experimental Pd−O bond length is 2.06(7) Å for 2357 samples. The shortest such distance reported is 1.86 Å for a square-planar, PdII bis(acac) derivative.27 Interestingly, the three other Pd−O bonds in this complex are 1.98−2.05 Å. Apart from that in ref 27, the next shortest reported Pd−O bond lengths are ca. 1.94−1.95 Å, primarily for phenolate and acac type ligands. The first complex studied (11, Figure 2) possesses a squarepyramidal geometry with the oxo in the apical position and a
COMPUTATIONAL METHODS
MCSCF and MP2 calculations were performed using GAMESS.17 Density functional simulations utilized Gaussian 09.18 Test calculations on models with B3LYP,19,20 M06,21 and MP222 methods using pseudopotentials and augmented valence basis sets did not yield appreciably different optimized structures. After evaluation at these levels of theory of the geometries, spin states, and energy Hessians (all were minima) of plausible linkage and coordination isomers, final candidates for geometry optimization at the 8-orbital, 8-electron complete active space self consistent field (CASSCF)/fully optimized reaction space (FORS) level of theory: CAS(8,8) CASSCF/FORS23 calculations were carried out using the occupation restricted multiple active spaces (ORMAS)24 code within GAMESS. Active space selection is delineated more fully in the Supporting Information. The Stuttgart effective core potential (28-electron core)25 and valence basis set was used for Pd in conjunction with the 6-31+G(d,p) allelectron basis set for the main-group elements. Hessians were calculated at all DFT-optimized geometries to characterize the stationary points as minima. All energetics are given in kcal mol−1 under standard conditions.
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RESULTS AND DISCUSSION A. Computations on the NH(CH)2NH Ligand. MCSCF calculations were performed on 1,4-diazabutadiene, a model of diimine ligands employed experimentally such as 2,2′bipyridine, using a CAS(4,4) active space. The 4 orbital active space corresponds to the two NCCN π orbitals and the two NCCN π* orbitals and the electrons contained therein. Also, ligand calculations focused on a cis arrangement, as this is the geometry needed to bind in a bidentate fashion to the palladium center. Full results of CAS(4,4) calculations on NH(CH)2NH are given in the Supporting Information. What are most salient for the following discussion are the computed natural orbital occupation numbers (NOONs) for the four π orbitals of L: π1 (1.92 e); π2 (1.89 e); π*3 (0.12 e); π*4 (0.07 e). B. Computations on the PdOCl2 Fragment. A DFT survey was conducted of spin states and molecular geometries for PdOCl2. These computations indicated two low-energy coordination geometries (T-shaped and trigonal planar) and spin states (singlet and triplet), details of which are in the Supporting Information. Utilization of the DFT optimized structures of PdOCl2 as starting guesses for CAS(4,4) geometry optimizations yielded three distinct stationary points (Figure 1). The singlet geometry changed little, retaining a Y shape. The final stationary point, computed to be the most stable, resulted from a triplet CAS(4,4) optimization (Figure 1, left). The PdO bond was broken, yielding linear PdCl2 and O(3P) (Figure 1). For both geometries with an intact Pd−O bond, the active
Figure 2. CAS(8,8)-optimized structure of 11 (left, Erel = 37.6 kcal/ mol) and 31 (right, Erel = 1.2 kcal/mol). Bond lengths are given in Å and bond angles in deg. The superscripts 1 and 3 denote the multiplicity.
singlet multiplicity. This coordination isomer most closely corresponds to proposed experimental intermediates (Scheme 1).3,4 Several points may be gleaned from CAS(8,8) computations. First, the relative energy of 11 is much higher in comparison to those of other Pd oxo isomers studied, Erel = 4994
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37.6 kcal/mol being above the lowest energy structure (vide infra) at the CAS(8,8) level of theory. Structure 11 has a calculated Pd−O bond length of 1.96 Å, with the oxygen slightly dislocated from an ideal apical location, likely resulting in poorer Pd(dπ)−O(pπ) overlap and an elongated Pd−O bond length. Thus, in terms of experimental exemplars (vide supra), a calculated Pd−O bond length of 1.96 Å seems to suggest a short Pd−O single bond, perhaps a reflection of a higher formal oxidation state (PdIII or PdIV) in 11 versus the great preponderance of PdII examples in the CCDC.26 A triplet square-pyramidal complex with oxo in the apical position (31) was also found. However, inspection of the geometry of 31 indicates that it more closely corresponds to a square-planar PdII complex with a dissociated O (3P) (Figure 2). The relative stability of this dissociated structure (Erel = 1.2 kcal/mol above the global minimum) clearly confirms the potent oxidizing ability of a Pd oxo complex if suitable supporting ligation could be identified to access such an entity. Taken together, however, the foregoing CAS(8,8)-derived observations argue against a formulation of a PdIV oxo intermediate as posited in earlier experimental studies3,4 of PdII-mediated oxy insertion. A triad of low-energy isomers of LPdOCl2 with intact Pd−O bonds were identified. The CAS(8,8)-optimized geometry of the lowest energy Pd oxo isomer (12) found is displayed in Figure 3. The Pd is square pyramidal, and O occupies a basal
Figure 4. CAS(8,8) natural orbitals for 12: isovalue = 0.045; NOON = 1.00 e for both. The superscript 1 denotes the multiplicity.
shell singlet. In this regard, 12 is reminiscent of copper nitrene intermediates discussed previously.8,9 Interestingly, while one of the natural orbitals is polarized toward the palladium, the other is polarized toward oxygen. This disposition is reminiscent of classic valence bond descriptions of O2 and RuO+.28 A final low-energy Pd oxo isomer was identified, 4.9 kcal/mol above the 12 ground state (Figure 5). This singlet (13) is quite
Figure 5. CAS(8,8)-optimized structure of 13. Erel = 4.9 kcal/mol. Bond lengths are given in Å and bond angles in deg. The superscript 1 denotes the multiplicity.
different from the other isomers computed and proposed.3,4 (Attempted optimizations of a corresponding triplet to 13 yielded 32.) Rather than square-pyramidal coordination around the palladium, 13 is closer to a trigonal-bipyramidal polytope. Also, 13 displays the shortest Pd−O bond length among the models studied, 1.86 Å. On the basis of the analysis of crystallographically determined Pd−O bond lengths discussed above, a bond length of 1.86 Å suggests significant multiplebond character. Analysis of the natural orbitals (Figure 6) also supports a multiple-bond description for 13. The natural orbitals most
Figure 3. CAS(8,8)-optimized structure of 12 (left, Erel = 0 kcal/mol) and 32 (right, Erel = 2.9 kcal/mol). Bond lengths are given in Å and bond angles in deg. The superscripts 1 and 3 denote the multiplicity.
coordination site. The computed Pd−O bond length is 2.09 Å, in line with the aforementioned experimental Pd−O average of 2.06 Å. Metric data thus indicate a Pd−O single bond with little evidence of multiple-bond character, so that one may hypothesize a bonding description for 12 more weighted toward a polarized Pd oxyl (Pdq+1O•−) than a Pd oxo (Pdq+2O2−), similar to deductions made by Pellarin et al.12 for platinum oxos. An oxyl description of 12 implies a related low-energy triplet state, and indeed such a structure was identified, 32 (Figure 3). The geometries of 12 and 32 are remarkably similar, the most interesting difference between them being a slightly longer (ca. 2.5%) Pd−O bond length in the former (2.09 Å) versus the latter (2.04 Å). Inspection of the natural orbitals suggests an origin for the structural similarity of 32 and 12 (Figure 3). Both are nearly identical in their active space orbital composition. Most interesting are two largely nonbonding orbitals with NOON values of ∼1.00 e. These orbitals are plotted in Figure 4 for 12; those for 32 are very similar. The orbitals in Figure 4 make it clear that 12 is biradical in nature and corresponds to an open-
Figure 6. CAS(8,8) natural orbitals for 13: isovalue = 0.045. NOON = 1.67 e for PdO π and 0.34 e for PdO π*. The superscript 1 denotes the multiplicity.
localized on the PdO moiety show obvious Pd(dπ)−O(pπ) character, bonding and antibonding. The NOON values are 1.67 e for the bonding component and 0.34 e for its antibonding complement, suggesting significant biradical character for 13, albeit less than for the open-shell singlet 12.
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SUMMARY AND CONCLUSIONS Analysis of the bonding of PdO complexes is reported utilizing MCSCF16 theory. Models include hard (N) and soft (Cl) donors to mimic proposed Pd oxo intermediates3,4 (Scheme 1). Calculations argue against formulation of a Pd IV oxo 4995
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intermediate as posited in earlier studies3,4 of PdII-mediated oxy insertion: i.e., a square-pyramidal complex with an apical oxo ligand. Such an isomer (11) was computed to be 37.6 kcal/mol above the lowest energy structure. The lowest energy PdO structure (12) is a singlet, square pyramid with oxo in a basal site; the long Pd−O bond (2.09 Å) of 12 suggests a single-bond description. In light of recent work by Pellarin et al.12 it is reasonable to assume that such a system would be easily protonated to yield PdOH complexes. However, an oxyl (O•−) structure (13) with a very short PdIII−O bond (1.86 Å) was isolated, lying only 4.9 kcal/mol above 12. The choice of oxidant is an important consideration in the hunt for such species. Kamaraj and Bandyopadhyay4 found oxidant-dependent selectivity, e.g., Pd−C oxidation with C6F5IO, and thioether oxidation with a ferryl porphyrin oxidant. They proposed that the oxidant nucleophilicity was key, which thus defines an area for future experimental and computational research in this area. Computations show little evidence of charge transfer between L π/π* and PdO orbitals. However, supporting ligands must play some role in stabilization of the Pd−O bond, as MCSCF calculations on isolated PdOCl2 indicate that the lowest energy “isomer” is PdIICl2 + O(3P). A PdII isomer (31) for LPdOCl2 was 1.2 kcal/mol higher than the global minimum 1 2. While this is a small difference, stabilization of bound PdO complexes is significant, as it implies that ligand sets, e.g., harder σ donors that favor higher oxidation states of Pd (III/IV vs II) or stronger π acids (to diminish excess d electron density at Pd), are of interest. The latter may be hard to implement, given the propensity of π acids such as CO, PR3 and NCR to be oxidized. A more extensively conjugated ligand or one that displays redox noninnocence may enhance PdO to L π/π* charge transfer and help stabilize PdIV oxo/PdIII oxyl intermediates. Powers et al.29 and Deprez and Sanford30 explored the role of high-valent Pd intermediates in the oxidation of aromatic C−H bonds. Interestingly, N∼C chelates such as benzo[h]quinolone and 2-phenylpyridine figure prominently in both studies. Similar ligand design strategies may also identify high-valent, multiply bonded Pd imide/imidyl complexes.31−33 Regardless, such complexes need to retain sufficient activity due to their proximity to the Oxo Wall34 to achieve the two tough chemical tasks inherent in methane functionalization: activating strong C−H bonds and encouraging the reluctant methyl group to migrate from a metal to oxygen (or nitrogen) ligand in order to complete a functionalization cycle.
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ACKNOWLEDGMENTS This work was supported as part of the Center for Catalytic Hydrocarbon Functionalization, a U.S. Department of Energy Frontier Research Center (Grant DE-SC0001298).
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
Text, figures, and tables giving theoretical details, optimized coordinates, and full citations for refs 5, 6 and 18. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail for T.R.C.:
[email protected]. Notes
The authors declare no competing financial interest. 4996
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