Palladium Acetate Revisited: Unusual Ring-Current Effects, One

Communication Cite This: Inorg. Chem. 2018, 57, 8046−8049

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Palladium Acetate Revisited: Unusual Ring-Current Effects, OneElectron Reduction, and Metal−Metal Bonding Ryan J. Pakula,† Monika Srebro-Hooper,‡ Charles G. Fry,† Hans J. Reich,† Jochen Autschbach,*,§ and John F. Berry*,† †

Department of Chemistry, University of WisconsinMadison, 1101 University Avenue, Madison, Wisconsin 53706, United States Faculty of Chemistry, Jagiellonian University, Gronostajowa 2, 30-387 Krakow, Poland § Department of Chemistry, University at Buffalo, State University of New York, Buffalo, New York 14260, United States Downloaded via DURHAM UNIV on July 19, 2018 at 19:40:12 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

To enable more complete solution characterization of the Pd3(O2CR)6 structure, we prepared chelate-stabilized analogues of 1 using the dicarboxylate ligand H2esp (H2esp = α,α,α′,α′tetramethyl-1,3-benzenedipropionic acid; Figure 1). H2esp has advanced Rh2 catalysis by stabilizing the Rh2(O2CR)4 structure to highly oxidizing conditions.4 Similar esp paddlewheel structures of Ru2,5 Re2,6 Cu2,7 and BiRh have been reported,8 as well as a palladium(III) half-paddlewheel complex.9 A 1:3 mixture of Pd3(OAc)6 and H2esp in CHCl3 reacts producing >95% yield of an orange powder with an elemental analysis consistent with the formula Pd3(esp)3 (Figure 1). Crystallization from CH2Cl2 and hexanes yields red conical crystals and orange plates, which contain two isomers of Pd3(esp)3. The red crystals contain a C3h-symmetric isomer of Pd3(esp)3 (C3h-2; Figure 2a). The molecule lies on the

ABSTRACT: Palladium(II) acetate (1) and two new complexes of the ligand α,α,α′,α′-tetramethyl-1,3-benzenedipropionate (esp2−), Cs-Pd3(esp)3 (Cs-2) and C3hPd3(esp)3 (C3h-2), are studied in the solid state and in solution. Variable-temperature NMR and DFT studies of Cs-2 reveal an unusual shielding region above the Pd atoms. The compounds show a surprising quasi-reversible reduction between −880 and −1200 mV versus Fc/Fc+, and the Pd3(esp)3 complexes may be cleanly reduced electrochemically. EPR spectra of reduced samples show pseudo-axial signals with 105Pd hyperfine coupling, consistent with unprecedented, isostructural Pd35+ species with a valence-trapped PdII−PdII−PdI electronic structure.

P

alladium(II) acetate (1) is used industrially on a multiton scale as a precatalyst for organic transformations.1 In the solid state, 1 and other PdII carboxylates have a triangular Pd3 core supported by six peripheral carboxylate ligands (Figure 1).

Figure 2. Thermal ellipsoid plots with ellipsoids drawn at the 50% probability level of (a) C3h-2 and (b) Cs-2. Most H atoms are omitted.

intersection of a site of crystallographic C3h symmetry. Each esp ligand has an equivalent chemical environment in which the phenylene groups lie on the periphery of the Pd3 triangular edges and are bisected by the Pd3 plane. The orange plates contain a Cs-symmetric isomer of Pd3(esp)3 (Cs-2; Figure 2b). In this structure, one esp ligand (α) is oriented as in C3h-2, but the other two esp ligands (β) are positioned such that the phenylene groups lie either above or below the Pd3 plane. Table S1 contains selected bond lengths comparing Pd3(OAc)6 and both isomers of Pd3(esp)3. The four ligand environments seen in the 1H NMR spectrum of crude Pd3(esp)3 are explained by the presence of both isomers in solution. The 1H NMR spectrum of C3h-2 (Figure S1b)

Figure 1. Structures of Pd3(OAc)6, the two isomers of Pd3(esp)3, and the esp2− ligand. Synthesis of Cs-2 and C3h-2.

The labile carboxylates facilitate the use of 1 in synthesis but hinder a full understanding of its solution properties.2 Furthermore, although reduction of 1 is widely used for the preparation of Pd nanoparticles,3 the electrochemical properties of PdII carboxylates have never been reported. Also, the nature of Pd−Pd bonding within the compound and the electronic consequences of having a triangle of three PdII atoms in such close proximity have not been analyzed. In this work, we note an unusual chemical shift effect around the Pd3 core and report that one-electron reduction of Pd3 complexes generates rare palladium(I) species that contain stronger Pd−Pd bonds than are present in 1. © 2018 American Chemical Society

Received: May 21, 2018 Published: July 3, 2018 8046

DOI: 10.1021/acs.inorgchem.8b01369 Inorg. Chem. 2018, 57, 8046−8049

Communication

Inorganic Chemistry reflects the high symmetry, with only one set of signals for the one ligand environment. The 1H NMR spectrum of Cs-2 (Figure S1c) possesses three methyl resonances: one from the α ligand and two from the β ligands, which have diastereotopic methyl and methylene protons, with one set pointing away from the Pd3 unit and the other set pointing toward it. The unusual Hb resonance at 5.96 ppm in the 1H NMR spectrum of Cs-2 (Figure 2b, Figure S1c) prompted further inquiry because it is highly shielded compared with typical aryl resonances. For comparison, Ha resonates at 6.89 ppm, 0.93 ppm higher than Hb,10 and free H2esp resonates at 6.99 ppm. Such strong shielding typically requires multiple electrondonating groups on the arene ring, such as for the H atom at the 2 position of 1,3-phenylenediamine (5.94 ppm)11 and the aryl H atom in 1,3,5-triaminobenzene (5.52 ppm).12 Thus, we hypothesized that the shielding of Hb may be due to its proximity to the Pd36+ core. However, the observed δ value of Hb is due to the interconversion shown in Figure 3.

signal intensity decreases because of the exchange shown in Figure 3. These cw saturation-transfer NMR experiments were carried out at three temperatures, and υOUT was located by a drop in the signal of IN at 7.05 ppm, within the normal range for an aryl resonance (Figure 4b). Its apparent absence is consistent with its calculated population of only 19% at room temperature (even lower at −94 °C) and its exchange broadening. The electronic reason for the highly shielded resonance of Hb,in prompts theoretical study. Major contributing factors to the δ values of Ha and Hb,in (Table S3) were identified from natural bond orbital (NBO) analysis.14 The results for (1) the Pd3 complex, (2) models with Pd atoms replaced by Mg or point charges, and (3) unbound ligands in their “constrained” configurations (i.e., as in the complex) all reproduce the observed ca. 1 ppm Δδ between Ha and Hb,in (see the SI for details). When the constrained ligands are compared to an optimized (unconstrained) structure, Δδ is due to the influence of bond and lone-pair orbitals of the proximal carboxylate groups, likely via through-space-induced ring-current effects, along with changes in the shielding contributions from the C−H σ-bonding orbital. While all models reflect Δδ rather well, only the Pd3 model accurately reproduces the absolute experimental chemical shifts, an effect that NBO analysis attributes to covalency in the Pd−O bonds. Although the Pd orbitals clearly participate in the strongly delocalized electronic structure of the complex and therefore also contribute directly to the Ha and Hb,in shielding, the main influence of the metal atoms on Δδ is to stabilize the crowded arrangement of the carboxylate ligands. The absence of electrochemical experiments on 1 may be partially due to the high lability of Pd2+, with exogenous ligands and even trace water leading to fragmentation of the complex.2 Thus, we performed cyclic voltammetry on Pd3(OAc)6 and both Pd3(esp)3 isomers under stringently water- and oxygen-free conditions. The three Pd3(O2CR)6 structures show irreversible oxidative features around Ea = +1400 mV versus Fc/Fc+ (Figure S3). Surprisingly, quasi-reversible reductive waves are found at E1/2 = −880 mV for 1, −1120 mV for Cs-2, and −1200 mV for C3h-2 (Figure 5). The lower E1/2 values of the esp2− complexes

Figure 3. Representation of the two extreme conformers of the β esp ligands in Cs-Pd3(esp)3, highlighting the position of Hb in each. For clarity, only one of the two β ligands is shown.

Upon cooling of a CD2Cl2 solution of Cs-2, the signal of Hb passes through coalescence at Tc ≈ −51 °C (Figure 4a). At T ≫

Figure 4. (a) Variable-temperature 1H NMR spectra (in CD2Cl2) of Cs2. (b) Plot of the integration of Hb,in as the irradiated frequency was swept from 6.25 to 8.00 ppm in 0.05 ppm increments. Integrations are given as a percentage of the same peak’s integration using a standard 1H pulse sequence.

Tc (e.g., +24 °C), fast exchange gives an averaged signal at avg υIN,OUT = PINυIN + POUTυOUT, where IN is the major conformation with fractional population PIN and resonance frequency υIN and OUT is the minor conformation with POUT and υOUT.13 Well below Tc (e.g., −93 °C), slow exchange should give rise to signals from the two conformations at υIN and υOUT, one avg upfield of υIN,OUT and one downfield. While the upfield resonance at υIN sharpens as expected, reaching 5.67 ppm at −93 °C, no downfield resonance was directly detectable. This observation implies that the exchange is in the slow-intermediate (si) regime,13 where the low population and exchange broadening of Hb,out adversely affect its line width because k si . ΔυOUT,1/2 = OUT π The OUT conformation can be indirectly detected by saturating its resonance while it undergoes exchange with the IN conformer. When a low-power continuous-wave (cw) radiofrequency pulse is applied at υOUT, Hb,out saturates and the Hb,in

Figure 5. Cyclic voltammograms at 100 mV/s in the reductive direction of the three Pd3 species in 0.1 M NBu4PF6 in THF (1 mM analyte).

versus 1 are consistent with the greater electron-donating ability of the esp2− ligand. Quasi-reversible behavior is present at scan rates ranging from 20 to 1000 mV/s and implies that the oneelectron-reduced species are at least transiently stable, although the large peak-to-peak separation suggests slow electron-transfer kinetics due to a significant geometric distortion in the reduced complexes. Chemical reductions do not yield a stable species, but bulk electrolysis monitored by electronic absorption spectroscopy yields cleaner results. While a potential of −1300 mV versus an Ag pseudoreference is maintained, a clean isosbestic point is 8047

DOI: 10.1021/acs.inorgchem.8b01369 Inorg. Chem. 2018, 57, 8046−8049

Communication

Inorganic Chemistry observed by UV−vis when 1 equiv of electrons is added to Cs-2 in tetrahydrofuran (THF; Figure S4b). The isosbestic behavior suggests the formation of an isostructural, monoanionic species. An isosbestic point is also present upon the reduction of C3h-2 (Figure S4a), but 1 has more complex behavior. In THF, the reduction of 1 forms Pd nanoparticles, as evidenced by a broadband rise in absorption as well as the formation of a sootlike solid (Figure S4c). In CH2Cl2, the reduction of 1 still does not show isosbestic behavior (Figure S4d), but no solid forms. During the reductions, aliquots were analyzed by EPR spectroscopy to learn about the nature of the reduced species. The reduced Pd3(esp)3 samples show roughly axial EPR signals with g values of ∼2.6, 2.1, and 2.095 (Figures 6 and S5 and Table

orbital (SOMO; inset, Figure 6), and the longer Pd−O distances for this Pd atom (Table S1). This result is consistent with our EPR data, which show hyperfine coupling from one 105Pd nucleus (I = 5/2). Upon reduction, the PdI center in each case is pulled closer to the two PdII centers, and the PdII···PdII distances increase. Consistently, the PdI···PdII BOs increase to ∼0.21, compared with ∼0.14 for the PdII···PdII interactions in the neutral species (PdII···PdII BOs in the anions are 3 Å, but the Pd atoms pucker inward toward each other, suggesting the presence of weak d8···d8 interactions, similar to those seen in PdII dimers.17 Mayer bond orders (BOs) for Pd···Pd interactions are in the range 0.12−0.19, consistent with this hypothesis. The Pd 4d orbitals combine in these complexes to form metal−metal bonding and antibonding combinations of all sets of d orbitals (z2, x2 − y2, xy, xz, and yz; Figure S6), and the dz2 combinations overlap in the center of the Pd3 triangle. Mixing of the Pd 5s and 5p orbitals with the 4d orbitals stabilizes the antibonding combinations of Pd dz2 orbitals (approximately 5% s, 1% p, and 94% d character for the E′-derived highest occupied molecular orbitals in Figure S6), leading to a net increase in bonding. The degree of Pd···Pd bonding is found to increase upon reduction. The unpaired electron in the [Pd3(O2CR)6]− systems is localized on one Pd atom, as evidenced by its Mulliken spin population of 0.84, the nature of the singly occupied molecular

CCDC 1556707−1556710 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: jochena@buffalo.edu (J.A.). *E-mail: [email protected] (J.F.B.). ORCID

Monika Srebro-Hooper: 0000-0003-4211-325X Charles G. Fry: 0000-0002-9049-0781 Jochen Autschbach: 0000-0001-9392-877X John F. Berry: 0000-0002-6805-0640 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the U.S. Department of Energy for support through Grant DE-SC0016442. The NMR facilities were supported by the National Science Foundation (NSF; Grant CHE-1048642) and a bequest from P. J. and M. M. Bender. Mass spectrometry facilities were funded by the National Institutes ofHealth (NIH; Grant 1S10OD020022-1). J.A. acknowledges NSF Grant CHE1560881 for supporting the computational NMR analysis.M.S.H. acknowledges an “Outstanding Young Scientist” scholarship from the Ministry of Science and Higher Education in Poland.

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REFERENCES

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