Stability of Dinuclear Phosphane Palladium(III) Complexes: A DFT

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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Stability of Dinuclear Phosphane Palladium(III) Complexes: A DFT Approach Francisco Estevan,† Pipsa Hirva,*,‡ Mercedes Sanaú,† and MaAngeles Ú beda*,† †

Departament de Química Inorgànica, Universitat de València, Dr. Moliner 50, 46100 Burjassot, Valencia, Spain Department of Chemistry, University of Eastern Finland, Joensuu Campus, P.O. Box 111, FI-80101 Joensuu, Finland

‡

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S Supporting Information *

ABSTRACT: Computational density functional theory studies have been carried out for the dinuclear ortho-metalated palladium(III) compounds [Pd2{μ-(C6H4)PPh2}2{μ-(X1-X2)}2Cl2]. These studies have shown that the electronic and steric properties of the auxiliary ligands (X1-X2 = bridging (carboxylato) or chelating (phenolato/acetylacetonato) O,O-donor ligands, bridging N,N-donor ligands (triazenido/formamidinato/pyrazolato), and bridging N,S-donor ligands) lead to systematic trends in their stability, highlighting that (a) the electronic nature of the donor atoms trans to the P has a clear trend, the replacement of hard donor atoms (O, N) by softer S donors generally reducing the stability of the compounds, (b) the geometrically flexible ligands with bulky substituents partially blocking the axial sites (formamidinato and triazenido ones) diminish the stability, except in cases where additional intramolecular interactions provide extra stabilization, and (c) the axial Cl−Pd···Pd−Cl interactions play a major role in the stability of the studied Pd(III) complexes. The presence of a Pd···Pd bond in these compounds was verified by analyzing the UV−vis spectra simulated via TDDFT calculations. As supported by DFT calculations, palladium(III) intermediates have been suggested in the catalytic 2-phenylation of indoles with (Ph2I)PF6. Detailed analysis of the Pd···Pd, Pd−X, and axial Pd− Cl and Pd−C(Ph) interactions was executed by calculating the properties of the electron density according to QTAIM methods in order to reveal the factors affecting the overall stability of the compounds.

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INTRODUCTION Even though in the last 10 years the chemistry of palladium in oxidation state III has undergone some major advances, the topic continues to be not well-known.1 With regard to the chemistry of the dinuclear palladium(III) compounds, only a few of them have been well characterized since Cotton and coworkers reported in 1998 the synthesis in low yield of the first dinuclear paddle-wheel palladium(III) compound, Pd 2 (hpp) 4 Cl 2 (hpp = hexahydro-2H-pyrimido[1,2-α]pyridinate).2 Ritter and co-workers have characterized not only discrete dinuclear3 and 1D wire4 palladium(III) complexes with metal−metal bonds but also a transition state analogue for the oxidation of a palladium(II) compound to a palladium(III) compound.5 Furthermore, Mirica and coworkers6a and Bauer and co-workers6b reported new dinuclear compounds with PdIII−X−PdIII bridges in their structures. In the last few years our research has focused on the chemistry of this type of palladium(III) compound with a Pd26+ core, being the first group that reported the synthesis in high yield of these complexes.7a As a result of this research, 16 ortho-metalated palladium(III) compounds with bridging (carboxylato) or chelating (phenolato/acetylacetonato) O,Odonor ligands,7a,b,d,g,8,9 bridging N,N-donor ligands (triazenido/formamidinato/pyrazolato),7c,f,h and bridging N,S-donor ligands7e have been synthesized and characterized (Figure 1). We present here new DFT calculations that have allowed us to compare systematically the geometrical and electronic © XXXX American Chemical Society

Figure 1. Ortho-metalated palladium(III) compounds.

properties of these compounds. In particular, this study has been focused on the nature of the bridging/chelating ligands X1-X2 and axial ligands Y (Cl, Ph) and their effect on the stability of the synthesized Pd(III) compounds and their Pd(II) precursors, by calculating the properties of the electron density in selected representatives from each group of compounds by utilizing Bader’s quantum theory of atoms in molecules (QTAIM) approach. 10 Furthermore, in the computational studies, we added related complexes which have not been synthesized, to follow a systematic approach. Additionally, UV−vis spectra were simulated to verify the existence of the Pd···Pd bonding interaction in the Pd(III) complexes. The computational properties have been correlated with the experimental properties in order to find the relative trends in the dinuclear compounds upon modification of the ligand core. DFT calculations have also been carried out in the Received: May 22, 2018

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DOI: 10.1021/acs.organomet.8b00342 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

donor atoms will affect not only the coordination to metal atoms but also the electronic properties of the whole complexes. Dinuclear Palladium(II) and -(III) Compounds. General Trends in Geometry. The isolated and characterized palladium(III) compounds as well as the palladium(II) precursors studied are collected in Figure 2 and Figures S1− S10. The palladium(II) compounds 1 and 4−7 showed a paddlewheel structure with the dinuclear unit bonded by four bridging ligands, while for 2 and 3 with chelating ligands, the dinuclear unit is only supported by two bridging ligands. All of them show two points of chirality,11,12 being in general, compounds with stereochemistry SM/RP (Tables S1 and S2). Palladium(III) compounds 8, 10, 13, and 14 retained the same ligand arrangement and stereochemistry as their precursors.7 Even though the cores of the Pd(II) and Pd(III) complexes showed similar overall geometries (Figure 2), the nature of the auxiliary ligands had a rather large effect on the Pd···Pd and Pd−X distances. Figure 3 compares the computational and experimental Pd···Pd distances for the Pd(II) precursors as well as the corresponding Pd(III) complexes. The experimental Pd···Pd distances (from the available X-ray structures)7 are given in Table S3, and other geometrical parameters are given in Tables S4−S15. The computational values along with the optimized structures for the Pd(II) precursors are presented in Figures S11−S15. The overall geometries of the Pd(III) compounds follow the structures of the precursors with the chlorido ligands attached to the axial sites. For both palladium(II) and palladium(III) compounds, the Pd···Pd distances are shorter than the sum of the van der Waals palladium radii (3.26 Å). It is noteworthy that the available experimental data agree very well with the computational values and show the same trends. Another notable trend can be seen in the slightly larger Pd···Pd distance for complexes 5a,d in comparison to the related complexes 4. The trend can be explained by the different electronic nature of the formamidinato and triazenido ligands, which has also been noted in the experimental electrochemical studies of the Pd(II) compounds. These studies highlighted that the potential values are higher for triazenide derivatives than for the equivalent formamidinate species, supporting the fact that, even though both ligands are isoelectronic, the triazenidos are weaker donor ligands.7f The weaker donor capacity of the triazenido ligands was also obtained in the computational AIM charges of the free ligands (Table 1) and, as will be seen later, in the amount of donated charge density in the complexes (Figure 4). In addition, the compounds with chelating ligands with the dinuclear metal unit supported by only two bridging ortho-metalated phosphanes (2 and 3) stand out with their notably longer Pd···Pd distances (2.9762−2.9899 Å, 2; 2.9730−3.0940 Å, 3). Compounds 2 and 3 also exhibit the largest reduction in the distance upon oxidation to the corresponding Pd(III) complexes, in agreement with the experimental findings. Pd···Pd distances in palladium(III) compounds are shorter than those of their palladium(II) counterparts, being the shortest for the pyrazolate derivatives. The experimental value of 2.5053 Å in 13a is not only the shortest among these compounds but also among all the dinuclear palladium(III) described in the literature.1−5,7 Compound 10b, with only two bridging ligands supporting the dinuclear unit, has the longest distance. Even though there are only limited data on the

search for a possible dinuclear palladium(III) intermediate as the first step in the 2-phenylation of indoles with (Ph2I)PF6.

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RESULTS AND DISCUSSION X1-X2 Ligands. Electronic characteristics of the X1-X2 ligands (Figure 1) were compared by calculating the total charges of the donor atoms by the QTAIM method. Table 1 Table 1. Anionic Ligands and Total Atomic Charges (q) of the Donor Atoms (X1, X2) Calculated with the QTAIM Method

gives the charges of the X1 and X2 atoms in a selected set of free ligands. For clarity, the charge values are given only to ligands with R = CH3. As could be expected, the charge of the free ligands follows the general trend q(O) > q(N) > q(S), but the surrounding ligands/substituents can have a major effect: for example, when the charge of the donor nitrogens in triazenidos (q(N) = −0.58) is compared with those in structurally similar formamidinatos (q(N) = −1.25), which makes nitrogens in formamidinatos much “harder”. It can be suggested that the difference in the electronic nature of the B


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Figure 2. Palladium(II) and (III) compounds. The asterisks indicate compounds which were not synthesized but were included in the computational study.

C


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experimental crystal structures of Pd(III) compounds, the general trends in the metal−metal distance follow well the computationally obtained values. Characterization by NMR Spectroscopy. The experimental 31P{1H} NMR spectra of the palladium(II) compounds showed a single signal, and their δ (ppm) values as well as the corresponding 13C chemical shifts of the metalated carbon, are given in Table 2. It was observed that while 13C chemical shift values remain practically constant, the 31P{1H} values showed a wide dispersion (δ 37.7−0.3 ppm). Since the phosphorus atom has d orbitals with capacity to accept and retrodonate electron density, the electron density in P depends on the donor properties of the ligand atom trans to P, which suggests that the electron distribution of the ligands can explain the obtained trends in the 31P{1H} chemical shifts. Therefore, we compared the AIM charges of the atom trans to phosphorus (X1) with the experimental δ (31P{1H}). The results are depicted in Figure S16. For the complexes with ligands containing “harder” X1 atoms, O or N, the correlation between q(X1) and δ(31P{1H}) is rather good. This applies even for the formamidinate compounds 5, where, as discussed earlier, the nitrogen is even harder than oxygen in the O,O complexes (even in carboxylate complexes, although the free carboxylate exhibits clearly harder oxygen atoms; Table 1). However, the chemical shift values for the complexes with X1 = S do not follow the same absolute trend. Obviously, the computational charges are much smaller for less electronegative sulfur atoms in complexes 1f,g* and 7, but the chemical shifts show values similar to those of the complexes with hard oxygen X1 atoms. On the other hand, it is possible that the donor ability of sulfur is nevertheless fairly good because of its larger amount of electrons. We calculated the amount of donated electron density upon complexation by comparing the donor atom charge in free ligands to that of the complexes. There is a much better correlation between the amount of donated electron density of the atom trans to P and the experimental phosphorus chemical shift values (Figure 4).

Figure 3. Pd···Pd distances (Å) in the selected DFT optimized Pd(II) and Pd(III) compounds with available experimental data. The asterisks indicate compounds which were not synthesized but were included in the computational study.

Figure 4. Correlation between the amount of charge donated by the atom trans to phosphorus with the 31P{1H} chemical shift values for selected Pd(II) complexes. Δq(X1) is the difference between the atomic charge of X1 in free ligands and in complexes. The asterisks indicate the compounds which were not synthesized but were included in the computational study.

Table 2.

31

P{1H} and 13C NMR Spectroscopy Data at 298 K for Compounds 1−7 31

compound 1a 1b 1c 1d 1e 1f

P{1H} NMR δ (ppm) 19.3 18.8 18.7 20.3 20.2 13.8

C NMR δ (ppm) metalated C

13

31

compound

P{1H} NMR δ (ppm)


13

31

compound

Palladium(II) Compounds with O,O-Donor Ligands7a,b,d,g 2a 27.2 161.7 (m) 2b 26.5 161.8 (m) 2c 26.2 160.5 (m)

160.2 160.8 165.4 156.8 158.2 158.9

4a o-4b o-4c o-4d p-4b p-4c p-4d

3.3 0.7 2.5 −0.3 3.8 3.4 3.4

162.8 165.4 166.0 165.7 161.6 163.2 163.3

(t) (t) (t) (t) (t) (t) (t)

7a 7b 7c

14.5 12.6 13.5

164.0 (m) 166.1 (m) 165.6 (m)

Palladium(II) Compounds with N,N-Donor Ligands7c,f,h 5a 16.2 165.5 (m) 5b 15.8 166.0 (m) 5c 17.0 166.3 (m)

Palladium(II) Compounds with N,S-Donor Ligands7e 7d 20.3 162.9 (m) 7e 19.3 165.3 (m) 7f 19.2 165.3 (m) D

P{1H} NMR δ (ppm)


13

3a 3b 3c

26.8 26.7 22.3

163.7 (m) 158.9 (m) 162.3 (m)

6a 6b 6c 6d 6e

37.4 37.7 35.7 34.4 35.4

164.3 162.6 164.9 167.5 165.8

7g 7h

16.2 18.3

163.5 (m) 161.6 (m)

(d) (d) (d) (d) (d)


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Formation of a Pd···Pd Bonding Interaction. The formation of the Pd···Pd bonding interaction can be expected to change the absorption characteristics of the compounds upon oxidation, which would be seen in the lowest energy excitation signals of the UV−vis spectrum. Therefore, we analyzed the simulated absorption spectra obtained via TDDFT methods for both the Pd(II) and Pd(III) complexes. The compositions of the lowest energy excitations for Pd(II) and Pd(III) compounds are given in Tables 4 and 5, respectively.

Clearly, the chemical shift value follows the general trend in the donating ability of X1; a larger electron donation leads to a shift in the δ value to higher field. According to calculations, sulfur donates a larger amount of its charge, which is why the O,S complexes 1f,g* and N,S complexes 7 exhibit δ(31P{1H}) values at higher field in comparison to other Pd(II) compounds. The largest deviation from this trend is for triazenide compounds 4, which show their δ(31P{1H}) values at much higher field than expected, indicating that the electron donation from the triazenido ligand nitrogens is notably underestimated by the computational method. In the case of the 13C NMR spectroscopy the δ values for the metalated C for all the compounds are very similar, which indicates only a small trans effect. This was verified by calculating the amount of charge transfer from the ligand atom trans to the metalated carbon (X2). Even though there is a similar trend in the calculated charges in comparison to the case of X1, the amount of transfer is much smaller (Figure S17), which is in agreement with the suggestion that carbon without d orbitals should maintain the electron density. For palladium(III) compounds 8−14 the 31P{1H} NMR spectra also showed a single signal, shifted toward higher fields between 18 and 34 ppm in relation to that observed for the precursors. Carboxylate (thiocarboxylate), triazenide, and formamidinate derivatives showed the highest shifts of the signal (∼30 ppm), while the pyrazolate derivatives shows the smallest shifts (∼20 ppm). Their δ (ppm) values are given in Table 3.

Table 4. Interpretation of the Lowest Energy Absorption Signals According to TD-DFT Calculations for Selected Pd(II) Compounds

Table 3. 31P{1H} NMR Spectroscopy Data at 223 K for Compounds 8, 10−14 31

compound

P{1H} NMR δ (ppm)

31

compound

P{1H} NMR δ (ppm)

31

compound

P{1H} NMR δ (ppm)

Palladium(III) Compounds with O,O(S)-Donor Ligands 8a(Cl) −13.8 8c −8.8 10b 4.6 8a(Br) −15.0 8d −13.0 8b −13.0 8f −16.9 Palladium(III) Compounds with N,N-Donor Ligands 11a −35.6 12a −16.3 13a 17.3 p-11b −34.7 12b −16.4 13b 18.2 Palladium(III) Compounds with N,S-Donor Ligands 14f −5.1 14g −13.1 14h −9.0

compound

λmax (nm)

f

transition

1a 1c 1f 1g* 2a

480 481 463 459 428;402

2d*

448; 404

3a 4a 4e* 5a 5d* 6b 6c 6d

403 481 450 468 425 417 412 416

0.034 0.030 0.012 0.016 0.030; 0.043 0.014; 0.072 0.074 0.023 0.026 0.016 0.030 0.023 0.022 0.030

6f* 7b 7c

416 473 476

0.019 0.006 0.007

7e

432

0.017

7d

436

0.013

HOMO → LUMO (93%) HOMO → LUMO (94%) HOMO → LUMO+2 (94%) HOMO → LUMO (94%) HOMO → LUMO (91%); HOMO → LUMO+2 (76%) HOMO → LUMO (97%); HOMO → LUMO+2 (77%) HOMO → LUMO (87%) HOMO → LUMO (73%) HOMO → LUMO (89%) HOMO → LUMO (86%) HOMO → LUMO (81%) HOMO → LUMO+1 (85%) HOMO → LUMO+1 (80%) HOMO → LUMO+1 (68%) + HOMO → LUMO+2 (23%) HOMO → LUMO+1 (83%) HOMO → LUMO+1 (94%) HOMO → LUMO+1 (74%) + HOMO → LUMO+2 (22%) HOMO → LUMO+1 (25%) + HOMO → LUMO+2 (67%) HOMO → LUMO (91%)

Table 5. Interpretation of the Lowest Energy Absorption Signals According to TD-DFT Calculations for Selected Pd(III) Compounds

The coordination of chlorido ligands to the palladium in the palladium(III) compounds increases the electronic density in the metal; therefore, the signal in 31P{1H} NMR spectroscopy moves to higher fields. Otherwise the correlation between the amount of donated charge of the atom trans to phosphorus, Δq(X1), and the experimental δ(31P{1H}) values remains the same; as can be seen from Figure S18, again the largest donation comes from ligands containing sulfur atoms trans to P. Stability of the Palladium(III) Compounds. The Pd···Pd distances in all of the compounds are shorter than the sum of the van der Waals palladium radii, indicating an intermetallic interaction that is stronger for the diamagnetic palladium(III) compounds. We investigated the effect of the stronger Pd···Pd interaction on the stability of the Pd(III) compounds by analyzing the differences in the absorption characteristics, nature of the interactions, and stabilization energies.

compound

λmax (nm)

f

transition

8a(Cl) 8g* 9a* 10a* 11e 12d* 13c* 14b

534 610 943 770 581 989 561 1033

0.091 0.049 0.023 0.034 0.013 0.010 0.002 0.012

HOMO → LUMO (53%) HOMO → LUMO (82%) HOMO-1 → LUMO (97%) HOMO-1 → LUMO (89%) HOMO-2 → LUMO (98%) HOMO-1 → LUMO (98%) HOMO-2 → LUMO (84%) HOMO → LUMO (98%)

An example of the appearance of the molecular orbitals involved in the excitations is presented in Figure 5. The complete presentation of the excitations is given in Figures S19−S23 in the Supporting Information. The lowest energy excitations were found to originate from transitions between the HOMO (or HOMO-1) and LUMO (or LUMO+1). In the Pd(II) compounds, the HOMO consisted mainly of an antibonding combination of palladium d orbitals (70−85%) and the LUMO expanded over the ligand π* system (phosphanes and X1-X2), with a large contribution from the palladium d orbitals (40−60%). This signal was also E


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Table 6. Properties of the Electron Density at the Pd···Pd Bond Critical Point According to the QTAIM Method for the Selected Pd(II) Compoundsa

Figure 5. Change in the nature of the frontier molecular orbitals involved in the lowest energy excitations upon oxidation of Pd(II) compounds (1a) to corresponding Pd(III) compounds (8a(Cl)).

clearly visible in the experimental spectra and indicated substantial interaction between the two metal atoms even in the Pd(II) complexes. Furthermore, the signal shifted to higher wavelengths in the Pd(III) complexes, which verified the shortening and strengthening of the Pd···Pd interaction. The nature of the HOMOs changed to a bonding combination of the palladium d orbitals with a notable contribution from the axial chlorides, and the LUMOs were formed out of antibonding Pd(d) and axial Cl(p) orbitals. The change in the nature of the FMOs is clearly seen in Figure 5. Nature of the Pd···Pd Interactions. The nature of the Pd··· Pd interaction was further studied via calculating the properties of the electron density at the Pd···Pd bond critical point by the QTAIM method. Table 6 gives the electron density ρ, the ratio of potential energy density and kinetic energy density |V|/G, and the interaction energy EINT for the palladium(II) complexes. The nature of the Pd···Pd interaction is similar in all groups of complexes. Slightly smaller values of electron density are found at the Pd···Pd BCP for the paddle-wheel compounds containing sulfur (1f,g* and 7). On the other hand, the strongest interactions are found for complexes which include more nitrogen: namely, triazenide complexes. Chelating complexes 2 and 3 exhibit notably smaller interaction energies, which is in agreement with their longer interaction distances. In all cases, the Pd···Pd interaction is partially covalent, as can be judged from the value of |V|/G, which falls between 1 and 2. In palladium(III) compounds, the effect of the axial chlorido ligands was studied by analyzing the nature of the Pd···Pd and Pd−Cl interactions via the QTAIM method (Table 7). In comparison to the corresponding Pd(II) complexes (Table 6), the strong axial interaction of the chlorido ligands indeed increases the electron density around the palladium metal atoms, which explains the shift to higher field in the 31 1 P{ H} NMR spectra. Furthermore, the Pd···Pd interaction strengthens and exhibits more covalent nature than in compounds 1−7. Otherwise the trends remain the same as with the Pd(II) complexes: the chelating complexes 9a* (phenolato ligands) and 12a* (acetylacetonato ligands) have slightly weaker Pd···Pd interactions than the other Pd(III) compounds. Furthermore, the O,S compound 8g* exhibits a

compound

ρ (e Å−3)

|V|/G

EINT (kJ mol−1)

1a 1c 1f 1g* 2a 2d* 3a 4a 4e* 5a 5d* 6b 6c 6d 6f* 7b 7c 7e 7f

0.300 0.300 0.277 0.269 0.166 0.156 0.164 0.311 0.309 0.288 0.281 0.308 0.309 0.306 0.309 0.270 0.276 0.255 0.257

1.23 1.23 1.23 1.23 1.17 1.16 1.17 1.23 1.23 1.23 1.23 1.24 1.23 1.23 1.23 1.23 1.23 1.23 1.23

−64 −64 −57 −57 −26 −24 −26 −67 −67 −60 −58 −67 −67 −66 −67 −55 −57 −50 −51

The properties are ρ (e Å−3) = electron density at the BCP, |V|/G = ratio between potential energy density and kinetic energy density, and EINT (kJ mol−1) = interaction energy at the BCP. a

Table 7. Properties of the Electron Density at the Pd···Pd and Pd−Cl Bond Critical Points According to the QTAIM Method for the Selected Pd(III) Compoundsa compound 8a(Cl) 8g* 9a 10a* 11e* 12d* 13c* 14b* 8a(Cl) 8g* 9a 10a* 11e* 12d* 13c* 14b*

ρ (e Å−3)

|V|/G

Pd···Pd Interaction 0.451 1.44 0.405 1.45 0.364 1.47 0.346 1.47 0.452 1.44 0.422 1.45 0.459 1.44 0.413 1.44 Pd−Cl Interaction 0.437 1.21 0.423 1.21 0.417 1.20 0.409 1.20 0.409 1.20 0.401 1.19 0.444 1.21 0.404 1.19

EINT (kJ mol−1) −79 −66 −55 −51 −79 −70 −82 −59 −87 −83 −81 −79 −81 −79 −90 −79

a

For clarity, only compounds with R = CH3 were chosen for the detailed analysis.

weaker Pd···Pd interaction in comparison to the corresponding O,O compound 8a(Cl). The Pd−Cl interactions, on the other hand, show only small differences between various types of compounds. The most notable result is the strength of the axial interaction in pyrazolate compounds (13c*), which increases the stability of the Pd(III) complexes more than in the other compound. Stabilization Energies. Favorability of forming the dichloro complexes from the Pd(II) precursors was investigated by F


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are based on a possible metal−metal cooperation that can lower the activation barrier to the redox process.14b Direct C−H arylation has emerged as a viable and attractive alternative for the traditional cross-coupling reaction.15a,b Following the protocol reported by Sanford and co-workers,15c compounds 1−7 have been tested in the direct 2-phenylation of indoles with (Ph2I)PF6 (Scheme S1). While 1−7 have shown to be competent precatalysts in these catalytic reactions, 8 and 9 were not. The results are displayed in Tables S16 and S17. Additional experiments allowed us to conclude that (a) no free radicals are involved in the reaction mechanism,7d (b) the reaction quickly started with the addition of the oxidant, (Ph2I)PF6, indicating that the interaction of the precatalysts with the oxidant gave catalytically active oxidized intermediates whose instability does not allow their recording by 31P{1H} NMR spectroscopy at 298 K (the acetic acid as solvent prevented the use of low temperatures),7d and (c) with orthometalated mononuclear palladium precatalysts the reactions proceeded with much slower rates although with high yields (Table S18).7d,f Previously, we have shown that one plausible route to direct 2-phenylation of indoles is initiated with the formation of a dimetallic Pd(III) complex, where the axial sites act as active sites for binding.7d,g,f In the present work, we explored the relative stability of the different Pd(III) intermediates in order to explain why the pyrazolates and the N,S compounds have not been found to be competent in this catalytic reaction. We adopted a approach similar to that previously by studying the interaction of the palladium(II) compounds with the phenyl groups at the axial sites by DFT calculations. Table 9 summarizes the relative stability of the selected representatives of the Pd(III) compounds with two axial

calculating the relative energies of the model reaction 1. The reaction energies are given in Table 8. [Pd 2{μ‐(C6H4)PPh 2}2 {μ‐(X1‐X2)}2 ] + Cl 2 → [Pd 2{μ‐(C6H4)PPh 2}2 {μ‐(X1‐X2)}2 Cl 2]

(1)

Table 8. Reaction Energies (ΔEr) According to Model Reaction 1 for Selected Pd Complexes compounds ΔEr (kJ mol−1) 1a 1c 1f 1g* 2a 2d* 3a 4a 4e* 5a 5d* 6b 6c 6d 6f* 7b 7c 7e 7f

8a(Cl) 8c 8f 8g* 9a* 9d* 10a* 11a 11e* 12a 12d* 13b 13b* 13d* 13f* 14b* 14c* 14e* 14f

−143 −146 −140 −133 −99 −90 −84 −89 −181 −84 −178 −183 −192 −187 −186 −75 −79 −154 −117

It is noteworthy that the most stable Pd(III) complexes are formed from the Pd(II) complexes with pyrazolate ligands (13b,c*−f*). The different electronic nature of the aromatic system in the ligands causes different donating properties, as can be seen in the case of N,S ligands in 14e*,f, which show larger reaction energies in comparison to the corresponding N,S compounds 14b*,c. Compounds 3−5 generally have larger steric bulk than other auxiliary ligands, and hence they exhibit smaller stabilization energies, giving 10−12, because of the partial blocking of the axial sites. However, in some cases additional intramolecular H···Cl interactions are able to stabilize the Pd(III) structures, as in compounds 11e* and 12d*, which exhibit considerable stabilization in comparison to 11a and 12a (−181 and −178 kJ mol−1 vs −89 and −84 kJ mol−1, respectively). Generally, the complexes with large stability have been successfully synthesized and vice versa. In particular, the chelating complexes 2 and 3 were found to have poorer stability energies, which is in good agreement with the experimental difficulties in their chemical oxidation. Dinuclear Palladium(III) Intermediates in Catalysis. The synthetic accessibility of palladium compounds in a high oxidation state (III or IV) demonstrated in the last few years has allowed the development of new strategies in which these compounds are considered as intermediates in some catalytic processes. Sanford and Hickman described the advantages of a “high valent” organometallic palladium intermediates over the more common “low valent analogues”.13 Dinuclear palladium(III) compounds have been recently considered as intermediates in some catalyzed C−H oxidation reactions.1a.3.5,14 The advantages of the dinuclear palladium(III) intermediates

Table 9. Reaction Energies (ΔEr) According to Model Reaction 2 for Selected Pd Complexes Pd(II) compound

Pd(III) compound

ΔEr (kJ mol−1)

1a 1g* 2a 3a 4e* 5d* 6c 7b

1a-2Ph 1g*-2Ph 2a-2Ph 3a-2Ph 4e*-2Ph 5d*-2Ph 6c-2Ph 7b-2Ph

−82 −48 −4 −18 −10 +2 −103 +37

phenyl ligands according to model reaction 2, which would represent the formation of the intermediate in the catalytic 2phenylation. The Pd(II) compounds were selected with R = CH3 to maintain similar substituent effects and to reveal the different electronic nature of the ligands. [Pd 2{μ‐(C6H4)PPh 2}2 {μ‐(X1‐X2)}2 ] + 2Ph− → [Pd 2{μ‐(C6H4)PPh 2}2 {μ‐(X1‐X2)}2 Ph 2]

(2)

The reaction energies show that generally the Pd(II) complexes with the O,O ligands form more stable intermediates in comparison to the complexes with other types of auxiliary ligands, with one exception: the pyrazolate compound 6c formed a very stable Pd(III) compound with two axial phenyl ligands. This trend was also seen in the large stability of the pyrazolates with axial chlorides (Table 8). On G


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Organometallics the other hand, formation of the Pd(III) intermediate from compound 7b with N,S ligands was not favorable, also reflecting the results of the dichloride compounds. Experimentally, compounds with pyrazolato or N,S-ligands are not competent precatalysts. The calculations show that the reasons for their poorer ability to act as precatalysts are different in compounds 6 and 7. The pyrazolate compounds 6 form such stable intermediates that the catalysis reaction is not likely to proceed to products. In contrast to this, the first step in the catalytic reaction for the N,S compounds 7 is not favorable enough, in comparison to the other studied Pd(II) compounds. Furthermore, the other observed experimental trends can be explained via the formation of the Pd(III) catalytic intermediate. The O,O ligands lead to more favorable intermediates in comparison to the O,S compounds (1a vs 1g*); hence, the 2-phenylation proceeds with higher rate and better yield. Likewise, the triazenides are greater potential as precatalysts in comparison to formamidinates (4e* vs 5d* in Table 9). In order to study the cooperative effect of the axial phenyls and the interaction between the palladium atoms, we calculated the properties of the electron density at the Pd··· Pd and Pd−C(Ph) BCPs (Table 10) for the representatives of the Pd(III) compounds.

2Ph. On the other hand, all compounds form rather similar, very strong Pd−C(Ph) interactions with substantial electron density and degree of covalency at the corresponding BCP, but this strong interaction is not entirely responsible for the overall stability of the intermediates, which seems to be more affected by the Pd···Pd interaction than by the small differences in the axial Pd−C(Ph) interactions. Thus, the cooperative interactions at the C(Ph)−Pd···Pd−C(Ph) axis play a major role in the formation of the intermediates with suitable stability and reactivity. Further information on the axial interactions was sought by studying the frontier molecular orbitals of the intermediates. Figure 6 shows an example of the appearance of the HOMO

Table 10. Properties of the Electron Density at the Pd···Pd and Pd−C(Ph) Bond Critical Points According to the QTAIM Method for the Selected Pd(III) Compoundsa

Figure 6. Appearance of the HOMO and LUMO orbitals for the Pd(II) complex 1a-2Ph. The percentage values represent different contributions in the FMOs.

compound 1a-2Ph 1g*-2Ph 2a-2Ph 3a-2Ph 4e*-2Ph 5d*-2Ph 6c-2Ph 7b-2Ph 1a-2Ph 1g*-2Ph 2a-2Ph 3a-2Ph 4e*-2Ph 5d*-2Ph 6c-2Ph 7b-2Ph

ρ (e Å−3)

|V|/G

Pd···Pd Interaction 0.326 1.45 0.287 1.45 0.271 1.48 0.261 1.46 0.353 1.45 0.335 1.47 0.380 1.45 0.298 1.46 Pd−Cl Interaction 0.698 1.50 0.658 1.47 0.661 1.47 0.635 1.43 0.629 1.43 0.613 1.42 0.670 1.45 0.666 1.46

EINT (kJ mol−1)

and LUMO orbitals of compound 1a-2Ph. The appearance of FMOs for other complexes has been included in Figures S24− S26 in the Supporting Information. Again the most striking feature in the FMOs is the change to a bonding Pd···Pd interaction in HOMO and the concentration of electron density at the axial level in the LUMO, which emphasizes the importance of the axial interactions in the stability of the complexes. Axial phenyl interactions in the Pd(III) complexes change the nature of the Pd···Pd interaction from antibonding to a bonding one, similarly to Pd(III) compounds with axial chlorido ligands (Figure 5 and Figures S21−S23). However, axial chlorido ligands are able to create a greater energetic effect on the FMOs in comparison to axial phenyl groups. On the other hand, the large contribution of the phenyl groups in the HOMOs indicates strong interaction with the palladium atoms, which in turn reduces the involvement of the Pd d orbitals in the highest occupied MOs. The sulfur donors in the N,S ligands in compounds 7 also participate in both HOMO and LUMO via p orbitals, further reducing the metal contribution. These observations support the importance of the strength and stability of the axial interactions in the formation of the catalyst intermediates.

−52 −40 −36 −33 −56 −51 −64 −43 −143 −131 −132 −126 −126 −121 −139 −136

a

For clarity, only compounds with R = CH3 were chosen for the detailed analysis.

Oxidation of the Pd(II) centers increases the electron density at the Pd···Pd bond critical point, but the increase is smaller than that with the axial chlorido ligands (Tables 6 and 7). However, unlike the case for complexes 8−14, formation of the catalyst intermediate with two axial phenyl groups does not lead to strengthening of the Pd···Pd interaction in comparison to the Pd(II) precursors, except in the case of complexes 2a and 3a with the chelating O,O ligands. For example, the interaction energies at the Pd···Pd BCP are −64, −79, and −52 kJ mol−1 in compounds 1a, 8a(Cl), and 1a-2Ph, respectively, but −26, −51, and −36 kJ mol−1 in the series 2a, 9a*, and 2a-

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CONCLUSIONS This study supported by DFT calculations has allowed us to offer an overview of stability and electronic properties of dinuclear phosphane palladium(III) chemistry, providing an important contribution to the palladium chemistry in this unusual oxidation state. Computationally optimized structures were found to agree well with the experimental crystal structures of the synthesized H


Article

Organometallics

Axial phenyl interactions change the nature of the Pd···Pd interaction from antibonding to bonding, similarly to the Pd(III) complexes with axial chlorido ligands. However, the truly strong Pd−C(Ph) interaction in comparison to the Pd− Cl interaction concentrates electron density at the Pd−C(Ph) BCP and the electron density between the metal atoms is much smaller than in the case of complexes 8−14. On the other hand, the overall stability of the compounds seems to be more affected by the strength of the Pd···Pd interactions, and therefore, the cooperative interactions at the C(Ph)−Pd···Pd− C(Ph) axis play a major role in the formation of the intermediates with suitable stability and reactivity.

compounds, which made it feasible to include additional presentatives in the computational analysis and systematically study the structural and electronic effect of the auxiliary ligands on the properties of the complexes. Although the charge of the donor atoms in free ligands follows the expected order q(O) > q(N) > q(S), the surrounding ligands/substituents also have a major effect on the charge distribution and, therefore, on the donating ability of the ligands upon complexation. The sulfur atoms in the O,S complexes (thiocarboxylates 1f,g*) and N,S ligand (7) show the highest electron donation, which can be seen, for example, in a considerable shift to higher field in the 31 1 P{ H} NMR signals of these compounds. Clear correlation between the δ(31P{1H}) values and the amount of electron donation from the atom trans to phosphorus was found in the computational analysis. The stability of the Pd(III) compounds varied according to the electronic and steric properties of the auxiliary ligands. The most stable palladium(III) compounds contain pyrazolato ligands in a coordination of a four-membered ring. The formation of the Pd−Cl interaction in these compounds increases their stability, in most cases increasing also the strength and the degree of covalency of the Pd···Pd interactions. Replacement of hard O or N donor atoms with the softer sulfur generally leads to slightly reduced stability, which would most probably increase the reaction times and reduce the yield of the chemical oxidation product. Ligands with larger steric bulk, such as amidinatos and triazenidos, show weaker stability in comparison to the more “open” structures. However, in some cases additional intramolecular interactions between the axial chlorido ligands and hydrogens of the auxiliary ligands (or their substituents) can stabilize the Pd(III) complex. A detailed study of the Pd···Pd interaction indicates that in palladium(II) compounds the nature of this interaction is similar in all the complexes, being slightly smaller in compounds with ligands containing sulfur (1f, 7) and stronger for triazenide derivatives (4). Complexes with chelating ligands (2, 3) exhibit notably smaller interaction energies at the Pd··· Pd bond critical points. Values of |V|/G between 1 and 2 show that this interaction is partially covalent. In palladium(III) compounds, the shortened Pd···Pd distances and their diamagnetism suggest a Pd···Pd bonding interaction. According to the DFT analysis, the nature of the HOMOs and LUMOs changes from palladium(II) to palladium(III) compounds. Upon oxidation, the HOMO changes from an antibonding combination of palladium d orbitals (70−85%) to a bonding combination of the palladium d orbitals with a notable contribution from the axial chloride ligands. It can be concluded that the combined Cl−Pd···Pd−Cl axial interactions affect mostly the stability of the Pd(III) complexes. The dinuclear palladium(II) compounds with the exception of 1f, 6, and 7 have been competent in the direct 2-phenylation of indoles. In the search for an intermediate in this catalytic process, the formation of a palladium(III) compound has been explored via consideration of the interaction of the palladium atoms with the phenyl groups at the axial sites. DFT calculations support the experimental fact that pyrazolate (6) and N,S (7) compounds are not competent precatalysts: the first (6) form very stable intermediates, and for 7, the formation of the intermediate is not favored enough. Therefore, in both cases the catalysis reaction is not likely to proceed.

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

Computational Details. All molecular compounds were fully optimized with the Gaussian09 program package16 at the DFT level of theory. A hybrid density functional B3PW9117 was utilized together with the basis set consisting of the Stuttgart-Dresden effective core potential with an additional p-polarization function for Pd atoms (SDD(p)),18 and the standard all-electron basis set 6-31G(d) for all other atoms. Frequency calculations with no scaling were conducted to ensure optimization to true minima. None of the optimized structures gave imaginary frequencies. Moreover, the absorption properties in dichloromethane (CH2Cl2) media were calculated by TDDFT19 with the conductor-like polarized continuum model (CPCM) for the solvent effects.20 The computational approach was tested for the performance and the methodology was found to produce very well the overall absorption characteristics compared to the available experimental spectra, even though the wavelengths were somewhat overestimated. Examples of the experimental and simulated UV−vis spectra are provided as Supporting Information (Figure S27). Topological charge density analysis was performed with the QTAIM (Quantum Theory of Atoms in Molecules)8 methods, which allowed us to access the nature of the bonding via calculating different properties of the electron density at the bond critical points (BCPs). The analysis was done with the AIMAll program21 using the wave functions obtained from the DFT calculations with the computationally fully optimized structures. S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00342. Perspective view of compounds of type 1−8, 10, 13, and 14, DFT optimized structures of the Pd(II) complexes 1−7, results associated with the validation of the computational models and methods, crystallographic data for compounds 1−7 and 8−14, comparison of the experimental UV−vis spectra and spectra simulated via TD-DFT calculations for complexes 1c,f, and results on the catalytic 2-phenylation of indoles (PDF) Cartesian coordinates and absolute energies of all the points presented in the paper (XYZ) Accession Codes

CCDC 1052931−1052933, 1501673−1501676, 1823385, 1823543, 612060−612062, 628882−628883, 713165− 713166, 849919−849921, 941940−941947, 944600−944605, and 996514−996520 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. I


Article

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for P.H.: pipsa.hirva@uef.fi *E-mail for M.A.Ú .: [email protected]. ORCID

MaAngeles Ú beda: 0000-0003-3193-3033 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge grants of computer capacity from the Finnish Grid and Cloud Infrastructure (persistent identifier urn:nbn:fi:research-infras-2016072533).

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REFERENCES

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Stability of Dinuclear Phosphane Palladium(III) Complexes: A DFT

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