Suppressed Phosphine Dissociation by Polarization Effects on the

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Suppressed Phosphine Dissociation by Polarization Effects on the Donor−Acceptor Bonds in [Ni(PEt3)4−n(ECp*)n] (E = Al, Ga) Julius Hornung,†,‡,§ Jana Weßing,†,‡ Paul Jerabek,§ Christian Gemel,†,‡ Alexander Pöthig,†,‡ Gernot Frenking,∥ and Roland A. Fischer*,†,‡ †

Department of Chemistry, Technical University Munich, Lichtenbergstrasse 4, D-85748 Garching, Germany Catalysis Research Centre, Technical University Munich, Ernst-Otto-Fischer Strasse 1, D-85748 Garching, Germany § The New Zealand Institute of Advanced Study, Massey University, Private Bag 102904, 0632 Auckland, New Zealand ∥ Fachbereich Chemie, Philipps-Universität Marburg, D-35032 Marburg, Germany

Inorg. Chem. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 09/24/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: A series of heteroleptic complexes [Ni(PEt3)4−n(ECp*)n] (E = Al, Ga, Cp* = pentamethylcyclopentadienyl, n = 0−4) was prepared and characterized by experimental as well as computational means. The series of compounds was studied with respect to ligand dissociation processes which are fundamental for reactivity. In contrast to the homoleptic complexes [Ni(PR3)n] phosphine dissociation is remarkably suppressed in the heteroleptic title complexes. Single crystal X-ray structures as well as density functional theory calculations reveal a gradual decrease of the Ni−PEt3 distances with increasing number of coordinated group-13 ligands ECp*. Accordingly, variabletemperature UV−vis studies of [Ni(PEt3)4−n(AlCp*)n] in solution indicate no ligand dissociation equilibrium for n ≥ 2. Energy decomposition analysis with the natural orbital for chemical valence extension shows higher Ni−P interaction energies for [Ni(PEt3)4−n(AlCp*)n] than for [Ni(PEt3)4] which is mainly a result of an increase in columbic attraction forces induced by Ni−PEt3 bond polarization upon ECp* coordination.

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INTRODUCTION Transition-metal fragments [LnM] with free coordination sites at the metal center (or 2σ(I))

RESULTS AND DISCUSSION Synthesis, Spectroscopic, and Structural Characterization of [Ni(PEt3)4−n(ECp*)n]. The complexes 1a-2b, 4a,3 4b,14 and 513 were synthesized in one-pot procedures following known protocols by reaction of the stoichiometric amounts of ECp* and/or PEt3 with [Ni(cod)2] in n-hexane or toluene (Scheme 1). Scheme 1. Synthesis of [Ni(PEt3)4−n(ECp*)n] by Ligand Replacement from Ni(cod)2a

a

Numbering scheme of the complexes refers to the number n of ECp* coordinated to Ni: 1a−4a, E = Al and n = 1−4; 1b−4b, E = Ga and n = 1−4; 5, n = 0.

2b

3a

C32H60Ga2NiP2 704.87 yellow fragment monoclinic P2/n 14.567(8) 16.615(10) 29.858(19) 90 90.19(2) 90 7227(7) 8 1.296 2976 100 2.103 12776/532/912 1.036 0.0291, 0.0561

C36H60Al3NiP 663.44 yellow fragment orthorhombic Pna21 17.5103(7) 11.9425(5) 18.4956(8) 90 90 90 3867.7(3) 4 1.139 1432 100 0.633 6841/1/389 1.040 0.0316, 0.0656

For full details see Supporting Information. bR1 = ∑(||F0 | − |Fc||)/ ∑|F0|. cwR2 = {∑[w(F02 − Fc2)2]/∑[w(F02)2]}1/2.

a

All attempts to synthesize 3a/3b by the same procedure failed and lead to inseparable mixtures of [Ni(PEt3)4−n(ECp*)n] with n = 2−4 (see Figures S24 and S25). However, 3a can be synthesized via an alternative approach starting from a precursor complex with predefined Ni/AlCp* ratio of 1:3. Treatment of [Ni(H)(SiEt3)(AlCp*)3]3 with PEt3 at elevated temperatures (60 °C) selectively yields 3a by reductive elimination of HSiEt3 and coordination of PEt3. Since no analogous GaCp* containing precursor is known, 3b was not accessible by this latter route. It was only observed in reaction mixtures but could never be isolated in pure form. 1H NMR spectra of 1a−4b reveal in all cases three signal groups with the expected intensity ratios, a singlet for the Cp*-methyl protons as well as two multiplets for the ethyl substituents at the phosphines (see Table S1). The Cp* resonances of 1−4 show a small upfield shift with increasing number of ECp* ligands. Within the series of complexes [Ni(PEt3)1(AlCp*)3] (3a), [Ni(PEt3)2(AlCp*)2] (2a), [Ni(PEt3)3(AlCp*)1] (1a), and [Ni(PEt3)4] (5), the 31P NMR resonances experience a characteristic upfield shift from 61.9 ppm in 3a to 17.8 ppm in 5. Single crystals suitable for X-ray diffraction studies were obtained by slow cooling of saturated solutions of 2b and 3a in n-hexane to −30 °C. In both compounds, a distorted tetrahedral coordination around the central nickel atom can be observed. In 2b, the P−Ni−P angle (120°) is considerably larger than the Ga−Ni−Ga angle (92°), which is most probably an effect of the higher steric demand of PEt3 with respect to GaCp*. This reasoning is in line with the respective structural data of the literature known compound [Ni(PMe3)2(GaCp*)2] in which the Ga−Ni−Ga angle is larger than the P−Ni−P angle (120.8 compared to 106.9°), presumably due to the lower steric demand of the PMe3 ligand.7 The Ni−ECp* bond lengths as well as the E− Cp*centroid distances are in accordance to known [Ni(PR3)4−n

These changes in the Ni−PEt3 bond lengths are nicely reproduced by DFT (BP86-D3/TZVPP) calculations, showing that successive introduction of ECp* is reflected by a stepwise Ni−P bond shortening. The optimized gas-phase structures of 1−5 are in good agreement with the experimentally determined structures from single-crystal X-ray diffraction data, which was also shown for other [TMx(ECp*)y(L)z] compounds using the BP86 functional.34 For example, the calculated Ni−PEt3 bonds in 2b, 3a, and 5 nicely reproduce the experimental results with a maximum deviation of 0.03 Å for 5 (for 2b and 3a only 0.01 Å). The Ni−ECp* bonds are calculated to be about 0.03 Å longer in 2b as compared to the X-ray structures. Ligand Dissociation from [Ni(PEt3)4‑a(ECp*)n] Studied by UV−vis Spectroscopy. As discussed in the Introduction, we were interested to study the ligand dissociation behavior of the series of compounds [Ni(PEt3)4−n(ECp*)n] described above in comparison with [Ni(PEt3)4] as the reference (Scheme 2). In the following we focus on the series of Alcompounds 1a−4a. Variable-temperature (VT) UV−vis spectroscopy in solution of the Al-series of complexes [Ni(PEt3)4−n(AlCp*)n] (1a−4a) was performed to investigate the dissociation behavior of either PEt3 or AlCp*. Classic studies by Tolman et al. on the dissociation of [Ni(PEt3)4] (λmax = 290 nm) revealed extensive PEt3 dissociation by the appearance of a band at λmax = 502 nm, which was attributed to the electronically unsaturated 16ve species [Ni(PEt3)3].9 Accordingly, the room-temperature UV−vis spectrum of 1a (see Figure S28) shows an intensive band at 320 nm and a weak absorption at 500 nm. Upon heating, the absorption intensity at 500 nm increases, whereas the band at 320 nm decreases. In accordance to the experimental UV−vis spectra of 5, the increase of the band C

DOI: 10.1021/acs.inorgchem.8b01817 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 2. Comparison of Selected Structural Parameters of [Ni(PR3)4−n(ECp*)n]a 533 d(Ni−P) d(Ni−ECp*) d(E−Cp*centroid) ∠(P−Ni−P) ∠(E−Ni−E)

2.212 (2.23)

107.0 (110.2)

2b

3a

2.138 (2.15) 2.242 (2.27) 2.052 (2.09) 121.1 (125.0) 98.9 (98.1)

2.1170 (2.12) 2.185 (2.19) 1.942 (1.95) 103.4 (102.9)

[Ni(AlCp*)4]32 2.1727 (2.18) 1.933 (1.93) 109.43 (110.1)

[Ni(GaCp*)2(PMe3)2]7 2.139 2.251 2.045 106.9 120.8

Experimental bond lengths are given in Å, bond angles in °. If applicable, average values are given for bond distances and angles. Calculated values (BP86-D3(BJ)/def2-TZVPP) are added in brackets below for comparison.

a

Figure 1. Molecular structures of 2b (left) and 3a (right) in the solid state. Due to reasons of clarity, Cp* ligands are presented in wireframe style. Hydrogen atoms and disorders are omitted for clarity. For 2b, only one of the two independent molecules of the asymmetric unit is shown. The displacement ellipsoids are shown at the 50% probability level. Selected interatomic distances (Å) and angles (deg): 2b: Ni−P: 2.1361(14)− 2.1401(14), Ni−Ga: 2.2375(11)−2.2462(10), P−Ni−P: 119.85(4)−122.43(4) Ga−Ni−Ga: 98.80(4)−98.92(4); 3a: Ni−P: 2.1170(1), Ni−Al: 2.18451(1)−2.1863(1), Al−Ni−Al: 102.047(2)−104.714(2).

Scheme 2. Two Concurring Ligand Dissociation Pathways for the Heteroleptic Complexes [Ni(PEt3)4−n(ECp*)n] (1− 5)

located at 500 nm can be attributed to dissociation processes. However, under the experimental conditions, this unsaturated species seems to be quite unstable or too reactive, as small traces of moisture or oxygen cannot be rigorously excluded at the low concentrations necessary for the UV−vis spectroscopic measurements. Thus, a thermodynamic analysis of a fully reversible dissociation process was not possible. We assign the absorption at 500 nm to PEt3 dissociation from 1a to yield [Ni(PEt3)2(AlCp*)1] based on the high purity of the employed sample 1a, as evidenced by elemental analysis and 31 P NMR spectrum of this batch of 1a which rules out the presence of traces of 5, and thus the presence of the respective dissociation product [Ni(PEt3)3] is also ruled out. The room-temperature UV−vis spectrum of 2a in n-hexane exhibits three different features: two bands at 260 and 302 nm and a small shoulder at around 380 nm (Figure 2). TDDFT calculations show that the experimental spectrum can be nicely reproduced (see Figure S30 for clarification), and thus the shoulder at 380 nm is an intrinsic feature (which can be attributed to the HOMO−1 to LUMO/LUMO+1 transition, see Figure S31) of the absorption spectrum of 2a. The two possible ligand dissociation products of 2a, either PEt3 dissociation to yield [Ni(PEt3)1(AlCp*)2] or AlCp*

Figure 2. VT UV−vis spectra (line diagram) of 2a in n-hexane with absorption bands at 260, 302, and 380 nm compared with the calculated absorptions (black bars) of [Ni(AlCp*)2(PEt3)2] on the TDDFT (BP86/TZVPP) level of theory.

dissociation to yield [Ni(PEt3)2(AlCp*)1], should exhibit electronic transitions at longer wavelengths as compared to [Ni(PEt3)3], which reasoning is qualitatively supported by TDDFT calculations (shift to about 580 and 540 nm, see Figures S32 and S33). Stepwise heating of the solution of 2a from 25 °C up to 61 °C (Figure 1), however, does not lead to significant changes in the UV−vis spectra. Together, these investigations confirm that 2a, even at low concentrations (about 5 × 10−5 mol/L) and moderately elevated temperatures, shows no sign of any ligand dissociation, and thus 2a is kinetically inert under usual conditions. This is also the case for D

DOI: 10.1021/acs.inorgchem.8b01817 Inorg. Chem. XXXX, XXX, XXX−XXX

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character of involved Ni−L bonds in [Ni(PEt3)4−n(ECp*)n]. Using the qualitative σ-type donation/π-type back-donation concept, one could argue that the strong electron-donating effect of the ECp* ligands35 increases the electron density at the central Ni atom which in turn would lead to stronger Ni → P back donation into low-lying P−C σ*-bonds.8 However, previous investigations revealed that M−ECp* and M−PR3 bonds are governed by electrostatic interactions, and the pure covalent orbital-based bonding picture described above may not be applicable.36 The bonding analysis of 1a−5 was performed employing energy decomposition analyses with the “natural orbitals for chemical valence” extension (EDA-NOCV).26b26c Therefore, complexes 1a−5 were fragmented into [Ni(PEt3)3−n(ECp*)n] and PEt3 (to investigate the Ni−PEt3 donor−acceptor bond) or into [Ni(PEt3)4−n(ECp*)n−1] and ECp* (to investigate the Ni−ECp* donor−acceptor bond), respectively. In the homoleptic phosphine complex [Ni(PEt3)4] (5), the interaction energy of the Ni−PEt3 bond is calculated to be about −55 kcal/mol. This energy consists of Pauli repulsion (129 kcal/mol), electrostatic interaction (−102 kcal/mol), orbital interaction (−53 kcal/mol), and dispersive attractions (−29 kcal/mol). The attractive interaction of the Ni−PEt3 bond of 5 quite substantially derives from electrostatic contributions (55%). Analysis of the orbital interaction term shows contributions from σ-type donation (−18 kcal/mol), π-type back-donation (2× −10 kcal/mol), and σ-type back-donation (−6 kcal/mol) (see S37 for details) on the decomposition of orbital interactions. Comparison of the Ni−ECp* bonds in the homoleptic complexes 4a/4b reveals that the intrinsic Ni− AlCp* bond energy (around −60 kcal/mol) is significantly higher than for the Ni−GaCp* bond (−48 kcal/mol) (see Figures S35 and S36 for more details). Orbital interactions of the Ni-ECp* bonds in 4a/4b closely resemble the orbitals contributions to the bond Ni−PEt3 in 5. After having discussed the bonding situation in the homoleptic systems 4a/4b and 5, now let us shift attention to the heteroleptic complexes. Stepwise introduction of ECp* in the series 5 to 3a/3b leads to a steady strengthening of the Ni−PEt3 bond (Figure 4 and Figure S34) from −55.4 kcal/ mol up to −64.1 kcal/mol in 3a. The biggest increase in the Ni−PEt3 bond strength is observed when a second PEt3-ligand is replaced by AlCp* to yield [Ni(PEt3)2(AlCp*)2] (2a). Another intriguing feature is the sharp rise in Pauli repulsion (+27.7 kcal/mol) and attractive electrostatic interactions (−31.5 kcal/mol) when one AlCp* is introduced (from 5 to 1a). Further replacement of PEt3 by AlCp* leads to a slight decrease of the Pauli repulsion, but the electrostatic interactions remain constantly high. The dispersive forces as well as the orbital interactions vary only slightly with minor changes in the σ/π-bonding ratio. The analysis of the orbital interactions of the Ni−AlCp* and Ni-PEt3 bonds in 2a reveals similar values for the σ-type and π-type interaction showing that both ligands are rather equal within the classical covalent σ-donation/π-back-donating model. From 1a/1b to 4a/4b, the interaction energy of the Ni−ECp* bond increases irrespective of E. Since it was found that there are no changes in the values of the covalent σ/π bonding contributions for the Ni−P bonds in 1a−5, these theoretical results suggest that the introduction of ECp* leads to polarization of the Ni−PEt3 bond which in turn rises the electrostatic interaction and thus strengthens the NiPEt3 bond, irrespective of the nature of E. This polarization is

3a and 4a which show absorption bands at 305 and 315 nm, respectively (see Figures S31 and S32). In summary, the VT UV−vis investigations reveal that only 1a, in contrast to 2a− 4a, is kinetically labile. These findings can be further supported by results from ligand exchange reactions (followed by 1H/31P NMR). Whereas treatment of kinetically labile [Ni(PEt3)4] with 1 equiv of AlCp* leads to the selective formation of [Ni(PEt3)3(AlCp*)1] (1a), the addition of 4 equiv of AlCp* does not yield the fully substituted product [Ni(AlCp*)4] (4a), but only the formation of [Ni(PEt3)2(AlCp*)2] (2a) is observed (Figure 3).

Figure 3. 31P NMR spectra of the reaction of Ni(cod)2 with 1 equiv of AlCp* selectively yielding [Ni(AlCp*)1(PEt3)3] (brown spectrum) as well as the reaction of Ni(cod)2 with 4 equiv of AlCp* selectively yielding [Ni(AlCp*)2(PEt3)2] (blue spectrum).

In accordance to this observation, treatment of [Ni(PEt3)2(GaCp*)2] with excess GaCp* does also not lead to the formation of 3b or 4b (Scheme 3). Therefore, we Scheme 3. Reaction Scheme Showing the Phosphine Replacement That Is Possible for [Ni(PEt3)4] and [Ni(PEt3)3(AlCp*)1]a

a

However, after introduction of two ECp* ligands, a further phosphine replacement is suppressed.

summarize that the extensive dissociation of [Ni(PEt3)4] is surprisingly suppressed by coordination of ECp* and that the coordination of ECp* leads to a shortening of the respective Ni−PEt3 bond as evidenced by single crystal X-ray structures as well as by DFT calculations. Both results could be interpreted as an ECp*-induced Ni−PEt3 bond strengthening. Theoretical Investigations. This unexpectedly significant effect on the Ni−PEt3 bond strength upon coordination of the strong σ-donor ligands ECp* prompted us to perform further theoretical analyses in order to get more insight into the E

DOI: 10.1021/acs.inorgchem.8b01817 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. EDA-NOCV decomposition of the Ni−P bonds for the different members of the series Ni(AlCp*)n(PEt3)4−n. The total bonding energy increases steadily with higher number of AlCp* ligands. Incorporation of one AlCp* ligand leads to an increase in Pauli repulsion as well as in the attractive electrostatic interaction.

contrast to the covalent σ/π donor−acceptor orbital bonding picture that is often used to rationalize effects on M−L bond strengths, it was found that electrostatic interactions are the dominant contribution to the Ni−PEt3/Ni−ECp* bonds also in the heteroleptic compounds, and these effects govern the ligand dissociation behavior. This result is in accordance to previous results of EDA-NOCV bonding analysis for homoleptic M−PR3 and M−ECp* compounds.36 Therefore, we conclude that ECp* ligands cause a significant polarization as the main reason for the bond Ni−PEt3 strengthening which leads to the kinetic inertness of most of the heteroleptic compounds. It follows that less polarizable ligands than PR3, for example, N-donor ligands C5H5N, NMe3, TMEDA, should be less susceptible to undesired bond strengthening effects due to ECp* coordination. Indeed, preliminary EDA-NOCV studies support such a strategy for deriving coordinatively unsaturated fragments [M(ECp*)n] by predissociation of Ndonor ligands.

probably the result of the higher electrophility of the ligating atoms E with respect to P. This is well in line with the experimentally determined 31P NMR shifts. Inclusion of higher numbers of ECp* ligands results in a significant upfield shift of the respective 31P signals, induced by the polarizing effects of the ECp* ligands. The Ni−AlCp* bond was found to be much stronger than the Ni−GaCp* bond which is in accordance to previously published results.35

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CONCLUSION Phosphine dissociation, which is known for [Ni(PR3)4]9 and is important for allowing reactive transformations of the unsaturated intermediates [Ni(PR3)3] is strongly suppressed by introduction of ECp* (E = Al, Ga) in the series of compounds [Ni(PEt3)4−n(ECp*)n] (n = 1−3). This was revealed by variable-temperature UV−vis measurements as well as by PEt3/ECp* ligand displacement reactions followed by NMR spectroscopy. All compounds of the investigated homologous series [Ni(PEt3)4−n(ECp*)n] with n ≥ 2 are kinetically inert, only [Ni(PEt 3 ) 3 (ECp*) 1 ] shows PEt 3 dissociation. The initial idea of deriving electron rich, however coordinatively unsaturated and reactive, fragments [Ni(ECp*)3] by predissociation of PR3 from [Ni(PR3)(ECp*)3] is not feasible. These experimental observations are supported by theoretical analysis of the donor−acceptor bonding situation using energy decomposition analyses with the “natural orbitals for chemical valence” extension (EDANOCV). A successive strengthening of the Ni−PEt3 bond was found with increasing number of ECp* ligands. The biggest increase in the bond strength was found progressing from [Ni(PEt3)3(AlCp*)1] (1a) to [Ni(PEt3)2(AlCp*)2] (2a), which is in accordance with the experimental findings. In

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01817. Additional experimental details and crystallographic information (PDF) Accession Codes

CCDC 1852315−1852316 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. F

DOI: 10.1021/acs.inorgchem.8b01817 Inorg. Chem. XXXX, XXX, XXX−XXX

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AUTHOR INFORMATION

Corresponding Author

*E-mail: roland.fi[email protected]. Fax: +49 (0)89 289 13194. ORCID

Alexander Pöthig: 0000-0003-4663-3949 Gernot Frenking: 0000-0003-1689-1197 Roland A. Fischer: 0000-0002-7532-5286 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Deutsche Forschungsgemeinschaft (grant Fi 502/23-2). REFERENCES

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DOI: 10.1021/acs.inorgchem.8b01817 Inorg. Chem. XXXX, XXX, XXX−XXX