Ligand-Controlled Electron Structure of Catalytically Active Ni

Article Cite This: J. Phys. Chem. A XXXX, XXX, XXX−XXX

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Ligand-Controlled Electron Structure of Catalytically Active Ni Complexes Published as part of The Journal of Physical Chemistry virtual special issue “Manuel Yáñez and Otilia Mó Festschrift”. M. Teresa Quirós, Daniel Collado-Sanz, Elena Buñuel, and Diego J. Cárdenas* Departamento de Química Orgánica, Facultad de Ciencias, Universidad Autónoma de Madrid, Institute for Advanced Research in Chemical Sciences (IAdChem), Av. Francisco Tomás y Valiente 7, Cantoblanco, 28049 Madrid, Spain S Supporting Information *

ABSTRACT: We have performed a systematic study of the electron structure of a series of Ni(I) and Ni(II) iodo and methyl complexes with a variety of di- and tridentate nitrogen ligands to study the influence of these ligands in the structure of catalytically active complexes in cross-coupling reactions. Ni(II) compounds show the expected square-planar configuration typical of complexes of d8 metals, regardless of the kind of coordinating nitrogen atom (sp2 or sp3) found in ligands derived from either trialkylamines or pyridines. In contrast, Ni(I) complexes show different structures. Thus, the absence of orbitals capable of delocalizing the unpaired electron (such as in TMEDA and PMDTA derivatives) leads to nonplanar iodo or methyl tetracoordinate complexes. In contrast, the presence of ligands derived from pyridine allows delocalization of the unpaired electron on the ligand. This delocalization is especially effective for terpyridine species.

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INTRODUCTION The use of Ni complexes as reaction catalysts and promoters has experienced a renaissance during the last years, especially in the context of cross-coupling reactions for the formation of carbon−carbon bonds.1−4 Ni has shown several advantages compared to other metals both from a synthetic point of view and for the discovery of new mechanistic processes. In contrast to other metals of the platinum group, Ni shows a wider variety of oxidation states that are accessible along the catalytic cycles for organometallic complexes involved in synthetically useful reactions. Consequently, novel catalytic processes involving less common activation modes of organic molecules have been developed. Noteworthy, the importance of Ni(I) derivatives in activation of small molecules in enzymatic, stoichiometric, and catalytic reactions has been recently highlighted and reviewed.5,6 Regarding metal-catalyzed cross-couplings, initial formation of the catalytically active organo−Ni(I) species has been proposed to take place by comproportionation of Ni(0) and Ni(II) complexes. Regeneration of the active Ni(I) catalyst at the end of the cycles most probably takes place by transmetalation from the nucleophile to a Ni(I)−halide intermediate. Reduction of precursor Ni(II) complexes can be achieved by in situ reduction with alkyl organometallic reagents in the presence of suitable ligands. The alkyl−Ni(I) complexes formed this way are able to activate haloarenes and, more interestingly, haloalkanes by homolytic C(sp3)−halogen bond cleavage. Others and we have taken advantage of this reactivity to develop Ni-catalyzed coupling reactions involving radical © XXXX American Chemical Society

intermediates that have also allowed cascade reactions with formation of several alkyl−alkyl bonds in smooth conditions.7−16 Depending on the organic halide used as electrophile (aryl or alkyl) and the kind of nucleophile (organozinc or organomagnesium halide) different polydentate nitrogen ligands have shown the best performance. The first isolation of a Ni(I)−methyl complex and the study of its properties was possible due to stabilization by terpyridine (tpy) ligand.17−20 This complex is better explained as a Ni(II) derivative containing a formally anionic tpy, as shown by its square planar structure and the localization of the unpaired electron on the π*-orbitals of the polydentate ligand. Therefore, tpy acts as a redox-active ligand that favors the formation of a relatively low formal oxidation state derivative. Redox-active ligands are important in stabilization and tuning of the reactivity of intermediate metal complexes, and their participation has enabled new reactions catalyzed by different metals.21 In this work, we wish to report our results on the systematic study of the electron structure and properties of a series of Ni(I) and Ni(II) iodo and methyl complexes with a variety of di- and tridentate nitrogen ligands, especially centered on the influence of these ligands in the stabilization of the complexes. Received: November 28, 2017 Revised: January 24, 2018 Published: February 14, 2018 A

DOI: 10.1021/acs.jpca.7b11713 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

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METHODS Calculations were performed with Gaussian 09 at DFT and ab initio levels. The geometries of all complexes here reported were optimized using both the B3LYP hybrid functional22−24 and MP2 method. Optimizations were carried out using 631G(d) basis set for C, H, and N. The LANL2DZ basis set, which includes the relativistic effective core potential (ECP) of Hay and Wadt and employs a split-valence (double-ζ) basis set, was used for Ni. Harmonic frequencies were calculated at the same level to characterize the stationary points and to determine the zero-point energies (ZPE). Molecular orbitals were calculated for the minima obtained after geometry optimization at MP2 level.

(PMDTA) or 2,2′:6′,2″-terpyridine (tpy) ligands show very different structures (Figure 3). PMDTA complexes 2 and 7 present a geometry between tetrahedral and square planar, with dihedral angle defined by the four coordinated atoms around 40° (Table 2). In contrast, Ni(II) analogous complexes (12 and 17) show a more planar arrangement, in accord with the expected geometry for d8 Ni(II) compounds. Deviation from planarity in 12 and 17 is due to the geometrical restrictions imposed by the tridentate chelating ligand. The dihedral angles defined by the four coordinated atoms are 27° and 14° for 12 and 17, respectively. The more distorted geometry for Ni(I) compounds 2 and 7 has to do with the localization of the unpaired electron on Ni, as we will discuss below. Comparing iodo and methyl complexes of Ni(I), we find the latter are more planar and exhibit longer Ni−N distances with the N atoms cis to the methyl ligand. Structures of tpy derivatives are very different. Perfect square-planar minima are obtained for complexes 4, 9, 14, and 19, regardless the Ni oxidation state and the nature of the other ligand (iodo or methyl). Metal to ligand distances are of course shorter for the Ni(II) complexes 14, and 19, compared to their corresponding Ni(I) partners 4 and 9 (around 0.2 Å, Table 1). The planarity of Ni(I)−tpy complexes is due to the special electron structure that will be discussed below. Finally, nonchelated complexes containing three pyridine ligands (5, 10, 15, and 20) were calculated to explore the combined influence of the chelation and the conjugation of aromatic rings, in comparison with their tpy analogues. Ni(II) derivatives (15 and 20) again exhibit the expected square-planar configuration around Ni, with relatively short Ni−ligand distances. Instead, Ni(I) derivatives have astonishing differential structures. Thus, the iodo complex 5 presents a perfect square-planar configuration with shorter Ni− N distance for the pyridine ligand trans to iodide compared to that for cis ligands. In contrast, in complex 10 the N for the ligands arranged in cis with respect to the methyl ligand are slightly pyramidalized, suggesting a light redox-active behavior for these two pyridines (see electron structure below). In summary, Ni(I) tetracoordinate complexes show different geometries depending on the nature of the ligands, exhibiting the typical square-planar coordination of Ni(II) derivatives for tpy compounds. A previous DFT study suggested that this is due to the delocalization of the unpaired electron on the antibonding π orbitals of the ligand. Accordingly, the structure of these complexes could be considered as the result of a formally anionic tpy ligand to Ni(II), for which square-planar coordination is expected on the basis of the high energy of the empty d orbital contained within the coordination plane. Coordination of THF to Tricoordinate Complexes. We hypothesized that tricoordinate formally Ni(I) complexes containing π-accepting bidentate ligand bpy could be prone to coordinate a fourth ligand, as expected for Ni(II) derivatives. In fact, coordination of THF to Ni(II) complexes 11, 13, 16, and 18 is thermodinamically favorable, as expected (Table 3). In contrast, coordination to Ni(I)−I derivatives (1 and 3) shows positive reaction ΔG, although coordination of the oxygen ligand is exothermal. In addition, longer Ni−O distances are observed compared to the case of Ni(II) derivatives (Table 1). However, coordination to Ni(I)−methyl complexes is more difficult. Thus, no minimum could be located for the reaction of THF with TMEDA derivative 6. This complex is more stable without an additional ligand. For the related bpy analogue 8-THF, the reaction calculated at DFT level is practically thermoneutral, but it has a positive ΔG

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RESULTS AND DISCUSSION We intended to compare the properties of Ni(I) and Ni(II) model complexes with bi- and tridentate nitrogen ligands containing either iodo or methyl as reactive groups (Figure 1).

Figure 1. Complexes calculated in this study.

Alkyl−Ni(I) complexes can be formed from the corresponding halogen derivatives along the catalytic cycles. Calculations have been extended to polydentate ligands derived from both pyridine and trimethylamine, to compare their properties depending on the kind of sp2 or sp3 coordinating atoms, and the presence of low-lying antibonding π-orbitals in the ligand. These derivatives are accurate models for the proposed intermediate complexes in carbon−carbon cross-coupling reactions. Additionally, tris(pyridine) derivatives have been included to determine the influence of extended conjugation in the stabilization. Geometry of Complexes. Tricoordinate complexes derived from bidentate N-ligands show different structures depending on the type of ligand, N,N-tetramethylethylenediamine (TMEDA) or 2,2′-bipyridine (bpy), the coordination of iodo or methyl, and the Ni oxidation state. As an expected general rule, Ni−N distances are shorter in Ni(II) derivatives than the Ni(I) analogues (Table 1). Ni(I)−iodide derivatives 1 and 3 (with TMEDA and bpy ligands, respectively) show a symmetrical structure with similar N−Ni−I angles (Figure 2). In contrast, methyl derivatives 6 and 8 are better described as distorted T-shaped complexes, with N−Ni−C angles around 160° and 120° in both cases. In contrast, Ni(II) tricoordinate complexes 11, 13, 16, and 18 are T-shaped derivatives reminiscent of the structure of square-planar complexes lacking one ligand. This is especially evident in methyl derivatives 16 and 18. As an exception, the structure of TMEDA iodo derivative 11 is different with optimization at B3LYP or MP2 levels. For the former calculation level, a symmetrical structure is obtained, whereas MP2 leads to a distorted T-shaped structure. Regarding the tetracoordinate complexes, Ni(I) derivatives containing N,N,N′,N″,N″-pentamethyldiethylenetriamine B


Article

The Journal of Physical Chemistry A Table 1. Bond Lengths for Minima Calculated at MP2 and B3LYP Levels

Ni−N1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 1-THF 3-THF 8-THF 11-THF 13-THF 16-THF 18-THF

Ni−N2

Ni−N3

MP2

DFT

MP2

DFT

2.112 2.173 2.050 2.146 1.907 2.180 2.267 2.088 2.001 2.274 1.903 1.996 1.883 1.957 1.885 1.898 1.983 1.870 1.936 1.885 2.189 2.122

2.111 2.193 2.015 2.133 2.107 2.208 2.393 2.074 1.962 2.259 1.950 2.009 1.897 1.974 1.931 1.938 2.038 1.889 1.973 1.942 2.200 2.088 1.909 2.000 1.950 2.001 1.945

2.111 2.163 2.050 2.049 1.868 2.123 2.175 2.048 1.963 2.069 1.932 1.970 1.917 1.880 1.942 2.052 2.044 1.994 1.899 2.032 2.160 2.095

2.113 2.177 2.016 1.993 2.156 2.156 2.204 2.016 1.887 2.068 1.949 2.014 1.913 1.887 2.005 2.060 2.087 1.990 1.918 2.083 2.174 2.050 1.982 2.056 1.973 2.132 2.024

1.969 1.942 1.954 1.923

2.005 1.959 2.075 2.001

Ni−X

MP2

DFT

2.190

2.217

2.146 1.907

2.133 2.107

2.227

2.302

2.001 2.258

1.955 2.292

1.967

2.034

1.957 1.885

1.974 1.931

1.964

2.019

1.936 1.887

1.978 1.938

due to the entropy change corresponding to an association process (we could not optimize this structure at MP2 level). The impossibility to delocalize the unpaired electron in the Ni complex with the N(sp3) ligand TMEDA seems to be the reason for this differential behavior compared to the behavior of bpy, as it will be shown below. Molecular Orbital Studies. We have performed molecular orbital calculations on the MP2 optimized structures to get insight into the effect of the different kinds of ligands on the location of the unpaired electron in Ni(I) complexes. Regarding tricoordinate complexes (Figure 4), Ni(I)− TMEDA derivatives 1 and 6 show a high spin density (close to 1, Table 4) on the Ni atom. The SOMO of the iodo derivative 1 is located on the Ni−I fragment, whereas for the methyl derivative 6 the electron density is mainly located along the vacant coordination site. In contrast, the spin density on the metal atom is significantly lower in bpy derivative 3 (0.12), and the SOMO is located on bpy, indicating that it is behaving as a redox-active ligand and that the structure can be better explained as an anionic bpy coordinated to Ni(II). The natural charge on Ni is also higher compared to that in 1 and 6 (0.7 vs 0.3−0.4). Regarding methyl complex 8, although the shape of the SOMO is similar to that of the iodo analogue 3, the spin density value close to 1 indicates that the unpaired electron is located on Ni. This behavior may be a consequence of the different geometry of 3 and 8. Coordination of THF to 1 and 3 affords complexes with similar spin densities on Ni (in contrast

MP2 2.518 2.629 2.511 2.624 2.459 1.980 2.037 1.983 1.979 2.033 2.421 2.522 2.420 2.505 2.473 1.785 1.865 1.782 1.866 1.871 2.655 2.653

(I) (I) (I) (I) (I) (C) (C) (C) (C) (C) (I) (I) (I) (I) (I) (C) (C) (C) (C) (C) (I) (I)

2.535 2.523 1.875 1.871

(I) (I) (C) (C)

Ni−O DFT 2.526 2.662 2.518 2.645 2.773 1.968 2.001 1.974 1.932 2.009 2.431 2.566 2.422 2.544 2.552 1.869 1.921 1.861 1.912 1.932 2.689 1.682 1.936 2.579 2.568 1.925 1.918

(I) (I) (I) (I) (I) (C) (C) (C) (C) (I) (I) (I) (I) (I) (I) (C) (C) (C) (C) (C) (I) (I) (C) (I) (I) (C) (C)

MP2

DFT

2.230 2.202

2.255 2.230 1.984 1.956 1.930 1.970 1.951

1.932 1.904 1.936 1.916

with the unsolvated derivatives) and with the SOMO located mainly on Ni and I (Figure 5). The electron structure differences found for TMEDA and bpy complexes may be the reason for the different catalytic activity observed for these complexes. Thus, TMEDA derivatives have been proposed to promote the activation of iodoalkanes in the reaction with alkyl−Mg compounds,3a whereas bpy analogues are the putative intermediates in the reaction of aryl halides with alkyl−Zn halides.3 However, tetracoordinate complexes with Ni bound to three nitrogen atoms show very different electronic structure features depending on the kind of ligands (Figure 6). PMDTA derivatives 2 and 7 exhibit the expected behavior, with unpaired electron residing on the metal atom in a more localized SOMO. This is also the case for tris(pyridine) methyl complex 10, although in this case partial delocalization on the cis-tomethylpyridines is observed, in accord with the pyramidalization of the nitrogen atoms mentioned above. In contrast, tris(pyridine) iodo complex 5 fully delocalizes the unpaired electron in the antibonding orbitals of pyridine ligand trans to iodide. A very low spin density on Ni (0.10) is observed in complex 9, containing the terpyridine ligand. Delocalization involves the three coplanar aromatic rings of the ligand. In contrast, related iodo complex 4 has a high spin density on Ni (0.94). The effect of the iodide ligand seems to compensate the delocalizing effect of the terpyridine ligand. The SOMO is populated by Ni and the central orbitals in this case. C


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Figure 2. Geometry of tricoordinate complexes.

Figure 3. Coordination geometry for Ni(I) and Ni(II) tetracoordinate complexes. Methyl groups on TMEDA and PMDTA ligands have been omitted for clarity.

activation and reaction energies for the reaction of model 2iodopropane with complexes bearing different ligands that have been used in catalytic reactions (6, 8, and 9, Figure 7). Reactions are practically thermoneutral for starting tricoordinate complexes 6 and 8, and show moderate activation

Coupling reactions of alkyl halides with organomagnesium or organozinc reagents catalyzed by Ni(I) complexes have been proposed to proceed through initial activation of the alkyl halide by reaction of alkyl−Ni(I) complexes to give a free carbon radical and a Ni(II) complex. We have calculated D


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The Journal of Physical Chemistry A Table 2. Dihedral Angles N1−N2−N3−X for the Tetracoordinate Complexes

Table 4. Natural Charges and Atomic Spin Densities on Ni for the Ni(I) Complexes from NBO Analysis (MP2)

α(N1−N2−N3−X)

spin density

natural charge

complex

B3LYP

MP2

complex

MP2

B3LYP

MP2

B3LYP

2 4 5 7 9 10 12 14 15 17 19 20

39.2 0.0 0.1 30.5 1.1 2.4 26.4 0.01 0.0 16.1 2.3 1.0

41.0 −0.0 0.0 36.6 −3.9 4.0 26.6 0.2 0.0 14.5 6.5 0.5

1 2 3 4 5 6 7 8 9 10 1-THF 3-THF

0.945 0.954 0.121 0.940 −0.018 0.994 0.983 0.996 0.100 0.965 0.945 0.125

0.923 0.913 0.955 0.998 0.887 0.969 0.950 1.084 0.049 0.922 0.908 0.948

0.345 0.278 0.672 0.318 0.377 0.392 0.411 0.400 0.614 0.406 0.345 0.650

0.265 0.288 0.35 0.406 0.375 0.362 0.388 0.480 0.493 0.434 0.344 0.407

Table 3. Reaction Energy for THF Coordination (kcal· mol−1)

B3LYP initial complex

Δ(E+ZPE)

1 3 6 8 11 13 16 18

−4.7 −6.7 −0.1 −29.1 −34.4 −29.5 −32.8

MP2 ΔG

Δ(E+ZPE)

7.5 −10.4 7.5 −11.6 THF does not coordinate 14.3 −16.1 −41.0 −20.4 −45.6 −15.9 −38.9 −19.7 −40.6

ΔG 2.9 1.3

−26.5 −32.2 −25.1 −27.4

Figure 5. Molecular structure and the SOMO of Ni(I)−THF complexes.

energies. The haloalkane approaches the metal along the vacant coordination site and generate transition states TS_6−21 and TS_8−22 (Figure 8).

Figure 4. Molecular structure and the SOMO of tricoordinate complexes. E


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Figure 6. SOMO orbitals for Ni(I) complexes with three nitrogen ligands.

pentacoordinate geometry. IRC studies showed the endothermal formation of a previous association complex (ΔG = 24.03 kcal mol−1),8 which is the main factor that renders this process unfavorable (Supporting Information). It is important to note that experimental results8−19 show that complexes containing tricoordinate ligands such as tpy or pybox derivatives are more convenient in terms of reactions yields and better reaction performance. For this reason, we have searched for an alternative reaction pathway avoiding this high-energy process. Thus, we have calculated that dissociation of one of the pyridines from Ni to give a tricoordinate derivative similar to complex 8, has an energy cost of only 9.2 kcal mol−1 (ΔG, Supporting Information). Therefore, a reaction pathway involving dissociation of one of the ligands followed by reaction with the iodoalkanes along the vacant coordination site generated, avoiding TS_9−23, is more favorable. According to our calculations, we propose that the effect of these ligands is to stabilize the intermediate alkyl−Ni complexes by formation of tetracoordinate derivatives such as 9, precluding catalyst decomposition. Tetracoordinate intermediates most probably dissociate one ligand before activating iodoalkanes, but they are more convenient for their especial stability related to unpaired electron delocalization. The effect of hemilabile ligands has been reported for related Ni complexes.25 In summary, tpy derivatives comprise both good stability and reactivity, which explains their good behavior as cross-coupling catalysts.

Figure 7. Activation and reaction energies for the reaction of 2iodopropane with methyl−Ni(I) complexes.

Figure 8. Transition states for the activation of 2-iodopropane with methyl−Ni(I) complexes.

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CONCLUSIONS The study of a variety of Ni complexes with catalytic activity bearing N-based ligands has allowed us to analyze the factors involved in determining their different electronic structures. Ni(II) compounds show the expected square-planar configuration typical of complexes of d8 metals having 16 valence

In contrast, the reaction involving tetracoordinate complex of tpy ligand 9 is endothermal and shows a much higher activation energy. The electrophile approaches the metal complex along an axis approximately perpendicular to the coordination plane, and the resulting transition state (TS_9−23) exhibits a F


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electrons, regardless of the kind of coordinating nitrogen atom (sp2 or sp3). The only minor deviations are observed for polydentate ligands, because of the geometrical restrictions imposed by chelation. However, Ni(I) complexes exhibit very different features that depend on the properties of the ligands, comprising the type of N atom, chelation, conjugation, and the presence of iodo or methyl as additional ligands. The absence of orbitals capable to delocalize the unpaired electron (such as TMEDA and PMDTA) leads to nonplanar structures for tetracoordinate complexes, and to T-shaped or C2v geometries for tricoordinate derivatives. Spin density measurements suggest that the SOMO is located mainly on Ni, or on the Ni−I fragment. Coordination of THF to Ni(I) complexes of TMEDA, which would afford 17 valence electron complexes, is not favorable. In contrast, the presence of ligands derived from pyridine allows delocalization of the unpaired electron. This delocalization is especially effective for terpyridine species, compared with bpy or pyridine. Comparison with tris(pyridine) analogues indicates that both the extended conjugation and the geometry restrictions imposed by chelation are responsible for the differences observed. As a whole, our results are in accord with the observed catalytic activity and versatility of Ni complexes containing bi- and tridentate N(sp2) ligands (bpy tpy, pybox). These ligands act as redox-active fragments that force formally Ni(I) species to exhibit geometries typical of Ni(II) derivatives. Therefore, these complexes should be considered as constituted by anionic ligands bound to Ni(II). Finally, our calculations suggest that activation of alkyl halides involving tricoordinate nitrogen ligands, such as tpy, probably involves dissociation of one of the ligands prior to interaction with the alkyl halide rather that direct reaction through a pentacoordinate complex. The good performance shown by these complexes can derive from the stabilization of tetracoordinate alkyl−Ni(I) intermediates along the catalytic cycle by the effect of the redox-active ligand.

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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.jpca.7b11713. Atomic coordinates and energies for all stationary points along with the main molecular orbitals (PDF) Orbital diagrams (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Diego J. Cárdenas. E-mail: [email protected]. ORCID

Diego J. Cárdenas: 0000-0002-1707-6445 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Spanish MECD for a FPU fellowship to D. ColladoSanz, as well as the MINECO for funding (Grants CTQ201342806-R and CTQ2016-79826-R) and for a Juan de la Cierva contract to M. T. Quirós.

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REFERENCES

(1) Phapale, V. B.; Cárdenas, D. J. Nickel-catalysed Negishi Crosscoupling Reactions: Scope and Mechanisms. Chem. Soc. Rev. 2009, 38, 1598−1607. G


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Ligand-Controlled Electron Structure of Catalytically Active Ni

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