Structural Diversity and Dynamics of Nickel Complexes with

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

Structural Diversity and Dynamics of Nickel Complexes with Ambidentate Phosphorus Heterocycles Shamil K. Latypov,* Yulia S. Ganushevich, Svetlana A. Kondrashova, Sergey V. Kharlamov, Vasily A. Milyukov, and Oleg G. Sinyashin Arbuzov Institute of Organic and Physical Chemistry, FRC Kazan Scientific Center of RAS, Arbuzov str. 8, Kazan, Tatarsan, Russian Federation 420083

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S Supporting Information *

ABSTRACT: The reaction of [Ni(C2H4)(dtbpe)] with 1-alkyl-1,2-diphospholes afforded [Ni(1-alkyl-1,2-diphosphole)(dtbpe)] complexes. Both DNMR experimental and DFT calculation results have shown that in solution these complexes are in an equilibrium of two P−P side-on η2-coordinated 1-alkyl-1,2-diphosphole isomers. In one form, unusual η2 bonding occurred at a P2−P1 side, presumably at the expense of a vicinal P2−C1 π bond, and it could be characterized as a phosphametallacycle with the formal oxidation state of Ni(II). In the second isomer with the Ni(0) state, Ni bonds to the ligand through two σ interactions with phosphorus lone pairs. The P2 chemical shift, being heavily dependent on structure, can be used to monitor these isomer populations.

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INTRODUCTION The effectiveness of many transition-metal complexes as catalysts for reactions comes from the ability of these metals to complex reversibly with a variety of functional groups.1−5 Generally, metal complexes assist in modifying different molecular species via the formation and transformation of something like a “super complex”, which comprises an auxiliary metal complex, source reagents, products, and intermediates. This activity is ascribed to their intrinsic capability to adopt multiple oxidation states and form a diversity of structural forms with ligands (i.e., reagents, intermediates, products) due to the specific properties of the outermost d orbitals, which are incompletely filled with electrons and can easily give and take electrons.6 Thus, the numerous local minima on the potential energy surfaces of the complexes open the possibility to convert one structural organization to another through a lowbarrier pathway. The variation of auxiliary ligands may be helpful to this end in tuning the structure, equilibrium, and dynamics of the “super complex” with regard to the target product and thereby facilitate the design of more effective catalytic systems.7,8 In this regard, compounds with electron-donor heteroatoms and/or unsaturated bonds are of particular interest as ligands due to the presence of lone pairs (LPs) and π bonds, which provide binding sites to transition metals.9−13 There are many such examples in the literature, e.g., ligands that incorporate CC,9−13 CN,7,14−18 CO,19 CP,20−27 PN,28,29 and PP30,31 bonds. Generally, a diversity of bonding motifs can be realized in such systems due to specific interactions: simple η 1 coordination with the LP of a heteroatom and η2 coordination (π and σ complexes) with unsaturated bonds. In the case of cyclic diene systems, particularly in heteroaromatic ligands, a © XXXX American Chemical Society

number of bonding modes essentially arise and therefore the structure and dynamics of such complexes become more complicated, particularly in solution.26,32 Among the aforementioned unsaturated bonds, the PC bond is of particular interest: typically the phosphorus LP σ orbital and the π orbital of the P−C bond are HOMOs of nearly equal energies, and the level of the π* LUMO is low.22−25 Therefore, it might be expected that phosphaalkenes in complexes with transition metals will exhibit either η1 coordination via the phosphorus LP or η2 coordination of the Dewar−Chatt−Duncanson (DCD) type via the P−C bond.33,34 Significant progress has been made to this end in the coordination chemistry of the phosphorus analogues of usual unsaturated organic ligands that demonstrate abundant coordination chemistries partially due to the phosphorus LP. For example, there are η1- through η6-coordinated, phosphorus-substituted ligands.35−38 In this context, 1-alkyl-1,2-diphosphole five-membered cycles are of particular interest because many of the aforementioned particularities may be well presented (Scheme 1). These systems can, in principle, be considered as eightelectron-donor ligands (two phosphorus LPs and two π bonds). Therefore, several η2-coordinated complexes with transition metals might be expected. Additionally, the η1 or metal-bridged types (μ) of complexes may also have an effect due to soft donating phosphorus LPs. The situation becomes complicated due to the vicinal position of these LPs that may operate simultaneously and/or not independently. Therefore, Received: May 16, 2018

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DOI: 10.1021/acs.organomet.8b00319 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

DPFGNOE45 techniques were used to measure NOEs. Related 1D/2D NMR spectra are given in the Supporting Information. The chemical structure of the title complexes could be established almost “directly” through various internuclear NMR connectivities. First, the (dtbpe)Ni and 1-alkyl-1,2diphosphole moieties were revealed, and then the 31P−31P COSY correlations allow “linking” two fragments into a single whole. While the assignment of the dtbpe fragment nuclei in the 1H, 13C ,and 31P spectra can be done easily, the assignment of the 1-alkyl-1,2-diphosphole fragments, particularly their heterocyclic nuclei, is not straightforward. To that end, the NOEs between the P1−R substituent and vicinal aromatic protons are particularly helpful in discriminating three aromatic systems step by step and then assigning corresponding heterocyclic carbons. On the whole, most proton, carbon, and phosphorus chemical shifts (CSs) are in the typical range of values. The exception is P2, which resonates at a field essentially higher than would be expected if simple Ni(0) η1- or η2-π complexes were formed (e.g. the 13P NMR spectrum of 5 is shown in Figure 1b while, for comparison, the free ligand’s (1) spectrum is also given in Figure 1a). These deviations are even more pronounced for complexes with an i-Pr group at P1 (Table 1). In general, essentially similar 31P spectral patterns are observed as wekk for other Ni complexes with para-substituted aromatic systems (Table 1). It is also worth mentioning that at room temperature halves of the dtbpe moiety are equivalent in terms of NMR, implying fast (on the NMR time scale) interchange of these fragments. Additionally, a detailed analysis of NMR spectra at room temperature shows that there are notable line broadenings on certain nuclei of the dtbpe fragment. For example, the line width of P3/P4 (Δν1/2 = 12 Hz) is approximately 2 times larger than that of P1 (Δν1/2 = 6 Hz). This is a clear indication that an exchange process may take place in these systems and that its barrier may be high enough to be frozen on the NMR time scale at moderately lower temperatures. To get an idea about a presumable process, we carried out low-temperature NMR experiments with the title complexes. At decreasing temperature the most spectacular changes were observed for 31P spectra and, to a lesser extent, for the heterocyclic carbons. First, upon a temperature decrease the P2 signal shifts dramatically to high field (e.g., for 5 in Figure 1c). Additionally, for P3/P4 phosphorus coalescence was observed in 31P NMR spectra at approximately 233 K (Figure 1c), and then two signals with different multiplicities were seen for these phosphorus atoms at lower temperature. It is important that the multiplicity of P1 is also modified. This indicates that one process involving a P3/P4 exchange became slow and that these phosphorus atoms are nonequivalent and have different spin−spin couplings (SSCs) with P1/P2 phosphorus. A further decrease of temperature to 193 K produces only a high-field shift of P2, but other phosphorus signals are not significantly changed (Figure 1d). These features are even more pronounced for compounds bearing i-Pr at P1 (6−8) (Table 1). The para substitution has only a slight influence on the 31P spectra and their temperature evolution (Table 1). It is important that 1-alkyl-1,2-diphosphole carbon signals also change notably with the temperature, indicating that some processes influence their CSs as well. These changes are differently affected by the temperature decrease: while the C2 carbon’s CS was modified only slightly, C1 and C3 are shifted by approximately 2−4 ppm to high fields (Table 1).

Scheme 1. Binding Modes of 1,2-Diphospholes to Metal Centers

the overall balance of the binding modes and their preference is not clear. To get insight into the structural diversity of such complexes, several 1-alkyl-1,2-diphospha-3,4,5-triarylcyclopenta-2,4-diene (1-alkyl-1,2-diphosphole) ligands were synthesized and their nickel complexes based on bulky 1,2-bis(di-tertbutylphosphino)ethane (dtbpe) were prepared. Their solution structures and dynamics were investigated by DNMR experiments and quantum chemical methods. Here we report our results on the synthesis, structure, and electronic properties of the title complexes and the key factors that control their behavior in solution.

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RESULTS Synthesis. The nickel ethylene complex [Ni(C2H4)(dtbpe)] was prepared according to the literature.39 1-Alkyl1,2-diphospholes 1−4 were prepared by alkylation40 of the corresponding 3,4,5-triaryl-1,2-diphosphacyclopentadienides (aryl = Ph,41 p-ClC6H4,42 p-FC6H442). Syntheses of complexes 5−8 (Scheme 2) were carried out in situ directly in an NMR Scheme 2

tube because of their extreme sensitivity to oxygen and moisture. An NMR tube was charged with 1.05 equiv of [Ni(C2H4)(dtbpe)] (0.09−0.11 mmol) and 1 equiv of 1-alkyl1,2-diphosphole (0.08−0.10 mmol). After addition of deuterated solvent (0.6 mL), the NMR tube was sealed. Structure, Dynamics, and DNMR Data. Complete structure elucidation of the title compounds was accomplished by a variety of 1D/2D correlation NMR experiments (1H−1H COSY, 1H−13C HSQC, 1H−13C/1H−31P HMBC).43,44 1D B


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Figure 1. 31P NMR spectra of 1-ethyl-1,2-diphosphole 1 (a) and complex 5 in toluene-d8 at different temperatures (b−d) (■, tBu2P(O)CH2CH2PtBu2; ▲, tBu2P(O)CH2CH2P(O)tBu2).

At the same time the η2-type coordination to the C1−C2 (SP5) side and the η1 complex at P1 (TP-2) do not correspond to energy minima. The P1−R group prefers (by ca. 1−3 kcal/ mol) to orient with its bulkier site inward toward the 1,2diphosphole ring rather than toward another (Table1). On first sight, some of these forms should be essentially different in electronic structure, which may be specifically reflected in their NMR CSs, of the heterocyclic ring nuclei in particular. To check this hypothesis, GIAO CS calculations were carried out for optimized structures (Table 1). Results of calculations for the title complexes demonstrated that, indeed, the CSs of nuclei in the 1-alkyl-1,2-diphosphole five-membered cycle are dramatically varied in these isomers (Table 1; e.g. for 5 see Figure 2). A particularly strong difference is predicted for P2: its CS varies from an extremely low field (ca. 220 ppm in the TP-1 form) up to a high field (ca. −127 ppm in the SP-1 form) (Figure 2). Moreover, 13C CSs of C1−C3 carbons are also expected to differ significantly in these forms (Table 1). In general, the analysis of theoretical CSs reveals that the observed NMR parameters, P2 CS in particular, cannot be ascribed to one form only and can only be explained if there is an exchange between at least two forms with close populations. These isomers can be divided into three groups according to the CS of P2: (i) in the TP-1 isomer its value is predicted to be at a very low field and close to a value in free ligand (ca. 220 ppm, Figure 2); (ii) within the second group (DT, SP-2, and SP-3) P2 resonates at a moderately low field (5−90 ppm); (iii) finally, there is the SP-1 isomer, in which P2 is expected to resonate at a high field (ca. −127 ppm, Figure 2). The first conclusion is that the only notable effect of the SP-1 isomer with an inherently strong high-field shift for the P2 explains the experimentally observed high-field shift for this phosphorus. Thus, one form is the SP-1 isomer, while the second form should have CSs (31P and 13C) such that, when they are averaged with the values for the SP-1 form, they are close to the experimental values.

The observed line-shape evolution indicates that there are at least two processes: the “slow” process is rotation46 of the (dtbpe)Ni moiety that results in the mutual exchange of dtbpe halves; the “fast” process is an exchange between at least two forms in which the magnetic environment of heterocyclic nuclei, particularly P2, differs essentially. Moreover, these indications are somehow stronger for i-Pr-bearing (5−8) compounds. Unfortunately, no conclusion about the fine structure of the title complexes in solution and intramolecular dynamics can be derived from experimental data solely due to the fact that a slow exchange regime on the NMR time scale cannot be achieved because of the low barrier of the second process. Therefore, to analyze possible structures and their NMR parameters in detail, quantum chemical methods were invoked. Results of DFT Calculations. In principle, there can be seven isomeric forms due to the different binding modes of (dtbpe)Ni to the 1-alkyl-1,2-diphosphole ring (Scheme 3). The first five structures represent the η2 coordination of the (dtbpe)Ni with the heterocycle (square-planar forms, SP-1− SP-5), and there are two with the η1 mode (trigonal-planar forms, TP-1 and TP-2). Additional conformational diversity may arise due to P1 substituent rotation. According to the DFT optimization some of these forms correspond to minima on a potential energy surface (PES) (Table 1). Moreover, optimization for the first isomer (η2 coordination to the P2−P1 side) starting from different orientations of the (dtbpe)Ni plane relative to the 1-alkyl1,2-diphosphole units reveals two energy minimums which formally correspond to the η2 mode (e.g., for 5 see Figure 2) with roughly square planar (SP-1) and distorted tetragonal (DT) coordination of the Ni. According to the calculations the SP-1 and DT forms are close in energy (the energy gap is less than 1.2 kcal), while the η1 type at P2 (TP-1) and the η2 type at P2−C1 (SP-2) isomers are notably higher (>5 kcal/mol). The η2-type complex bindings to the P1−C3 (SP-3) and C2−C3 (SP-4) sides are essentially higher in energy (>24 kcal/mol). C


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Organometallics Table 1. Key Experimental and Calculated NMR CSs (ppm) of 1−8 for the Main forms (with Energies)

a

The relative computed energies (in kcal/mol) of isomers. bChloroform-d. cToluene-d8. dSee Scheme 3. eTHF-d8. fHexane-d14.

Scheme 3

From the isomers outlined above (Figure 2), the TP-1 complex with an extremely low field shift for the P2 (220.4

ppm) can be excluded from consideration because of its strong deviation from the experimental value that cannot be D


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Figure 2. Schematic representation of the main isomers of the title complexes with the corresponding calculated and experimental 31P CSs for 5.

Figure 3. Schematic representation of the main isomers of the title complexes with the corresponding calculated and experimental 13C CSs for 5.

According to calculations the DT isomer is slightly lower in energy than the SP-1 form. Some minor discrepancy in isomeric preference between the experiment and the results of calculations might be due to solvent effects. To check this hypothesis, calculations with inclusion of the solvent effects (PCM model)47−49 were also run. According to these calculations, indeed, the energy gap drops in CHCl3 (0.5 kcal/mol) and vanishes in DMSO. Thus, the solvent may bias the equilibrium in favor of the more polar SP-1 planar form. Additional NMR experiments in solvents with lower (hexane) and higher (THF) polarity supported this hypothesis as well. Namely, while P1, P3, and P4 phosphorus atoms in 31P NMR spectra of compound 6 resonate at approximately the same fields as in the toluene solution (Table 1), the P2 signal is shifted to high (by ca. 8 ppm) or to low (by ca. 20−25 ppm) field in THF or hexane, respectively (Table 1). To further support the above structural hypothesis, PES calculations were carried out for the rotation of the (dtbpe)Ni moiety that allows conversion of one isomer into another and superimposition of the form where the (dtbpe)Ni nuclei are interchanged. These calculations demonstrated that indeed the energy barrier between these two forms (SP-1 and DT) is

compensated by the value for the SP-1 isomer. Some of the other isomers can be excluded from consideration from the 13C data (Figure 3). Namely, in the SP-2−SP-4 isomers strong high-field shifts for the C3 or C1 carbons (up to 85.7 ppm, Table 1) are expected, being essentially higher than those observed experimentally, and this cannot be explained even by the equilibrium with another isomer with low-field shifts for these carbons. Finally, although 13C CSs in the SP-1 and DT isomers differ notably from experimental values, the agreement with experiment will be observed if there is a fast exchange between them and they have close populations. In this case exchange-averaged values of 13C CSs are in reasonable agreement with experimental data. Thus, square-planar (SP-1) and distorted-tetragonal (DT) nickel complexes with η2-type coordination of 1-alkyl-1,2diphospholes seem to be the two major forms in solution. If at room temperature the populations of these forms are close, then the P2 CS has an intermediate value. A decrease of the temperature biases the equilibrium in favor of the SP-1 form, and as a result the P2 resonance is shifted to a higher field (Table 1). In the same way 13C CSs and their evolution with temperature are explained (Table 1). E


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bond now and two electron pairs were found between C1 and C2. P2−Ni and P1−Ni localized electron pairs are mainly donated by phosphorus atoms and are formed presumably at the expense of the P2−C1 double-bond character. Thus, at first sight this structure resembles a phosphametallacycle. The situation for the DT isomer with distorted-tetragonal geometry at Ni is different. Although the NBO analysis revealed two localized electron-pair orbitals between the ligand’s phosphorus and Ni atom (Figure 4c), the P2−Ni bond has low electron density (1.28 e), mainly being donated by the Ni atom (63%) in this case. For the diphosphole moiety, similar to the case for a free ligand, two double bonds were found. Thus, the nature of the bonds in the 1-ethyl-1,2diphosphole and its DT isomer are similar, which suggests weaker interactions between the (dmpe)Ni and the heterocyclic ligand. Some additional information on the bonding character can be found from an analysis of frontier orbitals. Attention was focused basically on the HOMO and HOMO-1, as only these orbitals were invoked in the DCD model33,34,51,52 to explain the interaction of the transition metal with unsaturated systems. For comparison, orbitals for the reference system, e.g., the Ni complex of C6H6, were also analyzed. Optimization for [Ni(η2-C6H6)(dmpe)] complexes produced the wellknown structure of the π complex.10 Visualization of frontier orbitals demonstrates that the HOMO-1 combines the ligand’s conjugated p orbitals of σ symmetry with the d orbital of Ni. At the same time the HOMO orbital seems to be responsible for the back-donation from the d orbital of Ni with π symmetry to the p orbitals of the ligand (Figure 5a).

relatively low (ca. 2.5 kcal/mol, Figure S2) while the barrier to full rotation is very high (9.6 kcal/mol, Figure S2) to freeze rotation on the NMR time scale that was observed experimentally (ΔG⧧233 = 9.8 kcal/mol, Table S2). To summarize, these complexes, in solution, are in an equilibrium of two isomeric forms. Both forms formally represent the η2-P2−P1 coordination mode of the 1-alkyl-1,2diphosphole, while the mutual orientation of the phosphorus cycle and dtbpe plane is different: in the SP-1 isomer the (P2− P1)/(P3−P4) dihedral angle is ca. 0°, and in the second form (DT) this interplanar angle is ca. 50−55°. At first glance, there is not much difference in the structures of these forms. The sharp distinction is only in the orientation of the (dtbpe)Ni plane, but it has a dramatic effect on their NMR parameters, which presumably reflects a strong difference in electronic structure. Thus, a question arises: what is the key difference between these forms that has such dramatic effects on, for example, P2 CS? Intuitively, the difference in these two forms seems to be due to changes concerning the hybridization of this phosphorus and/or its vicinity: e.g., exchange between σ2λ3 versus σ3λ3 P2 phosphorus. In the DT form, P2 seems to have a σ2λ3 character with a formal double bond with a vicinal carbon, and therefore a low-field resonance is expected for it. However, in the SP-1 form P2 may be converted into σ3λ3 type and therefore would resonate at a high field. In order to obtain support for this hypothesis and gain insight into the electronic factors governing the coordination of the 1-alkyl-1,2-diphosphole, some theoretical analysis has been undertaken. Molecular Orbital Analysis.50 To reveal chemical bonding features, a natural bond orbital (NBO) analysis was carried out. For simplification, calculations were carried out on [Ni(dmpe)] as a model for the [Ni(dtbpe)] fragment and with methyl groups replacing the Ph groups in the 1-ethyl-1,2diphosphole moiety. To start with, we analyzed the free 1ethyl-1,2-diphosphole model. These calculations generated the idealized Lewis structure in which there are two double bonds in the cycle and two phosphorus atoms have lone pairs (Figure 4a). It is interesting to note that there is a double bond between the P2−C1 atoms with conjugated p orbitals.

Figure 5. Highest occupied orbitals of the [Ni(η2-C6H6)(dmpe)] complex (a) and the 1-ethyl-1,2-diphosphole (model) complex in the two isomeric forms SP-1 (b) and DT (c).

An interesting situation occurred in the case of the 1-ethyl1,2-diphosphole complex. In the SP-1 isomer, the HOMO and HOMO-1 orbitals (Figure 5b) look very similar (in the bonding region) to those observed for the (dmpe)Ni complex of C6H6: the HOMO-1 mainly consists of the ligand’s conjugated p orbitals of σ symmetry with the d orbital of Ni, while the HOMO seems to be the π* orbital of the ligand interacting with the d orbital of Ni with π symmetry. It is worth noting that there is a very strong overlapping of orbitals in the resulting HOMO-1 in comparison to the similar C6H6 complex, presumably thanks to π system disruption between P2 and C1 atoms. Thus, this is also an indirect indication of the π complex formation.

Figure 4. Principal data of NBO analysis for the free ligand model (a) and its [Ni(dmpe)] complex (SP-1 (b) and DT (c) isomers). Populations of some localized electron-pair orbitals and the weight of each atom in some of it (in %) are shown.

Two main isomers (SP-1 and DT) of the model complex were then analyzed. For the isomer with square-planar geometry at Ni, the NBO search finds two localized electron-pair orbitals between both phosphorus and Ni atoms (Figure 4b) with high electron density (ca. 1.7−1.8 e). It is important that the P2−C1 bond is identified as a single F


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Organometallics A different picture is observed for the DT form. First, the HOMO-1 is basically due to interactions of the Ni d orbital and the LPs of both phosphorus atoms (Figure 5c). Furthermore, in contrast to the (dmpe)Ni complex of C6H6 the HOMO is somewhat delocalized in this case. Thus, the main interaction is the σ coordination of Ni with both LPs of two phosphorus atoms of 1-ethyl-1,2-diphosphole, which acts as a chelating ligand.

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DISCUSSION Diversity of Bonding Motifs: σ−π Rearrangement of the 1-Alkyl-1,2-diphosphole Ligand. The majority of cases in the literature concerning transition-metal complexes with coordination to the LP of heteroatom (η1 type) or to the π system (η2, η3, η4 type etc.) of a heterocycle have been described.26,30−32,35−38 When both types of binding modes to the same unsaturated bond are presented, e.g., the XC unit (X = N, P), there is an equilibrium between both coordination types, which can be biased toward either, depending on the ligands.20,21,24,25,28,52 In some cases, η2 bonding may lead to a metallacycle with the creation of two covalent bonds with the atoms at the expense of a double-bond disruption between them.7,11,13,15−17,29 In the case comprising several π systems in the ligand, an exchange between complexes with coordination to different π sites may also occur.9,10,12,17,26,32 In the 1-alkyl-1,2-diphospholes all of these potential coordination modes are well presented, and therefore a diversity of structural forms with equilibria among them can be realized. The situation becomes more intriguing with the presence of two LPs in the same ligand because those LPs belong to directly bonded phosphorus atoms. Analysis of the frontier orbitals of the 1-alkyl-1,2-diphosphole ligand demonstrates that its π system mainly constitutes the HOMO (Figure 6a). Therefore, π bonds of the cycle (C

Figure 7. Schematic representation of the dominant orbital interactions in η1,η1-type complexes (a) and π-type complexes (b).

Figure 6. Highest occupied orbitals (a−c) of the 1-alkyl-1,2diphosphole model ligand with energies for 1 and schematic representation of the possible reaction sides (d).

mentioning that tetragonal coordination is typical for Ni(0) complexes that are formed when the metal bonds to the LP’s phosphorus. Distortion from the ideal tetragonal geometry might be due to steric hindrance or due to specific orientations (bite angles) of LPs in a ligand such as 1-alkyl-1,2-diphosphole. The situation of the bonding mode that occurred in the SP-1 isomer is very unusual and ambiguous. Although there is no pronounced π system that can be associated with the P2−P1 bond in a free ligand, in the complex it is activated and operates as a relay to transmit electron density from a P2C1 bond to the d orbitals of the transition metal (Figure 7b, top). Then, back-donation from the metal d orbital to the P2−P1 bond π* system may occur by analogy with “classical” aromatic complexes. Therefore, in this case there is the possibility of π bonding to the P2−P1 side (B, Figure 7). However, if we consider the fact that the bonding in zero- and divalent transition-metal complexes differs only in magnitude but not in mode,51,52 the resonance structure C should not be neglected. Moreover, a number of their key parameters (first, there are two highly populated bonds localized by NBO analysis; second, these bonds are longer in the SP-1 isomer than in the DT; third, the P2 signal in the SP-1 isomer is very close to those of P(III)-containing three-membered rings,53 which corresponds to the resonance structure C) are consistent with being determined primarily by their σ effects. These results suggest the dominance of a resonance “metallacyclic” structure (C, Figure 7). Thus, the formal oxidation state of the Ni in this isomer should be assigned as 2+.

P, CC) must be prone to the donation of electrons, and in doing so they tend to bond to transition metals. However, these sides are sterically hindered due to bulky groups of the 1alkyl-1,2-diphosphole substituents (aromatic rings and P1−R group; see Figure 6d). Therefore, only the P2−P1 side is relatively easily accessible for transition metals. Two phosphorus LPs on this side can be involved in coordination. These LPs belong to HOMO-1 and HOMO-2, which are only slightly lower in energy than the HOMO (Figure 6b,c). Therefore, when the PC (CC) π system is not accessible, a transition-metal d orbital of corresponding symmetry may effectively interact with both LPs of the ligand (A, Figure 7a). Thus, there are two donor−acceptor interactions between the metal and the ligand. Although this isomer is formally an η2 complex, in fact the 1-alkyl-1,2diphosphole serves as a chelating ligand, and consequently this isomer would be correctly defined as η1,η1 type. It is worth

CONCLUSIONS In solution [Ni(1-alkyl-1,2-diphosphole)(dtbpe)] complexes are in equilibrium of two P−P side-on η2-coordinated 1-alkyl1,2-diphosphole isomers. In the lower-energy isomer, unusual η2 bonding occurred at the P2−P1 side, presumably at the expense of a vicinal P2−C1 π-bond. The latter leads to a change in the hybridization of the P2 from σ2λ3 to σ3λ3 and consequently to a high-field shift of its resonance in the 31P NMR spectra. This form with square-planar Ni can be characterized as a phosphametallacycle with two localized bonds between Ni and two phosphorus atoms of the 1-alkyl1,2-diphosphole ligand. In the second isomer (with a distortedtetragonal Ni coordination plane), Ni bonds to the ligand through two σ interactions between the metal and the LPs of the ligand. Although formally this isomer is a η2 complex, in fact the 1-alkyl-1,2-diphosphole serves as a chelating ligand. Accordingly, this isomer should be defined as the type η1,η1−.

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Hz, i-Ar1), 143.6 (d, JCP = 11 Hz, i-Ar3), 141.6 (d, JCP = 13 Hz, i-Ar2), 131.7 (s, o-Ar2), 131.1 (m, C1), 130.1 (d, JCP = 11 Hz, o-Ar1), 128.8 (m, o-Ar3), 127.5−127.0 (s, m-Ar1, s, m-Ar2 and s, m-Ar3), 125.0 (s, pAr2), 122.0−121.5 (s, p-Ar1 and s, p-Ar3), 117.9 (m, C3), 34.6 (m, (tBu)2), 34.3 (m, (t-Bu)2), 30.6 (s, (t-Bu)2), 30.1 (s, (t-Bu)2), 25.9 (m, CH-(CH3)2), 22.7−22.0 (m, (CH2-P)2 and CH-(CH3)2). 31P{1H} NMR (toluene-d8, 242.9 MHz, 193 K): δP 84.5 (d, 2JPP = 33 Hz, P4), 74.1 (dd, 2JPP = 81 Hz, 2JPP = 33 Hz, P3), 25.8 (dd, 1JPP = 365 Hz, 2JPP = 81 Hz, P1), −105.6 (br d, 1JPP = 365 Hz, P2). [Ni(1-isopropyl-3,4,5-tris(p-fluorophenyl)-1,2-diphosphole)(dtbpe)] (7). 1H NMR (toluene-d8, 600.1 MHz, 303 K): δH 7.15 (m, 2H, o-Ar3), 7.09 (m, 2H, o-Ar1), 6.96 (m, 2H, o-Ar2), 6.94 (t, J = 8.6 Hz, 2H, m-Ar3), 6.67 (m, 4H, m-Ar1 and m-Ar2), 2.44 (m, 1H, CH(CH3)2), 1.42 (m, 2H, CH2-P), 1.24 (m, 2H, CH2-P), 1.20−1.00 (overlapped, 39H, (t-Bu)4 and CH-CH3), 0.98 (m, 3H, CH-CH3). 31 1 P{ H} NMR (toluene-d8, 242.9 MHz, 293 K): δP 81.9 (d, 2JPP = 36 Hz, P3,4), 17.9 (dt, 1JPP = 344 Hz, 2JPP = 36 Hz, 2JPP = 36 Hz, P1), −57.3 (d, 1JPP = 344 Hz, P2). 13C{1H} NMR (toluene-d8, 150.9 MHz, 303 K): δC 161.2 (d, JCF = 244 Hz, p-Ar2), 159.4 (d, JCF = 241 Hz, pAr1), 159.2 (d, JCF = 241 Hz, p-Ar3), 149.6 (d, JCP = 18 Hz, C2), 143.5 (d, JCP = 24 Hz, i-Ar1), 139.4 (d, JCP = 11 Hz, i-Ar3), 136.9 (d, JCP = 8 Hz, i-Ar2), 132.9 (d, JCF = 7 Hz, o-Ar2), 130.9 (m, o-Ar1), 129.7 (m, oAr3), 128.4 (m, C1), 115.1 (m, C1), 114.4 (d, JCF = 21 Hz, m-Ar2), 114.3 (d, JCF = 21 Hz, m-Ar3), 114.0 (d, JCF = 21 Hz, m-Ar1), 34.7 (t, JCP = 4 Hz, t-Bu), 34.0 (t, JCP = 6 Hz, (t-Bu)3), 30.6 (m, t-Bu), 30.4 (t, JCP = 3 Hz (t-Bu)3), 25.7 (m, CH-(CH3)2), 22.5−22.0 (m, CH(CH3)2, (CH2-P)2). 31P{1H} NMR (toluene-d8, 242.9 MHz, 203 K): δP 84.9 (d, 2JPP = 31 Hz, P4), 74.7 (dd, 2JPP = 76 Hz, 2JPP = 31 Hz, P3), 24.3 (dd, 1JPP = 358 Hz, 2JPP = 76 Hz, P1), −99.6 (br d, 1JPP = 358 Hz, P2). [Ni(1-isopropyl-3,4,5-tris(p-chlorophenyl)-1,2-diphosphole)(dtbpe)] (8). 1H NMR (toluene-d8, 600.1 MHz, 303 K): δH 7.11−7.02 (m, 6H, o-Ar3, o-Ar1 and o-Ar2), 6.99 (overlapped, 2H, m-Ar3), 6.96− 6.89 (overlapped, 4H, m-Ar1 and m-Ar2), 2.58 (m, 1H, CH-(CH3)2), 1.37 (m, 2H, CH2-P), 1.21 (m, 2H, CH2-P), 1.18−1.00 (overlapped, 39H, (t-Bu)4 and CH-CH3), 0.98 (m, 3H, CH-CH3). 31P{1H} NMR (toluene-d8, 242.9 MHz, 293 K): δP 82.0 (d, 2JPP = 25 Hz, P3,4), 20.2 (dt, 1JPP = 347 Hz, 2JPP = 25 Hz, 2JPP = 25 Hz, P2), −70.6 (d, 1JPP = 347 Hz, P1). 13C{1H} NMR (toluene-d8, 150.9 MHz, 303 K): δC 149.4 (d, JCP = 18 Hz, C2), 145.9 (d, JCP = 22 Hz, i-Ar1), 141.4 (d, JCP = 11 Hz, i-Ar3), 139.5 (d, JCP = 11 Hz, i-Ar2), 132.7 (s, o-Ar2), 131.2 (d, JCP = 11 Hz, o-Ar1), 129.3 (d, JCP = 7 Hz, o-Ar3), 128.1−127.4 (s, m-Ar1, s, m-Ar2 and s, m-Ar3), 125.5 (m, C1), 112.9 (m, C3), 34.6 (m, (t-Bu)2), 34.0 (m, (t-Bu)2), 30.4 (s, t-Bu), 30.3 (s, (t-Bu)2), 30.1 (s, tBu), 25.0 (m, CH-(CH3)2), 22.7 (s, CH-CH3), 22.4−22.0 (m, (CH2P)2 and CH-CH3). 31P{1H} NMR (toluene-d8, 242.9 MHz, 193 K): δP 85.6 (d, 2JPP = 30 Hz, P4), 73.7 (dd, 2JPP = 87 Hz, 2JPP = 30 Hz, P3), 26.9 (dd, 1JPP = 365 Hz, 2JPP = 87 Hz, P2), −117.7 (br d, 1JPP = 365 Hz, P1).

EXPERIMENTAL SECTION

NMR Spectroscopy. All NMR experiments were performed with 600 and 400 MHz (600 and 400 MHz for 1H NMR; 150 and 100 MHz for 13C NMR; 243 and 162 MHz for 31P NMR) spectrometers equipped with a 5 mm diameter gradient inverse broad-band probe head and a pulsed gradient unit capable of producing magnetic field pulse gradients in the z direction of 53.5 G cm−1. 13C{1H,31P} NMR experiments were performed with 6a 00 MHz spectrometer equipped with 5 mm diameter triple-resonance probes (TXI). Chemical shifts (δ in ppm) are referenced to the solvent toluene-d8 (δ 2.09 ppm for 1 H and 21.3 ppm for 13C NMR) or to external H3PO4 (0.0 ppm for 31 P NMR). DNMR experiments were carried out using a Bruker BVT3000 variable-temperature unit (with BTO2000, accuracy ±0.1 K calibrated using a methanol reference). The samples were allowed to equilibrate for 15 min at each temperature. Line-shape analysis of signals broadened due to chemical exchange was carried out by a DNMR module of the Bruker TopSpin 2.1 software package. Activation parameters were calculated according to the Eyring equation.54 Calculations. The quantum chemical calculations were performed using the Gaussian 0355 software package. Full geometry optimizations have been carried out within the framework of the DFT (PBE1PBE) method using 6-31G+(d) basis sets. 1H, 13C, and 31P NMR chemical shifts were calculated by the GIAO method at the PBE1PBE/6-311G(2d,2p) level. 1H and 13C chemical shifts were referred to TMS chemical shifts, which were calculated under the same conditions. 31P chemical shifts were referred to H3PO4, and a linear scaling procedure was applied.56 To take into account solvent effects, energies were then computed using a PCM model49 at the same level of theory as for the other steps. The PESs were obtained using a potential-energy scan for modification of some dihedral angle with 10° steps at the PBE1PBE/6-31G+(d) level of theory. The natural bonding orbital (NBO) calculations of the model compound were carried out by using the NBO 3.157 program as implemented in the Gaussian 03 package at the PBE1PBE/6-31G+(d) level. NMR Data. [Ni(1-ethyl-3,4,5-triphenyl-1,2-diphosphole)(dtbpe)] (5). 1H NMR (toluene-d8, 399.9 MHz, 303 K): δH 7.42 (d, 2H, J = 7.6 Hz, o-Ar3), 7.37 (d, 2H, J = 7.6 Hz, o-Ar1), 7.26 (d, 2H, J = 6.0 Hz, oAr2), 7.09 (t, 2H, J = 7.4 Hz, m-Ar3), 7.06−6.99 (overlapped, 5H, pAr2, m-Ar2 and m-Ar1), 6.91 (m, 2H, p-Ar3 and p-Ar1), 2.66 (m, 1H, CH2-CH3), 1.58−1.42 (overlapped, 3H, CH2-CH3 and CH2-P), 1.25 (overlapped, 2H, CH2-P), 1.21−1.02 (overlapped, 36H, (t-Bu)4), 0.93 (dt, 1H, 3JHH = 7.2 Hz, 3JHP = 15.5 Hz, CH2−CH3). 31P{1H} NMR (toluene-d8, 242.9 MHz, 293 K): δP 84.4 (d, 2JPP = 42 Hz, P3,4), 5.9 (dt, 1JPP = 342 Hz, 2JPP = 42 Hz, 2JPP = 42 Hz, P1), −37.8 (d, 1JPP = 342 Hz, P2). 13C{1H} NMR (toluene-d8, 100.6 MHz, 303 K): δC 150.7 (d, JCP = 18 Hz, C2), 148.8 (d, JCP = 22 Hz, i-Ar1), 143.8 (d, JCP = 11 Hz, i-Ar3), 142.9 (d, JCP = 13 Hz, i-Ar2), 140.1 (m, C1), 132.9 (s, i-Ar2), 131.1 (d, JCP = 13 Hz, o-Ar1), 130.3 (d, JCP = 9 Hz, o-Ar3), 128.5−128.0 (s, m-Ar1, s, m-Ar2 and s, m-Ar3), 125.9 (s, p-Ar2), 123.2−123.0 (s, p-Ar1 and s, p-Ar3), 122.1 (m, C3), 36.2 (m, (t-Bu)2), 34.9−34.6 (overlapped, (t-Bu)2), 32.1 (t, JCP = 3 Hz, t-Bu), 31.6 (t, JCP = 3 Hz, t-Bu), 30.8 (s, t-Bu), 27.3 (s, t-Bu), 23.6 (m, (CH2-P)2), 19.5 (q, JCP = 18 Hz, CH2-CH3), 14.5 (dd, 2JCP = 8 Hz, 3JCP = 3 Hz, CH2−CH3. 31P{1H} NMR (toluene-d8, 242.9 MHz, 183 K): δP 87.0 (d, 2JPP = 42 Hz, P4), 82.1 (dd, 2JPP = 102 Hz, 2JPP = 42 Hz, P3), 5.8 (dd, 1JPP = 352 Hz, 2JPP = 102 Hz, P1), −53.2 (br d, 1JPP = 352 Hz, P2). [Ni(1-isopropyl-3,4,5-triphenyl-1,2-diphosphole)(dtbpe)] (6). 1H NMR (toluene-d8, 600.1 MHz, 303 K): δH 7.42 (d, 2H, J = 7.2 Hz, oAr3), 7.37 (d, 2H, J = 7.2 Hz, o-Ar1), 7.28 (d, 2H, J = 6.5 Hz, o-Ar2), 7.10 (t, 2H, J = 8 Hz, m-Ar3), 7.06−6.97 (overlapped, 5H, p-Ar2, mAr2 and m-Ar1), 6.91 (m, 2H, p-Ar1 and p-Ar3), 2.56 (m, 1H, CH(CH3)2), 1.42 (m, 2H, CH2-P), 1.24 (m, 2H, CH2-P), 1.20−1.15 (m, 21H, (t-Bu)2 and CH-CH3), 1.13 (m, 18H, (t-Bu)2), 1.05 (m, 3H, CH-CH3). 31P{1H} NMR (toluene-d8, 242.9 MHz, 293 K): δP 81.8 (d, 2JPP = 37 Hz, P3,4), 18.3 (dt, 1JPP = 344 Hz, 2JPP = 37 Hz, 2JPP = 37 Hz, P1), −54.6 (d, 1JPP = 344 Hz, P2). 13C{1H} NMR (toluene-d8, 150.9 MHz, 303 K): δC 151.1 (d, JCP = 18 Hz, C2), 147.8 (d, JCP = 22

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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.organomet.8b00319. Calculated energies, the key NMR CS’s of 5−8, and all 1D/2D NMR spectra of the new compounds (5−8) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*S,K.L.: tel, +7 (843) 2731892; fax, +7 (843) 2732253; e-mail, [email protected]. ORCID

Shamil K. Latypov: 0000-0003-4757-6157 Notes

The authors declare no competing financial interest. H


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