Redox Activity, Ligand Protonation, and Variable Coordination Modes

Mar 19, 2018 - groups for the acyl functionality.8 A p-tolyl group was chosen as a substituent that ..... The authors declare no competing financial i...
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Redox Activity, Ligand Protonation, and Variable Coordination Modes of Diimino-Pyrrole Complexes of Palladium Andrew J. McNeece, Mu-Chieh Chang, Alexander S. Filatov, and John S. Anderson* Department of Chemistry, The University of Chicago, Chicago, Illinois 60637, United States S Supporting Information *

ABSTRACT: Ligand-based functionality is a prominent method of increasing the reactivity or stability of metal centers in coordination chemistry. Some of the most successful catalysts use ligand-based redox activity, pendant protons, or hemilability in order to specifically accelerate catalysis. Here we report the diimino-pyrrole ligand Tol,CyDIPyH (Tol,CyDIPy = 2,5bis(N-cyclohexyl-1-(p-tolyl)methanimine)pyrrolide), which exhibits all three of these ligand properties. Metalation of Tol,Cy DIPy to Pd gives the pseudo-square planar complex (Tol,CyDIPy)PdCl, which upon reduction forms a mixture of products, including a Pd(I)−Pd(I) dimer wherein Tol,CyDIPy bridges the dimeric unit. Upon addition of PMe3, the imine arms of (Tol,CyDIPy)PdCl are displaced to yield (Tol,CyDIPy)Pd(PMe3)2Cl, where the Tol,CyDIPy ligand binds in a monodentate fashion. This complex can be reduced to generate a ligandbased radical, as shown by EPR spectroscopy. Finally, (Tol,CyDIPy)PdCl also can be protonated at the imine arm, exhibiting a total of three different coordination modes across this series of complexes. Taken together, these studies show that Tol,CyDIPy exhibits notable flexibility in its coordination and redox chemistry.

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fashion.7 Part of the challenge with these chelates is that they frequently bind in a bidentate manner, where one of the imine arms can rotate away from the metal center along the N−C− C−N dihedral angle. We wanted to modify these ligands by installing larger steric protection around the imine carbon which would disfavor this arm rotation. This strategy has allowed us to synthesize a new diiminopyrrole pincer ligand dubbed Tol,CyDIPyH (Tol,CyDIPy = 2,5-bis(N-cyclohexyl-1-(ptolyl)methanimine)pyrrolide), which can be readily metalated with Pd. These Pd complexes display redox activity, pendant protonation sites, and multiple coordination modes. These features make this ligand attractive for further investigations in stoichiometric and catalytic reactivity.

INTRODUCTION The redistribution of reactivity traditionally associated with metal centers to organic ligands is a powerful method for expanding the scope of coordination chemistry.1 Many approaches in this area have proven effective, most notably ligand-centered redox activity,2 pendant proton donors,3 and coordinative hemilability.4 There have been detailed studies of these myriad properties in a wide variety of systems, but studies that look at the combination of these properties into single scaffolds have been more limited.5 We have been interested in discovering new systems that combine these properties with the goal of examining their catalytic reactivity. Pyrrole-based pincer ligands are potentially attractive targets in this regard, as a fivemembered conjugated ring may enable concerted two-electron chemistry. Furthermore, the arms of the pincer may also provide proton donor or hemilabile properties to the ligand. While pyridine-based pincer ligands have been extensively studied, pyrrole-based systems are comparatively more rare, in some part due to synthetic challenges associated with pyrrole heterocycles. Nevertheless, tridentate pyrrole ligands have received recent attention.6 Within the broader class of pyrrole pincer ligands, diimino pyrrole ligands are an appealing target, as they bear close parallels to the well-studied class of diiminopyridine ligands.2 Notably, diiminopyrrole scaffolds should have an extended π-system capable of redox activity as well as pendant imine groups that could serve as proton relays. While some of these ligands have been investigated, their coordination chemistry has only been explored in a limited © XXXX American Chemical Society

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RESULTS AND DISCUSSION Previously reported methods for the 2,5-diacylation of pyrroles with benzoxathiolium salts were adapted, as the tunability of this methodology allowed us to choose a variety of potential R groups for the acyl functionality.8 A p-tolyl group was chosen as a substituent that would provide sufficient steric protection while still enabling facile subsequent condensation reactivity, and the corresponding diacyl pyrrole was synthesized via extension of the previously reported procedures (Scheme 1).8 Subsequent condensation with cyclohexylamine gave Tol,CyDIPyH in 75% yield. The metalation of Tol,CyDIPyH proceeds Received: March 19, 2018

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DOI: 10.1021/acs.inorgchem.8b00737 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Synthesis of

Tol,Cy

DIPyH and Metalation To Form 1

Scheme 2. Protonation of 1 To Give 2 and Addition of Phosphines To Form 4 and Subsequent Reduction to 5

as protonation sites. Addition of HCl·Et2O to 1 results in conversion to (Tol,CyDIPyH)PdCl2 (2; Scheme 2). Unlike 1, complex 2 shows signals consistent with an asymmetric ligand environment in its 1H NMR spectrum, suggesting that the ligand is no longer tridentate. Single-crystal XRD confirms that the ligand binds to Pd in a κ2 fashion through the pyrrole N and one imine N while two Cl atoms complete the expected squareplanar geometry (Figure 2). The structure shows lengthening of the Npyrrole−Pd bond and shortening of the Nimine−Pd bond to accommodate the new binding mode and allow for the extra chlorine atom bound to Pd (Table 1). Despite the protonation on the ligand arm, rotation around the N−C−C−N dihedral angle is not observed, which supports the efficacy of the steric protection the tolyl group provides in preventing this isomerization. Indeed, the protonated imine arm is still positioned near the Pd center which could be suitable for proton-transfer reactivity. The structure is disordered with each Pd half-occupied in both bidentate binding pockets. Nevertheless, the structure provides sufficient accuracy that bond lengths within the conjugated system can be used to analyze the resonance state of the complex (Table 1). These bond lengths support the resonance structure shown in Scheme 2, as the C8−C9 bond contracts from 1.44 to 1.38 Å and the C10−C11 bond at the pyrrole 3,4-positions contracts from 1.38 to 1.28 Å, consistent with greater double-bond character. This form of tautomeric amine−azafulvene structure is reminiscent of that recently reported in related pyrrole-based ligands.10 Finally, we note that complex 2 can also be accessed in good yield (97%) by the direct reaction of free Tol,CyDIPyH with (COD)PdCl2. The protonation of 2 is also reversible, as reaction with KOtBu results in clean conversion to 1. Complexes 1 and 2 show that Tol,Cy DIPy complexes can react with acid to protonate on the imines of the ligand and that this chelate supports both κ3 and κ2 binding modes. With evidence for at least two coordination modes and ligand protonation, we began to probe the potential redox activity of this system. Several reductions of 1 were attempted, but a mixture of products was invariably obtained. While these mixtures have been difficult to characterize, at least one product obtained by reduction with 1 equiv of sodium naphthalenide is a Pd(I)−Pd(I) dimer (3), as determined by single-crystal XRD and 1H NMR spectroscopy (see the Supporting Information). Complex 3 could not be isolated in large enough quantities for thorough characterization, but the crystal structure exhibits a bridging coordination mode of Tol,CyDIPy and a short Pd−Pd

smoothly via initial deprotonation with KN(SiMe3)2 followed by dropwise addition to a stirred solution of (COD)PdCl2 (COD = 1,5-cyclooctadiene), resulting in (Tol,CyDIPy)PdCl (1) (Scheme 2) as an orange solid in good yield. The 1H NMR spectrum of 1 is consistent with a symmetric diamagnetic product, suggestive of a planar κ3 binding mode for the Tol,Cy DIPy ligand. Single-crystal X-ray diffraction (XRD) of 1 corroborates this assignment and reveals a distorted square planar geometry around Pd, with an Nimine−Pd−Nimine angle of 152° (Figure 1). The Npyrrole−Pd bond length is also very short,

Figure 1. XRD structure of 1, with ellipsoids set to 50% probability. See Table 1 for selected bond lengths.

at 1.87 Å (Table 1), among the shortest reported for N−Pd.9 Both of these structural parameters reflect the strained metallacycle formed by planar chelation in 1 and likely arise from the larger bite angle enforced by the pyrrole backbone. This apparently strained binding mode prompted us to investigate the ability of the imine arms of Tol,CyDIPy to behave Table 1. Bond Lengths (Å) for Complexes 1−4 (Figures 1 and 2)a N2−Pd N1−Pd C8−N1 C10−C11 C8−C9

1

2

3

4

1.879(3) 2.151(2) 1.312(3) 1.391(5) 1.444(3)

2.080(5) 2.250(5) 1.296(7) 1.286(12) 1.383(8)

2.001(3) 2.300(3) 1.292(4) 1.388(6) 1.450(6)

2.030(14) 3.09 1.286(2) 1.392(2) 1.462(2)

a

Note that complexes 1 and 2 have a crystallographic mirror plane containing the pyrrole N and bisecting the C10−C11 bond. B

DOI: 10.1021/acs.inorgchem.8b00737 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. XRD structures of 2 (left), 3 (middle), and 4 (right) with ellipsoids set to 50% probability. Hydrogen atoms in all structures and two cyclohexyl groups in 3 have been omitted for clarity. Selected bond lengths can be found in Table 1.

bond of 2.47 Å, a distance similar to that observed in many examples of Pd(I)−Pd(I) dimers (Figure 2).11 The other imine of the Tol,CyDIPy ligand bridges to the second Pd atom of the dimer with a bond length of 2.049(3) Å, illustrating an additional bridging binding mode for this system. The bond lengths in the Tol,CyDIPy ligands of 3 suggest a pyrrolide structure similar to that of 1, rather than an amine−azafulvene resonance structure similar to 2 (Table 1). Given this observation, the addition of an exogenous ligand was targeted to prevent formation of dimeric species and lead to either monomeric Pd(I) or a ligand-based radical. Addition of 2 equiv of PMe3 to 1 results in clean conversion to (Tol,CyDIPy)Pd(PMe3)2Cl (4) (Scheme 2). The XRD structure of 4 shows a square-planar Pd center where the imine arms of the Tol,CyDIPy have been displaced by two PMe3 ligands such that the PdP2Npyrrole square plane is orthogonal to the conjugated system of the Tol,CyDIPy ligand (Figure 2). The Npyrrole−Pd bond is substantially lengthened in comparison with 1, reflecting the release of the strain from the metallacycle, and the average Nimine−Pd distance is 3.02 Å, suggesting minimal interaction with the Pd center (Table 1). This observation further demonstrates the lability of the imine arm of the ligand, going from κ3 to κ1 upon addition of external ligands. It is still noteworthy that arm rotation is not observed and that the imine arms remain poised near the Pd center. Unlike attempts with 1, reduction of 4 with sodium metal results in the formation of a new species which displays a deep blue color in solution (5; Scheme 2 and Figure 3, bottom). This newly formed species is silent by 1H NMR spectroscopy, suggesting the formation of a paramagnetic product. Analysis of solutions of 5 by EPR spectroscopy shows an intense signal at g = 2.003, attributed to a radical species (Figure 3 top). This signal is strongly suggestive of an organic radical, and simulations of this spectrum support this assignment. The broadness of the main peak obscures any phosphorus or nitrogen coupling, but there is resolved coupling to 105Pd which results in small but significant satellite peaks as shown. The 105 Pd coupling is 55 MHz, which is large for a ligand-based radical Pd(II) species, but still significantly smaller than would be observed in a bona fide Pd(I) species.12 Complex 5 is unstable at room temperature, and its half-life depends on the solvent utilized, varying from 240 min in THF to under 10 min in benzene or petroleum ether (Figure 3 bottom, and the Supporting Information). This observation suggests that the

Figure 3. (top) X-band EPR spectrum of 5 at 15 K, with a simulation shown in blue. Simulation parameters: g = 2.003, APd = 55 MHz, HStrain = 40 MHz. (bottom) UV−vis spectra of the decay of 5 in Et2O at room temperature, with scans taken every 12 min.

coordinating ability of the solvent may play a role in the stability of 5. The decomposition of 5 has precluded structural characterization; therefore, DFT calculations were used to model potential structures. The results of these calculations are given in Table S1, but the most stable structure is the fourC

DOI: 10.1021/acs.inorgchem.8b00737 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry coordinate complex (Tol,CyDIPy)Pd(THF) with Tol,CyDIPy bound in a κ3 fashion similar to that for compound 1 and a THF filling the fourth coordination site on Pd (Scheme 2). The computationally predicted electronic structure shows that the majority of the spin density is ligand centered but that there is a Mulliken spin population of ∼0.35 on Pd. Other structures featuring a bound PMe3, or one vacant coordination site, are also energetically reasonable and have comparable spin localization. These computational results are consistent with our assignment of an organic radical but also suggest that Pd(I) resonance structures are also important contributors to the electronic structure of 5. Even with these calculations, the structure of 5 depicted in Scheme 2 is speculative, and other potential assignments cannot be excluded. It is likely that the exact nature of 5 depends strongly on the potential ligands in solution and that a ligand equilibrium is possible in this species. To test this hypothesis, the possible fluxionality of PMe3 binding was examined by 31P NMR spectroscopy. No 31P signal is observed in freshly prepared solutions of 5, presumably due to paramagnetic broadening via interaction with the radical species (see the Supporting Information). Further addition of excess PMe3 does not result in any observable signal, supporting exchange of the PMe3 ligands in solution. Vacuum transfer of the volatiles from a freshly prepared solution of 5 indicates that all of the PMe3 can be removed, as monitored by 31 P NMR integration (see the Supporting Information). These studies are consistent with a weakly coordinating, potentially fluxionally bound, PMe3 ligand in solution. In sum, we cannot concretely assign the structure of 5, but its properties are consistent with the depiction in Scheme 2. As a final probe of the properties of 5, its redox chemistry was examined electrochemically. Cyclic voltammetry (CV) shows a quasi-reversible redox couple at approximately −0.8 V vs Fc/Fc+ (see the Supporting Information). This is significantly more positive and reversible than the couples observed in both 1 and 4, which show only irreversible features in the CV at substantially more negative potentials (