Synthesis, Characterization, and Variable-Temperature NMR Studies

Article pubs.acs.org/IC

Synthesis, Characterization, and Variable-Temperature NMR Studies of Silver(I) Complexes for Selective Nitrene Transfer Minxue Huang, Joshua R. Corbin, Nicholas S. Dolan,† Charles G. Fry, Anastasiya I. Vinokur, Ilia A. Guzei, and Jennifer M. Schomaker* Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706, United States S Supporting Information *

ABSTRACT: An array of silver complexes supported by nitrogen-donor ligands catalyze the transformation of CC and C−H bonds to valuable C−N bonds via nitrene transfer. The ability to achieve high chemoselectivity and site selectivity in an amination event requires an understanding of both the solid- and solution-state behavior of these catalysts. X-ray structural characterizations were helpful in determining ligand features that promote the formation of monomeric versus dimeric complexes. Variable-temperature 1H and DOSY NMR experiments were especially useful for understanding how the ligand identity influences the nuclearity, coordination number, and fluxional behavior of silver(I) complexes in solution. These insights are valuable for developing improved ligand designs.

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INTRODUCTION

The C−N bond is present in an array of diverse molecules with significant biological and therapeutic value. One effective strategy for streamlining amine synthesis involves the direct introduction of a new C−N bond at a specific CC or C−H group in a substrate through metal-catalyzed nitrene transfer to yield an aziridine or an amine product, respectively.1−8 However, the limited ligand scaffolds that support metalcatalyzed nitrene transfer have resulted in a strong element of substrate control over the chemoselectivity and site selectivity of an amination event.3,4b,5,8 In contrast, our recent work using silver(I) complexes supported by nitrogen-donor ligands enables unprecedented flexibility and tunability in nitrene transfer that proceed under the auspices of catalyst control.9−15 However, the design of second-generation catalysts is challenging because of the lability of Ag−N bonds and their dynamic behavior, which can influence the nuclearity of the complex in solution.16−22 Figure 1 illustrates various nitrogen-donor ligands that are known to support efficient silver-catalyzed nitrogen-atom transfer into CC and C−H bonds.9−15 These complexes give differing chemoselectivity and site selectivity in an amination event, depending on the reaction conditions, the silver/ligand ratio, the identity of the metal counteranion, and the nature of the ligand; however, clear structure−activity relationships for driving the design of improved catalysts are not immediately apparent. In this paper, we describe our efforts to improve the overall understanding of how the ligand identity exerts control over both the solid- and solution-state behavior of diverse silver(I) complexes through the synthesis, X-ray characterization, and NMR studies of catalysts competent for nitrene transfer. © 2017 American Chemical Society

Figure 1. Selection of nitrogen-donor ligands supporting silver(I) complexes capable of metal-catalyzed nitrene transfer.

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RESULTS AND DISCUSSION Bipyridine- and Phenanthroline-Based Ligands for Chemoselective Nitrene Transfer. Changing the silver/ ligand ratio using bidentate nitrogen-donor ligands, including tert-butylbipyridine (tBubipy), 1,10-phenanthroline (phen), and 3,4,7,8-tetramethylphenanthroline (Me4phen), influences the chemoselectivity of silver-catalyzed amination of the homoallylic carbamate 1 (Table 1) to furnish 1a and 1b.10,11 All three ligands gave good yields and selectivities for aziridination (entries 1, 3, and 5); however, the phen-based ligands were significantly more effective for promoting C−H insertion (compare entries 4 and 6 vs 2). To ascertain whether the rigidity of the bidentate ligand impacts the coordination geometry around silver, these complexes were studied by Xray crystallography. Received: March 31, 2017 Published: May 16, 2017 6725

DOI: 10.1021/acs.inorgchem.7b00838 Inorg. Chem. 2017, 56, 6725−6733

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Inorganic Chemistry Table 1. Effect of the Ligand and AgOTf/Ligand Ratio on the Chemoselectivity of Silver-Catalyzed Nitrene Transfer

0.125 mmol of substrate, 10 mol % AgOTf, 12.5 mol % ligand, 2.0 equiv of PhlO, 4 Å MS, 0.05 M, rt. 0.125 mmol of substrate, 10 mol % AgOTf, 30 mol % ligand, 3.5 equiv of PhlO, 4 Å MS, 0.05 M, rt.

Single-crystal X-ray structures of compounds resulting from 1:1 and 1:2 mixtures of AgOTf/Me4phen were obtained (Figure 2). Complex 2 forms a three-dimensional framework in the solid state (Figure 2A,B), where the basic unit of the extended structure is a dimer, (Me4phen)Ag(μ-OTf)2Ag(Me4phen), with two triflates serving as bridging ligands. Each silver center coordinates to two nitrogen atoms of the Me4phen ligand and two bridging oxygen atoms from two different triflates. The geometry at silver is best described as a distorted seesaw with index τ4′ = 0.32.23 The value τ4′ = 0 describes square-planar complexes, whereas τ4′ = 1 corresponds to a tetrahedral geometry. The Ag−N distances are substantially different at 2.2795(18) and 2.3870(17) Å. The shorter bond is opposite to the shorter Ag1−O1 distance [2.2730(16) Å], whereas the longer Ag−N distance is trans to the substantially elongated Ag1−O3 (1.5 − x, 0.5 − y, 1 − z) bond of 2.6609(16) Å. There are also dative metal−π interactions between the silver center and the phen ligands of neighboring molecules positioned on each side of the N1−N2− Ag−O1−O2 coordination plane. The Ag1···C1 (1 − x, y, 0.5 − z) and Ag1···C6 (1 − x, 1 − y, 1 − z) separations span 3.133(2) and 3.234(2) Å, lengths that are shorter than the sum of the silver van der Waals (vdW) radius and half-thickness of an aromatic ring (3.45 Å). Taking into account the latter interactions, the local geometry about the silver center may appropriately be considered disordered octahedral. These distal Ag···π interactions connect the dimers into a three-dimensional structure (Figure 2B). In contrast, higher ligand loadings resulted in the crystallization of the 2:1 Me4phen/AgOTf complex 3 (Figure 2C), which does not dimerize. The Ag1 center is ligated by two Me4phen ligands, whereas the triflate counterions are outer-sphere. The all-nitrogen metal-coordination environment about the silver center in 3 corresponds to a distorted seesaw (τ4′ = 0.43). Both bidentate Me4phen ligands bind asymmetrically, with one Ag−N distance being ∼0.16 Å shorter than the other; however, all Ag−N distances fall within the expected range. Crystal structures of analogues of our Ag(Me4phen)2(OTf) (3) complex have been reported in the literature, most notably [AgL2]PF6, where L = octachloro-1,10-phenanthroline,24 [AgL2]BF4, where L = 2,9-dimethyl-1,10-phenanthroline,25 and [Ag(phen)2] O2CtBu.26 These complexes show distorted tetrahedral geometries, with Ag−N bond lengths similar to those of 2, but differing angles for N−Ag−N, depending on the substitution pattern of the phen ligand. These analogues were not tested in our chemistry because substitution at the 2 and 9

Figure 2. (A) Single-crystal X-ray structure of a dimer of [Ag(Me4phen)(OTf)]n (2) shown with 50% probability ellipsoids. Selected bond distances (Å) and angles (deg): Ag1−O1, 2.2730(16); Ag1−N1, 2.3870(17); Ag1−N2, 2.2795(18); O1−Ag1−N1, 114.87(6); O1−Ag1−N2, 157.43(6); N2−Ag1−N1, 71.56(6); S1− O1−Ag1, 127.08(10). Symmetry code: (i) 1.5 − x, 0.5 − y, 1 − z. (B) Molecular drawing of 2, showing metal−π interactions between the silver center and phen ligands of neighboring molecules above and below. Symmetry code: (i) 1 − x, y, 0.5 − z; (ii) 1 − x, 1 − y, 1 − z. (C) Molecular structure of 3 shown with 50% probability ellipsoids. Selected bond distances (Å) and angles (deg): Ag1−N1, 2.249(3); Ag1−N2, 2.398(3); Ag1−N3, 2.246(3); Ag1−N4, 2.410(3); N1− Ag1−N2, 71.69(12); N1−Ag1−N4, 145.21(12); N2−Ag1−N4, 95.20(11); N3−Ag1−N1, 132.42(11); N3−Ag1−N2, 151.81(11); N3−Ag1−N4, 71.87(11).

positions of phen was shown to shut down nitrene-transfer chemistry.10 The combination of excess tBuBipy ligand and AgOTf delivers a complex, Ag(tBuBipy)2OTf (4), that is not as selective for the amination of allylic C−H bonds (Table 1, entry 2) compared to 3. An single-crystal X-ray structure has been previously reported for 4.27 A comparison of the single-crystal X-ray structures of 3 and 4 (Figures 2 and 3) shows that both structures exhibit a distorted seesaw geometry of the allnitrogen four-coordinate environment about the metal center, but 3 displays geometry index τ4′ = 0.43 vs 0.38 for 4, slightly favoring the tetrahedral geometry. In 4, the two tBuBipy ligands bind more symmetrically than Me4phen does in 3. In the case of 3, there are two short [2.246(3) and 2.249(3) Å] and two long [2.398(3) and 2.410(3) Å] bonds, whereas the average of the four Ag−N bond lengths in 4 is 2.300(14) Å. These variances may reflect the different conformational nature of the two ligands: in 3, the Me4phen moieties remain nearly planar, with the dihedral angle between the two pyridine planes 6726


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Inorganic Chemistry

and we suspected this might be the case for 8.28 Indeed, variable-temperature (VT) NMR studies on 8 carried out in CD2Cl2 over a temperature range of +24 to −90 °C (Figure 4)

Figure 3. Single-crystal X-ray structure of 4 drawn with 50% probability ellipsoids. Selected bond distances (Å) and angles (deg): Ag1−N1, 2.279(2); Ag1−N2, 2.305(2); Ag1−N3, 2.312(2); Ag1−N4, 2.303(2); N1−Ag1−N2, 73.23(8); N1−Ag1−N3, 112.68(9); N1− Ag1−N4, 154.17(8); N2−Ag1−N3, 151.69(8); N4−Ag1−N2, 114.63(9); N4−Ag1−N3, 72.81(8).

spanning 1.73(14) and 5.49(14)° and the ligand bite angles averaging 71.78(13)°. In contrast, the dihedral angles between the pyridine planes of the tBuBipy ligands in 4 measured 22.77(9) and 28.73(10)°, whereas the ligand bite angles averaged 73.0(3)°. The dihedral angles between the planes defined by Ag1−N1−N2 and Ag1−N3−N4 comprised 42.74(9)° and 46.32(9)° in 3 and 4, respectively, and the difference is statistically significant. Tris(2-pyridylmethyl)amine (tpa) Ligands. The silver(I) complex Ag(tpa)OTf (8) supported by a tpa ligand shows modest selectivity for nitrene insertion into the benzylic C−H bond of 5 over a 3° alkyl C(sp3)−H bond (Table 2, entry 1).12 Table 2. Surprising Effect of the tpa Ligand on the Site Selectivity of Nitrene Insertion Figure 4. VT NMR data for complex 8.

showed clear evidence of dynamic behavior. From +24 to −40 °C, the 1H NMR spectra showed only one set of resonances for each of the aryl and (H1−H4) and methylene (H5) protons. At −60 °C, significant line broadening is observed, while in the temperature range of −60 to −90 °C, all of the aryl and methylene peaks are resolved. The H1 peak was clearly resolved into three distinct resonances (H1a−1c), while some of the other proton resonances were overlapping. There are two different fluxional processes in 8 that may potentially influence the site selectivity of nitrene transfer. One such dynamic behavior involves the equilibrium between monomeric structures represented by 8a/8a′ and the dimeric structure 8b. A DOSY NMR study of 8 in CD2Cl2 at three different temperatures (+25, −20, and −90 °C) and a concentration mimicking actual reaction conditions showed the monomer/dimer equilibrium to be 60:40, 55:45, and 19:81, respectively (see Figure S-3 for further details and calculations of the monomer/dimer equilibrium). A second fluxional behavior in 8 is related to the rapid association and dissociation of one of the pyridine arms of the tpa ligand on and off the silver center, as represented by the interconversion of 8a and 8a′ (Figure 4), perhaps through the intermediacy of 8. At low temperatures, distinct aryl protons H1-4a-c for the magnetically different pyridine rings, as well as distinct methylene group resonances labeled as H5a-c (only one set of protons is labeled in 8b in Figure 4 for clarity), are seen. As the temperature is increased, the individual proton signals begin to coalescence, indicating either a preference for the monomer over the dimer or sufficiently rapid exchange of the pyridine arms of the complex on the NMR time scale, such

0.125 mmol of substrate 5, 10 mol % catalyst, 3.5 equiv of PhlO, 4 Å MS, CH2CI2, rt. Isolated yields.

The preference for a reaction at the benzylic C−H site increases as steric congestion around the 3° alkyl C(sp3)−H bond increases. However, the site selectivity of C−H amination did not respond to simple electronic modifications to the tpa ligand because catalysts based on ligands 9 and 10 (entries 2 and 3) displayed little impact on the 6/7 ratio. Surprisingly, placement of a methyl group ortho to the pyridine nitrogen atoms in (o-Me)3tpaAgOTf (11) reversed the expected selectivity in favor of 7 (entry 4). These results argued against electronic effects as the primary mode for tuning the site selectivity and suggested that steric or conformational effects are more important for manipulating the site selectivity of silver-catalyzed nitrene transfer. Attempts to obtain suitable crystals for single-crystal X-ray crystallographic determination of the structure of 8 were not successful. Analogous copper(I) complexes supported by similar ligands exhibit highly fluxional behavior in solution, 6727


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Inorganic Chemistry that all of the methylene and aryl peaks on the three ligand arms appear as magnetically equivalent protons. With the features of the fluxional behavior of 8 established, we turned our attention to AgOTf supported by an (o- Me)3tpa ligand to see how it might differ from 8 in both the solid and solution states. In contrast to 8, complex 11 favored amination of a tertiary alkyl C(sp3)−H bond over a benzylic C−H bond in 5 (Table 2, entry 4). In the solid state, complex 11· 1 CH2Cl2 2 crystallized as a dicationic dimer (Figure 5), showing a five-

Figure 5. Molecular structure of [Ag(o-Me3)tpa(OTf)]2· 1 CH2Cl2 2

11· 1 CH2Cl2 from (o-Me)3tpa and AgOTf shown with 50% probability 2 ellipsoids. The dichloromethane molecule was omitted for clarity. Selected bond distances (Å) and angles (deg): Ag1−N8, 2.231(2); Ag1−N3, 2.377(2); Ag1−N1, 2.403(2); Ag1−N4, 2.501(2); Ag1− Ag2, 3.0366(3); Ag2−N2, 2.224(2); Ag2−N5, 2.381(2); Ag2−N6, 2.434(2); Ag2−N7, 2.483(2); N8−Ag1−N3, 140.49(7); N8−Ag1− N1, 142.83(7); N3−Ag1−N1, 75.43(7); N8−Ag1−N4, 114.17(7); N3−Ag1−N4, 78.75(6); N1−Ag1−N4, 75.06(7); N8−Ag1−Ag2, 79.08(5); N3−Ag1−Ag2, 107.05(5); N1−Ag1−Ag2, 80.21(5); N4− Ag1−Ag2, 152.29(5); N2−Ag2−N5, 140.89(7); N2−Ag2−N6, 119.56(7); N5−Ag2−N6, 74.77(7); N2−Ag2−N7, 139.75(7); N5− Ag2−N7, 76.34(7); N6−Ag2−N7, 77.44(7); N2−Ag2−Ag1, 76.67(5); N5−Ag2−Ag1, 79.34(5); N6−Ag2−Ag1, 152.71(5); N7− Ag2−Ag1, 104.81(5).

Figure 6. VT NMR data for complex 11.

11 differs significantly from that of 8. From +24 to −20 °C, the spectra showed one set of resonances for each of the methylene (H4) and methyl protons; we propose that this results from the monomeric form 11, which contains all four ligand nitrogen atoms bound to the silver center, with an outer-sphere OTf counteranion. In contrast, when the temperature is in the range of −40 to −60 °C, two unique sets of resonances are observed. We postulate that these signals are due to a mixture of the monomeric and dimeric forms of the complex. The individual protons in dimer 11b are not indicated for clarity; further details are found in the Supporting Information. A DOSY NMR study of 11 in CD2Cl2 at three different temperatures (+25, −20, and −90 °C) showed, at 25 °C, complex 11 mostly existed as a monomer, while lowering the temperature to −90 °C resulted in the corresponding dimer becoming the dominant species (see Figure S-4 for further details and calculations of the monomer/dimer equilibrium). At −40 °C, the methyl singlet at 2.62 ppm corresponds to monomer 11, where the broadening of the signals may result from rapid interconversion between the tridentate 11a and tetradentate 11. The three additional singlets in a 1:1:1 ratio at 2.50, 2.27, and 2.16 ppm are postulated to arise from dimer 11b. This same behavior and resolution was observed with the aryl and methylene protons. As the temperature is lowered to −90 °C, the singlets corresponding to the equivalent Me and H4 sites in 11 are replaced with signals showing inequivalent pyridine protons because of the silver complex residing largely in the dimeric form 11b at low temperatures. To provide further corroboration, the proposed coordination geometries at the silver center are dependent on the temperature, and a 1H−15N HMBC for complex 11 was collected at both room temperature (rt) and −80 °C (see Figure S-1 for the spectra). At rt, the data show that the pyridine arms and methyl groups are all equivalent; a single 1 H−15N HMBC correlation is seen between the methyl group

coordinate geometry at each of the two silver atoms. This is interesting because a related complex of (o-Me)3tpa with AgNO3 showed a distorted square-pyramidal geometry with only two of the pyridyl nitrogen atoms bound to silver, in addition to the alkylamine nitrogen atom. The two oxygen atoms from the nitrate group were bound to the silver atom as a bidentate ligand.29 This difference when the silver salt is changed from triflate in our system to nitrate points to the sensitivity of the solid-state structure to the counteranion. Fivecoordinate geometries are commonly characterized with τ5.30 A C4v square-pyramidal geometry is characterized with τ5 = 0, whereas τ5 = 1 corresponds to the D3h trigonal-pyramidal arrangement. In complex 11, the τ5 values are 0.16 and 0.20 for atoms Ag1 and Ag2; the coordination environment about the metal centers is substantially distorted square-pyramidal. The Ag···Ag separation in the dimer of 11· 1 CH2Cl2 [3.0366(3) Å] 2 is shorter than the sum of two silver vdW radii (3.244 Å) and reflects an attractive interaction between the two metal atoms. The four-coordinate (o-Me)3tpa ligands serve as bridging ligands, binding to one silver center with two pyridines and the amine nitrogen atom and to the second metal center with the remaining bridging pyridine. The two Ag−N(bridging) distances [average of 2.228(5) Å] are substantially shorter than the other six Ag−N bond lengths that range from 2.377(2) to 2.501(2) Å. To assess this possibility of complex 11 remaining dimeric in solution, VT NMR studies were carried out. The VT NMR experiments (Figure 6) showed that the dynamic behavior of 6728


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Inorganic Chemistry and the nitrogen atom of the pyridine arm. In contrast, the spectrum at −80 °C shows interaction between the methyl groups of two arms of the (o-Me)3tpa ligand and the pyridine nitrogen atom, while the other methyl group is not correlated. This supports the idea that one of the pyridine “arms” either is not coordinated to silver at all or is perhaps attached to another silver atom, as in dimer 11b. Because complexes 8 and 11 showed a propensity to dimerize, a feature that could complicate the site selectivity in silver-catalyzed nitrene transfer, we were interested in identifying a silver complex that would exhibit minimal fluxional behavior in both the solid and solution states. Thus, [Ag(o-Ph)3tpa(OTf)] (12) was prepared from a 1:1 mixture of AgOTf and (o-Ph)3tpa (Figure 7). In contrast to 11, complex

Figure 8. Ligand shielding of the metal centers in the cation [Ag(oPh)3tpa)]+ of 12 and the model complex [Ag(o-Me)3tpa)]+. The blue shadows on the spheres show projections of the ligands if they were “illuminated” by a light source positioned at the metal center. (a) [Ag(o-Ph)3tpa)]+ views down the Ag−N(amine) axis; (b) side view of [Ag(o-Ph)3tpa)]+; (c) [Ag(o-Me)3tpa)]+ viewed from the unobstructed side; (d) [Ag(o-Me)3tpa)]+ viewed perpendicularly to the Ag−N(amine) bond.

Figure 7. Molecular drawing of 12· 4 CH2Cl2 shown with 50% 3

is believed to be mainly responsible for the dimeric nature of 11 and the monomeric nature of 12. Complex 12 showed no tendency to form dimeric species in solution over the temperature range of −90 °C to rt (Figure 9).

probability ellipsoids. The OTf anion and solvent molecules are omitted. Selected bond distances (Å) and angles (deg): Ag1−N1, 2.474(3); Ag1−N2, 2.349(3); Ag1−N3, 2.364(3); Ag1−N4, 2.346(3); N2−Ag1−N1, 72.91(13); N2−Ag1−N3, 111.03(10); N3−Ag1−N1, 72.39(9); N4−Ag1−N1, 72.48(11); N4−Ag1−N2, 111.65(10); N4− Ag1−N3, 111.68(10).

12· 3 CH2Cl2 crystallized with two symmetry-independent 4

discrete monomeric cationic complexes with near-identical geometries, but similarly to 11· 1 CH2Cl2, the OTf anions were 2 outer-sphere. In each cation of 12, all four (o-Ph)3tpa nitrogen atoms ligate to the silver and form a nearly ideal tetrahedron (τ4′ = 0.97). The silver atoms are displaced from the plane of the pyridine nitrogen atoms away from the amine nitrogen atom by an average of 0.700(7) Å. The Ag−N(amine) distances [average of 2.474(3) Å] are substantially longer than the Ag− N(py) bond lengths that average 2.352(8) Å, but all fall within the expected range. Undoubtedly, the steric bulk of the (oPh)py moieties is critical to the monomeric nature of 12. An interesting, and perhaps nonintuitive, insight into the ligand behavior in 11 and 12 is provided by analysis of the “crowdedness” of the metal-coordination environment. The total metal shielding of the central silver atom by the ligands can be represented with a G parameter, which is the percentage of the metal-coordination sphere unavailable to a nucleophilic attack.31 For the two symmetry-independent complexes [Ag(oPh)3tpa]+ in 12· 3 CH2Cl2, the G parameters are 77% (Figure 4 8a,b). The central metal is shielded by the (o-Ph)py ligand to a larger extent along the 3-fold axis looking down the Ag− N(amine) bond (Figure 8a) than from the sides (Figure 8b), and the (o-Ph)pyridine arms are not flexible enough to afford a conformation suitable for dimerization. For the model complex [Ag(o-Me)3tpa]+, the G parameter is 61%. Thus, the silver center is considerably more accessible (Figure 8c,d), and there is sufficient room in the metal-coordination sphere to accommodate changes in the conformations of the ligand. The size difference between the methyl and phenyl substituents

Figure 9. VT NMR data for complex 12.

The bulky phenyl groups present a significant energetic barrier to the dissociation of a pyridine arm of the ligand, forcing the complex to adopt a tetradentate coordination geometry in both the solid and solution states. As further support for the assignment of the silver coordination geometry as tetradentate in the solution state, the H−15N HMBC of 12 (Figure S-2) was collected. The data showed one correlation between the equivalent CH2 groups and the equivalent pyridine nitrogen atoms, indicating the symmetry in the complex. Unfortunately, although increasing the steric bulk of the tpa ligand to control 6729


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Inorganic Chemistry the fluxional behavior of the silver complex was effective, it also shut down the desired nitrene-transfer reactivity. The use of 12 as a catalyst proceeded sluggishly or not at all, presumably as a result of the difficulty in forming the necessary metal nitrene because of steric congestion around the metal. Taken together, the X-ray crystallography and VT NMR studies carried out on 8, 11, and 12 provide important insights into the factors controlling the behavior of silver-catalyzed nitrene transfer. First, the steric environment of a given ligand has a significant impact on the nature of the major conformer in solution, particularly with regard to the coordination geometry around the silver center. Second, less bulky ligands result in complexes that exhibit fluxionality; this behavior must be manipulated in a predictable way if more selective secondgeneration ligands are to be designed. Finally, VT NMR studies yield information about the effect of the temperature and concentration on fluxional behavior, where the data thus far suggest that tetradentate complexes favor the reaction of 3° alkyl C(sp3)-H bonds, perhaps because of the generation of more electrophilic silver nitrene species or a more favorable trajectory of the approach of the C−H bond to the putative silver nitrene. In contrast, lower-coordinate silver complexes appear to prefer to aminate secondary benzylic C−H bonds, perhaps because of the extra open coordination site available at the metal in the tridentate-coordinated silver complex. This open site may enable beneficial noncovalent interactions between the substrate and catalyst to drive the selectivity toward C−H bonds that reside adjacent to π bonds. Minimizing the Dynamic and/or Fluxional Behavior of Silver Complexes. We hypothesized that ligands that were able to favor more open coordination environments at silver could lead to catalysts that improve the preference for the activation of benzylic C−H bonds over more electron-rich 3° alkyl C(sp3)−H bonds. To test this proposal, we required a ligand design that was capable of minimizing the fluxional behavior of the resulting silver complexes. Thus, two new ligands for silver(I) were synthesized based on a piperidine scaffold, resulting in the compounds (α-Me)-syn-Py3Pip (13) and (α-Me)-anti-Py3Pip (14). Scheme 1 describes the final step

Figure 10. Single-crystal X-ray structure of 15 drawn with 50% probability ellipsoids. The two triflate counterions are omitted for clarity and are not bound directly to the silver centers. Selected bond distances (Å) and angles (deg): Ag1−Ag2, 2.8769(8); Ag1−N1, 2.3541(14); Ag1−N2, 2.3387(14); Ag1−N3, 2.5008(13); Ag1−N5, 2.2924(14); Ag2−N4, 2.3089(14); Ag2−N6, 2.5243(14); Ag2−N7, 2.3434(15); Ag2−N8, 2.3409(14); N1−Ag1−Ag2, 88.75(3); N1− Ag1−N3, 73.85(5); N2−Ag1−Ag2, 146.96(4); N2−Ag1−N1, 110.06(5); N2−Ag1−N3, 73.14(4); N3−Ag1−Ag2, 87.25(3); N5− Ag1−Ag2, 82.49(3); N5−Ag1−N1, 106.20(5); N5−Ag1−N2, 115.74(5); N5−Ag1−N3, 169.74(4); N4−Ag2−Ag1, 89.35(3); N4− Ag2−N6, 169.59(4); N4−Ag2−N7, 106.40(5); N4−Ag2−N8, 116.03(5); N6−Ag2−Ag1, 87.07(3); N7−Ag2−Ag1, 141.66(4); N7−Ag2−N6, 71.12(5); N8−Ag2−Ag1, 102.23(3); N8−Ag2−N6, 74.32(5); N8−Ag2−N7, 101.62(5).

Scheme 1. Synthesis of New Ligands 13 and 14

Figure 11. (Top) Single-crystal X-ray structure of the monomeric unit of [Ag((α-Me)-anti-Py3Pip)OTf]n in complex 16, shown with 50% probability ellipsoids. A solvent molecule of dichloromethane was omitted for clarity. (Bottom) Molecular drawing of the polymeric structure of 16. Selected bond distances (Å) and angles (deg): N1− Ag1, 2.303(2); N4−Ag1, 2.342(2). Symmetry codes: (i) x, 1.5 − y, 0.5 − z; (ii) x, 1.5 − y, z − 0.5.

of the synthesis, where treatment of the previously reported dimesylate32 with 1-(pyridin-2-yl)ethylamine yields a mixture of 13 and 14. The ratio of 13/14 was typically around 1.7:1, and the two diastereomers could be readily separated by column chromatography. Interestingly, the reactions of 13 and 14 with 1 equiv of AgOTf furnished two new silver complexes, [(Ag(α-Me)-synPy3Pip)OTf]2 (15) and Ag[(α-Me)-anti-Py3Pip]OTf (16), that exhibited very different behaviors in both the solid and solution states. The crystallographically characterized 15 and 16 possess dramatically different structures in the solid state (Figures 10 and 11): while complex 15 crystallizes as dimer [Ag(α-Me)-synPy3Pip]2[OTf]2, complex 16 crystallizes as a dichloromethanesolvated, one-dimensional polymer, [Ag(α-Me)-anti-

Py3Pip]n[OTf]n(CH2Cl2)n. In the dimeric dication [Ag(αMe)-syn-Py3Pip]22+ of 15, both of the silver atoms are fivecoordinate, with τ5 = 0.37 and 0.48 for Ag1 and Ag2, respectively. This corresponds to a seesaw geometry that is intermediate between square-pyramidal and trigonal-pyramidal. The Ag−Ag bond length of 2.8769(8) Å is shorter than that shown in 11 [3.0366(3) Å] and substantially shorter than the sum of the silver covalent radii (3.244 Å). Each ligand 13 was shown to be tetradentate in nature, with binding occurring between the amine and two pyridine nitrogen atoms to one silver center and the third pyridine nitrogen atom to the other 6730


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Inorganic Chemistry metal. The Ag−N(amine) distances average 2.513(16) Å. The Ag−N(bridging pyridine) bonds are positioned trans to the Ag−N(amine) bonds and are shorter [average of 2.301(12) Å] than the other Ag−N(py) distances [average of 2.344(7) Å]; this difference is statistically significant. In the polymeric structure of [Ag(α-Me)-anti-Py3Pip]+n (Figure 11, bottom), there are some features similar to those in [(α-Me)-syn-Py3PipAg]22+: two pyridine nitrogen atoms from the same moiety 14 ligate to the central metal with distances of 2.303(2) and 2.342(2) Å, whereas the atom N3(bridging pyridine) from an adjacent molecule binds stronger, with an Ag−N bond length of 2.261(2) Å. Similarities between the two structures end here. These three pyridine nitrogen atoms form a distorted triangle about the silver center, but the coordination is only formally trigonal-planar because the metal achieves coordinative saturation by two distal interactions. One apical site is occupied by the amine nitrogen atom with a Ag···N separation of 2.643(2) Å. The other apical position is capped by the π system of the pyridine ring containing atom C4 from the ligand that bears the bridging pyridine nitrogen atom. The Ag1···C4 separation spans 3.311(2) Å. Thus, the five-coordinate environment about the silver metal center, characterized by τ5 = 0.71, can be described as severely distorted trigonal-bipyramidal. The relative stereochemistry of the two chiral piperidine carbon atoms and the αmethyl carbon atom of one enantiomer of 16 was determined to be (R,R,S). A small amount of the (R,R,R) isomer of 16 was also produced but could be readily separated from 16. With the single-crystal X-ray structures of 15 and 16 in hand, we turned our attention to understanding how the relative stereochemistry of the pyridine substituents on the piperidine rings of the (α-Me)-Py3Pip ligands impacts the fluxional behavior of the corresponding silver complexes in solution. Complex 15 is dimeric in the solid state (Figure 10), while both VT and DOSY NMR studies of 15 in solution suggested that the dimeric and monomeric forms are in equilibrium (see Figure S-6 for details). In contrast, 16, formed from a 1:1 mixture of ligand 14 and AgOTf, yielded a complex that forms a one-dimensional cationic chain in the solid state. It was clear from the VT NMR spectra of 16 (Figure 12) that it exhibits far less fluxional behavior than either 8 or 11. At low temperatures, the three protons α to the pyridine arms of the ligands all exhibit distinct signals in the 1H NMR. The Ha proton resides in an equatorial position, where the small Jeq/ax and Jeq/eq couplings are not resolved in the broad singlet. The Hb proton residing in the axial position shows a large Jax,ax coupling, although the smaller Jax,eq is not resolved. Finally, Hc appears as a broad quartet. No evidence of dimer formation is seen throughout the entire temperature range of the VT NMR experiment (see Figure S-7); however, broadening of the peaks as the temperature is increased indicates rapid exchange of the pyridine arms appended to the piperidine on and off the silver center. The site selectivity for competing nitrene transfer between benzylic and 3° alkyl C(sp3)−H bonds was examined using both of the catalysts 15 and 16 (Table 3). 15, with its propensity to dimerize in both the solid and solution states, gave lower selectivity for 6 over 7 (entry 1), with the preference for 6 nearly doubling with 16 (entry 2). Lowering the temperature (entries 3 and 4) slightly improved the selectivity in excellent yields, especially compared to our earlier efforts with tpa-based catalysts (Table 2). While these results represent proof-of-concept, efforts are ongoing to strengthen the

Figure 12. VT NMR data for 16.

Table 3. Effect of 15 versus 16 on the Site Selectivity of C−H Insertion

0.125 mmol of substrate 5, 10 mol % catalyst, 3.5 equiv of PhIO, 4 Å MS, 0.05 M, rt. NMR yield.

noncovalent interactions that drive the preference for benzylic C−H amination in nitrene transfer catalyzed by 16. Our efforts to develop an efficient synthetic method based on this catalyst scaffold will be detailed in a subsequent publication.

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CONCLUSION In summary, the solid- and solution-state behaviors of a series of silver(I) complexes supported by an array of nitrogen-donor ligands have been described; these complexes are unique in that they display the ability to promote chemoselective and siteselective nitrene-transfer reactions. These studies clarified how the identity of the ligand influences the coordination number and geometry of the silver center in both the solid and solution states. Differing ratios of AgOTf/bipyridine/phen-based ligands gave silver complexes with distinct solid-state geometries and dynamic behaviors in solution10 that display tunable chemoselectivity in reactions of 1. In the context of site-selective C−H amination, VT 1H and DOSY NMR experiments of silver(I) complexes 8 and 11 show equilibrium between tetra- and tridentate monomeric species, as well as dimeric species. The steric environment of the ligand impacts the coordination geometry around the silver center in solution, with competition 6731


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Inorganic Chemistry

(e) Karila, D.; Dodd, R. H. Recent progress in iminoiodane-mediated aziridination of olefins. Curr. Org. Chem. 2011, 15, 1507. (f) Jung, N.; Bräse, S. New catalysts for the transition-metal-catalyzed synthesis of aziridines. Angew. Chem., Int. Ed. 2012, 51, 5538. (g) Degennaro, L.; Trinchera, P.; Luisi, R. Recent advances in the stereoselective synthesis of aziridines. Chem. Rev. 2014, 114, 7881. (2) Selected examples of rhodium-catalyzed nitrene transfer: (a) Roizen, J. L.; Harvey, M. E.; Du Bois, J. Metal-catalyzed nitrogen-atom transfer methods for the oxidation of aliphatic C-H bonds. Acc. Chem. Res. 2012, 45, 911. (b) Zalatan, D. N.; Du Bois, J. Metal-catalyzed oxidations of C-H to C-N bonds. Top. Curr. Chem. 2009, 292, 347. (c) Dequirez, G.; Pons, V.; Dauban, P. Nitrene chemistry in organic synthesis: still in its infancy? Angew. Chem., Int. Ed. 2012, 51, 7384. (d) Espino, C. G.; Du Bois, J. A Rh-Catalyzed C-H Insertion Reaction for the Oxidative Conversion of Carbamates to Oxazolidinones. Angew. Chem., Int. Ed. 2001, 40, 598. (e) Fiori, K. W.; Du Bois, J. Catalytic Intermolecular Amination of C−H Bonds: Method Development and Mechanistic Insights. J. Am. Chem. Soc. 2007, 129, 562. (f) Collet, F.; Lescot, C.; Liang, C. G.; Dauban, P. Studies in catalytic C-H amination involving nitrene C-H insertion. Dalton Trans. 2010, 39, 10401. (g) Espino, C. G.; Fiori, K. W.; Kim, M.; Du Bois, J. Expanding the scope of C-H amination through catalyst design. J. Am. Chem. Soc. 2004, 126, 15378. (h) Zalatan, D. N.; Du Bois, J. A Chiral Rhodium Carboxamidate Catalyst for Enantioselective C−H Amination. J. Am. Chem. Soc. 2008, 130, 9220. (i) Liang, C.; Robert-Peillard, F.; Fruit, C.; Muller, P.; Dodd, R. H.; Dauban, P. Efficient diastereoselective intermolecular rhodiumcatalyzed C-H amination. Angew. Chem., Int. Ed. 2006, 45, 4641. (j) Lescot, C.; Darses, B.; Collet, F.; Retailleau, P.; Dauban, P. Intermolecular C−H Amination of Complex Molecules: Insights into the Factors Governing the Selectivity. J. Org. Chem. 2012, 77, 7232. (k) Lebel, H.; Spitz, C.; Leogane, O.; Trudel, C.; Parmentier, M. Stereoselective Rhodium-Catalyzed Amination of Alkenes. Org. Lett. 2011, 13, 5460. (3) Harvey, M. E.; Musaev, D. G.; Du Bois, J. A Diruthenium Catalyst for Selective, Intramolecular Allylic C-H Bonds. J. Am. Chem. Soc. 2011, 133, 17207. (4) Selected references of iron-catalyzed nitrene transfer: (a) Halfen, J. A. Recent Advances in Metal-Mediated Carbon-Nitrogen Bond. Formation. Curr. Org. Chem. 2005, 9, 657. (b) Paradine, S. M.; White, M. C. Iron-catalyzed intramolecular allylic C-H amination. J. Am. Chem. Soc. 2012, 134, 2036. (c) Cramer, S. A.; Jenkins, D. M. Synthesis of Aziridines from Alkenes and Aryl Azides with a Reusable Macrocyclic Tetracarbene Iron Catalyst. J. Am. Chem. Soc. 2011, 133, 19342. (d) Hennessy, E. T.; Liu, R. Y.; Iovan, D. A.; Duncan, R. A.; Betley, T. A. Iron-mediated Intermolecular N-Group Transfer Chemistry with Olefinic Substrates. Chem. Sci. 2014, 5, 1526. (e) Fantauzzi, S.; Caselli, A.; Gallo, E. Nitrene transfer reactions mediated by metallo-porphyrin complexes. Dalton Trans. 2009, 5434. (f) Nakanishi, M.; Salit, A.-F.; Bolm, C. Iron-Catalyzed Aziridination Reactions. Adv. Synth. Catal. 2008, 350, 1835. (g) Hennessy, E. T.; Betley, T. A. Complex N-heterocycle synthesis via iron-catalyzed, direct C-H bond amination. Science 2013, 340, 591. (h) Liu, Y.; Guan, X.; Wong, E. L. M.; Liu, P.; Huang, J. S.; Che, C. M. Non-heme ironmediated amination of C (sp3)−H bonds. Quinquepyridine-supported iron-imide/nitrene intermediates by experimental studies and DFT calculations. J. Am. Chem. Soc. 2013, 135, 7194. (5) Selected examples of cobalt-catalyzed nitrene transfer: (a) Lu, H. J.; Subbarayan, V.; Tao, J. R.; Zhang, X. P. Cobalt (II)-Catalyzed Intermolecular Benzylic C−H Amination with 2, 2, 2-Trichloroethoxycarbonyl Azide (TrocN3). Organometallics 2010, 29, 389. (b) Lu, H.; Zhang, X. P. Catalytic C−H functionalization by metalloporphyrins: recent developments and future directions. Chem. Soc. Rev. 2011, 40, 1899. (c) Ruppel, J. V.; Kamble, R. M.; Zhang, X. P. Cobalt-Catalyzed Intramolecular C− H Amination with Arylsulfonyl Azides. Org. Lett. 2007, 9, 4889. (6) Copper-catalyzed nitrene transfer: (a) Bagchi, V.; Paraskevopoulou, P.; Das, P.; Chi, L.; Wang, Q.; Choudhury, A.; Mathieson, J. S.; Cronin, L.; Pardue, D. B.; Cundari, T. R.; Mitrikas,

experiments showing that higher coordination appears to favor the reaction of 3° alkyl C(sp3)−H bonds, while lowercoordinate silver complexes prefer to aminate 2° benzylic C− H bonds, perhaps because of the extra open coordination site available at the metal for beneficial noncovalent interactions between the aryl group of the substrate and the catalyst. Comparing two isomers of new piperidine- based ligands 15 and 16 supported this hypothesis in that the less fluxional complex gave preferentially amination of benzylic C−H bonds. The design principles obtained in these studies are being applied to the synthesis of new ligands with improved selectivity and the ability to promote asymmetric nitrenetransfer reactions.

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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.7b00838. Experimental procedures and characterization data for all new compounds (PDF) X-ray crystallography data for compounds 2−4, 11, 12, 15, and 16 (PDF) Accession Codes

CCDC 1503474−1503477 and 1548537−1548539 and 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.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jennifer M. Schomaker: 0000-0003-1329-950X Present Address †

N.S.D.: College of Chemistry, University of California, Berkeley, CA.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded through the Wisconsin Alumni Research Foundation to J.M.S. Dr. Jon Paretsky is thanked for the crystallization of 12. The NMR facilities at UWMadison are funded by the NSF (CHE-1048642, CHE-0342998) and NIH S10 OD012245. The National Magnetic Resonance Facility at Madison is supported by the NIH (Grants P41GM103399, S10RR08438, and S10RR029220) and NSF (Grant BIR0214394).

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REFERENCES

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Synthesis, Characterization, and Variable-Temperature NMR Studies

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