One Macrocyclic Ligand, Four Oxidation States: A 16-Atom Ringed

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One Macrocyclic Ligand, Four Oxidation States: A 16-Atom Ringed Dianionic Tetra-NHC Macrocycle and Its Cr(II) through Cr(V) Complexes Markus R. Anneser, Xian B. Powers, KatieAnn M. Peck, Isabel M. Jensen, and David M. Jenkins* Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States

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ABSTRACT: Despite chromium being among the first transition metals ever reported to bind to an NHC, chromium NHC complexes, especially in mid and high oxidation states, have received scant attention. Herein, the synthesis, characterization, and reactivity of a series of Cr(II) to Cr(V) complexes bearing a 16-atom ringed dianionic tetra-NHC macrocycle are reported. The Cr(II) dimer is diamagnetic and displays a very short Cr−Cr quadruple bond, unprecedented for Cr-NHC complexes to date. Oxidative cleavage of the Cr−Cr bond leads to the formation of a highly stable diamagnetic Cr(IV) oxo complex. Similar reactions with organic azides lead to paramagnetic Cr(IV) imide complexes. Notably, the Cr(IV) oxo can be oxidized in a reversible reaction to yield a Cr(V) cationic oxo complex, which is a very rare high oxidation state Cr-NHC-compound. This Cr(V) oxo undergoes stoichiometric oxygen atom transfer. Similar reactions were attempted with molybdenum and tungsten to form macrocyclic NHC complexes, but only a molybdenum dimer could be isolated.

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INTRODUCTION Chromium N-heterocyclic carbene (NHCs) complexes have a long history, going back to reports by Wanzlick, Schönherr, and Ö fele on monodentate Cr(0) complexes.1,2 Despite these early discoveries, few examples of NHC chromium complexes have been effective in catalysis that requires changes in redox states at the metal.3−6 Zeng reported a cross coupling reaction which utilized a chromium center, while both Landman and Yang were able to use Cr NHC complexes to convert glucose feedstocks into 5-hydroxymethylfurfural, a necessary intermediate in biofuels.3,4,7 Perhaps this result is not surprising since very few high oxidation state Cr NHC complexes have been prepared. Zhu and Yang have prepared the single example of a Cr(V) NHC complex, but the nitride ligand shows no group transfer activity.7 Nonetheless, our group has recently demonstrated the viability of midvalent chromium tetra-NHC complexes to perform NR group transfer in a both stoichiometric and catalytic fashion beginning from organic azides.5,8 Small changes in ligand design and steric effects are known to effect large changes in complex reactivity or catalysis.9,10 While these changes have predominately focused on the use of Cp and phosphine ligands,11−13 such changes have been studied less extensively with NHCs, particularly outside of monodentate NHCs (Figure 1A, recent example from Johnson).9 In particular, the “bite angle”, as related to isostructural bidentate phosphine ligands, is less well understood (Figure 1B, example from Kühn).14 This concept of © XXXX American Chemical Society

Figure 1. Selected examples of complexes where minor changes in ligand sterics (shown in red) around the NHC resulted in changes in reactivity. Top to bottom (A−C) shows monodentate, bidentate, and tetradentate NHC examples with C being discussed herein.

bridging ligand sterics is further complicated for macrocyclic tetradentate NHCs that surround the metal center. Our group has performed extensive work on catalytic aziridination with iron complexes using 18-atom ringed dianionic tetra-NHC Received: July 12, 2019

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DOI: 10.1021/acs.organomet.9b00476 Organometallics XXXX, XXX, XXX−XXX

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Organometallics macrocycles,15 but found that switching to the analogous 16atom ringed dianionic tetra-NHC macrocycle inhibits aziridination.16 Instead, the 16-atom macrocycle stabilizes Fe(IV) imide complexes.16,17 In light of our results on iron with the smaller ringed system, we wanted to test the oxidation reactivity on chromium with the same macrocycle (Figure 1C). In this manuscript, we assess the differences between our previously reported 18-atom macrocyclic chromium complexes5,8 and a new set of complexes supported by a 16-atom macrocycle. We isolated and characterized seven distinct Cr NHC complexes in the oxidation states 2+ through 5+. The Cr(II) complex forms an unbridged dimer with a very short quadrupole bond between the chromium centers. This dimer is reactive for oxidative group transfer to form imide and oxo complexes that are similar to variants that we previously reported. However, their reactivity is surprisingly dissimilar, in that the Cr(IV) imide complexes do not reductively release the nitrene moiety. Furthermore, the Cr(IV) oxo complex can be oxidized to a cationic Cr(V) oxo that performs group transfer. Finally, for comparison, we attempted to prepare similar complexes on molybdenum and tungsten and, in the former case, were successful.

= 1).8 Single crystal X-ray analysis (Figure 2A) offered an unexpected explanation for this striking discrepancy, revealing the dimeric structure of 1 featuring a Cr−Cr quadruple bond, a feature commonly found in Cr(II) compounds.18,19 To the best of our knowledge, 1 is the first Cr(II) NHC complex ever reported featuring a Cr−Cr quadruple bond. The Cr−Cr distance (1.933(4) Å) is among the shortest ever reported, and is much closer to a reported Cr(I)−Cr(I)-quintuple bond (1.8351(4) Å)20 rather than unbridged Cr(II)−Cr(II) quadruple bonds, which average 2.057 Å.19,21−23 Despite this short Cr−Cr bond, the chromium coordination number prohibits a fifth bond between the metals, since the dx2−y2 orbital is required for metal−ligand bonding.24 To accommodate the quadruple bond, the Cr(II) centers are displaced 0.598 Å out of plane spanned by the carbon atoms, in contrast to the almost square planar coordination of the Cr(II) in the 18-atom ringed macrocycle.8 Our group has previously demonstrated catalytic aziridination with Cr(III) NHC complexes,5 so we were interested in preparing a trivalent example with this 16-atom macrocyclic ligand. By employing CrCl3(THF)3 as the metal salt, we were able to employ similar reaction conditions as for 1. Deprotonation of the ligand with nBuLi followed by addition of CrCl3(THF)3 yielded a green solid which instantaneously became faintly rose when dissolved in acetonitrile, indicative of coordination of acetonitrile to the Cr(III) center. This product, (BMe2,MeTCH)Cr(Br)(NCCH3), 2, was isolated as a paramagnetic complex with S = 3/2. Rose crystals grown from acetonitrile/diethyl ether and single crystal analysis of the structure of 2 (Figure 2B), shows an acetonitrile bound opposite the bromide. The bromide presumably comes from the counteranion of the ligand and has exchanged during the reaction. In the paramagnetic 1H NMR, only one species is observed, suggesting that there is not a mix of halides. The Xray structure of 2 shows an octahedral coordination and the Cr−Br distance of 2.480(1) Å is of comparable length to other Cr(NHC)−Br bonds (2.368(3) Å) and is close to its iron analogue, (BMe2,MeTCH)FeBr (2.499(1) Å).8,17 The average Cr−C distance of 2.052(2) Å is similar to Cr−C distances on related NHC complexes.8 Since the axially bound halide ligand could interfere with group transfer reactions, we substituted the halide counterion with an inert PF6− counterion through addition of TlPF6 to 2, forming [(BMe2,MeTCH)Cr(NCCH3)2](PF6), 3 (Scheme 1). AgPF6 was not effective and resulted in rapid decomposition of 2 and the formation of black precipitate (metallic silver). Complex 3 is an orange powder that is stable in air and acetonitrile solution for several days. Crystals of 3 were grown from slow diffusion of diethyl ether into an acetonitrile solution of the complex; the solid state structure is shown in Figure 2C and is quite similar to 2. Complexes 1 and 3 are effective for group transfer reactions of oxygen or nitrenes (Scheme 2). Addition of trimethyl Noxide to 2 in THF leads to cleavage of the Cr−Cr quadruple bond and the formation of (BMe2,MeTCH)Cr(O), 4. By mixing with a slight excess of oxidant in THF, 1 can be cleanly oxidized to monomeric complex (BMe2,MeTCH)Cr(O), 4, in 88% yield. The reaction starts out as a yellow suspension, due to the low solubility of 1; it becomes a transparent green solution after the oxidation to 4 has finished. Crystallization from THF/pentane results in the formation of a crystalline product that is highly stable. The crystal structure of 4 (Figure 3A) reveals a square pyramidal coordination, similar to its

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RESULTS AND DISCUSSION Chromium complexes can be synthesized through direct deprotonation of H4(BMe2,MeTCH)Br2 with nBuLi at low temperature, followed by the addition of a chromium salt in a manner analogous to their previously reported iron complexes (Scheme 1).16 With CrCl2 as the metal salt, we Scheme 1. Synthesis of the ((BMe2,MeTCH)Cr)2 Dimer, 1, and (BMe2,MeTCH)Cr(Br)(NCCH3), 2, and [(BMe2,MeTCH)Cr(NCCH3)2](PF6), 3, from the 16-Atom Imidazolium Precursor H4(BMe2,MeTCH)Br2

formed a diamagnetic product, ((BMe2,MeTCH)Cr)2, 1. Complex 1 can be prepared in very good yields as a bright yellow powder that is slightly soluble in benzene, toluene, THF, and acetonitrile. The poor solubility in polar solvents, vastly improved stability, and diamagnetic character (S = 0) were contrary to observations for its 18-ringed analogue, (BMe2,EtTCH)Cr, which was very soluble in polar solvents, extremely air sensitive, and showed paramagnetic behavior (S B

DOI: 10.1021/acs.organomet.9b00476 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 2. Synthesis of 4, 5, 6, and 7 from 1 or 3

BMe2,Me

Figure 3. X-ray crystal structure of (A) (BMe2,MeTCH)CrO, 4, and (B) [(BMe2,MeTCH)CrO](PF6), 5. Pink, red, blue, gray, and olive ellipsoids (50% probability) represent Cr, O, N, C, and B atoms, respectively. Solvent molecules, counterions and H atoms are omitted for clarity. Selected interatomic bond distances (Å) and angles (deg) are as follows: (4) Cr1−C1 = 2.037(3); Cr1−C2 = 2.042(5); Cr1−C3 = 2.049(3); Cr1−C4 = 2.036(4); Cr1−O1 = 1.563(4); C4−Cr1−C2 = 146.40(15); C3−Cr1−C1 = 146.04(15). (5) Cr1−C1 = 2.023(1); Cr1−C2 = 2.015(1); Cr1−C3 = 2.049(1); Cr1−C4 = 2.031(1); Cr1−O1 = 1.546(1); C1−Cr1−C3 = 144.27(2); C2−Cr1−C4 = 144.37(2).

H

Figure 2. X-ray crystal structures of (A) (( TC )Cr)2, 1, (B) (BMe 2 ,Me TC H )Cr(Br)(NCCH 3 ), 2, and (C) [( BMe 2 ,Me TC H )Cr(NCCH3)2](PF6), 3. Pink, yellow, blue, gray, and olive ellipsoids (50% probability) represent Cr, Br, N, C, and B atoms, respectively. Solvent molecules, counterions, and H atoms are omitted for clarity. Selected interatomic bond distances (Å) and angles (deg) are as follows: (1) Cr1−Cr2 = 1.933(4) Cr1−C1 = 2.091(2); Cr1−C2 = 2.095(2); Cr1−C3 = 2.089(2); Cr1−C4 = 2.090(2); Cr2−C19 = 2.084(2); Cr2−C20 = 2.086(2); Cr2−C21 = 2.085(2); Cr2−C22 = 2.076(2); C1−Cr1−C3 = 145.88(7); C2−Cr1−C4 = 146.19(7); C19−Cr2−C21 = 147.06(7); C20−Cr2−C22 = 147.44(7). (2) Cr1− C1 = 2.051(2); Cr1−C2 = 2.055(2); Cr1−C3 = 2.053(2); Cr1−C4 = 2.051(2); Cr1−Br1 = 2.480(1); Cr1−N9 = 2.093(1); C1−Cr1−C3 = 170.23(6); C2−Cr1−C4 = 171.59(6); N9−Cr1−Br1 = 179.30(4). (3) Cr1−C1 = 2.048(2); Cr1−C2 = 2.049(2); Cr1−C3 = 2.044(2); Cr1−C4 = 2.047(2); Cr1−N9 = 2.050(2); Cr1−N10 = 2.031(2); C1−Cr1−C3 = 174.13(7); C2−Cr1−C4 = 173.72(7); N9−Cr1− N10 = 178.91(6).

pyramidal or octahedral coordination.25,26 While most Cr(IV) oxo complexes are paramagnetic (S = 1), Groves reported a set of diamagnetic (S = 0) Cr(IV) oxos supported by porphyrin ligands.26−28 This diamagnetic ground state for 4, somewhat surprisingly, does not have any influence on the Cr−O bond distance compared to the 18-atom ringed counterpart, which has a reported spin of S = 1 (1.563(4) Å vs 1.564(3) Å).8 Electrochemical investigations of 4 via cyclic voltammetry revealed a fully reversible oxidation at −290 mV and an

larger analogue (BMe2,EtTCH)Cr(O),8 but distinct from some of the other reported Cr(IV)O compounds, which depict trigonal C

DOI: 10.1021/acs.organomet.9b00476 Organometallics XXXX, XXX, XXX−XXX

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Organometallics irreversible reduction at −2350 mV (both vs Fc/Fc+, see Figure S18), thus suggesting that 4 can be oxidized to form a cationic oxo complex. Stoichiometric addition of ferrocenium hexafluorophosphate to a THF solution of 4 led to an immediate change in color from green to red. Cooling the resulting red solution to −33 °C overnight led to the formation of a dark red crystalline solid in 65% yield. The dark red crystals of [(BMe2,MeTCH)Cr(O)](PF6), 5, are quite stable and can be stored for several days at room temperature in air without any visual or spectroscopic signs of decomposition. Despite having only one unpaired electron (Evans method, μB = 1.84), 5 is completely 1H NMR silent with no observable signals (See Figure S9). Complex 5 is the first reported example of an isolated Cr(V)O complex with NHC ligands, and only the second Cr(V) NHC complex.7 The crystal structure of 5 shows the expected square pyramidal Cr center bound by a terminal oxo (Figure 3B). The Cr(V)−O distance is 1.564(1) Å, which is slightly shorter than the distance found in 4. The average Cr−C distance is 2.032(1) Å, which is significantly shorter than the average Cr−C distance (2.111(1) Å) of the other Cr(V) complex with NHC ligands that was previously reported by Yang.7 Complex 5 can also be synthesized by direct oxygen transfer of trimethyl N-oxide to 3 (Scheme 2). Due to the effectiveness of a previous macrocyclic NHC chromium complex we prepared for catalytic aziridination, we were particularly interested in the reactions of 1 and 3 with organic azides.8 Addition of diisopropylphenyl azide (N3DiPP) to 1 in acetonitrile led to a slow color change to orange-red. After extracting the excess azide with pentane, the Cr(IV) imide complex, (BMe2,MeTCH)Cr(NDiPP) (6), was isolated in 60% yield. This imide complex is paramagnetic (S = 1), which is consistent with our previously prepared chromium imide complexes such as (BMe2,EtTCH)Cr(NMes) (Mes = mesityl),8 but contrary to diamagnetic complex 4. Notably, West found similar paramagnetic Cr(IV) imide complexes with porphyrins as auxiliary ligands.29 The single crystal X-ray structure for 6 is quite similar to (BMe2,EtTCH)Cr(NMes) and features a relatively long Cr−N bond (1.6979(12) Å) and a slightly bent Cr−N−C angle (158.95(11)°) (Figure 4).8 Similarly, the addition of tertbutyl azide to 1 led to the isolation of (BMe2,MeTCH)Cr(NtBu), 7. Unlike the isoelectronic oxo complex 4, these two imide complexes could not be oxidized to the Cr(V) state. Furthermore, addition of these organic azides to 3 resulted in no reactions to form Cr(V) complexes. With the set of imide and oxo complexes synthesized, we desired to determine if they could undergo group transfer reactions either stoichiometrically or catalytically. Chromium oxo complexes reportedly range from being completely inert to demonstrating radical based hydrogen abstraction reactions.8,25,26 As previously mentioned, 4 is very stable and shows no ability to transfer its oxygen atom. However, by switching to 5, oxygen transfer is possible with reductants such as PPh3. Over the course of several hours, the formation of OPPh3 (26 ppm) can be observed via 31P NMR spectroscopy at room temperature. Additionally, after 8h, the formation of a second peak at 21.4 ppm in the 31P NMR arises. We attribute this second peak to a OPPh3 adduct forming, as both the concentrations of 3 and OPPh3 increase over time (Figure S12). Reactions with the imide complexes 6 and 7 showed no reductive reactivity with alkenes or phosphines, which is in contrast to (BMe2,EtTCH)Cr(NMes), showing once again that a

Figure 4. X-ray crystal structure of (BMe2,MeTCH)Cr(N(DiPP)), 6. Pink, blue, gray, and olive ellipsoids (50% probability) represent Cr, N, C, and B atoms, respectively. H atoms are omitted for clarity. Selected interatomic bond distances (Å) and angles (deg) are as follows: Cr1−C1 = 2.0674(14); Cr1−C2 = 2.0797(14); Cr1−C3 = 2.0498(14); Cr1−C4 = 2.0427(14); Cr1−N9 = 1.6979(12); Cr1− N9−C19= 158.95(11).

very small change in the macrocyclic ligand leads to different reactivity.8 Since the heavier group six metals, Mo and W, are also known for their ability to form M-M multiple bonds, we extended our studies with the 16-atom ligand to these elements.2,30,31 The first challenge was to find suitable M(II) precursors. One of the most common Mo(II) precursors, [Mo2(MeCN)10](BF4)4, did not yield any product, possibly due to the incompatibility of the acetonitrile ligands with the strong base that was necessary to deprotonate the ligand. However, (Mo2(OAc)4) was an effective Mo(II) source under the same conditions as the formation of 1 (Scheme 3). Despite only providing low yields (8%) of a purple solid, ((BMe2,MeTCH)Mo)2 (8), the reaction was reproducible over multiple attempts. Similar to the Cr analogue 1, 8 features a short M−M quadruple bond (Mo−Mo, 2.139(4) Å; Figure 5) and Scheme 3. Synthesis of the ((BMe2,MeTCH)Mo)2 Dimer, 8, from the 16-Membered Imidazolium Precursor H4(BMe2,MeTCH)Br2 and (Mo2(OAc)4)a

a

D

A similar reaction with (W2Cl4(PBu3)4) led to no isolable products. DOI: 10.1021/acs.organomet.9b00476 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

complex could be isolated. Despite a highly similar structure to the previously reported 18-atom ringed macrocycle, there are considerable differences in the reactivity and complexes that can be isolated on these group six metals.

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EXPERIMENTAL SECTION

All reactions were performed under a dry nitrogen atmosphere with the use of either a drybox or standard Schlenk techniques. Solvents were dried on an Innovative Technologies (Newburyport, MA) Pure Solv MD-7 Solvent Purification System and degassed by three freeze− pump−thaw cycles on a Schlenk line to remove O2 prior to use. Acetonitrile was additionally distilled over P2O5 under nitrogen atmosphere and stored over activated molecular sieves. Benzene-d6, acetonitrile-d3, and tetrahydrofuran-d8 were degassed by three freeze− pump−thaw cycles prior to drying over activated molecular sieves. These NMR solvents were then stored under N2 in a glovebox. H4(BMe2,MeTCH)Br2,16 (Mo2(OAc)4),33 and (W2Cl4(PnBu3)4)32 were prepared by previously reported procedures. All other reagents were purchased from commercial vendors and used without purification. 1 H, 13C{1H} NMR spectra were recorded at ambient temperature on a Varian Mercury 300 MHz or a Varian VNMRS 500 MHz narrowbore broadband system. 1H and 13C NMR chemical shifts were referenced to the residual solvent. All mass spectrometry analyses were conducted in acetonitrile at the Biological and Small Molecule Mass Spectrometry Core facility located in the Department of Chemistry at the University of Tennessee. The DART analyses were performed using a JEOL AccuTOF-D time-of-flight (TOF) mass spectrometer with a DART (direct analysis in real time) ionization source from JEOL USA, Inc. (Peabody, MA). Infrared spectra were collected on a Thermo Scientific Nicolet iS10 with a Smart iTR accessory for attenuated total reflectance. Elemental analyses were conducted by Midwest Microlabs (Indianapolis, IN). Synthesis of ((BMe2,MeTCH)Cr)2 (1). In a 100 mL Schlenk flask, H4(BMe2,MeTCH)Br2 (500 mg, 0.93 mmol) was suspended in 50 mL of THF. In a second 100 mL Schlenk flask, anhydrous CrCl2 (120 mg, 0.98 mmol) was suspended in 25 mL of THF and a small Strauss tube (10 mL) was filled with 1.6 mL of 2.5 M nBuLi solution in hexanes (3.90 mmol). All flasks were taken out of the glovebox and connected to a Schlenk line. The ligand suspension and the nBuLi solution were cooled to −80 °C in an acetone/dry ice cooling bath for 15 min. Under vigorous stirring the nBuLi solution was added to the ligand suspension and mixed at −80 °C for at least 1 h. The now yellow suspension was allowed to warm up to 0 °C in an ice bath and vigorously stirred for at least another hour or until a clear yellow solution is obtained. The clear solution was cooled back down to −80 °C together with the flask containing the CrCl2 and THF suspension. The carbene solution was transferred into the CrCl2 suspension at −80 °C, at which time the resulting mixture immediately changes its color to yellowish brown. The mixture was allowed to slowly warm up to room temperature and is stirred overnight. The solution was evaporated in a 30 °C water bath (vacuum