Synthesis, Structure, and Hydrogenolysis of Pyridine Dicarbene Iron

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

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Synthesis, Structure, and Hydrogenolysis of Pyridine Dicarbene Iron Dialkyl Complexes Stephan M. Rummelt, Jonathan M. Darmon, Renyuan Pony Yu, Peter Viereck, Tyler P. Pabst, Zoë R. Turner, Grant W. Margulieux, Shunlin Gu, and Paul J. Chirik* Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States

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ABSTRACT: Two methods for the synthesis of bis(imidazol2-ylidene)pyridine iron dialkyl complexes, (CNC)Fe(CH2SiMe3)2, have been developed. The first route consists of addition of 2 equiv of LiCH2SiMe3 to the iron dihalide complex (CNC)FeBr2, while the second relies on addition of the free CNC ligand to the readily prepared (py)2Fe(CH2SiMe3)2 (py = pyridine). With aryl-substituted CNC ligands, octahedral complexes of the type (ArCNC)Fe(CH2SiMe3)2(N2) (ArCNC = bis(arylimidazol-2-ylidene)pyridine) were isolated, where the dinitrogen ligand occupies the site trans to the pyridine of the CNC chelate. In contrast, the alkyl-substituted variant ( tBuACNC)Fe(CH 2 SiMe 3)2 (tBuACNC = 2,6-(tBu-imidazol-2-ylidene)2pyridine) was isolated as the five-coordinate compound lacking dinitrogen. Exposure of the (ArCNC)Fe(CH2SiMe3)2(N2) derivatives to an H2 atmosphere resulted in formation of the corresponding iron hydride complexes (ArCNC)FeH4. These compounds catalyzed hydrogen isotope exchange between the deuterated benzene solvent and H2, generating isotopologues and isotopomers of (ArCNC)Fe(Hn)(D4−n) (n = 0−4). When (3,5-Me2MesCNC)Fe(CH2SiMe3)2(N2) (3,5-Me2MesCNC = 2,6-(2,4,6-Me3-C6H2-imidazol-2-ylidene)2-3,5-Me2-pyridine) was treated successively with H2 and then N2, the corresponding reduced dinitrogen complex (3,5-Me2MesCNC)Fe(N2)2 was isolated. The same product was also obtained following addition of pinacolborane to (3,5-Me2MesCNC)Fe(CH2SiMe3)2(N2).

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INTRODUCTION The oxidative addition of nonpolar bonds such as H−H, B−H, Si−H, and C−H is a elementary substrate activation step in organometallic transformations1 and poses a fundamental challenge for catalysis with earth-abundant first-row transition metals.2 One strategy for enabling this reactivity with iron and cobalt is to introduce pincer ligands with strongly σ donating phosphines or N-heterocyclic carbenes to enforce a strong ligand field and promote formation of low-spin complexes prone to two-electron reactivity.2a,c Examples of concerted, two-electron oxidative addition with both iron and cobalt are now well documented.2 The aryl-substituted bis(arylimidazol-2-ylidene)pyridine iron dinitrogen compounds [(ArylCNC)Fe(N2)2], originally synthesized by Danopoulos and co-workers,3,4 are exemplary of the strong-field ligand strategy and promote the oxidative addition of C−H,3a Si−H,3b and H−H bonds.3c These complexes have also been shown to be highly active precatalysts for the hydrogenation of hindered, essentially unactivated olefins.5 More recently, a variant with saturated arylimidazolines, (H4-arylCNC)Fe(N2)2 (H4-arylCNC = bis(arylimidazolin-2-ylidene)pyridine), was synthesized and shown to be an effective catalyst for hydrogen isotope exchange (HIE) and the tritiation of active pharmaceutical ingredients.6 The catalyst exhibits both broad functional group © XXXX American Chemical Society

tolerance and high HIE activity in polar solvents such as Nmethylpyrrolidone (NMP) relies on sterically driven site selectivity that is complementary to state of the art iridium catalysts by ignoring directing groups. One challenge associated with these compounds is their synthesis and extreme air and moisture sensitivity (Scheme 1a). Typical routes to the dinitrogen complexes involve reduction of iron(II) dihalide precursors with strong reducing agents, typically sodium amalgam or sodium naphthalenide. The success and efficiency of this reaction also proved to be highly dependent on the nature of the aryl substituents, as iron complexes bearing smaller groups required stoichiometric sodium and catalytic naphthalene for reduction to the corresponding iron bis(dinitrogen) complex.5 No examples of iron bis(dinitrogen) complexes with N-alkyl rather than Naryl substituents on the carbene are known. Another limitation of the current synthetic routes is the reliance on Fe[N(SiMe3)2]2 as the iron precursor for preparation of the iron dihalides. This material is a low-melting, green solid that is highly air sensitive and requires distillation for purification.7 Alternative synthetic routes that rely on more accessible Received: June 5, 2019

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DOI: 10.1021/acs.organomet.9b00382 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Scheme 1. Synthesis of (CNC)Fe Precatalysts from (a) (CNC)Fe Dihalide Complexes and (b) (CNC)Fe Dialkyl Complexes

Scheme 2. Two Methods for the Synthesis of (iPrCNC)Fe(CH2SiMe3)2(N2)

The 1H NMR spectrum of 1-Ns2(N2) in benzene-d6 is consistent with a C2v-symmetric complex with resolved coupling of the pyridine protons and the methylene groups of the neosilyl ligand detectable at −1.11 ppm. The carbene carbon atoms give rise to a signal in the 13C NMR spectrum at 223.1 ppm, consistent with other (CNC)Fe complexes.3−6,11 The 13C resonances of the alkyl carbons directly bound to iron, however, were not observed by 13C NMR spectroscopy. Single crystals of 1-Ns2(N2) suitable for X-ray diffraction were obtained from a concentrated pentane solution at −35 °C. The solid-state structure of 1-Ns2(N2) is best described as an idealized octahedral complex with the two alkyl ligands oriented trans to each other, although slightly deviated from linearity (C40−Fe1−C33 = 163.26(14)°, Figure 1). The coordination site trans to the pyridine is occupied by a dinitrogen ligand, the coordination of which is maintained in solution as judged by IR spectroscopy (νN2(C6D6) = 2129

precursors that are compatible with a range of ligand substitution patterns are therefore valuable and of interest. The iron dialkyl complexes (CNC)Fe(CH2SiMe3)2 (CNC = bis(imidazol-2-ylidene)pyridine) are attractive targets for isolable catalyst precursors due to their potential synthetic accessibility and activation by straightforward treatment with H2, silanes, or boranes.8 The commercial availability of LiCH2SiMe3 and the synthetic accessibility of (pyridine)2Fe(CH2SiMe3)2 reported by Cámpora9 also motivated exploration of routes utilizing these reagents. Previous reports from our laboratory have demonstrated the utility of both iron and cobalt dialkyl complexes of this type as precatalysts for alkene hydrogenation, hydroboration, hydrosilylation, HIE, and C(sp2/3)−H borylation reactions.8 Here, we describe the synthesis and characterization of a family of (CNC)Fe(CH2SiMe3)2 complexes, their propensity to coordinate dinitrogen as a function of N-heterocyclic carbene substituent, and their reactivity with hydrogen.

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RESULTS AND DISCUSSION Our studies commenced with the preparation of dialkyl derivatives of [(iPrCNC)Fe], as this compound is the most studied in its class. Dialkylation of the iron dihalide (iPrCNC)FeBr2 was initially explored with the addition of 2 equiv of LiCH2SiMe3 in diethyl ether at −35 °C. A color change to dark violet was observed, and filtration followed by recrystallization from pentane at −35 °C furnished (iPrCNC)Fe(CH2SiMe3)2(N2) (1-Ns2(N2)) as a diamagnetic violet solid in 61% isolated yield (Scheme 2). An alternative route was inspired by the work of Cámpora, who demonstrated that addition of tridentate, aryl-substituted pyridine(diimine) ligands to (py)2Fe(CH2SiMe3)2 resulted in pyridine displacement and formation of the five-coordinate iron dialkyl complexes.9 To explore analogous chemistry with the iPrCNC chelate, the free carbene was prepared by treatment of the bis(imidazolium) salt iPrCNC(HBr)2 with KHMDS (HMDS = N(SiMe3)2).10 Addition of the resulting free chelate to a readily generated solution of (py)2Fe(CH2SiMe3)2 in pentane at −35 °C resulted in a dark violet solution, which upon concentration and filtration resulted in isolation of 1-Ns2(N2) in 67% yield based on iPrCNC.

Figure 1. Solid-state molecular structure of ( iPr CNC)Fe(CH2SiMe3)2(N2) (1-Ns2(N2)) with 30% probability ellipsoids. Hydrogen atoms and one aryl substituent are omitted for clarity. Selected bond distances (Å) and angles (deg): C10−Fe1 1.936(3), C11−Fe1 1.933(3), N3−Fe1 1.877(3), N6−Fe1 1.829(3), C33−Fe1 2.137(3), C40−Fe1 2.135(3); N6−Fe1−N3 177.39(13), N6−Fe1− C11 100.79(14); N3−Fe1−C11 79.80(14), N6−Fe1−C10 99.50(14), N3−Fe1−C10 79.88(13), C11−Fe1−C10 159.68(15), C40−Fe1−C33 163.26(14). B


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Organometallics

Figure 2. Zero-field 57Fe Mössbauer spectra of (a) (iPrCNC)Fe(CH2SiMe3)2(N2) (1-Ns2(N2)), (b) (MesCNC)Fe(CH2SiMe3)2(N2) (2-Ns2(N2)), (c) (3,5-Me2MesCNC)Fe(CH2SiMe3)2(N2) (3-Ns2(N2)), and (d) (tBuACNC)Fe(CH2SiMe3)2 (4-Ns2) at 80 K in the solid state. Simulated parameters (blue lines) are as follows: (a) δ = 0.13 mm/s, |ΔEQ| = 1.71 mm/s; (b) δ = 0.13 mm/s, |ΔEQ| = 1.62 mm/s; (c) δ = 0.11 mm/s, |ΔEQ| = 1.90 mm/s; (d) δ = 0.13 mm/s, |ΔEQ| = 3.89 mm/s.

cm−1). The bite of the CNC chelate results in a deviation of the C(11)−Fe(1)−C(10) angle from linearity (C11−Fe1− C10 = 159.68(15)°) despite a yaw angle of 15.7° (obtained from ((N1−C11−Fe1)−(N2−C11−Fe1))/2). Having two strong-field, mutually trans X-type ligands and a neutral ligand trans to the pyridine in the CNC plane around the iron center is a general structural feature of this class of complexes, as it has also been observed in related iron dihydride and silyl hydride complexes.3b,c The zero-field, solid-state 57Fe Mössbauer spectrum of 1-Ns2(N2) was measured at 80 K and exhibited a doublet with an isomer shift (δ) of 0.13 mm/s and a quadrupole splitting (|ΔEQ|) of 1.71 mm/s, consistent with a low-spin iron(II) complex (Figure 2a).12 For the synthesis of other iron dialkyl complexes of this type, the route involving pyridine displacement from (py)2Fe(CH2SiMe3)2 was preferred because it obviates the use of Fe[N(SiMe3)2]2. While the yields using the two routes are similar, the iron dialkyl precursor obviates the distillation of an air-sensitive iron precursor. Addition of the free pyridine dicarbene pincer MesCNC to a pentane solution of (py)2Fe(CH2SiMe3)2 followed by filtration furnished (MesCNC)Fe(CH2SiMe3)2(N2) (2-Ns2(N2), MesCNC = 2,6-(2,4,6-Me3C6H2-imidazol-2-ylidene)2pyridine) as a violet-brown solid in 72% yield (Scheme 3). Using a similar procedure, the related 3,5-dimethylpyridine-substituted iron dialkyl dinitrogen complex (3,5-Me2MesCNC)Fe(CH2SiMe3)2(N2) (3-Ns2(N2), 3,5Me2MesCNC = 2,6-(2,4,6-Me3-C6H2-imidazol-2-ylidene)2 3,5Me2-pyridine) was isolated as a brown solid in 90% yield (Scheme 3). The dimethyl substituents at the 3- and 5-

positions of the central pyridine ring were originally reported by Danopoulos and co-workers to prevent deleterious metalation at those sites and generally gave improved yields.3a The diamagnetic iron complexes 2-Ns2(N2) and 3-Ns2(N2) were both characterized by a combination of NMR, IR, and Mössbauer spectroscopies. 1H and 13C NMR spectra exhibit features similar to those of 1-Ns2(N2), with the exception that the carbene carbons bound to iron were not located by 13C NMR spectroscopy. Dinitrogen coordination was confirmed by benzene solution IR spectroscopy with strong bands located at 2126 and 2118 cm−1 for 2-Ns 2(N 2) and 3-Ns 2(N 2), respectively. The zero-field 57Fe Mössbauer spectra for both complexes exhibit parameters as expected for octahedral iron complexes in a strong ligand field (δ = 0.13 mm/s, |ΔEQ| = 1.62 mm/s, 2-Ns2(N2); δ = 0.11 mm/s, |ΔEQ| = 1.90 mm/s (3-Ns2(N2)) (Figure 2b,c).12 The infrared and Mössbauer spectroscopic data both support a more electron donating pincer upon introduction of methyl groups in the 3,5-positions of the central pyridine ring. The solid-state structure of 3Ns2(N2) was determined by X-ray diffraction of single crystals obtained from a pentane/Et2O solution at −35 °C, confirming the idealized octahedral geometry of the complex with trans alkyl ligands (Figure 3). With a reliable synthetic method in hand, variants with alkyl substituents on the imidazol-2-ylidenes of the CNC ligands were targeted (Scheme 4). Addition of tBuACNC (tBuACNC = 2,6-(tBu-imidazol-2-ylidene)2pyridine) to a pentane solution of (py)2Fe(CH2SiMe3)2 followed by filtration furnished a brown diamagnetic solid in 77% yield identified as (tBuACNC)Fe(CH2SiMe3)2 (4-Ns2). In contrast to the aryl-substituted derivatives described above, no infrared band assignable to a dinitrogen stretch was observed in the benzene solution infrared spectrum. The 1H NMR spectrum of 4-Ns2 exhibited signals for the methylene groups of the neosilyl ligand at 8.34 ppm, as identified by a cross peak between the methylene group and the carbon atom of the SiMe3 group in the 1H−13C HMBC (heteronuclear multiple bond correlation) NMR spectrum. The triplet assigned to the 4-pyridine proton is shifted significantly downfield to 11.01 ppm in comparison to 1-Ns2(N2) (7.18 ppm) and 2-Ns2(N2) (7.11 ppm). Although the signal for this carbon was not directly observed by 13C NMR spectroscopy, the HSQC (HSQC = heteronuclear single quantum coherence) NMR spectrum exhibited a cross peak at

Scheme 3. Synthesis of (MesCNC)Fe(CH2SiMe3)2(N2) and (3,5-Me2MesCNC)Fe(CH2SiMe3)2(N2)

C


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Organometallics

Figure 3. Solid-state molecular structure of (3,5-Me2MesCNC)Fe(CH2SiMe3)2(N2) (3-Ns2(N2)) with 30% probability ellipsoids. Hydrogen atoms and one aryl substituent are omitted for clarity. Selected bond distances (Å) and angles (deg): C11−Fe1 1.8969(14), C10−Fe1 1.8983(15), C32−Fe1 2.1587(14), C36−Fe1 2.1256(15), Fe1−N6 1.8241(13), Fe1−N3 1.8822(12); N6−Fe1−C11 99.29(6), N3−Fe1−C11 80.69(6), N6−Fe1−C10 99.56(6), N3−Fe1−C10 80.57(6), C11−Fe1−C10 160.77(7), N6−Fe1−N3 178.53(5), C36− Fe1−C32 169.46(6).

Scheme 4. Synthesis of (tBuACNC)Fe(CH2SiMe3)2

Figure 4. Solid-state molecular structure of ( tBu ACNC)Fe(CH2SiMe3)2 (4-Ns2) with 30% probability ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): C10−Fe1 1.9467(17), C11−Fe1 1.9392(17), C20−Fe1 2.0633(16), C24−Fe1 2.0716(17), Fe1−N3 1.8205(14); N3−Fe1− C11 81.53(6), N3−Fe1−C10 81.55(6), C11−Fe1−C10 163.03(7), C20−Fe1−C24 148.63(7).

81.7 ppm. A signal for the carbene carbon was again not observable. Although 4-Ns2 is diamagnetic, the anomalous NMR chemical shifts are likely a result of the population of a low-lying triplet state. Single crystals of 4-Ns2 suitable for X-ray diffraction were obtained from a diethyl ether solution stored at −35 °C, and a representation of the solid-state structure is depicted in Figure 4. The coordination environment around the iron is between square pyramidal and trigonal bipyramidal with the two neosilyl ligands in a mutually trans disposition (C20−Fe1− C24 = 148.63(7)°) and in plane with the two carbene ligands. The tBu groups shield the sixth coordination site, likely preventing coordination of dinitrogen and resulting in the isolation of 4-Ns2 as a 5-coordinate, 16-electron complex. The diamagnetism of 4-Ns2 is noteworthy, as other fivecoordinate iron dialkyl complexes with tridentate pincer ligands usually adopt a trigonal-bipyramidal geometry and have either S = 1 or S = 2 ground states.8j,9,13 To our knowledge, an octahedral coordination environment has thus far been a prerequisite for diamagnetism among iron dialkyl complexes.14 The higher anisotropy of the electron density around the Fe(II) center this coordination environment confers has implications for Mössbauer spectroscopy. The zero-field 57Fe Mössbauer spectrum of 4-Ns2 exhibits a doublet with an isomer shift of 0.13 mm/s and a quadrupolar splitting of 3.89 mm/s, the latter being significantly higher than that observed for the octahedral, N2-containing complexes as well

as trigonal-bipyramidal, paramagnetic iron bis(neosilyl) complexes (Figure 3d).8j Similar behavior was also observed with square-planar, ferrous complexes in comparison to their tetrahedral counterparts.15 Synthesis of (ACNC)Fe(CH2SiMe3)2 analogues with smaller alkyl substituents on the imidazol-2-ylidene were unsuccessful, as addition of MeACNC (MeACNC = 2,6-(Meimidazol-2-ylidene)2pyridine) to (py)2Fe(CH2SiMe3)2 yielded a black solid, which was insoluble in benzene-d6 and did not exhibit any assignable resonances in the THF-d8 1H NMR spectrum. Iron dialkyl complexes with pincer-containing saturated N-heterocyclic carbenes were not obtained using this method, as deprotonation of (H4-iPrCNC)(HBr)2 and (H4-MesCNC)(HBr)2 produced a complex mixture of unidentified products, likely due to the decreased stability of free imidazolin-2-ylidenes in comparison to their unsaturated counterparts.16 With the various (CNC)Fe dialkyl complexes in hand, reactivity studies were conducted, specifically focused on the generation of reduced or hydride complexes, with potential application as hydrogenation and HIE catalysts.3c,5,6 First, ligand-induced reductive elimination of the alkyl ligands was targeted.14a,17 Exposure of a diethyl ether solution of 1Ns2(N2) to a carbon monoxide atmosphere did not yield the reductive elimination product (iPrCNC)Fe(CO)23b,11 but resulted in clean substitution of the dinitrogen ligand (Scheme 5). The diamagnetic iron product ( i P r CNC)Fe(CH2SiMe3)2(CO) (1-Ns2(CO)) was isolated in 81% yield following recrystallization from a Et2O/pentane mixture and was stable under an N2 atmosphere. As expected, both the 1H and 13C NMR spectra exhibited features similar to those of 1Ns2(N2). The zero-field, solid-state 57Fe Mössbauer spectrum exhibited a doublet with an isomer shift of −0.03 mm/s and a D


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Organometallics Scheme 5. Synthesis of (iPrCNC)Fe(CH2SiMe3)2(CO)

comparable to the spectroscopic features observed with 1Ns2(CO). Given the performance of [(CNC)Fe] complexes in catalytic alkene hydrogenation and HIE chemistry,6 hydrogenolysis of the iron dialkyl complexes was studied (Scheme 6a). Exposure Scheme 6. (a) Reaction of (CNC)Fe(CH2SiMe3)2(N2) complexes with H2 in the Presence of C6D6 and (b) HighField Excerpts of the Corresponding 1H NMR Spectra

quadrupole splitting of 0.65 mm/s. Coordination of CO was also corroborated by infrared spectroscopy in toluene solution (νCO = 1919 cm−1). Single crystals of 1-Ns2(CO) suitable for X-ray diffraction were obtained from a pentane solution at −35 °C, and the representation of the solid-state structure presented in Figure 5

of a benzene-d6 solution of 1-Ns2(N2) to 4 atm of H2 resulted in a color change from violet to brown. Monitoring the progress of the reaction by 1H NMR spectroscopy revealed formation of SiMe4 along with a new diamagnetic product with upfield resonances between −8 and −10 ppm, consistent with the formation of iron hydrides (Scheme 6b). In analogy to spectroscopic features reported for (H4-iPrCNC)Fe(N2)2 under similar reaction conditions,3c these resonances were assigned as the isotopologues of (iPrCNC)FeH4, generated from hydrogenolysis of the iron alkyl ligands and subsequent HIE with the benzene-d6 solvent. Performing a similar procedure with 2Ns2(N2) and 3-Ns2(N2) also produced 1H NMR signals consistent with formation of isotopologues and isotopomers of the corresponding iron dihydride dihydrogen complexes.3c Exposure of 4-Ns2 to dihydrogen also liberated SiMe4, but no signals assignable to an organometallic iron compound were observed. Attention was also devoted to the synthesis of the reduced iron dinitrogen complexes (CNC)Fe(N2)2 using the dialkyl derivatives as precursors. Such routes would obviate harsh and potentially hazardous alkali-metal reduction reactions. The aryl-substituted compound 3-Ns2(N2) was chosen as a representative example (Scheme 7). Stirring a suspension of 3-Ns2(N2) in pentane successively under 4 atm of H2 followed by 1 atm of N2 resulted in the isolation of (3,5-Me2MesCNC)Fe(N2)2 (3-(N2)2) as a diamagnetic, brown solid in 96% yield. The same complex was also obtained in 94% isolated yield following addition of 1.2 equiv of HBPin (Pin = pinacolato) to

iPr

Figure 5. Solid-state molecular structure of ( CNC)Fe(CH2SiMe3)2(CO) (1-Ns2(CO)) with 30% probability ellipsoids. Hydrogen atoms and one aryl substituent are omitted for clarity. Selected bond distances (Å) and angles (deg): C44−Fe1 1.730(3), N3−Fe1 1.925(2), C10−Fe1 1.931(3), C11−Fe1 1.915(3), C36− Fe1 2.137(2), C40−Fe1 2.154(3); C44−Fe1−C11 100.45(12), C44−Fe1−N3 178.25(11), C11−Fe1−N3 79.81(10), C44−Fe1− C10 100.14(12), C11−Fe1−C10 159.37(12), N3−Fe1−C10 79.64(10), C36−Fe1−C40 170.16(11).

establishes an idealized octahedral geometry nearly identical with that of 1-Ns2(N2) with the CO ligand trans to the pyridine and two alkyl ligands mutually trans. While the Fe− carbene and Fe−alkyl bond lengths are similar to those in 1Ns2(N2), the Fe−pyridine bond length is significantly longer (1.925(2) Å in 1-Ns2(CO), 1.829(3) Å in 1-Ns2(N2)), likely a result of the stronger trans influence of the CO ligand in comparison to N2. Addition of the stronger field ligand carbon monoxide to 4Ns2 overcomes the apparent steric penalty associated with formation of a six-coordinate complex, as diamagnetic 4Ns2(CO) was isolated. CO coordination was confirmed by the observation of a strong band centered at 1919 cm−1 in the benzene-d6 solution infrared spectrum. The 1H NMR spectrum of 4-Ns2(CO) also differs slightly from that of 4-Ns2, with the methylene groups of the neosilyl ligand (−1.34 ppm) and 4pyridine (7.10 ppm) protons significantly shifted upfield, E


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Organometallics Scheme 7. Synthesis of (3,5-Me2MesCNC)Fe(N2)2

recorded in dilute solution (

Synthesis, Structure, and Hydrogenolysis of Pyridine Dicarbene Iron

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