Article Cite This: Organometallics XXXX, XXX, XXX−XXX
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Combined Effects of Backbone and N‑Substituents on Structure, Bonding, and Reactivity of Alkylated Iron(II)-NHCs Salvador B. Muñoz, III,† Valerie E. Fleischauer,† William W. Brennessel, and Michael L. Neidig* Department of Chemistry, University of Rochester, Rochester, New York 14627, United States
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ABSTRACT: Iron and N-heterocyclic carbenes (NHCs) have proven to be a successful pair in catalysis, with reactivity and selectivity being highly dependent on the nature of the NHC ligand backbone saturation and N-substituents. Four (NHC)Fe(1,3-dioxan-2-ylethyl)2 complexes have been isolated and spectroscopically characterized to correlate their reactivity to steric effects of the NHC from both the backbone saturation and N-substituents. Only in the extreme case of SIPr where NHC backbone and N-substituent steric effects are the largest is there a major structural perturbation observed crystallographically. The addition of only two hydrogen atoms is sufficient for a drastic change in product selectivity in the coupling of 1-iodo-3-phenylpropane with (2(1,3-dioxan-2-yl)ethyl)magnesium bromide due to resulting structural perturbations to the precatalyst. Mössbauer spectroscopy and magnetic circular dichroism enabled the correlation of covalency and steric bulk in the SIPr case to its poor selectivity in alkyl−alkyl cross-coupling with iron. Density functional theory calculations provided insight into the electronic structure and molecular orbital effects of ligation changes to the iron center. Finally, charge donation analysis and Mayer bond order calculations further confirmed the stronger Fe−ligand bonding in the SIPr complex. Overall, these studies highlight the importance of considering both N-substituent and backbone steric contributions to structure, bonding, and reactivity in ironNHCs.
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INTRODUCTION The ability to synthetically manipulate ligand additives for catalysis has enabled the development of a large selection of commercially available organometallic precatalysts and ligands for use in catalytic reactions. N-heterocyclic carbenes (NHCs) are a particularly popular class of ligand additives and have been used in a vast number of catalytic reactions as organocatalysts,1,2 and in tandem with transition metals3−9 and Group 13 elements.1,2,10 NHCs are available in the form of stable salts with a large number of different N-substituents, central ring size, and even central ring functionalization, making them very easy to access and work with.2,11 Furthermore, “smart NHCs” are under development which are redox-switchable, bifunctional, or hemilabile and have even been shown to exhibit backbone reactivity.12 Due to their structural diversity, NHCs have been utilized in many catalytic transformations including hydrosilylation, olefin metathesis, C−H activation, and, of particular note, cross-coupling reactions.2 The value of the NHC functionality is seen in some of the earliest reports of NHC cross-coupling with precious metals. The first Kumada cross-coupling reaction using NHCs was developed by Nolan and co-workers and showed a dependence on the bulkier IPr in catalysis over IMes (Scheme 1, right) for the successful coupling of aryl iodides with aryl Grignard reagents using palladium, but no backbone variations were © XXXX American Chemical Society
Scheme 1. Examples of Cross-Coupling Reactions with IronNHCs and the Results of IPr vs SIPr on Product Yields18,27,32
explored.13 Alternatively, Beller and co-workers reported the first aryl−alkyl cross-coupling reactions having success with IMes as a ligand additive instead of IPr in the presence of palladium.14 A simple N-substituent change allowed for the cross-coupling of two different sets of reagents. Along with their popularity in precious metal catalysis, NHCs have been used in a multitude of iron catalyzed reactions.15,16 In cross-coupling, aryl−aryl,17−20 aryl−alkenyl,21 alkyl−aryl,22−30 aryl−alkyl,31 and alkyl−alkyl32 transformations have all been achieved using a variety of NHCs (Scheme 1). In a popular Received: July 5, 2018
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DOI: 10.1021/acs.organomet.8b00466 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics example, Nakamura and co-workers reported in 2007 the Kumada coupling of aryl Grignard reagents with aryl halides using iron, nickel, and cobalt fluorides.17,18 Here, SIPr is the most effective NHC for catalysis, outperforming IPr with 98% yield compared to only 25% cross-coupled product (Scheme 1). In this case, and in several other reactions using NHCs and iron, a change in backbone saturation drastically alters selectivity.27,31 To better understand the selectivity of certain NHCs for different catalytic reactions, previous studies have explored the reactivity and electronic structure of independently synthesized iron-NHC compounds. For example, Deng and co-workers have investigated aryl and alkyl iron-NHCs,33−37 and Tonzetich and co-workers have explored the electronic structure and bonding effects in (NHC)FeXn and aryl containing species.38−40 More recently, the first direct observation of an in situ formed reactive iron-NHC complex was reported by the Neidig group.41 These studies, however, do not address the combined effects of NHC backbone structure and N-substitutions on electronic structure and bonding of reactive iron species in great detail. In this study, IMes, SIMes, IPr, and SIPr (Scheme 1, right) were used for the independent synthesis of iron-alkyl compounds to create a complete set of variations in both backbone and N-substituents. The effects of these changes on reactivity, electronic structure, and bonding are identified using gas chromatography (GC), X-ray diffraction, 57Fe Mössbauer spectroscopy, magnetic circular dichroism (MCD), and density functional theory (DFT) and show that iron-NHC reactivity can be influenced by steric effects deriving from both N-substituents and backbone saturation effects.
Table 1. Reactivity of NHC Variations in Alkyl−Alkyl CrossCoupling with Iron
NHC
A (%)
B (%)
C (%)
IMes·HCl SIMes·HCl IPr·HCl SIPr·HCl
78 77 69 23
15 15 23 56
7 8 8 21
cross-coupled product yield to only 23% is observed. The change from mesityl to diisopropylphenyl causes a loss of chelation in one of the alkyl ligands on iron for the isolated precatalyst with SIPr compared to IMes.41 Since backbone saturation has almost no effect on the reactivity of the mesityl NHCs, IPr was expected to have similarly poor catalytic performance to SIPr when used in this alkyl−alkyl crosscoupling reaction. However, the use of IPr results in the formation of 69% crosscoupled product, greatly out-performing SIPr, suggesting that the loss of reactivity seen with SIPr is more complex than a simple argument of added N-substituent steric bulk. Structural Characterization of (NHC)Fe(1,3-dioxan-2ylethyl)2 Compounds. The unexpected high selectivity observed for the IPr/SIPr couple inspired further characterization of the precatalysts formed during the reaction protocol to directly identify any structural changes caused by either backbone saturation or N-substituent variation. In the alkyl− alkyl cross-coupling of (2-(1,3-dioxan-2-yl)ethyl)magnesium bromide with 1-iodo-3-phenylpropane, (NHC)Fe(1,3-dioxan2-ylethyl)2 type species were found to form in situ during the precatalyst formation step of the reaction where excess Grignard reagent is added to iron and NHC·HCl in THF at 50−60 °C. The IMes derivative, 1IMes (Figure 1A), was shown to have two hemilabile alkyl ligands from the Grignard reagent. Therefore, initial efforts focused on the isolation of SIMes and IPr derivatives. To form (SIMes)Fe(1,3-dioxan-2-ylethyl)2, complex 2SIMes, 4 equiv of (2-(1,3-dioxan-2-yl)ethyl)magnesium bromide was added in one portion to 1 equiv each of Fe(OAc)2 and SIMes· HCl at room temperature. Following cooling to −80 °C, yellow crystals suitable for X-ray diffraction were obtained. The structure of 2SIMes (Figure 1B) is described as a distorted 5coordinate iron complex (τ5 = 0.43) with two chelating oxygen moieties from the alkyl ligands, a similar geometry to that of 1IMes (τ5 = 0.52).42 Oxygen interactions are at 2.5069(15) and 2.4553(14) Å to the iron center, slightly elongated in comparison with 1IMes. Fe−C bonds to the alkyl ligands are 2.106(2) and 2.101(2) Å, and the Fe−SIMes bond is 2.124(2) Å in length, consistent with the bischelated compound 1IMes (Table S1). The IPr precatalyst was isolated in a similar way to complex 2SIMes: 6 equiv of (2-(1,3-dioxan-2-yl)ethyl)magnesium bromide was added to 1 equiv each of Fe(OAc)2 and IPr·HCl at room temperature. After cooling to −80 °C, yellow crystals suitable for X-ray diffraction were acquired. Consistent with its comparable reactivity to 1IMes and 2SIMes, 3IPr (Figure 1C) is also described as a distorted 5-coordinate (τ5 = 0.49) iron complex with two chelating dioxane alkyl rings with oxygen interactions
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RESULTS AND DISCUSSION Changes in Catalytic Activity Resulting from NHC Variations. As described previously, catalytic reactions with iron have used different NHCs with varying levels of success depending on N-substituent steric effects or backbone saturation. Recently, reactivity differences due to changes in N-substituents were solidified in a study from the Neidig group.41 (IMes)Fe(1,3-dioxan-2-ylethyl)2 (1IMes) was identified as a reactive iron species with chelating dioxane moieties on the alkyl ligands in the coupling of (2-(1,3-dioxan-2-yl)ethyl)magnesium bromide with 1-iodo-3-phenylpropane.41 In this alkyl−alkyl cross-coupling reaction originally reported by Cárdenas,32 bulkier NHCs like SIPr and even more sterically encumbering bisphosphine ligands resulted in low cross-coupled product yield (Scheme 1).32 Therefore, reactivity differences were yet again solely attributed to the change in NHC Nsubstituent. Preliminary results using alternative nucleophiles were also explored,41 but poor cross-coupled yields were observed for all reactions that did not contain a rigid, dioxanetype chelating moiety and are therefore not discussed in this NHC focused work. Since initial alkyl−alkyl cross-coupling studies did not include detailed catalytic data for the use of a series of NHCs with varying N-substituents and backbone saturation under optimized reaction conditions, the catalytic performances of IPr, SIPr, IMes, and SIMes were evaluated using a modified procedure from Cárdenas and co-workers32 as reported previously;41 Table 1 highlights these results. The use of IMes as a ligand additive results in the highest amount of crosscoupled product at 78% yield. Consistent with original reports, SIMes produces almost identical results as the only perturbation to ligand structure is in the saturation of the backbone. When an N-substituent change is made in the case of SIPr, a severe drop in B
DOI: 10.1021/acs.organomet.8b00466 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
Figure 1. X-ray crystal structures of NHC iron-alkyl complexes (A) (IMes)Fe(1,3-dioxan-2-ylethyl)241 (1IMes), (B) (SIMes)Fe(1,3-dioxan-2-ylethyl)2 (2SIMes), (C) (IPr)Fe(1,3-dioxan-2-ylethyl)2 (3IPr), and (D) (SIPr)Fe(1,3-dioxan-2-ylethyl)241 (4SIPr). Hydrogen atoms omitted for clarity except for the NHC backbones and thermal ellipsoids are shown at the 50% probability level. (E) Corresponding steric maps for the iron-alkyl complexes representing NHC proximity to the iron center. %Vbur was calculated using alkyl ligands as the pseudo xy plane, a radius of 3.5 Å from the iron center, and included H atoms; axes labels are in Å.
at 2.49068(13) and 2.44736(14) Å. Fe−C bond lengths to the alkyl groups are at 2.1125(19) and 2.11558(19) Å, and the Fe− IPr bond length is 2.1571(17) Å (Table S1). The stabilizing chelation of both alkyl groups through oxygen observed in 1IMes, 2SIMes, and 3IPr reduces the opportunity for β-H elimination during catalysis and is consistent with their high selectivity. This series of bisalkyl iron compounds is completed with the comparison to the structure of the SIPr derivative, complex 4SIPr (Figure 1D). This precatalyst was found to have the structure (SIPr)Fe(1,3-dioxan-2-ylethyl)2 with pseudo tetrahedral geometry (τ4 = 0.82) where only one alkyl group is chelated to iron through oxygen, attributed to the added steric bulk of the diisopropylphenyl groups as opposed to mesityl in 1IMes and 2SIMes. The Fe−NHC bond length is shortened to 2.0920 (10) Å, and the single chelated oxygen is at a distance of 2.3130(9) Å from the iron center. Loss of reactivity was initially assigned to the steric bulk of the N-substituents, but with the addition of complex 3IPr to this series, it is clear that this is not the only origin of structural perturbations in alkyl iron complexes. So, in cross-coupling reactions where NHC backbone variation correlates to product selectivity such as in the biaryl system developed by Nakamura, it is important to consider the electronic and steric effects of backbone variations. Interestingly, although 3IPr is also a bischelate through the two dioxane oxygen atoms, the steric bulk of the diisopropylphenyl groups appears to cause one of the dioxane rings to be severely canted relative to those of 1IMes and 2SIMes. When the RMS (root-mean-square) plane of the dioxane rings relative to the plane normal to the Fe−NHC bond is considered, the tilt of the dioxane rings in these complexes can be quantified. These results are shown in Table 2. The first “set” of chelated dioxane rings have similar orientations relative to the plane described above. In the case of 2SIMes and 4SIPr, which both have saturated NHC backbones, this angle is ∼10° greater than in 1IMes and 3IPr (Table 2), indicating that the simple addition of two hydrogens to the
Table 2. Quantified Canted Angles of Dioxane Rings Chelated to Iron in 1−4 Relative to the RMS Plane of the Dioxane Rings and the Plane Normal to the Fe-C13 Line complex
dioxane ring O1-O2/C3-C6
dioxane ring O3-O4/C9-C12
1IMes 2SIMes 3IPr 4SIPr
29.2° 39.7° 29.0° 38.6°
28.5° 11.3° 74.1° n.a.
NHC structure can be enough to modify substrate interaction with an iron center. In the case of the second “set” of chelated dioxane rings, the distortion in 3IPr has a value of 74.1°, whereas 1IMes shows an almost identical angle to that of the first chelated ring. Complex 2SIMes also shows a change in ring tilt from 39.7° to 11.3°, further confirming that backbone saturation can affect chelation to iron. To address the varying degrees of alkyl bischelation in compounds 1IMes to 4SIPr, the steric components of each iron compound were quantified as a function of NHC using SambVca 2.0.43 While the Tolman cone angle is a useful method for quantifying steric properties in monodentate phosphine ligands, the percent buried volume (%Vbur) allows for the sterics of the NHC ligand that project toward the metal center to be quantified.5 Complex 4SIPr shows the highest %Vbur for the NHC ligand of 44.6%, while 3IPr has a %Vbur of only 34.1%. When the steric maps44 are considered (Figure 1E), 4SIPr clearly shows the distortion of an isopropylphenyl group at approximately 1.4 Å to the iron center which creates the increase in %Vbur. 3IPr lacks this tilted isopropylphenyl ring and shows a maximum distance of approximately 2.2 Å from the iron center in its steric map. 1IMes and 2SIMes show nearly identical results to 3IPr with respect to maximum distance of the NHC substituents to the iron center and %Vbur. The steric mapping and %Vbur for 4SIPr are consistent with its loss of alkyl chelation observed crystallographically and C
DOI: 10.1021/acs.organomet.8b00466 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
variation in side N-substituents on the electronic structure of these molecules. In the case of Evans method analysis of the isolated crystalline 2SIMes in THF-d8, the μeff value of 4.9(0.2) μb is consistent with an S = 2 iron(II) center like in 1IMes and 4SIPr.41 5 K solid Mössbauer of isolated crystalline material further confirms the spin state of the iron center with δ = 0.54 mm/s and ΔEQ = 2.56 mm/s (Table 3, Figure S1). The Evans method of the isolated
subsequent decrease in cross-coupled product selectivity when compared with 3IPr. In fact, when the two NHC segments are directly superimposed, structural differences can be even more readily visualized (Figure 2A). Shown in red, the SIPr ligand
Table 3. Summary of 57Fe Mössbauer and MCD Parameters for (NHC)Fe(1,3-dioxan-2-ylethyl)2 Compounds Mössbauer
NIR MCD
complex
δ (mm/s)
ΔEQ (mm/s)
LF bands (cm−1)
IMes
0.57 0.54 0.58 0.44
2.42 2.56 2.36 2.91
5310, 7600 5400, 7630
