Reactivity of (NHC)2FeX2 Complexes toward Arylborane Lewis Acids

Reactivity of (NHC)2FeX2 Complexes toward Arylborane Lewis Acids and Arylboronates. Jay J. Dunsford†, Ian A. Cade†, ... Chloe Johnson and Martin A...
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Reactivity of (NHC)2FeX2 Complexes toward Arylborane Lewis Acids and Arylboronates Jay J. Dunsford,† Ian A. Cade,† Kathlyn L. Fillman,‡ Michael L. Neidig,‡ and Michael J. Ingleson*,† †

School of Chemistry, University of Manchester, Manchester M13 9PL, U.K. Department of Chemistry, University of Rochester, Rochester, New York 14627, United States

‡

S Supporting Information *

ABSTRACT: (NHC)2FeCl2 complexes undergo methoxide transfer in preference to aryl transfer from [(Aryl)B(OR)3]−, while addition of ArylBY2 (Y = Cl, OR) to (NHC)Fe−methoxide compounds leads only to formation of (NHC)BY2Aryl. The addition of PhBCl2 to (NHC)2FeX2 compounds is introduced as a method for probing NHC dissociation from iron. Two expanded ring NHCs also undergo dissociation from iron in the respective (NHC)2FeCl2 complexes.

W

Scheme 1. Transmetalation Reactions between Boron Nucleophiles and Iron Complexes

ell-defined iron(II) complexes have become the subject of growing interest as (pre)catalysts in a number of important catalytic transformations.1−4 This is particularly true for carbon−carbon bond formation, where low associated cost and low toxicity combined with a unique reactivity profile make iron catalysts an attractive alternative to palladium- and nickelbased systems.5−7 Important breakthroughs have been recently reported in this area using both well-defined and in situ generated iron species primarily bearing phosphine or monodentate N-heterocyclic carbene (NHC) ligands.8−19 These catalysts have enabled the cross-coupling of alkyl and aryl halides with a range of organometallic nucleophiles. While these advances have been significant, challenges in ironcatalyzed cross-coupling still remain. One in particular is achieving facile transmetalation, the transfer of the hydrocarbyl group, from boron-based nucleophiles to iron. This transformation is essential to realize a viable iron-catalyzed Suzuki− Miyaura reaction. Methodologies that proceed efficiently in palladium catalysis, specifically aryl boronic acids plus base, do not effect transmetalation with iron catalysts.20 Instead, anionic tetraarylborates or aryl boronates activated by coordination of organometallic nucleophiles (e.g., tBuLi to form [(tBu)ArylB(OR)2]−) have proved essential (Scheme 1).21−24 Cocatalysts such as MgBr2 and Zn(Aryl)2 were also necessary to promote hydrocarbyl transmetalation from boron to iron even when using these anionic boron-based nucleophiles. Because of the considerable potential of an iron-catalyzed Suzuki−Miyaura reaction that is compatible with conventional boron-based nucleophiles, we were interested in gaining an increased understanding of the reactivity of iron compounds toward common boron transmetalation reagents. In palladium cross-coupling two competitive transmetalation pathways have been determined, involving (i) ArylB(OR)2 and a Pd−OR species and (ii) a Pd−X (X = halide) species and M[ArylB(OR)3], with the former being dominant.25 Thus, Fe−X and Fe−OR species of relevance to C−C bond formation © 2013 American Chemical Society

necessitate investigation with four-coordinate anionic arylborates and neutral three-coordinate arylboranes, respectively. Nakamura and co-workers have previously determined that one successful class of iron catalysts for carbon−carbon crosscoupling contains an iron center that is coordinatively unsaturated, is in the +2 oxidation state, and has sufficient spin density located at the iron center.26 Combining this precedence with the extensive reports on NHC−Fe complexes catalyzing C−C cross-coupling,27,28 we have explored the reactivity of coordinatively unsaturated NHC−FeII complexes toward boron-based aryl nucleophiles. This study was restricted to monodentate NHC ligands, as heterocoupling reactions using (NHC)2FeX2 as catalyst precursors were shown to be more effectively catalyzed using two monodentate carbenes than when using a single chelating bidentate biscarbene.17,18 To simplify our transmetalation studies, 2-(p-tolyl)-1,3,2dioxaborolane (Tol-Beg; eg = ethylene glycolato, −OCH2CH2O−) and [Tol-Beg(OMe)]− were utilized as arylboronic acid surrogates. Arylboronic acids were deliberately avoided due to their existence in equilibrium with boroxines and H2O; the latter would lead to rapid hydrolysis of NHC−Fe bonds. To further facilitate this work, we initially focused on two known (NHC)2FeCl2 complexes (1, NHC = 5-iPr; 2, Received: November 13, 2013 Published: December 16, 2013 370

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NHC = IMes; Scheme 2).29,30 The NHCs in 1 and 2 are attractive, as they have distinct steric properties based on

hexanes. In both cases 1 and 2 were entirely consumed (by 1H NMR spectroscopy) but the new iron compounds were not observable in the 1H NMR spectra and frustrated all attempts at crystallization. Elemental analyses of the products derived from 2 repeatedly gave data that were consistent with NHC: Fe ratios of 1.0 mm/s; Figure 1A). We attribute these

Scheme 2. Previously Reported Iron(II) Complexes of the General Formula (NHC)2FeCl2 with Calculated %Vbur

percent buried volume (%Vbur)31 values calculated from the reported structures. The ability to vary sterics is important, as it can have a significant effect on the ability to form the bridging Fe−(μ-Y)−B species essential for transmetalation.25 Combination of 1 or 2 in THF with Tol-Beg and 5 equiv of NaOMe added to approximate classic cross-coupling conditions led in each case to complete consumption of the NHC−Fe starting material and formation of new iron compounds that were not observable in the 1H NMR spectra. In the reaction starting with 2 the only boron-containing species observed in the 11B NMR spectrum was (IMes)Tol-Beg (the identity of all boron-containing compounds was confirmed by independent synthesis) formed by loss of NHC from an iron compound, which then coordinates to Tol-Beg. However, with 1 both (5iPr)Tol-Beg and Na[Tol-Beg(OMe)] were observed in an approximate 1:1 ratio at short reaction times. On standing this mixture converted to predominantly (5-iPr)Tol-Beg after 16 h. The complete absence of Beg(OMe) (or its NHC adduct) and/or [Beg(OMe)2]− precluded aryl transfer to iron in both reactions. This is consistent with the complete absence of crosscoupling on addition of cycloheptyl bromide to the mixtures derived from 1 (or 2), NaOMe, and Tol-Beg. To exclude the possibility that rapid formation of Fe−OMe species by metathesis with NaOMe is consuming all Fe−Cl species before any direct reaction of Fe−Cl with Na[Tol-Beg(OMe)] can occur, stoichiometric Na[Tol-Beg(OMe)] was added to 1 and 2. In both cases this also resulted in the formation of (NHC)Tol-Beg as the only new boron-containing species. The transfer of MeO− to Fe−Cl species therefore occurs in preference to the transfer of aryl from [Tol-Beg(OMe)]− (eq 1). Selective MeO− transfer is not limited to NHC−Fe

Figure 1. 57Fe Mossbauer spectra at 80 K of (A) the product mixture from reaction of 2 with 5 equiv of NaOMe and (B) the solid-state multimeric product formed upon reaction of 5-iPr and 2 equiv of FeCl2 in THF. For both spectra the raw data (black dots), total fits (black lines) and individual species fits are given. The best fit to spectrum (A) contains the individual components δ = 1.28 mm/s, ΔEQ = 3.06 mm/s (46%, red), δ = 1.17 mm/s, ΔEQ = 2.44 mm/s (29%, blue), δ = 1.15 mm/s, ΔEQ = 1.93 mm/s (14%, green) and δ = 0.17 mm/s, ΔEQ = 1.30 mm/s (11%, purple). The best fit to spectrum B contains the individual components δ = 0.88 mm/s, ΔEQ = 1.41 mm/s (77%, red), δ = 0.92 mm/s, ΔEQ = 2.81 mm/s (16%, blue) and δ = 1.06 mm/s, ΔEQ = 2.20 mm/s (7%, green, attributed to 1 on the basis of NMR data).

to the partially/fully metathesized monomers/oligomers (IMes)2FeClx(OMe)2−‑x and {(IMes)FeClx(OMe)2−x}2 (x = 0−2). The reaction of this isolated mixture with PhBCl2 (ca. 2 equiv) was informative, leading to Cl/OMe scrambling between iron and boron, with IMes(Ph)B(OMe)2 and PhB(OMe)2 observed in the 11B{1H} NMR spectrum (Scheme 3). One new iron compound was visible in the 1H NMR spectrum,

complexes with addition of stoichiometric Na[Tol-Beg(OMe)] to dppeFeCl2 (dppe = bis(diphenylphosphino)ethane) resulting in complete conversion of Na[Tol-Beg(OMe)] to Tol-Beg (by 11B NMR spectroscopy) consistent with methoxide transfer to iron. With no evidence for transmetalation of aryl to well-defined Fe−X complexes, the ability to preform Fe−OMe complexes was investigated for subsequent reaction with Tol-Beg. Both 1 and 2 were reacted with an excess of NaOMe followed by isolation by filtration to remove unreacted NaOMe (which is poorly soluble in THF), drying in vacuo, and washing with

Scheme 3. Metathesis of 2 with NaOMe and Subsequent Reactivity with PhBCl2

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identifiable as {(IMes)FeCl2}2 (3).30 Complete conversion of PhBCl2 to (IMes)PhB(OMe)2 and PhB(OMe)2 confirmed that significant metathesis of chloride in 2 for methoxide had originally proceeded. With partial metathesis confirmed, the transmetalation propensity of the Fe−OMe-containing complexes isolated from 2 and 5 equiv of NaOMe was examined by combination with excess Tol-Beg. No new iron products were visible in the 1 H NMR spectra from this reaction, and attempts at isolation and characterization failed. This reaction did, however, lead to the rapid formation of new 11B NMR resonances attributable to (Tol)-Beg(OMe) and (IMes)Tol-Beg. With no products from aryl transfer observed in the 11B NMR spectra, this precludes transmetalation for these NHC−iron complexes containing Fe−OMe moieties. Thus, (NHC)Fe−OMe complexes will undergo anion exchange with B−Cl but not with B−aryl, while anionic borates will transfer alkoxide to Fe−Cl species in preference to an aryl moiety. The dissociation of the NHC ligands continually observed during metathesis/attempted transmetalation indicated labile NHC→Fe bonds. NHC dissociation from iron is an increasingly recognized phenomenon that is particularly prevalent in sterically crowded Fe−NHC complexes.28,34−37 NHC dissociation significantly complicates identification of the catalytically active species in NHC−Fe catalyzed crosscouplings, which are generally run with >1 equiv of NHC relative to iron. Control of the coordination number at iron is a crucial parameter, as it can drastically affect reactivity. This is exemplified by a three-coordinate (NHC)Fe(NHR)2 complex that does not react with aryl iodides, whereas a related fourcoordinate (NHC)2Fe(NHR)2 complex rapidly reacted.38 With NHC dissociation from (NHC)2FeX2 observed using two sterically disparate NHCs (based on %Vbur values of 1 and 2), we sought a simple chemical method to determine if carbene dissociation is occurring in NHC−Fe complexes. This is important to provide insight into what catalytic species are viable in solution. Furthermore, a simple synthetic probe is desirable, as it is often challenging to unequivocally identify NHC dissociation in paramagnetic NHC−Fe complexes by spectroscopic methods. To probe Fe−NHC bond stability, the reaction of (NHC)2FeCl2 and PhBCl2 was selected, as it introduces a useful 11B NMR spectroscopic handle and takes advantage of previously well-characterized (NHC)xFeCl2 complexes. These include monomeric (NHC)2FeCl2 complexes and the dimer {(IMes)FeCl2}2 (3).30 {(NHC)FeCl2}n oligomers are relevant, as they are the ultimate byproducts expected on NHC dissociation from (NHC)2FeCl2. To expand the number of well-characterized oligomeric species, {(5-iPr)FeCl2}n was first targeted. It is noteworthy that a stoichiometric combination of 5-iPr and (THF)1.5FeCl2 in THF led to the formation of both 1 and a new (5-iPr)nFeCl2 complex in an approximate 2:1 ratio (by 1H NMR spectroscopy). The new iron compound could be synthesized as the major product (ca. 5% of 1 was always present by 1H NMR spectroscopy) in 38% isolated yield following combination of 5iPr (0.5 equiv) and FeCl2 (1 equiv) in THF.33 The new species was identified as the desired oligomeric monocarbene iron species by its solid-state structure (Figure 2), which revealed it to be {(5-iPr)FeCl2}4 (4). Compound 4 is a linear tetramer containing two “terminal” distorted-tetrahedral four-coordinate iron centers and two core five-coordinate iron centers. While the poor quality of the data prohibits a detailed discussion of metrics, the tetrameric nature of 4 is unambiguous and is in

Figure 2. ORTEP representation of {(5-iPr)FeCl2}4 (4), with ellipsoids at 50% probability and hydrogens omitted for clarity.

contrast to the dimeric structures reported for 3 and the DIPP (2,6-diisopropylphenyl-substituted NHC)30 analogue. This disparity can be attributed to the decrease in steric demand associated with 5-iPr relative to IMes and DIPP. Dissolution of bulk 4 produced a 1H NMR spectrum which consisted of only three resonances (excluding the 5% intractable impurity of 1), inconsistent with the solid-state tetrameric structure. The data are more consistent with a dimer presumably in equilibrium with the tetramer. The solution magnetic moment (Evans method)39,40 of 7.7 μB at 294 K for 4 was closely comparable to the dimer {(DIPP)FeCl2}2 (7.5 μB),30 further supporting a dimeric solution formulation dominating for 4. The 80 K 57Fe Mössbauer spectrum of a powder sample exhibits a major feature with δ = 0.88 mm/s, ΔEQ = 1.41 mm/s (Figure 1B, 77%) and a minor species with δ = 0.92 mm/s, ΔEQ = 2.81 mm/s (16%). These are tentatively attributed to the dimer and the five-coordinate iron centers of the tetramer, respectively. The similarity of the coordination environment of terminal Fe in the tetramer to that of iron in the dimer (both distorted tetrahedral with 1 × 5-iPr, 1 × Clterminal, and 2 × μ-Cl) may result in coincidence in the Mö ssbauer spectrum. This spectrum is thus consistent with the solution-state observations of a mixture of complexes in which the dimeric complex is predominant. With the NMR spectroscopic handles of both (NHC)2FeCl2 and the oligomeric {(NHC)FeCl2}x forms in hand for both IMes and 5-iPr, the combination of (NHC)2FeCl2 and PhBCl2 in C6D6 was investigated (Figure 3). Analysis of the 1H NMR spectrum recorded minutes after addition of 1 equiv of PhBCl2 to 1 revealed the disappearance of all signals corresponding to 1 and new signals corresponding to 4. Analysis of the 11B NMR spectrum revealed the complete consumption of PhBCl2 (δ11B 55 ppm) and the presence of a single new resonance at 2.8 ppm consistent with a four-coordinate environment at boron. The identity of the new four-coordinate boron species was confirmed as (5-iPr)PhBCl2 (5) on the basis of a comparison of the 11B NMR resonances with independently prepared 5.33 No other resonances were observed precluding aryl transfer which would have formed BCl3 or (5-iPr)BCl3.41 The greater steric bulk of IMes vs 5-iPr indicated by the % Vbur values in the respective (NHC)2FeCl2 structures suggested that reversible dissociation of IMes from 2 should also be 372

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attempted synthesis of 8 a mixture of 8 and a second paramagnetic species, presumably a monocarbene Cl-bridged oligomeric species, {(7-iPr)FeCl2}n, were obtained in an approximate 50:50 ratio (by 1H NMR spectroscopy). This suggested that an increase in steric constraint imparted by the larger seven-membered-ring NHC species leads to an equilibrium position shifted toward oligomeric monocarbene iron species. The change in the relative stability of (NHC)2FeCl2 versus {(NHC)FeCl2}n and free NHC on increasing sterics has been previously observed for the DIPP analogue. In this case the considerable steric bulk of DIPP results in monomeric (DIPP)2FeCl2 being sterically inaccessible and exclusive formation of the dimer {(DIPP)FeCl2}2.30 Solid-state analysis of 7 and 8 (Figure 5) further established their formulation. The expected increased steric impact of the

Figure 3. Reaction of (NHC)2FeCl2 with PhBCl2.

occurring. Indeed, combination of 2 and 1 equiv of PhBCl2 in C6D6 at 20 °C resulted in an outcome analogous to that observed with 1, generating (IMes)PhBCl2 (6) and the Clbridged dimer 3. Considering the above experiments, it would appear that mono-NHC dissociation from (NHC)2FeX2 followed by oligomerization of the iron(II) complex occurs rapidly with NHC ligands of different steric demand. The free NHC is trapped by the borane Lewis acid, driving the equilibrium fully toward {(NHC)FeCl2}n. This process presumably proceeds via an unobserved monomeric (NHC)FeCl2 complex, with one such species recently isolated,42 that rapidly oligomerizes. In an effort to probe both the generality of NHC dissociation from iron(II) complexes and the applicability of PhBCl2 as a chemical probe for this process, we investigated other (NHC) Fe complexes. We were particularly interested in NHC ligands with steric demand comparable to that of IMes and 5-iPr, as this would facilitate formation of (NHC)2FeCl2 complexes while maximizing the probability of an analogous NHC dissociation equilibrium. Recently, there have been a number of reports concerning the synthesis and steric/electronic properties of expanded-ring NHCs, which are noted to be stronger σ-donor ligands by comparison to their imidazoliumbased analogues. The greater σ donicity of expanded ring NHC ligands comes with an increase in their associated steric parameters (7 > 6 > 5 for a fixed wingtip substituent).43 On the basis of the respective %Vbur values calculated from the structures of (NHC)Rh(COD)X (COD = cyclooctadiene, X = halide, OH),44 the 6-iPr and 7-iPr systems were reasonable targets (Figure 4), being stronger σ donors than 5-iPr and IMes but possessing steric bulk intermediate between the two.

Figure 5. ORTEP representations of (left) (6-iPr)2FeCl2 (7) and (right) (7-iPr)2FeCl2 (8) at the 50% ellipsoid probability level. Hydrogens and metrically similar second molecules of 7 and 8 present in the respective asymmetric units are omitted for clarity.

expanded ring NHC ligands was confirmed by the %Vbur values of the (NHC)2FeCl2 complexes, with 5-iPr < 6-iPr < 7-iPr (Table 1). The CNHC−Fe bond distances also elongate with increasing heterocycle ring size with 5-iPr < 6-iPr < 7-iPr, demonstrating the greater steric impact afforded by iPr wingtip Table 1. Selected Distances (Å) and Angles (deg) for (NHC)2FeCl2 NHC Fe−C1 Fe−C2 Fe−Cl1 Fe−Cl2 C1−Fe−C2 N1−C1−N2 N3−C2−N4 Cl1−Fe−Cl2 C1−Fe−Cl1 C2−Fe−Cl1 C1−Fe−Cl2 C2−Fe−Cl2 torsion anglec %Vburd

Figure 4. %Vbur values31 calculated from the structures of (NHC)Rh(COD)X (X = halide, OH).

The six- and seven-membered NHC complexes (6iPr)2FeCl2 (7) and (7-iPr)2FeCl2 (8) were accessed via the addition of the appropriate in situ generated free carbene to 0.25 equiv of {(TMEDA)FeCl2}2 in toluene. Compound 7 was formed as a single compound in solution and displayed a solution magnetic moment (Evans method) of 5.5 μB, consistent with a high-spin FeII center. In contrast, in the

IMes (2)a

5-iPr (1)b

6-iPr (7)

7-iPr (8)

2.140(4) 2.144(3) 2.280(2) 2.300(2) 126.09(14) 102.6(3) 102.9(3) 107.28(7) 99.74(9) 94.97(10) 112.46(11) 115.77(10) 80.8 31.5 30.6

2.136(2) 2.130(2) 2.304(1) 2.298(1) 102.05 104.2(1) 104.0(1) 112.95(2) 104.68(4) 106.85(4) 115.62(4) 114.41(4) 34.5 26.3 26.1

2.174(3) 2.187(3) 2.330(1) 2.310(1) 105.69(10) 117.2(2) 117.3(2) 108.40(3) 100.04(7) 98.59(7) 124.26(7) 121.71(7) 82.0 29.7 30.7

2.198(5) 2.198(5) 2.328(2) 2.324(1) 110.14(17) 119.7(4) 119.7(4) 105.65(5) 94.49(14) 97.85(7) 126.12(14) 125.35(13) 87.5 31.5 31.1

a From ref 30. bFrom ref 29. cTorsion angle defined as the calculated angle between the N−CNHC−N planes of the NHC ancillary ligands. d %Vbur calculated utilizing the SambVca software.31

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C6D6 producing (5-iPr)PhBCl2 (along with a small quantity of unreacted PhBCl2 consistent with starting stoichiometry) and an insoluble precipitate, most probably {FeCl2}n. It is highly unlikely that dissociation of the NHC would proceed directly from the dimeric species {(5-iPr)FeCl2}2, as this would require the intermediacy of an unprecedented45 three-coordinate neutral iron(II) center ligated only by chlorides. With no evidence for solvent complexation (e.g., diamagnetic Fe−η6arene complexes)46 NHC dissociation from 4 most probably involves higher oligomers such as the tetramer {(5-iPr)FeCl2}4. Close inspection of compound 4 revealed FeII−CNHC bond lengths (2.094(12) Å for the four-coordinate Fe and 2.114(10) Å for the five-coordinate iron) that do not indicate any significant degree of elongation and thus weakening. Furthermore, while the two 5-iPr ligands coordinated to the five-coordinate iron centers in 4 are located on the same face, the yaw angles (the in-plane distortion of the NHC defined as the difference between the two Fe−C−N angles divided by 2) are both 4 is key in this case to facilitate NHC dissociation by avoiding the formation, post NHC dissociation, of low-coordinate iron complexes possessing little steric protection. A related oligomerization step prior to NHC dissociation is precluded in (NHC)2FeCl2 complexes due to the crowded steric environment around iron imparted by two NHC ligands. In this case we favor the “steric assisted” dissociation of the NHC and formation of transient threecoordinate (NHC)FeX2 species, which then subsequently oligomerize. In summary, transfer of methoxide from anionic arylboronates to iron proceeds in preference to the transmetalation of aryl groups; the Fe−OMe species generated then do not react further with ArylB(OR)2 species. These transmetalation studies were complicated by the surprising tendency of NHC ligands to dissociate from low-coordinate iron(II) systems. NHC dissociation can be readily detected using the change in δ11B of the boron Lewis acid PhBCl2 on coordination of the NHC. This phenomenon was found to be general across a range of NHC−Fe systems regardless of the steric constraint or σ-donor function imparted by the ancillary NHC ligands employed. This observation has implications with regard to the application of (NHC)2FeCl2 systems in catalytic transformations.

substituents in the larger ring NHCs due to the widening N− CNHC−N angle. The increasing steric demand across the iPr series also manifests itself in a progressively greater distortion away from an ideal tetrahedral environment at iron toward a trigonal-pyramidal geometry containing disparate solid-state chloride environments: pseudoaxial and pseudoequatorial. While the %Vbur value for the 7-iPr compound 8 is comparable to that of the IMes derivative 2, the observation of considerably longer Fe−C bonds for 8 is consistent with the inability to access this species pure in solution from addition of 1 equiv of free carbene to 0.25 equiv of {(TMEDA)FeCl2}2. The long solid-state Fe−C distances in 7 suggested that in solution this compound is also undergoing reversible NHC ligand dissociation in an fashion analogous to that for compounds 1 and 2. Indeed, subjection of complex 7 to stoichiometric PhBCl2 in C6D6 afforded outcomes identical with those for the imidazolium-based systems in affording the Cl-bridged dimer {(6-iPr)FeCl2}2 (9) and the borane adduct (6-iPr)PhBCl2 (10). Support for the dimeric nature of 9 was initially forthcoming from NMR studies on an independently prepared sample, which displayed resonances consistent with only one 6-iPr environment in solution. Further confirmation came from X-ray diffraction studies, which revealed a dimeric structure for 9 (Figure 6) closely comparable to that for 3 and

Figure 6. ORTEP representation of {(6-iPr)FeCl2}2, (9) at the 50% ellipsoid probability level with hydrogens and a disordered molecule of C6D6 omitted for clarity. Selected bond lengths (Å) and angles (deg): Fe1−C1 = 2.138(7), Fe−Cl1 = 2.370(2), Fe−Cl1A = 2.414(2), Fe− Cl2 = 2.253(2); Fe1−Cl1−Fe1A = 90.11(7), Cl1−Fe−Cl1A = 89.89(7).

{(DIPP)FeCl2}2 containing a Fe2Cl2 core with the 6-iPr ligands coordinated on opposite faces.30 The most notable differences between the dimers are the increased Fe−C bond lengths in 9 (2.138(7) Å) in comparison to those in the IMes and DIPP congeners (2.089(4) and 2.090(2) Å, respectively).30 This elongation occurs despite the relative %Vbur values, which for the {(NHC)FeCl2}2 dimers follow the expected order DIPP > IMes > 6-iPr (34.2%, 32.8%, and 30.7%, respectively). In contrast to the (NHC)2FeX2 complexes described above, the bisphosphine compound (dppe)FeCl2 does not react with PhBCl2 in C6D6 (by 11B NMR spectroscopy), indicating a more robust iron−ligand linkage. This is not simply a chelation effect increasing ligand binding strength, as addition of PhBCl2 to the previously reported chelating biscarbene complex methylenebis(N-DIPP-imidazole-2-ylidene)FeI232 resulted in a rapid reaction that led to an intractable precipitate insoluble in common organic solvents and no observable resonances in the 11 B NMR spectra. Surprisingly, the {(5-iPr)FeCl2}n oligomers are also prone to NHC dissociation, with addition of 2.2 equiv of PhBCl2 to 4 in

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EXPERIMENTAL SECTION

General Remarks. All manipulations were carried out using standard Schlenk techniques, or in a glovebox, under an atmosphere of argon. Solvents were distilled from appropriate drying agents (THF (potassium); hexanes and diethyl ether (NaK)), while toluene was dried by passage through activated alumina columns and thoroughly degassed prior to use. All solvents were stored over potassium mirrors, except for THF, which was stored over activated 3 Å molecular sieves. d6-Benzene was distilled from potassium, degassed by three consecutive freeze−pump−thaw cycles, and stored under argon. 5iPr, 47 IMes, 48 6-iPr·HBr, 43 7-iPr·HI, 43 [Fe(TMEDA)Cl 2 ] 2 , 49 [FeCl2(THF)1.5],50 [Fe(5-iPr)2Cl2] (1),29 [Fe(IMes)2Cl2] (2),3 and iPr·BCl341 were all prepared according to previously reported literature procedures. All other compounds were purchased from commercial sources and used as received, except for sodium methoxide, which was freshly sublimed prior to use. NMR spectra were recorded on Bruker AV-400 and Bruker AV-500 spectrometers. Chemical shifts are 374

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inseparable mixture of 8 and a second compound we assign as its corresponding Cl-bridged oligomer {(7-iPr)Fe(μ-Cl)Cl}n (ratio 50:50 by 1H NMR). 1H NMR (400 MHz, C6D6, 298 K): for [Fe(7-iPr)2Cl2] (8): 22.24 (bs), 10.70 (bs), 9.11 (bs), 3.70 ppm (bs); for {Fe(7iPr)(μ-Cl)Cl}n, 26.94 (bs), 2.30 (bs), 0.95 (bs), 0.30 ppm (bs). Crystals of 8 suitable for X-ray diffraction were grown at ambient temperature from the mixture by layering a C6D6 solution with hexanes. {(6-iPr)Fe(μ-Cl)Cl}2 (9). A flame-dried Schlenk tube was charged with 6-iPr·HBr (500 mg, 2.01 mmol) which was dried under vacuum for ca. 1 h prior to the addition of KHMDS (561 mg, 2.81 mmol) in the glovebox. Outside the glovebox was added THF (10 mL) and the suspension was stirred at ambient temperature for 1 h, after which time the free carbene solution was transferred via oven-dried filter cannula to a stirred THF solution (10 mL) of FeCl2 (509 mg, 4.02 mmol). Upon addition the solution was seen to immediately turn dark brown. The reaction mixture was stirred at ambient temperature for 3 h prior to filtration via oven-dried filter cannula and the solvent removed to afford a brown residue. The residue was washed with several portions of pentane to afford 9 as a free-flowing off-white solid (374 mg, 60%). 1H NMR (400 MHz, C6D6, 298 K): 32.45 (bs), 4.15 (bs), 2.12 (bs), 0.13 ppm (bs). Crystals of 9 suitable for X-ray diffraction were grown at ambient temperature by layering a C6D6 solution with pentane. Attempts to obtain analytically pure bulk material were repeatedly unsuccessful, with microanalytical data on material pure by NMR spectroscopy being repeatedly low on C, H, and N, suggesting FeCl2 impurities. (6-iPr)PhBCl2 (10). A flame-dried Schlenk tube was charged with 6iPr·HBr (75 mg, 0.30 mmol) which was dried under vacuum for ca. 1 h prior to the addition of KHMDS (84 mg, 0.42 mmol) in the glovebox. Outside the glovebox was added THF (5 mL) and the suspension was stirred at ambient temperature for 1 h, after which time the free carbene solution was transferred via oven-dried filter cannula to a separate flame-dried Schlenk tube and all volatiles were removed. The resulting white solid was taken up in hexanes (5 mL) and PhBCl2 (55 μL, 0.30 mmol) added, whereupon an immediate pale yellow suspension was observed and stirring was continued at ambient temperature for 1 h. The yellow suspension was isolated by filtration and dried to afford 10 as a pale yellow free-flowing solid (64 mg, 65%). 1 H NMR (400 MHz, C6D6, 298 K): 7.96 (2H, bs, o-CH), 7.141 (2H, bs, m-CH), 7.31 (1H, bs, p-CH), 5.40 (2H, m, CH(CH3)2), 2.29 (4H, t, 3JHH = 5.2 Hz, NCH2), 1.12 (2H, m, NCH2CH2), 0.75 ppm (12H, d, 3 JHH = 5.7 Hz, CH(CH3)2). 13C{1H} NMR (100 MHz, C6D6, 298 K; N−CNHC−N not observed): 131.8, 127.7, 126.1, 53.9, 39.3, 21.0, 20.1 ppm. 11B{1H} NMR (128.4 MHz, C6D6, 298 K): 3.0 ppm. (7-iPr)PhBCl2 (11). A flame-dried Schlenk tube was charged with 7iPr·HI (100 mg, 0.32 mmol) which was dried under vacuum for ca. 1 h prior to the addition of KHMDS (90 mg, 0.45 mmol) in the glovebox. Outside the glovebox was added THF (5 mL) and the suspension was stirred at ambient temperature for 1 h, after which time the free carbene solution was transferred via oven-dried filter cannula to a separate flame-dried Schlenk tube and all volatiles were removed in vacuo. The white solid residue was taken up in hexanes (5 mL) and PhBCl2 (34 μL, 0.26 mmol) added, whereupon an immediate pale yellow suspension was observed and stirring was continued at ambient temperature for 1 h. The yellow suspension was isolated by filtration and dried to afford 11 as a pale yellow free-flowing solid (84 mg, 77%). 1 H NMR (400 MHz, C6D6, 298 K): 8.01 (2H, d, 3JHH = 7.5 Hz, oCH), 7.33 (2H, t, 3JHH = 7.5 Hz, m-CH), 7.18 (1H, m, p-CH), 5.19 (2H, sept, 3JHH = 6.4 Hz, CH(CH3)2), 2.65 (4H, t, 3JHH = 5.3 Hz, NCH2), 1.18 (4H, m, NCH2CH2), 0.75 ppm (12H, d, 3JHH = 6.7 Hz, CH(CH3)2). 13C{1H} NMR (100 MHz, C6D6, 298 K; N−CNHC−N not observed): 132.7, 127.9, 126.6, 54.3, 44.4, 22.9, 20.7 ppm. 11B{1H} NMR (128.4 MHz, C6D6, 298 K): 3.5 ppm. 2-(p-tolyl)-1,3,2-dioxaborolane (p-TolBeg). This compound was prepared according to a slightly modified literature procedure.52 Under ambient conditions with no additional precautions taken to exclude air and moisture, p-tolylphenylboronic acid (600 mg, 4.41 mmol) was suspended in reagent grade dichloromethane (15 mL) prior to the addition of MgSO4 (531 mg, 4.41 mmol) and ethylene

referenced relative to residual protio impurities in the NMR solvent for 1 H and 13C{1H}, respectively, while 11B{1H} shifts are referenced relative to external BF3−etherate. Solution-state magnetic moments were calculated according to the Evans method39,40 at 298 K on a Bruker AV-400 spectrometer. Microanalysis was performed by the London Metropolitan University microanalytical service. {(5-iPr)Fe(μ-Cl)Cl}2 (4). In a glovebox a flame-dried J. Young ampule was charged with 5-iPr (300 mg, 1.66 mmol) and anhydrous FeCl2 (420 mg, 3.31 mmol). Outside the glovebox THF (20 mL) was added to afford a dark yellow homogeneous solution and the reaction mixture was stirred at 80 °C for 16 h. After this time all volatiles were removed to afford a dark yellow-brown residue, which was extracted into warm toluene (3 × 20 mL), furnishing a pale yellow solution. The solvent was removed to afford 4 as a white free-flowing solid (182 mg, 36%), containing a minor impurity of 1 (ca. 5% by 1H NMR). Anal. Calcd for C22H40Cl4Fe2N4: C, 43.03; H, 6.57; N, 9.12. Found: C, 42.88; H, 6.61; N, 8.97. 1H NMR (400 MHz, C6D6, 298 K): 22.01 (12H, bs, CH3), 3.42 (24H, bs, CH(CH3)2), 0.30 ppm (4H, bs, NCH(CH3)2). μeff (Evans method, d8-THF solution, protio-toluene capillary, concentration 0.015 g/mL, 298 K): 7.7(1) μB. Crystals suitable for X-ray diffraction were grown at ambient temperature by layering a C6D6 solution of 4 with hexanes. (5-iPr)PhBCl2 (5).51 In a glovebox a J. Young NMR tube was loaded with 5-iPr (15 mg, 0.083 mmol) and C6D6 (0.8 mL). Outside the glovebox PhBCl2 (10.8 μL, 0.083 mmol) was added under an argon atmosphere and the reaction mixture agitated for 2 min at ambient temperature to cleanly afford 5. 1H NMR (400 MHz, C6D6, 298 K): 8.16 (2H, d, 3JHH = 7.5 Hz, o-CH), 7.33 (2H, t, 3JHH = 7.5 Hz, m-CH), 7.19 (1H, t, 3JHH = 7.8 Hz, p-CH), 5.85 (2H, sept, 3JHH = 5.8 Hz, CH(CH3)2), 1.50 (6H, s, CH3), 0.96 ppm (12H, d, 3JHH = 6.8 Hz, CH(CH3)2). 13C{1H} NMR (125 MHz, C6D6, 298 K): 133.4, 128.9, 127.2, 126.6, 50.5, 21.2, 10.6 ppm. 11B{1H} NMR (128.4 MHz, C6D6, 298 K): 2.7 ppm. (IMes)PhBCl2 (6). In a glovebox a J. Young NMR tube was loaded with IMes (25 mg, 0.082 mmol) and C6D6 (0.8 mL). Outside the glovebox PhBCl2 (10.7 μL, 0.082 mmol) was added under an argon atmosphere and the reaction mixture agitated for 2 min at ambient temperature. 11B{1H} NMR (128.4 MHz, C6D6, 298 K): 2.3 ppm. (6-iPr)2FeCl2 (7). A flame-dried Schlenk tube was charged with 6iPr·HBr (400 mg, 1.61 mmol) which was dried under vacuum for ca. 1 h prior to the addition of KHMDS (449 mg, 2.25 mmol) in a glovebox. Outside the glovebox was added THF (10 mL) and the resulting suspension was stirred at ambient temperature for 1 h, after which time the in situ generated free carbene solution was transferred via oven-dried filter cannula to a stirred THF solution (10 mL) of {Fe(TMEDA)Cl2}2 (194 mg, 0.40 mmol). Upon addition the pale yellow {Fe(TMEDA)Cl2}2 solution immediately darkened and the reaction mixture was stirred at ambient temperature for 1 h. After this time removal of all volatiles afforded a dark yellow residue, which was subsequently washed with hexanes (3 × 20 mL) to afford 7 as a freeflowing white solid (192 mg, 52%). Anal. Calcd for C20H40Cl2FeN4: C, 51.85; H, 8.70; N, 12.09 Found: C, 51.62; H, 8.81; N, 11.93. 1H NMR (400 MHz, C6D6, 298 K): 26.56 (8H, bs, NCH2), 14.57 (4H, bs), 9.94 (24H, bs, NCH(CH3)2), 9.22 ppm (4H, bs). μeff (Evans method, d8THF solution, protio-toluene capillary, concentration 0.014 g/mL, 298 K): 5.5(6) μB. Crystals suitable for X-ray diffraction were grown at ambient temperature by layering a C6D6 solution of 7 with hexanes. (7-iPr)2FeCl2 (8) + {(7-iPr)Fe(μ-Cl)Cl}n. A flame-dried Schlenk tube was charged with 7-iPr·HI (500 mg, 1.61 mmol) which was dried under vacuum for ca. 1 h prior to the addition of KHMDS (449 mg, 2.25 mmol) in a glovebox. Outside the glovebox was added THF (10 mL) and the resulting suspension was stirred at ambient temperature for 1 h, after which time the in situ generated free carbene solution was transferred via oven-dried filter cannula to a stirred THF solution (10 mL) of {Fe(TMEDA)Cl2}2 (194 mg, 0.40 mmol). Upon addition the pale yellow {Fe(TMEDA)Cl2}2 solution immediately darkened and the reaction mixture was stirred at ambient temperature for 1 h. After this time removal of all volatiles afforded a dark yellow residue, which was washed with diethyl ether (3 × 20 mL) to afford 141 mg of an offwhite free-flowing solid that after 1H NMR analysis was found to be an 375

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glycol (271 μL, 4.85 mmol). The reaction mixture was stirred at ambient temperature for 1 h prior to filtration and the removal of all volatiles to afford 2-(p-tolyl)-1,3,2-dioxaborolane as a white microcrystalline solid of sufficient purity to be used without further purification (642 mg, 90%). 1H NMR (400 MHz, CDCl3, 298 K): 7.73 (2H, d, 3JHH = 7.8 Hz, ArH), 7.22 (2H, d, 3JHH = 7.8 Hz), 4.38 (4H, s, OCH2), 2.39 ppm (3H, s, p-CH3). 13C{1H} NMR (100 MHz, CDCl3, 298 K): 141.7, 134.8, 128.7, 65.9, 21.7 ppm. 11B{1H} NMR (128.4 MHz, C6D6, 298 K): 31.5 ppm. (5-iPr)-p-TolBeg. In a glovebox a J. Young NMR tube was loaded with iPr (11.2 mg, 0.062 mmol), 2-(p-tolyl)-1,3,2-dioxaborolane (pTolBeg) (10 mg, 0.062 mmol), and C6D6 (0.8 mL). The colorless, homogeneous solution was agitated at ambient temperature for ca. 5 min to cleanly afford the title compound 5-iPr-p-TolBeg. 1H NMR (400 MHz, C6D6, 298 K): 7.81 (2H, bs, ArH), 7.26 (2H, d, 3JHH = 6.8 Hz, ArH), 6.35 (2H, bs, CH(CH3)2), 2.29 (4H, bs, eg), 1.56 (6H, s, CH3), 1.30 (3H, bs, CH3-p-Tol), 1.10 ppm (12H, d, 3JHH = 6.8 Hz, CH(CH3)2). 13C{1H} NMR (125 MHz, C6D6, 298 K): 128.2, 127.9, 124.4, 65.8, 49.3, 21.6, 21.6, 10.1 ppm. 11B{1H} NMR (128.4 MHz, C6D6, 298 K): 6.4 ppm. (IMes)-p-TolBeg. In a glovebox a J. Young NMR tube was loaded with IMes (25 mg, 0.082 mmol), 2-(p-tolyl)-1,3,2-dioxaborolane (13.3 mg, 0.082 mmol), and C6D6 (0.8 mL). 11B{1H} NMR (128.4 MHz, C6D6, 298 K): 5.4 ppm.

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(11) Hatakeyama, T.; Hashimoto, S.; Ishizuka, K.; Nakamura, M. J. Am. Chem. Soc. 2009, 131, 11949. (12) Hatakeyama, T.; Nakamura, M. J. Am. Chem. Soc. 2007, 129, 9844. (13) Guisan-Ceinos, M.; Tato, F.; Bunuel, E.; Calle, P.; Cardenas, D. J. Chem. Sci. 2013, 4, 1098. (14) Cahiez, G.; Gager, O.; Buendia, J.; Patinote, C. Chem. Eur. J. 2012, 18, 5860. (15) Bedford, R. B.; Betham, M.; Bruce, D. W.; Danopoulos, A. A.; Frost, R. M.; Hird, M. J. Org. Chem. 2006, 71, 1104. (16) Mo, Z.; Zhang, Q.; Deng, L. Organometallics 2012, 31, 6518. (17) Gao, H.; Yan, C.; Tao, X.-P.; Xia, Y.; Sun, H.-M.; Shen, Q.; Zhang, Y. Organometallics 2010, 29, 4189. (18) Meyer, S.; Orben, C. M.; Demeshko, S.; Dechert, S.; Meyer, F. Organometallics 2011, 30, 6692. (19) Xiang, L.; Xiao, J.; Deng, L. Organometallics 2011, 30, 2018. (20) Jana, R.; Pathak, T. P.; Sigman, M. S. Chem. Rev. 2012, 111, 1417. (21) Bedford, R. B.; Hall, M. A.; Hodges, G. R.; Huwe, M.; Wilkinson, M. C. Chem. Commun. 2009, 6430. (22) Hatakeyama, T.; Hashimoto, T.; Kondo, Y.; Fujiwara, Y.; Seike, H.; Takaya, H.; Tamada, Y.; Ono, T.; Nakamura, M. J. Am. Chem. Soc. 2010, 132, 10674. (23) Hatakeyama, T.; Hashimoto, T.; Kathriarachchi, K. K. A. D. S.; Zenymo, T.; Seike, H.; Nakamura, M. Angew. Chem., Int. Ed. 2012, 51, 8834. (24) Hashimoto, T.; Hatakeyama, T.; Nakamura, M. J. Org. Chem. 2012, 77, 1168. (25) For the mechanism of Pd-catalyzed Suzuki−Miyaura crosscoupling see: (a) Carrow, B. P.; Hartwig, J. F. J. Am. Chem. Soc. 2011, 133, 2116. (b) Lennox, A. J. J.; Lloyd-Jones, G. C. Chem. Soc. Rev. 2014, 43, 412. (26) Noda, D.; Sunada, Y.; Hatakeyama, T.; Nakamura, M.; Nagashima, H. J. Am. Chem. Soc. 2009, 131, 6079. (27) (a) See references in: Bezier, D.; Sortais, J.-B.; Darcel, C. Adv. Synth. Catal. 2013, 355, 19. (b) For an example after this review see: Wu, J.; Dai, W.; Farnaby, J. H.; Hazari, N.; Le Roy, J. J.; Mereacre, V.; Murugesu, M.; Powell, A. K.; Takase, M. K. Dalton Trans. 2013, 42, 7404. (28) Ingleson, M. J.; Layfield, R. A. Chem. Commun. 2012, 48, 3579. (29) Louie, J.; Grubbs, R. H. Chem. Commun. 2000, 1479. (30) Przyojski, J. A.; Arman, H. D.; Tonzetich, Z. Organometallics 2012, 31, 3264. (31) Poater, A.; Cosenza, B.; Correa, A.; Giudice, S.; Ragone, F.; Scarano, V.; Cavallo, L. Eur. J. Inorg. Chem. 2009, 1759. (32) Zlatogorsky, S.; Muryn, C. A.; Tuna, F.; Evans, D. J.; Ingleson, M. J. Organometallics 2011, 30, 4974. (33) See the Supporting Information. (34) Layfield, R. A.; McDouall, J. J. W.; Scheer, M.; Schwarzmaier, C.; Tuna, F. Chem. Commun. 2011, 47, 10623. (35) Lavallo, V.; El-Batta, A.; Bertrand, G.; Grubbs, R. H. Angew. Chem., Int. Ed. 2011, 50, 268. (36) Day, B. M.; Pugh, T.; Hendriks, D.; Guerra, C. F.; Evans, D. J.; Bickelhaupt, F. M.; Layfield, R. A. J. Am. Chem. Soc. 2013, 135, 13338. (37) Musgrave, R. A.; Tubervill, R. S. P.; Irwin, M.; Goichoechea, J. M. Dalton Trans. 2013, DOI: 10.1039/C3DT52638K. (38) Wang, X.; Mo, Z.; Xiao, J.; Deng, L. Inorg. Chem. 2013, 52, 59. (39) Evans, D. F. J. Chem. Soc. 1959, 2003. (40) Sur, S. K. J. Magn. Reson. 1989, 82, 169. (41) Muthaiah, S.; Do, D. C. H.; Ganguly, R.; Vidovic, D. Organometallics 2013, 32, 6718. (42) Pranckevicius, C.; Stephan, D. W. Organometallics 2013, 32, 2693. (43) For recent papers on the steric/electronic properties of expanded ring NHCs see: (a) Dunsford, J. J.; Cavell, K. J. Dalton Trans. 2011, 40, 9131. (b) Dunsford, J. J.; Cavell, K. J.; Kariuki, B. M. Organometallics 2012, 31, 4118. (c) Dunsford, J. J.; Tromp, D. S.; Cavell, K. J.; Elsevier, C. J.; Kariuki, B. M. Dalton Trans. 2013, 42, 7318.

ASSOCIATED CONTENT

S Supporting Information *

Text, figures, a table, and CIF files giving full experimental and crystallographic details and characterization data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for M.J.I.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the Royal Society (M.J.I. for the award of a University Research Fellowship), the ERC (J.J.D.), and the EPSRC (I.A.C., grant number EPJ000973/1) for financial support. M.L.N. acknowledges the University of Rochester for financial support.

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REFERENCES

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(44) For the structures of (NHC)Rh(COD)X see: (a) IMes: Evans, P. A.; Baum, E. W.; Fazal, A. N.; Pink, M. Chem. Commun. 2005, 63. (b) 5-iPr: Truscott, B. J.; Slawin, A. M.; Nolan, S. P. Dalton Trans. 2013, 42, 270. (c) 6-iPr: Mayr, M.; Wurst, K.; Ongania, K.-H.; Buchmeiser, M. R. Chem. Eur. J. 2004, 10, 1256. (d) 7-iPr: Iglesias, M.; Beetstra, D. J.; Stasch, A.; Horton, P. N.; Hursthouse, M. B.; Coles, S. J.; Cavell, K. J.; Dervisi, A.; Fallis, I. A. Organometallics 2007, 26, 4800. The %Vbur value for 7-iPr is estimated from the 7-cyclohexyl structure, which sets an approximate upper limit for the 7-iPr value. (45) No neutral three-coordinate iron(II) compounds ligated only by chlorides are reported in the Cambridge Structural Database (Oct 2013). (46) (NHC)Fe(η6-arene) complexes have been recently reported and are robust in a range of oxidation states: (a) Zlatogorsky, S.; Ingleson, M. J. Dalton Trans. 2012, 41, 2685. (b) Blom, B.; Tan, G.; Enthaler, S.; Inoue, S.; Epping, J. D.; Driess, M. J. Am. Chem. Soc. 2013, 135, 18108. (47) Kuhn, N.; Kratz, T. Synthesis 1993, 561. (48) Bantreil, X.; Nolan, S. P. Nat. Protoc. 2011, 6, 69. (49) Davies, S. C.; Hughes, D. L.; Leigh, G. J.; Sanders, J.; de Souza, J. S. J. Chem. Soc., Dalton Trans. 1997, 1981. (50) Winter, G.; Thompson, D. W.; Loehe, J. R. Inorg. Synth. 1973, 99. (51) This compound has been previously reported as an intermediate, but no physical data were reported; see: Wang, Y.; Robinson, G. H. Inorg. Chem. 2011, 50, 12326−12337. (52) Iwai, Y.; Gligorich, K. M.; Sigman, M. S. Angew. Chem., Int. Ed. 2008, 47, 3219.

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