Synthesis and Characterization of Bidentate NHC-CAryl Nickel (II

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Synthesis and Characterization of Bidentate NHC‑CAryl Nickel(II) Complexes: Isocyanide Insertion To Form NHC‑η2‑iminoacyl Complexes Joseph W. Nugent, Gabriel Espinosa Martinez, Danielle L. Gray, and Alison R. Fout* School of Chemical Sciences, University of Illinois at UrbanaChampaign, 600 South Matthews Avenue, Urbana, Illinois 61801, United States S Supporting Information *

ABSTRACT: A bidentate monoanionic NHC-CAryl ligand framework was synthesized, and a host of Ni(II) complexes were prepared. Addition of isocyanides to these complexes led to the formation of NHC-η2-iminoacyl nickel complexes. These complexes were characterized by a suite of spectroscopic techniques, including X-ray crystallography. The η2iminoacyl was shown to be displaced from the nickel center with oxidant and could then be reattached with reductant.

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INTRODUCTION N-heterocyclic carbenes (NHCs) have become nearly ubiquitous in organometallic chemistry.1−3 Similar to the case for phosphines, NHCs offer a strongly σ-donating lone pair and lack significant π-acceptor properties.4 The advantages of incorporating NHCs in a ligand system are many, including an increased oxidative robustness in comparison to their phosphine counterparts. Furthermore, stabilization of metal complexes through the M−CNHC bond imparts a significant field strength which biases the metal toward low-spin configurations.5 NHCs have been explored extensively in pincer complexes6,7 and bidentate ligand frameworks,8 both of which facilitate many different catalytic transformations.9 Tridentate pincer complexes are generally more stable than their bidentate counterparts due to the extra stabilization offered by the third binding position;10 however, bidentate ligands offer more coordinative flexibility for substrate binding by occupying fewer sites of the metal center. Many bidentate transition-metal complexes featuring NHCs have been developed. These include the complexes featuring neutral bis-NHC platforms11,12 and “tethered NHCs”.13,14 Tethered NHCs incorporate an anionic tether usually containing sulfur,15,16 oxygen,17,18 or nitrogen19 to the NHC and can be advantageous for multiple reasons: (1) they offer a stronger binding affinity to the metal center to avoid complexation equilibria in solution, and (2) they provide an opportunity to include chirality in the tether which can be effective during asymmetric catalysis. NHCs with tethered aryl groups have been explored with late second- and third-row transition metals for a host of applications, including utility in phosphorescent Pt(II) complexes,20 water oxidation catalysts on complexation with iridium,21 and other C−H and C−C bond activation studies.22 Despite their use with second- and third-row transition metals, bidentate NHC-CAryl ligands on first-row transition metals are © XXXX American Chemical Society

rare in the literature. In fact, the Ni(II) structures reported herein represent the first crystallographically characterized examples of this type of bidentate ligand framework on a first-row transition metal. This dearth of examples is likely due to the difficulty of metalating aryl groups in comparison to other tethers used in the tethered bidentate ligand systems. With a continued push to target and strengthen our understanding of base metal catalysis, new ligand constructs are necessary to facilitate desired reactivity. Herein we report the synthesis and characterization of a variety of nickel complexes featuring a bidentate monoanionic NHC-CAryl ligand. Insertion of isocyanides into the ligand backbone allowed for several NHC-η2-iminoacyl nickel species to be isolated. Using a model complex, the iminoacyl moiety was shown to be removable from the nickel center on treatment with the appropriate oxidant and could then subsequently be reattached to the nickel center with reductant.

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RESULTS AND DISCUSSION Synthesis and Deprotonation of [H(BrCC)]Br. The strategy employed to synthesize the monoanionic, bidentate NHC-CAryl ligand was the nucleophilic substitution reaction of an N-aryl imidazole with a benzylic bromide (Scheme 1). The flanking group of the imidazole and substitution of the aryl group were judiciously chosen to aid in characterization and isolation of the metalated species to be formed. The 2,6diisopropylphenyl flanking group on the imidazole provides a convenient 1H NMR handle and confers more solubility to the metal complexes featuring the ligand framework. Furthermore, the aryl bromide was included to provide a CAryl−Br bond onto which Ni(0) can oxidatively add,23−25 while the bulky tBu Received: June 16, 2017

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DOI: 10.1021/acs.organomet.7b00463 Organometallics XXXX, XXX, XXX−XXX

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by the PPh3 ligands. Reacting a mixture of [H(BrCC)]Br and NiBr2(PPh3)2 with LiN(SiMe3)2 resulted in the isolation of (BrCC)NiBr2PPh3 (1) as a purple powder in excellent yield (95%) following hexanes workup (Scheme 2). The integration of the 1H NMR spectrum suggested the formation of the desired complex 1 with bound PPh3, further corroborated by a resonance at 17.45 ppm in the 31P NMR spectrum. To confirm this assignment, purple, platelike crystals suitable for X-ray crystallography were grown from a concentrated MeCN solution of 1 containing 1 drop of THF. Refinement of the structural data revealed a Ni(II) center in a distorted-squareplanar environment (τδ = 0.15)26 bound to two bromides, the NHC carbon (CNHC), and PPh3, which is trans to the CNHC (Figure 1). The CNHC−Ni1−P1 and Br1−Ni1−Br2 angles are 175.50(9) and 161.77(2)°, respectively. The Ni1−CNHC distance of 1.885(3) Å is slightly shorter than those found in analogous Ni(X)2PPh3 (X = Cl, Br) complexes (average 1.913 Å).27 With an NHC-bound Ni(II) complex in hand, activation of the CAryl−Br bond in the ligand backbone, via reduction, was explored. Addition of 2 equiv of potassium graphite (KC8) to 1 in a 50/50 THF/toluene mixture resulted in the isolation of (CC)NiBrPPh3 (2) as a brown-orange powder in modest yield (55%) after filtration through Celite and workup (Scheme 2). The integration of the 1H NMR spectrum was consistent with the presence of the (CC) ligand framework; however, the aromatic resonances expected for the bound PPh3 manifested themselves as a broad resonance at ∼7.28 ppm in C6D6. The broadness of this resonance, along with the inability to observe a 31P NMR signal and the concentration dependence of the chemical shifts in the 1H NMR resonances, suggested that the PPh3 may be fluxionally bound on the NMR time scale. Crystallization attempts of this complex proved unsuccessful, though observation of [(CC)Ni]+ by HRMS (ESI+) was consistent with the activation of the CAryl−Br bond to form the targeted metal complex, 2. Further evidence of the formulation of 2 was offered by synthesis of the PMe3 analogue, (CC)NiBrPMe3 (3), via addition of PMe3 to 2. The formation of 3 with concomitant liberation of PPh3 was confirmed by 1H NMR spectroscopy (in situ monitoring by 1H NMR spectroscopy, Figure S24 in the Supporting Information). Synthesis of (CC)NiBrPMe3 (3). Encouraged that the reduction to Ni(0) proved a viable route to activate the CAryl− Br bond in this ligand framework, metalation of the free carbene with bis(1,5-cyclooctadiene)nickel(0) (Ni(COD)2) was explored. With the propensity to form (BrCC)2NiBr2 complexes in mind, additive L-type ligands were also employed

Scheme 1. Synthesis of the Bidentate Ligand Framework, [H(BrCC)]Br

group provides steric protection for the newly formed Ni−CAryl bond upon metalation. To this end, the imidazolium salt of the target ligand was generated by reacting 1-(2,6-diisopropylphenyl)-1H-imidazole with 2-bromo-3,5-di-tert-butylbenzyl bromide, resulting in the formation of [H(BrCC)]Br ([H(BrCC)]Br = 3-(2-bromo-3,5-di-tert-butylbenzyl)-1-(2,6-diisopropylphenyl)-1H-imidazolium bromide) in excellent yield (93%) after workup (Scheme 1). The 1H NMR spectrum of [H(BrCC)]Br contained a downfield resonance at 10.20 ppm in CDCl3, indicative of successful formation of the imidazolium salt. With the ligand precursor in hand, the accessibility of the free carbene (BrCC) was probed. Accordingly, lithium hexamethyldisilazide (LiN(SiMe3)2) was added to [H(BrCC)]Br in THF. Upon addition, the ligand salt suspension was rapidly solubilized, resulting in a clear yellow solution consistent with the loss of the salt character of the starting ligand precursor. Additionally, the absence of the most downfield resonance observed in the 1 H NMR spectrum of [H( Br CC)]Br corroborated deprotonation of the imidazolium salt to form the free carbene, BrCC. Subsequent experiments confirmed that metalation with in situ generated carbene was successful. Synthesis of (BrCC)NiBr2PPh3 (1) and (CC)NiBrPPh3 (2). Given the accessibility of the free carbene in solution, metalation with Ni(II) sources was attempted. Treatment of Br CC with NiCl2 or NiCl2py4 resulted in the formation of (BrCC)2NiBr2 complexes, in which two carbene ligands were bound to a single nickel center, as confirmed by preliminary Xray crystallographic data (connectivity). To address the issue of coordination of two carbene ligands to the nickel center, bulkier L-type ligands were employed. The use of NiBr2(PPh3)2 as the Ni(II) source proved to be a fruitful choice for two reasons: increased solubility of the nickel source and the presence of endogenous steric protection around the nickel center provided

Scheme 2. Synthesis of (BrCC)NiBr2PPh3 (1), (CC)NiBrPPh3 (2), and (CC)NiBrPMe3 (3)

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Figure 1. Molecular structures of 1, 3, 4, 7, and 8 shown with 50% probability ellipsoids and 5 (one out of three symmetrically independent molecules) shown with 30% probability ellipsoids. Solvent molecules, hydrogen atoms, outer-sphere bromide (4), and BArF24 (8) have been excluded for clarity.

Scheme 3. Synthesis of [(CCNtBu)NitBuNC]Br (4), (CCNXyl)NiBr (5), and (CCNNapth)NiBr (6)

comparison to [H(BrCC)]Br and complexes 1 and 2. This was apparent by the presence of two septets integrating to 1H each corresponding to the methine protons of the isopropyl groups (2.82 and 2.60 ppm), four doublets integrating to 3H each (1.40, 1.32, 1.10, and 0.72 ppm) corresponding to the methyl groups of the isopropyl moieties, and two doublets integrating to 1H each (6.20 and 4.90 ppm) with coupling consistent with

in this synthetic route. Indeed, exclusion of an L-type ligand resulted in complex mixtures of products. Mixing excess PMe3 with Ni(COD)2 and [H(BrCC)]Br in THF, followed by the addition of LiN(SiMe3)2, resulted in the isolation of 3 as a yellow-orange powder in good yield (74%) after workup (Scheme 2). Characterization of this product by 1 H NMR spectroscopy in CDCl3 revealed a loss in symmetry in C

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Table 1. Selected Structural Parameters of Complexes 4, 5,a and 8

geminal protons (2JHH = 13.6 Hz) assigned to the benzylic position of the ligand backbone. This loss of symmetry suggested a more conformationally rigid complex in comparison to that of 2. The 1H NMR spectrum also featured a doublet at 0.90 ppm which integrates to 9H corresponding to the PMe3 methyl groups (2JHP = 8.3 Hz). The coordination of the PMe3 ligand was also confirmed by a resonance at −20.33 ppm in the 31P NMR spectrum. The predicted formulation of 3 was corroborated by X-ray structural analysis. Yellow, rodlike crystals suitable for diffraction were grown from slow evaporation of a diethyl ether (Et2O) solution of 3. The structure of 3 features a Ni(II) center in a “distorted-squareplanar/sawhorse” (τδ = 0.23)26 configuration bound to the ligand in a bidentate fashion (Figure 1). The PMe3 ligand is positioned trans to the CNHC with Ni1−CNHC and Ni1−P1 distances of 1.910(2) and 2.2052(6) Å, respectively. The bromide is trans to the CAryl carbon with Ni1−CAryl and Ni1− Br distances of 1.906(2) and 2.3674(3) Å, respectively. The CNHC−Ni1−P1 and CAryl−Ni1−Br angles are 149.65(6) and 173.30(6)°, respectively. The distortion away from square planarity is likely due to the steric clash of the PMe3 ligand with the tert-butyl group ortho to the CAryl−Ni bond. Synthesis of [(CCNtBu)NitBuNC]Br (4), (CCNXyl)NiBr (5), and (CCNNapth)NiBr (6). The synthesis of 3 demonstrated that direct metalation with a Ni(0) source, Ni(COD)2, was a viable metalation route for the (BrCC) ligand platform. To further explore the generalizability of this synthetic route with other strongly electron-donating ancillary ligands, isocyanides of varying steric profiles were surveyed. Addition of excess tBuNC to the Ni(0) synthetic route resulted in the isolation of [(CCNtBu)NitBuNC]Br (4) as an orange-yellow powder in excellent yield (91%) following hexanes workup (Scheme 3). Due to the decreased solubility of this product in comparison to 3, a saltlike species with an outer-sphere bromide was postulated. The 1H NMR spectrum in CDCl3 revealed a reduction in symmetry similar to that seen in 3. Two singlet resonances integrating to 9H each (beyond those corresponding to the (CC) ligand tBu groups) were consistent with the presence of two tBuNC moieties in the complex. To unambiguously confirm the structure of 4, yellow-orange crystals suitable for X-ray crystallography were grown from a concentrated Et2O solution of 4 at −35 °C. Refinement of the structural data confirmed the presence of an outer-sphere bromide in 4 (Figure 1). Interestingly, the crystal structure revealed insertion of one tBuNC into the Ni−CAryl bond ortho to the steric bulk of the tBu group, resulting in an η2-iminoacyl binding mode to the Ni(II) center. The mechanism of this insertion process is likely similar to that proposed by Figueroa and co-workers in which, upon oxidative addition of Ni(0) onto a CAryl−Br bond, α-migration of the aryl group to the coordinated isocyanide forms the η2-iminoacyl unit.28 Late transition metals typically favor η1 coordination to iminoacyl ligands;29 however, this species is likely biased toward η2 coordination since the formation of a 14e− coordinatively unsaturated complex would arise if η1 coordination was operative. The Ni1−C31 and Ni1−N3 distances of 1.838(2) and 1.8778(18) Å, respectively, are comparable to those found in analogous Ni(II) η2-iminoacyl complexes.28,30 The C31−N3 distance of 1.245(3) Å is elongated in comparison to that of the coordinated tBuNC ligand with a N4−C36 distance of 1.152(3) Å. The Ni(II) center is also bound to the NHC with a Ni1−CNHC distance of 1.887(2) Å. More detailed bond angles and distances can be found in Table 1. IR spectroscopy

[(CCNtBu) NitBuNC]Br (4) Ni1−CNHC Ni1−C31 Ni1−N3 Ni1−C31/N3 centroid C31−N3 Ni1−C36 Ni1−Br1 Ni1−N4 CNHC−Ni1−C31 CNHC−Ni1−N3 C31−Ni1−N3 CNHC−Ni1−C36 CNHC−Ni1−Br1 CNHC−Ni1−N4 a

(CCNXyl) NiBr (5)a

Bond Distances (Å) 1.887(2) 1.852(6) 1.838(2) 1.811(6) 1.8778(18) 1.918(5) 1.751(4) 1.758(2) 1.245(3) 1.245(7) 1.868(2) 2.3542(10)

[(CCNXyl) Nippy]BArF24 (8) 1.878(3) 1.818(3) 1.882(3) 1.741(5) 1.251(4)

1.965(3) Bond Angles (deg) 97.03(9) 100.5(3) 135.23(8) 138.9(3) 39.12(9) 38.9(2) 110.22(9) 106.5(2)

101.04(14) 140.45(13) 39.49(13)

109.62(12)

Only one molecule of the asymmetric unit is reported.

corroborated the presence of the bound tBuNC ligand with a stretching frequency of 2161 cm−1, shifted from that of free t BuNC (2135 cm−1).31 Features near 1737 cm−1 likely correspond to the inserted iminoacyl functionality; however, the presence of CC and CN vibrations in this region makes definitive assignment difficult without further isotopic labeling studies. Complex 4 could also be synthesized by the addition of 2 equiv of tBuNC directly to 3 in THF (Scheme 3), supporting the proposed insertion mechanism. Addition of stoichiometric amounts of tBuNC to the Ni(0) synthetic route failed to yield the corresponding neutral complex with an inner-sphere bromide, and instead resulted in a complex mixture of products, as observed by 1H NMR spectroscopy. Treatment of 3 with 1 equiv of tBuNC yielded a mixture of products consisting of 4 and other uncharacterized diamagnetic species by 1H NMR spectroscopy, highlighting the preference for t BuNC to form 4 (incorporating 2 tBuNC) over the corresponding neutral species. In contrast to tBuNC, the addition of 1 equiv of 2,6dimethylphenyl isocyanide (XylNC) to the Ni(0) synthetic route resulted in the isolation of the neutral complex (CCNXyl)NiBr (5) as a red powder in good yield (81%) upon workup (Scheme 3). Two singlets integrating for 3H at 2.41 and 1.51 ppm and an additional 3H in the aromatic region of the 1H NMR spectrum in CDCl3 suggested the presence of one XylNC in the complex. The diagnostic lack of symmetry in the isopropyl groups and benzylic backbone suggested formation of the Ni−CAryl bond to furnish a complex with the (CC) ligand bound in the desired bidentate fashion. IR spectroscopy suggested that the XylNC had inserted into the Ni−CAryl bond due to the lack of features in the 2200−2100 cm−1 region. To further confirm the connectivity of 5, crystals suitable for X-ray diffraction were grown by vapor diffusion of hexanes into a THF solution of 5. Refinement of the structural data revealed three symmetrically independent complexes of 5 in the asymmetric unit (Figure 1). Akin to 4, the XylNC inserted into the Ni−CAryl bond; however, the use of XylNC in stoichiometric amounts resulted in the formation of a neutral D

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Scheme 4. Synthesis of (XylNCCC)NiBr3 (7) and Reversibility to 5

complex with the bromide bound to the nickel center. The Ni(II) center is in a binding environment like that of 4 with the iminoacyl moiety bound in an η2 fashion (more detailed structural parameters can be found in Table 1). Moreover, this synthetic route was also amenable to the use of 2-naphthyl isocyanide (NapthNC). Upon optimization of reaction conditions, addition of 1.3 equiv of NapthNC to the Ni(0) synthetic route resulted in the formation of (CCNNapth)NiBr (6) as a purple-red powder in modest yield (51%) after recrystallization out of a THF/hexanes mixture (Scheme 3). The integration of the corresponding resonances in the 1H NMR spectrum was indicative of the presence of one NapthNC, and in addition, the loss of symmetry resembled that of 3−5. As observed for 5, the IR spectrum lacked the stretch expected for a bound isocyanide. Crystallization attempts of 6 proved unsuccessful; however, 1H NMR, 13C NMR, and IR spectroscopic data, along with the observation of [(CCN Napth )Ni] + by HRMS (ESI + ), agreed with the formulation of 6. Analogous to the case for 4, 6 could also be synthesized by the addition of 1 equiv of NapthNC to 3 (Scheme 3). The successful preparation of 4−6 highlights the ability of alkyl and aryl isocyanides of varying steric bulk to insert into the Ni−CAryl bond, effectively modulating the steric and electronic profile of this ligand framework. Redox Chemistry of 5: Formation of (XylNCCC)NiBr3 (7). To explore the redox chemistry of these bidentate NHC-η2iminoacyl Ni(II) complexes, 5 was employed due to the facile formation of the neutral complex. Chemical oxidation of 5 by treatment with 1 equiv of trityl bromide (Ph3CBr), a oneelectron oxidant, in MeCN yielded a brown powder after ethereal workup to remove Gomberg’s dimer. Analysis of this powder by 1H NMR spectroscopy revealed the presence of paramagnetic resonances as well as unreacted 5. Encouraged by these findings, 5 was exposed to 2 equiv of Ph3CBr, resulting in the isolation of a blue powder in good yield (76%). The 1H NMR spectrum indicated the complete conversion of 5 to a single paramagnetic product. It was postulated that a Ni(III) complex ((CCNXyl)NiBr2) was responsible for the paramagnetic resonances; however, the requirement of 2 equiv of Ph3CBr to fully form this species was inconsistent with this postulation. Moreover, treatment of 5 with 1 equiv of the twoelectron oxidant benzyltrimethylammonium tribromide (BTMABr3, a bromine surrogate) also yielded the same paramagnetic species by 1H NMR spectroscopy. To unambiguously assign the structure of this complex, crystals suitable for X-ray crystallography were grown from a concentrated MeCN solution of the paramagnetic species. Refinement of the structural data revealed (XylNCCC)NiBr3 (7) with a distorted-tetrahedral (τδ = 0.85)26 Ni(II) center bound to CNHC and three bromides (Figure 1). The 2e− oxidation of 5 resulted in the η2-iminoacyl moiety being displaced from the Ni center, forming a nitrilium side arm (Scheme 4). The assignment of a 2+ oxidation state to the nickel center arises from the presence of three bromides and the cationic nitrilium moiety. A tetrahedral Ni(II) center with two unpaired electrons (μeff = 3.7 ± 0.1 μB, Evans method32)33 is consistent with the observed paramagnetic 1H NMR resonances. The N3−C31 distance of 1.147(5) Å in the nitrilium moiety is similar to that found in an analogous N-aryl- and C-aryl-substituted nitrilium (1.140(4) Å).34 The Ni1−CNHC distance is 1.986(4) Å, and the Ni1−Br distances are 2.4036(7), 2.4154(7), and 2.3658(6) Å with the elongated distances belonging to the bromides in closer proximity to the cationic nitrogen of the CN moiety.

To probe the reversibility of this iminoacyl detachment, 7 was treated with 2 equiv of KC8 in THF. Graphite was liberated immediately, and upon filtration through Celite and removal of solvent, the 1H NMR spectrum confirmed the reduction of 7 to 5 (Scheme 4). Formation of the nitrilium moiety via oxidation of the η2-iminoacyl moiety provides this complex a route to oxidatively detach and reductively reattach the steric bulk provided by the inserted isocyanide to the metal center. Synthesis of [(CCNXyl)Nippy]BArF24 (8). In an attempt to access a cationic, coordinatively unsaturated Ni(II) center, 5 was treated with sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaBArF24) in DCM. The 1H NMR spectrum of the resulting product featured broadened resonances (see Figure S25 in the Supporting Information). To determine if ancillary ligands could sharpen the broadened resonances, 2phenylpyridine (ppy) was added to the reaction mixture of complex 5 with NaBArF24. This resulted in the isolation of [(CCNXyl)Nippy]BArF24 (8) as an orange powder in excellent yield (94%) after filtration through Celite and hexanes workup (Scheme 5). The integration of the 1H NMR spectrum and the Scheme 5. Synthesis of [(CCNXyl)Nippy]BArF24 (8)

observation of the [(CCNXyl)Nippy]+ cation by HRMS (ESI+) all supported the formation of complex 8. Orange, rodlike crystals suitable for X-ray crystallography were grown from a solution of 8 in a 50/50 (Me3Si)2O/Et2O mixture of at −35 °C. Upon refinement, the structural data revealed the desired cationic Ni(II) complex featuring a bound ppy ligand and an outer-sphere BArF24 anion (Figure 1). The binding environment around the Ni(II) center is like that of 4 and 5 with an NHC-η2-iminoacyl binding mode (Table 1). Complex 8 showcases the ability of this ligand platform to support cationic Ni(II) complexes and to accommodate relatively bulky ligands, relevant to future catalytic transformations.

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CONCLUSION The metalation strategies to synthesize a series of Ni(II) complexes featuring a monoanionic, bidentate NHC-CAryl ligand framework ([H(BrCC)]Br) are reported. The presence of a Ni−CAryl bond allowed for the modulation of the ligand E

DOI: 10.1021/acs.organomet.7b00463 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

reduced pressure, yielding [H(BrCC)]Br (3.281 g, 92.6%) as an offwhite powder. Further manipulations of [H(BrCC)]Br were carried out in a glovebox. Anal. Calcd for (C30H42BrN2): C, 61.02; H, 7.17; N, 4.74. Found: C, 60.97; H, 6.99; N, 4.88. 1H NMR (500 MHz, CDCl3): δ 10.20 (s, 1H), 7.80 (d, J = 2.4 Hz, 1H), 7.77 (t, J = 1.8 Hz, 1H), 7.57 (d, J = 2.5 Hz, 1H), 7.53 (t, J = 7.8 Hz, 1H), 7.30 (d, J = 7.8 Hz, 2H), 7.11 (t, J = 1.8 Hz, 1H), 6.19 (s, 2H, Ar-CH2), 2.35 (sept, J = 6.7 Hz, 2H, iPr-CH), 1.54 (s, 9H, tBu-(CH3)3), 1.33 (s, 9H, tBu-(CH3)3), 1.23 (d, J = 6.8 Hz, 6H, iPr-CH3), 1.14 (d, J = 6.8 Hz, 6H, iPr-CH3). 13 C{1H} NMR (126 MHz, CDCl3): δ 151.65, 149.13, 145.59, 138.41, 134.03, 132.08, 130.28, 128.40, 127.19, 124.86, 123.85, 122.97, 122.12, 56.07, 37.75, 35.18, 31.40, 30.16, 28.86, 24.59, 24.37. HRMS (ESI): calcd for C30H42BrN2+, [H(BrCC)]+, 509.2531; found, 509.2509. Preparation of (BrCC)NiBr2PPh3 (1). A 20 mL scintillation vial was charged with [H(BrCC)]Br (118.1 mg, 0.2 mmol), NiBr(PPh3)2 (152.3 mg, 0.205 mmol), and ca. 4 mL of benzene, and the mixture was stirred thoroughly. To this dark green suspension was added LiN(SiMe3)2 (34.3 mg, 0.205 mmol) dropwise. Upon addition of LiN(SiMe3)2, the mixture turned into a dark purple solution. After the solution was stirred for 2 h, benzene was removed under reduced pressure. The resulting purple powder was suspended in hexanes (2 mL) and filtered through a plug of Celite. The purple residue was washed with hexanes (2 × 2 mL) and then eluted with benzene (4 mL). The benzene solution was frozen and dried under reduced pressure, yielding a fine purple powder (187.4 mg, 0.189 mmol, 95%). Crystals suitable for X-ray diffraction were grown in a concentrated solution of MeCN with 1 drop of THF at −35 °C. Anal. Calcd for (C48H57Br3N2NiP): C, 58.15; H, 5.80; N, 2.83. Found: C, 58.28; H, 5.67; N, 3.06. 1H NMR (500 MHz, C6D6): δ 7.79 (d, J = 6.8 Hz, 6H), 7.61 (d, J = 2.3 Hz, 1H), 7.54−7.49 (m, 2H), 7.44 (d, J = 7.6 Hz, 2H), 7.01 (m, 9H), 6.79 (s, 2H), 6.45 (s, 1H), 6.33 (s, 1H), 3.29 (sept, J = 6.8 Hz, 2H, iPr-CH), 1.58 (s, 9H, tBu-(CH3)3), 1.43 (d, J = 6.6 Hz, 6H, iPr-CH3), 1.23 (s, 9H, tBu-(CH3)3), 0.99 (d, J = 6.8 Hz, 6H, iPrCH3). 13C{1H} NMR (126 MHz, C6D6): δ 150.22, 148.39, 147.71, 137.90, 135.97, 135.54, 130.71, 129.69, 128.59, 127.48, 125.18, 124.49, 121.55, 56.82, 37.68, 35.11, 31.32, 30.37, 29.10, 26.83, 23.01. 31P{1H} NMR (202 MHz, C6D6): δ 17.45 (PPh3). Preparation of (CC)NiBrPPh3 (2). A 20 mL scintillation vial was charged with complex 1 (99.0 mg, 0.1 mmol, 1.0 equiv) and ca. 3 mL of toluene and cooled to −35 °C. In a separate vial, KC8 (29.7 mg, 0.22 mmol, 2.2 equiv) was suspended in THF and then added to the cold purple solution of complex 1 (2 mL of THF total). Graphite was liberated immediately upon mixing. The reaction mixture was stirred for 30 min while it was warmed to room temperature and then filtered through a plug of Celite, yielding a dark orange solution. Solvents were removed under reduced pressure. The resulting orange residue was suspended in hexanes (2 mL) and filtered through a plug of Celite, where it appeared as a purple residue. The residue was washed further with hexanes (2 × 2 mL) and eluted with benzene, yielding a dark orange solution. The resulting benzene solution was frozen and then dried under reduced pressure, yielding complex 2 as a fine orangebrown powder in modest yield (46.1 mg, 0.554 mmol, 55%). 1H NMR (500 MHz, C6D6): δ 7.45−7.18 (m, 18h), 6.88 (d, J = 1.8 Hz, 1H), 6.65 (d, J = 1.8 Hz, 1H), 6.34 (s, 1H), 6.24 (s, 1H), 4.64 (s, 2H, ArCH2), 3.16 (sept, J = 6.6 Hz, 2H, iPr-CH), 1.59 (d, J = 6.7 Hz, 6H, iPrCH3), 1.39 (s, 9H, tBu-(CH3)3), 1.23 (s, 9H, tBu-(CH3)3), 0.83 (d, J = 6.8 Hz, 6H, iPr-CH3). 13C{1H} NMR (126 MHz, C6D6): δ 146.33, 137.29, 130.04, 125.36, 123.85, 121.75, 119.81, 119.28, 58.02, 57.94, 40.38, 34.19, 31.75, 31.19, 29.11, 29.07, 26.38, 23.37, 23.34. HRMS (ESI): calcd for C30H41N2Ni+, [(CC)Ni]+, 487.2623; found, 487.2639. Preparation of (CC)NiBrPMe3 (3). A 20 mL scintillation vial was charged with [H(BrCC)]Br (82.7 mg, 0.14 mmol, 1.0 equiv), Ni(COD)2 (38.5 mg, 0.14 mmol, 1.0 equiv), PMe3 (1.0 M THF, 0.6 mL, 0.6 mmol, 4.3 equiv), and ca. 5 mL of THF, and the mixture was stirred and then cooled to −35 °C. In a separate vial LiN(SiMe3)2 (23.4 mg, 0.14 mmol, 1.0 equiv) was dissolved in ca. 3 mL of THF and added dropwise to the cold yellow suspension, with the mixture being warmed to room temperature. Over the course of 10 min the mixture turned into an orange solution. The solution was stirred for 6 h, and then volatiles were removed under reduced pressure. The crude

backbone via insertion of isocyanides of various substitution, forming the corresponding NHC-η2-iminoacyl nickel complexes. Oxidation of 5 detached the η2-iminoacyl moiety from the nickel center to form a nitrilium species which upon reduction re-formed complex 5. Complex 8 also demonstrated that NHC-η2-iminoacyl nickel complexes are amenable to anion substitution with a BArF24 anion to create a low-coordinate, cationic Ni(II) complex.

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EXPERIMENTAL SECTION

General Considerations. All manipulations of air- and moisturesensitive compounds were carried out in the absence of water and dioxygen in an MBraun inert-atmosphere drybox under a dinitrogen atmosphere, except where specified otherwise. The MBraun drybox was equipped with a −35 °C freezer for cooling samples and crystallizations. All glassware was oven-dried for a minimum of 8 h and cooled in an evacuated antechamber prior to use in the drybox. Solvents for sensitive manipulations were dried and deoxygenated on a Glass Contour System (SG Water USA, Nashua, NH) and stored over 4 Å molecular sieves purchased from Strem following the literature procedure prior to use.35 NMR solvents (CDCl3, C6D6, and CD3CN) were purchased from Cambridge Isotope Laboratories and were degassed and stored over 4 Å molecular sieves prior to use. Celite 545 (J. T. Baker) was dried in a Schlenk flask for 24 h under dynamic vacuum while it was heated to at least 150 °C prior to use in a glovebox. NiCl2py4 and potassium graphite (KC8) were prepared according to a literature procedure.36,37 The ligand precursors 1-(2,6diisopropylphenyl)-1H-imidazole38 and 2-bromo-3,5-di-tert-butylbenzyl bromide39 were synthesized according to literature procedures. Lithium hexamethyldisilazide (LiN(SiMe3)2) was purchased from Sigma-Aldrich and recrystallized from toluene under an inert atmosphere prior to use. Nickel(II) chloride anhydrous (98%), bis(cyclooctadiene)nickel(0) (98%), and NiBr2(PPh3)2 (99%) were purchased from Strem and used as received. Trimethylphosphine (1.0 M in THF), tert-butyl isocyanide (98%), 2,6-dimethylphenyl isocyanide (96%), and 2-naphthyl isocyanide (95%) were purchased from Sigma-Aldrich and used as received. Triphenylphosphine (≥95% (GC)) was purchased from Sigma-Aldrich and recrystallized using ethanol and dried before use. 1 H, 13C, 19F, and 31P NMR spectra were recorded at room temperature on a Varian spectrometer operating at 500 MHz (1H NMR), 126 MHz (13C NMR), 202 MHz (31P NMR), and 470 MHz (19F NMR) and referenced to the residual solvent peaks (1H: C6D5H, δ 7.16 ppm; CHCl3, δ 7.26 ppm; CD2HCN, δ 1.94 ppm) as a standard and H3PO4 resonance (δ 0 ppm) for 31P. Magnetic measurements were carried out in triplicate in acetonitrile, measuring the resonance shift in the 19F signal of trifluorotoluene. Solid-state infrared spectra were recorded using a PerkinElmer Frontier FT-IR spectrophotometer equipped with a KRS-5 thallium bromide/iodide universal attenuated total reflectance (ATR) accessory. Electrospray ionization mass spectrometry (ESI) was recorded on a Water Q-TOF Ultima ESI instrument in the Mass Spectrometry Laboratory at the University of Illinois at UrbanaChampaign (UIUC). Elemental analyses were performed by the UIUC School of Chemical Sciences Microanalysis Laboratory. Elemental analysis was attempted for all compounds; however, complexes 2 and 4−6 were too unstable to provide satisfactory results. For these complexes, bulk purity was confirmed via NMR spectroscopy. X-ray crystallography was performed at the George L. Clark X-ray Facility at UIUC. Preparation of [H(BrCC)]Br. Outside of the glovebox, a 15 mL pressure vessel containing a Teflon stir bar was charged with 1-(2,6diisopropylphenyl)-1H-imidazole (1.370 g, 6.0 mmol), 2-bromo-3,5di-tert-butylbenzyl bromide (2.194 g, 6.06 mmol), and ca. 7 mL of toluene. The mixture was then heated at 140 °C overnight (∼18 h), resulting in the formation of an off-white precipitate within 30 min of heating. After 18 h, the resulting mixture was cooled to room temperature and triturated with hexanes (4 × 30 mL). Pentane (30 mL) was then added, and the suspension was filtered over filter paper. The solid was rinsed with copious pentane and dried thoroughly under F

DOI: 10.1021/acs.organomet.7b00463 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics residue was suspended with cold hexanes (2 mL) and then filtered through a plug of Celite. The orange-brown residue was washed with cold hexanes (2 × 2 mL) and then eluted with DCM (3 mL). Solvent was removed under reduced pressure to give an orange-brown powder (66.3 mg, 0.10 mmol, 74%). Crystals suitable for X-ray diffraction were grown from slow evaporation of a concentrated solution of 3 in Et2O. Anal. Calcd for (C33H51BrN2NiP): C, 61.42; H, 7.97; N, 4.34. Found: C, 61.35; H, 7.81; N, 4.51. 1H NMR (500 MHz, CDCl3): δ 7.39 (t, J = 7.8 Hz, 1H), 7.23 (t, J = 7.9 Hz, 2H), 7.15 (d, J = 1.8 Hz, 1H), 6.97 (d, J = 2.0 Hz, 1H), 6.87 (d, J = 2.1 Hz, 1H), 6.70 (t, J = 1.7 Hz, 1H), 6.20 (d, J = 13.5 Hz, 1H, Ar-CH2), 4.90 (d, J = 13.6 Hz, 1H, Ar-CH2), 2.82 (sept, J = 6.5 Hz, 1H, iPr-CH), 2.60 (sept, J = 6.9 Hz, 1H, iPr-CH), 1.83 (s, 9H, tBu-(CH3)3), 1.40 (d, J = 6.7 Hz, 3H, iPr-CH3), 1.32 (d, J = 6.8 Hz, 3H, iPr-CH3), 1.26 (s, 9H, tBu-(CH3)3), 1.10 (d, J = 6.9 Hz, 3H, iPr-CH3), 0.90 (d, 2JHP = 8.3 Hz, 9H, P(CH3)3), 0.72 (d, J = 6.7 Hz, 3H, iPr-CH3). 13C{1H} NMR (126 MHz, CDCl3): δ 178.48, 177.59, 157.22, 147.66, 145.01, 144.79, 144.29, 144.27, 137.70, 136.69, 129.65, 124.91, 124.57, 124.54, 124.03, 122.70, 122.27, 119.24, 118.57, 59.92, 37.17, 34.08, 33.30, 31.78, 28.95, 28.14, 26.96, 26.67, 23.33, 22.65, 14.45, 14.25. 31P{1H} NMR (202 MHz, CDCl3): δ −20.33 (P(CH3)3). HRMS (ESI): calcd for C30H41N2Ni+, [(CC)Ni]+, 487.2623; found, 487.2639. Preparation of [(CCNtBu)NitBuNC]Br (4). A 20 mL scintillation vial was charged with [H(BrCC)]Br (82.7 mg, 0.14 mmol, 1.0 equiv), Ni(COD)2 (38.5 mg, 0.14 mmol, 1.0 equiv), tert-butyl isocyanide (0.2 mL, 1.76 mmol, 12.6 equiv), and ca. 5 mL of THF; the mixture was stirred and then cooled to −35 °C. In a separate vial LiN(SiMe3)2 (23.4 mg, 0.14 mmol, 1.0 equiv) was dissolved in ca. 3 mL of THF and added dropwise to the cold yellow suspension, with the solution being warmed to room temperature. Over the course of 10 min the mixture turned into an orange solution. After ca. 3 h a yellow-orange precipitate became apparent. The solution was stirred for 6 h, and then volatiles were removed under reduced pressure. The crude mixture was washed with hexanes (2 × 5 mL) and dissolved in DCM (3 mL); the solution was filtered through a plug of Celite, and the solvent was removed under reduced pressure to give a yellow-orange powder (83.2 mg, 0.127 mmol, 91%). Crystals suitable for X-ray diffraction were grown from a concentrated solution of 4 in Et2O at −35 °C. 1H NMR (500 MHz, CDCl3): δ 8.35 (d, J = 1.9 Hz, 1H), 8.11 (d, J = 1.9 Hz, 1H), 7.48 (d, J = 1.9 Hz, 1H), 7.44 (t, J = 7.7 Hz, 1H), 7.31 (d, J = 7.3 Hz, 1H), 7.24 (d, J = 7.7 Hz, 1H), 6.81 (d, J = 1.8 Hz, 1H), 6.03 (d, J = 14.7 Hz, 1H, Ar-CH2), 5.44 (d, J = 14.7 Hz, 1H, Ar-CH2), 3.00 (sept, J = 6.9 Hz, 1H, iPr-CH), 2.24 (sept, J = 6.8 Hz, 1H, iPr-CH), 1.46 (d, J = 6.9 Hz, 3H, iPr-CH3), 1.40 (s, 9H), 1.37 (s, 9H), 1.24 (s, 18H), 1.15 (d, J = 6.8 Hz, 3H, iPr-CH3), 1.08 (d, J = 6.8 Hz, 3H, iPrCH3), 0.95 (d, J = 6.9 Hz, 3H, iPr-CH3). 13C{1H} NMR (126 MHz, CDCl3): δ 176.61, 171.18, 153.65, 147.09, 146.06, 145.93, 136.03, 135.87, 129.64, 127.01, 124.95, 124.08, 123.90, 119.62, 60.67, 54.58, 37.63, 35.47, 33.08, 31.31, 30.39, 29.95, 28.49, 28.04, 24.98, 24.81, 24.57, 24.42. HRMS (ESI): calcd for C40H59N4Ni+, [(CCNtBu)NitBuNC]+, 653.4093; found, 653.4096. Preparation of (CCNXyl)NiBr (5). A 20 mL scintillation vial was charged with [H(BrCC)]Br (88.6 mg, 0.15 mmol, 1.0 equiv), Ni(COD)2 (41.3 mg, 0.15 mmol, 1.0 equiv), 2,6-dimethylphenyl isocyanide (19.7 mg, 0.15 mmol, 1.0 equiv), and ca. 5 mL of THF, yielding a dark orange-red suspension which was stirred and then cooled to −35 °C. In a separate vial LiN(SiMe3)2 (25.1 mg, 0.15 mmol, 1.0 equiv) was dissolved in ca. 3 mL of THF and added dropwise to the cold solution, while it was warmed to room temperature. Over the course of 10 min the mixture turned into a red solution. After a couple of hours, a red precipitate became apparent. The solution was stirred for 18 h, and then volatiles were removed under reduced pressure. The crude mixture was washed with cold hexanes (2 × 5 mL) and dissolved in DCM (4 mL); the solution was filtered through a plug of Celite, and the solvent was removed under reduced pressure to give a red powder (85.0 mg, 0.122 mmol, 81%). Crystals suitable for X-ray diffraction were grown from diffusion of hexanes into a concentrated solution of 5 in THF. 1H NMR (500 MHz, CDCl3): δ 7.47 (d, J = 1.9 Hz, 1H), 7.44 (t, J = 7.8 Hz, 1H), 7.34 (dd, J = 7.8, 1.5 Hz, 1H), 7.32 (d, J = 1.9 Hz, 1H), 7.22 (dd, J =

7.7, 1.5 Hz, 1H), 7.13 (d, J = 1.8 Hz, 1H), 6.95 (m, 2H), 6.80 (d, J = 1.8 Hz, 1H), 6.75 (d, J = 7.2 Hz, 1H), 6.67 (d, J = 14.5 Hz, 1H, ArCH2), 4.73 (d, J = 14.5 Hz, 1H, Ar-CH2), 3.31 (sept, J = 6.9 Hz, 1H, i Pr-CH), 2.41 (s, 3H, Xyl-CH3), 2.33 (sept, J = 6.9 Hz, 1H, iPr-CH), 1.78 (d, J = 6.8 Hz, 3H, iPr-CH3), 1.51 (s, 3H, Xyl−CH3), 1.37 (s, 9H, t Bu-(CH3)3), 1.32 (d, J = 6.8 Hz, 3H, iPr-CH3), 1.10 (d, J = 6.8 Hz, 3H, iPr-CH3), 1.02 (d, J = 6.9 Hz, 3H, iPr-CH3), 0.98 (s, 9H, tBu(CH3)3). 13C{1H} NMR (126 MHz, CDCl3): δ 178.92, 177.96, 152.83, 148.81, 146.42, 145.34, 137.53, 136.87, 135.62, 133.15, 129.59, 129.20, 128.04, 126.27, 126.23, 126.20, 125.16, 124.75, 124.63, 123.91, 123.73, 120.57, 56.42, 36.42, 35.24, 31.26, 31.16, 28.60, 28.50, 25.86, 25.20, 24.58, 23.61, 19.45, 19.11. HRMS (ESI): calcd for C30H41N2Ni+, [(CCNXyl)Ni]+, 618.3358; found, 618.3374. Preparation of (CCNNapth)NiBr (6). A 20 mL scintillation vial was charged with [H(BrCC)]Br (23.6 mg, 0.04 mmol, 1.0 equiv), Ni(COD)2 (11.0 mg, 0.04 mmol, 1.0 equiv), 2-naphthyl isocyanide (6.1 mg, 0.052 mmol, 1.3 equiv), and ca. 3 mL of THF, yielding a dark orange-red suspension which was stirred and then cooled to −35 °C. In a separate vial LiN(SiMe3)2 (6.7 mg, 0.04 mmol, 1.0 equiv) was dissolved in ca. 2 mL of THF and added dropwise to the cold solution, while the mixture was warmed to room temperature. Over the course of 10 min the mixture turned into a dark red solution. The solution was stirred for 18 h, and then volatiles were removed under reduced pressure. The crude mixture was dissolved in 8 mL of 4/1 hexanes/ THF, filtered through a plug of Celite, and recrystallized at −35 °C. The mother liquor was pipetted off, and the purple-red residue was triturated with hexanes, yielding the pure complex as a purple-red powder (14.7 mg, 0.020 mmol, 51%). 1H NMR (500 MHz, CDCl3): δ 7.79−7.70 (m, 3H), 7.62 (d, J = 8.8 Hz, 1H), 7.56 (d, J = 1.9 Hz, 1H), 7.46 (t, J = 7.8 Hz, 1H), 7.44−7.39 (m, 2H), 7.36−7.31 (m, 2H), 7.30−7.25 (m, 1H), 7.16 (d, J = 1.9 Hz, 1H), 7.14 (dd, J = 8.7, 2.0 Hz, 1H), 6.82 (d, J = 1.8 Hz, 1H), 6.57 (d, J = 14.6 Hz, 1H, Ar-CH2), 4.73 (d, J = 14.6 Hz, 1H, Ar-CH2), 3.04 (sept, J = 6.9 Hz, 1H, iPr-CH), 2.41 (sept, J = 6.8 Hz, 1H, iPr-CH), 1.71 (d, J = 6.9 Hz, 3H, iPr-CH3), 1.43 (s, 9H, tBu-(CH3)3), 1.37 (d, J = 6.8 Hz, 3H, iPr-CH3), 1.13 (s, 9H, t Bu-(CH3)3), 1.09 (d, J = 6.9 Hz, 3H, iPr-CH3), 0.98 (d, J = 6.9 Hz, 3H, iPr-CH3). 13C{1H} NMR (126 MHz, CDCl3): δ 178.36, 177.69, 152.42, 147.76, 146.59, 145.40, 136.85, 135.76, 133.93, 132.86, 132.27, 129.67, 128.89, 128.48, 127.94, 127.50, 126.76, 126.10, 125.06, 124.70, 123.99, 123.85, 123.60, 121.90, 120.98, 120.51, 56.24, 37.60, 35.28, 31.90, 31.26, 28.70, 28.63, 26.09, 25.16, 24.73, 23.29. HRMS (ESI): calcd for C41H48N3Ni+, [(CCNNapth)Ni]+, 640.3202; found, 640.3174. Preparation of (XylNCCC)NiBr3 (7). A 20 mL scintillation vial was charged with complex 5 (27.9 mg, 0.04 mmol, 1.0 equiv) and ca. 4 mL of MeCN and then chilled to −35 °C. Trityl bromide (25.9 mg, 0.08 mmol, 2.0 equiv) was added dropwise to the cold solution with ca. 3 mL of MeCN while the solution was warmed to room temperature. The solution went from red to green over the course of a couple of minutes. The reaction mixture was stirred for 30 min, and then volatiles were removed under reduced pressure. The crude mixture was washed with Et2O (3 × 5 mL), removing Gomberg’s dimer. The resulting blue residue was dissolved in MeCN (4 mL), filtered through a plug of Celite, and the solvent was removed under reduced pressure to give a blue powder (26.2 mg, 0.030 mmol, 76%). Crystals suitable for X-ray diffraction were grown from slow evaporation of a MeCN solution of complex 7. Anal. Calcd for (C39H50Br3N3Ni): C, 54.52; H, 5.87; N, 4.89. Found: C, 54.53; H, 5.75; N, 5.09. 1H NMR (500 MHz, CD3CN): δ 87.29, 60.99, 20.15, 18.76, 11.74, 7.34, 6.05, 5.45, 4.87, 0.75, −0.36, −1.32, −2.88, −9.55, −16.09, −22.21. HRMS (ESI): calcd for C39H50N3+, (XylNCCC)+, 560.4005; found, 560.4019; calcd for C39H50Br2N3Ni+, [(XylNCCC)NiBr2]+, 776.1701; found, 776.1699. μeff = 3.7 ± 0.1 μB. Preparation of [(CCNXyl)Nippy]BArF24 (8). A 20 mL scintillation vial was charged with NaBArF24 (26.0 mg, 0.0294 mmol, 1.05 equiv) and ca. 1 mL of DCM. In a separate vial, complex 5 (19.6 mg, 0.028 mmol, 1.0 equiv), 2-phenylpyridine (4.6 mg, 0.0294 mmol, 1.05 equiv), and ca. 3 mL of DCM were combined and then transferred to the NaBArF24 solution. Over the course of a couple of minutes, the red solution turned orange-yellow. The reaction mixture was stirred for 3 h and then filtered through a plug of Celite, and the solvent was G

DOI: 10.1021/acs.organomet.7b00463 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

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removed under reduced pressure. The residue was triturated with hexanes (2 × 3 mL), and solvent was removed under reduced pressure again to give an orange powder (43.5 mg, 0.0263 mmol, 94%). Crystals suitable for X-ray diffraction were grown from a 50/50 mixture of Et2O and (Me3Si)2O at −35 °C. Anal. Calcd for (C82H71BF24N4Ni): C, 60.13; H, 4.37; N, 3.42. Found: C, 59.43; H, 4.35; N, 3.54. 1H NMR (500 MHz, CDCl3): δ 7.72 (br s, 10H), 7.60− 7.54 (m, 3H), 7.52 (br s, 4H), 7.47 (d, J = 1.9 Hz, 1H), 7.43 (t, J = 7.3 Hz, 1H), 7.34−7.26 (m, 5H), 7.20−7.14 (m, 2H), 7.03 (dd, J = 7.2, 2.0 Hz, 1H), 6.96 (t, J = 7.5 Hz, 1H), 6.91 (d, J = 1.8 Hz, 1H), 6.83− 6.77 (m, 2H), 6.69 (d, J = 7.5 Hz, 1H), 6.07 (d, J = 14.6 Hz, 1H, ArCH2), 4.78 (d, J = 14.6 Hz, 1H, Ar-CH2), 2.83 (sept, J = 6.7 Hz, 1H, i Pr-CH), 2.13 (sept, J = 6.7 Hz, 1H, iPr-CH), 1.68 (d, J = 6.9 Hz, 3H, i Pr-CH3), 1.34 (s, 3H, Xyl-CH3), 1.32 (s, 9H, tBu-(CH3)3), 1.14 (d, J = 6.9 Hz, 3H, iPr-CH3), 1.00 (d, J = 6.8 Hz, 3H, iPr-CH3), 0.87 (s, 12H, Xyl-CH3 and tBu-(CH3)3), 0.24 (d, J = 6.9 Hz, 3H, iPr-CH3). 13C{1H} NMR (126 MHz, CDCl3): δ 176.31, 173.97, 161.95, 161.86 (q, 1JCB = 49.9 Hz), 154.92, 150.24, 149.31, 146.87, 146.39, 139.67, 137.87, 137.07, 135.45, 134.96, 134.74, 131.16, 130.81, 130.28, 129.87, 129.42, 129.17, 128.91, 128.78, 128.63, 128.12, 127.96, 127.62, 127.34, 127.12, 126.89, 125.79, 125.75, 124.90, 124.83, 124.79, 124.70, 123.63, 123.49, 123.38, 122.47, 122.24, 121.46, 117.62, 55.89, 36.37, 35.46, 31.03, 30.94, 28.71, 28.31, 25.64, 25.06, 24.81, 21.48, 19.14, 16.90. 19F NMR (470 MHz, CDCl3) = −62.79 (BArF24). HRMS (ESI): calcd for C50H59N4Ni+, [(CCNXyl)Nippy]+, 773.4093; found, 773.4087.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00463. Spectral data and crystallographic data for the ligand and complexes 1−8 (PDF) Accession Codes

CCDC 1555887−1555892 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for A.R.F.: [email protected]. ORCID

Danielle L. Gray: 0000-0003-0059-2096 Alison R. Fout: 0000-0002-4669-5835 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support for this work was provided by the University of Illinois at UrbanaChampaign and the NSF (CHE1351961). J.W.N. is thankful for a Robert C. & Carolyn J. Springborn Fellowship. A.R.F. is an Alfred P. Sloan Research Fellow and a Camille Dreyfus Teacher-Scholar.

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REFERENCES

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DOI: 10.1021/acs.organomet.7b00463 Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.organomet.7b00463 Organometallics XXXX, XXX, XXX−XXX