Synthesis and Reactivity of Cyclic (Alkyl)(Amino) Carbene Stabilized

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Synthesis and Reactivity of Cyclic (Alkyl)(Amino)Carbene Stabilized Nickel Carbonyl Complexes Ursula S. D. Paul and Udo Radius* Institut für Anorganische Chemie, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany S Supporting Information *

ABSTRACT: Cyclic (alkyl)(amino)carbenes cAACcy (1b) and cAACmenthyl (1c) react with [Ni(CO)4] to give the 18 VE complexes [Ni(CO)3(cAACcy)] (2b) and [Ni(CO)3(cAACmenthyl)] (2c). With these in hand, the donor-strength and the steric profile of the respective cAAC ligands were evaluated. CAACcy and cAACmenthyl possess similar overall-donating properties (Tolman electronic parameter (TEP) = 2046 (1b) and 2042 (1c)) as common NHCs, though they are also known to be better π-acceptors. 3,3-Diamino-2-aryloxyacrylimidamide 3b, arising from the reaction of cAACcy (1b) with released CO molecules, was obtained as side-product of CO substitution reactions at nickel carbonyls. In contrast to cAACmenthyl (%Vbur = 42), the sterically less encumbered cAACcy (% Vbur = 38) undergoes a subsequent CO substitution at [Ni(CO)3(cAACCy)] (2b) to afford the 16 VE complex [Ni(CO)(cAACcy)2] (4b). Treatment of both [Ni(CO)3(cAACmethyl)] (2a) and [Ni(CO)(cAACmethyl)2] (4a) with allyl bromides led to the formation of cAAC-stabilized allyl nickel complexes [NiBr(η3-H2CCH−CH2)(cAACmethyl)] (5a) and [NiBr(η3-H2CCH−CMe2)(cAACmethyl)] (5b). The chloro complex [NiCl(η3-H2CCMe−CH2)(cAACmethyl)] (6) was synthesized from [Ni(COD)2] (COD = 1,5-cyclooctadiene) by consecutive treatment with allyl chloride and cAAC. The allyl halide complexes 5 and 6 are thermally labile and decompose in solution already within a few hours at room temperature. One of the decomposition products, the dinuclear nickel complex [Ni2(μBr)2(η3-(cAACmethyl)CH−CH(CH3))2] (7), was crystallographically characterized.

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properties of ligands.8 Accordingly, cAACmethyl (1a) was proposed to be a stronger net electron donor than common NHCs, such as IPr or IMes, as it gave rise to a slightly smaller TEP value of 2046 cm−1,7 while possessing significantly superior π-acceptor properties (Chart 1).7,9 The analysis of the percentage buried volume (%Vbur)5 allowed a quantification of the steric demand of cAACmethyl (1a) ligand, which is at 38% and has a similar steric demand as that of the bulky NHC ItBu (37%).6g

INTRODUCTION During the last decades, N-heterocyclic carbenes (NHCs) have found versatile applications as ligands in transition-metal chemistry due to their unique stereoelectronic properties.1,2 Recent investigations have shown3 that the replacement of one amino substituent of the NHCs by an alkyl group results in a carbene, the so-called cAACs (cyclic (alkyl)(amino)carbenes), with a smaller HOMO LUMO gap, and thus a more nucleophilic yet simultaneously more electrophilic carbene compared to the parent NHCs. Furthermore, the introduction of a quaternary carbon atom in α position to the carbene center allows a considerable variation of the steric bulk of these carbenes.3 To gain a deeper understanding of the fundamental electronic4 and steric factors5 which characterize cAACs and to compare it with the large array of data available for NHC ligands,6 we recently reported the first results on substitution reactions of the sterically least demanding cAACmethyl (1a) with selected nickel carbonyl complexes.7 These reactions afforded the novel mono- and bis-cAACmethyl coordinated complexes [Ni(CO)3(cAACmethyl)] (2a), [Ni(CO)(cAACmethyl)2] (4a), and side-product 3a, a diamino-2-aryloxyacrylimidamide arising from the reaction of cAACmethyl with released CO molecules in the course of such substitution reactions. The CO A1 stretching frequency of complexes of the type [Ni(CO)3(L)] (L = 2 VE donor ligand), known as the Tolman electronic parameter (TEP), is used to catalog the electronic © XXXX American Chemical Society

Chart 1. [Ni(CO)3(L)]-Based TEP and %Vbur Values and 31P Phosphinidene Shifts for Selected cAACs7,9a,10

Received: February 10, 2017

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DOI: 10.1021/acs.organomet.7b00109 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

yellow complex 2c without comparably distinct color changes of the reaction mixture and formation of ketenic or acrylimidamidic (3) species. Therefore, compound 2c was isolated in much better yield (65%) than [Ni(CO)3(cAACcy)] (31%). Compound 3b was selectively synthesized by passing CO through a solution of cAACcy (1b), resulting in a deep blue solution at low temperatures, which turns colorless upon warming to room temperature (Scheme 2). The isolated colorless solid 3b was fully characterized (including X-ray diffraction; Figure 1). All spectroscopic and analytical data are in line with those for the cAACmethyl substituted compound 3a.7

We report here (i) investigations on the reactivity of the bulkier derivatives cAACcy (1b, Chart 1) and cAACmenthyl (1c) with [Ni(CO)4] to explore the influence of the increasing steric demand of the cAACs on the outcome of substitution reactions in nickel carbonyl chemistry and (ii) first investigations concerning the synthesis and stability of cAAC-stabilized allyl nickel complexes.

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RESULTS AND DISCUSSION The complexes [Ni(CO)3(cAACRR′)] (RR′ = cy = cyclohexyl 2b, RR′ = menthyl 2c) can be synthesized in low to moderate yield from the reaction of cAACRR′ (RR′ = cy 1b, RR′ = menthyl 1c)3e,g with a slight excess of [Ni(CO)4] in hexane (Scheme 1). Scheme 1. Synthesis of [Ni(CO)3(cAACRR′)] (2)

As reported earlier, [Ni(CO)3(cAACmethyl)] (2a) was isolated in yields of up to 33% due to the formation of a 3,3diamino-2-aryloxyacrylimidamide 3a.7 The same side-reaction was observed for the reaction of [Ni(CO)4] with cyclohexylsubstituted cAACcy (1b) as reagent. The color of this reaction mixture changes to a significant deep blue immediately after addition of the cAAC to a solution of nickel tetracarbonyl and brightens to pale yellow (the color of product 2b) within 2 h. Compound 3b is formed as a side-product from the reaction of 2 equiv of cAAC with CO to give in a first step the biscarbene adduct of CO, which subsequently rearranges with migration of the 2,6-diisopropylphenyl (Dipp) entity (Scheme 2). In contrast, though cAACmenthyl (1c) is known to react with CO to give the corresponding blue ketene,12 no side reaction of 1c with CO was observed in course of the synthesis of [Ni(CO)3(cAACmenthyl)] (2c). Addition of cAACmenthyl (1c) to a solution of [Ni(CO)4] leads to the precipitation of pale

Figure 1. Molecular structure of 3,3-diamino-2-aryloxyacrylimidamide 3b with thermal ellipsoids drawn at the 50% probability level; hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): C1−C3 1.345(4), C2−C3 1.500(4), C3−O1 1.406(4), C1−N1 1.404(4), C2−N2 1.269(4); C1−C3−C2 126.7(6), N1−C1−C3 124.4(3), C3−C2−N2 120.8(3).

Complexes [Ni(CO) 3 (cAAC c y )] (2b) and [Ni(CO)3(cAACmenthyl)] (2c) were characterized by multinuclear NMR as well as IR spectroscopy and elemental analysis. The 13 C NMR spectra of 2b and 2c give rise to the characteristic low-field resonances for the carbene carbon atom coordinated at nickel at 277.1 (2b) and 274.1 ppm (2c), respectively, and show only one signal for the three chemically equivalent carbonyl ligands at 198.4 (2b) and 197.7 ppm (2c). The molecular structures of 2b and 2c were determined by singlecrystal X-ray diffraction (Figure 2) and reveal the expected tetrahedral geometry, with bond angles between 103 and 118° at the metal atom. The Ni1−C1 distances to the carbene carbon atoms (1.9613(13) Å in 2b, 1.996(4) Å in 2c) are in good agreement with that of [Ni(CO)3(cAACmethyl)] (2a, 1.9629(13) Å)7 or other NHC-containing nickel tricarbonyl complexes reported earlier, such as [Ni(CO)3(SIPr)] (1.962(4) Å) and [Ni(CO)3(SIMes)] (1.960(2) Å).6e The distances Ni1−C2, Ni1−C3, and Ni1−C4 to the carbonyl carbon atoms, which lie in the range between 1.7872(14) and 1.842(6) Å, are slightly shorter compared to Ni1−C1 but are unexceptional. The A1 carbonyl stretching frequencies observed in the IR spectra of 2b (2046 cm−1) and 2c (2042 cm−1) are identical or close to those of 2a (2046 cm−1)7 and similar to those of NHCligated complexes such as [Ni(CO)3(IPr)] (2052 cm−1) and [Ni(CO)3(SIPr)] (2052 cm−1) investigated earlier.6e The average carbonyl stretching frequency of square planar complexes of the type cis-[IrCl(CO)2(L)] and cis-[RhCl(CO)2(L)] (L = 2VE donor ligand, e.g., a carbene) can be

Scheme 2. Carbonylation of cAACRR 1a

a Leads to a ketene (for RR′ = menthyl)12 or 3,3-diamino-2aryloxyacryl-imidamide 3b as final product (for RR′ = cy).

B


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Organometallics

Chart 2. Comparison of 31P NMR Spectroscopic Data7,9a and TEP Values of Selected NHCs6e and the cAACs 1a,7 1b, and 1c Investigated within Our Studies, as Well as Their Calculated %Vbur11 from the Respective Nickel Carbonyl Complexes [Ni(CO)3(carbene)] (2a−c).6e,7

Figure 2. Molecular structures of [Ni(CO)3(cAACcy)] (2b, left) and [Ni(CO)3(cAACmenthyl)] (2c, right) with thermal ellipsoids drawn at the 50% probability level; hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg) for 2b: Ni1−C1 1.9613(13), Ni1−C2 1.7872(14), Ni1−C3 1.7957(14), Ni1−C4 1.8106(14), C1−N1 1.3102(17); C1−Ni1−C2 112.23(6), C1−Ni1− C3 104.79(5), C1−Ni1−C4 113.08(6) C2−Ni1−C3 108.82(6), C2− Ni1−C4 108.72(6), C3−Ni1−C4 109.04(6); 2c: Ni1−C1 1.996(4), Ni1−C2 1.801(6), Ni1−C3 1.803(6), Ni1−C4 1.842(6), C1−N1 1.320(6); C1−Ni1−C2 117.7(2), C1−Ni1−C3 105.1(2), C1−Ni1− C4 112.0(2) C2−Ni1−C3 108.4(3), C2−Ni1−C4 102.5(2), C3− Ni1−C4 111.3(2).

correlated by linear regression with the respective nickelderived TEP (eq I or II, refs 8g or 4, respectively). accessible from the reaction of the mono-cAAC nickel tricarbonyl complexes [Ni(CO)3(cAAC)] with additional cAAC or from the reaction of suitable NHC precursors such as [Ni(CO)2(ItBu)] with 2 equiv of the cAAC. Indeed, both pathways are feasible (Scheme 3).

νav/Ir(CO) = 0.923νav/Rh(CO) + 141 cm−1 TEP = 0.847νav/Rh(CO) + 336 cm−1

(I)

TEP = 0.800νav/Rh(CO) + 420 cm−1

(II)

or

Scheme 3. Synthesis of [Ni(CO)(cAACRR′)2] (4)

Using these equations, the A1 carbonyl stretching frequencies of [Ni(CO)3(cAACcy)] (2b) (TEP = νA1/Ni = 2046 cm−1) and [Ni(CO)3(cAACmenthyl)] (2c) (TEP = νA1/Ni = 2042 cm−1) observed here are in good to excellent agreement with results obtained earlier for cis-[RhCl(CO)2(cAACcy)] (TEP = 2049 cm−1, νav/Rh = 2036 cm−1)3c and cis-[IrCl(CO)2(cAACmenthyl)] (TEP = 2041 cm−1, νav/Ir = 2013 cm−1),3a respectively. We calculate for [Ni(CO)3(cAAC)] a buried volume (%Vbur)5 of 38% for cAACcy and 42% for cAACmenthyl. Our results are summarized in Chart 2. Although cAACcy and cAACmenthyl have a similar %Vbur compared to ItBu (or IAd), the chemical behavior of these molecules is different. For the bulkier NHCs like ItBu, it is known that one NHC ligand replaces two CO ligands in the reaction with [Ni(CO)4] to give three-coordinate complexes [Ni(CO)2(NHC)] (NHC = ItBu and IAd).6c cAACs 1a−c, however, lead irrespective of their steric demand to formation of saturated four-coordinated 18 VE complexes [Ni(CO)3(cAACRR′)] (2) when they were reacted with [Ni(CO)4]. It is also known that the corresponding NHC complexes [Ni(CO)3(NHC)], which are ligated with IMes or IPr, are usually reluctant to further replacement of a carbonyl.6e,7 Bis-carbene complexes [Ni(CO)2(NHC)2] have so far only been accessible either by carbonylation of bis-NHC complexes,6d,f by the reaction of Ni(CO)4 with sterically less demanding NHCs,6a or by addition of a carbene to coordinatively unsaturated complexes bearing a bulky NHC ligand, such as [Ni(CO)2(ItBu)].6g We were thus interested to learn more about the stability of [Ni(CO)3(cAACRR′)] (2) with respect to further substitution and investigated possible strategies for the synthesis of bis-cAAC complexes of the type [Ni(CO)(cAAC)2] (4). These complexes should be either

Treatment of complexes [Ni(CO)3(cAACmethyl)] (2a) and [Ni(CO)3(cAACcy)] (2b) with additional carbene 1a and 1b led to the substitution of two additional carbonyl ligands and the formation of the three-coordinated 16 VE complexes [Ni(CO)(cAACmethyl)2] (4a) and [Ni(CO)(cAACcy)2] (4b) (Scheme 3). In the case of the reaction of cAACmenthyl and [Ni(CO)3(cAACmenthyl)] (2c), no reaction took place, presumably due to the steric bulk of the cAAC employed. This finding again reflects the exceptional position of cAACmenthyl (1c) among the cAACs investigated herein. The reaction of cAACmethyl (2a) and cAACcy (2b) with NHC complex [Ni(CO)2(ItBu)] also led to formation of 4a and 4b with substitution of one CO and one ItBu ligand (Scheme 3). This outcome was somehow surprising in comparison with reactions of the most closely related NHCs, like IMes. These were reported to substitute only the ItBu ligand with formation of tetra-coordinated bis-carbene complexes [Ni(CO)2(NHC)2].6g However, since the cAAC ligands show lower TEP parameters and are thus better net donor ligands compared to NHCs (Chart 2), this substitution of the ItBu by the better donor cAAC can be understood. Furthermore, the reaction of [Ni(CO)2(ItBu)] with 3 equiv of the sterically more demanding cAACmenthyl again failed to yield bis-carbene C


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Organometallics complex [Ni(CO)(cAACmenthyl)2] or any other product of a replacement of the NHC with cAACmenthyl (1c). Complex [Ni(CO)(cAACcy)2] (4b) has been isolated and was fully characterized. The 13C NMR spectrum of 4b shows only one low-field resonance for both carbene carbon atoms at 272.5 ppm and the signal for the carbonyl ligand at 196.6 ppm. The molecular structure of 4b in the solid state was established by single-crystal X-ray diffraction (Figure 3) and reveals a

Nolan et al. reported earlier on the reactivity of complexes [Ni(CO)3(NHC)] and [Ni(CO)2(NHC)] toward organyl halides.6e While four-coordinate complexes [Ni(CO)3(NHC)] proved to be inert against any kind of substitution reaction, coordinatively unsaturated complexes [Ni(CO)2(ItBu)] and [Ni(CO)2(IAd)] showed reactivity toward substitution with (partial) loss of the carbonyl ligands when treated with electron-poor trifluoropropene and allyl halides.6e First investigations have shown that complexes [Ni(CO)3(cAACmethyl)] (2a) and [Ni(CO)(cAACmethyl)2] (4a) are inert toward CO substitution using neutral ligands, such as phosphines, e.g., PEt3, and olefins, e.g., styrene, at ambient temperatures. However, both compounds react readily with allyl bromides via oxidative decarbonylation of the nickel(0) carbonyls to yield nickel(II) cAAC allyl halide complexes [NiBr(η3-H2CCH−CH2)(cAACmethyl)] (5a) and [NiBr(η3H2CCH−CMe2)(cAACmethyl)] (5b) (Scheme 4). Scheme 4. Synthesis of [NiBr(η3-H2CCH− CH2)(cAACmethyl)] (5a) and [NiBr(η3-H2CCH− CMe2)(cAACmethyl)] (5b)

Figure 3. Molecular structure of 4b with thermal ellipsoids drawn at the 50% probability level; hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Ni1−C1 1.8737(14), Ni1−C2 1.728(2) C1−N1 1.3348(18); C1−Ni1−C2 110.49(4), C1− Ni1−C1′ 139.02(9).

distorted trigonal planar geometry at the metal center, with bond angles between 110 and 139°. The Ni1−C1 distance to the carbene carbon atom of 1.8737(14) Å in 4b is shorter than the Ni−CcAAC bond length of parent mono-cAAC complex 2b (1.9613(13) Å) and similar to that of the corresponding cAACmethyl substituted compound [Ni(CO)(cAACmethyl)2] (4a) (1.874(2) Å).7 Furthermore, the Ni1−C2 bond length to the carbonyl carbon atom of 1.728(2) Å is significantly shorter than the Ni−CO bond lengths observed for complex 2b (1.7872(14)−1.8106(14) Å). Infrared spectroscopy reveals an exceptionally low lying carbonyl stretching frequency (1914 cm−1). All these factors indicate enhanced π-backbonding from the nickel atom to both the carbonyl and the carbene ligands upon replacement of two carbonyls for a second cAAC ligand. This reflects the synergy of σ-donating and π-accepting properties13 of 1b, as recently reported for the sterically less demanding methyl substituted cAAC 1a.7 Group 10 metal complexes containing allyl ligands play an important role as precursors and as catalysts in metal-mediated organic synthesis. We were thus interested in the reactivity of cAAC-st abilized nickel carbonyl complexes [Ni(CO)3(cAACmethyl)] (2a) and [Ni(CO)(cAACmethyl)2] (4a) with respect to π-acidic substrates, ultimately allyl halides.14 We were especially interested in the reaction with allyl halides, since oxidative addition of these substrates to [Ni(CO)3(cAAC)] would provide a facile entry into novel allyl nickel cAAC chemistry. Analogous NHC-stabilized nickel allyl complexes6e,15 as well as the closely related cyclopentadienyl complexes16 have attracted considerable attention over the past few years, due to various catalytic applications17,18 and for giving access to very interesting, highly reactive but isolable Ni(I) radical species.19 Introducing cAACs as ligands instead in this kind of complexes seemed quite promising to us as these are known to stabilize a broad range of main group element radicals as well as unusual paramagnetic transition metal complexes.3

The 13C NMR spectra of 5a and 5b give rise to a low-field resonance for the carbene carbon atoms at 277.9 and 280.9 ppm, respectively. Furthermore, the characteristic signals for the η3-coordinated allyl ligands at 107.5 (−CH), 73.6 (−CH2), and 46.3 ppm (−CH2) for 5a and 104.4 (−CMe2), 104.2 (−CH), and 36.1 ppm (−CH2) for 5b are observed in the 13C NMR spectra. The allylic protons give rise in the 1H NMR spectra of 5a and 5b to characteristic signals at 4.55 (−CH), 3.79/2.48 (−CH2), and 2.79/1.16 ppm (−CH2) for 5a and 1.84/1.04 (−CMe2), 4.33 (−CH), and 2.10/1.23 ppm (−CH2) for 5b. The IR spectrum of 5b does not indicate the allyl ligand binding in an η1-mode as no bands are observed around 1600− 1650 cm−1, the typical range of CC double bond stretching frequencies. The proposed binding of the allyl ligands in an η3mode was further supported by the molecular structures of 5a and 5b in the solid state, which were established by singlecrystal X-ray diffraction (Figure 4). The complexes adopt a distorted square-planar geometry around the metal centers, with sums of the bond angles of 359.4° (5a) and 360.2° (5b), in which the cAAC coordinates in cis position to the bromide and the terminal allyl carbon atoms are trans to either the carbene or the halide ligand. The Ni1−C1 distance to the carbene carbon atom of 1.8802(15) Å in 5a and 1.875(3) Å in 5b is shorter than the Ni−CcAAC bond length of parent monocAAC complex [Ni(CO)3(cAACmethyl)] (2a) (1.9629(13) Å) but similar to that of respective bis-cAAC complex [Ni(CO)(cAACmethyl)2] (4a) (1.874(2) Å).7 D


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with allyl chloride to give the dinuclear allyl chlorido complex [Ni2(μ-Cl)2(η3-H2CCH−CH2)2] as intermediate, which is further transformed in situ to desired mononuclear complex 6 by addition of the cAAC (Scheme 5). Scheme 5. Synthesis of [NiCl(η3-H2CCMe− CH2)(cAACmethyl)] (6)

Figure 4. Molecular structures of [NiBr(η3-H2CCH−CH2)(cAACmethyl)] (5a) and [NiBr(η3-H2CCH−CMe2)(cAACmethyl)] (5b) with thermal ellipsoids drawn at the 50% probability level; hydrogen atoms except of those bound to the allyl ligand are omitted for clarity. Selected bond distances (Å) and angles (deg) for 5a: Ni1− Br1 2.3419(3), Ni1−C1 1.8802(15), Ni1−C2 2.035(10), Ni1−C3 2.000(3), Ni1−C4 1.980(12), C1−N1 1.310(2), C2−C3 1.392(10), C3−C4 1.427(11); C1−Ni1−Br1 94.88(4), C1−Ni1−C2 172.1(2), C1−Ni1−C4 100.9(3) C2−Ni1−C4 73.4(4), C2−Ni1−Br1 90.2(3), C2−C3−C4 116.7(6); 5b: Ni1−Br1 2.3339(6), Ni1−C1 1.875(3), Ni1−C2 2.194(4), Ni1−C3 2.027(4), Ni1−C4 1.964(5), C1−N1 1.317(4), C2−C3 1.401(6), C3−C4 1.418(7), C2−C5 1.512(6), C2− C6 1.471(6); C1−Ni1−Br1 97.43(9), C1−Ni1−C2 168.68(14), C1− Ni1−C4 98.50(18) C2−Ni1−C4 72.4(2), C2−Ni1−Br1 91.9(1), C2−C3−C4 121.8(4).

In accordance with bromide complexes 5a and 5b, the 13C NMR spectrum of 6 shows a low-field resonance for the carbene carbon atoms at 279.1 ppm besides the characteristic signals for the η3-bound methallyl ligand at 120.5 (−CMe), 74.8 (−CH2), 43.5 (−CH2), and 22.6 ppm (−CMe). The molecular structure of 6 in the solid state was established by single-crystal X-ray diffraction (Figure 5) and shows the expected distorted

Furthermore, the Ni1−C1 distances as well as the Ni1−Br1 bond lengths to the halide atom of 2.3419(3) Å (5a) and 2.3339(6) Å (5b) are significantly shorter than those observed for NHC-containing nickel allyl complex [NiBr(η3-H2CCH− CH2)(ItBu)] (Ni1−C1 = 1.957(4) Å, Ni1−Br1 = 2.3970(8) Å).6e,15c This contraction is presumably not due to the different steric size (as the %Vbur of cAACmethyl and ItBu is almost identical) but might rather be attributed to the different electronic, i.e., acceptor, properties of the cAAC ligands. This stands in line with the findings at related NHC stabilized allyl complexes, that the Ni1−C1 and the Ni1−Br1 distances of [NiBr(η3-H2CCH−CH2)(ItBu)] and [NiBr(η3-H2CCH− CH2)(IMe)] are similar, irrespective of the very different steric demand of the NHC ligands.15c However, N-alkyl substituted NHCs are known to be worse π-acceptors than N-aryl substituted NHCs,9a which is reflected in an elongation of the carbene carbon−nickel (and also the nickel−halide) bond length for complexes [NiCl(η 3 -H 2CCH−CH2 )(ItBu)] (Ni1−C1 = 1.9290(18) Å) and [NiCl(η3-H2CCH−CH2)(IPr)] (Ni1−C1 = 1.9025(16) Å).15b As cAACs exhibit superior acceptor properties, they accordingly give rise to the shortest Ni1−C1 and the Ni1−Br1 bond lengths when they are coordinated to nickel metal in complexes [NiX(η3-H2CCH− CH2)(carbene)] (X = halide). The distances from Ni1−C2 (2.035(10) Å 5a, 2.194(4) Å 5b) and Ni1−C4 (1.980(12) Å 5a, 1.964(5) Å 5b) to the allyl ligand are slightly shorter compared to the NHC stabilized analogue but unexceptional. In marked contrast to these results, no reaction was observed for the cAAC-stabilized nickel carbonyl complexes if allyl chlorides were employed instead of allyl bromides. However, a corresponding cAAC-stabilized nickel allyl chloro complex [NiCl(η3-H2CCMe−CH2)(cAACmethyl)] (6) was prepared following a similar procedure as reported by Sigman et al. earlier for the one-pot synthesis of a series of NHC-ligated allyl chlorido complexes, such as [NiCl(η3-H2CCH−CH2)(IPr)].15 [Ni(COD)2] is used as starting material and reacted

Figure 5. Molecular structure of [NiCl(η3-H2CCMe−CH2)(cAACmethyl)] (6) with thermal ellipsoids drawn at the 50% probability level; hydrogen atoms except of those bound to the allyl ligand are omitted for clarity. Selected bond distances (Å) and angles (deg): Ni1−Cl1 2.193(2), Ni1−C1 1.894(2), Ni1−C2 2.061(3), Ni1−C3 2.001(3), Ni1−C4 1.965(3), C1−N1 1.310(3), C2−C3 1.379(5), C3−C4 1.414(5), C3−C5 1.506(4); C1−Ni1−Cl1 97.19(8), C1− Ni1−C2 169.83(12), C1−Ni1−C4 98.56(12) C2−Ni1−C4 72.57(14), C2−Ni1−Cl1 91.55(12), C2−C3−C4 117.2(3).

square-planar geometry around the metal center, with a sum of the bond angles of 359.9°. The cAAC ligand coordinates in cis position to the halide, and the terminal allyl carbon atoms are trans to either the carbene or the chlorido ligand. Likewise, the Ni1−C1 distance to the carbene carbon atom of 1.894(2) Å is shorter than the Ni−CcAAC bond length of parent mono-cAAC complex [Ni(CO)3(cAACmethyl)] (2a) (1.9629(13) Å) but is similar to that of bis-cAAC complex [Ni(CO)(cAACmethyl)2] (4a) (1.874(2) Å)7 and in the range of those of bromido complexes 5a (1.8802(15) Å) and 5b (1.875(3) Å). Furthermore, the Ni1−C1 as well as the Ni1−Cl1 distance (2.193(2) Å) and the Ni−Callyl bond lengths are similar to those observed for NHC-containing nickel allyl complexes, such as [NiCl(η 3 -H 2 CCH−CH 2 )(IPr)] (Ni1−C1 = 1.9025(16) Å, Ni1−Cl1 = 2.1862(5) Å).15 It should be pointed out that we observed two main differences in the reactivity of the cAAC-stabilized nickel carbonyl complexes compared to those of their NHC analogs. Both 18 VE [Ni(CO) 3(cAACmethyl)] (2a) and 16 VE E


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Organometallics [Ni(CO)(cAACmethyl)2] (4a) were found to react with allyl bromides with oxidative addition and loss of CO and the second cAAC ligand, respectively. In the case of the NHC complexes, only the use of the coordinatively unsaturated, three-coordinate 16 VE complexes [Ni(CO)2(NHC)] led to reaction with allyl halides, whereas saturated 18VE complexes [Ni(CO)3(NHC)] were reported to be inert against any kind of CO substitution. However, NHC-stabilized nickel carbonyls [Ni(CO)2(NHC)] react with allyl chloride and allyl bromide.6e Within our studies, however, no reaction of cAAC-ligated nickel compounds 2a or 4a with allyl chlorides was observed so far. However, to obtain chlorido complex [NiCl(η3-H2CCMe− CH2)(cAACmethyl)] (6), the alternative synthetic strategy (oxidative addition of allyl chloride to [Ni(COD)2] followed by substitution with the cAAC) can be used. It should also be noted that in contrast to their NHC-ligated analogues allyl halide complexes [NiBr(η3-H2CCH−CH2)(cAACmethyl)] (5a), [NiBr(η3-H2CCH−CMe2)(cAACmethyl)] (5b), and [NiCl(η3-H2CCMe−CH2)(cAACmethyl)] (6) already decompose at room temperature in solution within a few hours as well as in the solid state upon prolonged storage. These transformations are accompanied by a color change from yellow or orange of the allyl nickel halides to a purple color. In accordance with the literature, we initially assumed that traces of O2 might have resulted in the oxidation and elimination of the allyl ligand with formation of a dinuclear bis-μ-hydroxo nickel(II) complex.15a,b However, in the case of compound 5a we succeeded in isolating and characterizing dark purple decomposition product 7 by growing crystals suitable for X-ray diffraction from a saturated solution in toluene at −30 °C. Its molecular structure in the solid state (Figure 6) reveals no such

Scheme 6. Decomposition of Allyl Bromido Complex [NiBr(η3-H2CCH−CH2)(cAACmethyl)] (5a)

adopt a distorted square-planar geometry with a sum of the bond angles of 358.7°. The two [Ni(μ-Br)(η3-(cAACmethyl)CH−CH(CH3))] fragments are linked by asymmetrically bridging bromide ligands with Ni1−Br1 and Ni1−Br1′ bond lengths of 2.3799(8) and 2.4008(8) Å, respectively. These are longer than the Ni−Br distances (2.3419(3) Å) of the parent mononuclear compound [NiBr(η3-H2CCH−CH2)(cAACmethyl)] (5a) and similar to other bromo-bridged η3-allyl complexes, such as [Ni2(μBr)2(η3-(SiMe3)HCCH−CH(SiMe3))2] (2.362(10) and 2.365(1) Å) 20b or [Ni 2 (μ-Br) 2 (η 3 -H 2CCH(COOEt)− CH2)2] (2.334(5) and 2.378(5) Å).20a Likewise, the Ni1−C2 (1.970(4) Å) and Ni1−C3 (1.957(5) Å) bond lengths to the cAAC-based allyl ligand are in good agreement with those of other complexes (5a, 1.980(12)−2.035(10); [Ni2(μ-Br)2(η3(SiMe3)HCCH−CH(SiMe3))2], 1.978(6)−2.062(7) Å,20b [Ni 2 (μ-Br) 2 (η 3 -H 2 CCH(COOEt)−CH 2 ) 2 ], 1.90(2)− 2.06(2) Å).20a The Ni−C1 distance (2.353(4) Å) is, however, remarkably longer, which can be explained with the high steric congestion around the former carbene carbon atom C1. The nickel−nickel distance of 3.3329(9) Å is rather large; therefore, no significant interaction of the two metal centers is expected here. Further investigations on the reactivity of the allyl complexes including the generality and mechanism of this decomposition reaction are part of current work in our group.

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CONCLUSIONS We provide herein insight into carbonyl substitution in nickel tetracarbonyl using cyclic (alkyl)(amino)carbenes of different sizes, i.e., cAACmethyl (1a) and sterically more demanding cAACcy (1b) and cAACmenthyl (1c). In order to compare their electronic and steric properties, we prepared complexes [Ni(CO)3(cAACcy)] (2b) and [Ni(CO)3(cAACmenthyl)] (2c). Like their methyl-substituted congener cAACmethyl (1a), cAACcy and cAACmenthyl possess similar overall-donating properties with respect to the nickel tricarbonyl complex fragment and thus have similar TEP values (TEP = 2046 (1a), 2046 (1b), and 2042 (1c)). Sterically less encumbered carbenes cAACmethyl (1a) (%Vbur = 38) and cAACcy (1b) (%Vbur = 38) undergo a second CO substitution at the monocarbene complexes [Ni(CO) 3 (cAAC)] to afford [Ni(CO)(cAACmethyl)2] (4a) and [Ni(CO)(cAACcy)2] (4b). In contrast, sterically more demanding cAACmenthyl (1c) (%Vbur = 42) is already too bulky for a further reaction. Furthermore, we demonstrate that the wide variety of NHC complexes might be suitable precursors for the synthesis of cAAC-substituted compounds. The reaction of cAACmethyl (2a) and cAACcy (2b) with the NHC complex [Ni(CO)2(ItBu)] also led to formation of 4a and 4b with substitution of one CO and one ItBu ligand. First studies on the reactivity of both [Ni(CO)3(cAACmethyl)] (2a) and [Ni(CO)(cAACmethyl)2] (4a) with substrates other

Figure 6. Molecular structure of [Ni2(μ-Br)2(η3-(cAACmethyl)CH− CH(CH3))2] (7) with thermal ellipsoids drawn at the 50% probability level; hydrogen atoms except of those bound to the allyl ligand are omitted for clarity. Selected bond distances (Å) and angles (deg): Ni1−Br1 2.3799(8), Ni1−Br1′ 2.4008(8), Ni1−C1 2.353(4), Ni1−C2 1.970(4), Ni1−C3 1.957(5), Ni1−Ni1′ 3.3329(9), C1−N1 1.379(6), C1−C2 1.398(6), C2−C3 1.419(6), C3−C4 1.486(7); C1−Ni1−Br1 162.52(11), C1−Ni1−Br1′ 100.29(11), C1−Ni1−C3 69.98(17) C3− Ni1−Br1 96.82(13), Br1−Ni1−Br1′ 91.60(3), C1−C2−C3 124.5(4), C2−C3−C4 120.5(4).

reaction of parent compound 5a with oxygen but rather the transfer and incorporation of cAACmethyl into the allyl ligand with concomitant [1,3]-H shift (from C2 to C4) and formation of dinuclear allyl nickel bromide complex [Ni2(μ-Br)2(η3(cAACmethyl)CH−CH(CH3))2] (7) (Scheme 6). Compound 7 is the reaction product of a presumably internal nucleophilic attack of the cAAC ligand to the allyl moiety of the complex. The metal atoms of this dinuclear nickel complex F


Article

Organometallics

temperature for 1 h, resulting in a color change to pale yellow and precipitation of a light yellow solid. After filtration and washing with cold hexane (3 × 5 mL), complex 2c was obtained as a pale yellow powder. Yield: 0.27 g (65%). Elem. Anal. Calcd (%) for C30H43NNiO3: C, 68.72; H, 8.27; N, 2.67; Found: C, 68.44; H, 8.56; N, 2.66. 1H NMR (500 MHz, 25 °C, C6D6, ppm, δ) 7.21 (t, 1H, 3 JHH = 7.7 Hz, para-CHarom), 7.10 (d, 2H, 3JHH = 7.7 Hz, metaCHarom), 3.10 (br s, 1H, CHCH3), 2.77 (sept, 1H, 3JHH = 6.6 Hz, CHCH3), 2.74 (sept, 1H, 3JHH = 6.5 Hz, CHCH3), 2.58 (qd, 1H, JHH = 13.1 Hz, JHH = 3.1 Hz, cyCH2), 2.01−1.93 (m, 4H, cyCH, cyCH2), 1.99 (d, 2JHH = 13.7 Hz, 1H, CH2), 1.67 (dq, 1H, JHH = 13.6 Hz, JHH = 3.4 Hz, cyCH2), 1.49 (d, 3H, 3JHH = 6.5 Hz, CHCH3), 1.40 (d, 1H, 3JHH = 13.7 Hz, CH2), 1.35 (d, 3H, 3JHH = 6.6 Hz, CHCH3), 1.28−1.25 (m, 1H, cyCH), 1.20 (d, 3H, 3JHH = 6.6 Hz, CHCH3), 1.15 (d, 3H, 3JHH = 6.3 Hz, CHCH3), 1.14 (d, 3H, 3JHH = 6.3 Hz, CHCH3), 1.10−1.05 (m, 2H, cyCH2), 1.03 (s, 3H, CH3), 1.00 (d, 3H, 3JHH = 6.9 Hz, CHCH3), 0.99 (d, 3H, 3JHH = 6.5 Hz, CHCH3), 0.91 (s, 3H, CH3). 13C{1H} NMR (126 MHz, 25 °C, C6D6, ppm, δ) 274.1 (cAACCq), 197.7 (CO), 145.8, 145.5, 139.4, 129.3, 126.0, 125.6, 76.2, 67.3, 55.2, 54.1, 52.9, 35.6, 32.3, 30.0, 29.2, 29.1, 29.0, 28.9, 27.4, 27.1, 25.3, 25.3, 24.1, 23.8, 22.8, 20.7. IR (ATR, cm−1) ν̅CO: 2042 (s), 1948 (vs). 3,3-Diamino-2-aryloxyacrylimidamide (3b). A solution of 0.13 g (0.73 mmol) of NaHMDS in 5 mL of benzene was added to (cAACcy)H+BF4− (0.30 g, 0.73 mmol) in 5 mL of benzene at room temperature and the reaction mixture stirred for 15 min. After removal of volatiles and extraction with hexane (10 mL), the solution containing the freshly prepared cAACcy (1b) was degassed by three freeze−pump cycles, and the Schlenk flask was pressurized with 2 bar of carbon monoxide, resulting in an immediate color change to blue. The solution was stirred at room temperature for 2 h, and the color changed to colorless. The volatiles were removed in vacuo, and the resulting residue was washed with hexane (5 mL) to give 3b as a colorless solid. Yield: 0.17 g (69%). Elem. Anal. Calcd (%) for C47H70N2O: C, 83.13; H, 10.39; N, 4.13. Found: C, 82.26; H, 10.41; N, 4.21. Although the presented results are outside the range viewed as establishing analytical purity, the compound is spectroscopically pure (see Supporting Information). 1H NMR (500 MHz, 25 °C, C6D6, ppm, δ) 7.16 (m, 1H, meta-CHarom), 7.13 (dd, 1H, 3JHH = 7.5 Hz, 4JHH = 1.9 Hz, meta-CHarom), 7.06 (t, 1H, 3JHH = 7.7 Hz, para-CHarom), 7.02 (dd, 1H, 3JHH = 7.6 Hz, 4JHH = 1.9 Hz, meta-CHarom), 6.93 (dd, 1H, 3 JHH = 7.5 Hz, 4JHH = 1.9 Hz, meta-CHarom), 6.88 (t, 1H, 3JHH = 7.6 Hz, para-CHarom), 4.99 (sept, 1H, 3JHH = 6.9 Hz, CHCH3), 3.78 (dt, 1H, 4 JHH = 3.5 Hz, 3JHH = 13.3 Hz, cyCH2), 3.67 (sept, 1H, 3JHH = 6.6 Hz, CHCH3), 3.64 (sept, 1H, 3JHH = 6.6 Hz, CHCH3), 2.93 (sept, 1H, 3 JHH = 6.9 Hz, CHCH3), 2.31 (d, 1H, 3JHH = 13.8 Hz, cyCH2), 2.22 (d, 1H, 3JHH = 12.3 Hz, CH2), 2.19 (dt, 1H, 4JHH = 3.1 Hz, 3JHH = 13.0 Hz, cyCH2), 2.09 (dt, 1H, 4JHH = 3.9 Hz, 3JHH = 12.7 Hz, cyCH2), 1.83 (br s, 1H, cyCH2), 1.74−1.64 (m, 5H, cyCH2), 1.65 (s, 1H, CH3), 1.61−1.39 (m, 10H, CH2, cyCH2), 1.56 (d, 3H, 3JHH = 6.5 Hz, CHCH3), 1.49 (s, 3H, CH3), 1.47 (d, 3H, 3JHH = 6.7 Hz, CHCH3), 1.46 (d, 3H, 3JHH = 6.7 Hz, CHCH3), 1.27 (d, 3H, 3JHH = 6.7 Hz, CHCH3), 1.23 (d, 3H, 3JHH = 6.7 Hz, CHCH3), 1.18 (s, 3H, CH3), 1.16 (d, 3H, 3JHH = 6.7 Hz, CHCH3), 1.12−1.02 (m, 2H, cyCH2), 0.80 (br d, 1H, 3JHH = 13.9 Hz, cyCH2), 0.70 (s, 3H, CH3), 0.57 (d, 3H, 3 JHH = 6.7 Hz, CHCH3), 0.54 (d, 3H, 3JHH = 6.5 Hz, CHCH3). 13 C{1H} NMR (126 MHz, 25 °C, C6D6, ppm, δ) 172.8 (C−CN), 151.5, 147.5, 147.3 (C−C−N), 147.2, 142.6, 139.8, 137.4, 128.4, 126.2 (CO), 126.0, 125.6, 124.2, 124.1, 122.5, 70.2, 63.9, 58.6, 51.8, 47.9, 47.8, 38.5, 35.4, 34.9, 34.2, 32.0, 31.6, 28.5, 27.3, 27.0, 26.8, 26.6, 26.4, 26.3, 26.3, 26.2, 25.9, 25.9, 25.5, 25.3, 25.1, 24.2, 23.9, 23.8, 23.5. IR (ATR, cm−1) ν̅CC: 1582 (m). [Ni(CO)(cAACcy)2] (4b). A solution of 44.0 mg (0.24 mmol) of NaHMDS in 5 mL of benzene was added to (cAACcy)H+BF4− (0.10 g, 0.24 mmol) in 5 mL of benzene at room temperature and the reaction mixture stirred for 15 min. After removal of volatiles and extraction with hexane (10 mL), the solution containing the freshly prepared cAAC cy (1b) was added to a hexane solution (5 mL) of [Ni(CO)3(cAACcy)] (2b) (0.02 g, 0.05 mmol). Within 2 h, the color of the resulting solution changed from green to red. The volatiles

than the carbene itself revealed that allyl bromides add to the cAAC-stabilized nickel complex to afford [NiBr(η3-H2CCH− CH2)(cAACmethyl)] (5a) and [NiBr(η3-H2CCH−CMe2)(cAACmethyl)] (5b). Analogous chloride-substituted compound [NiCl(η3-H2CCMe−CH2)(cAACmethyl)] (6) had to be prepared following a different synthetic procedure, as the cAAC-stabilized nickel carbonyls were found to be not reactive against allyl chlorides. Contrary to their NHC stabilized analogues, allyl halide complexes 5a,b and 6 are, however, not stable at room temperature, and one of the decomposition products, dinuclear nickel complex [Ni 2 (μ-Br) 2 (η 3 (cAACmethyl)CH−CH(CH3))2] (7), has been characterized. Complex 7 contains a novel cAAC-based allyl ligand which was confirmed by X-ray diffraction.

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EXPERIMENTAL SECTION

General. Compounds (cAACmethyl) (1a),3e (cAACcy) (1b),3e (cAACmenthyl) (1c),3g [Ni(CO)3(cAACmethyl)] (2a),7 [Ni(CO)(cAACmethyl)2] (4a),7 and [Ni(ItBu)(CO)2]6c were prepared according to literature procedures. All other starting materials were purchased from commercial sources and used without further purification. All solvents for synthetic reactions were HPLC-grade and were further treated to remove traces of water using an Innovative Technology Inc. Pure-Solv Solvent Purification System and deoxygenated using the freeze−pump−thaw method. All reactions and subsequent manipulations were performed under an argon atmosphere in an Innovative Technology Inc. glovebox or using standard Schlenk line techniques. NMR spectra were recorded, if not noted otherwise, on Bruker Avance 200 or Bruker Avance 500 spectrometers, using C6D6 as the solvent. Assignment of the 1H NMR data was supported by 1H,1H and 13C,1H correlation experiments. 13C NMR spectra were broad-band protondecoupled (13C{1H}). Assignment of the 13C NMR data was supported by 13C,1H correlation experiments. Chemical shifts are listed in parts per million (ppm) and were determined relative to internal C6D5H (1H, δ = 7.16; C6D6) or to natural-abundance carbon resonances C6D6 (13C, δ = 128.06; C6D6). Coupling constants are quoted in Hertz. Infrared spectra were recorded on a Nicolet 380 FTIR or a Bruker Alpha FT-IR spectrometer as solids by using an ATR unit or in solution and are reported in cm−1. Elemental analyses were performed in the microanalytical laboratory of the Institute of Inorganic Chemistry of the University of Würzburg with an Elementar vario micro cube. [Ni(CO)3(cAACcy)] (2b). A solution of 0.18 g (1.00 mmol) of NaHMDS in 5 mL of benzene was added to (cAACcy)H+BF4− (0.41 g, 1.00 mmol) in 5 mL of benzene at room temperature and the reaction mixture stirred for 15 min. After removal of volatiles and extraction with hexane (10 mL), the freshly prepared cAACcy (1b) was added in situ to a hexane solution (10 mL) of [Ni(CO)4] (0.38 g, 2.22 mmol, 2.2 equiv), where the color immediately changed to blue. The solution was stirred at room temperature for 2 h, and the color changed to light yellow. Subsequently, the volatiles were removed in vacuo, yielding a white solid, and the compound was washed with hexane (3 × 5 mL), affording complex 2b as a pale yellow powder. Yield: 0.15 g (31%). Elem. Anal. Calcd (%) for C26H35NNiO3: C, 66.69; H, 7.53; N, 2.99. Found: C, 66.81; H, 7.38; N, 2.89. 1H NMR (500 MHz, 25 °C, C6D6, ppm, δ) 7.21 (t, 1H, 3JHH = 7.6 Hz, para-CHarom), 7.10 (d, 2H, 3JHH = 7.6 Hz, meta-CHarom), 2.71 (sept, 2H, 3JHH = 6.7 Hz, CHCH3), 2.36 (t, 2H, 3JHH = 13.0 Hz, cyCH2), 1.58 (m, 2H, cyCH2), 1.56 (s, 2H, CH2), 1.40 (d, 6H, 3JHH = 6.7 Hz, CHCH3), 1.36 (m, 2H, cyCH2), 1.28−1.12 (m, 4H, cyCH2), 1.14 (d, 6H, 3JHH = 6.7 Hz, CHCH3), 0.95 (s, 6H, CH3). 13C{1H} NMR (126 MHz, 25 °C, C6D6, ppm, δ) 277.1 (cAACCq), 198.4 (CO), 145.7, 138.6, 129.3, 125.6, 78.5, 62.8, 45.7, 37.5, 29.8, 29.0, 26.1, 25.9, 24.5, 22.7. IR (ATR, cm−1) ν̅CO: 2044 (s), 1947 (vs); IR (CH2Cl2, cm−1) ν̅CO: 2046 (s), 1979 (vs), 1952 (vs). [Ni(CO)3(cAACmenthyl)] (2c). Addition of a hexane solution (5 mL) containing cAACmenthyl (1c) (0.30 g, 0.79 mmol) to a hexane solution (5 mL) of [Ni(CO)4] (0.18 g, 1.10 mmol, 1.4 equiv) led to an color change to pale turquoise. The solution was stirred at room G


Article

Organometallics

(10 mL) of 15.0 mg (0.02 mmol) of [Ni(CO)(cAACmethyl)2] (4a). The solution was stirred at room temperature for 1 h, resulting in a color change from green to orange and precipitation of an orange solid. The suspension was decanted and the solid residue washed with hexane (5 mL). The combined supernatants were concentrated in vacuo, where an orange solid precipitated, which was washed with cold hexane (0 °C, 3 × 1 mL), affording complex 5b as an orange powder. Yield: 5.0 mg (51%). Elem. Anal. Calcd (%) for C25H40BrNNi: C, 60.88; H, 8.18; N, 2.84. Found: C, 60.84; H, 8.18; N, 2.89. 1H NMR (500 MHz, 25 °C, C6D6, ppm, δ) 7.10 (br s, 3H, CHarom), 4.33 (br s, 1H, allylCH), 3.21 (br s, 2H, CHCH3), 2.10 (br s, 1H, allylCH2), 1.84 (br s, 3H, allylCH3), 1.63 (br s, 3H, CH3), 1.53−1.48 (br m, 11H, CH2/CH3/CHCH3), 1.23−1.14 (br m, 7H, allylCH2/CHCH3), 1.04 (br s, 9H, allylCH3/CH3). 13C{1H} NMR (126 MHz, 25 °C, C6D6, ppm, δ) 280.9 (cAACCq), 147.7, 147.0, 137.7, 129.1, 125.7, 125.2, 104.4 (allylCq), 104.2 (allylCH), 79.2, 57.2, 51.0, 36.1 (allylCH2), 33.3 (allylCH3), 32.6, 28.6, 28.5, 27.2, 26.2, 20.7 (allylCH3). IR (ATR, cm−1): 2865 (m), 2974 (s), 2952 (s), 2927 (m), 1591 (w), 1535 (w), 1473 (m), 1441 (vs), 1431 (vs), 1385 (m), 1364 (m). [NiCl(η3-H2CCMe−CH2)(cAACmethyl)] (6). Dropwise addition of 3chloro-2-methylpropene (63.0 μL, 58.0 mg, 0.64 mmol, 1.4 equiv) to a suspension of 177 mg (0.64 mmol, 1.4 equiv) of [Ni(COD)2] in 1 mL of COD led to an immediate color change to red. Subsequently, a solution of cAACmethyl (1a) (131 mg, 0.46 mmol) in 5 mL of benzene was added was added to the clear solution with a color change to orange and the precipitation of a gray solid. The suspension was stirred for 1 h at room temperature, decanted, and the solid residue washed with hexane (5 mL). The combined supernatants were concentrated in vacuo, and 10 mL of hexane were added. An orange solid precipitated, which was washed with cold hexane (0 °C, 3 × 1 mL), affording complex 6 as an orange powder. Yield: 125 mg (79%). Elem. Anal. Calcd (%) for C24H38ClNNi: C, 66.31; H, 8.81; N, 3.22. Found: C, 66.92; H, 9.15; N, 3.16. We have shown that compound 6 is not thermally stable (vide supra). Although the presented results are outside the range viewed as establishing analytical purity, they are provided to illustrate the best values obtained to date. 1H NMR (500 MHz, 25 °C, C6D6, ppm, δ) 7.07 (br s, 3H, CHarom), 3.62 (br s, 1H, allyl CH2), 3.51 (br s, 1H, CHCH3), 3.00 (br s, 1H, CHCH3), 2.83 (br s, 1H, allylCH2), 2.00 (br s, 1H, allylCH2), 1.73 (br s, 3H, CH3), 1.64−1.57 (br s, 9H, CH3/allylCH3/CHCH3), 1.47 (br s, 2H, CH2), 1.33 (br m, 3H, CHCH3), 1.20 (br m, 3H, CHCH3), 1.13 (br m, 3H, CHCH3), 1.07 (br s, 4H, allylCH2/CH3), 0.99 (s, 3H, CH3). 13C{1H} NMR (126 MHz, 25 °C, C6D6, ppm, δ) 279.1 (cAACCq), 147.7, 147.1, 137.4, 129.2, 125.8, 125.4, 120.5 (allylCq), 79.6, 74.8 (allylCH2), 57.0, 50.7, 43.5 (allylCH2), 32.4, 32.0, 28.8, 28.5, 26.9, 25.9, 22.6 (allylCH3). IR (ATR, cm−1): 3056 (w), 2973 (vs), 2952 (vs), 2929 (s), 2868 (m), 1591 (m), 1530 (m), 1473 (s), 1451 (vs), 1387 (s), 1363 (s). Crystallographic Details. The crystal data were collected on a Bruker X8-APEX II diffractometer with a CCD area detector and graphite monochromated Mo Kα radiation. The structures were solved using the intrinsic phasing method (ShelXT), refined with the ShelXL program,21 and expanded using Fourier techniques. All nonhydrogen atoms were refined anisotropically. Hydrogen atoms were included in structure factor calculations. All hydrogen atoms were assigned to idealized geometric positions. Crystal Data for 2b. C26H35NNiO3, Mr = 468.26, colorless block, 0.34 × 0.27 × 0.21 mm3, monoclinic space group P21/n, a = 9.4425(5) Å, b = 15.0132(8) Å, c = 16.8434(9) Å, α = 90°, β = 92.668(2)°, γ = 90°, V = 2385.2(2) Å3, T = 100(2) K, Z = 4, ρcalcd = 1.304 g·cm−3, μ = 0.840 mm−1, F(000) = 1000; 30 717 reflections in h(−11/11), k(−18/ 18), l(−20/20) measured in the range 1.82° < θ < 26.13°, completeness 100%, 4745 independent reflections, 4443 observed reflections (I > 2σ(I)), 286 parameters, 0 restraints; all data: R1 = 0.0269 and wR2 = 0.0688, I > 2σ(I): R1 = 0.0248 and wR2 = 0.0672, Goof 1.034, largest difference peak/hole 0.401/−0.222 e·Å−3. Crystal Data for 2c. C30H43NNiO3, Mr = 524.36, yellow needle, 0.27 × 0.06 × 0.04 mm3, orthorhombic space group P212121, a = 8.9590(6) Å, b = 16.4741(12) Å, c = 18.8994(13) Å, α = 90°, β = 90°, γ = 90°, V = 2789.4(3) Å3, T = 100(2) K, Z = 4, ρcalcd = 1.249 g·cm−3, μ = 0.725 mm−1, F(000) = 1128; 26 409 reflections in h(−9/11),

were removed in vacuo, and the resulting residue was washed with hexane (5 mL) to give 4b as a red solid. Yield: 12.0 mg (33%). Alternative preparation of 4b: A solution of 44.0 mg (0.24 mmol) NaHMDS in 5 mL of benzene was added to (cAACcy)H+BF4− (0.10 g, 0.24 mmol) in 5 mL of benzene at room temperature and the reaction mixture stirred for 15 min. After removal of volatiles and extraction with hexane (10 mL), the solution containing the freshly prepared cAACcy (1b) was added to a hexane solution (5 mL) of [Ni(CO)2(ItBu)] (24.0 mg, 0.08 mmol) to an immediate color change to green. The solution was stirred at room temperature for 2 h, resulting in a color change to orange and partial precipitation of a red to orange solid. Subsequently, the volatiles were removed in vacuo, and the compound was washed with cold hexane (0 °C, 3 × 5 mL), affording complex 4b as a reddish orange powder. Yield: 40.0 mg (68%). Elem. Anal. Calcd (%) for C47H70N2NiO: C, 76.52; H, 9.56; N, 3.80. Found: C, 77.40; H, 10.23; N, 3.52. Although the presented results are outside the range viewed as establishing analytical purity, the compound is spectroscopically pure (see Supporting Information). 1H NMR (500 MHz, 25 °C, C6D6, ppm, δ) 7.26 (d, 2H, 3JHH = 7.6 Hz, meta-CHarom), 7.21 (t, 2H, 3JHH = 7.7 Hz, para-CHarom), 7.09 (d, 2H, 3JHH = 7.6 Hz, meta-CHarom), 3.44 (sept, 2H, 3JHH = 6.5 Hz, CHCH3), 2.86 (sept, 2H, 3 JHH = 6.6 Hz, CHCH3), 2.53 (t, 2H, 3JHH = 13.4 Hz, cyCH2), 1.99 (d, 2H, 3JHH = 13.6 Hz, cyCH2), 1.93 (d, 2H, 3JHH = 12.7 Hz, CH2), 1.86 (d, 6H, 3JHH = 6.5 Hz, CHCH3), 1.75−1.68 (m, 4H, cyCH2), 1.49 (d, 2H, 3JHH = 12.7 Hz, CH2), 1.47−1.15 (m, 12H, cyCH2), 1.30 (d, 6H, 3 JHH = 6.5 Hz, CHCH3), 1.23 (br s, 12H, CH3, CHCH3), 1.20 (d, 6H, 3 JHH = 6.6 Hz, CHCH3), 1.00 (s, 6H, CH3). 13C{1H} NMR (126 MHz, 25 °C, C6D6, ppm, δ) 272.5 (cAACCq), 196.6 (CO), 148.7, 147.0, 139.0, 127.8, 125.5, 124.7, 75.5, 61.0, 47.7, 41.0, 40.0, 30.6, 29.8, 29.3, 29.1, 29.0, 28.4, 26.2, 26.1, 24.9, 24.0, 23.5. IR (ATR, cm−1) ν̅CO: 1914 (vs). [NiBr(η3-H2CCH−CH2)(cAACmethyl)] (5a). Addition of allyl bromide (9.0 μL, 10.0 mg, 0.11 mmol, 3 equiv) to a hexane solution (5 mL) of 15.0 mg (0.04 mmol) of [Ni(CO)3(cAACmethyl)] (2a) led to an immediate color change to yellow with the precipitation of an orange solid. The suspension was stirred for 1 h at room temperature, decanted, and the solid residue washed with hexane (5 mL). The combined supernatants were concentrated in vacuo, and a yellow solid precipitated, which was washed with cold hexane (0 °C, 3 × 1 mL), affording complex 5a as a yellow powder. Yield: 18.0 mg (84%). Alternative preparation of 5a: Allyl bromide (6.0 μL, 7.0 mg, 0.06 mmol, 3 equiv) was added to a hexane solution (10 mL) of 15.0 mg (0.02 mmol) [Ni(CO)(cAACmethyl)2] (4a). The solution was stirred at room temperature for 1 h, resulting in a color change from green to yellow and precipitation of an orange solid. The suspension was decanted and the solid residue washed with hexane (5 mL). The combined supernatants were concentrated in vacuo, where a yellow solid precipitated, which was washed with cold hexane (0 °C, 3 × 1 mL), affording complex 5a as a yellow powder. Yield: 6.0 mg (64%). Elem. Anal. Calcd (%) for C23H36BrNNi: C, 59.39; H, 7.80; N, 3.01. Found: C, 59.30; H, 7.78; N, 3.06. 1H NMR (500 MHz, 25 °C, C6D6, ppm, δ) 7.11−7.05 (br m, 3H, CHarom), 4.55 (br s, 1H, allylCH), 3.79 (br s, 1H, allylCH2), 3.32 (br s, 1H, CHCH3), 3.03 (br s, 1H, CHCH3), 2.79 (br s, 1H, allylCH2), 2.48 (br s, 1H, allylCH2), 1.64 (br s, 6H, CH3), 1.46−1.36 (br m, 8H, CH2/CHCH3), 1.17−1.14 (m, 7H, allylCH2/ CHCH3), 1.01 (br s, 3H, CH3), 0.94 (br s, 3H, CH3). 13C{1H} NMR (126 MHz, 25 °C, C6D6, ppm, δ) 277.9 (cAACCq), 147.5, 147.3, 136.4, 129.2, 125.6, 107.5 (allylCH), 79.5, 73.6 (allylCH2), 57.5, 50.9, 46.3 (allylCH2), 33.2, 32.3, 28.7, 28.6, 25.6, 25.5. [NiBr(η3-H2CCH−CMe2)(cAACmethyl)] (5b). Addition of 1-bromo3-methyl-2-butene (12.0 μL, 15.0 mg, 0.11 mmol, 3 equiv) to a hexane solution (5 mL) of 15.0 mg (0.04 mmol) of [Ni(CO)3(cAACmethyl)] (2a) led to an immediate color change to orange with the precipitation of an orange solid. The suspension was stirred for 1 h at room temperature, decanted, and the solid residue washed with hexane (5 mL). The combined supernatants were concentrated in vacuo, where an orange solid precipitated, which was washed with cold hexane (0 °C, 3 × 1 mL), affording complex 5b as an orange powder. Yield: 17.0 mg (86%). Alternative preparation of 5b: 1-Bromo-3-methyl-2-butene (8.0 μL, 10.0 mg, 0.06 mmol, 3 equiv) was added to a hexane solution H


Article

Organometallics k(−20/20), l(−22/23) measured in the range 1.64° < θ < 26.03°, completeness 100%, 5475 independent reflections, 4969 observed reflections (I > 22σ(I), 326 parameters, 0 restraints; all data: R1 = 0.0583 and wR2 = 0.1315, I > 2σ(I): R1 = 0.0510 and wR2 = 0.1264, Goof 1.020, largest difference peak/hole 2.769/−0.725 e·Å−3. Crystal Data for 3b. C47H70N2O, Mr = 679.05, colorless needle, 0.24 × 0.08 × 0.06 mm3, triclinic space group P1̅, a = 10.1998(9) Å, b = 20.1660(17) Å, c = 20.2899(18) Å, α = 80.047(4)°, β = 77.268(4)°, γ = 82.493(4)°, V = 3990.0(6) Å3, T = 100(2) K, Z = 4, ρcalcd = 1.130 g·cm−3, μ = 0.066 mm−1, F(000) = 1496; 46 264 reflections in h(−12/ 12), k(−25/25), l(−25/25) measured in the range 2.06° < θ < 26.36°, completeness 100%, 16 110 independent reflections, 7214 observed reflections (I > 2σ(I)), 925 parameters, 0 restraints; all data: R1 = 0.1731 and wR2 = 0.1772, I > 2σ(I): R1 = 0.0655 and wR2 = 0.1351, Goof 0.933, largest difference peak/hole 0.232/−0.296 e·Å−3. Crystal Data for 4b. C61H86N2NiO, Mr = 922.02, red block, 0.36 × 0.26 × 0.17 mm3, monoclinic space group C2/c, a = 21.2791(19) Å, b = 10.7542(10) Å, c = 24.220(3) Å, α = 90°, β = 112.737(3)°, γ = 90°, V = 5111.8(9) Å3, T = 100(2) K, Z = 4, ρcalcd = 1.198 g·cm−3, μ = 0.422 mm−1, F(000) = 2008; 30 143 reflections in h(−26/26), k(−13/ 13), l(−30/30) measured in the range 1.82° < θ < 26.27°, completeness 100%, 5171 independent reflections, 4644 observed reflections (I > 2σ(I)), 302 parameters, 0 restraints; all data: R1 = 0.0390 and wR2 = 0.0896, I > 2σ(I): R1 = 0.0336 and wR2 = 0.0856, Goof 1.046, largest difference peak/hole 0.955/−0.332 e·Å−3. Crystal Data for 5a. C23H36BrNNi, Mr = 465.15, yellow block, 0.14 × 0.11 × 0.09 mm3, monoclinic space group P21/n, a = 10.4387(6) Å, b = 14.1171(8) Å, c = 15.0076(9) Å, α = 90°, β = 96.393(2)°, γ = 90°, V = 2197.8(2) Å3, T = 100(2) K, Z = 4, ρcalcd = 1.406 g·cm−3, μ = 2.708 mm−1, F(000) = 976; 28 351 reflections in h(−12/2), k(−17/ 17), l(−18/18) measured in the range 1.99° < θ < 26.15°, completeness 100%, 4389 independent reflections, 4048 observed reflections (I > 2σ(I)), 271 parameters, 72 restraints; all data: R1 = 0.0225 and wR2 = 0.0506, I > 2σ(I): R1 = 0.0197 and wR2 = 0.0494, Goof 1.062, largest difference peak/hole 0.408/−0.313 e·Å−3. Crystal Data for 5b. C25H40BrNNi, Mr = 493.20, orange block, 0.14 × 0.12 × 0.10 mm3, monoclinic space group P21/n, a = 10.7904(9) Å, b = 15.2337(13) Å, c = 14.7013(13) Å, α = 90°, β = 98.366(2)°, γ = 90°, V = 2390.9(4) Å3, T = 100(2) K, Z = 4, ρcalcd = 1.370 g·cm−3, μ = 2.493 mm−1, F(000) = 1040; 22 678 reflections in h(−13/13), k(−18/ 18), l(−16/18) measured in the range 1.94° < θ < 26.10°, completeness 100%, 4728 independent reflections, 3982 observed reflections (I > 2σ(I)), 302 parameters, 68 restraints; all data: R1 = 0.0538 and wR2 = 0.0963, I > 2σ(I): R1 = 0.0410 and wR2 = 0.0896, Goof 1.150, largest difference peak/hole 0.912/−0.591 e·Å−3. Crystal Data for 6. C24H38ClNNi, Mr = 434.71, orange block, 0.16 × 0.12 × 0.09 mm3, orthorhombic space group Pbca, a = 15.7561(9) Å, b = 15.8387(9) Å, c = 18.7715(10) Å, α = 90°, β = 90°, γ = 90°, V = 4684.5(5) Å3, T = 100(2) K, Z = 8, ρcalcd = 1.233 g·cm−3, μ = 0.951 mm−1, F(000) = 1872; 31 349 reflections in h(−15/19), k(−13/19), l(−23/18) measured in the range 2.12° < θ < 26.06°, completeness 100%, 4636 independent reflections, 3601 observed reflections (I > 22σ(I), 325 parameters, 126 restraints; all data: R1 = 0.0532 and wR2 = 0.0869, I > 2σ(I): R1 = 0.0353 and wR2 = 0.0775, Goof 1.038, largest difference peak/hole 0.412/−0.359 e·Å−3. Crystal Data for 7. C46H72Br2N2Ni2, Mr = 930.29, violet block, 0.92 × 0.73 × 0.15 mm3, triclinic space group P1̅, a = 8.9013(13) Å, b = 9.3188(14) Å, c = 14.652(2) Å, α = 78.748(4)°, β = 88.349(4)°, γ = 64.816(4)°, V = 1076.7(3) Å3, T = 100(2) K, Z = 1, ρcalcd = 1.435 g· cm−3, μ = 2.763 mm−1, F(000) = 488; 9228 reflections in h(−11/10), k(−11/11), l(−18/17) measured in the range 2.47° < θ < 26.11°, completeness 99.5%, 4235 independent reflections, 3532 observed reflections (I > 22σ(I), 244 parameters, 0 restraints; all data: R1 = 0.0580 and wR2 = 0.1282, I > 2σ(I): R1 = 0.0476 and wR2 = 0.01241, Goof 1.101, largest difference peak/hole 1.149/−0.470 e·Å−3. Crystallographic data have been deposited with the Cambridge Crystallographic Data Center as supplementary publication nos. CCDC-1532122 (2b), -1532123 (2c), -1532124 (3b), -1532125 (4b), -1532126 (5a), -1532127 (5b), -1532128 (6), and -1532129 (7). These data can be obtained free of charge from The Cambridge

Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/ cif

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00109. Additional characterization information (PDF) Crystallographic data for compounds 2b, 2c, 3b, 4b, 5a, 5b, 6, and 7 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Udo Radius: 0000-0002-0759-1806 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Julius-Maximilians-Universität Würzburg and the Deutsche Forschungsgemeinschaft (DFG RA 720/12-1) is gratefully acknowledged.

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REFERENCES

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