Stoichiometric and Catalytic Reactivity of Ni(6-Mes)(PPh3)2

Article pubs.acs.org/Organometallics

Stoichiometric and Catalytic Reactivity of Ni(6-Mes)(PPh3)2 Sara Sabater, Michael J. Page, Mary F. Mahon, and Michael K. Whittlesey* Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, U.K. S Supporting Information *

ABSTRACT: The three-coordinate Ni(0) N-heterocyclic carbene complex Ni(6-Mes)(PPh3)2 (1; 6-Mes = 1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene) is formed in the reaction of Ni(cod)2 with a 1:2 mixture of 6-Mes and PPh3 or upon reduction of Ni(6-Mes)(PPh3)Br (2) with KOtBu. Facile substitution of PPh3 in 1 gave a range of Ni(6-Mes)(PPh3)(L) products (L = PhCCMe (3), PhCHCH2 (4), Ph2CO (5), PhCHO (6)). Oxidative addition of C6F6 gave Ni(6-Mes)(PPh3)(C6F5)F (7), while 1 was also oxidized by 4-BrC6H4F to afford a mixture of 2 and Ni(6-Mes)(PPh3)(C6H4F)Br (8). Surprisingly, 1 was also oxidized upon reaction with the small 5-membered ring NHC IMe4 to give the terminal Ni(II) phosphido complex Ni(IMe4)2(PPh2)Ph (9). Compounds 1 and 5 proved to be active as a precursors for the catalytic transfer hydrogenation of ketones.

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Scheme 1. Substitution Reactions of Ni(6-Mes)(PPh3)2 (1)a

INTRODUCTION It is now recognized that the limited supply of the platinum group metals will necessitate the development of their more earth-abundant congeners for future catalytic applications. In the case of the group 10 elements,1 nickel-mediated processes are very much dominated by catalyst precursors in the Ni(II) oxidation state on the grounds that their air/water tolerance facilitates their manipulation.2,3 Ni(0) precursors tend to be avoided precisely because their synthesis invariably requires the use of the highly air-sensitive reagent Ni(cod)2.4,5 Most Ni(0) catalysis therefore results from the in situ combination of this reagent with two-electron donor ligands, typically phosphines and/or N-heterocyclic carbenes (NHC). Although this in situ approach enhances the rate at which catalytic investigations can be undertaken,6 it does negate one of the main benefits of using well-defined starting materials, namely the ability to use stoichiometric reactions to understand mechanism and, hence, improve catalyst activity through rational alteration of the coordination sphere surrounding the metal. Only a relatively small number of Ni(0)−NHC complexes have been isolated, and their stoichiometric and/or catalytic reactivity investigated.4,7,8 In all cases,9 five-membered ring NHCs, are present with the majority of examples also containing bulky N-substituents in order to generate highly reactive low-coordinate Ni centers. During the course of our work with 6- and 7-membered ring carbenes (so-called ringexpanded carbenes or RE-NHCs) for the synthesis of two- and three-coordinate Ni(I) carbene complexes,10 we chanced upon the formation of the Ni(0) 6-membered ring carbene complex, Ni(6-Mes)(PPh3)2 (1; 6-Mes =1,3-bis(2,4,6-trimethylphenyl)3,4,5,6-tetrahydropyrimidin-2-ylidene). We now report our initial studies of both its stoichiometric and catalytic chemistry.

a

instance serendipitously following treatment of the Ni(I) complex Ni(6-Mes)(PPh3)Br (2) with an excess of KOtBu in THF. In contrast to the many well-known examples of zerovalent PdL3 and PtL3 complexes with donor L ligands,11 fully characterized examples for M = Ni are still not that common.7b,8c,12,13 Given that 1 could only be isolated in poor yield (27%) via a presumed disproportionation reaction,14 a more effective pathway to the product was developed that involved the straightforward addition of a 1:2 ratio of 6-Mes and PPh3 to a C6H6 solution of Ni(cod)2. This gave 1 in 56% isolated yield. The compound was characterized by a combination of multinuclear NMR spectroscopy and X-ray crystallography. 13C proved to be the most diagnostic NMR

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RESULTS AND DISCUSSION Synthesis and Substitution Chemistry of 1. Deep-red Ni(6-Mes)(PPh3)2 (1, Scheme 1) was isolated in the first © XXXX American Chemical Society

Isolated yields of products are shown in parentheses.

Received: February 20, 2017

A

DOI: 10.1021/acs.organomet.7b00129 Organometallics XXXX, XXX, XXX−XXX

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Organometallics active nucleus, with a high frequency15 triplet resonance (δ 228, 2 JCP = 27 Hz) observed for the carbenic resonance of the coordinated 6-Mes ligand. The 31P{1H} NMR spectrum displayed a single resonance at δ 16.8. An X-ray crystal structure (Figure 1) confirmed the expected trigonal planar

Oxidation Chemistry of 1. Given the important role that Ni(0)L2 species are proposed to play in cross-coupling reactions,19 the reactivity of 1 toward aryl halides was investigated (Scheme 2). As anticipated,20 the reaction with C6F6 gave the square planar Ni(II) pentafluorophenyl fluoride complex, Ni(6-Mes)(PPh3)(C6F5)F (7). NMR measurements indicated that the reaction was complete within 30 min at room temperature, comparable in rate to that reported for the “latent” Ni(NHC)2 precursor Ni2(IiPr2)4(cod) (IiPr2 = 1,3diisopropylimidazol-2-ylidene).7a The rapidity of reaction of both 1 and Ni2(IiPr2)4(cod) with C6F6 contrasts noticeably with that of the corresponding Ni(PR3)2 species, which typically require days, or even weeks, to reach completion.21,22 Compound 7 was characterized in solution by multinuclear NMR spectroscopy and, in the solid-state, by X-ray crystallography (Figure 3). The structure revealed a trans arrangement of the 6-Mes and phosphine ligands. The Ni− C6F5 (1.9008(17) Å) and Ni−F (1.8470(10) Å) distances are somewhat shorter than those in trans-Ni(IiPr2)2(C6F5)F (1.907(2), 1.891(1) Å,7a,e which could reflect the greater donor power of the RE-NHC ligand23 and/or the drive for π−π stacking between the C6F5 and PPh3 rings. A distance of 3.572 Å between the centroids of the C6F5 phenyl ring and that for the aromatic ring based on C29, in conjunction with an associated shift of 0.61 Å, places these two moieties in the range whereby one might expect the presence of intramolecular π−π interactions. Although these two rings are not coplanar, P1 lies 0.223(3) Å out of the mean-plane containing atoms C29−C34, such that the latter ring-face is facilitated in bending toward that of the fluorinated ligand. The perpendicular distances between atoms C29−34 and the C23−28 ring-plane span a range of 3.047(2)−3.977(3) Å, with the shortest contact involving C29. The ease of the reaction with the strong C−F bond in C6F6 made the rapid cleavage of the C−Br bond in 4-BrC6H4F by 1 not hugely surprising. However, the reaction failed to cleanly generate the Ni(II) oxidative product but instead gave a mixture composed predominantly24 of the Ni(I) complex 2 as well as what we tentatively assign (on the basis of multinuclear NMR spectroscopy; see the SI) as Ni(6-Mes)(PPh3)(C6H4F) Br (8). Formation of the analogous Ni(NHC)2X products occurs for both of the N-arylcarbene complexes Ni(IPr)2 and Ni(IMes)2 upon addition of a variety of aryl halides (for IPr, X  Cl;25 for IMes, X  Cl, Br, I),26 whereas Ni(NHC)2 species with smaller, N-alkyl substituents do indeed yield fourcoordinate Ni(II) oxidative addition products.3,7g,27 In light of there being only two reported Ni(NHC)3 species,7b,8c,28 1 was reacted with IMe4 (1,3,4,5-tetramethylimidazol-2-ylidene). The immediate disappearance of the starting material signal at ca. δ 17 in the 31P{1H} NMR spectrum was observed upon addition of 2 equiv of the NHC to a C6D6 solution of 1 and new singlet resonances appeared at ca. δ 15 and δ 48 for two intermediates assigned as Ni(6Mes)(IMe4)(PPh3) and Ni(IMe4)2(PPh3). Within 5 min, a third species, ultimately identified as the Ni(II) phosphido complex, Ni(IMe4)2(PPh2)Ph (9, Scheme 2), started to form. Over ca. 12 h, Ni(IMe4)2(PPh3) disappeared, although it was reformed when a further 2 equiv of IMe4 was added, concomitant with loss of Ni(6-Mes)(IMe4)(PPh3). After 72 h, only 9 was present in solution. Compound 9 displayed a square-planar geometry (Figure 4) with mutually trans IMe4 ligands and a trans arrangement of the pyramidal phosphido ligand and phenyl group. The Ni−PPh2 distance (2.2520(5) Å) was comparable to that in the very few other known terminal

Figure 1. Molecular structure of Ni(6-Mes)(PPh3)2 (1). Ellipsoids are shown at 30% probability with all hydrogen atoms removed for clarity. Selected bond lengths (Å) and angles (deg): Ni(1)−C(1) 1.905(6), Ni(1)−P(1) 2.1607(17), Ni(1)−P(2) 2.1477(18), C(1)−Ni(1)−P(1) 130.30(16), C(1)−Ni(1)−P(2) 121.85(16), P(1)−Ni(1)−P(2) 107.94(7).

geometry, with the sum of the angles at Ni being 360°. The Ni−PPh3 distances of 2.1477(18) and 2.1607(17) Å are comparable to those in Stephan’s all-phosphine analogue Ni(PPh3)3 (2.148(2)−2.156(1) Å),13 although 1 is devoid of any of the short Ni···ortho−H−Cphenyl interactions reported in Ni(PPh3)3. The Ni−C6‑Mes distance (1.905(6) Å) is noticeably shorter than that in 2 (1.942(2) Å).10a,b Although 1 proved to be air-sensitive, undergoing rapid decolorization in solution upon exposure to only trace amounts of air, it exhibited reasonable thermal stable under an inert atmosphere, only decomposing after ca. 20 h at 60 °C in C6D6. Facile phosphine substitution was apparent from the reaction with 1 equiv of PPh3-d15, which gave a mixture of 1, 1-PPh3-d15, and 1-(PPh3-d15)2 in a ca. 1:1.3:0.4 ratio (by 31P{1H} NMR spectroscopy) within 1 h at room temperature. The mono PPh3-d15 complex appeared as an AB resonance centered at δ 16.5, while the bis-PPh3-d15 isotopomer appeared further upfield at δ 15.9. Addition of 2 equiv of PPh3-d15 shifted the equilibrium further toward the bis PPh3-d15 isotopomer, although 1 was still observable. The results of this reaction, along with those to afford complexes 3−6 following substitution of a single phosphine ligand by alkyne, alkene, ketone, and aldehyde respectively are summarized in Scheme 1.16 The X-ray crystal structures of 3-6 are shown in Figure 2. The high degree of backbonding from the Ni center to the CC, CC and CO ligands in 3−6 was evident from the structural metrics and 13C NMR chemical shifts of the new ligands (Table 1).7h,17 The η2-coordination mode adopted by the benzophenone and benzaldehyde ligands in 4 and 5 is indicative of coordination to an electron-rich metal center.18 B


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Organometallics

Figure 2. Molecular structures of Ni(6-Mes)(PPh3)(PhCCMe) (3), Ni(6-Mes)(PPh3)(PhCHCH2) (4), Ni(6-Mes)(PPh3)(η2-Ph2CO) (5), and Ni(6-Mes)(PPh3)(η2-PhCHO) (6). Ellipsoids are represented at 30% probability. Hydrogen atoms, except for those attached to C(23) in 3 and 6 and C23 and C(24) in 4, have been omitted for clarity.

Scheme 2. Oxidation of 1 by C6F6, 4-BrC6H4F, and IMe4a

Table 1. Selected Bond Lengths (Å), Angles (deg), and 13C NMR Ligand Chemical Shifts (δL) for Complexes 3−6 Ni−CNHC Ni−P Ni−L: Ni−C Ni−O CNHC−Ni−P δL a

3

4

5

6

1.9174(15) 2.1484(4)

1.9167(13) 2.1911(4)

1.963(2) 2.1500(6)

1.948(2) 2.1354(6)

1.8945(16) 1.9017(16)

1.9628(14) 2.0365(13)

2.014(2)

1.970(2)

122.32(5) 136.1; 120.8

119.72(4) 58.6; 39.4a 57.8; 38.9b

1.8741(17) 109.12(7) 86.1

1.8681(15) 115.80(6) 85.8

C6D5CD3. bTHF-d8.

Ni−phosphido species,29 while the Ni−Caryl bond length of 1.9371(15) Å was identical to that in Ni(IMe4)2(o-tolyl)Br prepared by Cavell.3 Although P−C bond cleavage of phosphines is very well established,30 such reactions typically result in bi- or multimetallic metal products with μ-PR2 ligands.31 To the best of our

a

C

Isolated yields of products are shown in parentheses.


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Organometallics

Fahey and Mahan ca. 40 years ago to be an intermediate enroute to Ni(PEt3)3(PPh3), Ni(PEt3)2(μ-PPh2)2Ni(PEt3) and biphenyl in the reaction of Ni(PEt3)2(Ph)Br with LiPPh2·OEt2 but was described as being “too labile to be isolated”.32 Catalytic Transfer Hydrogenation with 1. In seeking catalytic applications for 1, we opted to look for processes for which Ni complexes have very limited precedence. Transfer hydrogenation (TH)33 provides a perfect example of this with only a handful of Ni phosphine/carbene-catalyzed TH reactions of CO, CN, and CC bonds described in the literature.34 At 2 mol % loading, 1 converted a range of aromatic ketones (Table 2, entries 1, 7, 9, 13, and 14) and Table 2. Nickel-Catalyzed Transfer Hydrogenation of COContaining Substratesa

Figure 3. Molecular structure Ni(6-Mes)(PPh3)(C6 F5)F (7). Ellipsoids are shown at 30% probability with all hydrogen atoms removed for clarity. Selected bond lengths (Å) and angles (deg): Ni(1)−C(1) 1.9574(17), Ni(1)−P(1) 2.2347(5), Ni(1)−C(23) 1.9008(17), Ni(1)−F(1) 1.8470(10), C(1)−Ni(1)−P(1) 170.30(5).

entry

Ni source

R

R′

yieldb,g (%)

1 2c 3d 4e 5d,e 6f 7 8c 9 10c 11 12 13 14 15 16

1

H H H H H H H H OMe OMe Br Cl F H H H

Ph Ph Ph Ph Ph Ph Me Me Me Me Me Me Me Et H Ph

95 54 15 13 14 97 90 40 56 30 0 0 70 60 40 95

1 1 1 1 1 1 1 1 1 1 1 5

a

Reaction conditions: NaOtBu (5 mol %), Ni precursor (2 mol %), 2 mL iPrOH, reflux for 20 h. bYields determined by 1H NMR spectroscopy using anisole as internal standard. cReactions run in the absence of Ni precatalyst. dReactions run in the absence of NaOtBu. eReactions run using EtOH as solvent. fReaction run with 2 mol % Ni precursor and 2 mol % NaOtBu. gAverage of two runs.

benzaldehyde (entry 15) to the corresponding alcohols in moderate to high yields in the presence of NaOtBu (5 mol %) in refluxing iPrOH. The need for both base and iPrOH was clearly shown as only low yields of products were formed upon (i) the reduction of benzophenone with 1 under base-free conditions (entry 3) and (ii) use of EtOH in both the presence and absence of 5 mol % of NaOtBu (entries 4 and 5). Moreover, given the use of such a strong base,35 control experiments in the absence of 1 were conducted (entries 2, 8 and 10). These also gave reduced product yields confirming a role for Ni in the catalysis. A mercury drop experiment resulted in no change of activity, supporting catalysis being homogeneous. No reduction of the ketone group in either bromo- or chloroacetophenone was found (entries 11 and 12), although in both cases, acetophenone was formed in ca. 5% yield, consistent with reaction with base. With the fluoro derivative, no acetophenone was formed and the substrate was successfully reduced to 4-fluoromethylbenzyl alcohol (entry 13).

Figure 4. Molecular structure of Ni(IMe4)2(PPh2)Ph (9). Ellipsoids are shown at 30% probability with all hydrogen atoms removed for clarity. Selected bond lengths (Å) and angles (deg): Ni(1)−C(1) 1.8928(15), Ni(1)−C(8) 1.8859(15), Ni(1)−P(1) 2.2520(5), Ni(1)− C(15) 1.9371(15), C(1)−Ni(1)−P(1) 90.77(5).

knowledge, isolation of a terminal phosphido product as a result of formal P−C oxidative addition at a single metal center has not been reported. It is worth noting that the analogous phosphine complex, Ni(PEt3)2(PPh2)Ph, was postulated by D


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Organometallics

C58H58N2P2Ni: C, 77.08; H, 6.47; N, 3.10. Found: C, 77.23; H, 6.56; N, 3.13. Ni(6-Mes)(PPh3)(PhCCMe) (3). PhCCMe (4 μL, 0.032 mmol) was added to a benzene (0.6 mL) solution of 1 (21 mg, 0.023 mmol) in a J. Youngs resealable NMR tube, resulting in an immediate color change from deep-red to orange. The solvent was removed in vacuo and the residue extracted into hexane. The product was obtained as a orange-yellow crystalline solid from a concentrated hexane solution at room temperature overnight. Yield: 10 mg (60%). 1 H NMR (500 MHz, C6D6): δ 7.79 (d, 3JHH = 7.7 Hz, 2H, CHAr), 7.36 (t, 3JHH = 7.5 Hz, 2H, CHAr), 7.30−7.13 (m, 5H, CHAr), 7.08−6.92 (m, 11H, CHAr), 6.81 (br s, 2H, CHAr), 6.67 (br s, 2H, CHAr), 2.98− 2.82 (m, 4H, NCH2), 2.36 (s, 6H, CH3), 2.21−2.14 (m, 7H, NCH2CHH and CH3), 1.84 (s, 6H, CH3), 1.55−1.49 (m, 4H, NCH2CHH and CH3). 31P{1H} NMR (202 MHz, C6D6): δ 38.6 (s). 13 C{1H} NMR (126 MHz, C6D6): δ 231.5 (d, 2JCP = 5 Hz, NCN), 145.1 (s), 137.6 (s), 136.5 (s), 136.1 (d, 2JCP = 7 Hz, PhCCMe), 136.0 (s), 133.7 (d, JCP = 13 Hz), 132.4 (d, JCP = 10 Hz), 130.7 (d, JCP = 4 Hz), 129.9 (s), 129.6 (s), 127.5 (s), 127.4 (d, JCP = 9 Hz), 123.8 (s) 121.1 (s), 120.8 (d, 2JCP = 11 Hz, PhCCMe), 45.4 (s, NCH2), 22.8 (s, NCH2CH2), 21.1 (s, CH3), 20.6 (s, CH3), 19.8 (s, CH3) 11.7 (d, 3JCP = 12 Hz, CH3). Anal. Calcd for C49H51N2PNi: C, 77.68; H, 6.78; N, 3.69. Found: C, 77.65; H, 6.76; N, 2.92. Ni(6-Mes)(PPh3)(PhCHCH2) (4). As for 3, but with styrene (3 μL, 0.0258 mmol) and 20 mg (0.022 mmol) of 1. Yield: 15 mg (91%). 1 H NMR (400 MHz, C6D5CD3, 246 K):* δ 7.13 (br s, 5H, CHAr), 6.80 (br s, 3H, CHAr), 6.70 (br s, 2H, CHAr), 6.10 (br s, 2H, CHAr), 2.86−2.58 (m, 4H, NCH2), 2.51 (s, 3H, CH3), 2.44 (s, 3H, CH3), 2.40 (s, 3H, CH3), 2.36−2.30 (m, 4H, CHHCH and CH3), 2.27− 2.21(m, 1H, CHHCH), 2.18 (s, 3H, CH3), 1.71- 1.60 (m, 1H, NCH2CHH), 1.52−1.41 (m, 1H, NCH2CHH), 1.35 (s, 3H, CH3). *1H−13C HSQC shows corresponding CH2CH 1H signal obscured by toluene solvent. 1H NMR (400 MHz, THF-d8, 246 K): δ 7.29−6.29 (br m, 22H, C6H5), 5.62 (br s, 2H, C6H5), 3.50−3.15* (m, 4H, NCH2), 2.61 (s, 3H, CH3), 2.47 (s, 3H, CH3), 2.43 (s, 3H, CH3), 2.37 (s, 3H, CH3), 2.30* (m, 2H, NCH2CH2), 2.13 (s, 3H, CH3), 1.91 (br m, 1H, CHCHH), 1.74* (br m, 1H, CHCHH), 1.43* (br m, 1H, CHCHH), 1.29 (s, 3H, CH3). *Assignments based on/confirmed by 1H COSY and 1H−13C HSQC/HMBC experiments. 31P{1H} NMR (162 MHz, C6D5CD3): δ 29.8 (s). 13C{1H} NMR (101 MHz, C6D5CD3, 246 K): δ 229.7 (d, 2JCP = 10 Hz, NCN), 150.2 (s), 145.0 (s), 144.6 (s), 136.8 (d, JCP = 8 Hz), 136.7 (s), 136.5 (s), 136.4 (s), 136.1 (s), 134.5 (d, JCP = 20 Hz), 132.8 (d, JCP = 10 Hz), 131.9 (s), 130.8 (s), 130.5 (s), 130.2 (s), 127.7 (s), 125.0 (s), 121.2 (s), 58.6 (d, 2 JCP = 3 Hz, CH2CH), 46.3 (s, NCH2), 45.9 (s, NCH2), 39.4 (d, 2 JCP = 19 Hz, CH2CH), 22.4 (s, NCH2CH2), 21.8 (s, CH3), 21.7 (s, CH3), 20.2 (s, CH3), 19.7 (s, CH3), 19.6 (s, CH3), 17.5 (s, CH3). 13 C{1H} NMR (126 MHz, THF-d8, 246 K): δ 230.0 (d, 2JCP = 9 Hz, NCN), 150.4 (s), 145.2 (d, 1JCP = 25 Hz), 137.2 (s), 136.9 (s), 136.8 (s), 136.6 (s), 131.0 (s), 130.6 (s), 130.4 (s), 129.7 (s), 128.3 (s), 127.8 (s), 127.4 (s), 124.8 (s), 120.7 (s), 57.8$; (s, CHCH2), 47.2$; (s, NCH2), 46.7$; (s, NCH2), 38.9$; (d, 2JCP = 19 Hz, CHCH2), 23.2 (s, NCH2CH2), 21.6 (s, CH3), 21.5 (s, CH3), 20.3 (s, CH3), 19.8 (s, CH3), 19.7 (s, CH3) 17.5 (s, CH3). $;Assignments based on/confirmed by 1 H− 13 C HSQC/HMBC experiments. Anal. Calcd for C48H51N2PNi: C, 77.32; H, 6.89; N, 3.75. Found: C, 77.29; H, 6.94; N, 3.58. Ni(6-Mes)(PPh3)(Ph2CO) (5). Benzophenone (5 mg, 0.027 mmol) was added to a benzene (0.6 mL) solution of 1 (25 mg, 0.027 mmol) in a J. Youngs resealable NMR tube, resulting in an immediate color change from deep-red to orange. After shaking for 10 min, the solvent was removed in vacuo, and the residue was washed with hexane and then recrystallized from benzene/hexane. Yield 16 mg (70%). 1H NMR (500 MHz, C6D6): δ 7.20−7.16 (m, 4H, CHAr), 7.04−6.99 (m, 4H, CHAr), 6.96−6.85 (m, 8H, CHAr), 6.84−6.77 (m, 11H, CHAr), 6.58 (s, 2H, CHAr), 2.98 (s, 6H, CH3), 2.89 (m, 2H, NCH2), 2.79 (m, 2H, NCH2), 2.30 (s, 6H, CH3), 1.93 (m, 1H, NCH2CHH), 1.61 (s, 6H, CH3), 1.39 (m, 1H, NCH2CHH). 31P{1H} (C6D6, 202 MHz): δ 35.6 (s). 13C{1H} NMR (126 MHz, C6D6): δ 229.8 (d, 2JCP = 7 Hz,

Although the exact mechanism of the reaction remains to be established, we can say that (i) the identical activity of 1 and the benzophenone complex 5 (entries 1 and 16) implies the rapid formation of a Ni(0)-ketone complex at the start of the reaction and (ii) the need for only a catalytic amount of base (entry 6) might suggest the intermediacy of a Ni(II) alkoxy hydride that could allow base to be continually regenerated and the catalytic cycle to propagate. Efforts to substantiate this pathway are ongoing.

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SUMMARY AND CONCLUSIONS

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EXPERIMENTAL SECTION

The stoichiometric and catalytic activity of the Ni(0) complex Ni(6-Mes)(PPh3)2 (1) has been investigated. 1 is substutionally labile, undergoing replacement of a single phosphine ligand to give Ni(0) alkyne, alkene, ketone, and aldehyde complexes. Loss of a phosphine and oxidative addition takes place upon treatment with C6F6, whereas reaction with the aryl bromide, 4BrC6H4F, leads mainly to the known Ni(I) species Ni(6Mes)(PPh3)Br. Such reactivity agrees with that seen for other bulky NHC-containing Ni(0) complexes.25,26 Isolation of Ni(IMe4)2(PPh2)Ph, a rare example of a terminal nickel phosphido complex, from the reaction of 1 with 1,3,4,5tetramethylimidazol-2-ylidene is without a doubt the most unexpected product formed in the stoichiometric reactions. Pleasingly, 1 proved to be active for the catalytic transfer hydrogenation of CO bonds. Both the mechanism of this reaction, as well as the use of derivatives of 1 with different RENHCs and phosphines, is the subject of continuing work.

All manipulations were carried out using standard Schlenk, highvacuum, and glovebox techniques. Solvents were purified using an MBraun SPS solvent system (hexane) or under a nitrogen atmosphere from sodium benzophenone ketyl (benzene, THF). C6D6, THF-d8 and C6D5CD3 were vacuum transferred from potassium. NMR spectra were recorded at 298 K (unless otherwise stated) on Bruker Avance 500 and 400 and Agilent 500 MHz NMR spectrometers and referenced to solvent signals (benzene: δ 7.16 (1H), δ 128.0 (13C); toluene; δ 2.09, δ 20.4; thf: δ 3.58, 23.6). 31P{1H} spectra were referenced externally to 85% H3PO4 (δ 0.0), 19F, externally to CFCl3 (δ 0.0). Elemental analyses were performed by Elemental Microanalysis Ltd., Okehampton, Devon, UK. Ni(6-Mes)(PPh3)Br (2),10a 6Mes,36 and IMe437 were prepared according to literature methods. Ni(6-Mes)(PPh3)2 (1). Method A: A solution of Ni(6-Mes)(PPh3) Br (90 mg, 0.125 mmol) and excess KOtBu (58 mg, 0.517 mmol) in THF (10 mL) was stirred for 1 h at room temperature to give a very dark red solution. The volatiles were removed in vacuo, and the residue was extracted into hexane (2 × 5 mL), filtered, and concentrated to ca. 2 mL to form a brick red precipitate of the product. Yield 30 mg (27%). Method B: 6-Mes (150 mg, 0.468 mmol) was placed in a J. Youngs resealable ampule with PPh3 (246 mg, 0.936 mmol) and Ni(cod)2 (129 mg, 0.468 mmol) in C6H6 (10 mL) and the solution stirred at room temperature for 3 h. The solvent was removed in vacuo and the residue extracted into hexane (20 mL). Pure, crystalline 1 was obtained from a concentrated solution of the compound in hexane at −35 °C. Yield: 237 mg (56%). 1H NMR (500 MHz, C6D6): δ 7.20−7.14 (m, 12H, CHAr), 6.99−6.94 (m, 6H, CHAr), 6.93−6.88 (m, 12H, CHAr), 6.78 (s, 4H, CHAr), 3.00 (t, 4H, 3JHH = 5.7 Hz, NCH2), 2.21 (s, 18H, CH3), 2.01−1.96 (m, 2H, NCH2CH2). 31 1 P{ H} NMR (202 MHz, C6D6): δ 16.8 (s). 13C{1H} NMR (126 MHz, C6D6): δ 228.2 (t, 2JCP = 27 Hz, NCN), 145.1 (s, CMes), 139.6 (t, JCP = 14 Hz, CPPh3), 136.6 (s, CMes), 135.7 (s, CMes), 134.0 (t, JCP = 7 Hz, CHAr(PPh3)), 130.3 (s, CHAr(Mes)), 127.2 (t, JCP = 4 Hz, CHAr(PPh3)), 127.0 (s, CHAr(PPh3)), 46.4 (s, NCH2), 23.6 (s, NCH2CH2), 21.2 (s, CH3), 20.1 (s, CH3). Anal. Calcd for E


Article

Organometallics

Ni(IMe4)2(PPh2)Ph (9). IMe4 (6 mg, 0.048 mmol) was added to a benzene (0.6 mL) solution of 1 (10 mg, 0.011 mmol) in a J. Youngs resealable NMR tube and the solution shaken at room temperature for 72 h. Over this time, a color change from deep-red to orange/yellow was observed. After removal of the solvent, the residue was washed with Et2O (2 × 1 mL) to leave a yellow microcrystalline solid. Yield 4 mg (64%). Recrystallization from benzene/hexane afforded material appropriate for X-ray crystallography. 1H NMR (500 MHz, C6D6): δ 7.63−7.58 (m, 2H, CHAr), 7.41−7.35 (m, 4H, CHAr), 7.09−7.05 (m, 2H, CHAr), 6.90−6.82 (m, 7H, CHAr), 3.78 (s, 12H, NCH3), 1.27 (s, 12H, CH3). 31P{1H} NMR (C6D6, 202 MHz): δ 16.5 (s). 13C{1H} NMR (126 MHz, C6D6): δ 188.0 (d, 2JCP = 16 Hz, NCN), 169.0 (d, JCP = 36 Hz, NiCPh), 150.7 (d, JCP = 31 Hz, i-C(PPh2)), 138.7 (d, JCP = 2 Hz), 133.9 (d, JCP = 17 Hz), 126.6 (d, JCP = 6 Hz), 126.0 (d, JCP = 3 Hz), 123.9 (s), 123.1 (s), 121.1 (s), 34.4 (s, NCH3), 34.3 (s, NCH3), 8.3 (CH3). Attempts to characterize 9 by elemental analysis repeatedly gave extremely low values for %C. Anal. Calcd for C32H39N4PNi: C, 67.51; H, 6.91; N, 9.84. Found: C, 58.77; H, 6.68; N, 9.40. Transfer Hydrogenation. In a J. Youngs resealable ampule, a mixture of ketone (0.4 mmol), nickel complex 1 or 5 (8 μmol), NaOtBu (0.02 mmol), and anisole (0.4 mmol, as internal standard) in i PrOH (2 mL) was heated at reflux for 20 h. The reaction mixture was analyzed by 1H NMR spectroscopy by diluting aliquots (0.1 mL) of the reacting mixture with CDCl3 (0.5 mL). X-ray Crystallography. Data for 1 were collected on an Oxford diffraction Gemini diffractometer, while those for 3, 4, 5, 6, 7, and 9 were obtained using an Agilent SuperNova instrument (Table S1). All experiments were conducted at 150 K using a Cu Kα source. Convergences were achieved using SHELXL38 via Olex239 were relatively straightforward, and only points of note are mentioned hereafter. The data for 1 were obtained from a very small crystal (with a smallest dimension of 0.02 mm) using a sealed X-ray tube. This was manifest in the need to truncate the data at higher Bragg angles (due to intensity falloff) and in the higher than desirable Rint value accompanying those reflections used in the refinement. Nonetheless, the result is unambiguous, and convergence was successful upon inclusion of anisotropic displacement parameter (ADP) restraints for C1, C5, C29 and C47. The hydrogen atoms attached to C23 and C24 in 4 were readily located and refined subject to being equidistant from the relevant parent atoms. H23 in 6 was similarly located and, in this instance, refined at a distance of 0.98 Å from C23. In 7, the hydrogens pertaining to the mesityl carbon, C12, were disordered over sites in a 50:50 ratio. Lastly, the phenyl ring based on C27 in 9 was modeled to in a manner that accounted for 50:50 disorder. Each component was treated as a rigid hexagon, and the P1−C27, P1−C27A distances were restrained to be similar in the final least-squares. Crystallographic data for compounds 1, 3−7, and 9 have been deposited with the Cambridge Crystallographic Data Centre as supplementary publications CCDC1530908−1530914. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [fax(+44) 1223 336033, e-mail: [email protected]].

NCN), 148.1 (d, JCP = 2 Hz), 144.2 (s), 139.1 (s), 136.4 (s), 134.5 (s), 134.3 (d, JCP = 13 Hz), 131.4 (s), 130.2 (s), 127.8 (s) 127.4 (d, JCP = 9 Hz) 127.2 (s), 123.2 (s), 86.1 (s, Ph2CO), 47.4 (s, NCH2), 22.4 (s, NCH2CH2), 21.3 (s, CH3), 20.7 (s, CH3), 18.8 (s, CH3). Anal. Calcd for C53H53N2PONi: C, 77.28; H, 6.48; N, 3.40. Found: C, 77.16; H, 6.56; N, 3.16. Ni(6-Mes)(PPh3)(PhCHO) (6). As for 5 but with benzaldehyde (3 μL, 0.029 mmol) and 25 mg (0.027 mmol) of 1. After the reaction, the solvent was removed in vacuo and the residue extracted into hexane. Compound 6 was obtained as orange-yellow crystalline solid upon precipitation from a concentrated hexane solution of the complex left at room temperature overnight. Yield: 15 mg (73%). 1H NMR (500 MHz, C6D6): δ 7.19 (br s, 1H, CHAr), 7.10 (br s, 1H, CHAr), 7.06− 6.89 (m, 17H, CHAr), 6.83 (t, 3JHH = 7.5 Hz, 1H, CHAr) 6.71 (br s, 2H, CHAr) 6.63 (d, 3JHH = 7.5 Hz, 2H, CHAr), 4.40 (d, 3JHP = 7.0 Hz, 1H, CHO), 2.93 (2 × s, 6H, CH3) 2.88−2.78 (m, 2H, NCH2), 2.75− 2.66 (m, 2H, NCH2), 2.40 (s, 3H, CH3), 2.37 (s, 3H, CH3), 2.06 (s, 3H, CH3), 1.60−1.46 (m, 2H, NCH2CH2), 1.26 (s, 3H, CH3). 31 1 P{ H} NMR (202 MHz, C6D6): δ 37.9 (s). 13C{1H} NMR (126 MHz, C6D6): δ 225.7 (d, 2JCP = 9 Hz, NCN), 150.5 (s), 144.4 (s), 144.3 (s), 139.0 (s), 138.6 (s), 136.5 (s), 136.4 (s), 135.9 (s) 134.8 (s), 134.5 (d, JCP = 13.0 Hz,), 132.5 (d, JCP = 10 Hz, 131.6 (d, JCP = 3 Hz), 130.7 (s), 129.8 (s), 129.6 (s), 129.2 (s), 127.6 (s), 127.4 (d, JCP = 9 Hz), 123.9 (s), 123.0 (s), 85.8 (d, 2JCP = 3 Hz, CHO), 45.6 (s, NCH2), 45.1 (s, NCH2), 21.7 (s, NCH2CH2), 21.4 (s, CH3), 21.4 (s, CH3), 19.6 (s, CH3), 19.4 (s, CH3), 19.0 (s, CH3), 16.7 (s, CH3). Anal. Calcd for C47H49N2PONi: C, 75.51; H, 6.60; N, 3.74. Found: C, 75.67; H, 6.66; N, 3.44. Ni(6-Mes)(PPh3)(C6F5)F (7). C6F6 (30 μL, 0.26 mmol) was added to a C6H6 (0.6 mL) solution of 1 (20 mg, 0.17 mmol) in a J. Youngs resealable NMR tube and the mixture shaken for 30 min at room temperature. The solvent was removed in vacuo, and the residue was washed with hexane and then recrystallized from benzene/hexane. Yield: 10 mg (55%). 1H NMR (500 MHz, C6D6): δ 7.52−7.47 (m, 5H, CHAr), 7.41−7.35 (m, 1H, CHAr), 7.28 (br s, 2H, CHAr), 7.07− 6.97 (m, 6H, CHAr), 6.95−6.89 (m, 5H, CHAr), 2.71−2.64 (m, 8H, NCH2 and CH3), 2.61−2.54 (m, 2H, NCH2), 2.34 (s, 6H, CH3), 1.85 (s, 6H, CH3), 1.49−1.40 (m, 1H, NCH2CHH), 1.25−1.14 (m, 1H, NCH2CHH). 31P{1H} NMR (C6D6, 202 MHz): δ 14.1 (d, 2JPF = 57 Hz). 19F NMR (C6D6, 470 MHz): δ −107.8 (m, 2F, o-C6F5), 163.8 (t, 3 JFF = 20 Hz, p-C6F5), −166.2 (m, 2F, m-C6F5), −376.2 (d, 2JPF = 57 Hz, NiF). 13C{1H} NMR (126 MHz, C6D6): δ 199.9 (d, 2JCP = 105 Hz, NCN), 143.4 (s), 138.1 (d, J = 12 Hz), 138.0 (d, J = 6 Hz) 136.8 (d, J = 3 Hz), 134.7 (d, JCP = 11 Hz), 134.2 (d, J = 20 Hz), 132.5 (d, J = 10 Hz), 131.7 (s), 131.5 (d, JCP = 3 Hz), 131.4 (s), 130.4 (s), 130.0 (s), 129.5 (d, JCP = 2 Hz), 128.9 (d, J = 2 Hz), 128.8 (s), 128.5 (d, J = 12 Hz), 127.7 (d, JCP = 9 Hz), 47.5 (d, J = 3 Hz, NCH2), 21.4 (s, CH3), 21.3 (s, NCH2CH2), 19.4 (d, J = 13 Hz, CH3), 18.3 (d, J = 4 Hz, CH3). Anal. Calcd for C46H43N2PF6Ni·C6H6: C, 68.96; H, 5.45; N, 3.09. Found: C, 68.54; H, 5.32; N, 3.37. Ni(6-Mes)(PPh3)(C6H4F)Br (8). 4-BrC6H4F (2 μL, 0.018 mmol) and 13 mg (0.014 mmol) of 1 were combined in C6D6 a J. Young’s resealable NMR tube. 1H and 31P{1H} NMR spectra recorded after 5 min showed the formation of both 2 (observed in 1H NMR spectrum) and 8 (observed in both 1H and 31P{1H} NMR spectra). Conducting a second experiment containing an internal capillary of PPh3 gave showed ca. 30% of 1 was converted to 8 on the basis of an inversegated 31P{1H} NMR spectrum. Spectroscopic characterization of 8 was achieved by generating the product over the course of ca. 2 h upon addition of 4-BrC6H4F (2 μL, 0.018 mmol) to 2 (11 mg, 0.015 mmol) in C6D6. As a second product, assigned as Ni(6-Mes)(PPh3)Br2, was also formed, 8 was spectroscopically characterized by 1- and 2-D NMR methods (see the SI), but could not be isolated. Selected 1H NMR (500 MHz, C6D6): δ 6.23* (t, 3 JHH = 9.0 Hz, 2H, CHAr), 2.88 (s, 6H, CH3), 2.29 (s, 6H, CH3), 1.73 (s, 6H, CH3). 31P{1H} NMR (C6D6, 202 MHz): δ 23.6 (s). 19F NMR (C6D6, 470 MHz): δ −127.5 (br s, 1F, p-C6H4F).*1H−1H COSY showed correlation of this signal to an aryl multiplet centered at δ 6.34 which was partially obscured by a signal for free 4-BrC6H4F.

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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.7b00129. X-ray data for compounds 1, 3−7, 9 (ZIP) Multinuclear NMR spectra (1 and 3−9) and crystal data/structural refinement details (1, 3-7, 9) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 44 1225 383748. ORCID

Michael K. Whittlesey: 0000-0002-5082-3203 F


Article

Organometallics Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Royal Society (Newton Fellowship to S.S.) and EPSRC (Grant No. EP/F029292/1 for M.J.P.) are thanked for financial support.

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