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, UKSupporting Information Placeholder

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 substi-tution of PPh3 in 1 gave a range of Ni(6-Mes)(PPh3)(L) products (L = PhCCMe (3), PhCH=CHPh (4), Ph2CO (5), PhCHO (6)), while PhBr and C6F6 brought about oxidation to 2 and Ni(6-Mes)(PPh3)(C6F5)F (7) respectively. 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 (8). 1 and 5 proved to be active as a precur-sors for the catalytic transfer hydrogenation of ketones.

INTRODUCTIONIt is now recognized that the limited supply of

the Platinum Group metals will necessitate the development of their more earth-abundant con-geners 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 invari-ably requires the highly air-sensitive reagent Ni(cod)2 to be used.4,5 Most Ni(0) catalysis there-fore results from the in-situ combination of this reagent with 2-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 ben-efits 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 stoichio-metric and/or catalytic reactivity investigated.4,7,8

In all cases,9 five-membered ring NHCs, are present with the majority of examples also con-taining bulky N-substituents in order to generate highly reactive low-coordinate Ni centres. During the course of our work with 6- and 7-membered ring carbenes (so-called ring expanded 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-tetrahy-dropyrimidin-2-ylidene). We now report our initial studies of both its stoichiometric and catalytic chemistry.

RESULTS AND DISCUSSIONSynthesis and Substitution Chemistry of 1.

Deep-red Ni(6-Mes)(PPh3)2 (1, Scheme 1) was iso-lated in the first instance serendipitously follow-ing 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 zero-valent 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 crystal-lography. 13C proved to be the most diagnostic NMR active nucleus, with a high frequency15 triplet resonance ( 228, 2JCP = 27 Hz) observed for the carbenic resonance of the coordinated 6-Mes ligand. The 31P{1H} NMR spectrum displayed a single resonance at . An X-ray crystal structure (Figure 1) confirmed the expected trigo-nal planar 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 Niortho-H-Cphenyl in-teractions reported in Ni(PPh3)3. The Ni-C6-Mes dis-tance (1.905(6) Å) is noticeably shorter than that in 2 (1.942(2) Å).10a,b

Although 1 proved to be air-sensitive, undergo-ing rapid decolorisation in solution upon exposure to only trace amounts of air, it exhibited reason-able 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 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 cen-tred at 16.5, while the bis-PPh3-d15 isotopomer appeared further upfield at 15.9. Addition of 2 equivalents of PPh3-d15 shifted the equilibrium fur-ther towards the bis PPh3-d15 isotopomer, al-though 1 was still observable.

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 (): 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).

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

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. The X-ray crystal structures of 3-6 are shown in Figure 2. The high degree of backbond-ing from the Ni(0) center to the CC, C=C and C=O lig-ands in 3-6 was evident from the structural metrics and 13C NMR chemical shifts of the new ligands (Table 1).7h,16 The 2-coordination mode adopted by the benzophenone and benzaldehyde ligands in 4 and 5 is indicative of coordina-tion to an electron-rich metal center.17

Scheme 2 Oxidation of 1 by PhBr, C6F6 and IMe4.

Oxidation chemistry of 1. Given the impor-tant role that Ni(0)L2 species are proposed to play in cross-coupling reactions,18 the reactivity of 1 towards aryl halides was investigated (Scheme 2). As anticipated,19 the reaction with C6F6 gave the square planar Ni(II) pentafluorophenyl fluoride complex, Ni(6-Mes)(PPh3)(C6F5)F (7). NMR mea-surements indicated that the reaction was com-plete within 30 min at room temperature, compa-rable in rate to that reported for the ‘latent’

Ni(NHC)2 precursor Ni2(IiPr2)4(cod) (IiPr2 = 1,3-di-isopropylimidazol-2-ylidene).7a The rapidity of re-action of both 1 and Ni2(IiPr2)4(cod) with C6F6 con-trasts noticeably with that of the corresponding Ni(PR3)2 species, which typically require days, or even weeks, to reach completion.20,21

Table 1. Selected bond lengths (Å), angles () and 13C NMR ligand chemical shifts (L) for complexes 3-6.

3 4 5 6Ni-CNHC

1.9174(15)

1.9167(13)

1.963(2) 1.948(2)

Ni-P 2.1484(4)

2.1911(4)

2.1500(6)

2.1354(6)

Ni-L:Ni-C 1.8945(1

6)1.9628(

14)2.014(2) 1.970(2)

1.9017(16)

2.0365(13)

- -

Ni-O - - 1.8741(17)

1.8681(15)

CNHC-Ni-P

122.32(5)

119.72(4)

109.12(7)

115.80(6)

L 136.1;120.8

58.6;39.4a 57.8;38.9b

86.1 85.8

aC6D5CD3. bTHF-d8.

7 was characterized in solution by multinuclear NMR spectroscopy and, in the solid-state, by X-ray crystallography (Figure 3). The structure re-vealed a trans arrangement of the 6-Mes and phosphine ligands. The Ni-C6F5

3 4

5 6

Figure 2. Molecular structures of Ni(6-Mes)(PPh3)(PhCCMe) (3), Ni(6-Mes)(PPh3)(PhCH=CH2) (4), Ni(6-Mes)(PPh3)(2-Ph2CO) (5) and Ni(6-Mes)(PPh3)(2-PhCHO) (6). Ellipsoids are represented at 30% probability. Hydro-gen atoms, except for those attached to C(23) in 3 and 6 and C23 and C(24) in 4, have been omitted for clar-ity.

(1.9008(17) Å) and Ni-F (1.8470(10) Å) distances are some-what shorter than those in trans-Ni(IiPr2)2(C6F5)F (1.907(2), 1.891(1) Å; IiPr2 = 1,3-diisopropylimidazol-2-ylidene),7a,e which could reflect the greater donor power of the RE-NHC ligand22 and/or the drive for - 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 - inter-actions. Although these two rings are not co-planar, 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 towards 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 PhBr by 1 not hugely surprising. How-ever, the reaction failed to yield an oxidative ad-dition product, but instead gave the Ni(I) complex 2. Formation of the analogous Ni(NHC)2X products occurs for both of the N-aryl carbene complexes Ni(IPr)2 and Ni(IMes)2 upon addition of a variety of aryl halides (for IPr, X = Cl;23 for IMes, X = Cl, Br, I)24 whereas Ni(NHC)2 species with smaller, N-alkyl substituents do indeed yield four-coordinate Ni(II) oxidative addition products.3,7g,25

Figure 3. Molecular structure Ni(6-Mes)(PPh3)(C6F5)F (7). Ellipsoids are shown at 30% probability with all hydrogen atoms removed for clarity. Selected bond lengths (Å) and angles (): 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).

In light of there being only two reported Ni(NHC)3 species,7b,8c,26 1 was reacted with IMe4 (1,3,4,5-tetramethylimidazol-2-ylidene). The im-mediate disappearance of the starting material signal at ca. 17 in the 31P{1H} NMR spectrum was observed upon addition of 2 equiv of the NHC to a C6D6 solution of 1 and singlet resonances at ca. 15 and 48 for two two intermediates as-signed as Ni(6- Mes)(IMe4)(PPh3) and Ni(IMe4)2(PPh3) appeared. Within 5 min, a third species, ultimately identified as the Ni(II) phos-phido complex, Ni(IMe4)2(PPh2)Ph (8, Scheme 2), started to form. Over ca. 12 h, Ni(IMe4)2(PPh3) dis-appeared, although was reformed when a further 2 equiv IMe4 were added, concomitant with loss of Ni(6-Mes)(IMe4)(PPh3). After 72 h, only 8 was present in solution. 8 displayed a square planar

geometry (Figure 4) with mutually trans IMe4 lig-ands and a trans arrangement of the pyramidal phosphido ligand and phenyl group. The Ni-PPh2 distance (2.2347(5) Å) was comparable to that in the very few other known terminal Ni-phosphido species,27 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

Figure 4. Molecular structure of Ni(IMe4)2(PPh2)Ph (8). Ellipsoids are shown at 30% probability with all hydrogen atoms removed for clarity. Selected bond lengths (Å) and angles (): 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).

Although P-C bond cleavage of phosphines is very well established,28 such reactions typically result in bi- or multi-metallic metal products with -PR2 ligands.29 To the best of our knowledge, iso-lation 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 Fahey and Mahan ca. 40 years ago to be an intermediate en-route to Ni(PEt3)2(-PPh2)2Ni(PEt3) and biphenyl in the reaction of Ni(PEt3)2(Ph)Br with LiPPh2OEt2, but was described as being ‘too labile to be iso-lated’.30

Table 2. Nickel Catalyzed Transfer Hydro-genation of C=O Containing Substrates.

Entry Ni R R Yield

source (%)a,f

1 1 H Ph 952b - H Ph 543c 1 H Ph 154d 1 H Ph 135c,d 1 H Ph 146e 1 H Ph 977 1 H Me 908b - H Me 409 1 OMe Me 56

10b - OMe Me 3011 1 Br Me 012 1 Cl Me 013 1 F Me 7014 1 H Et 6015 1 H H 4016 5 H Ph 95

Reaction conditions: NaOtBu (5 mol%), Ni precursor (2 mol%), 2 mL iPrOH, reflux for 20 h. aYields deter-mined by 1H NMR spectroscopy using anisole as in-ternal standard. bReactions run in the absence of Ni pre-catalyst. cReactions run in the absence of NaOtBu. dReactions run using EtOH as solvent. eRe-action run with 2 mol% Ni precursor and 2 mol% NaOtBu. fAverage of two runs.

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)31 provides a perfect example of this with only a handful of Ni phosphine/carbene catalysed TH reactions of C=O, C=N and C=C bonds described in the literature.32 At 2 mol% loading, 1 converted a range of aromatic ketones (Table 2, entries 1, 7, 13 and 14) and 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% NaOtBu (entries 4 and 5). Moreover, given the use of such a strong base,33 control ex-periments 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 catal-ysis. A mercury drop experiment resulted in no change of activity, supporting catalysis being ho-mogeneous.

No reduction of the ketone group in either bromo- or chloroacetophenone was found (entries 11 and 12), although in both cases, acetophe-none was formed in ca. 5%, consistent with reac-tion with base. With the fluoro derivative, no ace-tophenone was formed and the substrate was

successfully reduced to 4-fluoromethylbenzyl al-cohol (entry 13).

Although the exact mechanism of the reaction remains to be established, we can say that (i) the identical activity of 1 and the benzophenone com-plex 5 (entries 1 and 16) implies the rapid forma-tion 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 inter-mediacy of a Ni(II) alkoxy hydride that could allow base to be continually regenerated and the cat-alytic cycle to propagate. Efforts to substantiate this pathway are ongoing.

SUMMARY AND CONCLUSIONSThe stoichiometric and catalytic activity of the

Ni(0) complex Ni(6-Mes)(PPh3)2 (1) has been in-vestigated. 1 is substutionally labile, undergoing replacement of a single phosphine ligand to give Ni(0) alkene, alkyne, ketone and aldehyde com-plexes. Loss of a phosphine also occurs upon treatment with PhBr, although this does not lead to oxidative addition, but instead formation of the known Ni(I) species Ni(6-Mes)(PPh3)Br. Such reac-tivity agrees with that seen for other bulky NHC containing Ni(0) complexes.23,24 Isolation of Ni(IMe4)2(PPh2)Ph, a rare example of a terminal nickel phosphido complex, from the reaction of 1 with 1,3,4,5-tetramethylimidazol-2-ylidene is without doubt the most unexpected result from the stoichiometric reactions. Pleasingly, 1 proved to be active for the catalytic transfer hydrogena-tion of C=O bonds. Both the mechanism of this reaction, as well as the use of derivatives of 1 with different RE-NHCs and phosphines, is the subject of continuing work.

EXPERIMENTAL SECTIONAll manipulations were carried out using standard

Schlenk, high vacuum and glovebox techniques. Sol-vents were purified using an MBraun SPS solvent sys-tem (hexane) or under a nitrogen atmosphere from sodium benzophenone ketyl (benzene, THF). C6D6, THF-d8 and C6D5CD3 were vacuum transferred from potas-sium. NMR spectra were recorded at 298 K (unless oth-erwise stated) on Bruker Avance 500 and 400 and Agi-lent 500 MHz NMR spectrometers and referenced to sol-vent signals (benzene: 7.16 (1H), 128.0 (13C); toluene; 2.09, 20.4; thf: 3.58, 23.6). 31P{1H} spec-tra were referenced externally to 85% H3PO4 ( 0.0), 19F, externally to CFCl3 ( 0.0). Elemental analyses were per-formed by Elemental Microanalysis Ltd, Okehampton, Devon, UK. Ni(6-Mes)(PPh3)Br (2),10a 6-Mes34 and IMe4

35

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 extracted into hexane (2 x 5 mL), filtered and concen-trated 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 am-pule 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 re-moved in vacuo and the residue extracted into hexane (20 mL). Pure, crystalline 1 was obtained from a con-centrated solution of the compound in hexane at -35 ºC. 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). 31P{1H} 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 C58H58N2P2Ni: C, 77.08; H, 6.47; N, 3.10; found C, 77.23; H, 6.56; N, 3.13.

Ni(6-Mes)(PPh3)(PhCCMe) (3). PhCCMe (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 orange-yellow crystalline solid from a con-centrated hexane solution at room temperature overnight. Yield 10 mg (60%). 1H 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). 13C{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, PhCCMe), 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, PhCCMe), 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%). 1H 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, 2JCP = 3 Hz, CH2=CH), 46.3 (s, NCH2), 45.9 (s, NCH2), 39.4 (d, 2JCP= 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). 13C{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 1H-13C 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 sol-vent was removed in vacuo, the residue 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, 7H, CHAr), 6.84-6.77 (m, 10H, 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): 35.6 (s). 13C{1H} NMR (126 MHz, C6D6): δ 229.8 (d, 2JCP = 7 Hz, 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 ben-zaldehyde (3 μL, 0.029 mmol) and 25 mg (0.027 mmol) 1. After the reaction, the solvent was removed in vacuo and the residue extracted into hexane. 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). 31P{1H} 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 mix-ture shaken for 30 min at room temperature. The sol-vent was removed in vacuo, the residue 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): 14.1 (d, 2JPF = 57 Hz). 19F (C6D6, 470 MHz): -107.8 (m, 2F, o-C6F5), 163.8 (t, 3JFF = 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(IMe4)2(PPh2)Ph (8). 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 x 1 mL) to leave a yellow mi-crocrystalline 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): 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 8 by elemental analysis repeatedly gave extremely low values for %C (e.g. 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 mol), NaOtBu (0.02 mmol) and anisole (0.4 mmol, as internal standard) in iPrOH (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 8 were obtained using an Agilent SuperNova instrument (Table S1). All experiments were conducted at 150 K using a Cu-K source. Conver-gences were achieved using SHELXL36 via Olex237 rela-tively straightforward and only points of note are men-tioned 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 fall-off) and in the higher than desir-able Rint value accompanying those reflections used in the refinement. Nonetheless, the result is unambiguous and convergence was successful upon inclusion of an-isotropic 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 sub-ject to being equidistant from the relevant parent atoms. H23 in 6 was similarly located and, in this in-stance, 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 8 was modelled to in a manner that accounted for disorder 50:50 disorder. Each component was treated as a rigid hexagon, and

the P1-C27, P1-C27A distances were restrained to being similar in the final least-squares.

Crystallographic data for compounds 1 and 3-8 have been deposited with the Cambridge Crystallographic Data Centre as supplementary publications CCDC1530908-1530914. Copies of the data can be ob-tained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [fax(+44) 1223 336033, e-mail: deposit@ccdc.cam.ac.uk]. 

ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website. Multinuclear NMR spectra and crystal data/structural refinement details for 1 and 3-8.

AUTHOR INFORMATIONCorresponding Author* E-mail: m.k.whittlesey@bath.ac.uk. Tel.: 44 1225 383748

NotesThe authors declare no competing financial interest.

ACKNOWLEDGMENT The Royal Society (Newton Fellowship to SS) and EPSRC (grant EP/F029292/1 for MJP) are thanked for financial support.

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