POCOP-Ligated Nickel Siloxide Complexes: Syntheses

Article pubs.acs.org/Organometallics

POCOP-Ligated Nickel Siloxide Complexes: Syntheses, Characterization, and Reactivities Jingjun Hao, Boris Vabre, and Davit Zargarian* Département de Chimie, Université de Montréal, Montréal, Québec, Canada H3C 3J7 S Supporting Information *

ABSTRACT: This report describes the reactivities of POCOP-type pincer complexes of nickel bearing a trimethylsiloxide ligand. The complex {κP,κC,κP-2,6-(i-Pr2PO)2C6H3}Ni(OSiMe3) (1-OSiMe3) reacts with 2,4,6-Me3-C6H2OH (MesOH) to give the stable derivative 1-OMes, whereas the corresponding tert-butoxide derivative could not be prepared. Reaction of 1-OSiMe3 and its aliphatic analogue {κP,κC,κP-(iPr2POCH2)2CH}Ni(OSiMe3) (2-OSiMe3) with Ph3SiOH led to facile formation of the corresponding protonolysis products 1-OSiPh3 and 2-OSiPh3. Treating 1-OSiMe3 or 2-OSiMe3 with Me3SiN3 gave the azide derivatives 1-N3 or 2-N3 in high isolated yields, whereas the analogous reactions with Me3SiCF3 gave 2-CF3 but not 1-CF3. These observations allow us to compare the reactivities of the Ni−OSiMe3 moiety as a function of pincer ligand backbone. All new complexes have been characterized fully. The reaction of the azide derivatives with benzyl bromide led to the formation of benzyl azide and the corresponding bromo derivatives; 2-N3 proved to be much more reactive than 1-N3. The analogous reactions with the trifluoromethyl derivatives are much more complicated and appear to convert PhCH2Br to Me-PhCF3 and PhBr to PhF.

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INTRODUCTION

complexes based on PIMCOP-, PIMIOCOP-, and NHCCOPtype ligands.12 Of the ECE-type complexes mentioned above, the POCOPtype complexes of Ni(II) are arguably the most versatile in terms of the ease of ligand synthesis and nickelation;13,14 as a result, the catalytic reactivities of these compounds have been scrutinized most closely.1h,11,15 Given that the reactivities of a given complex can be intimately related to the integrity/ stability of its auxiliary ligand(s), we and others have also examined the structural stabilities of POCOP−Ni complexes under various reaction conditions.16 These studies, particularly those dealing with the less robust POCsp3OP complexes, have identified a number of decomposition pathways which must be taken into account when designing catalytic applications based on this family of complexes. As a follow-up to the above studies on the reactivities and stabilities of POCOP−Ni complexes, we have examined the impact of X ligands in (POCsp2OP)- and (POCsp3OP)NiX complexes (X = OSiR3, OR, N3, CF3). More specifically, we have studied the ease of synthesis and relative stabilities of the siloxide derivatives 1-OSiMe3 and 2-OSiMe3 (Chart 1), evaluated the structural impact of so-called “dπ−pπ” interactions between Ni and O atoms in these compounds, and examined their suitability as precursors for preparing other derivatives.

Interest in EXE-type pincer complexes of nickel has increased significantly over the past decade, as a growing number of such complexes have emerged as versatile catalysts and advanced materials.1 Nickel’s propensity to form stable and isolable complexes with a wide range of pincer ligands (e.g., PCP, NCN, POCOP, PNP, NNN, POCN, PSiP, CNN, etc.)2 combined with the facile modification of the coordination, steric, and electronic properties of these ligands has allowed an extensive modulation of chemical reactivities for the complexes [(EXE)NiL]n+ (L = monoanionic or charge-neutral ligand; n = 0, 1). Recently introduced new derivatives featuring unsymmetrical binding moieties (EXE′),3 differently sized fused metallacycles (4,5, 5,6, 6,6, etc.),3,4 and different ligand backbones (aromatic vs aliphatic; C- vs Si- vs N-centered)5 have allowed a broad investigation of structure−reactivity relationships, and it is hoped that the knowledge generated from such studies will lead to the development of new applications. Our group has been interested in developing synthetic routes to ECE-type pincer complexes of nickel and exploring their reactivities. In previous reports, we have described the chemistry of PCP-,6 POCOP-,7 and POCN-type8 complexes of Ni(II) and Ni(III)9 (Chart 1) and shown that some of these complexes catalyze transformations such as Kumada−Corriu coupling,9b hydroamination, and hydroalkoxylation of acrylonitrile derivatives,10 and Kharasch additions to olefins,7a as well as oligomerization of PhSiH3 and its addition to styrene.11 A recent report has also introduced a new family of divalent © 2014 American Chemical Society

Received: September 4, 2014 Published: November 11, 2014 6568

dx.doi.org/10.1021/om500916d | Organometallics 2014, 33, 6568−6576

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The 31P NMR spectrum of 1-OSiMe3 shows a singlet at 175.2 ppm, very close to the corresponding signal for 2OSiMe3 (174.8 ppm).16b The 1H and 13C NMR spectra of 1OSiMe3 showed a set of characteristic resonances for the POCsp2OP ligand framework, as well as the anticipated signals for the SiMe3 moiety. The latter were quite similar to the analogous resonances in 2-OSiMe3: 1H singlets at δ 0.23 and 0.24 and 13C singlets at δ 5.32 and 5.46.16b The solid-state structure of 1-OSiMe3 will be discussed later when we describe the structures of analogous OSiPh3 derivatives. Reactions of 1-OSiMe3 and 2-OSiMe3 with ArOH, tBuOH, and Ph3SiOH. Previous studies had shown that protonolysis of 2-OSiMe3 with aryl alcohols allows isolation of the corresponding aryloxides 2-OAr in a straightforward manner, whereas the corresponding alkoxides, including 2-O(tBu), were inaccessible (Scheme 2).16b Similar observations

Chart 1. Various Pincer−Ni Systems

Scheme 2. Reactivities of Ni-OSiMe3 with ROH and ROM

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RESULTS AND DISCUSSION Synthesis and Characterization of 1-OSiMe3. A previous report has described the preparation of (POCsp3OP)Ni(OSiMe3) (2-OSiMe3) by treating its chloro precursor with KOSiMe3.16b A similar protocol gave the aromatic analogue of this siloxide complex, 1-OSiMe3, as shown in Scheme 1. Scheme 1. Synthesis of 1-OSiMe3 were made with 1-OSiMe3 in the course of the present study. Thus, reaction of 1-OSiMe3 with 1 equiv of 2,4,6-Me3C6H2OH (MesOH) gave the stable derivative 1-OMes in a nearly quantitative yield, whereas the much more bulky aryl alcohol 2,6-t-Bu2-4-Me-C6H2OH did not react with 1-OSiMe3 over 5 days at 90 °C. The salt metathesis reaction between 1-Cl and NaOMes also gave 1-OMes (Scheme 2), the conversion appearing to be quantitative by 31P NMR, but 1-OMes was obtained in only 65−70% isolated yield. On the other hand, the corresponding tert-butoxide derivative could not be obtained via protonolysis (1-OSiMe3 + t-BuOH) or salt metathesis (1-Cl + t-BuOK); indeed, the latter approach led to degradation of the POCsp2OP ligand framework, as inferred from the appearance of a resonance at ca. 99 ppm in the 31P NMR spectrum of the reaction mixture.16b These observations underline the crucial influence of the O-substituent on the ultimate success of these reactions. 1-OMes was characterized fully by multinuclear NMR spectroscopy, elemental analyses, and X-ray diffraction studies. The 31P singlet at 175.9 ppm is comparable to the corresponding signal for 2-OMes (175.4 ppm),16b but the 1H and 13C NMR spectra of these complexes were different. For instance, the methyl groups of the mesityl moiety gave rise to two 1H and 13C singlets (1:2) for 1-OMes, whereas three singlets (1:1:1) were observed for 2-OMes. These differences are attributed to the absence of planar symmetry in 2-OMes due to the nonplanar conformation of the POCsp3OP backbone and/or restricted rotation about the O−Mes bond in the

Formation of this derivative was confirmed by the 31P NMR spectrum of the reaction mixture, but several byproducts were also noted in this reaction, in contrast to the straightforward synthesis of its aliphatic analogue, 2-OSiMe3. Optimization experiments showed that the amounts of KOSiMe3 and the reaction temperature are important factors for minimizing side reactions during the synthesis of 1-OSiMe3. Thus, using no more than 1 equiv of KOSiMe3 and conducting the synthesis at −78 °C allowed us to isolate pure samples of this derivative in ca. 50% yield (Scheme 1). In comparison to its POCsp3OP analogue 2-OSiMe3, 1OSiMe3 is less sensitive to oxygen and moisture, both in the solid state and in solution, and also shows greater thermal stability in C6D6 solutions kept under an inert atmosphere. For instance, little or no decomposition was observed when a benzene solution of 1-OSiMe3 was heated at 85 °C for 2 days, whereas 2-OSiMe3 underwent partial decomposition (ca. 10− 20%) under comparable conditions. It appears, therefore, that the complications encountered during the synthesis of 1OSiMe3, alluded to above, are kinetic in nature, not thermodynamic. 6569


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The thermal stabilities of 1-OSiPh3 and 2-OSiPh3 are greater than that of 2-OSiMe3 but comparable to that of 1-OSiMe3 in the range 85−100 °C. The difference between the thermal stabilities of 1-OSiR3 relative to their aliphatic counterparts 2OSiR3 is even more apparent at higher temperatures (∼120 °C), confirming the thermodynamically more robust character of the aromatic ligand framework. Similarly, the greater stability of the OSiPh3 derivatives relative to their OSiMe3 analogues is presumably because the Ph groups help stabilize the negative charge on the siloxide oxygen atom, thus moderating the nucleophilic character of this moiety. The spectroscopic features of 1-OSiPh3 and 2-OSiPh3 were straightforward: the 31P signal appeared at ca. 175 ppm in both compounds, and all anticipated 1H and 13C NMR resonances for POC2OP, POCsp3OP, and OSiPh3 moieties were identified easily. The solid-state structures of 1-OSiMe3, 1-OSiPh3, and 2-OSiPh were determined by single-crystal X-ray diffraction (Figure 2). The Ni center in all three structures adopts a square-planar geometry, and most structural parameters (Table 1) are comparable to the corresponding values observed in (POCsp2OP)NiX18 and (POCsp3OP)NiX7a,16b,19 families of compounds. 2 The observed Ni−O distance range of 1.848(1)−1.956(2) Å is consistent with the previously reported distances for covalent NiII−O bonds.20 The greater steric bulk of OSiPh3 is likely the cause of the slightly longer Ni−O bond distances in these derivatives relative to their OSiMe 3 analogues, whereas the generally longer Ni−O bond distances in 2-OR relative to 1-OR can be attributed to the stronger trans influence of the Csp3−Ni bond in POCsp3OP systems. The O− Si distances fall into a rather narrow range normally observed in complexes featuring a M−OSiMe3 moiety (1.58−1.59 Å).21 The Ni−O−Si angles of 156−166° observed in these complexes are significantly larger than the ideal value of 109.5° expected for angles based on an sp3 O. Similarly large angles have been observed in recently reported Ni−OSiR3 complexes22 and are consistent with a greater degree of O→Si electron delocalization that presumably serves to minimize the dπ−pπ destabilizing interactions anticipated in such squareplanar d8 systems featuring M−heteroatom linkages.23 A combination of steric factors (Ph3Si > Me3Si) and the impact of ligand backbone (POCsp2OP vs POCsp3OP) leads to the observed order for Ni−O−Si angles: 1-OSiPh3 (166°) > 1OSiMe3 (160°) > 2-OSiPh3 (156°) > 2-OSiMe3 (151°). This trend can be rationalized by considering that the aliphatic backbone in 2-OSiR3 allows sufficient flexibility to alleviate the steric interactions between the OSiR3 moiety and the rest of the complex, whereas the more rigid aromatic backbone in 1-OSiR3

solution. It is noteworthy that the equivalence of the two phosphinite moieties in 1-OMes implied by observation of a single 31P resonance indicates that in solution there must be free rotation around the Ni−O bond. To our surprise, this rotation appears to have a very low energy barrier, since the 31P signal showed no sign of decoalescence even down to −80 °C. The solid-state structure of 1-OMes is shown in Figure 1. Most parameters are comparable to those of its precursors 1-

Figure 1. ORTEP view of 1-OMes (50% probability ellipsoids; all H atoms and Me groups of i-Pr omitted for clarity).

Cl7a,9a and 1-OSiMe3.16b The Ni−O bond distance in 1-OMes is shorter than that of 2-OMes (1.864(6) Å vs 1.895(1) Å), and both of these distances are longer than the corresponding Ni− O distances in their trimethylsiloxide analogues: 1.864(6) Å in 1-OMes vs 1.8482(9) Å in 1-OSiMe3; 1.895(1) Å in 2-OMes vs 1.873(3) Å in 2-OSiMe3. The Ni−O−CMes angle is slightly smaller in 1-OMes than in 2-OMes (ca. 132° vs 136°).16b The Mes ring is nearly perpendicular to the Ar moiety of POCsp2OP (ca. 84°), consistent with the mirror plane symmetry deduced from the 1H and 13C NMR spectral features discussed above. Protonolysis reactions were also conducted with Ph3SiOH to establish the stabilities of siloxide derivatives as a function of Si substituents and ligand backbone. Reactions of 1-OSiMe3 and 2-OSiMe3 with a small excess of Ph3SiOH led to equilibria between the precursors and the corresponding OSiPh3 derivatives (Scheme 3), presumably because of the similar acidities of the corresponding silanols (ca. 10).17 Independent preparation of the target OSiPh3 derivatives via salt metathesis provided pure samples of these complexes (Scheme 3), which allowed us to confirm the above assignments. Scheme 3. Synthesis of Ni-OSiPh3 Derivatives

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Figure 2. ORTEP views of (left) 1-OSiMe3, (middle) 1-OSiPh3, and (right) 2-OSiPh3 (50% probability ellipsoids; all H atoms and Me groups of iPr omitted for clarity).

Table 1. Bonding Parameters of Complexes 1 and 2a C1(or C1A)−Ni C1B−Ni Ni−X Ni−P1 Ni−P2 O−Si P−Ni−P C1(or C1A)−Ni−X C1B−Ni−X P1−Ni−X P2−Ni−X Ni−O−Si a

1-OSiMe3

1-OSiPh3

2-OSiPh3

1-OMes

1.8762(12)

1.883(2)

1.9561(19) 2.010(2) 1.8826(11) 2.1587(4) 2.1632(4) 1.5807(11) 162.945(18 163.63(10) 169.27(9) 94.53(3) 101.01(3) 156.05(7)

1.967(4)

1.8762(12)

1.864(6) 2.1675(6) 2.1675(6)

1.8482(9) 2.1530(3) 2.1566(3)

1.8482(9) 2.1530(3) 2.1566(3) 1.5843(9) 164.110(15 179.12(5)

1.862(1) 2.1633(4) 2.1846(4) 1.5830(10) 163.556(17) 176.68(6)

97.91(3) 97.96(3) 159.68(7)

95.15(3) 101.13(3) 165.68(7)

1-N3

164.76(4) 176.9(2)

164.110(15 179.12(5)

97.48(2) 97.48(2)

97.91(3) 97.96(3)

2-N3 1.988(3) 1.939(2) 1.9205(15) 2.1549(5) 2.1615(4) 166.30(2) 172.3(3) 167.7(3) 98.57(5) 95.13(5)

2-CF3 1.986(2) 2.009(2) 1.9361(16) 2.1496(4) 2.1560(4) 165.941(19 171.78(16) 166.62(16) 96.89(5) 97.12(5)

C1A and C1B represent the disordered carbon atoms in 2-OSiPh3, 2-N3, and 2-CF3.

both 1-N3 and 2-N3 display high thermal and light stabilities, no significant decomposition being observed when samples are heated to 120 °C over several days or exposed to 254 nm UV light. In contrast to the facile activation of the polar Si−N bond in Me3SiN3 by 1-OSiMe3 and 2-OSiMe3, no reactivity was observed toward a large excess of the nonpolar substrate Me3Si−SiMe3; even heating to 100 °C over several days failed to induce a reaction. On the other hand, greater reactivity was observed with the more polar C−Si bond in CF3SiMe3: treating 2-OSiMe3 with 3 equiv of Me3SiCF3 at room temperature over 3 days facilitated the silyl group transfer to give 2-CF3 as the main product (>90% conversion; route a, Scheme 5). To our surprise, 1-OSiMe3 did not display the same reactivity toward Me3SiCF3, no reactivity being detected even at 100 °C (route b, Scheme 5). It is significant to mention that 1-CF3 is a thermodynamically stable species that can be prepared via its fluoro derivative 1-F (routes c−e, Scheme 5).25 On the other hand, this approach (Ni−F + CF3SiMe3) cannot be used for preparing 2-CF3 because the required precursor 2-F is inaccessible: in the presence of a slight excess of AgF, 2-Br gave a colorless solution from which an intractable black solid precipitated (route f, Scheme 5), whereas 2-OSiMe3 underwent decomposition via rupture of both C−O bonds to generate the previously characterized trinickel species [{(η3-allyl)Ni(κP,μOPR2O)2}2Ni] (route g, Scheme 5).16b

forces adjustments on the Ni−O−Si angle to alleviate steric repulsions. Reactions of 1-OSiMe3 and 2-OSiMe3 toward Me3SiN3 and Me3SiCF3. The strongly oxophilic character of silicon combined with the presumed ionic character of the Ni−O bonds in 1-OSiMe3 and 2-OSiMe3 should facilitate metathetic reactions between Ni−siloxides and X−SiR3. We have investigated this supposition by studying the reactivities of 1OSiMe3 and 2-OSiMe3 with Si−Si, Si−C, and Si−N bonds, as described below. 31 P NMR monitoring of the reactions of the trimethylsiloxide derivatives with Me3SiN3 showed complete conversion within minutes, and the corresponding azide species 1-N3 and 2-N3 could be isolated in high yields (Scheme 4). This method is, therefore, a highly effective and attractive alternative to using potentially explosive alkali-metal azide complexes for the preparation of Ni−azide derivatives.24 It is worth noting that Scheme 4. Synthesis of Ni-N3 Derivatives

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Scheme 5. Reactivities of Ni-OSiMe3 Derivatives toward CF3SiMe3 and AgF

Figure 3. ORTEP views of 1-N3, 2-N3, and 2-CF3 (50% probability ellipsoids; all H atoms and Me groups of iPr omitted for clarity).

Complexes 1-N 3, 2-N3, and 2-CF3 have been fully characterized by NMR, elemental analyses, and X-ray diffraction studies. The 1H and 13C NMR spectra displayed the anticipated resonances for the POCsp2OP or POCsp3OP ligand frameworks. The 31P resonance signals for 1-N3 and 2N3 (184.9 vs 184.1 ppm) were close to the corresponding signals for halide derivatives of these compounds.7a The 31P NMR spectrum of 2-CF3 displayed a quartet centered at ca. 194 ppm (3JF−P = 14 Hz), which was reminiscent of the corresponding resonances observed in 1-CF3 (δ 197, 3JF−P = 15 Hz).25 Comparison to the corresponding 31P resonances observed for other hydrocarbyl derivatives (POCOP)Ni(R) (e.g., ca. 192 ppm for R = Me, ca. 189 ppm for R = Et,7a and ca. 193 ppm for R = CCH, CCPh)16b indicates that the chemical shifts of these complexes are not very sensitive to the nature of the hydrocarbyl moiety. The 19F NMR spectrum of 2-CF3 consisted of a triplet centered at −6.2 ppm, but the anticipated triplet of quartets for Ni−CF3 was not observed, presumably due to the weakness of the signal.26 The solid-state structures of 1-N3, 2-N3, and 2-CF3 were elucidated by single-crystal X-ray diffraction (Figure 3). Most of the bonding parameters related to the POCOP frameworks are similar to those found in the corresponding series of complexes (Table 1). The Ni−N distances in 1-N3 (1.935(2) Å) and 2-N3

(1.9205(15) Å) are slightly shorter than that in (t-BuPCP)NiN3 (1.950(4) Å),27 probably due to the less sterically demanding phosphinite moieties in our system (i-Pr2P vs tBu2P). The geometries of the azide groups in 1-N3 and 2-N3 are nearly linear, with N1−N2−N3 angles of 176.4° in 2-N3 and 175.3° in 1-N3, characteristic of metal azide complexes.24,27 The Ni−CF3 distance of 1.9361(16) Å in 2-CF3 is comparable to those found in previously reported Ni−CF3 complexes28 but slightly longer than that in (POCsp3OP)Ni(CCH),16b as would be expected on the basis of the different hybridizations of the carbon atoms in these complexes. The average C4−F distance of 1.37 Å is comparable to observed C−F distances in M− CF3.28 A recent report has described the preparation and characterization of closely related (POCsp3OP)NiR complexes based on 1,3-cyclohexanediol;15h the latter appear to have slightly shorter Ni−Cipso bond distances by comparison to 2-N3 and 2-CF3 (1.94−1.97 Å vs 1.97−1.99 Å). Preliminary Reactivity Studies. The azido and trifluoromethyl derivatives were subjected to simple tests in order to delineate their reactivity patterns. Initial tests of the two azide derivatives toward PhCH2Br showed that 2-N3 is far more reactive than 1-N3 in a SN2-type substitution reaction. Thus, heating a mixture of 2-N3 and PhCH2Br (large excess) at 80 °C for 15 h completed the conversion from 2-N3 to the 6572


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corresponding 2-Br (31P δ 186 ppm), the 31P NMR spectrum showing no indication of any decomposition product. Under the same conditions, the corresponding reaction with 1-N3 gave only ca. 10% conversion to 1-Br (Scheme 6). The GC/MS

from the decomposition of the POCsp3OP backbone. A rather clean reaction was indicated by the 19F NMR spectrum of the final reaction mixture (Figure S27 in the Supporting Information), which showed a singlet at ca. −62.4 ppm representing the sole F-containing product. Comparison of the latter signal to the 19F resonance reported in the literature for 2,2,2-trifluoroethylbenzene (PhCH2CF3, −66 ppm)31 showed that the anticipated product had not formed. Curiously, we found that the observed signal is an identical match for trifluoroxylene (Me−PhCF3, −62.4 ppm),32 which is a structural isomer of PhCH2CF3 (Scheme 8). Unfortunately, however, we have not succeeded in unambiguously confirming this outcome (and the specific regioisomer of Me−PhCF3), because we have not succeeded in isolating appreciable quantities of pure samples. Indeed, the GC/MS analysis of the reaction mixture showed only trace quantities of several unidentified species as opposed to one major component that might correspond to the species in question (m/z=160). To test the possibility of forming Ar−CF3, we examined next the reaction of 2-CF3 with PhBr (ca. 80 °C, 2 days), with the following results. The 31P NMR spectrum of this mixture displayed no new peak, only the singlet at 194 ppm representing the unreacted 2-CF3 and the upfield signal for the frequently observed product of an oxidative decomposition of the POCsp3OP framework. Significantly, the anticipated bromo derivative was absent in this spectrum. For its part, the 19 F NMR spectrum of this reaction mixture (Figure S28, Supporting Information) showed the triplet resonance for the starting material plus two other resonances. One of the latter was a singlet in the chemical shift region for Ar−CF3, whereas the second was an upfield triplet of triplets at −113 ppm (J = 6 and 9 Hz). Both the multiplicity and chemical shift of this last resonance match the 19F NMR data for PhF, as confirmed by recording the spectrum of an authentic sample in C6D6 (Figure S29, Supporting Information). We speculate that PhCF3 is generated in situ and then converted to PhF (Scheme 8) via a reaction pathway involving C−C and C−F bond ruptures. We have not probed the mechanism of these unusual reactions, but it is worth noting that a recent report from Arnold’s group has shown how high-valent Nb species can promote multiple C−F bond ruptures via (η6-PhCF3)Nb and Nb-CF2Ph intermediates.33

Scheme 6. Reactivities of Ni-N3 Derivatives toward PhCH2Br and PhCCH

analysis of the main organic product in both reactions showed intense signals for the parent ion M+ (m/z 133) and [M − 1]+ (m/z 132) and also for the fragments [M − N2]+ (m/z 105) and [M − N 3 ] + (m/z 91) (Figure S24, Supporting Information), consistent with the MS of authentic PhCH2N3.29 The azido complexes were also treated with phenylacetylene to explore the possibility of affecting a 2 + 3 cyclization to furnish a Ni−triazole derivative. Whereas no reaction ensued between PhCCH and 1-N3, a reaction was observed with 2-N3 at 100 °C, but instead of the target triazole complex we obtained Ph3−C6H3, a product of selective cyclotrimerization of PhCCH (m/z 306), in addition to minor quantities of a dimerization product (m/z 204).30 GC/MS analyses of the reaction mixture revealed one isomer of each product (Figures S25a and S25b, Supporting Information). Given the potential applications of trifluoromethyl complexes of transition metals as catalysts in fluorochemistry, we were prompted to investigate the reactivities of our Ni−CF3 compounds. A recent study on the stoichiometric reactivity of 1-CF 3 with PhCH2 Br revealed that instead of giving PhCH2CF3, the expected product of simple SN2 substitution, this reaction leads to benzylation of the aromatic solvent ArH, giving PhCH2Ar (in 30−70% yields) and the bromo derivative 1-Br (as the exclusive P-containing species); the fate of the CF3 fragment was unknown (Scheme 7).25 The tests conducted in the context of the present study have revealed different reactivities for 2-CF3. For example, the NMRscale reaction of 2-CF3 with PhCH2Br in C6D6 (60 °C, 2 days) led to appearance of the 31P signal for 2-Br in addition to another resonance at ca. 63 ppm, which is presumably arising

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CONCLUSION This study has led to the preparation and characterization of three new nickel siloxide complexes based on POCsp2OP and POCsp3OP ligand frameworks. Structural analyses and reactivity tests have revealed that these siloxide derivatives are stable, but the siloxide moiety is fairly reactive. The trimethylsiloxide derivative based on the POCsp3OP ligand framework is more reactive in alcoholysis in comparison to its POCsp2OP counterpart, but it is also thermally less stable and more prone to various side reactions. Both Ni−OSiMe3 complexes are also good precursors for the preparation of the azido derivatives, whereas only 2-OSiMe3 gave access to 2-CF3. These metathetic reactions are driven by the formation of volatile Me3Si−OSiMe3. The azido derivatives react with PhCH2Br to give the corresponding Ni−Br derivatives and benzyl azide, much greater reactivity being observed with 2-N3. On the other hand, a much more complex reactivity was observed with 2-CF3, as had been observed earlier with 1CF3.25 Thus, the bromo derivative is generated in the reaction with PhCH2Br, but not the anticipated organic product

Scheme 7. Reactivities of 1-CF3 toward PhCH2Br

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Scheme 8. Reactivities of 2-CF3 toward PhCH2Br and PhBr

dryness followed by extraction of the solid residues with n-hexane (30 mL) gave a yellow-orange solution that was concentrated to 1 mL and stored at −40 °C for 2 days to afford orange crystals of 1-OMes (0.31 g, 63%). 1H NMR (δ, 25 °C, 400 M, C6D6): 1.17−1.21 (m, 24H, PCH(CH3)2), 2.01 (sept, 3JHH = 7.0, 4H, PCH(CH3)2), 2.31 (s, 3H, Ar-Me), 2.62 (s, 6H, Ar-Me), 6.49 (d, JHH = 8, 2H, Ar-H), 6.82 (t, JHH = 8, 1H, Ar-H), 6.90 (s, 2H, Ar-H). 13C{1H } NMR (δ, 25 °C, 100 M, C6D6): 16.63 (s, 4C, PCH(CH3)2), 16.84 (pst, 2JPC = 2.8, 4C, PCH(CH3)2), 18.94 (s, 2C, Ar-Me), 20.95 (s, 1C, Ar-Me), 28.30 (pst, JPC = 10.0, 4C, PCH(CH3)2), 105.45 (pst, 4JPC = 5.7, 2C, C3/C5), 121.35 (t, 2JPC = 23.5, 1C, C1), 122.58 (s, Ar-C), 125.64 (s, Ar-C), 129.06 (s, Ar-C), 167.72 (t, JPC = 3.7, Ar-C), 169.55 (pst, 3JPC = 10.0, 2C, C2/C6). 31P{1H} NMR (δ, 25 °C, 162 M, C6D6): 175.92 (s). Anal. Calcd for C27H42O3P2Ni (535.26): C, 60.59; H, 7.91. Found: C, 60.59; H, 8.05. Synthesis of (POCsp2OP)Ni(OSiPh3) (1-OSiPh3). A mixture of 1Cl (0.23 g, 0.5 mmol) and KOSiPh3 (0.32 g, 1 mmol) in THF (30 mL) was stirred for 3 h and worked up following a procedure similar to that described above for 1-OSiMe3. The resulting solution (1 mL) was stored at −40 °C for 3 days to give yellow crystals of 1-OSiPh3 (0.25 g, 74%). 1H NMR (δ, C6D6): 1.13 (dtv, 3JHH = 6.8 and vJHP = 6.8, 12H, PCH(CH3)2), 1.25 (dtv, 3JHH = 8.5 and vJHP = 8.1, 12H, PCH(CH3)2), 1.70 (sept, 3JHH = 7.0, 4H, PCH(CH3)2), 6.49 (d, JHH = 8, 2H, Ar-H), 6.82 (t, JHH = 8, 1H, Ar-H), 7.20−7.25 (m, 8H, Ar-H), 7.65 (br, m, 2H, Ar-H), 7.94 (d, JHH = 6, 5H, Ar-H). 13C{1H} NMR (δ, C6D6): 16.65 (s, 4C, PCH(CH3)2), 18.11 (pst, 2JPC = 3.4, 4C, PCH(CH3)2), 27.93 (pst, JPC = 9.6, 4C, PCH(CH3)2), 105.42 (pst, 4 JPC = 5.7, C3/C5), 120.74 (t, JPC = 23.6, Ar-C), 127.37 (s, Ar-C), 128.59 (s, Ar-C), 136.13 (s, Ar-C), 142.32 (s, Ar-C), 169.99 (pst, 3JPC = 10.3, C2/C6). 31P{1H} NMR (C6D6): 176.76 (s). Anal. Calcd for C36H42O3P2SiNi·0.5C7H8 (721.53): C, 65.75; H, 6.98. Found: C, 65.47; H, 7.09. Synthesis of (POCsp3OP)Ni(OSiPh3) (2-OSiPh3). A mixture of 2Br (0.45 g, 1 mmol) and KOSiPh3 (0.63 g, 2 mmol) in THF (30 mL) was stirred overnight. Following a workup and recrystallization procedure similar to that described above for the preparation of 1OSiPh3 gave yellow crystals of 2-OSiPh3 (0.35 g, 55%). 1H NMR (δ, C6D6): 1.11 (dtv, 3JHH = 6.5 and vJHP = 5.9, 6H, PCH(CH3)2), 1.16 (dtv, 3JHH = 6.9 and vJHP = 6.4, 6H, PCH(CH3)2), 1.22 (dtv, 3JHH = 9.5 and vJHP = 7.5, 6H, PCH(CH3)2), 1.43 (dtv, 3JHH = 9.2 and vJHP = 7.5, 6H, PCH(CH3)2), 1.55 (sept, 3JHH = 7.09, 2H, PCH(CH3)2), 1.65 (sept, 3JHH = 7.2, 2H, PCH(CH3)2), 2.49 (m, 1H, CH2CHCH2), 3.13 (dd, 3JHH = 11.6, 3JHH = 9.3, 2H, CH2CHCH2), 3.17−3.30 (m, 2H, CH2CHCH2). 13C{1H} NMR (C6D6): 16.33 (s, 2C, PCH(CH3)2), 16.78 (s, 2C, PCH(CH3)2), 18.46 (pst, 2JPC = 3.8, 2C, PCH(CH3)2), 19.27 (pst, 2JPC = 3.4, 2C, PCH(CH3)2), 27.78 (pst, JPC = 11.4, 2C, PCH(CH3)2), 28.78 (pst, JPC = 9.1, 2C, PCH(CH3)2), 42.36 (t, 2JPC = 13.6, 1C, C1), 76.38 (pst, 3JPC = 7.8, 2C CH2CHCH2), 127.24 (s, ArC), 128.38 (s, Ar-C), 136.18 (s, Ar-C), 142.83 (s, Ar-C). 31P{1H} NMR (C6D6): 175.94 (s). Anal. Calcd for C33H48O3P2SiNi (641.46): C, 61.79; H, 7.54. Found: C, 62.07; H, 7.80. Synthesis of (POCsp2OP)NiN3 (1-N3). To a room-temperature solution of 1-OSiMe3 (0.14g, 0.3 mmol) in toluene (15 mL) was added dropwise Me3SiN3 (0.069 g, 0.6 mmol), and the resulting mixture was stirred for 1 h. After all volatiles were removed, the solid residue was extracted into n-hexane (10 mL) and evaporated to give NMR-pure solid samples of 2-N3 (0.12 g, 91%). 1H NMR (δ, 25 °C, 400 M, C6D6): 1.09 (dtv, 3JHH = 7.3 and vJHP = 7.1, 12H, PCH(CH3)2), 1.30 (dtv, 3JHH = 10.3 and vJHP = 7.5, 12H, PCH(CH3)2), 2.11 (sept, 3 JHH = 7.1, 4H, PCH(CH3)2), 6.51 (d, 3JHH = 8.0, 2H, Ar-H), 6.81 (t, 3 JHH = 8.0, 1H, Ar-H). 13C{1H} NMR (δ, 25 °C, 100 M, C6D6): 16.62

PhCH2CF3, the only F-containing product of this reaction appearing to be trifluoroxylene. In contrast, the reaction with PhBr did not generate the anticipated 2-Br but gave two Fcontaining products that appear to be PhF and (more tentatively) trifluorotoluene. The unusual reactivities observed with 2-CF3 imply the involvement of Ni-based intermediates arising from the types of side reactions taking place, thanks to the fragile ligand backbone.16b Efforts are underway to test the reproducibility of these reactions, unequivocally confirm the identities of these products, and probe the seemingly very complex reactivity patterns observed. Future studies will also focus on the development of catalytic reactions involving the azido and siloxide derivatives reported herein.

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EXPERIMENTAL SECTION

General Procedures. All manipulations were carried out under nitrogen using Schlenkware, double-manifold Schlenk lines, a glovebox, and standard procedures. Solvents were dried by passage over activated alumina and stored over 4 Å molecular sieves. Me3SiCF3 (2 M in THF) was purchased from Aldrich and used as received. The preparations of KOSiPh 3 , 34 (POC sp 2 OP)NiCl (1-Cl) 35 and (POCsp3OP)Ni(OSiMe3) (2-OSiMe3)16b have been described in previously published reports. High-field spectrometers were used for recording the 1H (400 MHz), 31P (161.9 MHz), and 13C (100.56 MHz) NMR spectra at room temperature (ca. 25 °C) or lower temperatures. The chemical shift values are reported in ppm (δ) and referenced either internally to the residual solvent signals (1H 7.15 ppm and 13C 128.06 ppm for C6D6) or externally (31P, H3PO4 in D2O, δ 0); coupling constants are reported in Hz. The elemental analyses were performed by the Laboratoire d’Analyze Élémentaire, Département de Chimie, Université de Montréal. Synthesis of (POCsp2OP)Ni(OSiMe3) (1-OSiMe3). To a −70 °C THF solution of (POCsp2OP)NiCl (0.44 g, 1 mmol, 10 mL) was added dropwise a THF solution of KOSiMe3 (0.13 g, 1 mmol, 10 mL). The resulting mixture was slowly warmed to room temperature and stirred for an additional 1 h before evaporating all volatiles. Following a general workup procedure, including extracting the solid residues with n-hexane (15 mL) and filtration, the resulting orange solution was concentrated to ca. 1 mL and stored at −40 °C for 2 days to furnish yellow crystals of 1-OSiMe3 (0.25 g, 51%). 1H NMR (δ, C6D6): 0.23 (s, 9H, SiMe3), 1.21 (dtv, 3JHH = 6.9 and 3JHP = 6.9, 12H, PCH(CH3)2), 1.39 (dtv, 3JHH = 10.0 and vJHP = 7.5, 12H, PCH(CH3)2), 2.13 (sept, 3 JHH = 7.1, 4H, PCH(CH3)2), 6.49 (d, 3JHH = 8.0, 2H, Ar-H), 6.81 (t, 3 JHH = 8.0, 1H, Ar-H). 13C{1H} NMR (δ, C6D6): 5.32 (s, 3C, SiMe3), 16.71 (s, 4C, PCH(CH3)2), 17.69 (pst, 2JPC = 3.7, 4C, PCH(CH3)2), 27.75 (pst, 2JPC = 9.7, 4C, PCH(CH3)2), 105.39 (t, 4JPC = 5.9, 2C, C3/ C5), 122.37 (t, 2JPC = 23.7, 1C, C1), 128.31 (s, 1C, C4), 169.86 (pst, 3 JPC = 10.7, 2C, C2/C6). 31P{1H} NMR (δ, C6D6): 175.21 (s). Anal. Calcd for C21H40O3P2SiNi (489.27): C, 51.55; H, 8.24. Found: C, 51.73; H, 8.36. Synthesis of (POCsp2OP)Ni(OMes) (1-OMes). Method A. A mixture of 1-OSiMe3 (0.15 g, 0.3 mmol) and MesOH (0.041g, 0.3 mmol) in toluene (10 mL) was stirred at room temperature for 2 days and then evaporated to dryness. The solid residue was extracted with n-hexane (30 mL) and evaporated to give in a nearly quantitative yield an NMR-pure sample of 1-OMes. Method B. A mixture of 1-Cl (0.22 g, 0.5 mmol) and NaOMes (0.32 g, 2 mmol) in THF (20 mL) was stirred at room temperature for 4−6 h until the mixture turned from yellow to red. Evaporation to 6574


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(s, 4C, PCH(CH3)2), 17.22 (pst, 2JPC = 3.5, 4C, PCH(CH3)2), 28.03 (pst, JPC = 10.7, 4C, PCH(CH3)2), 105.76 (tps, 4JPC = 6.1, 2C, C3/ C5), 125.47 (t, 2JPC = 21.6, 1C, C1), 129.48 (s, 1C, Cpara), 169.32 (pst, 3 JPC = 10.0, 2C, C2/C6). 31P{1H} NMR (δ, 25 °C, 162 M, C6D6): 184.91 (s). Anal. Calcd for C18H31O2P2N3Ni (442.10): C, 48.90; H, 7.07; N, 9.50. Found: C, 49.06; H, 7.17; N, 9.06. Synthesis of (POCsp3OP)NiN3 (2-N3). A procedure similar to that described above for the synthesis of 1-N3 was employed using 2OSiMe3 (0.14g, 0.3 mmol). The target complex was obtained in 92% yield (0.11 g). 1H NMR (δ, 25 °C, 400 MHz, C6D6): 1.11 (dtv, 3JHH = 6.2 and vJHP = 7.0, 6H, PCH(CH3)2), 1.14 (dtv, 3JHH = 7.2 and vJHP = 7.0, 6H, PCH(CH3)2), 1.29 (dtv, 3JHH = 10.1 and vJHP = 7.3, 6H, PCH(CH3)2), 1.42 (dtv, 3JHH = 9.7 and vJHP = 7.3, 6H, PCH(CH3)2), 1.91 (m, 3JHH = 7.1, vJHP = 2.4, 2H, PCH(CH3)2)), 2.12 (sept, 3JHH = 7.0, 2H, PCH(CH3)2), 2.63 (m, 1H, CH2CHCH2), 3.11 (dd, 3JHH = 11.8, 3JHH = 9.3, 2H, CH2CHCH2), 3.27−3.40 (m, 2H, CH2CHCH2). 13 C{1H } NMR (δ, 25 °C, 100 MHz, C6D6): 16.13 (s, 2C, PCH(CH3)2), 17.00 (s, 2C, PCH(CH3)2), 17.19 (pst, 2JPC = 3.8, 2C, PCH(CH3)2), 18.22 (pst, 2JPC = 3.2, 2C, PCH(CH3)2), 27.88 (pst, JPC = 12.5, 2C, PCH(CH3)2), 28.73 (pst, JPC = 10.3, 2C, PCH(CH3)2), 49.71 (t, 2JPC = 11.6, 1C, C1), 76.28 (pst, 3JPC = 7.2, 2C CH2CHCH2). 31 1 P{ H} NMR (δ, 25 °C, 162 MHz, C6D6): 184.14 (s). Anal. Calcd for C15H33N3NiO2P2 (408.08): C, 44.15; H, 8.15; N, 10.30. Found: C, 44.13; H, 8.23; N, 10.47. Synthesis of (POCsp3OP)NiCF3 (2-CF3). To a solution of 2OSiMe3 (0.14g, 0.3 mmol) in toluene (15 mL) was added dropwise Me3SiCF3 (45 mL of a 0.2 M solution in THF, 0.9 mmol) at room temperature. The solution was stirred for 3 days until 31P NMR confirmed nearly complete conversion. Following the general workup procedure described above gave an orange solution, which was stored at −40 °C for 3 days to afford orange crystals of 2-CF3 (0.11 g, 36%). 1 H NMR (δ, 25 °C, 400 MHz, C6D6): 1.12 (dtv, 3JHH = 6.3 and vJHP = 6.7, 6H, PCH(CH3)2), 1.15 (dtv, 3JHH = 7.0 and vJHP = 7.0, 6H, PCH(CH3)2), 1.28 (dtv, 3JHH = 9.3 and vJHP = 7.5, 6H, PCH(CH3)2), 1.35 (dtv, 3JHH = 9.2 and vJHP = 7.5, 6H, PCH(CH3)2), 2.02 (sept, 3JHH = 7.3, 2H, PCH(CH3)2)), 2.17 (sept, 3JHH = 7.2, 2H, PCH(CH3)2), 2.54−2.64 (m, 1H, CH2CHCH2), 3.26 (dd, 3JHH = 11.9, 3JHH = 9.0, 2H, CH2CHCH2), 3.65−3.78 (m, 2H, CH2CHCH2). 13C{1H} NMR (δ, 25 °C, 100 M, C6D6): 16.42 (s, 2C, PCH(CH3)2), 16.96 (s, 2C, PCH(CH3)2), 18.05 (s, 2C, PCH(CH3)2), 18.64 (s, 2C, PCH(CH3)2), 28.96 (pst, JPC = 13.6, 2C, PCH(CH3)2), 30.17 (pst, JPC = 11.6, 2C, PCH(CH3)2), 56.98 (t, 2JPC = 10.8, 1C, C1), 75.18 (pst, 3JPC = 6.5, 2C CH2CHCH2). 31P{1H} NMR (δ, 25 °C, 162 M, C6D6): 194.12 (quartet, 3JFP= 14.1). 19F{1H} NMR (δ, 25 °C, 376 M, C6D6): −6.2 (triplet, 3JFP = 14.1). Anal. Calcd for C16H33F3NiO2P2 (435.07): C, 44.17; H, 7.65. Found: C, 44.32; H, 7.35. X-ray Crystallography. Single crystals of 1-OSiMe3, 1-OSiPh3, 2OSiPh3, 1-OMes, 1-N3, 2-N3, and 2-CF3 suitable for X-ray diffraction were obtained by keeping nearly saturated hexane solutions at −40 °C. The crystallographic data for complexes 1-OSiMe3, 1-OSiPh3, 2OSiPh3, and 1-N3 were collected on a Bruker APEX II equipped with an Incoatec I\muS Microsource and a Quazar MX monochromator. The crystallographic data for complexes 1-OMes, 2-N3, and 2-CF3 were collected on a Bruker microstar diffractometer equipped with a Platinum 135 CCD Detector, Helios optics, and a Kappa goniometer. Cell refinement and data reduction were done using SAINT.36 An empirical absorption correction, based on the multiple measurements of equivalent reflections, was applied using the program SADABS.37 The space group was confirmed by the XPREP routine38 in the program SHELXTL.39 The structures were solved by direct methods and refined by full-matrix least squares and difference Fourier techniques with SHELX-97.40 All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms were set in calculated positions and refined as riding atoms with a common thermal parameter.

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ASSOCIATED CONTENT

S Supporting Information *

Text, a table, figures, and CIF files giving details of the X-ray structure determination, crystal data and collection/refinement parameters, NMR spectra, and GC/MS traces. This material is available free of charge via the Internet at http://pubs.acs.org. Complete details of the X-ray analyses reported herein have been deposited at The Cambridge Crystallographic Data Centre (CCDC 1022823 (1-OSiMe3), 1022824 (1-OMes), 1022825 (1-OSiPh3), 1022826 (2-OSiPh3), 1022827 (1-N3), 1022828 (2-N3) and 1022829 (2-CF3)). These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, U.K. (fax +44 1223 336033).

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AUTHOR INFORMATION

Corresponding Author

*E-mail for D.Z.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The authors are grateful to the NSERC of Canada, FRQNT of Québec, and Université de Montréal for financial support of this study.

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POCOP-Ligated Nickel Siloxide Complexes: Syntheses

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