Palladium(II) Complexes with N-Phosphine Oxide-Substituted

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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Palladium(II) Complexes with N‑Phosphine Oxide-Substituted Imidazolylidenes (PoxIms): Coordination Chemistry and Catalysis Lorenzo Branzi,† Dario Franco,† Marco Baron,† Lidia Armelao,†,‡ Marzio Rancan,‡ Paolo Sgarbossa,§ and Andrea Biffis*,† †

Dipartimento di Scienze Chimiche, Università degli Studi di Padova, Via F. Marzolo 1, 35131 Padova, Italy ICMATE-CNR, Via F. Marzolo 1, 35131 Padova, Italy § Dipartimento di Ingegneria Industriale, Università degli Studi di Padova, Via F. Marzolo 9, 35131 Padova, Italy

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‡

S Supporting Information *

ABSTRACT: The coordination chemistry of N-phosphine oxide-substituted imidazolylidenes (PoxIms) as heteroditopic, hemilabile ligands toward palladium(II) has been investigated. Both bis-carbene and mono-carbene complexes can be prepared, but the latter are accessible only in the presence of ancillary ligands that do not easily dissociate from the metal center. The phosphanyl oxide group was found to often coordinate to palladium(II) as well, though this interaction appears to be weak as this group is indeed easily replaced by, e.g., a solvent molecule. The resulting complexes have been tested as catalysts in the low-temperature Suzuki reaction of 4-chloroanisole with phenylboronic acid and they show promise for further optimization studies.

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INTRODUCTION

Chart 1. Typical Structures of NHCP, ADCP, and PoxIm Ditopic Ligands

The 21st century has witnessed great advances in the chemistry of N-heterocyclic carbenes (NHCs) and their metal complexes.1,2 Such an effort is justified by the unique features of NHCs, by the rich variety of structures that are potentially accessible, by the recognition that structural changes markedly influence the steric and electronic properties of the NHC, and by the application possibilities of NHCs and especially of their metal complexes, which nowadays extend from catalysis to bioinorganic chemistry, materials science, and beyond.3 Among the several classes of NHCs and immediate precursors thereof that are commercially available or readily accessible by well-established synthetic routes, those linked to another moiety able to act as a ligand are particularly interesting from the point of view of their coordination chemistry. Indeed, the NHC and the other ligand will interact differently with a metal center, which will impact on the stability of the resulting complex, on its stereochemical properties as well on its reactivity; in particular, the second ligand function can be selected to make its coordination to the metal center easily reversible, in which case a so-called “hemilabile” bidentate ligand is obtained.4 In the course of the last two decades, several novel heteroditopic ligands and ligand precursors of this kind have been thoroughly studied.5 In this context, we and others have recently developed N-phosphanyl-N-heterocyclic carbenes (NHCPs),6,7 as well as N-phosphanyl acyclic diaminocarbenes (ADCPs)8 as a novel class of bidentate ligands at carbon and phosphorus (Chart 1). The coordination chemistry of these compounds as bridging or chelating ligands has been explored, © XXXX American Chemical Society

particularly with group 10 and 11 metals, and application possibilities have been preliminarily screened. In 2015, the group of Hoshimoto reported on yet another class of functional NHCs featuring an N−P bond, namely, Nphosphine oxide-substituted imidazolylidenes, which were termed PoxIms (Chart 1).9 Such carbenes appear particularly useful as ligands, since the two coordination sites (the carbene carbon and the oxide) possess widely different properties, e.g., the former being a soft donor (with some acceptor character as well), the latter a hard donor. Interestingly, to the best of our knowledge, there are just two examples on the use of PoxIms as ligands, i.e., one by the group of Nozaki concerning PoxIm chelate coordination to palladium(II)10 and the other by the group of Hoshimoto concerning weak complexes with alkaline metal ions in which PoxIms act as monodentate ligands through the phosphanyl oxide group.9d We have started a systematic work on the evaluation of the coordination chemistry of these ligands, and in the context of this Received: March 19, 2019

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DOI: 10.1021/acs.organomet.9b00185 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

preisolated and characterized free carbene ligand did not alter the reaction outcome. We speculate that the initially formed 1:1 neutral complex has the phosphanyl oxide moiety only weakly coordinated to palladium(II). The steric bulk of the NHC prevents coordination of a second carbene moiety in the cis position, but on the other hand, it labilizes the bond to the chloride in the trans position, which is displaced by the second NHC ligand. This substitution generates a positive charge on the complex, which strengthens the interaction of the metal with one of the phosphanyl oxide moieties. It is useful to remark that previously reported ditopic phosphane/phosphane oxide ligands12 with a similar structure behave analogously: for example, bis-(diphenylphosphano)amine monoxide was also reported to form a 2:1 complex under similar conditions, in which however both ligands are monocoordinated through the P(III) atom in a cis fashion and do not tend to chelate by substituting the chlorides with the phosphanyl oxide groups, as in the present case.13 On the other hand, o-phenylene-bis(diphenylphosphano) monoxide was reported to produce, beside the 1:1 chelate complex as main product, a cationic 2:1 complex with the same connectivity as 7 and 8 as minor product.14 Evidence for the 2:1 stoichiometry of 7 and 8 is obtained via the ESI-MS characterization, and in the case of 7 also upon determination of the crystal structure of the compound, which is displayed in Figure 1; selected bond distances and angles are reported in Table S3.

contribution, we will consider palladium(II) as the metal center.

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RESULTS AND DISCUSSION We have included in our study carbene precursors 1, 2, and 5, previously reported by Hoshimoto (Scheme 1).9 The Scheme 1. Synthesis of Ligands 3, 4, and 6

preparation of these precursors is straightforward and is accomplished by oxidation with hydrogen peroxide of the known parent N-phosphanyl imidazolium or N,N′-diphosphanyl-imidazolium salt to yield 1, 2, and 5 as air- and moisture-stable compounds. Subsequent deprotonation of the precursors at C-2 with KHDMS furnishes the free carbenes 3, 4, and 6, which can be isolated and characterized. In the preparation of 6, we found that starting from N-trimethylsilylimidazole was advantageous compared to simple imidazole, which was employed by Hoshimoto, as in our hands, the first reaction step provided higher yields with the former substrate.11 In the frame of the present investigation, we synthesized ligands 3, 4, and 6 in situ and immediately reacted them with 1 equiv of [PdCl2(BN)2] (BN = benzonitrile). Remarkably, this synthetic procedure, which is identical to that employed for the preparation of N-phosphanyl carbene complexes from the corresponding protonated ligand precursors,6,7b,8 did not produce the expected 1:1 complexes when applied to ligands 3 and 4, but instead the trans-2:1 complexes 7 and 8 (Scheme 2). Changing the ligand to metal ratio as well as the reaction conditions (solvent, reaction time) or starting with a

Figure 1. ORTEP drawing of complex 7. Ellipsoids are drawn at the 50% probability level. Hydrogen atoms and the triflate anion have been omitted for clarity. Selected bond distances (Å) and angles (deg) are reported in Table S3.

Scheme 2. Synthesis of Pd Complexes 7−10

Complex 7 crystallizes in the monoclinic space group P21/n; the asymmetric unit contains one complex molecule and a triflate anion. The Pd(II) metal center is in a slightly distorted square planar coordination environment, as expected for a second row transition metal in a d8 electronic configuration. This coordination geometry is adopted by all the Pd(II) complexes structurally characterized in this study (9, 9b, 10, and 11, vide infra). It is useful to highlight the different coordination modes of the two ligands in complex 7. One ligand coordinates as a C,O chelate with a Pd−O distance of 2.107(4) Å. Its bond distances and angles compare well with those reported by Nozaki for the same ligand, but with a 2,6B


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Organometallics diisopropylphenyl instead of a mesityl group at the imidazolylidene wingtip position, coordinated to a Pd(II) metal center.10 The phosphanyl oxide group of the second ligand is not interacting as strongly with the Pd center, but nevertheless points toward the free apical coordination site of the metal and is found at 2.814(4) Å, that is, at a distance much greater than that of the other oxygen atom, but still significantly shorter than the sum of the van der Waals radii of Pd and O (3.15 Å). Thus, there seems to be an interaction also between the second phosphanyl oxide group and the Pd(II) center. From a structural point of view, the different coordination mode of the ligands (chelating bidentate or monodentate) influences also the dihedral angle between the mean plane of the NHC donor and the Pd(II) coordination plane. In fact, these planes are almost coplanar, in the case of the chelating, or perpendicular, in the case of the monodentate ligand with dihedral angles of 14.6(2)° and 90.3(2)°, respectively. The second phosphanyl oxide group appears ideally positioned to substitute the first one by an associative mechanism. Indeed, complexes 7 and 8 in acetonitrile solution are fluxional: whereas a single, sharp resonance for the phosphanyl oxide groups is present in the 31P NMR spectrum of both compounds, the t-butyl doublet in the 1H NMR spectrum appears broadened in 7 and split into two signals in 8. In the latter case, the signals of the methyl groups of the two 2,6-diisopropylphenyl moieties are also different, half of the protons appearing as a doublet at 0.97 ppm, the other half split into two doublets at 1.06 and 1.30 ppm. Finally, most signals in the 13C NMR spectrum of 8 appear split as well. All of these spectral features point toward a much slower interconversion between the two forms in complex 8, which might be due to the higher steric bulk of the 2,6-diisopropylphenyl substituent. Removal of the last chloride ligand in the coordination sphere of complex 7 by reaction with AgOTf generates the corresponding dicationic palladium(II) complex 9, in which both phosphine oxide moieties should coordinate to Pd. In practice, this complex exhibits fluxional behavior in acetonitrile solution as well, and in this case, it is the 31P NMR spectrum that exhibits a single, but significantly, broadened signal (quite unexpectedly, the signal of the t-butyl groups in the 1H NMR spectrum appears instead as a sharp doublet). Thus, it seems that the Pd−O coordinative bond is rather weak also in this case, despite the +2 positive charge on the metal, and indeed crystallization of the complex produces crystals of two different kinds (Figure 2), which contain, on the one hand, the expected trans-dicationic complex with two chelating C,O ligands (9, Pd−O bond distance 2.032(2) Å) and, on the other hand, another dicationic complex in which one of the phosphane oxide moieties has been substituted by an acetonitrile molecule in the coordination sphere of the metal center (9b). In the latter structure, the displaced phosphanyl oxide group still points toward the free apical coordination site of the metal and is found at 2.698(4) Å, whereas the other phosphanyl oxide moiety is obviously much closer to the metal (2.066(3) Å). Thus, this structure appears very similar to that of 7, although both Pd−O distances are shorter due to the higher charge on the metal center. Selected bond distances and angles for 9 and 9b are reported in Table S3. Complex 9 crystallizes in the triclinic space group P1̅. The asymmetric unit contains half of the complex molecule and a triflate anion; the other half is generated through an inversion center located at the Pd atom. Complex 9b crystallizes in the monoclinic space group P21/c;

Figure 2. ORTEP drawings of complexes 9 (above) and 9b (below). Ellipsoids are drawn at the 50% probability level. Hydrogen atoms, triflate anions, and solvent molecules have been omitted for clarity. Selected bond distances (Å) and angles (deg) are reported in Table S3. Symmetry code for 9: ′ = −x, 1 − y, 1 − z.

the asymmetric unit contains one complex molecule and a triflate anion. The second anion was identified, but could not be satisfactorily modeled, even using several restraints and constraints. This contribution to the final difference density Fourier map was treated with the masking routine implemented in OLEX2 (see the Supporting Information, SI). In contrast to the results obtained with ligands 3 and 4, ligand 6 reacted with [PdCl2(BN)2] cleanly, producing the expected 1:1 complex 10 as a dimer with bridging chloride ligands. NMR analysis in CD3CN evidenced the highly symmetric, nonfluxional nature of the compound, and its structure was confirmed both by ESI-MS analysis and by determination of the crystal structure by single crystal X-ray diffraction studies (Figure 3). Complex 10 crystallizes in the monoclinic space group C2/m. The structure is highly symmetric; the asymmetric unit contains indeed only a quarter of the dinuclear unit. The rest of the molecule is generated by three symmetry operations, namely, a reflection through a mirror plane coincident with the Pd coordination plane, an inversion through an inversion center located at center of the Pd1-Cl2-Pd1′-Cl2′ metallacycle, and a rotation through a 2fold axis passing through the inversion center and running perpendicularly with respect the Pd coordination plane. Selected bond distances and angles are reported in Table S3. It can be appreciated that the trans influence of the NHC ligand causes an elongation of the Pd−Cl bond trans to it. Remarkably, the phosphanyl oxide groups, although not being strongly coordinated to Pd, interact nevertheless weakly with C


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Organometallics

attempts yielded either products of ligand decomposition or complex 7 as exclusive organometallic product. Consequently, we reasoned that use of a palladium(II) allyl chloride dimer as precursor should enable the production of 1:1 products, due to the fact that the allyl moiety is difficult to displace. Indeed, this proved to be the case. Reaction of the compound with ligands 3 and 4 breaks up the dimer with coordination of the carbene moiety (Scheme 3). However, quite unexpectedly, the oxide Scheme 3. Synthesis of Pd Complexes 11 and 12

Figure 3. ORTEP drawing of complex 10. Ellipsoids are drawn at the 50% probability level. Hydrogen atoms have been omitted for clarity. Selected bond distances (Å) and angles are reported in Table S3. Symmetry codes: ′ = 1 − x, −y, −z; ′′ = 1 − x, +y, −z; ′′′ = +x, −y, +z.

coordinates to the metal as well and displaces the chlorido ligand, thus furnishing monocationic products 11 and 12. This is in contrast to the known chemistry of diphosphane monoxides and diphosphanoamine monoxides, which are known to form P,O chelate complexes upon reaction with the same Pd precursor only after chloride removal with a silver salt.17 The structures of these compounds were again confirmed by ESI-MS and by a crystal structure of compound 11. The molecular structure of complex 11 is reported in Figure 4;

the metal center pointing toward the apical position of the complex (Pd−O distance 2.880(5) Å). The Pd−O distance is however longer than in complexes 7, 9, and 9b due to the neutral nature of 10. On the contrary, complex 10 is the one that presents the shortest Ccarbene−Pd bond distance (1.952(8) Å). Having no chelating ligands, complex 10 is also the one that deviates less from the ideal square planar coordination geometry, with all cis L−Pd−L angles (L = Ccarbene or Cl) in the range 88−93°. NMR characterization of 10 suggests that such conformation of the phosphanyl oxide groups with respect to Pd is maintained also in solution. In particular, the 1 H NMR spectrum presents the features of a symmetric, nonfluxional species and both the chemical shift of the imidazole protons (7.41 ppm) and their small coupling constant with P are compatible with a conformation in which the oxygens of the phosphanyl oxide groups all point toward the metal center. Indeed, in studies with gold(I) complexes of PoxIm ligands, which will be reported separately, we have found that an anti-conformation of the phosphanyl oxide group with respect to the metal causes the oxide to engage in an intramolecular hydrogen bond with an imidazole C-H, which in turns causes a much higher deshielding of the C-H proton and a larger coupling constant between proton and phosphorus.15 The different outcome in the reaction of 6 with [PdCl2(BN)2] compared with the reaction with 3 or 4 could be the results of steric and electronic differences between these ligands: ligand 6 is expected to be less electron-donating (due to the electron-withdrawing character of the two phosphanyl oxide groups); hence it should labilize to a lower extent the chlorido ligand trans to it for substitution with another carbene moiety, while, at the same time, preventing substitution of the benzonitrile moiety for steric reasons. Eventually, the benzonitrile moieties are nevertheless lost with formation of the chlorido bridged dimer. Since we had an interest in preparing 1:1 complexes also in the case of ligands 3 and 4, we thought about modifying the palladium precursor or the reaction conditions in order to achieve this goal. We considered first the production of PEPPSI-type complexes of general structure trans-[(3)(3chloropyridine)PdCl2], which are known to be easily formed by contacting free 3 (either preformed10 or generated in situ by reaction with an inorganic base such as K2CO316) with PdCl2 in the presence of 3-chloropyridine. Unfortunately, these

Figure 4. ORTEP drawing of one of the three crystallographic independent forms of complex 11. Ellipsoids are drawn at the 50% probability level. For clarity, only one of three crystallographic independent complexes is shown. Hydrogen atoms and the triflate anion have been omitted. Selected bond distances (Å) and angles (deg) are reported in Table S3.

selected bond distances and angles are reported in Table S3. The complex crystallizes in the monoclinic space group P21; three crystallographically independent cations together with three triflate anions are present in the crystal (Figure S5 in the SI). The three complex molecules do not present significant differences in their structural parameters. The complex exhibits a true coordinative bond between the metal and the oxide, with a Pd−O distance of only 2.163(5) Å. Again, the NMR spectral features of the compounds allow us to assume that the phosphanyl oxide group remains coordinated or at least oriented toward the metal also in solution, as the chemical shift difference between the C-H protons in the 4 and 5 positions in the imidazole ring and the coupling constant with phosphorus are small. The complexes are nevertheless fluxional, in that the allyl group undergoes the well-known η3-η1-η3 equilibrium. This equilibrium is however slow on the NMR time scale at room temperature in CD3CN and becomes appreciable only at 60 °C, where broadening of the signals of the terminal protons as well as simplification of D


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Organometallics the multiplicity of the signal of the meso proton to the expected quintet is observed (Figure 5).

Table 1. Suzuki Couplings Catalyzed by Pd Complexes Described in This Studya

entry

cat.

solvent

T (°C)

yield 2 h (%)

yield 22 h (%)

1 2 3b 4 5 6 7 8 9

11 11 11 11 12 7 8 7 8

EtOH EtOH EtOH EtOH/H2O EtOH/H2O EtOH/H2O EtOH/H2O EtOH EtOH

25 40 25 40 40 40 40 40 40

traces traces 25 32 traces 9 5 29 19

14 17 29 39 traces 32 22 58 46

a

Reactions conditions: 0.60 mmol of 4-chloroanisole, 0.66 mmol of phenylboronic acid, 1.2 mmol of K2CO3, 6.0 μmol of catalyst. b Reaction performed with KOtBu as the base Figure 5. Allylic portion of the 1H NMR spectrum of complex 11 in CD3CN and its temperature dependence.

hydroxide anions together with K2CO3 employed as base.22 Under these conditions, particularly upon water addition, complex 11 exhibits good initial activity, although the catalytic system apparently deactivates with time. Quite surprisingly, complex 12 turned out to be instead rather inactive (entry 5). We also briefly examined bis-carbene complexes 7 and 8, which exhibit lower initial activity (lower yields after 2 h). The yield however improves at longer reaction times, suggesting in this case the generation of a more robust, catalytically competent species. The obtained catalytic results are quite comparable to the best results previously obtained by us with NHCP-palladium(II) complexes (42% yield with 1 mol % catalyst in 5 h at 28 °C),23 and somewhat worse than those obtained with other palladium(II) complexes containing both NHC ligands and simple allyl groups (up to 70−80% yield with 0.3 mol % catalyst in 4 h), which are however generally employed at higher temperature (80 °C).18a,24 Remarkably, it has been demonstrated that significantly higher activities can be reached with complexes bearing substituted allyl groups,18b as in this case, the formation of catalytically inactive, dimeric Pd(I) complexes by comproportionation of Pd(0) and Pd(II) complexes can be suppressed or at least drastically reduced.20b We are going to investigate this possibility in more detail in order to develop a second generation of more active complexes.

In Figure 5, the strong chemical shift difference in the syn terminal protons of the allyl ligand is also evident, which is mirrored also in the signals of the anti protons (which are covered by other signals in CD3CN but are well discernible in CDCl3) and especially in the signals of the terminal CH2 groups (at ca. 49 and 74 ppm) in the 13C NMR spectrum. This is clearly the consequence of the strongly different trans influence of the carbene and the phosphanyl oxide groups. The structure of complexes 11 and 12 resembles the structure of [(NHC)Pd(allyl)Cl] complexes of type 13 developed by the group of Nolan, which currently rank among the most efficient precatalysts for C−C coupling reactions.18 This prompted us to investigate the catalytic performance of our complexes as well. Consequently, we set out to preliminarly evaluate the activity of the complexes in a standard Suzuki coupling reaction of a deactivated aryl chloride such as 4-chloroanisole with phenylboronic acid. The low reactivity of aryl chlorides substituted by an electron-donating group such as methoxide allows a more precise evaluation of the catalytic performance of the complexes at low temperature (up to 40 °C), since potential Pd-containing decomposition products (e.g., Pd colloids) do not exhibit significant activity under these conditions.19 The results are reported in Table 1. We screened different reaction conditions in order to individuate the best pathway for the activation of the precatalyst, i.e., Pd reduction to the zerovalent state. This can be accomplished (a) by the ethanol solvent, which can conveniently act as a reducing agent toward Pd(II);20 (b) by the phenylboronic acid reagent, which can transfer the phenyl group to palladium(II) by transmetalation with subsequent reductive elimination;21 and (c) in the case of the allyl complexes 11 and 12, by the employed base, which nucleophilically attacks the allyl group directly or with prior coordination to Pd(II) in a reductive elimination process.18 The results obtained with complex 11 show that activation of the catalyst is very slow in anhydrous ethanol, even at 40 °C (entries 1 and 2), which can be taken as an indication that ethanol reduction of the complex is slow and inefficient. On the other hand, catalyst activation can be accelerated by using a stronger base under anhydrous conditions, such as potassium tbutoxide, or by addition of water, which expectedly forms

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CONCLUSIONS In this paper, we have carried out the first comprehensive study on the coordination chemistry of N-phosphine oxidesubstituted imidazolylidenes (PoxIms) ligands with Pd(II). We have demonstrated that complexes with both 2:1 and 1:1 PoxIm:Pd stoichiometries are accessible and that PoxIms tend to behave as chelate ligands toward Pd(II), though the Pd−O coordinative bond is not very strong and can be easily displaced by solvent molecules and presumably also by reagents in solution; this feature renders these complexes potentially useful for catalytic applications. Indeed, the complexes do catalyze the Suzuki reaction of a deactivated aryl chloride such as 4-chloroanisole at room temperature. Work currently in progress aims at optimizing the other ligands present in these complexes for the catalysis of Suzuki couplings as well as of other reactions and at extending the study of the E


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Organometallics

Synthesis of Complex 9. To a stirred solution of complex 7 (0.101 g, 0.10 mmol) in acetonitrile (5 mL) was added AgOTf (0.0264 g, 0.10 mmol) in one portion. The reaction mixture was allowed to stir for 30 min and then filtered. The resulting clear solution was evaporated to dryness and treated with diethyl ether to yield the product as an orange yellow solid. The product could be recrystallized by slow diffusion of diethyl ether into an acetonitrile solution. Yield 0.063 g (59%). 1H NMR (CD3CN): δ = 1.33 (d, JHP = 16.4 Hz, 36H, tBu), 2.04 (s, 12H, CH3), 2.34 (s, 6H, CH3), 7.13 (s, 4H, CHaryl). 7.45 (s, 2H, CHim), 7.78 (s, 2H, CHim), 13C NMR (CD3CN): δ = 17.0 (s, CH3), 20.2 (s, CH3), 25.0 (s, CH3), 38.0 (d, JCP = 59 Hz, C), 123.2 (d, JCP = 3.8 Hz, CH), 127.4 (d, JCP = 3.8 Hz, CH), 129.6 (s, CH), 135.2 (s, C), 140.8 (s, C), carbene carbon not detected. 31P NMR (CD3CN): δ = 94.7. ESI-MS (m/z): 947.07 ([M − OTf]+, expected 947.29). Elemental analysis calcd. (%) for C42H62F6N4O8P2PdS2 (1097.43): C 45.97, H 5.69, N 5.11, S 5.84; found: C 45.71, H 5.49, N 5.51, S 0.5.60. Synthesis of Complex 10. To a stirred solution of precursor 5 (0.0505 g, 0.0937 mmol) in THF (10 mL) was added potassium hexamethyldisilazide (0.0280 g, 0.140 mmol) in one portion. The reaction mixture was allowed to stir for 30 min, and then [PdCl2(BN)2] (0.0370 g, 0.0965 mmol) was added. The reaction mixture was left under stirring for 3 h, after which it was filtered over Celite and evaporated to dryness. The residue was washed with diethyl ether (3 × 5 mL). Yield 0.037 g (75%). 1H NMR (CD3CN): δ = 1.36 (d, JHP = 15.3 Hz, 72H, tBu), 7.44 (m, 4H, Him), 31P NMR (CD3CN): δ = 65.01. 13C NMR (CD3CN): δ = 27.7 (s, CH3). 39.1 (d, JCP = 65.1 Hz, C), 124.8 (m, CH), 164.0 (s, NCN; detected as cross-peak in a 13C-HMBC experiment). ESI-MS (m/z): 1095.01 ([M − Cl]+, expected 1095.20), 1170.83 ([M + K]+, expected 1171.13). Elemental analysis calcd. (%) for C38H76N4P4O4Cl4Pd2 (1131.55): C 40.33, H 6.77, N 4.95; found: C 40.59, H 6.54, N 5.02. Synthesis of Complex 11. To a stirred solution of precursor 1 (0.101 g, 0.203 mmol) in THF (10 mL) was added potassium hexamethyldisilazide (0.0442 g, 0.222 mmol) in one portion. The reaction mixture was allowed to stir for 30 min, and then [(C3H5)PdCl]2 (0.0368 g, 0.101 mmol) was added. The reaction mixture was left under stirring for 6 h, after which it was filtered over Celite and evaporated to dryness. The residue was washed with diethyl ether (3 × 5 mL). Yield 0.114 g (88%). 1H NMR (CD3CN): δ = 1.40 (d, JHP = 16.1 Hz, 18H, tBu), 1.96 (m, 1H, CH2anti), 2.29 (m, 7H, CH3 and CH2syn), 2.35 (s, 3H, CH3), 3.27 (d, JHH = 13.8 Hz, 1H, CH2anti), 4.25 (d, JHH = 7.6 Hz, 1H, CH2syn), 5.29−5.16 (m, 1H, CHmeso), 7.08 (s, 2H, Haryl), 7.47 (m, 1H, Him), 7.75 (m, 1H, Him). 13 C NMR (CD3CN): δ = 17.7 (s, CH3). 21.2 (s, CH3), 26.2 (s, CH3), 38.4 (d, JCP = 63 Hz, C), 49.3 (s, CH2), 73.3 (s, CH2), 117.7 (s, CH), 123.4 (d, JCP = 4.1 Hz, Cim), 126.8 (d, JCP = 2.9 Hz, C), 130.0 (s, CH), 135.7 (s, C) 136.8 (s, C), 141.0 (s, C), 188.7 (d, JCP = 12.8 Hz, NCN), 31P NMR (CD3CN): δ = 83.66. ESI-MS (m/z): 493.05 ([MOTf]+, expected 493.16). Elemental analysis calcd. (%) for C24H36N2PO4SF3Pd (642.99): C 44.95, H 5.64, N 4.36, S, 4.99; found: C 45.07, H 5.90, N 4.19, S 4.69. Synthesis of Complex 12. To a stirred solution of precursor 2 (0.0799 g, 0.161 mmol) in THF (10 mL) was added potassium hexamethyldisilazide (0.0355 g, 0.1780 mmol) in one portion. The reaction mixture was allowed to stir for 30 min, and then [(C3H5)PdCl]2 (0.0295 g, 0.0806 mmol) was added. The reaction mixture was left under stirring for 4 h, after which it was filtered over Celite and evaporated to dryness. The residue was extracted with 3 mL of dichloromethane. The dichloromethane extracts were evaporated to dryness, and the solid residue was washed with diethyl ether (3 × 5 mL). Yield 0.099 g (90%). 1H NMR (CD3CN): δ = 1.13 (d, JHH = 7.0 Hz, 12H, iPr), 1.41 (d, JHP = 16.2 Hz, 18H, tBu), 2.19− 2.23 (m, 2H, CH2anti and CH2syn), 2.38 (br s, 2H, CH), 3.37 (d, JHH = 13.6 Hz, 1H, CH2anti), 4.29 (d, JHH = 7.6 Hz, 1H, CH2syn), 5.30−5.17 (m, 1H, CHmeso), 7.40 (d, JHH = 7.7 Hz, 2H, CHaryl), 7.59 (t, JHH = 7.7 Hz, 1H, CHaryl), 7.65 (m, 1H, CHim), 7.75 (m, 1H, CHim). 13C NMR (CD3CN): δ = 23.6 (s, CH3), 24.4 (s, CH3) 25.9 (s, CH3), 29.4 (s, CH), 38.1 (d, JCP = 61.7 Hz, C), 49.4 (s, CH2), 74.1 (s, CH2), 117.6 (s, CH2), 122.7 (d, JCP = 4.0 Hz, CH), 125.2 (s, CH), 128.0 (d,

coordination chemistry of PoxIm ligands to other metal centers.

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EXPERIMENTAL SECTION

All manipulations of air- and moisture-sensitive compounds were carried out in a glovebox or using standard Schlenk techniques under an atmosphere of argon or dinitrogen. The reagents were purchased by Aldrich as high-purity products and generally used as received. All solvents were purified and dried by standard methods. Ligand precursors 1, 2, and 59 as well as starting complex [PdCl2(BN)2]25 were prepared according to standard procedures. NMR spectra were recorded on Bruker Avance spectrometers working at 300 MHz (300.1 MHz for 1H, 75.5 MHz for 13C, 282.2 MHz for 19F, and 121.5 MHz for 31P); chemical shifts (δ) are reported in units of ppm relative to the residual solvent signals and to external 85% H3PO4 (for 31P). ESI-MS analyses were performed using a LCQ-Duo (ThermoFinnigan) operating in positive ion mode. Instrumental parameters: capillary voltage 10 V; spray voltage 4.5 kV; capillary temperature 200 °C; mass scan range from 150 to 2000 amu; N2 was used as sheath gas; the He pressure inside the trap was kept constant. The pressure directly read by an ion gauge (in the absence of the N2 stream) was 1.33 × 10−5 Torr. Sample solutions, prepared by dissolving the compounds in acetonitrile, were directly infused into the ESI source by a syringe pump at 8 μL/min flow rate. Elemental analyses were carried out with a Fisons EA 1108 CHNS-O apparatus or with a Carlo Erba analyzer. Synthesis of Complex 7. To a stirred solution of precursor 1 (0.1030 g, 0.207 mmol) in THF (10 mL) was added potassium hexamethyldisilazide (0.0494 g, 0.248 mmol) in one portion. The reaction mixture was allowed to stir for 15 min, and then [PdCl2(BN)2] (0.0392 g, 0.103 mmol) was added. The reaction mixture was left under stirring for 16 h, after which it was evaporated to dryness. The residue was washed with diethyl ether (3 × 5 mL) and then extracted with dichloromethane (10 mL); undissolved salts was filtered off and discarded, whereas the dichloromethane solution was evaporated to dryness to yield the product as an orange yellow solid. The product could be recrystallized by slow diffusion of diethyl ether into an acetonitrile solution. Yield 0.073 g (74%). 1H NMR (CD3CN): δ = 1.42 (br s, 36H, tBu), 2.19 (s, 12H, CH3), 2.33 (s, 6H, CH3), 6.98 (s, 4H, CHim). 7.20 (m, 2H, CHim), 7.57 (m, 2H, CHim), 13 C NMR (CD3CN): δ = 18.3 (s, CH3), 21.1 (s, CH3), 26.4 (s, CH3), 38.7 (d, JCP = 68 Hz, C), 124.2 (s, CH), 127.3 (s, CH), 129.6 (s, CH), 129.9 (s, C), 136.2 (s, C), 140,1 (s, C), carbene carbon not detected. 31P NMR (CD3CN): δ = 83.8. ESI-MS (m/z): 833.23 ([M − OTf]+, expected 833.31). Elemental analysis calcd. (%) for C41H62ClF3N4O5P2PdS (983.82): C 50.05, H 6.35, N 5.69, S 3.26; found: C, 49.71; H, 6.49; N, 5.51; S 3.11. Synthesis of Complex 8. To a stirred solution of precursor 2 (0.1001 g, 0.186 mmol) in THF (10 mL) was added potassium hexamethyldisilazide (0.0501 g, 0.251 mmol) in one portion. The reaction mixture was allowed to stir for 30 min, and then [PdCl2(BN)2] (0.0354 g, 0.093 mmol) was added. The reaction mixture was left under stirring for 16 h, after which it was filtered over Celite and evaporated to dryness. The residue was washed with diethyl ether (3 × 5 mL). Yield 0.064 g (64%). 1H NMR (300 MHz, CD3CN): δ = 0.97 (d, JHH = 6.8 Hz, 12H, iPr), 1.06 (d, JHH = 6.8 Hz, 6H, iPr), 1.27 (d, JHP = 15.9 Hz, 18H, tBu), 1.30 (d, JHH = 6.8 Hz, 12H, iPr), 1.38 (d, JHP = 15.7 Hz, 18H, tBu), 2.52 (sept, JHH = 6.8 Hz, 4H, CH), 7.25−7.22 (m, 4H, CHaryl), 7.29 (m, 2H, CHim), 7.41 (t, JHH = 7.7 Hz, 2H, CHaryl), 7.51 (m, 2H, CHim). 13C NMR (75 MHz, CD3CN): δ = 23.6 (s, CH3), 23.8 (s, CH3), 25.2 (s, CH3), 25.4 (s, CH3), 26.5 (s, CH3), 26.7 (s, CH3), 29.2 (s, CH), 29.3 (s, CH), 38.52 (d, JCP = 61 Hz, C), 122.1 (d, JCP = 4.5 Hz, CH), 124.3 (s, CH), 124.7 (s, CH), 129.3 (d, JCP = 3.0 Hz, CH), 130.9 (s, C), 146.3 (s, C), 146.6 (s, C), 155.1 (s, NCN). 31P NMR (121 MHz, CD3CN): δ = 83.24. ESI-MS (m/z): 917.30 ([M − OTf]+, expected 917.40). Elemental analysis calcd. (%) for C47H74ClF3N4O5P2PdS (1067.98): C 52.86, H 6.98, N 5.25, S 3.00; found: C 52.53, H 6.87, N 5.22, S 2.77. F


Article

Organometallics JCP = 2.8 Hz, CH), 131.9 (s, CH), 135.6 (s, C), 146.3 (s, C), 189.6 (d, JCP = 13.3 Hz, NCN). 31P NMR (121 MHz, CD3CN): δ = 91.38. ESI-MS (m/z): 535.13 ([M − OTf]+, expected 535.21). Elemental analysis calcd. (%) for C27H42N2PO4SF3Pd (685.07): C 47.34, H 6.18, N 4.09, S 4.68; found: C 47.55, H 6.23, N 3.99, S 4.56. Catalytic Tests in the Suzuki Reaction. Typical procedure: In a Schlenk tube equipped with a magnetic stirring bar were placed under an inert atmosphere: 81 mg (0.66 mmol) of phenylboronic acid, 167 mg (1.2 mmol) of anhydrous K2CO3, and 6 μmol (1 mol %) of catalyst. The tube was degassed and put under an inert atmosphere. 0.600 mmol of aryl halide and 5 mL of solvent were subsequently added. The flask was immediately placed in an oil bath preheated at the reaction temperature, and the reaction mixture was vigorously stirred for 5 h. 14 mg (0.063 mmol) of 1,4-bis-trimethylsilylbenzene as an internal standard was then added. The mixture was evaporated to dryness and taken up in CDCl3. Yields were determined by 1H NMR. X-ray Crystal Structure Determination of Complexes 7, 9, 10, 11. Data for compounds7, 9a, 9b, and 11 were collected using an Oxford Diffraction Gemini E diffractometer, equipped with a 2K × 2K EOS CCD area detector and sealed-tube Enhance (Mo) and (Cu) Xray sources. Single crystals of compounds have been fastened on the top of a Lindemann glass capillary. Data have been collected by means of the ω-scans technique using graphite-monochromated radiation. Detector distance has been set at 45 mm. The diffraction intensities have been corrected for Lorentz/polarization effects as well as with respect to absorption. Empirical multiscan absorption corrections using equivalent reflections have been performed with the scaling algorithm SCALE3 ABSPACK. Data reduction, finalization, and cell refinement were carried out through the CrysAlisPro software. Accurate unit cell parameters were obtained by least-squares refinement of the angular settings of the strongest reflections, chosen from the whole experiment. Data for complex 10 were collected on a single crystal X-ray diffractometer equipped with a CMOS detector (APEX III, κ CMOS), a TXS rotating anode with Mo Kα radiation (λ = 0.71073 Å), and a Helios optic using the APEX III software package.26 The crystals were fixed on the top of a kapton micro sampler with perfluorinated ether, transferred to the diffractometer, and frozen under a stream of cold nitrogen. A matrix scan was used to determine the initial lattice parameters. Reflections were merged and corrected for Lorentz and polarization effects, scan speed, and background using SAINT.27 Absorption corrections, including odd and even ordered spherical harmonics, were performed using SADABS.27 The structures were solved with Olex228 by using the ShelXT29 structure solution program by Intrinsic Phasing and refined with the ShelXL30 refinement package using least-squares minimization. In the last cycles of refinement, non-hydrogen atoms were refined anisotropically. Hydrogen atoms were included in calculated positions, and a riding model was used for their refinement. The specific refinement details for each compounds are reported in the Supporting Information. Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre (CCDC 1901288−1901291, 1902487). Crystal data and refinement parameters are reported in Tables S1 and S2.

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obtained free of charge via www.ccdc.cam.ac.uk/data_request/ cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: andrea.biffi[email protected]. ORCID

Marco Baron: 0000-0001-6762-2240 Marzio Rancan: 0000-0001-9967-5283 Andrea Biffis: 0000-0002-7762-8280 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS University of Padova is gratefully acknowledged for the financial support (P-DiSC BIRD 2017). REFERENCES

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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.9b00185. NMR spectra of complexes 7−12, crystal data, refinement parameters and details for complexes 7, 9, 9b, 10, and 11 (PDF) Accession Codes

CCDC 1901288−1901291 and 1902487 contain the supplementary crystallographic data for this paper. These data can be G


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Palladium(II) Complexes with N-Phosphine Oxide-Substituted

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