ARTICLE pubs.acs.org/Organometallics
Synthesis and Characterization of Imine-Palladacycles Containing Imidate “Pseudohalide” Ligands: Efficient SuzukiMiyaura Cross-Coupling Precatalysts and Their Activation To Give Pd0Ln Species (L = Phosphine) Jose Luis Serrano,*,† Luis García,† Jose Perez,† Eduardo Perez,† Joaquín García,‡ Gregorio Sanchez,‡ Petr Sehnal,§ Sara De Ornellas,§ Thomas J. Williams,§ and Ian J. S. Fairlamb*,§ rea de Química Inorganica, Universidad Politecnica de Cartagena, Departamento de Ingeniería Minera, Geologica y Cartografica, A 30203 Cartagena, Spain ‡ Departamento de Química Inorganica, Universidad de Murcia, 30071 Murcia, Spain § Department of Chemistry, University of York, Heslington, York YO10 5DD, U.K. †
bS Supporting Information ABSTRACT: Dinuclear palladacyclic complexes [{Pd(C∧N)(μ-NCO)}2] (C∧N = N-phenylbenzaldimine, Phbz) containing asymmetric imidato units (NCO = succinimidate (succ; 1), phthalimidate (phthal; 2), maleimidate (mal; 3), 2,3-dibromomaleimidate (2,3-diBrmal; 4), glutarimidate (glut; 5)) have been readily prepared by reaction between the di-μ-acetate precursor and cyclic imide ligands in a 1:2 molar ratio. Base treatment of the less acidic ligands 2-oxazolidone and δ-valerolactame with KOH/MeOH was required to give analogous NCO bridged complexes (6 and 7). Reactions of the dinuclear complexes with tertiary phosphines provide novel mononuclear N-bonded imidate derivatives of the general formula [Pd(Phbz)(imidate)(PR3)] (R = Ph (a), 4-F-C6H4 (b), 4-MeO-C6H4 (c), CH2CH2CN (d)). The application of these novel palladacyclic complexes as precatalysts for the SuzukiMiyaura cross-coupling reactions of both aryl and benzyl bromides with phenylboronic acid has been examined. The acetate adducts [Pd(Phbz)(CH3COO)(PR3)] (8a,c) were prepared to assess the role of imidate ligands in catalyst performance. The mononuclear imidate derivatives possess greater activity than the parent dinuclear complexes, exhibiting comparable performance in the cross-coupling of benzyl bromide with arylboronic acids to the best examples reported in the literature. The mononuclear imidate derivatives give a common Pd0Ln intermediate, as inferred by the release of the organic fragment (first reductive elimination product). Catalyst activation occurs by reaction of phenylboronic acid with the palladacycle in the absence of exogenous base (as shown by GC-MS and ESI-MS), with implications for the reliable comparison of catalyst performance across a series of related precatalysts (e.g., how catalyst/reagents are mixed and what is their order of addition). The single-crystal X-ray structures of compounds 4, 7, 1d, 3c, and 8a have been determined.
’ INTRODUCTION Pd-catalyzed processes that facilitate the formation of CC bonds have seen great progress,1 and reactions such as Stille and SuzukiMiyaura cross-couplings (hereafter Suzuki) are employed extensively in synthetic routes to a myriad of natural products, advanced materials, and therapeutic agents.2 The use of cyclometalated PdII complexes as precatalysts has led in part to some important advances. Phospha-containing palladacycles and N-heterocyclic carbene Pd complexes were successfully used as catalysts for CC coupling reactions, and since then other systems such as “pincer” palladacycles containing functional groups bound to palladium by N, P, or S atoms have also shown their efficiency in Suzuki among other cross-coupling reactions.3 With regard to imine-based palladacycles, Milstein4 reported that the use of a dinuclear phosphine-free complex in the Suzuki coupling of nonactivated aryl bromides and later phosphine adducts of the type [PdII(C∧N)(PR3)(X)] (C∧N = imine-based r 2011 American Chemical Society
ortho-metalated ligand; X = Cl, TFA) exhibited increased catalytic activity.5 Bedford and co-workers elegantly highlighted the role played by the phosphine and the anionic “X” ligand in terms of both catalyst activity and lifetime, while comprehensive mechanistic studies on the involvement of halide/pseudohalide anions in Pdcatalyzed cross-coupling were originally carried out by Amatore and Jutand6 and expanded upon by others, including our recent contributions (vide infra). During the past few years we have tried to understand, control, and exploit the diverse functions and role of imidate anionic ligands (rather unusual pseudohalides with a subtle blend of σ-donating and π-accepting ability)7 in topical and important Pd-catalyzed cross-coupling reactions.8 From the discovery of the useful precatalyst9 Received: March 21, 2011 Published: September 09, 2011 5095
dx.doi.org/10.1021/om2002443 | Organometallics 2011, 30, 5095–5109
Organometallics
ARTICLE
Scheme 1. Synthesis of New Palladacyclic Complexes Containing Imidate Ligands
[trans-Pd(Br)(N-succ)(PPh3)2] (Sigma-Aldrich; Cat. No. 643742) for Stille cross-couplings to its application for Suzuki cross-couplings of benzylic halides, by us10 and very recently others,11 synthetic routes to next-generation imidato PdII precatalysts have been sought. Among them, binuclear cyclometalated palladium(II) complexes and mononuclear phosphine adducts have been evaluated in Stille,12 Suzuki, and Sonogashira cross-couplings,13 and later phosphine-free anionic dinuclear imidate complexes containing a palladacyclopentadiene backbone have shown superb performance in Stille couplings.14 Most studies published on Pd-catalyzed couplings have involved reactions of aryl/heteroaryl or vinyl halides with the organometallic reagent, while benzylic halide couplings have been investigated (by comparison) to a lesser extent,15 despite the fact that diarylmethanes, and related compounds, are important building blocks in organic synthesis and are of considerable interest in medicinal chemistry.16 As mentioned above, trans-[Pd(Br)(N-succ)(PPh3)2] (I) allows efficient access to diarylmethanes by Suzuki couplings.10,15a Low palladium loadings, the use of readily available Na2CO3, and the reduction of the benzyl halide/organoboronic stoichiometric ratio to 1/1 are the major improvements in comparison with previous synthetic protocols.17 We report here the preparation of new dinuclear ortho-metalated palladium(II) derivatives possessing a N-phenylbenzaldimine backbone with different bridging imidate ligands, their use as a precursor to mononuclear compounds of the type [Pd(Phbz)(imidate)(PR3)], and further the application of the novel complexes as precatalysts for Suzuki coupling of aryl and benzyl bromides with arylboronic acids. The effect exerted by the imidate and phosphine ligands on catalyst performance is evaluated, as well as the influence of changing the cyclometalated ligand by comparison with the activity of recently reported oxazoline-based palladacycles.18 The activation/degradation of these precatalysts by phenylboronic acid without exogenous base, in the presence and absence of air (O2), has been investigated; GC-MS and ESI-MS were used to characterize reductive elimination products and PdII intermediates, respectively.
’ RESULTS Synthesis and Characterization. The acetato-bridged cyclometalated dimer [{PdII(μ-OOCMe)(Phbz)}2]19 reacts with succinimide, phthalimide, maleimide, 2,3-dibromomaleimide, and glutarimide in refluxing acetone to give dinuclear complexes in which the imidato ligands replace the bridging acetate group. The synthesis of the succinimidate derivative 1 has been previously described, which requires base, washing with water, and drying over MgSO4,7 although it can be prepared in a straightforward manner following our improved method (vide infra). For the less acidic ligands 2-oxazolidone and δ-valerolactam, only the acetate-bridged complex was recovered from a direct reaction. Treatment of the ligands with KOH/MeOH (as described in the Experimental Section) was required to give the analogous NCO bridged complexes (6 and 7), which are displayed in Scheme 1. The dinuclear Pd derivatives are air-stable yellow solids, and their infrared spectra show the characteristic absorptions for the cyclometalated ligand, partially overlapped with one (6 and 7) or two (15) strong bands attributed to the imidato-carbonyl stretching bands. Further evidence for the dinuclearity of PdII complexes 17 comes from mass spectrometry, as inferred from the m/z values for the observed fragments. The abundances of the signals found around the parent ion are consistent with the expected natural isotopic distribution. TG, DTG, and DSC studies on the dinuclear imidato complexes under a dynamic nitrogen atmosphere was used to characterize the PdII complexes (note that the thermal analysis of mononuclear imidate-13 and halide-bridged ortho-palladated (azb and phpy based) complexes20 and studies on the systematic thermal behavior of cyclopalladated pseudohalogen-bridged complexes21 have been previously described). The TG curves of the PdII complexes display a one-stage decomposition pattern that reveals a high thermal stability (confirmed by the corresponding exothermic peaks by DSC). As expected, the new complexes exhibit thermal behavior similar to that of the corresponding azobenzene compounds and related 7,8-benzoquinoline or 2-phenylpyridine derivatives.13 5096
dx.doi.org/10.1021/om2002443 |Organometallics 2011, 30, 5095–5109
Organometallics
ARTICLE
Table 1. Selected Bond Distances and Angles for 4 and 7 4
7
Bond Distances (Å) Pd(1)C(1)
1.969(3)
1.956(8)
Pd(1)N(1)
2.055(2)
2.058(6)
Pd(1)N(2)
2.022(2)
2.014(6)
Pd(1)O(3)
2.1768(17)
Pd(1)O(2)
2.122(6)
Pd(2)C(18)
1.975(2)
Pd(2)C(19)
1.978(8)
Pd(2)N(3) Pd(2)N(4)
2.042(2) 2.021(2)
2.046(6) 2.002(7)
Pd(2)O(1)
2.1736(17)
2.128(6)
Pd(1)Pd(2)
3.0118(3)
3.0987(7)
Bond Angles (deg)
Figure 1. Thermal ellipsoid plot of 4, drawn at the 40% probability level. The hydrogen atoms and the acetone solvent molecules have been omitted for clarity.
C(1)Pd(1)N(1)
81.92(9)
81.3(3)
C(1)Pd(1)N(2)
92.72(10)
92.7(3)
C(1)Pd(1)O(3)
177.89(8)
C(1)Pd(1)O(2 N(1)Pd(1)N(2)
174.37(8)
N(1)Pd(1)O(3)
97.08(7)
N(1)Pd(1)O(2) N(2)Pd(1)O(3)
97.3(2) 88.23(7)
N(2)Pd(1)O(2) C(18)Pd(2)N(3)
88.3(3) 81.36(10
C(19)Pd(2)N(3)
81.3(3)
C(18)Pd(2)N(4) C(19)Pd(2)N(4)
93.70(10)
C(19)Pd(2)O(1)
178.94(9)
92.8(3)
C(18)Pd(2)O(1)
Figure 2. Thermal ellipsoid plot of 7, drawn at the 40% probability level. The hydrogen atoms have been omitted for clarity.
In contrast to the low solubility of the related dinuclear imidato derivatives with an ortho-metalated backbone, the new complexes 17 were soluble in CDCl3 and their 1H/13C NMR spectra supported the proposed ligand coordination mode. It has been reported for complex 1 that the proximity of N-phenyl and methylene groups imposed by a U-shaped conformation is the reason for the multiplet found in the aliphatic region.7 As it was also possible to grow single crystals of complexes 4 and 7 suitable for X-ray diffraction analysis, the molecular structure and conformation was unambiguously determined, presented in Figures 1 and 2, respectively. Complex 7 is the first description of a PdII complex with δ-valerolactamate as the ligand (in a search of the Cambridge Structural Database (CSD) v. 5.32 updated to May 2011). Selected bond distances and angles are displayed in Table 1.
172.4(3) 173.4(3)
172.7(2)
N(3)Pd(2)N(4)
174.83(8)
172.3(3)
N(3)Pd(2)O(1)
97.60(7)
96.7(2)
N(4)Pd(2)O(1)
87.34(7)
88.6(3)
The unit cell of complex 4 contains two acetone molecules, while 7 contains 0.5 CH2Cl2. The arrangement of the Pd atoms can be described as planar; their deviation from a planar coordination geometry has been quantified by measurements of improper torsion angles.22 For example, for complex 4 w1 = 1.28° and w2 = 1.24° for Pd(1) and w1 = 0.14° and w2 = 0.99° for Pd(2); in complex 7 w1 = 5.07° and w2 = 2.25° for Pd(1) and w1 = 4.91° and w2 = 3.72° for Pd(2). These values correspond to a moderate pyramidal square distortion from the ideal square plane. The “NPdC” angle, 81.36(10) and 81.77(6)° in complex 4 and 81.3(3) and 81.3(3)° in 7, and the ortho-metalated moiety is similar to those found in related complexes. 5,7 Pd(1)Pd(2) distances of 3.0118(3) and 3.0987(7) Å for 4 and 7, respectively, are close to the generally accepted value, indicating a PdPd intramolecular interaction (
