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Structure and properties of arylnorbornyl palladacycles as stable models for catalytic intermediates
Inorganica Chimica Acta 431 (2015) 230–233
Contents lists available at ScienceDirect
Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Structure and properties of arylnorbornyl palladacycles as stable models for catalytic intermediates Nicola Della Ca’, Marta Catellani, Chiara Massera ⇑, Elena Motti ⇑ Dipartimento di Chimica and CIRCC, Università degli Studi di Parma, Parco Area delle Scienze 17/A, 43124 Parma, Italy
a r t i c l e
i n f o
Article history: Received 12 January 2015 Received in revised form 11 March 2015 Accepted 15 March 2015 Available online 30 March 2015
a b s t r a c t The solid state structure of two arylnorbornyl palladacycles containing phenanthroline as ancillary ligand was elucidated through X-ray diffraction analysis. Both compounds, the phenylnorbornyl palladacycle (1a) and the o-tolylnorbornyl palladacycle (1b), possess very similar geometrical parameters in spite of the presence of an ortho methyl group in 1b. Ó 2015 Elsevier B.V. All rights reserved.
Keywords: Palladium complexes Crystal structure Palladacycles C–H activation
1. Introduction Selective ortho-alkylation and arylation of aryl halides can be achieved by the joint action of palladium(0) and norbornene [1]. The process can be made catalytic by reaction with a great variety of molecules or groups able to readily interact with the final orthofunctionalized arylpalladium species, leading to the formation of the desired organic products and the regeneration of the palladium in the initial oxidation state. The catalytic methodology is very useful for the synthesis of biologically and pharmaceutically active compounds [1,2]. Key intermediates in these transformations are alkylaromatic palladacycles formed by reaction of an aryl halide, palladium(0) and norbornene (Scheme 1). While the alkylation of metallacycles 1, namely the reaction with an alkyl halide, is highly regioselective and leads to the ortho dialkylated aromatic ring of the initial metallacycle, the arylation, namely the reaction with an aryl halide, proceeds through different pathways involving the two palladium-coordinated carbons highlighted in Scheme 1. In order to shed light on this different behavior various palladacycles of type 1 [3] containing the phenyl ring or the phenyl ring substituted in ortho, meta or para position have been prepared, starting from their open precursors 2 (Scheme 2) [4a].
⇑ Corresponding authors. Tel.: +39 0521 905428; fax: +39 0521 905557 (C. Massera). Tel.: +39 0521 905414; fax: +39 0521 905472 (E. Motti). E-mail addresses: [email protected] (C. Massera), [email protected] (E. Motti). http://dx.doi.org/10.1016/j.ica.2015.03.022 0020-1693/Ó 2015 Elsevier B.V. All rights reserved.
Under catalytic conditions (Scheme 1), palladacycles of type 1 are formed through the following steps: (1) oxidative addition of the aryl halide to palladium(0) with formation of the well-known arylpalladium(II) intermediate; (2) stereoselective insertion of norbornene into the aryl–Pd bond to give the open precursor; (3) ring closure, in the presence of a base, to the five-membered palladacycle 1 by activation of the ortho C–H aryl bond. Starting from the open precursor 2 and the appropriate stabilizing ligand, in the presence of a base, palladacycles of type 1 were prepared in good yields and characterized by NMR spectroscopy (Scheme 2). The ligand of choice was phenanthroline [3a,3c]. These complexes proved to be excellent models for the alkylation reaction and have allowed to detect and follow the alkylation steps of the aryl ring by NMR. They readily undergo oxidative addition of reactive alkyl halides to give metallacycles of palladium(IV) [5], which show a strong tendency to reductive eliminate to the more stable palladium(II) complex. The reductive elimination step takes place by selective migration of the newly palladium-coordinated alkyl group onto the aromatic side of the metallacycle. The result is the formation of an ortho alkylated arylnorbornylpalladium species isolated and characterized by X-ray crystallography [6]. However, these five-membered palladacycles containing phenanthroline as ligand are too stable to react with aryl halides. The reactivity of the phenylnorbornyl palladacycle and corresponding ortho, meta and para substituted complexes, has been investigated by stoichiometric reactions carried out in a coordinating solvent, acting as ligand, using palladacycles formed in situ from the open precursors 2, in the presence of a base.
N. Della Ca’ et al. / Inorganica Chimica Acta 431 (2015) 230–233
R
L Pd
X
X L
+ Pd(0)L2
Pd
R
R
L
X
K2CO3
- KX - KHCO3
R
Pd L
L
231
diffraction methods. Intensity data and cell parameters were recorded at 293(2) K on a Bruker AXS Smart 1000 (Mo Ka radiation k = 0.71073 Å) equipped with a CCD area detector and a graphite monochromator. The raw frame data were processed using the programs SAINT and SADABS to yield the reflection data files [9]. The structures were solved by Direct Methods using the SIR97 program [10] and refined on F2o by full-matrix least-squares procedures, using the SHELXL-97 program [11] in the WinGX suite v.1.80.05 [12]. All non-hydrogen atoms were refined with anisotropic atomic displacements. The hydrogen atoms were included in the refinement at idealized geometry (C H 0.95 Å) and refined ‘‘riding’’ on the corresponding parent atoms. The weighting schemes used in the last cycle of refinement were w = 1/[r2Fo2 + (0.0427P)2] and w = 1/[r2Fo2 + (0.0250P)2], where P = (Fo2 + 2Fc2)/3, for compounds 1a and 1b, respectively.
1 Scheme 1. Palladacycle formation under catalytic conditions (L = solvent or coordinating substrates).
R
R PhOK, phen
1/2 Pd
Pd
- KCl, - PhOH Cl
R = H, 2a R= Me, 2b
2
N
N
R = H, 1a R= Me, 1b
Scheme 2. Formation of palladacycles 1 (R = H, Me) from their dimeric open precursors ( = phen).
Only the ortho substituted arylnorbornyl palladacycles, in contrast with the meta and para substituted isomers, has allowed the formation of the desired aryl-aryl coupling, suggesting that a steric effect is essentially at work [7a]. Further support to this conclusion has come from theoretical calculations [7b]. Here we report the X-ray crystallographic characterization of two palladacycles, one containing a phenyl ring (1a) and the other an ortho tolyl group (1b). 2. Experimental 2.1. Materials All chemicals and solvents were of reagent grade and used as received. 2.2. Syntheses Complexes 1a and 1b have been prepared as previously reported [3a,3c] starting from the corresponding open precursors 2. The dimer complexes 2a and 2b have been synthesized by a known procedure [8] and fully characterized by NMR spectroscopy and X-ray crystallography [4a]. Crystals suitable for X-ray diffraction analysis have been obtained by slow crystallization at 18 °C from a very diluted solution of methylene dichloride (ca. 0.02 10 3 M) containing a small amount of hexane (ca. 5%). 2.3. Structural determination Crystal data and experimental details for data collection and structure refinement are reported in Table S1 (see SI). The crystal structures of compounds 1a and 1b were determined by X-ray
3. Results and discussion The molecular structures of compounds 1a and 1b are shown in Fig. 1. Since under catalytic conditions ortho-substituted or unsubstituted complexes of type 1 behaved in different way, we wanted to ascertain whether this effect could be ascribed to differences in their structures, as reported for the corresponding precursors 2a and 2b. [4a] Mealli and co-workers compared the unsubstituted arylnorbornenyl complex [(C7H8Ph)PdI(PPh3)] reported by Cheng and co-workers [4b] and the o,p-dimethyl substituted dimer [C7H10(o,p-dimethylphenyl)PdCl]2 [4a] showing the influence of a substituent in the ortho position of the aryl ring on the overall geometry of the complexes. The coordination modes for ortho-substituted and ortho-unsubstituted complexes appear to be different: in the unsubstituted arylnorbornenyl complex [(C7H8Ph)PdI(PPh3)] the metal affects both the ipso and ortho carbons while in the o,p-methyl disubstituted dimer [C7H10(o,p-dimethylphenyl)PdCl]2 only the ipso carbon atom is affected, suggesting an g1 interaction between the palladium and the arene ring. On the contrary, in the present case, palladacycles 1a and 1b have similar structures; both compounds crystallize in the space group P21/a and show very similar geometrical parameters (see Table 1 for selected bond distances and angles, and Fig. 1 for the complete labeling scheme). Each palladium(II) ion is surrounded by two nitrogen atoms belonging to the ligand phenanthroline and by two carbon atoms of the arylnorbornyl moiety. The resulting geometry around the metal centers is distorted square planar, as can be seen from the values of the coordination angles (see Table 1). The Pd–C distances do not change significantly when a hydrogen atom of the aryl moiety is substituted with a methyl group [Pd1–C21 is of 2.014(5) and 1.989(4) Å for 1a and 1b, respectively]. The Pd–N distances are asymmetrical both for 1a [Pd1–N1 2.144(3) Å; Pd1–N2 2.216(3) Å] and 1b [Pd1–N1 2.128(3) Å; Pd1–N2 2.210(3) Å], probably due to the steric hindrance between the aromatic rings of the ligands (A and B in Fig. 1). In both cases the longest distance is the one trans to the bond of Pd with the norbornyl moiety. In both complexes, the ligand phenanthroline and the leastsquares plane passing through the five-membered ring which completes the coordination around palladium are tilted one with respect to the other (13.96(5)° and 18.09(5)° for 1a and 1b, respectively). In the lattice, the complexes pile one on top of the other along the a axis of the unit cell (see Fig. 2), oriented in an ABAB mode, with average centroid centroid distances of around 3.8 Å between couples of aromatic rings. Only very few examples exist in the literature reporting the crystal structure of intermediate compounds related to 1a and 1b
232
N. Della Ca’ et al. / Inorganica Chimica Acta 431 (2015) 230–233
Fig. 1. Molecular structure of 1a (left) and 1b (right). Solvent lattice molecules have been omitted for clarity.
Table 1 Selected bond distances (Å) and angles (°) for 1a and 1b. 1a Pd1–N1 Pd1–N2 Pd1–C13 Pd1–C21 N1–Pd1–N2 N1–Pd1–C13 C13–Pd1–C21 N2–Pd1–C21
1b 2.144(3) 2.216(3) 2.027(4) 2.014(5) 76.94(9) 94.09(9) 82.88(9) 106.74(9)
Pd1–N1 Pd1–N2 Pd1–C13 Pd1–C21 N1–Pd1–N2 N1–Pd1–C13 C13–Pd1–C21 N2–Pd1–C21
2.128(3) 2.210(3) 2.013(3) 1.989(4) 76.90(9) 94.98(9) 82.25(9) 106.75(9)
(Table 2). One example by Lautens and co-workers [13], describes a palladacycle [C7H10(C6H5)Pd(PPh3)2] analogous of 1a in which phenanthroline is replaced by two triphenylphosphine groups (PPh3). The effect of the ancillary ligand is reflected in the difference of the Pd–C bond distances, which are longer in the phosphine-substituted compound but maintain the same trend (2.080 and 2.094 Å compared to 2.014 and 2.027 Å for the phenyl and norbornyl units, respectively). Also the angles around the metal center
are quite different, mainly due to the chelating nature of phenanthroline which forms a very small angle N–Pd–N (see Table 1). Another example was provided by Bach and co-workers [2b], who obtained the crystal structure of an N-norbornene type palladacycle with phenanthroline. The bidentate alkylaromatic ligand comprises the norbornyl unit bonded to the nitrogen atom of the indole ring (see the molecular structure in Table 2). The Pd–N distances and angles around the metal center are all comparable, while the Pd–C bond formed with the norbornyl unit is shorter for 1a and 1b (2.027 and 2.013 compared to 2.039 Å). This type of complexes are key intermediates in catalytic reactions leading to the selective alkylation of the 2-position of the indole structure. Thus they behave as expected for the reaction of an alkylaromatic palladacycle of this type with an alkyl halide: the alkyl group selectively couples with the aryl carbon. This straightforward 2-alkylation of the indole ring was applied to natural product total synthesis [2b]. If compound 1a is caused to react with p-nitrobenzyl bromide, a new species is obtained, namely [C7H10(p-nitrobenzylphenyl) PdBr(phen)], in which palladium is bonded to phenanthroline,
Fig. 2. Packing of 1a (up) and 1b (bottom) along the a axis of the unit cell.
N. Della Ca’ et al. / Inorganica Chimica Acta 431 (2015) 230–233 Table 2 Selected distances (Å) in 1a, 1b, and the two [C7H10(C6H5)Pd(PPh3)2] and [(C7H10(C8H5N)Pd(phen)]. 1a
1b
[C7H10(C6H5)Pd(PPh3)2] [13] EXEDUH
related
palladacycles
Pd1–N1 2.144(3) Pd1–N2 2.216(3) Pd1–C13 2.027(4) Pd1–C21 2.014(5) Pd1–N1 2.128(3) Pd1–N2 2.210(3) Pd1–C13 2.013(3) Pd1–C21 1.989(4) Pd1–C1 2.094(4) Pd1–C13 2.080(4) Pd1–P1 2.408(1) Pd1–P2 2.381(1)
233
substituent in the ortho position of the aryl ring, and compared them with the structure of analogous intermediates. The two complexes possess very similar geometrical parameters in spite of the presence of the substituent, which does not affect the geometry of the palladium-bonded ligands. Thus the different reactivity of the two intermediates under catalytic conditions is not to be found in the structural data of compounds 1a and 1b. Acknowledgments We thank Parma University for financial support. Appendix A. Supplementary material CCDC 1042469 and 1042470 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ica.2015.03.022. References
[(C7H10(C8H5N)Pd(phen)] [2b]EDEKUV
Pd1–C1 2.039(2) Pd1–C15 1.977(2) Pd1–N2 2.124(1) Pd1–N3 2.147(1)
one bromide ion and one norbornyl group in a square-planar geometry [6]. The coordination angles are typical of a slightly distorted square planar environment, with the usual exception of the small value N–Pd–N due to the chelating ligand. As for 1a, also in this case the Pd–N distances are asymmetrical, with the longest bond trans to the norbornyl group. Due to the presence of the bulky phenanthroline ligand, the aryl ring is no longer coordinated to the metal center and it is tilted away from the coordination plane, the ortho aryl C–Pd distance being 3.187 Å. This long distance prevents the g-coordination of the aromatic ring and further C–H activation, as opposed to what found in analogous complexes, that is the open dimer [C7H10(o,pdimethylphenyl)PdCl]2 by Mealli and co-workers [4a] (ortho aryl C–Pd distance 2.569 Å and ipso aryl C–Pd distance 2.244 Å), the [C7H10(m-methoxyphenyl)PdI(PPh3)] by Lautens and co-workers [13] (ortho aryl C–Pd distance 2.517 Å) and the unsubstituted arylnorbornenyl complex [(C7H8Ph)PdI(PPh3)] reported by Cheng and co-workers [4b] (ortho aryl C–Pd distance 2.590 Å). In conclusion, we have here reported the solid state structure of two arylnorbornyl palladacycles which differ for the presence of a
[1] (a) M. Catellani, E. Motti, N. Della Ca’, Acc. Chem. Res. 41 (2008) 1512; (b) M. Catellani, E. Motti, N. Della Ca’, Eur. J. Org. Chem. (2007) 4153; (c) G.P. Chiusoli, M. Catellani, M. Costa, E. Motti, N. Della Ca’, G. Maestri, Coord. Chem. Rev. 254 (2010) 456; (d) M. Lautens, D. Alberico, C. Bressy, Y.-Q. Fang, B. Mariampillai, T. Wilhelm, Pure Appl. Chem. 78 (2006) 351; (e) A. Martins, B. Mariampillai, M. Lautens, Top. Curr. Chem. 292 (2010) 1. [2] (a) H. Weinstabl, M. Suhartono, Z. Qureshi, M. Lautens, Angew. Chem., Int. Ed. 52 (2013) 5305; (b) L. Jiao, E. Herdtweck, T. Bach, J. Am. Chem. Soc. 134 (2012) 14563; (c) Z. Dong, G. Dong, J. Am. Chem. Soc. 135 (2013) 18350; (d) X. Sui, R. Zhu, Z. Gu, Synlett 24 (2013) 2023; (e) E. Motti, N. Della Ca’, D. Xu, A. Piersimoni, E. Bedogni, Z.-M. Zhou, M. Catellani, Org. Lett. 14 (2012) 5792; (f) D. Xu, L. Dai, M. Catellani, E. Motti, N. Della Ca’, Z.-M. Zhou, Org. Biomol. Chem. 13 (2015) 2260. [3] (a) M. Catellani, G.P. Chiusoli, J. Organomet. Chem. 346 (1988) C27; (b) M. Catellani, G.P. Chiusoli, J. Organomet. Chem. 437 (1992) 369; (c) M. Catellani, G.P. Chiusoli, Gazz. Chim. Ital. 123 (1993) 1. [4] (a) M. Catellani, C. Mealli, E. Motti, P. Paoli, E. Perez-Carreno, P.S. Pregosin, J. Am. Chem. Soc. 124 (2002) 4336; (b) C.-S. Li, C.-H. Cheng, F.-L. Liao, S.-L. Wang, J. Chem. Soc., Chem. Commun. (1991) 710; (c) C.-S. Li, D.-C. You, C.-H. Cheng, Organometallics 12 (1993) 3945; (d) C.-H. Liu, C.-S. Li, C.-H. Cheng, Organometallics 13 (1994) 18. [5] (a) M. Catellani, B.E. Mann, J. Organomet. Chem. 390 (1990) 251; (b) A.J. Canty, Acc. Chem. Res. 25 (1992) 83; (c) J. Vicente, A. Arcas, F. Juliá-Hernández, D. Bautista, Angew. Chem., Int. Ed. 50 (2011) 6896. [6] G. Bocelli, M. Catellani, S. Ghelli, J. Organomet. Chem. 458 (1993) C13. [7] (a) M. Catellani, E. Motti, New J. Chem. (1998) 759; (b) G. Maestri, E. Motti, N. Della Ca’, M. Malacria, E. Derat, M. Catellani, J. Am. Chem. Soc. 133 (2011) 8574. [8] H. Horino, M. Arai, M. Inoue, Tetrahedron Lett. (1974) 647. [9] (a) SAINT, Software Users Guide, 6.0; Bruker Analytical X-ray Systems: 1999.; (b) G.M. Sheldrick, SADABS Area-Detector Absorption Correction, 2.03, University of Göttingen, Göttingen, Germany, 1999. [10] A. Altomare, M.C. Burla, M. Camalli, G.L. Cascarano, C. Giacovazzo, A. Guagliardi, A.G.G. Moliterni, G. Polidori, R. Spagna, J. Appl. Crystallogr. 32 (1999) 115. [11] G.M. Sheldrick, Acta Crystallogr., A A64 (2008) 112. [12] L.J. Farrugia, J. Appl. Crystallogr. 32 (1999) 837. [13] D.I. Chai, P. Thansandote, M. Lautens, Chem. Eur. J. 17 (2011) 8175.
Contents lists available at ScienceDirect
Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Structure and properties of arylnorbornyl palladacycles as stable models for catalytic intermediates Nicola Della Ca’, Marta Catellani, Chiara Massera ⇑, Elena Motti ⇑ Dipartimento di Chimica and CIRCC, Università degli Studi di Parma, Parco Area delle Scienze 17/A, 43124 Parma, Italy
a r t i c l e
i n f o
Article history: Received 12 January 2015 Received in revised form 11 March 2015 Accepted 15 March 2015 Available online 30 March 2015
a b s t r a c t The solid state structure of two arylnorbornyl palladacycles containing phenanthroline as ancillary ligand was elucidated through X-ray diffraction analysis. Both compounds, the phenylnorbornyl palladacycle (1a) and the o-tolylnorbornyl palladacycle (1b), possess very similar geometrical parameters in spite of the presence of an ortho methyl group in 1b. Ó 2015 Elsevier B.V. All rights reserved.
Keywords: Palladium complexes Crystal structure Palladacycles C–H activation
1. Introduction Selective ortho-alkylation and arylation of aryl halides can be achieved by the joint action of palladium(0) and norbornene [1]. The process can be made catalytic by reaction with a great variety of molecules or groups able to readily interact with the final orthofunctionalized arylpalladium species, leading to the formation of the desired organic products and the regeneration of the palladium in the initial oxidation state. The catalytic methodology is very useful for the synthesis of biologically and pharmaceutically active compounds [1,2]. Key intermediates in these transformations are alkylaromatic palladacycles formed by reaction of an aryl halide, palladium(0) and norbornene (Scheme 1). While the alkylation of metallacycles 1, namely the reaction with an alkyl halide, is highly regioselective and leads to the ortho dialkylated aromatic ring of the initial metallacycle, the arylation, namely the reaction with an aryl halide, proceeds through different pathways involving the two palladium-coordinated carbons highlighted in Scheme 1. In order to shed light on this different behavior various palladacycles of type 1 [3] containing the phenyl ring or the phenyl ring substituted in ortho, meta or para position have been prepared, starting from their open precursors 2 (Scheme 2) [4a].
⇑ Corresponding authors. Tel.: +39 0521 905428; fax: +39 0521 905557 (C. Massera). Tel.: +39 0521 905414; fax: +39 0521 905472 (E. Motti). E-mail addresses: [email protected] (C. Massera), [email protected] (E. Motti). http://dx.doi.org/10.1016/j.ica.2015.03.022 0020-1693/Ó 2015 Elsevier B.V. All rights reserved.
Under catalytic conditions (Scheme 1), palladacycles of type 1 are formed through the following steps: (1) oxidative addition of the aryl halide to palladium(0) with formation of the well-known arylpalladium(II) intermediate; (2) stereoselective insertion of norbornene into the aryl–Pd bond to give the open precursor; (3) ring closure, in the presence of a base, to the five-membered palladacycle 1 by activation of the ortho C–H aryl bond. Starting from the open precursor 2 and the appropriate stabilizing ligand, in the presence of a base, palladacycles of type 1 were prepared in good yields and characterized by NMR spectroscopy (Scheme 2). The ligand of choice was phenanthroline [3a,3c]. These complexes proved to be excellent models for the alkylation reaction and have allowed to detect and follow the alkylation steps of the aryl ring by NMR. They readily undergo oxidative addition of reactive alkyl halides to give metallacycles of palladium(IV) [5], which show a strong tendency to reductive eliminate to the more stable palladium(II) complex. The reductive elimination step takes place by selective migration of the newly palladium-coordinated alkyl group onto the aromatic side of the metallacycle. The result is the formation of an ortho alkylated arylnorbornylpalladium species isolated and characterized by X-ray crystallography [6]. However, these five-membered palladacycles containing phenanthroline as ligand are too stable to react with aryl halides. The reactivity of the phenylnorbornyl palladacycle and corresponding ortho, meta and para substituted complexes, has been investigated by stoichiometric reactions carried out in a coordinating solvent, acting as ligand, using palladacycles formed in situ from the open precursors 2, in the presence of a base.
N. Della Ca’ et al. / Inorganica Chimica Acta 431 (2015) 230–233
R
L Pd
X
X L
+ Pd(0)L2
Pd
R
R
L
X
K2CO3
- KX - KHCO3
R
Pd L
L
231
diffraction methods. Intensity data and cell parameters were recorded at 293(2) K on a Bruker AXS Smart 1000 (Mo Ka radiation k = 0.71073 Å) equipped with a CCD area detector and a graphite monochromator. The raw frame data were processed using the programs SAINT and SADABS to yield the reflection data files [9]. The structures were solved by Direct Methods using the SIR97 program [10] and refined on F2o by full-matrix least-squares procedures, using the SHELXL-97 program [11] in the WinGX suite v.1.80.05 [12]. All non-hydrogen atoms were refined with anisotropic atomic displacements. The hydrogen atoms were included in the refinement at idealized geometry (C H 0.95 Å) and refined ‘‘riding’’ on the corresponding parent atoms. The weighting schemes used in the last cycle of refinement were w = 1/[r2Fo2 + (0.0427P)2] and w = 1/[r2Fo2 + (0.0250P)2], where P = (Fo2 + 2Fc2)/3, for compounds 1a and 1b, respectively.
1 Scheme 1. Palladacycle formation under catalytic conditions (L = solvent or coordinating substrates).
R
R PhOK, phen
1/2 Pd
Pd
- KCl, - PhOH Cl
R = H, 2a R= Me, 2b
2
N
N
R = H, 1a R= Me, 1b
Scheme 2. Formation of palladacycles 1 (R = H, Me) from their dimeric open precursors ( = phen).
Only the ortho substituted arylnorbornyl palladacycles, in contrast with the meta and para substituted isomers, has allowed the formation of the desired aryl-aryl coupling, suggesting that a steric effect is essentially at work [7a]. Further support to this conclusion has come from theoretical calculations [7b]. Here we report the X-ray crystallographic characterization of two palladacycles, one containing a phenyl ring (1a) and the other an ortho tolyl group (1b). 2. Experimental 2.1. Materials All chemicals and solvents were of reagent grade and used as received. 2.2. Syntheses Complexes 1a and 1b have been prepared as previously reported [3a,3c] starting from the corresponding open precursors 2. The dimer complexes 2a and 2b have been synthesized by a known procedure [8] and fully characterized by NMR spectroscopy and X-ray crystallography [4a]. Crystals suitable for X-ray diffraction analysis have been obtained by slow crystallization at 18 °C from a very diluted solution of methylene dichloride (ca. 0.02 10 3 M) containing a small amount of hexane (ca. 5%). 2.3. Structural determination Crystal data and experimental details for data collection and structure refinement are reported in Table S1 (see SI). The crystal structures of compounds 1a and 1b were determined by X-ray
3. Results and discussion The molecular structures of compounds 1a and 1b are shown in Fig. 1. Since under catalytic conditions ortho-substituted or unsubstituted complexes of type 1 behaved in different way, we wanted to ascertain whether this effect could be ascribed to differences in their structures, as reported for the corresponding precursors 2a and 2b. [4a] Mealli and co-workers compared the unsubstituted arylnorbornenyl complex [(C7H8Ph)PdI(PPh3)] reported by Cheng and co-workers [4b] and the o,p-dimethyl substituted dimer [C7H10(o,p-dimethylphenyl)PdCl]2 [4a] showing the influence of a substituent in the ortho position of the aryl ring on the overall geometry of the complexes. The coordination modes for ortho-substituted and ortho-unsubstituted complexes appear to be different: in the unsubstituted arylnorbornenyl complex [(C7H8Ph)PdI(PPh3)] the metal affects both the ipso and ortho carbons while in the o,p-methyl disubstituted dimer [C7H10(o,p-dimethylphenyl)PdCl]2 only the ipso carbon atom is affected, suggesting an g1 interaction between the palladium and the arene ring. On the contrary, in the present case, palladacycles 1a and 1b have similar structures; both compounds crystallize in the space group P21/a and show very similar geometrical parameters (see Table 1 for selected bond distances and angles, and Fig. 1 for the complete labeling scheme). Each palladium(II) ion is surrounded by two nitrogen atoms belonging to the ligand phenanthroline and by two carbon atoms of the arylnorbornyl moiety. The resulting geometry around the metal centers is distorted square planar, as can be seen from the values of the coordination angles (see Table 1). The Pd–C distances do not change significantly when a hydrogen atom of the aryl moiety is substituted with a methyl group [Pd1–C21 is of 2.014(5) and 1.989(4) Å for 1a and 1b, respectively]. The Pd–N distances are asymmetrical both for 1a [Pd1–N1 2.144(3) Å; Pd1–N2 2.216(3) Å] and 1b [Pd1–N1 2.128(3) Å; Pd1–N2 2.210(3) Å], probably due to the steric hindrance between the aromatic rings of the ligands (A and B in Fig. 1). In both cases the longest distance is the one trans to the bond of Pd with the norbornyl moiety. In both complexes, the ligand phenanthroline and the leastsquares plane passing through the five-membered ring which completes the coordination around palladium are tilted one with respect to the other (13.96(5)° and 18.09(5)° for 1a and 1b, respectively). In the lattice, the complexes pile one on top of the other along the a axis of the unit cell (see Fig. 2), oriented in an ABAB mode, with average centroid centroid distances of around 3.8 Å between couples of aromatic rings. Only very few examples exist in the literature reporting the crystal structure of intermediate compounds related to 1a and 1b
232
N. Della Ca’ et al. / Inorganica Chimica Acta 431 (2015) 230–233
Fig. 1. Molecular structure of 1a (left) and 1b (right). Solvent lattice molecules have been omitted for clarity.
Table 1 Selected bond distances (Å) and angles (°) for 1a and 1b. 1a Pd1–N1 Pd1–N2 Pd1–C13 Pd1–C21 N1–Pd1–N2 N1–Pd1–C13 C13–Pd1–C21 N2–Pd1–C21
1b 2.144(3) 2.216(3) 2.027(4) 2.014(5) 76.94(9) 94.09(9) 82.88(9) 106.74(9)
Pd1–N1 Pd1–N2 Pd1–C13 Pd1–C21 N1–Pd1–N2 N1–Pd1–C13 C13–Pd1–C21 N2–Pd1–C21
2.128(3) 2.210(3) 2.013(3) 1.989(4) 76.90(9) 94.98(9) 82.25(9) 106.75(9)
(Table 2). One example by Lautens and co-workers [13], describes a palladacycle [C7H10(C6H5)Pd(PPh3)2] analogous of 1a in which phenanthroline is replaced by two triphenylphosphine groups (PPh3). The effect of the ancillary ligand is reflected in the difference of the Pd–C bond distances, which are longer in the phosphine-substituted compound but maintain the same trend (2.080 and 2.094 Å compared to 2.014 and 2.027 Å for the phenyl and norbornyl units, respectively). Also the angles around the metal center
are quite different, mainly due to the chelating nature of phenanthroline which forms a very small angle N–Pd–N (see Table 1). Another example was provided by Bach and co-workers [2b], who obtained the crystal structure of an N-norbornene type palladacycle with phenanthroline. The bidentate alkylaromatic ligand comprises the norbornyl unit bonded to the nitrogen atom of the indole ring (see the molecular structure in Table 2). The Pd–N distances and angles around the metal center are all comparable, while the Pd–C bond formed with the norbornyl unit is shorter for 1a and 1b (2.027 and 2.013 compared to 2.039 Å). This type of complexes are key intermediates in catalytic reactions leading to the selective alkylation of the 2-position of the indole structure. Thus they behave as expected for the reaction of an alkylaromatic palladacycle of this type with an alkyl halide: the alkyl group selectively couples with the aryl carbon. This straightforward 2-alkylation of the indole ring was applied to natural product total synthesis [2b]. If compound 1a is caused to react with p-nitrobenzyl bromide, a new species is obtained, namely [C7H10(p-nitrobenzylphenyl) PdBr(phen)], in which palladium is bonded to phenanthroline,
Fig. 2. Packing of 1a (up) and 1b (bottom) along the a axis of the unit cell.
N. Della Ca’ et al. / Inorganica Chimica Acta 431 (2015) 230–233 Table 2 Selected distances (Å) in 1a, 1b, and the two [C7H10(C6H5)Pd(PPh3)2] and [(C7H10(C8H5N)Pd(phen)]. 1a
1b
[C7H10(C6H5)Pd(PPh3)2] [13] EXEDUH
related
palladacycles
Pd1–N1 2.144(3) Pd1–N2 2.216(3) Pd1–C13 2.027(4) Pd1–C21 2.014(5) Pd1–N1 2.128(3) Pd1–N2 2.210(3) Pd1–C13 2.013(3) Pd1–C21 1.989(4) Pd1–C1 2.094(4) Pd1–C13 2.080(4) Pd1–P1 2.408(1) Pd1–P2 2.381(1)
233
substituent in the ortho position of the aryl ring, and compared them with the structure of analogous intermediates. The two complexes possess very similar geometrical parameters in spite of the presence of the substituent, which does not affect the geometry of the palladium-bonded ligands. Thus the different reactivity of the two intermediates under catalytic conditions is not to be found in the structural data of compounds 1a and 1b. Acknowledgments We thank Parma University for financial support. Appendix A. Supplementary material CCDC 1042469 and 1042470 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ica.2015.03.022. References
[(C7H10(C8H5N)Pd(phen)] [2b]EDEKUV
Pd1–C1 2.039(2) Pd1–C15 1.977(2) Pd1–N2 2.124(1) Pd1–N3 2.147(1)
one bromide ion and one norbornyl group in a square-planar geometry [6]. The coordination angles are typical of a slightly distorted square planar environment, with the usual exception of the small value N–Pd–N due to the chelating ligand. As for 1a, also in this case the Pd–N distances are asymmetrical, with the longest bond trans to the norbornyl group. Due to the presence of the bulky phenanthroline ligand, the aryl ring is no longer coordinated to the metal center and it is tilted away from the coordination plane, the ortho aryl C–Pd distance being 3.187 Å. This long distance prevents the g-coordination of the aromatic ring and further C–H activation, as opposed to what found in analogous complexes, that is the open dimer [C7H10(o,pdimethylphenyl)PdCl]2 by Mealli and co-workers [4a] (ortho aryl C–Pd distance 2.569 Å and ipso aryl C–Pd distance 2.244 Å), the [C7H10(m-methoxyphenyl)PdI(PPh3)] by Lautens and co-workers [13] (ortho aryl C–Pd distance 2.517 Å) and the unsubstituted arylnorbornenyl complex [(C7H8Ph)PdI(PPh3)] reported by Cheng and co-workers [4b] (ortho aryl C–Pd distance 2.590 Å). In conclusion, we have here reported the solid state structure of two arylnorbornyl palladacycles which differ for the presence of a
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