Organometallics 2009, 28, 6617–6620 DOI: 10.1021/om900686r
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Synthesis and Structure of the First Heterodinuclear PCP-Pincer-CDP Complex with a Pd-Au d8-d10 Pseudo-Closed-Shell Interaction Christian Reitsamer, Walter Schuh,* Holger Kopacka, Klaus Wurst, and Paul Peringer Institut f€ ur Allgemeine, Anorganische und Theoretische Chemie der Universit€ at Innsbruck, Innrain 52a, A-6020 Innsbruck, Austria Received August 3, 2009 Summary: The first heterodimetallic PCP pincer complex, [PdAu(Cl)2(C(dppm)2 )]Cl, was prepared from [PdCl(C(dppm)2)]Cl and [AuCl(tht)]. Both metals are attached to the carbon atom of the carbodiphosphorane (CDP) functionality and are connected via a very short d8-d10 pseudo-closedshell interaction (Pd-Au = 2.8900(3) A˚) which significantly modifies the ligand backbone conformation.
Weak interatomic forces, such as van der Waals interactions and hydrogen bonding, gained considerable attention during the development of supramolecular chemistry for the controlled architecture of metal-ligand assemblies.1 Furthermore, there is increasing interest in metal-metal contacts, which are also relevant in this field: Gold is the most prominent element that shows so-called “tangential” metal-metal bonding or “aurophilic attraction”, which indicates closed-shell interactions with a direction different from covalent valence bonds.2 The bonding energies of these interactions, which are attributable to relativistic effects, are in the range of hydrogen bridges and may significantly influence the supramolecular characteristics of transitionmetal complexes and lead to unexpected structural features.2 In addition to the extensively studied Au-Au d10-d10 contacts, there also exist examples of d10-d10 bonding with Pd(0), Cu(I), Ag(I), and Hg(II), including heterometallic examples,3 but also Au d10 interactions with metals in an electronically pseudo-closed d8 configuration, such as platinum(II) or, more infrequently, palladium(II).4 The d8 center Pd(II) is a prominent metal in the broad field of the chemistry of pincer complexes;5 however, no bimetallic examples exhibiting d8-d10 interactions seem to have been described up to now. We report here on the first PCP-
type pincer complex that displays a heterodimetallic d8-d10 closed-shell interaction which significantly influences the backbone conformation of the pincer ligand.
Results and Discussion Synthesis. Recent theoretical and experimental results indicate that carbodiphosphoranes (CDPs) are best described as compounds of carbon(0) with two electron lone pairs.6 The carbon-phosphorus bonds involve mainly PfC donor-acceptor interactions rather than covalent bonding as in carbenes.6 In keeping with the formulation of two electron lone pairs at the divalent carbon center, CDPs are known to interact with one or two Lewis acids: e.g., with one or two protons,7 one8,9 or two metal centers,9 or one proton and one metal.6,9,10 CDP complexes in which two metals are attached to the central carbon are scarce, and the sole structurally determined compound is [(AuCl)2C(PPh3)2].9 No heterodimetallic examples are described. Scheme 1
*Corresponding author. E-mail: schuh.walter@net4you. (1) Lehn, J.-M. Supramolecular Chemistry, Concepts and Perspectives; VCH: Weinheim, Germany, 1995. (2) (a) Schmidbaur, H. Gold: Progress in Chemistry, Biochemistry and Technology; Wiley: Chichester, New York, 1999. (b) Schmidbaur, H. Nature 2001, 413, 31. (c) Pyykk€o, P. Angew. Chem., Int. Ed. 2004, 43, 4412. (3) (a) Benard, M.; Bodensieck, U.; Braunstein, P.; Knorr, M.; Strampfer, M.; Strohmann, C. Angew. Chem., Int. Ed. Engl. 1997, 36, 2758. (b) Schuh, W.; Braunstein, P.; Benard, M.; Rohmer, M.-M.; Welter, R. Angew. Chem., Int. Ed. 2003, 42, 2161. (4) (a) Crespo, O.; Laguna, A.; Fernandez, E. J.; Lopez-de-Luzuriaga, J. M.; Jones, P. G.; Teichert, M.; Monge, M.; Pyykk€ o, P.; Runeberg, N.; Schutz, M.; Werner, H.-J. Inorg. Chem. 2000, 39, 4786. (b) Harvey, P. D.; Eichhofer, A.; Fenske, D. Dalton Trans. 1998, 3901. (c) Vicente, J.; Gonzalez-Herrero, P.; Prez-Cadenas, M.; Jones, P. G.; Bautista, D. Inorg. Chem. 2007, 46, 4718. (d) Xia, B. H.; Zhang, H. X.; Che, C. M.; Leung, K. H.; Phillips, D. L.; Zhu, N. Y.; Zhou, Z. Y. J. Am. Chem. Soc. 2003, 125, 10362. (e) Singh, A.; Sharp, P. R. Dalton Trans. 2005, 2080. (5) For a recent review see: The Chemistry of Pincer Compounds; Morales-Morales, D., Jensen, C. M., Eds.; Elsevier: Amsterdam, 2007.
(6) Tonner, R.; Oexler, F.; Neumueller, B.; Petz, W.; Frenking, G. Angew. Chem., Int. Ed. 2006, 45, 8038. (7) Ramirez, F.; Desai, N. B.; Hansen, B.; McKelvie, N. J. Am. Chem. Soc. 1961, 83, 3539. (8) (a) Petz, W.; Kutschera, C.; Neumueller, B. Organometallics 2005, 24, 5038 and references cited therein. (b) Petz, W.; Weller, F.; Uddin, J.; Frenking, G. Organometallics 1999, 18, 619. (9) Vicente, J.; Singhal, A. R.; Jones, P. G. Organometallics 2002, 21, 5887 and references cited therein. (10) (a) Gamper, S. F.; Schmidbaur, H. Organometallics 1992, 11, 2863. (b) Romeo, I.; Bardaji, M.; Gimeno, M. C.; Laguna, M. Polyhedron 2000, 19, 1837.
r 2009 American Chemical Society
Published on Web 10/27/2009
We have recently reported on a new class of PCP-type pincer complexes involving the neutral ligand C(dppm)2 (Ph2PCH2P(Ph2)dCd(Ph2)PCH2PPh2, dppm = Ph2PCH2PPh2),
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Organometallics, Vol. 28, No. 22, 2009
Reitsamer et al. Table 1. Crystal Data of 1c 3 2MeOH empirical formula formula wt cryst syst space group unit cell dimens a b c β V Z calcd density final R indices (I >2σ(I))
Figure 1. Thermal ellipsoid plot of the cation of 1c. Selected distances (A˚) and angles (deg): Au(1)-C(1) = 2.078(3), Au(1)Cl(2) = 2.2769(8), Au(1)-Pd(1) = 2.8900(3), Pd(1)-C(1) = 2.128(3), Pd(1)-P(1) = 2.2774(8), Pd(1)-P(4) = 2.3145(8), Pd(1)-Cl(1) = 2.3470(9), P(2)-C(1) = 1.786(3), P(3)-C(1) = 1.787(3); Au(1)-C(1)-Pd(1) = 86.80(11), C(1)-Au(1)-Cl(2) = 175.06(8), C(1)-Au(1)-Pd(1) = 47.33(8), Cl(2)-Au(1)Pd(1) = 128.31(2), P(1)-Pd(1)-P(4) = 177.43(3), C(1)-Pd(1)Cl(1) = 174.17(8), C(1)-Pd(1)-P(1) = 89.43(8), C(1)-Pd(1)P(4) = 90.24(8), P(1)-Pd(1)-Cl(1) = 87.95(3), P(4)-Pd(1)Cl(1) = 92.61(3), P(1)-Pd(1)-Au(1) = 89.98(2), P(4)-Pd(1)Au(1) = 91.62(2), Cl(1)-Pd(1)-Au(1) = 128.90(3), P(2)-C(1)P(3) = 120.66(17).
which is formally composed of two dppm units connected by one divalent carbon atom in a CDP functionality (Scheme 1, [PdCl(C(dppm)2)]Cl, 1a).11 Treatment of the compound 1a with hydrochloric acid has been shown to produce [PdCl(CH(dppm)2)]Cl2 (1b) via protonation at the central CDP carbon atom.11 In order to probe a formal exchange of Hþ by the isolobal AuCl fragment, we were able to synthesize the new heterodinuclear PCP-type pincer complex [PdAu(Cl)2(C(dppm)2)]Cl (1c), in which the palladium and gold centers are both attached to the carbon of the CDP functionality. The compound 1c is formed in high yield upon treatment of the complex 1a with 1 equiv of [AuCl(tht)] (tht = tetrahydrothiophene) according to Scheme 1. The complex 1c is stable in air and is readily soluble in mixtures of CHCl3 and MeOH but is almost insoluble in pure CHCl3 or MeOH. The new compound was characterized by single-crystal X-ray diffraction and multinuclear NMR spectroscopy. Crystallography. Crystallization of 1c from MeOH/ CH2Cl2 led to formation of a solvate with two cocrystallized MeOH molecules. A thermal ellipsoid plot of the cation of 1c with selected bond lengths and angles is provided in Figure 1. A summary of the crystallographic data is presented in Table 1. The crystal structures of 1a 3 2.5(toluene) 3 MeOH and 1b 3 3H2O, which are also referred to in the following discussion, were reported elsewhere.11 The structure of 1c shows a quasi-Cs symmetry of the dimetallic center and the CDP ligand backbone, where the Cl-Au-C(1)-Pd-Cl moiety forms the symmetry plane. The Pd-Au unit is ligated by the C(dppm)2 pincer system, where both metals are bound to the CDP carbon atom C(1). According to a search in the Cambridge Structural (11) Stallinger, S.; Reitsamer, C.; Schuh, W.; Kopacka, H.; Wurst, K.; Peringer, P. Chem. Commun. 2007, 510.
C53H52AuCl3O2P4Pd 1254.54 monoclinic P21/n (No. 14) 12.6502(1) A˚ 20.0992(3) A˚ 20.2354(2) A˚ 90.620(1)° 5144.73(10) A˚3 4 1.620 Mg/m3 R1=0.0273, wR2=0.0644
Database, there are no structures previously described in which both an Au and a Pd center are bound to a carbon atom. The coordination sphere of gold additionally includes one chloride ligand; that of palladium is completed by the two phosphine functionalities of the C(dppm)2 pincer ligand and one chloride. The geometry of the C(1)-Au-Cl(2) sequence is essentially linear (175.06(8)°), and that of the PdP(1)C(1)P(4)Cl(1) kernel is square planar (sum of angles around Pd 360°). The Pd-Au separation of 2.8900(3) A˚ indicates a d8-d10 interaction when compared with a van der Waals limit of 3.29 A˚.12 As far as we are aware, the complex 1c displays the shortest Pd 3 3 3 Au pseudo-closed-shell interaction currently reported. The close Pd-Au separation in 1c may be attributed to the positive charge of the complex, in keeping with shorter Au-Au distances in cationic complexes,13 compared with neutral complexes, e.g. [(AuCl)2C(PPh3)2].9 Most of the relatively few reports on Pd-Au interactions concern clusters, which contain a central Pd atom coordinated to several phosphine-ligated Au atoms;14 the distances between Au(I) and Pd(II) in dinuclear compounds are between 2.954 and 3.300 A˚.4 The Pd-C distance of 2.128(3) A˚ in 1c is somewhat longer compared with 1b (2.102(3) A˚); the Au-C separation (2.078(3) A˚) is almost identical with that observed for [(AuCl)2C(PPh3)2] (mean 2.076(3) A˚).9 The mean of the C(1)-P distances in the CDP functionality of 1.786 A˚ is slightly shorter compared with those in the protonated CDP complex 1b (mean 1.8025 A˚) and the diprotonated CDP ligand [CH2(dppm)2]Cl2 (1.804(2) A˚) but is similar to that in [(AuCl)2C(PPh3)2] (1.776(3) A˚), consistent with some delocalization of a negative charge on the central carbon atom.9,11 The Au-C(1)-Pd angle in 1c amounts to 86.80(11)°. This differs distinctly from a tetrahedral geometry for the central C atom and is markedly smaller than the Au-C-Au angle of 98.44(11)° in [(AuCl)2C(PPh3)2]9 but is comparable to those in complexes where the bridge between Pd and Au is formed by sulfur (Au-S-Pd = 81.75-91.92°).4c In comparison, the H-C(1)-Pd angle in 1b is 102(2)°,11 which clearly underlines the attractive nature of the d8-d10 interaction in 1c in contrast to the isolobal, protonated complex 1b. The P-C(1)-P angle of 120.66(17)° is comparable with 121.93(19)° for 1b and 117.30(15)° for [(AuCl)2C(PPh3)2].9,11 Two of the eight phenyl rings (at P(3) and P(4)) participate in intramolecular π-π stacking. The chloride counterion is disordered over two positions. In one position there are (12) Bondi, A. J. Phys. Chem. 1964, 68, 441. (13) (a) Vicente, J.; Chicote, M. T.; Guerrero, R.; Jones, P. G. J. Am. Chem. Soc. 1996, 118, 699. (b) Vicente, J.; Chicote, M. T.; Lagunas, M. C. Inorg. Chem. 1993, 32, 3748. (14) Tran, N. T.; Powell, D. R.; Dahl, L. F. Dalton Trans. 2004, 209 and references cited therein.
Note
Figure 2. Projections along Pd-C(1) showing the ligand conformations in the cations of 1c (top), 1b (middle), and 1a (bottom). All chloride ligands are omitted, and only the ipso carbon atoms of the phenyl groups are shown for clarity.
O-H 3 3 3 Cl interactions involving the cocrystallized MeOH molecules; the other position displays C-H 3 3 3 Cl hydrogen bonds involving the methylene groups of the C(dppm)2 ligand,15 in keeping with the observation that coordinated dppm and related ligands are capable of acting as C-H 3 3 3 X hydrogen bond donors.16 The coordination of the Au-Cl moiety and the concomitant d8-d10 interaction in 1c significantly influences the backbone conformation of the pincer ligand. The two fivemembered rings of 1c both adopt an envelope conformation: the atoms C(1), Pd, P(1/4), and C(2/3) are nearly coplanar, and the phosphorane P atoms project out of the plane. Both flaps point to the opposite side of the coordination plane of Pd with respect to the Au-Cl moiety. This Cs-symmetric structure of the ligand backbone is fairly remarkable in (15) The crystal structures of 1a 3 2.5(toluene) 3 MeOH, 1b 3 3H2O, and 1c 3 2MeOH all reveal very weak contacts of the ligand CH2 groups mainly with the Cl- counterions (C-H 3 3 3 Cl = 2.7 A˚ and longer; in the case of 1b the central CH group is also involved) and partly with the cocrystallized MeOH/H2O molecules (C-H 3 3 3 O = 2.3 A˚ and longer). Due to the long distances observed, these contacts are considered not to contribute significantly to the conformations of the pincer ligands. (16) Jones, P. G.; Ahrens, B. Chem. Commun. 1998, 2307.
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comparison to that of the related compounds 1a and, in particular, 1b (Figure 2). In the nonprotonated compound 1a the five-membered rings display twist conformations with the respective C and P atoms of the ligand backbone above and below the C-M-P plane and vice versa for the second five-membered ring, resulting in a quasi-C2 symmetry along the C-Pd-Cl axis. Protonation at the central C atom (1b) leads to formation of two inequivalent envelope conformations: one with the phosphorane P atom at the flap and the other one with the dppm-CH2 group at the flap. Both flaps point to the opposite side of the coordination plane of Pd with respect to the central C-H group.11 The structure found for 1c indicates that the favorable steric energies of the conformation in 1b are overruled by the energy of the Pd-Au d8-d10 interaction. NMR Spectroscopy. The 31P NMR spectrum of 1c consists of an [AX]2 pattern. The chemical shifts are between those of [PdCl(C(dppm)2)]Cl (1a) and [PdCl(CH(dppm)2)]Cl2 (1b), with the P(1)/P(4) shift closer to the related signal in 1a and the P(2)/P(3) shift closer to the related signal in 1b.11 The value of N (|J(P(1)P(2)) þ J(P(1)P(3))|) is reduced compared with 1a and is close to that of 1b. 1H NMR spectroscopy shows that the coordination of the AuCl moiety found in the solid-state structure of 1c is also retained in solution: as a result of the coordination of the AuCl fragment to C(1), two sets of chemically inequivalent protons (2J(H-C-H) = 15.6 Hz) are present in the spectrum for the CH2 group of the ligand backbone. In contrast to the related compounds 1a and 1b there is no indication for a dynamic process of the ligand backbone in 1c, since the quasi-Cs symmetry found in the solid state is consistent with the NMR spectral pattern found in solution. The compound 1a does not retain a rigid structure in solution but must be subject to fast conformational exchange processes on the NMR time scale at ambient temperature: the single 1H resonance found for the dppm-CH2 group does not reflect the quasi-C2 symmetry found in the solid state but indicates an inversion of the twist conformations of the two five-membered-ring systems. Also for 1b the 1H as well as the 31P NMR pattern is not compatible with the C1 symmetry found in the solid state, since only two methylene proton and two phosphorus resonances are observed, respectively, instead of four. A fast interconversion of the inequivalent envelope conformations may account for its 1H and 31P NMR spectrum found at room temperature.
Conclusion In conclusion, we were able to synthesize the first heterodimetallic PCP pincer complex, which is also the first example of a heterodimetallic CDP complex. Moreover, we could demonstrate the significance of tangential metallophilic interactions for the modification of structural and dynamic features in the ligand backbone of this class of organometallic compounds: this approach was introduced for the first time in pincer complex chemistry.
Experimental Section The complexes [PdCl(C(dppm)2)]Cl (1a),11 [PdCl(CH(dppm)2)]Cl2 (1b),11 and [AuCl(tht)]17 were prepared by literature (17) Uson, R.; Laguna, A.; Laguna, M.; Inorg. Synth. 1989, 26, 85.
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procedures; all other reagents were obtained from commercial suppliers. The solvents were not dried. All operations were carried out under atmospheric conditions. Elemental analyses were performed by the Institut f€ ur Physikalische Chemie der Universit€at Wien. Single-crystal X-ray data of 1c 3 2MeOH were collected on a Nonius Kappa CCD diffractometer using graphite-monochromated Mo KR radiation (λ = 0.710 73 A˚); relevant parameters are collected in Table 1. The structure was solved by direct methods.18 1H and 31P NMR spectra were recorded on a Bruker DPX 300 NMR spectrometer and were referenced against 1H solvent peaks and external 85% H3PO4, respectively. For 1c a 1H{31P} experiment was performed in order to determine the vicinal H-H coupling constant of the backbone CH2 group. [PdAu(Cl)2(C(dppm)2)]Cl (1c). [AuCl(tht)] (16 mg, 0.05 mmol) was added to a solution of 1a (48 mg, 0.05 mmol) in CH2Cl2 (0.6 mL). After 1 h, 31P NMR spectra showed that the reaction was complete. The volatiles were removed in vacuo to give a
yellowish microcrystalline solid. Crystals of 1c 3 2MeOH suitable for X-ray diffraction were obtained by slow evaporation of a solution in MeOH/CH2Cl2 (40 mg, 64%). 31P{1H} NMR (CD2Cl2/MeOH, 121 MHz): δ 16.0 (N (=|J(P(1)P(2)) þ J(P(1)P(3))|) = 64, P(1)/P(4)), 40.4 (P(2)/P(3)). 1H NMR (CDCl3/ MeOH, 300 MHz): δ 4.41 (m, 2J(H-C-H) = 15.6, H-C-H, 2H), 3.88 (m, 2J(H-C-H) = 15.6, H-C-H, 2H), 6.75 - 8.06 (m, Ph-CH, ca. 40H). Anal. Calcd for C51H44AuCl3P4Pd 3 2CH3OH: C, 50.74; H, 4.18. Found: C, 50.9; H, 4.1. [PdCl(CH(dppm)2)]Cl2 (1b). 31P{1H} NMR (CD2Cl2/ MeOH): δ 10.2 (N (=|J(P(1)P(2)) þ J(P(1)P(3))|) = 62.7, P(1)/ P(4)), 42.4 (P(2)/P(3)). 1H NMR (CD2Cl2/MeOH): δ 6.81 (2J(H(1)P(2)) = 18.6, CH, 1H), 5.23 (m, H-C-H, 2H), 4.53 (m, H-C-H, 2H), 7.05-8.01 (m, Ph-CH, ca. 40H). [PdCl(C(dppm)2)]Cl (1a). 31P{1H} NMR (CD2Cl2/MeOH): δ 19.6 (N (=|J(P(1)P(2)) þ J(P(1)P(3))|) = 83.8, P(1)/P(4)), 34.4 (P(2)/P(3)). 1H NMR (CD2Cl2): δ 3.95 (m, CH2, 4H), 7.10-7.80 (m, Ph-CH, ca. 40H).
(18) (a) Sheldrick, G. M. SHELXS-86: Program for Crystal Structure Solutions; Universit€at G€ottingen, G€ottingen, Germany, 1986; (b) Sheldrick, G. M. SHELXL-97: Program for Refinement of Crystal Structures; Universit€at G€ ottingen, G€ ottingen, Germany, 1997.
Supporting Information Available: CIF file giving crystallographic data for 1c 3 2MeOH. This material is available free of charge via the Internet at http://pubs.acs.org.
