Intramolecular Metal–Metal Bond Rearrangement in a Pt2PdHg

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Intramolecular Metal−Metal Bond Rearrangement in a Pt2PdHg Heterometallic Cluster Forming a HgI−PdI Covalent Bond Aya Hosokawa, Bunsho Kure, Takayuki Nakajima, Kanako Nakamae, and Tomoaki Tanase* Department of Chemistry, Faculty of Science, Nara Women’s University, Kitauoya-nishi-machi, Nara 630-8506, Japan S Supporting Information *

ABSTRACT: Reaction of the linear trinuclear complex [Pt2M(μ-dpmp)2(XylNC)2](PF6)2 (M = Pd (1b)) with HgX 2 afforded the Pt 2 PdHg mixed-metal complexes [Pt2PdHgX2(μ-dpmp)2(XylNC)2](PF6)2 (X = Cl (2), Br (3), I (4)), which included an unprecedented Hg I−PdI covalent bond formed via intramolecular metal−metal bond rearrangement. In contrast, reaction of 1a (M = Pt) with HgCl2 afforded the novel pentagonal-shaped Pt3Hg3 planar cluster [Pt3Hg3Cl4(μ-dpmp)2(XylNC)2]Cl2(PF6)2 (5).

M

3.28 Å),6 and notably, the shortest Hg−Pd contact (2.6915(8) Å) reported thus far was a HgII−Pd0 (d10−d10) interaction by Braunstein et al. in the Fe2PdHg heterometallic complex [Hg{Fe[Si(OMe) 3 ](CO) 3 (μ-dppm)} 2 Pd] (dppm = bis(diphenylphosphino)methane).7 The Hg−Pd bond length in the present report is unprecedentedly short (2.5830(5) Å) and is revealed to be the first example of a HgI−PdI covalent bond. The relevant reactions of the triplatinum complex 1a were also examined to find a novel Pt3Hg3 planar cluster.

etal−metal-bonded small-size clusters are fine-tunable building blocks for nanostructured molecular devices and metal surface mimetic catalytic systems. 1,2 In particular, expanding heterometallic conjugates is a challenging synthetic subject, with the aim of exploring new heterometallic bonding systems and synergistic effects exerted within the heterometallic centers.1,3 Taking this background into consideration, we have utilized linear tri- and tetraphosphines (bis[(diphenylphosphino)methyl]phenylphosphine (dpmp) and mesobis[((diphenylphosphino)methyl)phenylphosphino]methane (dpmppm)), in which each neighboring P atoms are connected by a methylene unit to support significant interactions between the metals and have synthesized a variety of linearly constrained multimetallic complexes.4,5 Among them, the linear Pt3 and Pt2Pd complexes [Pt2M(μ-dpmp)2(XylNC)2](PF6)2 (M = Pt (1a), Pd (1b); XylNC = 2,6-xylyl isocyanide),4e recognized as good building blocks for molecular wires, have been investigated to expand by a direct metal−metal bonding interaction, leading to the hexanuclear metal wires of [Pt2M2Pt2(μ-H)(μ-dpmp)4(XylNC)2](PF6)3 and [Pt2M2Pt2(μ-dpmp)4(XylNC)2](PF6)4 (M = Pt, Pd),4i and by bis-isocyanide linkers, forming the rigid-rod cluster polymers {[Pt3(μ-dpmp)2(bisNC)](PF6)2}n, where bisNC is 2,3,5,6tetramethylphenylene-1,4-bis-isocyanide.4j Further, expanding and assembling the trimetallic units by additional heterometals would be a promising strategy, since d10 metal ions with Lewis acidic properties, such as AuI, AgI, and HgII, are likely to form donor−acceptor interactions with low-valent metal centers. 3 In the present study, reactions of the Pt2Pd complex 1b with HgII ions were examined to find a HgI−PdI covalent bond formed through intramolecular metal−metal bond rearrangement in a Pt2PdHg mixed-metal system. The reported structures including Hg−Pd fragments have really been limited only to donor−acceptor and metallophilic interactions (2.74− © 2011 American Chemical Society

Scheme 1.

a

a

Phenyl groups of dpmp ligands are omitted for clarity.

When complex 1b was reacted with HgX2 in dichloromethane, dark violet microcrystals of the Pt2PdHg tetranuclear complexes [Pt2PdHgX2(μ-dpmp)2(XylNC)2](PF6)2 (X = Cl (2), Br (3)) were obtained in 50 and 44% yields, respectively (Scheme 1). A similar treatment with HgI2 in a THF−MeOH mixed solvent at 60 °C afforded [Pt 2 PdHgI 2 (μdpmp)2(XylNC)2](PF6)2 (4) in 19% yield. Complexes 2−4 were characterized by analytical, spectroscopic, and X-ray diffraction (for 2) techniques.8 The chloride anions of 2 were Received: September 10, 2011 Published: October 26, 2011 6063

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replaced by treatment with excess KI in a CH 2Cl2−MeOH mixed solvent to yield [Pt2PdHgI2(μ-dpmp)2(XylNC)2](HgI4) (4′), which was analyzed by X-ray crystallography (see the Supporting Information). The cluster cation of 2 (Figure 1) comprises a Pt2PdHg tetrametallic core bridged by two dpmp ligands, which is

dative bond, DFT single-point calculations were performed using the B3LYP method with LANL2DZ basis set on the Xray-determined structure (Figure 2). The HOMO (−9.27 eV)

Figure 2. MO drawings of 2 from the DFT calculations. Legend: Hg, violet; Pt, yellow; Pd, green; P, brown; N, light blue; C, gray.

and the LUMO (−5.87 eV) are composed mainly of σ-bonding and -antibonding interactions between the Hg s and Pd d orbitals, respectively, with a large HOMO−LUMO gap assuring the stability of the Hg−Pd bond. The next HOMO (HOMO-1, −9.98 eV) and the next LUMO (LUMO+1, −5.38 eV) are made up of the σ-bonding and antibonding interactions between the two Pt d orbitals, these also indicating the presence of a stable Pt−Pt covalent interaction. Wiberg bond indices from natural population analyses10 exhibit appreciable bonds existing for Pt1−Pt2 (0.344) and Pd1−Hg1 (0.344), whereas almost no bonding interactions are evaluated for Pt1− Hg1 (0.017), Hg1−Pt2 (0.062), and Pt2−Pd1 (0.032) (see the Supporting Information). On the basis of natural bond orbital analyses, the Pt−Pt bond is comprised of sd-hybridized orbitals of Pt1 (44%) and Pt2 (56%), and the Hg−Pd bond is composed of the s orbital of Hg1 (43%) and sd-hybridized orbitals of Pd1 (57%); these could be accommodated within the context of PtI−PtI and HgI−PdI covalent bonds. In the XPS of 2, the binding energies for Hg 4f5/2 and 4f7/2 were observed at 104.8 and 100.7 eV, which were between the values of Hg2Cl2 and HgCl2 (see the Supporting Information). The CN stretching vibration energy of the two XylNC ligands was accidentally degenerate at 2171 cm−1, corresponding to terminal xylyl isocyanides bound to PtI and PdI centers.4e,9 In light of the structural, theoretical, and spectroscopic results, the Hg−Pd bond found in 2 is the first example of a HgI−PdI covalent bond, whereas the analogous HgI−PtI bonds were already known to have very short contacts: e.g., 2.5759(2) Å in [ArHgPtAr(PPh3)2] (Ar = 2,6-dinitro-3,4,5-trimethoxyphenyl) and 2.572(1) Å in [(PPh3)2(C6Cl5)PtHgW(C5H5)(CO)3].11 The 31P{1H} NMR spectra of 2−4 exhibited A2B2C2 resonances in which two sets (AB) were accompanied by one-bond 195Pt satellite peaks (1JPtPA = 2418−2449 Hz, 1JPtPB = 2750−2876 Hz) and the other set (C) appeared with longrange 195Pt satellites (3JPtPC = 228−235 Hz) as well as two-bond 199 Hg satellites (2JHgPC = 661−714 Hz) (Figure 3a; see also the Supporting Information). The chemical shifts (δ A−C) showed linear correlations to the electronegativity (χ P) of the halide ions included in the clusters; the peaks were shifted upfield depending on the donating ability of X−.

Figure 1. (a) ORTEP view for the complex cation of 2. (b) View vertical to the Pt2PdHg plane. Xylyl rings are omitted, and phenyl carbons are drawn with arbitrary circles for clarity.

divided into two metal−metal bonding units. The Pt−Pt unit is terminally capped by XylNC and Cl− ligands (Pt1−Pt2 = 2.7061(3) Å, Pt1−C1= 1.991(9) Å, Pt2−Cl1 = 2.432(2) Å), and the Hg−Pd unit is terminated by Cl− and XylNC ligands, respectively (Hg1−Pd1 = 2.5830(5) Å, Hg1−Cl2 = 2.395(2) Å, Pd1−C2 = 2.014(7) Å); the two metal−metal bonds are approximately parallel as a result of cleavage of the Pt−Pd bond (Pt2−Pd1 = 3.2178(5) Å, Pt1−Pt2−Pd1 = 121.678(13)°, Pt2− Pd1−Hg1 = 56.895(12)°). The Pt−Pt bond distance indicates the presence of a PtI−PtI covalent bond in comparison with the dppm- and dpmp-bridged Pt2 complexes.4e,9a Remarkably, the Hg−Pd bond length is the shortest among the structures with Hg−Pd interactions reported thus far6,7 and demonstrates the presence of quite a strong bonding interaction. The Hg atom has a two-coordinate linear structure which is slightly tilted with respect to the Hg−Pd vector (Pd1−Hg1−Cl2 = 160.25(5)°), due presumably to a steric hindrance of the chloride and/or a weak interaction between Pt2 and Hg1 atoms (Hg1−Pt2 = 2.8191(3) Å). The Hg1−Pt1 distance of 3.2313(3) Å shows an absence of bonding interaction between them. In order to understand the electronic structure of 2, whether the Hg−Pd bond is a HgI−PdI covalent bond or a Pd0→HgII 6064

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Figure 4. ORTEP view of the complex cation of 5. The C and N atoms are drawn with arbitrary circles for clarity. Figure 3. (a) 31P{1H} NMR spectrum for the dpmp region of 2 and the assignments. (b) Differences of the chemical shifts (Δδ referenced to those of 4) for PA (blue ▲), PB (red ●), and PC (green ■) vs Pauling’s electronegativity χ P for the halogen involved in complexes 2−4. (c) 2JHgPC values vs χ P for the halogen involved in complexes 2− 4.

across the Pt1−Pt2 and Pt2−Pt3 bonds, resulting in an asymmetrical structure with the number of Cl− ions. The seven Hg−Pt interactions are recognized as two groups: three bonds to the central Pt atom and four bonds to the outer two Pt atoms. The former bond lengths (Hg1−Pt2 = 2.6819(7), Hg2− Pt2 = 2.6168(7), Hg3−Pt2 = 2.6856(7) Å, average 2.661 Å) are significantly shorter than the latter ones (Hg1−Pt1 = 2.9540(8), Hg2−Pt3 = 2.8761(8), Hg3−Pt1 = 2.9184(5), Hg3−Pt3 = 2.8548(7) Å, average 2.901 Å). The 31P{1H} NMR spectrum of 5 exhibited two resonances at δ 4.7 and −4.2 ppm with 195Pt satellite peaks in a 2:1 ratio, demonstrating that 5 takes a symmetrical structure or fluxional behavior in the solution state. The spectral pattern was mostly unchanged even at −60 °C, except for some broadening features. In conclusion, the linear Pt2Pd complex 1b readily reacted with HgX2 to form the Pt2PdHg heterometallic complexes [Pt2PdHgX2(μ-dpmp)2(XylNC)2](PF6)2, in which an unprecedented HgI−PdI covalent bond was unambiguously characterized. In contrast, reaction of the linear Pt3 complex 1a with HgCl2 resulted in formation of a planar Pt3Hg3 cluster, suggesting that the Pd center plays an important role in the intramolecular metal−metal bond rearrangement. Detailed mechanistic studies are being carried out. The present results could provide useful information in relation to the metal cluster architecture by developing new heterometal bonding systems.

The δ B values were greatly influenced by the halide ions directly engaged with the Pt2 center (δ B −0.3 (2), −4.2 (3), −11.4 (4)), and the δ C values also showed the next conspicuous shifts (δ C 15.0 (2), 12.4 (3), 8.1 (4)), although the δ A values barely changed due to the intact ligand of the Pt 1 center (δ A −3.6 (2), −4.2 (3), −5.2 (4)) (Figure 3b). Interestingly, the 2JHgPC values varied depending on the halide anion attached to the Hg atom (714 (2), 676 (3), 661 (4) Hz), demonstrating that the Hg−Pd bond should be weakened by the trans influence of the halide anions in the order I− > Br− > Cl− (Figure 3c). The expected tendency was confirmed by the X-ray structure of 4′,8 which exhibited an Pt2PdHg structure essentially identical with that of 2. The metal−metal bonds Pd1−Hg1 = 2.6118(6) Å and Pt1−Pt2 = 2.7384(4) Å are elongated by 0.0288 and 0.0323 Å, respectively, due to the stronger trans influence of I− ions. The deformation of the Pd1−Hg1−I2 unit from a linear array (158.314(18)°) slightly increases, with a slightly shorter contact between the Hg1 and Pt2 atoms (2.7722(3) Å) (see the Supporting Information). This weak interaction might contribute to stabilizing the Hg− Pd bond to some extent. To elucidate the role of the terminal PdI ion of 1b in the intramolecular metal−metal bond rearrangement, the reactivity of the triplatinum analogue 1a was investigated. Reaction of 1a with HgCl 2 gave the Pt 3 Hg 3 hexanuclear complex [Pt3Hg3Cl4(μ-dpmp)2(XylNC)2]Cl2(PF6)2 (5) in 20% yield. The complex cation of 5 consists of a pentagonal-shaped hexanuclear Pt3Hg3 structure with three Pt atoms bridged by two dpmp ligands (Figure 4). The Pt3 array is considerably deformed from linearity (Pt1−Pt2−Pt3 = 127.51(3)°) due to the coordination of three Hg ions. The two Pt−Pt distances (Pt1−Pt2 = 2.7572(6), Pt2−Pt3 = 2.7995(5), average 2.778 Å) are slightly longer than those of 1a (average 2.724 Å) but still fall within the Pt−Pt bonding range. Three HgII ions are bound to the bent triplatinum core; one HgCl− unit is connected to the three Pt atoms and the HgCl2 and HgCl− fragments are

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ASSOCIATED CONTENT

S Supporting Information *

Text giving experimental details of 2−5 and 4′, figures giving an ORTEP view of 4′, 31P{1H} NMR spectra of 2−4, XPS data of 2, UV−vis spectra of 2−4, and results of the NBO analysis with DFT calculations, and CIF files giving crystallographic data for 2, 4′, and 5. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]. Fax +81 742-20-3847.

ACKNOWLEDGMENTS This work was supported by Grant-in-Aid for Scientific Research and that on Priority Area 2107 (no. 22108521) from the Ministry of Education, Culture, Sports, Science and 6065

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= 21.242(7) Å, b = 14.623(5) Å, c = 36.374(13) Å, β = 101.971(4)°, V = 11 053(7) Å3, Z = 4, Dcalcd = 1.833 g cm−3, T = −120 °C, total of 24 870 unique reflections (Rint = 0.056) collected (6 < 2θ < 55.0°), R1 = 0.103 (13 331 reflections, I > 2σ(I)), wR2 = 0.289. CCDC-822716 (2), -822717 (4′), and -822718 (5) contain supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam. ac.uk/data_ request/cif. (9) (a) Bérubé, J.-F.; Gagnon, K.; Fortin, D.; Decken, A.; Harvey, P. D. Inorg. Chem. 2006, 45, 2812−2823. (b) Yamamoto, Y.; Yamazaki, H. Organometallics 1993, 12, 933−939. (10) (a) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899−926. (b) Wiberg, K. B. Tetrahedron 1968, 24, 1083−1096. (11) (a) Vicente, J.; Arcas, A.; Gálvez-López, M. D.; Jones, P. G. Organometallics 2004, 23, 3528−3537. (b) Braunstein, P.; Rossell, O.; Seco, M.; Torra, I.; Solans, X.; Miravltlles, C. Organometallics 1986, 5, 1113−1116.

Technology of Japan. T.T. is grateful to Nara Women’s University for a research project grant.

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