Electron-Deficient Pt2M2Pt2 Hexanuclear Metal Strings (M = Pt, Pd

Mar 3, 2014 - Martin Fleischmann , Luis Dütsch , Mehdi Elsayed Moussa , Gábor Balázs , Werner Kremer , Christophe Lescop , and Manfred Scheer...
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Electron-Deficient Pt2M2Pt2 Hexanuclear Metal Strings (M = Pt, Pd) Supported by Triphosphine Ligands Eri Goto, Rowshan Ara Begum, Chiaki Ueno, Aya Hosokawa, Chie Yamamoto, Kanako Nakamae, Bunsho Kure, Takayuki Nakajima, Takashi Kajiwara, and Tomoaki Tanase* Department of Chemistry, Faculty of Science, Nara Women’s University, Kitauoya-nishi-machi, Nara 630-8506, Japan S Supporting Information *

ABSTRACT: Electron-deficient Pt 2 M 2 Pt 2 hexanuclear clusters, [Pt4M2(μ-dpmp)4(XylNC)2](PF6)4 (M = Pt (7), Pd (8); dpmp = bis((diphenylphosphino)methyl)phenylphosphine), were synthesized by oxidation of hydride-bridged hexanuclear clusters [Pt4M2(μ-H)(μdpmp)4(XylNC)2](PF6)3 (M = Pt (2), Pd (3)) and were revealed to involve a linearly ordered Pt2M2Pt2 array joined by delocalized bonding interactions with 84 cluster valence electrons, which are discussed on the basis of DFT calculations. The central M−M distances of 7 and 8 are significantly reduced upon the apparent loss of a hydride unit from the M−H−M central part of 2 and 3, indicating that the bonding electrons in the adjacent M−Pt bonds migrate into the central M−M bond to result in a dynamic structural change during two-electron oxidation of the hexanuclear metal strings. A similar Pt6 complex terminated by two iodide anions, [Pt6I2(μ-dpmp)4](PF6)2 (9), was synthesized from [Pt6(μ-H)I2(μ-dpmp)4](PF6) (5) by treatment with [Cp2Fe][PF6]. Complexes 7 and 8 were readily reacted with the neutral two-electron donors XylNC, CO, and phosphines to afford the trinuclear complexes [Pt2M(μ-dpmp)2(XylNC)L](PF6)2 (M = Pt, L = XylNC (1a), CO (10), PPh3 (11); M = Pd, L = XylNC (1b)) through cleavage of the electron-deficient central M−M bond. When complex 7 was reacted with the diphosphines (PP) trans-Ph2PCHCHPPh2 (dppen) and Ph2P(CH2)2PPh2 (dppe), the diphosphine was inserted into the central M−M bond to afford [(XylNC)Pt3(μ-dpmp)2(PP)Pt3(μ-dpmp)2(XylNC)](PF6)4 (12), which was transformed by treatment with another 1 equiv of diphosphine into the asymmetric trinuclear complexes [Pt3(μ-dpmp)2(XylNC)(PP)](PF6)2 (13). A further ligand exchange reaction of 13a (PP = trans-dppen) provided the diphosphine-terminated symmetrical Pt3 complex [Pt3(μ-dpmp)2(L)2](PF6)2 (L = trans-dppen (14a)). Complexes 7 and 8 were also reacted with [AuCl(PPh3)] to yield the Pt2MAu heterotetranuclear complexes [Pt2MAuCl(μ-dpmp)2(PPh3)(XylNC)](PF6)2 (M = Pt (15), Pd (16)), in which the Pt2M trinuclear fragment is inserted into the Au−Cl bond in a 1,1-fashion on the central M atoms of the Pt2M2Pt2 string.

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PtIPt0MI for the metal centers, were synthesized by kinetically controlled site-selective incorporation of d10 M0 species into a diplatinum complex, syn-[Pt2(μ-dpmp)2(XylNC)2](PF6)2, and have proven to be good building blocks for further expanded metal strings because they are very reactive toward small organic molecules and metal species. The axial isocyanide ligands of 1a were exchanged by bisisocyanide ligands to afford the rigid-rod cluster polymers {[Pt3(μ-dpmp)2(bisNC)]2+}n (bisNC = 2,3,5,6-tetramethylphenyl-1,4-bisisocyanide, 2,2′,6,6′-tetramethyl-4,4′-biphenylene-1,1′-bisisocyanide).8f In addition, complex 1a was shown to react with HgCl2 to yield a planar Pt 3 Hg 3 hexanuclear cluster, [Pt 3 Hg 3 Cl 4 (μdpmp)2(XylNC)2]4+, and the same reaction of 1b led to intramolecular metal−metal bond rearrangement to afford [Pt2PdHgCl2(μ-dpmp)2(XylNC)2]2+ through a novel HgI−PdI covalent bond formation.8g

INTRODUCTION Low-oxidation-state, electron-rich “extended metal atom chains” (EMACs) have attracted growing attention as nanoscaled molecular wires in which electron-transporting properties along a single metal string should be important to elucidate quantum conducting phenomena.1,2 However, successful examples are quite limited due to difficulties in their syntheses and characterization,3−7 and designing metal-supporting multidentate ligands is crucial to establish such EMACs. We have developed transition-metal complexes by utilizing the linearly connected tri- and tetraphosphine ligands bis((diphenylphosphino)methyl)phenylphosphine (dpmp), 8 meso-bis(((diphenylphosphino)methyl)phenylphosphino)methane (dpmppm),9a−f and meso-1,3-bis(((diphenylphosphino)methyl)phenylphosphino)propane (dpmppp).9g,h During our studies, the linear trinuclear complexes [Pt2M(μ-dpmp)2(XylNC)2](PF6)2 (M = Pt (1a), Pd (1b); XylNC = 2,6-xylyl isocyanide),8a possessing an electronically unsaturated linear trinuclear core of 44 cluster valence electrons (CVEs) with apparent oxidation state © 2014 American Chemical Society

Received: December 19, 2013 Published: March 3, 2014 1893

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Scheme 1. Preparations of the Hydride-Bridged Pt2M2Pt2 Hexanuclear Clusters

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RESULTS AND DISCUSSION Two-Electron Oxidation of Hydride-Bridged Hexanuclear Clusters 2, 3, and 5. As reported in our previous paper,10b the hydride-bridged hexanuclear complexes 2 and 3 were shown in the cyclic voltammograms (CVs) to undergo two-step irreversible one-electron oxidations at Epa1 = −0.08 V (vs Ag/AgPF6) and Epa2 = +0.14 V for 2 and Epa1 = −0.18 V and Epa2 = +0.04 V for 3, and potentiostatic electrolysis at +0.35 V for 2 and at +0.10 V for 3 consumed 2 F/mol of the clusters to afford the two-electron-oxidized species [Pt4 M 2 (μdpmp)4(XylNC)2](PF6)4 (M = Pt (7), Pd (8)), respectively. In the present study, clusters 2 and 3 were chemically oxidized by treatment with 2 equiv of ferrocenium hexafluorophosphate ([Cp2Fe][PF6]) or HPF6 in CH2Cl2 to afford the two-electron oxidized clusters 7 and 8 in good isolated yields (Scheme 2). The oxidation reaction of 2 with HPF6 was monitored by UV− vis spectroscopy at room temperature every 2 min, in which the characteristic peak of 2 at 583 nm, assignable to the HOMO− LUMO transition concerning the central M−H−M part,10b was decreased with a gradual red shift to around 600 nm (Figure 1a). In contrast, complex 3 reacted with HPF6 much more quickly than 2, and this reaction was monitored every 5 s at −5 °C (Figure 1b); the characteristic absorption band for 3 at 674 nm quickly red-shifted to around 710 nm and then gradually decreased. Finally, these spectral changes converged to the spectral patterns of 7 and 8, in which the intense absorptions at 583 nm (2) and 674 nm (3) attributable to the M−H−M interactions completely disappeared. During the reactions, hydrogen gas generation was not observed and the intervening low-energy bands around 600 nm (2 to 7) and 710 nm (3 to 8) may correspond to the one-electron-oxidized intermediate species [Pt4M2(μ-H)(μ-dpmp)4(XylNC)2]4+ with 85 CVE, in light of the two-step one-electron-oxidation waves in the CVs, although they were not isolated and characterized. A similar

Recently, we have tried to assemble the trimetallic clusters 1a,b by generating direct metal−metal bonding interactions and successfully synthesized a series of linear Pt2M2Pt2 clusters, [Pt4M2(μ-H)(μ-dpmp)4(XylNC)2]3+ (M = Pt (2), Pd (3)), [Pt 6 (μ-H)(μ-dpmp) 4 (CO) 2 ] 3+ (6), and [Pt 6 (μ-H)X 2 (μdpmp)4]+ (X = H (4), I (5)), where a hydride bridges the central two M atoms (M = Pt, Pd) site selectively to form an unprecedented Pt2M−H−MPt2 linear structure with 86 CVEs (Scheme 1).10 Furthermore, the hexanuclear strings of 2 and 3 were quite stable even in the solution state and were interestingly shown to undergo two-electron oxidation, as monitored by cyclic voltammetry. Hence, investigation of the redox properties of the hexanuclear metal strings is quite valuable in developing low-valent functional metal wires. We wish to report herein the synthesis and characterization of two-electron-oxidized clusters with 84 CVEs, [Pt4M2(μdpmp)4(XylNC)2](PF6)4 (M = Pt (7), Pd (8)), in which the electron-deficient Pt2M2Pt2 strings were remarkably retained against the apparent hydride dissociation from the central part. The 84-CVE count of 7 and 8 is “two-electron deficient” from the electron-precise number of 86 CVEs, which is derived from 6 16-valence-electron metal units connected by metal−metal covalent bonds. Complexes 7 and 8 were quite reactive toward the series of 2-electron donors isocyanide, carbon monoxide, and mono- and diphosphines as well as [AuCl(PPh3)] to afford a variety of tri- and tetranuclear complexes. The electronic structures of 7 and 8 are discussed on the basis of DFT calculations, showing that the two electron-poor {Pt2M(μdpmp)2(XylNC)}2+ fragments are connected with delocalized bonding interactions in the central Pt−M−M−Pt part. The Pt2M2Pt2 hexanuclear chains could be a new foothold to construct further expanded, low-valent metal atom chains.11 1894

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Hz) in CD2Cl2 (Figure S1b, Supporting Information). The peaks at the highest frequency (δ 20.8 ppm (7), 5.9 ppm (8)) are assigned to the P atoms attached to the central M centers of the Pt2M2Pt2 strings (M = Pt (7), Pd (8)). The ESI-MS spectra in acetone showed an intense divalent peak at m/z 864.635 (7) and 820.107 (8) assignable to the fragment of {Pt2M(dpmp)2(XylNC)}2+ (m/z 864.633 (M = Pt), 820.103 (M = Pd)) (Figure S2, Supporting Information), which are derived from hemolytic cleavage of the cluster cations 7 and 8. The detailed structures of 7 and 8 were unambiguously determined by X-ray crystallography; perspective views for the cluster cations of 7 and 8 are illustrated in Figure 2 and Figure S3 (Supporting Information). Some selected structural parameters are given in Table 1. The crystals of 7 and 8 are isomorphous, and the asymmetric unit contains half parts of two independent but chemically equivalent cluster cations of A and B together with four hexafluorophosphate anions and solvated dichloromethane molecules. Each cluster cation has a 4+ charge with 84 CVEs, corresponding to a two-electronoxidized count from the 86 CVEs of 2 and 3. For the hexaplatinum complex 7, the cluster cation 7A has a crystallographically imposed C2 axis along the metal chain, resulting in a perfectly linear alignment of the Pt6 rod terminated by two collinear isocyanides, and the other cation 7B has a crystallographic C2 axis perpendicular to the Pt6 chain which is almost linearly arranged (Pt8−Pt7−C3 = 178.5(3)°, Pt7−Pt8−Pt9 = 179.66(2)°, Pt8−Pt9−Pt9* = 178.42(2)°). The overall structural features of 7A and 7B are quite similar to each other, which can be recognized by an ideal D2 symmetry. In comparison with the structure of 2, the most striking feature is that the central Ptcen−Ptcen separation (dcen = 2.823 Å (average); Pt3−Pt4 = 2.8229(6) Å, Pt9−Pt9* = 2.8222(4) Å) is drastically reduced by 0.486 Å from the corresponding distance of 2 (3.3093(4) Å),10b indicating the presence of a metal−metal bonding interaction in the central part despite the absence of bridging organic ligands between them. In contrast, the neighboring Ptinn−Ptcen bond distances are appreciably elongated by 0.064 Å (dinn = 2.801 Å (average); Pt2−Pt3 = 2.8034(6) Å, Pt4−Pt5 = 2.8002(6) Å, Pt8−Pt9 = 2.8005(4) Å) from the distance of 2 (2.737 Å (average)). The outer Ptout− Ptinn bond distances (dout = 2.701 Å (average); Pt1−Pt2 = 2.7039(6) Å, Pt5−Pt6 = 2.6974(6) Å, Pt7−Pt8 = 2.7009(4) Å) are slightly shorter than those of 2 (2.716 Å (average)), showing the presence of a Pt−Pt single bond as is found in the triplatinum complex 1a (2.724 Å (average)).8a The structural change from 2 to 7 as shown in Figure 3a clearly demonstrated that, upon the apparent hydride elimination from the Ptcen−H− Ptcen part, the bonding electrons in the adjacent Ptinn−Ptcen bonds partially migrate into the central Ptcen−Ptcen region to result in a delocalized Ptinn−Ptcen−Ptcen−Ptin bonding interaction with formally four σ valence electrons. The full length of the Pt6 string (13.826 Å (average)) also decreases from that of 2 (14.202 Å) according to the alternation of valence electrons. As a result of the significant bonding interaction in the central part, the P−Ptcen−P angles (168.5° (average)) exhibit outward deformation from the {Pt3(μ-dpmp)2} fragment and the two Pt3 units are twisted around the Pt6 axis with an average P− Ptcen−Ptcen−P torsion angle of 71.4° to avoid steric repulsions. The crystal structure of the heteronuclear cluster 8 is isomorphous with that of 7, except that two palladium atoms occupy the central positions without any disorder to form the linear Pt2Pd2Pt2 chain (Figure S3, Supporting Information). As observed in 7, the central Pd−Pd separation considerably

Scheme 2. Preparations of the Pt2M2Pt2 Hexanuclear Clusters 7−9

Figure 1. UV−vis spectral changes for the reactions with HPF6 for (a) 2 in CH2Cl2 at room temperature monitored every 2 min and for (b) 3 in CH2Cl2 at −5 °C monitored every 5 s.

treatment of the hydride-bridged hexaplatinum complex 5 with 2 equiv of [Cp2Fe][PF6] afforded an iodide-terminated complex, [Pt6I2(μ-dpmp)4](PF6)2 (9), which exhibited characteristic electronic absorption bands around 300−450 nm together with a very weak absorption at 691 nm. The IR spectra of 7 and 8 showed a CN stretching band for the terminal xylyl isocyanides at 2161 cm−1 (7) and 2162 cm−1 (8), the energy of which is higher by ca. 30 cm−1 than the values of 2131 cm−1 (2) and 2135 cm−1 (3) and is consistent with the two-electron oxidation of the hexanuclear metal cores. The 31P{1H} NMR spectrum of 7 in acetone-d6 exhibited three resonances accompanied by satellite peaks due to coupling to 195 Pt in 1:1:1 ratio at δ 20.8 ppm (1JPtP = 3413 Hz), −1.1 ppm 1 ( JPtP = 2741 Hz), and −6.9 ppm (1JPtP = 2797 Hz) (Figure S1a, Supporting Information). While the signals were shifted to lower field from those of 2, the spectral pattern is quite similar to that of 2, suggesting that a C2-symmetrical hexanuclear structure is retained in the solution state. The 31P{1H} NMR spectrum of 8 also showed the same spectral patterns at δ 5.9 ppm, 0.4 ppm (1JPtP = 2766 Hz), and −11.1 ppm (1JPtP = 2887 1895

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Figure 2. Perspective drawings for the cluster cations of (a) 7A and (b) 7B with atomic numbering schemes. The cation of 7A possesses a crystallographically imposed C2 axis along the hexaplatinum chain, and the cation of 7B has a C2 axis perpendicular to the metal chain. The Pt and P atoms are illustrated with thermal ellipsoids at the 40% probability level. The hydrogen atoms are omitted, and the C and N atoms are drawn as arbitrary spherical models for clarity. Color scheme: Pt (yellow), P (pink), N (blue), C (gray).

methods to converge at a linear Pt6 structure (Ptout−Ptinn = 2.790 Å (average), Ptinn−Ptcen = 2.979 Å (average), Ptcen−Ptcen = 3.042 Å), essentially consistent with the crystal structure of 2 apart from the systematic elongation of distances between heavy atoms (Figure S4 and Table S1, Supporting Information). The calculated structure of M8 by the same methods converged at a structure similar to M7 with Ptout−Ptinn = 2.772 Å (average), Ptinn−Pdcen = 3.021 Å (average), and Pdcen−Pdcen = 3.138 Å (Figure S5, Supporting Information). The simplified interaction diagram for the MOs including dσ orbitals of the metal atoms in M7 and M8 and their MO surface diagrams for M7 are depicted in Figure 4. The trinuclear fragments {Pt2M(C2H9P3)2(CNCH3)}2+ (M = Pt (F7), Pd (F8)) contain three essentially important fragment MOs with dσ orbitals (FMO-1,2,3), and upon combination of two fragments to form [Pt2M2Pt2(C2H9P3)4(CNCH3)2]4+ (M = Pt (M7), Pd (M8)), six MOs of dσ systems are generated with in-phase and out-ofphase interactions between the central two M atoms as in the MO-1a,b, MO-2a,b, and MO-3a,b for M7 (Figure 4, right). The HOMO (MO-2b) contains an antibonding interaction between the central two M atoms which is stabilized by mixing of pσ orbitals rendering nonbonding character to some extent, and the LUMO (MO-3a) is derived from the out-of-phase fragment MOs (FMO-3) with σ-bonding interaction between the two central M atoms. The calculated electronic structure corresponds to a two-electron-deficient state of the electron-precise structure (86 CVEs) by connecting 6 16-valence-electron metal units. The natural charge population and the Wiberg bond indices12 for M7 and M8, supplied in Table S1 and Figures S4 and S5 (Supporting Information), suggest electron-deficient and delocalized Ptinn−Mcen−Mcen−Ptinn bonding systems which are in contrast with the strong Ptout−Ptinn bonding interactions. Since some attempts to optimize the full structures of 7 and 8 with B3LYP or MPWB95 functionals were not successful owing to the long-range attractive interactions such as dispersion

decreased by 0.416 Å (dcen = 2.836 Å (average); Pd1−Pd2 = 2.8367(17) Å, Pd3−Pd3* = 2.8343(11) Å) from that of 3 (3.2514(7) Å),10b and the neighboring Pd−Ptinn distances are elongated by 0.032 Å (dinn = 2.789 Å (average); Pd1−Pt2 = 2.7945(13) Å, Pd2−Pt3 = 2.7864(13) Å, Pd3−Pt6 = 2.7866(9) Å), indicating the presence of a delocalized Ptinn−Pdcen−Pdcen− Ptin bonding interaction. The terminal Ptout−Ptinn bond lengths (dout = 2.685 Å (average); Pt1−Pt2 = 2.6857(8) Å, Pt3−Pt4 = 2.6821(8) Å, Pt5−Pt6 = 2.6866(6) Å) are slightly shorter than those of 3 (dout = 2.714 Å (average)) and fall within the normal values for Pt−Pt single bonds. Although the central Pd−Pd distances of 8 (dcen = 2.836 Å (average)) are slightly longer than the corresponding distances of 7 (dcen = 2.823 Å (average)), the overall core structural change from 3 to 8 is quite similar to that from 2 to 7 (Figure 3b). The full length of the Pt2Pd2Pt2 core (13.783 Å (average)) is also shorter than that of 3 (14.177 Å). Other structural features of 8 are almost identical with those observed in 7 (Table 1). These X-ray structures definitively revealed that dynamic contraction of the Pt 2 M 2 Pt 2 strings occurred without fragmentation, which is coupled with two-electron oxidation or apparent hydride elimination from the M−H−M central part of 2 and 3.10b Electronic Structures of the Hexanuclear Clusters 7 and 8. In order to understand the electronic structures of the hexanuclear Pt2M2Pt2 clusters of 7 and 8, gas-phase theoretical calculations with DFT methods were performed on the model compounds [Pt4M2(μ-C2H9P3)4(CNCH3)2]4+ (M = Pt (M7), Pd (M8)) as well as the X-ray-determined structures [Pt4M2(μdpmp)4(XylNC)2]4+ (M = Pt (7), Pd (8)). The initial coordinates of M7 and M8 were generated by modifications of the X-ray crystal structures of 7 and 8, in which the phenyl groups of dpmp were replaced by H atoms and the xylyl groups by methyl groups. At first, the structure of M7, a model compound for 7, was optimized with B3LYP/LANL2DZ 1896

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Table 1. Structural Parameters of [Pt2M2Pt2(μ-dpmp)2(XylNC)2](PF6)4 (M = Pt (7), Pd (8)) and [Pt2M2Pt2(μ-H)(μdpmp)2(XylNC)2](PF6)3 (M = Pt (2), Pd (3))

7a Mcen−Mcen, Å

Ptinn−Mcen, Å

Ptout−Ptinn, Å

Mcen−Pcen, Å Ptinn−Pinn, Å Ptout−Pout, Å Ptinn−Mcen−Mcen, deg

Ptout−Ptinn−Mcen, deg

Ptinn−Ptout−C, deg

Pcen−Mcen−Pcen, deg Pin−Ptin−Pin, deg Pout−Ptout−Pout, deg a

8b d

2.8229(6) (A) 2.8222(5) (B)e 2.823 (av)f 2.8034(6) (A) 2.8002(6) (A) 2.8005(4) (B) 2.801 (av) 2.7039(6) (A) 2.6974(6) (A) 2.7009(4) (B) 2.701 (av) 2.285 (av) 2.260 (av) 2.309 (av) 180 (A) 178.42(2) (B) 179.21 (av) 180 (A) 179.66(2) (B) 179.83 (av) 180 (A) 178.5(3) (B) 179.25 (av) 168.54 (av) 176.03 (av) 174.79 (av)

2.8367(17) (A) 2.8343(11) (B) 2.836 (av) 2.7945(13) (A) 2.7864(13) (A) 2.7866(9) (B) 2.789 (av) 2.6857(8) (A) 2.6821(8) (A) 2.6866(6) (B) 2.685 (av) 2.305 (av) 2.256 (av) 2.302 (av) 180 (A) 178.00(3) (B) 179.00 (av) 180 (A) 179.66(2) (B) 179.83 (av) 180 (A) 177.9(5) (B) 179.0 (av) 168.30 (av) 176.23 (av) 174.50 (av)

2c

3c

3.3093(4)

3.2514(7)

2.7365(4) 2.7375(4) 2.737 (av)

2.7555(5) 2.7569(6) 2.756 (av)

2.7208(4) 2.7103(4) 2.716 (av)

2.7174(3) 2.7098(3) 2.714 (av)

2.271 (av) 2.257 (av) 2.297 (av) 178.446(13) 179.325(13) 178.89 (av) 174.939(15) 179.511(15) 177.23 (av) 177.0(2) 176.7(2) 176.9 (av) 171.49 (av) 178.06 (av) 178.33 (av)

2.285 (av) 2.258 (av) 2.303 (av) 178.74(2) 179.77(2) 179.26 (av) 174.58(2) 179.85(2) 177.22 (av) 177.2(2) 177.0(2) 177.1 (av) 170.91 (av) 178.36 (av) 177.91 (av)

See Figure 2. bSee Figure S3 (Supporting Information). cReference 10b. dCluster cation A. eCluster cation B. fav = average.

single-point DFT calculations on the X-ray-determined structures of 7 and 8 were carried out to show the quite similar electronic structures as predicted in the preliminary calculations on M7 and M8 (Figures S6 and S7 and Table S1, Supporting Information). The surface diagrams of the HOMO and LUMO and the natural charge and Wiberg bond indices are shown in Figure 5. The natural charge population (average −0.22 on Ptout, −0.46 on Ptinn, and −0.57 on Ptcen in 7 and average −0.20 on Ptout, −0.47 on Ptinn, and −0.40 on Pdcen in 8) implied that the oxidation state of the outer platinum atoms (Ptout) is +1, while those for the central four metal atoms (PtinnMcenMcenPtinn) are averaged between the values of +1 and 0. The Wiberg bond indices (WBIs) also indicated that there are single bonds between the Pinn and Pout atoms as WBIs for Ptinn−Ptout = 0.467 (7) and 0.465 (8) and, on the other hand, the bonding interactions between the central four metal atoms are weak and delocalized as WBIs for Mcen−Ptinn = 0.281 (7) and 0.248 (8) and those for Mcen−Mcen = 0.223 (7) and 0.159 (8). Notably, the incorporation of palladium atoms in the hexanuclear core reduces the bond strengths of the delocalized Ptinn−Mcen−Mcen−Ptinn system, although the reason is not clear at present.

Figure 3. Structural changes of the hexanuclear cores (a) from 2 to 7 and (b) from 3 to 8 with the average metal−metal distances (Å).

force, ππ, and C−H/π interactions existing in the real systems, in order to obtain more precise theoretical considerations, 1897

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Figure 4. Interaction diagram of the σ MOs for model complexes M7 and M8 from the corresponding trinuclear fragments {Pt2M(C2H9P3)2(CNCH3)}2+ (left) and the MO surface diagrams of MO-1a,b, MO-2a,b, and MO-3a,b for M7 (right) by DFT calculations with B3LYP/LNL2DZ methods.

Figure 5. (a) MO diagrams for the HOMO and LUMO of 7 (left) and 8 (right) and (b) values of natural charge (red) and Wiberg bond indices (blue) for 7 (left) and 8 (right), derived from single-point DFT calculations with B3LYP/LANL2DZ methods.

Reactions of Electron-Deficient Hexanuclear Complexes 7 and 8 with XylNC, CO, PPh3, trans-dppen, and

dppe. Reactivity of the electron deficient clusters 7 and 8 was examined by treatment with the neutral two-electron donors 1898

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Scheme 3. Reactions of 7 and 8 with XylNC, CO, and PPh3

Scheme 4. Reactions of 7 with the Diphosphines trans-dppen and dppe

XylNC, CO, and PPh3 (Scheme 3). Complexes 7 and 8 readily reacted with 2 equiv of XylNC to afford the trimetallic complexes [Pt2M(μ-dpmp)2(XylNC)2](PF6)2 (M = Pt (1a), Pd (1b)), which were monitored by UV−vis spectral changes by successive addition of XylNC in dichloromethane (Figure S8, Supporting Information). In the reaction of 7, the characteristic absorption of 7 at 377 nm was diminished while the characteristic bands for 1a appeared at 359, 389, and 500 nm with isosbestic points around 321, 363, 385, and 415 nm (Figure S8a). Similarly, in the reaction of 8, the peaks of 8 at 344, 369, and 462 nm decreased while the absorptions for 1b increased with isosbestic points of 318, 381, and 455 nm (Figure S8b). These results were in contrast with the inertness of the hydride-bridged hexanuclear clusters 2 and 3 for XylNC and strongly suggested that 7 and 8 are actually electron deficient in the central M−M part. Complex 7 also reacted with

CO (1 atm) or PPh3 (2 equiv) to be transformed into the asymmetric triplatinum complex [Pt 3 (μ-dpmp) 2 (CO)(XylNC)](PF6)2 (10) or [Pt3(μ-dpmp)2(PPh3)(XylNC)](PF6)2 (11); these were characterized by UV−vis, 1H and 31 1 P{ H} NMR, and ESI mass spectra (Figures S9, S10a,b, and S11a,b, Supporting Information). These asymmetric triplatinum complexes were not obtained from 1a (or 2) through the usual terminal ligand exchange reactions. When complex 7 was treated with the diphosphines transPh 2 PCHCHPPh 2 (trans-dppen) and Ph 2 P(CH 2 ) 2 PPh 2 (dppe), the diphosphine was first inserted into the central Ptcen−Ptcen bond to afford [(XylNC)Pt3(μ-dpmp)2(PP)Pt3(μdpmp)2(XylNC)](PF6)4 (PP = dppen (12a), dppe (12b)), which reacted further with the diphosphine to result in the asymmetric trinuclear complexes [Pt3(μ-dpmp)2(XylNC)(PP)](PF6)2 (PP = dppen (13a), dppe (13b)) (Scheme 4). 1899

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NMR spectrum of the reaction mixture containing 7 and transdppen in a 1:1 ratio was very broad and complicated, but ESIMS for this solution exhibited a dicationic peak at m/z 2072.35 corresponding to [{Pt3 (dpmp) 2 (XylNC)} 2(trans-dppen)(PF6)2]2+ (Figure S10c). During the latter transformation with addition of 1.0−2.0 equiv of trans-dppen, the absorption at 475 nm shifted to 463 nm and that around 541 nm diminished, while the intensity of the absorption at 398 nm was not altered. These relatively small spectral changes suggested that the cluster core did not drastically change in the latter half of the process. The 31P{1H} NMR spectra of the reaction mixture containing 7 and trans-dppen in a 1:2 ratio, corresponding to the final state of the titration, showed five well-resolved resonances at δ −13.5 (1JPtP = 2856 Hz), −6.0 (3JPP = 19 Hz), −4.0 (1JPtP = 2782 Hz), 1.9 (1JPtP = 2950 Hz), and 6.6 (1JPtP = 3270 Hz) in a 2:1:2:1:2 ratio, which are consistent with the asymmetric structure of 13a (Figure S10c). The doublet peak at δ −6.0 is assignable to the noncoordinated P atom of transdppen due to the absence of 195Pt satellite peaks. The ESI-MS spectrum exhibited an intense dicationic peak at m/z 1062.76 corresponding to {Pt3(dpmp)2(trans-dppen)(XylNC)}2+ (Figure S10d). Similar UV−vis absorption spectral changes were observed for the titration of 7 with dppe (Figure S12). When 7 was reacted with excess trans-dppen for several days, a further ligand exchange reaction of 13a occurred to result in the formation of [Pt3(μ-dpmp)2(dppen)2](PF6)2 (14a), which was isolated in crystalline form and characterized by X-ray crystallography (Figure S13, Supporting Information). The complex cation has an inversion center and consists of linearly arranged three Pt atoms bridged by two dpmp ligands (Pt1− Pt2 = 2.7540(5) Å). Two dppens ligate to the axial sites of the {Pt3(μ-dpmp)2}2+ unit in a monodentate fashion (Pt1−P4 = 2.348(3) Å, Pt2−Pt1−P4 = 162.91(8)°), and the other P atoms of the dppen ligands are uncoordinated. The Pt−Pt bond length is slightly longer than those of 1a due to the stronger σdonating ability of the phosphines in comparsion with isocyanides. Reactions of 7 and 8 with [AuCl(PPh3)], Forming the Pt 2 MAu Heterotetranuclear Clusters [Pt2 MAuCl(μdpmp)2(PPh3)(XylNC)](PF6)2 (M = Pt (15), Pd (16)). Complexes 7 and 8 were treated with 2 equiv of [AuCl(PPh3)] in dichloromethane; homolytic cleavage of the electrondeficient clusters occurred to yield the heterotetranuclear Pt2MAu clusters [Pt2MAuCl(μ-dpmp)2(PPh3)(XylNC)](PF6)2 (M = Pt (15), Pd (16)), which were characterized by spectroscopic and X-ray crystallographic techniques (Scheme 5). Whereas the 31P{1H} NMR spectra were too complicated and were not fully characterized with several multiplets accompanied by 195Pt satellites around −20 to −30 ppm (15) and −25 to −20 ppm (16), the peaks for Au-bound

These transformations were monitored using UV−vis spectroscopy by titrating 7 with successive addition of diphosphine, as shown in Figure 6 and Figure S12 (Supporting

Figure 6. (a) UV−vis spectral changes in dichloromethane for the titration of 7 by successive addition of trans-dppen (portions of 0.2 equiv) up to a total amount of 2.0 equiv, (b) spectral changes on adding 0−1 equiv of trans-dppen, and (c) spectral changes on adding 1−2 equiv of trans-dppen.

Information). Figure 6a shows the spectral changes through addition of trans-dppen by amounts of 0−2.0 equiv, which can be divided into two reaction stages with 0−1.0 equiv of transdppen (Figure 6b) and 1.0−2.0 equiv (Figure 6c). During the former transition, the characteristic absorption at 377 nm for 7 steadily decreases and, instead, new absorption bands appeared at 398, 475, and 541 nm which were saturated in intensity at a condition with addition of 1 equiv of trans-dppen. The 31P{1H} Scheme 5. Reactions of 7 and 8 with [AuCl(PPh3)]

1900

dx.doi.org/10.1021/om401211d | Organometallics 2014, 33, 1893−1904

Organometallics

Article

= 1.957(12) Å, Pt3−Cl1 = 2.398(2) Å). The Pt1−Pt2 bond length of 2.6608(6) Å is shorter than those of 1a (2.72 Å (average)) and close to that of the d9−d9 dimer [Pt2Cl2(μdppm)2] (2.651(1) Å),15 indicating the presence of a Pt−Pt single bond. In the Au(PPh3)-bridged Pt2Au1Pt3 part, on the other hand, the Pt2−Pt3 distance of 2.8012(6) Å is significantly expanded with the Pt2Au asymmetric interactions of Pt3−Au1 = 2.6358(6) Å and Pt2−Au1 = 2.7424(6) Å. The Pt−Au distances in 15 are approximately comparable to those found in Pt−Au heteronuclear clusters,16 and exactly, the former is shorter and the latter is longer than the Pt−Au bond distances observed in the A-frame type diplatinum complexes [Pt2(C C-tBu)2(μ-AuI)(μ-dppm)2] (2.659 Å (average)),13b [Pt2(μAuCl)(μ-dppm)2(CN)2] (2.641 Å (average)),17 and [Pt2(μAuPPh3){C6H4P(Ph)CH2CH2CH2PPh2}2](BF4) (2.710 Å (average)),14 which are all indicative of typical three-center/ two-electron bonding interactions in the Pt2Au systems. The asymmetric Pt−Au distance may imply that the AuI ion interacts more strongly with the terminal Pt3 atom than with the inner Pt2 atom, which is in good agreement with the 2JPtP values for the Au-bound PPh3 signal in the 31P{1H} NMR spectrum. The overall structure of the Pt2PdAu cluster 16 (Figure S16, Supporting Information) is identical with that of 15, and it is worth noting that the X-ray structure of 16 clearly demonstrated that Au(PPh3)+ is added onto the Pt−Pd bond, which is consistent with the 31P{1H} NMR results discussed above (Figure S14, Supporting Information). In the bent Pt2Pd chain (Pt1−Pt2−Pd1 = 151.40(2)°), the terminal platinum atom is capped by a XylNC ligand (Pt1−C1 = 1.936(11) Å) and the palladium atom is coordinated by a chloride ion (Pd1− Cl1 = 2.382(3) Å). The Pt1−Pt2 bond length of 2.6363(6) Å is slightly shorter than that of 15, while the Pt2−Pd1 distance of 2.8006(9) Å is almost the same as the corresponding distance in 15. The Au1 ion is selectively incorporated into the Pt2−Pd1 bond to result in an asymmetric Pt2Au1Pd1 triangular structure with Pt2−Au1 = 2.6935(6) Å and Pd1−Au1 = 2.6174(9) Å; both are shorter than the corresponding values of 15 and still suggested that the AuI ion is more tightly bound to the Pd center coordinated by a chloride ion. These structural features are quite parallel to those observed in 15 and are consistent with the 31P{1H} NMR spectrum. The structures of 15 and 16 are apparently derived from 1,1-insertion of an electrondeficient {Pt2M(μ-dpmp)2L}2+ fragment into the Au−Cl bond of [AuCl(PPh3)] at the central M position of 7 and 8 and consequently suggested that the reaction may be initiated by nucleophilic attack of a chloride ion from [AuCl(PPh3)] and concomitant electrophilic addition of AuI(PPh3) fragment on the central M atoms of 7 and 8.

phosphorus atoms of PPh3 were clearly observed at 37.8 ppm (2JPtP = 310 Hz, 2JPt’P = 769 Hz) (15) and 35.7 ppm (2JPtP = 353 Hz) (16) accompanied by distinctive 195Pt satellites (Figure S14, Supporting Information). Although similar spectral features for Au-bound phosphorus atom of PPh3 are also observed in the symmetric A-frame complexes [Pt2AuCl2(μ-dppm)2(PPh3)](NO3) (dppm = diphenylphosphinomethane)13a (δ 36.5 ppm; 2JPtP = 600 Hz) and [Pt2Au{μ{o-C6H4P(Ph)(CH2)nPPh2}2(PPh3)](BF4) (n = 2, δ 26.9 ppm, 2 JPtP = 619 Hz; n = 3, δ 30.4 ppm, 2JPtP = 620 Hz),14 the Aubound phosphorus peak of 15 clearly showed two sets of 195Pt satellite peaks with 2JPtP = 769 and 310 Hz (Figure S14a), suggesting an asymmetric bonding interaction in the Pt−Au−Pt moiety, in which one is stronger and the other is weaker. In contrast, the corresponding peak of 16 showed only one set of 195 Pt satellites with 2JPtP = 353 Hz (Figure S14b), strongly demonstrating that the heterometallic Pt−Pd bond is exclusively bridged by the Au(PPh3) fragment to form a Pt− Au−Pd moiety, in which the Pd−Au bond is estimated to be stronger than that of the Pt−Au bond in the light of a comparison of the 2JPtP values to those of 15. The ESI mass spectra of 15 and 16 clearly demonstrated dicationic parent peaks for [Pt2MAuCl(dpmp)2(PPh3)(XylNC)]2+ (M = Pt (15), Pd (16)) at m/z 1111.67 and 1067.61, respectively, indicating that the tetranuclear cores of 15 and 16 are retained in the solution state (Figure S15, Supporting Information). It is noteworthy that complexes 15 and 16 were no longer obtained from similar reactions of the trinuclear complexes 1a and 1b and the hexanuclear complexes 2 and 3, all possessing electron -precise CVE counts. The lack of reaction of 2 and 3 with AuCl(PPh3) is ascribed to the fact that the electron-rich M− H−M central part is stabilized by a 3c/2e interaction and sterically protected by the phenyl groups of dpmp ligands. ORTEP diagrams for the complex cations of 15 and 16 are shown in Figure 7 and Figure S16 (Supporting Information), respectively. The complex cation of 15 is comprised of a bent Pt3 chain (Pt1−Pt2−Pt3 = 152.08(2)°), in which an Au(PPh3)+ fragment is incorporated into one Pt−Pt bond to form the Pt3Au tetranuclear unit. The Pt3 moiety is supported by two dpmp ligands, and each terminal platinum atom is asymmetrically coordinated by a XylNC ligand and chloride ion (Pt1−C1

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CONCLUSION In the present study, the hydride-bridged hexanuclear clusters [Pt4M2(μ-H)(μ-dpmp)4(XylNC)2](PF6)3 (M = Pt (2), Pd (3)) and [Pt6(μ-H)I2(μ-dpmp)4](PF6) (5) were oxidized by [Cp2Fe][PF6] or HPF6 to afford the electron-deficient hexanuclear clusters [Pt4M2(μ-dpmp)4(XylNC)2](PF6)4 (M = Pt (7), Pd (8)) and [Pt6I2(μ-dpmp)4](PF6)2 (5) with 84 CVEs. The structures of 7 and 8 have been characterized by X-ray crystallography to involve a nanoscaled, linear Pt2M2Pt2 string wrapped with four triphosphine ligands, in which the central Pt−M−M−Pt atoms are joined by a two-electron-deficient delocalized bonding interaction. These X-ray structures clearly revealed that dynamic contraction of the Pt2M2Pt2 strings by an

Figure 7. Perspective view of the complex cation of 15 with the atomic numbering scheme. The Au, Pt, Cl, and P atoms are illustrated as thermal ellipsoids at the 40% probability level. The hydrogen atoms are omitted, and the C and N atoms are drawn as arbitrary spherical models for clarity. Color scheme: Au (violet), Pt (yellow), Cl (light green), P (pink), N (blue), C (gray). 1901

dx.doi.org/10.1021/om401211d | Organometallics 2014, 33, 1893−1904

Organometallics

Article

(Xyl), 12 H), 3.8 (br m, CH2, 4H), 4.1 (br m, CH2, 4H), 4.7 (br m, CH2, 8H), 6.8−8.0 (m, Ar, 106H). 31P{1H} NMR (in CD2Cl2): δ −11.1 (m, 1JPtP = 2887 Hz, 4P), 0.4 (m, 1JPtP = 2766 Hz, 4P), 5.9 (m, 4P). ESI-MS (in acetone): m/z 820.107 (z2, {Pt 2 Pd(dpmp)2(XylNC)}2+ (820.103)). Preparation of [Pt6I2(μ-dpmp)4](PF6)2·CH2Cl2 (9·CH2Cl2). [Cp2Fe][PF6] (3.9 mg, 12 × 10−3 mmol) was added to a solution of 5·3.5CH2Cl2 (23 mg, 6.0 × 10−3 mmol) in CH3CN, the solution was stirred for 7.5 h at room temperature, and then the mixture was heated to 40 °C for 2.5 h. The solvent was removed under reduced pressure, and the residue was washed with Et2O and extracted with 15 mL of CH2Cl2. The extract was filtered and concentrated, to which addition of Et2O at 2 °C afforded dark bluish green crystals of 9· CH2Cl2 (7.2 mg, 1.9 × 10−3 mmol; yield 31%). Anal. Calcd for C129H118P14F12Cl2I2Pt6: C, 40.51; H, 3.11. Found: C, 40.17; H, 3.15. IR (KBr): ν 1483, 1436, 1100, 839 (PF6) cm−1. UV−vis (in CH2Cl2): λmax (log ε) 691 (3.32), 441 (4.39), 393 (4.71), 291 (4.61) nm. ESIMS (in CH2Cl2): m/z 1724.07 (z1, {Pt3(I)(dpmp)2}+ (1724.09)). 1H and 31P{1H} NMR spectra were not measured owing to the low solubility and high instability. Preparation of [Pt3(μ-dpmp)2(XylNC)(CO)](PF6)2 (10). In a solution of 7 (12 mg, 2.9 × 10−3 mmol) in dichloromethane or acetone-d6 was dissolved carbon monoxide (1 atm), and the mixture was stirred for 1 h. The solution immediately changed from dark green to orange, and the product of 10 was characterized by spectroscopic methods. UV−vis (in CH2Cl2): λmax 383, 357 nm. 1H{31P} NMR (in acetone-d6): δ 1.36 (s, o-Me (Xyl), 6H), 3.86 (d, CH2, 2H, 2JHH = 15 Hz), 4.50 (d, CH2, 2H, 2JHH = 14 Hz), 5.47 (d, CH2, 2H, 2JHH = 15 Hz), 6.24 (d, CH2, 2H, 2JHH = 14 Hz), 6.6−8.2 (m, Ar, 53H). 31P{1H} NMR (in CD2Cl2): δ −15.4 (m, 1JPtP = 2652 Hz, 1P), −3.2 (m, 1JPtP = 2774 Hz, 2P), −1.1 (m, 1JPtP = 2886 Hz, 1P), 3.5 (m, 1JPtP = 3090 Hz, 2P). ESI-MS (in CH2Cl2): m/z 799.11 (z2, {Pt3(dpmp)2}2+ (798.60)), 864.15 (z2, {Pt3(dpmp)2(XylNC)}2+ (864.13)). Preparation of [Pt3(μ-dpmp)2(PPh3)(XylNC)](PF6)2 (11). To a solution of complex 2·CH2Cl2 (21 mg, 5.0 × 10−3 mmol) in CH2Cl2 (5 mL) was added [Cp2Fe][PF6] (4.0 mg, 12 × 10−3 mmol). The solution was stirred for 2 h, and the solvent was removed under reduced pressure. The residue was washed with Et2O and extracted with CH2Cl2 (5 mL). PPh3 (3.3 mg, 13 μmol) was added to the solution. The solution immediately changed to reddish orange. The reaction mixture was filtered, and the filtrate was concentrated to 2.5 mL. After addition of Et2O, the solution was kept at 2 °C to give reddish orange crystals of 11 (6.3 mg, 2.8 μmol; yield 28%). Anal. Calcd for C91H82NP9F12Pt3: C, 47.90; H, 3.62; N, 0.61. Found: C, 47.78; H, 3.66; N, 0.65. IR (KBr): ν 2168 (NC), 1437, 1099, 839 (PF6), 741, 692, 557 cm−1. UV−vis (in CH2Cl2): λmax (log ε) 455 (4.50), 395 (4.73), 357 (4.64) nm. 1H NMR (in CD2Cl2): δ 1.41 (s, oMe (Xyl), 6H), 3.91 (d, CH2, 2H, 2JHH = 15 Hz), 5.11 (d, CH2, 2H, 2 JHH = 14 Hz), 5.50 (d, CH2, 2H, 2JHH = 15 Hz), 6.21 (d, CH2, 2H, 2 JHH = 14 Hz), 6.7−8.1 (m, Ar, 68H). 31P{1H} NMR (in CD2Cl2): δ −13.6 (m, 1JPtP = 2832 Hz, 2P), −4.5 (m, 1JPtP = 2782 Hz, 2P), 6.8 (m, 1 JPtP = 3570 Hz, 1P), 24.7 (m br, 1JPtP = 2040 Hz, 2JPtP = 649 Hz, 3JPtP = 185 Hz, 1P). ESI-MS (in CH 2 Cl 2 ): m/z 995.67 (z2, {Pt3(dpmp)2(PPh3)(XylNC)}2+ (995.68)). Preparation of [Pt3(μ-dpmp)2(trans-dppen)(XylNC)](PF6)2 (13a). To a solution of complex 2·CH2Cl2 (18 mg, 4.4 × 10−3 mmol) in CH2Cl2 (5 mL) was added [Cp2Fe][PF6] (3.8 mg, 11 × 10−3 mmol). The solution was stirred for 2 h, and the solvent was removed under reduced pressure. The residue was washed with Et2O and extracted with CH2Cl2 (5 mL). A portion of trans-dppen (4.3 mg, 11 μmol) was added to the solution. The solution immediately changed to reddish orange. The reaction mixture was filtered, and the filtrate was concentrated. After addition of Et2O, the solution was kept at 2 °C to give reddish orange crystals of 13a (9.1 mg, 3.8 μmol; yield 43%). Anal. Calcd for C99H89NP10F12Pt3: C, 49.22; H, 3.71; N, 0.58. Found: C, 49.23; H, 3.73; N, 0.31. IR (KBr): ν 2144 (NC), 1437, 1097, 839 (PF6), 740, 694 cm−1. UV−vis (in CH2Cl2): λmax (log ε) 545 (3.96), 465 (4.44), 399 (4.72), 358 (4.58) nm. 1H NMR (in acetone-d6): δ 1.25 (s, o-Me (Xyl), 6H), 3.56 (d, CH2, 2H, 2JHH = 15 Hz), 4.42 (d, CH2, 2H, 2JHH = 14 Hz), 5.34 (d, CH2, 2H, 2JHH = 15

influx of bonding electrons into the central part occurred upon two-electron oxidation or apparent hydride elimination from the M−H−M central part of 2 and 3. Complexes 7 and 8 are definitively electron-deficient in the central part to readily react with a series of neutral two-electron donors, XylNC, CO, PPh3, and diphosphines (trans-dppen, dppe), affording [Pt2M(μdpmp)2(XylNC)2](PF6)2 (M = Pt (1a), Pd (1b)) and [Pt3(μdpmp)2(L)(XylNC)](PF6)2 (L = CO (10), PPh3 (11), transdppen (13a), dppe (13b)) through cleavage of the central M− M bond. They also reacted with [AuCl(PPh3)] and are transformed into the Pt 2 MAu tetranuclear complexes [Pt2MAuCl(μ-dpmp)2(PPh3)(XylNC)](PF6)2 (M = Pt (15), Pd (16)), which are apparently derived from insertion of an electron-deficient {Pt2M(μ-dpmp)2L}2+ fragment into the Au− Cl bond at the central M positions. These results are quite interesting for the design of redox-active low-valent metal chains, and the Pt2M2Pt2 hexanuclear clusters could be new starting materials to construct further extended, low-valent metal atom chains.

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EXPERIMENTAL SECTION

General Procedures. All reactions were carried out under a nitrogen atmosphere by using standard Schlenk techniques. Solvents were dried by standard procedures and freshly distilled prior to use. Other reagents were of commercial grade, and no further purifications were performed. The complexes [Pt2M(μ-dpmp)2(XylNC)2](PF6)2· (CH 3 ) 2 CO (M = Pt (1a), Pd (1b)), 8a [Pt 4 M 2 (μ-H)(μdpmp)4(XylNC)2](PF6)3 (M = Pt (2), Pd (3)),10b and [Pt6(μH)I2(μ-dpmp)4](PF6) (5)10b were prepared by the reported methods. 1 H NMR spectra were recorded at 300 MHz; frequencies are referenced to the residual resonances of the deuterated solvent. 31 1 P{ H} NMR spectra were recorded at 121 MHz, with chemical shifts being calibrated to 85% H3PO4 as an external reference. Electronic absorption spectra were recorded on solution samples. IR spectra were recorded on KBr pellets. ESI-TOF-MS spectra were measured with positive ionization mode. Preparation of [Pt6(μ-dpmp)4(XylNC)2](PF6)4 (7). A portion of [Cp2Fe][PF6] (5.7 mg, 17 × 10−3 mmol) was added to a solution of 2· 3CH2Cl2 (35 mg, 8.4 × 10−3 mmol) in CH2Cl2 (15 mL), and the solution was stirred at room temperature for 2 h. The solvent was removed under reduced pressure, and the black residue was washed with Et2O (2 mL × 3) and extracted with CH2Cl2 (10 mL). The extract was concentrated to ca. 1.5 mL, and Et2O (ca. 0.2 mL) was carefully added. The solution was allowed to stand at 2 °C for 1 day to afford dark green block crystals of 7 (18 mg, 4.5 × 10−3 mmol; yield 53%). Anal. Calcd for C146H134N2P16F24Pt6: C, 43.42; H, 3.34; N, 0.69. Found: C, 43.17; H, 3.20; N, 0.78. IR (KBr): ν 2161 (NC), 840 (PF6) cm−1. UV−vis (in CH2Cl2): λmax (log ε) 420 (4.41), 377 (4.85), 310 (sh, 4.61), 289 (sh, 4.70) nm. 1H NMR (in acetone-d6): δ 1.57 (s, o-Me (Xyl), 12 H), 3.6 (br m, CH2, 4H), 4.3 (br m, CH2, 4H), 5.0 (br m, CH2, 8H), 6.7−8.3 (m, Ar, 106H). 31P{1H} NMR (in acetone-d6): δ −6.9 (m, 1JPtP = 2797 Hz, 4P), −1.1 (m, 1JPtP = 2741 Hz, 4P), 20.8 (m, 1JPtP = 3413 Hz, 4P). ESI-MS (in acetone): m/z 864.635 (z2, {Pt3(dpmp)2(XylNC)}2+ (864.633)). Preparation of [Pt4Pd2(μ-dpmp)4(XylNC)2](PF6)4 (8). A portion of [Cp2Fe][PF6] (7.4 mg, 22 × 10−3 mmol) was added to a solution of 3 (41 mg, 11 × 10−3 mmol) in CH2Cl2 (15 mL), and the solution was stirred at room temperature for 2 h. The solvent was removed under reduced pressure, and the black residue was washed with Et2O (2 mL × 3) and extracted with CH2Cl2 (10 mL). The extract was concentrated to ca. 2 mL, and Et2O (ca. 0.6 mL) was carefully added. The solution was allowed to stand at 2 °C for 1 day to afford dark green block crystals of 8 (24 mg, 6.2 × 10−3 mmol; yield 55%). Anal. Calcd for C146H134N2P16F24Pd2Pt4: C, 45.41; H, 3.50; N, 0.73. Found: C, 45.04; H, 3.68; N, 0.72. IR (KBr): ν 2162 (NC), 839 (PF6) cm−1. UV−vis (in CH2Cl2): λmax (log ε) 461 (4.54), 370 (4.88), 343 (4.74), 310 (sh, 4.71) nm. 1H NMR (in CD3CN): δ 1.48 (s, o-Me 1902

dx.doi.org/10.1021/om401211d | Organometallics 2014, 33, 1893−1904

Organometallics

Article

Hz), 6.18 (d, CH2, 2H, 2JHH = 14 Hz), 5.5 (m, CH (dppen), 2H), 6.5−8.1 (m, Ar, 73H). 31P{1H} NMR (in acetone-d6): δ −13.5 (m, 1 JPtP = 2856 Hz, 2P), −6.0 (d, 3JPP = 19 Hz, 1P), −4.0 (m, 1JPtP = 2782 Hz, 2P), 1.9 (m, 1JPtP = 2950 Hz, 1P), 6.6 (m, 1JPtP = 3270 Hz, 2P). ESI-MS (in CH2Cl2): m/z 1062.76 (z2, {Pt3(dpmp)2(trans-dppen)(XylNC)}2+ (1062.69)), 1874.42 (z1, {Pt3(dpmp)2(XylNC)PF6}+ (1874.23)), 2270.49 (z1, {Pt3(dpmp)2(trans-dppen)(XylNC)PF6}+ (2270.35)). Preparation of [Pt3(μ-dpmp)2(trans-dppen)2](PF6)2 (14a). A procedure similar to that for 13a with 4 equiv of trans-dppen afforded red crystals of 14a (5.4 mg, 2.0 μmol; yield 20%). Anal. Calcd for C116H102P12F12Pt3: C, 51.97; H, 3.83. Found: C, 51.94; H, 3.84. IR (KBr): ν 1436, 1096, 839 (PF6), 740, 694 cm−1. UV−vis (in CH2Cl2): λmax (log ε) 508 (4.63), 390 (4.65), 399 (4.72), 359 (4.71) nm. ESIMS (in CH2Cl2): m/z 1195.19 (z2, {Pt3(dpmp)2(trans-dppen)2}2+ (1195.22)). Titration of 7 with Diphosphine. A dichloromethane solution of 7 (2.3 × 10−2 mM) was titrated with dichloromethane solutions of diphosphines (7.6 × 10−1 mM). The solutions were monitored by UV−vis absorption and ESI mass spectra at room temperature; transdppen and dppe were used as the diphosphines. Preparation of [Pt3AuCl(μ-dpmp)2(PPh3)(XylNC)](PF6)2 (15). To a solution of complex 2·CH2Cl2 (44 mg, 11.0 × 10−3 mmol) in CH2Cl2 (10 mL) was added [Cp2Fe][PF6] (7.3 mg, 22.1 × 10−3 mmol). The solution was stirred for 2 h, and the solvent was removed under reduced pressure. The residue was washed with Et2O and extracted with CH2Cl2 (15 mL). [AuCl(PPh3)] (10.9 mg, 22.0 μmol) was added to the solution, which was stirred overnight. The reaction mixture was filtered, and the filtrate was concentrated to 2.5 mL. After addition of Et2O, the solution was kept at 2 °C, yielding reddish orange crystals of 15 (12.7 mg, 5.05 μmol; yield 23%). Anal. Calcd for C91H82NP9AuClF12Pt3: C, 43.47; H, 3.29; N, 0.56. Found: C, 43.11; H, 3.26; N, 0.63. IR (KBr): ν 2153 (NC), 840 (PF6) cm−1. UV−vis (in CH2Cl2): λmax (log ε) 449 (3.6), 387 (4.7), 346 (4.4) nm. 1H NMR (in CD2Cl2): δ 1.38 (s, o-Me, 6H), 3.1−5.5 (br, CH2, 8H), 6.3− 8.3 (m, Ar, 58H). 31P{1H} NMR (in CD2Cl2): δ 37.8 (m, PPh3, 2JPtP = 310 Hz, 2JPt’P = 769 Hz), complicated peaks were observed around −21 to 26 ppm for 6P. ESI-MS (in CH2Cl2): m/z 1111.67 (z2, {Pt3AuCl(dpmp)2(PPh3)(XylNC)}2+ (1111.65)). Preparation of [Pt2PdAuCl(μ-dpmp)2(PPh3)(XylNC)](PF6)2 (16). Compound 16 was prepared by the same procedure as 15 using 3 instead of 2 as a starting complex. Yield: 10%. Anal. Calcd for C91H82NP9AuClF12Pt2Pd: C, 45.06; H, 3.41; N, 0.58. Found: C, 44.91; H, 3.39; N, 0.71. IR (KBr): v 2157 (NC), 839 (PF6) cm−1. UV−vis (in CH2Cl2): λmax (log ε) 464 (4.0), 371 (4.5), 344 (4.3) nm. 1H NMR (in CD2Cl2): δ 1.39 (s, o-Me, 6H), 3.3−5.5 (br, CH2, 8H), 6.3− 8.3 (m, Ar, 58H). 31P{1H} NMR (in CD2Cl2): δ 35.7 (m, PPh3, 2JPtP = 353 Hz), complicated peaks were observed around −23 to 19 ppm for 6P. ESI-MS (in CH 2 Cl 2 ): m/z 1067.61 (z2, {Pt 2 PdAuCl(dpmp)2(PPh3)(XylNC)}2+ (1067.62)). X-ray Crystallographic Analyses of 7·3CH2Cl2, 8·5CH2Cl2· Et2O, 14a·6CH2Cl2, 15·3.5CH2Cl2, and 16·3CH2Cl2. The previously reported X-ray data10a for 8 were re-refined using SHELXL-97.18 The crystals of 7·3CH2Cl2, 8·5CH2Cl2·Et2O, 14a·6CH2Cl2, 15·3.5CH2Cl2, and 16·3CH2Cl2 were quickly coated with Paratone N oil and mounted on top of a loop fiber at room temperature. Reflection data were collected at low temperature with a Rigaku VariMax Mo/Saturn CCD diffractometer equipped with graphite-monochromated confocal Mo Kα radiation using a rotating-anode X-ray generator RA-Micro7 (50 kV, 24 mA). Crystal and experimental data are summarized in Tables S2 and S3 (Supporting Information). All data were collected at −120 °C, and a total of 2160 oscillation images, covering a whole sphere of 6° < 2θ < 55°, were corrected by the ω-scan method (−62° < ω < 118°) with a Δω value of 0.25°. The crystal-to-detector (70 × 70 mm) distance was set at 60 mm. The data were processed using the Crystal Clear 1.3.5 program (Rigaku/MSC)19 and corrected for Lorentz−polarization and absorption effects.20 The structures of complexes were solved by direct methods with SHELXS-9719 and were refined on F2 with full-matrix least-squares techniques with SHELXL-9718 using the Crystal Structure 3.7 package.21 All non-

hydrogen atoms were refined with anisotropic thermal parameters, and the C−H hydrogen atoms (except those for some solvent molecules) were calculated at ideal positions and refined with riding models. In the structures of 7, 8, 15, and 16, some dichloromethane molecules of crystallization and PF6 anions (15 and 16) are disordered. All calculations were carried out on a Windows PC with the Crystal Structure 3.7 package.21 Theoretical Calculations. Gas-phase DFT single-point calculations were performed on the experimentally determined structures of 7 and 8 using the B3LYP22 functionals with the LANL2DZ basis set.23 DFT optimization on the structures of the model complexes [Pt 6 (C 2 H 9 P 3 )(CNCH 3 )] 4+ (M7) and [Pt 2 Pd 2 Pt 2 (C 2 H 9 P 3 )(CNCH3)]4+ (M8) were also carried out with the same functional. The initial coordinates for optimization were derived from the crystal structures of 7 and 8 on replacing the xylyl and phenyl groups by methyl and hydrogen units. All calculations were carried out on a UNIX Station with the Gaussian 03 program package.24

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ASSOCIATED CONTENT

S Supporting Information *

Tables, CIF files, and figures giving crystallographic data of 7, 8, 14a, 15, and 16, structural parameters of 7, 8, 14a, 15, and 16, ORTEP diagrams of 7, 14a, and 16, results of DFT calculations on 7, 8, M7, and M8, 31P{1H} NMR spectra of 7, 8, 13a, 15, and 16, ESI-MS of 7, 8, 10, 11, 12a, 13a, 15, and 16, and electronic absorption spectral changes for reactions of 7 and 8 with XylNC and that of 7 with dppe. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*T.T.: e-mail, [email protected]; fax, +81 742-20-3847; tel, +81 742-20-3399. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Grant-in-Aid for Scientific Research and that on Priority Area 2107 (Nos. 22108521, 24108727) from the Ministry of Education, Culture, Sports, Science and Technology ofJapan. T.T. is grateful to Nara Women’s University for a research project grant.

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REFERENCES

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