Article pubs.acs.org/IC
Integration of Paramagnetic Diruthenium Complexes into an Extended Chain by Heterometallic Metal−Metal Bonds with Diplatinum Complexes Kazuhiro Uemura,*,† Naoyuki Uesugi,† Akina Matsuyama,† Masahiro Ebihara,† Hirofumi Yoshikawa,‡ and Kunio Awaga‡ †
Department of Chemistry and Biomolecular Science, Faculty of Engineering, Gifu University, Yanagido 1-1, Gifu 501-1193, Japan Department of Chemistry and Research Center for Materials Science (RCMS), Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan
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S Supporting Information *
ABSTRACT: We successfully obtained a paramagnetic onedimensional (1D) chain complex [{Ru 2 (O 2 CCH 3 ) 4 }{Pt2(piam)2(NH3)4}2]n(PF6)4n·4nH2O (2; piam = pivalamidate) extended by metal−metal bonds. Compound 2 comprises two types of metal species, ruthenium and platinum, where an acetate-bridged dinuclear ruthenium complex (i.e., [Ru2]) and a pivalamidate-bridged platinum complex (i.e., [Pt2]) are connected by axial metal−metal bonds, forming an attractive quasi-1D infinite chain that can be expressed as −{[Pt2]−[Ru2]−[Pt2]}n−. Such axial metal−metal bonds are attributed to the interaction between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) along the z axis, where both the HOMO in [Pt2II,II] and the LUMO in [Ru2II,II] are σ* orbitals associated with metal cores. The crystal structure and X-ray photoelectron spectrum for 2 reveal that metal oxidation states are −{[Pt2II,II]−[Ru2II,II]−[Pt2II,II]}n−, where [Ru2II,II] can have an electronic configuration of σ2π4δ2δ*2π*2 or σ2π4δ2π*4. The magnetic susceptibility of 2 has a μeff [∝(χT)1/2] value of 2.77 μB per [Pt2II,II]−[Ru2II,II]−[Pt2II,II] unit at 300 K, showing that two unpaired electrons lie on π*(Ru2). Magnetic measurements performed at temperatures of 2−300 K indicate that S = 1 Ru2II,II units are weakly antiferromagnetically coupled (zJ = −1.4 cm−1) with a large zero-field splitting (D = 221 cm−1).
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[Ru2II,II] oxidation state, there are two types of ground-state configurations, σ2π4δ2δ*2π*2 (S = 1) and σ2π4δ2π*4 (S = 0); on the other hand, mixed-valent [Ru2II,III] have the quadruplet ground state, σ2π4δ2(δ*π*)3 (S = 3/2), which exists because of a coincidental near degeneracy of the π* and δ* orbitals (Scheme 1).1 For assembled [Ru2] structures, the selection of linker ligands is very important for determining the overall electronic structure inducing a paramagnetic or diamagnetic compound because axial ligands impart a significant influence on the electronic structure. Although numerous crystal structures with axially coordinated bridging ligands have been reported,3−6 to the best of our knowledge, extended one-dimensional (1D) chains containing [Ru2] units linked by metal−metal bonds have not yet been achieved. Direct interactions between metal− metal bonds are interesting from the viewpoint of materials science,7 and in practice, radical molecules such as 2,2,6,6tetramethylpiperidine-1-oxyl show strong through-bond exchange interactions capable of mediating these bonds.8
INTRODUCTION Dinuclear paddlewheel complexes have a rich redox chemistry and show various electronic configurations based on metal− metal bond orbitals, with frontier orbital engineering being achievable via the modification of coordinated ligands.1 Among these complexes, ruthenium dinuclear complexes (e.g., [Ru2]) are the most popular for chemists to use because their dinuclear core has various possible oxidation states,1 making them useful as building blocks for the formation of novel conductive, optical, and magnetic materials.2 There are numerous unique assembled [Ru2] structures with various linker ligands3−6 such as organic molecules,3 halide ions,4 coordination compounds,5 and inorganic molecules,6 which are exploited in the design of molecular magnetic materials. For example, [Ru2] units linked by 7,7,8,8-tetracyanoquinodimethane derivatives produce characteristic donor−acceptor systems with ligand-controlled charge transfer,2c and [Ru2(O2CtBu)4]3[Cr(CN)6]·2H2O shows ferromagnetic behavior with a high Tc that is attributed to the availability of both σ and π interactions with metal centers.5c Fascinatingly, [Ru2] has a manifold of ground-state configurations depending on its surrounding ligands: for the © XXXX American Chemical Society
Received: March 26, 2016
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DOI: 10.1021/acs.inorgchem.6b00741 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
mg, 26 μmol) and [Pt2(piam)2(NH3)4](PF6)2 (47.1 mg, 50 μmol) were mixed in EtOH (10 mL), and hexane (30 mL) was gently layered onto the solution. After several days, brown platelike crystals with a metallic luster were obtained (5.0 mg, 2.1 μmol); the yield was 8%. For elemental analysis, the sample was dried in vacuo for 1 day. Elemental analysis calcd for C28H76F24N12O12P4Pt4Ru2: C, 14.40%; H, 3.28%; N, 7.20%. Found: C, 14.04%; H, 3.17%; N, 6.86%. X-ray Structure Determination. X-ray diffraction measurements were taken using a Rigaku AFC7R diffractometer equipped with a normal focus Mo-target X-ray tube (λ = 0.71070 Å) operated at 15 kW power (50 kV, 300 mA) and a Rigaku Mercury CCD two-dimensional detector. A total of 744 frames were collected with a scan width of 0.5°. Empirical absorption correction15 was performed for all data. The structure was determined by the direct method16 with subsequent difference Fourier syntheses and refinement using SHELX-97,17 controlled by the Yadokari-XG software package.18 The crystal data and structure refinement results are summarized in Tables S1−S3. Physical Measurements. X-ray photoelectron spectroscopy (XPS) measurements were performed using a Quantera-SXM spectrometer at room temperature. Binding energies were measured relative to the C 1s peak (284.8 eV) of internal hydrocarbons. Diffuse reflectance spectra were recorded using a Hitachi U-4000 spectrophotometer (range of 200−2500 nm) at room temperature. The obtained reflectance spectra were converted to absorption spectra using Kubelka−Munk function F(R∞). IR spectra were recorded using a PerkinElmer Spectrum 400 instrument (range of 400−2000 cm−1) at room temperature. Magnetic data were obtained in the range of 2−300 K using a Quantum Design MPMS superconducting SQUID susceptometer working at a 1 T field strength. Crude data were corrected for the contribution of a sample holder and diamagnetism of constituent atoms.
Scheme 1. Three Kinds of Schematic Electronic Structures for Paddlewheel Ruthenium Complexes
Previously, we successfully obtained [{Rh2(O2CCH3)4}{Pt2(piam)2(NH3)4}2]n(PF6)4n·6nH2O (1; piam = pivalamidate) and analogues consisting of [Rh2L4] (L = CH3CO2−, CF3CO2−, or CH3NHCO−) and various modified platinum complexes ([Pt2]).9 In these compounds, vacant σ* orbitals in [Rh2L4] and filled σ* orbitals in [Pt2] effectively overlap to form unbridged Rh−Pt bonds, resulting in extended heterometallic 1D chains. Interestingly, ligands can be modified not only in platinum parts of the chain but also in rhodium parts, which induces the modulation of the electronic structure of the heterometallic 1D chains9,10 and may be exploited to create new polymeric materials based on element blocks.11 Considering the electronic configuration of [Ru2], it is also possible to form unbridged Ru−Pt bonds between [Ru2] and [Pt2II,II] because [Ru2] also has a vacant σ* orbital as its lowest unoccupied molecular orbital (LUMO), as is the case for [Rh2]. Here, we show a 1D chain complex comprising diruthenium paddlewheel complexes and platinum complexes with direct metal−metal interactions. Such a chain complex is promising not only for its contribution to the formation of novel heterometallic 1D chains with different metal species but also for its magnetic behavior that can be attributed to the [Ru2] parts of the 1D chains.
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RESULTS AND DISCUSSION Reaction of Dinuclear Ruthenium Complexes with [Pt2(piam)2(NH3)4](PF6)2. To confirm whether [Ru2II,III] and
EXPERIMENTAL SECTION
Materials. Ruthenium(III) chloride trihydrate and potassium tetrachloroplatinate(II) were obtained from Tanaka Kikinzoku Co. [Ru2(O2CCH3)4Cl],12 [Ru2(O2CCH3)4],13 and [Pt2(piam)2(NH3)4](PF6)2·H2O14 were synthesized according to previously reported procedures. Synthesis of [{Ru2(O2CCH3)4}{Pt2(piam)2(NH3)4}2]n(PF6)4n· 4nH2O (2). In method 1, a MeOH solution (100 mL) of [Ru2(O2CCH3)4Cl] (0.480 g, 1.0 mmol) was stirred with AgNO3 (0.170 g, 1.0 mmol) overnight in the dark, and the resulting AgCl was removed by filtration. The red filtrate obtained was evaporated to acquire a brown powder of [Ru2(O2CCH3)4(NO3)] (88% yield). A portion of [Ru2(O2CCH3)4(NO3)] (24 mg, 48 μmol) was stirred with [Pt2(piam)2(NH3)4]2(PF6)2 (46 mg, 48 μmol) in MeOH (12 mL) for 1 h. The resulting dark brown solution was slowly evaporated in a glass tube [10 mm internal diameter (i.d.)] at room temperature to obtain brown crystals with a metallic luster after one month. A few single crystals suitable for X-ray analysis were successfully obtained; however, the yield was very low (
