Extended Metal-Atom Chains with an Inert Second Row Transition Metal: [Ru5(μ5-tpda)4X2] (tpda2? = tripyridyldiamido dianion, X = Cl and NCS)

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Extended Metal-Atom Chains with an Inert Second Row Transition Metal:

[Ru

5

(

µ

5

-tpda)

4

X

2

] (tpda

2-

) tripyridyldiamido dianion, X ) Cl and NCS)

Caixia Yin,†,‡,§Gin-Chen Huang,Ching-Kuo Kuo,Ming-Dung Fu,|

Hao-Cheng Lu,†Jhih-Hong Ke,† Kai-Neng Shih,†Yi-Lin Huang,Gene-Hsiang Lee,Chen-Yu Yeh,Chun-hsien Chen,*,†and

Shie-Ming Peng*,†,§

Department of Chemistry, National Taiwan UniVersity, Taipei 10617, Taiwan, Key Laboratory of Chemical Biology and Molecular Engineering of Ministry of Education, Institute of Molecular Science, Shanxi UniVersity, Taiyuan 030006, China, Institute of Chemistry, Academia Sinica, Taipei 11529, Taiwan, Department of Chemistry, National

Tsing Hua UniVersity, Hsinchu, Taiwan 30013, and Department of Chemistry, National Chung Hsing UniVersity, Taichung, Taiwan 40227, ROC

Received June 30, 2008; E-mail: chhchen@ntu.edu.tw; smpeng@ntu.edu.tw

Nearly two decades after the first reports1–3on EMACs (extended

metal atom chains), exciting findings4,5continue to flourish because

of the advances in synthesis to make possible a unique platform for fundamental studies of metal-metal multiple bonds beyond dinuclear complexes. The metal atom chains are collinear and supported by four amine-based ligands such as prototypical and derivatized oligo-R-pyridylamines4,5and naphthyridine-modified

ones.6–10By coordinating the ligands with a variety of metal centers,

systematic examination of spectroscopic, electrochemical, magnetic, and electric conductance properties of the homologous series of EMACs are feasible. However, the metal centers in EMACs hitherto are limited to first row transition metals such as Cr,11–14Co,6,15,16

Ni,1,7–10,16–19and Cu,2,3probably because of their lability over

those relatively inert ones with filled 4d or 5d shells.4,5Reported

herein are a synthetic strategy and characterizations for [Ru5(µ5 -tpda)4Cl2] (1, Figure 1) and [Ru5(µ5-tpda)4(NCS)2] (2, see Sup-porting Information for the ORTEP view), the first pentanuclear EMACs of the second-row transition metals.

The rich chemistry20,21of ruthenium enables its applications

associated with electron and energy transfer.22,23 Diruthenium

complexes bear four formal oxidation states21,24–26 and exhibit

profound spectroscopic features.27[Ru

3(µ3-dpa)4Cl2] (dpa ) dipy-ridylamido anion) is our first attempt to explore ruthenium EMACs. The characterization of the physical properties was, however, hampered by a low yield of 2%.28Very recently we improved the

yield to 53% that allowed the studies29of the Ru-Ru distances,

spin states, and the spectroelectrochemistry of [Ru3]6+/7+/8+. The key to the success was the addition of an excess amount of LiCl, prior to metalation, to reflux with the starting materials, H2dpa and Ru2(OAc)4Cl. The metalation was then activated by a base, t-BuOK, and ferrocenium tetrafluoroborate was employed to confer a stable product, [Ru3(µ3-dpa)4Cl2][BF4] (yield 53%). Subsequent reduction by hydrazine yielded the neutral triruthenium EMAC (63%).29

However, the preparation of [Ru5]10+EMACs by the same protocol resulted in a low yield (<1%) of an oxidized pentaruthenium, [Ru5(µ5-tpda)4Cl2] {[Ru2(OAc)4Cl]2Cl} (3, see Supporting Informa-tion for the ORTEP view).30In this present study we discover that

a 10% yield of 1 can be achieved by swapping the sequence of metalation and the introduction of LiCl under reflux conditions where the latter impedes the ruthenium-acetate complexation and facilitates the formation of [Ru5]10+.

The crystal structure of 1 (Figure 1) shows a linear [Ru5]10+ unit helically coordinated by four tpda2-ligands, the same as the

family of pentanuclear EMACs.4,5,16I4/m space group is used for

the final refinement with the Ru(3) atom sitting at the crystal-lographic 2-fold axis which appears slightly elongated. The atomic positions of the compound are averaged owing to the cocrystalli-zation of the right- and left-turn helical forms. Therefore, accurate Ru-N distances are unavailable. Nevertheless, the bond length is in the order of [Ru(1)-Npy,outer] > [Ru(2)-Namido] ≈ [Ru(3)-Npy,inner], ascribed to a smaller negative charge at Npy,outer than those at the other nitrogen atoms. The Ru-Cl distance is found to be 2.550(4) Å, shorter than the corresponding 2.596(1) Å of [Ru3(dpa)4Cl2].29 There are two types of Ru-Ru bond lengths where the outer and inner ones are, respectively, 2.2827(17) and 2.2759(13) Å, slightly longer than the 2.2537(5) Å of [Ru3(dpa)4Cl2].29

EPR of 1 was silent, consistent with an even number of electrons for the [Ru5]10+. Figure 2 displays the paramagnetic behavior that the thermal magnetic susceptibility (χM) increases at low temper-atures. At room temperature, the effective magnetic moment (µeff) is 2.82µB, corresponding to that of the triplet state composed of two unpaired electrons. Upon cooling, µeff decreases gradually, †

National Taiwan University. ‡

Shanxi University. §

Academia Sinica. |

National Tsing Hua University. ⊥National Chung Hsing University.

Figure 1. (Top) the metal atom chain is supported by four oligo-R-pyridylamine ligands. (Bottom) ORTEP view of [Ru5(µ5-tpda)4Cl2] (1). Label A represents symmetric related positions. Thermal ellipsoids are drawn at the 30% probability level. Ru, aqua blue; N, blue; Cl, green; C, gray. The hydrogen atoms are not shown for clarity.

Published on Web 07/10/2008

10.1021/ja8016818 CCC: $40.752008 American Chemical Society 100909J. AM. CHEM. SOC. 2008, 130, 10090–10092

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indicative of a strong zero field splitting parameter. Theoretical fitting was performed according to the full spin Hamiltonian for an S ) 1 system with an axial zero-field splitting, H ) guβSuHu+

D[Sz2- S(S + 1)/3], where the subscript u denotes the direction

of the applied magnetic field.31The equations derived from this

Hamiltonian are provided in the inset of Figure 2 where the averaged magnetic susceptibility of xav ) (xz+ 2xx)/3 is listed.

The fitting resulted in 2.16, 1.96, and -112 cm-1for g, g|, and the zero-field splitting parameter D, respectively. One possibility for such a strong D is that the experiments were carried out with powder samples due to the difficulty in growing sufficiently large crystals. Because the measurements are significantly affected by the relative orientation of the crystalline plane to the direction of the magnetic field, we are uncertain about whether the isotropic model is suitable although the fitting results appear superb (R2)

0.99952).

DFT/B3LYP analysis of 1 was carried out with Gaussian 03w package using LANL2DZ on Ru, D95V on C, H, and D95* on N, Cl. The orbitals at the metal centers are drawn in Figure 3. The ground-state of 1 is found to be3A

2with an electronic configuration of Q22π*4σ

nb2δ*1δ*1where Q22representsσ2π4σ2π4δ2πnb4δ2δ2

and the last two electrons in theδ*1δ*1are filled, corresponding

to the paramagnetic behavior found in Figure 2. The bond order between the adjacent Ru2+is 1. For comparison with its analogue,

[Ru3(dpa)4Cl2], our previous DFT study showed an electronic configuration ofσ2π4δ2π

nb4δnb2δ*2σnb2, resulting in the character-istics of diamagnetism and an Ru-Ru bond order of 1.5.29

The voltammogram of complex 1 exhibited five reversible waves at -0.71, -0.11, +0.52, +0.90, and +1.22 V (E1/2against EAg/ AgClin CH2Cl2containing 0.1 M TBAP as the supporting electrolyte, see Supporting Information). To assign the redox states, 1 was subjected to spectroelectrochemical study using a platinum gauze OTTLE (optically transparent thin-layer electrode). However, the neutral form 1 is not stable because the peak intensities in the UV-vis spectra changed as a function of time prior to electro-chemical control. The spectra exhibited isobestic points, suggest-ing the possibility of a redox reaction taksuggest-ing place in CH2Cl2. We thus prepared the one-electron oxidized form 3, [Ru5(µ5 -tpda)4Cl2]{[Ru2(OAc)4Cl]2Cl}, by our previous protocol. Because the [Ru2(OAc)4Cl]2Cl anion complicates the voltammogram and UV-vis spectra, it was displaced by PF6

-. The UV-vis spectra of the resulting [Ru5(µ5-tpda)4Cl2](PF6) are stable in CH2Cl2and the voltammogram is identical to that of the neutral form 1. The potential for the 1-e- oxidation taking place is assigned to the potential range that yields spectroelectrochemical spectra identical to that of [Ru5(µ5-tpda)4Cl2]

+

acquired without potential control. Subsequently, other redox states can be determined. With this, the first oxidation potential (i.e., E1/2(ox1) for [Ru5(µ5-tpda)4Cl2]0/+) is determined to be at -0.11 V (versus EAg/AgCl). E1/2,ox2(at +0.52 V) and E1/2,ox3(at +0.90 V) for 1 are less positive than the respective reactions for [Ru3(dpa)4Cl2] at +0.89 and +1.53 V,29consistent with the trend that the longer the EMAC is, the easier for it is to undergo oxidation.4,5

The electrical conductance of a single molecule of [Ru5]10+ EMAC is determined by the method of STM (scanning tunneling microscopy) break-junction.32–34 This requires anchoring of the

molecular termini at the electrodes upon applying a bias voltage (Ebias) and monitoring the conducting current within the molecular junction. Therefore, for this study isothiocyanate was designed to be the axial ligand (i.e., complex 2) and the anchoring group because of isothiocyanate’s reasonable affinity toward gold surface.32,33The

electrodes were the gold surface and a gold STM tip. Because of concerns of instability of 2 in solution, the measurements were carried out under an electrochemical environment. Panels a and b of Figure 4 were obtained when the working electrode was potentiostatted at +200 and -300 mV against EAg/AgCl, respectively, where the corresponding complexes were the one-electron oxidized and the neutral forms. Typical conductance traces are shown in the upper panels of Figure 4. The vertical axis is plotted in units of G0(∼(12.9 kΩ)-1), defined as the conductance quantum for a gold wire with the cross-section being only a single atom.34–36 The

conductance value decreases in a stepwise fashion while the STM tip is pulled away from the substrate. Each fall associated with the tip stretching indicates the loss of a molecule from the tip-substrate junction. The histogram of counts from more than one thousand traces shows local maxima at conductance values which are integer multiples of a fundamental one, suggesting that the number of molecules in the junctions was one, two, and so forth.32–34The

conductance of a single molecule of the neutral form 2 is thus 2.4(0.5)× 10-3G0, inferior to [Cr5(µ5-tpda)4(NCS)2] (3.9(0.8)× 10-3 G0) but is more conductive than [Co5(µ5-tpda)4(NCS)2] (1.2(0.2)× 10-3G0) and [Ni5(µ5-tpda)4(NCS)2] (0.6(0.1)× 10-3 Figure 2. Temperature-dependence of µeff (9, right axis) and molar

magnetic susceptibilityχM(0, left axis) for compound 1. (Inset) plot of

χMT vs T and its fit according to the Heisenberg model (solid line).

Figure 3. MO diagram of [Ru5(µ5-tpda)4(Cl)2]. Only orbitals at the [Cl-Ru5-Cl]8+moiety are highlighted. See Supporting Information for

computational details.

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G0). The conductance of 2 correlates quite well with the trend of bond orders which are 1.5, 0.5, and 0, respectively, for [Cr5]10+, [Co5]10+, and [Ni5]10+EMACs. For [Ru5(µ5-tpda)4(NCS)2]

+

, the conductance value of 2.3(0.5)× 10-3 is essentially the same as the neutral form. On the basis of the bond order argument, the similar conductance indicates that the oxidized electron was removed from an orbital that has an insignificant contribution to the metal-metal interaction. From the calculated MO diagram (Figure 3), electronic states (Scheme S2), and the relative energy (Table S1), the most probable orbital for this oxidation is b2(i.e.,

δ*, depicted in Figure 3). This is based on the following reasons:

(1) b2 (δ*) is the highest occupied orbital, (2) δ and δ* are considered nonbonding orbitals owing to the helical conformation of the tpda2-ligand, and (3) the removal of an electron from b

2 does not affect the molecular structure and thus the conductance much.

In summary, the first second-row metal EMAC was success-fully synthesized with improved yields which made possible detailed characterization of its spectroscopy, magnetic properties, electrochemical redox states, and electrical conductance. Further investigation of EMACs with other second-row transition metals and with mixed metal centers are currently underway in our laboratory.

Acknowledgment. The authors thank MOE and NSC (ROC) for financial support, Mr. S.-C. Wang for SQUID measurements, Mr. L.-A. Lee for electrochemical measurements, and Drs. R. H. Ismayilov, W.-Z. Wang, and C.-L Hsieh for the fruitful discussions.

Supporting Information Available: Experimental procedures,

spectroelectrochemical data, voltammograms, and the X-ray crystal-lographic files for compounds 1 and 2 (CIFs). This material is available free of charge via the Internet at http://pubs.acs.org.

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(30) An oxidized pentaruthenium, [Ru5(µ5-tpda)4Cl2]{[Ru2(OAc)4Cl]2Cl}, was

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Ru2(OAc)4Cl], H2tpda, and excess LiCl. The anion, {[Ru2(OAc)4Cl]2Cl},

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JA8016818

Figure 4. Electrical conductance of 2 measured by electrochemical STM break junction. The potentials were parked at (a) +200 mV and (b) -300 mV against EAg/AgClto produce predominantly one-electron oxidized and neutral forms of 2, respectively. Upper panels: typical conductance-distance traces, with arbitrary x axis offsets, acquired upon stretching the molecular junction. Controlled experiments in blank dichloroethane yield exponential tunneling decay,32–34confirming that the staircase waveforms arose from 2. Lower panels: the conductance histogram plotted from more than one

thousand traces. The solution was 1 mM 2 dissolved in dichloroethane containing 0.1 M TBAP. The STM tip was insulated by polyethylene except the end of the tip to reduce the background noise. A Gaussian function was used to fit the histograms. The peak position and standard deviation of the Gaussian curve were used to find the single-molecule conductance and the uncertainty, respectively.

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Figure 3. MO diagram of [Ru 5 (µ 5 -tpda) 4 (Cl) 2 ]. Only orbitals at the [Cl-Ru 5 -Cl] 8+ moiety are highlighted

Figure 3.

MO diagram of [Ru 5 (µ 5 -tpda) 4 (Cl) 2 ]. Only orbitals at the [Cl-Ru 5 -Cl] 8+ moiety are highlighted p.2

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