Supramolecular Architecture - American Chemical Society

Chapter 4

Synthesis and Properties of Novel LowDimensional Ruthenium Materials

Downloaded by NORTH CAROLINA STATE UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: July 14, 1992 | doi: 10.1021/bk-1992-0499.ch004

Self-Assembled Multilayers and Mixed Polymers

Inorganic—Organic

DeQuan Li, Sara C. Huckett, Tracey Frankcom, M. T. Paffett, J. D. Farr, M . E. Hawley, S. Gottesfeld, J. D. Thompson, Carol J. Burns, and Basil I. Swanson Inorganic and Structural Chemistry Group INC-4, Los Alamos National Laboratory, Los Alamos, NM 87545

Low-dimensional ruthenium materials, built eitherfrommolecular self– assemblies, by using multidentate ligands and Ru(H O)6 or Ru (O CR) polymeric building blocks, are reported. The molecular self-assemblies and their derivative Ru films have been characterized by ellipsometry, FTIR-ATR, UV absorption spectroscopy, XPS, SIMS and STM. Polymers built from Ru (O CR) and 2,5dimethyldicyanoquinonediimine (DMDCNQI) have been synthesized and characterized by elemental analysis and IR and UV-Vis spectroscopies. For DMDCNQI, these results indicate the formation of chains of oxidized Ru (O CR) centers symmetrically bridged by radical DMDCNQI anions. The magnetic properties of these polymers as a function of tuning via the carboxylate R-group are discussed. 2+

2

2

2

4

2

2

2

2

4

4

The design and synthesis of low-dimensional materials has become an active area of research in recent years. This is due in part to their potential applications in areas such as energy conservation, sensing, superconductivity ( i ) , and electromagnetic shielding materials (2). A basic understanding of these purposely tailored systems with respect to their magnetic, metallic, and optical properties will help design macromolecular architecture constructions based on molecular constituents (2). Two approaches to building macromolecular constructions are described in this paper. Thefirstis to develop strategies for producing surface-bound linear chains using self-assembly techniques. With this method, 1-D materials can be built with a large degree of control over the specific sequence of components. These techniques have been successfully employed to generate thin film nonlinear optical (NLO) materials (3). In addition, the production of mixed inorganic-organic polymers built from molecular components has been investigated. These approaches offer tremendous flexibility with respect to the organic and inorganic building blocks and, therefore, a chemically controlled local environment resulting in desired physical, magnetic, and optical properties. The intrinsic relationships between molecular structures and materials properties can then be probed by studying these low dimensional molecular assemblies. 0097-6156/92/0499-0033$06.00/0 © 1992 American Chemical Society

In Supramolecular Architecture; Bein, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

34

SUPRAMOLECULAR ARCHITECTURE


R u Molecular Self-Assemblies The design, construction and molecular architecture of artificial supermolecular self-assembled arrays with planned structures and physical properties has attracted growing interest recently (4-8). Covalently bound self-assembled monolayers (9) or multilayers (10) such as alkylsilanes on glass or alkylthiols on gold offer a potential starting point for fabricating highly ordered multifunctional thin films such as covalently bound multilayers for nonlinear optical materials (11,12). The central advantage of using self-assembly schemes is that the weak physical interaction between interfaces (as in the case of Langmuir-Blodgett films) is replaced with covalent bonds. Therefore, it would be an instructive and unique challenge to sequentially construct an aligned interlocked mixed valence Ru (II/III) supralattice structure on well defined surfaces in a self-assembled manner thereby generating ideal structures for low-dimensional materials. We report here the construction of covalently bonded self-assembled monolayers of N-[(3-trimethoxysilyl)propyl]ethylenediaminetriacetate (TMPEDTA) and their Ru (Π) pyrazine self-assembly derivatives. Experimental. A l l procedures were carried out under an A r atmosphere and solvents were degassed before use. A l l solutions are aqueous unless otherwise noted. TMPEDTA was obtained from HULS and used without any further purification. The fused quartz substrates were ultrasonically cleaned in a 10% detergent solution for 10 minutes and then refluxed in a 1% tetrasodium ethylenediaminetetraacetate solution for 10 minutes followed by another 10 minutes sonication at ambient temperature. Finally, the substrates were thoroughly rinsed with deionized water, acetone and then exposed to Ar plasma for several hours. n

Synthesis of [ R u ( H 2 0 ) 6 l - 2 t o s (tos = /Moluenesulphonate). The hexaaquaruthenium(II) complex was synthesized according to a slightly modified literature procedure (13,14). Functionalization of the Quartz Substrate with a T M P E D T A Monolayer. A cleaned S1O2 substrate was incubated at pH = 2-3 (adjusted with concentrated HC1, a catalyst for Si-OMe bond hydrolysis) with a 5.0 χ 10" M , N-[(3trimethoxysilyl)propyl]ethylenediaminetriacetate (TMPEDTA) solution at 70°C for 3 days. After thoroughly rinsing with deionized water, the quartz substrate was immersed in a pH = 5.5 H A c / K A c buffer for 2-3 hours with 10 minutes gentle sonication every 30 min. The substrate was then cleaned by sonication (1 minute) in water and finally rinsed with water and acteone. 3

n

n

Formation of the [ T M P E D T A ] R u ( H 0 ) and [ T M P E D T A ] R u P z Molecular Self-Assemblies. The quartz substrate coated with the T M P E D T A monolayer was immersed in a 10 ml degassed solution of Ru (H20)6 2tos (3.6 χ 10" M ) . The temperature of the purple solution was kept at less than 30°C for 6 hours with 20 minutes periodic sonication. The resulting substrates with the covalently attached monolayer of [TMPEDTA]Ru (H20) were quickly rinsed with deionized water and then dipped into a 3.6 χ 10' M pyrazine (Pz) solution. The solution again was sonicated 5 times with 1 hour duration. 2

n

#

2

n

2

Low Temperature (< 50°C) R u M i r r o r Formation. The covalently modified T M P E D T A quartz substrate was immersed in a 10 ml degassed solution of Ru (H20)6 2tos (3.6xlO' M). The temperature of the purple solution was slowly n

#

2


4. L I E T A L .

Novel Low-Dimensional Ruthenium Materials

35

increased to 35-40°C and maintained overnight A shiny, smooth, metallic mirror was formed on the quartz substrate. Surface Analysis. Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) were performed with commercial cylindrical mirror electron energy analyzers. Electron beam conditions for A E S were typically 3KeV and 1 nA total beam current in a spot size 0.1mm . X-Ray irradiation for XPS was performed using an unmonochromatic M g source (hv=l253.6 eV). Narrow scans were performed with a band pass of 50eV and satellite and inelastic background were removed from spectra using established procedures. AES depth profiling was performed using 3KeV A r ions with a nominal sputter removal rate of ~100Â min" for Ru. Secondary ion mass spectroscopy (SIMS) was performed in both static and dynamic modes using either 5 KeV Ar or Xe mass filtered ion beams of appropriate beam current density. Mass analysis of the sputtered material was performed using a quadrupole instrument with provision for detection of positive, negative, and sputtered neutrals species. Insulating surfaces (e.g., quartz substrates) were examined using appropriate application of a low energy (Ei (CH2) vibrations, respectively. The T M P E D T A layer was functionalized with Ru by reaction with Ru (H2Û)6 · 2tos at room temperature. Throughout this reaction the monolayer surface was maintained at pH«5.5 by immersing the ligand-functionalized substrate in an H A c / K A c buffer solution; this helps to avoid degredation reactions of the Ru (H20) '2tos reagent. A pyrazine (Pz) bridging ligand was then coordinated to the surface Ru metal center by substitution of the last Ru aquo ligand. Surface characterization of the [TMPEDTA]Ru Pz modified substrate was done using XPS and static SIMS. The Is transitions of N , C, and Ο were monitored along with the Ru 3p and 3d transitions. In addition, a rough estimate of the attached layer thickness of 10Â was obtained from attenuation of the Si 2p and 2s transitions. The position of the Ru 3 d / transition (281.7eV binding energy) of the monolayer attached [TMPEDTA]Ru Pz substrate was 0.5 eV higher in binding energy than that obtained from the pressed powder of the [TMPEDTA]Ru Pz salt (when referenced to the C Is peak) (Figure 1). The difference in the Ru 3 d / binding energies between the [TMPEDTA]Ru Pz salt and covalently attached monolayer indicates a slightly altered electronic environment for the latter entity. Note that the intensity of Ru 3 d transition is only -10% that of the C Is transition located at 284.6 eV binding energy. Positive ion SIMS spectra displayed the presence of RuO, Ru, SiO, Si, K , Na, Ο, N , CH, and C ions. Negative ion SIMS did not indicate any significant contribution due to surface CI. In an attempt to generate a Ru(II/m) mixed-valence self-assembly, the [TMPEDTA]Ru Pz monolayer was further reacted with a K2[Ru Cl5(H20)] solution at - 30°C. XPS spectra of the Ru 3 d / region indicates that the Ru concentration has roughly doubled relative to the [TMPEDTA]Ru Pz modified substrate. In addition, 1

1

-1

s

a

2+

n

6

n

5

2

n

n

5

2

n

5 / 2

n

m

5

2

n



*0 ^~OH

OK

OH-


N^=0

HO OH OH

i) TMPEDTA pH=2-3, H 0 , 70°C

(CH ) 2

3

2

/)/)//)/

0 0 ο

ii) HAc/KAc Buffer pH = 5.5

Oxide Surface

/)/)//)/ Oxide Surface 1 A

|ιR u ( H 0 ) « 2 t o s 2

6

.11/ QH

Pyrazine

(CH ) 2

(CH ) 2

2

3

3

SI ο'ό o /)/)//)/

0 0 0

s

/)/)//)/ Oxide

Surface

Oxide Surface u

K [Ru(H 0)CI] 2

Ru (H 0) *2tos 2

6

2

35-40°C

(CH ) 2

Ru III species

3

SI ο'ό\> /)/)//)/ Oxide Surface

Scheme I



LI ET AL.


'

1

1

1

If

1

1

C(1s)

/

1

/

1

\

1 Ru(3d5/2)

_

/Λ / /

/

y

— — Τ " '

294

I

290

\ »

// ι

J

»\

1

.

/

1

\ \ \ V \x

A ΒR

- —

],~—1—.

286 282 278 Binding Energy (eV)

274

Figure 1. X P S of the C(ls) and Ru(3d) Region for (A) the "metallic" R u Film (lightly sputtered to remove surface contamination), (B) the [ T M P E D T A ] R u P z powder and (C) the Covalently Attached [ T M P E D T A ] R u P z Complex. n

n


38


negative ion static SIMS shows the presence of significant surface CI. Ellipsometric measurements were also carried out on the thin films, Figure 2 (the ellipsometric parameters Δ and Ψ are plotted in degrees). Because Ψ is constant at about 16° there are no complicating strong absorption effects and, therefore, the 2-3 degree Δ change after each monolayer deposition can be interpreted as corresponding to a concomitant increase in film thickness (~10-15Â)(75) When the T M P E D T A monolayer, buffered at p H » 5.5, was heated to 3540°C in a Ru (H20)6 2tos solution, a shiny, conductive (p = 57.5 Ω/cm) film was formed on the T M P E D T A covalently attached adhesion layer. Scanning tunneling microscopy shows a fairly smooth surface with standard deviation of about 10 nm for a l x l μ π ι scan (Figure 3a) and 10-25 nm diameter domains were observed for higher resolution scans (Figure 3b). X P S data taken from the lightly sputtered cleaned film indicates that the Ru is predominately present in a metallic state (3d / transition at 279.7 eV). A significant concentration of Ο (0.08-0.18 a/o) was observed in the film from A E S and SIMS depth profiles. A representative A E S spectrum recorded in the interior of the film is shown in Figure 4, along with the depth profile (inset). Some indication that the TMPEDTA ligand is still present at the interface of the Ru film and the quartz substrate was obtained from positive ion SIMS depth profiles (e.g., increase in C and Ν positive ion yield seen when sputtering using high O2 gas pressures (Ρθ2=3.3χ10" mbar)). The chemical composition, structure, and physical properties of the Ru films will be the subject of continuing investigations. Of particular interest is the characterization of the potentially mixed-valent [TMPEDTA]Ru PzRu (H20)5 films and the nature of the conductive [TMPEDTA]Ru mirrors.


n

#

2

5

2

7

n

ra

Mixed Inorganic-Organic Polymers A n alternate approach to the preparation of low-dimensional materials as described above is to develop methods for constructing mixed inorganic-organic polymers. Our initial investigations have focused on linking R u 2 ( 0 2 C R ) 4 centers with D M D C N Q I bridging ligands in order to synthesize chains with extended overlap of π-symmetry orbitals. These materials should be complementary to the well known σ-systems, which include Wolfram's red salt derivatives (16) and halide bridged metal dimers (77), formed by σ-type interactions of M d 2

4

2

2

2 z

and halide p orbitals. .The z

2

Ru compounds, with the σ π δ π * δ * electronic configuration (18), have partially filled π symmetry orbitals and are known to coordinate axial ligands with back donation into ligand π* orbitals (19). Both of these factors are important with respect to setting up M - L π overlap and ultimately delocalized π-symmetry orbitals along the chain axis.

Me

DMDCNQI



4. LI ET AL.

39


|Si/SiO

?

|Si/SiO/TMPEDTA|

Si/Si0 n"MP(EDTA)Ru (pz)Ru CI l,

2

Τ

160

161

I

162

I

163

,l,

5

""Τ Ι 65

164

Δ (Degrees)

I

166

167

Figure 2. Plot of the Ellipsometric Parameters ψ vs Δ for TMPEDTA, [TMPEDTA]Ru Pz, and [ T M P E D T A ] R u P z R u C l Films on SiCtySi Substrates. n

n

m

5


40



Figure 3. S T M Images of R u mirrors formed by self-assembled technique at (a) 1 χ 1 μτη and (b) 100 χ 100 n m Scales. 2

2


4. LI ET AL.


41

The Ru2(02CR)4 entities are also easily oxidized (20). This then allows access to other oxidation states which would facilitate doping of the target one-dimensional materials.


#

Synthesis. Two routes can be used to obtain the [Ru2(02CR)4 DMDCNQI]x systems as shown in Figure 5 (Toi = P - C 6 H 4 C H 3 , Mes = 2,4,6-C6H2(CH3>3). However, the direct reaction of Ru2(02CR)4 and two equivalents of D M D C N Q I produces purer products, as indicated by IR and elemental analysis, and this route is currently exclusively employed. As the yellow T H F solution of D M D C N Q I is dropped into the red-brown solution of Ru2(02CR)4 the color of the solution changes to an intense blue. The blue to purple/black products then precipitate from the solution. These observations indicate that: (1) the redox reaction between the Ru2 center and D M D C N Q I bridge has occurred (the Na radical anion salt of D M D C N Q I has the same blue color as the reaction mixture) and (2) that the products are polymeric (they can not be redissolved in organic solvents including THF, Et20, or CH2CI2).

Characterization. Preliminary X-ray powder diffraction results for [Ru2(02CMe)4»DMDCNQI]x show that the microcrystalline material does diffract and current effort is being directed towards obtaining structural information from this data. We have formulated the [Ru2(02CR)4»DMDCNQI]x materials as chains of Ru2(02CR)4 centers bridged by radical anionic D M D C N Q I ligands. This is based on the solubility properties of the materials, the color, optical spectroscopy , and elemental analysis data (27). The elemental analysis results show that the compounds contain a 1:1 stoichiometry of Ru2(02CR)4 to DMDCNQI. Several features contained in the infrared (IR) spectroscopic data shown in Table I are important to note. First, +

the position of the vc=N hand of the D M D C N Q I ligand in the [Ru2(02CR)4»DMDCNQI]x compounds is shifted to lower frequency relative to the free ligands and the Na[DMDCNQI]2 salt. This indicates coordination of the ligand to the metal center and formal reduction of the C ^ N bond order. Additionally, only one VON band is observed, indicating that the DMDCNQI ligand is symmetrically bound. Even for the alkyl substituted carboxylates, bands in the aromatic region are observed; this is consistent with the formulation of the bridging D M D C N Q I ligand as a radical anion which would have contributions from aromatic resonance structures. Spectroscopic evidence for the oxidation state of the Ru2 centers has been obtained by diffuse reflectance spectroscopy. For [Ru2(02CPh)4*DMDCNQI]x, broad features are observed at ~600 and 950 nm, in addition to a weak band at 1158 nm. This is in the same position, -1100 nm, where δ -> δ* transitions are observed for Ru2(02CR)4Cl compounds (22) containing R u 2 metal centers, suggesting that the R u 2 center in the Ru2(02CR)4 starting material has been oxidized to Ru2 . 5+

4+

5+

Magnetic Measurements. Preliminary magnetic measurements for the [ R u 2 ( 0 2 C R ) 4 « D M D C N Q I ] x systems have been carried out. The [Ru2(02CTol)4«DMDCNQI]x exhibits apparent antiferromagnetic behavior with a TNeel = 46K at Η = 0.1 T, Figure 6, that does not change within our experimental uncertainty (TN ± IK) for Η = IT. The magnetization exhibits strong field dependence which is consistent with the proposed one-dimensional nature of the material. At 10K, dM/dH is linear to -2.6T and then increases more rapidly than linear. The inverse susceptibility is strongly nonlinear in temperature but for T>TN always extrapolates to a negative paramagnetic Curie temperature. On the basis of



42

^\ Downloaded by NORTH CAROLINA STATE UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: July 14, 1992 | doi: 10.1021/bk-1992-0499.ch004

_ 100

X I- -

Ru

c 80 I 60 40 20 Ru

0

1

2

Sputter Time (minutes)

_J 30

230

430

I

I

630

L

830

1030

Electron Energy (eV) Figure 4. Representative AES Spectrum, dN/dE versus Electron Energy, Recorded in the Interior of the "Metallic" Ru Film. Inset: Depth Profile of the Same Surface.

Ru (0 CR) + 1-2 DMDCNQI 2

2

Ru2(0 CR) CI + NafDMDCNQI]

4

2

4

OR

NaCI, DMDCNQI RR

RR

RR

RR

M M Ru — L — M M MM MM -Ru RuR u — L — R u — — R u ^ — L — Ru°·%€>° RR

RR

RR

R = Η, Me, Et, Pr, Bu, Ph, Toi, Mes

RR

L = DMDCNQI

Figure 5. Synthetic Routes to [Ru2(C>2CR)4«DMDCNQI]x.


2

4. LI E T A L .

43


Table I. IR Spectral Data for [Ru2(0 CR) *DMDCNQI]x. Downloaded by NORTH CAROLINA STATE UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: July 14, 1992 | doi: 10.1021/bk-1992-0499.ch004

2

4

Compound

1

vc=N (cm" )

DMDCNQI

2166 2154

Na[DMDCNQI]2 [Ru (02CR) 'DMDCNQI]x 2

0 005 I 0

4

.

R =H

2098

R = Me

2132

R = Et

2116

R = Pr

2136

R = Ph

2132

R = Tol

2138

1 .—ι .—ι 100 200 300 Temperature (K)

. 400

I 0

.

. 1 100 200 Temperature (K)

·

300

Figure 6. Magnetic Susceptibility of [Ru2(02CR)4»DMDCNQI]x (R=Tol, Me) at 0.1T.



44

these data it is postulated that either intrachain or interchain coupling mechanisms could be operative. The [Ru2(C>2CMe)4 DMDCNQI]x compound appeared to show analogous behavior at H= IT, TNeel = 27K, but for H = 0.1T an additional magnetic transition was noted at 17K. Again the inverse susceptibility is nonlinear above 40K, but between 50 and 120K l/χ is approximately linear and extrapolates to a positive Curie temperature of ~20K, suggesting the presence of both antiferromagnetic (T>150K) and ferromagnetic-like (T

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