Reductive Chloride Ion Loss and Electropolymerization Techniques in

9 R e d u c t i v e C h l o r i d e Ion L o s s and E l e c t r o p o l y m e r i z a t i o n T e c h n i q u e s in

Preparing

Downloaded by FUDAN UNIV on January 30, 2017 | http://pubs.acs.org Publication Date: July 2, 1982 | doi: 10.1021/bk-1982-0192.ch009

M e t a l l o p o l y m e r F i l m s on E l e c t r o d e S u r f a c e s J. M. CALVERT, B. P. SULLIVAN, and T. J. MEYER University of North Carolina, Department of Chemistry, Chapel Hill, NC 27514 Two approaches for modifying electrode surfaces with a variety of metal complexes will be discussed. In the first method, reductive chloride loss from a complex in a non-coordinating solvent near or within a previously deposited polymer film is coupled with capture by ligating groups of the polymer to form a covalently bound species. This procedure has been used to create multimetallic films containing rhenium and osmium. The second approach involves electroreductive polymerization of metal complexes with vinyl-containing ligands. This technique has now been generalized to include complexes of other metals, e.g., Os(bpy) (vpy)2 (bpy = 2,2'-bipyridine; vpy = 4-vinylpyridine) and different ligands such as BPE and substituted stilbazoles. The reactivity and properties of these complexes will be compared in a quantitative manner with regard to the type and quantity of polymerizable ligands. 2+

2

Surface chemistry, i n general, i s an area i n which the a b i l i t y to selectively modify the chemical and physical properties of an interface i s highly desirable. The synthetic chemistry of surfaces i s now i n a developing stage, p a r t i c u l a r l y with respect to the attachment of electroactive redox s i t e s to metal or semiconductor surfaces (1^3). Single component and bilayer (4) electroactive films have been a f i e l d of intense research a c t i v i t y since their applications are apparent i n c a t a l y s i s , solar energy conversion, directed charge transfer, electrochromic devices, and trace analysis. There are four broad methods of forming electroactive surface films: 1) Surface linkage of preformed electroactive s i t e s to reactive groupings on the surface. An example of this procedure involves s i l a n i z a t i o n of a metal oxide and subsequent reaction with an electroactive molecule 0097-6156/82/0192-0159 $7.25/0

© 1982 American Chemical Society Miller; Chemically Modified Surfaces in Catalysis and Electrocatalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

CHEMICALLY MODIFIED SURFACES

160

2) 3)


4)

bonded to a nucleophile-containing side chain; For example, see ref. 2. Deposition of preformed electroactive polymers. An example of this approach i s found i n refs. 3 and Id. Deposition of thin layers of insulating polymer on a metal or semiconductor surface followed by attachment of the electroactive s i t e s to polymer f u n c t i o n a l i t i e s ; see, for example, r e f . l a . Formation of a growing polymer at the electrode surface by an e l e c t r o i n i t i a t e d process, as demonstrated i n r e f . lg.

In this paper we wish to discuss new synthetic advances i n methods of preparation of electroactive polymer-coated electrodes which f a l l into categories 3 and 4 l i s t e d above. The technique which we w i l l discuss i n the opening section of this paper i s referred to as reductive C l ~ ion l o s s , which involves using a preformed polymer coating as a "pseudo-solvent" for performing an electroreduction of a transition metal complex that undergoes a f a c i l e loss of chloride ion. The reactive i n t e r mediate generated upon reduction then reacts with l i g a t i n g groups within the polymer f i l m , producing surface-bound electroactive transition metal complexes. Examples of this type of reaction have been found i n Os, Re, and Ru chemistry and appear to be r e l a t i v e l y general phenomena (5). The second section of this paper deals with the synthesis of new Ru and Os derivatives of 4-vinylpyridine (vpy) t r a n s - 4 - s t i l bazole ( s t i l b ) and trans-l,2-bis-(4-pyridyl)-ethylene (BPE) and their e l e c t r o i n i t i a t e d polymerization reactions. The electropolymerization (EP) reactions of the BPE and s t i l b complexes represent graphic examples of the broad scope of this surface derivatization technique that i s available with substituted v i n y l pyridine ligands (6). These studies have provided considerable insight into structural and electronic influences on thin f i l m formation. Experimental Chemicals and Solvents. A c e t o n i t r i l e (Burdick and Jackson) and dichloromethane (Fisher) were stored over Davisson 3& molecular sieves for at least 24 h. before use. Tetra-n-ethylammonium hexafluorophosphate (TEAH) was purchased from A l f a and used without further p u r i f i c a t i o n . Tetra-n-ethylammonium perchlorate (TEAP) was prepared from the corresponding bromide s a l t (Eastman) with the use of a previously published procedure (7). Tetra-nbutylammonium hexafluorophosphate (TBAH) was prepared by dissolving the iodide s a l t (Eastman) i n a hot, equivolume water/ethanol/ acetone mixture followed by addition of HPF (Alfa). The solution was reduced to approximately half i t s o r i g i n a l volume, then cooled to room temperature. The resulting white s o l i d was f i l t e r e d off D

Miller; Chemically Modified Surfaces in Catalysis and Electrocatalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

9.

CALVERT ET A L .

161

Metallopolymer Films

and r e c r y s t a l l i z e d three times from b o i l i n g ethanol. Following preparation, both TEAP and TBAH were dried i n a vacuum oven at 70°C for 10 h., then stored i n a dessicator. Electrolyte solutions were either 0.2 M TBAH/CH C1 or 0.1 M i n electrolyte i f CH3CN was used as the solvent. Poly-(4-vinylpyridine) was purchased from Polysciences. Analysis by membrane osmometry (Arro Laboratories; J o l i e t , 111.) yielded a molecular weight of 33,000. The following polymerizable ligands were employed i n the synthesis of the ruthenium and osmium complexes: 4-vinylpyridine (vpy) purchased from Aldrich Chemical Co. was d i s t i l l e d at reduced pressure (77°C (31 torr)) and stored t i g h t l y capped i n the freezer. BPE was used as received from Aldrich. The ligands trans-4-stilbazole ( s t i l b ) , various 4 -substituted 4-stilbazcles and the complex [Ru(bpy) (stilb) ](PF$)2 (8) were generously provided by Dr. David G. Whitten. (The stilbazole ligands w i l l henceforth be abbreviated as "4'-X-stilb" where X w i l l be replaced by the appropriate functional group such as CI, OCH3 or CN. I f X = "H" the ligand i s stilbazole i t s e l f and may be represented by the abbreviation " s t i l b " . ) Preparations for the complexes [Ru(bpy)o(vpy)o](PF ) ( l g ) , mer-Os(Me PhP) Cl (9), mer-Ru(Me PhP)3CI3 (I0\ OsrV(bpy)Cl (11), and [Ru(bpy) (vpy)Cl]PF (lg) (bpy = 2,2 -bipyridine; vpy = 4-vinylpyridine; Me - methyl; Ph » phenyl) have been described i n the l i t e r a t u r e . Samples of these particular vpy complexes were generously provided by W. R. Murphy. The remaining ruthenium complexes were synthesized according to the following general procedure. The appropriate starting material, i . e . , cis-Ru(bpy) C l (12), cis-Ru(phen) Cl (13), Ru(trpy)Cl (14), Ru(HC(pz) )Cl (15), and [Ru(trpy)(bpy)Cl]PF (3) (trpy = 2,2 ,2"-terpyridine; HC(pz) » tris-(pyrazolyl)-methane) was combined with an excess of the desired polymerizable ligand i n 1:1 ethanol/water (v/v). The mixture was then heated at reflux u n t i l spectral changes were no longer evident. The product was precipitated as a PF^~ s a l t and separated from the accompanying insoluble polymeric material by extraction using CH C1 or CH CN. The resulting s o l i d was purified by chromatography on a column of alumina with CH CN/ toluene or CHg0H/CH Cl mixtures as eluants. Osmium complexes were also prepared according to the above procedure with the exception of ethylene glycol being used i n place of ethanol i n the r e f l u x step. 2

2


f

2

2

6

2

3

3

2

2

4

,

2

6

2

2

2

2

3

3

3

f

6

3

2

2

3

3

2

2

Electrodes and Instrumentation. Electrodes were mechanically polished with one micron diamond paste (Buehler) u n t i l s a t i s f a c tory background voltammograms were obtained. A disposable, 20 ml s c i n t i l l a t i o n v i a l served as a convenient, one-compartment electrochemical c e l l . Electrochemical instrumentation included a PAR model 174A Polarographic Analyzer and a homebuilt waveform generator (16). A l l measurements were recorded versus the saturated sodium chlo-


162



ride electrode (SSCE) at 25+2°C and are uncorrected for junction potential effects. No IR compensation was employed regardless of whether or not the surface of the working electrode was coated by a polymeric f i l m . A platinum wire served as the counter electrode. General Procedure for C l " Ion Loss Experiments. Methanolic PVP solutions (1.86 mgs/50 mis CH3OH) were prepared so that a lOul aliquot applied to a v e r t i c a l l y mounted glassy carbon disk d e l i vered 5 x 10"^ moles of pyridyl sites per square cm of electrode area. The solution was allowed to a i r - d r y , forming a v i s i b l e f i l m on the electrode surface. The film-covered electrode was then rinsed i n s t i r r i n g methanol for five minutes to swell the polymer and re-dried. Introduction of the desired metal complex into the polymer f i l m was accomplished by performing electrochemistry i n an 0.2 M TBAH/CH2CI2 electrolyte solution which was 5 mM i n complex. Solutions of the osmium and rhenium phosphines were protected from l i g h t due to their photolytic i n s t a b i l i t y . A l l solutions were degassed using a stream of Ch^C^-saturated nitrogen. An N blanket was maintained during the course of the experiment to prevent subsequent aeration. Potential l i m i t s were set so that the cycle encompassed both the reduction of the solution species and the couple produced by polymer adduct formation. Cycling was continued u n t i l the s i z e of the product couple no longer i n c r e a s e d — t y p i c a l l y t h i r t y minutes duration. The working electrode was removed from the c e l l , rinsed with CH^CN and air-dried. Further experiments on the coated electrode were performed i n fresh electrolyte. Surface coverages, T, were determined by graphical integration of the area encompassed by the voltammetric wave due to the electroactive material of interest. 2

General Procedure of the Electropolymerization (EP) Experiments . Electrodes were Teflon-shrouded platinum disks (Engelhard) of known area. Otherwise, the electrochemical instrumentation and materials are i d e n t i c a l to those described i n the chlorideloss section. The concentration of electropolymerizable complex used i n an experiment varied from approximately 1 to 3 mM. In general, the complex concentration was inversely related to the number of polymerizable groups. For three groups, [complex] ^ 1 mM; for two groups, [complex] « 1-2 mM; for one group, [complex] « 2-3 mM. Prior to the electrochemical experiment, solutions were degassed using a stream of C^CN-saturated nitrogen, then protected by an N blanket. Solutions containing bis-bipyridine complexes were protected from l i g h t to prevent the f a c i l e photosubstitution reaction known for complexes of this type (17). 2


9.

CALVERT ET AL.

163


Potential l i m i t s for the EP process were chosen so that the cathodic l i m i t of the cycle was ca. 150 mV negative of the E^ for the reductive couple of interest. In cases where the anodic Component of a couple i s not well-defined, the cathodic l i m i t was set at a potential s u f f i c i e n t l y past E (18) (~ 100 mV) so that the reduction process would not be inhloited. The anodic l i m i t of the cycle was chosen to be at a convenient potential i n the range -0.8 to -1.0 V. The number of cycles used i n a particular polymerization depended upon the nature and concentration of the complex involved as w e l l as the potential settings and therefore was determined separately for each reaction. The scan rate employed i n a l l experiments was 200 mV/s, except as noted otherwise. After completion of the EP procedure the working electrode was removed from the c e l l , rinsed with acetone and allowed to a i r dry. The coated electrode was then examined i n a solution of fresh TEAP/CH3CN electrolyte.


c

Results and Discussion Reductive Chloride Ion Loss as a Preparative Technique for Electroactive Thin Films. Chloride Loss from Transition Metal Complexes Upon Reduction. Recently we have investigated the electrochemistry of a number of mixed polypyridyl and phosphine osmium complexes that contain halides as a n c i l l a r y ligands (5). Representative complexes span three different oxidation states of osmium, 0s (bpy)Cl4, mer-Os (PMe Ph)3CI3 and c i s - 0 s ( b p y ) C l 2 (bpy = 2,2'-bipyridine). In addition, the perhalo species [ 0 s C l ] " , also was found to undergo f a c i l e C l ~ ion loss upon reduction to Os(III). Figure 1 shows the c y c l i c voltammetry of three of these complexes i n CH3CN solution with 0.1 M TBAH as supporting e l e c t r o l y t e , a l l at a scan rate of 200 mV/sec. Under these conditions a l l complexes exhibit an ECE-coupled mechanism which i s associated with the i n i t i a l chemically i r r e v e r s i b l e osmiumlocalized reduction. For [ O s C l ^ ] " , the O s complex produced i n the f i r s t reductive step i s the l a b i l e product, (eq. 1-3) IV

111

II

?

?

IV

2

0

IV

[ O s C l ] " + eI V

2

6

I i : i

[Os

3

P>

f a S t

C l ] " + CH CN 6

E

m

3

,[Os ci ] ~

C

m

2

5

** > [Os (CH CN)Cl ]~ + e"

2

5

(1)

6

3

[Os (CH CN)Cl ] " 3

1 1 1

> [Os (CH CN)Cl ] ~ + Cl"

3

III

2

IV

(

3

III

5

(2) (3)

For both [ O s ( b p y ) C l ] ~ and O s ( P M e 2 P h ) C l , reduction to O s results i n rapid C l ~ ion loss. (eq. 4-6) m

4

3

[ O s ( b p y ) C l ] - + e" III

4

n

2

[ 0 s ( b p y ) C l 4 ] " + CH3CN II

[Os (bpy) ( C H C N ) C l r 3

3

(

II

2

• [Os (bpy)Cl ] -

(4)

4

II

• [Os (bpy)(CH CN)Cl ]" 3

3

• [Os (bpy) (CH3CIOCI3] + e" IIT

1 1

3

0


(5) (6)



Figure 1. Reductive CI' loss processes in Refill), Os(IV), and Os(III) complexes in CH CN solution. Key: bottom, mer-ReflUXMetPhPhCh; middle, mer-Os(III)(Me PhP) Cl ; and top, [Os(IV)Cl ] '. S

2

t

s

s

6

Labeled couples are due to parent complex specified in figure. Middle couple is from product (acetonitrile) complex. A second product couple (not shown) occurs at potentials positive of the more oxidizing parent couple. Cyclic voltammograms were taken with 0.1 M TBAH as supporting electrolyte at a sweep rate of 200 mV/s.


9.

CALVERT ET

165


AL.

The l a b i l i z a t i o n of CI"" upon reduction can be understood as a simple effect of putting electron density on the metal atom, thereby reducing the need for ir-donation from the C l " ligands, which weakens the Os-Cl bond resulting i n greater substitutional lability. This effect i s most dramatically i l l u s t r a t e d i n the case of Os (bpy) Cl2 l a b i l i z a t i o n of C l " occurs not upon metal reduction but on the second bpy reduction, (eq. 7-11) II

w n e r e

2

II

[Os (bpy) Cl ]° + e~ 2

2

II

* [Os (bpy)(bfy)Cl ]"

(

II

Il:

[Os (bpy)(bpy)Cl ]'

2

2


2

(8)

2

II

[ O s ( b p y ) C l ] " + CH CN 2

2

• [Os (bpy) Cl ] "

2

II

(7)

2

•[Os (bpy) (CH CN)Cl]" + C l "

3

2

II

3

II

0

[Os (bpy) (CH CN)Cl]"^ ^[Os (bpy)(bpy)(CH CN)Cl] + e" 2

3

z=

3

II

[Os^bpy) (bpy) (CH CN)C1]° 3

+

• [Os (bpy) (CH CN)Cl] + e"

(

2

3

(9) (10) (11)

TT — 2 The ligand-reduced complex i n eq. 8, [Os (bpy) Cl ] ~, can be viewed as a formal analog of [ O s ( b p y ) C l ] ~ , where both the C l " and bipyridine radical anion (bpy) strongly donate electron density to the metal center resulting i n f a c i l e loss of the unidentate C l " ligand. This chloride ion loss chemistry appears to be reasonably general (5), further examples being [ R u ( t r p y ) ( P P h ) C l ] (14), Ru (bpy) Cl and Re (PMe Ph)3CI3. Formation of Surface Complexes from PVP Coated Electrodes by Reductive C l " Loss. Preparation of thin, electroactive metallopolymer films on glassy carbon electrode surfaces was accomplished by performing the fast C l " loss process i l l u s t r a t e d i n equation 2, for example, at an electrode surface which had been previously modified with a coating of PVP. In a typical experiment a 5 mM solution of mer-Os(Me PhP) Cl i n CH C1 with 0.1 M TBAH as supporting electrolyte was deoxygenated with an N stream and then the PVP-coated electrode was used as the working electrode i n a usual three electrode c y c l i c voltammetric configuration. During multiple scans (ca. 10-50) into the 0 s / 0 s reduction the chloride ion loss and pyridine coordination reaction occurred as schematically i l l u s t r a t e d i n Figure 2. The re-oxidation of surface-bound osmium (eq. 12) proved to be a convenient method to monitor the amount of metal incorporation by the p y r i dine coordination s i t e s . 2

II

2

2

4

JI

+

3

I]:

m

2

2

2

2

3

3

2

2

2

2

I I I

• [Os (Me PhP) Cl ^)j

I]:

IIX

Os (Me PhP) Cl2lf^| 2

3

(

2

3

2

1 1

+

] + e"

(12)

Control of the rate of deposition could be achieved by scanning through only part of the reductive wave. Table 1 compares E , E^, and IE (18) values for several electrodes with the corresponding values for the non-polymer ot




166

trans- fvfCPUCI Figure 2. Molecular events in the preparation of trans-M(ll)(MetPhP) (PVP)Cl,coated electrodes (M(II) = Re, Os) in CH Cl, solution with 0.1 M TBAH as supporting electrolyte. s

g



(Me PhP) (PVP)Cl

(Me PhP) (PVP)C1

trans-Os

2

2

3

3

3

b

b

2

2

2

trans-Re

2

3

2

(Me PhP) (PVP)Cl

2

trans-Re

2

3

(Me PhP) (py)Cl

2

trans-Re

2

(Me PhP) (PVP)Cl

3

trans-Os

2

(Me PhP) (py)Cl

(a)

trans-Os

Complex or Electrode

Table 1.

E, or E°' (V)

0s

60 /0s

n i

50

50

50

50

5

20

60

70

0s /0s

15

1

I V

I V

/0s

/ B

11

couple

couple

couple

couple

couple

couple T a b l e 1 c o n t i n u e d o n n e x t page.

couple 11

couple

couple n

m

m

11

/0s Re " / R e

1

0s

n i

Re /Re

0s

P o

1

m

H

1 1 1

n

1 1 1

Re " / R e couple I V III Re /Re couple Re " / R e couple

I V

Re /Re

m

0s

20 /Os

/Os

17

IV

Os

Assignment and Comments

60

AE (mV)

Surface and Solution Potentials of Re and Os Pyridine Complexes.


f

i

1

> r

H W H

5

v©


2

3

2

3

3

3

65

-0.56 -0.96

60

+0.64

3

-0.38 +1.00

1

70

100

p

AE (mV)

60 — 120

+ 0

+ ( K 9 4

+1.35

E,^ or E°' (V)

IV

3

2

3

11

3

2

2

3

+

2

ni

2

2

2

3

3

3

2

2

3

+

-

3

2

2

same scheme as 0 s

111

analog (see above)

irreversible reduction of Os^fMegPhP^Cl

trans-Os (Me PhP) (CH^CN)Cl

11

[trans-0s (Me PhP) (CH CN)Cl ]

in

mer-0s (Me PhP) Cl

3

[trans-Os^ (Me PhP) (CH CN)Cl ]

3

[mer-0s (Me PhP) C1 ] + e'

IV

[trans-Os (Me PhP) (CH CN)Cl ] * + e

Assignment and Comments

a) Potentials were measured vs. SSCE in 0.2M TBAH/CH2CI2 or CH3CN solution with 0.1M supporting electrolyte (TEAP or TBAH). Solution couples (EjJ were determined using a Pt disk electrode. Surface couples (E°') were measured as thin film? mounted on glassy carbon disks. Sweep rate was 200 mV/s. b) This material was one component of a bimetallic osmium/rhenium, surface-bound metallopolymer film.

mer-Re (Me PhP) Cl

in

mer-0s (Me PhP) C1

ni

Complex or Electrode**)

Table 1 continued.


9.

CALVERT ET AL.


169

analogs. The potentials for both the solution and surface-immobil i z e d couples are very similar as has been observed by Murray et. a l . for a wide variety of surface bound species (19). Surface coverage values for trans-Os (Me2PhPT3 (PVP)Cl varied from 10"^ to ca. 2 x 10"^ mol/cm which, depending upon the PVP f i l m thickness, indicated that up to 50% of the pyridine groups were metallated. This value i s considered to be a lower l i m i t since the pre-soaking technique (see experimental section) probably removes a fraction of the PVP coating from the surface. Figure 3 shows the 0 s / 0 s surface couple at scan rates ranging from 1-500 mV/sec; the lower portion of the figure demonstrates the diffusional character of the surface couple at fast scan rates (v > 50 mV/sec) since a linear relationship between i and v i s observed. (20) The o r i g i n of the d i f f u s i o n a l response may arise from either: 1) an i n t r i n s i c a l l y slow rate of electron transfer between redox sites (self-exchange) or 2) fast electron transfer limited by the existence of a structural b a r r i e r due to incorporation of the sites into a polymeric f i l m . The l a t t e r effect could be brought about i n one of two ways: a) effective i s o l a t i o n of redox sites because of polymer network r i g i d i t y , or b) i n a b i l i t y of the f i l m to incorporate or expel a s u f f i c i e n t quantity of charge-compensating counterions during the redox process. The f i r s t explanation i s the least l i k e l y because i n cases where comparisons have been made between homogeneous solution self-exchange data ( k ) and charge transport rates (DCT) for analogous complexes immobilized i n redox polymer films ( l j , lm) i t has been generally found that k substantially exceeds DQ , implying that electron exchange between redox sites i s not the l i m i t i n g factor i n the overall rate at which charge i s transported through the f i l m . A crude calculation reveals that the redox s i t e concentration i n our films i s i n the molar region and Anson has shown (lk) that D values i n similar (although oppositely charged) metallopolymer films are unaffected even at redox s i t e concentrations 100 times more dilute than those used here. These results argue against explanation 2a and, by default, point to 2b, restricted counterion d i f f u s i o n , as the cause of the observed electrochemistry. However, Murray has determined the rate of d i f fusion of bromide ion through an electropolymerized metallopolymer f i l m to be more than 10^ times greater than D ( l j ) . Making the assumption that perchlorate (the counterion used i n his and our experiments as well) has a similar mobility to Br" he concludes that polymer l a t t i c e mobility (explanation 2a), not 2b, sets the upper l i m i t for D^. With the knowledge of these two c o n f l i c t i n g results we cannot, on the basis of our data, make an informed choice between explanation 2a and 2b but we can be reasonably certain that the d i f f u s i o n a l response at faster sweep rates of our surface-bound polymer f i l m has i t s o r i g i n i n a structural barrier rather than an i n t r i n s i c a l l y slow electron transfer rate. At slower sweep rates (v < 20 mV/s) i follows a linear 2

2

I I I

1 1

p


2

ex

e x

T

CT

CT

p


170



Figure 3. Scan rate dependence of the Os(lII)/Os(lI) surface couple for a typical Os(U)(Me PhP)(PVP)Clt glassy carbon electrode (top) and i versus v and i versus v plots for the anodic wave of this couple (bottom). Cyclic voltammograms were recorded with 0.1 M TEAP as supporting electrolyte. %

t

9


p

9.

CALVERT E T AL.


171

relationship with v which indicates that there are no longer k i netic limitations to the rate of charge transport through the polymer f i l m . We have also observed that there i s a dramatic dependence of the electrochemical response on the nature of the e l e c t r o l y t e — i n particular, the existence of a s p e c i f i c anion effect. Figure 4 shows the effect on the same polymer f i l m of changing only the electrolyte anion from perchlorate to hexafluorophosphate. Not only i s there a significant reduction i n the size of both waves i n the PF^" medium (note change i n current s e n s i t i v i t y ) , but there i s also a profound effect on the shapes of the waves. In particul a r , the extreme sharpness of the anodic O s / O s wave may be due, i n part, to the phaselike behavior of c r y s t a l l i n e elements i n the f i l m which form i n the presence of PF^" as opposed to C10^~. Similar behavior has been observed i n the effect of e l e c t r o l y t e cations on the response of an anionic f i l m of Prussian blue (lh) and also the effect of various solvents on a plasma-polymerized vinyIferrocene f i l m ( l e ) .


III

11

Synthesis of Multimetallic Thin Films. The C l " loss technique can be extended to give films that have two or more metals and as many as five separate electrochemically metal-centered redox processes. An example of this i s shown i n Figure 5 where an Os(Me2PhP)3(PVP)Cl f i l m , prepared as previously described, was cycled i n a solution of mer-Re (Me^PhP)3CI3. The resultant f i l m has four redox processes corresponding to the M^/M and M / M couples. In 0.1 M TBAH/CH3CN solution this f i l m was reasonably stable on repeated cycling through the M ^ V M couples, but ca. 10 cycles through the M^/M couples resulted i n the charact e r i s t i c c y c l i c voltammogram (Fig. 5B) of trans-[Os (MepPhP)~(PVP)C1 ], indicating that rapid solvation of [Re (Me PhP) (PVP)C l ] had occurred. Under current investigation are the synthesis and properties of multimetallic thin films containing Ru *, O s , and R e * i n a wide variety of coordination environments. Judicious choice of such materials may lead to creation of an electroactive polymer f i l m which would exhibit a bandlike spectrum of reversible, metalcentered redox processes extending from ca. -0.6 to +1.5 V. Electropolymerization of 4-Vinylpyridine Complexes. Investigations of Structural and Electronic Influences on Thin Film Formation. The recent discovery of the reductive polymerization of complexes containing v i n y l p y r i d y l ligands ( l g ) , such as Ru (bpy)2(vpy)2 > has l * to the preparation of homogeneous thin layers of very stable electroactive polymers. This method has been extended to 4-vinyl-4 -methyl-2,2 -bipyridine ( l g , 21a) and 4-vinyl-l,10-phenanthroline (21b) on both ruthenium and iron. In the following section we discuss our results on thin films derived from the polymerizable ligands BPE and the trans-4'-X-stilbazoles, ( 4 - X - s t i l b ; X = C l , OMe, CN and H). In addition, we have prepared the f i r s t electropolymerizable 2

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Reductive Chloride Ion Loss and Electropolymerization Techniques in

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