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Langmuir 1991, 7, 2376-2379
Electrostatic Interaction Effects on the Rate of Reductive Polymerization for Vinylbipyridine-Containing Metal Complexes Christopher J. Baldy, Donald L. Morrison, and C. Michael Elliott* Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523 Received December 26,1990. In Final Form: May 6, 1991 A series of mixed ligand metal complexescontaining the electrochemicallypolymerizable ligand 4-methyl4’-vinyL2,2’-bipyridine (vbpy) have been prepared and their electrochemical polymerization has been studied as a function of formal oxidation state. The complexes chosen for this study were initially either neutral or 1+ charged prior to electrochemicalreduction. This is in contrast to previously studied complexes containing vbpy ligands, moat of which have been initially 2+ charged. The results of this study confirm our earlier suggestion that the existence of a net electrostatic charge on polymerizing vbpy metal monomer complexes reduces the rate of electrochemically induced polymer formation.
Introduction We have recently reported on the structure and formation mechanism of polymers prepared by the electrochemical reduction of metal complexes containing the ligand 4-methy1-4’-vinyl-2,2’-bipyridine.l These results have indicated that polymerization occurs through a chain propagation mechanism. For example, in the case of [Fe(vbpy)g]2+which was polymerized by reduction at constant potential, a poly(viny1bipyridine) backbone was formed containing a median monomer repeat unit of seven.’ While addressing arguments which led to the previously held conclusion that thia type of polymerization occurred mainly through hydrodimerization, we proposed that electrostatic interactions between monomer complexes may be an important factor in determining the rate and efficiency of polymer formation.’ Here we provide results of our further investigation into these electrostatic interactions of vbpycontaining metal complexes. The specific argument in question relates to the fact that many vinyl-containing pyridine and bipyridine complexes polymerize much more rapidly and efficiently (often by more than a factor of 10)when the respective complex is reduced by two electrons rather than one. This fact originally seemed to be most consistent with a hydrodimerization mechanism, since hydrodimerization would require two anion radicals to join. In contrast, only a single reducing electron should be necessary to initiate a radicalchain propagation polymerization. An alternative explanation for the greatly enhanced rate of polymerization of these doubly reduced complexes lies in their overall electrostatic charge. The majority of previously studied complexes had a net 2+ charge in their original, unreduced state. Thus, when such a monomer was reduced.by one electron, it would still posses a net 1+ charge, whereas a total of two electrons are required to reach the neutral form. Herein we report the preparation and polymerization of several vbpy-containing metal complexes which have net charges of either 1+ or 0 in the original unreduced form. Consideration is given to how the charge residing on the monomer might affect the rate of electrochemical polymerization. Experimental Section Materials. Acetonitrile (Burdick & Jackson) for electrochemical measurements was stored under nitrogen and used without further purification. Tetra-n-butylammoniumhexaflu(1) Elliott, C. Michael; Baldy, Christopher, J.; Nuwayeir, Lydia M.; Wilkins, Charles L. Inorg. Chem. 1990, 29, 389.
orophosphate (TBAPFe) was prepared as previously reported.2 Electrochemical solutions were all 0.1 M TBAPFe in acetonitrile. Potentiostatic reduction was conductedwith a EG&GPrinceton Applied Research Model 173 potentiostat, Model 175 UniversalProgrammer, and a model 179digital coulometer with a Houston 2000 X-Y recorder. All experimenta use a threeelectrode arrangement (SCE reference, Pt wire auxiliary electrode) and a three-compartment cell under a Nz blanket. The glassy carbon working electrode (3.0 mm diameter) was polished prior to each scan unless otherwise noted. All potentials are reported vs the SCE reference and are uncorrected for junction potentials. Synthesis. The 4-methyl-4’-vinyl-2,2’-bipyridine (vbpy)was prepared from 4,4’-dimethyL2,2’-bipyridine(Mezbpy) (Reilley, Indianapolis) using the method of Guarr and Anson? The 4-methyl-4’-carboxylato-2,2’-bipyridine (mcbpy) was prepared by a modification of the method of Schanze et al.‘ The 5-sulfonato-2,2’-bipyridine (sbpy) and [Run(vbpy)2(sbpy)]+were prepared by the method of Seddon et al.6 The bis(4-methyl4’-vinyl-2,2’-bipyridine)dichlororuthenium(II)complex, [Ru(vbpy)&12]0,was synthesizedby the method of Sullivan,Salmon, and Meyerae The bis(4-methyl-4’-vinyl-2,2’-bipyridine) (Cmethyl-4'-car boxylato-2,2’-bipyridine)ruthenium(II) c o m p l e x , [Ru(vbpy)~(mcbpy)]+,was prepared by the method of Sprintschnik et ala7 Mcbpy is the 1- charged ligand, 4-methyl-4’-carboxylato-2,2‘-bipyridine,which was prepared by placing the protonated mcbpy ligand in a solution of NazCOs and rotary evaporatingto dryness. The white solid was extractedinto MeOH three timesandrotaryevaporated todryness. Ru(vbpy)&lr2HzO (35.2 mg, 6.21 X mol) was dissolved in 10 mL of McOH and 13.3mg of mcbpy (6.21 X 104mol) added. The reaction mixture was heated under reflux for 16 h. The solution color changed from a dark purple to a rust-orange. The product (as an orange precipitate)was collected by centrifugationfollowingthe addition of H20 and NH4PFe and the removal of MeOH by rotary evaporation. The bis(4-methyl-4’-vinyl-2,2’-bipyridine)dicyanoruthenium-
(11)complex, [Ru(~bpy)2(CN)~]O, was synthesized by the method of Demas, Turner, and Crosby.* A solution of 15mg of KCN (2.3 (2) Elliott, C. M.; Hershenhart, E.; Finke, R. G.; Smith, B. L. J. Am. Chem. SOC. 1981,103,5558. (3) Guarr, T. F.; Anaon, F. C. J. Phy8. Chem. 1987,91, 4037. (4) Teleer, J.; Cruickehanek, K. A.;Schanze, K. S.; Netzel, T. L. J. Am. Chem. SOC.1989,111,7221. (5) Anderson, Suean; Constable, Edwin C.; Seddon, Kenneth R.; Turp, Janet E. J. Chem. SOC.,Dalton Trans. 1985,2247. (6) Sullivan, B. P.; Salmon, D. J.; Meyer, T. J.Inorg. Chem. 1978,17, 3334. (7) Sprintachnik, G e r h d ; Sprintachnik, Hertha W.; Kirsch, Pierre, P.; Whittin, David G. J. Am. Chem. SOC.1977, 99,4947. (8) Demas, J. N.; Turner, T. F.; Crosby, G. A. Inorg. Chem. 1969,8, 674.
0743-7463I91/240~-2376$02.50/0 0 1991 American Chemical Societv
Rates of Reductive Polymerization X lo-' mol) in 2.5 mL of H20 was poured into a solution of 15 mg of Ru(vbpy)&H20 (2.64 X 1od mol) in 10 mL of MeOH. The solution was refluxed with stirring for a total of 16 h. The solution color changed from an initial dark purple to an orangered. The reaction was monitored by TLC on unactivated silica gel plates using methanol as the eluent. Three major spots corresponded to the starting material (purple, Rf = 0.651, the product (orange-yellow, Rf = 0.45),and byproducts (orange, Rf
= 0).
At this point, the solution was evaporated to dryness with a nitrogen stream. The residue was removed from the sides of the flask and boiling water added. The extracts were filtered hot with a Bilchner funnel, leavingthe product as bright red crystals. Thisproduct was chromatographed on silicaby using a 1:l mixture of acetone and CH2C12. The electrochemistry (videinfra) of this product suggests some impurity remains. (The presence of this impurity is considered subsequently in the discussion section.) The bis(4-methyl-4'-vinyl-2,2'-bipyridine)dicyanoiron(II) complex, [Fe(vbpy)z(CN)2I0,was obtained as a generous gift from Professor H. D. Abrufiae (prepared by a modified method of Schiltlo). All complexes are chosen so that the LUMO is localized predominantly on the vbpy ligands. Potential Step Sequences for Polymer Film Formation. In order to determine the relative efficiency of polymer formation as a function of monomer oxidation state, a series of polymerforming potential step experiments were performed. Additionally, to ensure that these data were not distorted by polymer instability, a series of multiple potential step controls were also performed. This rather complex set of experiments is described in detail below. First, a newly polished glassy carbon electrode was placed in a 5.0 mM solution of the respective monomer and the potential scanned twice at 100 mV/s from 0.00 V through the metal-based oxidation (nopolymer present, no polymer formed). The current voltage curve for the second scan (vide infra) was recorded. This "base scan" provides the background from which one could determine, in subsequent experiments, what part of the current was due to oxidation of monomer in solution and what part was due to deposited polymer. Second, an identical freshly polished electrode was placed into the monomer solution. This time the potential was stepped from 0.00 V to just beyond the potential offirst ligand-based reduction of the monomer, held at this potential for 1.0 min, and then stepped back to 0.00 V. The amount of electroactive polymer deposited during the step was determined by fiist stirring to remove any solution concentration gradients and then scanning twice through the metal-based oxidation. The integrated current from the second of these two scans minus the integrated current from the "base scan" was taken to be directly proportional to the amount of electroactive polymer deposited during the step. The reason for choosing the second scan (vide infra) rather than the first is to eliminate any contribution to the charge from species 'trapped" in a more highly reduced form.11J2 Third, the process was repeated exactly as above except that the potential was stepped just past the second reduction. As mentioned above,stabilities of the monomersand polymers are potentially a concern when attempting to compare amounts of polymer formed at different potentials. In other words, differencesin the amount of electroactivepolymer present could, in principle, arise from a more rapid chemical decomposition of one or the other reduced form of the complex. For that reason, experiments were conducted to check whether the amount of polymer present was influenced by the length of time that the polymer had remained in a more highly reduced form. The details of these step sequencing experiments are as follows: A polymer film was formed on a clean glassy carbon electrode at the potential of the first reduction of the monomer just as described in the second step above. However, after stepping (9) AbruAa, H. A., Come11 University, Ithaca, NY. (10) Schilt, Alfred A J. Am. Chem. SOC.1960,82,3000. (11) Denisevich, P.; Abrufia, H. D.; Leidner, C. R.; Meyer, T. J.; Murray, R. W. Inorg. Chem. 1982,21, 2153. (12) Denisevich,P.;Willman, K. W.;Murray,R.W. J.Am. Chem. SOC. 1981,103, 4121.
Langmuir, Vol. 7, No. 10,1991 2377 back to 0.00 V and stirring the solution, a second layer of polymer was deposited on top of the first by stepping the potential past the second reduction (as in the third step above). The total amount of polymer deposited in the two steps was then determined by cyclic voltammetry just as with the single films. Exactly the same procedure was repeated a second time with a clean electrode except that the orders of the deposition potentials were reversed (i.e. reduction past the second peak was carried out fiist followed by reduction past the first peak). It should follow that if the total amount of electroactive polymer in these two"bi1ayer" experimentsis the same (i.e. there is no dependence on the order in which they are deposited), then decomposition of the reduced polymer is not influencing the amount of polymer present in either of the single-stepexperiments. Otherwise,there should be less total polymer present when the first layer is deposited at the less reducing potential (presuming, of course, that instability increases in the more highly reduced forms) since only with that ordering are both layers subjected to the more negative potential step.
Rssults Figure 1shows examples of the voltammetry obtained for typical polymer films formed by stepping past the first and second reduction waves of [Ru(mcbpy)(vbpy)2]+.The results of the polymer formation experiments are summarized in Table I. A cyclic voltammogram of each of the complexes yielded the redox potentials of the vbpy ligand reductions which are responsible for polymerization (columns 3 and 4) and the metal-based oxidation wave (column 2) which is used to quantify the amount of polymer formed. The step potentials (columns 5 and 6) were selected to ensure that the majority of the species at the electrode surface was in the oxidation state of the singularly or doubly reduced complex, respectively. Because reproducibility is a problem, the electrochemical results relevant to the rates of polymer deposition are reported in a statistical manner in Table I. Despite some variability, the results are nevertheless statistically meaningful. The fact that all ratios in column 7 are substantially less than one shows that the species which are initially 1+ charged polymerize to a greater degree upon reduction by one electron (to the neutral form) than do the same complexes when reduced by two electrons (to the 1- form).13 Similarly, the complexes which are initially neutral, [Fe/Ru(vbpy)z(CN)2], polymerize to a greater degree upon reduction by one electron (tothe 1- form) than do the same complexes when reduced by two electrons (to the 2- form). In addition, these initially neutral complexes produce very little polymer (relative to the other two complexes) irrespective of whether the polymerization was carried out a t the first or second bipyridine reduction (column 9). In all cases except [Fe(vbpy)2(CN)2l0the amount of polymer generated for the two layer films lacked dependence on the order of the potential steps (all values in column 8 are close to one). As anticipated, some film degradation appeared to occur when stepping beyond the second reduction for [Fe(vbpy)2(CN)2l0,which is inherently more labile than the other complexes under study. For the [R~(vbpy)2(CN)2]~ there was initially someconcem that upon reduction, substitution of CN- by the solvent might result. (Such a ligand substitution reaction might also be invoked to rationalize the apparent impurity observed in the electrochemistry of the purified [Ru(vbpy)zCNz]O). It is unlikely, however, that significant CN- loss occurs on the polymerization time scale especially (13) It should be noted that the free ligand reduction potentiale of both anionic bipyridyla (sbpy and mcbpy) are more negative than the reduction potential of vbpy; thus both the first and second reducing equivalentsshould reside predominantly on vbpy ligands in these mixed ligand complexes.
Baldy et al.
2378 Langmuir, Vol. 7, No. 10,1991
Potcnlial slcppcd lo -1 64 Potcntial stcppcd to -1 44
Figure 1. Cyclic voltammogram for the reduction scan of an acetonitrile/O.lO M TBAPFe solution containing ca. 1 mM [Ru(vbpy)z(mcbpy)]+(scan from 0.00 to -1.80 V) and cyclic voltammograms in acetonitrile/O.lO M TBAPFe showing the metal 3+/2+ couple of polymers formed by potential steps to the indicated potentials (scans from 0.00 to +1.40 V).
Table I. Results of Electrostatic Argument complex Ru(vbp~)z(Mcbp~)+
Ru(vbp~)z(Sbp~)+ RU(V~PY)~(CN)Z~ Fe(vbpy)2(CN)a0
oxidation"
+1.24 +1.24
+0.74 +0.38
1st reducb -1.37
2nd reducb -1.57
-1.44
-1.30 -1.71 -1.64
-1.49 -1.93 -1.84
-1.39 -1.81 -1.74
1st stepc
2nd stepc
-1.64 -1.58
-2.03 -1.94
re1 2ndllstd 0.70 (O.O6)21 0.80 (0.06), 0.73 (0.0716 0.54 (0.08)r
-
re1 2 1 / 1 4 2e 0.97 (0.02)4 0.98 (0.03)4 1.06 (0.10)4 0.60 (0.14)4
ipt pA
14.8 15.3 3.2 2.9
a E1 2 for metal based oxidation wave used to determine amount of polymer formed. 6 First and second vbpy based reductions, E d in V vs SCd. c Potential step beyond the first and secondvbpy based reductions,E,bpin V vs SCE. Relative polymer formation (amount of polymer formed by stepping beyond second reduction, divided by amount of polymer formed stepping beyond first reduction). Reported as ,i 2nd red/& 1st red (on-&,. ,I umplr. e Relative polymer formation (amount of polymer formed by stepping beyond second reduction and then first reduction, divided by amount of polymer formed stepping in the opposite order). Reported as i,(2 l)/ip(l 2)(un&,. ofumpla. f Peak current associated with polymer formed by stepping beyond the first reduction wave in a solution of 5 mM monomer complex.
-
given the strong back-bonding of the CN- ligand. Additionally, Cooper and Wertz14 have shown, by bulk electrolysis, that in DMF the reduction product of [Ru(vbpy)2(CN)2l0is stable to solvent substitution on the time scale of minutes. For the iron complex, however, this mode of decomposition is more problematic and, as implied above, is likely the reason for the low value of the ratio in column 8 of Table I. I t should be noted that the lack of consistency in the amount of polymer formed from one step to the next follows a general trend. During initial testing, more polymer is consistently formed when stepping beyond the first reduction vs stepping beyond the second reduction. Following repeated electrolysis,however, this pattern tends to reverse. (All data reported above were recorded prior to this pattern reversal.) We suspect that this trend is due to the formation of reduced oligomer generated during previous electrolysis or possibly other products leaching from the auxiliary compartment generated during previous electrolysis. Irrespective of the exact cause of the change in behavior with previous solution history, however, the initial data is always consistent with the less highly charged form of the complex (be it either neutral or 1-) more efficiently forming polymer. (14) Cooper, J. B.; Wertz,I).W.Znorg. Chem. 1989,28, 3108.
-
Discussion When a formally electrically neutral complex polymerizes, there are two obvious differences from the case when charged species polymerize: First, there is no site-site electrostatic repulsion for neutral species. Charged species always experience electrostatic repulsion from other similarly charged species, and, if of sufficient magnitude, this electrostatic repulsion could reduce the rate of polymerization. Second, whenever a polymer is constructed from charged monomers, counterions must be incorporated into the polymer along with the polymerizing monomer. Counterions require space and thus could sterically hinder the approach of additional monomers to sites of polymer growth. For reasons which we will enumerate below, we feel that electrostatic interactions are the predominating factor influencing the rate of polymerization of monomers of the type under consideration here. We will, however, first consider the case against steric contributions from counterions. The major factor which argues against a large counterionbased steric inhibition to the polymerization process is the general electrochemical behavior of these polymers once formed. A polymer film formed from [Ru(vbpy)aI2+, for example, undergoes i b various oxidations and reductions a t potentials which are nearly the same as those of
Rates of Reductive Polymerization the respective monomer in solution. Furthermore, the potentials are relatively insensitive to the size of the electrolyte ions, a t least for nonpolyelectrolytes.15J6 Additionally, the rates of these polymer-based electron transfer reactions appear, in most instances, not to be limited by ion motion within the polymer matrix. These observations, taken together, tend to support a picture of an open polymer matrix where the charge compensating ions have relatively free motion. Were this not the case, one would expect, for example, significantshifts in polymer potentials relative to those of the analogous monomer in solution. Also, one would expect rates of electron hopping through the polymer that are dependent on electrolyte ion size and this is simply not observed for a wide range of "normal" sized electrolyte ions.l6Js Given a lack of evidence for ion-size steric factors in the electrochemistry of a formed polymer, it seems unlikely to us that the presence of ions in a growing polymer would present a significant steric barrier to either the initiation or growth process. We have previously proposed arguments supporting the position that electrostatic repulsion is responsible for the large difference in rates of polymerization observed for vinylpyridine- and -bipyridine-containing metal complexes. Previously, all of the complexes studied either had been initially 2+ charged, thus requiring reduction by two electrons to reach charge neutral forms, or were complexes which became substitutionally labile upon reduction. The basics of the electrostatic argument are as follows: Based on space filling molecular models, metal bipyridine complexes attached to a polyvinyl backbone are sterically forced into ositions where the metal centers are on the order of 8 from their nearest neighbors. Depending on the location within the polymer, a given complex might experience as many as six nearest neighbor interactions. An electrochemicalstudy of a related soluble dinuclear iron tris(bipyridine) complex,17where the metals have been crystallographicallydetermined to be separated by 7.65 A, indicates that a significant degree of electrostatic interaction exists between the two centers. The electrostatic term was found to be on the order of 70-80 mV per unit charge, which is 3 times kT a t room temperature." On the surface of a growing polymer one might expect an even larger effect when there are several nearest neighbors of like charge. In any event, the data reported here, and the data from our previous studies, are all consistent with the existence of a significant electrostatic effect in the
K
(15) Pickup, P. G.;Kutner, W.; Leidner, C. R.; Murray, R. W. J. Am. Chem. SOC. 1984,106,1991. (16) Ikeda, T.; Schmehl, R.; Denisevich, P.; Willman, K.; Murray, R. W. J. Am. Chem. SOC.1982.104. 2683. (17)Serr, B. R.; Andersen, K. A.; Elliott, C. M.;Anderson, 0. P. Inorg. Chem. 1988,27,4499.
Langmuir, Vol. 7, No. 10, 1991 2379 rates of polymer formation for this class of vinyl bipyridinecontaining complexes. Finally, some consideration of the charge distribution within these complexes is in order. Charge equilibration calculations1* were carried out by using Biograf and Polygraf molecular simulation programs (version 2.1)19 for [Ru(sbpy)(vbpy)z]+. This monomer was selected because it seemed most likely to possess the largest dipole of all the complexes under consideration. The calculations indicate that more than half of the formal 2+ charge of ruthenium is delocalized into the 7r system of the ligands, leaving it with a net +0.72 charge. Likewise, almost half of the formal 1- charge typically associated with the SO3 is also delocalized into the ligands. The general results of these calculations suggest that while there is indeed a dipole associated with [Ru(sbpy-)(vbpy)z]+, the charges are fairly diffuse. These calculations taken together with the results obtained from the above mentioned studies of analogous discrete dinuclear complexes1' suggest that treating all of these complexes as roughly spherical charges is a reasonable, zero order approximation.
Conclusion The data presented illustrate that electrostatic interactions play an important role in the polymerization of vbpy-containing metal complexes and may be the dominant consideration. [M(vbpy)3I2+,[M(vbpy)zL]+, and [M(vbpy)2L2l0complexes all conform to this electrostatic interaction principle. The requirements for the most rapid and efficient polymerization appear to be (1)the complex must be reduced so that some degree of anion radical character resides on the vinyl-containing bipyridine (as demonstrated previously1J1J2)and (2) the complex should have the minimum absolute charge consistent with (1). Acknowledgment. We acknowledge the generous financial support of this work through the US.Department of Energy Office of Basic Energy Sciences (Grant No. DEFG02-81ER13666). We thank Professor H. D. Abruiia, Department of Chemistry, Cornel1 University, for providing us with a sample of [Fe(vbpy)n(CN)z]and Professor A. K. Rapph and Ms. Susan Ferrere, Department of Chemistry, Colorado State University, for technical assistance with the charge equilibration calculation and helpful discussions, therewith. Registry No. Ru(vbpy)g(Mcbpy)+,135799-09-6; Ru(vbpy)a(Sbpy)+,135773-98-7; Ru(vbpy)2(CN)2O,135773-99-8; Fe(vbpy12(CN)2', 119058-91-2. ~
(18) RappB, A. K.; Goddard, W. A. J. Phya. Chem. 1991,96,3368. (19) Biografand Polygrafobtained from BioDesign,subsidiary of Molecular Simulations, Inc., 199 S. Robles Ave., Suite 540, Pasadena, CA 91101.
