Electrochemical Studies of Organometallic Complexes with Tetra-n

Sep 15, 1995 - Tetrakis[ 3,5-bis( trifluoromethy1)phenyll borate as the ... electrolyte/methylene chloride solvent system improves the electrochemical...
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Organometallics 1995, 14, 4879-4885

4879

Electrochemical Studies of Organometallic Complexes with Tetra-n-butylammonium Tetrakis[3,5-bis(trifluoromethy1)phenyllborate as the Electrolyte. X-ray Crystal Structure of [CdCFd (CH3)41Fe(C5Hd Paul G. Gassman,? John R. Sowa, Jr.,*p$ Michael G. Hill,$ and Kent R. Mann* Department of Chemistry, University of Minnesota, Kolthoff Hall, Minneapolis, Minnesota 55455 Received May 30, 1995@ The tetra-n-butylammonium tetrakis[3,5-bis(trifluoromethyl)phenyllborate(TBA+TFPB-) electrolyte/methylene chloride solvent system improves the electrochemical reversibility of pentamethylcyclopentadienyl (Cp*) ruthenocenes, Cp*RuCp' (Cp' = fluorenyl, indenyl, cyclopentadienyl (Cp), acetylcyclopentadienyl, pentachlorocyclopentadienyl), as indicated by ip,c/ip,aratios of 0.79- 1.0 as determined by cyclic voltammetry. The quasi-reversible potentials (E"') of the Cp*RuCp' complexes and the complete series of group 9 Cp2M and Cp*zM complexes (M = Fe, Ru, Os) are also reported in TBA+TFPB-/CH2Clz. In addition, a study of the E"' values of group 9 complexes containing the (trifluoromethy1)tetramethylcyclopentadienyl (Cp*) ligand indicate t h a t the Cp* complexes are slightly (0.06-0.08V per Cp*) more difficult to oxidize than the cyclopentadienyl (Cp) derivatives. The structure of [C5(CF3)(CH3)41Fe(C5H5)was determined at -101 "C by a single-crystal X-ray diffraction study. The structure shows eclipsed Cp* and Cp rings, and the iron to Cp* centroid distance (1.643A) is slightly shorter than the iron to Cp centroid distance (1.651A); otherwise, no extreme differences in the coordination of the Cp* and Cp rings are noted. An infrared spectroelectrochemistry study of truns-[Cp*Fe(CO)21~shows t h a t it is electrochemically oxidized to trans-[CpTe(CO)~l2+ in TBA+TFPB-/CHzC12.

Introduction Recently Mann and co-workers reported' quasireversible electrochemical potentials E"' of ruthenocene (1)and osmocene (2)with use of tetra-n-butylammonium tetrakis[3,5-bis(trifluoromethyl)phenyllborate (TBA+TFPB-) electrolyte in methylene chloride solvent. Previously, complexes 1 and 2 were previously known to give only irreversible oxidation processes. To explore the general utility of the TBA+TFPB-/CHzClz electrolyte/ solvent system, we have studied the electrochemistry of several pentamethylcyclopentadienyl (Cp*) ruthenocene complexes (Cp*RuCp'; Cp' = fluorenyl (3), indenyl (41,cyclopentadienyl (Cp) (5), acetylcyclopentadienyl (61, pentachlorocyclopentadienyl (7), Cp* ( 8 ) that, except for 8, were previously reported2 to give irreversible electrochemical oxidations. We also report the E"' values for the complete series of group 9 CpzM (M = Fe, Ru (l),Os (2))and Cp"2M (M = Fe, Ru (B), Os (9)) complexes in TBAfTFPB-/CH2C12. In addition, we were interested in determining the electronic properties of the recently discovered3(trifluoromethy1)tetramethylcyclopentadienyl (Cp') ligand by studying the electrochemistry of C p T e C p (lo),Cp*2Ru (ll),and Cp*2Os (12) Deceased April 21, 1993. J.R.S., Department of Chemistry, Seton Hall University, South Orange, NJ 07079; M.G.H., Department of Chemistry, Occidental College, Los Angeles, CA 90041. Abstract published in Advance ACS Abstracts, September 15,1995. (1)Hill, M. G.; Lamanna, W. M.; Mann, K. R. Znorg. Chem. 1991, 30,4687. (2)Gassman, P.G.; Winter, C. H. J. Am. Chem. Soc. 1988,110,6130. (3)Gassman, P.G.; Mickelson, J. W.; Sowa, J. R., Jr. J . Am. Chem. SOC.1992,114,6942. +

* Current addresses:

complexes. Previously, we demonstrated3 that the Cp* ligand is electronically equivalent to the cyclopentadienyl (Cp) ligand and sterically equivalent to the pentamethylcyclopentadienyl ligand (Cp*). Thus, Cp* can be used t o determine the importance of steric and electronic effects in Cp and Cp* complexes. As one demonstration of the utility of Cp*,we report an infrared spectroelectrochemical study of the oxidation of [Cp*Fe(C0)zIz (13) and a comparison to the previously reported4 studies of [CpFe(C0)23~(14) and [Cp*Fe(CO)21z (15)complexes.

Experimental Section The following compounds were purchased from commercial sources (Aldrich or Strem) and, unless otherwise stated, were used as received: ruthenocene (11, osmocene (2) (recrystallized from anhydrous methanol under an atmosphere of argon), dicarbonylcyclopentadienyliron dimer ( 14),ferrocene (purified by sublimation, 50 "C, 10+ mm), and cobaltocene. The complexes (&luorenyl)( r/5-pentamethylcyclopentadienyl)ruthenium (3): (~s-indenyl)(~6-pentamethylcyclopentadienyl)ruthenium (4),2(~5-cyclopentadienyl)(~~-pentamethylcyclopentadienyl)ruthenium (5),2(~s-acetylcyclopentadienyl)(~s-pentamethylcyclopentadieny1)ruthenium(6): ($-pentachlorocyclopenta&enyl)( ~5-pentamethylcyclopentadienyl)ruthenium (7x2bis($-pentamethylcyclopentadieny1)ruthenium his($-pentamethylcyclopentadieny1)osmium (9),5 (tls-cyclopentadienyl)($-(trifluoromethy1)tetramethylcyclopentadienyl)iron(10): his($-

@

0276-733319512314-4879$09.0010

(4)Bullock, J. P.;Palazotto, M. C.; Mann, K. R. Znorg. Chem. 1991, 30, 1284.

(5)Albers, M. 0.;Liles, D. C.; Robinson, D. J.; Shaver, A.; Singleton, E.; Wiege, M. B.; Boeyens, J . C. A.; Levendis, D. C. Organometallics 1986,5 , 2321.

0 1995 American Chemical Society

4880 Organometallics, Vol. 14, No. 10, 1995

Gassman et al.

Table 1. Electrochemical Data for Organometallic Complexes in T'BA+TFPB-/CH2Clpand TBA+C104-/ CHaCla* ElectrolytdSolvent Systems compd E"' (V)in TBA+TFPB- a AE (VPC ip,dip,aasd Eu2 (V)in TBA+C104ip,Jip,ab,dze CpzRu (1) CPZOS (2) Cp*Ru (fluorenyl)(3) Cp*Ru (indenyl)(4) Cp*RuCp (5) Cp*Ru (acetylcp) (6) Cp*Ru (C5c15) (7) Cp*& ( 8 ) CP*ZOS ( 9 ) Cp*FeCp (10) CpzFe Cp*zFe Cp*zRu(11) cp*zos (12) [Cp*Fe(CO)zlz(13) [CpFe(CO)zlz(14) [Cp*Fe(CO)zlz(15)

1.03" 0.83" 0.41 0.51 0.69 0.93 1.41 0.48 0.31 0.53 0.47f -0.12 1.15 0.98 0.70 0.64 0.28

0.089 0.085 0.0968 0.084 0.099h 0.075 0.083 0.080 0.074 0.090 0.090 0.079 0.097 0.084 0.10 0.085 0.082

0.94

0.97

0.60

0.888 1.0 OHh 0.86 0.79 1.0 0.97 1.0 1.0 0.99 0.92 0.99 0.90 1.0 0.81

0.51 0.60 0.71 0.91 1.47 0.59

0.38 0.68 0.55 0.42 0.29 0.88

1.o

0.48'

Potentials (E"') vs aqueous AgCVAg in 1.0 M KC1 calibrated using an E"' value of ferrocene of 0.47 V.' The electrolyteholvent system is 0.10 M TBAfTFPB-/CH2C12, and ca. 0.5 mM solutions of the metal complexes were used. Reference 2. Potentials (Ellz) vs SCE were calculated using an E"' value for ferrocene of 0.48 V. The electrolytdsolvent system is 0.10 M TBA+C104-/CH&12. Anodic potential (Ep+)minus cathodic potential (EPJ determined by CV. The scan rate was 100 mM s-l unless otherwise indicated. Ratio of cathodic current (iPJ to anodic current (iPJ. e Calculated from the original data. f Reference 1.g Scan rate 500 mV s-l. Scan rate 250 mV s-l. Reference l l a .

(trifluoromethy1)tetramethylcyclopentadienyl)ruthenium(11): bis(v5-(trifluoromethyl)tetramethylcyclopentadienyl)osmium (12): dicarbonyl(pentamethylcyclopentadieny1)in dimer (15); and decamethylferrocenes were prepared as previously reported. Unless otherwise stated, all solvents were purified by distillation from standard drying agents' under an atmosphere of argon. Compound characterization and purity were established by NMR (Varian VXR-300 MHz or Bruker W 3 0 0 ) and FTIR (Mattson Polaris) spectroscopy. Preparation of { [Ca(CFs)(CH9)4]Fe(CO)2}~(13). Previously3 the preparation of 13 from Cp*H and Fe(C0)5 in refluxing octane gave an impurity (identified as [Cp*(CO)Fe@-C0)2Fe(CO)Cp*l)which was detected in the cyclic voltammogram. As the Cp*H starting material does not contain Cp*H, the defluorination reaction apparently occurs as a side reaction during the reaction between Fe(C0)5 and Cp*H. Pure 13 is obtained by the following procedure. To a solution of impure 13 (1.1g, -1.8 mmol) in methylene chloride (100 mL) was added a solution of iodine (0.47 g, 1.9 mmol) in methylene chloride (50 mL) to give a mixture of Cp*Fe(CO)21and Cp*Fe(C0)21(1.6g). After removal of the solvent on a rotary evaporator, the mixture was chromatographed in air on a column (25 x 3 cm) of neutral alumina (Brockman activity I). The first brown band was eluted with a mixture of 10% methylene chloride in hexanes, and the solution was evaporated to dryness on a rotary evaporator to give pure Cp*Fe(C0)ZI (1.0 g, 2.3 mmol). Further elution with a mixture of 20-50% methylene chloride in hexanes gave a second brown band of pure Cp*Fe(C0)21 (0.17 g, 0.45 mmol). To a solution of Cp*Fe(CO)ZI (1.0 g, 2.3 mmol) in tetrahydrofuran (50 mL) was added cobaltoceneeab(0.45 g, 2.5 mmol), and the reaction mixture was stirred under an atmosphere of argon for 30 min, after which time the infrared spectrum indicated that the reaction was complete. The solvent was removed under vacuum, and the residue was dissolved in methylene chloride (30 mL); diethyl ether (30 mL) was added to precipitate the CpzCo+I-, and the solution was filtered. ARer evaporation of the solvent under vacuum, the purple residue was dissolved in a minimum amount of warm methylene ~~~

(6)King, R. B.; Bisnette, M. B. J . Organomet. Chem. 1967, 8, 287. (7) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of

Laboratory Chemicals, 2nd ed.; Pergamon: New York, 1980.

(8)For similar synthetic procedures see: (a) Connelly, N. G.; Manners, I. J. Chem. Soc., Dalton Trans. 1989, 283. (b) Field, L. D.; Masters, A. F.; Gibson, M.; Latimer, D. R.; Hambley, T. W.; Buys, I. E. Inorg. Chem. 1993,32,211.

chloride and the solution was filtered through a column (15 x 3 cm) of neutral alumina. Recrystallization from a mixture of 50%methylene chloride in hexanes at -15 "C overnight gave dark purple crystals of 13 (0.47 g, 0.78 mmol) in 67% yield based on Cp*Fe(C0)21. Complex 13 is sensitive to air in solution, but it is stable in air as a solid. Mp: 245 "C dec. Anal. Calcd for C~4H24F6Fe204(M,= 602.14): C, 47.87; H, 4.02. Found: C, 47.82; H, 4.07. 'H NMR (CDC13): 6 1.84 (q, JHF= 2 Hz, 2,5-CH3), 1.59 (5, 3,4-CH3). 13C NMR (CDC13): 6 240.8 (br S, CO), 124.9 (9, JCF= 272 Hz, CF3), 101.2 (5, 2,5C-CHs), 99.7 (s, 3,4-C-CH3), 86.7 (9, JCF= 36 Hz, C-CFs), 9.5 (s, 2,5-CH3), 8.0 (s, 3,4-CH3). "F NMR (CDC13): 6 (VS CFCl3) -52.79. IR (CHzC12): v(C0) 1952,1776cm-I. MS: mle calcd for C24H24F6Fe204 602.0278, found 602.0267. Electrochemical Experiments. The electrochemicalanalyses were performed with use of a Bioanalytical Systems (BAS) Model 100 electrochemical analyzer or a Princeton Applied Research Model 273 potentiostat. Cyclic voltammetry (CV) and Osteryoung square wave voltammetry (OSWV) were performed at room temperature (-20 "C) with a normal threeelectrode configuration consisting of a highly polished glassycarbon-disk working electrode (A = 0.07 cm2), an AgCUAg reference electrode containing 1.0 M aqueous KC1, and a platinum-wire counter electrode. The working component of the electrochemical cell was separated from the reference compartment by a modified Luggin capillary. All three compartments contained a 0.10 M solution of TBA+TFPBelectrolyte. Analyses were performed on 0.75-1.0 mL of 0.5 mM solutions of the organometallic complexes. The methylene chloride solvent used for all electrochemical and spectroelectrochemical (vide infra)experiments was distilled from phosphorus pentoxide under an atmosphere of argon. Tetra-n-butylammonium tetrakis[3,5-bis(trifluoromethy1)phenyllborate (TBA+TFPB-/CHZClz)was obtained as a gift from the 3M Co. Alternatively, the electrolyte was prepared by metathesis of TBA+Br- (Aldrich) and Na+TFPBin a solution of methanol. Deionized water was added dropwise to precipitate the TBA+TFPB-, which was further purified by filtration through a column of neutral alumina in methylene chloride solvent. The electrolyte (mp 97 "C) was dried under vacuum at 90 "C. Immediately prior to use, the electrolyte solutions were passed through a short column of neutral alumina (80-200 mesh, Fisher Scientific) which was (9)Brookhart, M.; Grant, B.; Volpe, A. F., Jr. Organometallics 1992, 11, 3920.

Electrochemical Studies of Organometallic Complexes

Table 2. Crystallographic Data for (10) [ca(cFs)(CHs),lFe(C~Ha) (a) Crystal Parameters empirical formula, wt C I ~ H I ~ F310.14 ~F~, cryst color, habit yellow, needle cryst dimens 0.550 x 0.200 x 0.150 mm cryst class triclinic no. rflns for unit cell (28 range) 23 (30.1-49.1") 8 scan fwhm 0,oo space group P1 (No. 2) a

b C

a

B Y

V 2 dcalc P M o Kn

diffractometer monochromator radiation temp attenuator takeoff angle scan type scan rate scan width 2em,

7.617(5) A 8.346(4) A 12.650(5) A 70.19(3)0 81.52(3)" 63.67(2)' 678W A3 2 1.519 g ~ m - ~ 11.27 cm-l

(b) Data Collection Enraf-Nonius CAD-4 graphite Mo K a (A = 0.710 69 A) -101 "C Zr foil (factor 17.8) 2.8" w

2.1-8.2" min-1 (in w ) (1.20 0.35 tan 0)" 54.0" 3094 2885

+

no. of rflns collected no. of unique rflns (c) Solution and Refinement no. of observns (Z > 2.000(Z)) (No) 1362 no. of variables (Nv) 200 NdNv 6.81 R,R w 0.056,0.067 goodness of fit 2.14 max peak in final diff map 0.67 A-3 min peak in final diff map -0.91 A-3 max shiwerror in final cycle 0.00

activated at 300 "C. Working solutions were prepared by recording background cyclic voltammograms of the electrolyte solution before addition of the organometallic compound. The working compartment of the cell was bubbled with solventsaturated argon to deaerate the solution; however, in some cases, the entire cell and electrolyte solution were prepared in a nitrogen atmosphere glovebox (Braun). Potentials were recorded vs aqueous AgCVAg and are not corrected for the junction potential.1° The redox potentials of the transition-metal complexes were calibrated with the ferroceniudferrocene (Fc+/Fc) couple" in TBA+TFPB-/CHzClz, which was defined at the previously reported' value of the Fc+/Fc couple (0.47 V) and was determined after each series of runs. The E"', AE,and ip,c/ip,avalues were determined by CV or OSWV and are reported as averaged values of three scans on the same solution; the maximum error in the reported E"' and AE values is 10.01 V, and the error in ip,c/ip,avalues is estimated to be approximately 5%. Spectroelectmchemical Experiments. Infrared changes accompanying the thin-layer bulk electrolysis of [Cp?e(CO)& (13) were measured with use of a flow-through spectrochemical thin-layer cell as previously described.12 The spectroelectrochemical cell, background (0.10 M TBA+TFPB-/CHZClz),and sample (3 mM of 13, 0.10 M TBA+TFPB-/CH2C12) solutions were prepared in a nitrogen atmosphere glovebox. Infrared (10) Gagn6, R. R.; Koval, C. A.; Lisensky, G. C. Inorg. Chem. 1980,

Organometallics, Vol. 14, No. 10, 1995 4881

Table 3. Atomic Coordinates and Equivalent Isotropic Thermal Parameters for the Non-Hydrogen Atoms in [C5(CFs)(CHs)41Fe(C5H5) (10) ~

atom Fe F1 F2 F3

c1 c2 c3 c4

c5 C11 C12 C13 C14 C15 c21 C22 C23 C24 C25

X

0.3168(1Ib -0.0461(8) -0.0850(7) -0.2774(6) 0.415(1) 0.434(1) 0.558(1) 0.6146(9) 0.530(1) 0.0369(8) 0.0474(9) 0.1716(9) 0.235(1) 0.1504(9) -0.090(1) -0.054(1) 0.223(1) 0.363(1) 0.178(1)

~~

Y

z

-0.1363(1) 0.3590(6) 0.2351(5) 0.2746(6) -0.0103(9) -0.1866(8) -0.3265(8) -0.2344(9) -0.042(1) 0.0332(8) -0.1390(8) -0.2831(8) -0.2031(9) -0.0052(8) 0.2217(9) -0.166(1) -0.491(1) -0.307(1) 0.131(1)

0.77108(7) 0.6658(4) 0.8371(3) 0.7153(5) 0.8477(6) 0.9215(5) 0.8702(5) 0.7639(5) 0.7512(5) 0.7241(5) 0.8040(5) 0.7556(5) 0.6491(5) 0.6275(5) 0.7357(5) 0.9154(5) 0.8109(6) 0.5700(6) 0.5203(5)

~

B(eq),= 1.43(8) 5.4(5) 4.3(4) 5.3(5) 2.3(6) 2.0(6) 2.1(6) 2.1(6) 2.3(6) 1.5(5)

1.6(5) 1.8(5) 2.0(6)

1.8(5) 2.2(6) 2.6(6) 3.0(7) 3.3(7) 2.7(6)

a Equivalent isotropic thermal parameter. * Standard deviations are given in parentheses.

Table 4. Selected Bond Lengths (A)and Bond Angles (de& for [CS(CFS)(CH~)~IF~(CSH~) (10) Bond Lengths (A) Fe-C(l) Fe-C(2) Fe-C(3) Fe-C(4) Fe-C(5) Fe-C(l1) Fe-C(l2) Fe-C(l3) Fe-C(l4) Fe-C(l5)

2.O51(9la 2.053(7) 2.054(6) 2.041(7) 2.048(9) 2.007(6) 2.041(7) 2.047(9) 2.061(8) 2.050(6)

C(l)-C(2)-C(3) C(2)-C(l)-C(5) C(2)-C(3)-C(4) C(3)-C(4)-C(5) C(l)-C(5)-C(4) a

C(l)-C(2) 1.407(9) C(ll)-C(21) 1.482(9) C(l)-C(5) 1.424(9) C(12)-C(22) 1.500(9) C(2)-C(3) 1.426(9) C(13)-C(23) 1.52(1) C(3)-C(4) 1.416(9) C(14)-C(24) 1.50(1) C(4)-C(5) 1.40(1) C(15)-C(25) 1.505(9) C(ll)-C(12) 1.425(8) F(l)-C(21) 1.330(9) C(ll)-C(15) 1.432(8) F(2)-C(21) 1.331(9) C(12)-C(13) 1.426(9) F(3)-C(21) 1.333(9) C(13)-C(14) 1.408(8) C(14)-C(15) 1.421(9) Bond Angles (deg) 108.2(5) C(l2)-C(ll)-C(l5) 109.1(5) 107.1(6) C(ll)-C(l2)-C(l3) 106.0(5) 107.8(5) C(12)-C(13)-C(14) 109.8(5) 107.7(6) C(13)-C(14)-C(15) 107.8(5) 109.1(6) C(ll)-C(l5)-C(l4) 107.2(5)

Standard deviations are given in parentheses.

data were collected on a Mattson Sirius 100 FTIR spectrophotometer. All spectra were corrected for a stray light error (13%)which originates from the inadvertent collection of light off the front of the CaFz plates of the spectroelectrochemical cell. Bulk electrolyses were performed with use of the BAS100 thin-layer bulk electrolysis program.

Structure Determination of [Ca(CFs)(CHs)41Fe(CrHa) (10). Yellow needles of 10 were grown from a mixture of ethanol and water in air a t room temperature. Data were collected on an Enraf-Nonius CAD-4 diffractometer with graphite-monochromated Mo Ka radiation. The structure was solved by direct methods, and the non-hydrogen atoms were refined ani~otropica11y.l~ All calculations were performed using the TEXSAN crystallographic software package.13cThe crystallographic data (Table 2), the atomic coordinates and isotropic thermal parameters of the non-hydrogen atoms (Table 31, and selected bond lengths and angles (Table 4) are listed in the respective tables in parentheses. Other crystallographic data are included in the supporting information.

19. 2854.

(11)The ferroceniudferrocene couple is a recommended standard for electrochemical measurement of E"' values; see: (a) Geiger, W. E. In Organometallic Radical Processes; Trogler, W. C., Ed.; Journal of Organometallic Chemistry Library 22; Elsevier: New York, 1990; p 142. (b) Gritzner, G.; Kuta, J. Pure Appl. Chem. 1984, 56, 461. (12) Bullock, J. P.; Mann, K. R. Inorg. Chem. 1989, 28, 4006.

(13) (a) Gilmore, C. J. J. Appl. Crystallogr. 1984, 17, 42. (b) Beurskens, P. T. DIRDIF; Technical Report 1984/1; Crystallographic Laboratory, Toernooiveld, 6525 Ed Nijmegen, The Netherlands. (c) TEXSAN-TEXRAYStructure Analysis Package, Molecular Structure Corp., College Station, TX,1985. (d) Johnson, C. K. ORTEPII; Report ORNL-5138;Oak Ridge National Laboratory, Oak Ridge, TN, 1976.

Gassman et al.

4882 Organometallics, Vol. 14,No.10,1995

n

EW L T )

Figure 1. Cyclic voltammogram of Cp*Ru(indenyl)(4;E"' = 0.51 V and ip,c/ip,a = 1.0, in 0.10 M TBA+TFPB-/CH2C12).

Results and Discussion Electrochemistry of Metallocene Complexes. We were interested in determining the general utility of the TBAfTFPB-/CH2C12 electrolytdsolvent system for obtaining redox potentials (E"') of metallocene complexes. The E"' values of substituted ruthenocene complexes 1 and 3-7, which were previously reported2 to undergo irreversible oxidation in TBA+C104-/CH2C12, were redetermined in TBA+TFPB-/CH2C12(Table 1).As previously shown, ruthenocene (1)gave a quasi-reversible redox potential (E"' = 1.03 V), indicated by the ip,c/ i , , ratio of 1.0. The electrochemical reversibilities of complexes 3-7 are remarkably improved, as indicated by ip,c/ip,a ratios which range from 0.79 for complex 7 to 1.0 for complex 4 (Figure 1). In contrast, the range of ip,c/ip,a ratios of 1 and 3-7 previously determined2 in TBA+C104-/CH2C12was 0.29-0.68 (Table 1). It was recently reported14 that addition of alumina t o the electrochemical cell compartment improved the electrochemical reversibility of (q6-arene)Cr(C0)3complexes; however, this offered no improvement in the iP,,lip,, ratio of complex 7. In addition to the improved i , , / i , , ratios in TBAfTFPB-/CH2C12, the peak separation between the anodic and cathodic peaks (AI3 = E p , a for 3-7 is small, as these values range from 0.075 V for complex 6 to 0.099 V for complex 6 (Table 1). Although we did not generally obtain completely reversible behavior for 3-7, it is readily apparent that use of the TBA+TFPB-1CHzClz electrolyte /solvent system results i n a general improvement in the quality of the CV scans of these complexes which, except for Cp*aRu (81, were previously reported t o be electrochemically irreversible. It is likely that the chemical inertnessg of the TFPB anion and hydrophobicity of TBA+TFPBcontribute to the improved stability of the substituted ruthenocenium cations. Recently Gassman and Deck reported14bthat the use of TBA+TFPB-/CH&12 resulted in major improvements in the reversibility of CV scans of (q6-arene)Cr(C0)3complexes with electron-withdrawing groups on the arene ligand. The good agreement between the quasi-reversible values of the ruthenocene complexes reported here and the irreversible potentials previously reported indicates that the follow-up reactions which lead to irreversible (14)(a) Hunter, A.D.; Mozol, V.; Tsai, S. D. Organometallics 1992, 11, 2251.(b) Gassman, P. G.; Deck, P. A. Organometallics 1994, 13, 1934.

behavior are rapid and the rates are similar for this set of substituted ruthenocene cations.15J6Nevertheless, for these complexes the E"' values for the oxidation process are likely more reliable than the irreversible values (Table 1). Thus, the recently reported16 E p , a values (values in parentheses are reported vs SCE using E"' of ferrocene in TBA+PF6-/CH2C12 as 0.48 V') for 7 (1.59 V) and 8 (0.63 V) are 0.08 and 0.06 V, respectively, higher than the reversible potentials (E"') which are reported in the same paper. This suggests that the potentials (EpJ reported for Cp*Ru(C5F5) (1.52 VI, Cp*Ru(C5H4N02)(1.18VI, and Cp*Ru(C5(CF3)4H)(1.78 V) determined in TBA+PF6-/CH2C12 are also shifted to slightly higher potential than the reversible potential. Although the E"' value of Cp*Ru(C5(CF3)4H)is likely too high in solutions containing the TBA+TFPB- electrolyte," it would be interesting to determine the E"' values of Cp"Ru(C5F5) and Cp*Ru(C5HdN02) in TBA+TFPB-ICH2C12. In addition to the electrochemical evaluation of the effect of substituted cyclopentadienyl ligands on the electron density of the ruthenium metal in ruthenocenes,1,2J6 X-ray photoelectron spectroscopy (XPS)2,3J8 and Fourier transform ion cyclotron resonance mass spectrocopy methodslghave also been used. To compare electrochemical redox potentials for substituted ruthenocene compounds that have been reported by different researchers, we have found that it is best to consider a relative error of at least f O . l V in the reported potential. Nevertheless, we find that there is a good correlation of relative ligand effects among the different methods of analysis, and we obtain the following trend listed in order of decreasing electron donation: fluorenyl > pentamethylcyclopentadienyl % indenyl > cyclopentadienyl x (trifluoromethy1)tetramethylcyclopentadienyl > acetylcyclopentadienyl nitrocyclopentadienyl > pentachlorocyclopentadienyl x pentafluorocyclopentadienyl > tetrakis(trifluoromethy1)cyclopentadienyl. The electronic equivalence of the cyclopentadienyl and (trifluoromethy1)tetramethylcyclopentadienyl ligands is discussed below. A comparison of the effect of permethylation of the cyclopentadienyl ligand20 on the quasi-reversible E"' values of group 9 metallocenes indicates that the relative electron-donating effect of the Cp* ligand slightly decreases as one goes down the period from Fe (0.59 V) to Ru (0.55 V) to Os (0.52 V). As previously shown, the sequential replacement of Cp by Cp* in ferrocene derivatives is additive;20however, this is not a general trend, as the replacement of one Cp by one Cp* to give Cp*RuCp (5) results in a 0.34 V change in (15)Kissinger, P. T. In Laboratory Techniques in Electroanalytical Chemistry; Kissinger, P. T., Heineman, W. R., Eds.; Dekker: New York,

1.1

v.

(18)Gassman, P. G.;Macomber, D. W.; Hershberger, J. W. Organometallics 1983, 2, 1470. (19) (a) Ryan, M. F.; Siedle, A. R.; Burk, M. J.; Richardson, D. E. Organometallics 1992, 11, 4231.(b) Richardson, D.E.; Ryan, M. F.; Khan, Md. N. I.; Maxwell, K. A. J.Am. Chem. SOC.1992,114, 10482. ( 2 0 ) A similar study was recently published; however, the reported Eli2 values for the oxidation of CpaRu and CpzOs are likely irreversible: Denisovich, L. I.; Peterleitner, M. G.; Kravtson, D. N.; Kreindlin, A. Z.; Fadeeva, S. S; Rybinskaya, M. I. Organomet. Chem. USSR (Engl. Transl.) 1988, 1 , 166;Metalloorg. Khim. 1988, 1 , 301.

Electrochemical Studies of Organometallic Complexes

Organometallics, Vol. 14, No. 10, 1995 4883

a)

Figure 2. ORTEP drawings of Cp'FeCp (10): (a) side view; (b) top view, which illustrates the eclipsed cyclopentadienyl rings and similar structures of the Cp' and Cp ligands except for differences in steric bulk. the E"' value but the second replacement, to give Cp*2Ru (8),only results in a 0.21 V change in the E"' value.2 Thus, the cause of the slight decrease in electrondonating ability of Cp* as one goes from Fe t o Ru to Os may be due to the weaker effect of the second Cp*. However, it is not clear whether this is due to electronic or steric properties. We believe that this is the first complete comparison of the redox potentials of all of the group 9 CpzM and Cp*zM complexes where all of the potentials are quasi-reversible.20 Electrochemistry of (Trifluoromethy1)cyclopentadienyl Complexes. R e ~ e n t l ywe , ~ demonstrated the electronic equivalence of the cyclopentadienyl (Cp) and the (trifluoromethy1)tetramethylcyclopentadienyl(Cp*) ligand as indicated by XPS. Thus, we were interested in comparing the reversible redox potentials of complexes of Cp* with those of Cp and Cp*. Table 1shows that the Cp* complexes 10-13 are slightly more difficult t o oxidize by 0.06-0.08 V per Cp* ligand than the respective Cp complexes. Although the electrochemical data indicate a slight difference between Cp and Cp* complexes, this difference is small when a comparison is made with the Cp* complexes, since relative to Cp*, the Cp* derivatives are more difficult to oxidize by 0.30.4 V per Cp* ligand. To understand more fully the nature of coordination of the Cp* ligand, we performed a single-crystal X-ray crystallographic study of [C5(CF3)(CH3)41Fe(C5H5)(Cp*FeCphzl The ORTEP13d drawing (Figure 2) clearly indicates that the Cp* and Cp rings are eclipsed. In contrast, the revised crystal structure of ferrocene reported by Seiler and Dunitzz2shows the Cp rings are staggered by 9" from the eclipsed geometry. In Cp*FeCp, the C11 ring carbon on which the CF3 group is located has a shorter Fe-C(ring) bond distance (2.007(21)The compounds Cp*zRu and Cp*zOsboth crystallize in isomorphous triclinic cells with Z = 1;however, the molecules appear to be orientationally disordered and we could not obtain accurate information about the light atoms in the molecules. (22)Seiler, P.; Dunitz, J. D. Acta Crystullogr., Sect. B 1979,B35, 1068.

(6) A) than that found for the C12-Cl5 (2.041-2.061 A) ring carbons. However, the Cp and Cp* rings are

essentially planar, as the dihedral angle between the least-squares planes is only 0.63", which can be compared to that calculated (0.46 ") for ferrocene. The intraring bond distances (1.408-1.432 A) and bond angles (106.0-109.8 ") within the Cp* ring are remarkably similar to the bond distances (1.40-1.426 A) and bond angles (107.1-109.1 ") in the unsubstituted Cp ring of Cp*FeC Finally, the iron to Cp*-centroid distance (1.643 is 0.016 8, shorter than the iron to unsubstituted Cp-centroid distance. Although crystal structures of ruthenocene complexes show shorter metal to centroid distances for more electron-withdrawing l i g a n d ~ , ~ J ~ ~ v ~ ~ which suggests that the Cp* ligand is slightly more electron withdrawing than Cp, steric effects may also account for the longer iron t o unsubstituted Cp-centroid distance, as the iron to Cp*-centroid distance (1.643 A) is similiar to that in ferrocene (1.648 A). In fact, the average iron to Cp'-centroid distance in Cp*FeCp (1.651 A) is, within experimental error, equivalent to that distance (1.648 b)in ferrocene. Therefore, this crystallographic comparison of the Cp* and Cp ligands in the same molecule shows no extreme differences (e.g., ring slippage24 ) between the coordination of these two ligands, and except for differences in steric bulk the structures of the two ligands are practically the same. Electrochemical and Spectroelectrochemical Studies of [CpVe(C0)212(13). The quasi-reversible electrochemical potentials of [Cp'Fe(CO)zlz complexes (Cp' = Cp* (13),Cp (14), Cp* (15))are readily deter-

1;

_

_

_

~

_

_ ~

~

~

~

(23)This may not be a general trend, as crystal structures of Cp*Ru( C ~ ( C F S ) ~and H ) ~C~~~R U ( C S ( C O ~show M ~ ) very ~ ~ ~similar Ru-Cp'(centroid) distances, within experimental error, in spite of large differences in electronic characteristics; see, for example: (a) Burk, M. J.;Arduengo, A. J., 111; Calabrese, J. C.; Harlow, R. L. J . Am. Chem. Soc. 1989,111, 8938. (b) Bruce, M.I.; Skelton, B. W.; Wallis, R. C.; Walton, J. K.; White, A. H.; Williams, M. L. J . Chem. SOC.,Chem. Commun. 1981,428. (24)(a)O'Connor, J. M.; Casey, C. P. Chem. Rev. 1987,87,307.(b) Byers, L. R.; Dahl, L. F. Inorg. Chem. 1980,19, 277. (c) Yu, M.; Struchkov, Y. T.; Chernega, A. N.; Meidine, M. F.; Nixon, J. F. J . Organomet. Chem. 1992,436,79.

4884 Organometallics, Vol. 14, No. 10,1995

Gassman et al.

Table 5. Infrared Carbonyl Stretching Frequencies of the Parent [Cp’Fe(CO)212(Cp’ = Cp, Cp*,Cp*) and Radical-Cation [Cp’Fe(CO)212+ Complexesa compd [Cp*Fe(C0)232(13) [CpFe(CO)zlz (14)b [Cp*Fe(CO)& (15Ib a

parent v(CO), cm-’ 1952,1776 1955; 1995: 1922,1747

1773

CF3

radical-cation v(CO), cm-’ 2018,1904 2023; 2055,d 1934 1987,1884

CHzClz solvent. Reference 4. Trans isomer. Cis isomer.

I

wavenumber (cm-’)

Figure 3. Infrared spectral changes upon oxidation of a solution of [Cp*Fe(CO)&(13)in 0.10M TBA+TFPB-/CHzC12 at ca. 1.0V.The peaks due to the trans parent dimer (v(C0) 1952, 1776 cm-l) decrease in intensity as the concentration of the trans radical-cation dimer, [Cp*Fe(CO)zlz+(v(C0) 2018,1904 cm-l), increases during oxidation.

mined in TBAfTFPB-/CH2C12 (Table 1). Thus, similar E”’ values are obtained for 13 (0.70V) and 14 (0.64V), compared to that of 15 (0.28V). Since Cp* and Cp are electronically similar ligands, this result indicates that the electrochemical potentials of complexes 13-15 are controlled by the electronic properties of the Cp’ ligand as opposed to the steric properties. An infrared spectroelectrochemistry study of [Cp*Fe(CO)212 (13) was performed in TBAfTFPB-/CH2C12 to determine the structure of the oxidation product and to compare the reactivity of the Cp* complex to that of the Cp and Cp* derivatives. The neutral starting material [Cp*Fe(CO)212exists only as the trans isomer, as for [Cp*Fe(C0)212;25however, because of the similar electronic properties of Cp* and Cp, the v(C0) values of 13 are similar to those of the trans isomer of [CpFe(CO)212 (Table 5).4 As 13 is oxidized (Figure 31, the bands a t 1952 and 1776 cm-l decrease in intensity as two bands a t 2018 and 1904 (br) cm-l grow in isosbestically. As previously observed for truns-[Cp*Fe(C0)212+PF6- (Table 51,the new bands indicate the presence of a single i ~ o m e rthus, ; ~ these are assigned to the terminal and bridging bands of truns-[Cp~e(C0)212+TFPB-(eq1). (25)Teller, R. G.; Williams, J. M. Znorg. Chem. 1980, 19, 2770. (26) (a) Blaha, J. P.; Bursten, B. E.; Dewan, J. C.; Frankel, R. B.; Randolph, C. L.; Wilson, B. A.; Wrighton, M. S. J . Am. Chem. SOC. 1985, 107, 4561. (b) Hooker, R. H.; Mahmoud, K. A,; Rest, A. J. J . Chem. SOC.,Chem. Commun. 1983, 1022. (27) Manning, A. R. J . Chem. SOC.A 1968, 1319. (28) See the supplementary material in ref 4 for the spectroelectrochemically generated infrared spectra of [Cp*Fe(CO)& and [Cp*Fe(CO)ZlZ+.

After complete formation of truns-[Cp*Fe(CO)zIz+TFPB- was observed, bulk reduction of the solution cleanly regenerated the trans isomer of [Cp*Fe(C0)212 (eq 1). Previous spectroelectrochemical studies4 of cis,trans-[CpFe(CO)212indicated the formation of cis,truns[CpFe(CO)zlz+PF6-. Since Cp* and Cp are nearly electronically equivalent ligand^,^ the steric bulk of the Cp* ligand probably precludes the formation of cis-[Cp*Fe(C0)2]2+TFPB-. The trans Cp and Cp* complexes have similar v(C0) stretching frequencies (Table 5);3thus, we can assign the small bands at 2122 and 2074 cm-’ (Figure 3) as due to trace byproducts which result from the disproportionation of [CpSFe(C0)2I2+TFPB- by comparison with the byproducts formed in the oxidation of [CpFe(C0)21~.~ These two bands are assigned to the species Cp*Fe(C0)3+TFPB- in comparison to the bands of CpFe(C0)3+PFs- (2126,2081 cm-’h4 However, the slightly more intense 2074 cm-l band overlaps with the band of another byproduct, which is likely [Cp*Feas addition of water causes an (CO)Z(OHZ)~+TFPB-, increase in the intensity of this band. The remaining band from the latter byproduct is located under the 2018 cm-l peak of [Cp*Fe(C0)212+TFPB-,as bulk reduction of the solution gave a band at 2023 cm-’ (cf. the v(C0) values4 of CpFe(CO)z(OHz)+PF6- at 2076 and 2030 cm-l). We also note that small shoulders occur on the high-energy side of the bridging v(C0) band of 13. It is unlikely that this is due to the triply bridging [Cp*Fe@-CO)3FeCp*lcomplex, due to the high-intensity photolytic conditions required to generate the related Cp and Cp* derivatives.26Previously, Manning27assigned the higher energy shoulder in [CpFe(CO)212to a weak symmetric stretch of the bridging CO groups in the cis isomer; however, as this band also occurs in the transonly derivatives 13 and [Cp*Fe(C0)212,28it is likely that this band also results from a symmetric stretch of the CO groups in the trans isomers. Conclusions We have shown that cyclic voltammetry in tetra-nbutylammonium tetrakis[3,5-bis(trifluoromethyl)phenyllborate electrolyte and methylene chloride solvent (TBA+TFPB-/CHzC12)improves the electrochemical reversibility of pentamethylcyclopentadienyl (Cp*) ruthenocenes, Cp*RuCp’ (Cp’ = fluorenyl, indenyl, cyclopentadienyl (Cp), acetylcyclopentadienyl, pentachlororatios of 0.79cyclopentadienyl), as indicated by ip,c/ip,a 1.0. Quasi-reversible electrochemical potentials of a total of 17 substituted cyclopentadienyl complexes were

Electrochemical Studies of Organometallic Complexes

determined in TBA+TFPB-/CH&12, including complexes containing the (trifluoromethy1)tetramethylcyclopentadienyl (Cp*) ligand. Although the Cp* complexes are slightly more difficult t o oxidize (0.06-0.08 V per Cp*)than the Cp derivatives, the redox potentials of the Cp* complexes are more similar t o those of the Cp derivatives than t o those of the Cp* derivatives. The crystal structure of [C5(CF3)(CH3)41Fe(C5H5)indicates no extreme differences in the coordination of the Cp* and Cp rings except for differences in steric bulk. In addition, an infrared spectroelectrochemical study of [Cp*Fe(CO)& illustrates two important characteristics of the Cp*ligand: (1)upon oxidation its steric properties cause structural changes in [Cp*Fe(CO)& so that it behaves like the related Cp* derivative and (2) its electronic properties cause the E"' values and the v(C0) values to be more similar to those of the Cp derivative.

Organometallics, Vol. 14, No. 10, 1995 4885

Acknowledgment. The crystal structure of Cp*FeCp was determined by Professor Doyle Britton. We thank Professor John Ellis for use of his FTIR spectrophotometer for the spectroelectrochemistryexperiments, and we also thank Tim Wilson for his help with some of those experiments. We thank Dr. William Lamanna at the 3M Co. for a sample of TBA+TFPB-. Finally, we are grateful to the NSF for its support of this work. Supporting Information Available: Crystal data for 10, including complete tables of atomic coordinates and thermal parameters for the hydrogen and non-hydrogen atoms, general temperature factor expressions (u), bond distances and angles, and least-squares planes (14pages). Ordering information is given on any current masthead page.

OM950397T