modulation of the electronic communication between two equivalent ferrocene centers by proton...

Modulation of the Electronic Communication Between TwoEquivalent Ferrocene Centers by Proton Transfer,

Solvent Effects and Structural Modifications

Julio Alvarez, Yuhua Ni, Tong Ren and Angel E. Kaifer*

Center for Supramolecular Science and Department of Chemistry, University of Miami, Coral Gables, FL 33124-0431, USA

Received 4 January 2001; accepted 8 January 2001

Abstract—A series of six new dinuclear ferrocene compounds with the bridging fragment CH2–N(R)–CH2 were prepared andcharacterized. Bis(ferrocenylmethyl)-t-butylamine (1), bis(ferrocenylmethyl)-isopropylamine (2), bis(ferrocenylmethyl)-hexylamine(3), bis(ferrocenylmethyl)-p-methoxyaniline (4), bis(ferrocenylmethyl)aniline (5) and bis(ferrocenylmethyl)-p-nitroaniline (6), as wellas the quaternized amine bis(ferrocenylmethyl)hexylmethylammonium hexaflurophosphate (3+.PF6

�), were investigated using elec-trochemical techniques, 1H NMR spectroscopy and single crystal X-ray diffraction analysis. The voltammetric data indicate thatthe extent of electronic communication between the equivalent ferrocene centers increases in low polarity solvents and when bulkyaliphatic groups are covalently attached to the central tertiary nitrogen. The observed experimental data are consistent withthrough-space communication effects, essentially electrostatic in nature. The extent of electronic communication can be modulatedby reversible and irreversible reactions in the solution phase. Protonation or methylation of the tertiary nitrogen in the middle ofthe bridge disrupts the electronic communication between the ferrocene residues. # 2001 Elsevier Science Ltd. All rights reserved.

Introduction

Electronic communication effects between equivalentredox centers have been the subject of extensive researchwork and led to numerous reports in the chemical lit-erature.1 Due to their synthetic accessibility, multi-nuclear metallocene compounds have played animportant role in this area of research.2 Recently, wereported a new type of dinuclear ferrocene compoundcontaining a CH2–N(R)–CH2 tether.3 These compoundsdisplay a moderate degree of electronic communicationbetween their two identical ferrocene residues, whichessentially disappears upon protonation or methylationof the central tertiary nitrogen in the tether.3 In order tobetter understand this finding, we have prepared a seriesof three compounds having aliphatic R groups withvariable degrees of steric bulk (compounds 1, 2 and 3;see Chart 1 for structures). We have also preparedanother three compounds (4, 5, and 6; Chart 1) having

aromatic R groups to investigate the influence of elec-tron withdrawing/releasing effects on the communicationbetween the ferrocene centers.

In spite of the abundance of dinuclear ferrocene com-pounds that are already described in the literature,2 thecompounds reported here are attractive due to the fol-lowing reasons: (i) their ease of preparation, (ii) thepresence of a centrally located nitrogen atom in thetether connecting the two ferrocene groups, and (iii) thepossibility to manipulate (reversibly or irreversibly) theextent of electronic communication between the ferro-cene centers using the reactivity of the tertiary aminefunctional group. In this paper, we provide a detailedaccount of the synthesis and electrochemical propertiesof these compounds, as well as relevant 1H NMRspectroscopic and single crystal X-ray diffraction data.

Results and Discussion

Synthesis

The preparation of the dinuclear ferrocene compounds1–6 is based on the known dissociation in polar solventsof the (ferrocenylmethyl)trimethylammonium ion to

1472-7862/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.PI I : S1472-7862(01 )00007-7

Journal of Supramolecular Chemistry 1 (2001) 7–16

Keywords: electrochemistry; electronic communication; ferrocene;switchable molecules.*Corresponding author. Tel:+1-305-284-3468; fax: +1-305-444-1777;e-mail: [email protected]

produce trimethylamine and the corresponding ferro-cenylmethylene carbocation.4 In the presence ofnucleophiles, this carbocation gives rise to ferrocenyl-methylated products.4,5 Therefore, reaction with a pri-mary amine easily yields the correspondingbis(ferrocenylmethyl) tertiary amine (see Scheme 1) plusresidual amounts of the mono(ferrocenylmethyl) deri-vative, which can be easily separated by chromato-graphy. A similar reaction scheme has been used toattach ferrocene residues to polyaza macrocyclicligands.6 Our experimental results suggest that the pro-duct distribution in this reaction is determined by thedegree of steric hindrance around the primary aminenitrogen and its nucleophilic character. As anticipatedfrom the pKa values of the primary amines used asstarting materials in this work, the longest reaction timeand lowest yield of bis(ferrocenylmethyl) tertiary aminewas observed with p-nitroaniline.

Electrochemistry

The electrochemical oxidation of any of the ferrocenedimers investigated here can be described as two rever-sible one-electron transfer events [eqs (1) and (2)]occurring sequentially at relatively closed formal poten-tials (Eo0). The difference between the two formalpotentials (�Eo0=2Eo0–1Eo0) is a measure of the extentof electronic communication between the two ferrocenecenters, that is, improved communication leads to larger�Eo0 values.7 The value of Kc for the comproportiona-tion equilibrium [eq (3)] can be readily calculated fromthe �Eo0 value using eq (4) and also affords a measureof the degree of communication. Throughout this workwe assume that the formal potentials are identical to thehalf-wave potentials calculated from the voltammetricdata, a very common approximation that introducesminimal errors.8

Chart 1.

Scheme 1.

8 J. Alvarez et al. / Journal of Supramolecular Chemistry 1 (2001) 7–16

Fc� Fc�! �Fcþ � Fcþ e; 1Eo0 ð1Þ

Fcþ � Fc�! �Fcþ � Fcþ þ e; 2Eo0 ð2Þ

Fc� Fcþ Fcþ � Fcþ�! �2Fcþ � Fc;Kc ð3Þ

Kc ¼ expð�Eo0F=RTÞ ð4Þ

Table 1 gives relevant electrochemical parametersobtained for compounds 1–3 in voltammetric experi-ments and Figure 1 shows typical cyclic voltammo-grams in CH3CN, CH2Cl2 and 1:1 mixtures of thesesolvents. In every solvent system surveyed, the Kc valuesincrease in going from 3 to 1, that is, the degree ofelectronic communication between the two ferrocenecenters improves as the aliphatic substituent on thebridging nitrogen becomes more branched. Bulkiersubstituents should decrease the average distancebetween the two ferrocene residues. Another significantobservation is that, for any given dinuclear ferrocene

compound, the Kc values increase as the solvent polarityis reduced from that of CH3CN to that of CH2Cl2.These findings are consistent with through space, elec-trostatic communication between the ferrocene residues,as reduced solvent polarity is expected to increase theelectrostatic repulsion forces between the two oxidizedferrocene centers. Similarly, bulkier R groups attachedto the central nitrogen should lead to decreased averagedistances between the ferrocene centers, thus increasingthe strength of electrostatic effects between thepositively charged, ferrocenium centers.

In contrast, compounds 4–6 exhibit small Kc values(Table 2). The nitro derivative 6 shows a Kc value of 6.7,which is very close to the value of 4.0 expected for twoindependent redox centers with null electronic commu-nication.7 Comparisons between the compound sets 1–3and 4–6 must be made with caution because of the evi-dent electronic and structural differences between the Rsubstituents. Obviously, the parent amines for com-pounds 4–6 exhibit different pKa values, whereas this is

Table 1. Formal potentials (1Eo0 and 2Eo0) and comproportionation constants (Kc) for compounds 1–3 at 25 �C in several solvent systems (con-

taining 0.2 M Bu4N+PF6

�)

Compound CH3CN CH3CN–CH2Cl2 (50:50 v/v) CH2Cl2

1Eo0 (mV) 2Eo0 (mV) Kc1Eo0 (mV) 2Eo0 (mV) Kc

1Eo0 (mV) 2Eo0 (mV) Kc

1a 372 479 63 356 475 101 352 492 2272a 381 473 35 374 477 54 350 464 833b 422 506 24 402 490 31 407 503 42

aFormal potentials determined by OSWV.bFormal potentials determined by digital simulations of the experimental CV plots. All potentials measured against a Ag/AgCl reference electrode.

Figure 1. Cyclic voltammetric response of compounds 1–3 in several solvent systems. All solutions contain 0.2 mM dimer and 0.2 M Bu4N+PF6

�.Scan rate=0.1 V s�1.

J. Alvarez et al. / Journal of Supramolecular Chemistry 1 (2001) 7–16 9

not the case for compounds 1–3.9 On the other hand,the aromatic substituents in 4–6 cannot exert the samekind of steric effects present in the 1–3 series.

Protonation and N-methylation effects

We have already reported preliminary results showingthat both protonation and N-methylation significantlyreduce the electronic communication in compound 3.3

In acetonitrile, the protonated and N-methylated formsof 3 exhibit Kc values of 6.0 and 9.0, respectively, whichare considerably smaller than the Kc value for neutral 3in this solvent system. Similar results were obtainedwith the rest of the dinuclear ferrocene compoundsreported here. The results are summarized in Table 3and Figure 2. Evidently, protonation leads to larger Eo0

values due to the increased electron withdrawing effectexerted by the positively charged bridge. The voltam-mograms in Figure 2 clearly demonstrate that protona-tion leads to decreased electronic communicationbetween the ferrocene centers, since the resolutionbetween the two voltammetric waves is lost upon HCladdition. N-Methylation of compound 3 leads to asimilar loss of intramolecular communication betweenits ferrocene groups. The decreased level of electroniccommunication induced by protonation and/orN-methylation reactions is not easy to rationalize. Bothreactions generate a positive charge on the nitrogenatom, which becomes a center for solvation and coun-terion association. Perhaps, the more strongly asso-ciated solvent molecules or counterions provide a shieldfor the electrostatic repulsion forces between the oxi-dized ferrocene centers, thus decreasing their level ofcommunication. While both reactions have very similareffects on the ferrocene dimers, it is appropriate to markan important difference between these processes.

Protonation is fully reversible, while N-methylation isan essentially irreversible process.

1H NMR spectroscopic experiments

We used 1H NMR spectroscopy to monitor structuralchanges induced by protonation of 3 in solution. Notsurprisingly, the resonances more sensitive to protona-tion correspond to the four methylenic protons in thetether between the two ferrocene units. In the absence ofacid, the four protons appear as a singlet, as anticipatedfrom the molecular structure. However, upon additionof acid, the singlet splits into an AB pattern (Fig. 3) as thetwo protons in each methylene become diasterotopic.The same trend is observed upon protonation of all othercompounds surveyed. Interestingly, the spectrum of themethylated compound 3+ shows the same AB spectralpattern for the -CH2 protons. Therefore, in excellent agree-ment with the electrochemical data, 1H NMR spectro-scopic data also support strong similarities between theprotonated and methylated forms of these compounds.

X-ray crystal structures of compounds 1, 2, 3+PF6� 5

and 6

Single crystals of these compounds were submitted toX-ray diffraction analysis and the resulting molecular

Table 2. Formal potentials (1Eo0 and 2Eo0) and comproportionation

constants (Kc) for compounds 4–6 at 25 �C in CH3CN/0.2 M

Bu4N+PF6

�a

Compound 1Eo0 (mV) 2Eo0 (mV) Kc

4 419 495 205 434 504 166 487 536 6.7

aFormal potentials determined by digital simulations of the experi-mental CV plots. All potentials measured against a Ag/AgCl referenceelectrode.

Table 3. Protonation effects on the average formal potentials (Eo0)and Kc values of dinuclear ferrocene compounds 1–6 at 25 �Ca

Compound Before protonation After protonation

Eo0 (mV) Kc Eo0 (mV) Kc

1b 416 101 612 8.52b 426 54 602 4.83b 446 31 611 5.54c 457 20 581 5.85c 469 16 561 5.46c 5.12 6.7 538 4.8

aAll potentials measured against a Ag/AgCl reference electrode.bSolvent: CH3CH/CH2Cl2 (50:50 vv)+0.2 M Bu4N

+PF6�.

cSolvent: CH2Cl2+0.2 M Bu4N+PF6

�.

Figure 2. Cyclic voltammetric response of compounds 1–3 before(continuous line) and after (dotted line) addition of aqueous HCl. Allsolutions contain 0.2 mM dimer and 0.2 M Bu4N

+PF6� in a 50:50 (v/v)

mixture of CH3CN/CH2CI2. Scan rate=0.1 V s�1.


structures are shown as ORTEP plots in Figures 4–8.Selected bond lengths and angles are given in Table 4.The structural features of the ferrocene units are similarto those reported in the literature.10 Of particular inter-est to us are the geometrical parameters around the -N-bridge, which might have some relevance to the varyingdegrees of electronic communication observed in thesolution phase. In all the neutral dimers, 1, 2, 5 and 6,the N-C(I) and N-C(2) bond lengths [C(1) and C(2) arethe methylene carbons adjacent to the ferrocene groups]are identical within experimental error [range: 1.458(5)to 1.477(5) A], indicating the insensitivity of these N–Cbond lengths towards the nature of the third N-sub-stituent (R). However, the N–C(3) bond length doesdepend on the nature of the substituent itself. It is aboutthe same to the other two N–C distances for compoundswith alkyl substituents (1 and 2), but significantlyshorter in the aryl substituted compounds (5 and 6),probably a result of the sp2 character of the adjacent

carbon in these compounds. In contrast to the neutraldimers, the N-methylated compound 3+ displayslonger N–C bonds [1.497(5)–1.537(5)A] and essentiallyperfect tetrahedral coordination geometry around thenitrogen.

Nitrogen bond angles also provide important structuralclues in these compounds. For instance, the C(1)–N–C(3) and C(2)–N–C(3) bond angles are larger in com-pound 1 than in 2, while the reverse is true of the C(1)–N–C(2) angle. This is consistent with the larger stericeffect exerted by the t-butyl group as compared to iso-propyl. The closing of the C(1)–N–C(2) bond angle in 1may lead to a smaller average distance between the fer-rocene centers, thus, providing a rationale for theincreased communication observed in this compound(relative to 2 and 3). In order to garner further supportfor this idea, we attempted to correlate the measured Kc

values with the Fe–Fe distances obtained from our solid

Figure 3. 1H NMR spectra (400 MHz, CD3CN) of compounds 3 and 2 before (top) and after (bottom) addition of a 10-fold excess of DCl.

Figure 4. ORTEP plot of 1 at 30% probability level. Figure 5. ORTEP plot of 2 at 30% probability level.


state crystal structures. With the exception ofcompound 6, a reasonable linear correlation(R=97.8%) was obtained in the ln(Kc) versus d(Fe–Fe)plot (Fig. 9A). An even better correlation (Fig. 9B,R=99.9 %) was achieved when the ln(Kc) values wereplotted versus d(edge–edge), defined as the shortest C–Cdistance between the two ferrocene units. Compound 6was excluded from this analysis because of the partialpositive charge on the nitrogen that results from thestrong electron-withdrawing character of its p-nitro-phenyl substituent. Compound 3+ has a fully developedpositive charge on the nitrogen, which makes its struc-ture depart even more from the general features com-mon to compounds 1, 2 and 5.

Structural comparisons between solid state and solutionstructures are always risky. However, our X-ray data(solid state) correlate well with our electrochemical and1H NMR spectroscopic data (solution phase) and addsupport to our conclusion on the through space char-acter of the communication between the ferrocene resi-dues in these compounds.

Figure 6. ORTEP plot of 3+ at 30% probability level. Figure 7. ORTEP plot of 5 at 30% probability level.

Figure 8. ORTEP plot of 6 at 30% probability level.

Table 4. Selected bond lengths (A) and angles (�)

1 2 3+ 5 6

Bond lengths and distances (A)

N–C(1) 1.469 (3) 1.461 (2) 1.537 (5) — 1.458 (5)N–C(2) 1.469 (3) 1.467 (2) 1.531 (5) — 1.477 (5)N–C(3) 1.489 (3) 1.486 (2) 1.497 (5) — 1.352 (5)N–C(51) — — 1.497 (5) — –N–C(11) — — — 1.465 (2) –N–C(12) — — — 1.382 (3) –Fe–Fe 6.34 6.70 7.68 7.90 6.42Edge–edge 3.579 3.999 4.410 4.609 4.080(C(n)–C(m)) (C15–C21) (C16–C24) (C11–C21) (C1–C1A) (C17–C26)Angles (�)C(1)–N–C(2) 110.65 (17) 112.51 (14) 109.1 (3) — 115.3 (4)C(1)–N–C(3) 115.12 (18) 111.30 (14) 109.1 (3) — 120.7 (4)C(2)–N–C(3) 114.13 (18) 113.32 (3) 108.8 (3) — 115.3 (4)C(1)–N–C(51) — — 112.7 (3) — –C(2)–N–C(51) — — 109.8 (3) — –C(3)–N–C(51) — — 107.2 (3) — –C(11)–N–C(12) — — — 121.52 (9) –C(11)–N–C(11A) — — — 116.96 (18) –


Experimental

Materials

All solvents and chemicals for synthesis were commer-cially available and were used without any further pur-ification. Solvents were removed on a rotary evaporatorconnected to a water-aspirator and the remaining traceswere evaporated on a vacuum oven typically set at 70 �Covernight. For electrochemistry, acetonitrile and di-chloromethane (99.9%, HPLC grade) were purchasedfrom Aldrich.

Methods and instrumentation

1H NMR spectra were recorded on a Varian VXR-400spectrometer and chemical shifts were measured withreference to the residual solvent signals. Mass spectra(FAB, nitrobenzyl alcohol matrix) were recorded on aVG Trio-2 FAB spectrometer. Electrochemical mea-surements were performed with a BAS 100W analyzerequipped with a standard three-electrode configuration.For all etectrochemical measurements, the referenceelectrode was Ag/AgCl. Cyclic (CV) and square wave(OSWV) voltammograms were obtained using a glassycarbon disk (ca. 0.018 cm2) as working electrode and aplatinum wire as auxiliary electrode. The pulse para-meters for OSWV were as follows: square wave fre-quency, 15 Hz; potential step, 4 mV; square waveamplitude, 25 mV. Therefore, the effective scan rate was60 mV/s.

For a typical electrochemical experiment, we prepared a0.2 mM solution of the corresponding ferrocene dimerand 0.2 M in tetrabutylammonium hexafluorophosphateusing pure acetonitrile, acetonitrile/dichloromethane(50:50 v/v) or pure dichloromethane as the solvent.Protonation and deprotonation experiments were per-formed by adding microliter aliquots of aqueous HCl IN and NaOH 0.5 N solutions (Fisher) to a 2-mL volumeof solution in the electrochemical cell. Protonation wasalso monitored by 1H NMR spectroscopy in deuteratedacetonitrile (Aldrich), using DCl 20 % in D2O (Aldrich)as the corresponding acid. Cyclic voltammograms were

fitted and simulated using DigiSim1 software version2.1.11 Digital simulations were conducted in the caseswhere �E� could not be directly measured from OSWvoltammograms.

Preparation of di(ferrocenylmethyl)-t-butyl amine (1).t-Butylamine (Across) (0.106 mL, 1 mmol) and (ferroce-nyl)trimethylammonium iodide (0.848 g, 2.2 mmol) weredissolved in 10 mL of acetonitrile and reacted in thepresence of potassium carbonate K2CO3 (1.4 g,10 mmol). After refluxing the reaction mixture for twodays, ether was added to the crude mixture to pre-cipitate any remaining ammonium salt and potassiumcarbonate. The precipitates were filtered off and washedwith dichloromethane. The collected filtrates were thenevaporated to dryness and redissolved in dichloro-methane to allow further precipitation of carbonate.After removal of the residual precipitate, the liquid wasallowed to evaporate and an orange product wasrecrystallized from a hexane–ethyl acetate mixture. Thefinal yield of the dimer was 60%. 1H NMR (CDCl3):4.15 (t, 4H, C5H4), 4.08 (t, 4H, C5H4), 4.07 (s, 10H,C5H5), 3.52 (s, 4H, CH2), 1.06 (s, 9H, CH3).

13C {1H}NMR (CDCl3): (77.21, 70.21, 67.44) C5H4, 68.41 C5H5,54.67 CH2, 46.84 C, 27.79 CH3. MS (FAB; m/z): 470(MH+). Calcd for C26H31NFe2: H, 6.66%; C, 66.52%.Found: H, 6.75%; C, 66.60%.

Preparation of di(ferrocenylmethyl)isopropyl amine (2).Isopropylamine (Across) (0.085 mL, 1.0 mmol), (ferro-cenyl)trimethylammonium iodide (0.848 g, 2.2 mmol)and potasium carbonate K2CO3 (1.4 g, 10 mmol) wereplaced in 10 mL of acetonitrile and refluxed for 2 days.After precipitation with ether the crude mixture was fil-tered and the collected filtrates were evaporated to dry-ness and the residue dissolved in dichloromethane toallow further precipitation of carbonate. After removalof the residual precipitate, the liquid was concentratedand loaded into a silica gel column using ethyl acetate aseluent to separate the dimer from the monomer. Sub-sequent isolation of the dimeric product as orange nee-dles produced a yield of 65 %. 1H NMR (CDCl3): 4.18(t, 4H, C5H4), 4.08 (t, 4H, C5H4), 4.07 (s, 10H, C5H5),

Figure 9. Plots showing the correlation between ln(Kc) and structural parameters obtained from X-ray diffraction data in the solid state for com-pounds 1, 2 and 5.


3.39 (s, 4H, CH2), 2.94 (m, 1H, CH), 0.97 (d, 6H, CH3).13C {1H} NMR (CDCl3): (77.20, 69.78, 67.48) C5H4,68.47 C5H5, 49.27 CH2, 48.43 CH, 18.58 CH3. MS(FAB; m/z): 456 (MH+). Calcd for C25H29NFe2: H,6.42%; C, 65.96%. Found: H, 6.46%; C, 66.07%.

Preparation of di(ferrocenylmethyl)hexyl amine (3). Thesynthesis and characterization of compound 3 and its N-methylated form as PF6 salt (3+) have already beenreported elsewhere.3 Here, we present the X-ray struc-ture of 3+ and its comparative analysis with other dimercrystal structures.

Preparation of di(ferrocenylmethyl)-p-methoxyphenylamine (4). p-Anisidine (Aldrich) (0.123 g, 1.0 mmol) and(ferrocenyl)trimethylammonium iodide (0.848 g,2.2 mmol) were dissolved in 10 mL of acetonitrile andreacted in the presence of potassium carbonate K2CO3

(1.4 g, 10 mmol). After refluxing for 2 days, the reactionwas stopped and ether was added to the crude mixtureto precipitate the remaining ammonium salt and potas-sium carbonate. The precipitate was filtered off andwashed with dichloromethane and the residue obtainedafter drying the collected filtrates was loaded on to asilica gel column. The column was eluted with ethylacetate and further isolation of the dimer as a yellowpowder gave a yield of 80%. 1H NMR (CDCl3): 7.20 (t,2H, C6H5), 6.83 (d, 2H, C5H5), 6.69 (t, 1H, C6H5),4.23(s, 4H, CH2), 4.17 (t, 4H, C5H4), 4.14 (s, 10H,C5H5), 4.09 (t, 4H, C5H4).

13C {1H} NMR (CDCl3):(152.30, 144.11, 117.20, 114.34) C6H4, (77.21, 69.40,67.61) C5H4, 68.55 C5H5, 55.66 CH3O, 49.99 CH2. MS(FAB; m/z): 520 (MH+). Calcd for C29H29NFe2O: H,5.63%; C, 67.08%. Found: H, 5.68%; C, 67.02%.

Preparation of di(ferrocenylmethyl)phenyl amine (5).Aniline (Aldrich) (0.092 mL, 1 mmol), (ferrocenyl)-trimethylammonium iodide (0.848 g, 2 mmol) and pota-sium carbonate K2CO3 (1.4 g, 10 mmol) were dissolvedin 10 mL of acetonitrile and refluxed for 4 days. Etherwas added to the crude mixture and the filtrate wasconcentrated to dryness. The residue was loaded on asilica gel column using ethyl acetate as eluent. Chroma-tographic isolation of the dimer as orange needles gavea yield of 70%. 1H NMR (CDCI3): 7.20 (t, 2H, C6H5),6.83 (d, 2H, C6H5), 6.69 (t, 1H, C6H5), 4.23(s, 4H,CH2), 4.17 (t, 4H, C5H4), 4.14 (s, 10H, C5H5), 4.09 (t,4H, C5H4).

13C {1H} NMR (CDCl3): (149.1, 128.95,116.76, 113.64) C6H5, (77.18, 69.14, 67.64) C5H4, 68.60C5H5, 48.72 CH2. MS (FAB; m/z): 490 (MH+). Calcdfor C28H27NFe2: H, 5.56%; C, 68.74%. Found: H,5.52%; C, 68.46%.

Preparation of di(ferrocenylmethyl)-p-nitrophenyl amine(6). p-Nitroaniline (Aldrich) (0.138 g, 1 mmol) and (fer-rocenyl)trimethylammonium iodide (0.848 g, 2.2 mmol)were dissolved in 10 mL of acetonitrile with potassiumcarbonate K2CO3 (1.4 g, 10 mmol). The reaction underreflux was left to run for 8 days. Ether was used to pre-cipitate any remaining ammonium salt and potassiumcarbonate. After filtration the liquid was evaporatedand loaded into a silica gel column for chromatographicelution with ethyl acetate. Subsequent isolation of the

dimer as an orange powder produced a yield of 30%. 1HNMR (CDCl3): 8.09 (d, 2H, C6H5), 6.75 (d, 2H, C6H5),4.39 (s, 4H, CH2), 4.20 (t, 4H, C5H4), 4.19 (s, 10H,C5H5), 4.16 (t, 4H, C5H4).

13C {1H} NMR (CDCl3):(126.50, 126.17, 111.01, 110.83) C6H5, (77.20, 68.84,68.63) C5H4, 68.81 C5H5, 48.87 CH2. MS (FAB; m/z):535 (MH+). Calcd for C28H26N2Fe2O2: H, 4.91%; C,62.95%. Found: H, 5.51%; C, 62.86%.

X-ray structure determinations of compounds 1, 2,3+PF6

�, 5 and 6. Single crystals of 1, 2, 3+PF6�, 5 and

6 were grown by slow evaporation in solutions of mixedsolvents (chloroform/hexane for 1 and 2 or ethyl acetate/hexane for 3+PF6

�, 5 and 6). Diffraction data werecollected at 300 K on a Bruker SMART1000 CCD-based X-ray diffractometer equipped with a Mo-targetX-ray tube (l=0.71073 A). Data were measured usingomega scans of 0.3� per frame during 10 s such that ahemisphere was collected. No decay was detected forany of the data sets collected, as indicated by the recol-lection of the first 50 frames at the end of each dataacquisition. The frames were integrated with the BrukerSAINT# sotfware package using a narrow-frame inte-gration algorithm, which also corrects for Lorentz andpolarization effects.12 Absorption corrections wereapplied using SADABS supplied by George Sheldrick.13

All the structures were solved and refined using theBruker SHELXTL# software package version 5.1.14�16

While the space group assignments for crystals 1, 2,3+PF6

� and 5 were unambiguous, both space groupsPnma (No. 62, centric) and Pna21, (No. 33, acentric)were acceptable for crystal 6 based on the statistics of itsintensity data, and the structure was solved and refinedin the latter space group. In all the cases, the positionsof non-hydrogen atoms were determined by directmethods. For crystals 1, 2, 5, and 6, the structures wererefined to convergence by least squares method on F2,SHELXL-93, incorporated in SHELXTL.PC V 5.03with all non-hydrogen atoms being anisotropic and allhydrogen atoms in calculated positions and ridingmode. For crystal 3+PF6

�, the asymmetric unit consistsof two independent cations, two PF6 anions, and oneCHCl3 molecule, and severe disorder was present for allmolecular fragments but one of the cations. One PF6

was disordered by a rotation around the vector linking apair of trans-F atoms, while the other PF6 was dis-ordered by a rotation around a molecular C3 axis pas-sing the phosphorus center. Disordered anions andsolvent CHCl3 were refined with distance constraints,and the distorted cyclopentadienyl ring (C46–C50) wasrefined as a rigid pentagon. The final least-squaresrefinements converged at the R-factors reported inTable 5, along with other procedure parameters. Fulltables of atomic coordinates, bond lengths and angles,and anisotropic displacements are provided as SupportingInformation (Tables S1–S15).

Conclusions

In the solution phase the dinuclear ferrocene com-pounds 1 and 2 display different degrees of electronic


Table 5. Crystal data and structure refinement for compounds 1,2,3+PF6�,5,6

1 2 3+PF6� 5 6

Empirical formula C26H31NFe2 C25H26NFe2 C59H76N2F12P2Cl3Fe4 C28H27NFe2 C28H26N2O2Fe2

Formula weight 496.22 455.19 1432.91 489.21 534.21Space group P1 P1 P1 C2/c Pna21

a (A) 7.5028 (7) 6.0571 (6) 12.7465 (13) 25.1551 (17) 13.9178 (8)b (A) 10.4720 (10) 12.4054 (12) 15.9579 (17) 9.3651 (6) 15.9552 (10)c (A) 14.9037 (13) 14.5643 (14) 17.6245 (18) 9.5279 (6) 10.4860(6)a (�) 75.678 (2) 74.203 (2) 109.322 (2)b (�) 82.795 (2) 88.940 (2) 103.940 (2) 104.423 (2)g (�) 89.272 (2) 87.787 (2) 93.359 (2)Volume (A3) 1125.42 (18) 1052.22 (18) 3245.7 (6) 2173.8 (2) 2328.5 (2)Z 2 2 2 4 4Dcalc (g cm�3) 1.385 1.437 1.466 1.495 1.524Crystal size (mm) 0.33�0.30�0.17 0.47�0.22�0.11 0.41�0.20�0.07 0.47�0.40�0.19 0.26�0.09�0.09m (mm�1) 1.300 1.389 1.121 1.350 1.275Tmax, Tmin 0.802, 0.644 0.858, 0.656 0.925, 0.588 0.774, 0.539 0.892, 0.722Data collection instrument SMART1000 CCD SMART1000 CCD SMART1000 CCD SMART1000 CCD SMART1000 CCDl(MoKa) (A) 0.71073 0.71073 0.71073 0.71073 0.71073Temperature (K) 300 (2) 300 (2) 300 (2) 300 (2) 300 (2)y range for data collection 1.42 to 27.50� 1.45 to 27.50� 1.50 to 25.00� 1.67 to 27.99� 1.94 to 27.99�

Reflections collected 7114 6561 16994 7844 16905Independent reflections 4930 [R(int)=0.0170] 4549 [R(int)=0.0128] 11242 [R(int)=0.0229] 2631 [R(int)=0.0168] 5103[R(int)=0.0548]Data/restraints/parameters 4930/0/266 4549/0/256 11242/126/846 2631/0/197 5103/1/307Refinement method Full-matrix least-squares on F2 Full-matrix least-squares on F2 Full-matrix least-squares on F2 Full-matrix least-squares on F2 Full-matrix least-squares on F2

Final R indices [I>2s(I)] R1=0.031, wR2=0.070 R1=0.027, wR2=0.062 R1=0.054, wR2=0.140 R1=0.024, wR2=0.058 R1=0.042, wR2=0.081R indices (all data) R1=0.046, wR2=0.073 R1=0.036, wR2=0.063 R1=0.088, wR2=0.151 R1=0.034,wR2=0.061 R1=0.082, wR2=0.095Goodness-of-fit on F2 0.860 0.896 0.897 0.947 1.027Largest diff.peak (hole), E/A3 0.326 (�0.273) 0.261 (�0.262) 0.389 (�0.645) 0.328 (�0.203) 0.439 (�0.353)

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communication that correlate well with geometric para-meters describing the average distance between theirferrocene centers in the solid state. Although we couldnot determine the crystal structure of 3, the unbranchednature of its hexyl substituent fits very well with thetrend that extrapolates from the relative steric bulk ofthe substituents in compounds 1 (t-butyl) and 2 (iso-propyl). Therefore, the extent of electronic communica-tion decreases monotonically in going from 1 to 3, asthe steric bulk around the-N-center decreases and theaverage distance between the ferrocene centers increa-ses. This trend and the saturated nature of the tetherbetween the ferrocene centers led us to conclude that themodest level of electronic communication observedbetween the ferrocene centers is established via throughspace interactions, a conclusion that is further sup-ported by the solvent effects found in our electro-chemical experiments. These conclusions cannot beeasily extended to the dinuclear ferrocene compounds4–6, due to the aromatic nature of their R substituents.In fact, 4 and 5 exhibit similar levels of electronic com-munication while 6 displays almost no communication,probably due to the considerable positive charge densitypresent on its central nitrogen atom. Overall, the mostinteresting finding in this work is that whatever extentof electronic communication is present in these ferro-cene dimers, it can readily be obliterated by protonationor N-alkylation, thus, providing a mechanism for exter-nal control of a ground state molecular property. Assuch, these compounds constitute a novel example ofswitchable molecules.

Supporting Information

Crystallographic data have been deposited with theCambridge Crystallographic Data Centre as CCDC155780–155784 for compounds 1, 2, 3+ 6 and 5,respectively. Tables of atomic coordinates, bond lengthsand angles, and anisotropic displacement parametersfor these five compounds (Tables S1–S15).

Acknowledgements

The authors are grateful to the NSF (to AEK, CHE-9982104) for the generous support of this research

work. T.R. acknowledges the CCD diffractometer fundfrom the University of Miami. The authors also wish tothank Ms. Eden Pacsial for assistance in the collectionof the X-ray data set for 3+.

References and Notes

1. (a) For representative reviews, see Robin, M. B.; Day, P.Adv. Inorg. Chem. Radiochem. 1967, 10, 247. (b) Taube, H.Angew. Chem. 1984, 23, 329. (c) Ward, M. D. Chem. Soc. Rev.1995, 34, 121. (d) Astruc, D. Acc. Chem. Res. 1997, 30, 383. (e)Nelsen, S. F. Chem. Eur. J. 2000, 6, 581. (f) Kaim, W.; Klein,A.; Glocke, M. Acc. Chem. Res. 2000, 33, 755.2. (a) Barlow, S.; O’Hare, D. Chem. Rev. 1997, 97, 637. (b)Nguyen, P.; Gomez-Elipe, P.; Manners, I. Chem. Rev. 1999,99, 1515.3. Alvarez, J.; Kaifer, A. E. Organometallics 1999, 18, 5733.4. Pennie, J. T.; Bieber, T. I. Tetrahedron Lett. 1972, 34, 3535.5. Marr., G.; Rockett, B. W.; Rushworth, A. TetrahedronLett. 1979, 16, 1317.6. Beer, P. D.; Nation, J. E.; McWhinnie, S. L.. W.; Harman,M. E.; Hurtsthouse, M. B.; Ogden, M. I.; White, A. H. J.Chem. Soc., Dalton Trans. 1991, 2485.7. Bard, A. J.; Faulkner, L. R. Electrochemical Methods:Fundamentals and Applications; Wiley: New York, 1980; pp232–235.8. Bard, A. J.; Faulkner, L. R. Electrochemical Methods:Fundamentals and Applications; Wiley: New York, 1980; p 160.9. The pKa values for p-anisidine, aniline and p-nitroanilineare 5.3, 4.6 and 1, respectively. The pKa values of t-butyla-mine, isopropylamine and hexylamine fall in the narrow range10.6–10.7.10. Haaland, A. Acc. Chem. Res. 1979, 12, 415.11. Rudolph, M.; Reddy, D. P.; Feldberg, S. W. Anal. Chem.1994, 66, 589A.12. SAINT V 6.035 Software for the CCD Detector System;Bruker-Axs Inc., 1999.13. Blessing, B. Acta Cryst. 1995, A51, 33.14. Sheldrick, G. M. Shelxs-90, Program for the Solutionof Crystal Structures, University of Gottigen, Germany,1990.15. Sheldrick, G. M. SHELXL-93, Program for the Refine-ment of Crystal Structures, University of Gottigen, Germany,1993.16. SHELXTL 5.03 (WINDOW-NT Version), ProgramLibrary for Structure Solution and Molecular Graphics, Bruker-AXS Inc., 1998.


modulation of the electronic communication between two equivalent ferrocene centers by proton...

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