ARTICLE pubs.acs.org/Organometallics
Synthesis and Structures of 10 ,20,30,40 ,50-Pentamethylazaferrocene Complexes with Lewis Acidic Boranes Tim J. Brunker,*,† Benjamin T. Roembke,† James A. Golen,‡ and Arnold L. Rheingold§ †
Department of Chemistry, Jess and Mildred Fisher College of Science and Mathematics, Towson University, 8000 York Road, Towson, Maryland 21252, United States ‡ University of Massachusetts—Dartmouth, North Dartmouth, Massachusetts 02747, United States § Department of Chemistry, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093-0358, United States
bS Supporting Information ABSTRACT: The syntheses, X-ray crystal structures, and electrochemistry of BH3 (2) and BF3 (3) adducts of 10 ,20 ,30 ,40 ,50 -pentamethylazaferrocene (1) are reported, together with an improved synthesis, an X-ray crystal structure and the electrochemistry of the known compound, 1. 13C NMR data reveal a similiar dependence of chemical shift on nitrogen lone-pair coordination to that seen for heteroarene analogs. X-ray structural data for 13 reveal subtle changes in the binding of the pyrrolyl ring to iron in the Lewis Acid adducts, and intersting short intermolecular contacts in the case of 1 and 3. In cyclic voltammetry studies, 1 displays a reversible metal-based oxidation allowing estimation of the effect of N for CH substitution in the oxidation potential of azaferrocenes as compared to ferrocenes. 2 and 3 do not show reversible electrochemistry.
I
soelectronic analogues of ferrocene in which a heteroatom, typically nitrogen or phosphorus, replaces a CH moiety in a cyclopentadienyl ring have been a recurrent subject of investigation since shortly after the discovery of ferrocene itself.13 Azaferrocenes4 have received comparatively less recent attention than phosphaferrocenes5 because of their lower stability, which arises from the greater ease of η5 to η1 ring slippage via nitrogen lone-pair coordination to the metal and subsequent decomposition reactions.6 Strategies to stabilize azaferrocenes relative to the unsubstituted parent, FeCpPyr (Cp = cyclopentadienyl, Pyr = pyrrolyl), include increasing the degree of methylation of the Cp and/or Pyr rings, which leads to greater thermodynamic and kinetic stability, and protection of the nitrogen lone pair, which attenuates the possibility of ring slippage.7 In the first case, the synthesis of 10 ,20 ,30 ,40 ,50 -pentamethylazaferrocene (1, FeCp*Pyr; Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl) by Fu et al. in 19968 led to a revival of interest in this area,9 particularly in the synthesis of azaferrocene-containing ligands and reagents (including planar-chiral derivatives) for synthetic,1013 materials chemistry,14,15 and biological applications.16,17 Kuhn et al. demonstrated the efficacy of the second approach in isolating the only known diazaferrocene, FePyr*2 (Pyr* = 1,2,3,4-tetramethylpyrrolyl), as bis(adducts) in which both nitrogen lone pairs are either coordinated to the Lewis acids BH3 and BF3 or hydrogen-bonded to Pyr*H.18,19 The mixed-sandwich species FeCpPyr*BH3 has also been reported and has been r 2011 American Chemical Society
crystallographically characterized,20 as has Fe(Pyr*BH3)2.21 However, these compounds contain permethylated pyrrolyl rings, hindering further elaboration of the pyrrolyl ring in the useful ways that have been reported for 1 and related compounds.2224 Therefore, we set out to examine similar adducts of 1, which could find synthetic utility in azaferrocene chemistry. Herein we report the synthesis of two such adducts and compare their structural, spectroscopic, and electrochemical properties with those of 1 (see Figure 1): we also disclose an improved synthetic procedure for 1, along with its crystal structure and electrochemical properties, which have not previously been reported.
’ EXPERIMENTAL SECTION All reactions were performed under a dry nitrogen atmosphere in oven-dried glassware using standard Schlenk techniques or in a VAC Omni inert atmosphere glovebox. Tetrahydrofuran and diethyl ether solvents were dried using a VAC solvent purification system, whereas hexanes and dichloromethane were distilled from calcium hydride and stored under nitrogen before use. d6-Benzene for NMR spectroscopy was dried over potassium and stored under nitrogen. NMR spectra were obtained using a JEOL ECS 400 MHz spectrometer at room temperature. Chemical shifts are reported in ppm and referenced via residual Received: January 20, 2011 Published: March 29, 2011 2272
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Figure 1. Structures of compounds synthesized and discussed in this paper. solvent resonances to Me4Si (1H and 13C); 11B is reported relative to BF3 3 Et2O. IR spectra were collected on a Nicolet 360 FTIR instrument as KBr disks. Elemental analyses were performed by QTI Intertek, Whitehouse, NJ. Cp*H and anhydrous FeCl2 were supplied by Strem Chemicals Inc.; all other reagents were supplied by Aldrich. 10 ,20 ,30 ,40 ,50 -Pentamethylazaferrocene, FeCp*Pyr (1). This synthesis is a modification of the reported procedure of Ruble and Fu.8 To a solution of Cp*H (2.00 g, 14.7 mmol) in THF at 0 °C was added nBuLi dropwise (9.2 mL, 14.7 mmol, 1.6 M solution in hexanes) to form a thick white suspension, which was then warmed to room temperature. The suspension was then recooled to 0 °C and slowly added to a stirred slurry of anhydrous FeCl2 (1.86 g, 14.7 mmol) in THF (50 mL) at 0 °C. At the end of the addition a forest green solution was obtained, which was warmed to room temperature. A slurry of KPyr (1.52 g, 14.7 mmol, prepared from KH and pyrrole) in THF (30 mL) was then added slowly to the green solution to form an orange-brown suspension. After 3 h of stirring all solvent was removed in vacuo, while avoiding heating the solution above room temperature. The sandy brown solid obtained was extracted with several portions of dry diethyl ether (100 mL total) and filtered through dry Celite under nitrogen to give a medium brown solution. The solvent was then removed in vacuo while avoiding heating the solution above room temperature to give a dark brown crystalline solid of crude 1 (3.153 g, 12.3 mmol, 83% yield). Pure 1 was obtained by recrystallization from hexanes at 20 °C to remove an unidentified brown insoluble residue. Typical recoveries of 1 from recrystallizations performed on 1 g scales were in the range of 6070% (three to four crops combined). Single crystals of 1 suitable for X-ray diffraction were obtained from a saturated hexanes solution at 20 °C. The spectroscopic data agree with those previously reported: full 1H and 13C NMR data used for comparison purposes are listed below. 1 H NMR (C6D6): δ 4.93 (s, 2H, Pyr HR), 3.84 (s, 2H, Pyr Hβ), 1.81 (s, 15H, CH3). 1H NMR (CD2Cl2): δ 4.88 (s, 2H, Pyr HR), 4.12 (s, 2H, Pyr Hβ), 1.92 (s, 15H, CH3). 13C NMR (C6D6): δ 92.94 (d, 1JCH = 192 Hz, Pyr CR), 80.91 (s, Cp* quat), 74.31 (d, 1JCH = 176 Hz, Pyr Cβ) 11.55 (q, 1JCH = 127 Hz, CH3). 13C{1H} NMR (CD2Cl2): δ 92.31, 81.14, 74.52, 11.34. Anal. Calcd for C14H19NFe: C, 65.37; H, 7.45; N, 5.45. Found: C, 65.14; H, 7.57; N, 5.32. FeCp*PyrBH3 (2). The first steps of the synthesis follow the procedure given above for 1. However, after the addition of KPyr, the solution was stirred at room temperature for 3 h and then cooled to 0 °C. BH3THF complex (17 mL, 17.0 mmol, 1.0 M solution in THF) was then added dropwise. After 3 h, H2O (20 mL) was added cautiously and the solution filtered to remove a black insoluble impurity. Diethyl ether (50 mL) was added, and the organic and aqueous layers were separated. The aqueous layer was extracted with additional ether (2 30 mL), and the combined organics were washed with brine (50 mL) and then dried over MgSO4. Filtration followed by removal of solvent in vacuo without heating gave 2.79 g (10.3 mmol, 70% yield) of an orange-brown solid that was essentially pure 2. Recrystallization of this solid from a 2/1 petroleum ether/ diethyl ether mixture at 20 °C gave analytically pure 2 (with typical recoveries of 60% when performed on a 1 g scale). Slow cooling of a
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saturated solution in 2/1 petroleum ether/diethyl ether gave single crystals suitable for X-ray diffraction. 1 H NMR (C6D6): δ 4.80 (s, 2H, Pyr), 3.49 (s, 2H, Pyr), 2.80 (br 1:1:1:1 q, JBH = 96 Hz, BH3), 1.65 (s, 15H, Cp*H). 13C{1H} NMR (C6D6): δ 89.0 (Pyr), 83.1 (Cp* quat), 74.5 (Pyr), 10.4 (Cp*-CH3). 11 B{1H} NMR (C6D6): δ 18.4. IR: ν 3021, 3014, 2980, 2905, 2450, 2300, 2246, 1473, 1457, 1423, 1369, 1280, 1177, 1154, 1030, 924, 826, 505, 470, 428 cm1. Anal. Calcd for C14H22NBFe: C, 61.99; H, 8.18; N, 5.17. Found: C, 62.22; H, 8.26; N, 5.01. Formation of 1 from 2. 2 (101 mg, 0.373 mmol) was dissolved in deoxygenated methanol (10 mL) under N2, and 195 mg of 10% Pd on C was added. Some bubbling was initially observed. After it was stirred at room temperature overnight, the solution was filtered and the methanol was removed in vacuo to given an orange solid, the IR and 1H NMR spectra of which were identical with those of 1. The isolated yield of 1 was 57 mg (60%, unoptimized). FeCp*PyrBF3 (3). 1 (400 mg, 1.56 mmol) was dissolved in dry diethyl ether (10 mL) and cooled to 0 °C; then BF3 3 Et2O (0.25 mL, 2.03 mmol) was added by syringe. An orange precipitate formed immediately, and the mixture was stirred for a further 20 min before being stored overnight at 20 °C. The resultant dark yellow solid was isolated and dried in vacuo and was shown to be essentially pure FeCp*PyrBF3 by 1H NMR spectroscopy (264 mg, 0.812 mmol, 52.2% yield). Recrystallization from a concentrated CH2Cl2 solution layered with hexanes stored at 35 °C for 1 week yielded analytically pure orange crystals suitable for single-crystal X-ray diffraction. 1 H NMR (C6D6): δ 5.02 (s, 2H, Pyr), 3.55 (s, 2H, Pyr), 1.60 (s, 15H, Cp*H). 13C{1H} NMR (C6D6): δ 84.4 (Cp* quat), 84.1 (Pyr CR), 75.7 (Pyr Cβ), 10.3 (Cp*-CH3). 19F NMR (C6D6): δ 146.5 (br). 11B{1H} NMR (C6D6): δ 0.16 (br q, 1JBF = 11.1). IR: ν 3135, 2957, 2910, 1480, 1456, 1427, 1379 (s), 1275, 1214, 1158, 1135, 1104, 1043, 966, 951, 876, 848, 821, 659, 511, 486, 475, 441 cm1. Anal. Calcd for C14H19NBF3Fe: C, 51.69; H, 5.89; N, 4.31. Found: C, 51.47; H, 5.67; N, 4.23. Electrochemistry. Cyclic voltammograms were recorded using a BAS 100B potentiostat, a glassy-carbon working electrode, a Pt-wire auxiliary electrode, and a Ag wire anodized in 1 M aqueous KCl as a pseudoreference electrode. Solutions of 0.1 M n-Bu4NPF6 in dry THF or CH2Cl2 were used as the electrolyte, and potentials were referenced to FeCp2þ/0 by using FeCp*2 as an internal reference (E1/2 = 0.45 and 0.55 V vs FeCp2þ/0 in THF and CH2Cl2, respectively). Selected cyclic voltammograms are displayed in the Supporting Information.
X-ray Data Collection, Structure Determination, and Refinement. Crystallographic Information on compounds 1-3 is summarized in the Supporting Information. Disorder was observed in the Cp* ring of 1, which was modeled as two equally occupied ring positions in which the methyl groups could be resolved into two sites, whereas the ring C atoms could not. The average bond distances, angles, parameters and esd’s for 2 reported in Table 2 were calculated by following the procedures described by Taylor and Kennard as the unweighted mean of 10 independent molecules.25
’ RESULTS AND DISCUSSION 10 ,20 ,30 ,40 ,50 -Pentamethylazaferrocene (1, FeCp*Pyr) was initially synthesized by following the procedure of Ruble and Fu by sequential addition of Cp*Li and KPyr to a slurry of FeCl2 in THF.8 We encountered some decomposition of 1 during chromatography and from thermal instability when following their workup procedure. These observations, which have also been noted elsewhere,23,24,26 prompted an investigation of alternative purification methods. We found that extraction of the crude reaction mixture with ether and recrystallization from hexanes allowed access to highly crystalline 1 in good yields, without recourse to chromatography. Modification of this procedure by 2273
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Table 1. Comparison of 1H and 13C NMR Chemical Shift and Coupling Constant Data of 24 with Those of 1 H Δδ (ppm)a
C Δδ(ppm)a
1
ΔJ (Hz)b
13
1
JCH(R)
1
JCH(β)
HR
Hβ
Cp*-CH3
CR
Cβ
Cp* q
Cp*-CH3
2
0.05
0.32
0.15
3.81
0.17
2.18
1.14
þ4
þ6
3c
0.09
0.31
0.22
8.51
1.40
3.22
1.3
þ9
þ5
4d
0.77
0.67
0.02
10.25
2.46
5.64
0.64
þ19
þ15
c
Δδ = δ(compound) δ(1). b ΔJ = J(compound) J(1). c Spectra measured in d6-benzene, compared to 1 in the same solvent. d Spectra measured in d2-methylene chloride, compared to 1 in the same solvent. a
Table 2. Selected Bond Lengths, Angles, and Structural Parameters for 13 1
2a
3
FeN (Å)
2.048(2)
2.016(2)
FeCR(Pyr)b (Å)
2.034(2)
2.022(1)
2.032(2)
FeCβ(Pyr)b (Å)
2.056(2)
2.067(2)
2.082(2)
1.580(3)
1.587(2)
FeCp*(Ct) (Å)
1.642
1.646
1.657
FePyr(Ct) (Å)
1.658
1.655
1.661
Cp*(Ct)FePyr(Ct) (deg)
179.2
NB (Å)
Pyr(Ct)NB (deg)
2.015(1)
177.8
176.9
173.9
172.7
a
Values are the unweighted mean of parameters from 10 independent molecules, see the Experimental Section. b Values are the average of both bond lengths in structure.
addition of BH3THF prior to aqueous workup and crystallization allowed easy access to 2, the highly crystalline BH3 adduct of 1. Treatment of isolated 1 with BF3 3 OEt2 in ether led to precipitation 3, the BF3 adduct of 1. Qualitatively 2 shows greater thermal and solution stability than 1 and does not streak on untreated silica TLC plates as 1 does. 3 is less stable than either 1 or 2 and was handled under anhydrous conditions at all times. We also examined the removal of BH3 from 2 to give 1 and found that this could easily be accomplished by stirring a methanol solution of 2 with catalytic (1050%) amounts of Pd/C with no discernible decomposition of 1, as shown by NMR and IR spectroscopy.27 This result suggests that BH3 could be a viable protecting group for use in the synthetic elaboration of azaferrocenes. The effect of lone-pair coordination was examined by comparison of 1H and 13C NMR spectroscopic data for 2 and 3 with those for 1; these data are summarized in Table 1. Data for 4, the conjugate acid of 1, were also obtained by addition of an excess (ca. 7 equiv) of trifluoroacetic acid to a solution of 1 in d2-methylene chloride. No consistent trends in the 1H chemical shifts can be seen across the series, although the deshielding effect of the nitrogen lone pair appears to be attenuated in both 2 and 3 relative to 1. More illuminating trends are observed in the 13C NMR spectra: the most significant change is the shielding of the RC resonances of 24 vs those in 1, whereas the β-C and Cp* quaternary resonances become deshielded, although to a lesser extent. The magnitudes of these changes follow the order of decreasing electron-withdrawing ability of the N substituents: i.e., 4 > 3 > 2 (Hþ > BF3 > BH3). Although the increased shielding of the R-C resonances of 24 is opposite to what might be expected, assuming simply a decrease in electron density of the Pyr ring on adduct formation, this same effect has also been observed for BX3 adducts of pyridine28 and imidazole29 and for pyrrole compared to
Figure 2. Thermal ellipsoid plot of one of the crystallographically independent molecules of 2 with ellipsoids shown at the 50% probability level.
its conjugate base30 and has been attributed to changes in πelectron density and its effect on the paramagnetic shielding term. The Pyr ring CH coupling constants show increases in 1JCH for 24 relative to 1, in accordance with a decrease in carbon electron density,29 and again these changes are the greatest for 4. The 11B NMR spectrum of 2 shows a quartet resonance at 18.4 (1JBH = 96 Hz): this is similar to those reported for Fe(Pyr*BH3)2 (20.2 ppm),21 FeCpPyr*BH3 (19.8 ppm),20 and imidazoleborane (17.8 ppm)29 but significantly more shielded than for pyridineborane (11.2 ppm).28 Shielding at tetracoordinate boron is usually observed to be greater for Bazole adducts than for Bazine adducts and has been attributed to the greater stabilization of the formal positive charge of the nitrogen via delocalization in azole rings.31 In this regard, then, it appears that azaferroceneboranes are more similar to azoleboranes than to azineboranes. The 11B NMR spectrum of 3 shows a quartet at 0.16 ppm (1JBF = 11 Hz), which is similar to those for imidazoleBF3 (0.03 ppm)29 and pyridineBF3 (0.05 ppm).28 Compounds 13 were characterized by single-crystal X-ray diffraction; details of the data collections appear in the Supporting Information and selected structural parameters in Table 2. A view of the structure of 2 is shown in Figure 2. All three molecules display the expected sandwich structure with η5-pyrrolyl binding, essentially parallel rings, and nearly linear Cp*(Ct)FePyr(Ct) angles (Ct = centroid). In each case, the FePyr(Ct) 2274
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Figure 3. View of the extended structure of 1, down the crystallographic a axis. Thermal ellipsoids are shown at 50% probability. Selected intermolecular contacts given as distance (Å), angle (deg): C14H14 3 3 3 N1 = 2.610, 139.4.
distance is slightly greater than the FeCp*(Ct) distance in the same molecule, whereas in other crystallographically characterized azaferrocenes with unsubstituted Cp rings, FeCpPyr0 32 (Pyr0 = 2,5-dimethylpyrollyl) and FeCpPyr*BH3,20 the FePyr(Ct) distance is slightly shorter than the FeCp(Ct) distance, reflecting the more weakly electron donating nature of Cp vs Cp*. Within the series 13, variations in the FeCp*(Ct) distances are seen, with that of 3 being significantly longer (1.657 Å) than that of 1 (1.642 Å); however, the FePyr(Ct) distances in 13 are all very similar. However, some more subtle differences are seen between the coordination of the Pyr ring to iron in the adducts in comparison to the coordination in 1: the FeN bond length contracts slightly on Lewis acid coordination of the nitrogen lone pair, being shorter in 2 and 3 vs 1; in 2 and 3 Δ(FeC(Pyr)), the difference between the average FeCβ(Pyr) and the average FeCR(Pyr) distances, increases relative to 1 (Δ(FeC(Pyr)) = 0.050 Å for 3, 0.045 Å for 2, 0.022 Å for 1). These parameters indicate that iron shifts slightly closer to nitrogen relative to the centroid of the Pyr ring, presumably as a result of some decrease in electronelectron repulsion on coordination of the lone pair. It is interesting to note that previously reported DFT calculations of the structures of FeCpPyr and FeCpPyrBH3 qualitatively agree with the crystallographically observed trends: viz., an increase in FePyr(Ct) and FeCβ(Pyr) distances and a decrease in FeN, and Fe CR(Pyr) distances on coordination of the borane.33 As expected, the boron centers are tetrahedral in both 2 and 3 and the NB distances are similar to the sum of covalent radii for boron and nitrogen (1.58 Å).34 The average NB bond length of
2 is 1.580(3) Å and is slightly shorter than in the other known azaferroceneBH3 adducts (FeCpPyr*BH3, 1.604(5) Å;20 Fe(Pyr*BH3)2, 1.599(2) Å21), presumably due to some steric crowding in derivatives with a Pyr* ring. The NB bond length of 3 (1.587(2) Å) is shorter than that found in pyridineBF3 (1.603(5) Å)35 but longer than that in imidazoleBF3 (1.544(7) Å).36 In these BF3 adducts, the percent tetrahedral character of the boron atom can also be calculated by H€opfl’s formula using the six bond angles around boron:37 the value of 87.8% for 2 is very close to that of pyridineBF3 (87.1%) but somewhat lower than that of imidazoleBF3 (95.7%). In both 2 and 3, the boron center is displaced above the plane of the Pyr ring away from the Cp*ring, as indicated by the BNPyr(Ct) angles presumably to minimize steric interactions with the Cp* methyl substituents. Examination of the extended packing arrangements of both 1 and 3 reveals some structurally important short intermolecular contacts, which are illustrated in Figures 3 and 4. In the extended structure of 1, chains of 1 run parallel to the crystallographic b axis, in which azaferrocene molecules alternate in the orientation of their Cp*(Ct)Pyr(Ct) vector. Each molecule of 1 forms a dimer-like contact with another molecule from a parallel chain through intermolecular N 3 3 3 HC(R-Pyr) contacts of length 2.610 Å (0.14 Å shorter than the sum of van der Waals radii for N and H). In crystals of 3, multiple contacts from all three fluorine atoms to both sp2 and sp3 CH bonds are observed that are shorter than the sum of van der Waals radii for F and H (2.67 Å). Two molecules of 3 form pairwise contacts with each other between F2 3 3 3 HC(R-Pyr) and F1 3 3 3 HC(Cp*CH3); F3 makes two contacts to two CH bonds of adjacent methyl 2275
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Figure 4. View of the extended structure of 3 with thermal ellipsoids shown at 50% probability. Selected intermolecular contacts given as distance (Å), angle (deg): C8H8B 3 3 3 F1 = 2.560, 146.8; C6H6A 3 3 3 F1 = 2.623, 161.9; C11H11A 3 3 3 F2 = 2.401, 152.3; C6H6C 3 3 3 F3 = 2.553, 173.0; C10H10B 3 3 3 F3 = 2.323, 178.3.
groups on the Cp* ring of a different molecule with F 3 3 3 H distances of 2.323 and 2.553 Å and F 3 3 3 HC angles of 178.3 and 173.0°, respectively. Both pyridineBF3 and imidazoleBF3 show similar contacts between fluorine and aromatic sp2 CH bonds; however, none of these are as short as the 2.323 Å contact observed between fluorine and an sp3 CH in 3. In summary, both 1 and 3 present highly polarized atoms (N and F) embedded in an otherwise nonpolar environment, leading to intermolecular contacts with both sp2 CH and sp3 CH bonds, some of which conform to the criteria outlined for weak hydrogen bonds.38 The electrochemistry of 13 was investigated by cyclic voltammetry. Compound 1 undergoes a one-electron oxidation with ΔEp very similar to that of the internal reference (decamethylferrocene) which appears to be fully reversible (ip,c/ip,a = 1) at fast scan rates (500 mV/s) but which shows reduced ip,c/ip,a values at lower scan rates (0.8 in CH2Cl2, 0.4 in THF at 50 mV/s). The half-wave potential, E1/2, was measured at 0.03 V vs FeCp2þ/ FeCp2 (in either solvent), while a second irreversible oxidation wave was observed in CH2Cl2 at Ep,a = þ0.63 V. In contrast, no reversible oxidation events are found for 2 or 3: irreversible oxidations are observed at Ep,a = þ0.33 and þ0.53 V, respectively, in CH2Cl2, and at Ep,a = þ0.28 and þ0.47 V in THF (all at 50 mV/ s vs FeCp2þ/FeCp2). Addition of 1% by volume of trifluoroacetic acid to a CH2Cl2 solution of 1 allowed the CV of 4 to be recorded, which displayed an irreversible oxidation at Ep,a = þ0.59 V. The electrochemistry of 1 gives features similar to those reported for FeCpPyr0 —a reversible couple corresponding to a metal-based oxidation (þ0.17 V vs FeCp2þ/FeCp2) and a second irreversible oxidation at higher potential presumably due to electron removal from the nitrogen lone pair.39 The 1þ cation displays some instability in these experiments, particularly in THF solution, which is in accord with the generally observed instability of azaferrocenium ions.40 The increasing difficulty of oxidation of
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24 mirrors the Lewis acidity of the substituent (Hþ > BF3 > BH3) and is in agreement with 13C NMR data described above. It is of note that the potential of 1 is very similar to that of ferrocene, implying that substitution of Cp by Cp* and substitution of Cp by Pyr are nearly equal and opposite in their effect on the oxidation potential. Quantitatively, comparison of the potential of 1 with reported potentials for FeCpCp* recorded in CH2Cl2 (0.27 V41 to 0.30 V42 vs FeCp2þ/FeCp2) allows us to calculate the effect of replacement of a CH by N as an anodic shift in the range 240270 mV. This is in good agreement with the value derived by Kowalski and Winter (þ260 mV) on the basis of comparisons of the potential of FeCpPyr0 with 1,10 dimethylferrocene39 and with the rule of thumb that there is a cathodic shift of ca. 50 mV in the oxidation potential of ferrocene per methyl substituent.43 Two independent reports of the electrochemistry of unsubstituted azaferrocene (FeCpPyr) give quite different values for its oxidation potential (þ0.17 V44 and þ0.325 V40 vs FeCp2þ/FeCp2), perhaps due to solvent and supporting electrolyte effects, making an accurate determination of the effect of N-for-CH substitution by direct comparison with ferrocene difficult. Our value for N-for-CH substitution is greater than the average value for P-for-CH substitution in phosphaferrocenes (196 mV),43 in accord with calculations predicting greater charge shift from iron to the coordinated heterocycle in azaferrocene as compared to phosphaferrocene.33
’ CONCLUSION The known azaferrocene 1 forms stable adducts with some borane Lewis acids. In particular the BH3 adduct 2 may prove to be a helpful starting material for the synthesis of functionalized or more complex azaferrocenes, given its ease of synthesis, crystallinity, and stability as compared to 1 and the knowledge that the BH3 group can be readily removed. We are currently investigating this possibility. Structural data allow comparisons within the series 13 and to heteroarene donoracceptor complexes. Electrochemical data for 1 compliment and bolster previously reported data on azaferrocene species. ’ ASSOCIATED CONTENT
bS
Supporting Information. Tables and CIF files giving X-ray diffraction data, atomic coordinates, thermal parameters, and complete bond distances and angles for compounds 13 and figures giving plots of cyclic voltammograms for 14. This material is available free of charge via the Internet at http://pubs. acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Tel: (410) 704-3118.
’ ACKNOWLEDGMENT We thank both Towson University and the Fisher College of Science and Mathematics at Towson University for funding (T.J. B. for support as the Jess and Mildred Fisher Professor of Chemistry, B.T.R. for undergraduate research grants). We thank the NSF for funding the purchase of the NMR spectrometer (NSF-MRI award 0923051). We thank Dr. Stephen Barlow (Georgia Institute of Technology) for collecting cyclic voltammetry data and for helpful discussions. 2276
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Organometallics
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