ARTICLE pubs.acs.org/Organometallics
Electronic Communications in (Z)-Bis(ferrocenyl)ethylenes with Electron-Withdrawing Substituents Pavlo V. Solntsev,† Semen V. Dudkin,†,‡ Jared R. Sabin,† and Victor N. Nemykin*,† † ‡
Department of Chemistry and Biochemistry, 1039 University Drive, University of Minnesota, Duluth, Minnesota 55812, United States Organic Intermediates and Dyes Institute, 1/4 B. Sadovaya Street, Moscow 103787, Russia
bS Supporting Information ABSTRACT: Orange dimethyl (Z)-2,3-bis(ferrocenyl)-2-butenedioate (1) and blue (Z)-2,3-bis(ferrocenyl)maleimide (2) were prepared and characterized using NMR, IR, and UVvis spectroscopy as well as X-ray crystallography. X-ray crystallographic studies of 1 and 2 revealed a locked anti conformation of the ferrocene substituents, which was explained on the basis of the steric properties of these groups. Electronic structures and solvatochromic properties of 1 and 2 were investigated by UVvis spectroscopy and polarized continuum model (PCM) density functional theory (DFTPCM) as well as time-dependent DFT (TDDFT-PCM) approaches. The calculated vertical excitation energies are consistent with the experimental data and clearly suggest the dominance of metal-to-ligand charge-transfer bands in the visible region of the UVvis spectra for 1 and 2. Redox properties of 1 and 2 were investigated using cyclic voltammetry, differential pulse voltammetry, and spectroelectrochemical and chemical oxidation approaches. In a DCM/(NBu4)[B(C6F5)4] system, ferrocene-centered oxidation processes in 1 and 2 are separated by 300 and 345 mV, respectively. Stepwise chemical and spectroelectrochemical oxidation of 1 and 2 allowed us to obtain spectroscopic signatures of the mixed-valence 1þ and 2þ cations. Hush analysis of the intervalence charge-transfer bands in 1þ and 2þ is suggestive of class II (in Robin and Day classification) behavior.
’ INTRODUCTION Preparation of nanometer-scale molecular modules with desired optoelectronic, redox, or conductivity properties is of great interest for modern technology.1 Because of their well-defined and robust redox properties, mono- and poly(ferrocene)-containing compounds were suggested as potential candidates for molecular electronics.2 Among these, poly(ferrocene)-containing compounds with a strong metalmetal coupling are especially interesting from fundamental and practical points of view.3 Specifically, these molecules were intensely studied because of their fundamentally interesting multiredox, magnetic coupling, and unpaired electron density migration properties.4 Such nanomeric-sized multinuclear switchable arrays are also interesting from a practical point of view, because they can be potentially used in molecular electronics, quantum cellular automata, and optoelectronic materials for application in high-speed photonic or redox devices.5 In many cases, formation of the mixed-valence (MV) states in poly(ferrocenyl)-containing complexes are responsible for the aforementioned desired properties. Multinuclear ferrocene derivatives with MV states have been known for many years, and the factors affecting their formation and stability have been thoroughly investigated.313 In most cases, iron centers should be located at a distance less than 56 Å to achieve effective metalmetal coupling between ferrocene units in the same molecule, while examples of long-range (∼10 Å) r 2011 American Chemical Society
metalmetal couplings in polyferrocenyl-containing systems are still rare.14,15 The most popular and well-explored diferrocenyl-containing complexes with well-defined MV properties include systems with direct ferroceneferrocene bonds, compounds in which two ferrocene units are separated by a single atom(s) (FcXFc; X = CR2, BR, SiR2, etc.),7,8,12 complexes with ethylene-bridged ferrocene substituents (Fc(R1)CdXFc; X = CR2, N),9,10 and acetyleneferrocenes (Fc[CtC]nFc).11 When ethylenebridged diferrocenes are considered, the additional substituents R1 and R2 include alkyl, alkenyl, alkynyl, aryl, or complex electron-donating groups,9,10 while the influence of the electron-withdrawing groups on the MV properties in these compounds has never been investigated (although some examples of the mono(ferrocenyl) to electron withdrawing group dyads are known).8e In order to investigate such an influence, in this paper, we report syntheses, spectroscopic characterization, and crystal structures for conformationally flexible (with respect to the ester group rotation around CC bond) dimethyl (Z)-2,3-bis(ferrocenyl)-2-butenedioate (1) and conformationally rigid (Z)-2,3-bis(ferrocenyl)maleimide (2) (Scheme 1). MV properties of these complexes were investigated using electrochemical, spectroelectrochemical, and chemical oxidation approaches. Received: February 9, 2011 Published: May 10, 2011 3037
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Table 1. Summary of Crystallographic Data for Compounds 1 and 2 1
2
empirical formula
C26H24Fe2O4
C25H21Cl2Fe2N1O2
formula wt cryst syst
512.17 triclinic
550.05 triclinic
space group, Z
P1, 2
P1, 2
a (Å)
8.993(2)
10.253(2)
b (Å)
11.639(2)
11.074(2)
c (Å)
11.748(2)
11.120(2)
R (deg)
93.01(3)
63.18(3)
β (deg)
102.02(3)
80.01(3)
γ (deg) V (Å3)
112.70(3) 1097.2(5)
87.34(3) 1108.9(5)
Fcalcd (g/cm3)
1.550
1.647
μ(Mo KR) (mm1)
1.353
1.573
θmax (deg)
27.52
27.53
GOF (F2)
0.9290
1.0307
R1a (F2 > 2σ(F2))
0.0325
0.0404
wR2b (all data)
0.0925
0.1006
ΔFmax/ΔFmin (e/Å3)
0.57/0.47
0.61/0.94
R1(F) =∑||Fo| |Fc||/∑|Fo|. b wR2(F2) = {∑[w(Fo2 Fc2)2]/∑w(Fo2)2]}1/2. a
Figure 1. ORTEP drawings of complex 1 (a) and 2 (b) with thermal ellipsoids set at the 50% probability level.
Experimental data discussed below for 1 and 2 allow for the first time an insight into the influence of the electron-withdrawing substituents on the MV properties of ethylene-bridged diferrocenes.
’ RESULTS AND DISCUSSION Complex 1 was prepared by the palladium-catalyzed reaction between diferrocenylmercury and the dimethyl ester of acetylenedicarboxylic acid following a procedure published earlier.16 We also explored several synthetic pathways to convert ester 1 into imide 2. Specifically, the following three approaches were considered: (i) reaction between 1 and concentrated ammonia at 0 °C as well as room temperature, (ii) reaction between 1 and formamide, and (iii) reaction between 1 and sodium amide in liquid ammonia. The first strategy results in no conversion of the orange ester 1 into the blue imide 2, while the second approach gives only trace amounts of the target compound 2 and large
quantities of black insoluble tar as the major reaction product. The reaction between ester 1 and sodium amide in liquid ammonia, however, allowed us to isolate imide 2 in 83% yield. The imide NH proton could be clearly identified as a singlet located at 7.14 ppm in the 1H NMR spectrum of 2. The stronger electron-withdrawing properties of an imide functional group compared to the ester groups in 1 result in downfield shifts of the proton signals of the ferrocene substituents in 1H NMR and signals of the carbonyl carbons in 13C NMR spectra of 2. In IR spectra, initial ester CdO signals initially located at 1728 cm1 in 1 shifted to 1718 cm1 upon transformation of 1 into 2, and similar shifts were observed for the CdC bond stretch. The ultimate determination of the structures of complexes 1 and 2 was further gained on the basis of their X-ray crystal structures (Figure 1 and Supporting Information Figure 1). Refinement parameters for 1 and 2 are presented in Table 1, while their selected bond lengths and angles are summarized in Table 2. The crystal structure of 1 represents another polymorph (referred to as 10 below) of dimethyl (Z)-1,2-bis(ferrocenyl)-2butenedioate reported earlier by Beletskaya and co-workers (see the Supporting Information for further discussion).16 ORTEP diagrams for 1 and 2 are shown in Figure 1. Complex 2 crystallizes with one molecule of dichloromethane per molecule of 2, while compound 1 has no solvent molecules in the unit cell. In the crystal structures of 1 and 2, ferrocene substituents are locked in the anti conformation with respect to each other, which represents the conformation with the smallest steric interactions between bulky ferrocene substituents. Ferrocenyl substituents adopt a typical geometry for monosubstituted ferrocenes: the cyclopentadienyl rings are essentially planar with distances from cyclopentadienyl centroids to Fe in the range of 1.649(2)1.661(1) Å for 1 and 1.640(1)1.661(2) Å for 2. The torsion angle Cipso(Fc)CdCCipso(Fc) in 1 (2.8(5)°) is close to that reported for 10 (6.0(7) and 8.7(7)° for two independent molecules in the unit cell). At the same time, the torsion angle 3038
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Table 2. Selected Bond Lengths (Å) and Angles (deg) for 1 and 2 Compound 1 Fe 3 3 3 π (centroid) C(1)C(21)
1.649(2)1.661(2)
C(21)C(24)
1.345(3)
1.475(3)
C(24)C(25)
1.503(3)
C(11)C(24)
1.478(3)
C(21)C(22)
1.503(3)
C(1)C(21)C(22) C(11)C(24)C(25)
116.4(2) 114.5(2)
a
C(1)C(21)C(24) C(11)C(24)C(21)
122.7(2) 124.5(2)
C(5)C(1)C(21)C(22)
49.0(4)
C(2)C(1)C(21)C(24)
48.1(4)
C(15)C(11)C(24)C(21)
45.4(5)
C(12)C(11)C(24)C(25) C(1)C(21)C(24)C(11)
48.4(4) 2.8(5)
C(21)C(24)C(25)O(4)
58.8(4)
C(24)C(21)C(22)O(2)
48.1(4) Compound 2
Fe 3 3 3 π (centroid) C(1)C(21)
1.640(1)1.661(2)
C(23)C(24)
1.513(4)
1.464(3)
O(1)C(22)
1.214(4)
C(23)C(11)
1.452(4)
O(2)C(24)
1.205(4)
C(21)C(23) C(21)C(22)
1.354(4) 1.496(3)
N(1)C(22) N(1)C(24)
1.386(4) 1.377(4)
b
C(1)C(21)C(23)
129.2(2)
C(24)C(23)C(11)
122.4(2)
C(1)C(21)C(22)
122.1(2)
C(23)C(24)N(1)
106.0(2)
C(23)C(21)C(22)
108.6(2)
C(24)N(1)C(22)
111.8(2)
C(21)C(23)C(24) C(21)C(23)C(11)
107.5(2) 130.1(2)
C(21)C(22)N(1)
105.9(2)
C(4)C(3)C(7)C(12)
36.2(6)
C(2)C(3)C(7)C(8)
35.6(6)
C(19)C(14)C(8)C(9) C(16)C(14)C(8)C(7)
20.4(6) 21.9(6)
C(3)C(7)C(8)C(14)
10.1(7)
C(9)C(8)C(7)C(12)
5.0(4)
a
Ring centroids were built on C1C2C3C4C5, C6C7C8C9C10, C11C12C13C14C15, and C16C17C18C19C20 atoms, respectively. b Ring centroids were built on C1C2C3C4C5, C6C7C8C9C10, C11C12C13C14C15, and C16C17C18C19C20 atoms, respectively.
Figure 2. Experimental (DCM, top) and TDDFT-PCM predicted (DCM, bottom) UVvis spectra of complexes 1 (left) and 2 (right). Excited state compositions are provided in Supporting Information Table 1.
Cipso(Fc)CdCCipso(Fc) in 2 is slightly larger (10.2(6)°). Average FeC distances are 2.05 and 2.04 Å for 1 and 2, respectively, and are consistent with those reported in the literature.17,18 Ester groups in 1 are not coplanar with the CdC plane. The corresponding dihedral angles between the
CdC plane and ester groups are 58.8(4) and 48.1(1)°, respectively. The imide fragment in 2 is essentially planar, with the largest deviation being 4.8(3)°. The most striking difference between complexes 1 and 2 is their UVvis spectra (Figure 2). In both cases, two intense bands 3039
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Table 3. Molecular Orbital Compositions of Compounds 1 and 2a Compound 1 composition, % MO
E, eV
Fe
Cp
CdC
CO2Me
123
7.111
6.50
87.66
2.02
3.82
124
7.038
5.66
88.33
2.03
3.98
125
6.913
5.59
93.02
0.87
0.52
126
6.529
72.04
20.45
6.63
0.88
127
6.483
89.09
9.84
0.63
0.44
128
6.294
28.72
52.61
15.85
2.82
129 130
5.697 5.677
66.40 66.76
32.45 31.78
0.72 0.62
0.43 0.84
131
5.655
70.32
29.23
0.19
0.26
132
5.548
62.75
32.14
4.45
0.66
133
1.606
12.8
23.68
43.66
19.86
134
0.367
44.67
52.59
0.37
2.37
135
0.296
41.59
54.62
1.93
1.86
136
0.281
38.63
55.04
1.20
5.13
137 138
0.205 0.166
47.16 86.41
45.35 9.35
2.01 0.93
5.48 3.31
Compound 2 amt, % MO
E, eV
Fe
Cp
CdC
C(O)NHC(O)
110
7.176
7.46
86.29
3.68
2.57
111
7.071
5.97
92.77
0.26
1.00
112 113
6.957 6.581
4.75 72.85
86.13 20.51
2.67 5.04
6.45 1.60
114
6.545
89.01
9.97
0.87
0.15
115
6.306
30.23
49.74
15.17
4.86
116
5.763
65.78
32.68
1.06
0.48
117
5.731
68.55
30.57
0.27
0.61
118
5.717
70.04
29.41
0.31
0.24
119
5.562
60.83
32.65
5.23
1.29
120 121
2.606 0.424
10.36 42.88
13.12 52.19
35.52 0.55
41.00 4.38
122
0.377
39.39
57.89
0.35
2.37
123
0.356
124 125
0.267 0.211
41.25 46.8
53.43 51.88
3.57 0.79
1.75 0.53
86.83
10.96
1.00
1.21
a
The HOMO is given in bold, while the LUMO is given in bold italics.
(labeled as band I and band II) have been observed in the UVvis region. The more intense band was observed at ∼26 600 cm1 in both complexes, while the energy of the lowenergy band (band I) is quite different. It is 21 415 cm1 (log ε = 3.13) for 1 and only 16 920 cm1 (log ε = 3.81) for 2. As a result, ester 1 is orange, while complex 2 is blue in solution and the solid state. Since vinylferrocenes substituted with electron-withdrawing groups usually exhibit strong solvatochromic behavior associated with the low-energy MLCT band, we tested the solvatochromic behavior of imide 2 in a variety of solvents (Supporting Information Figure 2). It has been found that solvatochromic
Figure 3. Molecular orbital energy diagram for complexes 1 (left) and 2 (right) calculated at the DFT-PCM level.
shifts of the low-energy MLCT band are rather small (∼600 cm1) and thus will not be discussed below. Solvatochromic studies allow us, on the other hand, to test the stability of complexes 1 and 2 in solution, and we found no detectable degradation of the target compounds in any solvent tested when they were kept in the dark for one week (Supporting Information Figure 3). In order to explain the dramatic difference (∼4500 cm1) in the UVvis spectra of 1 and 2 as well as get insight into the electronic structure and nature of bands I and II, DFT-PCM and TDDFT-PCM calculations were performed on target compounds, which have been proven to provide reliable electronic structures as well as transition energies and intensities in a large variety of ferrocene derivatives, including ferrocenes directly connected to electron-withdrawing groups.19 Molecular orbital contributions and energies for the targets 1 and 2 are shown in Table 3, Figure 3, and Supporting Information Figure 4, respectively, while key frontier MOs are also depicted in Figure 3. Similar to the case for other ferrocene compounds with electronwithdrawing groups, iron-centered MOs are dominant in the HOMO energy region. Specifically, two sets of the iron-centered dxy, dx2y2, and dz2 orbitals are observed in HOMO to HOMO-3 and HOMO-5 to HOMO-6 orbitals. Interestingly, HOMO-4 has predominantly π character and is largely delocalized over the cyclopentadiaenyl rings and CdC fragment. The redox-active HOMO has ∼95% and ∼94% contributions from ferrocene substituents in 1 and 2, respectively. The LUMO in complexes 1 and 2 has predominant π character and is delocalized over the entire molecule. The degree of such delocalization, however, is very different for complexes 1 and 2. Indeed, the ester groups in complex 1 are not coplanar with the CdC bond plane. As a result, the ester groups cannot effectively contribute to the LUMO π orbital (the total contribution of two ester groups to the LUMO is 20%) and, thus, it consists of a 43% contribution from the CdC bond and a 37% contribution from the ferrocene substituents. The imide fragment in complex 2, on the other hand, is coplanar with the CdC bond plane and, as a result, the contribution of the imide fragment to the LUMO is very significant (41%) and is actually larger than that provided by the CdC bond (36%), while the contribution of the ferrocene substituents is relatively small (23%). Despite large differences in the nature of the LUMO in complexes 1 and 2, their LUMOþ1 3040
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Figure 4. CV (solid lines) and DPV (dashed lines) data for complexes 1 (top) and 2 (bottom).
Table 4. Electrochemical Data for 1 and 2 E1/2I, mV
a
E1/2II, mV
ΔE1/2, mV
Kc
ΔGc°
1
40
340
300
119 209
28 946
2
96
441
345
688 265
33 287
3a
67
92
159
490
15 341
4a
77
93
170
752
16 403
5a
90
82
172
813
16 595
Reference 9a; in CH3CN/0.1 M [Bu4N]PF6.
to LUMOþ4 orbitals have almost identical compositions. It is interesting to see that the energies of LUMOþ1 to LUMOþ6 and HOMO to HOMO-8 orbitals are quite close in complexes 1 and 2. Because of the very dissimilar delocalization, however, the LUMO's in compounds 1 and 2 have a large (∼1 eV) energy difference. As a consequence, this leads to a large difference in HOMOLUMO energy gap in complexes 1 and 2 (Figure 3). This difference, in turn, is responsible for the different vertical excitation energies in these compounds and thus results in their different colors. Overall, although the electron-withdrawing strengths of the ester groups in complex 1 and imide fragment in compound 2 are quite comparable, a coplanar imide group in 2 significantly changes the electron density distribution in the LUMO region, which ultimately affects excited state energies. On the other hand, the energies of the HOMO region MOs in complexes 1 and 2 remain close to each other and are dominated by the ferrocene substituents. Experimental and TDDFT-PCM predicted UVvis spectra of target complexes 1 and 2 are presented in Figure 2. As mentioned above, two intense bands are observed in the 10 000 35 000 cm1 region of the UVvis spectra of complexes 1 and 2. From the DFT-PCM predicted electronic structure of complexes 1 and 2, it is possible to expect four principal types of transitions in their UVvis spectra: (i) HOMO-4 f LUMO predominantly π(Cp, CdC) f π*(CdC, ester/imide) transition, (ii) HOMO HOMO-3, HOMO-5 HOMO-6 f LUMO predominantly Fe(Fc) f π*(CdC, ester/imide) metal-to-ligand (MLCT) transitions, (iii) HOMO-4 f LUMOþ 1 LUMOþ4 predominantly π(Cp, CdC) f π* (Fc) transitions, and (iv) HOMO HOMO-3, HOMO-5 HOMO-6 f LUMOþ1 LUMOþ4 predominantly Fe(Fc) f π*(Fc) MLCT transitions. In agreement with its electronic structure,
TDDFT-PCM calculations suggest that the low-energy band I in complex 1 is dominated by three MLCT Fe(Fc) f π*(Cp) transitions (excited states 1, 2, and 5; Supporting Information Table 1 and Figure 2). Each of these three transitions has a complex composition. Excited states 1 and 2 predominantly consist of HOMO HOMO-3 f LUMOþ1 LUMOþ3 transitions, while excited state 5 is dominated by HOMO-5 HOMO-6 f LUMOþ1 LUMOþ3 transitions. It should be noted, however, that HOMO, HOMO-1 f LUMO (excited states 1 and 2) as well as HOMO, HOMO-6 f LUMO (excited state 5) MLCT transitions also make a significant contribution to the band I intensity. Similarly, the high-energy band II for complex 1 is dominated by excited states 9, 11, and 13 (Supporting Information Table 1, Figure 2). The first two mainly consist of HOMO HOMO-1 f LUMO MLCT (Fe(Fc) f π*(CdC, ester)) transitions, while the last state also has a significant contribution from the HOMO-4 f LUMO π f π* transition. Stabilization of the LUMO energy in imide 2 results in a situation where the low-energy band I is dominated by the first and second excited states, which have HOMO HOMO-1 f LUMO (MLCT Fe (Fc) f π*(CdC, imide)) transitions as major contributions. Similarly, excited state 13, which consists of an almost pure HOMO-4 f LUMO ππ* transition (Supporting Information Table 1 and Figure 2), provides the major contribution to the intensity of the high-energy band II. Overall, experimental and TDDFT-PCM data on complexes 1 and 2 are in agreement with the previously discussed cyanovinyl ferrocenes and suggestive MLCT character of the low-energy band I. In order to investigate redox properties and potential metal metal coupling in target complexes 1 and 2, we conducted cyclic voltammetry (CV) and differential pulse voltammetry (DPV) experiments in a low-polarity solvent (DCM) using a noncoordinating electrolyte ([NBu4][B(CF5)4]). From numerous publications and evidence collected by Geiger’s group, such a solvent/electrolyte combination provides the best possibility of reducing solvent and electrolyte interactions with the electrochemically generated oxidized forms of poly(metallocenyl)containing compounds and thus increases the accuracy for investigation of their mixed-valence properties.20 Experimental CV and DPV data for 1 and 2 are presented in Figure 4, while their redox potentials are given in Table 4. In both cases, two clearly defined reversible oxidation processes, which correspond to the sequential oxidation of two ferrocene groups, have been observed. Experimentally observed differences between the first and second oxidation waves are 300 and 340 mV for complexes 1 and 2, respectively. As expected, because of the electronwithdrawing nature of the substituents, the oxidation potentials for compounds 1 and 2 are higher than those reported for similar (E)-1,2-dimethyldiferrocenylethylene (3), (Z)-1,2-dimethyldiferrocenylethylene (4), and (Z)-1,2-diferrocenylcyclohexene (5), which were previously reported (Table 4).9a In addition, an irreversible reduction wave was observed in CV and DPV experiments of 2, which could be associated with the oneelectron reduction of the imide group followed by its chemical transformation into an unidentified product. The large separation between the first and second oxidation waves allowed us to characterize electrochemically generated mixed-valence cations 1þ and 2þ as well as dications 12þ and 22þ discussed below. Well-defined redox waves for complexes 1 and 2 also allow us to estimate the comproportionation constant Kc from electrochemical data, following well-known approaches.21 3041
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Figure 5. Spectroelectrochemical oxidation of complexes 1 (left) and 2 (right) in the DCM/TFAB system.
Indeed, the mixed-valence forms 1þ and 2þ are in equilibrium with homovalent forms (i.e., 1 and 12þ or 2 and 22þ, respectively; eq 1) and related to two redox potentials observed for these systems, i.e., E1/2I(FcFcþ/FcFc) and E1/2II(FcþFcþ/ FcFcþ). Therefore, one can evaluate the comproportionation constant Kc from the electrochemical data.22 Kc
Fc Fc þ Fcþ Fcþ S 2Fc Fcþ E1
E2
Fc Fc S Fc Fcþ S Fcþ Fcþ Kc ¼
½Fc Fcþ 2 ½Fc Fc½Fcþ Fcþ
ð1Þ
Specifically, the comproportionation constant, Kc, could be easily calculated using eq 2, while the free energy change for the comproportionation, ΔGc, could be calculated using eq 3, where ΔE1/2 = E1/21 E1/2II and F is the Faraday constant. Values of Kc and ΔGc° for complexes 1 and 2 calculated using such an approach are summarized in Table 4. For reasons of comparison these values for similar, previously reported diferrocene-containing compounds 35 are also included.9a ΔE1=2 F Kc ¼ exp ð2Þ RT ΔGc ¼ RT ln Kc ¼ ΔE1=2 F
ð3Þ
It is currently commonly accepted that the solvent polarity as well as the nature of the electrolyte can significantly change oxidation potentials in poly(transition metal) complexes, including poly(ferrocenyl)-containing compounds. This, in turn, can dramatically change values of Kc and ΔGc° calculated from electrochemical data.21,23 Thus, the electrochemically based values of Kc and ΔGc° discussed in Table 4 should be treated with great caution. This is especially true when the values of Kc and ΔGc° are discussed for experiments conducted in different solvents and electrolytes (i.e., experiments for compounds 1 and 2 were conducted in DCM/TFAB system, while those for compounds 35 were collected in the more polar CH3CN solvent and more coordinating [Bu4N]PF6 electrolyte). As has been shown before, on the other hand, parameters of the IVCT band(s) in MV compounds can be used for a more accurate description of the MV states.24,25 MV 1þ and 2þ complexes could be generated in situ by either stepwise chemical or electrochemical oxidation of the initial neutral 1 and 2.
Spectroelectrochemical oxidation data for complexes 1 and 2 are presented in Figure 5. During spectroelectrochemical oxidation of 1 into MV 1þ, the intensity of the low-energy MLCT band at 467 nm decreases, while the intensity of the higher energy, predominantly ππ* band increases. In addition, new bands at 579 nm (with a shoulder at ∼750 nm) and ∼1900 nm appear in the UVvisnear-IR spectrum. The former band and a low-energy shoulder might consist of overlapping MLCT and LMCT transitions. In particular, the classic ferrocenium CT band, usually located at ∼650 nm, might be expected for both mono- and dications of compounds 1 and 2. The near-IR band is located at the usual IVCT energy envelope and thus was tentatively assigned as an IVCT transition. In order to confirm this assignment, MV 1þ was further oxidized into 12þ. During this oxidation, the near-IR band at ∼1900 nm disappears, confirming its IVCT assignment. In addition, the intensity of the band at 579 nm decreases, and a new band characteristic of LMCT transitions of ferrocenium ion appears at 640 nm in the UVvisnear-IR spectrum of 12þ. Similar to the case for spectroelectrochemical oxidation data, chemical oxidation of 1 into 1þ and 12þ could be achieved by using silver triflate or nitrosonium tetrafluoroborate in DCM (Supporting Information Figure 5). It is important to note that both spectroelectrochemical and chemical oxidation data are virtually the same, confirming the near-IR IVCT band assignment. Spectroelectrochemical oxidation of complex 2 is consistent with data collected for compound 1. Indeed, transformation of 2 into 2þ results in the appearance of the IVCT band in the near-IR region, an increase in the intensity of the ππ* band at 375 nm, and a red shift of the initial MLCT band at 580 nm to a new intense band at 680 nm (Figure 5). Further oxidation of the MV imide 2þ cation into the 22þ dication leads to disappearance of the IVCT band and a further red shift of the band at 680 nm. Chemical oxidation of the initial imide 2 with nitrosonium tetrafluoroborate results in clean formation of the MV imide 2þ (Supporting Information Figure 6). Our further attempts to transform 2þ into the 22þ dication using the same oxidant failed because of side reactions, which could include possible interaction of the NH groups with NOþ cation.26 Similarly, titration of the initial 2 with silver triflate does not produce MV imide 2þ, probably because of the well-known coordination of the Agþ ion to the nitrogen atom.27 Again, when a comparison between spectroelectrochemical and chemical oxidation experiments is valid, the spectroscopic signature of the MV 2þ cation remains independent of the nature of the oxidant. In order to gain insight into the magnitude of metalmetal coupling in compounds 1 and 2, IVCT band deconvolution 3042
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Organometallics
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Table 5. Spectroscopic Data for the IVCT Band and Electronic Coupling Parameters for Mixed-Valence Species of 1 and 2 1þ þ
νmax, cm1
Δν1/2, cm1
εIVCT, L mol1 cm1
fa
5212
4127
903
1.71 102 2
|M|,a Å esu
Hab,a cm1
R
9.19 102
4.41 1011
1
D,a Å
425
0.0816
2.69 10
2.30 10
1.10 1010
536
0.1037
460
8.10 103
0.35
1.69 1010
284
0.047
340
5.96 103
0.30
1.46 1010
270
0.045
2
5166
2675
2184
3þb
6020
3830
5þb
5951
3811
Calculated from f = (4.6 109)εIVCTΔν1/2, D = [f/((1.085 105)νmax)]1/2, |M| = (4.803 103)D, and Hab = (2.06 102)(εIVCTΔν1/2νmax)1/2/rmm. b Data for compounds 35 were taken from ref 9a. a
analysis was applied for MV 1þ and 2þ cations. In both cases, IVCT can be modeled using a single Gaussian shaped band, suggesting that both of the cations belong to the weakly coupled systems (Supporting Information Figures 7 and 8). Spectroscopic parameters for the IVCT band of MV cations 1þ and 2þ are summarized in Table 5. Several other numeric parameters were also calculated using experimental spectroscopic data and are included in the table, including oscillator strength (f), dipole strength (D), transition dipole moment (|M|), and electronic coupling matrix element (Hab).24,25 Metalmetal distances used in the evaluation of the aforementioned parameters were taken from the X-ray data and are 6.75 and 6.68 Å for 1 and 2, respectively. Also, the ground state delocalization parameter R = Hab/νmax was evaluated as a diagnostic for the extent of electronic delocalization for mixed-valence 1þ and 2þ species. In spite of comparable electron-withdrawing effects of the substituents in 1 and 2, the IVCT band of the MV cation 1þ has low band intensity (ε) and half-width (Δν1/2). As the result, the estimated values of the electronic coupling matrix element and delocalization parameter are quite small (Hab = 425 cm1 and R = 0.0816, respectively). These parameters could reflect two key factors in the MV 1þ. First, since ester groups in 1 are rotated from the CdC bond plane, it could be expected that the degree of delocalization in 1þ will be smaller compared to that in 2þ. Second, it is expected that the bond order of the CdC bond in 1þ will be lower compared to that in 1 and thus 1þ can irreversibly invert from the Z to the E isomer similarly to Z/E inversion of the MV (Z)-1,2-dimethyldiferrocenylethylene cation.9a In order to eliminate the second possibility, we kept a solution of the chemically generated MV 1þ in DCM for 2 weeks and then compared its 1H NMR spectrum (after reduction of 1þ into 1 with triethylamine) to that of an authentic sample of 1. Since no change in the 1H NMR spectrum was observed, it seems unlikely that 1þ undergoes Z/E isomerization. In contrast to the MV 1þ, the MV cation 2þ has significantly higher values for Hab and R (536 cm1 and 0.1037, respectively). Both MV cations (1þ and 2þ) fall within the weakly coupling class II mixedvalence compounds in the Robin and Day classification.25 It is interesting to note that the values of Hab and R for 2þ are slightly higher than those reported for for (E)-1,2-dimethyldiferrocenylethylene (Hab = 284, R = 0.047) and (Z)-1,2-diferrocenylcyclohexene (Hab = 270, R = 0.045).
’ CONCLUSIONS Dimethyl (Z)-2,3-bis(ferrocenyl)-2-butenedioate (1) and (Z)-2,3-bis(ferrocenyl)maleimide (2) have been prepared and characterized using NMR, IR, and UVvis spectroscopy as well as X-ray crystallography. X-ray crystallography of 1 and 2 reveal a locked anti conformation of the ferrocene substituents, which was explained on the basis of the steric properties of ferrocene
groups. The ester groups in 1 are noncoplanar with the CdC bond plane, while the imide fragment is almost coplanar with it. Electronic structures and solvatochromic properties of 1 and 2 were investigated by UVvis spectroscopy and DFT-PCM and TDDFT-PCM approaches. It has been found that the contribution of the imide group to the LUMO of 2 is approximately 2 times greater than the contribution of the ester groups in 1 to the same orbital. Such delocalization results in stabilization of the LUMO energy in 2 compared to 1. Not surprisingly, all TDDFTPCM predicted vertical excitation energies for visible bands in 2 are significantly shifted to the low-energy region. The vertical excitation energies calculated using the TDDFT-PCM approach in complexes 1 and 2 are consistent with the experimental data and clearly suggest the dominance of metal-to-ligand chargetransfer bands in the visible region of the UVvis spectra for 1 and 2. Redox properties of 1 and 2 were investigated using CV, DPV, spectroelectrochemical, and chemical oxidation approaches and reveal two sequential oxidations of ferrocene substituents. The differences between the first and the second oxidation potentials in complexes 1 (300 mV) and 2 (345 mV) in the DCM/(NBu4)[B(C6F5)4] system are quite large, and thus spectroelectrochemical oxidation of 1 and 2 allowed us to obtain spectroscopic signatures of the mixed-valence 1þ and 2þ cations as well as dications 12þ and 22þ. When a comparison between chemical and spectroelectrochemical oxidation experiments is possible, all oxidized species have identical spectroscopic signatures. Hush analysis of the IVCT bands in 1þ and 2þ were suggestive of class II (in the Robin and Day classification) behavior. The estimated values of the matrix coupling element Hab and degree of delocalization R are higher for the conformationally rigid 2þ in comparison to the conformationally flexible 1þ complex.
’ EXPERIMENTAL SECTION General Considerations. All commercial reagents were ACS grade and were used without further purification. For all electrochemical and spectroelectrochemical experiments, dichloromethane (DCM) was distilled over calcium hydride under an inert atmosphere prior to experiments. Solvents used in the preparation of complexes 1 and 2 were purified by standard methods and distilled under an atmosphere of dry nitrogen before use. Elemental analysis was conducted by Atlantic Microlab, Inc. NMR spectra were recorded on a Varian INOVA NMR spectrometer at 500 MHz (protons) and 125 MHz (carbons) frequencies using TMS as a reference. UVvis spectra were collected on a JASCO J-670 spectrophotometer. IR spectra were obtained using a Perkin-Elmer FT-IR spectrometer. CV and DPV experiments were carried out in a 0.05 M solution of [NBu4][B(C6F5)4] in dichloromethane using a CH electrochemistry system and a standard threeelectrode scheme with carbon working and platinum-wire auxiliary and reference electrodes. Potentials were corrected using an internal standard (ferrocene) in all cases. All electrochemical experiments were 3043
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Organometallics conducted using 0.15 M solutions of [NBu4][B(C6F5)4] in dichloromethane with a custom-made spectroelectrochemical cell.
Preparation of Dimethyl (Z)-2,3-Bis(ferrocenyl)-2-butenedioate (1). Complex 1 was prepared by the palladium-catalyzed reaction between diferrocenylmercury and the dimethyl ester of acetylenedicarboxylic acid following a procedure described earlier.16 Selected spectroscopic data for 1: 1H NMR (δ, ppm, TMS, CDCl3) 4.23 (4H, br s, R-Cp), 4.20 (4H, br s, β-Cp), 4.05 (10H, s, Cp H), 3.89 (6H, s, CH3); IR (KBr, cm1) 3099 (Ar H), 3083 (Ar H), 2949 (CH3), 1728 (CdO), 1652 (CdC), 1465, 1430, 1306, 1277, 1217, 1196, 1179, 1104, 1075, 1049, 1030, 1004, 908, 817, 682; UVvis (DCM, cm1 (log ε)): 26 666 (3.35), 21 413 (3.13). Preparation of (Z)-2,3-Bis(ferrocenyl)maleimide (2). A solution of 1 (374 mg; 0.7 mmol) in 25 mL of dry THF was added dropwise at 33 °C to a solution of sodium amide (168 mg; 7.3 mmol) in 50 mL of liquid ammonia. The reaction mixture was stirred for an additional 2 h at 33 °C and then was slowly warmed to room temperature. The resulting solution was left overnight to remove all ammonia. After this period of time, 100 mL of a 10% solution of ammonium chloride was added and the reaction mixture was stirred for 1.5 h and then extracted with chloroform (3 80 mL). All blue organic layers were combined, washed with solutions of sodium thiosulfate, sodium bicarbonate, and water, and finally dried over sodium sulfate. A chloroform solution was filtered, concentrated, and chromatographed on a SiO2 column using CH2Cl2/hexane (8/2 v/v) as eluent. A third blue band was collected and the solvent evaporated under reduced pressure. The target compound 2 was recrystallized from DCM/hexane. Yield: 270 mg (83%). 1H NMR (δ, ppm, TMS, CDCl3): 7.14 (s, 1H, NH), 4.83 (4H, br s, R-Cp), 4.19 (4H, br s, β-Cp), 4.13 (10H, s, Cp H). 13C NMR (δ, ppm, TMS, CDCl3): 169.70 (CdO), 134.90 (CdC), 73.60 (Cipso), 70.51 (R-Cp), 70.41 (Cp C), 70.37 (β-Cp). IR (KBr, cm1): 3356 (NH), 3030 (Ar H), 1783, 1764, 1718 (CdO), 1684 (CdC), 1484, 1384, 1338, 1108, 1048, 1028, 1003, 914, 820, 753. UVvis (DCM, cm1 (log ε)): 26 595 (4.01), 16 920 (3.81). Anal. Calcd for 2 3 CH2Cl2: C, 54.59; H, 3.85; N, 2.55. Found: C, 55.82; H, 3.94; N, 2.64. Calcd for 2 3 0.8CH2Cl2: C, 55.88; H, 3.90; N, 2.63. Found: C, 55.82; H, 3.94; N, 2.64. DFT-PCM and TDDFT-PCM Calculations. The initial geometries of complexes 1 and 2 were taken from the X-ray analysis and optimized at the DFT-PCM level of theory, using a hybrid PBE1PBE exchange-correlation functional, which was based on the pure functional of Perdew, Burke, and Ernzerhof and modified by Adamo.28 This exchange-correlation functional provided the best agreement between theory and experiment for structures of complexes 1 and 2 when tested against pure GGA (BP8629 and BPW9130) and hybrid B3LYP31 exchange-correlation functionals. TDDFT-PCM calculations for vertical excitation energies in complexes 1 and 2 were performed using a hybrid B3LYP exchange-correlation functional, which in the series of test calculations outperformed pure BP86 and BPW91 GGA as well as the hybrid PBE1PBE functional. The first 50 states were calculated for all TDDFT-PCM runs. In all calculations, Wachter’s full-electron basis set was used for iron32 centers and 6-311G(d)33 for all other atoms. Equilibrium geometries were confirmed by frequency calculations and specifically by the absence of the image frequencies. Solvation effects were modeled using the polarized continuum model (PCM) approach.34 DCM was used as the solvent in all calculations. All calculations were performed using Gaussian 03 or Gaussian 09 software.35 Molecular orbital analysis was conducted using the VMOdes 8.1 program.36 X-ray Crystallography. Single crystals suitable for X-ray crystallographic analysis of 1 were obtained by slow evaporation of a concentrated chloroform solution. Crystals of 2 were obtained by slow diffusion of pentane into a DCM solution. X-ray diffraction data were collected on a Rigaku AFC-7R diffractometer using graphite-monochromated Mo KR radiation (λ = 0.710 73 Å) at 293 K. ψ-scan
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absorption corrections were applied to the data using the TeXsan 10.3b program.37 The structures were solved by direct methods implemented in SIR-9238 and refined by full-matrix least squares based on F2 using CRYSTALS for Windows software.39 Distances from Fe atoms to the η5-C5H5 ring centroids were computed using PLATON40 software. PLATON/PLUTON software was used for visualization of the results.
’ ASSOCIATED CONTENT Supporting Information. CIF files giving crystallographic data for complexes 1 and 2 and text, tables, and figures giving DFT-PCM optimized coordinates and TDDFT-PCM predicted expansion coefficients for compounds 1 and 2, solvatochromic and stability studies for complex 2, chemical oxidation investigations on compounds 1 and 2, and IVCT band deconvolution for 1þ and 2þ. This material is available free of charge via the Internet at http://pubs.acs.org.
bS
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT Generous support from the NSF CHE-0809203, NSF MRI CHE-0922366 (X-ray diffractometer), and Minnesota Supercomputing Institute to V.N.N. as well as University of Minnesota Duluth Undergraduate Research Opportunity Grants to J.R.S. and a Summer Undergraduate Research Grant to S.V.D. is greatly appreciated. ’ REFERENCES (1) (a) Heath, J. R.; Ratner, M. A. Phys. Today 2003, 56, 43–49. (b) Chen, J.; Lee, T.; Su, J.; Wang, W.; Reed, M. A. Encycl. Nanosci. Nanotechnol. 2004, 5, 633–662. (c) Weiss, J. Coord. Chem. Rev. 2010, 254, 2247–2248. (d) Nano and Molecular Electronics Handbook, Lyshevski, S. E., Ed.; CRC Press: New York, 2007. (e) Bayley, H. Nature (London, U.K.) 2010, 467, 164–165. (f) Giacalone, F.; Martin, N. Adv. Mater. (Weinheim, Ger.) 2010, 22, 4220–4248. (g) Jurow, M.; Schuckman, A. E.; Batteas, J. D.; Drain, C. M. Coord. Chem. Rev. 2010, 254, 2297–2310. (h) Chung, A.; Deen, J.; Lee, J.-S.; Meyyappan, M. Nanotechnology 2010, 21, 412001/1–412001/22. (i) Belosludov, R. V.; Farajian, A. A.; Baba, H.; Mizuseki, H.; Kawazoe, Y. Jpn. J. Appl. Phys., Part 1 2005, 44, 2823–2825. (j) Belosludov, R. V.; Farajian, A. A.; Kikuchi, Y.; Mizuseki, H.; Kawazoe, Y. Comput. Mater. Sci. 2006, 36, 130–134. (k) Lee, S. U.; Belosludov, R. V.; Mizuseki, H.; Kawazoe, Y. J. Phys. Chem. C 2007, 111, 15397–15403. (l) Lee, S. U.; Belosludov, R. V.; Mizuseki, H.; Kawazoe, Y. Small 2008, 4, 962–969. (2) (a) Fabre, B. Acc. Chem. Res. 2010, 43, 1509–1518. (b) Mizuseki, H.; Belosludov, R. V.; Uehara, T.; Lee, S. U.; Kawazoe, Y. J. Korean Phys. Soc. 2008, 52, 1197–1201. (c) Uehara, T.; Belosludov, R. V.; Farajian, A. A.; Mizuseki, H.; Kawazoe, Y. Jpn. J. Appl. Phys., Part 1 2006, 45, 3768–3771. (d) Kaim, W.; Lahiri, G. K. Angew. Chem., Int. Ed. 2007, 46, 1778–1796. (e) Kaim, W.; Sarkar, B. Coord. Chem. Rev. 2007, 251, 584–594. (e) Chisholm, M. H.; Patmore, N. J. Acc. Chem. Res. 2007, 40, 19–27. (f) Arumainayagam, C. R.; Lee, H.-L.; Nelson, R. B.; Haines, D. R.; Gunawardane, R. P. Surf. Sci. Rep. 2010, 65, 1–44. (3) (a) Astruc, D.; Ornelas, C.; Ruiz Aranzaes, J. J. Inorg. Organomet. Polym. Mater. 2008, 18, 4–17. (b) Santi, S.; Orian, L.; Donoli, A.; Bisello, A.; Scapinello, M.; Benetollo, F.; Ganis, P.; Ceccon, A. Angew. Chem., Int. Ed. 2008, 47, 5331–5334. (c) Cowan, D. O.; LeVanda, C.; Park, J.; Kaufman, F. Acc. Chem. Res. 1973, 6, 1–7. 3044
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