Density Functional Investigations on the Charge Distribution

Mar 4, 2010 - Electronic structure, charge distribution, and vibrational frequencies of cucurbit[n]uril, CB[n] (n = 5−12), hosts have been derived u...
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Density Functional Investigations on the Charge Distribution, Vibrational Spectra, and NMR Chemical Shifts in Cucurbit[n]uril (n ) 5-12) Hosts Vivekanand V. Gobre, Rahul V. Pinjari, and Shridhar P. Gejji* Department of Chemistry, UniVersity of Pune, Pune 411007, India ReceiVed: January 30, 2010; ReVised Manuscript ReceiVed: February 20, 2010

Electronic structure, charge distribution, and vibrational frequencies of cucurbit[n]uril, CB[n] (n ) 5-12), hosts have been derived using the density functional methods. CB[n] conformers with different orientations of methylene group connecting glycouril units have been investigated. The conformers that possess uniform CB[n] cavity turn out to be of lowest energy, and molecular electrostatic potential (MESP) herein engender shallow minima near ureido oxygens along the series. MESP topography has been utilized to estimate the cavity height and diameter; the ratio of which governs the shape (circular or elliptical) of the cavity. When this ratio is larger than unity (for CB[n] with n g 8), an elliptical host cavity is noticed. Calculated vibrational spectra reveal that carbonyl stretching frequency shift in successive CB[n] homologue decreases steadily from 1760 cm-1 in CB[5] to 1742 cm-1 in CB[12]. An increase in glycouril units along the CB[n] series influences significantly the intensity profile of CdO and C-N stretching vibrations in the calculated infrared spectra. Furthermore, calculated 1H chemical shifts predict that one of methylene protons directing outside the host cavity are deshielded, whereas the remaining proton near the carbonyl group exhibits downshifted signal in the NMR spectra. 1. Introduction Cucurbit[n]urils (CB[n]) have been the focus of attention as novel hosts in supramolecular chemistry owing to varying size and shape of their hydrophobic cavity, which facilitates the efficient and selective binding to cationic guests. The syntheses of CB[n] have been carried out by condensation of formaldehyde and glycouril units in acidic solution.1 A glycouril monomer unit of CB[n] has been shown in Figure 1 along with atomic numbering scheme. CB[6] has been one of the most illustrious cavitand,2 well noted for its remarkable features in host-guest complexation, molecular recognition, catalysis, and molecular switches phenomena.3 CB[6] forms 1:1 complexes with a number of alkylammonium salts where the guest was encapsulated within the host cavity.4 Complexation of CB[n] (n ) 5-8) with a variety of guests encompassing positively charged species and organic molecules has been well studied in the literature.5–7 The molecular attributes such as the rigid cage structure with two carbonyl-fringed portals and hollow interior of these hosts provide binding site with high specificity and make CB[n] fascinating hosts in molecular recognition.8 Recent experiments have shown that a variety of guests such as acids, alcohols, peptides, ferrocene, and cobaltocene can be encapsulated within CB[n] cavities.9–17 Lower CB[n] (n < 7) homologues form 1:1 host-guest complexes, whereas CB[7] and CB[8] are capable of forming 1:2 complexes as well. CB[n] hosts are thus explored in separation technology, supramolecular chemistry, drug delivery vehicles, and nanotechnology.18–30 Fourier transform infrared reflection absorption spectroscopy experiments31,32 have shown that formation of self-assembled monolayer of CB[7] or CB[8] on gold surface can be efficiently achieved. Syntheses and isolation of CB[n] (n ) 5-8 and 10) have been reported.33–35 It may further be remarked here that electron spray mass spectroscopy and 13C NMR spectroscopy * Corresponding author. Tel: +91 20 25691725. Fax: +91 20 25691728. E-mail: [email protected].

Figure 1. Atomic numbering scheme in glycouril unit.

experiments in reaction mixtures31,35 have identified the homologues up to n ) 16 along the CB[n] series. Recently, Isaacs and coworkers36 have isolated CB[10], which can accommodate CB[5] within its cavity and further shown that it is capable of encapsulating chemically and biologically important cationic molecules, as also noticed in lower CB[n] hosts. The multinuclear metal complexes partially encapsulated in CB[7-12] or analogs thereof have been utilized in cancer treatment methods.31 To understand binding patterns of CB[n] hosts to a variety of guests at the molecular level, the electron affinity and ionization potentials of CB[n] (n ) 5-10) macrocycles have been derived within the framework of local spin density approximation employing the minimal basis set.37 It was subsequently pointed out that uredio oxygens at the host portals become more negative with increasing size of the macrocycle. It should further be remarked here that CB[6] was predicted to be thermodynamically stable over the remaining CB[n] homologues, which was partially attributed to relatively planar ureido

10.1021/jp100904c  2010 American Chemical Society Published on Web 03/04/2010

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Figure 2. B3LYP optimized geometries of CB[12] isomers.

TABLE 1: Relative Stabilization Energies (in kJ mol-1) of Cucurbit[n]uril Conformers (n ) 8-12) ∆Erel CB[8]a 1,4-CB[8] 1,5-CB[8]

a

∆Erel CB[9]a 1,4-CB[9] 1,5-CB[9]

0.0 126.5 126.2

∆Erel 0.0 110.0 102.8

CB[10]a 1,4-CB[10] 1,5-CB[10] 1,6-CB[10]

∆Erel 0.0 98.6 88.5 85.1

CB[11]a 1,4-CB[11] 1,5-CB[11] 1,6-CB[11]

∆Erel 0.0 86.3 77.7 71.2

CB[12]a 1,4-CB[12] 1,5-CB[12] 1,6-CB[12] 1,7-CB[12]

0.0 75.8 70.4 62.8 61.2

Conformer with near circular cavity.

and methylene groups.38–40 The cavity dimensions and charge distribution within these hosts are of primary importance for understanding the host-guest interactions. As a pursuance to this, Pinjari and Gejji39,40 have recently analyzed systematically characteristic electronic distributions in the CB[n] (n ) 5-8) series and demonstrated how the shape and dimension of the host cavity can be gauged using the molecular electrostatic potential (MESP) topography that yields deeper insight into binding of guest at the molecular level. MESP investigations have led to the conclusion that host portals become less electronrich with the increasing number of glycouril units. To this end, encapsulation of ferrocene and its orientation within the CB[n] cavity has been analyzed recently39 within the framework of density functional theory. It should further be remarked here that theoretical reports on CB[n] hosts are primarily focused on n ) 5-8 only, and higher CB[n] homologues have not yet been explored. With this view, a systematic investigation on electronic structure, vibrational spectra, and 1H NMR chemical shifts of CB[n] (n ) 5-12) hosts have been carried out. The computational method is outlined below. 2. Computational Method Different conformers of CB[n] (n ) 5-12) homologues were optimized using the hybrid density functional theory incorporating the Becke’s three parameter exchange (B3) with Lee, Yang, and Parr’s (LYP) correlation functional.41,42 Optimizations were performed using the Gaussian 03 program.43 The internally stored 6-31G(d) basis set implemented within the Gaussian 03 program was employed. Stationary point geometries thus obtained were characterized as local minima on potential energy surface from the frequencies of normal vibrations, all of which turn out to be real. Normal vibrations were assigned by visualizing the displacement of atoms around their equilibrium positions.44 The MESP V(r) at a point r due to a molecular system with nuclear charges {ZA} located at {RA} and the electron density F(r) is given by N

V(r) )

Z

d r' ∑ |r -ARA| - ∫ F(r') |r - r'| 2

(1)

A

where N is the total number of nuclei in the molecule. The first and the second terms in the above equation refer to the bare nuclear potential and the electronic contributions, respectively. The balance of these two terms brings about the effective

localization of electron-rich regions in the molecular system. The negative regions in MESP represent sites for the interaction with electrophile. Topological analysis45,46 of V(r) based on the identification of the critical points (CPs) in MESP (where the gradient of V(r) vanishes) has been carried out subsequently. The rank of the CP was determined from the number of nonzero eigenvalues of the Hessian matrix. The CP is said to be nondegenerate if all eigenvalues of the Hessian matrix are nonzero. A nondegenerate CP can be characterized by an ordered pair (R, F) where R denotes the rank of Hessian matrix and F is the signature (algebraic sum of signs of the eigenvalues) Viz. (3,-1), (3, +1), and (3, +3) only. The (3, +3) CP represents the minimum, whereas the remaining (3,-1) or (3,+1) CPs correspond to a saddle points in the electrostatic potential. It may further be remarked here that the CP in electrostatic potential emerges as the signature of characteristic features of electronic distribution such as lone pairs, π-bonds, banana bonds, or aromatic character of the molecule. The function value at the CPs in the MESP topography can be correlated to reactivity toward electrophile. A program UNIVIS-2000 was used for visualization of the MESP topography.47–50 Nuclear magnetic shielding tensors of the atoms in CB[n] have been calculated using a gauge-independent atomic orbital method.51 The chemical shifts of protons (δH) in NMR spectra of CB[n] homologue estimated were by subtracting isotropic shielding constants of host protons from those in tetramethylsilane. The influence of solvent on energetics of CB[n] isomers and 1H chemical shifts in the NMR spectra was analyzed by employing the self-consistent reaction field theory incorporating the polarizable continuum model52 implemented in Gaussian 03 program. 3. Results and Discussion CB[n] conformers with (a) one of the methylene protons pointing outside the cavity (H1) and the other proton directing toward the portal (H3) (normal CB[n]) and secondly (b) protons H1 pointing toward the portals and H3 directing within the cavity, methylene distorted CB[n], were generated. The relative position of flipped methylene groups of glycourils in conformers (b) are denoted by “1,m-”. Following this notation, CB[12] conformers obtained from the B3LYP optimizations are shown in Figure 2 as a prototype example. The conformers of the remaining CB[n] hosts are displayed in Figure 1S of the Supporting Information. The relative stabilization energies of these conformers are reported in Table 1. As may readily be noticed, 1,4- and 1,5-CB[8] methylene distorted conformers are

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Figure 3. B3LYP optimized geometries of CB[n], n ) 5-12.

TABLE 2: Selected Optimized Geometrical Parameters of CB[n] Hostsa r(C3-H1) r(C4-H2) r(C3-H3) r(C2-O1) r(C4-C4′) r(C2-N3) r(C4-N3) a(N3C3N3) a(N3C4N5) d(N3C2N5′C5′) d(O1C2N3C3) a

CB[5]

CB[6]

CB[7]

CB[8]

CB[9]

CB[10]

CB[11]

CB[12]

1.098 1.092 1.100 1.211 1.575 1.393 1.447 115.5 117.6 179.4 3.4

1.100 1.101 1.093 1.213 1.572 1.392 1.446 115.4 117.1 -175.4 -1.8

1.101 1.102 1.093 1.213 1.570 1.393 1.446 115.3 116.9 -172.1 -5.2

1.102 1.102 1.092 1.213 1.568 1.393 1.446 115.2 116.8 -168.8 -8.2

1.103 1.102 1.092 1.213 1.567 1.393 1.445 115.2 116.7 -167.6 -10.9

1.104 1.102 1.092 1.213 1.566 1.394 1.446 115.0 116.6 -164.7 -11.6

1.104 1.102 1.192 1.213 1.565 1.394 1.446 115.0 116.6 -164.5 -12.6

1.104 1.102 1.192 1.213 1.564 1.394 1.446 114.9 116.5 -163.6 -13.6

r ) bond distance in angstroms, and a and d are bond and dihedral angles in degrees, respectively.

Figure 4. MESP isosurface (V) -184 kJ mol-1) mapped on CPs in cucurbit[n]urils (n ) 8-12). (Both side and top views are shown.)

TABLE 3: MESP Minima (in kJ mol-1) within the Cavity and near Portal Oxygens and Cavity Dimensions of CB[n] ESP at CP in cavity ESP at CP on portals cavity diameter (Å) cavity height (Å) a

CB[5]

CB[6]

CB[7]

CB[8]

CB[9]

CB[10]

CB[11]

CB[12]

-52.5a -299.0a,b 3.91a,b 7.35a,b

-68.5a -277.0a,b 5.50a,b 7.50a,b

-79.2a -260.5a,b 7.11a,b 7.63a,b

-85.1 -251.7 8.58c 7.71

-89.4 -244.1 10.47c 7.75

-90.1 -238.0 11.77c 7.81

-90.4 -232.9 13.20c 7.83

-91.0 -229.1 14.87c 7.85

Ref 40. b Ref 39. c Cavity is elliptical, and the data here refer to average values.

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Figure 5. MESP value at CP near nitrogen as a function of cavity diameter in CB[n].

Figure 6. MESP value at CP near portal oxygen as a function cavity diameter in CB[n].

Figure 7. Plot showing cavity height/diameter versus number of glycouril units.

destabilized by ∼126 kJ mol-1 over the lowest energy conformer possessing highest symmetry. The geometries of lowest energy CB[n] (n ) 5-12) conformers, within the B3LYP/6-31G(d) framework of theory, displayed in Figure 3 clearly show a wide range of cavity sizes offered by CB[n] family of hosts. The remaining conformers which are of relatively higher energy are shown in the Supporting Information. A decrease in the relative destabilization of 1,4- or 1,5- conformers over the corresponding lowest energy conformer can be noticed with the increase in cavity size and amounts to ∼65 kJ mol-1 in the case of CB[12]. It may further be inferred that flipping of -CH2- (methylene) group engenders large steric hindrance and yield relatively destabilized conformers. The symmetric arrangement of distorted methylene groups renders the least destabilization to the conformer. Therefore, the 1,7-CB[12] turns out to be lower energy among 1,m-CB[12] conformer. In general, the energy rank order for conformers has been noticed to be 1,4-CB[n] > 1,5-CB[n] > 1,6-CB[n] > 1,7-CB[n] > CB[n]. Selected geometrical parameters in CB[n] are reported in Table 2. The C4-C4′ bond distance (1.575 Å in CB[5]) decreases steadily with the increase in each glycouril unit and

TABLE 4: Selected Vibrational (ν ) Stretch and δ ) Bending) Frequencies of Curcurbit[n]urils (n ) 5-12)a assignment

CB[5]

CB[6]

2859(0) 2860(241) 2861(45) 1760(2359)

2950(26) 2840(431) 2834(9) 1754(2961)

CB[7]

2952(49) 2826(709) 2817(17) 1750(3617) [1751]b CH2 scissor 1382(506) 1384(264) 1387(286) δ(C-H3 + C-H2) + ν(C4-N3) 1395(1030) 1395(1185) 1394(1298) 1273(705) 1271(1039) 1268(1301) ν(C4-C4′) + δ(C-H2) 1362(0) 1358(0) 1355(0) 1177(711) 1174(782) 1170(1586) CH2 wag 1360(1) 1352(38) 1349(94) ν(C4-C4′ + C4-N3) + δ(C-H2) 1329(407) 1328(501) 1326(587) ν(C-N) + δ(N3-C2-N5′) 1249(566) 1248(668) 1246(714) 1231(183) 1233(197) 1232(204) ν(C-N) + δ(C-H2) 1204(231) 1202(307) 1199(178) ν(C4-N3) + ν(C4-C4′) 1085(6) 1094(16) 1100(42) CH2 rock 927(314) 930(477) 934(446) (N3-C4-N5) scissor 751(522) 757(764) 760(989) ν(C3-H1) ν(C3-H2) ν(C4-H3) ν(C2-O1)

a

CB[8]

CB[9]

CB[10]

CB[11]

CB[12]

2954(48) 2820(756) 2811(18) 1747(4098) [1746]c 1393(997) 1391(439) 1264(1717) 1352(0) 1164(1982) 1345(106) 1324(638) 1243(852) 1230(201) 1197(455) 1103(64) 938(677) 762(1225)

2956(46) 2817(675) 2803(99) 1745(4884)

2957(66) 2815(658) 2798(42) 1744(5536)

2958(94) 2814(658) 2790(255) 1743(6170)

2960(85) 2813(679) 2787(245) 1742(6784)

1395(462) 1391(631) 1263(1449) 1349(1) 1162(2542) 1343(234) 1323(617) 1242(833) 1230(163) 1194(492) 1106(115) 941(635) 764(1479)

1397(307) 1391(1600) 1260(2196) 1344(2) 1160(2916) 1341(368) 1322(716) 1240(1063) 1228(189) 1191(570) 1108(153) 944(755) 765(1723)

1399(413) 1391(1625) 1258(2513) 1341(521) 1157(2838) 1339(509) 1322(652) 1239(1181) 1228(167) 1190(594) 1109(183) 945(512) 765(1875)

1401(295) 1390(998) 1256(2574) 1340(576) 1155(3525) 1337(480) 1321(817) 1237(1296) 1227(166) 1188(665) 1111(282) 945(670) 765(2225)

The numbers in parentheses denote infrared intensities in km mol-1. b Ref 33. c Ref 32.

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Figure 8. Vibrational spectra (500-3200 cm-1) of (a) CB[5], (b) CB[6], (c) CB[7], (d)CB[8], (e) CB[9], (f) CB[10], (g) CB[11], and (h) CB[12] (intensity in kM mol-1).

is predicted to be 1.564 Å in CB[12]. A diminutive changes in the C-N and CdO bond distances along CB[n] series can be noticed. Bond angles are nearly insensitive to the number of glycouril units in CB[n] hosts, and the largest deviation within 1° can be noticed. Moreover, the methylene carbon near portal deviate by ∼14° from planarity relative to the ureido plane of CB[12], as evident from O1-C2-N3-C3 dihedral angle data reported in Table 2. As pointed out in the preceding section, the MESP brings out the effective localization of the electron-rich regions. An MESP isosurface of V ) -184 kJ mol-1 in Figure 4 (value chosen arbitrarily) facilitates a comparison of electron-rich region near CB[n] portals. The electron-rich regions are by and large localized near ureido oxygens of the host portals. The CPs in MESP have been identified, and the minima are rendered with blue in these MESP maps. Unlike its lower homologues, two distinct minima near nitrogen can be noticed

for CB[11] and CB[12] hosts. MESP at the minima near ureido oxygen and nitrogen are given in Table 3. A plot of MESP value near nitrogen as a function of effective host cavity diameter is displayed in Figure 5. MESP minima near nitrogen become deeper with increasing cavity diameter. Therefore, the potential V(r) ) -52.5 kJ mol-1 at CPs in CB[5] decreases gradually and attains a constant value ≈ -90 kJ mol-1 beyond CB[9]. Because the host-guest interactions are primarily governed by the interplay of size and shape of the cavity and charge distribution within the cavity that facilitates noncovalent (including electrostatic or hydrogen bonded) interactions, it may be inferred that the strength of electrostatic interactions within the cavity of CB[n] will remain nearly constant beyond CB[9] host. Therefore, varying affinity of larger hosts toward the guest may be dominated by their size and shape complimentarily. Furthermore, the minima near portal oxygens become shallow

Density Functional Investigations on CB[n] Hosts along the CB[n] series (cf. Figure 6). Accordingly, for CB[5], these minima were identified with V(r) ) -299.0 kJ mol-1, which finally reaches -229.1 kJ mol-1 in CB[12], which is ∼70 kJ mol-1 higher. In other words, the host portals become less electron-rich along the CB[n] series. MESP topography has further been utilized to estimate the dimensions of the host cavity.39 Cavity diameter and height along CB[n] hosts are compared in Table 3. A plot of cavity height and diameter versus number of glycouril units in CB[n] has been displayed in Figure 7. Therefore, it may be inferred that increasing the number of glycouril units engenders relatively large variation in effective cavity diameter on traversing along the CB[n] series. The diameter of CB[5] increases from 3.91 to 14.87 Å in CB[12]; the cavity height, however, increased indolently by 0.5 Å from 7.35 Å in CB[5]. It is worth to note here that beyond CB[7] the host cavity turns out to be elliptical where the effective cavity diameter becomes larger than the cavity height. The starred values in Table 3 denote elliptical cavity and effective cavity diameter refers to the mean (average) of major and minor diameters. For example, the effective cavity diameter 14.87 Å of CB[12] has been obtained as a mean of radial opposite distances of the MESP minima near ureido oxygen, which range from 14.73 to 15.02 Å. Frequencies of selected normal vibrations in the 500 to 3200 cm-1 region of CB[n] (n ) 5-12) are reported in Table 4. The frequencies displayed here are scaled by 0.9389. Vibrational spectra of CB[n] are depicted in Figure 8. As may readily be noticed, the methylene stretching of protons placed outside the host cavity (H1) assigned to the 2859 cm-1 band CB[5] shifts to higher wavenumber along the series, and in CB[12], it was predicted at 2960 cm-1. The C-H vibrations of protons directing toward the portals (H3) exhibit a frequency shift in the opposite direction from 2861 to 2787 cm-1 in CB[5] to CB[12], respectively. Frequency of C-H vibrations from methine protons (H2) also decreases steadily in wavenumber from 2860 to 2813 cm-1 along the CB[n] series. The intense CdO stretching assigned to 1760 cm-1 band in CB[5] downshifts to 1742 cm-1 in CB[12] and can be correlated to the MESP minima near carbonyl oxygen along the CB[n] series. Moreover, an increase in the number of glycouril units of the host along the CB[n] series led to enhanced intensity for certain vibrations of CB[n] hosts. Furthermore, a comparison of CdO stretching at 1751 and 1746 cm-1 in CB[7] and CB[8], respectively, with those measured from experiment is far from straightforward. Calculated vibration frequencies in CB[n] hosts refer to the isolated molecule, whereas the only experimental data from FTIR reflection absorption spectroscopy in the case of CB[7] and CB[8] for CdO stretching reported in the literature is for the self-assembled monolayer of these host on gold surface. The CdO and C-N stretching frequency in CB[7] was assigned to 1751 and 1474 cm-1 vibrations from these experiments, whereas the corresponding vibrations in CB[8] were assigned to those at 1748 and 1470 cm-1. B3LYP calculated wavenumbers of carbonyl in CB[7] as well as CB[8] agree well with the experimental measured spectra.31,32 It should further be remarked here that NCN bond angle bridging two five-member rings of CB[n] viz. N3C4N5 bending engender an intense band that corresponds to 765 cm-1 in CB[12], which has been downshifted to 751 cm-1 in the lower homologues of CB[n] series. Calculated 1H NMR chemical shifts (δH) of CB[n] (n ) 8-12) hosts turn out to be nearly insensitive to the cavity size, and the δH values follow the trend: H3 (6.1 ppm) > H2 (4.4 ppm) > H1 (3.2 ppm) in the gas phase. The presence of water

J. Phys. Chem. A, Vol. 114, No. 12, 2010 4469 as solvent engenders upfield signals for both the H3 and H2 (δH ) 3.8 and 5.1 ppm, respectively) protons unlike those directing outside the cavity which are shielded and exhibit signals those correspond to δH ) 5.6 ppm. Calculated δH values agree well with those in experimental NMR spectra.39,53The mean δH values of H1, H2, and H3 in CB[12] are reported in Table 1S of the Supporting Information. 4. Conclusions Electronic structure, charge distribution in terms of MESP topography, and normal vibrations of cucurbit[n]uril hosts, CB[n] (n ) 5-12), have been derived. It has been shown that the MESP minima near ureido oxygens become shallow with increasing number of glycouril units along the CB[n] series. The CB[n] (n > 10) hosts engender distinct minima near each nitrogen and facilitate more electron-rich regions inside the cavity. The ratio of cavity height to cavity diameter estimated from the MESP topography can be utilized to gauge either circular or elliptical host cavity. For CB[n], n g 8 hosts possessing elliptical cavity, the ratio turns out to be greater than unity. Normal vibrations derived within the B3LYP/6-31G(d) theory reveal that the carbonyl stretching frequencies decrease steadily from 1760 cm-1 in CB[5] to 1742 cm-1 in CB[12] with increasing number of glycouril units. 1H chemical shifts in the NMR spectra of CB[n] in water are insensitive along the CB[n] series. The methylene protons directing outside the cavity are deshielded, whereas those near carbonyl or within the host cavity exhibit upfield signals in the calculated 1H NMR spectra. Acknowledgment. S.P.G. acknowledges support from the University Grants Commission (UGC), New Delhi, India (research project F34-370/2008(SR)) and University of Pune. R.V.P. is grateful to Council of Scientific and Industrial Research, New Delhi, India for a Senior Research fellowship. We thank the Center for Network Computing, University of Pune, for providing computational facility. Supporting Information Available: Geometries of CB[n] and 1H NMR spectra of CB[n] (n ) 8-12) homologues. This material is available free of charge via the Internet at http:// pubs.acs.org References and Notes (1) Behrend, R.; Meyer, E.; Rusche, F. Justus Liebigs Ann. Chem. 1905, 339, 1. (2) Moran, J. R.; Karbach, S.; Cram, D. J. J. Am. Chem. Soc. 1982, 104, 5826. (3) Mock, W. L. Top. Curr. Chem. 1994, 17, 205. (4) Freeman, W. A.; Mock, W. L.; Shih, N. Y. J. Am. Chem. Soc. 1981, 103, 7368. (5) Hoffmann, R.; Knoche, W.; Fenn, C.; Buschmann, H. J. J. Chem. Soc., Faraday Trans. 1994, 90, 1507. (6) Buschmann, H. J.; Cleve, E.; Schollmeyer, E. Inorg. Chim. Acta 1992, 193, 93. (7) Buschmann, H. J.; Jansen, K.; Schollmeyer, E. Thermochim. Acta 1998, 317, 95. (8) Burnett, C. A.; Witt, D.; Fettinger, J. C.; Isaacs, L. J. Org. Chem. 2003, 68, 6184. (9) Buschmann, H. J.; Jansen, K.; Schollmeyer, E. Thermochim. Acta 1998, 317, 95. (10) Buschmann, H. J.; Jansen, K.; Schollmeyer, E. Thermochim. Acta 2000, 346, 33. (11) Lee, J. W.; Samal, S.; Selvapalam, N.; Kim, H. J.; Kim, K. Acc. Chem. Res. 2003, 36, 621. (12) Ong, W.; Kaifer, A. E. Organometallics 2003, 22, 4181. (13) Jeon, W. S.; Moon, K.; Park, S. H.; Chun, H.; Ko, Y. H.; Lee, J. Y.; Lee, E. S.; Samal, S.; Selvapalam, N.; Rekharsky, M. V.; Sindelar, V.; Sobransingh, D.; Inoue, Y.; Kaifer, A. E.; Kim, K. J. Am. Chem. Soc. 2005, 127, 12984.

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