Solid State and Solution Characterization of Chiral, Conformationally Mobile Tripodal Ligands

Article abstract of DOI:10.1021/ic9803098

The synthesis of the ligands N,N-bis[(2-pyridyl)methyl]-1-(2-pyridyl)ethanamine (1, α-MeTPA), N,N-bis[(6-phenyl-2-pyridyl)methyl]-1-(2-pyridyl)ethanamine (2, α-MePh₂TPA), N,N-bis[(2-quinolyl)methyl]-1-(2-pyridyl)ethanamine, (3, α-MeBQPA), and N,N-bis[(2-pyridyl)]methyl(phenyl)(2-pyridyl)methanamine (4, α-PhTPA) is described. The ligands form chiral, pseudo C₃-symmetric complexes with Zn^II and Cu^II salts that possess an available electrophilic coordination site. Xray crystallographic structures of the complexes [Zn(α-MeTPA)Cl](ClO₄), [Zn(α-MeBQPA)Cl](ClO₄), and [Zn(α-PhTPA)Cl](ClO₄) show that the spatial arrangement of the three pyridyl groups resembles a propeller whose directional sense is controlled by a substituent on one methylene arm. Chiroptical measurements provide supporting information that the complexes maintain similar structures in solution. Monte Carlo/stochastic dynamics (MC/SD) simulations of the [Zn(L)Cl]⁺ complexes indicate that only two conformers are populated at normal temperatures, suggest the presence of a synchronicity in the movement of the heteroaromatic rings during conformer interconversions, and provide an estimate of the energy difference between the conformers.

Full text of DOI:10.1021/ic9803098

Inorg. Chem. 1998, 37, 6255-6262
6255
Solid State and Solution Characterization of Chiral, Conformationally Mobile Tripodal
Ligands
James W. Canary,*,† Craig S. Allen,† Jesus M. Castagnetto,† Yu-Hung Chiu,†
Paul J. Toscano,‡ and Yihan Wang†
Department of Chemistry, New York University, New York, New York 10003, and Department of
Chemistry, SUNY Albany, Albany, New York 12222
ReceiVed March 19, 1998
The synthesis of the ligands N,N-bis[(2-pyridyl)methyl]-1-(2-pyridyl)ethanamine (1, R-MeTPA), N,N-bis[(6-phenyl-
2-pyridyl)methyl]-1-(2-pyridyl)ethanamine (2, R-MePh2TPA), N,N-bis[(2-quinolyl)methyl]-1-(2-pyridyl)ethanamine,
(3, R-MeBQPA), and N,N-bis[(2-pyridyl)]methyl(phenyl)(2-pyridyl)methanamine (4, R-PhTPA) is described. The
ligands form chiral, pseudo C3-symmetric complexes with ZnII and CuII salts that possess an available electrophilic
coordination site. X-ray crystallographic structures of the complexes [Zn(R-MeTPA)Cl](ClO4), [Zn(R-
MeBQPA)Cl](ClO4), and [Zn(R-PhTPA)Cl](ClO4) show that the spatial arrangement of the three pyridyl groups
resembles a propeller whose directional sense is controlled by a substituent on one methylene arm. Chiroptical
measurements provide supporting information that the complexes maintain similar structures in solution. Monte
Carlo/stochastic dynamics (MC/SD) simulations of the [Zn(L)Cl]+ complexes indicate that only two conformers
are populated at normal temperatures, suggest the presence of a synchronicity in the movement of the heteroaromatic
rings during conformer interconversions, and provide an estimate of the energy difference between the conformers.
Introduction
Chart 1
Chiral tripodal ligands related to tris(2-pyridylmethyl)amine
(TPA, 1) with an asymmetric center in one arm (e.g., R-MeTPA,
2) have been used as chiral solvating agents for sulfoxides1 and
for metal ion sensing with multiple signals.2 Copper(II)
complexes of the ligands have been shown to display redox-
switched chiroptical properties3 and have been applied to the
determination of absolute configurations of R-amino acids and
â-amino alcohols.4 These applications are possible as a result
of subtle conformational properties of the ligand-metal com-
plex. In this paper, we describe the synthesis, characterization
and conformational analysis of Zn(II), Cu(II), and Cd(II)
complexes of ligands 1-4 (Chart 1), which were the first
compounds studied in this series of experiments.1
The solid-state structures of tetradentate TPA coordination
complexes contain 5-, 6-, and 7-coordinate metal ions displaying
several coordination geometries.5-14 Two ligand conformations
predominate:7 the C3 conformation (Figure 1a), in which a C3
symmetry axis is present as exemplified by trigonal bipyramidal
metal ion complexes of TPA; and the Cσ conformation (Figure
1c), in which a plane of symmetry is present as displayed by
octahedral and square pyramidal metal-TPA complexes. As
shown by the C3 symmetric structure in Figure 1a, the ligand
occupies four coordination sites with the tertiary amine in an
axial position and the pyridyl nitrogens in three equatorial sites.
An additional apical site (not shown) is available for solvent or
anion coordination. Tetradentate TPA encompasses the metal
ion with a spatial arrangement of the three pyridyl groups, tilting
with respect to the metal-amine axis, which resembles a
† NYU.
‡ SUNY Albany.
(1) Canary, J. W.; Allen, C. S.; Castagnetto, J. M.; Wang, Y. J. Am. Chem.
Soc. 1995, 117, 8484.
(2) Castagnetto, J. M.; Canary, J. W. Chem. Commun. 1998, 203.
(3) Zahn, S.; Canary, J. W. Angew. Chem., Int. Ed. 1998, 37, 305.
(4) Zahn, S.; Canary, J. W., manuscript submitted for publication.
(5) Dong, Y.; Fujii, H.; Hendrich, M. P.; Leising, R. A.; Pan, G.; Randall,
C. R.; Wilkinson, E. C.; Zang, Y.; Que, L., Jr.; Fox, B. G.; Kauffmann,
K.; Mu¨nck, E. J. Am. Chem. Soc. 1995, 117, 2778.
(6) Toftlund, H.; Larsen, S.; Murray, K. S. Inorg. Chem. 1991, 30, 3964.
(7) Allen, C. S.; Chuang, C.-L.; Cornebise, M.; Canary, J. W. Inorg. Chim.
Acta 1995, 239, 29.
(8) Brownstein, S. K.; Plouffe, P. Y.; Bensimon, C.; Tse, J. Inorg. Chem.
1994, 33, 354.
(9) Mandel, J. B.; Maricondi, C.; Douglas, B. E. Inorg. Chem. 1988, 27,
2990.
(10) Goodson, P. A.; Oki, A. R.; Glerup, J.; Hodgson, D. J. J. Am. Chem.
Soc. 1990, 112, 6248.
(12) Wei, N.; Lee, D.-H.; Murthy, N. N.; Tyekla´r, Z.; Karlin, K. D.; Kaderli,
S.; Jung, B.; Zuberbu¨hler, A. D. Inorg. Chem. 1994, 33, 4625.
(13) Zang, Y.; Dong, Y.; Que, L., Jr.; Kauffmann, K.; Mu¨nck, E. J. Am.
Chem. Soc. 1995, 117, 1169.
(11) Murthy, N. N.; Karlin, K. D. J. Chem. Soc., Chem. Commun. 1993,
1236.
(14) Harata, M.; Jitsukawa, K.; Masuda, H.; Ensaga, H. J. Am. Chem. Soc.
1994, 116, 10817.
10.1021/ic9803098 CCC: $15.00 © 1998 American Chemical Society
Published on Web 11/11/1998

6256 Inorganic Chemistry, Vol. 37, No. 24, 1998
Canary et al.
and 4 differ in the size of the R substituent. Ligands 2 and 3
present greater steric hindrance on the heterocycle arms, which
was observed previously to increase the degree of twist observed
in crystallographic structures of Cu(I) and Cu(II) complexes.17-20
Semiempirical (MNDO) calculations21 of [Zn(R-MeTPA)Cl]+,
[Zn(R-PhTPA)Cl]+, and [Zn(R-MeBQPA)Cl]+ cations sup-
ported the predictions from examination of molecular models
that the anti conformation would be more stable than the syn
isomer by 1.5-2.5 kcal/mol, and that the complex with a carbon
center configuration of R would display a Λ (left-handed)
propeller-like twist.
Figure 1. [M(TPA)]n+ C3 and Cσ conformations. View is down the
metal-tertiary amine axis.
Inspection of the literature revealed that a few examples of
tripodal ligands with chiral arms had been reported.8,22-26 Our
objectives in the present study was to conduct a detailed analysis
of the conformational behavior of the ligands in the complexes,
including the factors that dictate the axial chirality of the
complexes.
Experimental Section
Syntheses. [Cd(r-MeTPA)I2]. A solution of CdI2 (58.0 mg, 0.158
mmol) in 0.5 mL of methanol was added dropwise via pipet to a hot
solution of 11 (R-MeTPA, 48.2 mg, 0.158 mmol) in 0.5 mL of methanol.
A white precipitate formed immediately and was collected and dried
in vacuo to yield 92.0 mg (86.6%). 1H NMR (300 MHz, DMSO-d6):
δ 8.70 (d, J ) 4.4 Hz, 1H), 8.62 (d, J ) 3.9 Hz, 1H), 8.43 (d, J ) 4.3
Hz, 1H), 8.05-8.00 (m, 1H), 7.89-7.84 (m, 1H), 7.67-7.38 (m, 5H),
7.26-7.22 (m, 1H), 7.10 (d, J ) 7.6 Hz, 1H), 4.71 (q, J ) 6.6 Hz,
1H), 4.48 (d, Jgem ) 15.0 Hz, 1H), 4.36 (d, Jgem ) 15.0 Hz, 1H), 3.86
(d, Jgem ) 16.5 Hz, 1H), 3.80 (d, Jgem ) 16.5 Hz, 1H), 1.76 (d, J ) 6.6
Hz, 3H). 13C NMR (75 MHz, DMSO-d6): δ 157.5, 154.7, 154.1, 147.8,
147.7, 147.3, 140.2, 139.6, 138.8, 125.0, 124.1, 123.9, 123.5, 123.2,
61.5, 59.1, 53.5, 10.9. Anal. Calcd for C19H20N4CdI2: C, 34.03; H,
3.01; N, 8.35. Found (R form): C, 34.16; H, 2.81; N, 8.19. Found (S
form): C, 33.81; H, 3.08; N, 8.32.
[Zn(r-MeTPA)Cl]ClO4. (Caution! Perchlorate salts of metal
complexes with organic ligands are potentially explosiVe. They should
be handled in small quantity and with caution.)27 A solution of
Zn(ClO4)2‚6H2O (60.5 mg, 0.163 mmol) in 0.4 mL of methanol was
added via pipet to a solution of R-MeTPA1 (49.5 mg, 0.163 mmol) in
0.4 mL of methanol. After stirring for about 10 min, a solution of
NaCl (9.53 mg, 0.163 mmol) in 0.4 mL H2O was added. The mixture
was left in refrigerator for several h during which time a white
precipitate formed. The precipitate was collected and dried (33.4 mg,
40.6%). 1H NMR (300 MHz, CD3COCD3): δ 9.19-9.10 (m, 3H),
8.33-8.20 (m, 3H), 7.94 (d, J ) 8.0 Hz, 1H), 7.84-7.76 (m, 5H),
4.83 (d, Jgem ) 16.7 Hz, 1H), 4.53 (d, Jgem ) 16.4 Hz, 1H), 4.38 (q, J
) 6.8 Hz, 1H), 4.21 (d, Jgem ) 16.7 Hz, 1H), 4.17 (d, Jgem ) 16.4 Hz,
1H), 1.94 (d, J ) 6.8 Hz, 3H). Anal. Calcd for C19H20N4ZnCl2O4‚
2H2O: C, 42.21; H, 4.47; N, 10.36. Found (R form): C, 42.39; H,
4.12; N, 10.43. Found (S form): C, 42.40; H, 3.98; N, 10.40.
The racemic precipitate was recrystallized from acetone/ether/toluene
to obtain X-ray quality crystals. Anal. Calcd for [Zn(MeTPA)Cl]ClO4‚
0.5C7H8‚0.5H2O: C, 48.28; H, 4.50; N, 10.01. Found: C, 48.46; H,
4.39; N, 10.13.
Figure 2. Conformations and numbering scheme.
propeller. The two possible propeller-like stereoisomers (Figure
1a,b) are conformational enantiomers.
The scarcity of chiral, C3 symmetric ligands15 led to the
proposal of modifying TPA by attaching substituents in ap-
propriate positions to influence the overall structure of the
corresponding coordination complexes. Our strategy for the
control of the propeller-like twist derives from the incorporation
of one chiral center on one arm of the tripodal ligand.
Examination of CPK (Corey-Pauling-Koltun) molecular mod-
els of trigonal bipyramidal [M(TPA)X]+ complexes indicated
that an alkyl or aryl substituent on one arm (“R” in Figure 2)
should influence the ligand in the coordination complex to adopt
a particular helical asymmetry dictated by the absolute config-
uration of the substituent-bearing carbon atom (Figure 2). Two
binding conformations are possible, the “anti” conformation in
which the R-substituent points away from the pyridyl groups,
and the “syn” conformation in which the R-substituent points
toward one pyridyl group. The syn conformation should be
less stable due to the presence of a syn-pentane type interaction
between the R-substituent and one of the pyridyl groups.16 The
R-substituted TPA complexes would demonstrate pseudo C3
symmetry with an asymmetric environment surrounding the fifth
coordination site in a [MII(R-R-TPA)X]n+ moiety. Subsequent
synthesis of other chiral TPA derivatives, containing substituents
attached to the 6-positions of the pyridyl rings (“R′” in Figure
2) or mixed tripodal ligands with pyridine and other heterocyclic
pendants should form pseudo C3 symmetric coordination
complexes with more highly asymmetric environments encom-
passing the available electrophilic apical site.
(17) Wei, N.; Murthy, N. N.; Karlin, K. D. Inorg. Chem. 1994, 33, 6093.
(18) Wei, N.; Murthy, N. N.; Chen, Q.; Zubieta, J.; Karlin, K. D. Inorg.
Chem. 1994, 33, 1953.
(19) Chuang, C.-L.; Lim, K.; Chen, Q.; Zubieta, J.; Canary, J. W. Inorg.
Chem. 1995, 34, 2562.
(20) Chuang, C.-L.; Lim, K.; Canary, J. W. Supramol. Chem. 1995, 5, 39.
(21) Spartan 3.01; Wavefunction, Inc.: 18401 Von Karman Ave., Suite
370, Irvine, CA.
(22) Ohtani, Y.; Utsuno, S. Chem. Lett. 1980, 465.
(23) Endo, I.; Horikoshi, S.; Utsuno, S. Chem. Commun. 1981, 296.
(24) Utsuno, S.; Miyamae, H.; Horikoshi, S.; Endo, I. Inorg. Chem. 1985,
24, 1348.
The initial set of ligands selected for this study were N,N-
bis[(2-pyridyl)methyl]-1-(2-pyridyl)ethanamine (1, R-MeTPA),
N,N-bis[(6-phenyl-2-pyridyl)methyl]-1-(2-pyridyl)ethanamine (2,
R-MePh2TPA), N,N-bis[(2-quinolyl)methyl]-1-(2-pyridyl)ethan-
amine (3, R-MeBQPA), and N,N-bis[(2-pyridyl)methyl](phen-
yl)(2-pyridyl)methanamine (4, R-PhTPA). TPA derivatives 1
(25) Sakurai, T.; Oi, H.; Nakahara, A. Inorg. Chim. Acta 1984, 92, 131.
(26) Hojland, F.; Toftlund, H.; Yde-Andersen, S. Acta Chem. Scand. A
1983, 37, 251.
(15) Moberg, C. Angew. Chem., Int. Ed. 1998, 37, 248.
(16) Wang, X.; Erickson, S. D.; Iimori, T.; Still, W. C. J. Am. Chem. Soc.
1992, 114, 4128.
(27) Chem. Eng. News 1983, 61, 4.

Conformationally Mobile Tripodal Ligands
Inorganic Chemistry, Vol. 37, No. 24, 1998 6257
[Cu(r-MeTPA)Cl]ClO4. A solution of Cu(ClO4)2‚H2O (73.8 mg,
0.200 mmol) in 0.4 mL of methanol was added via pipet to a solution
of R-MeTPA1 (60.7 mg, 0.200 mmol) in 0.4 mL of methanol. After
stirring for about 10 min, a solution of NaCl (11.7 mg, 0.200 mmol)
in 0.4 mL of water was added. Greenish crystals formed after cooling
the mixture in an ice bath for ca. 20 min. The crystals were collected
and dried (65.0 mg, 64.6%). Anal. Calcd for C19H20N4CuCl2O4: C,
45.38; H, 4.01; N, 11.14. Found (R form): C, 45.13; H, 4.26; N, 10.93.
Found (S form): C, 45.19; H, 3.99; N, 10.9.
6-Bromo-2-pyridylmethyl Bromide, 8. The compound 6-bromo-
2-pyridinemethanol (7, 21.8 g, 0.116 mol) was added to 300 mL of
48% HBr. The solution was heated to 120 °C for 4 h. After cooling,
the solution was made basic with sodium bicarbonate until pH ) 5-6,
extracted with ether, and dried over Na2SO4. Evaporation of the solvent
gave 24.8 g of 8 (85%). 1H NMR (200 MHz, CDCl3): 7.60-7.39 (m,
3H), 4.50 (s, 2H).
N,N-Bis[(6-bromo-2-pyridyl)methyl]-1-(2-pyridyl)ethanamine, 10.
Diisopropylethylamine (2.96 g, 22.9 mmol) and 8 (2.88 g, 11.5 mmol)
were added to 50 mL of THF.18 To this solution was added dropwise
1-(2-pyridyl)ethylamine1 (9, 0.70 g, 5.73 mmol). The mixture was
stirred at ambient temperature for 3 days, during which a white
precipitate formed. After the solution was filtered, the filtrate was
concentrated in vacuo to give a brown oil which was purified by column
chromatography on silica gel with CH2Cl2/ether (3:1, Rf ) 0.43) to
give pure product 10 (2.11 g, 74%). 1H NMR (200 MHz, CDCl3):
8.52-8.48 (m, 1H), 7.65-7.07 (m, 9H), 4.02 (q, J ) 6.9 Hz, 1H),
3.92 (d, Jgem ) 15.5 Hz, 2H), 3.72 (d, Jgem ) 15.5 Hz, 2H), 1.48 (d, J
) 6.9 Hz, 3H). 13C NMR (50 MHz, CDCl3): 162.6, 149.4, 141.6,
139.1, 136.6, 127.5, 126.5, 123.2, 122.5, 121.8, 61.3, 56.9, 15.6. MS
(ESI): 485 (100, M + Na+), 463 (4, M + H+).
N,N-Bis[(6-phenyl-2-pyridyl)methyl]-1-(2-pyridyl)ethanamine, 2
(r-MePh2TPA). To a stirred solution of 10 (0.46 g, 1.0 mmol) and
Pd(PPh3)4 (0.069 g, 0.06 mmol) in 4 mL of dry toluene under nitrogen
was added 2 mL of 2 M Na2CO3 and phenyl boronic acid (0.29 g, 2.4
mmol) in 1 mL of methanol. The vigorously stirred mixture was heated
to reflux for 8 h. It was then allowed to cool and was partitioned
between 100 mL of CHCl3 and 10 mL of 2 M Na2CO3 containing 1
mL of concentrated NH3. The organic layer was dried (Na2SO4) and
concentrated under reduced pressure. Flash chromatography on basic
alumina (CH2Cl2/EtOAc 20:1, Rf ) 0.32) gave 2 (0.34 g, 84.0%). The
product was further purified by recrystallization from methanol. 1H
NMR (200 MHz, CDCl3): 8.60-8.57 (m, 1H), 8.03-7.98 (m, 4H),
7.72-7.36 (m, 14H), 7.14-7.08 (m, 1H), 4.23 (q, J ) 6.8 Hz, 1H),
4.15 (d, Jgem ) 15.0 Hz, 2H), 3.96 (d, Jgem ) 15.0 Hz, 2H), 1.65 (d, J
) 6.8 Hz, 3H). 13C NMR (50.3 MHz, CDCl3): 163.2, 161.0, 156.9,
149.3, 140.0, 137.4, 136.5, 129.2, 129.1, 127.4, 123.4, 122.3, 121.6,
118.9, 61.1, 57.5, 15.4. MS (ES) m/e: 495 (45, M + K+), 479 (100,
M + Na+), 457 (57, M + H+). Anal. Calcd for C31H28N4: C, 81.55;
H, 6.18; N, 12.27. Found (R form): C, 81.63; H, 6.33; N, 12.41. Found
(S form): C, 81.76; H, 6.18; N, 12.51.
4 h. The reaction mixture was cooled and chilled water was added,
resulting in a white precipitate. The precipitate was isolated by filtration
and dried (16.5 g, 82.4%). The crude product was used directly in the
next step without further purification.28
The ketoxime (14.0 g, 0.0706 mol) was dissolved in 150 mL of
95% EtOH with heating. After cooling to ambient temperature, 75 g
of zinc dust and 75 mL of acetic acid were added over 3 h. The reaction
mixture was stirred overnight. After filtration, the filtrate was
concentrated in vacuo. Water was added and evaporated several times
to remove acetic acid. The mixture was basified with strong KOH
solution and extracted with ether. The organic layer was dried over
Na2SO4. After removal of the solvent, the crude product was distilled
using a Ku¨gelrohr apparatus, affording the colorless product 6 (10.7 g
82.3%). 1H NMR (300 MHz, CDCl3): 8.58 (d, J ) 4.5 Hz, 1H), 7.63-
7.12 (m, 8H), 5.26 (s, 1H), 2.35 (br s, 2H).
N,N-Bis(2-pyridylmethyl)-phenyl-2-pyridylmethanamine (r-Ph-
TPA), 4. Picolyl chloride hydrochloride (3.86 g, 23.6 mmol) was
dissolved in 6 mL of water. Amine 6 (2.17 g, 11.8 mmol) was then
added. The mixture was then warmed to 70 °C, and 4.6 mL of a 10M
NaOH solution was added slowly. Stirring was continued at this
temperature for another 4 h. After cooling, the reaction mixture was
extracted with CHCl3 and the organic layers were dried over Na2SO4.
The solvent was removed in vacuo and the crude product was
chromatographed on basic alumina (CH2Cl2/EtOAc ) 4:1, Rf ) 0.40).
The yield of pure 4 was 1.21 g (28.0%). 1H NMR (300 Hz, CDCl3):
8.59 (d, J ) 4.6 Hz, 1H), 8.49 (d, J ) 4.7 Hz, 2H), 7.67-7.07 (m,
14H), 5.20 (s, 1H), 3.93 (s, 4H). MS (ESI) m/e: 367 (100), 368 (30),
369 (3).
[Zn(r-PhTPA)Cl]ClO4. Compound 4 (523.5 mg, 1.43 mmol) was
dissolved in 0.5 mL of methanol. Zn(ClO4)2‚6H2O (531.4 mg, 1.43
mmol) in 0.8 mL of methanol was added. After stirring for a while
NaCl (83.4 mg, 1.43 mmol) in 0.5 mL of H2O was added slowly,
resulting in immediate precipitation. The precipitate was collected and
dried (0.771 g, 67.7%). Recrystallization from 50% methanol/H2O gave
X-ray-quality crystals. 1H NMR (300 MHz, CD3CN): 9.17 (d, J )
5.2 Hz, 1H), 9.30 (d, J ) 5.2 Hz, 1H), 8.98 (d, J ) 5.0 Hz, 1H), 8.12-
6.84 (m, 14H), 5.28 (s, 1H), 4.22 (d, Jgem ) 16.1 Hz, 1H), 4.13 (d, Jgem
) 16.5 Hz, 1H), 4.00 (d, Jgem ) 16.5 Hz, 1H), 3.77 (d, Jgem ) 16.1 Hz,
1H). Anal. Calcd for ZnC24H22N4Cl2O4: C, 50.86; H, 3.91; N, 9.89.
Found: C, 50.67; H, 3.72; N, 9.83.
X-ray Structures. Intensity data for [Zn(R-MeTPA)Cl](ClO4) and
[Zn(R-PhTPA)Cl](ClO4) were collected at -80 °C (to minimize crystal
decay) on a Rigaku AFC6R diffractometer equipped with a 12 kW
rotating anode generator, a graphite monochromator (Cu KR, λ )
1.541 78 Å) and a low-temperature apparatus which funneled a chilled
stream of nitrogen gas onto the mounted crystal. Structures were solved
by a combination of Patterson methods, PHASE,29 and direct methods,
DIRDIF,30 using the TEXSAN software package.31 Crystallographic
data are listed in Table 1. Details on the structure of [Zn(R-
MeBQPA)Cl](ClO4) were included in the preliminary report.1
Spectra. Solutions in methanol of the ligands and complexes were
prepared for the CD and the UV-vis studies. For CD spectra (1 mm
cell) the concentrations used were (0.8-2.2) × 10-4 M for the series
of solutions with R-MeBQPA as the ligand, (0.6-1.6) × 10-4 M for
R-MePh2TPA, and (0.9-2.3) × 10-4 M for R-MeTPA. For the UV-
vis studies (10 mm cell) the concentrations used were (6.6-7.7) ×
10-6 M for R-MeBQPA, (6.4-7.4) × 10-6 M for R-MePh2TPA, and
[Cd(r-MePh2TPA)I2]. A solution of CdI2 (80.6 mg, 0.220 mmol)
in 0.5 mL of MeCN was added dropwise via pipet to a hot solution of
2 (100.6 mg, 0.220 mmol) in 3.3 mL of MeCN. A white precipitate
formed after sitting overnight at 5 °C. The precipitate was collected
and washed with 2 mL of MeCN. The sample was dried in vacuo to
yield 123.5 mg (68.2%) of a white powder. Anal. Calcd for
C31H28N4CdI2: C, 45.25; H, 3.43; N, 6.81. Found (R form): C, 45.03;
H, 3.20; N, 6.94. Found (S form): C, 45.12; H, 3.22; N, 6.74.
[Zn(r-MePh2TPA)Cl]ClO4. A solution of Zn(ClO4)2‚6H2O (58.1
mg, 0.156 mmol) in 0.4 mL of H2O was added to a hot solution of 2
(71.3 mg, 0.156 mmol) in 3 mL of MeCN. While maintaining the
temperature a solution of NaCl (11.0 mg, 0.188 mmol) in 0.4 mL of
H2O was added. The solvent was partially evaporated at ambient
temperature and the clear solution was allowed to stand overnight. White
crystals formed which were collected and dried in vacuo. The yield
was 80.7 mg (78.7%). Anal. Calcd for C31H28N4ZnCl2O4: C, 56.68;
H, 4.30; N, 8.53. Found (R form): C, 56.90; H, 4.62; N, 8.80. Found
(S form): C, 56.94; H, 4.44; N, 8.77.
1
(7.2-10.5) × 10-6 M for R-MeTPA. Variable temperature H NMR
experiments were conducted on a 300 MHz Varian Gemini instrument.
Concentrations were 6-10 mg/0.75 mL of sovent. Frozen EPR spectra
of [Cu(R-MeTPA)Cl]ClO4 and [Cu(R-BQPA)Cl]ClO4 were determined
in glassy ethanol/methanol (1:9) and glassy DMF/CHCl3 (1:1) solutions,
respectively, at -196 °C utilizing a Varian E-4 spectrometer. The
(28) Niemers, E.; Hiltmann, R. Synthesis 1976, 593.
(29) Calabrese, J. C. Ph.D. Dissertation Thesis, University of Wisconsins
Madison, 1972.
(30) Beurskens, P. T. In Technical Report 1984/1; Crystallography Labora-
tory: Toernooiveld 1, 6525 ED Nijmegen, University of Nijmegen,
The Netherlands, 1984.
Phenyl-2-pyridylmethanamine, 6. 2-Benzoylpyridine (18.5 g,
0.101 mol) and hydroxylamine hydrochloride (13.5 g, 0.194 mol) were
dissolved in 90 mL of pyridine. The mixture was heated to reflux for
(31) Single-Crystal Structure Analysis Software, Version 5.0; Molecular
Structure Corporation: The Woodlands, TX, 1985.

6258 Inorganic Chemistry, Vol. 37, No. 24, 1998
Canary et al.
82
85
84
Figure 3. Synthesis of ligands.
Table 1. Synopsis of Structural Parameters Derived from X-ray
Crystallographic Data for Racemic [Zn(R-MeTPA)Cl]ClO4‚0.5C7H8
and [Zn(R-PhTPA)Cl]ClO4
[Zn(R-MeTPA)Cl]- [Zn(R-PhTPA)Cl]-
ClO4‚0.5C7H8
ClO4
empirical formula
fw
temp (°C)
cryst color, habit
cryst dimens (mm)
C22.5H20Cl2N4O4Zn C24H22Cl2N4O4Zn
546.72
-80
566.75
-80
colorless, block
0.450 × 0.075 ×
0.100
colorless, block
0.200 × 0.100 ×
0.100
Figure 4. X-ray crystallographic structure of [Zn(R-MeTPA)Cl](ClO4).
Some hydrogens omitted for clarity.
cryst system
space group
Z value
monoclinic
C2/c (No. 15)
8
monoclinic
P21/c (No. 14)
4
anol 7.32 Treatment with aqueous HBr followed by mild base
provided compound 8. Reaction of 1-(2-pyridyl)ethylamine 9
with 8 in DMF in the presence of diisopropylethylamine
(DIPEA) gave the dibromotripod 10. Suzuki coupling33,34 with
phenylboronic acid provided the target ligand 2 in good yield
(91%).
In general, coordination complexes were prepared simply by
mixing homogeneous solutions of ligand and metal salts under
conditions suitable for crystallization of the complexes. In most
cases, the solids were found to display correct elemental analysis
without further purification.
X-ray Structures. X-ray crystallographic studies were
undertaken on the racemic coordination complexes [Zn(R-
MeTPA)Cl]ClO4, [Zn(R-MeBQPA)Cl]ClO4,1 and [Zn(R-Ph-
TPA)Cl]ClO4 to determine the nuclearity, tripodal ligand
conformation and metal coordination geometry.
Colorless, block-shaped crystals of a zinc(II) complex of
R-MeTPA were obtained by recrystallization from a toluene/
diethyl ether/acetone solvent system. The structure, solved in
the monoclinic space group C2/c, was formulated as [Zn(R-
MeTPA)Cl]ClO4‚0.5C7H8. The perchlorate anion is disordered
in the structure and was partially modeled by two orientations
employing four pairs of oxygens around a central chlorine atom.
Highly disordered solvate molecules are present in the structure,
including a modeled toluene molecule (half-equivalent). ORTEP
representations of [Zn(R-MeTPA)Cl]+ cation are shown in
Figure 4. The [Zn(R-MeTPA)Cl]+ is structurally similar to
[Zn(TPA)Cl]+ 7 as a pentacoordinate zinc(II) cation resides in
lattice params
b ) 14.022(2) Å
c ) 13.331(3) Å
â ) 101.35(2)°
V ) 4881(2) Å3
Dcalcd (g/cm3)
F000
a ) 26.630(5) Å
b ) 13.12 (1) Å
c ) 21.47 (2) Å
â ) 97.93 (7)°
V ) 2349 (4) Å3
1.488
2232
37.80
2791
a ) 8.419(9) Å
1.602
1160
39.48
2307
µ
CuKR (cm-1
)
no. of observns
(I > 3.00σ(I))
no. of variables
reflcn/param ratio
residuals: R; Rw
366
7.63
0.066; 0.086
334
6.91
0.055; 0.064
1.96
0.44; -0.40
goodness-of-fit indicator 3.37
max/min peaks in final
0.94; -0.58
diff. map (e-/Å3)
concentration of the solutions were 0.0015 and 0.0010 M, respectively.
The microwave frequency was 9.22 × 109 Hz at 2 mW power, while
the modulation was 4.0 G at 100 kHz. The spectra were referenced to
external diphenylpicrylhydrazyl radical (DPPH).
Results
Synthesis. In general, the ligands reported in this paper were
prepared from the alkylation of primary amines with halo-
methylpyridines. Racemic samples were synthesized and
utilized for coordination chemistry studies including the prepa-
ration of samples for crystallographic analysis. Materials for
circular dichroism studies were obtained subsequently from
enantiomerically pure primary amines.
The syntheses of R-MeTPA26 and R-MeBQPA were described
in the previous report.1 The ligand R-PhTPA was prepared from
phenylpyridyl ketone 5 by formation of the oxime, reduction,
and alkylation with picolyl chloride. The synthesis of compound
2 (R-MePh2TPA, Figure 3) began with 6-bromo-2-pyridinemeth-
(32) Cai, D.; Hughes, D. L.; Verhoeven, T. R. Tetrahedron Lett. 1996, 37,
2537.
(33) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457.
(34) Aliprantis, A. O.; Canary, J. W. J. Am. Chem. Soc. 1994, 116, 6985.

Conformationally Mobile Tripodal Ligands
Inorganic Chemistry, Vol. 37, No. 24, 1998 6259
Figure 5. X-ray crystallographic structure of [Zn(R-MeBQPA)-
Cl](ClO4). Some hydrogens omitted for clarity.
a distorted trigonal bipyramidal environment with the pyridyl
nitrogens coordinated in the equatorial positions and the tertiary
amine nitrogen in an axial position. The zinc(II) cation is
perched 0.46 Å above the least-squares plane defined by the
pyridyl nitrogens. Intramolecular Zn-Npy bond distances range
from 2.05 to 2.09 Å and chelate angles (Nam-Zn-Npy) from
76 to 78° which are similar to the values in other trigonal
bipyramidal zinc/TPA complexes.7,11 The Zn-Nam bond (2.28
Å) is longer than the Zn-Npy bonds. The chlorine resides in
an apical site, nearly on the Zn-Nam axis as indicated by the
Nam-Zn-Cl angle of 178.0(1)°. The conformation of the ligand
in the complex is anti. The three pyridine rings are tilted in
the same direction, with angles between the planes containing
the pyridine rings and the Zn-Nam axis of 7.6, 7.0, and 10.2°.
The representation shown in Figure 4 shows that for a CR
configuration of R, a Λ propeller-like twist of the pyridine rings
is adopted.
Figure 6. X-ray crystallographic structure of [Zn(R-PhTPA)Cl](ClO4).
Some hydrogens omitted for clarity.
nitrogens. Intramolecular Zn-Nquin bond distances are in the
relatively narrow range of 2.14-2.16 Å, and the Zn-Nam bond
length is 2.20 Å. The average tilt angle of the pyridine planes
with the Nam-Zn axis is 15.9° with values 10.3, 23.1, and 14.2°.
The resulting twist is in the same direction relative to CR as
observed in the R-MeTPA complex.
The structure of racemic [Zn(R-PhTPA)Cl]ClO4 (Figure 6)
was solved in the monoclinic space group P21/c with the
asymmetric unit consisting of a [Zn(R-PhTPA)Cl]+ cation and
a disordered perchlorate anion which was partially modeled by
six oxygens emanating from a central chlorine atom. Intramo-
lecular Zn-Npy bond distances range from 2.05 to 2.10 Å and
the Zn-Nam bond is 2.28 Å. The tilt of the pyridine planes
with respect to the Nam-Zn axis is 11.4, 15.3, and 10.1°, forming
a propeller twist with the same directional sense relative to CR
as observed in the other two structures.
Spectra. The spectroscopic properties of the enantiomerically
pure ligands and their complexes with Zn(II), Cu(II), and Cd(II)
ions were studied by UV-vis and circular dichroism spectros-
copy. The spectra of the ligands and their complexes appear
in Figures 7-9, and some extinction coefficients are listed in
Table 2. The UV-vis spectra of R-MeTPA and its complexes
in methanol show that the π-π* transitions in the ligand are
affected only slightly when the ligand is complexed with Cu(II)
or Cd(II) which causes a hypsochromic shift of 2 nm; a
hyperchromic effect can also be observed. The maxima around
260-262 nm for the ligand and complexes correspond to the
pyridine B band. The n-π* is not observed as expected due
The structure of racemic [Zn(R-MeBQPA)Cl]ClO4 was solved
in the monoclinic space group C2/c with the asymmetric unit
consisting of a [Zn(R-MeBQPA)Cl]+ cation and a noncoordi-
nating perchlorate anion.1 The perchlorate anion was not
disordered in this structure. ORTEP diagrams of a [Zn(R-
MeBQPA)Cl]+ cation are shown in Figure 5. The figure reveals
a pentacoordinate zinc(II) in a distorted trigonal bipyramidal
environment with the heterocyclic nitrogens coordinated in the
equatorial positions and the tertiary amine nitrogen in an axial
site. Chloride resides in the available apical site but is not on
the Zn-Nam axis as the bond angle Nam-Zn-Cl is 166.2(2)°.
Steric hindrance between the chloride and the quinoline
8-hydrogens is evident as the chloride is forced to tilt away.
Steric hindrance influences the tilt of the heterocyclic rings
relative to the Zn-Nam axis. The zinc(II) cation is perched 0.48
angstrom above the least-squares plane defined by the pyridyl

6260 Inorganic Chemistry, Vol. 37, No. 24, 1998
Canary et al.
Figure 9. Circular dichroism and UV-vis spectra (×10-4) (methanol)
for S-R-MeBPPA, [Cd(S-R-MeBPPA)I2], and [Zn(S-R-MeBPPA)Cl]-
ClO4.
Figure 7. Circular dichroism and UV-vis spectra (×10-4) (methanol)
for S-R-MeTPA, [Cd(S-R-MeTPA)I2], [Zn(S-R-MeTPA)Cl]ClO4, and
[Cu(S-R-MeTPA)Cl]ClO4.
Table 2. Isotropic and Anisotropic Extinction Coefficients and
Dissymmetry Factors (g) of Free Ligands and Metal-Ligand
Complexes of (S)-R-MeTPA (265 nm), (S)-R-MePh2TPA (307 nm),
and (S)-R-MeBQPA (243 nm)
∆ꢀ
3.55
22.29
17.98
9.11
ꢀ (×10-4
1.33
1.45
1.68
1.75
)
g (×103)
0.27
1.54
1.07
0.52
R-MeTPA
[Zn(R-MeTPA)Cl]ClO4
[Cu(R-MeTPA)Cl]ClO4
[Cd(R-MeTPA)I2]
R-MePh2TPA
[Zn(R-MePh2TPA)Cl]ClO4
[Cd(R-MePh2TPA)I2]
-0.86
-17.59
-8.09
0.52
0.68
0.76
0.16
2.60
1.06
R-MeBQPA
0.34
-139.23
-111.54
-16.92
1.66
2.19
2.97
2.09
0.02
6.34
3.75
0.81
[Zn(R-MeBQPA)Cl]ClO4
[Cu(R-MeBQPA)Cl]ClO4
[Cd(R-MeBQPA)I2]
band) of the pyridine (278 and 282 nm) and benzene (250 nm)
chromophores. The latter is clearly distinguishable in the free
ligand and the Zn(II) complex; in the Cd(II) complexes the
transition due to the benzene appears as a shoulder. In this
case a hypochromic effect is seen when the ligand complexes
Zn(II) or Cd(II), and the Zn(II) complex shows also a batho-
chromic shift in the pyridine B band absorption.
The UV-vis spectra of R-MeBQPA and its metal complexes
show the strong E1 band of the quinoline rings (232 nm), the
B-band of the pyridine (263, 270 nm), and the B band of the
quinoline (303, 317 nm). As with R-MePh2TPA there is a small
hypochromic effect when the ligand is complexed with Zn(II)
or Cd(II), and a hyperchromic effect with Cu(II). The Cu(II)
and Cd(II) complexes show a bathochromic shift in the pyridine
B band, compared to the free ligand (from 263 to 270 nm).
The complex with Zn(II) does not show a distinguishable
absorption in this region. The B band from the quinoline is
Figure 8. Circular dichroism and UV-vis spectra (×10-4) (methanol)
for S-R-MeBQPA, [Cd(S-R-MeBQPA)I2], [Zn(S-R-MeBQPA)Cl]ClO4,
and [Cu(S-R-MeBQPA)Cl]ClO4.
to the strong B band. The spectra of R-MePh2TPA and its
complexes in methanol show the typical π-π* transition (B-

Conformationally Mobile Tripodal Ligands
Inorganic Chemistry, Vol. 37, No. 24, 1998 6261
clearly present in the free ligand and the Cu(II) and Zn(II)
complexes, but not in the Cd(II) complex.
The dissymmetry factor (also called optic anisotropy, g )
∆ꢀ/ꢀ × 103) was calculated for the ligands and complexes (Table
2). The trigonal bipyramidal complexes (Zn(II) or Cu(II)) give
relatively large values, whereas the values for the free ligand
and the Cd(II) complexes are small. In the case of S-R-
MeBQPA, the Zn(II) complex has a value of 6.3 and the Cu(II)
complex of 3.8 at λ ) 243, both being high values for aromatic
chromophores. For comparison, (+) hexahelicene presents a
g244 ) 7.7, and (+)-1,1′-binaphthyl-2,2′-diamine a g247 ) 3.39
In the case of S-R-MePh2TPA, the values are somewhat lower
for the Zn(II) complex (g307 ) 2.6). The relatively larger effect
observed for Zn(II) and Cu(II) complexes persists as it does
for R-R-MeTPA (Zn, g265 ) 1.5; Cu, g265 ) 1.1).
The magnitude of the ∆ꢀ and of the g number increases in
the order [Zn(R-MeTPA)Cl]ClO4 < [Zn(R-MePh2TPA)Cl]ClO4
< [Zn(R-MeBQPA)Cl]ClO4. This result is consistent with the
observation in the crystallographic data that the tilt of the
heteroaromatic rings and thus the propeller twist is larger in
the quinoline containing complex than in the R-MeTPA one.
Thus, the magnification of the chirality by metal ion complex-
ation is larger when the arms contain sterically demanding
substituents.
It should be noted that analysis of the spectra of the Cu(II)
complexes may be complicated by the possible presence of both
trigonal bipyramidal and square pyramidal complexes in solu-
tion. The absorbances at 872 and 725 nm are diagnostic of
partitioning between these two coordination geometries.17 In
[Cu(TPA)Cl]ClO4 (2 mM solution in methanol), the major
absorbance is at 872 nm (ꢀ ) 148) with a shoulder at 725 nm
(ꢀ ) 79). The complex [Cu(R-MeTPA)Cl]ClO4 is nearly
identical (ꢀ ) 156, 82) while [Cu(R-MeBQPA)Cl]ClO4 (two
peaks, ꢀ ) 157, 145) and [Cu(R-MePh2TPA)Cl]ClO4 (one broad
peak, ꢀ ) 146, 124) give patterns that indicate that a mixture
of trigonal bipyramidal and square pyramidal geometries are
present. However, the strong circular dichroic signals compared
with the corresponding zinc complexes, which should adopt only
trigonal bipyramidal coordination, suggests the possibility that
the copper complexes might give even more intense circular
dichroic signals if the coordination of the copper were more
predominantly trigonal bipyramidal. The frozen EPR spectra
In general the UV-vis data present all the electronic features
that are expected from these aromatic, tripodal tetradentate
ligands and complexes. There are small shifts and changes in
intensity upon complexation, which are dependent on the nature
of the metal ion. As expected, the metal ion coordination
geometry has little effect on the isotropic transitions.
The CD spectra of the ligands and their complexes in
methanol (Figures 7-9) show significant enhancement of the
signal when the Zn(II) and Cu(II) complexes are compared with
the free ligand or the Cd(II) complexes. In the case of [Zn(R-
MeBQPA)Cl](ClO4) and [Cu(R-MeBQPA)Cl](ClO4), the CD
spectra show Cotton effects centered approximately at the
wavelength of maximum absorption in the UV-vis spectra (232
nm), suggesting exciton coupling in light of the spatial proximity
of the heteroaromatic chromophores in a chiral environment.35,36
A separate spectroscopic study was published recently that
addresses this issue in more detail.37
Discussion
All three crystallographic structures reveal the substituted
ligand in the anti conformation. In each case, the three
heteroaromatic rings are tilted in the same direction with respect
to the central axis of the molecule (i.e., the line containing the
Zn-Nam bond). The magnitude of the tilt, which ranges from
7 to 26°, is influenced by several factors. No discernible trend
is apparent for the effect of the substituent on the saturated
carbon of the ligand arm; the average angle between the best
planes of the pyridine rings and the line containing the Nam
-
Zn bond is 11.9° for [Zn(TPA)Cl]+,7 8.3° for [Zn(R-Me-
TPA)Cl]+, and 12.3° for [Zn(R-PhTPA)Cl]+. The presence of
a bulky substituent at the pyridine position-6 appears to increase
this angle for [Zn(R-MeBQPA)Cl]+ (15.9°). Overall, crystal
packing forces probably account for much of the difference in
magnitude of the tilt angles as seen in the fact that even in
[Zn(TPA)Cl]+ one of the angles is double the other two (17.2,
9.7, and 8.9°).7 Taken together, the orientation of the rings in
a complex constitutes a propeller-like twist. The sense of this
twist is the same in each complex relative to the absolute
configuration of the asymmetric carbon atom of the molecule.
Recently, circular dichroism studies have yielded much
interesting information about solution structures,38 so this
technique was employed in an attempt to discern the behavior
of the present complexes in solution. For each ligand, the
Zn(Cl)(ClO4) and Cu(Cl)(ClO4) complexes give larger aniso-
tropic absorptions than the free ligand. This is the expected
result if the ligand adopts a similar conformation in solution as
that observed in the solid state. The free ligand has many
accessible conformations, so that no special chirality is present.
Upon presentation of a Zn(II) or Cu(II) ion, the ligand wraps
around the metal, with the heteroaromatic rings adopting a
helical configuration whose sense is dictated by the absolute
configuration of CR. The reduced effect observed in the Cd(II)
complexes serves as an important control experiment, since the
expected octahedral [Cd(L)I2]7 complexes would place the
ligand in a Cσ conformation, which would not possess the
propeller-like arrangement of the heteroaromatic moieties.
of [Cu(R-MeTPA)Cl]ClO4 (g ∼ 2.00, g ) 2.19, A ) 92 ×
||
10-4 cm-1) and [Cu(R-BQPPA)Cl]ClO4 (g ∼ 2.00, g ) 2.22,
||
A ) 124 × 10-4 cm-1) are consistent with those for Cu(II)
complexes with tripodal ligands that have trigonal bipyramidal
geometry.40
Monte Carlo/stochastic dynamics (MC/SD) simulations of the
conformational behavior of the [Zn(L)Cl]+ complexes were
conducted; the procedures and detailed results of the study are
included in the Supporting Information. Summarizing, the
simulations provided a picture of the complexes as dynamic
molecules interconverting between two energetically accessible
conformations (anti and syn). Energy minimization of structures
generated in the MC/SD simulation resulted in only two unique
structures (Λ and ∆). Under the conditions of the simulation,
the two conformations interconvert rapidly but (in the R-sub-
stituted ligand complexes) with a bias for the anti conformation
as anticipated from model examination and semiempirical
calculations. As the two ligand conformations interconvert, the
tilt of the pyridine rings and thus the axial chirality of propeller
reverses. This reversal is partially but not completely concerted
(35) Kirschner, S. Coor. Chem. ReV. 1967, 2, 461.
(36) Crabbe´, P. Optical Rotatory Dispersion and Circular Dichroism in
Organic Chemistry; Holden-Day: San Francisco, 1965.
(37) Castagnetto, J. M.; Xu, X.; Berova, N.; Canary, J. W. Chirality 1997,
9, 616.
(39) Eliel, E. L.; Wilen, S. H. Stereochemistry of Organic Compounds;
John Wiley & Sons: New York, 1994.
(38) Circular Dichroism: Principles and Applications; Nakanishi, K.,
Berova, N., Woody, R. W., Eds.; VCH Publishers: New York, 1994.
(40) Karlin, K. D.; Hayes, J. C.; Juen, S.; Hutchinson, J. P.; Zubieta, J.
Inorg. Chem. 1982, 21, 4106.

6262 Inorganic Chemistry, Vol. 37, No. 24, 1998
Canary et al.
such that all three heteroaromatic rings are tilted in the same
direction most but not all of the time. Thus, the simulations
support the presence of only two conformations as minimum
energy structures, and suggest that these limiting conformations
predominate at room temperature despite the fact that the ligand
arms are conformationally mobile and able to tilt in and out of
phase with the overall twist associated with the propeller-like
structure.
of the time, allowing strong anisotropic absorption despite
conformational mobility.
Conclusions
A combination of crystallographic, spectroscopic, and com-
putational methods provides a consistent picture of these
coordination complexes. The conformationally flexible ligands
wrap around trigonal bipyramidal ZnII and CuII ions, forming a
propeller-like complex due to the tilt of the heterocyclic rings
with respect to the central axis of the molecule. In the complex,
two conformations are available with opposite axial chirality,
with one being preferred. A single conformation is observed
in the solid state for three complexes that have been crystallized,
and a similar conformation appears to predominate in solution
as the circular dichroic data attest. The MC/SD simulations
provide a more detailed picture of the dynamic chirality of the
complexes that is consistent with all of the observed data.
The 1H NMR spectra of the complexes provide little specific
information about conformational behavior. Low-temperature
spectra of [Zn(TPA)](ClO4)2 show a singlet for the methylene
groups in CD3CN at -80 °C, whereas freezing of the inter-
conversion of the conformational enantiomers should result in
AB doublets. Spectra of [Zn(R-MeBQPA)Cl](ClO4) in CD2Cl2
as low as -90 °C failed to show any evidence that would
indicate slowly interconverting conformational isomers. This
latter result may result from a low population of the higher-
energy conformer at low temperature or fast exchange as
suggested by the mechanism of interconversion involving
rotation of single bonds sequentially in each arm.41
Thus, the Zn(II) and Cu(II) complexes of the chiral ligands
display pronounced circular dichroic spectra with amplitudes
that would be expected from much more rigid molecules. The
solid-state structures display propellers resulting in orientation
of the heterocycle chromophores in a manner consistent with
strong solution CD spectra. The MC/SD simulations connect
the solid state and spectroscopic data by showing that although
the arms of the tripod ligands are quite mobile, they remain in
an orientation similar to that observed in the solid-state most
Acknowledgment. We thank the National Institutes of
Health (GM 49170) for support of this research. We are grateful
to Professor Donald Wink for assistance with the crystal-
lographic structures. EI, FAB, and high-resolution mass spectra
were obtained at the Michigan State University Mass Spec-
trometry Facility (supported in part by grant DRR-00480, from
the Biotechnology Research Technology Program, NCRS, NIH).
Supporting Information Available: Molecular modeling compu-
tational procedures and results, and X-ray crystallographic details for
[Zn(RMeTPA)Cl](ClO4) and [Zn(R-PhTPA)Cl](ClO4), including ex-
perimental procedures, positional and thermal parameters, temperature
factors, bond distances and angles, and torsional angles (88 pages).
Ordering information is given on any current masthead page.
(41) Recently, the minor (syn) conformer of a copper complex of ligand 2
was observed crystallographically: Xu, X.; Canary, J. W., unpublished
results.
IC9803098