Multinuclear NMR spectra, 1H-T1 relaxation, conformational behavior, and intramolecular Hδ-····δ+H contacts of N-borane cyclic adducts in solution

Article abstract of DOI:10.1021/ic000890j

The VT ¹H NMR and ¹H-NOESY spectra revealed "frozen" envelope conformations in solutions of 1 and 2 with the BR₃ groups in equatorial positions. The ¹H-T₁ relaxation measurements provided determinatio

Full text of DOI:10.1021/ic000890j

Inorg. Chem. 2001, 40, 3243-3246
3243
1
were carried out by the standard inversion-recovery (180°-τ-90°)
method with the use of a Bruker-300 NMR spectrometer in deoxy-
genated solutions. Calculations of the relaxation times were completed
using the nonlinear three-parameter fitting routine of the spectrometer.
Adducts 1-4 were prepared by the room-temperature addition of
1.2 equiv of BH3‚S(CH3)2 or BF3‚OEt3 solutions to 1.0 equiv of a dry
amine under anhydrous conditions. An excess of BH3‚S(CH3)2 was
evaporated, and the resulting compounds were kept under N2. The BH3-
and BF3-adducts were obtained as white solids and viscous yellow
liquids, respectively.
Multinuclear NMR Spectra, H-T1 Relaxation,
Conformational Behavior, and Intramolecular
Hδ-‚‚‚‚δ+H Contacts of N-Borane Cyclic
Adducts in Solution
Marisol Gu1izado-Rodr´ıguez,† Angelina Flores-Parra,†
Sonia A. Sa´nchez-Ruiz,† Rafael Tapia-Benavides,‡
Rosalinda Contreras,*,† and Vladimir I. Bakhmutov*,†
1
2
Adduct 1. H NMR (CDCl3): δ 3.20 (m, 2H, H-1, J(H1-H2) )
Chemistry Department, Centro de Investigacio´n y de
Estudios Avanzados del IPN, A.P. 14-740, C.P. 07000,
Me´xico D.F, Mexico, and Centro de Investigaciones
Qu´ımicas, Universidad A. del Estado de Hidalgo.
Carretera Pachuca-Tulancingo Km 4.5. U. Universitaria,
Pachuca Hidalgo. C.P. 42074, Mexico
3
3
11.5 Hz, J(H1-H3) ) 6.0 Hz, J(H1-H4) ) 5.0 Hz); 2.65 (m, 2H,
H-2, 3J(H2-H3) ) 6.6 Hz, 3J(H2-H4) ) 7.0 Hz); 1.94 (m, 2H, H-3,
2J(H3-H4) ) 12.0 Hz); 1.80 (m, 2H, H-4); 4.6 (s, 1H, N-H); 1.4 (q,
3H, BH3). 13C NMR (CDCl3): δ 54.2 (t, C-2,5, 1J(C-H) ) 142.2 Hz);
24.6 (t, C-3,4, 1J(C-H) ) 133.9 Hz). 11B NMR (CDCl3): δ -17.2 (q,
BH3, 1J(B-H) ) 94.2 Hz). 15N NMR (C6D6): δ -331.3 (d, NH, 1J(N-
H) ) 71.3 Hz).
ReceiVed August 4, 2000
1
2
Adduct 2. H NMR (CDCl3): δ 3.16 (m, 2H, H-1, J(H1-H2) )
3
3
11.2 Hz), J(H1-H3) and J(H1-H4) ) 5.7 Hz); 2.97 (m, 2H, H-2,
Introduction
3J(H2-H3) and J(H2-H4) ) 6.7 Hz); 1.87 (m, 4H, H-3,4); 5.1 (s,
3
1H, N-H). 13C NMR (CDCl3): δ 47.0 (t, C-2,5, J(C-H) ) 146.1
1
Short proton-hydride (Hδ+...δ-H) contacts are organizing
interactions which initiate chemical reactions. For example, di-
hydrogen bonding in transition metal hydrides causes proton
transfer to yield dihydrogen complexes.1 The dihydrogen bonds,
M-Hδ-‚‚‚δ+H or even B-Hδ-...δ+H,2 are experimentally ob-
served by convenient methods in solution and solid state. Intra-
molecular contacts C-Hδ+‚‚‚‚-δH-B and C-Hδ+‚‚‚‚-δF-B
(2.2-2.5 Å), established in some cyclic borane adducts in solid
state,2b,3a can affect conformational states of such molecules in
solution.3 The aim of the present work was to determine these
C-Hδ+‚‚‚‚-δH-B contacts in solutions by the 1H-T1 relaxation
method.
Hz); 24.3 (t, C-3,4, 1J(C-H) ) 134.2 Hz). 11B{1H} NMR (CDCl3): δ
1
-1.0 (q, BF3, J(B-F) ) 16.9 Hz).19F NMR (C6D6): δ -156.9. 15N
1
1
NMR (C6D6): δ -324.0 (m, NH, J(N-H) ) 71.6 Hz, J(N-B) )
2
19.5, J(N-F) ) 19.5 Hz).
1
Adduct 3. H NMR (CDCl3): δ 3.12 (m, 2H, H-1); 2.71 (m, 2H,
H-2); 2.07 (m, 2H, H-3); 1.89 (m, 2H, H-4), 2.60 (s, 3H, Me-N). 13
C
NMR (CDCl3): δ 62.8 (t, C-2,5, 1J(C-H) ) 145.3 Hz); 23.0 (t, C-3,4,
1J(C-H) ) 131.5 Hz); 51.2 (q, Me-N, J(C-H) ) 141.4 Hz). 11B
1
1
NMR (CDCl3): δ -11.2 (q, BH3, J(B-H) ) 96.8 Hz).
1
Adduct 4. H NMR (CDCl3): δ 3.45 (m, 2H, H-2); 2.77 (m, 2H,
H-1); 1.97 (m, 4H, H-3, H-4); 2.60 (s, 3H, Me-N). 13C NMR
(CDCl3): δ 56.9 (t, C-2,5, 1J(C-H) ) 143.0 Hz); 23.1 (t, C-3,4,
1J(C-H) ) 133.4 Hz); 44.6 (q, Me-N, J(C-H) ) 140.7 Hz). 11B-
1
{1H} NMR (CDCl3): δ 0.1 (q, BF3, J(B-F) ) 15.7 Hz).19F NMR
1
Experimental Section
(CDCl3): δ -162.2.
Solvents and amines were freshly distilled and dried before use
according to convenient procedures. The NMR spectra were obtained
with JEOL-400 and Bruker-300 spectrometers. The T1 measurements
1
2
Adduct 7. H NMR (CDCl3): δ 3.04 (m, 2H, H-2, J(H1-H2) )
13.5 Hz, 3J(H2-H4) ) 11.0 Hz, 3J(H2-H3) ) 4.6 Hz); 2.81 (m, 2H,
3
3
H-1, J(H1-H3) and J(H1-H4) ) 3.9 Hz); 1.71 (m, 1H, H-5), 1.68
(m, 2H, H-3,4), 1.41 (m, 1H, H-6); 2.56 (m, 3H, CH3, J(H-11B) )
3
* To whom correspondence should be addressed. Fax: (internat. + 52-
5/747-7113). E-mail: rcontrer@mail.cinvestav.mx.
1.5 Hz). 13C NMR (CDCl3): δ 52.4 (t, C-2,6, J(C-H) ) 140.7 Hz);
1
1
1
19.0 (t, C-3,5, J(C-H) ) 129.1 Hz); 22.3 (t, C-4, J(C-H) ) 128.4
† Chemistry Department, Centro de Investigacio´n y de Estudios Avan-
zados del IPN.
Hz); 39.1 (q, Me-N, 1J(C-H) ) 141.4 Hz). 11B{1H} NMR (CDCl3):
1
δ 0.3 (q, BF3, J(B-F) ) 15.8 Hz).19F NMR (CDCl3): δ -163.4.
‡ Centro de Investigaciones Qu´ımicas, Universidad A. del Estado de
Hidalgo.
(1) (a) Richardson, T. B.; de Gala, S.; Crabtree, R. H.; Siegbahn, P. E.
M. J. Am. Chem. Soc. 1995, 117, 12875. (b) Shubina, E. S.; Belkova,
N. V.; Krylov, A. N.; Vorontsov, E. V.; Epstein, L. M.; Gusev, D.
G.; Niedermann, M.; Berke, H. J. Am. Chem. Soc. 1996, 118, 1105.
(c) Peris, E.; Wessel, J.; Patel, B. P.; Crabtree, R. H. J. Chem Soc.,
Chem. Commun. 1995, 2175. (d) Ayllo´n, J. A.; Gervaux, C.; Sabo-
Etienne, S.; Chaudret B. Organometallics 1997, 16, 2000. (e) Shubina,
E. S.; Belkova, N. V.; Bakhmutova, E. V.; Vorontsov, E. V.;
Bakhmutov, V. I.; Ionidis, A. V.; Bianchini, C.; Marvelli, L.; Peruzzini,
M.; Epstein, L. M. Inorg. Chim. Acta 1998, 280, 302.
(2) (a) Padilla-Mart´ınez, I. I.; Rosalez-Hoz, M. J.; Contreras, R. 49th
Southwest Regional ACS Meeting, October 24-27 1993. (b) Padilla-
Mart´ınez, I. I.; Rosalez-Hoz, M. J.; Tlahuext, H.; Camacho-Camacho,
C.; Ariza-Castolo, A.; Contreras, R. Chem. Ber. 1996, 129, 441. (c)
Epstein, L. M.; Shubina, E. S.; Bakhmutova, E. V.; Saitkulova, L.
N.; Bakhmutov, V. I.; Chistyakov, A. L.; Stankevich, I. V. Inorg.
Chem. 1998, 37, 3013. (d) Klooster, W. T.; Koetzle, T. F.; Siegbahn,
P. E. M.; Richardson, T. B.; Crabtree, R. H. J. Am. Chem. Soc. 1999,
121, 6337.
Results and Discussion
Borane adducts 1-4 were characterized by multinuclear NMR
spectra (Experimental Section). Simulation procedures, {1H}-,
{11B}-, and 1H-NOESY experiments provided the assignments
in Table 1.
1
The H NMR spectrum of adduct 1 (CDCl3, 25 °C) shows
four nonequivalent methylene protons, supporting the structure
in Chart 1. Protons 1 and 2 exhibit the different J(HCNH)
3
constants. Finally, the line of 2 is remarkably broadened (∆ν
) 2.5-3.0 Hz) due to a three-bond H-11B coupling. This
1
effect, by analogy with the 3J(H-H) coupling rule,4 results from
different dihedral angles H(1)-C-N-B and H(2)-C-N-B
(Chart 2). The same spectral features are detected in adduct 2.
In contrast, the “frozen” (ring-chair) conformations of the
cycles in 5-83 (Chart 1) show the equally broadened lines of
protons 1 and 2 due to their symmetrical location with respect
(3) (a) Flores-Parra, A.; Sa´nchez-Ruiz, S. A.; Guadarrama, C.; No¨th, H.;
Contreras, R. Eur. J. Inorg. Chem. 1999, 2069. (b) Flores-Parra, A.;
Farfa´n, N.; Herna´ndez-Bautista, A. I.; Ferna´ndez-Sa´nchez, L.; Con-
treras, R. Tetrahedron, 1991, 47, 6903. (c) Flores-Parra, A.; Cadenas-
Pliego, G.; Mart´ınez-Aguilera, L. M. R.; Garc´ıa-Nares, M. L.;
Contreras, R.; Chem. Ber. 1993, 126, 863.
(4) Haasnoot, C. A. G.; de Leeuw, F. A. A. M.; Altona, C. Tetrahedron,
1980, 36, 2783.
10.1021/ic000890j CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/23/2001

3244 Inorganic Chemistry, Vol. 40, No. 13, 2001
Notes
1
Table 1. The H-T1 NMR Data (300 MHz) for Adducts 1-8 in
Chart 1
CDCl3 at 25 °Ca
comp
proton
δ (ppm)
T1 (s)
1
3.20 (br),
3.1, 7.3, b0.24c
3J(HNCH) ) 6.3 Hz
2
2.65 (sbr),
3.4, 7.3, b0.27c
3.7, 7.9, b0.31c
3J(HNCH) ) 8.6 Hz
1
3
4
BH3
1
1.94
1.80
1.40
3.8, 8.7, b0.36c
1.5, 3.4, b0.11c
1.1, 2.0b
3.16 (br),
3J(HNCH) )5.3 Hz
2
2.97 (sbr),
1.2, 2.1b
2
3
3J(HNCH) ) 7.3 Hz
3
4
1.87
1.87
1.7, 2.0b
1.7, 2.0b
1
2
3
4
CH3
BH3
1
3.12
2.71
2.07
1.89
2.60(br)
1.50
5.6, 10.5,b0.93c
6.0, 10.6,b0.94c
5.9, 13.0,b0.96c
5.9, 11.0,b0.97c
3.6, 6.3,b0.50c
2.5, 4.0,b0.39c
4.8
2.85,
(3J(1H-11B) ) 0.8 Hz)
2
3.45,
5.1
(3J(1H-11B) ) 1.5 Hz)
4
5
3
4
CH3
2.06
2.06
2.60,
5.9
5.9
2.7
Chart 2
(3J(1H-11B) ) 1.4 Hz)
3.17,
1
2
1.6
1.7
3J(HNCH) ) 2.3 Hz
2.45,
3J(HNCH) ) 12.4 Hz
1.70, 1.50, 1.30
1.40,
3, 4, 5, 6
BH3
1.8, 1.6, 1.7
0.95
(3J(1HBN1H) ) 2.3 Hz)d
3.25 (br),
1
2
0.94
0.93
3J(HNCH) ) 3.0 Hz
2.59 (br),
6
7
3J(HNCH) ) 12.9 Hz
1.76, 1.57, 1.33
2.82 (br)
3, 4, 5, 6
1
2
3, 4, 5, 6
CH3
0.90, 0.75, 0.95
2.5
1.8
2.7, 2.6
2.3
3.05 (br)
1.75, 1.69, 1.41
2.56,
(3J(1H-11B) ) 1.5 Hz)
3.85
1
2
3
4
CH3
BH3
3.4
3.0
3.8
3.6
2.7
1.5
4.37
4.03
3.40
2.87
8
here axial protons 2 are high-frequency shifted (Table 1).
Usually five-membered cycles are flexible in solutions.5
1.65
1
However, the H and 13C NMR spectra of 1 (CD2Cl2) were
a Broadened due to a 1H-11B coupling (br), strongly broadened (sbr),
respectively b CD2Cl2 at 25 °C. c CD2Cl2 at -90 °C. d Measured by
{11B} - experiments.
temperature independent, and the 1H-NOESY experiments (25°
and -90 °C) revealed the cross-peaks 1-2, 3-4, 1-4, 2-3, and
1-NH. The VT 1H-NOESY spectra of adduct 2 (CD2Cl2) were
similar to that of 1.
The “frozen” conformations of 1, 2, and 83 are convenient
to use to determine contacts Hδ+‚‚‚‚-δH in solution by the 1H-
T1 relaxation method. The method provides a high accuracy
when 1H-T1 times reach minimal values (T1min) at low temper-
atures.8 Unfortunately, such measurements are not possible for
the investigated adducts because of their small inertia moments.
Actually, protons 1 and 2 in adduct 1 (CD2Cl2, -90 °C) show
the T1 values of 0.24 and 0.26 s, respectively, which are
to 11BR3 (Chart 2). On the basis of the data, one can conclude
that adducts 1 and 2 exist in solutions as envelope conforma-
tions5 with the equatorial BR3 groups (Chart 1). Note that
equatorial protons 1 are high-frequency shifted by the equatorial
BR3 groups.6 Six-membered cyclic adducts 5 and 6 show the
same spectral feature.3 In contrast, adducts 4 and 7, containing
the N(CH3)BF3 fragments, demonstrate the opposite tendency;
(5) (a) Pfafferott, G.; Oberhammer, H.; Boggs, J. E.; Caminati, W. J. Am.
Chem. Soc. 1985, 107, 2305. (b) Pfafferott, G.; Oberhammer, H.;
Boggs, J. E.; J. Am. Chem. Soc. 1985, 107, 2309.
(6) (a) Contreras, R.; Santiesteban, F.; Paz-Sandoval, M. A.; Wrackmeyer,
B. Tetrahedron 1984, 40, 3829. (b) Paz-Sandoval, M. A.; Santiesteban,
F.; Contreras, R. Magn. Reson. Chem. 1985, 23, 428. (c) Ariza-Castolo,
A.; Contreras, R. In Current Topics in the Chemistry of Boron;
Kabalka, G. W., Ed.; The Royal Society of Chemistry: Cambridge,
1994; p 90.
(7) Abragam, A. The Principles of Nuclear Magnetism; Oxford University
Press: New York; 1971.
(8) (a) Bakhmutov, V. I.; Vorontsov, E. V. Inorg. Chem. ReV. 1998, 18,
183. (b) Desrosiers, P. J.; Cai, L.; Lin, Z.; Richards, R.; Halpern, J. J.
Am. Chem. Soc. 1991, 113, 4173.

Notes
Inorganic Chemistry, Vol. 40, No. 13, 2001 3245
and H(2) (CDCl3, 25 °C). It is interesting that the T1 values are
more elongated for the axial protons which are closer to BH3
or BF3 (Chart 2). However, the difference disappears in CD2-
Cl2 at 25 °C and appears again at low temperatures (Table 1).
We assume that such effects (see also 3) are caused by
anisotropic motions and cannot be interpreted in distance terms.
Adducts 1, 2, and 8 show that averaged relaxation rates
(R1(1,2) ) 1/T1(1,2)), characterizing R-methylene protons, are
constantly higher than R1(3,4). This observation can be used to
determine contacts C-Hδ+‚‚‚‚-δH-B. According to the X-ray
structures of the cyclic NBH3 adducts2b,3a,c,9b (similar to 1, 2,
and 8), distances H...H in the NCH2 and SCH2 groups are
measured as 1.55-1.57 Å. These distances in the BH3 groups
are remarkably longer (1.84-1.93 Å). Finally, the B-H bond
lengths are lying between 1.10 and 1.15 Å. On the basis of the
structural data, the T1(3,4) value in adduct 8 gives, using eq 3a,
τc ) 0.44 10-11 s (CDCl3, 25 °C). In turn, this value predicts
T1 of the BH3 protons, relaxing by proton-proton and proton-
boron dipolar interactions (eq 4), as 1.30 s; this is in accord
with 1.54 s in
Figure 1. Inversion-recovery curves (Intensities I versus delay times
τ in sec), obtained for CH3 (3) and R-protons (b) in adduct 4 (CDCl3,
25 °C) and R-protons (o) in adduct 1 (CD2Cl2, 25 °C).
significantly greater than those calculated by eq 1, where ν is
1
8b
the H NMR frequency.
1/T1(BH3) ) 2(8.55 × 1011)(0.44 × 10-11)(1.84)-6+
(2.38 × 1011)(0.44 × 10-11)(1.10)-6 (4)
r(H-H)6 ) 5.8156 (T1min/ ν)
(1)
The calculation leads to T1min ) 0.10-0.15 s if r(H...H) ) 1.54-
1.64 Å and ν ) 300 MHz.
Table 1. One can think that BH3 in 8 is not a fast-spinning group
about the N-B bond because the rotation with τ , τc strongly
elongates 1H-T1 times.10 The same approach to 1 results in the
1
At room temperature, the dipole-dipole relaxation of H is
described as:
τc value of 0.43 × 10-11 s (CDCl3, 25 °C), 0.19 × 10-11
s
2
(CD2Cl2, 25 °C), and 4.9 × 10-11 s (CD2Cl2, -90 °C). Again,
the T1(BH3) time in 1 is well predicted as 1.4, 3.2, and 0.12 s,
respectively.
1/T1 ) (4/3)(µo/4π)2 p2γH γNS2S(S + 1)τcr(H-S)-6 (2)
where µo, S, and γ are known physical constants, and τc is a
correlation time of molecular reorientations.7 The equation
transforms to:
The equal T1(3) and T1(4) times in adduct 8 show that dipole-
dipole interactions 3-2(2’) are negligible. Hence, an additional
contribution to the total relaxation rate of R-protons, (R1(1,2)
- R1(3,4)), calculated as 0.0412 s-1, can be attributed to dipole-
dipole interactions with BH3.11 Then, 0.0412 s-1 and the τc value
of 0.44 × 10-11 s leads to a distance Hδ+‚‚‚‚-δH of 2.12 (
0.12 Å12a to account for one C-Hδ+‚‚‚‚-δH-B contact.
The faster relaxation of H(1,2) with respect to H(3,4) in 1
can be expressed as (R1(1,2) - R1(3,4))/R1(1,2) that is 16% of
R1(1,2). Thus, this contribution is higher than 10%, as discussed
above. The same approach to the T1 times in pirrolidine
(T1(1,2) ) 7.4 s, T1(3,4) ) 8.0 s, CDCl3, 25 °C) leads to 9%.
Hence the N-H proton also accelerates the relaxation of
H(1,2).12b Then, a proton-hydride contribution to the total
relaxation rate of H(1,2) in 1 is estimated as 7% or as 0.0228
s-1 (CDCl3, 25 °C), 0.0101 s-1 (CD2Cl2, 25 °C), and 0.291 s-1
1/T1(H-H) ) 8.55 × 1011τcr(H-H)-6
1/T1(H-11B) ) 2.38 × 1011τcr(H-B)-6
(3a)
(3b)
for the 1H-1H and 1H-11B (in account for the natural abundance
of 11B) interactions, respectively. Here T1 and τc are measured
in sec, and r is expressed in Å. In the solid cyclic N-BH3
adducts, the C-Hδ+‚‚‚‚-δH-B contacts (between the R-meth-
ylene and BH3 protons) are elongated up to 2.2-2.5 Å.2b,3a,c
1
Taking τc as 1 × 10-11 s, calculation by eq 3a leads to H-T1
g 13.3 s when a proton-hydride distance, r(H...H), is g2.2 Å.
The same τc value predicts T1 ) 1.54 s for two methylene
protons separated by 1.54 Å. Hence proton-hydride interactions
can be contributed to a total relaxation rate of the methylene
protons by e10%. Note that in a common case, T1 values are
determined with errors ≈5%.8a Thus, proton-hydride contribu-
tions (which can be quantitatively interpreted) are quite small
and correspond to 5-10% of measured magnitudes.
(CD2Cl2, -90 °C). In turn, these values give distances C-Hδ+
‚
‚‚‚-δH-B between 2.33 ( 0.14 and 2.29 ( 0.14 Å. Thus, 2.30
Å can be taken as a good estimation. Finally, it should be
emphasized that the spectral parameters and the relaxation
in adducts 1 and 2 are very similar. Therefore, the short
C-Hδ+‚‚‚‚-δF-B contacts can be also expected in solutions
of 2.
Eqs. (3) are valid for isotropic molecular motions, while the
adducts in Chart 1 are nonspherical. However, inversion-
recovery curves collected, for example, in solutions of 1 and 4
(Figure 1), do not reveal the features typical of anisotropic
motions.9a Table 1 shows that the T1(1) and T1(2) values in 5
and 6 are identical, corresponding well to the locations of
protons 1 and 2 with respect to BR3 (Chart 2). In contrast,
adducts 1 and 2 show the remarkable difference in T1 of H(1)
(10) Woessner, D. E. J. Chem. Phys. 1962, 36, 1
(11) This is reasonable because contacts between NCH3 and CH2 are not
shortened in the X-ray structures of the NCH3 heterocyclic derivatives.3a
In addition, R- and â-protons in N-methyl- pirrolidine showed the
practically equal T1 times (5.5 and 5.3 s, CDCl3, 25 °C) despite the
presence of the CH3 group.
(12) (a) The error is estimated on the basis of the averaged T1(1,2) values.
(b) The NH contribution is calculated as 0.0107 s-1. The r(H...H)
distance in the CH2 groups, taken as 1.57-1.55 Å (see the text), gives
τc ) 0.22-0.20 × 10-11 s. Then the contribution of 0.0107 s-1
corresponds to a very reasonable CH...HN distance of 2.4-2.3 Å.
(9) (a) Kratochwill, A.; Vold, R. L.; Vold, R. R. J. Chem. Phys. 1979,
71, 1319. (b) Brenchley, G.; Fedouloff, M.; Mahon, M. F.; Molloy,
K. C.; Wills, M. Tetrahedron 1995, 51, 10581.

3246 Inorganic Chemistry, Vol. 40, No. 13, 2001
Notes
Conclusions
contacts are close (or even shorter) to the sum of the van der
Waals radii of H. One can think that such contacts can control
the conformational states.
Borane adducts 1-4 and 7 were characterized by H, 13C,
1
15N, 19F, and 11B NMR spectra in CDCl3. The VT H NMR
1
and 1H-NOESY studies revealed “frozen” envelope conforma-
tions in solutions of 1 and 2. The BR3 groups occupy equatorial
positions.
Acknowledgment. V.I.B. thanks Conacyt-Mexico for a
Catedra Patrimonial and M.G.-R. for a scholarship. R.C. and
A.F. thank Conacyt for Grant G32710-F.
The relaxation study allowed the determination the Hδ+‚‚‚-δH
contacts in solutions of 1 (2.30( 0.14 Å) and 8 (2.12 ( 0.12
Å), which are similar to those found in the solid state. These
IC000890J