Fac-[Re(CO)3L] complexes containing tridentate monoanionic ligands (L-) having a terminal amido and two amine ligating groups

Article abstract of DOI:10.1007/s10870-007-9274-x

Treatment of fac-[Re(CO)₃(H₂O)₃] ⁺ with two unsymmetrical tridentate NNN donor ligands (LH) based on a diethylenetriamine (dien) moiety with a 2,4,6-trimethylbenzoyl (tmbCO) group linked to a terminal nitrogen of dien through an amide produced the neutral complexes [Re(CO)₃(tmbCO-N,N-Me₂dien)] and [Re(CO) ₃(tmbCO-dien)]. The [Re(CO)₃(tmbCO-N,N-Me ₂dien)] complex crystallized in the space group P2₁/c, a = 19.5793 (15), b = 7.4239 (5), c = 14.8071 (10) A, β = 104.425(5)°, V = 2084.4(3) A³, and the [Re(CO) ₃(tmbCO-dien)] complex crystallized in the space group C2/c, a = 36.861 (7), b = 7.3990 (10), c = 14.933 (3) A, β = 102.145 (8)°, V = 3981.6(12) A³. X-ray crystallographic and NMR analyses confirm that in both the solid and solution states the ligands are bound to the fac-Re(CO)₃ core in a tridentate fashion with the amide group being deprotonated. An important property in radiopharmaceuticals is shape, which in turn depends on ring pucker. For [Re(CO)₃(tmbCO-N,N-Me ₂dien)], the two chelate rings have different pucker chirality, as is commonly found for a broad range of metal complexes. However, for fac-[Re(CO)₃(tmbCO-dien)], the two chelate rings have the same pucker chirality, a finding attributable to two strong intermolecular hydrogen bonds from the NH₂ to two waters of crystallization. Impeded by methyl group clashes with chelate ring hydrogens, the tmb ring rotation about the C(amido)-C(tmb) bond is slow on the NMR time scale. fac -[Re(CO) ₃ L] Complexes Containing Tridentate Monoanionic Ligands (L ^- ) Having a Terminal Amido and Two Amine Ligating Groups Anna Maria Chirstoforou, Patricia A. Marzilli, Frank R. Fronczek, and Luigi G. Marzilli X-ray crystallographic and NMR analyses of fac-Re(CO)₃ complexes containing diethylenetriamine-based, NNN tridentate ligands with a terminal deprotonated amido donor group allow comparison to previously reported analogues with a terminal deprotonated sulfonamido donor group.

Full text of DOI:10.1007/s10870-007-9274-x

J Chem Crystallogr (2008) 38:115–121
DOI 10.1007/s10870-007-9274-x
ORIGINAL PAPER
fac-[Re(CO)3L] Complexes Containing Tridentate Monoanionic
Ligands (L@) Having a Terminal Amido and Two Amine Ligating
Groups
Anna Maria Christoforou Æ Patricia A. Marzilli Æ
Frank R. Fronczek Æ Luigi G. Marzilli
Received: 21 June 2007 / Accepted: 9 October 2007 / Published online: 27 October 2007
ꢀ Springer Science+Business Media, LLC 2007
Abstract Treatment of fac-[Re(CO)3(H2O)3]+ with two
unsymmetrical tridentate NNN donor ligands (LH) based on
a diethylenetriamine (dien) moiety with a 2,4,6-tri-
methylbenzoyl (tmbCO) group linked to a terminal nitrogen
of dien through an amide produced the neutral complexes
[Re(CO)3(tmbCO-N,N-Me2dien)] and [Re(CO)3(tmbCO-
dien)]. The [Re(CO)3(tmbCO-N,N-Me2dien)] complex
crystallized in the space group P21/c, a = 19.5793 (15),
Keywords Rhenium tricarbonyl Á Amide ligands
Introduction
Technetium-99m (99mTc) is the most widely used diagnostic
radionuclide in nuclear medicine [1–4]. The [99mTc(V)O]3+
core has been used most predominantly, but recently the
development of the advantageous [99mTc(CO)3(H2O)3]+
precursor has opened new directions for radiolabeling [5–7].
Schibli et al. showed that agents with the fac-99mTc(I)(CO)3+
core bearing a tridentate coordinated ligand are more robust
and have better pharmacokinetic profiles than agents bearing
bidentate ligands [8]. Frequently used chelating ligands
contain N, O, or S donor atoms in groups such as carboxyls
[9], amines [10, 11], nitrogen heterocycles [12], and thio-
ethers [13, 14]. A neutral metal-containing label is most
useful in the development of small (*5 kDa) metal nuclide
radiopharmaceuticals for bioconjugation to hormones, small
peptides, etc. because such neutral labels are most likely to
preserve the biological properties of the tagged species [15].
Complexes made from the non-radioactive fac-
[Re(CO)3(H2O)3]+ analogue of fac-[99mTc(CO)3(H2O)3]+
generally provide the necessary background to define the
chemistry of the 99mTc agents [13]. A neutral labeling unit
can best be achieved by using monoanionic tridentate
ligands. The most common negative donor, the carboxyl
group, does not allow bioconjugation, however. We recently
reported a series of fac-[Re(CO)3(NNN)] complexes of
monoanionic tridentate ligands with three nitrogen donors
(NNN) containing the seldom-used sulfonamido group [16].
In the names of the NNN ligands (LH) used here (cf. caption
of Fig. 1), the H’s designate the sulfonamide and amide NH
proton that is absent in the facially coordinated deprotonated
bound L@. The sulfonamido group offers diverse chemistry
˚
b = 7.4239 (5), c = 14.8071 (10) A, b = 104.425(5)ꢁ,
3
V = 2084.4(3) A , and the [Re(CO)3(tmbCO-dien)] com-
˚
plex crystallized in the space group C2/c, a = 36.861 (7), b =
˚
7.3990 (10), c = 14.933 (3) A, b = 102.145 (8)ꢁ, V =
3
3981.6(12) A . X-ray crystallographic and NMR analyses
˚
confirm that in both the solid and solution states the ligands
are bound to the fac-Re(CO)3 core in a tridentate fashion with
the amide group being deprotonated. An important property
in radiopharmaceuticals is shape, which in turn depends on
ring pucker. For [Re(CO)3(tmbCO-N,N-Me2dien)], the two
chelate rings have different pucker chirality, as is commonly
found for a broad range of metal complexes. However, for
fac-[Re(CO)3(tmbCO-dien)], the two chelate rings have the
same pucker chirality, a finding attributable to two strong
intermolecular hydrogen bonds from the NH2 to two waters
of crystallization. Impeded by methyl group clashes with
chelate ring hydrogens, the tmb ring rotation about the
C(amido)–C(tmb) bond is slow on the NMR time scale.
Electronic supplementary material The online version of this
article (doi:10.1007/s10870-007-9274-x) contains supplementary
material, which is available to authorized users.
A. M. Christoforou Á P. A. Marzilli Á F. R. Fronczek Á
L. G. Marzilli (&)
Department of Chemistry, Louisiana State University, Baton
Rouge, LA 70803, USA
e-mail: lmarzil@lsu.edu
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J Chem Crystallogr (2008) 38:115–121
and allows bioconjugation through the terminal ligating
group. The amido group is very commonly used in biocon-
jugation chemistry because of its significance in peptides.
Although the amido group has been widely used as a ligating
group in M(V)O3+ radiopharmaceuticals [17], very little use
of this group has been made for fac-99mTc(I)(CO)3 agents,
especially as a terminal ligating group [18]. Herein, we
describe the fac-[Re(CO)3L] complexes formed by fac-
[Re(CO)3(H2O)3]+ with tmbCO-N,N-Me2dienH and
tmbCO-dienH, containing the monoanionic deprotonated
amido ligand bound in a tridentate fashion. Since all com-
pounds described herein have a facial arrangement of the
three carbonyls, hereafter the fac designation will be used
only in referring to general types of compounds and will be
omitted when referring to specific compounds.
ligating atoms. Many of these structures have been ana-
lyzed by using principal component analysis (pca) and
other methods [19–26]. When diethylenetriamine-type
ligands bind to a metal center in a tridentate fashion, the
two five-membered chelate rings can have either d or k
chirality (Chart 1), leading to four possible combinations
of chelate ring chirality: dk, kd, kk and dd.
Pca studies on X-ray structures from the Cambridge
Structural Database (CSD) of M(dien) structures [19–21]
showed that the dk and kd conformation combinations
were found much more frequently than those with the kk
and dd pucker chirality.
Experimental Section
In fac-[99mTc(CO)3L] agents, the bioconjugation ema-
nates from an atom of one chelate ring of the tridentate
ligand. Since chelate ring atoms act as a fulcrum, the
configuration of asymmetric centers, the shape, and the
conformation of the rings can have a large effect on the
overall shape of the agent, and hence on its biological
activity. Therefore, it is clearly important to understand
how the ligand pucker depends on the various components
that can influence it (donor atoms, chains linking these
atoms, exocyclic substituents, etc.).
Starting Materials
N,N-dimethyldiethylenetriamine (N,N-Me2dien) from
Ames Laboratories, 2,4,6-trimethylbenzoyl chloride (tmb-
COCl) from Alfa Aesar, diethylenetramine (dien) and
Re2(CO)10 from Aldrich were all used as received. The
[Re(CO)3(H2O)3]OTf [13] (OTf = trifluoromethanesulfo-
nate) precursor was prepared by known methods.
Although chelate ring pucker and factors affecting
pucker have not been thoroughly examined for radiophar-
maceuticals, extensive structural information exists for a
large number of complexes with other metals containing
chelate rings with an ethylene group bridge between
NMR Spectroscopy
1H NMR spectra were recorded on either a 300 MHz or
400 MHz spectrometer. Peak positions are relative to TMS
or solvent residual peak, with TMS as reference. All NMR
data were processed with XWINNMR and Mestre-C
software.
X-ray Data Collection and Structure Determination
All single crystals suitable for X-ray crystallography were
obtained by slow evaporation from acetone or methanol.
Chart 1 d and k chelate ring conformations
Fig. 1 Ligands used in this study: N-[2-(2-dimethylaminoethylami-
no)-ethyl]-2,4,6-trimethylbenzamide (tmbCO-N,N-Me2dienH); and
N-[2-(2-aminoethylamino)-ethyl]-2,4,6-trimethylbenzamide (tmbCO-
dienH) and ligands referred to in the text: N-[2-(2-dimethyl-
aminoethylamino)-ethyl]-2,4,6-trimethylbenzenesulfonamide (tmbSO2
-N,N-Me2dienH) and N-[2-(2-aminoethylamino)-ethyl]-2,4,6-trimeth-
ylbenzenesulfonamide (tmbSO2-dienH). The H’s in the ligand names
designate a sulfonamide/amide proton lost upon Re binding
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J Chem Crystallogr (2008) 38:115–121
117
[Re(CO)3(tmbCO-N,N-Me2dien)] (1)
Single crystals were placed in a cooled nitrogen gas stream
at 90 K on a Nonius Kappa CCD diffractometer fitted with
an Oxford Cryostream cooler and graphite-monochromated
The general method, with 1 g of tmbCOCl and 6 mL of
˚
N,N-Me2dien, afforded the crude tmbCO-N,N-Me2dienH
1
ligand (650 mg, 45% yield). H NMR (ppm) in CDCl3:
Mo Ka (0.71073 A) radiation. Data reduction included
absorption corrections by the multi-scan method, with
HKL SCALEPACK [27]. All X-ray structures were solved
by direct methods and difference Fourier techniques and
refined by full-matrix least squares techniques using
SHELXL97 [28]. All non-hydrogen atoms were refined
anisotropically. All hydrogen atoms were visible in dif-
ference maps, but were placed in idealized positions with
Uiso = 1.2Ueq for the bonded atom (1.5 for methyl groups).
A torsional parameter was refined for each methyl group.
6.82 (s, 2H), 6.50 (b, 1H, NH), 3.52 (t, 2H, CH2), 2.84
(t, 2H, CH2), 2.68 (t, 2H, CH2), 2.35 (t, 2H, CH2), 2.29
(s, 3H, CH3), 2.26 (s, 6H, CH3), 2.28 (s, 3H, CH3), 2.15 (s,
6H, NCH3). Treatment of [Re(CO)3(H2O)3]+ (0.1 mmol)
with tmbCO-N,N-Me2dienH (27 mg) as described above
afforded [Re(CO)3(tmbCO-N,N-Me2dien)] as a white
powder, (12 mg, 22% yield). Crystals obtained by slow
evaporation from acetone were characterized by single-
1
crystal X-ray crystallography. H NMR (ppm) spectrum in
acetone-d6: 6.78 (s, 1H), 6.75 (s, 1H), 6.14 (b, 1H, N2H),
3.47 (m, 1H, CH2), 3.25 (m, 3H, CH2), 3.12 (m, 2H, CH2),
3.12 (s, 3H, NCH3), 2.97 (s, 3H, NCH3), 2.87 (m, 2H,
CH2), 2.21 (s, 3H, CH3), 2.18 (s, 3H, CH3), 2.13 (s, 3H,
CH3).
Synthesis of fac-[Re(CO)3L]
The crude ligands, LH, were synthesized by a slight
modification of Krapcho’s method [29]. A solution of the
acyl chloride (tmbCOCl) (*5 mmol, 100 mL of dioxane)
was added dropwise over the course of about 2 h to a
solution of the amine (*50 mmol, 100 mL of dioxane).
The reaction mixture was stirred at RT for 10 h. The
dioxane was completely removed under vacuum, and water
(50 mL) was added. The product was extracted into
CH2Cl2 (2 9 100 mL), and the solvent was removed under
rotary evaporation. The oil thus obtained was used to
synthesize the fac-[Re(CO)3L] complexes as follows: an
aqueous solution of [Re(CO)3(H2O)3]+ (10 mL, 0.1 mmol)
was treated with a methanol (5 mL) solution of the ligand
(0.1 mmol). The pH was adjusted to *5 and the reaction
mixture was heated at reflux for 10 h, then allowed to cool
at RT, and the pH was increased to *7. The resulting
white solid that precipitated was collected on a filter,
washed with water, and dried under vacuum. This proce-
dure succeeded with LH having an amide group, but the
procedure was less reliable than for the analogues with the
sulfonamide group.
[Re(CO)3(tmbCO-dien)] (2)
The general method, with tmbCOCl (1 g) and dien (5 mL),
yielded the crude tmbCO-dienH ligand (650 mg, 45%
yield). 1H NMR (ppm) in CDCl3: 6.80 (s, 2H), 6.27 (b, 1H,
NH), 3.53 (t, 2H, CH2), 2.85 (t, 2H, CH2), 2.77 (t,
2H, CH2), 2.67 (t, 2H, CH2), 2.28 (s, 3H, CH3), 2.26 (s, 6H,
CH3). Treatment of [Re(CO)3(H2O)3]+ (0.1 mmol) with
tmbCO-dienH (25 mg) as described above afforded
[Re(CO)3(tmbCO-dien)] as a white powder (15 mg, 29%
yield). Crystals obtained by slow evaporation from acetone
were characterized by single-crystal X-ray crystallography.
1H NMR (ppm) spectrum in acetone-d6: 6.79 (s, 1H), 6.75
(s, 1H), 6.16 (b, 1H, N2H), 4.86 (b, 1H, N1H), 4.52 (b, 1H,
N1H), 3.31 (m, 1H, CH2), 3.16 (m, 1H, CH2), 3.08–2.94
(m, 5H, CH2), 2.67 (m, 1H, CH2), 2.20 (s, 3H, CH3), 2.18
(s, 6H, CH3).
Fig. 2 Perspective drawings of
the S configuration of
[Re(CO)3(tmbCO-N,N-
Me2dien)] (1) and
[Re(CO)3(tmbCO-dien)] (2).
Thermal ellipsoids are drawn
with 50% probability
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J Chem Crystallogr (2008) 38:115–121
˚
Table 2 Selected bond and atom distances (A) and angles (deg) for
[Re(CO)3(tmbCO-N,N-Me2dien)] (1), and [Re(CO)3(tmbCO-dien)]
(2)
Results and Discussion
X-Ray Characterization
fac-[Re(CO)3L] complex
Both complexes reported here (Fig. 2) have a distorted
octahedral structure, with the three carbonyl ligands coor-
dinated facially. The remaining coordination sites are
occupied by two sp3 N amines and one sp2 N from an
amido group. Typically, the Re-C3 bonds, involving the
CO group trans to the amido group, are not different from
those of the other Re–CO bonds or the respective Re–C3
bonds of the relevant sulfonamido complexes [16]. Crystal
data and structural refinement details are summarized in
Table 1 for 1 and 2. Although we found that the aromatic
rings facilitated our efforts to obtain crystals, these rings
generally were not stacked.
1
2
Bond distances
Re–N1
2.2738 (19)
2.2087 (19)
2.1744 (17)
2.216 (3)
2.207 (3)
2.185 (3)
Re–N2
Re–N3
Bond angles
N1–Re–N2
N2–Re–N3
N1–Re–N3
N2–Re–C2
Non-bonded distances
N1 to N2
79.37 (7)
76.10 (7)
85.81 (7)
172.33 (9)
77.87 (10)
74.26 (10)
90.16 (10)
170.10 (13)
2.863 (2)
2.702 (2)
3.308 (3)
2.30
2.780 (4)
2.651 (4)
4.169 (4)
4.62
N2 to N3
Bond Lengths and Bite Angles
C4 to C7
C4H to C7H
C5 to C6
˚
The average Re–N bond length of *2.2 A (Table 2) found
2.510 (3)
2.28
2.468 (4)
2.59
in the amido complexes is consistent with distances found in
relevant fac-[Re(CO)3L] complexes [11, 14, 16]. The Re–N1
bond (N1Me2 group) in [Re(CO)3(tmbCO-N,N-Me2dien)]
(1) is longer than the other Re-N bonds of 1 and 2.
C5H to C6H
This lengthening is attributed to the bulkiness of the two
methyl groups on N1 (for 1) compared to the smaller H
group in 2. Similar results were observed for the respective
bonds of fac-[Re(CO)3L] (L = tmb-sulfonamido ligands)
complexes. It is known that the sp2 N donors can form
shorter Re–N bonds than the sp3 N donors [12, 30]. The
Re–N3 sp2 bond of the amido group in both 1 and 2 is
statistically shorter than the respective Re–N1 and Re–N2
bonds (which are Re–sp3N bonds). The shorter Re–N3 sp2
bond compared to the Re–N sp3 bonds was also observed in
similar fac-[Re(CO)3L] (L = tmb-sulfonamido ligands)
complexes [16]. The effect of the hybridized sp2 N3 should
also be seen in the N3–C7 bond. Indeed, the N3–C7 bond is
numerically (but not significantly) shorter than the other
sp3 N–C bonds (N1–C4, N2–C5, and N2–C6). Typical
Table 1 Crystal data and structural refinement for [Re(CO)3(tmbCO-
N,N-Me2dien)] (1), and [Re(CO)3(tmbCO-dien)] (2)
fac-[Re(CO)3L] complex
1
2
Empirical formula
CCDC deposition no.
fw
C19H26N3O4Re
651285
C17H22N3O4Re•H2O
651286
546.63
536.59
Crystal system
Space group
monoclinic
P21/c
monoclinic
C2/c
Unit cell dimensions
˚
a (A)
19.5793 (15)
7.4239 (5)
14.8071 (10)
104.425 (5)
2084.4 (3)
90.0(5)
36.861 (7)
7.3990 (10)
14.933 (3)
102.145 (8)
3981.6 (12)
90.0(5)
˚
b (A)
Re–sp2 N bond lengths range from 2.14 to 2.18 A, as
˚
˚
c (A)
measured in a several Re structures containing Re–N bonds
(aromatic sp2 N, e.g., in pyridyl ligands), whereas a typical
Re–sp3 N bond length is *2.2 A [12–14]. The sp
b (ꢁ)
3
V (A )
˚
2
˚
T (K)
hybridized N3 should affect the C–N3–C7, C–N3–Re, and
C7–N3–Re angles. However, the differences between
molecules, while generally consistent with expectations for
Re–sp2 N vs. Re–sp3 N centers, are small and thus are
masked by steric and solid state effects. The N–Re–N bite
angles in the amido complexes are between 74.2 and 79.3ꢁ.
The N2–Re–N3 bite angle (ring containing the amido
group) is smaller (*75ꢁ) than the N1–Re–N2 bite angle
(*78ꢁ), as found for the related angles in fac-[Re(CO)3L]
(L = tmb-sulfonamido ligands) complexes [16]. The N1 to
Z
4
8
qcalc (g/cm3)
l (mm@1
1.742
1.790
)
5.857
6.135
Transm. coeff.
0.387–0.474
73.8
0.373–0.837
65.2
2hmax (ꢁ)
R indicesa
0.027
0.032
wR2 = [I [ 2r(I)]b
Data/param
@3
0.060
0.077
9895/253
1.39, @2.14
7003/245
1.78, @1.88
˚
Resid. dens. (A
)
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J Chem Crystallogr (2008) 38:115–121
119
N2 and N2 to N3 non-bonded distances within five-mem-
bered chelate rings in 1 and 2 have a narrow range of
values (Table 2) and are similar to those of relevant fac-
[Re(CO)3L] complexes [16], as well as those of complexes
with higher valent metal centers [31].
comparisons indicate that the change from an amido to a
sulfonamido ligating group does not affect the backbone
structure, but the change in the nature of the linkage of the
tmb group to the coordinated nitrogen influences the
position of the pendant group.
Chelate Ring Chirality
Trimethylbenzyl Group Rotation
As mentioned in the Introduction, chelate ring shape and
conformation affect the overall shape of the bioconjugated
agent. In accordance with other relevant compounds [20,
21], and our findings for fac-[Re(CO)3L] sulfonamido
complexes [16], 1 has different chirality in each ring
pucker; kd for the S configuration at Re (The Re configu-
ration is S for structures shown in Fig. 2) [16]. In contrast,
both chelate rings have the same chirality in complex 2
(both d for the S configuration), a finding we attribute to
two strong intermolecular hydrogen bonds from the NH2 to
two waters of crystallization (N_O 2.924(4) at x,y,z and
In the X-ray structures of 1 and 2 (Fig. 2), the 3 and 5
protons and the 2 and 6 methyl groups of the tmbSO2 group
are not equivalent. Indeed, in the 1H NMR spectra of
complexes 1 and 2, the signals of the magnetically in-
equivalent two aromatic protons and two methyl groups of
the tmbCO group have similar shifts (see ‘‘Experimental’’
Section) but are fully resolved. The signals do not merge
when the spectrum in acetone-d6 is recorded at 40 ꢁC. In
contrast, for fac-[Re(CO)3L] complexes with a sulfon-
amido group instead of an amido group, these signals are
combined for the aromatic and for the methyl protons,
indicating fast rotation on the NMR time scale [16].
˚
2.941(4) A at 1@x, 1@y, 1@z). The fact that complex 2 has
the same chirality in both rings is an unusual but not
unprecedented finding [16, 19–21]. The non-bonded dis-
tances between the chelate rings agree with our previous
study [16] finding that C5 to C6 (Table 2) and C5H to C6H
distances are not very different whether the pucker chirality
of the two chelate rings is the same or different, whereas
the C4 to C7 and C4H to C7H distances (cf. Fig. 2 for the
atom numbering) are larger than when the chirality is
different.
In the X-ray structures of 1 and 2, the amido N3–C–O
group is planar. Therefore, it can adopt only two orienta-
tions with the oxygen facing away or towards the
coordination face; in both orientations serious clashes are
observed when the tmb group is rotated around the
C–C(tmb) bond (with the help of modeling software). The
conformer with the oxygen facing towards the coordination
face seems to be unfavorable because the tmb group is
placed very close to the carbonyl group and rotation of the
tmb group would overlap with the carbonyl group. In the
second conformer (which is observed in the X-ray struc-
tures of 1 and 2) clashes of the methyl group (of tmb) with
the ethylene chain of dien appear to hinder rotation around
the C(amido)–C(tmb) bond. Similar clashes were observed
also in the relevant fac-[Re(CO)3L] (L = tmb-sulfonamido
complexes) complexes [16] during rotation around the
C(sulfonamido)–C(tmb) bond. The sulfonamido group S
atom is part of longer bonds (N–S and S–C) and a more
acute N3–S–C angle compared to the respective bonds
We compared the position of the pendant group and the
shape and chirality of the ring pucker of complex 1 with its
relevant sulfonamido complex [16]. An overlay of the
structure of complex 1 and the [Re(CO)3(tmbSO2-N,N-
Me2dien)] complex (Fig. 3) illustrates that even though the
dien backbone of the ligand has a very good fit, the location
of the pendant tmb group is significantly different. These
˚
˚
(N3–C *1.32 A and C–C *1.51 A) and angle (N3–C–C
*120ꢁ) of the amido group in the [Re(CO)3(tmbCO-dien)]
and [Re(CO)3(N,N-Me2tmbCO-dien)] complexes. The
equivalence of the tmb signals of the sulfonamido com-
plexes despite the inequivalence in the X-ray structure [16]
was attributed to the relatively faster rotation of the tmb
group about the S(sulfonamido)–C(tmb) bond of the sul-
fonamido group compared to the C(amido)–C(tmb) bond of
the amido group [32], and that synchronous rotations about
the longer N–S and S–C bonds can explain the relatively
free rotation of the tmb group for sulfonamido complexes
[16]. We believe that for the fac-[Re(CO)3L] amido com-
plexes (1 and 2) the combination of the presence of only
Fig. 3 Overlay of the [Re(CO)3(tmbCO-N,N-Me2dien)] (red) and
˚
[Re(CO)3(tmbSO2-N,N-Me2dien)] (blue) complexes (RMS = 0.026 A
from overlaying the Re, N1, N2, and N3 atoms)
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J Chem Crystallogr (2008) 38:115–121
Fig. 4 Two views illustrating
the possible steric clashes
leading to restricted rotation
around the C(amido)–C(tmb)
bond for 2
one favorable conformer, the short bonds and slow rotation
of the amido group result in hindered rotation around the
C(amido)–C(tmb) bond, from clashes of the methyl group
(of tmb) with the ethylene chain (Fig. 4).
for fac-[Re(CO)3L] complexes with an exo-NH group. One
possibility is that steric effects obstruct formation of
H-bonds with the solvent, and therefore the downfield shift
effect of NH H-bonding to solvent is decreased.
NMR Characterization of [Re(CO)3L] (L@ = tmbCO-
N,N-Me2dien@ and tmbCO-dien@)
Conclusions
In our studies we find that the fac-[Re(CO)3L] amido
complexes are much more difficult to synthesize, compared
to the related sulfonamido complexes, consistent with the
scarcity of related amido complexes in the literature. An
inner coordination sphere with a neutral charge is achieved
upon binding of the tridentate monoanionic amido ligands
in the fac-[Re(CO)3(NNN)] complexes studied here. The
dangling tmb group connected through the amido bond
experiences hindered rotation on the NMR time scale, due
to the rigidity of the amido group (compared to the more
flexible sulfonamido group). Complex 1 has different chi-
rality in each chelate ring, as commonly found in the
literature, whereas complex 2 has the same chirality in each
chelate ring. The pendant tmb group has very different
The fac-[Re(CO)3L] complexes were characterized by
NMR spectroscopy in DMSO-d6, as well as in acetone-d6
(Table 3). The [Re(CO)3(tmbCO-dien)] (2) complex has a
terminal NH2 group with one NH exo and one NH endo to
the tricarbonyl face, which give rise to a relatively upfield
exo-NH signal (3.94 ppm) and a relatively downfield endo-
NH signal (4.99 ppm) in DMSO-d6. These shifts (Table 3)
are roughly comparable to those for the terminal NH2
group of relevant fac-[Re(CO)3L] (L@ = tmb-sulfonamido
ligands) complexes (3.30–3.54 ppm for upfield NH signal
and 5.21–5.26 ppm for downfield NH signal) [16]. The
exo-NH signal of [Re(CO)3(tmbSO2-dien)] is *0.5 ppm
more upfield compared to that of 2, a difference that might
be attributed to the different distance of the exo-NH pro-
tons to the nearest oxygens of the sulfonamido/amido
group. The distance of the exo-NH of 2 to the oxygen of the
positions
in
[Re(CO)3(tmbCO-N,N-Me2dien)]
and
[Re(CO)3(tmbSO2-N,N-Me2dien)]. Therefore, the different
linker components (CO and SO2) lead to different overall
shapes.
˚
CO group is 3.5 A, whereas the respective distance for the
˚
[Re(CO)3(tmbSO2-dien)] complex is 2.8 A. The explana-
tion for the relatively upfield shift of the exo-NH signal is
unclear, but such a signal appears to be a general finding
Supplementary Data
Table 3 Selected chemical shifts (ppm) of [Re(CO)3(tmbCO-N,N-
Me2dien)] (1), and [Re(CO)3(tmbCO-dien)] (2)
CCDC 651285 and CCDC 651286 contain the supplemen-
tary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_
request/cif, by e-mailing data_request@ccdc.cam.ac.uk, or
by contacting The Cambridge Crystallographic Data Cen-
tre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
+44(0)1223-336033.
1
2
DMSO-d6
exo-N1H/Me
endo-N1H/Me
N2H
2.79
3.02
6.72
3.94
4.99
6.69
acetone-d6
exo-N1H/Me
endo-N1H/Me
N2H
Acknowledgments This investigation was supported by funds from
LSU and by NIH Grant DK 38842 (to Andrew Taylor, Emory Uni-
versity, as PI). Purchase of the diffractometer was made possible by
Grant No. LEQSF(1999–2000)-ENH-TR-13, administered by the
Louisiana Board of Regents.
2.97
3.12
6.14
4.53
4.86
6.17
123

J Chem Crystallogr (2008) 38:115–121
121
16. Christoforou AM, Fronczek FR, Marzilli PA, Marzilli LG (2007)
Inorg Chem 46(17):6942–6949
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