Studies of the reactions of tripodal pyridine-containing ligands with Re(CO)5Br leading to rheniumtricarbonyl complexes with potential biomedical applications

Article abstract of DOI:10.1039/c1dt11020a

The complexes formed from the reaction of N-acylated tris-(pyridin-2-yl) methylamine (LH) with [Re(CO)₅Br] depend on the structure of the ligand and the reaction conditions. Thus, while N-[1,1,1-tris-(pyridin-2-yl) methyl]acetamide coordinates through the three pyridine nitrogens to give a stable cationic complex [LHRe(CO)₃Br], the analogous N-benzoyl ligand reacts under similar conditions to give a neutral complex [LRe(CO)₃] with coordination through two pyridine nitrogens and a deprotonated amide. To try to explain these different outcomes, the reactions of some structurally related N-acylated [1,1-bis(pyridin-2-yl)]methylamines (L′H) with [Re(CO)₅Br] have been studied and the reaction pathways identified. These studies indicate that a neutral complex [L′HRe(CO)₃Br] is initially formed in which the amide portion of the ligand is uncoordinated, but that this complex under appropriate conditions then rearranges to give a cationic complex [L′HRe(CO)₃]Br in which the coordinated amide nitrogen either remains protonated or is present in its imidic acid tautomeric form. Elimination of HBr from these complexes either thermally or in the presence of base then gives stable neutral complexes [L′Re(CO) ₃]. The impact of the N-acyl group and any substituent at the apex of the tripodal ligands (L′′H) on the relative stabilities of intermediate complexes on the reaction pathway helps provide an explanation for the observed difference in behaviour of the N-acylated tris(pyridin-2-yl) methylamines (LH). The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1dt11020a

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PAPER
Studies of the reactions of tripodal pyridine-containing ligands with
Re(CO)5Br leading to rheniumtricarbonyl complexes with potential
biomedical applications†
D. Vaughan Griffiths,* Yuen-Ki Cheong, Philip Duncanson and Majid Motevalli
Received 31st May 2011, Accepted 15th July 2011
DOI: 10.1039/c1dt11020a
The complexes formed from the reaction of N-acylated tris-(pyridin-2-yl)methylamine (LH) with
[Re(CO)5Br] depend on the structure of the ligand and the reaction conditions. Thus, while
N-[1,1,1-tris-(pyridin-2-yl)methyl]acetamide coordinates through the three pyridine nitrogens to give a
stable cationic complex [LHRe(CO)3Br], the analogous N-benzoyl ligand reacts under similar
conditions to give a neutral complex [LRe(CO)3] with coordination through two pyridine nitrogens and
a deprotonated amide. To try to explain these different outcomes, the reactions of some structurally
related N-acylated [1,1-bis(pyridin-2-yl)]methylamines (L¢H) with [Re(CO)5Br] have been studied and
the reaction pathways identified. These studies indicate that a neutral complex [L¢HRe(CO)3Br] is
initially formed in which the amide portion of the ligand is uncoordinated, but that this complex under
appropriate conditions then rearranges to give a cationic complex [L¢HRe(CO)3]Br in which the
coordinated amide nitrogen either remains protonated or is present in its imidic acid tautomeric form.
Elimination of HBr from these complexes either thermally or in the presence of base then gives stable
neutral complexes [L¢Re(CO)3]. The impact of the N-acyl group and any substituent at the apex of the
tripodal ligands (L¢¢H) on the relative stabilities of intermediate complexes on the reaction pathway
helps provide an explanation for the observed difference in behaviour of the N-acylated
tris(pyridin-2-yl)methylamines (LH).
(tpmaH) 1a and its N-benzoylated derivative (tpmbaH) 1b, we had
Introduction
found that both systems showed a preference for the formation
of rhenium complexes where coordination occurs through two
pyridine rings and the aliphatic nitrogen atom.
The development of convenient methods for forming rhenium(I)
tricarbonyl complexes1 for use in radiopharmaceutical applica-
tions has meant there is renewed interest in tridentate chelates
that are capable of coordinating in a facial manner.2 Since
we had previously used tripodal N-acylated 1,1,1-tris(pyridin-
2-yl)methylamine ligands to form metal complexes with both
copper and zinc in which coordination occurs facially via the three
pyridine nitrogen atoms,3 it was a logical extension to see whether
such tripodal systems might prove suitable for applications in
radiopharmaceutical imaging and therapy agents. The rationale
was that if coordination to the rhenium could be achieved via the
three pyridine rings then functionalisation of the uncoordinated
amide limb in the resulting rheniumtricarbonyl complex would
potentially provide a means for controlling its targeting and
biodistribution.
In contrast, when the N-acetylated tris(pyridin-2-yl)-
methylamine ligand 1c was used, a cationic complex formed in
which coordination to the rhenium centre had occurred via the
three pyridine rings.5 It is now clear from our studies that the
nature of the complexes isolated in the reaction of N-substituted
tris(pyridin-2-yl)methylamine ligands with [Re(CO)5Br] is depen-
dent on both the reaction conditions and the nature of the N-
substituent. To better understand the factors that influence the
nature and stability of the rhenium complexes formed we have now
broadened the scope of these studies to include the use of ligands
based on the bis(pyridin-2-yl)methylamine system (bpmaH) 2a
and its phenyl substituted analogue (Ph-bpmaH) 2a¢ (Fig. 1).
The phenyl substituted ligands 2a¢–c¢ were chosen to mimic
more closely the steric bulk of the corresponding tris(pyridin-
2-yl)methylamine ligands 1a–c while maintaining the donor set
found in the bis(pyridin-2-yl)methylamine ligands 2a–c. It was
hoped that a comparison of the complexing behaviour of these
three sets of ligands would enable us to get a better understanding
of the factors that influence the course of their reactions with
[Re(CO)5Br].
However, in a preliminary study4 of the reaction of [Re(CO)5Br]
with the parent tripodal tris(pyridin-2-yl)methylamine ligand
School of Biological and Chemical Sciences, Queen Mary University of
London, Mile End Road, London, UK E1 4NS. E-mail: d.v.griffiths@
qmul.ac.uk; Fax: +44 (0)20 7882 7427; Tel: +44 (0)20 7882 5389
† CCDC reference numbers 733190, 821715–821721. For crystallographic
data in CIF or other electronic format see DOI: 10.1039/c1dt11020a
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no strongly basic centres present in the cationic system 3b to
deprotonate the uncoordinated amide, it would seem likely that
deprotonation of the amide nitrogen occurs after its coordination
to the rhenium and hence that 4b lies on the pathway between 3b
and 5b (Scheme 2).
Fig. 1 Ligand structures.
Results and discussion
Ligand synthesis
We have previously reported the preparation of ligands 1a–c and
2a–b. For the present study we required the N-acetylated ligand
2c which was prepared by the action of acetic anhydride on 1,1-
bis(pyridin-2-yl)methylamine 2a.
The corresponding phenyl substituted ligands 2a¢–c¢ were
prepared from 2-benzoylpyridine as shown in Scheme 1.
Scheme 2 Proposed reaction pathway to the benzoylated complex 5b.
The facile formation of complex 5b containing an uncoordi-
nated pyridine ring, then posed the question whether a similarly
coordinated complex might be formed from the corresponding N-
benzoylated bis(pyridin-2-yl)methylamine ligand (bpmbaH) 2b.
This would be advantageous since with metal-based radiophar-
maceuticals it is desirable to keep the size of the metal complex
attached to the targeting vehicle as small as possible. Moreover,
if the basic ligand structure can be kept small this increases the
scope for subsequent modification to improve targeting.
However, the complex that precipitated from solution when
the bispyridyl system bpmbaH 2b was heated with [Re(CO)5Br]
in toluene was the neutral complex [(bpmbaH)Re(CO)3Br] 6b
(Scheme 3), in which no coordination of the amide group to the
rhenium had occurred.
Scheme 1 Synthesis of ligands 2a¢–c¢ from 2-benzoylpyridine.
Comparison of the reactions of tpmbaH 1b, bpmbaH 2b and
Ph-bpmbaH 2b¢ with [Re(CO)5Br]
In our earlier studies of the copper complexes of the tris(pyridin-
2-yl)methylamine ligands3 we had shown that it was possible to
prevent coordination at the aliphatic nitrogen in these ligands by
acylation of the primary amine group. However, as noted earlier,
the product isolated from the reaction of the N-benzoylated ligand
(tpmbaH) 1b with [Re(CO)5Br] in toluene was a neutral complex
[(tpmba)Re(CO)3] 5b in which the coordination had occurred via
two of the pyridine nitrogens and a deprotonated amide group.4
More recently5 we have shown that the complex 5b is formed
via the rearrangement of an initially formed complex 3b in which
coordination occurs via the three pyridine rings. Moreover, this
rearrangement of the complex 3b occurs under mild conditions,
since it occurred when attempts were made to crystallise the
complex 3b by the slow evaporation of a solution of this material
in methylene chloride at room temperature. Since there are
Scheme 3 Proposed reaction pathway to the benzoylated complexes 8b
and 8b¢.
The X-ray structure of the neutral complex 6b, shown in Fig. 2,
confirms that the molecule has adopted a distorted octahedral
10216 | Dalton Trans., 2011, 40, 10215–10228
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Table 1 Crystallographic parameters for 6b, 8b,5 8b¢.HBr and 8b¢
[(bpmbaH)Re(CO)3Br]
6b
[Re(CO)3(Ph-bpmbaH)]Br
8b¢·HBr
[Re(CO)3(Ph-bpmba)]
8b¢
Complex
[(bpmba)Re(CO)3] 8b
Formula
Mr (Da)
T (K)
C21H15BrN3O4Re·MeCN
680.52
160(2)
Triclinic
P1
7.602(3)
12.208(8)
13.094(5)
74.81(4)
80.92(4)
94.31(6)
1149.4(10)
1.966(2)
C27H18N3O4Re·HBr
C27H18N3O4Re
634.64
120(2)
Orthorhombic
Pc21n
11.0493(2)
12.7288(2)
16.2947(2)
90
C21H14N3O4Re·MeCN
579.56
120(2)
Triclinic
P1
10.7023(1)
12.8618(2)
15.0542(1)
109.990(1)
95.392(1)
91.556(1)
91.556(1)
1.954(4)
6.316
0.48 ¥ 0.30 ¥ 0.18
-13/13, -16/16, -19/19
40952
8865 (0.0373)
8280
0.3960 and 0.1515
8865/0/541
1.113
0.0237, 0.0553
0.0264, 0.0563
0.750 and -1.935
715.56
120(2)
Crystal system
Space group
Orthorhombic
Pcab
17.395(2)
13.956(2)
20.464(2)
90
90
90
4967.9(10)
1.913(8)
6.540
0.16 ¥ 0.06 ¥ 0.03
-21/21, -17/12, -25/25
25325
4955 (0.0897)
3630
0.8280 and 0.4209
4955/0/298
1.096
0.0841, 0.1816
0.1182, 0.2033
3.136 and -2.048
˚
a (A)
˚
b (A)
˚
c (A)
a (◦)
b (◦)
g (◦)
90
90
3
˚
V (A )
2291.76(6)
1.839(4)
5.343
0.26 ¥ 0.14 ¥ 0.12
-14/14, -16/16, -21/20
23097
5186 (0.0442)
4791
0.5665 and 0.3372
5186/1/292
1.097
0.0248, 0.0476
0.0286, 0.0488
0.716 and -1.159
DCalc (g cm -3) (Z)
mMo (mm-1)
7.062
Crystal size (mm3)
Index ranges for h, k, l
No. of reflections collected
No. of reflections unique (Rint
No. of reflections observed
Tmax/min
0.38 ¥ 0.30 ¥ 0.15
0/9, -14/14, -15/15
4336
4005 (0.0272)
3529
0.4173 and 0.1744
4005/0/291
1.024
0.0309, 0.0773
0.0403, 0.0799
1.829 and -2.056
)
Data/restraints/parameters
Goodness-of-fit (GOF)
R1, wR2 [I > 2s(I)]
(all data)
Largest diff. peak and hole
-3
˚
(e/A )
dissociation and reorientation of the ligand is occurring during
this process. It should also be noted that if the ligand 2b is heated
with [Re(CO)5Br] in DMF the complex 6b does not precipitate
from solution and only the formation of the final product 8b is
observed.5 Since the conversion of 6b to 8b can be achieved in the
absence of base, this process probably involves the intermediate
formation of the complex 7b where coordination to the amide NH
occurs prior to loss of HBr.
Support for this view comes from a study of the corresponding
phenyl substituted N-benzoylated bis(pyridin-2-yl)methylamine
ligand (Ph-bpmbaH) 2b¢ which was chosen to mimic the steric
bulk of the tris(pyridin-2-yl)methylamine ligand (tpmbaH) 1b
while maintaining the donor atom set present in the bis(pyridin-
2-yl)methylamine ligand (bpmbaH) 2b. When the ligand 2b¢ was
heated with [Re(CO)5Br] in toluene at 105–110 ◦C, the product
that precipitated was shown by NMR to be the intermediate
cationic complex 7b¢ where the amide nitrogen is coordinated to
Fig. 2 X-Ray crystal structure of the neutral complex 6b·MeCN.
arrangement around the rhenium with the amide portion of the
bispyridyl ligand orientated away from the coordinated bromide
ligand. Crystallographic parameters for 6b are presented in Table 1
and selected bond lengths and angles are shown in Table 2.
Although in the solid state the plane of the benzoyl phenyl
group is slightly rotated relative to the adjacent carbonyl group
[O(4)–C(15)–C(16)–C(17) dihedral angle 29.03◦], the NMR data
for 6b in solution shows a symmetrical complex with the two
pyridine rings having the same chemical shifts. The presence of
the amide N–H proton is also clearly visible at low field (dH 10.23
ppm) showing a strong coupling of 10 Hz to the adjacent CH (dH
7.16 ppm), this being consistent with their approximately coplanar
arrangement (H–N–C–H dihedral angle = -175.28◦) in the X-ray
structure (Fig. 2).6
Although the amide nitrogen is inappropriately orientated to
coordinate to the rhenium in the neutral complex 6b, it proved
possible to facilitate coordination of the amide group and loss
of HBr to give the neutral complex 8b if the neutral complex
6b was dissolved in DMF and the resulting solution heated at
ca. 118 ◦C for an extended period. This clearly indicates that
1
the rhenium as an N–H. Thus, the H NMR spectrum for this
complex in CDCl3 clearly showed an acidic N–H proton resonance
at dH 11.55 ppm, at significantly lower field than the uncoordinated
amide group in the free ligand [dH (CDCl3) 10.19 ppm]. The
presence of two sets of pyridine ring signals also reflected the
unsymmetrical nature of the complex.
It is interesting to note that the 1H NMR spectrum of 7b¢ has a
number of unusual features, the interpretation of which required
extensive use of 1H-1H and 1H-13C 2D correlation spectra. In
particular the two ortho protons on one of the phenyl rings gave
broad resonances at quite different chemical shifts (dH 7.96 and
8.93 ppm) indicating restricted rotation of this phenyl group. Since
the ortho protons on the other phenyl ring (dH 8.58 ppm) were
shown to have a long range coupling to the amide carbonyl carbon
it was clear that it was the phenyl ring at the apex of the tripodal
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Table 2 Selected bond lengths and angles for 6b, 8b,5 8b¢.HBr and 8b¢
[(bpmbaH) Re-
(CO)3Br] 6b
[(Ph-bpmbaH)Re-
(CO)3]Br 8b¢·HBr
[(Ph-bpmba)Re-
(CO)3] 8b¢
[(bpmba)Re(CO)3] 8b5
a
b
˚
˚
Bond lengths (A)
Bond lengths (A)
Re(1)–C(1)
Re(1)–C(2)
Re(1)–C(3)
Re(1)–N(1)
Re(1)–N(2)
Re(1)–N(3)
C(1)–O(1)
C(2)–O(2)
C(3)–O(3)
N(1)–C(4)
N(1)–C(15)
O(4)–C(15)
1.902(6)
1.898(6)
1.902(6)
—
2.198(4)
2.215(4)
1.135(7)
1.161(7)
1.156(7)
1.437(7)
1.363(7)
1.224(6)
1.890(17)
1.934(16)
1.924(18)
2.280(11)
2.169(13)
2.166(14)
1.182(19)
1.152(19)
1.17(2)
1.918(4)
1.925(5)
1.910(4)
2.159(3)
2.198(3)
2.173(2)
1.153(6)
1.150(5)
1.160(5)
1.492(5)
1.383(6)
1.233(5)
Re(1)–C(1)
Re(1)–C(2)
Re(1)–C(3)
Re(1)–N(2)
Re(1)–N(1)
Re(1)–N(3)
C(1)–O(1)
C(2)–O(2)
C(3)–O(3)
N(3)–C(9)
N(3)–C(15)
O(4A)–C(15A)
1.923(3)
1.92(3)
1.914(3)
2.1975(3)
2.1825(3)
2.1565(3)
1.153(4)
1.1505(4)
1.1585(4)
1.473(4)
1.3455(4)
1.249(4)
1.459(19)
1.446(19)
1.225(18)
Bond angles (◦)
Bond angles (◦)
C(1)–Re(1)–C(2)
C(1)–Re(1)–C(3)
C(2)–Re(1)–C(3)
C(1)–Re(1)–N(1)
C(1)–Re(1)–N(2)
C(1)–Re(1)–N(3)
C(2)–Re(1)–N(1)
C(2)–Re(1)–N(2)
C(2)–Re(1)–N(3)
C(3)–Re(1)–N(1)
C(3)–Re(1)–N(2)
C(3)–Re(1)–N(3)
N(1)–Re(1)–N(2)
N(1)–Re(1)–N(3)
N(2)–Re(1)–N(3)
C(4)–N(1)–C(15)
87.9(3)
86.7(3)
88.9(2)
—
94.4(2)
98.1(2)
—
176.43(18)
93.9(2)
—
93.9(2)
174.6(2)
—
87.2(6)
85.9(7)
89.9(6)
169.9(6)
98.3(6)
99.0(6)
100.5(5)
171.5(6)
93.2(6)
100.6(5)
96.9(6)
174.3(5)
73.4(4)
74.2(5)
79.6(5)
115.8(12)
87.62(18)
89.95(15)
93.98(17)
168.23(13)
98.21(16)
95.38(15)
98.33(16)
172.41(16)
93.88(19)
99.70(14)
90.89(14)
170.68(19)
75.08(12)
74.18(14)
80.80(16)
114.7(3)
C(1)–Re(1)–C(2
C(1)–Re(1)–C(3)
C(2)–Re(1)–C(3)
C(1)–Re(1)–N(1)
C(1)–Re(1)–N(2)
C(1)–Re(1)–N(3)
C(2)–Re(1)–N(1)
C(2)–Re(1)–N(2)
C(2)–Re(1)–N(3)
C(3)–Re(1)–N(1)
C(3)–Re(1)–N(2)
C(3)–Re(1)–N(3)
N(1)–Re(1)–N(2)
N(1)–Re(1)–N(3)
N(2)–Re(1)–N(3)
C(9)–N(3)–C(15)
89.4(14)
89.62(14)
89.77(14)
98.085(12)
94.08(12)
166.815(12)
92.185(11)
173.27(11)
101.535(12)
172.04(12)
95.99(12)
97.52(12)
81.64(10)
74.545(10)
74.335(10)
114.7(3)
—
83.17(16)
130.0(5)
Torsion angles (◦)
Torsion angles (◦)
O(4)–C(15)–N(3)–Re(1)
O(4)–C(15)–N(3)–C(9)
Re(1)–N(1)–C(4)–C(5)
Re(1)–N(1)–C(4)–C(10)
Re(1)–N(1)–C(4)–C(22)
Re(1)–N(1)–C(15)–C(16)
Re(1)–N(1)–C(15)–O(4)
Re(1)–N(2)–C(10)–C(4)
Re(1)–N(3)–C(5)–C(4)
O(4)–C(15)–N(1)–C(4)
O(4)–C(15)–C(16)–C(17)
N(1)–C(15)–C(16)–C(17)
N(1)–C(4)–C(22)–C(23)
—
—
—
—
-55.7(10)
56.0(10)
-175.7(8)
72.7(12)
-104.4(13)
6.2(18)
-8.0(13)
9.6(18)
-164.8(10)
18.2(16)
3.0(14)
-56.5(3)
50.7(3)
-177.4(2)
73.7(4)
-108.2(4)
-5.8(4)
-0.1(4)
12.2(6)
-151.5(3)
26.8(5)
—
8.7(6)
-4.5(6)
—
29.2(7)
-152.0(7)
—
73.7(3)
a Atom numbering is that shown in Fig. 4, this differs from cif file which shows inverse structure; b Average values from the two conformations present.
ligand that was exhibiting slow rotation on the NMR timescale.
The corresponding ortho carbon resonances on the other hand,
while showing the expected broadening, exhibited a less significant
chemical shift difference (cf. dC 131.9 to 133.1 ppm) consistent with
there being smaller changes in magnetic anisotropy nearer to the
axis of rotation.
If we assume that both the ligands 2b and 2b¢ follow the
same basic reaction pathway in their reaction with [Re(CO)5Br]
(see Scheme 3), then the precipitation of 7b¢ rather than 6b¢ in
the latter case suggests that the concentration of 7b¢ builds up
more quickly than that of 6b¢. If so, this would suggest that the
presence of the phenyl substituent is encouraging 6b¢ to adopt
a conformation that facilitates an interaction between the amide
nitrogen atom and the metal centre. It is interesting to speculate
whether a similar conformational effect in the structurally related
trispyridyl ligand 1b helps to explain the relative ease of formation
of the neutral complex 5b from the initially formed cationic
complex 3b.5
It is also interesting to note that while the solution NMR
spectra of the complex formed from the reaction of the ligand
2b¢ with [Re(CO)5Br] clearly identified it as being 7b¢, the X-
ray structure determined for the same crystals suggests that in
the solid state it dissociates to give the neutral complex 8b¢ and
HBr. Both components are clearly visible in the crystal structure
shown in Fig. 3. We must conclude that this dissociation permits
more favourable packing in the crystalline state since these two
molecules clearly readily re-associate to give 7b¢ when returned
to the solution state. It is, however, interesting to note that the
ATR infrared spectrum of the complex shows a weak band at
2568 cm-1 typical of H–N+ bonds and suggesting that some degree
of protonation of the amide nitrogen is present even in the solid
phase.
10218 | Dalton Trans., 2011, 40, 10215–10228
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two coordinated pyridine rings as being magnetically equivalent.
Moreover, in the analogous tris(pyridyl)methylamine complex 5b
we had observed restricted rotation of the uncoordinated pyridine
ring. This had led to exceptional broadening of the 3-H proton
resonances of the coordinated pyridine rings to the point that
they were not readily apparent at ambient temperatures.5 No such
effects were observed with the corresponding complex 8b¢, where
the uncoordinated pyridine ring in 5b has been replaced by a
phenyl group. This clearly confirms our earlier suspicions that the
presence of the uncoordinated pyridine nitrogen atom does have a
significant impact on the conformational behaviour of 5b on the
NMR timescale, and that the unsymmetrical nature of the rotating
pyridine ring is also responsible for the exceptional broadening of
the 3-H proton resonances observed in this compound.
We were also able to confirm that the neutral complex 8b¢ does
reform the cationic complex 7b¢ in solution in the presence of
HBr. Thus, bubbling a little HBr into the NMR solution of the
neutral complex 8b¢ produced changes in the NMR spectrum of
the resulting solution consistent with the formation of the cationic
complex 7b¢.
As expected the conversion of 7b¢ to 8b¢ can be carried out much
more quickly (<45 min) and at room temperature if a base, such as
triethylamine, is added to facilitate the deprotonation. This ready
deprotonation of 7b¢ in the presence of base also helps explain why
no evidence for the formation of the intermediate cationic complex
4b is observed from the trispyridyl ligand 1b when a solution of
the initially formed complex 3b is warmed (see Scheme 2). The
cationic complex 4b can be viewed as containing a built-in base
(the uncoordinated pyridine ring) and this would be expected to
facilitate the rapid removal of the amide N–H to give the observed
final complex 5b.
Fig. 3 X-Ray crystal structure suggesting the cationic complex 7b¢
dissociates into 8b¢·HBr in the solid state.
Selected bond lengths and angles from the X-ray crystal
structure of the complex formed from the reaction of the ligand
2b¢ with [Re(CO)5Br] (i.e. 8b¢·HBr) are shown in Table 2 and other
crystallographic parameters are presented in Table 1.
We earlier noted that the extended heating of a solution of the
neutral complex 6b in DMF resulted in loss of HBr and formation
of the neutral complex 8b. The complex 8b¢·HBr, formed from
the reaction of the ligand 2b¢ with [Re(CO)5Br], behaves similarly
under comparable conditions resulting in formation of 8b¢.
The X-ray structure of the neutral complex 8b¢ is shown in
Fig. 4. Its crystallographic parameters are presented in Table 1
with selected bond lengths and angles in Table 2. The most
obvious change between this structure and that of 8b¢·HBr is
the orientation of the uncoordinated phenyl ring in 8b¢, which
is rotated through ca. 71◦ relative to the case when the molecule
of HBr is present. However there are also significant changes in
the orientation of the benzoyl C–N bond relative to the axis of the
apical phenyl group and the orientation of the adjacent benzoyl
ring [D◦8b¢·HBr–8b¢¢ for C(15)–N(1)–C(4)–C(22) = 10.39 and for N(1)–
C(15)–C(16)–C(17) = -8.53]. As anticipated the structure adopted
by 8b¢ is also very similar to the analogous complex from the
N-benzoylated tris(pyridin-2-yl)methylamine ligands 1b.5
Comparison of the reactions of tpmaaH 1c, bpmaaH 2c and
Ph-bpmaaH 2c¢ with [Re(CO)5Br]
We next investigated the impact of changing the substituent on
the amide nitrogen from benzoyl to acetyl. We have already
commented on the marked difference in the behaviour of the
N-substituted tris(pyridin-2-yl)methylamine ligands 1b and 1c in
their reaction with [Re(CO)5Br]5 and it was hoped that a study
of the corresponding N-substituted bis(pyridin-2-yl)methylamine
ligands 2c and 2c¢ might help shed some light on the main factors
involved.
The previously unreported X-ray structure of the N-acetylated
trispyridyl complex 3c (Fig. 5) is shown in Fig. 6 and related data
is shown in Table 3. The structure shows some disorder about
the acetyl group but the bond lengths of the C–N and the C
O
˚
bonds (1.387 and 1.222 A, respectively) are consistent with those
expected for a normal amide and similar to those found in the free
7
˚
ligand (1.350 and 1.228 A, respectively).
It is interesting to note that while the corresponding benzoyl
complex 3b had readily undergone rearrangement and loss of HBr
on heating in solution (Scheme 2),5 the N-acetylated complex 3c
had proved resistant to rearrangement. We had earlier speculated
that this possibly reflected the greater acidity of the N–H proton
in 3b, thus facilitating its removal and encouraging coordination
of the deprotected amide group.5 However, it is interesting to note
that we have subsequently shown that the acetylated compound 3c
failed to rearrange to give the neutral complex 5c even when heated
Fig. 4 X-Ray crystal structure of the neutral complex 8b¢.
In contrast to the NMR spectrum of 7b¢, it is interesting to
note that the NMR spectra of the neutral complex 8b¢ shows the
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Table 3 Crystallographical parameters, selected bond lengths and angles for 3c, [(tpmaaH)Re(CO)3]Br
˚
Formula
Mr (Da)
T (K)
C21H16BrN4O4Re·H2O
672.50
120(2)
Trigonal
R3
29.8269(16)
29.8269(16)
15.6504(11)
90
90
120
12057.9(12)
1.667(18)
6.060
0.11 ¥ 0.10 ¥ 0.04
-25/35,-26/35,-19/19
19587
5025 (0.0550)
3987
0.7936 and 0.5553
5025/0/303
1.211
0.0820, 0.1731
0.01045, 0.1822
2.877 and -1.852
Bond lengths (A)
Re(1)–C(1)
Re(1)–C(2)
Re(1)–C(3)
Re(1)–N(1)
Re(1)–N(2)
Re(1)–N(3)
C(1)–O(1)
C(2)–O(2)
C(3)–O(3)
N(4)–C(4)
N(4)–C(15)
O(4)–C(15)
1.902(15)
1.921(18)
1.930(18)
2.168(11)
2.207(13)
2.166(12)
1.168(18)
1.13(2)
1.14(2)
1.44(2)
1.39(2)
1.22(2)
Crystal system
Space group
˚
a (A)
˚
b (A)
˚
c (A)
a (◦)
b (◦)
g (◦)
3
˚
V (A )
DCalc (g cm-3) (Z)
mMo (mm-1)
Crystal size (mm3)
Index ranges for h, k, l
No. of reflections collected
No. of reflections unique (Rint
No. of reflections observed
Tmax/min
Bond angles (◦)
C(1)–Re(1)–C(2)
C(1)–Re(1)–C(3)
C(2)–Re(1)–C(3)
C(1)–Re(1)–N(1)
C(1)–Re(1)–N(2)
C(1)–Re(1)–N(3)
C(2)–Re(1)–N(1)
C(2)–Re(1)–N(2)
C(2)–Re(1)–N(3)
C(3)–Re(1)–N(1)
88.4(7)
88.6(6)
88.4(8)
175.1(5)
93.2(5)
95.3(5)
96.4(6)
177.3(6)
95.0(7)
92.5(5)
)
Data/restraints/parameters
Goodness-of-fit (GOF)
R1, wR2 [I > 2s(I)]
(all data)
-3
˚
Largest diff. peak and hole (e/A )
2c and 2c¢ with those of the corresponding N-benzoylated ligands
2b and 2b¢ investigated earlier.
To maximise the chances of seeing reaction intermediates,
the reaction of [Re(CO)5Br] with the bispyridyl ligand 2c was
initially investigated under mild conditions in toluene. However,
because [Re(CO)5Br] is not particularly soluble in toluene at
room temperature, the reaction had to be carried out at about
80 ◦C. After heating at this temperature for about 20 min the
reaction mixture was shown by NMR spectroscopy to contain
three pyridine-containing compounds in a ratio of ca. 0.7 : 1 : 1.
These were subsequently identified as the starting ligand 2c,
the neutral complex 6c [(bpmaaH)Re(CO)3Br] and a cationic
complex 9c (Scheme 4). By repeating the reaction under more
vigorous conditions (heating under reflux for 5 h) it was possible
to demonstrate that 6c, the initially formed complex, undergoes
rearrangement to give the cationic complex 9c.
Fig. 5 Structure of the cationic complex 3c.
Because the neutral complex 6c slowly underwent conversion
to give the cationic complex 9c in solution it was not possible to
isolate a pure sample of 6c for NMR analysis, although sufficient
data could be obtained to show it shared the characteristic features
observed in the spectra of the N-benzoylated complex 6b. Thus, for
example, 6b exhibited an amide proton resonance at dH 10.23 ppm
showing coupling to the bridgehead methyne proton (d, JHH 10
Hz), and the complex assigned to 6c gave a similar resonance at
dH 10.25 ppm (d, JHH 10.5 Hz).
It is interesting to note that while the cationic complex [(Ph-
bpmbaH)Re(CO)3]Br 7b¢, derived from the phenyl-substituted N-
benzoylated ligand 2b¢, had shown the amide nitrogen coordinated
as an N–H, the X-ray structure of the cationic complex 9c (Fig. 7)
shows the amide portion of the ligand to be coordinated to the
rhenium via its imidate form.
Fig. 6 X-Ray crystal structure of the cationic complex 3c·H2O (the water
molecule has been omitted).
with triethylamine in DMSO for 16 h at 70 ◦C. The greater steric
bulk of the benzoyl group relative to the acetyl group therefore
appears to have a more significant role in determining whether
rearrangement of the initially formed complex 3 occurs.
The presence of an acidic hydroxyl group in 9c is supported by
1
the H NMR spectrum of the complex in CDCl3, which shows
a broad low field signal at ca. dH 13.5 ppm. Coordination of the
amide nitrogen via its imidic acid tautomer means that the complex
In view of this it was of interest to compare the coordination be-
haviour of the N-acetylated bis(pyridin-2-yl)methylamine ligands
10220 | Dalton Trans., 2011, 40, 10215–10228
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Fig. 8 Complexes with the acetyl group in its imidic acid tautomeric
form.
angles for the cationic complex 9c are shown in Table 5 and other
crystallographic parameters are presented in Table 4.
No spectral evidence for the formation of the intermediate
cationic complex 7c was observed during the conversion of 6c
into 9c.
Treatment of the complex 9c with triethylamine in chloroform at
room temperature resulted in the removal of HBr and formation
of the neutral complex 8c. The X-ray structure of 8c is shown
in Fig. 9, selected bond lengths and angles are shown in Table 5
and other crystallographic parameters are presented in Table 4.
As noted earlier the ‘amide’ bond lengths in 8c are typical of the
values seen in other complexes containing C–N and C O bonds
such as 5b and 7c¢.
Scheme 4 Proposed reaction pathway to the benzoylated complexes 8c
and 8c¢.
Fig. 7 X-Ray crystal structure of the cationic complex 9c.
Fig. 9 X-Ray crystal structure of the neutral complex 8c.
remains cationic with the bromide counterion hydrogen-bonding
to the ‘imidic acid’ proton. This tautomerism also explains why
Having established that the N-acetylated bispyridyl ligand 2c
reacts with [Re(CO)5Br] in the presence of base to give the neutral
complex 8c via a reaction pathway involving the intermediate
complexes 6c and 9c, it was of interest to see how the presence
of an additional phenyl group, as in 2c¢, would affect the reaction
pathway and the relative stabilities of the intermediate complexes.
Heating the ligand 2c¢ with [Re(CO)5Br] in toluene under reflux
led to the formation of a precipitate that was shown to be the
cationic complex 7c¢ (Fig. 10). Selected bond lengths and angles
for this complex are shown in Table 5 and other crystallographic
parameters are presented in Table 4. Interestingly, in this complex
the amide portion of the original ligand has been coordinated to
the rhenium as an NH without tautomerising to its imidate form.
This is supported by the NMR data in CDCl3 which shows the
presence of an acidic N–H resonance at dH 11.4 ppm analogous
to the chemical shift value seen in 7b¢ (dH 11.4 ppm). Addition
of a little d4-MeOD to the solution of 7c¢ in CDCl3 was found to
improve the appearance of the spectrum and aid interpretation.
˚
the ‘amide’ carbon–nitrogen bond length of 1.288 A is shorter
˚
in 9c than that observed in the trispyridyl system 5b (1.361 A)
˚
or the bispyridyl systems 7c¢/8c (1.442/1.340 A), and likewise the
˚
length of the ‘amide’ C–O bond is longer in 9c (1.312 A) than in the
˚
˚
carbonyl groups in 5b (1.242 A) or 7c¢/8c (1.196/1.249 A). We were
unable to find another example of this type of tautomerisation of
an acetyl group in a rhenium complex, although there are rare
examples of the coordination of an acetyl group via its imidic
acid tautomer in palladium,8 platinum,9,10 cobalt11 and iridium12
complexes. Fortunately, limited X-ray crystallographic data is
available for two of these complexes, 108 and 1112 to support the
proposed structures (Fig. 8). For the palladium complex 10 it is
interesting to note that the length of the C N bond is 1.288
˚
A, the same length as that seen for the corresponding bond in
˚
9c, while in the iridium complex 11 the C N bond (1.260 A)
˚
is slightly shorter and the C–O bond (1.321 A) slightly longer
than those observed in 9c. Further selected bond lengths and
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Table 4 Crystallographical parameters for 7c¢, 8c, 8c¢ and 9c
Complex
[(Ph-bpmaaH)Re(CO)3]Br 7c¢ [(bpmaa)Re(CO)3] 8c
[(Ph-bpmaa)Re(CO)3] 8c¢ [(Re(CO)3(bpmaaH)]Br 9c
Formula
Mr (Da)
T (K)
C22H17BrN3O4Re
653.50
120(2)
Monoclinic
C2/c
17.8235(5)
17.5400(3)
16.1051(4)
90
120.057 (1)
90
4357.79(18)
1.992 (8)
C16H12N3O4Re
496.49
120(2)
Triclinic
P1
8.6598(1)
8.7143(1)
11.5640(2)
77.870(1)
88.862(1)
70.192(1)
801.452(19)
2.057 (2)
7.606
C22H16N3O4Re·CH2Cl2
615.04
120(2)
Monoclinic
P2/n
14.2033(7)
11.9057(5)
26.0996(12)
90
92.1430(10)
90
4410.4(4)
1.853 (8)
5.666
C16H13BrN3O4Re
577.40
120(2)
Orthorhombic
Pnma
12.2711(4)
10.3063(3)
14.6466(5)
90
Crystal system
Space group
˚
a (A)
˚
b (A)
˚
c (A)
a (◦)
b (◦)
g (◦)
90
90
3
˚
V (A )
1852.35(10)
2.070 (4)
8.742
0.18 ¥ 0.04 ¥ 0.03
-15/15, -13/12, -18/18
21095
2242 (0.0439)
2018
0.7794 and 0.3021
2242/0/129
1.115
0.0248, 0.0492
0.0311, 0.0521
1.317 and -1.033
DCalc (g cm-3) (Z)
mMo (mm-1)
7.445
Crystal size (mm3)
Index ranges for h, k, l
No. of reflections collected
No. of reflections unique (Rint
No. of reflections observed
Tmax/min
0.22 ¥ 0.16 ¥ 0.05
-23/22, -22/21, -20/20
24328
5003 (0.0555)
4280
0.7072 and 0.2912
5003/0/273
1.059
0.0334, 0.0688
0.0435, 0.0728
0.80 ¥ 0.38 ¥ 0.28
-11/10, -11/11, -15/14 -17/17, -14/14, -32/32
0.40 ¥ 0.20 ¥ 0.12
18270
3673 (0.0389)
3620
0.2246 and 0.0643
3673/0/219
1.124
0.0162, 0.0388
0.0166, 0.0390
0.817 and -1.058
19785
7984 (0.0385)
7173
0.5496 and 0.2102
7984/13/547
1.060
0.0480, 0.1324
0.0532, 0.1392
2.966 and -2.303
)
Data/restraints/parameters
Goodness-of-fit (GOF)
R1, wR2 [I > 2s(I)]
(all data)
-3
˚
Largest diff. peak and hole (e/A ) 1.675 and -1.309
C
O had also been moved to higher wavenumber (nmax 1759 cm-1)
relative to its position in the free ligand 2c¢ (nmax 1675 cm-1), this
being consistent with an increase in the electronegativity of the
substituent on the carbonyl group. With this in mind the situation
shown in Fig. 11, with the major contribution coming from the
resonance structure on the right, would appear to be a better
representation of 7c¢ than that shown in Scheme 3.
Fig. 11 Improved representation of the cationic complex 7c¢.
As previously noted, the IR spectrum of the corresponding
benzoyl system 7b¢ also showed some evidence for the presence of
N+–H bonds although the movement of the C O band to higher
wavenumber in the complex relative to the free ligand was much
less marked in the case of 7b¢ (Dnmax 48 cm-1) than observed for 7c¢
(Dnmax 84 cm-1). This is also consistent with our earlier conclusion
that there is significant dissociation of 7b¢ into 8b¢ and HBr in the
solid phase.
Fig. 10 X-Ray crystal structure of the cationic complex 7c¢.
However, even so, the signals in the 1H and 13C NMR spectra of
7c¢ were broadened, reflecting the presence of an NMR exchange
process. Careful analysis of these NMR spectra showed effects
very similar to those previously discussed for the corresponding
benzoyl analogue 7b¢. In particular the ortho protons on the phenyl
ring at the apex of the tripod in 7c¢ were again non-equivalent (dH
8.35 and 7.82 ppm) indicating that the rotation of this phenyl
group around its axis was again very slow on the NMR timescale.
It is interesting to note that the IR spectrum of 7c¢ obtained with
an ATR accessory shows a broad band at nmax 2594 cm-1 consistent
with the presence of an N+–H bond which also explains the absence
of an amide N–H band in the region 3100–3600 cm-1. The adjacent
Heating a solution of the complex 7c¢ in DMF at 120 ◦C for
a protracted period resulted in loss of HBr and formation of the
neutral complex 8c¢. This conversion can also be carried out at
room temperature in the presence of a base such as triethylamine.
Crystallisation of the complex 8c¢ from a DCM/hexane mixture
gave crystals of formula [(Ph-bpmaa)Re(CO)3]·0.5CH2Cl2 suitable
for X-ray analysis.
10222 | Dalton Trans., 2011, 40, 10215–10228
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Table 5 Selected bond lengths and angles for 7c¢, 8c, 8c¢ and 9c
[(Ph-bpmaa)Re(CO)3] 8c¢
[(Re(CO)3(bpmaaH)]Br 9c
[(Ph-bpmaaH)Re-
(CO)3]Br 7c¢
[(bpmaa)Re-
(CO)3] 8c
A
B
˚
˚
Bond lengths (A)
Bond lengths (A)
Re(1)–C(1)
Re(1)–C(2)
Re(1)–C(3)
Re(1)–N(1)
Re(1)–N(2)
Re(1)–N(3)
C(1)–O(1)
C(2)–O(2)
C(3)–O(3)
N(1)–C(4)
N(1)–C(15)
O(4)–C(15)
1.912(5)
1.936(5)
1.916(5)
2.227(4)
2.173(4)
2.176(4)
1.157(6)
1.154(6)
1.157(6)
1.524(5)
1.442(6)
1.197(6)
1.929(3)
1.919(3)
1.920(3)
2.139(2)
2.194(2)
2.190(2)
1.151(3)
1.151(4)
1.153(4)
1.474(3)
1.340(3)
1.249(3)
1.919(8)
1.923(7)
1.932(8)
2.159(6)
2.160(6)
2.205(6)
1.152(10)
1.154(9)
1.147(10)
1.504(9)
1.377(9)
1.236(8)
1.925(7)
1.924(7)
1.931(8)
2.173(5)
2.147(5)
2.203(6)
1.147(9)
1.152(9)
1.152(9)
1.490(9)
1.352(9)
1.239(8)
Re(1)–C(1)
Re(1)–C(2)
Re(1)–C(2)#
Re(1)–N(1)
Re(1)–N(3)
Re(1)–N(3)#
C(1)–O(1)
C(2)–O(2)
C(2)#–O(2)#
N(1)–C(4)
1.942(6)
1.914(4)
1.914(4)
2.151(4)
2.195(3)
2.195(3)
1.140(7)
1.157(5)
1.157(5)
1.468(6)
1.288(6)
1.312(6)
N(1)–C(15)
O(3)–C(15)
Bond angles (◦)
Bond angles (◦)
C(1)–Re(1)–C(2)
C(1)–Re(1)–C(3)
C(2)–Re(1)–C(3)
C(1)–Re(1)–N(1)
C(1)–Re(1)–N(2)
C(1)–Re(1)–N(3)
C(2)–Re(1)–N(1)
C(2)–Re(1)–N(2)
C(2)–Re(1)–N(3)
C(3)–Re(1)–N(1)
C(3)–Re(1)–N(2)
C(3)–Re(1)–N(3)
N(1)–Re(1)–N(2)
N(1)–Re(1)–N(3)
N(2)–Re(1)–N(3)
C(4)–N(1)–C(15)
87.3(2)
89.52(19)
91.4(2)
90.57(12)
87.72(11)
90.04(13)
168.73(10)
97.02(10)
96.68(9)
97.46(10)
172.26(10)
96.51(11)
100.12(10)
91.76(10)
172.05(9)
74.81(8)
87.8(3)
89.5(3)
91.1(3)
168.0(3)
95.4(3)
98.5(3)
101.4(3)
171.8(3)
90.8(3)
98.0(3)
96.5(3)
171.9(3)
74.5(2)
73.9(2)
81.1(2)
115.0(6)
86.8(3)
89.3(3)
89.9(3)
167.9(3)
95.3(3)
98.2(3)
102.4(3)
172.3(3)
92.0(3)
98.5(3)
97.5(3)
172.3(3)
74.6(2)
73.9(2)
80.4(2)
115.5(5)
C(1)–Re(1)–C(2)
C(1)–Re(1)–C(2)#
C(2)–Re(1)–C(2)#
C(1)–Re(1)–N(1)
C(1)–Re(1)–N(3)
C(1)–Re(1)–N(3)#
C(2)–Re(1)–N(1)
C(2)–Re(1)–N(3)
C(2)–Re(1)–N(3)#
C(2)#–Re(1)–N(1)
C(2)#–Re(1)–N(3)
C(2)#–Re(1)–N(3)#
N(1)–Re(1)–N(3)
N(1)–Re(1)–N(3)#
N(3)–Re(1)–N(3)#
C(4)–N(1)–C(15)
90.50(18)
90.50(17)
88.7(2)
166.78(16)
97.74(17)
96.02(16)
100.62(17)
174.40(16)
95.71(17)
100.71(16)
91.03(17)
171.19(17)
73.95(13)
72.86(13)
81.47(14)
116.7(4)
170.2(2)
97.97(16)
97.97(16)
96.48(13)
95.31(15)
170.58(13)
96.48(13)
170.58(13)
95.31(15)
74.63(10)
74.63(10)
79.51(15)
120.0(4)
74.62(8)
81.15(8)
117.3(3)
Torsion angles (◦)
Torsion angles (◦)
Re(1)–N(1)–C(4)–C(5)
Re(1)–N(1)–C(4)–C(10)
Re(1)–N(1)–C(4)–C(17)
Re(1)–N(1)–C(15)–C(16)
Re(1)–N(1)–C(15)–O(4)
Re(1)–N(2)–C(10)–C(4)
Re(1)–N(3)–C(5)–C(4)
O(4)–C(15)–N(1)–C(4)
N(1)–C(4)–C(17)–C(18)
-55.5(3)
53.7(3)
-179.4(2)
89.6(4)
-90.7(4)
-0.9(5)
3.4(4)
-56.52(19)
57.96(19)
—
6.0(4)
-174.78(19)
6.7(2)
-58.7(5)
51.9(5)
59.5(5)
-51.5(5)
-169.2(4)
56.6(7)
-125.8(6)
6.9(7)
12.6(7)
1.0(9)
27.9(6)
Re(1)–N(1)–C(15)–O(3)
Re(1)–N(1)–C(4)–H(4)
Re(1)–N(1)–C(15)–C(16)
O(3)–C(15)–N(1)–C(4)
180.0
180.0
0.000(1)
0.000(1)
170.5(4)
-62.0(7)
121.9(6)
-7.3(7)
-11.2(7)
-3.6(9)
-21.6(7)
-0.8(2)
-1.7(4)
24.8(6)
The presence of one solvent molecule to two of the complex
resulted in the presence of two conformations of the complex (A
and B). The structure of 8c¢A is shown in Fig. 12 and that of 8c¢B in
Fig. 13. Selected bond lengths and angles for both conformations
are shown in Table 5 and other crystallographic parameters are
presented in Table 4.
The torsion angles shown in Table 5 indicate that the two
conformations 8c¢A and 8c¢B have an approximate mirror image
relationship, although there is a significant difference in the extent
of rotation of the phenyl group relative to the N(1)–C(4) bond in
these conformations. In solution the molecule is conformationally
mobile since the NMR spectra show only an averaged spectrum
with the two pyridine rings magnetically equivalent. Neither the
1H or 13C NMR spectra show any evidence for restricted rotation
around the apical phenyl group.
Conclusions
Our studies of the reactions of N-acetylated bispyridylmethy-
lamine ligands 2 with [Re(CO)5Br] have provided a much
clearer insight into the factors that influence the nature of the
complexes formed with this ligand system and the related N-
acetylated trispyridylmethylamine ligand system. These studies
had been initiated following the unexpected observation that
quite different rhenium tricarbonyl complexes were formed
from the reaction of [Re(CO)5Br] with N-[1,1,1-tris(pyridin-2-
yl)methyl]benzamide, tpmbaH, 1b than when using N-[1,1,1-
tris(pyridin-2-yl)methyl]acetamide, tpmaaH, 1c.5 This was sig-
nificant in the context of our efforts to develop rhenium-based
agents for nuclear medicine where it was essential to be able to
predict the nature of the final complex. In particular, we wanted
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tripodal ligand facilitates the interaction of the amide nitrogen
with the rhenium centre, so that while the unsubstituted ligands
2b and 2c showed the initial formation of the neutral complexes 6b
and 6c, the corresponding phenyl substituted ligands 2b¢ and 2c¢
quickly produced the cationic complexes 7b¢ and 7c¢. All four of
these rhenium complexes could be converted to the corresponding
neutral complexes 8 simply by heating although in some cases the
conditions required were rather vigorous. However, if a base was
added to the reaction mixture, much milder conditions could be
used to bring about these same conversions.
If we compare the structures of the trispyridyl ligands 1 with
the bispyridyl ones 2 we can see that the trispyridyl ligands 1 in
principle contain both elements needed to facilitate formation of
neutral complexes analogous to 8. The trispyridyl ligands 1 can
be considered as being bispyridyl ligands of type 2 that have a
pyridine group at the apex of the tripod. Moreover, this pyridine
ring is relatively large, certainly of comparable size to the phenyl
group present in 2b¢ and 2c¢, and it can also function as a base.
So it is reasonable to assume that both the trispyridyl ligands 1b
and 1c should readily form the corresponding neutral complexes
5b and 5c given the opportunity. The reason that the acetylated
ligand 1c does not do so is probably related to the stability
of the initially formed cationic complex 3c. In contrast, in the
corresponding benzoyl system 3b we would expect increased steric
crowding between the substituent at the apex of the tripod and
the coordinated pyridine rings. This would increase the likelihood
that one of these coordinated pyridine rings will twist and become
detached from the rhenium. This in turn would then provide access
to the reaction pathway seen in the bispyridyl ligands (Schemes 2
and 3) leading to the observed formation of the neutral complex 5b.
If stable complexes of type 4 are required we can therefore conclude
that it is essential to ensure that the nature of the substituent placed
at the apex of the tripod will contribute as little as possible to steric
interactions with the coordinated pyridine rings.
Fig. 12 X-Ray crystal structure of the neutral complex 8c¢A.(0.5)DCM
with the DCM molecule omitted.
Another interesting issue is why the N-acetyl group in the ligand
2c forms a complex 9c in which the ligand is bound in its imidic acid
tautomeric form, whereas the corresponding phenyl substituted
ligands 2c¢ and 2b¢ do not. We believe this is due to steric factors.
In the absence of a substituent at the apex of the tripodal ligand,
the imidic acid portion of the molecule 9c can readily adopt a
conformation where it is coplanar with the bond from the amide
nitrogen to the rhenium, minimising its interactions with the
adjacent carbonyl ligands and giving a symmetrical molecule. In
contrast, a similar conformation in the analogous complexes from
the substituted ligands 2c¢ and 2b¢ would produce a significant
steric interaction between the imidic oxygen and the phenyl group
at the apex of the ligand, requiring the imidic portion to adopt
the amide tautomeric form so that it can rotate to reduce this
interaction. Unfortunately, for the N-benzoylated ligand 2b the
vigorous conditions needed to get the initially formed complex
6b to react further meant that no intermediate complexes could
be observed en route to the formation of the final complex 8b
(Scheme 3). However, the N-benzoyl substituent in its imidic acid
tautomeric form would be expected to adopt a planar arrangement
to permit conjugation between the imidic acid N C bond and the
adjacent phenyl ring, so a symmetrical complex 9b analogous to
that seen in the corresponding N-acetylated complex 9c is feasible
providing there is no significant steric interaction between the
phenyl ring and the adjacent carbonyl groups on the rhenium.
Fig. 13 X-Ray crystal structure of the neutral complex 8c¢B.(0.5)DCM
with the DCM molecule omitted.
to investigate why the N-acetylated trispyridylmethylamine lig-
and (tpmaaH) 1c formed a stable cationic octahedral complex
[(tpmabaH)Re(CO)3]+ Br- 3c while the analogous complex from
the N-benzoylated ligand 1b readily rearranged to give a neutral
octahedral complex [(tpmba)Re(CO)3] 5c in which one of the
pyridine rings was uncoordinated.
We have now established that under appropriate conditions
bispyridyl ligands 2, structurally related to the N-acylated
trispyridylmethylamine ligands 1b and 1c studied earlier, can
form neutral octahedral complexes analogous to 5c via a reaction
pathway (see Schemes 2 and 3) involving several intermediate
complexes. The nature of these intermediates and their relative
stabilities has been shown to depend on the substituents on the
bispyridine ligand. Having a phenyl group at the apex of the
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n
max(film, cm-1) 3292 (br, NH), 1661 (C O), 1589, 1570, 1506,
Experimental
1472 (pyr); m/z (ESI) 266 (M+K+. C13H13KN3O requires 266).
General details
1-Phenyl-1,1-bis(pyridin-2yl)methylamine, Ph-bpmaH, 2a¢.
a) 1-Phenyl-1-(pyridin-2-yl)methylamine. 1-Phenyl-1-(pyridin-
2-yl)methylamine was prepared from 2-benzoylpyridine, hydroxy-
lamine hydrochloride, glacial acetic acid and zinc dust using the
method of Winthrop et al.15 and isolated as a pale brown oil (6.6 g,
90%); dH(270 MHz, CDCl3) 2.20 (2 H, br s, NH2), 5.16 (1 H, s,
a-CH), 6.98 (1 H, ddd, J 7.3, 4.9 and 0.8, 5-H), 7.10–7.18 (2 H, m,
3-H and 4¢-H), 7.23 (2 H, t, J 8, 3¢/5¢-H), 7.34 (2 H, d, J 8, 2¢/6¢-H),
7.45 (1 H, dd, J 8 and 2, 4-H), 8.48 (1 H, br, 6-H); dC(67.9 MHz,
CDCl3) 60.6 (a-CH), 121.4 (C-5), 121.7 (C-3), 126.7 (¥2)(C-3¢/5¢),
126.9 (C-4¢), 128.3 (¥2)(C-2¢/6¢), 136.4 (C-4), 144.2 (C-1¢), 148.6
(C-6), 162.9 (C-2).
NMR (1H, 13C, COSY, 13C-1H correlated) spectra were recorded
on JEOL EX-270 and Bruker AMX400, AV400 and AV600
spectrometers. 1H NMR spectra in CDCl3 are referenced to TMS,
those in d4-MeOD to the residual CD2H signal at dH 3.31 ppm and
those in d6-DMSO to the residual CD2H signal at dH 2.50 ppm
unless otherwise indicated.13C NMR spectra are referenced to
the solvent resonance, i.e. CDCl3 at dC 77.23 ppm, d4-MeOD at
dC 49.15 ppm or d6-DMSO at dC 39.51 ppm unless otherwise
indicated. J values are given in Hz. ‘J’ indicates an apparent
coupling constant in a second order spin system or in a resonance
showing substantial line broadening. IR spectra were recorded
using either a Shimadzu FTIR-8300 or a Perkin Elmer Spectrum
65 FT-IR spectrophotometer and selected bands are reported
below. Low resolution mass spectra were obtained using a Bruker
Esquire LC mass spectrometer equipped with an electrospray
ionisation source. High resolution mass spectrometry was carried
out by the facility at Kings College, London or the EPSRC
facility at Swansea. Elemental analyses were obtained from the
Department of Chemistry at UCL or Medac Limited. Melting
points were obtained on a Buchi SMP-20 capillary melting point
apparatus and are uncorrected. TLC was performed with alumina
backed silica gel 60 F254 eluting with the solvent system used for
the column chromatography unless otherwise stated and the plates
were visualised under UV light or developed in an iodine tank.
Column chromatography used silica gel with particle size 33–
50 mm and was purchased from BDH. Rheniumpentacarbonyl
bromide [Re(CO)5Br] was purchased from Strem Chemicals
UK. All other materials were purchased from Sigma–Aldrich Ltd.
and used as received unless indicated otherwise.
b) 1-Phenyl-1,1-bis(pyridin-2yl)methylamine, Ph-bpmaH, 2a¢.
A solution of n-butyl lithium in hexanes (2.5 M, 12 cm3, 30
mmol) was added dropwise over 30 min to a stirred solution of 1-
phenyl-1-(pyridin-2-yl)methylamine (5.35 g, 29 mmol) in dry THF
◦
(50 cm3) cooled to -78 C under dry argon. After allowing this
mixture to warm slowly to room temperature it was stirred for a
further 16 h. 2-Chloropyridine (3.4 g, 30 mmol) was then added
dropwise and the mixture stirred for 24 h.◦Volatile components
were removed under reduced pressure (40 C at 15 mmHg) and
water (50 cm3) added carefully to the residue. The mixture was
then extracted with dichloromethane (3 ¥ 50 cm3). The organic
extracts were combined, dried (MgSO4), filtered and the solvent
evaporated under reduced pressure to leave a brown viscous oil.
This product was purified by chromatography on silica using a
DCM/MeOH (95 : 5) mixture as the eluent to give the Ph-bpmaH
2a¢ (2.9 g, 38%) as a viscous oil, Rf 0.4; dH(270 MHz, CDCl3) 2.98
(2 H, br s, NH2), 7.05 (2 H, d, J 8.0, 3-H), 7.12 (2 H, ddd, J 8.0,
4.5 and 1.0, 5-H), 7.24–7.28 (5 H, m, Ph), 7.54 (2 H, td, J 8 and 2,
4-H), 8.59 (2 H, d, J 4.5, 6-H); dC(67.9 MHz, CDCl3) 69.2 (a-C),
121.8 (¥2)(C-5), 123.1 (¥2)(C-3), 127.1 (C-4¢), 128.2 (¥2)(C-3¢/5¢),
128.6 (¥2)(C-2¢/6¢), 136.1 (¥2)(C-4), 146.7 (C-1¢), 149.0 (¥2)(C-6),
165.7 (¥2)(C-2); Selected IR bands nmax(ATR)/cm-1 3370, 3300
(NH2), 1584, 1570, 1490, 1462 (pyr); m/z (ESI) 262.1339 (M+H+.
C17H16N3 requires 262.1339).
Ligand synthesis
The syntheses of 1,1,1-tris(pyridin-2-yl)methylamine, tpmaH,
1a,3 N-[1,1,1-tris(pyridin-2-yl)methyl]benzamide, tpmbaH, 1b,5
N-[1,1,1-tris(pyridin-2-yl)methyl]acetamide, tpmaaH, 1c,5 1,1-
bis(pyridin-2-yl)methylamine, bpmaH, 2a13,14 and N-[bis(pyridin-
2-yl)methyl]benzamide, bpmbaH, 2b5 are reported elsewhere.
N-[1-Phenyl-1,1-bis(pyridin-2-yl)methyl]benzamide,
Ph-
bpmbaH, 2b¢. A solution of benzoyl chloride (300 mg, 2.1
mmol) in dichloromethane (2 cm3) was added dropwise to
a stirred solution of Ph-bpmaH 2a¢ (330 mg 1.2 mmol) and
triethylamine (0.25 g 2.5 mmol) in dichloromethane (15 cm3) and
the mixture stirred at room temperature for 3 h. Water (20 cm3)
was then added and the layers separated. The organic layer was
then washed with further portions of water (2 ¥ 20 cm3) and then
dried (MgSO4) and filtered. Volatile components were removed
from the filtrate under reduced pressure (35 ◦C at 15 mmHg)
and the residue purified by chromatography on silica using a
DCM/MeOH (195 : 5) mixture as the eluent. Recrystallisation of
the product from a DCM/Hexane mixture gave the Ph-bpmbaH
2b¢ (Rf 0.15) as fawn coloured crystals (205 mg, 37%), m.p.
229–230 ◦C; dH(400 MHz, CDCl3) 7.09 (2 H, ddd, J 7.4, 4.8 and
0.8, 5-H), 7.14–7.22 (3 H, m, 3¢/5¢-H and 4¢-H), 7.29 (2 H, dm, J
8.0, 2¢/6¢-H), 7.34–7.42 (3 H, m, 3¢¢/5¢¢-H and 4¢¢-H), 7.43 (2 H,
dt, J 8.0 and 0.8, 3-H), 7.54 (2 H, ddd, J 8, 7.4 and 1.6, 4-H), 7.90
(2 H, d, J 8, 2¢¢/6¢¢-H), 8.50 (2 H, ddd, J 4.8, 1.6 and 0.8, 6-H),
10.19 (1 H, br s, NH); dC(67.9 MHz, CDCl3) 69.9 (a-C), 122.3
N-[Bis(pyridin-2-yl)methyl]acetamide, bpmaaH, 2c. Acetic an-
hydride (0.3 g, 2.9 mmol) was added dropwise to a solution of
bpmaH 2a (0.5 g, 2.7 mmol) in DCM (2 cm3) at room temperature
and the mixture stirred for 30 min. Dichloromethane (10 cm3)
was then added and the solution washed with saturated aqueous
sodium hydrogen carbonate (3 ¥ 10 cm3). The organic phase was
then dried (MgSO4), filtered and volatile components removed
by warming under reduced pressure (35 ◦C at 10 mmHg). The
resulting acetamide 2c was then purified by chromatography on
silica using mixtures of acetonitrile and acetone as the eluent.
The bpmaaH 2c was obtained as a viscous oil (0.5 g, 81%),
dH(270 MHz, CDCl3) 2.02 (3 H, s, Me), 6.16 (1 H, d, J 6.5, a-
CH), 7.05 (2 H, ddd, J 7.6, 4.8 and 1, 5-H), 7.35 (2 H, d, J 7.7,
3-H), 7.53 (2 H, td, J 7.7 and 1.7, 4-H), 7.92 (1 H, d, J 6.5, NH),
8.44 (2 H, ddd, J 4.8, 1.7 and 1, 6-H); dC(100.63 MHz, CDCl3) 22.6
(Me), 58.8 (CH), 121.6 (¥2)(C-5), 121.9 (¥2)(C-3), 136.3 (¥2)(C-4),
148.5 (¥2)(C-6), 158.6 (¥2)(C-2), 169.2 (C O); Selected IR bands
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(¥2)(C-5), 124.4 (¥2)(C-3), 127.3 (C-4¢), 127.5 (¥2)(C-2¢¢/6¢¢),
127.9 (¥2)(C-3¢/5¢), 128.6 (¥2)(C-3¢¢/5¢¢), 129.5 (¥2)(C-2¢/6¢),
131.6 (C-4¢¢), 135.4 (C-1¢¢), 136.7 (¥2)(C-4), 143.4 (C-1¢), 147.9
(¥2)(C-6), 161.6 (¥2)(C-2), 165.3 (C O); Selected IR bands
of the neutral complex [(bpmba)Re(CO)3] 8b which was identified
by comparison with an authentic sample.5
The reaction of [Re(CO)5Br] with Ph-bpmbaH 2b¢.
n
max(ATR)/cm-1 3335 (NH), 1659 (C O), 1585, 1560, 1505, 1465
Formation of [(Ph-bpmbaH)Re(CO)3]Br 7b¢. A mixture of
Ph-bpmbaH 2b¢ (103 mg, 0.28 mmol), [Re(CO)5Br] (105 mg, 0.26
mmol) and dry toluene (3 cm3) was heated at 105–110 ◦C for
16 h and then cooled. The resulti◦ng solid was filtered off, washed
with petroleum ether (bp 40–60 C) (20 cm3) and then air dried
(120 mg, 65%). Crystals of the cationic complex 7b¢ suitable for
X-ray diffraction studies were obtained from the slow evaporation
of a DCM/Hexane solution of the product, mp >250 ◦C; dH
(600 MHz, CDCl3)16 7.36 (1 H, m, 5∑-H), 7.42 (1 H, m, 5-H), 7.43–
7.51 (4 H, m, 5¢-H, 4¢-H, 3¢¢/5¢¢-H), 7.55 (1 H, t, J 7, 4¢¢-H), 7.60 (1
H, br, 3¢-H), 7.76 (1 H, d, J 8, 3-H), 7.95 (1 H, m, 4-H), 7.96 (1 H,
br, 6¢-H), 8.00–8.07 (2 H, m, 3∑-H, 4∑-H), 8.58 (2 H, d, J 7, 2¢¢/6¢¢-
H), 8.91 (1 H, d, ‘J’ 5, 6∑-H), 8.93 (1 H, br, 2¢-H), 8.98 (1 H, d,
‘J’ 4.5, 6-H), 11.55 (1 H, br s, NH); dC(100.64 MHz, CDCl3) 79.9
(a-C), 124.5 (C-5∑), 124.8 (C-3∑), 125.8 (C-5), 126.6 (C-3), 127.6
(C-4¢), 129.0 (¥2)(C-3¢¢/5¢¢), 130.0 (C-1¢¢), 130.0 (br, C-5¢), 130.4
(¥2)(C-2¢/6¢), 130.4 (br, C-3¢), 131.9 (br, C-6¢), 132.3 (C-1¢), 133.1
(br, C-2¢), 134.7 (C-4¢¢), 140.5 (C-4∑), 140.8 (C-4), 154.5 (C-6∑),
154.8 (C-6), 159.5 (C-2), 160.3 (C-2∑), 170.9 (C O), 193.5 (CO),
194.1 (CO), 195.4 (CO); Selected IR bands nmax (ATR)/cm-1 2568
(w), 2026, 1933, 1894 (CO), 1707 (C O), 1599, 1448, 1413 (pyr);
m/z (ESI) 636.0921 (M-Br+. C27H19N3O4Re requires 636.0933)
(pyr); m/z (ESI) 366.1601 (M+H+. C24H20N3O requires 366.1601).
N-[1-Phenyl-1,1-bis(pyridine-2-yl)methyl]acetamide, Ph-
bpmaaH, 2c¢. A mixture of Ph-bpmaH 2a¢ (500 mg, 1.9 mmol)
and acetic anhydride (3 cm3) was stirred at room temperature for
16 h. Volatile components were removed under reduced pressure
(65 ◦C at 15 mmHg) and the residue stirred with saturated sodium
bicarbonate solution (20 cm3) for 30 min. This mixture was then
extracted with DCM (3 ¥ 15 cm3). The combined organic layers
were dried (MgSO4), filtered and the solvent removed under
reduced pressure (30 ◦C at 15mmHg) to give a brown viscous
residue. Purification of this residue by chromatography on silica
gel using a mixture of DCM/MeOH (195 : 5) as the eluant gave
the Ph-bpmaaH 2c¢ (200 mg, 32%) as a waxy solid, Rf 0.2;
dH(270 MHz, CDCl3) 2.04 (3 H, s, Me), 7.04 (2 H, dd, J 7.5 and 5,
5-H), 7.15–7.23 (3 H, m, 3¢/5¢-H and 4¢-H), 7.27 (2 H, m, 2¢/6¢-H),
7.34 (2 H, d, J 8, 3-H), 7.49 (2 H, td, J 8 and 1.5, 4-H), 8.46 (2 H,
br d, J 5, 6-H), 9.11 (1 H, br s, NH); dC(67.9 MHz, CDCl3) 24.2
(Me), 69.6 (a-C), 121.9 (¥2)(C-5), 124.0 (¥2)(C-3), 126.9 (C-4¢),
127.5 (¥2)(C-3¢/5¢), 129.0 (¥2)(C-2¢/6¢), 136.2 (¥2)(C-4), 143.0
(C-1¢), 147.5 (¥2)(C-6), 161.2 (¥2)(C-2), 168.2 (C O); Selected
IR bands nmax(ATR)/cm-1 3325 (NH), 1675 (C O), 1586, 1569,
1499, 1464 (pyr); m/z (ESI) 304.1447 (M+H+. C19H18N3O requires
304.1444).
Formation of [(Ph-bpmba)Re(CO)3] 8b¢. Method 1—A mix-
ture of the cationic complex [(Ph-bpmbaH)Re(CO)3]Br 6b¢ (72 mg,
◦
0.1 mmol) and DMF (2 cm3) was heated at 120 C for 48 h and
volatile components were then removed under reduced pressure
(70 ◦C at 15 mmHg). The yellow residue was dissolved in
dichloromethane (1 cm3) and the resulting solution added to
hexane (20 cm3) with rapid stirring. The precipitated solid was
filtered off and then crystallised by the slow evaporation of a
DCM/Hexane solution. The neutral complex 8b¢ (40 mg, 63%)
was isolated as yellow crystals.
Synthesis of rhenium complexes
The reaction of [Re(CO)5Br] with tpmbaH 1b. The formation
of [(tpmbaH)Re(CO)3]Br 3b and [(tpmba)Re(CO)3] 5b from the
reaction of [Re(CO)5Br] with bpmbaH 1b has been described
elsewhere.5
The reaction of [Re(CO)5Br] with bpmbaH 2b.
Method 2—Triethylamine (100 mg, 1.0 mmol) was added
Formation of [(bpmbaH)Re(CO)3Br] 6b. A stirred mixture of
[Re(CO)5Br] (115 mg, 0.28 mmol), bpmbaH 2b (87 mg, 30 mol)
and toluene (10 cm3) was heated under reflux for 2 h by which
time the [Re(CO)5Br] had completely dissolved and had been
replaced by a colourless solid. This solid was filtered off, washed
with DCM (10 cm3) and dried. Slow crystallisation of this sample
from acetonitrile gave [(bpmbaH)Re(CO)3Br] 6b (140 mg, 79%) as
colourless crystals suitable for X-ray crystallography, mp >250 ◦C,
(Found: C, 40.1; H, 2.55; N, 8.0; C21H15BrN3O4Re. MeCN requires
C, 40.59; H, 2.67; N, 8.23%). dH(400 MHz, DMSO-d6, TMS)
7.16 (1 H, d, J 10, a-CH), 7.60–7.65 (4 H, m, 5-H and 3¢/5¢-H),
7.69 (1 H, t, J 8, 4¢-H), 8.03 (2 H, d, J 8, 3-H), 8.16 (2 H, d, J
7.5, 2¢/6¢-H), 8.20 (2 H, td, J 7.5 and 1.5, 4-H), 9.12 (2 H, dd,
J 5.5 and 1, 6-H), 10.23 (1 H, d, J 10, NH); dC(100.63 MHz,
DMSO-d6) 59.5 (a-CH), 122.0 (¥2)(C-5), 125.2 (¥2)(C-3), 128.0
(¥2)(C-3¢/5¢), 128.6 (¥2)(C-2¢/6¢), 132.3 (C-4¢), 133.2 (C-1¢), 140.7
(¥2)(C-4), 156.9 (¥2)(C-6), 157.0 (¥2)(C-2), 167.0 (C O), 189.7
(CO), 195.7 (x2)(CO); Selected IR bands nmax (MeCN, cm-1) 3325
(NH), 2026, 1923, 1895 (CO), 1674 (C O); m/z (ESI) 561.0623
(M-Br+. C21H15N3O4Re requires 561.0620).
dropwise to a solution of the cationic complex [(Ph-
bpmbaH)Re(CO)3]Br 6b¢ (115 mg, 0.16 mmol) dissolved in
dry dichloromethane (5 cm3) and the mixture stirred at room
temperature for 45 min. Dichloromethane (5 cm3) was added and
the solution washed with water (3 ¥ 5 cm3). The organic layer was
then dried (MgSO4), filtered and solvent removed under reduced
pressure to give the neutral complex 8b¢ as a fawn coloured solid.
Slow crystallisation of this sample from a DCM/hexane mixture
gave crystals (80 mg, 78%) suitable for X-ray study.
◦
The neutral complex [(Ph-bpmba)Re(CO)3] 8b¢, mp >250 C,
gave dH(400 MHz, d6-DMSO) 7.27 (2 H, t, J 7.5, 3¢¢/5¢¢-H), 7.32-
7.37 (2 H, m, 4¢-H and 4¢¢-H), 7.38–7.46 (4 H, m, 5-H and 3¢/5¢-H),
7.67 (2 H, d, J 7.5, 2¢/6¢-H), 7.83 (2 H, d, J 7.5, 2¢¢/6¢¢-H), 7.87 (2
H, d, J 8, 3-H), 8.05 (2 H, td, J 8 and 1.5, 4-H), 8.93 (2 H, br d, J
5.5, 6-H); dC(100.63 MHz, d6-DMSO) 82.5 (a-C), 124.0 (¥2)(C-5),
124.6 (¥2)(C-3), 127.1 (¥2)(C-3¢¢/5¢¢), 127.2 (C-4¢), 127.3 (¥2)(C-
3¢/5¢), 128.9 (¥2)(C-2¢/6¢), 129.9 (C-4¢¢), 130.4 (¥2)(C-2¢¢/6¢¢),
138.9 (C-1¢¢), 140.3 (¥2)(C-4), 140.5 (C-1¢), 153.9 (¥2)(C-6), 163.8
(¥2)(C-2), 175.8 (C O), 197.7 (CO), 199.4 (¥2)(CO); Selected IR
bands nmax(ATR)/cm-1 2007, 1906, 1878 (CO), 1599 (C O), 1564,
1460 and 1442 (pyr); m/z (ESI) 636.0918 (M+H+. C27H19N3O4Re
requires 636.0929).
Heating a sample of the [(bpmbaH)Re(CO)3]Br complex 6b in
DMF at 118 ◦C for 48 h resulted in the loss of HBr and formation
10226 | Dalton Trans., 2011, 40, 10215–10228
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The reaction of [Re(CO)5Br] with tpmaaH 1c.
89%) was shown to be the complex [(bpmaa)Re(CO)3] 8c in a
good state of purity. Recrystallisation of this product from an
acetonitrile/hexane mixture gave the pure complex 8c as orange
colourless crystals suitable for X-ray crystallography. (Found:
C, 38.56; H, 2.40; N, 8.40%; C16H12N3O4Re requires C, 38.71;
H, 2.44; N, 8.46%); dH(400 MHz, CDCl3) 2.20 (3 H, s, CH3),
7.19 (2 H, ddd, J 8, 5 and 1, 5-H), 7.22 (1 H, s, CH), 7.64 (2 H,
d, J 8, 3-H), 7.84 (2 H, td, J 8 and 1, 4-H), 8.82 (2 H, d, J 5,
6-H); dC(100.63 MHz, CDCl3) 27.1 (CH3), 70.7 (a-CH), 122.2
(¥2)(C-3), 123.9 (¥2)(C-5), 140.3 (¥2)(C-4), 154.1 (¥2)(C-6), 161.3
(¥2)(C-2), 176.2 (C O), 198.2 (CO), 198.5 (¥2)(CO); Selected IR
bands nmax(ATR)/cm-1 2017, 1879 (br) (CO), 1608 (C O), 1574,
1467, 1433 (pyr).
Formation of [(tpmaa)Re(CO)3] 3c. This complex was pre-
pared from tpmaaH 1c and [Re(CO)5Br] as previously described.
The slow evaporation of a solution of this compound in acetone
gave crystals suitable for X-ray diffraction studies.5
Investigation of the stability of the complex [(tpmaa)Re(CO)3]
5c. 1. In base: A sample of the cationic complex 3c (10 mg, 0.015
mmol) was heated with triethylamine (15 mg, 0.15 mmol) in d6-
DMSO (1.5 cm3) for 16 h at 70 ◦C. The solvent and triethylamine
were then removed under reduced pressure. Analysis of the product
by NMR spectroscopy confirmed that the complex 5c remained
unchanged.
2. Thermal stability: As previously reported,5 no changes were
observed when a solution of the complex 3c in DMF was heated
at 105–110 ◦C for 16 h.
The reaction of [Re(CO)5Br] with Ph-bpmaaH 2c¢.
Formation of [(Ph-bpmaaH)Re(CO)3]Br 7c¢. A mixture of
Ph-bpmaaH 2c¢ (104 mg, 0.34 mmol), [Re(CO)5Br] (115 mg, 0.28
mmol) and dry toluene (3 cm3) was heated at 105–110 ◦C for 16 h
and then cooled. The precipitated solid was filtered off, washed
with petroleum ether (bp 40–60 ◦C) and then air dried. This solid
(162 mg, 92%) was found to be the complex 7c¢ in a good state
of purity. Pure crystals of 7c¢ suitable for X-ray diffraction studies
were obtained by the slow evaporation of a solution of the complex
in DCM/Hexane; dH (600 MHz, CDCl3/d4-MeOD)18 7.41 (1 H,
m, 5∑-H), 7.48 (1 H, m, 5-H), 7.53-7.58 (2 H, m, 4¢-H, 5¢-H),
7.65 (1 H, br, 3¢-H), 7.71 (1 H, d, ‘J’HH 7, 3-H), 7.82 (1 H, br,
6¢-H), 7.97 (1 H, d, ‘J’HH 7, 3∑-H), 8.00–8.07 (2 H, m, 4-H, 4∑-
H), 8.35 (1 H, br, 2¢-H), 8.93 (1 H, d, ‘J’HH 3.5, 6∑-H), 9.00 (1
H, d, ‘J’HH 4.5, 6-H); dC (100.63 MHz, CDCl3) 25.7 (Me), 79.3
(a-C), 125.0 (br, C-3∑/5∑), 125.9 (br, C-5), 126.3 (br, C-3), 128.1
(br, C-4¢), 130.0 (br, C-3¢), 130.4 (br, C-5¢), 130.5 (C-1¢), 131.7 (br,
C-6¢), 132.0 (br, C-2¢), 140.9 (br, C-4/4∑), 154.8 (C-6/6∑), 159.3
(C-2), 175.8 (C O), 193.0 (CO), 195.8 (¥2)(br, CO); Selected IR
bands nmax (ATR)/cm-1 2594 (br, N+–H), 2026, 1927, 1906 (CO),
1759 (C O), 1602, 1497, 1460 (pyr); m/z (ESI) 574.0762 (M-Br+.
C22H17N3O4Re requires 574.0777)
The reaction of [Re(CO)5Br] with bpmaaH 2c. A mixture
of bpmaaH 2c (58 mg, 0.26 mmol), Re(CO)5Br (100 mg, 0.24
mmol) and toluene (2 cm3) was gradually heated until the
Re(CO)5Br dissolved. This occurred at about 80 ◦C, about 20 min
after heating had commenced.13C NMR analysis of the product
mixture at this time indicated the presence of three components,
subsequently identified as unreacted starting material bpmaaH
2c, and the isomeric complexes [(bpmaaH)Re(CO)3Br] 6c and
[(‘bpmaaH’)Re(CO)3]Br 9c in the ratio of 1 : 1 : 0.7. The initially
formed neutral complex 6c was observed to undergo rearrange-
ment in solution to give the cationic complex 9c preventing its
isolation in a pure state.
The neutral complex 6c gave dH(270 MHz, CDCl3) 2.05 (3 H, s,
CH3), 7.21 (1 H, d, J 10.5, CH), 7.24 (2 H, m, 5-H), 7.75 (2 H, td,
J 8 and 1, 4-H), 8.25 (2 H, d, J 8, 3-H), 9.17 (2 H, dd, J 5 and 1,
6-H), 10.25 (1 H, br d, J 10.5, NH); dC(100.63 MHz, CDCl3) 23.3
(CH3), 57.2 (a-CH), 123.2 (C-5), 124.6 (C-3), 140.1 (C-4), 156.5
(C-6), 158.8 (C-2), 174.3 (C O), 189.7 (CO), 195.6 (¥2)(CO).
Formation of [(‘bpmaaH’)Re(CO)3]Br 9c17. A mixture of
bpmaaH 2c (58 mg, 0.26 mmol), [Re(CO)5Br] (100 mg, 0.24
mmol) and toluene (5 cm3) was heated under reflux for 5 h. After
cooling, the brown precipitate was filtered off and dried to give
the cationic complex 9c (98 mg, 82%) in a good state of purity.
Slow crystallisation of this product from acetonitrile gave the pure
complex 9c as orange crystals suitable for X-ray crystallography,
mp >250 ◦C (Found: C, 33.45; H, 2.25; N, 7.35. C16H12BrN3O4Re
requires C, 33.34; H, 2.10; N, 7.29%); dH(400 MHz, MeOD-d4)
2.40 (3 H, s, Me), 7.38 (1 H, s, a-CH), 7.38 (2 H, ddd, JHH 7.8, 5
and 1, 5-H), 8.04 (2 H, td, JHH 7.8 and 1.4, 4-H), 8.20 (2 H, d, JHH
7.8, 3-H), 8.82 (2 H, d, JHH 5, 6-H); dC(100.63 MHz, d4-MeOD)
24.4 (Me), 70.3 (a-CH), 124.1 (¥2)(C-5), 125.8 (¥2)(C-3), 142.1
(¥2)(C-4), 154.6 (¥2)(C-6), 158.3 (¥2)(C-2), 172.3 (NC-O), 195.8
(CO), 196.3 (¥2)(CO); Selected IR bands nmax (KBr, cm-1) 3400
(br, OH), 2025, 1911 (br) (CO), 1653 (N C), 1533, 1474, 1437
(pyr).
Conversion of 7c¢ to [(Ph-bpmaa)Re(CO)3] 8c¢.
Thermally. A solution of the complex 7c¢ (75 mg, 0.11 mmol)
◦
in DMF (2 cm3) was heated at 120 C for 48 h. The solvent was
removed under reduced pressure (70 ◦C at 10 mmHg) to leave
a yellow residue. This was redissolved in DCM (1 cm3) and the
solution added to hexane (20 cm3) while rapidly stirring. The
yellow solid that precipitated was filtered off and crystallised by the
slow evaporation of a DCM/Hexane solution to give the complex
8c¢ as a yellow crystalline solid (35 mg, 53%).
In the presence of base. Triethylamine (55 mg, 0.5 mmol) was
added dropwise to a solution of complex 7c¢ (112 mg, 0.17 mmol)
dissolved in dry chloroform (10 cm3) and the mixture stirred at
room temperature for 16 h. The solution was then washed with
water (3 ¥ 5 cm3), dried (MgSO4), filtered and solvent evaporated
under reduced pressure to leave the complex 8c¢ (90 mg, 93%) in a
good state of purity. Slow evaporation of a DCM/Hexane solution
of 8c¢ yielded yellow crystals suitable for X-ray diffraction studies,
Formation of [(bpmaa)Re(CO)3] 8c. A mixture of
[(‘bpmaaH’)Re(CO)3]Br 9c (80 mg, 0.14 mmol), triethylamine
(70 mg, 0.7 mmol) and chloroform (10 cm3) was stirred for 3 h
at room temperature. Water (30 cm3) was added the mixture
separated and the aqueous layer extracted further with chloroform
(2 ¥ 10 cm3). The three organic extracts were combined, dried
(MgSO4), filtered and volatile components removed under
reduced pressure (60 ◦C at 10 mmHg). The residue (62 mg,
◦
mp >250 C; dH (400 MHz, DMSO-d6) 1.98 (3 H, s, Me), 7.37–
7.43 (3 H, m, 5-H2, 4¢-H), 7.44 (2 H, t, J 7.5, 3¢/5¢-H), 7.70 (2 H, d,
J 7.5, 2¢/6¢-H), 7.80 (2 H, d, J 8, 3-H), 8.00 (2 H, td, J 8 and 1.5,
4-H), 8.98 (2 H, dd, J 5.5 and 1, 6-H); dC (100.63 MHz, DMSO-
d6) 28.3 (CH3), 82.2 (a-C), 124.0 (¥2)(C-5), 124.6 (¥2)(C-3), 127.0
(C-4¢), 127.3 (¥2)(C-3¢/5¢), 130.3 (¥2)(C-2¢/6¢), 139.0 (C-1¢), 140.3
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The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 10215–10228 | 10227
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(¥2)(C-4), 154.0 (¥2)(C-6), 163.4 (¥2)(C-2), 177.4 (C O), 197.7
(CO), 200.2 (¥2)(CO); Selected IR bands nmax (ATR)/cm-1 2008,
1863(br) (CO), 1593 (C O), 1570, 1459, 1433 (pyr); m/z (ESI)
574.0760 (M+H+. C22H17N3O4Re requires 574.0777)
2 R. S. Herrick, C. J. Ziegler, D. L. Jameson, C. Aquina, A. Cetin, B. R.
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X-Ray structure determination
For all complexes, the crystals were glued to a glass fibre and
mounted on the diffractometer head. Except for complex 6b,
intensity data for all crystals were collected at 120 K, using either
a Bruker-Nonius Roper CCD diffractometer (8b, 8b¢, 8c and 7c¢)
or a Bruker-Nonius APEX II CCD diffractometer (8b¢·HBr, 3c,
9c and 8c¢), both equipped with a Mo-Ka rotating anode (l =
˚
0.71073 A), monochromated by graphite or confocal focusing
mirrors. The crystals were positioned 30 mm from the CCD and
all intensities were measured using a counting time of 20 s with
1.0◦ increments (ø and X) to fill the Ewald sphere. The unit cell
parameters were determined by least-squares refinement of 25
automatically centred reflections with setting angles of 2.91 £ 2q
£ 27.48◦.
For complex 6b, the crystal was mounted the same way and
intensity data were collected at 160 K on a Enraf-Nonius CAD-4
diffractometer equipped with a Mo-Ka fine-focus sealed tube (l =
12 J. C. Lee, A. L. Rheingold, B. Muller, P. S Pregosin and R. H. Crabtree,
J. Chem. Soc., Chem. Commun., 1994, 1021–1022.
13 P. J. Arnold, ‘Novel pyridine-containing ligands as models for the
copper centres in nitrite reductase’, PhD Thesis, University of London,
2001.
˚
0.71073 A) and a graphite monochromator. The data collection
method used was the same as mentioned above, except the setting
angles of 10.327 £ 2q £ 12.318◦ were used to determine unit cell
parameters.
14 C. Na´jera, J. Gil-Molto´ and S. Karlstrc¸m, Adv. Synth. Catal., 2004,
346, 1798–1811.
15 S. O. Winthrop and G. Gavin, J. Org. Chem., 1959, 24, 1936–
1939.
All intensities were collected using the programs CAD419 (for
complex 6b) or COLLECT20 Data reductions and refinements
were performed using XCAD421 (for compound 6b) or DENZO22
according to Lorentz and polarisation effects. Absorption correc-
tions were applied based on multi-scan method and were obtained
using SADABS.23 X-ray crystal structures were determined using
the DirAx24 program. The programs ORTEP-325 and PLATON26
were used for drawing the molecules. WINGX27 was used to
prepare material for publication.
16 Small couplings are not resolved in the spectrum due to line broadening
arising from NMR exchange effects, those signals denoted as broad (br)
are too broad even to resolve 3JHH ortho couplings.
17 ‘bpmaaH’ indicates the bpmaaH ligand is present in its imidic acid
form.
18 Addition of a little d4-MeOD helped improve the appearance of the
spectrum and aided interpretation but removed the NH signal at dH
11.41 ppm that is visible in CDCl3.
19 Enraf-Nonius CAD-4/PC Software. Version 1.5c, 1994, Enraf-Nonius,
Delft, The Netherlands.
20 R. Hooft, B. V. Nonius, COLLECT: a program for crystal data collection
and processing user interface, 1998.
All structures were solved by the heavy-atom method using
the DIRDIFF9928 program and refined anisotropically (non-
hydrogen atoms) by full-matrix least-squares technique against
F2 using the SHELXL-9729 program. All H atoms were calculated
geometrically and refined by a riding model.
21 K. Harms, XCAD4: a program for data reduction, 1996, Philipps-
Universita¨t, Marburg, Germany.
22 Z. Otwinowski and W. Minor, Methods in Enzymology, ed. C. W. Carter
Jr and R. M. Sweet, Academic Press, New York, 1997, 276, part A, 307.
23 G. M. Sheldrick (2007). SADABS. Version 2007/2. Bruker AXS Inc.,
Madison, Wisconsin, USA.
24 A. J. M. Duisenberg, Indexing in Single-Crystal Diffractometry with
an Obstinate list of reflections, J. Appl. Crystallogr., 1992, 25, 92–96.
25 L. J. Farrugia, ORTEP-3 for Windows, J. Appl. Cryst., 1997, 30,
565.
Acknowledgements
The authors would like to thank the EPSRC UK National Crys-
tallographic Service at Southampton and the EPSRC National
Mass Spectrometry Service Centre (NMSSC) at Swansea for their
help with the acquisition of some of the data presented.
26 A. L. Spek, PLATON: a multipurpose crystallographic tool, 1998,
Utrecht University, Utrecht, The Netherlands.
27 L. J. Farrugia, WinGX: a program for crystal structure analysis, 1998,
Univ. of Glasgow.
28 P. T. Beurskens, G. Beurskens, W. P. Bosman, R. de Gelder, S. Garcia-
Granda, R. O. Gould, R. Israel, J. M. M. Smits, DIRDIF99: a program
system, 1999, Crystallography Laboratory, University of Nijmegen,
The Netherlands.
Notes and references
1 R. Alberto, R. Schibli, R. Waibel, U. Abram and A. P. Schubiger,
Coord. Chem. Rev., 1999, 190–192, 901–919.
29 G. M. Sheldrick, SHELXL-97: Program for crystal structure refinement;
Acta Cryst., 2008, A64, 112.
10228 | Dalton Trans., 2011, 40, 10215–10228
This journal is
The Royal Society of Chemistry 2011
©