Internal hdyrogen bonding in tetrahedral and trigonal bipyramidal zinc(II) complexes of pyridine-based ligands

Article abstract of DOI:10.1039/b305476b

Polydentate ligands (6-R¹-2-pyridylmethyl)-R² (R¹ = NHCO^tBu, R² = bis(2-pyridylmethyl)amine L¹; R¹ = NH2, R² = bis-(2-pyridylmethyl)amine L²; R¹ = NHCO^tBu, R² = N(CH2CH2)2NMe L³; R¹ = NH2, R² = N(CH2CH2)2NMe L⁴; R¹ = NHCO^tBu, R² = N(CH2CH2)2O L⁵; R¹ = NH2, R² = N(CH2CH2)2O L⁶) were prepared as part of an effort to rationally design ligands that induce internal hydrogen bonding to other metal-bound ligands to be used as active site models of metallohydrolases and oxygenases. L¹, L³ and L⁵ were prepared by alkylation of the appropriate amine (bis(2-pyridylmethyl)amine, N-methylpiperazine or morpholine) with 2-(pivaloylamido)-6-bromomethyl)pyridine. L², L⁴ and L⁶ were prepared by acid hydrolysis of L¹, L³ and L⁵, respectively. L^1,2 were metallated with ZnCl2 to give [(L¹)Zn(Cl)](Cl) 1' and [(L²)Zn(Cl)](Cl) 2' salts, which after a metathesis reaction with NaBPh4 in MeOH afford [(L¹)Zn(Cl)](BPh4) 1 and [(L²)Zn(Cl)](BPh4) 2. The reaction of L^3-6 with ZnCl2, however, affords the neutral complexes [(L³)Zn(Cl)2] 3, [(L⁴)Zn(Cl02] 4, [(L⁵)Zn(Cl)2] 5 and [(L⁶)Zn(Cl)2] 6. X-Ray crystallographic studies of 1, 2 and 4-6 revealed that these complexes adopt trigonal bipyramidal (N4Cl) and tetrahedral (N2Cl2) geometries, respectively, with 'internal' N-H...Cl-Zn hydrogen bonding. 1H NMR, IR and X-ray crystallographic studies indicated that internal N-H...Cl-Zn hydrogen bonding in 4-6 is of similar strength and weaker than in the trigonal bipyramidal complexes 1 and 2. The chemical shift of the amine and amide NH proton associated with the internal N-H...Cl-Zn hydrogen bond is shifted downfield by 2.2-2.5 ppm in 1, 2 and by 1.1-1.2 ppm in 3-6 relative to in the corresponding ligand L^1-6. Thus, in the 1-6 series, the magnitude of the chemical shift changes experienced by the hydrogen bonded N-H can be correlated with the hydrogen bond energies determined by IR and 1H NMR variable temperature coalescence studies, and with the hydrogen bond geometries revealed by X-ray crystallography.

Full text of DOI:10.1039/b305476b

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Internal hydrogen bonding in tetrahedral and trigonal bipyramidal
zinc(II) complexes of pyridine-based ligands†
Juan C. Mareque Rivas,* Emiliano Salvagni, Rafael Torres Martín de Rosales
and Simon Parsons
School of Chemistry, The University of Edinburgh, Joseph Black Building, King’s Buildings,
West Mains Road, Edinburgh, UK EH9 3JJ. E-mail: Juan.mareque@ed.ac.uk
Received 15th May 2003, Accepted 8th July 2003
First published as an Advance Article on the web 29th July 2003
Polydendate ligands (6-R1-2-pyridylmethyl)-R2 (R1 = NHCOtBu, R2 = bis-(2-pyridylmethyl)amine L1; R1 = NH2,
R2 = bis-(2-pyridylmethyl)amine L2; R1 = NHCOtBu, R2 = N(CH2CH2)2NMe L3; R1 = NH2, R2 = N(CH2CH2)2NMe
L4; R1 = NHCOtBu, R2 = N(CH2CH2)2O L5; R1 = NH2, R2 = N(CH2CH2)2O L6) were prepared as part of an effort to
rationally design ligands that induce internal hydrogen bonding to other metal-bound ligands to be used as active site
models of metallohydrolases and oxygenases. L1, L3 and L5 were prepared by alkylation of the appropriate amine
(bis-(2-pyridylmethyl)amine, N-methylpiperazine or morpholine) with 2-(pivaloylamido)-6-(bromomethyl)pyridine.
L2, L4 and L6 were prepared by acid hydrolysis of L1, L3 and L5, respectively. L1,2 were metallated with ZnCl2 to give
[(L1)Zn(Cl)](Cl) 1Ј and [(L2)Zn(Cl)](Cl) 2Ј salts, which after a metathesis reaction with NaBPh4 in MeOH, afford
[(L1)Zn(Cl)](BPh4) 1 and [(L2)Zn(Cl)](BPh4) 2. The reaction of L3–6 with ZnCl2, however, affords the neutral
complexes [(L3)Zn(Cl)2] 3, [(L4)Zn(Cl)2] 4, [(L5)Zn(Cl)2] 5 and [(L6)Zn(Cl)2] 6. X-Ray crystallographic studies of
1, 2 and 4–6 revealed that these complexes adopt trigonal bipyramidal (N4Cl) and tetrahedral (N2Cl2) geometries,
respectively, with ‘internal’ N–H ؒ ؒ ؒ Cl–Zn hydrogen bonding. 1H NMR, IR and X-ray crystallographic studies
indicated that internal N–H ؒ ؒ ؒ Cl–Zn hydrogen bonding in 4–6 is of similar strength and weaker than in the
trigonal bipyramidal complexes 1 and 2. The chemical shift of the amine and amide NH proton associated with
the internal N–H ؒ ؒ ؒ Cl–Zn hydrogen bond is shifted downfield by 2.2–2.5 ppm in 1, 2 and by 1.1–1.2 ppm in 3–6
relative to in the corresponding ligand L1–6. Thus, in the 1–6 series, the magnitude of the chemical shift changes
experienced by the hydrogen bonded N–H can be correlated with the hydrogen bond energies determined by IR
and 1H NMR variable temperature coalescence studies, and with the hydrogen bond geometries revealed by
X-ray crystallography.
peptidase A,8 aminopeptidase A,9 the lethal anthrax factor
Introduction
(LF),10 thermolysin,11 Pseudomonas aeruginosa alkaline pro-
The great importance of hydrogen bonding has been widely
tease,12 and several matrix metalloproteinases13 are just a few
recognised and studied for many years.1 Elucidation of the
examples of zinc peptidases, amidases and nucleases in which
influence of metal centres on hydrogen bonding, however, has
XH groups (X = N, O) of arginine, lysine, histidine, tyrosine
become an area of intense interest only recently.2
Metals can strengthen hydrogen bonding by influencing the
and/or serine residues are hydrogen bonded to zinc()-bound
water and/or substrate groups. In these enzymes the cooperation
hydrogen bonding properties of hydrogen bond donors and
between metals and non-coordinating active-site residues could
acceptors through metal-coordination. A clear example of this
be involved in the activation, recognition and/or stabilization
is provided by M–H bonding where the hydrogen may be
mechanisms displayed by these important enzymes. In fact,
regarded as hydridic (HϪ) or protic (Hϩ), and thus allowing the
several very interesting recent results have shown the
importance of incorporating second sphere elements in
formation of M–H ؒ ؒ ؒ H–X or M–H ؒ ؒ ؒ X hydrogen bonds,
respectively.2c–e Another important example of considerable
synthetic models of several types of zinc() enzymes including
current interest are halogens, which, when metal-bound are
lyases,14 nucleases,15 phosphoryl transferases16 and oxido-
very effective hydrogen bond acceptors but bound to carbon
reductases.17
atoms are not.2f,3
Despite the great potential of exploiting and modelling
second-sphere hydrogen bonding features of metals centres,
Increasing research is also concerned with investigating the
functional roles and applications of hydrogen bonding inter-
actions mediated by transition metals. Much of the importance
very few studies have investigated factors affecting their
strength.18 Strategies to induce and manipulate the strength of
of hydrogen bonding involving metal centres arguably lies
hydrogen bonding to metal-bound ligands, however, is a key
in the numerous supporting roles that these may be able to play
aspect for any research concerned with exploiting or modelling
the effects of these interactions.
In this report we investigate the strength of internal
N–H ؒ ؒ ؒ Cl–Zn hydrogen bonding in tetrahedral and trigonal
bipyramidal geometries as model interactions. Factors that
affect the strength of the internal N–H ؒ ؒ ؒ Cl–Zn hydrogen
bond and how these changes are expressed spectroscopically
are also discussed. In addition we investigate strategies and
in important chemistry4–6 at metal sites and in the rapidly
developing field of crystal engineering.2b,3,7
For instance, recent studies have highlighted the potential
importance of H-bonding in organometallic reactions involving
electrophiles.4 There is also a growing awareness that hydrogen
bonding to metal-coordinated molecules may be able to play
key supporting roles in enzyme catalysis.5 Thus, carboxy-
principles to induce and manipulate hydrogen bonding inter-
actions in metal complexes.
Structural features, including hydrogen bonding interactions
found in the solid state by X-ray diffraction studies, are con-
trasted and correlated with NMR and/or IR studies in solution.
† Electronic supplementary information (ESI) available: Cambridge
structural database search and data. Histograms of H ؒ ؒ ؒ X distances
and VT NMR spectra. See http://www.rsc.org/suppdata/dt/b3/
b305476b/
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
D a l t o n T r a n s . , 2 0 0 3 , 3 3 3 9 – 3 3 4 9
3339

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with the Cl ligand occupying one of the axial positions.20 Here,
Results and discussion
we report an identical behavior for L2, from which the
[(L2)Zn(Cl)](BPh4) salt (L2, 2) was prepared (Scheme 3). This
result suggests that in the presence of Cl ligand(s) zinc() com-
plexes of L1,2, the trigonal bipyramidal [(L1,2)Zn(Cl)]ϩ cation is
preferred over trigonal bipyramidal [(L1,2)Zn](Cl)2, octahedral
[(L1,2)Zn(Cl)]ϩ and octahedral [(L1,2)Zn(Cl)2] coordination
environments. The reaction of the N-methyl-piperazine
and morpholine based ligands L3–6 with ZnCl2 affords neutral
[(L3–5)Zn(Cl)2] complexes (L3, 3; L4, 4; L5, 5; L6, 6), in which the
zinc() center is tetrahedrally N2Cl2 ligated. Interestingly, the
zinc() ion prefers tetrahedral N2Cl2 ligation over trigonal
bipyramidal N2Cl2O and N3Cl2 (or N2OCl2) coordination
environments (Scheme 3), despite the fact that chelate effects
and the possibility of coordinating an additional group to the
zinc() centre could be considered sources of stability.
Design and synthesis
The ligand fragment (6-R1-2-pyridylmethyl)amine (R1
=
NHCOR or NH2) has a suitable stereochemistry for an N–H
group to participate in an internal hydrogen bond to an
adjacent metal-bound ligand. This fragment can be incorpor-
ated into a variety of polydentate ligands thus allowing the
systematic investigation and exploitation of internal hydrogen
bonding in metal complexes of different geometries and
chemistry.
Thus, the bis-(2-pyridylmethyl)amine-based ligands L1 and
L2 (Scheme 1) provide a good ligand platform for investigating
how the nature of the hydrogen bonding group affects the
strength of hydrogen bonding to metal bound ligands in tri-
gonal bipyramidal and octahedral coordination environments.
The N-methylpiperazine and morpholine based ligands L3–6
(Scheme 1) offer the opportunity of investigating the same
effects in tetrahedral and trigonal bipyramidal ligand environ-
ments. As a group these six ligands can be used to assess and
compare the strength of internal hydrogen bonding at these
metal geometries. In addition, amine and amide groups differ in
terms of hydrogen bonding, steric and electronic properties.
The synthesis of L1–6 can be accomplished in 3–6 steps using
2-(pivaloylamido)-6-(bromomethyl)pyridine19 (L0) as common
reagent (Scheme 2).
X-Ray crystallography
Crystal data for [(L2)Zn(Cl)](BPh4) 2ؒCH3CN, [(L3)Zn(Cl)2]
3ؒCH3CN, [(L4)Zn(Cl)2] 4, [(L5)Zn(Cl)2] 5 and [(L6)Zn(Cl)2] 6, is
listed in Table 1. Single crystals suitable for X-ray diffraction
were grown by slow evaporation of acetonitrile or acetonitrile/
water solutions
Structures of 1ؒCH3CN and 2ؒ0.5CH3CN. Thermal ellipsoid
plots of the X-ray crystal structures of the [(L1,2)Zn(Cl)]ϩ
cations of 120 and 2 are shown in Fig. 1 and selected distances
and angles are given in Table 2.
Recently, we reported that reaction of L1 with ZnCl2 gives
[(L1)Zn(Cl)](Cl) 1Ј, which then combined with NaBPh4
afforded the [(L1)Zn(Cl)](BPh4) salt (L1, 1), in which the zinc()
centre was in a trigonal bipyramidal N4Cl ligand environment
In 1ؒCH3CN and 2ؒ0.5CH3CN the zinc() centre is in a
trigonal bipyramidal environment ligated to the three pyridine
nitrogen atoms in the trigonal plane, and to the bridgehead
nitrogen of the tripodal ligand and a chloride ion in the axial
positions. The Zn–N distances of complexes are in accordance
with other [(L)Zn(Cl)]ϩ cations.21 The longest and shortest
Zn–Nequatorial bonds in 1ؒCH3CN and 2ؒ0.5CH3CN are made to
the substituted pyridines, Zn–N(2) 2.1351(15) Å in 1ؒCH3CN
and 2.0704(19) Å in 2ؒ0.5CH3CN, presumably due to the
electron withdrawing and donating effects of the pivaloylamido
and amino groups, respectively. In 1ؒCH3CN, coordination of
the chloride dictates the positioning of the pivaloylamido
group, which seeks to optimise N–H ؒ ؒ ؒ Cl hydrogen bonding
(N(7) ؒ ؒ ؒ Cl 3.2127(19) Å; H(7N) ؒ ؒ ؒ Cl 2.22 Å; N(7)–
H(7N) ؒ ؒ ؒ Cl 168Њ for a N(7)–H(7N) bond extended to 1.01
Å).22 As a result of this, the angle between the pyridine plane
(N(2)C(2)C(3)C(4)C(5)C(6)) and the plane containing the
amide group (N(7)C(8)O(8)) is 31.5Њ. In L1 the same angle was
Scheme 1
t
Scheme 2 Reagents and conditions used for the synthesis of L0–6; (i) BuCOCl (1.5 equiv.), CH2Cl2, NEt3, 22 h; (ii) NBS (1.5 equiv.) AIBN (0.1
equiv.), CCl4, 80 ЊC, 3.5 h; (iii) potassium phthalamide (1 equiv.), DMF, 120 ЊC, 3 h; (iv)a N2H4ؒH2O (1 equiv.), EtOH, 60 ЊC, 3 h; (v)a and (iv)b
PyCH2Cl (2 equiv.) or HN(CH2CH2)2X (1 equiv.), Na2CO3, CH3CN, 60 ЊC, 20–48 h (vi)a; (v)b 2 M HCl(aq), 70 ЊC, 24–40 h
D a l t o n T r a n s . , 2 0 0 3 , 3 3 3 9 – 3 3 4 9
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Scheme 3
Table 1 Crystallographic data and structure refinement details for 2ؒ0.5CH3CN, 3ؒCH3CN, 4,5 and 6
2ؒ0.5CH3CN
C43H40.5 BClN5.5Zn
3ؒCH3CN
C18H29Cl2N5OZn
4
5
6
Empirical Formula
C11H18Cl2N4Zn
C15H23Cl2N3O2Zn
C10H15Cl2N3OZn
Formula
T/K
745.94
150(2)
467.73
150(2)
342.56
150(2)
413.63
150(2)
329.52
150(2)
Crystal system
Space group
Crystal size/mm
a/Å
b/Å
c/Å
α/Њ
β/Њ
Monoclinic
P21/n
0.91 × 0.6 × 0.58
9.9482(13)
16.374(2)
23.249(3)
90
101.631(2)
90
3709.2(8)
4
Triclinic
P1
Orthorhombic
P212121
0.54 × 0.32 × 0.20
9.0138(7)
11.1924(9)
14.6243(11)
90
90
90
1475.4(2)
4
1.542
Orthorhombic
Pbca
0.35 × 0.21 × 0.14
10.9906(7)
16.3898(10)
20.4387(12)
90
90
90
3681.7(4)
8
1.492
Monoclinic
P21/n
0.6 × 0.17 × 0.13
7.3993(4)
10.5846(6)
17.0626(10)
90
96.1520(10)
90
1328.62(13)
4
¯
0.81 × 0.76 × 0.20
7.9950(13)
10.2286(16)
14.220(3)
101.773(2)
100.503(2)
92.944(2)
1114.6(3)
2
γ/Њ
V/Å3
Z
Dc/g cmϪ3
1.336
0.773
1.394
1.359
1.647
2.237
µ/mmϪ1
2.015
1.635
Reflections measured,
unique
18202,6529
9678, 5113
9124, 3556
22085, 4586
8041, 3169
Rint
0.0321
0.0470
0.0995
1.047
0.0229
0.0363
0.0874
1.057
0.0284
0.0246
0.0554
1.024
0.0499
0.0691
0.1183
1.218
0.02
0.0283
0.0664
1.062
R1(F)a
wR2(F 2)a (all data)
S(F 2)a (all data)
Largest difference peak,
hole/e Å3
0.606, Ϫ0.327
0.745, Ϫ0.334
0.382, Ϫ0.355
0.585, Ϫ0.561
0.372, Ϫ0.301
4
a R1(F) = Σ(|Fo| Ϫ |Fc|)/Σ(|Fo|; wR2(F 2) = [Σw(Fo2 Ϫ Fc2)2/ΣwF0 ]1/2; S(F 2) = [Σw (Fo2 Ϫ Fc2)2/(n Ϫ p)]1/2
.
found to be 11.4Њ.20 This arrangement of the amide group,
however, still allows interaction between the carbonyl O-atom
and H5, (C(5) ؒ ؒ ؒ O(8) 2.83(3) Å, H(5A) ؒ ؒ ؒ O(8) 2.34 Å,
C(5)–H(5A) ؒ ؒ ؒ O8 112Њ with C(5)–H(5A) fixed to 0.95 Å),
although presumably weaker than in L1.
Since amide N–H groups are more acidic than those of amines,
one might have expected stronger internal N–H ؒ ؒ ؒ Cl–Zn
hydrogen bonding in 1 than in 2. The X-ray crystal structure
of 2ؒCH3CN, however, shows an internal N–H ؒ ؒ ؒ Cl–Zn
hydrogen bond with geometric features which are essentially
D a l t o n T r a n s . , 2 0 0 3 , 3 3 3 9 – 3 3 4 9
3341

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Fig. 1 Thermal ellipsoid plot drawn with 50% probability ellipsoids of (i) the [(L1)Zn(Cl)]ϩ cation of 1ؒCH3CN showing the internal N–H ؒ ؒ ؒ Cl–
Zn hydrogen bonding and (ii) [(L2)Zn(Cl)](BPh4) of 2ؒ0.5CH3CN showing the internal N–H ؒ ؒ ؒ Cl–Zn and external N–H ؒ ؒ ؒ π hydrogen bonding.
Selected distances (Å) for 1: Zn–N(2) 2.1351(15), Zn–N(12) 2.0885(16), Zn–N(22) 2.0651(17); for 2: Zn–N(2) 2.0671(19), Zn–N(12) 2.102(2),
Zn–N(22) 2.081(2) (X(1A) is the centroid of the phenyl ring C(51)–C(56)).
identical to in 1ؒCH3CN (Table 3) (N(7) ؒ ؒ ؒ Cl 3.213(3) Å;
H(7N) ؒ ؒ ؒ Cl 2.24 Å; N(7)–H(7N) ؒ ؒ ؒ Cl 160Њ for a N(7)–
H(7N) bond extended to 1.01 Å). An aspect that may account
for this observation is the fact that whereas the pivaloylamide
acts as an electron withdrawing group on the pyridine, the
amine is electron donating. This effect is reflected in the
Zn–NpyЈ distances (pyЈ = pyridine carrying the amino or
pivaloylamido group) and affects the distance between the
hydrogen bonded atoms (Zn–N(2) 2.1351(13) Å, N(7)–
2.0704(19) Å, N(7)–H(7NA) ؒ ؒ ؒ Cl–Zn 3.213(3)
0.5CH3CN; Tables 2 and 3).
Remarkably, the X-ray structure of 2 shows a very short
Å
2ؒ
external N–H ؒ ؒ ؒ π hydrogen bond with a phenyl group of the
BPh4 anion23 (N(7) ؒ ؒ ؒ X(1A) 3.26 Å, H(7NB) ؒ ؒ ؒ X(1A)
Ϫ
2.26 Å, N(7)–H(7NB) ؒ ؒ ؒ X(1A) 168Њ; Fig. 1(ii)).
Structures of 3ؒCH3CN and 5. Thermal ellipsoid plots for
3 and 5 are shown in Fig. 2 and selected distances and angles
are given in Table 4.
H(7NA) ؒ ؒ ؒ Cl–Zn 3.2127(19)
Å
1ؒCH3CN; Zn–N(2)
In both compounds the zinc() centre is in a tetrahedral
N2Cl2 environment. The position of the amide group in
3ؒCH3CN and 5 is such that allows the N–H group to form
internal N–H ؒ ؒ ؒ Cl hydrogen bonds (N(7) ؒ ؒ ؒ Cl(1)
3.3580(15) Å; H(7N) ؒ ؒ ؒ Cl(1) 2.38 Å; N(7)–H(7N) ؒ ؒ ؒ Cl(1)
162.1Њ for 3ؒCH3CN and N(7) ؒ ؒ ؒ Cl(1) 3.277(3) Å;
H(7N) ؒ ؒ ؒ Cl(1) 2.30 Å; N(7)–H(7N) ؒ ؒ ؒ Cl(1) 161.6Њ for 5).
The small angle between the pyridine (N2C2C3C4C5C6) and
amide planes (N7C8O8), 16.2Њ for 3ؒCH3CN and 15.2Њ for 5,
seems optimum for the interaction of the amide oxygen (O8)
with the hydrogen in the adjacent position of the pyridine ring
(H(5A)) (C(5) ؒ ؒ ؒ O(8) 2.823(2) Å; H(5A) ؒ ؒ ؒ O(8) 2.25 Å;
C(5)–H(5A) ؒ ؒ ؒ O(8) 117.8Њ for 3ؒCH3CN C(5) ؒ ؒ ؒ O(8)
2.842(4) Å; H(5A) ؒ ؒ ؒ O(8) 2.27 Å; C(5)–H(5A) ؒ ؒ ؒ O(8)
118.2Њ for 5). The chloride in 3 and 5 is therefore positioned so
that the amide plane can be oriented simultaneously to optimise
Table 2 Selected bond lengths (Å) and angles (Њ) for zinc() complexes
1ؒCH3CN20 and 2ؒ0.5CH3CN
1ؒCH3CN
2ؒ0.5CH3CN
Zn–N(2)
Zn–N(12)
Zn–N(22)
Zn–N(1)
Zn–Cl
2.1351(15)
2.0885(16)
2.0651(17)
2.1990(15)
2.2812(7)
2.0704(19)
2.1021(19)
2.081(2)
2.2247(18)
2.2959(7)
N(2)–Zn–N(12)
N(2)–Zn–N(22)
N(12)–Zn–N(22)
N(1)–Zn–N(2)
N(1)–Zn–N(12)
N(1)–Zn–N(22)
N(1)–Zn–Cl
Cl–Zn–N(2)
Cl–Zn–N(12)
Cl–Zn–N(22)
121.11(6)
109.84(6)
115.84(6)
78.02(5)
77.51(6)
77.75(6)
174.09(4)
106.20(4)
96.66(5)
104.24(5)
113.26(7)
113.34(7)
120.25(7)
79.11(7)
76.54(7)
77.75(7)
170.00(5)
110.50(5)
96.55(6)
100.10(6)
N–H ؒ ؒ ؒ Cl–Zn and C–H ؒ ؒ ؒ O᎐C interactions. In contrast, in
᎐
the trigonal bipyramidal complex 1 the angle between amide
and pyridine planes is 31.5Њ, which presumably optimises the
N–H ؒ ؒ ؒ Cl–Zn hydrogen bonding at expense of weakening the
Table 3 Geometric features of internal hydrogen bonding interactions in 1ؒCH3CN, 2ؒ0.5CH3CN, 3ؒCH3CN, 4, 5 and 6
Interaction
N–Ha/Å
N ؒ ؒ ؒ Cl/Å
H ؒ ؒ ؒ Cl/Å
N–H ؒ ؒ ؒ Cl/Њ
Ref.
Trigonal bipyramidal
1ؒCH3CN
N(7)–H(7N) ؒ ؒ ؒ Cl
N(7)–H(7NA) ؒ ؒ ؒ Cl
1.01a
1.01
3.2127(19)
3.213(3)
2.22
2.24
167.7
160.4
20
2ؒ0.5CH3CN
This work
Tetrahedral
3ؒCH3CN
N(7)–H(7N) ؒ ؒ ؒ Cl(1)
N(7)–H(7NA) ؒ ؒ ؒ Cl(1)
N(7)–H(7N) ؒ ؒ ؒ Cl(1)
N(7)–H(7NA) ؒ ؒ ؒ Cl(1)
1.01a
1.01a
1.01a
1.01a
3.3580(15)
3.402(2)
3.277(3)
2.38
2.44
2.30
2.71
162.1
158.3
161.6
152.6
This work
This work
This work
This work
4
5
6
3.6342(18)
a Extended distance.
D a l t o n T r a n s . , 2 0 0 3 , 3 3 3 9 – 3 3 4 9
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Table 4 Selected bond lengths (Å) and angles (Њ) for zinc() complexes 3ؒCH3CN and 4–6
3ؒCH3CN
4
5
6
Zn–Cl(1)
Zn–Cl(2)
Zn–N(1)
Zn–N(2)
2.2016(6)
2.2238(6)
2.0830(14)
2.0823(14)
2.2122(5)
2.2311(5)
2.0854(16)
2.0450(15)
2.2338(9)
2.1924(9)
2.071(3)
2.091(3)
2.2053(5)
2.2066(5)
2.1018(14)
2.0403(14)
N(2)–Zn–N(1)
N(2)–Zn–Cl(1)
N(1)–Zn–Cl(1)
N(2)–Zn–Cl(2)
N(1)–Zn–Cl(2)
Cl(1)–Zn–Cl(2)
82.90(6)
108.21(4)
125.43(4)
109.86(4)
104.00(4)
119.91(3)
83.55(6)
108.51(5)
124.96(4)
116.60(5)
106.93(4)
113.40(2)
83.06(10)
103.22(7)
118.62(8)
117.82(7)
111.12(8)
117.93(4)
82.90(5)
113.71(4)
118.44(4)
112.32(4)
107.03(4)
117.473(19)
Fig. 2 Thermal ellipsoid plot drawn with 50% probability ellipsoids of the molecular structure of [(L3)Zn(Cl)2] 3 (left) and [(L5)Zn(Cl)2] 5 (right).
Fig. 3 Thermal ellipsoid plot drawn with 50% probability thermal ellipsoids of the molecular structure of [(L4)Zn(Cl)2] 4 (left) and [(L6)Zn(Cl)2] 6
(right).
C–H ؒ ؒ ؒ O᎐C interaction. Internal N–H ؒ ؒ ؒ Cl–Zn hydrogen
bonding in the tetrahedral zinc() complexes 3 and 5, however,
is significantly longer (weaker) than in 1ؒCH3CN.
H(7NB) ؒ ؒ ؒ O(13) 164Њ). The different preference of the
external N–H for intermolecular hydrogen bonding may be
partly due to the different steric effects of the N(13)–CH3 (4)
and O(13) (6) and could affect the strength of the internal
N–H ؒ ؒ ؒ Cl–Zn hydrogen bonds in the solid state.
᎐
Structures of 4 and 6. Thermal ellipsoid plots of the molec-
ular structures of 4 and 6 are shown in Fig. 3 and a list with
selected distances and angles is given in Table 4. As in 3 and 5,
the zinc() centre is in a N2Cl2 tetrahedral environment. The
Zn–Npy distances in 4 and 6 are slightly shorter than in 3ؒ
CH3CN and 5 presumably reflecting the electron donating and
electron withdrawing nature of the amine and amide units,
respectively.
The amine N–H groups are involved in internal (intra-
molecular) and external (intermolecular) hydrogen bonding to
chloride ligands (Fig. 4). Internal N–H ؒ ؒ ؒ Cl hydrogen
bonding in 4 is only slightly longer than in 3ؒCH3CN even
though the latter contains a substantially stronger hydrogen
bond donor, and significantly shorter than in 6.
Comparison of structures. Trigonal bipyramidal N4ClZn
structures have slightly longer Zn–NpyЈ distances (pyЈ = pyridine
carrying the N–H hydrogen bond donor), ca. 2.07 Å in 2 and
2.13 Å in 1, than the tetrahedral N2Cl2Zn structures 3–6 (2.04–
2.08 Å). In principle this structural feature should favour a
slightly shorter (stronger) internal N–H ؒ ؒ ؒ Cl–Zn hydrogen
bond in 3–6, as it would bring the hydrogen bond donor and
acceptor closer. In addition, the zinc() centre in 3–6 should be
more strongly Lewis acidic than in 1, 2 as a result of having
a lower coordination number, a feature which would make the
N–H groups stronger hydrogen bond donors. This study shows
that internal hydrogen bonding is stronger in the trigonal
bipyramidal zinc() complexes 1 and 2 than in the tetrahedral
3–6. In idealised tetrahedral and trigonal bipyramidal geom-
etries the ClZnNpyЈ angle would be around 109 and 90Њ, respect-
ively. This structural feature would force the N–H groups
(hydrogen bond donor) and Cl ligand (hydrogen bond
Whereas the external N–H of 4 is involved in intermolecular
N–H ؒ ؒ ؒ Cl(2)–Zn hydrogen bonding (N(7) ؒ ؒ ؒ Cl(2) 3.34 Å;
H(7NB) ؒ ؒ ؒ Cl(2) 2.35 Å; N(7)–H(7NB) ؒ ؒ ؒ Cl(2) 169Њ), in 6, it
forms an intermolecular N–H ؒ ؒ ؒ O(13) hydrogen bond
(N(7) ؒ ؒ ؒ O(13) 3.01 Å; H(7NB) ؒ ؒ ؒ O(13) 2.02 Å; N(7)–
D a l t o n T r a n s . , 2 0 0 3 , 3 3 3 9 – 3 3 4 9
3343

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6-Pivaloylamido-2-pyridylmethyl derivatives. The arrange-
ment of the amide group of the metal-free ligands L1, L3 and L5
is such that allows the intramolecular interaction of H5Ј with
the amide oxygen. This deduction is consistent with the X-ray
crystal structure of L1 and the observed downfield position of
the H5Ј resonance (Fig. 5, Table 5).20
Fig. 4 Thermal ellipsoid plots drawn with 30% probability ellipsoids
showing the involvement of the external amine NH of 4 in inter-
molecular N–H ؒ ؒ ؒ Cl hydrogen bonding (top) and 6 in intermolecular
N–H ؒ ؒ ؒ O hydrogen bonding (bottom).
Fig. 5 Aromatic and NH region of the 1H NMR (360.1 MHz,
CD3CN, 293 K) of L1, [(L1)Zn(Cl)](BPh4) 1, L3 and [(L3)Zn(Cl)2] 3. See
Table 5 for chemical shift values and Scheme 1 for labelling explanation.
acceptor) to be further apart in tetrahedral ligand environments
and could be a priori interpreted as the plausible reason for the
observed weaker internal N–H ؒ ؒ ؒ Cl–Zn hydrogen bonding in
the tetrahedral structures. This X-ray crystallographic study,
however, shows that the ClZnNpyЈ angle in the trigonal
bipyramidal complexes 1 and 2 are in the 106–110Њ range,
compared to 103–114Њ in the tetrahedral complexes 3–6. Thus,
the main reason for the stronger internal N–H ؒ ؒ ؒ Cl–Zn
hydrogen bonding in the trigonal bipyramidal structures
appears to be in fact the smaller angle between the
ZnN(2)C(6)N(7) and ZnN(2)Cl planes (ZnN(2)Cl(1) for 3–6),
which is 25.1Њ in 1 and 20.6Њ in 2 compared to 34.5Њ in 3, 39.4Њ in
4, 44.0Њ in 5 and 43.9Њ in 6. This crystallographic analysis also
shows that the geometric features of the internal N–H ؒ ؒ ؒ Cl–
Zn hydrogen bond in 1 and 2 are approximately the same
despite amide N–H groups being presumably better hydrogen
bond donors. One structural feature that may partly account
for this somewhat ‘unexpected’ result is the shorter Zn–NpyЈ
distance of 2 compared to 1 due to the electron donating and
withdrawing effect of the amine pivaloylamido groups on the
pyridine ring, respectively. A similar effect was found in the
tetrahedral structures of 3 and 5. This could be a potentially
important result in that it implies that even a poor hydrogen
bond donor, if brought sufficiently close to the hydrogen
bond acceptor as the result of short (strong) metal–ligand
binding, should result in a short (strong) internal hydrogen
bond to another ligand. Moreover, it could suggest that the
same effect (the electron withdrawing nature) that makes the
amide a better hydrogen bond donor than an amine, when part
of a ligand, can cause it to be further away from the metal-
bound hydrogen bond acceptor. Thus, it appears to us that an
alternative strategy to induce strong internal hydrogen bonding
in coordination complexes could be to attach the hydrogen
bonding group to a ligand that induces short metal–ligand
distances.
In the zinc() chloride complexes 1, 3 and 5, proton
resonances of 6-pivaloylamido-2-pyridylmethyl unit (pyЈCH2–)
are shifted downfield by 0.2–0.3 ppm relative to the corre-
sponding ligand (L1, L3 or L5), a feature consistent with metal
binding (Fig. 5, Table 5). That the NH proton resonance under-
goes significantly more prominent downfield shifts of ca.
1.05–1.15 ppm in the tetrahedral complexes 3 and 5 and of ca.
2.5 ppm in 1 provides good evidence that internal N–H ؒ ؒ ؒ Cl–
Zn hydrogen bonding is retained in solution. The same data
clearly suggest that these hydrogen bonds are weaker in the
tetrahedral complexes 3 and 5 than in the trigonal bipyramidal
complex 1, in agreement with the X-ray crystal structures. The
strength of the N–H ؒ ؒ ؒ Cl–Zn hydrogen bonding can be
approximately quantified from the positions of the N–H
stretching vibration, νN–H, using IR spectroscopy.24 Thus, νN–H
band of the trigonal bipyramidal complex 1 is shifted to lower
energy values by 173 4 cmϪ1 relative to L1 (Table 6), which
corresponds to a hydrogen bond energy of 16.8 0.6 kJ molϪ1.
In tetrahedral complexes 3 and 5 the νN–H vibration is weakened
by 124 and 110 cmϪ1 relative to in the corresponding ligand,
respectively, which corresponds in this case to hydrogen bond
energies of 13.4 and 14.2 0.6 kJ molϪ1. Thus, the magnitude
of the downfield shifts experienced by the NH proton reson-
ance (NMR), changes in the νN–H vibration (IR) and hydrogen
bonding distances (X-ray diffraction) can be approximately
correlated.
1
6-Amino-2-pyridylmethyl derivatives. The H NMR spectra
(360.1 MHz, CD3CN, 293 K) of 2, 2Ј and L2 show broad
singlets due to the NH2 protons at 6.98, 7.02 and 4.80 ppm,
respectively (Table 5). The large downfield shift experienced by
the amine proton resonance of 2 or 2Ј relative to L2 of ca. 2.2
ppm is consistent with the formation of internal N–H ؒ ؒ ؒ Cl–
Zn hydrogen bonding in solution and it is comparable to the
downfield shift experienced by the amide NH proton of 1
relative to L1 (2.5 ppm). The fact that at room temperature the
NH2 protons appear as a broad singlet indicates that they are in
rapid exchange between the two chemical environments. At low
NMR and IR studies
NMR and IR studies were used to probe the solution structures
of L1–6 and the zinc() complexes 1–6 in acetonitrile and to
correlate these with the X-ray crystal structures.
D a l t o n T r a n s . , 2 0 0 3 , 3 3 3 9 – 3 3 4 9
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Table 5 Summary of selected 1H NMR (360 MHz, CD3CN, 293 K) chemical shift data for L1–6 and 1–6a
L1
1
2
L2
tBu
H10
NH
H7
PyCH2N
H1ЈA,B
Py (aromatic)
H3Ј
H4Ј
H5Ј
1.26
8.16
3.71
1.37 (ϩ0.11)
10.67 (ϩ2.51)
4.03 (ϩ0.32)
4.8
3.6
6.98 (ϩ2.18)
3.8 (ϩ0.2)
7.32
7.67
7.98
7.12 (Ϫ0.20)
7.92 (ϩ0.25)
8.32 (ϩ0.34)
6.82
7.37
6.35
6.58 (ϩ0.24)
7.51 (ϩ0.14)
6.58 (ϩ0.23)
L3, L5
3, 5
L4, L6
4, 6
tBu
H10
NH
H7
PyCH2N
H1ЈA,B
Py (aromatic)
H3Ј
H4Ј
H5Ј
1.26, 1.27
8.17, 8.17
3.48, 3.49
1.33 (ϩ0.07), 1.32 (ϩ 0.05)
9.32 (ϩ1.15), 9.21 (ϩ 1.04)
4.00 (ϩ0.52), 4.00 (ϩ0.51)
4.9, 4.8
5.89 (ϩ0.99), 5.87 (ϩ 1.07)
3.82 (ϩ0.40), 3.80 (ϩ0.45)
3.42, 3.35
7.14,7.17
7.69,7.70
7.97, 8.00
7.21 (ϩ0.07), 7.24 (ϩ0.07)
8.00 (ϩ0.31), 8.02 (ϩ0.32)
8.22 (ϩ0.25), 8.14 (ϩ 0.14)
6.63, 6.65
7.37, 7.37
6.35, 6.36
6.69 (ϩ0.06), 6.69 (ϩ 0.04)
7.61 (ϩ0.24), 7.63 (ϩ0.26)
6.69 (ϩ0.34), 6.69 (ϩ0.33)
a Chemical shifts are in ppm relative to CH3CN at 1.94 ppm. Values in parentheses denote chemical shifts downfield (positive) or upfield (negative)
versus values in the corresponding ligand. The symbol Ј refers to the 2-pyridylmethyl with the 6-pivaloylamido or amino group.
Table 6 Selected infrared vibrational data of L1,3,5 and 1,3 and 5 in
estimated to be ca. 44.2 kJ molϪ1 from the coalescence temper-
ature (Tc) and amine proton resonances at low temperatures.
From this barrier the energy of the N–H ؒ ؒ ؒ Cl–Zn hydrogen
bond was approximately calculated to be ca. 16.8 kJ molϪ1 by
substracting the activation barrier restricting rotation of the
NH2 group in the absence of hydrogen bonding.26 Concen-
tration experiments suggest that the ClϪ counter-ion weakly
hydrogen bonds to the external N–H in concentrated samples
of 2Ј and that its contribution to the rotational barrier is
acetonitrile solutions
νN–H a/cmϪ1
L1
3438
3264
(ϩ174)
3439
3315
(ϩ124)
3440
3330.0
(ϩ110)
[(L1)Zn(Cl)](BPh4) 1
(difference)
L3
[(L3)Zn(Cl)2] 3
(difference)
L5
1
negligible at lower concentrations (Table 7).27 Thus, these H
NMR studies suggest that N–H ؒ ؒ ؒ Cl–Zn hydrogen bonding
in 2 is at least as strong as in 1, in excellent agreement with the
above crystallographic results and discussion.26
[(L5)Zn(Cl)2] 5
(difference)
a
4 cmϪ1
.
The X-ray structure of 2 shows a very short external
Ϫ
N–H ؒ ؒ ؒ π hydrogen bond with a phenyl group of the BPh4
anion (Fig. 1(ii)). The fact that the 1H NMR spectra of 1.4–28
mM solutions of 2 in CD3CN resembles that of 1.4 mM
solutions of 2Ј suggests in this case that the external N–H ؒ ؒ ؒ π
hydrogen bond is not formed in solution.
temperatures, however, the NH2 resonance flattens and then
diverges into two singlets, of which the downfield resonance
corresponds to the internal N–H, which is downfield shifted to
ca. 7.91–7.98 ppm due to the N–H ؒ ؒ ؒ Cl–Zn hydrogen bond.
The strength of the internal N–H ؒ ؒ ؒ Cl–Zn hydrogen bond in
In the tetrahedral complexes 4 and 6 the NH2 protons appear
as a broad singlet at 5.9 ppm compared to 4.9 ppm in L4 and
4.8 ppm in L6 (Fig. 7, Table 5). The fact that the downfield shifts
1
the [(L2)Zn(Cl)]ϩ cation was approximately determined by H
NMR spectroscopy variable temperature coalescence studies25
of [(L2)Zn(Cl)](X) (X = BPh4 (2), Cl (2Ј)) (Fig. 6, Table 7). Thus,
the barrier restricting the rotation of the NH2 group of 2 was
Fig. 7 Aromatic and NH region of the 1H NMR (360.1 MHz, CD3CN,
293 K) of L4, [(L4)Zn(Cl)2] 4, L6 and [(L6)Zn(Cl)2] 6. See Table 5 for
chemical shift values and Scheme 1 for labelling explanation.
Fig. 6 1H VT NMR of [(L2)Zn(Cl)](Cl) 2Ј in CD3CN (1.8 mM)
showing aromatic and NH2 (*) resonances.
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1
Table 7 Summary of temperature and concentration dependency of the NH2 chemical shifts (δ, given in ppm) in the H NMR (360.1 MHz,
CD3CN) of [(L2)Zn(Cl)](Cl) 2Ј (i), and [(L2)Zn(Cl)](BPh4) 2 (ii), showing also the calculated rotational barriers of the NH2 group and estimated
H-bond energy of the internal N–H ؒ ؒ ؒ Cl–Zn H-bond in these compounds
(i) [(L2)Zn(Cl)](Cl) 2Ј
δ NHav (internal ϩ
external)/ppm
δ NHav (internal ϩ
external)/ppm
δ NHi
(internal)/ppm
δ NHe
(extenal)/ppm
Rotational
H-bond energya/
kJ molϪ1
Tcoalescence/K
barrier/kJ molϪ1
28 mM
14 mM
7 mM
3.5 mM
1.8 mM
T/K
7.10
7.04
7.00
6.97
6.97
323
7.29
7.20
7.12
7.07
7.03
293
7.91
7.93
7.94
7.95
7.96
223
7.37
7.08
6.82
6.56
6.36
223
248
243
243
243
243
47.2
45.9
45.4
44.9
44.6
19.8
18.5
18
17.5
17.2
(ii) [(L2)Zn(Cl)](BPh4) 2
δ NHav (internal ϩ
δ NHav (internal ϩ
external)/ppm
δ NHi
(internal)/ppm
δ NHe
(extenal)/ppm
Rotional
H-bond energya/
kJ molϪ1
external)/ppm
Tcoalescence/K
barrier/kJ molϪ1
28 mM
7 mM
1.8 mM
T/K
6.92
6.92
6.92
6.94
6.96
6.96
7.98
7.97
7.98
6.03
6.03
6.03
243
243
243
44.2
44.2
44.2
16.8
16.8
16.8
323
293
223
223
a Assuming an intrinsic C–N rotational barrier of 27.4 kJ molϪ1 (see text).
of the amine protons are very similar for 4 and 6 and to the
chemical shift changes experienced by the amide NH proton of
3 relative to L3 (1.15 ppm) and of 5 relative to L5 (1.04 ppm) can
be taken as indicative that internal N–H ؒ ؒ ؒ Cl–Zn hydrogen
bonding in 3–6 is of similar strength, in good agreement
with the X-ray crystal structures of 3, 4 and 5. This result also
suggests that the longer internal N–H ؒ ؒ ؒ Cl–Zn hydrogen
bond in the crystal structure of 6 may be due to close packing
effects. These data are consistent also with internal hydrogen
bonding in the tetrahedral complexes 4 and 6 being weaker than
in the trigonal bypiramidal complex 2 as implied by the X-ray
crystal structures.
structural features found in the solid state structures, except a
very short intermolecular external N–H ؒ ؒ ؒ π (arene) hydrogen
bonding in 2, are retained in solution and are clearly expressed
1
in the H, 13C NMR and IR spectra of these compounds. This
work also provides an upper limit of the strength of the internal
N–H ؒ ؒ ؒ Cl–Zn hydrogen bonding in acetonitrile solution
using IR and 1H NMR variable temperature coalescence
studies, which is ∼16–17 kJ molϪ1 and ∼13–14 kJ molϪ1 in the
trigonal bipyramidal (1,2 and 2Ј) and tetrahedral complexes
1
(3–6), respectively. The magnitude of changes in the H NMR
spectra of 1–6 can be correlated with the strength of the
hydrogen bond in acetonitrile solutions and corroborates the
conclusions derived from the X-ray crystallographic studies.
There is considerable current interest in elucidating the
role(s) of second-sphere hydrogen bonding to metal-bound
species in metallohydrolases, oxidases and peroxidases using
small-molecule models.6 This important chemistry requires
metal centres such as Zn(), Cu(/) and Fe(/). Based on this
work it is reasonable to suggest that strong internal hydrogen
bonding in models of these metalloenzymes could be effectively
pursued with ligands that induce short metal–L distances
(L = ligand carrying the hydrogen bonding group). This
suggestion may be particularly relevant to metal–dioxygen
chemistry, as strongly donating ligands carrying hydrogen
bonding groups will be more effective at reducing dioxygen and
inducing strong hydrogen bonds to the metal-bound oxygen/
dioxygen species. Moreover, internal hydrogen bonding is likely
to become stronger as the metal is oxidised and dioxygen
reduced. Important recent work has shown that internal
hydrogen bonding can stabilise high-valent Fe()–oxo species,
perhaps in analogy to some oxygenases.5c,6f
Conclusion
This study has explored the use N–H groups of the ligand
unit (6-X-2-pyridylmethyl)amine (X = NHCOR or NH2) as a
strategy to induce internal hydrogen bonding to an adjacent
metal-bound ligand in different coordination geometries. Three
ligands with a pivaloylamido group, L1,3,5, and three ligands
with an amino group, L2,4,6, were used in this study. Ligands
with the pivaloylamido group form zinc() complexes 1, 3 and 5
with internal N–H ؒ ؒ ؒ Cl–Zn hydrogen bonding. Ligands with
the amino group also form zinc() complexes 2, 2Ј, 4 and 6 with
internal H ؒ ؒ ؒ Cl–Zn hydrogen bonding and a variety of
external hydrogen bonding. The X-ray crystal structures of a
trigonal bipyramidal zinc() complex with N4Cl coordination
environment, 2, and four tetrahedral (N2Cl2), 3–6, were
reported. These five X-ray crystal structures together with the
structure of 120 show that the geometry of the internal
N–H ؒ ؒ ؒ Cl–Zn hydrogen bond is relatively insensitive to the
nature of the hydrogen bond donor. These structures show also
that internal N–H ؒ ؒ ؒ Cl–Zn hydrogen bonding is significantly
shorter (stronger) in the trigonal bipyramidal complexes. We
propose that a variety of structural parameters determine the
geometry of the internal N–H ؒ ؒ ؒ Cl–Zn hydrogen bond,
including the Zn–NpyЈ distance and the angle between the plane
containing the hydrogen bond donor and Zn–Cl vector of
the hydrogen bond acceptor. Thus, metal–ligand effects and
geometry are clear examples of the ‘inorganic’ factors that
affect the strength of hydrogen bonding interactions involving
coordination complexes. The main objective of this study was
to correlate the information extracted from X-ray studies with
structural and spectroscopic studies in solution. Thus, the
studies reported herein provide very good evidence that the
In addition, this work shows that hydrogen bonding in
tetrahedral complexes, which is the most common geometry
in metallohydrolases, can be weaker than in trigonal bi-
pyrimidal complexes.
Experimental
General
Reagents were obtained from commercial sources and used
as received unless otherwise noted. Solvents were dried and
purified under N2 by using standard methods28 and were
distilled immediately before use. All compounds were prepared
under N2 unless otherwise mentioned. The synthesis and
D a l t o n T r a n s . , 2 0 0 3 , 3 3 3 9 – 3 3 4 9
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characterisation of L1 and [(L1)Zn(Cl)](BPh4) 1 was reported
elsewhere.20 The NMR spectra were obtained using a Bruker
ARX 250 or Bruker DPX 360 at 20 ЊC in CD3CN unless
otherwise noted. 13C and1H chemical shifts are referenced with
respect to the carbon (δC 1.32 and 118.26 ppm) and residual
proton (δH 1.94 ppm) solvent peaks. Peak assignments are done
with the aid of 2-D NMR spectroscopy. Sample concentrations
for the NMR studies were 1.4–28 mM. Mass spectra were
performed on a micromass Platform II system operating in
Flow Injection Analysis mode with the electrospray method.
Elemental analyses were carried out by the microanalyses
service provided by the School of Chemistry at the University
of Edinburgh. Infrared spectra were recorded with a JASCO
FTIR-410 spectrometer between 4000 and 250 cmϪ1 as KBr
pellets (solid state) or as acetonitrile solutions in KBr cells.
The strength of N–H ؒ ؒ ؒ Cl–Zn hydrogen bonding was
estimated using solid-state and solution FTIR studies applying
Iogansen’s equation24 and/or variable temperature NMR
coalescence methods.25 The variable temperature 1H NMR
studies were repeated twice on freshly prepared samples and
gave reproducible results.
L4. L3 (8.8 mmol, 2.55 g) was dissolved in 2 M HCl (145 cm3).
The resulting yellow solution was heated at reflux overnight.
The solution was allowed to cool to room temperature, after
which, 1 M NaOH was added until ∼ pH 14. The product was
extracted with CH2Cl2 (3 × 100 cm3). The combined organic
phases were dried over Na2SO4 and dried to dryness under
reduced pressure to afford the ligand as a yellow solid (1.36 g,
75%) (Found: C, 63.12; H, 8.51; N, 26.22. Calc. for C11H18N4ؒ
0.2H2O: C, 63.13; H, 9.08; N, 26.29%).
1H NMR (CD3CN, 360.1 MHz): δH (ppm) 7.37 (dd, J = 7.9,
7.3 Hz, 1H, pyЈ-H4), 6.63 (d, J = 7.3 Hz, 1H, pyЈ-H3), 6.35 (d,
J = 7.9 Hz, 1H, pyЈ-H5), 4.9 (br, 2H, pyЈ-NH2) 3.42 (s, 2H,
NCH2-pyЈ), 2.41–2.30 (m, 8H, N(CH2CH2)2NCH3), 2.17 (s, 3H,
NCH3). 13C NMR (CD3CN, 90.5 MHz): δC (ppm) 159.9
and 158.0 (pyЈ-C2 and pyЈ-C6), 138.6 (pyЈ-C3), 112.9 and
107.2 (pyЈ-C4 and pyЈ-C5), 65.1 (NCH2-pyЈ), 55.9 and 54.1
(N(CH2CH2)2NCH3), 46.3 (NCH3). ESI-MS (ϩ ion): Found
207.2 (100%). Calc. 207.16 (100%) for [(L4)H]ϩ, and matches
theoretical isotope distribution.
L5. This ligand was prepared in the same way as L3 using
morpholine (2 mmol, 0.17 cm3) (0.480 g, 87%) (Found: C,
64.45; H, 8.25; N, 14.77. Calc. for C15H23N3O2: C, 64.95; H,
8.36; N, 15.15%).
Synthesis of ligands
1H NMR (CD3CN, 360.1 MHz): δH (ppm) 8.17 (br s, 1H,
pyЈ-NH), 8.00 (d, J = 8.2 Hz, 1H, pyЈ-H5), 7.70 (t, J = 7.6 Hz,
1H, pyЈ-H4), 7.17 (d, J = 7.6 Hz, 1H, pyЈ-H3), 3.49 (s, 2H,
NCH2-pyЈ), 3.62, 2.43 (m, 4H and 4H, N(CH2CH2)2O), 1.27 (s,
9H, C(CH3)3).13C NMR (CD3CN, 90.5 MHz): δC (ppm) 177.9
L2. L1 (4 g, 10.3 mmol) was dissolved in 2 M HCl(aq) (150 cm3)
and the solution was refluxed for 24 h. The solution was then
poured into 1 M NaOH(aq). The product was extracted with
dichloromethane (3 × 100 cm3) and the organic fractions were
dried over Na2SO4. The solvent was evaporated under vacuum
and washed with diethyl ether to yield the product as a brown
oil (3 g, 96.3%) (Found: C, 70.80; H, 6.29; N, 22.79. Calc. for
C18H19N5: C, 70.80; H, 6.27; N, 22.93%).
(C᎐O). 158.1 and 152.2 (pyЈ-C2 and pyЈ-C6), 139.4 (pyЈ-C3),
᎐
119.5 and 112.7 (pyЈ-C4 and pyЈ-C5), 65.1 (NCH2-pyЈ), 67.4
and 54.5 (N(CH2CH2)2O), 40.3 (C(CH3)3), 27.4 (C(CH3)3).
ESI-MS (ϩ ion): Found 277.8 (100%), Calc. 278.19 (100%) for
[(L5)H]ϩ, and matches theoretical isotope distribution.
1H NMR (CD3CN, 360.1 MHz): δH (ppm) 8.46 (m, 2H,
py-H6), 7.68 (td, J = 7.92, 1.8 Hz, 2H py-H4), 7.58 (d, J = 7.9
Hz, 2H, py-H3), 7.37 (dd, J = 8.2, 7.5 Hz, 1H, pyЈ-H4), 7.16 (m,
2H, py-H5), 6.82 (d, J = 7.5 Hz, 1H, pyЈ-H3), 6.35 (d, J = 8.2
Hz, 1H, pyЈ-H5), 4.8 (br, 2H, pyЈ-NH2) 3.8 (s, 4H, NCH2-py),
3.6 (s, 2H, NCH2-pyЈ).13C NMR (CD3CN, 90.5 MHz):
δC (ppm) 160.9 (py-C2), 160.0 and 159.0 (pyЈ-C2 and pyЈ-C6),
149.9 (py-C6), 138.8 (pyЈ-C3), 137.3 (py-C3), 123.9 and 123.0
(py-C4 and py-C5), 112.9 and 107.3 (pyЈ-C4 and pyЈ-C5),
61.0 (NCH2-py and NCH2-pyЈ). ESI-MS (ϩ ion): Found
306.1 (100%). Calc. 306.0 (100%) for [(L2)H]ϩ, and matches
theoretical isotope distribution.
L6. This ligand was prepared by acid hydrolysis of L5 (0.400
g, 1.44 mmol) in the same way as L4 (0.210 g, 75%) (Found:
C, 61.78; H, 7.75; N, 21.47. Calc. for C10H15N3O: C, 62.15; H,
7.82; N, 21.74%).
1H NMR (CD3CN, 360.1 MHz): δH (ppm) 7.37 (t, J = 7.6 Hz,
1H, pyЈ-H4), 6.65 (d, J = 7.6 Hz, 1H, pyЈ-H3), 6.36 (d, J = 7.9
Hz, 1H, pyЈ-H5), 4.8 (br, 2H, pyЈ-NH2), 3.35 (s, 2H, NCH2-
pyЈ), 3.61 and 2.41 (m, 4H and 4H, N(CH2CH2)2O). 13C NMR
(CD3CN, 90.5 MHz): δC (ppm) 159.9 and 157.5 (pyЈ-C2 and
pyЈ-C6), 138.6 (pyЈ-C3), 112.9 and 107.2 (pyЈ-C4 and pyЈ-C5),
65.5 (NCH2-pyЈ), 67.5 and 54.6 (N(CH2CH2)2O). ESI-MS
(ϩ ion). Found 194.0 (100%), Calc. 194.13 (100%) for [(L6)H]ϩ,
and matches theoretical isotope distribution.
L3. N-Methylpiperazine (0.23 cm3, 2 mmol) and Na2CO3
(2.12 g, 20 mmol) were dissolved in CH3CN (∼15 cm3). This
solution was then treated with 2-(pivaloylamido)-6-(bromo-
methyl)pyridine19 (L0) (0.542 g, 2 mmol) and the resulting
mixture was stirred for 24 h at 80 ЊC. The solution was cooled to
room temperature, and then poured in 1 M NaOH(aq) (20 cm3).
The product was extracted with CH2Cl2 (3 × 50 cm3). The com-
bined organic phases were dried over Na2SO4, and concentrated
under reduced pressure to afford an orange oil. This oil was
treated with diethyl ether and the white precipitate removed by
filtration. The filtrate was evaporated under vacuum to afford
the pure compound (0.389 g, 67%) (Found: C, 65.82; H, 8.91;
N, 19.00. Calc. for C16H26N4O: C, 66.17; H, 9.02; N, 19.29%).
1H NMR (CD3CN, 360.1 MHz): δH (ppm) 8.17 (br s, 1H,
pyЈ-NH), 7.97 (d, J = 7.9 Hz, 1H, pyЈ-H5), 7.69 (t, J = 7.8 Hz,
1H, pyЈ-H4), 7.14 (d, J = 7.6 Hz, 1H, pyЈ-H3), 3.48 (s, 2H,
NCH2-pyЈ), 2.50–2.30 (m, 8H, N(CH2CH2)2NCH3), 2.12 (s, 3H,
NCH3), 1.26 (s, 9H, C(CH3)3).13C NMR (CD3CN, 90.5 MHz):
Synthesis of zinc(II) complexes
[(L2)Zn(Cl)](Cl) 2Ј. L2 (0.25 g, 0.8 mmol) and ZnCl2 (0.11 g,
0.8 mmol) were dissolved in dry acetonitrile (10 cm3). The
solution was stirred for 1 h at room temperature. The solution
was filtered and the filtrate was evaporated under vacuum to
yield the crude material as a yellow solid. Addition of dry
diethyl ether (10 cm3) to the crude material formed a white
precipitate. The white precipitate was re-dissolved in dry
acetonitrile (10 cm3) and filtered through Celite. The solvent
was removed under vacuum to yield a white solid (0.09 g, 26%).
1H NMR (CD3CN, 360.1 MHz): δH (ppm) 9.03 (d, J = 5.1 Hz,
2H, py-H6), 7.96 (td, J = 8.0, 1.8 Hz, 2H py-H4), 7.52 (t, J = 6.8
Hz, 1H, py-H5), 7.44 (d, J = 7.9 Hz, 2H, py-H3), 7.39 (t, J = 7.9
Hz, 1H, pyЈ-H4), 7.16 (br, 2H, pyЈ-NH2), 6.66 (d, J = 8.2 Hz,
1H, pyЈ-H3), 6.55 (d, J = 7.2 Hz, 1H, pyЈ-H5), 4.13 (s, 4H,
NCH2-py), 3.9 (s, 2H, NCH2-pyЈ).13C NMR (CD3CN, 90.5
MHz): δC (ppm) 162.0 and 151.9 (pyЈ-C2 and pyЈ-C6), 155.7
and 149.4 (py-C2 and py-C6), 141.4 (py-C3), 141.0 (pyЈ-C3),
125.5 and 125.1 (py-C4 and py-C5), 112.3 and 112.2 (pyЈ-C4
and pyЈ-C5), 58.4 (NCH2-pyЈ) and 57.4(NCH2-py). ESI-MS
δC (ppm) 178.5 (C᎐O). 159.2 (pyЈ-C2), 152.9 (pyЈ-C6), 140.0
᎐
(pyЈ-C3), 120.1 and 113.2 (pyЈ-C4 and pyЈ-C5), 65.4 (NCH2-
pyЈ), 56.9 and 54.7 (N(CH2CH2)2NCH3), 46.9 (NCH3) 41.0
(C(CH3)3), 28.1 (C(CH3)3). ESI-MS (ϩ ion): Found 291.2
(100%). Calc. 291.22 (100%) for [(L3)H]ϩ, and matches
theoretical isotope distribution.
D a l t o n T r a n s . , 2 0 0 3 , 3 3 3 9 – 3 3 4 9
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(ϩ ion): Found 404.2 (100%), Calc. 404.1 (100%) for
[(L2)Zn(Cl)]ϩ, and matches theoretical isotope distribution.
7.24 (d, J = 7.5 Hz, 1H, pyЈ-H3), 4.00 (s, 2H, NCH2-pyЈ), 3.92,
3.10 and 2.66 (m, 4H, 2H and 2H, N(CH2CH2)2O), 1.32 (s, 9H,
C(CH3)3). 13C NMR (CD3CN, 62.9 MHz, 298 K): δC (ppm)
[(L2)Zn(Cl)](BPh4) 2. 2Ј (0.09 g, 0.2 mmol) was dissolved in
methanol (5 cm3). NaBPh4 (0.07 g, 0.2 mmol) was then added
and the solution was stirred for 2 h. The white precipitate
formed was collected by filtration, washed with ether (2 cm3),
and dried under vacuum (0.10 g, 69%) (Found: C, 68.69; H,
5.41; N, 10.03. Calc. for C44H39BClN5Zn, 2ؒ0.5CH3CN: C,
69.23; H, 5.47; N, 10.33%).
178.6 (C᎐O), 152.5 and 152.4 (pyЈ-C2 and pyЈ-C6), 144.0
᎐
(pyЈ-C3), 120.5 and 117.4 (pyЈ-C4 and pyЈ-C5), 63.1 (NCH2-
pyЈ), 66.1 and 55.0 (N(CH2CH2)2O), 40.9 (C(CH3)3), 27.2
(C(CH3)3). ESI-MS (ϩ ion): Found 376.2 (100%), Calc. 376.08
{(L5)ZnCl}ϩ, and matches theoretical isotope distribution.
[(L6)Zn(Cl)2] 6. This complex was prepared in the same way
as 3 using the ligand L6 (0.077 g, 0.4 mmol) to afford the pure
complex as a white solid (yield >90%) (Found: C, 36.57; H,
4.52; N, 12.68. Calc. for C10H15Cl2N3OZn: C, 36.45; H, 4.59; N,
12.75%).
1H NMR (CD3CN, 360.1 MHz): δH (ppm) [(L2)Zn(Cl)]ϩ 9.08
(d, J = 5.4 Hz, 2H, py-H6), 8.03 (td, J = 7.6, 1.5 Hz, 2H py-H4),
7.61 (t, J = 6.1 Hz, 1H, py-H5), 7.51 (dd, J = 8.3, 7.2 Hz, 2H,
pyЈ-H4), 7.48 (d, J = 7.5 Hz, 1H, py-H3), 6.98 (br, pyЈ-NH2),
6.58 (t, J = 7.9 Hz, 2H, pyЈ-H3 and pyЈ-H5), 4.06 (s, 4H, NCH2-
1H NMR (CD3CN, 360.1 MHz): δH (ppm) 7.63 (dd, J = 8.3,
7.2 Hz, 1H, pyЈ-H3), 6.69 (t, J = 7.9 Hz, 2H, pyЈ-H5 and
pyЈ-H4), 5.87 (br, 2H, pyЈ-NH2) 3.80 (s, 2H, NCH2-pyЈ), 3.94,
3.1 and 2.61 (m, 4H, 2H and 2H, N(CH2CH2)2O). 13C NMR
(CD3CN, 90.5 MHz): δC (ppm) 159.8 and 151.0 (pyЈ-C2 and
pyЈ-C6), 142.5 (pyЈ-C3), 113.1 and 111.8 (pyЈ-C4 and pyЈ-C5),
63.0 (NCH2-pyЈ), 66.2 and 54.8 (N(CH2CH2)2O). ESI-MS
(ϩ ion): Found 291.9 (100%), Calc. 292.02 {(L4)ZnCl}ϩ, and
matches theoretical isotope distribution.
Ϫ
py), 3.80 (s, 2H, NCH2-pyЈ); BPh4 7.27 (m, 8H, Ar-H2), 6.98
(t, J = 7.5 Hz, 8H, Ar-H3) and 6.83 (t, J = 7.2 Hz, Ar-H4). 13
C
NMR (CD3CN, 90.5 MHz, 298 K): δC (ppm) [(L2)Zn(Cl)]ϩ
162.1 and 152.1 (pyЈ-C2 and pyЈ-C6), 155.8 and 149.7 (py-C2
and py-C6), 142.1 (py-C3), 141.8 (pyЈ-C3), 125.9 and 125.5
(py-C4 and py-C5), 112.8 and 112.6 (pyЈ-C4 and pyЈ-C5), 57.3
Ϫ
(NCH2-pyЈ), 56.5 (NCH2-py); BPh4 164.6 (B-C1, JB–C = 34.2
Hz), 136.6 (C2), 126.5 (C3), 122.7 (C4). ESI-MS (ϩ ion): Found
404.0 (100%), Calc. 404.1 (100%) for [(L2)Zn(Cl)]ϩ, and
matches theoretical isotope distribution.
X-Ray crystallography
Crystals suitable for X-ray diffraction studies of 2–6 were
grown by slow evaporation of acetonitrile or acetonitrile–water
solutions at room temperature.
[(L3)Zn(Cl)2] 3. ZnCl2 (55 mg, 0.4 mmol) and L3 (116 mg, 0.4
mmol) were dissolved in dry CH3CN (20 cm3) and the reaction
mixture was stirred at room temperature for 1 h. The solution
was then filtered through Celite and the solvent removed under
vacuum to afford the pure compound as a yellow solid (yield
>90%) (Found: C, 44.14; H, 5.7; N, 12.69. Calc. for
C16H26Cl2N4OZnؒ0.5H2O: C, 44.11; H, 6.25; N, 12.86%).
Intensity data for 2–6 were collected at 150 K using a Bruker-
AXS SMART APEX area detector diffractometer with
graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å).
The structure of 2ؒCH3CN was solved by Patterson methods
using the program DIRDIF-9929 and refined to convergence
against F 2 data using the SHELXTL suite of programs.30 The
structures of 3–6 were solved by direct methods and refined
to convergence against F 2 data using the SHELXTL suite of
programs. Data were corrected for absorption applying
empirical methods using the program SADABS,31 and the
structures were checked for higher symmetry using the program
PLATON.32 All non-hydrogen atoms were refined aniso-
tropically unless otherwise noted. Hydrogen atoms were placed
in idealised positions and refined using a riding model with
fixed isotropic displacement parameters. The N–H hydrogens
were located in the difference map and refined isotropically.
Difference maps of the crystal structure of 2 revealed the
presence of a region of disordered solvent, which was modelled
as a molecule of acetonitrile disordered over an inversion
centre.
1H NMR (CD3CN, 360.1 MHz): δH (ppm) 9.32 (s, 1H, NH),
8.22 (d, J = 8.2 1H, pyЈ-H5), 8.00 (t, J = 7.9 Hz, 1H, pyЈ-H4),
7.21 (d, J = 7.8 Hz, 1H, pyЈ-H3), 4.00 (s, 2H, NCH2-pyЈ), 3.16
and 2.8–2.60 (m, 2H and 6H, N(CH2CH2)2NCH3), 2.28 (s, 3H,
NCH3), 1.33 (s, 9H, C(CH3)3).). 13C NMR (CD3CN, 62.9 MHz,
298 K): δC (ppm) 178.5 (C᎐O). 152.8 and 152.6 (pyЈ-C2 and
᎐
pyЈ-C6), 143.7 (pyЈ-C3), 120.1 (py-C4Ј), 117.3 (pyЈ-C5), 62.2
(NCH2-pyЈ), 54.7 and 54.0 (N(CH2CH2)2NCH3), 45.5 (NCH3),
40.9 (C(CH3)3), 27.3 (C(CH3)3). ESI-MS (ϩ ion): Found 389.3
(100%), Calc. 389.11 {(L3)ZnCl}ϩ. Found 427.3 (100%), Calc.
427.08 [(L3)Zn(Cl)2]Hϩ (10% intensity relative to {(L3)ZnCl}ϩ),
and matches theoretical isotope distribution.
[(L4)Zn(Cl)2] 4. This complex was prepared in the same way
as 3 using the ligand L4 (0.083 g, 0.4 mmol) to afford the pure
complex as a yellow solid (yield >90%) (Found: C, 39.57; H,
5.30; N, 16.49. Calc. for C11H18Cl2N4Znؒ0.25CH3CN: C, 39.15;
H, 5.36; N, 16.87%).
CCDC reference numbers 205581 and 210658–210661.
See http://www.rsc.org/suppdata/dt/b3/b305476b/ for crystal-
lographic data in CIF or other electronic format.
1H NMR (CD3CN, 360.1 MHz): δH (ppm) 7.61 (t, J = 7.2 Hz,
1H, pyЈ-H3), 6.69 (t, J = 7.9 Hz, 2H, pyЈ-H5 and pyЈ-H4), 5.89
(br, 2H, pyЈ-NH2) 3.82 (s, 2H, NCH2-pyЈ), 3.10, 2.77 and 2.57–
2.47 (m, 2H, 2H and 4H, N(CH2CH2)2NCH3), 2.24 (s, 3H,
NCH3). 13C NMR (CD3CN, 90.5 MHz): δC (ppm) 159.8
(pyЈ-C2), 151. 0 (pyЈ-C6), 142.4 (pyЈ-C3), 112.9 and 111.7
(pyЈ-C4 and pyЈ-C5), 62.6 (NCH2-pyЈ), 54.7 and 54.3 (N(CH2-
CH2)2NCH3), 45.8 (NCH3). ESI-MS (ϩ ion): Found 305.2
(100%), Calc. 305.05 {(L4)ZnCl}ϩ. Found 343.2 (100%), Calc.
343.02 [(L4)Zn(Cl)2]Hϩ (50% intensity relative to {(L4)ZnCl}ϩ),
and matches theoretical isotope distribution.
Acknowledgements
We gratefully acknowledge the EPSRC (GR/R25743/01 and
GR/M81830), the Royal Society (RSRG: 22702), the Nuffield
Foundation (NAL/00286/G) and The University of Edinburgh
for funding. We would like to thank Dr Andy Parkin for
processing X-ray data.
References and notes
[(L5)Zn(Cl)2] 5. This complex was prepared in the same way
as 3 using the ligand L5 (0.111 g, 0.4 mmol) to afford the pure
complex as a white solid (yield >90%) (Found: C, 43.86; H,
5.60; N, 10.12. Calc. for C15H23Cl2N3OZn: C, 43.55; H, 5.60; N,
10.16%).
1 G. A. Jeffrey and W. Saenger, Hydrogen Bonding in Biological
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26 The barrier restricting the rotation of the NH2 in 2-aminopyridine is
16.6 kJ molϪ1. We were unable to determine the barrier restricting
the rotation of the complex [(L2)Zn(NCCH3)](PF6)2 because the
coalescence temperature for the exchange of the internal and
external NH was below the melting point of the solvent (CD3CN or
CD2Cl2), which means that this barrier is very low. The barrier
restricting the rotation of the NH2 of the metal complex
[IrH2(X)(Y)(PPh3)](BF4) (X = CH3CN; Y = 2-aminopyridine) with
N–H ؒ ؒ ؒ X hydrogen bonding, however, was estimated to be <27.4
18
kJ molϪ1
.
In the absence of a better value, we have taken 27.4 kJ
molϪ1 as an upper limit estimate of the intrinsic barrier restricting
rotation of the NH2 group in the absence of hydrogen bonding, from
which we could at least determine the minimum energy of the
N–H ؒ ؒ ؒ Cl–Zn hydrogen bond in 2 and 2Ј.
27 The proton resonance of the external N–H moves downfield
and the barrier restricting rotation of the NH2 increases slightly
as the concentration increases, presumably due to some external
N–H ؒ ؒ ؒ ClϪ hydrogen bonding, which is very weak in concen-
trations <7 mM. See Table 7 and ESI.
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