Directed secondary interactions in transition metal complexes of tripodal pyrrole imine and amide ligands

Article abstract of DOI:10.1039/c2dt30539a

Tripodal pyrrole imine and amide ligands provide platforms for combined primary and secondary coordination sphere interactions in square planar Pd and Cu, and octahedral Ti complexes.

Full text of DOI:10.1039/c2dt30539a

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COMMUNICATION
Directed secondary interactions in transition metal complexes of tripodal
pyrrole imine and amide ligands†
John S. Hart, Gary S. Nichol and Jason B. Love*
Received 6th March 2012, Accepted 22nd March 2012
DOI: 10.1039/c2dt30539a
ligand remains available for, and involved in, secondary-sphere
interactions.
Tripodal pyrrole imine and amide ligands provide platforms
for combined primary and secondary coordination sphere
interactions in square planar Pd and Cu, and octahedral Ti
complexes.
The pyrrole–imine pro-ligand H3L1 was synthesised according
to the literature from the straightforwardly made, triply formy-
lated tripyrrole (Scheme 1).12,13 The amide variant H6L2 was
prepared in two high-yielding steps from the tripyrrole that
involved the formation of the 2-trichloroacyl derivative and its
subsequent nucleophilic substitution reactions with either cyclo-
Ligand designs that can satisfy the coordinative needs of tran-
sition metals and provide peripheral functionality to stabilise and
direct reactivity are becoming intrinsic to modern chemical cata-
lysis.1 This is especially apparent in the multiple proton–electron
transfer reactions that are requisite for the sustainable transform-
ations of small molecules such as O2, H2, CO2, and N2.2 Highly
specialised ligands such as picket-fence,3 crown,4 Pacman,5 and
Hangman porphyrins,6 have been developed to introduce axially
positioned metal binding sites or acid–base appendages and
provide correlation with the proton and electron delivery path-
ways found in metalloenzymes that carry out these important
transformations.7 Unfortunately, the multi-step synthetic routes
to many of these particular ligands can be inefficient and time-
consuming, so limiting their applicability. Organometallic
carbene complexes which incorporate N–H donors close to the
reactive metal site have also been prepared and show potential as
substrate recognition motifs.8
We have shown previously that Schiff-base condensation reac-
tions favour the ready formation of flexible polypyrrolic macro-
cycles that form well-defined, Pacman complexes reminiscent of
cofacial diporphyrins.9 Monometallic uranyl complexes were
prepared in which the secondary sphere can be addressed to
facilitate unique reduction and functionalisation chemistry of the
uranyl oxo-group.10 Ditopic pyrrole–crown ether macrocycles
were also prepared and form complexes in which the apical poly-
ether acts as a hydrogen-bond acceptor and promotes supramole-
cular structural assembly.11 Here, we report an alternative
molecular design strategy in which judicious choice of the tran-
sition metal in metallation reactions of the related, and poten-
tially tripodal pyrrole–imine ligand L1,12 and its amide variant
L2, results in the formation of well-defined metal complexes of
porphyrin-like N4-donors in which one arm of the tripodal
1
hexylamine or methylamine.14 The H NMR spectrum of H6L2a
(R′ = Cy) in CDCl3 has characteristic pyrrole and amide N–H
resonances at 10.9 and 7.78 ppm, respectively, the latter a
doublet due to coupling to the methine hydrogen of the cyclo-
hexyl substituent.
The reaction between pyrrole–imine H3L1 and M(OAc)2 (M =
Pd, Cu) in MeCN led to the formation of the complexes [M
(HL1)] in good yields. In the 1H NMR spectrum of [Pd(HL1)] in
CDCl3, it is clear that two different pyrrole–imine environments
are present in a 2 : 1 ratio, and are consistent with two arms of
the tripodal ligand coordinated to Pd while the other remains
pendant. This assignment of the solution structure is supported
by the solid state structure determined by X-ray crystallography
(Fig. 1),‡ which shows that Pd1 is bound by L1 in a square
planar geometry by two of the pyrrole–imine arms of the tripodal
ligand, while the third is pendant and positioned above the axial
coordination site. This ligand is therefore adopting a hypodentate
coordination mode similar to that seen for complexes of, for
example, polydentate N-donor and scorpionate ligands in which
the metal ion binds using less that the maximum number of
donor atoms.15 Unlike in [Co(H3L1)2] and [Zn2(H2L1)2], which
EaStCHEM School of Chemistry, University of Edinburgh, Joseph Black
Building, The King’s Buildings, West Mains Road, Edinburgh EH9 3JJ,
UK. E-mail: jason.love@ed.ac.uk; Fax: +44 (0) 131 6504743
†Electronic supplementary information (ESI) available: Full experimen-
tal data and crystal data. CCDC 866705–866708. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/
c2dt30539a
Scheme 1 Synthetic approaches to imine and amide donor-extended
tripodal pyrroles.
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Fig. 1 Solid-state structure of [Pd(HL1)] (top) and its space-filled rep-
resentation (bottom). For clarity, all hydrogen atoms, except those
involved in hydrogen-bonding interactions, are omitted (where shown,
displacement ellipsoids are drawn at 50% probability). Selected bond
lengths (Å) and angles (°): Pd1–N1 1.944(2); Pd1–N2 2.084(2); Pd1–
N3 1.946(2); Pd1–N4 2.092(2); ∑∠Pd1 = 359.76.
Fig. 2 Solid-state structures of [Pd(ClH2L1)](CHCl3) (top) and [CuCl
(H2L1)] (bottom). For clarity, all hydrogen atoms, except those involved
in hydrogen-bonding interactions, and some solvent of crystallisation are
omitted (displacement ellipsoids are drawn at 50% probability). Selected
bond lengths (Å) and angles (°): (i) Pd1–N1 1.952(4); Pd1–N2 2.108(4);
Pd1–N3 1.955(4); Pd1–N4 2.116(4); N5⋯Cl1 3.175(4); N6⋯Cl1 3.138
(5); C50⋯Cl1 3.425(6); ∑∠Pd1 = 359.9; (ii) Cu1–Cl1 2.583(1); Cu1–
N1 1.948(3); Cu1–N2 2.085(3); Cu1–N3 1.947(3); Cu1–N4 2.064(3);
Cl1⋯N5 3.270(3); Cl1⋯N6 3.099(3); Cl1⋯C20 3.711(5); Cl1⋯C402
3.81(1); N1–Cu1–Cl1 99.90(8); N2–Cu1–Cl1 89.85(8).
adopt structures in which pyrrole–imine tautomerisation occurs
to form the azafulvene–amine isomer,12 the geometrical data of
the pendant arm confirms that the pyrrole–imine tautomer is pre-
ferred in this case. The orientation of the arms of the tripodal
ligand in [Pd(HL1)] impacts on the extended structure in which
two molecules are organised such that the hydrophobic and
bulky cyclohexyl substituents interdigitate to form a dimeric
motif (Fig. 1, bottom) with shortest C–C contacts of 3.74 Å. Fur-
thermore, C–H bonds of the cyclohexyl group of the pendant
arm interact with the vacant Pd axial site of an adjacent
complex, with close C–C, C–N, and C–Pd contacts ranging from
3.69–3.72 Å, and which results in an infinite dimeric chain
assembly in the solid state with Pd⋯Pd separations of 8.38 and
10.9 Å. While no crystals of the Cu analogue were grown, the
ESI spectrum displayed a molecular ion at m/z 650 and the sol-
ution magnetic moment of 2.23 BM supports the presence of the
d9 metal centre.
Similar reactions between the metal halide salts MCl2 (M =
Pd, Cu) and H3L1 resulted in the ready formation of the mono-
meric complexes [MCl(H2L1)] in which the pendant arm of the
tripodal ligand is protonated. Unlike the metal acetate reactions
above, the addition of NEt3 did not neutralise the complex to
form [M(HL1)] complexes, which suggests that the pKa of the
imine is >10.8. The 1H NMR spectrum of [Pd(ClH2L1)] in
CDCl3 supports Pd coordination, with two pyrrole–imine
environments in a 2 : 1 ratio.‡ In this case, both pyrrole
(13.3 ppm) and imine (12.9 ppm) nitrogens are protonated, the
latter resonance a multiplet due to coupling to cyclohexyl and
imine CH protons. Crystals of both complexes were analysed by
X-ray crystallography and the solid state structures display
similar gross geometries in which the Pd or Cu metal centre is
bound by two of the three pyrrole–imine arms while the third is
pendant and interacts with the chloride (Fig. 2).‡ In [Pd
(ClH2L1)], the chloride Cl1 is hydrogen-bonded to both imine
and pyrrole hydrogens and a molecule of CHCl3 solvent of crys-
tallisation and does not interact with the metal centre. In contrast,
in the Cu analogue [CuCl(H2L1)], Cu1 adopts a square-pyrami-
dal geometry in which the chloride Cl1 is coordinated to Cu1 in
the Jahn–Teller distorted axial position and located in an
assembled hydrogen-bonding pocket comprising N–H and C–H
donors from the pendant pyrrole–imine arm, a cyclohexyl group,
and MeCN solvent of crystallisation. It is therefore evident that
for metals that favour square planar geometries, L1 can be used
to straightforwardly construct complexes which address both the
primary and secondary coordination spheres, and act as plat-
forms for combined cation and anion binding.16
While pyrrole–amide compounds have been keenly evaluated
as receptors for anions,17 they have been little used as ligands in
transition metal chemistry even though the presence of the amide
group would be expected to alter the structural and electronic
characteristics of the complex.18 As such, we carried out the pro-
tonolysis reaction between the tripodal pyrrole–amides H6L2a/b
5786 | Dalton Trans., 2012, 41, 5785–5788
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and Ti(OiPr)4 in THF, which resulted in the clean elimination of
two equivalents of HOiPr and the formation of [Ti(OiPr)2(H4L2a/
b)] in good yields; no further HOiPr elimination was observed,
even at elevated temperatures.‡ In a similar manner to the imine
and amide functionalities on the secondary sphere chemistry of
these complexes, and are investigating routes to the cyclisation
of the square-planar donor set to facilitate complexation of a
range of metals.
We thank the University of Edinburgh and the UK EPSRC for
financial support, and Dr Claire Wilson for collecting the X-ray
data for [Ti(OiPr)2(H4L2b)].
1
complexes [Pd(HL1)] and [Pd(ClH2L1)], the H NMR spectra of
both [Ti(OiPr)2(H4L2a/b)] display two sets of resonances in a
2 : 1 ratio that support the coordination of two arms of the tripo-
dal ligand to the metal, with one arm remaining pendant. This
geometry is supported by the X-ray crystal structure of [Ti
(OiPr)2(H4L2b)] (Fig. 3) in which the Ti centre adopts a distorted
octahedral geometry comprising a N2O2 equatorial donor set
arising from the tripodal ligand and axial OiPr groups.‡ The two
OiPr groups are canted away from the pyrrolides subtending a
non-linear O4–Ti1–O5 angle, presumably as a result of steric
pressure from the pendant pyrrole–amide arm and the presence
of MeCN hydrogen-bonded to O4 (not shown). The presence of
the electropositive, oxophilic Ti metal centre results in preferen-
tial coordination of the amide group through its O-donor, and
allows the amide N–H to hydrogen bond to MeCN solvent of
crystallisation. In contrast to [CuCl(H2L1)], the pendant pyrrole–
amide arm in [Ti(OiPr)2(H4L2b)] does not interact with the axial
O-ligands but instead forms N–H⋯O hydrogen-bonds to a
neighbour which results in an infinite linear zig-zag motif in the
solid state.
Notes and references
‡Representative synthetic and crystal data: [Pd(HL1)] yellow solid in
62% yield (0.39 g). Analysis. Found: C, 63.81; H, 6.93; N, 12.67%
C35H46N6Pd requires C, 63.96, H, 7.05, N, 12.79%. Single crystals were
grown by cooling a saturated MeCN solution: C35H46N6Pd1, M 657.18,
ˉ
triclinic P1, a = 11.6995(6), b = 13.0619(7), c = 13.1928(7) Å, α =
115.965(5), β = 113.289(5), γ = 94.036(4)°, V 1590.89(15) Å3, 150(2)
K, Z = 2, 7757 independent reflections, R(int) 0.047, R[F2 > 2σ(F2)]
0.032, CCDC 866705.
[Pd(ClH2L1)] yellow solid in 60% yield (0.075 g). Analysis. Found:
C, 60.47; H, 6.68; N, 11.92% C35H47ClN6Pd requires C, 60.60, H, 6.83,
N, 12.12%. Single crystals were grown by slow diffusion of hexane into
a saturated CH2Cl2 solution: C36H48Cl4N6Pd, M 813.00, monoclinic
P21/c, a = 17.130(2), b = 11.0690(14), c = 20.818(3) Å, α = γ = 90, β =
108.888(14)°, V 3734.7(8) Å3, 150(2) K, Z = 4, 4544 independent
reflections, R(int) 0.057, R[F2 > 2σ(F2)] 0.042, CCDC 866706.
[CuCl(H2L1)] Green solid in 65% yield (0.37 g). Analysis. Found: C,
64.64; H, 7.32; N, 12.84% C35H47CuClN6 requires C, 64.59, H, 7.28,
N, 12.91%. Single crystals were grown by slow diffusion of Et2O into a
saturated MeCN solution: C75.25H105.75Cl2Cu2N12.5O,
M 1399.45,
We have shown that the primary and secondary spheres in
complexes of the pyrrole–imine ligand L1 and the pyrrole–amide
ligand L2 can be managed through judicious choice of metal.
For L1, square planar CuII and PdII cations favour a porphyrin-
like N4-donor set made up of two of the three arms of the poten-
tially tripodal ligand, thus leaving one arm pendant and able to
define the acid–base characteristics of the secondary sphere
environment. A similar structural motif is favoured in a Ti alkox-
ide complex of L2, and suggests that the formation of truly tripo-
dal complexes of these ligands is generally inhibited, with the
N4- or N2O2 square-planar donor environments dominant. We
are currently evaluating the impact of the pendant pyrrole imine
orthorhombic Pna21, a = 16.9395(3), b = 16.1910(3), c = 26.7675(5) Å,
α = β = γ = 90°, V 7341.5(2) Å3, 150(2) K, Z = 4, 14 997 independent
reflections, R(int) 0.041, R[F2 > 2σ(F2)] 0.046, CCDC 866707.
[Ti(OiPr)2(H4L2b)] colourless solid in 66% yield (0.13 g). Analysis.
Found: C, 56.01; H, 6.37; N, 15.18% C26H35N6O5Ti requires C, 55.72,
H, 6.47, N, 14.99%. Single crystals were grown by slowly cooling a
saturated MeCN solution: C32H45N9O5Ti, M 683.67, monoclinic P21/c,
a = 11.704(10), b = 35.30(3), c = 9.483(12) Å, α = γ = 90, β = 112.407
(12)°, V 3622(7) Å3, 160(2) K, Z = 4, 6022 independent reflections, R
(int) 0.124, R[F2 > 2σ(F2)] 0.087, CCDC 866708.
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1.806(3); Ti1–O5 1.794(3); N2⋯N8 2.934(7); N4⋯N7 3.040(7);
O3⋯N6′ 2.793(7); O1–Ti1–O2 128.1(1); O4–Ti1–O5 150.3(2); O1–
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