Inorganic click (iClick) synthesis of heterotrinuclear Pt II/AuI2 complexes

Article abstract of DOI:10.1039/c3dt52105b

Metal-azide-metal-acetylide cycloaddition (iClick) reactions to synthesize heterotrimetallics and an unexpected novel tetranuclear gold(i) complex, are described. In addition, a discussion regarding the connection between traditional azide-alkyne cycloaddition reactions and iClick is presented focusing on applications towards linking multiple metal ions.

Full text of DOI:10.1039/c3dt52105b

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Inorganic click (iClick) synthesis of heterotrinuclear
II
I
Pt /Au complexes†
Cite this: Dalton Trans., 2013, 42, 14963
2
Received 2nd August 2013,
Andrew R. Powers, Xi Yang, Trevor J. Del Castillo, Ion Ghiviriga, Khalil A. Abboud
and Adam S. Veige*
Accepted 4th September 2013
DOI: 10.1039/c3dt52105b
www.rsc.org/dalton
Metal-azide–metal-acetylide cycloaddition (iClick) reactions to
Fig. 1 describes an analogy between iClick and the most
synthesize heterotrimetallics and an unexpected novel tetra- notable Click reaction, CuAAC. Importantly, it shows how this
nuclear gold(I) complex, are described. In addition, a discussion synthetic technique potentially translates to the construction
regarding the connection between traditional azide–alkyne cyclo- of multi-metallic materials. One notable difference is CuAAC
addition reactions and iClick is presented focusing on applications reactions normally provide disubstituted 1,4-triazoles, whereas
towards linking multiple metal ions.
in iClick reactions, trisubstituted 1,4,5-triazoles form selec-
tively. Fig. 1A depicts a prototypical CuAAC and the analogous
reaction between a metal-azide and a metal-acetylide. In
Fig. 1B, a metal-diacetylide reacts with two equivalents of
metal-azide or vice versa (not drawn) to give trimetallic com-
plexes, analogous to a double azide cycloaddition to an arene-
diyne. Another illustration of this concept appears in Fig. 1C.
A mixed metal-azide–acetylide combines to create a metallo-
1
I
Few reactions rival the utility of the Click classified Cu -cata-
lyzed azide–alkyne cycloaddition (CuAAC) for its ease of use
in building molecules with high yield, regiospecificity, and
atom economy. Multiple review articles detail the diverse appli-
2
3
cations of both the Huisgen 1,3-dipolar cycloaddition and
2
CuAAC, with special attention given to building macro-
4
molecules. It is exciting to imagine the extension of this
polymer, analogous to step-growth polymerization of azide–
powerful synthetic tool by incorporating metal ions. Azide–
4a–d
alkyne monomers.
In the case where M = M′ a homometal-
5
alkyne cycloadditions within a metal coordination sphere and
lic material forms; in the case of M ≠ M′ the result is a per-
6
pendant ligands are known, though only recently the direct
fectly alternating heterometallic material. In Fig. 1D, step
linking of two metal ions via azide–acetylide cycloaddition was
4a–d
growth of organo-diazides with diacetylenes
are akin to a
6
achieved. We reported the iClick reaction involving the cyclo-
reaction between a metal-diazide (M–(N ) ) and a metal-di-
3
2
addition of a metal-azide‡ with a metal-acetylide to yield a
2
acetylide (M′–(CuCR) ).
1
,4,5-substituted 1,2,3-triazolate bridged dimetallic complex.
Here, we present significant progress towards realizing this
new synthetic method for linking metal ions between triazo-
late bridges by extending, both literally and figuratively, iClick
to the synthesis and characterization of heterotrimetallic
I
I
Specifically, PPh
3
Au N
3
I
(1-Ph) reacts with PPh
3
Au CuCC
6
H
5
(
2) to give 1-5-[PPh Au ] (µ-N C -4-C H ) (3-Ph; where, N C =
3
2
3
2
6
5
3 2
6
1
,2,3-triazolate) in 88% isolated yield (eqn (1)).
II
I
Pt /Au complexes.
2
ð1Þ
University of Florida, Center for Catalysis, Department of Chemistry,
P.O. Box 117200, Gainesville, Florida 32611, USA. E-mail: veige@chem.ufl.edu;
Fax: +1 352-392-3255; Tel: +1 352-392-9844
†
Electronic supplementary information (ESI) available: NMR spectra, X-ray data,
and experimental details. CCDC 931002–931005. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c3dt52105b
Fig. 1 CuAAC analogy with metal-azide–metal-acetylide iClick reactions.
Dalton Trans., 2013, 42, 14963–14966 | 14963
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product forming first, followed by attack of the second equi-
valent of 5-Ph from the opposite, less sterically hindered, side
of the square plane.
ð2Þ
ð3Þ
II
In an example of case B (Fig. 1), treating cis-(PPh ) Pt (N )
3
2
3 2
7
I
(
4-Ph) with two equivalents of PPh
3
Au CuCC
6
H
4
NO
2
(5-Ph;
8
where C H NO = p-nitrophenyl) in benzene (50 °C) provides
6
4
2
II
the yellow heterotrimetallic complex cis-(PPh ) Pt -1,5-
3
2
Replacing all the PPh ligands with more basic, yet coni-
3
I
[
3 3 2 6 4 2 2
PPh Au (µ-N C -4-C H NO )] (cis-6-Ph) in 92% isolated yield
via convenient filtration from the reaction medium (eqn (1)). A
P{ H} NMR spectrum of cis-6-Ph exhibits two resonances, a
ligands bonded to Au , and a
singlet at 7.79 ppm featuring satellites ( JPtP = 3095.32 Hz), for
the PPh with platinum connectivity. This reaction embodies
the Click mantra, as it proceeds in very high yields, with high
regioselectivity, and provides an isolable product that is syn-
thesized from easily obtained starting materials. P NMR data
indicates a single cycloaddition occurs first; the phosphorous
resonance due to 4-Ph disappears with concomitant appear-
ance of two doublets at 8.09 ( JPP = 21.27; JPtP = 3093.47 Hz)
and 11.19 ppm ( JPP = 21.99; JPtP = 3573.66 Hz), indicating the
cally smaller PEt
way. Treating cis-(PEt
3
ligands alters the chemistry in a notable
) (4-Et) in benzene with two
3 2
equivalents of PEt Au CuCC H NO (5-Et) provides the yellow
II
9
3
I
)
2
Pt (N
3
1
1
3
6
4
2
I
singlet at 43.53 ppm for PPh
3
II
I
heterotrimetallic complex cis-(Et
-C NO )] (cis-6-Et) as a crystalline solid. However, two
other products also precipitate from the reaction mixture (in
3 2 3 3 2
P) Pt -1,5-[PEt Au (µ-N C -
4
6
H
4
2 2
3
31
1
the ratios depicted in eqn (3)), as determined by P{ H} NMR
spectroscopy (CDCl ), and both are identifiable (see ESI†)
eqn (3)). Fortunately, the major component of the mixture,
the double iClick product cis-6-Et, is isolable in 35% yield as
single crystals via fractional crystallization from benzene.
3
3
1
(
Again, the product contains a pseudo-C -symmetric square
planar Pt ion with cis-phosphines and triazolate links to anti
2
II
II
phosphorus nuclei bound to Pt are no longer equivalent. As
I
disposed Au PEt
3
units. A notable structural difference
the reaction proceeds, resonances attributable to the single
cycloaddition product diminish concomitant with the emer-
gence of cis-6-Ph signals.
between cis-6-Et (Fig. 3) and cis-6-Ph is that the triazolate rings
in cis-6-Ph are nearly perpendicular (∼83.71°) to the square
plane, whereas in cis-6-Et the rings are significantly canted
The X-ray crystal structure of cis-6-Ph, confirms the result of
a double iClick (Fig. 2). The asymmetric unit cell consists of
four chemically equivalent, but crystallographically indepen-
dent molecules, each of which have a very similar geometry (as
such, and for clarity, only one of these structures is shown in
(
∼66.20°), most likely a result of the relaxed steric congestion
within cis-6-Et.
A minor product that is the result of ligand exchange is
II
complex 8. Azide exchange for acetylide at the Pt ion, and a
single cycloaddition provides complex 8. Multinuclear and 2-D
NMR spectroscopy data provides reasonable evidence for the
identity of 8, though its isolation from the product mixture
was not possible. In a somewhat cleaner reaction, treating
trans-(PEt ) Pt(N ) with two equivalents of 5-Et provides 8;
3 2 3 2
however, a small amount of inseparable 7 (5%; NMR) also
Fig. 2). Complex cis-6-Ph comprises a pseudo-C -symmetric
2
II
square-planar Pt ion bearing cis-PPh groups and two triazo-
units. Interestingly, the triazolate-Au-
PPh ligands are positioned on opposite (anti) sides of the Pt
3
I
late bridges to Au PPh
3
II
3
square plane. This is consistent with a single cycloaddition
forms, thus precluding combustion analysis.
Fig. 2 Solid state molecular structure of cis-6-Ph. Hydrogen atoms and lattice
solvent molecules are removed for clarity. Only one of four chemically equi-
valent, but crystallographically independent complexes from the asymmetric
unit cell is depicted in the figure. Selected bond lengths (Å) and angles (°): Pt1–
N1 = 2.040(7), Pt1–N5 = 2.073(7), Au1–C1 = 2.036(10), Au2–C27 = 2.020(10),
N1–Pt1–N5 = 86.2(3), N5–Pt1–P4 = 89.3(2), P4–Pt1–P3 = 98.25(9), P3–Pt1–N1 =
Fig. 3 Solid state molecular structure of cis-6-Et. Hydrogen atoms and dis-
ordered atoms are removed for clarity. Selected bond lengths (Å) and angles (°):
Pt1–N1 = 2.064(7), Pt1–N5 = 2.057(7), Au1–C1 = 2.044(9), Au2–C27 = 2.017(9),
N1–Pt1–N5 = 87.1(3), N5–Pt1–P3 = 85.9(2), P3–Pt1–P4 = 102.05(8), P4–Pt1–N1
= 85.3(2), C1–Au1–P1 = 175.9(3), C27–Au2–P2 = 175.4(3).
8
6.3(2), C1–Au1–P1 = 176.0(3), C27–Au2–P2 = 173.6(3).
1
4964 | Dalton Trans., 2013, 42, 14963–14966
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The third component, complex 7, forms from Au -acetylide–
Communication
I
I
I
azide ligand exchange and subsequent homo Au –Au cyclo-
addition to provide the intermediate digold complex 9, which
then dimerizes to yield the unique tetragold cluster complex 7.
Evidence for this sequence of events comes from an alternative
ð5Þ
1
0
synthesis of 7 by employing 9 purposely generated in situ
I
(eqn (3)). Treating 5-Et with PEt
3 3
Au N (1-Et) provides 7 in 88%
Orange single crystals of 11 deposit from the reaction
medium (CHCl ) over the course of 48 h. Evidence for the
3
yield.
identity and purity of 11 comes from a X-ray diffraction experi-
ment performed on a single crystal (Fig. 5), NMR spectroscopy
in DMSO-d (11 has low solubility in benzene, chloroform, and
6
ethereal solvents), and combustion analysis. Complex 11 con-
ð4Þ
II
tains a square-planar Pt ion with cis-PEt3 ligands, and a
I
I
unique chelating bis-triazolate ligand that features a Au –Au
interaction within the chelating fragment. The average Au–Au
distance of 3.1878(10) Å is within the range of similar
I
I
crystallographically characterized Au –Au
An X-ray crystal structure of 7 provides its solid state struc- bridged complexes.11
ture, which comprises two dinuclear gold units held together
Complex 11 is C -symmetric in the solid state, but is C -
by aurophilic interactions to create a pseudo C -symmetric dis- symmetric in solution. A H NMR spectrum of 11 features dia-
diphosphine
1
s
1
2
I
torted tetrahedral geometry of Au ions (Fig. 4). The four Au– stereotopic methylene protons that are well separated and
Au interactions range from 3.0099(11) to 3.0923(11) Å with an appear as a doublet of triplets, indicative of the C -symmetry
s
average of 3.0527(11) Å, well within the acceptable length for a and their unique chemical environments (δ 5.20 ppm, dt, JPH
=
1
1
bond. Distortion from a pure tetrahedron is due to the triazo- 13.4 and 13.4 Hz; δ 3.70 ppm, dt, J = 14.5 and 10.9 Hz). A
3
1
1
late bridges that prevent close contacts between Au1–Au3
P{ H} NMR spectrum of 11 indicates each set of phos-
(
3.9357(11) Å) and Au2–Au4 (3.9121(11) Å), which are well phorous atoms are equivalent (Au–P = 36.4 ppm; Pt–P =
I
above the sum of two van der Waals radii for Au (3.80 Å).
−1.6 ppm); a fluxional ring flip occurs in solution resulting in
In both cis-6-Ph and cis-6-Et, the Au ions orient anti a mirror plane of symmetry that bisects the methylene carbon
I
I
I
II
relative to each other. To create a Au –Au interaction, and to and Pt ion.
demonstrate the synthetic diversity of iClick, we sought to
In conclusion this work establishes iClick as a convenient
method for linking multiple metal ions across triazolate
I
II
forcibly constrain the Au ions on the same side of the Pt
I
square plane. Tethering the Au -acetylide units across a bridges to build heterotrimetallic complexes. In the vein of
bis(diphenylphosphino)methane (dppm) bridge provides building multi-metallic materials, the iClick reaction is unique
the necessary structural constraint. Treating 4-Et with because the synthesis of well-defined heterometallic complexes
I
[
Au (CuCC
6
H
4
NO
2
)]
2
(µ-dppm) (10) provides the trimetallic is possible by simple choice of metal-azide–metal-acetylide
II
I
Pt /Au complex 11 in 36% yield.
pairing. In one case presented, smaller and more basic PR3
ancillary ligands lead to a mixture of products including the
unique tetragold cluster 7. A point worthy of discussion is the
observed exclusive formation of 1,4,5-triazolate isomers, in
2
Fig. 4 Solid state molecular structure of 7. Hydrogen atoms and disordered
atoms are removed for clarity. Selected bond lengths (Å) and angles (°): Au1–N1 =
Fig. 5 Solid state molecular structure of 11. Hydrogen atoms and disordered
atoms are removed for clarity. Selected bond lengths (Å) and angles (°): Au1–
2
.062(12), Au2–N5 = 2.052(13), Au3–C1 = 2.039(15), Au4–C9 = 2.061(18), Au1–
Au2 = 3.0245(11), Au2–Au3 = 3.0842(11), Au3–Au4 = 3.0099(11), Au4–Au1 =
Au2
= 3.1631(11), Pt1–N1 = 2.025(15), Pt1–N5 = 2.042(14), Au1–C11 =
3
.0923(11), Au1–Au3
=
3.9360(11), Au2–Au4
=
3.9120(11), P1–Au1–N1
=
2.055(14), Au2–C1 = 2.036(16), P1–Pt1–P2 = 94.4(3), P2–Pt1–N5 = 92.7(5), N5–
Pt1–N1 = 84.7(6), N1–Pt1–P1 = 88.5(4).
1
71.9(6), P2–Au2–N5 = 170.1(7), P3–Au3–C1 = 172.1(7), P4–Au4–C9 = 173.0(7).
This journal is © The Royal Society of Chemistry 2013
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I
which the Au ion is in the 5-position. At first this may appear
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CuAAC. However, in the generally accepted CuAAC mechanism,
a Cu substituted 1,4,5-triazolate intermediate forms; sub-
sequent hydrolysis places a proton in the 5-position producing
the 1,4-triazole isomer (eqn (5)). In iClick (eqn (6)), hydrolysis
does not occur (though it could, depending on the choice of
metal ion and conditions), thus arresting the reaction at the
I
1
,4,5-triazolate intermediate, which coincidentally provides
I
indirect support for the role of Cu in CuAAC. Work continues
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6
Notes and references
7
8
‡
Note: metal-azides are known to be explosive and should be handled with the
appropriate level of precaution and safety measures.
1
2
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1
4966 | Dalton Trans., 2013, 42, 14963–14966
This journal is © The Royal Society of Chemistry 2013