Toward permetalated alkyne/azide 3 + 2 or "click" cycloadducts

Article abstract of DOI:10.1021/om300542a

The Cu(I)-catalyzed reaction of the platinum butadiynyl complex trans-(C₆F₅)(p-tol₃P)₂Pt(C≡C) ₂H and rhenium cyclopentadienylazide complex (η⁵- C₅H₄N₃)Re(CO)₃ yields the 1,2,3-triazole trans-(C₆F₅)(p-tol₃P) ₂PtC≡CC=CHN((η⁵-C₅H ₄)Re(CO)₃)N=N (54%), which upon treatment with Re(CO) ₅OTf or Re(CO)₅Br affords Re(CO)₅ ⁺TfO^- or cis-Re(CO)₄Br adducts derived from attack at N(3) (84-98%). The crystal structures of solvates of these three complexes are compared, but attempts to metalate the =CH groups were unsuccessful. However, the reaction of monometallic trans-(C₆F ₅)(p-tol₃P)₂PtC≡CC=CHN(CH ₂C₆H₅)N=N and MeI gives an N(3)-methyl triazolium salt (81%), which upon addition of Ag₂O and [RhCl(cod)]₂ yields the N-heterocyclic carbene complex trans-(C ₆F₅)(p-tol₃P)₂PtC≡CC ^...C(RhCl(cod))^...N(CH₂C₆H ₅)^...N^...N(Me). Further experiments and analyses suggest that =CH derivatization is possible only when the ¹H NMR chemical shift is downfield of ca. δ 7.9 ppm. Electropositive metal fragments generally effect upfield shifts, meaning that insulating spacers, electronegative ligands, and/or new chemical methodologies will be required to construct tetrametallic arrays via click chemistry.

Full text of DOI:10.1021/om300542a

Communication
pubs.acs.org/Organometallics
Toward Permetalated Alkyne/Azide 3 + 2 or “Click” Cycloadducts
Melissa C. Clough, Paul D. Zeits, Nattamai Bhuvanesh, and John A. Gladysz*
Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, Texas 77842-3012, United States
S
* Supporting Information
ABSTRACT: The Cu(I)-catalyzed reaction of the platinum
butadiynyl complex trans-(C6F5)(p-tol3P)2Pt(CC)2H and rhe-
nium cyclopentadienylazide complex (η5-C5H4N3)Re(CO)3 yields
the 1,2,3-triazole trans-(C6F5)(p-tol3P)2PtCCCCHN((η5-
C5H4)Re(CO)3)NN (54%), which upon treatment with Re-
(CO)5OTf or Re(CO)5Br affords Re(CO)5 TfO− or cis-Re-
+
(CO)4Br adducts derived from attack at N(3) (84−98%).
The crystal structures of solvates of these three complexes are
compared, but attempts to metalate the CH groups were
unsuccessful. However, the reaction of monometallic trans-
(C6F5)(p-tol3P)2PtCCCCHN(CH2C6H5)NN and MeI
gives an N(3)-methyl triazolium salt (81%), which upon addition of Ag2O and [RhCl(cod)]2 yields the N-heterocyclic carbene
complex trans-(C6F5)(p-tol3P)2PtCCC...C(RhCl(cod))...N(CH2C6H5)...N...N(Me). Further experiments and analyses suggest
that CH derivatization is possible only when the 1H NMR chemical shift is downfield of ca. δ 7.9 ppm. Electropositive metal
fragments generally effect upfield shifts, meaning that insulating spacers, electronegative ligands, and/or new chemical
methodologies will be required to construct tetrametallic arrays via click chemistry.
olecular frameworks that can incorporate tailor-made
In recent work, Viege has demonstrated that the gold azide and
alkynyl complexes Ph3PAuN3 and PhCCAuPPh3 undergo an
uncatalyzed 3 + 2 cycloaddition at room temperature to give the
digold complex 4 shown in Scheme 1 (bottom) in 88% yield after
workup.7 In this study, we report the first click-derived trimetallic
triazole derivatives, attendant crystallographic data, and results
relevant to further metalations.
Accordingly, equimolar quantities of the platinum butadiynyl
complex trans-(C6F5)(p-tol3P)2Pt(CC)2H (2a)11 and the
rhenium cyclopentadienylazide complex (η5-C5H4N3)Re(CO)3
(5)12 were combined in DMF. As shown in Scheme 2,
CuSO4·5H2O (1.6 mol %), ascorbic acid (2.3 mol %), and
water were added. Workup gave the anticipated product, the
heterobimetallic cycloadduct 6, as a white solid in 54% yield.
Complex 6, and all other new compounds isolated below,
were characterized by NMR (1H, 13C, 31P) and IR spectros-
copy and microanalysis, as summarized in the Supporting
Information. The spectroscopic properties of 6 combined
features typical of monosubstituted cyclopentadienyl com-
plexes of rhenium tricarbonyl with those of the adduct 3a in
Scheme 1.
M
heteromultimetallic arraysto wit, M, M′, M″, M‴, M⁗
or, more ambitiously, aM, bM′, cM″, dM‴, eM⁗might serve as
precursors to unusual bulk or nanoscopic species. To our
knowledge, the most advanced efforts along these lines are
those of Lang,1,2 who has synthesized a molecule with one of
each of six different metal atoms: iron, ruthenium, rhenium, gold,
copper, and titanium (1). As shown in Scheme 1 (top), this
species features carbon-rich alkyne and/or arene bridging units.
However, certain metal atoms in 1 would be challenging to vary;
a number of limitations are obvious.
The copper catalyzed “click” 3 + 2 cycloaddition of terminal
alkynes and azides to give 1,2,3-triazoles3 is seeing ever-
increasing use in metal coordination spheres,4,5 and similar
reactions without added catalysts have also been reported.6,7 As
depicted in Scheme 1 (middle), we have shown that the platinum
polyynyl complexes trans-(C6F5)(p-tol3P)2Pt(CC)nH (2;
n = 2−6) efficiently undergo click reactions with organic azides
to give 3.8 Others have employed ethynyl moieties appended to
various metal ligands,4 as well as azido species that incorporate
metals.4e,6,7 In any case, the resulting triazoles have two
additional sites that are amenable to functionalization: (a) the
nucleophilic lone pair of the unsubstituted nitrogen atom (N3)
that is remote from the substituted nitrogen atom (N1) and
(b) the moderately acidic proton of the CH moiety,9
abstraction of which can lead to an “abnormal” or “mesionic”10
N-heterocyclic carbene complex.
The introduction of a third metal was sought. Accordingly, 6
was combined with 1.15 equiv of the rhenium triflate
Re(CO)5OTf13 in CH2Cl2. After 5 days at room temperature,
workup gave the triazolium triflate salt 7+TfO−, derived from
electrophilic attack at N(3), as a white solid in 84% yield.
Hence, 1,2,3-triazole cores could seemingly serve as versatile
templates for assemblies containing four different transition-
metal atoms, and we set out to explore protocols toward this end.
Received: June 15, 2012
Published: July 24, 2012
© 2012 American Chemical Society
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The NMR and IR properties of 7+TfO− were similar to those
of 6, but with signals characteristic of a Re(CO)5 fragment,
suggesting the structure depicted in Scheme 2. An analogous
reaction of 6 and the less electrophilic rhenium bromide
Re(CO)5Br was slower. After 10 days, workup gave the solvate
8·2CH2Cl2 as a white solid in 98% yield. The IR and other data
suggested the nonionic structure shown in Scheme 2.
Scheme 1. A Complex with Six Different Transition-Metal
Atoms (Top) and Some Previously Reported Alkyne/Azide
3 + 2 Cycloadditions in Metal Coordination Spheres (Middle,
Catalyzed; Bottom, Uncatalyzed)
Single crystals of the solvates 6·2CH2Cl2, 7+TfO−·CH2Cl2,
and 8·2CH2Cl2 were grown, and the X-ray structures were
determined as described in the Supporting Information. The
thermal ellipsoid diagrams in Figures 1−3 confirm the proposed
Figure 1. Thermal ellipsoid plot (50% probability level) of the
molecular structure of 6·2CH2Cl2 with the solvate molecule omitted.
Key bond lengths and angles: Pt1−C1 = 1.998(4), C1C2 = 1.210(5),
C2−C3 = 1.422(6), C3C4 = 1.368(5), C4−N1 = 1.345(5), N1−N2
= 1.348(4), N2N3 = 1.317(4), N3−C3 = 1.380(5); Cipso−Pt1−C1 =
170.00(15), Pt1−C1−C2 = 167.7(3), C1−C2−C3 = 171.8(4).
Scheme 2. Triply Metalated Alkyne/Azide 3 + 2
Cycloadducts
Figure 2. Thermal ellipsoid plot (50% probability level) of the
molecular structure of 7+TfO−·CH2Cl2 with the anion and solvate
molecule omitted. Key bond lengths and angles: Pt1−C1 = 2.001(7),
C1C2 = 1.202(9), C2−C3 = 1.414(9), C3C4 = 1.352(10), C4−
N1 = 1.351(9), N1−N2 = 1.316(7), N2N3 = 1.288(7), N3−C3 =
1.373(8); Cipso−Pt1−C1 = 175.8(3), Pt1−C1−C2 = 171.7(6), C1−
C2−C3 = 169.4(7).
formulations. In the last structure, the bromide ligand of the cis-
Re(CO)4Br moiety was disordered over two positions (84:16).
With the cationic complex 7+TfO−·CH2Cl2, the N1−N2 and
N2N3 bond lengths (1.316(7) and 1.288(7) Å) were slightly
shorter than those in the others (1.343(6)−1.348(4) and
1.323(6)−1.317(4) Å). Analogous N1−N2 contractions have
been observed upon alkylations of triazoles.14 As seen in most
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Scheme 3. Reactions Relevant to a Fourth Metalation of
Alkyne/Azide 3 + 2 Cycloadducts
Figure 3. Thermal ellipsoid plot (50% probability level) of the dominant
conformation of the molecular structure of 8·2CH2Cl2 with the solvate
molecules omitted. Key bond lengths and angles: Pt1−C1 = 1.995(5),
C1C2 = 1.207(7), C2−C3 = 1.407(7), C3C4 = 1.387(8), C4−N1
= 1.351(6), N1−N2 = 1.343(6), N2N3 = 1.323(6), N3−C3 =
1.387(7); Cipso−Pt1−C1 = 177.6(2), Pt1−C1−C2 = 173.4(4), C1−
C2−C3 = 166.6(5).
other platinum complexes in this series, C6H4CH3/C6F5/
C6H4CH3 stacking interactions were evident.8
Efforts were then directed at introducing a fourth metal at the
triazole CH moiety. Recipes were tested that successfully
afforded N-heterocyclic carbene complexes from 1,3,4-trisub-
stituted triazolium salts and related heterocycles.9 Unfortunately,
no reactions were observed upon treatment of 7+TfO− or
8·2CH2Cl2 with Ag2O or Cu2O in CH2Cl2 at 25−40 °C.
Combinations of Ag2O and [RhCl(cod)]2 or PdCl2(NCMe)2, or
[IrCl(cod)]2 and KOtBu,15 also gave no reactions. In view of the
structural data in Figures 1−3, and the successful metalation of
triazolium salts with analogous arrays of organic substituents,9 it
was considered unlikely that these failures were due to steric
effects. In order to probe for electronic effects, the monometallic
triazolium salts 9+TfO− and 10+I− were prepared as shown in
Scheme 3.
Table 1. CH 1H NMR Data for Compounds with the
Structural Unit I
The 1H NMR chemical shifts of the triazole or triazolium 
CH moieties in the preceding compounds proved to be
particularly informative. These are summarized in Table 1,
together with that of the purely organic salt C6H5CCHN-
(Et)NN(Me)]+I− (12+I−).9a This compound readily reacts
with Ag2O to give a cationic bis(triazolylidene) silver complex
that is easily transmetalated. Importantly, 12+I− exhibits the
most downfield CH signal (δ 9.52 ppm, CDCl3), sugges-
tive of the greatest acidity. In the remaining complexes, the
CH signal generally shifts upfield as the number of electro-
positive metal substituents increases (δ 8.01−6.15 ppm for one
metal fragment, δ 6.26 ppm for two, and δ 6.03−5.81 ppm for
three).
a
b
c
ArL2 = trans-(C6F5)(p-tol3P)2. In CDCl3. In CD2Cl2.
the bimetallic N-heterocyclic carbene complex 11 shown in
1
Scheme 3 occurred, as assayed by H NMR and evidenced
by a characteristic 13C NMR signal at δ 168.6 ppm (d, JCRh
=
1
46.9 Hz).9a,c Accordingly, 10+I− exhibits the second most
1
deshielded CH H NMR signal in Table 1 (δ 8.01 ppm,
The monometallic triazolium salt 9+TfO− (δ 7.83 ppm,
CDCl3) was similarly inert under the conditions investigated for
the further metalation of the trimetallic complexes 8·2CH2Cl2
and 7+TfO−. However, the salt 10+I−, in which the metal
fragment is insulated from the triazolium core by a CC unit,
reacted with Ag2O and [RhCl(cod)]2. Clean conversion to
CD2Cl2). Hence, a chemical shift of ca. δ 7.9 ppm seems to
represent an operational cutoff for elaborating this carbon atom
to a carbene complex under the conditions investigated.
In summary, this work has significantly expanded the scope
of “click” chemistry in metal coordination spheres. Given that
alkynyl, azide, and electrophilic derivatives of every transition
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assembly of virtually any hetereotrimetallic combination.
However, a comparatively versatile introduction of a fourth
metal onto the triazole or triazolium core must await one of two
developments. The most desirable would be the discovery of
new CH functionalization methodologies that are not as
sensitive to proton acidity. Alternatively, more electronegative
metal fragments could be employed, possibly in conjunction with
spacers that would enhance insulation from the heterocycle core.
Efforts toward these ends will be reported in due course.
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ASSOCIATED CONTENT
* Supporting Information
■
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Text, figures, and tables giving experimental procedures and
crystallographic data. This material is available free of charge via
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AUTHOR INFORMATION
Corresponding Author
*Tel: 979-845-1399. E-mail: gladysz@mail.chem.tamu.edu.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the U.S. National Science Foundation (CHE-0719267
and CHE-1153085) for support.
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