1,2,3-Triazolylidene based complexes via post-modification of pincer click ligands

Article abstract of DOI:10.1039/c1dt10264h

Preparation of 1,2,3-triazolylidene metal complexes by simple alkylation of triazolyl based parent species was demonstrated for the first time. Based on this post-modification approach, unprecedented tridentate Pd and Pt complexes bearing a 1,2,3-triazolylidene core were synthesized. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1dt10264h

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This article is published as part of the Dalton Transactions themed issue entitled:
Pincers and other hemilabile ligands
Guest Editors Dr Bert Klein Gebbink and Gerard van Koten
Published in issue 35, 2011 of Dalton Transactions
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Martin Albrecht and Monika M. Lindner
Dalton Trans., 2011, DOI: 10.1039/C1DT10339C
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CSC-pincer versus pseudo-pincer complexes of palladium(II): a comparative study on
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COMMUNICATION
1,2,3-Triazolylidene based complexes via post-modification of pincer click
ligands†
Elaine M. Schuster, Mark Botoshansky and Mark Gandelman*
Received 17th February 2011, Accepted 24th March 2011
DOI: 10.1039/c1dt10264h
Preparation of 1,2,3-triazolylidene metal complexes by simple
alkylation of triazolyl based parent species was demonstrated
for the first time. Based on this post-modification approach,
unprecedented tridentate Pd and Pt complexes bearing a
1,2,3-triazolylidene core were synthesized.
Since the inception of stable carbenes, they have received
extraordinary attention in homogeneous catalysis both as free
molecules4 and as ligands for transition metals.5 In addition to
classical N-heterocyclic carbenes 3, those with lesser degrees of
heteroatom stabilization, known as abnormal carbenes (species 4–
6), have been recently of great interest.6 Albrecht and coworkers
have recently extended the family of abnormal carbene ligands
by preparation of 1,2,3-triazolylidene metal complexes (Fig. 2,
ii) from the corresponding 1,2,3-triazolium salt.7,8 This type
of carbenes is especially attractive due to the simplicity and
versatility of preparation of the triazole scaffold by click chemistry.
Moreover, Bertrand and coworkers successfully isolated these
free “mesoionic carbenes” 5 by deprotonation of their triazolium
precursors, and probed their electronic properties.9
Tridentate pincer-type ligands D1CD2 have found a variety of
valuable applications in different areas of chemistry (Scheme
1, 1).1 The pincer structure affords its metal complex a high
stability, which is widely attributed to the protective, sheltered
environment in which the metal is situated. Additionally, steric
and electronic properties of the metal center can be fine-tuned
by versatile selection of the chelating arms of the pincer ligand.
The backbone of the pincer systems can also be used for remote
electronic modulation of metal center. All these intrinsic features
of the pincer system have generated extensive research into the use
of these complexes.
Recently, we reported the preparation of a new class of pincer
ligands utilizing a triazole unit as the central core motif (Fig. 1, 2).2
Apart from the conceptually distinct route to the preparation of
the tridentate ligands, our system 2 differs from the conventional
species 1 by bearing a functional triazole backbone. Indeed,
we have demonstrated that the triazole-based ligands 2 can
be complexed to late-transition metals in both tridentate and
bidentate (involving nitrogen atoms of triazole) coordination
modes.3 The “non-innocent” behavior of the triazole backbone
sparked our interest in further exploring the ability to modify this
core and thus affect its ligand properties.
Fig. 2 (i) Some selected examples of abnormal carbenes. (ii) Albrecht’s
Pd-triazolylidene complex from the triazolium salt.
While there exist a variety of methods to prepare conventional
and abnormal carbene ligands, most of them utilize corresponding
free heterocyclic derivatives as a starting material, which after
some manipulations result in a metal-carbene species. The most
popular approach in this respect is activation of C–H or C-halogen
bond of the heterocyclic salts. Although there are few instances
when carbenes are prepared by post-modification of a covalently
bound heterocyclic ring (e.g., alkylation or protonation of pyridyl
and pyrolyl ligands),10 the approach has not yet been explored for
the preparation of 1,2,3-triazolylidene complexes. Here we report
that 1,2,3-triazolylidene ligands can be prepared by simple post-
modification of triazolyl-transition metal complexes. Namely, we
Fig. 1 Typical phenyl- (1) and triazolyl-based (2) pincer ligands. D
represents donor groups.
Schulich Faculty of Chemistry, Technion- Israel Institute of Technology,
Haifa, 32000, Israel. E-mail: chmark@techunix.technion.ac.il; Fax: +972-
4-8293703; Tel: +972-4-8295951
† Electronic supplementary information (ESI) available. CCDC reference
number 812268. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c1dt10264h.
8764 | Dalton Trans., 2011, 40, 8764–8767
This journal is
The Royal Society of Chemistry 2011
©

demonstrate that the covalently metal s-bonded triazolyl ligand
can be converted to its carbene-type counterpart by alkylation of
the triazole ring. The palladium and platinum complexes prepared
by this route represent the first example of tridentate chelating
system bearing a mesoionic carbene.
analysis were obtained by slow cooling of a chloroform solution
of 11 (Fig. 3).
Ligands 8 and 9 were prepared based upon the Cu(I)-catalyzed
[2 + 3] cycloaddition of azides and alkynes (“click” chemistry),11
decorated with donor arms, to form triazole as the main tool for
ligand assembly (Scheme 1).12,2 Utilization of the “click” reaction
is advantageous because it allows for the simple and selective
preparation of a diverse family of ligands from a small group
of monomers.
Fig.
3
Perspective view of a molecule of 11. Hydrogen atoms
◦
˚
are omitted for clarity. Selected bond lengths [A] and angles [ ]:
Pd1–C1 1.902(9), Pd1–P1 2.306(2), Pd1–P2 2.334(2), Pd1–I1 2.6241(11),
P1–Pd1–P2, 159.24(9), C1–Pd1–I1, 179.2(3), P2–Pd1–I1 99.97(6),
P1–Pd1–I1 100.72(7).
Scheme 1 Synthesis of pincer click ligands.
Palladation of ligand 8 in a selective tridentate manner was
achieved by its thermal reaction with (tmeda)PdCl2 (tmeda =
tetramethylethylenediamine) in the presence of a base.2 In order
to address post-modification feasibility of the triazole backbone
of 10 we performed reactions with alkylating agents. Thus, when
compound 10 was exposed to an excess of methyl iodide at 70 ◦C,
smooth formation of triazolylidene complex 11 was observed
(Scheme 2).‡ The selective methylation of the triazole ring was
obtained, and no Pd(IV) methylated species was observed.13
Methylation of the triazole nitrogen in solution was confirmed by
NMR spectroscopy. The methyl group of 11 gives rise to a singlet
at 4.14 ppm in 1H NMR and dC 38.7 ppm in 13C NMR, which is
comparable to other reported methyl- triazolylidene systems.7,8 In
general, spectroscopic and structural (vide infra) characteristics of
the triazolylidene motif in complex 11 are in good agreement with
those of reported Pd complex 7.8 Complex 7 is most relevant for
comparison to our systems due to its similar trans triazolylidene-
Pd–I geometry. Specifically, the ipso carbon of 11 appears at
dC 148.6, as compared with the carbene resonance dC 154.6 in
complex 7.
The palladium center has a slightly distorted square planar
geometry, as expected for d8 Pd(II) complexes. The geometry of
the atoms coordinated around the palladium center is essentially
planar, with the sum of angles equal to 360.6◦. As compared with
the non-methylated parent species 10, in species 11 the N1–N2
and N2–N3 bonds equalize, which is anticipated for mesoionic
carbene complexes.
˚
Importantly, the C1–Pd bond is shortened from 1.929 A in 10
˚
to 1.902 A in 11, indicating the increased contribution of carbene
character of this bond. This Pd–C1 bond is significantly shorter
˚
˚
than the analogous Pd–C bond in complex 7 (1.902 A vs. 1.997 A).
This is likely due to the restriction imposed by coordination of
rigid phosphine arms of pincer framework to the palladium center.
However, the Pd–I bond in cationic complex 11 is shorter than that
˚
˚
of neutral complex 10 (2.624 A vs. 2.651 A).
Preparation of triazolylidene-metal species by post-
modification of triazole-metal precursors was also demonstrated
on platinum complexes. Complex 12 was prepared by reaction of
ligand 9 with a Pt(II) precursor in the presence of triethyl amine
in DMF (Scheme 3). Reaction of this complex with methyl iodide
caused precipitation (the putative cationic Pt methylated species
are likely to be low soluble). In order to increase solubility of the
desired triazolylidene product, 4-(trifluoromethyl)benzyl bromide
was used as an alkylating agent. Thus, when Pt complex 12 was
reacted with 4-(trifluoromethyl)benzyl bromide in acetonitrile,
selective formation of 13 was observed. This complex was fully
characterized by multinuclear NMR techniques. The benzyl
protons resonate at 5.91 ppm, indicative of an N-alkylated
species. This novel complex is the first example of a 1,2,3-
triazolylidene-Pt compound, extending the list of known Pd, Rh,
Ir and Au-triazolylidene species.7,8
Platinum-195 NMR proved useful in observing changes in the
electronic environment of the platinum center.14 Alkylated cationic
complex 13 exhibits a doublet of doublets due to its coupling to two
nonsymmetric phosphines at dPt -4347 ppm. This is an effective
upfield shift of approximately 100 ppm relative to starting neutral
complex 12 (Fig. 4a and b). Addition of one equivalent of silver
Scheme 2 Synthesis of Pd-triazolylidene complex11 by post-modification
of triazolyl-Pd 10.
The molecular structure of 11 was unambiguously confirmed by
X-ray spectroscopy of a single crystal. Crystals suitable for X-ray
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 8764–8767 | 8765
©

been highlighted as some of the strongest s-donors as compared
to other N-heterocyclic carbenes. The relatively high IR stretch
of the carbonyl ligand in complex 14 may be attributed to the
high positive charge on the biscationic Pt center.15 It would be
valuable to compare triazolylidene donor strength of complex
14 to the triazolyl ligand in parent complex 12. Unfortunately,
our attempts to remove the Cl ligand of 12 with a silver salt
in order to prepare the corresponding carbonyl complex were
unsuccessful, because of immediate formation of an insoluble
orange precipitate. We presume that this precipitate is an oligomer
formed by coordination of another triazole unit to the naked
cationic metal center.16
In summary, we have demonstrated the first example of 1,2,3-
triazolylidene preparation by alkylation of a metal-bound triazolyl
ligand. This is also the first example of pincer-based complexes
bearing triazolidene carbenes as a central motif. This method not
only expands the versatility of our triazole-based ligands as biden-
tate, tridentate, or carbene-type systems, but also highlights post-
modification as a method for altering the existing metal complex
properties. Further studies are underway in our laboratories to
explore the catalytic activity of these compounds.
Scheme 3 Synthesis of Pt-triazolylidene complex 13 by alkylation of
triazolyl-Pt 12.
Financial support from US-Israel Binational Science Founda-
tion (Grant No. 2008391) and the State of Lower-Saxony and the
Volkswagen Foundation is acknowledged. The authors wish to
thank Dr Yael Balazs for her valuable assistance with 195Pt NMR.
Experimental
Synthesis of 11
To a solution of 10 (13 mg, 0.028 mmol) in chloroform (1 mL)
was added MeI (3.2 mL, 0.051 mmol). The mixture was heated
to 70 ◦C in a closed vessel. After 18 h, the product had fully
precipitated from solution. The precipitate was filtered to give 11
Fig. 4 195Pt NMR of compounds (a) 12; (b) 13; (c) 15; and (d) 16.
triflate to 13 resulted in new complex 15 as was indicated by 31P
NMR. However, there was no observable change in 195Pt NMR
(Fig. 4b and c), indicating that the Br ion is not situated in the Pt
first coordination sphere. Further addition of a second equivalent
of silver triflate to 15 in acetonitrile gave complex 16 (Scheme 4).
The formation of this stable biscationic complex is confirmed by
an effective upfield 195Pt NMR shift of approximately 100 ppm
relative to alkylated species 13 and 15 (Fig. 4d).
When complex 13 was reacted with two equivalents of silver
triflate in THF, and subsequently exposed to a CO atmosphere,
biscationic Pt carbonyl complex 14 was obtained. This complex
was analyzed by IR and multinuclear NMR techniques. The
CO ligand located trans to the carbene unit exhibited a CO
stretch in IR at 2124 cm-1. Interestingly, triazolylidenes have
1
(12.5 mg, 92%) as a white crystalline solid. H NMR (CD3CN)
d: 1.15–1.38 (24H, m), 2.54–2.66 (4H, m, CH), 3.22 (2H, d, JHP
8.4 Hz), 4.14 (3H, s, CH3–N), 4.91 (2H, d, JHP = 5.7 Hz, CH2).
13C NMR (CD3CN) d: 17.1 (dd, JCP = 9.3, 2.0 Hz), 17.7 (dd, JCP
=
=
4.0, 10.0 Hz), 19.9 (d, JCP = 24.0 Hz), 24.7 (dd, JCP = 3.8, 19.4 Hz),
25.5 (dd, JCP = 3.8, 18.5 Hz), 38.7, 48.8 (d, JCP = 27.5 Hz), 148.6
(C-ipso). 31P NMR (CD3CN) d: 62.6 (1P, d, JPP = 400 Hz), 73.16
(1P, d, JPP = 400 Hz).
Synthesis of 13
To a solution of 12 (35.5 mg, 0.051 mmol) in acetonitrile
(1 mL) was added a solution of 4-(trifluoromethyl)benzyl bromide
Scheme 4 Preparation of Pt carbonyl complex 14 bearing a triazolylidene ligand trans to CO.
8766 | Dalton Trans., 2011, 40, 8764–8767 This journal is The Royal Society of Chemistry 2011
©

(13.4 mg, 0.102 mmol, 1.1 eq) in acetonitrile (0.5 mL). The
resulting solution was heated in a closed vessel at 70 ◦C for 48 h.
6 (a) O. Schuster, L. Yang, H. G. Raubenheimer and M. Albrecht,
Chem. Rev., 2009, 109, 3445; (b) M. Melaimi, M. Soleilhavoup and
G. Bertrand, Angew. Chem., Int. Ed., 2010, 49, 8810.
7 (a) M. Paulson, A. Neels and M. Albrecht, J. Am. Chem. Soc., 2008, 130,
13534. See also: (b) T. Karthikeyan and S. Sankararaman, Tetrahedron
Lett., 2009, 50, 5834; (c) K. J. Kilpin, U. S. D. Paul, A.-L. Lee and J.
D. Crowley, Chem. Commun., 2011, 47, 328.
8 A. Poulain, D. Canseco-Gonzalez, R. Hynes-Roche, H. Mu¨ller-
Bunz, O. Schuster, H. Stoeckli-Evans, A. Neels and M. Albrecht,
Organometallics, 2011, 30, 1021.
9 G. Guisado-Barrios, J. Bouffard, B. Donnadieu and G. Bertrand,
Angew. Chem. Int. Ed., 2010, 49, 4759.
10 (a) H. G. Raubenheimer and S. Cronje, J. Organomet. Chem., 2001,
617–618, 170; (b) H. Meguro, T.-A. Koizumi, T. Yamamoto and T.
Kanbara, J. Organomet. Chem., 2008, 693, 1109; (c) A. Weisman,
M. Gozin, H.-B. Kraatz and D. Milstein, Inorg. Chem., 1996, 35,
1792.
1
31P{ H} NMR showed quantitative formation of 13 as a single
product. The solvent was evaporated and the residue was washed
with hexane and toluene (3 ¥ 3 ml) and extracted with THF (3 ¥
3 ml). The combined fractions were evaporated resulting in pure
complex 13 (36 mg, 73%). 1H NMR (CD3CN) d: 4.0 (2H, d, JHP
=
9.9 Hz), 5.5 (2H, d, JHP = 7.2 Hz), 5.91 (2H, s), 7.52–7.89 (24H,
m). 13C NMR (CD3CN) d: 30.4 (d, JCP = 37.5 Hz), 44.7 (d, JCP
43.3 Hz), 125.8 (q, CF3), 129.1, 129.3, 129.5, 129.7, 132.3 (d, JCP
=
=
2.5 Hz), 132.7 (d, JCP = 2.6 Hz), 133.2 (d, JCP = 2.3 Hz), 133.3 (d,
JCP = 2.3 Hz), 133.5 (d, JCP = 2.3 Hz), 133.7(d, JCP = 2.4 Hz), 136.3,
147.3 (C-ipso). 19F NMR (CDCl3) d: -62.5. 31P NMR (CDCl3) d:
44.4 (1P, d, JPP = 436 Hz), 31.28 (1P, d, JPP = 436 Hz). m/z (HRMS-
ESI) M-H 931.0328, C36H29N3F3P2ClPt calcd: 931.0309.
11 (a) A. Meldal and C. W. Tornøe, Chem. Rev., 2008, 108, 2952; (b) P. Wu
and V. V. Fokin, Aldrichim. Acta, 2007, 40, 7.
12 For a review on the use of the Cu(I)-catalyzed azide-alkyne cycloaddi-
tion for adaptable ligand construction, see: H. Struthers, T. L. Mindt
and R. Schibli, Dalton Trans., 2010, 39, 675.
Notes and references
13 Although several instances of Pd(IV) pincer complexes prepared from
Pd(II) precursors have been reported, their preparation was achieved
through the use of strong oxidants such as trivalent iodine compounds.
For examples, see: (a) N. Selander, B. Willy and K. J. Szabo´, Angew.
Chem. Int. Ed., 2010, 49, 4051; (b) M. C. Lagunas, R. A. Gossage, A.
L. Spek and G. van Koten, Organometallics, 1998, 17, 731; (c) A. J.
Canty, Dalton Trans., 2009, 10409; (d) A. J. Canty, T. Rodemann, B.
W. Skelton and A. H. White, Organometallics, 2006, 25, 3996; (e) L.
T. Pilarski, N. Selander, D. Bo¨se and K. J. Szabo´, Org. Lett., 2009, 11,
5518.
‡ Crystal data for 11: pale-brown plates, C17H35I2N3P2Pd, M = 703.62,
˚
triclinic, a = 8.545(2)◦, b = 11.233(2), c = 13.533(3) A, a = 9.60(2), b =
3
¯
˚
88.03(2), g = 85.80(2) , U = 1294.7(5) A , T = 293 K, space group P1 (no
2), Z = 2, 10612 reflections were measured, 4442 unique (Rint = 0.0726)
which were used in all calculations. The final R-factors was 0.0569 for
reflections with I>2s(I) and 0.0934 (all data).
1 (a) G. van Koten and M. Albrecht, Angew. Chem., Int. Ed., 2001, 40,
3750; (b) M. E. van der Boom and D. Milstein, Chem. Rev., 2003, 103,
1759; (c) The Chemistry of Pincer Compounds, ed. Morales-Morales
and C. Jensen, Elsevier, 2007.
2 (a) E. M. Schuster, M. Botoshansky and M. Gandelman, Angew.
Chem., Int. Ed., 2008, 47, 4555; (b) E. M. Schuster, G. Nisnevich, M.
Botoshansky and M. Gandelman, Organometallics, 2009, 28, 5025.
3 E. M. Schuster, M. Botoshansky and M. Gandelman, Organometallics,
2009, 28, 7001.
14 For a review on 195Pt NMR, see: B. M. Still, P. G. A. Kumar, J. R.
Aldrich-Wright and W. S. Price, Chem. Soc. Rev., 2007, 36, 665.
15 Dependence of IR frequencies of CO ligands on the net ionic charge of
the metal is well-documented. See: R. H. Crabtree, The Organometallic
Chemistry of the Transition Metals, John Wiley & Sons, New York, 3rd
edn, 2001, ch. 10, pp. 282.
16 Facile coordination of 1,2,3-triazole derivative to cationic NCN–Pt and
Pd complexes has been documented: B. M. J. M. Suijkerbuijk, B. N. H.
Aerts, H. P. Dijkstra, M. Lutz, A. L. Spek, G. van Koten and R. J. M.
Klein Gebbink, Dalton Trans., 2007, 1273.
4 D. Enders, O. Niemeier and A. Hensler, Chem. Rev., 2007, 107, 5606.
5 N-Heterocyclic Carbenes in Transition Metal Catalysis, ed. F. Glorius,
Springer, 2006.
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 8764–8767 | 8767
©