A Convenient Solvothermal Synthesis of Group 6 PNP Pincer Tricarbonyl Complexes

Article abstract of DOI:10.1021/acs.organomet.5b00940

The solvothermal synthesis of a series of zerovalent Cr, Mo, and W complexes of the type [M(PNP)(CO)₃] featuring PNP pincer ligands based on 2,6-diaminopyridine is described. We demonstrate that the solvothermal synthesis technique presented provides a powerful, simple, and practical synthetic method resulting in high isolated yields in a short time. In particular, Cr and W complexes are not readily accessible via conventional methods. Moreover, this study allows a direct comparison of steric and electronic properties of group 6 metal PNP pincer tricarbonyl complexes.

Full text of DOI:10.1021/acs.organomet.5b00940

Article
pubs.acs.org/Organometallics
A Convenient Solvothermal Synthesis of Group 6 PNP Pincer
Tricarbonyl Complexes
†
†
†
‡
,†
̈
Matthias Mastalir, Sara R. M. M. de Aguiar, Mathias Glatz, Berthold Stoger, and Karl Kirchner*
†
‡
Institute of Applied Synthetic Chemistry and Institute of Chemical Technologies and Analytics, Vienna University of Technology,
Getreidemarkt 9, A-1060 Vienna, Austria
*
S Supporting Information
ABSTRACT: The solvothermal synthesis of a series of zerovalent Cr, Mo,
and W complexes of the type [M(PNP)(CO) ] featuring PNP pincer ligands
3
based on 2,6-diaminopyridine is described. We demonstrate that the
solvothermal synthesis technique presented provides a powerful, simple,
and practical synthetic method resulting in high isolated yields in a short time.
In particular, Cr and W complexes are not readily accessible via conventional
methods. Moreover, this study allows a direct comparison of steric and
electronic properties of group 6 metal PNP pincer tricarbonyl complexes.
INTRODUCTION
Scheme 1. Halo Carbonyl and Hydrido Carbonyl Mo(II)
and W(II) PNP Pincer Complexes as a Function of the PR2
Moieties and the NR′ Spacers
■
Among the many ligand systems that can be found in the
chemical literature, pincer ligands play an important role and
their complexes have attracted tremendous interest due to their
1
high stability, activity, and variability. These tridentate ligands
are often planar scaffolds consisting of an anionic or neutral
central aromatic backbone tethered to two, mostly bulky,
phosphine donors by different spacers. In this family of ligands
steric, electronic, and also stereochemical parameters can be
manipulated by modifications of the substituents at the donor
sites and/or the spacers. Accordingly, many applications of
mostly precious transition-metal pincer complexes in the fields
of catalysis, molecular recognition, and supramolecular
chemistry were discovered, turning this area into an intensively
investigated subject in organometallic chemistry.
a series of hydrido carbonyl and halo carbonyl tungsten pincer
complexes featuring a related PNP pincer-type ligand based on
silazane, viz. HN(SiMe CH PPh ) , were described by
2
2
2 2
8
Templeton and co-workers. Gambarotta et al. could show
Surprisingly, as far as group 6 pincer complexes are
that the Cr(II) and Cr(III) PNP complexes [Cr(PNP-Ph)Cl2]
a n d [ C r ( P N P - P h ) C l 2 ] ( P N P - P h
=
2 , 6 - b i s -
concerned, only a few examples have been reported in the
2
−11
(
diphenylphosphinomethyl)pyridine) are active catalysts for
literature.
For the first time, Haupt and co-workers
9
the oligomerization of ethylene. Recently, Schrock and co-
prepared PNP pincer complexes of the type [M(PNP-
10
workers showed that the molybdenum PCP pincer complex
based on the 1,3-bis(phosphinito)benzene scaffold is capable of
cleaving molecular dinitrogen to give a Mo(IV) PCP nitride
complex. Another impressive reaction of group 6 PNP pincer
Ph)(CO) ] (M = Cr, Mo, W; PNP-Ph = N,N′-bis-
3
(
diphenylphosphino)-N,N′-2,6-diaminopyridine), albeit in low
2
yields (19, 34, and 22%). We are focusing on the chemistry of
molybdenum and tungsten complexes containing PNP pincer
ligands based on the 2,6-diaminopyridine scaffold, where the
pyridine ring and the phosphine moieties are connected via
11
complexes was discovered by the group of Nishibayashi. They
found that dinuclear molybdenum and tungsten dinitrogen
complexes bearing bulky PNP pincer ligands (PNP = 2,6-
bis(dialkylphosphinomethyl)pyridines) are effective catalysts
for the formation of ammonia from molecular dinitrogen.
These few examples provide already a fortaste of the potential
4
NH, N-alkyl, or N-aryl linkers. These studies resulted in the
preparation of halocarbonyl and hydridocarbonyl complexes of
the types [M(PNP)(CO) X]X (M = Mo, W; X = I, Br, Cl) (I),
3
[
Mo(PNP)(CO) X ] (X = I, Br, Cl, F) (II), [Mo(PNP)(CO)-
2 2
+
X ] (X = I, Br, Cl) (III), and [M(PNP)(CO) H] (M = Mo,
W) (IV), as illustrated in Scheme 1.
2
3
5
−7
Analogous Cr
Received: November 11, 2015
complexes have, as yet, not been described. The synthesis of
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.5b00940
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Organometallics
Article
of group 6 pincer complexes with respect to stoichiometric and
catalytic reactions involving, for instance, small molecules.
Accordingly, in order to further develop the chemistry of
group 6 pincer systems, the availability of efficient synthetic
protocols is essential. The most common synthetic entries into
group 6 carbonyl complexes are the substitutionally inert
Scheme 2. Synthesis of [M(PNP)(CO) ] (M = Cr, Mo, W)
3
a
Complexes under Solvothermal Conditions in CH CN
3
hexacarbonyl complexes M(CO) , which are treated directly
6
with a ligand under refluxing conditions in CH CN as a
3
coordinating solvent stabilizing the labile [M(CO) (CH CN) ]
3
3
3
intermediate. This reaction requires typically several hours for
Cr and Mo but several days for W, with generally rather low
yields in the case of Cr and W. Under conventional reaction
conditions the use of M(CO) complexes is also hampered by
6
sublimation and deposition of the metal hexacarbonyl on the
reflux condenser. The synthesis of transition-metal carbonyl
complexes is sometimes significantly improved if the reactions
12−15
are performed under microwave conditions.
In some cases,
special Teflon-lined vessels rather than simple glass vials were
required.
In this paper, we report on the synthesis of the zerovalent Cr,
Mo, and W complexes [M(PNP)(CO) ] featuring PNP pincer
3
ligands based on the 2,6-diaminopyridine scaffold via a simple
and fast solvothermal approach with no need for microwave
1
6
equipment. All reactions are performed in standard
aluminum-capped, microwave glass vials which allow the
generation of higher pressures and superheating of the solvent.
This results in significantly decreased reaction times and highly
increased product yields in a clean fashion over methodologies
reported using traditional procedures. Moreover, this study
allows comparisons among a complete series of Cr, Mo, and W
PNP tricarbonyl complexes as far as electronic and steric
properties are concerned.
a
Labeling of complexes refers to letters of ligands depicted here.
Scheme 3. Three-Step Synthesis of W(0) Complexes via
[W(CO) (μ-Br)Br] and [W(PNP)(CO) Br]Br
4 2 3
Intermediates
RESULTS AND DISCUSSION
A suspension of hexacarbonyl complexes M(CO) and PNP
■
6
ligands 1a−j in CH CN were placed in a sealed microwave
3
glass tube and stirred for 2−5 h at 135−160 °C. After workup,
the analytically pure products 2−4 were obtained in 78−99%
isolated yields (Scheme 2). Several years ago Haupt and co-
workers reported the synthesis of [M(PNP-Ph)(CO )] (M =
3
2
Cr, Mo, W) (2e−4e).
We reported recently a comparatively time consuming three-
step synthesis of W(0) complexes [W(PNP)(CO) ] via the
3
dinuclear complex [W(CO) (μ-Br)Br] , prepared in situ from
4
2
W(CO) and stoichiometric amounts of Br . Treatment of
6
2
[
W(CO) (μ-Br)Br] with the PNP ligands 1b,d,e afforded the
significant upfield shift on going from Cr to Mo to W (Table
1). The IR spectra show, in most cases, the typical three strong
to medium absorption bands of a mer CO arrangement in the
4
2
tungsten(II) intermediates [W(PNP)(CO) Br]Br, which sub-
sequently were reduced with sodium amalgam to yield the
W(0) complexes 4b, 1d, and 4e in 56, 46, and 68% overall
yields, respectively (Scheme 3).
3
−1
range of 1985−1756 cm assignable to one weaker symmetric
9
and two strong asymmetric ν stretching modes. The νCO
CO
All complexes were fully characterized by a combination of
frequencies, in particular the symmetric CO stretch, is
indicative of increasing electron donor strengths of the PNP
ligands and follow roughly the order PNP-BIPOL < PNP-Ph <
1H, C{ H}, and P{ H} NMR spectroscopy, IR spectrosco-
py, and elemental analysis. Characteristic features comprise, in
the 13C{ H} NMR spectrum, two low-field triplet resonances
13
1
31
1
1
Me
Me
PNP -Ph < PNP-Cy < PNP-iPr ≈ PNP -iPr < PNP-Et <
(
1:2 ratio) in the range of 240−196 ppm assignable to the
PNP-tBu (Table 2).
carbonyl carbon atoms trans and cis to the pyridine nitrogen,
In addition to spectroscopic characterization, the solid-state
structures of 2a,b,d were determined by single-crystal X-ray
diffraction. Structural views are depicted in Figures 1 and 2 with
selected bond distances given in the captions (the structure of
2b is provided in the Supporting Information).
3
1
1
respectively. The P{ H} NMR spectra exhibit singlet
resonances with J coupling constants of 315−494 Hz in
the case of tungsten complexes (Table 1).
1
WP
The tungsten−phosphorus coupling was observed as a
1
83
doublet satellite due to W (14% abundance with I = 1/2)
superimposed over the dominant singlet. Both the carbonyl
resonances (δ ) and the phosphorus resonances (δ ) exhibit a
The coordination geometry around the chromium center, as
2
,6,7
in the case of analogous Mo and W complexes,
corresponds
to a distorted octahedron. In particular, the carbonyl−metal−
co
P
B
DOI: 10.1021/acs.organomet.5b00940
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Table 1. Selected 13C{ H} and P{ H} NMR Data of
Article
1
31
1
[
M(PNP)(CO) ] (M = Cr, Mo, W) Complexes
3
metal
Cr
Mo
W
ligand δCO, ppm δP, ppm δCO, ppm δP, ppm δCO, ppm δP, ppm
1
1
1
1
1
1
1
1
1
1
a
b
c
d
e
f
235.7
23.8
236.2
25.6
236.1
25.8
238.7
33.0
234.6
20.6
235.2
26.0
223.4
20.8
234.8
22.8
230.2
17.9
236.2
23.9
130.0
146.7
149.7
164.4
125.7
171.5
152.1
147.8
230.0
193.5
230.3
213.8
231.4
216.9
231.1
216.4
233.1
224.0
228.4
211.2
230.8
217.9
227.8
211.9
229.9
214.3
224.7
208.4
111.3
143.6
122.6
161.9
116.2
159.0
131.0
129.5
204.8
222.0
208.2
221.1
210.6
222.3
211.3
224.7
219.4
206.0
196.6
222.5
211.6
221.1
207.4
221.7
208.4
215.5
201.4
94.0
128.5
106.5
147.2
100.2
144.0
114.7
112.7
188.2
2
Figure 1. Structural view of [Cr(PNP-Et)(CO) ] (2a) showing 50%
thermal ellipsoids (H atoms omitted for clarity). Selected bond lengths
3
2
(
Å) and bond angles (deg): Cr1−P1 2.2678(3), Cr1−P2 2.2724(3),
Cr1−C14 1.8704(9), Cr1−C15 1.8295(10), Cr1−C16 1.18551(9),
2
Cr1−N1 2.139(1); P1−Cr1−P2 160.03(1), C14−Cr1−C16
1
66.68(4).
2
2
2
g
h
i
2
2
2
Figure 2. Structural view of [Cr(PNP-tBu)(CO)
CH CN) showing 50% thermal ellipsoids (H atoms and CH
3
3
]·CH
3
CN (2d·
j
3
CN
omitted for clarity). Selected bond lengths (Å) and bond angles (deg):
2
Cr1−P1 2.3627(4), Cr1−P2 2.3706(4), Cr1−22 1.864(1), Cr1−C23
1
1
.793(1), Cr1−C24 1.867(1), Cr1−N1 2.187(1); P1−Cr1−P2
54.76(1), C22−Cr1−C24 155.22(5).
carbonyl angles of the CO ligands trans to one another deviate
significantly from 180°. They vary with the bulkiness of the PR2
moiety and decrease from 166.68(4)° in [Cr(PNP-Et)(CO)3]
(
1
2a) to 164.04(1)° in [Cr(PNP-iPr)(CO) ] (2b) to
3
conventional methods. Moreover, this study also allows a direct
comparison of steric and electronic properties of group 6 metal
PNP pincer complexes.
55.22(5)° in [Cr(PNP-tBu)(CO) ] (2d). The same trend is
3
found in the analogous Mo and W complexes: 171.1(8)° in
[
(
1
[
Mo(PNP-Ph)(CO) ] (3d), 166.03(5)° in [Mo(PNP-iPr)-
3
Me
CO) ] (3b), 162.93(7)° in [Mo(PNP -iPr)(CO) ] (3f),
EXPERIMENTAL SECTION
3
3
■
56.53(4)° in [Mo(PNP-tBu)(CO) ] (3d), 165.7(2)° in
3
General Considerations. All manipulations were performed
under an inert atmosphere of argon by using Schlenk techniques or
in an MBraun inert-gas glovebox. The solvents were purified according
W(PNP-iPr)(CO) ] (4b), and 156.46(9)° in [W(PNP-
3
tBu)(CO) ] (4d). Accordingly, the bulkiness of the PNP
3
1
7
ligands follows roughly the order (PNP-Ph ≈ PNP-Et < PNP-
to standard procedures. The deuterated solvents were purchased
Me
from Aldrich and dried over 4 Å molecular sieves. The ligands PNP-Et
iPr < PNP -iPr < PNP-tBu).
2
1
8
4
19
4
(
(
1a), PNP-iPr (1b), PNP-Cy (1c), PNP-tBu (1d), PNP-Ph
2 7 5 19
In summary, we demonstrated that the solvothermal
synthesis technique provides a powerful, simple, and practical
synthetic method to afford group 6 PNP pincer carbonyl
Me
Me
Ph
1e), PNP -iPr (1f), PNP -Ph (1g), PNP -Et (1h), and PNP-
BIPOL (1i) were prepared according to the literature. The synthesis
4
1
of PNP-SPEA (1j) is described in the Supporting Information. H,
complexes of the type [M(PNP)(CO) ] (M = Cr, Mo, W) in
13
1
31
1
3
C{ H}, and P{ H} NMR spectra were recorded on Bruker
high isolated yields in a short time. It has to be emphasized that
in particular the short reaction times allow the use of thermally
more sensitive ligands. As far as Cr and W complexes are
concerned, these complexes are not readily accessible with
1
13
1
AVANCE-250 and AVANCE-400 spectrometers. H and C{ H}
NMR spectra were referenced internally to residual protio solvent and
solvent resonances, respectively, and are reported relative to
tetramethylsilane (δ 0 ppm). 31P{ H} NMR spectra were referenced
1
−1
Table 2. Carbonyl Stretching Frequencies (νCO, cm ) of [M(PNP)(CO) ] (M = Cr, Mo, W) Complexes
3
ligand
metal
Cr
1a
1b
1c
1d
1e
1f
1g
1h
1i
1j
1905
1923
1785
1913
1807
1793
1941
1828
1790
1933
1807
1790
1908
1792
1756
1922
1808
1771
1914
1799
1759
1973
1840
1807
1964
1858
1765
1955
1847
1759
1926
1795
1774
1936
1810
1795
1928
1890
1797
1954
1842
1819
1956
1911
1850
1954
1839
1801
1945
1807
1972
1860
1936
1827
1811
1
1
821
763
Mo
W
1929
1936
1809
1790
1929
1805
1784
1949
1815
1985
1876
1
1
840
780
1921
1934
1804
1979
1858
1
1
834
768
C
DOI: 10.1021/acs.organomet.5b00940
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
externally to H PO (85%) (δ 0 ppm). Aluminum-capped 20 mL
microwave vials from Biotage or VWR were used as reaction vessels.
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3
4
General Procedure for the Synthesis of [M(PNP)(CO) ] (M =
3
Cr, Mo, W). A suspension of the metal hexacarbonyl (0.60 mmol) and
(5) de Aguiar, S. R. M. M.; Stoger, B.; Pittenauer, E.; Allmaier, G.;
̈
1
.1 equiv of the respective PNP ligand (0.63 mmol) in acetonitrile (3
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C (unless otherwise noted; see the Supporting Information),
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°
̈ ̈
̈
6) de Aguiar, S. R. M. M.; Oztopcu, O.; Stoger, B.; Mereiter, K.;
(
whereupon a clear solution was obtained. The reaction mixture was
cooled to room temperature without stirring. In most cases the
product was obtained as a crystalline material and was decanted and
washed with n-hexane. In all other cases the solvent was removed
under reduced pressure. The remaining solid was washed with n-
hexane and dried under vacuum. Complexes 2a−j, 3a−i, and 4a−i
were obtained in 77−99% isolated yields.
Veiros, L. F.; Pittenauer, E.; Allmaier, G.; Kirchner, K. Dalton Trans.
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(
014, 43, 14669−14679.
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(
2
ASSOCIATED CONTENT
■
(
*
S
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.5b00940.
(
Chem. Commun. 2013, 49, 11215−11217. (b) Kinoshita, E.; Arashiba,
K.; Kuriyama, S.; Miyake, Y.; Shimazaki, R.; Nakanishi, H.;
Nishibayashi, Y. Organometallics 2012, 31, 8437−8443. (c) Arashiba,
A.; Sasaki, K.; Kuriyama, S.; Miyake, Y.; Nakanishi, H.; Nishibayashi, Y.
Organometallics 2012, 31, 2035−2041. (d) Arashiba, K.; Miyake, Y.;
Nishibayashi, Y. Nat. Chem. 2011, 3, 120−125. (e) Tanaka, H.;
Arashiba, K.; Kuriyama, S.; Sasada, A.; Nakajima, K.; Yoshizawa, K.;
Nishibayashi, Y. Nat. Commun. 2014, 5, 1−11.
Experimental procedures, characterization data of all
complexes, and crystallographic details of 2a, 2b·
CH CN, and 2d·CH CN (PDF)
3
3
Crystallographic data for 2a, 2b·CH CN, and 2d·
3
CH CN (CIF)
3
(12) VanAtta, S. L.; Duclos, B. A.; Green, D. B. Organometallics 2000,
1
9, 2397−2399.
AUTHOR INFORMATION
■
(13) Cesari, C.; Sambri, L.; Zacchini, S.; Zanotti, V.; Mazzoni, R.
Organometallics 2014, 33, 2814−2819.
14) Amarante, T. R.; Neves, P.; Coelho, A. C.; Gago, S.; Valente, A.
A.; Almeida Paz, F. A.; Pillinger, M.; Goncalves, I. S. Organometallics
Corresponding Author
K.K.: e-mail, kkirch@mail.tuwien.ac.at; tel, (+43) 1 58801
63611; fax, (+43) 1 58801 16399.
(
*
1
2
010, 29, 883−892.
Notes
(15) Ardon, M.; Hogarth, G.; Oscroft, D. T. W. J. Organomet. Chem.
2004, 689, 2429−2435.
The authors declare no competing financial interest.
(
(
16) Chen, X.-M.; Tong, M.-L. Acc. Chem. Res. 2007, 40, 162−170.
17) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
ACKNOWLEDGMENTS
■
Chemicals, 3rd ed.; Pergamon: New York, 1988.
Financial support by the Austrian Science Fund (FWF)
(
18) Glatz, M.; Bichler, B.; Mastalir, M.; Stoger, B.; Mereiter, K.;
̈
(Project No. P24202−N17) is gratefully acknowledged.
Weil, M.; Pittenauer, E.; Allmaier, G.; Veiros, L. F.; Kirchner, K. Dalton
Trans. 2015, 44, 281−294.
(
19) Glatz, M.; Holzhacker, C.; Bichler, B.; Mastalir, M.; Sto
̈
ger, B.;
sch-Zanetti, N. C.; Kirchner,
K. Eur. J. Inorg. Chem. 2015, 2015, 5053−5065.
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D
DOI: 10.1021/acs.organomet.5b00940
Organometallics XXXX, XXX, XXX−XXX