Synthesis and characterization of xylene-based group-six metal PCP pincer complexes

Article abstract of DOI:10.1007/s00706-019-02422-6

In the present study, the Cr(III) complex trans-[Cr(PCP^CH2-iPr)(CH₃CN)(Br)₂] as well as seven-coordinate cationic bromo carbonyl Mo(II) and W(II) complexes of the type [M(PCP^CH2-iPr)(CO)₃Br] (M=Mo, W)

Full text of DOI:10.1007/s00706-019-02422-6

Monatshefte für Chemie - Chemical Monthly
https://doi.org/10.1007/s00706-019-02422-6
ORIGINAL PAPER
Synthesis and characterization of xylene‑based group‑six metal PCP
pincer complexes
Wolfgang Eder1 · Berthold Stöger2 · Karl Kirchner1
Received: 16 February 2019 / Accepted: 1 April 2019
© The Author(s) 2019
Abstract
In the present study, the Cr(III) complex trans-[Cr(PCPCH2-iPr)(CH3CN)(Br)2] as well as seven-coordinate cationic bromo
carbonyl Mo(II) and W(II) complexes of the type [M(PCPCH2-iPr)(CO)3Br] (M=Mo, W) featuring PCP pincer ligands based
on a xylene scafold were prepared and characterized. The seven-coordinate bromo tricarbonyl complexes exhibit fuxional
behavior in solution due to rapid CO ligand interconversions. The mechanism of this process was studied by means of DFT
calculations. Structures of representative complexes were determined by single-crystal X-ray analyses.
Graphical abstract
Keywords Chromium · Molybdenum · Tungsten · Pincer complexes · Carbonyl ligands · DFT calculations
Introduction
[4–7]. Such complexes were shown to be promising can-
didates for the activation and splitting of small molecules
such as dinitrogen. Schrock and co-workers discovered [4]
that the Mo(III) PCP pincer complex [Mo(POCOP-tBu)(I)2]
forms upon reduction with NaHg in the presence of N2 the
anionic Mo(IV) nitride complex [Mo(POCOP-tBu)(N)(I)]−.
Here we utilize the oxidative addition of the C–Br bond of
the ligand precursor (2-bromo-1,3-phenylene)bis(methylene)
bis(diisopropylphosphane) (P(C-Br)PCH2-iPr) (1) to the hex-
acarbonyl complexes [M(CO)6] (M=Cr, Mo, W) as synthetic
entry into group-six metal PCP pincer complexes. This pro-
cedure has been successfully applied recently for the synthe-
sis of several Cr, Mo, W, Mn, and Fe PCP complexes bearing
NR (R=alkyl) linkers [8, 9]. X-ray structures of the Mo and
W complexes are presented.
PCP pincer complexes which feature an aromatic anionic
benzene backbone connected to phosphine donors via CH2,
O, or NR (R=H, alkyl, aryl) linkers are a very attractive
class of compounds [1–3]. With respect to group-six metals,
PCP complexes are exceedingly rare [1]. In fact, according
to our knowledge, there are only three established Mo and W
systems, and one Cr system described as shown in Scheme 1
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s00706-019-02422-6) contains
supplementary material, which is available to authorized users.
* Karl Kirchner
karl.kirchner@tuwien.ac.at
1
Institute of Applied Synthetic Chemistry, Vienna University
of Technology, Getreidemarkt 9/163-AC, 1060 Vienna,
Austria
2
X-ray Center, Vienna University of Technology,
Getreidemarkt 9/163-OC, 1060 Vienna, Austria
Vol.:(0123456789)
1 3

W. Eder et al.
Scheme 1
mol, which is in line with the fact that the analogous chloride
compound, featuring NEt linkers, adopts a trans-chloride
arrangement [7]. Due to the extreme air-sensitivity of this
complex characterization was accomplished only by IR spec-
troscopy and magnetic measurements in solution. A solution
magnetic moment of μef =3.7 μB (CH2Cl2, Evans method)
was determined which is consistent with three unpaired elec-
trons as expected for a d3 confguration. In the IR, the νCN
stretching frequency of the acetonitrile ligand was observed
at 2194 cm−1. The analogous compound bearing NEt linkers,
exhibits the νCN stretching frequency at 2207 cm−1 indicat-
ing that the xylene-based PCP pincer ligand is slightly more
electron donating [7].
Results and discussion
The xylene-based PCP ligand 1 was prepared in a two-step
process following a literature procedure [10]. 2-Bromo-1,3-
bis(bromomethyl)benzene (1a) was treated with one equiv of
PiPr2H to give the phosphonium intermediate 1b which was
subsequently deprotonated with NaOAc in water to aford
the desired ligand (P(C-Br)PCH2-iPr) 1 in 49% isolated yield
(Scheme 2).
A suspension of the hexacarbonyl complexes [M(CO)6]
(M=Cr, Mo, W) and one equivalent of P(C-Br)PCH2-iPr
(1) in CH3CN was heated in a sealed microwave glass tube
at 130 °C (Cr), 130 °C (Mo) and 160 °C (W) for 3–12 h.
In the case of chromium, a mixture of several complexes
was obtained. From this mixture, only the dibromo-Cr(III)
complex with the tentative formula [Cr(PCPCH2-iPr)-
(CH3CN)(Br)2] (2) could be obtained in 40% isolated yield
(Scheme 3). According to DFT calculations, the trans-bro-
mide complex is more stable than the cis-isomer by 36.0 kJ/
On the other hand, with molybdenum and tungsten
the expected seven-coordinate bromo tricarbonyl Mo(II)
and W(II) complexes [Mo(PCPCH2-iPr)(CO)3Br] (3) and
[W(PCPCH2-iPr)(CO)3Br] (4) were obtained (Scheme 3).
These compounds were isolated in 80 and 83% yield and
1
were characterized by H, 13C{1H}, and 31P{1H} NMR
Scheme 2
1 3

Synthesis and characterization of xylene-based group-six metal PCP pincer complexes
The mechanism of the dynamic processes observed for
Scheme 3
the bromo carbonyl complexes 3 and 4 was investigated
by means of DFT calculations (Fig. 1). The free energy
profles of these interconversions are depicted in Fig. 2.
All CO ligands are involved in interchange (pseudorota-
tion) processes that occur in single-step paths. The two
CO ligands located closer to the PCP plane (CO1 and CO2)
exchange positions going through barriers of 53.3 and
57.4 kJ/mol for Mo and W, respectively. This reaction is
the faster processes and corresponds to the one that could
not be stopped in the NMR experiments even at −50 °C
(400 MHz). Exchanging the apical CO (CO3, trans to the
bromide ligand) with one of the other two CO ligands is
a considerably more difcult rearrangement. The calcu-
lated barriers, 96.4 (M = Mo) and 98.9 kJ/mol (M = W)
are indicative of a process that should be slow under the
experimental conditions. Therefore, the DFT calculations
indicate that the apical CO (CO3) corresponds to the single
signal in the 13C{1H} NMR spectrum, while the two CO
ligands opposite to the PCP ligand and closer to its plane
(CO1 and CO2) are engaged in a fast exchange process and
give rise to the broad 13C{1H} NMR signals associated
with two of those ligands.
spectroscopy, IR spectroscopy and HRMS. As typical
for seven-coordinate complexes, 3 and 4 are fuxional
in solution. Thus, in the 31P{1H} NMR spectra only one
resonance is observed at 75.1 and 53.6 ppm, respectively.
In the case of 4, the tungsten–phosphorus coupling was
observed as a doublet satellite due to 183W (14% natural
abundance, I = ½) which is superimposed over the domi-
nant singlet (1JWP = 84 Hz). Characteristic features com-
prise in the 13C{1H} NMR spectrum of 3 and 4 triplet
resonances at 171.3 ppm (JPC = 8.0 Hz) and 168.9 ppm
(JPC = 6.8 Hz), respectively, assigned to the ipso carbon
atoms of the benzene moiety. Moreover, the CO ligands
give rise to one broad and one sharp low-field triplet
resonance in a 2:1 ratio in the range of 238–218 ppm.
The broad signals arise from a fast interchange process
between two CO ligands (vide infra). In the IR spectra,
they display the typical three strong νCO bands at 2008,
1923, and 1871 cm−1 for 3 and 2000, 1913, 1859 cm−1 for
The solid-state structures of [Mo(PCPCH2-iPr)(CO)3Br]
(3) and [W(PCPCH2-iPr)(CO)3Br] (4) were determined by
X-ray difraction. Molecular views of 3 and 4 are depicted
in Figs. 3 and 4 and selected bond lengths and angles are
provided in the captions. Complexes 3 and 4 are seven-
coordinate species where the tridentate PCP ligand is
bound in the typical meridional coordination mode and
three carbonyls ligands and one bromide ligand are flling
the remaining four sites.
4. For comparison, the analogous complexes [Mo(PCPNEt
-
Conclusion
iPr)(CO)3Cl] and [W(PCPNEt-iPr)(CO)3Cl], featuring NEt
linkers, display the three νCO bands at 2007, 1921, and
1899 cm−1 and 2002, 1915, and 1878 cm−1, respectively,
again indicating that the xylene-based pincer ligand is
slightly more electron donating than the one based on the
1,3-diaminobenzene moiety [7].
In sum, we have prepared xylene-based group-six PCP
metal complexes via a solvothermal approach starting from
[M(CO)6] (M=Cr, Mo, W) and (2-bromo-1,3-phenylene)
bis(methylene)bis(diisopropylphosphane) (P(C–Br)PCH2
-
iPr). Treatment of [Cr(CO)6] with P(C-Br)PCH2-iPr in
Fig. 1 Calculated structures of
trans- and cis-[Cr(PCPCH2-iPr)
(CH3CN)(Br)2]
1 3

W. Eder et al.
Fig. 2 Free-energy profle (kJ/
mol) for the “pseudorotations”
of the CO ligands in 3 and 4
CH3CN under solvothermal conditions resulted in the forma-
tion of the Cr(III) complex trans-[Cr(PCPCH2-iPr)(CH3CN)-
(Br)2] together with several unidentifed compounds. In con-
trast, with [M(CO)6] (M=Mo, W) seven-coordinate cationic
bromo carbonyl Mo(II) and W(II) complexes of the type
[M(PCPCH2-iPr)(CO)3Br] were formed in high isolated
yields. These seven-coordinate complexes are fuxional in
solution due to CO interconversions. The mechanism of this
dynamic behavior was studied by means of DFT calcula-
tions. All CO ligands are involved in pseudorotation pro-
cesses that occur in single-step paths. The structures of the
Mo and W complexes could be determined by single-crystal
X-ray analysis.
Fig. 3 Structural view of [Mo(PCPCH2-iPr)(CO)3Br] (3) with 50%
thermal ellipsoids (H atoms omitted for clarity). Selected bond
lengths (Å) and bond angles (°): Mo1-C1 2.286(4), Mo1-C21
1.987(6), Mo1-C22 2.023(6), Mo1-C23 1.952(8), Mo1-Br1 2.697(1),
Mo1-P1 2.524(1), Mo1-P2 2.523(1), P1-Mo1-P2 147.84(4)
Fig. 4 Structural views of [W(PCPCH2-iPr)(CO)3Br] (4) with 50%
thermal ellipsoids (H atoms omitted for clarity). Selected bond
lengths (Å) and bond angles (°): W1-C1 2.271(3), W1-C21 2.016(4),
W1-C22 1.991(3), W1-C23 1.964(5), W1-Br1 2.6871(7), W1-P2
2.5219(7), W1-P1 2.5236(7), P1-W1-P2 148.42(2)
1 3

Synthesis and characterization of xylene-based group-six metal PCP pincer complexes
J=7.4 Hz, 2H, CarH), 6.99 (t, J=7.4 Hz, 1H, CarH), 4.04
Experimental
(dt, J=15.5 Hz, 5.0 Hz, 2H, CH2P), 3.72 (dt, J=15.5 Hz,
3.9 Hz, 2H, CH2P), 2.91 (m, 2H, PCH(CH3)2), 2.47 (m,
2H, PCH(CH3)2), 1.32 (m, 18H, PCH(CH3)2), 1.10 (m,
6H, PCH(CH3)2) ppm; 13C{1H} NMR (101 MHz, CD2Cl2,
20 °C): δ=238.1 (br, CO), 224.5 (t, J=12.0 Hz, CO), 171.3
(t, J = 8.0 Hz, Mo-C), 149.1 (t, J = 7.8 Hz, CarC), 127.4
(CarH), 123.1 (t, J=8.0 Hz, CarH), 40.6 (vt, CH2P), 27.6 (t,
J=9.5 Hz, PCH(CH3)2), 27.2 (t, J=10.4 Hz, -PCH(CH3)2),
19.7 (m, PCH(CH3)2) ppm; 31P{1H} NMR (162 MHz,
CD2Cl2, 20 °C): δ=75.1 ppm; IR (ATR):=2008 (νCO), 1923
(νCO), 1871 (νCO) cm−1; HRMS (ESI+, CH3CN/MeOH+1%
H2O): m/z calcd for C23H35O3BrNaMoP2 ([M + Na]+)
621.0197, found 621.0181.
All manipulations were performed under an inert atmos-
phere of argon by using Schlenk techniques or in an MBraun
inert-gas glovebox. The solvents were purifed according to
standard procedures [11]. Deuterated solvents were dried
over 4 Å molecular sieves. The synthesis of (2-bromo-
1,3-phenylene)bis(methylene)bis(diisopropylphosphane)
(P(C–Br)PCH2-iPr) (1) was carried according to the literature
[8]. 1H, 13C{1H}, and 31P{1H} NMR spectra were recorded
1
on Bruker AVANCE-250 and 400 spectrometers. H and
13C{1H} NMR spectra were referenced internally to residual
protio-solvent, and solvent resonances, respectively, and are
reported relative to tetramethylsilane (δ=0 ppm). 31P{1H}
NMR spectra were referenced externally to H3PO4 (85%)
(δ = 0). Room temperature solution (CH2Cl2) magnetic
moments were determined by 1H NMR spectroscopy using
the method of Evans [12].
[(Bromo)[2,6‑bis[[bis(1‑methylethyl)phosphino‑κP]methyl]‑
phenyl‑κC](tricarbonyl)tungsten(II)], [W(PCPCH2‑iPr)(CO)3Br]
(4, C23H35BrO3P2W) This complex was prepared analogously
to 3 with 84.4 mg [W(CO)6] (0.24 mmol) and 100 mg 1
(0.24 mmol) as starting materials at 160 °C. Yield: 136 mg
High-resolution accurate mass data mass spectra were
recorded on a hybrid Maxis Qq-aoTOF mass spectrometer
(Bruker Daltonics, Bremen, Germany) ftted with an ESI
source. Measured accurate mass data of the [M]+ ions for
confrming calculated elemental compositions were typically
within 5 ppm accuracy. The mass calibration was done
with a commercial mixture of perfuorinated trialkyltria-
zines (ES Tuning Mix, Agilent Technologies, Santa Clara,
CA, USA).
1
(83%) as yellowish-brown crystals. H NMR (400 MHz,
CD2Cl2, 20 °C): δ=7.16 (d, J=7.5 Hz, 2H, CarH), 7.02 (t,
J = 7.5 Hz, 1H, CarH), 4.10 (dt, J = 15.6 Hz, 4.8 Hz, 2H,
CH2P), 3.74 (dt, J=15.5 Hz, 3.9 Hz, 2H, CH2P), 2.95 (m,
2H, PCH(CH3)2), 2.51 (m, 2H, PCH(CH3)2), 1.31 (m, 18H,
PCH(CH3)2), 1.11 (m, 6H, PCH(CH3)2) ppm; 13C{1H}
NMR (101 MHz, CD2Cl2, 20 °C): δ=232.2 (br, CO), 217.5
(t, J = 9.15 Hz, JWC = 73 Hz, CO), 168.9 (t, J = 6.80 Hz,
W–C), 150.7 (t, J=7.74 Hz, CarC), 127.6 (CarH), 123.1 (t,
J=7.42 Hz, CarH), 40.8 (vt, CH2P), 27.8 (vt, PCH(CH3)2),
26.8 (vt, PCH(CH3)2),19.8 (m, PCH(CH3)2) ppm; 31P{1H}
NMR (162 MHz, CD2Cl2, 20 °C): δ=53.6 (1JWP =84 Hz)
ppm; IR (ATR):=2000 (νCO), 1913 (νCO), 1859 (νCO) cm−1;
HRMS (ESI+, CH3CN/MeOH + 1% H2O): m/z calcd for
C23H35O3BrNaP2W ([M+Na]+) 707.0652, found 707.0620.
[(Dibromo)[2,6‑bis[[bis(1‑methylethyl)phosphino‑κP]‑
methyl]phenyl‑κC ](acetonitrile)chromium(III)],
[Cr(PCPCH2‑iPr)(CH3CN)(Br)2] (2, C22H38Br2CrNP2) A sus-
pension of 53 mg [Cr(CO)6] (0.24 mmol) and 100 mg 1
(0.24 mmol) in 2.5 cm3 acetonitrile in a sealed microwave
tube was heated at 130 °C for 12 h afording an orange solu-
tion. After evaporation of the solvent the remaining solid
was washed three times with n-pentane (10 cm3). The resi-
due was then taken up in 3 cm3 dichloromethane and precipi-
tated with 10 cm3 n-pentane giving 2 as reddish-brown solid.
Yield: 45 mg (40%); µef =3.7 µB (Evans method, CH2Cl2);
IR (ATR):=2194 (νCN) cm−1.
X‑ray structure determination
X-ray diffraction data of 3 and 4 (CCDC 1897494 and
1897495) were collected at T = 100 K in a dry stream of
nitrogen on a Bruker Kappa APEX II difractometer sys-
tem using graphite-monochromatized Mo-Kα radiation
(λ=0.71073 Å) and fne-sliced φ- and ω-scans. Data were
reduced to intensity values with SAINT and an absorp-
tion correction was applied with the multi-scan approach
implemented in SADABS [13]. The structure was solved
by the dual-space approach implemented in SHELXT [14]
and refned against F2 with SHELXL [15]. Non-hydrogen
atoms were refned anisotropically. The H atoms were placed
in calculated positions and thereafter refned as riding on
the parent atoms. The Br and CO ligands were refned as
disordered about all four positions. The total occupancy of
[(Bromo)[2,6‑bis[[bis(1‑methylethyl)phosphino‑κP]methyl]‑
phenyl‑κC](tricarbonyl)molybdenum(II)], [Mo(PCPCH2‑iPr)‑
(CO)3Br] (3, C23H35BrMoO3P2) A suspension of 100 mg 1
(0.24 mmol) and 63.2 mg [Mo(CO)6] (0.24 mmol) in 2.5 cm3
acetonitrile in a sealed microwave vial was heated at 130 °C
for 3 h afording a red-brown solution. After cooling to
room temperature orange-brown crystals precipitated from
the mother liquor. The supernatant solution was decanted
and the crystals were washed twice with MeOH (1 cm3) to
yield analytically pure 3. Yield: 115 mg (80%) as orange
crystals. 1H NMR (400 MHz, CD2Cl2, 20 °C): δ=7.13 (d,
1 3

W. Eder et al.
6. Tomsu G, Mastalir M, Stöger B, Pittenauer E, Allmaier G, Kirch-
ner K (2018) Organometallics 37:1919
the Br atom was restrained to one. Molecular graphics were
generated with the program MERCURY [16].
7. Himmelbauer D, Mastalir M, Stöger B, Veiros LF, Kirchner K
(2018) Organometallics 37:3631
Computational details
8. Himmelbauer D, Mastalir M, Stöger B, Pignitter M, Somoza V,
Veiros LF, Kirchner K (2018) Inorg Chem 57:7925
9. Himmelbauer D, Stöger B, Veiros LF, Kirchner K (2018) Orga-
nometallics 37:3475
Calculations were performed using the Gaussian 09 software
package [17] with the OPBE and PBE0 functionals without
symmetry constraints, the Stuttgart/Dresden ECP (SDD)
basis set to describe the electrons of the chromium, molyb-
denum and tungsten atoms and a standard 6-31G** basis
for all other atoms as already described previously [7, 18].
10. Rybtchinski B, Ben-David Y, Milstein D (1997) Organometallics
16:3786
11. Perrin DD, Armarego WLF (1988) Purifcation of Laboratory
Chemicals, 3rd edn. Pergamon, New York
12. Sur SK (1989) J Magn Reson 82:169
13. Bruker computer programs (2018) APEX2, SAINT, SADABS,
and TWINABS. Bruker AXS Inc, Madison
Acknowledgements Open access funding provided by Austrian Sci-
ence Fund (FWF). Financial support by the Austrian Science Fund
(FWF) is gratefully acknowledged (Project no. P29584-N28). The
X-ray center of the Vienna University of Technology is acknowledged
for fnancial support and for providing access to the single-crystal
difractometer.
14. Sheldrick GM (2015) Acta Crystallogr A 71:3
15. Sheldrick GM (2015) Acta Crystallogr C 71:3
16. Macrae CF, Edgington PR, McCabe P, Pidcock E, Shields GP,
Taylor R, Towler M, van de Streek J (2006) J Appl Cryst 39:453
17. Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA,
Cheeseman JR, Scalmani G, Barone V, Mennucci B, Petersson
GA, Nakatsuji H, Caricato M, Li X, Hratchian HP, Izmaylov AF,
Bloino J, Zheng G, Sonnenberg JL, Hada M, Ehara M, Toyota K,
Fukuda R, Hasegawa J, Ishida M, Nakajima T, Honda Y, Kitao
O, Nakai H, Vreven T, Montgomery JA Jr, Peralta JE, Ogliaro
F, Bearpark M, Heyd JJ, Brothers E, Kudin KN, Staroverov VN,
Kobayashi R, Normand J, Raghavachari K, Rendell A, Burant JC,
Iyengar SS, Tomasi J, Cossi M, Rega N, Millam JM, Klene M,
Knox JE, Cross JB, Bakken V, Adamo C, Jaramillo J, Gomperts
R, Stratmann RE, Yazyev O, Austin AJ, Cammi R, Pomelli C,
Ochterski JW, Martin RL, Morokuma K, Zakrzewski VG, Voth
GA, Salvador P, Dannenberg JJ, Dapprich S, Daniels AD, Farkas
Ö, Foresman JB, Ortiz JV, Cioslowski J, Fox DJ (2009) Gaussian
09, revision A.02. Gaussian Inc, Wallingford
Open Access This article is distributed under the terms of the Creative
Commons Attribution 4.0 International License (http://creativecom-
mons.org/licenses/by/4.0/), which permits unrestricted use, distribu-
tion, and reproduction in any medium, provided you give appropriate
credit to the original author(s) and the source, provide a link to the
Creative Commons license, and indicate if changes were made.
References
1. Murugesan S, Kirchner K (2016) Dalton Trans 45:4163
2. Albrecht M, van Koten G (2001) Angew Chem Int Ed 40:3750
3. van der Boom ME, Milstein D (2003) Chem Rev 103:1759
4. Hebden TJ, Schrock RR, Takase MK, Müller P (2012) Chem
Commun 48:1851
18. Schröder-Holzhacker C, Stöger B, Pittenauer E, Allmaier G, Vei-
ros LF, Kirchner K (2016) Monatsh Chem 147:1539
Publisher’s Note Springer Nature remains neutral with regard to
jurisdictional claims in published maps and institutional afliations.
5. de Aguiar SRMM, Stöger B, Pittenauer E, Allmaier G, Veiros LF,
Kirchner K (2016) Organometallics 35:3032
1 3