The cyclometallation of bis(di-p-methylbenzylphosphino)methane

Article abstract of DOI:10.1016/j.jorganchem.2005.04.020

The new diphosphine (4-MeC₆H₄CH₂) ₂PCH₂P(4-MeC₆H₄CH₂) ₂, L, was reacted with [MnMe(CO)₅] to give the novel cyclometallated compound [Mn{(4-MeC

Full text of DOI:10.1016/j.jorganchem.2005.04.020

Journal of Organometallic Chemistry 690 (2005) 3638–3640
www.elsevier.com/locate/jorganchem
The cyclometallation of bis(di-p-methylbenzylphosphino)methane
1
Alberto Fernandez , Jose M. Vila
*
´
´
´ ´
Departamento de Quımica Inorganica, Universidad de Santiago de Compostela, E-15782 Santiago de Compostela, Spain
Received 19 January 2005; received in revised form 8 April 2005; accepted 11 April 2005
Available online 8 June 2005
Abstract
The new diphosphine (4-MeC6H4CH2)2PCH2P(4-MeC6H4CH2)2, L, was reacted with [MnMe(CO)5] to give the novel cyclomet-
allated compound [Mn{(4-MeC6H3CH2)(4-MeC6H4CH2)PCH2P(4-MeC6H4CH2)2}(CO)3], as the mer isomer, and with the ligand in
a terdentate [C,P,P] fashion.
Ó 2005 Elsevier B.V. All rights reserved.
Keywords: Palladium; Cyclometallation; Phosphines
1. Introduction
2. Results and discussion
Transition metal cyclometallated compounds com-
prise a rich and fruitful research field within organome-
tallic chemistry [1–7], due in part to the numerous
available metals [8], but mainly to the ever growing ver-
satility of the organic ligands [8], which renders a nearly
limitless range of substrates that may be employed in the
metallation reaction; nitrogen-donor species, on the one
hand, and palladium and platinum, on the other, are the
more frequently used ligands and metals, respectively.
Cyclometallation may also be achieved through phos-
phorus-donor ligands, which likewise give rise to a great
range of compounds [9] and it has been shown that ter-
tiary phosphines with bulky groups on the phosphorus
atom [10a] as well as formation of five-membered metal-
lacycles [8] is paramount to the cyclometallation pro-
cess, although tertiary phosphines can also form
compounds with four-membered rings [10]. Herein, we
report the cyclometallation of the short chain diphos-
phine bis(di-p-methylbenzylphosphino)methane.
Addition of bis(dichlorophosphino)methane to an
ethereal solution of p-methylbenzylmagnesium bromide
gave the new diphosphine (4-MeC6H4CH2)2PCH2P(4-
MeC6H4CH2)2, L, as a white solid which was fully char-
acterised (see Section 3). The AA0XX0 pattern of the
four phenyl ring protons appear as a broad signal which
integrates for 16 H; four singlets at d2.36, d2.34, d2.33
and d2.31 were assigned to the p-Me groups.
Treatment of L with [MnMe(CO)5] in n-octane gave
the unprecedented cyclometallated manganese (I) com-
pound [Mn{(4-MeC6H3CH2)(4-MeC6H4CH2)PCH2P-
(4-MeC6H4CH2)2}(CO)3], 1, as a yellow solid, with the
diphosphine as terdentate [C, P,P], which was fully char-
acterised (see Section 3). The reaction was monitored by
close inspection of the carbonyl region in the infrared
spectrum. The final product consisted of only one of
the two possible isomers, i.e., the mer moiety, as op-
posed to the metallation of PMe2(CH2Ph) where both
isomers were obtained [11]. Thus, the IR spectrum
showed three t(CO) bands characteristic of a mer-
M(CO)3 group at 1980s, 1905s, 1880s cmꢀ1. The 1H
NMR spectrum showed the resonances due to the met-
allated ring protons: H3, H5 and H6, at d6.91s, d6.79d
and d6.35dd, respectively, the latter coupled to the 31P
*
Corresponding author. Tel.: +3481528073; fax: +3481595012.
E-mail address: qideport@usc.es (J.M. Vila).
1
Present address: Departamento de Qu´ımica Fundamental, Uni-
versidad de La Corun˜a, 15071 La Corun˜a, Spain.
0022-328X/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.04.020

´
A. Fernandez, J.M. Vila / Journal of Organometallic Chemistry 690 (2005) 3638–3640
3639
nucleus (see Section 3 and Scheme 1); a complex signal,
integrating for 12H, was assigned to three sets of
AA0XX0 protons of the remaining phenyl rings. The
31P–{1H} spectrum showed two doublets at d47.22 (P
trans to Ph) and d23.01 (P trans to CO), assigned to
the two inequivalent phosphorus atoms. The assignment
of the doublets was made on the assumption that a li-
gand of greater trans influence shifts the resonance of
the phosphorus atoms trans to it to lower frequency
[12]. The carbon-13 NMR spectrum was in agreement
with the assigned structure. The carbonyl resonances
were at d211.3 (due to the two mutually trans carbons)
and at d197.4 (trans to phosphorus) (see Section 3).
The CMn and C1 resonances were at d149.78, and
d139.15, respectively, downfield shifted as compared to
the other phenyl carbon signals consequent upon metal-
lation of the phenyl ring, the former appeared coupled
to both phosphorus nuclei; the CH2C6H3 carbon reso-
nance was also slightly downfield shifted, and coupled
to only one phosphorus nuclei.
As opposed to monodentate o-tolylphosphines,
which are readily metallated at the methyl group [13],
chelating phosphines R2P(CH2)nPR2 (n = 2, 3) are very
resistant to internal metallation by palladium or plati-
num, and only for n = 3, has this been reported [14];
for n = 1, metallation would seem even more difficult
due to the rigidity of the chelate ring, which restricts
the approach of the ortho-carbon atom of the phenyl
ring group towards the metal. However, the analogous
diphosphine, bis(di-p-methylbenzylphosphino)methane,
L, having greater flexibility induced by the methylene
group on the phosphorus atom, should enable the aro-
matic ring to approach the metal center, and to enhance
the easiness with which metallation is produced. As for
electronic effects it is known that substitution of a
methyl group in the para position on a tertiary aryl-
phosphine increases the tendency of the ligand to under-
go ortho-metallation with platinum (II) complexes [10a]
and that electron-withdrawing substituents activate the
ring to attack by nucleophilic reagents, whilst electron-
donating substituents activate the ring towards electro-
philic attack [15]. However, in the present case the
p-Me substituted ring could only be metallated by
Mn(I); attempts with Pd(II) and Pt(II) failed. Further-
more, methyl groups on metals such as rhodium or plat-
inum promote internal metallation of tertiary phosphine
ligands [16]; disappointingly, we have found this not to
be the case for [PtMe2(COD)] which failed to metallate
the title ligand, L; however, with [MnMe(CO)5] the
diphosphine ligand was readily metallated.
3. Experimental
All reactions were carried out in an atmosphere of
dry nitrogen. Solvents were purified by standard meth-
ods [17]. Chemicals were reagent grade. [MnMe(CO)5]
was prepared as reported [18]. The phosphine
Cl2PCH2PCl2 was purchased from Strem chemicals.
Microanalyses were carried out using a Carlo-Erba Ele-
mental Analyser, Model 1108. IR spectra were recorded
as Nujol mulls or polythene discs on a Perkin–Elmer
1330 and on a Mattson spectrophotometers. NMR spec-
tra were obtained as CDCl3 or (CD3)2SO solutions and
referenced to SiMe4 (1H, 13C) or 85% H3PO4 (31P–{1H})
and were recorded with Bruker AMX 300, AMX 500
and WM250 spectrometers. All chemical shifts were re-
ported downfield from standards.
3.1. Synthesis of (4-MeC6H4CH2)2PCH2P-
(4-MeC6H4CH2)2
Magnesium turnings (1.110 g, 45.58 mmol) and a cat-
alytic amount of iodine were added in anhydrous ethyl
Me
Me
Me
P
P
EtO2
PCl2
+
Cl2P
MgBr
Me
Me
MnMe(CO)5
n-octane
CO
(p-MeC6H4)
P(p-MeC6H4)2
CO
P
Mn
CO
6
3
5
Me
Scheme 1.

´
A. Fernandez, J.M. Vila / Journal of Organometallic Chemistry 690 (2005) 3638–3640
3640
ether (40 cm3). Then, a-bromo-p-xylene (8.470 g, 45.78
mmol) in diethyl ether (40 cm3) was added dropwise
and the resulting mixture was refluxed for 1.5 h. To
the cooled reaction bis(dichlorophosphino)methane
(1.458 g, 6.70 mmol) in diethyl ether (40 cm3) was added
dropwise and stirred for 12 h. Ammonium chloride in
water (9 g/100 cm3) was added, stirring continued for
a further 30 min, the mixture extracted with four 50
cm3 portions of ethanol and the combined extracts re-
duced to low volume under vacuum. The residue was
recrystallized from methanol to give a white solid, which
was filtered off and dried in vacuo. Yield: 70%. C33H38P2
(496.61). Anal. Found: C, 79.9; H, 7.6%. Calc.: C, 79.8;
(CH); 125.3 (CH); 124.7 (CH); 39.2 (d, CH2C6H3,
1J(PC) = 28.0 Hz); 32.6 (d, CH2, 1J(PC) = 27.5 Hz);
32.4 (d, CH2, 1J(PC) = 27.1 Hz); 32.0 (d, CH2,
1J(PC) = 28.2 Hz); 25.1 (d, PCH2 P, 1J(PC) = 26.5
Hz); 20.92, 20.85, 20.75, 20.70 (s, 4Me). 31P–{1H}
NMR: d47.22d, d23.01d (2J(PP) = 83.3 Hz).
Acknowledgements
´
We thank the Ministerio de Educacion y Cultura
(BQU2002-04533-C02-01/02) and the Xunta de Galicia
(Proyecto PGIDIT03PXIC1050PN), Spain.
1
H, 7.7. H NMR: d7.1 (m, 16H, C6H4) d3.0 (m, 10H,
PCH2P, PCH2Ph), d2.36 (s, 3H, Me), d2.34 (s, 3H,
Me), d2.33 (s, 3H, Me), d2.31 (s, 3H, Me). 31P–{1H}
NMR: d23.32s.
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1
m(C@O) 1980s, 1905s, 1880s cmꢀ1. H NMR: d6.91 (s,
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3
1H, H3), d6.79 (d, 1H, H5, J(H5H6) = 6.8 Hz), d6.35
4
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2
13C–{1H} NMR: d197.4 (dd, CO, J(PC) = 15.6, 9.5
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2
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3
CH, J(PC) = 3.1 Hz); 132.1 (CH); 132.0, 131.9, 131.7,
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