Functionalised N-pyrrolyl phosphines: Synthesis and molybdenum chemistry of a new ketophosphine

Article abstract of DOI:10.1039/b102193l

The synthesis and X-ray crystal structure of the new keto-functionalised N-pyrrolyl phosphine ligand PPh₂NC₄H₃{C(O)CH₃}-2 L is reported. The ligand coordinates to molybdenum(II) as either a uni- or a bi-dentate ligand, and the reversibility of the ketone coordination is demonstrated by the interconversion of the complexes [MoCl(CO)₂(L-κ¹P)(η⁵-C ₅H₅)] 1 and [Mo(L-κ² P,O)(CO)₂(η₅-C₅H₅)][BF ₄] 2 on abstraction or addition of chloride. The X-ray crystal structure of 1 is also reported.

Full text of DOI:10.1039/b102193l

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Functionalised N-pyrrolyl phosphines: synthesis and molybdenum
chemistry of a new ketophosphine
Christopher D. Andrews, Andrew D. Burrows,* Jason M. Lynam, Mary F. Mahon and
Mark T. Palmer
Department of Chemistry, University of Bath, Claverton Down, Bath, UK BA2 7AY .
E-mail: a.d.burrows=bath.ac.uk
Received (in Strasbourg, France) 5th March 2001, Accepted 27th March 2001
First published as an Advance Article on the web 15th May 2001
The synthesis and X-ray crystal structure of the new keto-functionalised N-pyrrolyl phosphine ligand
PPh NC H MC(O)CH N-2 L is reported. The ligand coordinates to molybdenum(II) as either a uni- or a bi-dentate
2
4 3
3
ligand, and the reversibility of the ketone coordination is demonstrated by the interconversion of the complexes
[
MoCl(CO) (L-j1P)(g5-C H )] 1 and [Mo(L-j2P,O)(CO) (g5-C H )][BF ] 2 on abstraction or addition of
5 5 5 5
chloride. The X-ray crystal structure of 1 is also reported.
2
2
4
There has recently been an upsurge of interest in N-pyrrolyl
phosphines1 due to their strongly electron withdrawing char-
acter, and the catalytic implications of this behaviour.2 While
functionalities have been added with a view to enhancing the
electronic character,3 very little use has been made of the pyr-
rolyl ring as a skeleton for phosphines with additional donor
groups.4 Since functionalised pyrroles are readily available, we
reasoned that N-pyrrolyl phosphines containing additional
functionalities were likely to be easily accessible ligands with
interesting properties.
The molecular structure of L is shown in Fig. 1 and selected
bond lengths and angles are given in Table 1.
The sum of angles around the nitrogen atom in L is 360¡,
similar to those in tri(N-pyrrolyl)phosphine. The PÈN distance
in
L is longer than those in tri(N-pyrrolyl)phosphine
[1.677(8)È1.710(8) A
character in contrast to aminophosphines of the type
Ž
],13 and consistent with no double bond
PPh NHR.12 The bond lengths observed for L suggest a sig-
2
niÐcant structural contribution from an enolate canonical
Ketophosphines such as PPh CH C(O)Ph have attracted
2
2
considerable interest, most notably from Braunstein and co-
workers,5 for their ability to act as uni- or bi-dentate ligands.
Bifunctional ligands containing both hard and soft donor
atoms are of interest catalytically, in part through the poten-
tial for hemilability,6 and also for the formation of hetero-
bimetallic compounds.7 The p-acceptor nature of N-pyrrolyl
phosphines a†ords the potential for enhancing the electronic
di†erence between the donor atoms in such a ligand.
The majority of studies with ketophosphines have concen-
trated on late transition metals, and a search of the Cam-
bridge Structural Database8 reveals only one example of a
Group 6 metal ketophosphine complex, a tungsten complex in
which the ketophosphine was formed in situ.9 Very recently,
molybdenum half-sandwich compounds of the amidophos-
phine PPh CH C(O)NPh have been reported,10 and a pre-
2
2
2
liminary account of molybdenum g6-arene complexes with b-
ketophosphines has appeared.11 In this paper we report the
synthesis of a new ketophosphine, based on 2-acetylpyrrole,
and the reactions of this ligand with molybdenum(II) com-
plexes.
Fig. 1 The molecular structure of L with thermal ellipsoids shown at
the 30% probability level.
Results and discussion
The keto-functionalised N-pyrrolyl phosphine PPh NC H -
Table 1 Selected bond lengths (A
Ž
) and angles (¡) for L
2
4 3
MC(O)CH N-2 L can be prepared in good yield from the reac-
3
tion of 2-acetylpyrrole and PPh Cl in the presence of base. If
P(1)ÈN(1)
1.7637(14)
1.365(2)
1.388(2)
1.358(3)
1.380(3)
C(15)ÈC(16)
C(16)ÈC(17)
C(17)ÈC(18)
C(17)ÈO(1)
1.375(3)
1.441(3)
1.504(3)
1.216(2)
2
NEt is used the reaction is slow, taking 48 h to reach com-
pletion. However, use of the stronger base DBU (1,8-
N(1)ÈC(13)
3
N(1)ÈC(16)
C(13)ÈC(14)
diazabicyclo[5.4.0]undec-7-ene) reduces the reaction time to 2
h. As in the case of functionalised aminophosphines,12 care
needs to be taken to exclude water to prevent the formation of
Ph PP(O)Ph . Compound L was characterised on the basis
C(14)ÈC(15)
C(13)ÈN(1)ÈC(16)
C(13)ÈN(1)ÈP(1)
C(16)ÈN(1)ÈP(1)
107.45(15)
126.74(12)
125.48(12)
N(1)ÈC(16)ÈC(17)
O(1)ÈC(17)ÈC(16)
122.14(17)
121.11(18)
2
2
of multinuclear NMR, IR and a single-crystal X-ray analysis.
824
New J. Chem., 2001, 25, 824È826
DOI: 10.1039/b102193l
This journal is ( The Royal Society of Chemistry and the Centre National de la Recherche ScientiÐque 2001

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[
*d \ 56.4 ppm], whereas non-coordination of the carbonyl
group is consistent with the small change in l(CO) from that
of L [*l(CO) \ ]11 cm~1]. The NMR parameters suggest
that 1 exists solely as the cis-isomer, in a similar manner to
[
MoCl(CO) (PPh )(g5-C H )].14 ConÐrmation of this struc-
2
3
5 5
ture was obtained by a single crystal X-ray structure analysis,
and the molecular structure of 1 is shown in Fig. 2, with selec-
ted bond lengths and angles given in Table 2. Compound 1
adopts a pseudo-square pyramidal metal geometry, with the
cis carbonyls, chloride and phosphorus atom forming the base
of the pyramid and the cyclopentadienyl ring the apex. The
bond lengths and angles within the ketophosphine are gener-
ally similar to those in the free ligand, though shortening of
the PÈN bond on coordination may be signiÐcant.
The reaction of L with [Mo(CH )(CO) (g5-C H )] and
3
3
5 5
HBF É OEt in dichloromethane gave [Mo(L-j2P,O)(CO) -
4
2
2
(
g5-C H )][BF ] 2 in good yield. The value of d(31P) for 2 is
5
5
4
similar to that observed for 1, though a large decrease in
l(CO) from that of L [*l(CO) \ [90 cm~1] is indicative of
coordination of the keto oxygen atom. Compounds 1 and 2
can be readily interconverted by the abstraction or addition of
Fig. 2 The molecular structure of complex 1 with thermal ellipsoids
shown at the 30% probability level.
chloride. Hence the reaction of 1 with AgBF in dichloro-
4
methane led to precipitation of AgCl, and coordination of the
keto group to give 2. Furthermore, the reaction of 2 with
[
NEt Bz]Cl in dichloromethane led to the displacement of the
form, thus C(16)ÈC(17) is very short for a CÈC single bond,
and C(15)ÈC(16) is signiÐcantly longer than C(13)ÈC(14). The
low value of the carbonyl stretching frequency [l(CO) \ 1643
cm~1] is also consistent with this, as is the fact that the car-
bonyl group lies approximately in the plane of the pyrrole
ring.
3
keto group by chloride and formation of 1. These reactions,
along with the syntheses of 1 and 2 are summarised in Scheme
1
.
In summary, we have demonstrated that the pyrrole ring
can serve as the skeleton for functionalised phosphines. The
new ketophosphine ligand L has been prepared and character-
ised, and shown to coordinate to molybdenum(II) as either a
uni- or a bi-dentate ligand. Interconversion of complexes 1
and 2 on abstraction or addition of chloride demonstrates the
reversible coordination of the ketone oxygen atom. Despite
the contribution from the enolate canonical form, coordinated
L is not readily deprotonated. This leads to di†erences in
reactivity between compounds of L and analogous com-
pounds of PPh CH C(O)Ph. These will be reported at a later
The reaction of L with [MoCl(CO) (g5-C H )] in reÑuxing
3
5 5
hexane gave [MoCl(CO) (L-j1P)(g5-C H )] 1 in good yield.
2
5 5
Coordination of the phosphorus atom in 1 is witnessed by the
large shift in d(31P) from that observed for the free ligand
Table 2 Selected bond lengths (A
Ž
) and angles (¡) for 1
Mo(1)ÈP(1)
2.5176(16)
2.4991(16)
1.961(5)
1.969(6)
1.748(4)
1.383(7)
1.407(6)
76.0(2)
80.03(15)
134.99(15)
111.62(16)
77.86(16)
76.42(5)
C(8)ÈC(9)
1.355(8)
1.395(8)
1.366(8)
1.451(7)
1.495(7)
1.222(6)
2
2
Mo(1)ÈCl(1)
C(9)ÈC(10)
C(10)ÈC(11)
C(11)ÈC(12)
C(12)ÈC(13)
C(12)ÈO(3)
date.
Mo(1)ÈC(6)
Mo(1)ÈC(7)
P(1)ÈN(1)
Experimental
N(1)ÈC(8)
N(1)ÈC(11)
Reactions were routinely carried out using Schlenk-line tech-
niques under pure dry dinitrogen using dry dioxygen-free sol-
vents unless noted otherwise. Microanalyses (C, H and N)
were carried out by Mr Alan Carver (University of Bath
Microanalytical Service). Infrared spectra were recorded on a
Nicolet 510P spectrometer as KBr pellets or dichloromethane
solutions in KBr cells. 1H and 31PM1HN NMR spectra were
recorded on a JEOL JNM-EX270 spectrometer operating at
C(6)ÈMo(1)ÈC(7)
C(6)ÈMo(1)ÈCl(1)
C(7)ÈMo(1)ÈCl(1)
C(6)ÈMo(1)ÈP(1)
C(7)ÈMo(1)ÈP(1)
Cl(1)ÈMo(1)ÈP(1)
C(8)ÈN(1)ÈC(11)
C(8)ÈN(1)ÈP(1)
C(11)ÈN(1)ÈP(1)
N(1)ÈC(11)ÈC(12)
O(3)ÈC(12)ÈC(11)
106.1(4)
125.5(4)
128.3(3)
122.2(5)
121.0(5)
2
70 MHz referenced to SiMe and 109.4 MHz referenced to
4
H PO , respectively. 13CM1HN NMR spectra were recorded
on a Varian Mercury 400 spectrometer operating at 100.4
3
4
MHz referenced to SiMe .
4
Synthesis of L
DBU (1.83 g, 12 mmol) and PPh Cl (2.41 g, 11 mmol) were
2
added sequentially with stirring to
a solution of 2-
acetylpyrrole (1.19 g, 11 mmol) in THF (40 cm3). The reaction
mixture was stirred for 2 h and the solution Ðltered to remove
[
DBUH]Cl. The resulting solution was evaporated under
reduced pressure to give a white solid, which was washed with
hexane and recrystallised from THFÈhexane to give block-
shaped crystals. Yield 2.34 g (73%) (found: C, 73.4; H, 5.62;
N, 4.75; C
H
NOP requires C, 73.7; H, 5.50; N, 4.78%).
1
8 16
3
1PM1HN NMR (CDCl ) d 55.8; 1H NMR (CDCl ) d 7.40È7.38
3
3
(
m, 6H, Ar), 7.32È7.27 (m, 4H, Ar), 7.16 (m, 1H, CH), 6.44 (m,
Scheme 1 (i) L; (ii) L, HBF ; (iii) AgBF ; (iv) Cl~.
1H, CH), 6.24 (m, 1H, CH), 2.43 (s, 3H, CH ); 13CM1HN NMR
4
4
3
New J. Chem., 2001, 25, 824È826
825

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(
[
CDCl ) d 187.4 (s, C2O), 25.7 (s, Me). IR (KBr, cm~1): 1643
arisation e†ects. The structure solution and reÐnement were
undertaken using SHELXS-8615 and SHELXL-9716 respec-
tively. The plots of the asymmetric units were produced using
ORTEX.17
CCDC reference numbers 161741 and 161742. See http://
www.rsc.org/suppdata/nj/b1/b102193l/ for crystallographic
data in CIF or other electronic format.
3
vs, l(C2O)].
Synthesis of 1
[
0
6
MoCl(CO) (g5-C H )] (0.150 g, 0.53 mmol) and L (0.149 g,
3
5 5
.53 mmol) were dissolved in hexane (20 cm3) and reÑuxed for
h. The crude product was recrystallised from
dichloromethaneÈtoluene to give 1 as a dark red powder
Acknowledgements
The EPSRC and the University of Bath are thanked for
funding.
(
0.281 g, 89%) (found: C, 54.9; H, 3.94; N, 2.64;
C
H
ClMoNO P requires C, 55.0; H, 3.88; N, 2.57%).
2
5 21
3
3
7
1
1PM1HN NMR (CDCl ) d 112.2; 1H NMR (CDCl ) d 7.72È
3
3
.38 (m, 10H, Ar), 7.20 (m, 1H, CH), 6.46 (m, 1H, CH), 6.21 (m,
References
H, CH), 5.61 (s, 5H, Cp), 2.31 (s, 3H, Me); 13CM1HN NMR
(
2
CDCl ) d 254.4 (d, MCO, 2J \ 31.4), 242.9 (d, MCO,
1
K. G. Moloy and J. L. Petersen, J. Am. Chem. Soc., 1995, 117,
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3
PC
J \ 1 Hz), 185.5 (s, C2O), 96.1 (s, Cp), 26.0 (s, Me). IR
PC
2
(
CH Cl , cm~1): 1970 [vs, l(CO)], 1888 [vs, l(CO)], 1654 [s,
2
l(C2O)].
1
7, 104; V. Rodriguez, B. Donnadieu, S. Sabo-Etienne and B.
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`

Synthesis of 2
2

lkowski, J.
[
Mo(CH )(CO) (g5-C H )] (0.133 g, 0.49 mmol) was dissolved
5 5
in CH Cl (10 cm3), cooled to [78 ¡C and HBF É OEt (81
3
2
3
2
4
2

lL, 0.49 mmol) added dropwise with stirring. The solution
was allowed to warm to room temperature, then cooled back
to [78 ¡C, and L (0.148 g, 0.50 mmol) added. The solution
was then allowed to warm back to room temperature, stirred
at ambient temperature for 16 h, and recrystallised from
dichloromethaneÈtoluene to give 2 as a dark red powder
1
3
4
5
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3
59.
(
0.250 g, 83%) (found C, 49.2; H, 3.73; N, 2.17;
S.-E. Bouaoud, P. Braunstein, D. Grandjean, D. Matt and D.
Nobel, Inorg. Chem., 1986, 25, 3765; P. Braunstein, D. Matt, D.
Nobel, F. Balegroune, S.-E. Bouaoud, D. Grandjean and J.
Fischer, J. Chem. Soc., Dalton T rans., 1988, 353; P. Braunstein, Y.
C
H
BF MoNO P É 0.25CH Cl requires C, 49.0; H, 3.50;
2
5 21
4
3
2 2
N, 2.27%). 31PM1HN NMR (CDCl ) d 114.6; 1H NMR
3
(
6
CDCl ) d 7.67È7.45 (m, 11H, Ar and CH), 7.29 (m, 1H, CH),
3
Chauvin, J. Nahring, A. DeCian, J. Fischer, A. Tiripicchio and F.
Ž
.56 (m, 1H, CH), 5.55 (s, 5H, Cp), 2.59 (s, 3H, Me); 13CM1HN
Ugozzoli, Organometallics, 1996, 15, 5551; J. Andrieu, P. Braun-
stein, F. Naud and R. D. Adams, J. Organomet. Chem., 2000, 601,
NMR (CDCl ) d 248.0 (d, MCO, 2J \ 30.0), 240.0 (d, MCO,
3
PC
2
J
\ 3.2), 199.3 (d, C2O, 3J \ 7 Hz), 97.7 (s, Cp), 27.8 (s,
4
3.
PC
PC
Me). IR (CH Cl , cm~1): 1997 [vs, l(CO)], 1928 [vs, l(CO)],
6
A. Bader and E. Lindner, Coord. Chem. Rev., 1991, 108, 27; C. S.
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Dalton T rans., 1984, 863.
2
2
1
553 [s, l(C2O)].
Crystallography
7
8
Single crystals of L and 1 were prepared by recrystallisation
from dichloromethaneÈhexane and dichloromethaneÈtoluene
respectively.
9
0
L:
C
H
NOP, M \ 293.29, monoclinic, P2 /a,
18 16
1
a \ 9.501(3), b \ 16.156(3), c \ 11.009(3) A
Ž
, b \ 112.91(2)¡,
1
J.-M. Camus, D. Morales, J. Andrieu, P. Richard, R. Poli, P.
Braunstein and F. Naud, J. Chem. Soc., Dalton T rans., 2000,
2577.
V \ 1556.6(7) AŽ 3, Z \ 4, T \ 293(2) K, crystal dimensions
0
.50 ] 0.50 ] 0.50 mm, k \ 0.175 mm~1, 3326 measured
reÑections, 3036 independent reÑections (R \ 0.0085),
11 N. G. Jones, M. L. H. Green, X. Morise, L. Rees and D. Watkin,
int
3
4th Conference on Coordination Chemistry, Edinburgh, 2000,
R1 \ 0.0348, wR2 \ 0.0988 [I [ 2p(I)].
poster abstract P0453.
1
: C
H
ClMoNO P É 0.5CH Cl , M \ 588.25, triclinic,
2
5 21
3
2
2
12 A. D. Burrows, M. F. Mahon and M. T. Palmer, J. Chem. Soc.,
6
P1, a \ 9.762(3), b \ 10.383(4), c \ 14.411(5) A
Ž
, a \ 110.50(3),
Dalton T rans., 2000, 1669.
J. L. Atwood, A. H. Cowley, W. E. Hunter and S. K. Mehrota,
b \ 93.69(3), c \ 100.85(3)¡, V \ 1330.2(8)
AŽ
3, Z \ 2,
1
3
T \ 170(2) K, crystal dimensions 0.20 ] 0.20 ] 0.20 mm,
k \ 0.780 mm~1, 5137 measured reÑections, 4686 independent
Inorg. Chem., 1982, 21, 1354.
14 D. L. Beach, M. Dattilo and K. W. Barnett, J. Organomet. Chem.,
1
977, 140, 47; L. J. Todd, J. R. Wilkinson, J. P. Hickey, D. L.
reÑections
I [ 2p(I)].
(R \ 0.0242),
R1 \ 0.0492,
wR2 \ 0.1225
int
Beach and K. W. Barnett, J. Organomet. Chem., 1978, 154, 151.
G. M. Sheldrick, Acta Crystallogr. Sect. A, 1990, 46, 467.
G. M. Sheldrick, SHELXL-97, Computer Program for Crystal
[
1
1
5
6
Data were collected on an Enraf-Nonius CAD4 automatic
four-circle di†ractometer in the ranges 2.01 \ h \ 25.96¡ (L)
and 2.13 \ h \ 25.02¡ (1) and corrected for Lorentz and pol-
Structure ReÐnement, University of Go
Ž
ttingen, 1997.
17 P. McArdle, J. Appl. Crystallogr., 1995, 28, 65.
826
New J. Chem., 2001, 25, 824È826