Selective cleavage of P-N bonds and the conversion of rhodium N-pyrrolyl phosphine complexes into diphosphoxane-bridged dimers

Article abstract of DOI:10.1021/ic0108989

Rhodium(l) complexes trans-[RhCl(CO)(PR₂ {NC₄H₃C(O)Me-2})₂] (R = Ph, NC₄H₄) react with water to give the diphosphoxane-bridged dimers [Rh₂Cl₂(CO)₂ (μ-PR₂OPR₂)₂] following cleavage of the P-N bonds to the 2-acetyl-N-pyrrolyl groups. The two dimers have been crystallographically characterized and show a number of structural differences, with the PPh₂OPPh₂ compound possessing semibridging chloride and carbonyl ligands whereas the P(NC₄H₄)₂- OP(NC₄H₄)₂ compound contains only terminal chlorides and carbonyls. No evidence for cleavage of the P-N bonds involving the unfunctionalized N-pyrrolyl groups in trans-[RhCl(CO)(P{(NC₄H₄ })₂-{NC₄H₃C(O)Me-2})₂] was observed.

Full text of DOI:10.1021/ic0108989

Inorg. Chem. 2002, 41, 1695−1697
Selective Cleavage of P−N Bonds and the Conversion of Rhodium
N-Pyrrolyl Phosphine Complexes into Diphosphoxane-Bridged Dimers
Andrew D. Burrows,* Mary F. Mahon, Mark T. Palmer, and Maurizio Varrone
Department of Chemistry, UniVersity of Bath, ClaVerton Down, Bath, U.K. BA2 7AY
Received August 24, 2001
-
1
1
Rhodium(I) complexes trans-[RhCl(CO)(PR
R ) Ph, NC ) react with water to give the diphosphoxane-
bridged dimers [Rh Cl (CO) (µ-PR OPR ] following cleavage of
the P−N bonds to the 2-acetyl-N-pyrrolyl groups. The two dimers
have been crystallographically characterized and show a number
2
{NC
4
H
3
C(O)Me-2})
2
]
of ν(CO) (1981 cm ) suggests L to be less electron
withdrawing than PPh (pyr), despite the presence of the acyl
(
4
H
4
2
substituent. Complex 1 is stable in the solid state, but
dissolution in dichloromethane at room temperature led to a
slow darkening of color and the precipitation of red crystals
of 2. The IR spectrum of 2 showed the presence of both
terminal and bridging carbonyl bands (1964, 1793 cm ),
but no peak in the ν(CdO) region. The H NMR spectrum
showed broad signals that can be assigned to the phenyl
groups and the absence of the characteristic signals of the
pyrrole ring, in agreement with the loss of the 2-acetylpyr-
rolyl functionality. The very low solubility of 2 prevented a
satisfactory P{ H} NMR spectroscopic analysis, and only
a weak broad resonance could be observed after 18 h of
acquisition time, suggesting fluxionality in solution. The
spectroscopic evidence indicated that a major reorganization
of the ligand had occurred, with the formation of a poly-
2
2
2
2
2 2
)
of structural differences, with the PPh
semibridging chloride and carbonyl ligands whereas the P(NC
OP(NC compound contains only terminal chlorides and
carbonyls. No evidence for cleavage of the P−N bonds involving
the unfunctionalized N-pyrrolyl groups in trans-[RhCl(CO)(P{NC
NC C(O)Me-2}) ] was observed.
2 2
OPPh compound possessing
-
1
4 4 2
H ) -
1
4 4 2
H )
4 4 2
H } -
{
4
H
3
2
3
1
1
Recently, N-pyrrolyl phosphines have attracted attention
due to their electronic properties, being excellent π-acceptor
1
ligands. Although functionalities have been added with a
2
view to enhancing this ability, exploitation of the pyrrolyl
nuclear species. Structural characterization by X-ray crystal-
lography identified 2 as the dimeric compound [Rh
group as a framework for bifunctional phosphines has been
5
2
Cl
2
-
3
4
relatively neglected. We recently reported the synthesis of
2
2
(CO)
2
(µ-L )
2
] (L ) PPh OPPh ) (Figure 1). The oxygen
2 2
1
the pyrrole-based ketophosphine PPh
2
{NC
4
H
3
C(O)Me-2} L
2
atom in L is believed to originate from adventitious water,
and this was confirmed by an increase in the rate of formation
of 2 from 1 on use of wet dichloromethane. The identity of
the solvent is not an important factor in the reaction, and 2
can also be prepared from 1 using wet toluene.
The coordination geometry around each rhodium center
in 2 is distorted trigonal bipyramidal, with two phosphorus
atoms from different bridging L ligands arranged trans to
and demonstrated that it can act as a hemilabile ligand on
molybdenum. In this communication we report how hy-
1
drolysis of the P-N bond in rhodium complexes of both L
3
and the related ligand P(pyr)
NC
ing bridging diphosphoxane ligands.
The yellow complex trans-[RhCl(CO)(L1)
2
{NC
H
4 3
C(O)Me-2} L (pyr )
H , N-pyrrolyl) lead to the formation of dimers contain-
4
4
2
2
] 1 was formed
in high yield from the reaction of [Rh
L in toluene. The value of ν(CdO) (1650 cm ) is consistent
with noncoordination of the oxygen atom whereas the value
2
(µ-Cl)
2
(CO)
4
] with
each other in the axial positions. The equatorial positions
are occupied by one terminal and two semi-bridging ligands,
which were identified as carbonyls and chlorides, disordered
1
-1
equally between the terminal and bridging positions.
*
Author to whom correspondence should be addressed. E-mail:
2
Although a 1:1 mixture of [Rh
2
(CO)
2
(µ-Cl)
2
(µ-L )
2
] and
a.d.burrows@bath.ac.uk.
1) (a) Moloy, K. G.; Petersen, J. L. J. Am. Chem. Soc. 1995, 117, 7696.
2
[
Rh
2
Cl
2
(µ-CO)
2
2
(µ-L ) ] cannot be eliminated on the basis
(
(
b) Trzeciak, A. M.; Glowiak, T.; Grzybek, R.; Zi o´ lkowski, J. J. J.
of the X-ray analysis, a simpler model contains one
Chem. Soc., Dalton Trans. 1997, 1831. (c) Huang, J.; Haar, C. M.;
Nolan, S. P.; Marshall, W. J.; Moloy, K. G. J. Am. Chem. Soc. 1998,
2
2 2
compound, [Rh Cl(CO)(µ-Cl)(µ-CO)(µ-L ) ], present in two
1
20, 7806. (d) Rodriguez, V.; Donnadieu, B.; Sabo-Etienne, S.;
positions with equal likelihood (2′ and 2′′ in Figure 2). The
Chaudret, B. Organometallics 1998, 17, 3809.
(2) Huang, A.; Marcone, J. E.; Mason, K. L.; Marshall, W. J.; Moloy, K.
G.; Serron, S.; Nolan, S. P. Organometallics 1997, 16, 3377.
(3) (a) Brunner, H.; Weber, H. Chem. Ber. 1985, 118, 3380. (b) Brunner,
H.; Li, W.; Weber, H. J. Organomet. Chem. 1985, 288, 359.
(4) Andrews, C. D.; Burrows, A. D.; Lynam, J. M.; Mahon, M. F.; Palmer,
M. T. New J. Chem. 2001, 25, 824.
(5) X-ray data for 2: C50H40Cl2O4P4Rh2‚CH2Cl2, M ) 1190.42, T ) 293-
(2) K, monoclinic, P21/n, a ) 14.394(2) Å, b ) 9.683(2) Å, c )
3
17.287(3) Å, â ) 92.93(2)°, V ) 2406.3(7) Å , Z ) 2, Fc ) 1.599 g
-
3
-1
cm , µ ) 1.081 mm ; 4539 reflections collected of which 4230
independent [Rint ) 0.0167]. Final R indices [I > 2σ(I)] R1 ) 0.0475,
wR2 ) 0.1165.
1
0.1021/ic0108989 CCC: $22.00 © 2002 American Chemical Society
Inorganic Chemistry, Vol. 41, No. 7, 2002 1695
Published on Web 03/15/2002

COMMUNICATION
linear conformation observed for the terminal carbonyl
O(1)-C(26)-Rh(1) 176.3(13)°]. Similar semibridging car-
bonyl ligands were observed in the structure of [Rh (µ-CO)-
]. The two diphosphoxane oxygen atoms are
pointed in opposite directions in 2, giving a chair conforma-
tion for the Rh ring, as opposed to the boat conforma-
[
2
9
(
CO)
2
(µ-dppm)
2
2 4 2
P O
1
0
2 4 2
tion observed in the majority of M P C rings.
1
Although uncoordinated L is slowly hydrolyzed to form
PPh
2 2
P(O)Ph , hydrolysis of 1 is much faster under similar
conditions, suggesting the reaction to be metal-promoted. The
2
chelate complex [RhCl(PPh
prepared from a metal-promoted rearrangement and disrup-
tion of mutually cis Ph PO CCHdCH
ligands.11 The trans
3
)(L )] has previously been
2
2
2
1
orientation of the L ligands in 1 disfavors formation of the
analogous monomer [RhCl(CO)(L )] on hydrolysis, instead
promoting formation of the dimer 2 in which the trans
orientation of the phosphorus donors is maintained.
Figure 1. Molecular structure of 2, with carbonyls and chlorides shown
in one of the two positions. Selected bond lengths (Å) and angles (deg)
Rh(1)-Rh(1)′ 2.8683(9), Rh(1)-P(1) 2.292(1), Rh(1)-P(2)′ 2.299(1), Rh-
2
(
1)-Cl(1) 2.361(5), Rh(1)-Cl(2) 2.464(5), Rh(1)-C(26) 1.83(2), Rh(1)-
C(27) 1.84(2), P(1)-O(3) 1.636(3), P(2)-O(3) 1.651(3), P(1)-O(3)-P(2)
25.9(2). Primed atoms generated by symmetry transformation -x + 1,
y, -z.
1
-
In order to examine the generality of diphosphoxane dimer
3
formation, the ligand P(pyr)
pared from the reaction of P(pyr)
the presence of base and the analogous rhodium chemistry
2
{NC
4
3
H C(O)Me} L was pre-
2
Cl with 2-acetylpyrrole in
3
studied. The reaction of L with [Rh
2
(µ-Cl)
2 4
(CO) ] gave
3
[
RhCl(CO)(L ) ] 3, and IR spectra demonstrated the presence
2
-
1
-1
of both ν(CO) (1997 cm ) and ν(CdO) (1653 cm ). On
stirring complex 3 in wet toluene at 60 °C, a darkening of
color was observed, and on standing, deep red crystals of 4
precipitated. The IR spectrum of 4 showed the presence of
2
Figure 2. Orientations of [Rh2Cl(CO)(µ-Cl)(µ-CO)(µ-L )2] required to
give the observed disordered structure of 2.
-
1
a terminal carbonyl (2031 cm ) and the absence of the acetyl
related cationic complexes [Rh
2
(CO)
2
(µ-Cl)(µ-CO)(µ-L)
]+
2
1
stretch. H NMR spectroscopy again showed loss of the
-acetylpyrrole functionality. The P{ H} NMR spectrum
(
L ) dppm, PPh NHPPh ) have previously been structurally
2
2
3
1
1
2
6
characterized.
Compound 2 is the first reported example of a rhodium
dimer containing bridging PPh OPPh ligands, though rhod-
ium dimers incorporating the related bridging ligand bis-
diphenylphosphino)methane (dppm) have been widely stud-
consists of a broad apparent doublet of triplets consistent
1
2
with a dimeric structure.
The identity of 4 was confirmed crystallographically as
2
2
13
4
4
[Rh
2
Cl
2
(CO)
2
(µ-L )
2
2 2
] [L ) P(pyr) OP(pyr) ] (Figure 3). In
(
7
contrast to 2, the chloride and carbonyl ligands are terminally
coordinated and there is no disorder in the structure. The
geometry around each rhodium center is square-planar, and
the Rh‚‚‚Rh distance, 3.1177(3) Å, is considerably longer
than that observed in 2 and clearly indicates the absence of
a Rh-Rh bond. This was also the case in the structure of
ied. Analysis of the Rh-Rh distances from complexes
containing the Rh (µ-dppm) skeleton shows that the distance
2
2
in 2 [2.8683(9) Å] lies within the ranges observed both for
compounds which contain a Rh-Rh bond [2.52-3.01 Å
8
(
mean 2.77 Å)] and for those in which a Rh-Rh bond is
absent [2.83-3.47 Å (mean 3.16 Å)]. The P‚‚‚P separation
between the phosphorus atoms of the same ligand (2.93 Å)
is longer than the Rh-Rh distance, indicating compression
along the Rh-Rh internuclear axis, though this may be due
to the presence of the bridging ligands. These bridges are
highly unsymmetric, with Rh-C(27) distances of 1.84(2) and
[
Rh
2
Cl
2
(CO)
2
2
(µ-dppm) ], though for this compound the
1
4
Rh‚‚‚Rh distance is still longer [3.2386(5) Å]. The angle
between the mean plane of the rhodium coordination sphere
[Rh(1), C(17), Cl(1), P(1), and P(2)′] and the mean plane of
the dimer framework [Rh(1), P(1), P(2), Rh(1)′, P(1)′, and
4
P(2)′] in 4 is 82°, and the deviation of the L oxygen atom
2.69(3) Å and Rh-Cl(2) distances of 2.464(5) and 2.889(6)
Å, respectively; hence the ligands are best described as
semibridging. This is further reflected in the bond angles,
with the bridging carbonyl ligands [Rh(1)-C(27)-O(2)
(
9) Woodcock, C.; Eisenberg, R. Inorg. Chem. 1985, 24, 1285.
10) Anderson, D. J.; Kramarz, K. W.; Eisenberg, R. Inorg. Chem. 1996,
5, 2688.
(
(
(
3
11) Irvine, D. J.; Glidewell, C.; Cole-Hamilton, D. J.; Barnes, J. C.; Howie,
A. J. Chem. Soc., Dalton Trans. 1991, 1765.
12) Davis, A. L.; Goodfellow, R. J. J. Chem. Soc., Dalton Trans. 1993,
2273.
165.7(12)°] showing a relatively small deviation from the
(6) (a) Cowie, M. Inorg. Chem. 1979, 18, 286. (b) Olmstead, M. M.;
Lindsay, C. H.; Benner, L. S.; Balch, A. L. J. Organomet. Chem. 1979,
(13) X-ray data for 4: C34H32Cl2N8O4P4Rh2‚C7H8, M ) 1109.44, T ) 293-
179, 289. (c) Liehr, G.; Szucs a´ nyi, G.; Ellermann, J. J. Organomet.
(2) K, monoclinic, C2/c, a ) 19.8250(2) Å, b ) 11.8230(2) Å, c )
3
Chem. 1984, 265, 95.
20.9580(3) Å, â ) 110.9461(7)°, V ) 4587.74(11) Å , Z ) 4, Fc )
-
3,
-1
(
(
7) Puddephatt, R. J. Chem. Soc. ReV. 1983, 12, 99.
8) (a) Fletcher, D. A.; McMeeking, R. F.; Parkin, D. J. Chem. Inf.
Comput. Sci. 1996, 36, 746. (b) Allen, F. H.; Kennard, O. Chem. Des.
Autom. News 1993, 8, 31.
1.606 g cm µ ) 1.025 mm ; 26407 reflections collected of which
5218 independent [Rint ) 0.0487]. Final R indices [I > 2σ(I)] R1 )
0.0283, wR2 ) 0.0752.
(14) Cowie, M.; Dwight, S. K. Inorg. Chem. 1980, 19, 2500.
1696 Inorganic Chemistry, Vol. 41, No. 7, 2002

COMMUNICATION
centers are bridged by two diphosphoxane ligands. While
structural comparisons are limited by the disorder in 2, it is
evident that replacing phenyl groups with N-pyrrolyl groups
has a marked effect on the structure of the rhodium dimer.
3
Despite the presence of three P-N bonds in L , the bond
to the 2-acetylpyrrolyl group is selectively cleaved during
conversion of 3 to 4. Both trans-[RhCl(CO){PPh
2 2
(pyr)} ]
and trans-[RhCl(CO){P(pyr) ] have been previously pre-
3 2
}
1
pared, and neither has been reported to undergo a reaction
to give 2 or 4. Partial delocalization of the negative charge
onto the carbonyl group leads to 2-acetylpyrrolyl being a
better leaving group than pyrrolyl. This observation is likely
to be significant in the development of chemistry based on
functionalized N-pyrrolyl phosphines.
Figure 3. Molecular structure of 4. Selected bond lengths (Å) and angles
(
(
deg) Rh(1)‚‚‚Rh(1)′ 3.1177(3), Rh(1)-C(17) 1.839(3), Rh(1)-P(1) 2.2664-
Acknowledgment. The authors thank the EPSRC (M.V.)
and University of Bath (M.T.P.) for funding, the EPSRC/
JREI for funds to buy an area detector, and Johnson Matthey
plc for a generous loan of rhodium chloride.
5), Rh(1)-P(2)′ 2.2624(5), Rh(1)-Cl(1) 2.3719(5), P(1)-O(2) 1.618(2),
P(2)-O(2) 1.627(2), P(2)-N(3) 1.695(2), P(2)-N(4) 1.672(2), P(1)-O(2)-
P(2) 132.4(1). Primed atoms generated by symmetry transformation -x +
1
1
/
2, -y + /2, -z.
Supporting Information Available: Synthetic information for
compounds L and 1-4. Crystallographic information for com-
plexes 2 and 4. This material is available free of charge via the
Internet at http://pubs.acs.org.
from this latter plane is 0.08 Å, much less than the analogous
3
distance (0.32 Å) in 2.
Searches of the Cambridge Crystallographic Database and
8
Chemical Abstracts suggest that compounds 2 and 4 are the
first examples of transition metal dimers in which metal
IC0108989
Inorganic Chemistry, Vol. 41, No. 7, 2002 1697