An uncommon bonding mode of a familiar ligand: A molybdenum complex with a four-electron donor chelating η3-PPh3 ligand, and its structural determination using synchrotron radiation

Article abstract of DOI:10.1039/a905092b

A crystallographic study using synchrotron radiation shows that a C=C bond of one phenyl group of a PPh₃ ligand bonds to Mo in [Cp(CO)₂(PPh₃)Mo]⁺, resulting in an unusual bonding mode in which PPh₃ functions as a chelating four-electron donor ligand.

Full text of DOI:10.1039/a905092b

An uncommon bonding mode of a familiar ligand:
a molybdenum complex with a four-electron donor chelating h -PPh3 ligand,
and its structural determination using synchrotron radiation
3
Tan-Yun Cheng, David J. Szalda† and R. Morris Bullock*
Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973-5000, USA.
E-mail: bullock@bnl.gov
Received (in Bloomington, IN, USA) 15th June 1999, Accepted 8th July 1999
A crystallographic study using synchrotron radiation shows
that a CNC bond of one phenyl group of a PPh3 ligand bonds
to Mo in [Cp(CO)2(PPh3)Mo]+, resulting in an unusual
bonding mode in which PPh3 functions as a chelating four-
electron donor ligand.
d 90.0. These resonances are assigned as the ipso and ortho
carbons of one of the CNC bonds of the Ph ring. The significant
upfield shift of these carbons from the other aromatic carbons (d
120 –135) suggests that one CNC bond of the ring is coordinated
to the molybdenum, as indicated in eqn. (1).
Remarkable progress has been made recently in the design
and utility of large, weakly coordinating counterions.5 A serious
drawback often encountered is the difficulty of obtaining single
crystals that are large enough and that diffract well. Initial
attempts to obtain a crystal structure of 1 using a conventional
X-ray diffractometer suffered from an insufficient number of
observed data to enable a complete refinement. The use of a
high intensity source of X-rays, such as a synchrotron, can
provide sufficient scattering to obtain a reliable structural
determination. The crystal structure of 1 was successfully
solved and refined6 using data collected from a beamline at the
National Synchrotron Light Source at Brookhaven. A sig-
nificant advantage of using a synchrotron source of X-rays is
that small crystals are suitable; the crystal we used had
dimensions of 0.02 3 0.08 3 0.08 mm. The use of synchrotron
radiation may offer advantages for single crystal diffraction
studies of other complexes of large, weakly coordinating
counterions, as well as other complexes for which only small
crystals suitable for diffraction can be obtained.
The ORTEP diagram in Fig. 1 shows that the Mo is located
out of the plane of the arene ring. The distortion of the PPh3
ligand is evidenced by the acute Mo–P–C(31) angle of 73.0(3)°,
compared to angles of 117.8(3)° and 131.3(4)° for the other two
Mo–P–C bonds. The dihedral angle between the Mo–C–C plane
and the plane of the phenyl ring is 62°. The bond distances of the
Ph ring bonded to Mo show some evidence of localization of
CNC and C–C bonds. Similar structural parameters were found
The triphenylphosphine ligand occupies a prominent role in
organometallic chemistry and homogeneous catalysis. The
traditional mode of bonding of PPh3 is as a two-electron donor
through the lone pair on the phosphorus atom. We have
prepared a new complex in which PPh3 functions as a chelating
four-electron donor ligand, with a CNC bond of one phenyl
group bonding to the metal. A structural determination by X-ray
crystallography was carried out using synchrotron radiation.
Weak C–F···H–C interactions are found between C–F groups on
2
the BArA4 [ArA = 3,5-bis(trifluoromethyl)phenyl] counterion
and ligand C–H groups on the cationic molybdenum com-
plex.
2
Hydride transfer from Cp(CO)2(PPh3)MoH to Ph3C+BF4
gives Cp(CO)2(PPh3)MoFBF3.1 In this reaction and many
others reported by Beck and co-workers, the 16-electron metal
cation resulting from removal of hydride is captured by a
weakly coordinating counterion such as BF42.2 We recently
measured the kinetics of this and several related hydride transfer
reactions using stopped-flow techniques.3 We have found that a
completely different product results when hydride abstraction
from Cp(CO)2(PPh3)MoH is carried out using a counterion that
is much more weakly coordinating than BF42. Reaction of
2
Ph3C+BArA4 with Cp(CO)2(PPh3)MoH at 230°C led to the
isolation of a new complex (1) in 84% yield [eqn. (1)].4
The 13C NMR spectrum of 1 at 260 °C exhibits a doublet
(JCP = 29 Hz) at d 81.5 and another doublet (JCP = 12 Hz) at
Fig. 1 ORTEP diagram of 1 showing 50% probability ellipsoids. Selected
bond lengths (Å) and angles (°): Mo–C(2) 1.968(12); Mo–C(1) 1.99(2);
Mo–P 2.429(3); Mo–C(31) 2.566(9); Mo–C(32) 2.645(9); C(31)–C(32)
1.418(13); C(31)–C(36) 1.445(14); C(32)–C(33) 1.424(14); C(33)–C(34)
1.361(14); C(34)–C(35) 1.404(13); C(35)–C(36) 1.368(13); C(2)–Mo-C(1)
79.7(5); C(2)–Mo–P 83.8(3); C(1)–Mo–P 115.7(3).
† Research Collaborator at Brookhaven National Laboratory. Permanent
Address: Department of Natural Sciences, Baruch College, New York, NY
10010, USA.
Chem. Commun., 1999, 1629–1630
1629

2
2
2 W. Beck and K. Sünkel, Chem. Rev., 1988, 88, 1405.
in other M(h -arene) complexes, such as Cp*(PMe3)Rh(h -
phenanthrene),7
Cp*(NO)Ru(h -naphthalene),8
and
2
3 (a) T.-Y. Cheng, B. S. Brunschwig and R. M. Bullock, J. Am. Chem.
Soc., 1998, 120, 13121; (b) T.-Y. Cheng and R. M. Bullock, J. Am.
Chem. Soc., 1999, 121, 3150.
[(NH3)5Os(h -naphthalene)]2+.9 An h -CNC bond of a p-tolyl
group was shown to be bound to tungsten in a series of
bimetallic compounds where the p-tolyl group was part of a
bridging alkylidene in a W[m-C(p-tolyl)(R)]Pd moiety.10
In several previously reported examples where a metal bonds
to the phenyl group of a PPhR2 ligand, the CH bond of the arene
2
2
4 Selected spectroscopic data for 1: 1H NMR (CD2Cl2, 260 °C): the
signals for BArA42 (d 7.74 and 7.53) overlapped with the signals of PPh3
(d 7.82 –7.20), d 6.12 (br t, 1 H, CH, 3JHH
≈
3JPH = 6.9 Hz), 5.62 (s,
5 H, Cp). 13C{1H} NMR (CD2Cl2, 260 °C): d 237.0 (d, JCP = 24.8,
CO), 231.4 (s, CO), 94.4 (s, Cp), 90.0 (d, JCP = 11.8 Hz, o-C of
PPhPh2), 81.5 (d, JCP = 28.6 Hz, ipso-C of PPhPh2). 31P{1H} NMR
(CD2Cl2, 260 °C): d 37.6 (s). IR (CH2Cl2): n(CO) 2009 (s), 1939 (s)
interacts with the metal in an h -CH agostic11 interaction, rather
2
2
than the h -CNC bonding found here. Structurally characterized
cm21
.
phosphine complexes with agostic C–H interactions include
Pd(PPhtBu2)2,12 RhHCl(SiCl3)(PPh3)2,13 RuCl2(PPh3)3,14 and
5 S. H. Strauss, Chem. Rev., 1993, 93, 927.
6 Crystal data for 1·toluene: C64H40BF24MoO2P, M = 1434.68, mono-
clinic; space group P21/n, a = 15.371(2) Å, b = 24.095(2) Å, c =
17.319(2) Å, b = 106.67(2)°, V = 6144.8(12) Å3, Z = 4, r(calc) =
1.551 g cm23, 2qmax = 74°, l = 1.100 Å, image plates with y scans,
Fourier absorption correction, T = 95(2) K, 18041 reflections collected,
5504 independent reflections, heavy atom solution, full-matrix least-
squares refinement on F2, 674 parameters, R1 = 0.0738, wR2 = 0.1899.
All non-hydrogen atoms were refined using anisotropic thermal
parameters except for the carbon atoms of the BArA4 anion. CCDC
182/1325. See http://www.rsc.org/suppdata/cc/1999/1629/ for crys-
tallographic files in .cif format.
7 R. M. Chin, L. Dong, S. B. Duckett, M. G. Partridge, W. D. Jones and
R. N. Perutz, J. Am. Chem. Soc., 1993, 115, 7685.
8 C. D. Tagge and R. G. Bergman, J. Am. Chem. Soc., 1996, 118,
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9 M. D. Winemiller, B. A. Kelsch, M. Sabat and W. D. Harman,
Organometallics, 1997, 16, 3672.
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Daran and J. Vaissermann, Organometallics, 1995, 14, 1637.
11 M. Brookhart, M. L. H. Green and L.-L. Wong, Prog. Inorg. Chem.,
1988, 36, 1.
12 S. Otsuka, T. Yoshida, M. Matsumoto and K. Nakatsu, J. Am. Chem.
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13 K. W. Muir and J. A. Ibers, Inorg. Chem., 1970, 9, 440.
14 S. J. La Placa and J. A. Ibers, Inorg. Chem., 1965, 4, 778.
15 (a) N. A. Bailey, J. M. Jenkins, R. Mason and B. L. Shaw, Chem.
Commun., 1965, 237; (b) N. A. Bailey and R. Mason, J. Chem. Soc. (A),
1968, 2594.
16 (a) Y. W. Yared, S. L. Miles, R. Bau and C. A. Reed, J. Am. Chem. Soc.,
1977, 99, 7076; (b) C. B. Knobler, T. B. Marder, E. A. Mizusawa, R. G.
Teller, J. A. Long, P. E. Behnken and M. F. Hawthorne, J. Am. Chem.
Soc., 1984, 106, 2990.
17 D. D. Pathak, H. Adams, N. A. Bailey, P. J. King and C. White,
J. Organomet. Chem., 1994, 479, 237.
18 (a) N. Feiken, P. S. Pregosin, G. Trabesinger and M. Scalone,
Organometallics, 1997, 16, 537. (b) N. Feiken, P. S. Pregosin, G.
Trabesinger, A. Albinati and G. L. Evoli, Organometallics, 1997, 16,
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19 L. Shimoni and J. P. Glusker, Struct. Chem., 1994, 5, 383.
20 (a) L. Brammer, W. T. Klooster and F. R. Lemke, Organometallics,
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Chem., 1997, 36, 4372.
PdI2(PPhMe2)2.15 These h -CH agostic complexes have con-
2
2
spicuous structural differences that contrast with h -arene
complexes: the metal is normally nearly coplanar with the arene
ring (dihedral angle of < 15° between M–C–C and arene
planes) and the M–P–C angles are typically 111–114° rather
than the acute Mo–P–C(31) angle of 73.0(3)° noted above.
Although it has been suggested11 that [Rh(PPh3)3]+ 16 has an
agostic interaction, its structural features (Rh–P–C angle of
75.6(5)°; dihedral angle of 59° between Rh–C–C and arene
planes) are much more similar to those found in 1. Accordingly,
2
we suggest that [Rh(PPh3)3]+ 16 has an h -arene interaction,
2
rather than agostic, and we believe that these structural criteria
2
will be diagnostic in distinguishing between agostic and h -
arene interactions in other complexes. Other than complex 1
reported here and [Rh(PPh3)3]+ which we interpret as having an
2
h -CH arene interaction, we are not aware of other examples
where a PPh3 ligand bonds to a metal as a four-electron donor
3
2
in an h -bonding mode. There are, however, examples of h -
CNC bonds in Ru complexes with chelating diphosphine ligands
BINAP17 or MeO-BIPHEP.18 These diphosphines are bridged
by substituted-biphenyl groups.
The structural determination of 1 reveals a series of weak C–
F···H–C interactions, with over twenty F···H distances in the
range of 2.6–3.0 Å. The presence of F···H–C interactions19 has
been observed in a variety of compounds recently,20 though
their assignment as hydrogen bonds is not without con-
troversy.21
Although 1 was isolated as a pure solid, it decomposes slowly
at room temperature (several days in CD2Cl2 solution or a few
months as a solid). The weak CNC bond of 1 is readily and
cleanly displaced by ligands such as CH3I and H2O. Further
spectroscopic and structural studies of these compounds will be
reported.
This research was carried out at Brookhaven National
Laboratory under contract DE-AC02-98CH10886 with the US
Department of Energy and was supported by its Division of
Chemical Sciences, Office of Basic Energy Sciences. We thank
Dr. Jonathan C. Hanson for assistance with the use of Beamline
X7B at the National Synchrotron Light Source.
21 J. D. Dunitz and R. Taylor, Chem. Eur. J., 1997, 3, 89.
Notes and references
1 K. Sünkel, H. Ernst and W. Beck, Z. Naturforsch., Teil B, 1981, 36,
474.
Communication 9/05092B
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Chem. Commun., 1999, 1629–1630