Four-electron-donor hemilabile η3-PPh3 ligand that binds through a C=C bond rather than an agostic C-H interaction, and displacement of the C=C by methyl iodide or water

Article abstract of DOI:10.1021/om800401d

Hydride transfer from Cp(CO)₂(PPh₃)MoH to Ph ₃C⁺BAr′₄^- [Ar′ = 3,5-bis(trifluoromethyl)phenyl] produces [Cp(CO)₂(η³- PPh₃)Mo]⁺[BArVP. Spectroscopic and crystallographic data indicate that one C=C of a Ph ring is weakly bound to the Mo, so that the PPh₃ ligand is a four-electron-donor ligand. Computations (DFT/B3LYP and MP2 on [Cp(CO)₂(η³-PPh₃)Mo]⁺ and [Cp(CO)₂(η-PH₂Ph)Mo]⁺, and DFT/B3LYP on [Cp(CO)₂(η³-PH^tBuPh)Mo]⁺ and [Cp(CO)₂(η³-PH₂Ph)Nb]) provide further information on the bonding and on the preference for bonding of the metal to the C=C bond rather than an agostic C-H interaction found in many related complexes. The hemilabile C=C bond is readily displaced by CH₃I or H₂O, and crystal structures are reported for [Cp(CO) ₂(PPh₃)Mo(ICH₃)]⁺ and [Cp(CO) ₂(PPh₃)Mo(OH₂)J⁺. The equilibrium constant for [Cp(CO)₂(η³-PPh₃)Mo] ⁺ + ICH₃ to give [Cp(CO)₂(PPh ₃)Mo(ICH₃)]⁺ is K_eq = 5.2 × 10² M^-1 in CD₂Cl₂ at 22 °C.

Full text of DOI:10.1021/om800401d

Organometallics 2008, 27, 3785–3795
3785
Four-Electron-Donor Hemilabile η3-PPh3 Ligand that Binds through
a CdC Bond Rather than an Agostic C-H Interaction, and
Displacement of the CdC by Methyl Iodide or Water
Tan-Yun Cheng,† David J. Szalda,†,‡ Jonathan C. Hanson,† James T. Muckerman,† and
R. Morris Bullock*,†,§
Chemistry Department, BrookhaVen National Laboratory, Upton, New York 11973-5000, and Chemical
Sciences DiVision, Pacific Northwest National Laboratory, P.O. Box 999, K2-57,
Richland, Washington 99352
ReceiVed May 5, 2008
-
Hydride transfer from Cp(CO)2(PPh3)MoH to Ph3C+BAr′4 [Ar′ ) 3,5-bis(trifluoromethyl)phenyl]
produces [Cp(CO)2(η3-PPh3)Mo]+[BAr′4]-. Spectroscopic and crystallographic data indicate that one
CdC of a Ph ring is weakly bound to the Mo, so that the PPh3 ligand is a four-electron-donor ligand.
Computations (DFT/B3LYP and MP2 on [Cp(CO)2(η3-PPh3)Mo]+ and [Cp(CO)2(η3-PH2Ph)Mo]+, and
DFT/B3LYP on [Cp(CO)2(η3-PHtBuPh)Mo]+ and [Cp(CO)2(η3-PH2Ph)Nb]) provide further information
on the bonding and on the preference for bonding of the metal to the CdC bond rather than an agostic
C-H interaction found in many related complexes. The hemilabile CdC bond is readily displaced by
CH3I or H2O, and crystal structures are reported for [Cp(CO)2(PPh3)Mo(ICH3)]+ and [Cp(CO)2-
(PPh3)Mo(OH2)]+. The equilibrium constant for [Cp(CO)2(η3-PPh3)Mo]+
[Cp(CO)2(PPh3)Mo(ICH3)]+ is Keq ) 5.2 × 102 M-1 in CD2Cl2 at 22 °C.
+ ICH3 to give
additional ligands are of much interest. Hemilabile ligands2 have
two different types of ligand sites, one of which is readily but
reversibly displaced. Weakly bound ligands can temporarily
occupy and protect a vacant coordination site, so structural and
reactivity studies on metal complexes with weakly bound ligands
can be useful in understanding the role of such ligands in
catalysis.
Introduction
Organometallic complexes with a 16-electron configuration
have a remarkable range of stability. Some electronically
unsaturated 16-electron complexes are indefinitely stable,
particularly square-planar four-coordinate complexes of Rh(I),
Ir(I), Pt(II), and Pd(II). Examples include familiar complexes
such as Wilkinson’s catalyst, RhCl(PPh3)3, Vaska’s complex,
IrCl(CO)(PPh3)2, and Pt and Pd complexes of the general
formula L2MX2 (M ) Pt, Pd, L ) phosphine, X ) halide).
In other cases, coordinatively unsaturated metal complexes
very rapidly bind another ligand to achieve an 18-electron count.
An example comes from time-resolved infrared studies by
Bergman, Moore, and co-workers, who found that flash pho-
tolysis of Cp*Rh(CO)2 (Cp* ) η5-C5Me5) in mixtures of Kr
and cyclohexane did not lead to the observation of the
unsaturated 16-electron complex Cp*Rh(CO), but rather pro-
duced Cp*Rh(CO)(Kr) and Cp*Rh(CO)(cyclohexane) prior to
the formation of Cp*Rh(CO)(H)(C6H11).1 This reaction docu-
ments the very strong affinity of some metal complexes for
ligands, even those as weakly bound as noble gases or alkanes.
Many complexes relevant to homogeneous catalysis lie
between these two extremes of reactivity. Since dissociation of
a ligand to create a vacant coordination site at the metal is a
required step in the mechanism of many homogeneously
catalyzed reactions, complexes that weakly and reversibly bind
Triphenylphosphine has a prominent role in the development
of organometallic chemistry and homogeneous catalysis. In the
vast majority of examples, PPh3 serves as a traditional two-
electron donor ligand through the lone pair on the phosphorus
atom. There was evidence as early as 1965 that it could function
as an overall four-electron donor, through formation of a three-
center, two-electron bond between one of the C-H bonds of a
Ph ring and the metal.3 Such agostic4,5 interactions became well-
recognized in later years, and additional examples of this type
of bonding were found for PPh3 and related phosphines.6–9 We
found that the PPh3 ligand in [Cp(CO)2(PPh3)Mo]+[BAr′4]- [Ar′
) 3,5-bis(trifluoromethyl)phenyl] is a chelating, bidentate four-
electron donor, not involving an agostic C-H bond, but rather
an interaction in which one CdC of one phenyl ring binds
(2) (a) Slone, C. S.; Weinberger, D. A.; Mirkin, C. A. Prog. Inorg. Chem.
1999, 48, 233–350. (b) Braunstein, P.; Naud, F. Angew. Chem., Int. Ed.
2001, 40, 680–699.
(3) La Placa, S. J.; Ibers, J. A. Inorg. Chem. 1965, 4, 778–783.
(4) Brookhart, M.; Green, M. L. H.; Wong, L.-L. Prog. Inorg. Chem.
1988, 36, 1–124.
(5) Brookhart, M.; Green, M. L. H.; Parkin, G. Proc. Natl. Acad. Sci.
U.S.A. 2007, 104, 6908–6914.
(6) Sato, M.; Tatsumi, T.; Kodama, T.; Hidai, M.; Uchida, T.; Uchida,
Y. J. Am. Chem. Soc. 1978, 100, 4447–4452.
(7) Muir, K. W.; Ibers, J. A. Inorg. Chem. 1970, 9, 440–447.
(8) Otsuka, S.; Yoshida, T.; Matsumoto, M.; Nakatsu, K. J. Am. Chem.
Soc. 1976, 98, 5850–5858.
* Corresponding author. E-mail: morris.bullock@pnl.gov.
† Brookhaven National Laboratory.
‡ Reasearch collaborator at Brookhaven National Laboratory. Permanent
address: Department of Natural Sciences, Baruch College, New York, NY.
§ Pacific Northwest National Laboratory.
(1) (a) Schultz, R. H.; Bengali, A. A.; Tauber, M. J.; Weiller, B. H.;
Wasserman, E. P.; Kyle, K. R.; Moore, C. B.; Bergman, R. G. J. Am. Chem.
Soc. 1994, 116, 7369–7377. (b) Bengali, A. A.; Schultz, R. H.; Moore,
C. B.; Bergman, R. G. J. Am. Chem. Soc. 1994, 116, 9585–9589. (c)
McNamara, B. K.; Yeston, J. S.; Bergman, R. G.; Moore, C. B. J. Am.
Chem. Soc. 1999, 121, 6437–6443.
(9) King, W. A.; Luo, X.-L.; Scott, B. L.; Kubas, G. J.; Zilm, K. W.
J. Am. Chem. Soc. 1996, 118, 6782–6783.
10.1021/om800401d CCC: $40.75
2008 American Chemical Society
Publication on Web 07/12/2008

3786 Organometallics, Vol. 27, No. 15, 2008
Cheng et al.
weakly to the Mo.10 The CdC ligand is readily displaced by
other ligands, so that the η3-PPh3 ligand functions as a
hemilabile ligand. In this paper we report the synthesis,
characterization, and structure of [Cp(CO)2(η3-PPh3)-
Mo]+[BAr′4]-, along with [Cp(CO)2(PPh3)Mo(ICH3)]+ and
[Cp(CO)2(PPh3)Mo(OH2)]+, in which an ICH3 or H2O ligand
has displaced the weakly bound CdC interaction. Computational
studies provide further insights into the nature of the weak
bonding.
Table 1. ν(CO) IR Data in CH2Cl2 Solution for Mo Complexes with
Weakly Coordinating Ligands
complex
ν(CO), cm-1
ref
-
[Cp(CO)2(η3-PPh3)Mo]+BAr′4
2009, 1939
1987, 1917
1985, 1909
1992, 1910
1989, 1905
this work
this work
this work
17
[Cp(CO)2(PPh3)Mo(ICH3)]+BAr′4
-
[Cp(CO)2(PPh3)Mo(OH2)]+BAr′4
-
[Cp(CO)2(PPh3)Mo(OdCEt2)]+BAr′4
-
Cp(CO)2(PPh3)MoFBF3
15
also lists ν(CO) bands of related compounds for comparison.
All of these cationic complexes have ν(CO) bands at higher
energy than those of the neutral hydride Cp(CO)2(PPh3)MoH
(1936, 1855 cm-1 in CH2Cl2).
Results and Discussion
Synthesis and Spectroscopic Characterization of an
η3-PPh3 Complex: Different Products Resulting from
-
Different Counterions. Commonly used anions such as BF4
and PF6- were classically considered noncoordinating, but are
now known to bind to some metal cations through the F atom.11
Observation and characterization of very weak bonding interac-
tions in cationic metal complexes requires the use of even more
weakly coordinating counterions. A variety of very weakly
coordinating anions12 have become available and have had an
impact in many homogeneous catalytic reactions, especially
olefin polymerizations.13 Beck and co-workers reported the
synthesis of many metal carbonyl complexes with coordinated
BF4-, PF6-, and related weakly coordinating anions.11 Hydride
The 1H NMR spectrum of the product is not very informative
at room temperature; a single Cp resonance indicated a pure
product, but the aromatic region exhibited multiplets for the
aromatic protons of the PPh3 ligand. An NMR spectrum at -60
°C, however, shows a triplet (J ) 6.9 Hz) at δ 6.12, which
integrates for one proton. Homonuclear decoupling of the
aromatic resonances at δ 7.4 results in the collapse of this triplet
to a doublet. We assign this resonance to one ortho CH on one
of the three phenyl groups of the PPh3 ligand, with the triplet
-
transfer from Cp(CO)2(PPh3)MoH to Ph3C+BF4 gives
Cp(CO)2(PPh3)MoFBF314 (eq 1). We measured the kinetics of
this and several related hydride transfer reactions using stopped-
flow techniques.15,16 When the reaction is carried out at -80
°C and monitored by 1H NMR, a 90:10 mixture of trans-
3
pattern being accounted for by 3JHH ≈ JPH. This suggests that
Cp(CO)2(PPh3)MoFBF3
and
[trans-Cp(CO)2(PPh3)Mo-
(ClCH2Cl)]+BF4- was initially observed. When the temperature
is raised, the weakly coordinating dichloromethane ligand is
displaced, and the thermodynamically favored product cis-
Cp(CO)2(PPh3)MoFBF3 forms cleanly.
one of the Ph rings has a substituent that interacts with the Mo.
Two plausible bonding configurations are coordination of one
of the CdC bonds of a phenyl ring to the Mo as shown in eq
2, and interaction of an ortho C-H bond of the phenyl ring to
the Mo in an agostic bond,4,5 as shown in Figure 1. While the
1
triplet observed in the low-temperature H NMR is consistent
with either of these two structural possibilities, the 13C NMR
spectrum at -60 °C has two resonances that are informative in
distinguishing between the two structures under consideration.
Since the specific bonding of a CdC of one of the Ph rings
renders the other two Ph rings inequivalent, a large number of
resonances are found between δ 120 and 135 in the aromatic
region of the 13C NMR spectrum. A doublet (JCP ) 29 Hz) at
δ 81.5 and a doublet (JCP ) 12 Hz) at δ 90.0 are assigned as
the ipso and ortho carbons of one of the CdC bonds of the Ph
ring. These two resonances are observed in the 13C NMR
spectrum at chemical shifts more consistent with coordinated
alkenes than a normal aromatic carbon. These data favor the
structure shown in eq 2, in which one CdC bond is coordinated
to the Mo, rather than the agostic structure involving one C-H
of the phenyl ring (Figure 1).
A completely different product results when hydride abstrac-
tion from Cp(CO)2(PPh3)MoH is carried out using a much more
weakly coordinating counterion (eq 2). Reaction of
-
Ph3C+BAr′4 with Cp(CO)2(PPh3)MoH at -30 °C led to the
isolation of a new complex, [Cp(CO)2(η3-PPh3)Mo]+[BAr′4]-,
in 84% yield. The IR spectrum of the isolated product exhibited
bands for the metal carbonyl stretches at 2009 and 1939 cm-1
.
As shown in Table 1, the bands of the product are shifted to
even higher energy than in Cp(CO)2(PPh3)MoFBF3, indicating
lower electron density at the metal in the new product. Table 1
(10) For a preliminary communication of these results, see: Cheng, T.-
Y.; Szalda, D. J.; Bullock, R. M. J. Chem. Soc., Chem. Commun. 1999,
1629–1630.
(11) Beck, W.; Su¨nkel, K. Chem. ReV. 1988, 88, 1405–1421.
(12) (a) Strauss, S. H. Chem. ReV. 1993, 93, 927-942. Reed, C. A.
Acc. Chem. Res. 1998, 31, 133–139. (b) Krossing, I.; Raabe, I. Angew.
Chem., Int. Ed. 2004, 43, 2066–2090.
(13) Chen, E. Y.-X.; Marks, T. J. Chem. ReV. 2000, 100, 1391–1434.
(14) Su¨nkel, K.; Ernst, H.; Beck, W. Z. Naturforsch. 1981, 36b, 474–
481.
(15) Cheng, T.-Y.; Brunschwig, B. S.; Bullock, R. M. J. Am. Chem.
Soc. 1998, 120, 13121–13137.
(16) (a) Cheng, T.-Y.; Bullock, R. M. J. Am. Chem. Soc. 1999, 121,
3150–3155. (b) Cheng, T.-Y.; Bullock, R. M. Organometallics 2002, 21,
2325–2331.
Figure 1. Possible agostic structure for [Cp(CO)2(PPh3)-
Mo]+[BAr′4]-.

Four-Electron-Donor Hemilabile η3-PPh3 Ligand
Organometallics, Vol. 27, No. 15, 2008 3787
Figure 2. CdC bonded to Ru in [(MeO-BIPHEP)Ru(η5-C8H11)]+.
Further support for our assignment of CdC bonding to the
metal comes from a comparison with complexes that exhibit a
similar interaction with a diphosphine. Pregosin and co-workers
reported detailed NMR studies of a series of cationic Ru
complexes that were shown to have a six-electron-donor
diphosphine ligand, with one CdC bond coordinated from an
arene ring.18 2D NMR studies of [(MeO-BIPHEP)Ru(η5-
C8H11)]+ provided further characterization of the bonding shown
in Figure 2. The 13C NMR resonances of the coordinated CdC
atoms appear at δ 74.5 and 95.1 and are distinctly upfield of
the analogous carbons in the other ring that are not coordinated
(δ 139.7 and 135.4). The resonances for the bound CdC in
their Ru complex appear at a similar chemical shift to those in
[Cp(CO)2(PPh3)Mo]+. Structural studies of closely related Ru
complexes show that the angle between the two aryl rings
distorts, facilitating the bonding by allowing the π-system to
come closer to the metal. Similar bonding of a CdC of an arene
ring of a diphosphine was found in CpRu[(R)-(BINAP)]+I.19
Crystal Structure of [Cp(CO)2(η3-PPh3)Mo]+[BAr′4]. Crys-
tallography of [Cp(CO)2(η3-PPh3)Mo]+[BAr′4]- was carried out
at the National Synchrotron Light Source, where the high
intensity of X-rays available enabled us to determine the crystal
structure of the small (0.02 × 0.08 × 0.08 mm) crystal available.
The use of weakly coordinating counterions12 has been very
beneficial in recent years, enabling the synthesis of complexes
that could not be obtained with traditional counterions and
enhancing some catalytic reactions. In some cases complexes
with such counterions may not easily form crystals that are well-
suited for X-ray diffraction, so the use of a high-intensity source
of X-rays can ameliorate these limitations by allowing reliable
structural determinations from small crystals.
Figure 3. ORTEP diagram (30% probability ellipsoids) of [Cp-
(CO)2(η3-PPh3)Mo]+ (hydrogen atoms not shown).
Table 2. Selected Bond Distances (Å) and Angles (deg) for
[Cp(CO)2(η3-PPh3)Mo]+
Bond Distances
Bond Angles
Mo-C(2)
1.968(12)
1.99(2)
C(2)-Mo-C(1)
79.7(5)
83.8(3)
115.7(3)
42.1(2)
65.5(2)
73.0(3)
117.8(3)
131.3(4)
Mo-C(1)
C(2)-Mo-P
C(1)-Mo-P
P-Mo-C(31)
P-Mo-C(32)
Mo-P-C(31)
Mo-P-C(21)
Mo-P-C(11)
Mo-P
2.429(3)
2.566(9)
2.645(9)
1.418(13)
1.445(14)
1.424(14)
1.361(14)
1.404(13)
1.368(13)
Mo-C(31)
Mo-C(32)
C(31)-C(32)
C(31)-C(36)
C(32)-C(33)
C(33)-C(34)
C(34)-C(35)
C(35)-C(36)
Rh(PPh3)3+ has one of the three PPh3 ligands bound as η3; the
-
structure was determined for the ClO4 salt20 and with a
carborane anion.21 The latter has Rh-C distances of 2.502(8)
and 2.611(9) Å, while those in CpRu(η3-PPh3)(PMeiPr2)+ are
shorter,22 2.371(3) and 2.459(3) Å. The expected M-C distances
will depend on the metal, but for all three of these complexes
having an η3-PPh3 ligand, the M-C distances are long,
compared to M-C distances in the range of about 2.1-2.25 Å
for Cp*(PMe3)Rh(η2-phenanthrene),23 Cp*(NO)Ru(η2-naph-
thalene),24 [(NH3)5Os(η2-naphthalene)]2+ 25 and [κ2-(Hpz*)-
,
BHpz*2]Pt(H)(η2-benzene)]+,26 in which the arene is bonded
η2 to the metal but is not bonded to a phosphine.
In the two Ph rings that do not interact with the Mo, the
average carbon-carbon bond length was 1.39(1) Å, while in
the coordinated ring there is a pattern of alternating long and
short bond lengths (see Table 2), indicating some localization.
The long bonds average 1.42 Å, and two of the short bonds
The ORTEP diagram shown in Figure 3 shows the η3-PPh3
bonding to the Mo, and Table 2 lists selected bond distances
and angles. The distances from Mo to the two carbons in the
CdC bond are long: Mo-C(31) ) 2.566(9) Å and Mo-C(32)
) 2.645(9) Å. Two other complexes with η3-PPh3 ligands that
have been crystallographically characterized also show a dispar-
ity in the M-C distances, with the distance to the ipso carbon
being shorter than that to the ortho carbon. The rhodium cation
(20) Yared, Y. W.; Miles, S. L.; Bau, R.; Reed, C. A. J. Am. Chem.
Soc. 1977, 99, 7076–7078.
(21) Knobler, C. B.; Marder, T. B.; Mizusawa, E. A.; Teller, R. G.;
Long, J. A.; Behnken, P. E.; Hawthorne, M. F. J. Am. Chem. Soc. 1984,
106, 2990–3004.
(17) Bullock, R. M.; Voges, M. H. J. Am. Chem. Soc. 2000, 122, 12594–
12595. (a) Voges, M. H.; Bullock, R. M. J. Chem. Soc., Dalton Trans.
2002, 759–770.
(18) (a) Feiken, N.; Pregosin, P. S.; Trabesinger, G.; Scalone, M.
Organometallics 1997, 16, 537–543. (b) Feiken, N.; Pregosin, P. S.;
Trabesinger, G.; Albinati, A.; Evoli, G. L. Organometallics 1997, 16, 5756–
5762. (c) Geldbach, T. J.; Pregosin, P. S. Eur. J. Inorg. Chem. 2002, 1907–
1918.
(22) Jime´nez-Tenorio, M.; Puerta, M. C.; Valerga, P. Organometallics
2002, 21, 628–635.
(23) Chin, R. M.; Dong, L.; Duckett, S. B.; Partridge, M. G.; Jones,
W. D.; Perutz, R. N. J. Am. Chem. Soc. 1993, 115, 7685–7695.
(24) Tagge, C. D.; Bergman, R. G. J. Am. Chem. Soc. 1996, 118, 6908–
6915.
(25) Winemiller, M. D.; Kelsch, B. A.; Sabat, M.; Harman, W. D.
Organometallics 1997, 16, 3672–3678.
(19) Pathak, D. D.; Adams, H.; Bailey, N. A.; King, P. J.; White, C. J.
Organomet. Chem. 1994, 479, 237–245.
(26) Reinartz, S.; White, P. S.; Brookhart, M.; Templeton, J. L. J. Am.
Chem. Soc. 2001, 123, 12724–12725.

3788 Organometallics, Vol. 27, No. 15, 2008
Cheng et al.
Chart 1. Complexes Having Agostic C-H Interactions of
P-Ph Groups
average 1.36(1) Å. The CdC bonded to the metal is elongated
to 1.42(1) Å.
There are 18 F · · · H distances in the range 2.6-3.0 Å between
C-H bonds (of the Ph and Cp rings) and C-F bonds of the
anion in [Cp(CO)2(η3-PPh3)Mo]+[BAr′4]-, possibly indicating
weak C-F · · · H-C hydrogen bonding interactions. There are
15 F · · · H distances in this range in [Cp(CO)2(PPh3)-
Mo(OH2)]+BAr′4-, and 11 were found in [Cp(CO)2(PPh3)-
Mo(ICH3)]+BAr′4- (Vide infra). The presence of C-F · · · H-C
interactions has been previously recognized in organometallic
complexes27 and in numerous organic compounds such as
fluorobenzenes.28 Surveys and analyses of crystallographic data
have been reported,29,30 and theoretical studies have also
examined the C-F · · · H-C interaction.31 Even though such
interactions are individually weak, collectively they can have
an influence on the crystal structure. The assignment of these
weak interactions as hydrogen bonds,32 however, remains
controversial.30
Comparison of PPh3 Ligands Containing CdC Bonds
to the Metal with Those Having an Agostic C-H
Interaction. In thousands of organometallic complexes, PPh3
functions as a simple two-electron donor through the phospho-
rus. Of those that have an additional interaction, there appears
to be a much higher prevalence of C-H agostic interactions
compared to weakly bound CdC bonds. We now consider
structural features that clearly distinguish between the two modes
of bonding. La Placa and Ibers reported evidence for a possible
interaction between the Ru and the H on the ortho carbon of
of bonding is the location of the metal compared to the plane
of the interacting Ph ring. Since the η3-PPh3 bonding uses the
p-orbitals of the CdC fragment of the Ph ring, the metal resides
above the plane of the ring. For [Cp(CO)2(η3-PPh3)Mo]+-
[BAr′4]-, an angle of 62.1° is found for the dihedral angle
between the plane of the Ph ring and the plane containing the
metal and the two interacting (CdC) carbons. This angle is
similar to the dihedral angles in [Rh(PPh3)3]+ and CpRu(η3-
PPh3)(PMeiPr2)+, which were shown by crystallography to have
an η3-PPh3 ligand (Table 3). These values are also similar to
transition metal complexes having η2-arene ligands that are not
constrained in their geometry by being bound to a phosphine,
such as the dihedral angle of 71.1° for Cp*(PMe3)Rh(η2-
phenanthrene),23 71.1° for Cp*(NO)Ru(η2-naphthalene),24 71.8°
3
one of the Ph rings in the crystal structure of RuCl2(PPh3)3
(Chart 1). They cautiously interpreted the evidence for this
interaction as being “extremely tenuous”, but subsequent studies
of other complexes have provided further support for the three-
center, two-electron agostic4,5 bonding. Hidai and co-workers
reported the structure of a Mo carbonyl complex where a
chelating diphosphine exhibited this type of interaction6 (Chart
1). Kubas and co-workers reported detailed NMR characteriza-
tion of the agostic C-H interaction in the closely related
complexes Mo(CO)[(PhCH2)2PCH2CH2P(CH2Ph)2]2.33
for [(NH3)5Os(η2-naphthalene)]2+ 25 and 73.0° for [κ2-
,
(Hpz*)BHpz*2]Pt(H)(η2-benzene)]+ 26 (pz* ) 3,5-dimethylpyra-
zolyl). In contrast, complexes containing an agostic C-H bond
of the P-Ph moiety have the metal nearly in the plane of the
ring, with dihedral angles in the range 0-20°.
Multiple agostic interactions to a single metal can occur, as
documented by the crystal structure of Pd(PtBu2Ph)2 which
showed that two C-H bonds of Ph rings were within agostic
bonding distance of the Pd, along with similar weak interactions
In addition to these striking differences in dihedral angles,
there is also a notable difference in the M-P-Cipso angles
(Table 3). The acute M-P-C angles of 70-76° for the η3-
PPh3 ligands contrast with the much less distorted angles
observed with the complexes having an agostic C-H interaction,
which are generally >107°. For comparison, the Rh-P-C
angles for the noninteracting Ph groups in [Rh(PPh3)3]+ are
123.3(4)° and 126.2(3)°.21 The Mo-P-C angle in [Cp(CO)2(η3-
PPh3)Mo]+ is smaller than the Mo-P-C angle of 87.4(1)° of
CpMo(η2-C6H4PMe2)(PMe2Ph), a complex in which the phos-
phine is ortho-metalated.34
The M · · · H distance is often used in consideration of possible
agostic interactions.4,5 In most cases these distances are inferred
on the basis of the heavy atom positions, since hydrogen atom
positions are usually not reliably located using X-ray diffraction.
The complexes in Table 3 with η3-PPh3 bonding have shorter
M · · · H distances than those with agostic interactions, which
fall in the range of about 2.7-3.0 Å. The bonding in [Rh(P-
Ph3)3]+ has been suggested to be agostic,4 but we believe that
the criteria of dihedral angles and the M-P-C angles as
t
with Bu substituents on the ligand8 (Chart 1). Kubas and co-
workers found that the crystal structure of Mn(CO)(Ph2-
+
PCH2CH2PPh2)2 had two agostic interactions to the same
metal, arising from different C-H bonds of different Ph rings.9
Diagnostic structural features of complexes exhibiting C-H
agostic interactions distinguish them from complexes having
CdC bonding of the η3-PPh3 ring. A conspicuous difference
in the structures of complexes exhibiting the two different types
(27) (a) Brammer, L.; Klooster, W. T.; Lemke, F. R. Organometallics
1996, 15, 1721–1727. (b) Teff, D. J.; Huffman, J. C.; Caulton, K. G. Inorg.
Chem. 1997, 36, 4372–4380.
(28) Thalladi, V. R.; Weiss, H.-C.; Blaser, D.; Boese, R.; Nangia, A.;
Desiraju, G. R. J. Am. Chem. Soc. 1998, 120, 8702–8710.
(29) (a) Shimoni, L.; Glusker, J. P. Struct. Chem. 1994, 5, 383–397. (b)
Brammer, L.; Bruton, E. A.; Sherwood, P. Cryst. Growth Des. 2001, 1,
277–290.
(30) (a) Dunitz, J. D.; Taylor, R. Chem.-Eur. J. 1997, 3, 89–98. (b)
Howard, J. A. K.; Hoy, V. J.; O’Hagan, D.; Smith, G. T. Tetrahedron 1996,
52, 12613–12622.
(31) Hyla-Kryspin, I.; Haufe, G.; Grimme, S. Chem.-Eur. J. 2004, 10,
3411–3422.
(32) Desiraju, G. R.; Steiner, T. The Weak Hydrogen Bond in Structural
Chemistry and Biology; Oxford University Press: Oxford, 1999.
(33) Luo, X.-L.; Kubas, G. J.; Burns, C. J.; Eckert, J. Inorg. Chem. 1994,
33, 5219–5229.
(34) Poli, R.; Krueger, S. T.; Abugideiri, F.; Haggerty, B. S.; Rheingold,
A. L. Organometallics 1991, 10, 3041–3046.

Four-Electron-Donor Hemilabile η3-PPh3 Ligand
Organometallics, Vol. 27, No. 15, 2008 3789
Table 3. Comparison of Dihedral Angles, M-P-Cipso Angles and M · · · H Distances for Complexes with η3-PPh3 or C-H Agostic P-Ph
Ligands
dihedral angle
M · · · H
complex
MCC/arene (deg)a
M-P-C (deg)
(Å)b
ref
Complexes with η3-PPh3 Ligands
[Cp(CO)2(η3-PPh3)Mo]+
[Rh(PPh3)3]+
62.1
59.0
58.2
73.0(3)
75.6(3)
69.56(10)
2.64
2.57
2.68
this work
21
22
CpRu(η3-PPh3)(PMeiPr2)+
Complexes with C-H Agostic P-Ph Ligands
Pd(PPhtBu2)2
RhHCl(SiCl3)(PPh3)2
RuCl2(PPh3)3
Mo(CO)(Ph2PCH2CH2PPh2)2
Mn(CO)[Ph2PCH2CH2PPh2]2
10
9.7
0.60
20.3
3.0, 4.8
113.1(3)
107-121
100-128
108.9
2.83(11)
2.8
8
7
3
6
9
2.7c
2.98(11)
2.89(6),2.98(6)
+
112.4, 115.1
a Dihedral angle between the plane of the Ph ring and the plane containing the metal and the two interacting (CdC) carbons. b The M · · · H separation
is the distance between the metal and the H on the ortho carbon of the arene ring. In most cases such distances are calculated and are subject to
inaccuracies of locating H atoms from X-ray diffraction. c The value of >2.7 Å for the M · · · H distance in RuCl2(PPh3)3 is our estimate. La Placa and
Ibers reported3 a M · · · H distance of 2.59 Å based on a rigid group refinement for the Ph ring, using a C-H bond length of 1.08 Å. In modern
structural studies, the individual carbon atoms would be refined, and a 0.95 Å C-H bond length would be used, which would give a M · · · H distance
estimated as >2.7 Å.
+
sites. A related complex with PiPr groups, Ir(H)2(PiPr2Ph)3
,
has no agostic interactions and is stable in a 16-electron
configuration. Caulton and co-workers concluded from these
studies that steric pressure can encourage the formation of
agostic interactions by forcing the appropriate C-H bond close
enough to the metal to facilitate the formation of an agostic
bond. We have carried out computations (Vide infra) on the
hypothetical complex [Cp(CO)2(PHtBuPh)Mo]+. This complex
is related to [Cp(CO)2(η3-PPh3)Mo]+, but it has a chance to
form an aliphatic agostic bond that might compete with arene
bonding.
Weak bonding of metals to CdC bonds is not limited to
aromatic rings of phosphines: we found that a CdC bond of a
mesityl group in an N-heterocyclic carbene ligand is weakly
-
bound to the tungsten in CpW(CO)2(IMes)+B(C6F5)4 (IMes
) 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene).36 This W
complex catalyzes the hydrogenation36 and hydrosilylation37 of
ketones.
Figure 4. An Ir complex that has two agostic C-H interactions
but no CdC (phenyl) bonds to Ir.
documented in Table 3 provide a more conclusive criterion of
the predominant bonding, rather than using the M · · · H distances
to distinguish between the two bonding modes. The H on the
ortho carbon of the Ph ring is drawn close to the metal by the
geometrical requirements of the η3-PPh3 bonding, so we
consider the relatively short M · · · H distances to be an inevitable
consequence of the η3-PPh3 bonding, rather than independent
evidence of an agostic interaction.
Caulton and co-workers reported extensive experimental and
computational studies on agostic interactions in a series of
cationic Ir complexes.35 As shown in Figure 4, Ir(H)2-
(PtBu2Ph)2+ has agostic interactions involving C-H bonds from
different tBu groups on two different phosphines. Bond distances
for the Ir-C bonds involved in these agostic interactions range
from 2.81 to 2.94 Å. This agostic interaction with an aliphatic
C-H is observed in preference to agostic (or CdC) bonding
involving the Ph ring; Ir · · · C(phenyl) separations were at least
3.48 Å. Structural and computational studies on related com-
plexes showed how sensitive the agostic interactions can be to
steric effects. The ortho-metalated complex IrH(η2-C6H4PtBu2)-
(PtBu2Ph)2+ has only one agostic interaction (again with a C-H
bond of a tBu group) even though it had two vacant coordination
Computational Studies on [Cp(CO)2(PPh3)Mo]+ and
[Cp(CO)2(PH2Ph)Mo]+. B3LYP hybrid DFT38 and ab initio
second-order Møller-Plesset perturbation theory (MP2)39 com-
putations, with the Gaussian 03 package of programs using the
Los Alamos ECP plus DZ basis40 for Mo and Nb, and the
6-31G(d,p) basis41 for all other elements, were carried out on
[Cp(CO)2(PPh3)Mo]+ and on the hypothetical complex
[Cp(CO)2(PH2Ph)Mo]+ to provide further understanding of the
factors influencing the bonding mode. We sought to determine
if steric pressure exerted by two Ph groups was necessary for
the formation of the bonding of the CdC to the metal. B3LYP
calculations were carried out on the hypothetical complexes
[Cp(CO)2(PHtBuPh)Mo]+ and [Cp(CO)2(PH2Ph)Nb]0 to address
the competition between agostic vs arene bonding and the effect
of the charge on the metal center.
(36) (a) Dioumaev, V. K.; Szalda, D. J.; Hanson, J.; Franz, J. A.; Bullock,
R. M. Chem. Commun. 2003, 1670–1671. (b) Wu, F.; Dioumaev, V. K.;
Szalda, D. J.; Hanson, J.; Bullock, R. M. Organometallics 2007, 26, 5079–
5090.
(37) Dioumaev, V. K.; Bullock, R. M. Nature 2003, 424, 530–532.
(38) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (b) Lee,
C. T.; Yang, W. T.; Parr, R. G. Phys. ReV. B 1988, 37, 785–789. (c) Becke,
A. D. Phys. ReV. A 1988, 38, 3098–3100.
(39) (a) Møller, C.; Plesset, M. S. Phys. ReV. 1934, 46, 618–622. (b)
Head-Gordon, M.; Pople, J. A.; Frisch, M. J. Chem. Phys. Lett. 1988, 153,
503–506.
(35) (a) Cooper, A. C.; Streib, W. E.; Eisenstein, O.; Caulton, K. G.
J. Am. Chem. Soc. 1997, 119, 9069–9070. (b) Ujaque, G.; Cooper, A. C.;
Maseras, F.; Eisenstein, O.; Caulton, I. G. J. Am. Chem. Soc. 1998, 120,
361–365. (c) Cooper, A. C.; Clot, E.; Huffman, J. C.; Streib, W. E.; Maseras,
F.; Eisenstein, O.; Caulton, K. G. J. Am. Chem. Soc. 1999, 121, 97–106.
(40) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270–283.
(41) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54,
724–728.

3790 Organometallics, Vol. 27, No. 15, 2008
Cheng et al.
Table 4. Bond Distances (Å) and Angles (deg) from the Computed
Structures of [Cp(CO)2(PPh3)Mo]+ and [Cp(CO)2(PH2Ph)Mo]+
(B3LYP and MP2)
[Cp(CO)2(PPh3)Mo]+
B3LYP
MP2a
2.48 2.50
[Cp(CO)2(PH2Ph)Mo]+
B3LYP
MP2c
2.44 2.45
parameterb
Mo-P
2.69
2.82
75.8
60.7
2.74
2.90
76.9
61.5
Mo-Cipso
2.72
2.97
78.1
60.1
2.73
2.94
78.0
61.1
Mo-Cortho
Mo-P-Cipso
dihedral angled
a Orbitals 98 through 518 and the 20 electrons occupying orbitals 98
b
through 107 were included in the correlation energy calculation. Cipso
) C(31) and Cortho ) C(32) in [Cp(CO)2(η3-PPh3)Mo]+ (Figure 3).
c Orbitals 59 through 310 and the 18 electrons occupying orbitals 59
through 67 were included in the correlation energy calculation.
d Dihedral angle between the plane of the Mo and the ipso and ortho
carbon atoms of the arene ring and the plane defined by the ipso, ortho,
and meta carbon atoms of the arene ring. This convention for defining
dihedral angles by four atoms is in widespread use in quantum
chemistry codes and should correspond quite closely to the
crystallographic convention employing the dihedral angle between the
plane of the Ph ring and the plane containing the metal and the two
interacting (CdC) carbons, since deviations of the Ph ring from
nonplanarity are very small ((0.015 Å).
Figure 6. Computed DFT/B3LYP energy of [Cp(CO)2-
(PH2Ph)Mo]+ as a function of the Mo-P-C angle. At each fixed
value of the Mo-P-C angle, all other degrees of freedom were
relaxed to minimize the energy. The insets show the optimized
structure at the equilibrium geometry (78°) and at 108°. The MCC/
arene dihedral angles (see text) are 60° and 42°, respectively.
Considering the much larger M-P-C angle found in
agostic structures compared to those adopting bonding to the
CdC (Table 3), we wondered whether a larger Mo-P-C
angle would favor the agostic C-H over CdC bonding to
the metal. To investigate this possibility, a series of calcula-
tions was performed in which the Mo-P-C angle was
scanned in increments of 5°, from 73° to 128°. At each fixed
value of this angle, all other degrees of freedom were relaxed
to minimize the energy.
A plot of the energy as a function of Mo-P-C angle is
shown in Figure 6. The MCC/arene dihedral angle (between
the plane of the Ph ring and the plane containing the metal
and the two interacting CdC carbons) changes from 60° at
the minimum of the curve to 42° for a Mo-P-C angle of
108°. As mentioned above, Mo-P-C angles greater than
100° are usually associated with C-H agostic bonding, and
the very slight inflection in the curve shown in Figure 6
between 103° and 108°, together with the decrease in MCC/
arene dihedral angle, might be suggestive of the possible
onset of an agostic interaction that slightly stabilizes the
molecule in that region. The Mo · · · Cortho and Mo · · · H
distances relevant to a possible agostic interaction at the 108°
bond angle are 4.009 and 3.858 Å, respectively, which are
much too large to be considered an agostic interaction. Weak
M · · · H-C interactions have been observed in many com-
plexes at M · · · H distances longer than those normally found
in agostic complexes. Such interactions have been designated
as pregostic43 or preagostic,44 meaning either that they are
on the way to becoming agostic or that they have a weak
M · · · H-C interaction as described by Albinati, Pregosin, and
co-workers.45 A recent review article on agostic interactions
favors the term “anagostic” to describe weak hydrogen
bonding46 and other interactions that are not truly agostic.5
Figure 5. Structures of [Cp(CO)2(PPh3)Mo]+ (left) and
[Cp(CO)2(PH2Ph)Mo]+ (right) computed by MP2.
The features of the computed minimized structures are similar
to those found experimentally in the crystallographic study. Our
experimental and computational values are also similar to those
computed recently by Costa, Calhorda, and Pregosin using the
Amsterdam Density Functional program.42 Metrical parameters
are provided in Table 4, and Figure 5 shows the geometry for
both complexes. Both have bonding of the CdC of the Ph ring
to the Mo. The B3LYP bond distances are (unexpectedly not
longer, but) generally shorter than those from the MP2 calcula-
tion, although both sets of calculated bond lengths are longer
than the experimental values by 0.05 to 0.25 Å. The larger
distance of the Mo to the ortho carbon compared to the ipso
carbon is also found in the computed structure. The calculated
M-P-C angle is within 4° of the experimental value, and it
has a very similar value in both [Cp(CO)2(PPh3)Mo]+ and the
hypothetical [Cp(CO)2(PH2Ph)Mo]+ complex which should
exhibit no steric pressure to force this angle to be so small (78°).
This demonstrates that the arene bonding interaction is suf-
ficiently strong to distort the Mo-P-C angle. While the
Mo-P-C angle of the phenyl group involved in the arene
interaction in [Cp(CO)2(PPh3)Mo]+ is 76°, the other two
Mo-P-C angles are 120° and 129°. In [Cp(CO)2(PH2Ph)Mo]+,
the M-P-C angle is 78°, while the two Mo-P-H angles are
127° and 128°.
(43) Bortolin, M.; Bucher, U. E.; Ruegger, H.; Venanzi, L. M.; Albinati,
A.; Lianza, F.; Trofimenko, S. Organometallics 1992, 11, 2514–2521.
(44) (a) Lewis, J. C.; Wu, J.; Bergman, R. G.; Ellman, J. A. Organo-
metallics 2005, 24, 5737–5746. (b) Zhang, Y.; Lewis, J. C.; Bergman, R. G.;
Ellman, J. A.; Oldfield, E. Organometallics 2006, 25, 3515–3519.
(45) (a) Albinati, A.; Anklin, C. G.; Ganazzoli, F.; Ruegg, H.; Pregosin,
P. S. Inorg. Chem. 1987, 26, 503–508. (b) Albinati, A.; Pregosin, P. S.;
Wombacher, F. Inorg. Chem. 1990, 29, 1812–1817. (c) Albinati, A.; Arz,
C.; Pregosin, P. S. Inorg. Chem. 1987, 26, 508–513.
(42) Costa, P. J.; Calhorda, M. J.; Pregosin, P. S. Collect. Czech. Chem.
Commun. 2007, 72, 703–714.

Four-Electron-Donor Hemilabile η3-PPh3 Ligand
Organometallics, Vol. 27, No. 15, 2008 3791
Table 5. Calculated Structural Parameters of the
[Cp(CO)2(PHtBuPh)Mo]+ and Cp(CO)2(PH2Ph)Nb Complexes
property
[Cp(CO)2(PHtBuPh)Mo]+ Cp(CO)2(PH2Ph)Nb
M-P (Å)
2.47
2.71
2.81
76.8
59.0
2.51
2.90
3.07
82.2
58.0
M-Cipso (Å)
M-Cortho (Å)
M-P-Cipso (Å)
MCC/arene dihedral
angle (deg)
M-HtBu (Å)
M-CtBu (Å)
3.89
4.32
N/A
N/A
Although no definitive evidence for an agostic interaction
was obtained in our computations, the energy at the Mo-P-C
angle of 108° is about 4.5 kcal/mol higher than that at the
Mo-P-C angle of 78° (Figure 6). The energy of the
observed interaction between the Mo and the CdC bond
would thus be at least 4.5 kcal/mol stronger than the putative
agostic interaction. Hoff and co-workers estimated the
strength of the agostic interaction in (CO)3W(PCy3)2 as 10
( 6 kcal/mol.47 A more recent theoretical study of
(CO)3W(PCy3)2 estimated the strength of the agostic bonds
as 7-9 kcal/mol.48 Taking 8 kcal/mol as representative of
an agostic interaction in metal carbonyl complexes, and
adding the 4.5 kcal/mol estimated for the difference in the
agostic vs CdC bonding to Mo, gives a rough estimate of
12.5 kcal/mol for the strength of the bond between the Mo
and the CdC bond in [Cp(CO)2(PPh3)Mo]+. This estimate
is comparable to the value of 13.4 kcal/mol computed by an
energy decomposition analysis of [Cp(CO)2(PPh3)Mo]+.42
Computational Studies on [Cp(CO)2(PHtBuPh)Mo]+
and Cp(CO)2(PH2Ph)Nb. Introducing a tertiary butyl group
in place of one of the hydrogen atoms of the PH2Ph ligand
allows for the possibility of an agostic interaction of a CH3 group
to compete with the arene interaction in the same molecule (cf.
Caulton’s Ir complex in Figure 4). Figure 8 shows the calculated
(DFT/B3LYP) minimum-energy structure of this complex,
which exhibits an arene CdC bond but no agostic interaction
to a C-H on either the tBu or the Ph ring. Table 5 lists the key
geometric features.
Figure 7. Structural view (top panel) and HOMO (bottom panel)
of [Cp(CO)2(η3-PH2Ph)Mo]+ with an isovalue of 0.015 showing
the arene bonding to the metal center through overlap of the above-
plane π-orbitals of the ipso and ortho carbon atoms of the phenyl
ring with a metal d-orbital.
Regardless of the nomenclature of these weak interactions,
they normally refer to complexes having M · · · H distances
less than about 3 Å. There was no Mo · · · H distance found
in any of the minimized structures from Figure 6 less than 3
Å, indicating that the interaction of the Mo with the CdC
bond of the arene is favored over any type of agostic
interaction. When the Mo-P-C angle is opened to 128°,
the MCC/arene dihedral angle falls to 26°, but the Mo-Cortho
and Mo-H distances are too large (4.461 and 4.140 Å,
respectively) for there to be an agostic interaction.
As the Mo-P-C angle opens, changes are also found in the
OC-Mo-P angles. At an Mo-P-C angle of 78°, the
OC-Mo-P angles are 83.2° and 114.3°; at the larger Mo-P-C
angle of 108°, the OC-Mo-P angles are 80.5° and 116.4°.
Figure 7 shows the origin of the relatively weak arene bond
to the metal center. It is clear in the figure that the diffuse parts
of the above-plane π-orbital of the ipso and ortho carbons of
the phenyl ring overlap the diffuse part of a metal d-orbital in
a bonding interaction. This interaction requires the proper
orientation of the phenyl ring relative to the metal and a
sufficiently short distance for the overlap to occur. It may be
that these requirements are less demanding than those for an
agostic interaction.
The three examples in Table 3 with a CdC interaction are
cationic complexes, while most of those with an agostic complex
are neutral. Considering the relatively small number of ex-
amples, this observed trend may not be significant, but we
wanted to address the question of the possible dependence of
arene bonding on charge by calculating the structure of
Cp(CO)2(PH2Ph)Nb, the neutral Nb analogue of the cationic
Mo complex. The computed structure of [Cp(CO)2(PH2Ph)Nb]0
in Figure 8 is seen also to exhibit an arene bond, with an acute
Nb-P-C angle. The key geometric features of the Nb complex
are also listed in Table 5.
Figure 8. (Left) Starting geometry of [Cp(CO)2(PHtBuPh)Mo]+ used when trying to favor an agostic interaction. The Mo · · · H distance
was 2.58 Å for a putative agostic interaction between the Mo and a C-H on the tBu group. (Middle) Optimized structure showing that the
Mo(CdC) arene bonding is favored over the agostic interaction. (Right) Minimum-energy structure of [Cp(CO)2(PH2Ph)Nb]0.

3792 Organometallics, Vol. 27, No. 15, 2008
Cheng et al.
similar to the Ir-I-C angles of 105.5(4)° and 108.2(5)° found49
in [Ir(H)2(PPh3)2(ICH3)2]+ and the Ru-I-C angle50 of 104.9(7)°
in [Cp(PPh3)(CNtBu)Ru(ICH3)]+. The Mo-I distance in
[Cp(CO)2(PPh3)Mo(ICH3)]+, with an ICH3 ligand, is essentially
equal to the Mo-I distance of 2.858(3) Å in the molybdenum
iodide complex trans-Cp(CO)2(PPh3)MoI.51 These distances are
also very similar to the Mo-I distance of 2.865(1) Å found in
Cp(CO)2Mo(µ-I)(µ-PPh2)Mn(CO)4, in which the iodide bridges
a Mo and Mn.52
Halogenated hydrocarbons were also once considered to be
noncoordinating, but the binding ability of haloalkanes has
become well-recognized.53 Even solvents such as CH2Cl2 are
now known to be ligands,54 and structural data have been
reported for several chlorocarbon complexes.55
Displacement of the CdC by Water to Give
[Cp(CO)2(PPh3)Mo(OH2)]+. The CdC ligand of [Cp(CO)2(η3-
PPh3)Mo]+ can also be displaced by water to produce the aqua
complex [Cp(CO)2(PPh3)Mo(OH2)]+. This complex was first
observed as an unintentional side product during preparations
of [Cp(CO)2(η3-PPh3)Mo]+. Despite efforts to use rigorously
dried solvents and glassware, the formation of aqua complexes
occurs in numerous cases. Hughes and co-workers reported the
synthesis and crystal structures of a series of Rh and Ir
complexes with water ligands.56 They found that the cationic
intermediates {Cp*(PMe3)M[CF(CF3)2]}+ (M ) Rh, Ir) are
“voracious scavengers of adventitious moisture, even from glass
surfaces”. Koelle reviewed organometallic aqua complexes,57
and an extensive compilation of literature references on such
complexes is given in a paper by Kubas and co-workers.58
Figure 9. ORTEP diagram (30% probability ellipsoids) of
[Cp(CO)2(PPh3)Mo(ICH3)]+ (hydrogen atoms not shown).
Table 6. Selected Bond Distances (Å) and Angles (deg) for
[Cp(CO)2(PPh3)Mo(ICH3)]+
Bond Distances
Bond Angles
C(2)-Mo-C(1)
Mo-C(1)
1.96(2)
73.5(5)
122.4(4)
77.1(3)
Mo-C(1)
Mo-P
1.967(11)
2.522(3)
2.8592(13)
2.149(11)
1.130(11)
1.15(2)
C(2)-Mo-P
C(1)-Mo-P
C(2)-Mo-I
C(1)-Mo-I
P-Mo-I
Mo-I
79.2(4)
I-C(8)
C(1)-O(1)
C(2)-O(2)
125.0(3)
78.85(7)
105.7(4)
C(8)-I-Mo
Displacement of the CdC by Methyl Iodide to Give
[Cp(CO)2(PPh3)Mo(ICH3)]+. The weak CdC bond of [Cp-
(CO)2(η3-PPh3)Mo]+ is readily displaced by other ligands,
including weakly binding ligands such as CH3I. Addition of
CH3I to a solution of [Cp(CO)2(η3-PPh3)Mo]+ results in the
formation of cis-[Cp(CO)2(PPh3)Mo(ICH3)]+ (eq 3). The equi-
librium constant was determined from 1H and 31P NMR
measurements to be Keq ) 5.2 × 102 M-1 in CD2Cl2 at 22 °C.
When the solvent (and volatile CH3I) is pumped off and the
residue is dissolved in CD2Cl2, most of the [Cp(CO)2(η3-
PPh3)Mo]+ is regenerated.
(46) Brammer, L. Dalton Trans. 2003, 3145–3157.
(47) Gonzalez, A. A.; Zhang, K.; Nolan, S. P.; de la Vega, R. L.;
Mukerjee, S. L.; Hoff, C. D.; Kubas, G. J. Organometallics 1988, 7, 2429–
2435.
(48) Muckerman, J. T.; Fujita, E.; Hoff, C. D.; Kubas, G. J. J. Phys.
Chem. B 2007, 111, 6815–6821.
(49) Burk, M. J.; Segmuller, B.; Crabtree, R. H. Organometallics 1987,
6, 2241–2246.
(50) Conroy-Lewis, F. M.; Redhouse, A. D.; Simpson, S. J. J. Orga-
nomet. Chem. 1989, 366, 357–367.
(51) Bush, M. A.; Hardy, A. D. U.; Manojlovic-Muir, L.; Sim, G. A.
J. Chem. Soc. (A) 1971, 1003–1009.
(52) Horton, A. D.; Mays, M. J.; Adatia, T.; Henrick, K.; McPartlin,
M. J. Chem. Soc., Dalton Trans. 1988, 1683–1688.
(53) Kulawiec, R. J.; Crabtree, R. H. Coord. Chem. ReV. 1990, 99, 89–
115.
(54) (a) Beck, W.; Schloter, K. Z. Naturforsch. 1978, 33b, 1214–1222.
(b) Ferna´ndez, J. M.; Gladysz, J. A. Organometallics 1989, 8, 207–219.
(c) Peng, T.-S.; Winter, C. H.; Gladysz, J. A. Inorg. Chem. 1994, 33, 2534–
2542.
(55) (a) Newbound, T. D.; Colsman, M. R.; Miller, M. M.; Wulfsberg,
G. P.; Anderson, O. P.; Strauss, S. H. J. Am. Chem. Soc. 1989, 111, 3762–
3764. (b) Colsman, M. R.; Newbound, T. D.; Marshall, L. J.; Noirot, M. D.;
Miller, M. M.; Wulfsberg, G. P.; Frye, J. S.; Anderson, O. P.; Strauss, S. H.
J. Am. Chem. Soc. 1990, 112, 2349–2362. (c) Bown, M.; Waters, J. M.
J. Am. Chem. Soc. 1990, 112, 2442–2443. (d) Arndtsen, B. A.; Bergman,
R. G. Science 1995, 270, 1970–1973. (e) Butts, M. D.; Scott, B. L.; Kubas,
G. J. J. Am. Chem. Soc. 1996, 118, 11831–11843. (f) Huang, D.; Huffman,
J. C.; Bollinger, J. C.; Eisenstein, O.; Caulton, K. G. J. Am. Chem. Soc.
1997, 119, 7398–7399. (g) Huang, D.; Bollinger, J. C.; Streib, W. E.; Folting,
K.; Young, V., Jr.; Eisenstein, O.; Caulton, K. G. Organometallics 2000,
19, 2281–2290. (h) Fang, X.; Huhmann-Vincent, J.; Scott, B. L.; Kubas,
G. J. J. Organomet. Chem. 2000, 609, 95–103. (i) Tellers, D. M.; Bergman,
R. G. J. Am. Chem. Soc. 2001, 123, 11508–11509. (j) Wu, F.; Dash, A. K.;
Jordan, R. F. J. Am. Chem. Soc. 2004, 126, 15360–15361. (k) Zhang, J.;
Barakat, K. A.; Cundari, T. R.; Gunnoe, T. B.; Boyle, P. D.; Petersen, J. L.;
Day, C. S. Inorg. Chem. 2005, 44, 8379–8390.
Analytically pure crystals of cis-[Cp(CO)2(PPh3)Mo(ICH3)]+
were obtained by addition of excess CH3I to [Cp(CO)2(η3-
PPh3)Mo]+, followed by crystallization at low temperature. The
IR spectrum of [Cp(CO)2(PPh3)Mo(ICH3)]+ (Table 1) shows
ν(CO) bands at lower energy compared to those for [Cp(CO)2(η3-
PPh3)Mo]+, consistent with CH3I being a stronger donor than
the CdC bond. The 13C NMR spectrum of [Cp(CO)2(PPh3)-
Mo(ICH3)]+ at -60 °C exhibits a doublet (3JCP ) 4.5 Hz) at δ
-6.0 for the CH3I ligand, compared to the singlet for free CH3I,
which appears at δ -21.2 under the same conditions. A
crystallographic study (Figure 9) shows the cis geometry of the
CH3I and PPh3 ligands; selected bond distances and angles are
given in Table 6. The Mo-I-C angle of 105.7(4)° is quite
(56) Hughes, R. P.; Lindner, D. C.; Smith, J. M.; Zhang, D.; Incarvito,
C. D.; Lam, K.-C.; Liable-Sands, L. M.; Sommer, R. D.; Rheingold, A. L.
J. Chem. Soc., Dalton Trans. 2001, 2270–2278.
(57) Koelle, U. Coord. Chem. ReV. 1994, 135/136, 623–650.
(58) Kubas, G. J.; Burns, C. J.; Khalsa, G. R. K.; Van Der Sluys, L. S.;
Kiss, G.; Hoff, C. D. Organometallics 1992, 11, 3390–3404.

Four-Electron-Donor Hemilabile η3-PPh3 Ligand
Organometallics, Vol. 27, No. 15, 2008 3793
between δ 2.87 and 3.31 for the H2O ligand. Many complexes
contain both phosphines and water ligands, yet P-H coupling
is seldom observed. We were able to determine the P-H
coupling between the H on the water ligand and the phosphorus
in [Cp(CO)2(PPh3)Mo(OH2)]+. Protonation of Cp(CO)2(PPh3)-
MoH with [H(Et2O)2]+BAr′4-59 in CD2Cl2 produced a mixture
of [Cp(CO)2(η3-PPh3)Mo]+[BAr′4]- and [Cp(CO)2(PPh3)-
Mo(OH2)]+[BAr′4]-. The 1H NMR resonance for the OH2
ligand was observed at δ 3.47 as a doublet (3JPH ) 3.0 Hz).
Evolution of H2 (δ4.59) was observed by NMR and results from
decomposition of the cationic dihydride [Cp(CO)2-
(PPh3)Mo(H)2]+ or the dihydrogen complex [Cp(CO)2(PPh3)Mo-
(η2-H2)]+. No intermediate dihydride or dihydrogen complex
was observed. The tungsten dihydride [Cp(CO)2(PMe3)W(H)2]+-
OTf- has been characterized by X-ray crystallographically.60
The related Mo complex might also be a dihydride, but Poli
and co-workers studied protonation of Cp(CO)2(PMe3)MoH and
found prompt evolution of H2 even at low temperature; they
suggested that an unstable dihydrogen complex was formed.61
Regardless of whether the product of protonation of
Cp(CO)2(PPh3)MoH is a dihydride or dihydrogen complex, H2
is produced upon treatment of the metal hydride with acid, and
the formation of [Cp(CO)2(PPh3)Mo(OH2)]+ results from
Figure 10. ORTEP diagram (30% probability ellipsoids) of
[Cp(CO)2(PPh3)Mo(OH2)]+ (hydrogen atoms not shown).
Table 7. Selected Bond Distances (Å) and Angles (deg) for
[Cp(CO)2(PPh3)Mo(OH2)]+
Bond Distances
Bond Angles
Mo-C(1)
1.955(8)
1.958(11)
2.526(2)
2.445(9)
C(1)-Mo-C(3)
C(1)-Mo-O(2)
C(3)-Mo-O(2)
C(1)-Mo-P
74.7(4)
77.3(3)
126.9(2)
120.8(2)
76.5(2)
Mo-C(3)
Mo-P
-
adventitious water. Brookhart’s acid,59 [H(Et2O)2]+BAr′4 is
Mo-O(2)
known to be notoriously difficult to obtain completely dry,62
so it is not so surprising that the aqua complex is formed.
When the solvent is pumped off of solutions containing
[Cp(CO)2(PPh3)Mo(OH2)]+, the water ligand is lost, and most
of the product is converted to [Cp(CO)2(η3-PPh3)Mo]+. We
were unable to reliably determine the Keq of this reaction, but
qualitatively it appears to be roughly similar to that observed
in eq 3. The observation of water as a ligand in this type of Mo
complex is relevant since we have shown that [Cp(CO)2(η3-
PPh3)Mo]+[BAr′4]- serves as a catalyst precursor for ionic
hydrogenation63 of ketones.17 While the main product of these
hydrogenations is the alcohol produced by hydrogenation of the
CdO bond of the ketone, we also find that small amounts of
the alcohol condense to form ethers. This condensation also
produces water; so if strong irreversible binding of the water
were to occur, then the catalytic activity would be impeded.
Facile displacement of water is therefore important in our
catalytic reactions.
C(3)-Mo-P
O(2)-Mo-P
80.68(12)
Intentional addition of water to a solution of [Cp(CO)2(η3-
PPh3)Mo]+[BAr′4]- led to the isolation of [Cp(CO)2(PPh3)-
Mo(OH2)]+[BAr′4]- as red crystals in 78% yield. The crystal
structure was determined at the National Synchrotron Light
Source. There is some static disorder in this crystal, as the
coordinated water and one of the carbonyl ligands (C(3)-O(3))
are disordered between the two coordination sites. Attempts to
refine a model using a water molecule and a carbonyl ligand
sharing the same coordination site (in an approximately 75:25
ratio for one site and 25:75 for the other position) did not result
in a better model because the atoms sharing the coordination
site in the disordered model were only about 0.6 Å apart. This
disordering and the choice of including only the predominant
ligand in each site is responsible for the elongated thermal
ellipsoid of O(2).
An ORTEP diagram is shown in Figure 10, and selected bond
distances and angles are listed in Table 7. Hydrogen bonding
between water ligands and counterions is almost invariably
found with anions such as BF4- or OTf-, but is expected to be
less prevalent with counterions such as BAr′4-. The closest
distance between O(2) and an F on one of the CF3 groups of
Conclusion
PPh3 exhibits a remarkable versatility of bonding modes as
a ligand in organometallic complexes. While it most often
functions as a traditional two-electron donor, it is also capable
of bonding as a chelating ligand in the η3-PPh3 mode with a
CdC bond to the metal or through interaction of the metal with
an agostic C-H bond. Spectroscopic, structural, and compu-
tational studies on [Cp(CO)2(η3-PPh3)Mo]+ show bonding of
the Mo to a CdC of the Ph ring. Caulton has found examples
of Ir complexes where the influence of steric bulk favors agostic
interactions, in which bulky ligands push a C-H bond close
enough to the metal to engage in an agostic bond.35 In contrast,
-
the BAr′4 anion is 3.17 Å. Much shorter F · · · O contact
distances of 2.804 and 2.805 Å were found between the water
ligand and fluorine atoms of the CF(CF3)2 ligand in the crystal
structure of {Cp*(PMe3)Ir[CF(CF3)2]}+ · 0.5 OH2[BAr′4]-.56
The structure of [Cp(CO)2(PPh3)Mo(OH2)]+[BAr′4]- has a
carbonyl oxygen [O(1)] that is 3.1 Å from O(2), again indicating
at most a very weak hydrogen bond. Our interpretation of these
data is that hydrogen bonding of the OH2 ligand to either O or
F in the crystal structure of [Cp(CO)2(PPh3)Mo(OH2)]+[BAr′4]-
is not present or is, at most, a very weak interaction.
(59) Brookhart, M.; Grant, B.; Volpe, A. F., Jr. Organometallics 1992,
11, 3920–3922.
(60) Bullock, R. M.; Song, J.-S.; Szalda, D. J. Organometallics 1996,
15, 2504–2516.
(61) Quadrelli, E. A.; Kraatz, H.-B.; Poli, R. Inorg. Chem. 1996, 35,
5154–5162.
1
The H NMR resonance for H2O ligands in organometallic
complexes is often broad, and its chemical shift is variable and
dependent on temperature and concentration. Exchange between
free and bound water can cause broadening of the bound water
peak, and hydrogen bonding of the bound water ligand can affect
its chemical shift. Our 1H NMR spectra of [Cp(CO)2-
(PPh3)Mo(OH2)]+[BAr′4]- generally showed a broad resonance
(62) (a) Hughes, R. P.; Lindner, D. C.; Rheingold, A. L.; Yap, G. P. A.
Inorg. Chem. 1997, 36, 1726–1727. (b) Stahl, S. S.; Labinger, J. A.; Bercaw,
J. E. Inorg. Chem. 1998, 37, 2422–2431.
(63) Bullock, R. M. Chem.-Eur. J. 2004, 10, 2366–2374.

3794 Organometallics, Vol. 27, No. 15, 2008
Cheng et al.
of PPhPh2), 128.3 (br, q, JCF ) 31 Hz, m-C of BAr′4-), 126.2 (d,
JCP ) 53.0 Hz, ipso-C of PPhPh2), 124.1 (q, JCF ) 272 Hz, CF3 of
BAr′4-), 121.7 (d, JCP ) 60 Hz, ipso-C of PPhPh2), 117.2 (br s,
p-C of BAr′4-), 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, -60 °C): δ 37.6 (s).
steric pressure is not required to facilitate an interaction of
[Cp(CO)2(PPh3)Mo]+ with the CdC bond; computational
studies on [Cp(CO)2(PH2Ph)Mo]+ show that this complex (with
a less sterically hindered phosphine than PPh3) also has a
bonding interaction of the CdC with the metal. Computations
on [Cp(CO)2(PHtBuPh)Mo]+ show that it maintains the bond
of a CdC to the metal, rather than forming an agostic interaction
Crystal Formation of [Cp(CO)2(PPh3)Mo]+[BAr′4]-. In a
glovebox, ∼70 mg of [Cp(CO)2(PPh3)Mo]+[BAr′4]- was dissolved
in toluene (∼15 mL) to make a saturated solution. The mixture
was filtered into a Schlenk tube and cooled to 0 °C. After 20 days,
red crystals were collected and dried by blowing Ar over them.
One toluene molecule was found in the crystal structure.
t
of the metal to a C-H of the Bu group. The requirements for
the different modes of bonding for agostic C-H ligands vs CdC
result in distinct structural preferences, though both are relatively
weak bonding interactions. When a C-H of a phenyl group of
a PPh3 ligand engages in an agostic bond, the metal is generally
located close to the plane of the arene ring. In contrast, in the
interaction of the metal with the CdC fragment of an arene
ring, the dihedral angle between the plane of the aromatic ring
and the plane containing the metal and the two interacting CdC
carbons is about 60°. Our results show that bonding of a metal
to the CdC bond of an arene can be significantly favored over
the more commonly observed agostic C-H bonding.
Synthesis and Crystal Growth of [Cp(CO)2(PPh3)Mo-
(ICH3)]+[BAr′4]-. In a glovebox, [Cp(CO)2(PPh3)Mo]+[BAr′4]-
(45 mg; 0.033 mmol) was dissolved in toluene (5 mL) and CH2Cl2
(1.5 mL). Then CH3I (15 µL, 0.24 mmol, 7.3 equiv) was added,
and the solution was filtered into a Schlenk tube. Hexane (∼7 mL)
was layered on top, and the solution was cooled in a freezer (-20
°C) for 6 days. The red crystals (32 mg, 64% yield) were collected
by filtration and dried by blowing Ar over them (not dried under
vacuum). Anal. Calcd for C58H35BF24IO2PMo: C, 46.93; H, 2.38.
Found: C, 47.31; H, 2.30. 1H NMR (CD2Cl2, 22 °C): δ 7.72 (br, 8
H, o-H of BAr’4-), 7.73-7.50 (br m, 13 H, p-H of BAr′4- + PPh3),
7.28-7.22 (m, 6 H, PPh3), 5.55 (s, 5 H, Cp), 2.38 (s, 3 H, CH3).
31P{1H} NMR (22 °C): δ 46.5 (s). 13C NMR (-60 °C): 241.7 (d,
Experimental Section
General Procedures. All manipulations were carried out under
an atmosphere of argon using Schlenk or vacuum-line techniques,
or in a Vacuum Atmospheres drybox. 1H NMR chemical shifts
were referenced to the residual proton peak of CD2Cl2 at δ 5.32.
Elemental analyses were carried out by Schwarzkopf Microana-
lytical Laboratory (Woodside, NY). NMR spectra were recorded
JCP ) 29.0 Hz, CO), 240.9 (s, CO), 161.4 (1:1:1:1 quartet, JCB
)
49.7 Hz, ipso-C of BAr′4-), 134.2 (s, o-C of BAr′4-), 133.0-129.1
(m, PPh3), 128.2 (br q, JCF ) 30.1 Hz, m-C of BAr′4-), 124.0 (q,
JCF ) 272.4 Hz, CF3 of BAr′4-), 117.2 (s, p-C of BAr′4-), 94.6 (s,
Cp), -6.0 (d, 3JCP ) 4.5 Hz, ICH3). IR (CH2Cl2): ν(CO) 1987 (s),
1917 (s) cm-1. IR (Nujol): ν(CO) 1992 (s), 1927 (s) cm-1. (Spectra
1
on a Bruker AM-300 spectrometer (300 MHz for H). IR spectra
were recorded on
a Mattson Polaris FT-IR spectrometer.
-65
Cp(CO)2(PPh3)MoH,64 Ph3C+BAr′4
[Ar′ ) 3,5-bis(trifluoro-
1
of free CH3I in CD2Cl2 for comparison: H NMR (22 °C) δ 2.16
methyl)phenyl], and [H(Et2O)2]+BAr′4
were prepared by lit-
-59
(s); 13C{1H} NMR (-60 °C) δ -21.2 (s).)
erature methods. Ph3C+BF4 was purchased from Aldrich and
purified by recrystallization from CH2Cl2/Et2O. THF, Et2O, and
hexane were distilled from Na/benzophenone, and CH2Cl2 was
distilled from P2O5.
-
When a solution of [cis-Cp(CO)2(PPh3)Mo(ICH3)]+ was pumped
1
to dryness and redissolved in CD2Cl2, the H NMR showed that
71% of cis-Cp(CO)2(PPh3)Mo(ICH3)+ remained, but trans-
Cp(CO)2(PPh3)Mo(ICH3)+ (4%) and [Cp(CO)2(PPh3)Mo]+[BAr’4]-
(25%)wereformed.Partial1HNMRfor[trans-Cp(CO)2(PPh3)Mo(ICH3)]+:
δ 5.33 (d, Cp, JPH ) 1.9 Hz), 2.60 (s, CH3). 31P{1H} NMR of
[trans-Cp(CO)2(PPh3)Mo(ICH3)]+[BAr′4]-: δ 54.1 (s). After 22 h,
the NMR spectrum showed a mixture of [cis-Cp(CO)2(PPh3)-
Mo(ICH3)]+ (45%), [trans-Cp(CO)2(PPh3)Mo(ICH3)]+ (30%), [Cp-
(CO)2(PPh3)Mo]+[BAr′4]- (18%), and unidentified decomposition
products (7%).
Preparation of [Cp(CO)2(PPh3)Mo]+[BAr′4]. A solution of
Cp(CO)2(PPh3)MoH (300 mg, 0.625 mmol) in CH2Cl2 (12 mL)
was added to a cold (-30 °C) solution of Ph3C+BAr′4- (665 mg,
0.600 mmol) in CH2Cl2 (10 mL). The reaction mixture turned dark
red and was stirred at -30 °C for 20 min. The solvent was reduced
to 5 mL, and purple solids precipitated. Hexane (20 mL) was added,
and the precipitate was collected by filtration, washed with hexane
(3 × 10 mL), and dried under vacuum to give [Cp(CO)2-
(PPh3)Mo]+[BAr′4]- (700 mg, 84%) as a tan solid. 1H NMR
(CD2Cl2, 22 °C): δ 7.80-7.20 (br m, 27 H, BAr′4 + PPh3), 5.62
(s, 5 H, Cp). 31P{1H} NMR (CD2Cl2, 295 K): δ 38.3 (s). IR
(CH2Cl2): ν(CO) 2009 (s), 1939 (s) cm-1. IR (Nujol): ν(CO) 2019
(s), 1956 (s) cm-1. Anal. Calcd for C57H32BF24O2PMo: C, 50.99;
H, 2.40. Found: C, 50.45; H, 2.24. This complex decomposes to
unidentified products over several days at room temperature in
Determination of the Keq for Formation of [Cp(CO)2(PPh3)-
Mo(ICH3)]+[BAr′4]-. [Cp(CO)2(PPh3)Mo]+[BAr′4]- (30.0 mg,
0.0223 mmol) was placed in a NMR tube along with CD2Cl2 and
1,2-dichloroethane (0.6 µL, internal standard) to give a total volume
of 0.53 mL, with the concentration of [Cp(CO)2(PPh3)Mo]+-
[BAr′4]- ) 42 mM. After the initial spectra were taken, CH3I (2.5
µL, 0.040 mmol, 1.80 equiv) was added. 1H and 31P NMR spectra
recorded at 22 °C indicated equilibrium concentrations of 40 mM
-
CD2Cl2 solution. 1H NMR (CD2Cl2, -60 °C): the signals for BAr′4
[cis-Cp(CO)2(PPh3)Mo(ICH3)]+[BAr′4]-,
2.3
mM
(δ 7.74 and 7.53) overlapped with the signals of PPh3 (δ
[Cp(CO)2(PPh3)Mo]+[BAr′4]-, and 34 mM free ICH3; Keq ) 5.2
3
3
7.82-7.20), δ 6.12 (br t, 1 H, CH, JHH ∼ JPH ) 6.9 Hz) (with
1H decoupling at the aromatic protons δ 7.37, the triplet at δ 6.12
collapsed to a doublet with JPH ) 7.7 Hz), 5.62 (s, 5 H, Cp).
13C{1H} NMR (CD2Cl2, -60 °C): δ 237.0 (d, JCP ) 24.8, CO),
231.4 (s, CO), 161.4 (1:1:1:1 quartet, JCB ) 50.9 Hz, ipso-C of
BAr′4-), 135.2 (s, p-C of PPhPh2), 134.3 (br s, o-C of BAr′4-),
134.0 (d, JCP ) 11.6 Hz, o-C of PPhPh2), 134.0 (s, p-C of PPhPh2),
133.6 (d, JCP ) 12.1 Hz, o-C of PPhPh2), 133.1 (d, JCP ) 13.7 Hz,
m-C of PPhPh2), 132.2 (d, JCP ) 10.9 Hz, o-C’ of PPhPh2), 129.9
(d, JCP ) 11.8 Hz, m-C of PPhPh2), 129.6 (d, JCP ) 12.4 Hz, m-C
× 102 M-1
.
Synthesis and Crystal Growth of [Cp(CO)2(PPh3)Mo-
(OH2)]+[BAr′4]-. In a glovebox, [Cp(CO)2(PPh3)Mo]+[BAr′4]-
(50 mg, 0.037 mmol) was dissolved in toluene (5 mL) and CH2Cl2
(2.5 mL). The solution was filtered into a Schlenk tube, which was
removed from the glovebox. H2O (1.5 µL, 0.083 mmol, 2.3 equiv)
was added, and the solution was layered with hexane (3 mL) cooled
at -20 °C for 7 days. Red crystals (40 mg, 0.029 mmol, 78% yield)
were collected by filtration and dried by blowing Ar over them
(not dried under vacuum). Anal. Calcd for C57H34BF24O3PMo: C,
50.32; H, 2.52. Found: C, 50.36; H, 2.50. NMR (CD2Cl2, 22 °C):
(64) Bainbridge, A.; Craig, P. J.; Green, M. J. Chem. Soc. (A) 1968,
2715–2718.
(65) Bahr, S. R.; Boudjouk, P. J. Org. Chem. 1992, 57, 5545–5547.
-
δ 7.72 (br, 8 H, o-H), 7.73-7.50 (br m, 13 H, p-H of BAr′4
+
PPh3), 7.28-7.22 (m, 6 H, PPh3), 5.58 (s, 5 H, Cp). In isolated

Four-Electron-Donor Hemilabile η3-PPh3 Ligand
Organometallics, Vol. 27, No. 15, 2008 3795
Table 8. Crystallographic Data Collection and Refinement Information
[Cp(CO)2(PPh3)Mo]+[BAr′4]-
[Cp(CO)2(PPh3)Mo-(OH2)]+[BAr′4]-
[Cp(CO)2(PPh3)Mo-(ICH3)]+[BAr′4]-
formula
fw
C64H40BMoF24O2P
1434.68
C64H42BMoF24O3P
1452.70
C58H35BMoF24O2IP
1484.48
temp (K)
cryst syst
space group
a (Å)
b (Å)
c (Å)
95(2)
95(2)
293(2)
monoclinic
P21/n (No. 14)
15.371(2)
24.095(2)
17.319(2)
triclinic
triclinic
j
j
P1 (No. 2)
P1 (No. 2)
13.792(4)
15.032(6)
16.414(7)
69.08(2)
72.550(10)
82.62(2)
3032(2)
2
9.53
1.008
12.421(2)
13.151(2)
18.319(5)
96.53(2)
96.34(2)
90.11(2)
2954.5(10)
2
0.889
0.71073
1.669
R (deg)
ꢀ (deg)
γ (deg)
V (Å3)
Z
106.67(2)
6144.8(12)
4
12.2
1.100
1.551
0.08 × 0.08 × 0.02
2.31 to 37.00
18 041
µ (mm-1
λ (Å)
)
Fcalc (g cm-3
)
1.591
cryst size (mm)
0.33 × 0.27 × 0.03
3.67 to 38.43
17 076
0.52 × 0.30 × 0.11
1.13 to 24.99
10 322
2θ range (deg)
total no. of reflns
no. of indep reflns, I g 3.0σ(Ι)
5504 [R(int) ) 0.076]
6736 [R(int) ) 0.061]
10 322
795
no. of params
674
851
final R indices [I > 2σ(I)]a
R1 ) 0.0738,
wR2 ) 0.1899
R1 ) 0.0981,
wR2 ) 0.2097
1.016
R1 ) 0.0689,
wR2 ) 0.2088
R1 ) 0.0863,
wR2 ) 0.2409
1.112
R1 ) 0.0609,
wR2 ) 0.1377
R1 ) 0.2345,
wR2 ) 0.1981
0.999
R indices (all data)
goodness-of-fit on F2
extinction coeff
absorp corr
none
0.0064(9)
none
none
Gaussian
Fourier(XABS2)b
2
2
2 2 1/2
a R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) {∑[w(|Fo | - |Fc |)2]/∑[w|Fo | ]
.
b Parkin, S.; Moezzi, B.; Hope, H. J. Appl. Crystallogr. 1995, 28, 53-56.
samples and NMR tube preparations, the position of the resonance
of the OH2 ligand varied between δ 2.87 and 3.31 (br, 2 H, OH2).
It was usually broad (except as described for one case below), but
the integration (2H) confirms its assignment. 31P{1H} NMR of [cis-
Cp(CO)2(PPh3)Mo(OH2)]+BAr′4-: δ 55.1 (s). 13C NMR (CD2Cl2,
233 K): 250.5 (d, JCP ) 29.4 Hz, CO), 248.2 (s, CO), 161.6 (1:1:
1:1 quartet, JCB ) 49.6 Hz, ipso-C of BAr′4-), 134.5 (o-C of
BAr′4-), 133.0 (d, JC-P ) 11.2 Hz, o-C of PPh3), 132.1 (s, p-C of
PPh3), 131.2 (br d, JCP ) 54.3 Hz, ipso-C of PPh3), 129.8 (d, JCP
) 10.3 Hz, m-C of PPh3), 128.5 (br q, JCF ) 32.5 Hz, m-C of
BAr′4-), 124.3 (q, JCF ) 272.5 Hz, CF3 of BAr′4-), 117.4 (p-C of
coated with Vaseline and sealed inside a glass capillary, which was
transferred to an Enraf Nonius CAD-4 diffractometer. Diffraction
data indicated monoclinic symmetry and systematic absences
consistent with space group P21/n for [Cp(CO)2(PPh3)-
Mo]+[BAr′4]-; triclinic symmetry for [Cp(CO)2(PPh3)Mo(OH2)]+-
[BAr′4]- and [Cp(CO)2(PPh3)Mo(ICH3)]+[BAr′4]- consistent with
j
the space groups P1 and P1. In the least-squares refinement,
anisotropic temperature parameters were used for all the non-
hydrogen atoms in [Cp(CO)2(PPh3)Mo(OH2)]+[BAr′4]- and
[Cp(CO)2(PPh3)Mo(ICH3)]+[BAr′4]-. For [Cp(CO)2(PPh3)Mo]+-
[BAr′4]-, anisotropic temperature parameters were used for all the
non-hydrogen atoms except the boron and carbons in the anion.
Hydrogen atoms (except those on the toluene of crystallization for
[Cp(CO)2(PPh3)Mo]+[BAr′4]-, which were not included) were
placed at calculated positions and allowed to “ride” on the atom to
which they were attached.
BAr′4-), 96.1 (s, Cp). IR (CH2Cl2): ν(CO) 1985 (s), 1909 (s) cm-1
.
IR (Nujol): ν(CO) 1984 (s), 1887 (s) cm-1
.
NMR Tube Reaction of Cp(CO)2(PPh3)MoH with H(Et2O)2-
BAr′4- in CD2Cl2. A 5 mm NMR tube equipped with a Teflon J.
Young valve was charged with Cp(CO)2(PPh3)MoH (7.7 mg, 0.016
mmol) and H(Et2O)2BAr′4- (19.6 mg, 0.019 mmol). CD2Cl2 (∼0.5
mL) was added into this tube to give a red solution. The formation
of H2 (δ 4.59) was observed. The NMR spectra indicated the
formation of [Cp(CO)2(PPh3)Mo(OH2)]+[BAr′4]- (65%) and
[Cp(CO)2(PPh3)Mo]+[BAr′4]- (35%). 1H NMR for [Cp(CO)2(PPh3)-
Mo(OH2)]+[BAr′4]- were as described above, with the notable
exception of the H2O ligand, which appeared at δ 3.47 (d, 2 H,
3JPH ) 3.0 Hz, H2O). The solvent was removed under vacuum,
and the residue was redissolved in CD2Cl2. The NMR spectra
showed that most (89%) of the product was [Cp(CO)2-
(PPh3)Mo]+[BAr′4]-, with 11% of [Cp(CO)2(PPh3)Mo(OH2)]+-
[BAr′4]- remaining.
Determination and Refinement of the Crystal Structures.
Attempts to determine the structures of [Cp(CO)2(PPh3)-
Mo]+[BAr′4]- and [Cp(CO)2(PPh3)Mo(OH2)]+[BAr′4]- with a
conventional X-ray source (sealed tube) were not successful, since
the crystals were small and poor diffractors of X-rays. These crystals
were coated with perfluoropolyether oil and mounted on the end
of a glass capillary; data were collected at 95 K using X-rays from
the National Synchrotron Light Source at Brookhaven National
Laboratory. For [Cp(CO)2(PPh3)Mo(ICH3)]+[BAr′4]- crystals were
Acknowledgment. Research at Brookhaven National
Laboratory was carried out under contract DE-AC02-
98CH10886 with the U.S. Department of Energy and was
supported by its Division of Chemical Sciences, Office of
Basic Energy Sciences. Research at Pacific Northwest
National Laboratory was funded by the Division of Chemical
Sciences, Office of Basic Energy Sciences, U.S. Department
of Energy, and by an LDRD grant. Pacific Northwest
National Laboratory is operated by Battelle for the U.S.
Department of Energy.
Supporting Information Available: Coordinates of optimized
[Cp(CO)2(η3-PPh3)Mo]+,[Cp(CO)2(PHtBuPh)Mo]+,[Cp(CO)2(PH2Ph)-
Mo]+, and [Cp(CO)2(PH2Ph)Nb] and crystallographic information
(cif files) for [Cp(CO)2(η3-PPh3)Mo]+[BAr′4]-, [Cp(CO)2(PPh3)Mo-
(ICH3)]+[BAr′4]-, and [Cp(CO)2(PPh3)Mo(OH2)]+[BAr′4]-. This
materialisavailablefreeofchargeviatheInternetathttp://pubs.acs.org.
OM800401D