H+/AuPPh3+ Exchange for the Hydride Complexes CpMoH(CO)2(L) (L = PMe3, PPh3, CO). Formation and Structure of [Cp(CO)2(PMe3)Mo(AuPPh3)2] +[BF4]-

Article abstract of DOI:10.1021/ic961265a

The reaction of CpMoH(CO)₂L with AuPPh₃ ⁺BF₄^- in THF at -40 °C proceeds directly to the MoAu₂ cluster compounds [CpMo(CO)₂L(AuPPh₃)₂]⁺BF ₄^- (L = PMe₃ (1), PPh₃ (2)) with release of protons. A 1:1 reaction leaves 50% of the starting hydride unreacted. At lower temperature, however, the formation of a [CpMo(CO)₂-(PMe₃)(μ-H)(AuPPh₃)]⁺ intermediate is observed. This compound evolves to the cation of 1 and CpMoH(CO)₂-(PMe₃) upon warming and is deprotonated by 2,6-lutidine to afford CpMo(CO)₂(PMe₃)(AuPPh₃). The X-ray structure of 1 can be described as a four-legged piano stool with the PMe₃ and the η²-(AuPPh₃)₂ ligands occupying relative trans positions. [Cp(CO)₂(PMe₃)Mo(AuPPh₃)₂] ⁺[BF₄]^- (M_r = 1298.41): monoclinic, space group P2₁/ n, a= 18.1457(13) A?, b = 9.7811(7) A?, c = 26.096(2) A?, β= 105.086(5)°, V = 4472.0(5) ?³, Z = 4. The reaction of CpMoH(CO)₂(PMe₃) with 3 equiv of AuPPh₃⁺ affords a MoAu₃ cluster, [CpMo(CO)₂(PMe₃)-(AuPPh₃)₃] ²⁺ (3), in good yields under kinetically controlled conditions. Under thermodynamically controlled conditions, 3 dissociates extensively into 1 and free AuPPh₃⁺.. It is proposed that the hydride ligand helps build higher nuclearity Mo - Au clusters. The difference in reaction pathways for the interaction of AuPPh₃⁺ with CpMoH(CO)₂L when L = PR₃ or CO and for the interaction of CpMoH(CO)₂(PMe₃ with E⁺ when E = H, Ph₃C or AuPPh₃ is discussed. The lower acidity and greater aurophilicity of the [CpMo(CO)₂L(μ-H)(AuPPh₃)]⁺ intermediate when L = PMe₃ favor attack by AuPPh₃⁺ before deprotonation.

Full text of DOI:10.1021/ic961265a

Inorg. Chem. 1997, 36, 3001-3007
3001
H+/AuPPh3 Exchange for the Hydride Complexes CpMoH(CO)2(L) (L ) PMe3, PPh3,
+
CO). Formation and Structure of [Cp(CO)2(PMe3)Mo(AuPPh3)2]+[BF4]-
Rossana Galassi, Rinaldo Poli,*,† E. Alessandra Quadrelli, and James C. Fettinger
Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742
ReceiVed October 18, 1996X
-
The reaction of CpMoH(CO)2L with AuPPh3+BF4 in THF at -40 °C proceeds directly to the MoAu2 cluster
-
compounds [CpMo(CO)2L(AuPPh3)2]+BF4 (L ) PMe3 (1), PPh3 (2)) with release of protons. A 1:1 reaction
leaves 50% of the starting hydride unreacted. At lower temperature, however, the formation of a [CpMo(CO)2-
(PMe3)(µ-H)(AuPPh3)]+ intermediate is observed. This compound evolves to the cation of 1 and CpMoH(CO)2-
(PMe3) upon warming and is deprotonated by 2,6-lutidine to afford CpMo(CO)2(PMe3)(AuPPh3). The X-ray
structure of 1 can be described as a four-legged piano stool with the PMe3 and the “η2-(AuPPh3)2” ligands occupying
relative trans positions. [Cp(CO)2(PMe3)Mo(AuPPh3)2]+[BF4]- (Mr ) 1298.41): monoclinic, space group P21/
n, a ) 18.1457(13) Å, b ) 9.7811(7) Å, c ) 26.096(2) Å, â ) 105.086(5)°, V ) 4472.0(5) Å3, Z ) 4. The
+
reaction of CpMoH(CO)2(PMe3) with 3 equiv of AuPPh3 affords a MoAu3 cluster, [CpMo(CO)2(PMe3)-
(AuPPh3)3]2+ (3), in good yields under kinetically controlled conditions. Under thermodynamically controlled
conditions, 3 dissociates extensively into 1 and free AuPPh3+. It is proposed that the hydride ligand helps build
+
higher nuclearity Mo-Au clusters. The difference in reaction pathways for the interaction of AuPPh3 with
CpMoH(CO)2L when L ) PR3 or CO and for the interaction of CpMoH(CO)2(PMe3) with E+ when E ) H, Ph3C
or AuPPh3 is discussed. The lower acidity and greater aurophilicity of the [CpMo(CO)2L(µ-H)(AuPPh3)]+
+
intermediate when L ) PMe3 favor attack by AuPPh3 before deprotonation.
Introduction
Scheme 1
Electrophilic addition is a well-known type of reaction in
organometallic chemistry,1,2 and the addition of a variety of
electrophiles to transition metal hydride complexes has been
extensively investigated. A typical example of this reaction is
protonation, which can occur either at the metal center or at a
M-H bond, resulting in the formation of a classical (I, Scheme
1 path a) or nonclassical (II, path b) hydride complex,
respectively.3,4 The formation of I may be preceded by the
formation of II.5 The presence of other ligands (e.g. donor
solvents) may lead to the substitution of H2 from II (path d),
and the overall reaction is therefore classified as an electrophilic
abstraction. A second extensively investigated electrophilic
reagent is the trityl cation. In this case, intermediate II is not
observed, the reaction typically leading to electrophilic abstrac-
tion with formation of Ph3CH.6-9 However, depending on the
oxidation potential of the substrate, electron transfer may also
occur (path c) leading to a 17-electron hydride intermediate and
trityl radical, which then evolve according to a variety of
reaction pathways.10 Finally, metal-based electrophiles (e.g.
Cu+, Tl+, Ag+, AuL+) have received some attention, especially
the gold-based cation AuPPh3+. In this case, stable complexes
of type II have been isolated and characterized in numerous
occasions.11-24 Whenever these adducts are not stable, the
(10) Ryan, O. B.; Smith, K.-T.; Tilset, M. J. Organomet. Chem. 1991, 421,
315-322 and references therein.
(11) Alexander, B. D.; Johnson, B. J.; Johnson, S. M.; Casalnuovo, A. L.;
Pignolet, L. H. J. Am. Chem. Soc. 1986, 108, 4409-4417.
(12) Boyle, P. D.; Johnson, B. J.; Buehler, A.; Pignolet, L. H. Inorg. Chem.
1986, 25, 5-7.
† Current address: Laboratoire de Synthe`se et d’Electrosynthe`se Orga-
nometallique, Faculte´ des Sciences “Gabriel”, 6 Boulevard Gabriel, 21100
Dijon, France.
X Abstract published in AdVance ACS Abstracts, June 1, 1997.
(1) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles
and Applications of Organotransition Metal Chemistry; University
Science Books: Mill Valley, CA, 1987.
(13) Alexander, B. D.; Johnson, B. J.; Johnson, S. M.; Boyle, P. D.; Kann,
N. C.; Mueting, A. M.; Pignolet, L. H. Inorg. Chem. 1987, 26, 3506-
3513.
(14) Sutherland, B. R.; Folting, K.; Streib, W. E.; Ho, D. M.; Huffman, J.
C.; Caulton, K. G. J. Am. Chem. Soc. 1987, 109, 3489-3490.
(15) Albinati, A.; Lehner, H.; Venanzi, L. M.; Wolfer, M. Inorg. Chem.
1987, 26, 3933-3939.
(16) Alexander, B. D.; Gomez-Sal, M. P.; Gannon, P. R.; Blaine, C. A.;
Boyle, P. D.; Mueting, A. M.; Pignolet, L. H. Inorg.Chem. 1988, 27,
3301-3308.
(17) Albinati, A.; Demartin, F.; Venanzi, L. M.; Wolfer, M. K. Angew.
Chem., Int. Ed. Engl. 1988, 27, 563-563.
(18) Albinati, A.; Anklin, C.; Janser, P.; Lehner, H.; Matt, D.; Pregosin,
P. S.; Venanzi, L. M. Inorg. Chem. 1989, 28, 1105-1111.
(19) Albinati, A.; Demartin, F.; Janser, P.; Rhodes, L. F.; Venanzi, L. M.
J. Am. Chem. Soc. 1989, 111, 2115-2125.
(2) Crabtree, R. H. The Organometallic Chemistry of the Transition
Metals, 2nd ed.; Wiley: New York, 1994.
(3) Crabtree, R. H.; Hamilton, D. G. AdV. Organomet. Chem. 1988, 28,
299-338.
(4) Crabtree, R. H. Angew. Chem., Int. Ed. Engl. 1993, 32, 789-805.
(5) Parkin, G.; Bercaw, J. E. J. Chem. Soc., Chem. Commun. 1989, 255-
257.
(6) Su¨nkel, K.; Ernst, H.; Beck, W. Z. Naturforsch. 1981, 36b, 474-481.
(7) Ryan, O. B.; Tilset, M.; Parker, V. D. J. Am. Chem. Soc. 1990, 112,
2618-2626.
(8) Smith, K.-T.; Tilset, M. J. Organomet. Chem. 1992, 431, 55-64.
(9) Cheng, T.-Y.; Bullock, R. M. Organometallics 1995, 14, 4031-4033.
S0020-1669(96)01265-7 CCC: $14.00 © 1997 American Chemical Society

3002 Inorganic Chemistry, Vol. 36, No. 14, 1997
Galassi et al.
typical pattern is deprotonation (path e) resulting in the overall
+
replacement of an H+ with the isolobal AuPPh3 unit.
While each of the above mentioned electrophilic reagents
have been investigated with a variety of transition metal hydride
systems, there is apparently no single hydride complex for which
all of the above reagents have been tested. Thus, it is not clear
whether the different chemical reactivity should be attributed
to differences in the nature of the electrophile or to differences
in the hydride substrate. The class of compounds CpMoH-
(CO)2L (L ) PMe3, PPh3) has been investigated by us and others
in terms of oxidation (either chemical or electrochemical),8,25
protonation,25 and hydride abstraction with the trityl cation.8,9,25
Reactions with metal-based electrophiles have not, however,
been investigated. We report here our observations on the
interaction with the gold-based electrophile AuPPh3+. Interest-
ing differences have been observed with respect to the interac-
tion of H+ and Ph3C+ with the same substrates, as well as
+
relative to the interaction of AuPPh3 with CpMoH(CO)3.
Experimental Section
General Methods. All operations were carried out under an
atmosphere of dinitrogen or argon with standard Schlenk-line tech-
niques. Solvents were purified by conventional methods and distilled
under argon prior to use (THF and Et2O from Na/benzophenone,
heptane from Na, CH2Cl2 from P4O10, and MeCN from CaH2). FT-IR
spectra were recorded on a Perkin-Elmer 1880 spectrophotometer with
CaF2 or CsI cells. NMR spectra were obtained with Bruker WP200
and AF200 spectrometers. The peak positions are reported with positive
shifts downfield of TMS as calculated from the residual solvent peaks
(1H) or downfield of external 85% H3P04 (31P). For each 31P NMR
spectrum, a sealed capillary containing H3PO4 was immersed in the
same NMR solvent used for the measurement, and this was used as
reference. The elemental analyses were carried out by M-H-W
Laboratories, Phoenix, AZ, and by Atlantic Microlab, Norcross, GA.
Compounds Ph3PAuCl,26 CpMoH(CO)2(PMe3),27 CpMoH(CO)2(PPh3),28
and CpMo(CO)3(AuPPh3)29 were prepared as previously published.
AgBF4 was purchased from Aldrich and used without further purifica-
tion. All NMR data for the new compounds are collected in Table 1.
Preparation of [CpMo(CO)2(PMe3)(AuPPh3)2]BF4, 1. To a solu-
tion of (PPh3)AuCl (178 mg; 0.378 mmol) in 3 mL of THF was added
a solution of AgBF4 (70.1 mg; 0.378 mmol) in 5 mL of THF. The
AgCl precipitate was filtered off through Celite. The colorless solution
was cooled to -40 °C and solid CpMoH(CO)2(PMe3) (53 mg; 0.18
mol) was added, resulting in an immediate change to bright yellow.
After 1 h, the solution was filtered, concentrated by evaporation under
reduced pressure to a final volume of ca. 3 mL, and stored at -20 °C.
A yellow microcrystalline precipitate was obtained (97 mg, 42% yield).
Anal. Calc for C46H44Au2BF4MoO2P3: C 42.55; H 3.42. Found: C
42.37, H 3.54. IR (CH3CN, cm-1): 1855 s, 1792 s. IR (THF, cm-1):
1851 s, 1788 s. IR (Nujol mull, cm-1): 1850 br, 1757 br. Recrys-
(20) Alexander, B. D.; Mueting, A. M.; Pignolet, L. H. Inorg. Chem. 1990,
29, 1313-1319.
(21) Bianchini, C.; Meli, A.; Peruzzini, M.; Vacca, A.; Vizza, F.; Albinati,
A. Inorg. Chem. 1992, 31, 3841-3850.
(22) Bianchini, C.; Elsevier, C. J.; Ernsting, J. M.; Peruzzini, M.; Zanobini,
F. Inorg. Chem. 1995, 34, 84-92.
(23) Mueting, A. M.; Bos, W.; Alexander, B. D.; Boyle, P. D.; Casalnuovo,
J. A.; Balaban, S.; Ito, L. N.; Johnson, S. M.; Pignolet, L. H. New J.
Chem. 1988, 12, 505, and references therein.
(24) Mingos, D. M. P.; Watson, M. J. In AdVances in Inorganic Chemistry;
Sykes, A. G., Ed.; Academic Press: San Diego, CA, 1992; Vol. 39;
pp 327-399 and references therein.
(25) Quadrelli, E. A.; Kraatz, H.-B.; Poli, R. Inorg. Chem. 1996, 35, 5154-
5162.
(26) Bruce, M. I.; Nicholson, B. K.; Shawkataly, O. B. Inorg. Synth. 1989,
26, 324-328.
(27) Kalck, P.; Pince, R.; Poilblanc, R.; Roussel, J. J. Organomet. Chem.
1970, 24, 445-452.
(28) Bainbridge, A.; Craig, P. J.; Green, M. J. Chem. Soc. A 1968, 2715.
(29) Haines, R. J.; Nyholm, R. S.; Stiddard, M. H. B. J. Chem. Soc. A
1968, 46-47.

[Cp(CO)2(PMe3)Mo(AuPPh3)2]+[BF4]-
Inorganic Chemistry, Vol. 36, No. 14, 1997 3003
tallization from CH2Cl2/hepthane yielded yellow needles of 1 suitable
for X-ray analysis.
The mixture was filtered through Celite, and the filter cake was washed
with an additional 3 mL of THF. In another Schlenk tube was prepared
a solution of CpMoH(CO)2(PMe3) (32 mg, 0.108 mmol) and 2,6-lutidine
(12.6 µL, 0.108 mmol) in THF (2 mL). The AuPPh3 solution was
transferred to the solution of the Mo hydride and lutidine, resulting in
Preparation of [CpMo(CO)2(PPh3)(AuPPh3)2]BF4, 2. The syn-
thesis was carried out by a procedure identical to that described above
for 1, starting from CpMoH(CO)2(PPh3) (155 mg, 0.304 mmol) and 2
+
+
a color change to canary-yellow. After being stirred at room temper-
equiv of a AuPPh3 solution in THF, to yield compound 2 in 58%
1
ature for /2 h, the solution was evaporated to dryness under reduced
yield. Anal. Calc for C61H50Au2BF4MoO2P3: C 49.35, H 3.39.
Found: C, 49.15; H, 3.80. IR (THF, cm-1): 1868 s, 1807 s. The
CD3CN solution shows evidence for decomposition (formation of a
dark precipitate) after recording the NMR spectrum.
pressure to leave an oily residue which was washed with heptane (4 ×
5 mL) and dried. This material was investigated by 1H- and 31P-NMR
in CD3CN. The major resonances in the 1H- and 31P-NMR spectra are
due to compound 5. Strong resonances of the starting compound
CpMoH(CO)2(PMe3) are also visible (in a ca. 2:3 ratio relative to the
MoAu compound), as well as two resonances of the 2,6-lutidinium
ion at δ 8.14 (t, 1H, p-H, JHH ) 7.9 Hz) and 2.69 (s, 6H, Me). The
2,6-lutidinium resonance for the m-H protons, which should be observed
at δ 7.59 (d, JHH ) 8 Hz; from a genuine sample), is masked by the
stronger phenyl resonances. The isolation of the CpMo(CO)2(PMe3)-
(AuPPh3) compound in a pure crystalline state could not be achieved.
Reaction of CpMoH(CO)2(PMe3) with 3 equiv of [AuPPh3]BF4.
Preparation of [CpMo(CO)2(PMe3)(AuPPh3)3](BF4)2, 3. A colorless
solution of [AuPPh3]BF4 obtained from (PPh3)AuCl (207 mg, 0.418
mmol) and AgBF4 (81.4 mg, 0.418 mmol) in 15 mL of THF was filtered
through Celite, cooled to -40 °C, and added dropwise to an equally
cooled solution of CpMoH(CO)2(PMe3) (41 mg, 0.14 mmol) in 2 mL
of THF. All subsequent operations until isolation of the final product
were carried out at -40 °C. A lemon yellow solid formed initially
but redissolved upon further stirring for 1 h. After filtration, the solution
was evaporated under reduced pressure to a final volume of ca. 2 mL.
Addition of heptane (25 mL) yielded the product as a pale yellow
precipitate. The suspension was chilled to -80 °C for 2 h and filtered.
The solid was washed with heptane (2 × 5 mL) and dried in Vacuo.
Yield: 219 mg, 89%. The room-temperature 31P-NMR in CD3CN
showed the expected resonances. However, they were broader relative
to the low-temperature spectrum in acetone-d6, and decomposition
occurred upon prolonged data acquisition. The isolated solid was not
stable at room temperature under N2 for extensive periods of time,
changing color to a paler yellow and loosing crystallinity. A satisfactory
elemental analysis could not be obtained.
Reaction of CpMo(CO)3(AuPPh3) with [AuPPh3]BF4. Formation
of Complexes [CpMo(CO)3(AuPPh3)2]BF4 (6) and [CpMo(CO)3-
(AuPPh3)3](BF4)2 (7). A solution of [AuPPh3]BF4 was prepared in
situ as described above from (PPh3)AuCl (28 mg, 0.057 mmol) and
AgBF4 (11 mg, 0.057 mmol) in THF (7 mL). After filtration through
Celite, the colorless solution was added dropwise to a solution of CpMo-
(CO)3(AuPPh3) (40 mg, 0.057 mmol) in 1 mL of THF. The solution
changed color from orange-red to a paler orange while a gray solid
dropped out of solution. After being stirred at room temperature for 1
h and filtration, the solution was evaporated to dryness under reduced
pressure and the residue was investigated by NMR in acetone-d6. The
1H-NMR shows the phenyl signals in the δ 7.67-7.42 range and four
Cp resonances at δ 5.98, 5.87, 5.57, and 5.42 (starting compound).
The 31P-NMR shows resonances at δ 55.5 (starting compound), a
resonance at δ 50.5 (assigned to complex 6), two additional resonances
in a 2:1 ratio at δ 53.7 and 53.2 (assigned to complex 7), and a
resonance at δ 39.2 (AuPPh3+). These assignments are justified in the
Results section.
Interaction between 1 and AuPPh3+. A solution of [AuPPh3]BF4
was prepared in situ as described above from (PPh3)AuCl (5 mg, 0.01
mmol) and AgBF4 (2 mg, 0.01 mmol) in THF (3 mL). After filtration,
the colorless solution was chilled to -40 °C and compound 1 (13 mg,
0.01 mmol) was added. Compound 1 dissolved immediately to yield
1
a lemon-yellow solution. After being stirred at -40 °C for /2 h, the
solution was evaporated to dryness under reduced pressure. The residue
X-ray Analysis of Compound 1. A canary yellow crystal with
dimensions 0.50 × 0.20 × 0.075 mm was placed and optically centered
on the Enraf-Nonius CAD-4 diffractometer. The crystal final cell
parameters and orientation matrix were determined from 25 reflections
in the range 17.6 < θ < 19.3° and confirmed with axial photographs.
Data were collected with ω scans over the range 2.46 < θ < 24.96° in
the (h,-k,l octants, resulting in the measurement of 9060 reflections,
of which 7835 unique [R(int) ) 0.0674]. The data were corrected for
Lorenz and polarization factors and for absorption on the basis of eight
was investigated by 1H-NMR in CD3CN, revealing a mixture of 1 and
3 in a 1.25:1 ratio. The 31P-NMR spectrum also revealed the presence
+
of 1 and 3 and of unreacted AuPPh3
.
+
Reaction of CpMoH(CO)2(PMe3) with 1 equiv of AuPPh3 in
THF at Low Temperature and Subsequent Treatment with 2,6-
Lutidine. Formation of [CpMo(CO)2(PMe3)(µ-H)(AuPPh3)]BF4, 4,
and CpMo(CO)2(PMe3)(AuPPh3), 5. A solution of [AuPPh3]BF4 was
prepared in situ as described above from (PPh3)AuCl (42 mg, 0.085
mmol) and AgBF4 (16.5 mg, 0.085 mmol) in THF (6 mL). After
filtration, the colorless solution was cooled to -60 °C and CpMoH-
(CO)2(PMe3) (25 mg, 0.085 mmol) was added. A clear yellow solution
formed immediately. After being stirred at -60 °C for 1/2 h, the solution
was evaporated to dryness under reduced pressure and the oily residue
was washed with precooled (-60 °C) heptane (5 mL). This treatment
resulted in the crystallization of the residue, which was then investigated
ψ-scan reflections (transmission factors range 0.1433-0.2790).30
A
decay correction was not necessary. All crystallographic calculations
were performed on a personal computer with the SHELXTL package
of programs. The space group was uniquely determined as P21/n by
the systematic absences. Direct methods located the heavy atoms (Mo,
2 Au, 3 P) and the BF4 group. The structure was refined and completed
by alternating full-matrix least-squares cycles and difference-Fourier
maps. Hydrogen atoms were placed in calculated positions and used
for structure factor calculations but not refined; these positions were
constantly updated. All of the non-hydrogen atoms were refined
anisotropically, and the structure was refined to convergence (∆/σ e
0.002). A final difference-Fourier map possessed many peaks as large
as |∆F| e 2.50 e Å-3 near the heavy atoms, namely the Mo and Au
atoms. The remainder of the map was essentially featureless with |∆F|
e 0.86 e Å-3 indicating that the structure is both correct and complete.
1
by H-NMR after dissolution in precooled CD3CN (0.5 mL). After
dissolution, the solution was transferred into a thin-walled 5 mm NMR
tube which was frozen in a dry ice/acetone bath and thawed to room
1
temperature only prior to introduction into the NMR probe. The H-
NMR showed the presence of compound 4 as the major product; minor
resonances for compound 1 are also present. Following the recording
of this spectrum, an excess amount (ca. 30 µL) of 2,6-lutidine was
added to the NMR tube and the 1H-NMR spectrum was recorded again,
showing disappearance of the chemical shifts of 4 and appearance of
new chemical shifts due to compounds 5, CpMoH(CO)2(PMe3) [1H-
NMR: 5.21 (s, Cp), 1.48 (d, PMe3, JPH ) 10 Hz), and -6.24 (d, Mo-
H, JPH ) 60.5 Hz). 31P-NMR: 21.9 (s); these values correspond to
those reported in the literature],27 and AuPPh3(2,6-lutidine)+ [31P-NMR:
29.6]. From the 31P-NMR, the approximate relative ratio of these
2
The function minimized during the refinement was ∑w(Fo - Fc2),
2
2
where w ) 1/[σ2(Fo ) + (0.0857P)2 + 9.43P] and P ) (max(Fo ,0) +
2Fc2)/3. An empirical correction for extinction was also applied to
the data in the form (Fc2,corr) ) k[1 + 0.001xFc2λ3/sin(2θ)]-1/4 where
k ) 0.082 72 is the overall scale factor. The value determined for x
was 0.00043(9). The relevant crystal and refinement parameters are
collected in Table 2, and selected bond distances and angles are listed
in Table 3.
+
compounds is 10:1:5. Mixing fresh solutions of AuPPh3 and 2,6-
lutidine in CD3CN gave a resonance at δ 31.1.
Reaction of CpMoH(CO)2(PMe3) with AuPPh3+ and 2,6-Lutidine
in a 1:1:1 Ratio. Formation of Compound 5. A solution of [AuPPh3]-
BF4 was prepared in situ as described above from (PPh3)AuCl (53.8
mg, 0.108 mmol) and AgBF4 (21.2 mg, 0.108 mmol) in THF (3 mL).
(30) North, A. C. T.; Phillips, D. C.; Mathews, F. S. Acta Crystallogr.,
Sect. A 1968, A24, 351-359.

3004 Inorganic Chemistry, Vol. 36, No. 14, 1997
Table 2. Crystal Data for Compound 1
Galassi et al.
indicated by the stoichiometry of the reaction, by the analytical
data, and by NMR spectroscopy. In particular, two 31P-NMR
resonances in a 2:1 doublet:triplet ratio are observed and
attributed to the two AuPPh3 and the Mo(PR3) units, respec-
tively. Coupling between the two types of phosphorus nuclei
is weak, as expected from the three bond separation, but easily
observable (JPP is ca. 11 Hz for 1 and 9 Hz for 2). The IR
spectra exhibit two absorptions that are red shifted by ca. 60-
80 cm-1 relative to those of the corresponding starting material.
This shows that the two Au(PPh3)+ moieties are withdrawing
less electron density relative to a single H+ or, taking a different
point of view, that the [AuPPh3]2 unit donates more electron
density than the H- ligand. In addition, the structure of
compound 1 has been confirmed by a single-crystal X-ray
investigation (Vide infra).
formula
fw
space group
a, Å
b, Å
c, Å
C46H44Au2BF4MoO2P3
1298.41
P21/n
18.1457(13)
9.7811(7)
26.096(2)
105.086(5)
4472.0(5)
4
â, deg
V, Å3
Z
d
calc, Mg/m3
1.928
6.982
µ(Mo KR), mm-1
radiation (monochromated
in incident beam)
Mo KR (λ ) 0.710 73 Å)
temp, °C
153(2)
0.2790, 0.1433
R1 ) 0.0520, wR2 ) 0.1293
R1 ) 0.0650, wR2 ) 0.1364
transmissn factors: max, min
final R indices [I > 2σ(I)]a
R indices (all data)a
The PMe3 system allowed the synthesis of a trigold derivative,
[CpMo(CO)2(PMe3)(AuPPh3)3](BF4)2, 3. This product formed
selectively in high yields by reacting complex CpMoH(CO)2-
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. wR2 ) [∑w(|Fo| - |Fc|)2/∑w|Fo| ]1/2
Table 3. Selected Bond Lengths (Å) and Angles (deg) for 1
.
2
+
(PMe3) with 3 equiv of AuPPh3 at low temperatures; see eq
Au(1)-Au(2)
Au(1)-Mo(1)
Au(1)-P(1)
Au(2)-Mo(1)
Au(2)-P(2)
Mo(1)-P(3)
Mo(1)-CNT
2.7384(5)
2.7859(8)
2.284(2)
2.7791(8)
2.296(2)
2.457(3)
2.018(10)
Mo(1)-C(6)
Mo(1)-C(7)
C(6)-O(6)
1.989(9)
1.985(10)
1.151(11)
1.162(12)
2.414(9)
2.430(9)
2. Under the same conditions, the interaction of 1 with an
CpMo(CO)2(PMe3)H + 3AuPPh3+ f
C(7)-O(7)
Au(1)‚‚‚C(6)
Au(2)‚‚‚C(7)
[CpMo(CO)2(PMe3)(AuPPh3)3]2+ + H+ (2)
[CpMo(CO)2(PMe3)(AuPPh3)3]2+
a
Au(2)-Au(1)-Mo(1)
Au(2)-Au(1)-P(1)
Mo(1)-Au(1)-P(1)
Au(1)-Au(2)-Mo(1)
Au(1)-Au(2)-P(2)
Mo(1)-Au(2)-P(2)
Au(1)-Mo(1)-Au(2)
Au(1)-Mo(1)-P(3)
Au(1)-Mo(1)-C(6)
Au(1)-Mo(1)-C(7)
60.40(2) Au(2)-Mo(1)-C(6)
131.36(6) Au(2)-Mo(1)-C(7)
167.74(6) Au(2)-Mo(1)-CNT 110.6(3)
96.1(3)
58.5(3)
+
[CpMo(CO)2(PMe3)(AuPPh3)2]+ + AuPPh3 (3)
60.65(2) P(3)-Mo(1)-C(6)
124.46(6) P(3)-Mo(1)-C(7)
174.85(6) P(3)-Mo(1)-CNT
58.95(2) C(6)-Mo(1)-C(7)
133.52(7) C(6)-Mo(1)-CNT
75.7(3)
75.1(3)
+
additional 1 equiv of AuPPh3 also results in the formation of
109.6(3)
101.3(4)
133.6(4)
124.9(4)
169.8(8)
168.0(8)
3, but only an apparent equilibrium between 3 and unreacted 1
+
and AuPPh3 is reached (1/3 ca. 1.25:1). Compound 3 is not
stable either in solution or in the solid state. For this reason, a
satisfactory elemental analysis could not be obtained on this
compound. Nevertheless, the NMR investigation of the freshly
isolated material confirms its purity. Upon standing in solution,
57.9(3)
109.7(3)
C(7)-Mo(1)-CNT
Mo(1)-C(6)-O(6)
Mo(1)-C(7)-O(7)
Au(1)-Mo(1)-CNT 104.4(3)
Au(2)-Mo(1)-P(3)
130.55(7)
+
slow formation of 1 and AuPPh3 is observed by NMR,
Results
confirming the existence of equilibrium 3. Other decomposition
products, however, are also formed, including the known31
[Au8(PPh3)9]3+ (31P-NMR: δ 51 in CD3CN). This was obtained
as a precipitate from crystallization attempts of compound 3
and was identified by comparison with the 31P-NMR chemical
shift reported in the literature32 and by its characteristic green
color.
(a) Synthesis and NMR Characterization. The reaction
of CpMoH(CO)2(PR3) with [AuPPh3]BF4 in a 1:2 ratio in THF
solution at -40 °C yields compounds [CpMo(CO)2(PR3)-
(AuPPh3)2]BF4 (R ) Me (1), Ph (2)), see eq 1. The isolated
CpMoH(CO)2(PR3) + 2[AuPPh3]BF4 f
The main characteristic of the NMR spectrum of 3 is the
inequivalence of the AuPPh3 units (two resonances in a 2:1 ratio
are observed). This indicates Cs symmetry (either static or
dynamic) for the molecule and excludes a dynamic C3V
symmetry. Both types of symmetry are documented for MAu3
clusters.14 The PMe3 signal shows coupling to only two PPh3
units, but the two PPh3 resonances are broad and their coupling
pattern is not clearly discernible. This is not unusual, the same
phenomenon being observed in other cases, for instance [Au5-
[CpMo(CO)2(PR3)(AuPPh3)2]BF4 + HBF4 (1)
yields are only 42% (R ) Me) and 58% (R ) Ph), but an NMR
investigation indicates that substantial amounts of the products
are still present in solution and that no byproducts are formed
in the reactions. When the reactions are carried out with a 1:1
Mo/Au ratio, the same products are formed and 50% of the
Mo starting materials remain unreacted as shown by NMR. The
THF-solvated protons that form during the reaction are not
sufficiently acidic to protonate the residual hydride compound.
It has previously been shown25 that compounds CpMoH(CO)2-
(PR3) (R ) Me, Ph) are less basic than water and that their
protonation in dry, low-basicity solvents results in the evolution
of H2. No gas evolution, on the other hand, was observed for
reaction 1. IR and NMR monitoring of the reactions indicates
the replacement of the starting material with the final product
without the buildup of new resonances that could be attributed
to intermediates.
ReH4(PPh3)7]2+ 12
.
In view of the various mechanistic possibilities for the
formation of 1 and 2 which will be examined in the Discussion
section, additional experiments were carried out. When the
+
reaction between CpMoH(CO)2(PMe3) and AuPPh3 was car-
ried out in a 1:1 ratio in THF at -70°C, the formation of an
intermediate hydride-containing species was observed. Attempts
to isolate this compound were unsuccessful, as the complex
decomposes upon warming the THF solution. The formation
The products are soluble in most organic solvents and the
solutions are stable for brief periods of time. Slow decomposi-
tion, however, takes place, even at -80 °C, to yield unidentified
materials. The nature of the products as MoAu2 clusters is
(31) Cariati, F.; Naldini, L. J. Chem. Soc., Dalton Trans. 1972, 2286-
2287.
(32) Bruce, D. A.; Mueting, A. M.; Pignolet, L. H. Inorg. Chem. 1990,
29, 1313-1319.

[Cp(CO)2(PMe3)Mo(AuPPh3)2]+[BF4]-
Inorganic Chemistry, Vol. 36, No. 14, 1997 3005
of compound 1 as a minor product was also observed. The
downfield shift of the hydride signal (δ -5.63) and the reduced
JPH of 24 Hz with respect to the starting compound point to the
formulation of this derivative as [Cp(CO)2(PMe3)Mo(µ-H)-
(AuPPh3)]BF4, 4; see eq 4. The hydride resonance is coupled
CpMoH(CO)2(PMe3) + AuPPh3+ f
[Cp(CO)2(PMe3)Mo(µ-H)(AuPPh3)]+ (4)
to only one P nucleus. We suspect that this coupling is due to
the P nucleus of the PMe3 ligand. The lack of coupling with
the P nucleus of AuPPh3 could be due to fast chemical exchange
with free AuPPh3+. The 31P-NMR spectrum shows the PMe3
resonance at δ 19.3 [cf. 24.5 for CpMoH(CO)2(PMe3)] and
the PPh3 resonance at 50.1, the latter overlapping with the PPh3
resonance of compound 1.
When the reaction between CpMo(CO)2(PMe3)H and AuP-
Ph3+ in THF was carried out in a 1:1 ratio at room temperature
in the presence of the stoichiometric amount of 2,6-lutidine, a
single new compound was formed instead of the 1:1 mixture
of 1 and unreacted hydride complex (Vide supra). This product
is characterized by two 31P-NMR resonances in a 1:1 ratio at δ
47.7 and 21.5 and does not show any hydride resonance in the
1H-NMR, indicating the formation of CpMo(CO)2(PMe3)-
(AuPPh3), 5; see eq 5. The in situ generation of 4 followed by
the addition of lutidine also leads to the formation of 5.
Figure 1. Top view of the [CpMo(CO)2(PMe3)(AuPPh3)2]+ cation of
compound 1 with the numbering scheme employed. Hydrogen atoms
are omitted for clarity, and ellipsoids are drawn at the 40% probability
level.
+
CpMo(CO)2(PMe3)H + AuPPh3
+
NC5H3-2,6-Me2 f CpMo(CO)2(PMe3)(AuPPh3) +
[HNC5H3-2,6-Me2]+ (5)
The interaction between compound 1 and HBF4‚Et2O in
MeCN at room temperature or in acetone-d6 at -60 °C leads
to partial decomposition and formation of large amounts of
compound 3. Finally, the previously reported29 CpMo(CO)3-
(AuPPh3) compound was treated with 1 equiv of AuPPh3+. A
1
combination of H- and 31P-NMR monitorings indicated the
formation of both compounds [CpMo(CO)3(AuPPh3)2]BF4, 6,
and [CpMo(CO)3(AuPPh3)3](BF4)2, 7, while some [AuPPh3]+
remained unreacted (eq 6). Compound 6 is characterized by a
Figure 2. Side view of the cation of compound 1. Drawing parameters
are as in Figure 1.
2.728-3.028 Å having been found previously.24 Each Au atom
is further bonded to one terminal PPh3 ligand, while the Mo
atom is further bonded to a Cp ring, two CO ligands, and a
PMe3 ligand. An alternative way to view the structure, which
can better be appreciated from the top view in Figure 2, is to
consider the (AuPPh3)2 grouping as a ligand for the Mo(II)
center in a trans four-legged piano stool geometry which would
then be related to an hypothetical (η2-H2) complex. The “η2-
(AuPPh3)2” ligand adopts a parallel stereochemistry relative to
the plane of the Cp ring. When viewed in this light, the
distortions of the four “legs” follow the typical pattern,34 with
the two CO ligands being bent away from the Cp ligand to a
greater extent [average CNT-Mo-CO angle ) 129(5)°] than
the PMe3 and “η2-(AuPPh3)2” ligands [the bending of the “η2-
(AuPPh3)2” ligand is measured at the center point X of the Au-
Au bond as CNT-Mo-X ) 110.1(3)°].
CpMo(CO)3(AuPPh3) + AuPPh3+ f
[CpMo(CO)3(AuPPh3)2]+ + [CpMo(CO)3(AuPPh3)3]2+ (6)
single 31P-NMR resonance at δ 50.5 (cf. 49.5 for 1 and 47.6
for 2), while compound 7 is characterized by two resonances
in a 2:1 ratio at δ 53.7 and 53.2 (cf. 54.7 and 53.7 for compound
3). By comparison, the 31P-NMR resonance of free [AuPPh3]+
is at δ 39.2, and that of the starting complex CpMo(CO)3-
(AuPPh3) is at δ 55.5 in the same solvent (acetone-d6).
(b) X-ray Characterization. The chemical nature of com-
pound 1 is verified by an X-ray crystallographic study, which
also establishes the stereochemistry around the Mo center. A
side view of the cation is shown in Figure 1, and selected bond
distances and angles are in Table 3. The ion contains a MoAu2
triangle, with Mo-Au distances averaging 2.76(2) Å and an
Au-Au distance of 2.7384(5) Å. The Mo-Au distances are
longer than that reported for (η5-C5H4CHO)Mo(CO)3(AuPPh3)
[i.e. 2.7121(5) Å],33 as expected from the electron deficiency
of the MoAu2 unit in 1. The Au-Au distance is typical of
clusters containing a 3c-2e MAu2 unit, values in the range
Discussion
+
The reaction of AuPPh3 with compounds CpMoH(CO)2L
(L ) PMe3 or PPh3) follows a very different path relative to
the reactions of the isolobal electrophilic reagents H+ and Ph3C+.
While the H+ and Ph3C+ electrophiles lead to the formal
(33) Strunin, B. N.; Grandberg, K. I.; Andrianov, V. G.; Setkina, V. N.;
Perevalova, E. G.; Struchkov, Y. T.; N., K. D. Dokl. Akad. Nauk SSSR
1985, 281, 599-602.
(34) Poli, R. Organometallics 1990, 9, 1892-1900.

3006 Inorganic Chemistry, Vol. 36, No. 14, 1997
Galassi et al.
abstraction of the hydride ligand as H-, the interaction with
AuPPh3 leads to the ultimate departure of the hydride ligand
Scheme 2
+
as H+ as previously established for other transition metal hydride
complexes.23 Specific examples are the formation of CpM(CO)3-
(AuPPh3) from CpMH(CO)3 (M ) Cr, Mo, W),29 [Pt(AuPPh3)2-
Cl(PEt3)2]+ from trans-PtHCl(PEt3)2,35 and Os4(µ-H)2(AuPR3)2-
(CO)12 from Os4(µ-H)3(µ-AuPR3)(CO)12.36 In addition, the
+
reaction continues with further addition of the AuPPh3 elec-
trophilic fragments, building up digold- and even trigold-
molybdenum clusters. This behavior may be attributed to the
well-known aurophilicity of gold.37
The identification of compound 4 from the low-temperature
reaction (eq 4) is a clear indication that the first step of the
reaction leading to 1 and 2 is the addition of the electrophilic
+
AuPPh3 to the Mo-H bond. This also corresponds to the
accepted path of attack of other electrophilic reagents such as
H+ and Ph3C+ on the M-H bond, and in addition, many
analogous complexes containing a 3c-2e MHAu moiety are
sufficiently stable and have been isolated (see Introduction and
Scheme 1). However, compound 4 then undergoes exchange
+
of a proton with the isolobal AuPPh3 unit. This exchange
could in principle take place either via a dissociative mechanism
(i.e. deprotonation to form intermediate IV before addition of
the second AuPPh3+, see path a in Scheme 2) or via an
associative intermediate such as V (path b in Scheme 2).
Intermediate III corresponds to the cation of compound 4 when
L ) PMe3.
It is interesting to observe that IV is the stable observed
product of the reaction when L ) CO,29 whereas it is not
observed as an intermediate when L ) PMe3. However, such
system is indeed obtained (compound 5) when the reaction is
carried out in the presence of a strong base (eq 5). Since the
acidity of III should decrease in the order L ) CO < PMe3,
while the aurophilicity of III should correspondingly increase,
it seems most likely that the formation of 1 and 2 proceeds
according to path b of Scheme 2. A third possible pathway
would involve disproportionation of III to afford the product,
starting complex, and H+. This corresponds to the electrophilic
attack of III by another III and is therefore quite unlikely
relative to the proposed attack by AuPPh3+, because the latter
reagent is a stronger electrophile and is present at a higher
concentration.
Scheme 3
The formation of the trigold cluster 3 deserves greater
attention. The interaction between 1 and AuPPh3+ in a 1:1 ratio
forms the desired 3, but a substantial amount of starting materials
remain unreacted, indicating the presence of an equilibrium (eq
3). The direct interaction of CpMoH(CO)2(PMe3) and 3 equiv
of AuPPh3+, on the other hand, produces 3 in high yields without
the formation of significant amounts of 1. Once isolated,
+
compound 3 is rather unstable and dissociates AuPPh3 to
generate 1. These observations demonstrate that the formation
of 3 via the direct interaction of CpMoH(CO)2(PMe3) and 3
equiv of AuPPh3+ proceeds via a pathway that does not involve
the formation of 1. One possible rationalization that appears
attractive to us involves the interaction of the third AuPPh3
unit with an intermediate such as V (see Scheme 3). The
are present. For instance, the reductive elimination of R-H
from an alkyl-hydride complex is faster than that of R-R from
the corresponding dialkyl complex and the same is true for the
reverse oxidative addition process, these reactions involving the
formation of intermediates containing 3c-2e [MRX] (X ) H
or R) moieties.38-41 This rate differences are believed to be
+
hydrogen atom could kinetically facilitate the incorporation of
+
the third AuPPh3 group via an intermediate such as VI. No
detailed knowledge of stereochemistry for V, VI, and the final
trigold product is implied in the drawings of Scheme 3. 3c-
2e interactions are known to be established faster when H atoms
(38) Crabtree, R. H.; Dion, R. P.; Gibboni, D. J.; McGrath, D. V.; Holt, E.
M. J. Am. Chem. Soc. 1986, 108, 7222-7227.
(39) Periana, R. A.; Bergman, R. G. J. Am. Chem. Soc. 1986, 1986, 7346-
(35) Braunstein, P.; Lehner, H.; Matt, D.; Tiripicchio, A.; Tiripicchio-
Camellini, M. Angew. Chem., Int. Ed. Engl. 1984, 23, 304-305.
(36) Johnson, B. F. G.; Kaner, D. A.; Lewis, J.; Raithby, P. R.; Taylor, M.
J. Polyhedron 1982, 1, 105-107.
7355.
(40) Gozin, M.; Weisman, A.; Ben-David, Y.; Milstein, D. Nature 1993,
364, 699-701.
(41) Perthuisot, C.; Jones, W. D. J. Am. Chem. Soc. 1994, 116, 3647-
(37) Schmidbaur, H. Chem. Soc. ReV. 1995, 391-400.
3648.

[Cp(CO)2(PMe3)Mo(AuPPh3)2]+[BF4]-
Inorganic Chemistry, Vol. 36, No. 14, 1997 3007
related to a greater ease of H migration because of the spherical
symmetry of the 1s orbital used by H for bonding, whereas alkyl
groups must require rehybridization in the transition state.42,43
Similar considerations, therefore, may be applied when compar-
Scheme 4
+
ing H with AuPPh3. Other examples of additions of AuPPh3
to cationic hydride complexes, in many cases followed by
deprotonation, have been reported.23 To the best of our
knowledge, however, the system described here is the first one
leading, under kinetically controlled conditions, to a product
+
the same hydride substrates, i.e. CpMoH(CO)2L (L ) PMe3,
PPh3, or CO), is available. The observed pathways parallel those
previously reported for each reagent with other hydride sub-
strates. While the first step is presumably the same in all cases
with formation of a three-center-two-electron [MHE]+ moiety
(E ) electrophile), the harder electrophiles H+ and PPh3C+
prefer to remain bonded to the harder base H- (path a in Scheme
4),8,25 whereas the softer electrophile AuPPh3+ prefers to remain
bonded to the softer base [M]- (path b in Scheme 4). The nature
of the observed final product in this case is dictated by the
aurophilicity of the system. This study has also provided an
example of the ligand effect on the acidity of the 3c-2e
[MHAu] moiety: Spontaneous deprotonation occurs for
[Cp(CO)2LMo(µ-H)AuPPh3]+ when L ) CO, whereas the
complex with L ) PMe3 can only be deprotonated by strong
bases (e.g. 2,6-lutidine). Finally, evidence has been presented
for the kinetic role of a hydrogen atom in building transition
metal-gold clusters.
which is unstable toward elimination of AuPPh3
.
It is also interesting to observe the systematic increase in
stability upon replacement of a H atom with a AuPPh3 unit along
the series of compounds [Cp(CO)2LMoXY]+, where X and Y
are either H or AuPPh3 and the “MoXY” moiety establishes a
3c-2e bonding interaction. It was previously shown that the
[CpMo(CO)2(L)(η2-H2)]+ complexes (L ) PMe3 or PPh3), i.e.
the presumed first products of the interaction between CpMoH-
(CO)2L and H+, are not stable even at -80°C in noncoordinating
-
solvents (e.g. toluene or heptane), the BF4 counterion being
sufficiently basic to displace the H2 ligand from the coordination
sphere.25 On the other hand, complexes 1 and 2 are stable at
room temperature in the crystalline state, whereas compound 4
has an intermediate stability. There are many other examples
where a LnM-H compound is less stable than the isolobal LnM-
AuPPh3 analogue.24 Following the description of the system
with X ) Y ) H as a “dihydrogen” complex, compounds 1, 2,
and 4 could also be formally described as σ-complexes of the
hypothetical Ph3PAu-AuPPh3 and H-AuPPh3 ligands, respec-
tively.
Acknowledgment. Support for this work by the Department
of Energy (Grant No. DEFG059ER14230 to R.P.) is gratefully
acknowledged. R.G. thanks the Italian Ministery of Education
for a student fellowhip and the University of Camerino for a
leave.
Conclusion
For the first time, a comparison of the reactivity of the the
+
series of electrophilic reagents H+, Ph3C+, and AuPPh3 with
Supporting Information Available: An X-ray crystallographic file,
in CIF format, for compound 1 is available on the Internet only. Access
information is given on any current masthead page.
(42) Crabtree, R. H. Chem. ReV. 1985, 85, 245-269.
(43) Parkin, G.; Bercaw, J. E. Organometallics 1989, 8, 1172-1179.
IC961265A