Synthesis and structure of the stable paramagnetic cyclopentadienyl polyhydride complexes [Cp*MH3(dppe)]+ (M = Mo, W): Stronger M-H bonds upon oxidation

Full text of DOI:10.1021/ja9729488

J. Am. Chem. Soc. 1998, 120, 3257-3258
Synthesis and Structure of the Stable Paramagnetic
3257
Cyclopentadienyl Polyhydride Complexes
[Cp*MH3(dppe)]+ (M ) Mo, W): Stronger M-H
Bonds upon Oxidation
Brett Pleune,1a Rinaldo Poli,*,1b and James C. Fettinger1a
Department of Chemistry and Biochemistry
UniVersity of Maryland
College Park, Maryland 20742
Laboratoire de Synthe`se et d'Electrosynthe`se
Organome´tallique, Faculte´ des Sciences “Gabriel”
UniVersite´ de Bourgogne
6 BouleVard Gabriel, 21100 Dijon, France
ReceiVed August 22, 1997
Transition-metal polyhydride derivatives have been at the focus
of much experimental2 and theoretical3 work, especially dealing
with their high fluxionality4 and their structure type (i.e. classical
vs nonclassical).5 There is scarce information on how these
properties change upon oxidation to the 17-electron configuration
because of the paucity of stable complexes. Only a few
monohydride complexes have have been isolated,6 as facile
decomposition via deprotonation or disproportionation pathways
usually occurs.7 Isolable paramagnetic polyhydrides are even less
Figure 1. EPR spectra of complexes [Cp*MX3(dppe)]+ (M ) Mo, W;
X ) H, D). Solvent ) THF. The starred peak in the spectrum of [2-d3]+
is due to an impurity.
common. We are only aware of TaCl2H2L4 (L ) PMe3 or L2 )
- 6h
dmpe)6g and [WCl2H2(PMe3)4]+BF4
,
as well characterized,
unambiguous examples.8 Chemical reactivity studies of such
species are also rare.9 When these compounds are obtained by
one-electron oxidation of neutral precursors, H2 reductive elimina-
tion is facilitated by the decreased metal p basicity10 and adds to
the array of decomposition pathways available.11
We wish to report here the new complexes [Cp*MH3(dppe)]-
[PF6] (M ) Mo, ([1]PF6), W ([2]PF6)), which are accessible by
1-electron oxidation of the parent complexes 1 and 2 with FcPF6.12
Cyclovoltammetric studies show a reversible oxidation for both
1 and 2 in THF (-0.75 and -0.88 V vs Fc/Fc+, respectively).
Complex [1]+ exhibits a triplet of quartets in the EPR spectrum
(g ) 1.989, aP ) 28.9 G, aH ) 11.8 G; see Figure 1) consistent
with coupling to three equivalent H and two equivalent P ligands.
Chemical oxidation of Cp*MoD3(dppe) leads to the formation
of [1]+-d3, which is characterized by an EPR broad triplet (g )
1.991, aP ) 28.9 G; see Figure 1). IR investigations13 show the
expected isotope shift upon deuteration, and a 10-20 cm-1 blue
shift upon oxidation. From simple theory, the vibrational
frequency correlates directly with the bond energy.14 Thus, the
IR data indicate that the M-H/D bonds are stronger in the
oxidized materials, consistent with expectations on the basis of a
Mδ+-Hδ- bond polarity. Previous IR studies on Cp*FeH(dppe)
(1) (a) University of Maryland. (b) Universite´ de Bourgogne.
(2) (a) Arliguie, T.; Border, C.; Chaudret, B.; Devillers, J.; Poilblanc, R.
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1997, 16, 1179-1185.
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K. G. Inorg. Chem. 1996, 35, 6775-6783.
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J. C.; Sattelberger, A. P. Inorg. Chem. 1984, 23, 1718-1726. (h) Sharp, P.
R.; Frank, K. G. Inorg. Chem. 1985, 24, 1808-1813. (i) Hamon, P.; Toupet,
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J. Am. Chem. Soc. 1990, 112, 2618-2626. (d) Smith, K.-T.; Rømming, C.;
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1995, 117, 9473-9480.
(12) To a solution of 2 (115 mg, 0.160 mmol) in 4 mL of CH2Cl2 was
added FcPF6 (53 mg, 0.160 mmol) at room temperature. The yellow-orange
solution immediately turned red-orange. The solution was filtered and
concentrated under reduced pressure to ca. 0.5 mL. Orange single crystals of
-
[2]+PF6 were obtained by diffusion of a layer of diethyl ether into this
solution. Yield: 97 mg (70%). [1]+ can be obtained from 1 in an analogous
manner in either THF or CH2Cl2. The oxidized complexes do not give rise to
any observable NMR resonances.
(13) IR bands (THF, cm-1, all broad): 1, 1815 (m, sh), 1775 (m); [1]+,
1896 (w, sh), 1824 (m); 1-d3, 1307 (m); [1]+-d3, 1318 (m); 2, 1885 (w), 1815
(m); [2]+, 1897 (m), 1830 (w). W-D bands could not be located for
compounds 2-d3 and [2]+-d3 (no significant changes were observed upon
oxidation). The spectra of [1]+ and [1]+-d3 were recorded immediately after
charging the cell with the cold solution. The follow-up decompositions (see
text) generated a new weaker band at 1818 cm-1 from [1]+, assigned to 3. A
corresponding band for 3-d could not be observed.
(14) Berry, R. S.; Rice, S. A.; Ross, J. Physical Chemistry; Wiley: New
York, 1980; pp 266-268.
S0002-7863(97)02948-X CCC: $15.00 © 1998 American Chemical Society
Published on Web 03/19/1998

3258 J. Am. Chem. Soc., Vol. 120, No. 13, 1998
Communications to the Editor
and WH2Cl(PMe3)4 indicated either no change or a slight red shift
upon oxidation,6h,i,15 and a lowering of M-H BDE’s was indicated
for lower-valent carbonyl complexes by thermodynamic cycles
involving acidity and electrochemical measurements.16
Despite the supposedly greater Mo-H bond energy relative
to the neutral precursor, complex [1]+ decomposes rapidly (t1/2
) 2 min at 25 °C in either THF or CH2Cl2) with gas evolution,
to afford a new signal consisting of a doublet of triplets (g )
1.950, aP ) 16.5 G, aH ) 24.0 G; solvent independent), consistent
with the formation of Cp*MoH(dppe)(PF6), 3. [1]+-d3 cor-
respondingly decomposes to yield a triplet of 1:1:1 triplets (aD
) 4.0 G). This indicates that [1]+ decomposes by reductive
elimination of H2. The similar decomposition rate in THF and
CH2Cl2 suggests that H2 elimination may occur without solvent
precoordination. Solvent-independent R-R reductive elimina-
tions from dialkyl complexes have previously been reported.17
No solvent is coordinated to solid 3,18 which is therefore
formulated as either the salt of a rather uncommon19 15-electron
cation or a 17-electron complex with a coordinated FPF5 ligand.
Solvent coordination cannot be excluded in solution. Additional
studies on this compound are in progress and will be reported
later.
Figure 2. A view of the cation in compound [2]+PF6-. Selected bond
distances (Å) and angles (deg):W1-P1, 2.474(2); W1-P2, 2.506(2);
W1-CNT, 1.999(15); W1-H1, 1.71(2); W1-H2, 1.67(3); W1-H3,
1.69(3); CNT-W1-P1, 162.0(2); CNT-W1-P2, 119.1(2); CNT-W1-
H1, 109(2); CNT-W1-H2; 103(2); CNT-W1-H3, 113(2); P1-W1-
P2, 78.85(5).
Compound [2]+ exhibits a broad triplet resonance in the EPR
spectrum (g ) 2.017; see Figure 1), which decays slowly at room
temperature in THF (t1/2 ) 3 h). The nature of the decomposition
products is under current investigation. Cooling to -80 °C does
not affect the line shape of the signal. Oxidation of Cp*WD3-
(dppe) leads to [2]+-d3, which better reveals the phosphorus
coupling (g ) 2.022, aP ) 27.6 G; see Figure 1). A single crystal
of [2]PF6 could be investigated by X-ray diffraction,20 which
permitted the location and refinement of the three hydride ligands
(see Figure 2).
The cation and anion are well separated, the closest contacts
being between F atoms and Cp* and dppe C atoms (>3.0 Å).
Neglecting the hydrogen positions, the geometry of [2]+ is very
close to that of [Cp*WH4(dppe)]+, previously determined as the
BF4- salt.21 The major difference consists of a small displacement
(0.025(8) Å, as opposed to 0.002(9) Å in the tetrahydrido
complex) of the W atom from the plane defined by CNT, P(1),
and P(2) toward the side of the molecule occupied by H2 and
H3 (see Figure 2). The W-P and W-Cp* bond distances are
also very similar for [2]+ and [Cp*WH4(dppe)]+,21a suggesting
that the replacement of a W-H bond with a singly occupied W
orbital does not greatly affect the effective metal charge (i.e. the
W-H bond is highly covalent). Although the H positions are
not determined with high precision, the relatively long H‚‚‚H
separations (H1-H2 2.81 Å; H2-H3 2.11 Å) suggest a classical
formulation for [2]+. The geometry of [2]+ is intermediate
between I (a pseudotrigonal prism) and II (a pseudooctahedron),
i.e., the ideal geometries adopted by the isoelectronic d2 complexes
1 and [Cp*MoH(dppe)(MeCN)2]2+, respectively.21a,22
(15) Roger, C.; Hamon, P.; Toupet, L.; Rabaaˆ, H.; Saillard, J.-Y.; Hamon,
J.-R.; Lapinte, C. Organometallics 1991, 10, 1045-1054.
(16) (a) Tilset, M. J. Am. Chem. Soc. 1992, 114, 2740-2741. (b) Skagestad,
V.; Tilset, M. J. Am. Chem. Soc. 1993, 115, 5077-5083.
(17) (a) Pedersen, A.; Tilset, M. Organometallics 1993, 12, 56-64. (b)
Fooladi, E.; Tilset, M. Inorg. Chem. 1997, 36, 6021-6027.
The equivalence of the P and H hyperfine couplings in the
EPR spectrum of [1]+ clearly indicates fluxionality. A scrambling
mechanism involving a pseudo-Bailar twist between I and II, as
previously proposed for the precursor complexes 1 and 2, could
also be adopted by [1]+ and [2]+.21a Our future work will attempt
to define the chemical reactivity of this new class of 17-electron
polyhydride complexes and their decomposition products.
(18) Anal. for 3. Calcd for C36H40F6MoP3: C, 55.8; H, 5.2. Found: C,
55.1; H, 5.3. IR (Nujol mull, cm-1): 1820 [br, w, ν(M-H)], 840 [s, ν(PF6)].
The IR spectra of 3 crystallized from THF and from MeCN showed no
difference attributable to coordinated solvent. µeff ) 1.50 µB.
(19) Poli, R. Chem. ReV. 1996, 96, 2135-2204.
(20) Crystal data: Mr ) 856.46, monoclinic, P21/n, a ) 14.3763(8) Å, b
) 16.684(2) Å, c ) 15.0123(12) Å, â ) 100.424(6)°, V ) 3541.4(5) Å3, Z
) 4, Dx ) 1.623 g cm-3, λ(Mo KR) ) 0.71073 Å, µ(Mo KR) ) 3.454 mm-1
,
F(000) ) 1724, T ) 153(2) K, R(F) ) 7.13%, R(wF2) ) 8.61% for all 6226
independent reflections [R(F) ) 4.12%, wR(F2) ) 7.67% for 4577 data with
Fo > 4σ(Fo)]. Structure solution by direct methods located the W and P atoms
and several F atoms. All other non-hydrogen atoms were located by alternating
least-squares refinement cycles and difference Fourier maps (SHELXTL) and
refined anisotropically. All hydrogen atoms except the three hydrides were
placed in calculated positions (AFIX). The three hydride atoms were located
from the highest peaks near the W atom with W-peak distances ranging from
1.4 to 1.7 Å. Their refinement was assisted by restraining the W-H distances
to be near 1.70 Å (ADFIX with esd of 0.03 Å). Thermal parameters were
allowed to refine freely.
Acknowledgment. We are grateful to the DOE Office of Energy
Research (Grant No. DEFG059ER14230) for support of this work and
to a reviewer for useful suggestions.
Supporting Information Available: Tables of crystal and refinement
parameters, fractional coordinates, bond distances and angles, and
anisotropic displacement parameters (10 pages, print/PDF). See any
current masthead page for ordering information and Web access
instructions.
(21) (a) Pleune, B.; Poli, R.; Fettinger, J. C. Organometallics 1997, 16,
-
1581-1594. (b) Crystals of [2]+PF6 are orange, while those of [Cp*WH4-
JA9729488
(dppe)]+BF4- are colorless. Colorless crystals of [Cp*WH4(dppe)]+PF6- have
a significantly different unit cell from that of [2]+PF6-: a ) 14.439(4) Å, b
) 16.575(4) Å, c ) 15.080(4) Å, â ) 100.29(2)°, V ) 3551.0(14) Å3.
(22) Fettinger, J. C.; Pleune, B.; Poli, R. J. Am. Chem. Soc. 1996, 118,
4906-4907.