First structure of a cyclopentadienyl trihydride d2 system: A pseudotrigonal prism rather than the expected pseudooctahedron and its mechanism of hydrogen scrambling

Full text of DOI:10.1021/ja953408i

4
906
J. Am. Chem. Soc. 1996, 118, 4906-4907
First Structure of a Cyclopentadienyl Trihydride d2
System: A Pseudotrigonal Prism Rather Than the
Expected Pseudooctahedron and Its Mechanism of
Hydrogen Scrambling
James C. Fettinger, Brett A. Pleune, and Rinaldo Poli*
Department of Chemistry and Biochemistry
UniVersity of Maryland, College Park, Maryland 20742
ReceiVed October 10, 1995
Half-sandwich cyclopentadienyl polyhydride derivatives of
the transition metals have recently been the subject of intensive
1
-8
9-11
experimental
and theoretical
investigations, especially
with regard to their high fluxionality and the choice between
classical and nonclassical formulations and the consequent
effects on physical properties and reactivity. The particular
Figure 1. A view of the Cp*MoH
hydrogen atoms are omitted for clarity. Selected bonding parameters
CNT ) Cp* center of gravity): Mo-CNT, 2.006(5) Å; Mo-P (av),
.368(12) Å; Mo-H1, 1.65(5) Å; Mo-H2, 1.57(5) Å; Mo-H3, 1.55-
3
(dppe) molecule. Carbon-bonded
(
ring)MH3L2 class of derivatives (ring ) Cp or substituted
derivative; M ) Mo or W; L ) 2-electron neutral donor, e.g.
a tertiary phosphine ligand) has been known since 1979, when
Green reported the first synthesis of CpMoH3(dppe).1 Single-
crystal diffraction studies have never been reported, but corre-
sponding trihalides adopt a pseudooctahedral structure (when
considering the Cp ring as accupying a single coordination
position) which, depending on the nature of the L ligands, can
either be fac (I), e.g. CpMoCl3[P(OCH2)3CEt]2, cis-mer (II),
e.g. CpMoCl3(L-L) (L-L ) dppe or dmpe ), or trans-mer
(
(
2
(
2
5) Å; CNT-Mo-H1, 107(2)°; CNT-Mo-H2, 102(2)°; CNT-Mo-
H3, 114(2)°; CNT-Mo-P1, 140.3(1)°; CNT-Mo-P2, 132.6(1)°.
unreported Cp*Mo trihydride derivatives. We now report that,
in the solid state, the Cp*MoH3(dppe) compound adopts a novel
structure for the (ring)MH3L2 stoichiometry which is based on
the trigonal prism rather than the octahedron and the structure
of a protonation product, which suggests a likely path for the
fluxional process.
13
14
15
III), e.g. CpMoCl3(PMe2Ph)2.16 H-NMR studies have not
1
been helpful to elucidate the structure of these hydride com-
pounds: a single triplet resonance (due to 31P coupling) for the
i
Compound Cp*MoH3(dppe), 1, can be synthesized in 63%
yields from Cp*MoCl4 and LiAlH4 in THF in the presence of
three hydride protons is observed for (C5H4Pr )MoH3(PMe3)2
down to -90 °C.17 For neither of the geometries I-III where
1
9
dppe. Compound 1 has NMR properties similar to the other
previously reported trihydride derivatives, showing a single
hydride resonance (triplet, JPH ) 42 Hz) at δ -5.27 and a single
3
1
1
P{ H} resonance at δ 91.8, which do not decoalesce upon
cooling to -80 °C. Thus, hydride-scrambling processes are still
too rapid for this compound to afford structural information by
1
H-NMR spectroscopy.
The single-crystal X-ray structure of 120 is shown in Figure
1. The position of the three hydrides was directly located from
the difference Fourier synthesis and refined without contraints.
The geometry cannot be based on the pseudooctahedron (cf.
I-III). Rather, it can be idealized to a pseudotrigonal prism
with the center of the Cp* ligand and two hydride ligands
defining one triangular face and the dppe and the third hydride
ligand defining the opposite triangular face, as illustrated
schematically in IV. For a regular trigonal prism, identical
CNT-Mo-P angles should be observed. Such angles are quite
close at 132.6(1)° and 140.3(1)° indicating a small twist toward
X ) H nor any other conceivable geometry, would a single
hydride resonance be expected in the slow-exchange limit. Since
increasing the steric bulk has the effect of slowing down
1
fluxional processes (e.g. see the low-temperature H-NMR
18
properties of Cp*MoH(PMe3)3 ), we explored previously
(
1) Arliguie, T.; Border, C.; Chaudret, B.; Devillers, J.; Poilblanc, R.
Organometallics 1989, 8, 1308-1314.
2) Parkin, G.; Bercaw, J. E. J. Chem. Soc., Chem. Commun. 1989, 255-
57.
3) Herrmann, W. A.; Theiler, H. G.; Kiprof, P.; Tremmel, J.; Blom, R.
(
2
(
J. Organometal. Chem. 1990, 395, 69-84.
(
4) Jia, G.; Morris, R. H. J. Am. Chem. Soc. 1991, 113, 875-883.
5) Jia, G.; Lough, A. J.; Morris, R. H. Organometallics 1992, 11, 161-
(
(19) Cp*MoCl4 (734 mg, 1.97 mmol) and dppe (783 mg, 1.97 mmol) in
THF (60 mL) were stirred with LiAlH4 (820 mg, 19.7 mmol) at room
temperature for 12 h. After addition of MeOH (ca. 5 mL), evaporation to
dryness, and extraction with heptane, 782 mg of crude product (63% yield)
was recovered as a yellow-orange powder, which was recrystallized by
dissolving in heptane and cooling to -80 °C. Single crystals were obtained
from a saturated warm heptane solution upon cooling to room temperature.
Anal. calcd for C36H42MoP2: C, 68.35; H, 6.69. Found: C, 67.6; H, 7.0.
1
71.
(
6) Klooster, W. T.; Koetzle, T. F.; G., J.; Fong, T. P.; Morris, R. H.;
Albinati, A. J. Am. Chem. Soc. 1994, 116, 7677-7681.
7) Gross, C. L.; Wilson, S. R.; Girolami, G. S. J. Am. Chem. Soc. 1994,
16, 10294-10295.
(
1
(8) Le Husebo, T.; Jensen, C. M. Organometallics 1995, 14, 1087-1088.
(9) Lin, Z.; Hall, M. B. Organometallics 1992, 11, 3801-3804.
(10) Lin, Z.; Hall, M. B. Organometallics 1993, 12, 4046-4050.
(11) Lin, Z. Y.; Hall, M. B. Coord. Chem. ReV. 1994, 135, 845-879.
(12) Aviles, T.; Green, M. L. H.; R., D. A.; Romao, C. J. Chem. Soc.,
1
H-NMR (C6D6, δ): 7.8-7.0 (m, 20H, Ph), 1.85 (m, 4H, CH2), 1.83 (s,
15H, Cp*), -5.27 (t, JPH ) 42 Hz, 3H, MoH).
(20) Crystal data for 1: monoclinic, space group P21/c, a ) 10.5920(6)
Å, b ) 29.010(2) Å, c ) 11.3044(9) Å, â ) 114.402(5)°, V ) 3163.3(4)
Dalton Trans. 1979, 1367-1371.
13) Poli, R.; Kelland, M. A. J. Organometal. Chem. 1991, 419, 127-
36.
3
-3
(
Å , Z ) 4, Dx ) 1.328 g cm , λ(Mo KR) ) 0.71073 Å, µ(Mo KR) )
-1 2
1
0.539 mm , F(000) ) 1320, T ) 153 K, R(F) ) 0.0545, wR(F ) ) 0.1006
for 369 parameters and 4405 data with Fo > 4σ(Fo). The structure solution
by direct methods and refinement by full-matrix least-squares and different
Fourier syntheses were carried out with programs contained in the
SHELXTL package. At convergence for the Cp*Mo(dppe) model with all
non-hydrogen atoms anisotropic, a difference Fourier map showed the
position of the three Mo-bonded hydrogen atoms. These atoms were allowed
to refine freely (xyzU), converging to chemically reasonable positions. A
(
(
(
14) St a¨ rker, K.; Curtis, M. D. Inorg. Chem. 1985, 24, 3006-3010.
15) Owens, B. E.; Poli, R. Inorg. Chim. Acta 1991, 179, 229-237.
16) Abugideiri, F.; Gordon, J. C.; Poli, R.; Owens-Waltermire, B. E.;
Rheingold, A. L. Organometallics 1993, 12, 1575-1582.
(
17) Grebenik, P. D.; Green, M. L. H.; Izquierdo, A.; Mtetwa, V. S. B.;
Prout, K. J. Chem. Soc., Dalton Trans. 1987, 9-19.
18) Abugideiri, F.; Kelland, M. A.; Poli, R.; Rheingold, A. L. Orga-
nometallics 1992, 11, 1303-1311.
(
-
3
final difference Fourier map was featureless with |∆F| < 0.42 e Å
.
S0002-7863(95)03408-1 CCC: $12.00 © 1996 American Chemical Society

Communications to the Editor
J. Am. Chem. Soc., Vol. 118, No. 20, 1996 4907
an octahedral geometry, whereas the angles should be very
different from each other in a structure of type II, or much
smaller in a structure of type I. In addition, the Mo atom is
displaced significantly from the plane defined by CNT and the
two P atoms (by 0.283 Å), one hydride ligand being located on
one side of such plane and the other two hydride ligands being
located on the other side.
The average Mo-P distance of 2.368(12) Å is far shorter
than those in the isoelectronic CpMoCl3(dppe),14 which is of
type II (axial, 2.521(2) Å; equatorial, 2.688(4) Å). If these
distances are regulated mostly by steric interactions, the
replacement of three Cl with three H is more important than
the replacement of the Cp with Cp*. The less electronegative
H ligands could also have the effect of expanding the metal
orbitals and favoring stronger Mo-P covalent interactions. The
Mo-H distances average 1.59(5) Å, which compares with 1.52-
(dppe)]+ cation for
Figure 2. A view of the [Cp*MoH(MeCN)
2
compound 2. Carbon-bonded hydrogen atoms are omitted for clarity.
Selected bonding parameters (CNT ) Cp* center of gravity): Mo-
CNT, 1.998(7) Å; Mo-P1, 2.541(2) Å; Mo-P2, 2.554(2) Å; Mo-
H1, 1.69(6) Å; Mo-N51, 2.137(6) Å; Mo-N61, 2.122(6) Å; CNT-
Mo-H1, 100(2)°; CNT-Mo-N51, 103.1(2)°; CNT-Mo-N61,
104.8(2)°; CNT-Mo-P1, 159.5(2)°; CNT-Mo-P2, 121.6(2)°.
(
4) Å for (C5Me4Et)MoH5(PMe3).21 The distance between H2
Scheme 1
and H3 of 1.71 Å is much longer than the longest separation
reported for a “dihydrogen” ligand, e.g. 1.357(7) Å for ReH7-
22
[
P(p-tolyl)3]2, thus 1 can be considered as a classical trihydride
complex.
There are no previous examples of (ring)ML5 compounds
2
with a geometry based on the trigonal prism. For d systems,
the octahedral geometry should be electronically favored because
the six σ bonds formed by the s, p, and d (eg set) orbitals are
augmented by the two Mo-Cp π bonds that use the dxz and dyz
set, while the two metal electrons would reside in the dxy orbitals
which can engage in Mo-Cp δ back bonding. It is then likely
that the pseudotrigonal prismatic structure for 1 is enforced by
steric requirements.
distortion which compresses the P1 donor against the hydride
ligand (P1-Mo-H1 ) 59(2)°). The CNT-Mo-P angles are
in this case quite different from each other [104.8(2)° and 159.5-
(
2)°], and the Mo atoms lie essentially in the plane defined by
Protonation of 1 with HBF4‚OEt2 in MeCN, followed by
diffusion of ether, produces single crystals of [Cp*MoH-
the two P atoms and CNT (deviation 0.033 Å).
On the basis of these two structures, it is possible to propose
a mechanism of scrambling of the hydride positions in (ring)-
MoH3L2 complexes as illustrated in the Scheme 1, which can
be viewed as a special case of a Bailar twist. We shall next
seek solid state structural information of less sterically hindered
(
MeCN)2(dppe)][BF4]2 (2). The NMR properties of this com-
23
plex indicate a pseudooctahedral structure, which is confirmed
by an X-ray structural determination (see Figure 2).
2
4
In
particular, the structure is of type II, with the seric interaction
between the Cp* and equatorial PPh2 groups causing a severe
(ring)MoH3L2 compounds, to determine whether the pseudooc-
(
21) Shin, J. H.; Parkin, G. Polyhedron 1994, 13, 1489-1493.
tahedral or the pseudotrigonal prismatic structure is more favored
in general.
(
22) Brammer, L.; Howard, J. A. K.; Johnson, O.; Koetzle, T. F.; Spencer,
J. L.; Stringer, A. M. J. Chem. Soc., Chem. Commun. 1991, 241-243.
1
(
23) H-NMR (δ, CD3CN): 7.9-7.1 (m, 20H, Ph), 3.2-2.4 (m, 4H,
Acknowledgment. We gratefully acknowledge support of this work
by the DOE (Grant No. DEFG0592ER14230).
CH2), 1.82 (s, 15H, Cp*), -4.05 (dd, J ′HP ) 10 Hz; J ′′HP ) 84 Hz, 1H,
3
1
1
M-H). P{ H}-NMR (δ, CD3CN): 76.2 and 50.9 (both d, JPP ) 24 Hz).
(
24) Crystal data for 2: orthorhombic, space group Pna21, a ) 22.245-
3
(
2) Å, b ) 10.6805(9) Å, c ) 16.9237(11) Å, V ) 4020.9(6) Å , Z ) 4, Dx
Supporting Information Available: Tables of crystal data, atomic
coordinates, bond distances and angles, and anisotropic thermal
parameters (27 pages). Ordering information is given on any current
masthead page.
-3
-1
)
1.464 g cm , λ(Mo KR) ) 0.71073 Å, µ(Mo KR) ) 0.475 mm , F(000)
2
)
1816, T ) 153 K, R(F) ) 0.0338, wR(F ) ) 0.0735 for 347 parameters
and 2369 data with Fo > 4σ(Fo). Calculations were carried out as for
compound 1. The hydrogen atom bonded to molybdenum was located
directly from a difference Fourier map and refined freely. A final difference
-
3
Fourier map was featureless with |∆F| < 0.42 e Å
.
JA953408I