Neutron diffraction study of the 'elongated' molecular dihydrogen complex [(C5Me5)Os(H2)H2(PPh 3)]+

Article abstract of DOI:10.1039/a704997h

Protonation of the osmium(IV) hydrides (C₅Me₅)OsH₃(L) with HBF₄ in diethyl ether afforded the cationic dihydrogen complexes [(C₅Me₅)Os(H₂)H₂(L)][BF ₄], where L is PPh₃ 1 or AsPh₃ 2; a single-crystal neutron diffraction study of 1 reveals that the H-H distance is 1.014(11) A.

Full text of DOI:10.1039/a704997h

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Neutron diffraction study of the ‘elongated’ molecular dihydrogen
complex [(C5Me5)Os(H2)H2(PPh3)]؉ †
Christopher L. Gross,a Dianna M. Young,b Arthur J. Schultzb and Gregory S. Girolami*,a
a
School of Chemical Sciences, The University of Illinois at Urbana-Champaign,
601 South Goodwin, Urbana, IL 61801, USA
b Intense Pulsed Neutron Source, Argonne National Laboratory, Argonne, IL 60439, USA
order to establish more definitively the presence of a molecular
Protonation of the osmium() hydrides (C5Me5)OsH3(L) with
HBF4 in diethyl ether afforded the cationic dihydrogen
complexes [(C5Me5)Os(H2)H2(L)][BF4], where L is PPh3 1 or
AsPh3 2; a single-crystal neutron diffraction study of 1 reveals
that the H᎐H distance is 1.014(11) Å.
dihydrogen ligand, time-of-flight neutron diffraction data were
collected from a 4 mm3 single crystal of 1.
The neutron diffraction data clearly reveal the presence of a
dihydrogen ligand trans to the phosphine (Fig. 1). The H᎐H
distance of 1.014(11) Å shows that the dihydrogen ligand is of
the ‘elongated’ variety. (The value of 1.014 Å is not corrected
for librational motion of the H2 ligand. Corrections of this type
lengthen the H᎐H bond by ca. 0.02 to 0.1 Å.7a,8a) In contrast to
the X-ray results,|| the dihedral angle between the Os᎐H2 and
P᎐Os᎐Cn planes refined to a chemically reasonable value of
8.7Њ. The angle between the mutually ‘trans’ terminal hydrides
[H(1)᎐Os᎐H(3)] of 132.6(5)Њ is considerably larger than the cor-
responding 119(2)Њ angle measured for the neutral complex
(C5Me5)OsH3(PPh3) by X-ray diffraction.12 This difference sig-
nals a change in the hybridization of the metal–ligand bonding
orbitals upon protonation of the metal center.
Since their discovery, molecular dihydrogen complexes have
generated considerable interest because they represent a mid-
way point along the reaction coordinate that leads to the oxid-
ative addition of dihydrogen by transition-metal centers.1-4 The
H᎐H bond distance is a measure of the extent to which the H2
ligand has been activated. Distances less than 0.85 Å are typical
for ‘normal’ dihydrogen complexes while distances longer than
1.50 Å are characteristic of classical dihydrides.5,6 A few di-
hydrogen complexes, however, have ‘elongated’ H᎐H bond dis-
tances of approximately 1.0 to 1.4 Å that constitute an inter-
mediate situation.7 Herein we report the synthesis of the first
compounds of general stoichiometry [(C5R5)MH4L]ϩ. Single-
crystal neutron diffraction 7,8 shows that, when L = PPh3, two of
the hydrogen atoms form an ‘elongated’ dihydrogen ligand.
Protonation of the previously reported osmium() trihydride
(C5Me5)OsH3(PPh3)9 with HBF4ؒEt2O in diethyl ether affords a
white precipitate of stoichiometry [(C5Me5)OsH4(PPh3)][BF4] 1
in 91% yield. As we will show below, this ‘tetrahydride’ com-
plex is best formulated as the dihydrogen–dihydride species
[(C5Me5)Os(H2)H2(PPh3)][BF4].‡ The analogous triphenyl-
arsine compound [(C5Me5)Os(H2)H2(AsPh3)][BF4] 2 can be
prepared similarly.§ Interestingly, attempts to prepare the
analogous ruthenium compound, [(C5Me5)RuH4(PPh3)]ϩ, have
been unsuccessful, evidently because this complex is unstable
toward loss of H2.10
1
The room-temperature H NMR spectrum of complex 1
features a doublet at δ Ϫ9.61 [JH P(ave) = 14.2 Hz] for the four
osmium-bound hydrogen atoms; thus, exchange of the classical
and non-classical hydrogen atom sites is fast on the NMR time-
scale at 25 ЊC. The room-temperature 1H NMR spectrum of 2 is
similar except that the Os᎐H resonance appears as a singlet at δ
Ϫ9.88. For both compounds, the hydride resonance remains
sharp down to approximately Ϫ100 ЊC, at which point it begins
to broaden. At Ϫ140 ЊC (in CDFCl2), the hydride resonance of
1 remains a broad singlet, but the resonance of 2 decoalesces to
two broad equal-intensity features separated by 1.0 ppm. The
activation free energy for the dihydrogen–hydride exchange
process in 2 is 6.0 kcal molϪ1.
Additional insight into the solution structure can be obtained
from the NMR spectra of partially deuteriated isotopologs.
Approximately 2.5 of the 4 hydrogen atoms are deuteriated
upon stirring a CH2Cl2 solution of 1 under 2 atm of D2 for 24 h.
An X-ray crystallographic study of complex 1 conducted at
198 K strongly suggested that it adopts a four-legged piano
stool geometry in which the four legs are described by the phos-
phine ligand, two classical hydride ligands, and a non-classical
dihydrogen ligand.¶ The phosphine is ‘trans’ to the dihydrogen
ligand and the H᎐H distance within the latter refined to 0.8(1)
Å. The metric parameters of the dihydrogen ligand deduced
from X-ray data are, however, likely to have large errors,|| and in
1
A H NMR spectrum of the partially deuteriated material at
25 ЊC shows a doublet [2JH P(ave) = 14.7 Hz] of multiplets; the
multiplet splitting gives JH D (ave) = 3.6 Hz. These coupling con-
¶ Crystal data for complex 1 (M = 678.53). X-Ray (198 K): monoclinic,
space group P21/c, a = 10.5516(4), b = 27.681(1), c = 9.4129(4) Å, β =
99.062(1)Њ, U = 2715.0(2), Z = 4, µ = 4.8 mmϪ1, RwF = 0.0863 for 336
2
parameters and all 6468 data. The non-hydrogen atoms were refined
anisotropically. A difference map revealed four possible hydrogen atom
positions. The locations of these hydrogen atoms were refined without
constraints except that a common isotropic displacement parameter
was assigned. The positions of the atoms converged slowly, especially
that for hydrogen atom H(4). Neutron (20 K): monoclinic, space group
P21/c, a = 10.4294(16), b = 27.562(4), c = 9.3218(13) Å, β = 99.690(10)Њ,
U = 2641(1), Z = 4, RF = 0.093, RwF = 0.058 for 324 parameters and
3648 data with I > 3σ(I). All atoms were refined isotropically except
the four osmium-bound hydrogen atoms, which were refined aniso-
tropically. CCDC reference number 186/672.
† Dedicated to the memory of Geoffrey Wilkinson, whose work will
long stand as a landmark and an inspiration.
Supplementary data available (No. SUP 57272, 3 pp.): a plot of TϪ1/K
vs. ln(T1) for [(C5Me5)Os(H2)H2(PPh3)][BF4]. See instructions for
Authors, 1997.
Non-SI units employed: cal ≈ 4.18 J, atm = 101 325 Pa.
‡ Complex 1 (Found: C, 49.6; H, 5.05; P, 4.27. Calc: C, 49.6; H, 5.05; P,
4.56%). IR (Nujol, cmϪ1): 2108m (νOs᎐H ), 2063m (νOs᎐H ). 1H NMR
(CD2Cl2, 22 ЊC): δ 7.5 (m, o᎐CH and m᎐CH), 7.3 (m, p᎐CH), 2.06
(d, JPH = 1.4 Hz, C5Me5), Ϫ9.61 (d, JPH = 14.2 Hz, Os᎐H).
§ Complex 2 (Found: C, 46.7; H, 4.82; As, 10.2. Calc: C, 46.6; H, 4.74;
|| For example, the dihedral angle described by the Os᎐H2 plane and the
P᎐Os᎐Cn plane (Cn = centroid of the C5Me5 ring) obtained from the
X-ray data set was unexpected: it was not close to either 0 or 90Њ, but
inbetween (60.6Њ). Dihydrogen ligands with unusual dihedral angles
are known, however.8c
1
As, 10.4%). IR (Nujol, cmϪ1): 2088w (νOs᎐H ), 2029w (νOs᎐H ). H NMR
(CD2Cl2 22 ЊC): δ 7.5 (m, o᎐CH and m᎐CH), 7.4 (m, p᎐CH), 2.13
(s, C5Me5), Ϫ9.88 (s, Os᎐H).
J. Chem. Soc., Dalton Trans., 1997, Pages 3081–3082
3081

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1
agrees with the distance deduced from JH D and from the
neutron diffraction data.
We are continuing our studies of this new class of dihydrogen
complexes.
Acknowledgements
We wish to acknowledge the support of the US Department
of Energy under Grant No. DEFG02-91ER45439. The work
at Argonne National Laboratory was supported by the US
Department of Energy, Basic Energy Sciences-Materials
Sciences, under Contract No. W-31-109-ENG-38.
** The fast spinning correction is not necessary if the H2 ligand rotates
by 180Њ reorientation processes. In the present molecule, the M → H2
back bonding would be weakened upon rotation of the dihydrogen
ligand in 1 by 90Њ. This destabilizing effect, however, may be compen-
sated by H ؒ ؒ ؒ H interactions with the two classical hydride ligands.8c If
so, then the rotation may involve 90Њ reorientations so that the fast
rotation correction is applicable. Effects due to large torsional motion
may also be important.16
References
Fig. 1 An ORTEP 11 view of the molecular structure of the [(C5Me5)-
Os(H2)H2(PPh3)]ϩcation as determined by neutron diffraction; the 20%
probability density surfaces are shown for the Os᎐H ligands while
spheres of arbitrary size are shown for all other atoms. Selected bond
distances (Å) and angles (Њ) (all taken from the neutron study except
the first two): Os᎐P 2.334(1), Os᎐Cave 2.251(6), H(2)᎐H(4) 1.014(11),
Os᎐H(1) 1.654(9), Os᎐H(2) 1.659(9), Os᎐H(3) 1.631(9), Os᎐H(4)
1.680(9); H(2)᎐Os᎐H(4) 35.4(4), H(2)᎐Os᎐H(3) 68.0(5), H(2)᎐Os᎐
H(1) 71.2(5), H(1)᎐Os᎐H(3) 132.6(5), H(3)᎐Os᎐H(4) 84.1(5), H(1)᎐
Os᎐H(4) 76.2(5), H(1)᎐Os᎐P 77.3(3), H(2)᎐Os᎐P 83.0(4), H(3)᎐Os᎐P
74.9(3), H(4)᎐Os᎐P 117.9(4)
1 D. M. Heinekey and W. J. Oldham, jun., Chem. Rev., 1993, 93, 913.
2 P. G. Jessop and R. H. Morris, Coord. Chem. Rev., 1992, 121, 155.
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4 R. H. Morris, Can. J. Chem., 1996, 74, 1907.
5 D. G. Gusev, R. L. Kuhlman, K. B. Renkema, O. Eisenstein and
K. G. Caulton, Inorg. Chem., 1996, 35, 6775. Distances cited are
uncorrected for librational motion of the H2 ligand.
6 F. Maseras, A. Lledós, M. Costas and J. M. Poblet, Organo-
metallics, 1996, 15, 2947.
7 For dihydrogen compounds with H᎐H distances between 1.0 and 1.4
Å as determined by neutron diffraction see: (a) W. T. Klooster,
T. F. Koetzle, G. Jia, T. P. Fong, R. H. Morris and A. Albinati,
J. Am. Chem. Soc., 1994, 116, 7677; (b) A. Albinati, V. I.
Bakhmutov, K. G. Caulton, E. Clot, J. Eckert, O. Eisenstein, D. G.
Gusev, V. V. Grushin, B. E. Hauger, W. T. Klooster, T. F. Koetzle,
R. K. McMullan, T. J. O’Loughlin, M. Pélissier, J. S. Ricci, M. P.
Sigalas and A. B. Vymenits, J. Am. Chem. Soc., 1993, 115, 7300;
(c) P. A. Maltby, M. Schlaf, M. Steinbeck, A. J. Lough, R. J. Morris,
W. T. Klooster, T. F. Koetzle and R. C. Srivastava, J. Am. Chem.
Soc., 1996, 118, 5396; (d) T. Hasegawa, Z. Li, S. Parkin, H. Hope,
R. K. McMullan, T. F. Koetzle and H. Taube, J. Am. Chem. Soc.,
1994, 116, 4352; (e) L. Brammer, J. A. K. Howard, O. Johnson, T. F.
Koetzle, J. L. Spencer and A. M. Stringer, J. Chem. Soc., Chem.
Commun., 1991, 241.
8 For other dihydrogen complexes studied by neutron diffraction see:
(a) G. J. Kubas, C. J. Burns, J. Eckert, S. W. Johnson, A. C. Larson,
P. J. Vergamini, C. J. Unkefer, G. R. K. Khalsa, S. A. Jackson and
O. Eisenstein, J. Am. Chem. Soc., 1993, 115, 569; (b) J. S. Ricci, T. F.
Koetzle, M. T. Bautista, T. M. Hofstede, R. H. Morris and J. F.
Sawyer, J. Am. Chem. Soc., 1989, 111, 8823; (c) L. S. Van Der Sluys,
J. Eckert, O. Eisenstein, J. H. Hall, J. C. Huffman, S. A. Jackson,
T. F. Koetzle, G. J. Kubas, P. J. Vergamini and K. G. Caulton, J. Am.
Chem. Soc., 1990, 112, 4831; (d) J. Eckert, C. M. Jensen, T. F.
Koetzle, T. L. Husebo, J. Nicol and P. Wu, J. Am. Chem. Soc., 1995,
117, 7271.
9 C. L. Gross, S. R. Wilson and G. S. Girolami, J. Am. Chem. Soc.,
1994, 116, 10 294.
10 T. Arliguie, B. Chaudret, F. A. Jalon, A. Otero, J. A. Lopez and
F. J. Lahoz, Organometallics, 1991, 10, 1888.
11 C. K. Johnson, ORTEP, Report ORNL-5138, Oak Ridge National
Laboratory, Oak Ridge, TN, 1976.
12 C. L. Gross and G. S. Girolami, unpublished work.
13 D. G. Hamilton and R. H. Crabtree, J. Am. Chem. Soc., 1988, 110,
4126.
14 P. J. Desrosiers, L. Cai, Z. Lin, R. Richards and J. Halpern, J. Am.
Chem. Soc., 1991, 113, 4173.
15 M. T. Bautista, K. A. Earl, P. A. Maltby, R. H. Morris, C. T.
Schweitzer and A. Sella, J. Am. Chem. Soc., 1988, 110, 7031.
16 See, for example, R. H. Morris and R. J. Wittebort, Magn. Reson.
Chem., 1997, 35, 243.
stants are averages owing to the exchange process. If we assume
2
that the geminal JH D couplings are all between 0 and 1 Hz,5
then the intrinsic 1JH D coupling within the bound HD ligand is
between 20.6 and 21.6 Hz. [The thermodynamic site preferences
(i.e., deuterium in the dihydrogen vs. hydride sites) are small as
2
shown by the invariance of JH P(ave) to the extent of deuteri-
ation.] These values, when substituted into Morris’s empirical
equation dH H = Ϫ0.0167 JH D ϩ 1.42,4 yield H᎐H distances of
1.06 to 1.08 Å. The calculated distance is in good agreement
with that derived from the neutron diffraction data, especially
after correction for librational effects, and we conclude that the
structure of 1 in solution is similar to that seen in the solid state.
We have also carried out variable-temperature 1H NMR stud-
ies of the spin-lattice relaxation time of undeuteriated samples
of 1. At 500 MHz in CD2Cl2, the T1 of the Os᎐H resonance
reaches a minimum of 99 ms at Ϫ70 ЊC (SUP 57272). At this
temperature, the exchange between the Os᎐H and Os᎐H2 sites
is in the fast exchange limit, and thus the observed relaxation
time is an average given by the expression 2R(ave, min) = R(c,
min) ϩ R(n, min), where R(c, min) and R(n, min) are the relax-
ation rates (R = 1/T1) for the classical and non-classical hydro-
gen sites at Ϫ70 ЊC.13 By using Halpern’s method 14 to sum
dipole–dipole relaxation rates calculated from the interatomic
distances determined crystallographically, we can calculate that
R(c, min) is approximately 4.14 sϪ1 and that the relaxation rates
of the hydrogen atoms in the H2 ligand (excluding the dipole–
dipole interaction within the H2 ligand itself) are 3.94 sϪ1 for
hydrogen atom H(2) and 2.25 sϪ1 for hydrogen atom H(4).
These estimated relaxation rates lead to a value of 12.96 sϪ1 for
the relaxation rate due just to the dipole–dipole interaction
within the H2 ligand [i.e., T1(n, min) = 77 ms]. If we assume that
the dihydrogen ligand rotation rate is fast compared with the
molecular tumbling rate, then from the expression dH H = 5.81
6
T1(n, min)/4ν, where T1 is in seconds and ν is the spectrometer
√
frequency in MHz, we obtain dH H = 1.07 Å.**,4,15 The value
Received 14th July 1997; Communication 7/04997H
3082
J. Chem. Soc., Dalton Trans., 1997, Pages 3081–3082