A Molecular Rotor Possessing an H-M-H "spoke" on a P-M-P "axle": A Platinum(II) trans-Dihydride Spins Rapidly even at 75 K

Article abstract of DOI:10.1021/jacs.5b08213

A new class of low-barrier molecular rotors, metal trans-dihydrides, is suggested here. To test whether rapid rotation can be achieved, the known complex trans-H₂Pt(P^tBu₃)₂ was experimentally studied by ²H and ¹⁹⁵Pt solid-state NMR spectroscopy (powder pattern changes with temperature) and computationally modeled as a ^tBu₃P-Pt-P^tBu₃ stator with a spinning H-Pt-H rotator. Whereas the related chloro-hydride complex, trans-H(Cl)Pt(P^tBu₃)₂, does not show rotational behavior at room temperature, the dihydride trans-H₂Pt(P^tBu₃)₂ rotates fast on the NMR time scale, even at low temperatures down to at least 75 K. The highest barrier to rotation is estimated to be ～3 kcal mol^-1, for the roughly 3 ? long rotator in trans-H₂Pt(P^tBu₃)₂.

Full text of DOI:10.1021/jacs.5b08213

Subscriber access provided by The University Library | University of Sheffield
Communication
A Molecular Rotor Possessing an H-M-H ‘Spoke’ on a P-M-P
‘Axle’: A Platinum(II) trans-Dihydride Spins Rapidly Even at 75 K
Ernest Prack, Christopher A. O'Keefe, Jeremy Moore, Angel Lai, Alan J. Lough,
Peter M. Macdonald, Mark S. Conradi, Robert W. Schurko, and Ulrich Fekl
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b08213 • Publication Date (Web): 08 Oct 2015
Downloaded from http://pubs.acs.org on October 12, 2015
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Journal of the American Chemical Society is published by the American Chemical
Society. 1155 Sixteenth Street N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 5
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
A Molecular Rotor Possessing an H-M-H ‘Spoke’ on a P-M-P
‘Axle’: A Platinum(II) trans-Dihydride Spins Rapidly Even at 75 K
Ernest Pracka, Christopher A. O’Keefeb, Jeremy Moorec, Angel Laia, Alan J. Loughd,
Peter M. Macdonalda, Mark S. Conradic, Robert W. Schurko*,b and Ulrich Fekl*,a
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
a) University of Toronto, Mississauga Campus, 3359 Mississauga Road, Mississauga, Ontario, Canada L5L 1C6
b) University of Windsor, 401 Sunset Ave., Windsor, Ontario, Canada, N9B 3P4
c) Washington University St. Louis, One Brookings Drive St. Louis, MO 63130-4899, USA
d) University of Toronto, St. George Campus, 80 St George St, Toronto, Ontario, Canada M5S 3H6
Supporting Information Placeholder
the rotator is also organic; however, inorganic rotators are
ABSTRACT: A new class of low-barrier molecular rotors,
very promising as well. In 1994, Gray’s group reported a
trans-spanning diphosphine molybdenum(0) tetracarbonyl
complex capable of low-barrier random rotations at tempera-
tures as low as 253 K.12 In 2000, Gladysz’ group reported a
singly trans-spanning diphosphine platinum(II) perfluoro-
phenyl chloride complex where the perfluorophenyl and
chloride could potentially rotate.5 No rotation was observed
metal trans-dihydrides, is suggested here. To test whether
rapid rotation can be achieved, the known complex trans-
2
H2Pt(PtBu3)2 was experimentally studied by H and 195Pt sol-
id-state NMR spectroscopy (powder pattern changes with
t
temperature) and computationally modeled as a Bu3P-Pt-
PtBu3 stator with a spinning H-Pt-H rotator. Whereas the
related chloro-hydride complex, trans-H(Cl)Pt(PtBu3)2, does
not show rotational behavior at room temperature, the dihy-
dride trans-H2Pt(PtBu3)2 rotates fast on the NMR time scale,
even at low temperatures down to at least 75 K. The highest
barrier to rotation is estimated to be ca. 3 kcal mol-1, for the
roughly 3 Å long rotator in trans-H2Pt(PtBu3)2.
a
even at 368 K13 but partial rotation in the solid-state was
later seen for a derivative.13b Other rotor-like transition-metal
complexes followed which employed halides, nitriles, and
carbonyl ligands as the rotators.6 The multi-step syntheses of
the large stator ligands have pushed the boundaries of organ-
ic synthesis; however, many of the most impressive random
rotors have high barriers to rotation (≥ 10 kcal mol-1) and
become fixed in place (‘jammed’) in the solid state. 6b,d
The question arises: are there overlooked classes of
Inspired in part by nature’s molecular machinery,
current research is developing functional parts for future
human-made molecular machines.1 Among the key ‘ma-
chine parts’ are molecular rotors, devices that possess a func-
tional axis of rotation, which allows the mobile part, the ro-
tator, to rotate independently of a static part, the stator.2
Research towards the design of functional rotors can be
broadly divided into either developing rotors that move ran-
domly in both directions powered by thermal energy in equi-
librium, or devising unidirectional rotors that require a
ratchet mechanism and a source of free energy.2,3,4 Our focus
is on hindered random rotors, where lowering the barrier(s)
to rotation is the key challenge. While such devices will ini-
low-barrier rotors? Rapidly spinning ‘molecular machine
parts’ that are easily synthesized? We think there are, and
would like to draw attention to transition metal trans-
dihydrides. Such rotators would be rather large in one direc-
tion (ca. 3 Å for the H-H separation)14 but the small van-der-
Waals radius of hydrogen gives hope for a low barrier to rota-
tion. The H-M-H geometry in trans-dihydrides is linear for d8
metals. Here, we provide the first demonstration that a
trans-dihydride can act as a molecular rotor with a low rota-
tional energy barrier.
The known complex trans-
H2Pt(PtBu3)2 is shown here to exhibit rapid rotations of the
Pt-bound hydrogen positions, down to temperatures of
around 75 K.
, ,
tially not be unidirectional, uses as gyroscopes,5 6 7 for exam-
ple, can be envisioned, and unidirectional movement may
conceivably be added later, perhaps by making the rotor po-
lar and exploiting its dipole moment.8
Mislow’s solution-state triptycene rotors were
among the first (1980s) intensely studied rotors.9 Later, the
groups of Gladysz, Garcia-Garibay, and others have explored
factors that enable molecular rotation in the solid state as
well.5,6,8,10 The size, shape, and symmetry of the rotor and its
parts all influence its mobility. The synthesis of rotors with
high mobility in a solid-state structure (relevant for future
devices) is difficult to accomplish11 and requires careful de-
sign of a protective stator. Nearly all current examples of
molecular rotors involve large organic stators.2,5,6,8,10 Often,
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 2 of 5
Figure 1. Structures of rotor 1 (trans-H2Pt(PtBu3)2) and ‘jammed
rotor’ 2-D (trans-(D)ClPt(PtBu3)2). Bond distances (Å) and angles
(deg): for 1: Pt1-H1Pt, 1.35(4); Pt1-P1, 2.2783(5); P1-Pt1-P1', 180.0; P1-
Pt1-H1Pt, 91.0(16); P1'-Pt1-H1Pt, 89.0(16); for 2-D: Pt1-D1, 1.68(3); Pt1-
P1, 2.3260(5); Pt1-Cl1, 2.4344(7); P1-Pt1-P1', 164.01(2); P1-Pt1-D1,
82.004(12); P1-Pt1-Cl1, 97.996(11); Cl1-Pt1-D1, 180.
1
2
3
4
5
6
7
8
Trans-H2Pt(PtBu3)2 (1) was specifically chosen to
give a H-Pt-H rotator steric protection through a bulky
phosphine stator. Compound 1 was synthesized by a modi-
fied literature procedure in one step from [(COD)PtCl2], in
83 % yield (see SI).15 Recrystallization from toluene/hexanes
at –30 oC gave crystals suitable for X-ray crystallography. The
structure (at 147 K)16 is shown in Figure 1 (left). For compari-
son, the formally similar chloro-hydride complex trans-
H(Cl)Pt(PtBu3)2 (2), which turns out not to be a rotor, was
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 2. WURST-CPMG 195Pt SSNMR spectra and simulations of 1 at
298 K and 133 K. Dashed lines show relative positions of δ22 and δ33.
2
also
synthesized.
Its
deuterated
version,
trans-
The H EFG tensor is very sensitive to a variety of modes of
D(Cl)Pt(PtBu3)2 (2-D), was also crystallographically charac-
terized (Figure 1, right).
motion over a wide range of temperature-dependent ex-
2
change rates.22 Variable-temperature (VT) H SSNMR pro-
Close examination of the X-ray crystal structure of 1
suggested that the hydrides may be mobile. The apparent
Pt-H bond length in 1, from refinement, is 1.35(4) Å, which is
very short. Apparent Pt-H bond lengths, from X-ray crystal-
vides direct and convincing evidence for a rotating H-Pt-H
unit. The Pt-deuterated complex trans-D2Pt(PtBu3)2, 1-D2,
was prepared. Experimental VT 2H SSNMR spectra are shown
2
in Figure 3 (left). For comparison, a room-temperature H
,
lography, are commonly in the range of 1.5-1.8 Å,17 18 and a
SSNMR spectrum of the ‘jammed rotor’ complex trans-
D(Cl)Pt(PtBu3)2, 2-D, was obtained, showing a quadrupolar
coupling constant (CQ) of 110 kHz, indicating a static plati-
num hydride (Fig. S12).23 Notably, a similar CQ is observed for
1-D2 only at very low temperatures and about half of that
neutron diffraction study has yielded 1.55(3) Å in one exam-
ple of a terminal hydride coordinated to a Pt(II) trans-
diphosphine complex.19 The Pt-D bond distance in 2-D (1.68
Å) fits into the typical range of bond lengths. The short Pt-H
bond distance seen for 1 is likely an artifact from mobility
due to ‘smearing’ of the electron density around a section of
a circular path. Given the limitations of X-ray crystallography
in determining H positions, a disorder model was also tested:
three disordered positions (6 partial hydrides, each with 1/3
occupancy) resulted in no change to the (excellent) R-factor
(1.36%). Hence, X-ray data are consistent with mobility of the
platinum-bound hydrides, but do not provide strong evi-
dence.
2
value at room temperature: For 1-D2, the H SSNMR spec-
trum acquired at 30 K was simulated with a CQ of 88(2) kHz
2
and an asymmetry parameter (ηQ) of 0.0(1). These H EFG
tensor parameters (see SI for definitions) are in agreement
with those from DFT calculations, indicating that the mode
and/or rate of motion at 30 K does not result in a motional
averaging of the principal components of the 2H EFG tensors,
which would lead to a reduced apparent CQ and perhaps
even a distinct ηQ. The spectra remain relatively unchanged
until warming to 75 K, where the effects of motion begin to
result in increased values of ηQ (Figure 3, left). Further in-
creases in temperature result in further increases in ηQ and
accompanying decreases in CQ.
A similar conclusion (pointing towards mobility)
may be obtained from 195Pt solid-state NMR (SSNMR) data
(Figure 2). The 195Pt isotropic chemical shift, δiso, is extremely
sensitive to both the oxidation state of Pt and the local struc-
ture.20 195Pt SSNMR powder patterns are typically dominated
by the effects of chemical shift anisotropy (CSA), which is
described by the span (Ω) and the skew (κ) (see SI for defini-
tions).21 Using the WURST-CPMG pulse sequence,21 195Pt
SSNMR spectra for complex 1 were obtained at room temper-
ature and 133 K (Figure 2). The spectra collected at both low
and high temperatures have similar values of δiso and almost
identical values of Ω. However, skew values are different
enough (κ = -0.82(2) at 133 K and κ = -0.90(2) at 298 K) to
suggest that the motion in 1 at 298 K results in a partial aver-
aging of the δ22 and δ33 components of the 195Pt CS tensor,
resulting in a more axially symmetric spectrum (i.e. κ ap-
proaches -1, see Table S3). The orientation of the 195Pt CS
tensor in the molecular frame was determined using DFT
calculations (Fig. S14): the δ11 component is along the P-Pt-P
axis, δ22 is parallel to the H-Pt-H vector, and δ33 is perpendic-
ular to the P-Pt-H plane. This is consistent with the partial
averaging of the δ22 and δ33 components due to rapid rotation
of the hydrides. However, the small differences between the
low and high temperature 195Pt SSNMR spectra elucidate
neither the exact nature of this motion nor its rates.
Figure 3. Experimental VT 2H SSNMR of 1-D2 (left) and accompany-
ing simulations of the rotor motion using a n:1:1 population model of
the various rotational states (right). See text for details (a minor
impurity, indicated by the isotropic central ‘spike’, is not modeled).24
2H SSNMR is ideally suited to the study of molecu-
lar-level dynamics.
2
ACS Paragon Plus Environment

Page 3 of 5
Journal of the American Chemical Society
1
2
At 295 K, the highest temperature used in the experimental
study,25 CQ = 40 kHz and ηQ = 0.1 are observed.
3
4
5
6
7
8
9
A detailed analysis of the motion must take into ac-
count the symmetry of the molecule, as influenced in part by
crystal packing. Although the idealized symmetry of the R3P-
Pt-PR3 framework in 1 is D3d, its crystallographic symmetry is
only Ci. The H-Pt-H unit is exactly linear. Due to the inver-
sion center, the unique potential energy landscape that un-
folds upon a 0o-180o rotation of the rotator repeats exactly
from 180o-360o. Thus, while there are six minima along the
path of a 360o rotation of one of the hydrides, repetition due
to Ci symmetry allows for formal treatment as a system with
three minima. The minima, shown in Figure 4, are not de-
generate in the point group of the local symmetry (Ci), alt-
hough states 60/240 and 120/300, as it turns out (below), are
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 4. Newman projection along P-Pt-P, illustrating the minima
for hydride rotation. Carbon atoms are labeled as in Figure 1.
2
energetically very close. H SSNMR spectra were simulated
using the EXPRESS software package.26 Initially, simulations
involved simple 2-, 3-, and 6-site jump models (Fig. S18) with
exchange rates (νex) in the intermediate motional regime (103
Hz < νex < 107 Hz). However, none of these simulations agree
with the experimental data. Experimentally varying the echo
2
delay at 120 K results in very little change in the H powder
pattern shape, indicating that the exchange is in the fast mo-
tion limit (νex > 107 Hz) even at that temperature (Fig. S19).
The temperature-dependent spectral changes appear to arise
from different populations associated with each of the energy
minima (rotational states shown in Figure 4) at different
temperatures.27 The best model for the populations of the
states assumes that one conformation is unique and has the
lowest energy, while the remaining two conformations,
slightly higher in energy, are very similar in energy. The sim-
Figure 5. Energy landscape from DFT, using B3LYP. Basis
sets include 6-31G** for C, H, and P; LANL2DZ for Pt.
The landscape shown in Figure 5 appears qualita-
tively appropriate but would have to be scaled (y-axis, ener-
gy) by a factor of ca. 0.5 to 0.3 to be quantitatively correct.
Possible reasons for this discrepancy include the lack of dis-
persion forces in the DFT treatment (notoriously hard to
treat with DFT),28 in addition to the general problem that
very extensive optimization of functional and basis sets
would be needed to reduce the error to 1 kcal mol-1 or less.
In conclusion, we have demonstrated for the first
time that a linear trans-dihydride can be a molecular rotor
with a very low barrier, even at low temperatures, in the solid
state. We expect that this will open the door to more exam-
ples in this class of molecules, and enlarge the field of facile
rotors in the solid state. Further work will be directed to de-
signing systems with controlled motion using different sta-
tors and to incorporating a dipole moment.
2
ulated H SSNMR spectra using n:1:1 population distributions
(with n > 1; n representing the relative population of the
global minima states) are shown in Figure 3, right. When the
VT populations are plotted in a Van’t Hoff fashion (Fig. S24),
it is predicted that the higher-energy conformations are up-
hill from the global minimum states by ca. 0.27(3) kcal mol-1.
We modeled the energy landscape with DFT (Figure
5), using the Ci-symmetric heavy atom framework directly as
obtained from the crystal structure. The Pt-H distance was
fixed at the DFT-optimized value (1.657 Å), and the H-Pt-P
angle was fixed at 90o. The platinum hydride orientation was
varied by incrementing the H-Pt-P-C torsion angle and fixing
the other hydride exactly trans. In this restricted geometry
optimization, simulating the environment in the crystal, the
heavy atom framework was not allowed any internal move-
ment, but the tert-butyl methyl groups were allowed to ro-
tate. The result (Figure 5) is in qualitative agreement with
the experimental data: one global minimum (in fact, the
same as suggested by the initial crystallographic refinement)
and two other local minima per 180o rotation. The two uphill
minima are very similar in energy, consistent with the best
Supporting Information
Experimental and computational details, crystal data and
spectra are available free of charge via the Internet at
http://pubs.acs.org.
Author Information (Corresponding Authors)
*E-Mail: rschurko@uwindsor.ca; ulrich.fekl@utoronto.ca
2
model of the experimental H VT NMR data. The quantita-
tive agreement between experiment and theory is not per-
fect: the energetic separation between the minima is about
0.27(3) kcal mol-1 experimentally and 0.89 kcal mol-1 compu-
tationally. Also, the highest barrier between minima, to be
consistent with experimental information (fast exchange at
120 K), is ca. 3.0 kcal mol-1 or lower (page S26), while the
highest barrier computes as 4.9 kcal mol-1.
ACKNOWLEDGMENT
U.F. and P. M. M. thank the University of Toronto (UT) and
NSERC of Canada for support. We thank Professor Bob Mor-
3
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 4 of 5
ris (UT) for continuous valuable suggestions. David Arm-
1
2
3
4
5
6
7
8
strong (UT) is acknowledged for help with DFT computa-
tions. Computations were performed on the GPC supercom-
puter at the SciNet HPC Consortium. SciNet is funded by the
Canada Foundation for Innovation (CFI), NSERC, the Gov-
ernment of Ontario, Fed Dev Ontario, and UT. R.W.S.
thanks NSERC for discovery/accelerator grants and the Uni-
versity of Windsor for a 50th Anniversary Jubilee award. He
also thanks NSERC, the CFI, the Ontario Ministry of Re-
search and Innovation and the University of Windsor for
supporting the solid-state NMR facilities. C. A. O. thanks the
Ontario Ministry of Education and Training for graduate
scholarships.
Transition Metals’ in Hydrogen-Transfer Reactions (eds J. T.
Hynes, J. P. Klinman, H.ꢀH. Limbach and R. L. Schowen),
WileyꢀVCH, Weinheim, Germany. (b) Chinn, M. S.; Heiꢀ
nekey, D. M. J. Am. Chem. Soc. 1990, 112, 5166. (c) Facey,
G. A.; Fong, T. P.; Gusev, D.; Macdonald, P. M.; Morris, R.
H.; Schlaf, M.; Xu, W. Can. J. Chem. 1999, 77, 1899. See (a)
above also for polyhydride systems with very rapid interconꢀ
version between MꢀH and Mꢀ(H2) hydrogens.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
15 Goel, A. B.; Goel S. Inorg. Chim. Acta 1982, 65, L77.
16
A previous room temperature structure determination
REFERENCES
showed the same crystalline phase (just different orientaꢀ
tion of unit cell chosen: P21/c instead of P21/n) but the hyꢀ
dride positions were not detected: Ferguson, G.; Siew, P.
Y.; Goel, A. B. J. Chem. Res.(S) 1979, 362.
17
Robertson, G. B.; Tucker, P. A.; Wickramasinghe, W. A.
1 Balzani, V.; Credi, A.; Raymo, F. M.; Stoddart, J. F. Angew.
Chem. Int. Ed. 2000, 39, 3348.
Aust. J. Chem. 1986, 39, 1495.
18
Owston, P. G.; Partridge, J. M.; Rowe, J. M. Acta Cryst.
2
1960, 13, 246.
Kottas, G. S.; Clarke, L. I.; Horinek, D.; Michl, J. Chem.
19
Albinati, A.; Bracher, G.; Carmona, D.; Jans, J. H. P.;
Rev. 2005, 105, 1281.
3
Klooster, W. T.; Koetzle, T. F.; Macchioni, A.; Ricci, J. S.;
Thouvenot, R.; Venanzi, L.M. Inorg. Chim. Acta 1997, 265,
255.
Kelly, R. T.; De Silva H.; Silva, R. A. Nature 1999, 401,
150.
4
Koumura, N.; Zijlstra, R. W. J.; van Delden, R. A.; Harada,
20
Lucier, E. G.; Reidel, A. R.; Schurko, R. W. Can. J. Chem.
N.; Feringa, B. L. Nature 1999, 401, 152.
5
2011, 89, 919.
Bauer, E. B.; Ruwwe, J.; MartinꢀAlvarez, J. M.; Peters, T.
21
(a) O’Dell, L. A.; Schurko, R. W.; Chem. Phys. Lett. 2008,
B.; Bohling, J. C.; Hampel, F. A.; Szafert, S.; Lis, T; Gladysz,
464, 97ꢀ102. (b) Hamaed, H.; Ye, E.; Udachin, K.; Schurko,
R. W. J. Phys. Chem. B 2010, 114, 6014.
J. A. Chem. Commun. 2000, 2261.
6
(a) Shima, T.; Hampel, F.; Gladysz, J. A. Angew. Chem. Int.
22 Chandrakumar, N. Spin-1 NMR. Springer: Berlin, 1996.
Ed. 2004, 43, 5537. (b) Wang, L.; Hampel, F.; Gladysz, J. A.
Angew. Chem. Int. Ed. 2006, 45, 4372. (c) Nawara, A. J.;
Shima, T.; Hampel, F.; Gladysz, J. A. J. Am. Chem. Soc. 2006,
128, 4962. (d) NawaraꢀHultzsch, A. J.; Stollenz, M.;
Barbasiewicz, M.; Szafert, S.; Lis, T.; Hampel, F.; Bhuvanesh,
N.; Gladysz, J. A. Chem. Eur. J. 2014, 20, 4617.
23
Bakhmutov, V. I. (2002), Deuterium Spin-Lattice Relaxa-
tion and Deuterium Quadrupole Coupling Constants. In
Gielen, M.; Willem, R.; Wrackmeyer, B. (Eds.), Unusual
Structures and Physical Properties in Organometallic Chemis-
try (pp. 145ꢀ164). West Sussex, UK: Wiley & Sons.
2
24
7
A H SSNMR spectrum of nonꢀdeuterated 1 shows a
Krause, M.; Hulman, M.; Kuzmany, H; Dubay, O.; Kresse,
similar isotropic peak; also, temperature cycling of 1-D2
G.; Vietze, K; Seifert, G.; Wang, C.; Shinohara, H. Phys. Rev.
Lett. 2004, 93, 137403(4).
does not increase the amount of impurity.
25
8
While 1 is chemically stable at 295 K and below, and
Vogelsberg, C. S.; GarciaꢀGaribay, M. A. Chem. Soc. Rev.
while Xꢀray crystallography additionally indicates the same
crystalline phase at 147 K and at room temperature (this
work and Ref 16), we observed decomposition upon heatꢀ
ing to 325 K.
2012, 41, 1892.
9 Iwamura, H.; Mislow, K. Acc. Chem. Res. 1988, 21, 175.
10 (a) Khuong, T.ꢀA. V.; Nunnez, J. E.; Godinez, C. E.; Garciaꢀ
Garibay, M. A. Acc. Chem. Res. 2006, 39, 413. (b) Comotti,
A.; Bracco, S.; Ben, T.; Qiu, S.; Sozzani, P. Angew. Chem.,
Int. Ed. 2014, 53, 1043. (c) Setaka, W.; Yamaguchi, K. J. Am.
Chem. Soc. 2013, 135, 14560.
26 Vold, R. L.; Hoatson, G. L. J. Magn. Reson. 1995, 33, 57.
27
The temperatureꢀdependence of the 2H powder pattern of 1-
D2 is driven by the Boltzmann population weighting of the
slightly unequal minima, but barriers are surmounted rapidly
compared to the NMR static linewidth. This contrasts with the
more common situation where motional averaging is due to
multiple minima separated by tall energy barriers.
11
An exception is CH3 group rotation, which is rather comꢀ
monly found to have a low barrier: Wang, X.; Beckman, P. A.;
Mallory, C. W.; Rheingold, A. L.; DiPasquale, A. G.; Carroll,
P. J.; Mallory, F. B. J. Org. Chem. 2011, 76, 5170.
12 Gray, G. M.; Duffey, C. H. Organometallics 1994, 13, 1542.
13 (a) Shima, T.; Bauer. E. B.; Hampel, F.; Gladysz, J. A. Dal-
ton Trans. 2004, 1012. (b) Lewanzik, N.; Oeser, T.; Blümel,
J.; Gladysz, J. A. J. Mol. Catal. A 2006, 254, 20.
28
An attempt to improve the quality of the potential energy
landscape by adding dispersion corrections (Grimme, S.; Anꢀ
tony, J.; Ehrlich, S.; Krieg, H. J. Chem. Phys. 2010, 132,
154104) was only partially successful: while the span deꢀ
creased (from 4.9 kcal molꢀ1 to 2.3 kcal molꢀ1), the order of the
minima switched from “one low, two high” to “two low, one
high”, inconsistent with the experimental data (a 1:n:n fit is
inferior to an n:1:1 fit (for n>1, see Fig S20 for details)).
14
Ca. 3 x larger than the H2 rotor in dihydrogen complexes, a
wellꢀrecognized class of facile rotors; see: (a) Kubas, G. J.
(2006) ‘The Extraordinary Dynamic Behavior and Reactivity
of Dihydrogen and Hydride in the Coordination Sphere of
4
ACS Paragon Plus Environment

Page 5 of 5
Journal of the American Chemical Society
SYNOPSIS TOC
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ACS Paragon Plus Environment