Synthesis and characterization of dimethylbis(2-pyridyl)borate nickel(II) complexes: Unimolecular square-planar to square-planar rotation around nickel(ii)

Article abstract of DOI:10.1021/om500173j

The syntheses of novel dimethylbis(2-pyridyl)borate nickel(II) complexes 4 and 6 are reported. These complexes were unambiguously characterized by X-ray analysis. In dichloromethane solvent, complex 4 undergoes a unique square-planar to square-planar rotation around the nickel(II) center, for which activation parameters of ΛH^a = 12.2(1) kcal mol ^-1 and ΛS^a = 0.8(5) eu were measured via NMR inversion recovery experiments. Complex 4 was also observed to isomerize via a relatively slow ring flip: ΛH ^a = 15.0(2) kcal mol^-1; and ΛS^a = -4.2(7) eu. DFT studies support the experimentally measured rotation activation energy (cf. calculated ΛH^a = 11.1 kcal mol^-1) as well as the presence of a high-energy triplet intermediate (ΛH = 8.8 kcal mol^-1).

Full text of DOI:10.1021/om500173j

Article
pubs.acs.org/Organometallics
Open Access on 04/07/2015
Synthesis and Characterization of Dimethylbis(2-pyridyl)borate
Nickel(II) Complexes: Unimolecular Square-Planar to Square-Planar
Rotation around Nickel(II)
Jeff A. Celaje,† Megan K. Pennington-Boggio,† Robinson W. Flaig,† Michael G. Richmond,‡
and Travis J. Williams*,†
†Loker Hydrocarbon Institute and Department of Chemistry, University of Southern California, Los Angeles, California, 90089-1661,
United States
‡Department of Chemistry, University of North Texas, Denton, Texas, 76203-5017, United States
S
* Supporting Information
ABSTRACT: The syntheses of novel dimethylbis(2-pyridyl)-
borate nickel(II) complexes 4 and 6 are reported. These
complexes were unambiguously characterized by X-ray analysis.
In dichloromethane solvent, complex 4 undergoes a unique
square-planar to square-planar rotation around the nickel(II)
center, for which activation parameters of ΔH⧧ = 12.2(1) kcal
mol−1 and ΔS⧧ = 0.8(5) eu were measured via NMR inversion recovery experiments. Complex 4 was also observed to isomerize
via a relatively slow ring flip: ΔH⧧ = 15.0(2) kcal mol−1; and ΔS⧧ = −4.2(7) eu. DFT studies support the experimentally
measured rotation activation energy (cf. calculated ΔH⧧ = 11.1 kcal mol−1) as well as the presence of a high-energy triplet
intermediate (ΔH = 8.8 kcal mol−1).
dral configurations.5a,6 Nonetheless, no case has been carefully
documented wherein a diamagnetic square-planar metal
undergoes facile, unimolecular ligand rotation.
Herein we report the synthesis, characterization, and solution
INTRODUCTION
A variety of low-valent nickel complexes have important
■
reactivity in reactions ranging from the cycloisomerization of
CC1 and CO2 π systems to the reduction of CO2.3 In line
dynamics of the novel nickel(II) complexes (dimethylbis(2-
with the latter, we have recently observed that ammonia−
borane dehydrogenation catalysts 1 and 2 (Figure 1A),4 each
pyridyl)borate)(triphenylphosphine)nickel(II) chloride (4) and
(dimethylbis(2-pyridyl)borate)nickel(II) acetylacetonate (6)
(Scheme 1). Complex 4 undergoes isomerization via two
different mechanisms: (1) a unimolecular square-planar to
square-planar rotation around the nickel(II) center and (2) a
relatively slower ring flip. The activation parameters (ΔH⧧ and
1
ΔS⧧) for both mechanisms were measured using H NMR
inversion recovery experiments.7 Studies to show that rotation
around the metal center is indeed unimolecularnot the usual
associative or dissociative isomerization pathways that have
Figure 1. (A) Ru(II) complexes possessing an anionic bidentate
borate ligand. (B) The dimethylbis(2-pyridyl)borate ligand 3.
been studied for many four-coordinate square planar metals8
are discussed.
bearing an anionic bidentate borate ligand, are capable of CO2
reduction concurrent with ammonia−borane dehydrogenation.
We therefore became interested in investigating the reactivity of
other nickel complexes of the anionic bidentate ligand
dimethylbis(2-pyridyl)borate (3; Figure 1B). In developing
the synthesis of such complexes, we found an unexpected
unimolecular square-planar to square-planar mutarotation of
RESULTS
■
Synthesis and Characterization of Complexes 4 and 6.
Complexes 4 and 6 were synthesized via simple metathesis
reactions. The reaction of 1 equiv of bis(triphenylphosphine)-
nickel(II) chloride with borate 3 in dichloromethane gives
complex 4 in 71% isolated yield (Scheme 1). The formation of
complex 4 is accompanied by formation of a small amount of
the thermodynamically favored bis(borate) nickel complex 5,
which is known to form with facility when 3 is treated with
diamagnetic (borate)nickel(II) complex 4. Rotations of this
class, e.g. cis/trans isomerization of diamagnetic, square-planar
late-metal complexes, are generally known not to proceed
through unimolecular mechanisms.5 Nickel(II) has a possible
exception, however, in that these species can equilibrate
between diamagnetic square-planar and paramagnetic tetrahe-
Received: February 17, 2014
Published: April 7, 2014
© 2014 American Chemical Society
2019
dx.doi.org/10.1021/om500173j | Organometallics 2014, 33, 2019−2026

Organometallics
Article
4 is 47.9(2)°, indicating that the dimethylbis(2-pyridyl)borate
ligand is tilted farther out of plane in complex 4. Consistent
with the observation of the increased tilt is the decreased
distance between nickel and the carbon in the endo-configured
(methyl)boron group in complex 4 (3.143 Å versus 3.171 Å in
complex 6). Complex 4 exhibits the expected trans effect with
the Ni−N bond trans to PPh3 longer than that trans to the Cl−
(1.945(2) and 1.893(3) Å, respectively). On the other hand,
Ni−N bonds are equivalent (1.8943(8) and 1.8955(8) Å), and
the Ni−O bonds are similar (1.8567(7) and 1.8615(7) Å) in
complex 6. The Ni−N bond lengths in both 4 and 6 are
consistent with those observed for 5 (1.906(2) and 1.902(2)
Å).9
Scheme 1. Syntheses of 4 and 6
nickel(II) salts.9 The acetylacetonate-ligated nickel complex 6
was readily prepared by treating an excess of Ni(acac)2 (2.5
equiv) with sodium borate 3, which yielded complex 6 (67%,
Scheme 1). In contrast, the reactions of 1 equiv of either
nickel(II) acetate tetrahydrate (Ni(OAc)2·4H2O) or nickel(II)
acetylacetonate (Ni(acac)2) with 3 exclusively provide bis-
(borate) nickel complex 5.
Low-Temperature NMR Studies on Nickel Complex 4.
1
The H NMR spectrum of nickel complex 4 in CD2Cl2 at 25
°C shows that 4 is predominantly diamagnetic in solution.
Further, it undergoes a dynamic process in solution, which is
observed in variable-temperature (VT) NMR studies (Figure
3). Whereas the pyridyl α hydrogens of the bound dimethylbis-
Both 4 and 6 were characterized by single-crystal X-ray
crystallography (Figure 2). Both complexes are square planar.
1
Figure 3. Variable-temperature H NMR spectra of complex 4.
(2-pyridyl)borate ligand give a coalesced singlet (δ 8.55 ppm)
at 25 °C, two distinct singlets are observed at lower
temperatures (i.e., δ 8.12 and 8.82 ppm at −40 °C). This
means that complex 4 is isomerizing in solution (Figure 4A). A
similar isomerization has been observed for an analogous Bp
complex, 7 (Figure 4B).10
Isomerization of square-planar complexes usually proceeds
via either an associative or a dissociative mechanism (Scheme 2,
vide infra).8 However, we were intrigued by the possibility that
we were observing isomerization via a mechanism involving a
unimolecular (first-order) square-planar to square-planar
Figure 2. ORTEP diagrams of (A) complex 4 and (B) complex 6.
Ellipsoids are drawn at the 50% probability level. Complex 4
cocrystallizes with 0.5 equivalent of hexanes, which is omitted for
clarity.
However, complex 4 exhibits a slight distortion. Whereas in
complex 6 the sum of the bond angles around the nickel is
360.03(7)°, the sum of angles around nickel for complex 4 is
361.6(2)°, with the P−Ni−Cl plane tilted 14° relative to the
N−Ni−N plane. An analogous nickel complex of the bidentate
borate ligand dihydrobis(pyrazolyl)borate (Bp) has recently
been reported and similarly adopts a square-planar structure.10
As in all previous complexes of 3, the metal−chelate six-
membered ring exhibits a boat conformation in both 4 and
6.4,11 The dihedral angle between metal and pyridine planes in
complex 6 is 53.3(4)°. However, the dihedral angle in complex
Figure 4. (A) Observed isomerization in complex 4. (B) Isomerization
of the dihydrobis(pyrazolyl)borate (Bp) complex 7.10
2020
dx.doi.org/10.1021/om500173j | Organometallics 2014, 33, 2019−2026

Organometallics
Article
a
Scheme 2. Possible Mechanisms of Isomerization
Figure 6. ln/ln plots of the dependence of isomerization rate of 4 on
[PPh3] (left) and [Cl−] (right). For the plot on the left, inversion
recovery data were collected at −41.1 °C using a 12 mM CD2Cl2
solution of 4 with [PPh3] ranging from 0.1 to 10.0 equiv. Slope =
0.00(1). For the plot on the right, inversion recovery data were
collected at −43.0 °C using a 12 mM CD2Cl2 solution of 4 with [(n-
Bu)4NCl] ranging among 0.4, 0.7, 1.1, 1.5, and 2.0 equiv versus Ni
atom. Slope = −0.01(3).
a
Proposed associative and dissociative mechanisms involving PPh3 are
shown, but it is possible that the other two ligands, Cl− and 3, could
also be responsible for effecting either of these mechanisms.
with the observed rate of rotation had a slope of 0.0, which
indicates that PPh3 is of kinetic order 0.0 in the rotation
mechanism.
Also, to ensure that PPh3 exchange is not occurring, we
performed a 31P NMR inversion recovery study to see whether
magnetization transfer from coordinated PPh3 to free PPh3
would occur. No magnetization transfer was observed at −42.4
°C (Figure 7), indicating that PPh3 exchange is indeed not
occurring at a temperature where rotation is fast: t1/2 = 30 ms.
rotation around a metal center. Such a process could explain
the observation from the H NMR VT studies.
Kinetic Studies of the Conformational Dynamics of 4.
To show that neither an associative nor dissociative mechanism
is responsible for the isomerization of nickel 4, we analyzed the
dependence of the rate of isomerization with respect to the
three ligands present in 4: PPh3, chloride, and 3.
1
1
The H NMR spectrum of nickel 4 at −40 °C shows well-
differentiated pyridyl α hydrogens of the borate ligand (Figure
5). This presents an ideal opportunity to use 1H NMR
Figure 7. 31P NMR inversion recovery experiment at −42.4 °C. Data
were collected using a 12 mM CD2Cl231solution of 4 with 0.5 equiv of
added PPh3: ( ) coordinated PPh3, δ( P) 12.60, 31P T1 = 213(3) ms;
○
31
□
( ) free PPh3;, δ( P) −7.23; linear slope 0.00(0).
Rate Dependence on Chloride. The rate of isomerization
of 4 is also independent of the concentration of the chloride
ligand (Figure 6, right). Because of the insolubility of the
chloride anion in CD2Cl2, the isomerization is unlikely to be
proceeding via a chloride dissociation mechanism. We
nonetheless examined the rate of isomerization in the presence
of tetra-n-butylammonium chloride ((n-Bu)4NCl), a soluble
source of chloride. Addition of (n-Bu)4NCl leads to a reaction
wherein a small portion of an unidentified paramagnetic species
1
Figure 5. H NMR spectrum of nickel 4 at −40 °C. (Methyl)boron
groups are assigned by 1D-NOESY spectroscopy.
inversion recovery experiments to determine the rate of this
isomerization. For example, when the pyridyl proton at 8.82
ppm is pulsed (selectively labeled by inversion), magnetization
transfer to the pyridyl proton at 8.12 ppm can be observed. By
acquisition of data with different mixing times, a rate constant
for the isomerization can be obtained.7
1
is formed; regardless, 4 is observed in the H NMR spectrum,
Rate Dependence on PPh3. The rate constants for the
isomerization of 4 as a function of [PPh3] at −41.1 °C were
obtained by using 1H NMR inversion recovery experiments and
fitting the data into CIFIT.7 A plot of ln kobs vs ln [PPh3]
(Figure 6, left) shows that the rate of isomerization is
independent of [PPh3]. The rate of isomerization was
unchanged upon addition of up to 10 equiv of PPh3, and the
slope of the ln/ln plot comparing the concentration of PPh3
and we recorded rate constants for the rotation in the presence
of up to 2 equiv of (n-Bu)4NCl. The rate of isomerization was
unchanged by the presence of excess (n-Bu)4NCl (Figure 6,
right). We also examined the rate of isomerization in the
presence of excess LiCl, which is insoluble in CD2Cl2 and does
not react with 4. The rate of isomerization at −42.0 °C is
unaffected by addition of an excess of LiCl (see the Supporting
Information).
2021
dx.doi.org/10.1021/om500173j | Organometallics 2014, 33, 2019−2026

Organometallics
Article
While the dissociation of the chloride ligand to form a
positively charged tricoordinate intermediate in a noncoordi-
nating solvent (CD2Cl2) is unlikely, if the chloride ligand is
dissociating from 4, addition of thallium triflate should lead to
formation of TlCl and to the removal of chloride from the
coordination sphere of 4. To test for this situation, we treated a
Scheme 4. Ring Flip Places the (Methyl)boron Groups of 4
in a Different Chemical Environment, Whereas Rotation
a
Places Them in the Same Chemical Environment
1
solution of 4 with 2 mol equiv of Tl(OTf). The H NMR
spectrum of this sample is unaltered by the addition of
Tl(OTf): 4 is stable in the presence of Tl(OTf) at room
temperature for days, which indicates that the Cl− ligand does
not dissociate. Moreover, the presence of 2 equiv of Tl(OTf)
does not alter the rate of isomerization at −42.0 °C (23.1(4)
s−1 immediately after addition; 23.5(6) s−1 4 days after
addition).
Consideration of Borate 3 and Observation of a
Second, Relatively Slower Isomerization Pathway. A
mechanism of isomerization involving (a) dissociation of one of
the pyridine rings of ligand 3, followed by (b) rotation around
the Ni−N bond of the bound pyridine, and then (c)
recoordination of the free pyridine ring (Scheme 3) can be
a
Magnetization transfer between endo and exo (methyl)boron groups
is not observed at low temperatures because rotation places the methyl
groups in the same chemical environment (by symmetry). In contrast,
magnetization transfer can be observed at higher temperatures via ring
flip.
Scheme 3. Possible Mechanism of Isomerization Involving
a
Dissociation of Ligand 3
Figure 8. Eyring plots for unimolecular rotation (left) and ring flip
(right). For the plot on the left, inversion recovery data were collected
using a 12 mM CD2Cl2 solution of 4 at −61.6, −61.4, −53.7, −46.8,
−42.0, −32.6, −31.8, and −18.9 °C. ΔH⧧ = 12.2(1) kcal mol−1 and
ΔS⧧ = 0.8(5) eu.13 For the plot on the right, data were collected using
a 12 mM CD2Cl2 solution of 4 at 13.7, 18.8, 28.1, and 38.2 °C. ΔH⧧ =
15.0(2) kcal mol−1 and ΔS⧧ = −4.2(7) eu.13
a
Ring flip is possible with or without initial dissociation of a pyridyl
nitrogen. The measured rate of ring flip is slower than the rate of
rotation by greater than 2 orders of magnitude.
flip differ by greater than 2 orders of magnitude (ca. 3 kcal/
mol), with rotation being faster.
envisaged. This process leads to an isomerized product
DFT Studies. The potential energy surface for rotational
isomerization of the PMe3 analogue of nickel 4 was evaluated
computationally by DFT (Figure 9). The geometry-optimized
structure of singlet A shows an excellent correspondence with
the experimentally determined structure of 4, which reinforces
the use of this model as a probe for the isomerization in 4.
equivalent to an isomerized product of a ring flip. Since the
1
two (methyl)boron groups are differentiated in the H NMR
spectrum of 4 (singlets at 2.22 and 0.32 ppm, Figure 5),
inversion recovery experiments to measure the rate of this
1
process are possible. When an H NMR inversion recovery
experiment is performed at −40 °C, no magnetization transfer
is observed from one methyl group to the other. Even at 0 °C,
the magnetization transfer is too slow to permit measurement
of a rate constant. Only at 13.7 °C were we able to obtain a rate
constant: kobs = 2.69(4) s−1. This ring flip is therefore a
different, slower isomerization pathway for nickel 4 (Scheme
4). Since this separate process is much slower than the rotation,
we find that the observed rotation behavior cannot be
accounted for on the basis of a ring flip.
Activation Parameters for Square-Planar Rotation
and Ring Flip. To determine activation parameters, we used
Eyring plots constructed from rate constants obtained by NMR
inversion recovery experiments. For the square-planar rotation
of 4, we obtained values of ΔH⧧ = 12.2(1) kcal mol−1 and ΔS⧧
= 0.8(5) eu (Figure 8, left). For the ring flip, we measured
values of ΔH⧧ = 15.0(2) kcal mol−1 and ΔS⧧ = −4.2(7) eu
(Figure 8, right).12,13 Note that the rates for rotation and ring
Figure 9. PBE-optimized structures for the singlet (1A) and triplet
(3B) species [Me2B(2-py)2]NiCl(PMe3) and the potential energy
surface for the low-energy rotational isomerization of the PMe3 and Cl
3
ligands via TSBB′. Energy values (ΔH) are given in kcal mol−1.
2022
dx.doi.org/10.1021/om500173j | Organometallics 2014, 33, 2019−2026

Organometallics
Article
Diamagnetic A is the thermodynamically preferred isomer, and
the pseudotetrahedral triplet B lies 8.8 kcal mol−1 above A. The
computed free energy difference between A and B is 9.9 kcal
mol−1, leading to a Keq value of ca. 10−8 for the triplet at room
temperature, whose concentration is negligible and is in
keeping with the silent EPR spectrum and magnetic
susceptibility data recorded for 4 (vide infra). B is formed by
a directionally specific clockwise rotation of the PMe3 and Cl
ligands in A, and continued rotation of these groups furnishes
the transition structure TSBB’ that exhibits Cs symmetry. Here
the PMe3 ligand maintains a proximal orientation with respect
to the BMe2 moiety throughout the isomerization; PMe3
rotation away from the BMe2 moiety and under the pyridyl
rings leads to deleterious steric interactions. The computed
enthalpic barrier of 11.1 kcal mol−1 closely matches the
experimentally determined ΔH⧧ value for the process.
an anionic ligand to generate a cationic metal species or
dissociation of a neutral ligand to generate a neutral metal
species. This is followed by recoordination of the original ligand
and loss of the catalytic ligand. Some authors contend that
displacement of a neutral ligand would not result in
isomerization, due to the stereospecific nature of ligand
substitutions,8 while others claim to have definitively proven
the existence of the neutral intermediate.18 There is also debate
about the existence of complexes which isomerize via a
pseudorotation mechanism and disagreement about what
evidence definitively proves a pseudorotation mechanism over
a consecutive displacement mechanism.
While it is difficult to distinguish among the mechanisms of
associative isomerization, it is straightforward to classify an
isomerization as associative: by showing first-order rate
dependence on added ligand. In contrast, dissociative isomer-
ization proceeds through loss of a ligand to generate a three-
coordinate complex. Diminution of rate in the presence of
excess ligand, isomerization in the absence of potential ligands,
including coordinating solvents, and positive ΔS⧧ are often
taken as evidence of a dissociative process, but evidence of the
existence of a three-coordinate intermediate is required
confidently to conclude a dissociative mechanism.
Unimolecular Rotation of Nickel 4. Previous studies on
square-planar to tetrahedral equilibria of nickel(II) complexes,
which include molecular orbital analyses, indicate the possibility
of isomerization via a unimolecular square-planar to square-
planar rotation around a nickel center (i.e., a 90° rotation from
square planar to tetrahedral followed by another 90° rotation
from tetrahedral to square planar).14−16 However, no study that
systematically showsby excluding the associative or dis-
sociative isomerization pathwaysunimolecular rotation
around a diamagnetic square-planar nickel complex has been
reported. These cases involve equilibrium between two
observable geometric isomers at nickel: a diamagnetic square-
planar complex and a paramagnetic tetrahedral complex.
Moreover, ligands in some of these complexes are known to
be substitutionally labile. For example, in the presence of excess
phosphine, bis(n-alkyldiphenylphosphine)nickel(II) dihalides
undergo phosphine exchange to give a second-order ligand
exchange mechanism that contributes to the rate of isomer-
ization.14k Similarly, bis(salicylaldimine)nickel(II)14d and bis(β-
ketoimine)nickel(II)14g complexes undergo facile mixed ligand
exchange.
The studies mentioned in the Results show that the rotation
of complex 4 in a noncoordinating solvent involves neither an
associative nor a dissociative mechanism at the temperatures
studied. The most likely ligand to be involved in associative or
dissociative isomerization is PPh3. The observation that the rate
of isomerization is independent of [PPh3] excludes an
associative process and provides evidence against a dissociative
process. Moreover, 31P NMR inversion recovery experiments
show that, in the presence of excess PPh3, exchange between
coordinated and free PPh3 is not occurring, thus excluding a
dissociative mechanism involving PPh3.
DISCUSSION
■
Possible Mechanisms of Isomerization. While no
example of a unimolecular square-planar to square-planar
rotation around a diamagnetic late-metal complex has been
documented, prior studies show that such a rotation is possible
for some nickel(II) systems. In noncoordinating solvents, four-
coordinate d8 nickel(II) complexes that exist in equilibrium as
diamagnetic (S = 0) square planar and paramagnetic (S = 1)
tetrahedral isomers have been well documented.14 Eaton used
the Woodward−Hoffmann rules to obtain the selection rules
for isomerization of four-coordinate nickel(II) complexes and
showed that the isomerization from square planar to tetrahedral
should be a facile, thermally allowed process.15 Likewise, using
orbital correspondence analysis with maximum symmetry,
Halevi and Knorr showed that tetrahedral triplet to planar
singlet isomerizations of nickel(II) complexes accompanied by
a spin flip are thermally allowed.16 Indeed, the reported rates of
isomerizations for several nickel(II) species are fast. For
example, lifetimes of 10−5 s for each isomer have been
measured for nickel(II) aminotroponeiminates,14a bis-
(salicylaldimine)nickel(II),14b,c and a series of bis(n-
alkyldiphenylphosphine)nickel(II) dihalides.14k Moreover, the
racemization of optically active diastereomeric (Δ and Λ)
nickel(II) species was noted to be so fast that each
diastereomer could not be observed by NMR.14g,k Thermody-
namic studies show that this equilibrium is affected by ligand
sterics, with the presence of bulkier ligands favoring formation
of the usually less thermodynamically stable tetrahedral
complex.14 The effects on equilibrium by electronic factors
are also important.14a,j−l,p
Although the literature suggests that rotation around a
square-planar nickel(II) center is possible, only two classes of
isomerization have been mechanistically characterized by
studies of a large number of diamagnetic square-planar
complexes, (1) associative and (2) dissociative, with associative
mechanisms being more frequently reported.8,17 Associative
isomerization can proceed through two mechanisms: Berry
pseudorotation and consecutive displacement. These associa-
tive isomerizations initiate by coordination of a catalytic ligand
to metal to form a five-coordinate intermediate. This ligand can
be an added base, solvent, or any metal-coordinating group. In
the case of the Berry pseudorotation, the five-coordinate
intermediate isomerizes to a trigonal bipyramid in which
ligands can be isomerized, followed by dissociation of the
catalytic ligand. In the consecutive displacement mechanism,
coordination of the catalytic ligand is followed by either loss of
The rate of isomerization is also unaffected by the presence
of excess (n-Bu)4NCl or LiCl, which precludes an associative
mechanism with respect to the chloride ligand. This result is
not surprising, given the low dielectric constant of CD2Cl2,
which disfavors charge separation. A dissociative process is
ruled out by the fact that addition of 2 equiv of Tl(OTf) does
not alter the 1H NMR spectrum of complex 4. If dissociation of
the chloride ligand is occurring, the addition of thallium, which
2023
dx.doi.org/10.1021/om500173j | Organometallics 2014, 33, 2019−2026

Organometallics
Article
has strong affinity for halides, would sequester free chloride in
solution and alter the 1H NMR spectrum. Moreover, the rate of
isomerization at −42.0 °C was unaffected by the presence of 2
equiv of Tl(OTf), even after the solution was allowed to stand
at room temperature for 4 days. This shows that (a) the
structure and (b) the conformational dynamics of 4 are
unaffected by the presence of Tl(OTf), which is inconsistent
with chloride dissociation under these conditions.
phosphine crossover experiment shown in Figure 7 shows that,
under the isomerization conditions, the 31P NMR integrations
of 4 and PPh3 in solution match the portions added to the
sample. This provides evidence that there is not a large portion
of phosphine associated with an NMR-invisible paramagnetic
species. Furthermore, an EPR spectrum of 4 (X-band, 25 °C,
toluene) showed no signals.24 Moreover, whereas the para-
magnetic susceptibility of many nickel(II) complexes which
exhibit square-planar to tetrahedral equilibria can be measured
using the Evans method,14a,j−l we observe no paramagnetic
susceptibility of complex 4 even at high concentrations (127
mM, 25 °C; see the Supporting Information) using this
method.25
DFT studies confirm the presence of a high-energy triplet
intermediate on the potential energy surface for the rotational
exchange of the phosphine and chlorine ligands in 4. These
computational studies show that the enthalpic barrier for
rotation in the PMe3 analogue (ΔH⧧ = 11.1 kcal mol−1) of
nickel 4 is very close to that value experimentally measured for
nickel 4. The presence of a high-energy triplet intermediate
(ΔH = 8.8 kcal mol−1) was also corroborated by these studies.
On the basis of the forgoing evidence, we believe that the
internal rotation of 4 is the first documented case of a
unimolecular square-planar to square-planar rotation around a
late-metal center. We believe that this mechanism proceeds
through a high-energy paramagnetic tetrahedral (or pseudote-
trahedral) nickel(II) intermediate species. We propose,
however, that no ligand coordination or dissociation, including
agostic coordination of a (methyl)boron group, is requisite to
the mechanism.
Studies of the dependence of the rate of isomerization on [3]
are not possible because addition of excess 3 results in fast
formation of bis(borate) nickel complex 5. However, the
isomerization proceeds in the absence of excess 3; thus, an
associative process involving 3 is unlikely. Also, the ΔS⧧ value
for isomerization (0.8(5) eu) is inconsistent with ligand
association (which generally has ΔS⧧ = −10 to −15 eu).19
Likewise, the ΔS⧧ value is too small to be consistent with
ligand dissociation (which generally has ΔS⧧ = 10 to 15 eu).19
For example, the ΔS⧧ values for the dissociative substitution of
CO in Ni(CO)4 range from +8 to +13 eu, depending on
solvent.20 Moreover, the bond enthalpy of a Ni−N(pyridine)
bond (ca. 26 kcal mol−1)21 is much higher than the measured
ΔH⧧ value for the isomerization (12.2(1) kcal mol−1), which
indicates that the Ni−N bond is not broken during the
isomerization process. Furthermore, an isomerization involving
the dissociation of 3 (Scheme 3), which leads to a product
equivalent to a ring flip product, was considered. This process
was found to be a separate, slower isomerization pathway: ring
flip is >100-fold slower than rotation at a given temperature.
The ΔS⧧ for the ring flip (−4.2(7) eu) indicates a more
ordered transition state and thus argues against a dissociative
mechanism (i.e., the ring flip occurs without hemidissociation
of 3). If hemidissociation is not occurring at higher temper-
atures where the ring flip is observed (as high as 38.2 °C), then
it is unlikely that hemidissociation is occurring at temperatures
as low as −61.6 °C, where rotation is observed. Finally, the
presence of excess PPh3 should affect the rate of isomerization
if hemidissociation is occurring, but this is not observed. On the
basis of these observations, we believe that association or
dissociation of ligand 3 is not involved in the rotation
mechanism.
CONCLUSION
■
The novel dimethylbis(2-pyridyl)borate nickel complexes 4 and
6 were synthesized and characterized. The solution dynamics of
complex 4 were studied, and it was found to isomerize via two
different mechanisms: (a) a unimolecular square-planar to
square-planar rotation around the nickel center and (b) a
relatively slower ring flip. The activation parameters for each
isomerization pathway were measured using 1H NMR inversion
recovery experiments. The barrier for rotation (ΔH⧧ = 12.2(1)
kcal mol−1 and ΔS⧧ = 0.8(5) eu) is lower than the barrier for
the ring flip (ΔH⧧ = 15.0(2) kcal mol−1 and ΔS⧧ = −4.2(7)
eu), as expected. This study is the first to systematically show
(by excluding the associative and dissociative pathways) that a
180° rotation around a square-planar nickel center is a viable
mechanism of isomerization.
A further associative rotation mechanism that fits these
kinetic data is one that involves formation of an agostic Ni−HC
interaction22 to the endo (methyl)boron group to generate a
transient five-coordinate nickel center. We believe that such an
agostic interaction is not forming for three key reasons. (1) The
1H NMR peak width of the endo-positioned (methyl)boron
group in 4 is invariant over a range of 75 °C (6.9 1.0 Hz,
1
Figure 3). (2) The H chemical shift of the same is invariant
EXPERIMENTAL SECTION
■
over the same temperature range: 2.22 0.09 ppm. (3) The
solid-state Ni−H distance is 2.43 Å, which is longer than those
observed for agostic interactions between this ligand and
ruthenium(II) (1.72 Å)4a or platinum(IV) (2.02 Å).11b
Moreover, the agostic Ni−H distance in a (NacNac)Ni(κ2-
C2H5) complex is 1.66 Å,23 which is far smaller than our
observed Ni−H distance (NacNac = N,N′-(2,6-
(CH3)2C6H3)2CH3C(N)CHC(N)CH3 anion). Whereas none
of our observations are consistent with known agostic
complexes involving borate 3, we predict that no agostic
intermediate is involved in the unimolecular rotation of 4.
As corroborated by DFT calculations, the unimolecular
rotation of 4 involves the intermediacy of a paramagnetic,
tetrahedral (or pseudotetrahedral) complex, but this species is
not present in an observable concentration. For example, the
General Procedures. All air- and water-sensitive procedures were
carried out either in a Vacuum Atmospheres glovebox under nitrogen
(2−10 ppm O2 for all manipulations) or using standard Schlenk
techniques under nitrogen. Deuterated NMR solvents were purchased
from Cambridge Isotopes Laboratories. Dichloromethane, ethyl ether,
and hexanes were purchased from VWR and dried in a J. C. Meyer
solvent purification system with alumina/copper(II) oxide columns,
benzene (EMD) was dried by distillation over sodium−benzophenone
ketyl, and bis(triphenylphosphine)nickel(II) chloride and nickel(II)
acetylacetonate were purchased from Strem and used as received.
NMR spectra were recorded on a Varian VNMRS 500 or VNMRS
600 spectrometer. All chemical shifts are reported in units of ppm and
referenced to the residual 1H or 13C solvent peak and line-listed
according to (s) singlet, (bs) broad singlet, (d) doublet, (t) triplet,
(dd) doublet of doublets, etc. 13C spectra are delimited by carbon
peaks, not carbon count. 11B and 31P chemical shifts are referenced to
2024
dx.doi.org/10.1021/om500173j | Organometallics 2014, 33, 2019−2026

Organometallics
Article
the lock channel. Air-sensitive NMR spectra were taken in 8 in. J.
Young tubes (Wilmad or Norell) with Teflon valve plugs. Inversion
recovery kinetics data were fitted using CIFIT 2.0 by Alex Bain.7b
Temperatures for NMR experiments were calibrated using an external
methanol standard. Errors in Eyring parameters are calculated using
the equations derived by Girolami et al.13
MALDI mass spectra were obtained on an Applied Biosystems
Voyager spectrometer using the evaporated drop method on a coated
96-well plate. The matrix was anthracene. In a standard preparation,
ca. 1 mg of analyte and ca. 10 mg of matrix were dissolved in dry
benzene and spotted on the plate with a glass capillary. Infrared
spectra were recorded on a Bruker OPUS FTIR spectrometer. High-
resolution ESI mass spectra were recorded at the University of
California, Riverside. CHN elemental analyses were collected at the
University of Illinois at Urbana−Champaign at the School of Chemical
Sciences Microanalysis Laboratory.
Nickel Complex 4. In the drybox under nitrogen, (PPh3)2NiCl2
(65.4 mg, 0.10 mmol) was dissolved in 10 mL of dry dichloromethane
in a dry vial containing a Teflon stir bar. In another vial,
[(py)2BMe2]Na9 (22.0 mg, 0.10 mmol) was dissolved in 5 mL of
dry dichloromethane and added slowly to the (PPh3)2NiCl2 solution.
The [(py)2BMe2]Na vial was rinsed with 5 mL of dichloromethane
and added to the (PPh3)2NiCl2 solution. The dark green solution
turned brown and then orange upon addition of [(py)2BMe2]Na. The
solution was stirred for 2 h and then filtered. The solvent was removed
under reduced pressure. Dry diethyl ether was added to the residue,
and the vial was sonicated briefly. The residue was then cooled using a
cold well, and cold, dry hexanes was added. The suspension was
filtered and washed with cold, dry ether. The suspension was dissolved
with cold, dry benzene. The benzene was then removed by
lyophilization to yield 39.5 mg (0.071 mmol, 71%) of [(py)2BMe2]-
Ni(PPh3)Cl as a pale pink solid. Crystallization from dichloromethane
and hexanes produced crystals suitable for X-ray crystallographic
analysis.
C17H22BN2O2Ni+ [M]+ 354.1050. FT-IR (thin film/cm−1): ν 2891.92,
2821.81, 1586.41, 1532.39, 1388.63, 1289.47, 122.57, 1159.73,
1015.71, 934.76, 788.67, 753.86, 738.46. HR-MS (+ESI): m/z
355.1129, calcd for C17H22BN2O2Ni+ [MH]+ 355.1122.
Computational Methodology and Modeling Details. The
geometry optimizations reported here were performed with the DFT
gradient-corrected correlation functional PBE, as implemented by the
Gaussian 09 program package.26 The Ni atom was described by a
Stuttgart−Dresden effective core potential (ecp) and an SDD basis set,
while the 6-31G(d′) basis set was employed for the remaining Cl, P,
N, C, B, and H atoms.
All structures were fully optimized and evaluated for the correct
number of imaginary frequencies through calculation of the vibrational
frequencies, using the analytical Hessian (positive eigenvalues ground-
state minima and one negative eigenvalue for a transition state). The
computed frequencies were used to make zero-point and thermal
corrections to the electronic energies, and the reported enthalpies are
quoted in kcal/mol relative to singlet species A. The computed triplet
species B and TSBB′ revealed no significant spin contamination from
higher order spin states. The geometry-optimized structures have been
drawn with the JIMP2 molecular visualization and manipulation
program.27
ASSOCIATED CONTENT
* Supporting Information
■
S
Figures, tables, text, CIF, and .mol files giving graphical NMR
data, NMR inversion recovery data, CIFIT plots for
determination of rate constants, Eyring plots for determination
of activation parameters, magnetic susceptibility data, crystallo-
graphic data for complexes 4 and 6, complete ref 26, and
atomic coordinates of the optimized stationary points A and B
and the transition state TSBB′. This material is available free of
charge via the Internet at http://pubs.acs.org. CCDC 984289
(4) and 984290 (6) contain supplementary crystallographic
data for these compounds.
1H NMR (CD2Cl2, 600 MHz, 25 °C): δ 8.55 (br s, 2H), 7.77−7.61
(m, 6H), 7.44 (t, J = 7.3 Hz, 3H), 7.40 (d, J = 7.6 Hz, 2H), 7.33 (t, J =
7.6 Hz, 6H), 7.16 (br s, 2H), 6.41 (br s, 2H), 2.22 (s, 3H), 0.32 (s,
3H). 1H NMR (CD2Cl2, 600 MHz, −40 °C): δ 8.82 (s, 1H), 8.12 (s,
1H), 8.02−7.02 (m, 18H), 6.94 (s, 1H), 6.86 (s, 1H), 5.92 (s, 1H),
2.15 (s, 3H), 0.26 (s, 3H). 13C NMR (CD2Cl2, 150 MHz, 25 °C): δ
189.72 (br), 151.89, 135.07, 134.86, 131.08, 129.91, 128.91, 127.00,
119.75, 19.51 (br), 9.48 (br). 11B NMR (CD2Cl2, 192 MHz): δ
−12.68. 31P NMR (CD2Cl2, 243 MHz, −40 °C): δ 12.42. MALDI: m/
z 552.8941, calcd for C30H29BClNiN2P+ [M]+ 552.1203. FT-IR (thin
film/cm−1): ν 3067.76, 2912.82, 2823.99, 1963.28, 1896.13, 1817.39,
1592.78, 1553.13, 149.62, 1435.29, 1290.62, 1217.84, 1093.77,
1012.64, 744.01. Anal. Calcd for C30H29BClNiN2P: C, 65.1; H, 5.28;
N, 5.06. Found: C, 65.04; H, 5.47; N, 5.28.
AUTHOR INFORMATION
Corresponding Author
*E-mail for T.J.W.: travisw@usc.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was sponsored by the NSF (CHE-1054910), the
University of Southern California, the Loker Hydrocarbon
Research Institute, the Hydrocarbon Research Foundation, the
Robert A. Welch Foundation (Grant B-1093-MGR), and the
Anton Burg Foundation. We thank the NSF (DBI-0821671,
CHE-0741936, CHE-0840366, CHE-1048807) and the NIH
(S10 RR25432) for analytical instrumentation and computa-
tional facilities. R.W.F. acknowledges NSF-REU support (CHE-
1156836). We are grateful to A. Kershaw (NMR), Dr. R.
Haiges (X-ray crystallography), X. Wu (EPR), and Dr. S. Zhang
(EPR) for technical assistance. M.G.R. thanks Prof. Michael B.
Hall (TAMU) for providing us a copy of his JIMP2 program,
which was used to prepare the geometry-optimized structures
reported here, and Dr. David A. Hrovat for helpful DFT
discussions.
Nickel Complex 6. In the drybox under nitrogen, Ni(acac)2 (64.2
mg, 0.25 mmol) was suspended in 10 mL of dry dichloromethane in a
dry vial containing a Teflon stir bar. In another dry vial,
[(py)2BMe2]Na9 (22.0 mg, 0.10 mmol) was dissolved in 5 mL of
dichloromethane and added slowly to the solution of Ni(acac)2.
Another 5 mL of dichloromethane was used to rinse the vial and added
slowly to the solution of Ni(acac)2. The green solution turned orange
on addition of [(py)2BMe2]Na. The solution was stirred for 2 h at
room temperature and then filtered through Celite. The solvent was
removed in vacuo. Dry hexanes (10 mL) was added to the residue, and
the vial was treated with sonication briefly. The suspension was then
cooled using a cold well, filtered, and washed with cold, dry hexanes.
The solid was dried under vacuum to yield 23.7 mg (0.067 mmol,
67%) of [(py)2BMe2]Ni(acac) as an orange solid. Crystallization from
dichloromethane and hexanes produced crystals suitable for X-ray
crystallographic analysis.
1H NMR (CD2Cl2, 600 MHz): δ 8.30 (d, J = 5.6 Hz, 2H), 7.40 (d, J
= 7.5 Hz, 2H), 7.34 (t, J = 7.3 Hz, 2H), 6.83 (t, J = 6.3 Hz, 2H), 5.55
(s, 1H), 2.27 (br s, 2H), 1.88 (s, 6H), 0.27 (br s, 3H). 13C NMR
(CD2Cl2, 150 MHz): δ 189.81 (q, J = 48 Hz) 188.11, 149.89, 135.23,
125.93, 119.71, 102.04, 26.09, 18.93 (br), 8.93 (br). 11B NMR
(CD2Cl2, 192 MHz): δ −12.68. MALDI: m/z 354.2477, calcd for
REFERENCES
■
(1) (a) Jolly, P. W. Nickel-Catalyzed Oligomerization of Alkenes and
Related Reactions. In Comprehensive Organometallic Chemistry;
Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon: New
York, 1982; Vol. 8, pp 615−712. (b) Jolly, P. W.; Wilke, G. The
Organic Chemistry of Nickel; Academic: New York, 1974, 1975; Vols. 1
2025
dx.doi.org/10.1021/om500173j | Organometallics 2014, 33, 2019−2026

Organometallics
Article
and 2. (c) Heimbach, P.; Jolly, P. W. Wilke, G. In Advances in
Organometallic Chemistry; Stone, F. G. A., West, R., Eds.; Academic:
New York, 1970; Vol. 8, p 29. (d) Reed, H. W. B. J. Chem. Soc. 1954,
1931−1941. (e) Yamamoto, Y. Chem. Rev. 2012, 112, 4736−4769.
(2) (a) Modern Organonickel Chemistry; Tamaru, Y., Ed.; Wiley-
VCH: Weinheim, Germany, 2005. (b) Oblinger, E.; Montgomery, J. J.
Am. Chem. Soc. 1997, 119, 9065−9066. (c) Kimura, M.; Ezoe, A.;
Shibata, K; Tamaru, Y. J. Am. Chem. Soc. 1998, 120, 4033−4034.
(d) Takimoto, M.; Hiraga, Y.; Sato, Y.; Mori, M. Tetrahedron Lett.
1998, 4543−4546. (e) Kimura, M.; Fujimatsu, H.; Ezoe, A.; Shibata,
K; Shimizu, M.; Matsumoto, S.; Tamaru, Y. Angew. Chem., Int. Ed.
1999, 38, 397−400. (f) Kang, S.; Yoon, S. Chem. Commun. 2002,
2634−2635. (g) Miller, K.; Huang, W.; Jamison, T. F. J. Am. Chem.
Soc. 2003, 125, 3442−3443. (h) Montgomery, J. Angew. Chem., Int. Ed.
2004, 43, 3890−3908. (i) Ogoshi, S.; Oka, M.; Kurosawa, H. J. Am.
Chem. Soc. 2004, 126, 11802−11803. (j) Ogoshi, S.; Ueta, M.; Arai, T.;
Kurosawa, H. J. Am. Chem. Soc. 2005, 127, 12810−12811.
(3) (a) Chakraborty, S.; Zhang, J.; Krause, J. A.; Guan, H. J. Am.
Chem. Soc. 2010, 132, 8872−8873. (b) Chakraborty, S.; Patel, Y. J.;
Krause, J. A.; Guan, H. Polyhedron 2012, 32, 30−34. (c) Chakraborty,
S.; Zhang, J.; Patel, Y. J.; Krause, J. A.; Guan, H. Inorg. Chem. 2013, 52,
37−47. (d) Schmeier, T. J.; Hazari, N.; Incarvito, C. D.; Raskatov, J. A.
Chem. Commun. 2011, 47, 1824−1826. (e) Wu, J.; Hazari, N.;
Incarvito, C. D. Organometallics 2013, 32, 300−308. (f) Vogt, M.;
Rivada-Wheelaghan, O.; Iron, M. A.; Leitus, G.; Diskin-Posner, Y.;
Shimon, L. J. W.; Ben-David, Y.; Milstein, D. Organometallics 2013, 32,
300−308.
(4) (a) Conley, B.; Williams, T. J. J. Am. Chem. Soc. 2010, 132, 1764−
1765. (b) Conley, B. L.; Guess, D.; Williams, T. J. J. Am. Chem. Soc.
2011, 133, 14212−14215. (c) Lu, Z.; Williams, T. J. Chem. Commun.
2014, DOI: 10.1039/C3CC47384H.
(5) (a) Basolo, F.; Pearson, R. G. Mechanisms of Inorganic Reactions,
2nd ed.; Wiley: New York, 1958; pp 423−435. (b) Redfield, D. A.;
Nelson, J. H. J. Am. Chem. Soc. 1974, 96, 6219−6220.
(6) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry: A
Comprehensive Text, 4th ed.; Wiley: New York, 1980; pp 791−794.
(7) (a) Williams, T. J.; Kershaw, A. D.; Li, V.; Wu, X. J. Chem. Educ.
2011, 88, 665−669. (b) Bain, A. D.; Cramer, J. A. J. Magn. Reson.
1996, 118 A, 21−27.
R. E.; O’Connor, M. J.; Holm, R. H. J. Am. Chem. Soc. 1967, 89, 6104−
6113. (h) Everett, G. W.; Holm, R. H. Inorg. Chem. 1968, 7, 776−785.
(i) Pignolet, L. H.; Horrocks, W. E., Jr. J. Am. Chem. Soc. 1968, 90,
922−926. (j) Gerlach, D. H.; Holm, R. H. J. Am. Chem. Soc. 1969, 91,
3457−3467. (k) La Mar, G. N.; Sherman, E. O. J. Am. Chem. Soc.
1970, 92, 2691−2699. (l) Que, L., Jr.; Pignolet, L. H. Inorg. Chem.
1973, 12, 156−163. (m) Knoch, R.; Elias, H.; Paulus, H. Inorg. Chem.
1995, 34, 4032−4040. (n) Simon-Manso, E.; Valderrama, M.; Boys, D.
Inorg. Chem. 2001, 40, 3647−3649. (o) Brusko, V. V.; Rakhmatullin,
A. I.; Shtyrlin, V. G.; Krivolapov, D. B.; Litvinov, I. A.; Zabirov, N. G.
Polyhedron 2006, 25, 1433−1440. (p) Mori, Y.; Shirase, H.; Fukuda, Y.
Bull. Chem. Soc. Jpn. 2008, 81, 1108−1115. (q) Starikov, A. G.;
Minyaev, R. M.; Minkin, V. I. Chem. Phys. Lett. 2008, 459, 27−32.
(15) Eaton, D. R. J. Am. Chem. Soc. 1968, 90, 4272−4275.
(16) Halevi, E. A.; Knorr, R. Angew. Chem., Int. Ed. 1982, 21, 288−
289.
(17) For a few examples, see: (a) Albright, T. A.; Hoffmann, R.;
Thibeault, J. C.; Thorn, D. L. J. Am. Chem. Soc. 1979, 101, 3801−3812.
(b) Redfield, D. A.; Nelson, J. H. Inorg. Chem. 1973, 12, 15−19.
(c) Romeo, R.; Plutino, M. R.; Elding, L. I. Inorg. Chem. 1997, 36,
5909−5916. (d) Bartolome, C.; Espinet, P.; Martin-Alvarez, J. M.;
Villafane, F. Eur. J. Inorg. Chem. 2004, 2326−2337. (e) Yang, F.-Z.;
Wang, Y.-H.; Chang, M.-C.; Yu, K.-H.; Huang, S.-L.; Liu, Y.-H.; Wang,
Y.; Liu, S.-T.; Chen, J.-T. Inorg. Chem. 2009, 48, 7639−7644.
(18) Redfield, D. A.; Nelson, J. H. J. Am. Chem. Soc. 1974, 96, 6219−
6220.
(19) Crabtree, R. H. The Organometallic Chemistry of the Transition
Metals, 4th ed.; Wiley-Interscience: Hoboken, NJ, 2005; pp 104−112.
(20) 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; p 247.
(21) (a) Kappes, M. M.; Staley, R. H. J. Am. Chem. Soc. 1982, 104,
1813−1819. (b) Hettich, R. L.; Jackson, T. C.; Stanko, E. M.; Freiser,
B. S. J. Am. Chem. Soc. 1986, 108, 5086−5093. (c) Jacobson, D. B.;
Freiser, B. S. J. Am. Chem. Soc. 1983, 105, 7492−7500.
(22) Brookhart, M; Green, M. L. H.; Parkin, G. Proc. Natl. Acad. Sci.
U.S.A. 2007, 104, 6908−6914.
(23) Kogut, E.; Zeller, A.; Warren, T. H.; Strassner, T. J. Am. Chem.
Soc. 2004, 126, 11984−11994.
(24) This does not necessarily discount the presence of paramagnetic
nickel(II), because standard EPR spectroscopy at conventional
microwave frequencies can be ineffective for detecting transition
metals with integer spin ground states: Krzystek, J.; Park, J.-H.; Meisel,
M. W.; Hitchman, M. A.; Stratemeier, H.; Brunel, L.-C.; Telser, J.
Inorg. Chem. 2002, 41, 4478−4487.
(25) (a) Schubert, E. M. J. Chem. Educ. 1992, 69, 62. (b) Evans, D. F.
J. Chem. Soc. 1959, 2003−2005.
(26) Frisch, M. J. , et al. Gaussian 09, Revision A.02; Gaussian, Inc.,
Wallingford, CT, 2009.
(27) JIMP2, version 0.091, a free program for the visualization and
manipulation of molecules: (a) Hall, M. B.; Fenske, R. F. Inorg. Chem.
1972, 11, 768. (b) Manson, J.; Webster, C. E.; Hall, M. B. Texas A&M
University, College Station, TX, 2006; http://www.chem.tamu.edu/
jimp2/index.html.
(8) For a review, see: Anderson, G. K.; Cross, R. J. Chem. Soc. Rev.
1980, 9, 185−215.
(9) Hodgkins, T. G.; Powell, D. R. Inorg. Chem. 1996, 35, 2140−
2148.
(10) Gutierrez, E.; Hudson, S. A.; Monge, A.; Nicasio, M. C.;
Paneque, M.; Ruiz, C. J. Organomet. Chem. 1998, 551, 215−227.
(11) (a) Khaskin, E.; Zavalij, P. Y.; Vedernikov, A. N. J. Am. Chem.
Soc. 2006, 128, 13054−13055. (b) Khaskin, E.; Zavalij, P. Y.;
Vedernikov, A. N. Angew. Chem., Int. Ed. 2007, 46, 6309−6312.
(c) Khaskin, E.; Zavalij, P. Y.; Vedernikov, A. N. J. Am. Chem. Soc.
2008, 130, 10088−10089. (d) Khaskin, E.; Lew, D. L.; Pal, S.;
Vedernikov, A. N. Chem. Commun. 2009, 6270−6272.
(12) Because CD2Cl2 boils at 39 °C, we were only able to obtain
rates from 13.8 to 38.2 °C (a range of 25.1 °C) in this solvent. To
obtain reliable values for ΔS⧧, a range of at least 40 °C is
recommended. The activation parameters were thus measured in
benzene from rates obtained over a wider temperature range of 37.2
°C (7.0 to 44.2 °C), and similar values for the activation parameters
were obtained (ΔH⧧ = 15.5(3) kcal mol−1 and ΔS⧧ = −3.3(11) eu; see
the Supporting Information).
(13) Errors in the activation parameters were calculated using
previously derived equations: Morse, P. M.; Spencer, M. D.; Wilson, S.
R.; Girolami, G. S. Organometallics 1994, 13, 1646−1655.
(14) (a) Eaton, D. R.; Phillips, W. D.; Caldwell, D. J. J. Am. Chem.
Soc. 1963, 85, 397−406. (b) Sacconi, L.; Paoletti, P.; Ciampolini, M. J.
Am. Chem. Soc. 1963, 85, 411−416. (c) Holm, R. H.; Chakravorty, A.;
Dudek, G. O. J. Am. Chem. Soc. 1964, 86, 379−387. (d) Chakravorty,
A.; Holm, R. H. J. Am. Chem. Soc. 1964, 86, 3999−4004. (e) Everett,
G. W., Jr.; Holm, R. H. J. Am. Chem. Soc. 1965, 87, 2117−2128.
(f) Shupack, S. I. J. Inorg. Nucl. Chem. 1966, 28, 2418−2419. (g) Ernst,
2026
dx.doi.org/10.1021/om500173j | Organometallics 2014, 33, 2019−2026