Insights into the stability and structures of phosphine-boranes and their α-metalated derivatives

Article abstract of DOI:10.1021/om5005995

The reaction between iPr₂PCl and Ph₂P(BH₃)CH₂Li gives the mixed phosphine/phosphine-borane Ph₂P(BH₃)CH₂PiPr₂ (1a) in good yield. Thermolysis of 1a leads to borane migration and the formation of Ph₂PCH₂P(BH₃)iPr₂ (2a) along with small amounts of Ph₂P(BH₃)CH₂P(BH₃)iPr₂ (3a) and Ph₂PCH₂PiPr₂ (4a). Compound 3a may be synthesized directly from the reaction of 1a with BH₃·SMe₂, while 4a can be prepared cleanly by heating 1a in methanol under reflux. Kinetic studies on the conversion of 1a to 2a reveal the reaction to be apparently first order in 1a, suggesting a dissociative process, and yield the activation parameters δH^∥ = 63 ± 8 kJ mol^-1, δS^∥ = -145 ± 24 J K^-1 mol^-1, and δG^∥ = 106 ± 8 kJ mol^-1, the negative entropy of activation conversely suggesting an associative process. DFT studies suggest that concerted migration of borane within a molecule of 1a is disfavored, but that both the dissociative and associative mechanisms for borane migration operate simultaneously. Metalation of 1a-4a with nBuLi in the presence of tmeda gives the complexes [{Ph₂P(BH₃)}CHPiPr₂]Li(tmeda) (1b), [Ph₂PCH{P(BH₃)iPr₂}]Li(tmeda) (2b), [{Ph₂P(BH₃)}CH{P(BH₃)iPr₂}]Li(tmeda) (3b), and [Ph₂PCHPiPr₂]Li(tmeda) (4b), respectively, which adopt similar structures in the solid state. Analysis of the crystal structures suggests that the phosphine-borane groups stabilize the adjacent charge to a greater extent than the phosphine groups. This is supported by DFT calculations, which show that the greatest delocalization of negative charge from the carbanion is into the P-C(Ph) or P-C(Pr) σ-orbitals of the phosphine-borane substituents.

Full text of DOI:10.1021/om5005995

Article
pubs.acs.org/Organometallics
Insights into the Stability and Structures of Phosphine-Boranes and
Their α‑Metalated Derivatives
Keith Izod,* Corinne Wills,* Emma Anderson, Ross W. Harrington, and Michael R. Probert
Main Group Chemistry Laboratories, School of Chemistry, Newcastle University, Bedson Building, Newcastle upon Tyne, NE1 7RU,
U.K.
*
S Supporting Information
ABSTRACT: The reaction between iPr PCl and Ph P(BH )CH Li gives the mixed
2
2
3
2
phosphine/phosphine-borane Ph P(BH )CH PiPr (1a) in good yield. Thermolysis of 1a
2
3
2
2
leads to borane migration and the formation of Ph PCH P(BH )iPr (2a) along with small
2
2
3
2
amounts of Ph P(BH )CH P(BH )iPr (3a) and Ph PCH PiPr (4a). Compound 3a may be
2
3
2
3
2
2
2
2
synthesized directly from the reaction of 1a with BH ·SMe , while 4a can be prepared cleanly
3
2
by heating 1a in methanol under reflux. Kinetic studies on the conversion of 1a to 2a reveal
the reaction to be apparently first order in 1a, suggesting a dissociative process, and yield the
⧧
−1
⧧
−1
−1
⧧
activation parameters ΔH = 63 ± 8 kJ mol , ΔS = −145 ± 24 J K mol , and ΔG = 106
−1
±
8 kJ mol , the negative entropy of activation conversely suggesting an associative process.
DFT studies suggest that concerted migration of borane within a molecule of 1a is disfavored,
but that both the dissociative and associative mechanisms for borane migration operate
simultaneously. Metalation of 1a−4a with nBuLi in the presence of tmeda gives the complexes [{Ph P(BH )}CHPiPr ]Li-
2
3
2
(
tmeda) (1b), [Ph PCH{P(BH )iPr }]Li(tmeda) (2b), [{Ph P(BH )}CH{P(BH )iPr }]Li(tmeda) (3b), and [Ph PCHPiPr ]-
2 3 2 2 3 3 2 2 2
Li(tmeda) (4b), respectively, which adopt similar structures in the solid state. Analysis of the crystal structures suggests that the
phosphine-borane groups stabilize the adjacent charge to a greater extent than the phosphine groups. This is supported by DFT
calculations, which show that the greatest delocalization of negative charge from the carbanion is into the P−C(Ph) or P−C(Pr)
σ*-orbitals of the phosphine-borane substituents.
3
INTRODUCTION
methanol at elevated temperatures. For borane adducts of
electron-rich phosphines, which are not amenable to amine- or
alcohol-mediated deprotection, P−B cleavage is best achieved
■
Phosphine-borane adducts, R P-BH , rank among the most
3
3
useful precursor materials in organophosphorus chemistry and
are key intermediates in the synthesis of a wide array of both
chiral and achiral (poly)phosphines, many of which are
unsurpassed as ligands for the support of transition metal
by treatment of the adduct with strong acids such as HBF or
4
4
HOTf.
Perhaps of equal importance is the ability of phosphine-
borane groups to activate an α-CH group toward deprotona-
tion, to give the corresponding phosphine-borane-stabilized
1
catalysts. These adducts are characterized by several key
features: (i) the P−B bond is conveniently formed by the
addition of commercially available BH ·THF or BH ·SMe to
−
1
carbanion (PBC), [R P(BH )CR′ ] . This permits the
2
3
2
3
3
2
convenient elaboration of phosphine-borane adducts, providing
straightforward access to an impressive diversity of function-
alized mono- and diphosphines. Typically, such carbanions are
generated via treatment of the phosphine-borane adduct with
an organolithium reagent. In the vast majority of cases these
phosphine-borane-stabilized carbanion complexes are generated
and used in situ, and, until recently, little was known about the
structures of and bonding in these species. This is particularly
surprising given the isoelectronic and isosteric relationship
between Me P(BH ) and Me Si groups, the latter of which
the corresponding phosphine; (ii) the P−B bond is stable
under a wide variety of reaction conditions, and so the
phosphorus centers are effectively protected from oxidation;
(iii) formation of the adduct decreases the hydridic character of
the B−H hydrogen atoms, and so these compounds are
relatively inert; (iv) the formation of the P−B bond is
frequently reversible, and, for P-chiral adducts, cleavage of the
borane group proceeds with retention of configuration at
phosphorus; and (v) CH protons α to the phosphorus center
are activated toward deprotonation, affording highly stabilized
carbanions.
2
3
3
have played a fundamental role in the development of main
group, transition metal, and lanthanide organometallic
The straightforward formation and cleavage of the P−B bond
under relatively mild conditions is one of the most useful of
these features. For borane adducts of electron-poor phosphines,
P−B cleavage may be achieved by treatment of the phosphine-
borane with a large excess of amine, which acts to displace the
borane group and form the corresponding amine-borane
chemistry. It is noteworthy, however, that while Me P(BH )
2
3
and Me Si groups bear a strong resemblance to each other, the
3
former also afford the opportunity for the formation of
Received: June 5, 2014
2
adduct, or by alcoholysis of the adduct with ethanol or
©
XXXX American Chemical Society
A
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Article
moderately strong B−H···M contacts by virtue of the residual
phosphine/phosphine-borane Ph PCH P(BH )iPr (2a); the
2
2
3
2
hydridic character of the H atoms in the BH group. Such
identity of 2a was confirmed by multinuclear NMR spectros-
3
1
31
contacts are found in almost all of the small number of known
main group, lanthanide, and transition metal complexes of
PBCs, and these potentially hemilabile interactions afford a
mechanism by which electron density may be donated to the
copy and selective H{ P} NMR experiments. This rearrange-
ment is accelerated at higher temperatures: heating a solution
of 1a in toluene under reflux for 12 h gives 2a as a colorless oil,
after removal of solvent, in excellent yield. Below 0 °C
compound 1a is stable in the solid state and shows no sign of
isomerization to 2a even after several months.
5
−7
metal centers.
This has proven to be a unique feature of
these ligands, permitting the isolation of otherwise inaccessible
compounds; for example, we have shown that B−H···E
contacts significantly stabilize electron-deficient Sn(II) and
Pb(II) centers, enabling the isolation of a rich variety of
The clean isolation of 1a provides an excellent opportunity
to study the migration of a borane group between two
electronically different tertiary phosphine centers in the same
molecule. Perhaps surprisingly, given the ubiquity of BH3
adducts in phosphine chemistry, previous mechanistic studies
have focused on the cleavage of P−B bonds in adducts between
tertiary phosphines and substituted (e.g., alkyl, aryl, or alkoxy)
monomeric dialkylstannylenes and -plumbylenes R E (E = Sn,
2
7
Pb).
We now report the synthesis of a new mixed phosphine/
phosphine-borane and studies on the migration of the borane
group between the two phosphorus centers in this compound,
including a detailed study of the kinetics of this rearrangement.
We also report the metalation of both of these compounds and
the corresponding bis(phosphine) and bis(phosphine-borane)
and, with reference to accompanying DFT studies, comment
on the relative charge-stabilizing capacity of the two
phosphorus centers in each case.
8
−11
boranes;
to the best of our knowledge there has been no
prior investigation of the mechanism by which R P−BH bonds
3
3
are cleaved by Lewis bases. This is possibly a consequence of
the weaker P−B bonds, and hence more rapid exchange
processes, in phosphine-organoborane adducts, which allow for
straightforward investigation of exchange phenomena by
variable-temperature NMR studies.
For amine-mediated P−B cleavage both dissociative (S 1-
N
RESULTS AND DISCUSSION
Phosphine-Boranes: Synthesis and Mechanistic As-
pects. The low-temperature reaction between iPr PCl and one
■
type) and associative (S 2-type) mechanisms have been
N
proposed, the adopted mechanism appearing to depend on
the substituents at the B center. For example, variable-
2
equivalent of in situ generated Ph P(BH )CH Li in THF gives
1
2
3
2
temperature H NMR studies on R P−BMe and R P−
3
3
3
the mixed phosphine/phosphine-borane Ph P(BH )CH PiPr
1a) as a colorless, crystalline solid in excellent yield (Scheme
2
3
2
2
BPhMe [R P = Me P, PhMe P, tBuMe P, Me (Me N)P]
2 3 3 2 2 2 2
(
1
suggest a dissociative mechanism for exchange between these
). Since the iPr P moiety in this compound is more electron-
8
2
adducts and an excess of either donor or acceptor. In contrast,
the displacement of PPh from the B-chiral adduct Ph P-
3
3
Scheme 1. Syntheses of 1a−4a
BH(CN)(Ipc) with PPhMe2 appears to proceed via a
dissociative mechanism in THF, but an associative mechanism
9
in C D [Ipc = monoisopinocampheyl]. Theoretical calcu-
6
6
lations on the transfer of BH between various Lewis bases
3
indicate that the associative and dissociative processes may have
almost identical free energies of activation, such that both
10
mechanisms may operate in a single reaction.
We find that heating a solution of 1a in toluene to 80 °C
3
1
1
results in a gradual decrease in the intensity of the P{ H}
NMR signals due to 1a, accompanied by the appearance of a
sharp doublet at −26.7 (J = 38 Hz) and a broad multiplet at
PP
3
5.5 ppm, due to the PPh and PiPr centers of the borane-
2 2
exchange product 2a (Figure 1). In addition, after a short
induction period, additional, low-intensity, broad multiplets
rich than the Ph P moiety, there is the potential for migration
2
of the borane group from the latter to the former center during
1
1
1
31
1
this reaction. However, B{ H} and P{ H} NMR spectra of
the crude reaction solution confirm that 1a is the sole
phosphorus-containing product under these conditions. The
3
1
1
P{ H} NMR spectrum of 1a consists of a broad multiplet at
1
6.6 ppm and a sharp doublet at −8.3 ppm (J = 73 Hz),
PP
which we assign to the phosphine-borane and tertiary
phosphine centers, respectively. Selective decoupling of the
3
1
31
P nuclei resonating at −8.3 ppm leads to loss of P coupling
1
in the H NMR signal due to the iPr groups, while selective
31
decoupling of the P nuclei resonating at 16.6 ppm leads to
loss of 31P coupling in the aromatic region of the H NMR
spectrum, confirming that the borane group remains attached
1
to the Ph P center in 1a.
2
Although 1a is isolated cleanly through the above procedure,
the borane group slowly migrates from the Ph P to the more
2
electron-rich iPr P center over a period of several weeks in
toluene solution at room temperature, to give the alternative
2
Figure 1. 31P{ H} NMR spectra of 1a in d -toluene at 80 °C over 8 h.
1
8
B
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Scheme 2. Possible Pathways for the Migration of BH between the Two Phosphorus Centers
3
−
1
12
appear at 12.6 and 40.8 ppm, along with a low-intensity, sharp
previously been calculated as 166.5 kJ mol ), while the
⧧
pair of doublets at −3.0 and −18.6 ppm (J = 123 Hz), due to
strongly negative value of ΔS is inconsistent with a dissociative
process. Rather, these values suggest either an intramolecular
PP
the bis(phosphine-borane) Ph P(BH )CH P(BH )iPr (3a)
2
3
2
3
2
and the bis(phosphine) Ph PCH PiPr (4a), respectively; the
migration of the BH group between the Ph P and iPr P
2
2
2
3
2
2
identities of 3a and 4a were confirmed by comparison with data
from deliberately prepared samples of these compounds (see
below). At 80 °C the concentrations of 3a and 4a reach a
maximum of 7.8% and 6.1%, respectively, after 2 h, before
gradually decreasing to 5.7% and 3.4%, respectively, after 8 h.
Small amounts (<1%) of 3a and 4a remained in samples that
had been heated at this temperature for several days.
moieties, via a four-membered transition state (1intra), or an
intermolecular migration of BH between two molecules of 1a
3
(via transition state 1inter, pathway c, Scheme 2). However, the
former does not explain the formation of 3a and 4a during the
rearrangement process, while the latter is inconsistent with the
first-order dependence of the reaction on the concentration of
1a.
The rearrangement of 1a to 2a was monitored by 31P{ H}
NMR spectroscopy at 5 °C intervals between 60 and 90 °C.
The kinetic data obtained indicate that the reaction exhibits an
apparent first-order dependence on the concentration of 1a;
consistent with this, the rate of consumption of 1a exhibits a
linear relationship with the concentration of this compound
1
In an attempt to resolve this dichotomy, we have undertaken
a DFT study of the rearrangement of 1a to 2a. A brief
13
benchmarking exercise showed that the wB97XD functional,
which explicitly includes mid- to long-range dispersive
interactions, outperformed both the highly parametrized
14
15
M06-2X and the ever-popular B3LYP hybrid functionals
in generating an optimized geometry for 1a that correlated with
the data obtained by X-ray crystallography (see Supporting
Information). Optimizations and frequency calculations were,
therefore, conducted at the wB97XD/6-311+G(d,p) level of
(see Supporting Information). However, the formation of 2a,
3
a, and 4a does not obey simple first- or second-order kinetics.
The apparent first-order dependence of the migration of BH3
on 1a suggests that the rate-determining step of the
rearrangement of 1a to 2a is a unimolecular process and is
16
theory with solvation by toluene included implicitly using the
IEF polarizable continuum model (see Experimental Section
consistent with either (i) an S 1-type process involving
N
17
dissociation of BH from the PPh group (pathway a, Scheme
for further information).
3
2
2
) or (ii) intramolecular migration of the borane group between
The optimized geometry of 1a′ (calculated structures are
indicated throughout with a prime) shows a close correspond-
ence with the solid-state structure of 1a; calculated bond
lengths are typically overestimated by 0.01 Å or less. The global
minimum energy conformations of 1a′ and 2a′ lie relatively
the two phosphorus centers in the compound via transition
state 1intra (pathway b). Pathway a is also consistent with the
formation of 4a as an intermediate in the reaction and with the
formation of 3a (from the reaction of 1a with the free BH3
formed during such a dissociative process). However, both the
formation of 3a and 4a and the non-first-order kinetics for the
formation of 2a suggest that the rearrangement of 1a to 2a is
not straightforward and that it is unlikely to proceed through a
single process.
−1
close in energy, with 2a′ just 12.3 kJ mol more stable than 1a′
(Figure 2).
A relaxed potential energy surface scan indicates that
dissociation of BH from 1a′ (pathway a) proceeds smoothly
3
and does not pass through a saddle point. The calculated free
Consistent with the foregoing, while the formation of 3a and
a supports a dissociative process, a plot of ln(k/T) versus 1/T
energy of dissociation of the BH fragment from 1a′, corrected
3
−1
4
for basis set superposition error (BSSE), is 106.4 kJ mol ,
extremely close to the experimentally determined free energy of
for the decay of 1a gives a straight line, which yields the
⧧
−1
⧧
−1
activation parameters ΔH = 63 ± 8 kJ mol , ΔS = −145 ±
activation for the isomerization of 1a to 2a (106 ± 8 kJ mol ),
−1
−1
⧧
−1
2
4 J K mol , and ΔG = 106 ± 8 kJ mol . The value of
again suggesting that this is the likely rate-determining step in
⧧
ΔH is rather low for a process involving the breaking of a P−B
the BH migration process (Figure 3); however, the enthalpy
3
−1
bond (the bond dissociation energy of Me P−BH has
and entropy of activation for this process are 152.4 kJ mol
3
3
C
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Article
and 154.2 J K− mol , respectively (cf. 63 ± 8 kJ mol and
−
1
−1
−1
145 ± 24 J K− mol , respectively, for the experimentally
determined values).
1
−1
For comparison, a transition state (1intra′) was located for the
concerted migration of the borane group between the two
phosphorus centers in 1a′ (pathway b). The calculated free
energy barrier to BH migration via this transition state is 134.8
3
−1
kJ mol , suggesting that such a process is significantly
disfavored with respect to dissociation of BH from 1a′. The
3
enthalpy and entropy of activation for this process were
calculated to be 141.1 kJ mol− and 21.0 J K mol ,
1
−1
−1
Figure 2. Optimized global minimum energy geometries for 1a′ and
respectively.
2
a′ [wB97XD/6-311+G(d,p)].
We were unable to locate a stable transition state for the
simultaneous migration of the two borane groups between two
head-to-tail molecules of 1a′, which would generate two
−1
Figure 3. Relative free energies (kJ mol ) of (a) the dissociative and concerted migration pathways for conversion of 1a′ to 2a′ (pathways a and b)
and (b) the associative pathway for conversion of 1a′ to a mixture of 3a′ and 4a′ (pathway c′) [wB97XD/6-311+G(d,p) at 298.15 K, corrected for
ZPE and BSSE as appropriate].
D
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Scheme 3. Alternative Pathway to 2a via BH Elimination from 3a
3
molecules of 2a′ directly (pathway c, see Scheme 2). However,
we have modeled this process in a “slipped head-to-tail” pair,
on removal of the solvent. When this latter reaction was carried
out in the higher boiling ethanol, a mixture of 4a, 3a, and 2a
was isolated in the approximate ratio of 4:1:2, due to
competitive migration of the borane group under these
where a single BH group migrates between two molecules of
3
1
a′ to give a molecule each of 3a′ and 4a′ (pathway c′). The
overall energy difference between two molecules of 1a′ and a
conditions. Since “free” BH is likely to react rapidly with
3
−1
molecule each of 3a′ and 4a′ is just 3.2 kJ mol , suggesting
that this process will be reversible and, therefore, that, given a
sufficiently low barrier to activation, these species will be in
dynamic equilibrium. The transition state for this process
ethanol under these conditions, the formation of 2a and 3a in
this reaction is further evidence for at least a second borane
migration pathway, in addition to an S 1-type dissociation of
N
BH from 1a.
3
1
13
1
31
1
11
1
(
1
1inter′, Figure 3b) has a calculated free energy of activation of
The H, C{ H}, P{ H}, and B{ H} NMR spectra of
−1
31
1
19.7 kJ mol . This is again close to the experimentally
both 3a and 4a are as expected: the P{ H} NMR spectrum of
3a consists of a pair of broad multiplets at 12.6 and 40.8 ppm,
observed free energy of activation for the borane migration
−1
31
1
process (106 ± 8 kJ mol ), while the calculated enthalpy and
while the P{ H} NMR spectrum of 4a consists of a pair of
⧧
−1
⧧
entropy of activation (ΔH = +57.2 kJ mol , ΔS = −195.7 J
doublets at −19.2 and −3.8 ppm (J = 123 Hz). Both 3a and
PP
−
1
−1
K
mol ) are also of a similar magnitude to the
experimentally determined values (ΔH = +63 ± 8 kJ mol ,
ΔS = −145 ± 24 J K mol ). Such an intermolecular process
would also account for the formation of 3a and 4a during the
isomerization of 1a to 2a. However, this process is inconsistent
with the apparent first-order dependence on the concentration
of 1a observed experimentally.
4a were isolated cleanly from these reactions and were used
without further purification.
⧧
−1
⧧
−1
−1
Phosphine-Borane-Stabilized Carbanions: Synthesis,
Structures, and Stabilities. Treatment of each of 1a−4a with
one equivalent of nBuLi in THF, followed by one equivalent of
tmeda gives the complexes [{Ph P(BH )}CHPiPr ]Li(tmeda)
2
3
2
(1b), [Ph
PCH{P(BH )iPr }]Li(tmeda) (2b), [{Ph P(BH )}-
3 2 2 3
2
From the foregoing, we may reasonably rule out an
CH{P(BH )iPr }]Li(tmeda) (3b), and [Ph PCHPiPr ]Li-
3
2
2
2
intramolecular migration of the borane group (via transition
(tmeda) (4b), respectively, as colorless to yellow, crystalline
solids in good yield; the alternative adduct [{Ph P(BH )}-
state 1 ) as the principal mechanism for the transformation
intra′
2
3
of 1a′ into 2a′, on the grounds of the rather large calculated
free energy of activation for this process compared to the
alternatives. However, both dissociation of borane from 1a′ and
migration of borane between two molecules of 1a′ have similar
CHPiPr
]Li(THF) (1c) was also isolated as X-ray quality
2 2
crystals [tmeda = N,N,N′,N′-tetramethylethylenediamine].
1
13
1
11
1
31
1
7
1
The H, C{ H}, B{ H}, P{ H}, and Li{ H} NMR
7
1
spectra of 1c in d -toluene are as expected; the Li{ H} NMR
spectrum consists of a singlet at 0.3 ppm, while the P{ H}
8
−
1
31
1
free energies of activation (106.4 and 119.7 kJ mol ,
respectively), and both lie close to the experimentally
NMR spectrum consists of a doublet (JPP = 166 Hz) at 9.1 and
−1
31
7
determined value of 106 ± 8 kJ mol . It is likely, therefore,
that both processes operate simultaneously and that the
observed kinetic data (i.e., an apparent first-order dependence
on the concentration of 1a, but a strongly negative entropy of
activation) represent a weighted average for these. This
interpretation is consistent with earlier investigations into the
substitution behavior of related amine-boranes, where, for
adducts containing moderately sterically hindered borane
moieties, S 1 and S 2 processes have been observed to
a broad multiplet (JPB = 91 Hz) at 14.1 ppm, P− Li coupling
is not observed due to rapid P−Li exchange on the NMR time-
7
1
31
1
scale. However, the Li{ H} and P{ H} NMR spectra of 1b in
d
-toluene consist of a broad doublet (JPLi = 53 Hz) at 0.8 ppm
and a pair of multiplets at 9.1 (JPP = 165, JPLi = 53 Hz) and 13.9
ppm (JPP = 165, JPB = 94 Hz), attributed to the PiPr and
P(BH )Ph groups, respectively (Figure 4). The presence of
Li− P coupling indicates that P−Li exchange is slow on the
NMR time-scale and suggests that the molecular structure of 1b
8
2
3
2
7
31
N
N
1
1
7
1
31
1
operate simultaneously.
is maintained in solution. Similarly, the Li{ H} and P{ H}
In reality, the situation is likely to be even more complex, as
these processes may be supplemented by dissociation of BH3
from the small quantity of 3a produced as an intermediate by
spectra of 2b consist of a broad doublet at 0.9 ppm (JPLi = 62
Hz) and a pair of approximate doublets of quartets at −6.7 (J
PP
= 165, J = 62 Hz) and 32.2 ppm (J = 165, J = 95 Hz),
PLi
PP
PB
pathway c, to give 2a and free BH , the latter of which may
respectively, again indicating slow P−Li exchange on the NMR
time-scale. The slow P−Li exchange in these compounds
clearly derives from the incorporation of the tertiary phosphine
center into a chelate ligand and the partial delocalization of
charge from the carbanion center onto the phosphorus atom
(see below), which enhances the essentially electrostatic P−Li
3
combine with 4a to give a further equivalent of 2a (Scheme 3).
A relaxed potential energy surface scan indicates that this
dissociation proceeds smoothly and does not pass through a
saddle point; the free energy for this process is calculated to be
1
1
00.3 kJ mol− (corrected for BSSE), indicating that the
1
13
1
11
1
31
1
7
1
dissociation of 3a is competitive with the other processes
outlined above.
interaction. The H, C{ H}, B{ H}, P{ H}, and Li{ H}
NMR spectra of 3b and 4b are as expected.
A clean sample of 3a was prepared by treatment of 1a with
While 1a converts slowly to 2a on heating, we find that 1b is
unchanged after heating under reflux in toluene for several days.
The P−B bond in 1b is likely to be relatively unaffected by any
negative hyperconjugation of the carbanion lone pair, and so
the P−B bond strength in 1b should be similar to that in 1a
one equivalent of BH ·SMe in THF and was isolated as an
3
2
essentially pure, colorless powder on removal of the solvent.
The bis(phosphine) 4a was prepared by heating 1a under reflux
in degassed methanol for 24 h and was isolated as a colorless oil
E
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Organometallics
Article
7
1
31
1
Figure 4. (a) Li{ H} and (b) P{ H} NMR spectra of 1b in d -
8
toluene.
(
see DFT studies below). We therefore attribute the lack of
borane migration observed on heating 1b to the unavailability
of the PiPr lone pair, which remains coordinated to the lithium
2
cation in toluene solution. This hypothesis is supported by the
3
1
1
7
slow rate of P−Li exchange evident from the P{ H} and Li
NMR spectra of 1b (see above). Interestingly, we observe no
evidence for the migration of the borane group from
phosphorus to the carbanion center, such as that recently
reported by Langer and co-workers for a barium complex of a
18
phosphine-borane-stabilized carbanion.
Compounds 1b, 1c, 2b, 3b, and 4b were obtained as single
crystals suitable for X-ray crystallography. These compounds
present a unique opportunity to examine the effect of the
degree of borane substitution and the nature of the substituents
at phosphorus on the stabilities and structures of PBCs. Only
the structures of the directly comparable compounds 1b−4b
are discussed in detail here; details of the structures of 1a and of
the THF adduct 1c may be found in the Supporting
Information.
The molecular structures of 1b and 2b are shown in Figure 5,
along with selected bond lengths and angles. Compounds 1b
and 2b crystallize as discrete molecular species. In each case the
lithium ions are coordinated by the two nitrogen atoms of a
Figure 5. Molecular structures of (a) 1b and (b) 2b, with 40%
probability ellipsoids and with C-bound H atoms omitted for clarity.
Selected bond lengths (Å) and angles (deg): 1b Li−P(1) 2.583(3),
Li−N(1) 2.145(4), Li−N(2) 2.086(4), Li−H(B) 2.20(2), Li−H(C)
1
.97(2), Li···B 2.442(4), P(1)−C(1) 1.736(2), P(1)−C(2) 1.869(2),
P(1)−C(5) 1.856(2), P(2)−B 1.933(2), P(2)−C(1) 1.702(2), P(2)−
C(8) 1.8305(19), P(2)−C(14) 1.842(2), P(1)−Li···B 86.91(12),
P(1)−C(1)−P(2) 120.83(12); 2b Li−P(2) 2.621(3), Li−N(1)
2.128(3), Li−N(2) 2.088(3), Li−H(A) 2.043(17), Li−H(B)
1.923(18), Li···B 2.361(3), P(1)−B 1.925(2), P(1)−C(1)
chelating molecule of tmeda and by the tertiary phosphine
2
center and an η -BH contact with the phosphine/phosphine-
3
1
.7158(15), P(1)−C(2) 1.8494(16), P(1)−C(5) 1.8409(17), P(2)−
borane-stabilized carbanion. This affords a distorted pseudote-
trahedral geometry at the lithium centers and generates a
pseudo-five-membered Li−P−C−P−B ring in each case. There
is no contact in each case between the lithium cations and the
formal carbanion centers. The Li−P distances in 1b and 2b of
C(1) 1.7292(17), P(2)−C(8) 1.8439(16), P(2)−C(14) 1.8438(16),
P(2)−Li···B 86.72(10), P(1)−C(1)−P(2) 120.76(9).
3
3
adopts a μ -η :η bridging mode, range between 2.02(3) and
2
2
.621(3) and 2.583(3) Å, respectively, are typical of Li−P
2.12(3) Å, and the Li···B distances in this compound are
19
distances in lithium phosphinomethanides; for example, the
2.467(5) and 2.461(6) Å [pmdeta = N,N,N′,N″,N″-pentam-
22b
Li−P distances in [{(Me PhSi)(Me P) C}Li] range from
ethyldiethylenetriamine].
2
2
2
3
20
2
[
.573(7) to 2.641(7) Å. The Li−H and Li···B distances in 1b
Compound 3b crystallizes with two independent molecules
in the asymmetric unit, which differ only marginally in
structure; the structure of 3b is shown in Figure 6, along
with selected bond lengths and angles. Each lithium cation is
Li−H 1.97(2) and 2.20(2) Å, Li···B 2.442(4) Å] and 2b [Li−
H 1.923(18) and 2.043(17) Å, Li···B 2.361(3) Å] are typical of
2
η -BH −Li contacts; for example, the Li−H and Li···B distances
n
in [{Me P(BH )}Li(tmeda)] are 2.03(2) and 2.372(4) Å,
coordinated by the two N atoms of a chelating molecule of
2
3
∞
2
1
2
respectively. For comparison, the Li−H distances in [Li-
BH )(pmdeta)], in which the BH anions adopt an η
configuration, are 2.04(4) and 2.05(4) Å, while the Li−H
distances in [Li(BH )(tmeda)] , in which the BH anion
tmeda and by two η -BH contacts; once again there is no
3
2
(
contact between the lithium ion and the formal carbanion
center. This affords a pseudotetrahedral geometry at lithium
and generates a pseudo-six-membered Li−B−P−C−P−B
4
4
22a
4
2
4
F
dx.doi.org/10.1021/om5005995 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Figure 7. Molecular structure of 4b with 40% probability ellipsoids
and with C-bound H atoms omitted for clarity. Selected bond lengths
(Å) and angles (deg): Li−P(1) 2.537(4), Li−P(2) 2.568(4), Li−N(1)
2.080(4), Li−N(2) 2.073(4), Li···C(22) 2.786(4), Li···C(23)
2.760(4), P(1)−C(1) 1.738(2), P(1)−C(2) 1.870(2), P(1)−C(5)
1.867(2), P(2)−C(1) 1.7277(19), P(2)−C(8) 1.844(2), P(2)−C(14)
1.853(2), P(1)−Li−P(2) 70.09(10), P(1)−C(1)−P(2) 115.54(11).
Figure 6. Molecular structure of one of the two independent
molecules of 3b with 40% probability ellipsoids and with C-bound H
atoms omitted for clarity. Selected bond lengths (Å) and angles (deg):
molecule 1 Li−H(1A) 1.927(17), Li−H(1C) 2.079(18), Li−H(2A)
.082(17), Li−H(2B) 2.007(17), Li−N(1) 2.102(3), Li−N(2)
.115(3), Li···B(1) 2.402(3), Li···B(2) 2.424(3), P(1)−B(1)
.9335(18), P(1)−C(1) 1.7242(16), P(1)−C(2) 1.8445(16), P(1)−
C(5) 1.8487(17), P(2)−B(2) 1.9285(18), P(2)−C(1) 1.7061(16),
P(2)−C(8) 1.8342(16), P(2)−C(14) 1.8326(16), B(1)···Li···B(2)
2
2
1
Metalation of 1a results in a significant decrease in the
C(1)−P(1) and C(1)−P(2) distances [C(1)−P(1) 1.867(4)
(
1a), 1.736(2) Å (1b); C(1)−P(2) 1.815(5) (1a), 1.701(2) Å
9
2
5.29(11), P(1)−C(1)−P(2) 129.22(9); molecule 2 Li(2)-H(3A)
.051(19), Li(2)−H(3B) 1.941(19), Li(2)−H(4A) 2.007(17), Li(2)−
(1b)], consistent with the change in hybridization of C(1) from
3
2
H(4B) 2.092(17), Li(2)−N(3) 2.125(3), Li(2)−N(4) 2.113(3),
sp to sp and with partial C(1)−P multiple-bond character due
to the delocalization of charge from the carbanion center into
the P−C σ*-orbitals (negative hyperconjugation). Consistent
with this, the C(1)−P(1) and C(1)−P(2) distances in 2b, 3b,
and 4b are also significantly shorter than the corresponding
distances in 1a (see Table 1).
Li(2)···B(3) 2.418(3), Li(2)···B(4) 2.439(3), P(3)−B(3)
1
.9308(19), P(3)−C(26) 1.7223(17), P(3)−C(27) 1.8411(17),
P(3)−C(30) 1.8454(17), P(4)−B(4) 1.9290(18), P(4)−C(26)
1
B(3)···Li(2)···B(4) 94.75(11), P(3)−C(26)−P(4) 128.89(10).
.7070(16), P(4)−C(33) 1.8359(16), P(3)−C(39) 1.8304(16),
The C(1)−P(1) distance in 1a [1.867(4) Å] is significantly
longer than the C(1)−P(2) distance [1.815(5) Å], possibly as a
consequence of the inductive effect of the propyl groups, which
decreases the electronegativity, and therefore increases the
covalent radius, of P(1) in comparison to P(2). A similar,
although less pronounced, difference is seen for the
corresponding distances in the lithium complex 1b, the
bis(phosphine-borane)-stabilized carbanion complex 3b, and
the bis(phosphino)methanide complex 4b. In contrast, for 2b,
chelate ring. The Li−H distances range from 1.927(17) to
2
2
.092(17) Å, while the Li···B distances range from 2.402(3) to
2
.439(3) Å, consistent with an η -BH −Li contact (see above).
3
These distances are similar to the Li−H and Li···B distances in
the related compound [{Ph P(BH )} CH]Li(OEt ) , which
also exhibits η -BH −Li contacts.
2
3
2
2 2
2
5j
3
Compound 4b adopts a typical structure for a diphosphino-
methanide complex; the molecular structure of 4b is shown in
Figure 7, along with selected bond lengths and angles. The
lithium ion is coordinated by both of the phosphorus centers to
give a four-membered chelate ring [bite angle 70.09(10)°] and
by the two nitrogen atoms of the tmeda coligand, affording an
approximately tetrahedral geometry at the lithium center. In
addition, the lithium ion has short contacts with both of the
methylene carbon atoms of the tmeda coligand [Li···C(22)
in which there is a single borane group on the iPr P center, the
2
C(1)−P(2) distance [1.7292(17) Å] is longer than the C(1)−
P(1) distance [1.7158(15) Å]. This suggests that there is
greater delocalization of electron density from the carbanion
center toward a phosphine-borane, rather than a phosphine
group; this is supported by our DFT calculations (see below).
DFT Studies on PBC Complexes. The closely related
series of complexes 1b−4b provides a unique opportunity to
investigate the extent of negative hyperconjugation in
phosphine- and phosphine-borane-stabilized carbanions and
provides insight into the molecular orbitals involved: each of
these compounds has the same coligand (tmeda) and adopts a
2
.786(4), Li···C(23) 2.760(4) Å]. The two Li−P distances are
rather similar [Li−P(1) 2.537(4), Li−P(2) 2.568(4) Å] and are
typical of Li−P distances in lithium phosphinomethanide
1
9
complexes.
Although the precise location of hydrogen atom positions is
problematic, the carbanion centers in 1b−4b appear to adopt a
planar configuration [sums of angles at C(1) = 358.39−
similar structure, with solely Li−P and/or Li···BH contacts
3
between the lithium cation and the corresponding anion.
The calculated structures of 1b′−4b′ bear a very close
resemblance to the structures obtained by X-ray crystallog-
raphy; at the wB97XD/6-311+G(d,p) level of theory bond
lengths in the calculated structures are typically overestimated
by 0.02 Å or less. The calculated changes in geometric
parameters on metalation of the free phosphine-boranes 1a′−
4a′ are consistent with the observed differences between the
3
59.96°]. The P−C−P angle increases with increasing borane
substitution of the PBC ligand [P(1)−C(1)−P(2) 115.54(11)°
4b), 120.83(12)° (1b), 120.76(9)° (2b), 129.22(9)°/
28.89(10)° (3b)], consistent with the increase in steric
(
1
compression in each case and the change in chelate ring size
from a four- to a pseudo-five- to a pseudo-six-membered ring
on going from 4b to 1b/2b to 3b, respectively.
G
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Organometallics
Article
Table 1. Selected Bond Lengths (Å) and Angles (deg) for 1a and 1b−4b
1
a
1b
2b
3b
4b
P(1)−B(1)
P(2)−B(2)
C(1)−P(1)
C(1)−P(2)
C(2)−P(1)
C(5)−P(1)
C(8)−P(2)
C(14)−P(2)
P(1)−C(1)−P(2)
1.925(2)
1.9335(18)
1.9285(18)
1.7242(16)
1.7061(16)
1.8445(16)
1.8487(17)
1.8342(16)
1.8326(16)
129.22(9)
1.913(6)
1.867(4)
1.815(5)
1.869(5)
1.854(4)
1.820(4)
1.814(4)
99.3(2)
1.933(2)
1.736(2)
1.701(2)
1.869(2)
1.856(2)
1.8305(19)
1.842(2)
120.83(12)
1.7158(15)
1.7292(17)
1.8494(16)
1.8409(17)
1.8439(16)
1.8438(16)
120.76(9)
1.738(2)
1.7277(19)
1.870(2)
1.867 (2)
1.844(2)
1.853(2)
115.54(11)
Table 2. Comparison of Calculated Bond Lengths (Å) and Angles (deg) on Metalation of the Free Phosphine-Boranes
1
a′
Δ
1b′
2a′
Δ
2b′
3a′
Δ
3b′
4a′
Δ
4b′
P(1)−B(1)
P(2)−B(2)
C(1)−P(1)
C(1)−P(2)
C(2)−P(1)
C(5)−P(1)
C(8)−P(2)
C(14)−P(2)
P(1)−C(1)−P(2)
1.9301
+0.0133
1.9434
1.9322
1.9327
1.8505
1.8335
1.8498
1.8476
1.8150
1.8272
125.20
+0.0061
+0.0022
−0.1123
−0.1118
+0.0132
+0.0109
+0.0330
+0.0098
+2.53
1.9383
1.9349
1.7382
1.7217
1.8630
1.8585
1.8480
1.8370
127.73
1.9281
1.8802
1.8271
1.8731
1.8680
1.8219
1.8236
112.50
+0.0159
−0.1281
−0.1094
−0.0034
+0.0158
+0.0173
+0.0233
+7.04
1.9440
1.7521
1.7177
1.8697
1.8838
1.8392
1.8469
119.54
1.8366
1.8588
1.8442
1.8495
1.8424
1.8449
121.12
−0.1064
−0.1239
+0.0158
+0.0147
+0.0084
+0.0097
−0.32
1.7302
1.7349
1.8600
1.8642
1.8508
1.8546
120.80
1.8735
1.8549
1.8727
1.8688
1.8471
1.8435
110.94
−0.1188
−0.1197
−0.0005
+0.0138
+0.0078
+0.0114
+4.68
1.7547
1.7352
1.8722
1.8826
1.8549
1.8568
115.62
−1
Table 3. E(2) Energies (kJ mol ) of Selected Delocalizations, the Percentage p-Character of the Carbon Lone Pair, and Its
Occupancy for 1b′−4b′
1
b′
sum
2b′
sum
3b′
sum
4b′
sum
l.p.C(1)→P(1)−B(1)
l.p.C(1)→P(2)−B(2)
−
0.00
0.00
5.77
3.26
−
−
−
l.p.C(1)→P(1)−C(2)
l.p.C(1)→P(1)−C(5)
35.90
69.50
55.15
78.49
97.78
21.34
31.25
81.21
1
05.40
46.65
133.64
121.72
119.12
133.09
112.46
132.55
l.p.C(1)→P(2)−C(8)
l.p.C(1)→P(2)−C(14)
52.84
93.81
40.59
81.13
98.03
35.06
47.66
84.89
1
%
p char l.p.C(1)
97.5
1.68
99.1
1.69
96.7
1.71
98.7
1.65
occupancy l.p.C(1)
Table 4. Wiberg Bond Indices for 1a′−4a′ and 1b′−4b′
1a′
Δ
1b′
2a′
Δ
2b′
3a′
Δ
3b′
4a′
Δ
4b′
P(1)−B(1)
P(1)-Li
0.970
−0.007
0.963
0.975
−0.007
0.968
0.325
0.953
0.315
P(2)−B(2)
P(2)−Li
0.961
−0.008
0.965
−0.006
0.959
0.013
1.103
1.119
0.846
0.838
0.880
0.849
0.271
1.101
1.152
0.863
0.836
0.869
0.843
C(1)−P(1)
C(1)−P(2)
C(2)−P(1)
C(5)−P(1)
C(8)−P(2)
C(14)−P(2)
0.883
0.923
0.907
0.904
0.897
0.904
0.196
0.222
1.079
1.145
0.864
0.846
0.854
0.828
0.910
0.910
0.893
0.896
0.911
0.926
0.203
0.219
0.885
0.903
0.899
0.899
0.909
0.900
0.184
0.209
1.069
1.112
0.832
0.873
0.833
0.870
0.903
0.924
0.907
0.901
0.911
0.922
0.198
0.228
−0.043
−0.058
−0.043
−0.076
−0.047
−0.058
−0.031
−0.077
−0.057
−0.026
−0.076
−0.030
−0.044
−0.065
−0.042
−0.079
2
3
solid-state structures of 1a and 1b. In each case metalation leads
to a significant decrease in the C(1)−P distances of between
P(BH ) center in 1b′, natural bond orbital (NBO) analyses
3
indicate substantial C−P multiple-bond character in the anions
due to delocalization of the carbanion lone pair into the P−C
σ*-orbitals. NBO analyses of 1b′−4b′ reveal that the HOMO is
a lone pair at carbon, which has >96% p-character in each case,
and that this is extensively delocalized into the P−C(Ph) and
P−C(Pr) σ*-orbitals. This Cl.p. → (P−C)* negative hyper-
conjugation is significantly stabilizing, such that the E(2)
0
.11 and 0.13 Å (Table 2). While some of this decrease may be
2
attributed to the smaller covalent radius of the sp -hybridized
3
carbanion center in 1b′ compared to the sp -hybridized
methylene carbon in 1a′, and to an increased electrostatic
interaction between the negatively charged carbanion center
and the partially positively charged phosphorus atom in the
H
dx.doi.org/10.1021/om5005995 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
energies of these interactions lie between 21.3 and 98.0 kJ
CONCLUSIONS
■
−1
mol (Table 3); in each case, the greatest stabilization is
observed where the acceptor antibonding orbital is aligned with
the lone pair orbital at the carbanion center, as expected. In
accord with the foregoing, the Wiberg bond indices (WBIs) for
the C(1)−P bonds increase significantly (by between 0.184 and
The clean synthesis of the mixed phosphine/phosphine-borane
adduct 1a provides ready access to the corresponding isomer
a, the bis(phosphine-borane) 3a, and the unsymmetrical
bis(phosphine) 4a. Detailed kinetic and DFT studies reveal that
the thermally driven isomerization of 1a into 2a is not
straightforward, but appears to proceed via at least two
mechanisms simultaneously: (i) slow dissociation of BH3
2
0.228) on metalation (Table 4). Since the borane group
coordinates the lithium ion in each of 1b′−3b′, the p-type
carbanion lone pair lies orthogonal to the plane of the P−B σ*-
orbital, and so there is essentially no hyperconjugation between
these orbitals and, correspondingly, only a marginal change in
P−B WBI on metalation of the neutral precursors.
from the Ph P center in 1a, followed by rapid adduct formation
2
between the released BH and the iPr P center, and (ii)
intermolecular migration of BH between two molecules of 1a.
These processes may additionally be supplemented by the
3
2
3
These observations are in agreement with So and Mez
́
ailles’
dissociation of BH from the bis(phosphine-borane) 3a.
3
recent study of the isolated mono- and dicarbanions [Ph P-
Analysis of the solid-state structures of the lithium salts 1b−
b indicates that the phosphine/phosphine-borane-stabilized
2
−
2−
4
(
S)CHP(BH )Ph ] and [Ph P(S)CHP(BH )Ph ] , in which
3 2 2 3 2
carbanions are significantly stabilized by delocalization of the
carbanion lone pair into the P−C σ*-orbitals. DFT calculations
and NBO analyses support this proposal and indicate that the
greatest delocalization is into the P−C σ*-orbitals of the
phosphine-borane substituent in each case.
the major negative hyperconjugative interactions were found to
be between the (di)carbanion lone pair(s) and the P(S)−
C(Ph) σ*-orbitals, with smaller, but still significant delocaliza-
24
tions into the P(BH )−C(Ph) σ*-orbitals. In the present
3
case, the greatest delocalizations are observed to the P(BH )−
3
C(Ph/Pr) σ*-orbitals.
EXPERIMENTAL SECTION
The present study allows for a direct comparison between
the degree of negative hyperconjugation supported by an
adjacent phosphine-borane group in contrast to an adjacent
phosphine substituent. In 3b′ and 4b′, which possess either two
phosphine-borane or two phosphine groups, respectively, it is
notable that there is a greater degree of negative hyper-
conjugation into the P−C(Ph) compared to the P−C(Pr) σ*-
orbitals in each case [the sum of E(2) energies for the pair of
■
All manipulations were carried out using standard Schlenk techniques
under an atmosphere of dry nitrogen. THF, toluene, and diethyl ether
were dried prior to use by distillation under nitrogen from sodium,
potassium, or sodium/potassium alloy as appropriate; CH Cl and
2
2
hexamethyldisiloxane were distilled from CaH under nitrogen. THF,
2
CH Cl , and hexamethyldisiloxane were stored over activated 4A
2
2
molecular sieves; all other solvents were stored over a potassium film.
Deuterated toluene and C D6 were distilled from potassium and
6
−1
CDCl was distilled from CaH under nitrogen; all NMR solvents
interactions is 133.09 and 119.12 kJ mol , respectively, for 3b′
3
2
−1
were deoxygenated by three freeze−pump−thaw cycles and were
and 132.55 and 112.46 kJ mol for 4b′]. This is consistent
with a better energy match between the lone pair orbitals and
the P−C(Ph) σ*-orbitals in each case. In addition to this, in 1b′
and 2b′, where only one of the phosphorus atoms is bound to a
borane group, the carbanion electron density is more
extensively delocalized into the P−C σ*-orbitals of the
phosphine-borane substituent in comparison to the phosphine
substituent. For example, in 1b′ the sum of the E(2) energies of
stored over activated 4A molecular sieves. Ph P(BH )Me was prepared
2
3
25
by a previously published procedure; n-butyllithium was purchased
from Aldrich as a 2.5 M solution in hexanes. Tmeda was distilled from
CaH under nitrogen and was stored over activated 4A molecular
2
sieves. All other compounds were used as supplied by the
manufacturer.
1
13
1
H and C{ H} NMR spectra were recorded on a JEOL ECS500
spectrometer operating at 500.16 and 125.65 MHz, respectively, a
JEOL ECS400 spectrometer operating at 399.78 and 100.53 MHz,
respectively, or a Bruker Avance300 spectrometer operating at 300.15
and 75.47 MHz, respectively; chemical shifts are quoted in ppm
−
1
the C → (P(BH )−C)* delocalizations is 146.65 kJ mol ,
l.p.
3
whereas the sum of these energies for the C → (P−C)*
l.p.
−1
31
1
11
1
7
1
delocalizations is 105.40 kJ mol . For 2b′ there is at first
glance a less substantial difference in the E(2) energies of the
Cl.p. → (P(BH )−C)* and C → (P−C)* delocalizations
relative to tetramethylsilane. P{ H}, B{ H}, and Li{ H} NMR
spectra were recorded on a JEOL ECS500 spectrometer operating at
2
02.35, 160.16, and 194.38 MHz, respectively; chemical shifts are
3
l.p.
−
1
quoted in ppm relative to external 85% H PO , BF ·Et O, and 0.1 M
3
4
3
2
[
133.64 and 121.72 kJ mol , respectively]; however, the
1
31
LiCl, respectively. Signals due to BH3 protons and the H− P
underlying difference is masked by the greater delocalization of
electron density into the P−C(Ph) compared to the P−C(Pr)
σ*-orbitals in all of 1b′ to 4b′.
coupling constants for these signals were identified using selective
1
11
H{ B} experiments. Elemental analyses were obtained by the
Elemental Analysis Service of London Metropolitan University.
Consistent with the foregoing, in almost all cases metalation
of 1a′−3a′ results in a small increase in the P−C(Ph) and P−
C(Pr) distances of up to 0.034 Å, with a corresponding
decrease in WBI of between 0.026 and 0.079; in each case the
greatest increase in bond length/decrease in WBI is observed in
Preparation of Ph P(BH )CH PiPr (1a). To a cold (0 °C)
2
3
2
2
solution of Ph P(BH )Me (2.16 g, 0.01 mol) in THF (30 mL) was
2 3
added, dropwise, nBuLi (4.00 mL, 0.01 mol), and this solution was left
to stir for 1 h while warming to room temperature. This solution was
added to a cold (−78 °C) solution of iPr PCl (1.6 mL, 0.01 mol) in
2
THF (30 mL), and this mixture was stirred for 12 h, while warming
slowly to room temperature. The solvent was removed in vacuo to give
a pale yellow solid, which was extracted into dichloromethane (40 mL)
and filtered; the solvent was removed in vacuo from the filtrate to give
the P(BH )−C(Ph/Pr) bond in the substituent that lies
3
pseudoantiperiplanar to the carbanion lone pair. In contrast,
metalation has little effect on the P−B distance(s), which
increase by approximately 0.01 Å in each case, with a
corresponding decrease in WBI of between 0.006 and 0.008;
NBO analysis similarly indicates there is essentially no
interaction between the lone pair at carbon and the P−B σ*-
orbital(s) (the E(2) energies for this interaction range between
1
a as a colorless solid, which was sufficiently clean for use without
1
11
further purification. Yield: 2.67 g, 81%. H{ B} NMR (CDCl ): 0.94
3
(
dd, JHH = 7.0, JPH = 5.0 Hz, 6H, CHMeMe), 0.97 (dd, JHH = 7.0, JPH =
6
2
7
.5 Hz, 6H, CHMeMe), 1.04 (d, JPH = 16.3 Hz, 3H, BH ), 1.73 (m,
3
H, CHMeMe), 2.13 (dd, J = 11.5, JP′H = 7.0 Hz, 2H, CH ), 7.38−
PH
2
.80 (m, 10H, ArH). 13C{ H} NMR (CDCl ): δ 18.66 (dd, J = 38.2,
1
3
PC
−1
0
.00 and 5.77 kJ mol ).
JP′C = 37.6 Hz, CH ), 19.16 (d, J = 13.6 Hz, CHMeMe), 19.22 (d,
2
PC
I
dx.doi.org/10.1021/om5005995 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
JPC = 12.8 Hz, CHMeMe), 24.27 (dd, JPC = 15.6, JP′C = 6.4 Hz,
crystallized from cold (−30 °C) hexamethyldisiloxane (10 mL) to give
2b as a pale yellow, crystalline solid. Yield: 0.59 g, 35%. Anal. Calcd for
C H BLiN P (452.34): C 66.38, H 9.80, N 6.19. Found: C 66.47, H
CHMeMe), 128.67 (d, JPC = 9.8 Hz, Ar), 130.42 (dd, JPC = 52.2, JP′C
=
=
2
9
.4 Hz, Ar), 131.21 (d, J = 2.4 Hz, Ar), 132.72 (dd, JPC = 9.1, JP′C
PC
25 44
2 2
11
1
1
.1 Hz, Ar). B{ H} NMR (CDCl ): δ −39.3 (d, J = 49 Hz).
9.70, N 6.04. H NMR (d -benzene): δ 0.87 (dd, J = 11.0, JP′H = 3.0
3
PB
6 PH
3
1
1
P{ H} NMR (CDCl ): δ −8.3 (d, J = 73 Hz, PiPr ), 16.6 (br m,
Hz, 1H, P CH), 1.29 (dd, J = 14.5, JHH = 7.5 Hz, 6H, CHMeMe),
2 PH
3
PP
2
PPh2).
1.31 (t, JPH/HH = 7.6 Hz, 6H, CHMeMe), 1.99 (m, 2H, CHMeMe),
1
3
1
Preparation of [Ph P(BH )CHPiPr ]Li(tmeda) (1b). To a cold
0 °C) solution of 1a (0.42 g, 1.27 mmol) in THF (25 mL) was added
7.10 (m, 2H, ArH), 7.25 (m, 4H, ArH), 7.83 (m, 4H, ArH). C{ H}
NMR (d -benzene): δ 3.08 (dd, JPC = 84.3, JP′C = 23.9 Hz, P CH),
17.04 (d, JPC = 1.7 Hz, CHMeMe), 18.05 (CHMeMe), 25.73 (dd, JPC
= 40.9, JP′C = 5.3 Hz, CHMeMe), 45.80 (NMe ), 56.42 (CH N),
126.48 (ArC), 127.70 (d, JPC = 6.7 Hz, ArC), 131.94, 132.07 (ArC),
2
3
2
(
6
2
nBuLi (0.51 mL, 1.27 mmol). This mixture was allowed to attain room
temperature and was stirred for 1 h. One equivalent of tmeda (0.19
mL, 1.27 mmol) was added to the solution, and the resulting mixture
was stirred for 10 min. Solvent was removed in vacuo, leaving a sticky
orange solid, which was crystallized from cold (−30 °C) diethyl ether
2
2
7
1
148.79 (dd, JPC = 7.7, JP′C = 4.9 Hz, ipso-ArC). Li{ H} NMR (d -
6
11
1
benzene): δ 0.9 (d, JPLi = 62 Hz). B{ H} NMR (d
-benzene): δ
-benzene): δ −6.7 (m, JPP
), 32.2 (m, JPP = 165, JPB = 95 Hz, PiPr ).
P(BH )CH P(BH )iPr (3a). To a solution of
1a (0.51 g, 1.54 mmol) in THF (25 mL) was added BH ·SMe (0.77
6
31
1
(
10 mL) as yellow blocks. Yield: 0.53 g, 92%. Anal. Calcd for
−41.1 (d, JPB = 95 Hz). P{ H} NMR (d
165, JPLi = 62 Hz, PPh
Preparation of Ph
6
=
C H BLiN P (452.34): C 66.38, H 9.80, N 6.19. Found: C 66.49, H
9
2
2
2
5
44
2 2
1
11
.70, N 6.08. H{ B} NMR (d -toluene,): δ 1.06 (dd, J = 15.5, JHH
2
3
2
3
2
8
PH
=
7.0 Hz, 6H, CHMeMe), 1.12 (d, JPH = 10.5 Hz, 3H, BH ), 1.17 (dd,
3
2
3
JPH = 10.5, JHH = 6.5 Hz, 6H, CHMeMe), 1.74 (m, 2H, CHMeMe),
.76 (s, 4H, CH N), 1.93 (s, 12H, NMe ), 7.06 (m, 2H, ArC), 7.18
mL, 1.54 mmol), and this solution was stirred for 1 h. The solvent was
1
removed in vacuo to give 3a as a white solid. Yield: 0.49 g, 92%.
2
2
13
1
1
11
(
(
m, 4H, ArC), 8.06 (m, 4H, ArC). C{ H} NMR (d -toluene): δ 2.45
dd, JPC = 92.6, JP′C = 16.5 Hz, P CH), 18.47 (d, J = 3.8 Hz,
H{ B} NMR (CDCl
): δ 0.22 (d, JPH = 14.8 Hz, 3H, BH ), 1.08 (dd,
3 3
8
JPH = 15.1, JHH = 6.9 Hz, 6H, CHMeMe), 1.15 (dd, JPH = 15.1, JHH
=
2
PC
CHMeMe), 21.21 (d, JPC = 15.0 Hz, CHMeMe), 25.86 (dd, JPC = 8.3,
JP′C = 4.2 Hz, CHMeMe), 46.29 (NMe ), 56.93 (CH N), 127.54 (d,
6.9 Hz, 6H, CHMeMe), 1.17 (d, JPH = 15.1 Hz, 3H, BH ), 2.42 (m,
3
2H, CHMeMe), 2.59 (dd, JPH =1 11.8, J = 10.2 Hz, 2H, P CH ),
2
2
P′H
2
2
3
1
JPC = 8.5 Hz, ArC), 131.84 (d, JPC = 8.7 Hz, ArC), 143.86 (dd, JPC
=
7.44−7.76 (m, 10H, ArH). C{ H} NMR (CDCl ): δ 17.58
3
5
7.4, JP′C 7= 5.5 Hz, ipsoArC) [remaining aromatic signal obscured by
(CHMeMe), 17.96 (CHMeMe), 18.54 (dd, JPC = 23.9, JP′C = 18.9
Hz, CHMeMe), 129.04 (d, JPC = 10.1 Hz, ArC), 129.94 (dd, JPC
56.6, JP′C = 2.5 Hz, ipsoArC), 131.78 (d, JPC = 2.5 Hz, ArC), 132.29 (d,
1
11
1
solvent]. Li{ H} NMR (d -toluene): δ 0.8 (d, J = 53 Hz). B{ H}
NMR (d -toluene): δ −35.5 (d, J = 94 Hz). P{ H} NMR (d -
toluene): δ 9.1 (m, J = 165, J = 53 Hz, PiPr ), 13.9 (m, J = 165,
=
8
PLi
31
1
8
PB
8
11
1
JPC = 10.1 Hz, ArC). B{ H} NMR (CDCl ): δ −43.3 (d, JPB = 42
PP
PLi
2
PP
3
Hz), −39.0 (d, JPB = 56 Hz). 31P{ H} NMR (CDCl
1
): δ 12.6 (br m),
JPB = 94 Hz, PPh2).
Preparation of [Ph P(BH )CHPiPr ]Li(THF) (1c). To a cold (0
3
40.8 (br m).
2
3
2
2
°C) solution of 1a (0.82 g, 2.06 mmol) in THF (30 mL) was added
Preparation of [{Ph P(BH )}CH{P(BH )iPr }]Li(tmeda) (3b). To
2 3 3 2
nBuLi (0.68 mL, 2.06 mmol). This mixture was allowed to warm to
room temperature and was stirred for 1 h. The solvent was removed in
vacuo, leaving a sticky orange solid, which was crystallized from cold
a cold (0 °C) solution of 3a (0.50 g, 1.48 mmol) in THF (25 mL) was
added nBuLi (0.59 mL, 1.48 mmol), and this solution was stirred for 1
h while warming to room temperature. To the resulting solution was
added tmeda (0.22 mL, 1.48 mmol), and this mixture was stirred for
10 min. The solvent was removed in vacuo to leave a yellow oil, which
was crystallized from cold (5 °C) diethyl ether (10 mL) to give 3b as a
pale yellow, crystalline solid. Yield: 0.31 g, 45%. Anal. Calcd for
(
−30 °C) diethyl ether (10 mL) as yellow-orange blocks. Yield: 0.47 g,
4
8%. Anal. Calcd for C H BLiO P (480.34): C 67.51, H 9.23.
27
44
11
2 2
1
Found: C 67.42, H 9.13. H{ B} NMR (d -toluene): δ 0.61 (br s, 3H,
8
BH ), 1.12 (dd, J = 15.5, JHH = 7.0 Hz, 6H, CHMeMe), 1.16 (dd, JPH
3
PH
=
11.0, JHH = 7.0 Hz, 6H, CHMeMe), 1.36 (m, 8H, THF), 1.79 (m,
C
25
H
47
B
2
LiN
2 2
P (466.17): C 64.41, H 10.16, N 6.01. Found: C 64.29,
1
11
2
H, CHMeMe), 1.97 (dd, JPH = 1.5, JP′H = 11.5 Hz, 1H, P CH), 3.50
H 10.01, N 5.85. H{ B} NMR (d
8
-toluene): δ 0.33 (dd, JPH = 10.5,
CH), 0.53 (d, JPH = 15.2 Hz, 3H, BH ), 1.17 (d,
PH = 17.0 Hz, 3H, BH ), 1.23 (dd, JPH = 7.0, JHH = 6.8 Hz, 6H,
2
(
m, 8H, THF), 7.04 (m, 2H, ArH), 7.18 (m, 4H, ArH), 8.06 (m, 4H,
J
J
P′H = 12.6 Hz, 1H, P
2
3
13
1
ArH). C{ H} NMR (d -toluene): δ 2.74 (dd, J = 93.4, JP′C = 14.3
3
8
PC
Hz, P CH), 18.40 (d, J = 3.8 Hz, CHMeMe), 20.88 (d, JPC = 15.2
CHMeMe), 1.24 (dd, JPH = 6.9, JHH = 5.3 Hz, 6H, CHMeMe), 1.74 (s,
4H, CH N), 1.83 (m, 2H, CHMeMe), 1.97 (s, 12H, NMe ), 7.07 (m,
2H, ArH), 7.18 (m, 4H, ArH), 8.02 (m, 4H, ArH). C{ H} NMR (d
toluene): δ −1.52 (dd, JPC = 90.6, JP′C = 78.1 Hz, P CH), 16.58 (d, JPC
= 1.9 Hz, CHMeMe), 17.44 (d, JPC = 0.6 Hz, CHMeMe), 26.56 (dd,
PC = 41.8, JP′C = 4.4 Hz, CHMeMe), 45.64 (NMe ), 56.74 (CH N),
2
PC
Hz, CHMeMe), 25.87 (dd, JPC = 8.6, JP′C = 4.5 Hz, CHMeMe), 25.20
THF), 68.15 (THF), 127.26 (d, JPC = 9.4 Hz, ArC), 131.57 (d, JPC
.7 Hz, ArC), 143.71 (dd, JPC = 58.7, JP′C = 7.0 Hz, ipso-ArC)
2
2
1
3
1
(
=
-
8
8
2
7
1
[
remaining aromatic signal obscured by solvent]. Li{ H} NMR (d -
toluene): δ 0.3 (s). B{ H} NMR (d -toluene): δ −36.1 (d, J = 91
8
11
1
J
8
PB
2
2
Hz). 31P{ H} NMR (d -toluene): δ 9.1 (d, J = 166 Hz, PiPr ), 14.1
1
127.39 (d, JPC = 9.6 Hz, ArC), 128.00 (d, JPC = 2.4 Hz, ArC), 131.57
(d, JPC = 9.1 Hz, ArC), 143.34 (dd, JPC = 61.0, JP′C = 4.4 Hz, ipsoArC).
8
PP
2
(
m, JPB = 91, JPP = 166 Hz, P(BH )Ph ).
3
2
7
1
11
1
Preparation of Ph PCH P(BH )iPr (2a). Compound 1a (1.32 g,
Li{ H} NMR (d
8
-toluene): δ 0.6 (s). B{ H} NMR (d -toluene): δ
8
2
2
3
2
−40.9 (d, J = 92 Hz), −35.7 (d, J = 88 Hz). 31P{ H} NMR (d -
toluene): δ 11.4 (br m), 26.6 (br m).
Preparation of Ph PCH PiPr (4a). A solution of 1a (1.04 g, 3.15
1
4
.07 mmol) was dissolved in toluene (30 mL), and this solution was
PB
PB
8
heated under reflux for 12 h. The solvent was removed in vacuo to give
2
1
11
a as a viscous oil. Yield: 1.32 g, 100%. H{ B} NMR (CDCl ): δ 0.37
3
2
2
2
(
d, JPH = 15.0 Hz, 3H, BH ), 1.12 (dd, J = 14.1, JHH = 7.3 Hz, 6H,
mmol) in degassed methanol (50 mL) was heated under reflux for 24
3
PH
CHMeMe), 1.16 (dd, JPH = 14.7, JHH = 7.1 Hz, 6H, CHMeMe), 2.10
h. The solution was cooled to room temperature, and volatiles were
removed in vacuo to leave 4a as a colorless oil. Yield: 0.88 g, 88%. H
1
(
(
m, 2H, CHMeMe), 2.37 (d, J = 9.5 Hz, 2H, P CH ), 7.33−7.51
PH
2
2
m, 10H, ArH). 13C{ H} NMR (CDCl ): δ 17.25 (d, J = 1.8 Hz,
1
NMR (CDCl ): δ 1.04 (m, 6H, CHMeMe), 1.09 (m, 6H, CHMeMe),
3
PC
3
CHMeMe), 17.31 (d, JPC = 2.8 Hz, CHMeMe), 19.87 (dd, JPC = 33.0,
JP′C = 26.7 Hz, CH ), 22.58 (dd, J = 32.2, JP′C = 4.0 Hz CHMeMe),
1.83 (m, 2H, CHMeMe), 2.06 (br, 2H, CH P ), 7.31−7.35 (m, 6H,
2
2
13
1
ArH), 7.45−7.49 (m, 4H, ArH). C{ H} NMR (CDCl ): δ 19.00 (d,
2
PC
3
1
28.74 (d, JPC = 7.3 Hz, ArC), 129.26 (ArC), 133.0 (d, J = 20.7 Hz,
J
= 10.1 Hz, CHMeMe), 19.77 (d, J = 14.9 Hz, CHMeMe), 20.87
PC
PC PC
1
1
1
ArC), 138.71 (dd, JPC = 14.3, JP′C = 5.6 Hz, ipso-ArC). B{ H} NMR
(m, CH P ), 24.34 (m, CHMeMe), 128.44 (d, J = 6.5 Hz, Ar),
128.68, 133.01 (Ar), 139.62 (br, ipso-Ar). P{ H} NMR (CDCl ): δ
2
2
P
C
CDCl ): δ −43.5 (d, J = 59 Hz). 31P{ H} NMR (CDCl ): δ −26.7
d, J = 38 Hz, PPh ), 35.5 (m, P(BH )iPr ).
1
31
1
(
(
3
PB
3
3
−19.2 (d, J = 123 Hz, PPh ), −3.8 (d, J = 123 Hz, PiPr2).
PP
2
3
2
PP
2
PP
Preparation of [Ph PCH{P(BH )iPr }]Li(tmeda) (2b). To a cold
Preparation of [Ph PCHPiPr ]Li(tmeda) (4b). To a solution of
2
3
2
2 2
(
0 °C) solution of 1b (1.24 g, 3.76 mmol) in THF (30 mL) was added
4a (0.54 g, 1.71 mmol) in THF (10 mL) was added nBuLi (0.7 mL,
1.75 mmol) and tmeda (0.2 mL, 0.26 g, 2.24 mmol), and this solution
was stirred for 1 h. Solvent was removed in vacuo, the sticky yellow
residue was dissolved in diethyl ether (5 mL), and this solution was
cooled to 5 °C for 16 h. The pale yellow crystals of 4b were isolated
nBuLi (1.50 mL, 3.76 mmol). This mixture was allowed to warm to
room temperature and was stirred for 1 h. To this solution was added
tmeda (0.57 mL, 3.76 mmol), and this mixture was stirred for 5 min.
Solvent was removed in vacuo, leaving an orange oil, which was
J
dx.doi.org/10.1021/om5005995 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
and washed with a little light petroleum. Yield: 0.65 g, 87%. Anal.
Calcd for C H LiN P (438.50): C 68.48, H 9.42, N 6.39. Found: C
ASSOCIATED CONTENT
Supporting Information
For 1a−4a H, C{ H}, B{ H} and P{ H} NMR spectra.
For 1a, 1b, 1c, 2b, 3b, and 4b details of structure
determination, atomic coordinates, bond lengths and angles,
and displacement parameters in CIF format; details of the
molecular structures of 1a and 1c. For the conversion of 1 into
■
25
41
2 2
1
*
S
6
8.37, H 9.29, N 6.28. H NMR (d -THF): 0.93 (dd, J = 14.6, JHH
=
1
13
1
11
1
31
1
8
PH
6
.7 Hz, 6H, CHMeMe), 0.95 (m, 1H, CHLi), 1.00 (dd, JPH = 10.7, JHH
7.0 Hz, 6H, CHMeMe), 1.52 (m, 2H, CHMeMe), 2.14 (s, 12H,
NMe ), 2.29 (s, 4H, CH N), 6.89 (m, 2H, ArH), 7.02 (m, 4H, ArH),
=
2
2
13
1
7
.52 (m, 4H, ArH). C{ H} NMR (d -THF): δ 9.52 (m, CHLi),
8
1
8.42 (d, JPC = 4.4 Hz, CHMeMe), 20.90 (d, JPC = 16.7 Hz,
2
a a plot showing the concentration dependence of the reaction
and an Eyring plot from whence the thermodynamic
CHMeMe), 26.51 (d, JPC = 14.7 Hz, CHMeMe), 45.35 (NMe ), 57.93
2
(
1
CH N), 124.30 (Ar), 126.33 (d, J = 5.6 Hz, Ar), 130.92 (d, J
=
2
PC
PC
7
parameters were derived. For 1a′−4a′, 1b′−4b′, 1intra′, and
5.2 Hz, Ar), 152.24 (dd, J = 15.1, JP′C = 10.4 Hz, Ar). Li NMR (d -
P
C
8
31
1
1
inter′ details of DFT calculations, final atomic coordinates, and
THF): δ 0.1 (s). P{ H} NMR: δ 0.4 (d, J = 381 Hz, PPh ), 20.9
PP
2
energies. The supplemental file combined.xyz contains the
computed Cartesian coordinates of all of the molecules
reported in this study. The file may be opened as a text file
to read the coordinates or opened directly by a molecular
modeling program such as Mercury (version 3.3 or later,
http://www.ccdc.cam.ac.uk/pages/Home.aspx) for visualiza-
tion and analysis. This material is available free of charge via
the Internet at http://pubs.acs.org.
(
d, JPP = 381 Hz, PiPr2).
Kinetic Studies. The kinetic studies on the isomerization of 1a to
a were carried out on 0.17 M solutions in toluene (taken from a stock
2
31
1
solution kept at −30 °C). P{ H} NMR spectra were recorded over 5
min at 20 min intervals with a repetition time of 3 s. The spin−lattice
relaxation times of the 31P nuclei of 1a and 2a were determined to be
nearly identical (T = 5.79 and 5.86 s, respectively), and so integrals
were measured directly from the spectra with no correction.
1
Crystal Structure Determinations of 1a, 1b, 1c, 2b, 3b, and
4
(
b. Measurements were made at 150 K on an Oxford Diffraction
Agilent Technologies) Gemini A Ultra diffractometer, using Cu Kα
radiation (λ = 1.541 78 Å; 1a) or Mo Kα radiation (λ = 0.710 73 Å;
b−4b, 1c). Cell parameters were refined from the observed positions
AUTHOR INFORMATION
Corresponding Authors
E-mail: keith.izod@ncl.ac.uk.
E-mail: corinne.wills@ncl.ac.uk.
Notes
■
*
1
*
of all strong reflections. Intensities were corrected semiempirically for
absorption, based on symmetry-equivalent and repeated reflections.
The authors declare no competing financial interest.
2
The structures were solved by direct methods and refined on F values
for all unique data (see the Supporting Information for further details).
All non-hydrogen atoms were refined anisotropically, and C-bound H
atoms were constrained with a riding model, while B-bound H atoms
were freely refined; U(H) was set at 1.2 (1.5 for methyl groups) times
Ueq for the parent C atom. Programs were Oxford Diffraction
CrysAlisPro for data collection and processing and SHELXTL for
ACKNOWLEDGMENTS
■
The authors are grateful to Newcastle University for financial
support and to Prof. R. A. Henderson (Newcastle) for helpful
discussions of the kinetic data and would like to acknowledge
the use of the EPSRC UK National Service for Computational
Chemistry Software (NSCCS) at Imperial College London in
carrying out this work.
26
structure solution, refinement, and molecular graphics.
DFT Calculations. Geometry optimizations were performed with
2
7
the Gaussian09 suite of programs (revision B.01, C.01, or D.01).
Ground-state optimizations were performed using the GGA meta-
REFERENCES
■
1
3
hybrid wB97XD functional, which includes a correction for
(1) For recent reviews on the chemistry of phosphine-borane adducts
and their applications see: (a) Ohff, M.; Holz, J.; Quirmbach, M.;
Borner, A. Synthesis 1998, 1391. (b) Brunel, J. M.; Faure, B.; Maffei,
M. Coord. Chem. Rev. 1998, 178−180, 665. (c) Carboni, B.; Monnier,
L. Tetrahedron 1999, 55, 1197. (d) Gaumont, A. C.; Carboni, B. In
Science of Synthesis; Kaufmann, D.; Matteson, D. S., Eds.; Thieme:
Stuttgart, 2004; Vol. 6, pp 485−512. (e) Staubitz, A.; Robertson, A. P.
M.; Sloan, M. E.; Manners, I. Chem. Rev. 2010, 110, 4023.
16
dispersion effects; the 6-311+G(d,p) all-electron basis set was used
on all atoms [default parameters were used throughout]. The global
minimum energy conformations of 1a′−4a′ were located by relaxed
potential energy surface scans at the HF/3-21G* level in which the P−
C−P−C(iPr) dihedral angle was increased in 10° steps through a full
3
60° rotation; the located minimum energy geometries were then
reoptimized at the wB97XD/6-311+G(d,p) level. The identity of
minima was confirmed by the absence of imaginary vibrational
frequencies in each case. Natural bond orbital analyses were performed
(
2) (a) Imamoto, T.; Oshiki, T.; Onazawa, T.; Kusumoto, T.; Sato,
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23
using the NBO 3.1 module of Gaussian09. For the rearrangement of
a′ to 2a′ the transition states 1 ′ and 1 ′ were initially located
1
intra
inter
28
29
using the QST3 method at the HF/3-21G* level; the geometries
obtained were then reoptimized at the wB97XD/6-311+G(d,p) level.
The nature of these transition states was confirmed by the presence of
a single imaginary vibrational frequency, which correlated with the
(b) Schro
7, 1931.
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der, M.; Nozaki, K.; Hiyama, T. Bull. Chem. Soc. Jpn. 2004,
7
(
(
expected displacement vector for migration of the BH group between
3
the two phosphine centers in each case. For the dissociation of 1a′ and
(
3
a′ and the migration of BH between two molecules of 1a′ via 1
′
3
inter
Tyson, B. V.; Clegg, W.; Harrington, R. W. Chem. Commun. 2004, 570.
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the free energy was corrected for basis set superposition error (using
30
the counterpoise method); the absence of a transition state for the
dissociation of both 1a′ and 3a′ was confirmed by performing a
relaxed potential energy surface scan, increasing the P−B distance in
2
2
0 increments of 0.1 Å, which showed no maximum on the pathway to
complete dissociation. In all calculations solvation by toluene was
1
7
included implicitly using the IEF Polarizable Continuum Model. All
energies were corrected to 298.15 K.
K
dx.doi.org/10.1021/om5005995 | Organometallics XXXX, XXX, XXX−XXX

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dx.doi.org/10.1021/om5005995 | Organometallics XXXX, XXX, XXX−XXX