Thermodynamic studies and hydride transfer reactions from a rhodium complex to BX3 compounds

Article abstract of DOI:10.1021/ja905287q

This study examines the use of transition-metal hydride complexes that can be generated by the heterolytic cleavage of H₂ gas to form B-H bonds. Specifically, these studies are focused on providing a reliable and quantitative method for determining when hydride transfer from transition-metal hydrides to three-coordinate BX₃ (X = OR, SPh, F, H; R = Ph, p-C ₆H₄OMe, C₆F₅, ^tBu, Si(Me)₃) compounds will be favorable. This involves both experimental and theoretical determinations of hydride transfer abilities. Thermodynamic hydride donor abilities (ΔG°_H-) were determined for HRh(dmpe)₂ and HRh(depe)₂, where dmpe ) 1,2-bis(dimethylphosphinoethane) and depe ) 1,2-bis(diethylphosphinoethane), on a previously established scale in acetonitrile. This hydride donor ability was used to determine the hydride donor ability of [HBEt₃]^- on this scale. Isodesmic reactions between [HBEt₃]^- and selected BX3 compounds to form BEt₃ and [HBX₃]^- were examined computationally to determine their relative hydride affinities. The use of these scales of hydride donor abilities and hydride affinities for transition-metal hydrides and BX₃ compounds is illustrated with a few selected reactions relevant to the regeneration of ammonia borane. Our findings indicate that it is possible to form B-H bonds from B-X bonds, and the extent to which BX₃ compounds are reduced by transition-metal hydride complexes forming species containing multiple B-H bonds depends on the heterolytic B-X bond energy. An example is the reduction of B(SPh)₃ using HRh(dmpe)₂ in the presence of triethylamine to form Et ₃N-BH₃ in high yields.

Full text of DOI:10.1021/ja905287q

Published on Web 09/15/2009
Thermodynamic Studies and Hydride Transfer Reactions from
a Rhodium Complex to BX3 Compounds
Michael T. Mock, Robert G. Potter, Donald M. Camaioni, Jun Li,§
William G. Dougherty,† W. Scott Kassel,† Brendan Twamley,‡ and Daniel L. DuBois*
Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352,
Department of Chemistry, VillanoVa UniVersity, VillanoVa, PennsylVania 19085, and
Department of Chemistry, UniVersity of Idaho, Moscow, Idaho 83844
Received June 26, 2009; E-mail: daniel.dubois@pnl.gov
Abstract: This study examines the use of transition-metal hydride complexes that can be generated by
the heterolytic cleavage of H2 gas to form B-H bonds. Specifically, these studies are focused on providing
a reliable and quantitative method for determining when hydride transfer from transition-metal hydrides to
t
three-coordinate BX3 (X ) OR, SPh, F, H; R ) Ph, p-C6H4OMe, C6F5, Bu, Si(Me)3) compounds will be
favorable. This involves both experimental and theoretical determinations of hydride transfer abilities.
-
Thermodynamic hydride donor abilities (∆G°H ) were determined for HRh(dmpe)2 and HRh(depe)2, where
dmpe ) 1,2-bis(dimethylphosphinoethane) and depe ) 1,2-bis(diethylphosphinoethane), on a previously
established scale in acetonitrile. This hydride donor ability was used to determine the hydride donor ability
of [HBEt3]- on this scale. Isodesmic reactions between [HBEt3]- and selected BX3 compounds to form
BEt3 and [HBX3]- were examined computationally to determine their relative hydride affinities. The use of
these scales of hydride donor abilities and hydride affinities for transition-metal hydrides and BX3 compounds
is illustrated with a few selected reactions relevant to the regeneration of ammonia borane. Our findings
indicate that it is possible to form B-H bonds from B-X bonds, and the extent to which BX3 com-
pounds are reduced by transition-metal hydride complexes forming species containing multiple B-H bonds
depends on the heterolytic B-X bond energy. An example is the reduction of B(SPh)3 using HRh(dmpe)2
in the presence of triethylamine to form Et3N-BH3 in high yields.
Introduction
various methods of hydrogen release from AB have been
published,3 however, only a few reports on the regeneration
As the need for clean and renewable sources of energy
grows, attention has increasingly focused on using hydrogen
as an alternative to hydrocarbon fuels.1 The development of
new hydrogen storage methods and materials has become an
area of intense research, and safe and convenient methods
to transport, store, and distribute hydrogen continue to evolve.
In particular, ammonia borane (AB, NH3BH3) with a gravi-
metric density of 19.6 wt % H2 is a promising candidate for
hydrogen storage.2 Several important studies addressing
of AB from dehydrogenated material have appeared in the
literature.4 These methods use energy intensive reagents such
as LiAlH4 or MgH2 to achieve B-H bond formation from
B-O or B-Cl bonds resulting from spent fuel digestion. As
a result, an energy efficient method for the regeneration of
AB remains a significant challenge for using AB as a
hydrogen storage material.
As an alternative approach to AB regeneration, we are
exploring the use of transition-metal hydride complexes that
can be generated by the heterolytic cleavage of H2 gas to
regenerate B-H bonds from B-X bonds in model com-
pounds potentially generated from the digestion of dehydro-
genated material with alcohols,5 phenols, or thiols6 to form
BX3 compounds (BX3 ) aryl, alkyl or thio borate esters). In
previous publications, we have focused on understanding the
structural and electronic features controlling the hydride
† Villanova University.
‡ University of Idaho.
§ Present address: Department of Chemistry, Tsinghua University,
Beijing, China.
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14454 J. AM. CHEM. SOC. 2009, 131, 14454–14465
10.1021/ja905287q CCC: $40.75  2009 American Chemical Society

A R T I C L E S
donor abilities of transition-metal hydride complexes with
chelating diphosphine ligands, e.g. HM(diphosphine)2 and
[HM′(diphosphine)2]+ complexes where M is Co or Rh and
M′ is Ni, Pd, or Pt.7 These complexes exhibit a large range
the hydride donor ability of [HBEt3]- on this scale. Third, a series
of isodesmic reactions between [HBEt3]- and BX3 compounds to
form BEt3 and [HBX3]- are studied computationally to determine
the relative hydride affinities of the BX3 compounds. The result is
a series of transition-metal hydride complexes of known hydride
donor abilities obtained from previous studies and a series of BX3
compounds with hydride acceptor abilities determined computa-
tionally. This information can be used to predict when hydride
transfer reactions between transition-metal complexes and three-
coordinate boron compounds should occur and what the driving
force for these reactions should be. The use of this extensive scale
of hydride donor abilities for transition-metal hydrides and BX3
compounds is illustrated with a few selected reactions relevant to
the regeneration of AB.
of hydride donor abilities with the free energies associated
8
-
with reaction 1 (∆G°H ) ranging from 34 to 76 kcal/mol,
-
with smaller values of ∆G°H representing better hydride
donors.
[HM(diphosphine)2]n+ f [M(diphosphine)2](n+1)+ + H- (1)
These studies revealed that strong hydride donors are
favored by electron-donating substituents on the diphosphine
ligands9 and ligands with small natural bite angles.10 In
addition, second row transition-metal hydride complexes are
better hydride donors than either the first or third row
complexes.11 This led us to study HRh(dmpe)2, where dmpe
) 1,2-bis(dimethylphosphinoethane), one of the strongest
hydride donors of this type, for its ability to transfer a hydride
ligand to triethylborane, BEt3, to form [HBEt3]-, Super-
Hydride.12 This is important because HRh(dmpe)2 can be
generated from H2 and [Rh(dmpe)2]+ in the presence of a
strong base, potentially providing a more direct route for the
production of boron hydride compounds from hydrogen.
Herein we describe our efforts to provide a reliable and
quantitative method for determining when hydride transfer from
transition-metal hydrides to three-coordinate BX3 compounds will
be favorable. This involves both experimental and theoretical
determinations of hydride transfer abilities. In addition to under-
standing the thermodynamic feasibility of transferring a hydride
ligand from a transition-metal hydride to boron compounds, it is
also important that the resulting [HBX3]- anions undergo ligand
redistribution reactions or further reaction with the transition-metal
hydride to form species containing multiple B-H bonds. To
achieve this goal, hydride transfer reactions from HRh(dmpe)2 to
B(SPh)3 were studied and shown to result in the cleavage of B-S
bonds and the formation of multiple B-H bonds.
Thermodynamic Studies of [(H)2Rh(dmpe)2][CF3SO3]. Previ-
ously, we studied the reactions of H2 with [Rh(dmpe)2][CF3SO3]
and [Rh(depe)2][CF3SO3], (depe ) 1,2-bis(diethylphosphinoet-
hane)) as shown on the left in reaction 2, and determined
equilibrium constants of 2.1 ( 0.1 atm-1 and 0.23 ( 0.02 atm-1,
respectively, using acetonitrile as the solvent.12 Reactions
of [Rh(dmpe)2][CF3SO3] with H2 in the presence of strong
bases provided a qualitative measure of the acidity of
[(H)2Rh(dmpe)2]+. For example, potassium tert-butoxide
(pKa tert-butanol ) 32.2 in Me2SO),13 deprotonates [(H)2-
Rh(dmpe)2]+, establishing an equilibrium between [(H)2Rh-
(dmpe)2]+ and HRh(dmpe)2. This result indicated that the acidity
of [(H)2Rh(dmpe)2]+ was low and quantitative pKa measurements
of [(H)2Rh(dmpe)2]+ would require base strengths greater than 30
pKa units in acetonitrile. Ideally, measurements of this type should
be performed in acetonitrile, where a large body of compara-
tive data is available and the effects of ion-pairing and self-
association can be minimized.14 Unfortunately, it was not
possible to measure pKa values of these complexes in
acetonitrile or benzonitrile, due to undesirable side reactions
of HRh(dmpe)2 and HRh(depe)2 with these solvents. After
surveying several bases, we found that the isopropyl deriva-
tive of Verkade’s base (VSB-ipr) (P(RNCH2CH2)3N, R ) iPr),
a proazaphosphatrane nonionic superbase, was capable of
meeting our requirements. It is sufficiently basic (pKa of 33.6
in acetonitrile15), the protonated and unprotonated forms of
the base exhibit complete solubility in THF, and the bulky
isopropyl groups surrounding the basic phosphorus site should
minimize self-association between protonated and unproto-
nated forms of the base.
Results and Discussion
The main objective of this research is to provide a reliable and
quantitative method for predicting when B-H bond formation from
transition-metal hydrides will be favorable. This can be achieved
-
in three steps. First, the hydride donor ability (∆G°H ) of HRh(d-
mpe)2 on the previously established scale in acetonitrile is
determined. Second, this hydride donor ability is used to determine
(7) (a) Berning, D. E.; Noll, B. C.; DuBois, D. L. J. Am. Chem. Soc.
1999, 121, 11432. (b) Curtis, C. J.; Miedaner, A.; Ellis, W. W.;
DuBois, D. L. J. Am. Chem. Soc. 2002, 124, 1918.
(19) Raebiger, J. W.; DuBois, D. L. Organometallics 2005, 24, 110.
(20) Grant, D. J.; Dixon, D. A.; Camaioni, D. M.; Potter, R. G.; Christe,
K. O. Inorg. Chem. 2009, DOI: 10.1021/ic901092x.
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N. J.; Marder, T. B. Organometallics 1990, 9, 3028.
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(8) (a) Price, A. J.; Ciancanelli, R.; Noll, B. C.; Curtis, C. J.; DuBois,
D. L.; DuBois, M. R. Organometallics 2002, 21, 4833. (b) Miedaner,
A.; Raebiger, J. W.; Curtis, C. J.; Miller, S. M.; DuBois, D. L.
Organometallics 2004, 23, 2670.
(9) Berning, D. E.; Miedaner, A.; Curtis, C. J.; Noll, B. C.; DuBois, M. R.;
DuBois, D. L. Organometallics 2001, 20, 1832.
(23) Marks, T. J.; Kolb, J. R. Chem. ReV. 1977, 77, 263.
(24) A search of the 2008 Cambridge Structural Data Center (CSDC)
revealed two coordinatively unsaturated metal complexes containing
an outer sphere, non-coordinating BH4 anion: (a) Antsyshkina, A. S.;
Porai-Koshits, M. A.; Makhaev, V. D.; Borisov, A. P.; Mal’tseva, N. N.
Koord. Khim. 1992, 18, 474. (b) Burgess, K.; van der Donk, W. A.;
Westcott, S. A.; Marder, T. B.; Baker, R. T.; Calabrese, J. C. J. Am.
Chem. Soc. 1992, 114, 9350.
(10) Raebiger, J. W.; Miedaner, A.; Curtis, C. J.; Miller, S. M.; DuBois,
D. L. J. Am. Chem. Soc. 2004, 126, 5502.
(11) Curtis, C. J.; Miedaner, A.; Raebiger, J. W.; DuBois, D. L. Organo-
metallics 2004, 23, 511.
(12) DuBois, D. L.; Blake, D. M.; Miedaner, A.; Curtis, C. J.; DuBois,
M. R.; Franz, J. A.; Linehan, J. C. Organometallics 2006, 25, 4414.
(13) Olmstead, W. N.; Margolin, Z.; Bordwell, F. G. J. Org. Chem. 1980,
45, 3295.
(25) Elliot, J. R.; Boldebuck, E. M.; Roedel, G. F. J. Am. Chem. Soc. 1952,
74, 5047.
(14) Jordan, R. F.; Norton, J. R. J. Am. Chem. Soc. 1982, 104, 1255.
(15) Kisanga, P. B.; Verkade, J. G. J. Org. Chem. 2000, 65, 5431.
(16) Qi, X.-J.; Liu, L.; Fu, Y.; Guo, Q.-X. Organometallics 2006, 25, 5879.
(17) Wayner, D. D. M.; Parker, V. D. Acc. Chem. Res. 1993, 26, 287.
(18) Qi, X.-J.; Fu, Y.; Liu, L.; Guo, Q.-X. Organometallics 2007, 26, 4197.
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2680.
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A R T I C L E S
Mock et al.
Upon bubbling H2 through a solution of [Rh(dmpe)2]-
[CF3SO3] in the presence of excess VSB-ipr, an equilibrium
was observed in THF (shown on right in reaction 2). This
reaction was monitored by 31P NMR spectroscopy and
resonances corresponding to [(H)2Rh(dmpe)2]+ and HRh(d-
mpe)2 were integrated to determine the ratios of these two
products. These ratios were used to calculate an equilibrium
constant of (1.2 ( 0.3) × 10-3. Assuming that this equilib-
rium constant in THF is a reasonable estimate of the
equilibrium constant in acetonitrile, a pKa value of 36.5 can
be calculated for [(H)2Rh(dmpe)2][CF3SO3] in acetonitrile.
The assumption of nearly equivalent equilibrium constants
for THF and acetonitrile solutions should be reasonable given
that there is no net change in the charge of the species from
one side of the equilibrium to the other and that the rhodium
cation and the cationic protonated base are of similar size.
Because of the limited choices of solvents available for this
reaction, this pKa value for [(H)2Rh(dmpe)2]+ in acetonitrile
provides the best possible experimental value available to
us at the present time. This value is in good agreement with
the computationally derived pKa value of 36.7 determined
by Liu and co-workers16 in a study that developed an ab initio
protocol to predict the pKa values of transition-metal hydride
complexes in acetonitrile.
HRh(dmpe)2 + H+ a [(H)2Rh(dmpe)2]+
(3)
[(H)2Rh(dmpe)2]+ a [Rh(dmpe)2]+ + H2
H2 f H+ + H-
(4)
(5)
(6)
(7)
HRh(dmpe)2 f [Rh(dmpe)2]+ + H-
∆GoH - 1.37pKa + ∆GH2 + 76.0
-
As noted above, we reported that HRh(dmpe)2 has a hydride
donor ability equal to that of Li[HBEt3], more commonly known
as Super-Hydride.12 This result was based on H NMR spectral
1
data indicating the formation of an equilibrium between [HBEt3]-
and BEt3 upon titrating HRh(dmpe)2 with BEt3. These results were
corroborated by the disappearance of HRh(dmpe)2 and the simul-
taneous formation of [Rh(dmpe)2]+ in the 31P NMR spectrum.
Furthermore, when Li[HBEt3] was added to [Rh(dmpe)2][CF3SO3],
HRh(dmpe)2 and [Rh(dmpe)2][CF3SO3] were observed by 31P
NMR, as expected for a true equilibrium. These results indicate
that the hydride donor ability of Li[HBEt3] should be approximately
26 kcal/mol on the acetonitrile hydride donor scale.
Determination of Hydride Affinities of BX3 Compounds. To
determine the relative hydride acceptor abilities or hydride
affinities of BX3 compounds, we performed electronic structure
calculations on the isodesmic reaction of BX3 with [HBEt3]-,
shown in reaction 8.
Hydride Donor Ability of HRh(dmpe)2 and [HBEt3]-. The
hydride donor ability of HRh(dmpe)2 can be determined using
reactions 3-5. Reaction 3 is the protonation of HRh(dmpe)2 to
form [(H)2Rh(dmpe)2]+. The free energy associated with this
reaction is -50.0 kcal/mol (-1.37 × pKa). Reaction 4, loss of
H2 from [(H)2Rh(dmpe)2][CF3SO3], is the reverse of the H2
addition reaction discussed above, and the free energy associated
with this reaction is +0.4 kcal/mol (RT ln KH2). Reaction 5 is
the heterolytic cleavage of H2 gas to form H+ and H- solvated
in acetonitrile. The free energy of this reaction is +76.0 kcal/
mol.17 The sum of reactions 3-5 is reaction 6, the heterolytic
cleavage of the Rh-H bond of HRh(dmpe)2 to form [Rh-
HBEt3- + BX3 ∆H°8 BEt3 + HBX3-
(8)
We follow the convention where the gas-phase hydride
affinity is defined as the negative of the ∆H° value for
addition of H- to a molecule in the gas-phase. The hydride
affinity of BH3 is known from accurate calculations20 to be
73.7 kcal/mol. From this value and ∆H° ) -15.5 calculated
for the hydride transfer from [HBEt3]- to BH3, we estimate
the hydride affinity of BEt3 to be 58.2 kcal/mol in the gas-
phase. Our results show that the hydride affinity for BX3
compounds varies over a wide range, depending on the nature
of X. Table 1 summarizes the computationally derived gas-
phase hydride affinity values for the BX3 compounds used
(dmpe)2]+ and H-. The free energy of reaction 6 (∆G°H ) is
-
26.4 kcal/mol, the sum of the free energies associated with
reactions 3-5 (eq 7). This experimentally derived value is
comparable to the computationally determined hydricity
-
(∆G°H ) of 23.6 calculated for HRh(dmpe)2 by Liu and co-
workers.18 To determine the hydride donor ability of
HRh(depe)2 we refer to a previous study12 where the equi-
librium constant for diphosphine ligand exchange reactions
between HRh(dmpe)2 and HRh(depe)2 was determined. This
measurement indicated that HRh(dmpe)2 is a better hydride
donor than HRh(depe)2 by 1.7 kcal/mol. On the basis of this
information, the free energy for the heterolytic cleavage of
the Rh-H bond of HRh(depe)2 to form [Rh(depe)2]+ and H-
is 28.1 kcal/mol. As a check on this value, the hydride donor
abilities of HRh(depx)2 (45 kcal/mol)19 and [HPd(depx)2]+
(61 kcal/mol)10 (depx ) 1,2-(Et2PCH2)2C6H4) differ by 16
kcal/mol. A similar difference would be expected between
HRh(depe)2 and [HPd(depe)2]+, for which a value of 43 kcal/
mol has been determined.9 On this basis we would estimate
Table 1. Hydride Affinity Values of BX3 Compounds Calculated
Using the B3LYP/6-311+G** Level of Theory
BX3 compound
hydride affinity -∆H° (kcal/mol)
B(OtBu)3
B(OSiMe3)3
BEt3
B(SMe)3
B(p-OC6H4OMe)3
BH3
B(OPh)3
HB(S2C6H4)
BF3
38.0
46.4
58.2
69.0
69.9
73.7
74.0
74.3
75.5
86.9
94.3
B(SPh)3
B(OC6F5)3
-
a value of 27 kcal/mol for ∆G°H for HRh(depe)2, in good
agreement with our measured value of 28.1 kcal/mol.
9
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A R T I C L E S
Figure 1. Scales of experimentally determined hydride donor abilities of selected transition-metal hydride complexes in acetonitrile (blue, bottom) and the
calculated gas-phase hydride affinity values for selected BX3 compounds (red, top).
Figure 2. 1H-coupled 11B NMR spectrum of the reaction between Li[HBEt3] and B(OPh)3 recorded immediately after mixing.
Figure 3. 1H-coupled 11B NMR spectrum of the reaction between Li[HBEt3] and B(OPh)3 in a 10% NEt3 ·THF mixture recorded after heating at 50 °C for
5 h.
in this study. Because HRh(dmpe)2 and [HBEt3]- have the
same hydride donor abilities, it is possible to relate these
two scales as shown in Figure 1, assuming constant T∆S and
solvent corrections. This quantitative approach to examining
hydride transfer reactions predicts that HRh(dmpe)2 and
Li[HBEt3] have the ability to transfer a hydride ligand to
BX3 compounds with hydride affinities greater than 58 kcal/
mol. In accord with this result, the selected transition-metal
hydrides included in Figure 1 should have the ability to
transfer a hydride ligand to BX3 compounds located to the
right of their position on the parallel hydride affinity scale.
Hydride Transfer Reactions from Triethylborohydride to
change (∆Hr° ) 20 kcal/mol) of this reaction. In contrast,
B(OPh)3 readily accepts hydride from Li[HBEt3] even when
less than stoichiometric amounts of hydride are used, consistent
with the calculated large negative enthalpy change (∆Hr ) -16
1
kcal/mol) of this reaction. Figure 2 shows the H-coupled 11B
NMR spectrum of the reaction mixture, indicating that Li[H-
B(OPh)3] is formed as well as products of ligand redistribution,
1
Li[B(OPh)4] and Li[BH4]. Furthermore, as shown by the H-
coupled 11B NMR spectrum in Figure 3, when the reaction is
carried out in the presence of triethylamine, a small amount of
Et3N-BH3 is formed instead of Li[BH4].
Hydride Transfer Reactions from Rhodium to Borate Esters
B(OR)3 (R ) Ph, C6F5, p-C6H4OMe, Si(CH3)3, tBu). To validate
that the juxtaposition of the hydride affinity and hydride donor
ability scales shown in Figure 1 is correct, the reactions between
HRh(dmpe)2 and selected B(OR)3 compounds were performed
in THF, according to reaction 9 where n ) 0, 2, or 3.
t
Borate Esters, B(OR)3 (R ) Bu, Ph). To determine whether
the calculated gas-phase hydride affinities may be useful in
predicting hydride transfer in solution between boron com-
pounds, we performed reactions of the subject borate esters with
Li[HBEt3]. The reagents were mixed in THF, and the reaction
was monitored by 11B NMR spectroscopy. B(OtBu)3, the poorest
hydride acceptor of the BX3 compounds in this study, does not
accept a hydride even with a 10-fold excess of Li[HBEt3]. This
result is consistent with the calculated large positive enthalpy
HRh(dmpe)2 + B(OR)3 THF8 [Rh(dmpe)2][HB(OR)3] +
[Rh(dmpe)2][HnB(OR)4-n
]
(9)
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A R T I C L E S
Mock et al.
of Li[HB(OPh)3] and further discussion of the 11B chemical
shifts for this complex are provided in the Supporting Informa-
tion.
As indicated by the examples in Table 1, the substitution of
electron-donating or electron-withdrawing functionalities on the
aryl rings can dramatically alter the hydride affinity of the
BX3 compound. In addition to B(OPh)3, selected B(OR)3 com-
pounds, reflecting a wide range of hydride affinity values, were
examined. Hydride transfer reactions between excess HRh(d-
mpe)2 and the borate esters B(OC6F5)3 and B(p-OC6H4OMe)3
resulted in similar B-H-containing products, while experiments
conducted using excess HRh(dmpe)2 with the borate esters
B(OSi(Me)3)3 and B(OtBu)3, provided no spectroscopic evidence
of hydride transfer even after heating at 50 °C for 3 days.
Notably, the formation of [B(OR)4]- was routinely encountered
as a major reaction product, likely resulting from redistribution
of the aryloxy group. Hydride transfer does not occur between
HRh(dmpe)2 and [B(OAr)4]-, highlighting the stability of the
four-coordinate borate ester. A description of these reactions
and discussion regarding [B(OAr)4]- formation is included in
the Supporting Information.
Figure 4. (A) 1H-coupled 11B NMR spectrum recorded at 50 °C from the
hydride transfer reaction of HRh(dmpe)2 to B(OPh)3. (B) The corresponding
11B{1H} NMR spectrum recorded at 50 °C.
Table 2. 11B NMR Data for Products Formed via Hydride Transfer
from HRh(dmpe)2
M ) [Rh(dmpe)2]+
11B NMR δ (ppm)
JB-H (Hz)
Hydride Transfer Reactions from Rhodium to Borane-THF.
We previously reported that attempts to synthesize HRh(dmpe)2
by treatment of [Rh(dmpe)2][CF3SO3] with bis(triphenylphos-
phine)iminium borohydride (PPNBH4) in THF were unsuccess-
ful.12 This indicated that the tetrahydroborate anion is not
sufficiently hydridic to transfer a hydride ligand to rhodium,
and that the reverse reaction, hydride transfer to BH3 ·THF from
HRh(dmpe)2, should be thermodynamically favorable. The
hydride affinity of BH3 is 73.7 kcal/mol,20 similar to the value
for B(OPh)3. A 11B{1H} NMR spectrum taken after the addition
of a 1.0 M BH3 ·THF solution to HRh(dmpe)2 in THF results
in the formation of a singlet observed at δ -37.4. The
1H-coupled 11B NMR spectrum reveals the expected quintet for
the tetrahydroborate anion assigned as [Rh(dmpe)2][BH4], (see
Figure S8 in the Supporting Information). A notable example
of a similar hydride transfer reaction was reported by Baker
and co-workers where HRh(dmpe)2 reacts with 0.5 equiv of
thexylborane ([BH(CMe2CHMe2)(µ-H)]2) via hydride transfer
to produce [Rh(dmpe)2][H3B(thexyl)] as determined by NMR
spectroscopy.21 In addition to hydride transfer product [Rh-
(dmpe)2][BH4], a competing reaction is frequently observed,
especially when excess quantities of BH3 · THF are added to
HRh(dmpe)2, resulting in the formation of dmpe-(BH3)2. This
side reaction can be attenuated by the addition of substo-
ichiometric amounts of BH3 · THF. Crystals of [Rh(dmpe)2]-
[BH4] suitable for an X-ray diffraction study were grown
from a concentrated THF solution. The molecular structure
of [Rh(dmpe)2][BH4], shown in Figure 5, contains a square-
planar cation formed by rhodium and two dmpe ligands. The
presence of discrete uncoordinated tetrahydroborate anions
as the counterion to the [Rh(dmpe)2]+ cations confirm that
hydride transfer from rhodium to boron has occurred. The
[Rh(dmpe)2]+ cation contains typical Rh-P bond distances
of 2.274(2) and 2.287(2) Å analogous to literature reports.22
The four hydrogen atoms of the tetrahydroborate anion were
located in the difference Fourier map. Selected bond lengths
and bond angles are listed in Table 3. Tetrahydroborate ions
commonly form covalent σ-complexes through bridging
hydrogen atoms with a variety of transition metals.23 This
can be especially common for complexes that are coordina-
tively unsaturated such as [Rh(dmpe)2]+. However, this is
a rare example24 where the molecular structure of [Rh-
M[HB(OPh)3]
M[B(OPh)4]
M[HB(OC6F5)3]
M[HB(p-OC6H4OMe)3]
M[BH4]
3.2
2.4
5.0
124
-
137
105
82
114
110
128
111
97
3.8
-37.4
Et3N-BH2F
3.0
M[H2B(S2C6H4)]
M[HB(SPh)3]
M[H2B(SPh)2]
M[H3B(SPh)]
M[B(SPh)]4
-11.5
-4.3
-14.5
-25.6
4.5
-6.5
-12.8
-
115
96
Et3N-BH2(SPh)
H3B-NEt3
Experimental results support the computational studies suggest-
ing that hydride transfer reactions are thermodynamically
favorable for boron compounds with calculated hydride affinities
greater than 58 kcal/mol. For example, the room temperature
reaction performed in THF between excess HRh(dmpe)2 and
B(OPh)3, with a hydride affinity of 74 kcal/mol, was monitored
by 31P NMR spectroscopy. Consumption of HRh(dmpe)2, (d, δ
1
26.5, JRhP ) 139 Hz) via hydride transfer, was indicated by
clean conversion to [Rh(dmpe)2]+, as indicated by 31P{1H} NMR
1
spectroscopy (d, δ 36.0, JRhP ) 125 Hz, identical to
[Rh(dmpe)2][CF3SO3]). The 11B{1H} NMR spectrum recorded
at 50 °C, (Figure 4, trace B) displays two major boron-containing
products, a sharp singlet at δ 2.4 and a broad singlet at δ 3.2.
The sharp singlet is assigned to [Rh(dmpe)2][B(OPh)4] based
on an X-ray diffraction and 11B{1H} NMR study of crystals
that precipitated from the reaction (see Figure S5 in the
Supporting Information). In the 1H-coupled 11B NMR spectrum
shown in trace A of Figure 4, the resonance at δ 3.2 is a broad
doublet (1JBH ) 124 Hz), confirming the formation of a B-H
bond. On the basis of this chemical shift, the B-H coupling
constant, and a B-H stretching frequency of 2350 cm-1
(KBr), the second product is attributed to the formation of
[Rh(dmpe)2][HB(OPh)3]. Table 2 summarizes the 11B chemical
shifts and B-H coupling constants for products formed via
hydride transfer reactions described in this study. The reaction
mixture described here and those discussed in the following
sections are generally stable for several days if stored in an
oxygen- and water-free environment. The independent synthesis
9
14458 J. AM. CHEM. SOC. VOL. 131, NO. 40, 2009

A R T I C L E S
solution was heated at 50 °C for one hour. Examination of
1
the H-coupled 11B NMR spectrum (trace A, Figure S9 in
the Supporting Information) revealed a quartet at δ -12.8
(JB-H ) 97 Hz), corresponding to Et3N-BH3, and a singlet
at δ -1.1, indicating the formation of [Rh(dmpe)2][BF4] that
slowly precipitates over the course of the reaction. In
addition, the mixed species Et3N-BH2F27 appears as an
overlapping doublet of triplets at δ 3.0 (JB-H ) 114 Hz),
corroborated by a doublet (JB-F ) 86 Hz) in the 11B{1H}
NMR spectrum. Gentle heating of the reaction mixture at 50
°C for 20 h leads to the disappearance of Et3N-BH2F and
produces [Rh(dmpe)2][BF4] and Et3N-BH3 as the major boron
species in solution (trace B of Figure S9, Supporting
Information). Broad features at ca. δ 20 are assigned to
species that are believed to be polymeric in nature and that
are caused by the ring-opening of THF by BF3 or other boron
intermediates produced in the reaction.28
Figure 5. Molecular structure of [Rh(dmpe)2][BH4] with atom numbering.
Thermal ellipsoids drawn at 30% probability. Hydrogen atoms omitted
except on the BH4 anion.
Hydride Transfer Reactions from Rhodium to 1,3,2-ben-
zodithiaborolane. 1,2-Dithiobenzene has been used to digest the
polymeric product resulting from dehydrogenation of AB
to form the ammonia adduct of 1,3,2-benzodithiaborolane,
(C6H4S2)BH·(NH3).6 We were interested to see if HRh(dmpe)2
would transfer a hydride ligand to a similar species, HB(S2C6H4).
The computed hydride affinity of 74 kcal/mol for HB(S2C6H4)
is nearly identical to that of B(OPh)3, and therefore hydride
transfer is expected. In fact, hydride transfer is observed upon
mixing a 1:1 ratio of HRh(dmpe)2 and HB(S2C6H4) in THF.
Initially this reaction produced a yellow precipitate in THF at
room temperature; however, the 1H-coupled 11B NMR spectrum
recorded at 50 °C exhibits a triplet at δ -11.5 (JB-H ) 110
Hz) (Figure S6 in the Supporting Information), corresponding
to the formation of [H2B(S2C6H4)]-. This value is nearly
identical to that of Li[H2B(S2C6H4)] (t, δ -11.0, JB-H ) 110
Hz) reported by No¨th and co-workers prepared by the reaction
of LiBH4 with 1,2-dithiobenzene in THF.29 Reactions performed
with a 3-fold excess of HRh(dmpe)2 produced identical results,
suggesting the chelate effect of the dithiolate ligand precludes
further B-H bond formation.
Table 3. Selected Bond Distances (Å) and Angles (deg) for
[Rh(dmpe)2][BH4], trans-[HRh(SPh)(dmpe)2][HB(SPh)3], and
[Rh(dmpe)2][SPh]
[Rh(dmpe)2][BH4]
Rh(1)-P(1)
Rh(1)-P(2)
B(1)-H1
2.2737 (17)
2.2870 (17)
1.04 (4)
P(1)-Rh(1)-P(2)
P(1)-Rh(1)-P(1A)
P(1)-Rh(1)-P(2A)
HB1-B(1)-HB2
83.97 (6)
95.73 (9)
177.78 (7)
111 (5)
B(1)-H2
1.04 (3)
trans-[HRh(SPh)(dmpe)2][HB(SPh)3]
Rh(1)-P(1)
Rh(1)-P(2)
Rh(1)-P(3)
Rh(1)-P(4)
Rh(1)-S(1)
Rh(1)-H
B(1)-S(2)
B(1)-S(3)
B(1)-S(4)
B(1)-H
2.3206 (4)
2. 3209 (3)
2.3224 (3)
2.3144 (3)
2.5083 (4)
1.492 (18)
1.9345 (17)
1.9162 (16)
1.9122 (16)
1.05 (2)
P(1)-Rh(1)-P(2)
P(1)-Rh(1)-P(3)
P(1)-Rh(1)-P(4)
P(2)-Rh(1)-P(3)
P(2)-Rh(1)-P(4)
P(3)-Rh(1)-P(4)
P(1)-Rh(1)-S(1)
P(2)-Rh(1)-S(1)
S(1)-Rh(1)-H(1R)
S(2)-B(1)-S(3)
S(2)-B(1)-S(4)
S(3)-B(1)-S(4)
S(2)-B(1)-HB(1)
84.580 (12)
95.109 (12)
176.691 (12)
167.956 (11)
95.167 (12)
84.447 (12)
92.901 (12)
105.080 (11)
172.1 (7)
107.60 (8)
117.79 (8)
108.21 (8)
110.0 (11)
Hydride Transfer Reactions from Rhodium to B(SPh)3. In
the preceding discussion, the utility of the hydride affinity and
hydride donor ability scales for predicting the feasibility of
hydride transfer reactions was demonstrated. However, the
inability of HRh(dmpe)2 to reduce [B(OPh)4]-, [BF4]-, and
[H2B(S2C6H4)]- by cleaving the B-O, B-F, and B-S bonds,
respectively, limits the extent of conversion of these species. A
second objective of this work was to demonstrate that transition-
metal hydrides, such as HRh(dmpe)2, could be used to generate
boron compounds containing multiple B-H bonds by converting
BX3 to BH3 derivatives in which all B-X bonds are cleaved.
We hoped to achieve this by selecting BX3 compounds with
appropriate heterolytic B-X bond strengths. In this section, a
series of reactions between HRh(dmpe)2 and B(SPh)3 are
described that demonstrate the feasibility of forming boron
products containing multiple B-H bonds (e.g., reaction 10,
where n ) 2, 3, 4) and forming Et3N-BH3 in the presence of
NEt3.
[Rh(dmpe)2][SPh]
Rh(1)-P(1)
Rh(1)-P(2)
Rh(2)-P(3)
Rh(2)-P(4)
S(1)-C(18)
2.2797 (5)
2.2767 (5)
2.2898 (5)
2.2884 (5)
1.740 (2)
P(1)-Rh(1)-P(2)
P(1)-Rh(1)-P(2A)
P(1)-Rh(1)-P(1A)
P(3)-Rh(2)-P(4)
P(3)-Rh(2)-P(3A)
P(3)-Rh(2)-P(4A)
84.43 (2)
177.663 (19)
95.60 (3)
84.384 (17)
179.998 (18)
95.614 (17)
(dmpe)2][BH4] clearly indicates the tetrahydroborate anion
resides in the outer sphere and does not interact with the
rhodium center.
Hydride Transfer Reactions from Rhodium to BF3 ·OEt2.
Early work of Schlesinger and Brown25,26 discovered that
diborane could be prepared in high yields by the reaction of
lithium or sodium hydride with boron trifloride diethylether-
ate. To determine if analogous reactivity was possible using
transition-metal hydrides, we investigated the reaction be-
tween HRh(dmpe)2 and BF3 · OEt2 in THF in the presence of
triethylamine in order to intercept Et3N-BH3. The hydride
affinity for BF3 was calculated to be 76 kcal/mol, slightly
greater than the value for BH3, indicating favorable thermo-
chemistry for hydride transfer from HRh(dmpe)2. BF3 · OEt2
was added to excess HRh(dmpe)2 and NEt3 in THF, and the
(28) (a) Edwards, J. D.; Gerrard, W.; Lappert, M. F. J. Chem. Soc. 1957,
348. (b) Gerrard, W.; Lappert, M. F. Chem. ReV. 1958, 58, 1081.
(29) Knizek, J.; No¨th, H. J. Organomet. Chem. 2000, 614-615, 168.
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A R T I C L E S
Mock et al.
HRh(dmpe)2 + B(SPh)3 THF8 [Rh(dmpe)2][HnB(SPh)4-n
]
(10)
Hydride transfer reactions from HRh(dmpe)2 to B(SPh)3 are
expected to be thermodynamically favorable on the basis of the
hydride affinity of B(SPh)3 of 87 kcal/mol. In addition, due to
a weaker B-S bond20 compared to B-O and B-F bonds, and
absent the chelate effect, thiolate ligand substitution was
anticipated to be facile, improving the likelihood of boron
products with multiple B-H bonds and decreased [B(SPh)4]-
formation. Hydride transfer reactions of HRh(dmpe)2 with
B(SPh)3 were performed in THF, resulting in rapid hydride
transfer to boron as described below. Although reports describ-
ing aryl- and alkyl-transfer30 between H2B(SPh) (a potential
hydride transfer product) and ethereal solvents such as THF
and diethyl ether have been noted, these types of side reactions
were not observed in our studies.
1
Figure 6. (A) H-coupled 11B NMR spectrum recorded at 50 °C of the
reaction between excess B(SPh)3 and HRh(dmpe)2. (B) The corresponding
31P{1H} NMR spectrum.
planar geometry of three-coordinate boron in the molecular
structure of B(SPh)3 (see Figure S7 in the Supporting Informa-
tion). Two of the thiophenolate ligands are endo with respect
to the hydride ligand, while the third thiophenolate ligand is
exo. The B-S bond lengths of the [HB(SPh)3]- anion are
1.916(2), and 1.935(2) Å and 1.912(2), respectively, while the
B-H bond length is 1.05(2) Å. These B-S bond distances are
slightly longer than the average of the B-S bond distances in
B(SPh)3 (1.80 Å). Selected metric parameters are listed in Table
3. The molecular structures determined for B(SPh)3 and [HB-
(SPh)3]- by X-ray diffraction are consistent with the optimized
molecular structures of these compounds determined computa-
tionally.
Reactions of a slight excess of B(SPh)3 with HRh(dmpe)2 in
THF were followed by 31P and 11B NMR spectroscopy. 31P{1H}
spectral data indicated complete consumption of HRh(dmpe)2,
1
resulting in the formation of [Rh(dmpe)2]+ (d, δ 35.7, JRhP
)
125 Hz) as the major phosphorus-containing product (Figure
6, trace B). In addition, a minor doublet located at δ 38.2, (1JRhP
) 90 Hz) was observed accounting for approximately 2% of
the total product mixture by integration. This minor species was
identified as trans-[HRh(SPh)(dmpe)2]+, and was prepared
independently by the reaction of thiophenol with [Rh(dmpe)2]-
[CF3SO3] to confirm the 31P NMR spectral data. The formation
of this six-coordinate rhodium species suggests trace amounts
of thiophenol in the B(SPh)3 oxidatively adds to [Rh(dmpe)2]+
1
after hydride transfer was complete. The H-coupled 11BNMR
Hydride transfer reactions performed with an excess of
1
HRh(dmpe)2 produced significantly different results. The H-
spectrum recorded at 50 °C immediately after mixing, shown
by trace A of Figure 6, indicated the presence of three major
boron-containing species, unreacted B(SPh)3 at δ 62.3, a sharp
resonance at δ 4.5 assigned to [B(SPh)4]-, and a doublet at δ
-4.3 (1JBH ) 128 Hz) assigned to [HB(SPh)3]-, indicating the
formation of a B-H bond.
coupled 11B NMR spectrum recorded immediately after mixing
a 3:1 ratio of HRh(dmpe)2 to B(SPh)3 is shown in Figure 8,
trace A. Similar to the reaction using a 1:1 ratio, [HB(SPh)3]-
and [B(SPh)4]- are observed at δ -4.3 and δ 4.5, respectively.
However, a new triplet is present in this reaction at δ -14.5,
JB-H ) 111 Hz, assigned to [H2B(SPh)2]-, indicating the
formation of two B-H bonds. The chemical shift of
[H2B(SPh)2]- is similar to that reported for Li[H2B(S2C6H4)],
Vide supra. Integration of the corresponding 31P NMR spectrum
indicates roughly one-half of HRh(dmpe)2 was consumed
producing [Rh(dmpe)2]+ as the only observable phosphorus-
containing product. The reaction mixture was heated at 50 °C
for 30 min and the 31P and 11B spectral data were recorded once
Slow diffusion of ether into the reaction mixture precipitated
a crop of yellow crystals accounting for the majority of the
material. Unfortunately, these crystals were not suitable for
X-ray diffraction studies. However, a minor component of this
recrystallization was suitable for analysis by X-ray diffraction.
The molecular structure of this material is shown in Figure 7.
The cation of the colorless crystals is the octahedral rhodium(III)
species, trans-[HRh(SPh)(dmpe)2]+, rather than the anticipated
[Rh(dmpe)2]+. The rhodium coordination sphere consists of two
dmpe ligands occupying the equatorial positions, while a
thiophenolate ligand and a hydride ligand occupy the apical
positions. The average Rh-P bond distance is 2.3196(3) Å,
comparable to other octahedral Rh(III) complexes with chelating
phosphine ligands.31 The Rh-S bond length is long at 2.5083(4)
Å, while the Rh-H bond length is 1.49(2) Å. The identity of
the anion supports our assignment of the upfield doublet as
[HB(SPh)3]-. The geometry of [HB(SPh)3]- can be regarded
as nearly tetrahedral about the boron center, compared to the
1
again. The H-coupled 11B NMR spectrum recorded at 50 °C,
Figure 8, trace B revealed the reaction mixture had significantly
changed. The products [B(SPh)4]- and [HB(SPh)3]- (δ -4.3)
were not observed and the reaction contained only [H2B(SPh)2]-
(t, δ -14.5) and a new quartet at δ -25.6, JB-H ) 97 Hz
assigned to [H3B(SPh)]- with an integration ratio of 3:1. The
31P NMR spectrum indicates ∼ 85% of HRh(dmpe)2 was
consumed and [Rh(dmpe)2]+ was the only observed product.
After heating the reaction at 50 °C for 20 h the 31P NMR data
indicated all of the HRh(dmpe)2 was consumed. The corre-
1
sponding H-coupled 11B NMR spectrum, shown in Figure 8,
spectrum C, indicates further changes in the boron-containing
product distribution, suggesting additional B-H bonds have
been formed. The amount of [H2B(SPh)2]- present after heating
has decreased significantly and the major product is [H3B-
(SPh)]-, integrated at nearly a 1:5 ratio. In addition, two new
products are observed, a complex multiplet centered at δ -38.8
and a quintet at δ -41.1 assigned to dmpe-(BH3)2 and [BH4]-,
(30) (a) Pasto, D. J. J. Am. Chem. Soc. 1962, 84, 3777. (b) Pasto, D. J.;
Cumbo, C. C.; Balasubramaniyan, P. J. Am. Chem. Soc. 1966, 88,
2187.
(31) (a) Simonsen, K. P.; Suzuki, N.; Hamada, M.; Kojima, M.; Ohba, S.;
Saito, Y.; Fujita, J. Bull. Chem. Soc. Jpn. 1989, 62, 3790. (b) Clegg,
W.; Elsegood, M. R. J.; Scott, A. J.; Marder, T. B.; Dai, C.; Norman,
N. C.; Pickett, N. L.; Robins, E. G. Acta Crystallogr., Sect. C 1999,
55, 733.
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A R T I C L E S
Figure 7. Molecular structure of trans-[HRh(SPh)(dmpe)2][HB(SPh)3]·THF with atom numbering. Thermal ellipsoids drawn at 30% probability. The THF
solvent molecule has been omitted as well as hydrogen atoms except for those bound to Rh and B.
Figure 8. 1H-coupled 11B NMR spectra from the hydride transfer reaction from HRh(dmpe)2 to B(SPh)3: (A) Immediately after mixing. (B) After heating
at 50 °C, 30 min. (C) After heating at 50 °C, 20 h.
respectively. These results indicate that mild heating can produce
the fully reduced tetrahydroborate anion, but also results in a
slight decomposition of [Rh(dmpe)2]+. A possible decomposition
pathway may involve the coordination of dmpe to BH3 formed
by dissociation of PhS- from [H3B(SPh)]-. A notable difference
between reactions with B(SPh)3 compared to B(OPh)3 is the
transitory formation of the [B(SPh)4]- species. Reactions with
B(OPh)3 produce [B(OPh)4]- as a final product, and hydride
transfer to this species was not observed. Although [B(SPh)4]-
was formed in the initial stages of the reaction of B(SPh)3 with
excess HRh(dmpe)2, [B(SPh)4]- was not observed as a final
product. This noteworthy result reflects how the heterolytic B-X
bond strength can dramatically affect the final product distribu-
tion and most importantly the formation of products that contain
multiple B-H bonds. Independent syntheses and 11B NMR
spectral characterization of the boron anions described in the
previous paragraphs were carried out to support the proposed
assignments and are described in the Supporting Information.
Reactions of B(SPh)3 with three equivalents of HRh(dmpe)2
yield products in which multiple B-H bonds are formed and
multiple B-S bonds are broken. In these reactions, ion pairing
of [HnB(SPh)4-n]- with [Rh(dmpe)2]+ occurs in THF, and some
9
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A R T I C L E S
Mock et al.
product accounting for roughly 90% of the total boron in the
sample is a quartet assigned to the formation Et3N-BH3. Identical
11B NMR spectral data of independently prepared Et3N-BH3
from the addition of triethylamine to BH3 ·THF confirms this
assignment. The remaining products identified in the 11B NMR
spectrum are [H3B(SPh)]- (δ -25.6) and trace amounts of
dmpe-(BH3)2 and [BH4]-. The presence of NEt3 changes the
identity of the final boron-containing products, and significantly
decreases the decomposition of the [Rh(dmpe)2]+ complex by
providing an alternative Lewis base to complex with BH3.
Furthermore, in the context of a regeneration process for AB,
the formation of Et3N-BH3 provides a material suitable for a
final ammoniation step, where substitution of NEt3 for NH3
produces NH3BH3 as the final product. Studies aimed at
providing mechanistic and thermochemical information regard-
ing the final ammoniation step are currently underway in our
laboratory.
Summary and Conclusions
Both transition metal hydrides and boron hydrides are well-
known for their ability to reduce a range of organic and
inorganic substrates. In this study, a previously developed
scale of hydride donor abilitities of a series of transition-
metal hydrides in acetonitrile has been cross referenced, using
HRh(dmpe)2 and [HBEt3]-, to a scale of computed gas-phase
hydride affinities of BX3 compounds. The result is a scale
that allows predictions regarding when hydride transfer
should or should not occur between members of these two
classes of compounds. Hopefully this scale will have ap-
plications to a broad range of problems and provide a starting
point for the comparison of the hydride donor abilities of
transition-metal hydrides and main group metal hydrides. We
have used this scale to guide our approach to the regeneration
of AB.
Figure 9. Molecular structure of [Rh(dmpe)2][SPh] with atom numbering.
Thermal ellipsoids drawn at 30% probability. Hydrogen atoms omitted.
of the displaced thiophenolate ligands form [Rh(dmpe)2][SPh].
The presence of free thiolate is supported by an X-ray diffraction
study of crystals grown from the reaction mixture. The molecular
structure of [Rh(dmpe)2][SPh], shown in Figure 9, consists of
a cationic square planar rhodium center formed by two dmpe
ligands. The thiophenolate anion, located in the outer sphere,
exhibits no interaction with the metal center. Metric parameters
are analogous to the other cationic rhodium(I) complexes
reported here. Selected metric parameters are listed in Table 3.
The facile hydride transfer from rhodium to B(SPh)3 led us
to explore these hydride transfer reactions in the presence of a
Lewis base such as triethylamine with the intent of producing
a neutral amine borane product, Et3N-BH3. Under these condi-
tions, the presence of NEt3 is anticipated to drive the reaction
to the products [Rh(dmpe)2][SPh] and Et3N-BH3, as depicted
in reaction 11.
Approaches to the regeneration of AB from dehydroge-
nated material typically involve a digestion step to generate
BX3 compounds,4b,5,6 which must be converted using H2 back
to AB. The results presented in this work demonstrate that
HRh(dmpe)2 can be generated from H2 in the presence of a
strong base, and that HRh(dmpe)2 has the ability to transfer
a hydride ligand to a variety of BX3 compounds, including
BH3, BF3, HB(S2C6H4), B(SPh)3 and aryl borate esters. In
the context of the regeneration of AB, choosing a BX3
compound for which the resulting [HBX3]- anions undergo
ligand redistribution reactions to form species containing
multiple B-H bonds is an important consideration. Our
results indicate that reactions between excess HRh(dmpe)2
and BX3, where X ) F or OAr, result in the formation of
B-H containing products, but are accompanied by formation
of [BX4]-, to which hydride transfer does not occur. In
contrast to these findings, when X ) SPh, the major products
contained multiple B-H bonds, and [B(SPh)4]- was not
observed as a final product. Of particular interest to AB
regeneration is the reaction of HRh(dmpe)2 with B(SPh)3 in
the presence of triethylamine to form Et3N-BH3 in high yields,
a reaction in which all B-S bonds are cleaved. Practical
systems for AB regeneration will require the use of transition
metal complexes using much less expensive metals than
o
THF, 50 C
3 HRh(dmpe)2 + B(SPh)3
8 Et3N-BH3 +
20 equiv NEt3
3 [Rh(dmpe)2](SPh) (11)
For example, in a typical reaction triethylamine (20 equiv vs
B(SPh)3) was added to a THF solution of HRh(dmpe)2 followed
1
by the addition of B(SPh)3. The initial H-coupled 11B NMR
spectrum recorded at 50 °C, Figure 10, trace A, indicated that
[H2B(SPh)2]- is the principle boron species formed. In addition,
several other products containing B-H bonds were identified
as in the previously described reactions, including [HB(SPh)3]-,
[H3B(SPh)]-, and a new product, tentatively assigned as Et3N-
BH2(SPh), which appears as a triplet at δ -6.5 (JB-H ) 115
Hz). While the initial 11B NMR spectrum exhibits nearly
identical product formation as the hydride transfer reaction in
the preceding paragraph, heating of the reaction mixture at 50
°C for 14 h produced greatly different results compared to
1
reactions without NEt3. After this time, integration of the H-
coupled 11B NMR spectrum (Figure 10, trace B) indicates the
9
14462 J. AM. CHEM. SOC. VOL. 131, NO. 40, 2009

A R T I C L E S
Figure 10. 1H-coupled 11B NMR spectra of the hydride transfer reaction from HRh(dmpe)2 to B(SPh)3 in the presence of NEt3: (A) Recorded after heating
sample for 10 min at 50 °C. (B) Recorded after heating 14 h at 50 °C, showing triethylamineborane, Et3N-BH3 at δ -12.8.
tadiene)rhodium(I) triflate, 1,2-bis(dimethylphosphino)ethane, and
rhodium, but the principles established here should be of use
in guiding this further development.
i
Verkades base (P(RNCH2CH2)3N (R ) Pr) (2,8,9-tri-i-propyl-
2,5,8,9-tetraaza-1-phosphabicyclo[3.3.3]undecane) were purchased
n
Experimental Section
from Strem Chemicals, Inc. BuLi (2.5 M in hexanes), borane-
dimethylsulfide, lithium hydride, and triethylamine were purchased
from Alfa-Aesar. Borane-THF, 1,2-benzenedithiol, boron trifluo-
ride diethyl etherate, and triethylamine were purchased from Sigma-
Aldrich. Triethylborane (1.0 M in THF) was purchased from Acros
Organics. High purity grade (99.9%) H2 gas was passed through a
column of Drierite before use. [Rh(dmpe)2][OTf] was prepared from
bis(1,5-cyclooctadiene)rhodium(I) triflate using literature methods.9,36
1,3,2-Benzodithiaborolane37 and lithium triethylborohydride38 were
prepared as described previously.
Computational Methods. Electronic structure calculations using
the NWChem package of programs32 were performed to determine
hydride affinities of BX3 compounds in Table 1. Minimum energy
structures of the boranes and their corresponding borohydrides were
optimized using density functional theory,33 the B3LYP func-
tional,34 and 6-311+G** basis set.35 Enthalpies and free energies
at 298 K of optimized structures were estimated from frequency
calculations using the harmonic oscillator-rigid rotor approximation.
Physical Measurements and General Procedures. NMR
spectra were recorded in thin-walled quartz NMR tubes (25 °C,
unless otherwise noted) on a Varian Inova 500 MHz spectrometer
equipped with a Sun workstation. 1H chemical shifts are referenced
to residual protons in deuterated solvent. 31P chemical shifts are
proton decoupled unless otherwise noted, and referenced to H3PO4
as an external reference. 11B chemical shifts are reported relative
to BF3-OEt2 as an external reference. 11B NMR spectra were
integrated to determine the ratio of total boron-containing products
in solution. Infrared spectra were recorded on a Nicolet Magna 860
FT-IR spectrometer at ambient temperature and under a purge
stream of nitrogen gas. Solid-state FT-IR samples were prepared
as KBr pellets. Integration of 31P NMR spectral data for the pKa
measurement of [(H)2Rh(dmpe)2]+ was performed using pro-Fit
software package developed by QuantumSoft.
Synthesis of BX3 Compounds. All aryl borates were synthesized
from BCl3 and the corresponding alcohol or thiol using a low-
temperature version of the Michaelis method39 developed by
Lappert and co-workers.40 B(OPh)3 was distilled over molten
sodium metal before use. B(SPh)3 was prepared by adding 5 mL
(48.8 mmol) of thiophenol to 16 mL (16 mmol) of 1.0 M BCl3 in
heptane at 0 °C. The reaction was warmed to room temperature
over one hour and stirred for an additional hour. The reaction was
heated at 50 °C for 12 h followed by removal of heptane under
reduced pressure. The crude mixture was washed with ether (3 ×
30 mL) and dried in vacuo, affording an off-white solid. Yield:
1.03 g, 19%. 11B NMR (THF): δ 62.3. 11B NMR (CDCl3): δ 58.1.
13C NMR (CDCl3): 134.9, 129.9, 129.0, 128.4. 1H NMR (CDCl3):
δ 7.50 (d, 3JHH ) 7.2 Hz, 6 H), 7.30 (t, 3JHH ) 7.9 Hz, 6 H), 7.23
3
(t, JHH ) 7.3 Hz, 3 H).
Synthesis and Materials. All synthetic procedures were per-
formed under an atmosphere of N2 using standard Schlenk or
glovebox techniques. Unless described otherwise, all reagents were
purchased from commercial sources and were used as received.
Solvents were dried by passage through activated alumina in an
Innovative Technology, Inc., PureSolv solvent purification system.
Deuterated THF was purchased from Cambridge Isotope Labora-
tories, dried over NaK, and vacuum transferred before use.
Triethylamine was distilled over CaH2 before use. Bis(1,5-cyclooc-
Preparation of HRh(dmpe)2. HRh(dmpe)241 was generated by
n
adding one equivalent of BuLi (2.5 M in hexanes) to ca. 20-30
mg [Rh(dmpe)2](CF3SO3) in THF. The reaction was stirred for 30
min, and the THF was removed under reduced pressure. The
remaining yellow solids were extracted with pentane or hexanes
to remove Li[CF3SO3], filtered through Celite, and dried under
(36) James, B. R.; Mahajan, D. Can. J. Chem. 1980, 58, 996.
(37) Hadebe, S. W.; Robinson, R. S. Eur. J. Org. Chem. 2006, 4898.
(38) Brown, H. C.; Kim, S. C.; Krishnamurthy, S. J. Org. Chem. 1980,
45, 1.
(39) Michaelis, A.; Hillringhaus, F. A. Ann. 1901, 315, 41.
(40) (a) Colclough, T.; Gerrard, W.; Lappert, M. F. J. Chem. Soc. 1955,
907. (b) Colclough, T.; Gerrard, W.; Lappert, M. F. J. Chem. Soc.
1956, 3006.
(41) Zargarian, D.; Chow, P.; Taylor, N. J.; Marder, T. B. J. Chem. Soc.,
Chem. Commun. 1989, 540.
(32) Bylaska E. J.; et al. NWChem, A Computational Chemistry Package
for Parallel Computers, Version 5.0; Pacific Northwest National
Laboratory: Richland, Washington, 2006.
(33) (a) Hohenberg, P.; Kohn, W. Phys. ReV. 1964, 136, B864. (b) Kohn,
W.; Sham, L. J. Phys. ReV. 1965, 140, A1133.
(34) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(35) Schafer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100, 5829.
9
J. AM. CHEM. SOC. VOL. 131, NO. 40, 2009 14463

A R T I C L E S
Mock et al.
1
1
vacuum. Average yield 85%, 1H NMR (THF-d8): δ 1.43 (m, dmpe-
CH2, 8 H), 1.32 (s, dmpe-CH3, 24 H), -11.5 (doublet of pentets).
δ 37.5 (d, JRhP ) 125 Hz, [Rh(dmpe)2]+), 26.5 (d, JRhP ) 139
Hz, HRh(dmpe)2.
1
31P{1H} NMR (THF-d8): δ 26.5 (d, JRhP ) 139 Hz).
Reaction of HRh(dmpe)2 with HB(S2C6H4). A solution of
HRh(dmpe)2 14 mg (0.034 mmol) in 2 mL THF was added to 5
mg (0.034 mmol) HB(S2C6H4). The contents were mixed and
transferred to an NMR tube affording a yellow solution with a
yellow precipitate. The reaction was characterized by 11B and 31P
NMR spectroscopy. Spectral data immediately after mixing:
11B{1H}, [1H-coupled] NMR (THF, 50 °C): 11.6 (s, [s], [B-
(S2C6H4)2]-, 4%), -4.7 (s, [m], unassigned, 16%), -11.5 (s, [t]1JBH
) 110 Hz, [H2B(S2C6H4)]-, 80%). 31P{1H} NMR (THF): δ 36.3
(d, 1JRhP ) 125 Hz, [Rh(dmpe)2]+, 95%), 26.5 (d, 1JRhP ) 139 Hz,
HRh(dmpe)2, 5%).
Reaction of Li[HBEt3] with B(OPh)3. Li[HBEt3] (2.0 mL, 1.0
mmol of a 0.50 M solution in THF) was added to 0.105 g (0.36
mmol) of solid triphenyl borate. The mixture was stirred, producing
a clear solution and the reaction was monitored by 11B NMR
spectroscopy. Spectral data immediately after mixing:11B{1H}, [1H-
coupled] NMR (THF): δ 76.5 (br, [br], Li[Et3B-H-BEt3]), 53.5 (br,
[br], unassigned), 3.2 (br, [d], 1JBH ) 130 Hz, [HB(OPh)3]-, 75%),
2.4 (s, [s], [B(OPh)4]-, 21%), -41.9 (s, [quintet], [BH4]-, 4%).
Reaction of Li[HBEt3] with B(OPh)3 in the Presence of
Triethylamine. Triethylamine, 0.10 mL, was added to 2.0 mL (1.0
mmol) of a 0.50 M solution of Li[HBEt3] in THF. This mixture
was added to 0.121 g (0.42 mmol) solid triphenyl borate. The
mixture was stirred, producing a cloudy solution. An additional
0.5 mL of THF was added to obtain a homogeneous mixture. The
reaction was monitored by 11B NMR spectroscopy. Spectral data
recorded after heating at 50 °C for 5 h: 11B{1H}, [1H-coupled] NMR
(THF): δ 74.5 (br, [br], Li[Et3B-H-BEt3]), 53.6 (br, [br], unas-
Reaction of HRh(dmpe)2 with Excess B(SPh)3. In an NMR
tube, 10.1 mg (0.030 mmol) of solid B(SPh)3 was added to 11 mg
(0.027 mmol) of HRh(dmpe)2 in 2 mL of THF. The tube was shaken
to produce a cloudy yellow solution, and the reaction was followed
by 11B and 31P NMR spectroscopy. Spectral data immediately after
mixing: 11B{1H}, [1H-coupled] NMR (THF, 50 °C): δ 63.2 (s, [s],
B(SPh)3), 4.5 (s, [s], [B(SPh)4]-, 16%), -4.3 (s, [d], JBH ) 128
1
Hz, [HB(SPh)3]-, 84%). 31P{1H} NMR (THF): δ 38.2 (d, JRhP
)
1
1
signed), 11.9 (br, [br], unassigned), 3.7 (br, [d], JBH ) 126 Hz,
90 Hz, trans-[HRh(SPh)(dmpe)2]+, 2%), 35.7 (d, JRhP ) 125 Hz,
1
[HB(OPh)3]-, 72%), 2.4 (s, [s], [B(OPh)4]-, 26%), -12.8 (s, [q],
Et3N-BH3, 2%).
[Rh(dmpe)2]+, 98%).
Reaction of Excess HRh(dmpe)2 with B(SPh)3. In an NMR
tube, 3.4 mg (0.010 mmol) of solid B(SPh)3 was added to 13 mg
(0.030 mmol) HRh(dmpe)2 in 1.5 mL THF. The tube was shaken
to produce a clear yellow solution. The reaction was monitored by
11B and 31P NMR spectroscopy while being heated at 50 °C over
a period of 20 h. Spectral data immediately after mixing: 11B{1H},
[1H-coupled] NMR (THF, 50 °C): δ 4.5 (s, [s], [B(SPh)4]-), -4.3
Reaction of HRh(dmpe)2 with B(OR)3, (R ) Ph, C6F5,
p-C6H4OMe). In a typical experiment, solid B(OR)3 was added to
a stirring solution of HRh(dmpe)2 (0.030 mmol) in 2 mL of THF.
The reaction was monitored by 11B and 31P NMR spectroscopy.
Spectral data immediately after mixing with (0.014 mmol)
B(OPh)3: 11B{1H}, [1H-coupled] NMR (THF, 50 °C): δ 3.2 (br,
1
[d], JBH ) 124 Hz, [HB(OPh)3]-, 62%), 2.4 (s, [s], [B(OPh)4]-,
1
1
(s, [d], JBH ) 128 Hz, [HB(SPh)3]-), -14.5 (s, [t], JBH ) 111
35%), -3.5 (d, [m], unassigned, 3%), -11.1 (s, [m], unassigned,
1
Hz, [H2B(SPh)2]-). 31P{1H} NMR (THF): δ 35.4 (d, JRhP ) 125
trace amount), -38.8 (d, (1JBP ) 57 Hz) [m], dmpe-(BH3)2, trace
1
Hz, [Rh(dmpe)2]+, 55%), 26.5 (d, JRhP ) 139 Hz, HRh(dmpe)2,
1
amount). 31P{1H} NMR (THF): δ 36.0 (d, JRhP ) 125 Hz,
45%). After heating at 50 °C, 30 min: 11B{1H}, [1H-coupled] NMR
(THF, 50 °C): δ -14.5 (s, [t], 1JBH ) 111 Hz, [H2B(SPh)2]-, 68%),
[Rh(dmpe)2]+, 65%), 26.5 (d, 1JRhP ) 139 Hz, HRh(dmpe)2, 35%).
Spectral data immediately after mixing with (0.010 mmol) B(p-
OC6H4OMe)3: 11B{1H}, [1H-coupled] NMR (THF, 50 °C): δ 3.8
1
-25.6 (s, [q], JBH ) 97 Hz, [H3B(SPh)]-, 32%). 31P{1H} NMR
1
(THF): δ 34.9 (d, JRhP ) 125 Hz, [Rh(dmpe)2]+, 85%), 26.5 (d,
1
(br, [d], JBH ) 105 Hz, [HB(p-OC6H4OMe)3]-, 40%), 2.4 (s, [s],
1JRhP ) 139 Hz, HRh(dmpe)2, 15%). After heating at 50 °C, 20 h:
[B(p-OC6H4OMe)4]-, 57%), -3.2 (d, (1JBP ) 57 Hz) [m], unas-
11B{1H}, [1H-coupled] NMR (THF, 50 °C): δ -14.5 (s, [t], 1JBH
)
1
signed, 3%). 31P{1H} NMR (THF): δ 36.2 (d, JRhP ) 125 Hz,
1
111 Hz, [H2B(SPh)2]-, 11%), -25.6 (s, [q], JBH ) 97 Hz,
[Rh(dmpe)2]+, 25%), 26.5 (d, 1JRhP ) 139 Hz, HRh(dmpe)2, 75%).
Spectral data immediately after mixing with (0.010 mmol)
B(OC6F5)3: 11B{1H}, [1H-coupled] NMR (THF, 25 °C): δ 5.0 (br,
[d], 1JBH ) 137 Hz, [HB(OC6F5)3]-, 38%), 1.1 (s, [s], [B(OC6F5)4]-,
50%), -5.4 (s, [q], 1JBH ) 97 Hz, [H3B(OC6F5)]-, 9%), -38.8 (d,
(1JBP ) 57 Hz), [m], (1JBH ) 97 Hz), dmpe-(BH3)2, 3%). 31P{1H}
NMR (THF): δ 36.2 (d, 1JRhP ) 125 Hz, [Rh(dmpe)2]+, 39%), 26.6
[H3B(SPh)]-, 68%), -38.5 (d, (1JBP ) 57 Hz), [m], (1JBH ) 95
1
Hz), dmpe-(BH3)2, 18%), -41.1 (s, [quintet], JBH ) 80 Hz,
1
Li[BH4], 3%). 31P{1H} NMR (THF): δ 36.4 (d, JRhP ) 125 Hz,
[Rh(dmpe)2]+).
Reaction of Excess HRh(dmpe)2 with B(SPh)3 in the Pres-
ence of NEt3. Triethylamine (50 µL, 0.36 mmol) was added to
20.0 mg (0.050 mmol) of HRh(dmpe)2 in 2 mL of THF and 5.8
mg (0.017 mmol) of solid B(SPh)3 was added to this mixture. The
tube was shaken to produce a clear yellow solution. The reaction
was immediately monitored by 11B and 31P NMR spectroscopy,
then again after being heated at 50 °C over a period of 14 h at
which time a small amount of a yellow precipitate had formed.
Spectral data recorded after heating 10 min at 50 °C: 11B{1H}, [1H-
1
(d, JRhP ) 139 Hz, HRh(dmpe)2, 61%).
Reaction of HRh(dmpe)2 with BH3 ·THF. BH3 ·THF (13 µL,
0.013 mmol of a 1.0 M solution) was added to a solution of 0.016
g (0.04 mmol) of HRh(dmpe)2 in 1 mL THF. The tube was shaken
to produce a clear yellow solution. The reaction was characterized
by 11B and 31P NMR spectroscopy. Spectral data immediately after
mixing: 11B{1H}, [1H-coupled] NMR (THF, 50 °C): -37.4 (s,
1
1
[quintet], JBH ) 82 Hz, [Rh(dmpe)2][BH4], 85%). 31P{1H} NMR
coupled] NMR (THF, 50 °C): δ -4.3 (s, [d], JBH ) 128 Hz,
[HB(SPh)3]-, 34%), -6.5 (s, [t], JB-H ) 115 Hz, Et3N-BH2(SPh),
5%), -14.5 (s, [t], 1JBH ) 111 Hz, [H2B(SPh)2]-, 58%), -25.6 (s,
[q], 1JBH ) 97 Hz, [H3B(SPh)]-, 3%). 31P{1H} NMR (THF): δ 35.1
(d, 1JRhP ) 125 Hz, [Rh(dmpe)2]+, 75%), 26.5 (d, 1JRhP ) 139 Hz,
HRh(dmpe)2, 25%). Spectral data after heating 14 h at 50 °C:
1
(THF): δ 37.5 (d, JRhP ) 125 Hz, [Rh(dmpe)2]+, 35%), 26.5 (d,
1JRhP ) 139 Hz, HRh(dmpe)2 (65%).
Reaction of HRh(dmpe)2 with BF3 ·OEt2. BF3 ·OEt2 (2.0 µL,
0.016 mmol) was added to a solution of 0.020 g (0.050 mmol)
HRh(dmpe)2 and 100 µL of NEt3 in 1.0 mL of THF. The tube was
shaken to produce a clear yellow solution. The reaction was
characterized by 11B and 31P NMR spectroscopy. [Rh(dmpe)2][BF4]
precipitates from THF during the course of the reaction affecting
the integrated quantity of this product versus other observed
products. Spectral data after heating 1 h at 50 °C: 11B{1H}, [1H-
1
11B{1H}, [1H-coupled] NMR (THF, 50 °C): δ -12.8 (s, [q], JBH
1
) 97 Hz, Et3N-BH3, 90%), -25.6 (s, [q], JBH ) 97 Hz,
[H3B(SPh)]-, 10%). 31P{1H} NMR (THF): δ 34.5 (d, 1JRhP ) 125
1
Hz, [Rh(dmpe)2]+, 95%), 26.5 (d, JRhP ) 139 Hz, HRh(dmpe)2,
5%).
1
coupled] NMR (THF): δ 3.0 (d, JBF ) 86 Hz, [dt], JB-H ) 114
Determination of pKa for [(H)2Rh(dmpe)2][CF3SO3]. In a
typical reaction, 1.0 mL of a 1.27 × 10-2 M stock solution of
[Rh(dmpe)2][CF3SO3] in THF was added to a predetermined amount
of VSB-ipr base. The solution was mixed, transferred to an NMR
tube, and sealed with a rubber septum. H2 gas was purged through
Hz, Et3N-BH2F, 37%), -1.1 (s, [s], BF4, 20%), -12.8 (s, [q], 1JBH
) 97 Hz, Et3N-BH3, 43%). Spectral data after heating 20 h at 50
°C: 11B{1H}, [1H-coupled] NMR (THF): δ -1.1 (s, [s], BF4, 25%),
-12.8 (s, [q], 1JBH ) 97 Hz, Et3N-BH3, 75%). 31P{1H} NMR (THF):
9
14464 J. AM. CHEM. SOC. VOL. 131, NO. 40, 2009

A R T I C L E S
Table 4. Crystallographic Data for [Rh(dmpe)2][BH4],
[Rh(dmpe)2][BH4]. The systematic absences in the diffraction
data were consistent with the centrosymmetric monoclinic space
group, P2/n. [Rh(dmpe)2]+ and [BH4]- were located on special
positions yielding Z ) 2, and Z′ ) 1/2. The rhodium cation was
disordered over two positions which were located from the
difference map and refined using a second free variable. All non-
hydrogen atoms were refined with anisotropic displacement pa-
rameters. Hydrogen atoms were treated as idealized contribution
except for the hydrogen atoms from the [BH4]- group, which were
located from the difference map. [BH4]- hydrogen atoms were
allowed to refine freely except their distances were made equivalent
to stabilize the refinement. The goodness of fit on F2 was 1.047
with R1(wR2) 0.0308 (0.0784) for [Iθ > 2(I)] and with largest
difference peak and hole of 0.962 and -0.520 e/Å3.
trans-[HRh(SPh)(dmpe)2][HB(SPh)3]·THF, and [Rh(dmpe)2][SPh]a
trans-[HRh(SPh)(dmpe)2]
[HB(SPh)3] ·THF
[Rh(dmpe)2][BH4]
[Rh(dmpe)2][SPh]
empirical formula
formula weight
color, habit
crystal system
space group
a, Å
C6H18B0.50P2Rh0.50 C40H62BOP4RhS4
C18H37P4RhS
512.33
yellow, plates
monoclinic
C2/c
30.0678 (5)
9.3277 (1)
17.4534 (3)
90
100.997 (1)
90
4805.2 (1)
1.54178
8
1.416
100 (2)
209.00
924.74
yellow, blocks
monoclinic
P2/n
colorless, blocks
triclinic
P1
j
10.4454 (1)
9.1995 (2)
11.4117 (2)
90
112.762 (1)
90
13.066 (1)
13.086 (1)
13.553 (1)
74.795 (3)
83.485 (3)
89.799 (3)
2221.1 (3)
(0.71073)
2
b, Å
c, Å
R, deg
ꢀ, deg
γ, deg
V(Å3)
1011.18 (3)
trans-[HRh(SPh)(dmpe)2][HB(SPh)3]·THF. The data was con-
λ, Å (Cu (Mo), KR) 1.54178
j
sistent with the centrosymmetric, triclinic space group P1. The
Z
4
trans-[HRh(SPh)(dmpe)2]+ cation and the [HB(SPh)3]- anion were
located on general positions yielding Z ) 2, and Z′ ) 1. The
[HB(SPh)3]- anion was disordered over two positions (85:15) which
were located from the difference map and refined using a second
free variable. All non-hydrogen atoms were refined with anisotropic
displacement parameters. Hydrogen atoms were treated as idealized
contributions except for the hydrogen atoms bonded to Rh(1) and
B(1) which were located from the difference map and allowed to
refine freely. The B-H proton on the minor component of the
disordered [HB(SPh)3]- anion could not be located and was
calculated using an AFIX command with the B-H distance fixed
to match that of the B-H bond in the major component. The
goodness of fit on F2 was 1.024 with R1(wR2) 0.0260(0.0600) for
[Iθ > 2(I)] and with largest difference peak and hole of 0.686 and
-0.509 e/Å3.
density (g/cm3)
temperature (K)
2θ range, deg
µ(Cu (Mo), KR),
mm-1
1.373
1.383
100 (2)
4.81-68.00
9.685
100 (2)
1.57-33.22
(0.747)
2.99-68.96
9.062
R(F), Rw(F)
0.0308, 0.0784
0.0260, 0.0600
0.0225, 0.0588
a
2
2
Quantity minimized ) R(wF2) ) Σ[w(Fo - Fc2)2]/ Σ[(wFo )2]1/2; R
) Σ∆/Σ(Fo), ∆ ) |(Fo - Fc)|, w ) 1/[σ2(Fo ) + (aP)2 + bP], P ) [2Fc2
+ Max(Fo,0)]/3.
2
the reaction mixture for 2 min, and the sample was immediately
inserted into an NMR probe maintained at 25.0 °C. The reaction
was monitored by 31P NMR spectroscopy until the integrated ratio
of 31P NMR signals for the products and reactants, HRh(dmpe)2,
protonated base and [(H)2Rh(dmpe)2]+, unprotonated base, respec-
tively, remained constant. Each reaction reached equilibrium in less
than 8 h after mixing but was monitored for an additional 24 h to
ensure equilibrium had been established. In order to accurately
[Rh(dmpe)2][SPh]. The systematic absences in the diffraction
data were consistent with the centrosymmetric monoclinic space
group, C2/c. Two [Rh(dmpe)2]+ cations were located on special
positions along with one thiophenolate anion located on a general
position yielding Z ) 8, and Z′ ) 1. All non-hydrogen atoms were
refined with anisotropic displacement parameters. All hydrogen
atoms were treated as idealized contributions. The goodness of fit
on F2 was 1.029 with R1(wR2) 0.0225 (0.0588) for [Iθ>2(I)] and
with largest difference peak and hole of 0.521 and -0.427 e/Å3.
determine the equilibrium constant, due to the overlapping 31P NMR
resonances of HRh(dmpe)2, [(H)2Rh(dmpe)2]+ and Rh(dmpe)2
,
+
each of the above rhodium species was fitted to a Lorentzian
function, and the best-fit data were integrated to quantify each
species. The experimental procedure described above was performed
using 5, 10, 20, and 40 equiv of VSB-ipr, resulting in an average
equilibrium constant of (1.2 ( 0.3) × 10-3. The pKa of
[(H)2Rh(dmpe)2]+ was calculated by using the reported pKa of the
VSB-ipr in acetonitrile (33.6)15 and the equilibrium constant
measured in THF to afford a pKa value of 36.5 in acetonitrile.
X-ray Diffraction Studies. X-ray diffraction data were collected
on a Bru¨ker-AXS Kappa APEX II CCD diffractometer with 0.71073
Å Mo KR radiation or 1.54178 Å Cu KR radiation. Selected crystals
were mounted using Paratone oil onto a glass fiber and cooled to
the data collection temperature of 100 K. For data collection using
Mo KR radiation, unit cell parameters were obtained from 60 data
frames, 0.5° Φ, from three different sections of the Ewald sphere.
For data collection using Cu KR radiation,unit cell parameters were
obtained from 90 data frames, 0.5° Φ, from three different sections
of the Ewald sphere. Cell parameters were retrieved using APEX
II software42 and refined using SAINT+43 on all observed
reflections. Each data set was treated with SADABS44 absorption
corrections based on redundant multiscan data. The structure was
solved by direct methods and refined by least-squares method on
F2 using the SHELXTL program package.45 Crystallographic data
for each structure are listed in Table 4. Details regarding specific
solution refinement for each compound are provided in the
following paragraphs.
Acknowledgment. This work was supported by the U.S.
Department of Energy’s (DOE) Office of Energy Efficiency and
Renewable Energy Center of Excellence for Chemical Hydrogen
Storage. A portion of the research was performed using EMSL, a
national scientific user facility sponsored by the Department of
Energy’s Office of Biological and Environmental Research and
located at Pacific Northwest National Laboratory (PNNL). PNNL
is operated by Battelle for DOE. The Bruker (Siemens) SMART
APEX diffraction facility was established at the University of Idaho
with the assistance of the NSF-EPSCoR program and the M. J.
Murdock Charitable Trust, Vancouver, WA.
Supporting Information Available: Experimental details and
NMR spectral data for the preparation of the compounds
Li[HB(OPh)3], Li[HB(SPh)3], Na[H3B(SPh)], Na[B(SPh)4]; full
X-ray diffraction refinement data (CIF) for [Rh(dmpe)2][BH4],
trans-[HRh(SPh)(dmpe)2][HB(SPh)3], [Rh(dmpe)2][SPh], [Rh-
(dmpe)2][B(OPh)4], B(SPh)3; NMR spectral data for hydride
transfer reactions to BH3 ·THF, BF3 ·OEt2 and HB(S2C6H4),
results of hydride transfer from rhodium to B(OC6F5)3 and B(p-
OC6H4OMe)3; discussion regarding formation of [B(OAr)4]-;
complete ref 32. This material is available free of charge via
the Internet at http://pubs.acs.org.
(42) APEX II, v. 2009.3; Bruker AXS: Madison, WI, 2009.
(43) SAINT+: Data Reduction and Correction Program, v. 7.56A; Bruker
AXS: Madison, WI, 2009.
(44) SADABS: An Empirical Absorption Correction Program, v. 2008/1;
Bruker AXS Inc.: Madison, WI, 2008.
(45) Sheldrick, G. M. SHELXTL: Structure Determination Software Suite,
v. 2008; Bruker AXS Inc.: Madison, WI, 2008.
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