Phosphines with N-Heterocyclic Boryl-Substituents: Ligands for Coordination Chemistry and Catalysis

Article abstract of DOI:10.1002/ejic.201801081

Boryl-substituted phosphines NHB–P(R)Ph (R = H, Ph, NHB = N-heterocyclic boryl substituent) react with Fe₂(CO)₉ to give isolable Fe(CO)₄ complexes, two of which were characterized by single-crystal XRD studies. The electronic and steric properties for a series of the boryl phosphines were further assessed by evaluation of TEPs for in-situ formed complexes [RhCl(NHB–PR¹R²)(CO)₂] (R¹, R² = H, Ph, Me, NMe₂), and calculations of buried volumes for Fe(CO)₄ complexes. The results imply that the NHB-phosphines exhibit due to their conformational flexibility some variability in their steric bulk, and that some specimens may exhibit similar electron releasing power and steric demand as tBu₃P. Studies of the amination of bromobenzene with 2,6-diisopropylaniline confirmed that these properties can be exploited to promote Pd-catalyzed C–N cross coupling reactions, and that formal replacement of a phenyl by a NHB substituent in the auxiliary phosphine has a beneficial effect on catalyst performance.

Full text of DOI:10.1002/ejic.201801081

A Journal of
Accepted Article
Title: Phosphines with N-heterocyclic Boryl-substituents: Ligands for
Coordination Chemistry and Catalysis
Authors: Manuel Kaaz, Ralf J C Locke, Luisa Merz, Mathis Benedikter,
Simon König, Johannes Bender, Simon H Schlindwein, Martin
Nieger, and Dietrich Gudat
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201801081
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10.1002/ejic.201801081
European Journal of Inorganic Chemistry
FULL PAPER
Phosphines with N-heterocyclic Boryl-substituents: Ligands for
Coordination Chemistry and Catalysis
Manuel Kaaz,[a] Ralf J. C. Locke,[a] Luisa Merz,[a] Mathis Benedikter,[a] Simon König,[a] Johannes
Bender,[a] Simon H. Schlindwein,[a] Martin Nieger,[b] and Dietrich Gudat*[a]
Dedicated to Professor Dr. Edgar Niecke on the occasion of his 80th and to Professor Dr. Koop Lammertsma on the occasion of his
70th birthday.
Abstract: Boryl-substituted phosphines NHB–P(R)Ph (R = H, Ph,
NHB = N-heterocyclic boryl substituent) react with Fe2(CO)9 to give
isolable Fe(CO)4-complexes, two of which were characterized by
single-crystal XRD studies. The electronic and steric properties for a
series of the boryl phosphines were further assessed by evaluation
of TEPs for in-situ formed complexes [RhCl(NHB–PR1R2)(CO)2] (R1,
R2 = H, Ph, Me, NMe2), and calculations of buried volumes for
Fe(CO)4-complexes. The results imply that the NHB-phosphines
exhibit due to their conformational flexibility some variability in their
steric bulk, and that some specimens may exhibit similar electron
releasing power and steric demand as tBu3P. Studies of the
amination of bromobenzene with 2,6-diisopropylaniline confirmed
that these properties can be exploited to promote Pd-catalyzed C–N
cross coupling reactions, and that formal replacement of a phenyl by
a NHB substituent in the auxiliary phosphine has a beneficial effect
on catalyst performance.
cases alkyl and aryl groups.[5] More recently, also heteroatom-
based substituents have moved into focus. For example,
Buchwald et al. discovered carboranyl-phosphines 1 (Scheme 1)
which were considered more electron rich than any other known
alkyl- or aryl-based phosphine.[6] Fu et al. described an anionic
boratabenzene-phosphide 2 which was found to be a more
strongly donating ligand than isosteric PPh3.[7] The groups of
Kuhn[8] and Dielmann[9] drew on bulky imidazolin-2-ylideneimino-
units[10] to create phosphines 3.[11] These species display a
similar electron donating ability as N-heterocyclic carbenes
(NHCs) which enables them to promote the Suzuki-Miyaura
cross-coupling of aryl chlorides with boronic acid, or to form
isolable adducts with CO2,[12] respectively. The high electron
donating power of 1 - 3 was inferred from structural and
spectroscopic studies and related to the strongly -donating
character of the guanidinato substituent and the electron
releasing properties of the anionic boratabenzene unit or the B-
bound carborane cage, respectively.[6],[7],[9] Pringle et al. reported
that an increase in the electron donating power of phosphine
ligands was likewise observable upon formal replacement of aryl
units by isoelectronic B-based aminoboranyl substituents, and
that a rhodium complex of 4 showed higher catalytic activity in
the hydrogenation of cyclohexene than the complex containing
an isoelectronic PCC-based phosphine.[13] These findings imply
that the idea to employ boryl substituents on phosphorus for
boosting the electron donating power of phosphines, which had
first been expressed in case of 1, is presumably more widely
applicable.
Introduction
The usefulness of phosphines in coordination chemistry and
catalysis is closely connected with the recognition that changing
substituents on phosphorus can cause marked variations in the
ligand behavior, which offer in turn options to manipulate the
electron density and steric accessibility of the metal center of
transition metal complexes with these ligands.[1] This flexibility is
of particular importance during the adaptation and optimization
of catalytic processes. A lucid example is the amination of aryl
halides with palladium phosphine complexes as catalysts that is
known as Buchwald-Hartwig coupling.[3],[4] The consequent
elaboration of phosphine ligands with high electron donating
power and steric demand permitted extending its scope to
enable reactions under activation of difficult substrates, at
ambient temperature, or with very low catalyst loadings.[4],[5]
The phosphorus substituents employed to advance the electron
donating power and steric bulk of phosphines include in most
R'
N
PR2
PR2
K
B PPh2
N
N
R'
1
2
3
PR2
R'
N
HN B
B PR2
[a]
M. Kaaz, L. Merz, M. Benedikter, S. König, J. Bender, S. H.
Schlindwein, Prof. Dr. Dr. h. c. D. Gudat
Institut für Anorganische Chemie, University of Stuttgart,
Pfaffenwaldring 55, 70550 Stuttgart, Germany
E-mail: gudat@iac.uni-stuttgart.de
N
R'
5
4
http://www.iac.uni-stuttgart.de/arbeitskreise/akgudat
Dr. M. Nieger
Department of Chemistry,
Scheme 1. Examples of P-heteroatom substituted electron rich phosphines (R,
R' = alkyl, aryl); open and closed circles in the structure of 1 denote B- and C-
centered vertices of a carboranyl (meta-C2B10H11) substituent.
[b]
University of Helsinki
P.O. Box 55, 00014 University of Helsinki, Finland
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejic.201801081
European Journal of Inorganic Chemistry
FULL PAPER
Some time ago, we had described the phosphines 5 (Scheme 1)
with N-heterocyclic boryl substituents (NHB-phosphines) and
noted that these molecules lack a significant boron-centered
electrophilic character but may potentially bind to metal centers
through their phosphorus atom.[14] Considering that the NHB
fragment embodies, like an alkyl group, essentially a strong -
donor substituent, the phosphines 5 may be regarded as
electron rich, strongly electron donating ligands. Starting from
this hypothesis, we initiated a study of the metal coordination
properties of these NHB-substituted phosphines. Herein, we
report on the formation of isolable metal complexes, a study of
the steric and electronic ligand properties, and a first application
as electron rich ligand in a catalytic Buchwald-Hartwig reaction.
Attempting to prepare isolable complexes with NHB-phosphine
ligands, we established that 5a,b and 6a,b react smoothly with
Fe2(CO)9 to give isolable iron pentacarbonyl and complexes
7a,b and 8a,b, which were unambiguously identified by their
NMR and IR spectral data. Recrystallization from pentane
produced single-crystals of complexes 7b and 8b which were
further characterized by single-crystal X-ray diffraction studies.
Attempts to produce single-crystals of 7a and 8a were as yet
unsuccessful. The crystals 7b and 8b contain isolated molecular
complexes (see Figure 1 for 7b; the molecular structure of 8b,
and selected metrical parameters of both species and the
ligands 5b and 6b are listed in Table S1 in the supporting
information). Although the crystals are not isotypic, the
molecular conformations and individual metrical parameters in
both species are closely similar.
Results and Discussion
The studies described in this work were performed using NHB-
phosphines carrying both CC-unsaturated 1,3,2-diazaborolyl
substituents (5a-d, Scheme 2) and CC-saturated 1,3,2-
diazaborolidinyl substituents (6a,b) on phosphorus. Ligands 5a-
c[14] and 6a,b[15] had been reported previously and were
synthesized as described. The P-alkyl derivative 5d was newly
prepared by analogy to 5a,b from the appropriate 2-bromo-
1,3,2-diazaborole and lithium dimethyl phosphide, and
characterized by multinuclear NMR spectroscopy. Like its
congeners,[14],[15] the new NHB-phosphine is thermally stable in
solution and the solid state, and can be shortly handled in air
without decomposition.
R3
N
X
B P
R1
N
R2
R3
R1
R2
X =
-- Fe(CO)4 Rh(CO)2Cl
Figure 1. Molecular structure of 7b in the crystal. Thermal ellipsoids are drawn
at the 50% probability level and hydrogen atoms except H30 were omitted for
clarity. Selected distances [Å] and angles [°]: B1-P30 1.948(2), P30-C31
1.822(2), P30-Fe37 2.2635(6), P30-H30 1.28(2), Fe37-C39 1.784(2), Fe37-
C40 1.791(3), Fe37-C38 1.798(2), Fe37-C37 1.799(3), C31-P30-B1 108.50(9),
C31-P30-H30 98.5(10), B1-P30-H30 97.3(10).
5a
5b
5c
5d
7a
7b
-
9a
9b
9c
9d
Ph
Ph
Ph
H
NMe2 NMe2
Me Me
-
R3
Interaction of the metal atoms with the carbon atoms of four
carbonyls and the phosphorus atom of the NHB-phosphine gives
rise to a trigonal-bipyramidal coordination geometry expected for
Fe(0) centers with a formal d8 electron configuration. There is no
indication for a 2-type interaction of the PB-unit with the metal
atom, which had been observed for a phosphinoborane featuring
a strongly electrophilic boryl substituent and high P–B double
bond character.[16] The phosphine ligands in 7b, 8b occupy axial
coordination sites as in the majority of known phosphine-iron
tetracarbonyl complexes.[17] The Fe–P distances (7b: 2.264(1) Å,
8b: 2.266(1) Å) lie slightly above the mean distance of 2.254(35)
Å for known phosphine-Fe(CO)4 complexes,[17] but are in the
normal range. The Fe–C distances are unexceptional.
N
X
B P
R1
N
R2
R3
R1
R2
X =
-- Fe(CO)4 Rh(CO)2Cl
6a
6b
8a
8b
10a
10b
Ph
Ph
Ph
H
Scheme 2. Molecular structures of NHB-substituted phosphines and their
complexes included in this study (R3 = 2,6-iPr2C6H3, Dipp).
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10.1002/ejic.201801081
European Journal of Inorganic Chemistry
FULL PAPER
[21]
Comparison of intra-ligand distances in 7b/8b and the free
ligands 5b/6b (Table S1) reveals that the P–B bonds lengthen
upon metal coordination whereas the P–C and P–H bonds
display a slight shortening that is, however, on the verge of
being not significant. The P–B bond lengthening is presumably
attributable to the fact that the weak PB -bonding contribution
in the free ligand[14] is further cut back when the lone-pair on
phosphorus gets involved in metal bonding. A further noteworthy
difference between free and metal bound ligands in the crystal
structures is a different alignment of the bulky NHB-moiety
(Figure 2), which can be traced back to torsional twists of the
heterocyclic ring and the N-aryl groups around the P–B and N–C
bonds, respectively. This conformational reorientation reduces
steric crowding in the vicinity of the metal centers and attests to
a notable conformational flexibility of the NHB-phosphines.
phenyl in PPh3 by more electron releasing tBu
or 1,3-di-
isopropyl-benzimidazol-2-ylidenimino-moieties,[9] respectively. In
contrast, the presence (in 5a,b) or absence (in 6a,b) of a CC-
double bond in the NHB unit has no visible effect on the TEP. In
view of the general non-additivity of substituent contributions to
the TEP [22] and the variance of 
̃, these results do not allow us
to extract a quantifiable and transferable substituent increment
which characterizes the influence of NHB-units on the electron
donor properties
a phosphine. Nonetheless, they confirm
qualitatively that the NHB substituent can, like a m-carboranyl
moiety,[6] boost its net electron donating power beyond the range
accessible for phosphines with only carbon-based substituents.
Table 1. Values of TEP and %Vbur for NHB-phosphines 5a-d, 6a,b and
selected reference compounds.
Ligand
5a
TEP [cm-1]
2055.5
%Vbur
-- (37)
-- (38)
27 (32)
29 (32)
-- (39)
-- (30)
29
6a
2054.9
5b
2058.4
6b
2058.2
5c
2053.7
5d
2052.8
Figure 2. Overlay of the molecular structures of ligand 5b (red, data from ref.
[14]) and complex 7b (light blue) viewed from two directions. Molecules are
represented using a wire model, and all but the phosphorus-bound hydrogen
atoms were omitted for clarity.
PPh3
2068.9 [d]
2073.3 [d]
2065.6 [e]
2065.3 [d]
2056.1 [d]
2052.0 [f]
2050.8 [f]
2050.1 [f]
2060.8 [g]
PPh2H
PPh(NMe2)2
PPhMe2
P(tBu)3
SIMes[a]
IMes[b]
ItBu[b]
P(NBiPr)Ph2
25
--
The molecular structures of 7b/8b confirm the ability of the NHB-
phosphines 5b/6b to act as ligands in transition metal
complexes, but the variations in individual bond distances and
angles do not allow inferring a quantifiable picture of their donor
properties. To this end, we turned to an analysis of trends in the
"Tolman electronic parameter" (TEP)[1] which remains, despite
the development of alternative or more sophisticated descriptors
or quantum chemically based concepts, still popular as a
measure of the electron density a ligand L in a complex imparts
on the metal center.[18] For practical reasons, all studies were
carried out on rhodium complexes [(L)RhCl(CO)2] (L = 5a-d,
6a,b) which were generated in situ from reactions of the ligands
25
34
32
32
35 [h]
40 [g]
[c]
[a] SIMes
=
1,3-dimesityl-imidazolin-2-ylidene. [b] IMes/ItBu
= 1,3-
with [{RhCl(cod)}2] and CO.[19] The wavenumbers of the (CO)
dimesityl/di-t-butyl-imidazol-2-ylidene. [c] NBiPr
=
1,3-diisopropyl-
benzimidazolin-2-ylidenimino. [d] data from ref. [1]. [e] data from ref. [22]. [f]
data from ref. [20]. [g] data from ref. [9]. [h] data for [(C5H3tBu2)Fe(PtBu3)I]
as the crystal structure of the Fe(CO)4 complex is unknown.
[20]
modes derived from IR spectra were converted
to the
commonly used scale [1] which refers to the wavenumbers of the
totally symmetric (CO) vibrational mode of the appropriate
Ni(CO)3 complexes.
The TEPs of all NHB-phosphines studied are similar or even
larger than those of alkyl phosphines (Table 1), and the value for
5d approaches even those reported for some NHCs. The net
electron donating ability of all ligands but 5b/6b may on this
scale be rated to exceed that of P(tBu)3, the strongest all-carbon
substituted phosphorus donor known so far. The depression of
the TEP upon formal exchange of the phenyl in a phosphine
The steric properties of phosphines are often assessed via their
cone angle.[1] However, the use of this descriptor, which goes
likewise back to the work of Tolman, faces known limitations
when the shape of a ligand cannot be well approximated by a
cone.[23] In view of the irregular shape of the NHB-phosphines 5,
6, we presumed that analysis of the percent buried volume %Vbur
R1R2PhP by a NHB unit amounts to 
̃
= 12-15 cm-1 and
[24]
can provide a more logical and realistic picture of trends in
steric properties. This descriptor was developed for analyzing
exceeds the shifts of 4.2 and 8.1 cm-1 induced by exchange of a
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10.1002/ejic.201801081
European Journal of Inorganic Chemistry
FULL PAPER
the sterics of NHCs, but is also routinely applied to other types
of ligands. Computation of %Vbur is in principle feasible on the
basis of both crystal structure data and molecular geometries
from quantum chemical calculations, but requires a careful
choice of the data source employed to model the ligand
geometry.[20][24] In particular, using structural data of a free ligand
and its metal complex can yield quite different results when the
metal coordination goes along with a conformational change that
alters the overall ligand shape. As this is the case for the NHB-
phosphines, we refrained from analyzing the structures of the
free ligands (even if a complete set of crystal structure data is
available). Instead, we computed buried volumes for 5b, 6b and
selected reference compounds using the crystal structure data
and 5c, which excel the other NHB-phosphines in both electron
releasing power and steric bulk. For a proof of principle, we
performed an exemplary study of the Pd-catalyzed Buchwald-
Hartwig coupling of bromobenzene with 2,6-diisopropylaniline
(Dipp-NH2) in refluxing toluene (reaction (1), Scheme 3). The
reaction protocol copies a known synthesis of target amine 11
using
a
bulky Buchwald-type phosphine as activating
ligand.[26][27] For our studies, phosphine 5a was deemed to
provide the best compromise between optimum ligand
properties, chemical stability, and easy synthetic accessibility.
cat. Pd(OAc)2/PR3
PhBr, 1.5 NaOtBu
Pr
i
Pr
i
i
of the iron complexes 7b, 8b and published crystallographic data
toluene
(1)
(2)
(3)
NH2
[17]
NHPh
- NaBr
- tBuOH
of appropriate Fe(CO)4 complexes in the CSD
as input. In
addition, we computed buried volumes for all NHB-phosphines
using energy optimized geometries obtained from DFT-
calculations (see experimental section and supporting
information for details). All results are collected in Table 1.
The buried volumes of ligands 5b, 6b in the complexes 7b, 8b
compare to that of PPh3 in [Fe(PPh3)(CO)4] but are clearly
smaller than the values derived with DFT-calculated molecular
geometries. This discrepancy can be traced to the presence of
visibly different ligand conformations in calculated and observed
structures of 7b, 8b. Since the calculations reproduce bond
distances and angles quite well, we explain this deviation as
reflecting mainly the impact of packing effects in the crystal, and
rate it as a further proof for the conformational flexibility of the
ligands. That the steric demand inferred from %Vbur-values can
be viewed as snapshot which may not fully account for the
flexibility of a ligand (with the implication that values computed
for different complexes with the same ligand, or even different
conformers of a complex, may be subject to sizable variation)
had been pointed out earlier.[20]
Pr
i
i
Pr
11
Pr
0.7 mol-% Pd(OAc)2
5a
Pr
1.1 mol-%
i
PhBr, 1.5 NaOtBu
toluene, 24 h
NHPh
NPh2
Pr
- NaBr
- tBuOH
Pr
i
i
11
12 (65% yield)
2 mol-% Pd(OAc)2
5a
3 mol-%
PhBr, 1.5 NaOtBu
toluene, 24 h
NH
NPh
- NaBr
- tBuOH
Scheme 3. Pd-catalyzed aminations of bromobenzene studied in this work.
Further details on the conditions for reaction (1) are given in Table 2.
Comparison of buried volumes of all ligands 5a-d/6a,b derived
from DFT-optimized molecular geometries for complexes 7a-
d/8a,b confirms the expected increase in steric bulk upon formal
replacement of the PhHPh-fragment by PPh2- or P(NMe2)2-units,
respectively. In contrast, formal exchange of the PHPh- by a
PMe2-moiety has practically no effect. In view of the sensitivity of
the buried volume toward conformational changes [20] and metal-
ligand distances,[24] we consider the small variations between
phosphines with unsaturated (5a,b) and saturated (6a,b) NHB
rings insignificant and refrain from an interpretation. In
comparison to other phosphines and NHCs (Table 1), the steric
bulk of 5b/6b is rated as similar to that of PPh3, whereas 5d
should be assessed as slightly smaller. The overall steric bulk of
5a,d and 6a may be considered as roughly comparable to that of
P(tBu)3 and N-mesityl substituted NHCs but does not reach that
of bulky Buchwald-type phosphines.[25]
Table 2. The effect of different reaction conditions on the amination of
bromobenzene with 2,6-diisopropylaniline.
Entry
1
n(Pd(OAc)2)
2 mol-%
2 mol-%
0.2 mol-%
0.2 mol-%
2 mol-%
--
Phosphine
3 mol-% 5a
3 mol-% PPh3
0.3 mol-% 5a
0.3 mol-% PPh3
3 mol-% 5a
--
Conditions
Conversion[a]
100(72)%[b]
100%
2 h, 115 °C
2 h, 115 °C
2 h, 115 °C
3 h, 115 °C
10 min, 200 °C
40 min, 270 °C
2
3
100%
4
100%
5[c]
6[c]
100%
67%
The electronic and steric descriptors listed in Table 1 suggest to
rate NHB-phosphines as ligands that may impart high electron
density and steric shielding to the metal center in a complex. As
these characteristics are also essential prerequisites for active
catalysts in Pd-catalyzed cross-coupling reactions,[5] NHB-
phosphines make promising candidates for application in cross-
coupling catalysis. The best performance is expected for 5b/6b
[a] determined spectroscopically in the crude reaction product. [b] isolated
yield after column chromatography in parentheses. [c] Reaction performed in
a closed vessel under microwave irradiation.
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10.1002/ejic.201801081
European Journal of Inorganic Chemistry
FULL PAPER
[26]
Performing the reaction with the reported
catalyst load (2
(entry 5 in Table 2), but 67% conversion to 11 was also
achieved when the reaction was carried out under more forcing
conditions (40 min at 270 °C; entry 6 in Table 2) in the absence
of both Pd(OAc)2 and 5a. No reaction at all was observed at
room temperature, either with or without catalyst. Transition
metal-free, MW-assisted amination of aryl halides has previously
been observed in other cases and was successfully incorporated
in amine syntheses,[29] but needs in the present case further
elaboration of reaction conditions before it can compete with the
Pd-catalyzed variant.
mol-% Pd(OAc)2 and 3 mol-% phosphine) led to quantitative
conversion of the aryl halide within two hours. Work-up afforded
the product in a yield 72% (Table 2, entry 1), which is slightly
smaller than the reported yield of 82% after 8 hours reaction
using a Buchwald-type phosphine. Unexpectedly, we found that
quantitative conversion is also achieved when PPh3 instead of
5a is used as activating ligand (Table 2, entry 2), indicating that
the use of tailored phosphines is not essential in this particular
case. Presuming that the seemingly equal performance of 5a
and PPh3 arises from an overloading with catalyst, we repeated
the reactions with a reduced catalyst load (0.2 mol-% Pd(OAc)2
and 0.3 mol-% phosphine) under otherwise identical conditions.
Full conversion of the aryl halide was still feasible within two
hours with 5a (Table 2, entry 3) and three hours with PPh3
(Table 2, entry 4). The different efficacy of the two phosphines
was established by a kinetic study which disclosed that the
activity of the catalyst based on 5a (TOF = 7.0(4)·103 h-1)
exceeds that of the PPh3-based system (TOF = 2.83(3)·103 h-1)
by a factor of 2.5 (Figure 3).
Conclusions
The reactions of NHB-phosphines with Fe2(CO)9 yield isolable
complexes which were spectroscopically identified and in two
cases fully characterized. The crystal structure data confirm that
the NHB-phosphines act, like most previously known boryl
phosphines,[30] as P-donor ligands. The net electron donor
capability, judged by the TEP, can be considered to match or
even exceed that of highly electron rich trialkyl phosphines,
including PtBu3. Comparisons between observed and computed
molecular structures of free NHB-phosphines and their iron
complexes suggest that the NHB-substituents exhibit some
conformational flexibility which can modulate the steric bulk of
the ligand. Even if this effect encumbers a precise assessment,
analysis of buried volumes for a series of Fe(CO)4-complexes
suggests that the steric bulk of NHB-phosphines is, depending
on the ancillary P-substituents, comparable to that of common
phosphines like PPh3 or PtBu3. The combination of strong
electron releasing power and appreciable bulk makes NHB-
phosphines appealing ligands for application in Pd-catalyzed
cross-coupling reactions. An exemplary evaluation of Buchwald-
Hartwig aminations of bromobenzene reveals that formal
Ph/NHB-exchange in the auxiliary phosphine used has a definite
accelerating effect, even if the performance of the ligand
employed remains still lower than that of PtBu3 which also
promotes C–N cross-coupling at ambient temperature or with
chlorobenzene as substrate.[5] Nonetheless, NHB-phosphines
with two bulky alkyl substituents or more than a single NHB-unit
on phosphorus promise a further increase in both electron donor
power and steric bulk and make thus interesting targets for
further research.
Figure 3. Plot of turnover numbers (TON = n(11)/n(Pd)) during the early
reaction stages vs. overall time for reaction (1) in Scheme 2. Blue diamonds
(♦) refer to reactions carried out using 5a and green squares (■) to reactions
carried out using PPh3 as auxiliary ligand, respectively. The straight lines
represent the result of linear fits to the data. Turnover frequencies (TOF) of
were obtained from the slopes of the regression lines.
Experimental Section
Expectedly, NHB-phosphine 5a promotes also the amination of
iodobenzene with Dipp-NH2 (quantitative conversion within <30
min), while no activity at all was observed with fluoro- or chloro-
benzene as substrates. The efficacy of 5a in syntheses of
tertiary amines is illustrated in the coupling of bromobenzene
with 11 (reaction (2)) and piperidine (reaction (3)); both reactions
mimic reported syntheses with Buchwald-type ligands as
auxiliary.[26] Because conducting C–N cross-coupling reactions
under microwave (MW) irradiation enables substantial time
savings,[26][28] we studied the synthesis of 11 in a MW reactor.
Interestingly, quantitative reaction occurred within 10 min at
200 °C in the presence of Pd(OAc)2 and 5a as pre-catalysts
General Conditions: All manipulations were carried out under dry argon
using Schlenk glassware. Solvents were purified and dried by standard
methods. Microwave syntheses were carried out using an Anton Paar
Monowave 400 reactor. NMR spectra: Bruker Avance 250 (1H: 250 MHz,
11B: 80.2 MHz, 13C: 62.9 MHz, 31P: 101.2 MHz) or Avance 400 (1H:
400.13 MHz, 11B: 128.3 MHz, 13C: 100.6 MHz, 31P: 162.9 MHz) in C6D6
or CDCl3 at 30 °C. Chemical shifts were referenced to ext. TMS (1H, 13C)
using BF3·OEt2 ( (
11B, Ξ = 32.083974 MHz), or 85% H3PO4 31P, Ξ =
40.480742 MHz) as secondary references. Coupling constants are given
as absolute values. Elemental analyses: Elementar Micro Cube.
This article is protected by copyright. All rights reserved.

10.1002/ejic.201801081
European Journal of Inorganic Chemistry
FULL PAPER
Synthesis of 5d: A solution of 2-bromo-1,3-bis(2,6-diisopropylphenyl)-
1,3,2-diazaborole (2.00 g, 3.28 mmol) in THF (20 mL) was added at r.t. to
a stirred solution of lithium dimethylphosphide (266 mg, 4.28 mmol) in
THF (20 mL). The mixture was stirred for 16 h before all volatiles were
evaporated under reduced pressure. The residue was dissolved in n-
pentane (50 mL) and the suspension was filtered through celite. The
filtrate was concentrated to a volume of 10 mL and stored at -20 °C to
yield 1.30 g (2.90 mmol, 68%) of colorless crystals of 5d. - 1H NMR (250
2JPC = 17.6 Hz, CO), 146.4 (s, i-C6H3), 139.9 (s, o-C6H3), 134.2 (d, 3JPH
=
9.8 Hz, o-Ph), 132.1 (d, 1JPC = 40.5 Hz, i-Ph), 129.8 (d, 5JPC = 2.7 Hz, p-
2
Ph), 128.0 (d, JPC = 10.1 Hz, m-Ph), 127.3 (s, p-C6H3), 124.1 (s, m-
3
C6H3), 55.8 (d, JPC = 6.5 Hz, NCH2), 29.0 (s, CH(CH3)2), 23.1 (s,
CH(CH3)2). – 31P{1H} NMR (101.2 MHz, CDCl3):  = 8.7 (br s). – IR (ATR,
CH2Cl2): ̃
= 2045, 1965, 1940, 1925 cm-1 (CO).
8b: Yield 150 mg (235 mol, 42%). – 1H NMR (400 MHz, CDCl3):  =
7.30-7.24 (m, 4 H, m-C6H3), 7.12 (m, 1 H, p-Ph), 7.04 (m, 2 H, p-C6H3),
4
MHz, C6D6):  = 6.98 (m, 6 H, C6H3), 6.04 (d, 2 H, JPH = 0.8 Hz, NCH,
3
3
1
3.11 (sept, 4 H, JHH = 6.9 Hz, CH(CH3)2), 1.14 (d, 12 H, JHH = 6.9 Hz,
6.92 (m, 2 H, m-Ph), 6.76 (m, 2 H, o-Ph), 4.63 (d, 1 H, JPH = 349.0 Hz,
3
2
CH(CH3)2), 1.00 (d, 12 H, JHH = 6.9 Hz, CH(CH3)2), 0.54 (d, 6 H, JPH
=
PH), 3.92-3.83 (m, 2 H, NCH2), 3.79-3.71 (m, 2 H, NCH2), 3.65 (sept, 2 H,
3JHH = 6.6 Hz, CH(CH3)2), 3.36 (sept, 2H, 3JHH = 6.6 Hz, CH(CH3)2), 1.49
(d, 6 H, 3JHH = 6.6 Hz, CH(CH3)2), 1.29 (d, 6 H, 3JHH = 6.6 Hz, CH(CH3)2),
1.7 Hz, PCH3). – 11B NMR (128.3 MHz, C6D6):  = 28.0 (br s). – 13C{1H}
NMR (62.9 MHz, CDCl3):  = 146.1 (s, o-C6H3), 139.0 (s, i-C6H3), 127.4
3
3
3
(s, p-C6H3), 123.2 (s, m-C6H3), 120.9 (d, JPC = 6.0 Hz, NCH), 28.4 (s,
1.21 (d, 6 H, JHH = 6.6 Hz, CH(CH3)2), 0.96 (d, 6 H, JHH = 6.6 Hz,
CH(CH3)2), 25.6 (s, CH(CH3)2), 23.1 (s, CH(CH3)2), 6.9 (d, 1JPC = 8.3 Hz,
PCH3). – 31P-NMR (250 MHz, C6D6):  = -120.9 (br s).
CH(CH3)2). – 11B NMR (128.3 MHz, CDCl3):  = 32.9 (br d). – 13C{1H}
2
NMR (62.9 MHz, CDCl3):  = 213.7 (d, JPC = 18.4 Hz, CO), 147.6 (s, o-
C6H3), 146.9 (s, o-C6H3), 138.4 (s, i-C6H3), 132.7 (d, 2JPC= 10.1 Hz, o-Ph),
129.4 (d, 4JPC = 3.1 Hz, p-Ph), 128.4 (d, 3JPC = 10.7 Hz, m-Ph), 128.0 (s,
i-Ph), 127.7 (s, p-C6H3), 124.4 (s, m-C6H3), 124.2 (s, m-C6H3), 54.2 (d,
3JPC = 6.7 Hz, NCH2), 28.6 (s, CH(CH3)2), 28.4 (s, CH(CH3)2), 27.0 (s,
CH(CH3)2), 26.7 (s, CH(CH3)2), 23.8 (s, CH(CH3)2), 22.8 (s, CH(CH3)2). –
Synthesis of iron complexes: The appropriate NHB-phosphine (0.55
mmol) and Fe2(CO)9 (0.20 g, 0.55 mmol) were suspended in THF (10
mL) and the mixture was stirred for 10 h at ambient temperature.
Filtration of the dark red solution formed through alumina produced a
yellow solution which was once more evaporated to dryness. The residue
was washed twice by suspension in pentane (5 mL) to produce a light
31P-NMR (101.2, CDCl3):  = -51.5 (br d). – IR (ATR, CH2Cl2): 
1959, 1931 (br) cm-1 (CO).
̃ = 2043,
yellow solution and
a yellow solid. The supernatant liquid was
subsequently removed with a syringe. Drying the residue under reduced
pressure produced the products as yellow, microcrystalline powders (no
yields determined).
Determination of TEPs. Rhodium complexes [RhCl(NHB-phosphine)
(CO)2] 9a-d, 10a,b were prepared according to the following procedure: a
NMR-tube equipped with a septum lid was charged with [Rh(cod)Cl]2 (25
mg, 51 μmol) and the appropriate NHB-phosphine (102 μmol). After
addition of CH2Cl2 (1 mL), CO was bubbled through the yellowish
solution using a cannula as a gas inlet. A color change to bright yellow
was observed. The gassing was stopped after 10 min, and 31P and 11B
NMR spectra as well as an IR spectrum were recorded. The spectral
data confirmed that selective formation of the target complexes had
occurred. Attempts to isolate the products were unsuccessful.[19] The
Tolman electronic parameter (TEP, see Table 1) was calculated as
reported by Glorius [20] using the formula
7a: Yield 141 mg (198 mol, 35%). – 1H NMR (400 MHz, CDCl3):  =
3
7.41 (m, 4 H, o-Ph), 7.18-7.07 (m, 6 H, m/p-Ph), 7.11 (t, 2 H, JHH = 7.6
Hz, p-C6H3), 6.98 (d, 4 H, 3JHH = 7.6 Hz, m-C6H3), 6.20 (d, 2 H, 4JPH = 1.9
Hz, NCH), 3.04 (sept, 4 H, 3JHH = 6.7 Hz, CH(CH3)2), 1.07 (d, 24 H, 3JHH
= 6.7 Hz, CH(CH3)2). – 11B NMR (128.3 MHz, CDCl3):  = 25.1 (br d, 1JPB
= 108 Hz). – 13C{1H} NMR (62.9 MHz, CDCl3):  = 213.8 (d, 2JPC = 18 Hz,
CO) 145.7 (s, i-C6H3), 139.1 (s, o-C6H3), 134.5 (d, 2JPC = 10.0 Hz, o-Ph),
130.0 (d, 4JPC = 2.5 Hz, p-Ph), 128.5 (s, p-C6H3), 128.2 (d, 3JPC = 10.2 Hz,
3
m-Ph), 124.0 (d, JPC = 6.3 Hz, NCH), 123.9 (s, m-C6H3), 29.3 (s,
CH(CH3)2), 26.8 (s, CH(CH3)2). – 31P-NMR (101.2, CDCl3):  = 8.08 (br d).
TEP [cm-1] = 0.8001∙
ã v(CO) + 420.0
– IR (ATR, CH2Cl2): 
̃
= 2044, 1965, 1940, 1925 cm-1 (CO).
where av(CO) represents the mean value of the wavenumbers of the two
CO modes observed in the IR spectra of the complexes. Observed
spectral data:
̃
7b: Yield 156 mg (245 mol, 44%).1H NMR (400 MHz, CDCl3):  = 7.35 (t,
2 H, JHH = 7.6 Hz, p-C6H3), 7.27 (dd, 2 H, JHH = 7.6 Hz, JHH = 1.6 Hz,
m-C6H3), 7.19 (m, 1 H, p-Ph), 7.15 (dd, 2 H, 3JHH = 7.6 Hz, 4JHH = 1.6 Hz,
3
3
4
4
m-C6H3), 7.07 (m, 2 H, m-Ph), 6.99 (m, 2 H, o-Ph), 6.38 (d, 2 H, JPH
=
=
1
(9a) 31P{1H} NMR:  = -23:4 (br). – 11B NMR:  = 22.3 (br d, JPB = 125
2.0 Hz, NCH), 5.05 (d, 1 H, 1JPH = 346.5 Hz, PH), 3.05 (sept, 2 H, 3JHH
Hz). – IR (ATR, CO in CH2Cl2): ̃
= 2083, 2006 cm-1.
3
6.8 Hz, CH(CH3)2), 2.83 (sept, 2 H, JHH = 6.8 Hz, CH(CH3)2), 1.30 (d, 6
H, 3JHH = 6.8 Hz, CH(CH3)2), 1.14 (d, 6 H, 3JHH = 6.8 Hz, CH(CH3)2), 1.05
(d, 6 H, 3JHH = 6.8 Hz, CH(CH3)2), 1.02 (d, 6 H, 3JHH = 6.8 Hz, CH(CH3)2).
(9b) 31P{1H} NMR:  = -73.4 (br m). – 11B{1H} NMR:  = 21.8 (br d, 1JPB
137 Hz). – IR (ATR, CO in CH2Cl2): 
= 2087 cm-1, 2008 cm-1.
=
=
=
̃
–
11B NMR (128.3 MHz, CDCl3):  = 25.4 (br d). – 13C{1H} NMR (62.9
MHz, CDCl3):  = 213.3 (d, 2JPC = 18.9 Hz, CO), 145.9 (d, 3JPC = 7.5 Hz,
i-C3H6), 137.6 (s, m-C3H6), 132.8 (d, 2JPC = 10.1 Hz, o-Ph), 129.7 (d, 4JPC
(9c) 31P{1H} NMR:  = 105.4 (br m). – 11B{1H} NMR:  = 22.2 (br d, 1JPB
105 Hz). – IR (ATR, CO in CH2Cl2): 
= 2080 cm-1, 2003 cm-1.
3
̃
= 2.6 Hz, p-Ph), 128.60 (d, JPC = 10.7 Hz, m-Ph), 128.55 (s, p-C3H6),
124.0 (d, 4JPC = 4.6 Hz, o-C3H6), 122.5 (d, 3JPC = 5.7 Hz, NCH), 28.72 (s,
CH(CH3)2), 28.69 (s, CH(CH3)2), 26.7 (s, CH(CH3)2), 26.4 (s, CH(CH3)2),
22.8 (s, CH(CH3)2), 22.4 (s, CH(CH3)2). – 31P-NMR (101.2 MHz, CDCl3):
1
(9d) 31P{1H} NMR:  = -73.4 (br). – 11B{1H} NMR:  = 22.8 (br d, JPB
101 Hz). – IR (ATR, CO in CH2Cl2): 
̃
= 2081 cm-1, 2000 cm-1.
 = -51.7 (br d). – IR (ATR, CH2Cl2): 
(CO).
̃
= 2047, 1967, 1938 (br) cm-1
(10a): 31P{1H} NMR:  = -20.8 (br). – 11B{1H} NMR:  = 29.6 (br). – IR
(ATR, CO in CH2Cl2): 
= 2083, 2004 cm-1.
̃
8a: Yield 132 mg (184 mol, 34%). – 1H NMR (400 MHz, CDCl3):  =
7.41 (m, 4 H, o-Ph), 7.15 (m, 2 H, p-Ph), 7.07 (m, 4 H, m-Ph), 7.03 (t, 2 H,
3JHH = 7.6 Hz, p-C6H3), 7.02 (d, 4 H, 3JHH = 7.6 Hz, m-C6H3), 3.76 (s, 4 H,
NCH), 3.47 (sept, 4 H, 3JHH = 6.6 Hz, CH(CH3)2), 1.23 (d, 12 H, 3JHH = 6.6
Hz, CH(CH3)2), 1.08 (br s, 12 H, CH(CH3)2). – 11B NMR (128.3 MHz,
CDCl3):  = 32.6 (br s). 13C{1H} NMR (62.9 MHz, CDCl3):  = 213.8 (d,
(10b): 31P{1H} NMR:  = -76.3 (br). – 11B NMR:  = 28.1 (br). – IR (ATR,
CO in CH2Cl2): 
= 2087, 2008 cm -1
̃
.
This article is protected by copyright. All rights reserved.

10.1002/ejic.201801081
European Journal of Inorganic Chemistry
FULL PAPER
3
3
Synthesis of 11 via Pd-catalyzed amination of bromobenzene with
2,6-diisopropylaniline
(sept, 2 H, JHH = 6.8 Hz, CH(CH3)2), 0.85 (d, 12 H, JHH = 6.8 Hz,
CH(CH3)2).
(a) With conventional heating: A 100 mL reaction vessel was charged
with Pd(OAc)2 (36 mg, 0.16 mmol, 2 mol-%), a phosphine (137 mg of 5a
or 63 mg of PPh3, 0.24 mmol, 3 mol-%), and NaOtBu (1.15 g, 12 mmol).
Toluene (30 mL) was added and the mixture refluxed for 10 min to yield a
yellowish orange solution. Bromobenzene (1.29 g, 0.86 mL, 8.22 mmol)
and 2.6-diisopropylaniline (1.77 g, 1.82 mL, 10 mmol) were added. The
solution was then refluxed under stirring for 2 h during which time a
greyish precipitate formed. The suspension was allowed to cool to
ambient temperature. An aliquot (0.10 mL) of the supernatant liquid was
withdrawn, quenched by addition of CDCl3 (0.50 mL), and the resulting
solution analyzed by recording a 1H NMR spectrum. Deconvolution of the
CH3(iPr) region allowed to determine the molar amount of product in
relation to all iPr-containing species in the solution. This value was then
used to calculate the degree of conversion. The bulk reaction mixture of
the run conducted with 5a was quenched by addition of saturated
aqueous NH4Cl (20 mL). EtOAc (30 mL) was added, the organic layer of
the resulting biphasic mixture separated, and the aqueous phase washed
with EtOAc (3 x 10 mL). The combined organic phases were dried with
Na2SO4, filtered, and evaporated to dryness. The remaining red oil was
purified by chromatography (silica, eluent 120 mL hexane/EtOAc 30:1).
Evaporation of the eluate to dryness afforded 1.49 g (5.9 mmol, 72%
yield) of 11 as a colorless, crystalline material.– 1H NMR (250 MHz,
Synthesis of phenyl piperidine: Reaction and work-up were carried out
by analogy to the synthesis of 10 using 36 mg (0.16 mmol, 2 mol-%) of
Pd(OAc)2, 137 mg (0.24 mmol, 3 mol-%) of 5a, 1.15 g (12 mmol) of
NaOtBu, 851 mg (0.95 mL, 10 mmol) piperidine, and 1.29 g (0.86 ml,
8.22 mmol) bromobenzene in 30 mL toluene. The crude oil obtained after
the extraction procedure solidified upon overnight storage at -15 °C. The
product was identified by 1H NMR; no yield was determined. 1H NMR
3
(250 MHz, CDCl3):  = 7.15 (m, 2 H, m-Ph), 6.90 (d, 2 H, JHH = 8.0 Hz,
o-Ph), 6.75 (t, 1 H, 3JHH = 7.3 Hz, 1 H, p-Ph), 3.08 (t, 4 H, 3JHH = 5.3 Hz,
NCH2), 1.65 (m, 4 H, NCCH2), 1.52 (m, 2 H, NCCCH2).
Computational studies: RI-DFT calculations were carried out on the
[31]
bwForCluster Justus with the TURBOMOLE
program suite (version
7.2.2017 [32]). Energy optimization of molecular structures was carried out
using the BP86 functional with a def2-svp basis set
to include dispersion effects. The buried volume of
ligands was calculated using the SambVca2.0 online tool
default settings provided.
[33]
and Grimme's
[34]
D3BJ formalism
[35]
with the
Crystal structure determinations. Diffraction studies were carried out
using a Bruker diffractometer equipped with a Kappa Apex II Duo CCD-
3
CDCl3):  = 7.25-7.12 (m, 4 H, o/m-Ph), 7.06 (t, 1 H, JHH = 7.7 Hz, p-
detector and a KRYO-FLEX cooling device with Mo-ܭ radiation ( =
3
3
஑
C6H3), 6.63 (t, 1 H, JHH = 7.3 Hz, p-Ph), 6.40 (d, 2 H, JHH = 7.7 Hz, m-
0.71073 Å) at T = 100(2) K for 7b and T = 130(2) K for 6b, 8b. The
structures were solved by direct methods (SHELXS-97[36]) and refined
with a full-matrix least-squares scheme on F2 (SHELXL-2014[36]). Semi-
empirical absorption corrections were applied for all structures. Non-
hydrogen atoms were refined anisotropically except when disordered, the
hydrogen atoms at phosphorus isotropically, and all other hydrogen
atoms with a riding model. One iPr-group in 8b is disordered over two
positions. Disordered atoms were refined isotropically and with restraints.
CCDC-1866063 (6b), CCDC-1866064 (7b), and CCDC-1866065 (8b)
contain the supplementary crystallographic data for this paper. This data
can be obtained free of charge from the Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
3
C6H3), 5.04 (s, 1 H, NH), 3.13 (sept, 2 H, JHH = 6.9 Hz, CH(CH3)2), 1.07
(d, 12 H, 3JHH = 6.9 Hz, CH(CH3)2). – Anal. for C18H23N (253.39 g mol-1):
calcd. C 85.32 H 9.15 N 5.53%; found C 85.31 H 9.23 N 5.44%.
For kinetic studies, the preparation of the reaction mixture and the
addition of reactants were carried out as described, except that only 0.2
mol-% of Pd(OAc)2 (3.7 mg, 16 mol) and 0.3 mol-% of phosphine (14.1
mg of 5a or 6.3 mg of PPh3, 25 mol) were employed. After all reagents
had been added, an aliquot of the reaction mixture was withdrawn and
subjected to 1H NMR analysis as described above. The mixture was then
refluxed for 2 h (reaction with 5a) or 3 h (reaction with PPh3). Aliquots
(0.10 mL) were withdrawn every five min during the first 90 min. In the
reaction with PPh3, additional samples were drawn after 120, 150, and
180 min. Analysis of all samples was carried out as described; the results
are included in the supporting information. No work-up was attempted.
Acknowledgments
(b) Under microwave irradiation: A microwave reaction vessel (volume
30 mL) was charged with bromobenzene (2.83 g, 1.89 mL, 18 mmol),
2,6-diisopropylaniline (3.55 g, 3.64 mL, 20 mmol), NaOtBu (2.00 g, 21
mmol), Pd(OAc)2 (81 mg, 0.36 mmol, 2 mol-%), ligand 5a (206 mg, 0.36
mmol, 3 mol-%), and toluene (10 mL). The vessel was placed into the
microwave reactor, heated to 200 °C and held at this temperature for 10
min before being cooled down to ambient temperature. The reaction
mixture was stirred during the whole procedure at a rate of 1200 rpm.
The greyish precipitate was allowed to settle down and the mixture then
analyzed as described under (a). No work-up was attempted.
The computational studies were supported by the bwHPC
initiative and the bwHPC-C5 project provided through the
associated compute services of the JUSTUS HPC facility at the
University of Ulm. bwHPC and bwHPC-C5 (http://www.bwhpc-
c5.de) are funded by the state of Baden-Württemberg and the
DFG (grant no. INST 40/467-1 FUGG). We further thank B.
Förtsch for elemental analyses and Dr. W. Frey (Institute of
Organic Chemistry, University of Stuttgart) for the collection of
X-ray data sets.
Synthesis of 12: A 100 mL reaction vessel was charged with Pd(OAc)2
(13 mg, 0.16 mmol, 0.7 mol-%), 5a (50 mg, 0.24 mmol, 1.1 mol-%),
NaOtBu (419 mg, 4.37 mmol), amine 11 (880 mg, 3.47 mmol), and
toluene (11 mL). The mixture was refluxed for 10 min, bromobenzene
(453 mg, 0.31 mL, 2.88 mmol) was added, and the reaction mixture
refluxed for 24 h. Stirring was maintained during the whole procedure.
Further analysis and work-up was carried out as described above. The
crude product obtained after extraction was recrystallized from MeOH to
afford 742 mg (2.25 mmol, 65%) of 12 as colorless crystalline material. –
1H NMR (250 MHz, CDCl3):  = 7.30 (m, 1 H, p-C6H3 ), 7.17 (m, 2 H, m-
C6H3), 7.10 (m, 4 H, m-Ph), 6.93 (m, 4 H, o-Ph), 6.79 (m, 2 H, p-Ph), 3.08
Keywords: Phosphane ligands • Ligand effects • Cross-coupling
• Palladium • Boranes
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This article is protected by copyright. All rights reserved.

10.1002/ejic.201801081
European Journal of Inorganic Chemistry
FULL PAPER
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10.1002/ejic.201801081
European Journal of Inorganic Chemistry
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Phosphines with an N-heterocyclic
boryl substituent at phosphorus
behave as strongly electron releasing
ligands that may impart despite their
conformational flexibility sizable steric
shielding to a transition metal atom.
This makes them suitable candidates
for application as ligands in cross-
coupling chemistry.
Electron rich phosphines
M. Kaaz, R. J. C. Locke, L. Merz, M.
Benedikter, S. König, J.Bender, S. H.
Schlindwein, M. Nieger and D. Gudat*
Page No. – Page No.
Phosphines with N-heterocyclic
Boryl-substituents: Ligands for
Coordination Chemistry and
Catalysis
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