Iridium-catalysed dehydrocoupling of aryl phosphine-borane adducts: Synthesis and characterisation of high molecular weight poly(phosphinoboranes)

Article abstract of DOI:10.1039/c6cc04363a

The thermal dehydrogenative coupling of aryl phosphine-borane adducts with iridium complexes bearing a bis(phosphinite) pincer ligand is reported. This catalysis produces high molecular weight poly(phosphinoboranes) [ArPH-BH₂]_n (Ar = Ph, ^pTol, Mes). Furthermore, we investigated the reactivity of these pincer complexes towards primary phosphines and their respective borane adducts on a stoichiometric scale.

Full text of DOI:10.1039/c6cc04363a

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Iridium-Catalysed Dehydrocoupling of Aryl Phosphine-Borane
Adducts: Synthesis and Characterisation of High Molecular Weight
Poly(phosphinoboranes)
Received 00th January 20xx,
Accepted 00th January 20xx
U. S. D. Paul, H. Braunschweig*, U. Radius*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
The thermal dehydrogenative coupling of aryl phosphine-borane coupling of phosphine-borane adducts (RPH2–BH3).
adducts with iridium complexes bearing a bis(phosphinite) pincer
O
PtBu2
ligand is reported. This catalysis produces high molecular weight
poly(phosphinoboranes) [ArPH–BH2]n (Ar = Ph, pTol, Mes).
H
[Fe] / [Ir] cat.
H
Ir
H
MeNH2 BH3
N
BH2
[Ir] cat.=
Me
Furthermore, we investigated the reactivity of these pincer
complexes towards primary phosphines and their respective
borane adducts on a stoichiometric scale.
n
O
PtBu2
1a
H
P
R
[Rh] / [Fe] cat.
PhPH 2 BH3
BH2
[Rh] cat.= [(1,5-cod)2Rh]OTf
[Fe] cat.= [Cp(CO)2FeI]
The formation of main group element-element bonds has
traditionally been achieved using methods such as salt
metathesis and reductive coupling. Catalytic routes to form
relatively strong homonuclear (E–E) und heteronuclear (E–E’)
n
2a
3a
Scheme 1. Examples for catalytic dehydropolymerisation of amine1h,4a and
phosphine3a,4b boranes.
bonds between lighter main group elements (E = B, N, Al, Si, P, Since the early 2000s, detailed studies on rhodium-catalysed
S), are mainly based on dehydrocoupling of main group dehydropolymerisation of a broad range of primary phosphine
element hydrides (E–H). The transition metal and main group boranes3 have been reported, giving polydisperse, soluble,
element catalysts used for this kind of bond-formation branched polymers (Mn > 10 000) as well as crosslinked,
reaction usually need to be highly reactive in order to facilitate swellable, but insoluble materials, depending on the specific
an element-hydrogen bond activation step, typically required conditions and substituents of the phosphine. Though
for the dehydrocoupling.
sufficient results were only obtained when the polymerisation
Iridium complexes bearing the bis(phosphinite) pincer ligand was carried out as a melt reaction at elevated temperatures
3g-h
tBuPOCOP (κ3-C6H3-1,3-(OPtBu2)2) have shown unique activities (ca. 130 °C).3a-e,
in the homogeneous (transfer)dehydrogenation of various reported recently an iron-catalysed dehydrocoupling of
substrates such as alkanes,1a alcohols1b and amines.1c PhPH2–BH3
2a) in solution to yield the respective soluble
In marked contrast, Manners et al.
(
Furthermore, the complex [(tBuPOCOP)IrH2] (1a) proved to be poly(phosphinoborane) (3a) with control over the molecular
an active (pre)catalyst in the dehydrogenation of ammonia weight (Mn 42 000 to 80 000) by varying the catalyst loading
borane (NH3–BH3)1d-h as well as in dehydrocoupling reactions (Scheme 1).4b The iron complex [Cp(CO)2FeI] investigated is
of primary amine boranes (RNH2–BH3) to yield soluble high one of the rare examples5 of a catalyst which was successfully
molecular weight poly(aminoboranes) [RNH-BH2]n.1h The latter applied in the dehydrogenation of both amine-4a and
result was of particular interest, as previous studies revealed phosphine-borane4b adducts, a fact that further encouraged us
that a variety of catalysts only led to the formation of to conduct our studies. Interestingly, the substrate scope of
borazines and/or poorly characterised insoluble and branched this dehydrogenative coupling reaction is limited to aryl
oligomeric material.2 This prompted us to investigate these substituted phosphine boranes, while alkyl substituted
iridium pincer complexes as catalysts for the dehydrogenative poly(phosphinoboranes) are accessible through metal-free
addition/polymerisation of suitable Lewis base stabilised
monomeric phosphinoboranes, as recently reported by Scheer
et al.6
Treatment of primary phosphine boranes ArPH2–BH3 (Ar = Ph
p
2a, Tol 2b, Mes 2c) with 5 mol% of [(tBuPOCOP)IrH2] (1a) in
toluene at 100 °C results in the complete consumption of the
starting materials within 24 hours and, as monitored by 11B
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and 31P NMR spectroscopy, in the selective formation of the We investigated the molecular weights of the polymers 3a-c
respective poly(phosphinoborane) [ArPH–BH2]n (Ar = Ph 3a through gel permeation chromatography (GPC) experiments
pTol 3b, Mes 3c) (Table 1).7 The polymeric products were (Table 1 and Figure S1-S3 in the SI).8 In the case of [PhPH–BH2]n
isolated as off-white solids by precipitation into cold pentane
3a) and [pTolPH–BH2]n (3b) the GPC traces show a very broad
and subsequently characterised by multinuclear NMR as well distribution of molecular weights (entries 1-2). The same
,
(
as IR spectroscopy and elemental analysis.
observation was made for [MesPH–BH2]n (3c) (entry 3 and
Figure S1). The high molecular weight obtained for the mesityl-
substituted polymer encouraged us to further optimise the
catalysis with MesPH2–BH3 (2c) as starting material (entries 4-7
and Figure S2). Lowering the catalyst loading to 2.5 mol%
results in a more-defined GPC trace with narrower molecular
weight distribution (entry 4) but lower overall weights.
Furthermore, quantitative dehydrocoupling of 2c already takes
place at 80 °C in benzene within 24 hours9 giving high
molecular weight 3c (entry 5 and Figure S3). When only
0.5 mol% of 1a is used as catalyst under similar conditions the
molecular weight of polymer 3c further decreases (entry 7).
This dependence of the molecular weight on the catalyst
loading (higher loadings lead to higher molecular weights)
would suggest a step-growth mechanism of the investigated
iridium catalysed polymerisation of phosphine boranes. The
presence of high molecular weight polymers even at low
conversion (6 h, ≈ 36 %, entry 6), however, rather indicates a
chain-growth character of this reaction.
Table 1. Iridium-catalysed dehydrocoupling of primary phosphine-borane
adducts 2a-c to polymers 3a-c
.
H
0.5 - 5 mol% 1a
ArPH 2 BH3
P
BH2
solvent, 80 - 100 °C
Ar
n
2
3
Ar = Ph a, pTol b, Mes c
Adduct
2a
Mol% 1a
Methoda
Time
24 h
24 h
24 h
24 h
24 h
6 h
Mn/Mw
PDI
21.6
13.2
30.0
2.6
1
2
3
4
5
6
7
5
A
A
A
A
B
B
B
10 000/ 216 000
5 000/ 66 000
10 000/ 300 000
5 000/ 13 000
33 000/ 59 000
20 000/ 116 000
5 000/ 10 000
2b
5
2c
5
2c
2.5
2.5
2.5
0.5
2c
1.8
2c
5.8
2c
24 h
2.0
a Reaction conditions for methods A: 5 mL of toluene, 100 °C and B: 0.6 mL of benzene,
80 °C.
In good agreement with previous reports for polymer 3a 3a,b
,
the 31P NMR spectra of the isolated products show one broad
singlet at δ(31P) = -48.7 (3a)/ -49.8 (3b)/ -73.0 ppm (3c), which
splits into a doublet (1JPH = 352 (3a)/ 355 (3b)/ 355 Hz (3c)) in
the 1H-coupled spectrum and thus confirms the presence of
one single hydrogen substituent at phosphorus (Figure 1). The
1H NMR spectra were also consistent with the assigned
structures and contain a broad doublet at δ(1H) = 4.64 (3a)/
4.73 (3b)/ 4.93 ppm (3c) for the PH group (1JPH = 352 (3a)/
353 (3b)/ 356 Hz (3c)) as well as a broad signal for the BH2
protons (δ(1H{11B}) = 2.12-1.56 ppm) besides the peaks for the
aromatic protons (δ(1H) = 7.39-6.50 ppm) and the methyl
groups bound to the aryl substituents (δ(1H) = 2.41-2.01 ppm),
in the case of 3b and 3c. The 11B NMR spectra show a single
broad resonance at δ(11B) = -33.9 (3a)/ -33.5 (3b)/ -34.3 ppm
These contrary results would be consistent with a two-stage
polymerisation mechanism, as already reported for the
dehydrocoupling of primary amine boranes.1h According to
this, in the first step dehydrogenation proceeds slowly through
hydrogen cleavage from the parent phosphine borane by the
iridium bis(phosphinite) pincer complex.1i The so formed
monomer
phosphinoborane
ArPH–BH2
subsequently
undergoes (conceivably metal assisted) fast chain-growth
polymerisaton to the respective poly(phosphinoborane)
[ArPH–BH2]n in the second step. In such a reaction sequence
the availability of the monomer phosphinoborane would be
the limiting factor (for the progress of the polymerisation),
thus, explaining the observed dependence of the polymer
molecular weight on the catalyst loading.
(
3c), which lies in the expected range of a four-coordinate
boron center and is slightly downfield shifted compared to the
starting materials 2a-c (Figure 1). The 13C NMR resonances
appear in the expected region. In the IR spectra of the
polymers 3a-c strong absorptions were observed around 2443
and 2382 cm-1, which is in the expected range for both P–H
and B–H stretching vibrations.
Figure 2. Time dependent 31P NMR spectra (162 MHz, C6D6, 298 K) of the
dehydrogenation reaction of adduct 2c with complex 1a as catalyst
.
To gain further insight into the mechanism of this
iridium-promoted dehydrogenation, we monitored the
reaction of MesPH2–BH3 (2c) with catalytic amounts of 1a by
31P{1H} NMR spectroscopy (Figure 2). The signal for 1a vanishes
Figure 1. 31P (162 MHz, C6D6, 298 K) and 11B NMR spectrum (128 MHz, C6D6,
298 K) of the isolated polymerisation-product 3c
.
2 | J. Name., 2012, 00, 1-3
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upon addition of the adduct MesPH2-BH3 (2c) and four These observations supported the assumption that
different iridium species can be observed, namely the iridium compounds 6c and 7c might be products upon coordination of
),1d minor free mesitylphosphine to the iridium center, resulting in the
borohydride complex [(tBuPOCOP)Ir(H)2(BH3)]
(
4
10
amounts of the tetrahydride complex [(tBuPOCOP)IrH4] (
5
)
formation
of
[(tBuPOCOP)Ir(PH2Mes)]
(
6c
)
or
and two additional species 6c and 7c. These complexes were [(tBuPOCOP)Ir(H)2(PH2Mes)] (7c).
considered to be [(tBuPOCOP)Ir(PH2Mes)]
6c and To confirm our assignment of complexes 6c and 7c, we studied
[(tBuPOCOP)Ir(H)2(PH2Mes)] (7c) as each of them show one the reactivity of [(tBuPOCOP)IrH2] (1a) towards different
(
)
doublet for the pincer ligand (δ(31P) = 185.2/173.4 ppm with primary phosphines ArPH2 (Ar = Ph 8a pTol 8b, Mes 8c
, )
2JPP = 9.2/11.9 Hz) and one triplet (δ(31P) = -101.6/-139.7 ppm (Scheme 3). Addition of one equivalent of MesPH2
(
8c) to a
2
with JPP = 9.2/11.9 Hz) in the region expected for the primary solution of complex 1a resulted in the liberation of dihydrogen
mesitylphosphine coordinating to a transition metal. Upon and formation of a mixture of two products, which were
heating the reaction mixture to 80 °C the signals for complexes identified by comparison of 31P{1H} NMR spectroscopical data
4
,
5
and 6c disappear completely and compound 7c can be as the two compounds already described above as iridium(I)
observed as the only iridium containing species in solution and phosphine [(tBuPOCOP)Ir(PH2Mes)]
6c and iridium(III)
therefore might be described as the resting state of the dihydrido phosphine complex [(tBuPOCOP)Ir(H)2(PH2Mes)]
(
)
catalytically active species. This is supported by the
observation that the catalysis can be resumed through
addition of a further equivalent of adduct 2c to this reaction
mixture and subsequent heating (Figure S4).
(7c).
O
PtBu2
O
O
PtBu2
O
PtBu2
+ ArPH 2 (8)
H
H
Ir
H
Ir PH2Ar
+
Ir PH2Ar
Similar results are obtained when reacting [(tBuPOCOP)IrH2]
H
O
O
PtBu2
PtBu2
O
PtBu2
(
1a) with a stoichiometric amount of the phosphine-borane
1a
6
7
adduct MesPH2-BH3 (2c) at room temperature (Scheme 2).
PtBu2
O
PtBu2
O
O
PtBu2
PtBu2
O
PtBu2
+ H2
O
+ ArPH 2 (8)
H
(1 atm)
+ 2c
H
Ir
H
H
Ir
H
BH2
+
Ir PH2Mes 6c
PtBu2
Ir Ph
Ir PH2Ar
PtBu2
H
PtBu2
O
PtBu2
O
PtBu2
O
O
Ar = Ph a, pTol b, Mes c
4
1a
16 hours
1b
6
O
PtBu2
Scheme 3. Reaction of primary phosphines 8a-c with complexes 1a and 1b to
give the iridium(I) phosphine 6a-c and iridium(III) dihydrido phosphine
complexes 7a-c, respectively.
H
"BH3"
+
Ir PH2Mes 7c
H
O
PtBu2
The same results were obtained for the analogous reactions of
p
PhPH2 (8a) and TolPH2 (8b) with 1a, giving rise to mixtures of
Scheme 2. Stoichiometric reaction of MesPH2-BH3 (2c) with complex
[(tBuPOCOP)IrH2] (1a).
[(tBuPOCOP)Ir(PH2Ar)]
(Ar = Ph
6a
,
pTol
6b
)
and
According to 31P{1H} NMR spectroscopical data, complex 1a is
immediately converted equally into [(tBuPOCOP)Ir(H)2(BH3)]
[(tBuPOCOP)Ir(H)2(PH2Ar)] (Ar = Ph 7a
,
pTol 7b). All of these
iridium phosphine complexes can be synthesised selectively by
(4) and compound 6c and within further 16 hours the latter
variation of the starting material. Reacting the phosphines 8a-c
complex 6c is quantitatively converted into compound 7c
.
with the bis(phosphinite) stabilised iridium complex
[(tBuPOCOP)IrHPh] (1b
(Figure 3) During this reaction, the 11B NMR spectrum reveals
10
)
instead, the phosphine complexes
[(tBuPOCOP)Ir(PH2Ar)] (Ar = Ph 6a pTol 6b, Mes = 6c) are
,
only signals for starting material 2c and borohydride complex
4.
Furthermore, with the formation of 7c the 1H NMR
formed quantitatively (Scheme 3). Subsequent pressurising of
the iridium(I) complexes 6a-c with dihydrogen (1 atm) at room
temperature yields the respective iridium (III) dihydrides
spectrum
reveals
a
new
hydride
resonance
(δ(1H) = -10.98 ppm) besides those assigned for compound
(δ(1H) = -20.36, -6.14, -4.95 ppm).
4
, , Mes 7c).
[(tBuPOCOP)Ir(H)2(PH2Ar)] (Ar = Ph 7a pTol 7b
Compounds 6a-c and 7a-c were characterised by multinuclear
NMR spectroscopy and single-crystal X-ray diffraction (Figure
4). Furthermore, the iridium (phosphine) complexes 1b, 6c and
7c serve as (pre)catalysts for the dehydrogenation of adduct 2c
(Figure S5), hence corroborating our assumption of the
phosphine complexes being a resting state rather than a
dormant form of the catalytically active species.
During the course of this study, we found no evidence for the
formation of boryl, phosphido or phosphido-borane
complexes, which were proposed as the catalytic
intermediates that undergo the P–B bond formation processes
in the respective rhodium11 and iron4 based systems. Thus,
further investigations are needed to shed light into the precise
Figure 3. Time dependent 31P NMR spectra (162 MHz, C6D6, 298 K) of the
stoichiometric reaction of adduct 8c with complex 1a
.
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Dietrich, K. I. Goldberg, D. M. Heinekey, T. Autrey and J. C.
Linehan, Inorg. Chem., 2008, 47, 8583-8585; h) A. Staubitz,
M. E. Sloan, A. P. M. Robertson, A. Friedrich, S. Schneider, P.
J. Gates, J. Schmedt auf der Günne and I. Manners, J. Am.
Chem. Soc., 2010, 132, 13332-13345; i) A. Paul and C. B.
Musgrave, Angew. Chem. Int. Ed., 2007, 46, 8153-8156.
a) C. A. Jaska, K. Temple, A. J. Lough and I. Manners, J. Am.
Chem. Soc., 2003, 125, 9424-9434; b) [Ru]: N. Blaquiere, S.
Diallo-Garcia, S. I. Gorelsky, D. A. Black and K. Fagnou, J. Am.
Chem. Soc., 2008, 130, 14034-14035; c) [Rh]: C. A. Jaska, K.
Temple, A. J. Lough and I. Manners, Chem. Commun., 2001,
962-963; d) [Ni]: R. J. Keaton, J. M. Blacquiere and R. T.
Baker, J. Am. Chem. Soc., 2007, 129, 1844-1845; e) [Pt
heterogen]: R. P. Shrestha, H. V. K. Diyabalanage, T. A.
Semelsberger, K. C. Ott and A. K. Burrell, Int. J. Hydrogen
Energy, 2009, 34, 2616-2621.
order of the P–H/B–H activation process and P–B bond
formation in particular.
2
3
a) H. Dorn, R. A. Singh, J. A. Massey, A. J. Lough and I.
Manners, Angew. Chem. Int. Ed., 1999, 38, 3321-3323; b) H.
Dorn, R. A. Singh, J. A. Massey, J. M. Nelson, C. A. Jaska, A. J.
Figure 4. Molecular structure of [(tBuPOCOP)Ir(PH2Mes)] 6c (left) and
[(tBuPOCOP)Ir(H)2(PH2Mes)] 7c (right); thermal ellipsoids set at 50 % probability
level. Some hydrogen atoms have been omitted for clarity. Selected bond
lengths [pm] and angles [°]: 6c: Ir1–C1 205.1(4), Ir1–P1 226.90(11), Ir1–P2
228.48(12), Ir1–P3 228.92(12); C1-Ir1-P3 171.88(12), P1-Ir1-P2 156.29(4), P1-Ir1-
C1 78.58(13), Ir1-P3-C23 130.08(14); 7c: Ir1–C1 206.5(2), Ir1–P1 229.35(6), Ir1–
P2 231.33(6), Ir1–P3 231.48(6); C1-Ir1-P3 171.88(6), P1-Ir1-P2 155.94(2), P1-Ir1-
C1 78.48(6), Ir1-P3-C23 126.85(7).
Lough and I. Manners, J. Am. Chem. Soc., 2000, 122
,
6669-6678; c) H. Dorn, E. Vejzovic, A. J. Lough and I.
Manners, Inorg. Chem., 2001, 40, 4327-4331; d) H. Dorn, J.
M. Rodezno, B. Brunnhöfer, E. Rivard, J. A. Massey and I.
Manners, Macromolecules, 2003, 36, 291-297; e) T. J. Clark,
J. M. Rodezno, S. B. Clendenning, S. Aouba, P. M. Brodersen,
A. J. Lough, H. E. Ruda and I. Manners, Chem. Eur. J., 2005,
11, 4526-4534; f) E. M. Leitao, T. Jurca and I. Manners, Nat.
In summary, we have demonstrated that the bis(phosphinite)
pincer ligated iridium complexes [(tBuPOCOP)IrH2] (1a) and
[(tBuPOCOP)IrHPh] (1b) can be used as catalysts in the
dehydrogenative coupling of primary phosphine-borane
adducts 2a-c in solution to yield the respective
poly(phosphinoboranes) 3a-c as readily soluble, high molecular
Chem., 2014, 5, 817-829; g) S. Pandey, P. Lönnecke and E.
Hey-Hawkins, Inorg. Chem., 2014, 53, 8242-8249; h) S.
Pandey, P. Lönnecke and E. Hey-Hawkins, Eur. J. Inorg.
Chem., 2014, 2456-2465; i) H. C. Johnson, T. N. Hooper and
A. S. Weller, Top. Organomet. Chem., 2015, 49, 153-220.
a) J. R. Vance, A. Schäfer, A. P. M. Robertson, K. Lee, J.
Turner, G. R. Whittell and I. Manners, J. Am. Chem. Soc.,
2014, 136, 3048–3064; b) A. Schäfer, T. Jurca, J. Turner, J. R.
Vance, K. Lee, V. An Du, M. F. Haddow, G. R. Whittell and I.
Manners, Angew. Chem. Int. Ed., 2015, 54, 4836-4841.
Further example for a rhodium-based system: a) R.
Dallanegra, A. P. M. Robertson, A. B. Chaplin, I. Manners and
A. S. Weller, Chem. Commun., 2011, 47, 3763-3765; b) M. A.
Huertos and A. Weller, Chem. Sci., 2013, 4, 1881-1888.
a) K.-C. Schwan, A. Y. Timoshkin, M. Zabel and M. Scheer,
Chem. Eur. J., 2014, 12, 4900-4908; b) C. Marquardt, A.
Adolf, A. Stauber, M. Bodensteiner, A. V. Virovets, A. Y.
Timoshkin and M. Scheer, Chem. Eur. J., 2013, 19, 11887-
11891; c) C. Thoms, C. Marquardt, A. Y. Timoshkin, M.
Bodensteiner and M. Scheer, Angew. Chem, Int. Ed., 2013,
52, 5150-5154; d) C. Marquardt, C. Thoms, A. Stauber, G.
Balázs, M. Bodensteiner and M. Scheer, Angew. Chem. Int.
Ed., 2014, 53, 3727-3730; e) C. Marquardt, T. Jurca, K.-C.
Schwan, A. Stauber, A. V. Virovets, G. R. Whittell, I. Manners
and M. Scheer, Angew. Chem. Int. Ed., 2015, 54, 13782-
13786.
4
weight
[(tBuPOCOP)IrH2] (1a) with ArPH2–BH3 (2c) lead to cleavage of
the phosphine with formation of
boranes11
[(tBuPOCOP)Ir(H)2(BH3)] (
), iridium(I) phosphine complexes
[(tBuPOCOP)Ir(PH2Ar)] (Ar = Ph 6a pTol 6b, Mes 6c) and
iridium(III) dihydrido phosphine
[(tBuPOCOP)Ir(H)2(PH2Ar)] (Ar = Ph 7a
polymers.
(Stoichiometric)
reactions
of
4
5
6
,
complexes
,
pTol 7b, Mes 7c). The
latter complexes have also been prepared directly from the
metal precursor and the corresponding phosphine. Due to the
observed dependence of the polymer molecular weight on the
catalyst loading we assume that the iridium catalysed
dehydrocoupling of phosphine-borane adducts proceeds
through a two-stage polymerisation mechanism, likewise
postulated for the respective reaction with amine boranes.1h
This work was supported by the Julius-Maximilians-
University
Würzburg
and
the
Deutsche
7
8
Similar to amine boranes, no dehydrogenative coupling was
Forschungsgemeinschaft (DFG).
observed for secondary phosphine boranes.1h
GPC analysis in THF as eluent needs to be considered
carefully, as GPC studies might be hampered by column
adsorption effects and/or the formation of aggregates in
solution. 3a-e
The catalytic dehydrocoupling of phosphine boranes 2a and
2b also proceeds at 80 °C in benzene, but requires 3 days for
completion.
Notes and references
1
a) I. Göttker-Schnettmann, P. S. White and M. Brookhart, J.
Am. Chem. Soc., 2004, 126, 1804-1811; b) A. V. Polukeev, P.
V. Petrovskii, A. S. Peregudov, M. G. Ezernitskaya and A. A.
Koridze, Organometallics, 2013, 32, 1000-1015; c) W. H.
9
10 I. Göttker-Schnettmann, P. S. White and M. Brookhart,
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