Intermediates in the Rh-catalysed dehydrocoupling of phosphine-borane

Article abstract of DOI:10.1039/c2cc32696e

Active species, product distributions and a suggested catalytic cycle are reported for the dehydrocoupling of the phosphine-borane H₃B· P^tBu₂H to give HP^tBu₂BH ₂P^tBu₂BH₃ using the [Rh(COD) ₂][BAr^F₄] pre-catalyst.

Full text of DOI:10.1039/c2cc32696e

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Intermediates in the Rh-catalysed dehydrocoupling of phosphine–boranewz
Miguel A. Huertos and Andrew S. Weller*
Received 16th April 2012, Accepted 15th May 2012
DOI: 10.1039/c2cc32696e
Active species, product distributions and a suggested catalytic
cycle are reported for the dehydrocoupling of the phosphine–
borane H3BꢀPtBu2H to give HPtBu2BH2PtBu2BH3 using the
[Rh(COD)2][BArF4] pre-catalyst.
recently reported complex pathways for dehydrocoupling that
revolve around Rh(I) and Rh(III) sigma complexes of amine–
boranes,11,12 and such insight leads to the rationalisation of
the relative efficiency of certain catalysts.13 As the direct study
of the species present in phosphine–borane dehydrocoupling
under the melt conditions required for catalysis is problematic,
we instead report complementary studies that interrogate
indirectly the melt. These studies demonstrate that species
closely related to those observed in analogous amine–borane
chemistry are likely to be present during catalysis.
The transition-metal mediated catalytic dehydrocoupling1 of
primary and secondary phosphine–boranes is potentially a
very useful process for delivering oligomeric and polymeric
materials with P–B bonds, for example polyphosphinoboranes
that find application as electron beam resists and as pre-ceramic
precursors to semi-conducting boron-phosphide.2 Such polymeric
materials are also exciting in that they are valence isoelectronic
with technologically ubiquitous polyolefins. Metal-catalysed
routes to their synthesis offer the potential for control over
molecular weight, stereochemistry and functional-group tolerance.
The current best catalyst, originally reported in 1999, uses the
[Rh(COD)2][OTf] precursor in neat phosphine–borane under melt
conditions (90–140 1C),3,4 although other catalyst systems have
been reported.4–6 For secondary phosphines, depending on the
substituent on phosphorus and the reaction temperature, dimeric
We initially chose to study the dehydrocoupling of H3Bꢀ
PPh2H, 1b, as this gives a single product, 2b, in high conversion
at 90 1C.3 However, indirect monitoring of this melt reaction of
[Rh(COD)2][BArF] by dissolving in 1,2-F2C6H4 solvent, using
electrospray ionisation mass spectrometry (ESI-MS) and
31P{1H} NMR spectroscopy, showed multiple organometallic
species; while attempts to synthesise possible intermediate
species also led to multiple products (vide infra). By contrast,
the tBu-substituted starting material, H3BꢀPtBu2H, 1a, afforded
generally well-defined species that allowed for the identification
of likely intermediates. Thus after 20 hours under melt conditions
(140 1C) catalysis using [Rh(COD)2][BArF] formed 2a in moderate
yield (B65%), as reported.7 Interestingly the bis-phosphine-
boronium [(tBu2HP)2BH2][BH4], 7[BH4],14 was also formed as
a co-product (B10%, Scheme 2). Interrogation of a catalyst-
mixture after 5 hours by 31P{1H} NMR spectroscopy and ESI-MS
(1,2-F2C6H4 solution) showed that the cations [Rh(PtBu2H)2-
(Z2-H3BꢀPtBu2BH2PtBu2H)]+, 3a+, and [Rh(PtBu2H)2(Z-F2-
C6H4)]+, 8+, were present in a B1 : 1 ratio.15 We presume
that 8+ is formed by reaction of a [Rh(PtBu2H)2]+ fragment
with the excess of solvent.11
t
HPR2BH2PR2BH3 (R = Bu, 2a; Ph, 2b) or cyclic (R = Ph)
oligomers are formed, Scheme 1.4,7
Although experimental evidence points strongly to a homo-
geneous, rather than heterogeneous, catalyst;8 due to the high
temperature melt conditions required, details regarding the
active species and mechanism are scarce.5,6,9 This is in contrast
to amine–borane dehydrocoupling where reactions occurring
at room temperature in common solvents have allowed for
mechanistic details to be resolved.10 For example we have
The observation of PtBu2H ligation in 3a+ suggests
that P–B bond cleavage in 1a, as commented on previously,6,8
had occurred. To further explore this, addition of 1a to
[Rh(PiBu3)2(C6H5F)][BArF4] in 1,2-F2C6H4 solution, a catalyst
we have successfully used in amine–borane dehydrocoupling,11
Scheme 1 Dehydrocoupling of H3BꢀPR2H using [Rh(COD)2][OTf]
t
as a catalyst (R = Bu, Ph) under melt conditions.
Department of Chemistry, Inorganic Chemistry Laboratories,
University of Oxford, Oxford, OX1 3QR, UK.
E-mail: andrew.weller@chem.ox.ac.uk
w This article is part of the ChemComm ‘Frontiers in Molecular Main
Group Chemistry’ web themed issue.
z Electronic supplementary information (ESI) available: Synthesis and
characterisation of new complexes and catalysis monitoring. CCDC
865050. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c2cc32696e
Scheme 2 Monitoring dehydrocoupling under melt conditions upon
dissolving in 1,2-F2C6H4.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 7185–7187 7185

Scheme 4 Routes to 5a[BArF4]. 1,2-F2C6H4 solution [BArF4] anions
not shown.
Placing 4a[BArF4] under a D2 atmosphere resulted in the
Scheme 3 Synthesis of 3a[BArF4] and 4a[BArF4]. [BArF4] anions not
formation of [Rh(PtBu2H)2(D)2(Z2-H3BꢀPtBu2H)][BArF
]
4
shown.
5a[BArF4]-d2. Over time slow H–D exchange occurred at
B–H and Rh–D but not at P–H, similar to that observed for
analogous amine–borane complexes.11 HD(dissolved) was also
observed by 2H NMR spectroscopy. Addition of H2 to
resulted in a mixture of complexes identified as [Rh(PiBu3)n-
(PtBu2H)2ꢁn(H3BꢀPR3)][BArF4] (PR3 = PiBu3, PtBu2H; n =
2–0; ESI). Complex 3a[BArF ] could be synthesised in good yield
4
by addition of 2a7 to [Rh(PtBu2H)2(Z-C6H5F)][BArF4] 9[BArF ],
Scheme 3. The equivalent complex of 1a can be synthesised:
4a[BArF4] is reversible, and passing Ar over 5a[BArF
]
4
4
(less than 5 minutes), or exposure to a hydrogen acceptor or
a vacuum regenerates 4a[BArF4]. Interestingly, addition of H2
to 3a[BArF4] did not result in any reaction. These data suggest
that cations 4a+/5a+ and 3a+ are all likely to be present
under catalytic conditions.
[Rh(PtBu2H)2(Z2-H3BꢀPtBu2H)][BArF4] 4a[BArF ].y NMR and
4
ESI-MS data are in full accord with the description of these
complexes as s-phosphine–borane adducts of the {Rh(PtBu2-
H)2}+ fragment, by comparison with the analogous amine–borane
complexes11,12 as well as other metal-complexes of phosphine–
boranes.16 In particular, a diagnostic upfield shifted signal
in the 1H NMR spectrum for the Rhꢀ ꢀ ꢀH3B interaction
(e.g. 4a[BArF4], d ꢁ1.89, 3 H), and down-field shifted signal
Addition of 1a to 4a[BArF4] results in the B10% formation
of 5a[BArF4] (5 hours, 60 1C, 1,2-F2C6H4) and unreacted
starting material. Full conversion to 5a[BArF4] (B95%) is
only achieved at 75 1C after 15 hours, Scheme 4, alongside
unidentified phosphine–borane products. Under these non-melt
conditions, use of higher temperatures (110 1C) results in the
formation of 7+ and 2a in an approximate 1 : 1 ratio, but also in
the decomposition of the metal fragment. This shows that
although melt conditions are not explicitly required for the
formation of 2a, they do influence the balance of final products.
We speculate that this reaction proceeds via formation of a very
reactive (and as yet unreported) H2BQPtBu2 species,20 by analogy
with amine–borane chemistry.11,21 In the absence of 1a at 75 1C
complex 4a[BArF ] only slowly (5 days) changes into 5a[BArF ]
in the 11B NMR spectrum (e.g. 4a[BArF ], d 0.5), are observed.
4
The solid-state structure of 4a[BArF ] is shown in Fig. 1. These
4
complexes add to the small number of transition metal complexes
reported to be sigma-bound with secondary phosphine–boranes.17
Those with tertiary phosphine–boranes are better represented.16,18,19
Attempts to prepare the analogous complexes using PPh2H were
not successful, leading to mixtures of intractable products. Starting
from isolated 3a[BArF ] or 4a[BArF ] as catalysts under melt
4
4
conditions (5 mol%, 140 1C) gives productive turnover and the
same mixture of products as with [Rh(COD2][BArF4] (2a, 7[BH4])
consistent with their role in catalysis.
Addition of H2 to 4a[BArF4] results in oxidative addition
and formation of the Rh(III) dihydride complex [Rh(PtBu2H)2-
4
4
and other as yet to be identified products, demonstrating the
requirement for excess 1a to cleanly form 5a[BArF ].
4
(H)2(Z2-H3BꢀPtBu2H)][BArF
] ] as characterised
4
5a[BArF
4
by NMR spectroscopy (Scheme 4), with diagnostic signals observed
in the 1H NMR spectrum at d ꢁ17.67 (2 H, dt, Rh–H, J(RhH) 20,
J(PH) 15 Hz) and d ꢁ0.94 (3 H, br, Rh–H–B, J(BH) 104 Hz).
Scheme 5 Reactivity of [Rh(COD)2][BArF4] with 1a. Conditions:
sealed NMR tube; 1,2-F2C6H4 solution. [BArF4] anions not shown.
Having established the base-line synthesis and reactivity for
complexes of 1a, we next explored how they might be formed,
albeit in solution rather than melt conditions. Addition of two
equivalents of 1a to [Rh(COD)2][BArF4] at 25 1C (1,2-F2C6H4
solution) formed the Rh(I) complex [Rh(COD)(PtBu2H)-
(Z2-H3BꢀPtBu2H)][BArF4] 6a[BArF ] in essentially quantitative
4
yield (Scheme 5), which was characterised by NMR spectroscopy,
by comparison with the chelating phosphine–borane complex
[Rh(COD)(k1,Z2-H3BPh2CH2PPh2)][PF6].18 Heating 6a[BArF ]
4
with two further equivalents of 1a at 75 1C in a sealed NMR
Fig. 1 Solid-state structure of 4a[BArF4]. Most hydrogen atoms and
the anion are omitted for clarity, as is the minor disordered component.
Displacement ellipsoids are shown at the 40% probability level. Selected
bond lengths [A] and angles [1]: Rh1–B1, 2.188(3); B1–P1, 1.946(3);
Rh1–P2, 2.2155(7); Rh1–P3, 2.2035(7); Rh1–H1, 1.87(3); Rh1–H2,
1.90(3); P2–Rh1–P3, 91.12(3).
tube resulted in the formation of 5a[BArF ] as the major (90%)
4
organometallic product, alongside unidentified phosphine–borane
materials. We suggest that this process likely proceeds via the
intermediate 4a[BArF ] (e.g., Scheme 4), and that the COD
4
ligands are lost as the hydroboration products.
c
7186 Chem. Commun., 2012, 48, 7185–7187
This journal is The Royal Society of Chemistry 2012

Taken together our observations lead to an outline catalytic
cycle as shown in Scheme 6 with 3a+ assigned to a resting-state
in the cycle, although the precise details of the mechanism
remain to be resolved.
Notes and references
y Crystallographic data for 4a: C59H74B2F25P3Rh, M = 1475.62,
%
triclinic, P1 (Z = 2), a = 12.8587(1) A, b = 13.3769(1) A, c =
20.2366(2) A, a = 87.9016(4)1, b = 80.6774(4)1, g = 82.2782(5)1. V =
3403.373(50) A3, T = 150(2) K, 15 457 unique reflections [R(int) =
0.0174]. Final R1 = 0.0427 [I > 2s(1)]. CCDC 865050.
1 T. J. Clark, K. Lee and I. Manners, Chem.–Eur. J., 2006, 12, 8634;
R. Waterman, Dalton Trans., 2009, 18.
2 A. Staubitz, A. P. M. Robertson, M. E. Sloan and I. Manners,
Chem. Rev., 2010, 110, 4023.
3 H. Dorn, R. A. Singh, J. A. Massey, A. J. Lough and I. Manners,
Angew. Chem., Int. Ed., 1999, 38, 3321.
4 H. Dorn, R. A. Singh, J. A. Massey, J. M. Nelson, C. A.
Jaska, A. J. Lough and I. Manners, J. Am. Chem. Soc., 2000,
122, 6669.
5 K. Lee, T. J. Clark, A. J. Lough and I. Manners, Dalton Trans.,
2008, 2732.
6 M. E. Sloan, T. J. Clark and I. Manners, Inorg. Chem., 2009,
48, 2429.
7 H. Dorn, E. Vejzovic, A. J. Lough and I. Manners, Inorg. Chem.,
2001, 40, 4327.
8 C. A. Jaska and I. Manners, J. Am. Chem. Soc., 2004, 126, 9776.
9 C. A. Jaska, A. J. Lough and I. Manners, Dalton Trans., 2005, 326.
10 A. Staubitz, A. P. M. Robertson and I. Manners, Chem. Rev.,
2010, 110, 4079; N. C. Smythe and J. C. Gordon, Eur. J. Inorg.
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11 T. M. Douglas, A. B. Chaplin, A. S. Weller, X. Z. Yang and
M. B. Hall, J. Am. Chem. Soc., 2009, 131, 15440.
12 L. J. Sewell, G. C. Lloyd-Jones and A. S. Weller, J. Am. Chem.
Soc., 2012, 134, 3598.
Scheme 6 Proposed and simplified catalytic cycle for the dehydro-
coupling of 1a.
As described (Scheme 2), alongside the starting material
1a and final product 2a, an additional phosphine–borane
species is also formed. This compound is characterised by a
quadrupolar broadened 1 : 1 : 1 : 1 quartet at d 36 in the
31P{1H} NMR spectrum, and a very strong molecular ion in
the ESI-MS at m/z = 305.27 with the correct isotope pattern
13 R. Dallanegra, A. P. M. Robertson, A. B. Chaplin, I. Manners and
A. S. Weller, Chem. Commun., 2011, 47, 3763.
for the bis-phosphine boronium [(PtBu2H)2BH2]+, 7+.22
14 We assume this is as the [BH4]ꢁ salt, although the characterisitc
signal at d(11B) ꢁ41.1 is coincident with that of H3Bꢀ
PtBu2BH2PtBu2H.
ꢁ
Complex 7+ (as [BArF
]
4
and [BH4]ꢁ salts) has been
independently synthesised (ESI) confirming this assignment.
We discount the formation of 7+ as coming from P–B bond
cleavage in 2a as heating (melt, 140 1C, 20 hours) 2a
with [Rh(PtBu2H)2(Z6-C6H5F)][BArF4] results in no further
reaction. 7[BArF] is not an intermediate and must be formed
by a parallel route to 2a, as using it as a substrate in catalysis
results in no reaction. We speculate that 7+ might arise
from reaction of H2BQPtBu2 with PtBu2H,2 and subsequent
protonation in the melt conditions. Stephan et al. have
reported a related compound by the addition of 4-tBu–C6H4N
to Cy2P–B(C6F5)2.20b We cannot discount, however, alternative
mechanisms similar to those discussed for the formation of
[(NH3)2BH2][BH4].23
15 There is also another species observed [31P d 93.8 J(RhP) 199 Hz]
that currently remains unidentified.
16 M. Shimoi, S. Nagai, M. Ichikawa, Y. Kawano, K. Katoh,
M. Uruichi and H. Ogino, J. Am. Chem. Soc., 1999, 121, 11704;
Y. Kawano, M. Hashiva and M. Shimoi, Organometallics, 2006,
25, 4420.
17 A. E. W. Ledger, C. E. Ellul, M. F. Mahon, J. M. J. Williams and
M. K. Whittlesey, Chem.–Eur. J., 2011, 17, 8704; Adducts of H3Bꢀ
PR2H with rhodium dimers have been suggested on the basis of
in situ NMR spectroscopy: J. Mattiza, D. Albert, M. Stankevic,
K. Dziuba, A. Szmiggielska, K. M. Pietusiewicz and H. Duddeck,
Tetrahedron: Asymmetry, 2006, 17, 2689.
18 M. Ingleson, N. J. Patmore, G. D. Ruggiero, C. G. Frost,
M. F. Mahon, M. C. Willis and A. S. Weller, Organometallics,
2001, 20, 4434.
19 N. Merle, G. Koicok-Kohn, M. F. Mahon, C. G. Frost,
G. D. Ruggerio, A. S. Weller and M. C. Willis, Dalton Trans.,
2004, 3883.
20 For related phosphinoborane complexes isolated using bulky
R groups see: (a) X. Feg, M. M. Olmstead and P. P. Power,
et al., Inorg. Chem., 1986, 25, 4615; (b) S. J. Geir, T. M. Gilbert and
D. W. Stephan, Inorg. Chem., 2011, 50, 336.
21 C. J. Stevens, R. Dallanegra, A. B. Chaplin, A. S. Weller,
S. A. Macgregor, B. Ward, D. McKay, G. Alcaraz and S. Sabo-
Etienne, Chem.–Eur. J., 2011, 17, 3011.
Polymeric phosphine–boranes, formed from transition-
metal-mediated dehydrocoupling, have significant potential
to be technologically interesting materials. However, catalyst
development for their production lags behind those used
for amine–borane dehydrocoupling, mainly due to the melt
conditions currently required. Our observations provide
compelling clues as to the species that may be present during
catalysis under melt conditions. With such information in
hand, the next steps of catalyst development, which must be
aimed at lowering dehydrocoupling temperatures, avoiding
the requirement for melt conditions and elucidation of the
initial dehydrogenation product, can now be realistically
tackled.
22 This complex has been reported previously as the Brꢁ salt:
Y. Yamamoto, T. Koizumi, K. Katagiri, Y. Furuya, H. Danjo,
T. Imamoto and K. Yamaguchi, Org. Lett., 2006, 8, 6103.
23 M. Bowden, D. J. Heldebrant, A. Karkamkar, T. Proffen,
G. K. Schenter and T. Autrey, Chem. Commun., 2010, 46, 8564;
H. K. Lingam, X. Chen, J.-C. Zhao and S. G. Shore, Chem.–Eur.
J., 2012, 18, 3490.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 7185–7187 7187