Amine-borane σ-complexes of rhodium. Relevance to the catalytic dehydrogenation of amine-boranes

Article abstract of DOI:10.1021/ja806582n

Rhodium amine-borane σ-complexes of H₃BNHMe₂ have been isolated which are potential intermediates in the catalytic dehydrogenation of H₃B·NHMe₂. Copyright

Full text of DOI:10.1021/ja806582n

Published on Web 10/10/2008
Amine-Borane σ-Complexes of Rhodium. Relevance to the Catalytic
Dehydrogenation of Amine-Boranes
Thomas M. Douglas, Adrian B. Chaplin, and Andrew S. Weller*
Department of Chemistry, Inorganic Chemistry Laboratories, UniVersity of Oxford, U.K., OX1 3QR
Received August 19, 2008; E-mail: andrew.weller@chem.ox.ac.uk
Chemical hydrogen storage in amine-boranes (e.g., H3B·NH3,
19.6 wt% H) is a possible solution to the transport of hydrogen for
future energy requirements due to their high hydrogen content.1
Although solution and solid-state dehydrogenations have been
reported, there is much interest in transition-metal-catalyzed de-
hydrogenation or hydrolytic2 reactions due to favorable kinetics
and lower reaction temperatures. Catalysts for dehydrogenation
include Cp2Ti derivatives,3,4 Re-nitrosyls,5 Ir-pincer complexes,6,7
and Ni-NHC complexes;8 colloidal-Rh has also been shown to be
an active catalyst,9,10 for which in situ EXAFS suggests that the
Figure 1. Cationic portion of 2-Me (left) and 3-H (right). Selected distances
(Å) and angles (deg): (2-Me): Rh1-B1, 2.180(4); P1-Rh1-P2, 97.35(4).
(3-H): Rh1-B1, 2.318(8); P1-Rh1-P2, 163.65(7).
active species might actually be smaller Rh4 and Rh6 “clusters”.11
Scheme 1. Amine-Borane Dehydrogenation
Addition of DMAB (2 equiv) to [Rh(PiBu3)2][BArF ] 118 (ArF
4
4
) 3,5-C6H3(CF3)2) in 1,2-difluorobenzene results in the immediate
formation of a purple Rh(I) species [Rh(PiBu3)2(η2-H3B·NHMe2)]-
[BArF ] 2-H. 2-H is short-lived (t1/2 ∼1 min) and evolves to give
4
yellow Rh(III) [Rh(H)2(PiBu3)2(η2-H3B·NHMe2)][BArF ] 3-H and
4
the cyclic dimer [BH2NMe2]2.10 Addition of smaller amounts of
DMAB to 1 resulted in the formation of mixtures of 2-H, 3H, and
1 in varying proportions, meaning that 2-H could not be isolated
free of 3-H. 2-H was longer-lived under these conditions allowing
for its full characterization. Both complexes have been characterized
by NMR spectroscopy, ESI-MS/MS, and, for 3-H, also in the solid
state (Figure 1).
Scheme 1 illustrates the accepted reaction course for homoge-
neous systems. Computational studies indicate a number of
mechanistic scenarios for the dehydrogenation step: NH proton
transfer to a coordinated ligand followed by transfer to the metal
(Ni-NHC),12 intermolecular stepwise transfer of NH then BH
(Cp2Ti-derivatives),13 and concerted NH/BH activation at the metal
center (Ir-pincer complexes).14 Oxidative addition of the BH bond
followed by NH ꢀ-elimination has also been suggested.8 All routes
implicate σ-complexes of amine-borane in the reaction, and while
details of intermediate species remain scarce,4,7 materials that
represent catalyst deactivation products have been isolated.4,6,7 We
report here η2-amine-borane σ-complexes of rhodium that are
models for such intermediate complexes, and also catalysts for the
dehydrogenation of H3B · NHMe2 (DMAB), a close analogue of
H3B·NH3. Borane σ-complexes have been reported previously15
as have σ-amine-borane complexes,16 but as far as we are aware,
there is only a brief report of such species’ involvement in catalytic
dehydrogenation.17 No examples involving Rh have been reported.
1H NMR spectra show the coordinated borane group as a broad
3H signal, relative to iBu and NH groups, at δ -2.13 (2-H) and δ
-0.77 (3-H), which sharpen on 11B decoupling. This suggests rapid
exchange of terminal and bound hydrides. Cooling 3-H to 190 K
arrests this process (δ -3.15, 2H, Rh-H-B). 2-H was not stable
in suitable low-temperature solvents (CD2Cl2). The two hydrido
ligands in 3-H are observed as a 2H dt, δ -17.42 [J(PH) 20, J(RhH)
17], while the NH signals appear at δ 4.67 (2-H) and δ 3.87 (3-
H). 31P{1H} NMR spectra indicate a Rh(I) species 2-H δ 35.9
[J(RhP) 174] and a Rh(III) species 3-H δ 22.3 [J(RhP) 105].
11B{1H} NMR spectroscopy shows broad signals at δ 19.3 (2-H)
and δ 2.23 (3-H), shifted significantly downfield from DMAB (δ
-13.4). The solid-state structure of 3-H shows a pseudo-octahedral
Rh(III) center with trans phosphines, cis hydrides, and an η2-
H3B·NMe2H ligand [Rh1-B1 2.318(8) Å] (Figure 1). NMR data
and structural metrics indicate a Rh(III) center ligated with a
σ-borane rather than an alternative Rh(V) tetrahydridoboryl struc-
ture;19 a bond-indices analysis on calculated structures confirms
this (see SI). 3-H probably forms Via dehydrogenation of the bound
Scheme 2. Synthesis of New Amine-Borane σ-Complexes
DMAB in 2-H to give [Rh(H)2(PiBu3)2][BArF ] 4,18 which com-
4
bines with a further equivalent of DMAB. Consistent with this,
3-H can be formed by addition of DMAB to 4. 3-H slowly loses
H2 under vacuum to reform (unstable) 2-H, establishing a plausible
dehydrogenation cycle for DMAB mediated by 1.
As complex 2-H is short-lived and undergoes dehydrogenation
to give 3-H by NH/BH scission, blocking this route should afford
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10.1021/ja806582n CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Figure 3. Cationic portion of 5 from asymmetric cell. Selected distance
(Å) and angle (deg): Rh1-B1, 2.161(6); P1-Rh1-P2, 98.31(6).
Figure 2. DMAB dehydrogenation by 1 (5 mol%, C6H4F2) in a sealed
NMR tube. Inset: 11B NMR spectrum after 90 min.
In conclusion we have isolated Rh(I) and Rh(III) σ-amine-borane
complexes of H3B·NMe2R, and although the details of the
dehydrogenation mechanism currently remain unresolved, these
complexes provide useful insight into the likely intermediates. Given
the isoelectronic relationship between alkane and amine-boranes,
complexes 2 and 3 are also analogues of σ-alkane complexes of
late-transition metals.16 5 is thus an analogue of a transition metal
bound to a cyclic alkane, complexes that have previously been
observed in solution at low temperatures by NMR spectroscopy or
by time-resolved IR spectroscopy.21
a stable complex. This is the case, with H3B·NMe3 (TMAB)
affording a stable (under Ar) analogue 2-Me. The resulting complex
2-Me has a solid-state structure that shows a coordinated TMAB
ligand with a pseudo square-planar Rh(I) center (Figure 1) and is
structurally similar to related hydridoborate complexes of Rh(I).20
3-Me can be prepared by adding H2 to 2-Me or addition of TMAB
to 4. Spectroscopic and ESI-MS/MS data are in full accord with
these structures and are also similar to 2-H/3-H underscoring their
own structural assignment. Interestingly 3-Me loses H2 much more
rapidly than 3-H (simply by flushing with Ar), and we speculate
that this is a steric effect arising from the additional N-methyl group,
forcing the Rh center to adopt a less crowded Rh(I) square-plane
configuration.
Complexes 1, 4, and 3-H are active catalysts for the dehydro-
genation of DMAB. In an open system under Ar a modest4,7,8
overall turnover frequency (34 h-1, 298 K, 5 mol%, 100%
conversion) is achieved to ultimately afford the cyclic dimer
[H2BNMe2]2 {δ(11Β) 5.4 [t, J(BH) 113]}.10 Repeating this reaction
in a sealed NMR tube resulted in a lower TOF (2 h-1) indicating
inhibition by H2 released during catalysis. Under these attenuated
conditions a time/concentration plot (Figure 2) showed no evidence
of sigmoidal kinetics. Addition of Hg did not inhibit catalysis. Both
observations suggest nanoparticle formation is not occurring in
catalysis.9 A species that shows characteristic intermediate time/
concentration dependence is also observed by 11B NMR spectros-
copy in both the open and closed systems, δ 2.4 [t, J(BH) 112],
tentatively identified as [H2BNMe2]3. This species has also been
identified during the dehydrogenation of DMAB by “Cp2Ti”.3 A
small amount of H2BdNMe2, δ 38 [d, J(BH) 123], following a
similar concentration/time profile, was also observed.11
Acknowledgment. The EPSRC for support. Dr. Simon Aldridge
for stimulating discussions and the gift of H2BdNCy2.
Supporting Information Available: Full experimental details,
kinetic and characterization data. This material is available free of
charge via the Internet at http://pubs.acs.org.
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Monitoring the “closed” system during catalysis by NMR
spectroscopy identified a number of metal containing species,
including 3-H (ca. 20%). Other species currently elude definitive
identification. At the end of catalysis only two compounds are
observed in a ca. 1:1 ratio: 3-H and another that is currently only
partially characterized. 31P{1H} NMR spectroscopy suggests a
Rh(III) center, while 1H NMR data indicate 2 Rh-H, 2 Rh-H-B
groups and no NH. These data fit an empirical formula [Rh(H)2-
(PiBu3)2(η2-H2BdNMe2)]+.15 In support of this assignment, addi-
tion of H2BdNCy2 to 4 results in a complex with similar NMR
spectroscopic characteristics (see Supporting Information). We
discount assignment as a [H2BNMe2]2 adduct, as addition of this
fragment10 to 4 is followed by immediate H2 loss and the isolation
of a different complex in quantitative yield: [Rh(PiBu3)2{η2-
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(H2BNMe2)2}][BArF ] 5 (Figure 3), a σ-complex of a cyclic amino-
4
borane. Addition of excess DMAB to 5 or the postcatalysis mixture
gives 3-H and the resumption of catalysis. Addition of H2 (1 atm)
to 5 gives a mixture of 5, 4, and [H2BNMe2]2.
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