Activation of H2 by phosphinoboranes R2PB(C 6F5)2

Article abstract of DOI:10.1021/ja805493y

Phosphinoboranes that combine bulky electron-rich phosphides and electron deficient B(C₆F₅)₂ fragments produce monomeric phosphinoboranes that undergo facile addition of H₂ to give the phosphine-borane adducts (

Full text of DOI:10.1021/ja805493y

Published on Web 08/28/2008
Activation of H2 by Phosphinoboranes R2PB(C6F5)2
Stephen J. Geier,† Thomas M. Gilbert,‡ and Douglas W. Stephan*,†
Department of Chemistry, UniVersity of Toronto, 80 St. George Street, Toronto, Ontario Canada, M5S 3H6 and
Department of Chemistry and Biochemistry, Northern Illinois UniVersity, DeKalb, Illinois 60115
Received July 22, 2008; E-mail: dstephan@chem.utoronto.ca
The potential of hydrogen storage materials1 has focused much
recent experimental2–14 and theoretical studies15–20 on main group
compounds and amino-boranes in particular. While the liberation
of H2 has been a focus, a critical yet less explored issue involves
the uptake of H2 by main group species. Early studies described
the interactions of H2 with main group compounds in an argon
matrix;21 however, it was not until the work of Power and co-
workers22 that a main group species was shown definitively to react
with H2. In that work, germynes were hydrogenated to give a
mixture of digermene, digermane, and primary germane products.
In 2006, we described the room temperature addition of H2 to
(C6H2Me3)2P(C6F4)B(C6F5)2 to give the phosphonium borate salt
(C6H2Me3)2PH(C6F4)BH(C6F5)2.23 In an elegant and related work,
Bertrand and co-workers24 subsequently showed that sterically
demanding alkyl-amino-carbenes activate H2 to convert the carbene
carbon to a methylene fragment. We further showed that simple
combinations of Lewis acids and bases such as tBu3P/B(C6F5)3 are
sterically precluded from forming a dative bond and effect
heterolytic cleavage of H2 to give [tBu3PH][HB(C6F5)3].25 Lewis
acid/base combinations, termed “frustrated Lewis pairs”, capable
of H2 activation have been extended to include alkyl-linked
phosphine-boranes,21 as well as mixtures of B(C6F5)3 with sterically
crowded imines,26,27 amines,26,27 and N-heterocyclic carbenes.28
In considering the application of this concept to systems more
closely related to amino-boranes, we note the recent computational
study by Manners and co-workers29 that suggested the combination
of electron-donating and electron-withdrawing groups on N and
B, respectively, would afford amido-boranes capable of reaction
with H2. Herein, we describe related phosphinoboranes with such
electronic features and demonstrate that the combination of bulky
electron-rich phosphides and electron-deficient B(C6F5)2 fragments
produces phosphinoboranes that undergo facile addition of H2 to
give the phosphine-borane adducts (R2PH)(HBR′2). This finding,
in combination with DFT calculations, sheds light on the uptake
of H2 across a group 13-group 15 bond, a critical requirement for
the development of a recyclable H2 storage device based on amine-
borane adducts.
Figure 1. Synthesis and reactivity of phosphinoboranes. POV-ray drawings
of 1, 4, and 6. C: black, F: pink, P: orange, B: light green, H: white. All
hydrogen atoms except BH and PH in 6 were omitted for clarity.
unambiguously confirmed via a crystallographic study. The products
3 and 4 exhibited simple doublet resonances at δ 39.5 and 41.8,
respectively, in the 11B NMR spectra with P-B coupling constants
of 142 and 150 Hz, respectively. Species 3 and 4 were formulated
as monomeric phosphinoboranes R2PB(C6F5)2. This was confirmed
by an X-ray crystallographic study of 4 (Figure 1). The geometries
about B and P are pseudotrigonal planar as the sum of the angles
about B and P are 360.0° and 359.07°, respectively. The B-P
distance in 4 is 1.786(4) Å, which is markedly shorter than that
observed for 1 (B-Pavg 2.057(3) Å) and a number of related B-P
species.31,32
Reactions of the phosphinoboranes 1-4 with H2 were examined.
Solutions of each phosphinoborane were pressurized under 4 atm
of H2 and allowed to stand at 25 °C. Monitoring of the reactions
by NMR spectroscopy over a period of 4 weeks revealed no reaction
of the dimeric phosphinoboranes 1 and 2. In contrast, the species
3 and 4 underwent slow addition of H2 to afford the phosphinebo-
rane adducts (R2PH)(BH(C6F5)2) (R ) Cy 5, tBu 6). In the case of
3, the formation of 5 was complete at 25 °C in 2 weeks, whereas
the corresponding reaction of 4 afforded 6 in 80% yield in 4 weeks.
In contrast, at 60 °C the reactions of 3 and 4 with H2 resulted in
the quantitative formation of 5 and 6, respectively, in 48 h.
Identification of 5 and 6 was confirmed by independent syntheses,
as treatment of the corresponding secondary phosphines with the
borane (C6F5)2BH30 yielded adducts 5 and 6 in 77 and 65% isolated
yields, respectively. NMR data for 5 and 6 were characteristic of
these species as doublets at δ 4.78 and 4.84 in the 1H NMR spectra
exhibiting P-H coupling constants of 381 and 375 Hz, respectively.
In addition, resonances attributable to the BH fragment were
observed at δ 3.33 and 3.48 for 5 and 6, respectively. The
corresponding 31P NMR resonances for 5 and 6 were observed at
δ 7.1 and 32.0, respectively, while the 11B NMR signals occurred
Secondary lithium phosphides (R2PLi R ) Et, Ph, Cy, tBu) were
treated with (C6F5)2BCl30 in toluene at -35 °C and allowed to stir
overnight. Subsequent workup afforded the colorless pentane
insoluble, crystalline materials 1-4, respectively, in isolated yields
1
ranging from 61 to 83%. In all cases the H and 13C{1H} NMR
data were consistent with the presence of the R2P fragment in the
product while the 19F NMR spectra revealed resonances attributable
to the C6F5 rings on B. In the case of 1 and 2, the 11B NMR spectra
showed triplet resonances at δ -12.9 and -2.2 with B-P coupling
constants of 72 and 66 Hz and broad 31P NMR signals at δ -23.4
and -0.8, respectively. These data support the formulation of 1
and 2 as [(µ-R2P)B(C6F5)2]2 dimers. In the case of 1 this was
† University of Toronto.
‡ Northern Illinois University.
9
12632 J. AM. CHEM. SOC. 2008, 130, 12632–12633
10.1021/ja805493y CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
insight on molecular features required for H2 uptake, a critical issue
for the design of a fully recyclable H2 storage material. We continue
to explore group 13/15 systems seeking to apply the concept of
“frustrated Lewis pairs”37 to uncover and apply new systems
capable of activation of H2 as well as other small molecules to
material design and catalysis.
Acknowledgment. The support of NSERC of Canada is
gratefully acknowledged.
Figure 2. Gaussview drawing of the (a) HOMO and (b) LUMO of 4. C:
gray, F: blue, P: orange, B: pink. Hydrogen atoms were omitted for clarity.
Supporting Information Available: Experimental and computa-
tional details. For crystallographic data, see: CCDC 691485-691487.
This material is available free of charge via the Internet at http://
pubs.acs.org.
at δ -28.1 and -30.0, respectively, with P-B coupling constants
of 68 and 48 Hz. Identification of 6 is further confirmed via X-ray
crystallography (Figure 1). The geometry about both B and P was
pseudotetrahedral with hydrogen atoms adopting an anti orientation.
The B-P distance in this adduct was found to be 1.966(9) Å,
dramatically longer than that seen in 4.
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