Synthesis and reactivity of o-benzylphosphino- and o-α- methylbenzyl(N, N-dimethyl)amine-boranes

Article abstract of DOI:10.1021/ic102044z

The series of o-benzylphosphino-boranes, o-(R₂B)C ₆H₄CH₂PtBu₂ (R = Cl 3, Ph 4, Cy 6, C₆F₅ 7, Mes 8) and o-(BBN)C₆H ₄CH₂PtBu₂ (5), were synthesized from reactions of the respective chloroboranes with the lithiated benzyl-phosphine. In an analogous fashion, the α-methylbenzyl(N,N-dimethyl)amine-boranes o-(R ₂B)C₆H₄CH(Me)NMe₂ (R = Cl 10, Ph 11, Cy 12, C₆F₅ 13, Mes 14) were prepared. While these species were inactive in the catalytic hydrogenation of tBuN≡CHPh, compounds 7, 8, and 14 did react with H₂ at elevated temperatures (100 °C), resulting in the elimination of HC₆F₅ and mesitylene, respectively. In the latter case, the species o-((Mes)HB)C ₆H₄CH(Me)NMe₂ 15 was isolated. 14 was also shown to react with H₂O to give the species o-((Mes)(HO)B)C ₆H₄CH(Me)NMe₂ 16 with the loss of mesitylene. The structure of these compounds and the nature of these reactions were also probed spectroscopically, crystallographically, and computationally. The energies for the products of hydrogenation, the phosphonium and ammonium hydridoborates, were computed. In all cases, these products were endothermic with respect to the precursor phosphine-boranes and amine-boranes and H ₂. The barriers to H₂ activation were found to be in the range of 24-38 kcal/mol. These theoretical studies also demonstrate that the steric bulk around the boron center dramatically affects the activation barrier for H₂ activation, while the Lewis acidity of the borane has the largest effect on the stabilization of the resulting onium-borohydride. In the case of the elimination reactions, the driving forces appear to be the loss of arene byproduct and formation of a strong donor-acceptor bond.

Full text of DOI:10.1021/ic102044z

1470 Inorg. Chem. 2011, 50, 1470–1479
DOI: 10.1021/ic102044z
Synthesis and Reactivity of o-Benzylphosphino- and
o-r-Methylbenzyl(N,N-dimethyl)amine-Boranes
Zachariah M. Heiden, Michael Schedler, and Douglas W. Stephan*
Department of Chemistry, University of Toronto, 80 St. George St., Toronto, Ontario M5S 3H6, Canada
Received October 8, 2010
The series of o-benzylphosphino-boranes, o-(R2B)C6H4CH2PtBu2 (R = Cl 3, Ph 4, Cy 6, C6F5 7, Mes 8) and
o-(BBN)C6H4CH2PtBu2 (5), were synthesized from reactions of the respective chloroboranes with the lithiated benzyl-
phosphine. In an analogous fashion, the R-methylbenzyl(N,N-dimethyl)amine-boranes o-(R2B)C6H4CH(Me)NMe2
(R = Cl 10, Ph 11, Cy 12, C6F5 13, Mes 14) were prepared. While these species were inactive in the catalytic
hydrogenation of tBuNdCHPh, compounds 7, 8, and 14 did react with H2 at elevated temperatures (100 °C), resulting
in the elimination of HC6F5 and mesitylene, respectively. In the latter case, the species o-((Mes)HB)C6H4CH(Me)NMe2
15 was isolated. 14 was also shown to react with H2O to give the species o-((Mes)(HO)B)C6H4CH(Me)NMe2 16 with
the loss of mesitylene. The structure of these compounds and the nature of these reactions were also probed
spectroscopically, crystallographically, and computationally. The energies for the products of hydrogenation, the
phosphonium and ammonium hydridoborates, were computed. In all cases, these products were endothermic with
respect to the precursor phosphine-boranes and amine-boranes and H2. The barriers to H2 activation were found to be
in the range of 24-38 kcal/mol. These theoretical studies also demonstrate that the steric bulk around the boron center
dramatically affects the activation barrier for H2 activation, while the Lewis acidity of the borane has the largest effect on
the stabilization of the resulting onium-borohydride. In the case of the elimination reactions, the driving forces appear to
be the loss of arene byproduct and formation of a strong donor-acceptor bond.
Introduction
In 2006, we demonstrated the first nonmetal system that
reversibly activates H2.6 This species which was an arene-
linked phosphine/borane species (Chart 1, A) was also shown
to be a metal-free catalyst for the hydrogenation of sterically
bulky imines.6-10 This development was based on the con-
cept of “frustrated” Lewis pairs (FLPs). Subsequently Erker
and co-workers applied the species Mes2PC2H4B(C6F5)2
(Chart 1, B) complex,11,12 in the catalytic hydrogenation of
imines, enamines, and silyl enol ethers. Rieger et al developed
a related N/B catalyst based on a linked tetramethylpiper-
idine and a borane center (Chart 1, C).13,14 In an effort to
The use of hydrogenation catalysis is critical to the pro-
duction of a large range of chemicals, materials, and pro-
ducts, including upgraded crude oil, production of bulk
commodity materials, and the preparation of food, agricul-
tural, and pharmaceutical products. In addition, hydrogena-
tion is an important process in synthetic organic chemistry.
While homogeneous or heterogeneous transition metal-
based catalysts are currently used in these processes, recent
research efforts have targeted metal free reducing agents.
While these efforts are interesting from a fundamental science
perspective, they also offer ways to avoid the use of expensive
transition metal catalysts1 and their removal from the reac-
tion products. The latter issue is generally a major concern
for the pharmaceutical industry where toxicity concerns are
of paramount importance.2,3 Known main-group reducing
reagents are typically stoichiometric reductions, and result in
tedious procedures and toxic chemical waste.4,5
(6) Welch, G. C.; San Juan, R. R.; Masuda, J. D.; Stephan, D. W. Science
2006, 314, 1124–1126.
(7) Chase, P. A.; Welch, G. C.; Jurca, T.; Stephan, D. W. Angew. Chem.,
Int. Ed. 2007, 46, 8050–8053.
(8) Chase, P. A.; Jurca, T.; Stephan, D. W. Chem. Commun. 2008, 1701–
1703.
(9) Stephan, D. W.; Erker, G. Angew. Chem., Int. Ed. 2010, 49, 46–76.
(10) Chen, D.; Klankermayer, J. Chem. Commun. 2008, 2130–2131.
(11) Spies, P.; Erker, G.; Kehr, G.; Bergander, K.; Froehlich, R.;
Grimme, S.; Stephan, D. W. Chem. Commun. 2007, 5072–5074.
(12) Spies, P.; Schwendemann, S.; Lange, S.; Kehr, G.; Froehlich, R.;
Erker, G. Angew. Chem., Int. Ed. 2008, 47, 7543–7546.
(13) Sumerin, V.; Schulz, F.; Atsumi, M.; Wang, C.; Nieger, M.; Leskelae,
M.; Repo, T.; Pyykkoe, P.; Rieger, B. J. Am. Chem. Soc. 2008, 130, 14117–
14119.
*To whom correspondence should be addressed. E-mail: dstephan@
chem.utoronto.ca.
(1) Enthaler, S.; Junge, K.; Beller, M. Angew. Chem., Int. Ed. 2008, 47,
3317–3321.
(2) Garrett, C. E.; Prasad, K. Adv. Synth. Catal. 2004, 346, 889–900.
(3) Elemental Impurities - Limits. In Pharmacopeial Forum; U.S. Pharma-
copeia: Rockville, MD, 2010; Vol. 36, p 1.
(14) Sumerin, V.; Schulz, F.; Nieger, M.; Atsumi, M.; Wang, C.; Leskelae,
M.; Pyykkoe, P.; Repo, T.; Rieger, B. J. Organomet. Chem. 2009, 694, 2654–
2660.
(4) Yaragorla, S. Synlett 2008, 3073–3074.
(5) Zhenjiang, L. Synlett 2005, 182–183.
r
pubs.acs.org/IC
Published on Web 01/26/2011
2011 American Chemical Society

Article
Inorganic Chemistry, Vol. 50, No. 4, 2011 1471
Chart 1. Metal-Free Bifunctional Hydrogenation Catalysts
degassed water. The solution was added to a solution of sodium
acetate (25 g) in degassed water (100 mL). The aqueous phase
was anaerobically extracted with ether. The mixed ether phases
were dried with MgSO4, and the ether removed under vacuum.
Distillation of the residue gave 1 (4.91 g, 15.6 mmol; 52%) as a
1
3
colorless oil. H NMR (C6D6) 1.08 (d, JPH = 10.9 Hz, 18 H,
tBu), 2.97 (d, 2JPH = 3.2 Hz, 2 H, CH2), 6.65 (td, 3JHH = 8.0 Hz,
5JHH = 1.9, 1 H, Ph), 6.96 (td, 3JHH = 7.7 Hz, 5JHH = 1.4 Hz,
1 H, Ph.), 7.41 (dd, 3JHH = 8.3 Hz, 5JHH = 1.5 Hz, 1 H, Ph), 7.79
(m, 1 H, Ph); 31P NMR (C6D6) 34.2 (br). Anal. Calcd for
C15H24BrP: C: 57.15%, H: 7.67%; Found: C: 57.59%, H:
7.95%.
Generation of LiC6H4CH2PtBu2 (2). This procedure is a slight
variation of the reported method.17 1 (1.60 g, 5.09 mmol) was
dissolved in hexanes (30 mL) and nBuLi in hexanes (2.45 mL,
6.10 mmol, 2.5 M) was added via syringe, resulting in the
immediate formation of a white precipitate. The mixture was
stirred 1 h at room temperature. The mixture was filtered in a
glovebox and 2 (1.20 g, 4.94 mmol; 97%) was obtained as a white
solid. The product was used without further purification. 2: 1H
NMR (C6D6) 1.03 (d, 3JPH = 11.0 Hz, 18 H, tBu), 3.20 (d, 2JPH
=
=
explore related bifunctional systems, herein, we report the
synthesis of a series of phosphine-borane and amine-borane
species in which a benzylic fragment links the Lewis acid and
donor sites. The impact of the nature of the donor as well as
variation of the substituents on B is examined. Reactions
with H2 are probed and the relationship between structure
and reactivity is probed experimentally and computationally.
The ramifications for the design of new bifunctional hydro-
genation catalysts are discussed.
3
4.4 Hz, 2 H, CH2), 7.03-7.17 (m, 3 H, Ph), 8.11 (d, JHH
6.5 Hz, 1 H, Ph); 31P NMR (C6D6) 32.5 (br s).
Synthesis of o-(Cl2B)C6H4CH2PtBu2 (3). Compound 2 (120 mg,
0.50 mmol) was dissolved in toluene (20 mL) and BCl3 in hexanes
(0.5 mL, 0.5 mmol, 1.0 M) was added with a syringe. An initially
purple solution became a colorless solution containing a slightly
yellow solid after stirring the solution for 4 days. The mixture
was filtered, and the solvent of the filtrate was removed under
reduced pressure resulting in a slightly yellow solid. The solid was
recrystallized from a toluene/hexanes mixture to give a colorless
Experimental Section
solid (60 mg, 0.18 mmol, 38%). 1H NMR (C7D8): 1.08 (d, 3JPH
=
13.2 Hz, 18 H, tBu), 2.72 (d, JPH = 9.4 Hz, 2 H, CH2), 6.85
2
Materials and Methods. All preparations and manipulations
were performed on a double manifold N2/vacuum line with
Schlenk-type glassware or in a N2-filled VAC glovebox. Sol-
vents were dried using an Innovative Technologies solvent
system, and degassed before use. NMR spectra were obtained
on a Bruker Avance 400 MHz spectrometer, and spectra were
referenced to residual solvent (1H, 13C) or externally (11B;
3
3
5
(d, JHH = 7.4 Hz, 1 H, Ph), 7.06 (td, JHH = 7.6 Hz, JHH
1.4 Hz, 1 H, Ph), 7.14 (t, 3JHH = 7.5 Hz, 1 H, Ph), 7.92 (d, 3JHH
=
=
P
7.3 Hz, 1 H, Ph); 11B NMR (C7D8) 0.02 (d, 1JBP = 96.5 Hz); 31
NMR (C7D8) 31.2 (br s). Anal. Calcd for C15H24BCl2P: C:
56.83%, H: 7.63%; Found: C: 56.99%, H: 7.75%.
Synthesis of o-(Ph2B)C6H4CH2PtBu2 (4). Method (i): This
species was prepared via reaction of (2) and ClBPh2 as described
below for compounds 5-8. Yield: 61%. Method (ii): 3 (45 mg,
0.14 mmol) was dissolved in 10 mL of toluene and PhLi (0.3 mL,
0.56 mmol, 1.8 M) in di-n-butylether was added with a syringe at
room temperature. The mixture turned deep red because of the
color of the phenyllithium solution. The mixture was stirred
overnight and turned nearly colorless. Removal of the solvent
BF3 OEt2, 19F; CFCl3, 31P; 85% H3PO4). Chemical shifts are
3
reported in parts per million (ppm). NMR solvents were pur-
chased from Cambridge Isotopes, dried over CaH2 or Na/
benzophenone, vacuum distilled prior to use and stored over
˚
4 A molecular sieves in the glovebox. B(C6F5)3 was gener-
ously provided by Nova Chemicals (from Boulder Scientific
Company). ClB(C6F5)2 was prepared according to the literature
procedure previously reported by Piers and co-workers.15 Com-
bustion analysis was performed in house on a Perkin-Elmer
CHN Analyzer. (()-R-methylbenzylamine was purchased from
Aldrich and used as received. o-LiC6H4CH(Me)NMe2 (9) was
prepared according the procedure previously reported by Payne
and Stephan.16
gave a slightly yellow solid. Complete conversion was confirmed
=
3
by NMR spectroscopy. 1H NMR (C7D8) 0.76 (d, JPH
12.0 Hz, 18 H, tBu), 2.99 (d, JPH = 8.8 Hz, 2 H, CH2), 7.13
2
3
(d, JHH = 7.1 Hz, 2 H, p-Ph), 7.17-7.21 (m, 3 H, Ph), 7.23
(t, 3JHH = 7.3 Hz, 4 H, p-Ph), 7.70 (br, 1 H, Ph), 7.79 (d, JHH
=
7.4 Hz, 4 H, o-Ph); 11BNMR (C7D8) -0.5 (br s); 31P NMR
(C7D8) 38.2 (br s). Anal. Calcd for C27H34BP: C: 81.00%, H:
8.56%; Found: C: 81.05%, H: 8.77%.
Synthesis of o-BrC6H4CH2PtBu2 (1). The known compound,
1,17 was prepared by a similar route that was used to prepare
C6H5CH2PtBu2.18,19 HPtBu2 (4.42 g, 30.23 mmol) was dissolved
in degassed acetone and a solution of o-BrC6H4CH2Br (7.56 g,
30.23 mmol) in 75 mL of degassed acetone was added. After
30 min of refluxing a white precipitate started to form. The
mixture was refluxed for 16 h, cooled to room temperature, and
100 mL of dry pentane was added to complete the precipitation.
The solid was separated via filtration and dissolved in 100 mL of
Synthesis of o-(BBN)C6H4CH2PtBu2 (5), o-(R2B)C6H4CH2-
PtBu2 (R = Cy 6, C6F5 7, Mes 8). These compounds were pre-
pared in a similar fashion using the corresponding boron-
chloride precursor and thus only one preparation is detailed. 2
(182 mg, 0.75 mmol) was dissolved in toluene (40 mL) and
Cy2BCl (0.75 mL, 0.75 mmol, 1.0 M) in hexanes was added with
a syringe. After 30 min the colorless solution became milky.
After 18 h of stirring at room temperature the mixture of a colorless
solution and a slightly yellow precipitate was filtered, and the
solvent of the filtrate was removed under reduced pressure. The
resulting colorless oil was dissolved in hexanes and stirred for 1 h.
Removal of the hexanes gave 246 mg (0.60 mmol, 80%) of 6, a
colorless solid.
(15) Parks, D. J.; Piers, W. E.; Yap, G. P. A. Organometallics 1998, 17,
5492–5503.
(16) Payne, N. C.; Stephan, D. W. Inorg. Chem. 1982, 21, 182–188.
(17) Abicht, H. P.; Issleib, K. J. Organomet. Chem. 1980, 185, 265–75.
(18) Abicht, H. P.; Baumeister, U.; Hartung, H.; Issleib, K.; Jacobson,
R. A.; Richardson, J.; Socol, S. M.; Verkade, J. G. Z. Anorg. Allg. Chem.
1982, 494, 55–66.
5: yield 140 mg, 0.37 mmol, 90%. 1H NMR (C7D8) 0.98
(d, 3JPH = 11.2 Hz, 18 H, tBu), 1.36-2.39 (br, 14 H, BBN), 2.8
(19) Abicht, H. P. J. Organomet. Chem. 1986, 311, 57–61.

1472 Inorganic Chemistry, Vol. 50, No. 4, 2011
Heiden et al.
(d, 2JPH = 7.5 Hz, 2 H, CH2), 6.99 (d, 3JHH = 6.0 Hz, 1 H, Ph),
7.10 (t, 3JHH = 4.0 Hz, 1 H, Ph), 7.19 (t, 3JHH = 7.5 Hz, 1 H,
o-F), -155.1 (t, 3JFF = 21.3 Hz, p-F), -158.1 (t, 3JFF = 21.3 Hz,
p-F), -161.7 (m, m-F). Anal. Calcd for C22H14BF10N: C: 53.54%;
H: 2.86%; N: 2.84%. Found: C: 53.59%; H: 3.36%; N: 2.80%.
14: yield 167 mg (65%). 1H NMR (C7D8) 0.96 (br s, 3H,
Me2NCHMe), 1.86 (br s, 6H, Me2NCHMe), 1.86 (br s, 3H,
C6Me3H2, 2.04 (br s, 3H, C6Me3H2, 2.12 (br s, 3H, C6Me3H2,
2.19 (br s, 9H, C6Me3H2, 3.95 (br s, 1H, Me2NCHMe), 6.64
(br s, 1H, Ph), 6.73 (br s, 2H, Ph), 6.80 (br s, 1H, Ph), 7.02
(m, 2H, Ph), 7.15-7.30 (br m, 2H, Ph), 7.42 (br d, J = 8.2 Hz,
1H, Ph). 11B NMR (C6D6) 73.9 (br s). Anal. Calcd for
C28H36BN: C: 84.62%; H: 9.13%; N: 3.52%. Found: C:
84.55%; H: 9.03%; N: 3.44%.
3
Ph), 7.95 (d, JHH = 7.7 Hz, 1 H, Ph); 11B NMR (C7D8) 7.2
(br s); 31P NMR (C7D8) 42.2 (br s) Anal. Calcd for C23H38BP:
C 77.53%, H 10.75%; Found: C 77.56%, H 10.46%.
6: yield 246 mg, 0.60 mmol, 80%. 1H NMR (C7D8) 0.99
=
(d, 3JPH = 10.8 Hz, 18 H, tBu), 1.2-2.1 (m, Cy), 2.84 (d, 2JPH
6.4 Hz, 2 H, CH2), 6.99 (d, JHH = 5.7 Hz, 1 H, Ph), 7.07
3
(td, 3JHH = 7.4 Hz, 5JHH = 1.4 Hz, 1 H, Ph), 7.15 (t, 3JHH
=
7.0 Hz, 1 H, Ph), 7.57 (d, 3JHH = 7.6 Hz, 1 H, Ph); 11B NMR
(C7D8) 13.5 (br s); 31P NMR (C7D8) 47.1 (br s). Anal. Calcd for
C27H40BP: C: 79.80%, H: 9.92%; Found: C: 79.43%, H: 9.95%.
7: yield 120 mg, 0.21 mmol, 61%. 1H NMR (C7D8) 0.75
(d, 3JPH = 12.4 Hz, 18 H, tBu), 2.87 (br, 2 H, CH2), 6.94-7.11
(m, 3 H, Ph), 7.45 (br, 1 H, Ph); 11B NMR (C7D8) -7.79 (br s);
19F NMR (C7D8) -163.4 (br, 2F, Fm), -157.1 (s, br., 1 F,
Fp), -127.5 (br, 2 F, Fo); 31P NMR (C7D8) 53.2 (br). Anal.
Calcd for C27H24F10BP: C: 55.89%, H: 4.17%; Found: C:
56.40%, H: 4.46%.
Synthesis of (() o-((Mes)HB)C6H4CH(Me)NMe2 (15). A
toluene solution of 100 mg (0.25 mmol) of 14 was pressurized
with 1 H2 at 77 K (∼ 5 atm at 373 K), after degassing the solution
through three freeze-pump-thaw cycles. The colorless solu-
tion was heated to 100 °C for 16 h. The solvent was removed
under reduced pressure, and the formed mesitylene was remov-
ed by a hexane wash. The colorless solid was recrystallized
from CH2Cl2 and hexanes. 15 was found to be air stable
over the period of weeks. Yield: 60 mg (85%). 1H NMR
(CDCl3) 1.53 (d, J = 7.2 Hz, 3H, minor-MeMeNCHMe),
1.55 (d, J = 7.3 Hz, 3H, major-MeMeNCHMe), 2.12 (d, J =
2.4 Hz, 6H, major-B(Mes)), 2.22 (s, 3H, minor-MeMeNC), 2.26
(s, 9H, minor-B(Mes)), 2.39 (s, 3H, minor-MeMeNC), 2.56
(s, 3H, major-B(Mes)), 2.60 (s, 3H, major-MeMeNC), 2.73
(s, 3H, major-MeMeNC), 4.18 (q, J = 7.0 Hz, 1H, major-Me-
MeNCHMe), 4.27 (q, J = 7.0 Hz, 1H, minor-MeMeNCHMe),
6.67-7.37 (12H, Ph). 11B NMR (CDCl3) 1.3 (br s). Anal. Calcd
for C19H26BN: C: 81.73%; H: 9.39%; N: 5.02%. Found: C:
81.52%; H: 9.22%; N: 5.09%.
8: yield 192 mg, 0.39 mmol, 72%. yellow solid. 1H NMR
(C7D8) 0.97 (br s, 18 H, tBu), 2.1-2.35 (br s, 18 H, Mes-CH3),
2.89 (br s, 2 H, CH2),6.71(brs,2H,Mes-H),6.96(d,3JHH =7.2Hz,
1 H, Ph), 7.21 (td, 3JHH = 7.4 Hz, 5JHH = 1.5 Hz, 1 H, Ph), 7.41
(d, 3JHH = 7.4 Hz, 1 H, Ph), 8.11 (dd, 3JHH = 7.9 Hz, 5JHH
=
4.8 Hz, 1 H, Ph); 11B NMR (C7D8) 37 (v br, s); 31P NMR (C7D8)
23.9 (br s) Anal. Calcd for C33H46BP: C: 81.81%, H: 9.57%;
Found: C: 81.43%, H: 9.67%.
Synthesis of (() o-(R2B)C6H4CH(Me)NMe2 (R = Cl 10, Ph
11, Cy 12, C6F5 13, Mes 14). These compounds were prepared in
a similar fashion using the corresponding boron-chloride pre-
cursor and thus only one preparation is detailed. A 0.6 mL
portion (0.6 mmol) of a 1.0 M BCl3 in hexanes solution was
added to a slurry of 0.10 g (0.6 mmol) of complex 9 in 15 mL of
toluene, precooled to -78 °C, resulting in an immediate color
change from a gray colored slurry to a white colored slurry. The
solution was allowed to warm to room temperature, and the
white precipitate was removed by filtration. The solvent was
removed of the resulting colorless filtrate under reduced pres-
sure. The colorless solid was recrystallized from diethyl ether.
o-((Mes)HOB)C6H4CH(Me)NMe2 (16). 16 was obtained as a
decomposition product of 2 in moist air/wet solvents. 1H NMR
(C6D6) 1.36 (d, J = 6.9 Hz, 3H, Me2NCHMe), 1.86 (s, 6H,
C6Me3H2), 2.30 (s, 3H, Me2NCHMe), 2.45 (br s, 6H, Me2N),
3.02 (q, J = 6.7 Hz, 1H, Me2NCHMe), 6.92 (s, 2H, C6Me3H2)
6.93-7.14 (m, 3H, Ph), 7.75 (d, J = 7.1 Hz, 1H, Ph), 13.87 (br s,
1H, BOH). 11B NMR (C6D6) 47.6 (br s).
X-ray Data Collection and Reduction. Crystals were coated in
Paratone-N oil in the glovebox, mounted on a MiTegen Micro-
mount and placed under an N2 stream, thus maintaining a dry,
O2-free environment for each crystal. The data were collected
on a Bruker Apex II diffractometer. The data were collected at
150((2) K for all crystals (Table 1). The frames were integrated
with the Bruker SAINT software package using a narrow-frame
algorithm. Data were corrected for absorption effects using the
empirical multiscan method (SADABS).
1
3
10: yield 185 mg (80%). H NMR (CDCl3) 1.55 (d, JHH
6.7 Hz, 3H, NCHMe), 2.41 (s, 3H, MeMeNC), 2.88 (s, 3H, Me-
=
MeNC), 4.60 (q, 3JHH = 6.7 Hz, 1H, NCHMe), 7.02 (d, 3JHH
=
7.2 Hz, 1H, Ph), 7.20-7.32 (m, 2H, Ph), 7.54 (d, 3JHH = 6.9 Hz,
1H, Ph). 11B NMR (CDCl3) 9.3 (s). Anal. Calcd for C10H14-
BCl2N: C: 52.39%; H: 6.16%; N: 6.11%. Found: C: 52.68%; H:
6.84%; N: 6.12%.
11: yield 160 mg (80%). 1H NMR (C6D6): 0.68 (d, 3JHH = 6.8
Hz, 3H, NCHMe), 1.44 (s, 3H, MeMeNC), 1.96 (s, 3H, MeM-
Structure Solution and Refinement. Non-hydrogen atomic
scattering factors were taken from the literature tabulations.20
The heavy atom positions were determined using direct methods
employing the SHELXTL direct methods routine. The remain-
ing non-hydrogen atoms were located from successive difference
Fourier map calculations. The refinements were carried out by
using full-matrix least-squares techniques on F, minimizing the
function ω (Fo - Fc)2 where the weight ω is defined as 4Fo2/2σ
(Fo2) and Fo and Fc are the observed and calculated structure
factor amplitudes, respectively. In the final cycles of each
refinement, all non-hydrogen atoms were assigned anisotropic
temperature factors in the absence of disorder or insufficient
data. In the latter cases atoms were treated isotropically. C-H
atom positions were calculated and allowed to ride on the
carbon to which they are bonded assuming a C-H bond length
eNC), 3.84 (q, 3JHH = 6.8 Hz, 1H, NCHMe), 6.89 (d, 3JHH
7.1 Hz, 1H, C6H4BPh2), 7.16-7.35 (m, 6H, Ph), 7.39 (t, 3JHH
=
=
=
8.3 Hz, 2H, Ph), 7.59 (d, 3JHH = 8.3 Hz, 2H, Ph), 7.76 (d, 3JHH
7.2 Hz, 1H, Ph), 8.01 (d, JHH = 8.3 Hz, 2H, Ph). 11B NMR
(C6D6) 6.7 (s). Anal. Calcd for C22H24BN: C: 84.29%, H:
7.72%, N: 4.47%; Found: C: 84.10%, H: 7.70%, N: 4.51%.
12: yield 210 mg (81%). 1H NMR (C6D6) 0.74 (d, 3JHH = 7.7
Hz, 3H, NCHMe), 0.78-2.05 (m, 22H, Cy), 1.67 (s, 3H,
3
3
MeMeNC), 1.98 (s, 3H, MeMeNC), 3.98 (q, JHH = 6.9 Hz,
3
1H, NCHMe), 6.81 (d, JHH = 8.0 Hz, 1H, Ph), 7.16 (m,
1H, Ph), 7.27 (tt, JHH = 1.13JHH = 7.1 Hz, 1H, Ph), 7.53
5
(d, JHH = 7.3 Hz, 1H, Ph). 11B NMR (C6D6) 8.7 (s). Anal.
3
Calcd for C22H36BN: C: 81.16%; H: 11.15%; N: 4.30%. Found:
C: 80.92%; H: 11.03%; N: 4.22%.
=
˚
13: yield 178 mg (56%). 1H NMR (C6D5Br) 1.19 (d, 3JHH
of 0.95 A. H-atom temperature factors were fixed at 1.10 times
the isotropic temperature factor of the C-atom to which they are
bonded. The H-atom contributions were calculated, but not
refined. The locations of the largest peaks in the final difference
6.6 Hz, 3H, NCHMe), 1.83 (s, 3H, MeMeNC), 2.57 (s, 3H, Me-
=
MeNC), 4.81 (q, 3JHH = 6.6 Hz, 1H, NCHMe), 6.87 (d, 3JHH
7.3 Hz, 1H, C6H4), 7.12 (t, JHH = 7.3 Hz, 1H, C6H4), 7.21
3
3
3
(t, JHH = 7.3 Hz, 1H, C6H4), 7.72 (br d, JHH = 7.3 Hz, 1H,
C6H4). 11B NMR (C6D5Br) 2.0 (s). 19F NMR (C6D5Br) -125.4 (d,
5JFF = 7.2, 25.4 Hz, o-F), -126.6 (dd, 5JFF = 7.2 3JFF = 25.4 Hz,
(20) Cromer, D. T.; Waber, J. T. Int. Tables X-Ray Crystallogr. 1974,
4, 71.

Article
Inorganic Chemistry, Vol. 50, No. 4, 2011 1473
Table 1. Crystallographic Data
3
4
5
6
7
8
formula
C15H24BCl2P
317.04
orthorhombic
Pbca
11.0177 (20)
12.8239 (21)
23.7562 (40)
90.00
90.00
90.00
3356.52 (146)
8
1.255
4.7
3814
C27H34BP
400.34
monoclinic
P2(1)/n
11.6783 (9)
14.2236 (10)
14.0628 (9)
90.00
93.380 (3)
90.00
2331.88 (46)
4
1.140
C23H38BP
356.33
orthorhombic
Pbca
10.0869 (6)
16.8594 (10)
24.3705 (15)
90.00
90.00
90.00
4144.404 (717)
8
1.142
C27H46BP
412.44
C27H24BF10
580.25
triclinic
P
C33H46BP
484.50
formula weight
crystal system
space group
monoclinic
P2(1)/c
8.7532(11)
16.0435(21)
17.5553(25)
90.00
90.7844 (90)
90.00
2465.102(693)
4
1.111
orthorhombic
P2(1)2(1)2(1)
10.7842 (5)
15.3950 (6)
17.7979 (8)
90.00
90.00
90.00
2954.871 (306)
4
1.089
P1
˚
a (A)
9.4512 (4)
9.7356 (4)
14.8664 (6)
104.5766 (25)
90.3618 (21)
99.9471 (22)
1302.193 (111)
2
1.480
1.9
7541
3955
358
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
3
˚
V (A )
Z
d(calc) g cm-1
abs coeff, μ, cm-1
data collected
1.3
5701
3872
268
1.4
4785
3599
232
1.2
5664
3463
268
1.1
6969
5102
328
data Fo2 > 3σ(Fo
2883
178
2
)
variables
R
Rw
0.0353
0.1228
0.839
0.0653
0.2394
0.754
0.0387
0.1085
0.957
0.1096
0.3268
1.119
0.0492
0.2162
0.779
0.0507
0.1104
1.014
goodness of fit
10
11
12
13
15
16
formula
C10H14BCl2N
229.94
orthorhombic
Pbca
14.1854(4)
20.6850(6)
23.3812(8)
90.00
90.00
90.00
6860.605(538)
24
1.336
C22H24BN
313.24
monoclinic
P2(1)
10.1559(5)
16.5857(8)
10.8949(6)
90.00
106.3827(18)
90.00
1760.665(256)
4
1.182
C22H36BN
325.34
monoclinic
P2(1)/n
8.5311(5)
17.7333(11)
13.3028(8)
90.00
105.6576
90.00
1937.822(303)
4
1.112
C22H14BF10
493.15
N
C19H26BN
279.23
monoclinic
P2(1)/c
9.6978(5)
7.7722(4)
22.4589(13)
90.00
93.3801(34)
90.00
1689.863(213)
4
1.097
C19H26BNO
295.23
formula weight
crystal system
space group
monoclinic
P2(1)/c
12.6394(4)
19.7683(7)
15.9329(5)
90.00
91.9819(20)
90.00
3978.617(297)
8
1.647
monoclinic
P2(1)/n
8.4770(4)
18.3487(9)
11.5037(5)
90.00
102.7405(23)
90.00
1745.263(193)
4
1.124
˚
a (A)
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
3
˚
V (A )
Z
d(calc) g cm-1
abs coeff, μ, cm-1
data collected
5.3
7868
6291
388
0.7
9266
8168
439
0.6
4014
2901
220
1.6
2874
2119
619
0.6
4254
3028
200
0.7
4244
3053
264
data Fo2 > 3σ(Fo
2
)
variables
R
Rw
0.0373
0.090
1.031
0.0460
0.1169
1.043
0.0463
0.1188
1.034
0.0285
0.0508
0.891
0.0688
0.2102
1.054
0.0541
0.1610
1.055
goodness of fit
Fourier map calculation as well as the magnitude of residual
electron densities in each case were of no chemical significance.
Additional details are provided in the Supporting Information.
Computational Studies. All calculated structures were mini-
mized using the Gaussian 03 program21 at the B3LYP/6-31 g(d)
level of theory. All minimized structures were found to contain
no imaginary frequencies, unless otherwise noted. Transition
states contained only one imaginary frequency, and the transi-
tion states were connected to the minimized structures via IRC
analysis, unless otherwise noted.
Results and Discussion
o-Benzylphosphino-boranes. Targeting benzyl linked
phosphino-boranes, the species o-BrC6H4CH2PtBu2 (1)
was prepared via simply refluxing an acetone solution of
ClPtBu2 with BrC6H4CH2Br for 16 h, followed by a basic
aqueous workup. Subsequent vacuum distillation re-
sulted in the isolation of 1 in 52% yield. Lithiation of 1
was readily accomplished with n-BuLi in hexanes,18 and
the resulting product LiC6H4CH2PtBu2 (2) precipitated
from solution, allowing its isolation. Interestingly, lithia-
tions of 1 in coordinating solvents such as Et2O gave no
conversion, inferring the insolubility of 2 in hexanes
drives the reaction to completion. The 31P NMR (C6D6)
of 2 was found to shift minimally from 34.2 ppm for 1 to
32.5 ppm for 2. Subsequent reaction of 2 with BCl3 in
hexanes gave initially a purple solution which gradually
became colorless with a white precipitate. This was
attributed to the initial formation of a P-B adduct 2-
BCl3, which gradually transforms to a thermodynamic
product, o-(Cl2B)C6H4CH2PtBu2 3 (Scheme 1). This
product was isolated in 38% yield. Although uncon-
firmed, the diminished yield was attributed to further
reaction of 3 with 2. The formulation of 3 was supported
by the observation of the 31P NMR signal at 31.2 ppm and
(21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; J. A. Montgomery, J.; Vreven, T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; ; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,
J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,
S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,
B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision
E.01; Gaussian, Inc.: Wallingford, CT, 2004.

1474 Inorganic Chemistry, Vol. 50, No. 4, 2011
Heiden et al.
Figure 1. ORTEP drawing of 3. Thermal ellipsoids are drawn at 50%
probability.
Figure 2. ORTEP drawing of 4. Thermal ellipsoids are drawn at 50%
probability.
Scheme 1. Syntheses of o-Benzylphosphino-boranes 3-8
of the t-butyl-groups. These signals coalesce at -13 °C
and show a single resonance at room temperature. Similar
behavior is observed for the benzylic protons, as two
distinct signals are seen at -60 °C, where coupling to the
phosphorus center is seen. In addition, one of the benzylic
protons also exhibits through-space coupling to a fluorine
atom (JFH = 13.3 Hz) of a C6F5 group. The 19F-NMR
spectrum of 7 shows 10 distinct signals for the fluorine
atoms at -60 °C. These signals coalesce to 3 signals at
10 °C. In contrast, the 11B and 31P NMR spectra showed
no temperature dependence. These data are consistent
with the fluxionality of a five-membered ring resulting
from a weak P-B bond. At low temperature the inequi-
valent C6F5 rings arise from axial and equatorial disposi-
tions together with inhibited rotation about the B-C
bonds.
the 11B resonance at 0.02 ppm. The observation of the
B-P coupling constant of 96.5 Hz suggested a P-B dative
bond. An X-ray crystallographic study (Figure 1) confirmed
that the B is coordinated to P with a P-B bond length of
2.024(2) A. This distance is shorter than that reported for
the related benzylic phosphine-borane tBu2BC6H4CH2-
PPh2 (2.108 A)22 consistent with the greater Lewis acidity
˚
The 1H NMR spectrum of 8 showed sharp peaks in the
phenyl region, with broad peaks for both the benzylic
and the tBu protons suggesting a less rigid structure.
This is reminiscent of 1-N-2,2,6,6-tetramethylpiperidine-
(CH2)[B(C6F5)2]C6H4.13,14 The upfield 31P NMR signal
and downfield 11B NMR peak for 8 are consistent with
three coordinate phosphine and boron centers, suggesting
the absence of a dative P-B bond in this case. Crystal-
lographic analyses for 5-7 (Figure 3-5) confirmed that
˚
and basicity of the B and P centers respectively in 3.
The lithiated species 2 also reacts with ClBPh2 to give 4
which was isolated in 61% yield. Alternatively, this
product can also be synthesized from the addition of 2
equiv of PhLi to 3 affording 4 in 80% yield (Scheme 1). 4
showed similar NMR spectra to 3, suggesting the pre-
sence of a P-B bond. X-ray analysis (Figure 2) confirmed
the structure of 4 is similar to that seen for 3 although the
˚
the B-P bond lengths of 2.119(2) A, 2.179(6) and 2.082(3)
˚
A for 5-7, are longer than those in 3 and 4. In contrast,
the structural data for 8 (Figure 6) confirmed the absence
of a P-B bond, as the tBu2P and the BMes2 groups were
oriented away from each other with a intramolecular
˚
P
B separation of 4.910 (5) A. This is presumably
3 3 3
˚
results from the combination of sterically demanding
substituents on the P and B centers.
P-B bond length (2.064(3) A) was found to lengthen by
˚
0.04 A in comparison to 3. This longer P-B bond is
r-Methylbenzyl(N,N-dimethyl)amine-Boranes. Employ-
ing similar synthetic strategies analogous amine-borane
systems were targeted. Use of rac-R-methylbenzyl(N,N-
dimethyl)amine as a ligand scaffold provides the additional
steric feature of the methyl substituent on the benzylic
carbon. Employing metalation of the benzyl-amine PhCH-
(Me)NMe2 to generate o-LiC6H4CH(Me)NMe2 (9), subse-
quent treatment with BCl3 affords o-(Cl2B)C6H4CH(Me)-
NMe2 (10) (Scheme 2). In a fashion similar to that used to
prepare 4, arylation of 10 with PhLi affords o-(Ph2B)C6H4-
CH(Me)NMe2 11. Alternatively 11 could be prepared from
consistent with the lesser Lewis acidity and increased
steric bulk about the B-center.
In a similar fashion the species o-(BBN)C6H4CH2-
PtBu2 (5), o-(R2B)C6H4CH2PtBu2 (R = Cy 6, C6F5 7,
Mes 8) were synthesized from hexane solutions of their
respective chloroboranes, in yields ranging from 61% to
99% (Scheme 1). The 1H, 11B, and 31P NMR of 5 and 6
exhibited similar spectral attributes to those of 3 and 4.
Interestingly, the NMR spectra of 7 are temperature
dependent (see Supporting Information). On cooling to
-60 °C, the 1H spectrum shows distinct signals for each

Article
Inorganic Chemistry, Vol. 50, No. 4, 2011 1475
Figure 3. ORTEP drawing of 5. Thermal ellipsoids are drawn at 50%
probability.
Figure 6. ORTEP drawing of 8. Thermal ellipsoids are drawn at 50%
probability.
Scheme 2. Syntheses of o-Phenethyldimethylamino-Boranes 9-14
Figure 4. ORTEP drawing of 6. Thermal ellipsoids are drawn at 50%
probability.
those described for the P-based analogues described above.
Crystallographic characterization of 10-13 (Figure 7-10)
˚
˚
revealed B-N distances of 1.637(2) A, 1.738(3) A, 1.754(6)
˚
˚
A, and 1.666(3) A, respectively, and a benzylic methyl group
in the axial position with respect to the five-membered
BNC3 rings. These bond distances reflect both the steric
demands and the electronic nature at the B center. In the
case of 10 and 13 the electron withdrawing nature of the
B-substituents results in comparatively short B-N bonds.
The converse is true for the cases of 11 and 12, where
electron donating substituents, lessen the electrophilicity
of the B centers.
Toyota and co-workers developed a concept to gauge
the strength of a donor-acceptor bonds in benzylic
amine-borane complexes23 known as percent tetrahedral
character (%THC). These initial studies examined the
deviation of the bond angles around boron from 109.5° to
quantify the B-N bond strength.23,24 Hopfl expanded
this approach further to include all of the angles around
boron and the donor (Figure 11), using eq 1 to correlate
%THC values with adduct strength.25 This approach was
Figure 5. ORTEP drawing of 7. Thermal ellipsoids are drawn at 50%
probability.
the reaction of 9 with ClBPh2. The corresponding species
o-(R2B)C6H4CH(Me)NMe2 (Cy 12, C6F5 13, Mes 14) were
prepared using 9 with the corresponding borane precursors,
XBR2 (Scheme 2). The spectroscopic data for these com-
pounds are consistent with these formulations and similar to
(22) Mueller, G.; Lachmann, J. Z. Naturforsch., B: Chem. Sci. 1993, 48,
1248–1256.
(23) Toyota, S.; Oki, M. Bull. Chem. Soc. Jpn. 1992, 65, 1832–1840.
(24) Toyota, S.; Oki, M. Bull. Chem. Soc. Jpn. 1992, 65, 2215–2220.
(25) Hopfl, H. J. Organomet. Chem. 1999, 581, 129–149.

1476 Inorganic Chemistry, Vol. 50, No. 4, 2011
Heiden et al.
Figure 7. ORTEP drawing of 10. Thermal ellipsoids are drawn at 50%
probability.
Figure 10. ORTEP drawing of 13. Thermal ellipsoids are drawn at 50%
probability.
Figure 11. Percent tetrahedral character (THC) of a Lewis acid-base
adduct.
shorter B-P distance In the same vein, the B-P bond in 5
is significantly shorter than in 6 and yet the %THC
is higher in the latter species. Calculations for the one
previously reported analogue, tBu2BC6H4CH2PPh2 gave
a %THC of 61.2%,22 while the related 5-member rings
species R2PC3H6B(C6F5)2 (R = Ph, tBu) gave %THCs of
57.8 and 70.8%, respectively).26 Collectively these data
demonstrated the impact of steric demands on B-P bond
lengths and the pyramidalization of B, although the %
THC does not strictly correlate with bond distance in
these systems.
Figure 8. ORTEP drawing of 11. Thermal ellipsoids are drawn at 50%
probability.
2
3
P
j109:5 - θnj°
90°
6
7
5
%THC ¼ 1 - n ¼ 1 - 6
ꢀ 100
ð1Þ
4
Calculation of the %THC from these structural data
reveals that 10-13 exhibits consistently greater pyrami-
dalization of B compared to the P-analogues 3, 4, 6 and 7.
This observation presumably relates to the presence of the
sterically less encumbered donor, in the amino-borane
derivatives.
Figure 9. ORTEP drawing of 12. Thermal ellipsoids are drawn at 50%
probability.
Reactivity of Benzylic Phosphino- and Amino-borane
Complexes. Efforts to affect the catalytic hydrogenation
of tBuN=CHPh at 100 °C for several days employing
5 mol % of 3, 4, 5, and 6 under a H2 atmosphere were
applied to the present compounds (Table 2). As expected
the two complexes, 3 and 4 with the shortest experimental
P-B bond distances resulted in the highest %THC con-
sistent with strong bonds. The P-B bond for 7 was found
to have a similar %THC to 6, despite the experimental
observation that the former species exhibits a somewhat
(26) Spies, P.; Froehlich, R.; Kehr, G.; Erker, G.; Grimme, S. Chem.;
Eur. J. 2008, 14, 333–343.

Article
Inorganic Chemistry, Vol. 50, No. 4, 2011 1477
Table 2. Experimental and Theoretical Bond Distances and %THC
experimental
theoretical
P(N)-B
distance (A)
P(N)-B
distance (A)
˚
˚
complex
%THC
%THC
3
4
5
6
2.024(2)
2.064(3)
2.119(2)
2.179(6)
2.082(3)
1.637(2)
1.738(3)
1.754(6)
1.666(3)
72.0
63.7
47.6
54.3
54.7
79.5
68.1
67.7
60.7
2.057
2.196
2.321
2.451
2.146
1.696
1.826
1.829
1.724
73.2
59.8
49.8
45.7
50.8
66.0
61.4
65.7
56.3
7
10
11
12
13
Scheme 3. Syntheses of 15 and 16
Figure 12. ORTEP drawing of 15. Thermal ellipsoids are drawn at 50%
probability.
unsuccessful. There are a number of possible explana-
tions for this observation. In contrast to other catalyti-
cally active phosphino-borane species, it could be that the
P-B bond in the present species is strong and fails to open
to permit the heterolytic activation of H2. Alternatively it
could be that inclusion of B centers in 3-6 are insuffi-
ciently Lewis acidic to stabilize the H2 adduct resulting in
rapid elimination of H2 reforming the P-B bond. In
marked contrast, 7 and 8 were found to react with H2 at
elevated temperatures (100 °C). In these cases, elimina-
tion of HC6F5 and mesitylene were evident from the
Figure 13. ORTEP drawing of 16. Thermal ellipsoids are drawn at 50%
probability.
1
characteristic H and 19F NMR signals; however, the
˚
the B-O and N-H distances were found to be 1.335(2) A
and 1.589(3) A, respectively.
nature of resulting B-P products could not be unam-
biguously confirmed as all efforts to isolate these pro-
ducts were unsuccessful. Treatment of 14 with H2 gave no
reaction at ambient conditions, but increasing the tem-
perature to 100 °C resulted in the formation of two new
diastereomeric complexes in a ratio of 3: 2. Crystallo-
graphic analysis confirmed the presence of chiral C and B
centers and thus identified the (S,S) isomer of o-((Mes)-
HB)C6H4CH(Me)NMe2 15 (Scheme 3, Figure 12), in
which the R-methyl group adopts an axial position in
the 5-membered chelate ring that results from coordina-
tion of N to B. The B-N bond length was determined to
˚
.
Computational Studies. To garner further insight into
the structure-reactivity relationship, the present phos-
phino-boranes 3-8 and amino-boranes 10-14 were ex-
amined computationally using the Gaussian 03 program
at the B3LYP/6-31 g(d) level of theory (Table 2).21 Pre-
vious literature infers that the limitations of the B3LYP/
6-31G* approach particularly in the computation of
interaction energies for donor-acceptor adducts can give
rise to large errors as these functionals do not properly
account for medium-range correlation effects.27-30 How-
ever these conclusions continue to be the subject of
controversy.31 Nonetheless, the minimized structures
were found to be very similar to those observed experi-
mentally. The largest deviation between experimental and
˚
be 1.674(3) A. Interestingly dissolution of the crystals of
15 gave rise to a diastereomeric mixture identical to that
obtained from the initial reaction mixture of 14 þ H2.
This suggests that epimerization of the chiral B center is
facile. In a similar vein, although 14 was stable in dry air,
exposure to small amounts of water over the course
of days at room temperature resulted in the slow conversion
of 14 to the species o-((Mes)HOB)C6H4CH(Me)NMe2 16
with the loss of mesitylene. Spectroscopic and crystallographic
data were consistent with the formulation of 16 (Scheme 3,
Figure 13). Strong hydrogen bonding between the ammo-
nium center and the boronic acid fragment is evident as
˚
theoretical B-P(N) distances, of about 0.2 A, was ob-
served in the structures 5 and 6. A similar observation was
(27) Grimme, S. Angew. Chem., Int. Ed. 2006, 45, 4460.
(28) Plumley, J. A.; Evanseck, J. D. J. Phys. Chem. 2007, 111, 13472.
(29) Schwabe, T.; Grimme, S. Acc. Chem. Res. 2008, 41, 569.
€
(30) Rakow, J. R.; Tullmann, S.; Holthausen, M. C. J. Phys. Chem. A.
2009, 113, 12035.
(31) Simon, L.; Goodman, J. M., Org. Biomol. Chem. 2010, DOI: 10.1039/
c0ob00477d.

1478 Inorganic Chemistry, Vol. 50, No. 4, 2011
Heiden et al.
Table 3. Calculated ΔG Energies for the Reaction H2 with o-Benzylphosphino-
boranes
Scheme 5. Reaction Coordinate Diagram for the Reaction of H2
with 14
ΔG‡ (H2)
(kcal/mol)
o-(R2BH)C6H4CH(Me)NHMe2
(kcal/mol)
precursor
3
4
5
6
7
8
38.0
32.1
28.3
28.3
21.1
21.0
26.3
25.7
13.2
31.7
31.6
no t.s. found
Table 4. Calculated ΔG Energies for the Reaction H2 with o-Methylbenzylamino-
boranes
ΔG‡ (H2)
(kcal/mol)
o-(R2BH)C6H4CH(Me)PHtBu2)
(kcal/mol)
precursor
10
11
12
13
14
43.8
30.6
24.3
30.4
24.3
25.6
24.2
21.2
14.3
19.5
Scheme 4. Reaction Coordinate Diagram for the Reaction of H2
with 7
experimental observation that these species do not react
with H2.
In the case of the reaction of 7 with H2, calculations
show a small stabilization of 3.0 kcal/mol is achieved by
the formation of the dative P-B bond. This is consistent
with the experimental observation of an equilibrium
between an open and closed forms in solution. The
activation energy for the reaction with H2 is computed
to be 31.6 kcal/mol (Scheme 4). Subsequent heterolytic
cleavage to generate a phosphonium borate results as the
product is 20 kcal/mol lower in energy (ΔG) than the
transition state, but endergonic by 13.2 kcal/mol
with regards to 7 and H2. From this salt the activation
barrier for proton transfer to a B-bound C6F5 group is
2 kcal/mol less than that for the elimination of H2. Thus,
loss of HC6F5 proceeds resulting in the formation of
o-(C6F5BH)C6H4CH2PtBu2 which is exergonic (ΔG) by
almost 19 kcal/mol in comparison to 7 and H2 (Scheme 3).
The loss of HC6F5 is thought to be entropy driven as the
loss of HC6F5 relieves much of the steric congestion
allowing formation of a stronger dative P-B bond in
o-(C6F5BH)C6H4CH2PtBu2. This computed reaction co-
ordinate is consistent with the experimental observations
that the reaction of 7 with H2 proceeds at elevated
temperatures to give HC6F5, although the P/B species
could not be isolated. The formation of the dative bond in
o-(C6F5BH)C6H4CH2PtBu2 precludes further reaction
with H2. A similar reaction is expected for 8 although
the loss of a mesitylene from the reaction of 8 and H2 was
not probed computationally.
made by Gilbert in his theoretical analysis of B-N
bonds.32 The reactivity of these species with H2 was also
probed computationally (Table 3 and Table 4). The
computed energies for the products of hydrogenation,
the phosphonium hydridoborates, were found to be
similar for the products from 5 and 6 while those derived
from 3 and 4 were slightly lower in energy. However, in all
cases these products were endothermic with respect to the
precursor phosphine-boranes and H2. The barriers to H2
activation were also computed and found to be in the
range of 28-38 kcal/mol, although a transition state was
not found for 8. In these cases, the high barrier and
endergonic nature of the products are in accord with the
Similar calculations for the reaction of 14 with H2
(Scheme 5) revealed a similar reaction path as observed
for 7. In the case of the amino-borane species, the ΔG‡
(32) Gilbert, T. M. J. Phys. Chem. 2004, 108, 2550–2554.

Article
Inorganic Chemistry, Vol. 50, No. 4, 2011 1479
associated with the activation barrier to the heterolytic
activation of H2 was computed to be 30.1 kcal/mol,
similar to that seen for the phosphino-borane above.
However, the subsequent loss of mesitylene and genera-
tion of 15 was much more exergonic, as 15 was computed
to be 28.0 kcal/mol (ΔG) lower in energy than 14.
The driving force for this proton transfer is that the loss
of mesitylene. The resulting homochiral 15 was found to
be 0.3 kcal/mol lower in energy than its diastereomer,
which is in accord with the experimentally observed 3:2
ratio of the diastereomers. The low activation barrier of
epimerization of the chiral B center of 14.8 kcal/mol is
consistent with observed rapid equilibration of the two
diastereomers.
These data, demonstrate that sterically demanding donor
fragments are essential for H2 activation. However, proxim-
ity of the base and acid sites can favor donor-acceptor
adduct formation as well as prompt substituent elimination
reactions. This latter issue can be potentially avoided with
judicious modification of the basicity of the Lewis base.
However, the design of new bifunctional metal-free hydro-
genation catalysts must also account for the ability of the
Lewis basic site to deliver a proton to the substrate during
hydrogenation catalysis. The study of the structure-reactivity
relationships and the design of new FLP hydrogenation
catalysts continue to be a subject of intense efforts in our
laboratories.
Acknowledgment. The authors thank NSERC of Canada
for financial support. D.W.S. is grateful for the award
of a Canada Research Chair and a Killam Research Fellow-
ship. This research was supported by NSERC. Z.M.H.
is grateful for the award of an Ontario Postdoctoral Fellow-
ship.
Conclusions
A series of linked phosphine and amine-boranes species
have been prepared and studied. Introduction of a dialkyl or
diaryl boron centers resulted in species that give rise to
intramolecular B-P or B-N dative bonds. Groups such as
C6F5 and mesityl substituents on B result in weaker or no
dative bonds. Reaction of these latter species with H2 are
shown to exhibit accessible activation barriers, resulting in
the activation of heterolytic cleavage of H2 and subsequent
loss of an aryl substituent from B and the formation of new
five-membered B-P ring derivatives.
Supporting Information Available: Crystallographic data in
CIF format, 3D coordinates and energies of all calculated
structures, variable temperature NMR of 7 and 14. This ma-
terial is available free of charge via the Internet at http://pubs.
acs.org.