Development of rhenium catalysts for amine borane dehydrocoupling and transfer hydrogenation of olefins

Article abstract of DOI:10.1021/om900458e

Five-coordinated rhenium(I) hydride complexes of the type [Re(Br)(H)(NO)(PR₃)₂] (R = Cy 2a, iPr 2b) were prepared from [Re(Br)₂(NO)(PR₃)₂(η²- H₂)] (R = Cy la,. iPr lb) via deprotonation of the η²-H₂ ligands with various bases. Filling the vacant site of 2a or 2b by various less bulky two-electron donors produced the 18-electron complexes [Re(Br)(H)(NO)(PR₃)₂(L)] (L = O ₂ 3, CH₂=CH₂ 4, acetylene 5, H₂ 6, CO 7, CH₃CN 8). The influence of the trans-coordinated ligand on the Re-H bond was examined. The ¹H NMR chemical shift of the hydride depends on L in the order O₂ > acetylene > CH ₂=CH₂ > H₂ > CO > CH₃CN. The reactions of 2a or 2b with the IMes or SIMes ligands afforded the five-coordinated complex [Re(Br)(H)(NO)(PR₃)(NHC)] (NHC = IMes 9 (IMes = 1,3bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), SIMes 10 (SIMes = l,3-bis(2,4,6-trimethylphenyl)4,5-dihydroimidazol-2-ylidene)) via replacement of one phosphine. The reaction of 2a or 2b with n-BuLi leads to the formation of the n-butene-coordinated dihydride complexes [Re(H)₂(NO)(PR ₃)₂(η²-n-CH₂=CHC ₂H₅)] (R = Cy 12a, iPr 12b). Species 1a and 1b reacted also with NaNMe₂BH₃, affording the tetrahydride complexes [Re(H)₄(NO)(PR₃)₂] (R = Cy 14a, iPr 14b) via the intermediacy of 2a and 2b. The molecular structures of complexes 8b, 10a, and 10b were established by single-crystal X-ray diffraction studies. The five-coordinated rhenium(I) hydride complexes 2a, 2b, 9a, and 9b catalyzed the dehydrocoupling of Me₂NHBH₃ and the transfer hydrogenation of olefins using Me₂NHBH₃ as a hydrogen donor, which showed high activities. Mechanistic studies were carried out indicating that these rhenium(I) hydride catalyses allowed formation of dihydrogen hydride complexes. A plausible catalytic cycle for both dehydrocoupling and transfer hydrogenation was proposed, which implies the ability of rhenium(I) complexes to activate B-H and N-H bonds by the facile redox interplay of Re(I) and Re(III) species.

Full text of DOI:10.1021/om900458e

Organometallics 2009, 28, 5493–5504 5493
DOI: 10.1021/om900458e
Development of Rhenium Catalysts for Amine Borane Dehydrocoupling
and Transfer Hydrogenation of Olefins
Yanfeng Jiang, Olivier Blacque, Thomas Fox, Christian M. Frech, and Heinz Berke*
€
€
Anorganisch-Chemisches Institut, Universitat Zurich, Winterthurerstrasse 190, CH-8037, Zurich,
€
Switzerland
Received May 29, 2009
Five-coordinated rhenium(I) hydride complexes of the type [Re(Br)(H)(NO)(PR3)2] (R=Cy 2a,
iPr 2b) were prepared from [Re(Br)2(NO)(PR3)2(η2-H2)] (R=Cy 1a, iPr 1b) via deprotonation of the
η2-H2 ligands with various bases. Filling the vacant site of 2a or 2b by various less bulky two-electron
donors produced the 18-electron complexes [Re(Br)(H)(NO)(PR3)2(L)] (L = O2 3, CH2dCH2 4,
acetylene 5, H2 6, CO 7, CH3CN 8). The influence of the trans-coordinated ligand on the Re-H bond
was examined. The 1H NMR chemical shift of the hydride depends on L in the order O2 > acetylene
> CH2dCH2 > H2 > CO > CH3CN. The reactions of 2a or 2b with the IMes or SIMes ligands
afforded the five-coordinated complex [Re(Br)(H)(NO)(PR3)(NHC)] (NHC=IMes 9 (IMes=1,3-
bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), SIMes 10 (SIMes = 1,3-bis(2,4,6-trimethylphenyl)-
4,5-dihydroimidazol-2-ylidene)) via replacement of one phosphine. The reaction of 2a or 2b with
n-BuLi leads to the formation of the n-butene-coordinated dihydride complexes [Re(H)2-
(NO)(PR3)2(η2-n-CH2dCHC2H5)] (R = Cy 12a, iPr 12b). Species 1a and 1b reacted also with
NaNMe2 BH3, affording the tetrahydride complexes [Re(H)4(NO)(PR3)2] (R=Cy 14a, iPr 14b) via
the intermediacy of 2a and 2b. The molecular structures of complexes 8b, 10a, and 10b were
3
established by single-crystal X-ray diffraction studies. The five-coordinated rhenium(I) hydride
complexes 2a, 2b, 9a, and 9b catalyzed the dehydrocoupling of Me2NH BH3 and the transfer
hydrogenation of olefins using Me2NH BH3 as a hydrogen donor, which showed high activities.
3
3
Mechanistic studies were carried out indicating that these rhenium(I) hydride catalyses allowed
formation of dihydrogen hydride complexes. A plausible catalytic cycle for both dehydrocoupling
and transfer hydrogenation was proposed, which implies the ability of rhenium(I) complexes to
activate B-H and N-H bonds by the facile redox interplay of Re(I) and Re(III) species.
Introduction
Recently mainly dehydrocoupling reactions of amine bor-
anes were studied for applications in dehydrogenative pro-
cesses utilizing transition metal catalyses with various Rh,
Ru, Ir, Pd, Ni, Re, and Ti compounds (Scheme 1).3-10 For
instance, the catalytic dehydrocoupling of amine boranes,
“Hydrogen” as an energy carrier would crucially involve
storage of hydrogen in suitable chemical compounds.
Furthermore “fuelling” and “refuelling” requires chemical
reactions that are dehydrogenations/hydrogenations or de-
hydrocoupling/hydrocoupling processes.1 Amine borane
compounds are considered valuable candidates for storage
materials due to their high H2 storage capacity and low
thermicity in their “fuelling” and “refuelling” reactions.2
such as Me2NH BH3, can be achieved in a homogeneous
3
(3) Douglas, T. M.; Chaplin, A. B.; Weller, A. S. J. Am. Chem. Soc.
2008, 130, 14432.
(4) (a) Jaska, C. A.; Temple, K.; Lough, A. J.; Manners, I. Chem.
Commun. 2001, 962. (b) Jaska, C. A.; Temple, K.; Lough, A. J.; Manners, I.
J. Am. Chem. Soc. 2003, 125, 9424. (c) Jaska, C. A.; Manners, I. J. Am.
Chem. Soc. 2004, 126, 2698.
*Corresponding author. E-mail: hberke@aci.uzh.ch.
(1) (a) Coontz, R.; Hanson, B. Science 2004, 305, 957. (b) Grochala,
W.; Edwards, P. P. Chem. Rev. 2004, 104, 1283. (c) Welch, G. C.; Juan, R. R.
S.; Masuda, J. D.; Stephan, D. W. Science 2006, 314, 1124. (d) Gutowska, A.;
Li, L.; Shin, Y.; Wang, C. M.; Li, X. S.; Linehan, J. C.; Smith, R. S.; Kay, B.
D.; Schmid, B.; Shaw, W.; Gutowski, M.; Autrey, T. Angew. Chem., Int. Ed.
2005, 44, 3578.
(2) (a) Hamilton, C. W.; Baker, R. T.; Staubitz, A.; Manners, I.
Chem. Soc. Rev. 2009, 38, 279. (b) Pons, V.; Baker, R. T. Angew. Chem.,
Int. Ed. 2008, 47, 9600. (c) Stephens, F. H.; Pons, V.; Baker, R. T. Dalton.
Trans. 2007, 2613. (d) Marder, T. B. Angew. Chem., Int. Ed. 2007, 46, 8116.
(e) Bluhm, M. E.; Bradley, M. G.; Butterick, R.; Kusari, U.; Sneddon, L. G. J.
Am. Chem. Soc. 2006, 128, 7748. (f) Ramachandran, P. V.; Gagare, P. D.
Inorg. Chem. 2007, 46, 7810. (g) Nguyen, V. S.; Matus, M. H.; Nguyen, M.
T.; Dixon, D. A. J. Phys. Chem. C 2007, 111, 9603. (h) Nguyen, V. S.;
Matus, M. H.; Grant, D. J.; Nguyen, M. T.; Dixon, D. A. J. Phys. Chem. A
2007, 111, 8844.
(5) (a) Clark, T. J.; Russell, C. A.; Manners, I. J. Am. Chem. Soc.
2006, 128, 9582. (b) Clark, T. J.; Lee, K.; Manners, I. Chem.;Eur. J. 2006,
12, 8634.
(6) Jiang, Y.; Berke, H. Chem. Commun. 2007, 3571.
(7) Pun, D.; Lobkovsky, E.; Chirik, P. J. Chem. Commun. 2007, 3297.
(8) (a) Keaton, R. J.; Blacquiere, J. M.; Baker, R. T. J. Am. Chem.
Soc. 2007, 129, 1844. (b) Yang, X.; Hall, M. B. J. Am. Chem. Soc. 2008,
130, 1798.
(9) (a) Denney, M. C.; Pons, V.; Hebden, T. J.; Heinekey, D. M.;
Goldberg, K. I. J. Am. Chem. Soc. 2006, 128, 12048. (b) Hebden, T. J.;
Denney, M. C.; Pons, V.; Piccoli, P. M. B.; Koetzle, T. F.; Schultz, A. J.;
Kaminsky, W.; Goldberg, K. I.; Heinekey, D. M. J. Am. Chem. Soc. 2008,
130, 10812. (c) Paul, A.; Musgrave, C. B. Angew. Chem., Int. Ed. 2007, 46,
8153.
(10) Staubitz, A.; Soto, A. P.; Manners, I. Angew. Chem., Int. Ed.
2008, 47, 6212.
r
2009 American Chemical Society
Published on Web 08/18/2009
pubs.acs.org/Organometallics

5494 Organometallics, Vol. 28, No. 18, 2009
Jiang et al.
Scheme 1
Concerning the synthetic access to transition metal hydrides,
there is, besides the classic ways of protonation of basic metal
complexes or halide replacement with hydride donors,15 the
prominent route of deprotonation of η2-H2 complexes. Most
dihydrogen ligands are acidic and can be deprotonated with
bases, leaving a hydride on the metal and a proton on the base,
resulting in heterolytic cleavage of the H2 molecule. In fact, the
discovery of dihydrogen complexes by Kubas in 1984 initiated
this research including catalytic hydrogen chemistry occurring
with formal heterolysis of the H2 ligand.16
In recent years our group focused on the exploration of a
related hydride chemistry with middle transition metal cen-
ters,17 in particular with respect to the development of polar
homogeneous catalyses involving H-H, H-Si, and H-B
activation.6,18,19 For example in the realm of rhenium, the
dihydrogen rhenium(I) complexes [Re(Br)2(NO)(PR3)2(η2-
H2)] (R=Cy 1a, iPr 1b) were found to catalyze dehydrocou-
fashion by a η2-amine borane σ-complex of Rh3 or hetero-
geneously by Rh(0) colloids4 and the Cp2TiCl2/n-BuLi sys-
tem.5 The latter two catalysts are also active in tandem
dehydrocoupling-hydrogenation reactions of amine bor-
anes and olefins, which in total are transfer hydrogenations.
Similar reactions in solution were observed with rhenium(I)
dibromides6 and titanocene dinitrogen catalysts.7 More
practical and active catalysts for hydrogen release from
NH3 BH3 are an NHC nickel8 (NHC = N-heterocyclic
3
carbene) and pincer-ligated iridium dihydride complexes,9
by which the formation of the quite volatile dehydrocoupling
product borazine is efficiently avoided. More recently, high
molecular weight polyaminoboranes were produced in the
dehydrocoupling of primary amine boranes catalyzed by the
pincer iridium dihydride.10 Ruthenium complexes activated
by KOtBu were found to be exceptionally active catalysts for
hydrogen release from NH3 BH3 and Me2NH BH3.11
pling of Me2NH BH3 and transfer hydrogenations of olefins
3
with Me2NH BH3 as a hydrogen donor.6 Recently, five-
3
coordinated rhenium(I) hydride complexes [Re(Br)(H)-
(NO)(PR3)2] (R=Cy 2a, iPr 2b) were isolated from dehy-
drogenative silylations of alkenes using 1a and 1b as cata-
lysts.19 Such mononitrosyl Re(I) hydrides, which resemble
the mentioned monocarbonyl Ru(II) or Os(II) hydrides,
were expected to bear great potential in particular with
regard to homogeneous catalysis. In extension of our earlier
work, we report here the organometallic chemistry of 2a and
2b as a new platform for catalysis including the preparations
of these compounds from dihydrogen complexes, their reac-
tions toward less bulky two-electron donors to give 18-
electron complexes, their transformations into more bulky
NHC derivatives, and their reactions with n-BuLi or NaN-
3
3
In all these reactions transition metal hydrides are thought
to play a pivotal role with their M-H bond strengths in a
range that they enable a wide variety of key transformations,
such as hydride transfers and insertions.12 These are not only
generally synthetically useful but also crucial to the homo-
geneous dehydrocoupling reactions of amine boranes.3,9-11
Most of the known transition metal hydrides obey the 18-
electron rule.12 Open-shell 16-electron complexes are less
abundant and usually bear early or late transition metal
centers. 16-Electron complexes with middle transition metal
elements need for the stabilization as catalytic intermediates
the help of the residual coordination sphere to create vacant
sites. Utilization of π-donor effects or the trans influence is
prevalent. For instance in the case of d6 square-pyramidal
hydride complexes [M(X)(H)(CO)(PR3)2] (M=Ru, Os, X=
halide) the coordinative vacancy is supported among other
effects by the electronic interplay of the π-donor X and the
strong σ-donor H.13 Metal hydride complexes possessing a
vacant site, like those with ruthenium and osmium, also
display catalytic chemistry.14 Therefore we attempted ex-
ploration of a related isoelectronic nitrosyl hydride chemis-
try of rhenium.
Me2 BH3 to generate dihydride and tetrahydride species.
3
Furthermore the catalytic activities of appropriate rhenium
hydrides in the dehydrocoupling of Me2NH BH3 and
3
NH3 BH3, as well as transfer hydrogenations of olefins
3
using Me2NH BH3, are probed.
3
Results and Discussion
Preparation of [Re(Br)(H)(NO)(PR3)2]. Previously we re-
ported that the five-coordinate 16-electron complex 2a or 2b
could be prepared from the dihydrogen complex 1a or 1b by
(15) James, B. R.; Ashby, M. T. In Inorganic Reactions and Methods;
Norman, A. D., Ed.; VCH Publishers: New York, 1991.
(16) (a) Kubas, G. J. Chem. Rev. 2007, 107, 4152. (b) Kubas, G. J. Adv.
Inorg. Chem. 2004, 56, 127. (c) Kubas, G. J. Metal Dihydrogen and σ-Bond
Complexes; Kluwer: New York, 2001. (d) Kubas, G. J. Acc. Chem. Rev.
1988, 21, 120. (e) Kubas, G. J.; Ryan, R. R.; Swanson, B. I.; Vergamini, P. J.;
Wasserman, H. J. J. Am. Chem. Soc. 1984, 106, 451.
(11) Blaquiere, N.; Diallo-Garcia, S.; Gorelsky, S. I.; Black, D. A.;
Fagnou, K. J. Am. Chem. Soc. 2008, 130, 14034.
(12) (a) Peruzzini, M.; Poli, R. In Recent Advances in Hydride
Chemistry; Elsevier: Amsterdam, Holland, 2001. (b) Crabtree, R. H. The
Organometallic Chemistry of the Transition Metals; Wiley-Interscience,
2001.
(17) (a) Choualeb, A.; Blacque, O.; Schmalle, H. W.; Fox, T.;
Hiltebrand, T.; Berke, H. Eur. J. Inorg. Chem. 2007, 5246. (b) Frech, C.
M.; Blacque, O.; Schmalle, H. W.; Berke, H.; Adlhart, C.; Chen, P. Chem.;
Eur. J. 2006, 12, 3325. (c) Frech, C. M.; Blacque, O.; Schmalle, H. W.;
Berke, H. Chem.;Eur. J. 2006, 12, 5199. (d) Kurz, P.; Rattat, D.; Angst, D.;
Schmalle, H.; Spingler, B.; Alberto, R.; Berke, H.; Beck, W. Dalton Trans.
2005, 804. (e) Jaunky, P.; Schmalle, H. W.; Blacque, O.; Fox, T.; Berke, H. J.
Organomet. Chem. 2005, 690, 1429. (f) Liu, X. Y.; Venkatesan, K.;
Schmalle, H. W.; Berke, H. Organometallics 2004, 23, 3153. (g) Messmer,
A.; Jacobsen, H.; Berke, H. Chem.;Eur. J. 1999, 5, 3341. (h) Frech, C. M.;
Blacque, O.; Schmalle, H. W.; Berke, H. Dalton Trans. 2006, 4590.
(18) (a) Choualeb, A.; Maccaroni, E.; Blacque, O.; Schmalle, H. W.;
Berke, H. Organometallics 2008, 27, 3474. (b) Llamazares, A.; Schmalle,
H. W.; Berke, H. Organometallics 2001, 20, 5277. (c) Gusev, D.; Llama-
zares, A.; Artus, G.; Jacobsen, H.; Berke, H. Organometallics 1999, 18, 75.
(d) Jiang, Y.; Berke, H. Chem. Commun. 2007, 3571.
(13) Esteruelas, M. A.; Oro, L. A. In Advances in Organometallic
Chemistry, Vol. 47; Academic Press Inc: San Diego, 2001.
(14) (a) Esteruelas, M. A.; Sola, E.; Oro, L. A.; Meyer, U.; Werner, H.
Angew. Chem., Int. Ed. Engl. 1988, 27, 1563. (b) Esteruelas, M. A.; Sola, E.;
Oro, L. A.; Werner, H.; Meyer, U. J. Mol. Catal. 1988, 45, 1. (c) Lay, P. A.;
Harman, W. D. Adv. Inorg. Chem. 1991, 37, 219. (d) Albeniz, M. J.; Buil, M.
L.; Esteruelas, M. A.; Lopez, A. M.; Oro, L. A.; Zeier, B. Organometallics
1994, 13, 3746. (e) Bakhmutov, V. I.; Bertran, J.; Esteruelas, M. A.; Lledos,
A.; Maseras, F.; Modrego, J.; Oro, L. A.; Sola, E. Chem.;Eur. J. 1996, 2,
815. (f) Esteruelas, M. A.; Gomez, A. V.; Lahoz, F. J.; Lopez, A. M.; Onate,
E.; Oro, L. A. Organometallics 1996, 15, 3423. (g) Esteruelas, M. A.; Lahoz,
F. J.; Onate, E.; Oro, L. A.; Sola, E. J. Am. Chem. Soc. 1996, 118, 89.
(h) Buil, M. L.; Elipe, S.; Esteruelas, M. A.; Onate, E.; Peinado, E.; Ruiz, N.
Organometallics 1997, 16, 5748. (i) Yi, D. C. S.; Lee, W. Organometallics
1999, 18, 5152. (j) Esteruelas, M. A.; Oro, L. A. Chem. Rev. 1998, 98, 577.
(19) Jiang, Y.; Blacque, O.; Fox, T.; Frech, C. M.; Berke, H. Chem.;
Eur. J. 2009, 15, 2121.

Article
Organometallics, Vol. 28, No. 18, 2009 5495
Scheme 2
Scheme 3
relatively high temperatures in solution (80 °C). We sus-
pected that the relatively high stability of both derivatives
possessing an oxidant (O2) and a reductant (H-) in the
coordination sphere at the same time is of kinetic origin
and caused by the trans position of both ligands at “maxi-
mum” separation and apparently hindered rearrangement of
the coordination sphere.17h 3a and 3b were fully character-
ized by elemental analysis and various spectroscopic meth-
ods. In the IR spectra the O-O stretching vibration was
observed at 869 cm-1 (3a) or 873 cm-1 (3b), which is
comparable in its position to that of the structurally analo-
gous η2-O2 complex of osmium.14a In the 1H NMR spectra
the Re-H signals were observed as triplets at δ 7.11 ppm
(2J(HP)=40.0 Hz) for 3a and 6.99 ppm (2J(HP)=39.0 Hz) for
3b, which are shifted strongly to low field in comparison with
the related signals of 2a (-17.6 ppm) and 2b (-16.7 ppm).
This indicates that the coordinated O2 acts as a π acceptor,
most probably oriented parallel to the P-Re-P axis in
compliance with the structure of the previously reported
η2-styrene adducts.19 The unusual low-field shift of the
hydride can be explained via strong d-electron back-dona-
tion from rhenium to the π* orbital of η2-O2, which decreases
the electron density at the trans hydride site.
The influence of trans-coordinated two-electron ligands
on the Re-H bond was further explored by 1H NMR
observation of changes in the hydride chemical shift. In
solution complexes 2a and 2b reacted readily with acetylene,
CO, and CH3CN to yield the corresponding 18-electron
complexes [Re(Br)(H)(NO)(PR3)2(η2-C2H2)] (R = Cy 5a,
iPr 5b), [Re(Br)(H)(NO)(PR3)2(CO)] (R = Cy 7a, iPr 7b),
and [Re(Br)(H)(NO)(PR3)2(CH3CN)] (R=Cy 8a, iPr 8b),
which were isolated as pure solids in excellent yields
(Scheme 3, Table 1). All the complexes appear to be stable
in the solid state, and decomposition did not take place even
after prolonged treatment under vacuum. The structure of
complex 8b was established by a single-crystal X-ray diffrac-
tion study, as shown in Figure 1.20 8b crystallizes with one
THF solvate molecule in the asymmetric unit. The coordina-
tion sphere of 8b is pseudo-octahedral, with the hydride trans
to CH3CN (N-Re-H angle of 180.00(1)o). The bromide
and nitrosyl group are trans to each other with a Br-Re-N
angle of 178.48(13)o. The phosphine ligands in 8b are bent
toward the hydride ligand (P-Re-P angle of 164.14(3)o)
the reaction with a hydride donor, such as Et3SiH or
Ph3SiH.19 We supposed that 2a or 2b could also be generated
via the heterolytic cleavage of the η2-H2 ligand of 1a or 1b by
hydrodebromination with various bases, such as HNiPr2,
Na[N(SiMe3)2], or Me2NH BH3 (Scheme 2). For example,
3
treatment of 1a with 2 equiv of HNiPr2 in benzene solution
afforded at room temperature 2a in 88% yield within 10 min;
1 equiv of HNiPr2 did not furnish satisfying conversions. In
comparison, the reaction of 1b with HNiPr2 was much more
sluggish and took 15 h at 80 °C to reach 80% conversion,
implying that the dihydrogen ligand of 1b is presumably less
acidic than that of 1a. Treatment of 1a with 2 equiv of the
stronger base Na[N(SiMe3)2] afforded 2a within 5 min at
room temperature and in 92% NMR yield. In contrast, no
reaction occurred between 1b and Na[N(SiMe3)2] at room
temperature. At the higher temperature of 80 °C the reaction
between 1b and Na[N(SiMe3)2] led within 2 h to the forma-
tion of free PiPr3 in 100% conversion probably with sub-
stitution by the amide anion. The relatively high dissociation
tendency of the PiPr3 in this complex was further demon-
strated by the reaction with BH3 THF, producing the
3
phosphine abstraction product H3B PiPr3 in 99% yield. In
3
contrast, the reaction of 1a with BH3 THF yielded 2a in 50%
3
yield. However, this pathway was observed concomitant
with the abstraction of a phosphine ligand to form
H3B PCy3. The reaction of BH3 THF with the ethylene
3
3
complex [Re(Br)2(NO)(η2-C2H4)(PCy3)2] did not yield the
hydride complex 1a, but solely a phosphine abstraction
product, H3B PCy3. This indicates that the reaction of 1a
3
with BH3 THF induces heterolytic cleavage of the η2-H2
ligand. It should be noted that such a phosphine borane
adduct H3B PR3 is also formed in the dehydrocoupling
3
3
reaction of Me2NH BH3 catalyzed by the complex 1a or
3
1b.6 We re-examined the stoichiometric reaction of 1a or 1b
with Me2NH BH3 (10 equiv) and found that besides the
3
main H3B PR3 products a small amount (less than 33%) of
the hydride complex 2a or 2b appeared within 30 min at 75
°C. We assume that either the B-H bond or the freed
3
Me2NH, generated by H3B PR3 formation, acts as a base
3
here.
Reaction of [Re(Br)(H)(NO)(PR3)2] with Less Bulky Two-
Electron Donors. The unsaturated hydrides 2a and 2b are
stable under N2 atmosphere. Exposure of hexane solutions
of 2a or 2b to air under ambient condition (23 °C, 1 bar) led to
the immediate formation of the 18-electron complexes
[Re(Br)(H)(NO)(PR3)2(η2-O2)] (R = Cy 3a, iPr 3b), which
were isolated as gray powders in 92% (3a) and 88% (3b)
yield. These saturated complexes not only withstand in the
solid state treatment with high vacuum but survive also
(20) X-ray crystal-structure data for 8b: yellow crystals,
C26H52BrN2OP2Re, M=736.75, crystal size 0.23 ꢀ 0.23 ꢀ 0.17 mm3,
˚
trigonal, space group P3221; a=11.22500(10) A, b=11.22500(10) A, c=
˚
3
˚
˚
21.6967(2) A, R=90°, β=90°, γ=120°, V=2367.54(4) A , Z=3, Fcalcd
=
1.550 Mg m-3, F(000) = 1110, μ = 5.236 mm-1, 25 459 reflections
3
(2θmax = 61°), 4814 unique (Rint = 0.0442), 175 parameters, R1 (I >
2σ(I))=0.0175, wR2 (all data)=0.0428, GooF=1.070, largest difference
peak and hole 0.701 and -0.821 e A
-3
˚
.
3

5496 Organometallics, Vol. 28, No. 18, 2009
Jiang et al.
Table 1. Yields for the Preparation of [Re(Br)(H)(NO)(PR3)2(L)]
Complexes and 1H NMR Chemical Shiftsa of the Hydride
Ligands
Scheme 4
1H δ
complex
yield (%)b (Re-H, ppm)
[Re(Br)(H)(NO)(PCy3)2(η2-O2)] (3a)
[Re(Br)(H)(NO)(PiPr3)2(η2-O2)] (3b)
[Re(Br)(H)(NO)(PCy3)2(η2-C2H4)] (4a)
[Re(Br)(H)(NO)(PiPr3)2(η2-C2H4)] (4b)
[Re(Br)(H)(NO)(PCy3)2(η2-C2H2)] (5a)
83
86
66
55
85
7.11
6.99
4.11
3.75
5.68
5.11
3.66
3.29
2.82
2.50
-1.53
-1.82
[Re(Br)(H)(NO)(PiPr3)2)2(η2-C2H2)] (5b) 91
[Re(Br)(H)(NO)(PCy3)2(η2-H2)] (6a)
[Re(Br)(H)(NO)(PiPr3)2)2(η2-H2)] (6b)
[Re(Br)(H)(NO)(PCy3)2(CO)] (7a)
[Re(Br)(H)(NO)(PiPr3)2)2(CO)] (7b)
[Re(Br)(H)(NO)(PCy3)2(CH3CN)] (8a)
99c
99c
87
90
90
[Re(Br)(H)(NO)(PiPr3)2)2(CH3CN)] (8b) 67
a In benzene-d6 solution. b Isolated yield. c Determined in situ by the
integral in the 31P NMR spectrum.
spectrum of compound 6a or 6b, recorded under H2 atmo-
sphere, showed a low-field resonance for the dihydrogen
ligand (3.46 ppm for 6a, T1=49 ms, and 3.38 ppm for 6b, T1=
87 ms). Both signals appear broad apparently due to fast
rotation of the H2 moiety and unresolved coupling with the
phosphorus nuclei. The long T1 time of 6b indicates an
elongated H2 ligand approaching dihydride character.22,16
Reactions of the [Re(Br)(H)(NO)(PR3)2] Complexes with
Bulky NHCs. At room temperature 2a or 2b did not undergo
ligand addition by treatment with 1.2 equiv of the bulkyl N-
heterocyclic carbene (NHCs) IMes (IMes = 1,3-bis(2,4,6-
trimethylphenyl)imidazol-2-ylidene); rather a ligand ex-
change reaction was initiated, affording the five-coordinate
rhenium(I) IMes hydrides [Re(Br)(H)(NO)(PR3)(IMes)]
(R=Cy 9a, iPr 9b) in good yield (71% for 9a and 86% for
9b). Complexes 9a and 9b were fully characterized. The IR
spectra of 9a and 9b showed a strong ν(NO) absorption at
1631 cm-1 (9a) and 1626 cm-1 (9b). In comparison with the
ν(NO) absorptions of 2a (1649 cm-1) or 2b (1640 cm-1), the
lower wavenumbers of 9a or 9b speak for a stronger electron-
donating ability of the IMes ligand in comparison with the
PCy3 and PiPr3 derivatives. The 31P{1H} NMR spectra
showed a singlet at δ 30.3 (9a) and 41.5 ppm (9b). In the
1H NMR spectra the methyl signals of the IMes ligand
were observed at δ 2.43, 2.36, 2.12 ppm (9a) and 2.43, 2.35,
2.12 ppm (9b), respectively, and the signals for the imidazole
ring protons were found at δ 6.24 (9a) or 6.25 ppm (9b). The
HRe resonance of 9a was observed at δ -18.18 ppm (doublet
with 2J(HP)=12.0 Hz), confirming the presence of only one
phosphine ligand. The relatively small JHP coupling constant
was consistent with the hydride cis to the PCy3 ligand. The
HRe signal of 9b, however, appeared as a broad resonance
at δ -17.94 ppm.
Figure 1. ORTEP drawing of the molecular structure of com-
plex 8b (thermal ellipsoids are shown at the 20% probability
level; the benzene solvent and selected hydrogen atoms are
˚
omitted). Selected bond lengths (A) and angles (deg): Re1-Br1,
2.5297(7); Re1-N1, 1.780(5); Re1-H1, 1.65(4); Re1-P1,
2.4368(5); Re1-N2, 2.174(3); N1-O1, 1.230(5). O1-N1-Re1,
178.2(5); N1-Re1-N2, 91.17(13); N1-Re1-P1, 90.02(12);
P1-Re1-P1i, 164.14(3); N2-Re1-H1, 180.0; N1-Re1-H1,
88.83(13); P1-Re1-H1, 82.1(9); N2-Re1-Br1, 178.48(13);
C1-N2-Re1, 180.0. Symmetry operation: (i) -xþ2, -xþyþ1,
-zþ2/3.
Interestingly, complex 9a or 9b could be prepared directly
from 1a or 1b by treatment with 2 equiv of IMes (Scheme 4).
The reaction proceeded via initial formation of 2a and 2b
with heterolytic cleavage of the η2-H2 ligand of 1a or 1b
induced by IMes as a strong base. HBr was removed with
mainly due to the optimization of d-π*CN orbital overlap
between rhenium and the acetonitrile. The CH3CN moiety in
8b is linear, with a Re-N-C angle of 180.00°. The
˚
Re-NCCH3 distance is 2.174(3) A and close to the average
21
˚
value of 2.098 A.
formation of the IMes HBr salt, which could be isolated and
3
In the presence of H2, the dihydrogen rhenium(I) hydride
complexes [Re(Br)(H)(NO)(PR3)2(η2-H2)] (R = Cy 6a, iPr
6b) could be generated in situ from 2a and 2b. However, upon
removal of the H2 atmosphere, 6a and 6b were immediately
transformed back into the starting materials 2a and 2b,
which also prevented isolation of 6a and 6b. The 1H NMR
proven by 1H NMR spectroscopy.
Similarly, 2a or 2b was reacted in a 1:1.2 molar ratio
with SIMes (SIMes = 1,3-bis(2,4,6-trimethylphenyl)-4,5-
dihydroimidazol-2-ylidene) in benzene at room tem-
perature, which gave purple solutions. The complexes
(21) Jacobsen, H.; Schmalle, H. W.; Messmer, A.; Berke, H. Inorg.
Chim. Acta 2000, 306, 153.
(22) (a) Heinekey, D. M.; Oldham, W. J. Chem. Rev. 1993, 93, 913. (b)
McGrady, G. S.; Guilera, G. Chem. Soc. Rev. 2003, 32, 383.

Article
Organometallics, Vol. 28, No. 18, 2009 5497
[Re(Br)(H)(NO)(PR3)(SIMes)] (R=Cy 10a, iPr 10b) were
separated as purple solids in 51% (10a) or 75% (10b) yield.
10a and 10b were fully characterized. The ν(NO) absorptions
of 10a and 10b were observed at 1636 (10a) and 1629 cm-1
(10b), respectively. The 31P{1H} NMR spectra showed sing-
letsatδ 32.5(10a) and 43.2 ppm (10b). Inthe 1H NMR spectra
the methyl signals of the SIMes ligand wereobserved atδ 2.58,
2.57, 2.12 ppm (10a) or 2.58, 2.43, 2.10 ppm (10b), respec-
tively. The signals for the dihydroimidazole ring protons
appeared as two multiplet resonances at δ 3.36, 3.24 ppm
(10a) and 3.44, 3.25 ppm (10b). The broad HRe signal of 10a
was observed at δ -16.15 ppm, and the HRe signal of 10b
appeared as a doublet at δ -15.50 ppm with 2J(HP)=14.8 Hz,
indicating the presence of only one phosphine ligand in the
coordination sphere.
The spectroscopically derived molecular structures of 10a
and 10b were confirmed by single-crystal X-ray diffraction
studies.23 Complex 10a adopts a square-pyramidal geometry
around the rhenium centers, with the hydride ligand occupying
apical positions, while 10b shows a trigonal-bipyrimid geome-
try, as shown in Figure 2 and Figure 3, respectively. In 10a the
bromide is approximately trans to the NO group
(Br1-Re1-N1average =174.9(2)o), as well as the SIMes and
the PCy3 ligands (C1-Re1-P1=171.73(6)o). From a struc-
tural comparison it became clear that the SIMes is sterically
more demanding than the PCy3 or the PiPr3 ligand. The
Figure 2. ORTEP drawing of the molecular structure of
[Re(Br)(H)(NO)(PCy3)(SIMes)] (10a). Thermal ellipsoids are
shown at the 20% probability level; the trans NO/Br disorder
and selected hydrogen atoms are omitted. Selected bond lengths
˚
(A) and angles (deg): Re1-H, 1.69(2); (Re1-N1)average
,
1.711(3); (Re1-Br1)average
(N1-O1)average 1.318(4); (O1-N1-Re1)average
(N1-Re1-Br1)average 174.9(2); H-Re1-N1a, 95.4(8);
,
2.4876(8); Re1-P1, 2.4307(5);
,
,
175.3(6);
,
P1-Re1-H, 89.2(7); H-Re1-Br1a, 87.0(7); C1-Re1-P(1),
171.73(6).
˚
average Re-NO bond distance of 10a (1.711(3) A) is signifi-
cantly shorter than that of 2a (1.746 A). This can be explained
˚
by the better σ-donation of SIMes than that of PCy3, resulting
in increased π-back-donation into the NO π* orbital of 10a
˚
and a longer NO bond distance in 10a (1.318(4) A) than in 2a
˚
˚
(1.258 A). The Re-P distance in 10a (2.4307(5) A) is slightly
shorter than that in 2a (2.4419 A).24 In 10b the SIMes and PiPr3
˚
groups are more bent toward the hydride ligand, with an
average C-Re-P angle of 167.9(1)o, which also accounts for
the small Br-Re-N angle (146.5(2)o) in 10b.
It should be noted at this point that still bulkier NHC
ligands, such as SIPr (SIPr=1,3-bis(2,6-isopropylphenyl)-
4,5-dihydroimidazol-2-ylidene), proved to be unreactive in
such ligand exchange reactions. Similar to the chemistry of
the bis(phosphine) complexes 2a and 2b, the vacant site of the
carbene complexes 9a and 9b can be occupied by less bulky
two-electron donors. For example, when solutions of 2a or
2b were treated with hydrogen at room temperature,
rhenium(I) IMes dihydrogen hydride complexes
[Re(Br)(H)(NO)(PR3)(IMes)(η2-H2)] (R=Cy 11a, iPr 11b)
were generated in situ in 100% yield, as indicated by NMR
spectroscopy. Complexes 11a and 11b are however stable
only under hydrogen atmosphere and will immediately
Figure 3. ORTEP drawing of the molecule structure of
[Re(Br)(H)(NO)(PiPr3)(SIMes)] (10b) (thermal ellipsoids are
drawn at the 30% probability level). Selected bond lengths
(A): N1a-O1a, 1.221(6); N1a-Re1, 1.721(5); P1-Re1,
2.4186(14); Br1-Re1, 2.5236(5); Re1-H1, 1.40(3). Selected
bond angles (deg): O1a-N1a-Re1, 173.7(6); C1-Re1-P1,
(23) X-ray crystal-structure data for 10a: purple crystals,
C39H60BrN3OPRe, M=883.98, crystal size 0.38 ꢀ 0.31 ꢀ 0.11 mm3,
˚
˚
monoclinic, space group P21/n; a=12.3151(1) A, b=17.3984(1) A, c=
˚
3
˚
˚
18.9751(2) A, R=90°, β=104.296(1), γ=90°, V=3939.76(6) A , Z=4,
F
calcd=1.490 Mg m-3, F(000)=1792, μ=4.172 mm-1, 54 723 reflections
3
168.42(12);
89.0(17); Br1-Re1-H1, 124.2(17).
N1a-Re1-Br1,
146.5(2);
N1a-Re1-H1,
(2θmax=61°), 14 324 unique (Rint=0.0460), 452 parameters, R1 (I >
2σ(I))=0.0265, wR2 (all data)=0.0585, GooF=0.992, largest difference
peak and hole: 1.816 and -0.823 e A-3. X-ray crystal-structure data for
˚
3
convert into the starting complexes 9a and 9b once they are
exposed to N2. The 1H NMR spectra of 11a and 11b,
recorded under H2 atmosphere, showed low-field-shifted
10b: purple crystals, C30H48BrN3OPRe, M=763.79, crystal size 0.38 ꢀ
3
˚
0.28 ꢀ 0.06 mm , triclinic, space group P1: a = 10.9984(4) A, b =
˚
˚
16.0869(5) A, c = 18.9301(4) A, R =103.099(2)°, β = 91.571(2)°, γ =
91.916(3) , V=3258.22(17) A , Z=4, Fcalcd=1.557 Mg m-3, F(000)=
o
3
˚
3
2
1528, μ=5.031 mm-1, 35 535 reflections (2θmax=61°), 13 300 unique (Rint
doublets for the hydride ligands (δ 4.63 ppm, J(HP)
=
2
=0.0511), 710 parameters, R1 (I > 2σ(I))=0.0377, wR2 (all data)=
26.2 Hz for 11a and 4.46 ppm, J(HP) =26.0 Hz for 11b) and
broad signals for the dihydrogen ligand (δ 4.08 ppm for 11a,
T1=44 ms, and 4.09 ppm for 11b, T1=59 ms). The 31P{1H}
NMR spectra showed singlets at δ30.4 (11a) and 40.2 ppm (11b).
0.07-973, GooF=0.952, largest difference peak and hole 1.666 and -1.568
˚
e A
.
(24) Lee, H. M.; Smith, D. C.; He, Z.; Stevens, E. D.; Yi, C. S.; Nolan,
3
S. P. Organometallics 2001, 20, 794.

5498 Organometallics, Vol. 28, No. 18, 2009
Jiang et al.
Scheme 5
Na[NMe2 BH3] in THF afforded immediately a yellow
3
solution accompanied by formation of a white precipitate.
The 11B NMR spectra indicated the presence of mainly
[Me2N-BH2]2 (δ 4.50 ppm, t, JBH=112 Hz) along with trace
amounts of Me2NdBH2 (δ 36.82 ppm, t, JBH=113 Hz) and
(Me2N)2BH (δ 28.07 ppm, d, JBH=134 Hz). The 1H NMR
and 31P{1H} NMR spectra indicated the formation of a
tetrahydride rhenium complex, [Re(H)4(NO)(PCy3)2] (14a),
in 43% spectroscopic NMR yield. Compound 14a was
known and was previously prepared via the reaction of the
hydroborate complex 13a with 2-propanol or H2.18c Re-
corded at room temperature the 1H NMR spectrum showed
in the hydride region two broad resonances at -1.71 and
-7.10 ppm in a ratio of 3:1. The 31P{1H} NMR spectra of the
resulting mixture displayed besides the typical signal at
44.6 ppm for 14a two other resonances at 51.6 and
28.6 ppm, which were assigned to two presumably isomeric
rhenium dihydride species. Further support for this assign-
ment came from the 1H NMR spectrum showing additional
broad signals in the chemical shift range from -8.0 to 0 ppm.
Reaction of [Re(Br)(H)(NO)(PR3)2] with n-BuLi. Treat-
ment of 2a or 2b with 1.5 equiv of n-BuLi afforded at room
temperature the n-butene-coordinated dihydride complexes
[Re(H)2(NO)(PR3)2(η2-n-CH2dCHC2H5)] (R=Cy 12a, iPr
12b), which were obtained as pure colorless oils in 78% (12a)
and 83% (12b) yield. The reaction presumably proceeded via
replacement of the bromide ligand with the n-butyl group to
establish a [Re(Bu)(H)(NO)(PR3)2] species, which is then
converted by β-H shift into the dihydride 12a or 12b
(Scheme 5). In the IR spectra the coordinated NO ligands
gave rise to ν(NO) bands at 1610 cm-1 (12a) and 1615 cm-1
(12b). The 31P{1H} NMR spectra of 12a and 12b displayed
Interestingly, by adding 1 equiv of Me2NH BH3 to this
3
mixture, the two dihydride species could be completely
transformed to 14a, which became then the unique organo-
metallic species in the reaction solution (100% NMR yield).
In the 11B NMR spectrum, the trimer [Me2N-BH2]3
(δ 1.23 ppm, t, JBH=96 Hz) was observed besides the dimer
[Me2N-BH2]2. Formation of the linear dimer Me2NH BH2-
3
signals with AB splitting patterns at 24.9, 21.6 ppm (2J(PP)
=
NMe2 BH3 was not detected.25 This indicated that the two
unknown dihydride species are not only active in the abstrac-
3
82 Hz) for 12a and 35.0, 33.7 ppm (2J(PP)=83 Hz) for 12b, in
agreement with the asymmetry induced by the alignment of
the n-butene ligand along the P-Re-P axis and hindered
rotation. In the 1H NMR spectra the two hydride ligands HA
(trans to NO) and HB (trans to n-butene) were observed as
doublets of triplets at -1.21 (Re-HA), and -5.24 ppm
(Re-HB) for 12a and -1.63 (Re-HA) and -5.89 ppm
(Re-HB) for 12b. Such doublets of triplet signals are in-
dicative of the given substitution pattern by both the J(PH)
couplings (38.5, 27.2 Hz for 12a and 38.0, 28.0 Hz for 12b)
and J(HH) couplings (7.5, 5.7 Hz for 12a and 6.9, 6.3 Hz for
12b). η2-Hydroborate complexes [Re(H)(NO)(PR3)2(η2-
BH4)] (R=Cy 13a, iPr 13b) can also be prepared from the
hydride 2 by treatment with NaBH4. For instance, the
reaction of 2a with 5 equiv of NaBH4 in THF-d8 solution
afforded complex 13a in 74% yield at 55 °C for 15 h. The
yield was determined on the basis of the integration of the
31P{1H} NMR spectrum. The 1H NMR spectroscopic data
were in accord with those reported.18c The relatively long
reaction time is most likely due to the poor solubility of
NaBH4 in THF.
tion of H2 from the added Me2NH BH3 but are in addition
3
mediating the conversion of [Me2N-BH2]2 into [Me2N-
BH2]3.
In fact, it turned out that the best route to prepare the
complexes 14a and 14b is the reaction of the dibromide
compounds 1a and 1b with Na[NMe2 BH3]. For example,
3
the reaction of 1b with 2 equiv of Na[NMe2 BH3] afforded
3
instantaneously [Re(H)4(NO)(PiPr3)2] (14b) in 99% spectro-
scopic yield. The 11B NMR spectrum indicated the forma-
tion of [Me2N-BH2]2 in 99% yield. The facile reaction can be
explained by the generation of hydrogen in the first step of
the reaction of 1a and 1b with 1 equiv of Na[NMe2 BH3],
3
which is then consumed in a subsequent step forming 14a and
14b by the reaction with one of the proposed isomeric
rhenium dihydride species (Scheme 6).
Dehydrocoupling of Me2NH BH3 and Transfer Hydroge-
3
nation of Olefins Using Me2NH BH3 As a Hydrogen Donor.
3
The catalytic activity of the various rhenium(I) hydrides was
tested with the catalytic reactions of Me2NH BH3 carried
3
out under comparable conditions (Scheme 7, Table 2). For
the dehydrocoupling reaction typically a solution of
Me2NH BH3 (12 mg, 0.02 mmol) in benzene (0.5 mL) was
Indeed, 12a or 12b could also be formed at room tempera-
ture by treatment of the dibromide complexes 1a or 1b with 3
equiv of n-BuLi. This reaction proceeded via the pri-
mary formation of the hydride species 2a or 2b, which
could be traced by NMR spectroscopy of the reaction
mixture. The intermediate n-butene-coordinated complexes
[Re(Br)(H)(NO)(PR3)2(η2-n-CH2dCHC2H5)] were spectro-
scopically detected in the reaction mixture.
3
treated with catalytic amounts of 2a (1.7 mg, 1.0 mol %) and
kept at 75 °C for 80 min. The 11B NMR spectrum indicated in
this case 100% conversion to the cyclic aminoborane dimer
[Me2N-BH2]2 (δ 4.50 ppm, t, JBH=112 Hz) with a TOF of 77
h
-1. This is a 3-fold enhancement in comparison with the
reactions catalyzed by the dibromide complex 1a.6 Rate
enhancement was also observed for the dehydrocoupling of
Reaction of [Re(Br)(H)(NO)(PR3)2] with Na[NMe2 BH3].
3
In order to gain more insight into the amine borane related
rhenium chemistry, we further examined the reaction be-
tween 2 and the sodium borane amide Na[NMe2 BH3],
the Me2NH BH3 compound using the hydride complex 2b as
3
a catalyst. The 18-electron rhenium(I) phosphine hydride
complexes [Re(Br)(H)(NO)(PR3)2(L)] (type 3 to 8) were also
3
which was prepared by treatment of Me2NH BH3 with
3
1.0 equiv of NaH in THF at room temperature. At room
temperature the reaction of 2a with 2.0 equiv of
€
(25) (a) Noth, H; Thomas, S. Eur. J. Inorg. Chem. 1999, 1373. (b)
Hahn, G. A.; Schaefer, R. J. Am. Chem. Soc. 1964, 86, 1503.

Article
Organometallics, Vol. 28, No. 18, 2009 5499
Scheme 6
Scheme 7
55 °C for over 15 h in THF solution. The most active hydride
species proved to be the IMes rhenium(I) hydrides 9a and 9b,
which catalyzed the dehydrocoupling of Me2NH BH3, ex-
3
hibiting TOFs of 100 and 92 h-1, respectively. This is nearly 4
times higher than the catalytic activity of the dibromide
complexes 1a and 1b in the same reaction. Surprisingly, the
rhenium(I) hydrides 10a and 10b, bearing the stronger
electron-donating carbene SIMes, showed an increase in
rates of only a factor of 2 with respect to 1a and 1b. These
values are even lower than those of the bis(phosphine)
hydrides 2a and 2b. In contrast to the unsaturated IMes,
the saturated SIMes carbene apparently is not only a stron-
ger σ-donor but a more efficient π-back-acceptor, which
makes the rhenium center a worse σ-acceptor, probably
resulting in weaker interaction with the B-H or N-H bond
Table 2. Dehydrocoupling of Me2NH BH3 Catalyzed by Re(I)
Complexesa
3
yield (%)b
TOF (/h)
in the substrate Me2NH BH3.26
entry
[Re]
time (h)
3
The most active catalyst among the prepared hydrides, 9a,
was also tested for the dehydrocoupling reaction of ammonia
1
2
3
4
5
6
7
8
1a
1b
2a
2b
9a
9b
10a
10b
12a
12b
13a
13b
1.0
4.0
1.3
1.5
1.0
1.0
3.0
2.0
1.0
1.0
1.0
1.0
11
96
100
83
100
92
99
88
47
28
<2
20
11
24
77
55
100
92
33
44
47
28
<2
20
borane NH3 BH3 but proved less active. At 65 °C, the THF
3
solution of NH3 BH3 afforded in the presence of 1.0 mol % of
3
9a a mixture of cyclotriborazane [NH2-BH2]3 (δ -11.1 ppm, t,
JBH = 109 Hz, 22% in situ yield) and borazine [NHdBH]3
(δ 29.8 ppm, d, JBH=122 Hz, 17% in situ yield) within 4 h.
We next probed the catalytic activity of rhenium(I) hy-
dride complexes in transfer hydrogenation of olefins using
9
10
11
12
Me2NH BH3 as a hydrogen donor. The activities of differ-
3
ent rhenium(I) hydrides were tested using n-octene as a
substrate (Table 3, entries 1-8). For example, reaction of
a 0.2 mmol of Me2NH BH3, 0.002 mmol of rhenium(I) complex, in
3
benzene (0.5 mL) at 75 °C. b Determined by integration of the 11B NMR
spectra.
n-octene with 1 equiv of Me2NH BH3 in the presence of
3
1.0 mol % of 2a led within 1 h at 75 °C to the formation of
octane in 99% yield. With respect to the related reaction
carried out with the dibromide 1a, the reaction rate was
found only slightly enhanced. Other rhenium(I) hydrides
were also tested as catalysts. Similar to the activity for the
tested as catalysts, but showed generally lower activities in
comparison with their unsaturated derivatives 2a and 2b (see
Supporting Information). The dioxygen-coordinated hy-
dride 3a showed no activity in the dehydrocoupling of
Me2NH BH3 due to a too strong coordination of the η2-
dehydrocoupling of Me2NH BH3, the bis(phosphine) hy-
3
3
dride complexes 2a and 2b and the IMes rhenium(I) hydrides
9a and 9b showed the highest activities, with a TOF of over
99 h-1. The SIMes rhenium(I) hydrides 10a and 10b, how-
ever, displayed lower activity than 1a and 1b.
O2 ligand, which prevents formation of a vacant site. The
butene-coordinated dihydride complexes 12a and 12b
(entries 9, 10) also showed enhanced activity in comparison
with that of 1a and 1b, but demonstrated lower activity than
the monohydride complexes 2a and 2b. The hydroborate
complexes 13a and 13b furnished rather poor catalytic
performance. 13a showed almost no activity in the dehydro-
coupling reaction (entries 11, 12). The in situ prepared
tetrahydride species 14a was also tested as a catalyst for
the dehydrocoupling reaction, but showed no activity at
(26) (a) Fantasia, S.; Petersen, J. L.; Jacobsen, H.; Cavallo, L.; Nolan,
S. P. Organometallics 2007, 26, 5880. (b) Hu, X.; Castro-Rodriguez, I.;
Olsen, K.; Meyer, K. Organometallics 2004, 23, 755. (c) Getty, K.; Delgado-
Jaime, M. U.; Kennepohl, P. J. Am. Chem. Soc. 2007, 129, 15774. (d) Hillier,
A. C.; Sommer, W. J.; Yong, B. S.; Petersen, J. L.; Cavallo, L.; Nolan, S. P.
Organometallics 2003, 22, 4322.

5500 Organometallics, Vol. 28, No. 18, 2009
Jiang et al.
Other olefins such as cyclooctene, vinylcyclohexane, and
ally(triethoxyl)silane have also been tested as substrates in
transfer hydrogenation reactions catalyzed by 2a or 9a
(Table 3, entries 9-20). Both catalysts performed in most
cases quite similarly. In the case of the sterically hindered
olefin R-methylstyrene (entries 17, 18), the NHC compound
9a displayed a distinctly higher activity than the bis-
(phosphine) compound 2a.
instance, when 2a was treated with 10 equiv of Me2NH BH3,
3
no reaction occurred at room temperature. After heating to
75 °C for 10 min, NMR spectroscopy indicated complete
formation of the dehydrocoupling product [Me2N-BH2]2
and the dihydrogen hydride complex 6a remained in solution
as the only organometallic species. Exposure of the solution
to vacuum led to full recovery of catalyst 2a. Complex 6a
apparently served as a catalytic precursor, since the dehy-
drocoupling reaction could be continued after addition of
In order to gain further insight into the reaction mechan-
ism, various exemplary experiments were carried out. For
another 10 equiv of Me2NH BH3 to the solution, demon-
3
strating the living nature of the catalyst. In contrast to the
reactions catalyzed by Ir, Ti, or Zr complexes,7,9 no deacti-
vation pathway of the catalyst proceeding with cleavage of
B-N bonds was observed in the reactions catalyzed by
hydride 2a. The poor activity of the borohydride complex
13 is also in accord with reported results.7
Table 3. Transfer Hydrogenation of Olefins Catalyzed by Re(I)
Complexes Using Me2NH BH3 as a Hydrogen Donora
3
entry
olefin
[Re] time (h) hydro yield (%)b TOF (/h)
1
2
3
4
5
6
7
8
n-octene
n-octene
n-octene
n-octene
n-octene
n-octene
n-octene
n-octene
cyclooctene
cyclooctene
vinylcyclohexane
vinylcyclohexane
cyclohexylvinyl ether 2a 2.0
cyclohexylvinyl ether 9a 2.0
4-methylcyclohexene 2a 1.2
4-methylcyclohexene 9a 1.2
R-methylstyrene
R-methylstyrene
ally(triethoxy)silane 2a 1.0
ally(triethoxy)silane 9a 1.0
1a 1.0
1b 1.0
2a 1.0
2b 1.0
9a 1.0
9b 1.0
10a 2.5
10b 4.0
2a 1.5
9a 1.5
2a 2.0
9a 2.0
84
75
99
100
99
100
95
100
87
88
93
95
90
99
99
99
65
96
98
99
84
75
99
100
99
100
38
25
58
59
46
47
45
50
83
83
32
48
98
99
In a similar way to the dehydrocoupling reaction, the
catalytic conversion of 10 equiv of n-octene and
Me2NH BH3 using 2a at 75 °C for 10 min allowed for
3
observation of transfer hydrogenation in 99% yield, and
complex 6a remained in solution as the main organometallic
species, as detected by NMR spectroscopy, along with a
small amount of 2a. The formation of 6a could be due to
9
10
11
12
13
14
15
16
17
18
19
20
some excess of Me2NH BH3 in solution, which underwent
3
dehydrocoupling, affording free H2.
In the cases of the 9a- and 9b-catalyzed dehydrocoupling
of Me2NH BH3 and the transfer hydrogenation of n-octene
3
using Me2NH BH3 as a hydrogen donor, the dihydrogen
3
hydride complexes 11a and 11b were finally present in both
catalytic processes as the only organometallic species in
solution. Exposure of the solution to vacuum led to full
recovery of the catalyst 9a or 9b. Both catalytic reactions
could be continued after the addition of additional amounts
of substrates to the completed solution, demonstrating the
living nature of catalysts 9a and 9b.
2a 2.0
9a 2.0
a 0.2 mmol of Me2NH BH3 and olefin, 0.002 mmol of rhenium(I)
3
complex, in benzene-d6 (0.5 mL) at 75 °C. b Hydrogenation yield was
determined by integration of the 1H NMR spectra.
Scheme 8

Article
Organometallics, Vol. 28, No. 18, 2009 5501
A mechanism for dehydrocoupling of Me2NH BH3 and
3
Schlenk techniques or in a glovebox (M. Braun 150B-G-II) filled
with dry nitrogen. Solvents were freshly distilled under N2 by
employing standard procedures and were degassed by freeze-
thaw cycles prior to use. The deuterated solvents were dried
with sodium/benzophenone (toluene-d8, benzene-d6, THF-d8)
and vacuum transferred for storage in Schlenk flasks fitted with
Teflon valves. 1H NMR, 13C{1H} NMR, and 31P{1H} NMR
data were recorded on a Varian Gemini-300, Varian Mercury
200, or Bruker DRX 500 spectrometers using 5 mm diameter
NMR tubes equipped with Teflon valves, which allow degassing
and further introduction of gases into the probe. Chemical
transfer hydrogenation of olefins using Me2NH BH3 as a
3
hydrogen donor is proposed in Scheme 8. The dehydro-
coupling of Me2NH BH3 is demonstrated in cycle A.
Me2NH BH3 coordinates initially to the vacant site of the
3
3
catalyst 2, leading to the σ-complex 15, which then oxida-
tively adds the amine borane to the rhenium center with
phosphine dissociation to form the Re(III) dihydride species
16. Then reductive elimination of hydrogen occurs, forming
the [Re-BH2NMe2H] species 17, which subsequently under-
goes β-H shift, affording initially the monomeric [Me2N-
BH2] species and 2. The catalytic cycle terminates at 2,
forming 6 as a resting state in the presence of hydrogen.
Another reaction cycle is also possible, which involves for-
mation of the σ-complex 150 bound through a Re-H-B
interaction, followed by N-H protonation of Re-H to give
H2, and finally B-H bond cleavage to generate the new
Re-H species and release Me2NdBH2. Similarly, the trans-
1
shifts are expressed in parts per million (ppm). H and 13C{1H}
NMR spectra were referenced to the residual proton or 13C
resonances of the deuterated solvent. All chemical shifts for the
31P{1H} NMR data are reported downfield in ppm relative
to external 85% H3PO4 at 0.0 ppm. Signal patterns are reported
as follows: s, singlet; d, doublet; t, triplet; m, multiplet. IR
spectra were obtained by using ATR methods with a Bio-Rad
FTS-45 FTIR spectrometer. Microanalyses were carried out at
the Anorganisch-Chemisches Institut of the University of
fer hydrogenation of olefins using Me2NH BH3 as a hydro-
Zurich. Me2NH BH3 and alkenes were purchased from Fluka;
NH3 BH3 was purchased from Aldrich and used without further
purification.
3
3
gen donor operates as demonstrated in cycle B. The olefin
adds to the vacant site of 2, giving the saturated species 18,
which undergoes intramolecular insertion into the Re-H
bond at higher temperature, producing the Re-alkyl species
3
Caution: When generating hydrogen gas in Young-tap NMR
tubes, the tubes should be vented for safety reasons after the
experiment.
20. Then oxidative addition of Me2NH BH3 to rhenium
3
Heterolytic Cleavage of the η2-H2 Bond in 1a or 1b by Various
Bases. In a 3 mL Young-tap NMR tube, 1a (7 mg, 0.01 mmol) or
1b (10 mg, 0.01 mmol) and an appropriate amount of base were
mixed in benzene (0.5 mL). The mixture was kept at a certain
temperature, and the yields of the formed 2a or 2b were
determined by 31P NMR spectroscopy (as seen in the Support-
ing Information).
occurs, leading to the Re(III) species 21, which undergoes
reductive elimination and formation of the hydrogenated
product and the same [Re-BH2NMe2H] species 17 as in cycle
A. Finally the monomeric [Me2N-BH2] species was released
from 17 via β-H shift, leading to 2 and closing up of the
catalytic cycle. However, a reaction path involving primary
N-H oxidative addition cannot be excluded. Nevertheless,
N-H reactivity seems less preferred for thermodynamic and
kinetic reasons, since N-H bonds are generally stronger (bond
dissociation energies: B-H 83 kcal/mol, N-H 104 kcal/mol)27
and show also less propensity for H-coordination.
[Re(Br)(H)(NO)(PCy3)2(η2-O2)] (3a). In a 30 mL Young-tap
Schlenk tube, [Re(Br)(H)(NO)(PCy3)2] (20 mg, 0.023 mmol)
was dissolved in 3 mL of hexane. The Young-tap Schlenk tube
was exposed to air for 10 s. The solution turned from violet to
light yellow with a precipitate forming. After stirring at room
temperature for 30 min, the supernatant solution was removed
and the residue was dried in vacuo. Yield: 17 mg, 83%. IR (ATR,
cm-1): ν(C-H) 2929, 2844, ν(Re-H) 2095, ν(NO) 1690,
Conclusion
1
ν(O-O) 869. H NMR (300.1 MHz, benzene-d6, ppm): δ 7.11
Five-coordinated rhenium(I) hydride complexes of the
type [Re(Br)(H)(NO)(PR3)2] (R=Cy 2a, iPr 2b) proved to
be robust precursors for the preparation of a wide variety of
organometallic species including the 18-electron hydride
complexes (3-8), dihydride species (12, 13), tetrahydride
species (14), and their NHC derivatives (9-11) and also
enabled exploration of effective homogeneous catalyses,
(t, 2J(PH)=40 Hz, 1H, Re-H), 1.23-2.81 (m, 66H, P(C6H11)3).
13C{1H} NMR (75.5 MHz, benzene-d6, ppm): δ 35.0 (t, J(PC)
=
12.8 Hz, P-CH), 29.6 (s), 29.4 (s), 27.9 (m), 26.8 (s). 31P{1H}
NMR (121.5 MHz, benzene-d6, ppm): δ 18.1 (s). Anal. Calcd for
C36H67BrNO3P2Re (889.98): C, 48.58; H, 7.59; N, 1.57. Found:
C, 48.27; H, 7.45; N, 1.69.
[Re(Br)(H)(NO)(PiPr3)2(η2-O2)] (3b). Similar to the synthesis
of 3a, compound 3b was obtained by exposure of the benzene
solution of [Re(Br)(H)(NO)(PiPr3)2] (10 mg, 0.016 mmol) to air.
Yield: 9 mg, 86%. IR (ATR, cm-1): ν(C-H) 2962, 2932, 2896,
ν(Re-H) 2096, ν(NO) 1687, ν(O-O) 873. 1H NMR (300.1
MHz, benzene-d6, ppm): δ 6.99 (t, 2J(PH)=39 Hz, 1H, Re-H),
2.80 (m, 6H, PCH(CH3)2), 1.21-1.32 (m, 36H, PCH(CH3)2).
13C{1H} NMR (75.5 MHz, benzene-d6, ppm): δ 25.4 (t, J=14
Hz, PCH(CH3)2), 19.1 (s, PCH(CH3)2), 18.7 (s, PCH(CH3)2).
31P{1H} NMR (80.95 MHz, benzene-d6, ppm): δ 27.4 (s). Anal.
Calcd for C18H43BrNO3P2Re (649.60): C, 33.28; H, 6.67; N,
2.16. Found: C, 33.64; H, 6.72; N, 2.08.
such as dehydrocoupling of Me2NH BH3 and transfer hy-
3
drogenation reactions of olefins using Me2NH BH3 as a
3
hydrogen donor. The five-coordinated complexes 2a, 2b, 9a,
and 9b turned out to perform best as catalysts for both types
of reactions. After the catalyses the catalysts survived as the
dihydrogen hydride complexes 6a, 6b, 11a, and 11b. A
plausible mechanistic cycle for both dehydrocoupling and
transfer hydrogenation was proposed, which implies the
ability of rhenium(I) complexes to effectively activate B-H
and N-H bonds by the facile redox interplay of Re(I) and
Re(III) species.
[Re(Br)(H)(NO)(PCy3)2(η2-C2H2)] (5a). In a 30 mL Young-
tap Schlenk tube, [Re(Br)(H)(NO)(PCy3)2] (34 mg, 0.04 mmol)
was dissolved in 5 mL of benzene. The nitrogen atmosphere
Experimental Section
was replaced with 640 mbar of acetylene by using
a
freeze-pump-thaw cycle. The mixture was stirred at room
temperature for 30 min, giving a light yellow solution. The
solvent was removed in vacuo. Yield: 30 mg, 85%. IR (ATR,
cm-1): ν(C-H) 2927, 2850, ν(Re-H) 2099, ν(η2-HCCH) 2014,
General Experimental Procedures. All manipulations were
performed under an atmosphere of dry nitrogen using standard
(27) (a) Nakajima, Y.; Kameo, H.; Suzuki, H. Angew. Chem., Int. Ed.
2006, 45, 950. (b) Martin, J. M. L.; Francois, J. P.; Gijbels, R. J. Chem. Phys.
1989, 91 (7), 4425.
1
ν(NO) 1655. H NMR (300.1 MHz, benzene-d6, ppm): δ 8.49
2
(m, 2H, η2-CHCH), 5.68 (t, J(PH) = 30 Hz, 1H, Re-H),

5502 Organometallics, Vol. 28, No. 18, 2009
Jiang et al.
1.26-2.69 (m, 66H, P(C6H11)3). 13C{1H} NMR (75.5 MHz,
benzene-d6, ppm): δ 101.0 (s, HCCH), 35.0 (t, J(PC) =12 Hz,
P-CH), 30.0, 29.7, 27.9, 27.7, 27.0. 31P{1H} NMR (121.5 MHz,
benzene-d6, ppm): δ 15.32 (s). Anal. Calcd for C38H69-
BrNOP2Re (884.02): C, 51.63; H, 7.87; N, 1.58. Found: C,
51.90; H, 8.01; N, 1.45.
PCH(CH3)2). 31P{1H} NMR (80.95 MHz, benzene-d6, ppm): δ
31.6 (s). Anal. Calcd for C19H43BrNO2P2Re (645.61): C, 35.35;
H, 6.71; N, 2.17. Found: C, 35.21; H, 6.62; N, 2.12.
[Re(Br)(H)(NO)(PCy3)2(CH3CN)] (8a). In a 20 mL vial in a
glovebox, [Re(Br)(H)(NO)(PCy3)2] (35 mg, 0.042 mmol) was
dissolved in 0.5 mL of benzene. Then 0.1 mL of CH3CN was
added dropwisely, and the color of the solution immediately
turned from violet to bright yellow. After 1 min, 15 mL of extra
CH3CN was added and a yellow precipitate was formed. The
supernatant solution was removed, and the precipitate was dried
in vacuo. Yield: 33 mg, 90%. IR (ATR, cm-1): ν(C-H) 2928,
2850, ν(Re-H) 2004, ν(NO) 1643. 1H NMR (300.08 MHz,
benzene-d6, ppm): δ 0.97-2.66 (m, 66H, P(C6H11)3), 1.86 (s,
[Re(Br)(H)(NO)(PiPr3)2(η2-C2H2)] (5b). Similar to the synth-
esis of 5a, compound 5b was obtained by the reaction of
[Re(Br)(H)(NO)(PiPr3)2] (7.8 mg, 0.013 mmol) with 880 mbar
of acetylene. Yield: 7.6 mg, 91%. IR (ATR, cm-1): ν(C-H)
2964, 2931, 2874, ν(Re-H) 2096, ν(η2-HCCH) 2018, ν(NO)
1649. 1H NMR (300.1 MHz, benzene-d6, ppm): δ 8.45 (m, 2H,
2
η2-CHCH), 5.11 (t, J(PH) =30 Hz, 1H, Re-H), 2.72 (m, 6H,
PCH(CH3)2), 1.16-1.33 (m, 36H, PCH(CH3)2). 13C{1H} NMR
3H, CH3CN), -1.53 (t, J(PH) =20 Hz, 1H, Re-H). 13C{1H}
2
(75.5 MHz, benzene-d6, ppm): δ 100.8 (s, CHCH), 25.0 (t, J(PC)
=
NMR (75.47 MHz, benzene-d6, ppm): δ 35.4 (t, J(PC)=10 Hz,
P-CH), 30.1, 30.0, 28.4, 27.6. 31P{1H} NMR (80.95 MHz,
benzene-d6, ppm): δ 16.57 (s). Anal. Calcd for C38H70-
BrN2OP2Re (898.37): C, 50.77; H, 7.85; N, 3.12. Found: C,
50.93; H, 7.98; N, 3.12.
11 Hz, PCH(CH3)2), 19.8 (s, PCH(CH3)2), 19.2 (s, PCH(CH3)2).
31P{1H} NMR (121.5 MHz, benzene-d6, ppm): δ 22.0 (s). Anal.
Calcd for C20H45BrNOP2Re (643.64): C, 37.32; H, 7.05; N, 2.18.
Found: C, 37.61; H, 7.11; N, 1.43.
In Situ Formation of [Re(Br)(H)(NO)(PCy3)2(η2-H2)] (6a). In
a 3 mL Young-tap NMR tube, [Re(Br)(H)(NO)(PCy3)2] (37 mg,
0.04 mmol) was dissolved in 0.5 mL of toluene-d8. The nitrogen
atmosphere was replaced with 1 bar of H2 by using a
freeze-pump-thaw cycle. After warming to room temperature,
the solution immediately turned from violet to light brown.
NMR spectroscopy indicated the formation of compound 6a.
Yield: 100% (based on the integral of the 31P NMR spectrum).
1H NMR (500.25 MHz, toluene-d8, ppm): δ 3.66 (t, 2J(PH)=26
Hz, 1H, Re-H, T1=239 ms), 3.46 (br, 2H, η2-H2, Δ=50 Hz,
T1 = 49 ms), 1.22-2.37 (m, 66H, P(C6H11)3). 13C{1H} NMR
(125.8 MHz, toluene-d8, ppm): δ 33.9 (t, J(PC)=12 Hz, P-CH),
29.9 (d, J(PC) = 4.5 Hz), 28.0 (m), 27.2. 31P{1H} NMR
(202.5 MHz, toluene-d8, ppm): δ 33.3 (s).
[Re(Br)(H)(NO)(PiPr3)2(CH3CN)] (8b). Similar to the synth-
esis of 8a, compound 8b was obtained by the reaction of
[Re(Br)(H)(NO)(PiPr3)2] (28 mg, 0.045 mmol) with CH3CN.
Yield: 20 mg, 67%. IR (ATR, cm-1): ν(Re-H) 2063, ν(NO)
1634. 1H NMR (300.08 MHz, benzene-d6, ppm): δ 2.78 (m, 6H,
PCH(CH3)2), 1.34-1.46 (m, 36H, PCH(CH3)2), 1.21 (s, 3H,
CH3CN), -1.82 (t, 2J(PH)=20 Hz, 1H, Re-H). 13C{1H} NMR
(75.47 MHz, benzene-d6, ppm): δ 25.6 (t, J(PC)=10 Hz, P-CH),
19.9 (s, PCH(CH3)2), 19.6 (s, PCH(CH3)2). 31P{1H} NMR
(80.95 MHz, benzene-d6, ppm): δ 26.91 (s). Anal. Calcd for
C20H46BrN2OP2Re (658.65): C, 36.47; H, 7.04; N, 4.25. Found:
C, 36.44; H, 6.99; N, 4.18.
[Re(Br)(H)(NO)(PCy3)(IMes)] (9a). In a 20 mL vial in glove-
box, [Re(Br)(H)(NO)(PCy3)2] (35 mg, 0.04 mmol) was dissolved
in 2 mL of benzene. Then IMes (15.0 mg, 0.05 mmol) was added,
and the solution immediately turned from violet to purple. After
stirring at room temperature for 2 h, the 31P{1H} NMR spec-
trum indicated that the starting material [Re(Br)(H)-
(NO)(PCy3)2] had been completely consumed and the free
PCy3 was liberated. The solvent was removed in vacuo. The
purple residue was washed with cold hexane (2 ꢀ 2 mL) and
again dried in vacuo. Yield: 26 mg, 71%. IR (ATR, cm-1):
ν(C-H) 2922, 2919, 2851, ν(Re-H) 2068, ν(NO) 1631. 1H
NMR (500.25 MHz, benzene-d6, ppm): δ 6.84 (s, 2H, meta-
CH), 6.78 (s, 2H, meta-CH), 6.24 (s, 2H, im-H), 2.43 (s, 6H,
para-CH3), 2.36 (s, 6H, ortho-CH3), 2.12 (s, 6H, ortho-CH3),
1.12-2.47 (m, 33H, P(C6H11)3), -18.18 (d, 2J(PH)=12 Hz, 1H,
Re-H). 13C{1H} NMR (125.8 MHz, benzene-d6, ppm): δ 138.4,
136.6, 136.0, 129.4 (s, meta-CH), 129.2 (s, meta-CH), 122.6
(s, im-CH), 36.2 (s, P-C), 36.0 (s, P-C), 30.9, 30.2, 28.1 (m),
27.0, 21.0 (s, ortho-CH3), 19.5 (s, ortho-CH3), 19.2 (s, para-CH3).
31P{1H} NMR (202.5 MHz, benzene-d6, ppm): δ 30.3 (s). Anal.
Calcd for C39H58BrN3OPRe (881.98): C, 53.11; H, 6.63; N, 4.76.
Found: C, 53.32; H, 6.80; N, 4.69.
[Re(Br)(H)(NO)(PiPr3)(IMes)] (9b). In a 20 mL vial in a
glovebox, [Re(Br)(H)(NO)(PiPr3)2] (35 mg, 0.05 mmol) was
dissolved in 2 mL of benzene. Then IMes (18 mg, 0.06 mmol)
was added and the solution immediately turned from violet to
purple. After stirring at room temperature for 90 min, the
31P{1H} NMR spectrum indicated that the starting material
[Re(Br)(H)(NO)(PiPr3)2] had been completely consumed and
free PiPr3 was formed. The solvent was evaporated in vacuo for 8
h. The purple residue was washed with cold hexane (1 ꢀ 2 mL)
and again dried in vacuo. Yield: 33 mg, 86%. IR (ATR, cm-1):
ν(C-H) 2955, 2916, 2865, ν(Re-H): 2095, ν(NO): 1626. 1H
NMR (500.25 MHz, benzene-d6, ppm): δ 6.85 (s, 2H, meta-CH),
6.78 (s, 2H, meta-CH), 6.25 (s, 2H, im-H), 2.50 (m, 3H, P-CH-
(CH3)2), 2.43 (s, 6H, para-CH3), 2.36 (s, 6H, ortho-CH3),2.12(s,6H,
ortho-CH3), 1.07 (m, 18H, PCH(CH3)2), -17.94 (br, 1H, Re-H).
13C{1H} NMR (125.8 MHz, benzene-d6, ppm): δ138.4, 136.6, 136.0,
129.3 (s, meta-CH), 129.2 (s, meta-CH), 122.6 (s, im-CH), 26.4
In Situ Formation of [Re(Br)(H)(NO)(PiPr3)2(η2-H2)] (6b). In
a 3 mL Young-tap NMR tube, [Re(Br)(H)(NO)(PiPr3)2] (12 mg,
0.02 mmol) was dissolved in 0.5 mL of toluene-d8. The nitrogen
atmosphere was replaced with 1 bar of H2 by using a
freeze-pump-thaw cycle. After warming to room temperature,
NMR spectroscopy indicated the formation of complex 6b.
Yield: 100% (based on the integral of 31P NMR spectrum). 1H
NMR (500.25 MHz, toluene-d8, ppm): δ 3.38 (br, 2H, η2-H2,
T1=87 ms), 3.29 (t, 2J(PH)=25 Hz, 1H, Re-H, T1=714 ms), 2.42
(m, 6H, PCH(CH3)2), 1.20 (m, 36H, PCH(CH3)2). 13C{1H}
NMR (125.8 MHz, toluene-d8, ppm): δ 24.3 (t, J(PC)=12 Hz,
PCH(CH3)2), 19.24 (s, PCH(CH3)2), 19.17 (s, PCH(CH3)2).
31P{1H} NMR (202.5 MHz, toluene-d8, ppm): δ 41.2 (s).
[Re(Br)(H)(NO)(PCy3)2(CO)] (7a). In a Young-tap NMR
tube, [Re(Br)(H)(NO)(PCy3)2] (20 mg, 0.024 mmol) was dis-
solved in 0.5 mL of benzene-d6. The nitrogen atmosphere was
replaced with 900 mbar of CO by using a freeze-pump-thaw
cycle. The color of the solution immediately turned from violet
to yellow. The solvent was removed in vacuo. Yield: 18 mg, 87%.
IR (ATR, cm-1): ν(C-H) 2929, 2852, ν(Re-H) 1999, ν(CO)
1982, ν(NO) 1680. 1H NMR (199.96 MHz, benzene-d6, ppm): δ
2
2.82 (t, J(PH) = 24 Hz, 1H, Re-H), 1.27-2.46 (m, 66H,
P(C6H11)3). 13C{1H} NMR (75.47 MHz, benzene-d6, ppm):
δ 209.6 (s, Re-CO), 34.4 (t, J(PC) = 12 Hz, P-CH), 29.9 (s),
29.7 (s), 28.1 (m), 27.2 (s). 31P{1H} NMR (80.95 MHz, benzene-
d6:acetone-d6 = 10:1, ppm): δ 22.7 (s). Anal. Calcd for
C37H68BrNO2P2Re (887.00): C, 50.10; H, 7.73; N, 1.58. Found:
C, 50.26; H, 7.99; N, 1.32.
[Re(Br)(H)(NO)(PiPr3)2(CO)] (7b). Similar to the synthesis of
7a, compound 7b was obtained by the reaction of
[Re(Br)(H)(NO)(PiPr3)2] (6.8 mg, 0.011 mmol) with 971 mbar
of CO. Yield: 6.4 mg, 90%. IR (ATR, cm-1): ν(C-H) 2963,
2932, 2872, ν(Re-H) 2095, ν(CO) 1995, ν(NO) 1678. 1H NMR
(199.96 MHz, benzene-d6, ppm): δ 2.50 (t, 2J(PH)=24 Hz, 1H,
Re-H), 2.50 (m, 6H, PCH(CH3)2), 1.20-1.31 (m, 36H, PCH-
(CH3)2). 13C{1H} NMR (75.47 MHz, benzene-d6, ppm): δ 24.6
(t, J(PC) = 12 Hz, PCH(CH3)2), 19.1 (s, PCH(CH3)2), 18.8 (s,

Article
Organometallics, Vol. 28, No. 18, 2009 5503
(s, PCH(CH3)2), 26.2 (s, PCH(CH3)2), 21.0 (s, ortho-CH3), 20.3
(s, PCH(CH3)2), 19.9 (s, PCH(CH3)2), 19.4 (s, ortho-CH3), 19.1
(s, para-CH3). 31P{1H} NMR (202.5 MHz, benzene-d6, ppm): δ41.5
(s).Anal.CalcdforC30H46BrN3OPRe (761.79): C, 47.30; H, 6.09; N,
5.52. Found: C, 47.52; H, 6.23; N, 5.65.
integral of 31P NMR spectrum). 1H NMR (500.25 MHz,
benzene-d6, ppm): δ 6.86 (s, 4H, meta-CH), 6.40 (s, 2H, im-H),
4.46 (d, 2J(PH)=26 Hz, 1H, Re-H, T1=544 ms), 4.05 (br, 2H, η2-
H2, T1 = 59 ms), 2.37 (m, 3H, P-CH(CH3)2), 2.33 (s, 12H,
ortho-CH3), 2.16 (s, 6H, ortho-CH3), 1.02 (m, 18H, P-CH-
(CH3)2). 13C{1H} NMR (125.8 MHz, benzene-d6, ppm): δ 140.1,
138.5, 136.7, 136.5, 129.4 (s, meta-CH), 129.3 (s, meta-CH),
122.5 (s, im-CH), 23.6, 23.3, 20.9, 18.8, 18.7. 31P{1H} NMR
(202.5 MHz, benzene-d6, ppm): δ 40.4 (s).
[Re(Br)(H)(NO)(PCy3)(SIMes)] (10a). In a 20 mL vial in a
glovebox, [Re(Br)(H)(NO)(PCy3)2] (43 mg, 0.05 mmol) was
dissolved in 2 mL of benzene. SIMes (17.1 mg, 0.05 mmol)
was then added, and solution was kept stirring at room tem-
perature for 15 h. The solvent was removed in vacuo, and the
purple residue was redissolved in 1 mL of hexane. After staying
at room temperature for 1 h, purple crystals precipitated out.
The solution was discarded, and the remaining crystals were
collected and again dried in vacuo. Yield: 23 mg, 51%. IR (ATR,
[Re(H)2(NO)(PCy3)2(η2-CH2dCHC2H5)] (12a). In a 3 mL
Young-tap NMR tube, [Re(Br)(H)(NO)(PCy3)2] (17.2 mg,
0.02 mmol) was dissolved in 0.5 mL of benzene. n-BuLi
(14 uL, 0.02 mmol, 1.6 M) in hexane was then added, and the
color of the solution turned from violet to light yellow within 1
min with a precipitate forming. The solvent was removed in
vacuo. The residue was extracted with hexane (2 ꢀ 2 mL) and
again dried in vacuo, giving a brown oily residue. Yield: 11 mg,
66%. IR (ATR, cm-1): ν(C-H) 2922, 2848, ν(Re-H) 1863,
1
cm-1): ν(C-H): 2912, 2846, ν(Re-H) 2072, ν(NO) 1636. H
NMR (500.25 MHz, benzene-d6, ppm): δ 6.87 (s, 2H, meta-CH),
6.81 (s, 2H, meta-CH), 3.39-3.43 (m, 2H, im-H), 3.22-3.26 (m,
2H, im-H), 2.58 (s, 6H, ortho-CH3), 2.57 (s, 6H, ortho-CH3),
2.12 (s, 6H, para-CH3), 1.08-2.39 (m, 33H, P(C6H11)3), -16.15
(br, 1H, Re-H). 13C{1H} NMR (125.8 MHz, benzene-d6, ppm):
δ 137.9, 136.5, 129.7 (s, meta-CH), 129.6 (s, meta-CH), 129.5 (s,
meta-CH), 51.5 (s, im-CH), 36.4 (s, P-CH), 36.2 (s, P-CH),
30.8, 30.3, 28.1 (m), 26.9, 21.0 (s, ortho-CH3), 19.5 (s, ortho-
CH3), 19.2 (s, para-CH3). 31P{1H} NMR (202.5 MHz, benzene-
d6, ppm): δ 32.5 (s). Anal. Calcd for C39H60BrN3OPRe (884.00):
C, 52.99; H, 6.84; N, 4.75. Found: C, 53.30; H, 6.98; N, 4.67.
[Re(Br)(H)(NO)(PiPr3)(SIMes)] (10b). In a 20 mL vial in a
glovebox, [Re(NO)(PiPr3)2(H)(Br)] (31.5 mg, 0.05 mmol) was
dissolved in 0.5 mL of benzene. Then SIMes (17.8 mg, 0.052
mmol) was added, and the solution was kept stirring at room
temperature for 18 h. The solvent was removed in vacuo. The
purple residue was washed with cold hexane (1 ꢀ 1 mL) and
again dried in vacuo. Yield: 29 mg, 75%. IR (ATR, cm-1):
ν(C-H): 2953, 2914, 2871, ν(Re-H): 2087, ν(NO) 1629. 1H
NMR (500.25 MHz, benzene-d6, ppm): δ 6.86 (s, 2H, meta-CH),
6.80 (s, 2H, meta-CH), 3.44 (m, J=6.8 Hz, 1H, im-H), 3.25 (m,
J=6.8 Hz, 1H, im-H), 2.58 (s, 12H, ortho-CH3), 2.43 (m, 3H,
PCH(CH3)2), 2.10 (s, 6H, para-CH3), 0.90-1.10 (m, 18H, PCH-
(CH3)2), -15.50 (d, J(H-P) = 14.8 Hz, 1H, Re-H). 13C{1H}
NMR (125.8 MHz, benzene-d6, ppm): δ 137.8, 137.7, 136.6,
129.7 (s, meta-CH), 129.6 (s, meta-CH), 51.5 (s, im-CH), 26.7
(s, P-CH(CH3)2), 26.3 (s, ortho-CH3), 21.0 (s, para-CH3), 20.2
(s, PCH(CH3)2), 19.9 (s, PCH(CH3)2). 31P{1H} NMR (202.5
1
ν(NO) 1610. H NMR (300.1 MHz, benzene-d6, ppm): δ 2.94
(m, 2H, CH2d), 2.61 (m, 1H, dCH-), 1.26-2.44 (m, 68H, CH2
3
and P(C6H11)3), 0.87 (t, J(HH)=7.2 Hz, 3H, CH3), -1.21 (dt,
2J(HH)=7.5 Hz, 2J(PH)=38.5 Hz, 1H, Re-H), -5.24 (dt, 2J(HH)
=
5.7 Hz, 2J(PH)=27.2 Hz, 1H, Re-H). 13C{1H} NMR (75.5 MHz,
benzene-d6, ppm): δ 40.4 (d, J(PC)=23.0 Hz), 39.7 (d, J(PC)=23.0
Hz), 33.1 (s, CH2d), 31.7 (m), 30.3 (d, J(PC)=9.0 Hz), 20.7 (d,
J
(PC) =18.7 Hz), 28.2 (m), 27.2 (s), 22.6 (s), 14.3 (s). 31P{1H}
NMR (121.5 MHz, benzene-d6, ppm): δ 24.9 (d, 3J=82 Hz, 1P),
21.6 (d, 3J=82 Hz, 1P). Unfortunately, elemental analysis could
not be obtained.
[Re(H)2(NO)(PiPr3)2(η2-CH2dCHC2H5)] (12b). In a 3 mL
Young-tap NMR tube, [Re(Br)(H)(NO)(PiPr3)2] (12.3 mg,
0.02 mmol) was dissolved in 0.5 mL of benzene. n-BuLi
(13 uL, 0.02 mmol, 1.6 M) in hexane was then added, and the
color of the solution turned from violet to light brown within 1
min with a precipitate forming. The solvent was removed in
vacuo. The residue was extracted with hexane (2 ꢀ 2 mL) and
dried again in vacuo, giving a light brown oily residue. Yield: 9.1
mg, 76%. IR (ATR, cm-1): ν(C-H): 2959, 2926, 2871,
1
ν(Re-H): 1825, ν(NO) 1615. H NMR (300.1 MHz, benzene-
d6, ppm): δ 2.76 (t, 3J(HH)=9.0 Hz, 2H, CH2d), 2.21-2.43 (m,
7H, dCH- and P-CH(CH3)2), 1.70 (m, 2H, CH2), 1.36 (t,
3J(HH)=6.9 Hz, 2H, CH3), 1.10 (m, 36H, P-CH(CH3)2), -1.63
2
2
(dt, J(HH)=6.9 Hz, J(PH)=38.0 Hz, 1H, Re-H), -5.89 (dt,
2J(HH)=6.3 Hz, 2J(PH)=28.0 Hz, 1H, Re-H). 13C{1H} NMR
(75.5 MHz, benzene-d6, ppm): δ 43.1 (s, CH2d), 33.1 (s,
MHz, benzene-d6, ppm): δ 43.2 (s). Anal. Calcd for
C30H48BrN3OPRe (763.81): C, 47.17; H, 6.33; N, 5.50. Found:
C, 47.29; H, 6.30; N, 5.40.
3
2
dCH-), 30.1 (dd, J(PH) = 3.6 Hz, J(PC) = 22.0 Hz, P-CH-
In Situ Formation of [Re(Br)(H)(NO)(PCy3)(IMes)(η2-H2)]
(11a). In a Young-tap NMR tube, [Re(Br)(H)(NO)(PCy3)-
(IMes)] (9 mg, 0.01 mmol) was dissolved in 0.5 mL of ben-
zene-d6. The nitrogen atmosphere was replaced with 950 mbar
of H2 by using a freeze-pump-thaw cycle. After warming to
room temperature, NMR spectroscopy indicated the formation
of a hydrogen-coordinated complex. Yield: 100% (based on the
(CH3)2), 29.2 (dd, J(PH) = 4.5 Hz, J(PC) = 22.0 Hz, P-CH-
(CH3)2), 23.1 (s), 20.6 (s), 20.0 (s), 19.6 (s). 31P{1H} NMR (121.5
MHz, benzene-d6, ppm): δ 35.0 (d, 3J=83 Hz, 1P), 37.0 (d, 3J=
83 Hz, 1P). Unfortunately, elemental analysis could not be
obtained.
3
2
Preparation of Na[NMe2 BH3]. In a 20 mL vial, Me2NH BH3
3
3
(80 mg, 1.33 mmol) and NaH (50 mg, 60% in mineral oil) were
mixed in 10 mL of THF. After stirring at room temperature for
20 min, visible hydrogen evolution had ceased. The solvent was
removed in vacuo, and the residue was quickly washed with 5 mL
of benzene and dried again in vacuo, giving a white product.
1
integral of the 31P NMR spectrum). H NMR (500.25 MHz,
benzene-d6, ppm): δ 6.86 (s, 4H, meta-CH), 6.38 (s, 2H, im-H),
4.63 (d, 2J(PH)=26 Hz, 1H, Re-H, T1=355 ms), 4.08 (br, 2H, η2-
H2, T1 =44 ms), 2.33 (s, 12H, ortho-CH3), 2.16 (s, 6H, para-
CH3), 1.16-2.23 (m, 33H, PCy3). 13C{1H} NMR (125.8 MHz,
benzene-d6, ppm): 140.1, 138.4, 136.6, 136.4, 129.5 (s, meta-CH),
129.4 (s, meta-CH), 122.6 (s, im-CH), 33.1 (s, P-CH), 32.8 (s,
P-CH), 29.4, 29.2, 27.7 (m), 26.7, 20.9 (s, para-CH3), 18.8 (s,
ortho-CH3), 18.7 (s, ortho-CH3). 31P{1H} NMR (202.5 MHz,
benzene-d6, ppm): δ 30.4 (s).
1
Yield: 62 mg, 58%. H NMR (199.96 MHz, THF-d8, ppm): δ
2.09 (s, 6H, CH3), 1.30 (q, J(BH)=86 Hz, 3H, BH3). 11B NMR
(64.15 MHz, THF-d8, ppm): δ -14.48 (q, J(BH)=85 Hz).
Catalytic Dehydrocoupling of Me2NH BH3 by Rhenium Hy-
dride Complexes. In a 30 mL Young-tap Schlenk tube,
Me2NH BH3 (12 mg, 0.20 mmol) and an appropriate amount
3
3
In Situ Formation of [Re(Br)(H)(NO)(PiPr3)(IMes)(η2-H2)]
(11b). In a Young-tap NMR tube, [Re(Br)(H)(NO)(PiPr3)-
(IMes)] (8 mg, 0.01 mmol) was dissolved in 0.5 mL of ben-
zene-d6. The nitrogen atmosphere was replaced with 950 mbar
of H2 by using a freeze-pump-thaw cycle. After warming to
room temperature, NMR spectroscopy indicated the formation
of a hydrogen-coordinated complex. Yield: 100% (based on the
of rhenium catalyst (0.002 mmol) were mixed in benzene-d6 (0.5
mL). The mixture was stirred at 75 °C for an appropriate
reaction time. The yield of [Me2N-BH2]2 was determined by
11B NMR spectroscopy.
Catalytic Transfer Hydrogenation Reaction of n-Octene Using
Me2NH BH3 as a Hydrogen Donor by Rhenium Hydride
Complex. In a 30 mL Young-tap Schlenk tube, Me2NH BH3
3
3

5504 Organometallics, Vol. 28, No. 18, 2009
Jiang et al.
(12 mg, 0.20 mmol), n-octene (32 μL, 0.2 mmol), and an
appropriate amount of rhenium catalyst (0.002 mmol) were
mixed in benzene-d6 (0.5 mL). The mixture was stirred at 75
°C for an appropriate reaction time. The yield of hydrogenated
product octane and dehydrocoupling product [Me2N-BH2]2 was
determined by 1H NMR and 11B NMR spectroscopy.
and the Funds of the University of Zurich are gratefully
acknowledged.
Supporting Information Available: CCDC-724862 (8b),
-724863 (10a), and -724864 (10b) contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif. This
material is also available free of charge via the Internet at http://
pubs.acs.org.
Acknowledgment. Support from the Swiss National
Science Foundation, the European COST Programme,