Dehydrogenation of saturated CC and BN bonds at cationic N-heterocyclic carbene stabilized M(III) centers (M = Rh, Ir)

Article abstract of DOI:10.1021/ja1043787

Chloride abstraction from the group 9 metal bis(N-heterocyclic carbene) complexes M(NHC)₂(H)₂Cl [M = Rh, Ir; NHC = IPr = N,N′-bis(2,6-diisopropylphenyl)imidazol-2-ylidene or IMes = N,N′-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene] leads to the formation of highly reactive cationic species capable of the dehydrogenation of saturated CC and BN linkages. Thus, the reaction of Ir(IPr)₂(H)₂Cl (1) with Na[BAr^f₄] in fluorobenzene generates [Ir(IPr)₂(H)₂]⁺[BAr^f ₄]^- (4) in which the iridium center is stabilized by a pair of agostic interactions utilizing the methyl groups of the isopropyl substituents. After a prolonged reaction period C-H activation occurs, ultimately leading to the dehydrogenation of one of the carbene ⁱPr substituents and the formation of [Ir(IPr)(IPr′′)(H) ₂]⁺[BAr^f₄]^- (5), featuring the mixed NHC/alkene donor IPr′′ ligand. By contrast, the related IMes complexes M(IMes)₂(H)₂Cl (M = Rh, Ir), which feature carbene substituents lacking β-hydrogens, react with Na[BAr ^f₄] in fluorobenzene to give rare examples of NaCl inclusion compounds, viz., [M(IMes)₂(H)₂Cl(Na)] ⁺[BAr^f₄]^- (M = Rh, 6; M = Ir, 7). Intercalation of the sodium cation between the mesityl aromatic rings of the two NHC donors has been demonstrated by crystallographic studies of 7. Synthetically, 6 and 7 represent convenient yet highly reactive sources of the putative 14-electron [M(NHC)₂(H)₂]⁺ cations, readily eliminating NaCl in the presence of potential donors. Thus 7 can be employed in the synthesis of the dinitrogen complexes [Ir(IMes) ₂(N₂)₂]⁺[BAr^f ₄]^- (8a) and [Ir(IMes)₂(N₂)THF] ⁺[BAr^f₄]^- (8b) (albeit with additional loss of H₂) by stirring in toluene under a dinitrogen atmosphere and recrystallization from the appropriate solvent system. The interactions of 6 and 7 with primary, secondary, and tertiary amineboranes have also been investigated. Although reaction with the latter class of reagent simply leads to coordination of the amineborane at the metal center via two M-H-B bridges {and formation, for example, of the 18-electron species [M(IMes)₂(H)₂(μ-H)₂B(H)NMe₃] ⁺[BAr^f₄]^- (M = Rh, 9; M = Ir, 10)}, the corresponding reactions with systems containing N-H bonds proceed via dehydrogenation of the BN moiety to give complexes containing unsaturated aminoborane ligands. Thus, for example, 6 catalyzes the dehydrogenation of R₂NHBH₃ (R = ⁱPr, Cy) in fluorobenzene solution (100% conversion over 6 h at 2 mol % loading) to give R₂NBH ₂; the organometallic complex isolated at the end of the catalytic run in each case is shown to be [Rh(IMes)₂(H)₂(μ-H) ₂BNR₂]⁺[BAr^f₄] ^- (R = ⁱPr, 11; R = Cy, 12). In contrast to isoelectronic alkene donors, the aminoborane ligand in these complexes (and in the corresponding iridium compounds 13 and 14) can be shown by crystallographic methods to bind in end-on fashion via a bis(s-borane) motif. Similar dehydrogenation chemistry is applicable to the primary amineborane ^tBuNH₂BH₃, although in this case the rate of rhodium-catalyzed dehydrogenation is markedly slower. This enables the amineborane complex [Rh(IMes)₂(H)₂(μ-H) ₂B(H)NH₂^tBu]⁺[BAr^f ₄]^- (15) to be isolated at short reaction times (ca. 6 h) and the corresponding (dehydrogenated) aminoborane system [Rh(IMes) ₂(H)₂(μ-H)₂BNH^tBu] ⁺[BAr^f₄]^- (16) to be isolated after an extended period (ca. 48 h). As far as further reactivity is concerned, aminoborane systems such as 14 show themselves to be amenable to further dehydrogenation chemistry in the presence of tert-butylethylene leading ultimately to the dehydrogenation of the boron-containing ligand and to the formation of a directly Ir-B bonded system described by limiting boryl (Ir-B) and borylene (Ir=B) forms.

Full text of DOI:10.1021/ja1043787

Published on Web 07/12/2010
Dehydrogenation of Saturated CC and BN Bonds at Cationic
N-Heterocyclic Carbene Stabilized M(III) Centers (M ) Rh, Ir)
Christina Y. Tang, Amber L. Thompson, and Simon Aldridge*
Inorganic Chemistry Laboratory, Department of Chemistry, UniVersity of Oxford, South Parks Road,
Oxford OX1 3QR, U.K.
Received May 20, 2010; E-mail: simon.aldridge@chem.ox.ac.uk
Abstract: Chloride abstraction from the group 9 metal bis(N-heterocyclic carbene) complexes M(NHC)2(H)2Cl
[M ) Rh, Ir; NHC ) IPr ) N,N′-bis(2,6-diisopropylphenyl)imidazol-2-ylidene or IMes ) N,N′-bis(2,4,6-
trimethylphenyl)imidazol-2-ylidene] leads to the formation of highly reactive cationic species capable of the
dehydrogenation of saturated CC and BN linkages. Thus, the reaction of Ir(IPr)2(H)2Cl (1) with Na[BArf4] in
fluorobenzene generates [Ir(IPr)2(H)2]+[BArf4]- (4) in which the iridium center is stabilized by a pair of agostic
interactions utilizing the methyl groups of the isopropyl substituents. After a prolonged reaction period C-H
i
activation occurs, ultimately leading to the dehydrogenation of one of the carbene Pr substituents and the
formation of [Ir(IPr)(IPr′′)(H)2]+[BArf4]- (5), featuring the mixed NHC/alkene donor IPr′′ ligand. By contrast, the
related IMes complexes M(IMes)2(H)2Cl (M ) Rh, Ir), which feature carbene substituents lacking ꢀ-hydrogens,
react with Na[BArf4] in fluorobenzene to give rare examples of NaCl inclusion compounds, viz.,
[M(IMes)2(H)2Cl(Na)]+[BArf4]- (M ) Rh, 6; M ) Ir, 7). Intercalation of the sodium cation between the mesityl
aromatic rings of the two NHC donors has been demonstrated by crystallographic studies of 7. Synthetically, 6
and 7 represent convenient yet highly reactive sources of the putative 14-electron [M(NHC)2(H)2]+ cations, readily
eliminating NaCl in the presence of potential donors. Thus 7 can be employed in the synthesis of the dinitrogen
complexes [Ir(IMes)2(N2)2]+[BArf4]- (8a) and [Ir(IMes)2(N2)THF]+[BArf4]- (8b) (albeit with additional loss of H2)
by stirring in toluene under a dinitrogen atmosphere and recrystallization from the appropriate solvent system.
The interactions of 6 and 7 with primary, secondary, and tertiary amineboranes have also been investigated.
Although reaction with the latter class of reagent simply leads to coordination of the amineborane at the metal
center via two M-H-B bridges {and formation, for example, of the 18-electron species [M(IMes)2(H)2(µ-
H)2B(H)·NMe3]+[BArf4]- (M ) Rh, 9; M ) Ir, 10)}, the corresponding reactions with systems containing N-H
bonds proceed via dehydrogenation of the BN moiety to give complexes containing unsaturated aminoborane
ligands. Thus, for example, 6 catalyzes the dehydrogenation of R2NH·BH3 (R ) iPr, Cy) in fluorobenzene solution
(100% conversion over 6 h at 2 mol % loading) to give R2NBH2; the organometallic complex isolated at the end
of the catalytic run in each case is shown to be [Rh(IMes)2(H)2(µ-H)2BNR2]+[BArf4]- (R ) iPr, 11; R ) Cy, 12).
In contrast to isoelectronic alkene donors, the aminoborane ligand in these complexes (and in the corresponding
iridium compounds 13 and 14) can be shown by crystallographic methods to bind in end-on fashion via a bis(σ-
borane) motif. Similar dehydrogenation chemistry is applicable to the primary amineborane tBuNH2 ·BH3, although
in this case the rate of rhodium-catalyzed dehydrogenation is markedly slower. This enables the amineborane
complex [Rh(IMes)2(H)2(µ-H)2B(H)·NH2 Bu]+[BArf4]- (15) to be isolated at short reaction times (ca. 6 h) and the
t
corresponding (dehydrogenated) aminoborane system [Rh(IMes)2(H)2(µ-H)2BNHtBu]+[BArf4]- (16) to be isolated
after an extended period (ca. 48 h). As far as further reactivity is concerned, aminoborane systems such as 14
show themselves to be amenable to further dehydrogenation chemistry in the presence of tert-butylethylene
leading ultimately to the dehydrogenation of the boron-containing ligand and to the formation of a directly Ir-B
bonded system described by limiting boryl (Ir-B) and borylene (IrdB) forms.
Introduction
chemical bonds and, in favorable cases, exploited in catalytic
processes for the functionalization of simple organic/inorganic
Electron-deficient late transition metal complexes stabilized,
for example, by labile intramolecular agostic or intermolecular
solvent molecule/counterion derived interactions1-4 have been
implicated in the coordination or activation of a range of
molecules.5–7 Within this area, low electron count complexes
of rhodium and iridium have been the subject of significant
interest, having been shown to be effective in both alkane
dehydrogenation and alkene hydrogenation chemistries.8,9
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9
10578 J. AM. CHEM. SOC. 2010, 132, 10578–10591
10.1021/ja1043787  2010 American Chemical Society

Dehydrogenation of Saturated CC and BN Bonds
A R T I C L E S
Highly unsaturated Rh(I) and Ir(I) complexes, both cationic
and charge-neutral, are often prone to side reactions involving
oxidative addition of ancillary C-H bonds;10 systems bearing
ꢀ-hydrogen atoms offer the additional possibility for further
steps leading to ligand dehydrogenation.11–14 The greater
resistance to oxidative addition of M(III) complexes, however,5,15
means that 14-electron systems of the type [(R3P)2M(H)2]+,
stabilized by secondary agostic interactions, are accessible from
related charge-neutral systems via halide abstraction; subse-
quently, such systems have been exploited in a number of a
catalytic dehydrogenation processes.10b,c,16 In recent work we
have been targeting analogous bis(NHC) ligated systems in the
hope of exploiting the very strong σ donor properties of the
NHC ligand class to stabilize M(III) complexes of the type
[(NHC)2M(H)2]+ (M ) Rh, Ir).17,18 A primary aim of this work
at the outset was the exploitation of such highly electrophilic
systems in the trapping of reactive species such as those
generated in the dehydrogenation of amineboranes.
The metal-catalyzed dehydrogenation of ammonia borane
(AB, H3N·BH3) and related primary and secondary amine
derivatives (RH2N·BH3 and R2HN·BH3) has been the subject
of significant recent interest, both with an ultimate aim of
unlocking the potential of such systems as hydrogen storage
media and with a view to developing new Group 13/15 materials
by dehydrocoupling approaches.19–21 Typically, in the absence
of significant steric bulk at R, the dehydrogenation of alkyl- or
dialkyl-amineboranes catalyzed by Group 9 metal systems leads
to the formation of oligo- or polymeric products.20 The
implication of metal complexes containing the unsaturated
aminoborane unit [i.e., LnM(H2BNR2)] in such processes has
been debated, but until recent preliminary reports by us and by
Sabo-Etienne,17,22 the structural characterization of these systems
had remained elusive. In targeting such complexes, the mono-
23,24
i
meric nature of R2NBH2 (R ) Pr, Cy)
led us to consider
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9
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A R T I C L E S
Tang et al.
[Rh(IMes)2(H)2(µ-H)2BNR2]+[BArf4]- (R ) iPr, 11; R ) Cy, 12);
[Ir(IMes)2(H)2(µ-H)2BNiPr2]+[BArf4]- (13), and [Rh(IMes)2(H)2(µ-
In this article, we therefore report on the reactivity of sources
of [(NHC)2M(H)2]+ (M ) Rh, Ir) toward R2HN·BH3 (R ) iPr,
Cy), together with comparative studies focusing on tertiary and
primary amineboranes, and on ammonia borane itself. Structural
characterization of a range of cationic rhodium and iridium
aminoborane complexes, together with that of a closely related
1,1-disubstituted alkene complex, allows direct experimental
comparison of the modes of binding of the formally isoelectronic
R2NBH2 and R2CCH2 ligand systems.
H)2B(H) ·NH2 Bu]+[BArf4]- (15) have been communicated by us
t
previously.17 NMR spectra were measured on a Varian Mercury
VX-300 or Bruker AVII 500 FT-NMR spectrometer. Residual
signals of solvent were used as reference for 1H and 13C NMR; 11
B
and 19F NMR spectra were referenced with respect to Et2O·BF3
and CFCl3, respectively. Infrared spectra were measured on a
Nicolet 500 FT-IR spectrometer. Elemental microanalyses were
carried out at London Metropolitan University. Abbreviations: br
) broad, s ) singlet, d ) doublet, q ) quartet, sept ) septet, m )
multiplet, IPr ) N,N′-bis(2,6-diisopropylphenyl)imidazol-2-ylidene,
IMes ) N,N′-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene, Dipp
)2,6-iPr2C6H3.
Experimental Section
General Considerations. Manipulations of air-sensitive reagents
were carried out under a nitrogen or argon atmosphere using
standard Schlenk line or drybox techniques. With the exception of
fluorobenzene (which was distilled from CaH2), nondeuterated
solvents were dried using a commercially available Braun solvent
purification system. C6D5CD3 (Goss) and 3,3-dimethyl-1-butene
were degassed and dried over potassium prior to use; CD2Cl2 (Goss)
was degassed and stored over molecular sieves. The known
compounds Na[BArf4], Ir(IPr)2(H)2Cl (1), and M(IMes)2(H)2Cl (M
) Rh, 2; M ) Ir, 3) were prepared by literature procedures.10a,11c,17,25
All other reagents were used as received from commercial sources.
Preliminary reports of the syntheses of [Ir(IPr)(IPr′′)(H)2]+[BArf4]-
(5), [M(IMes)2(H)2Cl(Na)]+[BArf4]- (M ) Rh, 6; M ) Ir, 7),
Syntheses. [Ir(IPr)2(H)2]+[BArf4]- (4). To a suspension of
Na[BArf4] (1.18 g, 1.33 mmol) in fluorobenzene (20 cm3) at -30
°C was added a solution of 1 (1.34 g, 1.33 mmol) also in
fluorobenzene (20 cm3) and the reaction mixture was warmed to
20 °C over a period of 1 h. After stirring for a further 3 h at 20 °C,
the resulting deep red solution was filtered and dried in vacuo, and
orange crystals suitable for X-ray diffraction were obtained by
1
concentration and storage at 0 °C. Isolated yield 1.59 g, 65%. H
NMR (C6D6, 500 MHz, 20 °C): δ -38.95 (s, 2H, IrH), 0.44 (d, J
) 6.6 Hz, 24H, CH3 of iPr), 0.86 (d, J ) 6.9 Hz, 24H, CH3 of iPr),
i
1.26 (overlapping m, 8H, CH of Pr), 6.20 (s, 4H, NCHCHN of
IPr), 6.86 (d, J ) 8.1 Hz, 8H, m-CH of IPr), 7.16 (t, J ) 6.6 Hz,
p-CH of IPr), 7.71 (s, 4H, p-CH of [BArf4]-), 8.32 (s, 8H, o-CH of
[BArf4]-). 13C NMR (C6D6, 126 MHz, 20 °C) (i) signals due to
cation: δ 23.3, 24.2 (CH3 of IPr), 34.5 (CH of IPr), 122.1
(NCHCHN of IPr), 130.7 (o-quaternary C of IPr), 137.3 (m-CH of
IPr), 145.1 (p-CH of IPr), 177.2 (br, carbene quaternary C of IPr),
N-bound aryl quaternary signals not observed; (ii) signals due to
anion, [BArf4]-: 118.0 (s, p-CH), 125.3 (q, J ) 275.0 Hz, CF3),
129.8 (q, J ) 31.5 Hz, m-quaternary C), 135.6 (s, o-CH of anion),
162.7 (q, J ) 48.0 Hz, ipso-quaternary C). 11B NMR (C6D5CD3,
96 MHz, 20 °C): δ -6.2. 19F NMR (C6D5CD3, 282 MHz, 20 °C):
δ -61.6. IR (cm-1): 2711 br [ν(agostic C-H)]. Anal. Calcd for
C86H86N4IrBF24: C, 56.27; H, 4.73 N, 3.05. Found: C, 56.15; H,
4.77; N, 2.92.
(20) For Group 9-based systems see, for example: (a) Jaska, C. A.; Temple,
K.; Lough, A. J.; Manners, I. J. Am. Chem. Soc. 2003, 125, 9424–
9434. (b) Jaska, C. A.; Manners, I. J. Am. Chem. Soc. 2004, 126,
9776–9785. (c) Jaska, C. A.; Clark, T. J.; Clendenning, S. B.; Grozea,
D.; Turak, A.; Lu, Z.-H.; Manners, I. J. Am. Chem. Soc. 2005, 127,
5116–5124. (d) Denney, M. C.; Pons, V.; Hebden, T. J.; Heinekey,
D. M.; Goldberg, K. I. J. Am. Chem. Soc. 2006, 128, 12048–12049.
(e) Fulton, J. L.; Linehan, J. C.; Autrey, T.; Balasubramanian, M.;
Chen, Y.; Szymczak, N. K. J. Am. Chem. Soc. 2007, 129, 11936–
11949. (f) Hebden, T. J.; Penney, M. C.; Pons, V.; Piccoli, P. M. B.;
Koetzle, T. F.; Schulz, A. J.; Kaminsky, W.; Goldberg, K. I.; Heinekey,
D. M. J. Am. Chem. Soc. 2008, 130, 10812–10820. (g) Dietrich, B. L.;
Goldberg, K. I.; Heinekey, D. M.; Autrey, T.; Linehan, J. C. Inorg.
Chem. 2008, 47, 8583–8585. (h) Douglas, T. M.; Chaplin, A. B.;
Weller, A. S. J. Am. Chem. Soc. 2008, 130, 14432–14433. (i) Staubitz,
A.; Soto, A. P.; Manners, I. Angew. Chem., Int. Ed. 2008, 47, 6212–
6215. (j) Rousseau, R.; Schenter, G. K.; Fulton, J. L.; Linehan, J. C.;
Engelhard, M. H.; Autry, T. J. Am. Chem. Soc. 2009, 131, 10516–
10524. (k) Douglas, T. M.; Chaplin, A. B.; Weller, A. S.; Yang, X.;
Hall, M. B. J. Am. Chem. Soc. 2009, 131, 15440–15456. (l) Sloan,
M. E.; Clark, T. J.; Manners, I. Inorg. Chem. 2009, 48, 2429–2435.
[Ir(IPr)(IPr′′)(H)2]+[BArf4]- (5). To a suspension of Na[BArf4]
(1.18 g, 1.33 mmol) in fluorobenzene (20 cm3) at -30 °C was added
a solution of 1 (1.34 g, 1.33 mmol) also in fluorobenzene (20 cm3)
and the reaction mixture was warmed to 20 °C over a period of
1 h. After further stirring for 12 h at 20 °C, the resulting deep
brown solution was filtered and concentrated in vacuo, and orange
crystals of 5 (as the pentane solvate) suitable for X-ray diffraction
were obtained by layering with pentane and storage at 20 °C.
Isolated yield: 1.89 g, 78%. 1H NMR (C6D5CD3, 300 MHz, 20 °C):
¨
(m) Zarmakiran, M.; Ozkar, S. Inorg. Chem. 2009, 48, 8955–8964.
(n) Dallanegra, R.; Chaplin, A. B.; Weller, A. S. Angew. Chem., Int.
Ed. 2009, 48, 6875–6878. (o) Chaplin, A. B.; Weller, A. S. Inorg.
Chem. 2010, 49, 1111–1121. (p) Chaplin, A. B.; Weller, A. S. Angew.
Chem., Int. Ed. 2010, 49, 581–584.
i
δ -33.98 (s, 2H, IrH), 0.41 (d, J ) 7.7 Hz, 3H, CH of Pr), 0.85
i
(d, J ) 7.0 Hz, 3H, CH of Pr), 0.88 (d, J ) 7.1 Hz, 12H, CH of
(21) For other metal systems see, for example: (a) Clark, T. J.; Russell,
C. A.; Manners, I. J. Am. Chem. Soc. 2006, 128, 9582–9583. (b)
Keaton, R. J.; Blacquiere, J. M.; Baker, R. T. J. Am. Chem. Soc. 2007,
129, 1844–1845. (c) Pun, D.; Lobhovsky, E.; Chirik, P. J. Chem.
Commun. 2007, 3297–3299. (d) Jiang, Y.; Berke, H. Chem. Commun.
2007, 3571–3573. (e) Pons, V.; Baker, R. T.; Szymczak, N. K.;
Heldebrant, D. J.; Linehan, J. C.; Matus, M. H.; Grant, D. J.; Dixon,
D. A. Chem. Commun. 2008, 6597–6599. (f) Blaquiere, N.; Diallo-
Garcia, S.; Gorelsky, S. I.; Black, D. A.; Fagnou, K. J. Am. Chem.
Soc. 2008, 130, 14034–14035. (g) Jiang, Y.; Blacque, O.; Fox, T.;
Frech, C. M.; Berke, H. Organometallics 2009, 28, 5493–5504. (h)
Friedrich, A.; Drees, M.; Schneider, S. Chem.sEur. J. 2009, 15,
10339–10342. (i) Ka¨ss, M.; Friedrich, A.; Drees, M.; Schneider, S.
Angew. Chem., Int. Ed. 2009, 48, 905–907. (j) Sloan, M. E.; Staubitz,
A.; Clark, T. J.; Russell, C. A.; Lloyd-Jones, G. C.; Manners, I. J. Am.
Chem. Soc. 2010, 132, 3831–3841.
i
iPr), 1.01 (d, J ) 7.9 Hz, 12H, CH of Pr), 1.08 (d, J ) 6.7 Hz,
12H, CH of iPr), 1.15 (s, 3H, CH3 of alkene), 1.25, 1.81 (br s, each
i
1H, CH of alkene), 2.10 (sept, J ) 7.9 Hz, 2H, CH of Pr), 2.15
i
i
(m, 1H, CH of Pr), 2.54 (sept, J ) 6.7 Hz, 2H, CH of Pr), 3.02
i
(sept, J ) 7.1 Hz, 2H, CH of Pr), 6.13, 6.42, 6.84, 7.01 (s, 4H,
NCHCHN of IPr and IPr′′), 6.87 to 7.24 (overlapping m, 12H,
aromatic CH of IPr and IPr′′), 7.69 (s, 4H, p-CH of [BArf4]-), 8.40
(s, 8H, o-CH of [BArf4]-). 13C NMR (C6D5CD3, 126 MHz, 20 °C)
(i) signals due to cation: δ 14.3, 22.7, 23.2, 23.3, 24.3, 25.5, 25.9,
26.7, 28.7, 28.8, 34.4, 53.1 (CH and CH3 of IPr and IPr′′; alkene
CH2 and alkene quaternary of IPr′′), 123.2, 123.9, 124.2, 124.7,
(22) Alcaraz, G.; Vendier, L.; Clot, E.; Sabo-Etienne, S. Angew. Chem.,
Int. Ed. 2010, 49, 918–920.
(26) (a) Cosier, J.; Glazer, A. M. J. Appl. Crystallogr. 1986, 19, 105. (b)
Denzo: Otwinowski, Z.; Minor, W. Methods in Enzymology; Carter,
C. W., Sweet, R. M., Eds.; Academic Press: New York, 1996; Vol.
276, p 307. (c) CrystalClear (Version 2.0); Rigaku: Americas, TX.
(d) Sir-92: Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi,
A.; Burla, M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994,
27, 435. (e) CRYSTALS: Betteridge, P. W.; Carruthers, J. R.; Cooper,
R. I.; Prout, J.; Watkin, D. J. J. Appl. Crystallogr. 2003, 36, 1487.
(23) Pasumansky, L.; Haddenham, D.; Clary, J. W.; Fisher, G. B.; Goralski,
C. T.; Singaram, B. J. Org. Chem. 2008, 73, 1898–1905.
(24) Euzenat, L.; Horhant, D.; Ribourdouille, Y.; Duriez, C.; Alcaraz, G.;
Vaultier, M. Chem. Commun. 2003, 2280–2281.
(25) Reger, D. L.; Wright, T. D.; Little, C. A.; Lamba, J. J. S.; Smith,
M. D. Inorg. Chem. 2001, 40, 3810.
9
10580 J. AM. CHEM. SOC. VOL. 132, NO. 30, 2010

Dehydrogenation of Saturated CC and BN Bonds
A R T I C L E S
126.4, 129.2, 130.6, 135.5, 137.9, 145.1, 146.4 (NCHCHN, aromatic
CH and aromatic quaternary C of IPr and IPr′′), 178.2, 184.9
(carbene quaternary C of IPr and IPr′′), N-bound aryl quaternary
signals not observed; (ii) signals due to [BArf4]- anion: 118.0 (s,
p-CH), 124.1 (q, J ) 275.0 Hz, CF3), 129.8 (q, J ) 33.1 Hz,
m-quaternary C), 135.5 (s, o-CH), 162.7 (q, J ) 48.0 Hz, ipso-
o-CH of anion), 162.3 (q, J ) 49.1 Hz, ipso-quaternary C). 11B
NMR (C6D5CD3, 96 MHz, 20 °C): δ -6.1. 19F NMR (C6D5CD3,
282 MHz, 20 °C): δ -62.5. IR (cm-1): 1987 br [ν(NtN)]. Anal.
Calcd for C74H60N8BF24Ir: C, 51.64; H, 3.52; N, 6.51. Found: C,
51.47; H, 3.37; N, 6.23.
Data for 8b. 1H NMR (C6D5CD3, 300 MHz, 20 °C): δ 1.52 (m,
4H, THF), 1.72 (s, 24H, o-CH3 of IMes), 2.19 (s, 12H, p-CH3 of
IMes), 3.39 (m, 4H, THF), 6.01 (s, 8H, NCHCHN of IMes), 6.70
(s, 4H, aromatic CH of IMes), 7.67 (s, 4H, p-CH of [BArf4]-), 8.32
(s, 8H, o-CH of [BArf4]-). 13C NMR (C6D5CD3, 126 MHz, 20 °C)
(i) signals due to cation: δ 17.6 (s, o-CH3 of IMes), 23.9 (para-
CH3 of IMes), 25.5, 68.1 (s, CH2 of THF), 123.1 (NCHCHN of
IMes), 129.7 (o-quaternary C of IMes), 135.5 (m-CH of IMes),
139.6 (p-quaternary C of IMes), 178.2 (carbene quaternary C of
IMes). (ii) signals due to anion: 118.0 (s, p-CH), 127.3 (q, J )
273.0 Hz, CF3), 129.9 (q, J ) 30.1 Hz, m-quaternary C), 134.7 (s,
o-CH), 162.8 (q, J ) 49.2 Hz, ipso-quaternary C). 11B NMR
(C6D5CD3, 96 MHz, 20 °C): δ -6.0. 19F NMR (C6D5CD3, 282
MHz, 20 °C): δ -58.0. IR (cm-1): 1985 [ν(NtN)]. Anal. Calcd
for C78H68N6BF24IrO: C, 53.07; H, 3.89; N, 4.76. Found: C, 52.98;
H, 3.65; N, 4.68.
quaternary C). 11B NMR (C6D5CD3, 96 MHz, 20 °C): δ -6.1. 19
F
NMR (C6D5CD3, 282 MHz, 20 °C): δ -61.4. Anal. Calcd for
C86H84N4BF24Ir: C, 56.36; H, 3.06 N, 4.62. Found: C, 56.25; H,
3.19; N, 4.52.
[M(IMes)2(H)2Cl(Na)]+[BArf4]- (M ) Rh, 6; M ) Ir, 7).
Compounds 6 and 7 were prepared by a common method given
here for 7. To a suspension of Na[BArf4] (1.06 g, 1.19 mmol) in
fluorobenzene (20 cm3) at -30 °C was added a solution of 2 (1.00
g, 1.19 mmol) also in fluorobenzene (20 cm3), and the reaction
mixture was warmed to 20 °C over a period of 1 h. After stirring
for a further 3 h at 20 °C, the resulting deep brown solution was
filtered and concentrated in vacuo, and orange crystals of 7 suitable
for X-ray diffraction were obtained by layering with pentane and
1
storage at 20 °C. Isolated yield 1.35 g, 72%. H NMR (C6D5CD3,
300 MHz, 20 °C): δ -34.21 (br, 2H, IrH), 1.85 (s, 24H, o-CH3 of
IMes), 2.13 (s, 12H, p-CH3 of IMes), 5.95 (s, 4H, NCHCHN of
IMes), 6.53 (s, 8H, aromatic CH of IMes), 7.64 (s, 4H, p-CH of
[BArf4]-), 8.32 (s, 8H, o-CH of [BArf4]-). 13C NMR (C6D5CD3,
126 MHz, 20 °C) (i) signals due to cation: δ 19.8 (br, o-CH3 of
IMes), 21.3 (p-CH3 of IMes), 121.5 (NCH of IMes), 124.8
(o-quaternary C of IMes), 127.7 (m-CH of IMes), 136.8 (p-
quaternary C of IMes), 172.3 (br, carbene quaternary of IMes),
N-bound aryl quaternary signal not observed; (ii) signals due to
[BArf4]- anion: 118.0 (s, p-CH), 125.2 (q, J ) 273.0 Hz, CF3),
129.9 (q, J ) 30.2 Hz, m-quaternary C), 135.5 (s, o-CH), 162.6 (q,
J ) 50.3 Hz, ipso-quaternary C). 11B NMR (C6D5CD3, 96 MHz,
20 °C): δ -6.1 ([BArf4]-). 19F NMR (C6D5CD3, 282 MHz, 20 °C):
δ -62.1. Anal. Calcd for C74H62N4ClBF24NaIr: C, 51.53; H, 3.62;
N, 3.25. Found: C, 51.52; H, 3.53; N, 3.21.
[M(IMes)2(H)2(µ-H)2B(H)·NMe3]+[BArf4]- (M ) Rh, 9; M
) Ir, 10). Compounds 9 and 10 were prepared by a common
method, illustrated here for compound 10. To a suspension of 7
(0.43 g, 0.25 mmol) in fluorobenzene (50 cm3) at -30 °C was added
excess Me3N·BH3 (0.37 g, 5.0 mmol), and the reaction mixture
was warmed to 20 °C over a period of 1 h. After stirring for a
further 6 h at 20 °C, the resulting pale yellow solution was filtered
and concentrated in vacuo, and very pale yellow crystals suitable
for X-ray diffraction were obtained by layering with pentane and
1
storage at 20 °C. Isolated yield 0.33 g, 75%. H NMR (C6D5CD3,
300 MHz, 20 °C): δ -21.80 (s, 2H, IrH), -2.28 (br, 3H, BH3),
1.53 (s, 9H, CH3 of NMe3), 1.63 (s, 24H, o-CH3 of IMes), 2.26 (s,
12H, p-CH3 of IMes), 6.04 (s, 4H, NCHCHN of IMes), 6.69 (s,
8H, aromatic CH of IMes), 7.71 (s, 4H, p-CH of [BArf4]-), 8.32
(s, 8H, o-CH of [BArf4]-). 13C NMR (C6D5CD3, 75 MHz, 20 °C)
(i) signals due to cation: δ 18.3 (o-CH3 of IMes), 51.1 (CH3 of
NMe3), 121.7 (NCHCHN of IMes), 134.7 (o-quaternary C of IMes),
136.9 (m-CH of IMes), 138.2 (p-quaternary C of IMes), 170.3 (br,
carbene quaternary C of IMes), p-CH3 of IMes obscured by toluene
solvent and N-bound aryl quaternary signals not observed; (ii)
signals due to anion: 117.6 (s, p-CH), 124.9 (q, J ) 272.9 Hz,
CF3), 129.5 (q, J ) 31.7 Hz, m-quaternary C), 135.5 (s, o-CH),
162.3 (q, J ) 50.3 Hz, ipso-quaternary C). 11B NMR (C6D5CD3,
96 MHz, 20 °C): δ -6.1 ([BArf4]-), 14.4 (br, Me3N·BH3). 19F NMR
(C6D5CD3, 282 MHz, 20 °C): δ -62.1. IR (cm-1): 2497, 2272,
2001 [ν(B-H)]. Anal. Calcd for C77H74N5B2F24Ir: C, 53.14; H, 4.29
N, 4.03. Found: C, 53.08; H, 4.32; N, 4.12.
1
Data for 6. Isolated yield 1.43 g, 68%. H NMR (C6D5CD3,
300 MHz, 20 °C): δ -35.24 (br, 2H, RhH), 1.93 (s, 24H, o-CH3
of IMes), 2.23 (s, 12H, p-CH3 of IMes), 6.19 (s, 4H, NCH of IMes),
6.70 (s, 8H, m-CH of IMes), 7.66 (s, 4H, p-CH of [BArf4]-), 8.32
(s, 8H, o-CH of [BArf4]-). 13C NMR (C6D5CD3, 126 MHz, 20 °C)
(i) signals due to cation: δ 18.7 (br, o-CH3 of IMes), 21.2 (p-CH3
of IMes), 121.8 (NCH of IMes), 124.7 (o-quaternary C of IMes),
127.5 (m-CH of IMes), 136.9 (p-quaternary C of IMes), 177.1 (br,
carbene quaternary of IMes), N-bound aryl quaternary signals not
observed; (ii) signals due to [BArf4]- anion: 118.1 (s, p-CH), 125.4
(q, J ) 273.1 Hz, CF3), 129.9 (q, J ) 32.2 Hz, m-quaternary C),
135.5 (s, o-CH), 162.9 (q, J ) 50.3 Hz, ipso-quaternary C). 11B
NMR (C6D5CD3, 96 MHz, 20 °C): δ -6.2. 19F NMR (C6D5CD3,
282 MHz, 20 °C): δ -62.3. Anal. Calcd for C74H62N4BClF24NaRh:
C, 54.35; H, 4.09; N, 3.43. Found: C, 54.30; H, 3.96; N, 3.51.
[Ir(IMes)2(N2)L]+[BArf4]- (L ) N2, 8a; L ) THF, 8b). The
two compounds were synthesized in closely related fashion. A
sample of 7 (0.21 g, 0.125 mmol) in toluene (20 cm3) was heated
to 80 °C for 5 h under an atmosphere of dinitrogen. The resulting
deep brown solution was filtered, volatiles were removed in vacuo,
and the resulting brown powder was recrystallized at 20 °C from
either fluorobenzene (to yield 8a) or THF/pentane (8b) as single
crystals suitable for X-ray diffraction. Isolated yields 81% (for 8a),
85% (for 8b). Data for 8a: 1H NMR (C6D5CD3, 300 MHz, 20 °C):
δ 1.64, (s, 24H, o-CH3 of IMes), 2.26 (s, 12H, p-CH3 of IMes),
5.96, (s, 4H, NCHCHN of IMes), 6.75 (s, 8H, aromatic CH of
IMes), 7.72 (s, 4H, p-CH of [BArf4]-), 8.35 (s, 8H, o-CH of
[BArf4]-). 13C NMR (C6D5CD3, 126 MHz, 20 °C) (i) signals due
to cation: δ 17.6 (o-CH3 of IMes), 22.5 (p-CH3 of IMes), 122.7
(NCHCHN of IMes), 129.7, (o-quaternary C of IMes), 135.8, (m-
CH of IMes), 139.3 (p-quaternary C of IMes), 172.3 (br, carbene
quaternary C of IMes), N-bound aryl quaternary signals not
observed; (ii) signals due to anion: 118.0 (s, p-CH), 125.4 (q, J )
273.0 Hz, CF3), 129.3 (q, J ) 30.1 Hz, m-quaternary C), 134.7 (s,
1
Data for 9. Isolated yield 0.28 g, 68%. H NMR (C6D5CD3, 300
MHz, 20 °C): δ -18.77 (d, 2H, J ) 27.0 Hz, RhH), -1.27 (br, 3H, BH3),
1.56 (s, 9H, CH3 of NMe3), 1.62 (s, 24H, o-CH3 of IMes), 2.25 (s, 12H,
p-CH3 of IMes), 6.03 (s, 4H, NCHCHN of IMes), 6.69 (s, 8H, aromatic
CH of IMes), 7.70 (s, 4H, p-CH of [BArf4]-), 8.32 (s, 8H, o-CH of
[BArf4]-). 13C NMR (C6D5CD3, 75 MHz, 20 °C) (i) signals due to cation:
δ 18.4 (o-CH3 of IMes), 22.0 (p-CH3 of IMes), 52.4 (CH3 of NMe3),
122.5 (NCHCHN of IMes), 135.0, (o-quaternary C of IMes), 137.0 (m-
CH of IMes), 138.7 (p-quaternary C of IMes), 184.3, (d, J ) 41.5 Hz,
carbene quaternary C of IMes), N-bound aryl quaternary signals not
observed; (ii) signals due to anion: 118.1 (s, p-CH), 125.3 (1 q, J ) 273.0
Hz, CF3), 129.9 (q, J ) 30.2 Hz, m-quaternary C), 135.5 (s, o-CH), 162.7
(q, J ) 49.8 Hz, ipso-quaternary C). 11B NMR (C6D5CD3, 96 MHz, 20
°C): δ -6.0 (BArf anion), 1.5 (br, Me3N·BH3). 19F NMR (C6D5CD3, 282
MHz, 20 °C): δ -62.2. IR (cm-1): 2475, 2175, 1998 [ν(B-H)]. Anal.
Calcd for C77H74N5B2F24Rh): C, 56.02; H, 4.52 N, 4.24. Found: C, 55.90;
H, 4.44; N, 4.18.
[Rh(IMes)2(H)2(µ-H)2BNR2]+[BArf4]- (R ) iPr, 11; R ) Cy, 12)
i
and [Ir(IMes)2(H)2(µ-H)2BNR2]+[BArf4]- (R ) Pr, 13; R ) Cy,
14). Compounds 11-14 were prepared from 6 or 7 by a common
method, illustrated here for compound 13. To a suspension of 7
9
J. AM. CHEM. SOC. VOL. 132, NO. 30, 2010 10581

A R T I C L E S
Tang et al.
(0.43 g, 0.25 mmol) in fluorobenzene (50 cm3) at -30 °C was added
[BArf4]-). 13C NMR (C6D5CD3, 75 MHz, 20 °C) (i) signals due to
cation: δ 17.8 (o-CH3 of IMes), 23.3 (p-CH3 of IMes), 23.5, 24.8,
25.9, 29.4, 29.6, 34.5 (CH2 of Cy), 55.8, 58.2 (CH of Cy) 122.1
(NCHCHN of IMes), 129.1 (o-quaternary C of IMes) 134.4 (meta-
CH of IMes), 138.6 (p-quaternary C of IMes), carbene quaternary
C of IMes and N-bound aryl quaternary signals not observed; (ii)
signals due to anion: 117.6 (s, p-CH), 124.9 (q, J ) 273.0 Hz,
CF3), 129.4 (q, J ) 31.7 Hz, m-quaternary C), 135.1 (s, o-CH),
162.2 (q, J ) 50.3 Hz, ipso-quaternary C). 11B NMR (C6D5CD3,
96 MHz, 20 °C): δ -6.1 (BArf4]-), 35.9 (br, H2BNCy2). 19F NMR
(C6D5CD3, 282 MHz, 20 °C): δ -61.2. IR (cm-1): 2236 br
[ν(B-H)]. Anal. Calcd for C86H86N5B2F24Ir: C, 55.52; H, 4.66, N,
3.77. Found: C, 55.24; H, 4.32; N, 3.66.
i
excess Pr2HN·BH3 (0.57 g, 5.0 mmol), and the reaction mixture
was warmed to 20 °C over a period of 1 h. After stirring for a
further 6 h at 20 °C, the resulting deep brown solution was filtered
and concentrated in vacuo, and yellow crystals of 13 suitable for
X-ray diffraction were obtained by layering with pentane and
storage at 20 °C. Isolated yield: 0.25 g, 56%. 1H NMR (C6D5CD3,
300 MHz, 20 °C): δ -15.50 (br, 2H, IrH), -5.83 (br, 2H, BH2),
i
0.75 (d, J ) 7.2 Hz, 12H, CH3 of Pr), 1.51 (s, 24H, o-CH3 of
IMes), 2.24 (s, 12H, p-CH3 of IMes), 2.79 (sept, J ) 7.2 Hz, 2H,
CH of iPr), 6.95 (s, 4H, NCHCHN of IMes), 6.65 (s, 8H, aromatic
CH of IMes), 7.71 (s, 4H, p-CH of [BArf4]-), 8.33 (s, 8H, o-CH of
[BArf4]-). 13C NMR (C6D5CD3, 75 MHz, 20 °C) (i) signals due to
i
t
cation: 14.3 (CH3 of Pr), 17.8 (o-CH3 of IMes), 23.4 (p-CH3 of
[Rh(IMes)2(H)2(µ-H)2B(H)·NH2 Bu]+[BArf4]- (15). To a sus-
IMes), 49.0 (CH of iPr), 122.1 (NCHCHN of IMes), 127.0
(o-quaternary C of IMes), 135.2 (m-CH of IMes), 138.7 (p-
quaternary C of IMes), 172.9 (br, carbene quaternary C of IMes)
N-bound aryl quaternary signal not observed; (ii) signals due to
[BArf4]- anion: 117.6 (s, p-CH), 123.1 (q, J ) 273.0 Hz, CF3),
129.6 (q, J ) 32.7 Hz, m-quaternary C), 134.3 (s, o-CH), 162.6 (q,
J ) 50.3 Hz, ipso-quaternary C). 11B NMR (C6D5CD3, 96 MHz,
20 °C): δ -6.0 ([BArf4]-), 37.9 (br, H2BNiPr2). 19F NMR (C6D5CD3,
282 MHz, 20 °C): δ -62.1. IR (cm-1): 2239 br [ν(B-H)]. Anal.
Calcd for C80H78N5B2F24Ir: C, 54.00; H, 4.42; N, 3.94. Found: C,
53.88; H, 4.33; N, 4.02.
pension of 6 (0.41 g, 0.25 mmol) in fluorobenzene (50 cm3) at -30
t
°C was added excess BuH2N·BH3 (0.44 g, 5 mmol), and the
reaction mixture was warmed to 20 °C over a period of 1 h. After
stirring for a further 6 h, the resulting deep brown solution was
filtered and concentrated in vacuo, and very pale yellow crystals
of 15 suitable for X-ray diffraction were obtained by layering with
pentane and storage at 20 °C. Isolated yield: 0.21 g, 51%. 1H NMR
(C6D5CD3, 300 MHz, 20 °C): δ -17.95 (d, 2H, J ) 29.7 Hz, RhH),
t
-1.66 (br, 3H, BH3), 0.49 (s, 9H, CH3 of Bu), 0.26, 0.43, 1.37,
2.07 (s, each 3H, p-CH3 of IMes), 1.58 (2 overlapping signals),
1.72, 2.14 (s, each 6H, ortho-CH3 of IMes), 5.76, 5.98 (s, each
2H, NCHCHN of IMes), 6.65, 6.67 (s, each 4H, aromatic CH of
IMes), 7.65(s, 4H, p-CH of [BArf4]-), 8.35(s, 8H, o-CH of [BArf4]-).
13C NMR (C6D5CD3, 126 MHz, 20 °C) (i) signals due to cation: δ
14.4 (CH3 of tBu), 17.1, 18.1, 21.0, 21.1, 22.8, 27.2 (2 overlapping
Data for 11. Isolated yield 0.27 g, 68%. 1H NMR (CD2Cl2, 300
MHz, 20 °C): δ -16.02 (br d, 2H, RhH), -1.40 (br, 2H, BH),
1.00 (d, J ) 6.6 Hz, 6H, CH3 of iPr), 1.01 (d, J ) 6.6 Hz, 6H, CH3
of iPr), 1.73 (s, 24H, o-CH3 of IMes), 2.42(s, 12H, p-CH3 of IMes),
i
t
3.14 (sept, J ) 6.6 Hz, 2H, CH of Pr), 6.90 (s, 8H, meta-CH of
signals), 34.1 (CH3 of IMes), 55.3 (quaternary C of Bu), 121.0,
IMes), 6.93 (s, 4H, NCH of IMes), 7.56 (s, 4H, p-CH of [BArf4]-),
7.73 (s, 8H, o-CH of [BArf4]-). 13C NMR (CD2Cl2, 76 MHz, 20
122.7 (NCHCHN of IMes), 127.8, 129.3 (o-quaternary C of IMes),
135.4 (2 overlapping signals, m-CH of IMes), 139.1, 140.2
(p-quaternary C of IMes), carbene quaternary of IMes and N-bound
aryl quaternary signal not observed; (ii) signals due to [BArf4]-
anion: 118.1 (s, p-CH), 125.3 (q, J ) 271.9 Hz, CF3), 129.7 (q, J
) 29.6 Hz, m-quaternary C), 134.6 (s, o-CH), 162.7 (q, J ) 50.2
Hz, ipso-quaternary C). 11B NMR (C6D5CD3, 96 MHz, 20 °C): δ
i
°C) (i) signals due to cation: δ 15.8 (CH3 of Pr), 18.8 (o-CH3 of
IMes), 22.1 (para-CH3 of IMes), 49.3 (CH of iPr), 121.9 (NCH of
IMes), 127.1 (o-quaternary C of IMes), 132.7 (meta-CH of IMes),
136.9 (p-quaternary C of IMes), 175.2 (br, carbene quaternary of
IMes), N-bound aryl quaternary signals not observed; (ii) signals
due to [BArf4]- anion: 115.4 (s, p-CH), 123.9 (q, J ) 272.1 Hz,
CF3), 127.1 (q, J ) 32.3 Hz, m-quaternary C), 134.3 (o-CH), 159.7
(q, J ) 49.8 Hz, ipso-quaternary C). 11B NMR (CD2Cl2, 96 MHz,
20 °C): δ -6.0 ([BArf4]-), 35.4 (br, H2BNiPr2). 19F NMR (CD2Cl2,
282 MHz, 20 °C): δ -62.1. IR (cm-1): 2049 br [ν(B-H)]. Anal.
Calcd for C80H78N5B2F24Rh: C, 56.86; H, 4.65 N, 4.14. Found: C,
56.74; H, 4.54; N, 4.09.
-6.2 ([BArf4]-), -35.1 (q, J ) 83.1 Hz, H3B·NH2 Bu). 19F NMR
t
(C6D5CD3, 282 MHz, 20 °C): δ -61.3. IR (cm-1): 2501, 1996,
1961 [ν(B-H)]. Anal. Calcd for C78H68N5B2F24Rh: C, 56.29; H,
4.06 N, 4.23. Found: C, 56.21; H, 4.13; N, 4.17.
[Rh(IMes)2(H)2(µ-H)2BNHtBu]+[BArf4]- (16). Prepared from
6 and a 20-fold excess of tBuH2N·BH3 in a similar manner to 15,
with the exception that the reaction mixture was left to stir for a
total of 48 h at 20 °C. The resulting fluorobenzene solution was
filtered, concentrated in vacuo, and intense yellow crystals suitable
for X-ray diffraction were obtained by layering with pentane and
1
Data for 12. Isolated yield 0.31 g, 71%. H NMR (C6D5CD3,
300 MHz, 20 °C): δ -15.90 (br d, J ) ca. 25 Hz, 2H, RhH), -1.74
(br, 2H, BH2), 0.88 to 2.51 (overlapping m, 22H, CH2 of Cy), 1.51
(s, 24H, o-CH3 of IMes), 2.24 (s, 12H, p-CH3 of IMes), 2.50 (m,
2H, CH of Cy), 5.96 (s, 4H, NCHCHN of IMes), 6.64 (s, 8H,
aromatic CH of IMes), 7.69 (s, 4H, p-CH of [BArf4]-), 8.33 (s,
8H, o-CH of [BArf4]-). 13C NMR (C6D5CD3, 126 MHz, 20 °C) (i)
signals due to cation: δ 17.6 (o-CH3 of IMes), 22.8 (p-CH3 of IMes),
25.3, 25.5, 26.2, 26.8, 34.9 35.2 (CH2 of Cy), 53.3, 60.6 (CH of
Cy) 122.0 (NCHCHN of IMes), 129.5 (o-quaternary C of IMes)
135.5 (meta-CH of IMes), 139.1 (p-quaternary C of IMes),176.5
(br, carbene quaternary C of IMes), N-bound aryl quaternary signals
not observed; (ii) signals due to [BArf4-] anion: 117.9 (s, p-CH),
125.8 (q, J ) 272.9 Hz, CF3), 130.1 (q, J ) 30.3 Hz, m-quaternary
C), 134.7 (s, o-CH), 162.6 (q, J ) 49.6 Hz, ipso-quaternary C).
11B NMR (C6D5CD3, 96 MHz, 20 °C): δ -6.0 (BArf4]-), 35.8 (br,
H2BNCy2). 19F NMR (C6D5CD3, 282 MHz, 20 °C): δ -62.2. IR
(cm-1): 2040 br [ν(B-H)]. Anal. Calcd for C86H86N5B2F24Rh: C,
58.35; H, 4.90 N, 3.98. Found: C, 58.28; H, 4.85; N, 3.90.
1
storage at 20 °C. Isolated yield 0.32 g, 72%. H NMR (C6D5CD3,
300 MHz, 20 °C): δ -21.69 (d, J ) 51 Hz, 2H RhH), -2.62 (br,
2H, BH2), 0.52 (s, 9H, CH3 of tBu), 1.63 (s, 24H, o-CH3 of IMes),
2.22 (s, 12 H, p-CH3 of IMes), 5.97 (s, 4H, NCHCHN of IMes),
6.58 (s, 8H, aromatic CH of IMes), 7.70 (s, 4H, p-CH of [BArf4]-),
8,34 (s, 8H, o-CH of [BArf4]-). 13C NMR (C6D5CD3, 126 MHz,
20 °C) (i) signals due to cation: δ 18.5 (o-CH3 of IMes), 23.2 (p-
CH3 of IMes), 31.0 (CH3 of tBu), 56.4 (quaternary C of tBu), 122.4,
(NCHCHN of IMes), 129.4 (o-quaternary C of IMes), 135.0 (m-
CH of IMes), 139.5 (p-quaternary C of IMes), carbene quaternary
of IMes and N-bound aryl quaternary signal not observed; (ii)
signals due to [BArf4]- anion: 118.4 (s, p-CH), 125.6 (q, J ) 273.0
Hz, CF3), 130.3 (q, J ) 33.9 Hz, m-quaternary C), 135.9 (s, o-CH),
163.0 (q, J ) 50.3 Hz, ipso-quaternary C). 11B NMR (C6D5CD3,
96 MHz, 20 °C): δ -6.1 ([BArf4]-), 37.4 (br, H2BNHtBu). 19F NMR
(C6D5CD3, 282 MHz, 20 °C): δ -61.9. IR (cm-1): 2171 [ν(B-H)].
Anal. Calcd for C78H74N5B2F24Rh: C, 56.34; H, 4.49 N, 4.21. Found:
C, 56.43; H, 4.38; N, 4.23.
1
Data for 14. Isolated yield 0.32 g, 69%. H NMR (C6D5CD3,
300 MHz, 20 °C): δ -15.44 (br s, 2H, IrH), -5.83 (br s, 2H, BH2),
0.87 to 2.64 (overlapping m, 22H, CH2 of Cy), 1.52 (s, 24H, o-CH3
of IMes), 2.17(s, 12H, p-CH3 of IMes), 2.48 (overlapping m, 2H,
CH of Cy), 5.96 (s, 4H, NCHCHN of IMes), 6.65 (s, 8H, aromatic
CH of IMes), 7.70 (s, 4H, p-CH of [BArf4]-), 8.32 (s, 8H, o-CH of
[Ir(IMes)(IMes′)(H)(µ-H)2BNCy2]+[BArf4]- (17). To a suspen-
sion of 7 (0.43 g, 0.25 mmol) in fluorobenzene (50 cm3) at -30
°C was added excess Cy2HN ·BH3 (0.98 g, 5 mmol), and the
reaction mixture was warmed to 20 °C over a period of 1 h. After
9
10582 J. AM. CHEM. SOC. VOL. 132, NO. 30, 2010

Dehydrogenation of Saturated CC and BN Bonds
A R T I C L E S
stirring for a further 6 h at 20 °C, 3,3-dimethylbutene (TBE; ca.
10 equiv) was added, and the reaction was allowed to continue for
a further 6 h. The resulting light brown solution was filtered and
concentrated in vacuo, and yellow crystals suitable for X-ray
diffraction were obtained by layering with pentane and storage at
20 °C. Isolated yield 0.29 g, 63%. 1H NMR (C6D5CD3, 300 MHz,
20 °C): δ -15.43 (s, 1H, IrH), -10.88 (br s, 1H, BH), -5.81 (br
s, 1H, BH), 0.85 to 2.50 (overlapping m, 20H, CH2 of Cy), 1.51
(s, 12H, o-CH3 of IMes), 1.61, 1.71, 1.85 (s, each 3H, o-CH3 of
IMes′), 1.92 (br, 2H, CH2Ir), 2.09, 2.14 (s, each 3H, p-CH3 of
IMes′), 2.24 (s, 6H, p-CH3 of IMes), 2.46 (overlapping m, 2H, CH
of Cy), 5.96 (s, 2H, NCHCHN of IMes), 6.01, 6.07 (br s, each 1H,
NCHCHN of IMes′), 6.56 (s, 2H, aromatic CH of IMes′), 6.65 (s,
4H, aromatic CH of IMes), 6.69, 6.75 (s, each 1H, aromatic CH of
IMes′), 7.70 (s, 4H, p-CH of [BArf4]-), 8.32 (s, 8H, o-CH of
[BArf4]-). 13C NMR (C6D5CD3, 75 MHz, 20 °C) (i) signals due to
cation: δ 17.3, 17.8, 17.9, 18.1 (o-CH3 of IMes and IMes′), 24.5,
24.8, 25.2 (p-CH3 of IMes and IMes′), 25.3, 25.9, 32.0, 32.8, 33.5
34.5, (CH2 of Cy), 53.6 (CH2Ir), 55.3, 58.2 (CH of Cy), 121.0,
122.0, 122.6 (NCHCHN of IMes and IMes′), 129.7, 130.3, 130.5,
131.4, 131.6 (o-quaternary C of IMes and IMes′), 134.4, 134.5,
135.2, 135.4, 136.6 (m-CH of IMes and IMes′), 138.6, 139.2, 140.1
(p-C of IMes and IMes′), carbene quaternary of IMes/IMes′ and
N-bound aryl quaternary signals not observed; (ii) signals due to
anion: 117.5 (s, p-CH), 124.9 (q, J ) 273.0 Hz, CF3), 129.9 (q, J
) 30.2 Hz, m-quaternary C), 135.1 (s, o-CH), 162.3 (q, J ) 48.3
Hz, ipso-quaternary C). 11B NMR (C6D5CD3, 96 MHz, 20 °C): δ
-6.0 ([BArf4]-), 36.2 (br, H2BNCy2). 19F NMR (C6D5CD3, 282
MHz, 20 °C): δ -61.2. IR (cm-1): 2256 [ν(B-H)]. Anal. Calcd
for C86H84N5B2F24Ir: C, 55.58; H, 4.56, N, 3.77. Found: C, 55.50;
H, 4.45; N, 3.64.
DENZO-SMN package, and reflection intensities were corrected
for absorption effects by the multiscan method, based on multiple
scans of identical and Laue equivalent reflections.26b Data for
[Rh(IMes)2(H)2(THF)]+[BArf4]- were collected on Beamline I19
(EH1) at the Diamond Light Source, and raw frame data were
processed using CrystalClear.26c All structures were solved using
SIR92, and refinement was carried out using full-matrix least-
squares within the CRYSTALS suite,26d,e on either F2 or F (the
latter for [Rh(IMes)2(H)2(THF)]+[BArf4]- only). In general, all non-
hydrogen atoms were refined with anisotropic displacement pa-
rameters. The nature of the [BArf4]- anion means that all structures
exhibited disordered CF3 groups, the most extreme examples of
which were modeled using two rotationally offset components with
distances and thermal parameters controlled using “same distance”
and rigid bond restraints. In one or two cases, out of plane motion
was also evident and the model was adjusted accordingly. For
compound 8a it was evident that one of the dinitrogen sites was
partially occupied by other components and a 3-fold disordered
model was used; similar disorder was observed in
[Rh(IMes)2(H)2(THF)]+[BArf4]-. Compounds 9, 14, and 16 also
exhibit disorder in the amineborane ligand; in the cases of 9 and
16, the orientational disorder affected the whole ligand, whereas
in 14 only the cyclohexyl groups were affected. In each case the
disorder was modeled appropriately and the components restrained
to give similar geometry and thermal behavior.
In general, hydrogen atoms were visible in the difference map,
but alkyl/aromatic hydrogen atoms were positioned geometrically,
and their positions and isotropic displacement parameters were
refined using restraints prior to inclusion into the model with riding
constraints. Other hydrogen atoms were located by examination of
the difference Fourier map. In cases where this was challenging,
the contribution to the scattering from the hydrogen atoms was
increased by truncating the data to ca. 1.1 Å, which had the effect
of enhancing peaks due to the presence of hydrogen atoms. Once
located in this way, hydrogen atoms were generally then treated as
described above; exceptions to this procedure were in the case of
the disordered components in compounds 8a, 16, and
[Rh(IMes)2(H)2(THF)]+[BArf4]-, where it was necessary to place
some hydrogen atoms at sensible geometric positions. For riding
hydrogen atoms, standard uncertainties were calculated from the
final refinement using the full variance covariance matrix for
distances where this was applicable.
[Ir(IMes)(IMes′)(H)BNCy2]+[BArf4]- (18). A suspension of 17
(0.19 g, 0.10 mmol) in toluene (50 cm3) was heated at 65 °C with
vigorous stirring for a period of 36 h. The reaction mixture was
concentrated and layered with pentane, yielding 18 as bright yellow
crystals suitable for X-ray diffraction. Isolated yield 0.15 g, 82%.
1H NMR (C6D5CD3, 300 MHz, 20 °C): δ -10.85 (s, fwhm 43 Hz,
1
fwhm H{11B} 12 Hz, 1H, IrHB), 0.45 to 1.80 (overlapping m,
20H, CH2 of Cy), 1.16, 1.88 (s, each 3H, o-CH3 of IMes′), 2.00,
2.09 (s, each 6H, o-CH3 of IMes), 2.28 (s, 3H, o-CH3 of IMes′),
1.75 (d J ) 12 Hz, 1H, CH2Ir), 2.34 (s, 3H, p-CH3 of IMes′), 2.36
(s, 6H, p-CH3 of IMes), 2.39 (s, 3H, p-CH3 of IMes′), 2.39 (d J )
12 Hz, 1H, CH2Ir), 2.50 (overlapping m, 2H, CH of Cy), 6.81,
6.84 (br s, each 1H, NCHCHN of IMes′), 6.91 (br s, 4H, aromatic
CH of IMes), 7.00 (br s, 2H, aromatic CH of IMes′), 7.03 (s, 2H,
NCHCHN of IMes), 7.06 (d J ) 2 Hz, 1H, aromatic CH of IMes′),
7.61 (d J ) 2 Hz, 1H, aromatic CH of IMes′), 7.58 (s, 4H, p-CH
of [BArf4]-), 7.74 (s, 8H, o-CH of [BArf4]-). 13C NMR (CD2Cl2,
75 MHz, 20 °C) (i) signals due to cation: δ 17.3, 17.6, 18.0, 18.4,
18.9 (o-CH3 of IMes and IMes′), 20.7, 21.1, 21.7 (p-CH3 of IMes
and IMes′), 25.0, 25.3, 30.4, 30.6, 32.3, 33.4, (CH2 of Cy), 34.2
(CH2Ir), 55.3, 58.2 (CH of Cy), 116.9, 117.7, 118.0 (NCHCHN of
IMes and IMes′), 128.7, 129.3, 129.8, 130.1, 130.4 (o-quaternary
C of IMes and IMes′), 134.4, 134,7, 134.8, 135.2, 135.5 (m-CH of
IMes and IMes′), 139.3, 139.3, 140.9 (p-C of IMes and IMes′),
174.4, 177.2 (carbene quaternary C of IMes and IMes′), N-bound
aryl quaternary signals not observed; (ii) signals due to anion: 117.1
(s, p-CH), 124.6 (q, J ) 272.0 Hz, CF3), 128.9 (q, J ) 32.2 Hz,
m-quaternary C), 135.0 (s, o-CH), 162.3 (q, J ) 46.0 Hz, ipso-
quaternary C). 11B NMR (CD2Cl2, 96 MHz, 20 °C): δ -6.0
([BArf4]-), 38.0 (br, HBNCy2). 19F NMR (C6D5CD3, 282 MHz, 20
°C): δ -62.8. IR (cm-1): 2254 [ν(B-H)]. Anal. Calcd for
C86H82N5B2F24Ir: C, 55.64; H, 4.46, N, 3.77. Found: C, 55.50; H,
4.45; N, 3.64.
All standard uncertainties were calculated using the full variance
covariance matrix within CRYSTALS unless otherwise stated. After
final convergence, dummy atoms were placed at the calculated
centroid positions where required. Distances and standard uncer-
tainties were then calculated after a single least-squares cycle with
all atoms constrained to ride within CRYSTALS; in each case the
maximum shift changed by less than 50% of the final refined value.
Selected structural details for the new compounds are included
in Table 1, and full crystallographic data for all structures have
been deposited with the Cambridge Crystallographic Data Centre,
CCDC 777346-777356. Structural details for compounds 5, 7, 11,
12, 13, and 15 were included in a preliminary communication of
part of this work17 and can be accessed from the CCDC using
reference numbers 758029-758034. Copies of these data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Results and Discussion
Halide abstraction is well established as a synthetic methodol-
ogy for the generation of highly reactive cationic species
stabilized, for example, by weak agostic or solvent molecule
derived interactions.16a,b We have employed this synthetic
approach in association with the strongly σ donating NHC class
of ligand18 to target systems of the type [ML2(H)2]+. Thus, the
reactions of M(IPr)2(H)2Cl (1) and M(IMes)2(H)2Cl (M ) Rh,
2; M ) Ir, 3) toward the potent halide abstraction agent
Crystal Structure Determination. Data for 4, 8a, 8b, 9, 10,
14, 16, 17, 18 and [(IMes)BH2NH3]+[BArf4]- were collected on a
Nonius KappaCCD diffractometer (Mo KR radiation; λ ) 0.71073
Å) with an Oxford Cryosystems N2 open-flow cooling device.26a
Unit cell parameters and intensity data were processed using the
9
J. AM. CHEM. SOC. VOL. 132, NO. 30, 2010 10583

A R T I C L E S
Tang et al.
Table 1. Crystallographic Data for 4, 8a, 8b, 9, 10, 14·0.37(C5H12), 16, 17, and 18
4
8a
8b
CCDC deposition number
empirical formula
formula weight
777346
C86H86B24IrN4
1834.64
777347
777348
C78H68BF24N6OIr
1764.42
C74H60.43BCl0.15F24N7.27Na0.15Ir
1719.28
temperature (K)
150
150
150
wavelength (Å)
0.71073
0.71073
0.71073
crystal system
space group
monoclinic
P21/c
monoclinic
P2/n
monoclinic
P2/n
unit cell lengths: a, b, c (Å)
20.8509(1)
17.1103(1)
24.2437(2)
90, 104.1813(2), 90
8385.72(9)
1.453
1.692
3712
0.06 × 0.21 × 0.23
5.100 to 27.493
-27 to 27, -22 to 22, -31 to 31
139522
19077 (0.045)
0.990
12.7677(1)
14.3005(1)
20.9203(2)
90, 95.129(1), 90
3804.43(5), 2
1.501
1.866
1715
0.11 × 0.16 × 0.21
5.19 to 27.49
16, 18, 27
60119
8668 (0.031)
99.2
12.9101(1)
14.4822(1)
20.7483(2)
90, 95.7575(4), 90
3859.67(5), 2
1.518
R, ꢀ, γ (deg)
volume (Å3), Z
density (calcd) (Mg/m3)
absorption coeff (mm-1
F(000)
)
1.836
1768
crystal size (mm3)
0.08 × 0.15 × 0.18
θ range for data collectn (deg)
index ranges (h, k, l)
no. of reflections collected
no. of indep reflns/Rint
completeness to θmax (%)
absorption correction
5.14 to 27.41
-16 to 16, -18 to 18, -26 to 26
71076
8701 (0.020)
99.0
multiscan
multiscan
multiscan
max and min transmission
refinement method
0.77 and 0.90
full matrix least-squares on F2
19077/648/1157
0.9577
0.81, 0.75
0.86, 0.80
full matrix least-squares on F2
8668/334/553
0.9531
full matrix least-squares on F2
8700/324/560
no. data/restraints/params
goodness-of-fit on F2
1.0001
final R indices [I > 2σ(I)]
R indices (all data)
R1 ) 0.0412, wR2 ) 0.0813
R1 ) 0.0674, wR2 ) 0.1007
2.59, -1.12
R1 ) 0. 0481, wR2 ) 0.1009
R1 ) 0.0621, wR2 ) 0.1165
1.09, -3.36
R1 ) 0. 0375, wR2 ) 0.0894
R1 ) 0.0431, wR2 ) 0.0961
1.99, -0.97
largest peak/hole (e Å-3
)
9
10
14·0.37(C5H12)
CCDC deposition number
empirical formula
formula weight
777349
C77H74B2F24N5Rh
1649.95
777350
C77H74B2F24IrN5
1739.26
777351
C87.84H90.41B2F24N5Ir
1885.99
temperature (K)
150
150
150
wavelength (Å)
0.71073
0.71073
0.71073
crystal system
triclinic
triclinic
triclinic
space group
P-1
P-1
P-1
unit cell lengths: a, b, c (Å)
12.3626(1)
12.5131(1)
13.0196(1)
18.0479(1)
18.0250(2)
16.2636(1)
18.2196(2)
18.3267(2)
41.958(3)
R, ꢀ, γ (deg)
85.750(1), 89.665(1), 73.417(1)
3884.84(6), 2
1.410
0.323
1684
92.264(1), 109.900(1), 92.529(4)
3876.41(7), 2
1.490
1.825
1748
86.177(1), 89.955(1), 84.565(1)
8824.6(6), 4
1.419
1.610
3821.8
0.11 × 0.16 × 0.25
5.10 to 27.52
-16 to 16, -20 to 20, 0 to 54
99224
38802 (0.029)
95.6
volume (Å3), Z
density (calcd) (Mg/m3)
absorption coeff (mm-1
F(000)
)
crystal size (mm3)
0.14 × 0.18 × 0.20
0.11 × 0.16 × 0.18
θ range for data collectn (deg)
index ranges (h, k, l)
no. of reflections collected
no. of indep reflns/Rint
completeness to θmax (%)
absorption correction
5.11 to 27.53
5.11 to 27.53
-16 to 16, -23 to 23, -23 to 23
69814
17698 (0.024)
98.9
-16 to 16, -22 to 23, -23 to 23
63853
17624 (0.029)
98.6
multiscan
multiscan
multiscan
max and min transmission
refinement method
0.96, 0.93
0.82, 0.74
0.84, 0.77
full matrix least-squares on F2
17697/552/1084
0.9992
full matrix least-squares on F2
17624/324/1038
0.9988
full matrix least-squares on F2
33463/1394/2421
0.9993
no. data/restraints/params
goodness-of-fit on F2
final R indices [I > 2σ(I)]
R indices (all data)
R1 ) 0.0505, wR2 ) 0.1192
R1 ) 0.0624, wR2 ) 0.1324
1.49, -1.19
R1 ) 0.0393, wR2 ) 0.0888
R1 ) 0.0523, wR2 ) 0.1030
2.06, -1.35
R1 ) 0. 0789, wR2 ) 0.1520
R1 ) 0.0963, wR2 ) 0.1652
4.54, -5.65
largest peak/hole (e Å-3
)
16
17
18
CCDC deposition number
empirical formula
formula weight
777352
C78H74B2F24N5Rh
1661.96
777353
777354
C86H82B2F24N5Ir
1855.42
C86H84B2F24N5Ir
1857.44
150
temperature (K)
150
150
wavelength (Å)
crystal system
space group
0.71073
triclinic
P-1
0.71073
triclinic
P-1
0.71073
monoclinic
P21/n
unit cell lengths: a, b, c (Å)
12.5092(1)
18.0131(2)
18.2799(2)
12.9838(2)
18.0619(2)
19.1755(3)
13.4156(1)
16.6035(2)
38.5024(4)
9
10584 J. AM. CHEM. SOC. VOL. 132, NO. 30, 2010

Dehydrogenation of Saturated CC and BN Bonds
A R T I C L E S
Table 1. Continued
16
17
18
R, ꢀ, γ (deg)
86.989(1), 70.691(1), 87.986(1)
3881.23(7), 2
1.422
0.324
1696
89.919(1), 72.222(1), 85.174(1)
4265.57(11), 2
1.446
1.664
1876
90, 95.1242(4), 90
8541.98(15), 4
1.443
1.662
3744
volume (Å3), Z
density (calcd) (Mg/m3)
absorption coeff (mm-1
F(000)
)
crystal size (mm3)
0.11 × 0.16 × 0.20
0.06 × 0.14 × 0.19
0.03 × 0.14 × 0.16
θ range for data collectn (deg)
index ranges (h, k, l)
no. of reflections collected
no. of indep reflns/Rint
completeness to θmax (%)
absorption correction
max and min transmission
refinement method
5.11 to 27.50
5.11 to 27.54
5.12 to 27.49
-15 to 16, -22 to 23, -23 to 23
49251
17537 (0.036)
98.4
multiscan
-16 to 16, -23 to 23, -24 to 24
64780
19371 (0.037)
98.4
multiscan
-17 to 17, 0 to 21, 0 to 50
20092
19437 (0.064)
99.1
multiscan
0.97 and 0.84
0.90 and 0.80
0.95 and 0.62
full matrix least-squares on F2
17537/1158/1177
0.9995
full matrix least-squares on F2
19370/648/1175
0.9999
full matrix least-squares on F2
19410/747/1194
0.9531
no. data/restraints/params
goodness-of-fit on F2
final R indices [I > 2σ(I)]
R indices (all data)
R1 ) 0.0653, wR2 ) 0.1604
R1 ) 0.0820, wR2 ) 0.1822
1.66, -1.23
R1 ) 0.0582, wR2 ) 0.1240
R1 ) 0.0788, wR2 ) 0.1405
1.80, -1.44
R1 ) 0.0654, wR2 ) 0.1085
R1 ) 0.1103, wR2 ) 0.1380
3.72, -3.25
largest peak/hole (e Å-3
)
Scheme 1. Isopropyl Group Dehydrogenation Initiated by Chloride Abstraction; Isolation of Bis(agostic) Intermediate 4a
a Key reagents and conditions: (i) Na[BArf4], fluorobenzene, 3 h at 20°C, 65%; (ii) from 1: Na[BArf4], fluorobenzene, 12 h at 20°C, 78%.
Na[BArf4] [Arf ) 3,5-C6H3(CF3)2] have been investigated; in
the event such chemistry leads to the formation of highly
reactive cationic species capable of the dehydrogenation of
saturated CC and BN linkages.
IPr Systems. The reaction of Ir(IPr)2(H)2Cl (1) with Na[BArf4]
in fluorobenzene over 3 h generates [Ir(IPr)2(H)2]+[BArf4]- (4)
in 65% isolated yield, with the iridium center stabilized by a
pair of agostic interactions utilizing the methyl groups of the
isopropyl substituents (Scheme 1). 1H NMR data for 4 in
toluene-d8 solution show only two iPr methyl resonances (even
at 200 K), consistent with rapid fluxional exchange; the IR
Figure 1. Molecular structures of the cationic components of 4 and 5;
hydrogen atoms [except those attached to Ir(1), C(15) and C(56) (for 4),
and Ir(1) and C(3) (for 5)] omitted for clarity, and thermal ellipsoids set at
the 40% probability level. Key distances (Å) and angles (deg): (for 4)
Ir(1)· · ·C(15) 3.049(5), Ir(1) · · ·C(56) 2.943(5), Ir(1)-H(152) 2.207(5),
Ir(1)-C(562) 2.117(5); (for 5) Ir(1)-C(2) 2.275(3), Ir(1)-C(3) 2.239(4),
C(7)-Ir(1)-C(31) 162.64(3).
spectrum of 4 in solution, however, reveals a C-H stretching
band at 2711 cm-1 in the region characteristic of C-H· · ·Ir
agostic systems.16a More definitive evidence for agostic stabi-
lization of the metal center comes from crystallographic
measurements in the solid state (Figure 1).
Geometrically, the structure of the cationic component of 4
features trans IPr [∠C(2)-Ir(1)-C(31) ) 177.3(2)o], cis hydride
and cis C-H agostic ligands (clearly visible in the difference
Fourier map) and is thus reminiscent of bis(phosphine) and
bis(NHC) iridium(III) complexes reported by Caulton and
Nolan.10b,c,16a,b The Ir · · ·C distances associated with the C-H
agostic ligands in 4 [2.943(5) and 3.049(5) Å] are slightly longer
than those reported for the bis(phosphine) system [Ir(PPhtBu2)2-
(H)2]+ (2.81-2.94 Å) and for the closely related rhodium
complex [Rh(PiBu3)2(H)2]+ [2.90(3) and 2.891(5) Å];16c the
corresponding distances reported for [Ir(ItBu)2(H)2]+ (2.653 Å)
are significantly shorter than any of these three systems.10b,c
After prolonged (12 h) reaction at 20 °C, C-H activation
leads to the dehydrogenation of one of the carbene iPr
substituents and the formation of [Ir(IPr)(IPr′′)(H)2]+[BArf4]-
(5), featuring the mixed NHC/alkene donor IPr′′ ligand (Scheme
1). This type of isopropyl dehydrogenation chemistry has
recently been observed for Ir(I) bis(NHC) systems and has wider
precedent in the chemistry of phosphine, ꢀ-ketiminate, and other
classes of donors featuring pendant substituents such as iso-
propyl, cyclohexyl or cyclopentyl.11–14 Spectroscopically, iso-
propyl dehydrogenation is signaled by a significant increase in
i
the complexity of the Pr Me and CH signals and by the
appearance of two alkene CH resonances and a singlet at δH
1.15 corresponding to the remaining unactivated methyl group
of the -(Me)CdCH2 moiety. While further characterization by
9
J. AM. CHEM. SOC. VOL. 132, NO. 30, 2010 10585

A R T I C L E S
Tang et al.
Scheme 2. Synthesis of NaCl Inclusion Compounds 6 and 7 and
Subsequent Reactivity of 7 with Dinitrogena
Figure 2. Molecular structure of the cationic component of 7; hydrogen
atoms (except those attached to the metal center) and second disorder
component omitted for clarity. Thermal ellipsoids set at the 40% probability
level. Key bond lengths (Å) and angles (deg): Ir(1)-Cl(2) 2.402(3),
Cl(2)-Na(3) 2.612(7), C(4)-Ir(1)-C(27) 180 (by symmetry).
a Key reagents and conditions: (i) Na[BArf4], fluorobenzene, 3 h at 20°C,
ca. 70%; (ii) dinitrogen atmosphere, toluene, 5 h at 80°C, recrystallization
from either fluorobenzene (8a) or THF/pentane (8b), ca. 80%.
[2.456(7), 2.463(9) and 2.504(7) Å for 7 (allowing for the
disorder)]29 are somewhat longer than those associated with
other reported NaCl inclusion compounds [e.g., 2.145, 2.171
Å for Zn2(ODipp)4(Na)Cl(THF)3].30 That said, Zn2(ODipp)4-
(Na)Cl(THF)3 features a Na+ cation bound to a single arene
ring; comparison of 7 with related systems in which a sodium
cation sits between two sterically encumbered π systems reveals
that the Na-centroid distances and centroid-Na-centroid angle
[143.7(3)o] are within the expected ranges (cf. 2.556, 2.535 Å
and 150.7° for Na[(2,6-trip2C6H3Ge)2] (trip )2,4,6-iPr3C6H3)).31
Moreover, the presence of the sodium cation in 7 is also signaled
by the effects it has on the conformation of the Ir(IMes)2 unit;
the two carbene heterocycles in 7 lie close to coplanar (torsion
angle between least-squares planes )12.0°) in contrast to the
more staggered geometry adopted by 3 (torsion ) 47.5°),10a
presumably on steric grounds. ·
multinuclear NMR and elemental microanalysis is consistent
with the proposed formulation for 5, definitive establishment
of connectivity was reliant on single crystal X-ray diffraction
(Figure 1). The geometry of the cationic component of 5 is
similar to that of the related square pyramidal complex
[Ir(IPr′)(IPr′′)H]+ (Scheme 1) with each cation featuring a basal
coordination plane composed of mutually trans NHC donors,
alkene, and hydride/alkyl ligands;11c the apical coordination site
is occupied by a hydride ligand in accordance with quantum
chemical predictions for d6 ML5 systems.27 In each case, the
alkene moiety lies effectively parallel with the basal coordination
plane (∠C(7)-Ir(1)-alkene centroid-C(3) ) 178.5°, cf. 174.4°
for the corresponding angle in [Ir(IPr′)(IPr′′)H]+); computational
studies are consistent, however, with a relatively low lying
alternative (perpendicular) ligand orientation (at +4.9 kcal mol-1
in the case of [Ir(IPr′)(IPr′′)H]+).11c
Synthetically, 6 and 7 represent useful, highly reactive sources
of the putative 14-electron cations [M(NHC)2(H)2]+, readily
eliminating NaCl in the presence of potential donors (vide infra).
Although both compounds are stable under an argon atmosphere,
the extremely facile loss of sodium chloride can readily be
demonstrated by the isolation of the THF adduct
[Rh(IMes)2(H)2(THF)]+[BArf4]- (see Supporting Information)
and the fact that exposure of solutions of 7 to N2 gas leads to
the formation of related Ir(I) dinitrogen complexes, albeit with
additional loss of H2. Thus, heating of a toluene solution of 7
at 80 °C under a dinitrogen atmosphere for 5 h leads to the
formation of [Ir(IMes)2(N2)2]+[BArf4]- (8a) in ca. 80% yield
after recrystallization from fluorobenzene. The closely related
complex [Ir(IMes)2(N2)THF]+[BArf4]- (8b) is obtained in
similar fashion after recrystallization from THF/pentane. Spec-
troscopic and microanalytical data for 8a and 8b are consistent
with the proposed compositions; in particular NtN stretching
IMes Systems. Studies of systems stabilized by ancilliary IPr
ligands reveal that although a bis(agostic)-stabilized species
[Ir(IPr)2(H)2]+ can be isolated from the reaction of 1 with
Na[BArf4], its subsequent reactivity toward B/N-containing
systems is complicated by the presence of carbene substituents
featuring ꢀ-hydrogen atoms. Dehydrogenation generates a
tethered geminally disubstituted alkene function that competes
with external donors for coordination at the iridium center. Thus,
we targeted instead the related IMes complexes 2 and 3, which
although offering slightly reduced steric shielding of the metal
center, also lack ꢀ-hydrogens within the carbene substituents.
Surprisingly, the reaction of either 2 or 3 with Na[BArf4] in
fluorobenzene at 20 °C (under an argon atmosphere) does not
proceed via salt metathesis but rather to the formation of the
sodium-containing cations [M(IMes)2(H)2Cl(Na)]+, (M ) Rh,
6; M ) Ir, 7) as the [BArf4]- salts (Scheme 2).
Compounds 6 and 7 represent rare examples of isolated NaCl
metathesis intermediates and can be shown to eliminate the
metal halide in the presence of a suitable donor ligand (vide
infra).28 Both compounds have been characterized by standard
spectroscopic techniques, by elemental microanalysis, and in
the case of 7, by single crystal X-ray diffraction. The cationic
component of compound 7 in the solid state (Figure 2) is
formally derived from 3 by intercalation of the sodium cation
between the mesityl rings of the two IMes ligands. The
Na-arene centroid distances characterizing these interactions
modes of vibration give rise to bands at 1993 and 1985 cm-1
,
respectively. The structures of 8a and 8b have also been
confirmed crystallographically (Figure 3) with each conforming
to the planar four-coordinate geometry typical of d8 Ir(I), and
(29) The crystal structure of 7 occupies a position on a 2-fold rotation axis
such that one IMes and the sodium ion are disordered over two
positions while the two halves of the second IMes are related by the
symmetry operator. Thus there are three Na-centroid distances and
two centroid-Na-centroid angles. Because the crystal structure is
averaged over time and space, it is not possible to know the relationship
between the disordered IMes and the sodium ion, however all distances
and angles are given.
(27) See, for example: (a) Rachidi, I. E.; Eisenstein, O.; Jean, Y. New
J. Chem. 1990, 14, 671. (b) Riehl, J. F.; Jean, Y.; Eisenstein, O.;
Pelissier, M. Organometallics 1992, 11, 729.
(30) Darensbourg, D. J.; Yoder, J. C.; Struck, G. E.; Holtcamp, M. W.;
Draper, J. D.; Reibenspies, J. H. Inorg. Chim. Acta 1998, 274, 115–
121.
(28) For a previous example of an isolated metathesis intermediate see,
for example: Patmore, N. J.; Ingelson, M. L.; Mahon, M. F.; Weller,
A. S. Dalton Trans. 2003, 289, 4–2904.
(31) Pu, L.; Phillips, A. D.; Richards, A. F.; Stender, M.; Simons, R. S.;
Olmstead, M. M.; Power, P. P. J. Am. Chem. Soc. 2003, 125, 11626–
11636.
9
10586 J. AM. CHEM. SOC. VOL. 132, NO. 30, 2010

Dehydrogenation of Saturated CC and BN Bonds
A R T I C L E S
Scheme 3. Reactions of 6 and 7 with Me3N·BH3; Formation of
Simple η2-Amineborane Adductsa
a Key reagents and conditions: (i) Me3N·BH3 (20 equiv), fluorobenzene,
6 h at 20°C, 68% (for 9), 75% (for 10).
Figure 3. Molecular structures of the cationic components of 8a and 8b;
hydrogen atoms and minor disorder component (for 8a) omitted for clarity.
Thermal ellipsoids set at the 40% probability level. Key bond lengths (Å)
and angles (deg) for 8b: Ir(1)-N(2) 1.840(6), N(2)-N(3) 1.109(8),
Ir(1)-O(4)2.128(5),Ir(1)-N(2)-N(3)180.0(bysymmetry),N(2)-Ir(1)-O(4)
180.0 (by symmetry), C(7)-Ir(1)-C(7′) 179.7(2).
featuring trans NHC and trans N2/N2 or N2/THF ligand pairs.
Although the structure of complex 8a represents, to our
knowledge, the first such report of a bis(dinitrogen) complex
of a group 9 metal, pernicious disorder prevents any meaningful
discussion of bond metrics. However, the metrical parameters
for 8b [Ir-N and N-N distances of 1.840(6) and 1.109(8) Å,
respectively] are similar to those reported for the related Ir(I)
bis(phosphine) system trans-Ir(PiPr3)2(N2)Cl [1.822(10) and
1.028(14) Å] with the slightly longer N-N distance presumably
reflecting, at least in part, the reduced degree of back-bonding
from the cationic Ir(I) center in 8b.32
Figure 4. Molecular structures of the cationic components of 9 and 10;
hydrogen atoms (except those attached to metal and boron centers) omitted
for clarity. Thermal ellipsoids set at the 40% probability level. Key bond
lengths (Å) and angles (deg): (for 9) Rh(1) · · ·B(48) 2.327(3), B(48)-N(49)
1.605(5), Rh(1)· · ·B(48)-N(49) 138.4(3), C(2)-Rh(1)-C(25) 166.6(1); (for
10) Ir(1) · · ·B(48) 2.230(2), B(48)-N(49) 1.581(1), Ir(1) · · ·B(48)-N(49)
138.2(1), C(2)-Ir(1)-C(25) 166.6(2).
As outlined above, a primary objective of this research
program is an appraisal of the potential of [(NHC)2M(H)2]+
systems for the coordination of B/N-containing substrates and
their subsequent activation toward dehydrogenation chemistry.
With this in mind, the reactions of 6 and 7 with primary,
secondary, and tertiary amineboranes (i.e., RnH3-nN·BH3, n )
1-3) have been investigated.33 While the latter class of
compound (exemplified in the current studies by Me3N·BH3)
offers nothing in the way of dehydrogenation chemistry, initial
studies sought to utilize this borane, not only to provide
confirmation of the ability of the [(IMes)2M(H)2]+ (M ) Rh,
Ir) fragments simply to bind B-H bonds but also as a basis for
comparison (i) with other metal systems known to bind
amineboranes, and (ii) with [(IMes)2M(H)2]+ complexes con-
taining dehydrogenated B/N ligand systems (vide infra). Ac-
cordingly, the reaction of Me3N·BH3 with 6 or 7 in fluoroben-
zene leads to the formation of the 18-electron species [M(IMes)2-
(H)2(µ-H)2B(H)·NMe3]+[BArf4]- (M ) Rh, 9; M ) Ir, 10) in
68 and 75% isolated yields, respectively (Scheme 3). Spectro-
scopically, each complex is characterized in the 1H NMR
spectrum by two high field resonances with relative intensities
2:3 assigned to the M-H and (fluxional) B-H hydrogens,
respectively [at δH -18.77, -1.27 for 9; δH -21.80, -2.28 for
10] and by a broad 11B signal shifted slightly from that of the
free borane, and which sharpens on broad-band 1H decoupling
(δB 1.5, 14.4, -7.3 for 9, 10, and Me3N·BH3, respectively).20o
Structurally, 9 and 10 are each characterized by a pseudo-
octahedral metal coordination geometry, featuring trans NHC,
cis hydride ligands (which were visible in the difference map),
and two additional hydrogens that constitute the points of
attachment of a η2-Me3N·BH3 ligand (Figure 4). The B-N
distances [1.605(5), 1.581(1) Å] and M · · ·B-N angles [138.4(3),
138.2(2)o] for 9 and 10 are consistent with a coordinated ligand
featuring four-coordinate boron and nitrogen centers. In each
case the M · · ·B distance [2.327(3) and 2.230(2) Å for 9 and
10, respectively] is significantly longer than the sum of the
respective covalent radii (2.13, 2.14 Å for Rh/B and Ir/B,
respectively),34 consistent with the presence of no significant
direct M-B interaction and with the small change in δB on
coordination of the borane.20o While significantly longer M · · ·B
distances have been reported for η1-Me3N·BH3 complexes (e.g.,
2.648(3) Å for [CpRu(PMe3)2(µ-H)BH2 ·NMe3]+),35 that mea-
sured for an η2-complex of Me2NH·BH3 is very similar to that
of 9 (2.318(8) Å for [Rh(PiBu3)2(H)2(µ-H)2BH·NMe2H]+).20k
Interestingly, the corresponding trimethylamineborane complex
[Rh(PiBu3)2(H)2(µ-H)2BH·NMe3]+ has been reported to be
unstable with respect to loss of H2 yielding the corresponding
four-coordinate Rh(I) complex;20k by contrast no such spontane-
ous hydrogen loss is observed for the bis(IMes) system 9, and
indeed even in cases where such dehydrogenation chemistry
(32) (a) Werner, H.; Ho¨hn, A. Z. Naturforsch. 1984, 39b, 1505. (b) Bottcher,
H.-C.; Graf, M.; Mayer, P.; Suenkel, K. Z. Anorg. Allg. Chem. 2008,
634, 1241.
(33) The reactivity of 6 towards the parent B/N system ammonia borane,
H3N·BH3, in fluorobenzene has also been investigated. These studies
are consistent with the formation of a mixture of dehydrogenated
products and with the ultimate formation of the cationic donor/acceptor
adduct [(IMes)BH2NH3]+ (as the [BArf4]- salt). Thus, although 6 does
appear to catalyze the dehydrogenation of ammonia borane, its use in
practice is limited by degradation processes involving ligand abstrac-
tion from the metal by strongly Lewis acidic borane species. Full
spectroscopic, analytical and crystallographic data for [(IMes)BH2NH3]+-
[BArf4]- are included in Supporting Information.
(34) Emsley, J. In The Elements; Oxford University Press: Oxford, 1991.
(35) (a) Shimoi, M.; Nagai, S.; Ichikawa, M.; Kawano, Y.; Katoh, K.; Uruichi,
M.; Ogino, H. J. Am. Chem. Soc. 1999, 121, 11704–11712. (b)
Kakizawa, T.; Kawano, Y.; Shimoi, M. Organometallics 2001, 20,
3211–3213. (c) Kawano, Y.; Hashiva, M.; Shimoi, M. Organometallics
2006, 25, 4420–4426. (d) Kawano, Y.; Yamaguchi, K.; Miyake, S.;
Kakizawa, T.; Shimoi, M. Chem.sEur. J. 2007, 13, 6920–6931.
9
J. AM. CHEM. SOC. VOL. 132, NO. 30, 2010 10587

A R T I C L E S
Tang et al.
Scheme 4. Synthesis of Aminoborane Complexes 11-14 by the
Although NMR data are consistent with an interaction of the
aminoborane molecule with the Group 9 metal center via
bridging B-H-M interactions, the fluxional behavior and
variable hapticity characteristic of many B-H containing ligand
systems39 means that definitive characterization of the mode of
ligation was reliant on single crystal X-ray diffraction studies
(Figure 5).
Dehydrogenation of Secondary Amineboranesa
The structures of the cationic components of 11-14 each
feature the expected trans arrangement of the NHC donors, cis
hydride ligands (which were visible in the difference Fourier
maps in each case), and an aminoborane ligand featuring a
planar C2NBH2 skeleton coordinated to the metal center via two
B-H-Minteractions,i.e.,actingasabis(σ-borane)ligand.21,37–39,41
The B-N distances [1.370(4), 1.379(8), 1.380 (mean) and
1.424(12) Å, respectively] are consistent with appreciable π
bond character [cf. 1.58 Å for the sum of the respective covalent
radii and 1.380(6) for the B-N separation in Ph2NBCl2].34,42
Moreover the Rh · · ·B separations for 11 and 12 [2.204(4) and
2.176 (mean) Å] are notably shorter than those measured for
related rhodium complexes containing an amineborane ligand
(i.e., featuring a four-coordinate boron center); the corresponding
distances determined for 9 and 15 (vide infra) are 2.327(3) and
2.305(4) Å, respectively. In a similar fashion, the Ir-B distances
measured for 12 and 14 [2.088(5) and 2.025(mean) Å] are
markedly shorter than that measured for η2-Me3N·BH3 complex
10 [2.230(2) Å]. By contrast, the M · · ·B distances determined
for rhodium complexes 11 and 12 are markedly longer than
those associated with η2-borane complexes featuring the iso-
electronic [Ru(PCy3)2(H)2] fragment.21,37–39,41 In the case of
these ruthenium systems, the presence of a direct Ru-B
interaction has been suggested, reflecting ruthenium-boron
separations [1.938(4), 1.934(2) and 1.956(2) Å for complexes
of MesBH2, tBuBH2, and NH2BH2, respectively], which are up
to 0.15 Å (7.2%) shorter than the sum of the respective covalent
radii (2.09 Å).21,34,37,38 In the cases of the isoelectronic rhodium
cations 11 and 12, however, the extent of any analogous M-B
interaction is likely to be significantly smaller, as indicated (i)
by M · · ·B separations that are actually longer than the sum of
the corresponding covalent radii (2.13 Å),34 and (ii) by the
relatively small shifts in the aminoborane 11B NMR resonance
on coordination in each case (∆δB < 3 ppm cf. ∆δB 10 ppm for
H2BtBu on coordination at ruthenium).40,43 Moreover, the longer
M--B distances measured for 11 and 12 are also consistent
with the idea of more weakly bound borane ligands implied by
a Key reagents and conditions: (i) R2HN·BH3 (20 equiv), fluorobenzene,
6 h at 20°C, 56-71%.
can be effected [e.g., by the addition of 3,3-dimethylbutene (tert-
butylethylene, TBE)], subsequent C-H activation chemistry
typically regenerates a six-coordinate metal complex (vide infra).
As such, these empirical observations for rhodium systems point
to the IMes ligand being somewhat less sterically demanding
than PiBu3.36 To our knowledge, 10 represents the first example
of an amineborane complex of iridium to be reported.
The corresponding reactions of 6 and 7 with systems
containing N-H bonds (i.e., primary and secondary aminebo-
ranes) ultimately result in dehydrogenation of the BN moiety
to give complexes containing unsaturated aminoborane ligands
(Scheme 4). Thus, 6 and 7 catalyze the dehydrogenation of the
i
secondary amineboranes R2HN·BH3 (R ) Pr, Cy) in fluo-
robenzene at room temperature (100% conversion over 4 h at
2 mol % loading for 6; synthetic runs typically performed at 5
mol %), leading to the generation of monomeric R2NBH2; the
metal-containing products isolated on completion of these
reactions (after recrystallization from fluorobenzene/hexanes)
are the aminoborane adducts [M(IMes)2(H)2(µ-H)2BNR2)]+-
[BArf4]- [M ) Rh, R ) iPr, 11; M ) Rh, R ) Cy, 12; M ) Ir,
i
R ) Pr, 13; M ) Ir, R ) Cy, 14]. The 11B NMR resonances
for all four cations (δB 35.4, 37.9, 35.8, and 35.9 ppm) are as
expected for species containing three-coordinate boron centers
formed by dehydrogenation of R2HN·BH3 and indeed are very
close to those of the free aminoboranes themselves (cf. δB 35.2,
35.4 for iPr2NBH2 and Cy2NBH2).20a,23,24 Moreover, the pattern
1
of the H NMR resonances in the hydride region in each case
[e.g., δH -1.40 (2H), -16.02 (2H) for the B-H-Rh and Rh-H
protons in 11; cf. δH 4.32 for the B-H hydrogens of iPr2NBH2
and -23.0 for rhodium-bound hydrides of Rh(IMes)2(H)2Cl],20a,10a
provides evidence for coordination of the aminoborane to the
[M(IMes)2(H)2]+ fragment in solution. By means of comparison,
the data reported for the high-field region of the 1H NMR spectra
of the related ruthenium η2-boranes Ru(PCy3)2(H)2(η2-H2BX)
are δH -5.90, -11.05 (for X ) Mes) and δH -6.80, -11.85
(for X ) NH2).21,37,38 A subtle difference between 11 and 12
and these related charge-neutral ruthenium systems is the lower-
field (less hydridic) chemical shift range observed for the
B-H-M hydrogens (δH ) -1.40, -1.74 ppm for 11 and 12,
respectively). This data, along with crystallographic evidence,
is consistent with weaker binding of the borane fragment in
the case of the cationic rhodium systems (vide infra).
1
the less hydridic Rh-H-B H NMR resonances.
The bis(σ-borane) mode of coordination observed for ami-
noborane molecules bound to the [M(IMes)2(H)2]+ fragment in
11-14 clearly contrasts with the classical side-on mode of
binding typically observed for isoelectronic alkene donors
toward transition metals.5 Moreover, the structural data obtained
for 5 confirm that a conventional side-on mode of binding of
an alkene to the [Ir(IPr)2(H)2]+ fragment is viable, with only
slight asymmetry in the Ir-C distances [2.239(4) and 2.275(3)
(39) See, for example: Marks, T. J.; Kolb, J. R. Chem. ReV. 1977, 77, 263–
293.
(40) (a) Gloaguen, Y.; Alcaraz, G.; Vendier, L.; Sabo-Etienne, S. J.
Organomet. Chem. 2009, 694, 2839–2841. (b) Alcaraz, G.; Grellier,
M.; Sabo-Etienne, S. Acc. Chem. Res. 2009, 42, 1640.
(41) Hesp, K. D.; Rankin, M. A.; McDonald, R.; Stradiotto, M. Inorg.
Chem. 2008, 47, 7471–7473.
(36) For a recent review examining comparative steric properties of tertiary
phosphine and NHC ligands see, for example:Clavier, H.; Nolan, S.
Chem. Commun. 2010, 46, 841–861.
(37) Alcaraz, G.; Clot, E.; Helmstedt, U.; Vendier, L.; Sabo-Etienne, S.
J. Am. Chem. Soc. 2007, 129, 8704–8705.
(42) Zettler, F.; Hess, H. Chem. Ber. 1975, 108, 2269–2273.
(43) For an analysis of the effects of metal-boron bonding on 11B chemical
shifts see, for example: Weller, A. S.; Fehlner, T. P. Organometallics
1999, 18, 447–450.
(38) Alcaraz, G.; Helmstedt, U.; Clot, E.; Vendier, L.; Sabo-Etienne, S.
J. Am. Chem. Soc. 2008, 130, 12878–12879.
9
10588 J. AM. CHEM. SOC. VOL. 132, NO. 30, 2010

Dehydrogenation of Saturated CC and BN Bonds
A R T I C L E S
Figure 5. Molecular structures of the cationic components of 11 and 13, and of one of the cationic components of the asymmetric unit of 12 and 14;
hydrogen atoms (except those attached to metal centers) omitted for clarity. Thermal ellipsoids set at the 40% probability level. Key bond lengths (Å) and
angles (deg): (for 11) Rh(1) · · ·B(48) 2.204(4), B(48)-N(49) 1.370(4), Rh(1)-B(48)-N(49) 179.6(3), C(2)-Rh(1)-C(25) 165.5(1); (for 12) Rh(101) · · ·B(148)
2.198(7), B(148)-N(149) 1.380(8), Rh(101) · · ·B(148)-N(149) 177.9(5), C(102)-Rh(101)-C(125) 161.4(2); (for 13) Ir(1) · · ·B(48) 2.088(5), B(48)-N(49)
1.379(8); Ir(1)-B(48)-N(49) 178.4(3), C(2)-Ir(1)-C(25) 165.6(2); (for 14) Ir(201) · · ·B(248) 2.027(10), B(248)-N(249) 1.424(12), Ir(201) · · ·B(248)-N(249)
177.2(8), C(202)-Ir(201)-C(225) 164.8(3).
Scheme 5. Isolation of Amine- and Aminoborane Complexes from
Å] reflecting closer approach of the less hindered terminal alkene
carbon. It therefore seems likely that the alternative binding
motif revealed for aminoboranes is reflective of the electronic
properties of this type of ligand, and in particular the more
hydridic nature of the B-H bonds (cf. C-H), and the
significantly lower energy of the BdN π system (cf. CdC).44
Consistent with this assertion, Alcaraz and Sabo-Etienne have
reported that the end-on bis(σ-borane) binding motif is the most
stable isomer of the model complex trans-Ru(PMe3)2(H)2-
(H2BNH2) with alternative modes of interaction utilizing the
BN π system being >14 kcal mol-1 higher in energy.21
a
t
the Complexation and Dehydrogenation of BuNH2 ·BH3
a Key reagents and conditions: (i) tBuH2N·BH3 (20 equiv), fluorobenzene,
t
6 h at 20°C, 51%; (ii) from 6: BuH2N·BH3 (20 equiv), fluorobenzene,
48 h at 20°C, 72%.
Similar dehydrogenation chemistry is applicable to the
primary amineborane BuNH2 ·BH3, although in this case the
rate of rhodium-catalyzed dehydrogenation appears to be slower.
t
system [Rh(IMes)2(H)2(µ-H)2BNHtBu]+[BArf4]- (16) (Scheme
5). 15 and 16 have been characterized by standard spectroscopic
and analytical techniques, with the 11B NMR chemical shifts
for the coordinated boranes (δB -35.1, and 37.4, respectively)
being particularly diagnostic of the coordination number at
boron, and hence on the extent of hydrogenation of the
coordinated borane fragment. The 1H NMR resonance associated
with the boron-bound hydrogens occurs at higher field for 16
(δH -2.62 cf. -1.66 ppm for 15), consistent with η2-ligated
boranes in each case and with fluxional averaging of one B-H
and two Rh-H-B protons in 15. Crystallographic authentica-
tion of the η2 mode of ligation has been achieved for both 15
and 16 (Figure 6); these structural studies allow, for the first
time, comparison of a saturated amineborane and the corre-
sponding unsaturated aminoborane coordinated to the same
transition metal fragment, i.e., [Rh(IMes)2(H)2]+.
Moreover, in common with the findings of Kawano, Shimoi
t
and co-workers, the dehydrogenation of BuNH2 ·BH3 under
such conditions leads to the formation of a mixture of products,
including B2H5(µ-NHtBu), (tBuNH)2BH, tBuNHBH2, and (tBuN-
BH)3.45 Not unexpectedly, the proportion of dehydrogenated
B/N containing species (as measured by 11B NMR spectroscopy)
increases as a function of time; moreover from the perspective
of the rhodium-containing species present in solution, short
reaction times (ca. 6 h) lead to the isolation of the amineborane
complex [Rh(IMes)2(H)2(µ-H)2B(H)·NH2 Bu]+[BArf4]- (15),
t
while extended reaction times (ca. 48 h) bring about the
formation of the corresponding (dehydrogenated) aminoborane
(44) Results derived from photoelectron spectroscopy imply that the BN π
bonding orbital in H2NdH2 is stabilized by 0.85 eV (ca. 80 kJ mol-1
)
The geometric differences between 15 and 16 associated with
the coordinated B/N fragment are largely as expected on the
basis of the lower coordination number at the boron and nitrogen
centers, i.e., shorter B-N and Rh · · ·B distances [1.368(7) vs
with respect to the corresponding orbital in H2CdCH2. See: Westwood,
N. P. C.; Westiuk, N. H. J. Am. Chem. Soc. 1986, 108, 891–894.
(45) Kawano, Y.; Uruichi, M.; Shimoi, M.; Taki, S.; Kawaguchi, T.;
Kakizawa, T.; Ogino, H. J. Am. Chem. Soc. 2009, 131, 14946–14957.
9
J. AM. CHEM. SOC. VOL. 132, NO. 30, 2010 10589

A R T I C L E S
Tang et al.
) 1.343 Å (riding)] consistent with other Ir(µ-H)B motifs [e.g.,
1.81 Å (mean) and 1.32 Å (mean) for 17]. Further evidence for
the presence of H(481) and for some degree of direct B-H
interaction can also be obtained from 11B and 1H NMR
measurements. The 11B resonance for 18 (δB 38.0 ppm) is at
much higher field than typical aminoborylene complexes [cf.
δB 67 ppm for Cp*Ir(CO)dBN(SiMe3)2],46,47 and the only high
field 1H NMR resonance (at δH -10.85 ppm) falls intermediate
between the Ir-H and Ir-H-B signals for 14 (δH -15.44 and
1
-5.83 ppm, respectively); moreover, this H signal narrows
markedly on 11B decoupling. As such, it seems likely that the
structure of 18 can be described in terms of significant
contributions from two limiting resonance forms, i.e., an iridium
borylene complex stabilized by intramolecular B-coordination
of an iridium-bound hydride or an R-agostic iridium boryl
system (Scheme 7). Consistent with this description, the atoms
Ir(1), H(481), B(48), and N(49), together with tertiary carbons
of the cyclohexyl groups [C(50) and C(56)] are approximately
coplanar, with the largest out-of-plane deviation being <0.07
Å. Interestingly, the bridging position of the hydride ligand in
18 implied by NMR and crystallographic measurements,
contrasts with that reported for a related ruthenium system,
which is described in terms of noninteracting hydride and
borylene ligands within an isoelectronic (but charge-neutral)
L2Ru(X)H(BR) system.38 Conceivably this reflects the greater
electrophilicity of the boron center within cationic borylene
systems such as 18.47 However, precedent for this type of
bridging interaction within strongly electrophilic cations such
as 18 comes from the structure of [Cp*Mo(dmpe)(H)SiMes]+
reported by Mork and Tilley.48
Figure 6. Molecular structures of the cationic components of 15 and 16;
hydrogen atoms (except those attached to metal centers) and minor disorder
component (for 16) omitted for clarity. Thermal ellipsoids set at the 40%
probability level. Key bond lengths (Å) and angles (deg): (for 15) Rh(1) · · ·B(48)
2.305(4), B(48)-N(49) 1.604(5), Rh(1)· · ·B(48)-N(49) 127.0(3), C(2)-Rh(1)-
C(25) 168.7(1); (for 16) Rh(1)· · ·B(480) 2.245(7), B(480)-N(490) 1.368(7),
Rh(1)· · ·B(480)-N(490) 152.7(4), C(2)-Rh(1)-C(25) 168.1(2).
1.604(5) Å; 2.245(7) vs 2.305(4) Å] and a wider Rh · · ·B-N
angle [152.7(4) vs 127.0(3)o] for the aminoborane complex 16.
That said, the geometry of the [Rh(IMes)2(H)2]+ fragment does
not appear to vary significantly between the two complexes
[d(Rh-CNHC) ) 2.044(3), 2.051(4) and 2.043(3), 2.045(3);
∠(CNHC-Rh-CNHC) ) 168.7(1), 168.1(2)o, for 15 and 16
respectively], implying that the pocket available for the coor-
dination of additional ligands is sufficient to accommodate both
the planar aminoborane ligand and the more bulky amineborane.
Such a finding contrasts with results reported for related systems
containing a bis(triisobutylphosphine) ligand set, in which case
coordination of bulkier amineboranes induces reductive elimina-
tion of H2 from the metal center in order to relieve steric
crowding.20k,o Indeed, in the case of the [M(IMes)2(H)2]+
systems reported here, further loss of dihydrogen can only be
effected by the addition of a sacrificial alkene such as tert-
butylethylene. In the case of iridium dicyclohexylaminoborane
system 14, loss of H2 is revealed by the generation of
tert-butylethane but is also accompanied by oxidative addition
of one of the IMes methyl C-H bonds to give [Ir(IMes)(IMes′)-
(H)(µ-H)2BNCy2]+[BArf4]- (17; Scheme 6).
Quantum chemical studies of the mechanism of secondary
amineborane dehydrogenation at Group 9 metal centers have
recently been reported, albeit for less bulky systems in which
the dehydrogenated aminoborane species ultimately couple to
give oligomeric products;20k notably, the binding energies of
Me2HN·BH3 and (monomeric) Me2NBH2 at cationic 14-electron
metal centers are reported not to be markedly different. In
systems where the dehydrogenated aminoborane product has a
more limited propensity to oligomerize [as with R2NBH2 (R )
Cy, iPr) and to a lesser extent tBuNHBH2], the identities of the
metal/boron-containing products isolated after different time
periods are therefore (at least in part) reflective of the relative
concentrations of the starting amineborane and dehydrogenated
aminoborane present in solution. Thus, the underlying reasons
for the different products obtained in the reactions of 6/7 with
The identity of 17 has been deduced from standard spectro-
scopic and analytical techniques, with additional confirmation
being provided by X-ray diffraction (Figure 7). Structurally, the
geometry of the Ir(µ-H)2BNCy2 unit varies little from that
determined for complex 14, with the Ir · · · B distances being
statistically indistinguishable [2.012(8) vs 2.027(10) Å for 14].
The structure of 17 is indicative of an alternative response to
the unsaturation brought about by loss of H2 from six-coordinate
18-electron σ-borane complexes, contrasting with the simple
change in metal coordination geometry revealed by Weller and
Hall for Rh(PiBu3)2 systems and the borane to borylene
conversion reported by Sabo-Etienne and Alcaraz for Ru(PCy3)2
complexes, each of which involves a change in the formal
oxidation state either of the metal or of the boron ligand.20k,38
Interestingly 17 can be further dehydrogenated under more
forcing conditions to give complex 18, which features a direct
Ir-B interaction, as revealed by a short Ir-B distance, similar
to that expected for an IrdB double bond [1.881(8), cf. 1.892(3)
for the iridium aminoborylene complex Cp*Ir(CO)dBN(SiMe3)2
and 2.012(8) Å for 17].46 Closer inspection, however reveals
that the Ir-B-N framework is distorted somewhat from
i
tBuH2N·BH3 and R2HN·BH3 (R ) Pr, Cy) appear to reflect
the relative rates of metal-catalyzed dehydrogenation of these
amineboranes in fluorobenzene and consequently the identities
of the predominant B/N-containing species in solution (at the
end of the 6 h reaction period) available to coordinate to the
[M(IMes)2(H)2]+ fragment. Thus, whereas R2HN·BH3 (R ) iPr,
Cy) is efficiently dehydrogenated by 6 over the time-period of
the synthetic experiment (6 h), effectively leaving R2NBH2 (R
)
iPr, Cy) as the only borane remaining in solution,
tBuH2N·BH3 is dehydrogenated much more slowly, such that
over the same time frame it remains a major B/N constituent
(46) Braunschweig, H.; Forster, M.; Kupfer, T.; Seeler, F. Angew. Chem.,
Int. Ed. 2008, 47, 5981.
(47) (a) Braunschweig, H.; Kollann, C.; Seeler, F. Struct. Bonding (Berlin)
2008, 130, 1. (b) Vidovic, D.; Pierce, G. A.; Aldridge, S. Chem.
Commun. 2009, 1157. (c) Braunschweig, H.; Dewhurst, R. D.;
Schneider, A. Chem. ReV. Published ASAP March 17, 2010, DOI:,
10.1021/cr900333n.
linearity [∠Ir-B-N
)
167.2(6) cf. 175.9(3)o for
Cp*Ir(CO)dBN(SiMe3)2] and that a bridging hydrogen atom
between Ir(1) and B(48) is visible in the difference Fourier map
with Ir-H and B-H distances [d(Ir-H) ) 1.613(8), d(B-H)
(48) Mork, B. V.; Tilley, T. D. Angew. Chem., Int. Ed. 2003, 42, 357.
9
10590 J. AM. CHEM. SOC. VOL. 132, NO. 30, 2010

Dehydrogenation of Saturated CC and BN Bonds
A R T I C L E S
Scheme 6. Dehydrogenation and C-H Activation in Aminoborane Complex 14a
a Key reagents and conditions: (i) 3,3-dimethylbutene (TBE; excess), fluorobenzene, 6 h at 20°C, 63%; (ii) toluene, 36 h at 65°C, 82%.
of the sodium cation between the mesityl aromatic rings of the
two NHC donors. Compounds 6 and 7 represent convenient yet
highly reactive sources of the putative 14-electron
[M(NHC)2(H)2]+ cations, readily eliminating NaCl in the
presence of potential donors. Thus 7 can be employed in the
synthesis of the dinitrogen complexes [Ir(IMes)2(N2)2]+[BArf4]-
(8a) and [Ir(IMes)2(N2)THF]+[BArf4]- (8b) (albeit with ad-
ditional loss of H2) by simple dissolution in fluorobenzene or
THF under a dinitrogen atmosphere. The reactivity of 6 and 7
toward primary, secondary, and tertiary amineboranes have also
been investigated; reaction with the latter class of reagent leads
to simple coordination of the amineborane at the metal center
via two M-H-B bridges and the formation of [M(IMes)2(H)2-
(µ-H)2B(H)·NMe3]+[BArf4]- (M ) Rh, 9; M ) Ir, 10). The
corresponding reactions with systems containing N-H bonds
proceed via dehydrogenation to give complexes of the corre-
sponding (unsaturated) aminoborane ligands. Thus, for example,
Figure 7. Molecular structure of the cationic components of 17 and 18;
hydrogen atoms (except those attached to the metal center) omitted for clarity.
Thermal ellipsoids set at the 40% probability level. Key bond lengths (Å) and
angles (deg): (for 17) Ir(1) · · ·B(48) 2.012(8), B(48)-N(49) 1.365(9),
Ir(1)· · ·B(48)-N(49) 168.4(5), C(6)-Ir(1)-C(25) 167.6(2); (for 18) Ir(1)-B(48)
1.881(8), B(48)-N(49) 1.372(10), Ir(1)-H(481) 1.613(8), B(48)-H(481) 1.343
(riding), Ir(1)-B(48)-N(49) 167.2(6), C(2)-Ir(1)-C(29) 168.1(2).
i
6 catalyzes the dehydrogenation of R2HN·BH3 (R ) Pr, Cy)
in fluorobenzene solution to give [Rh(IMes)2(H)2(µ-
Scheme 7. Limiting Valence Bond Descriptions of Complex 18
i
H)2BNR2]+[BArf4]- (R ) Pr, 11; R ) Cy, 12) as the ultimate
organometallic products. In contrast to isoelectronic alkene
donors, the aminoborane ligand in these complexes (and in the
corresponding iridium compounds 13 and 14) can be shown by
crystallographic methods to bind in end-on fashion via a bis(σ-
borane) motif. Similar dehydrogenation chemistry is applicable
to the primary amineborane tBuH2N·BH3, although in this case
the rate of rhodium-catalyzed dehydrogenation is slower,
thereby allowing the amineborane complexes [Rh(IMes)2(H)2(µ-
of the reaction mixture. Consistent with this assertion, extension
of the reaction period in the case of BuH2N·BH3 dehydroge-
nation leads to larger concentrations of the monomeric ami-
noborane tBuNHBH2 in solution (as judged by in situ 11B NMR
spectroscopy) and to the isolation of [Rh(IMes)2(H)2(µ-
H)2BNHtBu]+[BArf4]- (16) after ca. 48 h.
t
t
H)2B(H)·NH2 Bu]+[BArf4]- (15) to be isolated at short reaction
times (ca. 6 h) and the corresponding (dehydrogenated) ami-
noborane systems [Rh(IMes)2(H)2(µ-H)2BNHtBu]+[BArf4]- (16)
to be isolated after an extended period (ca. 48 h). Reactivity-
wise aminoborane systems such as 14 show themselves to be
amenable to further dehydrogenation chemistry, leading initially
to intramolecular C-H activation processes, but ultimately to
the dehydrogenation of the boron-containing ligand itself and
to the formation of a system described by limiting boryl (Ir-B)
and borylene (IrdB) forms.
Conclusions
Chloride abstraction from the Group 9 metal bis(N-hetero-
cyclic carbene) complexes M(NHC)2(H)2Cl has been shown to
generate highly reactive cationic species capable of the dehy-
drogenation of saturated CC and BN linkages. Thus, the reaction
of Ir(IPr)2(H)2Cl (1) with Na[BArf4] in fluorobenzene generates
[Ir(IPr)2(H)2]+[BArf4]- (4) in which the iridium center is
stabilized by a pair of agostic interactions; subsequent C-H
activation leads to the dehydrogenation of one of the carbene
iPr substituents and the formation of [Ir(IPr)(IPr′′)(H)2]+[BArf4]-
(5), featuring the mixed NHC/alkene donor IPr′′ ligand. The
closely related IMes complexes M(IMes)2(H)2Cl (M ) Rh, Ir),
which feature carbene substituents lacking ꢀ-hydrogens, react
with Na[BArf4] in fluorobenzene to give rare examples of NaCl
inclusion compounds [M(IMes)2(H)2Cl(Na)]+[BArf4]- (M ) Rh,
6; M ) Ir, 7); crystallographic studies of 7 reveal intercalation
Acknowledgment. We thank the EPSRC for support of this
work (EP/F01600X/1). We also gratefully thank the Diamond Light
Source for an award of beam-time on I19 (MT1880) and the
instrument scientists on the beamline for technical assistance.
Supporting Information Available: Synthetic, spectroscopic
and crystallographic data for [(IMes)BH2NH3]+[BArf4]-; crystal-
lographic data for [Rh(IMes)2(H)2(THF)]+[BArf4]-. This material
is available free of charge via the Internet at http://pubs.acs.org.
JA1043787
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J. AM. CHEM. SOC. VOL. 132, NO. 30, 2010 10591