Further exploring the "sting of the scorpion": Hydride migration and subsequent rearrangement of norbornadiene to nortricyclyl on rhodium(i)

Article abstract of DOI:10.1021/om900499v

A new boron-based flexible scorpionate ligand based upon 7-azaindole, Li[Ph(H)B(azaindolyl)₂] (Li[^phBai]), has been prepared. This ligand, together with the previously reported ligand K[HB(azaindolyl) ₃] (K[Tai]), have been used to prepare a range of monovalent group 9 transition-metal complexes. The complexes [M(COD){K³N,N,H-Ph(H) B(azaindolyl)₂}] (where M = rhodium, iridium and COD = 1,5-cyclooctadiene) and [Rh(NBD){K³N,N,H-HB(R)(azaindolyl) ₂}] (where NBD = 2,5-norbornadiene and R = Ph, azaindolyl) have been prepared. Structural characterization of [M(COD){K³NNH-Ph(H) B(azaindolyl)₂}] (where M = rhodium, iridium) and [Rh(NBD){k ³N, N, H-HB(azaindolyl)₃}] reveal strong interactions of the B-H functional group with the metal centers, particularly in the case of [Ir(COD){K³N,N,H-Ph(H)B(azaindolyl)₂}]. The complex [Rh(NBD){K³N,N,H-HB(azaindolyl)₃}] undergoes a further reaction, resulting from hydride migration from boron to the norbornadiene group. Subsequent rearrangement results in the formation of the rhodium-nortricyclyl complex [Rh(nortricyclyl){k⁴ N,N,-B,N-B(azaindolyl)₃}], providing the first nitrogen-based metallaboratrane complex to contain the tetradentate (K⁴N,N,B,N) coordination mode.

Full text of DOI:10.1021/om900499v

5222 Organometallics 2009, 28, 5222–5232
DOI: 10.1021/om900499v
Further Exploring the “Sting of the Scorpion”: Hydride Migration and
Subsequent Rearrangement of Norbornadiene to Nortricyclyl on
Rhodium(I)
Nikolaos Tsoureas, Thomas Bevis, Craig P. Butts, Alex Hamilton, and Gareth R. Owen*
The School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K.
Received June 12, 2009
A new boron-based flexible scorpionate ligand based upon 7-azaindole, Li[Ph(H)B(azaindolyl)2]
(Li[PhBai]), has been prepared. This ligand, together with the previously reported ligand K[HB-
(azaindolyl)3] (K[Tai]), have been used to prepare a range of monovalent group 9 transition-metal
complexes. The complexes [M(COD){κ3N,N,H-Ph(H)B(azaindolyl)2}] (where M = rhodium,
iridium and COD = 1,5-cyclooctadiene) and [Rh(NBD){κ3N,N,H-HB(R)(azaindolyl)2}] (where
NBD = 2,5-norbornadiene and R = Ph, azaindolyl) have been prepared. Structural characterization
of [M(COD){κ3NNH-Ph(H)B(azaindolyl)2}] (where M = rhodium, iridium) and [Rh(NBD){κ3N,
N,H-HB(azaindolyl)3}] reveal strong interactions of the B-H functional group with the metal
centers, particularly in the case of [Ir(COD){κ3N,N,H-Ph(H)B(azaindolyl)2}]. The complex
[Rh(NBD){κ3N,N,H-HB(azaindolyl)3}] undergoes a further reaction, resulting from hydride migra-
tion from boron to the norbornadiene group. Subsequent rearrangement results in the formation of the
rhodium-nortricyclyl complex [Rh(nortricyclyl){κ4N,N,B,N-B(azaindolyl)3}], providing the first ni-
trogen-based metallaboratrane complex to contain the tetradentate (κ4N,N,B,N) coordination mode.
Introduction
a “hydride source” following the discovery of the “metalla-
boratrane” complexes by Hill in 1999 (Figure 1).5
For some time now there has been great interest in finding
new methodologies for the generation of transition-metal
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transition-metal centers,3 and recent discoveries have pro-
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Hill, A. F.; White, A. J. P.; Williams, D. J. Organometallics 2004, 23, 913. (c)
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*To whom correspondence should be addressed. Tel: þ44 (0)117 928
7652. E-mail: gareth.owen@bristol.ac.uk.
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pubs.acs.org/Organometallics
Published on Web 08/12/2009
2009 American Chemical Society

Article
Organometallics, Vol. 28, No. 17, 2009 5223
Figure 1. Metallaboratrane complex [M{κ4S,S,B,S-B(mt)3L0L]
(1; N-S = mt).
Figure 3. Complexes [Rh(COD){κ3N,N,H-HB(azaindolyl)3]
(2) and [Ir(COD){κ3N,N,H-HB(azaindolyl)3] (3).
Scheme 1. Hydride Migration Induced by Addition of Carbon
Monoxide
Figure 2. Reversible migration of “hydride” between boron
and metal centers (N N represents the 7-azaindole rings).
-
Metallaboratrane complexes, which feature a transition-
metal-to-boron dative interaction, have since attracted much
attention.6,7 The originally postulated σ-acceptor coordina-
tion within these compounds has been called into question
and is currently a matter of debate.6c,d This debate has been
fueled by the unexpected geometries of some of the resulting
compounds.8c The majority of the reported examples have
been based on Tm [HB(mt)3] or similar derivatives.9,10 More-
over, recent examples by Bourissou8 based on phosphane
amphiphilic ligands have augmented the field by providing
further insight into the metal-boron bonding interaction.
Additionally we have very recently reported the first exam-
ples to be supported by a flexible ligand containing nitrogen
donors.11
Our focus lies in metal-borane complexes, since the boron
atom could potentially be utilized as a “hydride store” and
could have a number of catalytic applications (Figure 2).
Both Hill9i and ourselves12 have provided evidence for
reversible hydride migration between boron and transition-
metal centers. As part of our investigations we have recently
reported the coordination chemistry of the relatively un-
explored11,13-16 flexible tridentate nitrogen scorpionate
ligand hydrotris(7-azaindolyl)borate (Tai).14 We further re-
ported that this nitrogen based ligand, a more flexible
analogue of Trofimenko’s original scorpionate ligand
(Tp),17 could be utilized to provide facile hydride migration
leading to the generation of a new family of nitrogen based
metallaboratrane complexes.11
(3); COD = 1,5-cyclooctadiene) and their application as
catalysts for the transfer hydrogenation of ketones.14 In an
effort to explore the concept further, we began to investigate
the effect of changing one of the substituents at boron and, in
addition, of varying the coligands at the metal center. During
these investigations, it was found that hydride migration is
dependent on both of these factors. We therefore wish to
report the synthesis and characterization of group 9 transi-
tion-metal complexes containing 7-azaindole-based ligands
together with the results of our investigations exploring the
propensity of hydride migration between boron and transi-
tion-metal centers.
Results and Discussion
We have recently reported the synthesis and catalytic
investigations of rhodium and iridium cyclooctadiene com-
plexes, containing Tai.14 The isostructural complexes 2 and 3
were found to bind to the metal centers with a κ3N,N,H
coordination mode (Figure 3). Both complexes showed
fluxional behavior at room temperature due to the rapid
exchange between the free and coordinated azaindole rings.
Within these examples, there was no evidence to indicate that
hydride migration between the boron and the metal centers
occurred even at elevated temperatures (up to 80 °C).
It was shown, however, that hydride migration can be
induced by disturbing the coordination sphere at the metal
center.11 Addition of carbon monoxide to a solution of 3
results in the loss of one of the double bonds of the cyclo-
octadiene moiety from the coordination sphere, followed by
subsequent hydride migration from boron to the metal
center and then insertion of the olefin into the resulting
iridium-hydride bond. This led to the isolation of the
square-based-pyramidal complex Ir(CO)(C8H13){κ3N,N,B-
B(azaindolyl)3} (5) via the dicarbonyl complex 4 (Scheme 1).
In contrast to the case for the phosphine- and sulfur-based
systems, we had not observed the tetradentate coordination
mode (κ4N,N,B,N) for the azaindole-based ligands and had
attributed this to the rigid structure provided by the fused
bicyclic ring system of this heterocycle. With this in mind, it
Our studies with this ligand led to the synthesis of
[M(COD){κ3N,N,H-HB(azaindolyl)3}] (M = Rh (2), Ir
(10) (a) Landry, V. K.; Melnick, J. G.; Bucella, D.; Pang, K.; Ulichny,
J. C.; Parkin, G. Inorg. Chem. 2006, 45, 2588. (b) Pang, K.; Quan, S. M.;
Parkin, G. Chem. Commun. 2006, 5015. (c) Mihalcik, D. J.; White, J. L.;
Tanski, J. M.; Zakharov, L. N.; Yap, G. P. A.; Incarvito, C. D.; Rheingold, A.
L.; Rabinovich, D. Dalton Trans. 2004, 1626. (d) Blagg, R. J.; Charmant, J.
P. H.; Connelly, N. G.; Haddow, M. F.; Orpen, A. G. Chem. Commun. 2006,
2350. (e) Figueroa, J. S.; Melnick, J. G.; Parkin, G. Inorg. Chem. 2006, 45,
7056.
(11) Tsoureas, N.; Haddow, M. F; Hamilton, A; Owen, G. R. Chem.
Commun. 2009, 2538.
(12) Rudolf, G. C.; Hamilton, A.; Orpen, A. G.; Owen, G. R. Chem.
Commun. 2009, 553.
(13) Tsoureas, N.; Owen, G. R.; Hamilton, A.; Orpen, A. G. Dalton
Trans. 2008, 6039.
(14) Song, D.; Jia, W. L.; Wu, G.; Wang, S. Dalton Trans. 2005, 433.
(15) Saito, T.; Kuwata, S.; Ikariya, T. Chem. Lett. 2006, 35, 1224.
(16) Wagler, J.; Hill, A. F. Organometallics 2008, 27, 2350.
(17) (a) Trofimenko, S. In Scorpionates: The Coordination of Poly-
(pyrazolyl)borate Ligands,; Imperial College Press: London, 1999. (b)
Trofimenko, S. Chem. Rev. 1993, 93, 943. (c) Trofimenko, S. Polyhedron
2004, 23, 197.

5224 Organometallics, Vol. 28, No. 17, 2009
Tsoureas et al.
Figure 4. Monosubstituted borate ligand Na[(7-azain-7)BH3] (6).
Scheme 2. Synthesis of Lithium Hydrophenylbis-
(azaindole)borate (7, Li[PhBai])
Figure 5. Molecular structure of [Li{Ph(H)B(azaindolyl)2}]2.
Hydrogen atoms, with the exception of H100, have been omitted
for clarity (thermal ellipsoids are drawn at the 50% probability
was of interest to prepare the corresponding bis-substituted
compound [H2B(azaindolyl)2]- (Bai). Hill and Wagler have
recently reported the synthesis of a monosubstituted borate
anion (6) together with some preliminary complexes.16 In
this case, the 7-azaindole unit was found to bind to the boron
atom via the pyridine nitrogen following tautomerization of
the azaindole group (Figure 4).
Our attempts to synthesize and isolate the bis-substituted
ligand Bai, however, proved unsuccessful, leading to mix-
tures of the two possible bis-substituted products (bound via
both pyridine and pyrrole nitrogen atoms) and small quan-
tities of Tai. Although the target compound Na[Bai] was
found to be the major component (approximately >80% as
determined by 11B{1H} NMR), it was not possible to sepa-
rate it from the other components in the mixture.16 We
therefore targeted an analogous compound which also con-
tained two azaindole units.18 The lithium salt Li[Ph(H)B-
(azaindolyl)2] (7, Li[PhBai] could be readily synthesized by
˚
level). Selected bond lengths (A) and angles (deg): B(1)-H-
(100) = 1.20(3), Li(1)-H(100) = 1.90(3), Li(1i)-H(100) =
2.08(3), N(1)-B(1)=1.541(4), N(2)-Li(1)=1.984(6); N(1)-B-
(1)-H(100) = 103.6(16), Li(1i)-B(1)-H(100) = 52.0(16), N-
(2)-Li(1)-H(100)=85.0(10), B(1i)-Li(1)-H(100)=92.2(10),
H(100i)-Li(1)-H(100) = 65.1(17), N(2)-Li(1)-H(100i) =
138.8(10), B(1)-H(100)-Li(1)=140(2), B(1)-H(100)-Li(1i)=
100.9(19), N(1)-B(1)-Li(1) = 88.7(3), Li(1)-H(100)-Li(1i)=
111.03(5).
to the B-H group, the low stretching frequency of this band
suggesting a strong interaction of the B-H group with the
lithium center. The 7Li{1H} (s, 4.8 ppm (δ C7D8)/s, 5.1 ppm (δ
CDCl3)) and 13C{1H} NMR spectra were also consistent with
the formation of Li[PhBai], although in the latter the phenyl
aromatic carbon ipso to the boron center could not be located
even when longer acquisition times or relaxation delays were
employed.
reaction of Li[PhBH3] 2THF (see the Experimental Section)
3
with an excess (2.4 equiv) of 7-azaindole (Scheme 2). These
two components were heated together in toluene at 120 °C,
and the progress of the reaction was monitored by 11B{1H}
NMR spectroscopy. The reaction was complete within a
period of 48 h, and a single peak at -6.9 ppm (hhw 132 Hz)
was observed in the boron NMR spectrum. A proton-
coupled boron (11B) NMR experiment was carried out in
order to confirm the reaction of two azaindole units at the
boron center. The experiment was not conclusive, since the
signal was not fully resolved. Nevertheless, the signal had
broadened with respect to the boron-decoupled experiment
(hhw 198 Hz), suggesting coupling to a single adjacent
proton. Filtration of the reaction mixture while hot and
cooling at -30 °C overnight yielded the desired borate ligand
as a crystalline solid in moderate yield. The spectroscopic
and analytical data were consistent with the formation of
Li[PhBai]. Confirmation of bis-substitution was obtained
The formation of 7 was further confirmed by a single-
crystal diffraction study (Figure 5). The molecular structure
in the solid state reveals a dimeric structure involving two
lithium centers and two PhBai moieties. One of the azaindole
units from each ligand is connected to a lithium atom, and
the B-H unit from each ligand bridges the two lithium
˚
centers (Li-H distances 1.90(3) and 2.08(3) A). The short
lithium-hydrogen distances confirm the moderately strong
interactions of the B-H group with the metal centers. Addi-
tionally, there is a short distance between the ipso carbon of the
˚
boron-bound phenyl group and a lithium center (2.546(7) A).
The calculated molecular structure also confirmed that
the azaindole units were connected to boron center via the
“pyrrole” nitrogen atoms. A similar dimeric motif has also
been observed in the solid-state structure of K[Tai].14
When the reaction was repeated on a larger scale, full
consumption of Li[PhBH3] 2THF was observed after a
1
from the H{11B} NMR spectrum, which revealed a peak
3
at 5.68 ppm (δ C7D8) integrating for one proton and was
assigned as the hydrogen of the B-H group. This was not
observed in the standard H spectrum, as a result of quad-
period of 48 h. On this occasion after the same workup (see
the Experimental Section), the 11B{1H} NMR (δ CDCl3)
spectrum consisted of two new signals at -8.8 and -3.7 ppm.
1
11B NMR experiment indicated that both these peaks
rupolar broadening from the boron atom. The infrared spec-
trum of 7 showed a band at 2196 cm-1, which corresponded
A
corresponded to species containing one B-H bond with 1JBH
coupling constants of 63.5 Hz (-8.8 ppm) and 89.4 Hz (-3.7
ppm). The difference in the B-H coupling constants for
these species suggests an interaction of the B-H group with
(18) Garcia, R; Paulo, A.; Domingos, A.; Santos, I. J. Organomet.
Chem. 2001, 632, 41.

Article
Organometallics, Vol. 28, No. 17, 2009 5225
Scheme 3. Synthesis of [Rh(COD){Ph(H)B(azaindolyl)2}] (8)
and [Ir(COD){Ph(H)B(azaindolyl)2}] (9)
(H)B(azaindolyl)2}] and [Ir(COD){κ3N,N,H-Ph(H)B(azain-
dolyl)2}], respectively. In contrast to the analogous com-
1
plexes 2 and 3, the H NMR spectra of 8 and 9 showed no
fluxional behavior at room temperature due to the PhBai
ligand scaffold not having a free N-donor coordinating
moiety available to exchange with coordinated azaindolyl
rings. This observation also suggests that the coordination of
1
the B-H unit to the metal center is not fluxional. The H
NMR spectra of 8 and 9 consisted of eight signals between
6.25 and 8.55 ppm, corresponding to the 5 azaindole ring
proton environments (integrating for 10 protons) and 3
environments of the phenyl ring (integrating for 5 protons),
indicating a Cs symmetry. The 13C{1H} NMR spectra of 8
and 9 showed 10 signals in the aromatic region, 7 corre-
sponding to the azaindole ring carbons and 3 corresponding
to the phenyl ring (the resonance of the aromatic carbon ipso
to the boron center could not be located, even when longer
acquisition times or relaxation delays were employed). The
geometries of complexes 8 and 9 were further confirmed by
X-ray diffraction studies on single crystals obtained by
cooling their pentane solutions at -30 °C (Figures 6 and 7,
respectively).
the metal center in the former species and no interaction in
the latter. The 7Li{1H} NMR (δ CDCl3) spectrum also
consisted of two peaks at 5.8 and 4.1 ppm. The H NMR
1
(δ CDCl3) spectrum revealed a similar pattern, with some
differences in the chemical shifts in addition to the presence
of two THF molecules per molecule of ligand. Furthermore,
the signal corresponding to the BH proton had shifted
upfield to 4.91 ppm, suggesting a different chemical environ-
ment for this group. We tentatively assigned these differences
7
observed in the 11B{1H} and Li{1H} NMR spectra to the
THF solvent perturbing the B-H-Li interaction by coordi-
€
nation to the lithium center. Noth has previously shown that
Complexes 8 and 9 are isostructural (and their crystals are
isomorphous). Selected bond lengths and angles are high-
lighted in Table 2. Both complexes adopt a square-pyramidal
geometry with one COD ligand, two azaindolyl moieties
coordinating through the nitrogen atom of the pyridine
heterocycle, and a B-H interaction at the apical site on the
metal. The M-H bond distances in 8 and 9 are 2.00(4) and
both 11B and 7Li NMR chemical shifts are highly dependent
on the number of coordinated etherate solvents within
lithium hydroborate compounds.19 In order to confirm this,
a CDCl3 solution of crystalline material synthesized pre-
viously was prepared and the 1H NMR, 11B{1H} NMR
(-7.0 ppm), and 7Li{1H} NMR (5.1 ppm) spectra were
recorded. Upon addition of a few drops of THF, spectra
similar to those described above were observed, confirming
our assumption. We were unable to further verify this by a
crystallographic study, despite repeated attempts to obtain
single crystals.
˚
1.78(4) A, respectively. Even though these are similar (within
esds) to the corresponding M-H distances found in the
analogous complexes 2 and 3, there appears to be a greater
difference between the distance found in the case of complex
8 as compared to that found in 9. The elongation of the M-H
bond distances in 8 and 9 are consistent with the type 1 (for 9)
Once the identity of the new ligand (PhBai) was confirmed,
we investigated its coordination chemistry with group 9 transi-
tion-metal complexes containing diene coligands. The
complexes [Rh(COD){Ph(H)B(azaindolyl)2}] (8) and [Ir-
(COD){Ph(H)B(azaindolyl)2}] (9) were isolated, as orange
microcrystalline powders, employing a synthetic methodology
analogous to that for the preparation of 2 and 3 (Scheme 3).14
The compounds were fully characterized by spectroscopic
and analytical methods (Table 1). The 11B{1H} NMR spec-
tra revealed single peaks at -4.6 ppm (hhw 123 Hz) and -0.3
ppm (hhw 86 Hz) for 8 and 9, respectively (downfield from
the ligand (7), -6.9 ppm). A 11B NMR experiment resulted in
further broadening of the signals observed for 8 and 9
(hhw=180 and 156 Hz, respectively), suggesting coupling
to an adjacent proton. The BH resonances were located as
broad signals at 4.21 ppm (δ C6D6) for 8 and 3.51 ppm
(δ C6D6) for 9 in the 1H{11B} NMR spectra. The upfield shift
of this signal in comparison to that for the free ligand
(1H{11B}, δ(C7D8) 5.68 ppm) is suggestive of a strong inter-
action of the BH groups with the metal centers. This was also
confirmed by the IR spectra of 8 and 9 in the solid state (2117
and 2004 cm-1, respectively) and in solution (2088 and 1989
cm-1), both indicating a κ3N,N,H coordination which is
retained in solution (Table 1).
and type 2 (for 8) categorizations of 3c-2e M H-B
3 3 3
bonding interactions proposed by Spicer and Reglinski,20
using their criteria of d(M H) vs rcov (rcov = sum of the
P
3 3 3
covalent radii of the metal center and hydrogen).20b Accord-
˚
ingly, the Ir H bond distance in 9 is 1.78(4) A and is
3 3 3
P
˚
therefore 0.06 A greater than the rcov value, indicating
a significant covalent interaction with the metal center
(type 1). This is further reflected by a parallel elongation of
the B-H distance in 9. In the case of 8, the Rh H distance
3 3 3
P
˚
is 2.00(4) A, 0.27 A greater than the rcov value, consistent
˚
˚
with the type 2 category (where 0.25 A < d(M .H)
3 3 3
P
˚
-
rcov < 0.75 A). The B-H stretching frequencies in their
infrared spectra also reflect the structural data (cf. 2117 cm-1
(8) and 2004 cm-1 (9)), suggesting a marked reduction in the
bond order of the B-H bond in 9 compared to 8. This is
further supported by the chemical shift of the BH resonances
described above (4.21 ppm (8) and 3.51 ppm (9)). There also
appears to be a significant difference between the longest
metal-nitrogen bond distances for complex 9 and the pre-
˚
˚
viously reported complex 3 (cf. 2.138(3) A in 9 vs 2.163(3) A
in 3), which is not observed in rhodium examples 2 and 8.
Our studies with the new ligand architecture revealed no
evidence to suggest that hydride migration was occurring in
these complexes. We have recently shown that hydride
1
The H and 13C{1H} NMR data for both 8 and 9 were
consistent with the formation of [Rh(COD){κ3N,N,H-Ph-
(20) (a) Spicer, M. D.; Reglinski, J. Eur. J. Inorg. Chem. 2009, 1553.
ꢀ
Covalent radii were taken from: (b) Cordero, B.; Gomez, V.; Platero-Prats, A.
€
(19) (a) Giese, H.-H.; Noth, H.; Schwenk, H.; Thomas, S. Eur. J.
€
614-615, 168.
ꢀ
ꢀ
Inorg. Chem. 1998, 941. (b) Knizek, J.; Noth, H. J. Organomet. Chem. 200,
E.; Reves, M.; Echeverría, J.; Cremades, E.; Barragan, F.; Alvarez, S. Dalton
Trans. 2008, 2832.

5226 Organometallics, Vol. 28, No. 17, 2009
Table 1. 11B{1H} NMR Data for Scorpionate Complexes of 7-Azaindole-Based Ligands
11B{1H} NMR
Tsoureas et al.
complex
δ (ppm)a
1JB-H (Hz)
IR (B-H) power film (cm-1
)
ref
[Rh(COD)(κ3N,N,H-Tai)] (2)
[Rh(NBD)(κ3N,N,H-Tai)] (10)
[Ir(COD)(κ3N,N,H-Tai)] (3)
[Rh(COD)(κ3N,N,H-PhBai)] (8)
[Rh(NBD)(κ3N,N,H-PhBai)] (12)
[Ir(COD)(κ3N,N,H-PhBai)] (9)
-2.2
78
78
59
2106
2015
2102
2117
2022
2004
14
this work
14
this work
this work
this work
-1.6b, -1.5c
2.1
-4.6
unresolvedd
unresolvedd
unresolvedd
-3.5
-0.3
a In C6D6 unless otherwise stated. b In C7D8. c In CD2Cl2. d The B-H coupling constant was unresolved due to the broadness of the signal.
Table 2. Comparison of Structural Data for Complexes 2, 3, 8,
and 9
˚
bonding distance (A)/
angle (deg)
2a
3a
8
9
M-Nb
2.159(2)
2.134(2)
1.99(2)
1.17(3)
89.24(8)
138.9(18) 139.9(3)
83.4(7)
80.7(4)
110.7(2)
2.163(3)
2.125(3)
1.87(4)
1.25(5)
2.159(3)
2.139(3)
2.00(4)
1.15(4)
2.138(3)
2.119(3)
1.78(4)
M-Nc
M-H
B-H
N-M-N
B-H-M
N-M-Hb
N-M-Ha
N-B-N
1.281(19)
89.06(11) 89.22(11) 89.18(12)
136(3)
85.9(13)
80.5(14)
111.7(3)
143(5)
85.4(11)
82.4(11)
109.8(3)
85.2(19)
82.7(19)
109.9(3)
a Reference 14. b Longest M-N distance within structure. c Shortest
M-N distance within structure.
Table 3. 11B{1H} NMR Data for Scorpionate and Metallabora-
trane Complexes of 7-Azaindole-Based Ligands
complex
11B{1H} (ppm)a
[Rh(C7H9){κ4N,N,B,N-B(azaindolyl)3}] (11)
Ir(CO)2(C8H13){κ3N,N,B-B(azaindolyl)3}]b
Ir(CO)(C8H13){κ3N,N,B-B(azaindolyl)3}]b
5.0
5.8
-9.3
4.3
3.7
Figure 6. Molecular structure of 8. Hydrogen atoms, with the
exception of H100, have been omitted for clarity (thermal
ellipsoids are drawn at the 50% probability level).
Ir(CO)(CNC8H9)(C8H13){κ3N,N,B-B(azaindolyl)3}]b
Ir(CO)(CNC4H9)(C8H13){κ3N,N,B-B(azaindolyl)}]b
a In C7D8 solvent. b Reference 11.
[RhCl(NBD)]2 (NBD = bicyclo[2.2.1]hepta-2,5-diene) was
reacted with 2 molar equiv of K[Tai] and Li[PhBai].21
Two equivalents of K[Tai] was added to 1 equiv of
[Rh(NBD)Cl]2 in THF at -90 °C; the resulting mixture
was warmed to room temperature, during which time the
color of the solution changed from yellow to orange. A
11B{1H} NMR spectrum of the reaction mixture revealed
two signals at -1.6 and 5.0 ppm (THF, unlocked). The
former peak was in the region expected for the formation
1
of [Rh(NBD){κ3N,N,H-HB(azaindolyl)3}] (10) with a JBH
coupling constant of 78 Hz, as obtained from the 11B NMR
experiment (cf. [Rh(COD){κ3N,N,H-HB(azaindolyl)3}] (2):
δ(C6D6) 11B{1H} -2.2 ppm, 1JBH=78 Hz) (see also Table 1).
The signal at 5.0 ppm in the 11B{1H} NMR experiment
appeared as a doublet (J = 14 Hz, partially resolved hhw
25 Hz), suggesting coupling of the boron atom to the
rhodium center. Furthermore, the boron-coupled experi-
ment (11B) of this mixture revealed no changes for the signal
at 5.0 ppm, indicating that a direct B-H bond was absent
in this species. This chemical shift is also similar to our
Figure 7. Molecular structure of 9. Hydrogen atoms, with the
exception of H100, have been omitted for clarity (thermal
ellipsoids are drawn at the 50% probability level).
€
(21) (a) Bucher, U. E.; Currao, A.; Nesper, R.; Ruegger, H.; Venanzi,
migration was prompted when complex 3 was exposed to a
carbon monoxide atmosphere. We wondered whether hy-
dride migration could be facilitated by the use of a strained
olefin such as 2,5-norbornadiene in place of 1,5-cycloocta-
diene. In an attempt to test this, the rhodium complex
L. M.; Younger, E. Inorg. Chem. 1995, 34, 66. (b) Sanz, D.; Santa María,
M. D.; Claramunt, R. M.; Cano, M.; Heras, J. V.; Campo, J. A.; Ruíz, F. A.;
Pinilla, E.; Monge, A. J. Organomet. Chem. 1996, 526, 341. (c) Akita, M.;
Ohta, K.; Takahashi, Y.; Hikichi, S.; Moro-oka, Y. Organometallics. 1997,
16, 4121. (d) Adams, C. J.; Anderson, K. M.; Charmant, J. P. H.; Connelly, N.
G.; Field, B. A.; Hallett, A. J.; Horne, M. Dalton Trans. 2008, 2680.

Article
Organometallics, Vol. 28, No. 17, 2009 5227
Scheme 4. Synthesis of 10 and Subsequent Conversion to Complex 11
previously reported octahedral iridaboratranes containing
the κ3N,N,B coordination mode for Tai (Table 3). We were
therefore intrigued to discover whether the signal at 5.0 ppm
also corresponded to a product resulting from hydride
migration. The reaction mixture was heated to 65 °C, and
the 11B{1H} spectrum was recorded at regular intervals.
After 1 h, the 11B{1H} NMR spectrum revealed 97% con-
version to the product (11), corresponding to the peak at
5.0 ppm, while no further conversion occurred over longer
periods of heating or when higher temperatures were
employed (Scheme 4). Evaporation of all volatiles followed
by extraction in Et2O, filtration through diatomaceous
earth, and removal of diethyl ether gave a mixture of 10
(<3%, by 1H NMR integration) and 11 (>97%), in a good
yield (82%). Unfortunately, it was not possible to isolate 11
in spectroscopically pure form due to the contamination with
traces of complex 10.
experiments), indicating 3 different environments for the
azaindole rings. This was also verified by the presence of
21 signals in the aromatic region of the 13C{1H} NMR
spectrum. The 1H NMR spectrum also showed the absence
of (un)coordinated alkene protons (except for signals show-
ing 3% of 10), while the alkyl region of the spectrum
consisted of broad peaks at 2.17, 1.77, 1.58, 1.24, 1.10,
0.88, and -0.49 ppm (δ C7D8) integrating in total for 9
protons. The same region in the 13C{1H} NMR spectrum
consisted of 7 signals, 2 of which were determined to be
methylene groups and the other 5 as methyne by a DEPT-135
experiment. This evidence led us to the conclusion that the
hydride moiety had migrated from the boron center onto
the former norbornadiene group. However, the absence of
olefinic protons indicated that a further transformation had
occurred. The unusual rearrangement of the norbornadiene
ligand to a nortricyclyl group was further confirmed by
COSY, HMQC, and HMBC correlation experiments, which
agree with the assignment presented in Figure 8, and a
comparable rearrangement has previously been reported.23
Our assignment of the stereochemistry of the nortricyclyl
moiety was further confirmed by 1D-NOESY spectroscopy,
specifically NOE interactions between protons Ha and Hc, as
well as Hi and the aromatic He proton of the azaindole
heterocycle (Figure 12). Of particular note in the 13C{1H}
NMR spectrum of 11 was the presence of a doublet at 37.3
ppm corresponding to a CH with a 1JRhC coupling constant
of 28 Hz. This value is in agreement with the previously
reported examples.23
In order to confirm our assignment, an X-ray crystal-
lography study was carried out. Single colorless crystals were
obtained from a saturated pentane solution. The calculated
molecular structure of the resulting crystals is shown in
Figure 9. Crystallographic parameters are given below.
Complex 11 adopts a distorted-square-pyramidal geome-
try with all three azaindolyl moieties coordinating through
the nitrogen atom of the pyridine heterocycles, a boron atom
(at the apical site), and one nortricyclyl group (Figure 9).
There is no ligand in the site trans to boron. Complex 11
represents the first example of a κ4N,N,B,N coordination
mode for this flexible scorpionate ligand. In all previously
reported examples, only two out of the possible three azaindole
rings coordinated to the metal center in the metallaboratrane
Further spectroscopic analysis was carried out on the
isolated solid in order to confirm the identity of species 11.
The 11B NMR and 11B{1H} NMR (δ C7D8) spectra of the
isolated solid were identical with those of the reaction
mixture discussed above. The broad nature of the signals in
the boron spectra typically precludes the resolution of the
rhodium-boron coupling in the vast majority of complexes
containing a rhodium-boron bond. The coupling constant
only becomes resolved when the boron center is within a
highly symmetrical environment, where the line width of the
peak becomes small.22f There has been only one rho-
dium-borane complex where the 11B{1H} NMR signal
1
was sufficiently resolved to observe a JRhB coupling con-
stant of 80 Hz.10d It is unclear why the rhodium-boron
coupling constant is so small in the case of 11.
The infrared spectrum of the isolated solid showed no
observable bands in the region expected for B-H or Rh-H
functional groups. Furthermore, the 1H NMR spectrum (δ
C7D8) confirmed the absence of metal hydrides, since no
signals could be located in the upfield region of the spectrum.
1
The H NMR spectrum of 11 revealed 11 signals in the
downfield region of the spectrum, 7 signals integrating for 1
proton each and 4 other signals integrating for 2 pro-
tons (resulting from overlap of 2 independent signals,
respectively, and confirmed by 1H/13C{1H} correlation
(22) (a) Braunschweig, H. Angew. Chem., Int. Ed. 1998, 37, 1786. (b)
Irvine, G. I.; Lesley, M. J. G.; Marder, T. B.; Norman, N. C.; Rice, C. R.;
Robins, E. G.; Roper, W. R.; Whittell, G. R.; Wright, L. J. Chem. Rev. 1998,
98, 2685. (c) Smith, M. R.III Prog. Inorg. Chem. 1999, 48, 505. (d)
Braunschweig, H.; Colling, M. Coord. Chem. Rev. 2001, 223, 1. (e) Aldrich,
S.; Coombs, D. L. Coord. Chem. Rev. 2004, 248, 535. (f) Khattar, R.; Puga,
J.; Fehlner, T. P. J. Am. Chem. Soc. 1989, 111, 1877.
(23) (a) Mata, J. A.; Peris, E.; Incarvito, C.; Crabtree, R. H. Chem.
Commun. 2003, 184. (b) Dotra, R.; Konstantinovski, L.; Shimon, L. J. W.;
Ben-David, Y.; Milstein, D. Eur. J. Inorg. Chem. 2003, 70. (c) Mail, R. E.;
Garralda, M. A.; Hernandez, R.; Ibarlucea, L.; Pinilla, E.; Torres, M. R.;
Zarandona, M. Eur. J. Inorg. Chem. 2005, 1671. (d) Trauthwein, H.; Tillack,
A.; Beller, M. Chem. Commun. 1999, 2029.
ꢀ

5228 Organometallics, Vol. 28, No. 17, 2009
Tsoureas et al.
Figure 8. 1H and 13C{1H} NMR assignments of the nortricyclyl
group within complex 11.
Figure 10. Molecular structure of 10. Hydrogen atoms (with
the exception of H100) and two disordered molecules of toluene
have been omitted for clarity (thermal ellipsoids are drawn at the
˚
50% probability level). Selected bond lengths (A) and angles
(deg): Rh(1)-N(1)=2.160(3), B(1)-H(100)=1.15(5), Rh(1)-H-
(100)=1.98(5); N(1i)-Rh(1)-N(1)=93.93(14), N(1)-Rh(1)-H-
(100) = 82.6(10), N(2)-B(1)-N(2i) = 108.9(4).
Figure 11. Change in coordination mode for Tai induced by
carbon monoxide.
Figure 9. Molecular structure of 11. Hydrogen atoms have been
omitted for clarity (thermal ellipsoids are drawn at the 50%
˚
probability level). Selected bond lengths (A) and angles (deg):
Rh(1)-N(5) = 2.057(3), Rh(1)-N(1)=2.063(3), Rh(1)-B(1)=
2.064(4), Rh(1)-Cc = 2.135(11), Rh(1)-N(3) = 2.186(3); N-
(5)-Rh(1)-B(1) = 83.41(15), N(1)-Rh(1)-B(1) = 81.92(15),
N(5)-Rh(1)-N(3)=91.64(11), N(1)-Rh(1)-N(3)=94.30(11),
B(1)-Rh(1)-N(3) = 84.53(14), Cc-Rh(1)-N(3) = 178.0(3),
N(2)-B(1)-N(6)= 121.0(3), N(2)-B(1)-N(4) = 108.3(3), N-
(6)-B(1)-N(4) = 111.0(3), N(2)-B(1)-Rh(1) = 105.7(3), N-
(6)-B(1)-Rh(1) = 104.8(3), N(4)-B(1)-Rh(1) = 104.9(2),
N(5)-Rh(1)-N(3)=91.64(11), N(1)-Rh(1)-N(3)=94.30(11),
B(1)-Rh(1)-N(3) = 84.53(14), N(5)-Rh(1)-Cc = 90.3(3), N-
(1)-Rh(1)-Cc = 83.9(3), B(1)-Rh(1)-Cc = 96.0(4), N(5)-Rh-
(1)-N(1)=163.56(12). The carbons of the nortricyclyl moiety are
named after the protons attached to them, as shown in Figure 8.
Figure 12. Labeling scheme for compounds described herein.
(which range between 105.61(6) and 115.5(4)°). This strained
angle is accompanied by a displacement of the boron atom
from the axial site (Cc-Rh(1)-B(1) = 96.0(4)°). Such a
correlation between these two angles can also be seen in a
previous example reported by Hill in [RhCl{B(mt)3(PPh3)]
(13; Cl trans to Rh-B) where an even larger distortion is
observed (P-Rh1-B = 98.27(16)°).9d Significant distortion is
also observed in the plane of complex 11; for example,
N(1)-Rh-N(5) = 163.56(12)°. It is as yet unclear whether
this displacement from idealized geometry (as derived from
crystallographic data)9d,g,10d can be correlated to the chemical
shift and broadening of the resulting 11B{1H} NMR signals. In
compounds.11 We had attributed this fact to the rigid nature of
the fused bicyclic aromatic rings. Complex 11 highlights that
the tetradentate coordination mode is indeed possible,
although there appears to be some strain associated with its
coordination. The N(6)-B(1)-N(2) angle is 121.0(3)°, repre-
senting a considerable distortion with respect to the idealized
angle expected for a tetrasubstituted boron atom (cf. those
found in K[Tai]; 108.66(17), 108.92(17), and 109.32(17)°).14
This angle is also significantly larger than those found in the
sulfur-based rhodaboratranes reported by Hill9d,g and
[Rh{B(taz)3}(CO)(PPh3)]PF6 (12; taz = 4-ethyl-3-methyl-
1H-1,2,4-triazole-5(4H)-thione) reported by Connelly10d
˚
complex 11, the Rh(1)-B(1) bond length (2.064(4) A) is
significantly shorter than in the case of complexes 12 and 13
˚
˚
(2.155(5) A for 12; 2.132(6) and 2.122(7) A for two indepen-
dent molecules of 13 cocrystallized). Indeed, there is only
one reported example which contains a shorter metal-
boron distance, [Pd{κ4S,S,B,S-(mttBu)3}(PMe3)], where the
palladium-boron distance is 2.050(8) A.10b The Rh(1)-B(1)
˚

Article
Organometallics, Vol. 28, No. 17, 2009 5229
Scheme 5. Reaction of Li[PhBai] with [RhCl(NBD)]2, Revealing That No Hydrogen Migration Occurs with This Ligand
distance found in complex 11 is also within the range found for
structurally characterized rhodium-boryl complexes.22 Of the
three rhodium-nitrogen distances in 11, the Rh(1)-N(3)
azaindole rings could be substituted from [Ru{κ3N,N,H-
HB(azaindolyl)3}(Cp*)] in the presence of carbon monoxide
without the loss of the B-H interaction with the metal center
(Figure 11).
˚
distance is longer (2.186(3) A), as a result of the large trans
influence of the rhodium-alkyl σ-bond. Finally, the rho-
It was of interest to us whether the new ligand, PhBai,
would show a hydride migration reactivity similar to that
found for Tai. A different coordination mode would cer-
tainly be obtained, since the κ4N,N,B,N coordination mode
is not possible in this case. Upon reaction of 2 equiv of
Li[PhBai] with 1 equiv of [Rh(NBD)Cl]2 in THF at -90 °C
followed by equilibration to room temperature and standard
reaction workup (see the Experimental Section), the 11B{1H}
NMR spectrum of the reaction mixture revealed only one
signal at -3.9 ppm (s, hhw = 138 Hz; 11B NMR δ (C6D6)
-3.5 (s, Δν1/2 = 189 Hz)). The compound was isolated in
high yield and was fully characterized by spectroscopic and
analytical methods. A comparison with the analogous com-
plexes (see Table 1) suggested the formation of the nonacti-
vated complex [Rh(NBD){κ3N,N,H-Ph(H)B(azaindolyl)2}]
(12). This was further confirmed by the analysis of the
˚
dium-carbon distance (2.135(11) A) is similar to that in
the η1-nortricyclyl rhodium complex reported by Mail
(2.12(2) A).23c All other metric data for the nortricyclyl moiety
˚
are similar to those reported in the literature.23
The isolation of complex 10 was targeted in order to probe
the position of the hydride. We have previously reported the
structural characterization of [Ru{κ4H,B,S,S-HB(CH2Ph)-
(mt)2}(Cl)(PCy3)], where the hydride was located at an
intermediate point between the ruthenium and boron cen-
ters.12 Single crystals of 10 were obtained from a saturated
toluene solution containing a mixture of 10 and 11. Further
noncrystalline precipitate obtained from the mixture was
found to contain both complexes in an approximately 1:1
1
ratio (as determined by H and 11B NMR spectroscopy).
Upon standing, at room temperature, it was found that
complex 10 slowly converted into complex 11 over time
(see the Experimental Section). An X-ray diffraction study
was performed on the single crystals obtained. The calcu-
lated structure, shown in Figure 10, revealed the expected
structure assigned to species 10, [Rh{κ3N,N,H-HB(aza-
indolyl)3(NBD)].
1
spectroscopic data, most notably the H NMR spectrum,
which indicated the coordination of the norbornadiene
ligand in a η4 mode. The spectrum also suggests a Cs
symmetrical molecule, as it exhibits three signals for the
norbornadiene ligand at 0.85, 3.15, and 3.25 ppm (δ C6D6; in
a 2:4:2 integral ratio, respectively). The geometry around the
metal center is also verified by the aromatic region of the
spectrum, where signals attributed to the azaindolyl rings
show that these experience one chemical environment.
Unlike the case for complex 10, we were unable to obtain
any evidence for the migration of the hydride between boron
and the metal center in complex 12, in various solvents at
elevated temperatures. This observation is certainly due to
the replacement of one azaindolyl ring with a phenyl group.
This suggests that the κ4N,N,B,N coordination mode in the
final product is a requirement for hydride migration to
occur.
Complex 10 shows structural features similar to those of
complexes 3 and 8. The complex adopts a square-pyramidal
geometry with one NBD ligand, two azaindolyl moieties
coordinating through the nitrogen atom of the pyridine
heterocycle, and a B-H interaction at the apical site on the
˚
metal. The M-H bond distance of 1.98(5) A is similar to the
˚
cyclooctadiene examples (cf. 1.99(2) A (3), 2.00(4) A (8)) and
˚
˚
is also consistent with a type 2 M H-B interaction (0.25 A
3 3 3
P
greater than the
rcov).20 In contrast to the structural
evidence, the B-H stretching frequency for 10 (2015 cm-1
)
is somewhat lower than these 3 and 8 (2106 cm-1 (3) and
2117 cm-1 (8)). It is, however, similar to the stretching
frequency for the complex [Rh{κ3N,N,H-H(Ph)B(aza-
indolyl)3}(NBD)] (12), which is outlined below (2022 cm-1),
indicating a reduction in the bond order of the B-H bond in
the norbornadiene examples.
Concluding Remarks
In summary, we have reported the synthesis of the novel
analogue of the K[Tai] scorpionate ligand Li[PhBai] (7),
where an azaindolyl moiety has been replaced by a phenyl
ring, and have explored its coordination chemistry with
group 9 transition metals. The subtle differences of the two
ligand scaffolds were demonstrated by the structural and
spectroscopic comparison of their coordination compounds
with Rh(I) and Ir(I) metal centers. The effect of changing the
substituents around the boron center was highlighted by the
difference of reactivity of the two ligand sets toward
[Rh(NBD)Cl]2. When 7 was reacted with [Rh(NBD)Cl]2, a
product analogous to the η4-cyclooctadiene complexes 2 and
8 was obtained. On the other hand, when the same reaction
was performed using K[Tai], the novel rhodaboratrane
We found no evidence of a rhodium-hydride species (or
any other intermediate) during the reaction, suggesting that
insertion of the olefin into a transient rhodium-hydride
bond occurs very rapidly. A further possibility could involve
direct attack of the olefin by the “hydride” species. Finally, a
third possibility could involve dissociation of a second
azaindolyl ring from complex 10 to form a complex such as
[Rh{κ2N,H-HB(azaindolyl)3(NBD)]. This compound could
subsequently undergo a B-H activation followed by olefin
insertion and recoordination of the second azaindolyl ring.
There is precedent for the κ2N,H coordination mode for the
Tai ligand; Kuwata and Ikariya15 showed that one of the

5230 Organometallics, Vol. 28, No. 17, 2009
Tsoureas et al.
complex 11 was obtained in good yields. This complex is the
first example where a κ4N,N,B,N coordination mode is
observed with nitrogen-based scorpionate ligands. The for-
mation of the metallaboratrane complex is accompanied by
rearrangement of the η4-norbornadiene ligand to form a
rhodium-nortricyclyl σ bond. The fact that this transforma-
tion is not observed in the case of PhBai suggests that the κ4
coordination mode is necessary for this transformation to
occur. We are currently exploring the mechanism for the
formation of 11 as well as its reactivity toward σ-bases and π-
acids, and the results will be presented in a forthcoming
publication.
and the residue washed twice with 10 mL of Et2O. The filtrate
was stripped of volatiles under vacuum and extracted in Et2O
(15 mL), the extract was filtered through Celite, and the Celite
pad was washed (3 ꢀ 10 mL) with Et2O. During the washing the
product starts precipitating out of the filtrate. This was reduced
to half and the solid isolated by filtration. The filtrate was chilled
at -30 °C to give a second crop of Li[PhBH3] 2THF, which was
3
isolated by filtration. Combined yield: 1.2 g (47.7%). 1H NMR δ
=
(C7D8): 1.35 (8H, m, CH2, THF), 1.5 (3H, broad quart, 1JBH
77.6 Hz, LiBH3Ph), 3.61 (8H, m, CH2, THF), 7.17 (1H, virtual t,
aromatic), 7.35 (2H, virtual t, aromatic), 7.77 (2H, broad s,
aromatic). 11B{1H} NMR δ (C7D8): -27.1. 11B NMR δ (C7D8):
1
(quart, JBH =77.6 Hz). 13C{1H} NMR δ (C7D8): 25.1 (CH2,
THF), 68.1 (CH2, THF), 123.3, 126.8, 135.5 (aromatics). The
ipso B-Ph was not detected.
Experimental Section
Lithium Phenylbis(7-azaindolyl)borate (7; Li[PhBai]).
Schlenk tube was charged with Li[PhBH3] 2THF (0.5 g, 2.06
A
General Remarks. All manipulations were performed in a
Braun glovebox with an O2 and H2O atmosphere of below 5
ppm or by using standard Schlenk techniques. Solvents
(toluene, THF, Et2O) were dried using a Grubbs alumina system
and were kept in Young ampules under N2 over molecular sieves
3
mmol) and 7-azaindole (0.586 g, 4.96 mmol). Toluene (15 mL)
was added, and the reaction mixture was heated to 120 °C for
2 days under N2 or until all Li[PhBH3] 2THF was consumed, as
3
judged by 11B NMR and 11B{1H} NMR. Upon completion the
reaction mixture adopted a dark orange-brown coloration. It
was filtered hot through glass microfiber paper, and the filtrate
was chilled to -30 °C overnight to form white-pale yellow
crystals. These were separated by filtration and washed with
pentane (5 mL) to give 7. Li[PhBai] is soluble in chloroform,
methylene chloride, and toluene. Yield: 344.00 mg (44.6%).
ꢀ
˚
(4 A). Dry n-pentane (<0.05 ppm of H2O) was purchased from
Fluka and was kept in a Young ampule under N2 over molecular
sieves (4 A). [Ir(COD)Cl]224 and [Rh(COD)Cl]225 were synthe-
ꢀ
˚
sized according to standard literature procedures. Li[Ph-
BH3] 2THF was prepared via a modification of the published
3
procedure26 (see below). Deuterated solvents were degassed by
three freeze-thaw cycles, dried over the appropriate agent
(C7D8 refluxing over Na for 12 h; C6D6 refluxing over Na/
benzophenone for 12 h; CD2Cl2 refluxing over CaH2 for 12 h,
Anal. Calcd for C20H11N4LiB 0.25C7H8: C, 73.97; H, 5.14; N,
3
15.86. Found: C, 73.47; H, 5.60; N, 15.26. 1H NMR δ (CD2Cl2):
4.8-6.0 (1H, br, BH), 6.40 (2H, d, 3JHH=3.3 Hz, Hb azaindole),
6.65 (2H, dd, 3JHH=5.0 Hz, 3JHH = 7.8 Hz, Hd, azaindole), 6.94
(2H, d, JHH=3.3 Hz, Ha azaindole), 7.13 (2H, dd, 4JHH = 1.3
Hz, 3JHH = 5.00 Hz, Hc azaindole), 7.30 (5H, m, phenyl), 7.80
(2H, dd, 4JHH=1.3 Hz, 3JHH=7.8 Hz, He azaindole). 11B{1H}
NMR δ (CD2Cl2): -6.88 (s, br, Δv1/2 = 132.2 Hz). 11B NMR δ
(CD2Cl2): -6.88 (s, br, Δv1/2 = 198.4). 7Li{1H} NMR δ (C7D8):
4.79 (s). 1H{11B} NMR δ (C7D8): 5.68 (1H, BH). 13C{1H} NMR
δ (CD2Cl2): 100.2, 114.4, 124.1, 127.9, 129.6, 129.9, 132.4, 133.8,
139.9, 151.4. The ipso B-Ph was not detected. IR: 2196.4 cm-1
(νB-H). MS m/z (ESI-): 323.16 [M]-, 653.34 [2 M þ Li]-.
Details for Scale-Up Preparation of 7. A Schlenk tube was
charged with 7-azaindole (1.17 g, 9.91 mmol) and Li-
ꢀ
˚
CDCl3 stirred over 4 A molecular sieves), and then vacuum-
ꢀ
˚
distilled and kept in Young ampules over 4 A molecular sieves
under N2. 1H NMR, 11B{1H} NMR, and 11B NMR spectra
were recorded on a JEOL ECP300 spectrometer operating at
300 MHz (1H). 13C{1H} NMR, DEPT-135, and homo-/hetero-
nuclear correlation experiment spectra were recorded on a
Varian VNMR S500 instrument operating at 500 MHz (1H)
or a JEOL ECP400 spectrometer operating at 400 MHz (1H).
The spectra were referenced internally, to the residual protic
solvent (1H) or the signals of the solvent (13C). 11B{1H} NMR
and 11B NMR spectra were referenced externally relative to
BF3 OEt2. A labeling scheme for the hydrogen atoms is given in
3
[PhBH3] 2THF (1.00 g, 4.13 mmol) in the glovebox. Anhydrous
3
Figure 12. Mass spectra were recorded on a VG Analytic
Quattro instrument in ESIþ mode or on a 4700 Proteomics
Analyzer (Applied Biosystems) in MALDI mode. Elemental
analyses were performed at the microanalytical laboratory of
the School of Chemistry at the University of Bristol. Infrared
spectra were recorded on a Perkin-Elmer Spectrum One FT-IR
spectrometer (solution, NaCl cell) or a Perkin-Elmer Spectrum
100 FT-IR spectrometer (solid state, neat) from 4000 to
650 cm-1. Nortricyclyl refers to tricyclo[2.2.1.02,6]heptanyl.
toluene was added under N2, and the reaction mixture was
refluxed for 3 days until no Li[PhBH3] 2THF was observed by
3
11B NMR and 11B{1H} NMR. During this time a gray pre-
cipitate had formed. The solution was filtered hot through glass
microfiber filter paper, and the filtrate upon cooling to room
temperature and standing for 1 h gave a first crop of white solid
(250 mg). This was isolated by filtration as above, and the filt-
rate was reduced to approximately half-volume and chilled at
-30 °C overnight to give a second crop (729 mg) of the bis-THF-
solvated form of 7. The NMR spectra of the two crops were
identical and are described in the Results and Discussion.
Combined yield: 50%.
Li[PhBH3] 2THF. A three-neck 250 mL round-bottom flask
3
equipped with an N2 inlet, a magnetic stirring bar, and a
pressure-equalizing addition funnel was charged with 2,20-bi-
pyridine (1.2 mg, 0.0078 mmol) and degassed. To this was added
10 mL of a solution of LiAlH4 in Et2O (1.0 M) via syringe to give
a pink-purple solution. Phenylboronic acid (800 mg, 6.6 mmol)
was placed in a Schlenk tube, degassed, and then dissolved in a
mixture of 16 mL of Et2O and 10 mL of THF. This solution was
transferred to the pressure-equalizing addition funnel and
added slowly at room temperature to the LiAlH4 solution until
the coloration of the solution was discharged (a dusky solution
is formed). This was filtered through a glass-microfiber filter
[Rh(COD)(K3N,N,H-{Ph(H)B(azaindolyl)2})] (8). In a glove-
box a Schlenk tube was charged with [Rh(COD)Cl]2 (60 mg,
0.122 mmol) and 7 (80.4 mg, 0.244 mmol). The two solids were
cooled to -90 °C (acetone/liquid N2), and THF (15 mL) was
added with stirring. Upon addition a yellow solution formed,
which was allowed to equilibrate to room temperature. Volatiles
were removed under vacuum, and the residue was extracted with
hexane (4 ꢀ 15 mL) and filtered through Celite. Volatiles were
removed from the filtrate to leave a yellow precipitate, which
was washed with pentane (10 mL), filtered, and dried under
vacuum. The pentane washing was reduced under vacuum by
one-third and cooled to -30 °C and then at -80 °C to produce
crystals of 8 suitable for a single-crystal X-ray diffraction study.
(24) Herde, J. L.; Lambert, J. C.; Senoff, C. V. Inorg. Synth. 1974, 15,
18.
(25) Giordano, G.; Crabtree, R. H. Inorg. Synth. 1990, 28, 88.
(26) (a) Singaram, B.; Cole, T. E.; Brown, H. C. Organometallics
1984, 3, 774. (b) Dodds, C. A.; Garner, M.; Reglinski, J.; Spicer, M. D. Inorg.
Chem. 2006, 45, 2733.
Yield: 93.8 mg (72%). Anal. Calcd for C28H28N4RhB 0.1-
(pentane): C, 63.24; H, 5.45; N, 10.34. Found: C, 63.73; H,
3

Article
Organometallics, Vol. 28, No. 17, 2009 5231
5.45; N, 10.66. 1H NMR δ (C6D6): 1.55 (4H, m, JHH = 7.7 Hz,
COD), 2.31 (4H, m, COD), 3.73 (4H, broad s, COD), 6.25 (2H,
broad d, JHH = 2.9 Hz), 6.37 (1H, d, JHH = 8.1 Hz), 6.40 (1H, d,
of 11 started forming. When the washing was left for approxi-
mately 10 days, a second crop of 11 (10 mg) was isolated.
Combined yield: 211 mg (73%). The solid is contaminated by
approximately 3% 10. The two cocrystallize, and repeated
recrystallization from pentane, toluene, or their mixtures
resulted in the isolation of 11 contaminated by approximately
JHH = 7.3 Hz), 7.27 (2H, dd, JHH = 1.5 Hz, J = 8.1 Hz), 7.44
(1H, virtual t, JHH = 7.3 Hz), 7.62 (2H, virtual t, JHH = 7.3 Hz),
7.78 (2H, unresolved d, JHH=3.7 Hz), 8.21 (2H, unresolved d,
JHH = 6.6 Hz), 8.51 (2H, unresolved d, JHH = 5.1 Hz). 1H{11B}
NMR δ(C6D6): 4.21 (br, s, 1H, BH). 13C{1H} NMR δ(C6D6):
31.2 (CH2, COD), 75.9 (CH, COD) 101.2, 114.2, 124.6, 125.8,
127.9, 128.5, 132.9, 133.7, 143.1, 153.0. The ipso B-Ph was not
detected. 11B{1H} NMR δ (C6D6): -4.56 (s, br, Δv1/2 = 123.0
Hz). 11B NMR δ (C6D6): -4.56 (s, br, Δv1/2 =180.3 Hz). IR:
2116.9 cm-1 (νB-H). MS m/z (ESIþ): 534.14 [M þ H]þ.
3% 10. Anal. Calcd for C28H24BRhN6 1/3C5H7: C, 61.19; H,
3
4.85; N, 14.43. Found: C, 61.30; H, 4.93; N, 14.68. 1H NMR δ
(C7D8): -0.49 (1H, virtual t, JHH=5.1 Hz, He), 0.75 (2H, m, Hf
and Hd), 0.88 (1H, s, Ha), 1.10 (1H, broad d, Hb), 1.24 (1H,
broad s, Hc), 1.58 (1H, broad d, Hh), 1.77 (1H, broad s, Hg), 2.17
(1H, broad d, Hi), 6.02 (1H, d, 3JHH = 3.3 Hz, Hb), 6.30 (2H, m,
azaindole), 6.39 (1H, d, 3JHH = 3.0 Hz, azaindole), 6.42 (1H, d,
3JHH = 3.2 Hz, Ha), 6.56 (1H, dd, 3JHH = 5.1 Hz, 3JHH = 7.7
Hz, Hd), 7.15 (1H, d, 3JHH = 3.7 Hz, azaindole), 7.20 (2H, m,
[Ir(COD)(K3N,N,H-{Ph(H)B(azaindolyl)2}] (9). In a glove-
box a Schlenk tube was charged with [Ir(COD)Cl]2 (60 mg,
0.089 mmol) and 7 (58.9 mg, 0.178 mmol). The two solids were
cooled to -90 °C (acetone/liquid N2), and THF (8 mL) was
added with stirring. Upon addition an orange-red solution
formed, which was warmed to room temperature and stirred
at this temperature for 2 h. Volatiles were removed under
vacuum to leave an orange residue, which was extracted with
diethyl ether (3 ꢀ 10 mL) and filtered through Celite. The yellow
filtrate was reduced under vacuum to leave an orange solid,
which was washed with pentane (10 mL) and dried under
vacuum. The pentane washings were collected by filtration
and cooled to -30 °C to give orange crystals of 9 suitable for
a single-crystal X-ray diffraction study. Yield: 78.9 mg (71%).
Anal. Calcd for C28H28BIrN4: C, 53.93; H, 4.53; N, 8.98. Found:
3
azaindole), 7.24 (1H, broad d, JHH = 7.5 Hz, Hc), 7.58 (2H,
virtual t, 3JHH = 3.2 Hz, azaindole), 7.85 (2H, dd, 3JHH = 5.3
Hz, 3JHH = 13.0 Hz, azaindole), 8.30 (1H, dd, 3JHH = 5.1 Hz,
4JHH = 1.2 Hz, He) (further assignment was impeded due to the
overlap of protons of the two other azaindolyl rings located
trans to each other in the coordination sphere). 11B{1H} NMR δ
1
(C7D8): 5.00 (d, JBRh = 14.1 Hz, Δv1/2 = 25.2 Hz,
[Rh(NBD){κ4N,N,N,B-Tai}]). 13C{1H} NMR δ (C7D8): 12.9
and 13.2 (CHf/d), 17.2 (CHe), 35.5 (CHaHb), 36.2 (d, 1JRhC=2.0
Hz, CHg), 36.7 (CHhHi), 37.3 (d, 1JRhC = 28.4 Hz, CHc), 102.3,
103.6, 103.7, 114.8, 115.2, 115.4, 119.4, 120.4, 120.6, 127.2, 127.7
(behind d8-toluene residual peak but shows on the DEPT-135),
128.09, 128.10, 128.14, 128.5, 141.4, 143.8, 144.0, 152.2, 157.9,
158.0. MS m/z (ESIþ): 447.33 [M - NBD]þ, 559.13 [M þ H]þ.
[Rh(NBD){K3N,N,H-Ph(H)B(azaindolyl)2}] (12). In a glove-
box a Schlenk tube was charged with [Rh(NBD)Cl]2 (43.5 mg,
0.094 mmol) and 7 (62.1 mg, 0.188 mmol). The mixture was
cooled to -90 °C (acetone/liquid N2), and THF (10 mL) was
added with stirring. The reaction mixture was warmed to room
temperature, upon which time a yellow solution was obtained.
This was heated to 70 °C for 12 h, volatiles were removed under
vacuum and extracted with diethyl ether (3 ꢀ 10 mL), and the
extract was filtered through Celite. Volatiles were removed
under vacuum to give a yellow residue, which was washed with
pentane (10 mL) and dried under vacuum. The pentane wash-
ings were reduced to approximately half and chilled to -30 °C to
give 12 as an analytically pure and microcrystalline solid. Anal.
Calcd for C27H24BRhN4: C, 62.58; H, 4.67; N, 10.81. Found: C,
62.50; H, 5.01; N, 11.09. 1H NMR δ (C6D6): 0.85 (2H, m
(appears as virtual t), NBD), 3.15 (4H, dd, JHH = 2.0 Hz,
JHH = 4.9 Hz, CHdCH NBD), 3.25 (2H, m, CH NBD), 4.3
(1H, very broad, BH), 6.31 (2H, d, 3JHH=3.3 Hz, Hb azaindole),
6.45 (2H, dd, 3JHH = 5.3 Hz, 3JHH = 7.7 Hz, BC6H5), 7.34 (2H,
d, 3JHH = 1.5 Hz, 3JHH=7.7 Hz, Hc azaindole), 7.39 (1H, m,
aromatic), 7.52 (2H, m, aromatic), 7.69 (2H, d, 3JHH=3.3 Hz,
Ha azaindole), 8.07 (2H, unresolved d, JHH = 8.1 Hz, BC6H5),
1
C, 54.23; H, 4.96; N, 9.77. H NMR δ (C6D6): 1.52 (4H, m,
JHH=7.74 Hz, COD), 2.21 (4H, m, JHH=5.14 Hz, COD), 3.46
(4H, broad s, COD), 6.32 (2H, dd, JHH = 6.6 Hz, JHH = 7.3 Hz,
aromatic Ph), 6.40 (2H, d, 3JHH = 3.7 Hz, Hb azaindole), 7.25
(2H, d, 3JHH = 8.1 Hz, Hc azaindole), 7.37 (1H, virtual t, JHH
=
8.1 Hz, JHH = 6.6 Hz aromatic phenyl), 7.52 (2H, virtual t, 3JHH
=
7.3 Hz, 3JHH = 8.1 Hz, Hd azaindole), 7.63 (2H, d, 1JHH = 3.7 Hz,
Ha azaindole), 8.05 (2H, d, 3JHH=7.3 Hz, He azaindole), 8.55 (2H, d,
JHH =5.1 Hz, phenyl aromatic). 1H{11B} NMR δ(C6D6): 3.51 (br, s,
1H, BH). 11B{1H} NMR δ (C6D6): -0.32 (s, br, Δv1/2 = 86.42 Hz).
11BNMRδ(C6D6):-0.32 (s, br, Δv1/2 = 155.88 Hz). 13C{1H} NMR
δ (C6D6): 33.1 (CH2, COD), 58.5 0(broad, CH, COD), 102.9, 115.4,
125.0, 126.9, 128.5 (under solvent residual peak but appears in DEPT-
135), 129.1, 132.5, 134.3, 144.2, 155.1. The ipso B-Ph was not detected.
IR: 2004.3 cm-1 (νB-H). MS m/z (MALDI): 623.2 [M]þ, 515.1 [M -
COD]þ.
[Rh(NBD)(K3N,N,H-{HB(azaindolyl)3})] (10). This was iso-
lated as crystalline material from a saturated d8-toluene solution
of 10 standing at room temperature overnight. 1H NMR
δ(CD2Cl2): 1.24 (2H, virtual t, NBD), 3.63 (4H, virtual dd,
JHH = 2.2 Hz, JHH=5.1 Hz, CHdCH NBD), 3.81 (2H, m, CH
NBD), 6.43 (3H, broad s, azaindole), 6.99 (3H, broad m,
azaindole), 7.86 (3H, broad d, azaindole), 8.11 (3H, broad s,
azaindole), 8.46 (3H, broad d, azainole). 11B{1H} NMR δ
(CD2Cl2): -1.52. 11B NMR δ (CD2Cl2): -1.52 (d, 1JBH=77.6
Hz). IR: 2014.6 cm-1 (νB-H). Further spectroscopic analysis
was hindered, as the compound rearranges to 11 (∼97%)
standing at room temperature overnight as judged by 11B and
11B{1H} NMR.
3
8.26 (2H, d, JHH = 5.3 Hz, He azaindole). 11B{1H} NMR δ
(C6D6): -3.85 ppm (s, br, Δv1/2=137.8 Hz). 11B NMR δ (C6D6):
-3.51 ppm (s, br, Δv1/2=189.4 Hz). 13C{1H} NMR δ (C6D6):
41.9 (d, JRhC = 10.0 Hz, CHdCH), 48.0 (d, JRhC=3.1 Hz,
1
2
3
CH), 58.7 (d, JRhC = 6.1 Hz, CH2), 101.41, 114.18, 124.17,
126.21, 127.89, 128.47, 132.59, 134.22, 141.76, 153.22. The ipso
B-Ph was not detected. IR: 2021.6 cm-1 (νB-H). MS m/z
(ESIþ): 425.05 [M þ H - NBD]þ, 518.12 [M þ H]þ.
[Rh(nortricyclyl){K4N,N,B,N-B(azaindolyl)3}] (11). In a glo-
vebox a Schlenk tube was charged with [Rh(NBD)Cl]2 (120 mg,
0.26 mmol) and K[Tai] (210.6 mg, 0.52 mmol). The mixture of
the two solids was cooled to -90 °C (acetone/liquid N2), THF
(10 mL) was added with stirring, and the reaction mixture was
warmed to room temperature, upon which time it adopted an
orange-yellow coloration. The reaction mixture was then heated
at 65-70 °C under N2 for 1 h, and then the volatiles were
removed under vacuum, the residue was extracted with diethyl
ether (3 ꢀ 15 mL), and the extract was filtered through Celite.
Volatiles were removed under vacuum to leave a yellow-orange
solid, which was washed with pentane (10 mL). The pentane
washing was left standing under N2, and after ca. 30 min crystals
X-ray Crystallography. The data for 7 and 9-11 were col-
lected on a Bruker Kappa Apex II CCD detector diffractometer
˚
with a fine-focus sealed-tube Mo KR (wavelength 0.710 73 A)
radiation source and a Cryostream Oxford Cryosystems low-
temperature device, operating in ω scanning mode with ψ and
ω scans to fill the Ewald sphere. Data for 8 were collected
on a Bruker Microstar diffractometer with a Cu KR rotating
˚
anode (wavelength 1.541 78 A), equipped with a Platinum
135 CCD detector and a Cryostream Oxford Cryosystems
low-temperature device, operating in ω scanning mode with ψ
and ω scans to fill the Ewald sphere. Collections for 7, 8, and 11

5232 Organometallics, Vol. 28, No. 17, 2009
Tsoureas et al.
were performed at 100 K and those for 9 and 10 at 173 K. The
programs used for control and integration were APEXII,
SAINT v7.34A, and XPREP v2005/4.27 The crystals were
mounted on glass fibers with silicon grease, from Parafin oil.
All solutions and refinements were performed using the
Bruker SHELXTL software and all software packages within.28
All non-hydrogen atoms were refined using anisotropic thermal
parameters, and hydrogen atoms were added using a riding
model. A disordered toluene molecule was squeezed out of the
and refinement details is presented in the Supporting Informa-
tion (Table S1).
Acknowledgment. We thank the Royal Society (G.R.
O.), the Leverhulme Trust (N.T.) and the EPSRC (A.H.)
for funding and Johnson Matthey for the generous
loan of the platinum group metal salts. We also thank
Dr. M. F. Haddow for help with the crystal structure
determination of complex 11. G.R.O. is a Royal Society
Dorothy Hodgkin Research Fellow.
unit cell in both 7 and 10, using the program Platon.29
A
summary of the crystallographic data collection parameters
(27) Bruker-AXS, 2007; Bruker-AXS, 2005.
(28) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr.
2008, 64, 112.
(29) Spek, A. L. PLATON, A Multipurpose Crystallographic Tool;
Utrecht University, Utrecht, The Netherlands, 2008.
Supporting Information Available: CIF files and tables giving
crystallographic data for compounds 7-11, including collection
parameters and refinement details. This material is available free
of charge via the Internet at http://pubs.acs.org.