Revisiting the 1,2,4-triaza-3,5-diborolyl ligand: σ and π coordination modes in the alkali metal and rhodium complexes of a planar, 6-π-electron B2N3- ring

Article abstract of DOI:10.1021/ic7013286

Lithium (2a), sodium (2b), and potassium (2c) salts of 1-methyl-3,5- diphenyl-4-methylamino-1,2,4-triaza-3,5-diborolyl were prepared by deprotonation of the ring nitrogen in neutral precursor 1. The alkali metal derivatives were characterized by multinuclear NMR, mass spectrometry, and single-crystal X-ray diffraction. The structural determinations revealed extended 2D structures for 2a and 2b and an extended 1D structure for 2c. All three solvent-free structures are dominated by σ interactions, and π interactions are also present for the potassium derivative. Addition of triphenylborane to 2a, 2b, and 2c produced the adducts 3a, 3b, and 3c, respectively, and these were characterized by multinuclear NMR and mass spectrometry. Structural determinations have been performed for the lithium and potassium salt, showing that Ph₃B coordinates at the 2 position of the ring, whereas the alkali metal is coordinated by the pendant methylamino group. The lithium ion is additionally coordinated by three acetonitrile molecules in the monomeric structure of 3a, whereas the potassium ion is coordinated by three phenyl groups, forming the 1D polymeric structure of 3c. Reaction of 2a with [Rh(cod)Cl]₂ yielded the dimeric 4, containing two 1,2,4-triaza-3,5-diborolyl rings bridging two Rh(cod) fragments through the substituent-free ring nitrogen atoms.

Full text of DOI:10.1021/ic7013286

Inorg. Chem. 2007, 46, 9303−9311
Revisiting the 1,2,4-Triaza-3,5-diborolyl Ligand:
σ
and
π
Coordination
Modes in the Alkali Metal and Rhodium Complexes of a Planar,
-
6-
π
-Electron B2N3 Ring
Hanh V. Ly, Joanna H. Chow, Masood Parvez, Robert McDonald,† and Roland Roesler*
Department of Chemistry, UniVersity of Calgary, 2500 UniVersity Dr. NW, Calgary, AB, T2N 1N4
Canada, and Department of Chemistry, UniVersity of Alberta, 11227 Saskatchewan Dr. NW,
Edmonton, AB, T6G 2G2 Canada
Received July 4, 2007
Lithium (2a), sodium (2b), and potassium (2c) salts of 1-methyl-3,5-diphenyl-4-methylamino-1,2,4-triaza-3,5-diborolyl
were prepared by deprotonation of the ring nitrogen in neutral precursor 1. The alkali metal derivatives were
characterized by multinuclear NMR, mass spectrometry, and single-crystal X-ray diffraction. The structural
determinations revealed extended 2D structures for 2a and 2b and an extended 1D structure for 2c. All three
solvent-free structures are dominated by
σ interactions, and π interactions are also present for the potassium
derivative. Addition of triphenylborane to 2a, 2b, and 2c produced the adducts 3a, 3b, and 3c, respectively, and
these were characterized by multinuclear NMR and mass spectrometry. Structural determinations have been performed
for the lithium and potassium salt, showing that Ph3B coordinates at the 2 position of the ring, whereas the alkali
metal is coordinated by the pendant methylamino group. The lithium ion is additionally coordinated by three acetonitrile
molecules in the monomeric structure of 3a, whereas the potassium ion is coordinated by three phenyl groups,
forming the 1D polymeric structure of 3c. Reaction of 2a with [Rh(cod)Cl]2 yielded the dimeric 4, containing two
1,2,4-triaza-3,5-diborolyl rings bridging two Rh(cod) fragments through the substituent-free ring nitrogen atoms.
Introduction
cyclopentadienyl with RB-NR′ fragments.3 Structural in-
vestigations have shown that the ring carbon atoms play a
key role in the binding of these ligands to metals. Chrono-
logically, the heterocyclic anions C having a carbon-free
framework were studied before their analogues A and B,
mainly through the efforts of No¨th. The substituted B2N3
ring framework was first reported in 1963,4a and the
The tremendous success of cyclopentadienyl as a ligand
in organometallic chemistry and catalysis1 has inspired for
several decades now the search for heterocyclic analogues.
Among the numerous derivatives incorporating main-group
elements that were synthesized and characterized, the most-
successful have been the boron- and the phosphorus-
containing rings. The formal replacement of a HC-CH pair
in cyclopentadienyl with the isolobal fragment RB-NR′
produced the azaborolyl ligands A, and their coordination
chemistry and catalytic applications have been extensively
investigated.2 Aiming to tune the electronic properties of
cyclopentadienyl, we recently isolated sandwich complexes
containing the anionic, heterocyclic ligands B obtained
through the formal replacement of two HC-CH pairs in
(2) (a) Schulze, J.; Schmid, G. Angew. Chem., Int. Ed. Engl. 1980, 19,
54. (b) Schulze, J.; Boese, R.; Schmid, G. Chem. Ber. 1980, 113, 2348.
(c) Schmid, G. Comments Inorg. Chem. 1985, 4, 17. (d) Ashe, A. J.,
III; Fang, X. Org. Lett. 2000, 2, 2089. (e) Liu, S.-Y.; Hills, I. D.; Fu,
G. C. Organometallics 2002, 21, 4323. (f) Liu, S.-Y.; Lo, M. M.-C.;
Fu, G. C. Angew. Chem., Int. Ed. 2002, 41, 174. (g) Ashe, A. J., III;
Yang, H.; Fang, X.; Kampf, J. W. Organometallics 2002, 21, 4578.
(h) Liu, S.-Y.; Hills, I. D.; Fu, G. C. J. Am. Chem. Soc. 2005, 127,
15352.
(3) (a) Ly, H. V.; Forster, T. D.; Maley, D.; Parvez, M.; Roesler, R. Chem.
Commun. 2005, 4468. (b) Ly, H. V.; Forster, T. D.; Corrente, A. M.;
Eisler, D. J.; Konu, J.; Parvez, M.; Roesler, R. Organometallics 2007,
26, 1750. (c) Ly, H. V.; Forster, T. D.; Parvez, M.; McDonald, R.;
Roesler, R. Organometallics 2007, 26, 3516.
* To whom correspondence should be addressed. E-mail: roesler@
ucalgary.ca.
† University of Alberta.
(1) (a) Togni, A.; Halterman, R. L. Metallocenes; Wiley VCH: Weinheim,
Germany, 1998; Vol. I, II. (b) Long, N. J. Metallocenes: An
Introduction to Sandwich Complexes; Blackwell Science Ltd.: London,
1998. (c) Patil, A. O.; Hlatky, G. G. Beyond Metallocenes: Next-
Generation Polymerization Catalysts; American Chemical Society:
Washington DC, 2003.
(4) (a) No¨th, H.; Regnet, W. Z. Naturforsch. 1963, 18 B, 1138. (b)
Niedenzu, K.; Fritz, P.; Jenne, H. Angew. Chem., Int. Ed. Engl. 1964,
3, 506. (c) No¨th, H.; Regnet, W. Z. Anorg. Allg. Chem. 1967, 352, 1.
(d) No¨lle, D.; No¨th, H. Angew. Chem., Int. Ed. Engl. 1971, 10, 126.
(e) No¨lle, D.; No¨th, H. Z. Naturforsch. 1972, 27 B, 1425. (f) No¨lle,
D.; No¨th, H.; Winterstein, W. Z. Anorg. Allg. Chem. 1974, 406, 235.
10.1021/ic7013286 CCC: $37.00
Published on Web 09/19/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 22, 2007 9303

Ly et al.
Scheme 1
protonated precursors to the anionic ligands were described
1 year later.4b The first metal complexes,4c and an improved
synthesis of the ligand using boron thiolates followed shortly
thereafter.4d,e A ferrocene analogue containing ligands of type
C was prepared and extensively investigated; however a
crystal structure confirming its identity and structure was
bis(dimethylamino)borane to give 1,2,4-triaza-3,5-diborole
1 was documented, and the resulting compound was structur-
ally characterized.7b A similar reaction involving methylhy-
drazine and methyldi(methylthio)borane had been observed
before.7c Aside from 1, structural data is available for only
one other ring of this type, a dimeric 3,5-dichloro-1,2,4-
trimethyl-1,2,4-triaza-3,5-diborol.8 Being interested in the
π-coordinating ability of ligands with main-group frame-
works, we investigated and present herein the syntheses and
structures of alkali metal salts derived from 1. The assembly
of a planar, tricyclic B4N8 framework starting from 1 was
recently communicated.7d
1
not obtained. On the basis of H NMR, 11B NMR, and IR
spectroscopy, molecular weight determination and elemental
analysis, as well as taking into account the reduced reactivity,
a symmetric structure containing π-coordinating ligands was
postulated for the diamagnetic, crystalline compound.4c The
synthesis and reactivity of the protonated precursors to the
B2N3- rings was further investigated,4f but no references to
metal complexes incorporating these ligands were made in
the literature after 1972. Much later, theoretical studies
supported the sandwich structure proposed by No¨th,5a
predicting that this compound would be a fully inorganic
analogue of ferrocene.5b A fully inorganic analogue of
ferrocene, a sandwich complex containing no carbon atoms
in the ligand skeletons, remains elusive to date. No such
complex could be characterized beyond doubt using single-
crystal X-ray diffractometry, and the closest confirmed
structures were those of phosphorus-containing derivatives
such as [Cp*Fe(η5-P5)]6a,b and the fully inorganic titanocene
[Ti(η5-P5)2]2-.6c
Results and Discussion
Ligand precursor 1 was prepared using the aforementioned
1
reported procedure.7b The H and 13C NMR spectra of this
compound featured different signals for the chemically
inequivalent methyl and phenyl substituents. The singlet
resonance corresponding to the ring proton appears at 7.49
ppm, significantly downfield shifted with respect to the
quartet resonance, corresponding to the proton of the pendant
methylamino group (4.14 ppm). A much smaller difference
following the same trend is observed for the methyl groups
(3.21 at the ring nitrogen vs 2.45 ppm at the pendant
nitrogen). In the 11B NMR spectrum, however, the signals
corresponding to the two distinct boron atoms overlapped
into a broad singlet at 27.5 ppm. Deprotonation of 1 occurred
easily in THF in the presence of various metalating agents
(Scheme 1), yielding the alkali metal salts 2, with little
influence on the appearance of the NMR spectra. The
disappearance of the signal corresponding to the ring proton
was accompanied by a 0.3 ppm upfield shift of the resonance
corresponding to the proton on the pendant methylamino
group, and the lithium and sodium salts 2a and 2b featured
distinct 11B NMR resonances for the inequivalent boron
nuclei. The mass spectrum of 2a displayed signals corre-
sponding to protonated ligand 1 and the [L-Me]+ fragment.
The attempted double deprotonation at the ring and the
pendant methylamino group in presence of the same meta-
lating agents led to decomposition of the ring with the
formation of unidentified products.
Precursors to less-symmetric anionic rings D, having a ring
proton in the 2 position instead of the 4 position as required
for ligands of type C, have been reported as well.7a Most
notably, the self-assembly of methylhydrazine with phenyl-
(5) (a) Su, M.-D.; Chu, S.-Y. J. Phys. Chem. 1989, 93, 6043. (b)
Bridgeman, A. J.; Rothery, J. Inorg. Chim. Acta 1999, 288, 17.
(6) (a) Scherer, O. J.; Bru¨ck, T. Angew. Chem., Int. Ed. Engl. 1987, 26,
59. (b) Scherer, O. J.; Bru¨ck, T.; Wolmersha¨user, G. Chem. Ber. 1988,
121, 935. (c) Urne˘zˇius, E.; Brennessel, W. W.; Cramer, C. J.; Ellis, J.
E.; Schleyer, P. v. R. Science 2002, 295, 832.
(7) (a) Geschwentner, M.; Elter, G.; Meller, A. Z. Naturforsch. 1994, 49
B, 459. (b) Engelhardt, U.; Park, S. S. Acta. Cryst. 1996, C 52, 3248.
(c) No¨lle, D.; No¨th, H. Chem. Ber. 1978, 111, 469. (d) Ly, H. V.;
Tuononen, H. M.; Parvez M.; Roesler R. Chem. Commun., submitted
for publication.
Single-crystal X-ray analyses have been performed for 2a,
2b, and 2c (Table 1), and the results showed that the metric
parameters of the ligand vary very little between the three
structures (Table 2). The ring is basically planar, with the
sum of the pentagonal angles situated between 539.7 and
(8) Fuszstetter, H.; No¨th, H.; Peters, K.; von Schnering, H. G.; Huffman,
J. C. Chem. Ber. 1980, 113, 3881.
9304 Inorganic Chemistry, Vol. 46, No. 22, 2007

1,2,4-Triaza-3,5-diborolyl Ligand
Table 1. Selected Data and Structure Refinement Details for 2a, 2b, 2c, 3a(CH3CN)3, 3c, and 4‚THF
2a
2b
2c
3a(CH3CN)3
3c
4‚THF
empirical formula
fw
C14H17B2LiN4
269.88
C14H17B2N4Na
285.93
C14H17B2KN4
302.04
C38H41B3LiN7
635.15
C32H32B3KN4
544.15
C26H37B2N4ORh
546.13
cryst syst
space group
a (Å)
b (Å)
c (Å
R (deg)
â (deg)
γ (deg)
V (Å3)
monoclinic
P21/c
13.464(2)
10.0183(15)
11.2470(17)
90
100.283(3)
90
1492.7(4)
4
monoclinic
P21/c
13.419(7)
10.143(5)
11.526(8)
90
99.82(3)
90
1545.8(15)
4
orthorhombic
P212121
7.746(4)
11.973(4)
16.742(9)
90
monoclinic
C2/c
32.087(12)
11.022(5)
23.883(7)
90
118.10(2)
90
7451(5)
8
monoclinic
P21/c
9.111(4)
17.582(4)
18.059(6)
90
91.344(14)
90
2892.1(17)
4
triclinic
P1h
9.818(2)
10.130(3)
13.284(3)
77.906(13)
77.701(13)
87.761(15)
1262.2(5)
2
90
90
1552.7(13)
4
1.292
Z
d
calcd (g cm-3
)
1.201
1.229
1.132
1.250
1.437
2θmax (deg)
52.94
0.071
3072
55.00
0.098
3520
50.06
0.338
2711
50.00
0.067
6573
55.00
0.212
6566
55.20
0.703
5750
µ(Mo KR) (mm-1
)
independent reflns
(Rint ) 0.0922)
3072/1/239
1.038
[R(int) ) 0.076]
3520/0/193
1.01
[R(int) ) 0.0256]
2711/0/193
1.056
[R(int) ) 0.046]
6573/0/447
1.020
[R(int) ) 0.034]
6566/0/366
1.00
[R(int) ) 0.037]
5750/0/307
1.03
data/restraints/params
GOF on F 2
R1(F) [I > 2σ(I)]
0.0621
0.1995
0.0600
0.1820
0.0379
0.1014
0.0620
0.1770
0.0460
0.1260
0.038
0.098
wR2(F 2) (all data)
Table 2. Selected Bond Lengths (Angstroms) and Angles (Degrees) for 1, 2a, 2b, 2c, 3a(CH3CN)3, 3c, and 4‚THF
17b
2a (M ) Li)
2b (M ) Na)
2c (M ) K)
3a(CH3CN)3 (M ) Li)
3c (M ) K)
4‚THF (M ) Rh)
N(1)-N(2)
1.414(5)
1.441(4)
1.404(6)
1.399(6)
1.432(6)
1.446(6)
1.547(6)
1.538(6)
1.440(7)
1.463(7)
104.9(4)
130.0(4)
125.1(4)
104.2(4)
131.8(4)
124.0(4)
109.6(3)
133.8(4)
116.4(4)
110.3(3)
110.9(3)
124.7(3)
124.3(3)
110.4(3)
539.9
1.435(3)
1.439(3)
1.400(4)
1.407(4)
1.443(4)
1.446(4)
1.572(4)
1.571(4)
1.448(3)
1.465(4)
105.2(2)
125.9(2)
128.7(3)
109.3(2)
125.3(2)
125.4(2)
112.5(2)
130.4(2)
116.6(2)
105.06(19)
108.0(2)
126.5(2)
124.8(2)
108.42(19)
540.0
1.433(3)
1.437(3)
1.411(4)
1.400(4)
1.446(4)
1.448(4)
1.561(4)
1.587(4)
1.444(3)
1.458(4)
104.7(2)
127.1(3)
128.2(3)
109.9(2)
126.0(2)
124.1(2)
112.7(2)
131.1(2)
115.9(2)
104.9(2)
107.8(2)
127.4(2)
124.1(2)
110.8(2)
540.0
1.430(3)
1.440(3)
1.410(4)
1.401(4)
1.454(3)
1.470(4)
1.574(4)
1.579(4)
1.455(4)
1.468(4)
104.6(2)
129.7(2)
125.6(2)
109.7(2)
126.3(2)
124.0(2)
113.4(2)
131.1(2)
114.6(2)
104.9(2)
107.1(2)
125.4(2)
127.3(2)
110.5(2)
539.7
1.448(3)
1.440(3)
1.412(4)
1.420(4)
1.422(4)
1.452(4)
1.573(4)
1.581(4)
1.450(3)
1.465(3)
105.9(2)
129.5(2)
124.5(3)
107.6(2)
120.4(2)
131.9(2)
110.9(2)
126.3(2)
120.5(2)
106.10(19)
109.4(2)
125.5(2)
124.8(2)
111.2(2)
539.9
1.447(2)
1.439(2)
1.405(2)
1.414(2)
1.437(3)
1.452(2)
1.575(3)
1.580(3)
1.447(3)
1.435(3)
1.406(4)
1.460(4)
1.446(4)
1.419(4)
1.566(4)
1.584(4)
1.444(4)
1.453(4)
105.7(2)
129.2(3)
125.0(3)
108.7(2)
123.3(3)
128.0(3)
111.9(2)
131.5(2)
116.1(2)
104.3(2)
109.3(2)
124.3(2)
126.4(2)
111.0(3)
539.9
N(3)-N(4)
B(1)-N(1)
B(2)-N(2)
B(1)-N(3)
B(2)-N(3)
B(1)-C(3)
B(2)-C(9)
N(1)-C(1)
1.451(2)
1.466(3)
N(4)-C(2)
N(3)-B(1)-N(1)
N(3)-B(1)-C(3)
C(3)-B(1)-N(1)
N(3)-B(2)-N(2)
N(3)-B(2)-C(9)
C(9)-B(2)-N(2)
B(1)-N(1)-N(2)
B(1)-N(1)-C(1)
C(1)-N(1)-N(2)
N(1)-N(2)-B(2)
B(1)-N(3)-B(2)
B(1)-N(3)-N(4)
B(2)-N(3)-N(4)
N(3)-N(4)-C(2)
105.87(16)
128.53(16)
125.49(17)
108.06(16)
121.82(16)
129.88(16)
111.09(14)
127.53(15)
120.78(13)
106.21(13)
108.72(14)
126.61(15)
124.59(15)
110.80(15)
539.9
∑
pentagn angles
540.0°, and the ring substituents show little deviation from
the ring plane. The intra- and extraannular N-N bonds are
equal (1.44 Å) and comparable in length to the N-N single
bond in hydrazine (1.45 Å),9 clearly indicating no significant
multiple bond character. The two B-N bonds involving the
hydrazine unit that contributes with both nitrogen atoms to
the ring skeleton are equal and relatively short (1.40 Å) in
comparison with the other two B-N bonds (1.45 Å). The
length of the B-N bonds in borazines is usually situated
between these two values (1.42-1.44 Å),10 whereas the B-N
bonds observed in 1,2-azaborolinyl (A) complexes (1.46-
1.50 Å)2 and alkali metal 1,2-diaza-3,5-diborolyl (B) salts
(ca. 1.46 Å)3b are generally slightly longer. The alternating
bond lengths in the ring framework suggest that the cyclo-
pentadiene-like resonance structure E has a major contribu-
tion to the structure of salts 2. With 1.56-1.59 and 1.44-
1.47 Å, respectively, the B-C and N-C bonds lengths are
situated in the expected single-bond ranges.
The structures of 2a and 2b are isomorphous and will be
discussed together. In the crystal, each ligand coordinates
in a σ fashion to three alkali metals, and each alkali metal is
coordinated by three nitrogen atoms (Figure 1). Two ligands
bridge two alkali metals in a dimeric arrangement through
the substituent-free ring nitrogen atoms N(2), which achieve
in this way a distorted tetrahedral coordination environment.
The resulting M2N2 rhombs feature relatively short M-N
distances (2.013(5) and 2.068(5) for M ) Li and 2.350(3)
and 2.416(3) Å for M ) Na) and the M2N2 planes are
perpendicular to the planes of the B2N3 rings (82.8 for M )
(10) (a) Boese, R.; Maulitz, A. H.; Stellberg, P. Chem. Ber. 1994, 127,
1887. (b) Jaska, C. A.; Temple, K.; Lough, A. J.; Manners, I. J. Am.
Chem. Soc. 2003, 125, 9424. (c) Ja¨schke, T.; Jansen, M. Z. Anorg.
Allg. Chem. 2004, 630, 239.
(9) Kohata, K.; Fukuyama, T.; Kuchitsu, K. J. Phys. Chem. 1982, 86,
602.
Inorganic Chemistry, Vol. 46, No. 22, 2007 9305

Ly et al.
ment of each lithium ion.12 The lithium salt of the sterically
more-demanding 2,3-ditertbutylpyrrole features a monomeric
σ-bonded structure with a Li-N distance of 1.932(7) Å and
two THF molecules coordinated to the trigonal-planar lithium
ion.13 For sodium, the crown-ether-containing indolyl salt
is similar to the sodium pyrrolyl, and the Na-N distance
measures 2.331(2) Å.14 In the presence of tertiary amines
completing the tetrahedral coordination environment of the
metal, lithium and sodium indolyl form the same type of
dimeric associates as described above, with M-N distances
measuring 2.004(7) and 2.231(10) for M ) Li and 2.356-
(5)-2.481(5) Å for M ) Na.15 An array of σ-bonded
carbazolyl sodium salts containing various ethers have been
structurally characterized, with Na-N bond lengths situated
between 2.30 and 2.53 Å.16 Some of these structures
contained [Na(carbazole)]2 dimers similar to those observed
in 2b, and the Na-N distances for these compounds were
situated at the higher end of the range, between 2.40 and
2.34 Å. It can therefore be concluded that the coordination
behavior of the 1,2,4-triaza-3,5-diborolyl ligand toward alkali
metals is very similar to that of its organic analogues,
pyrrolyl, indolyl, and carbazolyl. These ligands coordinate
to lithium and sodium in a σ fashion as well, and dimeric
associates containing M2N2 rhombs with M-N bonds very
similar in length have been observed in some of their solid-
state structures. A π-coordinating ligand (η5) was reported
in the 1D polymeric structure of the sodium salt of the
electron-rich tetramethylpyrrolyl in the solid state, but even
there, Na-N σ interactions were present (2.351(1) and 2.411-
(2) Å), and the THF solutions contained dimeric, likely
σ-bonded units.17 Indolyl-metal π interactions have also
been observed in the sodium and potassium salts, featuring
a pendant macrocyclic ligand attached to the indolyl moiety.18
Lithium salts of 2,4,6-triorganoborazines crystallized with
various Lewis bases have been structurally characterized, and
all of them contained σ interactions between lithium and
nitrogen.19 Although dimers formed through Li2N2 bridges,
similar to those observed in 2a, were present only in three
out of nine structures, in six of the examples the deprotonated
nitrogen in borazine was connected to two lithium ions,
resulting in a distorted tetrahedral environment of the metal.
In all of the derivatives, the Li-N bonds ranged between
1.96 and 2.16 Å. Upon deprotonation, an elongation of the
borazine skeleton was observed in these compounds as a
Figure 1. Fragment from the 2D polymeric structure of 2a with 50%
probability level thermal ellipsoids, illustrating the environment of the ligand.
Figure 2. Fragment from the 2D polymeric structure of 2b with 50%
probability level thermal ellipsoids. For clarity, only the ipso carbon atom
of the phenyl groups is represented, and all of the hydrogen atoms on the
organic substituents have been omitted.
Li and 88.9° for M ) Na). The pyramidal coordination
environment of each metal (sum of the angles at the metal
of 353.8 for M ) Li and 336.6° for M ) Na) is completed
by an exocyclic nitrogen atom N(4) from a different ligand.
These M-N(4) bonds, involving the pendant methylamino
group, are only slightly longer than the M-N(2) bonds
(2.100(5) for M ) Li and 2.449(3) Å for M ) Na) and
connect the dimers, forming a 2D structure (Figure 2). A
good comparison for 2a and 2b is provided by the structures
of the alkali metal salts of the isoelectronic pyrrolyl, which
were crystallized with the aid of crown ethers.11 The solid-
state structures of pyrrolyl lithium, sodium, and potassium
are very similar to each other, featuring the pyrrolyl ligand
σ-coordinated to the alkali metal that is additionally coor-
dinated by the crown ether. The M-N distances measure
1.950(4) and 2.319 Å for lithium and sodium, respectively,
being slightly shorter than the shortest M-N distances in
2a and 2b. In the solid state, the lithium carbazolyl forms
σ-bonded dimers very similar to those of 2a, with N-Li
distances measuring 2.012(2)-2.165(2) Å, and two THF
molecules, completing the tetrahedral coordination environ-
(12) Hacker, R.; Kaufmann, E.; Schleyer, P. v. R.; Mahdi, W.; Dietrich H.
Chem. Ber. 1987, 120, 1533.
(13) Westerhausen, M.; Wieneke, M.; No¨th, H.; Seifert, T.; Pfitzner, A.;
Schwarz, W.; Schwarz, O.; Weidlein J. Eur. J. Inorg. Chem. 1998,
1175.
(14) Heldt, I.; Borrmann, T.; Behrens, U. Z. Anorg. Allg. Chem. 2003, 629,
1980.
(15) Gregory, K.; Bremer, M.; Bauer, W.; Schleyer, P. v. R.; Lorenzen,
N. P.; Kopf, J.; Weiss, E. Organometallics 1990, 9, 1485.
(16) (a) Bock, H.; Arad, C.; Na¨ther, C.; Havlas, Z. HelV. Chim. Acta 1997,
80, 606. (b) Bock, H.; Arad, C.; Na¨ther, C.; Havlas, Z. J. Organomet.
Chem. 1997, 548, 115.
(17) Kuhn, N.; Henkel, G.; Kreutzberg, J. Angew. Chem., Int. Ed. Engl.
1990, 29, 1143.
(18) Hu, J.; Barbour, L. J.; Gokel, G. W. Proc. Natl. Acad. Sci. U.S.A.
2002, 99, 5121.
(19) (a) No¨th, H.; Rojas-Lima, S.; Troll, A. Eur. J. Inorg. Chem. 2005,
1895. (b) No¨th, H.; Troll, A. Eur. J. Inorg. Chem. 2005, 3524.
(11) Heldt, I.; Behrens, U. Z. Anorg. Allg. Chem. 2005, 631, 749.
9306 Inorganic Chemistry, Vol. 46, No. 22, 2007

1,2,4-Triaza-3,5-diborolyl Ligand
Figure 4. Molecular structure of 3a(CH3CN)3 with 50% probability level
thermal ellipsoids. Hydrogen atoms on the organic substituents have been
omitted for clarity.
Figure 3. Fragment from the 1D polymeric structure of 2c with 50%
probability level thermal ellipsoids. For clarity, only the ipso carbon atom
of the phenyl group on B(1) is represented, and all of the hydrogen atoms
on the organic substituents have been omitted.
result of the reduction in the width of B-N--B and the
opposite N-B-N angles. This was explained through the
greater electronic repulsions between the B-N bonds and
the σ lone pair on nitrogen, according to the VSEPR theory
of bonding. A similar effect is observed upon deprotonaton
of the 1,2,4-triaza-3,5-diborole, with the N(1)-N(2)-B(2)
angle contracting from 110 in the neutral ligand 1 to 105°
in the alkali metal salts 2a, 2b, and 2c.
Potassium salt 2c features a 1D polymeric structure in the
solid state, with the ligand coordinating in both σ and π
fashions (Figure 3). The potassium ion is σ-coordinated by
the amide nitrogen, N(2), and the nitrogen of the pendant
methylamino group, N(4), of the successive ligand, with the
K-N bonds measuring 2.681(3) and 2.889(3) Å, respec-
tively, leading to the formation of linear chains. The
potassium ion is additionally coordinated by the ipso carbon
of a phenyl group in the ligand, with a K-C contact distance
of 3.127 Å. Two such linear chains are connected by η3
interchain interactions involving the metal and the B(2)-
N(2)-N(1) fragment in the B2N3 ring. The B(2)-K, N(2)-
K, and N(2)-K distances measure 3.149(3), 2.800(3), and
3.009(3) Å, respectively. The coordination environment of
the potassium ion could be described as a three-legged piano
stool. The polymeric double-stranded arrangement observed
in 2c is somewhat similar to the aforementioned structure
of tetramethylpyrrolyl sodium17 and to the structure of 2,3-
dimethylindolyl potassium.20 Exclusively, σ interactions
between potassium and the pyrrolyl ring are rather rare and
have been observed only for pyrrolyl and carbazolyl salts
having a crown ether hosting the metal (K-N distance 2.712-
(3), 2.774(2) Å).11,21 More commonly, the pyrrolyl,22-24
Figure 5. Fragment from the monodimensional polymeric structure of 3c
with 50% probability level thermal ellipsoids. Hydrogen atoms on the
organic substituents have been omitted for clarity.
indolyl,18,20 and carbazolyl25 ligands coordinate to potassium
in a similar fashion to that of the triazadiborolyl ligand in
2c, through σ (K-N 2.676(3)-2.826(2) Å) and π interactions
(potassium to ring plane distance 2.80-3.06 Å). By com-
parison, the distance between the potassium center and the
plane of the B2N3 ring in 2c measures 2.79 Å.
The above results show that the alkali metal coordination
chemistry of ligand D is dominated by σ interactions
involving the amide ring nitrogen, just like the chemistry of
the organic analogues pyrrole, indole, and carbazole. At-
tempting to enforce π coordination, the alkali metal salts
2a-c were reacted with BPh3 (Scheme 1). The involvement
of the nonparticipating electron pair on nitrogen in a dative
bond to boron was successful in ferrocene analogues
incorporating pyrrolyl26,27 and in lithium pyrrolyl salts.28 The
formation of the 1:1 adducts 3a-c was immediate and
quantitative, as indicated by the singlet resonance charac-
teristic for a borate ion in the 11B NMR spectra (δ ) -4 to
-7). The singlet resonances corresponding to the two
(20) Evans, W. J.; Brady, J. C.; Ziller, J. W. Inorg. Chem. 2002, 41, 3340.
(21) Esbak, H.; Behrens, U. Z. Anorg. Allg. Chem. 2005, 631, 1581.
(22) Reid, S. D.; Blake, A. J.; Wilson, C.; Love, J. B. Inorg. Chem. 2006,
45, 636.
(23) Love, J. B.; Blake, A. J.; Wilson, C.; Reid, S. D.; Novak, A.;
Hitchcock, P. B. Chem. Commun. 2003, 1682.
(25) Gregory, K.; Bremer, M.; Schleyer, P. v. R.; Klusener, P. A. A.;
Brandsma, L. Angew. Chem., Int. Ed. Engl. 1989, 28, 1224.
(26) Kuhn, N.; Schulten, M.; Zauder, E.; Augart, N.; Boese, R. Chem. Ber.
1989, 122, 1891.
(27) Kuhn, N.; Horn, E.-M.; Boese, R.; Augart, N. Angew. Chem., Int. Ed.
Engl. 1989, 28, 342.
(24) Athimoolam, A.; Gambarotta, S.; Korobkov, I. Can. J. Chem. 2005,
83, 832.
(28) Kehr, G.; Roesmann, R.; Fro¨hlich, R.; Holst, C.; Erker, G. Eur. J.
Inorg. Chem. 2001, 535.
Inorganic Chemistry, Vol. 46, No. 22, 2007 9307

Ly et al.
Scheme 2
inequivalent ring borons overlap into a broad signal at 25-
27 ppm. Except for the additional signals corresponding to
the BPh3 moiety, the 1H and 13C NMR spectra of the adducts
are not significantly different from the corresponding spectra
of the alkali metal salts 2a-c.
derivatives described above as starting materials. Various
transition-metal reagents were employed, such as FeCl2(thf)2,
[Cp*RuCl]4, and CuBr. The major product of most reactions
between the alkali metal salts 2a-c and the transition-metal
reagent was 1, along with a complex mixture of byproducts
that was not further investigated. With the most likely source
of protons necessary for the regeneration of 1 being the
pendant methylamino group of the ligand itself, it is possible
that the presence of transition-metal reagents promotes a
disproportionation of the monoanionic salts 2 to 1 and the
dianionic analogues (Scheme 2). The doubly deprotonated
form of 1 is indeed not isolable (vide supra), and the driving
force for the process could be the formation of transition-
metal methylhydrazides.
The crystallographic analysis of 3a(CH3CN)3 reveals a
monomeric structure featuring the BPh3 unit coordinated to
the amide ring nitrogen, N(2), of the 1,2,4-triaza-3,5-diborolyl
ligand and the lithium ion σ-coordinated by the nitrogen atom
of the pendant methylamino group (Figure 4). The distorted
tetrahedral coordination sphere of the lithium ion is com-
pleted by three acetonitrile molecules, and no π interactions
between the metal and the cyclic ligands are present. The
metric parameters of the ligand do not suffer considerable
changes upon the coordination of the triphenyl borane. As
expected for a tetracoordinated boron, the extraannular B-N
bond (1.579(3) Å) is significantly longer than the intraannular
B-N bonds (1.412(4)-1.452(4) Å). The Li-N distances fall
in a narrow range, between 1.979(6) and 2.072(6) Å,
comparable to the corresponding distances observed in 2a.
The crystal structure of 3c features polymeric chains
containing potassium ions alternating with triphenylborane
moieties and triazadiboryl ligands (Figure 5). As observed
in 3a, the triphenylborane molecule is coordinated by the
amide nitrogen of the cyclic ligand through a long B(3)-
N(2) bond (1.576(2) Å), and this coordination induces little
change in the metric parameters of the triazadiborolyl ligand.
The potassium center is σ-coordinated by the pendant amino
group (K-N distance 2.8334(18) Å) and π-coordinated by
three phenyl groups, one belonging to the ligand and two
belonging to the phenylborane moiety. The distances between
potassium and the phenyl planes measure 2.84 Å for the
ligand phenyl and 2.91 and 2.99 Å for the phenyl groups in
triphenylborane, with the shortest K‚‚‚C contacts involving
the ipso (3.04), ortho (2.99), and ipso (3.11 Å) carbon atoms,
respectively. Such π coordination by aryl groups is typical
for potassium and is encountered in K[BPh4], which is used
for the selective precipitation of this ion from aqueous
solutions.29 The K‚‚‚C contacts observed in 3c are short; for
comparison, the distances between potassium and the aryl
planes measure 2.98 in K[BPh4] and range between 2.92 and
3.02 Å in K[B(C6H4OC6H5)4].30,31
The reaction of 2a with [Rh(cod)Cl]2 in THF, however,
1
gave 4 in a moderate yield. The H and 13C NMR spectra
indicated that 4 consisted of Rh(cod) fragments and triaza-
diborolyl ligands (L) in a ratio of 1:1, and the heaviest ion
featured in the electron-impact mass spectrum corresponded
to the formulation Rh(cod)L. Only one broad resonance was
observed in the 11B NMR spectrum, but the 1H and 13C NMR
spectra displayed signals for two sets of nonequivalent
cyclooctatetraene vinyl and methylene groups. Single-crystal
X-ray diffractometry revealed that 4 has a centrosymmetric
dimeric structure [Rh(cod)L]2 in the solid state, with two
ligands bridging two Rh(cod) fragments through the amide
nitrogen N(2) (Figure 6). The plane of the Rh2N2 rhomb is
practically perpendicular to the planes of the planar B2N3
rings (dihedral angle 88.4°). The only mentionable changes
in the metric parameters of the triazadiborolyl are the slight
lengthening of the B(2)-N(2) bond and the shortening of
the B(2)-N(3) bond, and as a consequence, the ligand is
better described by resonance structure F. The lengths of
the Rh-N bonds (2.141(2) and 2.168(2) Å) fall in the same
range observed for cyclooctadienerhodium complexes con-
taining 2-aminomethylpyrrolidines32 and 2-aminomethylpyr-
rolyl,33 and the Rh-N-Rh and N-Rh-N angles measure
98.33(9) and 81.67(9)°, respectively. The cyclooctadiene
ligand is oriented with the CdC bonds coordinating to the
metal trans to the amide ring nitrogen in a pseudo-square
planar geometry, and the Rh-C distances measure 2.115-
(3)-2.168(2) Å. This type of geometry is common for
molecules containing the Rh(cod)N2 moiety, such as 2-ami-
The transition-metal coordination chemistry of the triaza-
diborolyl ligand was investigated using the alkali metal
(32) (a) Raja, R.; Thomas, J. M.; Jones, M. D.; Johnson, B. F. G.; Vaughan,
D. E. W. J. Am. Chem. Soc. 2003, 125, 14982. (b) Jones, M. D.; Raja,
R.; Thomas, J. M.; Johnson, B. F. G.; Lewis, D. W.; Rouzaud, J.;
Harris, K. D. M. Angew. Chem., Int. Ed. 2003, 42, 4326. (c) Rouzaud,
J.; Jones, M. D.; Raja, R.; Thomas, J. M.; Johnson, B. F. G.; Thomas,
J. M.; Duer, M. J. HelV. Chim. Acta 2003, 86, 1753.
(33) de Bruin, B.; Kicken, R. J. N. A. M.; Suos, N. F. A.; Donners, M. P.
J.; den Reijer, C. J.; Sandee, A. J.; de Gelder, R.; Smits, J. M. M.;
Gal, A. W.; Spek A. L. Eur. J. Inorg. Chem. 1999, 1581.
(29) (a) Wittig, G.; Keicher, G.; Ru¨ckert, A.; Raff, P. Liebigs Ann. 1949,
563, 110. (b) Wittig, G. Angew. Chem. 1950, 62, 231. (c) Cassaretto,
F. P.; McLafferty, J. J.; Moore, C. E. Anal. Chim. Acta 1965, 32,
376.
(30) Hoffmann, K.; Weiss, E. J. Organomet. Chem. 1974, 67, 221.
(31) Alaviuhkola, T.; Bobacka, J.; Nissinen, M.; Rissanen, K.; Ivaska, A.;
Pursiainen, J. Chem.sEur. J. 2005, 11, 2071.
9308 Inorganic Chemistry, Vol. 46, No. 22, 2007

1,2,4-Triaza-3,5-diborolyl Ligand
The coordination behavior of the investigated 1,2,4-triaza-
3,5-diborolyl ligand strongly resembles the coordination
behavior of its organic analogues pyrrolyl, indolyl, and
carbazolyl. All of these carbon-based nitrogen ligands feature
σ-coordination modes in their alkali metal salts and π-co-
ordination modes in many of their transition-metal com-
plexes. It can be therefore concluded that the 1,2,4-triaza-
3,5-diborolyl ligands are promising candidates for the
formation of inorganic sandwich compounds, as claimed by
No¨th four decades ago.4c The pendant amino group in 1
provides an additional binding site, allowing for a more-
varied coordination chemistry. According to our investiga-
tions, the replacement of the active hydrogen of the pendant
methylamino group with a less-reactive organic group is a
mandatory condition for the successful use of heterocycles
derived from 1 as π ligands in transition-metal chemistry.
Figure 6. Molecular structure of 4 with 50% probability level thermal
ellipsoids. For clarity, only the ipso carbon atom of the phenyl group on
boron is represented, and all of the hydrogen atoms on the organic
substituents have been omitted.
Experimental Section
General Considerations. All of the operations were performed
under an argon atmosphere using standard Schlenk and glove-box
techniques. Solvents were dried and deoxygenated prior to use and
methylhydrazine was dried over CaH2. Methylated hydrazines are
highly toxic and probable carcinogens, and their handling requires
special precautions; all of the residues were neutralized using
commercial bleach solution. 1-methyl-4-methylamino-triaza-3,5-
diphenyldiborole 1 was prepared according to a reported procedure.7b
NMR spectra were recorded on Bruker Advance DRX-400 and
AMX-300 spectrometers and calibrated with respect to CDHCl2
(1H, 5.32), THF-d7 (1H, 3.58), CD2Cl2 (13C, 54.00), THF-d8 (13C,
67.57), and BF3‚Et2O (11B, 0 ppm). The mass spectrum of 2 was
performed by the Analytical Instrumentation Laboratory, Depart-
ment of Chemistry, University of Calgary, using a Micromass
VG7070F instrument.
nomethylpyrrolidines31 and 2-aminomethylpyrrolyl.32 The
structure reveals the reason for the nonequivalence of the
methylene and vinyl groups in the coordinated cyclooctatet-
raene, which occupy different positions with respect to the
asymmetric triazadiborolyl ligand. It is worth mentioning
that, although transition-metal complexes featuring π-coor-
dinating pyrrazolyl-type ligands are not uncommon, no such
derivatives of rhodium have been reported, and hence
σ-bonded 4 is not an exception.
Conclusions
1-methyl-3,5-diphenyl-4-methylamino-1,2,4-triaza-3,5-di-
borole 1 can be selectively deprotonated at the ring nitrogen,
yielding salts that have polymeric bidimensional (lithium,
2a, and sodium, 2b) and monodimensional (potassium, 2c)
solid-state structures dominated by σ M-N interactions
between the carbon-free cyclopentadienyl analogue and the
alkali metals. In the potassium derivative, π interactions
involving the metal and the B2N3 ring are present as well.
The coordination site at the amido ring nitrogen can be
conveniently blocked using triphenylborane, and this change
does not induce π coordination through the B2N3 ring in the
resulting alkali metal derivatives. Instead, the lithium ion is
coordinated by the pendant methylamino group of the ligand
and three acetonitrile molecules in 3a(CH3CN)3, whereas the
potassium ion is coordinated by the pendant methylamino
group of the ligand and three phenyl groups in 3c. Reaction
of the alkali metal salts 2a-c with most transition-metal
reagents led to reprotonation of the ligand in a potential
disproportionation process that also produced an intractable
mixture of byproducts. However, [Rh(cod)Cl]2 reacted with
2a and yielded dimeric 4, featuring Rh(cod) fragments
bridged by σ-coordinating 1,2,4-triaza-3,5-diborolyl moieties.
The B2N3 ring is planar in all of the compounds described
herein, and its metric parameters indicate that the most-
accurate descriptions of the bonding in this ligand are
provided by resonance structures E and F.
3,5-Diphenyl-1-methyl-4-methylamino-1,2,4-triaza-3,5-di-
borole, 1.7b 1H NMR (400 MHz, THF-d8, 25 °C): δ ) 2.45 (d,
3H, JHH ) 6.2 Hz, HNCH3), 3.21 (s, 3H, BNCH3), 4.14 (q, 1H,
3
3JHH ) 6.2 Hz, HNCH3), 7.27-7.36 (m, 6H, C6H5), 7.49 (s, br,
HNB), 7.60 (d, 2H, o-C6H5), 7.93 (d, 2H, o-C6H5); 13C NMR (100
MHz, THF-d8, 25 °C): δ ) 35.6 (s, BNCH3), 43.3 (s, HNCH3),
128.3 (s, m-C6H5), 128.7 (s, m-C6H5), 129.0 (s, p-C6H5), 129.3 (s,
p-C6H5), 134.3 (s, o-C6H5); 11B NMR (128 MHz, THF-d8, 25 °C):
δ ) 27.5 (s, br, LW1/2 ) 330 Hz). The NMR data was in good
agreement with the reported values.7b
3,5-Diphenyl-1-methyl-4-methylamino-1,2,4-triaza-3,5-diboro-
lyllithium, 2a. A yellow solution of lithium 2,2,6,6-tetramethylpi-
peridide, LiTMP, was prepared from n-butyllithium in hexane (3.11
mL, 1.6 M, 4.99 mmol) and tetramethylpiperidine, HTMP, (0.704
g, 4.99 mmol) in THF (3 mL). Solutions of 1 (1.32 g, 4.99 mmol)
in THF (3 mL) and LiTMP were pre-cooled to -35 °C, mixed and
kept at this temperature for 2 h. The resulting yellow solution was
allowed to warm to room temperature overnight, and the solvent
was removed under a vacuum, leaving behind a yellow residue that
was washed twice with hexane (30 mL) and dried under a vacuum.
The product was obtained as a colorless powder (1.24 g, 92.1%).
1H NMR (400 MHz, THF-d8, 25 °C): δ ) 2.36 (d, 3H, 3JHH ) 6.4
3
Hz, HNCH3), 3.22 (s, 3H, BNCH3), 3.75 (q, 1H, JHH ) 6.4 Hz,
HNCH3), 7.05-7.23 (m, 6H, m- and p-C6H5), 7.60 (d, 2H, o-C6H5),
7.77 (d, 2H, o-C6H5); 13C NMR (100 MHz, THF-d8, 25 °C): δ )
39.0 (s, BNCH3), 44.0 (s, HNCH3), 125.9 (s, p-C6H5), 126.7 (s,
p-C6H5), 127.5 (s, m-C6H5), 127.9 (s, m-C6H5), 134.5 (s, o-C6H5),
134.6 (s, o-C6H5), 139.9 (s, br, i-C6H5), 144.0 (s, br, i-C6H5); 11B
Inorganic Chemistry, Vol. 46, No. 22, 2007 9309

Ly et al.
NMR (128 MHz, THF-d8, 25 °C): δ ) 24.2 (s, br), 27.8 (s, br);
7Li NMR (155 MHz, THF-d8, 25 °C): δ ) -1.24 (s); MS (EI+,
70 eV): m/z(%): 264(36) [M - Li + H]+, 179.2(100) [M - Li -
Me + H]+. Colorless crystals suitable for X-ray diffractometry were
obtained by slow diffusion of hexane into a THF solution of 2a.
3,5-Diphenyl-1-methyl-4-methylamino-1,2,3-triaza-3,5-diboro-
lylsodium, 2b. A solution of 1 (340 mg, 1.28 mmol) in THF (20
mL) was added to a stirring solution of sodium bis(trimethylsilyl)-
amide, NaHMDS, (236 mg, 1.28 mmol) in THF (15 mL). The
resulting yellow mixture was stirred at ambient temperature for 3
h to ensure completion of the reaction. The solvent was removed
under a vacuum, leaving behind a light-yellow residue that was
washed twice with hexane (30 mL) and dried under a vacuum. The
br, i-BC6H5), 162.9 (s, br, i-B(C6H5)3); 11B NMR (128 MHz, THF-
d8, 25 °C): δ ) -4.60 (s, br, BPh3), 27.3 (s, br, BPh). Colorless
crystals of 3a(CH3CN)3 suitable for single-crystal X-ray diffraction
were obtained by cooling a concentrated solution of 3a in a mixture
of acetonitrile and hexane to -30 °C.
3,5-Diphenyl-1-methyl-4-methylamino-2-triphenylboryl-1,2,3-
triaza-3,5-diborolylsodium, 3b. A colorless solution of BPh3 (150
mg, 0.619 mmol) in THF (15 mL) was added dropwise to a stirred-
yellow solution of 2b (177 mg, 0.619 mmol) in THF (20 mL). The
pale-yellow mixture was stirred at ambient temperature for 12 h,
and the volatiles were subsequently removed in vacuo. The residue
was washed twice with hexane and dried under a vacuum, yielding
1
the desired product as a colorless powder (280 mg, 85.9%). H
1
3
product was obtained as a colorless powder (340 mg, 92.4%). H
NMR (400 MHz, THF-d8, 25 °C): δ ) 2.19 (d, 3H, JHH ) 6.4
3
3
NMR (400 MHz, THF-d8, 25 °C): δ ) 2.35 (d, 3H, JHH ) 6.4
Hz, HNCH3), 2.71 (s, 3H, BNCH3), 3.54 (q, 1H, JHH ) 6.4 Hz,
3
Hz, HNCH3), 3.25 (s, 3H, BNCH3), 3.80 (q, 1H, JHH ) 6.4 Hz,
HNCH3), 6.62 (t, 3H, p-B(C6H5)3), 6.74 (t, 6H, m-B(C6H5)3), 7.39
(d, 6H, o-B(C6H5)3), 6.70 (m, 3H, p- and m-BC6H5), 6.96 (m, 2H,
o-BC6H5), 7.13 (t, 1H, p-BC6H5), 7.21 (t, 2H, m-BC6H5), 7.66 (d,
2H, o-BC6H5); 13C NMR (100 MHz, THF-d8, 25 °C): δ ) 37.1
(s, BNCH3), 43.7 (s, HNCH3), 122.6 (s, p-B(C6H5)3), 125.7 (s,
m-B(C6H5)3), 124.7 (s, p-BC6H5), 127.1 (s, p-BC6H5), 126.6 (s,
m-BC6H5), 127.7 (s, m-BC6H5), 134.1 (s, o-BC6H5), 135.1 (s,
o-BC6H5), 136.7 (s, o-B(C6H5)3), 138.9 (s, br, i-BC6H5), 142.5 (s,
br, i-BC6H5), 162.9 (s, br, i-B(C6H5)3); 11B NMR (128 MHz, THF-
d8, 25 °C): δ ) -6.45 (s, br, BPh3), 25.0 (s, br, BPh).
HNCH3), 7.08-7.28 (m, 6H, m- and p-C6H5), 7.56 (d, 2H, 3JHH
)
6.6 Hz, o-C6H5), 7.83 (d, 2H, JHH ) 6.6 Hz, o-C6H5); 13C NMR
(100 MHz, THF-d8, 25 °C): δ ) 39.5 (s, BNCH3), 44.0 (s,
HNCH3), 126.6 (s, p-C6H5), 127.1 (s, p-C6H5), 128.1 (s, m-C6H5),
128.2 (s, m-C6H5), 133.9 (s, o-C6H5), 134.4 (s, o-C6H5), 139.3 (s,
br, i-C6H5), 142.7 (s, br, i-C6H5); 11B NMR (128 MHz, THF-d8,
25 °C): δ ) 24.7 (s, br), 27.4 (s, br). Colorless crystals suitable
for X-ray diffractometry were obtained by slow diffusion of hexane
into a THF solution of 2b.
3
3,5-Diphenyl-1-methyl-4-methylamino-1,2,3-triaza-3,5-diboro-
lylpotassium, 2c. A solution of 1 (1.50 g, 5.68 mmol) in THF (30
mL) was added slowly under stirring to a suspension of potassium
hydride (0.228 g, 5.68 mmol) in THF (20 mL). The hydride
dissolved with concomitant evolution of gas, yielding a yellow
solution. The mixture was stirred at ambient temperature for 3 h to
ensure completion of the reaction, and the solvent was subsequently
removed under a vacuum. The yellow residue was washed twice
with hexane (30 mL) and dried under a vacuum, yielding a light
yellow powder (1.68 g, 98%). 1H NMR (400 MHz, THF-d8,
3,5-Diphenyl-1-methyl-4-methylamino-2-triphenylboryl-1,2,3-
triaza-3,5-diborolylpotassium, 3c. A colorless solution of BPh3
(160 mg, 0.662 mmol) in THF (10 mL) was added dropwise to a
stirred-yellow solution of 2c (200 mg, 0.662 mmol) in THF (15
mL). The pale-yellow mixture was stirred at ambient temperature
for 12 h, and the volatiles were subsequently removed in vacuo.
The colorless solid residue was washed with hexane and dried under
1
a vacuum, yielding a colorless powder (300 mg, 83%). H NMR
3
(400 MHz, THF-d8, 25 °C): δ ) 2.24 (d, 3H, JHH ) 6.4 Hz,
3
HNCH3), 2.69 (s, 3H, BNCH3), 3.56 (q, 1H, JHH ) 6.4 Hz,
3
25 °C): δ ) 2.40 (d, 3H, JHH ) 6.4 Hz, HNCH3), 3.27 (s, 3H,
HNCH3), 6.65 (t, 3H, p-B(C6H5)3), 6.78 (t, 6H, m-B(C6H5)3), 7.43
(d, 6H, o-B(C6H5)3), 6.72 (m, 3H, p- and m-BC6H5), 6.70 (m, 2H,
o-BC6H5), 7.15 (t, 1H, p-BC6H5), 7.23 (t, 2H, m-BC6H5), 7.65 (d,
2H, o-BC6H5); 13C NMR (100 MHz, THF-d8, 25 °C): δ ) 37.1
(s, BNCH3), 43.6 (s, HNCH3), 122.6 (s, p-B(C6H5)3), 125.7 (s,
m-B(C6H5)3), 124.8 (s, p-BC6H5), 127.1 (s, p-BC6H5), 126.6 (s,
m-BC6H5), 127.6 (s, m-BC6H5), 134.1 (s, o-BC6H5), 135.1 (s,
o-BC6H5), 136.6 (s, o-B(C6H5)3), 138.9 (s, br, i-BC6H5), 142.1 (s,
br, i-BC6H5), 162.9 (s, br, i-B(C6H5)3); 11B NMR (128 MHz, THF-
d8, 25 °C): δ ) -6.58 (s, br, BPh3), 25.3 (s, br, BPh). Colorless
crystals suitable for single-crystal X-ray diffraction were obtained
by slow evaporation of a solution of 3c in a mixture of THF and
Et2O.
3
BNCH3), 3.87 (q, 1H, JHH ) 6.4 Hz, HNCH3), 7.06-7.27 (m,
6H, m- and p-C6H5), 7.56 (d, 2H, 3JHH ) 6.6 Hz, o-C6H5), 7.89 (d,
3
2H, JHH ) 6.6 Hz, o-C6H5); 13C NMR (100 MHz, THF-d8,
25 °C): δ ) 39.4 (s, BNCH3), 44.0 (s, HNCH3), 126.3 (s, p-C6H5),
127.1 (s, p-C6H5), 128.2 (s, m-C6H5), 128.3 (s, m-C6H5), 133.5 (s,
o-C6H5), 134.4 (s, o-C6H5), 139.0 (s, br, i-C6H5), 143.1 (s, br,
i-C6H5); 11B NMR (128 MHz, THF-d8, 25 °C): δ ) 26.6 (s, br).
Yellow crystals suitable for single-crystal X-ray diffraction were
obtained by slow diffusion of hexane in a THF solution of 2c.
3,5-Diphenyl-1-methyl-4-methylamino-2-triphenylboryl-1,2,3-
triaza-3,5-diborolyllithium, 3a. A solution of BPh3 (475 mg, 1.96
mmol) in THF (15 mL) was added dropwise to a stirred-yellow
solution of 2a (530 mg, 1.96 mmol) in THF (20 mL), and the pale-
yellow mixture was stirred at ambient temperature for 12 h.
Subsequent removal of the volatiles in vacuo left behind a colorless
solid that was washed with hexane and dried under a vacuum,
3,5-Diphenyl-1-methyl-4-methylamino-1,2,3-triaza-3,5-diboro-
lylrhodiumcyclooctadiene Dimer, 4. A solution of the 2a (100
mg, 0.370 mmol) and [Rh(cod)Cl]2 (91 mg, 0.185 mmol) in THF
(10 mL) was stirred for 2 h at ambient temperature. The bright-red
solution was concentrated in vacuo to 3 mL and subsequently cooled
to -35 °C for 2 days to afford a yellow-crystalline solid that was
separated by decantation, washed with pentane, and dried under a
vacuum (42%). Yellow, thin-prismatic crystals were obtained by
cooling a concentrated solution of 4 in a mixture of hexane and
1
yielding a colorless powder (930 mg, 93%). H NMR (400 MHz,
3
THF-d8, 25 °C): δ ) 2.22 (d, 3H, JHH ) 6.4 Hz, HNCH3), 2.69
3
(s, 3H, BNCH3), 3.56 (q, 1H, JHH ) 6.4 Hz, HNCH3), 6.62 (t,
3H, p-B(C6H5)3), 6.75 (t, 6H, m-B(C6H5)3), 7.39 (d, 6H, o-B-
(C6H5)3), 6.71 (m, 3H, p- and m-BC6H5), 6.96 (m, 2H, o-BC6H5),
7.13 (t, 1H, p-BC6H5), 7.21 (t, 2H, m-BC6H5), 7.66 (d, 2H,
o-BC6H5); 13C NMR (100 MHz, THF-d8, 25 °C): δ ) 37.1 (s,
BNCH3), 43.6 (s, HNCH3), 122.6 (s, p-B(C6H5)3), 125.7 (s,
m-B(C6H5)3), 124.8 (s, p-BC6H5), 126.6 (s, m-BC6H5), 127.1 (s,
p-BC6H5), 127.6 (s, m-BC6H5), 134.1 (s, o-BC6H5), 135.1 (s,
o-BC6H5), 136.6 (s, o-B(C6H5)3), 138.9 (s, br, i-BC6H5), 142.1 (s,
1
THF to -35 °C. H NMR (400 MHz, CD2Cl2, 25 °C): δ ) 1.30
(m, 2H, CH2), 1.89 (m, 2H, CH2), 2.20 (s, 3H, HNCH3), 2.23 (m,
2H, CH2), 2.77 (m, 2H, CH2), 2.90 (s, 3H, BNCH3), 3.30 (q, br,
2H, CHCH2), 3.46 (t, br, 2H, CHCH2), 3.63 (s, br, 1H, HNCH3),
7.24-7.35 (m, 5H, BC6H5), 7.55 (t, 1H, p-BC6H5), 7.67 (t, 2H,
m-BC6H5), 8.86 (d, 2H, o-BC6H5); 13C NMR (100 MHz, CD2Cl2,
9310 Inorganic Chemistry, Vol. 46, No. 22, 2007

1,2,4-Triaza-3,5-diborolyl Ligand
25 °C): δ ) 28.9 (2, CH2), 32.8 (s, CH2), 39.3 (s, BNCH3), 43.3
authors wish to thank Dr. Farideh Jalilehvand and Ms. Vicky
Mah for the Raman measurements.
1
1
(s, HNCH3), 76.6 (d, JRhC ) 13.0 Hz, CHCH2), 84.3 (d, JRhC
)
13.0 Hz, CHCH2), 127.9 (s, m-BC6H5), 128.0 (s, m-BC6H5), 128.5
(s, p-BC6H5), 128.8 (s, p-BC6H5), 133.8 (s, o-BC6H5), 135.2 (s,
o-BC6H5); 11B NMR (128 MHz, CD2Cl2, 25 °C): δ ) 30.6 (s, br);
MS (EI+,70 eV): m/z(%): 474 (10) [(cod)RhL]+, 364 (5) [RhL-
2H]+, 264 (100) [L + H]+.
Supporting Information Available: Complete crystallographic
data in table format. This material is available free of charge via
the Internet at http://pubs.acs.org. CIF files are available online from
the Cambridge Crystallographic Data Centre (CCDC Nos. 645592
(2a), 645593 (2b), 645594 (2c), 645595 (3a), 645596 (3c), and
645597 (4)).
Acknowledgment. This work was supported by the
Natural Sciences and Engineering Research Council of
Canada, the Canada Foundation for Innovation, and the
Alberta Science and Research Investments Program. The
IC7013286
Inorganic Chemistry, Vol. 46, No. 22, 2007 9311