Synthesis, structure, and reactivity of β-diketiminate boron(III) complexes

Article abstract of DOI:10.1016/S0277-5387(99)00137-0

The chemistry of boron supported by the β-diketiminate ligand, tolylnacnac (tolylnacnacH=2-N-p-tolylamino-4-N-p-tolylimino-2-pentene), has been investigated. (tolylnacnac)Li reacted with one equivalent of BF₃·OEt₂ to afford (tolylnacnac)BF₂ (1) in 46% yield. The structure of compound 1 was solved indicating that the diketiminate ligand is η²-bound to B to form a six-membered heterocycle. While alkylation of compound 1 can be effected with alkyl lithium or Grignard reagents, nucleophilic addition to the diketiminate ligand occurred in the reaction between compound 1 and MeLi to afford [η²-(Me)₂C(Ntolyl)CH=C(Ntolyl)Me]BMe (2). For Me₃SiCH₂Li, deprotonation of the diketiminate ligand afforded [η²-CH₂=C(Ntolyl)CH=C(Ntolyl)Me]BCH ₂SiMe₃ (3). Conversely, alkyl Grignard reagents selectively delivered two alkyl groups to the boron center, and several pseudo-tetrahedral (tolylnacnac)BR₂ (4a-d; a, R=Me, b, R=ⁿPr, c, R=vinyl, d, R=allyl) complexes have been prepared. The structures of compounds 2, 3, and 4a were solved, and variations in B-N and B-C metrical data for compounds 2 and 4a were correlated to bond order, inductive effects of the co-ligands, and hybridization of the boron center. The reaction between compound 4a and tris(pentafluorophenyl)boron gave [(tolylnacnac)BMe]⁺[MeB(C₆F₅)₃] ^- (5). Compound 5 reacted with pyridine to give an adduct, [(tolylnacnac)B(py)Me]⁺[MeB(C₆F₅) ₃]^- (6). Elsevier Science Ltd.

Full text of DOI:10.1016/S0277-5387(99)00137-0

Polyhedron 18 (1999) 2405–2414
Synthesis, structure, and reactivity of b-diketiminate boron(III) complexes
*
Baixin Qian, Sung W. Baek, Milton R. Smith III
Department of Chemistry, Michigan State University, East Lansing, MI 48824, USA
Received 6 November 1998; accepted 5 May 1999
Abstract
The chemistry of boron supported by the b-diketiminate ligand, tolylnacnac (tolylnacnacH52-N-p-tolylamino-4-N-p-tolylimino-2-
pentene), has been investigated. (tolylnacnac)Li reacted with one equivalent of BF3?OEt2 to afford (tolylnacnac)BF2 (1) in 46% yield.
The structure of compound 1 was solved indicating that the diketiminate ligand is h2-bound to B to form a six-membered heterocycle.
While alkylation of compound 1 can be effected with alkyl lithium or Grignard reagents, nucleophilic addition to the diketiminate ligand
occurred in the reaction between compound 1 and MeLi to afford [h2-(Me)2C(Ntolyl)CH=C(Ntolyl)Me]BMe (2). For Me3SiCH2Li,
deprotonation of the diketiminate ligand afforded [h2-CH2=C(Ntolyl)CH=C(Ntolyl)Me]BCH2SiMe3 (3). Conversely, alkyl Grignard
reagents selectively delivered two alkyl groups to the boron center, and several pseudo-tetrahedral (tolylnacnac)BR2 (4a–d; a, R5Me, b,
R5nPr, c, R5vinyl, d, R5allyl) complexes have been prepared. The structures of compounds 2, 3, and 4a were solved, and variations in
B–N and B–C metrical data for compounds 2 and 4a were correlated to bond order, inductive effects of the co-ligands, and hybridization
of the boron center. The reaction between compound 4a and tris(pentafluorophenyl)boron gave [(tolylnacnac)BMe]1[MeB(C6F5)3]2 (5).
Compound 5 reacted with pyridine to give an adduct, [(tolylnacnac)B(py)Me]1[MeB(C6F5)3]2 (6).
1999 Elsevier Science Ltd. All
rights reserved.
Keywords: Three-coordinate boron; Four-coordinate boron; X-ray structure; b-Diketiminate ligand; Abstraction reaction
1. Introduction
and transition metal complexes supported by the tolylnac-
nac ligands (tolylnacnacH52-N-p-tolylamino-4-N-p-
Recently, several research groups have examined nitro-
gen-based ligands as complements to cyclopentadienyl
ligands. Popular chelating ligands that contain two nitro-
gen coordination sites include bis(amide) [1], bis(amidi-
nate) [2], a- and b-iminoamine [3,4], and aminotroponimi-
nate [5] ligands. b-Diketiminates have been known for
many years, and were initially employed in spectroscopic
studies of coordination compounds [6,7]. In synthetic
applications, the chemistry of this ligand class is domi-
nated by dianionic tetraazamacrocyclic analogs [8], with
examples of monoanionic diketiminate complexes having
been described more recently. The facile synthesis of
acyclic b-iminoamines from 2,4-pentanedione and primary
aryl amines [9] makes b-diketiminate ligands attractive
candidates in stoichiometric and catalytic applications
since steric and electronic requirements of the ligand can
be fine tuned by varying the amine source [4,10–15].
We have begun to explore chemistry of the main-group
tolylimino-2-pentene) [16–18]. We recently reported that
the tolylnacnac ligand stabilizes aluminum alkyl complex-
es [19]. Remarkably, (tolylnacnac)AlMe2 does not react
with moisture at room temperature and can be stored in the
air without decomposition for months, illustrating the
kinetic stability imparted by diketiminate ligands. Herein,
we want to describe chemistry and structures of related
boron complexes.
2. Experimental
2.1. General considerations
All manipulations were carried out using standard
Schlenk techniques. Solvents were freshly distilled over
sodium/benzophenone ketyl and were saturated with di-
nitrogen before use. Elemental analyses (C, H, N) were
performed by Desert Analytics, Tucson, Arizona or Atlan-
tic Microlabs, Inc. Varian VXR-300 NMR spectrometer
*Corresponding author. Tel.: 11-517-355-9715; fax: 11-517-353-
1793.
E-mail address: smithmil@pilot.msu.edu (M.R. Smith III)
1
was used to record H (299.96 MHz), 11B (96.23 MHz),
13C (75.43 MHz) and 19F (282.203 MHz) NMR spectra
0277-5387/99/$ – see front matter
PII: S0277-5387(99)00137-0
1999 Elsevier Science Ltd. All rights reserved.

2406
B. Qian et al. / Polyhedron 18 (1999) 2405–2414
1
unless noted otherwise. H and 13C chemical shifts were
referenced to the residual solvent peaks. 11B chemical
shifts were referenced to a neat BF3?OEt2 (0 ppm) external
standard. 19F NMR spectra were referenced to a neat CFCl3
(0 ppm) external standard. CDCl3 was dried over activated
warmed to room temperature. After stirring for 12 h, the
mixture was filtered and the solvent was removed from the
filtrate to give a yellow oil. The oil was extracted with
pentane and the solvent volume was reduced to |2 ml.
Compound 2 crystallized upon standing overnight at
2788C as pale yellow crystals (0.52 g, 61%). mp 87–908C
˚
4-A molecule sieves, and vacuum transferred to an air-free
flask. C6D6 was dried over activated 4-A molecule sieves,
1
˚
(dec); H NMR (500 MHz, C6D6) d 20.009 (s, 3H), 1.33
(s, 6H), 1.58 (d, J51.0 Hz, 3H), 2.07 (s, 3H), 2.12 (s, 3H),
4.45 (q, J51.0 Hz, 1H), 6.82 (d, J58.3 Hz, 2H), 6.90 (m,
4H), 7.05 (d, J58.3 Hz, 2H); 13Ch1Hj NMR (75 MHz,
C6D6) d 1.7 (y1 / 2550 Hz), 20.87, 20.91, 21.23, 31.87,
54.94, 109.0, 129.1, 129.4, 130.1, 132.0, 135.1, 135.2,
143.0, 143.2; 11B (96 MHz, CDCl3) d 30.7 (y1 / 25283 Hz).
Anal. Calcd. for C21H37BN2: C, 79.25; H, 8.55; N, 8.80.
Found C, 79.03; H, 8.52; N, 8.78.
and vacuum transferred to a sodium-mirrored air-free flask.
Uncorrected melting points of crystalline samples in sealed
capillaries (under an argon atmosphere) were reported as
ranges. Low resolution mass spectra were obtained on a
portable Trio-1 VG Masslab Ltd. mass spectrometer and
were reported in the form (M, %I), where M was the
highest mass observed for a molecular ion or fragment
peak, and %I was the intensity of the peak relative to the
most intense peak in the spectrum. A YSI model 31A
conductivity bridge with an Orion conductivity cell
01801A was used to measure conductivities at room
temperature.
2.2.3. [h2-CH2=C(Ntolyl)CH=C(Ntolyl)Me]BCH2SiMe3
(3)
A 100-ml Schlenk tube was charged with 1 (0.46 g, 1.4
mmol) and LiCH2SiMe3 (0.26 g, 2.8 mmol) in a glovebox.
Diethyl ether (20 ml) was added to the mixture at 08C with
stirring. After 30 min, the mixture was filtered and the
solvent was removed under vacuum. The resultant orange
oil was taken into pentane, and compound 3 crystallized as
a pale yellow solid (0.25 g, 47%) upon standing at 2808C
overnight. mp 78–808C (dec); 1H NMR (300 MHz,
CDCl3) d 20.44 (s, 9H), 20.13 (s, 2H), 1.57 (s, 3H), 2.34
(m, 6H), 2.83 (s, 1H), 3.51 (s, 1H), 5.40 (s, 1H), 6.98 (d,
J58.1 Hz, 2H), 7.05 (d, J58.1 Hz, 2H), 7.16 (d, J58.1
Hz, 2H), 7.20 (d, J58.1 Hz, 2H); 13Ch1Hj NMR (75 MHz,
CDCl3) d 1.07, 5.06 (y1 / 2543 Hz), 20.90, 20.97, 21.10,
79.34, 104.7, 129.3, 129.6, 130.0, 135.8, 136.0, 140.4,
141.4, 142.1, 149.4; 11B NMR (96 MHz, CDCl3) d 33
(y1 / 25475 Hz). LRMS 373 (M1–H, 55). Anal. Calcd. for
C23H31BN2Si: C, 73.78; H, 8.55; N, 7.48. Found C, 73.52;
H, 8.29; N, 7.45.
nBuLi and BCl3 were purchased from the Aldrich
Chemical Co. and were used as received. BF3?OEt2 was
distilled over calcium hydride under reduced pressure
before
use.
2-p-tolylamino-4-p-tolylimino-2-pentene
(tolylnacnacH) was prepared by straightforward modifica-
tion of the literature method [6]. B(C6F5)3 was prepared
from BCl3 and C6F5Li [38]. MeLi was prepared from Li
and ClCH3 and was stored as a 1.4 M solution in ether.
LiCH2(SiMe3) was prepared from Li and ClCH2SiMe3.
Grignard reagents were prepared from magnesium turnings
and corresponding organic halides in ether. Concentrations
of the ethereal solutions were determined by titration
before use.
2.2. Syntheses of compounds
2.2.1. (Tolylnacnac)BF2 (1)
A 10-ml toluene solution of Li(tolylnacnac) (7.0 g, 25
mmol) was added to a stirred 20-ml toluene solution of
freshly distilled BF3?OEt2 (3.5 g, 25 mmol) at 08C. Upon
warming to room temperature, a precipitate formed. After
stirring for 12 h, the mixture was filtered and the filtrate
was concentrated to afford compound 1 as yellow crystals
2.2.4. (Tolylnacnac)BMe2 (4a)
A stirred suspension of 1 (0.48 g, 1.5 mmol) in 15 ml
diethyl ether was treated with an ethereal solution of
MeMgI (1.2 M, 2.4 ml, 2.9 mmol) at 08C. After 5 min, the
mixture was filtered and the solvent was removed under
vacuum. The crude product was extracted with pentane,
and compound was isolated as yellow crystals (0.31 g,
66%) upon standing at 2808C overnight. mp 115–1188C;
1H NMR (300 MHz, CDCl3) d 20.44 (s, 6H), 1.66 (s,
6H), 2.31 (s, 6H), 4.82 (s, 1H), 6.90 (d, J58.1 Hz, 4H),
7.09 (d, J58.1 Hz, 4H); 13Ch1Hj NMR (75 MHz, C6D6) d
8.22 (br, s, y1 / 2564 Hz), 20.92, 21.77, 95.18, 127.54,
129.46, 135.5, 143.36, 162.23; 11B NMR (96 MHz, C6D6)
d 1.07 (s, y1 / 25259 Hz). LRMS 318 (M1 1), 303 (M1–
Me, 100). Anal. Calcd. for C21H27BN2: C, 79.25; H, 8.55;
N, 8.80. Found C, 79.32; H, 8.61; N, 8.76.
1
(3.7 g, 46%). mp 189–1908C (dec); H NMR (300 MHz,
CDCl3) d 1.87 (s, 6H), 2.33 (s, 6H), 5.16 (s, 1H), 7.08–
7.17 (m, 8H); 13Ch1Hj NMR (75 MHz, CDCl3) d 21.03,
21.31, 95.15, 127.2, 129.4, 136.8, 138.5, 163.5; 11B (96
MHz, CDCl3) d 2.0 (t, 1:2:1 J529.1 Hz); 19F NMR (282
MHz, CDCl3) d 2128.9 (q, 1:1:1:1, J529.8 Hz). LRMS
326.2 (M1, 45). Anal. Calcd. for C19H21BF2N2: C, 69.96;
H, 6.49; N, 8.58. Found C, 69.74; H, 6.45; N, 8.44.
2.2.2. [h2-(Me)2C(Ntolyl)CH=C(Ntolyl)Me]BMe (2)
A stirred suspension of 1 (0.88 g, 2.7 mmol) in 20 ml
diethyl ether was treated with LiMe (1.4 M, 3.8 ml, 5.4
mmol) ether solution at 08C and the reaction mixture was
2.2.5. (Tolylnacnac)BnPr2 (4b)
Compound 4b was prepared in a similar fashion to

B. Qian et al. / Polyhedron 18 (1999) 2405–2414
2407
n
compound 4a from 1 and PrMgBr in 56% yield as bright
214.8 (y1 / 2530 Hz), 37.1 (y1 / 251200 Hz); 19F NMR (282
MHz, CDCl3) d 2167.2 (m), 2164.5 (m), 2132.9 (m).
Anal. Calcd. for C39H27B2F15N2: C, 56.42; H, 3.28; N,
3.37. Found C, 56.10; H, 3.24; N, 3.33. For conductivity
measurements, solutions were prepared in the glovebox.
The molar conductivity of a methylene chloride solution of
[(tolylnacnac)BMe]1[MeB(CF5)3]2 (4.731023 M) was
1
yellow crystals. mp 98–1028C (dec); H NMR (300 MHz,
CDCl3) d 0.11 (m, 4H), 0.73 (t, J56.9 Hz, 6 H), 1.20 (m,
4H), 1.63 (s, 6H), 2.34 (s, 6H), 4.59 (s, 1H), 6.97 (d,
J58.1 Hz, 4H), 7.10 (d, J58.1 Hz, 4H); 13Ch1Hj NMR
(75 MHz, CDCl3) d 18.52, 19.35, 20.98, 22.03, 27.8
(y1 / 2534 Hz), 93.56, 127.47, 128.8, 135.4, 142.5, 163.8;
11B NMR (96 MHz, CDCl3) d 4.3 (y1 / 25377 Hz). Anal.
Calcd. for C25H35BN2: C, 80.21; H, 9.42; N, 7.48. Found
C, 80.44; H, 9.54; N, 7.34.
1.631022 Sm2 mol21
[nBu4N]1Br2 solution at the same concentration was
LM51.231022 Sm2 mol21
. The molar conductivity of
.
2.2.9. [(Tolylnacnac)B(py)Me]1[MeB(C6F5 )3 ]2 (6)
A stirred suspension of compound 5 (0.30 g, 0.36
mmol) in 5 ml toluene was treated with an excess of
pyridine (0.5 ml, 6.2 mmol) at 08C. Upon addition, the
mixture turned yellow. The volatile materials were re-
moved under vacuum, and the resulting yellow oil was
triturated with pentane to give compound 6 as yellow solid
2.2.6. (Tolylnacnac)B(C2H3 )2 (4c)
Compound 4c was prepared in a similar fashion to
compound 4a from compound 1 and (C2H31)MgBr in 50%
yield as yellow solid. mp 85–888C (dec); H NMR (300
MHz, CDCl3) d 1.79 (s, 6H), 2.29 (s, 6H), 5.03 (s, 1H),
5.02 (dd, J54.5, 13.2 Hz, 2H), 5.28 (dd, J54.5, 13.2 Hz,
2H), 5.87 (dd, J54.5, 13.2 Hz, 2H), 6.93 (d, J58.1 Hz,
4H), 7.04 (d, J58.1 Hz, 4H); 13Ch1Hj NMR (75 MHz,
CDCl3) d 21.04, 21.88, 96.70, 121.4, 127.4, 128.8, 135.4,
142.4, 148.1 (y1 / 2530 Hz), 162.1; 11B NMR (96 MHz,
1
(0.25 g, 76%). mp 107–1098C (dec); H NMR (300 MHz,
CDCl3) d 0.073 (s, 3H), 0.48 (s, br, y1 / 2510 Hz, 3H), 1.97
(s, 6H), 2.32 (s, 6H), 5.76 (s, 1H), 6.53 (d, J58.1 Hz, 4H),
7.13 (d, J58.1 Hz, 4H), 7.63 (dd, J55.1, 7.5 Hz, 2H),
8.11 (t, J57.5 Hz, 1H), 8.49 (d, J55.1 Hz, 2H); 13Ch1Hj
CDCl3)
d
21.2 (y1 / 25240 Hz). Anal. Calcd. for
C23H27BN2: C, 80.52; H, 8.16; N, 8.17. Found C, 80.44;
H, 8.04; N, 8.07.
NMR (75 MHz, CDCl3) d 5.26 (y1 / 2547 Hz), 10.2 (y1 / 2
141 Hz), 20.88, 22.30, 101.8, 125.8, 126.1, 129.0 (y1 / 2
5
5
150 Hz), 130.6, 136.1 (d, 1JC–F5247 Hz), 137.4 (d, 1JC–F5
240 Hz), 138.5, 139.0, 141.8, 145.7, 148.2 (d, 1JC–F5227
Hz), 168.4; 11B NMR (96 MHz, CDCl3) d 215.23 (s,
y1 / 2565 Hz), 24.19 (s, y1 / 2580 Hz); 19F NMR (282 MHz,
CDCl3) d 2167.2 (m), 2164.6 (m), 2132.7 (m). Anal.
Calcd. for C44H32B2F15N3: C, 58.11; H, 3.55; N, 4.62.
Found C, 58.22; H, 3.63; N, 4.54.
2.2.7. (Tolylnacnac)B(C3H5 )2 (4d)
Compound 4d was prepared from compound 1 and
freshly prepared (C3H5)MgBr in 69% as a pale yellow
1
solid. mp 53–588C; H NMR (300 MHz, CDCl3) d 1.12
(d, J57.5 Hz, 4H), 1.64 (s, 6H), 2.32 (s, 6H), 4.59 (m,
4H), 4.73 (s, 1H), 5.78 (m, 1H), 7.02 (d, J58.1 Hz, 4H),
7.09 (d, J58.1 Hz, 4H); 13Ch1Hj NMR (75 MHz, CDCl3)
d 20.80, 21.99, 30.98 (y1 / 2532 Hz), 95.27, 110.6, 127.9,
129.0, 135.8, 142.0, 142.4, 163.9; 11B NMR (96 MHz,
CDCl3) d 0 (y1 / 2531 Hz). Anal. Calcd. for C25H31BN2:
C, 81.08; H, 8.44; N, 7.56. Found C, 80.77; H, 8.40; N,
7.50.
2.3. X-ray analysis
X-ray quality crystals of 1 were grown from a concen-
trated toluene solution at 2308C. X-ray quality crystals of
2, 3, and 4 were grown from concentrated pentane
solutions at 2308C.
2.2.8. [(Tolylnacnac)BMe]1[MeB(C6F5 )3 ]2 (5)
Toluene solutions of B(C6F5)3 (0.54 g, 1.0 mmol) and
4a (0.34 g, 1.0 mmol) were combined at 08C with stirring.
After 10 min, the reaction mixture was concentrated to |2
ml and layered with pentane. After cooling to 2308C
overnight, an oily solid deposited at the bottom of the
Schlenk flask. The mother liquor was decanted and the
solid was washed with pentane. After drying under high
vacuum, compound 5 was collected as colorless solid (0.61
Crystals of 1, 2, 3 and 4a were coated with Paratone-N
oil and suitable single crystals were selected under a
microscope and mounted on a glass fiber. The crystals
were then transferred to the goniometer of a Siemens
SMART CCD diffractometer using Mo Ka radiation (l5
˚
0.71073 A). Data were collected as 30 s per frame at 173
K. initial cells were calculated by the Smart from three sets
of 15 frames. All data sets were collected over a hemi-
sphere of reciprocal space. SAINT was used to integrate
1025 frames and to generate the raw file [39]. Final unit
cell parameters were obtained by least-squares refinement
of strong reflections obtained. Absorption correction and
time decay were applied to the data by SADABS. In all
structures, the non-hydrogen atoms were found using
SHELXS-86. Atomic coordinates and thermal parameters
were refined using the full-matrix least-squares program,
1
g, 70%). mp 83–878C (dec); H NMR (300 MHz, CDCl3)
d 0.27 (s, 3H), 0.41 (s, br, 3H), 2.25 (s, 6H), 2.42 (s, 6H),
6.73 (s, 1H), 6.94 (d, J58.1 Hz, 4H), 7.34 (d, J58.1 Hz,
4H); 13Ch1Hj NMR (75 MHz, CDCl3) d 1.43 (br, y1 / 2530
Hz, 10.7 (br, y1 / 25113 Hz), 21.03, 22.67, 111.62, 124.95,
1
128.0 (B–C, y1 / 25120 Hz), 131.4, 136.5, (d, JC–F5246
Hz), 137.3, 137.4 (d, 1JC–F5242 Hz), 140.5, 148.2 (d,
1JC–F5236 Hz), 170.6; 11B NMR (96 MHz, CDCl3) d

2408
B. Qian et al. / Polyhedron 18 (1999) 2405–2414
SHELXL-97, and calculations were based on F2 data. All
non-hydrogen atoms were refined using anisotropic ther-
mal parameters. All hydrogen atoms were placed in
calculated positions using HFIX. All crystallographic
computations were performed on Silicon Graphics Indigo
computers.
and B are essentially coplanar. The similar bond distances
˚
˚
for N(1)–B (1.550(3) A) and N(2)–B (1.553(3) A) the
˚
C–C and N–C pairs (C(2)–C(3)51.384(3) A, C(3)–
˚
˚
C(4)51.401(3) A; N(1)–C(2)51.339(3) A, N(2)–C(4)5
˚
1.347(3) A) suggest that the C3N2 backbone is delocalized
[21]. Delocalization is confirmed by chemical equivalence
of symmetry related protons between 250 and 1508C.
˚
The average B–F bond distances of 1.404(4) A in com-
˚
˚
pound 1 (B–F(1), 1.411(3) A; B–F(2), 1.396(3) A) are
very close to those of 2,2-difluoro-1,3,4,6-tetramethyl-3-
aza-1-azonia-2-bora-4,6-cyclohexadiene (Chart 1 A) (B–F
3. Results and discussion
When BF3?OEt2 was treated with Li(tolylnacnac) in
toluene, the b-diketiminate boron difluoride complex,
(tolylnacnac)BF2 (1) was isolated in 46% yield (Eq. (1)).
Compound 1 was characterized by conventional spectro-
˚
1.403(2) A) [21] and 4,4-difluoro-1,3,5,7,8-pentamethyl-
3a,4a-diaza-4-bora-s-indacene (Chart 1 B) (B–F 1.394(3)
˚
A) [22,23].
1
scopic methods. Variable temperature (VT) H NMR data
(toluene-d8, 250 to 508C) are consistent with the presence
of a C2 axis containing B and the methine carbon of the
diketiminate ligand in compound 1. The 11B NMR spec-
trum of compound 1 exhibits a 1:2:1 triplet (1JB–F529.1
Hz) at d 2.0, and the 19F NMR spectrum reveals a 1:1:1:1
1
quartet at d 2128.9 with a similar value of JB–F. The
solution NMR data are consistent with a monomeric boron
species [20].
3.1. Alkylation chemistry
Related difluoride compounds related to compound 1
have been described in the literature. For example, Vin-
amidine boron difluoride (A) has been used as a ligand,
binding in h5-fashion, to stabilize the tricarbonylchromium
fragment [10]. Since our interests centered on the reactivity
at boron, alkylation of compound 1 with various lithium
and magnesium reagents was examined (Scheme 1).
When compound 1 was treated with two equivalents of
MeLi in ether, [h2-(Me)2C(Ntolyl)CH=C(Ntolyl)Me]BMe
(2) was isolated in 61% yield. The desired dimethyl boron
compound could not be detected in the crude reaction
mixture. The inequivalent methyl resonances for the ligand
backbone in the 1H NMR spectrum for compound 2
initially suggested a structure where the tolylnacnac ligand
was h1-bound to the BMe2 moiety. However, spectro-
scopic data did not support the ‘arm-off’ structure since the
The solid state structure of compound 1 was solved (Fig.
1). Cell parameters and refinement details for compound 1
are listed in Table 1, which contains data for all structures
in this paper. The structure contains a pseudo-tetrahedral
boron center and the diketiminate ligand is h2-bound to
boron through the nitrogen atoms. The atoms in the
diketiminate backbone (C(2), C(3), C(4), N(1) and N(2))
˚
Fig. 1. ORTEP of 1 (ellipsoids drawn at 50% probability level) and atom-labeling scheme. Selected bond length (A) and bond angles (8): F(1)–B,
1.411(3); F(2)–B, 1.396(3); N(1)–B, 1.550(3); N(2)–B, 1.553(3); N(1)–C(2), 1.339(3); N(2)–C(4), 1.347(3); C(4)–C(3), 1.401(3) C(3)–C(2), 1.384(3).
F(2)–B–F(1), 107.8(2); F(2)–B–N(1), 110.6(2); F(1)–B–N(1), 109.4(2); F(2)–B–N(2), 110.4(2); F(1)–B–N(2), 110.0(2).

B. Qian et al. / Polyhedron 18 (1999) 2405–2414
2409
Table 1
Data collection parameters for compounds 1, 2, 3, and 4a
Compound
1
2
3
4a
Formula
Formula weight
Temperature (K)
C19H21BF2N2
326.19
173(2)
0.71073
Monoclinic
P21 /n
C21H27BN2
318.26
173(2)
0.71073
Monoclinic
C2/c
C23H31BN2Si
374.40
173(2)
0.71073
Monoclinic
P21 /c
C21H27BN2
318.26
173(2)
0.71073
Triclinic
¯
P1
˚
Wavelength (A)
Crystal system
Space group
Unit cell
˚
a (A)
13.133(3)
7.312(2)
18.537(4)
40.617(8)
6.1689(12)
15.096(3)
6.2891(13)
25.017(5)
14.543(3)
7.679(2)
7.899(2)
17.726(4)
99.83(3)
92.59(3)
115.42(3)
948.3(3)
2
˚
b (A)
˚
c (A)
a (8)
b (8)
103.14(3)
97.77(3)
95.73(3)
g (8)
3
˚
V (A )
1733.4(6)
4
1.250
0.088
3747.7(13)
8
1.128
0.065
2276.7(8)
4
1.092
0.112
Z
dcal (mg/m3)
Abs. coeff. (mm21
F(000)
1.115
0.064
344
)
688
1376
808
Size (mm)
u range (8)
Index ranges
0.2030.1830.18
1.73–28.20
216#h#16
24#k#9
223#l#23
10 162
3993 [R(int)50.0261]
3993/0/217
1.444
0.3030.2230.20
2.02–28.31
250#h#45
25#k#8
214#l#20
6635
3797 [R(int)50.0848]
3797/0/217
1.022
0.2830.2630.26
1.63–28.23
28#h#8
233#k#33
219#l#19
26 000
5489 [R(int)50.0374]
5489/0/244
1.437
03030.2530.25
2.35–28.21
29#h#10
210#k#10
223#l#23
10 662
4366 [R(int)50.0313]
4366/0/217
1.420
Reflections collected
Independent reflections
Data/restraints/parameters
GOF/F2
R [I.2s(I)]
R (all data)
Lgst. dif. pk. & hole (e A
R1 50.0662, wR2 50.2070
R1 50.0959, wR2 50.2215
0.440 and 20.400
R1 50.0997, wR2 50.2060
R1 50.2401, wR2 50.2595
0.288 and 20.327
R1 50.0636, wR2 50.1962
R1 50.0873, wR2 50.2082
0.468 and 20.473
R1 50.0722, wR2 50.2176
R1 50.1108, wR2 50.2349
0.443 and 20.432
23
˚
)
high field peak (d 20.01, C6D6) assigned to the boron
methyl groups integrated as three hydrogen atoms instead
of the expected six. Methylation at the imine-carbon was
confirmed by a quaternary carbon resonance in the 13C
NMR spectrum (d 54.94, CDCl3) [24]. Although imines
are generally less electrophilic than corresponding alde-
Scheme 1.

2410
B. Qian et al. / Polyhedron 18 (1999) 2405–2414
Table 2
hydes or ketones, nucleophilic addition in an imine ligand
has literature precedent. For example, Jordan et al. [25]
reported a similar methylation on the dibenzotetraazaan-
nulene ligand of a zirconium complex.
˚
Selected bond distances (A) and angles (8) for compounds 1, 2, 3 and 4a
1
2
3
4a
B–C
B–C
N1–B
N2–B
N1–B–N2
N1–B–C
N2–B–C
N1–B–C
N2–B–C
1.563(7)
1.567(3)
1.619(4)
1.626(3)
1.610(3)
1.615(3)
105.0(2)
109.9(2)
110.2(2)
109.6(2)
109.6(2)
The structure for compound 2 was confirmed by single
crystal X-ray diffraction analysis (Fig. 2), and selected
bond lengths and bond angles are listed in Table 2. The
boron center in compound 2 is planar, as indicated by the
sum of angles about boron (360.0(7)8). The bond distances
1.550(3)
1.553(3)
108.6(2)
1.431(6)
1.425(6)
118.5(4)
119.6(4)
121.9(4)
1.438(3)
1.445(3)
116.9(2)
121.9(2)
121.1(2)
˚
˚
of B–N (B–N(1), 1.431(6) A; B–N(2), 1.425(6) A) and
˚
B–C (B–C(21), 1.563(7) A) 2 resemble those in 1,2,3-tri-
methyl-1,3,2-diazaborolidine (Chart 1 C) [26]. As ex-
pected, the C–C distances in the C3N2B ring in compound
˚
˚
2 (C(2)–C(3), 1.305(6) A; C(3)–C(4), 1.488(6) A) are no
longer equivalent, since the delocalization observed in
compound 1 is broken by the quaternary carbon C(4).
chemical shifts for compounds 2 (d 31) and 3 (d 33) are
similar to that for (MeBNMe)3 (d 35.9) [27].
Three potential reaction pathways could account for the
formation of compound 2 (Scheme 2). The first involves
alkylation at boron followed by nucleophilic addition to
the imine-carbon and subsequent LiF elimination (pathway
i). Conversely, nucleophilic addition to carbon could
precede alkylation at boron (pathway ii). Lastly, compound
2 could result from methyl migration in the dimethyl
complex, (tolylnacnac)BMe2 (4a) (pathway iii). This
possibility can be excluded since compound 4a can be
independently prepared and is stable under the reaction
conditions (vide infra). To distinguish between pathways i
and ii, compound 1 and MeLi were reacted in a 1:1 molar
ratio at 2788C. Under these conditions, a mixture of 2 and
unreacted 1 formed. Thus, we cannot determine whether
the initial methylation occurs at the boron or the ligand
backbone.
When
a
more hindered alkyl lithium reagent,
LiCH2SiMe3, was used, alkylation at boron and deprotona-
tion of the tolylnacnac group occurred, giving [h2-
CH2=C(Ntolyl)CH=C(Ntolyl)Me]BCH2SiMe3 (3) in 47%
yield (Scheme 1). Presumably since CH2SiMe3 is more
sterically demanding than Me, deprotonation of an acti-
vated diketiminate methyl group is favored over nu-
cleophilic attack at the imine-carbon in the reaction
between compound 1 and LiCH2SiMe3. Compound 3 was
characterized by spectroscopic methods and single crystal
X-ray diffraction, and an ORTEP diagram for compound 3
is shown in Fig. 3. Like compound 2, boron is three-
coordinate, and B–C and B–N distances in structures for
compounds 2 and 3 are similar. The biggest structural
difference between 2 and 3 is apparent delocalization along
the diene backbone of the chelating ligand in 3 as indicated
by the C–C distances for carbons from C(1) through C(5):
When compound 1 was treated with two equivalents of
freshly prepared MeMgI in Et2O, the desired dimethyl
product, (tolylnacnac)BMe2 (4a), was isolated (Scheme 1).
In this case, methylation occurred exclusively at boron,
C(1)–C(2)51.413(3),
C(2)–C(3)51.395(3),
C(3)–
C(4)51.389(3), and C(4)–C(5)51.427(3) A. The 11B
˚
˚
Fig. 2. ORTEP of 2 (ellipsoids drawn at 50% probability level) and atom-labeling scheme. Selected bond length (A) and bond angles (8): C(21)–B,
1.563(7); N(1)–B, 1.431(6); N(2)–B, 1.425(6); N(1)–C(2), 1.423(6); N(2)–C(4), 1.491(5); C(3)–C(2), 1.305(6); C(3)–C(4), 1.488(6). N(2)–B–N(1),
118.5(4); N(2)–B–C(21), 121.9(4); N(1)–B–C(21), 119.6(4).

B. Qian et al. / Polyhedron 18 (1999) 2405–2414
2411
˚
Fig. 3. ORTEP of 3 (ellipsoids drawn at 50% probability level) and atom-labeling scheme. Selected bond length (A) and bond angles (8): B–C(20),
1.567(3); B–N(1), 1.438(3); B–N(2), 1.445(3); C(1)–C(2), 1.413(3); C(2)–C(3), 1.395(3); C(3)–C(4), 1.389(3); C(4)–C(5), 1.427(3); Si(1)–C(20),
1.892(2); N(1)–C(2), 1.422(3); N(2)–C(4), 1.420(3). N(1)–B–N(2), 116.9(2); N(1)–B–C(20), 121.9(2); N(2)–B–C(20), 121.1(2).
and compound 2 was not detected in the reaction mixture.
In addition to diagnostic ligand peaks, the 1H NMR
spectrum for compound 4a contains a high field singlet (d
20.44, 6H) which is assigned to BMe2 protons. The 11B
NMR data of 4a (d 1.07, y1 / 2 5259 Hz) are consistent with
a tetrahedral boron center. Inter-conversion between com-
pounds 2 and 4a did not occur after prolonged heating of 2
or 4a in toluene at 708C. Attempts to make (tolylnac-
nac)B(F)(Me) by mixing 1 with one equivalent of MeMgI
at different temperatures invariably led to 4a and unreacted
1 [28]. Other dialkyl complexes, (tolylnacnac)BR2 (4b–d,
b, R5nPr, c, R5C2H3, d, R5C3H5), were similarly
synthesized from compound 1 and the corresponding
Grignard reagents (Scheme 1).
The solid state structure of compound 4a was deter-
mined and its ORTEP is shown in Fig. 4. Its structure
contains a pseudo-tetrahedral boron center with bond
angles (8): N(2)–B–N(1), 105.0(2); N(2)–B–C(21),
109.6(2); N(1)–B–C(21), 109.6(2); N(2)–B–C(20),
110.2(2); N(1)–B–C(20), 109.9(2). The average B–C
˚
bond length of 1.623(4) A in compound 4a is longer than
˚
that of B–C in compound 2 (1.563(7) A) (Table 2). The
elongated B–C bonds in 4a are consistent with rehybridi-
zation from sp2 boron in compound 2 to sp3 boron in
Scheme 2.

2412
B. Qian et al. / Polyhedron 18 (1999) 2405–2414
˚
Fig. 4. ORTEP of 4a (ellipsoids drawn at 50% probability level) and atom-labeling scheme. Selected bond length (A) and bond angles (8): B–N(2),
1.610(3); B–N(1), 1.615(3); B–C(21), 1.626(3); B–C(20), 1.619(4); N(1)–C(2), 1.333(3); N(1)–C(6), 1.448(3); N(2)–C(4), 1.331(3); N(2)–C(13),
1.451(3); C(1)–C(2), 1.507(3); C(2)–C(3), 1.395(3); C(3)–C(4), 1.403(3); C(4)–C(5), 1.509(3). N(2)–B–N(1), 105.0(2); N(2)–B–C(21), 109.6(2);
N(1)–B–C(21), 109.6(2); N(2)–B–C(20), 110.2(2); N(1)–B–C(20), 109.9(2); C(21)–B–C(20), 112.3(2); C(2)–N(1)–C(6), 120.0(2); C(2)–N(1)–B,
125.2(2); C(6)–N(1)–B, 114.7(2); C(4)–N(2)–C(13), 119.4(2).
compound 4a. The longer B–N bond distances in com-
compound with a discrete cation and anion, or as a methyl-
bridged zwitterion [29]. The 11B NMR spectrum of
compound 5 contains two peaks at d 214.8 (s, y1 / 2 530
Hz) and 37.1 (y1 / 2 51200 Hz), respectively. The resonance
at d 214.8 is assigned to MeB(C6F5)3. The narrow
linewidth is consistent with tetrahedral boron; however, the
resonance is shifted downfield significantly from the ‘free’
MeB(C6F5)3 anion. For the diketiminate boron, the 11B
resonance at d 37.1 is shifted downfield substantially from
the resonance for compound 4a (d 1.07) and appears
slightly downfield from resonances for compounds 2 and 3
(|d 32). Coupled to the observation that the line-width for
the resonance at d 37.1 is typical for three-coordinate
boron, the 11B data support three- and four-coordinate B
˚
pound 4a (B–Naverage 51.615(3) A) relative to compound
˚
1 (B–Naverage 51.552(3) A) are consistent with the weaker
inductive effect of Me relative to F.
3.2. Alkyl abstraction reactions
Methyl abstraction from compound 4a to generate a
cationic boron center was examined. Unlike the aluminum
analogue, (tolylnacnac)AlMe2, which underwent aryl-Me
exchange with B(C6F5)3 (Eq. (2)) [19],
centers
expected
for
the
discrete
pair,
[(tolylnacnac)BMe]1[MeB(C6F5)3]2 [30]. The molar con-
compound 4a reacted with B(C6F5)3 to give
[(tolylnacnac)BMe]1[MeB(C6F5)3]– (5) in 70% yield
(Scheme 3). Compound 5 can be formulated as an ionic
ductivity for a CH2Cl2 solution of compound 5 (LM 5
1.631022
S
m2 mol21
)
was similar to that for
[nBu4N]1Br2 (LM 51.231022 S m2 mol21). Thus, com-
Scheme 3.

B. Qian et al. / Polyhedron 18 (1999) 2405–2414
2413
pound 5 is largely dissociated in CH2Cl2. Although the
resonance assigned to the MeB(C6F5)3 protons (d 0.41) is
close the reported value for free [MeB(C6F5)3]2(d 0.4)
[31]; the fact that this resonance shifts to d 0.48 when
abstraction from compound 4a by B(C6F5)3 gives a
cationic boron diketiminate compound, 5. Compound 5 is
less Lewis acidic than B(C6F5)3; however, a stable adduct,
[(tolylnacnac)B(py)Me1] [MeB(C6F5)32] (6), forms upon
addition of pyridine to compound 5 in toluene.
pyridine is added to solutions of
5 implies that
(tolylnacnac)(Me)B???MeB(C6F5)3 interactions are pres-
ent. Presumably, the association implied by the shift of the
protons for [MeB(C6F5)3]2 upon pyridine addition is
weak.
Supplementary data
Inspection of the 1H NMR spectra for compounds 4a
and 5 provides clues regarding the low electrophilicity of
the boron center in compound 5. Specifically, the methine
proton in compound 4a (d 4.82) shifts downfield substan-
tially in compound 5 (d 6.73). This suggests enhanced
aromaticity for the diketiminate ring. While the sp3 hybrid-
ized boron center cannot participate in pp-delocalization,
methyl abstraction from B provides an additional sp2
center that completes a delocalized six-membered ring
[32–34]. A possible explanation for the abcense ion-pair-
ing in compound 5 is that stablization from increased
aromaticity outweighs coulombic contributions.
Four crystal data sets were deposited at Cambridge
Crystallographic Data Center. The crystallographic data
center deposition codes are: 102912 for compound 1,
102913 for compound 2, 102914 for compound 3, and
102915 for compound 4a.
Acknowledgements
We are grateful for support from the National Science
Foundation (CHE-9817230) and the Center for the Fun-
damental Material Research at MSU. S.W.B. thanks the
REU program supported by the National Science Founda-
tion. B.Q. is grateful for a Carl H. Brubaker Endowed
Fellowship. The single crystal X-ray equipment at MSU
was supported by the National Science Foundation (CHE-
9634638), and the NMR equipment was provided by the
National Science Foundation (CHE-8800770) and the
National Institutes of Health (1-S10-RR04750-01).
We surveyed reactivity of compound 5 with olefins and
Lewis bases. In contrast to some cationic aluminum alkyl
compounds, compound 5 did not polymerize olefins
[35,36]. More surprisingly, compound 5 did not bind
diethyl ether or propylene oxide. Nonetheless, compound 5
reacted
with
pyridine
to
form
[(tolylnacnac)B(py)Me]1[MeB(C6F5)3]2, (6) (Scheme 3).
1
The H NMR spectrum of compound 6 contains a reso-
nance at d 0.48 for the CH3B(C6F5)3 anion. The 19F NMR
data for compound 6 (d 2167.2, 2164.6, 2132.7) are
virtually identical to those for compound 5 (d 2167.2,
2164.5, 2132.9), and the 11B NMR spectrum of com-
pound 6 contains two peaks at d 215.23 (s, y1 / 2 565 Hz)
and d 24.19 (s, y1 / 2 580 Hz). Both resonances have
narrow line widths, indicating pseudotetrahedral boron
centers, and the diketiminate resonance at d 24.19 is
shifted to higher field relative to compound 5. The 1H
NMR spectra for mixtures of 6 and pyridine exhibit one set
of pyridine resonances that differ from those of pure 6 or
pyridine. Thus, exchange between the bound pyridine in 6
and free pyridine is rapid on the NMR time scale [37]. The
fact that coordination occurs only with a strong Lewis base
supports the notion that stabilization from aromaticity in
compound 5 is significant.
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