4552 Organometallics, Vol. 28, No. 15, 2009
Arrowsmith et al.
of 2 h, the solution turned bright yellow. Removal of volatiles,
addition of hexane, and cooling to -30 °C for 2 h provided
colorless crystals, which were isolated by filtration (8.87 g,
77%). 1H NMR ppm (CDCl3): 2.90 (s, 9H, CH3), 4.24 (q, 1H,
BH, 1J11BH=156 Hz). 13C NMR ppm (CDCl3): 50.6 (CH3). 11B-
{1H} NMR ppm (CDCl3): -0.73.
[{HB(ImtBu)3}Sr{N(SiMe3)2}(THF)], 30. Compound 1 (1.33
g, 2.45 mmol), strontium iodide (0.31 g, 0.92 mmol), [KN-
(SiMe3)2] (0.73 g, 3.67 mmol), filtration through filter cannula
only, all glassware prewashed with a basic 2-propanol solution
of potassium hydroxide before drying 1 day at 150 °C: colorless
crystals after 1 day at -20 °C (0.26 g, 40%). A second crystal-
lization fraction yielded 0.15 g of compound 3. 1H{11B} NMR
ppm (d8-tol, 298 K): 7.20 (d, 3H, ring-CHdCHNtBu, 3J = 1.2
Hz), 6.48 (d, 3H, ring-CHdCHNtBu, 3J=1.2 Hz), 4.86 (s, 1H,
BH), 3.55 (m, 4H, THF), 1.39 (m, 4H, THF), 1.37 (s, 27H, CH3),
0.40 (s, 18H, Si(CH3)3). 13C{1H} NMR ppm (d8-tol, 298 K):
198.7 (carbene-C), 126.8 (ring-CHdCHNtBu), 113.8 (ring-
CHdCHNtBu), 68.3 (THF), 55.3 (CCH3), 31.3 (CCH3), 25.5
(THF), 6.7 (Si(CH3)3). 11B{1H} NMR ppm (d8-tol, 298 K): 2.40
(BH). Anal. Calcd for C31H60BN7OSi2Sr (701.5): C, 53.08; H,
8.62; N, 13.98. Found: C, 52.99; H, 8.55; N, 13.88.
[HB(ImtBu)3]Br2, 1. 1-tert-Butylimidazole (8.00 g, 64.4 mmol)
and Me3N BHBr2 (4.95 g, 21.5 mmol) were refluxed in dry
3
chlorobenzene (25 mL) under nitrogen for 18 h. Upon cooling to
room temperature, a white precipitate formed, which was iso-
lated by filtration. The crude white solid was dissolved in
CH2Cl2, precipitated with Et2O, and dried consecutively in air
and invacuo for a day to removeresidual traces of chlorobenzene.
Crystallization from CHCl3 afforded a colorless solid (8.26 g,
71%, lit.11a 68%), mp 270 °C (lit. 273-275 °C).11a 1H NMR
ppm (CDCl3): 9.85 (s, 1H, ring-NCHN), 8.45 (s, 1H, ring-CH=
CHNtBu), 7.26 (s, 1H, ring-CHdCHNtBu), 1.73 (s, 9H, CH3).
13C NMR ppm (CDCl3): 138.1 (ring-NCHN), 126.0 ring-
CHdCHNtBu), 120.1 (ring-CHdCHNtBu), 60.1 (CCH3), 30.7
(CH3). 11B NMR ppm (CDCl3): -2.16 (BH).
[{HB(ImtBu)3}Ba{N(SiMe3)2}(N-ImtBu)1.5], 4. Compound 1
(1.00 g, 1.84 mmol), barium iodide (0.72 g, 1.84 mmol), [KN-
(SiMe3)2] (1.46 g, 7.34 mmol): colorless crystals after 7 days at
-20 °C (0.77 g, 48%). In solution each barium center is solvated
by two tert-butylimidazole fragments. 1H{11B} NMR ppm (d8-
tol, 298 K): 7.34 (br s, 2H, coordinated ImtBu NCHN), 7.30 (s,
3H, ligand ring-CHdCHNtBu), 6.98 (s, 2H, coordinated ImtBu
CH dCHNtBu), 6.54 (s, 3H, ligand ring-CHdCHNtBu), 6.42 (s,
2H, coordinated ImtBu CHdCHNtBu), 4.98 (s, 1H, BH), 1.30
(s, 27H, ligand CH3), 0.97 (s, 27H, coordinated ImtBu CH3),
0.51 (s, 18H, SiCH3). 13C{1H} NMR ppm (d8-tol, 298 K): 205.9
(carbene-C), 135.5 (coordinated ImtBu NCHN), 129.2 (coordi-
nated ImtBu CHdCHNtBu), 126.5 (ligand ring-CHd
CHNtBu), 115.5 (coordinated ImtBu CHdCHNtBu), 113.1
(ligand ring-CHdCHNtBu), 55.0 (ligand CCH3), 53.9 (coordi-
nated ImtBu CCH3), 31.2 (ligand CCH3), 29.7 (coordinated
ImtBu CCH3), 6.5 (SiCH3). 11B{1H} NMR ppm (d8-tol, 298 K):
1.32 (BH).
Synthesis of Tris(imidazolin-2-ylidenyl)borate Alkaline Earth
Compounds 2-5. General Procedure. In a glovebox, compound
1, MI2 (M=Ca, Sr, Ba), and potassium bis(trimethylsilyl)amide
were weighed at a 1:1:3 or 4 ratio into a dry Schlenk flask. Dry
THF was added at -78 °C, and the mixture was stirred at room
temperature overnight or until the metal iodide beads had been
consumed. The solvent was removed in vacuo and toluene
added. The resultant milky solution was stirred for another
hour and then allowed to settle prior to cannula and Celite
filtration and concentration of the solution to incipient crystal-
lization. After between 3 and 7 days at -20 °C, the mother liquor
was filtered into a second Schlenk flask and the transparent
crystals completely dried on a vacuum line.
[{HB(ImtBu)3}Ca{N(SiMe3)2}], 2. Compound 1 (0.5 g, 0.92
mmol), calcium iodide (0.27 g, 0.92 mmol), [KN(SiMe3)2] (0.73
g, 3.67 mmol): colorless crystals after 2 days at -20 °C (0.62 g,
[{HB(ImtBu)3}CaI(N-ImtBu)], 5. Compound 1 (0.5 g, 0.92
mmol), calcium iodide (0.27 g, 0.92 mmol), [KN(SiMe3)2] (0.55
g, 2.76 mmol): off-white powder after 2 days at -20 °C (0.42 g,
68%). 1H{11B} NMR ppm (d8-tol, 298 K): 7.91 (br s, 1H,
coordinated ImtBu NCHN), 7.19 (d, 3H, ligand ring-CHd
CHNtBu, 3J = 1.5 Hz), 6.55 (br s, 1H, coordinated ImtBu
1
88%). H{11B} NMR ppm (d8-tol, 298 K): 7.33 (s, 3H, ring-
CHdCHNtBu), 6.65 (s, 3H, ring-CHdCHNtBu), 4.97 (s, 1H,
BH), 1.37 (s, 27H, CH3), 0.27 (s, 18H, Si(CH3)3). 13C{1H} NMR
ppm (d8-tol, 298 K): 198.5 (carbene-C), 127.3 (ring-CHd
CHNtBu), 113.9 (ring-CHdCHNtBu), 55.8 (CCH3), 31.2
(CCH3), 2.6 (Si(CH3)3). 11B{1H} NMR ppm (d8-tol, 298 K):
2.53 (BH). Anal. Calcd for C27H52BCaN7Si2 (581.8): C, 55.74;
H, 9.01; N, 16.85. Found: C, 55.69; H, 8.96; N, 16.91.
3
CHdCHNtBu), 6.44 (d, 3H, ligand ring-CHdCHNtBu, J =
1.5 Hz), 6.10 (br s, 1H, coordinated ImtBu CHdCHNtBu), 4.88
(s, 1H, BH), 1.31 (s, 27H, ligand CH3), 0.84 (s, 9H, coordinated
ImtBu CH3). 13C{1H} NMR ppm (d8-tol, 298 K): 195.5 (car-
bene-C), 129.0 (coordinated ImtBu NCHN), 125.4 (coordinated
ImtBu CHdCHNtBu), 125.0 (ligand ring-CHdCHNtBu), 115.3
(coordinated ImtBu CHdCHNtBu), 113.4 (ligand ring-
CHdCHNtBu), 54.3 (ligand þ coordinated ImtBu CCH3),
29.9 (ligand CCH3), 28.8 (coordinated ImtBu CCH3). 11B{1H}
NMR ppm (d8-tol, 298 K): 1.7 (BH). Anal. Calcd for
C28H46BCaNI8 (672.5): C, 50.01; H, 6.89; N, 16.66. Found: C,
49.93; H, 6.87; N, 16.58.
[Ca{N(SiMe3)2}(N-ImtBu)2], 8. 10% byproduct of the above
1
reaction, isolated in the first crystallization fraction. H{11B}
NMR ppm (d8-tol, 298 K): 8.02 (br s, 1H, ring-NCHN), 6.98 (br
s, 1H, ring-CHdCHNtBu), 6.42 (br s, 1H, ring-CHdCHNtBu),
1.03 (s, 9H, CH3), 0.44 (s, 18H, Si(CH3)3). 13C{1H} NMR ppm
(d8-tol, 298 K): 135.7 (ring-NCHN), 124.9 (ring-CHd
CHNtBu), 114.8 (ring-CHdCHNtBu), 53.5 (CCH3), 28.7
(CCH3), 4.8 (Si(CH3)3).
[{HB(ImtBu)3}Sr{N(SiMe3)2}(N-ImtBu)], 3. Compound 1
(1.00 g, 1.84 mmol), strontium iodide (0.62 g, 1.84 mmol),
[KN(SiMe3)2] (1.46 g, 7.34 mmol): colorless crystals after 1
[{HB(ImtBu)3}CaI(THF)], 50. 5-10% byproduct of the above
reaction. Although colorless single crystals were isolated for
X-ray crystallography, sufficient pure material was not avail-
able for NMR or even elemental analysis.
1
day at -20 °C (0.55 g, 64%). H{11B} NMR ppm (d8-tol, 298
K): 7.47 (br s, 1H, coordinated ImtBu NCHN), 7.33 (d, 3H,
ligand ring-CHdCHNtBu, 3J = 1.5 Hz), 7.19 (br s, 1H, coordi-
nated ImtBu CHdCHNtBu), 6.65 (d, 3H, ligand ring-
CHdCHNtBu, 3J = 1.5 Hz), 6.55 (br s, 1H, coordinated ImtBu
CHdCHNtBu), 5.00 (s, 1H, BH), 1.39 (s, 27H, ligand CH3), 0.99
(s, 9H, coordinated ImtBu CH3), 0.27 (s, 18H, Si(CH3)3). 13C-
{1H} NMR ppm (d8-tol, 298 K): 201.5 (carbene-C), 137.1
(coordinated ImtBu NCHN), 129.3 (coordinated ImtBu CHd
CHNtBu), 127.6 (ligand ring-CHdCHNtBu), 113.6 (ligand þ
coordinated ring-CHdCHNtBu), 55.7 (ligand þ coordinated
ImtBu CCH3), 31.2 (ligand CCH3), 30.0 (coordinated ImtBu
CCH3), 2.59 (Si(CH3)3). 11B{1H} NMR ppm (d8-tol, 298 K):
3.35 (BH). Anal. Calcd for C34H64BN9Si2Sr (753.5): C, 54.19; H,
8.56; N, 16.73. Found: C, 54.29; H, 8.56; N, 16.79.
Synthesis of Tris(imidazolin-2-ylidenyl)borate Alkaline Earth
Compounds 6 and 7. General Procedure. In a glovebox, com-
pound 1 and [M{N(SiMe3)2}2(THF)2] (M = Ca, Sr) were
weighed at a 1:1.5 ratio into a dry Schlenk flask. Dry THF
was added at -78 °C, and the mixture was stirred at room
temperature overnight. The solvent was removed in vacuo and
toluene added. The resultant milky solution was stirred for
another hour and then allowed to settle prior to cannula
filtration and concentration of the solution to incipient crystal-
lization. After 3-5 days at -20 °C, the mother liquor was
filtered into a second Schlenk flask, and the transparent crystals
were completely dried on vacuum line.
[{HB(ImtBu)3}CaBr]2, 6. Compound 1 (0.5 g, 0.92 mmol) and
[Ca{N(SiMe3)2}2(THF)2] (0.70 g, 1.38 mmol): colorless crystals