The synthesis, characterization, and alkylation of nacnac chromium triflate derivatives

Article abstract of DOI:10.1016/j.ica.2010.08.010

A Cr(III) triflate coordinated by the bulky β-diketiminate ^MeL^iPr (^MeL^iPr = 2,4-pentane N,N′-bis(2,6-diisopropylphenyl)diketiminate) was synthesized from the corresponding bridging iodide complex [^MeL

Full text of DOI:10.1016/j.ica.2010.08.010

Inorganica Chimica Acta 364 (2010) 138–143
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
The synthesis, characterization, and alkylation of nacnac chromium
triflate derivatives
⇑
John F. Young, Leonard A. MacAdams, Glenn P.A. Yap, Klaus H. Theopold
Department of Chemistry and Biochemistry, Center for Catalytic Science and Technology’, University of Delaware, Newark, DE 19716, USA
a r t i c l e i n f o
a b s t r a c t
A Cr(III) triflate coordinated by the bulky b-diketiminate MeLiPr
(
MeLiPr = 2,4-pentane N,N0-bis(2,6-diisopro-
-I)]2
Article history:
Available online 12 August 2010
pylphenyl)diketiminate) was synthesized from the corresponding bridging iodide complex [MeLiPrCr(
l
by ligand substitution and subsequent oxidation with silver triflate (AgOTf). MeLiPr CrIII(OTf)2 exhibits rare
trigonal bipyramidal geometry about Cr(III). Attempts to alkylate this triflate synthon with 1,4-dilithiobu-
tane (Li(CH2)4Li) led to reduction, while reaction with dimethylzinc (ZnMe2) led to a mono-alkylated prod-
uct; only the reaction with methyl lithium (MeLi) was successful in generating a dialkyl.
Ó 2010 Elsevier B.V. All rights reserved.
Dedicated to Arnie Rheingold, the fastest
crystallographer in the East and (now) West.
Keywords:
Chromium
Triflates
b-Diketiminates
Lithium alkyls
Alkylation
1. Introduction
atmosphere of N2. Solvents were purchased from Fisher Scientific,
degassed, and dried by passing through activated alumina. THF-d8
b-Diketiminates have been utilized to synthesize low-[1,2] and
high-valent [3] chromium compounds and they stabilize unusual
coordination environments such as complexes with distorted
Y-shaped geometry [4]. A homogeneous model system for the het-
erogeneous Phillips Catalyst has been reported using b-diketimi-
nate ligands [5]. Synthesis of neutral Cr dialkyls provides a viable
and C6D6 were purchased from Cambridge Isotopes Laboratory and
stored under vacuum over Na/K alloy. All other reagents were pur-
chased from Aldrich or Acros and dried using standard procedures
when necessary. [MeLiPrCr(
l-I)]2 was synthesized according to the
literature procedure [1,2]. 1H NMR spectra were taken on a Bruker
DRX-400 spectrometer and were referenced to the residual protons
of the solvent. FT-IR spectra were taken on a Mattson Alpha Cent-
auri or Mattson Genesis Series spectrometers. UV–Vis spectra were
taken using a HP 8453 spectrophotometer. Mass spectra were ob-
tained by the University of Delaware Mass Spectrometry Facility.
Elemental analyses were performed by Columbia Analytics and
Robertson Microlit Laboratories. Room-temperature molar mag-
synthetic route to cationic complexes [5] and dinuclear
enes [6]. Cr(III) dialkyls (alkyl = CH3, CH2C6H5, and CH2Si(CH3)3)
with the smaller b-diketiminate MeLMe MeLMe = 2,4-pentane N,N0-
l-alkylid-
(
bis(2,6-dimethylphenyl) ketiminate) have been synthesized [1,2];
however, there is no reported synthesis using the larger MeLiPr
ligand. Alkylation of [MeLiPrCr(III)(Cl)(
l-Cl)]2 with benzyl Grignard
was reported to reduce the Cr(III) to a Cr(II) chloride dimer [7]
Cr(III) monoalkyls featuring Cp (Cp = cyclopentadienyl) and MeLiPr
ligands have also been prepared [8]. Although precedent suggests
that the synthesis of a b-diketiminato Cr(III) dialkyls bearing the
bulky MeLiPr ligand is synthetically challenging, the first example,
i.e., a Cr(III) dimethyl compound is described herein.
netic susceptibilities (vm) in the solid state were determined using
a Johnson Matthey magnetic susceptibility balance. They were cor-
rected for diamagnetism using Pascal constants [9] and converted
into effective magnetic moments (leff).
2.2. MeLiPrCr(OTf)(THF) (1)
To a solution of [MeLiPrCr(
l-I)]2 (0.200 g, 0.168 mmol) in 50 mL
2. Experimental
of THF, solid AgOTf (0.084 g 0.336 mmol) was added. The reaction
was stirred for 20 min and the solvent was removed under vac-
uum. The crude mixture was extracted with pentane, filtered
through a pad of Celite, to yield a blue-green filtrate. Concentrating
the solution followed by slow cooling to ꢀ30 °C yielded X-ray qual-
ity blue-green crystals (0.176 g, 76%). 1H NMR (400 MHz, d8-THF):
118.8 (b), 10.8 (b), 9.6 (s), 4.6 (s), 2.6 (s), 1.17 (s), ꢀ1.30 (s) ppm. IR
(KBr): 3567 (m), 3060 (m), 2960 (s), 2869 (s), 1527 (s), 1437 (s),
2.1. Materials and methods
All manipulations of compounds were carried out using stan-
dard Schlenk, high vacuum line, or glovebox techniques under an
⇑
Corresponding author.
E-mail address: theopold@udel.edu (K.H. Theopold).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.08.010

J.F. Young et al. / Inorganica Chimica Acta 364 (2010) 138–143
139
1387 (s), 1317 (s), 1260 (s), 1205 (s), 1146 (m), 1101 (s), 1024 (s),
935 (w), 796 (s), 758 (m), 645 (m), 598 (w), 523 (w) cmꢀ1. UV–Vis
dark green crystals of marginal X-ray diffraction quality of 5b
(0.460 g, 88%). 1H NMR (CD2Cl2): 14.3 (b, 6H), 8.3 (b, 4H), 4.4 (vb,
6H), 1.7 (b, 24H) ppm. IR (KBr): 3059 (w), 2965 (s), 2931 (m),
2871 (m), 1532 (s), 1466 (m), 1436 (m), 1379 (s), 1339 (s), 1312
(m), 1261 (m), 1234 (m), 1199 (s), 1138 (w), 1109 (w), 1004 (s),
935 (w), 864 (w), 796 (m), 759 (w), 632 (m) cmꢀ1. UV–Vis (Et2O):
(Pentane): kmax (e leff = 4.8(1) lB
) = 602 (174 Mꢀ1 cmꢀ1) nm.
(294 K). Mp = 166–168 °C dec. Mass Spectrum m/z (%): 618 (100)
[M+ꢀC4H8O]. Anal. Calc for C34H49N2O3F3SCr: C, 59.15; H, 7.15; N,
4.05. Found: C, 58.68; H, 7.12; N, 4.19%.
kmax
(e =
) = 535 (458 Mꢀ1 cmꢀ1), 629 (435 Mꢀ1 cmꢀ1) nm. leff
2.3. MeLiPrCr(OTf)2. (2)
4.0(1) lB (294 K). Mp: 184–186 °C. Mass Spectrum, m/z (%): 618
(100) [M+ꢀCH3]. Anal. Calc for C34H44N2O3F3SCr: C, 58.75; H, 6.84;
N, 4.42. Found: C, 58.43; H, 6.90; N, 4.67%. X-ray quality crystals
of MeLiPrCrCH3(OTf)(THF) (5a) can be prepared by recrystallizing
5b from THF.
To a solution of [MeLiPrCr(
l-I)]2 (0.200 g, 0.168 mmol) in 50 mL
of toluene, solid AgOTf (0.169 g, 0.672 mmol) was added. The solu-
tion was stirred for 20 min and filtered through a pad of Celite. The
solvent was removed under vacuum. The crude solid was recrystal-
lized from pentane at ꢀ30 °C yielding X-ray quality brown crystals
(0.190 g, 74%). 1H NMR (400 MHz, CD2Cl2): 70 (vb), 23.9 (b), 1.18
(b) ppm. IR (KBr): 3063 (w), 2968 (s), 2873 (s), 1645 (m), 1525
(m), 1465 (m), 1387 (s), 1204 (s), 1106 (w), 1022 (s), 799 (m),
2.7. MeLiPrCr(CH3)2(THF) (6)
A solution of 2 (0.322 g, 0.419 mmol) in 10 mL of diethyl ether
was chilled to ꢀ30 °C. MeLi (0.600 mL, 0.839 mmol) was then
added dropwise to the cold solution. The solution gradually dark-
ened in color as it stirred for two hours at room temperature. The
solvent was then removed under vacuum. The crude solid was
extracted with pentane, filtered, and the pentane solvent was then
removed under vacuum. The crude solid was recrystallized by slow
cooling of a THF solution to ꢀ30 °C, which yielded brown crystals of
marginal X-ray diffraction quality (0.203 g, 85%). 1H NMR
(400 MHz, C6D6): 144 (b), 46.2 (b), 13.0 (b), 5.79 (b), 4.58 (b),
3.097 (b), 1.744 (b), 1.26 (s) ppm. IR (KBr): 3056 (w), 2959 (s),
2926 (m), 2867 (m), 1521 (s), 1462 (s), 1435 (s), 1392 (s), 1310
(s), 1229 (w), 1171 (m), 1098 (m), 1054 (m), 933 (m), 868 (m),
792 (m), 760 (m), 747 (w), 643 (w), 517 (w), 450 (w) cmꢀ1. UV–
634 (m), 593 (w) cmꢀ1
(857 Mꢀ1cmꢀ1 nm, 600 (263 Mꢀ1cmꢀ1) nm.
.
UV–Vis (Pentane): kmax
(e) = 460
)
leff = 3.9(1) lB
(294 K). Mp = 246–248 °C dec. Mass Spectrum m/z (%): 767 (4)
[M+], 618 (100) [M+ꢀSO3CF3]. Anal. Calc for: C31H41N2S2O6F6Cr: C,
48.49; H, 5.38; N, 3.65. Found: C, 48.24; H, 5.13; N, 3.50%.
2.4. MeLiPrCr(OTf)2(py) (3)
To a solution of 2 (0.200 g, 0.260 mmol) in 25 mL of pentane,
pyridine (0.021 mL, 0.260 mmol) was added dropwise. The reac-
tion was stirred for 20 min and the solvent was removed under
vacuum. The crude solid mixture was recrystallized from pentane
at ꢀ30 °C to yield a purple microcrystalline powder. X-ray quality
crystals were obtained by slow evaporation of a toluene solution at
ambient temperature. (0.181 g, 82%). 1H NMR (400 MHz, CD2Cl2):
44.1 (vb), 36.9 (vb), 19.4 (vb), 10.5 (b), 8.55 (b), 7.3 (s), 7.1 (s),
6.3 (b), 3.1 (s), 1.18 (b) ppm. IR (KBr): 3060 (w), 2965 (s), 2930
(s), 2872 (m), 1610 (m), 1529 (s), 1447 (s), 1386 (s), 1335 (s),
1201 (s), 1108 (w), 1017 (s), 935 (m), 800 (m), 762 (m), 702 (m),
648 (s), 632 (m), 590 (w), 519 (w) cmꢀ1. UV–Vis (Pentane): kmax
Vis (Pentane): kmax (e leff = 3.8(1)
) = nm) 668 (142 Mꢀ1 cmꢀ1) nm.
lB (294 K). Mp = 192–194 °C. Anal. Calc for C31H55N2OCr: C, 73.52;
H, 9.69; N, 4.90. Found: C, 73.85; H, 9.56; N, 4.76%.
2.8. X-ray crystallography
Details of the crystallographic date collection and refinement
parameters are given in Table 1. Suitable crystals were selected,
mounted with viscous oil and cooled to the data collection temper-
ature. Data were collected on a Brüker-AXS APEX CCD diffractome-
(e
) = 441
(1837 Mꢀ1 cmꢀ1) nm,
522
(542 Mꢀ1 cmꢀ1) nm.
l
eff = 3.7(1) lB (294 K). Mp = 227–230 °C dec. Mass Spectrum m/z
(%): 767 (5) [M+ꢀC5H5N], 618 (65) [M+ꢀC6H5NSO3F3]. Anal. Calc
for C36H46N3O6F6S2Cr: C, 51.06; H, 5.47; N, 4.96. Found: C, 50.49;
H, 5.94; N, 4.96%.
ter with graphite-monochromated Mo K
a radiation (k = 0.71073 Å).
Unit cell parameters were obtained from 60 data frames, 0.3°
x
,
from three different sections of the Ewald sphere. The systematic
absences and equivalent reflections in the diffraction data are
consistent with Cc and C2/c for 3, with P21 and P21/m for 5b, and,
uniquely, with I41/a for 6. No symmetry higher than triclinic was
observed for the other data sets. Solution in the centrosymmetric
space group options yielded chemically reasonable and computa-
tionally stable results of refinement. The data-sets were treated
with SADABS absorption corrections based on redundant multiscan
data [11]. Compounds 5b and 6 consistently deposited as clusters of
small, multiple or cracked crystals and the data reported herein
represent the best of several attempts. The data crystal for structure
4 was modeled as compound 4 co-crystallized with 3% iodide start-
ing material. The structures were solved using direct methods and
refined with full-matrix, least-squares procedures on F2. A coordi-
nated THF molecule each in 1 and 5a was located disordered in
two conformations with 53/47 and 52/48, respectively, refined site
occupancy. One molecule of co-crystallized solvent molecule was
located in each asymmetric unit of 2 (pentane) and 4 (toluene). Se-
verely disordered solvent molecules in 1 (two pentane molecules
per unit cell) and in 3 (12 toluene molecules per unit cell) were trea-
ted as diffused contributions [12]. Three of the isopropyl groups in
5a were located disordered in two positions with 64/36, 57/43, and
71/29 refined site occupancies. Disordered contributions were re-
fined with equal atomic displacement parameters and with similar
1,2 and 1,3 distances between chemically equivalent atoms. Atomic
2.5. MeLiPrCr(OTf)(py) (4)
To a solution of 1 (0.232 g, 0.336 mmol) in 25 mL of pentane,
pyridine (0.027 mL, 0.336 mmol) was added dropwise, producing
a reddish orange color. Concentrating and slow cooling of the solu-
tion to ꢀ30 °C yielded a microcrystalline powder. X-ray quality
crystals were grown from toluene (0.183 g, 78%). 1H NMR
(400 MHz, C6D6) 9.85 (b), 8.26 (b), 3.60, 2.09, 1.17, 0.28 ppm. IR
(KBr): 3057 (w), 2965 (s), 2929 (s), 2869 (m), 1608 (m), 1526 (s),
1443 (s), 1410 (s), 1361 (s), 1261 (s), 1239 (s), 1213 (s), 1175 (s),
1032 (s) 933 (w), 791 (m) 754 (m), 700 (m), 629 (s), 597
(w) cmꢀ1. UV–Vis (Pentane): kmax ) = 459 (959 Mꢀ1 cmꢀ1) nm.
(e
l
eff = 4.9(1) lB (294 K). Mp = 180–182 °C dec. Mass Spectrum m/z
(%): 618 (100) [M+ꢀC5H5N]. Anal. Calc for C35H46N3O3F3SCr: C,
60.24; H, 6.64; N, 6.02. Found: C, 58.84; H, 6.89; N, 5.95%.
2.6. MeLiPrCrCH3(OTf)(THF) (5a) and MeLiPrCrCH3(OTf) (5b)
To a solution of [MeLiPrCr(
l-Me)]2 [10] (0.400 g, 0.413 mmol) in
30 mL of diethyl ether solid AgOTf (0.212 g, 0.413 mmol) was
added. The color of the solution immediately became dark green.
After stirring for 45 min the reaction mixture was filtered. Concen-
tration of the filtrate followed by slow cooling to ꢀ30 °C, yielded

140
J.F. Young et al. / Inorganica Chimica Acta 364 (2010) 138–143
Table 1
Crystal and experimental data for compounds 1–6.
Compound
1
2
3
4
5a
5b
6
Formula
MW
Crystal system
Space group
a (Å)
C
39H61N2O4F3SCr C36H53N2O6F6S2Cr C64H78N3O6F6S2Cr C41.97H54N3O2.91F2.91S0.9I0.03Cr C35H52N2O4F3SCr C31H44N2O3F3SCr C35H55N2OCr
762.96
Triclinic
P-1
839.92
Triclinic
P-1
1215.41
Monoclinic
C2/c
31.480(5)
22.264(3)
18.766(3)
90
789.28
Triclinic
P-1
705.85
Triclinic
P-1
633.74
Monoclinic
P2(1)/m
9.0288(2)
20.1030(4)
10.1482(2)
90
571.81
Tetragonal
I4(1)/a
27.329(3)
27.329(3)
17.714(4)
90
10.503(3)
12.731(3)
16.824(4)
82.727(4)
78.876(4)
68.399(4)
2048.5(9)
2
11.693(2)
12.257(2)
14.885(2)
97.709(2)
92.878(2)
96.122(2)
2093.4(6)
2
10.396(2)
11.703(3)
18.772(4)
77.964(3)
81.997(3)
65.882(3)
2034.9(8)
2
10.280(7)
12.017(8)
15.904(2)
75.707(2)
84.685(2)
70.612(2)
1796(2)
2
b (Å)
c (Å)
a
(°)
b (°)
120.270(5)
90
115.262(1)
90
90
90
c
(°)
V (Å3)
Z
11359(3)
8
1665.8(1)
2
13231(4)
16
T (K)
120
120
120
120
170
213
171
l
(Mo K
(mmꢀ1
F (0 0 0)
Dcalc (g cmꢀ3
a
)
)
0.383
0.443
0.352
0.408
0.431
0.455
0.373
816
1.237
9252/426
1.036
882
5128
1.421
13041/497
1.026
835
1.288
8799/493
1.031
750
1.305
8800/472
1.032
670
4976
1.148
8268/364
1.046
)
1.333
9381/490
1.020
1.263
3485/202
1.020
Data/parameters
Goodness-of-fit
(GOF) on F2
R(F) (%)a
0.0632
0.1200
0.0600
0.1558
0.0487
0.1313
0.0578
0.1258
0.0480
0.1303
0.0643
0.2514
0.0751
0.1532
R(F2) (%)a
a
2
Quantity minimized: Rw(F2) =
R
[w(F0 ꢀ Fc2)2]/
R
[wFo2)2]1/2; R =
R
D
/
R
(Fo),
D
= |Fo ꢀ Fc|.
Scheme 1. Synthesis of chromium compounds.
scattering factors are contained in the SHELXTL 6.12 program library
[13].
the desired MeLiPrCr(OTf)2 (2). To avoid the coordinating effects of
THF the reaction was repeated in toluene solvent. Upon addition
of AgOTf, the solution immediately changed color from a dark
green to brown and a distinct silver mirror was observed. The X-
ray structure of 2 (see Fig. 2) confirmed the successful oxidation
and the presence of two coordinated triflate ligands. Compounds
containing both coordinated j1 and j2 bound triflates have been
observed on other metals [15–19]; however, a search of the Cam-
bridge Structural database showed that 2 is the first example for
chromium. The triflate bonded j1 has a Cr–O bond distance of
1.945(2) Å and the other triflate bonded j2 features longer Cr–O
bond distances of 2.069(2) Å and 2.117(2) Å. The coordination
geometry of 2 is best described as trigonal bipyramidal and is quite
rare for Cr(III), which overwhelming favors octahedral six-coordi-
nation [20]. In the reported 5-coordinate structures, trigonal
bipyramidal geometry is enforced by bulky ligands or by ligands
with constrained geometry as also seen in 2 (O(1)–Cr(1)–O(2)
67.57(8)°). Menjón and co-workers have reported that five-coordi-
nate Cr(III) compounds prefer to be square pyramidal versus trigo-
nal bipyramidal [21]. The trigonal bipyramidal geometry in 2 is
defined by the O(4)–Cr–O(2) axis (angle = 159.51(9)°) and the tri-
gonal plane (N1,N2,O1) (sum of the bonding angles around the
Cr center is 357.2(2)°).
3. Results and discussion
We have reasons to be interested in mononuclear Cr(III) dialkyls
featuring the sterically hindered MeLiPr ligand; such compounds
were not known heretofore. To possibly avoid reduction and the
formation of a stable Cr(II) dimer, as previously reported by Gibson
[7], it was hypothesized that using a monomeric Cr(III) synthon
with excellent leaving groups might mitigate this problem. To gen-
erate this Cr(III) synthon it was anticipated that silver triflate
(AgOTf (OTf = O3SCF3)) would readily oxidize [MeLiPrCr(
yield MeLiPrCrI(OTf). However, although the reaction mixture of [Me-
LiPrCr(
-I)]2 with two equivalents of AgOTf in THF immediately
l-I)]2 to
l
changed color from green to blue, no silver mirror was observed.
Characterization of the product did not reveal the anticipated oxi-
dation product, but instead showed that ligand substitution had
occurred (see Scheme 1) as confirmed by the X-ray structure
(Fig. 1). (MeLiPrCr(OTf)(THF) (1) has distorted square planar geome-
try which deviates somewhat from more prototypical Cr(II) com-
plexes [14].
Four equivalents of AgOTf were then used to oxidize [Me-
As our interest in Cr(III) dialkyls was sparked by the potential
LiPrCr(
l
-I)]2 in an attempt to prepare a Cr(III) bis(triflate) com-
preparation of metallacycles, alkylation of
2 was initially
pound. Initial reactions in THF solvent yielded mixtures of 1 and
attempted with Li(CH2)4Li [22]. The product of this reaction was

J.F. Young et al. / Inorganica Chimica Acta 364 (2010) 138–143
141
Fig. 1. The molecular structure of 1. 2,6-Diisopropyl groups and hydrogen atoms
are omitted for clarity. Thermal ellipsoids are shown at the 30% probability level.
Cr–N(1) 2.005(3) Å, Cr–N(2) 2.015(3) Å, Cr–0(4) 2.096(2) Å Cr–O(1) 2.039(2) Å,
N(1)–Cr(1)–O(4) 156.93(10)°, N(2)–Cr(1)–O(1) 159.99(11)°.
Fig. 2. The molecular structure of 2. 2,6-Diisopropyl groups, hydrogen atoms, and
solvent are omitted for clarity. Thermal ellipsoids are shown at the 30% probability
level. Cr–N(1) 1.924 (2) Å, Cr–N(2) 1.921(2) Å, O(4)–Cr(1)–O(2) 159.51(9)°.
determined to be reduced, namely [MeLiPrCr(
l-I)]2. Mechanistically,
it is envisioned that 2 is first reduced to a Cr(II) triflate intermedi-
ate either via homolysis or electron transfer, which then undergoes
ligand substitution with lithium iodide. Li(CH2)4Li was synthesized
by lithium/halogen exchange of 1,4-diiodobutane, thus two equiv-
alents of inseparable lithium iodide are present with each equiva-
lent of Li(CH2)4Li.
To disfavor reduction, 2 was treated with pyridine to make the
Cr more electron rich. The change in the coordination environment
around Cr was apparent from the solution’s instant color change
from brown to purple, to form the octahedral MeLiPrCr(OTf)2(py)
(3). The crystal structure of 3 (Fig. 3) confirmed the presence of a
coordinated pyridine. As a result of 3 being a crowded octahedral
complex the Cr-atom bond distances all increase compared to less
crowded five-coordinate 2. The
j
2-triflate Cr–O bond distances
increased by 0.046(2) Å and 0.080(1) Å, the j1 triflate Cr–O bond
distance is longer by 0.047(2) Å. Finally, the Cr–N bond distances
of the b-diketiminate moiety increased in length by 0.071(2) Å
and 0.077(2) Å, respectively.
Unfortunately, the reaction of 3 with Li(CH2)4Li also resulted in
reduction. X-ray crystallography revealed that the product of
this reaction was the pyridine adduct of the Cr(II) triflate, i.e.,
MeLiPrCr(OTf)(py) (4) (see Fig. 4). In contrast to the reaction of 2 with
Li(CH2)4Li, [MeLiPrCr(
l-I)]2 was not obtained, but the crystal struc-
ture of 4 did contain 3% of an iodide species. Compound 4 differs
from 1 only in the coordinated donor ligand; however, the Cr–O
bond distance of 2.026(2) Å indicated that the triflate is bound with
similar strength in both complexes. Compound 4 can also be pre-
pared independently by treating 1 with one equivalent of pyridine.
Attempts were made to avoid the facile reduction by using less
reducing alkylating agent. Thus, 2 was reacted with one equivalent
of ZnMe2. X-ray quality crystals grown in THF revealed that
although reduction was averted, 2 only underwent mono-alkyl-
ation to form MeLiPrCrCH3(OTf)(THF) (5a) (see Fig. 5). Compound
Fig. 3. The molecular structure of 3. 2,6-Diisopropyl groups and hydrogen atoms
are omitted for clarity. Thermal ellipsoids are shown at the 30% probability level.
Cr–N(1) 1.979(2) Å, Cr–N(2) 2.001(2) Å, Cr–N(3) 2.001(2) Å.
without coordinated THF (5b) by oxidizing [MeLiPrCr(
l-CH3)]2 with
two equivalents of AgOTf in ether [10]. The coordination of THF
caused the triflate to be bound j1 in 5a, whereas, without THF
the triflate is bound j2 1
.
5a is a square pyramidal Cr(III) species containing a j
1-triflate with
an apically positioned methyl group (the basal plane is defined by
N(1), N(2), O(1), and O(4)). Compound 5a can also be isolated
1
See supplementary material for structure.

142
J.F. Young et al. / Inorganica Chimica Acta 364 (2010) 138–143
Fig. 6. The molecular structure of 6. Hydrogen atoms are omitted for clarity.
Thermal ellipsoids are shown at the 30% probability level. N(1)–Cr–N(1) 89.5(2)°,
N(1)–Cr–O(1) 90.7(2)°, O(1)–Cr–C(2) 85.5(2)°, C(2)–Cr–N(2) 93.6(2)°, N(1)–Cr–C(1)
98.8(2)°, N(2)–Cr–C(1) 93.4(2)°, O(1)–Cr–C(1) 88.1(2)°, C(1)–Cr–C(2) 109.2(2)°.
Fig. 4. The molecular structure of 4. 2,6-Diisopropyl groups, hydrogen atoms, and
solvent are omitted for clarity. Thermal ellipsoids are shown at the 30% probability
level. Cr–N(1) 2.016(2) Å, Cr–N(2) 2.034(2) Å, Cr–N(3) 2.133(2) Å, N(1)–Cr(1)–O(1)
165.23(9)°, N(2)–Cr(1)–N(3) 158.52(9)°.
similar to the apical methyl carbon to Cr bond distance in 5a.
The methyl group (C2) in the basal plane (defined by N(1), N(2),
O(1), and C(2)) is located 2.084(4) Å from the Cr. The longer Cr–C
bond distance in the basal plane is presumably a result of the trans
influence of the nitrogen atom (N1) from the b-diketiminate
ligand. This trans influence is also evident in the Cr–N bond
distances. The Cr–N(2) has a bond distance of 2.032(2) Å, while
the trans nitrogen atom (N1) has a Cr–N(1) bond distance of
2.104(3) Å. Thus
2 suffered reduction when reacted with
Li(CH2)4Li, but the analogous reaction with two equivalents of MeLi
produced the dialkyl species. It is, of course, difficult to predict a
priori the outcome of such reactions, since there are various factors
that could affect the reduction potential. It is well documented that
organolithium compounds can vary in their reactivity as they exist
as aggregates and these aggregation states can change with coordi-
nating ligands [26]. Methyllithium is known to exist as a tetramer
in ether [27–29]; however, the state of Li(CH2)4Li in ether is
unknown.
4. Conclusions
This work surveys the synthesis and reactivity of several chro-
mium triflate compounds supported by the sterically extremely
encumbering b-diketiminate ligand MeLiPr. As expected based on
previous results, these b-diketiminato Cr(III) complexes are easily
susceptible to reduction by alkylating reagents. However, the
supreme lability of the triflate ligands has allowed for the synthesis
of MeLiPrCr(Me)2THF, the first example of a neutral Cr(III) dialkyl
featuring this particular diketiminate ligand.
Fig. 5. Structure of 5a. 2,6-Diisopropyl groups and hydrogen atoms are omitted for
clarity. Thermal ellipsoids are shown at the 30% probability level. Cr–N(1)
2.025(2) Å, Cr–N(2) 2.018(2) Å, Cr–O(1) 2.045(2) Å, Cr–O(4) 2.010(2) Å, Cr–C(1)
2.049(3) Å, N(2)–Cr–O(1) 158.70(8)°, C(1)–Cr–O(1) 105.22(9)°.
The reduction potential of Li(CH2)4Li is not known; however,
lithium aryls and alkyls (LiR) have estimated E° values which vary
widely depending on the identity of R (e.g., R = tBu, ca. ꢀ2.7 V;
R = Cp, ca. ꢀ0.8 V) [23–25]. Accordingly, 2 was then reacted with
two equivalents of MeLi. The use of MeLi was auspicious in yield-
ing the desired bis(methyl) species MeLiPrCr(CH3)2 (6) (see Scheme
1). The molecular structure of 6 (see Fig. 6) was determined by
X-ray diffraction and confirmed the presence of two methyl
groups. Compound 6 is a square pyramidal complex with a coordi-
nated molecule of THF and also is the first reported Cr dialkyl coor-
dinated by MeLiPr. The methyl group (C1), located in the apical site
of the pyramid, has a Cr–C bond distance of 2.045(4) Å, which is
5. Supplementary material
CCDC 764574 – 764580 contain the supplementary crystallo-
graphic data for 1, 2, 3, 4, 5a, 5b, and 6, respectively. These data
can be obtained free of charge from the The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgement
We thank the National Science Foundation (Grants CHE-
0616375 and CHE-0911081) for financial support.

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