Nickel complexes of a bulky Β-diketiminate ligand

Article abstract of DOI:10.1021/ic025986n

Nickel(II) chloride forms a complex with tetrahydrofuran, NiCl₂(THF)_1.5, that can be used to prepare nickel chloride complexes of a bulky β-diketiminate ligand L^Me. [L^MeNiCl]₂ and L^MeNiCl₂LiTHF₂, which have tetrahedral geometries in the solid state, are in equilibrium with three-coordinate L^MeNiCl. Thermodynamic parameters for the equilibrium between [L^MeNiCl]₂ and L^MeNiCl are ΔH = 51(5) kJ/mol and ΔS = 116(11) J/(mol·K). L^MeNiCl forms a tetrahydrofuran complex with a binding constant of 1.2(2) M^-1 at 21 °C. The chloride complexes were used to generate a three-coordinate nickel(II)-amido complex. This amido complex, L^MeNiN(SiMe₃)₂, is compared with L^MeMN(SiMe₃)₂ (M = Mn, Fe, Co) (Panda, A.; Stender, M.; Wright, R. J.; Olmstead, M. M.; Klavins, P.; Power, P. P. Inorg. Chem. 2002, 41, 3909-3916). Trends in the metrical parameters of the three-coordinate L^MeM^II amido compounds are similar to the trends in three-coordinate L^tBuM^II chloride compounds.

Full text of DOI:10.1021/ic025986n

Inorg. Chem. 2003, 42, 1720−1725
Nickel Complexes of a Bulky â-Diketiminate Ligand
Nathan A. Eckert, Emily M. Bones, Rene J. Lachicotte, and Patrick L. Holland*
Department of Chemistry, UniVersity of Rochester, Rochester, New York 14627
Received August 29, 2002
Nickel(II) chloride forms a complex with tetrahydrofuran, NiCl2(THF)1.5, that can be used to prepare nickel chloride
complexes of a bulky â-diketiminate ligand LMe. [LMeNiCl]2 and LMeNiCl2LiTHF , which have tetrahedral geometries
2
in the solid state, are in equilibrium with three-coordinate LMeNiCl. Thermodynamic parameters for the equilibrium
between [LMeNiCl]2 and LMeNiCl are ∆H ) 51(5) kJ/mol and ∆S ) 116(11) J/(mol‚K). LMeNiCl forms a tetrahydrofuran
complex with a binding constant of 1.2(2) M-1 at 21 °C. The chloride complexes were used to generate a three-
coordinate nickel(II)−amido complex. This amido complex, LMeNiN(SiMe3)2, is compared with LMeMN(SiMe3)2 (M )
Mn, Fe, Co) (Panda, A.; Stender, M.; Wright, R. J.; Olmstead, M. M.; Klavins, P.; Power, P. P. Inorg. Chem. 2002,
II
41, 3909−3916). Trends in the metrical parameters of the three-coordinate LMeM amido compounds are similar to
tBu II
the trends in three-coordinate L M chloride compounds (Holland, P. L.; Cundari, T. R.; Perez, L. L.; Eckert, N.
A.; Lachicotte, R. J. J. Am. Chem. Soc. 2002, 124, 14416−14424).
Introduction
â-Diketiminate ligands have experienced a resurgence in
inorganic and organometallic chemistry.1 Compounds with
bulky â-diketiminate ligands (Figure 1, right) have been
synthesized for use in small-molecule activation and catalytic
studies and include examples from all of the late first-row
and several second- and third-row transition metals.2 How-
ever, nickel compounds with bulky â-diketiminate ligands
are extremely rare,1 despite the fact that group 10 metals
with bulky bidentate ligands are utilized as catalysts for
transformations such as olefin polymerization3 and alkene
and alkyne amination.4 Because ligand loss from four-
coordinate complexes is an initial step in catalytic reactions,
Figure 1. Literature â-diketimine compound7 (left) and the â-diketiminate
t
ligands described in this study (right; LMe, R ) Me, and LtBu, R ) Bu).
a number of groups have studied the structure, spectroscopic
properties, and reactivity of three-coordinate Ni(II) com-
pounds.5 â-Diketiminate complexes are a key part of this
continuing effort.
Several bis(â-diketiminato)nickel complexes have been
reported6 and Feldman et al. synthesized a Ni(II) compound
with a neutral â-diketimine ligand7 (Figure 1, left), but no
1:1 nickel/â-diketiminate complexes were known until our
recent report of three-coordinate LtBuNiCl.8 LtBuNiCl and its
* To whom correspondence should be addressed. E-mail: holland@
chem.rochester.edu.
(1) Review: Bourget-Merle, L.; Lappert, M. F.; Severn, J. R. Chem. ReV.
2002, 102, 3031-3066.
(2) (a) Budzelaar, P. H. M.; de Gelder, R.; Gal, A. W. Organometallics
1998, 17, 4121-4123. (b) Holland, P. L.; Tolman, W. B. J. Am. Chem.
Soc. 1999, 121, 7270-7271. (c) Holland, P. L.; Tolman, W. B. J.
Am. Chem. Soc. 2000, 122, 6331-6332. (d) Fekl, U.; Kaminsky, W.;
Goldberg, K. I. J. Am. Chem. Soc. 2001, 123, 6423-6424. (e) Smith,
J. M.; Lachicotte, R. J.; Holland, P. L. Chem. Commun. 2001, 1542-
1543. (f) Dai, X.; Warren, T. H. Chem. Commun. 2001, 1998-1999.
(g) Yokota, S.; Tachi, Y.; Itoh, S. Inorg. Chem. 2002, 41, 1342-
1344. (h) Spencer, D. J. E.; Aboelella, N. W.; Reynolds, A. M.;
Holland, P. L.; Tolman, W. B. J. Am. Chem. Soc. 2002, 124, 2108-
2109. (i) Panda, A.; Stender, M.; Wright, R. J.; Olmstead, M. M.;
Klavins, P.; Power, P. P. Inorg. Chem. 2002, 41, 3909-3916.
(3) (a) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. ReV. 2000, 100,
1169-1203. (b) Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew.
Chem., Int. Ed. 1999, 38, 428-447.
(5) (a) Mathew, M., Carty, A. J.; Palenik, G. J. J. Am. Chem. Soc. 1970,
92, 3198-3199. (b) Hope, H.; Olmstead, M. M.; Murray, B. D.; Power,
P. P. J. Am. Chem. Soc. 1985, 107, 712-713. (c) Conroy-Lewis, F.
M.; Mole, L.; Redhouse, A. D.; Litster, S. A.; Spencer, J. L. J. Chem.
Soc., Chem. Commun. 1991, 1601-1603. (d) Hay-Motherwell, R.;
Wilkinson, G.; Sweet, T. K. N.; Hursthouse, M. B. Polyhedron 1996,
15, 3163-3166. (e) Schebler, P. J.; Mandimutsira, B. S.; Riordan, C.
G.; Liable-Sands, L. M.; Incarvito, C. D.; Rheingold, A. L. J. Am.
Chem. Soc. 2001, 123, 331-332. (f) dtbpe ) bis(di-tert-butylphos-
phino)ethane. Mindiola, D. J.; Hillhouse, G. L. J. Am. Chem. Soc.
2001, 123, 4623-4624. (g) Ge, P.; Rheingold, A. L.; Riordan, C. G.
Inorg. Chem. 2002, 41, 1383-1390.
(6) (a) McGeachin, S. G. Can. J. Chem. 1968, 739-740. (b) Parks, J. E.;
Holm, R. H. Inorg. Chem. 1968, 7, 1408-1416.
(7) Feldman, J.; McLain, S. J.; Parthasarathy, A.; Marshall, W. J.;
Calabrese, J. C.; Arthur, S. D. Organometallics 1997, 16, 1514-1516.
(4) Mueller, T. E.; Beller, M. Chem. ReV. 1998, 98, 675-703.
1720 Inorganic Chemistry, Vol. 42, No. 5, 2003
10.1021/ic025986n CCC: $25.00 © 2003 American Chemical Society
Published on Web 02/13/2003

Nickel Complexes of a Bulky â-Diketiminate Ligand
Table 1. Data Collection and Refinement Parameters
[LMeNiCl]2
LMeNiCl2Li(THF)(OEt2)
LMeNiN(SiMe3)2
empirical formula
fw
cryst system
space group
a (Å)
C58H82N4Ni2Cl2
1023.59
monoclinic
P21/n
14.324(1)
13.516(1)
14.797(1)
105.411(1)
2761.7(3)
4
1.231
0.818
0.0479, 0.0979
0.969
C37H59N2O2NiLiCl
700.49
monoclinic
P21/n
19.708(2)
9.798(1)
21.389(2)
109.993(2)
3881.3(8)
4
1.181
0.669
0.0853, 0.2168
1.047
C35H59N3NiSi2
636.74
monoclinic
P21/n
9.1530(5)
20.257(1)
20.197(1)
90.785(1)
3744.4(3)
4
1.130
0.608
0.0643, 0.1244
1.135
b (Å)
c (Å)
â (deg)
V (Å3)
Z
F (g/cm3)
µ (mm-1
)
R1, wR2 (I > 2σ(I))
GOF
three-coordinate analogues LtBuFeCl and LtBuCoCl form a
well-characterized series of trigonal planar complexes.8 While
these compounds are interesting in that the third ligand can
be substituted while maintaining a three-coordinate geometry,
the synthesis of LtBu entails several steps.8,9 Three-coordinate
complexes of the very similar ligand LMe would be preferable
because of the easy synthesis of LMeH from 2,4-pentanedi-
one and 2,6-diisopropylaniline.7,10 The copper(II) chloride
complex of LMe is three-coordinate.11 Here we report that,
in an interesting contrast, nickel(II) chloride complexes of
LMe are four-coordinate. Despite the higher coordination
number in the halide complexes, the LMeNiII salts are
convenient starting materials for three-coordinate nickel(II)
compounds.
Figure 2. ORTEP drawing of [LMeNiCl]2. Thermal ellipsoids are shown
at 50% probability. Hydrogen atoms are omitted for clarity. Key distances
(Å) and angles (deg): Ni1-N11 1.938(3), Ni1-N12 1.946(3), Ni1-Cl1
2.324(1), Ni1-Cl2 2.349(1), Ni-Ni 3.458(3), N11-Ni-N12 93.7(1), Cl1-
Ni-Cl2 84.56(4).
Results and Discussion
Synthesis of NiCl2THF1.5. In recent work, we found FeCl2-
12
THF1.5 to be a convenient source of iron(II), because it is
more soluble than FeCl2.2e We desired an analogous nickel
starting material, but Kern reported that NiCl2 does not react
THFn were stirred in hot toluene, a bright blue solution was
formed. When extracted into CH2Cl2, the solution was dark
green, but dark blue crystals could be isolated in moderate
yield, and one was subjected to crystallographic analysis.
The molecular structure of [LMeNiCl]2 is shown in Figure 2
(Table 1). There is a crystallographic C2 axis of symmetry
that makes the two LMeNiCl units equivalent. The Ni-Cl
distances of 2.324(1) and 2.349(1) Å are longer than typical
Ni(II)-µ-Cl bonds,16 perhaps due to the sterically imposing
â-diketiminate and the high-spin Ni(II). The Ni-Ni distance
is 3.458(1) Å, suggesting that there is no direct metal-metal
interaction. The bite angle (N-Ni-N) of 93.7(1)° is typical
for â-diketiminate complexes.1,2,8-11
13
with THF.12 In our hands, heating anhydrous NiCl2 in
boiling THF under N2 did form an adduct, which was isolated
simply by removing solvent under vacuum. To establish the
chemical formula, the Ni content of the tan product was
determined by precipitation with dimethylglyoxime.14 De-
pending on the length of reflux time, the amount of
coordinated THF varied from 0.7 to 1.6 molecules, with a
reaction time of 24 h or more reproducibly giving a
stoichiometry of NiCl2THF1.5.15 The precise amount of
coordinated THF, however, did not influence the products
of the following reactions.
Syntheses and Solid-State Structures of Ni(II) Chloride
Complexes. When equimolar amounts of LiLMe and NiCl2-
Interestingly, when the reaction of LiLMe and NiCl2THFn
was carried out in THF at room temperature, deep purple
crystals were obtained from Et2O. The molecular structure
is shown in Figure 3 (Table 1). The product, LMeNiCl2Li-
(THF)(OEt2), is structurally analogous to the recently
described Fe(II) and Co(II) compounds LMeMCl2Li(ether)2.2e,i
The Ni-Cl distances are slightly shorter in LMeNiCl2Li-
(THF)(OEt2) (average 2.29 Å) than [LMeNiCl]2. The geom-
etry about nickel in LMeNiCl2Li(THF)(OEt2) is tetrahedral,
like that of [LMeNiCl]2, and the bond angles are similar
(8) Holland, P. L.; Cundari, T. R.; Perez, L. L.; Eckert, N. A.; Lachicotte,
R. J. J. Am. Chem. Soc. 2002, 124, 14416-14424.
(9) Budzelaar, P. H. M.; van Oort, A. B.; Orpen, A. G. Eur. J. Inorg.
Chem. 1998, 1485.
(10) Stender, M.; Wright, R. J.; Eichler, B. E.; Prust, J.; Olmstead, M. M.;
Roesky, H. W.; Power, P. P. J. Chem. Soc., Dalton Trans. 2001, 23,
3465-3469.
(11) Holland, P. L.; Tolman, W. B. J. Am. Chem. Soc. 1999, 121, 7270-
7271.
(12) Kern, R. J. J. Inorg. Nucl. Chem. 1962, 24, 1105.
(13) Anhydrous NiCl2 was prepared by boiling NiCl2‚6H2O with SOCl2:
Pray, A. R. Inorg. Synth. 1990, 28, 321-323.
(14) Vogel, A. I. Textbook of QuantitatiVe Inorganic Chemistry, 3rd ed.;
Wiley & Sons: New York, 1961; pp 479-481.
(15) The reported synthesis of LtBuNiCl (ref 8) used NiCl2THF0.7, but NiCl2-
THF1.5 works equally well.
(16) Orpen, A. G.; Brammer, L.; Allen, R. H.; Kennard, O. J. Chem. Soc.,
Dalton Trans. 1989, S1.
Inorganic Chemistry, Vol. 42, No. 5, 2003 1721

Eckert et al.
Figure 4. UV-vis spectra of [LMeNiCl]2 in 0.3 mM solutions. The solid-
Figure 3. ORTEP drawing of LMeNiCl2Li(THF)(OEt2). Thermal ellipsoids
are shown at 50% probability. The diethyl ether ligand was modeled with
two disorder components in a 55:45 ratio, and the major component is
shown. Hydrogen atoms are omitted for clarity. Key distances (Å) and angles
(deg): Ni-N1 1.950(5), Ni-N2 1.943(5), Ni-Cl1 2.284(2), Ni-Cl2 2.295-
(2), Ni-Li 3.144(1), N1-Ni-N2 94.7(2), Cl1-Ni-Cl2 95.15(7).
state spectrum is on an arbitrary scale.
Scheme 1
including the bite angle of the â-diketiminate. The Ni-Li
distance of 3.144(1) Å again suggests that there is no direct
metal-metal interaction.
It is interesting to observe the structural differences
enforced by different backbone substituents on â-diketiminate
ligands. LtBu gives a three-coordinate, trigonal planar chloride
complex, with a Ni-Cl distance of 2.137(2) Å and an
average Ni-N distance of 1.815(3) Å.8 These are 0.1-0.2
Å shorter than the analogous distances in the nickel chloride
complexes of LMe, consistent with the lower coordination
number of the metal. LtBuNiCl possesses a bite angle of 97.3-
(2)°, while LMeNiCl2Li(THF)(OEt2) and [LMeNiCl]2 have
much smaller bite angles, 94.2 ( 0.5°. This suggests that
larger backbone substituents increase the bite angle of
diketiminates. With LtBu, the tert-butyl groups push the aryl
groups toward the metal, forcing the metal deeper into the
NN binding pocket and enlarging the bite angle relative to
the LMe analogue. We can further observe the effect of
backbone substituents on diketiminate bite angle by compar-
ing the C-N-C bond angles of the diketiminates. LMeNiCl2-
Li(THF)(OEt2) has a C-N-C angle of 121.1(5)°, with an
analogous angle of 116.4(3)° for [LMeNiCl]2. For LtBuNiCl,
the angle is 130.4(4)°, nearly 10° larger than in either
complex of LMe! Similar bite angle perturbations have been
observed in the analogous Fe diketiminate complexes.2e
Properties of Ni(II) Complexes in Solution. The nickel
chloride complexes show interesting solvent-dependent
Table 2. Populations of ∼10 mM Samples at Room Temperature
equilib A
equilib B
% 1
% 2
% 2 + LiCl
% 3
solid
C6H6/toluene
CH2Cl2
100
50
0
0
50
100
0
20
95
100
80
5
spectra similar to three-coordinate LtBuNiCl (Figure 4).8 1
H
NMR spectra of CD2Cl2 solutions of LtBuNiCl8 and of [LMe-
NiCl]2 are similar, although the signals for [LMeNiCl]2 are
spread over a larger frequency range. For example, the
isopropyl methyl proton sets appear at 5.6 and 4.1 ppm in
LtBuNiCl, while they are found at 9.7 and 8.5 ppm in CD2-
Cl2 solutions of [LMeNiCl]2. The signals from the ligand
appear in the same order for both compounds, except the
different backbone alkyl substituents. The similar spectro-
scopic properties of [LMeNiCl]2 and LtBuNiCl in CH2Cl2
strongly suggest that [LMeNiCl]2 dissociates into mono-
meric LMeNiCl, which has an intense electronic absorption
at 765 nm.
In benzene or toluene, blue solutions of [LMeNiCl]2 had
1
two sets of signals in their H NMR spectra. One set was
1
behavior, which has been monitored using H NMR spec-
nearly identical to that assigned as LMeNiCl above, and the
other set had slightly shifted peaks. These are assigned as
LMeNiCl and [LMeNiCl]2, respectively. The ratio between the
two sets of peaks was dependent on concentration and
temperature, as expected for a monomer/dimer equilibrium.
At room temperature with low concentrations we observed
only the monomer and could not attain high enough
concentrations to observe [LMeNiCl]2 without some LMeNiCl
present. UV-vis spectra of [LMeNiCl]2 in aromatic solvents
at low concentration support the presence of both monomeric
troscopy (all the compounds have distinct, paramagnetically
shifted spectra) and UV-vis spectrophotometry. Scheme 1
shows our interpretation of the interconversions, and Table
2 lists estimates of the equilibrium populations in ∼10 mM
solution at room temperature. The rationale for our assign-
ments follows.
In the solid state, [LMeNiCl]2 is blue (λmax ) 580, 710 nm;
Figure 4). However, dissolving [LMeNiCl]2 in methylene
1
chloride gave green solutions with H NMR and UV-vis
1722 Inorganic Chemistry, Vol. 42, No. 5, 2003

Nickel Complexes of a Bulky â-Diketiminate Ligand
Figure 6. ORTEP drawing of LMeNiN(SiMe3)2. Thermal ellipsoids are
shown at 50% probability. Hydrogen atoms are omitted for clarity. Key
distances (Å) and angles (deg): Ni-N11 1.911(2), Ni-N21 1.953(2), Ni-
N14 1.873(2), N11-Ni1-N21 94.86(9), N11-Ni1-N14 133.25(9), N21-
Ni1-N14 128.86(9).
Figure 5. van’t Hoff plot of data from a 10.5 mM sample of [LMeNiCl]2
in toluene-d8 from -49 to 46 °C.
the association constant of THF to LMeNiCl (Ka; equilibrium
C in Scheme 1), we utilized UV-vis spectrophotometry in
CH2Cl2. Linear Scatchard plots19 of data at several concen-
trations yielded Ka ) 1.2(0.2) M-1 at 21 °C.20 Thus our data
support the observation that [LMeNiCl]2 is fully converted
to LMeNiCl(THF) when dissolved in THF.
The two Ni(II) chloride complexes were easily intercon-
verted. When solutions of [LMeNiCl]2 in THF were treated
with excess LiCl (followed by ultrasound irradiation to
facilitate LiCl incorporation), 1H NMR and electronic spectra
suggested formation of LMeNiCl2Li(THF)(OEt2). Conversely,
when LMeNiCl2Li(THF)(OEt2) was dissolved in CD2Cl2, we
observed 1H NMR signals from both LMeNiCl and LMeNiCl2-
Li(ether)2 (Supporting Information). On a preparative scale,
the conversion of LMeNiCl2Li(THF)(OEt2) to [LMeNiCl]2 may
be effected by heating in toluene at 100 °C for several hours,
extracting, and crystallizing from CH2Cl2.
LMeNiCl (λmax ) 740 nm) and dimeric [LMeNiCl]2 (λmax
)
580 nm) (Figure 4). To further examine this equilibrium in
solution, we monitored 1H NMR spectra at various temper-
atures. Above 46 °C only the monomer was detected, and
below -49 °C only the dimer was observed. Between these
temperatures, the measured equilibrium constants for the
dissociation of dimer into monomer (Keq; equilibrium A in
Scheme 1) gave ∆H ) +51(5) kJ/mol and ∆S ) +116(15)
J/(mol‚K) (Figure 5). As observed with other transition metal
dimer-monomer equilibria, equilibrium A (cleavage of
dimer) is enthalpically disfavored but entropically favored.17
The monomer/dimer equilibrium bears a resemblance to
those recently reported in (â-diketiminato)copper(II) halide
complexes. Both LtBuCuCl and LMeCuCl are monomers at
all temperatures in aromatic solvents.2b,2h With diketiminate
L′ (substantially smaller than LMe, with ortho-methyl sub-
stitution rather than ortho-isopropyl substitution on the aryl
rings), a dimer was crystallographically characterized, and
the equilibrium between monomer and dimer was monitored
using EPR and UV-vis techniques.2h,18 [L′CuCl]2 is a
monomer above -55 °C in toluene, a temperature similar
to that at which [LMeNiCl]2 begins to dissociate. Thus,
roughly the same dissociation temperature is evident in
electronically similar complexes of copper with a smaller
ligand and nickel with a larger ligand. We conclude that
nickel â-diketiminate complexes have a greater tendency to
dimerize than their copper analogues.
Scheme 2
Synthesis of a Three-Coordinate Nickel(II) Amido
Complex. The suitability of [LMeNiCl]2 and LMeNiCl2Li-
(THF)(OEt2) as starting materials for low-coordinate nickel-
(II) complexes was evaluated by adding lithium bis-
(trimethylsilyl)amide to ethereal solutions of these nickel(II)
complexes (Scheme 2). Starting from either complex, this
reaction cleanly gave a single product as judged by 1H NMR
spectroscopy. X-ray crystallographic analysis of the product
In THF solution, [LMeNiCl]2 became purple and the H
1
NMR and UV-vis spectra were similar to those of LMeNiCl2-
Li(THF)(OEt2) in THF. We suggest that this more drastic
spectroscopic change results from dissociation of the dimer
and THF coordination to form LMeNiCl(THF). To determine
(19) Connors, K. A. Binding Constants: The Measurements of Molecular
Complex Stability; John Wiley & Sons: New York, 1987; pp 59-
101 and 141-187.
(20) Scatchard plots from data collected at -5 and -20 °C gave Keq values
within experimental error of Keq at room temperature. This suggests
that ∆H for the reaction is roughly zero and the bond dissociation
energy (BDE) for the Ni-O(THF) bond is equal to the sum of the
decreases in other bond energies from increased coordination number.
(17) Minas da Piedade, M. E.; Simo˜es, J. A. M. J. Organomet. Chem. 1996,
518, 167-180.
(18) Spencer, D. J. E.; Reynolds, A. M.; Holland, P. L.; Jazdzewski, B.
A.; Duboc-Toia, C.; Le Pape, L.; Yokota, S.; Tachi, Y.; Itoh, S.;
Tolman, W. B. Inorg. Chem. 2002, 41, 6307-6321.
Inorganic Chemistry, Vol. 42, No. 5, 2003 1723

Eckert et al.
Table 3. Relevant Distances and Angles for Three-Coordinate M(II)
There is an equilibrium that shifts between the monomer,
the dimer, and four-coordinate adducts, depending on solvent.
Equilibrium constants and thermodynamic data are consistent
with the proposed equilibria. Structural comparisons to the
recently reported LtBuNiCl8 show that the structural differ-
ences can be attributed to the steric effects of the backbone
substituents. Due to the leaving ability of the chloride groups
in [LMeNiCl]2 and LMeNiCl2Li(THF)(OEt2), these compounds
are expected to be useful precursors for three-coordinate Ni-
(II) â-diketiminate complexes.
Diketiminate Bis(trimethylsilyl)amido Compoundsa
Mn
Fe
Co
Ni
M-N(diketiminate) (Å)
M-N(amido) (Å)
2.073(1) 2.008(1) 1.965(1) 1.932(2)
1.992(1) 1.928(1) 1.903(1) 1.873(2)
bite angle (N-M-N) (deg) 92.57(5) 94.33(4) 96.60(4) 94.86(9)
fold angleb (deg)
164.2
0.394
164.8
0.365
165.3
0.340
168.8(2)
0.281(3)
M-N2C3 distc (Å)
a All data for Mn, Fe, and Co compounds taken from ref 2i. b Fold angle
is defined as the angle between MN2 and N2C3 planes. c Distance from M
to the N2C3 plane.
Experimental Section
General Considerations. All manipulations were performed
under a nitrogen atmosphere by standard Schlenk techniques or in
an M. Braun glovebox maintained at or below 1 ppm of O2 and
H2O. Glassware was dried at 150 °C overnight. NMR data were
recorded on a Bruker Avance 400 spectrometer (400 MHz) at 22
°C and referenced to residual protiated solvent (C6D5H, 7.15 ppm;
CDHCl, 5.32 ppm; C7D7H, 2.09 ppm; C4D7HO, 1.73 ppm). IR
spectra were recorded on a Mattson Instruments 6020 Galaxy Series
FTIR using solution cells with CsF windows. UV-vis spectra were
measured on a Cary 50 spectrophotometer, using screw-cap or
Schlenk-adapted cuvettes. Solution magnetic susceptibilities were
determined by the Evans method.21 Elemental analyses were
determined by Desert Analytics, Tucson, AZ.
Figure 7. Molecular orbitals of LMeMX (adapted from ref 8).
Pentane, diethyl ether, tetrahydrofuran (THF), dichloromethane,
and toluene were purified by passage through activated alumina
and “deoxygenizer” columns obtained from Glass Contour Co.
(Laguna Beach, CA). Deuterated benzene was first dried over CaH2,
then over Na/benzophenone, and then vacuum transferred into a
storage container. Before use, an aliquot of each solvent was tested
with a drop of sodium benzophenone ketyl in THF solution. Celite
was dried overnight at 200 °C under vacuum. CD2Cl2 was dried
over CaH2 and used without further testing.
showed that it was LMeNiN(SiMe3)2 (Figure 6, Table 1),
which has a three-coordinate geometry. A similar procedure
was employed by Mindiola and Hillhouse to synthesize a
three-coordinate Ni(II) amido complex from [(dtbpe)NiCl]2.5f
The Ni-N(amido) distance of 1.873(2) Å is similar to that
of the homoleptic amido compound [Ni(NPh2)3]- (1.889(3)
Å)5b and the heteroleptic (dtbpe)NiNH(diisopropylphenyl)
(1.881(2) Å).5f LMeNiN(SiMe3)2 is isoleptic with Power’s
very recently reported compounds LMeMN(SiMe3)2 (M )
Mn, Fe, Co), and these compounds possessing identical
ligand sets show a monotonic decrease in M-N(amido)
distance from Mn to Ni.2i This trend is similar to the one
we have found in three-coordinate LtBuMCl complexes.8 The
Ni(II) amido compound is nearly planar at the metal (356.97-
(9)°) as well as the amido nitrogen (358.9(2)°), as found for
the -N(SiMe3)2 compounds of the other metals.2i Table 3
compares key distances and angles for the LMeMN(SiMe3)2
series. There is an increase in diketiminate bite angle (N-
M-N) from Mn to Co and then a dropoff for Ni. The
decreased nickel bite angle in three-coordinate chloride
complexes was previously explained through either onset of
diketiminate ligand folding or through occupation of mo-
lecular orbitals with N---N bonding character.8 In the amido
complexes, it is possible to distinguish between these two
explanations. Because the ligand folding is similar in all of
the amido complexes (Table 3), the major influence on bite
angle is likely to be occupation of the a1-symmetry molecular
orbital that has some N---N bonding (Figure 7).
[LMeNiCl]2. NiCl2THF0.7 (230 mg, 1.28 mmol) and LiLMe (0.60
g, 1.4 mmol) were dissolved in toluene (20 mL). The flask was
heated at 100 °C for 24 h, and then the volatile materials were
removed in vacuo. The blue residue was extracted with CH2Cl2
(10 mL), and the dark green solution was filtered through Celite.
[LMeNiCl]2 was crystallized at -35 °C from CH2Cl2 and isolated
1
as dark blue crystals in two crops (291 mg, 44% yield). H NMR
(CD2Cl2, 400 MHz): δH 58 (4H, CH(CH3)2 or m-Ar), 36 (4H,
CH(CH3)2 or m-Ar), 9.7 (12H, CH(CH3)2), 8.5 (12H, CH(CH3)2),
-27 (2H, p-Ar), -90 (6H, CH3), -284 (1H, backbone-H) ppm.
At room temperature in toluene-d8 both monomer and dimer were
observed in a ∼1:1 ratio on the basis of integration. 1H NMR (tol-
d8, 400 MHz): δH 57 (4H, CH(CH3)2 or m-Ar), 45 (4H, CH(CH3)2
or m-Ar), 36 (4H, CH(CH3)2 or m-Ar), 22 (4H, CH(CH3)2 or m-Ar),
12 (24H, CH(CH3)2), 9.3 (24H, CH(CH3)2), -16 (2H, p-Ar), -27
(2H, p-Ar), -81 (6H, CH3), -89 (6H, CH3), -268 (1H, CH), -273
(1H, CH) ppm. 1H NMR (THF-d8, 400 MHz): δH 43 (4H,
CH(CH3)2 or m-Ar), 25 (CH(CH3)2 or m-Ar), 6.5 (12H, CH(CH3)2),
6.1 (12H, CH(CH3)2), -19 (2H, p-Ar), -56 (6H, CH3), -154 (1H,
CH) ppm. UV-vis (solid): 580, 710 nm. UV-vis (CH2Cl2): 575
(680 M-1 cm-1), 765 (2800 M-1 cm-1) nm. UV-vis (toluene):
580 (1200 M-1 cm-1), 740 (2800 M-1 cm-1) nm. UV-vis (THF):
520 (1300 M-1 cm-1), 670 (830 M-1 cm-1) nm. µeff (CD2Cl2): 2.4
( 0.3 µB/Ni. µeff (THF-d8): 3.3 ( 0.3 µB/Ni. µeff (tol-d8): 3.1 (
0.3 µB/Ni. Anal. Calcd: C, 68.06; N, 5.47; H, 8.07. Found: C,
66.37; N, 5.58; H, 8.02.
Conclusion
We have synthesized Ni(II) complexes of a bulky â-diketim-
inate ligand and examined their solvatochromic behavior.
(21) Schubert, E. M. J. Chem. Ed. 1992, 69, 62.
1724 Inorganic Chemistry, Vol. 42, No. 5, 2003

Nickel Complexes of a Bulky â-Diketiminate Ligand
LMeNiCl2Li(THF)(OEt2). NiCl2THF0.7 (3.23 g, 17.9 mmol) and
LiLMe (8.00 g, 18.8 mmol) were dissolved in THF (40 mL). The
mixture was stirred at room temperature for 24 h. The volatile
materials were removed in vacuo. The purple residue was dissolved
in Et2O (40 mL) and filtered through Celite. The purple Et2O
solution was concentrated (15 mL) and cooled to -35 °C. The
product was isolated as dark purple crystals in three crops (8.4 g,
green needlelike crystals (1.3 g, 71%). LMeNiN(SiMe3)2 can be
synthesized from [LMeNiCl]2 using the same workup procedure, in
81% yield. 1H NMR (C6D6, 400 MHz): δH 49 (4H, m-Ar), 35 (4H,
CH(CH3)2), 8.1 (12H, CH(CH3)2), 5.5 (12H, CH(CH3)2), -3.9 (18H,
Si(CH3)3, -11 (2H, p-Ar), -66 (6H, CCH3) ppm. UV-vis
(pentane): 655 nm (1200 M-1 cm-1). µeff (C6D6): 2.5 ( 0.3 µB.
Anal. Calcd: C, 66.10; N, 6.61; H, 9.36. Found: C, 65.49; N, 6.36;
H, 9.13.
1
64% yield). H NMR (C6D6, 400 MHz): δH 4 5 (4H, CHMe2 or
m-Ar), 26 (4H, CH(CH3)2 or m-Ar), 8.3 (12H, CH(CH3)2), 6.8 (12H,
CH(CH3)2), -19 (2H, p-Ar), -62 (6H, CH3), -81 (1H, CH) ppm.
UV-vis (THF): 535 (910 M-1 cm-1), 665 nm (400 M-1 cm-1).
µeff (THF-d8): 3.1 ( 0.3 µB. Anal. Calcd: C, 63.45; H, 8.49; N,
4.00. Found: C, 63.42; H, 8.66; N, 3.96.
Acknowledgment. The authors thank the University of
Rochester and the National Science Foundation (Grant CHE-
0134658) for funding.
LMeNiN(SiMe3)2. LMeNiCl2Li(THF)(OEt2) (2.0 g, 2.9 mmol) and
LiN(SiMe3)2 (0.49 g, 2.9 mmol) were dissolved in Et2O (15 mL)
and stirred for 12 h. The dark green solution was filtered through
Celite, and all volatile materials were removed in vacuo. The residue
was extracted with pentane (40 mL), filtered, concentrated to 20
mL, and cooled to -35 °C. LMeNiN(SiMe3)2 was isolated as dark
Supporting Information Available: Proton NMR and UV-
vis spectra of complexes and crystal data in CIF format. This
material is available free of charge via the Internet at http://
pubs.acs.org.
IC025986N
Inorganic Chemistry, Vol. 42, No. 5, 2003 1725