Organochromium complexes bearing noninnocent diimine ligands

Article abstract of DOI:10.1002/ejic.201100803

We have prepared several anionic and neutral organochromium complexes featuring noninnocent α-diimine ligands. All neutral and anionic complexes refrain from insertion of the C=N bonds of the diimine ligand into the chromium-carbon bonds, presumably due to the reduced nature of the diimine ligand. However, insertion can be facilitated by one-electron oxidation. Thus, oxidation of [(^HL^iPr)CrR(THF)] [^HL ^iPr = Ar-N=C(H)-(H)C=N-Ar in which Ar = 2,6-diisopropylphenyl and R = CH₂SiMe₃ or CH₃] with [(C₅H ₅)₂Fe]⁺ presumably forms [(^HL ^iPr)CrR(THF)]⁺, which contains a neutral diimine ligand coordinated to Cr^II. This cationic complex apparently undergoes immediate insertion of the C=N bond of the diimine ligand into the Cr-alkyl bond, followed by C-H activation of the isopropyl substituent of an N-aryl group on the diimine ligand. DFT calculations confirm that [(^HL ^iPr)CrR(THF)]⁺ contains a neutral innocent diimine ligand that undergoes C=N bond insertion into the Cr-R bond. Copyright

Full text of DOI:10.1002/ejic.201100803

FULL PAPER
DOI: 10.1002/ejic.201100803
Organochromium Complexes Bearing Noninnocent Diimine Ligands
Kevin A. Kreisel,[a] Glenn P. A. Yap,[a] and Klaus H. Theopold*[a]
Keywords: Chromium complexes / Diimine ligands / N ligands / Alkylation / Insertion
We have prepared several anionic and neutral organochrom- forms [(HLiPr)CrR(THF)]+, which contains a neutral diimine
ium complexes featuring noninnocent α-diimine ligands. All
neutral and anionic complexes refrain from insertion of the
C=N bonds of the diimine ligand into the chromium–carbon
bonds, presumably due to the reduced nature of the diimine
ligand. However, insertion can be facilitated by one-electron
oxidation. Thus, oxidation of [(HLiPr)CrR(THF)] [HLiPr = Ar–
N=C(H)–(H)C=N–Ar in which Ar = 2,6-diisopropylphenyl
and R = CH2SiMe3 or CH3] with [(C5H5)2Fe]+ presumably
ligand coordinated to CrII. This cationic complex apparently
undergoes immediate insertion of the C=N bond of the di-
imine ligand into the Cr–alkyl bond, followed by C–H acti-
vation of the isopropyl substituent of an N–aryl group on the
diimine ligand. DFT calculations confirm that [(HLiPr)-
CrR(THF)]+ contains a neutral innocent diimine ligand that
undergoes C=N bond insertion into the Cr–R bond.
Most relevant to our work are reports of the reactions of
zirconium- and hafnium-alkyl compounds with diimine li-
gands and their use as catalysts in olefin polymerization.[7]
Thus, the reaction of M(CH2Ph)4 (M = Zr or Hf) with N-
aryl-substituted diimine ligands results in alkyl migration
to the carbon atom of the diimine ligand and a 1,2-hydride
shift, thereby leaving an amido–imine ligand coordinated to
the metal. However, the mechanism of this rearrangement
is still not entirely clear.
Introduction
The recent interest in transition-metal complexes con-
taining redox-active ligands has produced a remarkable
number of reports detailing unusual electronic and geomet-
ric structures as well as unprecedented reactivity.[1] In most
cases, the latter is best rationalized based on an assignment
of the most appropriate electronic structure. Work by
Wieghardt et al. and others has greatly improved our un-
derstanding of the noninnocence of a variety of ligands
such as pyridinebis(imines),[2] catechols, 1,2-dioxolenes,[1l]
and α-diimines (also known as diazadienes).[3] Among the
most frequently used of these ligands are the α-diimines,
due to the ease of varying the sterics and electronics of these
ligands. Pioneering work by van Koten showed the diversity
in the reactivity of diimine ligands and established the po-
tential noninnocence of the ligand by means of radical C–C
coupling reactions between coordinated diimine ligands.[4]
Arguably more interesting and perhaps mechanistically re-
lated to our work was the discovery of regioselective alky-
lation of diimine ligands by diorganozinc reagents.[5] Since
then, the reactions of diimines with organozinc, organom-
agnesium, and organoaluminum reagents have been studied
extensively.[5b,6] In many cases, these reactions lead to alkyl-
ation of the diimine ligand at various positions (see
Scheme 1). More recently, there have been reports of transi-
tion metals mediating the alkylation of diimine ligands.
Scheme 1. Possible regioisomers produced by alkylation of a di-
imine ligand.
We recently reported a thorough investigation of the co-
ordination chemistry of chromium-bearing diimine ligands
utilizing both X-ray crystallography and density functional
theory (DFT) calculations;[8] the present work extends this
work to organometallic derivatives. An understanding of
the electronic structure of these complexes facilitates a ra-
tionalization of their reactivity. Furthermore, the results
presented here provide insight into the mechanism of alkyl
[a] Department of Chemistry and Biochemistry,
Center for Catalytic Science and Technology,
University of Delaware,
Newark, Delaware 19716, USA
Fax: +1-302-831-6335
E-mail: theopold@udel.edu
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201100803 or from
the author.
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Organochromium Complexes Bearing Noninnocent Diimine Ligands
migrations that have plagued organometallic diimine chem-
istry for decades.
Herein, we summarize our study of the organometallic
chemistry of chromium supported by the diimine ligand,
Ar–N=C(H)–(H)C=N–Ar (HLiPr; Ar = 2,6-diisopropyl-
phenyl). The original intent of this work – in the context of
Phillips catalysis of ethylene polymerization[9] – was to pre-
pare cationic CrII-monoalkyls supported by bidentate nitro-
gen ligands, and to compare their catalytic activity for eth-
ylene polymerization to alkyl-CrIII compounds.[10] Al-
though this goal was not realized, the results of our study
shed light on a characteristic reaction pattern of diimine
metal–alkyl complexes.
Scheme 2. Synthesis of complexes 2 and 3.
Results and Discussion
Our initial attempts to prepare dialkyl-CrII complexes of
the type [(HLiPr)CrR2] by the reaction of [{(HLiPr)Cr(μ-
Cl)(Cl)}2] with four equivalents of alkylating agents, such
as PhMgCl or (Me)3SiCH2Li, indeed gave novel organome-
tallic compounds, but in low yield. Full characterization of
the reaction products revealed them to be formally CrI–di-
alkyl “ate” complexes of the general formula [(HLiPr)-
CrR2]–. Seeking to circumvent the apparent reduction of
chromium by the electron-rich main-group alkyls, we
eventually prepared the precursor [{(HLiPr)Cr(μ-Cl)}2] (1).
As discussed elsewhere, the latter is best described as con-
taining divalent chromium (CrII), coordinated by a singly
reduced α-diimine radical anion.[8] Compound 1 is also an
easily accessible synthon for the preparation of the afore-
mentioned dialkyls in good yield (see Scheme 2). Upon ad-
dition of alkylating reagent (4 equiv.), excellent yields of
[Li(THF)4][(HLiPr)Cr{CH2Si(CH3)3}2] (2) and [PhMg-
(THF)][(HLiPr)CrPh2] (3a) or [Li(THF)2][(HLiPr)CrPh2] (3b)
could be obtained. The molecular structures of 2 and 3a
were determined by X-ray crystallographic analysis; the
molecular structures are depicted in Figures 1 and 2, respec-
tively. Complex 2 crystallizes in the monoclinic space group
P21/n and displays a significantly distorted square-planar
geometry. Its Cr–C bond lengths of 2.131(5) and 2.136(5) Å
are in good agreement with other reported alkyl complexes
of chromium. The Cr–N bond lengths of 1.994(4) and
2.004(4) Å are also similar to those of previously reported
chromium diimine complexes. In addition, the THF-sol-
vated lithium cation shows no signs of interaction with the
chromate anion in the solid state [closest contact distance
is 3.506(11) Å]. The phenyl derivative 3a crystallizes in the
monoclinic space group P21/c and also exhibits distorted
square-planar coordination geometry about chromium. The
Cr–C bond lengths in 3a, at 2.164(2) and 2.203(2) Å, are
longer than those in 2, presumably reflecting the pro-
nounced bonding interaction of the phenyl groups with the
magnesium counterion. The Cr–N bond lengths are also
slightly elongated [2.0362(19) and 2.0613(19) Å]. The cation
in 3a {i.e., [PhMg(THF)]+} is positioned close to the ipso-
carbon atoms of both phenyl groups of the organochrom-
Figure 1. ORTEP plot of [Li(THF)4][(HLiPr)Cr{CH2Si(CH3)3}2] (2)
at the 30% probability level. Hydrogen atoms and the [Li(THF)4]+
cation have been omitted for clarity. Selected structural parameters
(distances in Å, angles in degrees): Cr–N1 2.004(4), Cr–N2
1.993(4), Cr–C27 2.131(5), Cr–C31 2.136(5), N1–C13 1.335(7), N2–
C14 1.352(6), C13–C14 1.372(8); N1–Cr–N2 79.27(16), N1–Cr–
C27 96.60(18), N1–Cr–C31 156.87(18), N2–Cr–C27 148.69(18),
N2–Cr–C31 97.93(17), C27–Cr–C31 97.22(19).
Figure 2. ORTEP plot of [PhMg(THF)][(HLiPr)CrPh2] (3a) at the
30% probability level. Hydrogen atoms have been omitted for clar-
ity. Selected structural parameters (distances in Å, angles in de-
grees): Cr–N1 2.0362(19), Cr–N2 2.0613(19), Cr–C32 2.164(2), Cr–
C38 2.203(2), N1–C13 1.335(3), N2–C14 1.335(3), C13–C14
1.383(3); N1–Cr–N2 78.61(8), N1–Cr–C32 92.21(8), N1–Cr–N38
172.45(8), N2–Cr–C32 170.54(8), N2–Cr–C38 94.01(8), C32–Cr–
ium complex ion. When prepared from PhLi (see C38 95.23(9).
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K. A. Kreisel, G. P. A. Yap, K. H. Theopold
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Scheme 2), we assume that the lithium cation of 3b also lies hydrides of chromium.[14] The Cr–Cr distance of 5 is sim-
between the aryl rings and is coordinated by two THF li- ilar, although slightly shorter than that of 4, measuring
gands; this assertion is consistent with the elemental analy- 2.4490(7) Å. The Cr–N distances in 5 are considerably
sis (see the Exp. Section). X-ray quality crystals of 3b were shorter than in the previously mentioned complexes
not attainable; however, the reaction was cleaner and re- [1.952(2) to 1.957(2) Å]. Similar to 4, the countercation of
sulted in higher yields. The diimine backbones of both 2 5 is not closely associated with the chromate anion, with a
and 3a show signs of reduction, consistent with one-elec- distance of 3.594(5) Å between them. Both 4 and 5 show
tron reduced ligand-centered radicals.[8] Considering these considerable disruption of the diimine π system and, based
bond lengths and the square-planar coordination favored solely on bond lengths (4: C–Nav. 1.375 Å; C–Cav. 1.355 Å;
by divalent chromium, the most appropriate assignment of 5: C–Nav. 1.371 Å; C–Cav. 1.352 Å), the ligands appear to
an electronic structure to these complexes would be CrII be closest to the dianionic enediamide electronic configura-
(d4, S = 2) with strong antiferromagnetic coupling to a li- tion. The latter would result in the assignment of +III as
gand-centered radical, analogous to [(HLiPr)CrCl2]– (S = a formal oxidation state of chromium. Room-temperature
3/2). Accordingly, room-temperature measurements of the magnetic measurements of 4 and 5 gave μeff values of 1.4(1)
effective magnetic moments (μeff) of 2 and 3b gave 3.7(1) μB and 1.5(1) μB per Cr, respectively. These low values (the ex-
for both complexes, close to the spin-only moment for three
unpaired electrons (3.87 μB).[11]
Interestingly, alkylation of 1 with MeLi (4 equiv.) did not
give the expected [Li(THF)4][(HLiPr)Cr(CH3)2], but instead
yielded dinuclear [Li(THF)4][{(HLiPr)Cr}2(μ-CH3)3] (4). In
a similar manner, the reaction of 1 with an excess amount
of Li[BEt3H] gave [Li(THF)4][{(HLiPr)Cr}2(μ-H)3] (5),
which is isostructural to 4. Both 4 and 5 could also be pre-
pared in good yield by addition of three equivalents of the
Scheme 3. Synthesis of bridging alkyl species 4 and 5.
appropriate alkylating reagent to 1 according to Scheme 3.
Crystallizations from Et2O at –30 °C gave blue crystals of
4 and purple crystals of 5, respectively. The molecular struc-
tures of 4 and 5 are shown in Figures 3 and 4, respectively,
along with selected interatomic distances and angles. Com-
plex 4 crystallizes in the monoclinic space group P21/c and
contains two diimine chromium fragments joined by three
bridging methyl ligands. The coordination geometry about
each chromium atom is best described as trigonal bipyrami-
dal, with the threefold axes approximated by C3–Cr1–N1
[168.23(16)°] and C1–Cr2–N3 [157.77(16)°]. The Cr–C dis-
tances range from 2.176(5) to 2.243(5) Å and are thus
longer than those in the monomeric complexes described
above, presumably due to three-center two-electron bond-
ing. The bridging methyl ligands also fix the chromium
atoms at a metal–metal distance of 2.469(1) Å, raising the
possibility of metal–metal bonding. Indeed, the [Cr2(μ-
CH3)3] core of 4 is remarkably similar in structure to that
of [Cp*2Cr2(μ-CH3)3]BF4 (e.g., Cr–Cr 2.42 Å; Cp* = penta-
methylcyclopentadienyl), to which we have attributed
metal–metal bonding between its CrIII centers many years
ago.[12] The Cr–N distances in 4 [1.988(3) to 2.006(4) Å] are
consistent with those in 1. The THF-solvated lithium cation
does not exhibit any close contacts with the anion; the clos-
Figure 3. ORTEP plot of [Li(THF)4][{(HLiPr)Cr}2(μ-CH3)3] (4) at
the 30% probability level. Hydrogen atoms and the [Li(THF)4]+
cation have been omitted for clarity. Selected structural parameters
est internuclear distance between the two measures is (distances in Å, angles in degrees): Cr1–Cr2 2.4690(10), Cr1–N1
2.006(4), Cr1–N2 2.002(3), Cr1–C1 2.192(4), Cr1–C2 2.235(5),
Cr1–C3 2.202(5), Cr2–N3 1.990(4), Cr2–N4 1.988(3), Cr2–C1
3.843(9) Å. The isostructural complex 5 crystallizes in the
triclinic space group P1, with each chromium also residing
¯
2.243(5), Cr2–C2 2.176(5), Cr2–C3 2.221(5), N1–C16 1.366(6), N2–
C17 1.393(6), N3–C42 1.367(5), N4–C43 1.374(6), C16–C17
1.349(6), C42–C43 1.361(7); Cr1–C1–Cr2 67.65(13), Cr1–C2–Cr2
68.07(14), Cr1–C3–Cr2 67.87(15), N1–Cr1–N2 79.51(14), N1–Cr1–
C1 102.91(16), N1–Cr1–C2 93.16(17), N1–Cr1–C3 168.23(16), N2–
Cr1–C1 116.96(17), N2–Cr1–C2 146.72(16), N2–Cr1–C3 90.37(16),
N3–Cr2–N4 79.69(15), N3–Cr2–C1 157.77(16), N3–Cr2–C2
105.70(18), N3–Cr2–C3 93.41(16), N4–Cr2–C1 94.06(16), N4–
in a trigonal-bipyramidal coordination sphere – the axial
vectors being defined by H1C–Cr1–N2 [168.8(9)°] and
H1C–Cr2–N4 [168.8(9)°]. Although there is always a sys-
tematic error involved with hydride bond lengths estab-
lished by X-ray crystallography,[13] the apparent Cr–H dis-
tances for the bridging hydride ligands range from 1.61(3)–
1.78(3) Å and are comparable to other reported bridging Cr2–C2 106.69(17), N4–Cr2–C3 160.06(17).
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Organochromium Complexes Bearing Noninnocent Diimine Ligands
3
pected spin-only value for S = /2 is 3.88 μB) are consistent
with antiferromagnetic coupling between the chromium
ions, metal–metal bonding, or a combination of the two.
Figure 5. ORTEP plot of [Li2(THF)3][(HLiPr)Cr(CH3)3] (6) at the
30% probability level. Hydrogen atoms have been omitted for clar-
ity. Selected structural parameters (distances in Å, angles in de-
grees): Cr–N1 2.081(3), Cr–N2 2.066(3), Cr–C27 2.159(5), Cr–C28
2.150(5), Cr–C29 2.107(4), N1–C13 1.402(4), N2–C14 1.401(4),
C13–C14 1.344(5); N1–Cr–N2 76.7(1), N1–Cr–C27 89.7(2), N1–
Cr–C28 161.7(2), N1–Cr–C29 103.9(1), N2–Cr–C27 159.1(2), N2–
Cr–C28 90.2(2), N2–Cr–C29 102.6(2), C27–Cr–C28 99.0(2), C27–
Cr–C29 96.0(2).
Figure 4. ORTEP plot of [Li(THF)4][{(HLiPr)Cr}2(μ-H)3] (5) at the
30% probability level. With the exception of the bridging hydride
ligands, all hydrogen atoms and the [Li(THF)4]+ cation have been
omitted for clarity. Selected structural parameters (distances in Å,
angles in degrees): Cr1–Cr2 2.4490(7), Cr1–N1 1.9544(18), Cr1–N2
1.9524(19), Cr1–H1A 1.78(3), Cr1–H1B 1.72(3), Cr1–H1C 1.71(3),
Cr2–N3 1.957(2), Cr2–N4 1.955(2), Cr2–H1A 1.61(3), Cr2–H1B
2.159(5) Å] are shorter than in the dinuclear complex 4 and
more closely approximate the Cr–C bond lengths in 2 and
1.69(3), Cr2–H1C 1.68(3), N1–C13 1.374(3), N2–C14 1.372(3), 3. Notably, the two lithium cations in 6 are closely associ-
N3–C39 1.3733, N4–C40 1.366(3), C13–C14 1.349(3) C39–C40
ated with the chromate anion; one interacts with the two
1.355(4); Cr1–H1A–Cr2 92.3(9), Cr1–H1B–Cr2 91.8(9), Cr1–H1C–
equatorial methyl ligands and the other is coordinated in
Cr2 92.3(9), N1–Cr1–N2 80.42(8), N1–Cr1–H1A 147.5(9), N1–
a η4 fashion by the reduced diimine ligand. This diimine
Cr1–H1B 141.0(9), N1–Cr1–H1C 97.9(9), N2–Cr1–H1A 103.5(9),
coordination of the cation is rare, but some examples do
exist.[15] Like complexes 4 and 5, the C–N distances of
1.401(4) and 1.402(4) Å and the C–C distance of 1.344(5) Å
are in accord with a heavily reduced diimine ligand, most
likely to the extent of a dianionic, enediamide ligand. A
magnetic susceptibility measurement of 6 gave a μeff of
N2–Cr1–H1B 112.7(9), N2–Cr1–H1C 168.8(9), N3–Cr2–N4
80.56(9), N3–Cr2–H1A 124.7(9), N3–Cr2–H1B 160.7(9), N3–Cr2–
H1C 97.1(9), N4–Cr2–H1A 113.1(9), N4–Cr2–H1B 101.3(9), N4–
Cr2–H1C 168.8(9).
Reaction of 1 with an excess amount of LiCH3 resulted
in the formation of a red product that was ultimately deter-
mined to be [Li2(THF)3][(HLiPr)Cr(CH3)3] (6) by X-ray
crystallography (see Scheme 4). Interestingly, reaction of 1
with an excess amount of Li[BEt3H] only gave 5 in low
yield; [Li2(THF)3][(HLiPr)CrH3] was not isolated. Figure 5
depicts the results of the X-ray structure determination of
6.
3
3.8(1) μB, in accord with an S = /2, CrIII bound to a dia-
magnetic enediamide ligand.
Condensation of alkyl complexes 2, 4, and 6 with 1 in the
proper stoichiometry readily furnishes neutral, monoalkyl
complexes. Thus, reaction of two equivalents of 2 with 1
gives [(HLiPr)Cr{CH2Si(CH3)3}(THF)] (7) in good yield (see
Scheme 5). The results of an X-ray structure determination
can be found in Figure 6.
Scheme 4. Preparation of [Li2(THF)3][(HLiPr)Cr(CH3)3] (6).
The trialkyl crystallizes in the monoclinic space group
C2/c. The geometry about the chromium in 6 is best de-
scribed as square pyramidal with the angles between the
Scheme 5. Preparation of [(HLiPr)Cr{CH2Si(CH3)3}(THF)] (7).
Complex 7 crystallizes in the monoclinic space group
apical ligand (C29) and the basal ones ranging from 91.2(2) P21/n with two crystallographically distinct, but chemically
to 103.9(1)°. The Cr–C distances [2.107(4), 2.150(5), and equivalent molecules (only one will be discussed). The dis-
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K. A. Kreisel, G. P. A. Yap, K. H. Theopold
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Figure 6. ORTEP plot of [(HLiPr)Cr{CH2Si(CH3)3}(THF)] (7) at
the 30% probability level. Hydrogen atoms have been omitted for
clarity. Selected structural parameters (distances in Å, angles in de-
grees): Cr1–N1 1.993(4), Cr1–N2 2.034(4), Cr1–C27 2.102(5), Cr1–
O1 2.104(4), N1–C13 1.338(6), N2–C14 1.347(6), C13–C14
1.376(7); N1–Cr1–N2 78.90(18), N1–Cr1–C27 96.6(2), N1–Cr1–O1
164.59(16), N2–Cr1–C27 160.3(2), N2–Cr1–O1 95.26(16), C27–
Cr1–O1 93.5(2).
Figure 7. ORTEP plot of [{(HLiPr)Cr(μ-CH3)}2] (8) at the 30%
probability level. With the exception of the hydrogen atoms on the
methyl ligands, all hydrogen atoms have been omitted for clarity.
Selected structural parameters (distances in Å, angles in degrees):
Cr–CrA 2.6649(8), Cr–N1 2.043(2), Cr–N2 2.026(2), Cr–C1
2.192(3), Cr–C1A 2.197(3), N1–C14 1.347(3), N2–C15 1.347(3),
C14–C15 1.377(3); N1–Cr–N2 79.6(1), N1–Cr–C1 163.9(1), N1–
Cr–C1A 93.6(1), N2–Cr–C1 95.3(1), N2–Cr–C1A 164.1(1), C1–
Cr1–C1A 95.0(1).
torted square-planar geometry around the Cr in 7 is similar
to that of its precursor, 2. However, the Cr–C bond length
of 2.098(6) Å is shorter than those in 2, presumably due to
the relief of the anionic charge buildup on the complex. The
difference in the Cr–N distances of 1.995(5) Å (Cr–N1) and
2.035(5) Å (Cr–N2) can be attributed to a trans influence
imparted by the alkyl group that is trans to N2. Like 2, the
reduction of the diimine ligand in 7 can be seen through
the C–N bond lengths of 1.336(6) and 1.344(6) Å and the
C–C bond length of 1.373(7) Å. This, and the room-tem-
perature magnetic moment of 3.8(1) μB, are consistent with
a S = 2, CrII complex strongly antiferromagnetically cou-
pled to a ligand-centered radical.
(see Figure 7), which is likely due to some measure of Cr–
Cr bonding, supported by the short metal–metal distance
[Cr–Cr 2.6649(8) Å].[14d] The methyl ligands are symmetri-
cally bridged with bond lengths of 2.192(3) and 2.197(3) Å.
As in 4, these long Cr–C distances are presumably a conse-
quence of three-center two-electron bonding. Complex 8
has average C–N bond lengths of 1.347(3) Å and a diimine
C–C bond length of 1.377(3) Å, making it most consistent
with a high-spin CrII complex (S = 2) coupled to a ligand-
centered radical. The room-temperature effective magnetic
moment [1.8(1) μB per Cr] indicates appreciable metal–
metal interactions, either Cr–Cr bonding or antiferromag-
netic coupling or more likely a combination of both, much
like in 4 and 5. The solution behavior of 8 in C6D6 is in
accord with the solid-state structure shown in Figure 7,
with a very broad, isotropically shifted resonance at δ =
17.4 ppm and sharp resonances at δ = 8.49, 3.88, and
In a similar manner, condensation of 1 with complex 4
or 6 in the proper stoichiometry yields the neutral, methyl-
bridged dimer, [{(HLiPr)Cr(μ-CH3)}2] (8) according to
Scheme 6. The results of an X-ray analysis of 8 can be
found in Figure 7.
1
1.64 ppm in the H NMR spectra. In [D8]THF a dramati-
cally different spectrum is obtained; thus, the broad reso-
nance is shifted to –13.1 ppm while the others are located
at 14.4, 2.69, and –1.91 ppm. We attribute these spectral
Scheme 6. Preparation of [{(HLiPr)Cr(μ-CH3)}2] (8).
Complex 8 crystallizes in the monoclinic space group C2/ differences to the formation of mononuclear [(HLiPr)-
1
c with a C2 axis running perpendicular to the Cr–Cr vector Cr(Me)(THF)] in THF solution. The H NMR spectrum
and between the bridging methyl ligands. The geometry of 8 in [D8]THF is similar in appearance to that of its pre-
around each chromium atom is best described as distorted sumed analogue, complex 7 (see Exp. Section). A reaction
square planar with a dihedral angle of 20.1° between the of 5 with 1 was also attempted to generate a hydride species
N1–Cr1–N2 plane and the C1–Cr1–C1A plane. The two similar to 8, namely, [{(HLiPr)Cr(μ-H)}2]. However, we
diimine chromium fragments are “folded” toward each found that the stronger Cr–H bonds in 5 would not give
other at the bridging methyl ligands at an angle of 53.0° way as the Cr–C bonds in 4 and 6 had done. Other methods
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Organochromium Complexes Bearing Noninnocent Diimine Ligands
of generating [{(HLiPr)Cr(μ-H)}2] from 1 (e.g., AlEt2Cl, tent with the S = 3/2 spin state of a typical CrIII compound.
through β-hydrogen elimnation of an ethyl intermediate) Oxidation of the methyl-bridged 8 in THF also gave a
produced a brown material with 1H NMR spectroscopic brown solution that was proposed to be [(H,MeL*)Cr-
1
resonances at 21.0, 2.17, 8.72, and –24.3 ppm. However, the (THF)2][BARF] (10) and has a similar H NMR spectrum
reaction consistently produced intractable materials and to that of 9 (see the Exp. Section).
structural characterization of the product was not achiev-
able in this context {note, however, the initial isolation of
[{(HLiPr)Cr}2] from a related reaction}.[16]
With neutral alkyl complexes 7 and 8 in hand, oxidation
was attempted to generate cationic alkylchromium com-
plexes. CrIII complexes of this type have proved to be very
successful homogeneous ethylene polymerization cata-
lysts.[10] However, oxidation of 7 with [(C5H5)2Fe][BARF]
[BARF = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate] in
Et2O, intended to produce the cationic, formally CrII com-
plex, [(HLiPr)Cr(CH2SiMe3)(THF)]+, instead led to the
product of a tortuous alkyl/hydride migration and C–H ac-
tivation (see Scheme 7).
Figure 8. ORTEP plot of [(H,TMSML*)Cr(THF)(Et2O)][BARF] (9)
at the 30% probability level. With the exception of the hydrogen
atoms on C13 and C30, all hydrogen atoms and the [BARF]– anion
have been omitted for clarity. Selected structural parameters (dis-
tances in Å, angles in degrees): Cr–N1 1.920(4), Cr–N2 2.013(5),
Cr–C30 2.053(6), Cr–O1 2.065(4), Cr–O2 2.144(4), N1–C13
1.454(8), N2–C14 1.308(7), C13–C14 1.486(8), C14–C15 1.493(7);
Scheme 7. Preparation of [(H,TMSML*)Cr(THF)(Et2O)][BARF] (9).
N1–Cr–N2 81.7(2), N1–Cr–C30 117.2(2), N1–Cr–O1 96.74(18),
N1–Cr–O2 145.41(18), N2–Cr–C30 86.6(2), N2–Cr–O1 177.90(19),
N2–Cr–O2 92.56(18), O1–Cr–O2 87.95(16), O1–Cr–C30 95.3(2),
The ionic product isolated from the oxidation reaction,
brown [(H,TMSML*)Cr(THF)(Et2O)][BARF] (9, in which
H,TMSML denotes alkyl migration to the backbone of what
is now an iminoylamide ligand and L* denotes C–H acti-
O2–Cr–C30 96.3(2).
Two mechanisms have been proposed for alkyl migration
vation of an isopropyl methyl group of an aryl substituent), to C=N bonds in diimine ligands. The first involves homo-
¯
crystallized in the triclinic space group P1; the result of the lytic cleavage of the metal–alkyl bond followed by recombi-
structure determination is depicted in Figure 8. The struc- nation of the alkyl radical with a diimine ligand-centered
ture of 9 is best described as square pyramidal with apical radical. The second mechanism assumes 1,2-insertion of a
(C30) to basal bond angles ranging from 86.7(3) to C=N bond of the diimine ligand into a metal–alkyl bond.
117.1(2)°. The Cr–C bond length is 2.053(6) Å and is thus Either reaction step is then followed by a 1,2-hydride shift
consistent with typical CrIII–C bond lengths. More interest- to engender unsaturation at the more substituted C–N
ing, however, is the apparent migration of the (trimethyl- bond. Assuming the HOMO of 7 houses the ligand-cen-
silyl)methyl group to the backbone of the diimine ligand. tered unpaired electron, oxidation would presumably pro-
Close inspection of the bond lengths shows that the N1– duce [(HLiPr)Cr(CH2SiMe3)(THF)]+, which contains a neu-
C13 and C13–C14 bond lengths have lengthened to 1.451(8) tral diimine ligand and CrII (see Scheme 8). This intermedi-
and 1.480(9) Å, respectively, whereas the N2–C14 bond ate could undergo either a 1,2-insertion of the alkyl ligand
length is considerably shorter [1.306(7) Å] than the bond into the C–N double bond or homolytic cleavage of the
lengths in the precursor, complex 7. Furthermore, the IR alkyl and single-electron transfer to the diimine ligand, fol-
spectrum of 9 shows the appearance of a band at 1610 cm–1 lowed by radical recombination. Either of these steps is
consistent with a C=N vibration. These observations would then followed by a hydride shift to the remaining imine car-
suggest that the backbone of the ligand is in an imine– bon and reestablishment of the C–N double bond at the
amide form with one C–N double bond, one C–N single bulkier, alkyl-substituted side of the ligand. The resulting
bond, and a C–C single bond. Thus, the oxidation state cationic CrII complex activates a C–H bond of an isopropyl
ambiguity in the previous complexes has been removed and substituent and a bimolecular reductive elimination of H2
straightforward electron counting reveals a 13-electron, d3, finally furnishes 9. The C–H bond-activation step is an ap-
CrIII complex ion. Accordingly, the experimentally deter- parent consequence of the formation of a highly reactive,
mined magnetic moment was found to be 4.0(1) μB, consis- three-coordinate, cationic, CrII species.
Eur. J. Inorg. Chem. 2012, 520–529
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525

K. A. Kreisel, G. P. A. Yap, K. H. Theopold
FULL PAPER
Scheme 8. Mechanism of formation of oxidation product 9 (S = THF, Et2O).
For a better understanding of the relative propensity of lated to be a quintet (S = 2) CrII complex, this spin state
metal alkyls such as 7 or its oxidized form towards alkyl being lower in energy than the S = 1 and S = 0 spin states.
migration, unrestricted DFT calculations at the B3LYP/6- Unlike the 7Ј to 7ЈЈ rearrangement, the 8Ј to 8ЈЈ transforma-
311g level of theory were run on model complexes [(HLMe)- tion is calculated to be energetically favorable by approxi-
Cr(Me)(THF)] (7Ј), [(H,Me
L
Me)Cr(THF)] (7ЈЈ), [(HLMe)- mately 10 kcalmol–1. These calculations are wholly consis-
Me)Cr(THF)]+ (8ЈЈ), in tent with our experimental observations, and they suggest
Cr(Me)(THF)]+ (8Ј), and [(H,Me
L
which the 2,6-diisopropylphenyl substituents and the alkyl that cationic alkyl-CrII complexes are subject to facile mi-
ligand have all been replaced by methyl groups. A geometry gratory insertions of unsaturated molecules into the chro-
optimization of 7Ј gave metric parameters that are in excel- mium–carbon bond, at least in an intramolecular fashion.
lent agreement (ΔC–N Ͻ 0.020 Å; ΔC–C Յ 0.022 Å; ΔCr–
N Յ 0.048 Å, ΔCr–C = 0.035 Å, ΔCr–O = 0.020 Å) with
those of actual 7, and its electronic structure is consistent
with our assignment that chromium is in the +II oxidation
state and the diimine ligand is reduced to a ligand-centered
3
radical resulting in a S = /2 ground state (see the Support-
ing Information).
An alkyl migration and subsequent hydride shift in 7Ј
would furnish 7ЈЈ (see Figure 9). Single-point calculations
5
3
of geometry-optimized structures of the S = /2, /2, and
1/2 spin-state possibilities for 7ЈЈ showed the quartet (S =
3/2) to be most stable. Complex 47ЈЈ shows ligand bond
lengths that are in accord with an iminoamide-type struc-
ture with unsaturation at the alkyl-substituted carbon posi-
tion. Not only does this formulation result in an apparent
reduction of the metal to CrI, a relatively unstable oxidation
4
state for Cr, but most importantly, 7ЈЈ is calculated to be
approximately 42 kcalmol–1 less stable than 7Ј. In silico oxi-
dation of 7Ј to 8Ј results in oxidation of the ligand-centered
radical and formation of a high-spin diimine complex of
CrII (d4, S = 2). This spin state was found to be lower in
energy than the corresponding triplet and singlet states.
Figure 9. Relevant energies (not to scale), structures, and spin states
for model complexes 7Ј, 7ЈЈ, 8Ј, and 8ЈЈ.
5
Complex 8Ј has bond lengths that are in accord with a
neutral diimine ligand, that is, C–N distances of 1.2986 and
1.2964 Å and a C–C distance of 1.4617 Å. Furthermore, the
Cr–OTHF and Cr–CH3 bond lengths are shorter than those
Conclusion
Several open-shell organometallic derivatives of chro-
in 7Ј due to the increase in formal charge on Cr. The calcu- mium supported by a redox noninnocent diimine ligand
lation on the migratory insertion product of 8Ј, namely 8ЈЈ, have been prepared. These compounds feature reduced di-
produced metric parameters for the diimine ligand that are imine ligands, with the extent of electron transfer depending
essentially identical to those in 7ЈЈ; thus the ligand is clearly on the particular compound. As the metal – which is only
an iminoamide ligand. Like 8Ј, complex 8ЈЈ is also calcu- formally in the CrI oxidation state – finds itself in a more
526
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Organochromium Complexes Bearing Noninnocent Diimine Ligands
1042 (m), 888 (w), 858 (m), 759 (m) cm–1. UV/Vis (THF): λmax (ε,
–1 cm–1) = 467 (3674), 569 (2972), 741 (4560) nm. μeff (294 K)
and more electron-rich coordination environment (i.e., the
more alkyl ligands it has and the more negatively charged
it is), the diimine radical anion accommodates the extra
electron density to the point where it oxidizes even CrII,
resulting in a formal CrIII/enediamide pairing. Further-
more, we have shown that alkyl chromate and neutral
organochromium complexes resist alkyl migration to the li-
gand, perhaps due to the reduced nature of the latter. How-
ever, upon oxidation to cationic alkyls, which feature neu-
tral diimine ligands, migratory insertion of the C=N double
bond into the metal–carbon bond becomes favorable and
indeed facile. This suggests that organometallic diimine
complexes of early transition metals will be most stable in
the presence of appreciably reduced diimine ligands.
While α-diimines make excellent ancillary ligands for
homogeneous olefin polymerization catalysts containing
late-transition metals,[17] we anticipate that they will be less
than “ancillary” when coordinated to metals on the left side
of the periodic table.
m
= 3.7(1) μB. Elemental analysis consistently gave values that were
significantly low in carbon and hydrogen and high in nitrogen. The
exact cause is unknown, but could be due to incomplete combus-
tion of the cation/anion pair. Cation exchange with Et4NCl led to
poor solubility causing difficulty in separating the product from
LiCl.
[Li(THF)2][(HLiPr)Cr(Ph)2] (3b):
A
sample of
1
(0.195 g,
0.210 mmol) in THF (15 mL) was cooled to –30 °C. 1.8 m LiPh
(0.44 mL) was added and the solution warmed to room temp. and
stirred for 2 h. The resulting solution was stripped of THF, washed
with pentane, and extracted into Et2O. The ether solution was con-
centrated and cooled to –30 °C to give brown 3 in 56% yield
(0.173 g). M.p. 184 °C (decomp.). 1H NMR ([D8]THF): δ = 17.0
(16 H, THF), 9.42 (2 H, aromatic), 6.54 (4 H, iPr), 4.34 (24 H, iPr)
ppm. IR (KBr disc): ν = 3042 (m), 3033 (m), 2957 (s), 2925 (s),
˜
2865 (s), 1464 (s), 1432 (s), 1382 (m), 1358 (s), 1318 (m), 1246 (m),
1197 (m), 1109 (w), 1039 (s), 887 (w), 759 (m), 730 (m), 708 (s)
cm–1. C46H62CrLiN2O2 (733.94): calcd. C 75.28, H 8.51, N 3.82;
found C 75.36, H 8.38, N 3.50. UV/Vis (THF) λmax (ε, m–1 cm–1) =
477 (1007), 557 (841), 871 (450) nm. μeff (294 K) = 3.7(1) μB.
[Li(THF)4][{(HLiPr)Cr}2(μ-Me)3] (4): Compound
1
(0.223 g,
Experimental Section
0.241 mmol) was placed in THF and cooled to –30 °C. A solution
of LiMe (1.6m, 0.45 mL) was added and the solution was stirred
for 2 h at room temp. The solvent was removed from the blue solu-
tion, then it was washed with pentane and extracted into Et2O. The
solution was then concentrated and cooled to –30 °C overnight to
yield 4 (0.182 g, 63%) as dark blue crystals. M.p. 184 °C (decomp.).
1H NMR ([D8]THF): δ = 7.47 (2 H, aromatic), 5.50 (4 H, iPr), 1.19
General: All manipulations were carried out with standard Schlenk,
vacuum line, and glovebox techniques. Pentane, diethyl ether, and
tetrahydrofuran were dried by passing the solvent through a col-
umn of activated alumina and sparging with N2 to remove residual
O2 prior to use. [D8]THF was predried with potassium metal and
stored under vacuum over Na/K. C6D6 was predried with sodium
metal and stored under vacuum over Na/K.
(12 H, iPr), 0.92 (12 H, iPr) ppm. IR (KBr disc): ν = 3052 (w),
˜
2959 (s), 2925 (m), 2865 (m), 1460 (m), 1437 (m), 1380 (m), 1359
(w), 1320 (m), 1253 (m), 1212 (w), 1105 (w), 1042 (m), 886 (w), 799
(w), 758 (m) cm–1. C71H117Cr2LiN4O4 (1201.65): calcd. C 70.97, H
9.81, N 4.66; found C 70.67, H 9.68, N 4.58. UV/Vis (THF): λmax
(ε, m–1 cm–1) = 491 (3528), 631 (4737), 882 (3683) nm. μeff (294 K)
= 1.4(1) μB per Cr.
Starting Materials: [(Trimethylsilyl)methyl]lithium was purchased
as a 1 m solution in pentane from Aldrich, was crystallized from
solution at –30 °C, and was isolated as a white crystalline solid.
All other alkyllithium and Gignard reagents were purchased from
Aldrich and used as received. The preparations of
1 and
[(C5H5)2Fe][BARF] have been previously reported.[8]
[Li(THF)4][{(HLiPr)Cr}2(μ-H)3] (5):
A solution of 1 (0.112 g,
Instrumentation: 1H NMR spectra were recorded on a Bruker
DRX-400 spectrometer and were referenced to the residual protons
of the solvent ([D8]THF, 1.73 and 3.58 ppm; C6D6, 7.15 ppm).
FTIR spectra were recorded on a Mattson Alpha Centauri spec-
trometer with a resolution of 4 cm–1. UV/Vis spectra were recorded
on a Hewlett–Packard 8453 spectrophotometer. Mass spectro-
scopic data were collected at the University of Delaware Mass
Spectrometry Facility in electron ionization mode (+15 eV), how-
ever no chromium containing fragments were detected for any of
the complexes. Elemental analyses were performed by Desert Ana-
lytics. Room-temperature magnetic susceptibility measurements
were carried out using a Johnson Matthey magnetic susceptibility
balance. Molar magnetic susceptibilities were corrected for diamag-
netism using Pascal constants.
0.121 mmol) in THF was cooled to –30 °C and a solution of LiB-
Et3H (0.36 mL, 1.0m) was added to it. The solution was allowed
to stir for 2 h at room temp. resulting in a violet solution. The
THF was then removed, the residue was washed with pentane and
extracted into Et2O. The solution was then concentrated and co-
oled to –30 °C overnight to yield 5 (0.107 g, 77%) as dark violet
crystals. M.p. 164 °C (decomp.).1H NMR ([D8]THF): δ = 7.76 (2
H, aromatic), 4.07 (4 H, iPr), 1.88 (24 H, iPr) ppm. IR (KBr disc):
ν = 3048 (w), 2954 (s), 2928 (m), 2861 (m), 1460 (m), 1438 (m),
˜
1375 (m), 1347 (w), 1320 (m), 1254 (m), 1210 (w), 1107 (w), 1080
(m), 1053 (m), 1037 (m), 756 (m) cm–1. C60H95Cr2N5 {after
[Li(THF)4]+ for [Et4N]+ exchange} (990.43): calcd. C 72.76, H 9.67,
N 7.07; found C 72.31, H 9.59, N 7.28. UV/Vis (THF): λmax (ε,
m
–1 cm–1)]: 490 (4332), 631 (3931), 816 (4042) nm. μeff (294 K) =
[Li(THF)4][(HLiPr)Cr(CH2SiMe3)2] (2): Complex
1
(0.150 g,
1.5(1) μB per Cr.
0.162 mmol) was dissolved in THF (15 mL) and cooled to –30 °C.
LiCH2SiMe3 (0.061 g, 0.649 mmol) was then added and the solu-
tion was stirred at room temp. for 2 h. The THF was removed, and
the residue was washed with pentane and extracted into Et2O. The
Et2O was then removed to give analytically pure 2 in 72% yield
(0.209 g). Green crystalline samples of 2 can be prepared by cooling
a concentrated solution to –30 °C overnight. M.p. 110 °C (de-
comp.). 1H NMR ([D8]THF): δ = 15.5 (2 H, aromatic), 4.70 (24 H,
[Li2(THF)3][(HLiPr)CrMe3] (6):
A solution of LiMe (1.6 m,
0.83 mL) was added to a –30 °C solution of 1 (0.176 g, 0.190 mmol)
in THF (10 mL). The resulting red solution was allowed to stir at
room temp. for 1 h. The solvent was then removed and the crude
material was dissolved in Et2O and filtered. The Et2O was removed
and the crude red material was dissolved in pentane and crys-
tallized overnight at –30 °C to give a 81% yield of 6 (0.267 g) as
iPr), –13.2 (4 H, iPr) ppm. IR (KBr disc): ν = 3050 (m), 2955 (s), red needles. M.p. 121 °C (decomp.). 1H NMR ([D8]THF): δ = 9.12
˜
2928 (s), 2881 (s), 1460 (s), 1435 (s), 1380 (m), 1318 (m), 1250 (m), (8 H, THF), 3.91 (24 H, iPr), 2.71 (8 H, THF) ppm. IR (KBr disc):
Eur. J. Inorg. Chem. 2012, 520–529
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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527

K. A. Kreisel, G. P. A. Yap, K. H. Theopold
FULL PAPER
ν = 3048 (w), 2958 (s), 2928 (m), 2864 (m), 1458 (m), 1435 (s), 1379
˜
collected on a Bruker APEX CCD diffractometer using graphite-
(w), 1353 (m), 1315 (m), 1248 (s), 1202 (m), 1104 (m), 1076 (m), monochromated Mo-Kα radiation (λ = 0.71073 Å). Unit-cell pa-
1041 (s), 890 (w), 759 (m) cm–1. UV/Vis (THF): λmax (ε, m–1 cm–1)
= 517 (928), 669 (334), 845 (449) nm. μeff (294 K) = 3.8(1) μB. Ele-
mental analysis consistently resulted in values that were low in car-
bon and hydrogen content and high in nitrogen content. Upon cat-
ion exchange with an excess amount of Et4NCl in THF, the solu-
tion turned blue and the anion of complex 4 was identified as the
rameters were obtained from three sets of 20 frames using 0.3° ω
scans from different sections of the Ewald sphere. Data sets were
corrected for absorption using SADABS multiscan methods.[18] No
symmetry higher than triclinic was observed for complexes 5 and
9. Systematic absences in the diffraction data and unit-cell param-
eters are consistent with P21/n (=P21/c; no. 14), uniquely, for com-
plexes 2, 7, 3, and 4; and, Cc (no. 9) and C2/c (no. 15) for complexes
6 and 8. The centrosymmetric space group options yielded chemi-
cally reasonable and computationally stable results of refinement.
Structures were solved by direct methods and refined with full-
matrix least-squares methods based on F2. Two symmetry unique
but chemically similar molecules are located in the asymmetric unit
for complex 7. The compound molecule is located at a twofold axis
in 8. Four structures display cocrystallized solvent molecules: 2 (0.5
Et2O); 5 (Et2O); 8 (0.5 pentane); and 9 (pentane), each per asym-
metric unit. Solvent molecules in 8 and 9 were treated as diffused
contributions using Squeeze.[19] All non-hydrogen atoms were re-
fined anistropically. All hydrogen atoms were treated as idealized
contributions except for the bridging hydrides in 5, the bridging
methyl protons in 4, 6, and 7, and the hydrogen atoms of the meth-
ylene C30 in 9, which were located in difference maps. Located
hydrogen atoms were positionally refined but with isotropic param-
eters constrained to 1.2Ueq of the attached carbon atom for the
methyl and methylene groups, and of the chromium atoms for the
hydrides. All atomic scattering factors are included in the
SHELXTL program library.[18]
1
only product by H NMR spectroscopy.
[(HLiPr)Cr(CH2SiMe3)(THF)] (7): Compounds
1
(0.067 g,
0.072 mmol) and 2 (0.130 g, 0.145 mmol) were added to pentane
(20 mL). The slurry was allowed to stir at room temp. overnight.
The solvent was then removed and the residue was extracted with
pentane, concentrated, and cooled to –30 °C overnight to give
green 7 (0.169 g, 59% yield). M.p. 116 °C (decomp.).1H NMR
([D8]THF): δ = 19.5 (2 H, aromatic), 11.6 (9 H, SiMe3), 2.15 (24
H, THF), –23.2 (4 H, iPr) ppm. IR (KBr disc): ν = 3054 (w), 2957
˜
(s), 2928 (s), 2865 (m), 1456 (m), 1439 (s), 1383 (w), 1360 (w), 1320
(m), 1254 (s), 1222 (w), 1102 (w), 860 (m), 800 (w), 757 (s) cm–1.
C34H55CrN2OSi (587.90): calcd. C 69.46, H 9.43, N 4.77; found C
69.56, H 9.59, N 5.05. UV/Vis (THF): λmax (ε, m–1 cm–1) = 499
(2672), 643 (2835) nm. μeff (294 K) = 3.8(1) μB.
[{(HLiPr)Cr(μ-Me)}2] (8): Pentane (25 mL) was added to 6 (0.155 g,
0.220 mmol) and 1 (0.208 g, 0.224 mmol). The slurry was allowed
to stir at room temp. overnight. The solvent was removed from the
resulting green solution and extracted with pentane, concentrated,
and cooled to –30 °C overnight to give 8 (0.152 g, 52% yield). M.p.
1
162 °C (decomp.). H NMR ([D8]THF): δ = 14.4 (2 H, aromatic),
2.69 (24 H, iPr), –1.91 (4 H, iPr), –13.1 (6 H, CH3) ppm. 1H NMR
(C6D6): δ = 17.2 (6 H, CH3), 8.49 (2 H, aromatic), 3.88 (4 H, iPr),
CCDC-837092 (for 2), -837093 (for 3), -837094 (for 4), -837095 (for
5), -837096 (for 6), -837097 (for 7), -837098 (for 8), and -837099
(for 9) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
1.64 (24 H, iPr) ppm. IR (KBr disc): ν = 3059 (w), 2959 (s), 2925
˜
(m), 2866 (w), 1460 (m), 1441 (s), 1422 (m), 1383 (w), 1361 (w),
1323 (m), 1260 (s), 1222 (w), 1100 (w), 798 (w), 758 (s) cm–1.
C59H90Cr2N4 (959.37): calcd. C 73.87, H 9.46, N 5.84; found C
73.23, H 9.06, N 5.85. UV/Vis (pentane): λmax (ε, m–1 cm–1) = 491
(3421), 615 (1048) nm. μeff (294 K) = 1.8(1) μB per Cr.
Supporting Information (see footnote on the first page of this arti-
cle): X-ray crystallographic data for complexes 2–9, the details of
the DFT calculations of 7Ј, 7ЈЈ, 8Ј, and 8ЈЈ and the complete ci-
tation for the Gaussian 03 software package.
[(H,TMSML*)Cr(THF)(Et2O)][BARF] (9): Compound 7 (0.113 g,
0.192 mmol) was placed in Et2O (20 mL). To this was slowly added
a solution of [(C5H5)2Fe][BARF] (0.200 g, 0.191 mmol) in Et2O
(10 mL). The solution was stirred for 2 h, then the Et2O was re-
moved, the residue was washed with pentane to remove ferrocene,
then crystallized from Et2O at –30 °C overnight to give 9 (0.120 g,
Acknowledgments
1
We thank the University of Wisconsin-Madison’s Chemistry Com-
puter Center for computational assistance. This research was sup-
ported by grants from the Nation Science Foundation (NSF) (to
K. H. T., grant numbers CHE-0616375 and CHE-0911081).
41% yield). M.p. 112 °C (decomp.). H NMR ([D8]THF): δ = 22.4
(2 H, aromatic), 7.69 (8 H, BARF), 7.49 (4 H, BARF), 3.76 (24 H,
iPr), –5.25 (9 H, SiMe ), –23.1 (4 H, iPr) ppm. IR (KBr disc): ν =
˜
3
3071 (w), 2968 (m), 2932 (m), 2874 (m), 1610 (m), 1462 (m), 1440
(m), 1356 (s), 1277 (s), 1136 (s), 887 (m), 840 (s), 801 (w), 714 (m),
682 (m), 670 (m) cm–1. C70H76BCrF24N2O2Si (1524.23): calcd. C
55.16, H 5.03, N 1.84; found C 55.01, H 4.86, N 2.00. UV/Vis
(Et2O): λmax (ε, m–1 cm–1) = 496 (748), 614 (350) nm. μeff (294 K) =
4.0(1) μB.
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8
(0.011 g,
0.012 mmol) and [(C5H5)2Fe][BARF] (0.024 g, 0.024 mmol) were
placed in a J. Young NMR spectroscopy tube followed by addition
of [D8]THF. The tube was then closed and a 1H NMR spectrum
1
was recorded. H NMR ([D8]THF): δ = 22.2 (2 H, aromatic), 7.78
(8 H, BARF), 7.57 (4 H, BARF), –0.41 (24 H, iPr), –22.9 (4 H,
iPr) ppm.
Crystallographic Structure Determinations: A summary of the crys-
tal data collection and refinement parameters for compounds 2–9
can be found in the Supporting Information. Suitable crystals were
selected, mounted with viscous oil, and cooled to 120 K. Data were
528
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Published Online: September 27, 2011
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