Synthesis, characterization, and electronic structure of diimine complexes of chromium

Article abstract of DOI:10.1021/ic800302w

We have prepared and structurally characterized several complexes of chromium coordinated by diimine (or 1,4-diazadiene) ligands, that is, Ar-N=C(R)-(R)C=N-Ar (^RL^Ar) (where Ar = 2,6-diisopropylphenyl ("iPr") or 2,6-dimethylphenyl ("M

Full text of DOI:10.1021/ic800302w

Inorg. Chem. 2008, 47, 5293-5303
Synthesis, Characterization, and Electronic Structure of Diimine
Complexes of Chromium
Kevin A. Kreisel,† Glenn P. A. Yap, and Klaus H. Theopold*
Department of Chemistry and Biochemistry, Center for Catalytic Science and Technology,
UniVersity of Delaware, Newark, Delaware 19716
Received February 18, 2008
We have prepared and structurally characterized several complexes of chromium coordinated by diimine (or 1,4-
diazadiene) ligands, that is, ArsNdC(R)s(R)CdNsAr (RLAr) (where Ar ) 2,6-diisopropylphenyl (“iPr”) or 2,6-
dimethylphenyl (“Me”) and R ) H or Me). The reaction of CrCl2 with HLiPr gave dinuclear [(HLiPr)Cr]2(µ-Cl)3(Cl)(THF)
when isolated from Et2O; in THF solution, however, the product exists as mononuclear (HLiPr)CrCl2(THF)2. Two
isostructural derivatives, (MeLMe)CrCl2(THF)2 and (HLMe)CrCl2(THF)2, have also been prepared. Furthermore, the
bis-ligand complex, (HLiPr)2Cr, has been prepared along with its reduction product, Li(THF)4[(HLiPr)2Cr]. We have
H
also synthesized the tetracarbonyl complex, (HLiPr)Cr(CO)4, by addition of LiPr to Cr(CO)4(NCCH3)2. The structure
and variable temperature magnetic susceptibility of the previously reported Cr halide dimer, [(HLiPr)Cr(µ-Cl)]2, is
also discussed in detail. All of the diimine complexes have been characterized structurally, spectroscopically, and
magnetically, and their electronic structures are discussed with the aid of density-functional theory calculations.
Chart 1. Possible Electronic Structures of Diimine Complexes
Introduction
As part of our investigation of chromium alkyls as
homogeneous models for commercially used heterogeneous
chromium catalysts for the polymerization of ethylene and
R-olefins,1,2 we required a class of neutral, bidentate nitrogen
ligands to compare the reactivity of their chromium com-
plexes with that of the structurally related “nacnac” (i.e.,
R-diketiminate) derivatives. Inspired by the extensive use
of diimine complexes of late transition metals in polymer-
ization catalysis,3 we chose to initiate a study of diimines
(or 1,4-diazadienes) as ancillary ligands in organochromium
chemistry.
Diimine ligands are popular because of the ease of varying
the steric and electronic effects that they exert upon the metal
in their complexes.4 Furthermore, the redox properties of
these ligands provide an interesting electronic relationship
between the ligand and the metal (see Chart 1). Diimine
complexes can be classified as having electronic structures
A, B, or C in Chart 1.5 In the case of A, the diimine ligand
is neutral and structurally essentially unperturbed; that is, it
* To whom correspondence should be addressed. E-mail: theopold@
udel.edu.
†
Current address: University of Wisconsin-Madison, Department of
Chemistry, 1101 University Ave. Madison, WI 53706.
(1) (a) Richeson, D. S.; Hsu, S. W.; Fredd, N. H.; Van Duyne, G.;
Theopold, K. H. J. Am. Chem. Soc. 1986, 108, 8273. (b) Thomas,
B. J.; Noh, S. K.; Schulte, G. K.; Sendlinger, S. C.; Theopold, K. H.
J. Am. Chem. Soc. 1991, 113, 893. (c) Thomas, B. J.; Theopold, K. H.
J. Am. Chem. Soc. 1988, 110, 5902. (d) Thomas, B. J.; Mitchell, J. F.;
Leary, J. A.; Theopold, K. H. J. Organomet. Chem. 1988, 348, 333.
(e) White, P. A.; Calabrese, J.; Theopold, K. H. Organometallics 1996,
15, 5473. (f) Liang, Y.; Yap, G. P. A.; Rheingold, A. L.; Theopold,
K. H. Organometallics 1996, 15, 5284.
(3) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. ReV 2000, 100, 1169;
and references therein.
(4) van Koten, G.; Vrieze, K. AdV. Organomet. Chem. 1982, 21, 151.
(5) (a) Milione, S.; Cavallo, C.; Tedesco, C.; Grassi, A. J. Chem. Soc.,
Dalton Trans. 2002, 1839. (b) Hannant, M. D.; Schormann, M.;
Bochmann, M. J. Chem. Soc., Dalton Trans. 2002, 4071. (c) Gibson,
V. C.; Tomov, A.; Wass, D. F.; White, A. J. P.; Williams, D. J.
J. Chem. Soc., Dalton Trans. 2002, 2261. (d) Schleis, T.; Spaniol,
T. P.; Okuda, J.; Heinemann, J.; Mulhaupt, R. J. Organomet. Chem.
1998, 569, 159.
(2) (a) Kim, W.; Fevola, M. J.; Liable-Sands, L. M.; Rheingold, A. L.;
Theopold, K. H. Organometallics 1998, 17, 4541. (b) MacAdams,
L. A.; Kim, W.; Laible-Sands, L. M.; Guzei, I. A.; Rheingold, A. L.;
Theopold, K. H. Organometallics 2002, 21, 952. (c) MacAdams, L. A.;
Buffone, G. P.; Incarvito, C. D.; Rheingold, A. L.; Theopold, K. H.
J. Am. Chem. Soc. 2005, 127, 1082.
10.1021/ic800302w CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 12, 2008 5293
Published on Web 04/29/2008

Kreisel et al.
shows C-N bond distances that are in accord with double
bonds and a central C-C single bond distance. Transfer of
one electron from the metal to the ligand gives B, which
can best be described as featuring a delocalized ligand-
centered radical, typically with very strong antiferromagnetic
coupling between any unpaired electrons on the metal and
the ligand centered spin.6 Structurally, B would show
lengthened C-N bond distances and a shortened C-C
distance compared to A.7 Transfer of a second electron from
the metal to the diimine ligand results in the electronic
structure of C, that is, a coordinated enediamide.8 C would
show longer C-N distances yet and a short C-C distance
that approaches typical double bond length. However, bond
distances alone are too blunt a tool to accurately assess the
degree of reduction of the ligand(s) and hence the formal
oxidation state of the metal.9 Furthermore, the truth may lie
somewhere between the idealized structures of A, B, and C,
whichsafter allsare but limited valence bond descriptions
of particular points of a continuum.
To complement our work with nacnac chromium
complexes,2we have recently taken an interest in the orga-
nometallic chemistry of chromium supported by neutral
ligands.13 One aim of this undertaking was to synthesize
cationic Cr(II) alkyl complexes to compare their reactivity
with that of structurally related cationic nacnacCr(III) alkyls.
Such a direct comparison might shed light on the longstand-
ing conundrum concerning the formal oxidation state of
chromium during these catalytic processes when activated
by alkyl aluminum cocatalysts.14 Diimine chromium com-
plexes have been synthesized and studied in the past,15 but
there have been no examples of N-Aryl substituted ligands
coordinated to chromium(II) except for a recent report,16
which we believe to be in error. Herein we summarize the
coordination chemistry of the diimine ligands ArsNdC(R)s
(R)CdNsAr (RLAr) (where Ar ) 2,6-diisopropylphenyl
(“iPr”) or 2,6-dimethylphenyl (“Me”) and R ) H or Me)
when bound to a variety of low-valent chromium fragments.
Organometallic derivatives of these compounds will be the
focus of a separate report.
Work on nickel diimine complexes by Wieghardt and others
has greatly improved our understanding of the unique interplay
between these ligands and transition metals.10 Specifically, it
has been shown that a combination of structural, spectroscopic,
and computational techniques is profitably employed to fully
understand the complex electronic structure of these systems,
and in some cases there remains some room for argument.11
In any event, from a reactivity viewpoint one of the attractive
features of complexes of this sort is their electronic malleability;
in other words, depending on the nature of the other ligands
and the overall charge of the complex, the diimine ligand may
relieve the metal of excess electron density or supply it when
required. Functioning as a sort of “electronic buffer”, coordi-
nated diimines may thus facilitate transformations that would
otherwise strain the metal’s tolerance for oxidation or reduc-
tion.12 With this idea in mind, we are exploring the reactivity
of chromium coordinated by aryl-substituted diimines.
Results and Discussion
Syntheses and Structures of (RLAr)Cr Complexes. Our
initial foray into chromium diimine chemistry employed the
HLiPr ligand and CrCl2 (see Scheme 1). Thus, to a slurry of
CrCl2 in tetrahydrofuran (THF) was added one equivalent
of HLiPr, to give a dark brown solution. After stirring
overnight, solvent removal, extraction with, and crystalliza-
tion from Et2O produced dark green crystals of [(HLiPr)Cr]2-
(µ-Cl)3(Cl)(THF) (1′), which were structurally characterized
by X-ray diffraction. The molecular structure of 1′ is shown
in Figure 1, and selected interatomic distances and angles
are listed in Table 1. The asymmetric unit of the crystal
contains two independent, but chemically equivalent mol-
(12) (a) Blackmore, K. J.; Ziller, J. W.; Heyduk, A. F. Inorg. Chem. 2005,
44, 5559. (b) Haneline, M. R.; Heyduk, A. F. J. Am. Chem. Soc. 2006,
128, 8410. (c) Blackmore, K. J.; Ziller, J. W.; Heyduk, A. F. Inorg.
Chem. 2008, 47, 265. (d) Stanciu, C.; Jones, M. E.; Fanwick, P. E.;
Abu-Omar, M. M. J. Am. Chem. Soc. 2007, 129, 12400.
(6) Tsukahara, Y.; Kamatani, T.; Iino, A.; Suzuki, T.; Kaizaki, S. Inorg.
Chem. 2002, 41, 4363.
(13) (a) Kreisel, K. A.; Yap, G. P. A.; Theopold, K. H. Organometallics
2006, 25, 4670. (b) Kreisel, K. A.; Yap, G. P. A.; Theopold, K. H.
Chem. Commun. 2007, 1510.
(7) (a) Bart, S. C.; Hawrelak, E. J.; Schmisseur, A. K.; Lobkovsky, E.;
Chirik, P. J. Organometallics 2004, 23, 237. (b) Ionkin, A. S.;
Marshall, W. J. J. Organomet. Chem. 2004, 689, 1057. (c) Gardiner,
M. G.; Hanson, G. R.; Henderson, M. J.; Lee, F. C.; Raston, C. L.
Inorg. Chem. 1994, 33, 2456.
(14) (a) Budzelaar, P. H. M.; van Oort, A. B.; Orpen, A. G. Eur. J. Inorg.
Chem. 1998, 1485. (b) Rahim, M.; Taylor, N. J.; Xin, S. X.; Collins,
S. Organometallics 1998, 17, 1315. (c) Lee, L. W. M.; Piers, W. E.;
Elsegood, M. R. J.; Clegg, W.; Parvez, M. Organometallics 1999,
18, 2947. (d) McGuinness, D. S.; Brown, D. B.; Tooze, R. P.; Hess,
F. M.; Dixon, J. T.; Slawin, A. M. Z. Organometallics 2006, 25, 3605.
(e) Jabri, A.; Temple, C.; Crewdson, P.; Gambarotta, S.; Korobkov,
I.; Duchateau, R. J. Am. Chem. Soc. 2006, 128, 9238.
(8) (a) Richter, B.; Scholz, J.; Neumu¨ller, B.; Weimann, R.; Schumann,
H. Z. Anorg. Allg. Chem. 1995, 621, 365. (b) Fedushkin, I. L.; Skatova,
A. A.; Chudakova, V. A.; Fukin, G. K. Angew. Chem., Int. Ed. 2003,
42, 3294.
(9) (a) Rindorf, G.; Thorup, N.; Bjornhom, T.; Bechgaard, K. Acta. Cryst.,
Sect. C 1990, C46, 1437. (b) Ray, K.; Weyhermu¨ller, T.; Goossens,
A.; Craje´, M. W. J.; Wieghardt, K. Inorg. Chem. 2003, 42, 4082.
(10) (a) Balch, A.; Holm, R. H. J. Am. Chem. Soc. 1966, 88, 5201. (b)
Walther, D. Z. Anorg. Allg. Chem. 1974, 405, 8. (c) Khusniyarov,
M. M.; Harms, K.; Burhaus, O.; Sundermeyer, J. Eur. J. Inorg. Chem.
2006, 2985. (d) Herebain, D.; Bothe, E.; Neese, F.; Weyhermu¨ller,
T.; Wieghardt, K. J. Am. Chem. Soc. 2003, 125, 9116. (e) Herebain,
D.; Wieghardt, K.; Neese, F. J. Am. Chem. Soc. 2003, 125, 10997. (f)
Chlopek, K.; Bothe, E.; Neese, F.; Weyhermu¨ller, T.; Wieghardt, K.
Inorg. Chem. 2006, 45, 6298. (g) Muresan, N.; Chlopek, K.;
Weyhermu¨ller, T.; Neese, F.; Wieghardt, K. Inorg. Chem. 2007, 46,
5327. (h) Muresan, N.; Weyhermu¨ller, T.; Wieghardt, K. Dalton Trans.
2007, 4390.
(15) (a) Franz, K. D.; tom Dieck, H.; Krynitz, U.; Renk, I. W. J. Organomet.
Chem. 1974, 64, 361. (b) tom Dieck, H.; Kinzel, A. Angew. Chem.,
Int. Ed. Engl. 1979, 18, 324. (c) Balk, R. W.; Snoeck, T.; Stufkens,
D. J.; Oskam, A. Inorg. Chem. 1980, 19, 3015. (d) Bell, A.; Walton,
R. A. Polyhedron 1986, 5, 951. (e) Bell, A.; Walton, R. A. Polyhedron
1986, 5, 1321. (f) Holzmeier, P.; Goerner, H.; Knoch, F.; Kisch, H.
Chem. Ber. 1989, 122, 1457. (g) Grevels, F.-W.; Kayran, C.; Ozkar,
S. Organometallics 1994, 13, 2937. (h) Bildstein, B.; Malaun, M.;
Kopacka, H.; Fontani, M.; Zanello, P. Inorg. Chim. Acta 2000,
300-302, 16.
(16) Turki, T.; Guerfel, T.; Bouachir, F. Inorg. Chem. Commun. 2006, 9,
1023; We believe that the diamagnetism of the tetrahedral (di-
imine)MCl2 complexes as well as the crystal structure of 2b2
reported in this paper are more consistent with zinc complexesszinc
was used as a reducing agent. We have communicated our view to
the authors.
(11) Bart, S. C.; Chlopek, K.; Bill, E.; Bouwkamp, M. W.; Lobkovsky,
E.; Neese, F.; Wieghardt, K.; Chirik, P. J. J. Am. Chem. Soc. 2006,
128, 13901.
5294 Inorganic Chemistry, Vol. 47, No. 12, 2008

Diimine Complexes of Chromium
Scheme 1. Synthesis of Diimine Cr Halide Complexes 1, 2, 3, and 1′
ecules; only one of these is shown in Figure 1. In the solid
state, 1′ is an unsymmetric, dinuclear complex with both
chromium atoms exhibiting distorted octahedral coordination.
Each chromium atom is coordinated by a diimine ligand and
three bridging chlorides; the dissymmetry is introduced by
a THF molecule coordinated to Cr(1), while Cr(2) is bonded
to a terminal chloride ligand. The Cr-Cl distances to the
bridging chlorides range from 2.350(1) to 2.452(1) Å and
fall within the range of other bridging chloride complexes
of chromium (2.293-2.813 Å; avg ) 2.397 Å).17 The shorter
Cr-Cl distances of the terminal chlorides are also unexcep-
tional at 2.279(1) and 2.291(1) Å. The Cr-N distances range
from 1.964(4) to 1.993(4) Å and are slightly shorter than
most other reported Cr(II)-N bond lengths (avg ) 2.079
Å).18
More interesting are the C-N and C-C bond distances
of the diimine ligands, which deviate substantially from those
of the free ligand (C-N, 1.2632(2) Å; C-C, 1.468(2) Å).7c
The average C-N distance in 1′ is 1.333(5) Å, whereas the
average C-C distance is only marginally longer at 1.391(6)
Å. These distances suggest some reduction of the diimine
ligand, possibly amounting to the formation of a ligand
centered radical (i.e., B in Chart 1).10g,h Consequently, the
Cr atoms in 1′ may be best described as adopting the Cr(III)
oxidation state. The room temperature magnetic moment of
1′ (µeff ) 2.8(1) µB per Cr) is consistent either with quartet
Cr(III) strongly antiferromagnetically coupled to a ligand
centered radical (S ) 1/2) or, however improbable, with low
spin octahedral Cr(II) (d4, S ) 1). The distance between the
two chromium atoms (Cr(1)-Cr(2) ) 3.090(4) Å) militates
against metal-metal bonding, and the agreement between
measured and expected magnetic moment renders significant
antiferromagnetic coupling between the Cr atoms unlikely.
Indeed, the effective magnetic moment of 1′ is similar to
that of the monomeric species 3 (vide infra), strongly
suggesting that each Cr atom in 1′ is acting as an isolated
paramagnet. We suggest that the most appropriate description
of the electronic structure of 1′ is as a dinuclear complex
containing two noninteracting Cr(III) ions, each coordinated
by, and antiferromagnetically coupled to a diimine radical
anion.
Figure 1. ORTEP plot of one of the two independent molecules of
[(HLiPr)Cr]2(µ-Cl)3(Cl)(THF) (1′) at the 30% probability level. Hydrogen
atoms have been omitted for clarity.
with overlapping peaks at 16.1, 0.51, -1.9, -7.4, and -21.2
ppm. In contrast, solutions of 1′ in THF-d8 were brown and
revealed a dramatically different spectrum with resolved
1
resonances at 20.0, 0.27, -1.9, and -10.2 ppm. These H
NMR data, as well as the structures of the related complexes
2 and 3 suggest that in THF solution 1′ dissociates into
mononuclear (HLiPr)CrCl2(THF)2 (1). Unfortunately, the high
solubility of 1 in THF precluded its crystallization and
structural characterization.
Reaction of MeLMe or HLMe with CrCl2 in THF gave similar
brown solutions (see Scheme 1). Crystallization of these
products from THF afforded (MeLMe)CrCl2(THF)2 (2) and
(HLMe)CrCl2(THF)2 (3). The molecular structure of 2, as
determined by X-ray diffraction, is shown in Figure 2; its
interatomic distances and angles are listed in Table 2. 2
features slightly distorted octahedral coordination, with cis
bond angles ranging from 79.68(6)° to 96.48(5)° and trans
angles ranging from 170.87(2)° to 174.95(6)°. The terminal
chloride distances in 2 (2.309(1) and 2.340(1) Å) are similar
to those in 1′, as are the Cr-N distances (1.983(2) and
1.969(2) Å). The Cr-OTHF distances of 2.148(2) and 2.154(2)
Å are also on par with other reported THF complexes of
chromium.19 Like 1′, 2 shows evidence of diimine reduction
with C-N distances of 1.347(2) and 1.343(2) Å and a C-C
distance of 1.407(2) Å. Once again, this suggests the
existence of a radical centered on the diimine ligand. The
molecular structure and metric data of 3 are similar to those
of 2 and therefore the full structural details have been
relegated to the Supporting Information. 3, and by analogy
2 (see Experimental Section), has a magnetic moment of
2.9(1) µB which is consistent with an S ) 1 spin state. The
same options as described above for 1′ exist, and akin to
the latter, the electronic structures of 2 and 3 are most
straightforwardly rationalized by invoking strong antiferro-
Green CD2Cl2 solutions of 1′ showed very broad and
1
isotropically shifted resonances in the H NMR spectrum,
(17) A search of the Cambridge Structural Database for chloride ligands
bridging two chromium atoms resulted in 116 hits. Allen, F. H. Acta
Crystallogr. 2002, B58, 380.
(19) (a) Bazan, G. C.; Rogers, J. S.; Fang, C. C. Organometallics 2001,
20, 2059. (b) Bhandari, G.; Kim, Y.; McFarland, J. M.; Rheingold,
A. L.; Theopold, K. H. Organometallics 1995, 14, 738. (c) Dionne,
M.; Jubb, J.; Jenkins, H.; Wong, S.; Gambarotta, S. Inorg. Chem. 1996,
35, 1874.
(18) A search of Cambridge Structural Database for Cr(II) complexes with
single Cr-N bonds resulted in 832 hits. Allen, F. H. Acta Crystallogr.
2002, B58, 380.
Inorganic Chemistry, Vol. 47, No. 12, 2008 5295

Kreisel et al.
Table 1. Selected Interatomic Distances (Å) and Angles (°) for One of the Two Independent Molecules of [(HLiPr)Cr]2(µ-Cl)3(Cl)(THF) (1′)
Distances (Å)
2.381(1)
Cr(1)-Cl(1)
Cr(1)-N(2)
Cr(2)-Cl(2)
Cr(2)-N(4)
N(4)-C(40)
2.350(1)
1.986(4)
2.406(1)
1.972(4)
1.334(5)
Cr(1)-Cl(2)
Cr(1)-O(1)
Cr(2)-Cl(3)
N(1)-C(13)
C(13)-C(14)
Cr(1)-Cl(3)
Cr(1)-Cr(2)
Cr(2)-Cl(4)
N(2)-C(14)
C(39)-C(40)
2.373(1)
3.090(4)
2.279(1)
1.343(5)
1.396(6)
Cr(1)-N(1)
Cr(2)-Cl(1)
Cr(2)-N(3)
N(3)-C(39)
1.987(4)
2.432(1)
1.964(4)
1.328(5)
2.065(3)
2.452(1)
1.328(5)
1.386(6)
Angles (°)
N(1)-Cr(1)-N(2)
N(1)-Cr(1)-Cl(1)
O(1)-Cr(1)-Cl(1)
N(3)-Cr(2)-N(4)
N(3)-Cr(2)-Cl(3)
81.0(2)
N(1)-Cr(1)-O(1)
N(1)-Cr(1)-Cl(2)
O(1)-Cr(1)-Cl(2)
N(3)-Cr(2)-Cl(1)
N(3)-Cr(2)-Cl(4)
94.1(1)
96.0(1)
87.19(9)
99.5(1)
91.2(1)
N(2)-Cr(1)-O(1)
95.1(1)
177.4(1)
86.05(9)
95.9(1)
95.9(1)
165.80(9)
81.1(2)
N(1)-Cr(1)-Cl(3)
O(1)-Cr(1)-Cl(3)
N(3)-Cr(2)-Cl(2)
179.4(1)
magnetic coupling between a Cr(III) ion and an adjacent
diimine ligand radical anion.
the diimine ligands. The C-N distances of 6 are significantly
longer than those in 1-5 at 1.382(3), 1.377(3), 1.375(4), and
1.379(3) Å. At the same time, the C-C distances are
shortened to 1.344(4) and 1.348(4) Å, essentially CdC
double bond distances. These distances might suggest that
the diimine ligands in 6 have now been reduced to the
dianionic, enediamide form (C in Chart 1), which would then
require the metal to be Cr(III). Besides the counterintuitive
notion of such a “reductively induced oxidation” of the metal,
the nearly identical coordination geometries of 5 and 6 also
argue against a change in metal oxidation state. An alternative
description of 6 would maintain chromium in the +II
oxidation state, coordinated by one dianionic enediamide
ligand and one monoanionic, ligand centered radical, similar
to K(THF)4[Zn(HLtBu*)2] (where HLtBu* is tBuNdCHsCHd
NtBu).7c,10g,22 Unlike the zinc example, it is apparent from
the C-N and C-C bond distances that the radical is not
localized on either one of the two ligands.
In an effort to access lower oxidation state complexes of
chromium, we began metalating with the one electron
reduced form of the diimine ligand, Na[HLiPr], which was
formed in situ by reduction with sodium metal (see Scheme
2).20 Addition of dark red THF solutions of Na[HLiPr] to a
slurry of CrCl2 in THF did not produce the expected
[(HLiPr)Cr(µ-Cl)]2 (4, vide infra), but rather the red-brown
bis-ligand complex (HLiPr)2Cr (5) in <30% yield along with
unreacted CrCl2. Accordingly, 5 could be readily prepared
in much better yield by slow addition of two equivalents of
Na[HLiPr] to CrCl2 in THF. Interestingly, complexes 1-3 can
be synthesized by mixing Na[RLAr] and CrCl3(THF)3 in
equimolar ratios. The solid-state structure, distances, and
angles for 5 can be found in Figure 3 and Table 3. Formally
a Cr(0) complex, 5 exhibits distorted square planar coordina-
tion (31.5° dihedral angle between the diimine ligand planes)
with a crystallographically imposed inversion center situated
at Cr; it is isomorphous to a previously reported nickel
complex.21 Despite the lower coordination number, the Cr-N
bond distances of 2.030(4)Å and 2.035(4)Å are longer than
those in complexes 1-3. Once again, the diimine ligands
show clear signs of reduction, with C-N distances of
1.334(10) and 1.328(11) Å and a C-C distance of 1.406(12)
Å. These metric parameters suggest that 5 contains a Cr(II)
ion (S ) 2) coordinated by two diimine radical anions rather
than Cr(0) with neutral diimine ligands, as was suggested
previously for a N,N′-dicyclohexyl substituted bis-diimine
complex of chromium.15b
Complex 5 was cleanly reduced by one electron in a
reaction with LiCH2SiMe3 to produce what is formally a Cr-
(-I) “ate” complex, namely Li(THF)4[(HLiPr)2Cr] (6) (see
Scheme 2). Green crystals of 6 were grown from Et2O at
-30 °C, and the molecular structure as determined by X-ray
diffraction is shown in Figure 4; structural data can be found
in Table 4. Similar to 5, complex 6 exhibits distorted square
planar geometry, with the dihedral angle of the diimine
ligands being nearly identical at 32.3°. Of considerable
interest are the Cr-N bond distances, which range from
1.995(2) to 2.013(2) Å, that is, marginally shorter than those
in 5. However, the more remarkable difference between the
structures of 5 and 6 lies in the C-N and C-C distances of
Variable temperature magnetic measurements were carried
out on 5 and 6 (see Supporting Information). Complex 5
displayed a slightly temperature dependent moment increas-
ing with temperature from 2.2(1) to 2.7(1) µB (30-300 K);
the room temperature value is close to the spin only value
for two unpaired electrons (µeff ) 2.83 µB for S ) 1). The
cause of the temperature dependence is not entirely clear,
but it could be due to weak intermolecular coupling between
adjacent molecules. Complex 6 showed no temperature
dependence with a moment of 3.9(1) µB (10-300 K)
indicating the presence of three unpaired electrons. Neither
5 nor 6 showed any obvious signs of antiferromagnetic
coupling between the metal and a ligand-centered radical,
but this is to be expected as the relevant metal-ligand
coupling constants have been calculated to be very large.23
Indeed, both complexes adhered to the Curie-Weiss law
with Curie constants of 0.773 emu K/mol and 1.98 emu
K/mol and Weiss constants of -7.7 and -1.9 K for 5 and
6, respectively. Unfortunately, these magnetic measurements
do not differentiate between Cr(II) and Cr(III) for either 5
or 6. We suggest that 5 exists as Cr(II) (S ) 2) coordinated
1
by two ligand-centered radicals (S ) /2), which results in
an overall spin state that matches well with the room
temperature moment of 2.7(1) µB. The electronic structure
for 6 is more difficult to discern; however, the unchanged
(20) Scholz, A.; Thiele, K.-H.; Scholz, J.; Weimann, R. J. Organomet.
Chem. 1995, 501, 195.
(22) Rijnberg, E.; Richter, B.; Thiele, K.-H.; Boersma, J.; Lee, F. C.; Raston,
C. L. Inorg. Chem. 1998, 37, 56.
(21) Bonrath, W.; Po¨rschke, K. R.; Mynott, R.; Kru¨ger, C. Z. Naturforsch.
B 1990, 45B, 1647.
(23) Kapre, R. R.; Bothe, E.; Weyhermu¨ller, T.; DeBeer George, S.;
Muresan, N.; Wieghardt, K. Inorg. Chem. 2007, 46, 7827.
5296 Inorganic Chemistry, Vol. 47, No. 12, 2008

Diimine Complexes of Chromium
Figure 2. ORTEP plot of (MeLMe)CrCl2(THF)2 (2) at the 30% probability level. Hydrogen atoms have been omitted for clarity.
Table 2. Selected Interatomic Distances (Å) and Angles (°) for (MeLMe)CrCl2(THF)2 (2)
Distances (Å)
Cr-Cl(1)
2.310(1)
1.983(2)
1.407(2)
Cr-Cl(2)
Cr-N(2)
2.340(1)
1.969(2)
Cr-O(1)
N(1)-C(10)
2.154(2)
1.347(2)
Cr-O(2)
N(2)-C(11)
2.148(2)
1.343(2)
Cr-N(1)
C(10)-C(11)
Angles (°)
O(1)-Cr-O(2)
N(1)-Cr-N(2)
Cl(1)-Cr-Cl(2)
N(2)-Cr-O(1)
170.86(2)
173.27(6)
90.98(5)
79.68(6)
N(1)-Cr-O(2)
O(1)-Cr-O(2)
174.95(6)
90.98(5)
istry have been communicated previously,24 a detailed
structural and electronic analysis has not been presented and
this is done here. The Cr-Cl bond lengths in 4 of 2.375(1),
2.375(1), 2.377(2), and 2.379(2) Å are similar to those for
1′, as are the Cr-N distances of 1.984(4)Å, 1.989(4)Å,
1.989(3)Å, and 1.990(3)Å. In addition, the diimine ligands
show signs of reduction to ligand-centered radicals with an
average C-N distance of 1.341(6) Å and an average C-C
distance of 1.388(6) Å. This assignment would then require
two Cr(II) ions (d4, S ) 2), which would presumably couple
antiferromagnetically to two ligand centered radicals giving
Scheme 2. Synthesis of Complexes 5 and 6
dihedral angle between the diimine ligands around Cr
suggests Cr(II) as well, thus requiring one diamagnetic,
dianionic, enediamide ligand (S ) 0) and one anionic, ligand-
centered radical (S ) 1/2) giving an overall S ) 3/2 spin state.
The targeted chloride dimer [(HLiPr)Cr(µ-Cl)]2 (4) was
ultimately prepared by reacting a THF solution of the dianion
of HLiPr, Na2[HLiPr] (also prepared in situ), with CrCl3(THF)3
according to Scheme 3.21 Although 4 and its magnetochem-
3
a local net S ) /2 spin state per Cr and an expected spin
only moment of 3.88 µB per Cr.
A room temperature magnetic susceptibility measurement
on 4 gave a lower than expected moment of 4.8(1) µB (i.e.,
3.4(1) µB per Cr). In view of the long Cr-Cr distance in 4
(3.431(1)Å), which precludes any meaningful Cr-Cr bond-
ing, this points to weak antiferromagnetic coupling between
the metals as the cause of the lowered moment. Accordingly,
a variable temperature magnetic susceptibility measurement
showed typical antiferromagnetic coupling between two
paramagnetic centers. To correctly fit the ꢀm vs T curve,25
the spin multiplicity of the individual halves of 4 was
required. Therefore, Et4N[(HLiPr)CrCl2] (7) was synthesized
from 4 by the addition of excess NEt4Cl in THF. Single
crystals of 7 were grown from Et2O at -30 °C, and Figure
(24) Kreisel, K. A.; Yap, G. P. A.; Dmitrenko, O.; Landis, C. R.; Theopold,
K. H. J. Am. Chem. Soc. 2007, 129, 14162.
(25) (a) Griffith, J. S. Struct. Bonding (Berlin) 1972, 10, 87. (b) The Theory
of Electric and Magnetic Susceptibilities; Van Vleck, J. H., Ed.; Oxford
University Press: London, 1932. (c) Magnetic Properties of Coordina-
tion and Organometallic Tranistion Metal Compounds, Landolt-
Bornstein Series; Hellwege, K.-H., Hellwege, A. M., Eds.; Springer-
Verlag: Berlin, 1981; Vol. 11.
Figure 3. ORTEP plot of (HLiPr)2Cr (5) at the 30% probability level.
Hydrogen atoms have been omitted for clarity.
Inorganic Chemistry, Vol. 47, No. 12, 2008 5297

Kreisel et al.
Table 3. Selected Interatomic Distances (Å) and Angles (°) for (HLiPr)2Cr (5)
Distances (Å)
Cr-N(1)
2.035(4)
1.406(12)
Cr-N(2)
2.030(4)
1.382(12)
N(1)-C(13)
1.334(10)
N(2)-C(16A)
1.328(11)
102.3(2)
C(13)-C(14)
C(15A)-C(16A)
Angles (°)
N(1)-Cr-N(2A)
N(1)-Cr-N(1A)
N(2)-Cr-N(2A)
81.8(2)
81.0(2)
159.5(2)
N(1)-Cr-N(2)
Table 4. Selected Interatomic Distances (Å) and Angles (°) for Li(THF)4[(HLiPr)2Cr] (6)
Distances (Å)
Cr-N(1)
N(1)-C(13)
C(13)-C(14)
2.013(2)
1.382(3)
1.344(4)
Cr-N(2)
N(2)-C(14)
C(39)-C(40)
2.001(2)
1.377(3)
1.348(4)
Cr-N(3)
N(3)-C(39)
1.995(2)
1.375(3)
Cr-N(4)
N(4)-C(40)
2.007(2)
1.379(3)
Angles (°)
N(1)-Cr-N(3)
N(2)-Cr-N(4)
N(1)-Cr-N(2)
N(2)-Cr-N(3)
81.46(9)
99.27(9)
158.6(1)
159.4(1)
N(1)-Cr-N(4)
N(3)-Cr-N(4)
105.81(9)
81.02(9)
Scheme 3. Synthesis of the Cr Halide Dimer, Complex 4
2.330(2) and 2.345(2)Å, whereas the Cr-N distances are
longer than in the previous examples at 2.013(2) and
2.020(2)Å. 7 apparently contains a reduced diimine ligand
much like that of 4, with C-N distances of 1.346(3) and
1.347(3) Å and a C-C distance of 1.374(4) Å. Using the
same reasoning as before, 7 should be considered a high spin
Cr(II) with an antiferromagnetically coupled ligand-centered
radical, identical to the situation for each paramagnetic center
in 4. The room temperature effective magnetic moment of 7
was 3.9(1) µB, consistent with an S ) 3/2 system. Using this
value, a fit of the variable temperature magnetic data for 4
with S1, 2 ) 3/2 gave a coupling constant of J ) -17 cm-1.24
Like the THF-d8 solutions of 1-3, complex 4 exhibited a
simple paramagnetic 1H NMR spectrum in C6D6 with
resonances at 17.7, 1.56, and -17.3 ppm. 4 can also bind
one or two THF ligands. A crystal structure of the unsym-
metric, mono-THF complex (4 · THF) can be found in the
Supporting Information. The mono-THF complex can be
5 and Table 5 show the results of the X-ray diffraction study.
The Cr atom in 7 is set in a slightly distorted square planar
geometry with coordination by two terminal chloride ligands
and a diimine ligand. Similar to the previously described
halide complexes 1-4, the Cr-Cl bond distances in 7 are
1
identified by H NMR resonances at 17.2, 2.07, 0.45, and
-18.9 ppm while the bis-THF complex featured resonances
at 17.9, 6.52, 0.70, and -16.9 ppm. We believe that the
latter complex should also be dimeric by comparison to
the structurally similar ꢁ-diketiminate chromium complex
[((iPr2C6H3)2nacnac)Cr(µ-Cl)(THF)]2.26
Electronic Structures of Complexes 1-7. Owing to the
redox properties of diimine ligands, the assignment of the
electronic structures of the diimine ligands, and the associated
metal oxidation states in 1-7, are somewhat problematic. Table
6 shows the C-N and C-C bond distances of the diimine
ligands for 1-7, along with the bond distances for the three
oxidation levels of HLiPr.7c,27 With the exception of complexes
6 and 8 (vide infra), all of the C-N bond distances fall within
0.021 Å of each other, and the C-C distances are within 0.032
Å of one another. Comparison of these distances with the values
for the free ligand makes it apparent that the diimine ligands
are reduced roughly to the extent of a one electron reduced,
ligand-centered radical. Examination of the C-N and C-C
bond lengths of 6 reveal that its diimine ligands have a different
electronic structure than the other diimine complexes; the
ligands here are even further reduced.27
Figure 4. ORTEP plot of Li(THF)4[(HLiPr)2Cr] (6) at the 30% probability
+
level. The Li(THF)4 cation and hydrogen atoms have been omitted for
clarity.
Figure 5. ORTEP plot of one of the two independent molecules of
Et4N[(HLiPr)CrCl2] (7) at the 30% probability level. The Et4N+ cation and
hydrogen atoms have been omitted for clarity.
(26) Gibson, V. C.; Newton, C.; Redshaw, C.; Solan, G. A.; White, A. J. P.;
Williams, D. J. Eur. J. Inorg. Chem. 2001, 1895.
(27) Chen, R.; Tatsumi, K. J. Coord. Chem. 2002, 55, 1219.
5298 Inorganic Chemistry, Vol. 47, No. 12, 2008

Diimine Complexes of Chromium
Table 5. Selected Interatomic Distances (Å) and Angles (°) for One of the Two Independent Molecules of Et4N[(HLiPr)CrCl2] (7)
Distances (Å)
Cr(1)-Cl(1)
N(1)-C(13)
2.330(2)
1.346(3)
Cr(1)-Cl(2)
N(2)-C(14)
2.345(2)
1.347(3)
Cr(1)-N(1)
C(13)-C(14)
2.020(2)
1.374(4)
Cr(1)-N(2)
2.013(2)
Angles (°)
N(1)-Cr(1)-N(2)
Cl(1)-Cr(1)-Cl(2)
Cl(2)-Cr(1)-N(2)
94.83(4)
168.14(7)
79.2(1)
Cl(1)-Cr(1)-N(1)
168.71(7)
Table 6. C-N and C-C Bond Distances (Å) for Complexes 1′-8,
tally determined structure of 6 (see Table 4), including the
ligand dihedral angle about the metal (calc ) 28.1°, exp )
32.3°). A population analysis of 6′ showed three singly
occupied, mostly metal-based orbitals and one orbital of
mixed metal/ligand character. In addition, two ligand-based
MOs were found to be populated: one doubly occupied,
entirely ligand-based MO and one singly occupied MO that
includes some metal character (see Figure 7).
-2
HLiPr, [HLiPr
compound
]
-1, and [HLiPr
]
C-N distance
C-C distance
HLiPr
1.263(2)
1.32(1), 1.33(1)
1.423(4), 1.409(4), 1.403(4), 1.431(4) 1.352(4), 1.355(4)
1.328(5), 1.343(5), 1.328(5), 1.334(5) 1.386(6), 1.396(6)
1.347(2), 1.343(2)
1.326(5), 1.340(5)
1.331(5), 1.350(6), 1.339(6), 1.342(5) 1.387(6), 1.389(6)
1.334(10), 1.328(11) 1.406(12)
1.382(3), 1.377(3), 1.375(4), 1.379(3) 1.344(4), 1.348(4)
1.346(3), 1.347(3)
1.298(3), 1.297(3)
1.468(2)
1.41(1), 1.40(1)
a
-1
[HLtBu*
]
b
[HLiPr
1′-dimer
]
-2
2
3
4
5
6
7
8
1.407(2)
1.385(6)
Finally, a geometry optimization of 7′ produced metric
parameters that were generally in excellent agreement
(∆Cr-N < 0.03 Å, ∆C-N < 0.02 Å, ∆C-C < 0.02 Å)
with the X-ray data (see Figure 5 and Table 5), except for
the Cr-Cl distances, which were poorly reproduced with a
∆Cr-Cl of nearly 0.1 Å. A population analysis of 7′ gave
results that resembled those for 5′, namely a Cr(II) ion (d4,
S ) 2) coupled to a ligand-centered radical, giving an overall
1.374(4)
1.433(3)
a From reference 7c; t-butyl groups are bound directly to the diimine
nitrogen atoms. b From reference 27.
To gain a more complete understanding of the electronic
structures of these molecules, density-functional theory
(DFT) calculations were carried out on bis-ligand complexes
5 and 6, as well as dichloride 7 at the B3LYP/6-311 g level
using simplified model complexes in which the 2,6-diiso-
propylphenyl groups were replaced with methyl groups, that
is, (HLMe*)2Cr (5′), [(HLMe*)2Cr]- (6′) and [(HLMe*)CrCl2]-
(7′).28 A spin-unrestricted calculation on 5′ resulted in metric
parameters that were in good agreement (∆Cr-N < 0.02
Å, ∆C-N < 0.03 Å, ∆C-C < 0.01 Å) with the X-ray data
of 5 (see Table 3). Even with the sterically less demanding
ligands, the calculated dihedral angle between the diimine
ligands about the Cr atom was 29.0°, very close to the
experimental value of 31.5°, indicating an electronic prefer-
ence for this particular geometry. A self-consistent field
(SCF) population analysis showed four singly occupied
metal-based orbitals in the spin-up configuration while each
of the diimine ligands carried one electron in the spin-down
configuration (see Figure 6). The Mulliken spin density also
supported this description, with ∼3.9 electrons at Cr and
∼1.9 electrons of opposite spin distributed over the diimine
ligands. With respect to the oxidation state formalism, these
calculations nicely corroborate our hypothesis that 5 indeed
contains Cr(II) coordinated by two ligand-centered diimine
radicals.
3
spin state of S ) /2 (see Figure 8).
One caveat to the electronic description of 7′ is that the
metal-based orbitals are not pure; they are mixed with
p-orbitals of the chloride ligands. Nevertheless, the calculated
Mulliken spin density shows ∼3.9 electrons centered at Cr
with ∼0.9 electrons spread over the diimine ligand and
carrying the opposite spin. Thus, it would seem that 7, and
by extension 4, are only formally Cr(I) complexes; an
As suggested earlier, the electronic structure of anionic 6
is more complex and at least two electronic situations are
conceivable. The first couples Cr(II) with one ligand-centered
radical anion and one dianionic, enediamide ligand. The
second posits Cr(III) coordinated by two enediamide ligands.
Both descriptions would be consistent with the experimen-
tally confirmed quartet spin ground state. Once again, a spin-
unrestricted structure optimization of 6′ gave metric param-
eters that were in good agreement (∆Cr-N e 0.025 Å,
∆C-N < 0.025 Å, ∆C-C < 0.025 Å) with the experimen-
Figure 6. Qualitative MO diagram of 5′ showing the singly occupied
molecular orbitals (SOMOs, labeled with percentage of major contribution)
that give rise to the S ) 1 spin state of 5.
(28) The calculations were performed using Gaussian 03 software: Frisch,
M. J.; et al. Gaussian 03, revision B.05; Gaussian, Inc.: Pittsburg,
PA, 2003 (see Supporting Information for complete citation).
Inorganic Chemistry, Vol. 47, No. 12, 2008 5299

Kreisel et al.
Figure 9. ORTEP plot of (HLiPr)Cr(CO)4 (8) at the 30% probability level.
Hydrogen atoms have been omitted for clarity.
with strong antiferromagnetic coupling between the two. For
dinuclear 4, one consequence is that the weak magnetic
coupling between the metal centers should be modeled
assuming quartet ground states (S ) 3/2) of the two
interacting halves of the dimer. The apparent success of this
end justifies the means.
Figure 7. Qualitative MO diagram of 6′ showing the SOMOs (labeled
To further explore the electron accepting nature of diimine
ligands, we have synthesized the octrahedral tetracarbonyl
complex (HLiPr)Cr(CO)4 (8), the assignment of which as a
Cr(0) diimine complex should be noncontroversial (see
Figure 9 and Table 7 for the structure determination).29 The
C-N and C-C bond distances of the diimine ligand in 8
effectively split the difference between those in the free
(neutral) ligand and the radical anion (see Table 6), while
the CO stretching frequencies in the IR spectrum of 8 reveal
a blue shift from νCO,ave ) 1913 cm-1 in the starting material,
Cr(CO)4(NCCH3)2, to νCO,ave ) 1932 cm-1 in 8. This shift
indicates less back-bonding from the metal to the CO ligands
in 8 than in the starting material. Presumably, the diimine
ligand is effectively competing with the CO ligands as a
π-acid, being reduced to some extent. The “real” oxidation
state of chromium in 8 is left for the reader to ponder.
3
with percentage of major contribution) that give rise to the S )
state of 6.
/ spin
2
Conclusion
We have prepared several chromium diimine complexes
in formal oxidation states of the metal ranging from II to -I.
These apparently low-valent complexes are stabilized by the
electron-withdrawing nature of the diimines. We have shown
that the π-acidity of these ligands allows for redistribution
of electron density from the Cr center to the diimine ligand.
The extent of reduction of the diimine ligands in the
complexes varies anywhere between slight and approaching
transfer of two electrons. The concomitant oxidation of the
metal center creates some conceptual discomfort, as the
assignment of metal oxidation state is no longer trivial. It is
well to remember that the very concept of the latter is an
(over)-simplificationsinteger numbers cannot capture the
minute shifts of electron density engendered by charge,
coordination number, geometry, coligands, and so forth.
Figure 8. Qualitative MO diagram of 7′ showing the SOMOs (labeled
3
with percentage of major contribution) that give rise to the S )
state of 7.
/ spin
2
incrementally better description of both complexes acknowl-
edges electron transfer from the metal to the easily reducible
diimine ligands. As the assignment of an (integer) oxidation
state of the metal is considered desirable and useful, a
description most closely approximating the real distribution
of the valence electrons is appropriate. For 7, and 4, that
representation involves Cr(II) ions and diimine radical anions,
(29) tom Dieck, H.; Kuehl, E. Z. Naturforsch. B 1982, 37B, 324.
5300 Inorganic Chemistry, Vol. 47, No. 12, 2008

Diimine Complexes of Chromium
Table 7. Selected Interatomic Distances (Å) and Angles (°) for One of the Two Independent Molecules of (HLiPr)Cr(CO)4 (8)
Distances (Å)
Cr-N(1)
Cr-C(28)
N(1)-C(13)
2.051(2)
1.863(2)
1.298(3)
Cr-N(2)
Cr-C(29)
N(2)-C(14)
2.049(2)
1.911(2)
1.297(3)
Cr-C(27)
1.890(2)
1.865(2)
1.433(3)
Cr-C(30)
C(13)-C(14)
Angles (°)
N(1)-Cr-C(27)
N(2)-Cr-C(30)
N(1)-Cr-N(2)
N(2)-Cr-C(29)
C(28)-Cr-C(30)
75.78(6)
97.83(8)
97.5(1)
97.54(8)
93.11(8)
N(1)-Cr-C(28)
C(27)-Cr-C(29)
93.70(8)
158.4(1)
d8, 1.73 and 3.58 ppm; CD2Cl2, 5.32 ppm; CDCl3, 7.27 ppm). FTIR
spectra were taken on a Mattson Alpha Centauri spectrometer with
a resolution of 4 cm-1. UV/vis spectra were taken on a Hewlett-
Packard 8453 spectrophotometer. Elemental analyses were per-
formed by Desert Analytics. Room-temperature magnetic suscep-
tibility measurements were carried out using a Johnson Matthey
magnetic susceptibility balance unless otherwise stated.
However, to the inorganic, and especially coordination
chemist, oxidation state (along with dn configuration) is a
powerful predictive device. It is therefore defensible, and
even profitable, to utilize a variety of experimental and
computational techniques to “determine” the oxidation state
that most closely reflects the electronic structure of any given
compound. The result reported herein should be viewed in
that context.
With these starting materials in hand and a grasp of their
electronic structure in place, we plan to extend our prepara-
tive work to organometallic derivatives. We hope that in
doing so we will be able to exploit the synergy between the
metal and the diimine ligand and to discover interesting
molecules and reactions. The first fruit of our labors, in the
form of a Cr-Cr quintuple bond, has recently been com-
municated.24 We believe that there is more where it came
from.
Preparation of [(HLiPr)Cr]2(µ-Cl)3(Cl)(THF) (1′). To a slurry
of CrCl2 (0.034 g, 0.276 mmol) in THF (10 mL) was added 0.104
g of HLiPr (0.276 mmol) at RT. The brown solution was allowed to
stir overnight after which time the THF was removed, and the
residue was crystallized from Et2O at -30 °C to give 1 in 82%
1
yield (0.121 g). H NMR (CD2Cl2): 16.1 (br), 0.514 (br), -1.93
(br), -7.42 (br), -21.7 (vbr) ppm. 1H NMR (THF-d8): 20.028 (2H,
aromatic), 0.217 (12H, iPr), -1.978 (12H, iPr), -10.274 (4H, iPr)
ppm. IR (KBr; cm-1): 3055 (m), 2946 (s), 2923 (s), 2863 (s), 1460
(s), 1443 (s), 1381 (m), 1359 (m), 1321 (m), 1248 (m), 1220 (m),
1175 (w), 1107 (w), 856 (m), 798 (m), 755 (m). Anal. Calcd. for
C56H80N4Cr2Cl4O: C, 62.79; H, 7.54; N, 5.23. Found: C, 62.65; H,
7.47; N, 5.00. UV/vis (THF; λmax, nm (ε, M-1cm-1)): 550 (2507),
638 (1067). µeff (294 K) ) 3.9 (1) µB or 2.8(1) µB per Cr. Mp: 192
°C dec.
Experimental Section
General Considerations. All manipulations were carried out
using standard Schlenk, vacuum line, and glovebox techniques.
Pentane, diethyl ether, tetrahydrofuran, and toluene were distilled
from purple Na benzophenone/ketyl solutions. THF-d8 was predried
over potassium metal and stored under vacuum over Na/K. CD2Cl2
was predried with P2O5 and stored under vacuum over 4 Å
molecular sieves. C6D6 was predried with sodium metal and stored
under vacuum over Na/K. CrCl3 (anhydrous), CrCl2 (anhydrous),
and sodium metal were purchased from Strem Chemical Co., and
Et4NCl was purchased from Acros Organics. ((Trimethylsilyl)m-
ethyl) 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. CrCl3(THF)3,30 Cr(CO)4(NCCH3)2
(Caution! Pyrophoric),31 and all diimine ligands were prepared
by literature procedures.32 The synthesis of [(HLiPr)Cr(µ-Cl)]2 has
been communicated recently.24
The AC susceptibility measurements were performed using the
ACMS accessory for a Quantum Design PPMS (Physical Properties
Measurement System).33 The ACMS consists of a primary winding
that causes the AC excitation field, as well as a pair of secondary
windings. Each data point is the result of five measurements of the
sample inside the instrument in a “five-point BTBCC (Bottom-
Top-Bottom-Center-Center)” configuration. The samples were run
at a constant field of 1 T.
Preparation of (MeLMe)CrCl2(THF)2 (2). To a slurry of CrCl2
(0.067 g, 0.545 mmol) in THF (10 mL) was added 0.159 g of MeLMe
(0.545 mmol) at RT. The brown solution was allowed to stir
overnight after which time the THF was concentrated and cooled
to -30 °C overnight to give 2 in 42% yield (0.128 g) after washing
with pentane. 1H NMR (THF-d8): 16.4 (2H, aromatic), -6.66 (12H,
aryl Me), -9.52(6H, backbone Me) ppm. IR (KBr; cm-1): 3011
(w), 2962 (s), 2931(s), 1465 (s), 1426 (m), 1377 (m), 1309 (w),
1257 (w), 1221 (s), 1162 (w), 1095 (w), 987 (w), 765 (m). UV/vis
(THF; λmax, nm (ε, M-1cm-1)): 551 (304), 641 (229). Mp: 188 °C
dec. Elemental analysis and magnetic measurements were not
performed because of the inability to separate this complex from
CrCl2 produced by an, as of yet, uncharacterized ligand dissociation
equilibrium.
Preparation of (HLMe)CrCl2(THF)2 (3). To a slurry of CrCl2
(0.048 g, 0.390 mmol) in THF (10 mL) was added 0.103 g of HLMe
(0.390 mmol) at RT. The brown solution was allowed to stir
overnight after which time the THF was concentrated and cooled
to -30 °C overnight to give 3 in 77% yield (0.160 g) after washing
with pentane. 1H NMR (THF-d8): 21.0 (2H, aromatic), -11.1 (12H,
aryl Me) ppm. IR (KBr; cm-1): 3053 (m), 2946 (s), 2925 (s), 2855
(m), 1460 (s), 1444 (s), 1380 (m), 1357 (m), 1258 (m), 1234 (s),
1179 (w), 1097 (w), 1017 (m), 915 (w), 878 (m), 859 (m), 768
(m). Anal. Calcd. for C26H36N2CrCl2O2: C, 58.75; H, 6.84; N, 5.27.
Found: C, 58.67; H, 6.51; N, 5.34. UV/vis (THF; λmax, nm (ε,
M-1cm-1)): 550 (832), 644 (356). µeff (294 K) ) 2.9(1) µB. Mp:
200 °C dec.
NMR spectra were taken on a Bruker DRX-400 spectrometer
and were referenced to the residual protons of the solvent (THF-
(30) Herwig, W.; Zeiss, H. H. J. Org. Chem. 1958, 9, 1404.
(31) Tate, D. P.; Knipple, W. R.; Augl, J. M. Inorg. Chem. 1962, 1, 433.
(32) (a) tom Dieck, H.; Svoboda, M.; Greiser, T. Z. Naturforsch. B 1981,
36B, 823. (b) Arduengo, A. J., III; Krafczyk, R.; Schmutzler, R.; Craig,
H. A.; Goerlich, J. R.; Marshall, W. J.; Unverzagt, M. Tetrahedron
1999, 55, 14523.
Preparation of (HLiPr)2Cr (5). Solid Na (0.029 g, 1.261 mmol)
was added to a THF (25 mL) solution of HLiPr (0.469 g, 1.245
mmol). After stirring for 1 day, the deep red solution was slowly
added to 0.075 g of CrCl2 (0.610 mmol) suspended in 5 mL of
THF. The reaction was allowed to stir at RT overnight. The resulting
(33) For details see: http://www.qdusa.com/products/ppms.html (accessed
April 16, 2008).
Inorganic Chemistry, Vol. 47, No. 12, 2008 5301

Kreisel et al.
Table 8. Crystallographic Data for 1-3 and 5-8
1′
2
3
5
6
7
8
formula
fw
space group
color
a, Å
b, Å
C63H97.5Cl4-Cr2N4O2.75 C28H40Cl2-CrN2O2 C26H36Cl2-CrN2O2 C52H72-CrN4 C68H106Cr-LiN4O4 C38H66Cl2-CrN3O C30H36Cr-N2O4
1200.75
P21/n
559.52
531.47
P21/n
805.14
C2/c
1102.51
P21/n
703.84
540.61
P21/n
j
P1
j
P1
green
brown
brown
9.533(2)
22.328(5)
12.676(3)
90
105.701(3)
90
2597.6(10)
4, 1
red
green
green
blue
15.3679(15)
40.969(4)
22.124(2)
90
108.121(2)
90
8.231(5)
11.436(7)
15.594(9)
77.112(8)
78.381(9)
82.698(8)
1396.3(14)
2, 1
24.802(10)
9.081(3)
22.496(9)
90
112.71(1)
90
22.054(9)
13.047(5)
23.299(9)
90
103.602(6)
90
11.389(8)
17.618(12)
22.492(16)
70.338(11)
81.626(12)
77.203(12)
4132(5)
4, 2
10.908(3)
13.495(4)
20.150(6)
90
100.339(3)
90
c, Å
R, deg
ꢁ, deg
γ, deg
V, Å3
Z, Z′
13239(2)
8, 2
4674(3)
4, 1/2
6516(4)
4, 1
2918.0(14)
4, 1
D(calcd), g cm-3 1.205
1.331
1.359
1.144
1.124
1.131
1.231
µ(Mo KR), mm-1 0.533
0.628
0.672
0.283
0.223
0.436
0.427
temp, K
120
120
120
120
120
120
120
no. data/params 11 185/988
19 82/161
1.021
4.29
15 1/76
1.037
6.50
13 91/152
1.047
8.53
19 41/45
1.070
7.92
22 769/811
1.014
5.81
17 7/57
1.015
4.23
GOF on F2
R(F), %a
Rw(F2), %a
a
1.078
4.61
7.61
4.46
7.66
10.64
9.71
11.28
5.72
2
2
2
Quantity minimized: Rw(F2) ) ∑[w(Fo - Fc )2]/∑[(wFo )2]1/2; R ) ∑∆/∑(Fo), ∆ ) |Fo - Fc|.
brown solution was filtered and washed with copious amounts of
THF followed by solvent removal. The crude material was washed
with Et2O and dried under vacuum to give 5 in 67% yield (0.333
g). Crystals of 5 can be grown from concentrated THF solutions at
-30 °C. 1H NMR (THF-d8): 16.6 (8H, aromatic), 0.11 (24H, iPr),
-1.70 (24H, iPr), -11.4 (8H, iPr), -14.2 (4H, aromatic) ppm. IR
(KBr; cm-1): 3058 (w), 2959 (s), 2920 (m), 2866 (m), 1458 (m),
1438 (s), 1382 (w), 1360 (w), 1315 (m), 1250 (m), 1220 (m), 1178
(w), 1097 (w), 797 (m), 755 (m). Anal. Calcd. for C52H72N4Cr: C,
77.56; H, 9.03; N, 6.46. Found: C, 77.40; H, 8.96; N, 6.72. UV/vis
(THF; λmax, nm (ε, M-1cm-1)): 510 (4000), 730 (3297). µeff (300
K) ) 2.7(1) µB (1 T, PPMS). Mp: 189 °C dec.
8.62; N, 6.56. UV/vis (THF; λmax, nm (ε, M-1cm-1)): 492 (4684),
632 (5588). µeff (294 K) ) 3.9(1) µB. Mp: 178 °C dec.
Preparation of [(HLiPr)Cr(CO)4 (8). To a THF (20 mL) solution
of 0.104 g of Cr(CO)4(NCCH3)2 (0.423 mmol) was added 0.160 g
(0.426 mmol) of HLiPr. The resulting blue solution was allowed to
stir overnight. The THF was then removed, and the crude material
taken up in pentane. The pentane was concentrated and cooled to
-30 °C to give 8 in 81% yield (0.186 g). 1H NMR (CDCl3): 8.050
(s, 2H, backbone), 7.217 (m, 6H, aromatic), 2.787 (m, 4H, iPr)
1.314 (d, 12H, iPr), 1.064 (d, 12H, iPr) ppm. IR (KBr; cm-1): 3060
(w), 2964 (s), 2924 (m), 2865 (w), 2004 (vs), 1933 (vs), 1905 (vs),
1885 (vs), 1655 (w), 1436 (s), 1359 (w), 1323 (w), 1306 (w), 1178
(w), 1047 (w), 754 (w), 622 (m), 612 (m). Anal. Calcd. for
C30H36N2CrO4: C, 66.65; H, 6.71; N, 5.18. Found: C, 66.24; H,
6.71; N, 5.19. UV/vis (THF; λmax, nm (ε, M-1cm-1)): 608 (6211).
Mp: 98 °C dec.
Preparation of Li(THF)4[(HLiPr)2Cr] (6). A THF (20 mL)
solution of 5 (0.180 g, 0.224 mmol) was cooled to -30 °C after
which 0.031 g (0.330 mmol) of LiCH2SiMe3 was added. The green
solution was allowed to stir for 2 h. The THF was removed, and
the crude material washed with pentane, taken up in Et2O and
filtered. The filtrate was concentrated, layered with pentane, and
cooled to -30 °C to give 6 in 71% yield (0.175 g). 1H NMR (THF-
d8): 15.5 (8H, aromatic), 10.2 (24H, iPr), 1.73 (24H, iPr), -6.28
(8H, iPr), -7.47 (4H, aromatic) ppm. IR (KBr; cm-1): 3049 (w),
2954 (s), 2922 (m), 2866 (w), 1544 (m), 1459 (s), 1431 (s), 1376
(m), 1314 (s), 1245 (s), 1205 (m), 1178 (s), 1112 (m), 1041 (s),
886 (w), 846 (w), 798 (w), 756 (m). Anal. Calcd. for
C68H104N4CrO4Li: C, 74.20; H, 9.54; N, 5.09. Found: C, 72.31; H,
8.96; N, 5.52. UV/vis (THF; λmax, nm (ε, M-1cm-1)): 492 (1029),
741 (1464). µeff (300 K) ) 3.9(1) µB (1 T, PPMS). Mp: 119 °C
dec. Elemental analysis consistently gave results that were low in
carbon and hydrogen content and high in nitrogen content. This is
presumably due to decomposition of the lithium coordinated THF.
Cation exchange with Et4NCl produced an intractable material that
could not be purified.
Preparation of Et4N[(HLiPr)CrCl2] (7). To a THF (20 mL)
solution of 4 (0.319 g, 0.688 mmol) was added 0.342 g (2.065
mmol) of Et4NCl. The solution was allowed to stir overnight. The
THF was then removed, and the crude material taken up in Et2O
and filtered. The green filtrate was concentrated and cooled to -30
°C to give 7 in 88% yield (0.381 g). 1H NMR (THF-d8): 18.6 (2H,
aromatic), 1.01 (24H, iPr), -20.8 (4H, iPr) ppm. IR (KBr; cm-1):
3050 (w), 2957 (s), 2927 (m), 2862 (m), 1655 (w), 1441 (s), 1393
(w), 1358 (w), 1321 (m), 1254 (s), 1219 (m), 1174 (m), 1102 (w),
1055 (w), 1000 (w), 797 (m), 758 (m). Anal. Calcd. for
C34H56N3CrCl2: C, 64.84; H, 8.98; N, 6.67. Found: C, 64.64; H,
Crystallographic Structure Determinations. A summary of
crystal data collection and refinement parameters for compounds
1′-3 and 5-8 can be found in Table 8. Suitable crystals were
selected, mounted with viscous oil and cooled to 120 K. Data
were collected on a Bruker-AXS APEX CCD diffractometer
using graphite monochromated Mo KR radiation (λ ) 0.71073
nm). Unit cell parameters 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.34 No symmetry higher than triclinic was
observed for complexes 2 and 7. Systematic absences in the
diffraction data and unit cell parameters are consistent with P21/n
for complexes 1′, 3, 6, and 8. The C-centered space group C2/c
was found to be consistent for complex 5. In all of the structures,
the centrosymmetric space group options yielded chemically
reasonable and computationally stable results of refinement.
Structures were solved using 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 complexes 1′ and 7. Two structures display
cocrystallized solvent molecules: 1′ has 1.75 molecules of
disordered Et2O per asymmetric unit that have been modeled as
diffuse contributions,35 and 7 has a Et2O molecule per asym-
metric unit. Except for compound 3, the isopropyl moieties
(34) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112.
(35) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7.
5302 Inorganic Chemistry, Vol. 47, No. 12, 2008

Diimine Complexes of Chromium
display varied degrees of unresolvable disorder resulting in large
Ueq ranges, and apparent Hirshfeld test failures. Complex 5
suffers severe disorder including a disordered diimine backbone
that has been modeled such that the C-C and C-N bond
distances are chemically reasonable. Various attempts to grow
crystals in other solvents all resulted in the same disorder.
Compound 8 consistently deposits as weakly diffracting, multiple
crystals resulting to less than ideal data coverage. All non-
hydrogen atoms were refined anisotropically. All hydrogen atoms
were treated as idealized contributions. All structure factors are
included in the SHELXTL program library.34 Details of crystal
structure data are available from the Cambridge Structural
Database under depository numbers: 1′, 644255; 2, 644260; 3,
644263; 5, 644257; 6, 644258; 7, 644262; 8, 644256; and
[(HLiPr)Cr(µ-Cl)]2(THF) (i. e., 4 · THF), 644259.
Acknowledgment. We thank Mr. Thomas Ekiert and
Professor Karl M. Unruh for variable temperature magnetic
measurements and the University of Wisconsin-Madison’s
Chemistry Computer Center for computational assistance.
This research was supported by a grant from the Nation
Science Foundation (Grant CHE-0616375 to K.H.T.).
Supporting Information Available: X-ray crystallographic files
in CIF format for complexes 1-3, 5-8, full structural data for
4·THF, the details of the DFT calculations of 5′, 6′ and 7′, and
the complete citation for the Gaussian 03 software package (PDF).
This material is available free of charge via the Internet at
http://pubs.acs.org.
IC800302W
Inorganic Chemistry, Vol. 47, No. 12, 2008 5303