Amine, amido, and imido complexes of tantalum supported by a pyridine-linked bis(phenolate) pincer ligand: Ta - N π-bonding influences pincer ligand geometry

Article abstract of DOI:10.1021/ic802234x

A series of tantalum imido and amido complexes supported by a pyridine-linked bis(phenolate) ligand has been synthesized. Characterization of these complexes via X-ray crystallography reveals both C_s and C ₂ binding modes of the bis(

Full text of DOI:10.1021/ic802234x

5096 Inorg. Chem. 2009, 48, 5096–5105
DOI: 10.1021/ic802234x
Amine, Amido, and Imido Complexes of Tantalum Supported by a Pyridine-Linked
Bis(phenolate) Pincer Ligand: Ta-N π-Bonding Influences Pincer Ligand
Geometry
Ian A. Tonks, Larry M. Henling, Michael W. Day, and John E. Bercaw*
Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology, Pasadena,
California 91125
Received November 25, 2008
A series of tantalum imido and amido complexes supported by a pyridine-linked bis(phenolate) ligand has been synthesized.
Characterization of these complexes via X-ray crystallography reveals both Cs and C2 binding modes of the bis(phenolate)
pyridine ligand, with complexes containing two or fewer strong π-donor interactions from ancillary ligands giving Cs
symmetry, whereas three strong π-donor interactions (e.g., three amido ligands or one amido ligand and one imido ligand)
give C2-symmetric binding of the bis(phenolate)pyridine ligand. DFT calculations and molecular orbital analyses of the
complexes have revealed that the preference for Cs-symmetric ligand binding is a result of tantalum-phenolate π-bonding,
whereas in cases where tantalum-phenolate π-bonding is overridden by stronger Ta-N π-bonding, C2-symmetric ligand
binding is preferred, likely because conformationally this is the lowest-energy arrangement. This electronically driven change
in geometry indicates that, unlike analogous metallocene systems, the bis(phenolate)pyridine pincer ligand is not a strong
enough π-donor to exert dominant control over the electronic and geometric properties of the complex.
Introduction
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defined, mono- and polydentate “non-metallocene” ligand sets
to support a diverse range of organometallic complexes and
transformations, catalysis, and small molecule activation
studies.1-13 Nonmetallocene ligand sets offer a wide variety of
symmetries and donor groups; these traits are particularly
desirable for developing catalysts capable of affecting new
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*To whom correspondence should be addressed. E-mail: bercaw@caltech.edu.
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2009 American Chemical Society
pubs.acs.org/IC
Published on Web 05/07/2009

Article
Inorganic Chemistry, Vol. 48, No. 12, 2009 5097
Scheme 1
a R, R0 = alkyl or aryl; linker = thiophene, furan, pyrrole, pyridine, NHC, or phenyl; L = C (neutral or anionic), N, O, or S.
Recently, our group34 and others35,36 have been investigat-
achieving a wider range of symmetries (C1, C2, C2v, and Cs)
depending on the twist angles of the phenolate ligands
(Scheme 1).35
ing the use of arene- and heterocycle-linked bis(phenolate)
donor ligand sets (heterocycle = pyridine, furan, thiophene)
to support titanium and zirconium polymerization cata-
lysts34 and as ancillary ligands for tantalum to explore other
organometallic transformations.37,38 These nonmetallocene
ligand sets are connected through sp2-sp2 aryl-aryl, or
aryl-heterocycle linkages39 instead of more flexible
sp3-sp3 linkages, imparting increased rigidity of the back-
bone, which could result in more thermally robust catalysts
that are less prone to undergo ligand C-H activation.36 One
advantage of these LX2-type ligand systems is that they
easily accommodate higher oxidation states commonly
found for compounds of the early transition metals. Addi-
tionally, these metal complexes with bis(phenolate)donor
ligands are particularly attractive, because they have frontier
orbitals similar to metallocenes38 yet could be capable of
With the intent to further develop the organometallic
chemistry of early transition metals supported by these
pincer ligands, a series of tantalum amine, amido, and imido
complexes has been synthesized. These complexes have
been particularly instructive for furthering our understanding
of the importance of phenolate-metal π-bonding on
the preferred [(ONO)Ta] symmetry and have provided
insights into the differences between the [(ONO)Ta]
and [Cp2Ta] platforms. Herein, we describe the synthesis
of an unusual amino-amido-imido tantalum species
supported by a pyridine-linked bis(phenolate) ligand and
discuss its structural preferences in relation to other
new dimethylamido and phenylimido tantalum complexes
having the bis(phenolate)pyridine ([(ONO)Ta]) ligand
platform.
(30) Mehrkhodavandi, P.; Schrock, R. R. J. Am. Chem. Soc. 2001, 123,
10746–10747.
Results
(31) Mehrkhodavandi, P.; Schrock, R. R.; Pryor, L. L. Organometallics
2003, 22, 4569–4583.
(32) Nakayama, Y.; Saito, H.; Ueyama, N.; Nakamura, A. Organome-
tallics 1999, 18, 3149–3158.
(33) Sernetz, F. G.; Mulhaupt, R.; Fokken, S.; Okuda, J. Macromolecules
Reaction of (ONO)TaMe3 (ONO = pyridine-2,6-bis
(4,6-tBu2-phenolate)) with 3 equiv of aniline at 90 °C over
the course of 12 h yields the imido-amido-amino tanta-
lum complex, (ONO)Ta(NPh)(NHPh)(NH2Ph) (1) (eq 1).40
Complex 1 crystallizes from a concentrated solution of
benzene upon cooling from 90 °C to room temperature and
was characterized by X-ray crystallography.
1997, 30, 1562–1569.
(34) Agapie, T.; Henling, L. H.; DiPasquale, A. G.; Rheingold, A. L.;
Bercaw, J. E. Organometallics 2008, 27, 6245–6246.
(35) (a) Chan, M. C. W.; Tam, K. H.; Zhu, N. Y.; Chiu, P.; Matsui, S.
Organometallics 2006, 25, 785. (b) Chan, M. C. W.; Tam, K. H.; Pui, Y.
L.; Zhu, N. Y. J. Chem. Soc., Dalton Trans. 2002,
3085–3087.
(40) In all of the schemes in this paper, amido (LX) ligands will be
represented as double bonds and imido ligands (LX2) as triple bonds since
these representations most accurately describe the bonding within these
systems (as evidenced by simple electron counting and DFT calculations),
despite this notation not being the normal convention. Additionally, all Ta-
O bonds will be drawn as single bonds, although some of the Ta-O bonds in
these complexes are best described as having three-center-four-electron
bonds and bond orders greater than 1. See Table 1 for a full description of the
Ta-O bond order in all of these complexes.
(36) (a) Sarkar, S.; Abboud, K. A.; Veige, A. S. J. Am. Chem. Soc. 2008,
130, 16128–16129. (b) Sarkar, S.; Carlson, A. R.; Veige, M. K.; Falkowski, J.
M.; Abboud, K. A.; Veige, A. S. J. Am. Chem. Soc. 2008, 130,
1116–1117.
(37) Agapie, T.; Bercaw, J. E. Organometallics 2007, 26, 2957–2959.
(38) Agapie, T.; Day, M. W.; Bercaw, J. E. Organometallics 2008, 27,
6123–6142.
(39) Steinhauser, S.; Heinz, U.; Sander, J.; Hegetschweiler, K. Z. Anorg.
Allg.Chem. 2004, 630, 1829–1838.

5098 Inorganic Chemistry, Vol. 48, No. 12, 2009
Tonks et al.
C2 twist of the ligand. Additionally, the pyridine linker of
1 binds in a much more linear fashion: the Ta1-N4-C9
angle is 170.8°, whereas in the Cs-symmetric cases the angle
is typically 150-160°. Interestingly, the NH protons on the
1
L-type aniline appear as a broad singlet in the H NMR
despite being diastereotopic. We believe this is a result of
equilibration due to rapid interconversion of enantiomers
of the C2 geometry in solution, probably progressing through
a C2v-symmetric intermediate. The average Ta-O bond
˚
˚
distance in 1 is 2.002(1) A, which is significantly (∼0.1 A)
longer than the distances observed earlier.38 These structural
features prompted further investigation of the underlying
reasons for a preference for C2 versus Cs ligand geometry and
the preferences for the ligand trans to the pyridine in these
complexes.
A distinctive feature of 1 (an (ONO)Ta(L)(LX)(LX2)-type
complex), as opposed to all previously studied (ONO)TaX3
complexes, is the presence of three strong Ta-X π bonds, one
for the amido and two for the imido ligand. We therefore
hypothesized that these Ta-N π bonds were responsible for
the observed C2 rather than Cs ligand geometry. A series of
[(ONO)Ta] amides and imides with varying X-type, LX-type,
and LX2-type ligands, and hence differing numbers of Ta-N
π bonds, were synthesized (eq 2-4).
Figure 1. Thermal ellipsoid drawing of 1. Side view and top-down view
˚
showing C2 symmetric bisphenolate ligand. Selected bond lengths (A)
andangles(deg):Ta-O1, 2.0045(10);Ta-O2, 1.9999(11);Ta-N1, 2.4796
(13); Ta-N2, 2.0271(11); Ta-N3, 1.7865(13); Ta-N4, 2.3126(10);
Ta-N4-C34, 127.21(9); Ta-N2-C40, 139.07(10); Ta-N3-C46,
176.07(11). N-bound hydrogens in calculated positions; all others omitted
for clarity.
(ONO)H2 reacts cleanly with Ta(NMe2)5 at room tem-
perature in benzene to yield the tris(amide) complex (ONO)
Ta(NMe2)3 (2). The 1HNMR spectrum of 2 shows two sharp
singlets at 2.975 and 3.741 ppm in a 12:6 ratio, indicative of
two distinctive types of dimethylamido groups;one type cis
to pyridine and the othertrans, respectively. In contrast to the
trimethyl complex38 (ONO)TaMe3, 2 shows no fluxional
exchange between the dimethylamido groups even at elevated
temperatures, likely a result of the increased preference for
octahedral rather than trigonal prismatic geometry (the
proposed intermediate for ligand site exchange),38 resulting
from strong TadN π-bonding. X-ray-quality crystals of 2
were grown from a saturated diethyl ether solution cooled
to -30 °C. The crystal structure (Figure 2) of 2 reveals that
the bis(phenolate)pyridine ligand binds to tantalum meri-
dionally in a C2-symmetric fashion, as expected for a complex
The Ta-N bond lengths for 1 (Figure 1) are indicative of
˚
three types of Ta-N bonding: 2.480(1) A for the L-type
˚
˚
aniline, 2.027(1) A for the LX-type anilide, and 1.786(1) A for
the LX2-type phenylimide. The bis(phenolate)pyridine ligand
binds in a meridonal fashion, with the anilide ligand trans to
the pyridine linker and aligned perpendicular to the O-Ta-
O plane;opposite alignment to what was observed in the
benzylidene complex (ONO)Ta(CHPh)(Bn)(PR3).38 This
meridional binding mode is typical for early metal complexes
of the bis(phenolate)pyridine ligand; however, it is the first
six-coordinate tantalum complex to exhibit a C2-symmetric
binding of the ligand instead of the typically observed Cs-
symmetric binding, as, for example, for the trimethyl tanta-
lum starting material in eq 1. The dihedral angle of 58.7°
between the two phenolate groups indicates a significant

Article
Inorganic Chemistry, Vol. 48, No. 12, 2009 5099
Figure 2. Thermal ellipsoid drawing of 2. Front view, top-down view showing C2 symmetry, and side-on view showingthe bis(phenolate)pyridinedihedral
˚
angle. Selected bond lengths (A) and angles (deg): Ta1-O1, 1.9908(18); Ta1-O2, 1.9788(18); Ta-N1, 2.3111(23); Ta1-N2, 2.0325(18); Ta1-N3, 1.9941
(24); Ta1-N4, 2.0289(18); Ta1-N1-C9, 178. H atoms admitted for clarity.
with three strong Ta-N π bonds, similar to complex 1. In
fact, the bis(phenolate)pyridine ligand in 2 shares all of the
new characteristic properties observed for 1. The dihedral
angle between the two phenyl rings of the bis(phenoxide)
ligand is 59.7°, again indicating a significant C2 twist of the
ligand. The Ta1-N1-C9 angle is 178°, indicating an almost
linear binding of the pyridine linker, and finally, the average
implicating a greater degree of phenolate π-bonding in this
complex. Indeed, this is expected, as one phenolate π bond is
required to complete an 18-electron count at tantalum.
Reaction of 1 equiv of aniline with 3 in benzene at 90 °C
for one week quantitatively yields the phenylimide chlo-
ride, (ONO)Ta(NPh)(HNMe2)Cl (4). A similar reaction
with excess aniline yields (ONO)Ta(NPh)(H2NPh)Cl (5),
although only limited crystallographic identification of
5 has been obtained due to twinned, unstable crystals.
X-ray-quality crystals of 4 were grown by slow evaporation
of a saturated benzene solution. Complex 4, as for 3, has a
Cs-symmetric, meridionally bound bis(phenolate)pyridine
ligand (Figure 4). It is interesting to note that one of the
phenolate arms is twisted 13.3° away from perfect Cs sym-
metry, possibly due to crystal packing forces. The pyridine
linker is canted out of the O-Ta-O plane by 23.3°, which is
typical in the Cs-symmetric binding mode. The Ta-N dis-
˚
Ta-O bond distances are long at 1.985(2) A. The Ta-N
bond lengths of the amides cis to pyridine are slightly longer
than that for the amide trans to pyridine, 2.029(2) and 2.032
˚
(2) versus 1.994(2)A, respectively, indicative of a slightly
larger amide trans influence, but well within the range of
Ta-N double bonds.
When complex 2 is treated with 1 equiv of TMSCl in
benzene, the monochlorinated product (ONO)Ta(NMe2)2Cl
(3) is generated quantitatively over the course of four days at
room temperature, or in 2 h at 90 °C (see Figure 3). Complex
3 exhibits sharp singlets at 3.029 and 3.919 ppm correspond-
ing to the dimethylamido groups cis and trans to pyridine,
respectively, and similar to complex 2, no fluxionality is
observed even at elevated temperatures. X-ray-quality crys-
tals of 3 were grown by slow evaporation from a saturated
diethyl ether solution. The bis(phenolate)pyridine ligand in
complex 3 binds meridionally, as for 1 and 2, but in a Cs-
symmetric fashion where the mirror plane in the molecule
bisects the phenolate-tantalum-phenolate angle. One of the
dimethylamides lies cis to the pyridine, while the other is
trans, consistent with 1H NMR, and their bond lengths are
˚
tance of 1.791(3) A is typical for tantalum imides, and the
Ta1-N3-C36 angle of 174.2° indicates that it is an LX2-type
˚
donor. The average Ta-O bond distance of 1.954(2) A is
again shorter than observed in complexes 1 and 2 and more
comparable to those in complex 3, indicating some degree of
Ta-O multiple-bond character.
Reaction of an excess of HNMe2 with (ONO)TaCl2Me in
benzene rapidly forms the mono(dimethylamido) complex,
(ONO)Ta(NMe2)MeCl (6), along with the concomitant pre-
cipitation of H2NMe2Cl. X-ray-quality crystals were ob-
tained by cooling a saturated solution of 6 in diethylether
to -30 °C overnight. Complex 6 (Figure 5) displays the usual
meridonal binding of the bis(phenolate)pyridine ligand and,
like complexes 3-5, is Cs-symmetric. The Ta-O average
bond length is 1.887(2), and the Ta-N distance of 1.975(2)
˚
˚
1.979(2) A and 1.982(2) A, respectively;again, consistent
with TadN double bonds. The average Ta-O phenolate
˚
bond distances in 3 are 1.924(3) A, significantly shorter
than seen in 1 and 2 (2.002(1) and 1.985(2) A, respectively),
˚

5100 Inorganic Chemistry, Vol. 48, No. 12, 2009
Tonks et al.
1
for dimethylamide is typical of a TadN double bond.
Interestingly, the π-bonding amide in 6 lies cis to the pyridine
linker, whereas in the previously reported benzylidene38 the
π-bond lies trans to pyridine. The H NMR spectrum is
consistent with a cis amide as well, as the dimethylamide
singlet resonates at 3.214 ppm, in the range of the cis
dimethylamide peaks seen in complexes 2 and 3. Because a
tantalum dπ orbital (dyz, vide infra) is available in both cis
and trans positions (vide infra), there does not appear to be a
strong electronic preference. Thus, we attribute this cis
preference of the amide to steric interactions for the alternate
geometry; the bulkiest ligand (NMe2) would be best accom-
modated in one of the cis positions. In the benzylidene
example, the π-bonding benzylidene is the least bulky sub-
stituent. This could also be a result of the strongest trans-
influencing ligand (Me or Bn) preferring to be trans to the
weak pyridine ligand.
Discussion
From the structural data for complexes 1, 2, 3, 4, and 6, as
well as for other previously reported tantalum complexes
having the bis(phenolate)pyridine ancillary ligand, it is ap-
parent that the number of strong tantalum-nitrogen π bonds
for the remaining three ligands significantly affects the degree
of Ta-O π-bonding for two phenolates. The amount of Ta-
O π-bonding can be quantified by examining the distances in
the solid-state structures (Table 1). In complexes where there
is no π-bonding, that is, the remaining three ligands are X-
˚
type, the average Ta-O bond length is roughly 1.9 A. As the
number of π-bonding ligands is increased, a corresponding
increase in the Ta-O bond length is also observed, reaching
˚
as high as 2.0 A in cases where there are three strong π-
donating ancillary ligands, as for example is the case with 1
(L, LX, and LX2 ligands) and 2 (three LX ligands). Thus, it is
apparent that the Ta-O bond order decreases as additional
strong π-bonders are introduced into the system;going
from a Ta-O bond order of 2 in the case of zero π-donating
ligands down to an order of 1 when there are three strong
π-donating ligands. The rather small change in the Ta-O
˚
bond length over all of the compounds (∼0.1 A) is likely due
to the generally poor π-donating ability of electronegative
oxygen and the inherent rigidity of the system, which should
limit the overall change in bond lengths.
From a molecular orbital perspective, a Cs-symmetric bis
(phenolate)pyridine ligand should be able to π-bond to the
metal center using lone pairs on each oxygen, making bonds
Figure 3. Thermal ellipsoid drawing of 3. Front view (left) and top-down
view (right) showing the Cs symmetry of the bis(phenolate)pyridine ligand.
Selected bond lengths (A) and angles (deg): Ta1-O1, 1.9282(15); Ta1-O2,
1.9207(17); Ta-N1, 2.3922(18); Ta1-N2, 1.9708(22); Ta1-N3, 1.9848(21);
Ta1-Cl1, 2.4686(6); Ta-N1-C9, 147.78(2). H atoms omitted for clarity.
˚
Figure 4. Thermal ellipsoid drawing of 4. Front view (left) and top-down view (right) showing the Cs geometry of the bis(phenolate)pyridine ligand.
˚
Selected bond lengths (A) and angles (deg): Ta1-O1, 1.9437(14); Ta1-O2, 1.9643(13); Ta-N1, 2.2918(17); Ta1-N2, 2.4568(17); Ta1-N3, 1.7841(14);
Ta1-Cl1, 2.3920(5); Ta-N1-C9, 156.82(8) Ta-N3-C36, 174.2(2). H atoms removed for clarity.

Article
Inorganic Chemistry, Vol. 48, No. 12, 2009 5101
Figure 5. Thermal ellipsoid drawing of 6. Front view (left) and top-down view (right) showing Cs symmetric binding of the bis(phenolate)pyridine ligand.
˚
Selected bond lengths (A) and angles (deg): Ta1-O1, 1.8936(25); Ta1-O2, 1.8810(26); Ta-N1, 2.3916(31); Ta1-N2, 1.9746(37); Ta1-C34, 2.1908(51);
Ta1-Cl1, 2.4683(11); Ta-N1-C9, 149.6(2). H atoms removed for clarity.
Table 1. Observed Solid State Ligand Symmetries and Average Ta-O Bond Length
in Selected Tantalum (Bis)phenolate Complexes
energetically unfavorable filled-filled repulsions. Second,
this twist could be a geometric relaxation effect; the
C2 symmetry can better accommodate the large tantalum
# of
TadL
average
TaO bond d(TaO)ave
atom because twisting lengthens its Ta-O bonds. As
evidenced by the canted pyridine rings, Cs symmetry, while
increasing Ta-O bonding, also forces an unnatural
Ta-pyridine bond length. The implication of either explana-
tion is that any oxygen π-bonding encourages Cs-symmetric
binding, since the overlap between the tantalum d orbitals
and occupied oxygen p orbitals is essentially lost in the
C2 geometry.
L3 for (ONO)TaL3
symmetry π bonds
ordera
(A)
˚
(NPh)(NHPh)(NH2Ph) (1)
(NMe2)3 (2)
(NMe2)2Cl (3)
C2
C2
Cs
Cs
Cs
Cs
Cs
Cs
3
3
2
2
2
1
1
0
1
1
2.002(1)
1.985(2)
1.924(3)
1.954(2)
1.948(3)b
1.887(2)
1.922(1)
1.906(1)
1.5
1.5
1.5
2
(NPh)(HNMe2) Cl (4)
(NPh)(NH2Ph) Cl (5)
(NMe2)(CH3)Cl (6)
(dCHPh)(CH2Ph)(PR3)c
2
2d
DFT calculations (B3LYP, 6-31G**, LANL2DZ Ta pseu-
dopotential) were performed on complex 1 in order to affirm
the bonding description described above. A structural opti-
mization starting from the crystal structure coordinates was
performed and produced a structure very similar to the
experimentally observed one. The calculated HOMO,
HOMO-1, and HOMO-2 distinctly show the anilide-dxz,
imide-dyz, and imide-dxy π-bonding interactions, consistent
with the bond distances observed in the crystal structure
(Figure 6). Notably absent in the calculated results is any
phenolate-tantalum three-center-four-electron π-bonding,
which is observed in other calculations performed on com-
plexes supported by this ligand set.38 The absence of pheno-
late π-bonding is, however, not entirely surprising; assuming
that the phenylimide and anilide are stronger π donors than
the phenolate, no phenolate π-bonding would be necessary to
complete the 18-electron count at tanatalum. This bonding
picture, however, is very different from its earlier-reported
metallocene counterpart Cp*2Ta(dNPh)(H).42 In the metal-
locene case, rather than displace the strong Cp*-Ta bonds,
the nitrogen lone pair remains nonbonding;demonstrating
one key difference between the bis(phenolate)pyridine
and metallocene ligand systems: unlike metallocenes, pheno-
lates are not strong enough π donors to effectively com-
pete with strong π-donating ancillary ligands such as imido
and amido.
b
(CH3)3
a To complete the 18-electron count at Ta. b Complex not anisotro-
pically refined. c Taken from ref 38. d A 16-electron maximum.
with dxz and dxy on tantalum (the bonding linear combina-
tion of oxygen pz’s (and py’s) and tantalum dxz (and dxy) are
shown at the top of Scheme 2). In the case of the mono
(amide) 6 and the previously reported benzylidene complex,38
where there exists only one ancillary ligand π bond, these
Ta-O π interactions force the amide (or benzylidene) ligand
to π-bond with dyz, despite the steric consequence of forcing
the amide methyls or phenyl of the benzylidene toward a
phenolate tert-butyl group(s). However, in the case where
there are two ligand π bonds such as the bis(amide) 3 or the
phenylimide-chlorides 4 and 5, one of the two Ta-O π bonds
must be sacrificed to make the second ancillary ligand π-
bond. For 4 and 5 (shown in Scheme 2), the remaining Ta-O
π interaction occurs through dxz; for 3 (not shown), interac-
tion occurs with dxy.41 Finally, in the case where there are
three strong ancillary ligand π bonds, all Ta-O π-bonding is
precluded to accommodate the other ligands. In these cases,
the bis(phenolate) ligand twists from Cs symmetry to C2
symmetry. This symmetry switch is likely due to two factors.
First, twisting to C2 symmetry reduces overlap between the
oxygen p orbitals and the filled N-to-Ta π bonds, reducing
In order to address the possibility of steric repulsion
driving the Cs-to-C2 geometry switch, DFT calculations
(41) One might question why the imido ligand of complexes 4 and 5 does
not occupy the site trans to pyridine in order to preserve the phenolate-
to-dxy π-bonding. We do not fully understand the preference for a cis
imido, but the weakest trans influencing L-type amine ligand may direct
the imido.
(42) Parkin, G.; van Asselt, A.; Leahy, D. J.; Whinnery, L.; Hua, N. G.;
Quan, R. W.; Henling, L. M.; Schaefer, W. P.; Santarsiero, B. D.; Bercaw, J.
E. Inorg. Chem. 1992, 31, 82–85.

5102 Inorganic Chemistry, Vol. 48, No. 12, 2009
Tonks et al.
(B3LYP, 6-31G**, LANL2DZ Ta pseudopotential) were
also carried out on simpler complexes with significantly
less steric bulk than the synthesized complexes. In all of
these cases, the ortho and para tert-butyl groups in the ligand
backbone were removed. Calculations were performed on
(ONO)Ta(NH3)(NH2)(NH) and (ONO)Ta(NMe2)3, which
are expected to be C2-symmetric according to the bonding
arguments above. The expected C2 symmetry is obtained
upon optimization of both C2v and C2 unoptimized starting
structures. Similarly, calculations on (ONO)Ta(NH2)2Cl,
(ONO)Ta(NH3)(NH)Cl, and (ONO)Ta(NMe2)2Cl, expec-
ted to display Cs symmetry, also yielded this symmetry
upon optimization from either C2v or Cs unoptimized
starting structures. The optimized structures from these
calculations are shown in Figure 7. It is noteworthy that,
in (ONO)Ta(NMe2)3 and (ONO)Ta(NMe2)2Cl, which
differ from their synthesized analogues only by removal
of the tert-butyl bulk, the optimized structures do not
dimethylamide is twisted away from perpendicular. We
attribute this twist to a small steric effect which is also seen
in complexes 2 and 3; it is notable that, while the amide
ligands rotate to reduce steric repulsion, no perturbation of
˚
significantly (<0.01 A Ta-O bond length and <0.7°
phenolate torsion angle) differ from the structures obtained
from X-ray diffraction. Additionally, small steric effects can
be seen by comparing the optimized structures of (ONO)Ta
(NH2)2Cl and (ONO)Ta(NMe2)2Cl. In the case of (ONO)Ta
(NH2)2Cl, the NH2 trans to pyridine lies perpendicular to
the O-Ta-O plane, while in (ONO)Ta(NMe2)2Cl, the
Figure 6. DFT calculations (B3LYP, 6-31G**, LANL2DZ Ta pseudo-
potential)showing HOMO, HOMO-1, and HOMO-2 (L-R), representing
the three Ta-N π bonds in complex 1.
Scheme 2

Article
Inorganic Chemistry, Vol. 48, No. 12, 2009 5103
The bis-μ-oxo complex 7 represents the first crystallogra-
phically characterized tantalum bis(phenolate)pyridine com-
plex that exhibits a facial binding of the ligand (Figure 8).
This complex, along with a related titanium complex,43
supports the proposed intermediacy of a facially bound
isomer in the exchange of methyl groups in (ONO)TaMe3,
as proposed earlier.38
Conclusions
A series of imido and amido complexes supported by
a pyridine-linked bis(phenolate) ligand have been synthe-
sized and structurally characterized via NMR and X-ray
crystallography. These complexes exhibit either a Cs- or
C2-symmetric meridional binding of the bis(phenolate)
ligand. The preferred geometry appears to be determined
by the degree of Ta-O π-bonding in the phenolate ligands.
When Ta-O π bonds are required to complete the 18-
electron count, the bis(phenolate)pyridine ligand binds in a
Cs-symmetric fashion. In cases where strong π donation from
ancillary ligands precludes Ta-O π-bonding (that is, when
there are three strong π-donor interactions from the other
ligands), the bis(phenolate) ligand twists in a C2 fashion to
reduce filled-filled repulsions between the Ta-X π bonds
and the oxygen lone pairs and leads to a less strained (ONO)
geometry. This electronically driven change in geometry
indicates that, unlike analogous metallocene systems, the
bis(phenolate) pincer ligand is not a strong enough π donor
to exert total control over the electronic and geometric
properties of the complex.
Figure 7. DFT (B3LYP, 6-31G**, LANL2DZ Ta pseudopotential)
optimized structures (see text) of calculated complexes displaying
C2- and Cs-symmetric structures with significantly smaller ligands.
Experimental Section
General Considerations and Instrumentation. All air- and
moisture-sensitive compounds were manipulated using stan-
dard high-vacuum and Schlenk techniques or were manipulated
in a glovebox under a nitrogen atmosphere. Solvents for air- and
moisture-sensitive reactions were dried over sodium benzophe-
none ketyl and stored over titanocene, where compatible, or
dried by the method of Grubbs et al.44 Benzene-d6 and toluene-
d8 were purchased from Cambridge Isotopes and dried over
sodium benzophenone ketyl, while methylene chloride-d2, also
purchased from Cambridge Isotopes, was dried over CaH2 and
filtered through a plug of activated alumina. TaCl5 purchased
from Strem Chemicals was sublimed prior to use. Amines were
purchased from Sigma-Aldrich and were distilled from CaH2
and degassed prior to use. All other materials were used as
received. 1H and 13C spectra were recorded on Varian Mercury
300 or Varian INOVA 500 spectrometers, and chemical shifts
are reported with respect to residual protio-solvent impurity
Figure 8. Thermal ellipsoid drawing of 7. Front view showing fac-
˚
binding of the bis(phenolate)pyridine ligand. Selected bond lengths (A):
Ta1-O1, 1.9502(5); Ta1-O2, 1.9622(5); Ta-O3, 1.9431(5); Ta1-O3b,
1.9328(5); Ta1-O4, 2.0733(8); Ta1-N1, 2.3120(5); Ta1-Ta1b, 2.9917
(1). H atoms removed for clarity.
the (ONO) framework is observed. These results, as well as
earlier observations that other quite bulky complexes based
on this ligand set such as (ONO)Ta(Bn)(CHPh)(PMe2Ph)
still exhibit Cs symmetry,38 lead us to believe that, at least
when examining similar series of compounds, the steric
demands of the ligands are not a major cause of geometric
rearrangement.
Attempts to make the analogous tantalum oxo series
of compounds have been unsuccessful to date. Instead,
the reaction of (ONO)TaMe3 with degassed H2O yields
oxo-bridged dimer products (eq 5).
1
for H (s, 7.16 ppm for C6D5H; t, 5.32 ppm for CDHCl2) and
solvent carbons for 13C (t, 128.39 for C6H6; s, 20.40 for CD2-
Cl2). 2,6-(HOC6H2-tBu2)2C5H3N, (2,6-(OC6H2-tBu2)2C5H3N)
(43) Golisz, S.; Bercaw, J. Unpublished results.
(44) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518.

5104 Inorganic Chemistry, Vol. 48, No. 12, 2009
Tonks et al.
TaMe3, and (2,6-(OC6H2-tBu2)2C5H3N)TaCl2Me were prepared
as previously reported.38 Despite repeated attempts, acceptable
elemental analyses for complexes 1, 4, and 5 were not obtained,
likely a result of the labile L donors or the loss of some solvent
from the crystal lattices.
reaction was stirred for 4 h, resulting in a mustard-colored
precipitate and a darker yellow solution. After 4 h, the solvent
and HNMe2 were removed in vacuo, yielding 311 mg (95%
yield) of 2 as a yellow solid. X-ray-quality crystals were obtained
by the cooling of a saturated Et2O solution of 3 to -30 °C
overnight. 1H NMR (300 MHz, C6D6): δ 1.429 (s, 18H, C
(CH3)3); 1.749 (s, 18H, C(CH3)3); 2.975 (s, 12H, Ta-(N-
(CH3)2)2); 3.741 (s, 6H, Ta-N(CH3)2) 7.078 (t, 1H, 4-C5NH3);
7.227 (d, 2H, 3,5-C5NH3); 7.520 (d, 2H, aryl-H); 7.637 (d, 2H,
aryl-H). 13C NMR (125 MHz, C6D6): δ 30.67 (C(CH3)3); 32.48
(C(CH3)3); 34.88 (C(CH3)3); 36.09 (C(CH3)3); 45.28 (N(CH3)2);
47.57 (N(CH3)2); 123.26, 123.99, 124.99, 127.52, 137.76, 138.57,
140.76, 155.14, 159.96 (aryl). Anal. calcd for C39H61N4O2Ta:
C, 58.63; H, 7.70; N, 7.01. Found: C, 58.40; H, 7.10; N, 6.79%.
(2,6-(OC6H2-tBu2)2C5H3N)Ta(NMe2)2Cl (3). In an in-
ert atmosphere glovebox, 2 (69 mg, 0.087 mmol, 1 equiv) was
dissolved in 4 mL of C6H6, and 11 μL of TMSCl (0.087 mmol, 1
equiv) was syringed in. The reaction was sealed and stirred for
48 h. After the reaction was complete, volatile coproducts were
removed in vacuo, quantitatively yielding 3 as a yellow solid. X-
ray-quality crystals were grown from the slow evaporation of a
saturated Et2O solution of 3. 1H NMR (300 MHz, C6D5CD3): δ
1.325 (s, 18H, C(CH3)3); 1.763 (s, 18H, C(CH3)3); 3.029 (s, 6H,
N(CH3)2); 3.919 (s, 6H, N(CH3)2); 7.121 (t, 1H, 4-C5NH3);
7.338 (d, 2H, aryl-H); 7.477 (d, 2H, 3,5-C5NH3); 7.704 (d, 2H,
aryl-H). 13C NMR (125 MHz, C6D6): δ 31.06 (C(CH3)3); 32.19
(C(CH3)3); 34.99 (C(CH3)3); 36.09 (C(CH3)3); 47.59 (N(CH3)2);
49.45 (N(CH3)2); 123.65, 126.57, 128.95, 138.69, 139.29, 143.37,
154.62, 157.88 (aryl). Anal. calcd for C37H55ClN3O2Ta: C,
56.23; H, 7.02; N, 5.32. Found: C, 57.09; H, 6.33; N, 4.74%.
(2,6-(OC6H2-tBu2)2C5H3N)Ta(NPh)(HNMe2)Cl (4).
Complex 3 (11 mg, 0.014 mmol, 1 equiv) and aniline (1.3 μL,
0.014 mmol, 1 equiv) were mixed together in a J-Young NMR
tube with 0.7 mL of C6D6. The vessel was sealed and heated to
90 °C in an oil bath. The reaction was monitored by 1H NMR,
and after 3 days at 90 °C, the reaction was complete, as
confirmed by the disappearance of the Ta-NMe2 peaks. The
solvent and HNMe2 was removed in vacuo, yielding 4 quantita-
tively. X-ray-quality crystals were grown from slow evapora-
tion of a saturated solution of 4 in benzene. 1H NMR (300 MHz,
C6D6): δ 1.37 (s, 18H, C(CH3)3); 1.71 (s, 6H, N(CH3)2); 1.78
(s, 18H, C(CH3)3); 6.33 (d, 2H, aryl-H); 6.46 (t, 1H, aryl-H); 6.88
(t, 2H, aryl-H); 6.94 (t, 1H, 4-C5NH3); 7.25 (d, 2H, 3,5-C5NH3);
7.30 (d, 2H, aryl-H); 7.74 (d, 2H, aryl-H). 13C NMR (125 MHz,
C6D6): δ 30.8 (C(CH3)3); 32.3 (C(CH3)3); 34.9 (C(CH3)3); 36.0
(C(CH3)3); 38.5 (N(CH3)2); 123.3, 123.9, 125.2, 126.3, 127.2,
127.3, 127.8, 138.6, 139.3, 142.2, 156.4, 158.2, 159.5 (aryl).
(2,6-(OC6H2-tBu2)2C5H3N)Ta(NPh)(NH2Ph)Cl (5).
Complex 3 (12 mg, 0.0152 mmol, 1 equiv) and aniline (2.8 μL,
0.031 mmol, 2.1 equiv) were mixed together in a J-Young NMR
tube with 0.7 mL of C6D6. The vessel was sealed and heated to
90 °C in an oil bath. The reaction was monitored by 1H NMR,
and after 3 days at 90 °C, the reaction was complete, as
confirmed by the disappearance of the Ta-NMe2 peaks. The
solvent, excess aniline, and HNMe2 were removed in vacuo,
yielding 5 quantitatively. X-ray-quality crystals were grown
from slow evaporation of a saturated solution of 5 in toluene.
1H NMR (300 MHz, C6D6): δ 1.37 (s, 18H, C(CH3)3); 1.77
(s, 18H, C(CH3)3); 2.92 (br s, 2H, NH2); 6.31 (2d, 4H, aryl-H);
6.46 (t, 1H, aryl-H); 6.69 (t, 1H, aryl-H); 6.88 (t, 2H, aryl-H);
6.98 (m, 3H, 4-C5NH3 + aryl-H); 7.25 (d, 2H, 3,5-C5NH3); 7.29
(d, 2H, aryl-H); 7.73 (d, 2H, aryl-H). 13C NMR (125 MHz,
C6D6): δ 30.9 (C(CH3)3); 32.2 (C(CH3)3); 34.9 (C(CH3)3); 36.1
(C(CH3)3); 123.3, 123.5, 125.2, 125.4, 126.3, 127.1, 127.2, 127.8,
129.6, 138.6, 139.1, 139.3, 142.2, 157.3, 158.2, 159.4 (aryl).
Computational Details. Density functional calculations
were carried out using Gaussian 03, revision D.01.45 Calcula-
tions on the model systems (with minimal steric bulk) were
performed using the nonlocal exchange correction by Becke46,47
and nonlocal correlation corrections by Perdew,48 as implemen-
ted using the b3lyp49,50 keyword in Gaussian. The following
basis sets were used: LANL2DZ51-53 for Ta atoms and 6-31G**
basis set for all other atoms. Pseudopotentials were utilized for
Ta atoms using the LANL2DZ ECP. All optimized structures
were verified using frequency calculations and did not contain
any imaginary frequencies. Iso-surface plots were made using
the Gaussian 03, revision D.01 program.45
(2,6-(OC6H2-tBu2)2C5H3N)Ta(NPh)(NHPh)(NH2Ph)
(1). In an inert atmosphere glovebox, a 25 mL Schlenk flask
fitted with a Teflon screwcap valve was charged with (2,6-
(OC6H2-tBu2)2C5H3N)TaMe3 (100 mg, 0.141 mmol, 1 equiv)
and 10 mL of C6H6. PhNH2 (38.5 μL, 0.4225 mmol, 3 equiv) was
syringed in, and the vessel was sealed and placed in a 90 °C bath
for 14 h. After 14 h, the solvent was removed in vacuo, and the
resulting yellow solid was washed with petroleum ether and
collected on a sintered glass funnel as 68 mg (51% yield) of a
1
white powder. H NMR (300 MHz, CD2Cl2): δ 1.065 (s, 1H,
Ta-NHPh); 1.394 (s, 18H, C(CH3)3); 1.428 (s, 18H, C(CH3)3);
4.106 (s, 2H, Ta-NH2Ph); 5.96 (d + t, 3H, aryl-H); 6.358
(d, 2H, aryl-H); 6.505 (t, 1H, aryl-H); 6.575 (d, 2H, aryl-H);
6.853 (t, 2H, aryl-H); 6.893 (t, 2H, aryl-H); 7.035 (t, 1H, aryl-H);
7.213 (t, 2H, aryl-H); 7.596 (s, 1H, aryl-H); 7.635 (s, 1H, aryl-H);
7.892 (d, 2H, 3,5-C5NH3); 8.038 (t, 1H, p-C5NH3). 13C NMR
(125 MHz, CD2Cl2): δ 29.87 (C(CH3)3); 31.71 (C(CH3)3); 34.62
(C(CH3)3); 35.38 (C(CH3)3); 118.47, 119.26, 119.43, 122.07,
123.51, 124.07, 124.31, 125.50, 126.68, 127.17, 127.47, 128.15,
129.59, 138.71, 139.29, 141.16, 141.34, 153.50, 115.83, 156.30,
159.73 (aryl). Anal. calcd for C51H61N4O2Ta: C, 64.96; H, 6.52;
N, 5.94. Found: C, 66.26; H, 6.44; N, 5.96%.
(2,6-(OC6H2-tBu2)2C5H3N)Ta(NMe2)3 (2). In an inert
atmosphere glovebox, 2,6-(HOC6H2-tBu2)2C5H3N (200 mg,
0.4115 mmol, 1 equiv) and Ta(NMe2)5 (165 mg, 0.4115 mmol,
1 equiv) were mixed in a 20 mL vial with 2 mL of Et2O. The
(45) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,
J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,
S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,
B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A.; Gaussian 03, revision
C.02; Gaussian, Inc.: Wallingford, CT, 2004.
(46) Becke, A. D. Phys. Rev. A: At., Mol., Opt. Phys. 1988, 38, 3098–3100.
(47) Becke, A. D. J. Chem. Phys. 1988, 88, 1053–1062.
(48) Perdew, J. P. Phys. Rev. B: Condens. Matter Mater. Phys. 1986, 33,
8800–8802.
(49) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B: Condens. Matter Mater.
Phys. 1988, 37, 785.
(2,6-(OC6H2-tBu2)2C5H3N)Ta(NMe2)MeCl (6). (2,6-
(OC6H2-tBu2)2C5H3N)TaCl2Me (159 mg, 0.212 mmol, 1 equiv)
was dissolved in 10 mL of C6Hc in an inert atmosphere glovebox,
transferred to a 50 mL round-bottom flask equipped with a
(50) Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Chem. Phys. Lett. 1989,
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Article
Inorganic Chemistry, Vol. 48, No. 12, 2009 5105
Table 2. Crystal and Refinement Data for Complexes 1, 2, and 3
Table 3. Crystal and Refinement Data for Complexes 4, 6, and 7
1
2
3
4
6
7
empirical
formula
C
51H61N4O2
Ta 11/2
(C6H6)
1060.15
100(2)
C39H61
N4O2Ta
C
37H55N3O2
ClTa 0.75
empirical
formula
fw
C
41H55N3O2
ClTa
838.28
100(2)
9.9628(5)
C36H52N2O2
ClTa C4H10
C
66H86N2
O8Ta2 4(CH2Cl2)
O
3
3
3
3
(C4H10O)
845.83
100(2)
14.7164(6)
18.3370(7)
31.2188(13)
835.32
100(2)
47.432(2)
13.7447(7)
12.4876(6)
1736.97
100(2)
16.0570(7)
12.8407(6)
18.4561(9)
fw
T (K)
798.87
100(2)
9.8149(5)
29.0840(14)
14.0826(6)
T (K)
˚
a, A
˚
˚
b, A
a, A
25.7250(8)
17.8546(5)
23.6141(7)
10.6118(5)
18.9240(9)
80.804(3)
89.355(3)
81.578(3)
1953.54(16)
2
˚
b, A
˚
c, A
˚
c, A
R, deg
β, deg
γ, deg
volume, A
Z
cryst syst
R, deg
β, deg
γ, deg
volume, A
Z
cryst syst
100.510(3)
103.542(3)
105.9790(10)
104.357(3)
101.802(2)
3
˚
8004.5(7)
8
monoclinic
C2/c
1.386
1.54 to 27.85
2.850
none
3699.5(3)
2
monoclinic
P21/c
1.559
1.95 to 51.56
3.297
semi emp.
1.629
0.0250, 0.0388
3
˚
10427.1(5)
8
monoclinic
C2/c
1.351
2.07 to 43.26
2.154
none
1.117
0.0299, 0.0520
3894.4(3)
4
monoclinic
P21/c
1.363
1.65 to 33.20
2.858
semi emp.
1.426
0.0350, 0.0448
8246.4(6)
8
triclinic
P1
1.425
monoclinic
P21/n
1.363
1.68 to 47.98
2.767
semi emp.
2.760
0.0731, 0.1059
space group
dcalc, g/cm3
θ range, deg
μ, mm1
space group
dcalc, g/cm3
θ range, deg
μ, mm1
1.97 to 35.14
2.919
none
1.281 1.678
0.0326, 0.0452 0.0358, 0.0563
abs. correction
GOF
abs. correction
GOF
R1, a wR2
b
R1, a wR2
[I > 2σ(I)]
b
P
P
P
P
[I > 2σ(I)]
a R1 = ||Fo|- |Fc||/ |Fo|. b wR2 = [ [w(Fo2 - Fc ) ]/ [w(Fo2)2]1/2
2 2
.
P
P
P
a R1 = ||Fo| - |Fc||/Σ|Fo|. b wR2 = [ [w(Fo2 - Fc ) ]/ [w(Fo2)2]1/2
.
2 2
X-Ray Crystal Data: General Procedure. Crystals were
removed quickly from a scintillation vial to a microscope slide
coated with Paratone N oil. Samples were selected and mounted
on a glass fiber with Paratone N oil. Data collection was carried
out on a Bruker KAPPA APEX II diffractometer with a 0.71073
Teflon needle valve, sealed, and degassed on a high-vacuum line.
A total of 4.7 mmol of degassed HNMe2 was then vac-trans-
ferred into the round-bottomed flask, and the reaction was
thawed and stirred for 1 h at room temperature. The orange
˚
solution turned dark yellow with
a white precipitate
A Mo KR source. The structures were solved by direct methods.
(H2NMe2Cl). After the reaction was complete, the reaction
was filtered through a sintered glass funnel in an inert atmo-
sphere glovebox, and the filtrate was dried in vacuo, yielding 148
mg (89.5%) of 6 as a yellow powder. 1H NMR (300 MHz,
C6D5CD3): δ 1.319 (s, 18H, C(CH3)3); 1.659 (s, 3H, Ta-CH3);
1.808 (s, 18H, C(CH3)3); 3.214 (s, 6H, N(CH3)2); 7.031 (t, 1H, 4-
C5NH3); 7.290 (d, 2H, aryl-H); 7.375 (d, 2H, 3,5-C5NH3); 7.767
(d, 2H, aryl-H). 13C NMR (125 MHz, C6D5CD3):δ 31.10 (C
(CH3)3); 32.10 (C(CH3)3); 35.05 (C(CH3)3); 36.03 (C(CH3)3);
48.69 (N(CH3)2); 48.90 (Ta-CH3); 123.77, 126.86, 126.96,
128.95, 139.14, 139.27, 144.86, 152.77, 157.98 (aryl). Anal. calcd
for C36H52ClN2O2Ta: C, 56.80; H, 6.89; N, 3.68. Found: C,
55.58; H, 6.08; N, 3.36%.
[(2,6-(OC6H2-tBu2)2C5H3N)Ta(OH)]2(μ-O)2 (7). In an
NMR tube, (2,6-(OC6H2-tBu2)2C5H3N)TaMe3 (7.7 mg,
0.011 mmol, 1 equiv) was dissolved in 1 mL of C6D6, and
4 equiv of H2O were quantitatively gas-transferred onto the
solution on a high vacuum line. The yellow solution imme-
diately turned clear with a significant amount of white
precipitate. The solvent was decanted off in an inert atmo-
sphere glovebox and the product dried in vacuo; crystals
were obtained from a saturated solution of the mixture of
products in CH2Cl2.
All non-hydrogen atoms were refined anisotropically. Some
details regarding refined data and cell parameters are available
in Tables 2 and 3. Selected bond distances and angles are
supplied in the corresponding figures.
Acknowledgment. We thank Dr. Nilay Hazari (Caltech) for
assistance with the DFT calculations. DFT calculations were
carried out using the Molecular Graphics and Computation
Facility, College of Chemistry, University of California, Berke-
ley, California, with equipment support from NSF grant CHE-
0233882. This work has been supported by USDOE Office of
Basic Energy Sciences (Grant No. DE-FG03-85ER13431). The
Bruker KAPPA APEXII X-ray diffractometer was purchased
via an NSF CRIF:MU award to the California Institute of
Technology, CHE-0639094.
Supporting Information Available: Tables of bond lengths,
angles, and anisotropic displacement parameters for the pre-
sented solid-state structures. Structural drawing of 5, which due
to its twinned nature cannot be refined anisotropically. X-ray
crystallographic data files (CIF) of compounds 1-7. This
material is available free of charge via the Internet at http://
pubs.acs.org.