Aryl-containing chelates and amine debenzylation to afford 1,3-di-2-pyridyl-2-azaallyl (smif): Structures of {κ-C,N,N py2-(2-pyridylmethyl)2N(CH2(4- tBu-phenyl-2-yl))}FeBr and (smif)CrN(TMS)2</su

Article abstract of DOI:10.1021/ic901329z

Aryl-bromide ligand precursors have been prepared with the potential to afford tetradentate chelates (2-pyridylmethyl)_3-xN(CH ₂-2-Aryl)_x (x = 1, 2) containing metal-aryl linkages that promise to impart stronger fields abou

Full text of DOI:10.1021/ic901329z

11576 Inorg. Chem. 2009, 48, 11576–11585
DOI: 10.1021/ic901329z
Aryl-Containing Chelates and Amine Debenzylation to Afford
1,3-Di-2-pyridyl-2-azaallyl (smif): Structures of {K-C,N,Npy -(2-
2
pyridylmethyl)2N(CH2(4-tBu-phenyl-2-yl))}FeBr and (smif)CrN(TMS)2
Brenda A. Frazier, Peter T. Wolczanski,* and Emil B. Lobkovsky
Department of Chemistry & Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853
Received July 9, 2009
Aryl-bromide ligand precursors have been prepared with the potential to afford tetradentate chelates (2-pyri-
dylmethyl)3-xN(CH2-2-Aryl)x (x = 1, 2) containing metal-aryl linkages that promise to impart stronger fields about first
row transition metals. Oxidative addition to Ni(COD)2 afforded two diamagnetic Ni(II) complexes, {κ-C,N,Npy-(2-
pyridylmethyl)N(CH2(4-tBu-phenyl-2-yl))(CH2(4-tBu-phenyl-2-Br))}NiBr (1-Ni) and {(κ-C,N,Npy2-(2-pyridylmethyl)2N-
(CH2(4-tBu-phenyl-2-yl))}NiBr (2-Ni) in 96% and 67% yield, respectively. Extending these synthetic efforts to iron
provided {κ-C,N,Npy2-(2-pyridylmethyl)2N(CH2(4-tBu-phenyl-2-yl))}FeBr (2-Fe, X-ray) in 91% yield via reduction of
an adduct, {κ-N,Npy2-(2-pyridylmethyl)2N(CH2(4-tBu-phenyl-2-Br))}FeBr2 (3-Fe). 5-Coordinate 2-Fe possessed a
pseudo-tbp structure, and SQUID magnetometry showed it to be S = 2 with significant zero field splitting (ZFS). 2-
Fe was initially prepared via oxidative addition to Fe{N(TMS)2}2(THF) upon disproportionation to “Fe(0)” and 2
Fe{N(TMS)2}3, but when this approach was attempted with Cr{N(TMS)2}2(THF)2, the azaallyl complex {κ-N,Npy
-
2
1,3-dipyridyl-2-azaallyl}CrN(TMS)2 ((smif)CrN(TMS)2, 4-Cr, X-ray), formed instead (>50%) via amine debenzylation.
An alternative route consisting of addition of 1,3-di-2-pyridyl-2-azapropene to Cr{N(TMS)2}2(THF)2 afforded 4-Cr in
74% yield. Pseudo-square planar 4-Cr was also S = 2 (SQUID) with marked ZFS. The dipyridylazaallyl ligand “smif”
imparts a remarkable optical density to 4-Cr via intraligand bands at 675 nm (ε ∼ 15 000 M-1cm-1) and 396 nm
(ε ∼ 27 000 M-1cm-1). The effective fields of the chelate complexes are discussed, and a comparison of smif to
isoelectronic NHC ligands is given.
Introduction
laboratories1-8 and others.9-17 Greater (n þ 1)s/ndz2 mixing
in the third row tends to decrease the density of states (DOS)
in 5d complexes, whereas lesser mixing in the 4d elements
results in a relatively greater DOS. As a consequence, the
path from reactant to product for a second row transition
metal species may utilize electronic surfaces other than those
of the respective ground states, thereby facilitating chemical
Electronic features that distinguish the structure and
reactivity of second row transition metals from their third
row congeners have been the focus of recent activity in these
*To whom correspondence should be addressed. E-mail: ptw2@cornell.
edu.
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Published on Web 09/21/2009
2009 American Chemical Society

Article
Inorganic Chemistry, Vol. 48, No. 24, 2009 11577
Figure 2. Initial ligand target precursors for heterolytic CH activation
(HAr2pyN and HArpy2N) and oxidative addition (BrAr2pyN and
BrArpy2N).
Scheme 1
Figure 1. Angular overlap arguments show that both the interaction
energy and orbital overlap favor C-based over N- and O-based ligands in
terms of field strength.
reactivity.1 Particular coordination geometries may also be
impacted by (n þ 1)s/ndz2 mixing; 4-coordinate Mo(IV) deri-
vatives have been shown to be pseudo-tetrahedral, while
related W complexes tend toward square planar.2,17
In principle, and certainly from a spectroscopic stand-
point,18 first row transition metal species should have the
greatest DOS within a column, and should be best suited for
chemical catalysis applications because of their anticipated
rapid reactivity. Unfortunately, first row transition metal
complexes are not as widely applied in catalysis even though
theyhave two significant advantages: (1) trace contamination
(e.g., drugs, food containers, etc.) is usually less of a health
concern when compared to second and third row species,
and (2) they are considerably less expensive.19-21 Utilization
of first row transition metal complexes is often problematic
because of weaker field strengths that promote a multitude
of states and 1e- changes, which can be detrimental to
catalysis.22-24 Critical catalytic events typically rely on the
formal shuttling of 2e-, such as in oxidative addition and
reductive elimination.21,24
arguments portray C-based orbitals as having very good
interaction energies because of their relative proximity to
appropriate metal orbitals.18,30 In contrast, N- and O-based
ligands are less well matched energetically because of their
electronegativity, and overlap arguments also favor carbon
(SC > SN > SO) for related reasons, thereby contributing to
the logic that carbon-based ligands are strong field in nature.
Herein are described some initial studies incorporating aryl
ligands into potential tetradentate chelates to help promote
stronger fields for first row transition metals. Figure 2 illus-
trates precursors to the ligands based on apical amine and
pyridine “hooks” to enable formation of the metal-aryl bond.
Two reaction types were envisioned for ligand attachment: (1)
heterolytic CH-bond activation to secure metal aryl bonds
from HAr2pyN and HArpy2N,31-34 and (2) oxidative addition
of aryl-Br bonds in BrAr2pyN and BrArpy2N.25 Both means of
attachment would lead to κ-C,C,N,Nax and κ-C,N,N,Nax liga-
tion under the appropriate scenarios. During the course of
these studies, the 1,3-di-2-pyridyl-2-azaallyl ligand35 was seren-
dipitously produced from an unusual C-N bond cleavage
process, an amine debenzylation. A related degradation repor-
ted by Westerhausen led to the characterization of (smif)2Zn.36
Carbon-based ligands can impart strong fields to a first row
transition metal.25-29 As Figure 1 illustrates, angular overlap
(18) Figgis, B. N.; Hitchman, M. A. Ligand Field Theory and Its Applica-
tions; Wiley-VCH: New York, 2000.
(19) de Meijere, A. Chem. Rev. 2000, 100, 2739–2740.
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13863.
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2009, 48, 1291–1295. (b) Dhuri, S. N.; Seo, M. S.; Lee, Y. M.; Hirao, H.; Wang,
Y.; Nam, W.; Shaik, S. Angew. Chem., Int. Ed. 2008, 47, 3356–3359.
(23) De Angelis, F.; Jin, N.; Car, R.; Groves, J. T. Inorg. Chem. 2006, 45,
4268–4276.
(24) For elucidation of catalysis involving 1e- changes, see: Poli, R.
Angew. Chem., Int. Ed. 2006, 45, 5058–5070.
(30) Richardson, D. E. J. Chem. Educ. 1993, 70, 372.
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J. Organomet. Chem. 2007, 692, 4774–4783.
(32) (a) Hull, K. L.; Anani, W. Q.; Sanford, M. S. J. Am. Chem. Soc. 2006,
128, 7134–7135. (b) Hull, K. L.; Lanni, E. L.; Sanford, M. S. J. Am. Chem. Soc.
2006, 128, 14047–14048.
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(34) Ryabov, A. D. Chem. Rev 1990, 90, 403–424.
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Am. Chem. Soc. 2009, 131, 3428–3429.
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Principles and Applications of Organotransition Metal Chemistry; University
Science Books: Mill Valley, CA, 1987. (b) Crabtree, R. H. The Organometallic
Chemistry of the Transition Metals, 4th ed.; John Wiley & Sons: New York,
2005.
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763–766.

11578 Inorganic Chemistry, Vol. 48, No. 24, 2009
Frazier et al.
Scheme 2
Scheme 3
Results
characterized, and further synthetic efforts focused on
potential oxidative addition routes.
Tetradentate Ligands: (2-pyridylmethyl)3-xN(CH2-
Aryl)x (x=1, 2). Scheme 1 illustrates the ligand precur-
sor syntheses, which were essentially benzylations of
2-pyridylmethylamine and di(2-pyridylmethyl)amine.
The 4-tert-butylbenzylbromides37 were used to aid in
the eventual solubility of the metal complexes, and
the tert-butyl was positioned meta to X = H, Br to not
interfere with the bond activation, yet provide some
modest steric protection of the incipient metal-aryl
bond. Yields were consistently around 70% for the
benzylation processes, which were conducted on 4-15 g
scales.
Heterolytic CH Bond Activation Attempts. Efforts to
affect CH-bond activation with ligands HAr2pyN and
HArpy2N failed despite treatment with numerous M(II)
equivalents (typically halides or triflates) under a variety
of conditions. Adduct formation was noted for certain
halides (e.g., {κ-N,Nap- HAr2pyN}MCl2; M = Fe, Co,
Ni), but since subsequent CH-bond activations were not
observed, even upon heating, these species were not
Metalation via Oxidative Addition. 1. Nickel. In
complexing first row transition metals via oxidative addi-
tion, nickel typically proves to be the easiest substrate
metal because of the variety of non-carbonyl Ni(0) pre-
cursors available.25 Metal carbonyls were avoided in the
context of this study because of the potential for undesir-
able CO insertion into the putative M-Ar bonds. As
38
Scheme 2 shows, treatment of Ni(COD)2 with BrAr2-
pyN afforded the orange, diamagnetic Ni(II) oxidative
addition product, {κ-C,N,Npy-(2-pyridylmethyl)N(CH2-
(4-tBu-phenyl-2-yl))(CH2(4-tBu-phenyl-2-Br))}NiBr (1-
Ni) in excellent yield (96%). Attempts to connect the
second aryl bond via reduction only resulted in diaryl
coupling to the tertiary amine derivative shown, presum-
ably with concomitant formation of Ni(0).
Given the history of nickel aryl-aryl couplings,39-42
and the various oxidation states implicated,25 the inabil-
ity to affect the ultimate Ni-Ar bond formation was not
(38) Schunn, R. A. Inorg. Synth 1974, 15, 5–9.
(39) Phapale, V. B.; Cardenas, D. J. Chem. Soc. Rev. 2009, 38, 1598–1607.
(40) Denmark, S. E.; Butler, C. R. Chem. Commun. 2009, 20–33.
(41) Terao, J.; Kambe, N. Acc. Chem. Res. 2008, 41, 1545–1554.
(42) Frisch, A. C.; Beller, M. Angew. Chem., Int. Ed. 2008, 44, 674–688.
(37) Mallory, F. B.; Butler, K. E.; Berube, A.; Luzik, E. D.; Mallory, C.
W.; Brondyke, E. J.; Hiremath, R.; Ngo, P.; Carroll, P. J. Tetrahedron 2001,
57, 3715–3724.

Article
Inorganic Chemistry, Vol. 48, No. 24, 2009 11579
surprising, hence the application of BrArpy2N was war-
ranted. The related treatment of Ni(COD)2 with the mono-
arylbromide provided another orange Ni(II) oxidative
addition product, {(κ-C,N,Npy2-(2-pyridylmethyl)2N(CH2-
(4-tBu-phenyl-2-yl))}NiBr ((Arpy2N)NiBr, 2-Ni) in 67%
yield. This product manifested Cs symmetry in its 1H and
13C{1H} NMR spectra, and is thus portrayed as a κ-C,N,
Table 1. Select Crystallographic and Refinement Data for (Arpy2N)FeBr (2-Fe)
and (smif)CrN(TMS)2 (4-Cr)
2-Fe
4-Cr
formula
formula wt
space group
Z
C23H26N3BrFe
480.23
Pbca
C18H28N4Si2Cr
408.62
P1
8
2
Npy 5-coordinate complex, although a rapid 2-pyridyl-
˚
2
a, A
13.0576(12)
16.9655(11)
20.0495(18)
90
90
90
4441.5(6)
1.436
2.491
8.0101(4)
11.5495(5)
11.9921(7)
95.2630(10)
91.093(4)
107.584(4)
1051.84(9)
1.290
0.666
173(2)
0.71073
R1 = 0.0405
wR2 = 0.0783
R1 = 0.0689
wR2 = 0.0928
1.001
˚
b, A
methyl exchange in the pseudo-square planar alternative
cannot be ruled out. It was somewhat surprising to find that
even this monoarylated complex disproportionated upon
thermolysis to afford the diaryl coupling product shown.
While this material may be useful as a neutral ligand
framework for other, perhaps dinuclear, chelation studies,
these results prompted a change away from nickel.
˚
c, A
R, deg
β, deg
γ, deg
3
˚
V, A
F
calc, g cm-3
3
μ, mm-1
temp, K
173(2)
2. Iron. Noncarbonyl Fe(0) precursors such as “Fe-
(PMe3)4”,43 and complexes with the potential to act as
Fe(0) sources such as (dmCh)2Fe (dmCh=dimethylcy-
clohexadienyl)44 failed to afford clean products, nor did
thermolyses of Fe(II) halides, which could disproportion-
ate to 1/3Fe(0) and 2/3Fe(III). Reasoning that solubility
issues could play a major factor in disproportionation of
salts or pseudo-salts, Fe{N(TMS)2}2(THF)45 was chosen
as a soluble Fe(II) complex capable of disproportiona-
tion. As Scheme 3 reveals, treatment of BrArpy2N with
Fe{N(TMS)2}2(THF) did indeed cause disproportiona-
˚
λ (A)
0.71073
R1 = 0.0501
wR2 = 0.0906
R1 = 0.1101
wR2 = 0.1100
1.000
R indices [I > 2σ(I)]a,b
R indices (all data)a,b
GOFc
P
P
P
P
2 1/2
] .
a R1 = ||Fo| - |Fc||/ |Fo|. b wR2 = [ w(|Fo| - |Fc|)2/ wFo
P
c GOF (all data) = [ w(|Fo| - |Fc|)2/(n - p)]1/2, n = number of
independent reflections, p = number of parameters.
46
tion to green Fe{N(TMS)2}3 and red, crystalline {κ-
C,N,Npy2-(2-pyridylmethyl)2N(CH2(4-tBu-phenyl-2-yl))}-
FeBr ((Arpy2N)FeBr, 2-Fe). While isolated in only
16%, this is roughly 50% of the expected yield based on
the disproportionation. Once characterized by X-ray
crystallography, an alternative synthesis of 2-Fe was
sought. Adduct formation to give {κ-N,Npy2-(2-pyridyl-
methyl)2N(CH2(4-tBu-phenyl-2-Br))}FeBr2 (3-Fe) was
accomplished via the combination of FeBr2 and BrAr-
py2N (97%), and a subsequent Na/Hg reduction afforded
2-Br in 91% yield. A 1H NMR spectrum of 2-Fe in THF-
d8 revealed 17 different hydrogens in addition to the
tert-butyl, indicative of plausible solvent binding.
Characterization of (Arpy2N)FeBr (2-Fe). 1. X-ray
Crystal Structure. Selected crystallographic data and
refinement details for (Arpy2N)FeBr (2-Fe) may be found
in Table 1, and a pertinent view of the molecule is given in
Figure 3. A slight twist in the aminomethylpyridine chains
prevents 2-Fe from having true Cs symmetry, but the
compound is quite close to a trigonal bipyramid. The apical
nitrogen and bromide span a 169.10(11)ꢀ angle about the
iron, and the amine is pyramidal, with the Fe1N1C angles
averaging 106.7(13)ꢀ. The bite angles of the chelate are
less than 90ꢀ (—N1Fe1C15 = 80.07(17)ꢀ, —N1Fe1N2=
75.47(15)ꢀ, —N1Fe1N3=75.19(15)ꢀ), and the correspond-
ing Br1Fe1C15, -N2 and -N3 angles are accordingly obtuse:
—Br1Fe1C15 =110.75(14)ꢀ, —Br1Fe1N2 = 99.77(11)ꢀ,
—Br1Fe1N3=97.86(12)ꢀ. A modest strain of chelation is
transmitted to each of the equatorial ligands, as each is
t
Figure 3. Molecular view of (Arpy2N)FeBr (2-Fe); the disordered Bu
group (two staggered positions, one shown) was refined isotropically.
˚
Selected bond distances (A) and angles (deg): Fe1N1, 2.276(4); Fe1N2,
2.149(4); Fe1N3, 2.156(4); Fe1C15, 2.068(5); Fe1Br1, 2.5352(9); N1C1,
1.461(6); N1C7, 1.466(6); N1C13, 1.467(6); N1Fe1C15, 80.07(17);
N1Fe1N2, 75.47(15); N1Fe1N3, 75.19(15); N1Fe1Br1, 169.10(11);
C15Fe1N2, 117.34(18); C15Fe1N3, 117.07(18); N2Fe1N3, 110.77(16);
Br1Fe1C15, 110.75(14); Br1Fe1N2, 99.77(11); Br1Fe1N3, 97.86(12);
Fe1C15C14, 113.0(4); Fe1C15C16, 131.8(4); Fe1N2C2, 116.5(3);
Fe1N2C6, 125.3(4); Fe1N3C8, 117.6(4); Fe1N3C12, 122.5(3); Fe1N1C1,
106.7(3); Fe1N1C7, 108.0(3); Fe1N1C13, 105.5(3).
“tipped away” from the amine: —Fe1C15C14=113.0(4)ꢀ,
—Fe1C15C16=131.8(4)ꢀ; —Fe1N2C2=116.5(3)ꢀ, —Fe-
1N2C6 =125.3(4)ꢀ; —Fe1N3C8 = 117.6(4)ꢀ, —Fe1N3-
C12=122.5(3)ꢀ). The bond distances are all normal, with
˚
the axial amine 2.276(4) A awayfrom the iron in contrast to
the pyridine nitrogens, which are closer at 2.149(4) and
˚
2.156(4) A. The iron-carbon and iron-bromide bond
lengths of 2.068(5) A and 2.5352(9) A are typical.
(43) Rathke, J. W.; Muetterties, E. L. J. Am. Chem. Soc. 1975, 97, 3272–
3273.
(44) DiMauro, P. T.; Wolczanski, P. T. Organometallics 1987, 6, 1947–
1954.
(45) (a) Olmstead, M. M.; Power, P. P.; Shoner, S. C. Inorg. Chem. 1991,
30, 2547–2551. (b) Andersen, R. A.; Faegri, K.; Green, J. C.; Haaland, A.;
Lappert, M. F.; Leung, W. P.; Rypdal, K. Inorg. Chem. 1988, 27, 1782–1786.
(46) Bradley, D. C.; Copperwaithe, R. G. Inorg. Synth 1978, 18, 112–120.
˚
˚
2. Magnetism. The molar susceptibility and corre-
sponding magnetic moment of (Arpy2N)FeBr (2-Fe) are
given as a function of temperature in Figure 4. The mea-
surements are consistent with an S=2 center for iron and
1
the observed paramagnetic H NMR spectrum, but are

11580 Inorganic Chemistry, Vol. 48, No. 24, 2009
Frazier et al.
Figure 4. a. Molar Susceptibility (χM) of (Arpy2N)FeBr (2-Fe) as a function of T(K). b. Magnetic moment (μeff in μB) of 2-Fe as a function of T(K).
Scheme 4
slightly high relative to the typical spin-only value of
4.90 μB. Unfortunately, the existence of one aryl-iron
bond is not sufficient to increase the field strength enough
to even achieve an intermediate spin system. The sub-
stantial decline in μeff below 50 K is attributed to the
effects of zero field splitting (ZFS),18 which can be
significant in systems far afield from spherical symmetry,
and the low symmetry of 2-Fe undoubtedly contributes
greatly.
Fe{N(TMS)2}3,46 the related chromium diamide, Cr{N-
(TMS)2}2(THF)2,47,48 was exposed to BrArpy2N in dieth-
yl ether over the course of a day at ∼23 ꢀC. Instead of
the desired 5-coordinate complex, the azaallyl complex
{κ-N,Npy2-1,3-dipyridyl-2-azaallyl}CrN(TMS)2 ((smif)-
CrN(TMS)2, 4-Cr), formed instead in >50% yield. In
addition to HN(TMS)2, 4-tert-butyl-2-bromotoluene was
easily identified as a byproduct, since it is a precursor to
the benzyl bromide used to prepare BrArpy2N.37 The
reaction mixture was also unusual in the intensity of the
emerald green color that increased as 4-Cr was produced.
NMR spectroscopic studies and subsequent investiga-
tions into the magnetism of 4-Cr revealed its S=2 ground
state at 23 ꢀC. Once it was identified by X-ray crystal-
lography, an alternative route consisting of the addition
C-N Bond Cleavage and Generation of (smif)CrN-
(TMS)2 (4-Cr). In reference to the proposed dispropor-
tionation of Fe{N(TMS)2}2(THF)45 to “Fe(0)” and 2
(47) Kern, R. J. J. Inorg. Nucl. Chem 1962, 24, 1105–1109.
(48) Bradley, D. C.; Hursthouse, M. B.; Newing, C. W.; Welch, A. J. J.
Chem. Soc., Chem. Commun. 1972, 567–568.

Article
Inorganic Chemistry, Vol. 48, No. 24, 2009 11581
of 1,3-(di-2-pyridyl)-2-azapropene49 to Cr{N(TMS)2-
(THF)2 was used to prepare 4-Cr in 74% yield as shown
in Scheme 4.
Characterizationof (smif)CrN(TMS)2 (4-Cr). 1. X-ray
Crystal Structure. Selected details regarding the collection
and refinement of data pertaining to (smif)CrN(TMS)2 (4-
Cr) are given in Table 1, and its molecular view can be seen
in Figure 5. The complex is nearly planar, with the N-
(TMS)2 group essentially perpendicular to the (smif)Cr
plane, but there is a slight distortion out of the plane by
the amide nitrogen, with an N2Cr1N4 angle of 168.76(6)ꢀ.
There is an additional asymmetry to the amide, as the
Cr1N4Si2 angle of 122.23(7)ꢀ is roughly 11ꢀ greater than
Cr1N4Si1 (110.97(7)ꢀ). Despite significant differences in
nitrogen type, the CrN bonds of 4-Cr are remarkably close
in distance, with the pyridine nitrogens farther away from
˚
the chromium (2.0864(15), 2.0887(14) A) than those of the
˚
˚
azaallyl (2.0416(14) A) and amide (2.0260(12) A). When
the entire N(TMS)2 group tips relative to the (smif)Cr
plane, both σ- and π-bonding effects are at play. The low
symmetry allows mixing of the 4s, 4px, and 3dx2-y2 orbitals
with 3dxz, thereby moderating the opposing Cr1N2 and
Cr1N4 bonds. By attenuating the trans-influence of the
opposing amide and azaallyl nitrogens via the distortion,
the chromium bond distance is allowed to be shorter,
thereby increasing N(pπ)-3dxy overlap for the half-order
π-bond. This overcomes the slight loss of π-overlap due to
being out of the (smif)Cr plane. Similar influences govern
the cant of alkylidenes, which is often misinterpreted in the
context of agostic effects.50
Figure 5. Molecular view of (smif)CrN(TMS)2 (4-Cr). Selected bond
˚
distances (A) and angles (deg): Cr1N1, 2.0887(14); Cr1N2, 2.0416(14);
Cr1N3, 2.0864(15); Cr1N4, 2.0260(12); N2C6, 1.332(2); N2C7, 1.320(2);
N4Si1, 1.6967(14); N4Si2, 1.6913(14); N1Cr1N3, 157.07(6); N2Cr1N4,
168.76(6); N1Cr1N2, 78.80(6); N1Cr1N4, 101.04(6); N2Cr1N3, 78.35(6);
N3Cr1N4, 101.74(6); Cr1N1C1, 129.05(12); Cr1N1C5, 112.12(12);
Cr1N3C12, 129.57(13); Cr1N3C8, 113.02(12); Cr1N2C6, 115.65(13);
Cr1N2C7, 115.77(13); C6N2C7, 128.48(17); Cr1N4Si1, 110.97(7);
Cr1N4Si2, 122.23(7).
The constraints of the smif ligand35 are revealed by the
N1Cr1N3 angle of 157.07(6)ꢀ, and the cant of the pyr-
idines that is related to that described for 2-Fe, with outer
CrNC angles of 129.3(4)ꢀ (ave) and inner CrNC angles of
112.6(6)ꢀ (ave). The bite angles of the smif ligand are
78.80(6)ꢀ (N1Cr1N2) and 78.35(6)ꢀ (N2Cr1N3), and the
corresponding NpyCrN(amide) angles are 101.04(6)ꢀ and
101.74(6)ꢀ, respectively. While the strain of the bound
smif is certainly evident, the bond distances suggest that
the ligand imparts a relatively strong field.
2. UV-vis Spectrum. The UV-vis spectrum of (smif)-
CrN(TMS)2 (4-Cr) is shown in Figure 6, and features
strong absorptions in the blue/violet and red regions of
the spectrum, giving rise to the intense green transmission
observed in solution. Like the previously communicated
(smif)2M (M = Fe, Co, Ni) and [(smif)2Co]OTf com-
plexes,35 and (smif)2M (M = V, Cr, Mn) and [(smif)2Cr]-
OTf compounds still undergoing investigation,51 the
spectra are likely dominated by intraligand (IL) bands,
and these may be accompanied by related metal-to-ligand
charge transfer bands (MLCT). The strongest feature at
396 nm (ε ∼ 27 000 M-1cm-1) is assigned to an IL transi-
tion from the azaallyl CNCnb orbital to pyridine π*
orbitals.
Figure 6. UV-vis spectrum of (smif)CrN(TMS)2 (4-Cr): 396 nm (ε ∼
27000 M-1 cm-1), 424 (sh, ε ∼ 12500 M-1 cm-1), 461 nm (ε ∼ 9 500 M-1
cm-1), 581 nm (ε ∼ 17000 M-1 cm-1), 627 nm (ε ∼ 19 000 M-1 cm-1),
675 nm (ε ∼ 15000 M-1 cm-1).
difficult to assess, but are in the region expected for the
lower energy IL band, which is also CNCnbfpy-π*.
Calculations on related complexes (e.g., (smif)2Cr)51 sug-
gest that the CNCnb orbital is energetically near the lower
lying 3d orbitals of the square plane; hence it is possible
that some of the bands may be MLCT in character.
However, related features have been observed in several
complexes containing the rigid CNC backbone, and the
separations between the bands of roughly 1130 and 1260
cm-1 suggests that they may be vibronic components of a
single electronic absorption. The fact that the separations
between absorption maxima are slightly different would
be expected if other, weaker absorptions, such as MLCT
and d-d bands, were underlying, thereby skewing the
progression. The IR spectrum of 4-Cr manifests a sharp
absorption at 1140 cm-1, which likely corresponds to a
The lower energy features at 675 nm (14,820 cm-1),
627 nm (15,950 cm-1), and 581 nm (17,210 cm-1) are
(49) Incarvito, C.; Lam, M.; Rhatigan, B.; Rheingold, A. L.; Qin, C. J.;
Gavrilova, A. L.; Bosnich, B. J. Chem. Soc., Dalton Trans. 2001, 3478–3488.
(50) Goddard, R. J.; Hoffmann, R.; Jemmis, E. D. J. Am. Chem. Soc.
1980, 102, 7667–7676.
(51) Frazier, B. A.; Lobkovsky, E. B.; Wolczanski, P. T.; Doucette, S.;
Mossin, S.; Meyer, K.; DeBeer George, S.; Cundari, T. R.; Hachmann, J.;
Chan, G. K.-L., manuscript in preparation.

11582 Inorganic Chemistry, Vol. 48, No. 24, 2009
Frazier et al.
Figure 7. a. Molar Susceptibility (χM) of (smif)CrN(TMS)2 (4-Cr) as a function of T(K). b. Magnetic moment (μeff in μB) of 4-Cr as a function of T(K).
CNC bending vibration that has the appropriate symme-
try to couple with the electronic IL transition. It is not
uncommon for IL bands to show vibrational components
in solution at room temperature.52,53
metal-aryl bond will be necessary to generate the fields
equated with low spin complexes.
Serendipitous (smif)CrN(TMS)2 Formation. The most
interesting discovery in this work concerns the serendipi-
tous formation of the 1,3-di-2-pyridyl-2-azaallyl or
“smif” complex (smif)CrN(TMS)2 (4-Cr), whose rela-
tively clean formation from Cr{N(TMS)2}2(THF)2 and
BrArpy2N remains mysterious. In Scheme 4, a mechanism
is shown that avoids redox changes that are expected to be
deleterious to the ultimate generation of a chromous
product. It is expected that any Cr(III) intermediate(s)
would be difficult to reduce back to Cr(II), and the
oxidation to Cr(IV) by an aryl-halide has only modest
precedent; hence a process that holds the chromous
oxidation state constant has considerable merit.54-57 In
the proposed process shown, adduct formation to give
(κ-N,Npy2-BrArpy2N)Cr{N(TMS)2}2 is followed by a
β-abstraction58 of a methylene hydrogen adjacent to
the amine by a basic N(TMS)2 group, rendering the
smif precursor monoanionic. A simple elimination of
4-tert-butyl-2-Br-toluene from the ligand backbone
would directly generate 4-Cr, but it is difficult to con-
ceive of how the chromium could facilitate this process,
which is basically not a standard organic reaction. In-
stead, an R-migration of the benzyl group is proposed,25
thereby returning the chelate to neutrality as (smif)H,
that is, the bound 1,3-(di-2-pyridyl)-2-azapropene, yet
keeping the chromium in its þ2 oxidation state. A se-
cond β-abstraction of the remaining methylene hydro-
gen;this time by the benzyl;generates 4-Cr via the
release of 4-tert-butyl-2-Br-toluene. A similar sequence
can be envisaged for Westerhausen’s preparation of
(smif)2Zn from ZnMe2 and (2-pyridylmethyl)2NH;36 it
is certainly likely that no redox changes occur in the
zinc reaction.
3. Magnetism. Magnetic susceptibility measurements
on (smif)CrN(TMS)2 (4-Cr) show a μeff of ∼4.7 μxB at
23 ꢀC that is fully consistent with the expected S = 2
chromium(II) center (Figure 7). Note that the susceptibility
is strongly attenuated below 50 K. While it is conceivable
that strong intermolecular interactions may be present, the
more likely explanation for the dramatic downturn in χM
and μeff below 50 K is the effect of the extremely low
symmetry in the system coupled with spin-orbit coupling,
that is, zero field splitting (ZFS). While it may not be
common to find ZFS effects so pronounced, it is prudent to
recognize that the gross deviation from spherical symmetry
that characterizes this system is exactly the instance in
which low symmetry effects are maximized. It is interesting
to compare 4-Cr with (Arpy2N)FeBr (2-Fe, Figure 4); both
S = 2 systems show significant ZFS, but the onset of the
downturn occurs at a much higher temperature for the
lower symmetry, pseudo-square planar derivative.
Discussion
Aryl-Containing Chelates. The concept of using aryl-
based chelates to impart strong fields to first row transi-
tion metal complexes has not been realized by the exam-
ples above, which are ambiguous at best in addressing this
issue. While S = 0 derivatives of square planar (and
possibly tbp) Ni(II) were prepared, high spin species are
typically found only for truly weak field ligands, or in
specific steric situations; hence their diamagnetism is not
enough to fully support the contention of significantly
greater fields. (Arpy2N)FeBr (2-Fe) was characterized as
an S = 2 species, thus the monoarylated ligand frame-
work failed to even produce an intermediate spin (i.e.,
S = 1) system that might be expected for a stronger field
ligand. Clearly, these results suggest that more than one
NHC Analogue: smif. From the X-ray crystal structure
of (smif)CrN(TMS)2 (4-Cr), it can be inferred that the
(54) Okazoe, T.; Takai, K.; Utimoto, K. J. Am. Chem. Soc. 1987, 109,
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(58) Hirsekorn, K. F.; Veige, A. S.; Marshak, M. P.; Koldobskaya, Y.;
Wolczanski, P. T.; Cundari, T. R.; Lobkovsky, E. B. J. Am. Chem. Soc. 2005,
127, 4809–4830.
(52) (a) Mack, J.; Stillman, M. J.; Kobayashi, N. Coord. Chem. Rev. 2007,
251, 429–453. (b) Mack, J.; Stillman, M. J. Coord. Chem. Rev. 2001, 219-221,
993–1032.
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(53) (a) Gudel, H. U.; Zilian, A. Coord. Chem. Rev. 1991, 111, 33–38. (b)
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Colombo, M. G.; Brunold, T. C.; Riedener, T.; Gudel, H. U.; Fortsch, M.; Burgi,
H.-B. Inorg. Chem. 1994, 33, 545–550.

Article
Inorganic Chemistry, Vol. 48, No. 24, 2009 11583
Figure 8. Lewis structures for N-heterocyclic carbenes (NHC) and smif showing isoelectronic features.
azaallyl backbone of the smif ligand possesses a relatively
strong field character, as its CrN bond, whose distance is
red spectra were recorded on a Nicolet Avatar 370 DTGX
spectrophotometer interfaced to an IBM PC (OMNIC soft-
ware). UV-vis spectra were obtained on a Shimadzu UV-2102
interfaced to an IBM PC (UV Probe software). GC-MS spectra
were obtained on a JEOL GCMate 2 mass spectrometer coupled
to an Agilent 689 N GC with EI ionization under standard
conditions. Solution magnetic measurements were conducted
via Evans’ method in toluene-d8.61 Solid state magnetic mea-
surements were performed using a Johnson Matthey magnetic
susceptibility balance calibrated with HgCo(SCN)4. Elemental
analyses were performed by Robertson Microlit Laboratories,
Madison, NJ, and by services at the University of Erlangen-
Nuremberg, Germany.
˚
2.0416(14) A, effectively competes with the trans-amide
˚
interaction (2.0258(13) A). While the pyridine portions of
the chelate undoubtedly help keep the azaallyl interaction
close, the charged nature of the ligand must provide an
ionic contribution to the bond that renders it stronger
than a typical pyridine or imine. Figure 8 illustrates the
relationship between smif and N-heterocyclic carbenes
(NHC), which are generally considered strong field li-
gands.59,60 The NHC ligands contain a neutral NCN
linkage that is isoelectronic to the anionic CNC- back-
bone of smif. While the N-donor smif presumably has
lesser overlap and a greater ΔE (Figure 1) relative to the
NHC’s carbon-donor, it is reasonable to assume that the
charge helps compensate.
Procedures. 1. N,N-bis(4-tert-butylbenzyl)-1-(pyridine-2-yl)-
methanamine. (HAr2pyN). To a 0 ꢀC biphasic solution of
2-(aminomethyl)pyridine (1.500 g, 13.87 mmol) in 10 mL of
H2O and 4-tert-butylbenzyl bromide (6.302 g, 27.74 mmol) in
70 mL of CH2Cl2 was slowly added a solution of NaOH (1.110 g,
27.75 mmol) in 10 mL of H2O. The biphasic reaction mixture
was slowly warmed to 23 ꢀC, while stirring vigorously, and
monitored by pH. When the reaction mixture was neutral, the
product was extracted from the aqueous layer with CH2Cl2 (4 ꢀ
15 mL). Organic layers were combined, dried over MgSO4, and
filtered. Rotary evaporation of the filtrate yielded a red-orange
oil. After the addition of hexanes, a brown impurity was filtered.
The hexanes filtrate was concentrated, cooled, and yielded
Conclusions
One aryl-metal bond imbedded into a chelate system
otherwise composed of metal-nitrogen bonds does not
appear to be enough to engender strong fields about first
row transition elements. The elaboration of multiple M-Ar
bonds into chelates is ongoing. The azaally backed ligand
smif shows considerable promise as a moderate field strength
ligand capable of supporting low symmetry complexes with
unusual optical properties.
1
yellow crystals of HAr2pyN. (4.198 g, 76%). H NMR (C6D6,
400 MHz): δ 1.25 (s, C(CH3)3,18 H), 3.64 (s, CH2, 4 H), 3.93 (s,
CH2, 2 H), 6.63 (t, py-C4H,1 H, J = 4.8 Hz), 7.14 (t, py-C5H,
1 H, J=4.8 Hz), 7.31 (d, C2,6H, 4 H, J=8 Hz), 7.43 (d, C3,5H,
4 H, J=8 Hz), 7.55 (d, py-C3H, 1 H, J=8 Hz), 8.48 (d, py-C6H,
1H,J=5 Hz).13C{1H} NMR (C6D6,100MHz):δ31.89 (C(CH3)3),
34.81 (C(CH3)3), 58.41 (CH2), 60.21 (CH2), 122.05 (py-C5H),
122.97 (py-C3H), 125.82 (C3H), 129.19 (C2H), 136.22 (py-C4H),
137.24 (C1H), 149.62 (py-C6H), 150.07 (C4H), 161.29 (py-C2H).
2. N-(4-tert-butylbenzyl)-1-(pyridine-2-yl)-N-(pyridine-2-yl-
methyl)methanamine (HArpy2N). To a 0 ꢀC biphasic solution of
2-dipicolylamine (5.000 g, 25.10 mmol) in 100 mL of CH2Cl2
and 4-tert-butylbenzyl bromide (5.700 g, 25.10 mmol) in 40 mL
of H2O was slowly added a solution of NaOH (1.004 g, 25.10
mmol) in 10 mL of H2O. The canary yellow biphasic reaction
mixture was slowly warmed to 23 ꢀC, while stirring vigorously,
and monitored by pH. When the reaction mixture was neutral,
the product was extracted from the aqueous layer with CH2Cl2
(4 ꢀ 15 mL). Organic layers were combined, dried over MgSO4,
and filtered. Rotary evaporation of the filtrate yielded an
orange-yellow oil. After the addition of hexanes, a brown
impurity was filtered. The hexanes filtrate was concentrated,
cooled, and yielded yellow crystals of HArpy2N (6.499 g, 75%).
1H NMR (C6D6, 400 MHz): δ 1.23 (s, C(CH3)3, 9 H), 3.69 (s,
CH2, 2 H), 3.97 (s, CH2, 4 H), 6.63 (t, py-C4H, 2 H, J = 5.6 Hz),
7.13 (t, py-C5H, 2 H, J = 7.6 Hz), 7.29 (d, C2,6H, 2 H, J = 8 Hz),
7.42 (d, C3,5H, 2 H, J = 8 Hz), 7.50 (d, py-C3H, 2 H, J = 7.9 Hz),
Experimental Section
General Considerations. All manipulations were performed
using either glovebox or high vacuum line techniques. Hydro-
carbon solvents containing 1-2 mL of added tetraglyme, and
ethereal solvents were distilled under nitrogen from purple
sodium benzophenone ketyl and vacuum transferred from same
prior to use. Benzene-d6 and toluene-d8 were dried over sodium,
vacuum transferred and stored under N2. THF-d8 was dried
over sodium benzophenone ketyl. Methylene chloride-d2
was dried over CaH2, vacuum transferred and stored over ac-
38
tivated 4 A molecular sieves. Ni(COD)2, CrCl2(THF),47 Fe-
˚
{N(TMS)2}2(THF),45 2-bromo-4-tert-butylbenzyl bromide,37
and 1,3-(2-pyridyl)-2-azapropene ((smif)H)49 were prepared
according to literature procedures. All other chemicals were
commercially available and used as received. All glassware was
oven-dried.
NMR spectra were obtained using Varian XL-400, INOVA
400, and Unity-500 spectrometers. Chemical shifts are reported
relative to benzene-d6 (1H δ 7.16; 13C{1H} δ 128.39), toluene-d8
(1H δ 2.09; 13C{1H} δ 20.4), thf-d8 (1H δ 3.58; 13C{1H} δ 67.57),
and methylene chloride-d2 (1H δ 5.32; 13C{1H} δ 54.00). Infra-
(59) Jacobsen, H.; Correa, A.; Poater, A.; Costabile, C.; Cavallo, L.
Coord. Chem. Rev. 2009, 253, 687–703.
(60) de Fremont, P.; Marion, N.; Nolan, S. P. Coord. Chem. Rev. 2009,
253, 862–892.
(61) (a) Evans, D. F. J. Chem. Soc. 1959, 2003–2005. (b) Schubert, E. M. J.
Chem. Educ. 1992, 69, 62.

11584 Inorganic Chemistry, Vol. 48, No. 24, 2009
Frazier et al.
8.48 (d, py-C6H, 2 H, J = 4.6 Hz). 13C{1H} NMR (C6D6, 100
MHz) δ: 31.86 (C(CH3)3), 34.80 (C(CH3)3), 58.56 (CH2), 60.48
(CH2), 122.10 (py-C5H), 123.11 (py-C3H), 125.81 (C3H), 129.27
(C2H), 136.21 (py-C4H), 137.06 (C1H), 149.75 (py-C6H), 150.09
(C4H), 160.91 (py-C2H).
64.93 (CH2), 70.92 (CH2), 119.45 (C6H), 120.95 (C1H), 121.90
(py-C5H), 122.33 (py-C3H), 125.35 (C6H), 125.99 (C4H), 129.20
(C3H), 131.46 (C3H), 136.12 (C4H), 136.59 (C2H), 140.30 (py-
C4H), 141.96 (C2H), 147.91(C5H), 148.71(C5H), 150.42 (py-
C6H), 153.84 (py-C2H), 160.90 (C1H). Anal. Calcd H28C34-
N2Br2Ni: C, 54.50; H, 5.55; N, 4.54. Found: C, 54.65, 54.71;
H, 5.64, 5.64; N, 4.03, 4.07.
3. N,N-bis(2-bromo-4-tert-butylbenzyl)-1-(pyridine-2-yl)methan-
amine (BrAr2pyN). To a 0 ꢀC biphasic solution of 2-(amino-
methyl)pyridine (1.000 g, 9.25 mmol) in 10 mL of H2O and
2-bromo-4-tert-butylbenzyl bromide (5.660 g, 18.50 mmol) in
60 mL of CH2Cl2 was slowly added a solution of NaOH (0.740 g,
18.50 mmol) in 10 mL of H2O. The biphasic reaction mixture
was slowly warmed to 23 ꢀC, while stirring vigorously, and
turned red within 2 h. The organic layer became orange upon
complete conversion to product. The product was extracted
from the aqueous layer with CH2Cl2 (3 ꢀ 15 mL). Organic layers
were combined, dried over Na2SO4, and filtered. Rotary eva-
poration of the filtrate yielded an orange oil. After the addition
of hexanes, a brown impurity was filtered. The hexanes filtrate
was concentrated and cooled to form orange-yellow crystals of
BrAr2pyN (16.16 g, 62%). 1H NMR (C6D6, 400 MHz): δ 1.06 (s,
C(CH3)3, 18 H), 3.96 (s, CH2, 6 H), 6.59 (t, py-C4H, 1 H, J = 5.8
Hz), 7.06 (t, py-C5H, 1 H, J = 7.6 Hz), 7.10 (d, C4H, 2H, J = 8
Hz), 7.33 (d, py-C3H, 1 H, J = 8 Hz), 7.62 (s, C6H, 2 H), 7.76 (d,
C3H, 2 H, J = 8 Hz), 8.48 (d, py-C6H, 1 H, J = 5.6 Hz). 13C{1H}
NMR (C6D6, 100 MHz) 31.46 (C(CH3)3), 34.73 (C(CH3)3),
58.53 (CH2), 60.60 (CH2), 122.17 (py-C5H), 123.19 (py-C3H),
125.08 (C1H), 125.15 (C4H), 130.43 (C6H), 130.78 (C3H), 136.18
(py-C4H), 136.35 (C2H), 149.86 (py-C6H), 152.23 (C5H), 160.26
(py-C2H). MS: m/z (%) 464, 466, 468 (58), 331, 333 (36), 225, 227
(50), 93 (100), Mþ not observed.
6. 2,10-Di-tert-butyl-6-(pyridine-2-ylmethyl)-6,7-dihydro-
5H-dibenzo[c,e]azepine (Coupled BrAr2pyN). To a 50 mL round-
bottom flask charged with 1-Ni (0.350 g, 0.57 mmol) and 0.95%
sodium amalgam (0.026 g Na, 1.13 mmol) was vacuum trans-
ferred 25 mL of THF at -78 ꢀC. Upon warming to 23 ꢀC and
stirring for 30 min, the orange reaction mixture turned orange-
brown. After stirring at 23 ꢀC for 7 h, volatiles were removed in
1
vacuo leaving a brown solid (0.164 g, 73%). H NMR (C6D6,
400 MHz): δ 1.30 (s, C(CH3)3, 18 H), 3.51 (s, CH2, 4 H), 3.98 (s,
CH2, 2 H), 6.71 (t, py-C4H, 1 H, J = 5.5 Hz), 7.19 (t, py-C5H,
1 H, J = 7.3 Hz), 7.30 (s, C5,6H, 4 H), 7.60 (d, py-C3H, 1 H, J =
7.8 Hz), 7.71 (s, C3H, 2 H), 8.57 (d, py-C6H, 1 H, J = 4 Hz).
13C{1H} NMR (C6D6, 125 MHz): δ 31.03 (C(CH3)3), 35.07
(C(CH3)3), 55.55 (CH2), 61.85 (CH2), 122.22 (py-C5H), 123.65
(py-C3H), 125.16 (C5H), 125.28 (C3H), 130.55 (C6H), 133.21
(C1H), 136.41 (py-C4H), 142.32 (C2H), 149.75 (py-C6H), 151.27
(C4H), 161.30 (py-C2H). MS: m/z (%) 306 (100), 93 (30), 57 (15),
Mþ not observed.
7. {K-C,N,Npy2-(2-pyridylmethyl)2N(CH2(4-tBu-phenyl-2-
yl))}NiBr (2-Ni). To a 100 mL round-bottom flask charged with
Ni(COD)2 (0.324 g, 1.18 mmol) and BrArpy2N (0.500 g, 1.18
mmol) was vacuum transferred 50 mL of benzene. The red-
orange reaction mixture was stirred at 23 ꢀC for 24 h while
orange needles precipitated from solution. The reaction was
filtered to yield microcrystalline orange needles of 2-Ni (0.380 g,
67%). 1H NMR (THF-d8, 400 MHz): δ 1.27 (s, C(CH3)3, 9 H),
4.39 (d, CH2, 2 H, J = 13.8 Hz), 4.45 (s, CH2, 2 H), 4.87 (d, CH2,
2 H, J = 13.9 Hz), 6.76 (d, C4H, 1 H, J = 7.7 Hz), 6.82 (d, C3H, 1
H, J = 7.3 Hz), 7.07 (t, py-C4H, 2 H, J = 6.6 Hz), 7.63 (t, py-
C5H, 2 H, J = 7.8 Hz), 7.86 (d, py-C3H, 2 H, J = 7.5 Hz), 8.34 (s,
C6H, 1 H), 8.57 (d, py-C6H, 2 H, J = 4.7 Hz). 13C{1H} NMR
(THF-d8, 100 MHz) δ: 32.34 (C(CH3)3), 64.17 (CH2), 69.75
(CH2), 121.34 (C3H), 122.16 (py-C5H), 124.02 (py-C3H), 125.26
(C5H), 138.01 (py-C4H), 141.01(C4H), 150.40 (py-C6H), 158.63
(C1H). Anal. Calcd H26C23N3BrNi: C, 57.19; H, 5.42; N, 8.70.
Found: C, 56.43, 56.26; H, 5.26, 5.20; N, 8.45, 8.33.
8. N,N0-(5,50-di-tert-butylbiphenyl-2,20-diyl)bis(methylene)bis-
(1-pyridin-2-yl)-N-(pyridine-2-ylmethyl)methanamine) (Coupled
BrArpy2N). Thermolysis of 2-Ni in pentane at 85 ꢀC for 2 d
led to the formation of coupled ligand, and, presumably NiBr2
and Ni0. 1H NMR (C6D6, 400 MHz): δ 1.22 (s, C(CH3)3, 9 H),
3.70 (s, CH2, 2 H), 3.97 (s, CH2, 2 H), 6.62 (t, py-C4H, 2 H, J =
5.8 Hz), 7.11 (t, py-C5H, 2 H, J = 7.3 Hz), 7.36 (s, C3H, 1 H),
7.43 (d, C5,6H, 2 H, J = 8.1 Hz), 7.51 (d, py-C3H, 2 H, J = 7.7
Hz), 8.52 (d, py-C6H, 2 H, J = 4 Hz). 13C{1H} NMR (C6D6, 125
MHz): δ 31.87 (C(CH3)3), 35.00 (C(CH3)3), 58.54 (CH2), 60.44
(CH2), 122.07 (py-C5H), 122.15 (py-C3H), 123.29 (C5H), 124.76
(C3H), 125.80 (C6H), 136.23 (py-C4H), 137.18 (C1H), 149.69
(C2H), 149.76 (py-C6H), 149.90 (C4H), 157.93 (py-C2H).
9. {K-C,N,Npy2-(2-pyridylmethyl)2N(CH2(4-tBu-phenyl-2-
yl))}FeBr (2-Fe). a.To a 25 mL round-bottom flask charged
with Fe{N(SiMe3)2}2(THF) (0.250 g, 0.56 mmol) and BrArpy2N
(0.473 g, 1.11 mmol) was vacuum transferred 10 mL of Et2O at
-78 ꢀC. The dark green reaction mixture was slowly warmed to
23 ꢀC. The reaction stirred for 14 h prior to removing all volatiles
and triturating with pentane. The green solid was taken up in
benzene and filtered through a Celite plug. Red, rod-shaped
crystals of 2-Fe were obtained in 16% yield (0.043 g). b. To a
100 mL round-bottom flask containing 3-Fe (0.900 g, 1.41
mmol) and 0.95% sodium amalgam (0.066 g, 2.88 mmol Na)
was vacuum transferred 50 mL of THF at -78 ꢀC. The reaction
mixture became red after it was stirred at 23 ꢀC for 2 h. Volatiles
4. N-(2-bromo-4-tert-butylbenzyl)-1-(pyridine-2-yl)-N-(pyri-
dine-2-ylmethyl)methanamine. (BrArpy2N). To a 0 ꢀC biphasic
solution of 2-dipicolylamine (10.000 g, 50.19 mmol) in 100 mL
of CH2Cl2 and 2-bromo-4-tert-butylbenzylbromide (15.36 g,
50.19 mmol) in 50 mL of H2O was slowly added a solution of
NaOH (2.01 g, 50.3 mmol) in 20 mL of H2O. The orange
biphasic reaction mixture was slowly warmed to 23 ꢀC, while
stirring vigorously, and monitored by pH. When the mixture
was neutral, the product was extracted from the aqueous layer
with CH2Cl2. Organic layers were combined, dried over MgSO4,
and filtered. Rotary evaporation of the filtrate yielded a tan-
yellow solid. After hot filtration of the solid in hexanes, tan-
yellow crystals were isolated from the filtrate a BrArpy2N
(15.713 g, 74%). 1H NMR (C6D6, 400 MHz): δ 1.06 (s, C(CH3)3,
9 H), 3.98 (s, CH2, 6 H), 6.60 (t, py-C4H, 2 H, J = 8 Hz), 7.10 (t,
py-C5H, C4H, 3 H, J = 8 Hz), 7.45 (d, py-C3H, 2 H, J = 7.8 Hz),
7.63 (s, C6H, 1 H), 7.78 (d, C3H, 1 H, J = 8 Hz), 8.47 (d, py-C6H,
2 H, J = 4.6 Hz). 13C{1H} NMR (C6D6, 100 MHz): δ 31.47
(C(CH3)3), 34.73 (C(CH3)3), 58.62 (CH2), 60.52 (CH2), 122.16
(py-C5H), 123.37 (py-C3H), 125.07 (C1H), 125.25 (C4H), 130.43
(C6H), 131.18 (C3H), 136.20 (py-C4H), 136.50 (C2H), 149.79
(py-C6H), 152.21 (C5H), 160.43 (py-C2H).
5. {K-C,N,Npy-(2-pyridylmethyl)N(CH2(4-tBu-phenyl-2-yl))-
(CH2(4-tBu-phenyl-2-Br))}NiBr (1-Ni). To a 100 mL flask
charged with Ni(COD)2 (1.000 g, 3.64 mmol) and BrAr2pyN
(2.03 g, 3.64 mmol) was vacuum transferred 60 mL of THF at
-78 ꢀC. After slowly warming to 23 ꢀC, the reaction mixture was
stirred for 1 h and filtered. The filtrate was concentrated, cooled
to -78 ꢀC, and filtered to yield 2.15 g 1-Ni as a crystalline orange
1
solid (96%). H NMR (C6D6, 400 MHz): δ 0.88 (s, C(CH3)3,
9 H), 1.47 (s, C(CH3)3, 9 H), 3.38 (d, CH2, 1 H, J = 14.4 Hz),
3.43 (d, CH2, 1 H, J = 16 Hz), 3.49 (d, CH2, 1 H, J = 13.2 Hz),
4.15 (d, CH2, 2 H, J=13.2 Hz), 5.18 (d, CH2, 1 H, J=14 Hz),
6.03 (t, py-C4H, 1 H, J=6 Hz), 6.23 (d, 1 H, J = 7.6 Hz), 6.49 (t,
py-C5H, 1 H, J = 7.6 Hz), 6.69 (d, C4H, 1 H, J= 7.6 Hz), 6.97 (s,
C6H, 1 H), 7.04 (t, C3,4H, 2 H, J = 7.6 Hz), 8.58 (s, C6H, 1 H),
8.82 (d, py-C6H, 1 H, J = 4.8 Hz), 10.13 (d, C3H, 1 H, J = 8 Hz).
13C{1H} NMR (C6D6, 100 MHz): δ 31.04 (C(CH3)3), 32.40
(C(CH3)3), 34.72 (C(CH3)3), 35.40 (C(CH3)3), 63.67 (CH2),

Article
Inorganic Chemistry, Vol. 48, No. 24, 2009 11585
were removed in vacuo yielding an orange-red solid. The solid
was dissolved in THF and filtered through Celite. The filtrates
were concentrated to yield 2-Fe as an orange powder (0.617 g,
91%). 1H NMR (THF-d8, 400 MHz): δ -37.15 (υ1/2 ≈ 26 Hz,
1 H), -11.47 (υ1/2 ≈ 29 Hz, 1 H), 0.45 (υ1/2 ≈ 15 Hz, 1 H), 5.75
(υ1/2 ≈≈ 49 Hz, 1 H), 6.35 (υ1/2 ≈ 40 Hz, 1 H), 10.96 (υ1/2 ≈ 22
Hz, C(CH3)3, 9 H), 33.52 (υ1/2 ≈ 462 Hz, 1 H), 41.68 (υ1/2 ≈ 45
Hz, 1 H), 51.45 (υ1/2 ≈ 71 Hz, 1 H), 51.61 (υ1/2 ≈ 44 Hz, 1 H),
57.56 (υ1/2 ≈ 67 Hz, 1 H), 63.61 (υ1/2 ≈ 174 Hz, 1 H), 67.96 (υ1/2
≈ 215 Hz, 1 H), 81.49 (υ1/2 ≈ 351 Hz, 1 H), 86.69 (υ1/2 ≈ 397 Hz,
1 H), 103.50 (υ1/2 ≈ 52 Hz, 1 H), 117.53 (υ1/2 ≈ 536 Hz, 1 H),
128.64 (υ1/2 ≈ 464 Hz, 1 H). Anal. Calcd H26C23N3BrFe: C,
57.52; H, 5.46; N, 8.75. Found: C, 58.17, 58.03; H, 5.61, 5.46; N,
8.42, 8.42. μeff (SQUID, 293 K) = 5.28 μB.
filtered, and washed with cold Et2O to isolate 0.237 g 4-Cr as
green crystals (74%). H NMR (C6D6, 400 MHz) δ: -78.71
1
(υ1/2 ≈ 2000 Hz, py-CH,1 H), -74.23 (υ1/2 ≈ 300 Hz, CH, 1 H),
-37.81 (υ1/2 ≈ 520 Hz, py-CH, 1 H), -18.60 (υ1/2 ≈ 580 Hz, py-
CH,1 H), 23.28 (υ1/2 ≈ 500 Hz, py-CH, 1 H), 57.60 (υ1/2 ≈ 5800
Hz, Si(CH3)3, 9 H). Anal. Calcd H28C18N4Si2Cr: C, 52.91; H,
6.91; N, 13.71. Found: C, 51.90; H, 6.78; N, 14.02. μeff (SQUID,
293 K) = 4.9 μB.
Magnetic Susceptibility Measurements. Magnetic susceptibil-
ity measurements of crystalline powdered samples (10-30 mg)
were performed on a Quantum Design MPMS-5 SQUID mag-
netometer at 10 kOe between 5 and 300 K for all samples. All
sample preparations and manipulations were performed under
an inert atmosphere because of the air sensitivity of the samples.
The samples were either measured in a flame-sealed NMR tube
or a custom machined sealed Teflon capsule. The diamagnetic
contribution from the sample container was subtracted from the
experimental data. Pascal’s constants62 were used to subtract
diamagnetic contributions, yielding paramagnetic susceptibil-
ities.
10. {K-N,Npy2-(2-pyridylmethyl)2N(CH2(4-tBu-phenyl-2-
Br))}FeBr2 (3-Fe). To a 25 mL round-bottom flask containing
FeBr2 (0.127 g, 0.59 mmol) and BrArpy2N (0.250 g, 0.59 mmol)
was vacuum transferred 10 mL of THF at -78 ꢀC. The yellow
suspension was warmed to 23 ꢀC and darkened to orange-yellow
after 1 h. The reaction mixture was stirred for 6 h, and the
volatiles were removed. The yellow solid was taken up in Et2O,
filtered and washed to yield 3-Fe (0.365 g, 97%). 1H NMR
(CD2Cl2, 400 MHz) δ: 0.84 (υ1/2 ≈ 9 Hz, C(CH3)3,9 H), 1.36 (υ1/2
≈ 37 Hz, CH2,2 H), 5.54 (υ1/2 ≈ 39 Hz, CH, 1 H), 6.21 (υ1/2 ≈ 24
Hz, CH, 1 H), 23.96 (υ1/2 ≈ 425 Hz, CH, 1 H), 27.18 (υ1/2 ≈ 432
Hz, py-CH, 2 H), 50.48 (υ1/2 ≈ 51 Hz, CH2, 2 H), 55.80 (υ1/2 ≈ 44
Hz, CH2, 2 H), 102.43 (υ1/2 ≈ 417 Hz, py-CH, 2 H), 105.92 (υ1/2
≈ 231 Hz, py-CH, 2 H), 116.06 (υ1/2 ≈ 461 Hz, py-CH, 2 H).
Anal. Calcd H26C23N3Br3Fe: C, 43.16; H, 4.09; N, 6.57. Found:
C, 43.18, 42.95 H, 4.23, 4.05; N, 6.43, 6.37. μeff (Gouy balance,
295 K) = 4.5 μB.
11. Cr{N(TMS)2}2(THF)2. A modified literature synthesis
was used.48 To a 250 mL flask charged with CrCl2(THF) (5.325
g, 27.31 mmol) and sodium hexamethydisilazide (10.000 g, 54.53
mmol) was vacuum transferred 150 mL of THF at -78 ꢀC. The
reaction mixture immediately became a deep indigo and was
slowly warmed to 23 ꢀC. After stirring at 23 ꢀC for 12 h, the
reaction was filtered, yielding a lavender filtrate and a green
filter cake that was washed with THF. The filtrates were
concentrated, cooled to -78 ꢀC, and filtered to yield 10.050 g
of Cr{N(TMS)2}2(THF)2 as a crystalline lavender solid (71%).
12. {K-N,Npy2-(1,3-dipyridyl-2-azaallyl}CrN(TMS)2 ((smif)Cr-
N(TMS)2, 4-Cr). a. To a 25 mL round-bottom flask containing
Cr{N(SiMe3)2}2(THF)2 (0.250 g, 0.48 mmol) and BrArpy2N
(0.206 g, 0.48 mmol) was vacuum transferred 15 mL of Et2O
at -78 ꢀC. The reaction mixture immediately turned green and
was slowly warmed to 23 ꢀC. After stirring at 23 ꢀC for 1 d, the
volatiles were removed in vacuo. The green solid was filtered in
pentane to yield (smif)CrN(SiMe3)2 as a green solid (0.103 g,
52%). b. A solution of (smif)H (0.153 g, 0.78 mmol) in 10 mL
Et2O was added dropwise to a stirred solution of Cr{N-
(SiMe3)2}2(THF)2 (0.400 g, 0.77 mmol) in Et2O (10 mL) at
23 ꢀC. The solution became emerald green. The reaction was
degassed and allowed to stir for 12 h at 23 ꢀC while green crystals
precipitated from solution. The suspension was concentrated,
Single Crystal X-ray Diffraction Studies. Upon isolation, the
crystals were covered in polyisobutenes and placed under a 173
K N2 stream on the goniometer head of a Siemens P4 SMART
CCD area detector (graphite-monochromated Mo KR radia-
˚
tion, λ = 0.71073 A). The structures were solved by direct
methods (SHELXS). All non-hydrogen atoms were refined
anisotropically unless stated, and hydrogen atoms were treated
as idealized contributions (Riding model).
13. 2-Fe. A red rod (0.20 ꢀ 0.03 ꢀ 0.03 mm) was obtained
from toluene. A total of 15,951 reflections were collected with
3,192 determined to be symmetry independent (Rint = 0.1178),
and 1,896 were greater than 2σ(I). A semiempirical absorp-
tion correction from equivalents was applied, and the refine-
ment utilized w-1 = σ2(Fo2) þ (0.0499p)2 þ 0.0000p, where p =
2
2
((Fo þ 2Fc )/3).
14. 4-Cr. A dark green hexagonal block (0.40 ꢀ 0.30 ꢀ 0.20
mm) was obtained from pentane. A total of 7,678 reflections
were collected with 7,678 determined to be symmetry indepen-
dent (Rint = 0.0000), and 5,121 were greater than 2σ(I). A
semiempirical absorption correction from equivalents was ap-
plied, and the refinement utilized w-1 = σ2(Fo2) þ (0.0400p)2 þ
2
2
0.0000p, where p = ((Fo þ 2Fc )/3).
Acknowledgment. We thank the National Science Founda-
tion (CHE-0718030, PTW) and Cornell University for financial
support. We thank Dr. Suzanne Doucette, and Dr. Susanne
Mossin for obtaining SQUID data, and Dr. Karsten Meyer and
Dr. Jay Winkler for helpful discussions.
Supporting Information Available: CIF files of 2-Fe and 4-Cr.
This material is available free of charge via the Internet at http://
pubs.acs.org.
(62) Carlin, R. L. Magnetochemistry; Springer-Verlag: Berlin; New York,
1986.