Ketiminato and ketimine Co, Eu, Cu and Fe complexes

Article abstract of DOI:10.1016/j.ica.2010.02.019

A series of metal complexes have been prepared using two similar ketiminato ligands, ArL¹H (([RN(H)(C(Me))₂C(Me)O], ((R = 2,6-diisopropylphenyl (Dipp) or 2,6-dimethylphenyl, (Dmp)) and the less bulky, multidentate molecule L²H, ([RN(H)C(Me)CHC(Me)O] (R = C ₂H₄NEt₂)). Reaction of ArL¹H or ArL¹Na with Co(SCN)₂, Eu(OTf)₃ and Cu(OTf) ₂, afforded [Co(DippL¹H)₂(NCS)₂] (1), [Eu(DippL¹H)₃(OTf)₃] (2), and [Cu(OTf)₂(DippL¹H)₂] (3). The coordination preferences of ArL¹ and L² with FeBr₂ were investigated and yielded crystals of [Fe(DippL¹)₂] (4), [L²FeBr·LiBr(THF)₂] (5), and [{(DmpNC(Me)) ₂C(H)Me}FeBr₂] (6). Complexes 1-6 have been structurally characterized by X-ray crystallography, and other pertinent techniques.

Full text of DOI:10.1016/j.ica.2010.02.019

Inorganica Chimica Acta 363 (2010) 2104–2112
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Ketiminato and ketimine Co, Eu, Cu and Fe complexes
Audra F. Lugo (neé Gushwa) a, Anne F. Richards b,
*
a Department of Chemistry, Texas Christian University, Fort Worth, TX 76129, United States
b Department of Chemistry, La Trobe Institute for Molecular Science, La Trobe University, Bundoora, VIC 3086, Australia
a r t i c l e i n f o
a b s t r a c t
A
series of metal complexes have been prepared using two similar ketiminato ligands, ArL1H
Article history:
Received 10 December 2009
Received in revised form 9 February 2010
Accepted 17 February 2010
Available online 24 February 2010
(([RN(H)(C(Me))2C(Me)@O], ((R = 2,6-diisopropylphenyl (Dipp) or 2,6-dimethylphenyl, (Dmp)) and the
less bulky, multidentate molecule L2H, ([RN(H)C(Me)CHC(Me)@O] (R = C2H4NEt2)). Reaction of ArL1H
or ArL1Na with Co(SCN)2, Eu(OTf)3 and Cu(OTf)2, afforded [Co(DippL1H)2(NCS)2] (1), [Eu(DippL1H)3(OTf)3]
(2), and [Cu(OTf)2(DippL1H)2] (3). The coordination preferences of ArL1 and L2 with FeBr2 were investi-
gated and yielded crystals of [Fe(DippL1)2] (4), [L2FeBrꢀLiBr(THF)2] (5), and [{(DmpN@C(Me))2C(H)Me}-
FeBr2] (6). Complexes 1–6 have been structurally characterized by X-ray crystallography, and other
pertinent techniques.
Keywords:
X-ray crystal structures
Ketiminate
Ó 2010 Elsevier B.V. All rights reserved.
Ketiminato
Transition metal
1. Introduction
rials [6]. b-Ketiminate d and f block complexes have been particu-
larly sought after as improved thin film CVD precursors [7] and are
Structurally and electronically related to b-diketiminates,
([{N(Ar)C(Me)}2CH]ꢁ), b-ketiminates (Fig. 1) have been widely
studied as monoanionic, p-conjugated, bidentate chelating ligands
highly attractive catalysts in a wide range of processes [1c,8]. The
desirability of ketiminates is, in part, attributed to the tunable
steric protection of the metal center by the bulky nitrogen substi-
tuent on one side, while leaving the metal more exposed to further
reactivity on the other side [9]. Moreover, N,O ligands are particu-
larly useful as they typically exhibit stronger metal–oxygen coordi-
nation with slightly weaker metal–nitrogen bonds, resulting in
ligand hemilability and potentially interesting catalytic behavior
[10].
Herein we report the synthesis of one lanthanoid and several
first-row transition metal ketimine and ketiminato complexes. In
complexes 1–3, DippL1H acts as a non-chelating oxygen donor li-
gand in heteroleptic complexes, alongside isothiocyanate = NCS
(in Co), 1, or trifluoromethanesulfonate = triflate = OTf (with Eu,
2, and Cu, 3). Such heteroligated complexes [11] are of interest
as have been targeted as polymerization catalysts with enhanced
productivity, offering greater insight into the structure and cata-
lytic performance relationships. In addition we report the synthe-
sis and characterization of three iron(II) complexes, 4–6 that
feature bidentate and tridentate chelation of ArL1 and L2,
respectively.
[1]. An advantage of b-ketiminates is their asymmetry which con-
fers unique chelating properties to the metal center and is attribut-
able to the two different donor atoms, N and O. Recently; we have
been examining the coordination behavior of the two ligands,
ArL1H (([RN(H)(C(Me))2C(Me)@O], ((R = 2,6-diisopropylphenyl
(Dipp) or 2,6-dimethylphenyl, (Dmp)) and the less bulky, multid-
entate molecule L2H, ([RN(H)C(Me)CHC(Me)@O] (R = C2H4NEt2))
[2]. We observed that the reaction of ArL1H/Li (Ar = Dipp = 2,6-
diisopropylphenyl, Dmp = 2,6-dimethylphenyl), with several main
group elements afforded a variety of products but the tridentate
ligand L2 consistently formed monomeric NNO-chelated com-
plexes [2]. We wished to continue our investigation of the coordi-
nation preferences of these related ketiminate ligands with d and f
block elements to determine whether a more definitive trend could
be observed.
2. Background
In addition to the fundamental knowledge gained by comparing
the structures derived from the two ligands, ArL1H and L2H, ketim-
inato complexes have proven to be quite versatile in a range of
applications, having been exploited as single-molecule magnets
[3] and as luminescent [4], liquid crystalline [5], and sensing mate-
3. Results and discussion
3.1. Synthesis of 1 [Co(DippL1H)2(NCS)2], 2 [Eu(DippL1H)3(OTf)3],
and 3 [Cu(OTf)2(DippL1H)2]
The synthesis of cobalt and europium complexes were
* Corresponding author.
E-mail address: a.richards@latrobe.edu.au (A.F. Richards).
attempted by reaction of MXn (M = Eu, X = OTf, n = 3; M = Co,
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.02.019

A.F. Lugo (neé Gushwa), A.F. Richards / Inorganica Chimica Acta 363 (2010) 2104–2112
2105
Fig. 1. Structures of ketimine ligands ArL1H and L2H.
X = SCN, n = 2) with neutral DippL1H and L2H via elimination of HX
with or without the presence of NEt3. Crystals suitable for X-ray
diffraction could be not obtained from these reactions. To ascertain
whether salt elimination was possible using these same metal pre-
cursors, the reactions with Eu(OTf)3 and Co(SCN)2 were repeated
with DippL1Na (Scheme 1), which had been isolated from reaction
of the neutral ligand DippL1H and NaH. Surprisingly, instead of salt
elimination, coordination of the neutral ligand to the metal center
occurred via a dative bond from the ligand oxygen to form com-
plexes 1 and 2. The source of the neutral ligand was likely the pres-
ence of DippL1H that remained after reaction with NaH. This trace
of unreacted neutral ligand in the batch of DippL1Na was con-
firmed by the presence of the N–H peak resonating at 13.10 ppm
in the 1H NMR spectrum [12]. Despite numerous subsequent at-
tempts at isolating 1 and 2 by the deliberate reaction of DippL1H
with the metal salt under the same conditions, X-ray quality crys-
tals of neither 1 nor 2 could be obtained. It is unclear why coordi-
nation of the neutral ligand occurs, yet crystals of these complexes
could not be isolated from reaction with DippL1H. In order to dem-
onstrate that an analogous heteroleptic metal complex could in-
deed be obtained through coordination of the neutral ligand with
a metal precursor the reaction of copper(II) triflate and the neutral
ligand was performed (Scheme 1). As expected, [Cu(OTf)2(Dip-
pL1H)2] (3), was formed in excellent yield.
Fig. 2. Molecular structure of 1. Hydrogen atoms and a toluene solvent molecule
are omitted for clarity. Thermal ellipsoids are drawn to 30% probability.
DippL1H with Co(1) in complex 1. Hence, the Co–N bond lengths
show better agreement with those of the [Co(NCS)4]2ꢁ dianion of
1
2[K{Cu(acen)}3][Co(NCS)4]ꢀ 4CH3OH (acen = acetylacetonethylen-
ediamine anion) (1.946(6)–1.966(6) Å) [14]. In complex 1, each
NCS ligand maintains
a near-linear geometry, 179.5(4)° and
179.2(5)°, around the central carbon. The Co–O interactions of 1
(1.975(3) and 1.965(2) Å) are intermediate values in the range for
other 4-coordinate (N,N,O,O)CoII systems (1.850(2)–2.07(1) Å)
[15]. The Co(II) metal center adopts a distorted tetrahedral geome-
try, with angles around the cobalt atom ranging from 102.19(12)° to
114.54(16)° (Table 1). The bond lengths of the DippL1H ligands (Ta-
ble 2) all indicate delocalization along the ligand backbone, as is
typically seen with ketiminato and related ligands [16]. The same
delocalization is displayed in complexes 2 and 3, and the results
also shown in Table 2 for comparison. Hydrogen atoms on the nitro-
gen atoms were located on the difference map and for complexes 1–
3, hydrogen bonding exists between the DippL1H N–H hydrogen
and the ligand oxygen atom. The presence of the NH group in 1–3
is confirmed in the IR spectra with broad N–H stretches at 3167
(1), 3170 (2), and 3165 cmꢁ1 (3). Because the DippL1H ligands in
1–3 were all protonated, N,O-chelation to the respective metal cen-
ꢀ
Complex 1 (Fig. 2) crystallizes in the triclinic space group, P1 as
green plates. Solid-state structural analysis of 1 reveals that the
thiocyanate ligands of the cobalt(II) thiocyanate starting material
had rearranged such that the nitrogens were the donor atoms,
rather than sulfur (Co–N: 1.949(3), 1.934(4) Å). These Co–N lengths
are shorter than related Co–NCS systems (1.957(5)–2.1646(16) Å)
[13], but these systems all involve more sterically encumbered,
chelating co-ligands than the simple monodentate interaction of
Scheme 1. Synthesis of heteroleptic complexes [Co(DippL1H)2(NCS)2] (1), [Eu(DippL1H)3(OTf)3] (2), and [Cu(OTf)2(DippL1H)2] (3).

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Table 1
Select angles (°) around the metal center for complexes 1 and 2.
Complex: 1
2
O(1)–Co(1)–O(2)
O(1)–Co(1)–N(3)
O(1)–Co(1)–N(4)
O(2)–Co(1)–N(3)
O(2)–Co(1)–N(4)
N(3)–Co(1)–N(4)
107.21(12)
114.34(13)
103.88(13)
113.49(13)
102.19(12)
114.54(16)
O(1)–Eu(1)–O(2)
O(1)–Eu(1)–O(4)
O(1)–Eu(1)–O(5)
O(1)–Eu(1)–O(7)
O(1)–Eu(1)–O(8)
O(1)–Eu(1)–O(10)
88.53(18)
68.54(17)
122.22(17)
150.99(18)
144.67(18)
73.57(18)
O(1)–Eu(1)–O(11)
O(4)–Eu(1)–O(5)
O(4)–Eu(1)–O(7)
O(4)–Eu(1)–O(8)
O(4)–Eu(1)–O(10)
O(4)–Eu(1)–O(11)
80.85(18)
53.69(17)
119.47(19)
129.61(19)
115.0(2)
69.8(2)
Table 2
Selected bond lengths (Å) for complexes 1–3. The atom labels correspond to the following figure in which OC0C00C000N represents the ligand backbone of each DippL1H, and XR0
represents the co–ligand:
Complex
1M = Co
2M = Eu
3M = Cu
1st DippL1H
2nd DippL1H
1st DippL1H
2nd DippL1H
3rd DippL1H
DippL1H
M–O
1.975(3)
1.299(4)
1.398(6)
1.411(5)
1.337(5)
1.965(2)
1.297(5)
1.393(5)
1.413(5)
1.328(5)
2.312(5)
1.306(8)
1.378(10)
1.401(10)
1.334(9)
2.333(5)
1.270(9)
1.379(10)
1.443(11)
1.303(9)
2.334(5)
1.269(8)
1.396(11)
1.388(12)
1.333(10)
1.9067(13)
1.295(2)
1.400(3)
1.415(3)
1.330(2)
O–C0
C0–C00
C00–C000
C
000–N
X = N (NCS)
1st Co-ligand
1.949(3)
X = O (OTf)
X = O (OTf)
Co-ligand
1.9619(13)
2nd Co-ligand
1.934(4)
1st Co-ligand
2nd Co-ligand
3rd Co-ligand
M–X
2.592(6)
2.562(5)
2.631(5)
2.536(5)
2.720(6)
2.495(5)
ters was averted, accounting for the monodentate M–O interactions
observed in all three complexes.
Isomerization between thiocyanate (ꢁSCN) and isothiocyanate
(ꢁNCS) derivatives is well documented [17], and reaction temper-
atures (40–80 °C) have been reported to promote the isomerization
from the thiocyanate intermediate to the thermodynamically more
stable isothiocyanate compound [18]. Therefore, the isolation of
the isothiocyanate complex 1 was seemingly due to the elevated
reaction temperature. The UV–Vis spectrum of 1 displays one
absorption band at 324 nm which compares well to kmax of
315 nm obtained from the green crystals of previously reported
examples, e.g. [CoIIL0]ꢀ(H2O)3, L0 = 1,2-di[4-(2-imino 4-oxo pen-
tane)phenyl]ethane [19]. The m(C@N) and m(C@O) absorption bands
at 1565 and 1625 cmꢁ1, respectively, in the IR spectrum of this
same L0 bis(ketiminate) complex [19] also show good agreement
with those of 1 (m , m
(C@N): 1561 cmꢁ1 (C@O): 1607 cmꢁ1).
X-ray crystallographic analysis of 2 (Fig. 3), which crystallizes in
the monoclinic space group P21/c, shows a distorted tricapped tri-
gonal prismatic geometry around the europium(III) center, a com-
mon nine-coordinate geometry for lanthanoid metals [20]. O(1),
O(2), and O(3) of the DippL1H ligands make up one set of axial li-
gands, forming the vertices of one triangle (Eu(1)–O(1) 2.312(5),
Eu(1)–O(2) 2.333(5), Eu(1)–O(3) 2.334(5) Å). The other set of axial
donor atoms making up the other triangle comes from one oxygen
from each of the three bidentate triflate ligands (Eu(1)–O(5)
2.562(5), Eu(1)–O(8) 2.536(5), and Eu(1)–O(11) 2.495(5) Å). The
remaining oxygen donor atoms from each triflate group (Eu(1)–
O(4) 2.592(6), Eu(1)–O(7) 2.631(5), and Eu(1)–O(10) 2.720(6) Å)
comprise the equatorial donor atoms. Therefore, each triflate sup-
plies one axial and one equatorial oxygen donor. The distortion is
then derived from the acute O–S–O angles (105.4(4)°, 107.8(3)°,
108.1(3)°) of the triflate groups, causing the equatorial oxygen
atoms to deviate from planarity with the Eu atom (Eu(1)–O(10)–
O(7)–O(4) torsion angle = 16.4°).
Fig. 3. Molecular structure of 2. Hydrogen atoms are omitted for clarity. Thermal
ellipsoids are drawn to 30% probability.
Of the reported crystallographically characterized europium
complexes with Eu–O(triflate) contacts [21], only two other mono-
nuclear complexes, [Eu(L)(OTf)3] (L = (1S,2S,10S,11S)-1,2,10,11-
tetraphenyl-1,11-dimethoxy-3,5,9-trioxaundecane) [21a] and
[Eu(tpa)(OTf)3(H2O)2] (tpa = tris(2-pyridylmethyl)amine) [21b]
have been reported that incorporate three triflate groups on the
Eu center, but each of these incorporates only one co-organic li-
gand. Thus, 2, to the best of our knowledge, represents the first
complex of the formula [EuL3(OTf)3] (L = organic ligand) reported
in the solid state. Furthermore, only one europium complex includ-

A.F. Lugo (neé Gushwa), A.F. Richards / Inorganica Chimica Acta 363 (2010) 2104–2112
2107
bond in 3, it is fairly short for a Cu–O(triflate) bond, which usually
lies in the range of (1.947(7)–2.267(11) Å) [24]. This short Cu–
O(triflate) bond length is consistent with the assignment of a Cu–
O single bond (as opposed to the dative interaction assigned for
the Cu–O(carbonyl) bond).
Despite the ambiguity in assigning covalent versus dative inter-
actions based solely on the Cu–O bond lengths in 3 (due to the fact
that they are all, for the most part, shorter than literature values),
the more compelling data is the spectroscopic observation by IR of
the neutral DippL1H ligand (the N–H stretch mentioned earlier and
the C@N and C@O stretches described in the following compari-
son), which necessitates a dative bond to the Cu(II) center for
charge balance. The C@N (1538 cmꢁ1) and C@O (1575 cmꢁ1
)
stretching frequencies of 3 are observed at somewhat lower wave-
numbers than those of [Co(DippL1H)2(NCS)2] (1), but compare well
with those of [CuCl2(H2L0)] (1522 and 1580 cmꢁ1, respectively),
L0 = 1,2-di[4-(2-imino 4-oxo pentane)phenyl]ethane [19]. Similar
lowering of these wavenumbers was observed with the latter L0–
Cu2+ complex, relative to the L0–Co2+ analog in that study [19] as
well.
Fig. 4. Molecular structure of 3. Hydrogen atoms are omitted for clarity. Thermal
ellipsoids drawn at 30% probability. Symmetry transformations used to generate
equivalent atoms: ꢁx + 1, ꢁy + 1, ꢁz.
ing a bidentate triflate ligand has been characterized by XRD,
[Eu(biqO2)(OTf)]2+2[OTf]ꢁꢀCH3CNꢀH2O (biqO2 = 3,30-biisoquino-
line-2,20-dioxide) [21c] with 2 being the first with more than
one such O,O0-chelating triflate group. The Eu–O(triflate) bond
3.2. Discussion of 4 (A and B) [(DippL1)2Fe], 5 [L2FeBrꢀLiBr(THF)2],
and 6 [{(DmpN@C(Me))2C(H)Me}FeBr2]
lengths of
2
compare well with those of [Eu(biqO2)-
(OTf)]2+2[OTf]ꢁꢀCH3CNꢀH2O (2.6498(17), 2.6750(17) Å) [21c].
The oxidation of Fe(II) to Fe(III) upon exposure to very small
traces oxygen is known to be a facile process [25]. In our previous
investigations of ArL1Li and L2Li with Mn(II) halides we observed
that despite rigorous anaerobic conditions, oxidation from Mn(II)
to Mn(III) occurred during the reaction affording [(DippL1)2MnCl]
and [L2MnBrꢀLiBr(THF)2 [2]. In light of this we wanted to compare
the outcome of the manganese complexes with those of iron to
determine whether the same oxidation would occur. The results
obtained from the reaction of FeBr2 with DippL1Li and L2Li in
THF (Scheme 2) were very similar to those observed with manga-
nese, except that oxidation of the Fe(II) center did not occur and in-
stead resulted in complex 4A [(DippL1)2Fe]ꢀTHF. Initially, orange
crystals of 4A were obtained from the refluxed reaction of DippL1Li
with FeBr2 (in 37% yield), but in order to establish whether slowing
the reaction would stabilize the formation of mono-ligated [Dip-
pL1FeBr], the reaction was repeated at ꢁ78 °C, again resulting in
crystals of 4A in similar yield (33%).
Due to disorder that could not be resolved in a THF solvent mol-
ecule in the crystal lattice of 4A, the reaction was repeated using a
different solvent system. The lithiation step was carried out in hex-
ane at low temperature, with stirring overnight at RT. The hexane
solution of DippL1Li was then added to an acetonitrile suspension
of FeBr2. Following removal of the volatiles and workup in toluene
as before, [(DippL1)2Fe] (4B), was obtained in similar yield (35%).
The difference between 4A and 4B is that 4B has a solvent free
The heteroleptic ketimine complex, [Cu(OTf)2(DippL1H)2],
(complex 3, Fig. 4), crystallizes in the triclinic space group P1 as
ꢀ
red plates. Examination of the crystal structure of 3 shows that
the copper(II) center lies on an inversion center at half occupancy;
the asymmetric unit of 3 consists of one DippL1H and one triflate,
each ligated to the Cu atom. The O(1)–Cu(1)–O(2) angle between
the two ligands in the asymmetric unit is nearly a right angle
(89.33(6)°), giving rise to a square planar Cu(II) complex with
matching ligands trans to each other in the complete structure.
The DippL1H ligands in 3 are oriented such that the aryl groups
are pointing in opposite directions to each other. The same is true
of the triflate groups.
The copper(II) center adopts only one DippL1H interaction on
the sterically unhindered oxygen side, thus allowing the ligand to
form a closer metal–oxygen contact than if the Cu atom were in a
strained 6-membered N,O-chelate ring. Therefore, the Cu–O(car-
bonyl) bond length of 1.9067(13) Å is shorter than the Cu–O single
bonds of reported N,O-chelated ketiminate Cu(II) complexes
(1.907(3)–1.95(4) Å) [22] and of those in other systems with four
O
atoms around the copper(II) center (1.9131(6)–1.955(2) Å)
[23]. It is identical to the Cu–O bond distance (1.9067(13) Å) in
square planar CuII(PhquinH)2 (PhquinH2 = 1H-2-phenyl-3-hydro-
xy-4-oxoquinoline) [23a]. Although the Cu–O(triflate) bond length
of 1.9619(13) Å is substantially longer than the Cu–O(carbonyl)
Scheme 2. Formation of complexes 4A [(DippL1)2Fe]ꢀTHF, 4B [(DippL1)2Fe], and 5 [L2FeBrꢀLiBr(THF)2].

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A.F. Lugo (neé Gushwa), A.F. Richards / Inorganica Chimica Acta 363 (2010) 2104–2112
Fig. 6. Molecular structure of 5. Hydrogen atoms are omitted for clarity. Thermal
ellipsoids are drawn to 30% probability. Fe(1)–N(1) 2.065(3), Fe(1)–O(1) 2.089(3),
Fe(1)–N(2) 2.295(3), Fe(1)–Br(1) 2.4597(7), Fe(1)–Br(2) 2.5680(7), Br(2)–Li(1)
2.545(7), Li(1)–O(1) 1.894(8), N(1)–C(3) 1.312(5), N(1)–Fe(1)–O(1) 85.44(12),
Fig. 5. Molecular structure of 4B, representative of 4A (4B is the same as 4A but is
solvent free, 4A has disordered THF in the crystal lattice). Hydrogen atoms are
omitted for clarity. Thermal ellipsoids are drawn to 30% probability. Fe(1)–O(1)
1.9065(11), Fe(1)–N(1) 2.0326(12), O(1)–C(1) 1.298(2), N(1)–C(3) 1.323(2), O(1)–
Fe(1)–N(1) 89.92(5), O(1)–Fe(1)–N(1A) 106.75(5), O(1)–Fe(1)–O(1A) 141.18(8),
N(1)–Fe(1)–N(1A) 128.86(7). Symmetry transformations used to generate equiva-
lent atoms: ꢁx + 1, y, ꢁz + ½.
N(1)–Fe(1)–N(2)
79.77(12),
O(1)–Fe(1)–Br(1)
96.10(8),
O(1)–Fe(1)–N(2)
163.14(11), Br(1)–Fe(1)–Br(2) 112.86(3).
lattice while 4A has a disordered THF molecule present. Complex
4B is shown in Fig. 5 and is representative of complex 4A, as the
metric parameters of the molecule remain unchanged. The DippL1
ligands are symmetry-related, with the Fe(II) center existing at half
occupancy in the asymmetric unit. The room-temperature reaction
of L2Li and FeBr2 (Scheme 2) led to complex 5, [L2FeBrꢀLiBr(THF)2],
(Fig. 6) which is isostructural to [L2MnBrꢀLiBr(THF)2] reported pre-
viously by our group [2].
The newly synthesized Fe(II) complexes have different geome-
tries around the iron(II) centers, with [(DippL1)2Fe, 4B, having tet-
rahedral geometry around Fe(II) and [L2FeBrꢀLiBr(THF)2] (5)
exhibiting trigonal bipyramidal geometry. Indeed, 4-coordinate
tetrahedral [26] and 5-coordinate trigonal bipyramidal [26a,27]
Fe(II) centers are each quite common, indicating that attaining
either of these geometries is very favorable. Aside from the differ-
ence in metal oxidation state, [(DippL1)2Fe] is similar to [(Dip-
pL1)2MnCl] [2A] in that it is yet another example of the inability
of DippL1 to sufficiently stabilize a complex containing only one
ketiminato ligand, instead resulting in the bis complex. And once
more, L2, with its tridentate functionality, stabilizes the mono-li-
gated [L2FeBrꢀLiBr(THF)2].
Fig. 7. Molecular structure of 6. Hydrogen atoms are omitted for clarity. Thermal
ellipsoids are drawn to 30% probability. Select bond lengths (Å) and angles (°):
Fe(1)–N(1) 2.073(8), Fe(1)–N(2) 2.074(8), Fe(1)–Br(1) 2.3921(18), Fe(1)–Br(2)
2.3999(18), N(1)–C(1) 1.298(12), N(2)–C(3) 1.284(11), C(1)–C(2) 1.502(13), C(2)–
C(3) 1.520(13), N(1)–Fe(1)–N(2) 89.4(3), N(1)–Fe(1)–Br(1) 113.9(2), N(2)–Fe(1)–
Br(1) 115.9(2), Br(1)–Fe(1)–Br(2) 113.59(7), C(1)–C(2)–C(3) 120.7(8), C(1)–C(2)–
C(5) 107.7(8), C(3)–C(2)–C(5) 107.1(8), C(1)–N(1)–C(7) 120.1(8), C(1)–N(1)–Fe(1)
125.1(7), C(7)–N(1)–Fe(1) 114.8(6), C(3)–N(2)–C(15) 120.9(8), C(3)–N(2)–Fe(1)
127.2(6), C(15)–N(2)–Fe(1) 111.9(6).
To investigate the effects of varying the aryl substituent of Dip-
pL1, the diisopropylphenyl group was replaced with dimethyl-
phenyl, (Dmp), and the reaction with FeBr2 performed under
similar conditions. Surprisingly, solid-state structural analysis of
the resulting yellow crystalline plates revealed that the product
was [{(DmpN@C(Me))2C(H)Me}FeBr2] (complex 6, Fig. 7), the FeBr2
adduct of a trace of the diimine tautomer of neutral b-diketimine
present in the reaction flask (Scheme 3). Because the synthesis of
b-diketimines involves a stepwise condensation reaction between
a dione and two equivalents of a primary amine [1], we observed
the occasional formation of b-diketimine during routine ketimine
synthesis (which involves only a 1:1 stoichiometric dione to pri-
mary amine ratio) [16] even when using an excess of dione.
Complex 6 crystallizes in the monoclinic space group P21/c. The
long Fe–N bond lengths of 2.074(8) and 2.073(8) Å are consistent
with two dative interactions and are accordingly longer than in re-
ported deprotonated b-diketiminate Fe(II) halide structures [28];
(nacnac = [{N(Dipp)C(R)}2CH]ꢁ, R = Me, tBu). These Fe–N single
bond lengths lie between 1.948(2) and 2.021(4) Å. On the other
hand, the Fe–N bond distances of 6 are shorter than the dative
bonds in compounds [Fe(g5
N(CH2CH2N@C(CH3)CHC(CH3)O)–(CH2CH2CH2O)(CH2CH2O)
,l
-L00)]2 (2.2251(15), 2.130(2) Å) L00 =
[29]
and [(2,6-bis(imino)pyridyl)FeBr2] (2.103(6), 2.260(6), 2.271(6) Å)
[27], but this disparity is likely a result of the more sterically
demanding ligands in these particular complexes.
The ligand backbone bond lengths are consistent with the loss
of delocalization to favor the diimine tautomer, with C@N double
bond lengths of 1.298(12) and 1.284(11) Å and C–C single bond
distances (1.502(13), 1.520(13) Å) on either side of
c-position
C(2). Also, the angles around C(2) of 107.1(8)°, 107.7(8)°, and
nacnacFe(l-Cl)2Li(THF)2, Mg(THF)4[LFeCl(l-Cl)]2, and nacnacFeCl
120.7(8)° are indicative of a sp3 hybridized carbon, and the C(2)–

A.F. Lugo (neé Gushwa), A.F. Richards / Inorganica Chimica Acta 363 (2010) 2104–2112
2109
dried using an MBraun-SPS solvent purification system. The li-
gands, L1H and L2H were prepared according to published proce-
dures [12,31]. All other reagents were purchased from Aldrich
and used as received. Crystal data were collected with a Bruker
SMART 1000 diffractometer, molybdenum radiation (k =
0.7107 Å). Crystals were mounted on glass fibers using paratone
oil. The data were corrected for absorption. Structures were solved
by direct methods [32] and refined [32] via full-matrix least
squares. Hydrogen atoms were placed in idealized positions except
hydrogen atoms on NH groups that were localized from the elec-
tron density map and refined with distant restraints. Crystal data
for complexes 1–6 is presented in Tables 3 and 4. The 1H and 13C
NMR spectra were recorded at 300.05 MHz and 75.45 MHz, respec-
tively, on a Varian XL-300, IR analysis was conducted as Nujol
Mulls with NaCl plates on a MIDAC M4000 Fourier transform infra-
red (FT-IR) spectrometer, and mass spectrometry analysis was car-
ried out using a Bruker Esquire 6000 Mass Spectrometer and North
Carolina, Mass Spectrometry Facility. Melting points were deter-
Scheme 3. Synthesis of 6 from diimine tautomer of b-diketimine ligand.
mined in capillaries under
uncorrected.
a nitrogen atmosphere and are
H hydrogen atom was located using the electron density difference
map and freely refined. The following confirm the presence of two
C@N double bonds: an absence of electron density around the
nitrogen atoms in the difference map indicating no N–H hydrogen
atoms, angles around N(1) (114.8(6)–125.1(7)°) and N(2)
(111.9(6)–127.2(6)°) indicative of trigonal planar geometry, and
the C@N double bond stretching frequency observable by an
absorption of 1625 cmꢁ1 in the infrared spectrum All of these
parameters demonstrate that the diimine tautomer of the neutral
b-diketimine ligand has formed two dative interactions with one
molecule of iron(II) bromide, giving rise to a tetrahedral Fe(II)
center (89.4(3)–115.9(2)°). The relatively acute N–Fe–N angle of
89.4(3)° compared to the related b-diketiminate systems
(92.15(12)–96.35(11)°) [28] and is a result of the greater distances
between the nitrogen atoms and iron atom. Finally, the absorptions
at 247 and 324 nm in the UV–Vis spectrum compare well with kmax
(242 and 355 nm) observed in another Fe(II) complex, [FeL(SC6F5)],
L = hydrotris(3,5-diisopropyl-1-pyrazolyl)borate [30].
5.1. Synthesis of 1, [Co(DippL1H)2(NCS)2])3]
DippL1Na (0.5 g, 1.7 mmol) and cobalt(II) thiocyanate (0.30 g,
1.7 mmol) were combined in a Schlenk flask, and toluene was
added. The reaction mixture was refluxed overnight, resulting in
a green solution with unreacted red-brown cobalt(II) thiocyanate
solid. After filtration, concentration, and storage of the PhMe solu-
tion at room temperature, green crystals of 1 were obtained. Yield:
0.12 g, 9%. M.p. 142–147 °C. IR (Nujol Mull):
(w), 971 (m), 1058 (m), 1172 (m), 1301 (w), 1561 (s), 1607 (s),
2067 (w), 3167 (w, br). UV–Vis (THF, 25 °C): k (nm) 324.
m
(cmꢁ1) 767 (w), 935
5.2. Synthesis of 2, [Eu(DippL1H)3(OTf)3]
Three equivalents of DippL1Na (0.30 g, 1.0 mmol) dissolved in
THF was quickly added by cannula to a THF suspension of Eu(OTf)3
(0.20 g, 0.33 mmol) at room temperature, forming a clear dark yel-
low solution. Stirring was maintained overnight, after which time
the solvent was removed in vacuo, leaving a foamy orange solid.
After washing with hexane and extracting into toluene, the solu-
tion was stored at ꢁ30 °C for several days to afford orange crystals
of 2. Yield: 0.16 g, 32%. M.p. 158–162 °C (decomp), 168–170 °C
4. Conclusion
A series of metal complexes have been prepared using two sim-
ilar ketiminato ligands, ArL1H and the less bulky, multidentate
molecule L2H. The reaction of these ligands with transition metals
have led to more consistent observations with two cases of neutral
ligand coordination occurring ([Co(DippL1H)2(NCS)2])3] (1), and
[Eu(DippL1H)3(OTf)3] (2), instead of the intended acid or salt elim-
ination. A similar copper(II) adduct, [Cu(OTf)2(DippL1H)2] (3), was
intentionally synthesized in response to this through neutral li-
gand coordination. X-ray crystallographic analysis on complexes
1–3 and three Fe complexes, 4–6, reinforces the premise that it is
the steric differences of the ligand that governs the formation of
a mono- or bis-ketiminate product. It was furthermore revealed
that achieving the preferred metal geometry is the predominating
factor in the solid-state. We have demonstrated that either the
NNO ligand L2, with its superior steric protection to the metal cen-
ter, or the NO ligand ArL1, with its ability to leave the metal center
open to further ligand attack, can be employed for a range of appli-
cations, depending on the desirability of protection to the metal
center.
(melted). IR (Nujol Mull):
m
(cmꢁ1) 767 (w), 896 (w), 1169 (m),
1534 (m), 1561 (m), 1580 (m), 1606 (m), 3170 (w, br). MS (m/z;
(found (calc.)): 449.6 (450.1) Eu+2OTf, 274.3 (274.4) DippL1H,
160.2 (161.3) Dipp, 148.7 (149.0) OTf. UV–Vis (THF, 25 °C): k
(nm) 620, 627, 644.
5.3. Synthesis of 3, [Cu(OTf)2(DippL1H)2]
Two equivalents of DippL1H (0.35 g, 1.3 mmol) were combined
with copper(II) trifluoromethanesulfonate (0.23 g, 0.64 mmol) in a
Schlenk flask. THF was added rapidly via cannula at ambient tem-
perature and stirring was continued overnight. The solvent was re-
moved from the resultant dark red solution, leaving a foamy
maroon solid. After extraction into toluene and filtration, dark
red crystalline plates of 3 were obtained. Yield: 0.54 g, 93%. M.p.
123–124 °C, crystals became waxy in appearance; 133-139 °C,
melted. IR (Nujol Mull):
m
(cmꢁ1) 1169 (m), 1538 (w), 1575 (w),
3165 (w, br). UV–Vis (THF, 25 °C): k (nm) 326, 488.
5. Experimental
5.4. Synthesis of 4A, [Fe(DippL1)2]ꢀTHF
All manipulations were performed under anaerobic conditions
using standard Schlenk techniques. Hexane and Toluene were
A colorless THF solution of DippL1H (0.35 g, 1.3 mmol) was
cooled to ꢁ78 °C and n-BuLi (2.5 M in hexanes, 0.51 mL, 1.3 mmol)

2110
A.F. Lugo (neé Gushwa), A.F. Richards / Inorganica Chimica Acta 363 (2010) 2104–2112
Table 3
Crystal data and refinement details for 1–3.
Compound name
[Co(DippL1H)2(NCS)2])3] (1)
[Eu(DippL1H)3(OTf)3] (2)
[Cu(OTf)2(DippL1H)2] (3)
Chemical formula
Formula weight
Crystal system
Space group
T (K)
C83H115Co2N8O4S4
1534.93
C57H81EuF9N3O12S3
1419.39
monoclinic
P21/c
C38H54CuF6N2O8S2
908.49
triclinic
triclinic
ꢀ
ꢀ
P1
P1
213(2)
213(2)
213(2)
a (Å)
b (Å)
c (Å)
9.0615(19)
15.794(13)
15.900(3)
74.927(4)
82.832(4)
80.504(4)
2159.0(8)
1
14.4874(17)
17.874(2)
32.448(4)
90.00
105.329(5)
90.00
8.8963(5)
10.3450(6)
11.9037(7)
86.1850(10)
88.9270(10)
79.2940(10)
1074.05(11)
1
a
(°)
b (°)
c
(°)
V (Å3)
8103.4(17)
4
Z
Reflections collected
Independent reflections
Data/restraints/parameter ratio
Rint
10 609
7619
7619/3/485
0.0313
1.181
67 741
14 663
14 663/0/783
0.0871
1.163
9146
3834
3834/1/266
0.0176
1.405
Dcalc (Mg/m3)
Absorption coefficient (mmꢁ1
F(0 0 0)
)
0.531
819
0.919
2928
0.682
475
R indices (all data)
Final R indices [I > 2r(I)]
R1 = 0.1131wR2 = 0.1629
R1 = 0.0556wR2 = 0.1294
R1 = 0.1167wR2 = 0.2313
R1 = 0.0811wR2 = 0.2113
R1 = 0.0355wR2 = 0.0915
R1 = 0.0311wR2 = 0.0871
Table 4
Crystal data and refinement details for 4–6.
Compound name
[Fe(DippL1)2] (4B)
[L2FeBrꢀLiBr(THF)2] (5)
[{(DmpN@C(Me))2C(H)Me}FeBr2] (6)
Chemical Formula
Formula weight
Crystal system
Space group
T (K)
C36H52FeN2O2
600.65
monoclinic
C2/c
C19H36Br2FeLiN2O3
563.11
monoclinic
P21/c
C22H28Br2FeN2
536.13
monoclinic
P21/c
223(2)
223(2)
218(2)
a (Å)
b (Å)
c (Å)
21.719(2)
13.6613(16)
11.9919(14)
90.00
10.3263(11)
12.6720(13)
19.6830(18)
90.00
11.539(2)
14.896(3)
14.167(3)
90.00
a
(°)
b (°)
104.754(2)
90.00
3440.8(7)
4
106.738(5)
90.00
2466.5(4)
4
107.675(4)
90.00
2320.1(8)
4
c
(°)
V (Å3)
Z
Reflections collected
Independent reflections
Data/restraints/parameter ratio
Rint
14 359
3121
3121/0/193
0.0266
1.159
12 730
4453
4453/0/257
0.0412
1.516
11 626
4194
4194/0/252
0.1121
1.535
Dcalc (Mg/m3)
Absorption Coefficient (mmꢁ1
F(0 0 0)
)
0.470
1296
3.871
1148
4.103
1080
R indices (all data)
Final R indices [I > 2r(I)]
R1 = 0.0382wR2 = 0.0893
R1 = 0.0312wR2 = 0.0823
R1 = 0.0570wR2 = 0.1036
R1 = 0.0374wR2 = 0.0913
R1 = 0.1732wR2 = 0.1947
R1 = 0.0585wR2 = 0.1279
was added dropwise, resulting in a clear bright yellow solution,
which was warmed to room temperature gradually and stirred
overnight. The following day, the lithiated ligand solution was
added by cannula to a stirring suspension of FeBr2 (0.28 g,
1.3 mmol), and the reaction mixture was refluxed overnight. The
following day, the resulting dark yellow–brown solution was con-
centrated and filtered. Repeated concentration and filtration for 1
week led to very large orange-brown crystalline blocks of 4A.
Yield: 0.32 g, 37%. Alternatively, 4A was also prepared by the same
method as described above, except by adding the lithiated ligand
solution to a THF solution of FeBr2 (same scale) that had been
cooled to ꢁ78 °C, allowing the solution to warm to RT quickly, stir-
ring overnight, and working up as previously described. The prod-
uct of the reaction performed at low temperature was determined
by a unit cell check. Yield: 0.27 g, 33%. M.p. 91–98 °C. IR (Nujol
5.5. Synthesis of 4B, [Fe(DippL1)2]
Complex 4B was prepared as described above in the synthesis of
4A, except by first substituting hexane for THF during the lithiation
step at 0 °C and then allowing the hexane solution of DippL1Li to
stir overnight at RT. The hexane solution was then added to an ace-
tonitrile (instead of THF) suspension of FeBr2, and worked up the
following day in toluene as described above. Yield: 0.27 g, 35%.
M.p. 73–77 °C. IR (Nujol Mull):
Vis (THF, 25 °C): k (nm) 238, 319.
m
(cmꢁ1) 1156 (m), 1647 (w). UV–
5.6. Synthesis of 5, [L2FeBrꢀLiBr(THF)2]
Complex 5 was prepared by the same procedure as that of 4 on
the same scale: FeBr2 (0.28 g, 1.3 mmol), leading to yellow crystals
of 5. Yield: 0.29 g, 40%. M.p. 109–112 °C (decomp). IR (Nujol Mull):
Mull):
244, 326.
m
(cmꢁ1) 1154 (w), 1652 (w). UV–Vis (THF, 25 °C): k (nm)
m
(cmꢁ1) 1598. UV–Vis (THF, 25 °C): k (nm) 302.

A.F. Lugo (neé Gushwa), A.F. Richards / Inorganica Chimica Acta 363 (2010) 2104–2112
2111
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5.7. Synthesis of 6, [{(DmpN@C(Me))2C(H)Me}FeBr2]
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After combining solid DmpL1Li (0.30 g, 1.3 mmol) and FeBr2
(0.29 g, 1.3 mmol) in a Schlenk flask, THF was added rapidly via
cannula at RT, resulting in a clear yellow–brown solution which
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6. Yield: 0.091 g, 13%. M.p. 114–115 °C (decomp). IR (Nujol Mull):
m
(cmꢁ1) 892 (w), 967 (w), 1154 (m), 1304 (m), 1625 (w). UV–Vis
(c) T.J. Boyle, M.A. Rodriguez, D. Ingersoll, T.J. Headley, S.D. Bunge, D.M.
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Acknowledgements
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