The synthesis and characterization of aluminum, gallium and zinc formamidinates

Article abstract of DOI:10.1016/j.poly.2010.01.020

Reactions of the formamidinate ligand, RN(H)C(H)NR, L^H, (R = 2,6-diisopropylphenyl), with AlMe₃, AlMe₂Cl, GaMe₃ and ZnEt₂ were investigated to examine potential coordination modes of the ligand and th

Full text of DOI:10.1016/j.poly.2010.01.020

Polyhedron 29 (2010) 1411–1422
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The synthesis and characterization of aluminum, gallium and zinc formamidinates
*
Leslie A. Lesikar, Anne F. Richards
Department of Chemistry, Texas Christian University, Fort Worth, TX 76129, United States
Department of Chemistry, La Trobe University, Melbourne 3086, Australia
a r t i c l e i n f o
a b s t r a c t
Reactions of the formamidinate ligand, RN(H)C(H)NR, LH, (R = 2,6-diisopropylphenyl), with AlMe3,
AlMe2Cl, GaMe3 and ZnEt2 were investigated to examine potential coordination modes of the ligand
and the effect of hydrolysis on the products. Nine new complexes have been fully characterized by X-
ray crystallography and other spectroscopic techniques and highlight the diverse coordination modes
of the formamidinate ligand.
Article history:
Received 15 November 2009
Accepted 12 January 2010
Available online 20 January 2010
Keywords:
Formamidinate
Ó 2010 Elsevier Ltd. All rights reserved.
Aluminum and zinc alkyls
Crystal structures
1. Introduction
concluded that the different structural outcomes of the amidinate
ligand (Fig. 1) were dependent on steric factors, with the idealized
Amidinate ligands, [R1NC(R2)NR3]ꢀ, have been used extensively
for an increasingly large number of main group, transition, and lan-
thanide metal complexes [1]. The steric and electronic properties
of the amidine ligands can be easily tailored by modifying the R
group attached to the carbon and nitrogen atoms of the ligand
backbone [2,3]. More specifically when R2 = H the ligands are
classed as formamidines [1b]. An advantage of these ligands is
amidinate structure (B) having 120° angles at the C and N centers
allowing the nitrogen sp2 orbitals to project in parallel directions.
Increased steric interactions between the amidinate substituents
will tend to decrease the NCN angle and will favor either chelated
structures or l1g1g2 structures. Other related aluminum and zinc
amidinate complexes have been reported [9e,11–15] including the
acetamidinate aluminum complexes, [{MeC(NiPr)2}2AlMe] and
[{MeC(NPh)2}2AlMe] [14]. Bisamidinate aluminum alkyl com-
plexes incorporating the formamidinate ligand, LH, are less com-
mon. To the best of our knowledge only a similar aluminum
hydride bisformamidinate complex, [{HC(NDipp)2}2AlH], has been
reported by Jones and co-workers [7]. In contrast to the numerous
reports of aluminum amidinate complexes, relatively few amidi-
nate complexes of zinc(II) have been reported. Examples include
mono-oxygenated tetra nuclear zinc clusters [16], mixed Li/Zn
amidinate oxide oligomers [17], bisamidinate [1a,18], and polynu-
clear zinc structures [19]. Among these results, Gibson, Marshall,
and co-workers completed a study on zinc amidinate complexes
which demonstrated the importance of steric bulk in order to form
mono-chelated zinc(II) triazenide and amidinate complexes [20].
The variety of compounds reported included a bisacetamidinate
zinc(II) complex, [{MeC(NDipp)2}2Zn] (I), a mono-oxygenated tet-
ranuclear zinc complex, [{MeC(NDipp)2}Zn2Me2]2O (II), and the
the presence of
p-bonds which allows the negative charge to be
delocalized across the N–C–N backbone [2,4]. As a result of this
charge delocalization, a variety of possible coordination modes to
metals can exist, including the monodentate (A), bidentate chelat-
ing (B), and bimetallic bridging (C), modes, Fig. 1 [1–5]. While rel-
atively few examples of the monodentate binding mode have been
reported [6,7], the other two are more commonly observed [8].
Interest in metal amidinate complexes has been fueled by their
relative efficacy for a variety of catalytic applications including ole-
fin polymerization and organic transformations [5,9]. Furthermore,
amidinate complexes have become useful precursors for chemical
vapor deposition (CVD) [9] and atomic layer deposition (ALD) [10].
More specifically aluminum amidinate complexes are significant
due to their ability to act as catalysts; for example in olefin poly-
merization, [5,9a,9c,9d] polymerization of ethylene [9d], and C–H
bond activation [9f]. As a result, several monoamidinate and bis-
amidinate aluminum alkyl and halide complexes have been re-
ported [9a,9c–f,10–14]. Jordan and co-workers [9a–d] performed
a study of aluminum amidinate complexes that afforded monoami-
dinate, bis(amidinate), and dinuclear cationic complexes. They
first reported examples of mono(g
2-amidinate) zinc(II) complexes
including, [{tBuC(NDipp)2}ZnN(TMS)2] (III) where the parent
t
homoleptic ligand was butylamidinate, Fig. 2.
Zinc amidinate complexes have potential applications as cata-
lysts for polymerization [19], since the related zinc guanidinate
complexes have been found to act as catalysts in ring-opening
polymerization (ROP) of lactides [19,20]. Additionally, similar zinc
* Corresponding author. Tel.: +1 817 257 6220; fax: +1 817 257 5851.
E-mail address: a.richards@latrobe.edu.au (A.F. Richards).
0277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2010.01.020

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L.A. Lesikar, A.F. Richards / Polyhedron 29 (2010) 1411–1422
Fig. 1. Coordination bonding modes including: (A) monodentate, (B) bidentate chelating, (C) bridging.
Fig. 2. Examples of recently reported zinc amidinate complexes.
complexes with N,N0-chelating ligands, such as the b-diketiminate
ligand, have also shown to act as catalysts in ROP of lactides and
the copolymerization of CO2 with a variety of epoxides [21]. How-
ever, the application potential of zinc amidinates is still unexplored
due to their fairly recent discovery.
The equatorial positions of the trigonal bypyramid are occupied
by the nitrogen and carbon atoms N1, N3, and C51 and, as ex-
pected, result in shorter Al–N bond distances as compared to the
Al–N distances from the N2 and N4 atoms in the axial positions.
These distances are comparable to 1.925(12) Å and 2.096(12) Å re-
ported for Neq–Al and Nax–Al, respectively, in the related complex
[{MeC(NiPr)2}2AlCH3] [14], as well as 1.914(2) Å and 2.041(2) Å ob-
served in [{MeC(NiPr)2}2AlCl] [9c]. The bond angles N3–Al1–C51,
N1–Al1–C51, and N3–Al1–N1 are close to the 120° expected for tri-
gonal bipyramidal geometry. Similarly to the related aluminum
systems [{MeC(NiPr)2}2AlCl] [9c], [{MeC(NiPr)2}2AlCH3] [14],
[{PhC(NSiMe3)2}2AlH] [9f], and [{HC(NDipp)2}2AlH] [7], the N2–
Al1–N4 bond angle of 156.46(10)°, shows a distortion from the ex-
pected 180° which can be attributed to the rather acute bite angles
of 65.89(10)° and 66.29(10)° for N1–Al1–N2 and N3–Al1–N4
respectively. Spectroscopic data confirmed the solid state analysis
with distinct Al–Me resonances at ꢀ0.36 ppm and 12.9 ppm in the
1H and 13C NMR spectra [9a–c,12,14].
In light of the recent developments of N,N0-chelating ligands
supporting low valent metal complexes [22,23], and as a continua-
tion of our earlier work employing chelating N,N0-donor ligands
[24], we were interested investigating the coordination preferences
of the smaller formamidinate ligand RN(H)C(H)NR (R = Dipp = 2,6-
diisopropylphenyl), LH, which lacks a bulky substituent on the car-
bon atom of the NCN backbone for the preparation of aluminum,
gallium and zinc complexes that are potential catalysts and as pre-
cursors to low valent zinc and aluminum complexes. In addition, gi-
ven the continued interest in the effect of water on organometallic
compounds [25] and oxide and hydroxide incorporated organome-
tallic complexes we were interested in examining the effect of
hydrolysis on aluminum and gallium alkyls supported by the form-
amidinate ligand. To this end we investigated the reactions of LH
with AlMe3, AlMe2Cl, GaMe3 and ZnEt2. Our goal was to specifically
focus on the stoichiometric ratio of LH to metal alkyl in an attempt
to obtain varied ligand coordination modes and to examine the ef-
fect of water/oxygen contamination using the same reaction condi-
tions, with the aim being the isolation of metal oxide/hydroxide
complexes. Herein we report our results.
Repeating the reaction of AlMe3 with LH under similar condi-
tions as were employed for the synthesis of complex 1 (Scheme
1) but using solvents directly from the bottle, resulted in the for-
mation of 2, Scheme 2, Fig. 4.
X-ray crystallographic analysis on complex 2, revealed an alu-
minum dimer, with each Al center supported by a deprotonated li-
gand. A terminal methyl group can be observed on each aluminum
center and methoxy groups bridge the metal centers. The presence
of oxygen can be traced to moisture from the undried solvent, as no
methanol was used in the synthesis of 2. We postulate that the
methoxy group was produced by the reaction of a methyl ligand
with a trace amount of oxygen in the reaction medium. As the
methoxy group has a stronger tendency than a methyl group to
act as a bridging ligand the dimeric structure of 2, is not too unex-
pected [27]. Each aluminum center is five-coordinate and can be
described as a distorted trigonal bypyramid in which the nitrogen
atoms occupy the axial positions.
2. Results and discussion
The formamidinate ligand DippN(H)C(H)NDipp, LH, was pre-
pared according to literature procedures [26].
2.1. Discussion of 1, [{HC(NDipp)2}2AlMe]
The room temperature, 2:1 reaction of LH with AlMe3 in toluene
afforded colorless crystals of an Al(III) bisformamidinate complex,
1, that crystallized in the monoclinic space group P21/n, Scheme 1.
Structural analysis revealed that the aluminum atom adopts
distorted trigonal biypyramidal geometry, Fig. 3.
As in complex 1, the negative charge of the ligand is delocalized
over the NCN backbone, which is reinforced by N–C bond lengths
of 1.311(4) and 1.320(4) Å and similar Al–N bonds of 1.973(3)

L.A. Lesikar, A.F. Richards / Polyhedron 29 (2010) 1411–1422
1413
Scheme 1. Aluminum product from the reaction of AlMe3 with LH.
Fig. 3. Molecular structure of 1. Thermal ellipsoids drawn at 30% probability level,
hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles
(°): Al1–N1 1.928(3), Al1–N2 2.093(3), Al1–N3 1.923(3), Al1–N4 2.113(3), N1–C38
1.325(3), N2–C38 1.300(4), N3–C13 1.336(4), N4–C13 1.294(4), Al1–C51 1.963(3),
N2–C38–N1 111.7(3), N4–C13–N3 113.1(3), N1–Al1–N2 65.89(10), N3–Al1–N4
66.29(10), N3–Al1–N2 102.79(11), N3–Al1–N1 121.11(11), N2–Al1–N4 156.46(10),
N3–Al1–C51 119.17(14), N1–Al1–C51 119.71(14).
Fig. 4. X-ray crystal structure of 2. Thermal ellipsoids drawn at 30% probability,
hydrogen atoms are removed for clarity. Selected bond lengths (Å) and angles (°):
Al1–O1 1.875(3), Al1–O1# 1.830(3), Al1–C26 1.965(4), Al1–N1 2.071(3), Al1––N2
1.973(3), Al1–Al1a 2.919(2), N1–C13 1.311(4), Al1–N2 1.973(3), N1–Al1–C26
108.94(15), N2–Al1–N1 65.84(11), O1–Al1–O1# 76.02(11).
reaction of LH with a 1.2 equiv. of AlMe2Cl afforded the monoden-
tate complex 3 in moderate yield, Scheme 3, Fig. 5.
and 2.071(3) Å. The bridged oxygen from the methoxy group has a
typical bond length of 1.875(3) Å, and angle of 76.02(11)°. The N–
Al–N bond angle of 65.84(11) is similar to the values of 65.89(10)°
for N3–Al1–N4 as seen in 1. The Al–Al separation of 2.919(2) Å is
considerably shorter than that of 3.460(3) Å in the related methoxy
bridged, Al4( 3-8-quinolylimide)2(CH3)7( -OCH3)] [28]. In the 1H
NMR a chemical shift associated with the terminal Al–Me groups
is observed at ꢀ0.45 ppm while the methoxy group can be as-
signed to the signal at 0.79 ppm. It is noteworthy to mention that
our attempts to add stoichiometric quantities of water or oxygen to
anhydrous reactions were unsuccessful.
Complex 3 is interesting as despite numerous literature reports
of amidinate complexes, only a handful exhibit the monodentate
coordination mode, with examples of aluminum fairly rare [29].
The crystal structure of 3 indicates that the metal complex crystal-
lizes preferentially as the E isomer with respect to the N1–C13
bond, Fig. 5. The formation of the geometric isomer is predicted
to be a result of the hydrogen attached to the NCN backbone, rather
than a more bulky alkyl group. For example, the formamidinate
l
l
gallium complex, [{HC(NDipp)2H}GaCl3], crystallizes as the
E
isomer, [29] whereas the acetamidinate aluminum [29] and
molybdenum [30] complexes, [{MeC(NDipp)2H}AlI3] and [{MeC(N-
Dipp)2H}Mo(CO)5], crystallize in the Z form. The aluminum center
in 3 has distorted tetrahedral geometry with the Al1–N1 bond
length of 1.915(4) Å in the normal range [1]. A methyl group and
a chlorine atom occupy two of the four tetrahedral coordination
sites and are refined at full occupancy, while the remaining sites
consist of a dative nitrogen bond and a chlorine atom which is dis-
ordered over 3 positions, each at 1/3 occupancy. The attachment of
2.2. Discussion of 3, [{HC(NDipp)2H}AlMeCl2], and 4,
[{HC(NDipp)2H}AlCl1.4I1.6]
Continuing with the reactions of LH with aluminum alkyls, LH
was reacted with dimethyl aluminum chloride. In contrast to the
formation of the bisamidinate complex 1, the room temperature
Scheme 2. The effect of water on the formation of 1 affording complex 2.

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L.A. Lesikar, A.F. Richards / Polyhedron 29 (2010) 1411–1422
Scheme 3. Synthesis of [{HC(NDipp)2H}AlMeCl2].
Fig. 5. X-ray crystal structure of LHAlMeCl2, complex 3. Thermal ellipsoids drawn at
30% probability level, hydrogen atoms and have been omitted for clarity. Selected
bond lengths (Å) and angles (°): Al1–Cl2 2.137(2), Al1–C26 1.960(4), Al1–N1
1.915(4), N1–C13 1.304(5), N2–C13 1.316(5), C26–Al1–Cl2 116.17(17), N1–Al1–Cl2
104.86(13), N1–Al1–C26 111.21(18), N1–C13–N2 128.7(4).
Fig. 6. Crystal structure of the monodentate complex, 4. Thermal ellipsoids drawn
at 30% probability level, hydrogen atoms have been omitted for clarity. Selected
bond lengths (Å) and angles (°): Cl1–Al1 2.146(4), Al1–Cl2 2.050(12), I1–Al1
2.550(4), I2–Al1 2.506(3), Al1–N1 1.908(7), N1–C13 1.316(9), N2–C13 1.306(10),
Cl2–Al1–Cl1 117.1(5), Cl1–Al1–I1 116.06(17), Cl1–Al1–I2 111.97(15), Cl2–Al1–I2
104.5(5), I1–Al1–I2 110.73(14), N1–Al1–Cl1 109.6(2), N1–Al1–Cl2 109.3(5), N1–
Al1–I1 104.0(2), N1–Al1–I2 103.4(2), N2–C13–N1 127.0(7).
the second chlorine, rather than a methyl group, is most likely as a
result from the reaction conditions, employing a slight excess of Al-
Me2Cl to LH at room temperature and given that [Me2AlCl]2 can
disproportionate to provide Me3Al and [MeAlCl2]. Such dispropor-
tionation of aluminum alkyls in the presence of base has been pre-
viously noted, and was initially suggested by Ziegler in the 1950’s
[31]. Comparison of the N1–C13 and N2–C13 bond lengths:
1.304(5) Å and 1.316(5) Å, respectively, suggest delocalization of
the double bond. The hydrogen atom on N2 was located from the
electron density difference map and confirmed by 1H NMR spec-
troscopy with a resonance peak at 8.46 ppm which corresponds
well with chemical shifts of 8.98 and 8.67 ppm reported for the
aluminum acetamidinate and gallium formamidinate examples
[{MeC(NDipp)2H}AlI3] and [{HC(NDipp)2H}GaCl3], respectively
[29]. Similarly, the 1H NMR and 13C NMR confirm the presence of
only one methyl group attached to the aluminum metal center
which is additionally confirmed by mass spectral analysis.
In an attempt to examine the chemistry of this system, we
wished to synthesize [{HC(NDipp)2H}AlClI2] for further reactions.
To this end, the reaction of [{HC(NDipp)2H}AlClMe2] with I2 was
performed, Scheme 4 [32]. Colorless crystals of 4 were isolated in
58% yield and found to crystallize in the monoclinic space group
P21/c, Fig. 6.
Similarly to complex 3, the Al atom has distorted tetrahedral
geometry. The Al1–N1 bond length of 1.908(7) Å and Al–X (X = ha-
lide) bond lengths are within the range for normal values
[1,9c,13,29]. X-ray crystallographic analysis revealed that complex
4 has an iodine and chlorine ion sharing a position with partial
occupancy at 60/40 (I:Cl), which is probably the result of the
in situ reaction of LH with AlMe2Cl to form the desired LAlMe2Cl
Scheme 4. Synthesis of [{HC(NDipp)2H}AlCl1.4I1.6], complex 4.

L.A. Lesikar, A.F. Richards / Polyhedron 29 (2010) 1411–1422
1415
with subsequent treatment with one equivalent of I2. Therefore, it
is likely the reaction mixture contains a mixture of LAlMe2Cl and
LAlCl2Me [31]. The neutral adduct, 4, shows delocalization across
the NCN backbone (N1–C13 is 1.316(9) Å and N2–C13 is
1.306(10) Å). The solid state analysis is confirmed by solution
NMR and IR spectroscopy with indicative N–H and C–H peaks from
the NCN backbone observed. Additionally, the N–H stretch at
3302 cmꢀ1 is observed in the infrared spectra. The positive-mode
mass spectrum of the aluminum adduct complex 4 gave the parent
ion peak at m/z = 644.1 as compared to the calculated m/z = 644.2.
2.3. Synthesis and discussion of [{DippNCHDipp}2Al3l3O(OH)2(Cl/
OH)ClMe], 5
The reaction of AlMe2Cl with LH was repeated with the goal of
isolating an alumoxane or aluminum hydroxide complex; there-
fore, solvents were used directly from the bottle, Scheme 5.
Complex 5 was crystallized from hexane in the monoclinic
space group, P21/c. Structural determination revealed a bridged
amidinate structure in which two deprotonated ligands support
three aluminum centers, Fig. 7.
The aluminum atom, Al1, is coordinated to one nitrogen atom,
N1, and is connected to Al2 and Al3 through a l3 oxygen atom
O1 that links the three aluminum centers. The second aluminum
center, Al2, is coordinated to two nitrogen atoms from two differ-
ent ligands, N2 and N3. The structure can be best described as two
6 membered rings, that share Al2 and O1. Each Al center has
slightly distorted tetrahedral geometry, and Al–N distances and
NCN angles that are similar to those in neutral chelated amidinate
complexes [9c]. As observed in complexes 1–4 the bond lengths of
the ligand: N3–C38, N4–C38 and N1–C13, N2–C13 are intermedi-
ate between that of single and double bond distances, indicating
Fig. 7. Structure of Complex 5. Selected bond lengths and angles: Al1–O1 1.782(4),
Al1–N1 1.928(5), Al1–C51 1.954(6), Al1–Cl1 2.125(3), Al2–O1 1.772(4), Al2–N3
1.916(4), Al3–N4 1.926(4), Al3–O3 1.96(2), Al3–O4 2.035(4), Al3–Cl2 2.17(3), N1–
C13 1.311(7), N2–C13 1.334(6), N3–C38 1.333(6), N4–C38 1.319(6), O1–Al1–N1
99.15(19), O1–Al1–C51 118.8(2), N1–Al1–C51 109.3(2), O1–Al1–Cl1 107.51(15),
N1–Al1–Cl1 105.30(17), C51–Al1–Cl1 114.9(2), O1–Al2–N3 101.72(19), N3–Al2–N2
120.5(2), N2–Al2–O2 109.99(17), C13–N1–Al1 124.2(4), C13–N2–Al2 120.5(3),
C39–N4–Al3 113.0(3).
2.4. Synthesis and discussion of complex 6, [{DippNCHDipp}2
Ga3(lO)2(Me)7AlLi]
Gallium amidinate complexes have previously been synthe-
sized by the reaction of gallium halides with preformed amidinate
reagents [11,13,29]. Typically the reactions with bulky amidinates
favor a chelating bonding mode because of the steric crowding
[11,13,29,34]. Given the limited numbers of gallium amidinate
complexes [35] and their potential as volatile precursors to nitride
materials [36] we were interested in studying the reactions of
GaMe3 with LH. To this end, GaMe3 was prepared from the reaction
of GaCl3 and 3MeLi. The in situ generated GaMe3 was reacted with
LH at room temperature in toluene. Despite numerous attempts
and various reaction conditions, we were unable to obtain crystals
from this particular reaction. However, our attempts to yield an Al/
Ga, bimetallic complex through the reaction of GaMe3 with LH with
sequential addition of 1 equivalent of AlMe3, (Scheme 6) afforded
complex 6, Fig. 8.
significant
p delocalization within the NCN unit.
Comparing complex 5 with dimeric 1, [{HC(NDipp)2}2AlMe] and
monomeric AlMeCl2LH, 3, it can be observed that each Al center is
coordinated to at least one nitrogen atom, therefore it seems likely
that the presence of moisture and/or oxygen promoted hydrolysis
of a chloride or methyl groups and promoted further reactions to
occur. The overall cationic charge of +9, from 3 Al3+ ions is balanced
by two ligands, a terminal methyl groups, two terminal hydroxide
groups, a chloride atom, bridged oxygen atom and a mixed occu-
pancy site (Cl2/O3) comprising of a chloride and hydroxide which
was successfully refined as 1/3Cl, 2/3 O. The terminal methyl and
hydroxyl groups are confirmed by 1H NMR with an AlMe resonance
observed at ꢀ0.11 ppm and 3 signals between 0.39–0.91 for the
hydroxide groups, which are in the range for other reported Al–
OH signals [33]. In the infrared spectra of 5, an OH stretch can be
found at 3349 cmꢀ1 which is similar to that observed in related
complexes [33].
The crystal structure of 6 was found to consist of two bridging
ligands that support three gallium atoms and one aluminum atom.
The N1–C13–N2 backbone bridges the gallium atoms Ga1 and Ga2
Scheme 5. Synthesis of Complex 5.

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L.A. Lesikar, A.F. Richards / Polyhedron 29 (2010) 1411–1422
Scheme 6. Synthesis of complex 6.
while Ga3 is coordinated to the second amidinate ligand through
N3. The three gallium atoms and the aluminum atom show slightly
distorted tetrahedral geometry. The tetrahedral sites around Ga1
are occupied by a nitrogen atom, N1, two terminal methyl groups
and a bridged oxygen atom O2 which bridges all three gallium
atoms. Ga3 also has a terminal methyl group and is further coordi-
nated to a second oxygen atom O1 to which there is an AlMe3 frag-
ment attached. Ga2 is coordinated to the second formamidinate
ligand and two methyl groups. The second nitrogen atom of this li-
gand, N4, has a lithium atom coordinated, which in turn has coor-
dination to the oxygen atoms O1, O2 and Ga3. One can notice a
distinct difference in the Ga–N bond lengths. The Ga3–N3 distance
of 2.003(3) is comparable to that observed in neutral Ga imine
complexes (2.007(2) Å) (e.g., N-methylsalicylaldiminate gallium
dimethyl [37] while the N1–Ga1 distance (2.026(3) Å) is compara-
ble to those in bulky gallium amide complexes [38], e.g.
{Me2Ga(NHPh)}2 (2.03(7) Å) [27a]. Other bond distances and an-
gles are unexceptional, for example, the average Ga–C(Me) dis-
tance of ꢁ1.95 Å in 6 is comparable to 1.957(8) Å observed in
hydroxyl(methyl)gallium bis(pyrazolyl)dimethylboron [39a] and
If one considers the structure of 6 but without the lithium atom
and AlMe3 fragment present then 6 is structurally very similar to
the Al3 cluster, 5. Therefore, it is possible that a similar product
is formed that but does not crystallize out. As with complex 2,
the synthesis of 5 and 6 was attempted using stoichiometric
amounts of water and oxygen but were unsuccessful.
For charge balance, the +13 charge from three Ga atoms, one Al
atom and one Li+ is offset by eight terminal methyl groups, two an-
ionic ligands, an oxygen atom (O1). Presumably the oxygen atom,
O2, is protonated although the hydrogen atom could not be located
from the difference map. The solid state structure of 6 is confirmed
by NMR and mass spectral analysis. The 1H NMR spectra of 6 shows
three resonances at – 0.46, ꢀ0.48–0.52 ppm associated with the
methyl groups located on the aluminum atom. The methyl groups
from the gallium atoms are observed at similar values of 0.41 ppm
and 0.39 ppm and overlapping single signals at 0.5–0.6 ppm. These
values are similar to those reported for {tBuC(NiPr)2}GaMe.{tBuC-
+
(NiPr)2}GaMe2 that displays Ga–Me resonances at 0.55, 0.45,
0.35 [9a] and the related [LGaOH(Me)], (L = [HC{(CMe)(2,6-
iPr2C6H3N)}2]) that shows two resonances (d +0.08 and ꢀ0.57
ppm), which can be attributed to the protons of OH and GaMe
groups, respectively [41a]. The mechanism for the formation of 6
is unclear but work is underway to elucidate a pathway and to pre-
pare a series of bimetallic complexes.
in [(2,6-Mes2C6H3Ga(Me)(l2-OH)]2 (average 1.947 Å) [39b]. Simi-
larly the Ga–O distances ꢁ1.96 Å are comparable to reported liter-
ature values [40].
2.5. Discussion of 7, [{HC(NDipp)2}2Zn]
In order to compare the coordination preferences and products
obtained from the aluminum alkyl reactions with LH, the reactions
with ZnEt2 were performed under varying conditions.
Colorless crystals of complex 7 were obtained by reacting 1
equiv. of ZnEt2 with 2 equiv. of LH at room temperature, Scheme 7.
As was previously observed in complex 1, a homoleptic zinc
complex was isolated through elimination of both ethyl groups,
Fig. 9. Similar bisformamidinate zinc complexes have been re-
ported, examples including [{PhC(NSiMe3)2}2Zn] and [{MeC(N-
Dipp)2}2Zn] [17–20].
Structural analysis of complex 7 shows the zinc atom achieves a
coordination number of four with N–Zn–N bond angles between
123.77(12)° and 150.08(17)° which can be attributed to the rather
acute bite angle 65.97(12)° of the formamidinate ligand. The bond
lengths within the ligand backbone N1–C13 and N2–C13 are sim-
ilar at 1.326(5) Å and 1.306(4) Å and are indicative of delocaliza-
tion over the NCN backbone. The Zn–N bond distances of
2.013(3) Å and 2.036(3) Å are similar to the Zn–N bond distances
that have been reported for similar structures, for example; the
acetamidinate zinc(II) complex [{MeC(NDipp)2}2Zn] [20] has Zn–
N bond distances of 2.031(5) Å and 2.038(4) Å. The 1H and 13C
Fig. 8. Solid state structure of the bimetallic amidinate cluster, 6. Selected bond
lengths (Å) and angles (°): N1–Ga1 2.026(3), Ga1–C26 1.959(4), Ga1–O1 1.937(3),
Ga1–C27 1.973(4), N2–Ga2 1.985(3), Ga2–C28 1.955(5), Ga2–Li1 2.678(8), Li1–O1
2.026(8), Li1–O2 1.923(9), Al1–O2 1.641(3), Al1–C61 1.860(6), O1–Ga3 1.990(2),
Ga3–N3 2.003(3), Ga3–C58 1.983(5), N1–C13 1.315(5) C13–N2 1.329(5), N3– C44
1.338(5), N4–C44 1.302(5), Ga3–O1 1.990(2), N1–Ga1–C26 102.32(16), N2–Ga2–
C28 113.78(17), N3–Ga3–C57 110.29(18), C13–N2–Ga2 119.1(3), C59–Al1–C60
109.9(3).

L.A. Lesikar, A.F. Richards / Polyhedron 29 (2010) 1411–1422
1417
Scheme 7. Synthesis of the zinc bisformamidinate 7.
spectra are consistent with the solid state structures. Mass spec-
tra analysis of crystalline 7 displayed the parent ion peak at
m/z = 793.4, while infrared analysis displayed the characteristic
peaks of 1667, 1634 (C@N), 1557 (C@C), and 1319 cmꢀ1 (C–N).
2.6. Discussion of 8, [{HC(NDipp)2}Zn2Et2]2(O), and 9,
[{HC(NDipp)2}2Zn3Et2](OEt)2
Continuing with the reactions of LH and ZnEt2 with the aim of
isolating a heteroleptic Zn(II) complex, the reaction stoichiometery
was altered. From the 1:1 reaction of LH with ZnEt2 colorless crys-
tals of complexes 8 and 9 were isolated in low yield. Both were
ꢀ
found to crystallize in the triclinic space group P1, Scheme 8.
Single crystal X-ray analysis of complex 8 revealed a [L2Zn4-
(Et4)O] complex that has a Zn4O core in which there is a four-coor-
dinate central oxygen atom surrounded by four zinc atoms, Fig. 10.
The formamidinate ligand, LH, exhibits a bimetallic bridging coor-
dination mode with each nitrogen atom coordinated to a zinc
atom. The overall +8 charge of the zinc cations is balanced by a
ꢀ1 charge observed for each NCN ligand, (ꢀ2), as well as a ꢀ2
charge on the central oxygen atom and a ꢀ1 charge for each ethyl
group, (ꢀ4). The central oxygen atom O1 adopts distorted tetrahe-
dral geometry with O–Zn bond angles ranging from 98.74(9)° to
Fig. 9. Molecular structure of 7. Thermal ellipsoids drawn at 30% probability level,
hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles
(°): Zn1–N1 2.013(3), Zn1–N2 2.036(3), N1–C13 1.326(5), N2–C13 1.306(4), N1–
Zn1–N2 65.97(12), N1–Zn1–N2#1 123.77(12), N2–C13–N1 113.8(3).
Dipp
C
Dipp
C
N
N
Zn
Zn
Zn
Zn
N
N
H
H
H
O
Dipp
C
Dipp
N
H
N
2
Dipp
Dipp
Dipp
N
H
Dipp
C
Zn
N
N
N
O
Dipp
Zn
H
C
O
Zn
Dipp
Scheme 8. Preparation of the zinc–oxygen cluster formamidinates: 8 and 9.

1418
L.A. Lesikar, A.F. Richards / Polyhedron 29 (2010) 1411–1422
Fig. 10. Molecular structure of complex 8. On the left hand side is the asymmetric unit and on the right hand side is the central Zn4O core of the molecule. Thermal ellipsoids
drawn at 30% probability level, hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (°): Zn1–N2 1.965(2), Zn2–N4 1.966(2), Zn3–N3 1.988(2),
Zn4–N1 1.969(2), Zn1–O1 1.966(2), Zn2–O1 1.965(2), Zn3–O1 1.948(2), Zn4–O1 1.960(2), Zn1–Zn2 2.9835(5), Zn2–Zn4 3.1083(6), N1–C13 1.317(4), N2–C13 1.323(4), Zn2–
O1–Zn1 98.74(9), Zn3–O1–Zn4 109.72(10), Zn4–O1–Zn1 114.41(9), Zn3–O1–Zn2 114.69(10), O1–Zn1–C28 119.11(13), O1–Zn2–C57 119.34(16), O1–Zn3–C55 123.44(14),
C26–Zn4–O1 118.20(16).
114.69(10)° and bond lengths in the range of 1.948(2) Å (Zn3–O1)–
1.966(2) Å (O1–Zn1) and 1.965(2) Å (Zn1–N2) to 1.988(2) Å (Zn3–
N3) for N–Zn. These bond lengths compare well to three related
tetrahedral Zn4O cluster amidinate compounds which exhibit bond
lengths in the range of 1.912(4)–1.927(3) Å (O–Zn) and 2.023(5)–
2.045(4) Å (N–Zn) for [{HC(NPh)2}6Zn4](O) [16a], 1.923(15) Å (O–
Zn) and 2.015(6) Å (N–Zn) for [{HC(Np-Tol)2}6Zn4](O) [17], and
1.948(13)–1.953(14) Å (O–Zn) and 1.962(17)–1.974(18) Å (N–Zn)
for [{MeC(NDipp)2}Zn2R2]2(O) (R = Me, Et) [20]. Complex 8 exhibits
Zn1???Zn2 and Zn2–Zn4 distances of 2.9835(5) Å and 3.1083(6) Å,
respectively, which correspond closely to the [{HC(NPh)2}6Zn4](O)
and [{HC(Np-Tol)2}6Zn4](O) complexes with distances at 3.135 Å
(mean average) and 3.145(25) Å respectively, [16] and are slightly
longer than the sum of van der Waals radii for Zn–Zn, 2.78 Å [42].
In an attempt to eliminate oxygen contamination, the reaction
was performed numerous times and it was found that complex 8
is reproducibly isolated. However, on one attempt complex 9 was
isolated as colorless needles, Fig. 11. The asymmetric unit of 9
contains two crystallographically independent units. There are no
significant geometric differences between the two units, however
no higher symmetry space group could be determined by PLATON
[43]. Similarly to 8, the crystal structure of 9 reveals a bimetallic
bridging binding mode, however, in contrast 9 contains two four-
coordinate oxygen atoms within the central core each with a dis-
torted tetrahedral geometry. The coordination sites around each
oxygen atom are occupied by 3 zinc atoms and 1 ethyl group.
Examples of a four-coordinate oxygen center with bonds to three
zinc atoms and one ethyl group are rare but not unprecedented
[44]. The ethyl group attached to the oxygen (O–Et) is likely a
result from the elimination of ethane and subsequent coordination
to the oxygen atom; indeed the reactions of ZnMe2 with N,N0-
diphenyl benzamidine are found to react with molecular oxygen
[18]. Each zinc atom in
9 displays distorted tetrahedral
geometry. The Zn–Zn separation falls within the range of
Fig. 11. Molecular structure of 9. On the left hand side is the asymmetric unit and on the right hand side is the Zn3O2 core of the molecule. Thermal ellipsoids drawn at 30%
probability level, hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (°): Zn1–N1 1.986(3), Zn1–N3 1.984(3), Zn2–N2 2.010(3), Zn3–N4
2.026(3), Zn1–O1 2.043(3), Zn1–O2 2.036(3), Zn2–O1 2.112(3), Zn2–O2 2.179(3), Zn3–O1 2.177(3), Zn3–O2 2.111(3), Zn1–Zn2 2.9905(7), Zn1–Zn3 2.9990(7), Zn2–Zn3
2.9454(8), N1–C13 1.331(5), N4–C38 1.305(5), Zn2–O1–Zn3 86.73(11), C51–O1–Zn1 126.0(3), C53–O2–Zn2 128.4(3), N3–Zn1–O2 110.85(13), N1–Zn1–O1 113.10(13), N2–
Zn2–O1 103.29(13), C57–Zn2–O1 119.9(2), N4–Zn3–O1 101.28(12), C55–Zn3–O2 122.56(17).

L.A. Lesikar, A.F. Richards / Polyhedron 29 (2010) 1411–1422
1419
2.9454(8)–2.9990(7) Å, which correspond closely to the Zn–Zn dis-
4. Experimental
tances of complex 8 and other previously mentioned examples
[42]. However, only three zinc atoms are present in complex 9 with
one zinc atom attached to both formamidinate ligands via N,N0-
chelation. Additionally, the Zn–N, Zn–O and N–C–N bond lengths
for complexes 8 and 9 are similar to those observed for related
amidinate analogs, such as [{HC(NPh)2}6Zn4]O, [16a] [{HC(N-p-
CH3C6H4)2}6Zn4]O, [16b] and [{MeC(NDipp)2}2Zn4]O [20]. The
source of the oxygen contamination in both complexes 8 and 9 is
not confirmed, but likely from atmospheric oxygen/water moisture
during the reaction. Tetranuclear zinc amidinate clusters contain-
ing oxygen have been previously reported and are known to be
highly oxophilic [17,20]. Spectroscopic data confirmed the struc-
tures of 8 and 9. Specifically, distinctive peaks for CN, C@C, and
C–N were observed in the infrared spectroscopy with stretching
bands at 1667, 1597, 1557, and 1320 cmꢀ1, respectively, for com-
plex 8, and 1665, 1597, 1561, 1287 cmꢀ1 for complex 9.
All manipulations were performed under anaerobic conditions
using standard Schlenk techniques. Hexane and Toluene were
dried using an MBraun-SPS solvent purification system. The amidi-
nate ligand, LH = (Dipp)N(H)C(H)N(Dipp), was prepared according
to published procedures [26]. All other reagents were purchased
from Aldrich and used as received. Crystal data were collected with
a Bruker S0MART 1000 diffractometer, molybdenum radiation
(k = 0.7107 ÅA) at ꢀ60 °C. Crystals were mounted on glass fibers
using paratone oil. The data were corrected for absorption. Struc-
tures were solved by direct methods [45] and refined [45] via
full-matrix least squares. Hydrogen atoms were placed in idealized
positions except those for M–CH3 groups (M = Al, Ga, Zn) and OH
groups that were localized from the electron density map and re-
fined isotropically with distant restraints. Crystal data for 1–9 is gi-
ven in Tables 1 and 2. The 1H and 13C NMR spectra were recorded
at 300.05 MHz and 75.45 MHz respectively on a Varian XL-300, IR
analysis was conducted as Nujol Mulls with NaCl plates on a MID-
AC M4000 Fourier transform infrared (FT IR) spectrometer, and
mass spectrometry analysis was carried out using a Bruker Esquire
6000 Mass Spectrometer. Melting points were determined in cap-
illaries under a nitrogen atmosphere and are uncorrected.
3. Conclusion
The reactions of the neutral formamidinate, DippN(H)C
(H)NDipp, with aluminum, gallium and zinc alkyls have produced
nine complexes featuring different coordination modes. These nine
compounds expand on the large volume of amidinate group 13
complexes. The chelating binding mode is observed which is in
accordance with the sterically large ‘Dipp’ substituents attached
to the nitrogen atoms of the NCN backbone. Yet, X-ray crystallo-
graphic analysis revealed that steric constraints do not govern
coordination modes entirely. Through manipulation of reaction
stoichiometry a variety of coordination compounds that feature
4.1. Synthesis of 1, [{HC(NDipp)2}2AlMe]
LH 0.35 g (0.96 mmol) was dissolved in ꢁ20 mL of toluene. At
room temperature 0.24 mL of AlMe3 (2.0 M solution) was added
by syringe. The evolution of gas could be observed by small bub-
bles within the reaction mixture. Stirring was maintained for 2 h,
after which time, all volatiles were removed in vacuo. The remain-
ing white sold was extracted into hexanes and stirred. The solution
was filtered and stored at room temperature overnight yielding
colorless crystals of 1.
monodentate and
g
2-bridging coordination as well as cluster com-
plexes were observed. The monomeric complexes and aluminum
are adducts, as previously seen with AlX3 due to their vacant orbi-
tal. However, the
g
2-bridging and cluster type zinc complexes are
likely the result of the lack of steric protection on the carbon of the
NCN backbone. Future work will focus on the further chemistry of
the reported structures and will investigate the steric influence of
the formamidinate ligand and reaction conditions in metal com-
plex formation.
Yield: 0.13 g, 25%. M.p. >250 °C. 1H NMR (C6D6, 25 °C): d (ppm)
ꢀ0.36 (s, 3H, AlCH3), 0.57–1.15 (m, 48H, CH(CH3)2), 2.33 (septet,
1
8H, JH–H = 6.9 Hz, CH(CH3)2), 6.58–6.98 (m, 12H, Haryl), 7.33 (s,
2H, NCHN); 13C NMR (C6D6, 25 °C): d (ppm) 12.9 (AlCH3), 21.1
(CH(CH3)2), 22.3 (CH(CH3)2), 26.9 (CH(CH3)2), 117.5 (m-ArC),
Table 1
Crystal data for compounds 1–5.
Compound name
1
2
3
4
5
Chemical formula
Formula weight
Crystal system
Space group
T (K)
C51H73AlN4
769.11
monoclinic
P21/n
C54H82Al2N4O2
873.20
monoclinic
P21/c
C26H39N2AlCl2
477.47
monoclinic
P21/c
C25H36N2AlCl1.4I1.6
644.21
monoclinic
P21/c
C51H75Al3Cl1.33N4O3.67
930.91
monoclinic
P21/c
213(2)
223(2)
213(2)
213(2)
213(2)
a (Å)
b (Å)
c (Å)
14.6183(11)
16.3071(13)
21.2633(18)
90
12.641(3)
16.579(4)
14.282(4)
90
10.4346(12)
20.253(2)
14.0541(16)
90
10.8346(12)
14.7019(16)
21.1937(18)
90
18.885(3)
12.3056(19)
29.332(4)
90
a
(°)
b (°)
110.087(2)
90
4760.5(7)
4
114.804(4)
90
2717.0(12)
2
108.602(2)
90
2814.9(6)
4
118.063(4)
90
2979.0(5)
4
127.304(7)
90
5422.1(14)
4
c
(°)
V (Å3)
Z
Reflections collected
Independent reflections
Data/restraints/parameter ratio
Rint
22909
8591
8591/0/522
0.0847
1.073
18353
4876
4876/0/293
0.1149
1.067
14010
5057
5057/1/288
0.0490
1.127
24781
5363
5363/1/302
0.0622
1.436
41773
9790
9790/0/577
0.0915
1.140
Dcalc (Mg/m3)
Absorption coefficient (mmꢀ1
F(0 0 0)
)
0.079
1680
0.094
952
0.277
1024
1.866
1286
0.178
2000
R indices (all data)
R1 = 0.1568
wR2 = 0.1761
R1 = 0.0610,
wR2 = 0.1321
0.210 and -0.271
R1 = 0.1673
wR2 = 0.2109
R1 = 0.0828,
wR2 = 0.1753
0.477 and ꢀ0.309
R1 = 0.1309
wR2 = 0.2317
R1 = 0.0746,
wR2 = 0.1954
1.146 and ꢀ0.893
R1 = 0.1365
wR2 = 0.2096
R1 = 0.0598,
wR2 = 0.1462
0.666 and ꢀ0.929
R1 = 0.1753
wR2 = 0.2186
R1 = 0.0835
wR2 = 0.2186
1.002 and ꢀ0.562
Final R indices [I > 2r(I)]
Largest difference in peak and hole
(e Åꢀ3
)

1420
L.A. Lesikar, A.F. Richards / Polyhedron 29 (2010) 1411–1422
Table 2
Crystal data for compounds 6–9.
Compound name
6
7
8
9
a
Chemical formula
Formula weight
Crystal system
Space group
T (K)
C58H94AlGa3LiNO2
1122.45
C50H70N4Zn
792.47
orthorhombic
C2221 [46]
213(2)
C58H90N4OZn4
1120.82
C58H90N4O2Zn3
1071.45a
triclinic
triclinic
triclinic
ꢀ
ꢀ
ꢀ
P1
P1
P1
223(2)
213(2)
213(2)
a (Å)
b (Å)
c (Å)
10.7199(9)
15.3561(14)
19.2095(17)
95.399(2)
93.933(2)
100.289(2)
3085.7(5)
2
14.4081(18)
20.4135(18)
16.0402(17)
90
90
90
12.1524(6)
13.9272(7)
19.6143(10)
70.5200(10)
83.7430(10)
71.4230(10)
2966.6(3)
2
10.6164(8)
22.9140(17)
23.9395(19)
97.0060(10)
96.0100(10)
90.703(2)
5746.5(8)
2
a
(°)
b (°)
c
(°)
V (Å3)
4717.7(9)
4
Z
Reflections collected
Independent reflections
Data/restraints/parameter ratio
Rint
25026
11067
11067/0/626
0.0501
1.208
10204
4500
4500/0/258
0.0589
1.116
17076
12818
12818/0/604
0.0221
1.255
45775
20620
20620/4/1215
0.0499
1.235
Dcalc (Mg/m3)
Absorption Coefficient (mmꢀ1
F(0 0 0)
)
1.355
1190
0.556
1712
1.636
1188
1.284
2276
R indices (all data)
R1 = 0.0941
wR2 = 0.1157
R1 = 0.0457, wR2 = 0.0950
0.523 and ꢀ0.460
R1 = 0.1067
wR2 = 0.1102
R1=0.0507, wR2 = 0.0948
0.413 and ꢀ0.617
R1 = 0.0719
wR2 = 0.1097
R1 = 0.0410, wR2 = 0.0914
0.530 and ꢀ0.485
R1 = 0.1016
wR2 = 0.1352
R1=0.0519, wR2 = 0.1117
0.968 and ꢀ1.150
Final R indices [I > 2
r(I)]
Largest difference in peak and hole (e Åꢀ3
)
a
Formula based on one molecule in the asymmetric unit.
121.7 (m-ArC), 122.0 (p-ArC), 122.3 (p-ArC), 124.5 (o-ArC), 130.8
(o-ArC), 139.1 (ArC-N), 143.0 (ArC-N), 166.4 (NCN); IR (
cmꢀ1, Nu-
jol mull): 1666 (m), 1635 (w), 1553 (m), 1324 (w), 1292 (w), 1179
(m), 966 (m), 934 (m), 755 (m), 664 (w), 636 (w); MS (m/z; found
(calc.)): 769.4 (769.1) M+, 365.3 (364.5) LH, 176.1 (176.2) LH–
(Dipp)NC(H), 160.3 (159.4) M–AlMe–(DippN)2C(H)–Dipp–iPr.
Yield: 0.15 g, 46%. M.p. 243–249 °C (decomp). 1H NMR (C6D6,
25 °C): d (ppm) ꢀ0.85 (s, 3H, AlCH3), 1.15–1.28 (m, 24H, CH(CH3)2),
2.82–3.23 (m, 4H, CH(CH3)2), 7.09–7.25 (m, 6H, Haryl), 7.33 (s, 1H,
NCHN), 8.46 (s, 1H, NH); 13C NMR (C6D6, 25 °C): d (ppm) ꢀ2.0
(AlCH3), 19.1 (CH(CH3)2), 19.6 (CH(CH3)2), 20.7 (CH(CH3)2), 21.1
(CH(CH3)2), 22.8 (CH(CH3)2), 27.3 (CH(CH3)2), 120.5 (m-ArC),
121.2 (m-ArC), 121.8 (p-ArC), 123.5 (p-ArC), 133.8 (o-ArC), 135.8
t
(o-ArC), 141.4 (ArC-N), 143.3 (ArC-N), 166.5 (NCN); IR (t
cmꢀ1, Nu-
4.2. Synthesis of 2, [{HC(NDipp)2}AlMe(lOMe)]2
jol mull): 3067 (shoulder, N–H stretch), 1680 (m), 1639 (m), 1596
(m), 1556 (s), 1528 (m), 1335 (m), 1199 (m), 934 (m), 854 (m), 776
(m), 682 (m); MS (m/z; found (calc.)): 478.5 (477.5) M+, 365.3
(364.5) M–AlCl2Me, 176.2 (176.3) M–AlCl2Me–(Dipp)NC(H).
LH 0.35 g (0.96 mmol) was dissolved in toluene. AlMe3 (0.6 mL
of a 2.0 M solution) was added by neat with stirring. Stirring was
maintained for 1 h, the toluene was removed under vacuum and
the product extracted into hexane (direct from the bottle). The pale
yellow solution was concentrated and stored at room temperature
over night affording colorless crystals of 2. Yield: 0.26 g, 30%. M.p.
248–252 °C (decomp). 1H NMR (C6D6, 25 °C): d (ppm) ꢀ0.45 (s, 6H,
AlCH3), 0.79 (s, 6H, OCH3), 1.55 (d, 12H, CH(CH3)2, 1JH–H = 6.90 Hz),
4.4. Synthesis of 4, [{HC(NDipp)2H}AlCl1.4I1.6]
To a 25 °C toluene solution (ꢁ20 mL) of LH (0.35 g, 0.96 mmol)
was added 1.2 equiv. of AlMe2Cl (1.2 mL, 1.2 mmol) drop-wise.
Evolution of gas was observed. Stirring was maintained for 3 h,
after which time freshly sublimed I2 (0.24 g, 0.96 mmol) was added
quickly to the reaction mixture. The dark purple reaction was then
allowed to stir for 4 days until the reaction mixture became a clear
pale tan color. The toluene was then removed in vacuo and the pale
tan solid was extracted into hexane, concentrated, and filtered.
Overnight storage of the hexane solution at room temperature
afforded colorless crystalline plates of 4. Yield (based on ligand):
0.36 g, 58%. M.p. 231–235 °C (decomp). 1H NMR (C6D6, 25 °C): d
1
1.69 (d, 36H, CH(CH3)2, JH–H = 6.90 Hz), 3.97–4.11 (m, 8H,
1
CH(CH3)2, JH–H = 6.90 Hz), 7.52–7.62 (m, 12H, Ar–H), 8.06 (s, 2H,
NC(H)N). 13C NMR (C6D6, 25 °C): d (ppm) 1.7 (OCH3), 12.9 (AlCH3),
21.9 (CH(CH3)2), 22.0 (CH(CH3)2), 23.0 (CH(CH3)2), 23.3 (CH(CH3)2),
24.6 (CH(CH3)2), 24.7 (CH(CH3)2), 26.6 (CH(CH3)2), 27.0 (CH(CH3)2),
28.8 (CH(CH3)2), 30.5 (CH(CH3)2), 122.3 (m-ArC), 122.9 (m-ArC),
125.0 (p-ArC), 126.3 (p-ArC), 126.6 (ipso-ArC), 126.9 (ipso-ArC),
143.1 (o-ArC), 144.2 (o-ArC), 165.6 (NCN), 168.5 (NCN). MS (m/z;
found (calc.)): 435.1 (436.5), M+, 420.5 (421) M–Me, 365.2
1
t
(364.5) LH, IR ( cmꢀ1, Nujol mull): 3348 (w), 1667 (m), 1588
(ppm) 0.80 (d, 24H, JH–H = 7.2 Hz, CH(CH3)2), 2.87 (septet, 4H,
1
1JH–H = 6.8 Hz, CH(CH3)2), 6.60 (d, 4H, JH–H = 7.5 Hz, m-Haryl), 6.78
(m). 1538 (w), 1287 (w), 936 (w), 753 (w).
1
(t, 2H, JH–H = 7.8 Hz, p-Haryl), 6.86 (s, 1H, NCHN), 12.05 (s, 1H,
NH); 13C NMR (C6D6, 25 °C): d (ppm) 22.2 (CH(CH3)2), 27.7
(CH(CH3)2), 122.9 (m-ArC), 128.8 (p-ArC), 130.0 (o-ArC), 144.4
4.3. Synthesis of 3, [{HC(NDipp)2H}AlMeCl2]
(ArC-N), 157.7 (NCN); IR (
t
cmꢀ1, Nujol mull): 3302 (shoulder,
LH 0.5 g (1.37 mmol) was dissolved in ꢁ20 mL of toluene. At
room temperature, one equivalent of a 1.0 M solution of AlMe2Cl
(1.4 mL) was added drop-wise. The reaction was stirred at ambient
temperature for 2–3 h after which time the toluene was removed
under reduced pressure and the remaining white solid was ex-
tracted into hexanes. After 14 h at room temperature colorless
crystals of 3 were isolated.
N–H stretch), 1682 (m), 1638 (m), 1595 (m), 1557 (m), 1337 (m),
935 (s), 670 (m); MS (m/z; found (calc.)): 644.1 (644.2) M+, 422.3
(422.6) M+–I–Cl–Cl–iPr, 365.4 (364.5) LH, 144.2 (144.1) LH–
(Dipp)NH–iPr–H, 130.2 (130.1) LH–(Dipp)NHC(H)–iPr–2H, 117.2
(117.1) LH–Dipp–iPr–iPr. The positive-mode mass spectrum of
complex 3 gave the parent ion peak at m/z = 644.1 as compared
to the calculated m/z = 644.2, which confirms the iodine and

L.A. Lesikar, A.F. Richards / Polyhedron 29 (2010) 1411–1422
1421
chlorine at 60:40 partial occupancy. The parent peak is followed by
4.7. Synthesis of 7, [{HC(NDipp)2}2Zn]
sequential loss of an iPr, I and Cl (at full occupancy), and Cl (at par-
tial occupancy) to give a peak at m/z = 422.3.
LH 0.35 g (0.96 mmol) was dissolved in toluene and at room
temperature 0.48 mL of ZnEt2 (1.0 M solution) was added by
syringe. The colorless reaction mixture was stirred at room tem-
perature for 2 h after which time the volatiles were removed and
the white solid extracted into hexanes. Overnight storage of the
reaction mixture at room temperature yielded colorless crystals
of 7.
4.5. Synthesis of 5, [{DippNCHDipp}2Al3l3O(OH)2(Cl/OH)ClMe]
To a ꢁ20 mL hexane solution of LH (0.35 g, 0.96 mmol), AlMe2Cl
was added neat (1.2 mL, 1.2 mmol). The reaction mixture was stir-
red at room temperature for 1 h. The pale yellow solution was fil-
tered from the white solid and stored at room temperature over
night to yield crystals of 5.
Yield: 0.18 g, 33%. M.p. 209–211 °C. 1H NMR (C6D6, 25 °C): d
(ppm) 0.75 (d, 1JH–H = 6.9 Hz, 12H, CH(CH3)2), 0.84 (d,
1JH–H = 6.6 Hz, 36H, CH(CH3)2), 3.12 (br, 8H, CH(CH3)2), 6.60–6.90
(m, 12H, Haryl), 6.96 (s, 2H, NCHN); 13C NMR (C6D6, 25 °C): d
(ppm) 21.1 (CH(CH3)2), 22.2 (CH(CH3)2), 27.2 (CH(CH3)2), 121.8
(m-ArC), 123.7 (p-ArC), 140.0 (o-ArC), 142.3 (ArC-N), 165.9
Yield: 0.15 g, 46%. M.p. 268–274 M.p. 243–249 °C (decomp). 1H
NMR (C6D6, 25 °C): d (ppm) ꢀ0.11 (s, Al–CH3), 0.39 (s), 0.76 (s),
1
0.91 (s, Al–OH), 1.40 (d, 6H, CH(CH3)2, JH–H = 6.90 Hz), 1.73 (d,
(NCN); IR (t
cmꢀ1, Nujol mull): 1667 (w), 1634 (w), 1597 (m),
1557 (m), 1319 (m), 1177 (m), 934 (m), 865 (m), 756 (m), 722
(m); MS (m/z; found (calc.)): 793.4 (792.5) M+, 365.3 (364.5) LH,
176.1 (176.2) LH–(Dipp)NC(H).
1
6H, CH(CH3)2, JH–H = 6.60 Hz), 1.83–1.87 (overlapping d, CH
(CH3)2), 4.17–4.35 (overlapping multiplets, 4H, CH(CH3)2), 7.48–
7.57 (m, 12H, Ar–H), 8.02 (s, 1H, NC(H)N) 8.03 (s, 1H, NC(H)N).
13C NMR (C6D6, 25 °C): d (ppm) 22.3 (CH(CH3)2), 22.8 (CH(CH3)2),
22.9 (CH(CH3)2), 23.8 (CH(CH3)2), 24.4 (CH(CH3)2), 25.2 (CH(CH3)2),
25.4 (CH(CH3)2), 25.7 (CH(CH3)2), 25.8 (CH(CH3)2), 25.9 (CH(CH3)2),
26.2 (CH(CH3)2), 27.7 (CH(CH3)2), 28.1 (CH(CH3)2), 28.2 (CH(CH3)2),
28.3 (CH(CH3)2), 28.5 (CH(CH3)2), 29.2 (CH(CH3)2), 29.6 (CH(CH3)2),
30.0 (CH(CH3)2), 31.5 (CH(CH3)2), 31.8 (CH(CH3)2), 32.1 (CH(CH3)2),
34.8 (CH(CH3)2), 38.4 (CH(CH3)2), 123.9 (m-ArC), 125.1 (p-ArC),
4.8. Synthesis of 8, [{HC(NDipp)2}Zn2Et2]2(O)
LH 0.35 g (0.96 mmol) was dissolved in toluene and at room
temperature 0.96 mL of ZnEt2 (1.0 M solution) was added by syr-
inge. The colorless reaction mixture was stirred at room tempera-
ture for 2 h after which time the volatiles were removed and the
white solid extracted into hexanes. The reaction mixture was
stored at room temperature overnight and yielded colorless crys-
tals of 8.
137.9 (ipso-ArC), 144.8 (o-ArC), 146.9 (o-ArC), 170.3 (NCN). IR (
t
cmꢀ1, Nujol mull): 3349 (m), 1683 (m), 1604 (m), 1593 (m),
1544 (s), 1350 (s), 917 (w), 902 (m), 835 (m), 749 (w), 732 (m),
691 (w), 660 (w). MS (m/z; found (calc.)): 365.1 (364.5) LH, 765.3
(765.1) M–iPr, –O–CH3, –Cl, –2OH, –Cl/O, ( = 2LH(–iPr)2Al)).
Yield: 0.18 g, 23%. M.p. 210–213 °C. 1H NMR (C6D6, 25 °C): d
(ppm) 0.52–0.57 (m, 8H, ZnCH2CH3), 0.74–0.96 (m, 48H,
1
CH(CH3)2), 1.06 (t, JH–H = 6.5 Hz, 12H, ZnCH2CH3), 3.03–3.23
4.6. Synthesis of 6, [{DippNCHDipp}2Ga3(lO2)AlLiMe7]
(br. m, 8H, CH(CH3)2), 6.60–6.86 (m, 12H, Haryl), 7.02 (s, 2H,
NCHN); 13C NMR (C6D6, 25 °C): d (ppm) 9.7 (ZnCH2CH3), 22.3
(CH(CH3)2), 26.9 (ZnCH2CH3), 37.7 (CH(CH3)2), 122.0 (m-ArC),
GaCl3 (0.5 g, 2.83 mmol) in 10 mL of toluene was cooled to
ꢀ60 °C. Three equivalents of 1.6 M MeLi (5.3 mL) were added
drop-wise. The resultant reaction mixture was allowed to stir for
2 h, after which time a toluene solution of LH was added drop-wise.
The reaction mixture was warmed to room temperature and stir-
red for 1 h after which time, 1.4 mL of AlMe3 was added by syringe.
Stirring was maintained for a further 2 h after which time the solu-
tion was filtered and extracted into hexanes (direct from the bot-
tle), storage of the colorless solution at 5 °C overnight, yielded
colorless crystals of 6. Yield: 0.18 g, 33%. M.p. 209–211 °C. 1H
NMR (C6D6, 25 °C): d (ppm) ꢀ0.52, ꢀ0.48, ꢀ0.46, (Al–CH3) 0.39,
0.41, 0.5–0.6 (overlapping), (s, Ga–Me), 1.19 (d, 12H, CH(CH3)2,
122.3 (p-ArC), 141.9 (o-ArC), 144.6 (ArC-N), 153.9 (NCN); IR (
t
cmꢀ1, Nujol mull): 1667 (m), 1597 (m), 1557 (m), 1320 (w),
934 (m); MS (m/z; found (calc.)): M+ not observed, 1004.8
(1005.1) M–4Et, 365.3 (364.5) LH.
4.9. Synthesis of 9, [{HC(NDipp)2}2Zn3Et2](OEt)2
Complex 9 was isolated using the same procedure as described
for complex 8, however the reaction was stirred at room tempera-
ture for ꢁ16 h.
1
1JH–H = 6.90 Hz), 1.33 (d, 12H, CH(CH3)2, JH–H = 6.90 Hz), 1.42 (d,
Yield: 0.25 g, 35%. M.p. 232–235 °C. 1H NMR (C6D6, 25 °C):
1
1
1
12H, CH(CH3)2, JH–H = 6.60 Hz), 1.51 (d, 12H, CH(CH3)2, JH–
d
(ppm) 0.23 (q, JH–H = 8.1 Hz, 4H, ZnCH2CH3), 0.62 (d,
1
1
H = 6.60 Hz), 3.61–3.73 (septet, 4H, CH(CH3)2, JH–H = 6.60 Hz),
1JH–H = 6.6 Hz, 12H, CH(CH3)2), 0.72 (d, JH–H = 7.2 Hz, 12H, CH(CH3)2),
1
1
1
3.76–3.90 (septet, 4H, CH(CH3)2, JH–H = 6.90 Hz), 7.16–7.28 (m,
0.83 (t, JH–H = 6.3 Hz, 6H, OCH2CH3), 0.89 (d, JH–H = 7.2 Hz, 12H,
12H, Ar–H), 7.73 (s, 1H, NC(H)N), 7.75 (s, 1H, NC(H)N). 13C NMR
(C6D6, 25 °C): d (ppm) ꢀ8.9, ꢀ7.8, ꢀ7.3, ꢀ4.9, ꢀ4.2, 0.78, 1.28,
(Al/Ga CH3), 21.6 (CH(CH3)2), 22.1 (CH(CH3)2), 23.0 (CH(CH3)2),
23.2 (CH(CH3)2), 23.6 (CH(CH3)2), 24.2 (CH(CH3)2), 24.6 (CH(CH3)2),
24.7 (CH(CH3)2), 26.5 (CH(CH3)2), 30.5 (CH(CH3)2), 121.9 (m-ArC),
122.2 (m-ArC), 122.7 (p-ArC), 125.9 (p-ArC), 139.5 (ipso-ArC),
141.4 (ipso-ArC), 142.4 (ipso-ArC), 143.2 (ipso-ArC), 144.1 (o-
ArC), 144.9 (o-ArC), 162.8 (NCN), 166.3 (NCN).
CH(CH3)2), 1.11 (d, JH–H = 6.9 Hz, 12H, CH(CH3)2), 1.32 (t,
1
1JH–H = 6.9 Hz, 6H, ZnCH2CH3), 2.82–2.98 (m, 4H, CH(CH3)2), 3.20–
1
3.33 (m, 4H, CH(CH3)2), 4.16 (q, JH–H = 6.8 Hz, 4H, OCH2CH3),
6.60–6.86 (m, 12H, Haryl), 7.00 (s, 2H, NCHN); 13C NMR (C6D6,
25 °C): d (ppm) 5.5 (ZnCH2CH3), 9.7 (ZnCH2CH3), 12.8 (OCH2CH3),
21.1 (CH(CH3)2), 21.5 (CH(CH3)2), 22.3 (CH(CH3)2), 23.2
(CH(CH3)2), 26.9 (ZnCH2CH3), 27.8 (ZnCH2CH3), 28.8 (CH(CH3)2),
29.4 (CH(CH3)2), 33.3 (CH(CH3)2), 37.7 (CH(CH3)2), 66.6
(OCH2CH3), 121.7 (m-ArC), 122.0 (m-ArC), 122.3 (p-ArC), 122.6
(p-ArC), 129.3 (o-ArC), 137.5 (o-ArC), 144.6 (ArC-N), 145.4 (ArC-
MS (m/z; found (calc.)): 1121.3 (1122.45), M+, 1023.5 (1023.7),
M–6Me, ꢀLi, 990.0 (991.6), M–8Me, ꢀLi, ꢀ2H, 929.7, 856.9,
600.3, 547.8 (548) M–DippNC(H)NDipp–8Me, ꢀLi, ꢀGa, ꢀAl
463.2 (462) M–DippNCHNDipp, ꢀ8Me, ꢀLi, ꢀGa, ꢀAl, ꢀ2iPr,
300.9 (299), M–DippNCHNDipp, ꢀ8Me, ꢀLi, ꢀGa, ꢀAl, ꢀDipp, O–
O(1.83).
N), 166.1 (NCN); IR (
t
cmꢀ1, Nujol mull): 1665 (m), 1597 (m),
1561 (m), 1287 (m), 1180 (m), 756 (m); MS (m/z; found (calc.)):
M+ not observed, 1042.6 (1042.4) M–Et, 447.2 (447.1) M–2OEt–
2Et–Zn–2Dipp–2iPr–2H, 365.3 (364.5) LH.

1422
L.A. Lesikar, A.F. Richards / Polyhedron 29 (2010) 1411–1422
[21] (a) M.P. Coles, P.B. Hitchcock, Eur. J. Inorg. Chem. (2004) 2662;
Supplementary Material
(b) A.P. Dove, V.C. Gibson, E.L. Marshall, A.J.P. White, D.J. Williams, Dalton
Trans. (2004) 570;
The crystal structure data has been deposited with the CCDC,
reference numbers 754580-754588 and is available free of charge
at http://www.ccdc.cam.ac.uk.
(c) M. Cheng, E.B. Lobkovsky, G.W. Coates, J. Am. Chem. Soc. 120 (1998) 11018.
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