Synthetic and structural studies of ethyl zinc β-amidoenoates and β-ketoiminates

Article abstract of DOI:10.3390/molecules26113165

A set of heteroleptic ethyl zinc β-amidoenoates (1, 2) and β-ketoiminates (3) of the form [LZnEt]₂ with varying steric bulk have been synthesised via the reaction of diethylzinc with β-aminoenoate ligands HL¹ and HL² and β-ketoimine HL³. These complexes have been characterised via¹ H and¹³ C NMR, mass spectrometry and single-crystal X-ray diffraction, which unambigu-ously determined all three structures as dimeric species in the solid state. We observe the unusual dimerisation of 1 and 2 through coordination of the central zinc atom to the methine carbon of the second monomer, which gives these complexes high reactivity. The thermal properties of complex 3 are explored via thermal gravimetric analysis (TGA), to investigate their potential as single-source precursors to zinc oxide, which shows that 3 has a significantly lower decomposition temperature as compared to its bis-ligated counterpart [Zn(L³)₂ ], which gives 3 promise as a single-source precursor to zinc oxide.

Full text of DOI:10.3390/molecules26113165

molecules
Article
Synthetic and Structural Studies of Ethyl Zinc β-Amidoenoates
and β-Ketoiminates
Malavika A. Bhide , Joe A. Manzi, Caroline E. Knapp and Claire J. Carmalt *
Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, UK;
malavika.bhide.14@ucl.ac.uk (M.A.B.); j.manzi@alumni.ucl.ac.uk (J.A.M.); caroline.knapp@ucl.ac.uk (C.E.K.)
* Correspondence: c.j.carmalt@ucl.ac.uk
Abstract: A set of heteroleptic ethyl zinc β-amidoenoates (1, 2) and β-ketoiminates (3) of the form
[LZnEt]2 with varying steric bulk have been synthesised via the reaction of diethylzinc with β-
aminoenoate ligands HL1 and HL2 and
β
-ketoimine HL3. These complexes have been characterised
via H and 13C NMR, mass spectrometry and single-crystal X-ray diffraction, which unambigu-
1
ously determined all three structures as dimeric species in the solid state. We observe the unusual
dimerisation of
second monomer, which gives these complexes high reactivity. The thermal properties of complex
are explored via thermal gravimetric analysis (TGA), to investigate their potential as single-source
precursors to zinc oxide, which shows that has a significantly lower decomposition temperature as
1 and 2 through coordination of the central zinc atom to the methine carbon of the
3
3
compared to its bis-ligated counterpart [Zn(L3)2], which gives
to zinc oxide.
3 promise as a single-source precursor
Keywords: precursor; dimerisation; materials; zinc oxide; chemical vapour deposition
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Citation: Bhide, M.A.; Manzi, J.A.;
Knapp, C.E.; Carmalt, C.J. Synthetic
and Structural Studies of Ethyl Zinc
β-Amidoenoates and β-Ketoiminates.
Molecules 2021, 26, 3165. https://
doi.org/10.3390/molecules26113165
1. Introduction
Zinc oxide (ZnO) is widely used as a transparent conducting oxide (TCO) material.
TCOs combine high electrical conductivity with optical transparency, making them an es-
sential and industrially important class of material. Although TCOs such as indium-doped
tin oxide (ITO) and fluorine-doped tin oxide (FTO) are the most industrially produced for
electronic applications, the scarcity and subsequent rising cost of indium requires a shift
towards TCO materials incorporating more abundant elements, such as ZnO [1,2].
Deposition of ZnO thin films has been undertaken via several methods such as sputter-
Academic Editor: Charles L.
B. Macdonald
Received: 29 April 2021
Accepted: 20 May 2021
Published: 25 May 2021
ing [
3
], pulsed vapour deposition [
4], spray pyrolysis [5] and chemical vapour deposition
(CVD) [6–8
]. The advantage of using solution-based methods such as aerosol-assisted CVD
is that precursors only have to be soluble, which therefore allows for a greater range of pre-
cursors to ZnO. Diethylzinc is the most widely used precursor to ZnO thin films via vapour
deposition methods [9,10]. An in situ reaction with an oxygen source, commonly methanol,
produces highly conductive ZnO films with excellent transparency. The drawback of using
diethylzinc is its highly pyrophoric nature, making its use and handling difficult, especially
for scale-up processes. A shift towards single-source precursors (SSPs) for ZnO thin films
has led to several complexes being reported in the literature, including zinc acetate [11],
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zinc β-diketonates [12], zinc β-ketoiminates [13–15], zinc β-amidoenoates [16], zinc ox-
anes [17] and zinc alkoxides [18], to name a few. These complexes are mostly homoleptic
bis-ligated species, and offer the advantage of stability and ease of handling. Although
they are still air- and moisture-sensitive, they are relatively stable compared to pyrophoric
diethylzinc. This however comes at a cost—the thermal decomposition temperatures of
these complexes tend to be higher, as well as having greater carbon contamination in the
resultant deposits upon decomposition [19,20].
Copyright:
© 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
In this work, we explore the synthesis of a set of heteroleptic ethyl zinc
β-amidoenoates
and
β-ketoiminates with a view to using them as SSPs to ZnO. We hypothesise that the
Molecules 2021, 26, 3165. https://doi.org/10.3390/molecules26113165
https://www.mdpi.com/journal/molecules

Molecules 2021, 26, 3165
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heteroleptic nature of these complexes will allow for both the high reactivity from the ethyl
group while also maintaining the stability offered from a chelating ligand.
2. Results and Discussion
i
The
β
-aminoenoates ligands MeC(NHiPr)CHC(O)OEt (HL1) and MeC(NH(CH2)2 Pr)
CHC(O)OEt (HL2) were synthesised via 1:1 acid-catalysed condensation reactions between
ethyl acetoacetate and isopropylamine and isopentylamine, respectively, using K-10 mont-
morillonite clay as the acid catalyst. The ligands were afforded in excellent yields using this
method. Following standard synthetic routes for the preparation of β-ketoimine ligands,
MeC(NHiPr)CHC(O)Me (HL3) was synthesised via a simple 1:1 condensation reaction of
acetylacetone and isopropylamine, and isolated as an orange oil of a high purity and with
a good yield (Figure 1).
Figure 1. Ligands used in this work.
The ethyl zinc
plex 3, were synthesised via 1:1 reactions of diethylzinc with the respective
or -ketoimine in toluene. After reduction of the solvent in vacuo, all three compounds crys-
tallised out of concentrated toluene solutions after being stored at ) and
18 ◦C for 24 h (
48 h ( ). Compounds and
were all characterised by 1H and 13C NMR spectroscopy,
as well as single-crystal X-ray diffraction (XRD), which confirmed the structures as dimeric
species in the solid state (Figure 2). Peaks observed for the -amidoenoate protons in the
1H NMR spectrum of
and were all shifted as compared to the free ligands, confirming
β
-amidoenoate complexes
1
and
2
, and the ethyl zinc
β
-ketoiminate com-
β
-aminoenoate
β
−
3
1,
2
1
,
2
3
β
1
2
coordination to the zinc centre, and we also saw triplet–quartet peaks corresponding to the
bound ethyl groups appearing as broad signals.
The ethylzinc
β
-amidoenoates
1 and 2 both crystallised out as centrosymmetric crystals
in the triclinic space group P1, each with two zinc centres; one of which was unique and
one of which was symmetrically generated about an inversion centre. Both the complexes
dimerise analogously, with dative interactions between the zinc atom on one monomeric
unit and the methine carbon on the ligand of the second monomeric unit (Figure 3). This
dimerisation is uncommon in zinc alkyl complexes, with most dimerisation occurring
through a [Zn–L]2 (L = donor atom) ring interaction (Figure 3, left) [21
heterocyclic carbene) zinc ethyl complex previously reported dimerises in a similar fashion
to and , though this was due to the unusual deprotonation of the methylene proton of
–24]. A bis (N-
1
2
the ligand backbone by diethylzinc, and the formation of a formal Zn–C bond (Figure 3,
right) [25].

Molecules 2021, 26, 3165
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Figure 2. Solid state structures of
1
(
above) and
2
(below) with thermal ellipsoids drawn at 50%
probability and with hydrogen atoms omitted for clarity.
Figure 3. Schematic representation of ethyl zinc complex dimerisation through a [Zn–L]2 ring; L represents a donor atom of
a bidentate ligand (left). Bis(N-heterocyclic carbene) zinc ethyl complex reported by Rit et al. exhibiting similar dimerisation
to 1 and 2 (right).

Molecules 2021, 26, 3165
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In both
1
and
2
(Figure 2), two coordination sites around Zn1 are occupied by the
coordination of the appropriate bidentate ligand L1/L2 which bonds through the O10
and N10 atoms, and the bonding of L1/L2 forms a puckered six-membered ZnOC3N ring.
Zn1 forms a dative interaction with C3 on the symmetry equivalent monomer unit, and
as such,
two monomer units, in which Zn1 adopts a highly distorted tetrahedral geometry. Taking
compound as an example, the longest interaction made by Zn1 is the Zn1–C3 interaction
1 and 2 exist as dimers in the solid state with two dative interactions between
1
between Zn1 on one monomer unit to C3 on the other (2.408(3) Å). The Zn1–C10 bond
measures 1.980(3) Å, which is typical of a Zn–C bond. The increased value of the Zn1–
C3 distance is highly indicative of a dative interaction between Zn1 and C3. Zn1 also
bonds to O10 and N10 of the ligand L1, with bond lengths of 2.046(2) Å and 2.013(2) Å,
respectively (Table 1). The largest bond angle about the Zn atom in
1
is 138.12(13)◦ for
N10–Zn1–C10, which is over 40◦ larger than the N10–Zn1–O10 angle (93.15(9)◦). The large
N10–Zn1–C10 bond angle redu0ces the steric interaction between the terminal ethyl group
0
0
on Zn1 and the iPr group on N1 . However, the O1 –Zn1–N1 angle is small due to the steric
0
0
constraints of the formed ring system. The other angles about Zn1 of O1 –Zn1–C3, O1 –Zn1–
0
◦
◦
C10, N1 –Zn1–C3 and C3–Zn1–C10 have intermediate bond angles of 97.02(9) , 114.23(11) ,
98.26(9)◦ and 108.42(12)◦, respectively. The deviations from the ideal tetrahedral angle of
109.5◦ are expected in complexes of this type and result from the inflexibility of the bound
chelate ligand and the requirement to reduce the steric interaction between the iPr groups
and terminal ethyl groups. The trends for bond angles and lengths are analogous for
to both complexes having very similar structures in the solid state.
2 due
◦
Table 1. Selected bond lengths (Å) and bond angles ( ) for compounds 1, 2 and 3.
1
2
3
Bond Lengths/Å
Zn1-O1
Zn1-O2
Zn1-N1
2.046(2)
-
2.013(2)
1.980(3)
1.252(4)
2.0278(14)
-
1.9964(18)
1.962(2)
1.254(3)
2.0233(10)
2.1329(10)
2.0262(11)
1.9806(14)
1.3141(17)
Zn1-CH2CH3
O1-C
◦
Bond Angles/
O1-Zn1-N1
O1-Zn1-CH2CH3
N1-Zn1-CH2CH3
93.15(9)
114.23(11)
138.12(13)
92.99(7)
121.22(9)
133.75(10)
83.47(4)
123.96(5)
130.03(5)
The bond angles around the four coordinate Zn centres in
1
and
2
both have a range
◦
that deviates significantly from the 109.5 that would be observed for a centre with perfect
tetrahedral geometry. This is due to the inflexibility of the chelating ligand and the relatively
small bite angle resulting from the (OC(OEt)CHC(Me)N(R)) chain, where R is iPr in
1 and
i
(CH2)2 Pr in
2
. The N1–Zn1–CH2CH3 bond angle is the largest in both complexes as to
reduce the steric conflict in the complexes and place distance between the organic ligand
0
on the N atom and the ethyl group on the Zn atom. The N1 –Zn1–C10 angle in
1
measures
138.12(13)◦, compared to the N1–Zn1–C12 bond angle of 133.75(10)◦ in
2
. The iPr group
on the N in 1 has a short carbonyl chain and less rotation available about its bonds. This
results in the steric bulk of the ligand being in closer proximity to the ethyl group on the
Zn, requiring an increased bond angle between them. However, the isopentyl group has a
longer carbonyl chain with increased rotation available, which causes a reduction in the
steric frustration with the ethyl group on the Zn allowing for a smaller bond angle. This is
compensated for by the angle between the O, Zn and C on the ethyl group, which measures
114.23(11)◦ for the O10–Zn1–C10 bond angle in
1
and 121.22(9)◦ for the O1–Zn1–C12 bond
angle in 2.
The Zn centre in both
forming two bonds to form a puckered ZnOC3N ring from the
one dative bond to the second monomer unit. Delocalisation in the rings in both complexes
1
and
2
retains one ethyl group from the diethylzinc while
β
-aminoenoate ligand and

Molecules 2021, 26, 3165
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is evidenced by the shorter length of the C–O, C–N and C–C bonds within the formed
rings when compared to those outside of it. Most bond lengths are slightly shorter in
2
, for example the Zn1–O1 and Zn1–N1 bonds measure 2.0278(14) Å and 1.9964(18) Å,
respectively, which compares to 2.046(2) and 2.013(2) Å for the Zn1–O10 and Zn1–N10
bonds, respectively, in 1. However, 2 does have a slightly longer dative bond between the
two monomer units, with the Zn1–C30 measuring 2.452(2) Å compared to the equivalent
Zn1–C3 bond measuring 2.408(3) Å in 1.
0
The structural parameter τ4 was used to quantify the degree of distortion at the zinc
0
centre. The zinc centres in
1
and
2
adopted highly distorted tetrahedral geometries with τ4
0
values of 0.69 and 0.71, respectively. It can be argued that the further the τ4 value deviates
from the ideal value of 0 (square planar) or 1 (tetrahedral), the more unstable the molecule
will be and the more likely it will be that it decomposes at a lower temperature, making it a
better contender as a precursor. However, deposition is also dependent on the similarity
between the molecular geometry of the precursor and the arrangement0of atoms in the
desired bulk material, in which case it would be favourable to have a τ4 value closer to
ideal values. This is especially true for zinc oxide, where it has been shown that certain
geometries of precursors lead to films with certain preferred orientations [26,27]. For zinc
oxide, which exists most commonly in the wurtzitic form where both zinc and oxygen
atoms adopt tetrahedral geometries, it could be favourable to have precursors with values
closer to 1, so as to match the geometries of the precursor to the bulk material.
Similarly to
1 and 2 3 were shifted as
, peaks observed in the 1H NMR spectrum of
compared to the free ligand, indicating that the ligand was coordinated to the zinc centre.
The protons corresponding to the bound ethyl group appeared as broad signals, similarly
to 1 and 2. Compound 3 crystallised out of a concentrated toluene solution as a dimeric
species in the tetragonal space group P42/n, with two crystallographically non-equivalent
zinc centres (Figure 4). This dimerisation is 0much more common in zinc complexes, as
discussed above. The structural parameter τ4 for
3 was calculated to be 0.73, indicating a
highly distorted tetrahedral zinc centre. Each zinc centre is bound to two oxygen atoms in
a [Zn-O]2 ring structure. All bond lengths including the Zn-O bond lengths are comparable
to those in
those in and
angle for a six-membered ring (109.5◦). However, the τ4 value for
tetrahedral geometry of 1 as complexes and
have greater than 109.5◦ bond angles for
1
and
2
2
. The O1–Zn1–N1 bite angle is significantly narrower (by ~10◦) than
1
, measuring 83.47(4)◦, which is further from the ideal internal tetrahedral
0
3
is closer to the ideal
1
2
O1–Zn1–CH2CH3 and N1–Zn1–CH2CH3, causing the coordination geometry around the
zinc centres to be more distorted.
Figure 4. Solid state structure of
3 with thermal ellipsoids drawn at 50% probability and with
hydrogen atoms omitted for clarity.

Molecules 2021, 26, 3165
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The 1:1 reaction of HL3 with diethylzinc resulted in the formation of
3
, along with the
formation of the bis-ligated compound [Zn(L3)2], even when conducting a cooled dropwise
addition. The ratios of
3
to [Zn(L3)2] from 1H NMR was found to be 2:1 for the 1:1 reaction.
Repeats of the reaction with 1.6 eq. and 2.5 eq. excess of diethylzinc still resulted in the
formation of [Zn(L3)2], with ratios of 3:1 of
3
:[Zn(L3)2], which shows the stability of the
bis-ligated complex. The bis-ligated complexes [Zn(L1)2] and [Zn(L2)2] did not form when
conducting these reactions and we believe that this is due to electronic differences in the
ligands used. The single-crystal structure of [Zn(L3)2] has been reported previously, which
shows that the compound is monomeric [28].
In
of ZnO as there are more pre-formed Zn–O bonds as required in the bulk material. Even
though there is a greater atomic percentage of oxygen in and , each Zn atom is only
bound to one oxygen atom. As such, thermal gravimetric analysis (TGA) was used to
evaluate the efficacy of as a single source precursor to ZnO. Furthermore, TGA analysis
was not able to be carried out on and due to their high moisture and air sensitivity.
Due to the inseparable mixture obtained from the reaction of diethylzinc with HL3, the
3, each Zn atom is bound to two O atoms, which is advantageous for deposition
1
2
3
1
2
TGA for this 3:1 mixture of 3
:[Zn(L3)2] was carried out. The thermal profile shows a two-
step decomposition pathway, with an onset decomposition temperature of 150 ◦C, and a
relatively wide decomposition window of ~200 ◦C (Figure 5).
Figure 5. Thermal decomposition profile for the 3:1 mixture of 3:[Zn(L3)2].
The TGA profile for a similar bis-ligated zinc
β
-ketoiminate complex, with an nBu
◦
group on the N atom, showed the onset decomposition to be at 250 C, with complete
◦
decomposition at 400 C [13]. From this, we may hypothesise that the initial decomposition
starting at 150 ◦C is of compound
3
, with the mass loss at the higher temperature of ~300
◦C attributed to the decomposition of [Zn(L3)2]. The same authors also reported the TGA
profile for the bis-ligated complex [Zn(L1)2], which had an onset decomposition tempera-
ture of ~250 ◦C, with complete decomposition at 400 ◦C. From the trend seen between
3
and its bis-ligated counterpart, we can infer that
1 and 2 will also have significantly lower
decomposition temperatures than their bis-ligated counterparts, perhaps even more so
than 3, as their sensitivity did not allow for TGA analysis to be undertaken. Given that a
mixture of 3:1 of
mass% of 32.0% should remain, which is in line with the observed 32.8% final mass% in
3
:[Zn(L3)2] was used, it can be calculated that if ZnO was to be formed, a

Molecules 2021, 26, 3165
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the TGA, suggesting that this precursor does decompose to ZnO. This was corroborated by
XRD analysis of the thermal decomposition product, which confirmed its identity as ZnO
(Figure 6).
Figure 6. XRD pattern of the thermal decomposition product, ZnO, of the 3:1 mixture of 3
:[Zn(L3)2].
3. Materials and Methods
All preparations were performed under an inert argon atmosphere using standard
Schlenk techniques or using an MBraun nitrogen-filled glovebox. All chemicals were
obtained from commercial sources. All solvents were obtained from a solvent purification
system and stored over molecular sieves. C6D6 and CDCl3 were dried using freeze–pump–
thaw cycles and stored over molecular sieves. HL1 and HL3 were synthesised according to
procedures in the literature [14,16].
Single-crystal X-ray diffraction (XRD) data were collected using a SuperNova Atlas
(Dual) diffractometer using Cu Kα radiation of wavelength 1.54184 Å. Suitable crystals
were selected and mounted on a nylon loop and the crystal was kept at 150 K during
data collection. Nuclear magnetic resonance (NMR) data were recorded in C6D6 solutions
1
using a Bruker Advance III 500 MHz instrument at ambient temperature. H and 13C{1H}
NMR assignments were confirmed by 1H–1H (COSY and NOESY) and 1H–13C (HSQC and
HMBC) experiments where necessary. Thermogravimetric analysis (TGA) measurements
were made using a PerkinElmer STA6000 TGA instrument, with a sensitivity of 0.1 mg and
◦
◦
using N2 as the shield gas. The samples were heated from 30 C to 500 C, at a heating rate
of 10 ◦C min−1 under the flow of shield gas. XRD patterns were recorded using a Bruker
D8 Discover diffractometer.
i
3.1. Synthesis of OC(OEt)CHC(Me)NH(CH2)2 Pr (HL2)
Isopentylamine (6.96 cm3, 60 mmol) was added dropwise to ethyl acetoacetate (3.97 cm3,
30 mmol) dispersed over K-10 montmorillonite clay (20 g) in a 3-necked round-bottom
flask fitted with an overhead mechanical stirrer. The reaction slurry initially gave out heat
and was stirred at room temperature for 15 h. The product was extracted by washing with

Molecules 2021, 26, 3165
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dichloromethane (3
dark orange liquid (yield: 5.43 g, 90%). H NMR
×
30 cm3) and filtered. The solvent was removed in vacuo to yield a
1
δ
/ppm (CDCl3): 0.89 (6H, d, J = 6.7 Hz,
(CH3)2CH), 1.22 (3H, t, J = 7.1 Hz, CH3CH2), 1.43 (2H, q, J = 7.1 Hz, CHCH2), 1.67 (1H, m,
(CH3)2CH), 1.89 (3H, s, CH3C), 3.18 (2H, q, J = 6.5 Hz, CH2NH), 4.05 (2H, q, J = 7.1 Hz,
CH3CH2), 4.40 (1H, s, CHCO), 8.50 (1H, s (broad), NH). 13C{1H} NMR
δ/ppm (CDCl3): 14.8
(CH3CH2), 19.5 (CH3C), 22.5 ((CH3)2CH), 25.6 ((CH3)2CH), 39.3 (CH2CH), 41.2 (CH2NH),
58.3 (CH3CH2O), 81.8 (CHCO), 162.1 (q) (CNH), 170.8 (q) (CO). MS: m/z [M + H]+: 200.11.
3.2. Synthesis of [(Zn(OC(OEt)CHC(Me)NiPr)(Et))2] [L1ZnEt]2 (1)
A solution of HL1 (1.00 g, 5.84 mmol) in toluene (15 cm3) was added dropwise to a
cooled solution of diethylzinc (1.1 M in toluene) (5.26 cm3, 5.84 mmol) in toluene (15 cm3).
The resulting solution was brought to room temperature and stirred for 24 h. Toluene was
partially removed in vacuo and the remaining solution was left at
−
18 ◦C for 48 h. The
product crystallised out as yellow tinted crystals suitable for analysis via single-crystal
1
XRD (yield: 1.17 g, 75%). H NMR
δ
/ppm (C6D6, 500 MHz): 0.32 (4H, br, Zn–CH2CH3),
0.93 (12 H, d, J = 6.4 Hz, (CH3)2CH), 1.09 (6H, t, J = 7.1 Hz, OCH2CH3), 1.28 (6H, br, Zn–
CH2CH3), 1.61 (6H, s, CH3C), 3.44 (2H, m, (CH3)2CH), 4.03 (4H, q, J = 7.1 Hz, OCH2CH3),
4.57 (2H, s, CHCO). 13C{1H} NMR
δ/ppm (C6D6, 500 MHz): 4.8 (Zn–CH2CH3), 11.2
(Zn–CH2CH3), 14.9 (OCH2CH3), 22.2 (CH3C), 25.6 ((CH3)2CH), 49.4 ((CH3)2CH), 59.5
(OCH2CH3), 79.6 (CHCO), 170.3 (q) (CN), 172.2 (q) (CO). MS: m/z [M]+·: 530.25.
i
3.3. Synthesis of [(Zn(OC(OEt)CHC(Me)N(CH2)2 Pr)(Et))2] [L2ZnEt]2 (2)
A solution of HL2 (1.00 g, 5.02 mmol) in toluene (15 cm3) was added dropwise to a
cooled solution of diethylzinc (1.1 M in toluene) (4.52 cm3, 5.02 mmol) in toluene (15 cm3).
The resulting solution was brought to room temperature and stirred for 42 h. Toluene was
partially removed in vacuo and the remaining solution was left at
−
18 ◦C for 48 h. The
product crystallised out as yellow tinted crystals suitable for analysis via single-crystal
1
XRD (yield: 0.98 g, 67%). H NMR
δ
/ppm (C6D6, 500 MHz): 0.21 (4H, q (br), J = 8.1 Hz,
Zn–CH2CH3), 0.81 (12H, d, J = 6.6 Hz, (CH3)2CH), 1.09 (6H, t, J = 7.1 Hz, OCH2CH3), 1.20
(6H, t (br), J = 8.1 Hz, Zn–CH2CH3), 1.25 (4H, m, NCH2CH2), 1.41 (4H, m, NCH2CH2),
1.61 (6H, s, CH3C), 2.98 (2H, m, (CH3)2CH), 4.04 (4H, q, J = 7.1 Hz, OCH2CH3), 4.62 (2H, s,
CHCO). 13C{1H} NMR
δ/ppm (C6D6, 500 MHz): 5.7 (Zn–CH2CH3), 10.8 (Zn–CH2CH3),
14.9 (OCH2CH3), 21.8 (CH3C), 22.8 ((CH3)2CH), 26.6 (NCH2CH2), 41.9 (NCH2CH2), 49.2
((CH3)2CH), 59.6 (OCH2CH3), 79.6 (CHCO), 172.6 (q) (CN), 172.7 (q) (CO).
3.4. Synthesis of [(Zn(OC(Me)CHC(Me)NiPr)(Et))2] [L3ZnEt]2 (3)
A solution of HL3 (0.84 g, 6.0 mmol) in toluene (15 cm3) was added dropwise to a
cooled solution of diethylzinc (1.1 M in toluene) (5.40 cm3, 6.0 mmol) in toluene (15 cm3).
The resulting solution was brought to room temperature and stirred overnight. Toluene
was partially removed in vacuo and the remaining solution was left at
after which clear crystals suitable for analysis via single-crystal XRD had formed. H NMR
−
18 ◦C for 24 h
1
δ
/ppm (C6D6, 500 MHz): 0.67 (2H, br, Zn–CH2CH3), 0.93 (6H, d, J = 6.3 Hz, NCH(CH3)2),
1.49 (3H, s, NCCH3), 1.56 (3H, br, Zn–CH2CH3), 2.01 (3H, s, OCCH3), 3.38 (1H, hept, J
= 6.3 Hz, NCH(CH3)2), 4.78 (1H, s, NCCH). 13C{1H} NMR
/ppm (C6D6, 500 MHz): 1.9
δ
(Zn–CH2CH3), 12.8 (Zn–CH2CH3), 21.6 (NC(CH3)), 24.6 (NCH(CH3)2), 27.5 (OC(CH3)),
50.4 (NCH), 98.8 (NCCH), 169.6 (CN), 179.9 (CO).
4. Conclusions
We have presented the synthesis of three ethyl zinc complexes as potential SSPs for
ZnO thin films. The unusual dimerisation of
whilst the more common dimerisation seen in
the most suitable complex for use as a precursor. As such, TGA was carried out on
and its decomposition temperature was significantly lower (~250 ◦C) than its bis-ligated
counterpart [Zn(L3)2], and this relatively low temperature makes
suitable for use as an
1
and
2 led to highly unstable complexes,
3
offered greater stability and was therefore
3
3

Molecules 2021, 26, 3165
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SSP in CVD processes. Although this reaction yielded a mixture, CVD is usually an in
situ process and as such complexes are not isolated. Because the bis-ligated compound
[Zn(L3)2] is an SSP to ZnO in its own right, we do not believe that the inclusion of a small
amount of this would hinder the growth of ZnO when undertaking the deposition of 3 to
form ZnO.
The usage of chelating
group remaining from the diethylzinc has given rise to a set of heteroleptic complexes
with properties intermediate to diethylzinc and bis-ligated zinc -amidoenoates and β-
β-amidoenoate and β-ketoiminate ligands along with one ethyl
β
ketoiminates. They are not pyrophoric due to the chelate ligand, but they are still highly
sensitive to air and moisture due to the remaining ethyl group, and are therefore promising
as SSPs for ZnO, as evidenced by the TGA data of
confirmed as ZnO by XRD analysis.
3 and its thermal decomposition product
Author Contributions: M.A.B. and J.A.M. carried out the experimental work, supervision C.E.K.
and C.J.C. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by the Engineering and Physical Sciences Research Council (EP-
SRC), grant EP/R513143/1 (M.A.B.) and grant EP/G036675 (J.A.M.), and the grants EP/K001515 and
EP/L017709.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The data presented in this study are available on request from the
corresponding author.
Conflicts of Interest: The authors declare no conflict of interest.
Sample Availability: Samples of the compounds are not available from the authors.
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