Reactions of zirconium amide amidinates with dioxygen. Observation of an unusual peroxo intermediate in the formation of oxo compounds

Article abstract of DOI:10.1039/c4cc04032e

Reaction of d⁰ Zr(NMe₂)₂[MeC(N ⁱPr)₂]₂ (1) with O₂ at -30°C gives three Zr containing products: a peroxo trimer {(μ-η²: η²-O₂)Zr[MeC(NⁱPr)₂] ₂}₃ (2), an oxo dimer {(μ-O)Zr[MeC(N ⁱPr)₂]₂}₂ (3), and an oxo polymer {(μ-O)Zr[MeC(NⁱPr)₂]₂}_n (4). 2 is a rarely observed peroxo complex from the reaction of a d⁰ complex with O₂. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc04032e

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Reactions of zirconium amide amidinates with
dioxygen. Observation of an unusual peroxo
intermediate in the formation of oxo compounds†
Cite this: Chem. Commun., 2014,
50, 10517
Received 26th May 2014,
Accepted 21st July 2014
Adam C. Lamb, Zheng Lu and Zi-Ling Xue*
DOI: 10.1039/c4cc04032e
www.rsc.org/chemcomm
i
Reaction of d0 Zr(NMe2)2[MeC(NiPr)2]2 (1) with O2 at À30 8C gives coworkers have shown that [FeII(TP Pr )(O2CC(O)Me)] reacts
2
three Zr containing products: a peroxo trimer {(l-g2:g2-O2)Zr- with O2 to form a FeIII–O–O–FeIII peroxo dimer.2k
[MeC(NiPr)2]2}3 (2), an oxo dimer {(l-O)Zr[MeC(NiPr)2]2}2 (3), and an
oxo polymer {(l-O)Zr[MeC(NiPr)2]2}n (4). 2 is a rarely observed peroxo complexes and O2 focused on O insertion into the Zr–Si bond in
complex from the reaction of a d0 complex with O2.
Earlier studies of the reactions between d0 transition metal
Cp2Zr(SiMe3)Cl,1b and M–C bonds in Cp2ZrRCl/Cp2ZrR2 (R = alkyl),1c
(RO)2TiMe2,1d (ArO)2TaMe3,1e and (ArNQ)2MoMe2.1f Abu-Omar
Early transition metal complexes are often sensitive to O2 and et al. have reported the preparation and structure of a d0 Zr(IV)
water, and these complexes are handled under an inert atmo- bisperoxo complex (6) containing two redox-active diimine ligands
sphere.1 Many early transition metal complexes contain no (Scheme 1).1a The redox-active ligands each donate two electrons to
valence d electrons (d0) and their reactivities toward O2 are O2 to give the peroxo ligands. Bercaw and coworkers have shown
different from complexes with valence d electrons (dn).2 Reac- that O2 reacts with d2 permethyltantalocene alkyl and phenyl com-
tions of O2 with d0 complexes have been used to make micro- plexes, which are in equilibrium with corresponding d0 alkylidene
electronic metal oxide (MOn) materials through chemical vapor hydride, olefin hydride, or benzyne hydride, yielding peroxo alkyl
deposition (CVD) and atomic layer deposition (ALD) processes.3 complexes (Scheme 1).1i In other words, the d2 metal ion is oxidized
The metal oxides, as gate dielectric materials, have replaced in the reactions, as in typical reactions of dn complexes with O2.
SiO2, allowing further device miniaturization and prolonging
We have studied reactions of O2 with d0 Groups 4 and 5
the life of batteries in, e.g., tablet computers after each charge.4 homoleptic amide complexes.1n–p M(NMe2)4 (M = Zr, Hf) react
d0 amides, e.g., have been used as precursors.4b,c Even though with O2, giving oxo aminoxides M3(NMe2)6(m-NMe2)3(m3-O)-
the CVD/ALD reactions have been successfully used to make (m3-ONMe2) and Me2NNMe2.1n Group 5 M(NMe2)5 (M = Nb,
metal oxides, the mechanism of the reactions is not well Ta) react with O2 to yield several products.1o,p It should be
understood. This is in part because O2 is a diradical, and many noted that a metal-peroxo complex was not observed in any of
of its reactions are radical in nature. Homoleptic amide com- the reactions of O2 with the amide-containing complexes.
plexes M(NR2)n are too sensitive towards air to handle.4b,c,5
Since both amide and amidinate ligands are widely used in
Ancillary ligands such as amidinates have thus been used to CVD/ALD precursors, we report two new d0 zirconium amide
make precursor compounds more stable.3b–f
In the reactions of O2 with dn transition metal complexes,
the metals are often oxidized. For example, Theopold and
coworkers found that O2 reacted with [(iPr2Ph)2nacnacCrI]2-
(m-N2) to give [(iPr2Ph)2nacnacCrV](QO)2.2e Karlin, Nam and
coworkers reported that the reaction between O2 and [CrII(12-
TMC)]2+ {(12-TMC) = 12-membered tetraazamacrocyclic N-tetra-
methylated cyclam} yielded a CrIV-peroxo complex.2b Que and
Department of Chemistry, The University of Tennessee, Knoxville, Tennessee 37996,
USA. E-mail: xue@utk.edu; Tel: +1-865-974-3443
† Electronic supplementary information (ESI) available: Experimental details and
characterization of 1, 4 and 7. CIF file for the crystal structure of 2. CCDC
Scheme 1 Synthesis of 61a and the formation of Ta peroxo complexes
through d2 species.1i
1005137. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c4cc04032e
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Scheme 2 Preparation of 1 and 7.
amidinate complexes and studies of their reactions with dioxy-
gen. The new Zr amide amidinates 1 and 75 have been prepared
through the aminolysis of Zr(NR2)4 (R = Me, Et) with the
i
amidine PrN(H)C(Me)QNiPr (Scheme 2). Characterization by
1H and 13C NMR, DEPT, HMBC, HSQC and elemental analysis
is consistent with its structure in Scheme 2. When a solution of
1 in pentane at –30 1C was exposed to O2 in air over a period of
a few weeks, crystals of peroxo trimer {(m-Z2:Z2-O2)Zr[MeC-
(NiPr)2]2}3 (2) were obtained as a solid mixture with powders
of unreacted 1, oxo dimer {(m-O)Zr[MeC(NiPr)2]2}2 (3), and oxo
polymer {(m-O)Zr[MeC(NiPr)2]2}n (4, Scheme 3).6 When the solid
mixture was dissolved in benzene-d6 at 23 1C, only oxo dimer 3,
oxo polymer 4, CH2(NMe2)2, HNMe2, and unreacted 1 were
observed (Scheme 3), indicating that peroxo 2 is unstable in
solution, a point discussed later.
The crystal structure of 2 is given in Fig. 1. The Zr atoms are
linked by peroxo ligands that bind to the metals in the m-Z2:Z2
mode. The three O–O bond lengths of 1.5147(13)–1.5172(13) Å
in 2 are similar to 1.504(4)–1.512(4) Å in the Zr(IV) bisperoxo
complex 6 (Scheme 1),1a and those in [M2(O2)2(XW11O39)2]12À
[X = Si, 1.491(11) Å; Ge, 1.529(13) Å],7 but are longer than
1.394(4) and 1.401(4) Å in {Zr[N(iPr2PO)2]2}2(m-Z2:Z2-O2)2 (8).8
Fig. 1 ORTEP view of 2 (hydrogens omitted).6 Selected bond lengths (Å) and
angles (1): O1–O2 1.5172(13), O3–O4 1.5147(13), O5–O6 1.5150(13),
O1–Zr3 2.1239(10), O1–Zr1 2.1992(10), O2–Zr1 2.1162(10), O2–Zr3 2.2014(10),
O3–Zr1 2.1320(10), O3–Zr2 2.2038(10), O4–Zr2 2.1179(10), O4–Zr1 2.2082(10),
O6–Zr3 2.1148(10), O6–Zr2 2.2007(10), O5–Zr2 2.1242(10), O5–Zr3 2.2026(10);
O2–O1–Zr3 72.23(5), O2–O1–Zr1 66.51(5), Zr3–O1–Zr1 125.06(5), O1–O2–Zr1
72.38(5), O1–O2–Zr3 66.75(5), Zr1–O2–Zr3, 125.34(4), O4–O3–Zr1 72.30(5),
O4–O3–Zr2 66.47(5), Zr1–O3–Zr2 124.13(4), O3–O4–Zr2 72.56(5), O3–O4–
Zr1 66.90(5), Zr2–O4–Zr1 124.61(5), O5–O6–Zr3 72.60(5), O5–O6–Zr2
66.80(5), Zr3–O6–Zr2 126.15(4), O6–O5–Zr2 72.23(6), O6–O5–Zr3 66.38(5),
Zr2–O5–Zr3 125.57(5), O2–Zr1–O3 104.96(4), O2–Zr1–O1 41.11(3), O3–Zr1–
O4 40.80(4), O5–Zr2–O6 40.96(4), O4–Zr2–O3 40.97(3), O1–Zr3–O2 41.02(3),
O6–Zr3–O5 41.02(4).
The O atoms form slightly unsymmetrical bridges between the are 66.47(5)–72.60(5)1 and 40.80(4)–41.13(3)1, respectively. The
Zr atoms: one O–Zr length [2.1162(10)–2.320(10) Å] is shorter Zr–O–Zr angles are in the range of 124.13(5)–126.15(4)1.
than the other [2.1992(10)–2.2082(10) Å]. The Zr–O bonds in 2
Characterization of the solid mixture of peroxo trimer 2,
are longer than those in 6,1a but are similar to those in 8.8 The unreacted 1, oxo dimer 3, and oxo polymer 4 by mass spectro-
O–O–Zr and O–Zr–O angles for each pair of peroxo ligands in 2 metry (MS) gave a spectrum showing the dimeric peroxo
complex {(m-O2)Zr[MeC(NiPr)2]2}2 (5, Fig. 2), indicating that
trimer 2 had converted to smaller dimer 5 during the MS
process. Zr is an element with five stable isotopes and has a
unique pattern in MS. The identification of 5 is based on the
isotopic pattern and a comparison with the predicted MS
spectrum for 5 (Fig. 2). Although the MS spectrum in Fig. 2
could also be assigned to the adduct O2-3 formed during the
DART process, this species is unlikely. When 3 was character-
ized by MS in air, the peaks in Fig. 2 were not observed,
indicating that the adduct O2-3 was not formed between 3
and O2 during the DART process. The crystal structure of
peroxo trimer {(m-Z2:Z2-O2)Zr[MeC(NiPr)2]2}3 (2) and observa-
tion of peroxo dimer {(m-O2)Zr[MeC(NiPr)2]2}2 (5) in MS are
significant as, to our knowledge, a Group 4 peroxo complex,
yielded from O2 and not supported by a redox-active ligand (as
in 6, Scheme 1), has not been reported. Oxo dimer 3 was also
observed in the MS of the solid mixture, and its MS spectrum is
discussed below.
Peroxo complexes derived from H2O2 have been reported. For
example, Kortz and coworkers have obtained peroxo heteropoly-
tungstates [M2(O2)2(XW11O39)2]12À (M = Zr, X = Si, Ge; M = Hf,
Scheme 3 Reactions of 1 and 7 with O2 or H2O, and conversion of a solid
mixture of 1 and 2 to 3, 4, CH2(NMe2)2, and HNMe2.
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Scheme 4 Reactions of 1 with H2O2.
added to 1 in a THF solution, oxo dimer 3, CH2(NMe2)2, and
HNMe2 were observed, along with unreacted 1. The observation of
CH2(NMe2)2 here is consistent with the formation of the peroxo
intermediate 2 (or its monomeric derivative or dimeric peroxo
complex 5) in this reaction of 1 with H2O2 (Scheme 4).
Reaction of O2 with Zr(NEt2)2[MeC(NiPr)2]2 (7), a diethylamide
analog of 1, also yielded oxo dimer 3 and oxo polymer 4 (Scheme 3).6
HNEt2 was the organic product. Both 1 and 7 react with H2O as well,
quickly yielding 3, 4 and HNR2 (Scheme 3).6 The fact that both
dimethylamide and diethylamide complexes Zr(NR2)2[MeC(NiPr)2]2
(R = Me, 1; Et, 7) yield the same metal-containing products, oxo
dimer 3 and oxo polymer 4, supports the structural assignment of 3
Fig. 2 (top) Calculated and (bottom) observed MS of [5 + H+].
X = Si) through reactions of ZrCl4/HfCl4 with [XW11O39]8À
,
followed by treatment with H2O2.7 Exchanges of the O2À ligands and 4. In other words, the dialkylamide ligands NR2 are removed in
with O22À lead to the peroxo ligands in the products. Leung and the reactions of O2 with 1 and 7, respectively, both yielding 3 and 4. 4
coworkers found that the reaction of Zr(OtBu)4 with iPr2P(QO)– does not dissolve in benzene-d6, DMF, Et2O, hexanes, pentane,
NH–P(QO)iPr2 yielded peroxo {Zr[N(iPr2PO)2]2}2(m-Z2:Z2-O2)2 (8), methanol, THF, or toluene-d8. The solid sample of 4 from the
although the authors did not know how the complex was formed reaction of 7 with O2 was used for its characterization by elemental
and what led to the peroxo ligands.8
analysis, solid-state 13C{1H} CPMAS NMR, IR, and thermal gravi-
Attempts have been made to observe trimeric peroxo 2 by other metric analysis (TGA).6 IR spectrum of 4 reveals a peak at 1641 cmÀ1
,
spectroscopies. The solid mixture of 1–4, containing crystals of 2, is and this peak was assigned to the CQN stretch.6 Solid-state 13C{1H}
mostly non-crystalline, unreacted amidinate 1. When this solid CPMAS NMR spectrum of 4 shows peaks at 14.19 ppm for NC(Me)N,
mixture was dissolved in benzene-d6, oxo dimer 3, oxo polymer 4, 23.27 ppm for CHMe2, 46.34 ppm for CHMe2, and 152.35 ppm for
CH2(NMe2)2, and HNMe2 were observed as products.6 These results NC(Me)N.6 Characterization of the solid of 4 by powder X-ray
suggest that 2 is unstable in solution. We have not been able to diffraction indicates that the solid is amorphous.
obtain pure 2 and tested its stability in solution. Thus we cannot
Solids of 1 or its diethylamide analog 7 were exposed to air briefly
rule out that 2 decomposes on its own whether 1 is present or not. during their MS characterization with a He gas source at 200 1C.6
It is, however, more likely that, once 2 is formed, it reacts with 1 to Neither 1 nor 7 was observed by MS. However, the oxo monomer
give oxo dimer 3 and the organic products. In a separate test, a (OQ)Zr[MeC(NiPr)2]2 (9), dihydroxyl Zr(OH)2[MeC(NiPr)2]2 (10), and
solution of 1 in benzene-d6 was added 1 equiv. of 0.5 atm O2 and oxo dimer 3 were observed (Scheme 5). Since the reaction of 1 (or 7)
the reaction at 23 1C was monitored by NMR. It took ca. 3 weeks for with O2 is slow, 3, 9 and 10 are likely the products of the reaction
1 to disappear. In another NMR experiment, a solution of 1 in between moisture in air and 1 (or 7). The oxo monomer 9 may come
toluene was added 2 equiv. of 1.0 atm O2, and then heated at 80 1C from dehydration of 10 or splitting of the dimer 3. Also, the cations
for ca. 5 days when 1H NMR monitoring showed that the reaction {Zr(NR2)[MeC(NiPr)2]2}+ (R = Me, Et) were observed and are likely
was completed. No intermediate was observed in either test. The formed by the MS process. The MS spectra are given in the ESI.†
amount of the oxo polymer 4 in the reaction product mixture
gradually grew. Attempts to grow crystals of the oxo dimer 3 instead are apparently oxidized to radicals NMe2. Several studies have
yielded the oxo polymer 4. These observations suggest that oxo demonstrated that, once NMe2 radicals are formed, their reac-
In the formation of 2, the anionic amide ligands NMe2 in 1
ꢀ
ꢀ
dimer 3 polymerizes into 4. CH2(NMe2)2 and HNMe2, the organic tions lead to the formation of HNMe2 and CH2(NMe2)2 involving
products of the reactions, were characterized by comparing their 1H
and 13C NMR spectra with authentic samples, and both were also
confirmed by GC-MS. Oxo dimer 3 was characterized by 1H and 13
C
NMR spectroscopies and MS.6
Reaction of H2O2 (96%)6 with 1 has also been studied in an
attempt to prepare 2. The high purity H2O2 was prepared by
evacuating 20 mL of 30% aqueous H2O2 solution at 23 1C until
the volume was ca. 2 mL, and the concentration of H2O2 was
determined by titration with KMnO4.9 Such an approach has been
used to obtain high purity H2O2 and D2O2.10 When H2O2 was
Scheme 5 Complexes observed in the MS spectra when 1 or 7 was
exposed to air.
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Scheme 6 Hypothesized pathways in the formation of 2.
ꢀ
intermediates such as CH3 , Hꢀ, CH2QNH, CH3NQCH2, and
11
ꢀ
(CH3)2N–CH2 . Such a pathway may operate in the current
reaction of 1 with O2, giving CH2(NMe2)2 and HNMe2. Our earlier
studies of M(NMe2)n (M = Zr, Hf; n = 4;1n M = Nb, Ta; n = 51o,p) did
not give a peroxo product. DFT studies did not point to a peroxo
complex as a likely intermediate/transition structure.1n–p
Two proposed pathways are given in Scheme 6. Coordination of
O2 to 1 yields triplet A. In Pathway I, A undergoes Zr–NMe2
ꢀ
ꢀ
homolysis to yield superoxo ꢀC and NMe2. The radical C then
undergoes another Zr–NMe2 homolysis, forming the monomer of
peroxo 2. In this step, the superoxo ligand in ꢀC takes in an electron
from the remaining Zr–NMe2 ligand to become a peroxo ligand in 2.
Since ꢀC is fairly crowded, it is perhaps difficult for the ꢀNMe2
radical to attack the Zr–NMe2 ligands (to give Me2NNMe2). Instead
the radicals react among themselves,11 giving CH2(NMe2)2 and
HNMe2. In Pathway II, the triplet A undergoes O2 insertion into a
Zr–NMe2 bond with a spin state conversion to give B. The O–O bond
in monomeric B is probably stronger than the O–NMe2 bond, and
the homolytic cleavage of the O–NMe2 bond yields the ꢀC and the
ꢀNMe2 radicals. DFT studies are currently underway to probe the
pathway in the formation of the peroxo complex 2.
Observation and characterization of 2, 3 and 4 help understand
the nature of the reactions between O2 and d0 transition metal
complexes and pathways in CVD/ALD of metal oxides from amide
amidinates 1 and 7. Peroxo Zr complexes from O2 are rare.1a In the
peroxo 2, there is no redox-active ligand to support the complex.
The work is supported by the U. S. National Science Founda-
tion (Grant CHE-1012173). The authors would like to thank
Stephen C. Gibson and Dr Liguo Song for their assistance with
MS studies, Matthew D. Dembo for his help with the solid-state
NMR work, and Tabitha M. Callaway for her assistance with the
purification of H2O2.
5 7 was disclosed anonymously in ‘‘Zirconium and hafnium amidi-
nates metal–organic compounds and method of forming hafnium
and zirconium containing thin films using the same’’ (http://ip.
com/IPCOM/000161957). However, the details of the synthesis were
not given, and 7 was not characterized in the disclosure.
Notes and references
6 See ESI† for details.
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