Preparation of all N-coordinated zirconium amide amidinates and studies of their reactions with dioxygen and water

Article abstract of DOI:10.1016/j.poly.2015.07.045

Zr(NR₂)₂[MeC(NⁱPr)₂]₂ (R = Me, 1; Et, 2) have been prepared through aminolysis and their reactions with O₂ and water have been studied. Two major products from the reactions are the oxo dimer {(μ-O)Zr[MeC(NⁱPr)₂]₂}₂ (3) and its insoluble polymer {(μ-O)Zr[MeC(NⁱPr)₂]₂}_n (4). Over time the dimer 3 polymerizes to 4. Zr peroxo trimer {(μ-η²:η²-O₂)Zr[MeC(NⁱPr)₂]₂}₃ (5) was also observed from the reaction of 1 with O₂ and its crystal structure is reported. DFT calculations show that the reaction of 1 with O₂ follows a radical process, yielding the peroxo trimer 5. Mass spectrometric studies of the reactions of water in air with 1 and 2 show the formation of the oxo monomer (O=)Zr[MeC(NⁱPr)₂]₂ (6), oxo dimer {(μ-O)Zr[MeC(NⁱPr)₂]₂}₂ (3), and the dihydroxy monomer (HO)₂Zr[MeC(NⁱPr)₂]₂ (7). In addition, the cations {Zr(NR₂)[MeC(NⁱPr)₂]₂}⁺ (R = Me, Et) were observed. 2 revealed an interesting dynamic NMR behavior. Variableerature (VT) NMR spectroscopy has been used to study the Bailar twist process in 2, giving activation parameters ΔH^? = 10.9(1.1) kcal mol^-1, ΔS^? = -11(4) eu and ΔG^?_{303 K} = 14(2) kcal mol^-1.

Full text of DOI:10.1016/j.poly.2015.07.045

Polyhedron 103 (2016) 2–14
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Preparation of all N-coordinated zirconium amide amidinates
and studies of their reactions with dioxygen and water
Adam C. Lamb a, Zheng Wang b, Tabitha M. Cook a, Bhavna Sharma a, Shu-Jian Chen a, Zheng Lu a,
Carlos A. Steren a, Zhenyang Lin b, , Zi-Ling Xue a,
⇑
⇑
a Department of Chemistry, The University of Tennessee, Knoxville, TN 37996, USA
b Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong, China
a r t i c l e i n f o
a b s t r a c t
Zr(NR2)2[MeC(NiPr)2]2 (R = Me, 1; Et, 2) have been prepared through aminolysis and their reactions
Article history:
Received 15 May 2015
Accepted 17 July 2015
Available online 4 August 2015
with O2 and water have been studied. Two major products from the reactions are the oxo dimer
{(
l l
-O)Zr[MeC(NiPr)2]2}2 (3) and its insoluble polymer {( -O)Zr[MeC(NiPr)2]2}n (4). Over time the dimer
3 polymerizes to 4. Zr peroxo trimer {(l- :g
g2 2-O2)Zr[MeC(NiPr)2]2}3 (5) was also observed from the reac-
tion of 1 with O2 and its crystal structure is reported. DFT calculations show that the reaction of 1 with O2
follows a radical process, yielding the peroxo trimer 5. Mass spectrometric studies of the reactions of
water in air with 1 and 2 show the formation of the oxo monomer (O=)Zr[MeC(NiPr)2]2 (6), oxo dimer
Dedicated to Professor Malcolm Chisholm
on the occasion of his 70th birthday.
{(l
-O)Zr[MeC(NiPr)2]2}2 (3), and the dihydroxy monomer (HO)2Zr[MeC(NiPr)2]2 (7). In addition, the
Keywords:
cations {Zr(NR2)[MeC(NiPr)2]2}+ (R = Me, Et) were observed. 2 revealed an interesting dynamic NMR
Zirconium amide amidinates
Peroxo complexes
Oxygen
Water
DFT studies
behavior. Variable-temperature (VT) NMR spectroscopy has been used to study the Bailar twist process
in 2, giving activation parameters
D
Hà = 10.9(1.1) kcal molꢀ1
,
D
Sà = ꢀ11(4) eu and
D
Gà303
K = 14(2) kcal molꢀ1
.
Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction
adventitious O2 [19]. Rapid autoxidation of M(CH2R)4 by O2 has
been studied [10].
Reactions of early transition metal complexes with O2 [1–20]
are typically different from those of late transition metal com-
plexes [21–42]. In early transition metal complexes, metals are
usually at their highest oxidation states with no valence d elec-
trons (d0). In comparison, metals in late transition metal com-
plexes generally have valence d electrons (dn). When such a
complex reacts with O2, the metal is often oxidized and its oxida-
tion state is increased.
Metal oxide (MOn) thin films are important microelectronic
materials and they have been prepared through chemical vapor
deposition (CVD) and atomic layer deposition (ALD) processes to
replace SiO2 as gate insulator materials [43–51]. Metal oxides such
as HfO2 (k = 25) and Ta2O5 (k = 26) with high dielectric constants
(high-k dielectrics) are of intense interest. The recent use of thinner
and better insulating MOn thin films allows the number of transis-
tors on integrated circuits to increase. d0 Groups 4 and 5 amide
[46–51] complexes M(NR2)n, many of which originally prepared
by Bradley and Thomas [52], are used as precursors to make metal
oxide thin films through their reactions with O2 or water. These
amide complexes are very air-sensitive requiring their handling
under inert atmosphere [52–61]. Ancillary bidentate ligands such
as amidinate [62–75] and guanidinate [76] have been used to make
complexes less reactive toward oxygen sources. Amidinate ligands
[RC(NR0)2]ꢀ bond in a bidentate fashion through conjugation and
donate four electrons to a metal center. They are considered steri-
Previous studies of the reactions between d0 early transition
metal complexes and O2 focused on O insertion into the Zr–Si bond
in Cp2Zr(SiMe3)Cl [1], and M–C bonds in Cp2ZrRCl/Cp2ZrR2
(R = alkyl) [2,3], (RO)2TiMe2 [4], (ArO)2TaMe3 [5], and (ArN=)2-
MoMe2 [6]. Also, a Zr(IV) bisperoxo complex was obtained as a pro-
duct from the reaction of O2 with a Zr(IV) complex that contains
redox-active diimine ligands (Scheme 1) [17]. Complexes with no
valence d electrons have also been prepared using O2 as a reactant.
For example, d0 Mo(NMe2)6 was prepared from d2 Mo(NMe2)4 and
cally equivalent to
g p–
5-cyclopentadienyl but isoelectronic with
allyl ligands [62]. Lone pair electrons contribute to the conjugation
to the metal center. Amidinate complexes have been used in catal-
ysis [62,66,67], N2 activation [68], Si–H/Si–Cl bond activation [69]
⇑
Corresponding authors. Tel.: +852 2358 7379 (Z. Lin), +1 865 974 3443
(Z.-L. Xue).
E-mail addresses: chzlin@ust.hk (Z. Lin), xue@utk.edu (Z.-L. Xue).
http://dx.doi.org/10.1016/j.poly.2015.07.045
0277-5387/Ó 2015 Elsevier Ltd. All rights reserved.

A.C. Lamb et al. / Polyhedron 103 (2016) 2–14
3
Scheme 1. Synthesis of a Zr peroxo complex with two redox-active ligands [17].
and CVD [70–75]. Devi and coworkers have studied the reaction of
a hafnium amidinate with O2 in CVD to give a HfO2 thin film
(Scheme 2) [73]. Pallem and Dussarrat have used ALD to give a
ZrO2 film by reacting an amidinate with H2O (Scheme 2) [75].
However, pathways in the reactions of the amidinate complexes
with O2 are not understood. We have prepared two new d0 Zr
amide amidinates and studied their reactions with dioxygen and
water. A rare peroxo complex from the reaction of d0 Zr(NMe2)2[-
MeC(NiPr)2]2 (1) with O2 was observed [77]. These studies are
reported here.
Scheme 2. Amidinates used as CVD/ALD precursors [73,75].
dom uncertainty in the rate constants was combined with the
estimated systematic uncertainty of 5%. The total uncertainties in
the rate constants k were used in the ln(keq/T) vs. 1000/T plot in
Fig. S11 and error calculations. The NMR probe temperature
measurements contribute to an estimated uncertainty of 1 K.
2. Materials and methods
The uncertainties in
following error formulas derived from the Eyring equation
Hà/T + Sà [82]:
D D
Hà and Sà were calculated from the
All manipulations were carried out under a dry nitrogen atmo-
sphere with the use of either a glovebox or standard Schlenk tech-
niques. Glassware was flame-dried under vacuum. Solvents were
purified by distillation from a potassium benzophenone ketyl.
NMR solvents were dried and stored over 5 Å molecular sieves.
Potassium oxalate monohydrate, KMnO4, and Na2SO4 (All Certified
ACS, Fischer Scientific) were used without further purification.
N,N0-Diisopropylcarbodiimide, LiNMe2 and MeLi (1.6 M in Et2O)
were purchased from Acros and used without further purification.
ZrCl4 (Strem) was sublimed at 170 °C before use. Zr(NMe2)4 and
Zr(NEt2)4 were prepared according to the literature methods [78].
Deionized H2O was degassed and stored in a Schlenk flask. O2 (Air-
gas) was purified by passing it through a P2O5 column. Li[MeC(Ni-
Pr)2] was prepared by the reaction of N,N0-diisopropylcarbodiimide
with MeLi using the procedure of Hessen et al. [79].
iPrN(H)C(Me) = NiPr was prepared by the reaction of Li[MeC(NiPr)2]
with H2O by an approach similar to that of Oshima [80] and Miller
[81]. High purity H2O2 was prepared from 30% aqueous H2O2 solu-
tion (Fischer Scientific, reagent grade) and its purification is dis-
cussed below. 1H, 13C{1H}, 2D-13C-HSQC and 2D-13C-HMBC NMR
spectra were recorded on a Bruker Avance 400 MHz or a Varian
VNMRS-500 MHz spectrometer. The VT-NMR experiments were
performed on the Bruker Avance 400 MHz spectrometer. Solid-
state NMR spectrum was recorded on a Varian INOVA 400 MHz
spectrometer equipped with a Chemagnetics 5 mm CPMAS probe
and 13C{1H} spectrum was referenced to adamantane. Elemental
analyses were conducted by Complete Analysis Laboratories, Inc.,
Parsippany, NJ. Mass spectra were recorded on a JEOL AccuTOF™
DART Mass Spectrometer. To record IR spectra, solid samples were
grounded with KBr, which had been dried at 100 °C and under vac-
uum, and then pressed into pellets. IR spectra were recorded on a
Varian 4100 Excalibur spectrometer. Thermal gravimetric analysis
was recorded on a Q-50 TGA.
Rln(kh/kbT) = ꢀ
D
D
(
"
ꢀ
ꢁ
ꢀ
ꢁ
2
2
R2T2maxT2min
r
T
T
DL
ð
r
D
H–Þ2 ¼
1 þ Tmin
D
T
D
T2
#
)
ꢀ
ꢁ
ꢀ
ꢁ
2
2
D
L
rk
k
þ
1 þ Tmax
þ 2
D
T
(
"
ꢀ
ꢁ
ꢀ
ꢁ
2
2
R2
r
T
T
D
L
ðr
D
S–Þ2 ¼
T2max 1 þ Tmin
T2
D
T
D
#
)
ꢀ
ꢁ
ꢀ
ꢁ
2
2
ꢂ
ꢃ
D
L
rk
þT2min 1 þ Tmax
þ
T2max þ T2min
D
T
k
where
Tmin).
D
L = [ln(kmax/Tmax) ꢀ ln(kmin/Tmin)] and
D
T = (Tmax
ꢀ
2.2. Synthesis of Zr(NMe2)2[MeC(NiPr)2]2 (1)
Zr(NMe2)4 (1.601 g, 5.984 mmol) in pentane (50 mL) at ꢀ30 °C
was added dropwise with stirring iPrN(H)C(Me) = NiPr (1.758 g,
12.36 mmol) in pentane (50 mL). The solution was stirred for
12 h and volatiles were removed in vacuo, affording an analytically
pure yellow solid of 1 (2.764 g, 5.99 mmol, 95.6% yield). 1H NMR
(benzene-d6, 399.17 MHz, 23 °C): d 3.45 (m, 4H, CHMe2), 3.23 (s,
12H, NMe2), 1.58 (s, 6H, NC(Me)N), 1.18 (d, 24H, CHMe2); 13C{1H}
NMR (benzene-d6, 100.63 MHz, 23 °C):
d 175.07 (NC(Me)N),
48.34 (CHMe2), 46.10 (NMe2), 25.61 (CHMe2), 11.31 (NC(Me)N).
1H NMR (chloroform-d1, 400.00 MHz, 23 °C): d 3.49 (m, 4H,
CHMe2), 2.93 (s, 12H, NMe2), 1.83 (s, 6H, NC(Me)N), 1.03 (d, 24H,
CHMe2). 13C{1H} NMR (chloroform-d1, 100.58 MHz, 23 °C):
d
174.21 (NC(Me)N), 47.58 (CHMe2), 45.23 (NMe2), 24.89 (CHMe2),
11.04 (NC(Me)N). It should be noted that, in chloroform-d1, slow
decomposition of 1 was observed. The 1H and 13C{1H} NMR assign-
ments were confirmed by DEPT, HMBC and HSQC experiments.
Anal. Calcd: C, 52.01; H, 10.04. Found: C, 51.86; H, 9.98.
Caution: Extreme care should be taken in using hydrogen peroxide
in the reactions. A shield should be used for protection.
2.1. Calculating errors in VT-NMR studies
The activation parameters, determined from the Eyring plot,
were obtained by using the average rate constants which were
obtained through two separate experiments at a given tempera-
ture and their averages are listed in Table S1. The maximum ran-
2.3. Synthesis of Zr(NEt2)2[MeC(NiPr)2]2 (2)
Zr(NEt2)4 (2.344 g, 6.171 mmol) in pentane (50 mL) at ꢀ30 °C
was added dropwise with stirring iPrN(H)C(Me) = NiPr (1.761 g,

4
A.C. Lamb et al. / Polyhedron 103 (2016) 2–14
12.38 mmol) in pentane (50 mL). The solution was stirred for 12 h
and volatiles removed in vacuo affording an analytically pure yel-
low solid of 2 (2.954 g, 5.702 mmol, 92.4% yield). 1H NMR (ben-
zene-d6, 400.17 MHz, 23 °C): d 3.71 (br, 8H, N(CH2CH3)2), 3.46
(br, 4H, CHMe2), 1.59 (s, 6H, NC(Me)N), 1.20 (br, 24H, CHMe2),
1.10 (t, 12H, N(CH2CH3)2). 13C{1H} NMR (benzene-d6,
100.62 MHz, 23 °C): d 173.35 (NC(Me)N), 48.04 (CHMe2), 41.67
(N(CH2CH3)2), 25.62 (CHMe2), 13.69 (N(CH2CH3)2), 11.90
(NC(Me)N). 1H NMR (toluene-d8, 400.08 MHz, 23 °C): d 3.68 (br,
8H, N(CH2CH3)2), 3.46 (br, 4H, CHMe2), 1.61 (s, 6H, NC(Me)N),
1.26 (br, 24H, CHMe2), 1.09 (t, 12H, N(CH2CH3)2). 13C{1H} NMR
(toluene-d8, 100.60 MHz, 23 °C): d 172.64 (NC(Me)N), 48.27 (br,
CHMe2), 41.65 (N(CH2CH3)2), 25.67 (br, CHMe2), 13.68 (N(CH2-
CH3)2), 11.81 (NC(Me)N). 1H NMR (toluene-d8, 400.08 MHz,
ꢀ60 °C): d 3.95 (m, 4H, N(CH2CH3)2), 3.62 (m, 4H, N(CH2CH3)2),
3.47 (m, 2H, CHMe2), 3.32 (m, 2H, CHMe2), 1.53 (s, 6H, NC(Me)N),
1.42 (d, 6H, CHMe2), 1.36 (d, 6H, CHMe2), 1.14 (d, 6H, CHMe2),
1.13 (d, 6H, CHMe2), 1.13 (t, 12H, N(CH2CH3)2). 13C{1H} NMR
(toluene-d8, 100.60 MHz, ꢀ60 °C): d 173.14 (NC(Me)N), 48.15
(CHMe2), 47.55 (CHMe2), 41.11 (N(CH2CH3)2), 25.93 (CHMe2),
25.91 (CHMe2), 25.53 (CHMe2), 23.52 (CHMe2), 13.30 (N(CH2CH3)2),
11.27 (NC(Me)N). The 1H and 13C{1H} NMR assignments were con-
firmed by HMBC (23 °C) and HSQC (213 K) experiments. Anal.
Calcd: C, 55.65; H, 10.51; N, 16.23. Found: C, 55.49; H, 10.40; N,
16.17.
were taken and after 3 weeks, 1 disappeared, forming 3, HNMe2,
CH2(NMe2)2, and insoluble 4.
In a separate test, O2 (1 atm, 0.0396 mmol) was added to 1
(18.3 mg, 0.0396 mmol) in benzene-d6. The volume of the head-
space in the Young’s NMR tube was 2.1 mL. The NMR tube was
heated at 70 °C. After ca. 12 days, the 1H NMR spectrum showed
the disappearance of 1 and appearance of 3 and two byproducts:
HNMe2 (0.13 mg, 7.0% yield based on NMR) and CH2(NMe2)2
(1.1 mg, 27% yield based on NMR). HNMe2 and CH2(NMe2)2 were
confirmed by GC–MS. A light yellow precipitate formed which
was identified to be the polymeric 4 (9 mg, 58% yield).
Reaction in a Schlenk flask. In another experiment, O2 (1 atm,
2.014 mmol) was added to 1 (0.465 g, 1.007 mmol) in toluene in
a Schlenk flask. The headspace of the flask was 20.0 mL. The solu-
tion was heated at 80 °C for ca. 5 days. A pale yellow solid had pre-
cipitated from the solution. Volatiles were removed in vacuo and
pentane was added. The solution was filtered and concentrated
in attempts to grow crystals. Over time, a light yellow solid precip-
itated out. Attempts to grow crystals of 3 in Et2O, hexanes and
toluene were not successful. The characterization of 4 is discussed
below.
1H NMR (benzene-d6, 400.17 MHz, 23 °C): d 4.26 (m, 2H,
CHMe2), 3.47 (m, 2H, CHMe2), 1.34 (s, 6H, NC(Me)N), 1.25 (d,
12H, CHMe2) 1.02 (d, 12H, CHMe2). 13C{1H} NMR (benzene-d6,
100.63 MHz, 23 °C): d 151.38 (NC(Me)N), 49.20 (CHMe2), 41.40
(CHMe2), 25.60 (CHMe2), 22.90 (CHMe2), 15.38 (NC(Me)N). The 1H
and 13C{1H} NMR assignments were confirmed by HSQC.
2.4. Synthesis of Zr(NMe2)3[MeC(NiPr)2] (8)
2.7. Reaction of Zr(NEt2)2[MeC(NiPr)2]2 (2) with O2
Zr(NMe2)4 (1.10 g, 4.13 mmol) in pentane (50 mL) at ꢀ50 °C
was added dropwise with stirring iPrN(H)C(Me) = NiPr (0.585 g,
4.11 mmol) in pentane (50 mL). The solution was warmed to room
temperature and stirred for an additional 12 h. Volatiles were
removed in vacuo affording a yellow solid of 8 (0.805 g, 2.21 mmol,
53.5% yield). 1H NMR (benzene-d6, 399.17 MHz, 23 °C): d 3.34 (m,
2H, CHMe2), 3.09 (s, 18H, NMe2), 1.52 (s, 3H, NC(Me)N), 1.04 (d,
12H, CHMe2); 13C{1H} NMR (benzene-d6, 100.63 MHz, 23 °C): d
175.42 (NC(Me)N), 47.87 (CHMe2), 42.62 (NMe2), 25.02 (CHMe2),
10.48 (NC(Me)N). The 1H and 13C{1H} NMR assignments were con-
firmed by DEPT experiments. 8 was found to be an intermediate in
the formation of 1, and was not characterized further.
NMR-scale reaction. In
a Young’s NMR tube, 2 (16.5 mg,
0.0319 mmol) was dissolved in benzene-d6, leaving 2.3 mL of
headspace. Before adding O2, the solution was frozen in liquid
nitrogen and nitrogen gas in the tube was removed in vacuo. O2
(1 atm, 0.0319 mmol) was then added. The solution was then
heated at 70 °C for 7 days. 1H NMR revealed the formation of 3
and HNEt2 (14% yield). HNEt2 was confirmed by GC–MS. A light
yellow precipitate was identified as {(
7.8 mg, 63% yield).
l
-O)Zr[MeC(NiPr)2]2}n (4,
Reaction in a Schlenk flask. O2 (1 atm, 1.06 mmol) was added to 2
(0.549 g, 1.06 mmol) in toluene in a Schlenk flask with 53.6 mL of
headspace. The solution was heated at 80 °C for 4 days, yielding a
pale yellow solid of 4. Volatiles were removed in vacuo and pen-
tane was added. The solution was filtered and concentrated in an
attempt to grow crystals. Over time a cloudy solution formed, pre-
2.5. Synthesis of Zr(NEt2)3[MeC(NiPr)2] (9)
This reaction was conducted in a Young’s tube and reagents
were mixed in a glovebox. Zr(NEt2)4 (0.0280 g, 0.0737 mmol) in
benzene-d6 (1 mL) was added dropwise iPrN(H)C(Me) = NiPr
(0.0102 g, 0.0717 mmol) in benzene-d6 (1 mL) with shaking. The
volatiles were removed in vacuo affording a yellow solid of 9
(0.0302 g, 0.0673 mmol, 91.3% yield). 1H NMR (benzene-d6,
499.73 MHz, 23 °C): d 3.46 (q, 12H, N(CH2CH3)2), 3.37 (m, 2H,
CHMe2), 1.53 (s, 3H, NC(Me)N), 1.20 (t, 18H, N(CH2CH3)2), 1.12 (d,
12H, CHMe2); 13C{1H} NMR (benzene-d6, 125.67 MHz, 23 °C): d
175.08 (NC(Me)N), 47.82 (CHMe2), 44.49 (N(CH2CH3)2), 25.22
(CHMe2), 15.90 (N(CH2CH3)2), 10.89 (NC(Me)N). The 1H and
13C{1H} NMR assignments were confirmed by an HSQC experiment.
9 was found to be an intermediate in the formation of 2 and not
characterized further.
sumably as {(
terization of 4 is discussed below.
l
-O)Zr[MeC(NiPr)2]2}n (4) precipitated out. Charac-
2.8. Characterization of {(l
-O)Zr[MeC(NiPr)2]2}n (4)
For the characterization here, the polymer 4 was prepared from
the reaction of 2 with O2. The solid-state 13C NMR spectrum of 4
shows peaks at 14.19 ppm for NC(Me)N, 23.27 ppm for –CHMe2,
46.34 ppm for –CHMe2, and 152.35 ppm for NC(Me)N. The IR spec-
trum shows a C@N peak at 1641 cmꢀ1. In a TGA experiment under
N2, the sample was heated from 23 to 1000 °C at a rate of
10 °C/min. There was a 36.21% loss which accounted for one amid-
inate ligand to be removed. Anal. Calcd: C, 49.31; H, 8.79; N, 14.38.
Found: C, 49.24; H, 8.71; N, 14.29.
2.6. Reaction of Zr(NMe2)2[MeC(NiPr)2]2 (1) with O2
2.9. Reactions of Zr(NR2)2[MeC(NiPr)2]2 (R = Me, 1; Et, 2) with H2O
NMR-scale reaction. In
a Young’s NMR tube, 1 (28.9 mg,
0.0626 mmol) was dissolved in benezene-d6, leaving 2.6 mL of
headspace in the NMR tube. The solution was frozen in liquid
nitrogen and nitrogen gas in the tube was removed in vacuo. O2
(0.5 atm, 0.0626 mmol) was then added. Several 1H NMR spectra
NMR-scale reaction. In a Young’s NMR tube, 2 (35.3 mg,
0.0682 mmol) was dissolved in benezene-d6. In a separate Young’s
NMR tube, H2O (1.2 mg, 0.0611 mmol) was dissolved in 0.5 mL of
THF. The solution of 2 in benezene-d6 was cooled with an ice bath

A.C. Lamb et al. / Polyhedron 103 (2016) 2–14
5
and the H2O/THF solution was added dropwise. 1H and 13C{1H}
2.13. Computational details
NMR spectroscopies showed the formation of 3 and HNEt2. {(l-
O)Zr[MeC(NiPr)2]2}n (4) precipitated out of the solution. This was
the same procedure used for the reaction of 1 with H2O to confirm
3 and 4.
Molecular geometries of complexes were optimized via DFT cal-
culations using B3LYP functional [85]. In the DFT calculations, the
effective core potentials (ECPs) of Hay and Wadt with double f
valence basis sets (LanL2DZ) were chosen to describe Zr [86,87].
In addition, polarization functions were added for Zr (ff = 0.875)
[88]. The 6-31G⁄ basis set was used for C, H, N and O. Frequency
analyses have been performed to obtain the Gibbs free energies
and identify all of the stationary points as minima (zero imaginary
frequency) or transition states (one imaginary frequency) on the
potential energy surfaces (PES). Intrinsic reaction coordinate
(IRC) calculations were also calculated to confirm that transition
state structures indeed connect two relevant minima [89,90]. Since
reactions of O2 involve state crossing between triplet and singlet
states, we employed the code developed by Harvey and co-workers
to optimize structures of minimum energy crossing points (MECP)
between different spin states [91,92]. An MECP is not a minimum
on the potential energy surface, and therefore there is a difficulty
in calculating its relative free energy because the entropy contribu-
tion cannot be calculated. To approximately estimate the relative
free energies for the two MECPs calculated and presented in
Scheme 9, we took the average (14.7 kcal/mol) of the entropy con-
tributions calculated for the intermediates H, C, and F as the
entropy contribution to the relative free energy for an MECP struc-
ture. Here, we consider the three intermediates for our estimation
because their structures are closely related to the two MECP
structures.
B3LYP, which contains 20% of Hartree–Fock exchange, was
reported to overestimate the stability of high spin states for many
systems, and it was suggested to make the percentage of Hartree–
Fock exchange as 15% (labelled as B3LYP⁄) to alleviate the overes-
timation [93,94]. We performed single-point energy calculations
for the key intermediates B, H, C, and F using B3LYP⁄. The results
show that the relative electronic energies for the intermediates
B, H, C, and F are ꢀ1.9, ꢀ17.5, 4.8, and ꢀ16.8 kcal/mol, respectively,
using B3LYP⁄ versus ꢀ1.4, ꢀ17.6, ꢀ4.0, and ꢀ17.4 kcal/mol using
B3LYP. These additional calculations suggest that B3LYP does not
show the overestimation in the systems we studied here.
2.10. Purification of H2O2
Caution: Extreme care should be taken when using hydrogen
peroxide.
High purity H2O2 was prepared by evacuating at <0.1 torr a
20 mL of 30% aqueous H2O2 solution at 23 °C until the volume of
the remaining solution is ca. 2 mL [83,84]. Since boiling points of
H2O2 and water are 150.2 and 100 °C, respectively, water was
expected to be preferentially removed during the process. The resi-
due, containing mostly H2O2, was then vacuum-transferred at
<0.1 torr and condensed by liquid nitrogen in another Schlenk flask
in order to remove the stabilizer in the H2O2 solution. The con-
densed H2O2 liquid was titrated three times with a 0.05 M aqueous
KMnO4 solution which had been standardized with potassium oxa-
late. The H2O2 liquid was found to be 96% pure.
To standardize the KMnO4 solution, potassium oxalate
(0.2447 g, 1.47 mmol) in deionized water (15 mL) was heated to
ꢁ70 °C. (Do not boil the solution!) Concentrated H2SO4 (1 mL)
was then added. Titrations gave the concentration of the KMnO4
solution, 0.506 M.
To determine the concentration of the H2O2 liquid, H2O2
(0.0818 g, 2.40 mmol) was dissolved in deionized water (15 mL)
and concentrated H2SO4 (1 mL). Titrations with KMnO4 (0.506 M,
18.2 mL, 9.21 mmol) gave the concentration of 95.5–96.6%.
2.11. Reaction of 1 with H2O2
Caution: Extreme care should be taken when using hydrogen
peroxide.
NMR-scale reaction. In a Young’s tube, 1 (13.6 mg, 0.0294 mmol)
in benzene-d6 was added H2O2 (0.70
l
L, 0.030 mmol). The 1H and
13C{1H} NMR spectra revealed 3, HNMe2, and CH2(NMe2)2. A pre-
cipitate formed and was presumed to be {(l
-O)Zr[MeC(NiPr)2]2}n
(4).
2.14. Determination of the crystal structure of 5
Reaction conducted in a Schlenk flask. In a large scale experiment,
H2O2 (26.5 mg, 0.780 mmol) in THF (20 mL) at 0 °C was added to 1
(357 mg, 0.772 mmol) in THF (15 mL) and cooled to ꢀ50 °C. It was
allowed to stir overnight and volatiles were removed in vacuo. The
1H NMR spectrum revealed the oxo-bridged dimer 3. A precipitate
The X-ray crystal structure of 5 was determined on a Bruker
AXS Smart 1000 X-ray diffractometer equipped with a CCD area
detector and a graphite-monochromated Mo source (Ka radiation,
formed and was presumed to be {(
Reactions of solid 1 with H2O2. High purity H2O2 (0.40
l
-O)Zr[MeC(NiPr)2]2}n (4).
0.71073 Å) and fitted with an upgraded Nicolet LT-2 low tempera-
ture device. A suitable crystal was coated with paratone oil (Exxon)
and mounted on a fiber loop under a stream of nitrogen at
100(2) K. The structure was solved by direct methods. All non-hy-
drogen atoms were anistropically refined. Empirical absorption
correction was performed with SADABS [95]. Global refinements
for the unit cells and data reductions were performed under the
Saint program (Version 6.02). All calculations were performed
using SHELXTL (Version 5.1) proprietary software package [96].
lL,
0.017 mmol) was added directly on powders of Zr(NMe2)2[MeC(Ni-
Pr)2]2 (1, 17.1 mg, 0.0370 mmol) in a glovebox. This was an
attempt to minimize the contact of the product, peroxo 5, with
unreacted 1 by not using a solvent. As soon as the liquid H2O2
was added via syringe to the powders of 1, a spark with smoke
occurred. This observation suggests that {(
Pr)2]2}3 (5) is a reactive compound (Scheme 6).
l- :g
g2 2-O2)Zr[MeC(Ni-
2.12. Reaction of 2 with O2 in the presence of azobisisobutyronitrile
3. Results and discussion
(AIBN)
3.1. Synthesis and characterization of amide amidinates 1 and 21
In a Young’s tube, AIBN (13.6 mg, 0.0828 mmol) was added to a
benzene-d6 solution of 2 (45.0 mg, 0.0869 mmol). The solution
was frozen with liquid nitrogen and O2 (1 atm, 0.0869 mmol)
was added. (Volume of the headspace in the Young’s tube was
2.1 mL.) The Young’s tube was heated at 72 °C. After 2 days, the
reaction was completed. 1H NMR spectrum showed the oxo-dimer
3 had formed.
Zr(NMe2)2[MeC(NiPr)2]2 (1) has been prepared by three
different routes: aminolysis (Scheme 3a) and two salt metathesis
1
2 was disclosed anonymously in ‘‘Zirconium and hafnium amidinates 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 2 was not characterized in the disclosure.

6
A.C. Lamb et al. / Polyhedron 103 (2016) 2–14
Scheme 3. Preparation of 1 and 2.
reactions [97] (Scheme 3b and c). When the reaction was carried
out by aminolysis using iPrN(H)C(Me) = NiPr [80,81] and
Zr(NMe2)4, the yield of 1 was the highest. Removal of volatiles gave
analytically pure 1 in 95% yield. This route was thus chosen to
make 1 and 2.
The 1H NMR spectrum (Fig. S3) of 1 in benzene-d6 shows a dou-
blet at 1.18 ppm for the CHMe2 groups. Two singlets at 1.58 and
3.23 ppm were assigned to the –C(Me) group on the amidinate
and NMe2 ligands, respectively. A mulitiplet for the CHMe2 groups
appears at 3.45 ppm. These observations suggest that 1 undergoes
Scheme 4. Bailar twist mechanism for the exchange in 1 and 2 [98,99].
a
rapid exchange at room temperature by the Bailar twist
mechanism [98], the exchange of two enantiomers through a
trigonal-prismatic intermediate (Scheme 4). In this case the
exchange makes all four isopropyl groups equivalent. We have
studied such twist mechanism with a bulkier amide amidinate,
Zr(NMe2)2[MeC(NCy)2]2 (Cy = cyclohexyl) by VT-NMR [99]. In the
current work, the exchange in the diethylamide analog 2 was
investigated, as discussed below. The exchange in 1 was thus not
studied further.
In the 13C{1H} NMR spectrum (Fig. S4) of 1, peaks at 11.31 and
175.07 ppm were assigned to –C(Me) and –C(Me)– groups on the
amidinate, respectively. The CHMe2 groups appear at 25.61 ppm.
The NMe2 ligands appear at 46.10 ppm. The CH carbon atom on
the isopropyl groups appears at 48.34 ppm.
At room temperature some 1H and 13C{1H} NMR peaks of
Zr(NEt2)2[MeC(NiPr)2]2 (2) were broad (Figs. S5 and S6). One con-
tributing factor to the fluxionality is the bulkier NEt2 groups which
slow down the exchanges in comparison to its NMe2 analog 1. We
have studied the dynamic behavior of 2 by 1H VT-NMR and
obtained the activation parameters for the process. As the temper-
ature was lowered to ꢀ60 °C, de-coalescence of all peaks in the 1H
NMR spectrum occurred (Fig. S7). At ꢀ60 °C, four doublets appear
for the isopropyl (CHMe2) groups at 1.42, 1.36, 1.14, and 1.13 ppm.
Two peaks for CHMe2 appear at 3.47 and 3.32 ppm, and two peaks
for NCH2CH3 were observed at 3.96 and 3.64 ppm. The methyl
groups on CHMe2 are separated into four doublets in the 1H NMR
spectrum of 2 at ꢀ60 °C, two of which overlap with N(CH2CH3)2.
At 23 °C, the 13C{1H} NMR spectrum of 2 in toluene-d8 also shows
a dynamic feature in which the CH and CHMe2 resonances are
broad at 48.27 and 25.67 ppm, respectively (Fig. S6). However, at
ꢀ60 °C, two sharp CH peaks appear at 48.14 and 47.55 ppm in
toluene-d8 (Fig. S8). Four sharp CHMe2 peaks at 25.93, 25.91,
25.52 and 23.52 ppm were also observed. 1H and 13C{1H} NMR
assignments were confirmed by HSQC (ꢀ60 °C, Fig. S9) and HMBC
(23 °C, Fig. S10) studies.
Fig. 1. Partial 1H VT-NMR spectra of 2 in toluene-d8.
Since there were broad peaks at room temperature, VT-NMR
experiments were performed in order to calculate its activation
parameters. Partial 1H VT-NMR spectra of 2 (213–303 K) for the
CHMe2 and N(CH2CH3)2 region are given in Fig. 1. As the tempera-
ture increases, the exchange based on the Bailar twist process
accelerates [98], reaching the coalescence at 30 °C [100]. The rate

A.C. Lamb et al. / Polyhedron 103 (2016) 2–14
7
constants (Table S1) of this interconversion at various tempera-
tures were calculated from Eq. (1):
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
v02
ꢀ
D
v
Þ
ð1Þ
2
k ¼
p
2ð
D
where
Dm0 and Dm are frequency differences (Hz) between the sites
of the slow exchange limit and the exchange-broadened sites at
temperature T, respectively [101–103].2
Scheme 5. Synthesis of Zr(NR2)3[MeC(NiPr)2] (R = Me, 8; Et, 9).
The frequency differences (Hz) were taken from the two CHMe2
on the isopropyl group and used in Eq. (1). The activation parame-
ters of the exchange were determined from an Erying plot
and 13C{1H} NMR spectra and GC–MS of the volatiles from the reac-
tion mixture. It should be pointed that Me2NNMe2 was not
observed in the NMR spectra or GC–MS of the volatiles. The struc-
ture of 3 is based on its 1H and 13C{1H} NMR (Figs. S16 and S17) and
MS spectra. As the reaction progressed, more precipitate formed
while the peaks of the oxo-dimer 3 stayed at about the same
intensity.
(Fig. S11) to give
The calculated
D
D
Hà = 10.9(1.1) kcal molꢀ1 and
D
Sà = ꢀ11(4) eu.
Gà303 K = 14(2) kcal molꢀ1 is close to those in
Zr(NEt2)2[MeC(NCy)2]2 [99].
3.2. Synthesis and characterization of the monoamidinates
Zr(NR2)3[MeC(NiPr)2] (R = Me, 8; Et, 9)
When a solution of 1 in pentane at ꢀ30 °C was exposed to O2 in
Compounds 8 and 9 were prepared in order to obtain their 1H
and 13C{1H} NMR spectra as they are expected intermediates in
the synthesis of diamidinates 1 and 2 from Zr(NR2)4 (R = Me, Et;
Scheme 3a). The NMR spectra were then used to monitor the for-
mation of 1 and 2. 8 and 9 were indeed observed as intermediates
in the reactions in Scheme 3a and then converted to 1 and 2,
respectively.
air over a period of a few weeks, crystals of peroxo trimer {(l-
g2
:g
2-O2)Zr[MeC(NiPr)2]2}3 (5) were obtained as a solid mixture
with powders of unreacted 1, {(
-O)Zr[MeC(NiPr)2]2}2 (3), and
{(
-O)Zr[MeC(NiPr)2]2}n (4, Scheme 6). When the solid mixture
l
l
was dissolved in benzene-d6 at 23 °C, only 3, 4, CH2(NMe2)2,
HNMe2, and 1 were observed (Scheme 6).
Peroxo complexes derived from H2O2 have been reported
[104,105]. For example, Kortz and coworkers have obtained peroxo
Zr(NMe2)3[MeC(NiPr)2] (8) was prepared by adding 1 equiv of
iPrN(H)C(Me) = NiPr in pentane to Zr(NMe2)4 at ꢀ50 °C (Scheme 5).
The addition was slow to avoid the formation of the bisamidinate
1, giving raw product in 53% yield. 1H NMR spectrum of
Zr(NMe2)3[MeC(NiPr)2] (8) in benzene-d6 (Fig. S12) showed a dou-
blet at 1.04 ppm for the CHMe2 groups. Two singlets at 1.52 and
3.09 ppm were assigned to –C(Me) group on the amidinate and
NMe2, respectively. A mulitiplet for the CHMe2 groups appears at
3.34 ppm. In the 13C{1H} NMR spectrum (Fig. S13), peaks at 10.48
and 175.42 ppm were assigned to –C(Me) and –C(Me)– groups on
the amidinate, respectively. The peak at 25.02 ppm accounts for
the CHMe2 groups. The NMe2 ligands appear at 42.62 ppm.
The ethyl analog Zr(NEt2)3[MeC(NiPr)2] (9) was prepared by
reacting 1 equiv of iPrN(H)C(Me) = NiPr with Zr(NEt2)4 (Scheme 5).
Unlike its NMe2 analog, this reaction was carried out at room tem-
perature to give a crude product as a yellow solid in 91% yield. Its
1H NMR spectrum of 9 (Fig. S14) showed a doublet at 1.12 ppm for
the CHMe2 groups. A triplet for the N(CH2CH3)2 groups appeared at
1.20 ppm. A singlet at 1.53 ppm was assigned for MeC(NiPr)2. Mul-
tiplets for the CHMe2 and N(CH2CH3)2 groups are at 3.37 ppm and
3.46 ppm, respectively. In the 13C{1H} NMR spectrum (Fig. S15),
peaks at 10.89 and 175.08 ppm were assigned to –C(Me) and –
C(Me)–, respectively. The peaks at 15.90, 25.22, and 44.49 ppm
were assigned to the N(CH2CH3)2, N(CH2CH3)2, and CHMe2 groups,
respectively. The CH carbon atoms of the CHMe2 groups appeared
at 47.82 ppm.
heteropolytungstates [M2(O2)2(XW11O39)2]12ꢀ (M = Zr, X = Si, Ge;
8ꢀ
M = Hf, X = Si) through reactions of ZrCl4/HfCl4 with [XW11O39
]
,
followed by treatment with H2O2 [104]. Exchanges of the O2ꢀ
ligands with O22ꢀ lead to the peroxo ligands in the products.
In attempts to synthesize 5 independently from O2, we have
found that it is unstable. When 1 is present, they reacted together
to give 3. Reaction of H2O2 (96%) with 1 has also been studied in an
attempt to prepare 5. The high purity H2O2 was prepared by evac-
uating 20 mL of 30% aqueous H2O2 solution at 23 °C until the vol-
ume was ca. 2 mL, and the concentration of H2O2 was determined
by titration with KMnO4 [106]. Such an approach has been used to
obtain high purity H2O2 and D2O2 [83,84]. When H2O2 was added
to 1 in a THF solution, 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 5 (or
its monomeric derivative or dimeric peroxo complex 10) in this
reaction of 1 with H2O2 (Scheme 7).
The 1H NMR spectrum of 3 (Fig. S16) shows two doublets at
1.02 and 1.25 ppm for CHMe2. A singlet at 1.34 ppm was assigned
to (iPrN)(Me)C(NiPr). Two multiplets for CHMe2 are assigned at
3.47 and 4.26 ppm. In the 13C{1H} NMR spectrum (Fig. S17), two
peaks at 22.90 and 25.60 ppm were assigned to CHMe2, and their
CH atoms appeared at 41.40 and 49.20 ppm. The quaternary C
atom peak appeared at 151.38 ppm. These observations suggest
that the amides are replaced with an oxide ligand to give 3 in
Scheme 6. After the reaction was completed, attempts to crystallize
the soluble 3 were unsuccessful, as it formed the insoluble polymer
3.3. Reaction of Zr(NMe2)2[MeC(NiPr)2]2 (1) with O2
{(l
-O)[Zr[MeC(NiPr)2]2}n (4). 4 does not dissolve in benzene-d6,
1 reacts slowly with 1 atm of O2 at 70 °C in benzene-d6
(Scheme 6), taking ca. 12 days to complete the reaction. Through-
out the process, 1H NMR peaks of 1 gradually decreased and a
new set of peaks appeared that are consistent with an oxo-bridged,
DMF, Et2O, hexanes, methanol, pentane, THF, or toluene-d8.
It should be noted that the NMR spectra of the oxo-dimer 3 also
support the structures of the oxo monomer 6, trimer 11, tetramer
12, or the polymer 4. The following observations and analysis sup-
port the assignment of 3: (1) 3 was also observed in the reaction
between 1 and H2O (discussed below); (2) 3 was observed in MS
in the reaction of 1 or 2 with H2O in air; (3) Reactions between
Zr(NEt2)2[MeC(NiPr)2]2 (2) and O2 or H2O also give 3; (4) Elemental
analysis of the polymer 4 is consistent with the composition that
this species is an oligomer of 6. Although 6 was observed in MS
and is discussed later, it is unlikely the observed product in the
NMR spectra because it is penta-coordinated and perhaps less
soluble dimer {(l
-O)Zr[MeC(NiPr)2]2}2 (3). In this process, an insol-
uble precipitant {(l
-O)Zr[MeC(NiPr)2]2}n (4) formed. Two organic
byproducts, HNMe2 and CH2(NMe2)2, were also observed in 1H
2
It should be noted that this experiment used a 400 MHz NMR spectrometer. The
coalesence and static temperatures are dependent on the magnetic field. Thus NMR
spectra at other frequencies would show slightly different temperatures of
coalescence.

8
A.C. Lamb et al. / Polyhedron 103 (2016) 2–14
Scheme 6. Reactions of 1 or 2 with O2 or H2O.
Scheme 7. Attempts to make 5. Trace water in high-purity H2O2 may also lead to the formation of 3.
spectroscopy was used in an attempt to characterize the Zr–O
bonds but due to florescence of the sample no signals were
observed. Also, a powder X-ray study of the sample did not give
a diffraction pattern, probably due to the lack of crystallinity of
4. Mass spectroscopy was also used in several attempts to charac-
terize 4 using different ionizing sources: direct analysis in real time
(DART at 200, 300, and 400 °C), electrospray ionization (ESI) and
matrix-assisted laser desorption ionization time-of-flight
(MALDI-TOF). However, no Zr isotopic patterns were observed in
the MS spectra and the observations are consistent with the pro-
posed polymeric structure of 4.
Scheme 8. Hexacoordinated trimer 11 and heptacoordinated tetramer 12.
Three tests were performed to determine the stability of the
polymer product 4. The first test was to add a solvent that could
donate a proton and possibly protonate the amidinate ligand.
Methanol was added to a solution of 4 in benzene-d6 and moni-
tored by 1H NMR. Spectra of the mixture were collected at 5 min
and then 12, 24, and 96 h. There was no change in the 1H NMR
stable than the hexa-coordinated 3. We cannot rule out the
observed species is the trimer 11 (Scheme 8), which is also hexa-
coordinated. But it is unlikely to be the hepta-coordinated tetramer
12 as Zr centers are perhaps too crowded.
Characterization of the polymer{(l
-O)Zr[MeC(NiPr)2]2}n (4) was
i
spectra. We also did not observe the amidine PrN(H)C(Me) = NiPr.
based on elemental analysis, IR, and solid-state NMR spectroscopy.
Solid-state 13C{1H} NMR CPMAS spectrum of 4 (Fig. S18) shows
peaks at 14.19 ppm for NC(Me)N, 23.27 ppm for –CHMe2,
46.34 ppm for –CHMe2, and 152.35 ppm for NC(Me)N. IR spectrum
of 4 in a KBr pellet (Fig. S19) revealed a peak at 1641 cmꢀ1, and this
peak was assigned to the C@N stretch, as the stretching
frequencies of C@N are in the range of 1690–1640 cmꢀ1. Raman
TGA was performed to see how thermally stable the amidinate
ligands are (Fig. S20). At 900 °C, the removal of one amidinate
ligand was observed by a 36.21% weight loss. Since the tempera-
ture is so high, 4 is considered thermally stable. Air stability of 4
was studied by making an IR pellet of 4 in KBr and left it in air.
IR spectra were recorded before exposure and 2 and 16 h after

A.C. Lamb et al. / Polyhedron 103 (2016) 2–14
9
mode. The three O–O bond lengths of 1.5147(13)–1.5172(13) Å in
5 are similar to 1.504(4)–1.512(4) Å in the Zr(IV) bisperoxo com-
plex (Scheme 1) [17], and those in [M2(O2)2(XW11O39)2]12ꢀ
[X = Si, 1.491(11) Å; Ge, 1.529(13) Å] [104], but are longer than
1.394(4) and 1.401(4) Å in {Zr[N(iPr2PO)2]2}2( g2 2-O2)2 (13)
l- :g
[105]. The O atoms form slightly unsymmetrical bridges between
the Zr atoms: One O–Zr length [2.1162(10)–2.320(10) Å] is shorter
than the other [2.1992(10)–2.2082(10) Å]. The Zr–O bonds in 5 are
longer than those in Zr bisperoxo complex (Scheme 1) [17], but are
similar to those in 13 [105]. The O–O–Zr and O–Zr–O angles for
each pair of peroxo ligands in 5 are 66.47(5)–72.60(5)° and
40.80(4)–41.13(3)°, respectively. The Zr–O–Zr angles are in the
range of 124.13(5)–126.15(4)°.
Characterization of the solid mixture of 5, unreacted 1, 3, and 4
by MS gave a spectrum showing the dimeric peroxo {(
l
-O2)-
had
Zr[MeC(NiPr)2]2}2 (10, Fig. 3), indicating that trimer
5
converted to smaller 10 during the MS process. Zr is an element
with five stable isotopes (Table 1) and has a unique pattern in
MS. The identification of 10 is based on the isotopic pattern and
a comparison with the predicted MS spectrum for 10 (Fig. 3). When
3 was characterized by MS in air, the isotopic pattern in Fig. 3 was
not observed, indicating that the adduct O2 ? 3 was not formed
between 3 and O2 during the DART process. The crystal structure
l- :g
g2
2-O2)Zr[MeC(NiPr)2]2}3 (5) and observa-
Fig. 2. ORTEP view of 5 (hydrogens omitted). Selected bond lengths (Å) and angles
(°): 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).
of peroxo trimer {(
tion of peroxo dimer {(
l
-O2)Zr[MeC(NiPr)2]2}2 (10) in MS are sig-
nificant as, to our knowledge, a Group 4 peroxo complex, yielded
from O2 and not supported by a redox-active ligand (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.
3.5. DFT studies of the reaction between Zr(NMe2)2[MeC(NMe)2]2 (A)
and O2
The identification of HNMe2 and CH2(NMe2)2 as byproducts in
the reaction of 1 with O2 suggests this reaction is radical in nature
[107–109]. Several papers have reported that, once NMe2 radicals
Å
exposure to air. Little change in the C@N stretching frequency was
observed, indicating that 4 is stable to air.
are formed, their reactions lead to the formation of HNMe2 and
Å
CH2(NMe2)2 (5) involving intermediates such as CH3, CH2@NH,
3.4. Crystal structure and MS studies of 5
ÅH, CH3N = CH2, and ÅCH2N(CH3)2 [107,108]. With the aid of DFT cal-
culations, a plausible pathway has been proposed to account for
the formation of the peroxo trimer complex (5). In the DFT calcu-
lations, the model complex Zr(NMe2)2[MeC(NMe)2]2 (A), in which
The crystal structure of 5 is given in Fig. 2. The Zr atoms are
linked by peroxo ligands that bind to the metals in the
l- :
g2 g2
Fig. 3. (Top) calculated and (Bottom) observed MS of [10+H+].

10
A.C. Lamb et al. / Polyhedron 103 (2016) 2–14
Table 1
Stable isotopes of zirconium.
intermediate H having a Zr–O–O–NMe2 linkage via a minimum
energy crossing point (MECP) (Scheme 9). The MECP is less stable
than the TS for the single-electron transfer process. An alternative
O2 insertion into a Zr–NMe2 bond could also occur in the triplet
biradical intermediate C. The MECP for this alternative O2 insertion
is less stable than the TS for the release of an amide radical (ÅNMe2)
to give D. Therefore, the most plausible pathway is the one dis-
cussed above, i.e., A ? B ? C ? D ? E.
Isotope
Accurate mass
% Natural abundance
90Zr
91Zr
92Zr
94Zr
96Zr
89.9047026(26)
90.9056439(26)
91.9050386(26)
93.9063148(28)
95.908275(4)
51.45(2)
11.22(2)
17.15(1)
17.38(2)
2.80(1)
This helps explain the formation of the major products, 3 and 4,
in the reaction of 1 with O2 (Scheme 6), giving HNMe2 and
CH2(NMe2)2. We earlier reported that the reactions of M(NMe2)4
iPr is simplified to Me, is used to model the amide amidinate com-
plex 1.
(M = Zr, Hf) with O2 give M3(NMe2)6(l-NMe2)3(l3-O)(l3-ONMe2)
In the plausible pathway (Scheme 9), a triplet van der Waals
species (B) is first formed between A and O2. We found that the
van der Waals complex B can undergo an internal single-electron
transfer (an electron being transferred from an amide ligand to
the dioxygen moiety) to form a triplet biradical intermediate (C)
which contains a superoxide (Oꢀ2 ) ligand and adopts a pseudo pen-
tagonal bipyramidal structure if the superoxide (Oꢀ2 ) ligand is con-
sidered as occupying one single coordinate site. The transition
state (TS) for this single-electron transfer process is 21.3 kcal/mol
in free energy above the energy reference point (A+O2). Electron
density analysis of this triplet biradical intermediate shows that
the two oxygen atoms of the superoxide ligand each carry a spin
density of 0.54e while the N atom of the equatorial amide ligand
carries a spin density of 0.92e. The triplet biradical intermediate
C can easily release an amide radical (ÅNMe2) to give a doublet
intermediate (D). The doublet intermediate D can further release
a second amide radical (ÅNMe2) to form E containing a peroxide
ligand, from which trimerization leads to the eventual formation
of the peroxo trimer complex (5). Scheme 9 shows that the overall
free energy barrier for the formation of E is 21.3 kcal/mol and the
rate-determining transition state is related to the internal single-
electron transfer process.
and Me2NNMe2 [53]. However, Me2NNMe2 was not observed in
the current reaction of Zr(NMe2)2[MeC(NiPr)2]2 (1) with O2. In
addition, the reaction of Zr(NMe2)4 with O2, occurring at 23 °C
overnight, is much faster than the current reaction of Zr diamide
diamidinate 1, which requires heating at 70–80 °C for several days.
These interesting differences perhaps could be explained by the
intermediates postulated by the DFT calculations of the two reac-
tions (Schemes 9 and 10). DFT calculations of the reaction of
Zr(NH2)4 with O2 using models suggest a mechanism involving a
ÅNMe2 radical from the homoleptic cleavage of a MO–NMe2 bond
in I in Scheme 10, yielding the oxo-radical II, a doublet. II is
expected to be much more reactive than the superoxide-radical
D, also a doublet (Scheme 9), from the reaction of the diamidinate
1 with O2. This is perhaps understood in view of the fact that
superoxides are well-known, relatively stable species. The much
more reactive oxo-radical II contributes to the ‘‘immediate’’ attack
of a nearby Zr–NH2 bond by the newly formed ÅNMe2 radical, yield-
ing H2NNH2 and III in Scheme 10 [53]. In comparison, because the
superoxide-radical D (Scheme 9) in the current case is much less
reactive, the ÅNMe2 radical formed along with D does not react with
D. Instead it undergoes decomposition to give CH2(NMe2)2 and
HNMe2. Two other factors may play roles here: (a) 6-coordinated
D in the current work with two bulky amidinates (Scheme 9) is
much sterically crowded than 5-coordinated II (Scheme 10). (b)
We also examine the possibility of O2 insertion into a Zr–NMe2
bond in the van der Waals complex B to give the singlet
Scheme 9. Pathways leading to the formation of E from the reaction of A with O2. Relative free energies and electronic energies (in parentheses) are given in kcal/mol. The
relative free energies for MECPs were approximately estimated (by addition of 14.7 kcal/mol to the relative electronic energies; see the computational details section) and
rounded up to the nearest integers.

A.C. Lamb et al. / Polyhedron 103 (2016) 2–14
11
with O2 and see if 3 and 4 were the Zr-containing products. Indeed
the reaction yielded 3 and 4 (Scheme 6). The only organic product
was HNEt2 as confirmed by 1H and 13C{1H} NMR spectroscopies
(Figs. S21 and S22) and GC–MS. It should also be noted that this
reaction was completed more quickly (70 °C for 7 days) than its
methyl analog 1 (70 °C for 12 days). It is not clear why the reaction
of 2 with O2 is faster.
3.7. Reactions of Zr(NR2)2[MeC(NiPr)2]2 (R = Me, 1; Et, 2) with H2O
The reactions were monitored by 1H and 13C{1H} NMR spectro-
scopies. Addition of 1 equiv of H2O to 2 was found to give soluble
products HNEt2 and 3 (Scheme 6). The reaction, much faster than
that of 1 or 2 with O2, was completed in a few minutes. The pro-
duct mixture turned cloudy, presumably as the result of the precip-
itation of 4. 1H and 13C{1H} NMR chemical shifts of 3 (Figs. S23 and
S24) are identical to those from the reactions of O2 with 1 and 2
(Figs. S16, S17, S21 and S22). The amount of the solid was too small
to be isolated. Since the reactions here with H2O likely give oxo
products, the observations of 3 and 4 in the reactions in Scheme 6
support the assignments of 3 and 4 as the dimeric and polymeric
oxo products.
Scheme 10. Excerpt of the pathway in the reaction between Zr(NH2)4 and O2 from
DFT studies [53]. The energies and free energies (in parenthesis) are in kcal/mol.
Bernasek and Schwartz reported earlier that the reactions of
Zr(OtBu)4 with surface OH groups are faster than those of its alkyl
t
analog Zr(CH2 Bu)4 [110]. Lone pair electrons on the OtBu ligands
assist the H+ migration from the OH groups to the alkoxide ligand,
removing the latter as the alcohol HOtBu. The amide ligands NR2 in
Zr(NR2)2[MeC(NiPr)2]2 (R = Me, 1; Et, 2) contain lone pair electrons
as well and they may assist the H+ migration and removal of the
amide ligands (Scheme 11).
3.8. MS studies of the reactions between Zr(NR2)2[MeC(NiPr)2]2 and
H2O
Scheme 11. Lone-pair electrons on amide ligands assist in H+ transfer.
Since the reactions of Zr(NR2)2[MeC(NiPr)2]2 (R = Me, 1; Et, 2)
with O2 or H2O have been studied, it is of interest to see what prod-
ucts would be detected in MS. In MS experiments, there is a brief
exposure to air for the sample to enter into the gas phase. The
exposure to O2 was not of concern because it takes days for 1 or
2 to complete their reactions with O2. Since zirconium has five
stable isotopes (Table 1), it is easy to see the isotopic pattern of a
Zr complex in mass spectra. Solid powders of 1 or 2 were kept in
a sealed vial under nitrogen until the mass spectrometer was
heated to 200 °C. The powders were collected on the sealed end
There is only one NMe2 ligand in D while there are six NH2 ligands
Å
in II to potentially react with the NMe2 radical. Both make it not
Å
likely that the NMe2 radical reacts with D.
3.6. Reaction of the diethylamide analog Zr(NEt2)2[MeC(NiPr)2]2 (2)
with O2
One study to support the identity of {(
and {(
-O)Zr[MeC(NiPr)2]2}n (4) was to investigate the reaction of 2
l
-O)Zr[MeC(NiPr)2]2}2 (3)
l
Fig. 4. (Top) calculated and (Bottom) observed MS for [6+H+].

12
A.C. Lamb et al. / Polyhedron 103 (2016) 2–14
Scheme 12. Observed species by MS in the reactions of 1–2 with water in air.
Fig. 5. (Top) calculated and (Bottom) observed MS for [7+H+].
Fig. 6. (Top) Calculated and (Bottom) observed MS for [3+H+].
of a capillary tube and added quickly to the heated stream of He
gas in air. 1 or 2 reacted with H2O in air and the volatile products
[M+H+] (species + H+) were then analyzed by MS. Since the reac-
tions of both 1 and 2 with water give the same products, it was
not surprising that the same products are observed in MS. The
MS analyzer detected three Zr and O containing products: oxo
monomer
6
(Fig. 4), dihydroxyl (OH)2Zr[MeC(NiPr)2]2 (7,
Scheme 12, Fig. 5), and 3 (Fig. 6). One possible pathway for the
formation of 6 is dehydration of 7 (Scheme 12). In addition,
Scheme 13. Thermal decomposition of AIBN.

A.C. Lamb et al. / Polyhedron 103 (2016) 2–14
13
{Zr(NR2)[MeC(NiPr)2]2}+ (R = Me, Et) cations were observed
(Figs. S25 and S26).
[4] T.V. Lubben, P.T. Wolczanski, J. Am. Chem. Soc. 109 (1987) 424.
[5] R. Wang, K. Folting, J.C. Huffman, L.R. Chamberlain, I.P. Rothwell, Inorg. Chim.
Acta 120 (1986) 81.
[6] V.C. Gibson, C. Redshaw, G.L.P. Walker, J.A.K. Howard, V.J. Hoy, J.M. Cole, L.G.
Kuzmina, D.S. De Silva, J. Chem. Soc., Dalton Trans. (1999) 161.
[7] C.J. Schaverien, A.G. Orpen, Inorg. Chem. 30 (1991) 4968.
[8] X. Liu, D. Cui, Dalton Trans. (2008) 3747.
[9] A. Van Asselt, B.D. Santarsiero, J.E. Bercaw, J. Am. Chem. Soc. 108 (1986) 8291.
[10] P.B. Brindley, J.C. Hodgson, J. Organomet. Chem. 65 (1974) 57.
[11] S.J. Kim, D.W. Choi, Y.J. Lee, B.H. Chae, J.J. Ko, S.O. Kang, Organometallics 23
(2004) 559.
[12] A.M. Morris, C.G. Pierpont, R.G. Finke, Inorg. Chem. 48 (2009) 3496.
[13] W. Adam, J. Putterlik, R.M. Schuhmann, J. Sundermeyer, Organometallics 15
(1996) 4586.
[14] W.M. Vetter, A. Sen, Organometallics 10 (1991) 244.
[15] A. Van Asselt, M.S. Trimmer, L.M. Henling, J.E. Bercaw, J. Am. Chem. Soc. 110
(1988) 8254.
3.9. Reaction between 2 and O2 in the presence of AIBN
Since the reaction between 2 and O2 (Scheme 6) is slow, a rad-
ical initiator was used to increase the rate of the reaction. The ini-
tiator was azobisisobutyronitrile (AIBN). It decomposes under
heating into two radicals and N2 (Scheme 13). These radicals are
expected to speed up the reaction between 2 and O2. AIBN was
added to a solution of 2 in a 1:1 M ratio. O2 at 1 atm was then
added and the reaction was completed in 2 days at 72 °C. Without
AIBN, the reaction took 7 days to complete. The observations sup-
port that the reaction between 2 and O2 is radical in nature.
[16] B.J. Boro, R. Lansing, K.I. Goldberg, R.A. Kemp, Inorg. Chem. Commun. 14
(2011) 531.
[17] C. Stanciu, M.E. Jones, P.E. Fanwick, M.M. Abu-Omar, J. Am. Chem. Soc. 129
(2007) 12400.
[18] F. Lu, R.A. Zarkesh, A.F. Heyduk, Eur. J. Inorg. Chem. (2012) 467.
[19] M.H. Chisholm, C.E. Hammond, J.C. Huffman, J. Chem. Soc., Chem. Commun.
(1987) 1423.
[20] S.-J. Chen, J.K.C. Abbott, C.A. Steren, Z.-L. Xue, J. Clust. Sci. 21 (2010) 325.
[21] A.N. Campbell, S.S. Stahl, Acc. Chem. Res. 45 (2012) 851.
[22] K.H. Theopold, O.M. Reinaud, S. Blanchard, S. Leelasubeharoen, A. Hess, S.
Thyagarajan, ACS Symp. Ser. 823 (2002) 75.
4. Conclusions
Two diamidinates Zr(NR2)2[MeC(NiPr)2]2 (R = Me, 1; Et, 2) have
been prepared and characterized. Their reactions with O2 and
water have been studied. Zr-containing products from the reac-
tions of both 1 and 2 with O2 or H2O are oxo {(
Pr)2]2}2 (3) and its insoluble polymer {(
The crystal structure of a rarely observed Zr peroxo complex (5)
was obtained from the reaction of 1 with O2 at ꢀ30 °C. To our
knowledge this is the first d0 Zr peroxo complex that contains no
redox active ligands. DFT calculations show that the reaction
between Zr(NMe2)2[MeC(NMe)2]2 (A) and O2 proceeds through
several intermediate species and ultimately two amide radicals
(ÅNMe2) are released forming a species that contains a peroxo
ligand from which trimerization leads to the formation of 5. In both
reactions, the observation of CH2(NMe2)2 and 3 suggests that a per-
oxo species is forming, presumably unstable 5.
l
-O)Zr[MeC(Ni-
l
-O)Zr[MeC(NiPr)2]2}n (4).
[23] L. Que, W.B. Tolman, Nature 455 (2008) 333.
[24] R.A. Himes, K.D. Karlin, Curr. Top. Chem. Biol. 13 (2009) 119.
[25] R.L. Shook, A.S. Borovik, Chem. Commun. (2008) 6095.
[26] R.A. Sheldon, in: B. Meunier (Ed.), Biomimetic Oxidations Catalyzed by
Transition Metal Complexes, Imperial College Press, 2000, pp. 613–662.
[27] Metal-Dioxygen Complexes:
(1994) Issue 3 567–856.
A Perspective. Special Issue, Chem. Rev. 94
[28] W.H. Monillas, G.P.A. Yap, L.A. MacAdams, K.H. Theopold, J. Am. Chem. Soc.
129 (2007) 8090.
[29] A.C. McQuilken, Y. Jiang, M.A. Siegler, D.P. Goldberg, J. Am. Chem. Soc. 134
(2012) 8758.
[30] K.A. Prokop, D.P. Goldberg, J. Am. Chem. Soc. 134 (2012) 8014.
[31] M.L. Scheuermann, U. Fekl, W. Kaminsky, K.I. Goldberg, Organometallics 29
(2010) 4749.
[32] L. Boisvert, M.C. Denney, S. Kloek Hanson, K.I. Goldberg, J. Am. Chem. Soc. 131
(2009) 15802.
[33] J.R. Khusnutdinova, N.P. Rath, L.M. Mirica, J. Am. Chem. Soc. 134 (2012) 2414.
[34] K.T. Nguyen, S.P. Rath, L. Latos-Grazynski, M.M. Olmstead, A.L. Balch, J. Am.
Chem. Soc. 126 (2004) 6210.
[35] J. Maury, L. Feray, S. Bazin, J.-L. Clement, S.R.A. Marque, D. Siri, M.P. Bertrand,
Chem. Eur. J. 17 (2011) 1586.
[36] J. Lewinski, M. Koscielski, K. Suwala, I. Justyniak, Angew. Chem., Int. Ed. 48
(2009) 7017.
[37] D. Mukherjee, A. Ellern, A.D. Sadow, J. Am. Chem. Soc. 134 (2012) 13018.
[38] S. Jana, R.J.F. Berger, R. Fröhlich, T. Pape, N.W. Mitzel, Inorg. Chem. 46 (2007)
4293.
The reactions of 1 or 2 with water are much faster that
those with O2 probably due to the H+ transfer from water to the
lone-pair electrons on NR2. When Zr(NR2)2[MeC(NiPr)2]2 (R = Me,
1; Et, 2) was briefly exposed to air, several important products
have been identified by MS, including (O=)Zr[MeC(NiPr)2]2 (6),
{(l
-O)Zr[MeC(NiPr)2]2}2 (3), (HO)2Zr[MeC(NiPr)2]2 (7), and
{Zr(NR2)[MeC(NiPr)2]2}+ (R = Me, Et). The amidinate ligands in the
polymer 4 binds to the metal atom very strongly, requiring
900 °C to remove.
[39] C.-M. Lee, C.-H. Chuo, C.-H. Chen, C.-C. Hu, M.-H. Chiang, Y.-J. Tseng, C.-H. Hu,
G.-H. Lee, Angew. Chem., Int. Ed. 51 (2012) 5427.
Acknowledgments
[40] M.R. Kelley, J.-U. Rohde, Chem. Commun. 48 (2012) 2876.
[41] S.N. Brown, J.M. Mayer, Inorg. Chem. 31 (1992) 4091.
[42] G. Parkin, J.E. Bercaw, J. Am. Chem. Soc. 111 (1989) 391.
[43] J. Robertson, Rep. Prog. Phys. 69 (2006) 327.
[44] A.C. Jones, H.C. Aspinall, P.R. Chalker, R.J. Potter, K. Kukli, A. Rahtu, M. Ritala,
M. Leskelae, J. Mater. Chem. 14 (2004) 3101.
[45] R. Arghavani, G. Miner, M. Agustin, Semicond. Internal. 30 (2007) 32. 36, 38.
[46] K.-A. Son, A.Y. Mao, Y.-M. Sun, B.Y. Kim, F. Liu, A. Kamath, J.M. White, D.L.
Kwong, D.A. Roberts, R.N. Vrtis, Appl. Phys. Lett. 72 (1998) 1187.
[47] B.-C. Kan, J.-H. Boo, I. Lee, F. Zaera, J. Phys. Chem. A 113 (2009) 3946.
[48] M.K. Wiedmann, M.J. Heeg, C.H. Winter, Inorg. Chem. 48 (2009) 5382.
[49] C.L. Dezelah IV, J. Niinisto, K. Kukli, F. Munnik, J. Lu, M. Ritala, M. Leskela, L.
Niinisto, Chem. Vap. Deposition 14 (2008) 358.
The work is supported by the U.S. National Science Foundation
(CHE-1362548 to Z.X.) and the Hong Kong Research Grants Council
(GRF 16303614 to Z.L.). The authors would like to thank Stephen C.
Gibson and Dr. Liguo Song for their assistance with MS studies and
Matthew D. Dembo for his help with the solid-state NMR work.
Appendix A. Supplementary data
[50] C.L. Dezelah IV, O.M. El-Kadri, I.M. Szilagyi, J.M. Campbell, K. Arstila, L.
Niinistoe, C.H. Winter, J. Am. Chem. Soc. 128 (2006) 9638.
[51] S. Consiglio, R.D. Clark, G. Nakamura, C.S. Wajda, G.J. Leusink, J. Vacuum Sci.
Technol. A 30 (2012). 01A119/1.
[52] See, e.g. D.C. Bradley, I.M. Thomas, Proc. Chem. Soc. (1959) 225.
[53] R. Wang, X. Zhang, S.-J. Chen, X. Yu, C. Wang, D.B. Beach, Y. Wu, Z. Xue, J. Am.
Chem. Soc. 127 (2005) 5204.
[54] S. Chen, X. Zhang, X. Yu, H. Qiu, G.P.A. Yap, I.A. Guzei, Z. Lin, Y. Wu, Z. Xue, J.
Am. Chem. Soc. 129 (2007) 14408.
[55] S.-J. Chen, X.-H. Zhang, Z. Lin, Y.-D. Wu, Z.-L. Xue, Sci. China Chem. 52 (2009)
1723.
CCDC 1005137 contains the supplementary crystallographic
data for 5. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2015.07.045.
[56] S.-J. Chen, G.P.A. Yap, Z.-L. Xue, Sci. China Chem. 52 (2009) 1583.
[57] S.-J. Chen, J. Zhang, X. Yu, X. Bu, X. Chen, Z. Xue, Inorg. Chem. 49 (2010) 4017.
[58] H. Qiu, S.-J. Chen, C.-S. Wang, Y.-D. Wu, I.A. Guzei, X.-T. Chen, Z.-L. Xue, Inorg.
Chem. 48 (2009) 3073.
[59] X. Yu, X.-T. Chen, Z.-L. Xue, Organometallics 28 (2009) 6642.
[60] S.-J. Chen, Z.-L. Xue, Organometallics 29 (2010) 5579.
References
[1] T.D. Tilley, Organometallics 4 (1985) 1452.
[2] T.F. Blackburn, J.A. Labinger, J. Schwartz, Tetrahedron Lett. 16 (1975) 3041.
[3] J.A. Labinger, D.W. Hart, W.E. Seibert, J. Schwartz, J. Am. Chem. Soc. 97 (1975)
3851.

14
A.C. Lamb et al. / Polyhedron 103 (2016) 2–14
[61] Z. Wu, H. Cai, X.-H. Yu, J.R. Blanton, J.B. Diminnie, H.-J. Pan, Z.-L. Xue,
K.N. Kudin, V.N. Staroverov, T. Keith, R. Kobayashi, J. Normand, K.
Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N.
Rega, J.M. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J.
Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C.
Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth,
P. Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, O. Farkas, J.B. Foresman,
J.V. Ortiz, J. Cioslowski, D.J. Fox, Gaussian 09, revision D.01, Gaussian, Inc.,
Wallingford CT, 2009.
Organometallics 21 (2002) 3973.
[62] F.T. Edelmann, Adv. Organom. Chem. 57 (2008) 183.
[63] S.-D. Bai, H.-B. Tong, J.-P. Gou, M.-S. Zhou, D.-S. Liu, Inorg. Chim. Acta 362
(2009) 1143.
[64] E. Otten, P. Dijkstra, C. Visser, A. Meetsma, B. Hessen, Organometallics 24
(2005) 4374.
[65] J. Hong, L. Zhang, K. Wang, Z. Chen, L. Wu, X. Zhou, Organometallics 32 (2013)
7312.
[86] W.R. Wadt, P.J. Hay, J. Chem. Phys. 82 (1985) 284.
[66] S. Collins, Coord. Chem. Rev. 255 (2011) 118.
[87] P.J. Hays, W.R. Wadt, J. Chem. Phys. 82 (1985) 299.
[67] D.A. Kissounko, M.V. Zabalov, G.P. Brusova, D.A. Lemenovskii, Russ. Chem.
Rev. 75 (2006) 351.
[68] P.P. Fontaine, B.L. Yonke, P.Y. Zavalij, L.R. Sita, J. Am. Chem. Soc. 132 (2010)
12273.
[88] A.W. Ehlers, M. Böhme, S. Dapprich, A. Gobbi, A. Höllwarth, V. Jonas, K.F.
Köhler, R. Stegmann, A. Veldkamp, G. Frenking, Chem. Phy. Lett. 208 (1993)
111.
[89] K. Fukui, J. Phys. Chem. 74 (1970) 4161.
[69] P.J. Tiong, A. Nova, E. Clot, P. Moundtford, Chem. Commun. 47 (2001) 3147.
[70] T.B. Thiede, M. Krasnopolski, A.P. Milanov, T. Arcos, A. Ney, H.-W. Becker, D.
Rogalla, J. Winter, A. Devi, R.A. Fischer, Chem. Mater. 23 (2011) 1430.
[71] M. Eleter, L.G. Hubert-Pfalzgraf, S. Daniele, G. Pilet, B. Tinant, Polyhedron 29
(2010) 2522.
[72] Z. Li, R.G. Gordon, V. Pallem, H. Li, D.V. Shenai, Chem. Mater. 22 (2010) 3060.
[73] K. Xu, A.P. Milanov, M. Winter, D. Barreca, A. Gasparotto, H.-W. Becker, A.
Devi, Eur. J. Inorg. Chem. 11 (2010) 1679.
[74] Z. Li, S.T. Barry, R.G. Gordon, Inorg. Chem. 44 (2005) 1728.
[75] V.R. Pallem, C. Dussarrat, PCT Int. Appl., 2012, WO 2012138332 A1.
[76] B. Sharma, T.M. Callaway, A.C. Lamb, C.A. Steren, S.-J. Chen, Z. Xue, Inorg.
Chem. 52 (2013) 11409.
[77] A.C. Lamb, Z. Lu, Z.-L. Xue, Chem. Commun. 50 (2014) 10517.
[78] G.M. Diamond, R.F. Jordan, J.L. Petersen, Organometallics 15 (1996) 4030.
[79] S. Ge, A. Meetsma, B. Hessen, Organometallics 27 (2008) 3131.
[80] K.-I. Tada, K. Inaba, T. Furukawa, T. Yamakawa, N. Oshima, PCT Int. Appl.,
2007, WO 2007015436 A1 20070208, The patent is in Japanese with an
English abstract.
[90] K. Fukui, Acc. Chem. Res. 14 (1981) 363.
[91] J.N. Harvey, M. Aschi, Phys. Chem. Chem. Phys. 1 (1999) 5555.
[92] J.N. Harvey, M. Aschi, H. Schwarz, W. Koch, Theor. Chem. Acc. 99 (1998) 95.
[93] M. Reiher, B.A. Hess, Theor. Chem. Acc. 107 (2001) 48.
[94] O. Salomon, M. Reiher, B.A. Hess, J. Chem. Phys. 117 (2002) 4729.
[95] G.M. Sheldrick, SADABS, A Program for Empirical Absorption Correction of
Area Detector Data; University of Göttingen, Göttingen, Germany, 1996.
[96] G.M. Sheldrick, SHELXL-97, A Program for the Refinement of Crystal Structures,
University of Göttingen, Göttingen, Germany, 1997.
[97] S. Ren, Z. Qiu, Z. Xie, Organometallics 32 (2013) 4292.
[98] J.C. Bailar, J. Inorg. Nucl. Chem. 8 (1958) 165.
[99] J. Sun, S.-J. Chen, Y. Duan, Y. Li, X. Chen, Z. Xue, Organometallics 28 (2009)
3088.
[100] V.S. Dimitrov, J.A. Ladd, Magn. Reson. Chem. 23 (1985) 529.
[101] J. Sandstrom, Dynamic NMR Spectroscopy, Academic Press, London, 1982.
[102] R.S. Macomber, A Complete Introduction to Modern NMR Spectroscopy,
Wiley, New York, 1998. pp. 158–160.
[103] The rate constants here are the chemical rate constants, and they differ from
the observed magnetization transfer rate constants by a factor of 2. See, e.g.
M.L.H. Green, L.L. Wong, A. Sella, Organometallics 11 (1992) 2660.
[104] S.S. Mal, N.H. Nsouli, M. Carraro, A. Sartorel, G. Scorrano, H. Oelrich, L. Walder,
M. Bonchio, U. Kortz, Inorg. Chem. 49 (2010) 7.
[81] J.H. Forsberg, V.T. Spaziano, T.M. Balasubramanian, G.K. Liu, S.A. Kinsley, C.A.
Duckworth, J.J. Poteruca, P.S. Brown, J.L. Miller, J. Org. Chem. 52 (1987) 1017.
[82] P.M. Morse, M.D. Spencer, S.R. Wilson, G.S. Girolami, Organometallics 13
(1994) 1646.
[83] A.U. Grunewald, K.H. Gericke, F.J. Comes, J. Chem. Phys. 89 (1988) 345.
[84] J. Lee, N.D. Chasteen, G. Zhao, G.C. Papaefthymiou, S.M. Gorun, J. Am. Chem.
Soc. 124 (2002) 3042.
[85] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, G.M.A. Robb, J.R.
Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H.
Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G.
Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.A.
Montgomery, J.E. Peralta Jr., F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers,
[105] G.-C. Wang, H.H.Y. Sung, I.D. Williams, W.-H. Leung, Inorg. Chem. 51 (2012)
3640.
[106] C.E. Huckaba, F.G. Keyes, J. Am. Chem. Soc. 70 (1948) 1640.
[107] J. Seetula, K. Kalliorinne, J. Koskikallio, J. Photochem. Photobiol., A.: Chem. 43
(1988) 31.
[108] K.G. Hancock, D.A. Dickinson, J. Org. Chem. 40 (1975) 969.
[109] X.-H. Zhang, S.-J. Chen, H. Cai, H.-J. Im, T. Chen, X. Yu, X. Chen, Z. Lin, Y.D. Wu,
Z.-L. Xue, Organometallics 27 (2008) 1338.
[110] J.B. Miller, S.L. Bernasek, J. Schwartz, J. Am. Chem. Soc. 117 (1995) 4037.