Benzimidazolin-2-iminato Hafnium Complexes: Synthesis, Characterization, and Catalytic Addition of Alcohols to Carbodiimides

Maxim Khononov, Heng Liu, Natalia Fridman, Matthias Tamm, and Moris S. Eisen*

Article

Benzimidazolin-2-iminato Hafnium Complexes: Synthesis,

Characterization, and Catalytic Addition of Alcohols to

Carbodiimides

Cite This: https://dx.doi.org/10.1021/acs.organomet.0c00384

Read Online

ACCESS

Metrics & More

Article Recommendations

sı Supporting Information

ABSTRACT: A series of asymmetric imidazolin-2-iminato and benzimidazo-

lin-2-iminato hafnium(IV) complexes were synthesized and fully characterized

including single-crystal X-ray diﬀraction. The asymmetric imidazolin-2-iminato

hafnium complex exhibits a shorter Hf−N bond length as compared with the

corresponding benzimidazolin-2-iminato hafnium complex. These complexes

were successfully utilized as catalysts for the addition of alcohols to

carbodiimides. The synthesis of several active intermediates and kinetics,

thermodynamics, and stoichiometric reaction studies allowed us to propose a

mechanism for the reaction. The X-ray crystal structure of the monomeric Hf(O Bu) is presented.

INTRODUCTION

Benzamidinate ligands can be deemed steric equivalents of Cp

■

or Cp* (Cp = C H ; Cp* = C Me ) moieties; however, they

Hafnium complexes have mainly been used for the catalytic

homo- and copolymerization of α-oleﬁns

catalysts in various organic transformations including the

polymerization of cyclic esters.

6,84,93,85−92

−18

retain distinctive electronic properties.

The

and as Lewis acid

activation of the complexes containing these systems has

provided crucial insights into the tailoring of new ancillary

ligands, which are isolobal to the cyclopentadienyl ligands yet

possess a higher level of steric control. Imidazolin-2-iminato

19−46

More recently, Hf−MOF

complexes have been utilized in the catalytic activation of small

7−51

molecules.

Among the group-IV metals, hafnium is

ligands are among the leading fascinating examples of such

considered to be the most oxophilic metal when compared

with zirconium and titanium, as illustrated by their

corresponding metal−oxygen bond dissociation energies

94−96

non-Cp ancillary ligands, as presented in Scheme 1A.

Scheme 1. (A) Imidazolin-2-imine and (B) Benzimidazolin-

2-imine Neutral Ligands

(

Ti−O = 666 kJ/mol, Zr−O = 766 kJ/mol, and Hf−O =

01 kJ/mol). Hence, the strong metal−oxygen bond and the

high electrophilicity of the metal have precluded the use of

organometallic complexes of hafnium in processes involving

53−56

oxygen-containing moieties with acidic protons.

We have

postulated that if, during a stoichiometric or a catalytic process,

we start with a metal−oxygen bond and after the trans-

formation we end up with a diﬀerent metal−oxygen bond,

then, theoretically, we are not paying thermodynamically for

the M−O bond; however, we need to have the energy of

simple and general method for the synthesis of this class of 2-

iminoimidazoline N-heterocyclic N-donor ligands, which

behave as 2σ, 4π-electron donors, has been already introduced

57−64

activation to be able to execute the process.

An additional challenge is encountered when comparing the

hafnium and zirconium sizes. Because of the lanthanide

contraction, both metals have virtually the same covalent

97−103

in the literature.

Catalytic additions of E−H (NH, PH, SH, CH, etc.)

moieties to carbodiimides have received substantial interest in

recent years, and various types of catalysts have been used such

2,65,66

radii,

and hence we can expect the same behavior as

related to coordination numbers. Therefore, when designing a

catalytic complex that will operate with oxygen-containing

substrates, ﬁne-tuning of the electronic and steric properties of

the ligands is imperative to avoid the formation of strong and

nonactive Hf−O bonds.

Received: June 5, 2020

During the past decade, our group has been interested in

non-Cp ancillary ligands. In particular, we have been attracted

,14,17,18,67−83

by chelating amido-type benzamidinate ligands.

XXXX American Chemical Society

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

105

106

as main-group metal centers,

actinides,

copper,

lanthanides,

RESULTS AND DISCUSSION

■

7,61,63,64,107

108

and titanium.

Inspired by the

The synthesis of complexes 1, 2, and 3 was carried out by

reacting a solution of the homoleptic hafnium tetrabenzyl

complex with one equivalent of the corresponding benzimida-

zolin-2-imine or imidazolin-2-imine ligand precursor (Scheme

observation that hafnium−oxygen moieties can be an active

9,20,25−28,30,35−37,43,44,109−115

motif in catalytic processes,

started to design our ligands toward the synthesis of hafnium

complexes. On the one hand, we need a ligand that is highly

nucleophilic to allow the stabilization of the metal complex and

to ensure that the ligand will remain attached to the metal after

the catalytic process. On the other hand, a less sterically

demanding ligand (i.e., smaller cone angle, vide infra) will

allow us to achieve the unprecedented hafnium-catalyzed

addition of alcohols to carbodiimides. We integrate a phenyl

ring to the backbone of the imidazolin-2-iminato moiety to

tailor its electronic and steric properties and synthesized a

number of benzimidazolin-2-iminato ligands, as shown in

Scheme 1B.

). The reaction mixture was allowed to stir overnight at room

Scheme 3. Synthesis of Complexes 1−3

These imidazolin-2-iminato ligands can be represented by

the two mesomeric structures shown in Scheme 2i, which, in

Scheme 2. (i) Mesomeric Structures (A and B) in

Imidazolin-2-iminato Ligands; (ii) Diﬀerent Cone Angles of

the (C) Symmetric and (D) Asymmetric Ligands as a

Consequence of the Diﬀerent Substitution Groups

temperature, followed by solvent evaporation under vacuum.

Crystalline materials were obtained by recrystallization of the

crude solids from a mixture of toluene and hexane at −30 °C

(

1:3, respectively).

The solid-state structures of all complexes were established

by X-ray diﬀraction analyses (Figure 1). All three complexes

showed shorter Hf−N

bond lengths as compared with the

bonds as well as a near-linearity for

CN

amido

corresponding Hf−N

the Hf−N−C angles, corroborating a higher bond order for the

Hf−N

bonds. With respect to the Hf−N

bond

CN

distances, complexes 1 and 2 displayed values of 1.915(4)

and 1.917(4) Å, whereas complex 3 displayed a shorter metal−

nitrogen bond of 1.818(12) Å, which is ∼0.1 Å shorter as

compared with complexes 1 and 2, respectively, and among the

shortest bonds known in the literature to date (HfN bonds

are between 1.82 and 1.86 Å),

due to the stronger

turn, should endow the highly basic ligands with a strong

electron-donating capacity. Moreover, to create more acces-

sible, less sterically encumbered metal atoms, a nonsymmetric

ligand was designed to allow the incorporation of a reduced

cone angle, resulting in a more open coordination sphere as

compared with the symmetric imidazolin-2-iminato ligand; see

nucleophilicity of the imidazolin-2-iminato ligand as compared

with that of the benzimidazolin-2-iminato motif. In regards to

the imine CN double-bond lengths, all complexes 1−3 have

very similar values of 1.293(6), 1.293(6), and 1.288(19) Å and

Hf−N−C angles close to linearity, with values of 176.7(3),

174.9(4), and 169.5(11)°, respectively, indicating that the

double-bond character of the CN bond is maintained

7,116,117

Scheme 2ii.

19,20,43

In addition, to examine the electronic eﬀect of the phenyl

moiety on the nonsymmetric benzimidazolin-2-iminato com-

plexes, we have also synthesized the nonsymmetric imidazolin-

regardless of what ligand is being utilized.

These

diﬀerences allowed us to conclude that the existence of the

phenyl ring on the backbone has a large inﬂuence on the

electronic structure of the ligand attached to the metal.

Despite the electronic structural diﬀerences, a comparison of

the cone angles, which allows us to assess the proximity of the

-iminato ligand, as presented in Scheme 1A. In a previous

work from our group, we have shown that the incorporation of

the highly basic and nucleophilic ligands such as the

benzimidazolin-2-iminato moiety helps to stabilize highly

oxophilic uranium and thorium complexes (U−O: 758 kJ/

48,49

ligand substituents to the metal center,

was performed.

When compared with the cone angles of ca. 248° in the

57,61

Dipp

119

mol; Th−O: 854 kJ/mol).

Hence, in this work, we present

symmetric [(Im N)TiMe₃]

and 262° in the symmetric

Dipp

120

the synthesis of three new hafnium complexes, with two of

these complexes bearing the benzimidazolin-2-iminato ligand

and three benzyl groups and an additional complex that

contains an asymmetric imidazolin-2-iminato ligand and three

benzyl groups for comparison. We have decided to tackle the

question of whether these highly oxophilic hafnium complexes

are able to operate as catalysts in the presence of alcohols, as

required for the addition of alcohols to carbodiimides.

[(Im N)HfBn₃], considerably smaller cone angle values

were displayed for the hafnium complexes (160, 120, and 125°

for 1, 2, and 3, respectively). It can be inferred that these new

hafnium complexes exhibited a more open coordination sphere

that will be available to the metal toward the incoming

21−124

substrates.

On the basis of our hypothesis, we strived to explore the

intermolecular addition of alcohols to 1,3-di-p-tolylcarbodii-

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

substrates, we continued our studies with complex 1. The

scope of the reaction with regards to the alcohol substrate was

studied by varying the steric demand and the acidity of the

alcohol (Table 2). The catalytic addition of the less sterically

Table 2. Insertion Reaction of Alcohol and DTC Mediated

by Complex 1

Figure 1. ORTEP drawings of complexes 1−3 are presented with

thermal displacement parameters at 50% probability. Hydrogen atoms

are omitted for clarity. Bond lengths for complex 1 (Å): Hf−N =

.917(4), Hf−C = 2.233(6), Hf−C = 2.273(5), Hf−C

.275(5), and C −N = 1.293(6). Bond angles (deg): Hf−N −C =

76.6(4), N −Hf−C = 105.4(2), N −Hf−C = 105.85(19), and

N −Hf−C = 103.14(18). Cone angle (deg): 160. Bond lengths for

complex 2 (Å): Hf−N = 1.915(4), Hf−C = 2.285(6), Hf−C

.265(6), Hf−C = 2.259(5), and C −N = 1.293(6). Bond angles

(deg): Hf−N −C = 174.9(4), N −Hf−C = 103.7(2), N −Hf−C

Reaction conditions: catalyst 5.8 μmol, Cat/MeOH/DTC 1:50:50,

103.9(2), and N −Hf−C = 103.6(2). Cone angle (deg): 120.

the solvent is C D , the total volume is 600 μL, RT. The yield was

Bond lengths for complex 3 (Å): Hf−N = 1.818(12), Hf−C

determined by H NMR spectroscopy of the crude reaction mixture.

.238(16), Hf−C = 2.288(15), Hf−C = 2.233(17), and C −N =

.288(19). Bond angles (deg): Hf−N −C = 169.5(11), N −Hf−C

105.1(6), N −Hf−C = 109.5(6), and N −Hf−C = 110.3(6).

Cone angle (deg): 125.

encumbered alcohols, that is, MeOH, to diisopropylcarbodii-

mide (DIC) and dicyclohexylcarbodiimide (DCC) by

precatalysts 1 did not lead to any turnovers. The reaction of

more acidic alcohols (phenols) with DIC even proceeded

without any catalysts. Nevertheless, this reaction did not

operate when DTC was employed, and the aid of the hafnium

complexes was needed for the fast formation of the isourea

products. Similar to the aliphatic alcohols, changing the size of

the phenol substrates has an important impact on this

intermolecular addition reaction.

mide (DTC) (Table 1). To our delight, all hafnium complexes

proved active in this addition reaction, aﬀording the respective

Table 1. Addition Reaction of ROH and DTC Mediated by

Complexes 1−3

The scope of the reaction with regards to the alcohol

substrate was studied by the variation of the steric demand and

the acidity of the alcohols. One of the more surprising results

encountered is that the aliphatic alcohols were added much

faster than the corresponding aromatic ones. This result

indicates the need for a higher electron density at the alkoxo

moiety to induce eﬃcient addition (entries 1−5). Moreover,

the bulkiness of the aliphatic chain was found to have no major

eﬀect on the addition, as can be expected for a four-centered

transition state. For aromatic phenols, the reaction time to

reach ∼90% yield was 6 h, unless a bulky phenol was utilized,

as with 2,6-dimethylphenol, which needed 24 h (entry 10).

entry

cat

R = CH (%)

R = C(CH ) (%)

3 3

Reaction conditions: catalyst 5.8 μmol, Cat/ROH/DTC 1:50:50, the

solvent is C D , the total volume is 600 μL, 2 h, at RT. The yield was

determined by H NMR spectroscopy of the crude reaction mixture.

isourea products in moderate to very high yields. It is

important to point out that no reaction was observed in the

absence of the hafnium complexes. Moreover, the free ligands

alone do not induce the addition reaction, and freshly

recrystallized complexes were used in all catalytic studies.

Because the thermal stability and the reactivity of complex 1

were found to be the best for both methanol and tert-butanol

The use of the sterically hindered 3,5- Bu C H OH (entry 12)

resulted in lower activities as compared with 2,6-Me C H OH

(entry 10). The use of aromatic phenols induces a drop in the

catalytic activity of the complexes, presumably due to the

displacement of the ancillary ligand (see later) from the active

metal site (complexes 6 and 7).

Organometallics XXXX, XXX, XXX−XXX

Organometallics

as can be seen in Scheme 4 .

Article

A stronger steric eﬀect was found for diﬀerent carbodiimides

Table 3). Larger NR substituents at the carbodiimide (RN

Scheme 4. Schematic Drawing of Complex 4

(

CNR) resulted in lower reaction rates. For example, in a

comparison of the ortho- and para-carbodiimides (entries 14

and 15, respectively), methanol added to the ortho-substituted

carbodiimides much slower than to the corresponding DTC.

In addition, the use of sterically hindered, nonsymmetrical

carbodiimides (entries 16 and 17) induced a larger inhibition

of the reactivity of the catalyst, and two isomers were obtained.

The main product was the one formed from the more

kinetically stable amido intermediate precursor, whereas the

less steric amido moiety attached to the metal center before

the protonolysis (vide infra).

The imidazolin-2-imine ligand exhibits an elongated Hf−N

bond length of 2.385(7)Å, whereas the N−C is 1.305(10)

ipso

125

Å, similar to that found for the other Hf complexes. This

elongation is clearly the result of the protonation of the ligand

(the imido is more basic than the alkoxo); however, even in

the presence of a large excess of alcohol, the ligand remains

attached to the metal atom, inﬂuencing the catalytic reaction

Table 3. Insertion Reaction of Methanol and Carbodiimides

Mediated by Complex 1

(

Figure 2). As a result of the protonolysis, the ligand changes

from a linear to a bent orientation toward the metal with a Hf−

N−C angle of 145.0(5)°, as can be seen in Scheme 4; the

ipso

NH hydrogen atom is involved in hydrogen bonding with one

of the oxo moieties in the equatorial plane of the complex.

To further assess the hydrogen bonding in 4, IR studies were

performed. Complex 4 shows the N−H stretching vibration at

−

328 cm , which is shifted from the corresponding free ligand

−

N−H vibration that appears at 3316 cm . To conﬁrm the N−

H signal/interaction on the IR measurements, a reaction

between complex 1 and BuOD was performed. The

corresponding reaction product showed a sharp band at

2464 cm , attributed to bridged N−D−O (2487 cm )

moiety.

−

126,127

−1

Reaction conditions: catalyst 5.8 μmol, cat/MeOH/carbodiimide

:50:50, the solvent is C D , the total volume is 600 μL, at RT. The

6 6

To check the inﬂuence of the benzimidazolin-2-iminato

moiety on the geometry of complex 4 (Figure 2), we decided

to examine the resulting geometry of the homoleptic tert-

butoxide hafnium complex. The reaction of hafnium

yield was determined by H NMR spectroscopy of the crude reaction

mixture.

To investigate the fate of complex 1 in the presence of

alcohols and to identify possible reaction intermediates, 1 was

treated with 20 equiv of BuOH. Upon the addition, a release

tetrabenzyl with 4 and 5 equiv of BuOH resulted in the

consistent trimer 5a. Complex 5a has been reported in the

literature in the synthesis of the evasive Hf(O Bu) . In complex

of three benzyl groups was monitored by H NMR

5a, three hafnium atoms are bridged by a μ -O Bu and by a μ -

spectroscopy. The reaction mixture was set for crystallization

at −30 °C in toluene overnight, leading to the formation of

white crystals. The X-ray diﬀraction analysis of complex 4

O moiety. In addition, the hafnium atoms are bridged by a μ -

O Bu unit, and each hafnium has two additional O Bu

128,129

motifs.

The formation of the oxo species is always due

(

Figure 2) indicated the formation of a trigonal−bipyramidal

to a ubiquitous amount of water found in the crystallizing

129

complex with the protonated benzimidazolin-2-imine ligand in

an axial position in addition to four more tert-butoxide groups,

solvent.

Hence, to avoid trace amounts of water, the

crystallization was performed inside a glovebox having a

maximum of 0.1 ppm of water. Under these conditions, in the

crystallization process, a cocrystalline mixture of 5a and the

evasive homoleptic Hf(O Bu) (5b) was obtained (Figure 3).

The trimer part (5a) in the cocrystal is like that reported in the

literature; however, the homoleptic Hf(O Bu) shows a

hafnium in a distorted tetrahedral geometry. A simpliﬁed

schematic drawing of the cocrystals 5a and 5b can be found in

Scheme 5.

The slightly distorted tetrahedral structure of 5b displays

angles of 108.9(8) (O −Hf −O 8, O −Hf −O , O₁₂

−

Hf −O , and O −Hf −O ) and 110.7(17)° (O −Hf −

and O₁₂−Hf −O ) and bond lengths of 1.92(3) Å for

all Hf −O bonds. A comparison of the Hf−O Bu bond

lengths (not bridging moieties) between structures 5a and 5b

shows alike bond distances, Hf −O = 1.937(13) and Hf −O

Figure 2. ORTEP drawings of complex 4, with thermal displacement

parameters at 50% probability. Hydrogen atoms are omitted for

clarity. Bond lengths for complex 4 (Å): Hf−N = 2.385(7), Hf−O =

1.92(2) Å, in 5a. Interestingly, when comparing other bond

.950(6), Hf−O = 1.933(6), Hf−O = 1.935(5), Hf−O = 1.934(6),

lengths of Hf−O Bu in diﬀerent complexes bearing diﬀerent

ancillary ligands and recrystallized in diﬀerent solvents, similar

and N −C = 1.305(10). Hf−N −C = 145.0(5)°.

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

Scheme 6. Reactivity Permutation of the Reaction of

Hf(Bn) with BuOH and DTC

Figure 3. ORTEP drawings of complexes 5a and 5b, with thermal

displacement parameters at 50% probability. Hydrogen and carbon

atoms are omitted for clarity. Bond lengths for complex 5a (Å): Hf −

O = 2.162(19), Hf −O = 2.20(2), Hf −O = 2.42(2), Hf −O =

.03(2), Hf −O = 1.937(13), and Hf −O = 1.92(2). Bond lengths

for complex 5b (Å): Hf −O = 1.92(3).

Scheme 5. Schematic Drawing of Complexes 5a and 5b

hafnium complex and allows the formation of the two

amidinates, as shown in Scheme 6.

Interestingly, the structure of complex 1 changes when

reacted with an excess of phenol or 2,6-xylenol. In these cases,

it resulted in a dimerized complex 6 for the less steric phenol;

however, for the bulky phenol, the monomeric complex 7 was

obtained. Amazingly, even when an excess of the aromatic

alcohols is used, in both complexes, the ligand is present in the

crystal structure! (See Figure 4 for complexes 6 and 7.) In

Hf−O Bu bond lengths between 1.92 and 1.94 (Å) are always

obtained.

Complex 5 shows that the hafnium complexes in the

presence of the tert-butoxide moiety form clusters and bridges

that might cause a less reactive/available metal center/complex

due to a compact closed protective shell of the metal center.

To test this hypothesis, we used complex 5 in the addition

reaction of BuOH toward DTC. This reaction was slower by

0% compared with that when using complex 4. This result

indicates the importance of incorporating a highly nucleophilic

ligand in this type of complex to achieve better reactive

catalytic complexes.

To understand the formation of some of the active species

using hafnium tetrabenzyl as the starting material, various

stoichiometric reactions were studied. It is clear that the

formation of complexes 5a and 5b takes place in the solid state,

however, in the solution, we were interested in studying how

many molecules of BuOH react with the hafnium tetrabenzyl.

Hence, we have found that 5 equiv of BuOH reacts with

Figure 4. ORTEP drawing of complex 6, with thermal displacement

parameters at 50% probability. Hydrogen atoms are omitted for

clarity. Bond lengths for complex 6 (Å): Hf−O = 1.962(6), Hf−O =

Hf(CH Ph) to form, in solution, the pentacoordinate complex

Hf(O Bu) ( BuOH), presenting only one signal for the Bu

.071(5), Hf−O = 2.007(5), Hf−O = 2.175(5), and Hf−O =

groups on the NMR, indicating that the additional proton is

rapidly scrambled among all of the butoxide groups. The

addition of 1.2 equiv of DTC produces the monoisourea

.942(5).

trialkoxo hafnium complex with a molecule of BuOH still

complex 6, each hafnium is coordinated to three phenoxy

groups and two bridging phenoxide moieties (Scheme 7),

closing the charge balance. In addition, each hafnium is also

attached to a bent phenol moiety, giving rise to a distorted

octahedral coordination for each metal center. The two neutral

phenol moieties are disposed in an anti-coordination on

diﬀerent sides of the plane of the hafnium moieties.

attached to the hafnium center and a small amount of the free

isourea product (Scheme 6, pathway i). However, the

stoichiometric reaction between hafnium tetrabenzyl and 4

equiv of DTC (Scheme 6, pathway ii) proceeds, allowing three

benzyl groups to add to the corresponding DTC to form the

corresponding benzyl tris(amidinate) hafnium complex, and 1

equiv of DTC remains unreacted. The consecutive addition of

Complex 6 (Figure 4) crystallized in a dimeric distorted

octahedral structure. The complex exhibits two hafnium metal

centers, where each hafnium center is bonded to four phenols

.2 equiv of BuOH to the “in situ” formed complex produces

the corresponding mono tert-butoxide tris(carbodiimide)

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

Scheme 7. Schematic Drawing to Obtain Complex 6

Figure 6. ORTEP drawing of complex 7, with thermal displacement

parameters at 50% probability. Hydrogen atoms are omitted for

clarity. Bond lengths for complex 7 (Å): Hf−O = 1.989(5), Hf−O =

.922(4), Hf−O = 1.983(3), Hf−O = 2.072(4), and Hf−O =

.002(3).

and two bridging phenol units in addition to two

benzimidazolin-2-iminato ligands in the peripheral sphere.

The angle between Hf−O −C showed a bent angle of 128.3°

Scheme 8. Schematic Drawing to Obtain Complex 7

Figure 5 ) as compared with the other Hf −O bonds (not the

Figure 5. Hydrogen bonding between N −H−O in part of the

molecule of complex 6. The ORTEP drawing is presented with

thermal displacement parameters at 50% probability. Hydrogen atoms

are omitted for clarity, except for H . Hydrogen bond length in

complex 6 (Å): H −O = 1.875 Å.

bridging moieties). This bent angle suggested a hydrogen

Figure 7. Hydrogen bonding in N −H−O . ORTEP drawing of

bonding with the N−H of the ligand and O . In addition, we

complex 7, with thermal displacement parameters at 50% probability.

can see the displacement of the ligand not bonded to the metal

center producing the hydrogen bonding (Figure 5) with a

bond length of 1.875 Å. This displacement could imply the

lower catalytic activity using the phenols (entries 10 and 11).

Thus, to understand the role of the ligand, an additional

reaction was performed to obtain complex 7.

Hydrogen atoms are omitted for clarity, except for the relevant

hydrogen H1A. Hydrogen bond length for complex 7 (Å): H1A−O

2.048.

toluene and a benzimidazolin-2-imine penta(tert-butoxide)

hafnium complex (Scheme 9). Then, we added 1.2 equiv of

Complex 7 (Figure 6) crystallized in the distorted trigonal

bipyramidal conﬁguration. The structure exhibits a hafnium

center, surrounded by four molecules of 2,6-xylenol and one

neutral 2,6-xylenol molecule, as can be seen in Scheme 8. The

DTC, and the reaction was monitored by H NMR

spectroscopy, revealing the full consumption of the free

DTC (the disappearance of the signal at 1.94 ppm and the

appearance of the product signal at 5.79 ppm (N−H) as

opposed to the NH of the ligand, which is located at the 5.29

ppm).

bond length of Hf−O (neutral ligand) was the longest

compared with the other Hf−O bonds, as expected. The

angle of Hf −O −C is bent (137.3(3)), implying a hydrogen-

In addition, the H NMR analysis revealed the appearance of

tBuO

bonding N −H −O with the benzimidazolin-2-imine ligand

the benzimidazolin-2-iminato monoisourea (DTC

)

(

Dipp,Me)

moiety stabilizing the complex structure (Figure 7).

trialkoxo hafnium species (BenzIm

NH)Hf-

OtBu

Furthermore, to elaborate on our understanding regarding

the role of the benzimidazolin-2-iminato ligand in the active

species of complex 1, the complex was reacted with 5 equiv of

(O Bu) (DTC ). The integration of the tert-butoxide

moieties corresponded to three tert-butoxide groups attached

to the metal, one tert-butoxide moiety attached to the

carbodiimibe and additional tert-butoxide group from the

BuOH. This reaction resulted in the formation of 3 equiv of

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

Scheme 9. Reaction of Complex 1 with BuOH and DTC

Scheme 10. Plausible Mechanism for Catalytic Insertion of

Methanol into DTC

amidinate product. Furthermore, when reacting the obtained

complex from the reaction of complex 1 and 5 equiv of

complex BuOH (a similar complex to that of complex 4) with

equiv of DTC, the same products were obtained; however, 2

equiv of the amidinate product was obtained.

Kinetic studies on the addition of methanol to DTC using

the precatalyst 1 were conducted by changing the concen-

tration of one of the substrates or the catalyst (one order of

magnitude) while keeping the other substrates constant, and

EXPERIMENTAL SECTION

■

the progress of the reaction was monitored by in situ H NMR

General Considerations. All manipulations of air-sensitive

materials were performed with the rigorous exclusion of oxygen and

moisture in ﬂamed Schlenk-type glassware on a high vacuum line

spectroscopy. The initial rate of the reaction was found to ﬁt a

linear dependence on the concentrations, revealing that the

reactions display a ﬁrst-order dependence on MeOH, DTC,

and complex 1, following the kinetic rate law according to eq 1.

−

(

10 Torr) or in nitrogen-ﬁlled MBraun and Vacuum Atmospheres

gloveboxes with a medium capacity recirculator (1 to 2 ppm oxygen).

Argon and nitrogen were puriﬁed by passage through an MnO

oxygen-removal column and a Davison 4 Å molecular sieve column.

Analytically pure solvents were dried and stored with Na/K alloy and

degassed by three freeze−pump−thaw cycles prior to use (hexane,

toluene, benzene-d , toluene-d ). BenzImDippNH, BenzImMesNH,

(

See the SI .)

k_o_b_s[cat][DTC][MeOH]

(1)

The activation parameters were determined from the

ImMesNH, and the metal complex precursor, hafnium tetrabenzyl,

were synthesized according to published literature proce-

Arrhenius and Eyring plots, aﬀording the values of

‡

1,131,132

E = 9.62 (4) kcal/mol, ΔH = 9.02(4) kcal/mol, and ΔS =

−

dures.

1,3-Diisopropylcarbodiimide (Sigma-Aldrich) was

44.3(4) e.u. The large negative entropy value supports a

dried by vacuum transfer. MeOH, EtOH, iPrOH, and BuOH were

highly ordered transition state at the rate-determining step.

When performing the reaction with MeOD, a primary kinetic

isotope eﬀect (KIE) of 2.78(5) was observed, suggesting that

the protonolysis is the rate-determining step. Interestingly, for

distilled under CaH and stored over 4 Å molecular sieves. PhOH,

2,6-Me

PhOH, 2,4- Bu

PhOH, and 1,3-di-p-tolylcarbodiimide

−

(

Sigma-Aldrich) were dried for 12 h on a high vacuum line (10

Torr) and stored in a glovebox prior to use. MeOD and BuOD were

purchased from Sigma-Aldrich. NMR spectra were recorded on

Avance 200, Avance 300, Avance II 400, Avance 500, and Avance II

BuOD, a smaller KIE of 1.31(6) was measured, as expected,

when the steric inﬂuence started to be operative. A plausible

mechanism is presented in Scheme 10 for the addition of

methanol to DTC.

Complex 1 reacts with an excess of methanol to form

complex 4. DTC inserts into complex 4 in a rapid equilibrium,

forming the intermediate B. The protonolysis of the product

from complex B, by methanol, is the rate-determining step,

regenerating the active species 4.

00 Bruker spectrometers. Chemical shifts for H NMR and

NMR measurements are reported in ppm and referenced using

residual proton or carbon signals of the deuterated solvent relative to

tetramethylsilane. For all complexes, the formation of the correspond-

ing carbides precludes the elemental analysis (with or without V O ).

2 5

HRMS experiments were performed at 200 °C (source temperature)

on a Maxis Impact (Bruker) mass spectrometer using an atmospheric

pressure chemical ionization (APCI) solid-probe methodology. For

X-ray crystallographic measurements, the single-crystalline material

was immersed in perﬂuoropolyalkylether oil and was quickly ﬁshed

with a glass rod and mounted on a Kappa charge-coupled device

(CCD) diﬀractometer under a cold stream of nitrogen. Data

collection was performed using monochromated Mo Kα radiation

using φ and ω scans to cover the Ewald sphere. The structure was

solved by SHELXS-97 direct methods and reﬁned by the SHELXL-97

CONCLUSIONS

■

We have synthesized and characterized seven new hafnium

complexes bearing benzimidazolin-2-iminato and imidazolin-2-

iminato ligands. Their X-ray crystal structures revealed short

Hf−N bond lengths and a linear Hf−N−C angle, implying

high bond orders. These complexes were utilized for the

successful catalytic addition of alcohol to carbodiimides. This

reaction showed the ability of the highly electrophilic hafnium

complexes to participate in catalytic reactions involving

diﬀerent aliphatic and aromatic phenols due to the integration

of the highly nucleophilic imidazolin-2-iminato ligands.

133−135

program package.

Synthesis of Mono(benzimidazolin-2-iminato) Hafnium(IV)

Complexes 1 and 2. A toluene solution of the respective

benz(imidazolin-2-imine) Im NH (56.5 mg, 0.184 mmol) in 5 mL

of toluene was added dropwise to a preprepared solution of hafnium

tetrabenzyl (100 mg 0.184 mmol) in toluene (5 mL) at room

temperature. The reaction mixture was stirred overnight at room

Organometallics XXXX, XXX, XXX−XXX

Organometallics

The Supporting Information is available free of charge at

Article

temperature. The solvent was removed under vacuum to aﬀord the

crude complexes 1 and 2. In each case, the product was recrystallized

from a concentrated toluene solution at −35 °C to yield compounds 1

and 2 as crystalline materials.

Synthesis of 1-(2,6-Diisopropylphenyl)-3-methylbenzimi-

dazolin-2-imine Hafnium Tetra(phenoxy) Phenol

Dipp

[

(BenzIm NH)Hf(PhO) (PhOH)] (Complex 6). A solution of

phenol (24.8 mg, 0.263 mmol) in toluene (5 mL) was added to a

prepared solution of the respective complex 1 (40 mg, 0.052 mmol)

in 5 mL of toluene at room temperature. The reaction mixture was

stirred overnight at room temperature. The solvent was removed

under vacuum to aﬀord crude compound 6. The crude product of 6

was recrystallized from a concentrated toluene solution at −35 °C to

yield compound 6 as a crystalline material. Yield: 0.045 g, 0.042

-(2,6-Diisopropylphenyl)-3-methylbenzimidazolin-2-imine haf-

Dipp

nium tribenzyl [(BenzIm N)HfBn ], (Complex 1): Yield: 0.128

g, 0.168 mmol, 92%. H NMR (600 MHz, C D ) δ 7.27 (t, 1H, H−

Ar), 7.18 (m, 9H, H−Ph), 7.15 (m, 3H, H−Ph), 6.97 (m, 6H, H−

Ph), 6.85 (s, 1H, H−Ar), 6.75 (s, 1H, H−Ar), 6.60 (d, J = 7.6 Hz, 1H,

H−Ar), 6.38 (d, J = 7.4 Hz, 1H, H−Ar), 2.97 (s, 3H, CH ), 2.72 (m,

mmol, 90%. H NMR (500 MHz, C D ) δ 7.19, 6.97 (m, 20H, H−

H, CH(CH ) ), 1.61 (s, 6H, CH Ph), 1.24 (d, J = 6.9 Hz, 6H,

3 2 2

Ph), 6.75−6.57 (m, 5H, H−Ar), 6.12 (d, 1H, H−Ar), 6.03 (d, 1H,

H−Ar), 3.03 (s, 3H, CH ), 2.03 (m, 2H, CH(CH ) ), 1.02 (d, 6H,

CH ), 0.68 (d, 6H, CH ). C NMR (126 MHz, C D ) δ 156.2,

CH(CH ) ), 0.92 (d, J = 6.9 Hz, 6H, CH(CH ) ). C NMR (151

3 2

MHz, C D ) δ 148.1, 145.5, 142.7, 131.0, 130.3, 129.4, 128.8, 124.2,

122.3, 108.7, 108.0, 72.3, 28.6, 27.5, 24.1, 23.6. MS (APCI) for

47.7, 132.0, 129.0, 125.4, 119.9, 115.2, 31.4, 28.6, 23.7, 23.4, 22.6,

3.8. MS(APCI) for C H HfN O + H + N (from the APCI) (M +

C H HfN = 759.7492.

-(2,4,6-Trimethylphenyl)-3-methylbenzimidazolin-2-imine haf-

H + N) − C H = 890.8096.

Mes

nium tribenzyl [(BenzIm N)HfBn ], (Complex 2): Yield: 0.114 g,

Synthesis of 1-(2,6-Diisopropylphenyl)-3-methylbenzimi-

.160 mmol, 87%. H NMR (500 MHz, C D ) δ 7.31 (d, 1H, H−Ar),

Dipp

dazolin-2-imine Hafnium Penta-2,6-xylenol [(BenzIm NH)-

.13−6.99 (m, 15H, H−Ar) 6.79 (d, 1H, H−Ar), 6.63 (d, 1H, H−

Hf(2,6-dimethylphenoxy )(2,6-dimethylphOH)] (Complex 7).

Ar), 6.36 (d, 1H, H−Ar), 2.88 (s, 3H, CH ), 2.10 (s, 9H, CH −Ar),

A solution of 2,6-xylenol (32.1 mg, 0.263 mmol) in toluene (5 mL)

was added to a prepared solution of the respective complex 1 (40 mg,

0.052 mmol) in 5 mL of toluene at room temperature. The reaction

mixture was stirred overnight at room temperature. The solvent was

removed under vacuum to aﬀord crude compound 7. The crude

product of 7 was recrystallized from a concentrated toluene solution

at −35 °C to yield compound 7 as a crystalline material. Yield: 0.050

.00 (s, 6H, CH −Ph). C NMR (126 MHz, C D ) δ 146.10, 142.5,

39.0, 137.5, 130.4, 129.2, 125.4, 122.3, 107.9, 71.2, 27.3, 20.6, 17.3.

MS (APCI) for C H HfN + H (M + H) − C H = 642.3962

Synthesis of Mono 1-(2,4,6-Trimethylphenyl)-3-methylimi-

Mes

dazolin-2-imine-hafnium Tribenzyl [(Im N)HfBn ] (Complex

). A toluene solution of the respective (imidazolin-2-imine)

Im NH (19.79 mg, 0.092 mmol) in 5 mL of toluene was added

dropwise to a preprepared solution of hafnium tetrabenzyl (50 mg

Mes

g, 0.04 mmol, 87%. H NMR (300 MHz, C D ) δ 6.96 (d, 10H, H ),

6.90−6.86 (t, 5H, H ), 6.76−6.65 (m, 5H, H ), 6.17 (dd, 2H, H ),

0.092 mmol) in toluene (5 mL) at room temperature. The solvent

5.68 (S, 1H, NH), 4.02 (S, 1H, OH), 3.35 (S, 3H, CH ), 2.39 (S, 3H,

CH ), 2.32 (m, 2H, CH(CH3) ), 1.96 (S, 6H, CH ), 0.95 (d, J = 6.7

Hz, 6H, CH ), 0.76 (d, J = 6.8 Hz, 6H, CH ). C NMR (75 MHz,

C D ) δ 159.6, 155.8, 148.2, 131.5, 131.3, 130.9, 128.4, 128.2, 126.3,

125.2, 125.1, 120.0, 119.2, 115.2, 108.8, 107.0, 29.0, 28.4, 24.4, 22.8,

17.4, 15.4. MS (APCI) for C60

1093.9998).

H70HfN O (low-intensity signal:

3 5

Kinetic Studies on the Addition of MeOH into DTC Using

Complex 1. All of the kinetic experiments were performed in the

following manner. A J. Young NMR tube was charged with a

measured amount of a preprepared solution of DTC, MeOH, and

catalyst in C D , and additional solvent was added to reach a total

volume of 0.6 mL. The solution was added to the glovebox, and the

tube was sealed. The NMR tube was taken out of the glovebox, and

the H NMR experiment began at 25 °C. All of the experiments were

performed by changing only one of the substrates (0.11 to 1.1 mM)

or the catalyst (0.55 to 5.5 μM) while the other reagents were

constant. The product concentrations were measured by the area ratio

of the methyl groups at 3.00 and 3.68 ppm, which were assigned to

the starting material and the product, respectively. Reaction rates were

determined by the least-squares ﬁt of the initial product concentration

versus time, and the plots are shown in Figures S34 −S36 . Activation

‡

parameters (ΔH ), (ΔS ), and (E ) were calculated from the kinetic

data using the Eyring and the Arrhenius plots (Figures S37 and S38 ).

Labeling Experiment. All KIE experiments were performed by

charging a J. Young NMR tube with catalyst 1, DTC, MeOH, or

MeOD and BuOH or BuOD. For MeOH, the product concentration

was measured by the area ratio of the methyl group at 3.00 and 3.68

ppm, assigned to the starting material and product, respectively. For

BuOH, the product concentration was measured by the area ratios of

the BuOH group at 1.03 and 1.55 ppm, assigned to the starting

material and product, respectively. Reaction rates were determined by

the least-squares ﬁt of the initial product concentration versus time,

and the plots are shown in Figures S40 and S41 . The KIE values were

calculated from the ratio of the two slopes: KIE_M_e_O_H= 2.78(5) and

KIE_t_B_u_O_H= 1.31(6).

ASSOCIATED CONTENT

Supporting Information

https://pubs.acs.org/doi/10.1021/acs.organomet.0c00384.

■

sı

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

5) Klosin, J.; Fontaine, P. P.; Figueroa, R.; McCann, S. D.; Mort, D.

Synthesis of complexes, NMR spectra, kinetic studies,

stoichiometric experiments, deuterium labeling studies,

and crystallographic data for complexes 1−7. (PDF)

Preparation of New Olefin Polymerization Precatalysts by Facile

Derivatization of Imino-Enamido ZrMe ₃ and HfMe ₃ complexes.

Organometallics 2013, 32, 6488−6499.

(6) Mehrkhodavandi, P.; Schrock, R. R.; Pryor, L. L. Living

Accession Codes

Polymerization of 1-Hexene by Cationic Zirconium and Hafnium

Complexes That Contain a Diamido/Donor Ligand of the Type

H CC(2-C H N)(CH NMesityl) ] . A Comparison of Methyl and

CCDC 1945241 −1945246 and 1945362 contain the supple-

mentary crystallographic data for this paper. These data can be

obtained free of charge via www.ccdc.cam.ac.uk/data_request/

cif, or by emailing data_request@ccdc.cam.ac.uk, or by

contacting The Cambridge Crystallographic Data Centre, 12

Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

‑

[

3 5 4 2 2

Isobutyl Initiators. Organometallics 2003, 22, 4569−4583.

(7) Elkin, T.; Kulkarni, N. V.; Tumanskii, B.; Botoshansky, M.;

Shimon, L. J. W.; Eisen, M. S. Synthesis and Structure of Group 4

Symmetric Amidinate Complexes and Their Reactivity in the

Polymerization of α -Olefins. Organometallics 2013, 32, 6337−6352.

AUTHOR INFORMATION

Corresponding Author

Moris S. Eisen − Schulich Faculty of Chemistry, Technion-Israel

Institute of Technology, Technion City 32000, Israel;

orcid.org/0000-0001-8915-0256 ; Phone: +972-4-

(8) Macosko, C. W.; Thurber, C. M.; Coates, G. W.; Eagan, J. M.;

■

Bates, F. S.; Di Girolamo, R.; Xu, J.; LaPointe, A. M. Combining

Polyethylene and Polypropylene: Enhanced Performance with PE/

iPP Multiblock Polymers. Science 2017, 355, 814−816.

(9) Tshuva, E. Y.; Groysman, S.; Goldberg, I.; Kol, M.; Goldschmidt,

Z. [ONXO]-Type Amine Bis(Phenolate) Zirconium and Hafnium

Complexes as Extremely Active 1-Hexene Polymerization Catalysts.

Organometallics 2002, 21, 662−670.

292680; Email: chmoris@technion.ac.il

Authors

10) Cohen, A.; Kopilov, J.; Lamberti, M.; Venditto, V.; Kol, M.

Maxim Khononov − Schulich Faculty of Chemistry, Technion-

Same Ligand, Different Metals: Diiodo-Salan Complexes of the

Group 4 Triad in Isospecific Polymerization of 1-Hexene and

Propylene. Macromolecules 2010, 43, 1689−1691.

Israel Institute of Technology, Technion City 32000, Israel

Heng Liu − Schulich Faculty of Chemistry, Technion-Israel

Institute of Technology, Technion City 32000, Israel;

orcid.org/0000-0001-6064-6610

11) Tsurugi, H.; Ohnishi, R.; Kaneko, H.; Panda, T. K.; Mashima,

K. Controlled Benzylation of α -Diimine Ligands Bound to Zirconium

and Hafnium: An Alternative Method for Preparing Mono- and

Bis(Amido)M(CH Ph) (n = 2, 3) Complexes as Catalyst Precursors

Natalia Fridman − Schulich Faculty of Chemistry, Technion-

Israel Institute of Technology, Technion City 32000, Israel

Matthias Tamm − Institut fu

r Anorganische und Analytische

Braunschweig, 38106

for Isospecific Polymerization of α -Olefins. Organometallics 2009, 28,

680−687.

Chemie, Technische Universitat

12) Gibson, V. C.; Spitzmesser, S. K. Advances in Non-Metallocene

Braunschweig, Germany; orcid.org/0000-0002-5364-0357

Olefin Polymerization Catalysis. Chem. Rev. 2003 , 103 , 283 −316.

13) Kaminsky, W. New Polymers by Metallocene Catalysis.

Macromol. Chem. Phys. 1996, 197, 3907−3945.

14) Kulkarni, N. V.; Elkin, T.; Tumaniskii, B.; Botoshansky, M.;

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.organomet.0c00384

(

Notes

Shimon, L. J. W.; Eisen, M. S. Asymmetric Bis(Formamidinate)

Group 4 Complexes: Synthesis, Structure and Their Reactivity in the

Polymerization of α -Olefins. Organometallics 2014, 33 , 3119 −3136.

The authors declare no competing ﬁnancial interest.

Email: m.tamm@tu-bs.de (M.T.).

(15) Suo, H.; Solan, G. A.; Ma, Y.; Sun, W.-H. Developments in

Compartmentalized Bimetallic Transition Metal Ethylene Polymer-

ACKNOWLEDGMENTS

■

ization Catalysts. Coord. Chem. Rev. 2018 , 372 , 101 −116.

This work was supported by the Israel Science Foundation

administered by the Israel Academy of Science and Humanities

under Contract No. 184/18 and by the PAZY Foundation

Fund (ID 128-2020) administered by the Israel Atomic Energy

Commission.

(16) Collins, S. Polymerization Catalysis with Transition Metal

Amidinate and Related Complexes. Coord. Chem. Rev. 2011, 255,

18−138.

(17) Smolensky, E.; Eisen, M. S. Design of Organometallic Group Iv

Heteroallylic Complexes and Their Catalytic Properties for Polymer-

izations and Olefin Centered Transformations. J. Chem. Soc. Dalt.

Trans. 2007, 48, 5623−5650.

REFERENCES

■

18) Elkin, T.; Eisen, M. S. Amidinate Group 4 Complexes in the

Polymerization of Olefins. Catal. Sci. Technol. 2015, 5, 82−95.

19) Lamb, J. V.; Buffet, J.-C.; Matley, J. E.; Wright, C. M. R.;

1) Nakata, N.; Saito, Y.; Watanabe, T.; Ishii, A. Completely

Isospecific Polymerization of 1-Hexene Catalyzed by Hafnium(IV)

Dichloro Complex Incorporating with an [OSSO]-Type Bis-

Phenolate) Ligand. Top. Catal. 2014, 57, 918−922.

2) Hamaki, H.; Takeda, N.; Nabika, M.; Tokitoh, N. Catalytic

(

Turner, Z. R.; O ’Hare, D. Group 4 Permethylindenyl Complexes for

the Polymerisation of L-, D- and Rac -Lactide Monomers. Dalt. Trans.

Activities for Olefin Polymerization: Titanium(III), Titanium(IV),

019, 48, 2510−2520.

Zirconium(IV), and Hafnium(IV) β -Diketiminato, 1-Aza-1,3-Buta-

dienyl-Imido, and 1-Aza-2-Butenyl-Imido Complexes Bearing an

Extremely Bulky Substituent, the Tbt Group (Tbt = 2,4,6-.

Macromolecules 2012, 45, 1758−1769.

20) Mandal, M.; Ramkumar, V.; Chakraborty, D. Salen Complexes

of Zirconium and Hafnium: Synthesis, Structural Characterization

and Polymerization Studies. Polym. Chem. 2019 , 10 , 3444 −3460.

(21) Gurubasavaraj, P. M.; Charantimath, J. S. Group 4 Hydroxides

and Their Multimetallic Assemblies for Ring Opening Polymerization

ROMP) of Rac-Lactide without Solvent. Adv. Mater. Proc. 2017, 2,

(

3) Gies, A. P.; Kuhlman, R. L.; Zuccaccia, C.; MacChioni, A.;

Keaton, R. J. Mass Spectrometric Mechanistic Investigation of Ligand

Modification in Hafnocene-Catalyzed Olefin Polymerization. Organo-

metallics 2017, 36, 3443−3455.

643−647.

(22) Byers, J. A.; Biernesser, A. B.; Delle Chiaie, K. R.; Kaur, A.;

Kehl, J. A. Catalytic Systems for the Production of Poly(Lactic Acid).

Adv. Polym. Sci. 2017, 279, 67−118.

4) Bochmann, M.; Lancaster, S. J. Cationic Group IV Metal Alkyl

Complexes and Their Role as Olefin Polymerization Catalysts: The

Formation of Ethyl-Bridged Dinuclear and Heterodinuclear Zirco-

nium and Hafnium Complexes. J. Organomet. Chem. 1995, 497, 55−

(23) Chakraborty, D.; Mandal, D.; Ramkumar, V.; Subramanian, V.;

Vijaya Sundar, J. A New Class of MPV Type Reduction in Group 4

Alkoxide Complexes of Salicylaldiminato Ligands: Efficient Catalysts

59.

Organometallics XXXX, XXX, XXX−XXX

Organometallics

for the ROP of Lactides, Epoxides and Polymerization of Ethylene.

Article

(39) Roymuhury, S. K.; Chakraborty, D.; Ramkumar, V. Synthesis

and Characterization of Group 4 Metal Alkoxide Complexes

Containing Imine Based Bis-Bidentate Ligands: Effective Catalysts

for the Ring Opening Polymerization of Lactides, Epoxides and

Polymerization of Ethylene. Dalt. Trans. 2015, 44, 10352 −10367.

(40) Stirling, E.; Champouret, Y.; Visseaux, M. Catalytic Metal-

Based Systems for Controlled Statistical Copolymerisation of Lactide

with a Lactone. Polym. Chem. 2018, 9, 2517−2531.

Polymer 2015, 56, 157−170.

(24) Chakraborty, D.; Chokkapu, E. R.; Mandal, M.; Gowda, R. R.;

Ramkumar, V. Zwitterionic Complexes of Group 4 Metal Chlorides

Containing a Bis(Imino)Phenoxide Scaffold: Synthesis, Character-

ization and Polymerization Studies. ChemistrySelect. 2016, 1, 5218−

25) Chuang, H. J.; Wu, B. H.; Li, C. Y.; Ko, B.-T. T. Benzotriazole

229.

Phenoxide Hafnium Complexes as Efficient Catalysts for the Ring-

Opening Polymerization of Lactide: Synthesis, Characterization, and

Kinetics of Polymerization Catalysis. Eur. J. Inorg. Chem. 2014, 2014,

(41) Stopper, A.; Rosen, T.; Venditto, V.; Goldberg, I.; Kol, M.

Group 4 Metal Complexes of Phenylene-Salalen Ligands in Rac

Lactide Polymerization Giving High Molecular Weight Stereoblock

Poly(Lactic Acid). Chem. - Eur. J. 2017, 23, 11540−11548.

42) Su, C.; Chuang, H.; Li, C.; Yu, C.; Ko, B.; Chen, J.; Chen, M.-J.

Oxo-Bridged Bimetallic Group 4 Complexes Bearing Amine-Bis-

Benzotriazole Phenolate) Derivatives as Bifunctional Catalysts for

(

239−1248.

S.; Capacchione, C. Group 4 Metal Complexes Bearing Thioether-

26) Della Monica, F.; Luciano, E.; Roviello, G.; Grassi, A.; Milione,

phenolate Ligands. Coordination Chemistry and Ring-Opening

Polymerization Catalysis. Macromolecules 2014, 47, 2830−2841.

Ring-Opening Polymerization of Lactide and Copolymerization of

Carbon Dioxide with Cyclohexene Oxide. Organometallics 2014, 33,

(27) Duan, Y.-L.; Hu, Z.-J.; Yang, B.-Q.; Ding, F.-F.; Wang, W.;

Huang, Y.; Yang, Y. Zirconium and Hafnium Complexes Bearing

091−7100.

Pyrrolidine Derived Salalen-Type {ONNO} Ligands and Their

Application for Ring-Opening Polymerization of Lactides. Dalt.

Trans. 2017, 46, 11259−11270.

43) Sun, Y.; Jia, Z.; Chen, C.; Cong, Y.; Mao, X.; Wu, J. Alternating

Sequence Controlled Copolymer Synthesis of α -Hydroxy Acids via

Syndioselective Ring-Opening Polymerization of O-Carboxyanhy-

drides Using Zirconium/Hafnium Alkoxide Initiators. J. Am. Chem.

Soc. 2017, 139, 10723−10732.

(28) He, J. X.; Duan, Y. L.; Kou, X.; Zhang, Y. Z.; Wang, W.; Yang,

Y.; Huang, Y. Dinuclear Group 4 Alkoxides: Excellent Initiators for

Ring-Opening Polymerization of Cyclic Esters. Inorg. Chem. Commun.

(44) Turner, Z. R.; Buffet, J.-C. C.; O ’Hare, D. Chiral Group 4

Cyclopentadienyl Complexes and Their Use in Polymerization of

29) Higuchi, M.; Kanazawa, A.; Aoshima, S. Concurrent Cationic

015, 61, 144−148.

Lactide Monomers. Organometallics 2014, 33, 3891−3903.

(45) Wang, X.-Y.; Li, Y.-G.; Mu, H.-L.; Pan, L.; Li, Y.-S. Efficient

Vinyl-Addition and Coordination Ring-Opening Copolymerization

via Orthogonal Propagation and Transient Merging at the

Propagating Chain End. ACS Macro Lett. 2017, 6, 365−369.

Synthesis of Diverse Well-Defined Functional Polypropylenes with

High Molecular Weights and High Functional Group Contents via

Thiol-Halogen Click Chemistry. Polym. Chem. 2015, 6, 1150 −1158.

(30) Hu, M.; Han, F.; Zhang, W.; Ma, W.; Deng, Q.; Song, W.; Yan,

(46) Zhao, N.; Hou, G.; Deng, X.; Zi, G.; Walter, M. D. Group 4

H.; Dong, G. Preparation of Zirconium and Hafnium Complexes

Containing Chiral N Atoms from Asymmetric Tertiary Amine

Ligands, and Their Catalytic Properties for Polymerization of Rac-

Lactide. Catal. Sci. Technol. 2017, 7, 1394−1403.

Metal Complexes with New Chiral Pincer NHC-Ligands: Synthesis,

Structure and Catalytic Activity. Dalt. Trans. 2014 , 43 , 8261 −8272.

(47) Rojas-Buzo, S.; García-García, P.; Corma, A. Catalytic Transfer

Hydrogenation of Biomass-Derived Carbonyls over Hafnium-Based

(31) Huang, B.-H.; Tsai, C.-Y.; Chen, C.-T.; Ko, B.-T. Metal

Metal-Organic Frameworks. ChemSusChem 2018, 11, 432−438.

Complexes Containing Nitrogen-Heterocycle Based Aryloxide or

Arylamido Derivatives as Discrete Catalysts for Ring-Opening

Polymerization of Cyclic Esters. Dalt. Trans. 2016, 45, 17557−17580.

(48) Otake, K.; Cui, Y.; Buru, C. T.; Li, Z.; Hupp, J. T.; Farha, O. K.

Single-Atom-Based Vanadium Oxide Catalysts Supported on Metal-

Organic Frameworks: Selective Alcohol Oxidation and Structure-

Activity Relationship. J. Am. Chem. Soc. 2018, 140 , 8652 −8656.

(32) Jones, M. D.; Brady, L.; McKeown, P.; Buchard, A.; Schafer, P.

M.; Thomas, L. H.; Mahon, M. F.; Woodman, T. J.; Lowe, J. P. Metal

Influence on the Iso- and Hetero-Selectivity of Complexes of

Bipyrrolidine Derived Salan Ligands for the Polymerisation of Rac-

Lactide. Chem. Sci. 2015, 6, 5034−5039.

(49) Bakuru, V. R.; Davis, D.; Kalidindi, S. B. Cooperative Catalysis

at Metal MOF Interface: Hydrodeoxygenation of Vanillin over Pd

Nanoparticles Covered with UiO-66(Hf) MOF. Dalt. Trans. 2019, 48,

(

573−8577.

33) Kremer, A. B.; Mehrkhodavandi, P. Dinuclear Catalysts for the

Ring Opening Polymerization of Lactide. Coord. Chem. Rev. 2019,

80, 35−57.

34) Kricheldorf, H. R.; Mix, R.; Weidner, S. M. Poly(Ester

50) Otake, K.; Ye, J.; Mandal, M.; Islamoglu, T.; Buru, C. T.; Hupp,

J. T.; Delferro, M.; Truhlar, D. G.; Cramer, C. J.; Farha, O. K.

Enhanced Activity of Heterogeneous Pd(II) Catalysts on Acid-

Functionalized Metal-Organic Frameworks. ACS Catal. 2019, 9,

Urethane)s Derived from Lactide, Isosorbide, Terephthalic Acid, and

Various Diisocyanates. J. Polym. Sci., Part A: Polym. Chem. 2014, 52,

51) Du, X. D.; Yi, X. H.; Wang, P.; Zheng, W.; Deng, J.; Wang, C.

383−5390.

C. Robust Photocatalytic Reduction of Cr(VI) on UiO-66-NH (Zr/

67−875.

35) Luciano, E.; Buonerba, A.; Grassi, A.; Milione, S.; Capacchione,

Hf) Metal-Organic Framework Membrane under Sunlight Irradiation.

C. Thioetherphenolate Group 4 Metal Complexes in the Ring

Opening Polymerization of Rac-β -Butyrolactone. J. Polym. Sci., Part A:

Polym. Chem. 2016, 54, 3132−3139.

Chem. Eng. J. 2019, 356, 393−399.

(52) CRC Handbook of Chemistry and Physics, 96th ed.; Haynes, W.

M.; CRC Press: Boca Raton, FL, 2016; p 1585.

(36) Pappuru, S.; Chakraborty, D.; Vijaya Sundar, J.; Roymuhury, S.

(53) Weiss, C. J.; Marks, T. J. Organo-f-Element Catalysts for

K.; Ramkumar, V.; Subramanian, V.; Chand, D. K. Group 4

Complexes of Salicylbenzoxazole Ligands as Effective Catalysts for

the Ring-Opening Polymerization of Lactides, Epoxides and

Copolymerization of ε -Caprolactone with L-Lactide. Polymer 2016,

Efficient and Highly Selective Hydroalkoxylation and Hydro-

thiolation. Dalt. Trans. 2010, 39, 6576−6588.

(54) Hessen, B.; Teuben, J. H.; Blenkers, J.; Helgesson, G.; Jagner, S.

Reaction of Electron-Deficient Group 4 Metal s-Cis-Butadiene

Complexes with CO. Organometallics 1989 , 8 , 2809 −2812.

(

02, 231−247.

37) Rajashekhar, B.; Roymuhury, S. K.; Chakraborty, D.;

(55) Wang, H.; Chan, H.-S.; Xie, Z. Pentavalent vs Trivalent

Phosphorus-Bridged Ligands. Synthesis and Structural Character-

ization of Unexpected Group 4 Metal Complexes Incorporating an

Ramkumar, V. Group 4 Metal Complexes of Trost ’s Semi-Crown

Ligand: Synthesis, Structural Characterization and Studies on the

Ring-Opening Polymerization of Lactides and ε -Caprolactone. Dalt.

Trans. 2015, 44, 16280−16293.

Indenylide Unit, [σ :σ - Pr NP(O)(C H )(C B H )]M(NR ) .

2 2

Organometallics 2006, 25, 2569−2573.

(38) Richter, D.; Gooßen, S.; Wischnewski, A. Celebrating Soft

(56) Ziegler, T.; Tschinke, V.; Versluis, L.; Baerends, E. J.; Ravenek,

W. A Theoretical Study of Metal Ligand Bond Strengths (M-L: L =

OH, OCH , SH, NH , PH , CH , SiH , CN and H) in the Early

Matter ’s 10th Anniversary: Topology Matters: Structure and

Dynamics of Ring Polymers. Soft Matter 2015, 11, 8535−8549.

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Transition Metal Systems Cl ML (M = Ti, Zr and Hf) and Late

Article

(75) Gornshtein, F.; Kapon, M.; Botoshansky, M.; Eisen, M. S.

Titanium and Zirconium Complexes for Polymerization of Propylene

and Cyclic Esters. Organometallics 2007, 26, 497 −507.

Transition Metal Systems LCo(CO) . Polyhedron 1988, 7, 1625−

57) Liu, H.; Fridman, N.; Tamm, M.; Eisen, M. S. Catalytic

637.

(76) Volkis, V.; Tumanskii, B.; Eisen, M. S. Unusual Synergetic

Effect of Cocatalysts in the Polymerization of Propylene by a

Zirconium Bis(Benzamidinate) Dimethyl Complex. Organometallics

Addition of Alcohols to Carbodiimides Mediated by Benzimidazolin-

-Iminato Actinide Complexes. Organometallics 2017, 36, 4600−

610.

(58) Liu, H.; Eisen, M. S. Facile Coupling of Aldehydes with

006, 25, 2722−2724.

77) Volkis, V.; Smolensky, E.; Lisovskii, A.; Eisen, M. S. Solvent

Alcohols: An Evolved Tishchenko Process for the Preparation of

Effects in the Polymerization of Propylene Catalyzed by Octahedral

Unsymmetrical Esters. Eur. J. Org. Chem. 2017 , 2017 , 4852 −4858.

Complexes. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 4505−4516.

(59) Liu, H.; Eisen, M. S. Selective Actinide-Catalyzed Tandem

(78) Smolensky, E.; Kapon, M.; Woollins, J. D.; Eisen, M. S.

Proton-Transfer Esterification of Aldehydes with Alcohols for the

Formation of Elastomeric Polypropylene Promoted by a Dynamic

Octahedral Titanium Complex. Organometallics 2005, 24, 3255−

Production of Asymmetric Esters. Organometallics 2017, 36, 1461−

60) Liu, H.; Khononov, M.; Fridman, N.; Tamm, M.; Eisen, M. S.

464.

265.

79) Elkin, T.; Aharonovich, S.; Botoshansky, M.; Eisen, M. S.

Synthesis and Characterization of Group 4 Fluorinated Bis-

Amidinates) and Their Reactivity in the Formation of Elastomeric

Polypropylene. Organometallics 2012, 31, 7404−7414.

80) Aharonovich, S.; Kulkarni, N. V.; Zhang, J. S.; Botoshansky, M.;

Synthesis, Characterization and Catalytic Performances of Benzimi-

dazolin-2-Iminato Actinide (IV) Complexes in the Tishchenko

Reactions for Symmetrical and Unsymmetrical Esters. J. Organomet.

Chem. 2018, 857, 123−137.

(61) Liu, H.; Khononov, M.; Fridman, N.; Tamm, M.; Eisen, M. S.

Kapon, M.; Eisen, M. S. Polymerization of Propylene Promoted by

Catalytic Addition of Alcohols into Carbodiimides Promoted by

Zirconium Benzamidinates. Dalt. Trans. 2013, 42, 16762−16772.

Organoactinide Complexes. Inorg. Chem. 2017, 56, 3153−3157.

(81) Elkin, T.; Botoshansky, M.; Waymouth, R. M.; Eisen, M. S.

(62) Ghatak, T.; Drucker, S.; Fridman, N.; Eisen, M. S. Thorium

Titanium Bis(Amidinates) Bearing Electron Donating Pendant Arms

as Catalysts for Stereospecific Polymerization of Propylene. Organo-

metallics 2014, 33, 840−843.

Complexes Possessing Expanded Ring N-Heterocyclic Iminato

Ligands: Synthesis and Applications. Dalt. Trans. 2017, 46, 12005−

(82) Karmel, I. S. R.; Elkin, T.; Fridman, N.; Eisen, M. S.

2009.

63) Ghatak, T.; Fridman, N.; Eisen, M. S. Actinide Complexes

Dimethylsilyl Bis(Amidinate)Actinide Complexes: Synthesis and

Reactivity towards Oxygen Containing Substrates. Dalt. Trans.

2014, 43, 11376−11387.

Possessing Six-Membered N-Heterocyclic Iminato Moieties: Syn-

thesis and Reactivity. Organometallics 2017, 36, 1296−1302.

(64) Batrice, R. J.; Kefalidis, C. E.; Maron, L.; Eisen, M. S. Actinide-

(83) Karmel, I. S. R.; Fridman, N.; Eisen, M. S. Actinide Amidinate

Catalyzed Intermolecular Addition of Alcohols to Carbodiimides. J.

Am. Chem. Soc. 2016, 138, 2114−2117.

Complexes with a Dimethylamine Side Arm: Synthesis, Structural

Characterization, and Reactivity. Organometallics 2015 , 34 , 636 −643.

(84) Jayaratne, K. C.; Sita, L. R. Direct Methyl Group Exchange

between Cationic Zirconium Ziegler-Natta Initiators and Their Living

Polymers: Ramifications for the Production of Stereoblock Poly-

olefins. J. Am. Chem. Soc. 2001, 123, 10754−10755.

(65) Kozak, C. M.; Mountford, P. Zirconium & Hafnium: Inorganic

Coordination Chemistry. In Encyclopedia of Inorganic and

Bioinorganic Chemistry; John Wiley & Sons, Ltd: Chichester, U.K.,

011.

66) Patnaik, P. In Handbook of Inorganic Chemicals; The McGraw-

Hill Companies, 2003; pp 330−332.

67) Heins, S. P.; Morris, W. D.; Wolczanski, P. T.; Lobkovsky, E.

(

(85) Littke, A.; Sleiman, N.; Bensimon, C.; Richeson, D. S.; Yap, G.

P. A.; Brown, S. J. Bulky Bis(Alkylamidinate) Complexes of Group 4.

Syntheses and Characterization of M(CyNC(R ‘)NCy)₂ Cl ₂ and

Zr(CyNC(Me)NCy) Me (R ‘= Me, M = Ti, Zr, Hf; R ’ = t Bu, M =

(

B.; Cundari, T. R. Nitrene Insertion into C-C and C-H Bonds of

Diamide Diimine Ligands Ligated to Chromium and Iron. Angew.

Chem., Int. Ed. 2015, 54, 14407−14411.

Zr). Organometallics 1998, 17, 446−451.

(68) Aharonovich, S.; Botoshansky, M.; Balazs, Y. S.; Eisen, M. S.

(86) Duchateau, R.; Van Wee, C. T.; Meetsma, A.; Van Duijnen, P.

Elucidation of Substituent Effects in the Polymerization of Propylene

T.; Teuben, J. H. Ancillary Ligand Effects in Organoyttrium

Chemistry: Synthesis, Characterization, and Electronic Structure of

Bis(Benzamidinato)Yttrium Compounds. Organometallics 1996, 15,

Promoted by Titanium Amidinates. Organometallics 2012, 31, 3435−

69) Volkis, V.; Aharonovich, S.; Eisen, M. S. Deuterium Labeling

438.

87) Hagadorn, J. R.; Arnold, J. Titanium(II), -(III), and -(IV)

279−2290.

Complexes Supported by Benzamidinate Ligands. Organometallics

998, 17, 1355−1368.

88) Hagadorn, J. R.; Arnold, J. Preparation of New Zirconium

and Mechanistic Insights in the Polymerization of Propylene

Promoted by Benzamidinate Complexes. Macromol. Res. 2010, 18,

70) Chen, F.; Kapon, M.; Woollins, D.; Eisen, M. S. Bis-

67−973.

Imidodithiodiphosphinato) Titanium and Zirconium Complexes:

Benzamidinates: Alkyl Derivatives and Low-Valent Chemistry

Yielding Metallacycles via Coupling of Alkynes and Ethylene. J.

Chem. Soc., Dalton Trans. 1997, 18 , 3087 −3096.

Synthesis, Characterization, and Their Catalytic Activity in the

Polymerization of α -Olefins. Organometallics 2009, 28, 2391−2400.

(71) Volkis, V.; Lisovskii, A.; Eisen, M. S. Stereoerrors Formation in

(89) Hagadorn, J. R.; Arnold, J. Low-Valent Chemistry of Titanium

the Polymerization of Deuterated Propylene. In Progress in Oleﬁn

Polymerization Catalysts and Polyoleﬁn Materials; Shiono, T., Nomura,

K., Terano, M., Eds.; Elsevier B.V., 2006; pp 105 −112.

Benzamidinates Leading to New Ti μ -N , μ -O, Alkyl Derivatives, and

the Cyclometalation of TMEDA. J. Am. Chem. Soc. 1996, 118, 893−

(72) Aharonovich, S.; Volkis, V.; Eisen, M. S. Octahedral Complexes

94.

90) Edelmann, F. T. N-Silylated Benzamidines: Versatile Building

in the Polymerization of α -Olefins. Macromol. Symp. 2007, 260, 165−

71.

73) Gueta-Neyroud, T.; Tumanskii, B.; Kapon, M.; Eisen, M. S.

Blocks in Main Group and Coordination Chemistry. Coord. Chem.

Rev. 1994, 137, 403−481.

(91) Edelmann, F. T. Rare Earth Complexes with Heteroallylic

Synthesis and Characterization of Dichlorotitanium Alkoxide

Complex and Its Activity in the Polymerization of α -Olefins.

Macromolecules 2007, 40, 5261−5270.

Ligands. Organolanthoid Chem. Synth. Struct. Catal. 1996, 179, 113−

148.

(74) Gueta-Neyroud, T.; Tumanskii, B.; Botoshansky, M.; Eisen, M.

(92) Hagadorn, J. R.; Arnold, J. Zirconium Chemistry Involving

S. Synthesis, Characterization and Catalytic Activity of the Complex

Titanium Bis(Dimethylmalonate)-Bis(Diethylamido) in the Polymer-

ization of Propylene. J. Organomet. Chem. 2007, 692, 927−939.

Benzamidinate Ligands. Reduction of [PhC(NSiMe ) ] ZrCl To

3 2 2 2

Form an Imido-Iminoacyl Compound by Carbon-Nitrogen Bond

Cleavage. Organometallics 1994, 13, 4670−4672.

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

93) Herskovics-Korine, D.; Eisen, M. S. Bis(Trimethylsilyl)

Complexes in the Ring-Opening Polymerization of Rac-Lactide. Inorg.

Chem. 2017, 56, 3447−3458.

Benzamidinate Zirconium Dichlorides. Active Catalysts for Ethylene

Polymerization. J. Organomet. Chem. 1995, 503, 307−314.

(

Boese, R. Synthese Und Struktur von 2-Imino-l,3-Dimethylimidazolin.

Z. Naturforsch., B: J. Chem. Sci. 1995, 50, 1779−1784.

(

(112) De Leseleuc, M.; Collins, S. K. Direct Synthesis of

́ ́

94) Kuhn, N.; Fawzi, R.; Steimann, M.; Wiethoff, J.; Blas

er, D.;

Macrodiolides via Hafnium(Iv) Catalysis. Chem. Commun. 2015, 51,

10471−10474.

(113) McKeown, P.; Davidson, M. G.; Lowe, J. P.; Mahon, M. F.;

Thomas, L. H.; Woodman, T. J.; Jones, M. D. Aminopiperidine Based

Complexes for Lactide Polymerisation. Dalt. Trans. 2016, 45, 5374−

5387.

95) Kuhn, N.; Gohner, M.; Grathwohl, M.; Wiethoff, J.; Frenking,

G.; Chen, Y. 2-Iminoimidazoline  Starke Stickstoffbasen Als

Koordinationspartner in Der Anorganischen Chemie. Z. Anorg. Allg.

Chem. 2003, 629, 793−802.

(114) Mandal, D.; Chakraborty, D.; Ramkumar, V.; Chand, D. K.

Group 4 Alkoxide Complexes Containing [NNO]-Type Scaffold:

Synthesis, Structural Characterization and Polymerization Studies.

RSC Adv. 2016, 6, 21706−21718.

96) Kuhn, N.; Fawzi, R.; Steimann, M.; Wiethoff, J. Derivate Des

Imidazols. XXII. Imidazoliniminato-Komplexe Des Titans. Synthese

Und Kristallstrukturen von [(ImN)TiCl ₃ ]₂ Und [(ImN)-

TiCl 2MeCN] (ImN = 1,3-Dimethylimidazolin-2-Imino). Z. Anorg.

(115) Ishitani, H.; Suzuki, H.; Saito, Y.; Yamashita, Y.; Kobayashi, S.

3 ·

Allg. Chem. 1997, 623, 769−774.

Hafnium Trifluoromethanesulfonate [Hf(OTf) ] as a Unique Lewis

(97) Tamm, M.; Beer, S.; Herdtweck, E. Trioxorhenium(VII)

Acid in Organic Synthesis. Eur. J. Org. Chem. 2015 , 2015 , 5485 −5499.

(116) Wu, X.; Tamm, M. Transition Metal Complexes Supported by

Highly Basic Imidazolin-2-Iminato and Imidazolin-2-Imine N-Donor

Ligands. Coord. Chem. Rev. 2014, 260, 116−138.

(117) Karmel, I. S. R.; Fridman, N.; Tamm, M.; Eisen, M. S.

Mono(Imidazolin-2-Iminato) Actinide Complexes: Synthesis and

Application in the Catalytic Dimerization of Aldehydes. J. Am.

Chem. Soc. 2014, 136, 17180−17192.

(118) Hamaki, H.; Takeda, N.; Tokitoh, N. Reduction of

Tetravalent Group 4 Metal Complexes Supported by an Extremely

Bulky, Unsymmetrically Substituted β -Diketiminato Ligand Leading

to the Regioselective CN Bond Cleavage Giving Ring-Contracted

Metal-Imido Complexes. Organometallics 2006, 25, 2457−2464.

(119) Shoken, D.; Sharma, M.; Botoshansky, M.; Tamm, M.; Eisen,

M. S. Mono(Imidazolin-2-Iminato) Titanium Complexes for Ethylene

Polymerization at Low Amounts of Methylaluminoxane. J. Am. Chem.

Soc. 2013, 135, 12592−12595.

(120) Khononov, M.; Fridman, N.; Tamm, M.; Eisen, M. S.

Hydroboration of Aldehydes, Ketones and Carbodiimides Promoted

by Mono(Imidazolin-2-Iminato) Hafnium Complexes. Eur. J. Org.

Chem. 2020, 2020, 3153−3160.

Complexes with Imidazolin-2-Iminato Ligands. Z. Naturforsch., B: J.

Chem. Sci. 2004, 59, 1497−1504.

98) Tamm, M.; Randoll, So.; Bannenberg, T.; Herdtweck, E.

Titanium Complexes with Imidazolin-2-Iminato Ligands. Chem.

Commun. 2004, 4, 876−877.

(99) Beer, S.; Hrib, C. G.; Jones, P. G.; Brandhorst, K.; Grunenberg,

J.; Tamm, M. Efficient Room-Temperature Alkyne Metathesis with

Well-Defined Imidazolin-2-Iminato Tungsten Alkylidyne Complexes.

Angew. Chem., Int. Ed. 2007, 46, 8890−8894.

(100) Beer, S.; Brandhorst, K.; Grunenberg, J.; Hrib, C. G.; Jones, P.

G.; Tamm, M. Preparation of Cyclophanes by Room-Temperature

Ring-Closing Alkyne Metathesis with Lmidazolin-2-Iminato Tungsten

Alkylidyne Complexes. Org. Lett. 2008, 10, 981−984.

(101) Sharma, M.; Yameen, H. S.; Tumanskii, B.; Filimon, S. A.;

Tamm, M.; Eisen, M. S. Bis(1,3-Di-Tert-Butylimidazolin-2-Iminato)

Titanium Complexes as Effective Catalysts for the Monodisperse

Polymerization of Propylene. J. Am. Chem. Soc. 2012, 134, 17234−

(

7244.

102) Panda, T. K.; Trambitas, A. G.; Bannenberg, T.; Hrib, C. G.;

Randoll, S.; Jones, P. G.; Tamm, M. Imidazolin-2-Iminato Complexes

of Rare Earth Metals with Very Short Metal-Nitrogen Bonds:

Experimental and Theoretical Studies. Inorg. Chem. 2009, 48,

(121) Sharma, M.; Andrea, T.; Brookes, N. J.; Yates, B. F.; Eisen, M.

S. Organoactinides Promote the Dimerization of Aldehydes: Scope,

Kinetics, Thermodynamics, and Calculation Studies. J. Am. Chem. Soc.

2011, 133, 1341−1356.

(

462−5472.

103) Glockner, A.; Bannenberg, T.; Daniliuc, C. G.; Jones, P. G.;

Tamm, M. From a Cycloheptatrienylzirconium Allyl Complex to a

Cycloheptatrienylzirconium Imidazolin-2-Iminato “Pogo Stick ” Com-

plex with Imido-Type Reactivity. Inorg. Chem. 2012, 51, 4368 −4378.

(122) Stubbert, B. D.; Marks, T. J. Mechanistic Investigation of

Intramolecular Aminoalkene and Aminoalkyne Hydroamination/

Cyclization Catalyzed by Highly Electrophilic, Tetravalent Con-

strained Geometry 4d and 5f Complexes. Evidence for an M-N σ -

Bonded Insertive Pathway. J. Am. Chem. Soc. 2007, 129 , 6149 −6167.

(123) Stubbert, B. D.; Stern, C. L.; Marks, T. J. Synthesis and

Catalytic Characteristics of Novel Constrained-Geometry Organo-

actinide Catalysts. The First Example of Actiniae-Mediated Intra-

molecular Hydroamination. Organometallics 2003, 22, 4836−4838.

(124) Dash, A. K.; Gourevich, I.; Wang, J. Q.; Wang, J.; Kapon, M.;

Eisen, M. S. The Catalytic Effect in Opening an Organoactinide Metal

Coordination Sphere: Regioselective Dimerization of Terminal

104) Imberdis, A.; Lefevre, G.; Thuery, P.; Cantat, T. Metal-Free

̀ ́

and Alkali-Metal-Catalyzed Synthesis of Isoureas from Alcohols and

Carbodiimides. Angew. Chem., Int. Ed. 2018 , 57 , 3084 −3088.

(105) Williams, A.; Ibrahim, I. T. Carbodiimide Chemistry: Recent

Advances. Chem. Rev. 1981, 81, 589−636.

(106) Hu, L.; Lu, C.; Zhao, B.; Yao, Y. Intermolecular Addition of

Alcohols to Carbodiimides Catalyzed by Rare-Earth Metal Amides.

Org. Chem. Front. 2018, 5, 905−908.

(107) Karmel, I. S. R.; Tamm, M.; Eisen, M. S. Actinide-Mediated

Catalytic Addition of E-H Bonds (E = N, P, S) to Carbodiimides,

Isocyanates, and Isothiocyanates. Angew. Chem., Int. Ed. 2015, 54,

Alkynes and Hydrosilylation of Alkynes and Alkenes with PhSiH

Promoted by Me SiCp ″ Th Bu . Organometallics 2001, 20, 5084−

(

2422−12425.

5104.

108) Bhattacharjee, J.; Harinath, A.; Banerjee, I.; Nayek, H. P.;

(125) Lubben, T. V.; Wolczanski, P. T. Dioxygen Activation by

−

Panda, T. K. Highly Active Dinuclear Titanium(IV) Complexes for

Group 4 Tritox Alkyls [Tritox = (Tert-Bu) CO ]: Insertion and

the Catalytic Formation of a Carbon-Heteroatom Bond. Inorg. Chem.

Oxygen Atom Transfer. J. Am. Chem. Soc. 1987 , 109 , 424 −435.

(126) Limbach, H. H.; Hennig, J.; Stulz, J. IR-Spectroscopic Study of

Isotope Effects on the NH/ND-Stretching Bands of Meso-

Tetraphenylporphine and Vibrational Hydrogen Tunneling. J. Chem.

Phys. 1983, 78, 5432−5436.

(127) Gilli, P.; Bertolasi, V.; Ferretti, V.; Gilli, G. Evidence for

Intramolecular N-H ···O Resonance-Assisted Hydrogen Bonding in β -

Enaminones and Related Heterodienes. A Combined Crystal-

Structural, IR and NMR Spectroscopic, and Quantum-Mechanical

Investigation. J. Am. Chem. Soc. 2000, 122, 10405−10417.

(128) Asano, S.; Tada, K. Preparation and Crystal Structure of

Trinuclear Hafnium Oxo Alkoxo Complex. JP 2017-178833 A, 2017.

109) Chakraborty, D.; Rajashekhar, B.; Mandal, M.; Ramkumar, V.

018, 57, 12610−12623.

Group 4 Metal Complexes Containing the Salalen Ligands: Synthesis,

Structural Characterization and Studies on the ROP of Cyclic Esters.

J. Organomet. Chem. 2018, 871, 111−121.

(110) Luo, H. Y.; Consoli, D. F.; Gunther, W. R.; Roman-Leshkov,

Y. Investigation of the Reaction Kinetics of Isolated Lewis Acid Sites

in Beta Zeolites for the Meerwein-Ponndorf-Verley Reduction of

Methyl Levulinate to γ -Valerolactone. J. Catal. 2014, 320, 198−207.

(111) Lapenta, R.; Buonerba, A.; De Nisi, A.; Monari, M.; Grassi, A.;

Milione, S.; Capacchione, C. Stereorigid OSSO-Type Group 4 Metal

Organometallics XXXX, XXX, XXX−XXX

Organometallics

(129) Boyle, T. J.; Ottley, L. A. M.; Hoppe, S. M.; Campana, C. F.

Article

Series of Comparable Dinuclear Group 4 Neo-Pentoxide Precursors

for Production of PH Dependent Group 4 Nanoceramic Morphol-

ogies. Inorg. Chem. 2010, 49, 10798−10808.

(130) Boyle, T. J.; Steele, L. A. M.; Burton, P. D.; Hoppe, S. M.;

Lockhart, C.; Rodriguez, M. A. Synthesis and Structural Character-

ization of a Family of Modified Hafnium Tert -Butoxide for Use as

Precursors to Hafnia Nanoparticles. Inorg. Chem. 2012, 51, 12075−

131) Levens, A.; An, F.; Breugst, M.; Mayr, H.; Lupton, D. W.

2092.

Influence of the N-Substituents on the Nucleophilicity and Lewis

Basicity of N-Heterocyclic Carbenes. Org. Lett. 2016, 18, 3566−3569.

(132) Wang, K. T.; Wang, Y. X.; Wang, B.; Li, Y. G.; Li, Y. S. Novel

Zirconium Complexes with Constrained Cyclic β -Enaminoketonato

Ligands: Improved Catalytic Capability toward Ethylene Polymer-

ization. Dalt. Trans. 2016, 45, 10308−10318.

(

133) Kappa CCD Server Software; Nonius BV: Delft, The

Netherlands, 1997.

134) Otwinowski, Z.; Minor, W. [20] Processing of X-Ray

Diffraction Data Collected in Oscillation Mode. Methods Enzymol.

997, 276, 307−326.

135) Sheldrick, G. M. Crystal Structure Refinement with SHELXL.

Acta Crystallogr., Sect. C: Struct. Chem. 2015, 71, 3−8.