Preparation and characterization of terdentate [C,N,N] acetophenone and acetylpyridine hydrazone platinacycles: A DFT insight into the reaction mechanism

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D. Vazquez-Garcia, J. J. Fernández Sánchez, A. Fernández, M. Lopez-Torres, J. Lado and J. M. VILA,

Dalton Trans., 2017, DOI: 10.1039/C7DT03418K.

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Preparation and characterization of terdentate [C,N,N]

acetophenone and acetylpyridine hydrazone platinacycles. A DFT

insight into the reaction mechanism

Received 00th January 20xx,

Accepted 00th January 20xx

Ismael Marcos,^aVicente Ojea,^aDigna Vázquez-García,^a* Jesús J. Fernández,^aAlberto Fernández,^a

Margarita López-Torres,^aJorge Lado^aand José M. Vila.^b*

DOI: 10.1039/x0xx00000x

www.rsc.org/

Reaction of N-orto-chlorophenyl substituted acetylpyridine hydrazones (a and d) with K₂[PtCl₄] (n-butanol/water, 100 °C)

gave mononuclear complexes 1a and 1d with the ligands as [N,N] bidentate. Contrastingly, reaction of N-phenyl or N-

meta-chlorophenyl hydrazones (b and c, respectively) under analogous reaction conditions gave the cycloplatinated

species 2b and 2c with the ligand as [C,N,N] terdentate. Treatment of the mononuclear complexes 1a and 1d with NaOAc

(n-butanol, 100 °C) gave the corresponding cycloplatinated complexes 2a and 2d. Acetophenone hydrazone platinacycle

2e was prepared in a similar fashion and its reaction with tertiary mono and triphosphines gave mono or trinuclear species

depending on the reaction conditions. The X-ray crystal structures of some of these complexes showed interesting π-π

slipped stacking interactions between metallacyclic rings which, according to NCI analyses, showed aromatic character.

Aimed to the rationalization of the different reactivity showed by acetylpyridine hydrazones and the precise role of

acetate anion, the energy profiles for the three main steps for the cycloplatination (iminoplatinum complex formation,

chelation and cyclometallation) have been determined by DFT (M06) methods. Calculations indicate that cycloplatination

of 1b proceeds via electrophilic substitution, involving the direct replacement of chloride anion at the Pt(II) centre by the

N-phenyl moiety as the rate-determining step, to give an agostic intermediate 5b⁺that, subsequently, undergoes

elimination of a proton as hydrogen chloride. When present as an “external” base, acetate enters the coordination sphere

around the Pt(II) centre and facilitates hydrazone N-H deprotonation and electrophilic C-H activation through a

dissociative route, leading to a Wheland-type σ-complex intermediate 9ac.

platinum(II) cyclometallated complexes bearing tridentate

6

ligands and, particularly, cyclopalladated pyrimidin- and

Introduction

pyridazinhydrazones (examples i, ii in Chart 1) with endo

In preceding decades, the chemistry of cyclometallated

transition metal complexes has attracted much attention, in

geometry (i.e., with the carbon-nitrogen double bond inside

the metallacycle) and acetylpyridine phenylhydrazones (iii in

chart 1) with the exo coordination geometry. In these

complexes the position of the C=N bond seemed to have no

influence in the cyclometallation reaction, showing no

evidence of the so-called “endo effect”.⁷We also studied the

1

part due to their promising applications. The most well-

studied examples are five-membered metallacycles containing

2

palladium,

being platinum complexes less frequently

reported in spite of their interesting properties as, for

example, electroluminescent properties useful in the design of

light emitting devices found in compounds bearing tridentante

ligands. ³The use of platinum complexes as anti-cancer

reactivity of complexes i-iii with different nucleophiles such as

,

tertiary phosphines. ⁸Other palladium(II) examples with

9

4

cyclometallated acetylpyridine phenylhydrazones acting as

,

metallodrugs is also well known (e.g. the activity of

[C,N,N’] ligands ¹⁰or structurally related ligands ¹¹have also

cyclometallated species with ligands structurally related to

hydrazones has been widely studied).⁵

In the past, we have been interested in palladium(II) and

been reported. Examples with other metals ¹²also appear in

the literature but, to the best of our knowledge, no [C,N,N]

Pt(II) analogues are yet known.

The crystal structures of i-iiI and several of their derivatives

showed interesting intermolecular between metallacyclic rings

(i.e., rings involving at least one metal atom).⁸Ghedini et al.

reported one of the scarce examples showing such interactions

^a.Departamento de Química & Centro de Investigaciones Científicas Avanzadas

(CICA), Universidade da Coruña. E-15008 La Coruña, Spain.

^b.Departamento de Química Inorgánica, Universidad de Santiago de Compostela. E-

15782 Santiago de Compostela, Spain.

,

in which the metallacycle is a cyclometallated ring; ¹³these

†

Electronic Supplementary Informaꢀon (ESI) available: Syntheꢀc and

interactions could also be a structural evidence ¹⁴of the so

called “metalloaromaticity”.¹⁵

characterization details, Cartesian coordinates for all the computed molecules in

XYZ format, Additional DFT computational results, Additional crystallographic data

including inter- and intramolecular stacking parameters; CCDC 1536508 and

1536509 . See DOI: 10.1039/x0xx00000x

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R³

R²

R¹

exo

endo

Cl

N

Cl

M

Pd

HN

N

HN

Cl

Me

CF₃

i

ii, M = Pd

iv, M = Pt

iii, M = Pd

v, M = Pt

Scheme 1

R¹, R², R³= H, Cl

endo-Platinacycles. Reaction of N-(2-chlorophenyl) hydrazone

, with K₂[PtCl₄] in n-butanol/water at 100°C gave the non-

cyclometallated complex [Pt{2-Cl-

Chart 1

a

These type of interactions are attracting growing interest.^16,17

For example, inter or intramolecular, interactions ^{18 , 19}are

known to modulate the luminescent behaviour of

cyclometallated complexes and π stacking interactions are also

paramount to explain the action mechanism of metallo-

intercalators, ²⁰one type of antineoplastic drugs.

C₆H₄N(H)N=CMe(C₅H₄N)}(Cl₂)] 1a (See Scheme 1). Other

reaction conditions (K₂[PtCl₄] in ethanol/water at 60°C and

[Pt(η³-allyl)(μ-Cl)]₂in dry toluene at 60°C) also led to the [N,N´]

coordination complex 1a

.

Following the pioneering discovery by Shaw et al. that the

addition of NaOAc accelerated cyclometallation reactions,²¹

coordination complexes [PtCl₂(k²-N_imino,N_amino)], which are

analogous to 1a, were converted into the corresponding

terdentate [C,N,N´] cycloplatinated derivatives by reflux in a

donor solvent in the presence of NaOAc.²²Therefore, we

Thus, in view of the foreseeable applications of

cyclometallated complexes with tridentate hydrazones and the

lack of examples with platinum as central atom we sought to

study the synthesis and reactivity of platinum(II) hydrazone

cyclometallated complexes with the endo (iv) and exo (v)

decided to test whether it was possible to obtain

cyclometallated derivative from the coordination complex 1a

To our satisfaction, the synthesis of the desired

a

disposition of the imino group. The occurrence of inter- or

intramolecular interactions in such complexes was also among

the aims being pursued.

.

cyclometallated

monomer

[Pt{2-Cl-

Consequently, herein we report the reaction of acetophenone

and acetylpyridine hydrazones with [PtCl₄]^2-, to yield Pt(II)

C₆H₃N(H)N=CMe(C₅H₄N)}(Cl)] 2a

,

was accomplished by

treatment of 1a with a stoichiometric amount of NaOAc in n-

butanol at 100°C. Thus, the effect of NaOAc on the C-H

activation mechanism might be thermodynamic, by

neutralizing the HCl formed during the reaction, kinetic by

cyclometallated complexes with the ligand bonded in

a

tridentate [C,N,N’] fashion. Furthermore, since small structural

differences, among other influential factors, seemed to convey

rather large changes in the reactivity of the C‒H activation

lowering the 1a/2a activation barrier or by concurrence of

patterns, we felt that the corresponding mechanism involved

in the process should deserve a more detailed consideration,

whereby we decided to study the feasible reaction pathways

with the help of DFT calculations, which should also allow

control of the factors governing the reactivity.

both. In any case, no oxidative addition reaction of the C-Cl

bond to give a Pt(IV) complex, as has been reported for similar

complexes,²³was observed.

Contrastingly, reaction of the N-phenyl hydrazone

b with

K₂[PtCl₄] in n-butanol/water at 100°C for over 2 hours gave

[Pt{C₆H₄N(H)N=CMe(C₅H₄N)}(Cl)] 2b (see Scheme 2). Reaction

In addition, the reactivity of the acetophenone derivatives iv

was also studied and complexes with the (Ph₂PCH₂CH₂)₂PPh

times under

2

hours gave

a

mixture of 2b and

(triphos) ligand acting as [P,P,P] tridentate or the less common

19

[Pt{C₆H₅N(H)N=CMe(C₅H₄N)}(Cl₂)] 1b, from which the latter

could not be isolated. NaOAc was added to the mixture of 1b

and 2b in n-butanol with the intention of shift the reaction

toward 2b but heating of this solution led to the immediate

formation of black platinum.

ꢀ₃bridging mode

structure analysis for the latter case.

are described inclusive of X‒ray crystal

Where appropriate, the existence of intermolecular and

intramolecular π–π stacking interactions involving metal

chelate rings is discussed.

1

The signals corresponding to 1b could be identified in the H

NMR spectrum of the reaction mixture and showed similar

characteristics to those described for the analogue, 1a (see

Experimental Section and SI). Incidentally, compound 2b could

Results and discussion

also be prepared by reaction of

dry toluene at 60 °C.

b

with [Pt(η³-allyl)(μ-Cl)]₂in

Preparation of the cyclometallated complexes.

For the convenience of the reader the compounds and

reactions are shown in Schemes 1-5. The compounds

described in this paper were characterized by elemental

analysis (C, H, N), by IR spectroscopy, and by H, ³¹P-{¹H} and,

1

¹³C-{¹H} NMR spectroscopy, mass spectrometry and X-ray

single crystal diffraction (data in the Experimental Section). All

the spectroscopic and analytical data were in agreement with

the proposed formulations (see Supplementary Information

(SI) for an extended discussion).

Scheme 2

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These results seem to indicate that the non-cyclometallated [ethanol/water at 60°C or n-butanol/water at 100°C for 24h]

complexes 1a and 1b are formed in first place. However, whilst the cyclometallated complex [Pt{3,4-Me₂-

C-H activation of 1b happens spontaneously with longer C₆H₂C(Me)=NN(H)(4’-Cl-C₄H₂N₂)}(Cl)] 2e with the imine group

reaction times, the 1a/2a activation barrier seems to be higher in the endo position was obtained (see Scheme 4). Non-

and NaOAc is needed to lower this barrier.

cyclometallated complexes analogous to 1a and 1b were not

The different behaviour led us to study the detailed observed.

mechanism and role of acetate in the process by DFT methods

(vide infra). The understanding gained with the computational

model prompted us to further explore the same reaction

process with other ligands characterized by the presence of a

3-chlorophenyl (ligand

(ligand ) at the amino group (see Scheme 3). Under similar

reaction conditions, reaction of with K₂[PtCl₄] in n-

butanol/water at 100°C for 24 hours afforded the

cyclometallated complex [Pt{3-Cl-

C₆H₃N(H)N=CMe(C₅H₄N)}(Cl)], 2c. Thus, metallation of was

c) or a 2,4-dichlorophenyl substituent

d

c

Scheme 4

c

The low solubility of 2e precluded its characterizations in

direct and regioselective at position 6, in the less hindered site

solution, however, the structure of the derivatives 1e and 3e

-

and in para relationship with the chlorine substituent.

5e, vide infra, was in agreement with the structure proposed.

Moreover, recrystallization of 2e from dimethyl sulfoxide gave

crystals of the new complex 2e-dmso with the chloride ligand

replaced by a dimethyl sulfoxide molecule and deprotonation

of the hydrazinic nitrogen atom.

Conversely, reaction of

d with K₂[PtCl₄] in n-butanol/water at

100°C gave [Pt{2,4-Cl-C₆H₃N(H)N=CMe(C₅H₄N)}(Cl₂)], 1d which,

upon treatment with a stoichiometric amount of NaOAc in n-

butanol/water at 100°C for 24h, gave the corresponding

cyclometallated

C₆H₂N(H)N=CMe(C₅H₄N)}(Cl)], 2d, in a similar manner to 2a

Complexes 2c 1d and 2d showed similar spectroscopic

complex

[Pt{2,4-Cl-

Reaction of 2e with triphenylphosphine, gave [Pt{(3,4-Me₂-

C₆H₂C(Me)=NN(H)(4’-Cl-C₄H₂N₂)}(Cl)(PPh₃)], 1e, in which the

phosphine is coordinated to the metal atom after ring opening

(see Scheme 5, Experimental Section and SI for

characterization details). Ring opening could be avoided by

treatment of 2e with silver trifluoromethanesulfonate, prior to

reaction with the phosphine. Under these reaction conditions

[Pt{(3,4-Me₂-C₆H₂C(Me)=NN(H)(4’-Cl-C₄H₂N₂)}(PPh₃)][F₃CSO₃],

3e, was obtained. Compound 3e was also prepared by reaction

of 1e with silver trifluoromethanesulfonate in acetone.

.

,

characteristics to those described for the

(see Experimental Section and SI).

a and b derivatives

Therefore, ligands could be grouped in two categories

according to their behaviour toward C-H activation by

K₂[PtCl₄].

b

and

c

yielded directly the metallated species 2b

did not produce C H activation under

and 2c, but

a

and

d

‒

similar conditions, and treatment of the resulting coordination

compounds 1a and 1d with NaOAc was necessary in order to

metallate the ligands. This seems to indicate that C-H

activation in meta to one or two electron-withdrawing chlorine

substituents may not be accomplished by treatment with

K₂[PtCl₄] in n-butanol/water. However, C-H activation could be

promoted by NaOAc, in the second reaction stage.

Scheme 3

exo-Platinacycles. In order to synthesize Pt(II) complexes

Scheme 5. i) PPh₃(acetone, 1:1 molar ratio), ii) 1: AgCF₃SO₃, 2: PPh₃(acetone, 1:1 molar

containing a tridentate [C,N,N´] hydrazone ligand but showing

ratio), iii) triphos (acetone, 1:1 molar ratio), iv) triphos (acetone, 3:1 molar ratio).

an exo geometry, we decide to study the reactivity of ligand

e

with K₂[PtCl₄]. Regardless the reaction conditions

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Reaction of 2e with

the tertiary triphosphine (see SI, Scheme S4). ³¹Next, the reaction of [PtCl₃]^-with ligand

(Ph₂PCH₂CH₂)₂PPh (triphos), in a 1:1 molar ratio, gave [Pt{(3,4-

Me₂-C₆H₂C(Me)=NN(H)(4’-Cl-C₄H₂N₂)}(Ph₂PCH₂CH₂)₂PPh-

b

takes place by coordination to the phenyl group (see Scheme

6). Going through transition structure b/3b⁻, such coordination

P,P,P][Cl], 4e, as air-stable 1:1 electrolyte, which was fully requires an activation energy of 10.3 kcal/mol and gives rise to

characterized. Treatment of 2e with (Ph₂PCH₂CH₂)₂PPh, in a a η²-(C,C)-bonded ³²intermediate complex 3b⁻which lies 11.9

3:1 molar ratio, gave a green solid, sparingly soluble in most of kcal/mol higher in energy than the separate starting

the common solvents (see Experimental Section and SI for compounds (

characterization details). Fortunately, a small number of energy profile) but results 1.6 kcal/mol lower in energy than

crystals of 5e suitable for X-ray diffraction were grown from a chloride anion and [PtCl₃]^-(see Figures 1 and 3). Subsequently,

b

and [PtCl₄]^2-, selected as reference for the

b

,

chloroform solution of this complex (vide infra)

intermediate 3b⁻rearranges to the mono-coordinated

33,34,

iminoplatinum(II) derivative 4b⁻

,

³⁵which was found 2.1

Computational study of the cyclometallation of acetylpyridine

hydrazones promoted by [PtCl₄]^2-

kcal/mol above the starting reagents. Going through transition

structure 3/4b⁻, this rearrangement can proceed in one step

Due to its industrial relevance and synthetic applications, with an activation energy of 12.2 kcal/mol. Competitive

mechanistic aspects of C-H activation by metal complexes have pathways to intermediate 4b⁻, involving other dissociative

inspired

a

number of experimental and theoretical routes, nucleophilic substitutions at [PtCl₄]^2-or solvated [PtCl₃]^-

24

investigations,

including studies on methane activation (via five-coordinate transition states) ³⁶or direct activation of

reactions mediated by [PtCl₂(H₂O)₂], [PtCl₂(NH₃)₂], [Pt(bpym) the C-H bond were found higher in energy (see SI, Schemes S5-

Cl₂] (bpym = 2,2’-bipyrimidine) or [Pt(HPzm)₂Cl₄] (HPzm = S8).

25

pyrazolium) complexes (Shilov system)

and arene C-H

activation ²⁶or roll-over cyclometallations ²⁷promoted by Pt(II)

complexes with chelating ligands bearing nitrogen donor

atoms. Nevertheless, computational studies on the

cycloplatination reaction mechanism are scarce, and are

mainly related to electron-rich platinum(II) complexes which

cyclometallate with liberation of methane. For these

processes, multistep mechanisms have been proposed,

involving ligand dissociation to a coordinatively unsaturated

intermediate which subsequently undergoes an intramolecular

28

oxidative-addition/reductive elimination sequence. In this

manner, although [PtCl₄]^2-and [PtCl₂(solv)] are common metal

2-

Scheme 6. Reaction of ligand a,b with PtCl₄to form iminoplatinum complex 4a,b⁻.

5a,

precursors in cycloplatination reactions, ²⁹the energy profile

Relative free energies (kcal/mol) calculated in butanol solution at 373 K are shown in

brackets.

for arene cyclometallation reactions promoted by [PtCl₄]^2-has

not been previously computed.³⁰In an effort to gain useful

insight and understanding of the factors accounting for the

differential behavior of acetylpyridine hydrazones a-d in

cyclometallations promoted by [PtCl₄]^2-, we sought out to

study the possible reaction pathways by using M06 DFT

methods in conjunction with the polarizable continuum

solvation model (see Computational Methods in the SI).

From mono-coordinate intermediate 4b⁻two possible reaction

pathways have been computed: the first one, the

cyclometallation at the phenyl ring, involves a considerably

energy penalty (30.7 kcal/mol or higher, see SI, Scheme S9).

Instead, chloride substitution at Pt(II) by the pyridine nitrogen,

via the five-coordinate transition structure 4/1b⁻, showed a

lower activation energy (19.1 kcal/mol, see Scheme 7) and led

to chelate complex 1b, with the acetylpyridine hydrazone

acting as [N,N] bidentate ligand.

Cycloplatination of ligand b. Firstly, we carried out a study on

the reaction between ligand

aforementioned conditions,

b

and [PtCl₄]^2-in the

considering

associative,

dissociative and concerted (interchange) mechanisms for the

chloride substitution on Pt(II) centre, with either pyrido, imino,

amino or benzene moieties acting as nucleophiles. For the C-H

bond cleavage event we considered the electrophilic

substitution (via Wheland-type intermediate), the

σ-bond

metathesis and the oxidative addition pathways. For

a

complete representation of all the mechanistic possibilities

that have been computed see Schemes S3A,B in the SI.

Scheme 7. Formation of intermediate 1a,b by chelation of iminoplatinum complex

4a,b⁻. Relative free energies (kcal/mol) calculated in butanol solution at 373 K are

shown in brackets.

According to DFT calculations, reaction initiates with

dissociation of [PtCl₄]^2-to yield chloride anion and [PtCl₃]^-, in a

solvent assisted process, through

a Pt(II) square-planar

intermediate. Such dissociation was unfavorable by 13.5

kcal/mol, and required an activation energy of 30.1 kcal/mol

Chelate intermediate 1b was characterized by an

intramolecular hydrogen bond between the hydrazone

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hydrogen and a chloride ligand, and was calculated as the have been implicated in other theoretical works on C-H

most stable species in the reaction pathway, lying 10.6 activation catalyzed by Pt(II) complexes with chelating N-based

kcal/mol below the starting compounds. ³⁷^,Thus, according to ligands, ⁴¹but this is, to the best of our knowledge, the first

DFT calculations, cycloplatination of ligand

b

with [PtCl₄]^2-time a trans relationship between the incoming C-H bond and

takes place by initial iminoplatinum complex formation the ligand being displaced from the metal centre is reported.

followed by chelation. These two steps were calculated as Other alternative cyclometallation pathways have also been

exergonic and did not determine the rate of the taken into account but required higher energy barriers (see SI,

cycloplatination process (vide infra).

Schemes S10-S15).

Lowest energy pathway for the cyclometallation of chelate

complex 1b followed an electrophilic substitution process

involving the cationic intermediate 5b⁺, which was calculated

14.7 kcal/mol above starting compounds (see Scheme 8).

Scheme 8. Formation of agostic intermediate 5a,b⁺from the chelate 1a,b, by direct

replacement of the chloride anion at Pt(II) by the N-phenyl moiety. Relative free

energies (kcal/mol) calculated in butanol solution at 373 K are shown in brackets.

The associative route provided the most favoured transition

structures for the electrophilic activation, by direct

nucleophilic displacement of a chloride ligand from the Pt(II)

centre by the N-phenyl group. Such transition structures, cis

-

1/5b and trans-1/5b, and the resulting ion-pair intermediates

cis-5b and trans-5b, were characterized by either a cis or a

trans relationship between the activated C-H bond and the

leaving chloride. The ion-pairs (cis-5b and trans-5b) and the

cationic intermediate (5b⁺) exhibit a close proximity of the C-H

bond to the metal, revealed by a short Pt-H contact (1.96-2.04

Å), an elongated C-H distance (1.13-1.17 Å) and a wide Pt-H-C

angle (79.0-86.0°), which are indicative of an agostic

interaction (see Figure 1). ³⁸Such agostic interaction seems to

Figure 1. Optimized geometries of intermediates and transition structures in the lowest

energy route to 2b. Bond lengths and interaction distances are in Angstroms.

The preference for transition structure trans-1/5b over the cis

-

be incipient in the transition structures cis-1/5b and trans

-

1/5b counterpart may be originated by an incipient non-

1/5b and may contribute to reduce the energy barrier for the

cycloplatination. ³⁹Moreover, in transition structure cis-1/5b

the agostic interaction is sufficient to polarize the C-H bond

and allow the leaving chlorine atom to establish an

intramolecular hydrogen-bond like contact that becomes fully

covalent interaction between the leaving chloride ligand and

the

π-system of the N-phenyl hydrazone moiety, which is

becoming acidic in the associative transition structures.

Accordingly, in trans-1/5b the distances between the leaving

chloride and N1, N2, N3, C1, C5, C7 and C12 of the N-phenyl

developed at the resulting ion-pair intermediate cis-5b

.

hydrazone moiety (3.43

-

4.47 Å) are significantly shorter than

4.73 Å, see SI, Table

Nevertheless, calculations indicate that transition structure

trans-1/5b, which required an activation energy of 32.8

kcal/mol from the chelate intermediate, ⁴⁰was favoured over

cis-1/5b by 2.1 kcal/mol. Similar associative transition states

the corresponding values in cis-1/5b (3.56

-

S2). Description of the charge-transfer interactions in

transition structures cis-1/5b and trans-1/5b was obtained by

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+3Cl

[17.5]

[14.7]

the second-order perturbation theory analyses of Fock matrix

in the natural bond orbital (NBO) basis ⁴²(see SI, Table S2).

To obtain further insight into these non-covalent interactions

(NCI) the NCI index was computed for trans-1/5b.

⁴³Simple

5a⁺

5b⁺

+2Cl

Cl

HCl+2Cl

+2Cl

visual inspection of the NCI isosurfaces calculated for trans-

1/5b (see Figure 2) reveals red cigars for repulsive steric

clashes inside the rings, a deep blue almond-shaped surface

for the incipient stabilizing agostic interaction with a small red

line inside (due to steric strain of the forming 3-membered

ring), a light blue pill-like surface for the breaking Pt-Cl bond

(weaker than regular stabilizing interaction) and green flat

isosurfaces for the van der Waals interactions. In particular, a

light blue region in the greenish extended form is clearly

identified next to the leaving chloride atom, revealing an

attractive dispersion interaction with the C1 atom at the N-

Cl

H

R

Cl

Pt

N

Pt

N

H

N

H

N

H

N

5/2a

5a

cis- [18.5]

cis- [16.6]

[18.6]

5/2b [17.1]

2a [-1.6]

2b

5b

[-1.8]

Scheme 9. Elimination of HCl from agostic intermediate 5a,b⁺to give the

cycloplatinated complex 2a,b. Relative free energies (kcal/mol) calculated in butanol

solution at 373 K are shown in brackets.

phenyl moiety (corresponding to an attractive peak at

0.006 a.u. in the s(p) diagram, see SI, Figure S1).

ρ =

−

Cycloplatination of ligands a, c and d. With comparative

purposes, the cyclometallation of ligand a, with the chlorine

substituent at the phenyl ring, has also been computed (see

Schemes 6-9). In solution, lowest energy route to

cyclometallated complex 2a also involved initial formation of

an iminoplatinum complex 4a, subsequent chelation to 1a and

then cycloplatination by an electrophilic substitution, with

direct displacement of a chloride atom at the Pt(II) centre,

going through trans-1/5a in the rate-determining step to the

agostic ion-pair intermediate trans-5a. Even though the energy

(a)

(b)

profiles in solution are similar for series

a and b (see Figure 3),

Figure 2. NCI analysis of transition structure trans-1/5b. Gradient isosurfaces are

colored on a BGR scale, according to the values of sign(λ₂blue for attractive

interaction, green for weak interaction and red for repulsive steric clashes. Region of

weakly non-bonding interactions between leaving chloride and N-phenylhydrazone

moiety highlighted in green. (a) NCI isosurface with s = 0.3, color scale in the −0.07 <

some quantitative differences were found. Firstly, the

activation energies for electrophilic attack of [PtCl₃]^-to ligands

)ρ:

a

and b )

(going through transition structures a/3a⁻and b/3b⁻

are 24.2 and 23.8 kcal/mol, respectively. Secondly, the relative

energies computed for chelate intermediates 1a and 1b are

very similar, but transition structure trans-1/5a and

intermediate trans-5a are calculated higher in energy than

trans-1/5b and trans-5b by 1.4 and 2.5 kcal/mol, respectively.

Thus, it seems that the presence of the electron-withdrawing

chlorine substituent reduces the nucleophilic character of the

N-phenyl group in the ligand and the chelate intermediate and

also destabilizes the agostic intermediate. ⁴⁶Thirdly, the

presence of the chlorine also reduces the speed of the final

elimination step. In this manner, the activation energy for

deprotonation of intermediate cis-5a (leading to transition

structure 5/2a) was calculated 1.5 kcal/mol higher than that

sign(λ₂

0.008, color scale in the shorter −0.011 < sign(λ₂

differences between attractive and repulsive dispersion interactions.

)

ρ

< 0.05 au range. (b) NCI isosurface with s = 0.5 with a density cut-off of

ρ =

)

ρ

< 0.011 au range, to highlight the

Finally, a chloride anion acts as a base and intermediate 5b⁺

losses the C-H hydrogen and yields the cyclometallated

product 2b and hydrogen chloride (see Scheme 9). Although

the association of chloride to the activated C-H bond in 5b⁺to

form the intermediate ion-pair complex cis-5b was endergonic

in 1.9 kcal/mol, most stable transition structure for the

elimination step (5/2b) was located only 2.4 kcal/mol above

the cationic precursor 5b⁺

.

Calculations indicate that cyclometallated complex 2b is 8.8

kcal/mol higher in energy than the coordination intermediate

1b, assuming that all the reaction products are isolated within

the solvent reaction field and hydrogen chloride is not ionized

(see Schemes 8 and 9). Thus, considering the microscopic

reversibility of the process, protonolysis of the C-Pt bond in 2b

would be significantly more rapid than the C–H activation step.

Therefore, the formation of the cyclometallated complex with

good yield could be only explained if the hydrogen chloride

formed as side-product is ionized and partially neutralized by

required to reach transition structure 5/2b

Considering that ligands and may probably follow in their

cycloplatination processes the same route as ligands and b,

.

c

d

a

we have also located the most favoured structures for the

chelated intermediates 1c and 1d, the agostic intermediates

5c⁺and 5d⁺and the transition structures connecting them

through

a

direct electrophilic substitution process. As

and , transition structures

previously found for the series

a

b

trans-1/5c and trans-1/5d, with a trans relationship between

the leaving chloride and the incoming C-H bond, were

calculated lower in energy than the cis counterparts (see SI,

Schemes S16A,B). In agreement with the experimental trend,

the activation energies calculated for the cyclometallation of

,

45

the solvent mixture. ⁴⁴

ligands

b

and

c

by direct electrophilic substitution (going

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through trans-1/5b and trans-1/5c, respectively) resulted very carboxylate-assisted C-H activations at platinum centres.⁴⁹

similar (33.0±0.2 kcal/mol) and lower than those calculated for Sasaki et al. reported the key features in the C-H bond

the corresponding cyclometallations of ligands

a

and

d

(34.1 activation of bencene by [Pt(κ²-O₂CH)₂].⁵⁰The process involves

and 35.8 kcal/mol, respectively) which were not initial κ²

-

κ¹displacement of formate ligand to give a

σ-

experimentally observed in 1-butanol at 373 K.

complex intermediate, followed by heterolytic cleavage of the

C-H bond through a six-membered transition structure, where

the dangling oxygen of the monodentate acetate ligand

abstracts the proton (AMLA-6). A similar two-step mechanism

was reported by Goddard, Periana et al. for benzene activation

at

[Pt(bpym)TFA₂]

and

[Pt(pic)TFA₂]⁻

(pic

=

κ²-N,O-

picolinate

),

^26bwhere the trifluoroacetate ligand facilitated the

activation event.⁵¹

Thus, for the computational study of the cycloplatination of

chelated intermediate 1a in the presence of NaOAc, in addition

to electrophilic substitution and oxidative addition pathways,

we also considered concerted metallation and proton

abstraction (ambiphilic activation) to acetate, which could

either coordinate previously to the Pt(II) centre displacing

chloride (intramolecular assisted path) or not (intermolecular

assisted path). Computations indicate that cyclometallation of

chelate 1a in the presence of acetate anion initiates with a

hydrogen bond interaction between acetate and the

hydrazone hydrogen leading to complex 5ac⁻, which lies 4.3

kcal/mol above the separate precursors (see Scheme 10).

Next, intermediate 5ac⁻undergoes internal displacement of

chloride by acetate at the Pt(II) centre, to afford the complex

6ac in an exergonic process.³⁶The energy penalty for this

ligand exchange via transition structure 5/6ac⁻was 22.2

kcal/mol, and the chelate intermediate 6ac was favoured by

1.2 kcal/mol over the separate precursors, acetate and chelate

1a. Formation of intermediate 6ac by direct displacement of

chloride by acetate on chelate 1a was calculated higher in

energy (see SI, Scheme S17). In this manner, computational

results are consistent with previous experimental findings for

related cyclometallations with platinum group precursors:

when present in the reaction medium, acetate enters the

coordination sphere around the Pt(II) centre in the chelated

intermediate 1a and gives rise to an intermediate acetate

Figure 3. Schematic energy profiles for the reaction of acetylpyridine hydrazones a,b

with [PtCl₄]^2-in butanol at 373K.

Cycloplatination of 1a in the presence of acetate anion. Next

we explored the possible reaction pathways for the

cycloplatination of chelate intermediate 1a in the presence of

acetate anion (denoted as series ac in the Schemes 10-12 and

Figures 3 and 4).

Acetate anion has been used as an external base to assist

cyclometallations on coordination complexes [PtCl₂(κ²

-

N_imino,N_amino)] which are analogous to 1a.

²²In particular, for the

cycloplatination of aryl imines promoted by [cis-Pt(dmso)₂Cl₂]

and NaOAc in MeOH, kinetic studies at variable temperature

and pressure indicated the formation of an acetate-substituted

intermediate complex [PtCl(OAc)(κ²-N_imino,N_amino)], that could

be only detected by NMR spectroscopy.^22cThe kinetic and

activation parameters were interpreted in terms of a C-H

activation event via electrophilic substitution, through a highly

ordered and compressed transition state, involving the C-H

and the Pt-O(acetate) bonds. Such four-centered transition

complex 6ac

.

state should be very similar to that proposed for equivalent

47

cyclopalladations.

Cycloplatinations

of

N,N´,N´´-

triarylguanidines with cis-[Pt(dmso)₂Cl₂] and NaOAc in MeOH

have also been reported to produce intermediate acetato

complexes [PtCl(dmso)(OAc)(κ¹-N_imine)], that could be isolated

and crystallographically characterized.^35bFrom these acetate-

bound Pt(II) complexes, by analogy with cyclopalladations on

related systems,^24c,⁴⁸the metallation event was presumed to

follow either an ambiphilic metal-ligand activation (AMLA) or a Scheme 10. Entrance of acetate in the coordination sphere of Pt(II). Relative free

energies (kcal/mol) calculated in butanol solution at 373 K are shown in brackets.

solvent-assisted concerted metalation-deprotonation (CMD)

pathway, involving six-membered transition states. The

Once coordinated to the Pt(II) centre, acetate acts as an

internal proton-abstracting base and facilitates the

cyclometallation by an electrophilic substitution involving the

mechanistic proposals for these cycloplationations were not

supported by computational methods. In fact, until now, only

two theoretical investigations have been devoted to the

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neutral Wheland-type σ-complex intermediate 9ac, which lies Pt(II) centre may account for the inversion of the prevalent

only 2.7 kcal/mol above starting compounds plus acetate (see reaction sequence, so that the acetate-mediated

Scheme 11).⁵²

deprotonation (of the N-H bond) and departure of the acetic

acid ligand takes place previously to the C-H activation event.

The zwitterionic intermediate 8ac shows the carbonyl group of

the acetic acid moiety pointing to the metal, with the carbonyl

oxygen placed 3.51 Å away from the Pt(II) centre (see Figure

4), which may be indicative of a non-covalent interaction.⁵⁶

The hydrogen bond interaction between the acetic acid and

the hydrazonate nitrogen, which places the acetic acid moiety

in the second coordination sphere of the Pt(II) centre, was also

present in the subsequent associative transition structure

+3Cl

O

H

O

Cl

+3Cl

Cl

N

Pt

N

6ac [-11.7]

N

7ac [7.7]

6/7ac [8.4]

+3Cl

8/9ac and the resulting

σ-complex intermediate 9ac, with O-

+3Cl

…

H N distances close to 1.8 Å (see Figure 4). Most stable

transition structure 8/9ac for the nucleophilic attack of the

phenyl group to the coordinatively unsaturated Pt(II) centre

was located early in the bond making process, only 0.4

O

H

O

H

Cl

O

Cl

Pt

N

Cl

N

Pt

AcOH

Cl

N

Pt

N

kcal/mol above precursor 8ac

.

8ac [12.4]

8/9ac [12.8]

7/8ac [15.6]

+3Cl

O

N

1.87

1.65

H

2.63

1.72

H

O

Cl

AcOH +3Cl

2a [-21.4]

Cl

H

Cl

1.72

Pt

N

Pt

N

AcOH

N

6ac

5/6ac

5ac

9ac [2.7]

9/2ac [15.3]

Scheme 11. Cyclometalation of the acetate bound chelate intermediate 6ac through

the coordinatively unsaturated complex 8ac and the Wheland intermediate 9ac.

Relative free energies (kcal/mol) calculated in butanol solution at 373 K are shown in

brackets.

1.86

2.45

2.82

Calculations indicate that the dissociative pathway, leading to

the 14-electron, T-shaped intermediate 8ac is favoured over

the associative ones by more than 5.3 kcal/mol (see SI,

Schemes S18A,B).⁵³In this manner, from the acetate bound

Pt(II) complex 6ac, an intramolecular proton transfer from the

hydrazone to the carbonyl oxygen, going through the six-

membered transition structure 6/7ac, requires an activation

energy of 20.1 kcal/mol and affords the acetic acid bound

intermediate 7ac. Next, the lowest energy channel to

zwitterionic intermediate 8ac involves the seven-membered

transition structure 7/8ac, with an activation energy of only

7.9 kcal/mol, where the acetic acid ligand dissociates from the

Pt(II) centre but maintains a hydrogen bond interaction with

the hydrazonate nitrogen. An intramolecular proton transfer

from an N-H group to acetate has been previously

characterized by DFT methods for Pd(OAc)₂catalyzed C-H bond

6/7ac

7ac

7/8ac

3.67

2.10

2.97

1.39

1.79

1.34

1.84

1.40

2.44

1.78

3.51

9ac

8ac

8/9ac

2.04

1.35

1.32

1.67

1.02

9/2ac

54

functionalization but has no precedents in the platinum

Figure 4. Optimized geometries of intermediates and transition structures for the

cycloplatination of chelate 1a in the presence of acetate anion. Bond lengths and

interaction distances are in Angstroms

group metal-mediated cyclometallations. It appears that the

proton transfer increases the leaving group ability of the

acetate ligand as well as the electron-donating character of the

bidentate hydrazone ligand, and thus, opens a dissociative

pathway of lower activation energy for the electrophilic

activation for the resulting acetic acid/hydrazonate bound

Pt(II) complex. 7^c,⁵⁵It can be concluded that the presence of an

acidic N-H bond in the neighbourhood of the acetate bound

Transition structure 8/9ac showed a wide C-Pt bond forming

distance (2.97 Å, without any incipient Pt-H contact) and a cis

relationship between the hydrogen-bound acetic acid moiety

and the activated C-H bond. Alternative cyclometallation

pathways involving other dissociative routes for electrophilic

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substitution or oxidative C-H addition followed by reductive a sufficient extension to enable accumulation and isolation of

elimination have also been taken into account but required the intermediate

.

higher energy barriers (see SI, Schemes S19-S20). Despite (4) In the presence of NaOAc as an auxiliary base, the

repeated attempts, transition structures for the routes cyclometallation of 1a to 2a and acetic acid as side-product

involving concerted metalation and proton abstraction to becomes

acetate (CMD) were not located. present in the reaction medium, acetate enters the

-complex intermediate 9ac was assigned coordination sphere around the Pt(II) centre in the rate-

a thermodynamically favoured process. When

57

Wheland-type

σ

arenonium ion (cyclohexadienyl cation) character due to its determining step and facilitates subsequent hydrazone N-H

shortened C-N(amine) bond length (1.34 Å), small but deprotonation and electrophilic C-H activation following a

noticeable puckering of the phenyl moiety (C(1)C(2)C(3)C(4) dissociative route of lower activation energy. Thus, initial

dihedral angle of 10.9°), short Pt-C bond length and weak Pt-H dissociation of acetic acid ligand leading to a coordinatively

contact (2.10 and 2.26 Å, respectively), probably consequence unsaturated Pt(II) complex is followed by association of the

of the strong donating ability of the hydrazonate moiety.⁵⁸phenyl group, formation of

a Wheland-type σ-complex

Wheland-type intermediate 9ac was also characterized by the intermediate and final aromatization.

close proximity of the C-H bond to the carbonyl oxygen of the

Crystal structures of 2e-dmso and 5e.

acetic acid moiety, which may be indicative of a second

…

hydrogen bond-like interaction (H O distance of 2.44 Å and C- The molecular structures of 2e-dmso and 5e are illustrated in

…

H O angle of 135.0 °). Hence, aromatization of Wheland-type Figures 5 and 9, respectively. Crystal data, and selected bond

-complex intermediate 9ac and reprotonation of the distances and angles (Table S6) are given in the SI. Complete

σ

hydrazonate nitrogen can take place in one step, going intramolecular π–π stacking parameters are also in the SI

through the eight-membered transition structure 9/2ac, with (Table S3).

an energy penalty of only 12.6 kcal/mol. In transition structure The asymmetric unit of 2e-dmso contains one-half molecule of

9/2ac the hydrogen bound acetic acid molecule donates a the complex with all its non-hydrogen atoms, except the dmso

proton to the hydrazonate nitrogen and simultaneously methyl, lying in the (0 4 0) crystallographic mirror plane. In the

accepts the proton from the activated C-H bond. Finally, the molecule the platinum atom is bonded in a slightly distorted

cyclometallation of 1a to 2a in the presence of 1 equivalent of square-planar geometry to the C(1) carbon atom of the phenyl

acetate as auxiliary base becomes exergonic in 10.9 kcal/mol.

ring, the N(1) and the pyridazine N(3) nitrogen atoms, and to

Accordingly with the computational results we can conclude the S(1) sulphur atom of the coordinated dmso ligand.

the following:

(1) Cyclometallation of acetylpyridine N-phenylhydrazones a,b

promoted [PtCl₄]^2-take place in three consecutive processes,

involving initial iminoplatinum complex formation, followed by

chelation and then final cycloplatination. While formation of

the intermediate chelate complexes 1a and 1b is

thermodynamically favoured, cycloplatinations to 2a and 2b

are calculated as endergonic processes. Hence, isolation of

cyclometallated complex 2b may be explained if the hydrogen

chloride formed as side-product is ionized and partially

neutralized by the solvent mixture.

(2) With difference to related cycloplatinations which take

place with liberation of methane,²⁸lowest energy pathway for

the cyclometallation of chelate complex 1b in butanol at 373 K

involves an electrophilic substitution process. In this manner,

initial nucleophilic displacement of chlorine from the Pt(II)

centre by the phenyl group in the rate-determining step leads

to an intermediate agostic complex that subsequently losses

HCl in the final aromatization step. Most favoured associative

transition structure located for the arene activation event

showed an unprecedented trans relationship between the

incoming C-H bond and the ligand being displaced from the

metal centre.

Figure 5. Molecular structure of [Pt{3,4-Me₂-C₆H₂C(Me)=NN(4’-Cl-C₄H₂N₂)}((CH₃)₂S=O)],

2e-dmso. Hydrogen atoms have been omitted for clarity.

Bond distances and angles about the platinum atom are within

the expected values. ⁵⁹

π-π interactions, between the parallel pyridazine groups and

the cyclometallated rings (separation distance of the ring

centroids is 3.607(3) Å) were found.

The metal chelate rings [Pt(1), N(1), N(2), C(8), N(3)] of

neighbouring molecules were also parallel and within the

distance expected for a π-π “slipped stacking” interaction

(separation between ring centroids 3.599(3) Å) (see Figure 6).

The fact that we have previously observed similar interactions

in an analogous complex with Pd(II) instead of Pt(II) as central

(3) Isolation of the non-cyclometallated complexes 1a (or 1d

)

in the same reaction conditions can be understood as a

consequence of an increased activation energy for the

nucleophilic attack of the N-phenyl moiety to the Pt(II) centre,

due to the presence of the electron withdrawing chlorine

substituent(s), that may reduce the rate of cyclometallation in

atom

8 (see SI) is indicative that the intermolecular

interactions do not depend on the metal atom.

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Figure S4). In addition, intermolecular σ−σ dispersions

interactions between methyl groups of hydrazone moiety and

dmso (C-H14^…H15-C, deep blue color) and hydrogen bond-like

interactions between amine nitrogen and methyl group of

dmso and between Cl and methyl group at hydrazone (C-

H15^…N2 and C-H14^…Cl) are clearly visualized in the NCI

analysis, and may contribute to strengthen the stacking

interaction in the crystal structure.

(a)

(b)

Figure 6. (a) view nearly perpendicular to b axis showing intermolecular π-π stacking

interactions

between

molecules

of

[Pt{3,4-(Me)₂C₆H₂C(Me)=NN(4’-

ClC₄H₂N₂)}[(CH₃)₂S=O]], 2e-dmso, dashed lines link the centroids of the rings involved in

each stacking interaction. (b) View along

stacking.

b axis showing the antiparallel slipped

The molecules are stacked in an anti-parallel disposition that

could be explained by de tendency of strongly polarized rings

to adopt such disposition in order to maximize the dipole-

dipole interactions. ⁶⁰

(d)

(c)

In order to evaluate the energy of stacking interaction

between the molecules in the crystal we decided to perform

17

DFT calculations according to the method used by D. Zarić,

using the TPSS-D3 functional with the def2-TZVP basis set and

the counterpoise correction to eliminate the basis set

superposition error. In particular, we performed the

calculations over two stacked molecules of 2e-dmso taken

from the crystal structure without geometry optimization. At

TPSS-D3/def2-TZVP level of theory, the counterpoise-corrected

interaction energies calculated for 2e-dmso and the palladium

analogous were -25.05 and -22.94 kcal/mol, respectively. To

gain further insight into the nature and origin of the staking

interactions, NCI index was computed for the crystal structure

Figure 7. NCI analysis for the monomer and the dimer formed by two parallel

molecules taken from crystal structure of 2e-dmso. Gradient isosurfaces are colored on

a BGR scale, according to the values of sign(λ₂)ρ: blue for attractive interaction, green

for weak interaction and red for repulsive steric clashes. Region of weak hydrogen

bond-like interactions highlighted in cyan, region of σ−σ dispersion interactions

highlighted in magenta. (a) NCI isosurfaces for an isolated molecule with s = 0.3, color

scale in the −0.07 < sign(λ₂

0.3, color scale in the −0.03 < sign(λ₂

isosurfaces with s = 0.3, color scale in the shorter −0.01 < sign(λ₂

)

ρ

< 0.05 au range. (b) NCI isosurfaces in the dimer with s =

< 0.03 au range. (c and d) Intermolecular NCI

< 0.01 au range, to

)

ρ

of 2e-dmso

.

)

ρ

Thus, NCI isosurfaces were obtained from promolecular

densities calculated for an isolated molecule and a dimer

formed by two stacked molecules within the crystal.^43bFigure

7(a) displays the computed NCI isosurfaces for the monomer,

revealing repulsive interactions at the centre of the rings (red

surfaces) and for the steric clashes of the methyl groups at the

ligand (flat green-orange surfaces) as well as weak hydrogen

bond-like attractive interactions involving the oxygen and the

methyl groups of the dmso molecule (flat blue-green surfaces,

for C-H2^…O1 and C-H15^…N4 contacts).^43cFigure 7(b) displays

the NCI analysis computed for the dimer, which also includes a

green sheet-like extended isosurface between the overlapping

regions of the molecules, corresponding to the staking

interactions. Figures 7c and 7d only display the intermolecular

interactions, with a shorter color scale to differentiate the

attractive (bluish) from the repulsive (reddish) ones. The NCI

analysis reveal the dispersion interactions between the Pt

centre and C8-C9 moiety (at the pyridazine ring) and between

N4-C10-C11 and C1-C6-C7 moieties (at pyridazine and

cyclometallated rings, respectively) as the main attractive

contributions to the delocalized π−π stacking interactions. The

NCI peaks associated with these dispersion interactions appear

highlight the attractive and repulsive dispersion interactions (b: view along the b axis,

d: view near perpendicular to b axis).

Extended intramolecular isosurfaces as the one depicted in

Figure 7, are characteristic of π-π stacked polycyclic aromatic

systems such as naphthalene or anthracene.^43bIn our case

(Figure 7) the surface included the two metal chelate rings

which could be indicative of the aromatic nature of such rings.

Further evidence of metalloaromaticity was also supported on

structural parameters according to Ghedini et al ⁶¹(see SI

Figures S5 and S6).

In the molecular cation of 5e each platinum atom is bonded, in

a slightly distorted square-planar geometry to four atoms: one

carbon atom of the phenyl ring and two nitrogen atoms, one

from the C=N group and another from the heterocyclic ring.

The fourth coordination position is occupied by one

phosphorus atom from the (Ph₂PCH₂CH₂)₂PPh (triphos) ligand

which is acting as a bridging ligand binding three [Pt(C,N,N’)]

moieties. All bond distances and angles are within the

expected values.¹⁹

at

ρ values of -0.008 and -0.006 a.u., respectively (see SI,

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chloro substituted one (

c

) afforded directly cyclometallated

complexes (2b and 2c

, respectively). Contrastingly, the

reaction of the 2-chloro substituted N-phenyl hydrazone (

a) or

the 2,4-dichloro substituted one (d) under similar reaction

conditions gave the non-cyclometallated complexes (1a and

1d, respectively) regardless the reaction time. DFT studies

have shown that cyclometallation takes place in three

consecutive processes, involving initial iminoplatinum complex

formation, followed by chelation and then final

cycloplatination. With difference to related cycloplatinations

which take place with liberation of methane,²⁸lowest energy

pathways for the cyclometallation of chelate complexes 1a,b

involves an electrophilic substitution pathway. DFT data

indicate that formation of the intermediate chelate complexes

1a and 1b is thermodynamically favoured whilst

cycloplatinations to 2a and 2b are calculated as endergonic

processes. Hence, isolation of cyclometallated complex 2b may

only be explained if the hydrogen chloride formed as side-

Figure 8. Molecular structure of the cation of 5e. Hydrogen atoms and phosphine

phenyl rings have been omitted for clarity reasons. Ellipsoids drawn at 40% of

probability.

The cyclometallated moieties bonded to Pt(2) and Pt(3) adopt

an almost anti-parallel disposition with a distance between the

platinum atoms of 3.392(1) Å. This arrangement is stabilized by

π-π stacking interactions between the metallated phenyls and

the heterocyclic rings (distance between the ring centroids of

approximately 3.4 Å) displaying the so-called “slipped

stacking”. On the other hand, the centroid distances between

the cyclometallated rings and the coordination rings are within

the values expected for π-π stacked rings [3.264(8), and

3.271(9) Å]. This observation supports the existence of some

degree of “metalloaromaticity” due to active charge

delocalization (vide supra).

product is ionized and partially neutralized by the solvent

44,45

mixture.

However, under the same reaction conditions 2a

could not be prepared even though stabilization of the

released HCl is also reasonable. Thus the reluctance of 1a to

experiment C-H activation may be explained in kinetic terms as

a consequence of an increased activation energy for the

nucleophilic attack of the N-phenyl moiety to the Pt(II) centre

due to the presence of the electron withdrawing chlorine

substituent. This increase in activation energy reduces the rate

of cyclometallation in

a sufficient extension to enable

accumulation and isolation of the intermediate. The

cyclometallation of 1a to 2a may be accomplished upon

addition of NaOAc. The role of the acetate is dual; neutralizes

the acidic media stabilizing the final products and enters the

coordination sphere around the Pt(II) centre in the rate-

determining step and facilitates subsequent hydrazone N-H

deprotonation and electrophilic C-H activation.

Similar “slipped stacking”π-π stacking interactions have also

been found between the pyridazine rings bonded to Pt(1) and

Pt(3), with a distance between the ring centroids of 3.503(9) Å

(see Figure 9).

On the other hand, reaction of ligand

e derived from

cloropyridazinhydrazine with K₂[PtCl₄] in n-butanol/water at

100°C gave directly the cyclometallated complex 2e with the

imine group in the endo position. 2e reacts with tertiary mono

or triphosphines with or without opening of the coordination

ring, depending on the denticity of the phosphine and the

molar ration used. In dmso solution 2e exchanges the chloride

ligand by a solvent molecule to give 2e-dmso which crystal

structure showed π-π “slipped stacking” interaction with a

separation between ring centroids of 3.599(3) Å. The NCI

analyses seems to indicate that such interactions are probably

due to the metalloaromatic nature of the chelate rings. Similar

inter- and intramolecular interactions were also observed

within the crystal of the trinuclear complex 5e and in an

analogue 8 to 2e-dmso with palladium instead of platinum as

metal centre, indicating that such interactions might be

ubiquitous among polycyclic complexes containing metal

chelate rings.

Figure 9. View of the intermolecular (in red) and intramolecular (in black) π,π-stacking

interactions within the crystal of 5e, dashed lines link the centroids of the rings

involved in each stacking interaction

Conclusions

Acetylpyridine N-phenylhydrazones react with K₂[PtCl₄] in n-

butanol/water at 100 °C showing

a markedly different

Experimental

behaviour; the unsubstituted N-phenyl hydrazone (b) or the 3-

J. Name., 2013, 00, 1-3 | 11

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ARTICLE

complete description of general methods, synthetic

Journal Name

A

procedures, characterization details, computational methods

and x-ray diffraction details are given in the SI.

General procedure for the cyclometallation of hydrazones

with K₂[PtCl₄]: A pressure tube containing ligand a-e (4.07

mmol), K₂[PtCl₄] (3.39 mmol) and 0.1 cm³of water in 50 cm³of

1-butanol was sealed under argon. The resulting mixture was

stirred for 2-72 h at 100°C. After cooling to room temperature

the precipitate formed was filtered off, triturated with Et₂O

and dried in vacuo.

General procedure for the acetate-assisted cycloplatination:

A pressure tube containing chelated complex 1a,d (0.195

mmol), NaOAc•3H₂O (0.195 mmol) and 50 cm³of 1-butanol

was sealed under argon. The resulting mixture was stirred for

24 h at 100 °C. After cooling to room temperature the brown

precipitate formed was filtered off, triturated with Et₂O and

dried in vacuo.

S. Bernhard, Chem. Eur. J., 2006, 12, 7970-7977; (h) D.-L. Ma,

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Typical procedure for the reaction of platinacycle 2e with

phosphines: PPh₃(14 mg, 0.053 mmol) was added to a

suspension of 2e (28 mg, 0.055 mmol) in acetone (10 cm³). The

mixture was stirred for 3 h and the solvent removed to give a

2

red

solid

which

was

recrystallized

from

dichloromethane/ether.

Acknowledgements

We thank the Xunta de Galicia, Spain, for financial support

(projects EM2014/056, PRGRC2014/042). The authors are

indebted to Centro de Supercomputación de Galicia for

providing the computer facilities.

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have been shown to proceed at room temperature to yield

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34 Initial dissociation of a dmso ligand from cis-[PtCl₂(dmso)₂]

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afforded the corresponding iminoplatinum(II) complexes,

which have been chrystallographically characterized, see: (a)

A. D. Ryabov, S. Otto, P. V. Samuleev, V. A. Polyakov, L.

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30 (a) The mechanism of benzo[h]quinolone cyclopalladation

promoted by [PdCl₄]^2-was studied by DFT methods, and the

C-H activation has been predicted to proceed via

deprotonation by internal or external base, see: X. Ke, T. R.

Cundari, Organometallics, 2010, 29, 821-834. (b) DFT

calculations supporting the crystallographic evidences for

the electrophilic nature of the cycloplatination of aryloximes 36 The energetics and reaction path in

a series of SN2

with cis-[PtCl₂(dmso)₂] has also been reported, see: S. Otto,

A. Chanda, P. V. Samuleev, A. D. Ryabov, Eur. J. Inorg. Chem.

2006, 2561-2565.

substitutions at square-planar Pt(II) complexes has been

studied by DFT methods, see: J. Cooper, T. Ziegler, Inorg.

Chem., 2002, 41, 6614-6622.

14 | J. Name., 2012, 00, 1-3

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37 2-Pyridylhydrazone-diacetyl platinum(II) complexes have A. Zehnacker-Rentien, M. Mons, J.-P. Piquemal, Phys. Chem.

been crystallographically characterized, showing Chem. Phys., 2014, 16, 9876-9891.

intramolecular hydrogen bonds, see: T. Kluge, E. Bette, M. 44 The ionization and neutralization of HCl in water are known

Bette, J. Schmidt, D. Steinborn, J. Organomet. Chem., 2014,

762, 48-57.

to be exothermic processes, see: Handbook of Chemistry and

Physics, 59^thEd., CRC Press: Boca Raton, FL, 1978. Calculation

of the contribution of the ionization and neutralization of HCl

in n-BuOH/H₂O to the energetic balance for the

cyclometallation of 1a to 2a is beyond the scope of the

present work. For recent progress in the theorethical

determination of bond dissociation energies of small water

and HCl clusters, see: A. K. Samanta, G. Czako, Y. Yang, J. S.

38 The geometry of the agostic interaction calculated for

intermediate 5b⁺is similar to that determined by single-

crystal X-ray diffraction for a rhodium pincer complex (Rh-C

2.27 Å and Rh-H 1.95 Å), see: (a) A. Vigaloc, O. Uzan, L. J. W.

Shimon, Y. Ben-David, J. M. L. Martin, D. Milstein, J. Am.

Chem. Soc., 1998, 120, 12539-12544. For other complexes

characterized by X-ray chrystallography showing agostic

interactions of aromatic C-H bonds, see: (b) A. C. Albeniz, G.

Mancini, J. M. Bowman, H. Reisler, Acc. Chem. Res., 2014, 47

2700-2709.

,

Schulte, R. H. Crabtree, Organometallics, 1992, 11, 242-249; 45 The increase in the reaction rate observed by the addition of

(c) P. Dani, T. Karlen, R. A. Gossage, W. J. J. Smeets, A. L.

Spek, G. van Koten, J. Am. Chem. Soc., 1997, 119, 11317-

11318. For other T-shaped Pt(II) agostic complexes that have

been chrystallographically characterized see: (d) M. J.

Ingleson, M. F. Mahon, A. S. Weller, Chem. Commun., 2004,

water as solvent and the need to ionize HCl in

cycloplatination reactions of diphenylpyridine using K₂[PtCl₄]

has been reported, see: G. W. V. Cave, F. P. Fanizzi, R. J.

Deeth, W. Errington, J. P. Rourke, Organometallics, 2000, 19

1355-1364.

,

2398-2399; (e) W. Baratta, S. Stoccoro, A. Doppiu, E. 46 One referee has suggested that the different reactivity

Herdtweck, A. Zucca, P. Rigo, Angew. Chem. Int. Ed., 2003,

42, 105-108.

observed for 1a and 1b might derive from the involvement

of the chlorine substituent at the N-phenyl group in 1a in a

hydrogen bond with the hydrazine hydrogen. Nevertheless,

such hydrogen bond-like interaction is present along the

39 The energy of an incipient agostic interaction in the cis-trans

isomerization for cationic Pt(II) complexes was estimated by

kinetic and computational data to be in the range of 5–8

kcal/mol: see R. Romeo, G. D’Amico, E. Sicilia, N. Russo, S.

Rizzato, J. Am. Chem. Soc., 2007, 129, 5744–5755.

whole reaction coordinate in series

distances between the chlorine substituent and the

hydrazine hydrogen for ligand

, intermediates 1a and 5a⁺

a, and the calculated

a

,

40 The activation energy for the electrophilic attack of the Pt(II)

centre to the N-phenyl group on the chelate intermediate 1b

(32.8 kcal/mol) is lower than that calculated for the

key transition structure 1/5a and product 2a are very similar

(2.5-2.6 Å). Thus, there is not any computational evidence

supporting the correlation between such hydrogen bond

interaction and the increased activation energy for the

nucleophilic attack of the N-(2-chlorophenyl) moiety to the

Pt(II) centre.

corresponding

process

on

the

mono-coordinate

intermediate 4b⁻(40.3 kcal/mol, see Supplementary

Information, Scheme S9). Thus, chelation of the Pt(II) centre

by the acetylpyridine hydrazone as a [N,N] bidentate ligand 47 M. Gómez, J. Granell, M. Martínez, Organometallics, 1997,

appears to facilitate the cyclometallation process. 16, 2539-2546.

41 DFT calculations for the initial steps of methane activation 48 (a) L. Ackermann, Chem. Rev., 2011, 111, 1315-1345. (b) J.

catalysed by [(HPzm)₂PtCl₄] or [(bpym)PtCl₂] revealed the

involvement of associative transition structures and

intermediate ion-pair methane complexes showing weak Pt-

H interactions which resemble to transition structure cis-

1/5b and ion-pair complex cis-5b, see ref 25a and ref 25e.

Granell, M. Martínez, Dalton Trans. 2012, 41, 11243−11258.

For related palladium-catalyzed arylations, see: (c) S. I.

Gorelsky, D. Lapointe, K. Fagnou, J. Am. Chem. Soc., 2008,

130, 10848; (d) D. Garcia-Cuadrado, P. de Mendoza, A. A. C.

Braga, F. Maseras, A. M. Echavarren, J. Am. Chem. Soc., 2007,

129, 6880-6886.

42 A. E. Reed, L. A. Curtiss, F. Weinhold, Chem. Rev., 1988, 88

899–926.

,

49 For a recent review about computational studies of

43 The NCI method allows identification and visualization in real

space of inter and intramolecular non-covalent interactions,

based on the peaks that appear in the reduced density

carboxylate-assisted C-H activation and functionalization at

group 8-10 transition metal centers, see: D. L. Davies, S. A

Macgregor, C. L. McMullin, Chem. Rev. 2017, 117, 8649-

8709.

gradient (s) at low densities (ρ), see: (a) E. R. Johnson, S.

Keinan, P. Mori-Sánchez, J. Contreras-García, A. J. Cohen, W. 50 B. Biswas, M. Sugimoto, S. Sakaki, Organometallics, 2000, 19

Yan, J. Am. Chem. Soc., 2010, 132, 6498-6506; (b) M. Alonso, 3895-3908.

T. Woller, F. J. Martín-Martínez, J. Contreras-García, P. 51 Computational studies by Davis et al. on cyclometallation

,

Geerlings, F. De Proft, Chem. Eur. J., 2014, 20, 4931-4941; (c)

R. Chaudret, B. de Courcy, J. Contreras-García, E. Gloaguen,

reactions of dimethylbenzylamine with palladium or iridium

precursors in the presence of stoichiometric amounts of

J. Name., 2013, 00, 1-3 | 15

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Journal Name

acetate also indicated a two-step mechanism involving initial

slow κ²κ¹displacement of acetate ligand and facile

subsequent C-H activation via AMLA-6 transition structures, 58 In contrast, the molecular structure of the agostic

Louwen, J. G. Noltes, A. L. Spek, H. J. C. Ubbels, J. Am. Chem.

Soc., 1982, 104, 6609-6616.

-

see: (a) D. L. Davies, S. M. A. Donald, S. A. Macgregor, J. Am.

Chem. Soc. 2005, 127, 13754-13755; (b) D. L. Davies, S. M. A.

Donald, O. Al-Duaij, S. A. Macgregor, M. Pölleth, J. Am.

Chem. Soc. 2006, 128, 4210-4211.

intermediate 5a⁺showed a longer C-N(amine) bond (1.38 Å,

analogous to that shown in the free ligand ), no appreciable

puckering of the phenyl moiety, longer Pt-C bond length and

stronger Pt-H contact (2.20 and 1.93 Å, respectively).

a

52 Wheland intermediates were first invoked by Ryabov et al. 59 B. Cordero, V. Gómez, A. E. Platero-Prats, M Revés, J.

Echeverría, E. Cremades, F. Barragán, S. Alvarez, Dalton

Trans., 2008, 2832–2838.

to rationalize detailed kinetic and activation parameter

meassurements for the orto-palladations of dimethylbenzyl-

amine, see: A. D. Ryabov, I. K. Sakodinskaya, A. K Yatsimirsky, 60 C. Janiak, J. Chem. Soc., Dalton Trans., 2000, 3885-3896.

J. Chem. Soc., Dalton Trans. 1985, 2629-2638. Wheland-type 61 A. Crispini, M. Ghedini, J. Chem. Soc., Dalton Trans., 1997,

intermediates have been characterized in cyclopalladations

by DFT methods, see for example: A. J. Canty, A. Ariafard, B.

F. Yates, M. S. Sanford, Organometallics, 2015, 34, 1085-

1090.

75-80.

53 The number of well-characterized three-coordinate, 14-

electron Pt(II) complexes is very low, due to the necessary

blockage of the vacancy at the metal center to avoid intra- or

intermolecular interactions such as agostic bonds,

counteranions or solvent coordination that would mask the

T-shaped structure (see ref. 44d and 44e). Nevertheless,

coordinatively unsaturated Pt(II) species have been proposed

as intermediates in numerous intramolecular C-H bond

activations. For a review about true and masked T-shaped

Pt(II) complexes, see: M. A. Ortuño, S. Conejero, A. Lledós,

Belstein J. Org. Chem. 2013, 9, 1352-1382.

54 The calculated mechanistic cycle for the alkoxylation of N-

metoxibenzamides catalyzed by Pd(OAc)₂in MeOH includes

the coordination of Pd(II) acetate to the nitrogen followed by

intramolecular proton transfer from the N-H group to

acetate, also leading to an acetic acid bound intermediate,

see: M. Anand, R. B. Sunoj, Org. Lett., 2011, 13, 4802−4805.

55 Acceleration of the cyclopalladation of imines by protonation

of the abstracting ligand have been reported, see: J. Granell,

M. Martínez, Dalton Trans., 2012, 41, 11243-11258.

56 Accordingly with this proposal, second-order perturbation

theory analyses of Fock matrix in the NBO basis for 8ac

provided an estimated stabilization energy of 2.8 kcal/mol

for the donor-acceptor interaction between a lone pair

localized on the carbonyl oxygen and an empty lone pair on

the coordinatively unsaturated Pt(II) centre. Moreover, the

NCI surfaces calculated for 8ac enabled the visualization of

greenish forms around the Pt(II) centre, revealing attractive

dispersion interactions with the acetic acid moiety and also

with the chloride substituent at the phenyl ring (see SI,

Figure S3A,B).

57 Wheland-type

σ-complexes derived from Pt(II) and Pd(II)

precursors have been crystallographically characterized, see:

(a) K. Yamamoto, S. Kimura, T. Murahashi, Angew. Chem, Int.

Ed. 2016, 55, 5322-5326; (b) D. M. Grove, G. van Koten, J. N.

16 | J. Name., 2012, 00, 1-3

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Preparation and characterization of terdentate [C,N,N] acetophenone

and acetylpyridine hydrazone platinacycles. A DFT insight into the

reaction mechanism

Ismael Marcos,^aVicente Ojea,^aDigna Vázquez-García,^a* Jesús J. Fernández,^aAlberto Fernández,^a

Margarita López-Torres,^aJorge Lado^aand José M. Vila.^b*

Synthesis of Pt(II) organometallic complexes bearing tridentate hydrazones by direct and

carboxylate-assisted C-H activation and mechanistic DFT studies.

Article Doi

Preparation and characterization of terdentate [C,N,N] acetophenone and acetylpyridine hydrazone platinacycles: A DFT insight into the reaction mechanism

DOI: 10.1039/c7dt03418k

Source and publish data:

Authors:

Article abstract of DOI:10.1039/c7dt03418k

Full text of DOI:10.1039/c7dt03418k

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