Mechanistic insight into protonolysis and Cis - Trans isomerization of benzylplatinum(II) complexes assisted by weak ligand-to-metal interactions. A combined kinetic and DFT study

ARTICLE

pubs.acs.org/IC

Mechanistic Insight into Protonolysis and Cis-Trans Isomerization of

Benzylplatinum(II) Complexes Assisted by Weak Ligand-to-Metal

Interactions. A Combined Kinetic and DFT Study^†

Emanuela Guido,^‡Giuseppina D’Amico,^‡Nino Russo,^§Emilia Sicilia,^§Silvia Rizzato,^{^}Alberto Albinati,^{^}

Andrea Romeo,^‡M. Rosaria Plutino,*^,zand Raﬀaello Romeo^‡

^‡Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica, Universitꢀa di Messina, Viale F. Stagno D'Alcontres 31, Vill. S.

Agata, I-98166 Messina, Italy

^§Dipartimento di Chimica, Universitꢀa della Calabria, I-87030 Arcavacata di Rende (Cs), Italy

^{^}Dipartimento di Chimica Strutturale e Facoltꢀa di Farmacia, via Venezian 21, Universitꢀa di Milano, Milano, Italy

^zIstituto per lo Studio dei Materiali Nanostrutturati, ISMN-CNR, Unitꢀa di Messina-Sezione di Palermo, c/o Dip. di Chim. Inorg., Chim.

Anal., Chim. Fis., Universitꢀa di Messina, Viale F. Stagno D'Alcontres 31, Vill. S. Agata, I-98166 Messina, Italy

S Supporting Information

b

ABSTRACT: Low-temperature NMR measurements showed

that protonolysis and deuterolysis by H(D)X acids on meta-

and para-substituted dibenzylplatinum(II) complexes cis-[Pt-

(CH₂Ar)₂(PEt₃)₂] (Ar = C₆H₄Y^-; Y = 4-Me, 1a; 3-Me, 1b; H,

1c; 4-F, 1d; 3-F, 1e; 4-Cl, 1f; 3-Cl, 1g; 3-CF₃, 1h) in CD₃OD

leads directly to the formation of trans-[Pt(CH₂Ar)(PEt₃)₂-

(CD₃OD)]X (4a-4h) and toluene derivatives. The reaction

obeys the rate law k_obsd= k_H[H^þ]. For CH₂Ar = CH₂C₆H₅^-,

k_H= 176 ( 3 M^-1s^-1and k_D= 185 ( 5 M^-1s^-1at 298.2 K,

ΔH^q= 46 ( 1 kJ mol^-1and ΔS^q= -47 ( 1 J K^-1mol^-1. In

contrast, in acetonitrile-d₃, three subsequent stages can be distinguished, at diﬀerent temperature ranges: (i) instantaneous formation of new

benzylhydridoplatinum(IV) complexes cis-[Pt(CH₂Ar)₂(H)(CD₃CN)(PEt₃)₂]X (2a-2h, at 230 K), (ii) reductive elimination of 2a-2h

to yield cis-[Pt(CH₂Ar)(CD₃CN)(PEt₃)₂]X (3a-3h) and toluene derivatives (in the range 230-255 K), and ﬁnally (iii) spontaneous

isomerization of the cis cationic solvento species to the corresponding trans isomers (4a-4h, in therange 260-280 K). All compounds were

detected and fully characterized through their ¹H and ³¹P{¹H} NMR spectra. Kinetics monitored by ¹H and ³¹P{¹H} NMR and isotopic

scrambling experiments on cis-[Pt(CH₂Ar)₂(H)(CD₃CN)(PEt₃)₂]X gave some insight onto the mechanism of reductive elimination of

2a-2h. Systematic kinetics of isomerization of 3a-3h were followed in the temperature range 285-320 K by stopped-ﬂow techniques. The

process goes, as expected, through the relatively slow dissociative loss of the weakly bonded solvent molecule and interconversion of two

geometrically distinct T-shaped three-coordinate intermediates. The dissociation energy depends upon the solvent-coordinating ability. DFT

optimization reveals that along the energy proﬁle the “cis-like”[Pt(CH₂Ar)(PMe₃)₂]^þintermediate is strongly stabilized by a Pt η²-C1-

3 3 3

C_ipsobond between the unsaturated metal and benzyl carbons. The value of the ensuing stabilization energy was estimated by computational

data to be greater than that found for similar β-agostic Pt η²-CH interactions with alkyl groups containing β-hydrogens. An observed

3 3 3

consequence of the strong stabilization of “cis”-[Pt(η²-CH₂Ar)(PMe₃)₂]^þis the remarkable acceleration of the rate of isomerization, greater

than that produced by the so-called “β-hydrogen kinetic eﬀect”. Kinetic and DFT data concur to indicate that electron donation by

substituents on the benzyl ring leads to further stabilization of the “cis”-[Pt(η²-CH₂Ar)(PMe₃)₂]^þcationic species.

’ INTRODUCTION

species has been inferred simply from the observation of mass-law

retardation eﬀects. A more complicated protocol involving the

measurement and rationalization of a multiplicity of kinetic eﬀects

(rates of ligand exchange and substitution, saturation plots, electron-

ic, steric, solvent, and isotopic eﬀects, activation parameters, etc.) has

been rarely applied. Whatever the degree of sophistication of the

kinetic approach is, direct proof of the existence of three-coordinate,

For many years¹“14-electron”, T-shaped, three-coordinate inter-

mediates, formed upon ligand dissociation from d⁸square-planar

organometallic complexes, have been cursorily depicted in catalytic

cycles, bond activation reactions, substitution reactions, geometrical

isomerizations, transmetalations, etc. In particular, such coordina-

tively and electronically unsaturated species play a vital role as key

intermediates in many catalytic transformations of organic sub-

strates. Looking at the literature data, it is possible to see that very

often the presence of such compounds as elusive and transient

Received: September 13, 2010

Published: February 22, 2011

r

2011 American Chemical Society

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Inorganic Chemistry

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Chart 1. Variety of Metal-Benzyl Linkages

14-electron compounds in solution is diﬃcult to ﬁnd. Because of the

exceptionally high Lewis acidity inherent in these 14-electron metal

complexes, they tend to interact with even the weakest of nucleo-

philes, and the empty coordination sites can be ﬁlled by counterions

or by even very weakly coordinating solvent molecules.²The forma-

tion of dimers could also well be a reasonable alternative.³Recent

reports on the isolation and structural characterization of a few three-

coordinate, T-shaped rhodium(I),⁴cobalt(I),⁵nickel(II),⁶palladium-

(II),⁷and platinum(II)⁸compounds have fortunately helped to have

an insight into the structural and bonding characteristics of such

compounds that appear to be stabilized by auxiliary bonding inter-

actions. One of the oldest examples of this kind concerns the cation

Rh(PPh₃)₃^þ, which achieves a 16-electron as a consequence of interac-

tion with a CdC bond of a dangling phenyl group.^3bOther examples

can be found in the literature in which the immediately adjacent double

bond of a PPh₃donor or a variety of remote oleﬁns is involved in

bonding with the metal.⁹The large majority of the isolated three-

coordinate d⁸ML₃species show C-H bonds involved in agostic

interactions to the empty coordination site of otherwise 14-electron,

T-shaped d⁸complexes. In platinum(II) chemistry, these examples

include the cationic [Pt(R)(P-P)]^þcomplexes (P-P = chelat-

ing ligand; R = ethyl, norbornyl),^8a,8bthe very recent [Pt(CH₃)-

(ⁱPr₃P)₂]^þ,^8cand the novel “T-shaped, 14-electron” platinum(II)

cations trans-[Pt(CH₃)L₂]^þ[L =PR₂(2,6-Me₂C₆H₃); R= Ph, Cy].^8d

However, no notable C-H agostic interaction with the metal center

was detected in a series of cationic T-shaped boryl complexes of the

type trans-{(Cy₃P)₂Pt[B(X)X⁰]}^þ,^8e,8fin a series of palladium(II)

complexes [PdArXL]^7c(Ar = aryl group; X= amido group; L=

phosphane), and in the rhodium(I) complex [Rh(NNN)]BArF.^4e

Unprecedented examples of bare 14-electron planar four-coordinate d⁶

RhL₄and d⁶IrL₄metal complexes have been reported by Nolan

et al.¹⁰π donation, from a lone pair on a donor atom already

bound to the metal (e.g., R₂N-M), is crucial in understanding

the remarkable stability as well as the lack of agostic interactions in

the 14-electron compounds.^10,8e,3a

Thus, the challenge to isolate “14-electron”, T-shaped com-

pounds seems to rely on the use of ligands that guarantee some

form of protection of the fourth-coordination site and of electron

transfer to the vacant σ orbital of the metal. To what extent such

electron transfer occurs is a matter of debate.²However, when

occurring within a transition state (TS) or a reaction intermediate,

agostic or “preagostic” interactions are shown to produce the so-

called “β-hydrogen kinetic eﬀect”, a sharp acceleration of the reaction

rate.¹¹The combined kinetic and density functional theory (DFT)

study of the uncatalyzed isomerizat_þion of cationic solvent complexes

of the type cis-[Pt(R⁰)(S)(PR₃)₂] to their trans isomers, perhaps

the best-documented example¹²of a process driven by ligand

dissociation in platinum(II) chemistry,¹³has shown that when R⁰

is a linear alkyl the “cis-like” three-coordinate [Pt(R⁰)(PEt₃)₂]^þ

reaction intermediate was stabilized by a classical Pt-η²-CH agostic

interaction with the electron pair of a hanging C-H bond. The bond

energy was estimated by both kinetic and computational data to be in

the range of 21-33 kJ mol^-1. The structure and bonding features of

cis-[Pt(R⁰)(PMe₃)₂]^þhave been discussed in detail and have been

taken as a model of the ﬁrst approach of a C-H bond to an

unsaturated metal center prior to C-H activation.

In searching for similar eﬀects, we were prompted to analyze

the behavior of benzyl complexes because it is known that the

benzyl ligand diﬀers from simple alkyl groups in its capability of

adopting a variety of metal-benzyl linkages ranging from the

classical η¹bonding to the rather unusual η⁵bonding (Chart 1).

All of these structural types have been reported in the literature

and, in some cases, authenticated by X-ray crystallography.¹⁴

A

relevant feature of the present work is the discovery of the formation

of an η²-Pt-C1-C_ipsobond within the transient intermediate of

formula [Pt(η²-CH₂C₆H₅)(PMe₃)₂]^þ, which is somewhat stronger

than that found for the agostic [Pt(η²-CH₂CH₃)(PR₃)₂]^þspecies.

This strong stabilization is at the origin of a marked rate acceleration.

Tuning the energy of reaction intermediates can be a powerful

mechanistic tool in the control of organometallic reactions, especially

those involving bond activation.

’ RESULTS

Synthesis and Characterization of the Complexes. All the

dibenzyl complexes cis-[Pt(CH₂Ar)₂(PEt₃)₂] (Ar = C₆H₄Y^-;

Y = 4-Me, 1a; 3-Me, 1b; H, 1c; 4-F, 1d; 3-F, 1e; 4-Cl, 1f; 3-Cl,

1g;3-CF₃, 1h; shown in Chart 2) were synthesized from the reaction

of cis-[PtCl₂(PEt₃)₂] with benzylmagnesium halides following es-

sentially the synthetic procedure described by Chatt and Shaw for

compound 1c.¹⁵The desired products were obtained as solids in

high yield and purity and were characterized by elemental analysis

1

and H and ³¹P{¹H} NMR spectroscopy. The spectral character-

istics of these compounds do not differ much from those of other

dialkyl complexes.¹⁶In the ¹H NMR spectra, the phosphorus-

coupled methylene resonance appears as an apparent triplet

(³J_PH≈ 17 Hz) due to virtual coupling. The ³¹P{¹H} NMR spectra

show a single resonance and low values of ¹J_PtP(≈1850 Hz) that are

diagnostic for a cis stereochemistry, with the phosphane ligands trans

to the benzyl groups.¹⁷Selected NMR data for complexes 1a-1h are

reported in Table 1. A complete list and assignments of the NMR data

are given in the Experimental Section.

Solid-State Structures of the Complexes cis-[Pt(CH₂C₆H₅-

4-Me)₂(PEt₃)₂] (1a) and cis-[Pt(CH₂C₆H₅-4-Cl)₂(PEt₃)₂] (1f).

Table 2 lists selected bond distances and angles of the platinum(II)

species 1a, and an ORTEP view of the molecule is given in

Figure 1. The immediate coordination sphere around the Pt

center consists of the two η¹-bonded benzyl groups, cis to each

other, and the P atoms of the phosphane moieties. The Pt atom

lies on a crystallographic symmetry element so that only half of

the molecule is independent. The benzyl groups are on opposite

sides of the coordination plane with the same orientation,

imposed by symmetry, with a value of the C1⁰-Pt-C1-C2

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Chart 2. Structural Formula for the Platinum(II) Complexes

1a-1h, 2a-2h, 3a-3h, and 4a-4h

Table 1. Selected ¹H and ³¹P{¹H} NMR Data for Starting

cis-[Pt^II(CH₂Ar)₂(PEt₃)₂] (1a-1h) Complexes and the Cor-

responding Benzylhydridoplatinum(IV) Species cis-[Pt^IV-

(H)(CD₃CN)(CH₂Ar)₂(PEt₃)₂]X (2a-2h) Obtained by

Oxidative Addition with HX

cis-[Pt^II(CH₂Ar)₂(PEt₃)₂]^a

¹H NMR

³¹P NMR

¹J_PtP

no.

Ar

δ(Pt-CH₂)

²J_PtH

δ

1a^b

1b^c

1c^b

1d^c

1e^c

1f^c

4-MeC₆H₄

3-MeC₆H₄

C₆H₅

4-FC₆H₄

3-FC₆H₄

4-ClC₆H₄

3-ClC₆H₄

3-CF₃C₆H₄

2.39

2.21

2.46

2.21

2.17

2.20

2.39

2.44

83.4

83.8

83.7

82.4

84.6

83.8

83.1

82.5

12.5

12.4

12.5

12.6

12.8

1943

1961

1963

1954

2012

1988

2004

2008

1g^c

1h^c

cis-[Pt^IV(H)(CD₃CN)(CH₂Ar)₂(PEt₃)₂]X^d

¹H NMR

³¹P NMR

no.

Ar

δ(Pt-H) ¹J_PtHδ(Pt-CH₂) ²J_PtH

δ

¹J_PtP

2a^b4-MeC₆H₄

2b^c3-MeC₆H₄

2c^bC₆H₅

-21.4

1271

2.35

65.5 -3.5

-3.5

1188

1192

1198

1200

1216

1210

1218

1205

-21.7

-21.5

1253

1260

1256

1253

2.30

2.25

2.28

2.25

53.6 -3.4

56.6 -3.6

53.6 -3.4

59.5 -3.3

65.5 -3.2

-3.4

2d^c4-FC₆H₄

2e^c3-FC₆H₄

2f^c4-ClC₆H₄

2g^c3-ClC₆H₄

2h^c3-CF₃C₆H₄

^aIn acetonitrile-d₃at 298 K. ^bX = BArF. ^cX = OTf. ^dIn acetonitrile-d₃at

238 K.

Table 2. Selected Bond Distances (Å) and Angles (deg) for

Compounds 1a^aand 1f

dihedral angle of 71.1(2)ꢀ. The ipso-C atoms of the aryl rings,

closest to the Pt center, are at a distance of ca. 3.5 Å. The Pt-C

and Pt-P bond lengths of 2.127(2) and 2.2945(8) Å, respec-

tively, are unexceptional. As for the corresponding dibutyl

complex,¹¹the Pt-P separations are in the upper range of the

values found for other phosphanedialkyl complexes of platinum-

(II) as a result of the strong trans influence of the carbon ligand.

An ORTEP view of compound 1f is given in Figure 2, while

selected bond distances and angles are listed in Table 2. As for 1a, the

immediate coordination sphere around the Pt center consists of two

η¹-bonded benzyl groups, cis to each other, and the P atoms of the

phosphane moieties. The benzyl groups lie on opposite sides of the

coordination plane with slightly diﬀerent orientations compared to

compound 1a, as can be judged from the values of the torsion angles

(see Table 2), a diﬀerence due to the absence of symmetry cons-

traints. Moreover, we note that the orientation of the C2 aryl ring is

such that the H atom bonded to the ipso-carbon C7 points toward the

Pt center at a distance of 2.83(1) Å in a pseudoapical position,

1a

1f

Pt-C1

Pt-C8

Pt-P1

Pt-P2

2.127(2)

2.2945(8)

164.26(7)

104.66(5)

87.19(8)

83.3(2)

2.123(6)

2.110(6)

2.297(2)

2.301(2)

173.4(2)

166.2(2)

99.78(6)

91.2(2)

86.3(2)

83.2(3)

-96.5

P1-Pt-C1

P2-Pt-C8

P1-Pt-P2

P1-Pt-C8

P2-Pt-C1

C1-Pt-C8

C1-Pt-C8-C9

C8-Pt-C1-C2

Pt-C1-C2-C7

Pt-C8-C9-C14

-71.1(2)

58.3(2)

-87.4(5)

13.1(3)

41.7(4)

58.3(2)

^aThe numbering scheme is based on the less symmetric compound 1f for

easier comparison. The symmetry-related values in 1a appear as double entries.

Acidolysis of Dibenzyl Complexes in Methanol. Upon the

addition of a sufficient amount of an ethereal solution of HBF₄to

a solution of the platinum complex in CD₃OD, there is an

immediate and sharp change of the NMR spectrum. Cleavage of

the Pt-C(benzyl) σ bond takes place according to the reaction

H^þ

suggesting a weak nonconventional M H interaction (see

3 3 3

Figure S1 in the Supporting Information).¹⁸

The two Pt-P and Pt-C distances are equal and comparable

to those found in 1a.

cis-½PtðCH₂ArÞ₂L₂ꢀ

f

trans-½PtðCH₂ArÞðCD₃ODÞL₂ꢀ^þþ CH₃Ar

ð1Þ

The similar values of the Pt-C and Pt-P separations and the

absence of signiﬁcant deformations in the bond angles for 1a, 1f, and

cis-[Pt(ⁿBu)₂(PEt₃)₂]¹¹indicate that the presence of an aryl ring

(even with para-substituent groups) does not aﬀect signiﬁcantly

the molecular structure that is comparable to that of an alkyl

compound (cf. Table 2).

CD₃OD

The ﬁnal trans product can be recognized particularly through

the single ³¹P{¹H} resonance due to 2 equiv of phosphane

ligands. Attempts to detect by NMR at low temperature (230 K)

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the pseudo-ﬁrst-order rate constants k_obsdon the proton concentra-

tion (Table S1 in the Supporting Information) in methanol at 297 K

is described by a straight line with zero intercept, i.e., k_obsd= k_H[H^þ].

The same pattern of behavior is observed when the kinetic runs are

carried out with DOTf in CH₃OD (Table S1 in the Supporting

Information), i.e., k_obsd= k_D[D^þ]. The values of k_H= 176 ( 3

M^-1s^-1and k_D= 185 ( 5 M^-1s^-1were obtained from linear

regression analysis of the kinetic data. The values of k_Hat diﬀerent

temperatures are set forth also in Table S1 in the Supporting

Information, and the calculated activation parameters were ΔH^q=

46 ( 1 kJ mol^-1and ΔS^q= -47 ( 1 J K^-1mol^-1.

Acidolysis of Dibenzyl Complexes in Acetonitrile-d₃: Forma-

tion of Benzylhydridoplatinum(IV) Complexes. The addition of

acid (HOTf, HBArF, or HBF₄Et₂O) to 1a-1h at low temperature

3

in acetonitrile-d₃, according to the procedure described in the

experimental part, was expected to lead to cationic solvento species

cis- or trans-[Pt(CH₂C₆H₄Y)(CH₃CN)(PEt₃)₂]^þand toluene de-

rivative liberation, as described above in methanol or found before in

a series of alkylphosphane complexes of the type cis-[PtMe₂L₂],

cis-[PtMe₂(L-L)], and cis-[PtMeClL₂].¹⁶In contrast, at 230 K, the

³¹P{¹H} NMR spectra showed a single upfield resonance with values

of ¹J_PtP(≈1200 Hz) much lower than those of the precursors 1a-

1h (≈1980 Hz). A characteristic Pt-H resonance appears far upfield

in the ¹HNMRspectra(δ∼-21 ppm) with the corresponding plati-

num satellites (¹J_PtH∼ 1250 Hz), the Pt-CH₂R resonance remains

almost unchanged with respect to that of 1a-1h (δ ∼ 2.30 ppm),

showing a lower value of the ¹⁹⁵Pt satellite signal (²J_PtH≈ 60 Hz),

and the ethyl groups of the phosphanes show the typical splitting

patterns of a triplet (δ ∼ 0.80 ppm) and a quartet (δ ∼ 1.60 ppm),

each integrating for 18 and 12 protons, respectively. There was no

evidence for the buildup of toluene derivatives in solution. All these

findings indicate that the addition of HX to the planar 1a-1h

substrates generates rather stable benzylhydridoplatinum(IV) com-

plexes according to the reaction

Figure 1. ORTEP view of a molecule of compound 1a showing 50%

probability ellipsoids. Primed atoms are obtained from those unprimed

3

by the symmetry operation -x - 1, y, -z þ /₂.

H^þ

cis-½PtðCH₂ArÞ L₂ꢀ f cis-½PtðCH₂ArÞ ðHÞðCD₃CNÞL₂ꢀ^þ

2

CD₃CN

ð2Þ

The ¹H and ³¹P NMR resonances of the hydride compounds

2a-2h showed no signiﬁcant changes upon a change in the

nature of the acid used (HOTf, HBF₄, HBArF, or even HCl) so

that we can safely conclude that the sixth-coordinated group is a

molecule of acetonitrile, which concurs to confer particular

stability to the octahedral species. The magnitudes of the

1

coupling constants J_PtPare consistent with a stereochemical

arrangement in which H and CD₃CN reside trans each other, for

example, in the apical positions of the octahedral structure, while

phosphanes and benzyl groups maintain their original position in

the equatorial plane resembling the starting square-planar ar-

rangement. The magnetic equivalence of the two benzyl moieties

removes from consideration the possibility that a secondary

interaction between the metal and a benzyl group, of the type

discussed in the introductory part of this work, can be held

responsible for the stability of an otherwise ﬁve-coordinate

species. Selected NMR data for complexes 2a-2h are reported

in Table 1. A complete list and assignments of the NMR data are

given in the Experimental Section.

Figure 2. ORTEP view of a molecule of compound 1f showing 50%

probability ellipsoids.

the presence of the cis derivative or of possible alkylhydridoplati-

num(IV) intermediates were unsuccessful. Subsequent spec-

trophotometric studies conﬁrmed that the rate of cis-trans

isomerization of the cationic solvento species cis-[Pt(PEt₃)₂-

(CH₂R)(CD₃OD)]^þis too high to be followed even by rapid

scanning spectrophotometric techniques.¹⁹At relatively low acid

concentrations, the reaction in eq 1 went to completion in a

single stage and excellent ﬁts were obtained from regression

analysis of the absorbance versus time data. The dependence of

Reductive Elimination of Benzylhydridoplatinum(IV)

Complexes. At 230 K, acid addition to 1a-1h, according to

eq 2, leads to the quantitative formation of platinum(IV) hydrido

compounds 2a-2h. The reductive-elimination reactions of these

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Table 3. Selected ¹H and ³¹P{¹H} NMR Data for Cis

(3a-3h) and Trans (4a-4h) Monoalkyl Cationic Solvento

Complexes [Pt(CH₂Ar)(CD₃CN)(PEt₃)₂]^þObtained upon

Reductive Elimination from 2a-2h and Subsequent

Isomerization^a

cis-[Pt(CD₃CN)(CH₂Ar)(PEt₃)₂]^þ

¹H NMR

³¹P NMR

¹J_PtP

no.

Ar

δ(Pt-CH₂)

²J_PtH

δ

3a

4-MeC₆H₄

2.29

2.30

2.33

2.31

2.33

2.31

2.30

60.2

16.9

12.2

16.9

12.3

16.9

12.2

16.7

12.1

16.7

11.7

16.8

11.9

16.7

11.9

16.8

12.2

1790

4142

1789

4196

1775

4185

1788

4177

1806

4156

1797

4168

1828

4141

1796

4178

3b

3c

3d

3e

3f

3-MeC₆H₄

C₆H₅

58.5

61.8

64.8

61.8

61.2

61.8

60.8

4-FC₆H₄

3-FC₆H₄

4-ClC₆H₄

3-ClC₆H₄

3-CF₃C₆H₄

3g

3h

trans-[Pt(CD₃CN)(CH₂Ar)(PEt₃)₂]^þ

¹H NMR

³¹P NMR

no.

Ar

δ(Pt-CH₂)

²J_PtH

δ

¹J_PtP

4a

4b

4c

4d

4e

4f

4-MeC₆H₄

3-MeC₆H₄

C₆H₅

2.60

2.44

2.65

2.61

2.49

2.63

2.62

2.24

98.1

91.2

98.7

98.4

99.3

98.7

99.0

20.8

20.7

20.6

20.4

20.5

20.4

20.5

2783

2778

2769

2756

2736

2740

2730

2713

Figure 3. ³¹P{¹H} NMR spectra (122 MHz) of 2d (C_Pt= 29 mM in

CD₃CN) at 238.5 K as a function of time. ³¹P{¹H} signal of compound

2d (δ = -3.6 ppm, ¹J_PtP= 1200 Hz); ³¹P{¹H} signals of compound 3d

(δ = 16.7 ppm, ¹J_PtP= 1788 Hz; δ = 12.1 ppm, ¹J_PtP= 4177 Hz).

4-FC₆H₄

3-FC₆H₄

4-ClC₆H₄

3-ClC₆H₄

3-CF₃C₆H₄

latter compounds takes place easily in acetonitrile-d₃, in the

temperature range 230-255 K, according to eq 3, and can be

followed by means of ¹H and ³¹P NMR.

4g

4h

^aRecorded in CH₃CN-d₃as the solvent at 238.5 K for the cis solvento

cis-½PtðCH₂ArÞ ðHÞðCD₃CNÞL₂ꢀ^þf cis-½PtðCH₂ArÞðCD₃CNÞL₂ꢀ^þ

complexes and at 298 K for the trans solvento derivatives.

2

þ CH₃Ar

ð3Þ

at a controlled low temperature (T = 231.7 K). Selected NMR

data for the ﬁnal complexes 3a-3h are reported in Table 3.

A complete list and assignments of the NMR data are given in the

Experimental Section. Alternatively, the reaction can be mon-

Taking as an example the case of compound 2d, in Figure 3,

we see that there was no evidence in the NMR spectra for the

buildup of signiﬁcant amounts of any other intermediate

species, and only the starting platinum(IV) complex and the

solventoplatinum(II) product 3d were found in solution. The

progress of the reaction can be monitored through a decrease

of the intensity of the phosphorus resonance of 2d and a

parallel matching increase of the two magnetically nonequi-

valent phosphorus resonances of 3d. The latter are character-

ized by completely diﬀerent values of the coupling constant

with ¹⁹⁵Pt, as a consequence of the diﬀerent trans inﬂuence of

the benzyl or CD₃CN ligands (¹J_PtP= 1788 Hz for P_A, in a

trans position to the C atom, and ¹J_PtP= 4177 Hz for P_B, in a

trans position to CD₃CN).

1

itored by H NMR spectra, measuring the integrals of selected

peaks (of the growing toluene derivative, for instance) as a

function of time and using the calculated concentrations in the

ﬁtting procedure for calculation of the rate constant. The

temperature dependence of the reductive-elimination reaction

1

of 2d was measured by both H and ³¹P NMR, as described

above. Exponential curves for the change with time of the

molar fraction of the reactant and of the product are illustrated

in Figure S2 in the Supporting Information. The pseudo-ﬁrst-

order rate constants, k_obsd/s^-1, are collected in Table S2 in the

Supporting Information and were ﬁtted to the Eyring equation to

yield the values of the activation parameters ΔH^q= 63.8 ( 3.5 kJ

mol^-1and of ΔS^q= -29.0 ( 14.6 J K^-1mol^-1. The Eyring plot is

The procedure adopted for 2d was repeated for the entire set

of 2a-2h compounds, leaving the reactions to go to completion

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Table 4. Eﬀect of Variation of the Nature of Ar on the Rates

and Activation Parameters of the Spontaneous Isomerization

of cis-[Pt(CH₂Ar)(CH₃CN)(PEt₃)₂]^þ(3a-3h) to 4a-4h in

an Acetonitrile Solution

no.

Ar

k_i^a

ΔH^qb

ΔS^qc

ΔG^qd

3a

3b

3c

3d

3e

3f

4-MeC₆H₄

3-MeC₆H₄

C₆H₅

1.83

81.8 ( 2

89.9 ( 1

83.8 ( 2

97.1 ( 1

104.0 ( 2

103.4 ( 1

105.0 ( 4

95.7 ( 2

34 ( 5

60 ( 2

35 ( 4

76 ( 4

86 ( 7

87 ( 3

86 ( 10

52 ( 6

71.6

72.0

73.4

74.4

78.4

77.1

79.4

80.2

1.48

0.820

0.585

0.115

0.159

0.0758

0.0555

4-FC₆H₄

3-FC₆H₄

4-ClC₆H₄

3-ClC₆H₄

3-CF₃C₆H₄

3g

3h

^aFirst-order rate constants (s^-1) for isomerization at 298.2 K.

^bEnthalpies of activation (kJ mol^-1). Entropies of activation (J K^-1

c

mol^-1). ^dCalculated free-energy of activation (kJ mol^-1) at 298.2K.

with spectral changes showing well-deﬁned isosbestic points. The

systematic kinetics of isomerization were followed by stopped-ﬂow

spectrophotometric techniques at ﬁxed wavelengths, where the

diﬀerence of absorbance between cis and trans compounds was

largest. An example of a stopped-ﬂow kinetic plot is illustrated in

Figure S4 in the Supporting Information. The pseudo-ﬁrst-order

rate constants k_ifor the reaction of eq 4 at various temperatures are

reported in Table S3 in the Supporting Information and were

analyzed by least-squares regression of linear Eyring plots. The

values of k_iat 298.2 K, together with the associated activation

parameters, are listed in Table 4.

Computational Data. The analyzed processes, in the frame-

work of DFT, were the geometrical conversion of the benzylsol-

vento complexes cis-[Pt(CH₂Ar)(S)(PMe₃)₂]^þ(Ar = C₆H₅and

4-ClC₆H₄; S = acetonitrile and methanol) and of cis-[Pt(CH₂Ar)-

(CH₃CN)(PMe₃)₂]^þ(Ar = 4-MeC₆H₄). The aim was to obtain

insight into the way in which the different nature of the substituent

group on the aromatic ring of the labile coordinated solvent could

affect the various steps along the reaction coordinate. Geometry

optimizations were performed for the initial and final complexes and

for the main intermediates along the energy profile. The relevant

computational data listed in Table 5 refer to the energetics of the

isomerization of various species cis-[Pt(CH₂Ar)(S)(PMe₃)₂]^þin

which the nature of the benzyl group and of the leaving group has

been changed. Relative energies, calculated with respect to the

reactant cationic solvento complex, of the intermediates, of the TS

for their conversion, and of the product state are in kilojoules per

mole. Cartesian coordinates of the B3LYP-optimized, T-shaped cis-

and trans-[Pt(CH₂C₆H₄Y)(PMe₃)₂]^þ(Y = H, 4-Cl, 4-Me) com-

plexes are given in Table S4 in the Supporting Information.

Figure 4. ³¹P NMR spectral changes associated with the geometrical

isomerization of the cis-solventoplatinum(II) complex 3d to 4d in

CD₃CN at 260 K.

illustrated in Figure S3 in the Supporting Information. Kinetic

measurements, carried out in CD₂Cl₂at 233 K, in the presence of

variable amounts of CD₃CN, showed that the rate of reductive

elimination decreases with an increase in the CD₃CN concentration.

Geometrical Isomerization in Acetonitrile. The spontaneous

geometrical isomerization of 3a-3h can be monitored by NMR

techniques in the temperature range 260-280 K through a decrease

of the two associated ³¹P signals and a parallel and matching increase

in the signal of the corresponding trans complex, which appears as a

singlet with platinum satellites (Figure 4).

Therefore, the isomerization process in acetonitrile-d₃is des-

cribed by the reaction

cis-½PtðCH₂ArÞðCD₃CNÞL₂ꢀ^þf trans-½PtðCH₂ArÞðCD₃CNÞL₂ꢀ^þ

ð4Þ

’ DISCUSSION

NMR Features of Benzylhydridoplatinum(IV) Complexes.

As discussed before, among the six possible stereoisomers for an

octahedral species of the type Ma₂b₂cd, only one is compatible

with a single ³¹P resonance (two magnetically equivalent P atoms)

1

and with its very low J_PtPcoupling constant (the presence of a

couple of strong trans-activating C atoms), as a result of a trans

addition of acid to the original set of donor atoms in the starting cis-

platinum(II) complex. The question of whether the position in the

octahedron opposite to the apical hydride is occupied by the anion

X^-(OTf^-, BArF^-, BF₄^-) or by the solvent (CD₃CN) has been

solved by observation of the strict similarity of the values of chemical

Selected NMR data for the ﬁnal reaction products 4a-4h are

reported in Table 3. A complete characterization and assign-

ments of the NMR data are given in the Experimental Section.

A control, at suﬃciently low temperature, by rapid scanning

spectrophotometry ensured that conversion was 100% complete,

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shifts and coupling constants obtained using acids with different

anions. A factor of overriding importance in dictating the different

patterns in methanol and acetonitrile seems to be the good electron-

donor ability of CD₃CN that, as the sixth ligand, concurs to a strong

stabilization of the six-coordinate oxidative product. The use of

acetonitrile-d₃to trap five-coordinate transient reaction intermedi-

ates onto octahedral species is known.²⁰

cis-[PtMe₂(PR₃)₂], cis-[PtMe(R)(PR₃)₂], and cis-[PtMeCl-

(PR₃)₂] complexes, the form of the rate law, the positive large

values of the primary isotopic eﬀect, the lack of isotope scram-

bling with the H atoms belonging to the coordinated alkyl group,

and, ﬁnally, the failure to observe the formation of platinum(IV)

alkylhydride intermediate species in solution were all in favor of a

mechanism that entails a rate-determining proton transfer to the

Pt-C bond.¹⁶An identical pattern is observed in this work, when

the acidolysis of a cis-[Pt(CH₂Ar)₂(PEt₃)₂] (Ar = C₆H₅, 1c)

complex is carried out in methanol and, therefore, metal oxida-

tion and quantitative formation of alkylhydridoplatinum(IV)

complexes upon the addition of HOTf or HBArF to cis-[Pt-

(CH₂Ar)₂(PEt₃)₂] (Ar = C₆H₄Y) complexes (1:2, in a CD₃CN

solution) came as a surprise to us. Scheme 1 summarizes in a

simpliﬁed form the two possible alternative reaction pathways

and emphasizes the possibility of an easy changeover of the

mechanism dictated by rather small diﬀerences in the structural

properties of the complexes (trans vs cis geometry) or reaction

conditions (CD₃OD vs CD₃CN). The basic point is that the

energies of B and B⁰must have comparable values.

1

Interestingly, the values of J_PtPof the platinum(IV) com-

plexes exhibit an excellent correlation with the electron-donating

ability of the Y substituent in the aromatic ring of the benzyl

groups, as can be seen from the plot in Figure S5 in the

Supporting Information. The best line of the Hammett plot

(¹J_PtP= Fσ þ C), in which a rogue point has been omitted, has a

slope F of 56.3 ( 1.5 Hz, with a correlation coeﬃcient r = 0.996.

The positive value of F suggests that electron withdrawal from

substituents results in an increase of the ¹J_PtPvalues. An increase

in the ¹J_PtPvalues has been correlated with a decrease of the Pt-

P distance and presumably with an increase of the Pt-P bond

strength.^16,21Similar correlations have been observed for a

variety of phosphaneplatinum(II) complexes, but a survey of

the literature data indicates that it is diﬃcult to predict the

Thus, in methanol, the S_E2 mechanism seems a reasonable

hypothesis for cis-[Pt(CH₂Ar)₂(PEt₃)₂] (Ar = C₆H₄Y) com-

plexes, while trapping 100% of the product as a platinum(IV)

complex in acetonitrile-d₃is consistent with the initial proton-

ation occurring exclusively at the metal.

1

direction and extent of electronic eﬀects on the J_PtPcoupling

constants that appear to depend on a multiplicity of variables,

such as the nature of the trans-activating group, the nature

of groups attached to phosphorus,²²the chelate ring size,²³the

platinum oxidation state,²⁴the Pt-P bond length,²⁵and the

geometry and charge of the complex.^12a,12c,26

Reductive-Elimination Reactions. The principle of micro-

scopic reversibility dictates that the deprotonation step mechan-

ism must occur from the benzylhydridoplatinum(IV). Therefore,

reductive elimination of complexes 2a-2h, C, evolves along the

following steps: (i) dissociation of CD₃CN from the octahedral

benzylhydridoplatinum(IV), C, to generate a cationic, five-coordi-

nate platinum(IV) intermediate, B⁰, (ii) reductive C-H bond

formation producing a platinum(II) alkane σ complex, D, and

(iii) loss of alkane either through an associative or dissociative

substitution pathway to yield the cis-[Pt(CH₂C₆H₄Y)(CD₃CN)-

(PEt₃)₂]^þsolvento complex, E. This multistep mechanism, invol-

ving σ-complex-assisted metathesis, also known as the σ-CAM

process,^29ahas gained a general consensus^29bbased on the observa-

tion that reductive elimination from C, except rare cases,³⁵requires

ligand dissociation^27,36with the formation of a square-pyramidal

five-coordinate structure B⁰.³⁷

Reductive elimination results in a σ complex that so far is

supported by kinetic data and isotopic labeling experiments³⁸

and only by a little other indirect experimental evidence, such as

time-resolved IR spectroscopy on the nanosecond scale³⁹or the

detection of metastable minima in several theoretical investiga-

tions of C-H bond activation.⁴⁰The observation of a mass-law

retardation of the rates of reductive elimination with an increase

in the amount of CD₃CN in CD₂Cl₂/CD₃CN mixtures is consis-

tent with dissociation as a preliminary step for the reaction. The

lack of isotope scrambling with the methylene H atoms belong-

ing to a coordinated benzyl group, observed during the deuter-

olysis measurements, leads to the conclusion that the nucleo-

philic attack by CD₃CN at the alkane σ complex D must be very fast

and, in any case, much faster than the reverse process, preventing a

scrambling equilibrium. There is no evidence here, as in many other

cases,⁴¹of any involvement as intermediates of coordinatively un-

saturated three-coordinate, T-shaped platinum(II) species D⁰. The

latter, however, were shown to play a fundamental role in initiating

the concerted oxidative addition of the alkane C-H bonds in some

electron-rich platinum(II) organometallic complexes.^42,30

Oxidative-Addition Reactions. Alkyl- and arylhydridoplati-

num(IV) complexes were first proposed as reaction intermediates in

early kinetic investigations on the protonolysis of platinum com-

pounds, which were focused almost exclusively on haloalkyl-, diaryl-,

and alkylarylplatinum(II) complexes containing soft donor phos-

phanes.²⁷Protonation was thought to take place by (i) a stepwise

prior oxidative addition on the central metal followed by reductive

elimination [S_E(ox) mechanism] or (ii) a concerted attack at the

metal-Cbond(S_E2 mechanism), as for electrophilic substitution on

main-group organometallics.²⁸Because of the microscopic reversi-

bility principle, this issue becomes of overriding importance in the

mechanistic elucidation of the activation of alkanes or arenes accom-

plished using cationic complexes of the type [PtMe(S)(N-N)]^þ,²⁹

and in some orthometalation reactions using electron-rich organo-

platinum(II) reagents.³⁰Kinetic studies were unable to solve the

problem of the selectivity of the site attack at that time when the

intermediates were not detected directly. However, this uncertainty

still persists nowadays after the discovery that the protonation of

[PtMe₂(N-N)] complexes (with N-N typically a bidentate di-

amine or dimine ligand) with HX leads to the formation of alkyl-

hydridoplatinum(IV) species³¹and other stable methylhydrido-

platinum(IV) complexes can be prepared, especially with the use

of strongly coordinating fac-tridentate ligands.³²However, the ob-

servation of platinum(IV) species could not represent a clear-cut

indication of a S_E(ox) mechanism. Tilset et al.²⁰have objected that

unobserved kinetic products could precede a deceptive observation of

such hydrides, which could only identify the thermodynamic site of

protonation rather than the preferred site of protonation. They have

suggested a competitive trapping technique to solve the ambiguity.²⁰

The choice of the reaction pathway [S_E(ox) or S_E2 mechan-

ism] is still less easy for complexes containing phosphane ligands

that are less able to stabilize platinum(IV) species.³³While the

addition of acid to trans-[PtCl(alkyl)(PR₃)₂] complexes in a

suitable solvent is reported to lead to oxidative-addition products

detectable at low temperature,^34,16for an extensive series of

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Scheme 1. Alternative Reaction Pathways for Acidolysis of an Alkylplatinum(II) Complex

We found that the rate constant for 4-F-toluene elimination

from 2d is 7.78 ꢁ 10^-4s^-1at 233.4 K in acetonitrile-d₃. The tem-

perature dependence of this reaction was studied over the range

230-255 K, and the pseudo-ﬁrst-order rate constants k_obsdwere ﬁtted

to the Eyring equation to yield the values of the activation parameters

ΔH^q= 63.8 ( 3.5 kJ mol^-1and ΔS^q= -29.0 ( 14.6 J K^-1mol^-1.

These activation parameters can be compared with those of reductive

elimination of toluene from [(tmeda)Pt(CH₂Ph)(H)Cl₂] (tmeda =

N,N,N⁰,N⁰-tetramethylethylenediamine) in CD₂Cl₂, ΔH^q= 58.6 (

10 kJ mol^-1and ΔS^q= -77.4 ( 30 J K^-1mol^-1reported by Stahl

et al.^31cThe observed large negative activation entropies contrast

with the idea that elimination reactions should be accompanied by

very positive activation entropies and conﬁrm a mechanism other than

a simple direct reductive elimination from six-coordinate platinum-

(IV). Electrostriction eﬀects and ordering of solvent molecules

accompanying dissociation of a ligand from the octahedral six-

Figure 5. Eyring plot for the isomerization reaction of 3d constructed

by the use of rate data obtained with diﬀerent techniques (empty squares

from stopped-ﬂow spectrophotometric experiments; full squares from

¹H NMR experiments).

coordinate platinum(IV) can well account for a negative value of the

activation entropy.⁴³

Isomerization: Kinetic Studies. In acetonitrile-d₃, the sponta-

neous cis-to-trans geometrical conversion of 3a-3h, resulting from

the reductive elimination of the corresponding benzylhydridoplati-

num(IV) compounds, can be monitored quite easily in the 260-

280 K temperature range by NMR measurements (see Figure 4).

For all of the compounds, the conversion is complete, stable trans

isomers are the only species at the end of the reaction, and the

kinetics can be examined by using the time dependence of the

fraction of reaction (F) of the label. However, the systematic kinetics

were obtained by stopped-flow spectrophotometric measurements

in a temperature range (285-320 K) where the oxidative-addition/

reductive-elimination process is very fast and acidolysis of the

precursors dibenzyl complexes 1a-1h gives directly the cationic

cis-[Pt(CH₂Ar)(CH₃CN)(PEt₃)₂]^þsolvento complexes. The

variable-temperature rate constants for compound 3d obtained with

the two techniques were fitted to the Eyring equation (Figure 5),

leading to ΔH^q= 97 ( 2 kJ mol^-1, ΔS^q= 76 ( 4 J K^-1mol^-1, and

ΔG^q₂₉₈= 74.4 ( 0.8 kJ mol^-1.

The whole set of kinetic data and activation parameters for

3a-3h compounds is collected in Table 5. Typically, the

geometrical conversion of these solvento complexes appears

to be characterized by large positive values of the entropy of

activation, conﬁrming previous ﬁndings on similar systems.

Therefore, the well-established dissociative mechanism illu-

strated in Scheme 2 applies, which involves dissociative loss of

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Table 5. Eﬀect of Variation of the Nature of Ar and of the

Solvent on the Computed Relative Energies (kJ mol^-1) of the

Optimized Structures of the Reactant, the T-Shaped Reaction

Intermediates, the TSs for Their Fluxionality, and the

Product during Isomerization of cis-[Pt(CH₂Ar)(S)(PEt₃)₂]^þ

(S = Solvent) Complexes^a

no.

Ar

S

T-shaped cis TS T-shaped trans trans product

3c C₆H₅

CH₃CN

36.2

39.7

29.1

10.9

10.4

66.7

70.8

60.5

41.3

41.6

50.6

54.7

45.5

25.2

25.4

-18.6

-16.9

-19.7

3f 4-ClC₆H₄CH₃CN

3a 4-CH₃C₆H₄CH₃CN

3c C₆H₅

MeOH

3.30

-3.47

Figure 6. Calculated DFT energy proﬁle for isomerization of the

cationic solvento complex [Pt(CH₂C₆H₅)(CH₃CN)(PMe₃)₂]^þ.

3f 4-ClC₆H₄MeOH

^aAll relative energy values are calculated with respect to the original

cationic solvento complex.

agostic interaction caused by interactions of the unsaturated metal

with the benzyl moiety. On the basis of the values of the free

activation energies, the stabilization energy of an agostic TS relative

to that of a nonagostic TS (as for the methyl compound) is about

20 kJ mol^-1, while the extra TS energy stabilization induced by

Scheme 2. Mechanism for the Isomerization of Cationic

Solventoplatinum(II) Complexes

interactions with a benzyl group is in the range of 10-15 kJ mol^-1

.

Isomerization: DFT Studies. The energy profile for the geome-

trical isomerization of cis-[Pt(CH₂C₆H₅)(CH₃CN)(PMe₃)₂]^þis

given in Figure 6 as a typical example of the shape obtained from

DFT calculations for a variety of systems. Relative energies are cal-

culated with respect to the energy of the cis-[Pt(CH₂C₆H₅)(CH₃-

CN)(PMe₃)₂]^þcomplex indicated as I. In Figure 7 are shown the

fully optimized geometrical structures of minima and TS along with

the most relevant calculated geometrical parameters. The process is

assumed to evolve along the same key steps as those indicated in

Scheme 2, that is, (i) ligand dissociation, (ii) conversion of a three-

coordinate, T-shaped intermediate from a “cis-like” to a “trans-like”

geometry, and (iii) final uptake of the ligand by the latter.

The overall set of DFT data for the studied systems is given in

Table 5, which reports the relative energies of intermediates, of

the TS corresponding to the cis-to-trans rearrangement and of

the product.

Upon comparison of these data with those of the agostic

analogues reported in ref 11, we see that the ﬁrst reaction step,

namely, extrusion of the molecule of solvent from cis-[Pt(R)-

(CH₃CN)(PEt₃)₂]^þcomplexes, is energetically easier when R =

CH₂C₆H₅than when R = C₂H₅, combining a greater lability of

the CH₃CN solvent with respect to methanol with a greater

stability of the three-coordinate cis-[Pt(CH₂C₆H₅)(PEt₃)₂]^þ

complex.

Conversion from a cis-like to a trans-like T-shaped conﬁguration

occurs via a TS that possesses a Y-shaped conﬁguration and usually

requires a very low energy.⁴⁶The energy barrier associated with the

rearrangement of cis-[Pt(R)(PEt₃)₂]^þ, when the R group does not

contain β-hydrogens (R = Me), is very small (average = 9.1 ( 0.4 kJ

mol^-1).¹¹It is doubled (average =18.9 ( 2 kJ mol^-1) when the R

group does contain β-hydrogens (R = Et, Bu) and, when R = benzyl,

becomes still greater (average 30.9 ( 0.4 kJ mol^-1). Thus, conver-

sion of an agostic “cis-like” three-coordinate form to its trans form

requires a supplementary consumption of energy. This additional

energy request is increased further when a -CH₂Ph-metal bond is

involved, and it becomes comparable with the dissociation energy of

the solvent from the coordination sphere of the metal. Another

peculiarity of the potential energy surface (PES) shown in Figure 6,

still depending upon the strong stability conferred by the benzyl-

metal bond to the “cis-like” intermediate, is given by the fact that

intermediate II is about 15 kJ mol^-1more stable than intermediate

III, while methyl and ethyl complexes show a reverse trend.

the solvent molecule and interconversion of two geometrically

distinct three-coordinate, T-shaped, 14-electron intermediates.

A rate law of the form

k_D

k_i¼

ð5Þ

1 þ ðk_-D=k_TÞ½Sꢀ

can be derived, in which the term (k_-D/k_T)[S] measures the

retardation due to capture of the ﬁrst intermediate by the bulk

solvent.⁴⁴

The mechanism and the rate law account for the crucial role

played by the nature of the solvent in such reactions whose rate can

be ﬁnely tuned through appropriate control of the coordinating

properties of the leaving group.⁴⁵Other main factors that usually

combine in aﬀecting the rate of conversion are (i) the stereoelec-

tronic properties of the phosphane ligands^12a,12cand (ii) interac-

tion of the β-H atoms of the alkyl chain with the coordinatively

unsaturated intermediate. The sharp acceleration of the reaction

rate produced by such agostic interactions has been deﬁned as the

“β-hydrogen kinetic eﬀect”.¹¹We observe that the rate of isomer-

ization of the benzyl cis-solvento [Pt(CH₂Ar)(CH₃CN)(PEt₃)₂]^þ

complexes (Ar = C₆H₅; k_i= 0.820 s^-1, ΔH^q= 83.8 ( 1.3 kJ mol^-1

,

ΔS^q= 34.6 ( 4 J K^-1mol^-1, and ΔG^q₂₉₈= 73.5 ( 0.1 kJ mol^-1) is

higher than that of the ethyl [Pt(C₂H₅)(CH₃CN)(PEt₃)₂]^þcom-

plex (k_i= 4.9 ꢁ 10^-3s^-1, ΔH^q= 100.5 kJ mol^-1, ΔS^q= 48.2 J K^-1

mol^-1, and ΔG^q= 86.1 kJ mol^-1) and is several orders of magni-

tude higher than that of the methyl [Pt(CH₃)(CH₃CN)(PEt₃)₂]^þ

complex (k_i= 2.42 ꢁ 10^-6s^-1, ΔH^q= 147.7 kJ mol^-1, ΔS^q= 139 J

K

^-1mol^-1, and ΔG^q= 106.3 kJ mol^-1).¹¹Thus, the rate increases

markedly along the series R, CH₃, C₂H₅< CH₂C₆H₅, in the ratio

1:2000:340 000, giving a clear-cut indication of a stabilization

energy of the reaction TS/intermediate higher than that of the

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from this value, (iii) the angle at the methylene C atom (Pt-

CH₂-C_ipso) is well below the value expected for a tetrahedral sp³

C environment, being 80.3ꢀ, and (iv) its two M-C_orthodistances

(2.73 and 3.21 Å) are much longer than those commonly

observed for platinum(II), palladium(II), or nickel(II) com-

plexes containing η³-CH₂Ph groups consistent with an allyl type

of bonding where the Pt-C distances to the outer allylic C atoms

range from 2.110 to 2.449 Å.⁴⁷In Table 6, we report for

comparison some significant M-C_ipso, M-C_ortho, and M-C₁

distances (Å) for selected platinum(II), palladium(II), or nickel(II)

η³-benzyl complexes. In some of these complexes, the benzyl group

is bonded to the metal in an unsymmetrical fashion, with the

metal-C bond length increasing in the order M-CH₂< M-C_ipso

<

M-C_ortho; however, the M-C_orthodistance does not exceed

2.446 Å. Other interesting platinum(II) compounds with an allyl

type of bonding are the dinuclear [{Pt(Et₂PC₂H₄PEt₂)}₂(μ-

biphenyl)][BArF]₂complex,⁴⁸and some η³-bonded platinum

benzyl cations [Pt(N-N)(CH₂C₆H₅]^þ, where N-N represents

a bidentate diimine ligand, that have been identified by NMR in

solution.⁴⁹We performed a DFT optimization of one simplified

form of such cationic complexes, shown in Figure 8, which

confirms that the benzyl anion is in an η³-bonded mode, with

M-C distances increasing slightly along the series M-CH₂, 2.10

Å < M-C_ipso, 2.26 Å < M-C_ortho, 2.35 Å.

The energy stabilization associated with the presence of a

metal-η²-benzyl linkage in the transient cis-like T-shaped

Table 6. Comparison of the Intramolecular Dimensions in

Some Structurally Characterized Metal η³-Benzyl Complexes

M-

CH₂

M-

C_ipso

M-

c

a

b

complex

C_orthoref

Figure 7. Fully optimized geometrical structures of stationary points

intercepted along the energy proﬁle for isomerization of the cationic

solvento complex [Pt(CH₂C₆H₅)(CH₃CN)(PMe₃)₂]^þ. Bond lengths

are in angstroms and angles in degrees.

[Pt(dbpp)(η³-anti-1-MeCHC₆H₄-p-Br)]

[Pt(BAB)(η³-1-MeCHC₆H₅)]

[Pt(acac)(η³-CPh₃)]

2.163

2.118

2.088

2.117

2.242

2.145

2.120

2.135

2.446 47a

2.110 47b

2.148 47c

2.251 47d

[Pd(phen)(anti-η³-

In Figure 7 are shown the optimized structures of the

stationary points of Figure 6.

CH(CH₂CH₃)C₆H₅)]BArF

[Pd(dipy)(η³-syn-R-(CH₃)CHC₆H₅)]

[Ni(η³-CH₂C₆H₄-o-Me)ClPMe₃]

[Ni(η³-CH₂C₆H₄-p-CF₃)(dippm)]

Ni(η³-CH₂C₆H₅)[Ph₂PC₆H₄C-

(O-B(C₆F₅)₃)O-κ²-P,O]

2.08

2.13

2.28 47e

2.318 47f

2.185 47g

2.198 47i

1.930

1.981

1.944

2.050

2.044

2.064

Both I and IV, the starting cis-solvento and the ﬁnal trans-

solvento [Pt(R)(CH₃CN)(PMe₃)₂]^þcationic complexes, show

the expected geometrical features with bond separations and

angles comparable to those of similar structurally characterized

compounds, includingthedibenzylprecursorsshown inFigures1

and 2. The benzyl ligand in each of the complexes is normal,

exhibiting a M-CH₂single bond length of 2.160 Å in I and 2.092

Å in IV and a pseudotetrahedral environment around its

methylene C atom. Jahn-Teller instability favors a T-shaped

conﬁguration for both “cis-like” I and “trans-like” three-coordi-

nate [Pt(CH₂C₆H₅)(PMe₃)₂]^þ(IV) species, which mutually

convert passing through a Y-shaped transition state TS.

^aMetal to methylene carbon bond length. ^bMetal to ipso-carbon bond

length. ^cShortest metal to o-carbon bond distance.

Stabilization of Transient Three-Coordinate Species. It is

established that the labile existence of unsaturated 14-electron

intermediates is favored by several factors such as strong σ-

donating ligands, overcrowding of coordination sphere, and

agostic interactions. The most chemically interesting feature of

the computed structures displayed in Figure 7 is that in the

present case formation of an η²bond that involves the benzyl

ligand and the unsaturated metal is responsible of stabilization of

the “cis-like” T-shaped, three-coordinate intermediate II. The

“dihapto” bonding mode is characterized by the following

distinctive parameters: (i) the metal-methylene bond length

(2.116 Å) is in the range expected for a normal Pt^II-C single

bond, (ii) the M-C_ipsodistance (2.361 Å) is not very far away

Figure 8. Optimized DFT structure of [(MeNdC(Me)C(Me)d

NMe)Pt(η³-CH₂C₆H₅)]^þ.

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Inorganic Chemistry

ARTICLE

intermediate has been estimated starting from the fully optimized

structure of the cis-[Pt(CH₂C₆H₅)(MeOH)(PEt₃)₂]^þsolvent

complex and performing a relaxed scan calculation of the Pt-O

bond length, while MeOH leaves the coordination sphere of the

metal.⁵⁰The stabilization energy amounts to 31.8 kJ mol^-1and

compares well with the value of the agostic stabilization energy

(24.5 kJ mol^-1) calculated using a similar method.¹¹Thus, there

is a substantial agreement between the kinetic and computational

data, indicating in ≈10-15 kJ mol^-1the energy diﬀerence

between the incipient β-agostic and the incipient ipso-carbon

interactions shown in Figure 9.

Table 7 gives a summary of the kinetic and calculated activa-

tion data. The presence of a preequilibrium and of a retardation

term (k_-D/k_T)[S] in the rate law complicates the kinetics, and

the resulting k_obs, from which the kinetic activation data are

derived, is a composite rate constant that contains equilibrium

constants and rate constants for elementary steps. In constrast,

theoretical calculations refer only to elementary steps. Thus, we

cannot expect a strict analogy between kinetic and computational

data, in their absolute values but only good correlations and

similar trends. As an example, an excellent straight line correlates

the calculated (cis-[Pt(R)(CH₃CN)(PMe₃)₂]^þ) and experimen-

tal values of free energy of activation for isomerization of

cis-[Pt(R)(CH₃CN)(PEt₃)₂]^þ(R = Me, Et, Bz). The plot is

reported as Figure S6 in the Supporting Information.

Leaving-Group and Substituent Effects. The theoretical

data in Table 6 confirm the experimental findings. The energy

involved in the formation of the “cis-like” T-shaped, 14-electron,

three-coordinate intermediate (see data) depends on the nature

of the leaving group, with the sequence of lability being CH₃CN

, MeOH. This confirms the key role that dissociation of the

solvent from platinum(II) plays in the energetics of the overall

process and in the different observed reaction pathways.

Figure 10. Dependence of the rates of isomerization (k_iat 298.2 K)

of cis-[Pt(PEt₃)₂(CH₂C₆H₄Y)(CD₃CN)]^þcomplexes upon the elec-

tron-donor properties of Y substituents on the aromatic ring.

Table 8. Experimental Data for the X-ray Diﬀraction Study of

Compounds 1a and 1f

1a

1f

formula

mol wt

data collecn T, K

diﬀractometer

cryst syst

space group

a, Å

C₂₈H₄₈P₂Pt

641.69

295(2)

C₂₆H₄₂Cl₂P₂Pt

682.53

295(2)

Bruker SMART CCD

monoclinic

C2/c (No. 15)

16.596(2)

10.149(1)

17.844(2)

100.179(2)

2958(2)

4

monoclinic

Cc (No. 9)

12.592(1)

31.223(3)

8.4416(9)

120.802(2)

2850.6(9)

4

b, Å

c, Å

β, deg

V, Å³

Z

F

_calcd, g cm^-3

1.441

4.864

1.590

5.234

μ, cm^-1

radiation

Mo KR (graphite monochromator,

λ = 0.710 73 Å)

Both kinetic and DFT data indicate that isomerization is

strongly aﬀected by the amount of electron density at the metal

θ range, deg

2.32 < θ < 26.02

12964

2907

1.99 < θ < 26.02

12487

5573

no. of data collected

no. of independent data

no. of obsd reﬂns (n_o)

[ |F_o|²> 2.0σ(|F|²) ]

no. of reﬁned param (n_v)

R_int

R (obsd reﬂns)

wR2 (obs reﬂns)

GOF

2742

5000

141

280

0.0310

0.0174

0.0434

1.113

0.0286

0.0270

0.0601

1.047

absolute structure param (Flack)

-0.009(7)

P

2

^aR_int

=

|F_o- ÆF_oæ|/ ΣF_o. ^bR = (|F_o- (1/k)F_c|)/ |F_o|.

P

^cwR2 = { [w(F_o²- (1/k)F_c²)²]/ w|F_o| ]}^1/2. ^dGOF = [ _w(F_o

2 2

2

Figure 9. Optimized structures of the agostic [Pt(PMe₃)₂(ethyl)]^þand

of [Pt(PMe₃)₂(η²-benzyl)]^þintermediates.

- (1/k)F_c²)²/(n_o- n_v)]^1/2

.

Table 7. Summary of Kinetic and Theoretical Data for the

Geometrical Isomerization of cis-[PtL₂(R)(CH₃CN)]^þ

Complexes

atom, brought about by substituents on the aromatic ring. Electron-

donating substituents on the aromatic ring enhance the rate of

isomerization of the complexes cis-[Pt(PEt₃)₂(CH₂C₆H₄Y)(CD₃-

CN)]^þ, while electron-attracting groups have the reverse eﬀect. The

kinetic data for the isomerization ﬁt a rather good Hammett plot

ﬁtting the equation log (k/k_H) = Fσ þ C. The best line has a slope F

of -2.71, with a correlation coeﬃcient r = -0.993 (Figure 10).

The constant F measures the sensitivity of the isomerization to

electronic eﬀects transmitted by meta and para substituents on the

aryl ring, and it is of the expected sign and magnitude because

electron-donating (negative σ) substituents facilitate the departure

of CD₃CN with its previously bonding electron pair by stabilizing a

TS similar to that of the electron-deﬁcient platinum cation.

stabilization

energy ^f,g

R

k_i^a,b

ΔH ^qa,cΔS^qa,d

ΔG^qa,e

ΔH ^qc,f,g

Me 2.42 ꢁ 10^-6148

139

48

106

86

116

91

Et 4.90 ꢁ 10^-3100

25

35

Bz 0.820

84

35

73

67

^aL = PEt₃. ^bFirst-order rate constant in s^-1at 298.2 K. ^cEnthalpies of

activation in kJ mol^-1^dEntropies of activation in J K^-1mol^-1. ^eFree

.

energy of activation in kJ mol^-1at 298.2 K. ^fL = PMe₃. ^gFrom

theoretical calculations.

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ARTICLE

An analogous result stems from the DFT study, which indicates

that the energy of the TS for isomerization of cis-[Pt(PEt₃)(CH₂-

C₆H₄Y)(CD₃CN)]^þdecreases in the order of the electron-donor

capacity of the substituent Y (Y = 4-CH₃> H > 4-Cl), reﬂecting the

eﬃciency of the substituents in stabilizing the three-coordinate

platinum(II) cationic TS (Figure S8 in the Supporting Information).

open the way to control of the rates and the stereospeciﬁcity of

many fundamental processes.

’ EXPERIMENTAL SECTION

General Procedures and Chemicals. All manipulations were

performed in a drybox under a dry and oxygen-free dinitrogen atmo-

sphere in glassware that had been oven-dried or by using standard

Schlenk techniques. Unless otherwise noted, all reagents were commer-

cially obtained and, where appropriate, purified prior to use. Tetrahy-

drofuran, diethyl ether, and toluene (analytical reagent grade, Lab-Scan

Ltd.) were distilled under nitrogen from sodium benzophenone ketyl.

Dichloromethane was distilled from barium oxide.⁵¹Phosphane ligands

(from Strem), acetonitrile spectrophotometric grade for kinetic studies,

CD₃CN, CD₃OD, CH₃OD, and DOTf for NMR measurements were

used as received from Aldrich Chemical Co. HB{[3,5-C₆H₃-(CF₃)₂)₄]}

(HBArF) was prepared by the Brookhart method.⁵²Platinum dibenzyl

complexes were stored in a drybox under a dinitrogen atmosphere.

Instrumentation and Measurements. Microanalyses were

performed by the Microanalytical Laboratory, University of Dublin,

’ CONCLUSIONS

In this study, the diﬀerent kinetic behavior of meta- and para-

substituted dibenzylplatinum(II) complexes cis-[Pt(CH₂Ar)₂-

(PEt₃)₂] upon protonolysis and deuterolysis by H(D)X acids

in methanol or in acetonitrile-d₃as solvents is described. Low-

temperature NMR spectra recorded in CD₃OD show that the

reactions lead directly to the formation of the corresponding

solvento species trans-[Pt(CH₂Ar)(PEt₃)₂(CD₃OD)]X and

toluene derivatives. Instead, in acetonitrile-d₃, a suitable tem-

perature increase allows one to kinetically distinguish three sub-

sequent stages, such as the instantaneous formation of new

benzylhydridoplatinum(IV) complexes cis-[Pt(CH₂Ar)₂(H)-

(CD₃CN)(PEt₃)₂]X (at 230 K), followed by reductive elimina-

tion to yield cis-[Pt(CH₂Ar)(CD₃CN)(PEt₃)₂]X and toluene

derivatives (in the range 230-255 K), and ﬁnally spontaneous

isomerization of the cis-cationic solvento species to the corre-

sponding trans isomers (in the range 260-280 K).

Furthermore, this work shows by a kinetic and theoretical

approach that the energy of T-shaped, three-coordinate species,

formed upon ligand dissociation from a square-planar platinum-

(II) complex, can be modulated by control of electron donation

to the metal through secondary interactions. Because of its

coordinative and electronic unsaturation, the metal shows a great

tendency to interact with that part of a coordinated ligand that

can satisfy this requirement, such as the C-H bond of a dangling

coordinated alkyl or the ipso-C of a benzyl group. The onset of

such secondary interactions is indicated by a marked increase of

the rate of reaction and can be rationalized by geometry

optimization and DFT energy calculation of the species inter-

cepted at the stationary points of the PESs. We observe that there

is an energetic hierarchy in such interactions, with the bond with

an ipso-C being stronger than that with a C-H bond, even

though for both of them it should be more appropriate to speak

in terms of “incipient” interactions. As a matter of fact, the

β-hydrogen kinetic eﬀect is characterized by an intrinsic ener-

getic weakness of the agostic bond in the ethyl intermediate,

while in the “ipso-carbon kinetic eﬀect”, the presence of an η²-

benzyl bond in the benzyl intermediate reﬂects the diﬃculty of

developing a stronger η³-allyl-type bond. In light of the extreme

electrophilicity of the metal in a coordinatively unsaturated metal

d⁸three-coordinate 14-electron species, and of its tendency to

stabilize intramolecular interactions, its fate in solution is quite

unpredictable. As a matter of fact, such labile intermediate species

can undergo a number of diﬀerent processes as follows: (i) easy

attack or reentry in the coordination sphere by even very weak

nucleophiles such as a solvent molecule; (ii) a topological

process that changes the T-shaped geometry; (iii) intraligand

processes such as C-H bond activation, β-hydrogen elimina-

tion, η¹to η³ﬂuxional changes, etc. Because of the great interest

in the possibility of considering alternative low-energy pathways

involving these unsaturated electronically and coordinavely unsatu-

rated ML₃species, the ﬁne-tuning of the electronic request by the

metal, through a rationale choice of the coordinated ligands, can

1

Dublin, Ireland. H and ³¹P NMR spectra were recorded on a Bruker

AMX-300 spectrometer. The ¹H resonance of the solvent was used as an

internal standard, but chemical shifts are reported with respect to Me₄Si.

³¹P{¹H} NMR shifts are referenced to external 85% H₃PO₄. The data

are reported as follows: chemical shift in ppm (δ) units, multiplicity,

coupling constants (Hz), and integration. The temperature within the

probe was checked using the methanol method.⁵³Electronic spectra of

the solutions were taken with a rapid-scanning Hewlett-Packard model

8452A spectrophotometer or a Jasco V-560 UV-vis spectrophot-

ometer. Fast reactions required the use of an Applied Photophysics

Bio Sequential SX-17 MX stopped-flow ASVD spectrofluorometer. Rate

constants were evaluated by the Applied Photophysics software

package⁵⁴and are reported as average values from five to seven

independent runs.

Dibenzyl Complexes. The complex cis-[Pt(CH₂Ar)₂(PEt₃)₂]

(Ar = C₆H₅, 1c) was prepared according to the literature method.¹²

The complexes (Ar = 4-Me-C₆H₄, 1a; 3-Me-C₆H₄, 1b; 4-F-C₆H₄, 1d;

3-F-C₆H₄, 1e; 4-Cl-C₆H₄, 1f; 3-Cl-C₆H₄, 1g; 3-CF₃-C₆H₄, 1h) are new

and were synthesized according to a general procedure as follows. An

ethereal suspension of cis-[PtCl₂(PEt₃)₂] (400 mg, 0.8 mmol) at 0 ꢀC

and under an inert atmosphere was treated with a stoichiometric amount

of Grignard reagent, prepared from magnesium and the appropriate

benzyl halide. The mixture was stirred at 20 ꢀC for 2 h and then

hydrolyzed with ice. The crude product isolated from the dried

(Na₂SO₄) organic layer by evaporation of the residual solvent under

reduced pressure was recrystallized from a dichloromethane/methanol

mixture. The identity and purity of the these compounds were estab-

1

lished by elemental analysis and by H and ³¹P{¹H} NMR spectra.

Elemental analyses were consistent with the theoretical formulas.

cis-[Pt(CH₂C₆H₄-4-Me)₂(PEt₃)₂] (1a). Yield: 88.0%. Anal. Calcd

for C₂₈H₄₈P₂Pt: C, 52.41; H, 7.54. Found: C, 51.21; H, 7.36. ¹H NMR

(CH₃CN-d₃, T = 298 K): δ 7.06 (d, 4H, ³J_HH= 7.5 Hz, H_2,6), 6.83 (d,

4H, ³J_HH= 7.5 Hz, H_3,5), 2.39 (m, 4H, ²J_PtH= 83.4 Hz, ³J_PH= 16.9 Hz,

3

Pt-CH₂), 2.12 (s, 6H, Ph-CH₃), 1.74 (m, 12H, J_HH= 7.5 Hz, P-

CH₂), 0.96 (m, 18H, ³J_HH= 7.5 Hz, P-CH₃). ³¹P{¹H} NMR (CH₃CN-

d₃, T = 298 K): δ 12.5 (¹J_PtP= 1943 Hz).

Alkylhydridoplatinum(IV) Species 2a-2h. A solution of an

appropriate amount of 1a-1h (5-6 mg, ca. 10 μmol) in CH₃CN-d₃(ca.

400 μL) was cooled to -78 ꢀC in an NMR tube and accurately layered

with a solution of 2 equiv of acid (HOTf, HBArF, or HBF₄Et₂O) in

3

CH₃CN-d₃. While keeping the tube as cool as possible, it was capped and

shaken to mix the contents immediately before transfer to a precooled

NMR probe. Relevant ¹H and ³¹P NMR data for cis-[Pt(CH₃CN)(H)-

(CH₂C₆H₄Y)₂(PEt₃)₂]^þspecies are listed in Table 1.

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Inorganic Chemistry

ARTICLE

cis-[Pt(CH₂C₆H₄-4-Me)₂(H)(CD₃CN)(PEt₃)₂](BArF) (2a). ¹H

NMR (CH₃CN-d₃, T = 238.5 K): δ 7.14-6.65 (m, 8H, H_2,6þ H_3,5),

2.35 (m, 4H, ²J_PtH= 65.5 Hz, ³J_PH= 14.9 Hz, Pt-CH₂), 1.79 (m, 12H, ³J_HH

= 8.9 Hz, P-CH₂), 2.08 (s, 6H, Ph-CH₃), 0.96 (m, 18H, ³J_HH= 8.9 Hz,

P-CH₃), -21.4 (m, 1H, ¹J_PtH= 1271 Hz, ²J_PH= 26.8 Hz, Pt-H). ³¹P{¹H}

NMR (CH₃CN-d₃, T = 238.5 K): δ -3.5 (¹J_PtP= 1188 Hz).

Structural Study of 1f. The space group was determined from the

systematic absences and the successful refinement, while the cell constants

were refined by least squares, at the end of the data collection, using 1024

reflections (2θ_maxe 51.92ꢀ). The data were collected by using ω scans, in

steps of 0.3ꢀ. For each of the 1850 collected frames, the counting time was 40 s.

The least-squares reﬁnement was carried out using anisotropic dis-

placement parameters for all non-H atoms, while the H atoms were

included in the reﬁnement as described above. Reﬁning the Flack’s

parameter⁶¹tested the handedness of the structure.

Monoalkyl Solvento Complexes cis-[Pt(CH₂C₆H₄Y)(CD₃CN)-

(PEt₃)₂]^þ(3a-3h). Upon a gentle increase of the temperature in the

range 230-255 K, the alkylhydridoplatinum(IV) species 2a-2h of the

previous experiment undergo easy reductive elimination. The ¹H and ³¹

P

Kinetic Measurements. Protonolysis and deuterolysis reactions

of 1a-1f were followed by conventional spectrophotometric techniques

in methanol and methanol-d₄, respectively, under pseudo-first-order

conditions and were started by injecting solutions of HOTf or DOTf to

solutions of dibenzyl complexes with acid in excess. The second-order rate

constants for protonolysis, k_H, at 297 K were obtained by least-squares

regression analysis of the linear plots of the pseudo-first-order rate constants

versus the concentration of the acid. At other temperatures, the values of k_H

were obtained from the ratio of the measured pseudo-first-order rate con-

stants k_obsdto [H^þ]. The kinetics of isomerization of the solvento complexes

3a-3h in acetonitrile were followed in the UV region by stopped-flow

spectrophotometric techniques. Fresh solutions of the complexes (0.05-0.1

mM) were used for all kinetic runs. The reactions were carried out in the

thermostatted cell of the instrument, with a temperature accuracy of

(0.02 ꢀC, and were started by injecting acetonitrile solutions of HBF₄into

solutions of dibenzyl complexes (in a 100:1 acid-to-complex ratio). The acid

concentration does not affect the isomerization reaction and was calculated

to produce a very fast cleavage of the Pt-benzyl bond. All reactions obeyed a

first-order rate law until well over 90% of the reaction, and the rate constants,

k_obsd/s^-1, were obtained from a nonlinear least-squares fit of the experi-

mental data to D_t=D_¥þ (D₀-D_¥) exp(-k_obsdt) withD₀, D_¥, andk_obsdas

the parameters to be optimized (D₀= absorbance after mixing of the

reagents; D_¥= absorbance at completion of the reaction). Activation

parameters were derived from a linear least-squares analysis of ln(k_obsd/T)

vs T^-1data according to the linear expression of the Eyring equation

NMR spectra of the ensuing toluene derivatives and cis-monobenzylsolvento

species were recorded and resonances assigned.

cis-[Pt(CH₂C₆H₄-4-Me)(CD₃CN)(PEt₃)₂]^þ(3a). ¹H NMR (CH₃-

CN-d₃, T = 238.5 K): δ 6.97 (d, 2H, ³J_HH= 8.8 Hz, H_2,6), 6.87 (d, 2H,

³J_HH= 8.8 Hz, H_3,5), 2.29 (m, 2H, ²J_PtH= 60.2 Hz, ³J_PH=8.8 Hz, Pt-CH₂),

2.04 (s, 3H, Ph-CH₃), 1.74 (m, 6H, ³J_HH= 7.5 Hz, P_A-CH₂), 1.63 (m,

6H, ³J_HH= 7.5 Hz, P_B-CH₂), 0.90 (m, 9H, ³J_HH= 7.5 Hz, P_A-CH₃), 0.80

(m, 9H, ³J_HH= 7.5 Hz, P_B-CH₃). ³¹P{¹H} NMR (CH₃CN-d₃, T = 238.5

K): δ 16.9 (d, ¹J_PtP= 1790 Hz, ²J_{P P}= 17.80 Hz, P_Atrans to the benzyl

B

group), 12.2 (d, ¹J_PtP= 4142 Hz, ²J_P^A_P= 17.80 Hz, P_Btrans to CD₃CN).

A

B

Monoalkyl Solvento Complexes trans-[Pt(CH₂C₆H₄Y)(CD₃-

CN)(PEt₃)₂]^þ(4a-4h). Spontaneous conversion of the cis-solvento

isomers 3a-3h into their corresponding trans derivatives was obtained

easily by setting aside at room temperature the solution resulting from the

electrophilic attack on the corresponding dibenzyl precursors. The process

goes to completion in a few seconds.

trans-[Pt(CH₂C₆H₄-4-Me)(CD₃CN)(PEt₃)₂]^þ(4a). ¹H NMR

3

(CH₃CN-d₃, T = 298 K): δ 7.22 (d, 2H, J_HH= 7.5 Hz, H_2,6), 6.98

3

2

3

(d, 2H, J_HH= 7.5 Hz, H_3,5), 2.60 (m, 2H, J_PtH= 98.1 Hz, J_PH

=

16.9 Hz, Pt-CH₂), 2.27 (s, 3H, Ph-CH₃), 1.73 (m, 12H, ³J_HH= 7.5 Hz,

P-CH₂), 1.06 (m, 18H, J_HH= 7.5 Hz, P-CH₃). ³¹P{¹H} NMR

3

(CH₃CN-d₃, T = 298 K): δ 20.8 (¹J_PtP= 2783 Hz).

All details concerning characterization of 1b-1h, 2b-2h, 3b-3h,

and 4b-4h (i.e., synthesis yield, elemental analysis, and ¹H/³¹P NMR)

are given in the Supporting Information.

!

ꢀ ꢁ

k

k_BΔS^qΔH^q

X-ray Data Collection and Structure Determination for 1a

and 1f. Light-yellow prismatic crystals of 1a and (1f) suitable for X-ray

diffraction were obtained by the slow diffusion of hexane into a concen-

trated toluene solution and are air-stable. Crystals were mounted on a Bruker

SMART diffractometer, equipped with a CCD detector, for the unit cell and

space group determination and data collection. Selected experimental

crystallographic and other relevant data are listed in Table 8 and in the

Supporting Information. Data were corrected for Lorentz and polarization

factors with the data reduction software SAINT⁵⁵and empirically for

absorption using the SADABS program.⁵⁶The structures were refined by

ln

¼

þ

ð5Þ

T

h

R

RT

and are listed in Table 4.

Computational Details. Geometry optimizations as well as

frequency calculations for all of the model complexes considered here

were performed at the DFT level, employing Becke’s three-parameter

hybrid functional⁶²combined with the Lee, Yang, and Parr (LYP)⁶³

correlation functional, denoted as B3LYP within the GAUSSIAN03/

DFT package.⁶⁴The LANL2DZ effective-core potential⁶⁵was used for

the metal center. In LANL2DZ, the valence shell is explicitly represented

using a double-ζ and includes both n = 5(s,pd) and n = 6(s) orbitals. The

standard 6-31þG** basis set⁶⁶was employed for the rest of the atoms.

No symmetry restrictions were imposed during the geometry opti-

mizations, whereas for each optimized stationary point, vibrational analysis

was performed to determine its character (minimum or saddle point) and to

evaluate the zero-point vibrational energy corrections, which are included in

all relative energies. For all of the reported TSs, it was carefully checked that

the vibrational mode associated with the imaginary frequency corresponds to

the correct movement of involved atoms. All of the minima connected by a

given TS were conﬁrmed by intrinsic reaction coordinate⁶⁷driving calcula-

tions (in mass-weighted coordinates), as implemented in the Gaussian03

program.

full-matrix least-squares analysis⁵⁷(with the function minimized being

P

[

w(F_o²- (1/k)F_c²)²]). For all structures, no extinction correction was

deemed necessary. The scattering factors used, corrected for the real and

imaginary parts of the anomalous dispersion, were taken from the literature.⁵⁸

All calculations were carried out by using the WINGX⁵⁹and SHELX-97 and

ORTEP⁶⁰programs.

Structural Study of 1a. The space group was unambiguously

determined from the systematic absences, while the cell constants were

refined by least squares, at the end of the data collection, using 1024

reflections (2θ_maxe 51.83ꢀ). The data were collected by using ω scans,

in steps of 0.3ꢀ. For each of the 1860 collected frames, the counting time

was 30 s.

The Pt atom lies on a crystallographic symmetry element; thus, only

half of the molecule is the asymmetric unit. The least-squares reﬁnement

was carried out using anisotropic displacement parameters for all non-H

atoms. The contribution of the H atoms, in their calculated positions,

was included in the reﬁnement using a riding model [C-H = 0.96 Å;

B(H) = axB(C_bonded) Å²], where a = 1.5 for the H atoms of the methyl

groups and a = 1.2 for the others).

’ ASSOCIATED CONTENT

S

Supporting Information. Experimental Section giving

b

¹H/³¹P{¹H} NMR complex characterization, yields, and elemental

analysis for the starting complexes 1b-1h, the intermediate species

2236

dx.doi.org/10.1021/ic101879s |Inorg. Chem. 2011, 50, 2224–2239

Inorganic Chemistry

ARTICLE

2b-2h and 3b-3h, and the ﬁnal compounds 4b-4d, tables giving

rate constants for protonolysis and deuterolysis of the Pt-C bond

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reductive-elimination rate constants for 3d, all primary kinetic data

for isomerization of 3a-3h in CH₃CN, and Cartesian coordinates

of the B3LYP-optimized T-shaped cis- and trans-[Pt(CH₂C₆H₄Y)-

(PMe₃)₂]^þ(Y = H, 4-Cl, 4-Me) species, ﬁgures showing a view of a

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molecule of compound 1f with a weak nonconventional Pt

H

3 3 3

interaction with H7, exponential curves for molar fraction variation

as a function of time and an Eyring plot for the reductive-elimination

reaction, a typical kinetic run for the isomerization reaction and for

reductive elimination, dependence of the coupling constants ¹J_PtP

for the benzylhydridoplatinum(IV) complexes 2a-2h on Ham-

mett’sσof substituent groups, correlation between the experimental

and computed free energies of activation for isomerization, DFT-

optimized geometrical structures of the V and VI [Pt(CH₂C₆H₅)-

(MeOH)(PEt₃)₂]^þ, PES plots showing the electronic eﬀects on the

activation energy for isomerization of cis-[Pt(CH₂C₆H₄-4Y)(PEt₃)₂]

by para-substituent groups on the benzyl rings, and crystallographic

data for structures 1a and 1f in CIF format. This material is available

free of charge via the Internet at http://pubs.acs.org.

(9) Pregosin, P. S. Chem. Commun. 2008, 4875–4884 and references

cited therein.

’ AUTHOR INFORMATION

(10) Scott, N. M.; Dorta, R.; Stevens, E. D.; Correa, A.; Cavallo, L.;

Nolan, S. P. J. Am. Chem. Soc. 2005, 127, 3516–3526.

(11) Romeo, R.; D’Amico, G.; Sicilia, E.; Russo, N.; Rizzato, S. J. Am.

Corresponding Author

*Phone: þ39-090-6765734. Fax: þ39-090-3974108. E-mail:

Chem. Soc. 2007, 129, 5744–5755.

rosaria.plutino@pa.ismn.cnr.it.

(12) (a) Romeo, R. Comments Inorg. Chem. 2002, 23, 79–100 and

references cited therein. (b) Romeo, R.; Plutino, M. R.; Elding, L. I.

Inorg. Chem. 1997, 36, 5909–5916. (c) Romeo, R.; Alibrandi, G. Inorg.

Chem. 1997, 36, 4822–4830. (d) Alibrandi, G.; Minniti, D.; Monsꢀu

Scolaro, L.; Romeo, R. Inorg. Chem. 1988, 27, 318–324. (e) Romeo, R.;

Minniti, D.; Lanza, S.; Uguagliati, P.; Belluco, U. Inorg. Chem. 1978, 17,

2813–2818.

(13) For a discussion on dissociative pathways in platinum(II)

chemistry, see: (a) Romeo, R. Comments Inorg. Chem. 1990, 11, 21–57.

(14) Carmona, E.; Marin, J. M.; Paneque, M.; Poveda, M. L.

Organometallics 1987, 6, 1757–1765 and references cited therein.

(15) Chatt, J.; Shaw, B. L. J. Chem. Soc. 1959, 705, 4020–4033.

(16) Romeo, R.; D’Amico, G. Organometallics 2006, 25, 3435–3446.

(17) (a) Allen, F. H.; Pidcock, A. J. Chem. Soc. A 1968, 2700–2704.

(b) Appleton, T. G.; Clark, H. C.; Manzer, L. E. Coord. Chem. Rev. 1973,

10, 335–422.

’ ACKNOWLEDGMENT

We are grateful to the MURST (PRIN 2007), University

of Messina (PRA 2004), and CNR for funding this work.

E. Guido, G. D’Amico, A. Romeo, and M. R. Plutino wish to

thank Mr. G. Irrera for technical assistance.

DEDICATION

^†Dedicated to the memory of Professor Raﬀaello Romeo, who

died on February 23, 2009.

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1

free and coordinated acetonitrile in [Pt(dppe)(CH₃)(CH₃CN]^þby H

NMR magnetization transfer experiments have shown that the exchange is

very fast (“rate” = k₂[Complex][CH₃CN], with k₂= 0.711 s^-1at 298 K in

CD₂Cl₄). The exchange takes place with an associative mode of activation.

The presence of a similar process on cis-[Pt(PEt₃)₂(CH₂R)(CH₃CN)]^þis

obvious, but it is fruitless in aﬀecting isomerization. Rather, the exchange

gives an independent clear-cut indication that the rate of reentry of CH₃CN

on cis-[Pt(PEt₃)₂(CH₂R)]^þmust be very high and that the condition (k_-D

. k_T) for a preequilibrium treatment in deriving the rate law is totally

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Inorganic Chemistry

ARTICLE

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Mechanistic insight into protonolysis and Cis - Trans isomerization of benzylplatinum(II) complexes assisted by weak ligand-to-metal interactions. A combined kinetic and DFT study

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