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Cationic olefin complexes of platinum(II): Aspects of availability and reactivity
Accepted Manuscript Research paper Cationic Olefin Complexes of Platinum(II): Aspects of Availability and Reactivity Michele Benedetti, Carmen R. Barone, Sara de Pinto, Federica De Castro, Giovanni Natile, Francesco P. Fanizzi PII: DOI: Reference:
S0020-1693(17)30289-X http://dx.doi.org/10.1016/j.ica.2017.04.015 ICA 17520
To appear in:
Inorganica Chimica Acta
Received Date: Revised Date: Accepted Date:
28 February 2017 6 April 2017 10 April 2017
Please cite this article as: M. Benedetti, C.R. Barone, S. de Pinto, F. De Castro, G. Natile, F.P. Fanizzi, Cationic Olefin Complexes of Platinum(II): Aspects of Availability and Reactivity, Inorganica Chimica Acta (2017), doi: http://dx.doi.org/10.1016/j.ica.2017.04.015
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Cationic Olefin Complexes of Platinum(II): Aspects of Availability and Reactivity Michele Benedetti,a* Carmen R. Barone,b,c Sara de Pinto,b,d Federica De Castro,a Giovanni Natile,b* Francesco P. Fanizzi.a a
Dipartimento di Scienze e Tecnologie Biologiche ed Ambientali, Università del Salento, Prov.le Lecce-Monteroni,
Centro Ecotekne, I-73100 - Lecce, Italy. b
c
Dipartimento di Chimica, Università degli Studi di Bari, Via E. Orabona 4, I-70125 Bari, Italy. Present address: Istituto d’Istruzione Secondaria Superiore “E. Majorana” Via Montebello 11, I-72100 Brindisi,
Italy. d
Present address: Merck Serono, Via delle Magnolie n. 15, I-70026 Bari, Italy.
Abstract The series of complexes of formula [PtCl(η2-olefin)(N^N)]+, previously investigated for N^N = N,N,N’,N’-tetramethyl-ethylenediamine (Me4en), has been extended to the case of aromatic diimines 1,10-phenanthroline (phen) and 3,4,7,8-tetramethyl-1,10-phenanthroline (Me4phen) and to a variety of olefins (ethene, propene and styrene). The complexes have been prepared by reaction of the [PtCl3(η2-olefin)]− anions (K[PtCl3(η2-styrene)] reported here for the first time) with N^N in basic methanol. The initial [PtCl{η1-CH2−CH(R’)−OMe}(N^N)] (R’ = Me, Ph) complexes are formed in quantitative yield and as pure Markovnikov isomer. The reaction of the alkoxylic species with non coordinating acids, results in the quantitative formation of the desired cationic π-olefin complexes [PtCl(η2-olefin)(N^N)]+. The phenanthroline ligand confers peculiar properties to the new compounds. In particular, by reaction with triethylamine, [PtCl{η2-CH2=CH(Me)}(N^N)]+ species, undergo deprotonation of the olefin and formation of the dimeric species [{PtCl(N^N)}2(µ1
η1:η2-CH2CH=CH2)]+ which could be isolated and characterized. Interestingly such product in acetonitrile gives a disproportionation with precipitation of [PtCl2(phen)] and formation in solution of the new η3-allyl complex [Pt(η3-C3H5)(phen)]ClO4.
Keywords Platinum; coordination chemistry; organometallic chemistry; square planar complex; olefin deprotonation; allylic complex; ligand substitution.
List of Abbreviations Olefins: CH2=CH2 (a); CH2=CH(Me) (b); CH2=CH(Ph) (c). N^N dinitrogen ligands: N,N,N’,N’-tetramethyl-ethylenediamine (Me4en, x); 1,10-phenanthroline (phen, y); 3,4,7,8-tetramethyl-1,10-phenanthroline (Me4phen, z). Zeise’s salt analogues: [PtCl3(η2-olefin)]− (1a-1c); trans-[PtCl2(OMe)(η2-olefin)]− (1a’-1c’); Pentacoordinate complexes: [PtCl2(η2-olefin)(N^N)] (2ax-2cz). Cationic complexes: [PtCl(η2-olefin)(N^N)]+ (3ax-3cz). Dichlorido derivatives: [PtCl2(N^N)] (4x-4z). Methoxylated derivatives: [PtCl2(η1-olefin−OMe)(N^N)] (5ax-5cz). Dimeric species: [{PtCl(N^N)}2(µ-η1:η2-CH2CH=CH2)]+ (6by). Allylic derivatives: [Pt(η3-C3H5)(N^N)]+, (7by).
2
Introduction Several cationic complexes of platinum(II) containing olefins have been prepared in the years. 1 In particular we have been interested to species of formula [PtCl(η2-ethene)(N^N)]+, generally having perchlorate or tetrafluoroborate as counter anions, where N^N is a dinitrogen chelator, such as an aliphatic diamine
1b,d-g,o,p
and, more recently, an aromatic diimine.
1m,n
The overall synthetic
procedures starting from Zeise’s salt (K1a), is summarized in Scheme 1 [in the Schemes and throughout the text each type of complex is indicated by a number followed by letters that indicate the type of olefin (a, b and c for ethene, propene and styrene, respectively) and the type of N^N ligand (x, y and z for Me4en, phen and Me4phen, respectively)]. The Zeise’s anion (1a) reacts with the N^N ligand, forming a 5-coordinate species (2a) which is the end product only in the case of great steric bulk close to the donating atoms (such as in the case of 2,9-dimethyl-1,10-phenthroline). 2 Otherwise 2a spontaneously evolves into the cationic complex 3a and chloride anion. The positively charged species 3a can be preserved via precipitation of a sparingly soluble salt with a non coordinating anion,
1f
otherwise 3a reacts with
released Cl– forming the neutral dichlorido complex 4. Interestingly, in the analogous very reactive [PtBr(η2-ethyne)(N-N)]Br species also the nucleophilic attack of the bromide anion on the coordinated
alkyne,
leading
CH=CHBr)(N^N)], was observed.
to 2h
the
sigma
coordinated
alkenyl
species
[PtBr(E-η1-
When the transformation of 2a into 3a is performed in basic
methanol, MeO– adds to the coordinated olefin of the cationic complex and the alkoxy derivative 5a is formed.
1b,d,m,n
By acid hydrolysis with a non coordinating acid, 5a can be reconverted to 3a.
Following a different order in the mixing of the reagents, that is adding a strong base to the methanol solution of Zeise’s salt before the addition of the N^N coordinating ligand, an alternative reaction path, leading always to 5a, is activated (lower row of Scheme 1). This second procedure allows the obtainment of 5a (at least in part) also in the case of ligands bearing sterically demanding substituents close to the nitrogen donor atoms (following the first procedure the reaction would stop 3
to the formation of 2a).
1m,n,p,q,s
However, in the latter case the acid hydrolysis (using HCl) of 5a
does not produce the corresponding cationic complexes 3a but the 5-coordinate species 2a. 1m,n In the cationic complex [PtCl(η2-ethene)(Me4en)]+ (Me4en = N,N,N’,N’-tetramethylethylenediamine), which has been studied in more detail, the olefin has a high electrophilic character being able to add even inorganic anions, tertiary amines and weak carbon nucleophiles.3 In the family of analogous complexes, born by substitution of a higher alkene or styrene for ethene, the reactivity is almost the same, but tertiary amines produce deprotonation rather than addition to the coordinated unsaturated ligand. 4 DFT
calculations,
performed
on
[PtCl(η2-ethene)(Me4en)]+
(3ax)
and
[PtCl(η2-
propene)(Me4en)]+ (3bx), have also shown that both complexes are intrinsically activated towards nucleophilic attack (their LUMO is lower in energy with respect to the π* orbital of the free alkene). In the propene complex, however, the electrophilic character of the olefin is masked by the Brönsted acidity of its allylic protons. 3g In the cationic complexes containing an aromatic diimine, [PtCl(η2-CH2=CH2)(phen)]+ and [PtCl(η2-CH2=CHCH3)(bpy)]+ (bpy = 2,2’-bipyridyl), the decreased Lewis basicity of the N-donor ligand, as compared to that of diamines, renders the platinum atom more electrophilic and able to compete with the coordinated olefin for the addition of soft nucleophiles like carbanions. Therefore, unlike the case of [PtCl(η2-ethene)(Me4en)]+ where only the olefin addition product was observed, 3e
reaction of [PtCl(η2-ethene)(N^N)]+ (N^N = diimine) with acetylacetonate, leads to a mixture of
substitution and addition products (Scheme 2A). 1n Unlike carbanions, secondary aliphatic amines (NHR2, R = alkyl) always add to ethene forming the 2-aminoethanide derivative, whatever is the N^N ligand in the cationic [PtCl(η2ethene)(N^N)]+ (3a) complex. In the addition product a base-assisted intramolecular rearrangement can take place with nucleophilic substitution of the cis coordinated halido ligand by the aminic 4
nitrogen of the 2-aminoethanide pendant, leading to an aza-platinacyclobutane species (Scheme 2B).
1n,5
A stronger base is required (KOH) when N^N is an aliphatic diamine,
5
whereas for the
complexes bearing a diimine is sufficient a second stoichiometric amount of the secondary amine (Scheme 2B).
1n
A number of other compounds containing aza-metallacyclobutane rings are
reported in the literature.
6,7
The ring-closing step for secondary amines is favored by the Thorpe-
Ingold effect and though enhanced by substituents on the olefinic carbons.
7b,c
Other aiding factors
for this step are the presence of a labilizing ligand in trans position with respect to the outgoing halido ligand. 6,7a Cationic complexes of type [PtCl(η2-ethene)(N^N)]+ (3a), with N^N = diamine or diimine, are also able to alkylate the aromatic nucleus of a phenolate ion (Scheme 2C), 3g,8 but only in the case of diimine complexes takes place the ring closing step leading to formation of a metallachromane derivative (Scheme 2C). 8a We have now enlarged the family of cationic Pt(II)-η2-olefin complexes containing aromatic diimines, going beyond the ethene derivatives, with the aim of developing useful species in view of the potential biological or pharmacological activity of platinum(II) complexes containing aromatic diimines. Platinum complexes with 2,2’-bpy and 1,10-phen (and their ring-substituted analogues) have attracted much attention over the years because of their biological and/or photophysical properties. For instance, several high luminescent square-planar platinum(II) complexes contain variously-substituted 1,10-phenanthrolines as ligands.
9
Moreover, square-planar platinum(II)
phenanthroline complexes can be cytotoxic and thus represent a family of potential anticancer agents. The cytotoxic activity arises from the ability of such compounds to interact with cellular DNA: the intercalating properties of the planar phenanthroline ligand can synergize with the ability of the metal to form stable adducts with DNA.
10
Platinum(II)-phenanthroline complexes can also
promote different mechanisms of cytotoxicity. For instance, in a recent paper Reed et al. have shown that some Pt(II) phenanthroline complexes can act as quadruplex-DNA stabilizers. 11
5
Pt(II) phenanthroline complexes have also proved to be able to inhibit Amyloid-β peptide (Aβ) aggregation and as such could be potentially useful for the treatment of Alzheimer’s disease. 12 These complexes have high affinity for the metal-binding sites of the protein, thus modulating peptide aggregation and toxicity. It is worth noting that the classic anticancer drug cisplatin has no effect upon Aβ, suggesting a crucial role for the planar aromatic phenanthroline ligand. Some new cationic olefin complexes of platinum(II), containing 1,10-phenanthroline (y) and 3,4,7,8-tetramethyl-1,10-phenanthroline (z) and different olefins, together with some aspects of their reactivity, are described in this paper.
Results and Discussion Preparation of the cationic complexes [PtCl(η η2-olefin)(N^N)]+ (3). There is a striking difference between cationic complexes of the type [PtCl(η2-olefin)(N^N)]+ (3) containing aliphatic diamines and those containing diimines. While in the case of N^N = Me4en 3x species with different olefins can be prepared by simple olefin metathesis starting from the η2-ethene derivative, in contrast in the case of N^N = diimine the metathetic exchange of the coordinated ethene with other olefins affords only a very low yield of the desired product. For this reason, we looked for an alternative synthetic route in which the desired olefin is introduced in the metal coordination sphere prior to the complexation of the N^N ligand. Thus Zeise’s anion, [PtCl3(η2-ethene)]− (1a), was converted into its propene (1b) or styrene (1c) analogue by reaction in methanol (0 °C) with an excess of the desired olefin. The products, K[PtCl3(η2-propene)] (Κ1 1 b) and K[PtCl3(η2-styrene)] (K1c) (the latter reported for the first time) were obtained in quantitative yields and characterized via elemental analysis, ESI-MS and NMR spectroscopy (see Experimental section and Figure 1S).
6
Then the propene and styrene analogues of the Zeise’s salt (1b and 1c) were reacted with phen or 3,4,7,8-tetramethyl-1,10-phenanthroline (Me4phen) in basic methanol ([KOH] or [NaOMe] § 1M), leading to formation of [PtCl{η1-CH2−CH(R’)−OMe}(N^N)] (R’ = Me or Ph; N^N = phen or Me4phen) in almost quantitative yield. The isolated products were characterized by elemental analysis, ESI-MS and NMR spectroscopy (see Experimental and Supporting Information). In all cases, the methoxy derivative is formed as pure Markovnikov isomer as clearly indicated by the 1 H NMR spectrum in CDCl3 (presence of the Pt-CαH2 −CβH(R)−OMe moiety, Figure 1 and Table 1). In such compounds, due to the chirality of the β carbon of the platinum bound alkyl chain, the two α protons are diastereotopic. The reaction of the [PtCl{η1-CH2−CH(R’)−OR}(N^N)] species with acids bearing not coordinating anions (e.g. HClO4 and/or HBF4) results in the quantitative formation of the cationic η2-olefin complexes of type 3 (isolated as perchlorate and/or tetrafluoroborate salts). 1 For this last step the nature of the solvent is also important. In the case of the ethene complex, it was possible to perform the reaction in CH2 Cl2 ([PtCl(η2-ethene)(phen)](ClO4), 3ay, prepared by addition of a stoichiometric amount of HClO4, conveyed in a small volume of water or ethanol, to a suspension of [PtCl(η1-CH2CH2−OMe)(phen)], 5ay).
1n
In contrast, in the case of the propene and styrene
derivatives and even in the case of the ethene derivative when N^N = Me4phen, probably because of a steric/electronic destabilization of the η2-coordinated olefin, in chlorinated media (where traces of HCl are generally present) displacement of the olefin by Cl− occurs giving rise to formation of the [PtCl2(N^N)] (4) complex. However, if methanol is used instead of the chlorinated solvent, quantitative conversion of the methoxy derivative [PtCl{η1-CH2CH(R)−OMe}(N^N)] (5) into the corresponding cationic [PtCl{η2-CH2=CH(R)}(N^N)]+ (3) species is observed. All cationic complexes [PtCl{η2-CH2=CH(R)}(N-N)]+ (3) were characterized via elemental analysis, IR, ESIMS and NMR spectroscopy, see Experimental.
7
The collected resonance frequencies for the vinyl protons of the cationic compounds [PtCl{η2-CH2=CH(R)}(N-N)]+ (3) are reported in Table 2. The 1 H NMR spectra of [PtCl{η2CH2=CH(Me)}(phen)]+ (3by) and [PtCl{η2-CH2=CH(Ph)}(phen)]+ (3cy) are shown in Figure 2. Unlike the cationic π-olefin complexes bearing the aliphatic diamine Me4en, where the resonance frequencies of the vinyl protons move to lower NMR chemical shift with respect to those of the free olefin, the phenanthroline cationic species exhibit a higher chemical shifts for the vinyl protons.
1
The latter feature was already noticed in the previously reported ethene compound 3ay
(5.46 ppm for the η2-ethene, as compared to 5.37 ppm of the uncoordinated one, in acetone-d 6) and is particularly evident in the presently investigated propene complex 3by. However, in the same compounds the 13C NMR resonance frequencies show the usual chemical shift decrease (Tables 23), thus indicating that the observed NMR deshielding of the vinyl protons upon coordination might be caused by the aromatic ring current of the phenanthroline moiety. Reactivity towards nucleophiles: addition to, as opposed to substitution of the olefin. The newly obtained cationic species were subjected to reactivity tests towards some nucleophiles. Among the employed nucleophiles, methoxide anion was the only one able to add quantitatively to the coordinated olefin, regenerating the parent addition product. In contrast, nitrogen and carbon donors (like secondary amines and the acetylacetonate anion) largely prefer the substitution path (Scheme 3). In Figure 3, the 2D (1H,1H) COSY spectrum of the product obtained by reaction of [PtCl(η2styrene)(phen)](ClO4), 3cy(ClO4), with diethylamine is reported. Signals assignable to a platinumcoordinated NHEt2 molecule are present. Moreover, the loss of the unsaturated ligand from the metal coordination sphere is confirmed by the presence of signals assignable to free styrene (5-7 ppm). The ease of olefin substitution by an incoming nucleophile confirms the steric/electronic destabilization of the π-olefin in complexes with phenanthrolines, as compared to those with aliphatic diamines (Me4en). 8
Allylic deprotonation. By using a bulky nucleophile, such as triethylamine, both the addition to and the substitution of the π-olefin are destabilized. Under these conditions the reaction of [PtCl{η2-CH2=CH(Me)}(phen)]+ (3by) with NEt3 leads to an allylic deprotonation. A base-induced deprotonation of the coordinated propene was already reported for the analogous complex with Me4en, [PtCl{η2-CH2=CH(Me)}(Me4en)]+ (3bx),
4a,b
where the isolated product was the π-allyl-
bridged platinum dimer [{PtCl(Me4en)}{Pt(Me4en)}(µ-η1:η3-CHCHCH2)]+. In the case of the phenanthroline derivatives (such as 3by) the reaction with NEt3 is quite different. Starting with a suspension of 3by in chloroform, and using a moderate excess of triethylamine (NEt3/Pt = 2.5), a brick-red, sparingly soluble solid is formed. It corresponds to the perchlorate salt of the dimeric cation [{PtCl(phen)}2(µ-η1:η2-CH2CH=CH2)]+ (6by), Scheme 4. The IR spectrum of the isolated solid indicates the persistence of the Pt−Cl unit (ν Pt-Cl = 336 cm−1) and the presence of a perchlorate anion. The elemental analysis indicates that the organic part, besides phenanthroline, contains a C3H5− unit per each pair of platinum atoms. The brick-red solid is reasonably soluble in acetonitrile where it was possible to acquire the 1 H NMR spectrum. This showed an asymmetric phenanthroline molecule and an AX4 pattern for the olefin protons, in accord with a rapid interchange between η1 and η2 bonding modes, with signals at 4.15 (d, 4H, 3JH-H = 10 Hz, 2JPt-H = 75 Hz) and 7.21 ppm (m, 1H). In the case of the Me4en complex, the analogous 6bx dimer is only a transient species which is further deprotonated by NEt3, present in the reaction pot, to form the final µ-η1:η3-allyl product quoted above. In the phenanthroline case the dimeric species [{PtCl(phen)}2(µ-η1:η2CH2CH=CH2)]+ (6by) can be isolated, probably because of its very low solubility in the reaction medium. However, in acetonitrile solution, the isolated dimer undergoes a further transformation: at room temperature, the solution turns from orange to yellow in a few hours and a yellow precipitate (the highly insoluble neutral complex [PtCl2(phen)] is formed. The NMR spectrum of the remaining yellow solution is fully consistent with the presence of the η3-allyl complex [Pt(η3-C3H5)(phen)]+ (7by), which could also be isolated and characterized. Its 9
2D (1H,1H) COSY spectrum (Figure 4) shows signals characteristic of a symmetrical molecule: two doublets at 3.10 and 4.37 ppm (2JPt-H of ca. 74 and 32 Hz, respectively) arise from unequivalence of the two protons within each methylene of the η3-allyl moiety, while the central proton gives rise to a multiplet at 5.24 ppm. The structure was also confirmed by ESI-MS spectrometry. In Figure 5 it is shown the good fitting between the experimental ESI-MS spectrum in CH3CN of [Pt(η3C3H5)(phen)]+, 7by, (A) and the calculated pattern (B). These findings indicate that the acidity of the propene methyl protons, present in the already reported [PtCl{η2-CH2=CH(Me)}(Me4en)]+ (3bx) complex, is retained also in the presently investigated 1,10-phenanthroline analogue. In the latter case, however, the low solubility of the phenanthroline derivative 6by (formed by deprotonation of 3by) allowed, for the first time, its isolation and characterization in a pure form. Moreover, unlike the analogous compound with Me4en (6bx), 6by does not undergo further deprotonation to form the µ-η1:η3-allyl product, but in acetonitrile undergoes disproportionation to [PtCl2(phen)] (4y) and [Pt(η3-CH2CHCH2)(phen)]+ (7by). The low solubility of 4y played a pivotal role for the isolation of the η3-allyl complex (7by) in a pure form.
Conclusions The extension to phenanthrolines of the family of cationic π-olefin complexes of Pt(II) of formula [PtCl(η2-olefin)(N^N)]+ (3), initially investigated for N^N = Me4en, gave us the opportunity to better understand the peculiar properties conferred to such species by the steric/electronic properties of the diimine. In the complexes with phenanthrolines there is: i) much lower tendency to undergo olefin metathesis as compared to the complexes with Me4en; ii) the propene (3by and 3bz) and styrene (3cy and 3cz) complexes have a marked tendency to undergo substitution of the unsaturated ligand 10
in the reaction with nucleophiles, which prevents the functionalization of the coordinated olefin; iii) the propene complexes (3by and 3bz) retain the acidic character of the propene methyl protons, already reported for the analogous complex with Me4en, but form a µ-η1:η2-CH2CH=CH2 dinuclear species [{PtCl(phen)}2(µ-η1:η2-CH2CH=CH2)]+ which does not have tendency to undergo further deprotonation to give a µ-η1:η3-CHCHCH2 product of the type [{PtCl(Me4en)}{Pt(Me4en)}(µη1:η3-CHCHCH2)]+ as observed in the Me4en case; iv) the dinuclear [{PtCl(phen)}2(µ-η1:η2CH2CH=CH2)]+ species undergoes in solution disproportionation to [PtCl2(phen)] (sparingly soluble) and [Pt(η3-CH2CHCH2)(phen)]+ allowing an easy and clean procedure for the preparation of η3-allyl complexes of platinum(II) with phenanthrolines.
Acknowledgements We gratefully acknowledge Prof. Luciana Maresca (University of Bari, Italy) for inspiring this work and for helpful discussion. The University of Salento, Lecce (Italy), and the Consorzio Interuniversitario di Ricerca in Chimica dei Metalli nei Sistemi Biologici (CIRCMSB), Bari (Italy), are acknowledged for financial support.
Experimental Reagents and methods. Reagents and solvents were commercially available and used as received. Elemental analyses were performed with a CHN Eurovector EA 3011. 1H and
13
C spectra were
recorded with a 300 MHz Mercury Varian and a DPX 300 Avance Bruker instruments equipped with probes for inverse detection and with z gradient for gradient-accelerated spectroscopy. 1H and 13
C NMR spectra were referenced to TMS; the residual proton signal of the solvent was used as
internal standard. 1H/13C inversely detected gradient-sensitivity enhanced heterocorrelated 2D NMR spectra for normal coupling (INVIEAGSSI) were acquired using standard Bruker automation 11
programs and pulse sequences. Each block of data was preceded by eight dummy scans. The data were processed in the phase-sensitive mode. The ESI-MS spectra were recorded on an Agilent 1100 Series LC-MSD Trap System VL. IR spectra were recorded as KBr or high density polyethylene pellets on a Perkin-Elmer Spectrum One. All solvents and reagents, unless otherwise stated, were commercially available (purchased from Aldrich Chemical Company) and used as received. Chlorinated solvents were dried over activated molecular sieves (beads, 4-8 mesh). Zeise’s salt was prepared from potassium tetrachloroplatinate and ethene through a modification 1n of the method proposed by Chock et al.. 13 Synthetic Procedures. CAUTION: the use of perchlorates may be extremely dangerous (explosive), and should be avoided when possible. K[PtCl3{η η2-CH2=CH(Me)}], K1b. K[PtCl3(η2-CH2=CH2)]·H2O, (300 mg, 0.77 mmol) was dissolved in methanol (3-4 mL) and treated with 20-fold excess of propene. The reaction mixture was left under stirring for 24 hours at room temperature. Evaporation of the solvent under reduced pressure afforded a yellow powder, which was identified as compound K1b. The yield (referred to platinum) was nearly quantitative. Anal. Calcd. for C3H6Cl3KPt: C, 9.42; H, 1.52 %. Found: C, 9,39; H, 1.63 %. Peak in ESI-MS: m/z = 344.6 = [M - K]−. NMR (acetone-d 6, 298 K, ppm): δH 1.54 (d, 3H, 3JPt-H = 36 Hz, CH2=CHCH3), 4.11 (m, 2H, 2JPt-H ca. 60 Hz, CH2=CHCH3), 5.00 (m, 1H, 2
JPt-H ca. 66 Hz, CH2=CHCH3).
K[PtCl3{η η2-CH2=CH(Ph)}], K1c. K[PtCl3(η2-CH2=CH2)]·H2O, (300 mg, 0.77 mmol) was dissolved in methanol (3-4 mL) and the solution placed in an ice bath. A 20-fold excess of styrene (1764 µL, 15.4 mmol) was then added. The reaction mixture was left under stirring for about 7 hours at 0 °C. Evaporation of the solvent, under reduced pressure, left an oily residue, which was triturated with diethyl ether affording a yellowish powder. After removal of the organic phase, the 12
obtained solid was dried in vacuo and identified as the desired product K1c. The isolated yield, referred to platinum, was nearly quantitative. Anal. Calcd. for C8H8Cl3KPt: C, 21.61; H, 1.81 %. Found: C, 21.56; H, 1.78 %. Peak in ESI-MS: m/z = 404.7 = [M - K]−. NMR (CD3OD, 298 K, ppm): δH 4.31 (d,
1
H,
2
JPt-H ca. 74 Hz, CHH=CHC6H5), 4.95 (d,
1
H,
2
JPt-H ca. 60 Hz,
CHH=CHC6H5), 6.32 (m, 1H, 2JPt-H ca. 75 Hz, CH2=CHC6H5), 7.2-7.7 (m, 5H, CH2=CHC6H5). Complexes of type 5 reported in Scheme 1. [PtCl(η1-CH2CH2-OMe)(phen)] (5ay) was prepared according to reference 1n. [PtCl(η1-CH2CH2-OMe)(Me4phen)] (5az), [PtCl{η1-CH2CH(Me)OMe}(phen)]
(5by),
[PtCl{η1-CH2CH(Me)-OMe}(Me4phen)]
(5bz),
[PtCl{η1-CH2CH(Ph)-
OMe}(phen)] (5cy), and [PtCl{η1-CH2CH(Ph)-OMe}(Me4phen)] (5cz) were prepared in a way completely analogous to that of the already reported 5ay. In a typical experiment, 0.40 mmol of K1b, or K1c, were dissolved in methanol (4 mL) and the solution was placed in an ice bath. The stoichiometric amount of the di-imine (phen or Me4phen) was then added and the instantaneous formation of a yellow precipitate, corresponding to the five-coordinate species (2 in Scheme 1), did occur. At this stage a 20-fold excess of KOH (§3 mL, §2.5 M in MeOH) was added to the mixture. The reaction vessel was kept under stirring for 4 hours, meanwhile a deepening in the yellow color of the precipitate was also observed. The solid was separated by filtration of the solution through a sintered glass filter, washed with water until the filtrate reached pH = 7 and dried under vacuum. In all cases it turned out to be the desired compound of type 5 and the yield, referred to platinum, was always > 95%. 5az: Anal. Calcd. for C19H23N2ClOPt: C, 43.39; H, 4.41 %. Found: C, 43.36; H, 4.21 %. Peak of greatest intensity in ESI-MS: m/z = 548.9 = [M + Na]+. NMR (CDCl3, 298 K, ppm): δH 2.28 (app t, 2H, 2JPt-H ca. 88 Hz, Pt-CαH2), 3.42 (s, 3H, OCH3), 3.70 (app t, 2H, CβH2-O). 5by: Anal. Calcd. for C16H17N2ClOPt: C, 39.72; H, 3.54 %. Found: C, 39.70; H, 3.53 %. Peak of greatest intensity in ESI-MS: m/z = 506.9 = [M + Na]+. NMR (CDC13, 298 K, ppm); δH 1.44 (d, 3H, CβH(CH3)-O), 2.27 (app t, 1H, 2JPt-H ca. 88 Hz, Pt-CαHH), 2.55 (dd, 1H, 2JPt-H ca. 92 Hz, PtCαHH), 3.44 (s, 3H, OCH3), 3.71 (m, 1H, CβH(CH3)-O). 5bz: Anal. Calcd. for C20H25N2ClOPt: C, 13
44.49; H, 4.67 %. Found: C, 44.36; H, 4.65 %. Peak of greatest intensity in ESI-MS: m/z = 538.9 = [M + Na]+. NMR (CDCl3, 298 K, ppm): δH 1.33 (d, 3H, CβH(CH3)-O), 1.77 (app t, 1H, 2JPt-H ca. 86 Hz, Pt-CαHH), 2.33 (dd, 1H, 2JPt-H ca. 90 Hz, Pt-CαHH, 3.40 (s, 3H, OCH3), 3.58 (m, 1H, CβH(CH3)-O), 7.85, 8.95 (3JPt-H ca. 60 Hz) and 9.11 (4H, 3,4,7,8-Me4-1,10-phen). 5cy: Anal. Calcd. for C21H19N2ClOPt: C, 46.20; H, 3.51 %. Found: C, 46.15; H, 3.49 %. Peak of greatest intensity in ESI-MS: m/z = 568.9 = [M + Na]+. NMR (CDC13, 298 K, ppm): δH 2.28 (app t, 1H, 2JPt-H ca. 87 Hz, Pt-CαHH), 2.79 (dd, 1H, 2JPt-H ca. 88 Hz, Pt-CαHH), 3.31 (s, 3H, OCH3), 4.60 (m, 1H, CβH(Ph)-O). 5cz: Anal. Calcd. for C25H27N2ClOPt: C, 49.88; H, 4.52 %. Found: C, 49.79; H, 4.48 %. Peak of greatest intensity in ESI-MS: m/z = 601.8 = [M + Na]+. NMR (CDCl3, 298 K, ppm): δH 1.95 (app t, 1H, 2JPt-H ca. 84 Hz, Pt-CαHH), 2.91 (dd, 1H, 2JPt-H ca. 91 Hz, Pt-CαHH), 3.34 (s, 3H, OCH3), 4.65 (m, 1H, CβH(C6H5)-O), 7.97, 8.04, 8.46 (3JPt-H ca. 60 Hz) and 9.50 (4H, 3,4,7,8-Me4-1,10-phen). Complexes of type 3 in Scheme 1. [PtCl(η2-CH2=CH2)(phen)](ClO4) [3ay(ClO4)] was prepared as reported
in
reference
1n.
[PtCl(η2-CH2=CH2)(Me4phen)](ClO4)
[3az(ClO4)],
[PtCl{η2-
CH2=CH(Me)}(phen)](ClO4) [3by(ClO4)], [PtCl{η2-CH2=CH(Me)}(Me4phen)](ClO4) [3bz(ClO4)], [PtCl{η2-CH2=CH(Ph)}(phen)](ClO4) [3cy(ClO4)], and [PtCl{η2-CH2=CH(Ph)}(Me4phen)](ClO4) [3cz(ClO4)] were prepared by acid hydrolysis of the corresponding compounds of type 5 (5az, 5by, 5bz, 5cy, and 5cz). In a typical experiment 0.20 mmol of each compound 5 were suspended in methanol (1 mL) and treated with the stoichiometric amount of HClO4 (0.5 mL of a 0.4 M methanolic solution). The mixture was kept under stirring for 30 minutes at room temperature. The solid was separated by filtration through a sintered glass filter, washed twice with water, and dried under vacuum. It turned out to be the desired salt 3(ClO4); the yield, referred to platinum, was always > 90%). 3az(ClO4): Anal. Calcd. for C18H20N2Cl2O4Pt: C, 36.37; H, 3.39; N, 4.71 %. Found: C, 36.15; H, 3.41; N, 4.60 %. Molecular peak in ESI-MS: m/z = 494.8 = [M - ClO4]+. NMR 14
(acetone-d6, 298 K, ppm): δH 5.37 (s, 4H, 3JPt-H not evaluated, CH2=CH2). 3by(ClO4): Anal. Calcd. for C15H14N2Cl2O4Pt: C, 32.62; H, 2.56; N, 5.07 %. Found: C, 32,75; H, 2.43; N, 5.07 %. Molecular peak in ESI-MS: m/Z = 453.13 = [M-ClO4]+. NMR (acetone-d6, 298 K, ppm): δH 2.13 (d, 3H, CH2=CHCH3), 5.22 and 5.43 (m, 2H, η2-CH2=CHCH3), 6.4 (m, 1H, η2-CH2=CHCH3); δC 21.54 (η2-CH2=CHCH3), 73,07 (η2-CH2=CHCH3), 107.53 (CH2=CHCH3). 3bz(ClO4): Anal. Calcd. for C19H23N2Cl2O4Pt: C, 37.51; H, 3.64; Cl, 11.66 %. Found: C, 37.39; H, 3.44; Cl, 10.87 %. Molecular peak in ESI-MS: m/z = 508.32 = [M-ClO4]+. NMR (acetone-d6, 298 K ppm): d H 1.68 (d, 3H, CH2=CHCH3), 4.94 (m, 2H, CH2=CHCH3), 5.8 (m, 1H, CH2=CHCH3). 3cy(ClO4): Anal. Calcd. for C20H16N2Cl2O4Pt: C, 39.10; H, 2.63; N, 4.56 %. Found: C, 39.21; H, 2.54; N, 4.44 %. Molecular peak in ESI-MS: m/z = 515.28 = [M-ClO4]+. NMR (acetone-d6, 298 K, ppm): δH 5.24 (d, 1H, CHH=CHC6H5), 5.80 (d, 1H, CHH=CHC6H5), 6.74 (m, 1H, CHH=CHC6 H5). 3cz(ClO4): Anal. Calcd. for C24H25N2Cl2O4Pt: C, 43.00; H, 3.61; Cl, 10.58 %. Found: C, 42.38; H, 3.58; Cl, 10.9 %. Molecular peak in ESI-MS: m/z = 571.28 = [M - ClO4 ]+. NMR (acetone-d 6, 298 K, ppm): δH 5.23 (d, 1H, CHH=CHC6 H5), 5.81 (d, 1H, CHH=CHC6H5), 6.76 (m, 1H, CH2=CHC6H5). Reaction of [PtCl{η η2-CH2=CH(Me)}(phen)]+ (3by) with NEt3. In a typical experiment, the complex [PtCl{η2-CH2=CH(Me)}(phen)](ClO4), 3by(ClO4) (100 mg, 0.18 mmol), was suspended in chloroform (10 mL) and treated with a 2.5-fold excess of triethylamine (625 µL of a 0.72 M chloroform solution). The reaction mixture was kept under stirring for two hours at room temperature, meanwhile the formation of a red solid was observed. The solid was separated by filtration through a sintered glass filter, washed twice with chloroform and water, and dried in vacuo. The obtained red powder corresponded to the formula [{PtCl(phen)}2(µ-η1:η2CH2CH=CH2)](ClO4) (6by) (isolated yield, referred to platinum, of ca. 70%). Anal. Calcd. for C27H21N4Cl2O4Pt2: C, 35.00; H, 2.28; N, 6.05 %. Found: C, 34.44; H, 2.10; N, 5.74 %. NMR (CD3CN, 292 K, ppm): δH 4.15 (d, 4H, 3JH-H = 10 Hz, 2JPt-H = 75 Hz, CH2CHCH2), 7.21 (m, 1H, CH2CHCH2). 15
[Pt(η η3-C3H5)(phen)](ClO4),
5b(ClO4).
The
platinum
dimer
[{PtCl(phen)}2(µ-η1:η2-
CH2CH=CH2)]+ was suspended in 20 mL of CH3CN and kept under stirring for twelve hours at room temperature. A yellow precipitate of [PtCl2 (phen)] (4y) was formed. The solution was then filtered and the mother liquor evaporated under reduced pressure. The obtained yellow powder was the complex [Pt(η3-C3H5)(phen)] ClO4, 5b (isolated yield, referred to platinum, of ca. 75%). Molecular peak in ESI-MS: m/z = 416.0 = [M - ClO4 ]+. NMR (CD3CN, 298 K, ppm): δH 3.10 (d, 2H, 2JPt-H ca. 74 Hz, η3-CHHCHCHH), 4.37 (d, 2H, 2JPt-H ca. 32 Hz, η3-CHHCHCHH), 5.24 (m, 1H, 2JPt-H not evaluated, η3-CHHCHCHH), 8.04, 8.20, 8.91 and 9.47 (3JPt-H = 39 Hz) (8H, phen).
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[12] K. J. Bamham, V. B. Kenche, G. D. Ciccotosto, D. P. Smith, D. J. Tew, X. Liu, K. Perez, G. A. Cranston, T. J. Johansen, I. Volitakis, A. I. Bush, C. L. Masters, A. R. White, J. P. Smith, R. A. Cherny, R. Cappai, Proc. Natl. Acad. Sci. U.S.A. 105 (2008) 6813. [13] P. B. Chock, J. Halpern, F. E. Paulik, S. I. Shupack, T. P. Deangelis, Inorg. Synth., 28 (1990) 349.
Highlights -
[PtCl(η2-olefin)(N^N)]+ complexes with N^N = aromatic diimine; olefin = propene, styrene. The K[PtCl3(η2-styrene)] Zeise’s salt analogue is reported for the first time. [{PtCl(N^N)}2(µ-η1:η2-CH2CH=CH2)]+ and first reported [Pt(η3-C3H5)(phen)]+ species.
Cationic Olefin Complexes of Platinum(II): Aspects of Availability and Reactivity Michele Benedetti, Carmen R. Barone, Sara de Pinto, Federica De Castro, Giovanni Natile, Francesco P. Fanizzi.
The [PtCl{η2-CH2=CH(Me)}(N^N)]+ complex can deprotonate, in the presence of triethylamine, with formation of the dimeric [{PtCl(N^N)}2(µ-η1:η2-CH2CH=CH2)]+ species which can disproportionate with formation of the [PtCl2(phen)] and η3-allyl [Pt(η3-C3H5)(phen)]+ complexes.
19
Scheme 1
Scheme 2
Scheme 3.
Scheme 4. Reaction pathway for complex [PtCl(h2-propene)(phen)]+, 3by, reacting with NEt3.
Figure 1. 1H NMR spectrum of complex [PtCl{h1-CH2CH(CH3)-OCH3}(phen)], 5by, (300 MHz, CDCl3, 298 K).
A)
B)
Figure 2. Portion of 1H-NMR spectra (300 MHz, acetone-d6, 298 K) showing the resonance peaks of vinyl protons of: A) [PtCl(h2-propene)(phen)]+, 3by; B) [PtCl(h2-styrene)(phen)]+, 3cy.
Figure 3. 2D (1H,1H) COSY spectrum (300 MHz, THF-d8, 298 K) of the product obtained by reacting [PtCl(h2-stirene)(1,10-phen)](ClO4), 3cm(ClO4), with NHEt2. The cross peaks for protons of the platinum-coordinated amine molecule have been highlighted.
Figure 4. 2D (1H,1H) COSY spectrum (300 MHz, CD3CN, 298 K) of complex [Pt(h3C3H5)(phen)]+, 7by. Cross peaks for protons of the h3-allyl moiety have been highlighted.
A)
B)
Figure 5. A) Isotopic pattern of the molecular ion peak for the complex [Pt(h3-C3H5)(1,10phen)]+, 7by, in the ESI-MS spectrum (solvent: CH3CN); B) calculated isotopic pattern.
A)
B)
Figure 1S. 1H NMR spectra of complexes: A) K[PtCl3(h2-CH2=CHCH3)], (K)1b, (300 MHz, acetone-d6, 298 K); B) K[PtCl3(h2-CH2=CHC6H5)], (K)1c, (300 MHz, CD3OD, 298K).
TABLE 1. 1H NMR chemical shifts (ppm, downfield from Si(CH3)4, CDCl3), 2JPt-H in Hz in parentheses, for the platinum bound alkyl moiety in [PtCl(η1-CH2CH2OMe)(phen)] (5ay), [PtCl(η1CH2CH2OMe)(Me4phen)] (5az), [PtCl{η1-CH2CH(Me)OMe}(phen)] (5by), [PtCl{η1CH2CH(Me)OMe}(Me4phen)] (5bz), [PtCl{η1-CH2CH(Ph)OMe}(phen)] (5cy), [PtCl{η1CH2CH(Ph)OMe}(Me4phen)] (5cz), complexes. Compound −CH2− −CH(R’)− 5ay* 5az 5by 5bz 5cy 5cz
2.41 (90) 2.28 (88) 2.27 (88) 2.55 (92) 1.77 (86) 2.33 (90) 2.28 (87) 2.79 (88) 1.95 (84) 2.91 (91)
3.70 3.70 3.71 3.58 4.60 4.65
* See Ref. 1n
TABLE 2. 1H NMR chemical shifts (ppm, downfield from Si(CH3)4, acetone-d6), 2JPt-H in Hz in parentheses, for the platinum bound alkyl moiety in [PtCl(η2-CH2=CH2)(phen)]+ (3ay), [PtCl(η2CH2=CH2)(Me4phen)]+ (3az), [PtCl{η2-CH2=CH(Me)}(phen)]+ (3by), [PtCl{η2+ 2 + CH2=CH(Me)}(Me4phen)] (3bz), [PtCl{η - CH2=CH(Ph)}(phen)] (3cy), [PtCl{η2+ CH2=CH(Ph)}(Me4phen)] (3cz), complexes. Compound CH2= =CH(R’) 3ay 3az 3by 3bz 3cy 3cz
5.46 5.38 5.21, 5,40 4.89, 5.03 5.23, 5.78 5.23, 5.78
6.40 5.81 6.77 6.77
20
TABLE 3. Comparison between 1H and [13C] NMR resonance frequencies of the free propene (b) and η2-coordinated propene in the [PtCl{η2-CH2=CH(Me)}(phen)]+ (3by) complex (ppm, downfield from Si(CH3)4, acetone-d6). CH2= =CH(R’) Compound
CH2=CH(Me), (b) [PtCl{η2-CH2=CH(Me)}(phen)]+, (3by)
4.88, 4.98 [115.6] 5.21, 5.40 [73.07]
5.79 [133.7] 6.40 [107.53]
21
S0020-1693(17)30289-X http://dx.doi.org/10.1016/j.ica.2017.04.015 ICA 17520
To appear in:
Inorganica Chimica Acta
Received Date: Revised Date: Accepted Date:
28 February 2017 6 April 2017 10 April 2017
Please cite this article as: M. Benedetti, C.R. Barone, S. de Pinto, F. De Castro, G. Natile, F.P. Fanizzi, Cationic Olefin Complexes of Platinum(II): Aspects of Availability and Reactivity, Inorganica Chimica Acta (2017), doi: http://dx.doi.org/10.1016/j.ica.2017.04.015
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Cationic Olefin Complexes of Platinum(II): Aspects of Availability and Reactivity Michele Benedetti,a* Carmen R. Barone,b,c Sara de Pinto,b,d Federica De Castro,a Giovanni Natile,b* Francesco P. Fanizzi.a a
Dipartimento di Scienze e Tecnologie Biologiche ed Ambientali, Università del Salento, Prov.le Lecce-Monteroni,
Centro Ecotekne, I-73100 - Lecce, Italy. b
c
Dipartimento di Chimica, Università degli Studi di Bari, Via E. Orabona 4, I-70125 Bari, Italy. Present address: Istituto d’Istruzione Secondaria Superiore “E. Majorana” Via Montebello 11, I-72100 Brindisi,
Italy. d
Present address: Merck Serono, Via delle Magnolie n. 15, I-70026 Bari, Italy.
Abstract The series of complexes of formula [PtCl(η2-olefin)(N^N)]+, previously investigated for N^N = N,N,N’,N’-tetramethyl-ethylenediamine (Me4en), has been extended to the case of aromatic diimines 1,10-phenanthroline (phen) and 3,4,7,8-tetramethyl-1,10-phenanthroline (Me4phen) and to a variety of olefins (ethene, propene and styrene). The complexes have been prepared by reaction of the [PtCl3(η2-olefin)]− anions (K[PtCl3(η2-styrene)] reported here for the first time) with N^N in basic methanol. The initial [PtCl{η1-CH2−CH(R’)−OMe}(N^N)] (R’ = Me, Ph) complexes are formed in quantitative yield and as pure Markovnikov isomer. The reaction of the alkoxylic species with non coordinating acids, results in the quantitative formation of the desired cationic π-olefin complexes [PtCl(η2-olefin)(N^N)]+. The phenanthroline ligand confers peculiar properties to the new compounds. In particular, by reaction with triethylamine, [PtCl{η2-CH2=CH(Me)}(N^N)]+ species, undergo deprotonation of the olefin and formation of the dimeric species [{PtCl(N^N)}2(µ1
η1:η2-CH2CH=CH2)]+ which could be isolated and characterized. Interestingly such product in acetonitrile gives a disproportionation with precipitation of [PtCl2(phen)] and formation in solution of the new η3-allyl complex [Pt(η3-C3H5)(phen)]ClO4.
Keywords Platinum; coordination chemistry; organometallic chemistry; square planar complex; olefin deprotonation; allylic complex; ligand substitution.
List of Abbreviations Olefins: CH2=CH2 (a); CH2=CH(Me) (b); CH2=CH(Ph) (c). N^N dinitrogen ligands: N,N,N’,N’-tetramethyl-ethylenediamine (Me4en, x); 1,10-phenanthroline (phen, y); 3,4,7,8-tetramethyl-1,10-phenanthroline (Me4phen, z). Zeise’s salt analogues: [PtCl3(η2-olefin)]− (1a-1c); trans-[PtCl2(OMe)(η2-olefin)]− (1a’-1c’); Pentacoordinate complexes: [PtCl2(η2-olefin)(N^N)] (2ax-2cz). Cationic complexes: [PtCl(η2-olefin)(N^N)]+ (3ax-3cz). Dichlorido derivatives: [PtCl2(N^N)] (4x-4z). Methoxylated derivatives: [PtCl2(η1-olefin−OMe)(N^N)] (5ax-5cz). Dimeric species: [{PtCl(N^N)}2(µ-η1:η2-CH2CH=CH2)]+ (6by). Allylic derivatives: [Pt(η3-C3H5)(N^N)]+, (7by).
2
Introduction Several cationic complexes of platinum(II) containing olefins have been prepared in the years. 1 In particular we have been interested to species of formula [PtCl(η2-ethene)(N^N)]+, generally having perchlorate or tetrafluoroborate as counter anions, where N^N is a dinitrogen chelator, such as an aliphatic diamine
1b,d-g,o,p
and, more recently, an aromatic diimine.
1m,n
The overall synthetic
procedures starting from Zeise’s salt (K1a), is summarized in Scheme 1 [in the Schemes and throughout the text each type of complex is indicated by a number followed by letters that indicate the type of olefin (a, b and c for ethene, propene and styrene, respectively) and the type of N^N ligand (x, y and z for Me4en, phen and Me4phen, respectively)]. The Zeise’s anion (1a) reacts with the N^N ligand, forming a 5-coordinate species (2a) which is the end product only in the case of great steric bulk close to the donating atoms (such as in the case of 2,9-dimethyl-1,10-phenthroline). 2 Otherwise 2a spontaneously evolves into the cationic complex 3a and chloride anion. The positively charged species 3a can be preserved via precipitation of a sparingly soluble salt with a non coordinating anion,
1f
otherwise 3a reacts with
released Cl– forming the neutral dichlorido complex 4. Interestingly, in the analogous very reactive [PtBr(η2-ethyne)(N-N)]Br species also the nucleophilic attack of the bromide anion on the coordinated
alkyne,
leading
CH=CHBr)(N^N)], was observed.
to 2h
the
sigma
coordinated
alkenyl
species
[PtBr(E-η1-
When the transformation of 2a into 3a is performed in basic
methanol, MeO– adds to the coordinated olefin of the cationic complex and the alkoxy derivative 5a is formed.
1b,d,m,n
By acid hydrolysis with a non coordinating acid, 5a can be reconverted to 3a.
Following a different order in the mixing of the reagents, that is adding a strong base to the methanol solution of Zeise’s salt before the addition of the N^N coordinating ligand, an alternative reaction path, leading always to 5a, is activated (lower row of Scheme 1). This second procedure allows the obtainment of 5a (at least in part) also in the case of ligands bearing sterically demanding substituents close to the nitrogen donor atoms (following the first procedure the reaction would stop 3
to the formation of 2a).
1m,n,p,q,s
However, in the latter case the acid hydrolysis (using HCl) of 5a
does not produce the corresponding cationic complexes 3a but the 5-coordinate species 2a. 1m,n In the cationic complex [PtCl(η2-ethene)(Me4en)]+ (Me4en = N,N,N’,N’-tetramethylethylenediamine), which has been studied in more detail, the olefin has a high electrophilic character being able to add even inorganic anions, tertiary amines and weak carbon nucleophiles.3 In the family of analogous complexes, born by substitution of a higher alkene or styrene for ethene, the reactivity is almost the same, but tertiary amines produce deprotonation rather than addition to the coordinated unsaturated ligand. 4 DFT
calculations,
performed
on
[PtCl(η2-ethene)(Me4en)]+
(3ax)
and
[PtCl(η2-
propene)(Me4en)]+ (3bx), have also shown that both complexes are intrinsically activated towards nucleophilic attack (their LUMO is lower in energy with respect to the π* orbital of the free alkene). In the propene complex, however, the electrophilic character of the olefin is masked by the Brönsted acidity of its allylic protons. 3g In the cationic complexes containing an aromatic diimine, [PtCl(η2-CH2=CH2)(phen)]+ and [PtCl(η2-CH2=CHCH3)(bpy)]+ (bpy = 2,2’-bipyridyl), the decreased Lewis basicity of the N-donor ligand, as compared to that of diamines, renders the platinum atom more electrophilic and able to compete with the coordinated olefin for the addition of soft nucleophiles like carbanions. Therefore, unlike the case of [PtCl(η2-ethene)(Me4en)]+ where only the olefin addition product was observed, 3e
reaction of [PtCl(η2-ethene)(N^N)]+ (N^N = diimine) with acetylacetonate, leads to a mixture of
substitution and addition products (Scheme 2A). 1n Unlike carbanions, secondary aliphatic amines (NHR2, R = alkyl) always add to ethene forming the 2-aminoethanide derivative, whatever is the N^N ligand in the cationic [PtCl(η2ethene)(N^N)]+ (3a) complex. In the addition product a base-assisted intramolecular rearrangement can take place with nucleophilic substitution of the cis coordinated halido ligand by the aminic 4
nitrogen of the 2-aminoethanide pendant, leading to an aza-platinacyclobutane species (Scheme 2B).
1n,5
A stronger base is required (KOH) when N^N is an aliphatic diamine,
5
whereas for the
complexes bearing a diimine is sufficient a second stoichiometric amount of the secondary amine (Scheme 2B).
1n
A number of other compounds containing aza-metallacyclobutane rings are
reported in the literature.
6,7
The ring-closing step for secondary amines is favored by the Thorpe-
Ingold effect and though enhanced by substituents on the olefinic carbons.
7b,c
Other aiding factors
for this step are the presence of a labilizing ligand in trans position with respect to the outgoing halido ligand. 6,7a Cationic complexes of type [PtCl(η2-ethene)(N^N)]+ (3a), with N^N = diamine or diimine, are also able to alkylate the aromatic nucleus of a phenolate ion (Scheme 2C), 3g,8 but only in the case of diimine complexes takes place the ring closing step leading to formation of a metallachromane derivative (Scheme 2C). 8a We have now enlarged the family of cationic Pt(II)-η2-olefin complexes containing aromatic diimines, going beyond the ethene derivatives, with the aim of developing useful species in view of the potential biological or pharmacological activity of platinum(II) complexes containing aromatic diimines. Platinum complexes with 2,2’-bpy and 1,10-phen (and their ring-substituted analogues) have attracted much attention over the years because of their biological and/or photophysical properties. For instance, several high luminescent square-planar platinum(II) complexes contain variously-substituted 1,10-phenanthrolines as ligands.
9
Moreover, square-planar platinum(II)
phenanthroline complexes can be cytotoxic and thus represent a family of potential anticancer agents. The cytotoxic activity arises from the ability of such compounds to interact with cellular DNA: the intercalating properties of the planar phenanthroline ligand can synergize with the ability of the metal to form stable adducts with DNA.
10
Platinum(II)-phenanthroline complexes can also
promote different mechanisms of cytotoxicity. For instance, in a recent paper Reed et al. have shown that some Pt(II) phenanthroline complexes can act as quadruplex-DNA stabilizers. 11
5
Pt(II) phenanthroline complexes have also proved to be able to inhibit Amyloid-β peptide (Aβ) aggregation and as such could be potentially useful for the treatment of Alzheimer’s disease. 12 These complexes have high affinity for the metal-binding sites of the protein, thus modulating peptide aggregation and toxicity. It is worth noting that the classic anticancer drug cisplatin has no effect upon Aβ, suggesting a crucial role for the planar aromatic phenanthroline ligand. Some new cationic olefin complexes of platinum(II), containing 1,10-phenanthroline (y) and 3,4,7,8-tetramethyl-1,10-phenanthroline (z) and different olefins, together with some aspects of their reactivity, are described in this paper.
Results and Discussion Preparation of the cationic complexes [PtCl(η η2-olefin)(N^N)]+ (3). There is a striking difference between cationic complexes of the type [PtCl(η2-olefin)(N^N)]+ (3) containing aliphatic diamines and those containing diimines. While in the case of N^N = Me4en 3x species with different olefins can be prepared by simple olefin metathesis starting from the η2-ethene derivative, in contrast in the case of N^N = diimine the metathetic exchange of the coordinated ethene with other olefins affords only a very low yield of the desired product. For this reason, we looked for an alternative synthetic route in which the desired olefin is introduced in the metal coordination sphere prior to the complexation of the N^N ligand. Thus Zeise’s anion, [PtCl3(η2-ethene)]− (1a), was converted into its propene (1b) or styrene (1c) analogue by reaction in methanol (0 °C) with an excess of the desired olefin. The products, K[PtCl3(η2-propene)] (Κ1 1 b) and K[PtCl3(η2-styrene)] (K1c) (the latter reported for the first time) were obtained in quantitative yields and characterized via elemental analysis, ESI-MS and NMR spectroscopy (see Experimental section and Figure 1S).
6
Then the propene and styrene analogues of the Zeise’s salt (1b and 1c) were reacted with phen or 3,4,7,8-tetramethyl-1,10-phenanthroline (Me4phen) in basic methanol ([KOH] or [NaOMe] § 1M), leading to formation of [PtCl{η1-CH2−CH(R’)−OMe}(N^N)] (R’ = Me or Ph; N^N = phen or Me4phen) in almost quantitative yield. The isolated products were characterized by elemental analysis, ESI-MS and NMR spectroscopy (see Experimental and Supporting Information). In all cases, the methoxy derivative is formed as pure Markovnikov isomer as clearly indicated by the 1 H NMR spectrum in CDCl3 (presence of the Pt-CαH2 −CβH(R)−OMe moiety, Figure 1 and Table 1). In such compounds, due to the chirality of the β carbon of the platinum bound alkyl chain, the two α protons are diastereotopic. The reaction of the [PtCl{η1-CH2−CH(R’)−OR}(N^N)] species with acids bearing not coordinating anions (e.g. HClO4 and/or HBF4) results in the quantitative formation of the cationic η2-olefin complexes of type 3 (isolated as perchlorate and/or tetrafluoroborate salts). 1 For this last step the nature of the solvent is also important. In the case of the ethene complex, it was possible to perform the reaction in CH2 Cl2 ([PtCl(η2-ethene)(phen)](ClO4), 3ay, prepared by addition of a stoichiometric amount of HClO4, conveyed in a small volume of water or ethanol, to a suspension of [PtCl(η1-CH2CH2−OMe)(phen)], 5ay).
1n
In contrast, in the case of the propene and styrene
derivatives and even in the case of the ethene derivative when N^N = Me4phen, probably because of a steric/electronic destabilization of the η2-coordinated olefin, in chlorinated media (where traces of HCl are generally present) displacement of the olefin by Cl− occurs giving rise to formation of the [PtCl2(N^N)] (4) complex. However, if methanol is used instead of the chlorinated solvent, quantitative conversion of the methoxy derivative [PtCl{η1-CH2CH(R)−OMe}(N^N)] (5) into the corresponding cationic [PtCl{η2-CH2=CH(R)}(N^N)]+ (3) species is observed. All cationic complexes [PtCl{η2-CH2=CH(R)}(N-N)]+ (3) were characterized via elemental analysis, IR, ESIMS and NMR spectroscopy, see Experimental.
7
The collected resonance frequencies for the vinyl protons of the cationic compounds [PtCl{η2-CH2=CH(R)}(N-N)]+ (3) are reported in Table 2. The 1 H NMR spectra of [PtCl{η2CH2=CH(Me)}(phen)]+ (3by) and [PtCl{η2-CH2=CH(Ph)}(phen)]+ (3cy) are shown in Figure 2. Unlike the cationic π-olefin complexes bearing the aliphatic diamine Me4en, where the resonance frequencies of the vinyl protons move to lower NMR chemical shift with respect to those of the free olefin, the phenanthroline cationic species exhibit a higher chemical shifts for the vinyl protons.
1
The latter feature was already noticed in the previously reported ethene compound 3ay
(5.46 ppm for the η2-ethene, as compared to 5.37 ppm of the uncoordinated one, in acetone-d 6) and is particularly evident in the presently investigated propene complex 3by. However, in the same compounds the 13C NMR resonance frequencies show the usual chemical shift decrease (Tables 23), thus indicating that the observed NMR deshielding of the vinyl protons upon coordination might be caused by the aromatic ring current of the phenanthroline moiety. Reactivity towards nucleophiles: addition to, as opposed to substitution of the olefin. The newly obtained cationic species were subjected to reactivity tests towards some nucleophiles. Among the employed nucleophiles, methoxide anion was the only one able to add quantitatively to the coordinated olefin, regenerating the parent addition product. In contrast, nitrogen and carbon donors (like secondary amines and the acetylacetonate anion) largely prefer the substitution path (Scheme 3). In Figure 3, the 2D (1H,1H) COSY spectrum of the product obtained by reaction of [PtCl(η2styrene)(phen)](ClO4), 3cy(ClO4), with diethylamine is reported. Signals assignable to a platinumcoordinated NHEt2 molecule are present. Moreover, the loss of the unsaturated ligand from the metal coordination sphere is confirmed by the presence of signals assignable to free styrene (5-7 ppm). The ease of olefin substitution by an incoming nucleophile confirms the steric/electronic destabilization of the π-olefin in complexes with phenanthrolines, as compared to those with aliphatic diamines (Me4en). 8
Allylic deprotonation. By using a bulky nucleophile, such as triethylamine, both the addition to and the substitution of the π-olefin are destabilized. Under these conditions the reaction of [PtCl{η2-CH2=CH(Me)}(phen)]+ (3by) with NEt3 leads to an allylic deprotonation. A base-induced deprotonation of the coordinated propene was already reported for the analogous complex with Me4en, [PtCl{η2-CH2=CH(Me)}(Me4en)]+ (3bx),
4a,b
where the isolated product was the π-allyl-
bridged platinum dimer [{PtCl(Me4en)}{Pt(Me4en)}(µ-η1:η3-CHCHCH2)]+. In the case of the phenanthroline derivatives (such as 3by) the reaction with NEt3 is quite different. Starting with a suspension of 3by in chloroform, and using a moderate excess of triethylamine (NEt3/Pt = 2.5), a brick-red, sparingly soluble solid is formed. It corresponds to the perchlorate salt of the dimeric cation [{PtCl(phen)}2(µ-η1:η2-CH2CH=CH2)]+ (6by), Scheme 4. The IR spectrum of the isolated solid indicates the persistence of the Pt−Cl unit (ν Pt-Cl = 336 cm−1) and the presence of a perchlorate anion. The elemental analysis indicates that the organic part, besides phenanthroline, contains a C3H5− unit per each pair of platinum atoms. The brick-red solid is reasonably soluble in acetonitrile where it was possible to acquire the 1 H NMR spectrum. This showed an asymmetric phenanthroline molecule and an AX4 pattern for the olefin protons, in accord with a rapid interchange between η1 and η2 bonding modes, with signals at 4.15 (d, 4H, 3JH-H = 10 Hz, 2JPt-H = 75 Hz) and 7.21 ppm (m, 1H). In the case of the Me4en complex, the analogous 6bx dimer is only a transient species which is further deprotonated by NEt3, present in the reaction pot, to form the final µ-η1:η3-allyl product quoted above. In the phenanthroline case the dimeric species [{PtCl(phen)}2(µ-η1:η2CH2CH=CH2)]+ (6by) can be isolated, probably because of its very low solubility in the reaction medium. However, in acetonitrile solution, the isolated dimer undergoes a further transformation: at room temperature, the solution turns from orange to yellow in a few hours and a yellow precipitate (the highly insoluble neutral complex [PtCl2(phen)] is formed. The NMR spectrum of the remaining yellow solution is fully consistent with the presence of the η3-allyl complex [Pt(η3-C3H5)(phen)]+ (7by), which could also be isolated and characterized. Its 9
2D (1H,1H) COSY spectrum (Figure 4) shows signals characteristic of a symmetrical molecule: two doublets at 3.10 and 4.37 ppm (2JPt-H of ca. 74 and 32 Hz, respectively) arise from unequivalence of the two protons within each methylene of the η3-allyl moiety, while the central proton gives rise to a multiplet at 5.24 ppm. The structure was also confirmed by ESI-MS spectrometry. In Figure 5 it is shown the good fitting between the experimental ESI-MS spectrum in CH3CN of [Pt(η3C3H5)(phen)]+, 7by, (A) and the calculated pattern (B). These findings indicate that the acidity of the propene methyl protons, present in the already reported [PtCl{η2-CH2=CH(Me)}(Me4en)]+ (3bx) complex, is retained also in the presently investigated 1,10-phenanthroline analogue. In the latter case, however, the low solubility of the phenanthroline derivative 6by (formed by deprotonation of 3by) allowed, for the first time, its isolation and characterization in a pure form. Moreover, unlike the analogous compound with Me4en (6bx), 6by does not undergo further deprotonation to form the µ-η1:η3-allyl product, but in acetonitrile undergoes disproportionation to [PtCl2(phen)] (4y) and [Pt(η3-CH2CHCH2)(phen)]+ (7by). The low solubility of 4y played a pivotal role for the isolation of the η3-allyl complex (7by) in a pure form.
Conclusions The extension to phenanthrolines of the family of cationic π-olefin complexes of Pt(II) of formula [PtCl(η2-olefin)(N^N)]+ (3), initially investigated for N^N = Me4en, gave us the opportunity to better understand the peculiar properties conferred to such species by the steric/electronic properties of the diimine. In the complexes with phenanthrolines there is: i) much lower tendency to undergo olefin metathesis as compared to the complexes with Me4en; ii) the propene (3by and 3bz) and styrene (3cy and 3cz) complexes have a marked tendency to undergo substitution of the unsaturated ligand 10
in the reaction with nucleophiles, which prevents the functionalization of the coordinated olefin; iii) the propene complexes (3by and 3bz) retain the acidic character of the propene methyl protons, already reported for the analogous complex with Me4en, but form a µ-η1:η2-CH2CH=CH2 dinuclear species [{PtCl(phen)}2(µ-η1:η2-CH2CH=CH2)]+ which does not have tendency to undergo further deprotonation to give a µ-η1:η3-CHCHCH2 product of the type [{PtCl(Me4en)}{Pt(Me4en)}(µη1:η3-CHCHCH2)]+ as observed in the Me4en case; iv) the dinuclear [{PtCl(phen)}2(µ-η1:η2CH2CH=CH2)]+ species undergoes in solution disproportionation to [PtCl2(phen)] (sparingly soluble) and [Pt(η3-CH2CHCH2)(phen)]+ allowing an easy and clean procedure for the preparation of η3-allyl complexes of platinum(II) with phenanthrolines.
Acknowledgements We gratefully acknowledge Prof. Luciana Maresca (University of Bari, Italy) for inspiring this work and for helpful discussion. The University of Salento, Lecce (Italy), and the Consorzio Interuniversitario di Ricerca in Chimica dei Metalli nei Sistemi Biologici (CIRCMSB), Bari (Italy), are acknowledged for financial support.
Experimental Reagents and methods. Reagents and solvents were commercially available and used as received. Elemental analyses were performed with a CHN Eurovector EA 3011. 1H and
13
C spectra were
recorded with a 300 MHz Mercury Varian and a DPX 300 Avance Bruker instruments equipped with probes for inverse detection and with z gradient for gradient-accelerated spectroscopy. 1H and 13
C NMR spectra were referenced to TMS; the residual proton signal of the solvent was used as
internal standard. 1H/13C inversely detected gradient-sensitivity enhanced heterocorrelated 2D NMR spectra for normal coupling (INVIEAGSSI) were acquired using standard Bruker automation 11
programs and pulse sequences. Each block of data was preceded by eight dummy scans. The data were processed in the phase-sensitive mode. The ESI-MS spectra were recorded on an Agilent 1100 Series LC-MSD Trap System VL. IR spectra were recorded as KBr or high density polyethylene pellets on a Perkin-Elmer Spectrum One. All solvents and reagents, unless otherwise stated, were commercially available (purchased from Aldrich Chemical Company) and used as received. Chlorinated solvents were dried over activated molecular sieves (beads, 4-8 mesh). Zeise’s salt was prepared from potassium tetrachloroplatinate and ethene through a modification 1n of the method proposed by Chock et al.. 13 Synthetic Procedures. CAUTION: the use of perchlorates may be extremely dangerous (explosive), and should be avoided when possible. K[PtCl3{η η2-CH2=CH(Me)}], K1b. K[PtCl3(η2-CH2=CH2)]·H2O, (300 mg, 0.77 mmol) was dissolved in methanol (3-4 mL) and treated with 20-fold excess of propene. The reaction mixture was left under stirring for 24 hours at room temperature. Evaporation of the solvent under reduced pressure afforded a yellow powder, which was identified as compound K1b. The yield (referred to platinum) was nearly quantitative. Anal. Calcd. for C3H6Cl3KPt: C, 9.42; H, 1.52 %. Found: C, 9,39; H, 1.63 %. Peak in ESI-MS: m/z = 344.6 = [M - K]−. NMR (acetone-d 6, 298 K, ppm): δH 1.54 (d, 3H, 3JPt-H = 36 Hz, CH2=CHCH3), 4.11 (m, 2H, 2JPt-H ca. 60 Hz, CH2=CHCH3), 5.00 (m, 1H, 2
JPt-H ca. 66 Hz, CH2=CHCH3).
K[PtCl3{η η2-CH2=CH(Ph)}], K1c. K[PtCl3(η2-CH2=CH2)]·H2O, (300 mg, 0.77 mmol) was dissolved in methanol (3-4 mL) and the solution placed in an ice bath. A 20-fold excess of styrene (1764 µL, 15.4 mmol) was then added. The reaction mixture was left under stirring for about 7 hours at 0 °C. Evaporation of the solvent, under reduced pressure, left an oily residue, which was triturated with diethyl ether affording a yellowish powder. After removal of the organic phase, the 12
obtained solid was dried in vacuo and identified as the desired product K1c. The isolated yield, referred to platinum, was nearly quantitative. Anal. Calcd. for C8H8Cl3KPt: C, 21.61; H, 1.81 %. Found: C, 21.56; H, 1.78 %. Peak in ESI-MS: m/z = 404.7 = [M - K]−. NMR (CD3OD, 298 K, ppm): δH 4.31 (d,
1
H,
2
JPt-H ca. 74 Hz, CHH=CHC6H5), 4.95 (d,
1
H,
2
JPt-H ca. 60 Hz,
CHH=CHC6H5), 6.32 (m, 1H, 2JPt-H ca. 75 Hz, CH2=CHC6H5), 7.2-7.7 (m, 5H, CH2=CHC6H5). Complexes of type 5 reported in Scheme 1. [PtCl(η1-CH2CH2-OMe)(phen)] (5ay) was prepared according to reference 1n. [PtCl(η1-CH2CH2-OMe)(Me4phen)] (5az), [PtCl{η1-CH2CH(Me)OMe}(phen)]
(5by),
[PtCl{η1-CH2CH(Me)-OMe}(Me4phen)]
(5bz),
[PtCl{η1-CH2CH(Ph)-
OMe}(phen)] (5cy), and [PtCl{η1-CH2CH(Ph)-OMe}(Me4phen)] (5cz) were prepared in a way completely analogous to that of the already reported 5ay. In a typical experiment, 0.40 mmol of K1b, or K1c, were dissolved in methanol (4 mL) and the solution was placed in an ice bath. The stoichiometric amount of the di-imine (phen or Me4phen) was then added and the instantaneous formation of a yellow precipitate, corresponding to the five-coordinate species (2 in Scheme 1), did occur. At this stage a 20-fold excess of KOH (§3 mL, §2.5 M in MeOH) was added to the mixture. The reaction vessel was kept under stirring for 4 hours, meanwhile a deepening in the yellow color of the precipitate was also observed. The solid was separated by filtration of the solution through a sintered glass filter, washed with water until the filtrate reached pH = 7 and dried under vacuum. In all cases it turned out to be the desired compound of type 5 and the yield, referred to platinum, was always > 95%. 5az: Anal. Calcd. for C19H23N2ClOPt: C, 43.39; H, 4.41 %. Found: C, 43.36; H, 4.21 %. Peak of greatest intensity in ESI-MS: m/z = 548.9 = [M + Na]+. NMR (CDCl3, 298 K, ppm): δH 2.28 (app t, 2H, 2JPt-H ca. 88 Hz, Pt-CαH2), 3.42 (s, 3H, OCH3), 3.70 (app t, 2H, CβH2-O). 5by: Anal. Calcd. for C16H17N2ClOPt: C, 39.72; H, 3.54 %. Found: C, 39.70; H, 3.53 %. Peak of greatest intensity in ESI-MS: m/z = 506.9 = [M + Na]+. NMR (CDC13, 298 K, ppm); δH 1.44 (d, 3H, CβH(CH3)-O), 2.27 (app t, 1H, 2JPt-H ca. 88 Hz, Pt-CαHH), 2.55 (dd, 1H, 2JPt-H ca. 92 Hz, PtCαHH), 3.44 (s, 3H, OCH3), 3.71 (m, 1H, CβH(CH3)-O). 5bz: Anal. Calcd. for C20H25N2ClOPt: C, 13
44.49; H, 4.67 %. Found: C, 44.36; H, 4.65 %. Peak of greatest intensity in ESI-MS: m/z = 538.9 = [M + Na]+. NMR (CDCl3, 298 K, ppm): δH 1.33 (d, 3H, CβH(CH3)-O), 1.77 (app t, 1H, 2JPt-H ca. 86 Hz, Pt-CαHH), 2.33 (dd, 1H, 2JPt-H ca. 90 Hz, Pt-CαHH, 3.40 (s, 3H, OCH3), 3.58 (m, 1H, CβH(CH3)-O), 7.85, 8.95 (3JPt-H ca. 60 Hz) and 9.11 (4H, 3,4,7,8-Me4-1,10-phen). 5cy: Anal. Calcd. for C21H19N2ClOPt: C, 46.20; H, 3.51 %. Found: C, 46.15; H, 3.49 %. Peak of greatest intensity in ESI-MS: m/z = 568.9 = [M + Na]+. NMR (CDC13, 298 K, ppm): δH 2.28 (app t, 1H, 2JPt-H ca. 87 Hz, Pt-CαHH), 2.79 (dd, 1H, 2JPt-H ca. 88 Hz, Pt-CαHH), 3.31 (s, 3H, OCH3), 4.60 (m, 1H, CβH(Ph)-O). 5cz: Anal. Calcd. for C25H27N2ClOPt: C, 49.88; H, 4.52 %. Found: C, 49.79; H, 4.48 %. Peak of greatest intensity in ESI-MS: m/z = 601.8 = [M + Na]+. NMR (CDCl3, 298 K, ppm): δH 1.95 (app t, 1H, 2JPt-H ca. 84 Hz, Pt-CαHH), 2.91 (dd, 1H, 2JPt-H ca. 91 Hz, Pt-CαHH), 3.34 (s, 3H, OCH3), 4.65 (m, 1H, CβH(C6H5)-O), 7.97, 8.04, 8.46 (3JPt-H ca. 60 Hz) and 9.50 (4H, 3,4,7,8-Me4-1,10-phen). Complexes of type 3 in Scheme 1. [PtCl(η2-CH2=CH2)(phen)](ClO4) [3ay(ClO4)] was prepared as reported
in
reference
1n.
[PtCl(η2-CH2=CH2)(Me4phen)](ClO4)
[3az(ClO4)],
[PtCl{η2-
CH2=CH(Me)}(phen)](ClO4) [3by(ClO4)], [PtCl{η2-CH2=CH(Me)}(Me4phen)](ClO4) [3bz(ClO4)], [PtCl{η2-CH2=CH(Ph)}(phen)](ClO4) [3cy(ClO4)], and [PtCl{η2-CH2=CH(Ph)}(Me4phen)](ClO4) [3cz(ClO4)] were prepared by acid hydrolysis of the corresponding compounds of type 5 (5az, 5by, 5bz, 5cy, and 5cz). In a typical experiment 0.20 mmol of each compound 5 were suspended in methanol (1 mL) and treated with the stoichiometric amount of HClO4 (0.5 mL of a 0.4 M methanolic solution). The mixture was kept under stirring for 30 minutes at room temperature. The solid was separated by filtration through a sintered glass filter, washed twice with water, and dried under vacuum. It turned out to be the desired salt 3(ClO4); the yield, referred to platinum, was always > 90%). 3az(ClO4): Anal. Calcd. for C18H20N2Cl2O4Pt: C, 36.37; H, 3.39; N, 4.71 %. Found: C, 36.15; H, 3.41; N, 4.60 %. Molecular peak in ESI-MS: m/z = 494.8 = [M - ClO4]+. NMR 14
(acetone-d6, 298 K, ppm): δH 5.37 (s, 4H, 3JPt-H not evaluated, CH2=CH2). 3by(ClO4): Anal. Calcd. for C15H14N2Cl2O4Pt: C, 32.62; H, 2.56; N, 5.07 %. Found: C, 32,75; H, 2.43; N, 5.07 %. Molecular peak in ESI-MS: m/Z = 453.13 = [M-ClO4]+. NMR (acetone-d6, 298 K, ppm): δH 2.13 (d, 3H, CH2=CHCH3), 5.22 and 5.43 (m, 2H, η2-CH2=CHCH3), 6.4 (m, 1H, η2-CH2=CHCH3); δC 21.54 (η2-CH2=CHCH3), 73,07 (η2-CH2=CHCH3), 107.53 (CH2=CHCH3). 3bz(ClO4): Anal. Calcd. for C19H23N2Cl2O4Pt: C, 37.51; H, 3.64; Cl, 11.66 %. Found: C, 37.39; H, 3.44; Cl, 10.87 %. Molecular peak in ESI-MS: m/z = 508.32 = [M-ClO4]+. NMR (acetone-d6, 298 K ppm): d H 1.68 (d, 3H, CH2=CHCH3), 4.94 (m, 2H, CH2=CHCH3), 5.8 (m, 1H, CH2=CHCH3). 3cy(ClO4): Anal. Calcd. for C20H16N2Cl2O4Pt: C, 39.10; H, 2.63; N, 4.56 %. Found: C, 39.21; H, 2.54; N, 4.44 %. Molecular peak in ESI-MS: m/z = 515.28 = [M-ClO4]+. NMR (acetone-d6, 298 K, ppm): δH 5.24 (d, 1H, CHH=CHC6H5), 5.80 (d, 1H, CHH=CHC6H5), 6.74 (m, 1H, CHH=CHC6 H5). 3cz(ClO4): Anal. Calcd. for C24H25N2Cl2O4Pt: C, 43.00; H, 3.61; Cl, 10.58 %. Found: C, 42.38; H, 3.58; Cl, 10.9 %. Molecular peak in ESI-MS: m/z = 571.28 = [M - ClO4 ]+. NMR (acetone-d 6, 298 K, ppm): δH 5.23 (d, 1H, CHH=CHC6 H5), 5.81 (d, 1H, CHH=CHC6H5), 6.76 (m, 1H, CH2=CHC6H5). Reaction of [PtCl{η η2-CH2=CH(Me)}(phen)]+ (3by) with NEt3. In a typical experiment, the complex [PtCl{η2-CH2=CH(Me)}(phen)](ClO4), 3by(ClO4) (100 mg, 0.18 mmol), was suspended in chloroform (10 mL) and treated with a 2.5-fold excess of triethylamine (625 µL of a 0.72 M chloroform solution). The reaction mixture was kept under stirring for two hours at room temperature, meanwhile the formation of a red solid was observed. The solid was separated by filtration through a sintered glass filter, washed twice with chloroform and water, and dried in vacuo. The obtained red powder corresponded to the formula [{PtCl(phen)}2(µ-η1:η2CH2CH=CH2)](ClO4) (6by) (isolated yield, referred to platinum, of ca. 70%). Anal. Calcd. for C27H21N4Cl2O4Pt2: C, 35.00; H, 2.28; N, 6.05 %. Found: C, 34.44; H, 2.10; N, 5.74 %. NMR (CD3CN, 292 K, ppm): δH 4.15 (d, 4H, 3JH-H = 10 Hz, 2JPt-H = 75 Hz, CH2CHCH2), 7.21 (m, 1H, CH2CHCH2). 15
[Pt(η η3-C3H5)(phen)](ClO4),
5b(ClO4).
The
platinum
dimer
[{PtCl(phen)}2(µ-η1:η2-
CH2CH=CH2)]+ was suspended in 20 mL of CH3CN and kept under stirring for twelve hours at room temperature. A yellow precipitate of [PtCl2 (phen)] (4y) was formed. The solution was then filtered and the mother liquor evaporated under reduced pressure. The obtained yellow powder was the complex [Pt(η3-C3H5)(phen)] ClO4, 5b (isolated yield, referred to platinum, of ca. 75%). Molecular peak in ESI-MS: m/z = 416.0 = [M - ClO4 ]+. NMR (CD3CN, 298 K, ppm): δH 3.10 (d, 2H, 2JPt-H ca. 74 Hz, η3-CHHCHCHH), 4.37 (d, 2H, 2JPt-H ca. 32 Hz, η3-CHHCHCHH), 5.24 (m, 1H, 2JPt-H not evaluated, η3-CHHCHCHH), 8.04, 8.20, 8.91 and 9.47 (3JPt-H = 39 Hz) (8H, phen).
References [1] a) M. H. Chisholm, H. C. Clark, Inorg. Chem., 9 (1973) 2428; b) L. Maresca, G. Natile, G. Rizzardi, Inorg. Chim. Acta, 38 (1980) 53; c) H. Kurosawa, N. Asada, A. Urabe, M. Emoto, J. Organomet. Chem., 272 (1984) 321; d) G. Gervasio, S. A. Mason., L. Maresca, G. Natile, Inorg. Chem., 25 (1986) 2207; e) F. P. Fanizzi, L. Maresca, G. Natile, C. Pacifico, Gazz. Chim. Ital., 124 (1994) 261; f) F. P. Intini, L. Maresca, G. Natile, A. Pasqualone, Inorg. Chim. Acta, 124 (1994) 261; g) L. Maresca, G. Natile, Comments Inorg. Chem., 16 (1994) 95; h) G. S. Hill, L. M. Rendina, R. J. Puddephatt, J. Chem. Soc., Dalton Trans., (1996) 1809; i) I. Orabona, A. Panunzi, F. Ruffo, J. Organomet. Chem., 525 (1996) 295; j) M. Fusto, F. Giordano, I. Orabona, F. Ruffo, A. Panunzi, Organometallics, 16 (1997) 5981; k) J. P. Albietz Jr, K. Yang, R. J. Lachicotte, R. Eisenberg, Organometallics, 19 (2000) 3543;
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M. Vecchio, A. Ienco, L. Maresca, G. Natile, F. P. Fanizzi, Dalton Trans. 41 (2012) 3014; q) M. Benedetti, C. R. Girelli, D. Antonucci, S. A. De Pascali, F. P. Fanizzi, Inorg. Chim. Acta 413 (2014) 109; r) M. Benedetti, C. R. Girelli, D. Antonucci, F. P. Fanizzi, J. Organomet. Chem. 771 (2014) 40; s) M. Benedetti, C. R. Barone, C. R. Girelli, F. P. Fanizzi, G. Natile, L. Maresca, Dalton Trans. 43 (2014) 3669. [2] a) F. P. Fanizzi, L. Maresca, G. Natile, M. Lanfranchi, A. Tiripicchio, G. Pacchioni, J. Chem. Soc., Chem Commun. (1992) 333; b) L. Maresca, G. Natile, Comments Inorg. Chem. 15 (1993) 349; c) V. G. Albano, G. Natile, A. Panunzi, Coord Chem. Rew. 133 (1994) 67; d) S. A. De Pascali, D. Migoni, P. Papadia, A. Muscella, S. Marsigliante, A. Ciccarese, F. P. Fanizzi, Dalton Trans. 42 (2006) 5077; e) P. Papadia, A. Ciccarese, J. A. Miguel-Garcia, P. M. Maitlis, F. P. Fanizzi, J. Organomet. Chem. 690 (2005) 2097; f) M. Benedetti, D. Antonucci, S. A. De Pascali, C. R. Girelli, F. P. Fanizzi, J. Organomet. Chem. 714 (2012) 60; g) M. Benedetti, D. Antonucci, S. A. De Pascali, G. Ciccarella, F. P. Fanizzi, J. Organomet. Chem. 714 (2012) 104; h) M. Benedetti, V. Lamacchia, D. Antonucci, P. Papadia, C. Pacifico, G. Natile, F. P. Fanizzi, Dalton Trans. 43 (2014) 8826; i) M. Benedetti, P. Papadia, C. R. Girelli, F. De Castro, F. Capitelli, F. P. Fanizzi, Inorg. Chim. Acta 428 (2015) 8; l) M. Benedetti, D. Antonucci, C. R. Girelli, F. P. Fanizzi, Eur. J. Inorg. Chem. 2015 (2015) 2308; m) M. Benedetti, F. De Castro, D. Antonucci, P. Papadia, F. P. Fanizzi, Dalton Trans. 44 (2015) 15377. [3] a) L. Maresca, G. Natile, A. M. Manotti Lanfredi, A. Tiripicchio, J. Am. Chem. Soc. 104 (1982) 7661; b) L. Maresca, G. Natile, J. Chem. Soc., Chem. Comm. (1983) 40; c) G. Annibale, L. Maresca, G. Natile, A. Tiripicchio, M. Camellini, J. Chem. Soc., Dalton Trans. (1982) 1587; d) F. P. Fanizzi, F. P. Intini, L. Maresca, G. Natile, F. Gasparrini, J. Chem. Soc., Dalton Trans. 3 (1990) 1019; e) F. P. Fanizzi, F. P. Intini, L. Maresca, G. Natile, J. Chem. Soc., Dalton Trans. (1992) 309; f) L. Maresca, G. Natile, F. P. Fanizzi J. Chem. Soc., Dalton Trans. (1992) 1867; g) C. R. Barone,
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R. Cini, S. De Pinto, N. G. Di Masi, L. Maresca, G. Natile, G. Tamasi, Inorg. Chim. Acta 363 (2010) 205. [4] a) G. Bandoli, A. Dolmella, F. P. Fanizzi, N. G. Di Masi, L. Maresca, G. Natile, Organometallics 20 (2001) 805; b) G. Bandoli, A. Dolmella, F. P. Fanizzi, N. G. Di Masi, L. Maresca, G. Natile, Organometallics 20 (2002) 4595; c) G. Lorusso, G. Boccaletti, F. P. Fanizzi, N. G. Di Masi, L. Maresca, G. Natile, Eur. J. Inorg. Chem. (2004) 4751. [5] G. Lorusso, C. R. Barone, N. G. Di Masi, C. Pacifico. L. Maresca, G. Natile, Eur. J. Inorg. Chem. (2007) 2144. [6] A. De Renzi, B. Di Blasio, G. Morelli, A. Vitagliano, Inorg Chim. Acta 63 (1982) 233 and quoted references. [7] a) M. Green, J. K. K. Sarhan, I. M. Al-Najjax, J. Chem. Soc. (1981) 1565; b) J. K. K. Sarhan, M. Green, I. M. A1-Najjar, J. Chem. Soc. (1984) 771; c) M. Green, J. K. K. Sarhan, I. M. Al-Najjar, Organometallics 3 (1984) 520. [8] a) V. M. Vecchio, M. Benedetti, D. Migoni, S. A. De Pascali, A. Ciccarese, S. Marsigliante, F. Capitelli, F. P. Fanizzi, Dalton Trans. 48 (2007) 5720; b) M. Benedetti, D. Antonucci, C. R. Girelli, F. Capitelli, F. P. Fanizzi, Inorg. Chim. Acta 409 (2014) 427. [9] a) W. B. Connick, D. Geiger, R. Eisenberg, Inorg. Chem. 38(14) (1999) 3264; b) T. J. Wadas, S. Chakraborty, R. J. Lachicotte, Q.-M. Wang, R. Eisenberg, J. Inorg. Chem. 44 (2005) 2628; c) J. K.W. Lee, C.-C. Ko, K. M.-C. Wong, N. Zhu, V. W.-W. Yam, Organometallics 26 (2007) 12. [10] a) S. Kemp, N. J. Wheate, S. Wang, J. G. Collins, S. F. Ralph, A. I. Day, V. J. Higgins, J. R. Aldrich-Wright, J. Biol. Inorg. Chem. 12 (2007) 969; b) S. Kemp, N. J. Wheate, D. P. Buck, M. Nikac, J. G. Collins, J. R. Aldrich-Wright, J. Inorg. Biochem. 101 (2007) 1049. [11] J. E. Reed, A. J. P. White, S. Neidle, R. Vilar, Dalton Trans. (2009) 2558.
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[12] K. J. Bamham, V. B. Kenche, G. D. Ciccotosto, D. P. Smith, D. J. Tew, X. Liu, K. Perez, G. A. Cranston, T. J. Johansen, I. Volitakis, A. I. Bush, C. L. Masters, A. R. White, J. P. Smith, R. A. Cherny, R. Cappai, Proc. Natl. Acad. Sci. U.S.A. 105 (2008) 6813. [13] P. B. Chock, J. Halpern, F. E. Paulik, S. I. Shupack, T. P. Deangelis, Inorg. Synth., 28 (1990) 349.
Highlights -
[PtCl(η2-olefin)(N^N)]+ complexes with N^N = aromatic diimine; olefin = propene, styrene. The K[PtCl3(η2-styrene)] Zeise’s salt analogue is reported for the first time. [{PtCl(N^N)}2(µ-η1:η2-CH2CH=CH2)]+ and first reported [Pt(η3-C3H5)(phen)]+ species.
Cationic Olefin Complexes of Platinum(II): Aspects of Availability and Reactivity Michele Benedetti, Carmen R. Barone, Sara de Pinto, Federica De Castro, Giovanni Natile, Francesco P. Fanizzi.
The [PtCl{η2-CH2=CH(Me)}(N^N)]+ complex can deprotonate, in the presence of triethylamine, with formation of the dimeric [{PtCl(N^N)}2(µ-η1:η2-CH2CH=CH2)]+ species which can disproportionate with formation of the [PtCl2(phen)] and η3-allyl [Pt(η3-C3H5)(phen)]+ complexes.
19
Scheme 1
Scheme 2
Scheme 3.
Scheme 4. Reaction pathway for complex [PtCl(h2-propene)(phen)]+, 3by, reacting with NEt3.
Figure 1. 1H NMR spectrum of complex [PtCl{h1-CH2CH(CH3)-OCH3}(phen)], 5by, (300 MHz, CDCl3, 298 K).
A)
B)
Figure 2. Portion of 1H-NMR spectra (300 MHz, acetone-d6, 298 K) showing the resonance peaks of vinyl protons of: A) [PtCl(h2-propene)(phen)]+, 3by; B) [PtCl(h2-styrene)(phen)]+, 3cy.
Figure 3. 2D (1H,1H) COSY spectrum (300 MHz, THF-d8, 298 K) of the product obtained by reacting [PtCl(h2-stirene)(1,10-phen)](ClO4), 3cm(ClO4), with NHEt2. The cross peaks for protons of the platinum-coordinated amine molecule have been highlighted.
Figure 4. 2D (1H,1H) COSY spectrum (300 MHz, CD3CN, 298 K) of complex [Pt(h3C3H5)(phen)]+, 7by. Cross peaks for protons of the h3-allyl moiety have been highlighted.
A)
B)
Figure 5. A) Isotopic pattern of the molecular ion peak for the complex [Pt(h3-C3H5)(1,10phen)]+, 7by, in the ESI-MS spectrum (solvent: CH3CN); B) calculated isotopic pattern.
A)
B)
Figure 1S. 1H NMR spectra of complexes: A) K[PtCl3(h2-CH2=CHCH3)], (K)1b, (300 MHz, acetone-d6, 298 K); B) K[PtCl3(h2-CH2=CHC6H5)], (K)1c, (300 MHz, CD3OD, 298K).
TABLE 1. 1H NMR chemical shifts (ppm, downfield from Si(CH3)4, CDCl3), 2JPt-H in Hz in parentheses, for the platinum bound alkyl moiety in [PtCl(η1-CH2CH2OMe)(phen)] (5ay), [PtCl(η1CH2CH2OMe)(Me4phen)] (5az), [PtCl{η1-CH2CH(Me)OMe}(phen)] (5by), [PtCl{η1CH2CH(Me)OMe}(Me4phen)] (5bz), [PtCl{η1-CH2CH(Ph)OMe}(phen)] (5cy), [PtCl{η1CH2CH(Ph)OMe}(Me4phen)] (5cz), complexes. Compound −CH2− −CH(R’)− 5ay* 5az 5by 5bz 5cy 5cz
2.41 (90) 2.28 (88) 2.27 (88) 2.55 (92) 1.77 (86) 2.33 (90) 2.28 (87) 2.79 (88) 1.95 (84) 2.91 (91)
3.70 3.70 3.71 3.58 4.60 4.65
* See Ref. 1n
TABLE 2. 1H NMR chemical shifts (ppm, downfield from Si(CH3)4, acetone-d6), 2JPt-H in Hz in parentheses, for the platinum bound alkyl moiety in [PtCl(η2-CH2=CH2)(phen)]+ (3ay), [PtCl(η2CH2=CH2)(Me4phen)]+ (3az), [PtCl{η2-CH2=CH(Me)}(phen)]+ (3by), [PtCl{η2+ 2 + CH2=CH(Me)}(Me4phen)] (3bz), [PtCl{η - CH2=CH(Ph)}(phen)] (3cy), [PtCl{η2+ CH2=CH(Ph)}(Me4phen)] (3cz), complexes. Compound CH2= =CH(R’) 3ay 3az 3by 3bz 3cy 3cz
5.46 5.38 5.21, 5,40 4.89, 5.03 5.23, 5.78 5.23, 5.78
6.40 5.81 6.77 6.77
20
TABLE 3. Comparison between 1H and [13C] NMR resonance frequencies of the free propene (b) and η2-coordinated propene in the [PtCl{η2-CH2=CH(Me)}(phen)]+ (3by) complex (ppm, downfield from Si(CH3)4, acetone-d6). CH2= =CH(R’) Compound
CH2=CH(Me), (b) [PtCl{η2-CH2=CH(Me)}(phen)]+, (3by)
4.88, 4.98 [115.6] 5.21, 5.40 [73.07]
5.79 [133.7] 6.40 [107.53]
21