Dibromo–Isonitrile and N-acyclic Carbene Complexes of Platinum(II): Synthesis and Reactivity

: A series of dibromo-N-acyclic (NAC) carbene complexes of platinum(II) were synthesized, starting from trans-[Pt( µ -Br)Br(PPh 3 )] 2 and according to a protocol previously optimized for the preparation of analogous chlorinated compounds. In the ﬁrst step of the synthesis, the ring opening of the dinuclear precursor was carried out using suitable isonitrile ligands, while the following step consisted of the addition of N , N -diethylamine to the products obtained in the ﬁrst step. The two reactions were separately investigated, and attention was given to the differences between brominated and chlorinated systems.


Synthesis of Isocyanide Complexes [PtBr 2 (PPh 3 )(CNR)]
The preparation of isonitrile derivatives [PtBr 2 (PPh 3 )(CNR)] was carried out according to the reaction depicted in Scheme 1.The dinuclear brominated precursor was prepared according to a convenient reported procedure, starting from the easily available [PtCl 2 (NCMe) 2 ] [45] (see Supplementary Material for the synthesis), while the chosen isonitrile ligands were commercially available.

Synthesis of Isocyanide Complexes [PtBr2(PPh3)(CNR)]
The preparation of isonitrile derivatives [PtBr2(PPh3) (CNR)] was carried out according to the reaction depicted in Scheme 1.The dinuclear brominated precursor was prepared according to a convenient reported procedure, starting from the easily available [PtCl2(NCMe)2] [45] (see Supplementary Material for the synthesis), while the chosen isonitrile ligands were commercially available.The ring-opening reaction of the dinuclear precursor was carried out in 1,2-dichloroethane (1,2-DCE) and was followed by TLC or 31 P nuclear magnetic resonance (NMR) spectroscopy.The isonitrile ([ligand]/[Pt] = 2.0 molar ratio) was dissolved in 1,2-DCE and the addition was made at 0 °C to avoid any further substitution by the nucleophile.In all cases, the initially orange suspension turned into a light yellow, clear solution in a few hours and the chromatographic or spectroscopic analysis evidenced the disappearance of the precursor and the presence of products in solution.Only in the case of tert-butylisocyanide, a [ligand]/[Pt]= 3.0 molar ratio was necessary to obtain the complete conversion of the precursor, occurring anyway within a few hours at room temperature.The reaction is directed by the trans effect exerted by the phosphine ligand, so that the expected kinetic product is trans-[PtBr2(PPh3) (CNR)].However, being both isonitrile and triphenylphosphine π-acid ligands, the initial formation of the kinetic product can be followed by a fast isomerization process in solution, to afford a mixture of isomers, where the cis complex is the most abundant [9].Meanwhile, for the analogous chloro-complexes [9], we observed the complete conversion of the kinetic ring-opening products into cis-[PtCl2(PPh3) (CNR)]; in this case, a mixture of the two geometric isomers was obtained in most of the studied cases, both during the reaction and on the isolated samples, most likely for the higher steric hindrance of cis bromide ligands.Isolated yields in cis,trans complexes were quite good and the composition of the equilibrium mixtures could be conveniently studied using 31 P NMR spectroscopy.
Isolated yields and percentage compositions at equilibrium in solution are indicated in Table 1.Cis,trans percentages were calculated by integrating the corresponding 31   The ring-opening reaction of the dinuclear precursor was carried out in 1,2-dichloroethane (1,2-DCE) and was followed by TLC or 31 P nuclear magnetic resonance (NMR) spectroscopy.The isonitrile ([ligand]/[Pt] = 2.0 molar ratio) was dissolved in 1,2-DCE and the addition was made at 0 • C to avoid any further substitution by the nucleophile.In all cases, the initially orange suspension turned into a light yellow, clear solution in a few hours and the chromatographic or spectroscopic analysis evidenced the disappearance of the precursor and the presence of products in solution.Only in the case of tert-butylisocyanide, a [ligand]/[Pt]= 3.0 molar ratio was necessary to obtain the complete conversion of the precursor, occurring anyway within a few hours at room temperature.The reaction is directed by the trans effect exerted by the phosphine ligand, so that the expected kinetic product is trans-[PtBr 2 (PPh 3 )(CNR)].However, being both isonitrile and triphenylphosphine π-acid ligands, the initial formation of the kinetic product can be followed by a fast isomerization process in solution, to afford a mixture of isomers, where the cis complex is the most abundant [9].Meanwhile, for the analogous chloro-complexes [9], we observed the complete conversion of the kinetic ring-opening products into cis-[PtCl 2 (PPh 3 )(CNR)]; in this case, a mixture of the two geometric isomers was obtained in most of the studied cases, both during the reaction and on the isolated samples, most likely for the higher steric hindrance of cis bromide ligands.Isolated yields in cis,trans complexes were quite good and the composition of the equilibrium mixtures could be conveniently studied using 31 P NMR spectroscopy.
Isolated yields and percentage compositions at equilibrium in solution are indicated in Table 1.Cis,trans percentages were calculated by integrating the corresponding 31   The coordination of isonitrile ligands to the platinum center was evidenced in 31 P NMR spectra by the presence of satellites, with 1 J P-Pt coupling constants within 3310-3380 Hz for both isomers, in agreement with previous results [2,9,45].In the 195 Pt NMR spectra, doublet signals were observed in the −4180-−4600 ppm spectral zone, with the same 1 J PPt coupling constants measured in the 31 P NMR spectra.In comparing these values with those previously observed for the chlorinated counterparts [9], a shift towards high fields is evident, coherently with the substitution of chlorido ligands with bromido ones [46][47][48][49][50][51][52].Coordination was evident in the infrared (IR) spectrum as well, where very strong absorption bands were observed around 2200-2240 cm −1 , with an hypsochromic shift of about 90-100 cm −1 from the position of the same band in the free ligand [53].
In the case of complex 3, well-shaped single crystals were obtained by slow diffusion of pentane vapors into a chloroform solution of the compound, and the molecular structure was determined using single crystal X-ray diffraction.The structure of 3 is reported in Figure 1, while the most significant bond lengths and angles are listed in Table 2.The compound crystallized in the triclinic P-1 space group, and two independent molecules were observed in the unit cell, together with a molecule of chloroform.The coordination is square planar around the metal and the configuration is cis, with small deviations from ideality.The structure is in very good agreement with that previously described [9] for the chlorinated analogue cis-[PtCl 2 (CNC 6 H 4 (OCH 3 ))(PPh 3 )], where the most important differences in bond lengths have been ascribed to the larger size of bromido ions.
Inorganics 2023, 11, x FOR PEER REVIEW 3 of 13 The coordination of isonitrile ligands to the platinum center was evidenced in 31 P NMR spectra by the presence of satellites, with 1 JP-Pt coupling constants within 3310-3380 Hz for both isomers, in agreement with previous results [2,9,45].In the 195 Pt NMR spectra, doublet signals were observed in the −4180-−4600 ppm spectral zone, with the same 1 JPPt coupling constants measured in the 31 P NMR spectra.In comparing these values with those previously observed for the chlorinated counterparts [9], a shift towards high fields is evident, coherently with the substitution of chlorido ligands with bromido ones [46][47][48][49][50][51][52].Coordination was evident in the infrared (IR) spectrum as well, where very strong absorption bands were observed around 2200-2240 cm −1 , with an hypsochromic shift of about 90-100 cm −1 from the position of the same band in the free ligand [53].
In the case of complex 3, well-shaped single crystals were obtained by slow diffusion of pentane vapors into a chloroform solution of the compound, and the molecular structure was determined using single crystal X-ray diffraction.The structure of 3 is reported in Figure 1, while the most significant bond lengths and angles are listed in Table 2.The compound crystallized in the triclinic P-1 space group, and two independent molecules were observed in the unit cell, together with a molecule of chloroform.The coordination is square planar around the metal and the configuration is cis, with small deviations from ideality.The structure is in very good agreement with that previously described [9] for the chlorinated analogue cis-[PtCl2(CNC6H4(OCH3))(PPh3)], where the most important differences in bond lengths have been ascribed to the larger size of bromido ions.The synthesis of the NAC derivatives was carried out in 1,2-DCE solution, according to an experimental procedure previously applied to the successful preparation of chlorinated models.In each experiment, the chosen isocyanide complex was dissolved in 1,2-DCE and treated with a solution of N,N-diethylamine in the same solvent (Scheme 2) at 0 • C, following the reaction spectroscopically ( 31 P NMR).When complexes one and three were used, the reaction proceeded smoothly to afford the expected NAC product in a good, isolated yield.

Synthesis of Carbene Complexes [PtBr2(PPh3)(Et2N(H)CNR)]
The synthesis of the NAC derivatives was carried out in 1,2-DCE solution, according to an experimental procedure previously applied to the successful preparation of chlorinated models.In each experiment, the chosen isocyanide complex was dissolved in 1,2-DCE and treated with a solution of N,N-diethylamine in the same solvent (Scheme 2) at 0 °C, following the reaction spectroscopically ( 31 P NMR).When complexes one and three were used, the reaction proceeded smoothly to afford the expected NAC product in a good, isolated yield.The unprecedented 5 and 6 bromocomplexes were spectroscopically characterized.In the attenuated total reflectance IR spectra, the strong absorption band due to the stretching of isonitrile functional group was no longer observable, while a typical absorption band appeared, in both cases, around 1550 cm −1 , which could be ascribed to NCN stretching.In the 31 P NMR spectra, the disappearance of signals due to the isocyanido precursors was accompanied by the presence of new signals with satellites (JP-Pt ≈ 4000 Hz), which could be ascribed to the carbene species.In the case of benzyl derivative 5, a single signal was observed both in 31 P-and 195 Pt NMR spectra, indicating the stereoselectivity of the process towards the formation of a single isomer, to which a cis geometry was assigned for analogy with the analogous chlorinated system [9].In the case of the 4-methoxyphenyl derivative 6, a mixture of carbene products was observed, as indicated by the presence of two distinct signals with satellites in the 31 P NMR spectrum and of two doublet signals in the 195 Pt NMR one.The equilibrium composition of the mixture was 80/20 and it seems reasonable to assign the cis geometry to the main component of the mixture, taking into account the complete stereoselectivity observed in the synthesis of the analogous chlorinated compound [9].The 1 H NMR spectra of the carbene complexes 5 and 6 were quite typical of rigid systems, with non-equivalent hydrogen atoms affording distinct signals.
As an example, we report the 1 H NMR spectrum of complex 5 (Figure 2).The unprecedented 5 and 6 bromocomplexes were spectroscopically characterized.In the attenuated total reflectance IR spectra, the strong absorption band due to the stretching of isonitrile functional group was no longer observable, while a typical absorption band appeared, in both cases, around 1550 cm −1 , which could be ascribed to NCN stretching.In the 31 P NMR spectra, the disappearance of signals due to the isocyanido precursors was accompanied by the presence of new signals with satellites (J P-Pt ≈ 4000 Hz), which could be ascribed to the carbene species.In the case of benzyl derivative 5, a single signal was observed both in 31 P-and 195 Pt NMR spectra, indicating the stereoselectivity of the process towards the formation of a single isomer, to which a cis geometry was assigned for analogy with the analogous chlorinated system [9].In the case of the 4-methoxyphenyl derivative 6, a mixture of carbene products was observed, as indicated by the presence of two distinct signals with satellites in the 31 P NMR spectrum and of two doublet signals in the 195 Pt NMR one.The equilibrium composition of the mixture was 80/20 and it seems reasonable to assign the cis geometry to the main component of the mixture, taking into account the complete stereoselectivity observed in the synthesis of the analogous chlorinated compound [9].The 1 H NMR spectra of the carbene complexes 5 and 6 were quite typical of rigid systems, with non-equivalent hydrogen atoms affording distinct signals.As an example, we report the 1 H NMR spectrum of complex 5 (Figure 2).In the aliphatic portion of the spectrum, benzyl hydrogen atoms Ha and Hb originate two distinct double doublet signals at 5.65 and 4.44 ppm, where a typical geminal coupling constant (12 Hz) can be measured.Analogously, each of the four methylene hydrogen atoms of the diethylamino moiety (Hd-g) gives rise to a distinct multiplet signal (at 4.79, 3.64, 2.85 and 2.80 ppm).Finally, two triplet signals are observable at 1.19 and 0.88 ppm, which can be attributed to the two methyl groups of the diethylamino residue.The non-equivalence of the ethyl groups of the diethylamino residue is well observable in the 13 C NMR spectrum as well, where each carbon atom originates a distinct signal.A very similar spectral profile, although complicated by the presence of two geometric isomers, was observed in the NMR spectra of complex 6.
The reaction of tert-butylisocyano derivative 2 with N,N-diethylamine did not afford the expected products.Indeed, 31 P NMR analyses carried out on samples of the reaction mixture at different time spans revealed only the presence of the precursor.A greater excess of the nucleophile was added and the mixture was refluxed longer; nonetheless, the composition of the solution did not change and the precursor was recovered after the usual work-up procedures.It seems reasonable to ascribe this different behavior to the steric hindrance of the tert-butyl group.As a matter of fact, a sample of cis-[PtCl2(PPh3)(CNtert-Bu)], prepared in this work (see Supplementary Material for the synthesis) showed the same reactivity as the bromo complex.
Attempts were made to prepare the diethylamino NAC carbene of functionalized isocyano derivative 4. Unfortunately, repeated experiments afforded complex mixtures that could not be purified.It has to be noted that 4 is characterized by the presence of enolizable hydrogen atoms, in alfa position to diethyl carboxylate group.Indeed, this ligand is a synthetic equivalent of glycine and it has been used successfully to synthesize complex aminoacidic derivatives in experimental procedures based mostly upon the acidity of hydrogen atoms in alfa position to the ester group [54].It is likely that, in the presence of N,N-diethylamine, enolization equilibria are established, leading to the formation of byproducts.The same behavior was observed when a sample of cis-[PtCl2(PPh3)(CNCH2COOEt)] (see Supplementary Material for the synthesis) was reacted with N,N-diethylamine.

Stability of Complexes in DMSO
As anticipated, one of the possible applications of the prepared carbene complexes concerns their possible activity as anticancer agents.In view of the study of their In the aliphatic portion of the spectrum, benzyl hydrogen atoms H a and H b originate two distinct double doublet signals at 5.65 and 4.44 ppm, where a typical geminal coupling constant (12 Hz) can be measured.Analogously, each of the four methylene hydrogen atoms of the diethylamino moiety (H d-g ) gives rise to a distinct multiplet signal (at 4.79, 3.64, 2.85 and 2.80 ppm).Finally, two triplet signals are observable at 1.19 and 0.88 ppm, which can be attributed to the two methyl groups of the diethylamino residue.The nonequivalence of the ethyl groups of the diethylamino residue is well observable in the 13 C NMR spectrum as well, where each carbon atom originates a distinct signal.A very similar spectral profile, although complicated by the presence of two geometric isomers, was observed in the NMR spectra of complex 6.
The reaction of tert-butylisocyano derivative 2 with N,N-diethylamine did not afford the expected products.Indeed, 31 P NMR analyses carried out on samples of the reaction mixture at different time spans revealed only the presence of the precursor.A greater excess of the nucleophile was added and the mixture was refluxed longer; nonetheless, the composition of the solution did not change and the precursor was recovered after the usual work-up procedures.It seems reasonable to ascribe this different behavior to the steric hindrance of the tert-butyl group.As a matter of fact, a sample of cis-[PtCl 2 (PPh 3 )(CNtert-Bu)], prepared in this work (see Supplementary Material for the synthesis) showed the same reactivity as the bromo complex.
Attempts were made to prepare the diethylamino NAC carbene of functionalized isocyano derivative 4. Unfortunately, repeated experiments afforded complex mixtures that could not be purified.It has to be noted that 4 is characterized by the presence of enolizable hydrogen atoms, in alfa position to diethyl carboxylate group.Indeed, this ligand is a synthetic equivalent of glycine and it has been used successfully to synthesize complex aminoacidic derivatives in experimental procedures based mostly upon the acidity of hydrogen atoms in alfa position to the ester group [54].It is likely that, in the presence of N,N-diethylamine, enolization equilibria are established, leading to the formation of byproducts.The same behavior was observed when a sample of cis-[PtCl 2 (PPh 3 )(CNCH 2 COOEt)] (see Supplementary Material for the synthesis) was reacted with N,N-diethylamine.

Stability of Complexes in DMSO
As anticipated, one of the possible applications of the prepared carbene complexes concerns their possible activity as anticancer agents.In view of the study of their antiprolif-erative properties in vitro, the stability of the derivatives in media used for the biological tests is mandatory.The compounds here prepared are not soluble in water or ethanol and are well soluble in DMSO; however, it is well known [55] that the coordination properties of DMSO towards platinum can severely affect the nature of the tested compounds.Indeed, the coordination of this solvent to the metal center can be competitive with certain ligands, displacing them or, in the presence of traces of water, assisting metal-halogen hydrolysis processes [56,57].The occurrence of these side reactions is often enhanced by the presence of strongly trans-directing ligands.Thus, the behavior of derivatives 1-6 in DMSO was studied spectroscopically.In particular, the stability of the complexes was conveniently checked by 31 P NMR, as the possible substitution product of the isonitrile or NAC ligand by DMSO (cis-[PtBr 2 (PPh 3 )(SOMe 2 )]) is known to afford a signal at 17.2 ppm in d 6 -DMSO ( 1 J P-Pt = 3730 Hz) [45].In a typical experiment, a sample of the NAC complex 5 (about 10 mg) was dissolved in d 6 -DMSO and analyzed at different time spans (t = 0, 24 and 72 h).A single signal was observed in the freshly prepared sample (8.59 ppm, 1 J P-Pt = 4084 Hz, Figure S1), well in agreement with the 31 P NMR characterization previously registered in CDCl 3 .Analogously, in the 1 H NMR, all the signals attributed to 5 were present (Figure S2).No changes were observed in the spectra registered on the same sample after 24 and 72 h (Figures S3-S6).Other complexes afforded analogous results, thus proving their stability in DMSO.

Conclusions
The synthetic protocol previously used for the preparation of [PtCl 2 (PPh 3 )(CNR)] and [PtCl 2 (PPh 3 )(NAC)] derivatives proved suitable for the synthesis of the analogous brominated systems.Starting from trans-[Pt(µ-Br)Br(PPh 3 )] 2 , the corresponding isonitrile complexes [PtBr 2 (PPh 3 )(CNR)] (R = Bz (1), tert-Bu (2), 4-MeOC 6 H 4 (3) and CH 2 COOEt (4)) were obtained with very good yields (75-98%), although the reaction was not as stereoselective as for the chlorinated counterparts and mixtures of geometric isomers, generally enriched in the cis isomer, were observed in chloroform solution.This behavior can be reasonably ascribed to the steric hindrance of bromido ligands.In the case of R = 4-MeOC 6 H 4 (3), the cis isomer was crystallized, and its molecular structure was determined using single crystal X-ray diffraction.The reaction of the isonitrile derivatives 1 and 3 with N,Ndiethylamine afforded the desired NAC compounds in good yields (69-87%), while the reaction failed when substrates 2 and 4 were used.In the case of complex 2, the complete lack of reactivity observed seems to have been caused by the steric hindrance exerted by the tert-butyl residue on the isonitrile functional group, which makes it scarcely accessible by the attacking N,N-diethylamine.As for complex 4, it is reasonable to ascribe the side reactions observed to the high reactivity of hydrogen atoms in alfa position to the ethyl carboxylate group in a basic environment.Indeed, the easily enolizable hydrogen atoms of ethyl-2-isocyanoacetate are commonly exploited to synthetize glycine derivatives [54].Finally, both isonitrile and NAC complexes proved stable in DMSO solution, where they are all well soluble; thus, their antiproliferative properties will be investigated in vitro and compared with their chlorido counterparts.

Materials and Methods
General.All manipulations were carried out under inert (Ar) atmosphere, if not otherwise stated.Usual procedures were followed to purify and dry solvents [58,59].Solid, commercially available reagents were used with no further purification.Samples of [PtBr 2 (NCMe) 2 ] [45], trans-[Pt(µ-Br)Br(PPh 3 )] 2 [45], trans-[Pt(µ-Cl)Cl(PPh 3 )] 2 [10] were prepared according to reported procedures.Samples of 4-methoxyphenylisocyanide, benzylisocyanide, tert-butylisocyanide and ethyl isocyanoacetate were purchased from ™Merck and used without further purification.N,N-diethylamine was distilled over KOH and filtered over dry alumina immediately before use.An elemental analyzer "Vario MICRO CUBE" CHNOS was used for elemental analysis.IR spectra were recorded on an Agilent "Cary 630" spectrometer, equipped with an ATR accessory; absorption peak ( ν, cm −1 ) intensities and shapes were described by the following abbreviations: s = strong, m = medium, w = weak, br = broad and sh = shoulder. 1H-, 13 C-, 31 P-and 195 Pt NMR spectra were recorded on JEOL YH 400 MHz and JEOL CZR 500 MHz spectrometers, in CDCl 3 solution (™Deutero GmbH, stored over Ag) if not otherwise stated.When samples of the reaction mixtures were analyzed using 31 P NMR in non-deuterated solvents, a sealed capillary containing C 6 D 6 was inserted into the sample to lock the instrument.Chemical shifts (δ ppm) referred to: Si(CH 3 ) 4 for 1 H and 13 C, H 3 PO 4 (85% in D 2 O) for 31 P and H 2 PtCl 6 for 195 Pt.The observed signals were described according to the following abbreviations: s = singlet, d = doublet, t = triplet, dd = doublet of doublets, q = quadruplet and m = multiplet.

General Procedure for the Synthesis of [PtBr 2 (PPh 3 )(CNR)]
In a Schlenk tube equipped with a magnetic stirrer, an orange suspension of trans-[Pt(µ-Br)Br(PPh 3 )] 2 [45] (0.200-0.400 g) in 1,2-DCE (10-15 mL) was cooled (0 • C) and treated, under stirring, with a solution of the suitable isocyanide in the same solvent ([isocyanide]/[Pt] = 2.0 molar ratio).The temperature was raised (25 • C) and a clear, light yellow solution was obtained (2-12 h).The proceeding of the reaction was checked by TLC and/or 31 P NMR.The mixture was stirred until the maximum conversion of the precursor was obtained; then, the solution was concentrated under a vacuum up to a quarter of the original volume and treated with n-heptane (20-30 mL).A waxy-oily solid precipitated, which turned into a colorless powder upon prolonged stirring (3-12 h).The product was filtered, washed with n-heptane (2 × 3 mL) and dried under a vacuum.For each complex, the used isocyanide ligand, the yield, the elemental analysis and the spectroscopic (IR and NMR) characterizations are reported.

Single-Crystal X-ray Diffraction
Single-crystal X-ray diffraction was performed with a Bruker D8 Venture instrument equipped with microfocus Mo source (Kα radiation, λ = 0.71073 Å) and a 2D Photon III detector.The main experimental details regarding the determination of the structure of 3 by single-crystal X-ray diffraction are reported in Table 3.In detail, a specimen of C 53 H 45 Br 4 Cl 3 N 2 O 2 P 2 Pt 2 , approximate dimensions 0.100 mm × 0.200 mm × 0.400 mm, was used for the X-ray crystallographic analysis.The integration of the data using a triclinic unit cell yielded a total of 100,548 reflections to a maximum θ angle of 28.27 • (0.75 Å resolution), of which 13,634 were independent (average redundancy 7.375, completeness = 98.4%,R int = 5.02%, R sig = 3.59%) and 12,364 (90.69%) were greater than 2σ(F2).The final cell constants of a = 10.6145(3)Å, b = 14.8776(4)Å, c = 18.7841(4)Å, α = 103.8190(10) • , β = 103.3390(10) • , γ = 90.3250(10) • and volume = 2796.98(13)Å 3 are based upon the refinement of the XYZ-centroids of reflections above 20 σ(I).The calculated minimum and maximum transmission coefficients (based on crystal size) are 0.1400 and 0.4980.The final anisotropic full-matrix least-squares refinement on F2 with 615 variables converged at R1 = 3.67% for the observed data and wR2 = 11.14% for all data.The goodness-of-fit was 1.099.The largest peak in the final difference electron density synthesis was 1.183 e − /Å 3 and the largest hole was −2.282 e − /Å 3 with an RMS deviation of 0.199 e − /Å 3 .On the basis of the final model, the calculated density was 1.924 g/cm 3 and F(000), 1540 e − .
P NMR signals in CDCl3 solution.
P NMR signals in CDCl 3 solution.
a Calculated by integration of 31 P NMR signals in solution.Scheme 1. Synthesis of isonitrile complexes 1-4.
a Calculated by integration of 31 P NMR signals in solution.

Table 2 .
Most significant bond lengths and angles for complex 3.

Table 2 .
Most significant bond lengths and angles for complex 3.