Platinum(II)-Cyclometalated “Roll-over” - American Chemical Society

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Platinum(II)-Cyclometalated “Roll-over” Complexes with a Chiral Pinene-Derived 2,20-Bipyridine Antonio Zucca,*,† Diletta Cordeschi,† Sergio Stoccoro,† Maria Agostina Cinellu,† Giovanni Minghetti,† Giorgio Chelucci,† and Mario Manassero*,‡ † ‡

Dipartimento di Chimica, Universit
a di Sassari, via Vienna 2, 07100 Sassari, Italy Dipartimento di Chimica Strutturale e Stereochimica Inorganica, Universit
a di Milano, Centro CNR, Via Venezian 21, I-20133 Milano, Italy ABSTRACT: The first nonracemic cyclometalated “roll-over” complex of platinum(II), [Pt(L-H)Me(DMSO)], has been synthesized in good yields by reaction of the chiral pinene-derived ligand (5S,7S)-5,7-methane-6,6-dimethyl-2-(pyridin-2-yl)5,6,7,8-tetrahydroquinoline, L, with the electron-rich platinum complex [Pt(DMSO)2Me2]. Adducts have also been obtained, namely, [Pt(L)(Me)Cl] and [Pt(L)Cl2], when electron-poorer platinum complexes such as [Pt(DMSO)2(Me)Cl] or [Pt(DMSO)2Cl2] were used as reagents. The structure of [Pt(L)Cl2] has been solved by X-ray diffraction. Reaction of the mononuclear species [Pt(L-H)Me(DMSO)] with [Pt(Me)2(DMSO)2] under strictly controlled reaction conditions gives the dinuclear “double roll-over” complex [(DMSO)(Me)Pt(μ-L-2H)Pt(Me)(DMSO)], in which the doubly deprotonated ligand L-2H acts as a bridging dianionic ligand with a four fused ring core. Protonation of the uncoordinated nitrogen ligand in [Pt(L-H)Me(DMSO)] gives the uncommon [Pt(L*)Me(DMSO)]þ cationic species, in which L* is the zwitterionic ligand originated from L-H by protonation of the uncoordinated nitrogen atom. Some aspects of the reactivity of the mononuclear roll-over species [Pt(L-H)Me(DMSO)] with acids and neutral ligands are also discussed.

’ INTRODUCTION Transition metal complexes with sp2-nitrogen donors constitute an important class of derivatives with a great array of potential applications.1 In this context particular attention has been devoted to square-planar geometry (SP-4), usually exhibited by d8 noble metals, such as Pt(II), Pd(II), and Au(III). SP-4 is basically an achiral structure that becomes chiral in some circumstances, for instance when chiral ligands are involved.2 Chiral SP-4 complexes have found widespread interest in asymmetric catalysis,3 cyclometalation,4 medicinal chemistry,5 and materials.6 Among nitrogen donors, 2,20 -bipyridines play a key role. The attractiveness of these ligands is mainly due to their rich and intriguing coordination chemistry, enriched by the easiness of functionalization of the pyridine ring and by the high stability of their complexes against moisture and oxygen. The range of applications is so wide that bipyridines have been recently defined as “the most widely used ligands”.7 2,20 -Bipyridines, despite their abundant and well-documented coordination chemistry, have received attention as chiral inducers only in recent times.8 Generally, 2,20 -bipyridines act as N^N chelating ligands, i.e., the coordinating behavior shown since their first appearance, at the end of the nineteenth century.9 Only recently has a new coordinating behavior, called “roll-over” cyclometalation, appeared in the literature. This roll-over reaction involves a C(3)H bond activation and formation of a C(3)M bond. [We are aware that the “roll-over” cyclometalated ligand is not a 2,20 -bipyridine but a C(3)-deprotonated, formally anionic, r 2011 American Chemical Society

(2,20 -bipyridine-H) ligand. We wish here to point out this rare chemical behavior opposed to the classical bidentate coordination.]

This particular reaction is still rare, and only a few Ir(III),10 Rh(III),11 Pd(II),12 Pt(II),13 and Au(III)14 species have been described in the literature. This coordinating mode has interesting potentialities, mainly due to the presence in the roll-over complex of an uncoordinated nitrogen atom, available for coordination, protonation, or other chemical reactions. The reactivity of the second nitrogen has been investigated only in the case of platinum(II) complexes, which are able to produce bimetallic and polymeric systems.13b,15 Very recently, interesting applications of platinum(II) rollover derivatives have been reported. A combined experimental/ theoretical investigation explored the roll-over cyclometalation mechanism in the gas phase, showing that methane is formed through a combination of the C(3)-H proton with the Pt-Me group. Evidence for an unprecedented reversibility of the rollover process was also reported.16 Roll-over platinum(II) species Received: February 22, 2011 Published: May 17, 2011 3064

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Scheme 1

have shown to be active in the dehydrosulfurization and oxidative CC bond coupling of thioethers in the gas phase.17 As a part of our efforts on the study of the reactivity of cyclometalated 2,20 -bipyridines18 and on the synthesis of asymmetric square-planar complexes,19 we have now extended investigations to a chiral bipyridine ligand containing a rigid pinene-type framework, namely, (5S,7S)-5,7-methane-6,6-dimethyl-2-(pyridin-2-yl)-5,6,7,8-tetrahydroquinoline, L.20 This ligand has recently shown activity in asymmetric allylic substitution, allylic oxidation, and cyclopropanation.21

The pinene framework is useful for its capacity in addressing asymmetric transformations,22 not only in neutral bi- or tridentate N donor ligands, such as 2,20 -bipyridines, 1,10-phenanthrolines,23 or terpyridines,24 but also in N∧C cyclometalated ligands, such as pinene-substituted phenylpyridines (A, Scheme 1),25 thienylpyridines (B, Scheme 1),26 and N∧C∧N bis-pyridyl-phenyl pincer ligands (C, Scheme 1).19a The coordinating behavior of L toward platinum(II) has never been investigated. Herein we describe the synthesis of platinum(II) species in which L behaves both as a N∧N classical neutral chelated ligand and as a N∧C roll-over cyclometalated anionic ligand, originated from a CH bond activation. Some aspects of the reactivity of the new species are also reported.

Chart 1

adjacent pyridine ring. The same effect is present for the two H9 protons, which present very different chemical shifts (δ 1.32 ppm, H9 endo; 2.71 ppm, H9 exo). A long-range coupling is observed between the H5 and the H7 protons (4JHH = 5.6 Hz), as previously observed in related species,19a likely due to the sterically fixed “W” configuration. The diastereotopic H8 protons are casually overlapping and appear as a “doublet” at 3.20 ppm. The reactivity of L toward the platinum(II) reagents [Pt(DMSO)2Cl2], [Pt(DMSO)2(Me)Cl], and [Pt(DMSO)2 (Me)2] showed to be dependent on the nature of the starting complex. From the less electron-rich species [Pt(DMSO)2Cl2] and [Pt(DMSO)2(Me)Cl] only the adducts [Pt(L)Cl2] (1) and [Pt(L)(Me)Cl] (2) could be isolated in good yields.

’ RESULTS AND DISCUSSION Synthesis of Adducts and Cyclometalated Species. The chiral ligand (5S,7S)-5,7-methane-6,6-dimethyl-2-(pyridin-2yl)-5,6,7,8-tetrahydroquinoline, L, has been synthesized from (þ)-pinocarvone according to a literature procedure.20 Its coordinative behavior has been studied so far only in the case of palladium(II) complexes, which showed to be catalytically active in asymmetric allylic substitution.21 A complete 1H NMR assignment of L was first performed in order to help us in the characterization of its metal derivatives. In particular, 2D-COSY and NOE-1d experiments were carried out. In the 1H spectrum one of the methyl groups on C6 (Me endo) is strongly shielded (δ 0.69 ppm) with respect to the other one (Me exo, δ 1.45 ppm), likely due to the shielding cone of the

Coordination of L in 1 and 2 is clearly proved by 1H NMR spectroscopy. In the 1H NMR spectrum of complex 1 the chelated coordination of the bipyridine is confirmed by the 195 PtH60 coupling (3JPtH60 = 41 Hz) and by the deshielding of the H60 and H8 protons with respect to the free ligand (δ 9.75, 4.18, and 3.90 ppm in 1, with Δδ values of 1.08, 0.98, and 0.70 ppm, respectively) due to the adjacent coordinated chloride.27 Owing to coordination, the signals of the H8 protons are no longer coincident: a NOE-1d experiment showed that irradiation of the endo C6-methyl gave enhancement of the more shielded 3065

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Table 1. Selected Bond Distances (Å) and Angles (deg) for [Pt(L)CL2]a molecule 1

Figure 1. ORTEP view of complex 1. Ellipsoids are drawn at the 30% probability level.

H8 proton, at 3.90 ppm, namely, the proton on the same side of the C6-methyls. In the case of complex 2 only one of two possible geometric isomers was observed and isolated. Its geometry in solution was ascertained by a NOE-1d experiment, which shows contact between the coordinated methyl at δ 1.37 ppm and the H60 proton, which is also strongly coupled to the 195Pt nucleus (3JPtH = 65 Hz).

Moreover, the H8 protons appear strongly deshielded (δ 3.80 and 4.12 ppm) as in complex 1, and the coordinated methyl group at 1.37 ppm has a 2JPtH value (81 Hz) in line with a Pt-Me trans to an sp2 nitrogen atom. Our efforts to grow crystals suitable for X-ray analysis were successful only in the case of complex 1. Platinum(II) adducts with 2,20 -bipyridine are well known. The chloride adduct [Pt(2,20 -bipy)Cl2] exists in red and yellow crystalline forms, whose major differences arise from a diversity in the stacking mode of the molecules: the Pt atoms are well separated in the yellow form (4.435(1) Å),28 but in the red one, which is intensely luminescent even at room temperature, the square-planar complexes stack to form an approximately linear Pt---Pt chain with a spacing of 3.45 Å.29 In the yellow form two inequivalent PtCl distances are also noted, in contrast to the red form. Complex 1 was characterized in the solid state by X-ray diffraction. The structure consists of the packing of [Pt(L)Cl2] molecules in the chiral monoclinic space group P21 with no unusual van der Waals contacts (but see later for a borderline Pt 3 3 3 Pt interaction). The asymmetric unit of 1 contains two crystallographically independent molecules, very similar to each other. For ease of comparison, we have labeled the atoms of one of the two independent molecules in the usual way, whereas for the second molecule we have added a prime symbol to the labels of corresponding atoms. So, if in the first molecule (hereinafter molecule 1) we have labels Pt, Cl(1), Cl(2), and so on, in the second molecule (hereinafter molecule 2) we have labels Pt0 ,

molecule 2

average

PtCl(1)

2.293(1)

2.298(1)

2.295

PtCl(2) PtN(1)

2.304(1) 2.010(3)

2.297(1) 2.014(3)

2.300 2.012

PtN(2)

2.072(3)

2.079(3)

2.075

N(1)C(1)

1.359(5)

1.345(5)

1.352

N(1)C(5)

1.351(5)

1.356(5)

1.353

N(2)C(6)

1.383(6)

1.368(5)

1.375

N(2)C(10)

1.354(5)

1.351(5)

1.352

C(5)C(4)

1.388(6)

1.388(6)

1.388

C(5)C(6) C(6)C(7)

1.461(6) 1.376(6)

1.477(6) 1.381(5)

1.469 1.378

C(9)C(8)

1.381(6)

1.378(6)

1.379

C(9)C(10)

1.401(5)

1.411(5)

1.406

C(9)C(14)

1.507(6

1.504(6)

1.505

C(10)C(11)

1.501(6)

1.499(6)

1.500

Cl(1)PtCl(2)

84.04(4)

83.33(4)

83.68

Cl(1)PtN(1)

93.1(1)

93.5(1)

93.3

Cl(1)PtN(2) Cl(2)PtN(1)

172.6(1) 175.4(1)

173.0(1) 175.0(1)

172.8 175.2

Cl(2)PtN(2)

102.3(1)

102.9(1)

102.6

N(1)PtN(2)

80.3(1)

80.5((1)

80.4

PtN(1)C(1)

125.0(3)

125.8(3)

125.4

PtN(1)C(5)

115.7(3)

115.6(3)

115.6

C(1)N(1)C(5)

119.3(4)

118.4(3)

118.9

PtN(2)C(6)

111.7(2)

112.3(3)

112.0

PtN(2)C(10) C(6)N(2)C(10)

130.8(3) 117.4(3)

129.6(3) 118.0(3)

130.2 117.2

N(1)C(5)C(4)

121.1(4)

121.7(4)

121.4

N(1)C(5)C(6)

115.0(3)

114.6(3)

114.8

C(4)C(5)C(6)

123.9(4)

123.6(3)

123.8 115.8

N(2)C(6)C(5)

115.5(3)

116.2(3)

N(2)C(6)C7)

122.0(4)

122.1(4)

122.0

C(5)C(6)C7)

122.4(4)

121.7(4)

122.0

C(8)C(9)C(10) C(8)C(9)C(14)

119.7(4) 123.3(4)

119.2(4) 123.1(4)

119.4 123.2

C(8)C(9)C(14)

123.3(4)

123.1(4)

123.2

C(10)C(9)C(14)

116.7(4)

117.6(3)

117.1

N(2)C(10)C(9)

121.8(4)

121.7(4)

121.7

N(2)C(10)C(11)

120.6(3)

122.1(3)

121.3

C(9)C(10)C(11)

117.6(3)

116.2(4)

116.9

a

Bond parameters of molecules 1 and 2 have their estimated standard deviations (esd’s) on the last figure in parentheses; their average is reported without esd.

Cl(1)0 , Cl(2)0 , and so on. An ORTEP view of molecule 1 is shown in Figure 1. This figure could represent molecule 2 equally well, because the two independent molecules are practically indistinguishable. This can be seen in Table 1, where a list of selected bond lengths and angles for the two molecules is reported, together with the average values of corresponding interactions. In both molecules the two platinum atoms display a square-planar coordination, with slight tetrahedral distortions. Thus, in molecule 1, maximum displacements from the leastsquares plane of Pt are þ0.069(4) and 0.089(3) Å for atoms 3066

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Organometallics C(6) and N(2), respectively, whereas the same values for the least-squares plane of Pt0 are þ0.076(3) and 0.069(3) Å for atoms N(1)0 and N(2)0 , respectively. The dihedral angle between these two planes is only 4.3(4)°, which means that the metal least-squares planes of the two molecules are almost parallel. The two PtN(1)C(5)C(6)N(2) pentaatomic rings are also substantially planar, with a more marked degree of planarity in molecule 1. The four pyridinic rings are each strictly planar, but the two rings of each molecule are not coplanar. Thus, the dihedral angles between the planes of N(1) and N(2) are 12.7(3)° in molecule 1 and 8.3(4)° in molecule 2. This fact explains why neither intra- nor intermolecular π-stacking between different [Pt(L)Cl2] molecules is present. The structure of 1 may be compared with those of [Pt(2,20 -bipy)Cl2] (red and yellow forms)2830 and [Pt(bipyet)Cl2]31 (bipyet = 6-ethyl-2,20 -bipyridine). In complex 1 the PtN(2) distance, average 2.075 Å, is longer than the PtN(1) one, average 2.012 Å, in line with the corresponding average distances found in [Pt(bipyet)Cl2], 2.050 and 1.996 Å, respectively (also in [Pt(bipyet)Cl2] there are two independent, very similar molecules, in the asymmetric unit).31 The mentioned PtN(2) distances are markedly longer than those found in both the yellow and red forms of [Pt(2, 20 -bipy)Cl2],2830 whereas those of PtN(1) are very similar. Thus, in the yellow form28 the two values are 2.011(10) and 2.006(10) Å, respectively, whereas in the red form, in which the two PtN distances are equal by symmetry,29,30 the observed unique values are 2.001(6)29 and 2.009(6) Å.30 Although ref 29 is a pioneering work of 1974, whereas ref 30 reports a much more recent redetermination of 1996, the two values are in “excellent agreement”, as Connick and co-workers30 say themselves (page 6262), pointing out, however, that their redetermination is based on a much higher number of data. For this reason, we will use their redetermination30 as a reference. The lenghtening of the PtN(2) bond lengths, observed both in 1 and in [Pt(bipyet)Cl2],31 is likely due to the steric hindrance of the two different substituents that are bonded to the N(2) pyridinic rings of the two compounds. Another effect of these substituents is the enlargement of the Cl(2)PtN(2) angles with respect to the Cl(1)PtN(1) ones, 102.6° vs 93.3° in 1 and 100.5° vs 92.7° in [Pt(bipyet)Cl2]. Both effects (distances and angles) are greater in 1, in line with the different bulkiness of the two substituents. For comparison, the unique ClPtN angle observed in [Pt(bipy)Cl2], red form,30 is 95.4(2)°. On the contrary, the PtCl(1) and PtCl(2) distances observed in 1 are very similar to each other (average values 2.295 and 2.300 Å, respectively), and both are in good agreement with the PtCl bond lengths found in [Pt(bipyet)Cl2]31 (average bond lengths 2.297 and 2.284 Å, respectively). Corresponding values observed in [Pt(2,20 -bipy)Cl2] (yellow form) are 2.281(4) and 2.300(3) Å, respectively.28 In [Pt(2,20 -bipy)Cl2] (red form), the unique PtCl distance is 2.302(2) Å.30 In 1, the distance between atoms Pt and Pt0 of the same asymmetric unit is 3.4805(2) Å. This Pt 3 3 3 Pt distance is not very different from that found in the red form of [Pt(bipy)Cl2], 3.449(1) Å.30 There has been extensive discussion on whether these contacts are weakly bonding in nature, and a semiquantitative study indicates a bonding energy of about 12 kJ mol1.32 In the yellow form of [Pt(2,20 -bipy)Cl2],28 the shortest Pt 3 3 3 Pt distance is definitely nonbonding (4.435 Å). In contrast to [Pt(DMSO)2Cl2] and [Pt(DMSO)2(Me)Cl], reaction of the electron-rich complex [Pt(DMSO)2(Me)2] did

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not provide the related adduct [Pt(L)(Me)2] 3. This is due to the successive formation of a new species, originated from 3 through the C3H bond activation and identified as the “roll-over” derivative [Pt(L-H)(Me)(DMSO)], 4.

The adduct 3, which was observed only in solution by 1H NMR spectroscopy, is likely an intermediate in the formation of 4, according to previous observations.13a In order to verify this hypothesis, the reaction between [Pt(DMSO)2(Me)2] and L was monitored in deuterated acetone by means of 1H NMR spectroscopy. As expected, the spectra indicate an initial N∧N coordination, shown by a species with two different coordinated methyls (δ 1.14 ppm, JPtH = 87 Hz; 1.05 ppm, JPtH = 89 Hz) and an H60 proton, slightly deshielded and coupled to 195Pt, as expected (δ 9.15 pm, JPtH = 25 Hz). All signals in the aromatic region are shifted to low field with respect to the ligand, with the only exception of H30 . On the whole, the spectra showed the formation of complex 3, followed by rapid conversion to 4. The reaction is such that only mixtures of starting compounds, 3 and 4, can be isolated in the solid state. Compounds 3 and 4 are not easily separable by simple methods. When the mixture is heated to 50 °C for 4 h, the reaction is driven to completion and complex 4 is the only product isolated. The coordination in complex 4, called “roll-over metalation”, is a rare behavior of 2,20 -bipyridines. In the case of platinum(II), it needs electron-rich Pt(II) precursors to occur and is favored by the presence of substituents in the 6 position.18a,c The structure of 4 is based on analytical and spectroscopic data. In particular its 1H NMR spectrum shows the absence of the H3 signal in the aromatic region and the presence of a singlet at 7.51 ppm flanked by satellites, due to the H4 proton. The 195 Pt1H coupling constant value (3JPtH = 53 Hz) is consistent with a DMSO coordinated in trans position to the C3 atom.13a The signals at 0.68 (3H, Pt-Me, 2JPtH = 81.6 Hz) and 3.24 ppm (6H, DMSO, 3JPtH = 18.3 Hz) agree with this formulation, which is further confirmed by a NOE-1d experiment showing a contact between the coordinated methyl and the H4 proton.

The CH bond activation is regioselective: no indication was found for the roll-over metalation at the unsubstituted pyridine ring, a sign of the importance of steric factors. Complex 4 is the first chiral roll-over complex reported until now. 3067

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Scheme 2

Reactivity. Double “Roll-over” Cyclometalation. Mononuclear roll-over derivatives, such as 4 or the analogous 2,20 -bipyridine complex [Pt(bipy-H)(DMSO)Me] (A, Scheme 2), are fivemembered cyclometalated species that are similar to those obtained from 2-phenylpyridine (B, Scheme 2). The main difference between A and B species is the presence in A of an uncoordinated nitrogen atom, which is potentially able to coordinate, giving dinuclear (compound C, Scheme 2) or polymeric species (D, Scheme 2). Our first interest in the study of the chemical properties of the rollover species 4 was to investigate the behavior of the nitrogen lone pair. First, we studied its coordinative behavior; it is worth remembering that in the mononuclear roll-over species 4 the crowded part of the ligand is far from the metal center, so a minor influence on the reactivity should be expected. Our preceding experience indicated that only the unsubstituted 2,20 -bipyridine is able to give double-“roll-over” metalation, behaving as a tetradentate N∧CN∧C bridging ligand. When a substituent is located ortho to the nitrogen, only single roll-over metalation occurs, with the only exception being phenyl substituents, for which a further metalation occurs, to give C∧N∧C pincer compounds.33 In the presence of an aliphatic substituent we never observed a second metalation, likely due to steric hindrance.13a After several attempts we were able to isolate the dinuclear complex [(DMSO)(Me)Pt(μ-L-2H)Pt(Me)(DMSO)], 5, by reaction of 4 with [Pt(DMSO)2(Me)2] in 20% excess under strictly controlled conditions.

The difficulty in the synthesis of the dinuclear species 5 can be ascribed to the steric hindrance of the aliphatic pinene substituent. Unsubstituted 2,20 -bipyridine encounters major difficulties in the first roll-over metalation compared to L, but the formed mononuclear complex undergoes an easier second rollover metalation than 4.35 Protonated Species. Having ascertained the coordinative capacity of 4, the behavior toward protonation was next investigated. Species 4, which is an air- and moisture-stable complex, contains in addition to the uncoordinated nitrogen atom also two PtC bonds. The protonation reaction of complex 4 may therefore follow several routes: (i) electrophilic attack on the PtC(sp3) bond, (ii) electrophilic attack on the PtC(sp2) bond, (iii) oxidative addition of the platinum center to give a Pt(IV) hydride, and (iv) protonation of the nitrogen atom. Two different type of acids were used: a first one with a coordinating anion, HCl, and a second one having a weakly coordinating anion, [18-crown-6 H][BF4]. The reactions followed different routes. In the reaction of 4 with 18-crown-6 3 HBF4 3 H2O both the PtC bonds present in the complex were not affected by the acid, and the reaction resulted in protonation of the uncoordinated nitrogen atom, to give the cationic derivative [Pt(L*)(Me)(DMSO)][BF4], 6, as the only product,

where L* is the zwitterionic ligand originated from L by C(3)H deprotonation and protonation of the uncoordinated nitrogen atom.

The 1H NMR spectrum of 5 shows only four aromatic protons. The absence of the H3 and H30 protons clearly accounts for a double CH bond activation. Noteworthy, the diastereotopic methyls of the DMSO ligand coordinated to a platinum atom close to the chiral centers appear as two resonances, whereas the methyls of the other DMSO are casually coincident due to the distance from the chiral centers. In compound 5 the doubly deprotonated ligand L-2H acts as a bridging dianionic ligand with four fused rings. The π-system that connects the platinum atoms is likely partially delocalized, on account of a potential aromatic character of the five-membered cyclometalated rings (metalloaromaticity).34

The formulation of complex 6 is supported by the broad signal at 13.36 ppm in the 1H NMR spectrum, which indicates protonation of the uncoordinated nitrogen atom; this signal disappears by addition of D2O, confirming the assignment. The protonation is reversible; thus addition of Na2CO3 to an acetone-D6 solution of 6 gives 4 as the only product (1H NMR criterion). The cationic nature of the complex was confirmed by conductivity 3068

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Organometallics Scheme 3

data (ΛM 78 Ω cm2 mol1 in acetone; see Experimental Section). In addition, the IR spectrum shows bands in the region 31003300 cm1 (NH bond) and a strong broad band around 1060 cm1, typical of the counterion [BF4]. Reaction with HCl. In contrast, reaction of the complex with HCl gives the corresponding chloride [Pt(L-H)Cl(DMSO)], 7, with loss of methane. The reaction, which is likely to proceed toward oxidative addition of HCl, was followed in solution by means of 1H NMR spectroscopy from room to low temperature, but no intermediate, such as Pt(IV) hydrides or N-protonated compounds, was observed in solution. The reaction occurs with concomitant shift of the DMSO group; the DMSO-methyl protons in 7 show a coupling with the 195 Pt nucleus, 3J(PtH)= 23.1 Hz, indicative of coordination trans to a nitrogen atom. In addition, the H60 proton is strongly deshielded (δ 9.53 ppm) due to the close proximity of the chloride. The shift may be due to the low trans influence of the chloride, which places its position trans to the ligand with the strongest trans influence (i.e., the C3 carbon) to give the most stable geometric isomer (Scheme 3). Substitution Reactions. Starting from the roll-over species 4, 6, and 7, a new series of compounds may be obtained by substitution of DMSO with two-electron neutral ligands, such as PPh3.

In complex 4 the DMSO ligand is particularly labile, being coordinated trans to a carbon atom. The substitution takes place rapidly at room temperature, leading in almost quantitative yield to complex 8, characterized analytically and spectroscopically. In particular, the 31P NMR spectrum shows one signal in agreement with a phosphorus coordinated trans to a high trans-influence donor, such as a C(sp2), as indicated by the low value of the 1JPtP (2212 Hz). In addition, in the 1H NMR spectrum the signal of the coordinated methyl is split by a cis coupling with the 31 P nucleus (3JPH = 7.5 Hz, 2JPtH = 83.8 Hz). The proposed geometry is also confirmed by the shielding of the H60 proton, on account of the anisotropic effect of the aromatic rings in the adjacent triphenylphosphine ligand. In addition, the 13C NMR spectrum shows the metalated carbon atoms at 12.48 (JPC = 4.7 Hz, JPtC = 726.5 Hz, CH3Pt) and 155.14 ppm (JPC = 119.5 Hz, JPtC = 964.5 Hz, C(sp2)Pt); both chemical shift and coupling constant values agree with the proposed formulation.

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The cationic derivative [Pt(L*)(Me)(PPh3)][BF4], 9, may be easily isolated as the only product both from complex 8 (by reaction with 18-crown-6 3 HBF4 3 H2O) and from complex 6 (by substitution of DMSO with PPh3 at room temperature).

The substitution reaction of the DMSO ligand in 6 does not change the nature of the ligand L*, as indicated by 1H NMR spectroscopy (NH, δ 13.26 ppm, which disappears after addition of D2O), IR spectra (NH and BF4 bands, see Experimental Section), and conductivity data (ΛM ca. 62 Ω cm2 mol1 in acetone). In the 31P NMR spectrum the 31P195Pt coupling constant is slightly larger in 9 than in complex 8, as already seen comparing the DMSO signals in 4 and 6 (see later). Finally, the substitution reaction of the DMSO group by PPh3 in complex 7 is more difficult than in 4 and 6, as expected, by a DMSO trans to a nitrogen instead than to a carbon atom. Nevertheless, also in this case the phosphane complex, [Pt(LH)(Cl)(PPh3)], 10, may be synthesized at room temperature. In this case the coordination of PPh3 causes, in the 1H NMR spectrum, a strong shielding of the protons located in its proximity, e.g., the H4 (from 8.14 ppm in 7 to 6.28 ppm in 10) and the endo H9 and CH3 (1.05 and 0.41 ppm, respectively). On the contrary, the H60 proton is strongly deshielded (9.77 ppm) due to the proximity of the chloride and coupled to the phosphorus atom. In addition the H4 proton is strongly coupled to 195Pt (49 Hz) as expected because of the low trans influence of the chloride. It is worth noting that the reaction of the methyl complex [Pt(L-H)(Me)(PPh3)], 8, with HCl gives 10 in a mixture of other species.

This observation indicates that the reaction of the neutral rollover species with HCl is dependent on the nature of the neutral ligand. 3069

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Organometallics Chart 2

Furthermore, this synthetic approach enables the synthesis of a series of mononuclear compounds having different electronic and steric properties depending on the nature of the neutral and/ or the anionic ligand. Nature of the Ligand L*. The nature of the ligand L* in complexes 6 and 9 deserves comment. According to the literature this ligand may be regarded as a zwitterionic or a nonclassical heterocyclic carbene ligand.36 The chemistry of N-heterocyclic carbenes (NHCs) was initially focused on 2-imidazolylidenes and their derivatives.37 In addition to the classical “normal” NHC ligands a variety of “abnormal” and “remote” carbene complexes containing the heteroatom in a remote position from the carbenemetal bond have appeared in the literature. The definition of “abnormal carbenes” is commonly used for complexes for which a canonical valence bond representation requires additional formal charges in some nuclei, whereas the term “remote” indicates that no heteroatom is present in R-position to the carbenemetal bond36,38 (Chart 2). Pyridylene ligands may generate a particularly attractive subclass of NHCs, having the ability to form normal, abnormal, and remote carbenes38,39 (compounds D, E, and F, Chart 2), enriched by the versatility of the pyridine ring toward functionalization. Pyridine-based NHCs have shown to be generally stronger σ-donors and π-acceptors than imidazole-2-ylidenes40 and were reported to have interesting catalytic activity in cross-coupling reactions,41 higher than that of two-nitrogen five-membered NHCs.42 The increase of the π-accepting capability distinguishes pyridine-2-ylidene donors from their formally anionic phenyl analogues and makes them particularly well suited for accelerating associative ligand substitution reactions.43 In the particular case of C3-bonded pyridylenes (compound E, Chart 2), to which complex 6 may be related, a controversial debate is present on the classification as “carbenes” instead of “zwitterionic” ligands. In some cases a zwitterion canonical representation containing a carbanionic and an iminiun center may be appropriate, but the borderline between the two representations is subtle and may be semantic.36 To shed light on the nature of compounds 6 and 9, we started an investigation on their chemical behavior and spectroscopic properties. In the absence of structural data we tried to get 13C NMR data in order to determine the signal of the platinum-bonded C(sp2) atom. The low solubility of complex 6 did not allow us to obtain

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data from the 13C spectrum. Complex 9 is more soluble, and its 13C NMR spectrum clearly shows the signal of the metalated C(3) atom as a doublet with satellites at 160.27 ppm (2JPC = 120.9 Hz, 1JPtC = 990 Hz). These data, compared to those of the corresponding neutral species 8 (δ 155.14 ppm, 2JPC= 119.5 Hz, 1JPtC = 964.5 Hz), indicate that the protonation induces a downfield shift of ca. 5 ppm. This signal is not particularly deshielded but may be compared to platinum(II) C2-pyridilene complexes reported by Bercaw and co-workers43 (δ ca. 160170 ppm), who reported greater values of 1JPtC (ca. 12001300 Hz) but with DMSO in trans position instead of PPh3, i.e., with a ligand with a much lower trans influence. As a comparison, 3 J(PtH4) values decrease from 59 Hz in complex 6 (DMSO PtC(3) trans arrangement) to 50 Hz in complex 9 (PPh3 PtC(3) trans arrangement). In addition, it has recently been reported that “abnormal” palladium(II) C3-pyridilenes show lower chemical shifts (δ ca. 165175 ppm) than the “normal” homologues (δ >190 ppm) and was stated that chemical shift data for these abnormal carbenes must be considered with great care, being affected by several factors,36 and cannot be described in terms of simple valence bond theory.39b Nevertheless, it is worth mentioning that both 1J(PtC3) and 3 J(PtH4) coupling constants show a significant increment with protonation (1J from 964.5 to 990 Hz in 8 and 9, 3J from 53 to 59.2 Hz in 4 and 6), suggesting a stronger PtC(3) bond in the protonated species. On the other hand analysis of the 1H NMR spectra gave further indications: the ligand trans to the C(sp2) appears to be more strongly bonded in the cationic protonated species 6 and 9 than in neutral complexes 4 and 8. This is suggested by the coupling constant values of the coordinated DMSO (3JPtH 20.3 vs 18.3 Hz in 6 and 4) and PPh3 (1JPtP 2469 vs 2212 Hz in 9 and 8, respectively). Interestingly, the 2J(195PtCH3) and 3J(195PtH60 ) values are smaller in the protonated species (Table 2), suggesting weaker PtN and PtC bonds cis to the C(3) coordinated atom. In the absence of other data we are unable to give clear indications about the nature of these species, whose chemical behavior is currently under investigation. Anyhow, even though the real nature of L* is not completely clear, this neutral ligand may have some points of interest. Nitrogen ligands containing NH bonds recently received much attention, being able to respond to changes in the solution environment, such as variations in pH, and tune the properties of the transition metals to which they are bonded. Such ligands with multiple personalities44 are showing intriguing perspectives in prominent fields, such as CH bond activation45 and the design of molecular devices.46

’ EXPERIMENTAL SECTION The ligand (5S,7S)-5,7-methane-6,6-dimethyl-2-(pyridin-2-yl)5,6,7,8-tetrahydroquinoline, L, and [Pt(DMSO)2(Me)2] were synthesized according to refs 20 and 47. All solvents were purified and dried according to standard procedures.48 Elemental analyses were performed with a Perkin-Elmer elemental analyzer 240B by Mr. Antonello Canu (Dipartimento di Chimica, Universit
a degli Studi di Sassari, Italy). Infrared spectra were recorded with a FT-IR Jasco 480P using Nujol mulls. 1H, 13C{1H}, and 31P{1H} NMR spectra were recorded with a Varian VXR 300 spectrometer operating at 300.0, 75.4, and 121.4 MHz, respectively. Chemical shifts are given in ppm relative to internal TMS for 1H and 13C{1H} and external 85% H3PO4 for 31P{1H}. J values are 3070

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Organometallics

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Table 2. Selected NMR and IR Dataa L0

PtMe

8.67 dd

7.34 d

4

3.24b (18.3)

0.68 (81.6)

9.64 (15)

7.51 (53)

8

33.66c (2212)

0.72 (83.8)

ov

ov

7

3.28b (20.3)

0.74 (81)

9.86 (14)

8.18 (59.2)

8.57

3.03

3.45

2.82

1.33

1.48

0.70

13.36

9

32.32c (2469)

0.78 (82.8)

7.82 (15)

8.40 (50)

8.53

3.02

3.40

2.82

1.32

1.46

0.72

13.26

6

3.64 (23.1)

9.53 (38.8)

8.14 (42)

8.17

2.84

3.03

2.66

1.28

1.39

0.70

10

23.51c (4291)

9.77 (28)

6.28 (49)

8.20

1.85

2.89

2.40

1.05

1.18

0.41

L

H60

H4

H30

8.26

H5

H8

H9 exo

H9 endo

CH3 exo

CH3 endo

2.82 t

3.20 d

2.71 dt

1.32 d

1.43 s

0.69 s

2.79

3.07

2.67

1.29

1.41

0.68

2.80

3.06

2.63

1.28

1.40

0.72

NH

a Multiplicities are indicated only for L, 195Pt1H, and 195Pt31P coupling constants in parentheses. For the numbering scheme see below, ov = overlapping signals. b Coordinated DMSO. c 31P NMR data.

given in Hz. NOE difference, 2D-COSY, and 13C apt (attached proton test) experiments were performed by means of standard pulse sequences. Conductivities were measured with a Philips PW 9505 conductimeter.

6,6-Dimethyl-(5S,7S)-methane-2-(pyridin-2-yl)-5,6,7,8-tetrahydroquinoline, L. NMR data (assignments based on COSY and

NOE-1d experiments): 1H NMR (CDCl3 δ, ppm) 8.67 (ddd, 1H, H60 , JHH = 4.8 Hz, JHH = 1.8 Hz, JHH = 0.9 Hz); 8.34 (dt, 1H, H30 , JHH = 7.9 Hz, JHH = 0.9 Hz, JHH = 0.9 Hz); 8.03 (d, 1H, H3, JHH = 7.8 Hz); 7.79 (td, 1H, H40 , JHH = 7.8 Hz, JHH = 1.8 Hz); 7.34 (d, 1H, H4, JHH = 7.8 Hz); 7.27 (m, 1H, H50 , JHH = 4.8 Hz, JHH = 7.8 Hz, JHH = 0.9 Hz); 3.20 (d br, 2H, CH2 (H8) JHH = 2.8 Hz); 2.82 (t, 1H, H5, JHH = 5.6 Hz); 2.71 (dt, 1H, H9 exo, JHH = 9.5 Hz, JHH = 5.6 Hz); 2.41 (m, 1H, H7); 1.43 (s, 3H, CH3 (exo)); 1.32 (d, 1H, H5, JHH = 9.5 Hz); 0.69 (s, 3H CH3 (endo); 13 C NMR (CDCl3) (apt 13C NMR) 21.51 (CH3), 26.26 (CH3), 32.13 (CH2), 36.93 (CH2), 39.69 (Cq), 40.41 (CH), 46.61 (CH), 118.07 (CH), 120.99 (CH), 123.22 (CH), 133.94 (CH), 136.92 (CH), 142.39 (Cq), 149.31 (C60 H), 153.71 (Cq), 156.64 (Cq), 156.95 (Cq). Synthesis of [Pt(L)Cl2], 1. To a solution of [Pt(DMSO)2Cl2] (84 mg, 0.199 mmol) in acetone (25 mL) was added 53 mg of L (0.212 mmol, 5% excess).The stirred solution was heated to 50 °C. After 4 h the mixture was cooled to room temperature, concentrated to small volume, and treated with Et2O to form a precipitate. The solid was filtered off, washed with Et2O, and vacuum-pumped to give the analytical sample as an orange solid. Yield: 75%. Mp: >260 °C. Anal. Calcd for C17H18Cl2N2Pt: C, 39.55; H, 3.51; N, 5.43. Found: C, 39.72; H, 3.56; N, 5.23. 1H NMR (300 MHz;CDCl3; Me4Si): δ 9.75 (dd, 1H, JHH = 6.2 Hz, JPtH = 41 Hz, H60 ), 8.12 (td, 1H, JHH = 7.4 Hz, JHH = 1.4 Hz, H40 ), 7.93 (dd, 1H, JHH = 8.2 Hz, H30 ), 7.82 (d, 1H, JHH = 7.9 Hz, H3), 7.65 (d, 1H, JHH = 7.9 Hz, H4), 7.50 (m, 1H, JHH = 7.4 Hz, JHH = 1.4 Hz, H50 ), 4.18 (dd, 1H, JHH = 19.5 Hz, JHH = 3.1 Hz, CH2 (H8)), 3.90 (dd br, 1H, JHH = 19.5 Hz, CH2 (H8)), 2.94 (t, 1H, JHH = 5.8 Hz, H5), 2.71 (dt, 1H, JHH = 9.8 Hz, JHH = 5.4 Hz, H9 exo), 2.50 (m, 1H, H7), 1.44 (s, 3H, CH3 exo), 1.28 (d, 1H, JHH = 9.9 Hz, H9 endo), 0.69 (s, 3H, CH3 endo).

Synthesis of [Pt(L)Cl(Me)], 2. To a solution of [Pt(DMSO)2 ClMe] (80 mg, 0.198 mmol) in acetone (25 mL) was added 50 mg of L (0.199 mmol). The stirred solution was heated to 50 °C and kept under a nitrogen atmosphere. After 4 h the mixture was cooled to room temperature, concentrated to small volume, and treated with Et2O to form a precipitate. The solid was filtered off, washed with Et2O, and vacuum-pumped to give the analytical sample as an orange solid. Yield: 85%. Mp (dec): 200 °C. Anal. Calcd for C18H21ClN2Pt: C, 43.60; H, 4.27; N, 5.65. Found: C, 43.68; H, 3.94; N, 5.59. 1H NMR (300 MHz; CDCl3; Me4Si) δ 9.10 (d, 1H, JHH = 5.5 Hz, JPtH = 65 Hz, H60 ), 8.08 (td, 1H, JHH = 8.1 Hz, H40 ), 7.93 (d, 1H, JHH = 8.1 Hz, H30 ), 7.81 (d, 1H, JHH = 7.8 Hz, H3), 7.54 (d,1H, JHH = 7.8 Hz, H4), 7.37 (m, 1H, JHH = 5.5 Hz, H50 ), 4.12 (dd, 1H, JHH = 20.4 Hz, JHH = 1.6 Hz, CH2 (H8)), 3.80 (d, 1H, JHH = 20.4 Hz, CH2 (H8)), 2.90 (t, 1H, JHH = 5.7 Hz, H5), 2.66 (m, 1H, H9 exo), 2.50 (m, 1H, H7), 1.41 (s, 3H, CH3 exo), 1.37 (s, 3H, JPtH = 81 Hz, CH3-Pt), 1.27 (d, 1H, H9 endo), 0.66 (s, 3H, CH3 endo). 1 H NMR Data for Compound 3, [Pt(L)(Me)2]. 1H NMR (300 MHz; acetone D6; Me4Si): δ 9.15 (dd, 1H, JHH = 5.5 Hz, JPtH = 25 Hz, H60 ), 8.32 (d, 1H, JHH = 8.1 Hz, H30 ), 8.24 (td, 1H, JHH = 8.1 Hz, H40 ), 8.17 (d, 1H, JHH = 7.8 Hz, H3), 7.77 (d,1H, JHH = 7.8 Hz, H4), 7.57 ((m, 1H, JHH = 5.5 Hz, H50 ), 3.42 (m br, 2H, CH2 (H8)); 2.98 (t, 1H, H5,); 2.75 (dt, 1H, H9 exo, JHH = 9.5 Hz, JHH = 5.6 Hz); 2.45 (m, 1H, H7); 1.43 (s, 3H, CH3 (exo)), 1.28 (d, 1H, H5, JHH = 9.5 Hz), 1.14 (s, 3H, JPtH = 87 Hz, CH3-Pt), 1.05 (s, 3H, JPtH = 89 Hz, CH3-Pt), 0.68 (s, 3H CH3 (endo). Synthesis of [Pt(L-H)(Me)(DMSO)], 4. To a solution of [Pt (DMSO)2Me2] (114 mg, 0.299 mmol) in acetone (25 mL) was added 75 mg of L (0.299 mmol). The stirred solution was heated to 50 °C and kept under a nitrogen atmosphere. After 4 h the mixture was cooled to room temperature, concentrated to small volume, and treated with nhexane to form a precipitate. The solid was filtered off, washed with nhexane, and vacuum-pumped to give the analytical sample as an orange solid. Yield: 90%. Mp (dec): 190 °C. Anal. Calcd for C20H26N2OPtS: C, 44.68; H, 4.87; N, 5.21. Found: C, 44.45; H, 4.76; N, 5.21. 1H NMR (300 MHz; CDCl3; Me4Si): δ 9.64 (d, 1H, JHH = 4.8 Hz, JPtH = ca. 14 Hz, 3071

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Organometallics H60 ), 8.26 (d, 1H, JHH = 8.1 Hz, H30 ), 7.88 (t, 1H, JHH = 7.8 Hz, JHH =1.6 Hz, H40 ), 7.51 (s, 1H, JPtH = 53 Hz, H4), 7.28 (m, 1H, H50 ), 3.24 (s, 6H, JPtH = 18.3 Hz, CH3 (DMSO)), 3.07 (d, 2H, JHH = 2.3 Hz, CH2 (H8)), 2.79 (t, 1H, JHH = 5.6 Hz, H5), 2.67 (dt, 1H, JHH = 9.5 Hz, JHH = 5.6 Hz, H9 exo), 2.38 (m, 1H, H7), 1.41 (s, 3H, CH3 exo), 1.29 (d, 2H, JHH = 9.5 Hz, H9 endo), 0.68 (s, 6H, JPtH = 81.6 Hz, CH3 endoþ CH3-Pt). Synthesis of [(DMSO)(Me)Pt(μ-L-2H)Pt(Me)(DMSO)], 5. To a solution of [Pt(L-H)(DMSO)Me] (100 mg, 0.186 mmol) in anhydrous toluene (25 mL) was added 85 mg of [Pt(DMSO)2Me] (0.223 mmol). The stirred solution was heated to 100 °C and kept under a nitrogen atmosphere. After 1 h the mixture was concentrated to small volume and treated with Et2O; the precipitate formed was filtered off, washed with Et2O, and vacuum-pumped to give the analytical sample as a brown solid. Yield: 71%. Mp (dec): 200 °C. Anal. Calcd for C23H34N2O2Pt2S: C, 33.49; H, 4.15; N, 3.40. Found: C, 33.15; H, 3.94; N, 3.56. 1H NMR (300 MHz;CDCl3; Me4Si): δ 9.05 (d, 1H, JHH = 4.8 Hz, H60 ), 8.33 (d, 1H, JHH = 7.5 Hz, H40 ), 7.53 (s, 1H, JPtH = 48 Hz, H4), 7.05 (dd, 1H, JHH = 4.8 Hz, JHH = 7.5 Hz, H50 ), 3.23, 3.22, 3.20 (3 singlets, partially overlapping, 12H, DMSO), 3.12 (d, 1H, JHH = 18.6 Hz, CH2 (H8)), 2.89 (d, 1H, JHH = 18.6 Hz, CH2 (H8)), 2.77 (t, 1H, JHH = 5.8 Hz, H5), 2.60 (dt, 1H, H9 exo), 2.11 (m, 1H, H7), 1.31 (s, 3H, CH3 exo), 1.17 (d, 1H, JHH = 10.5 Hz, H9 endo), 0.69 (s, 3H, CH3-Pt, JPtH = ca. 80 Hz), 0.68 (3H, JPtH = 80 Hz, CH3-Pt), 0.52 (s, 3H, CH3 endo). Synthesis of [Pt(L*)(DMSO)(Me)][BF4], 6. To a solution of 4 (50 mg, 0.093 mmol) in CH2Cl2 (25 mL) was added 37.8 mg of 18crown-6 3 HBF4 3 H2O (0.102 mmol, 10% excess). After 2 h the mixture was concentrated to small volume and treated with Et2O to form a precipitate. The solid was filtered off, washed with Et2O, and vacuumpumped to give the analytical sample as a green solid. Yield: 92%. Mp (dec): 220 °C. Anal. Calcd for C20H27BF4N2OPtS: C, 38.41; H, 4.35; N, 4.48. Found: C, 38.68; H, 3.71; N, 4.28. 1H NMR (300 MHz; CDCl3; Me4Si): δ 13.36 (s br, 1H, N-H), 9.86 (d, 1H, JHH = 5.4 Hz, JPtH = 14 Hz, H60 ), 8.57 (d, 1H, JHH = 8.1 Hz, H30 ), 8.23 (m, 1H, JHH = 7.9 Hz, JHH = 1.3 Hz, H40 ), 8.18 (s, 1H, JPtH = 59.2 Hz, H4), 7.57 (m, 1H, H50 ), 3.45 (m, 2H, CH2 (H8)), 3.28 (s, 6H, JPtH = 20.3 Hz, CH3 (DMSO)), 3.03 (t, 1H, JHH = 5.4 Hz, H5), 2.82 (dt, 1H, H9 exo), 2.52 (m, 1H, H7), 1.48 (s, 3H, CH3 exo), 1.33 (d, 1H, JHH = 10.2 Hz, H9 endo), 0.74 (s, 3H, JPtH = 81 Hz, CH3-Pt), 0.70 (s, 3H, CH3 (endo)). FT-IR ((Nujol, νmax/cm1): 36003500 v br, 3271 m (NH), 3184 m (NH), 3123 m (NH), 1626 m, 1080 s br (BF4)). ΛM (5.104 M, acetone): 78 Ω cm2 mol1. Synthesis of [Pt(L-H)(DMSO)(Cl)], 7. To a solution of [Pt (L-H)(DMSO)(Me)] (100 mg, 0.186 mmol) in acetone (25 mL) were added under vigorous stirring 1 mL of DMSO and 1.8 mL of HCl (0.1 M, 0.186 mmol). After 4 h the solution was concentrated to small volume, and the complex was extracted with CH2Cl2, dried with Na2SO4, and concentrated to small volume. The residue was then treated with n-pentane to form a precipitate, which was filtered, washed with n-pentane, and vacuum-pumped to give the analytical sample as a yellow solid. Yield: 76%. Mp: 143147 °C. Anal. Calcd for C19H23ClN2OPtS: C, 40.90; H, 4.15; N, 5.02. Found: C, 41.24; H, 4.18; N, 5.03. 1H NMR (300 MHz; CDCl3; Me4Si): δ 9.53 (d, 1H, JHH = 5.5 Hz, JPtH = 38.8 Hz, H60 ), 8.17 (d, 1H, H30 ), 8.14 (s, 1H, JPtH = 42 Hz, H4), 7.90 (td, 1H, JHH = 7.6 Hz, JHH = 1.6 Hz, H40 ), 7.30 (m, 1H, H50 ), 3.64 (s, 6H, JPtH = 23.1 Hz, CH3 (DMSO)), 3.03 (m, 2H, CH2 (H8)), 2.84 (t, 1H, JHH = 5.6 Hz, H5), 2.66 (dt, 1H, JHH = 9.6 Hz, JHH = 5.6 Hz, H9 exo), 2.36 (m, 1H, H7), 1.39 (s, 3H, CH3 exo), 1.28 (d, 2H, JHH = 9.6 Hz, H9 endo), 0.70 (s, 3H, CH3 endo). Synthesis of [Pt(L-H)(PPh3)(Me)], 8. To a solution of [Pt(LH)(DMSO)Me] (95.7 mg, 0.178 mmol) in CH2Cl2 (25 mL) was added 46.7 mg of PPh3 (0.178 mmol). The stirred solution was kept under a nitrogen atmosphere. After 1 h the mixture was concentrated to small

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volume and treated with n-hexane to form a precipitate. The solid was filtered off, washed with n-hexane, and vacuum-pumped to give the analytical sample as a yellow solid. Yield: 98%. Mp: (dec) 210 °C. Anal. Calcd for C36H35N2PPt: C, 59.91; H, 4.89; N, 3.88. Found: C, 59.90; H, 4.48; N, 4.04. 1H NMR (300 MHz; CDCl3; Me4Si): δ 8.29 (d, 1H, JHH = 8.3 Hz), 7.787.68 (m, 9H), 7.407.38 (m, 9H), 6.59 (m, 1H), 3.06 (m br, 2H, CH2 (H8)), 2.80 (t, 1H, JHH = 5.4 Hz, H5), 2.63 (m, 1H, H9 exo), 2.36 (m, 1H, H7), 1.40 (s, 3H, CH3 (exo)), 1.28 (d, 2H, JHH = 9.5 Hz, H9 endo), 0.72 (6H, JPtH = 83.8 Hz, JHH = 7.5 Hz CH3endoþ CH3-Pt). 31P NMR (121.4 MHz; CDCl3, H4PO4): δ 33.66 (s, JPtP= 2212 Hz, PPh3). 13C NMR (75.42 MHz; CDCl3, 13C NMR and apt 13C NMR, ppm): δ 12.48 (d with sat, JPC = 4.7 Hz, JPtC = 726.5 Hz, CH3-Pt); 21.59 (CH3), 26.25 (CH3), 32.16 (CH2), 36.29 (CH2), 39.69 (Cq), 40.40 (CH), 47.14 (CH), 120.96 (s with sat, JPtC = 20.1 Hz, CH); 122.85 (s with sat, JPtC = 11.4 Hz, CH); 128.25 (d, JPC = 9.7 Hz, CH PPh3); 130.18 (d, JPC = 1.6 Hz, CH PPh3); 132.36 (d with sat, JPC = 43.3 Hz, JPtC = 16.8 Hz, Cq PPh3); 135.04 (d with sat, JPC = 12.1 Hz, JPtC = 16.4 Hz, CH PPh3); 137.20 (s with sat, JPtC = 83.6 Hz, CH); 137.27 (s, CH); 142.80 (s with sat, JPC = 5.4 Hz, JPtC = 52.7 Hz, Cq); 150.42 (d, JPC = 4.1 Hz, CH); 151.97 (s, Cq); 155.14 (d with sat, JPC = 119.5 Hz, JPtC = 964.5 Hz, C-Pt); 163.05 (s with sat, JPtC = 21.1 Hz, Cq); 163.25 (d with sat, JPC = 3.4 Hz, JPtC = 49.0 Hz, Cq). Synthesis of [Pt(L*)(PPh3)(Me)][BF4], 9. To a solution of [Pt(LH)(PPh3)(Me)] (85.5 mg, 0.118 mmol) in CH2Cl2 (25 mL) was added 48.3 mg of 18-crown-6 3 HBF4 3 H2O (0.130 mmol, 10% excess). The stirred solution was kept under a nitrogen atmosphere. After 2 h the mixture was concentrated to small volume and treated with Et2O to form a precipitate. The solid was filtered off, washed with Et2O, and vacuumpumped to give the analytical sample as a yellow solid. Yield: 80%. Mp: 150156 °C. Anal. Calcd for C36H36BF4N2PtS: C, 53.41; H, 4.48; N, 3.46. Found: C, 53.32; H, 4.09; N, 3.09. 1H NMR (300 MHz; CDCl3; Me4Si): δ 13.26 (s br, 1H, N-H) 8.53 (d, 1H, JHH = 7.9 Hz, H3 0 ), 8.40 (d, 1H, JPH = 5.3 Hz, JPtH = 50 Hz, H4), 8.04 (t, 1H, JHH = 7.5 Hz, H40 ), 7.82 (d, 1H, JHH = 5.4 Hz, JPtH = ca. 15 Hz H6 0 ), 7.787.64 (m, 6H), 7.527.38 (m, 9H), 6.85 (m, 1H), 3.40 (m, 2H, CH2 (H8)), 3.02 (t, 1H, JHH = 5.6 Hz, H5), 2.82 (dt, 1H, H9 exo), 2.50 (m, 1H, H7), 1.46 (s, 3H, CH3 exo), 1.32 (d, 1H, JHH = 10.3 Hz, H9 endo), 0.78 (d, 3H, JPH = 7.3 Hz, JPtH = 82.8 Hz, CH3-Pt), 0.72 (s, 3H, CH3 endo). 31P NMR (121.4 MHz; CDCl3, H4PO4): δ 32.32 (s, JPtP= 2469 Hz, PPh3). 13 C NMR (75.42 MHz; CDCl3, 13C NMR and apt 13C NMR, ppm): δ 11.68 (d with sat, JPC = 5.6 Hz, JPtC = 714.1 Hz, CH3-Pt); 21.41 (CH3), 25.75 (CH3), 31.71 (CH2), 31.95 (CH2), 39.09 (CH), 39.95 (Cq), 46.37 (CH), 121.37 (s with sat, JPtC = 16.3 Hz, CH); 126.36 (s with sat, JPtC = 8.5 Hz, CH); 128.80 (d, JPC = 10.2 Hz, CH PPh3),); 130.91 (d, JPC = 2.2 Hz, CH PPh3); 130.95 (d with sat, JPC = 46.5 Hz, JPtC = 20.1 Hz, Cq PPh3); 134.88 (d with sat, JPC = 11.5 Hz, JPtC = 18.8 Hz, CH PPh3); 139.46 (s, CH); 145.94 (s with sat, JPtC = 73.8 Hz, CH); 146.07 (d with sat, JPC = 5.6 Hz); 147.34 (s); 151.65 (d, JPC = 3.8 Hz, CH); 154.10 (d with sat, JPC = 1.7 Hz, JPtC = 23.5 Hz, Cq); 156.18 (d with sat, JPC = 4.3 Hz, JPtC = 44.9 Hz, Cq); 160.27 (d with sat, JPC = 120.9 Hz, JPtC = 990 Hz, CPt). FT-IR ((Nujol, νmax/ cm1): 36003500 v br, 3280 m (NH), 3195 m (NH), 3129 m (NH), 1627 m, 1095 s (PPh3), 1080 s (BF4)). ΛM (5.104 M, acetone): 62 Ω cm2 mol1. Synthesis of [Pt(L-H)(Cl)(PPh3)], 10. To a solution of [Pt (L-H)(Cl)(DMSO)] (50.0 mg, 0.089 mmol) in CH2Cl2 (25 mL) was added 28.2 mg of PPh3 (0.107 mmol). The stirred solution was kept under a nitrogen atmosphere. After 36 h the mixture was concentrated to small volume and treated with Et2O to form a precipitate. The solid was filtered off, washed with Et2O, and vacuum-pumped to give the analytical sample as a yellow solid. Yield: 70%. Mp: >250 °C. Anal. Calcd for C35H32ClN2PPt: C, 56.64; H, 4.35; N, 3.77. Found: C, 55.79; H, 3.97; N, 3.39. 1H NMR (300 MHz; CDCl3; Me4Si): δ 9.77 (m, 1H, 3072

dx.doi.org/10.1021/om200172h |Organometallics 2011, 30, 3064–3074

Organometallics

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Table 3. Crystallographic Data for Compound 1 formula

C17H18Cl2N2Pt

M

516.31

color

yellow

cryst syst

monoclinic

space group a/Å

P21 9.2967(6)

b/Å

12.5844(8)

c/Å

13.6271(8)

R/deg

90

β/deg

97.901(1)

γ/deg

90

U/Å3

1579.15(17)

Z F(000)

4 984

Dc/g cm3

2.172

T/K

150

λ (Mo KR)

0.71073

cryst dimens/mm

0.12  0.21  0.35

μ(Mo KR)/cm1

92.21

min. and max. transmn factors

0.6151.000

scan mode frame width/deg

ω 0.40

time per frame/s

10

no. of frames

4050

detectorsample distance/cm

6.00

θ-range/deg

328

reciprocal space explored

full sphere

no. of reflns (total; indep)

43 391, 8260

Rint final R2 and R2w indicesa (F2, all reflns)

0.0261 0.036, 0.047

conventional R1 index [I > 2σ(I)]

0.021

reflections with I > 2σ(I)

7935

no. of variables

397

goodness of fitb

0.979

R2 = [∑(|Fo2  kFc2|)/∑Fo2], R2w = [∑w(Fo2  kFc2)2/∑w(Fo2)2]1/2. [∑w(Fo2  kFc2)2/(No  Nv)]1/2, where w = 4Fo2/σ(Fo2)2, σ(Fo2) = [σ2(Fo2) þ (0.03Fo2)2]1/2, No is the number of observations, and Nv is the number of variables. a b

JPtH = 28 Hz, H6 0 ), 8.20 (d, 1H, H30 ), 7.90 (t, 1H, JHH = 7.5 Hz, H40 ), 7.827.75 (m, 5H), 7.447.37 (m, 11H), 6.28 (s, 1H, JPtH = 49 Hz, H4), 2.89 (m, 2H, CH2 (H8)), 2.40 (dt, 1H, JHH = 9.2 Hz, JHH = 5.6 Hz, H9 exo), 2.22 (m, 1H, H7), 1.85 (t, 1H, JHH = 5.6 Hz, H5),1.18 (s, 3H, CH3 exo), 1.05 (d, 1H, JHH = 9.5 Hz, H9 endo), 0.41 (s, 3H, CH3 endo). 31P NMR (121.4 MHz; CDCl3, H4PO4): δ 23.51 (s, JPtP= 4291 Hz, PPh3). X-ray Data Collection and Structure Determination. Crystal data are summarized in Table 3. The diffraction experiment was carried out on a Bruker APEX II CCD area-detector diffractometer at 150 K, using Mo KR radiation (λ = 0.71073) with a graphite crystal monochromator in the incident beam. No crystal decay was observed, so that no time-decay correction was needed. The collected frames were processed with the software SAINT,49 and an empirical absorption correction was applied (SADABS)50 to the collected reflections. The calculations were performed using the Personal Structure Determination package51 and the physical constants tabulated therein.52 The structure was solved by direct methods (SHELXS)53 and refined by full-matrix least-squares using all reflections and minimizing the function

∑w(Fo2  kFc2)2 (refinement on F2). There are two independent molecules in the asymmetrici unit, very similar to each other (see description of the structure and discussion). All the non-hydrogen atoms were refined with anisotropic thermal factors. The hydrogen atoms were placed in their ideal positions (CH = 0.97 Å), with the thermal parameter U being 1.10 times that of the carbon atom to which they are attached, and not refined. As the space group (P21) is chiral, full refinements of both the enantiomorphs were carried out. The refinement of the correct enantiomorph led to R2 = 0.036 and R2w = 0.047; the refinement of the wrong one led to R2 = 0.074 and R2w = 0.122. In the final Fourier map the maximum residual was 1.97(21) e Å3 at 1.01 Å from Pt. The minimum peak (hole) was 2.10 (21) e Å3. CCDC810593 contains the supplementary crystallographic data for this compound.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: (A.Z.) [email protected], (M.M.) mario.manassero@ fastwebnet.it.

’ ACKNOWLEDGMENT Financial support from Universit
a di Sassari (FAR) and Ministero dell’Universit
a e della Ricerca Scientifica (MIUR, PRIN 2007) is gratefully acknowledged. We thank Johnson Matthey for a generous loan of platinum salts. ’ REFERENCES (1) Redijk, J. In Comprehensive Coordination Chemistry; Wilkinson, G.; Gillard, R. D.; McCleverty, J. A., Eds.; Pergamon: Oxford, 1987; Vol. 2, p 73. (2) See for example: (a) Gianini, M.; Forster, A.; Haag, P.; von Zelewsky, A.; Stoeckli-Evans, H. Inorg. Chem. 1996, 35, 4889–4895. (b) Biagini, M. C.; Ferrari, M.; Lanfranchi, M.; Marchi
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