Heteroleptic Chini-type Platinum Clusters by the ... - ACS Publications

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Synthesis of [Pt12(CO)20(dppm)2]2− and [Pt18(CO)30(dppm)3]2− Heteroleptic Chini-type Platinum Clusters by the Oxidative Oligomerization of [Pt6(CO)12(dppm)]2− Beatrice Berti, Cristiana Cesari, Francesco Conte, Iacopo Ciabatti, Cristina Femoni, Maria Carmela Iapalucci, Federico Vacca, and Stefano Zacchini* Dipartimento di Chimica Industriale “Toso Montanari”, Università di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy S Supporting Information *

ABSTRACT: The reactions of [Pt6(CO)12]2− with CH(PPh2)2 (dppm), CH2C(PPh2)2 (P^P), and Fe(C5H4PPh2)2 (dppf) proceed via nonredox substitution and result in the heteroleptic Chini-type clusters [Pt6(CO)10(dppm)]2−, [Pt6(CO)10(P^P)]2−, and [Pt6(CO)10(dppf)]2−, respectively. Conversely, the reactions of [Pt6(CO)12]2− with Ph2P(CH2)4PPh2 (dppb) and Ph2PCCPPh2 (dppa) can be described as redox fragmentation that afford the neutral complexes Pt(dppb)2, Pt2(CO)2(dppa)3, and Pt8(CO)6(PPh2)2(CCPPh2)2(dppa)2. The oxidation of [Pt6(CO)10(dppm)]2− results in its oligomerization to yield the larger heteroleptic Chini-type clusters [Pt12(CO)20(dppm)2]2−, [Pt18(CO)30(dppm)3]2−, and [Pt24(CO)40(dppm)4]2− (for the latter there is only IR spectroscopic evidence). All the clusters were characterized by means of IR and 31P NMR spectroscopies and electrospray ionization mass spectrometry. Moreover, the crystal structures of [NEt4]2[Pt6(CO)10(dppm)]·CH3CN, [NEt4]2[Pt12(CO)20(dppm)2]·2CH3CN· 2dmf, [NEt 4 ] 2 [Pt 12 (CO) 20 (dppm) 2 ]·4dmf, [NEt 4 ] 2 [Pt 6 (CO) 10 (dppf)]·2CH 3 CN, Pt 2 (CO) 2 (dppa) 3 ·0.5CH 3 CN, Pt8(CO)6(PPh2)2(CCPPh2)2(dppa)2·2thf, and Pt(dppb)2 were determined by single-crystal diffraction (dmf = dimethylformamide; thf = tetrahydrofuran).

1. INTRODUCTION CO substitution is a general strategy to functionalize metal carbonyl clusters.1−10 Nonetheless, a competition among CO substitution with retention of the cluster structure, cluster rearrangement, and/or cluster breakdown is generally observed.1−10 This point has been well-exemplified by the studies recently performed by our group on the reactions of [Pt3n(CO)6n]2− (n = 2−5) Chini clusters with phosphine ligands.11−14 These homoleptic Chini clusters are composed of stacks of [Pt3(μ-CO)3(CO)3] triangular units arranged in trigonal prismatic structures.15,16 Their redox chemistry has been extensively investigated,17−19 and their employment as conductive materials,20−22 catalysts,23−25 precursors of platinum nanoclusters and bimetallic clusters,26−31 and platinum nanoparticles and nanowires32−38 has been studied. A competition between the nonredox substitution with retention of the nuclearity of the cluster and the redox fragmentation is observed in the case of the reactions of homoleptic Chini clusters with monodenate and bidentate phosphines (Figure 1).11−14 The nonredox substitution results in [Pt3n(CO)6n−x(L)x]2− (n = 1−5; x = 1 − n) heteroleptic analogues of anionic Chini clusters. Conversely, redox fragmentation (elimination) reactions lead to lower nuclearity homoleptic species [Pt3(n−1)(CO)6(n−1)]2− as well as widely varied neutral complexes Ptx(CO)y(L)z. The outcome of these reactions depends on (a) the nuclearity of the cluster, (b) the nature of the ligand, and (c) the stoichiometry of the reaction. © XXXX American Chemical Society

(a) The nonredox substitution is favored by lower nuclearity clusters, whereas larger clusters usually prefer redox fragmentation. This may be explained on the basis of the fact that the intertriangular bonding of Chini clusters is mainly based on the negative charge of the clusters. Since the charge (2−) is constant, the intertriangular Pt−Pt bonds become weaker as the nuclearity of the cluster increases.15−19 Thus, the nonredox substitution is favored over redox fragmentation in the order [Pt6(CO)12]2− > [Pt9(CO)18]2− > [Pt12(CO)24]2− > [Pt15(CO)30]2−. (b) Monodentate (PPh3 and 1,3,5-triaza-7-phosphaadamantane (PTA))12,14 and flexible bidentate phosphines (Ph2PCH2CH2PPh2 (dppe); R-Ph2PCH(Me)CH2PPh2 (R-dppp))11 favor the nonredox substitution, whereas more rigid bidentate ligands (CH2C(PPh2)2 (P^P); CH2(PPh2)2 (dppm); o-C6H4(PPh2)2 (dppBz)) often result in redox fragmentation affording neutral Pt complexes.13 (c) The nonredox substitution is favored by the use of stoichiometric amounts of the ligands, whereas an excess of phosphine leads to redox fragmentation. Thus, by employing stoichiometric amounts of PPh3, PTA, dppe, and R-dppp, heteroleptic anionic Chini-type clusters can Received: February 19, 2018

A

DOI: 10.1021/acs.inorgchem.8b00447 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Some examples of the reactions of the [Pt12(CO)24]2− homoleptic anionic Chini cluster with phosphines: (a) nonredox substitution and (b) redox fragmentation.

Figure 2. (a) Molecular structure of [Pt6(CO)10(dppm)]2− and (b) its Pt−P core (Pt, purple; P, orange; C, gray, O, red). H atoms were omitted for clarity.

cases retained with local deformations; in some cases an inversion of the cage from trigonal prismatic to octahedral occurs, or eventually the reciprocal rotation of two trigonal prismatic units with the loss of a Pt−Pt contact is observed. For the purposes of this paper, it is remarkable that the structure of [Pt12(CO)20(dppe)2]2− may be viewed as a dimer composed of two monoanionic [Pt6(CO)10(dppe)]− units joined by two Pt−Pt interactions (Figure 1). 11 This resemblance is not only formal, since [Pt12(CO)20(dppe)2]2− can be obtained by the oxidation of [Pt6(CO)10(dppe)]2−. Herein, we report a detailed study of the reactions of [Pt6(CO)12]2− with widely varied bidentate phosphines, that is, dppm, P^P, Fe(C5H4PPh2)2 (dppf), Ph2P(CH2)4PPh2 (dppb), and Ph2PCCPPh2 (dppa). Some of these ligands (dppm, P^P) have been previously employed in the reactions with

be obtained through nonredox substitution. Conversely, when used in excess, redox fragmentation occurs affording mixtures of lower nuclearity anionic clusters and zerovalent species such as Pt3 (CO) 3 (PPh 3 ), Pt 6 (CO) 6 (dppe) 3 , Pt 4 (CO) 4 (dppe) 2 , and Pt(dppe)2.11−14 The heteroleptic Chini clusters so far structurally characterized are [Pt6(CO)10(PPh3)2]2−, [Pt6(CO)10(dppe)]2−, [Pt9(CO)16(PPh3)2]2−, [Pt9(CO)16(dppe)]2−, [Pt9(CO)16(Rdppp)]2−, [Pt12(CO)22(PPh3)2]2−, [Pt12(CO)20(PTA)4]2−, [Pt12(CO)20(dppe)2]2−, and [Pt15(CO)30(PTA)5]2−.11−14 Several other species have been spectroscopically identified but not structurally characterized. In all these clusters, the triangular Pt3 units present in the parent homoleptic species are preserved, whereas their overall trigonal prismatic structures are in some B


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Inorganic Chemistry Table 1. Main Bond Distances of [Pt6(CO)10(dppm)]2− and [Pt6(CO)10(dppf)]2− Compared to [Pt6(CO)12]2−, [Pt6(CO)10(dppe)]2−, and [Pt6(CO)10(PPh3)2]2− Pt−Pt intratriangular [Pt6(CO)10(dppm)] [Pt6(CO)10(dppf)]

2−

2−

[Pt6(CO)10(dppe)]2− b [Pt6(CO)10(dppe)]2− c [Pt6(CO)10(PPh3)2]2− d [Pt6(CO)10(PPh3)2]2− e [Pt6(CO)12]2− f

2.6336(12)−2.6634(10) average 2.646(3) 2.6533(4)−2.6703(4) average 2.6620(10) 2.6513(3)−2.6839(3) average 2.6625(9) 2.6573(8)−2.6758(8) average 2.6644(14) 2.6558(6)−2.6743(6) average 2.6644(10) 2.6584(6)−2.6787(6) average 2.6694(10) 2.644(7)−2.659(3) average 2.653(10)

Pt−Pt intertriangulara

Pt−P

structure

2.9715(14)−3.0202(14) average 2.991(2)g 2.9659(4)−3.3550(4) average 3.0962(7) h 2.9839(4)−3.1797(3) average 3.1285(8) I 3.0437(10)−3.359(2) average 3.202(2) j 3.0353(6) k

2.228(2)−2.238(2) average 2.233(3) 2.2525(17)−2.2531(17) average 2.253(2) 2.2456(13)

trigonal prismatic

2.251(4)

distorted octahedral

2.240(3)


3.1380(6)−3.2079(6) average 3.1729(8) l 3.026(16)−3.049(17) average 3.03(3)

2.247(3)


distorted trigonal prismatic distorted octahedral

trigonal prismatic

a

Only Pt−Pt interactions less than or equal to 3.36 Å were included. bAs found in [NMe4]2[Pt6(CO)10(dppe)]. From ref 11. cAs found in [NEt4]2[Pt6(CO)10(dppe)]. From ref 11. dAs found in [NBu4]2[Pt6(CO)10(PPh3)2]. From ref 12. eAs found in [NBu4]2[Pt6(CO)10(PPh3)2]·2thf. From ref 12. fFrom ref 15. gThe shortest diagonal intertriangular Pt−Pt contacts are 3.628(5) and 3.411(2) Å. hThe shortest diagonal intertriangular Pt−Pt contacts are 3.625 (3), 3.722(3), and 3.762(3) Å. IFour intertriangular Pt−Pt bonding contacts are present. The remaining two Pt−Pt contacts are 3.526(2) Å. jSix intertriangular Pt−Pt bonding contacts are present. kThree sets of intertriangular Pt−Pt contacts are present: 3.0353(6), 3.3581(6), 3.5070(6) Å. lThree sets of intertriangular Pt−Pt contacts are present: 3.1380(6), 3.2079(6), 3.4310(6) Å.

Conversely, in the case of the more flexible dppe and the monodentate PPh3 ligands, the [Pt6(CO)10(dppe)]2− and [Pt6(CO)10(PPh3)2]2− preferred to adopt distorted octahedral structures, probably to release steric repulsion. Moreover, it must be noticed that both [Pt 6 (CO) 10 (dppe)] 2− and [Pt6(CO)10(PPh3)2]2− were crystallized into two different salts, showing some slight differences in their octahedral cores due to different packing effects. This points out that the intertriangular bonds in Chini clusters are rather weak and easily deformed by small changes in the van der Waals forces within the crystals or by variations of the properties of the ancillary ligands. Crystals of [NEt4]2[Pt6(CO)10(dppm)]·CH3CN displayed νCO bands in the IR spectrum recorded as mineral oil mull at 2000(s), 1980(s), 1949(s), 1778(m), 1756(m), and 1750(m) cm−1, in accord to the solution spectrum. Their ESI-MS spectrum in CH3CN solution (Figure S.2a) showed a main peak in the negative mode at m/z 917, corresponding to the molecular ion [Pt6(CO)10(dppm)]2−. Interestingly, a minor peak was observed at m/z 1833, which was assigned to [Pt12(CO)20(dppm)2]2−. This was likely to be formed by the oxidation of [Pt6(CO)10(dppm)]2− operated by air (see next section for details). This was confirmed by letting some air enter into the sample and repeating the analysis. As a consequence, the peak at m/z 1833 considerably increased (Figure S.2b). In keeping with the solid-state structure, the 31P{1H} NMR spectrum of [Pt6(CO)10(dppm)]2− recorded in CD3CN at 298 K displayed a resonance centered at δP 51.0 ppm, in view of the equivalence of the two P atoms of the unique dppm ligand (Figure 3 and Table 2). This resonance showed a large 1JPtP coupling to one Pt atom (4960 Hz) as well as a second-order 2 JPP (88 Hz).39−45 In addition, two very different 2JPtP coupling constants were observed; the larger one (602 Hz) corresponded to two equivalent Pt atoms, and the smaller one (15 Hz) corresponded to a single Pt atom. On the basis of the crystal structure, the larger 2JPtP values correspond to coupling to the two Pt atoms of the triangle to which the P atom is directly bonded, whereas the smaller 2JPtP values correspond to intertriangle coupling to the Pt atom bonded to the other P of

higher nuclearity Chini clusters and resulted in redox fragmentation. 13 Thus, we focused our attention on [Pt6(CO)12]2−, which is more prone to nonredox substitution. Indeed, the heteroleptic clusters [Pt6(CO)10(dppm)]2−, [Pt6(CO)10(dppf)]2−, and [Pt6(CO)10(P^P)]2− were obtained and will be described in the following sections. Then, the oxidative oligomerization of [Pt6(CO)10(dppm)]2− affording [Pt12(CO)20(dppm)2]2− and [Pt18(CO)30(dppm)3]2− (as well as IR spectroscopic evidence of [Pt24(CO)40(dppm)4]2−) will be presented.

2. RESULTS AND DISCUSSION 2.1. Synthesis and Structure of [Pt6(CO)10(dppm)]2−. The reaction of [Pt6(CO)12]2− (νCO 2000(vs), 1800(s) cm−1) with a slight excess of dppm in CH3CN afforded the new dianionic cluster [Pt 6 (CO) 10 (dppm)] 2− (ν CO 2006(s), 1980(vs), 1780(s), 1770(s) cm−1) in high yields (eq 1). The reaction was monitored by IR spectroscopy (Figure S.1), and the reaction was completed after ca. 2 h. The IR spectrum of [Pt6(CO)10(dppm)]2− in the νCO region was very similar to t h a t p r e v i o u s l y r e p o r t e d fo r t h e re l a t e d a n i o n [Pt6(CO)10(dppe)]2−.11 [Pt6(CO)12 ]2 − + dppm → [Pt6(CO)10 (dppm)]2 − + 2CO (1) 2−

The new cluster [Pt6(CO)10(dppm)] was characterized by means of electrospray ionization mass spectrometry (ESI-MS) and IR and 31P NMR spectroscopies, and its molecular structure was determined via single-crystal X-ray diffractometry as the [NEt4]2[Pt6(CO)10(dppm)]·CH3CN salt (Figure 2 and Table 1). The structure of the heteroleptic anion [Pt6(CO)10(dppm)]2− can be formally derived from that of [Pt6(CO)12]2−, after replacing two terminal CO ligands, one per Pt3-triangular unit, with dppm. The Pt6-cage of the cluster retained the trigonal prismatic structure of the parent homoleptic Chini cluster,1,2 whereas both [Pt6(CO)10(dppe)]2− and [Pt6(CO)10(PPh3)2]2− displayed distorted octahedral structures.11,12 Probably, in the case of [Pt6(CO)10(dppm)]2−, the presence of a single −CH2− group in the dppm ligand hampered the rotation of the Pt3-triangle. C


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Inorganic Chemistry

PPh3. Conversely, redox fragmentation was observed, with concomitant formation of lower-nuclearity anionic Chini clusters and neutral species such as Pt6(CO)6(dppm)3. For instance, the reaction of [Pt12(CO)24]2− with 2−3 equiv of dppm afforded [Pt9(CO)18]2− and Pt6(CO)6(dppm) 3.13 Similarly, the reaction of [Pt9(CO)18]2− with a slight excess of dppm resulted in a mixture of [Pt 6 (CO) 12 ] 2− , [Pt6(CO)10(dppm)]2−, and Pt6(CO)6(dppm)3. Thus, the heteroleptic clusters [Pt3n(CO)6n−2(dppm)]2− (n = 3−5) could not be directly obtained by substitution, but they must be synthesized by means of redox condensations. For instance, [Pt9(CO)16(dppm)]2− was conveniently obtained by reacting [Pt12(CO)20(dppm)2]2− (see below for its synthesis) and [Pt6(CO)12]2− in a 1:1 ratio in accord to reaction (2). Conversely, the redox condensation of equimolar amounts of [Pt6(CO)10(dppm)]2− and [Pt12(CO)24]2− resulted in a 1:1 mixture of [Pt9(CO)16(dppm)]2− and [Pt9(CO)18]2−, as depicted in reaction (3). The nature of [Pt9(CO)16(dppm)]2− was established on the basis of its IR and 31P NMR spectra (see experimental and Figure S.3), which were very similar to those previously reported for the related [Pt9(CO)16(dppe)]2− anion. [Pt12(CO)20 (dppm)2 ]2 − + [Pt6(CO)12 ]2 − → 2[Pt 9(CO)16 (dppm)]2 −

(2)

[Pt6(CO)10 (dppm)]2 − + [Pt12(CO)24 ]2 − → [Pt 9(CO)16 (dppm)]2 − + [Pt 9(CO)18 ]2 −

Figure 3. 31P{1H} NMR spectrum of [Pt6(CO)10(dppm)]2− in CD3CN at 298 K: (a) experimental; (b) simulated. δ (ppm): 51.0 (1JPtP = 4960 Hz (1Pt), 2JPtP = 602 Hz (2Pt), and 15 Hz (1Pt), 2JPP = 88 Hz).

(3)

2.2. Oxidation of [Pt6(CO)10(dppm)]2−: Synthesis of [Pt12(CO)20(dppm)2]2− and [Pt18(CO)30(dppm)3]2−. As reported in the previous section, [Pt6(CO)10(dppm)]2− was readily oxidized by air resulting in the formation of [Pt12(CO)20(dppm)2]2−. Alternatively, the same reaction was performed by employing HBF4·Et2O as an oxidant, in accord to reaction (4). The reaction was reversed by adding [NBu4][OH] under CO to a solution of [Pt12(CO)20(dppm)2]2− in dimethylformamide (dmf), in agreement with eq 5.

the same dppm ligand. The assignment of the 31P{1H} NMR spectrum was fully corroborated by simulation with gNMR 5.0.6.0.46 A similar coupling pattern was observed in the case of [Pt6(CO)10(dppe)]2−. The most significant difference was the fact that, because of the presence of two CH2 groups in the backbone of dppe, the 3 J P P coupling constant of [Pt6(CO)10(dppe)]2− was too small to be resolved. This suggested that the P−P coupling observed in the case of [Pt6(CO)10(dppm)]2− (2JPP = 88 Hz) occurred through the P− CH2−P backbone of the ligand and not the metal skeleton of the cluster. The reaction of higher-nuclearity Chini clusters [Pt3n(CO)6n]2− (n = 3−5) with dppm did not afford the expected isonuclear heteroleptic anionic clusters, as previously observed in the case of analogous reactions with dppe and

2[Pt6(CO)10 (dppm)]2 − + 2H+ → [Pt12(CO)20 (dppm)2 ]2 − + H 2

(4)

[Pt12(CO)20 (dppm)2 ]2 − + 2OH− + CO → 2[Pt6(CO)10 (dppm)]2 − + CO2 + H 2O

(5)

It is well-known that Chini clusters are oxidized by strong acids by means of the H+/H2 redox couple and reduced by CO

Table 2. 31P NMR Data of [Pt6(CO)10(dppm)]2−, [Pt12(CO)20(dppm)2]2−, [Pt18(CO)30(dppm)3]2−, [Pt9(CO)16(dppm)]2−, [Pt6(CO)10(P^P)]2−, [Pt6(CO)10(dppf)]2− δP 2−

[Pt6(CO)10(dppm)] [Pt12(CO)20(dppm)2]2− [Pt18(CO)30(dppm)3]2−

[Pt9(CO)16(dppm)]2− [Pt6(CO)10(P^P)]2− [Pt6(CO)10(dppf)]2−

51.0 46.2 38.7 46.1 43.8 36.2 47.3 39.0 62.2 49.3

1

2

JPtP

4960 4962 5144 5105 4952 5095 4941 5047 5059 5247

JPtP (intratriangle) 602 546 626 548 540 601 562 697 453 543

D

2

JPtP (intertriangle) 15

15

17

2

JPP

88 83 83 70 78 78 85 85 120


Article

Inorganic Chemistry Scheme 1. Proposed Mechanism for the Oxidation of [Pt6(CO)10(dppm)]2− to [Pt12(CO)20(dppm)2]2−

Figure 4. Molecular structure of (a) [Pt12(CO)20(dppm)2]2− and (b) its Pt−P core. (Pt, purple; P, orange; C, gray, O, red). H atoms were omitted for clarity.

Table 3. Main Bond Distances of [Pt12(CO)20(dppm)2]2− Compared to [Pt12(CO)24]2−, [Pt12(CO)20(dppe)2]2−, and [Pt12(CO)20(PTA)4]2− 2− b

[Pt12(CO)20(dppm)2] [Pt12(CO)20(dppm)2]2− c [Pt12(CO)20(PTA)4]2− d [Pt12(CO)20(dppe)2]2− e [Pt12(CO)20(dppe)2]2− f [Pt12(CO)22(PPh3)2]2− g [Pt12(CO)24]2− h

Pt−Pt intratriangular

Pt−Pt intertriangulara

Pt−P



2.241(3)−2.263(3) average 2.252(4) 2.239(16)−2.262(14) average 2.25(2) 2.222(9)−2.276(9) average 2.24(2) 2.254(4)−2.272(4) average 2.263(6) 2.251(2)−2.270(2) average 2.260(3) 2.281(4)

a Only Pt−Pt interactions less than or equal to 3.36 Å were included. bAs found in [NEt4]2[Pt12(CO)20(dppm)2]·2CH3CN·2dmf. cAs found in [NEt4]2[Pt12(CO)20(dppm)2]·4dmf. dFrom ref 14. eAs found in [NMe3(CH2Ph)]2[Pt12(CO)20(dppe)2]·CH3CN. From ref 11. fAs found in [NMe4]2[Pt12(CO)20(dppe)2]·2CH3CN. From ref 11. gFrom ref 12. hFrom ref 15.

in the presence of a base.15−19 Nonetheless, the direct oxidation of [Pt 6 (CO) 1 2 ] 2 − afforded [Pt 9 (CO) 1 8 ] 2 − and not [Pt12(CO)24]2−. In addition, we previously observed that, under the same experimental conditions, the oxidation of [Pt6(CO)10(dppe)]2− afforded [Pt12(CO)20(dppe)2]2−.11 Thus, it is likely that, in the presence of a bidentate ligand, the oxidation of [Pt6(CO)10(LL)]2− (LL = dppm, dppe) proceeds through a [Pt6(CO)10(LL)]− radical monoanion, which immediately dimerizes resulting in [Pt 12(CO) 20 (LL)2 ]2− (Scheme 1). The new compound [Pt12(CO)20(dppm)2]2− was fully characterized through IR (Figure S.4), ESI-MS (Figure S.5), and 31P NMR spectroscopy (Figure S.6). Its molecular

structure was determined through single-crystal X-ray crystallography as the [NEt4]2[Pt12(CO)20(dppm)2]·2CH3CN·2dmf and [NEt4]2[Pt12(CO)20(dppm)2]·4dmf solvated salts, which contained the same cluster anion with very similar geometries and bonding parameters (Figure 4 and Table 3). The structure and spectroscopic data of [Pt12(CO)20(dppm)2]2− are very similar to those previously reported for [Pt12(CO)20(dppe)2]2−.11 The cluster may be viewed as composed of two trigonal prismatic [Pt6(CO)10(dppm)] units rotated by 180° and joined by the two symmetry-equivalent Pt−Pt bonds [2.9798(6) and 2.986(5) Å, for the two salts, respectively]. As a consequence, there is one μ-CO ligand per [Pt6(CO)10(dppm)] unit that weakly interacts with one Pt E


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Inorganic Chemistry

Scheme 2. Oxidation of [Pt6(CO)10(dppm)]2− Resulting in [Pt12(CO)20(dppm)2]2− and [Pt18(CO)30(dppm)3]2− (ox = H+ and/ or air; red = CO/OH−)

Figure 5. Molecular structure of (a) [Pt6(CO)10(dppf)]2− and (b) its Pt−P core. (Pt, purple; P, orange; C, gray, O, red; Fe, blue). H atoms were omitted for clarity.

is reversibly reduced to [Pt12(CO)20(dppm)2]2− by reaction with [NBu4][OH] under CO atmosphere in dmf. The nature of [Pt18(CO)30(dppm)3]2− was fully corroborated through ESIMS (Figure S.8) and 31P NMR spectroscopy (Figures S.9 and S.10). Unfortunately, all the attempts to obtain single crystals of [Pt18(CO)30(dppm)3]2− suitable for X-ray crystallography failed. In particular, the IR spectrum of [Pt18(CO)30(dppm)3]2− displayed νCO bands at 2032(vs), 1871(m), 1856(s), 1843(m), and 1821(m) cm−1, considerably moved toward higher wavenumbers compared to the parent [Pt12(CO)20(dppm)2]2−. Its nature was further corroborated by the presence of a main peak in the ESI-MS spectrum (negative mode) at m/z 2752 attributable to the [Pt18(CO)30(dppm)3]2− molecular ion. Moreover, the 31P{1H} NMR spectrum recorded in dmf-d7 at 298 K displayed three resonances attributable to the three nonequivalent types of P atoms present within the structure, each type consisting of two P atoms. The resulting spectrum was rather complex (Figures S.9 and S.10), in view of the presence of different isotopomers, second-order effects, and the rich P−Pt and P−P coupling patterns, and its quality was not very high due to the reduced solubility of this cluster compared

atom of the second [Pt6(CO)10(dppm)] unit [Pt···C(O) 3.226(12) and 3.285(18) Å, for the two salts, respectively]. [Pt12(CO)20(dppm)2]2− displayed νCO bands in the IR spectrum recorded as mineral oil mull at 2013(vs), 1856(m), 1819(s), and 1810(s) cm−1 and at 2020(vs), 1851(m), 1829(s), and 1808(m) cm−1 in CH3CN solution. The ESI-MS spectrum of the same solution displayed, in the negative mode, the expected peak of the molecular ion at m/z 1834. The 31P NMR spectrum of [Pt12(CO)20(dppm)2]2− (Figure S.6) was fully consistent with its solid-state structure and displayed the presence of two nonequivalent P atoms in a 1:1 ratio. Thus, it displayed two resonances at δP 46.2 and 38.7 ppm. Both resonances showed 1JPtP coupling to one Pt atom (4962 and 5144 Hz, respectively), a 2JPtP coupling constant to two Pt atoms (546 and 626 Hz, respectively), and a smaller 2JPP (83 Hz). The reaction of [Pt12(CO)20(dppm)2]2− in dmf with a slight excess of HBF4·Et2O in the presence of air afforded [Pt18(CO)30(dppm)3]2− (Scheme 2) as indicated by the shift of the νCO bands in the IR spectrum toward higher wavenumbers (Figure S.7). In turn, [Pt18(CO)30(dppm)3]2− F


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Inorganic Chemistry

Figure 6. Comparison of the Pt6 metal cores of (a) [Pt6(CO)12]2−, (b) [Pt6(CO)10(dppm)]2−, (c) [Pt6(CO)10(dppf)]2−, (d, e) [Pt6(CO)10(dppe)]2− (as found in [NMe4]2[Pt6(CO)10(dppe)] and [NEt4]2[Pt6(CO)10(dppe)]), and (f, g) [Pt6(CO)10(PPh3)2]2− (as found in [NBu4]2[Pt6(CO)10(PPh3)2] and [NBu4]2[Pt6(CO)10(PPh3)2]·2thf). Only Pt−Pt contacts less than or equal to 3.36 Å were drawn as bonds.

to [Pt12(CO)20(dppm)2]2− and [Pt6(CO)10(dppm)]2−. The full assignment and interpretation of the spectrum was possible only by simulation with gNMR 5.0.6.0.46 It must be remarked that, by reacting [Pt18(CO)30(dppm)3]2− in air with an excess of HBF4·Et2O, there was evidence by IR spectroscopy of a new species displaying νCO bands at 2045(vs), 1859(s), and 1826(m) cm−1. The instability and low solubility of this species hampered its isolation as well as its characterization by ESI-MS and/or 31P NMR spectroscopy. Nonetheless, on the basis of its IR spectrum in the νCO region (Figure S.7), we can tentatively formulate it as [Pt24(CO)40(dppm)4]2−. These observations indicated the possibility that heteroleptic Chini clusters containing bidentate ligands may grow after oxidation by the sequential addition of {Pt6(CO)10(LL)} units resulting in [{Pt6(CO)10(LL)}x]2− (x = 1,2,3,4) molecular clusters. This should be contrasted with the “normal” growing mode of homoleptic Chini clusters, which involve the addition of {Pt3(CO)6} triangular units.15−19 2.3. Reactivity of [Pt6(CO)12]2− with Other Bidentate Phosphine Ligands. The reactions of [Pt6(CO)12]2− with P^P and dppf were very similar to that described in Section 2.1 with dppm, resulting in the formation of [Pt6(CO)10(P^P)]2− and [Pt6(CO)10(dppf)]2−, respectively. These compounds were characterized through IR and 31P NMR spectroscopies (Figures S.11 and S.12), and the molecular structure of [Pt6(CO)10(dppf)]2− was determined by single-crystal X-ray diffraction (Figure 5 and Table 1). The spectroscopic data of [Pt6(CO)10(P^P)]2− and [Pt6(CO)10(dppf)]2− were consistent

with the proposed structures and comparable to those previously reported for the closely related [Pt 6 (CO) 10 (PPh 3 ) 2 ] 2− , 12 [Pt 6 (CO) 10 (dppe)] 2− , 11 and [Pt6(CO)10(dppm)]2−. In particular, all these [Pt6(CO)10(LL)]2− clusters displayed two main IR absorptions in the νCO region, one attributable to the terminal carbonyls (LL = P^P 1979 cm−1, dppm 1979 cm−1,dppe 1976 cm−1, PPh3 1973 cm−1, dppf 1993 cm−1) and the other to the μ-CO ligands (LL = P^P 1774 cm−1, dppm 1779 cm−1, dppe 1765 cm−1, PPh3 1756 cm−1, dppf 1782 cm−1). It must be remarked that the wavenumbers of all these νCO bands were very similar, except the dppf derivative, which showed slightly higher wavenumbers, probably because of the larger electron-withdrawing character of the ferrocene backbone of the ligand. In agreement with the equivalence of the two P atoms in the [Pt6(CO)10(LL)]2− clusters, their 31P NMR spectra showed a single multiplet with the typical coupling pattern of a single P atom bonded to a Pt3 triangle: a larger 1JPtP coupling constant (4960−5301 Hz) to a single Pt, and a smaller 2JPtP coupling constant (453−602 Hz) to two equivalent Pt atoms. Moreover, in the case of [Pt6(CO)10(P^P)]2−, [Pt6(CO)10(dppm)]2−, and [Pt6(CO)10(dppe)]2− a second and even smaller 2JPtP coupling constant (17, 15, and 21 Hz, respectively) to one Pt was observed. This is an intertriangular coupling made possible by the bidentate nature of the P^P, dppm, and dppe ligands, which keeps the two P-bonded Pt atoms in close proximity and in a relative cis position [(P)Pt−Pt(P) 2.9715(14) Å for dppm, 3.0437(10) Å for dppe; the crystal structure of [Pt6(CO)10(P^P)]2− was not determined]. Conversely, no G


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Figure 7. Molecular structure of (a) Pt(dppb)2 and (b) a simplified view. (Pt, purple; P, orange; C, gray). H atoms were omitted for clarity.

intertriangular 2JPtP coupling was observed in the case of monodentate ligands, that is, [Pt6(CO)10(PPh3)]2− (which adopted a trans geometry of the PPh3 ligands), and [Pt6(CO)10(dppf)]2− [(P)Pt−Pt(P) 3.3550(4) Å]. In addition, a 2JPP coupling constant was present in the spectra of [Pt6(CO)10(P^P)]2− and [Pt6(CO)10(dppm)]2− (120 and 88 Hz, respectively) but not in the other clusters. As discussed in Section 2.1, this coupling was likely to occur through the backbone of the bidentate phosphine. Thus, it was larger in the case of P^P, whose backbone contained a single sp2 carbon, smaller in the case of dppm (a single sp3 carbon in the backbone) and absent in the case of dppe (two sp3 carbons in the backbone), as well as diphosphines with longer backbones. From a structural point of view, the metal core of [Pt6(CO)10(dppf)]2− can be described as a distorted Pt6 trigonal prism, which is intermediate between the almost regular trigonal prismatic structures found in [Pt6(CO)12]2− and [Pt6(CO)10(dppm)]2− and the distorted octahedral structures displayed by [Pt 6 (CO) 1 0 (dppe)] 2 − and [Pt6(CO)10(PPh3)2]2− (Figure 6). This observation further points out that the intertriangular bonds in Chini clusters are easily deformable and adapt themselves to the ancillary ligands or even to packing effects. The reactions of dppa and dppb with [Pt6(CO)12]2− were rather different from those reported for dppm, P^P, and dppf (as well as dppe, PPh3, and PTA, described in previous papers),11−14 resulting in the formation of Pt2(CO)2(dppa)3 and Pt(dppb)2, respectively. As previously noticed, a competition between the nonredox substitution with retention of the nuclearity and the redox fragmentation was observed in the case of the reactions of [Pt3n(CO)6n]2− Chini clusters with phosphine ligands.11−14 The nonredox substitution resulted in heteroleptic analogues of anionic Chini clusters, whereas redox fragmentation (elimination) reactions afforded lower-nuclearity homoleptic species [Pt3(n−1)(CO)6(n−1)]2− as well as widely varied neutral complexes. In the case of PPh3, PTA, dppe, and R-dppp ligands, it was possible to favor substitution over fragmentation by employing stoichiometric amounts of the ligands in the reactions with [Pt3n(CO)6n]2− (n = 2−4), whereas redox fragmentation was observed in the case of the higher nuclearity cluster [Pt15(CO)30]2−. In the cases of dppm, P^P, and dppf, the nonredox substitution was observed in the case of [Pt6(CO)12]2−, whereas redox fragmentation was observed for all higher nuclearity Chini clusters. Finally, by employing dppa, dppb, and dppBz13 redox fragmentation was always observed, independently of the Chini cluster employed.

Pt(dppb)2 showed a tetrahedral structure as expected for a zero-valent Pt(LL) 2 complex (Figure 7).47,48 Similarly, Pt2(CO)2(dppa)3 is composed of two tetrahedral Pt(0) centers bonded each to a CO ligand and three P atoms of three different dppa ligands (Figure 8). The structure of Pt 2 (CO) 2 (dppa) 3 was similar to that reported for Pt2(dppaO)2(dppa)3, where the two CO ligands were formally replaced by two Ph2P−CC−P(O)Ph2 (dppaO) ligands.49

Figure 8. Molecular structure of (a) Pt2(CO)2(dppa)3 and (b) its Pt− P core. (Pt, purple; P, orange; C, gray, O, red). H atoms were omitted for clarity.

Finally, the reaction of [Pt9(CO)18]2− with a slight excess of dppa afforded [Pt6(CO)12]2− in mixture with the neutral complex Pt8(CO)6(PPh2)2(CCPPh2)2(dppa)2 in poor yields. Crystals of Pt8(CO)6(PPh2)2(CCPPh2)2(dppa)2·2thf (thf = tetrahydrofuran) were obtained by slow diffusion of n-hexane on the reaction mixture (Figure 9). This compound was not soluble in any organic solvent, hampering any spectroscopic characterization, except recording the IR spectrum in the solid state as mineral oil mull (νCO 2023(vs), 2008(ms) cm−1). The metal part of the structure was composed of two isolated Pt atoms (formally zero valent) and two V-shaped Pt3 units. The ligands present were six terminal carbonyls, two neutral dppa ligands (each bonded to three different Pt atoms via the two P atoms and the CC triple bond), two edge-bridging phosphido [PPh2]− anions, and two [CCPPh2]− acetylides (each bonded to four Pt atoms, via the phosphine group, the CC triple bond and the terminal C atom). Thus, we can H


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Figure 9. Two views of the molecular structure of (a) Pt8(CO)6(PPh2)2(CCPPh2)2(dppa)2 and (b) simplified views with the same orientations. (Pt, purple; P, orange; C, gray, O, red). H atoms were omitted for clarity.

unit after each oxidation step, instead of a Pt6 trigonal prismatic unit. From a mechanistic point of view, it is likely that the oxidative oligomerization of [Pt6(CO)10(dppm)]2− proceeds via [Pt6(CO)10(dppm)]− radical intermediates. Similarly, monoanionic radicals of the type [Pt3n(CO)6n]− are sought to be intermediates in the oxidation of homoleptic Chini clusters. 5 0 In this respect, a monoanionic [Pt 3 (μCO)3(CNR)3]− (R = 2,6-(2,6-(i-Pr)2C6H3)2C6H3) cluster with terminal isocyanide ligands has been recently fully charactreized.51

assign a +2 charge to each of the two Pt3 units. The formation of Pt8(CO)6(PPh2)2(CCPPh2)2(dppa)2 suggested that partial cleavage of some dppa molecules occurred during the reaction, resulting in [PPh2]− and [CCPPh2]− with concomitant oxidation of the Pt-cluster.

3. CONCLUSIONS The stronger intertriangular Pt−Pt bonds of [Pt6(CO)12]2− compared to larger Chini clusters favored the nonredox substitution reactions over redox fragmentation. This allowed the synthesis and characterization of the heteroleptic anionic Chini clusters [Pt6(CO)10(dppm)]2−, [Pt6(CO)10(P^P)]2−, and [Pt6(CO)10(dppf)]2−. Conversely, the reactions of dppm, P^P, and dppf with [Pt3n(CO)6n]2− (n = 3−5) resulted in redox fragmentation. Because of the steric properties of dppb (very long backbone) and dppa (perfectly linear and rigid backbone), their reactions with any Chini cluster resulted in redox fragmentation. [Pt6(CO)10(dppm)]2− was employed for the synthesis of larger heteroleptic Chini clusters via redox condensation and oxidative oligomerization. The latter process is rather interesting, since it represents a new growing mode for Chini-type clusters. This consists in the growth of [Pt 6 (CO) 10 (dppm)]2− via the formal addition of one [Pt6(CO)10(dppm)] fragment after each oxidation step and results in the formation of [Pt6(CO)10(dppm)]n2− (n = 2−4) heteroleptic clusters. Further oxidation should afford infinite molecular wires, as already found while studying the oxidation of homoleptic Chini clusters.15−19 These reactions can be reversed by the addition of reducing agents. Conversely, the usual growing mode upon oxidation of homoleptic Chini clusters consists in the addition of a single Pt3(CO)6 triangular

4. EXPERIMENTAL SECTION 4.1. General Procedures. All reactions and sample manipulations were performed using standard Schlenk techniques under nitrogen and in dried solvents. All the reagents were commercial products (Aldrich) of the highest purity available and used as received, except [NEt4]2[Pt6(CO)12], which was prepared according to the literature.15 Analyses of C, H, and N were obtained with a Thermo Quest FlashEA 1112NC instrument. IR spectra were recorded on a PerkinElmer Spectrum One interferometer in CaF2 cells. 31P{1H} NMR measurements were performed on a Varian Mercury Plus 400 MHz instrument. The phosphorus chemical shifts were referenced to external H3PO4 (85% in D2O). 31P{1H} NMR spectra were simulated with gNMR 5.0.6.0, using the experimental parameters (δ in ppm, J in Hz).46 ESI mass spectra were recorded on a Waters Micromass ZQ4000 instrument. Structure drawings were performed with SCHAKAL99.52 4.2. Synthesis of [NEt4]2[Pt6(CO)10(dppm)]·CH3CN. A portion of dppm (0.310 g, 0.806 mmol) was added as a solid to a solution of [NEt4]2[Pt6(CO)12] (0.700 g, 0.396 mmol) in CH3CN (15 mL). The resulting mixture was stirred at room temperature for 2 h, and then, the solvent was removed in vacuo. The residue was washed with toluene (20 mL) and extracted with CH3CN (20 mL). Crystals of [NEt4]2[Pt6(CO)10(dppm)]·CH3CN suitable for X-ray analyses were I


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62.2 (1JPtP = 5059 Hz (1Pt), 2JPtP = 453 Hz (2Pt) and 17 Hz (1Pt), JPP = 120 Hz). 4.7. Synthesis of [NEt4]2[Pt6(CO)10(dppf)]·2CH3CN. A portion of dppf (0.695 g, 0.806 mmol) was added as a solid to a solution of [NEt4]2[Pt6(CO)12] (0.700 g, 0.396 mmol) in CH3CN (15 mL). The resulting mixture was stirred at room temperature for 2 h, and, then, the solvent was removed in vacuo. The residue was washed with toluene (20 mL) and extracted with CH3CN (20 mL). Crystals of [NEt4]2[Pt6(CO)10(dppf)]·2CH3CN suitable for X-ray analyses were obtained by layering n-hexane (4 mL) and diisopropyl ether (30 mL) on the CH3CN solution (yield 0.558 g, 60% based on Pt). C64H74FeN4O10P2Pt6 (2347.60): calcd. C 32.74, H 3.18, N 2.39; found: C 32.92, H 2.89, N 2.51%. IR (CH3CN, 293 K) νCO: 2005(sh), 1993(vs), 1802(sh), 1782(s) cm−1. 31P{1H} NMR (CD3CN, 298 K) δ (ppm): 49.3 (1JPtP = 5247 Hz (1Pt), 2JPtP = 543 Hz (2Pt). 4.8. Synthesis of Pt(dppb)2. A portion of dppb (0.511 g, 1.200 mmol) was added as a solid to a solution of [NEt4]2[Pt6(CO)12] (0.700 g, 0.396 mmol) in CH3CN (15 mL). The resulting mixture was stirred at room temperature for 4 h affording a microcrystalline solid, which was recovered by filtration. The solid was extracted with CH2Cl2 (10 mL). Crystals of Pt(dppb)2 suitable for X-ray analyses were obtained by layering n-hexane (25 mL) on the CH2Cl2 solution (yield 0.489 g, 20% based on Pt). C56H56P4Pt (1047.97): calcd. C 64.17, H 5.39; found: C 64.44, H 5.61%. 31P{1H} NMR (CD2Cl2, 298 K) δ (ppm): 0.5 (1JPtP = 3750 Hz). 4.9. Synthesis of Pt2(CO)2(dppa)3·0.5CH3CN. A portion of dppa (0.473 g, 1.200 mmol) was added as a solid to a solution of [NEt4]2[Pt6(CO)12] (0.700 g, 0.396 mmol) in CH3CN (15 mL). The resulting mixture was stirred at room temperature for 4 h affording a microcrystalline solid, which was recovered by filtration. The solid was extracted with CH2Cl2 (10 mL). Crystals of Pt2(CO)2(dppa)3· 0.5CH3CN suitable for X-ray analyses were obtained by layering nhexane (2 mL) and diisopropyl ether (30 mL) on the crude CH3CN solution before any workup (yield 0.392 g, 20% based on Pt). C81H61.5N0.5O2P6Pt2 (1649.80): calcd. C 58.97, H 3.76, N 0.42; found: C 59.19, H 3.51, N 0.62%. IR (mineral oil, 293 K) νCO: 1928(vs) cm−1. IR (CH2Cl2, 293 K) νCO: 1930(vs) cm−1. IR (toluene, 293 K) νCO: 1935(vs) cm−1. 31P{1H} NMR (CD2Cl2, 298 K) δ (ppm): −19.9 (1JPtP = 3391 Hz). 4.10. Synthesis of Pt8(CO)6(PPh2)2(CCPPh2)2(dppa)2·2thf. A portion of dppa (0.473 g, 1.200 mmol) was added as a solid to a solution of [NEt4]2[Pt9(CO)18] (0.895 g, 0.396 mmol) in thf (15 mL). The resulting mixture was stirred at room temperature for 4 h and layered with n-hexane (15 mL). After slow diffusion of the solvent, a few crystals of Pt8(CO)6(PPh2)2(CCPPh2)2(dppa)2·2thf suitable for X-ray analyses were obtained (yield 0.077 g, 5% based on Pt). These crystals are completely not soluble in any organic solvent. C118H96O8P8Pt8 (3450.42): calcd. C 41.07, H 2.80; found: C 41.31, H 2.68%. IR (mineral oil, 293 K) νCO: 2023(vs), 2008(ms) cm−1. 4.11. X-ray Crystallographic Study. Crystal data and collection details for [NEt4]2[Pt6(CO)10(dppm)]·CH3CN, [NEt4]2[Pt12(CO)20(dppm)2]·2CH3CN·2dmf, [NEt 4 ] 2 [Pt 12 (CO)20 (dppm) 2 ]·4dmf, [NEt4 ] 2 [Pt 6 (CO) 10 (dppf)]· 2CH3 CN, Pt 2(CO)2 (dppa)3 ·0.5CH3 CN, Pt 8 (CO)6 (PPh2 )2 (C CPPh2)2(dppa)2·2thf, and Pt(dppb)2 are reported in Table 1. The diffraction experiments were performed on a Bruker APEX II diffractometer equipped with a PHOTON100 detector using Mo Kα radiation. Data were corrected for Lorentz polarization and absorption effects (empirical absorption correction SADABS).53 Structures were solved by direct methods and refined by full-matrix least-squares based on all data using F2.54 Hydrogen atoms were fixed at calculated positions and refined by a riding model. All non-hydrogen atoms were refined with anisotropic displacement parameters, unless otherwise stated. [NEt4]2[Pt6(CO)10(dppm)]·CH3CN. The asymmetric unit of the unit cell contains one cluster anion, two [NEt4]+ cations, and one CH3CN molecule (all located on general positions). All the C, O, and N atoms were restrained to isotropic behavior (ISOR command in SHELXL, s.u. 0.01). Similar U restraints were applied to the dppm ligands, the

obtained by layering n-hexane (4 mL) and diisopropyl ether (30 mL) on the CH3CN solution (yield 0.550 g, 65% based on Pt). C53H63N3O10P2Pt6 (2134.54): calcd. C 29.82, H 2.97, N 1.97; found: C 30.05, H 3.11, N 1.74%. IR (mineral oil, 293 K) νCO: 2000(s), 1980(s), 1949(s), 1778(m), 1756(m), 1750(m) cm−1. IR (CH3CN, 293 K) νCO: 2006(s), 1980(vs), 1780(s), 1770(s) cm−1. 31 1 P{ H} NMR (CD3CN, 298 K) δ (ppm): 51.0 (1JPtP = 4960 Hz (1Pt), 2JPtP = 602 Hz (2Pt), and 15 Hz (1Pt), 2JPP = 88 Hz). 4.3. Synthesis of [NEt4]2[Pt9(CO)16(dppm)]. A solution of [NEt4]2[Pt12(CO)20(dppm)2] (0.823 g, 0.198 mmol) in CH3CN (15 mL) was added to a solution of [NEt4]2[Pt6(CO)12] (0.350 g, 0.198 mmol) in CH3CN (15 mL). The resulting mixture was stirred at room temperature for 2 h, and, then, the solvent was removed in vacuo. The residue was washed with toluene (20 mL) and extracted with CH3CN (20 mL). The resulting CH3CN solution was employed for the IR and 31 1 P{ H} NMR analyses. IR (CH3CN, 293 K) νCO: 2018(vs), 1828(s) cm−1. 31P{1H} NMR (CD3CN, 298 K) δ (ppm): 47.3 (1JPtP = 4941 Hz (1Pt), 2JPtP = 562 Hz (2Pt), 2JPP = 85 Hz), 39.0 (1JPtP = 5047 Hz (1Pt), 2JPtP = 697 Hz (2Pt), 2JPP = 85 Hz). 4.4. Synthesis of [NEt4]2[Pt12(CO)20(dppm)2]·2CH3CN·2dmf. A solution of HBF4·Et2O (80 μL, 0.588 mmol) was added dropwise to a solution of [NEt4]2[Pt6(CO)10(dppm)]·CH3CN (0.550 g, 0.258 mmol) in dmf (15 mL). The reaction was monitored by IR, and the final product was precipitated by the addition of a saturated solution of [NEt4]Br in H2O (40 mL). After filtration, the solid was washed with H2O (40 mL), dried under reduced pressure, and extracted with dmf (10 mL). Crystals of [NEt4]2[Pt12(CO)20(dppm)2]·2CH3CN·2dmf suitable for X-ray analyses were obtained by layering isopropanol (30 mL) on the dmf solution diluted with some CH3CN (5 mL) (yield 0.319 g, 59% based on Pt). Crystals of [NEt4]2[Pt12(CO)20(dppm)2]·4dmf can be obtained by following an analogous procedure, without the final addition of CH3CN. C96H104N6O22P4Pt12 (4158.81): calcd. C 27.73, H 2.52, N 2.02; found: C 27.51, H 2.89, N 1.91%. IR (mineral oil, 293 K) νCO: 2013(vs), 1856(m), 1819(s), 1810(s) cm−1. IR (CH3CN, 293 K) νCO: 2020(vs), 1851(m), 1829(s), 1808(m) cm−1. 31P{1H} NMR (dmf-d7, 298 K) δ (ppm): 46.2 (1JPtP = 4962 Hz (1Pt), 2JPtP = 546 Hz (2Pt), 2 JPP = 83 Hz), 38.7 (1JPtP = 5144 Hz (1Pt), 2JPtP = 626 Hz (2Pt), 2JPP = 83 Hz). 4.5. Synthesis of [NEt4]2[Pt18(CO)30(dppm)3]. A solution of HBF4·Et2O (20 μL, 0.147 mmol) was added dropwise to a solution of [NEt4]2[Pt12(CO)20(dppm)2]·4dmf (0.258 g, 0.061 mmol) in dmf (10 mL). The reaction was monitored by IR, and the final product was precipitated by the addition of a saturated solution of [NEt4]Br in H2O (40 mL). After filtration, the solid was washed with H2O (40 mL), dried under reduced pressure, and extracted with dmf (10 mL). The resulting dmf solution was employed for the IR and 31P{1H} NMR analyses. All attempts to crystallize [NEt 4 ] 2 [Pt 18 (CO) 30 (dppm) 3 ] failed, affording crystals of [NEt4]2[Pt12(CO)20(dppm)2] instead. IR (dmf, 293 K) νCO: 2032(vs), 1871(m), 1856(s), 1843(m), 1821(m) cm−1. 31P{1H} NMR (dmf-d7, 298 K) δ (ppm): 46.1 (1JPtP = 5105 Hz (1Pt), 2JPtP = 548 Hz (2Pt) and 15 Hz (1Pt), 2JPP = 70 Hz), 43.8 (1JPtP = 4952 Hz (1Pt), 2JPtP = 540 Hz (2Pt), 2JPP = 78 Hz), 36.2 (1JPtP = 5095 Hz (1Pt), 2JPtP = 601 Hz (2Pt), 2JPP = 78 Hz). 4.6. Synthesis of [NEt4]2[Pt6(CO)10(P^P)]. P^P (0.319 g, 0.806 mmol) was added as a solid to a solution of [NEt4]2[Pt6(CO)12] (0.700 g, 0.396 mmol) in CH3CN (15 mL). The resulting mixture was stirred at room temperature for 2 h, and, then, the solvent was removed in vacuo. The residue was washed with toluene (20 mL) and extracted with CH3CN (20 mL). An amorphous powder of [NEt4]2[Pt6(CO)10(P^P)] was obtained after CH3CN was removed under reduced pressure from the filtrate (yield 0.517 g, 62% based on Pt). C52H62N2O10P2Pt6 (2107.47): calcd. C 29.64, H 2.97, N 1.33; found: C 29.38, H 3.12, N 1.14. IR (CH3CN, 293 K) νCO: 2006(s), 1979(vs), 1774(s) cm−1. 31P{1H} NMR (CD3CN, 298 K) δ (ppm):

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[NEt4]+ cations, and the CH3CN molecule (SIMU command in SHELXL, s.u. 0.02). All the aromatic rings were constrained to fit regular hexagons (AFIX 66 command in SHELXL). [NEt4]2[Pt12(CO)20(dppm)2]·2CH3CN·2dmf. The asymmetric unit of the unit cell contains half of a cluster anion (located on an inversion center), one [NEt4]+ cation, one CH3CN, and one dmf molecule (located on general positions). All the aromatic rings were constrained to fit regular hexagons (AFIX 66 command in SHELXL). Similar U restraints were applied to the [NEt4]+ cation and the dmf and CH3CN molecules (SIMU command in SHELXL, s.u. 0.02). Restraints to bond distances were applied as follows (s.u. 0.02): 1.47 Å for C−N and 1.53 Å for C−C in [NEt4]+. [NEt4]2[Pt12(CO)20(dppm)2]·4dmf. The asymmetric unit of the unit cell contains half of a cluster anion (located on an inversion center), one [NEt4]+ cation, and two dmf molecules (located on general positions). The crystals appeared to be nonmerohedrally twinned. The TwinRotMat routine of PLATON55 was used to determine the twinning matrix and to write the reflection data file (.hkl) containing the two twin components. Refinement was performed using the instruction HKLF 5 in SHELXL and one BASF parameter, which refined as 0.333(5). All the C, O, and N atoms were restrained to isotropic behavior (ISOR command in SHELXL, s.u. 0.005). All the aromatic rings were constrained to fit regular hexagons (AFIX 66 command in SHELXL). Similar U restraints were applied to the [NEt4]+ cation and to the dmf molecules (SIMU command in SHELXL, s.u. 0.02). Restraints to bond distances were applied as follows (s.u. 0.02): 1.47 Å for C−N and 1.53 Å for C−C in [NEt4]+. [NEt4]2[Pt6(CO)10(dppf)]·2CH3CN. The asymmetric unit of the unit cell contains one cluster anion, two [NEt4]+ cations, and two CH3CN molecules (all located on general positions). Pt2(CO)2(dppa)3·0.5CH3CN. The asymmetric unit of the unit cell contains two cluster molecules and one CH3CN molecule (all located on general positions). Similar U restraints were applied to all the C, N, and O atoms (SIMU command in SHELXL, s.u. 0.01). Pt8(CO)6(PPh2)2(CCPPh2)2(dppa)2·2thf. The asymmetric unit of the unit cell contains half of a cluster molecule (located on an inversion center) and one thf molecule (located on a general position). Similar U restraints were applied to all the C atoms (SIMU command in SHELXL, s.u. 0.005). Restraints to bond distances were applied as follows (s.u. 0.01): 1.43 Å for C−O and 1.53 Å for C−C in thf. Pt(dppb)2. The asymmetric unit of the unit cell contains half of a molecule located on a twofold axis.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +39 051 2093711. Web: https://www.unibo.it/sitoweb/stefano.zacchini/en. ORCID

Cristina Femoni: 0000-0003-4317-6543 Stefano Zacchini: 0000-0003-0739-0518 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Univ. of Bologna for financial support. REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00447. IR spectra of [Pt6(CO)10(dppm)]2−, [Pt12(CO)20(dppm)2]2−, [Pt18(CO)30(dppm)3]2−, and [ P t 2 4 ( C O ) 4 0 ( d p p m ) 4 ] 2 − . E S I - M S sp e c t r a o f [Pt6(CO)10(dppm)]2−, [Pt12(CO)20(dppm)2]2−, and [Pt18(CO)30(dppm)3]2−. Experimental and simulated 31 P{ 1 H} NMR spectra of [Pt 9 (CO) 16 (dppm)] 2− , [Pt 12 (CO) 20 (dppm) 2 ] 2− , [Pt 18 (CO) 30 (dppm) 3 ] 2− , [Pt6(CO)10(P^P)]2−, and [Pt6(CO)10(dppf)]2−. Crystals and experimental details (PDF) Accession Codes

CCDC 1824418−1824424 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. K


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