Multiple Bonds Between Metal Atoms / 14-Nickel, Palladium and Platinum Compounds

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Fig. 14.7. The structure of the cis-[Pt2(hp)2(NH3)4(NO3)(H2O)]3+ with a head-to-head arrangement of the _-pyridonate ligands.

While the structure determination of the compound [Pt2(hp)2(NH3)4(H2O)(NO3)]- (NO3)3·2H2O was the first to be published130 for a diplatinum(III) derivative of the socalled platinum blues, an earlier report135 on a complex whose composition was purported to be (H5O2)[Pt2(1-MeC)2(NH3)4(NO2)2](NO3)2, where 1-MeC represents the monoanion of N,N-bound 1-methylcytosine, i.e. a Pt25+ complex, is in reality probably that of a Pt26+ complex. It was pointed out by Lippard130,133 that the Pt–Pt distance of 2.584(1) Å accords with it being a single bond. Subsequently, it has been confirmed136 that the correct formulation is cis-[Pt2(l-MeC)2(NH3)4(NO2)2](NO3)2·2H2O (see Table 14.1). More recently, several compounds containing 1-methylcytosinate ligands and Pt26+ cores have been characterized with various axial ligands including nucleobases.137,138 For example, the structure of the head-tail cis-[Pt2(1-MeC)2(NH3)2(gly-N,O)2](NO3)2 has been reported137 but an L-alanine analog of the latter compound has not been crystallographically characterized. In solution, two diastereomeric forms are observed in the NMR spectrum.137 Other compounds have the nucleobase 9-ethylguanine (EtguaH),138,139 NO3- or water or a mixture of NO2- and water occupying the two axial positions.138 The Pt–Pt distances are in the range 2.55–2.60 Å as shown in Table 14.1. Some compounds of this family can interact with DNA and exhibit antitumor activity.140,141

Another series of diplatinum(III) complexes that are closely related to those that contain a pair of bridging hp ligands have been prepared in which the monoanion of 1-methyluracil (1-MeU) replaces hp.142-147 Bernhard Lippert and co-workers142-144,146 have generally obtained these compounds by the oxidation of diplatinum(II) precursor complexes. The compounds that have been structurally characterized are listed in Table 14.1. The unsymmetrical mixed aquo-nitrito and aquo-nitrato species cis-[Pt2(1-MeU)2(NH3)4X(H2O)]3+, where X = NO2 or NO3,142,143 have a head-to-tail disposition of the 1-MeU ligands and therefore differ structurally from cis-[Pt2(hp)2(NH3)4(NO3)(H2O)]3+ and cis-[Pt2(hp)2(NH3)4(NO2)(NO3)]2+ which are in their head-to-head isomeric forms.132,134 All other diplatinum(III) 1-MeU complexes that have been structurally characterized contain head-to-head arrangements, including the 1:1 adducts with nitrite144 and carbon-bound 1-methyluracilato (bound through its deprotonated C(5) position).146 In both structures the single axial ligand is bound to the Pt atom that has the PtN2O2 ligand set. The cis-[Pt2(l-MeU)2(NH3)4(1-MeU)]3+ cation146 has a short strong Pt–C(axial) bond and a long Pt–Pt distance (2.685(1) Å) (see Table 14.1) which reflects a high structural transinfluence of this C-bound 1-MeU ligand.

654Multiple Bonds Between Metal Atoms Chapter 14

In aqueous solution, axial ligands such as Cl-, ONO2- and NO2- readily undergo solvolysis with the resulting formation of [Pt2(1-MeU)2(NH3)4(H2O)2]4+. The lability of the axial ligands has been taken advantage of in the conversion of the 1:1 head-to-head nitrito complex cis-[Pt2(l-MeU)2(NH3)4(NO2)](NO3)3·H2O to cis-[Pt2(l-MeU)2(NH3)4Cl2]Cl2·3.5H2O upon its reaction with aqueous HCl.145,147 This dichloride has also been prepared by chlorination of [Pt2(l-MeU)2(NH3)4]Cl2·H2O,143 but this reaction is complicated and other products, including mononuclear ones, are formed. Also, chlorination of the 1-MeU ligand can occur at the 5- position.143 Ligand lability is not restricted to the axial ligands, as illustrated by the reaction of cis-[Pt2(1-MeU)2(NH3)4Cl2]Cl2·3.5H2O with HCl over a period of several days145,147 to give the neutral complex cis-Pt2(l-MeU)2(NH3)4Cl2, in which a pair of cis-NH3 ligands have been replaced by Cl-.

Another example of a series of diplatinum(III) complexes with monoanionic N,O bridging ligands is that of the _-pyrrolidonato-containing compounds with the core cis-[Pt2(pyrr)2(NH3)n]4+, where pyrr = the anion of C4H6(O)NH. The earliest compound structurally characterized is cis- [Pt2(pyrr)2(NH3)4(NO2)(NO3)](NO3)2·H2O. This is formed148,149 by the nitric acid oxidation of the tetranuclear complex [Pt4(pyrr)4(NH3)8](NO3)6·2H2O and resembles structurally other compounds of this type. Crystallographic data149-151 for a total of six compounds of this family, including one with one axial position occupied by the deoxyribonucleoside 2N-deoxyguonosine,152 are given in Table 14.1. The Pt–Pt distances are generally over 2.6 Å except for the latter and for the dimer of dimers cis-[Pt2(pyrr)2(NH3)3(H2O)(µ-OH)]2(NO3)6·4H2O (H,T)150 in which it is only 2.553(1) Å. The exceptionally short distance has been attributed to hydrogen bonding between an amine group and the oxygen atom of the bridging OH unit between the dimer. Kinetic and equilibrium studies on the axial-ligand substitution reactions of the head-to-head and head-to-tail _-pyridonate-bridged dinuclear compounds have been done.153,154

A final example of a series of diplatinum(III) complexes with monoanionic N,O bridging ligands is that containing 1-methylthyminato (1-MeT) and 1-ethylthyminato (1-EtT) nucleobase ligands.155 These have been studied in solution and they are of general composition cis- [Pt2L2(amine)4XY]n+ where L = thymine nucleobase, amine = NH3 or NH3CH3 and the axial groups X and Y are ligands such NO2-, Cl-, water or no ligand at all. These are made by oxidation of Pt24+ precursors. The reaction proceeds via purple and blue-green intermediates, which are likely mixed-valence species. Unfortunately, synthetic procedures are sometimes poorly reproducible as a consequence of facile substitution reactions of the axial ligands and X-ray crystallography is usually the only reliable method of establishing the nature of the X and Y ligands. The compounds cis-[Pt2(1-MeT)2(NH3)4(NO2)](NO3)3 and cis-[Pt2(1-MeT)2- (NH2CH3)2Cl3]ClO4 have been crystallographically characterized and have Pt–Pt distances of 2.6507(6) and 2.612(2) Å, respectively.155 The most relevant structural feature is the head–head arrangement. This is similar to that of the 1-methyluracil described above. Thus one platinum atom is six-coordinate while the other is five-coordinate. The two units are held together by hydrogen bonding as shown in Fig. 14.8 for the 1-Met complex.

A few compounds having four bridging nucleobases have been made in low yield by heating the heteronuclear complex trans-[(NH3)2Pt(N4-1-MeC-N3)2Cu(H2O)2](ClO4)2 in water.156 By small modification of the reaction conditions, cis-2,2-[Pt2(1-MeC)4Ln](ClO4)2 complexes have been isolated and structurally characterized for Ln = (NH3)2, NH3/H2O. Another complex has only one axial NO2- group. The Pt–Pt distances are in the range of 2.452(1) to 2.498(1) Å (Table 14.1). These are about 0.1 Å shorter than those in species with only two bridging nucleobases such as [cis-Pt2(1-MeC)2(NH3)4(NO3)2]2+ but similar to those in Pt2(ButCONH)4Cl2 (2.448(2) Å).

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Fig. 14.8. The structure of the centrosymmetric pair of cations joined by hydrogen bonding in the 1-methylthiminato derivative cis-[Pt2(1-MeT)2(NH3)4(NO2)](NO3)3 (HH). Note that the platinum atom binds to the NO2 group through the N atom.

Several compounds that contain monoanionic bridging ligands with N,S donor sets are known and a few of these have been structurally characterized (Table 14.1).157-159 The reactions of aqueous solutions of K2PtX4 (X = Cl, Br or I) with methanol or ethanol solutions of pyrimidine- 2-thione (pymSH) lead to oxidation of the platinum to produce diplatinum(III) compounds of the type Pt2(pymS)4X2 (X = Cl, Br or I).157,158 A compound of composition Pt2(pymS)5Cl has also been prepared,158 and similar synthetic methods to these have been used158 to prepare Pt2(4-MepymS)4X2 (4-MepymS is the anion of 4-methylpyrimidine-2-thione; X = Cl or I) and Pt2(2-TU)4I2 (2-TU is the anion of 2-thiouracil). Crystal structure determinations of the iodo complexes Pt2(pymS)4I2157 and Pt2(2-TU)4I2158 show similar structures with cis-PtN2S2 geometries present about each Pt center. This cis-2,2 arrangement contrasts with the less symmetric structure of Pt2(pymS)4Cl2, in which there is a 3,l ligand arrangement, i.e. PtN3S and PtNS3 ligand atom sets about the two Pt atoms in the dimer.158 All three complexes have rotational geometries that are twisted considerably from the fully eclipsed arrangement (ρ in the range 25˚ to 29˚).157,158 A partial structure determination has been carried out158 on a crystal of composition Pt2(pymS)4Br1.2(pymS)0.8, in which the axial sites of Pt2(pymS)4Br2 are partially occupied by pymS. The Pt–Pt distance of 2.554(1) Å is the same as that of the di-iodide.

The diplatinum(II) complexes Pt2(pyS)4 and Pt2(4-MepyS)4, where pyS and 4-MepyS represent the monanions of 2-mercaptopyridine and 4-methyl-2-mercaptopyridine, are oxidized by chloroform to give Pt2(pyS)4Cl2 and Pt2(4-MepyS)4Cl2, respectively.159 When this CHCl3 oxidation of Pt2(pyS)4 is carried out in the presence of NaBr, NaI or NaSCN, then exchange of the axial ligands occurs to give Pt2(pyS)4X2, where X = Br, I or SCN.159 The reaction of Pt2(pyS)4 with CHBr3 also gives Pt2(pyS)4Br2.159 The structure of Pt2(pyS)4Cl2 is that of the cis- 2,2 isomer; the Pt–Pt distance of 2.532(1) Å159 is a little longer than that in Pt2(pymS)4Cl2.158 The electrochemical behavior of Pt2(pyS)4 and Pt2(4-MepyS)4 in DMF are characterized159 by quasi-reversible two-electron processes that result in oxidation to [Pt2(pyS)4(DMF)2]2+ and

[Pt2(4-MepyS)4(DMF)2]2+. The E1/2 values are +0.28 and +0.26 V versus Ag/Ag[Cryp(2,2)]+. A detailed study159 of the cyclic voltammetry of Pt2(pyS)4Cl2 shows that it conforms to a four-

component scheme which involves an ECEC mechanism. More recently, these compounds have been isolated from reactions of K2[PtCl4] with mercaptopyridine in hot alcohol.160 Reaction of Pt2(5-MepyS)4X2, X = Cl or Br, and WS42- or S22- in CHCl3 forms compounds of the type [Pt2(5-MepyS)4X]S4[Pt2(5-MepyS)4X] where the two Pt26+ units are held together by a chain of sulfur atoms from an S42- anion.161 In refluxing acetonitrile, these compounds decompose into Pt2(5-MepyS)4X2, Pt2(5-MepyS)4 and S8. The Pt–Pt distances for the corresponding

656Multiple Bonds Between Metal Atoms Chapter 14

chloride and bromide compounds are 2.556(2) and 2.560(2) Å, respectively.161 Treatment of Pt2(5-MepyS)4Cl]S4[Pt2(5-MepyS)4Cl with H2 in DMF at 150 ˚C produces 3-picoline which was generated by C–S bond cleavage of the bridging 5-MepyS ligands.162

14.4.5 Unsupported Pt–Pt bonds

A growing number of unbridged diplatinum(III) complexes has emerged. These are often unstable towards disproportionation to PtII and PtIV species. The first such complex was cis- Pt2Cl6[HN=C(OR)CR'3]4 where R = H and R' = CH3 (Pt–Pt distance of 2.694(1) Å).163 Here, each Pt atom has two mutually cis iminoether groups in equatorial positions. The other equatorial and the axial positions are occupied by Cl atoms. Later, two analogous compounds were made by reacting the PtII-iminoether precursors cis- and trans-PtCl2[HN=C(OR)CR'3]4 (R = CH3 and R' = H) with Cl2 at low temperature in the dark producing the corresponding cis- and trans-Pt2Cl6[HN=C(OR)CR'3]4 complexes.164 At room temperature these compounds readily disproportionate. In this case the iminoether ligands can have either E or Z configuration depending upon the relative position of the methoxy and platinum residues with respect to the C=N double bond. The crystals are stabilized by N–H···Cl bonds and there are long Pt–Pt distances (2.765(2) and 2.758(3) Å for the cis and trans isomers, respectively).

Reaction of Pt(phpy)2 (phpy is the anion of phenylpyridine) with an equimolar amount of AuCl(SMe2) in dichloromethane yields metallic gold and unsupported Pt2(phpy)4Cl2.165 The dimer has an approximate 2-fold axis bisecting the metal-metal bond and each phpy adopts a chelating mode. However, one is ax-eq while the other is eq-eq. The Pt–Pt distance of 2.7269(3) Å is similar to those mentioned above. In solution it is stable in the dark for several days.

Controlled oxidation with PhICl2 of a PtII compounds having two C8 carbocyclic _-dioxi- mato ligands produces Pt2(C8H12(=NO)H)4Cl2.166 The unsupported metal-metal bond length is 2.6964(5) Å. The two square units are capped by axially coordinated chloride ions. A Raman absorption at 139 cm-1 has been assigned to the Pt–Pt stretch.

There is also an unusual Pt26+ compound devoid of bridging and axial ligands. This is obtained by oxidation of Pt(OBQDI-H)2 (OBQDI = o-benzoquinodiimine) with AgO3SCF3.167 In spite of the long Pt–Pt distance of 3.031(1) Å in Pt2(OBQDI-H)4(CF3SO3)2, the Pt–Pt unit appears to be stable in solution, as shown by NMR spectroscopy. In the solid state, the diamagnetic molecule has an important number of hydrogen bonds between the triflate anions and each of the N–H groups in the cation.

Oxidation by controlled addition of chlorine to chilled solutions of various PtII-substituted acetylacetonates having formulae Pt(acacRR')2 with R/R' = Me/Me, Ph/Ph and Me/CF3 produces coupling of the planar Pt(acacRR')2 moieties.168 These compounds have not been crystallographically characterized but the 195Pt NMR resonance at around 1300 ppm (which is between those of -216 to -771 ppm for the PtII precursors and those of about 1900 ppm for the corresponding PtIV compounds) supports the presence of Pt26+ species. The Raman stretch at 144 cm-1 is very similar to that of Pt2(C8H12(=NO)H)4Cl2.166 There is also a very strong absorption band at 24,000 cm-1. This band is very weak in Pt(acacRR')2 compounds and disappears upon further oxidation to PtIV species.

Another important species having unsupported the Pt–Pt units is that of the dimeric anion [Pt2(CN)10]4- which has been made in solution but not isolated in the solid state by addition of an aqueous solution of Tl(ClO4)3, NaCN at a pH of 2 to a solution of Na2Pt(CN)4.169 The dimeric nature has been determined by EXAFS that shows a Pt–Pt distance of 2.729(3) Å and Pt–C distances of 2.008(2) Å.170 Each platinum atom has one axial and four equatorial CN groups. Thus the structure is similar to that of the [Co2(CN)10]6- anion171 which is discussed in Section 11.3.2. The Pt–Pt Raman stretching frequency of 144 cm-1 is similar to those of other

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unsupported Pt26+ units as well as those in [Pt2(pop)4X2]4- species15 but significantly lower than those in the anion [Pt2(SO4)4(H2O)2]2- and Pt2(SO4)4X2, X= Cl and Br, where such a vibration is at 333 cm-1.28

14.4.6 Dinuclear Pt25+ species

The understanding of the chemistry and electronic structure of dinuclear Pt25+ units is not as well-developed as that for other M25+ paddlewheel species. When a compound with an M2 core has four bridging mononegative ligands, the additional anion occupies an axial position, and an unsymmetrical species would be expected to form. However, that is not commonly found as the fifth mononegative ligand, e.g., a halide, is often shared and an infinite chain of the type

···M–M···X···M–M···X forms. Generally, these are paramagnetic compounds and EPR spectra clearly show that the unpaired electron is localized in the corresponding M2 unit, as has been shown for example for Cr25+,172 Mo25+,172 W25+ 173 and other dimetal units (see Chapters 8 and 9). The metal-metal bond distance is generally between those of the corresponding M24+ and M26+ species.

Undoubtedly, this model can be applied to the formamidinate derivative [Pt2(DTolF)4]PF6.174 The bond distance of 2.5304(6) Å is between 2.649(2) Å in Pt2(DPhF)4 174 and 2.5169(7) Å in Pt2(DPhF)4Cl2.86 This is in agreement with a bond order of 0.5 and an electronic configuration of μ2/4β2β*2/*4μ*. In the crystal, the molecules pack in such a way that there is a PF6 anion between each [Pt2(DTolF)4]+ unit as shown in Fig. 14.9. The Pt to F separation of 4.34 Å is too long to imply the presence of bonds and thus the cations are essentially isolated from each other.

Fig. 14.9. Packing of the mixed-valence [Pt2(DTolF)4]PF6 species along the c axis. The Pt···F separations of over 4 Å are too long to imply Pt to F bonding.

The bonding situation appears more complex in a class of mixed-valence Pt25+ complexes that have attracted a lot of attention.15,18,23,175 These are salts of composition A4[Pt2(pop)4X]·nH2O (X = C1, Br, I), A = Li, K, Cs or NH4 and n = 2 or 3. These are prepared by the partial oxidation of A2[Pt2(pop)4] (see Section 14.4.2) in aqueous solution using chlorine water, bromine water and KI3, respectively,61 or by the comproportionation reaction between K2[Pt2(pop)4] and K2[Pt2(pop)4X2] in water.61,72 These materials, which are linear-chain semiconductors, have a golden metallic appearance and while the finer points of their structures have prompted considerable debate,60,61,72,176 the situation now seems to have been clarified. Based upon the results of resonance Raman spectral studies,72,176 their electronic absorption spectra,72 and magnetic susceptibility properties,176 as well as a series of X-ray crystal structure determinations at room temperature (X = Cl60,176 and Br61) and at c. 20 K (X = Cl and Br),176 some differences are seen

658Multiple Bonds Between Metal Atoms Chapter 14

in the structures of the chloride and bromide. For the chloride, the translational symmetry is such that there appears to be a combination of equal amounts of Pt24+ and Pt2Cl24+ units alternating along the chains (as in 14.10) both at 300 K and 22 K. This gives rise176 to inequivalent Pt–Pt distances (2.685(2) and 2.969(2) Å at 22 K) and sets of Pt–Cl (short) and Pt–Cl (long) distances. The bromide, on the other hand, is best modeled as containing equivalent Pt–Pt bonds (2.793(1) Å at 300 K and 2.781(1) Å at 22 K).61,176 These measured distances are actually the average given by superimposing PtII–PtII and PtIII–PtIII distances. While the Pt–Br distances appear the same at 300 K, i.e. the Br atoms are equidistant between adjacent pairs of Pt2 units,61 at 19 K they resolve into a short–long disposition (2.579(4) Å and 2.778(4) Å).176

···Pt2+–Pt2+···Cl–Pt3+–Pt3+–Cl···

14.10

A structure determination on (NH4)4[Pt2(pop)4Cl] at room temperature likewise reveals an arrangement as in 14.10.177 The ‘averaged’ Pt–Pt distance is 2.830(1) Å, and the bridging Cl atom displays positional disorder over two sites so that the short Pt–Cl distance is 2.363(4) Å and the long one is 3.022(4) Å in the chain. The dihydrates of K4[Pt2(pop)4X] (X = C1, Br) have also been structurally characterized.178 Structure determinations at room temperature (X = Cl) and 125 K (X = Br) accord with the results of the previous studies; the only significant differences are that the chains deviate slightly from linearity and the short and long Pt–X distances differ by an amount greater than in the corresponding trihydrates.

In Section 14.4.3 the preparation of the singly bonded Pt2(S2CCH3)4I2 complex was mentioned. By adjusting the stoichiometry of the reaction, the mixed-valence compound Pt2(S2CCH3)4I can be isolated.93 It has also been prepared from the reaction of Pt2(S2CCH3)4 and Pt2(S2CCH3)4I2 in toluene at reflux.93 It is a semiconductor material and has a linear chain structure of the type ···Pt2S8···I···Pt2S8···I··· in which the Pt–Pt distance is 2.677(2) Å and the Pt–I distances are essentially identical.93 The latter feature is different from the disparity in Pt–X distances encountered in compounds that contain the {[Pt2(pop)4X]4-} chains. The [PtS4] units are twisted ȵ21˚ away from the full eclipsed conformation.93 Although the crystallographic data accord with an essentially symmetrical structure, an intervalence band is observed93 in the electronic absorption spectrum at 7800 cm-1. The compound exhibits metallic conduction above room temperature.179 Theoretical investigations indicate a noticeable reduction of electron–phonon coupling through the bridging halogen atoms.180 Crystallographic studies on Pt2(S2CC2H5)4I at temperatures ranging from 115 to 377 K show only small variations in the Pt–Pt distances with bond distances changing from 2.680(1) Å at 115 K to 2.686(1) Å at 377 K.95 The Pt–I distances change from 2.954(1) Å to 2.989(1) Å but they are essentially the same for each of the two axial iodide ligands. This compound shows a relatively high electrical conductivity at room temperature (5–30 C cm-1) and undergoes a metal–semiconductor transition at TM-S = 205 K.95

There is also a class of tetranuclear compounds with a formal oxidation state of Pt2.5+ but these are best described with the platinum blues in the following section.

14.4.7 The platinum blues

While the oligomeric compounds that encompass this very important class of molecules all formally contain at least one PtIII atom, the average oxidation state is usually far less than 3.0 and the average Pt–Pt bond order is therefore less than one. Accordingly, while a discussion of these materials is appropriate in order to point out their relationship to the diplatinum(III) compounds that have been described in previous sections (especially Section 14.4.4), a compre-

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hensive and detailed coverage of the literature falls outside the scope of this text. Many review articles that touch upon the platinum blues can be consulted for additional details.21,22,181-183

The oxidation state nuclearity relationships that commonly exist between diplatinum(II) and diplatinum(III) species and the platinum blues can be summarized as follows:

2[Pt(2+)]2

-e-

[Pt(2.25+)]4

-e-

[Pt(2.5+)]4

-e-

[Pt(2.75+)]

-e-

2[Pt(3+)]

-

+e

+e-

+e-

4

2

+e-

However, a full understanding of these relationships has taken most of the twentieth century to evolve. The first platinum blues, the platinum-acetamide blues, were reported in 1908 and formulated as Pt(CH3CONH)2(H2O).184 This same formula was proposed again in 1964185 augmented by some speculation as to the presence of Pt–Pt bonds. On the other hand two more studies186,187 then appeared in which it was proposed that the platinum blues are PtIV compounds, e.g., Pt(CH3CONH)2(OH)2. In connection with the anticancer action of platinum compounds,181 a second set of blue platinum compounds were made and studied in the 1970s. The reaction between a solution of cis-PtCl2(NH3)2 which has wholly or partially undergone aquation and various pyrimidines such as thymine, uridine, uracil, 1-methyluracil and polyuracil gives deep-blue products188,189 which also have anticancer activity.190,191 These developments led to a resurgence of efforts to obtain a better characterization of platinum blues, or at least some of the compounds that have been included under this name.

By using 2-hydroxypyridine (_-pyridone or Hhp), Lippard and co-workers191 were able to isolate and structurally define what is now recognized as being an analog of the previously described platinum blues. A combination of X-ray crystallography,191,192 XPS data,106 magnetic susceptibility,192 EPR spectroscopy,192 optical spectroscopy and scattered-wave X_ calculations193 have been applied to the characterization of the paramagnetic complex cis- [Pt4(hp)4(NH3)8(NO3)2](NO3)3·H2O in which the mean oxidation state is +2.25 and S = ½. The structure is as shown in 14.11. The Pt–Pt distances are 2.774(1) Å for the pair of outer bonds and 2.877(1) Å for the inner Pt–Pt bond.192 While the net μ-bonding interaction between the end pairs of Pt atoms in the chain is stronger than between the middle pair,193 it is not possible for either type of bond to be a full single bond. Note that this _-pyridone blue and other closely related tetranuclear analogs (see below) bear a structural relationship to the mixed-valence tetranuclear compounds Ir4(µ-C7H4NS2)4I2(CO)8 (Section 11.4.4) and [Rh4(1,3- di-isocyanopropane)8Cl]5+ (Section 12.5.2), both of which also contain linear M4 units.

3+

	N			O		O			N
			N		O			O	N
O2NO	Pt			Pt		Pt			Pt ONO2
H3N			H3N		H3N			H3N
	NH3			NH3		NH3			NH3
				14.11

The reactions that ultimately give rise to the formation of cis-diammine-platinum _-pyri- done blue from the reaction of cis-[Pt(NH3)2(H2O)2]2+ and 2-hydroxypyridine (_-pyridone) have been shown to involve a variety of mononuclear PtII and Pt IV complexes194-196 as well as the head-to-tail and head-to-head isomers of [Pt2(hp)2(NH3)4]2+.194,196

660Multiple Bonds Between Metal Atoms Chapter 14

The chemistry of the cis-diammineplatinum _-pyridone blue is representative of that of other closely related platinum blues. The analogous ethylenediamineplatinum _-pyridone blue [Pt4(hp)4(en)4(NO3)3](NO3)3·H2O has been prepared and characterized,197,198 and several cis-diammineplatinum _-pyrrolidone ([cis-Pt2(C4H6NO)2]mn+) species have been obtained.150,199-202 The latter include phases that have been described as ‘_-pyrrolidone green’199 and ‘_-pyrrolidone violet’200 and which have the compositions [Pt4(pyrr)4(NH3)8](NO3)5.48·3H2O and [Pt4(pyrr)4(NH3)8](PF6)2(NO3)2.56·5H2O, respectively. These ‘non-stoichiometric’ crystalline materials have been purported to be mixtures of the [Pt(2.25+)]4/[Pt(2.5+)]4 and [Pt(2+)]4/ [Pt(2.25+)]4 species, respectively. The so-called ‘_-pyrrolidone tan’, which is of composition [Pt4(pyrr)4(NH3)8](NO3)6·3H2O,201,202 has a structure similar to 14.11 but without axial coordination by nitrate. In the solid state hydrogen bonding between the two Pt2 units appear to stabilize the structure. The Pt–Pt distances are very similar to one another (2.702(6) Å, 2.710(5) Å and 2.706(6) Å in sequence down the chain), but are shorter than those in the _-pyridone blue. This is consistent with the _-pyrrolidone tan being in the higher average oxidation state of Pt(2.5+). Kinetic studies indicate that in solution this compound disproportionates into [PtIII2(NH3)4(pyrr)2]4+ and [PtII2(NH3)4(pyrr)2]2+, both of which are diamagnetic.203 This “tan” complex can be oxidized further204 by [S2O8]2- in a strongly acidic medium to give the yellow tetranuclear cation [Pt4(pyrr)4(NH3)8]8+ in which each platinum is formally PtIII. This diamagnetic species has been isolated in various salts in which there are mixtures of anions present (e.g., [Pt4(pyrr)4(NH3)8](SO4)2(ClO4)4·6H2O), and it has the interesting property of oxidizing water to molecular oxygen.204 The reaction of [Pt4(pyrr)4(NH3)8](NO3)6·2H2O with excess pyrazine in water has given the [PtIII]4 complex [(NO3)(NH3)2Pt(pyrr)2Pt(NH3)- (µ-NH2)]2(NO3)4 in very low yield.205 The Pt–Pt distances are 2.608(1) Å for the outer Pt–Pt bonds and 3.160(2) Å for the interdimer separation.

An early study206 of the spectroscopic, redox and chemical properties of cis-diammineplatinum _-pyridone blue established its close relationship to various other platinum blues, including the platinum acetamide blue and cis-diammineplatinum uracil blue. Such materials were shown to share the properties of mixed valence and oligomeric structure. More recent studies have confirmed these conclusions. Of special note is the isolation and structural characterization of a linear octanuclear platinum acetamide complex [Pt8(NHCOCH3)8(NH3)16](NO3)10·4H2O, in which the average oxidation state is Pt(2.25+).207 The Pt–Pt bonds are alternately supported (short) and unsupported (long) by bridging acetamido ligands and have lengths of 2.880(2) Å, 2.900(1) Å, 2.778(1) Å and 2.934(1) Å in this centrosymmetric structure. The complex is diamagnetic and shows no EPR signal.207 Another important result has been the confirmation that the tetranuclear uracil blue [Pt4(1-MeU)4(NH3)8](NO3)5·H2O, where 1-MeU is the monoanion of 1-methyluracil, has the expected structure, and a chemistry that closely mirrors that of the _-pyridone blues.197,208-210 Platinum uridine blue and uridine green compounds have also been prepared and characterized by a variety of spectroscopic methods.140 The uridine green species exhibits antitumor activity. Cationic platinum blues based on isonicotinamide, malonamide and biuret have been described.211

With the base 1-methylthymine, a complex of composition {[Pt2(MeT)2(NH3)2Cl2]2Cl}- (PtCl6)·6H2O has been obtained in which the average oxidation state is Pt(2.75+).212 However, this contains two cis-[(NH3)2Pt(µ-MeT)2PtCl2] units that are linked by a single chloride bridge and so is not a tetranuclear Pt4 linear cluster of the type usually encountered. The Pt–Pt distance within the individual Pt2 units is 2.699(1) Å.212 Platinum blue compounds containing acetamide and bypyridine groups have been described213 and the structure of a tetranuclear compound shows long outer distances of 2.908(2) Å and even longer inner distances of 3.209(4) Å.

Nickel, Palladium and Platinum Compounds 661

Murillo

Recently, a series of partially oxidized 1-D platinum chain complexes consisting of carboxylate-bridged cis-diammineplatinum dimer units have been crystallographically characterized. These compounds have the formulae [Pt(2.2+)2(acetato)2(NH3)4](NO3)2.4·2H2O and [Pt(2.2+)2(propionato)2(NH3)4](NO3)2(ClO4)0.4·2H2O. In both compounds the intra Pt–Pt distances range from 2.81 to 2.85 Å while the interdimer distances are c. 3.0 Å.214 The dimerdimer associations are stabilized by four hydrogen bonds formed between the ammine groups and the carboxylate anions.

A strategy has been developed215,216 for the synthesis of trinuclear mixed Pt2Pd complexes of composition cis-[A2PtL2PdL2PtA2]X2 and cis-[A2PtL2PdL2PtA2]X3, where L = 1-methylura- cilato (1-MeU) or 1-methylthyminato (1-MeT), A2 = 2NH3 or en, and X = NO3- or ClO4-. In the paramagnetic tri-cations the average metal oxidation state is M(2.33+), so that one of the three metals is MIII; this is believed to be PdIII. These intensely purple-blue colored compounds are considered models of a trinuclear platinum pyrimidine blue.

While it should be emphasized that the platinum blues which are the most thoroughly characterized, and whose chemistry is the most fully developed, are those that contain monoanionic bridging ligands with N,O donor atoms, other blues have been described. Examples include diammine platinum blues that are said to contain bridging monohydrogenphosphato and nitrito ligands,217 although full structural details of these compounds remain to be elucidated.

14.4.6 Other compounds

Some unusual molecules contain C-bound quinone or quinone-like bridges between the singly-bonded Pt26+ units such (Bun4N)2[Pt2(µ-C6F4O)2(C6F5)4] and (Bun4N)2[Pt2(µ- C6F4O(CH3)2)2(C6F5)4]. The Pt–Pt distances are 2.570(1) and 2.584(1) Å, respectively.218 Reaction of mesotetraphenylporphyrin dimethyl ether, H2MP, and K2[PtCl6] in boiling pyridine produces a compound of composition ClPtIIIMP,219 which has not been structurally characterized. It is possible that it might resemble compounds such as those of Ru and Ir which are known to contain unsupported metal–metal bonds. Several hydroxide and peroxide containing platinum compounds have been claimed to be diplatinum(III) species, but confirmation is lacking.220 As yet, there is no proof of their identity from X-ray crystallographic studies, their structures having been inferred from microanalytical data, infrared spectroscopy and potentiometric titrations. Finally, electrochemical and chemical oxidation using NO+ of the antitumor, organometallic compound [Pt{((p-HC6F4)NCH2)2}(py)2] have produced some moderately stable diamagnetic species that have been attributed to the formation of bridged complexes containing Pt–Pt bonds but no structural characterization has been offered.221

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