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

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replaced by NH3, py, OH- or NO2-, was first reported in 1980.39 Compounds with the chiral alcohol R(-)-2-amino-1-butanol (Amb), similar to that of the sulfate reported above,38 are also known to have formulas K2[Pt2(HPO4)4(Amb)2](Amb), NH4(Amb)[Pt2(HPO4)4(Amb)2]- (Amb) and (AmbH)2[Pt2(HPO4)4(Amb)2](Amb). The synthetic strategy for the preparation of [Pt2(HPO4)4L2]2-, which is similar to that used to prepare [Pt2(SO4)4]2-, involves the reaction of cis- or trans-Pt(NO2)2(NH3)2 with concentrated H3PO4, usually with heating.32,39,40 The use of K2[Pt(NO2)4] in place of Pt(NO2)2(NH3)2 has also been advocated.31 The structural identity of these compounds has been established from crystal structure determinations32,41-43 on several salts of the type M2I[Pt2(HPO4)4(L)2], the results of which are summarized in Table 14.1. Detailed studies have been made of the stepwise displacement of the H2O molecules in [Pt2(HPO4)4(H2O)2]2- by halide (Cl- and Br-),44 and various amine, thioether and thiolato ligands43 and rate constant data and equilibrium constants determined for several of these systems. In these studies,43,44 use was made of the sensitivity of the metal-based μΑμ* transition to the nature of the axial ligand. For example, in the cases where L is H2O, py, Cl-, Br- and I-, this band is at 224, 294, 296, 342 and 410 nm, respectively.43,44 Luminescence from the dμ*excited state of [Pt2(HPO4)4(H2O)2]2- and [Pt2(HPO4)4X2]4- (X = Cl or Br) has been studied45 for solids and low-temperature solution glasses. 195Pt NMR spectral measurements on adducts of [Pt2(HPO4)4]2-, including those where the two axial ligands are different, show31,37 that 1J(Pt–Pt) is always larger in magnitude than for the corresponding sulfato-bridged complexes, although ι(Pt–Pt) from the Raman spectra and X-ray structural data do not indicate a stronger Pt–Pt bond.

In a few instances, diplatinum(III) complexes have been isolated that contain monoanionic dihydrogenphosphato [H2PO4]- bridges. Complexes of the type (BH)[Pt2(H2PO4)- (HPO4)3(B)2]·H2O, where B = pyridine, 4-methylpyridine or 3,4-dimethylpyridine, are apparently present as minor contaminants in the complexes of stoichiometry (BH)2[Pt2(HPO4)4(B)2] that are formed by reacting these heterocyclic tertiary amines with phosphoric acid solutions of [Pt2(HPO4)4]2-.40 However, when 4-phenylpyridine is used as the base (4-PhpyH)- [Pt2(H2PO4)(HPO4)3(4-Phpy)2]·H2O is the major product.40 During attempts to grow single crystals of (pyH)[Pt2(HPO4)4(py)2], a crystalline sample of (pyH)[Pt2(H2PO4)- (HPO4)3(py)2]·H2O was obtained and structurally characterized.40 Subsequently, crystals of the bis-dihydrogenphosphato complex (Et4N)2[Pt2-(H2PO4)2(HPO4)2Cl2]·H2O were obtained upon treating (NH4)2[Pt2(HPO4)4(H2O)2] with Et4NCl in water.32 The measured Pt–Pt distance (Table 14.1) is one of the longest of all the structurally characterized phosphato (and sulfato) bridged diplatinum(III) anions.

The reaction of Na2[Pt2(HPO4)4(H2O)2] with the ligand guanine (guH2) gives the complex Na2[Pt2(HPO4)4(guH2)2],43 which upon dissolution in aqueous NaOH has been found46 to afford

crystals of composition Na10[Pt2(PO4)4(C5H3N5O)2]·22H2O, where C5H3N5O is the dianion of guanine. This complex is the first example46 of a structurally characterized diplatinum(III)

complex with a fully deprotonated phosphate bridge.

Other than studies of the substitutional lability of the axial ligands of the aforementioned diplatinum(III) phosphate complexes, investigations of their reactivity have been limited. The salt (NH4)2[Pt2(HPO4)4(H2O)2] reacts with concentrated H2SO4 to afford the corresponding sulfate derivative (NH4)2[Pt2(SO4)4(H2O)2].40 Other reactions have demonstrated that the Pt–Pt bond is subject to reductive cleavage. Thus, the reaction of (pyH)2[Pt2(HPO4)4(py)2] with PPh3 in water gives a product that has been formulated as Pt2(HPO4)3(PPh3)3(H2O)2, whereas in refluxing glacial acetic acid this reaction proceeds further to give mononuclear Pt(O2CCH3)2(PPh3)2.40 The complex Pt(CN)2(CNBut)2 is formed40 when (pyH)2[Pt2(HPO4)4(py)2] is treated with ButNC in refluxing methanol.

644Multiple Bonds Between Metal Atoms Chapter 14

14.4.2 Complexes with pyrophosphite and related ligands

Another very important class of diplatinum(III) complexes are those that contain the dianionic pyrophosphite ligand [P2O5H2]2- shown as 14.3. This chemistry has its origins in the important discovery of the non-metal-metal-bonded diplatinum(II) complex K4[Pt2(P2O5H2)4]·2H2O by Roundhill and co-workers47 in 1977. This compound, which is usually referred to as “platinum pop”, is prepared by the reaction of K2PtCl4 with phosphorous acid.47,48 Some minor modifications in the original procedure have been recommended,49 and this procedure has also been adapted for the synthesis of other salts of the type MI4[Pt2(pop)4] (MI = Na+, Bun4N+, Ph4As+ or ½Ba2+)49 as well as the corresponding derivative with the dianion of methylenebis(phosphinic acid), CH2[PH(O)(OH)]2, (pcpH), which is of composition K4[Pt2(pcp)4]·6H2O.50,51 Several properties of these complexes have proven to be of great interest, including the excited-state chemistry of [Pt2(pop)4]4- which appears to be18,52,53 the richest of that exhibited by any d8-d8 complex. However, of most significance to the subject of our monograph is the ease of the oxidation of [Pt2(pop)4]4- and [Pt2(pcp)4]4- to diplatinum(III) species.

HO O OH

PP

OO

14.3

The thermal and photochemical oxidative-additions of halogens (Cl2, Br2, I2)49,54-56 and alkyl54,57 and aryl halides57 to [Pt2(pop)4]4- provide a ready route to diplatinum(III) complex anions of the types [Pt2(pop)4X2]4- and [Pt2(pop)4(R)X]4-, several of which have been structurally characterized (Table 14.1).54,58-62 The electrochemical oxidation of [Pt2(pop)4]4- in the presence of X- also provides a means of generating [Pt2(pop)4X2]4-,63 while the photolysis of [Pt2(pop)4]4- in methanolic solutions of CHCl3 and CCl4 produces [Pt2(pop)4Cl2]4-,64 as does the thermal reaction of [Pt2(pop)4]4- with NOCl.65 The analogous halogen-containing pcp species [Pt2(pcp)4X2]4- are prepared from the reactions of [Pt2(pcp)4]4- with X2 (X = Cl, Br or I), and the complex K4[Pt2(pcp)4Cl2]·8H2O has been structurally characterized by X-ray crystallography.51 The lability of the axial halide ligand sites in [Pt2(pop)4X2]4- has been used49,55,56 as a means to generate mixed-halide species of the type [Pt2(pop)4XY]4-.

Other diplatinum(III)-pop complexes have been prepared from the reactions between [Pt2(pop)4]4- and various nucleophiles under oxidizing conditions (e.g., the presence of O2 or H2O2). By this means, salts of the types [Pt2(pop)4X2]4- (X = NO2- or SCN-) and [Pt2(pop)4L2]2- (L = H2O, nicotinamide, py or CH3CN) have been prepared,49,66,67 and several have been crystallographically characterized (Table 14.1).66,67 As an alternative means of synthesizing the bis-nitrito complex, the reaction between [Pt2(pop)4]4- and NO2 can be used.65 Interestingly, when attempts were made to grow single crystals of a salt of this complex anion by the reaction of an acidic solution of K4[Pt2(pop)4]·2H2O with NaNO2 in a sealed tube over a period of 30 days, the product turned out to be of composition Na8[Pt2(pop-H)4(NO2)2]·18H2O, and contained the monodeprotonated pop ligand.65 However, the anion is structurally very similar to that of [Pt2(pop)4(NO2)2]4- as characterized in the salt K4[Pt2(pop)4(NO2)2]·2KNO2·2H2O.66 The reactions of [Pt2(pop)4(NO2)2]4- with Cl- or Br- give [Pt2(pop)4(NO2)X]4-, while the reaction of 1 equiv of N-iodosuccinimide produces [Pt2(pop)4(NO2)I]4-.65 When [Pt2(pop)4(NO2)2]4- is treated with CO the following redox reaction is believed65 to occur:

[Pt2(pop)4(NO2)2]4- + 2CO Α [Pt2(pop)4]4- +2NO + 2CO2

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There is no evidence for the stabilization of diplatinum(III) by the nitrosyl ligand.65 Indeed, the reaction of [Pt2(pop)4]4- with NOCl produces only [Pt2(pop)4Cl2]4-.

Crystal structure data for the pyrophosphito-bridged and methylenebis(phosphito)-bridged diplatinum(III) complexes (Table 14.1) reveal closely related structures in all cases (see, for example, Fig. 14.4). These differ only to the extent of having either P–O–P (for pop) or P–CH2–P (for pcp) bridgehead units. The Pt–Pt distances, which vary over a range of about 0.1 Å (i.e. 2.676(1) Å to 2.782(1) Å), are shorter than the distances of 2.925(1) Å and 2.9801(2) Å in the diplatinum(II) analogs K4[Pt2(pop)4]·2H2O68,69 and K4[Pt2(pcp)4]·6H2O,51 respectively. The variations in Pt–Pt distances reflect a trans influence of the axial ligands. The P–O bond distances fall into three classes: P–OH, P=O and P–O(bridging). The terminal P–O groups are linked through O–H···O hydrogen bonds around the periphery of each planar PtP4 unit (see Fig. 14.4). In the case of Na8[Pt2(pop-H)4(NO2)2]·18H2O, which contains singly deprotonated pop ligands,65 the hydrogen-bonding network is more complicated and the Na+ ions are variously O–bonded to terminal P–O and H2O groups.

Fig. 14.4. The structure of the [Pt2(pop)4I2]4- anion as viewed down the Pt–Pt bond showing the O–H···O bonding around the periphery of each PtP4 unit.

The spectroscopic properties of these diplatinum(III) complexes have proven to be of considerable interest. Several studies have focused on the electronic absorption49,51,54,57,66,67 (including the MCD)70 and vibrational spectra,59,71,72 with a particular focus upon the dμ Α dμ* electronic transition and the influence of the axial ligands upon it and the vibrations of the X–Pt–Pt–X unit. Both ι(Pt–Pt) and ι(Pt–X) (X = halide) vibrations have been assigned. NMR spectral characterizations (especially the 195Pt and 31P spectra) have also attracted attention.51,57,65 Although electronic emission from singly-bonded d7-d7 complexes does not normally occur, salts of [Pt2(pop)4X2]4- (X = Cl, Br, SCN) and [Pt2(pop)4(py)2]2- have been found73 to exhibit strong red luminescence at 77 K.

The dμ*pμ (3A2u) excited state of [Pt2(pop)4]4- is a powerful one-electron reductant.18 Among its many interesting reactions are those with various hydrogen donors74-77 to give [Pt2(pop)4H2]4-. Detailed NMR and IR spectroscopic studies77 show that like other diplatinum(III)-pop complexes with X–Pt–Pt–X units, this compound possesses a linear H–Pt–Pt–H unit. Among the reactions of the electronically excited state of [Pt2(pop)4]2- is that with HNO3 in which NO2- is formed.78 The species [Pt2(pop)4]3- and [HNO3]- have been identified as intermediates. This same mixed-valence Pt25+ species [Pt2(pop)4]3- has also been formed by the photoionization of [Pt2(pop)4]4- in aqueous solution and by its thermal reaction with OH• radicals generated by pulse radiolysis.79 This unstable species has been spectroscopically characterized but it rapidly disproportionates to [Pt2(pop)4]4- and [Pt2(pop)4]2-.79 Interestingly, pulse radiolysis

646Multiple Bonds Between Metal Atoms Chapter 14

has also been used80 to reduce aqueous solutions of several diplatinum(III) complexes to their Pt25+ congeners.81 The species [Pt2(pop)4X2]4- (X = Cl, Br, SCN, imidazolyl) and [Pt2(pop)4(l- MeIm)2]2- (1-MeIm = 1-methylimidazole) have been reduced by this means; they exhibit a very characteristic, intense μ Α μ* transition in the near-UV region (X = 1-MeIm or Im, 310; Cl, 330; Br, 370; SCN, 390 nm).80 The UV-visible irradiation of these [Pt2(pop)4X2]4- species in methanol leads to reduction to [Pt2(pop)4]4- in essentially quantitative yield.82 In a similar context, irradiation at 313 nm into the μ Α dμ* absorption band of [Pt2(pop)4H2]4- quantitatively produces [Pt2(pop)4]4- and H2.77 The thermal chemistry of [Pt2(pop)4H2]4- includes its reactions with HCl and DCl to produce H2 and HD.77

A class of mixed-valence Pt25+ complexes that have attracted attention are the potassium salts of composition K4[Pt2(pop)4X]·3H2O (X = C1, Br, I). These are described in Section 14.4.6.

14.4.3 Complexes with carboxylate, formamidinate and related ligands

In contrast to the proclivity of the singly-bonded Rh24+ species to be stabilized by a wide range of carboxylate bridging ligands, the isoelectronic Pt26+ species are rare. The only authentic, simple diplatinum(III) carboxylates that have been structurally characterized are of the type [Pt2(O2CCH3)4(H2O)2]X2, X = ClO483,84 and CF3SO3.83 The cation is represented as 14.4. The corresponding Pt–Pt distances of 2.391(1) and 2.393(1) Å are the shortest distances known for any Pt26+ species (Table 14.1). The perchlorate compound has been prepared by reaction of K2[Pt2(NO2)4] and a 1:1 mixture (by volume) of acetic acid and 1 M perchloric acid while carefully controlling the temperature at 100 ˚C for 4 h:

K2[Pt(NO2)4]	CH3COOH/HClO4	[Pt2(CH3COO)4(H2O)2](ClO4)2
	100 °C

CH3

2+

CH3

C C

O O O O

H2O		Pt		Pt		OH2

O O O O

CC

CH3

CH3

14.4

It is noteworthy that use of a mixture of acetic acid and perchloric acid is very important for the success of the reaction. Otherwise mixtures of compounds having Pt in oxidation states of 2, 3 and 4 are observed.85 Nitric acid is also useful but the yield of the corresponding nitrate is lower and the product is sometimes contaminated with K2[Pt(NO2)6]. As in the sulfate and phosphate analogs, the axial water molecules can be substituted by a series of donor molecules such as DMF, SMe2 and pyridine or anions such as Cl- and Br-. The substitution reactions have been followed by 195Pt and 13C NMR spectroscopy.83 Preparation of Pt2(CH3COO)4Cl2 has been accomplished in nearly 100% yield by reaction of [Pt2(CH3COO)4(H2O)2](CF3SO3)2 and SOCl2.86

The compound Pt2(O2CCH3)6 has also been claimed87 as a product of the reaction of K2Pt(OH)6 with formic acid in glacial acetic acid. The analogous trifluoroacetate, Pt2(O2CCF3)6·4H2O, as

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well as the mixed acetate-trifluoroacetate of composition Pt2(O2CCH3)3(O2CCF3)3 have also been reported.88 The reaction conditions that are used for the preparation of Pt2(O2CCH3)6 can apparently be modified to produce materials that have been formulated as Pt4(O2CCH3)4(OH)8(H2O)4,89 Pt4(O2CCH3)10(OH)290 and Pt4(O2CCH3)4(OH)8(H2O)2.90 The last two of these have been characterized by the EXAFS technique.89 These tetranuclear formulations, if correct, may accord with a similar nuclearity for the ‘parent’ platinum(III) acetate, viz Pt4(O2CCH3)12, a possibility that is supported87 by molecular weight measurements. It is noteworthy that platinum(II) acetate is tetranuclear Pt4(µ-O2CCH3)8, with short Pt–Pt distances (2.49-2.50 Å).91 The correctness of these earlier structural conclusions remains clouded.

Another structurally characterized diplatinum(III) acetate complex is encountered in the salt Cs3[Pt2(µ-O2CCH3)2(µ-CH2COO-C,O)2Cl2]C1·3H2O,92 (14.5) in which there are two cis bridging O,O-bound acetate ligands as well as two singly deprotonated ones that have an unusual C,O-bridging mode. This complex, which was isolated in very low yield from a K2PtCl4/CH3CO2Ag/CH3CO2H–H2O (10:1) reaction mixture,92 has 1H and 13C NMR spectra that are fully consistent with this structural result. The Pt–Pt distance of 2.451(1) Å is slightly longer than those mentioned above where all carboxylate groups are bound through the oxygen atoms.

14.5

Many dithiocarboxylates Pt2(S2CR)4X2 (X = Cl, Br, I) have been prepared and structurally characterized.93-95 Treatment of the diplatinum(II) complexes Pt2(S2CR)495,96 with the halogens gives diamagnetic Pt2(S2CR)4X2 compounds (R = CH3, for X = Cl and Br, and R = CH3, (CH3)2CH, CH2Ph or C2H5 for X = I).93-95 The iodo complexes where R = (CH3)2CH,94 CH2Ph,94 and C2H595 have been structurally characterized and found to have Pt–Pt distances of 2.598(2), 2.578(1) and 2.582(2) Å, respectively. By adjusting the stoichiometry of the Pt2(S2CCH3)4/I2 reaction, the mixed-valence compound Pt2(S2CCH3)4I can be isolated.93,95 This work is described in Section 14.4.6.

Several diplatinum(III) complexes of stoichiometry Pt2(O2CR)2R'4(SR"2)2 that contain a pair of cis bridging carboxylate ligands (R = CH3, CF3 or (CH3)2CH), four equatorial alkyl or aryl groups (R' = CH3, Ph or p-tolyl) and two axial thioether ligands (R" = Et, Prn or Pri) have been prepared97,98 by the oxidation of Pt2(µ-SR"2)2(CH3)4 with AgO2CR, Hg(O2CR)2 or Tl(O2CCH3)3. The axial thioether ligands can be replaced by pyridine, 4-methylpyridine, PhNH2 or Cl-, giving Pt2(O2CR)2R'4L2 complexes. The structural identity of these compounds has been substantiated by crystal structure determinations of cis-Pt2(O2CCF3)2(CH3)4(4-Mepy)299 and cis- Pt2(O2CCH3)2(CH3)4(py)2.100 When PEt3 or P(OMe)3 are reacted with Pt2(O2CCH3)2R'4(SR"2)2, the 1:1 adducts Pt2(O2CCH3)2R'4(PR3) are formed.98 These probably have asymmetric structures with one Pt center six coordinate and the other five coordinate.

648Multiple Bonds Between Metal Atoms Chapter 14

There are only two paddlewheel compounds having a Pt26+ core surrounded by four bridging ligands having all-N donor atoms. These are the formamidinate complex Pt2(DPhF)4Cl2 which has a rather long Pt–Pt distance of 2.517(1) Å86 and the guanidinate compound Pt2(hpp)4Cl2 that has a significantly shorter Pt–Pt distance of 2.438(1) Å.101,102 The latter is just slighly longer than those in the carboxylate analogs mentioned above (Table 14.1). The formamidinate compound has been prepared using a reaction of Pt2(CH3COO)4Cl2 and molten HDPhF according to:

Pt2(CH3COO)4Cl2 + 4HDPhF 6 Pt2(DPhF)4Cl2 + 4CH3COOH

14.4.4 Complexes containing monoanionic bridging ligands with N,O and N,S donor sets

There are some amidate analogs of the Pt2(CH3COO)4X2 compounds described above. This is a group of compounds for which the existence of a Pt–Pt bond had been proposed but direct proof had been lacking until recently. This story goes back to the early 1950s when a series of what were then assumed to be acetamido-Pt(II) complexes was reported.103 Formulae such as 14.6 were suggested. However, in 1967, it was proposed,104 without evidence, that the compounds are in fact diplatinum(III) compounds with structures such as 14.7. Subsequently, XPS studies105 provided evidence that the platinum atoms are equivalent and in oxidation state +3. Thus, in six compounds the binding energies (Pt 4ƒ7/2, eV) were 75.0 ± 0.2, while the typical values for PtII and PtIV are 73.6 ± 0.8 and 76.3 ± 1.5, respectively. A value of 75.2 eV has been found for the sulfato compound K2[Pt2(SO4)4(H2O)2].106 An analysis of the radial distribution functions, obtained from the X-ray powder diffraction patterns, has been used107 to determine the structure of the nitrito derivative Pt2(CH3CONH)4(NO2)2. The structure was concluded to be as represented in 14.7, except that the disposition of equatorial ligands about each Pt atom gives a trans-PtN2O2 geometry. The Pt–Pt distance was reported to be 2.455 Å.107

CH3

C

CH3

HN

O

C

O

NH

X

O

X

Pt

Pt

X

HN

O

CO

NH

O

Pt

NCCH3

H3C

C

H

C

NH2

CH3

CH3

14.6

14.7

Another synthetic procedure for the preparation of Pt2(CH3CONH)4Cl2·2H2O involves the reaction at 90 ˚C for 16 h of an amide and a 1:1 mixture of K2PtCl6 and K2PtCl4 in aqueous solution.108 In these complexes axial ligands are labile and they can be replaced by other donor groups, e.g., pyridine.

The first compound of this class to be fully characterized crystallographically was Pt2(CH3CONH)4I2. It has a Pt–Pt distance of 2.473(2) Å.109 The chloro derivative having pivaloamide bridges is also known and has a metal-metal distance of 2.448(2) Å.110 These distances are short but slightly longer than those in the acetate analogs (Table 14.1). In the

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two compounds interstitial water molecules help stabilize the crystal by forming a complex network of hydrogen bonds. In the solid state the compounds are symmetrical with two cis amidate ligands having the nitrogen atoms pointing in one direction and the other two cis ligands having the oxygen atoms pointing in the same direction as the nitrogen atoms of the other ligands. However, isomers are observed in the NMR spectra. In the presence of halogens, the acetamidate ligands retain the binuclear Pt26+ cores at low temperature but halogenation occurs at the methyl and NH group of the acetamidate ligand.111 At higher temperature further oxidation to PtIV species occurs.

The axially coordinated halide ions are substituted by the neutral ligands triphenylphosphine or water after reaction with AgNO3.112 In this way, compounds with formulae [Pt2(PPh3)2(RCONH)4](NO3)2, R = CH3, But and [Pt2(H2O)(PPh3)(ButCONH)4](NO3)2 have been prepared. The corresponding Pt–Pt distances for the But derivatives are 2.504(1) and 2.468(1) Å.112 In solution, these compounds form isomers differing in the arrangement of the sets of equatorial ligands around each platinum atom, N3O/NO3 or N2O2, and for the latter there are cis and trans arrangements.

Reactions of Pt2(CH3CONH)4X2, X = Cl and I, with aqueous ethylenediamine solutions give the mixed acetamido-ethylenediamine complexes cis-[Pt2(CH3CONH)4(en)2]X2. The crystal structure of the iodide has been determined (Fig. 14.5).113 There are two acetamidato bridges in a cis disposition to one another, two monodentate, monodeprotonated acetamide ligands and two chelating en ligands that show equatorial and axial binding. The Pt–Pt distance of 2.566(1) Å is significantly longer than those of the precursors, a common occurrence in dimetal complexes that have less than four bridging ligands. An even longer Pt–Pt distance of 2.609(1) Å is found in cis-[Pt2(ButCONH)2(NH3)4(NO2)(NO3)].114 The interaction of cis- [Pt2(CH3CONH)4(en)2]Cl2 with chlorine has been studied at room temperature and on heating. In both cases the Pt26+ core is retained but chlorination occurs at the methyl group at room temperature but on heating replacement of the H atom of the NH group occurs also.115 Several platinum acetamide complexes, including some where the ethylenediamine ligands have been replaced by bipyridine, have been studied by XAFS photoelectron spectroscopy and other techniques such as XANES and EXAFS.116

Fig. 14.5. The structure of the [Pt2(CH3CONH)4(en)2]2+ cation, where en = ethylendiamine.

There is also an extensive organometallic chemistry of amidate-bridged, singly-bonded Pt26+ species, particularly where the bridges are pivalamidate (ButCONH). These compounds commonly contain cations of the type cis-[Pt2(ButCONH)2(NH3)4(alkyl)]3+. In one axial position

650Multiple Bonds Between Metal Atoms Chapter 14

there is an alkyl group with a strong Pt–C bond; the other axial position is generally empty or occupied by a very weakly bound anion such as nitrate. These studies have been done primarily in K. Matsumoto’s laboratory and have been reviewed recently.117,118 Most of these compounds are made by reaction of the platinum blue species [Pt4(ButCONH)4(NH3)4]5+ (see Section 14.4.7). For example, reaction with ketones in the presence of either HNO3 or Na2S2O8 gives the corresponding ketonylplatinumIII complexes according to:

When the ketone has two _-C–H bonds, a mixture of isomers is possible. Several of these compounds have been structurally characterized (Table 14.1).114,119,120 One example is shown in

Fig. 14.6. In general, there is a strong trans effect of the Pt–Pt bond that makes the Pt–O(nitrate) separation very long (more than 2.6 Å.) This is significantly longer than typical Pt–O(nitrate) distances of 2.16-2.36 Å found in other complexes with Pt26+ cores.

Fig. 14.6. The structure of the cation in the organometallic compound cis-[Pt2(ButCONH)2(NH3)4(CH2COCH3)(NO3)](NO3)2.

Structurally characterized Pt–C bonds can be made also by reaction of cis-Pt2- (ButCONH)2(NH3)4(NO3)2 with olefins,121 1,3 conjugated dienes122 and alkynes123 in aqueous solution. With dienes, the 4-hydroxy-(E)-2-alkenyl-PtIII dinuclear complexes are formed. In these compounds, the _-carbon atom of the axially coordinated alkenyl ligand bound to the

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PtIII atom is electrophilic and is easily attacked by water to release (E)-2-alkene-1,4,diol. Some reactions are summarized in the equation below:

With monoolefins such as cyclopentane or cyclohexane, double nucleophilic attacks by methanol and water occurs according to:

Mechanistic studies of ketone and alcohol formation from alkenes and alkynes,124 and of axial ligand substitution reactions of the olefin derivatives with p-styrenesufonate or 4-pentane- diol,125 and of replacement of various axial ligands with halide ions126 have been published.

The type of structure shown in Fig. 14.5 in which there are only two bridging ligands in a cis arrangement is actually quite common for diplatinum(III). Thus it is not surprising that the bis-µ-carboxylato complexes of the general type Pt2(O2CR)2R'4L2 that were mentioned in the preceding section have analogs in which various 2-hydroxypyridinato (also known as _-pyri- donato) ligands replace the acetates.100,127-129 The reactions of Pt2(µ-SEt2)2(CH3)4 and Ag(Xhp), where Xhp represents the monoanion of 2-hydroxypyridine (hp), 2-hydroxy-6-fluoropyridine (fhp), 2-hydroxy-6-chloropyridine (chp), 2-hydroxy-6-bromopyridine (bhp) or 2-hydroxy- 6-methylpyridine (mhp), in benzene followed by filtration and the addition of pyridine affords100,127 the pyridine adducts Pt2(Xhp)2(CH3)4(py)n, with n = 2 for X = H or F and n = 1 for X = Cl, Br or CH3. For all five complexes there is a cis arrangement of bridging Xhp ligands, with the details of the geometry and stoichiometry being dependent upon the size of X.100,127 When X is relatively small (H or F) the complex contains a non-polar arrangement (head-to- tail) of bridging ligands and has both axial positions occupied (14.8). An increase in the size of X (to Cl, Br or CH3) leads to a polar (head-to-head) arrangement of cis bridging Xhp ligands with only one axial site occupied by pyridine, namely, that which is less sterically congested and involves the Pt center which is coordinated by two O atoms from the Xhp ligands (14.9).

652Multiple Bonds Between Metal Atoms Chapter 14

When the bis-pyridine adducts Pt2(hp)2(CH3)4(py)2 and Pt2(fhp)2(CH3)4(py)2 are subjected to chromatography on a silica gel column they convert to the polar head-to-head monopyridine adducts,127 a conversion that can be reversed upon treatment of the latter with pyridine. The analogous 1:1 adducts with diethylsulfide have also been isolated for the bis-hp, fhp and mhp complexes, and all have been structurally characterized (Table 14.1). These have the expected head-to-head arrangement of Xhp ligands. NMR spectral properties have been used129 to determine the formation constants of the 1:2 pyridine adducts from the reactions of the 1:1 head-to-head isomers with pyridine. It has been suggested129 that the head-to-head Α head- to-tail rearrangement may involve a pre-equilibrium with pyridine, and the opening of a Xhp bridge followed by its rearrangement. A mechanism involving rupture of the Pt–Pt bond is not favored.

N

O

N

O

O

N

N

O

L

Pt

Pt

L

Pt

Pt

L

H3C

H3C

H3C

H3C

CH3

CH3

CH3

CH3

14.8

14.9

It should be noted that the beginnings of this chemistry had its origins in efforts during the early 1980s to unravel the structural secrets of the oligomeric platinum blues (see Section 14.4.7). In a series of reports that were published in the period 1981-83, Stephen J. Lippard and co-workers described130-133 the synthesis of several diplatinum(III) species of the type [Pt2-

(hp)2(NH3)4XY]n+,where n = 3 when X = H2O and Y = NO3, and n = 2 when X = Y = NO3, NO2, Cl or Br, by the chemical oxidation of cis-diammineplatinum _-pyridone blue with nitric acid,130,132 or the oxidation of the diplatinum(II) complex [Pt2(hp)2(NH3)4](NO3)2·2H2O with nitric acid, either alone130-132 or in the presence of NO2-, Cl- or Br-.133 The structural characterization of these compounds (Table 14.1) shows that all of them have very similar structures, with cis bridging hp ligands that are in a head-to-tail arrangement except when the axial ligands are different (i.e. X = H2O when Y = NO3). The structure of the latter complex cation is shown in Fig. 14.7. The Pt–Pt bond lengths are dependent upon the nature of the axial ligands and follow the order Br- 5 NO2- > Cl- > NO3-,133 a trend that parallels the known trans-influ- ence for these ligands. An electrochemical study on the head-to-tail nitrate isomer shows132 that it undergoes a concerted two-electron reversible reduction, the Pt26+ and Pt24+ species being cleanly interconverted upon controlled potential electrolysis. For the head-to-head isomer the reduction takes place in two one-electron steps which differ by 50 mV; exhaustive electrolytic reduction of this complex forms the corresponding _-pyridone blue.132

The mixed nitrito-nitrato complex cis-[Pt2(hp)2(en)2(NO2)(NO3)](NO3)2·0.5H2O, which possesses a head-to-head arrangement of cis bridging hp ligands, is formed134 upon oxidation of [Pt2(hp)2(en)2](NO3)2 with nitric acid or NaNO2 in nitric acid; the second method gives the higher yield. The complex retains its structure in freshly prepared aqueous or DMF solutions as shown by 195Pt NMR spectroscopy.134 However, decomposition to the diplatinum(II) species slowly occurs by a mechanism that is believed134 to involve the reductive elimination of the capping nitrito ligand as the nitronium ion, NO2+.