Multiple Bonds Between Metal Atoms / 08-Rhenium Compounds

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tal structure determination of a salt of the paramagnetic [Re2Cl6(µ-dppm)2]+ cation, showed that the structure of the parent Re2Cl6(µ-dppm)2 is retained, although the Re–Re bond distance increases to 2.6823(6) Å. The monocation and monoanion are believed to each possess Re–Re bond orders of 1.5 with ground state configurations of μ2/2β*2β1 and μ2/2β*2β2/*1, respectively.206

8.14

Several other metal-metal bonded edge-sharing bioctahedral compounds have been prepared from (Bu4N)2Re2Cl8 including Re2Cl6(+-dppa)2,203 Re2Cl6(µ-Ph2Ppy)2207 and Re2(µ-SEt)2- Cl4(dto)2;208 they all very likely contain Re=Re bonds.

In the case of neutral sulfur donors, a few complexes in which a Re–Re quadruple bond is present have been obtained. The reactions of tetramethylthiourea and 2,5-dithiahexane with (Bu4N)2Re2X8 (X = Cl or Br) form complexes of the type Re2X6L2 under mild reaction conditions.97 The formation of quadruply bonded Re2X6(tmtu)2 (X = Cl or Br) contrasts with the corresponding reactions of [Re2X8]2- with thiourea in acetone or acidified methanol (HCl or HBr) whereupon cleavage of the Re–Re bond occurs to give ReX3(tu)3 (Section 8.7).97

While the 2,5-dithiahexane compounds Re2X6(dth)2 (X = Cl or Br), which can be prepared from (Bu4N)2Re2X8,97 have yet to be structurally characterized by X-ray crystallography, there is no doubt that they are authentic derivatives of the quadruple Re–Re bond. Studies on their reactivity have established209 that they can be converted in very high yield to other dirhenium(III) complexes is which quadruple bonds are present, namely, Re2X6(PPh3)2 and Re2(O2CCH3)4X2, thereby implying that such a bond is also present in Re2X6(dth)2. Although spectroscopic studies62,210 failed to resolve the structural question, the subsequent structure characterizations of closely related systems suggests that the structure of Re2X6(dth)2 is that of a symmetrical Re26+ complex, with chelating dth ligands and sulfur atoms coordinated in both axial and equatorial positions. With use of relatively mild reaction conditions Powell and coworkers211 have been able to isolate the salts (Bu4N)Re2Cl7(dth) and (Bu4N)2Re2Cl7(dto). Both have the structure represented in 8.15, and this group of compounds therefore bears a close relationship to those of the types Re2X6(PR3)2 and [Re2Cl7(PR3)2]- (vide supra), although the sulfur ligands are bidentate. The axial Re–S bond distances in (Bu4N)Re2Cl7(dth) and (Bu4N)Re2Cl7(dto) are longer by c. 0.4 Å than the corresponding equatorial Re–S bonds. The compounds (Bu4N)Re2Cl7(SS) (SS = dth or dto) are probably intermediates in the reduction of (Bu4N)2Re2Cl8 to the triply-bonded, paramagnetic complexes Re2Cl5(SS)2 (see Section 8.5.1).

8.15

302Multiple Bonds Between Metal Atoms Chapter 8

8.5Dirhenium Compounds with Bonds of Order 3.5 and 3

Much of this chemistry has been developed through a careful and deliberate mapping-out of the redox chemistry of compounds that contain a Re–Re quadruple bond.212 The explicit recognition that quadruple metal-metal bonds exist and that they may be represented by the ground state electronic configuration μ2/4β2, provides a framework upon which to consider bonds of lower orders. We shall see that there are two classes of molecules possessing metalmetal triple bonds, namely, those that contain two electrons less (i.e. μ2/4, ‘electron-poor’ triple bonds), or two more (i.e. μ2/4β2β*2, ‘electron-rich’ triple bonds) than is necessary for a full quadruple bond. In the case of dirhenium chemistry, the electron-rich triple bond is much more commonly encountered, as is the related odd-electron configuration μ2/4β2β*1, in which the metal-metal bond order is 3.5. Since the publication of the second edition of this text,10 it is this area of dirhenium chemistry that has experienced the most dramatic growth.

8.5.1 The first metal–metal triple bond: Re2Cl5(CH3SCH2CH2SCH3)2 and related species

The first redox reaction involving the [Re2X8]2- anions, which also generated the first metalmetal triple bond, was encountered97 in the reaction between (Bu4N)2Re2Cl8 and 2,5-dithia- hexane. While reaction in methanol was found to afford Re2Cl6(dth)2, upon refluxing these reagents in acetonitrile beautiful red-black dichroic crystals were obtained that exhibited spectroscopic properties quite different from those of this dirhenium(III) complex. The crystal structure revealed213 that this was in fact a truly remarkable substance which, as shown in Fig. 8.13, has two unique features. First, in spite of the retention of a very short Re–Re bond (2.293(2) Å)213 the molecule is surprisingly unsymmetrical, being composed of [ReCl4] and [Re(dth)2Cl] units. This result, when taken in conjunction with the paramagnetism of the

complex (1.72 BM per dimetal unit), led originally to the suggestion213 that it be considered - +

as the ‘zwitterion’ Cl4Re(III)–Re(II)(dth)2Cl, i.e., Cl4Re–Re(dth)2Cl. Second, the molecule possesses a staggered rotational configuration, the first such instance to be encountered for a dimetal complex then recognized as possessing a metal-metal multiple bond. Another point of interest is the way in which weak intermolecular Re–Cl···Re bridges link the dimetal units together to form a ‘molecular wire’. The absence of a β bond in this structure led to the conclusion213 that Re2Cl5(dth)2 was an example (the first one known) of a molecule containing a metal-metal triple bond. Incidentally, the original suggestion of a ‘zwitterionic’ formulation213 arose out of the anticipation that the alternative Re(IV)–Re(I) formulation (i.e. non-zwitterionic) would be less likely because of the great disparity in oxidation numbers. However, as was pointed out subsequently,212 the Re(IV)–Re(I) case is certainly not untenable since this would simply entail making one of the components to the triple bond a Re(IV)–Re(I) dative contribution (i.e. ReRe). This possibility seems reasonable in light of the electronic and molecular structure of the mixed-valent Re(IV)–Re(II) complexes (RO)2X2ReReX2(PPh3)2 (Section 8.4.2).98,141,143 In any event, the electronic structure can best be represented as μ2/4, i.e. a bond of order 3, with an additional unpaired electron occupying a singly degenerate orbital localized on that rhenium atom which is bound to four chloride ligands. While many other complexes that contain the Re25+ core have subsequently been prepared, with few exceptions these possess a μ2/4β2β*1 electronic configuration, with the unpaired electron in a singly degenerate orbital delocalized over both metal nuclei, and thus having a metal-metal bond order of 3.5. Consequently, Re2Cl5(dth)2 is accorded special attention in our present discussion because of its historical significance and since it remains to this day something of a curiosity. It is only relatively recently that other compounds of this type have been prepared and characterized. The pair of paramagnetic complexes Re2X5(dto)2 (X = Cl or Br) have been prepared from (Bu4N)2Re2X8 and structurally characterized.68 The Re–Re distances are 2.2772(8) Å (X = Cl)

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and 2.2826(6) Å (X = Br), comparing closely with that reported for Re2Cl5(dth)2. The only significant structural difference from Re2Cl5(dth)2 is the absence of weak axial intermolecular Re–X···Re interactions in the case of Re2X5(dto)2. Cyclic voltammetric measurements on solutions of Re2X5(dto)2 in 0.1 M Bu4NPF6-CH2Cl2 show the presence of reversible one-electron

reductions at E1/2 = -0.61 V (X = Cl) and E1/2 = -0.42 V (X = Br) vs. Ag/AgCl.68 There is good evidence that compounds of these type Re2X5(SS)2 (SS = dth or dto) are formed via the intermediacy of the Re26+ complexes (Bu4N)Re2X7(SS) (see Section 8.4.4).211(b)

Fig. 8.13. The structure of Re2Cl5(dth)2.

8.5.2Simple electron-transfer chemistry involving the octahalodirhenate(III) anions and related species that contain quadruple bonds

Studies of the electron transfer chemistry on quadruply bonded dirhenium complexes have been carried out, utilizing both electrochemical techniques and chemical redox reagents. The earliest attempt to study the electrochemistry of the [Re2X8]2- anions involved the polarographic reduction of acetonitrile solutions of (Bu4N)2Re2X8 (X = Cl, Br or NCS).214 This study was important because it demonstrated the feasibility of using electrochemical techniques to study these species and, furthermore, it revealed that the reduction [Re2Cl8]2-+ e Α [Re2Cl8]3-, occurring at an E1/2 of -0.82 V (vs. SCE), gave a rhenium species that was analogous to that of the already structurally characterized [Tc2Cl8]3- anion. Six years after the publication of this paper,214 the results of more detailed electrochemical studies of [Re2Cl8]2- and [Re2Br8]2- were reported;215 dc polarograms for acetonitrile solutions of these two species were in accord with the earlier results. Cyclic voltammograms (CV) were also recorded and, with HMD (hanging mercury drop) and Pt electrodes, reduction waves were found for [Re2Cl8]2- at -0.85 V and approximately -1.45 V. Controlled potential electrolysis experiments215 gave a value of n close to 1 for the first reduction. Although it was claimed215 that this reduction represents a reversible process, the ip,a/ip,c current ratio does not appear to be unity for any of the sweep rates used (between 50 and 500 mV s-1). The reduction which is at the more negative potential is clearly electrochemically irreversible. Two reduction processes were also detected by cyclic voltammetry for acetonitrile solutions of (Bu4N)2Re2Br8.215

An independent polarographic and cyclic voltammetric study of (Bu4N)2Re2Cl8 confirmed216 the presence of the quasi-reversible reduction close to -0.85 V, but evidence for a second reduction was not obtained thereby throwing doubt upon the existence of [Re2Cl8]4-. In the CVs of

(Bu4N)2Re2Cl8 shown in Fig. 8.14, the ip,a/ip,c ratios for the process at -0.85 V was found to approach a value of unity for a sweep rate of 500 mV s-1 but decreased rapidly with decreasing

sweep rate. The variation of ip,a/ip,c is due to the rapid and irreversible decomposition of the

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reduced product [Re2Cl8]3-. The resulting (unidentified) chemical product is characterized by Ep,a -0.3 V (Fig. 8.14). A suggestion by Hendriksma and van Leeuwen215 that the electrochemically reduced solutions of [Re2Cl8]2- exhibit electronic absorption spectra with features due to [Re2Cl8]3- was later refuted.217

Fig. 8.14. The cyclic voltammogram of (Bu4N)2Re2Cl8 in acetonitrile (104 M) using a Pt electrode and 0.1 M Bu4NClO4 as supporting electrolyte. Sweep rates are

(A) 20 mV s-1 and (B) 200 mV s-1

Measurements of the CV’s of acetonitrile and dichloromethane solutions of (Bu4N)2Re2Cl8, with Bu4NPF6 as the supporting electrolyte, have also revealed a reversible looking process that can be attributed to the [Re2Cl8]1-/2- couple.218-221 This is best shown for the measurements

in dichloromethane, where E1/2 values for the [Re2Cl8]1-/2- and [Re2Cl8]2-/3- couples of +1.20 V and -0.87 V (vs. Ag/AgCl),220 and +1.25 V and -0.85 V (vs. SCE),221 were obtained in two

independent studies. By resorting to low temperature spectroelectrochemical measurements, Heath and Raptis222 were able to show conclusively that in 1:1CH2Cl2/CH3CN solution at 220 K successive reversible oxidations of [Re2Cl8]2- to [Re2Cl8]- and [Re2Cl8]0 occur. Both the paramagnetic species [Re2Cl8]- and [Re2Cl8]3- have been characterized by electronic absorption spectroscopy at low temperatures and their βΑβ* transitions (at 4650 cm-1 and 6950 cm-1, respectively) identified.222,223 There is a close relationship between the various [Re2Cl8]n- and [Re2Cl9](n-1) species that will be dealt with in Section 8.5.3.

The [Re2Cl8]3- anion has also been generated and characterized in a molten salt medium. Electrochemical measurements on solutions of [Re2Cl8]2- in the aluminum chloride-1-methyl- 3-ethylimidazolium chloride molten salt have shown224 that it can be reduced to [Re2Cl8]3- at a glassy-carbon electrode in a reversible electrode process (E1/2 c. -0.58 V vs. the Al3+/Al couple). Bulk electrolysis with the use of a Pt-gauze electrode gives solutions of [Re2Cl8]3- that are stable in the absence of oxygen and which have been characterized by electronic absorption and EPR spectroscopy.225 These properties confirm the μ2/4β2β*1 electronic configuration.

Another important example of simple electron-transfer reactions involving quadruply bonded dirhenium(III) complexes is provided by the [Re2(NCS)8]2- anion. The polarographic reductions of acetonitrile solutions of (Bu4N)2Re2(NCS)8, which were seen214 at -0.04 V and -0.71 V vs. SCE (with 0.5 M Bu4NClO4 as supporting electrolyte), are apparently genuine since CV measurements on dichloromethane solutions of (Bu4N)2Re2(NCS)8 at room temperature (with Bu4NPF6 as supporting electrolyte) revealed226 electrochemical reductions with E1/2 = -0.10 V and E1/2 = -0.82 V versus SSCE, as well as an oxidation at E1/2 = +1.03 V. Low temperature spectroelectrochemical characterizations of [Re2(NCS)8]- and [Re2(NCS)8]3- by IR and electron-

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ic absorption spectroscopic techniques were carried out subsequently.70,223 These studies, which were carried out on THF70 or n-PrCN223 solutions of (Bu4N)2Re2(NCS)8, also addressed the formation of the second one-electron reduced species, [Re2(NCS)8]4-, which was characterized by IR spectroscopy.70 The temperature dependence of the chemically reversible [Re2(NCS)8]3-/4- couple at temperatures of 290 K and below was interpreted223 in terms of the 3- (9e) anion being eclipsed and the 4- (10e) anion having a staggered rotational geometry. Companion electrochemical studies70,227 on the electrochemical properties of [Re2(µ-NCS)2(NCS)8]n- species (n = 1-4) has helped provide an explanation for the ease with which [Re2(NCS)8]2- is chemically oxidized to [Re2(NCS)10]3- (see Section 8.4.1).63

The electrochemical results we have discussed so far pertain to anions that contain ligands with no particular ability to stabilize low oxidation states, at least to the extent that these species can be isolated in the solid state. In contrast, the cyclic voltammograms of dichloromethane solutions of the phosphine derivatives Re2X6(PR3)2 (X = Cl or Br and PR3 = PEt3, PPrn3, PEt2Ph, PMePh2 and PEtPh2)228 exhibit an electrochemically reversible reduction with an E1/2 value between +0.06 and -0.13 V versus SCE. These data, along with results for other Re2X6(PR3)2 complexes, and for a few closely allied [Re2Cl7(PR3)]- anions that have been reported since these early studies,228 are listed in Table 8.2. All data given for the Re2X6(PR3)2 complexes are presumably for the 1,7-isomers, except in the case of Re2Cl6(dppf), which is a 1,3-isomer type.190 Clearly, the reductions of Re2X6(PR3)2 to [Re2X6(PR3)2]- occur at much more positive potentials than does the reduction of [Re2Cl8]2- to [Re2Cl8]3-, in accord with the greater ability of phosphines (compared to halide) to stabilize low oxidation states. Furthermore, the electrochemically generated anions [Re2X6(PR3)2]- were found to have reasonable stability as evidenced by EPR spectral meaurements.228 Subsequently, it was found possible to prepare salts of some of the [Re2Cl6(PR3)2]- anions through the use of cobaltocene as a oneelectron reducing agent.178,190 These reactions, when carried out in acetone or dichloromethane, proceed as follows:

Re2Cl6(PR3)2 + (δ5-C5H5)2Co Α [(δ5-C5H5Co][Re2Cl6(PR3)2]

PR3 = PEt3, PPrn3, PMePh2, PEtPh2 or dppf

Interestingly, in a few instances several compounds of the type (Bu4N)Re2Cl6(PR3)2, where PR3 = PPrn3, PEt2Ph or ½(Ph2P(CH2)3PPh2), have been prepared directly by the reaction of the phosphine with (Bu4N)2Re2Cl8; these kinetic products must proceed via dirhenium(III) phosphine intermediates.182,189 They are discussed further in Section 8.5.4.

Several of the Re2X6(PR3)2 complexes exhibit a second reduction at more negative potentials (E1/2 -0.9 V),178,228 but the resultant species [Re2X6(PR3)2]2- are not very stable chemically. Further mention is made of the redox chemistry of Re2X6(PR3)2 when the related properties of compounds such as those of the types Re2X5(PR3)3 and Re2X4(PR3)4 are discussed in Section 8.5.4.

In contrast to the behavior presented in Table 8.2 for the Re2X6(PR3)2 and [Re2Cl7(PR3)]- species, the alkoxide-containing complexes of the type Re2X4(OR)2(PAr3)2 display markedly different electrochemical properties, with a one-electron oxidation between +0.76 and +1.01 V and a one-electron reduction between -0.63 and -0.38 V versus Ag/AgCl, the E1/2 values depending on the nature of the X, R and Ar groups.98,143 This difference clearly reflects pronounced differences in the electronic structures of these two sets of complexes. In the case of the ‘mixed-valent’ Re(IV)–Re(II) alkoxide complexes, i.e. (RO)2X2ReReX2(PAr3)2, the oxidation may be associated formally with a metal-based orbital that has more ‘Re(II)’ character and the reduction with the ‘Re(IV)’ center.

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Table 8.2. Voltammetric E1/2 values for the dirhenium(III) complexes Re2X6(PR3)2 and related species in dichloromethane

Compound	E1/2(red)(1)a	E1/2(red)(2)a	ref.
Re2Cl6(PMe3)2	+0.01b	-1.03b	179
Re2Cl6(PEt3)2	-0.10	-1.17c	228(b)
Re2Cl6(PPrn3)2	-0.11		228(b)
Re2Cl6(PBun3)2	-0.13		228(b)
Re2Cl6(PMe2Ph)2	-0.05d	-0.92d	243
Re2Cl6(PEt2Ph)2	0.00	-0.95	228(b)
Re2Cl6(PMePh2)2	+0.02	-0.95	228(b)
Re2Cl6(PEtPh2)2	-0.02	-0.99	228(b)
Re2Cl6(PBunPh2)2	-0.37d		194
Re2Cl6(dppf)	-0.03b		190
Re2Br6(PEt3)2	+0.02		228(b)
Re2Br6(PMePh2)2	+0.06	-0.85	228(b)
Re2Br6(PEtPh2)2	+0.03		228(b)
(Bu4N)4[Re2Cl7(PMe3)]2Re2Cl8	-0.39b,e		185
(Ph4As)Re2Cl7(PBunPh2)	-0.34d		194
(Bu4N)Re2Cl7(µ-bdppp)	+0.03b	-0.75b	196

a In volts vs. the saturated sodium chloride calomel electrode (SSCE) with a Pt–bead working electrode; 0.1 M Bu4NPF6 (TBAH) or similar salt as supporting electrolyte.

b vs. Ag/AgCl.

cThis is an Ep,c value which can be inferred from data reported for the [Re2Cl6(PEt3)2]- anion (ref. 182); vs. Ag/AgCl.

d vs. SCE.

e The [Re2Cl8]2- anion in this complex has processes at E1/2(ox) = 1.21 V and E1/2(red) = -0.87 V.

Just as the phosphine-containing complexes Re2X6(PR3)2 exhibit a very accessible and reversible reduction (Table 8.2), so also do the quadruply bonded carboxylates of the type Re2(O2CR)4X2 (R = an alkyl or aryl group; X = Cl, Br or I),84,229 as well as the analogous 2-hydroxypyridinato and 2-mercaptopyridine complexes Re2(hp)4X2 (X = Cl, Br or I)155 and Re2(mp)4X2 (X = Cl or Br)157 (see Table 8.3). For the carboxylate complexes, the reduction potentials show a linear dependence upon the nature of the halogen (becoming more negative in the order I < Br < Cl) and upon the Taft μ* parameter for R.84 The reduced anions [Re2(O2CR)4X2]- and [Re2(hp)4X2]- are quite stable and EPR spectral measurements show that they all possess the μ2/4β2β*1 ground state electronic configuration.84,155 Cobaltocene can be used to prepare the salts [(δ5-C5H5)2Co][Re2(O2CR)4Cl2] (R = Prn, CMe3 or Ph)178 and [(δ5-C5H5)2Co][Re2(hp)4X2] (X = Cl or Br),155 thereby demonstrating the considerable stability of these paramagnetic species and the ready accessibility of Re–Re bonds of order 3.5. Note that in the case of Re2(mp)4X2, a one-electron oxidation is observed near the limit of the CV scans; for X = Cl, E1/2(ox) = +1.21 V but for X = Br the process is irreversible with Ep,a +1.30 V.157 Recent measurement of the CV of the heterometallic complex Re2[O2CCCHCo2(CO)6]4Cl2 (i.e. R = CCHCo2(CO)6) has shown that the one-electron reduction (measured using a vitreous carbon electrode) have an E1/2 value of -0.33 V versus SCE.88

Electrochemical studies of the [Re2]6+/[Re2]5+ couple have been reported for other groups of dirhenium(III) complexes, but in none of these cases has the simple one-electron reduced species been isolated. Cyclic voltammetric data for cis-Re2(O2CR)2X4L2 complexes (recorded

in CH2Cl2 or CH3CN) show98 that the E1/2 values occur over the range -0.47 V to -0.27 V vs. Ag/AgCl, the value being dependent primarily on the nature of X (Cl or Br), with only a small

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dependence on L. For the diaryl formamidinate complexes Re2(µ-DArF)4Cl2, two sequential electrochemical reductions in dichloromethane have often been observed, the second presumably corresponding to the [Re2]5+/[Re2]4+ process.161,162 The dependence of this electrochemistry on the nature of the aryl substituents has been examined in some detail,162 and in the case of Re2(DTolF)4Cl2 the chemical reductions to Re2(DTolF)4Cl and Re2(DTolF)4 have been accomplished; these reductions involve the stepwise loss of a terminal Cl- ligand (see Section 8.5.5).161 Cyclic voltammetric data have also been reported for Re2(DPhF)3Cl3,164 Re2(DPhF)2Cl4165 and the µ-amidato complexes Re2(RNCHO)4Cl2 (R = Ph or Cy);165 a one-electron reduction is observed in dichloromethane in each case.

Table 8.3. Voltammetric E1/2 values for the dirhenium(III) carboxylates, Re2(O2CR)4X2, and related complexes Re2(hp)4X2 and Re2(mp)4X2 in dichloromethanea

Rb	Cl	Br	I
Me3C	-0.42	-0.35	-0.31
C2H5	-0.34	-0.27	-0.20
C3H7	-0.34	-0.28	-0.21
PhCH2	-0.24	-0.18	-0.13
p-CH3OC6H4	-0.42	-0.35	-0.31
p-CH3C6H4	-0.35	-0.29	-0.26
Ph	-0.27	-0.22	-0.18
hpc	-0.73	-0.67	-0.55
mpd	-0.54	-0.51	–

aData taken from ref. 84 unless otherwise stated; in volts vs. SCE with a Pt–bead working electrode and 0.1 M Bu4NPF6 (TBAH) as supporting electrolyte.

b R is the alkyl or aryl substituent except in the case of the hp and mp complexes. c Data for Re2(hp)4X2 taken from ref. 155; vs. Ag/AgCl.

d Data for Re2(mp)4X2 taken from ref. 157; vs. Ag/AgCl.

Quite different redox behavior is encountered in the case of Re2(µ-hpp)4Cl2. Rather than an accessible reduction to Re25+ being observed, the cyclic voltammogram of this complex in dichloromethane shows two one-electron oxidations at E1/2 = +0.058 V and +0.733 V vs Ag/AgCl.230 Oxidation with [(δ5-C5H5)2Fe]PF6 produces [Re2(µ-hpp)4Cl2]PF6, which is the first paddlewheel complex with an Re27+ core and a bond order of 3.5.230 The Re–Re bond distance of 2.2241(4) Å is a little longer than that for the quadruple bond in Re2(µ-hpp)4Cl2.166

8.5.3 Oxidation of [Re2X8]2- to the nonahalodirhenate anions [Re2X9]n- (n = 1 or 2)

The first study undertaken to explore the consequence of oxidizing the [Re2Cl8]2- and [Re2Br8]2- anions was that carried out by Bonati and Cotton who, in 1966, investigated the products obtained by the action of halogens (Cl2 and Br2). Treatment of [Re2Cl8]2- and [Re2Br8]2- with chlorine and bromine, respectively, in dichloromethane or acetonitrile leads to the dirhenium(IV) complex anions [Re2X9]- which are dark green (X = Cl) or dark red (X = Br) in color.231 The salts (Bu4N)Re2X9 are quite stable in the solid-state but their solutions are easily reduced (under a variety of conditions) to produce either (Bu4N)2Re2X9, containing rhenium (+3.5), or the (Bu4N)2Re2X8 starting materials.231 The ‘intermediate’ oxidation state anion [Re2Cl9]2- is readily reoxidizable to [Re2Cl9]-. The various methods that were discovered in this early study231 were subsequently refined in some cases.221,232

The close relationship that exists between various [Re2Cl8]n- and [Re2Cl9](n-1)- species has been well documented by the elegent low temperature spectroelectrochemical studies of Heath and Raptis.222 Not only have the coupled chemical-electrochemical relationships been mapped

308Multiple Bonds Between Metal Atoms Chapter 8

out as shown in Fig. 8.15, but dichloromethane solutions of many of the unstable species have been characterized by electronic absorption spectroscopy.222,223,233 The electrochemical and spectroscopic properties of solutions of the [Re2Cl9]- and [Re2Cl9]- anions in basic aluminum chloride-1-methyl-3-ethylimidazolium chloride room temperature molten salts have also been measured.224,234

Fig. 8.15. Summary of the relationship between [Re2Cl8]n- and [Re2Cl9](n-1)- species as demonstrated by low temperature spectroelectrochemical techniques. The potentials are versus a Ag/AgCl reference electrode.

Another close relationship between [Re2Cl8]2- and the [Re2Cl9]n- anions (n = 1 or 2) was encountered during studies of the photochemistry of [Re2Cl8]2- which involves the electrontransfer chemistry of the luminescent excited state [Re2Cl8]2-*. This state is an ββ* singlet and behaves as a strong oxidant and moderately good reductant.218,219 Various electron acceptors (e.g. TCNE and chloroanil) quench the [Re2Cl8]2-* luminescence in non-aqueous solvents to produce [Re2Cl8]- and the reduced acceptor; the products back-react rapidly to give starting materials. The luminescence is also quenched by electron donor secondary and tertiary aromatic amines (e.g. N,N,N',N'-tetramethyl-p-phenylenediamine) in acetonitrile solution.218 Thus the ββ* singlet provides a facile route to the powerful oxidant [Re2Cl8]-, a species that has its own interesting chemistry. For example, it reacts with Cl- to generate [Re2Cl9]2-; this demonstrates that the Cl- trapping reaction efficiently competes with the very fast back-reaction between [Re2Cl8]- and [TCNE]- or [chloranil]-.221 In these experiments, the reaction stops at the [Re2Cl9]2- stage, because these particular quenchers cannot oxidize [Re2Cl9]2- to [Re2Cl9]-. However, with quenchers such as 2,3-dichloro-5,6-dicyano-1,4-benzoquinone the oxidized species [Re2Cl9]- is produced.221

Several structural studies on (Bu4N)Re2Cl9 have been carried out.233,235,236 The [Re2Cl9]- anion has long been viewed as a derivative of `-ReCl4, the latter containing Re2Cl9 units which are strung together by sharing terminal chlorine atoms (the structure can be represented as Re2Cl7Cl2/2).186 The expectation of a close structural relationship between `-ReCl4 and [Re2Cl9]- has been confirmed by a crystal structure determination on (Bu4N)Re2Cl9.233,235,236 This revealed that the anion possesses a confacial bioctahedral metal-metal bonded structure; the Re–Re distance is 2.704(1) Å. The structure of the dianion has been determined in the salt (Et4N)2Re2Cl9.233 Interestingly, the Re–Re distance in the lower oxidation state species, which formally has the lower bond order, is shorter by c. 0.23 Å (2.473(4) Å). An explanation for this shortening may lie in the occurrence of enhanced d-orbital overlap and diminished electrostatic repulsion in the more reduced species.233 The synthesis and structural characterization of (PCl4)Re2Cl9,237 (SCl3)Re2Cl9,238 and (Ph4P)Re2Cl9239 have been reported more recently; the Re–Re bond distances are 2.724(2) Å, 2.722(2) Å and 2.780 Å, respectively.

The close relationship between the reactivities of the nonachlorodirhenate anions [Re2Cl9]n-, [Re2Cl8]2- and `-ReCl4 has been well documented.115,240 Many of the reactions of `-ReCl4 are typical of those of [Re2Cl8]2- itself; most noteworthy is the relative ease of converting `-ReCl4 into [Re2Cl8]2- and [Re2Cl9]2-.115,240

Rhenium Compounds 309

Walton

8.5.4 Re25+ and Re24+ halide complexes that contain phosphine ligands

The single most important class of complexes that contain the electron-rich metal-metal triple bond (μ2/4β2β*2 electronic configuration) are Re24+ complexes of stoichiometry Re2X4(PR3)4 (X = Cl, Br or I; PR3 is a monodentate tertiary phosphine) and the analogous compounds Re2X4(LL)2, where LL represents a bidentate phosphine and/or arsine ligand. These compounds, and closely related ones such as Re2X5(PR3)3, constitute the topic of the first part of the present section. Note that the Re2X5(PR3)3 and Re2X4(PR3)4 compounds are formally derivatives of the [Re2X8]3- and [Re2X8]4- anions, neither of which has yet been stabilized in the solid state.

Monodentate phosphines

We have previously considered two important cases of the redox activity of the [Re2X8]2- anions where the structural integrity of the dirhenium unit is retained in the products, one a reduction (involving the dth ligand), the other involving halogen oxidation to [Re2X9]-. However, by far the most extensive series of redox reactions investigated to date are those that involve the reduction of [Re2X8]2- in the presence of tertiary phosphines. This work was originally an outgrowth of studies of the reactions of phosphines with the trinuclear rhenium(III) cluster Re3Cl9. In the reaction between triethylphosphine and this chloride, using forcing reaction conditions, the major product177 was glittering black crystals of stoichiometry [ReCl2(PEt3)2]n, that proved to be the dinuclear complex Re2Cl4(PEt3)4. Similar products were isolated in reactions between Re3Cl9 and PPrn3 and PEt2Ph, whereas with PMePh2 and PEtPh2 the intermediate oxidation state complexes Re2Cl5(PRPh2)3 (R = Me or Et) were formed.177 Such trinuclear to dinuclear transformations were subsequently found to occur upon reacting Re3Br9 (or Re3Br9(THF)3) with PMe3, PPrn3 and PEtPh2,241,242 and Re3I9 with PPrn3,127 to afford the corresponding dirhenium(II) complexes Re2X4(PR3)4. When trimethylphosphine is added to solutions of the mixed chloride-alkyl cluster Re3Cl3(CH2SiMe3)6 in light petroleum or diethylether reductive cleavage occurs to give Re2Cl2(CH2SiMe3)2(PMe3)4,196 a reaction clearly analogous to the reductive cleavage of Re3Cl9 by tertiary phosphines that leads to Re2Cl4(PR3)4.177

Since it seemed at the time177 that the synthesis of dinuclear complexes of the types Re2Cl4(PR3)4 and Re2Cl5(PR3)3 could be more logically approached via the quadruply bonded [Re2Cl8]2- anion, such a possibility was explored. It was noted in Section 8.4.4 that monodentate phosphines react with the [Re2Cl8]2- and [Re2Br8]2- ions to yield the simple substitution products Re2X6(PR3)2, when mild reaction conditions are used. However, in refluxing acetone or alcohol reduction was indeed found to occur,177 to an extent that appeared to depend upon the basicity of the phosphine, to give either Re2Cl4(PR3)4 or Re2Cl5(PR3)3 as products. In the period between the original discovery177 and the early 1990’s, this chemistry was developed to include the isolation and characterization of a range of compounds of the type Re2X4(PR3)4, with X = Cl, Br or I and PR3 = PMe3, PEt3, PPrn3, PMe2Ph, PEt2Ph, PMePh2 or PEtPh2.25,177,178,243-246 To access the PMePh2 and PEtPh2 complexes of this type, rather than Re2Cl5(PRPh2)3 (R = Me or Et),177 NaBH4 was added to the reaction mixtures to serve as a reducing agent.245,246 Even with the more basic phosphines, this reagent enhances the rate of formation of the Re2Cl4(PR3)4 products. In the reaction that led to Re2Cl4(PEtPh2)4, the red brown polyhydride complex Re2(µ-H)4H4(PEtPh2)4 was also formed.246

The first structure determination on a Re24+ complex of the type Re2X4(PR3)4 was carried out on Re2Cl4(PEt3)4, which was shown to have the eclipsed non-centrosymmetric structure represented in 8.16.247 With the use of the nomenclature first suggested by Cotton (see 8.12)248 this structure is that of a 1,3,6,8-Re2X4(PR3)4 isomer. This structure was later redetermined249 with the use of a different space group. The Re–Re distance of 2.250(4) Å is consistent with a triple bond. In this structure,247,249 there is a three-fold orientational disorder of the Re–Re unit. This

310Multiple Bonds Between Metal Atoms Chapter 8

may or may not be the case in other compounds of this type or closely similar molecules; for other structures that are mentioned in this section and are based upon the L4ReReL4 geometry, the original references can be consulted to see whether this type of disorder is present or not. The structure determination of Re2Cl4(PEt3)4 was followed by those of Re2Cl4(PMe2Ph)4,243 Re2Cl4(PMePh2)4,245 Re2Cl4(PMe3)4250 and Re2X4(PPrn3)4 (X = Cl or Br).250 In all cases, these have the 1,3,6,8 structure (8.16) and similar Re–Re bond distances (see Table 8.4). As we shall discuss shortly, isomers of the 1,2,7,8-type, as represented in structure 8.17, have been obtained in more recent studies. A compilation of all the mixed halide-phosphine complexes with multiply bonded Re24+ and Re25+ cores that have been structurally characterized is available in Table 8.4, together with a listing of the Re–Re distances.

8.16 8.17

Table 8.4. Structural data for mixed halide-phosphine complexes of Re24+ and Re25+ that contain Re–Re bonds of order 3 or 3.5

		Rotational
Compound	r(Re–Re)(Å)a	Geometryb	ref.
A. Re24+ Compounds
1,3,6,8-Re2Cl4(PMe3)4	2.247(1)	eclipsed	250
1,2,7,8-Re2Cl4(PMe3)4	2.414(8)	eclipsed	179
1,2,7,8-Re2Cl4(PMe3)3(PEt2H)	2.253(2)	eclipsed	262
1,3,6,8-Re2Cl4(PEt3)4	2.250(4)	eclipsed	249
1,3,6,8-Re2Cl4(PPrn3)4	2.252(2)	eclipsed	250
1,2,7,8-Re2Cl4(PEt2H)4	2.2533(8)	eclipsed	183
1,3,6,8-Re2Cl4(PMe2Ph)4	2.241(1)	eclipsed	243
1,2,7,8-Re2Cl4(PMe2Ph)4	2.261(1)	eclipsed	180
	2.258(1)	eclipsed	180
1,2,7,8-Re2Cl4(PMe2Ph)3(PEt2H)·CH2Cl2	2.247(1)	staggered	262
1,3,6,8-Re2Cl4(PMePh2)4·C6H6	2.260(1)	eclipsed	245
1,3,6,8-Re2Cl4(PMePh2)4·(CH3)2CO	2.255(0)	eclipsed	245
1,3,6,8-Re2Br4(PPrn3)4	2.253(4)	eclipsed	250
1,3,6,8-Re2I4(PMe3)4·CH2Cl2	2.2541(8)	eclipsed	184
1,3,6,8-Re2I4(PMe2Ph)4	2.258(1)	staggered	184
1,3,6,8-Re2I4(PEt2Ph)4	2.2698(7)	eclipsed	184
(Bu4N)[1,2,7-Re2Cl5(PMe3)3]	2.2354(7)	staggered	261
(Bu4N)[1,2,7-Re2Cl5(PMe2Ph)3]	2.2388(7)	staggered	262
_-Re2Cl4(dppe)2·4C6H6	2.2650(6)	eclipsed	277
_-Re2Cl4(dppe)2·dppe	2.2544(8)	eclipsed	277
`-Re2Cl4(dppe)2	2.244(1)	staggered	266
_-Re2Cl4(dppee)2·PrOH	2.250(1)	eclipsed	276
	2.265(1)	eclipsed	276