Multiple Bonds Between Metal Atoms / 08-Rhenium Compounds

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8.47

very different. An X-ray crystal structure of Re2Cl4(µ-dppm)2(CO)(CNXyl) has shown that the CO and XylNC ligands are cis to one another and the Re–Re distance accords with a double bond (Table 8.7); this structure is in all important respects the same as that of Re2Cl4(µ- dppm)2(CO)2 (see 8.39 in Fig. 8.26) and is shown in 8.48 (X = Cl). The structures of the ButNC and PriCN derivatives are almost certainly like that shown in 8.49 (X = Cl), based upon the spectroscopic properties of these complexes and their similarity to the bromide analog Re2Br4(µ-dppm)2(CO)(CNBut), which has been prepared by a similar procedure and characterized by X-ray crystallography (Table 8.7).335 This structure contains a Re–Re triple bond rather than the double bond that is present in compounds with structure 8.48. This is shown by the difference between the Re–Re bond distances in Re2Cl4(µ-dppm)2(CO)(CNXyl) (8.48) and Re2Br4(µ-dppm)2(CO)(CNBut) (8.49) which are 2.581(1) Å and 2.3805(14) Å, respectively. The compound Re2Br4(µ-dppm)2(CO)(CNXyl) has also been prepared when acetone is used as the reaction solvent and has this same open bioctahedral structure.335 When the precursor compound Re2Cl4(µ-dppm)2(CO) is reacted with an equivalent of XylNC in acetonitrile (instead of acetone) it forms Re2Cl4(µ-dppm)2(CO)(CNXyl) which has structure 8.49 rather than 8.48.353 This was the first case of structural isomerism with compounds of the type Re2X4(µ-LL)2(CO)(CNR).

Those isomers of Re2X4(µ-dppm)2(CO)(CNR) that have the open bioctahedral structure 8.49 (X = Cl or Br; R = But or Xyl) react with TlO3SCF3 in the absence of another donor molecule to give the cationic species [Re2X3(µ-dppm)2(CO)(CNR)]+ with a structure like that of their bis-carbonyl or bis-nitrile analogs (see 8.41).335,354 This transformation occurs through labilization of the Re–X bond that is trans to the XylNC ligand and CO transfer from the adjacent Re atom. The structure of [Re2Br3(µ-dppm)2(CO)(CNXyl)]+ has been determined crystallographically.335

When the monocarbonyls Re2X4(µ-dppm)2(CO) (X = Cl or Br) are reacted with stoichiometric quantities of nitrile ligands RCN (R = Me or Ph) in the presence of TlY (Y = PF6 or O3SCF3) the compounds [Re2X3(µ-dppm)2(CO)(NCR)]Y and [Re2X3(µ-dppm)2(CO)(NCR)2]Y

352Multiple Bonds Between Metal Atoms Chapter 8

are formed in high yield.333,355 Crystal structure determinations on salts of [Re2Cl3(µ- dppm)2(CO)(NCMe)]+ and [Re2Cl3(µ-dppm)2(CO)(NCMe)2]+ (see Table 8.7) have shown355 that the structures resemble closely 8.41 and 8.40, respectively; in the case of the bis-nitrile complex, the CO ligand is bridging with the acetonitrile molecules cis to it. These mixed CO/acetonitrile complexes readily interconvert upon the addition or loss of CH3CN.355 When salts of the [Re2X3(µ-dppm)2(CO)(NCMe)2]+ cations are oxidized with X2, the products are the same as those formed by Re2X4(µ-dppm)2(CO), viz, [Re2(µ-X2)(µ-dppm)2X3(CO)]+.336

The lability of one of the Re–Cl bonds of Re2Cl4(µ-dppm)2(CNR) (R = But or Xyl) has been demonstrated by the conversion of these 1:1 adducts to [Re2Cl3(µ-dppm)2(CNR)(NCR')]PF6 upon their reaction with R'CN (R' = Me, Et or Ph) and KPF6.343 The resulting complexes have very similar electrochemistry and electronic absorption and NMR spectral properties to those of the structurally characterized bis-nitrile salts (vide supra). The available evidence supports a structure closely akin to 8.41, with the isocyanide and nitrile ligands being coordinated to the same rhenium atom. They can be oxidized chemically with NOPF6 to yield the paramagnetic dications [Re2Cl3(µ-dppm)2(CNR)(NCR')](PF6)2, which show343 complex EPR spectra comparable to those of the oxidized bis-nitrile analogs.

The incorporation of mixed-sets of three CO,RNC and/or RCN ligands can easily be accomplished by the reactions of Re2X4(µ-dppm)2(CO)2 (8.39 in Fig. 8.26) and the isomers of Re2X4(µ-dppm)2(CO)(CNR) (8.48 and 8.49) with additional equivalents of RNC or RCN ligands in the presence of Tl+ salts that can labilize a Re–X bond, as in the conversion of Re2Cl4(µ-dppm)2(CO)2 to [Re2Cl3(µ-dppm)2(CO)3]+ (Fig. 8.26). Thus, salts of stoichiometry [Re2Cl3(µ-dppm)2(CO)2(NCR)]PF6 (R = Me, Et or Ph)356 have been prepared by reacting the dicarbonyl Re2Cl4(µ-dppm)2(CO)2 with an excess of nitrile in the presence of TlPF6. In the case of the acetonitrile derivative, it has also been obtained by reacting the all-cis complex [Re2Cl3(µ-dppm)2(CO)(NCMe)2]PF6 with CO in dichloromethane.357 An X-ray crystal structure of [Re2Cl3(dppm)2(CO)2(NCEt)]PF6 shows an all-cis arrangement of chloride ligands. The structure is just like that of [Re2Cl3(µ-dppm)2(CO)3]+ (8.40) except a terminal carbonyl ligand cis to the µ-CO ligand has been replaced by a propionitrile molecule.356 The salts [Re2Cl3(µ- dppm)2(CO)2(NCR)]PF6 can be reduced to the paramagnetic, EPR-active neutral species Re2Cl3(µ-dppm)2(CO)2(NCR) upon their reaction with cobaltocene in acetone.356 The analogous mixed carbonyl-isocyanide complexes [Re2Cl3(µ-dppm)2(CO)2(CNR)]PF6 (R = Pri, But or Xyl) have been prepared by reacting Re2Cl4(µ-dppm)2(CO)2 with RNC in the presence of TlPF6,356,358 or the displacement of the nitrile ligand of [Re2Cl3(µ-dppm)2(CO)2(NCR)]PF6 by RNC.356,357 The isocyanide complexes possess a well defined electrochemistry just like that of their nitrile analogs, including a very accessible reversible reduction. The reduction of these mixed carbonyl-isocyanide salts to the neutral, paramagnetic species Re2Cl3(dppm)2(CO)2(CNR), has been achieved chemically using cobaltocene, and also electrochemically. The complexes of stoichiometry [Re2Cl3(µ-dppm)2(CO)2(CNR)]PF6 that have been prepared by the aforementioned methods, possess the all-cis structure 8.50 that is shown in Fig. 8.28. This is the same basic structure as determined for the propionitrile derivative,356 as well as for the reduced complex Re2Cl3(µ-dppm)2(CO)2(CNPri).358 The dicarbonyl complex Re2Cl4(µ-dppE)2(CO)2 reacts with XylNC and 2,5-dimethylbenzonitrile in the presence of TlO3SCF3 to give this same type of complex with structures like 8.50.287

Other routes to complexes of stoichiometry [Re2Cl3(µ-dppm)2(CO)2(CNR)]PF6 involve exposing the monoisocyanide Re2Cl4(µ-dppm)2(CNR) or the mixed carbonyl-isocyanide Re2Cl4(µ-dppm)2(CO)(CNR) to an atmosphere of CO in the presence of TlPF6.358 However, these methods can give rise to geometric isomers as shown in Fig. 8.28. The isomer derived from Re2Cl4(µ-dppm)2(CNR) (R = But or Xyl) and Re2Cl4(µ-dppm)2(CO)(CNBut) possesses

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Fig. 8.28. Reaction scheme showing the syntheses, structures and isomerization of mixed carbonyl-isocyanide complexes [Re2Cl3(µ-dppm)2(CO)2(CNR)]PF6.

an unsymmetric arrangement of ligands in the equatorial plane (8.51) as shown by an X-ray crystal structure of [Re2Cl3(µ-dppm)2(CO)2(CNBut)]PF6.358 The Re–Re distance is similar to those of all the other edge-shared bioctahedral complexes listed in Table 8.7. This isomer is also formed by the carbonylation of the complex [Re2Cl3(µ-dppm)2(CO)(CNXyl)(NCMe)]O3SCF3, which has an open bioctahedral structure and a labile acetonitrile ligand.354 Isomerization to the more thermodynamically stable all-cis form occurs upon heating 1,2-di- chloroethane solutions of these species over a period of several hours (Fig. 8.28). A bromo analog [Re2Br3(µ-dppm)2(CO)2(CNXyl)]O3SCF3 has also been structurally characterized and shown to have structure 8.50, with Br in place of Cl.359 It is prepared by the reaction of CO with [Re2Br3(µ-dppm)2(CO)(CNXyl)]O3SCF3 (structure similar to 8.41).359 Different isomers of [Re2Cl3(µ-dcpm)2(CO)2(CNXyl)]O3SCF3 have been obtained starting from Re2Cl4(µ-dcpm)2(CNXyl), but their structures have not yet been determined crystallographically although one of the isomers probably has a structure resembling 8.51.360

Structural isomerism has also been encountered in the case of salts of [Re2Cl3(µ- dppm)2(CO)(CNR)2]+, where R = But or Xyl and the two RNC ligands can be the same or different. Examples were first encountered by reacting the structurally dissimilar isomers of Re2Cl4(µ-dppm)2(CO)(CNR) (R = But or Xyl), which can have structures 8.48 or 8.49, with nitriles (R'CN) and isocyanides (R'NC) in the presence of TlPF6, whereby complexes of the types [Re2Cl3(µ-dppm)2(CO)(CNR)(NCR')]PF6 and [Re2Cl3(µ-dppm)2(CO)(CNR)(CNR')]PF6 (R&R') are formed.357 The crystal structure of the acetonitrile complex [Re2Cl3(µ-dppm)2(CO)- (CNXyl)(NCCH3)]O3SCF3 has confirmed that it has an open bioctahedral structure; this isomer is formed from the isomer of Re2Cl4(µ-dppm)2(CO)(CNXyl) with structure 8.49.354 The nitrile ligands R'CN are labile and are readily displaced by CO and R'NC.354,357,361 This chemistry is quite extensive, and leads to complexes that can exist in several isomeric forms, e.g. [Re2Cl3(µ-dppm)2(CO)(CNBut)(CNXyl)]PF6 has been isolated and characterized in four forms, two of which are edge-shared bioctahedra (with µ-CO or µ-CNXyl ligands)357,362 and two are open bioctahedra.354,357,361 These structures are represented in Fig. 8.29 along with the Re–Re bond order that is present in each. The XylNC-bridged isomer I converts to the more thermodynamically stable II upon refluxing solutions in 1,2-dichloroethane.357 As we shall see,

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similar isomers are encountered when both isocyanides are the same (namely XylNC) but in this case the chemistry is even more complex.

Fig. 8.29. Structural isomers of the dirhenium cation [Re2Cl3(µ-dppm)2(CO)(CNBut)- (CNXyl)]+ with the bridging dppm ligand omitted for clarity.

The complex [Re2Cl3(µ-dppm)2(CO)(CNBut)2]+ has so far been identified in only one isomeric form and this has an open bioctahedral structure like those for isomers III and IV in Fig. 8.29. The Re–Re bond distance is typical of a triple bond (see Table 8.7).357,363 It is prepared by the reaction of [Re2Cl3(µ-dppm)2(CO)(CNBut)(NCMe)]PF6 with ButNC and of Re2Cl4(µ-dppm)2(CO)(CNBut) (8.49) with ButNC (1 equiv) and TlPF6.357,358 Because Re2Cl4(µ-dppm)2(CO)(CNXyl) exists as two stable isomers (structures 8.48 and 8.49),333,353 this leads to a quite extensive isomer chemistry in the case of the bis-XylNC cation [Re2Cl3(µ-dppm)2(CO)(CNXyl)2]+ which is formed by reacting these isomers with XylNC in the presence of K+ or Tl+ salts although, as we shall see, other precursors and procedures can also be used. Three of the isomers have a very close structural relationship to those of [Re2Cl3(µ-dppm)2(CO)(CNBut)(CNXyl)]+ that are shown in Fig. 8.29. The isomers of [Re2Cl3(µ-dppm)2(CO)(CNXyl)2]+, along with those of the bromo analog that have been identified, are shown in Fig. 8.30.353,354,359,361,362,364-367 The formal Re–Re bond orders are also indicated for each of the structures, and it can be seen that these are 3, 2, 1 or 0. Isomers V (yellow) and VI (green) are formed as separable mixtures by reacting Re2Cl4(µ-dppm)2(CO) or Re2Cl4(µ-dppm)2(CO)(CNXyl) (8.48) with XylNC.358,364 These reactions give the chloride salts which can be exchanged with [PF6]-, [O3SCF3]- or [ReO4]-.358,364 Both isomers have a rich redox chemistry that consists of a one-electron oxidation and two one-electron reductions, the first reduction being very accessible (E1/2 ca -0.1 V and c. -0.25 V vs. Ag/AgCl for V and VI, respectively).358,364 In the case of V, reduction with cobaltocene has been used to prepare the neutral complex Re2Cl3(µ-dppm)2(CO)(CNXyl)2 which can be reoxidized by [(δ5-C5H5)2Fe]PF6 (with preservation of structure).362 Crystal structure determinations on this one-electron reduction product of V,362 and of a salt of VI,364 have established the structures shown in Fig. 8.30 for these two isomers (see also Table 8.7). The neutral reduced complex Re2Cl3(µ-dppm)2(CO)(CNXyl)2 (isomer V) has a formal Re–Re bond order of 1.5. Isomer V converts irreversibly to VI when solutions in 1,2-dichloroethane are refluxed.358,364

The third isomer of [Re2Cl3(µ-dppm)2(CO)(CNXyl)2]+ (labeled VII in Fig. 8.30) is prepared by the reaction of Re2Cl4(µ-dppm)2(CO)(CNXyl) (8.49) with XylNC in the presence of TlO3SCF3.353 It is also formed when the acetonitrile complex [Re2Cl3(µ-dppm)2(CO)(CNXyl)- (NCMe)]O3SCF3, which has an open bioctahedral structure, is reacted with XylNC.354 The

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bromo analog is formed in low yield along with isomeric forms IX and X, by reacting [Re2Br3(µ- dppm)2(CO)(CNXyl)]Y (Y = PF6 or O3SCF3) with XylNC (1 equiv).359 A higher yield route involves the reaction of [Re2Br3(µ-dppm)2(CO)(CNXyl)(NCMe)]O3SCF3 with XylNC.361 Its crystal structure has been determined (Table 8.7). The thermal isomerization of VII to VIII occurs in essentially quantitative yield when solutions of VII (X = Cl or Br) in 1,2-dichloro- ethane (and other solvents) are heated at reflux.361,365 The paramagnetic mixed-valence isomers VIII contain no Re–Re bond; for X = Cl, the Re–Re distance is 3.321(1) Å. The most likely mechanism for this isomerization is a “merry-go-round” process.361,365

Isomer IX (Fig 8.30), which is obtained only in the case of X = Br, is formed as the major product in the reactions of [Re2X3(µ-dppm)2(CO)(CNXyl)]Y (Y = PF6 or O3SCF3) with XylNC and of Re2Br4(µ-dppm)2(CO)(CNBut) with XylNC and TlY.359 This product is actually a mixture of isomers, both of which have the basic structure shown for IX in Fig. 8.30, but they differ in having either boat or chair conformations for the Re2(µ-dppm)2 unit.366 These conformational isomers have been separated,366 and both have been shown to have long Re–Re single bonds (3.03 - 3.05 Å).359,366

Fig. 8. 30. Structural isomers (V-X) of the dirhenium cations [Re2Cl3(µ-dppm)2- (CO)(CNXyl)2]+ that have been identified. All have been characterized by X-ray crystallography except X whose proposed structure is based upon its spectroscopic properties and chemical reactivity. The bridging dppm ligands are omitted for clarity.

The final isomer of [Re2X3(µ-dppm)2(CO)(CNXyl)2]+ is X, which has been identified for both X = Cl and Br. It has been obtained in only very small amounts in the case of X = Br,359 but is obtained in very high yield when [Re2Cl3(µ-dppm)2(CO)(CNXyl)]O3SCF3 is reacted with 1 equiv of XylNC in dichloromethane. It has not yet been characterized by X-ray crystallography, so the structure given in Fig. 8.30 is tentative.367 Interestingly, it reacts with a further equivalent of XylNC in the presence of TlO3SCF3 to form367 one of several known isomers of [Re2Cl2(µ-dppm)2(CO)(CNXyl)3]+, as we shall shortly discuss.

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The only other system that gives complexes of composition [Re2X3(µ-LL)2(CO)(CNR)2]Y is [Re2Cl3(µ-dcpm)2(CO)(CNBut)2]Y (Y = Cl or O3SCF3), which can be isolated in two isomeric forms by the carbonylation of Re2Cl4(µ-dcpm)2(CNBut)2.360 Both isomers have been characterized on the basis of electrochemical and spectroscopic measurements but in neither case is the structure known for certain.

The reactions of Re2Cl4(µ-dppm)2(CO) (8.38), Re2Cl4(µ-dppm)2(CO)(CNXyl) (8.49) and [Re2Cl3(µ-dppm)2(CO)(CNXyl)2]O3SCF3 (isomer VII in Fig. 8.30) in dichloromethane with the requisite number of equivalents of TlO3SCF3 and XylNC that are necessary to give a compound of stoichiometry [Re2Cl2(µ-dppm)2(CO)(CNXyl)3](O3SCF3)2, successfully produce such a product but in each case a different isomer is formed.368 These isomers do not interconvert and each undergoes two reversible one-electron reductions when reacted with cobaltocene; for one of these isomers, the reduced products are similar structurally to the parent, while for the other two the first one-electron reduction is followed by isomerization to a different structure.368(b) These redox processes can be reversed chemically with the use of the oxidants [(δ5-C5H5)2Fe]PF6 or NOPF6. In some cases the reduced products undergo further slow isomerization in solution to give additional isomers which, in turn, have their own reversible redox chemistry.368(b) In total, the [Re2Cl2(µ-dppm)2(CO)(CNXyl)3]n+ species (n = 2, 1, or 0) have been found to exist in seven distinct forms which possess Re–Re bond orders of 3, 2, 1.5, 1 or 0.368(b) These bond orders depend on the specific bioctahedral structure that each species has and on its charge. In all cases, the crystallographic characterizations have shown that there is a large variation in the degree of Cl, CO and XylNC ligand bridging in the different complexes. The structural data for these complexes are not listed in Table 8.7; the original literature reference 368(b) should be consulted for full details.

It has been possible to increase the number of /-acceptor ligands bound to the dirhenium core by reacting several of the fully reduced neutral compounds of the type Re2Cl2(µ- dppm)2(CNXyl)3 with 1 equiv each of XylNC and TlO3SCF3.369 These reactions give the same symmetrical edge-shared bioctahedral complex [Re2(µ-Cl)(µ-CO)(µ-dppm)2(CNXyl)4]O3SCF3 which has a Re–Re single bond. When acetonitrile is used in place of XylNC these reactions give rise to three different isomeric forms of [Re2Cl(µ-dppm)2(CO)(CNXyl)3(NCMe)]O3SCF3.369

Another group of compounds are those that contain CO and/or RNC ligands in combination with anionic or neutral cyanide-containing ligands that have the potential to act as linkers to form polymetallic assemblies.291,370,371 The reactions of Re2Cl4(µ-dppm)2(L) (L = CO or XylNC) (8.38), the edge-shared bioctahedral complexes Re2Cl4(µ-dppm)2(CO)(L) (L = CO or XylNC) and Re2Cl4(µ-dppE)2(CO)2 (8.39 and 8.48), and the open bioctahedral complex Re2Cl4(µ-dppm)2(CO)(CNXyl) (8.49) with Na[N(CN)2] and K[C(CN)3] in methanol result in the substitution of one Re–Cl bond except in the case of Re2Cl4(µ-dppm)2(CNXyl) for which a second bond can be substituted to form Re2Cl2[N(CN)2]2(µ-dppm)2(CNXyl) and Re2Cl2[C(CN)3]2(µ-dppm)2(CNXyl).291,370 In all instances the [N(CN)2]- and [C(CN)3]- ligands coordinate through a single cyano group as shown by single crystal structure determination on representative complexes from each group (see Table 8.7).291

The structures of the complexes derived from Re2Cl4(µ-dppm)2(L) are shown in 8.52 and 8.53 (where X = N(CN)2 or C(CN)3) and are similar to that of Re2Cl4(µ-dcpm)2(CNXyl) (8.42). The compounds that are formed from Re2Cl4(µ-dppm)2(CO)(L) (L = CO or XylNC) and Re2Cl4(µ- dppE)2(CO)2 have structures that resemble those of the parent edge-shared bioctahedra (8.39 in Fig. 8.26 and 8.48) with substitution of the Re–Cl bond cis to the µ-CO ligand, while for the open bioctahedron Re2Cl4(µ-dppm)2(CO)(CNXyl)(8.49) the Re–Cl bond that is trans to XylNC is replaced by [N(CN)2]- or [C(CN)3]-.291,370 These reactions resemble those in which these same precursors are reacted with RNC or RCN ligands in the presence of Tl+ (vide supra).

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The reactions of acetylene with Re2X4(µ-dppm)2 (X = Cl or Br) give dirhenium complexes that contain δ2 and/or µ:δ2,δ2 bound ethyne molecules (see Fig. 8.27). When the compounds Re2X4(µ-dppm)2(L) (L = CO or CNR) and Re2Cl4(µ-dppm)2(CO)(L) (L = CO (8.39) or CNXyl (8.48)) are reacted with alkynes the chemistry becomes more complicated. The Re–Re triple bond is retained in the adducts of the type [Re2X3(µ-dppm)2(L)(δ2-RCCR')]Y that are formed in the reactions of RCCR' with Re2X4(µ-dppm)2(CO) and Re2X4(µ-dppm)2(CNR) (R = But or Xyl) in the presence of TlPF6 or TlO3SCF3.372,373 With the monocarbonyl complex, both internal and terminal alkynes were used372 while the mono-isocyanide complexes were reacted with terminal alkynes only.373 The ethyne complexes [Re2X3(µ-dppm)2(CO)(δ2-HCCH)]PF6 have also been prepared by the treatment of the mixed CO/nitrile compound [Re2X3(µ-dppm)2(CO)(NCR)2]PF6 with acetylene.355 The structural similarities of these 1:1 alkyne adducts was shown by infrared and NMR spectroscopy, by cyclic voltammetric measurements, and by representative X-ray structures on crystals of composition [Re2Cl3(µ-dppm)2(CO)(δ2-MeCCEt)]PF6372 and [Re2Cl3(µ-dppm)2(CNBut)(δ2-HCCH)]O3SCF3·CH3C(O)OC2H5.373 The Re–Re distances are 2.3407(4) Å and 2.3171(5) Å, respectively, and the structures of the cations involve different coordination numbers for the two Re centers (i.e. [(L)X2Re(µ-dppm)2ReX(δ2-RCCR')]+) and an anti arrangement of the L and δ2-RCCR' ligands (see 8.54). NMR spectroscopy has shown that these complexes are sterochemically rigid at room temperature.372,373

+

PP

LX

XReRe RC

XC

PP R'

8.54

Whentheδ2-alkyneadducts[Re2Cl3(µ-dppm)2(CO)(δ2-RCCH)]Y(R=H,Prn,BunorPh;Y=PF6 or O3SCF3) are reacted with tertiary phosphines PR3 (R3 = Me3, Et3, Me2Ph or MePh2) resonance stabilized ylides are formed that are of composition [Re2Cl3(µ-dppm)2(CO){C(R)CH(PR3)}]Y.374 A Re–Re triple bond is retained as shown by an X-ray structure determination of a crystal of composition [Re2Cl3(µ-dppm)2(CO){C(Prn)CH(PMe2Ph)}]O3SCF3·0.87C7H8 (Re–Re distance 2.311(1) Å).374 The structure resembles that of 8.54 except that the δ2-RCCH ligand is converted to the δ1-bound ylide C(R)CH(PR3).

When the edge-shared bioctahedral complexes Re2X4(µ-dppm)(CO)(L) (X = Cl or Br; L = CO or XylNC) (8.39 and 8.48) react with terminal alkynes RCCH (R = H, Prn, Bun, Ph or p-tol) at room temperature in the presence of TlPF6 they convert to the diamagnetic complexes [Re2(µ-X)(µ-COC(R)CH)X2(L)(µ-dppm)2]PF6 (structure 8.55; µ-dppm ligands omitted for clarity), in which the reductive coupling of the µ-CO ligand and the alkyne leads to a 3-metallafuran ring.375 These reactions, which are regiospecific, proceed through reaction intermediates of the type [Re2X3(µ-dppm)2(CO)(L)(δ2-RCCH)]+.375 Structure determinations have shown that the Re–Re distances are in the range 2.55-2.57 Å, but the assignment of bond order is not clear-cut.375 Under certain conditions the 3-metallafuran ring can undergo ring opening to afford paramagnetic mixed-valence dirhenium alkylidyne complexes of the type represented by 8.56.376 These reactions have been carried out only in the case of X = Cl, and the resulting complexes (one unpaired electron) can be reduced by cobaltocene to their neutral diamagnetic congeners, which probably do not contain a Re–Re bond.376(b)

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A few examples of mixed CO/PR3 (or P(OR)3) and CO/RNC/PR3 (or P(OR)3) complexes are also known. These are prepared from Re2Cl4(µ-dppm)2(CO),377 Re2Cl4(µ-dppm)(CO)2378 and Re2Cl4(µ-dppm)2(CO)(CNR) (R = But or Xyl),378 and structures that have been determined by X-ray crystallography are listed in Table 8.7. The structures of [Re2X3(µ-dppm)2(CO)(PMe3)2]Y (X = Cl or Br; Y = Cl, PF6 or BPh4) and [Re2Cl3(µ-dppm)2(CO){P(OR)3}2]PF6 (R = Me or Et) have been established by a single crystal X-ray structure analysis of a salt of [Re2Cl3(µ- dppm)2(CO)(PMe3)2]+ (Table 8.7).377 The yellow-green diamagnetic complexes [Re2X3(µ- dppm)2(CO)2(PMe3)]PF6 and [Re2X3(µ-dppm)2(CO)2{P(OR)3}]PF6 (R = Me or Et), and the dark blue, paramagnetic, one-electron reduced neutral complex Re2Cl3(µ-dppm)2(CO)2(PMe3) have been prepared.378 Single crystal X-ray structure analyses (Table 8.7) have shown that [Re2Cl3(µ- dppm)2(CO)2(PMe3)]PF6 and [Re2Cl3(µ-dppm)2(CO)2{P(OEt)3}]PF6 have an all-cis structure like 8.50 with unsymmetrical carbonyl bridges.378 The reactions of Re2Cl4(µ-dppm)2(CO)(CNR) (R = But or Xyl) with PMe3 and TlPF6 yield [Re2Cl3(µ-dppm)2(CO)(CNR)(PMe3)]PF6 in which the structure of the neutral precursor is retained, i.e., when R = But the structure is similar to isomer IV in Fig. 8.29, with PMe3 in place of XylNC, while for R = Xyl the structure resembles 8.50 (Fig. 8.28), with PMe3 in place of the terminal CO.378

A few complexes that are derived from Re2X4(µ-dppm)2 (X = Cl or Br) and contain CS ligands have been prepared by their reaction with carbon disulfide.379 A similar reaction occurs with Re2Br4(µ-dpam)2.379 These oxidative addition reactions afford the edge-shared bioctahedral dirhenium(III) complexes Re2(µ-S)(µ-X)X3(µ-dppm)(CS), which can be derivatized by reaction with organic nitriles, isocyanides, and CO in the presence of TlPF6 to give [Re2(µ-S)(µ-X)X2(µ-dppm)2(CS)(L)]PF6.379,380 The crystal structure of the complex with X = Br and L = EtCN shows a long Re–Re distance (2.949(1) Å) that implies the presence of a surprisingly weak metal-metal bond.379 The complexes of the types Re2(µ-S)(µ-X)X3(µ-LL)2(CS) and [Re2(µ-S)(µ-X)X2(µ-dppm)(CS)(L)]PF6 are converted to the analogous µ-SO2 complexes when reacted with NOPF6 in the presence of O2.380,381 These oxygenation reactions are catalytic in NOPF6. The µ-SO2 complexes possess two reversible one-electron reductions both of which can be accessed in some cases with the use of cobaltocene as the reductant.380,381 When Re2Cl4(µ-dppm)2 is treated with SO2 in tetrahydrofuran, a major product is the paramagnetic complex Re2(µ-Cl)(µ-SO2)Cl4(µ-dppm)2, which has a Re–Re bond distance of 2.6289(3) Å.382 The mechanism of the reaction that leads to this product is clearly quite complicated since it involves oxidation of the Re24+ core and the incorporation of an additional Cl- ligand, presumably through the sacrifice of some of the Re2Cl4(µ-dppm)2 starting material.

8.5.5 Other Re25+ and Re24+ complexes

A few additional examples of authentic triply bonded Re24+ complexes are known of which the homoleptic allyl complex Re2(C3H5)4 is one of the most thoroughly characterized. Rhenium(V) chloride is reacted with allylmagnesium chloride in diethyl ether to give a yel- low-brown solution from which orange crystals of Re2(C3H5)4 may be isolated.383 The crystal

360Multiple Bonds Between Metal Atoms Chapter 8

structure of this complex showed384 that an important difference exists from that of the isostructural pair Cr2(C3H5)4 and Mo2(C3H5)4. Unlike the latter complexes, which possess terminal and symmetrically bridging allyl groups, Re2(C3H5)4 has four chemically equivalent terminal Re(δ3-C3H5) bonds (D2d symmetry). The Re–Re distance of 2.225(7) Å is consistent with those of other Re24+ derivatives (Table 8.4). The He I photoelectron spectrum of Re2(C3H5)4 has been recorded; the data confirm the prediction from relativistic SCF-X_-SW calculations that this complex has a μ2/4β2β*2 configuration, but with a substantial amount of Re-to-allyl / back donation occurring primarily via interaction of the filled Re β and β* orbitals with the allyl /* levels.385

The novel [Re2(NCCH3)10]4+ cation has been isolated as its BF4- salt, following protonation

of [Re2Cl8]2- or Re2Cl4(PPrn3)4 with HBF4·Et2O in CH3CN/CH2Cl2, and its [Mo6O19]2- salt has been structurally characterized.292,386 The Re–Re bond length in this dirhenium(II) complex is

2.259(4) Å and the two halves of the cation are almost perfectly staggered with respect to one another (ρav = 44.5°). The kinetics of exchange of the eight equatorial acetonitrile ligands has been determined in CD3CN by 1H NMR spectroscopy.387

The vacuum pyrolyses of the rhenium(II) porphyrin complexes Re(Por)(PEt3)2 , where Por is the dianion of octaethylporphyrin (OEP) or tetra(p-tolyl)porphyrin, produce the triply bonded complexes Re2(Por)2.388,389 Like their halide-phosphine analogs of the type Re2X4(PR3)4, they can be oxidized in two one-electron steps to give the corresponding Re25+ and Re26+ derivatives. Thus, the treatment of Re2(OEP)2 with [(δ5-C5H5)2Fe]BF4 in acetonitrile and AgBF4 in toluene affords [Re2(OEP)2]BF4 and [Re2(OEP)2](BF4)2, respectively.388 Both of these oxidations are believed to be metal-centered, the first giving rise to the expected paramagnetic ground state.388 The resonance Raman and infrared spectra of the [Re2(OEP)2]n+ species (n = 0–2) have been measured. In the case of [Re2(OEP)2]+, exitation at 514.5 nm gives a weak Raman peak at 290 cm-1 that has been assigned to ι(Re–Re); a Re–Re bond distance of 2.20 Å has been estimated from this stretching frequency.390 The synthesis of the porphyrin complex Re2(AHEDMP)2, where H2AHEDMP is 5-(4-methoxyphenyl)-2,3,7,8,13,17-hexaethyl-12,18-dimethylporphyrin,391 as well as the phthalocyaninato complex Re2(pc)2,392 are similar to those reported for Re2(Por)2. The compound Re2(pc)2 has an eclipsed rotational geometry, a Re–Re distance of 2.285(2) Å, and a Raman-active ι(Re–Re) mode at 240 cm-1.392

The reduction of quadruply bonded complex Re2(DTolF)4Cl2 (Section 8.4.3) by Na/Hg gives the complexes Re2(DTolF)4Cl and Re2(DTolF)4.161 Dark purple crystals of composition Re2(DTolF)4·C6H6 have been characterized by X-ray crystallography; the Re–Re distance is 2.344(2) Å.161 This complex may have the novel triple bond configuration μ2/4β2/*2 based upon the results of SCF-X_ calculations.161

8.5.6 Other dirhenium compounds with triple bonds

In addition to the chloride phase `-ReCl4 and the nonahalo species [Re2(µ-X)3X6]- (see Section 8.5.3), all of which contain bridging halide ligands, a few compounds of Re28+ are known in which bridging ligands are not present and the Re–Re bonding can be considered in terms of the electron-poor μ2/4 ground state configuration.

Rhenium tetrafluoride is believed to exist as Re2F8 molecules in the vapor phase with an eclipsed D4h conformation and a Re>Re bond.393 There are several structurally characterized ternary oxides containing a lanthanide metal together with rhenium, with the latter in an oxidation state of +4 to +5. In two cases,394,395 there are Re(IV) ions present in Re2O8 units having D4h symmetry and Re–Re distances consistent with the presence of triple bonds between the rhenium atoms. In La4Re2O10,394 the overall structure can be thought of as distorted fluorite structure, with each La3+ and each Re28+ ion occupying a distorted cube of eight oxide ions.