Multiple Bonds Between Metal Atoms / 08-Rhenium Compounds

Rhenium Compounds 331

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The reaction of (Bu4N)2Re2Cl8 with the potentially tridentate P,N,P ligand 2,6-bis(diphenyl- phosphino)pyridine in refluxing methanol affords the complex Re2Cl4(µ-bdppp)2, in which the pair of bdppp ligands bridge in a trans head-to-tail fashion through N and P atoms.197 The other Ph2P group on each bdppp ligand is uncoordinated, and these are positioned so as to block access to the axial sites; the non-bonding Re···P distances are 2.98 Å and 3.11 Å. The conversion of (Bu4N)2Re2Cl8 to Re2Cl4(µ-bdppp)2 proceeds via (Bu4N)Re2Cl7(bdppp) (see Section 8.4.4).197

Another ligand that possesses a bridging N,P donor set is 6-diphenylphosphino-2- pyridone (pyphosH). However, it differs from Ph2Ppy and bdppp in being able to bond in both neutral and anionic forms. It reacts with (Bu4N)2Re2Cl8, Re2(µ-O2CCH3)4Cl2 and cis-Re2(µ-O2CCH3)2Cl4(H2O)2 in refluxing acetonitrile to give the diamagnetic Re24+ complex Re2Cl2(µ-pyphos)2(µ-pyphosH), in which the three bridging pyphos/pyphosH ligands are N,P bound. The two cis head-to-tail anionic pyphos ligands have their O atoms located in the vicinity of the axial sites of the dirhenium core (Re···O distances of 2.42 and 2.56 Å).302 The uncoordinated OH group of the pyphosH ligand forms a strong intermolecular hydrogen bond with the uncoordinated O atom of an adjacent symmetry related molecule such that the dirhenium units are linked into dimers-of-dimers (see Fig. 8.22).302

Fig. 8.22. The structure of Re2Cl2(µ-pyphos)2(µ-pyphosH) showing how pairs of these molecules are linked into dimers-of-dimers by intermolecular H-bonds.

Redox chemistry of Re2X4(µ-LL)2 compounds when LL = R2PXPR2

Of all the mixed halide-phosphine complexes of Re24+, those that contain a bridging bidentate phosphine with a single bridgehead group between the two donor atoms possess the richest reaction chemistry. Like most triply bonded dirhenium(II) complexes, those of the type Re2X4(µ-LL)2, where X = Cl, Br or I and LL = dppm, dppa, dppE, dcpm or dpam, exhibit well-defined electrochemical behavior with two reversible one-electron oxidations in the cyclic voltammograms of solutions in 0.1 M Bu4NPF6-CH2Cl2 (Table 8.5). In the case of Re2Cl4(µ-dppm)2, this redox chemistry has been coupled with the reaction chemistry of the cations that are generated. Thus, the reaction of Cl- with the one-electron oxidation product [Re2Cl4(µ-dppm)2]+ (E1/2 = +0.27 V vs. SCE) produces Re2Cl5(µ-dppm)2.203 If the latter complex is in turn oxidized to [Re2Cl5(µ-dppm)2]+, which can be accomplished at a potential of c. +0.6 V vs. SCE (Table 8.5), and this oxidation product reacted with Cl-, then the dirhenium(III) complex Re2Cl6(µ-dppm)2 is formed.203 The latter compound, which can also be synthesized by other means (see Sec. 8.4.4), possesses a µ-dichloro-bridged structure (8.14) with a Re–Re double bond.203 The Re25+ complex Re2Cl5(µ-dppm)2 has also been prepared from the reactions of (Bu4N)2Re2Cl8 with dppm in reagent grade acetone303 and of Re2Cl6(PPrn3)2 with dppm in

332Multiple Bonds Between Metal Atoms Chapter 8

diethyl ether.203 It is paramagnetic, and shows a broad and complex X-band EPR spectrum that is in accord with the unpaired electron being coupled to two Re nuclei, each with a spin 5/2.303 A crystal structure determination confirms that the dppm ligands are bridging, and that the structure most closely resembles that of Re2Cl4(µ-dppm)2 with the additional (fifth) chloride ligand being co-linear with the Re–Re bond. The Re–Re distance of 2.263(1) Å, which is about 0.03 Å longer than that in Re2Cl4(µ-dppm)2,274 reflects the presence of an axial Re–Cl bond.

The conversion of Re2Cl4(µ-dppm)2 to Re2Cl5(µ-dppm)2 and Re2Cl6(µ-dppm)2 can be summarized as follows:

The second step, namely, the conversion of Re2Cl5(µ-dppm)2 to Re2Cl6(µ-dppm)2, represents an interesting case of a reaction in which the oxidation of the Re25+ core to Re26+ results in a reduction in the Re–Re bond order, rather than an increase in bond order to 4.

The net two-electron oxidation of Re2Cl4(µ-dppm)2 to Re2Cl6(µ-dppm)2 has also been accomplished by the direct chlorination of the former complex in THF.304 This type of 2-electron oxidative addition to the Re>Re bond of Re2X4(µ-LL)2 compounds has been encountered in several other instances. Thus, a related oxidation to give an edge-shared bioctahedral complex occurs when Re2Cl4(µ-dppm)2 is reacted with Ph2Se2 in toluene to form Re2Cl4(µ-SePh)2(µ-dppm)2.305 The Ph2PH ligand also oxidatively adds to the Re>Re bond of Re2X4(µ-dppm)2 (X = Cl or Br) and Re2Cl4(µ-dpam)2 to give Re2(µ-X)(µ-PPh2)HX3(µ-LL)2 which contains a terminal Re–H bond.306 Monophenylphosphine likewise reacts with Re2X4(µ-dppm)2 to give Re2(µ-X)(µ- PHPh)HX3(µ-dppm)2,306(b) but when H2S is used the edge-shared bioctahedral compounds that are formed have the structure Re2(µ-H)(µ-SH)X4(µ-dppm)2, in which the hydride ligand bridges the two metal centers.307 More complicated redox chemistry is involved in the oxidation of Re2X4(µ-dppm)2 by dioxygen.308 The initial products are the weakly paramagnetic, edge-shared bioctahedral complexes Re2(µ-O)(µ-X)(O)X3(µ-dppm)2 in which a Re–Re bond is absent. The formation of these compounds precedes further oxidation to dinuclear and mononuclear oxo-Re(V) compounds.308-310

While NOPF6 can be used to oxidize Re2X4(dmpm)3 (X = Cl or Br) to [Re2X4(dmpm)3]PF6, the resultant chloride complex reacts with an additional equivalent of NOPF6 to produce the diamagnetic nitrosyl complex Re2Cl5(µ-dmpm)2(NO) in which the Re–Re distance is 2.379(1) Å and one of the dmpm ligands from the precursor complex has been lost.311 This compound can be treated formally as a Re24+ derivative if we consider the nitrosyl ligand as NO+.

A different type of redox behavioir is encountered when Re2Cl4(µ-dppm)2 is reacted with 7,7'8,8'-tetracyano-p-quinodimethane and 2,5-dimethyl-N,N'-dicyanoquinonediimine to afford the complexes [Re2Cl4(µ-dppm)2]2(µ-TCNQ) and [Re2Cl4(µ-dppm)2]2(µ-DM-DC- NQI).312,313 Both complexes have been characterized by X-ray crystallography and were shown to contain organocyanide bridges linking two Re2Cl4(dppm)2 molecules through equatorial positions. Based upon the spectroscopic, magnetic and electrochemical properties of these two complexes it is reasonable to conclude312,313 that significant change transfer occurs between the dirhenium units and both of these polycyano acceptor molecules. The Re–Re distances in the crystals of composition [Re2Cl4(µ-dppm)2]2(µ-L)·10THF are 2.2895(4) Å (L = TCNQ) and 2.2986(5) Å (L = DM-DCNQI),313 both of which are longer than the distance in Re2Cl4(µ- dppm)2 by c. 0.05 Å.

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Complexes that contain carboxylate and other anionic ligands in conjunction with halides and phosphines

A quite extensive chemistry has been developed in the last few years for Re24+ and Re25+ complexes that contain mixed carboxylate/halide/phosphine ligand sets. The most thoroughly studied compounds are of the types Re2(µ-O2CR)2X2(µ-LL)2 and Re2(µ-O2CR)X4(µ-LL), where LL represents a ligand such as dppm. These complexes were first encountered during studies that involved the reactions of tetrakis(carboxylato)dirhenium(III) complexes Re2(µ-O2CR)4X2 (R = CH3, C2H5 or C6H5; X = Cl or Br) with dppm in methanol or ethanol.271 The reaction products consisted of mixtures of cis and trans-Re2(µ-O2CR)2X2(µ-dppm)2 (structures 8.27 and 8.28). Because of the quite different redox properties of these isomers (Table 8.6) they could be separated and purified by making use of different oxidants to selectively oxidize them to their paramagnetic monocations. This is shown by the scheme in Fig. 8.23.271 When the bis-carbox- ylate complexes Re2(µ-O2CR)2X4L2 are used as starting materials instead of Re2(µ-O2CR)4X2, the chemistry becomes more complex and involves the formation of the Re25+ complexes Re2(µ-O2CR)X4(µ-dppm)2 as intermediates; these species can undergo reductive decarboxylation to give Re2X4(µ-dppm)2 or react with more carboxylate ion in hot methanol to afford cis-Re2(µ-O2CR)2X2(dppm)2.271 The structure of Re2(µ-O2CCH3)Cl4(µ-dppm)2, which has been determined by X-ray crystallography, is that shown in 8.29. The paramagnetic, EPRactive complexes Re2(µ-O2CR)X4(dppm)2 have a well-defined redox chemistry with a reversible one-electron oxidation and an irreversible reduction (see Table 8.6).271 The reactions of (Bu4N)2Re2X8 (X = Cl or Br) with various combinations of dppm and the appropriate carboxylic acid/anhydride or carboxylate salt in alcohol solvents have also been used as a means by which Re2(µ-O2CR)X4(µ-dppm)2, cis-Re(µ-O2CR)2X2(µ-dppm)2 and Re2X4(µ-dppm)2 can be formed.271 Compounds analogous to Re2(µ-O2CR)Cl4(µ-dppm)2 have been obtained in the case of the amidate complexes Re2[µ-HNC(R)O]Cl4(µ-dppm)2 when R = CH3 or Ph (see Section 8.4.3).171,173 Both are paramagnetic Re25+ species and they have been characterized by X-ray crystallography (see Table 8.4).

8.27

8.28

8.29

Fig. 8.23. Reaction scheme showing the products of the reactions of Re2(µ-O2CR)4X2 (R = CH3, C2H5, C6H5; X = Cl or Br) with dppm.

Footnotes: (a) The formation of the cis isomer is favored by long reaction times (several days) and Re2(µ-O2CR)4X2:dppm stoichiometric ratios of c. 1:6. (b) The formation of the trans isomer is favored by shorter reaction times (one day or less) and stoichiometric ratios of 1:<4.

334Multiple Bonds Between Metal Atoms Chapter 8

Table 8.6. Voltammetric E1/2 values for mixed carboxylate-halide-phosphine complexes of Re24+ and Re25+ in dichloromethanea

Compound	E1/2(ox) (2)	E1/2(ox) (1)	Ep,c	ref.
A. Re24+ Compounds
cis-Re2(µ-O2CCH3)2Cl2(µ-dppm)2	+1.34	+0.28	-	271
cis-Re2(µ-O2CC2H5)2Cl2(µ-dppm)2	+1.38	+0.30	-	271
cis-Re2(µ-O2CCCl3)2Cl2(µ-dppm)2	b	+0.60	-	271
cis-Re2(µ-O2CC6H5)2Cl2(µ-dppm)2	+1.38	+0.30	-	271
cis-Re2(µ-O2C-4-C5H4N)2Cl2(µ-dppm)2	b	+0.45	-	320
cis-Re2(µ-O2CC6H4-4-PPh2)2Cl2(µ-dppm)2	+1.40	+0.33	-	321(b)
cis-Re2(µ-O2C-4-quin)2Cl2(µ-dppm)2	b	+0.49	-	321(b)
cis-Re2(µ-O2CC6H4-4-PPh2)2Cl2(µ-dppm)2(AuCl)2	+1.44c	+0.40	-	321(b)
cis-Re2Cl2(µ-dppm)2[µ-O2CC6H4-4-N-	+1.25	+0.31	-	321(b)
(CH2PPh2)2(PdCl2)]2
cis-Re2(µ-O2CC6H10CO2Et)2Cl2(µ-dppm)2	+1.39	+0.32	-	323
cis-Re2Cl2(µ-dppm)2[(µ-O2CC5H4)2Fe]	+1.29	+0.31	-	323
cis-Re2(µ-O2CCH3)]2Br2(µ-dppm)2	+1.41	+0.35	-	271
cis-Re2(µ-O2CC2H5)2Br2(µ-dppm)2	+1.41	+0.35	-	271
cis-Re2(µ-O2CCCl3)2Br2(µ-dppm)2	b	+0.67	-	271
cis-Re2(µ-O2CC6H5)2Br2(µ-dppm)2	+1.37	+0.36	-	271
cis-Re2(µ-O2CCH3)2(NCBH3)2(µ-dppm)2	b	+0.60	-1.29,	325
			-1.48
cis-Re2(µ-O2CCH3)Cl2(µ-dppa)2	+1.39	+0.35	-1.58	284
cis-Re2(µ-O2CCH3)2Br2(µ-dppa)2	+1.40	+0.39	-1.52	284
cis-Re2(µ-O2CCH3)2Br2(µ-dpam)2	+1.38	+0.28	-	314
cis-Re2(µ-O2CC6H5)2Br2(µ-dpam)2	+1.31	+0.20	-	314
cis-Re2(µ-O2CCH3)2Cl2(µ-Ph2Ppy)2	+1.20	+0.11	-	319
cis-Re2(µ-O2CC2H5)2Cl2(µ-Ph2Ppy)2	+1.18	+0.10	-	319
cis-Re2(µ-O2CC2H5)2Br2(µ-Ph2Ppy)2	+1.22	+0.14	-	319
trans-Re2(µ-O2CCH3)2Cl2(µ-dppm)2	+0.93	-0.27	-	271
trans-Re2(µ-O2CC2H5)2Cl2(µ-dppm)2	+0.93	-0.29	-	271
trans-Re2(µ-O2CC6H5)2Cl2(µ-dppm)2	+1.00	-0.25	-	271
trans-Re2(µ-O2C-4-C5H4N)2Cl2(µ-dppm)2	+1.17	-0.08	-	314
trans-Re2(µ-O2C-3-C5H4N)2Cl2(µ-dppm)2	+1.11	-0.12	-	315
trans-Re2(µ-O2CCH3)2Br2(µ-dppm)2	+0.97	-0.21	-	271
trans-Re2(µ-O2CC6H5)2Br2(µ-dppm)2	+1.00	-0.22	-	271
trans-Re2(µ-O2CCH3)2Cl2(µ-dppa)2	+0.97d	-0.22d	-	284
trans-Re2(µ-O2CCH3)2Br2(µ-dppa)2	+0.99d	-0.19d	-	284
trans-Re2(µ-O2CCH3)2Cl2(µ-dmpm)2	+0.76d	-0.42d	-	317
trans-Re2(µ-O2CCH3)2Cl2(µ-dippm)2	+0.79d	-0.50d	-	293
trans-Re2(µ-O2CCH3)2Cl2(µ-dcpm)2	+0.78	-0.51	-	138
trans-Re2(µ-O2CCH3)2Cl2(µ-dppE)2	+0.97	-0.27	-	287
trans-Re2(µ-O2CC6H4-4-PPh2)2Cl2(µ-dppE)2	+1.05	-0.23	-	321(b)
trans-Re2(µ-O2CC6H4-4-PPh2)2Cl2(µ-dppE)2(AuCl)2	+1.06	-0.15	-	321(b)
trans-Re2(µ-O2C-4-quin)2Cl2(µ-dppE)2	+1.13	-0.12	-	321(b)
trans-Re2(µ-O2CCH3)2Cl2(µ-cdpp)2	+0.96	-0.33	-	286
trans-Re2(µ-O2CCH3)2Br2(µ-dpam)2	+1.01	-0.24	-	314

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Compound	E1/2(ox) (2)	E1/2(ox) (1)	Ep,c	ref.
trans-Re2(µ-O2CC6H5)2Br2(µ-dpam)2	+1.03	-0.25	-	314
[Re2(µ-O2CCH3)Cl2(µ-dmpm)3]Cl	-	+0.75	-1.48	317
[Re2(µ-O2CC2H5)Cl2(µ-dmpm)3]Cl	-	+0.69	-1.55	317
[Re2(µ-O2CCH3)Br2(µ-dmpm)3]Br	-	+0.74	-1.52	317
[Re2(µ-O2CC2H5)Br2(µ-dmpm)3]Br	-	+0.73	-1.48	317
	B. Re25+ Compounds
Re2(µ-O2CCH3)Cl4(µ-dppm)2	-	+0.52	-0.60	271
Re2(µ-O2CC2H5)Cl4(µ-dppm)2	-	+0.49	-0.58	271
Re2(µ-O2CC5H4N)Cl2(µ-dppm)2	-	+0.57	-0.53	323
Re2(µ-O2CCH3)Br4(µ-dppm)2	-	+0.55	-0.52	271
Re2(µ-O2CCH3)Cl4(µ-dmpm)2	-	+0.39	-0.69	139(b)
Re2(µ-O2CC2H5)Cl4(µ-dmpm)2	-	+0.40	-0.70	139(b)
Re2(µ-O2CCH3)Br4(µ-dmpm)2	-	+0.42	-0.64	139(b)
Re2(µ-O2CCH3)Cl4(µ-dppa)2	-	+0.49	-0.53	284
Re2(µ-O2CCH3)Br4(µ-dppa)2	-	+0.57	-0.39	284
Re2(µ-O2CCH3)Cl4(PPh3)2	-	+0.56c	-0.62e	318
Re2(µ-O2CCH3)Br4(PPh3)2	-	+0.58c	-0.58e	318
Re2(µ-O2CCH3)Cl4[P(CH2Ph)3]2	-	+0.54	-0.67e	143
Re2(µ-O2CCH3)Cl4(PMePh2)2	-	+0.48	-0.65e	142(b)
Re2(µ-O2CCH3)Cl4[P(C6H4-4-OMe)3]2	-	+0.47	-0.67e	142(b)
Re2(µ-O2CCH3)Cl4(Ph2Ppy)2	-	+0.47	-0.56e	318
Re2(µ-O2CC2H5)Cl4(Ph2Ppy)2	-	+0.49	-0.49e	318
Re2(µ-O2CCH3)Br4(Ph2Ppy)2	-	+0.58	-0.40e	318
Re2(µ-O2CCH3)Cl4(δ3-L1)f	-	+0.42	-1.03e	191(b)
Re2(µ-O2CCH3)Cl4(δ3-L2)f	-	+0.42	-1.00e	191(b)
Re2(µ-O2CCH3)Cl4(δ3-L3)f	-	+0.36	-1.09e	191(b)
Re2(µ-O2CCH3)Cl4(δ3-L4)f	-	+0.30	-1.10e	191(b)
Re2(µ-O2CCH3)Cl4(δ3-L5)f	-	+0.33	-1.11e	191(b)
Re2(µ-O2CC6H4-4-PPh2)Cl4(δ3-L1)f	-	+0.39	-1.02	191(b)
Re2(µ-O2CC6H4-2-PPh2)Cl4(δ3-L1)f	-	+0.37	-1.06	191(b)
Re2(µ-O2C-4-quin)Cl4(δ3-L1)f	-	+0.44	-0.97	191(b)
Re2(µ-O2CC6H4-4-PPh2)Cl4(δ3-L3)f	-	+0.37	-1.06	191(b)
Re2(µ-O2CC6H4-2-PPh2)Cl4(δ3-L3)f	-	+0.34	-1.10	191(b)
Re2(µ-O2C-quin)Cl4(δ3-L3)f	-	+0.40	-1.01	191(b)
[Re2Cl4(δ3-L1)]2(µ-O2CC6H4CO2)f	-	+0.43g	-1.00g	191(b)
[Re2Cl4(δ3-L3)]2(µ-O2CC6H4CO2)f	-	+0.39g	-1.05g	191(b)

aUnless otherwise stated, data are in volts vs the Ag/AgCl electrode with a Pt-bead working electrode and 0.1 M Bu4NPF6(TBAH) as supporting electrolyte.

b	E1/2(ox)(2) beyond the limits of the measurement.
c	Ep,a value
d	Potentials are based upon CV measurements on a salt of the monocation of the neutral Re24+ complex. In this
e	instance the neutral trans-Re2(µ-O2CCH3)2Cl2(µ-LL)2 complex has not been isolated.
	E1/2(red) value.
f	This compound contains a tridentate donor designated as Ln, the identity of which is given in the text (see also
	ref. 191(b)).
g	These processes are broadened due to weak electronic coupling between the pairs of dirhenium units.

336Multiple Bonds Between Metal Atoms Chapter 8

A related chemistry has been developed in the case of the dppa complexes.284 The main differences from the dppm system are as follows. First, in the dppa system, complexes of the type trans-[Re2(µ-O2CCH3)2X2(µ-LL)2]X are important intermediates in the formation of cis- Re2(µ-O2CCH3)2X2(µ-dppa)2 but the same does not appear to be the case with the analogous dppm complexes. Second, the reductive decarboxylation of Re2(µ-O2CR)X4(µ-dppm)2 to give Re2X4(µ-dppm)2 occurs more rapidly than does their reaction with carboxylate ion to give trans-[Re2(µ-O2CR)2X2(µ-dppm)2]X.271 This is in contradistinction to the behavior of Re2(µ- O2CCH3)X4(µ-dppa)2, which readily converts to trans-[Re2(µ-O2CCH3)2X2(µ-dppa)2]X in the presence of an excess of dppa.284

The most direct and easiest means of obtaining the cis and trans isomers of Re2(µ- O2CR)2X2(µ-LL)2 is the most obvious one, namely, the reaction of Re2X4(µ-LL)2 with a source of the appropriate carboxylate anion. This has been demonstrated in the case of the reactions of

Re2Cl4(µ-dppm)2 and Re2Br4(µ-dpam)2 with [PPN]O2CR (PPN = (Ph3P)2N+; R = CH3, C2H5, C6H5 or 4-C5H5N) in CH2Cl2 at room temperature give trans-Re2(µ-O2CR)2X2(µ-LL)2.314 If the

reactions are carried out in refluxing ethanol, the cis isomers are obtained in the case of R = CH3 or C2H5, a mixture of cis and trans for R = C6H5, and only the trans form when R = 4-C5H5N. Upon heating trans-Re2(µ-O2CR)2Cl2(µ-dppm)2 (R = CH3 or C2H5) in ethanol, isomerization to cis-Re2(µ-O2CR)2Cl2(µ-dppm)2 occurs, signifying that this is indeed the thermodynamically favored form when LL = dppm.314 Recently, the trans isomer that contains nicotinate (pyridine- 3-carboxylate) has been prepared by this same method.315

When Re2Cl4(µ-dppm)2 is reacted with pyridine-2-carboxylic acids, or their [PPN]+ salts, more complicated structures are obtained because the pyridine N is involved in forming N,O chelate rings.315 With py-2-CO2H, py-2,3-(CO2H)2 and py-2,4-(CO2H)2, complexes of the type Re2(δ2-N,O)Cl3(µ-dppm)2 are formed in which the µ-dppm ligands are bound in a trans, cis fashion, with the chelating pyridine carboxylate ligand being coordinated to the Re atom that has the trans disposition of P atoms.315 With the [PPN]+ salt of pyridine-2,6-dicarboxylic acid (dipicH2), both kinetic and thermodynamic products of composition Re2(dipic)Cl2(µ-dppm)2 are formed; the former species (8.30) is favored at room temperature and converts quantitatively to the thermodynamically stable form (8.31) when refluxed in ethanol. This conversion occurs by a partial “merry-go-round” process that results in a switch from a trans,trans to trans,cis coordination of the dppm ligands. Both isomers have been structurally characterized; they are labeled as isomer A and B, respectively, in Table 8.4. Note that a third isomer (C) has been isolated by the reaction of cis-Re2(µ-O2CCH3)2Cl2(µ-dppm)2 with dipicH2 and contains a cis,cis coordination of the µ-dppm ligands.316

8.30

8.31

While the synthesis of Re2(µ-O2CR)2X2(µ-LL)2 compounds directly from Re2X4(µ-LL)2 is the most logical synthetic route, compounds such as Re2Cl4(µ-dppm)2 are often themselves best prepared from (Bu4N)2Re2Cl8 via the intermediacy of the carboxylate complexes Re2(µ-

O2CCH3)Cl4(µ-dppm)2 (see earlier discussion in Section 8.5.4).271 Accordingly, the reactions

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of Re2(µ-O2CCH3)4Cl2 and cis-Re2(µ-O2CCH3)2X4L2 (X = Cl or Br; L = H2O, py or 4-Mepy) have continued to be used to prepare Re24+ and Re25+ carboxylate complexes. The phosphines dcpm,138 dippm,293 dppE,287 and cdpp286 have been reacted with Re2(µ-O2CCH3)4Cl2 in refluxing methanol to produce either neutral trans-Re2(µ-O2CCH3)2Cl2(µ-LL)2 (LL = dppE or cdpp)286,287 or the cationic species trans-[Re2(µ-O2CCH3)2Cl2(µ-LL)]+ (LL = dcpm or dippm).138,293 These compounds have been structurally characterized and their structures resemble closely those of the neutral and cationic cis and trans isomers in the case of LL = dppm or dppa (Table 8.4). The cyclic voltammetric data for these complexes are compared in Table 8.6.

Various mixed carboxylate-dmpm complexes have been prepared through the use of Re2(µ-O2CR)4X2 (X = Cl or Br) and Re2Cl4(µ-dmpm)3 as starting materials. Salts of the trans- [Re2(µ-O2CR)2X2(µ-dmpm)2]+ and [Re2(µ-O2CR)X2(µ-dmpm)3]+ cations have been obtained,317 and the acetate complexes [Re2(µ-O2CCH3)X2(µ-dmpm)3]PF6 have been oxidized by NOPF6 to the paramagnetic 1:2 salts [Re2(µ-O2CCH3)X2(µ-dmpm)3](PF6)2. When dmpm is reacted with cis-Re2(µ-O2CR)2X4L2 (X = Cl or Br; R = CH3 or C2H5; L = H2O or py) the compounds Re2(µ-O2CCH3)X4(µ-dmpm)2 and Re2(µ-O2CC2H5)Cl4(µ-dmpm)2 can be isolated.139 Note that Re2(µ-O2CCH3)Cl4(µ-dmpm)2 reacts further with dmpm to afford Re2Cl4(µ-dmpm)3,139(b) and it has been reacted with Ph2PH as a route to Re2(µ-Cl)(µ-PPh2)HCl3(µ-dmpm)2.306(b)

A comparison has also been made of the reactions between cis-Re2(µ-O2CCH3)2X4L2 (X = Cl or Br; L = H2O, py or 4-Mepy) and Ph2Ppy or PPh3 in refluxing ethanol or acetone.318,319 Both ligands form similar dark red paramagnetic complexes Re2(µ-O2CCH3)X4(Ph2Ppy)2 and Re2(µ-O2CCH3)X4(PPh3)2, which are in turn related to Re2(µ-O2CR)X4(µ-LL)2 (LL = dppm or dppa). A single crystal X-ray structure determination on Re2(µ-O2CCH3)Cl4(PPh3)2 shows it to contain an eclipsed rotational geometry as in 8.32, and a short Re–Re distance (Table 8.4).318 This structure is related to that in 8.29, except that with only two monodentate phosphines present the two axial Re–Cl bonds (in 8.29) now switch to become equatorial bonds. In the case of the Ph2Ppy ligand, longer reaction times favor the formation of cis-Re2(µ-O2CR)2X2(µ-Ph2Ppy)2 (X = Cl when R = CH3 or C2H5 and X = Br when R = C2H5).319 These same compounds are formed when the dirhenium(II) complexes Re2X4(µ-Ph2Ppy)2(PEt3) (X = Cl or Br) are reacted with NaO2CR in refluxing methanol for 1 day.319 Oxidation of cis-Re2(µ-O2CCH3)2Cl2(µ- Ph2Ppy)2 with [(δ5-C5H5)2Fe]PF6 gives the paramagnetic monocationic species which has been characterized by X-ray crystallography (Table 8.4).319

8.32

The triphenylphosphine ligands in Re2(µ-O2CCH3)Cl4(PPh3)2 are easily replaced by certain monodentate phosphines (i.e. P(CH2Ph)3, P(C6H4-4-OMe)3 and PMePh2) to form other Re2(µ-O2CCH3)Cl4(PR3)2 complexes, and also by bidentate dppm, dppa and dppE to give Re2(µ-O2CCH3)Cl4(µ-LL)2, all in high yield.142(b) The aforementioned tribenzylphosphine complex Re2(µ-O2CCH3)Cl4[P(CH2Ph)3]2 has also been prepared by the reaction between cis-Re2(µ-O2CCH3)2Cl4(H2O)2 and this phosphine in refluxing methanol.143

338Multiple Bonds Between Metal Atoms Chapter 8

Of all the Re24+ and Re25+ mixed carboxylate/halide/phosphine complexes that have been isolated, the two that have generated the greatest interest are cis-Re2(µ-O2CCH3)2Cl2(µ- dppm)2 and Re2(µ-O2CCH3)Cl4(µ-dppm)2, primarily as a result of the lability of their acetate ligands and, consequently, their use as synthons. The displacement of the µ-acetato ligands in cis-Re2(µ-O2CCH3)2X2(µ-dppm)2 upon reaction with trichloroacetic acid provides a route to cis-Re2(µ-O2CCCl3)2X2(µ-dppm)2,271 while the reaction of the chloro complex with isonicotinic acid gives cis-Re2(µ-O2C-4-C5H4N)2Cl2(µ-dppm)2, which has in turn been used to prepare the hybrid mixed-metal molecular squares [cis-Re2(µ-O2CC5H4N)2X2(µ-dppm)2PtL2]2(O3SCF3)4, where X = Cl for L = PEt3 and X = O3SCF3 for L = dbbpy.320 The latter compounds, which contain triply bonded Re24+ and planar Pt(II) units at the corners, contain bridging isonicotinate as the linker ligands; the connectivity (8.33) has been established by an X-ray crystal structure determination of the supramolecular assembly with L = PEt3.320 A variety of behavior has been found upon the reaction of cis-Re2(µ-O2CCH3)2Cl2(µ-dppm)2 with the picolinic (picH), dipicolinic (dipicH2), 2-hydroxynicotinic (HnicOH) and 6-hydroxypicolinic (HpicOH) acids. Picolinic acid gives Re2(pic)Cl3(µ-dppm)2, which is identical to the product formed upon the reaction of [PPN]pic with Re2Cl4(µ-dppm)2,315 while the other acids form Re2(dipic)Cl2(µ- dppm)2, cis-Re2(HnicO)2Cl2(µ-dppm)2 and cis-Re2(picO)2(µ-dppm)2, respectively.316 The dipicolinate complex is the third isomeric form of this compound (isomer C), and the compound formed from 6-hydroxypicolinic acid involves the coordination of two cis tridentate picO2- ligands that displace both the acetate groups and the two terminal chloride ligands. All these compounds have been characterized by X-ray crystallography (Table 8.4).316

8.33

The reactions of cis-Re2(µ-O2CCH3)2Cl2(µ-dppm)2 and trans-Re2(µ-O2CCH3)2Cl2(µ-dppE)2 with substituted benzoic acids of the type 4-XC6H4CO2H (X = Ph2P, Ph2P(O), Ph2P(S) or Ph2P(O)CH2) and with quinoline-4-carboxylic acid lead to displacement of the acetato ligands and retention of the cis and trans stereochemistries.321 The electrochemical properties of these products are similar to those of the parent molecules (representative data only are given in Table 8.6), and the X-ray crystal structures of the complexes cis-Re2(µ-O2CC6H4-4-PPh2)2Cl2(µ- dppm)2 and trans-Re2(µ-O2C-4-quin)2Cl2(µ-dppm)2 have been determined (Table 8.4).321 The use of the pendant donor atoms in these complexes to coordinate other metal centers has been demonstrated in the case of the two structurally characterized compounds cited above through the synthesis of mixed-metal complexes that contain Au(I) and Pd(II). The isolated products include the interesting mixed Re2Pd2 complex cis-Re2(µ-O2C6H4-4-PPh2)2Cl2(µ- dppm)2(Pd2Cl4), that has a structure (Fig. 8.24) which can be considered to be that of a molecular “tweezer”.321 A different type of Re2Pd2 assembly has been obtained by the reaction of cis-Re2(µ-O2CCH3)2Cl2(µ-dppm)2 with PdCl2[(Ph2PCH2)2N-4-C6H4CO2H], the latter reagent containing a free carboxylic acid group.321(b)

Studies have been made of the reactions between dicarboxylic acids and cis-Re2(µ- O2CCH3)2Cl2(µ-dppm)2 and Re2(µ-O2CCH3)Cl4(µ-dppm)2.322-324 The bis-acetate reacts with terephthalic acid to give the triangular assembly {[Re2Cl2(µ-dppm)2](µ-O2CC6H4CO2)}3 (Fig. 8.25), in which the three [Re>Re]4+ units have very similar Re–Re bond distances (Table 8.4).322 Electrochemical measurements show322,323 that the Re2 units are only very weakly coupled. While a

Rhenium Compounds 339

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similar triangular structure is probably formed with trans-1,4-cyclohexanedicarboxylate when 1:1 proportions of reagents are used,323 the use of a higher proportion of this diacid gives

the complex [cis-Re2(µ-O2CC6H10CO2H)2Cl2(µ-dppm)2]2(µ-O2CC6H10CO2). In the solid-state its structure consists of this “dimer-of-dimers” unit which is linked into an infinite zig-zag

chain-like polymer through intermolecular hydrogen-bonds involving the “free” carboxylic acid groups of the cis µ-O2CC6H10CO2H ligands.322,323 The diacid 1,1'-ferrocenedicarboxylic acid reacts with cis-Re2(µ-O2CCH3)2Cl2(µ-dppm)2 to give exclusively the trimetallic compound cis-Re2Cl2(µ-dppm)2[(µ-O2CC5H4)2Fe],323 rather than a compound in which the acid serves to bridge the dirhenium units, as occurs with dimolybdenum(II) to form a “dimer-of-dimers” or supramolecular square. The structure of this complex has been determined (see Table 8.4) as well as its electrochemical properties (Table 8.6).

Fig. 8.24. The structure of the Re2Pd2 moleculer “tweezer” complex cis-Re2(µ-O2CC6H4-4-PPh2)2Cl2(µ-dppm)2(Pd2Cl4).

Fig. 8.25. The structure of the molecule {[Re2Cl2(µ-dppm)2](µ-O2CC6H4CO2)}3 with the phenyl groups omitted.

340Multiple Bonds Between Metal Atoms Chapter 8

The paramagnetic mono-acetate complex Re2(µ-O2CCH3)Cl4(µ-dppm)2 also reacts with carboxylic acids in a fashion similar to that of cis-Re2(µ-O2CCH3)2Cl2(µ-dppm)2. Isonicotinic acid gives the expected product Re2(µ-O2CC5H4N)Cl4(µ-dppm)2 (see Tables 8.4 and 8.6),323 while with terephthalic acid the “dimer-of-dimers” complex [Re2Cl4(µ-dppm)2]2(µ-O2CC6H4CO2) is formed.322 Its structure is represented in 8.34 (with the trans sets of dppm ligands omitted).322 Similar dicarboxylate-bridged complexes are formed with the use of adipic acid, 4,4'- biphenyldicarboxylic acid and fumaric acid.323 When trans-1,4-cyclohexanedicarboxylic acid is reacted with Re2(µ-O2CCH3)Cl4(µ-dppm)2 in refluxing ethanol, the only product isolated was the reduced Re24+ complex cis-Re2(µ-O2CC6H10CO2Et)2Cl2(µ-dppm)2 (see Tables 8.4 and 8.6).323 The interactions between the paramagnetic centers in [Re2Cl4(µ-dppm)2]2(µ-L), where L = terephthalate, adipate, 4,4'-biphenyldicarboxylate or fumarate, have been probed by magnetic susceptibility and/or cyclic voltammetric and differential pulsed voltammetric measurements.323 Only in the case of the terephthate and fumarate bridged complexes is there evidence for weak coupling.

8.34

When the alkyne carboxylic acids HO2CC>CCO2H and CH3C>CCO2H are refluxed with Re2(µ-O2CCH3)Cl4(µ-dppm)2, decarboxylation occurs to give the paramagnetic µ-alkyne and diamagnetic µ-carbyne complexes Re2(µ-Cl)(µ-δ2-HCCH)Cl4(µ-dppm)2 and Re2(µ-Cl)(µ- CCH2CH3)Cl4(µ-dppm)2.324 Both reactions are believed to proceed via the formation of the Re25+ alkynoates. The crystal structures of the edge-sharing bioctahedral products show that the Re–Re bond distances are 2.6567(5) Å and 2.5277(6) Å, respectively; these values are consistent with Re–Re bond orders of 1.5 and 2.

Other examples of substitution reactions involving cis-Re2(µ-O2CR)2Cl2(µ-dppm)2 include that of the acetate complex with NaBH3CN in THF which gives cis-Re2(µ-O2CCH3)2(NCBH3)2(µ- dppm)2, while the reactions of cis-Re2(µ-O2CC2H5)2Cl2(µ-dppm)2 with nitriles RCN in the presence of HBF4·Et2O or HPF6(aq) provide a route to salts of the cis-[Re2Cl2(µ-dppm)2(NCR)4]2+ cations.325 The exposure of cis-Re2(µ-O2CR)2Cl2(µ-dppm)2 (R = Me or Et) to gaseous H2S in the presence of HBF4·Et2O gives either cis-Re2(µ-SH)2Cl2(µ-dppm)2, when THF or CHCl3 is used as the solvent, or the gem-dithiolato complexes cis-Re2(µ-S2CR1R2)Cl2(µ-dppm)2 and cis-Re2(µ-S2CHR2)Cl2(µ-dppm)2 in the presence of ketones (R1R2CO) and aldehydes (R2CHO).326 Single crystal X-ray structural characterizations of cis-Re2(µ-SH)2Cl2(µ-dppm)2 and cis-Re2(µ- S2CMe2)Cl2(µ-dppm)2 show that both complexes possess similar cradle-like geometries and short Re–Re distances that accord with retention of the electron-rich Re–Re triple bond (Table 8.4). The electrochemical properties of this series of compounds are very similar, with two oxidations (Ep,a 1.4 V and E1/2(ox) 0.65 V versus Ag/AgCl) and a one-electron reduction with E1/2(red) -1.6 V versus Ag/AgCl being observed in all cases.326

A different structural type of mixed carboxylate/halide/phosphine complex is obtained when the synthon cis-Re2(µ-O2CCH3)2Cl4(H2O)2 is reacted with tridentate ligands that contain P2O and P2N donor sets.191 The complexes that are formed are derivatives of the Re25+ core and all have the same unsymmetrical structure shown in 8.35, in which an O or N atom is weakly bound in