Multiple Bonds Between Metal Atoms / 08-Rhenium Compounds
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Rhenium Compounds 311
Walton
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Rotational |
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Compound |
r(Re–Re)(Å)a |
Geometryb |
ref. |
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`-Re2Cl4(dppee)2 |
|
2.242(3) |
staggered |
276 |
_-Re2Cl4(depe)2 |
|
2.2608(6) |
eclipsed |
279 |
`-Re2Cl4(depe)2 |
|
2.211(1) |
staggered |
278 |
_-Re2Cl4(dmpe)2·CH3OH |
|
2.266(1) |
eclipsed |
274 |
_-Re2Cl4(dppp)2 |
|
2.264(1) |
eclipsed |
273 |
_-Re2Cl4(dppp)2·4CH2Cl2 |
|
2.2559(8) |
eclipsed |
189 |
`-Re2Cl4(dpae)2 |
|
2.231(2) |
staggered |
272 |
Re2Cl4(µ-dppm)2 |
|
2.234(3)e |
staggered |
274 |
|
|
2.2497(4)e |
staggered |
294 |
|
|
2.2368(5) |
staggered |
294 |
Re2(CH3)4(µ-dppm)2 |
|
2.284(7) |
staggered |
289 |
Re2(NCBH3)4(µ-dppm)2(H2O)2·2THF |
2.2874(5) |
staggered |
290 |
|
Re2[N(CN)2]4(µ-dppm)2(DMF)2·3DMF |
2.2960(5) |
staggered |
291 |
|
Re2Cl4(µ-dppa)2·(CH3)2CO |
|
2.2417(5) |
staggered |
288 |
Re2Cl4(µ-dppE)2·CH2Cl2 |
|
2.2448(5) |
staggered |
287 |
Re2Cl4(µ-dcpm)2 |
|
2.2256(4) |
staggered |
294 |
|
|
2.2267(4) |
staggered |
294 |
Re2Cl4(µ-dmpm)3 |
(orthorhombic form) |
2.309(2) |
staggered |
295 |
|
(monoclinic form) |
2.3157(4) |
staggered |
295 |
[Re2Cl3(dpmp)2]Cl |
|
2.307(1) |
staggered |
297 |
[Re2Cl3(dpmp)2]PF6 |
|
2.300(1) |
staggered |
297 |
Re2Cl4(µ-dppm)(PMe3)2·0.75C7H8 |
|
2.238(1) |
staggered |
285 |
|
|
2.242(1) |
staggered |
285 |
`-Re2Cl4(dppm)(dppe) |
|
2.237(1) |
staggered |
298 |
Re2Cl3(Ph2Ppy)2[(C6H5)(C6H4Ppy] |
|
2.336(2) |
staggered |
207,301 |
Re2Cl4(Ph2Ppy)2(PEt3) |
|
2.270(1) |
staggered |
207 |
[Re2Cl2(Ph2Ppy)4](PF6)2·2(CH3)2CO |
2.300(1) |
staggered |
207 |
|
Re2Cl4(bdppp)2 |
|
2.2342(6) |
eclipsed |
197 |
Re2Cl2(pyphos)2(pyphosH)·CH3CN |
2.2693(3) |
eclipsed |
302 |
|
cis-Re2(µ-O2CCH3)2Cl2(µ-dppm)2 |
|
2.315(1) |
staggered |
271 |
cis-Re2(µ-O2CCH3)2(NCBH3)2(µ-dppm)2·CH2Cl2 |
2.2938(7) |
staggered |
325 |
|
cis-Re2(µ-O2CC5H4N)2Cl2(µ-dppm)2·2C2H5OH |
2.3271(4) |
staggered |
320 |
|
[cis-Re2(µ-O2CC5H4N)2(O3SCF3)2(µ-dppm)2Pt(dbbpy)]2- |
2.2839(15) |
staggered |
320 |
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(O3SCF3)4·2.37CH2Cl2·1.18H2O |
|
|
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cis-Re2(µ-O2CC6H10CO2Et)2Cl2(µ-dppm)2·2C2H5OH |
2.3120(5) |
staggered |
323 |
|
[cis-Re2(µ-O2CC6H10CO2H)2Cl2(µ-dppm)2]2- |
2.3172(9) |
staggered |
322 |
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(µ-O2CC6H10CO2)·7C2H4Cl2 |
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{[cis-Re2Cl2(µ-dppm)2](µ-O2CC6H4CO2)}3·2C6H6·H2O |
2.3192(12)f |
staggered |
322 |
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2.3186(13)f |
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2.3185(12)f |
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cis-Re2Cl2(µ-dppm)2[(µ-O2CC5H4)2Fe]·1.43C2H5OH |
2.3218(3) |
eclipsed |
323 |
|
cis-Re2(µ-O2CCH3)2Cl2(µ-dppa)2 |
|
2.3067(5) |
staggered |
284 |
cis-Re2(µ-O2CC6H4-4-PPh2)2Cl2(µ-dppm)2·2C2H5OH |
2.3040(2) |
staggered |
321 |
|
312Multiple Bonds Between Metal Atoms Chapter 8
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Rotational |
|
Compound |
r(Re–Re)(Å)a |
Geometryb |
ref. |
|
cis-Re(µ-O2CC6H4-4-PPh2)2Cl2(µ-dppm)2(Pd2Cl4) |
2.3295(6) |
staggered |
321 |
|
trans-Re2(µ-O2CCH3)2Cl2(µ-dppm)2·C7H8·2H2O |
2.2763(7) |
eclipsed |
314 |
|
trans-Re2(µ-O2C-3-C5H4N)2Cl2(µ-dppm)2 |
2.2931(3) |
eclipsed |
315 |
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trans-Re2(µ-O2CCH3)2Cl2(µ-dppE)2·CH3OH |
2.2861(6) |
staggered |
287 |
|
trans-Re2(µ-O2C-4-quin)2Cl2(µ-dppE)2 |
2.2808(4) |
staggered |
321(b) |
|
trans-Re2(µ-O2CCH3)2Cl2(µ-cdpp)2·CH3OH |
2.2871(5) |
eclipsed |
286 |
|
|
|
2.2858(5) |
eclipsed |
286 |
[Re2(µ-O2CCH3)Cl2(µ-dmpm)3]Cl·2H2O·CH2Cl2 |
2.304(1) |
staggered |
317 |
|
Re2(pic)Cl3(µ-dppm)2 |
|
2.2816(4) |
staggered |
315 |
Re2(pic)Cl3(µ-dppm)2·CH2Cl2 |
|
2.2937(4) |
staggered |
315 |
2Re2(pic)Cl3(µ-dppm)2·Re2Cl6(µ-dppm)2·2.172CH2Cl2 |
2.2841(5) |
staggered |
315 |
|
Re2[(O2C-2-(EtO2C-3-)py]Cl3(µ-dppm)2·CH2Cl2 |
2.2752(2) |
staggered |
315 |
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Re2[(O2C-2-(HO2C-4-)py]Cl3(µ-dppm)2·CH2Cl2·C6H6 |
2.2713(5) |
staggered |
315 |
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Re2(dipic)Cl2(µ-dppm)2·3C6H6 (Isomer A) |
2.2750(10) |
staggered |
315 |
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Re2(dipic)Cl2(µ-dppm)2·0.5CH2Cl2 (Isomer B) |
2.2512(3) |
staggered |
315 |
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Re2(dipic)Cl2(µ-dppm)2·2CH2Cl2 (Isomer C) |
2.2583(3) |
staggered |
316 |
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Re2(HnicO)2Cl2(µ-dppm)2 |
|
2.3035(6) |
staggered |
316 |
Re2(picO)2(µ-dppm)2·H2O |
|
2.3139(3) |
staggered |
316 |
Re2(µ-SH)2Cl2(µ-dppm)2·0.5CH2Cl2 |
2.2577(5) |
eclipsed |
326 |
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Re2(µ-S2CMe2)Cl2(µ-dppm)2·CH2Cl2 |
2.2544(6) |
eclipsed |
326 |
|
Re2(acac)Cl3(µ-dppm)2 |
|
2.2542(5) |
staggered |
329 |
Re2(acac)2Cl2(µ-dppm)2·(C2H5)2O |
|
2.2968(3) |
staggered |
329 |
Re2Cl5(µ-dmpm)2(NO) |
|
2.379(1) |
staggered |
311 |
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B. Re25+ Compounds |
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[1,3,6,8-Re2Cl4(PMe3)4]ReO4 |
|
2.205(1) |
eclipsed |
250 |
[1,3,6,8-Re2Cl4(PMe3)4]Cl·CH2Cl2 |
|
2.2152(9) |
eclipsed |
260 |
[1,3,6,8-Re2Cl4(PMe3)4]I·CH2Cl2 |
|
2.2122(7) |
eclipsed |
184 |
[1,3,6,8-Re2Cl4(PMe2Ph)4]PF6·0.5THF |
2.218(1) |
eclipsed |
243 |
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1,3,6-Re2Cl5(PMe3)3 |
|
2.2182(7) |
eclipsed |
248 |
1,3,6-Re2Cl5(PMe3)3·0.5CH2Cl2 |
|
2.211(1) |
eclipsed |
259 |
1,2,7-Re2Cl5(PMe3)3·Bun4NCl |
|
2.2274(8) |
staggered |
259 |
1,3,6-/1,2,7-Re2Cl5(PMe3)3 |
|
2.2183(8)c |
eclipsed |
259 |
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2.2261(5)d |
staggered |
259 |
1,2,7-Re2Cl5(PMe3)3 |
|
2.226(1) |
staggered |
248 |
1,3,6-Re2Cl5(PMe2Ph)3 |
|
2.219(1) |
eclipsed |
180 |
1,2,7-Re2Cl5(PMe2Ph)3 |
|
2.2313(4) |
staggered |
180 |
1,3,6-Re2Cl5(PEt3)2 |
|
2.221(2) |
eclipsed |
248 |
1,3,6-Re2Cl5(PPrn3)3 |
(trigonal form) |
2.2284(9) |
eclipsed |
189 |
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(orthorhombic form) |
2.220(1) |
eclipsed |
182 |
1,3,6-Re2Cl5(PPrn3)3·0.25C6H14 |
|
2.2224(7) |
eclipsed |
182 |
1,3,6-Re2Cl5(PCy2H)3·CH2Cl2·0.5C6H14 |
2.223(1) |
staggered |
181 |
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1,3,6-Re2Cl5(PEt2Ph)3 |
|
2.2262(3) |
eclipsed |
253 |
1,3,6-Re2Cl5(Ph2PCH2CO2Me)3 |
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2.2318(3) |
eclipsed |
263 |
Rhenium Compounds 313
Walton
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Rotational |
|
Compound |
r(Re–Re)(Å)a |
Geometryb |
ref. |
1,3,6-Re2I5(PMe3)3 |
2.235(1) |
eclipsed |
184 |
(Bu4N)[1,6-Re2Cl6(PPrn3)2] |
2.2211(6) |
staggered |
189 |
(Bu4N)[1,7-Re2Cl6(PPrn3)2 |
2.2141(4) |
staggered |
189 |
(Bu4N)[1,7-Re2Cl6(PEt2Ph)2] |
2.2273(4) |
eclipsed |
189 |
|
2.2278(5) |
eclipsed |
189 |
(Bu4N)[1,7-Re2I6(PEt3)2]·0.33C6H6 |
2.233(1) |
staggered |
184 |
|
2.240(1) |
eclipsed |
184 |
(Bu4N)[1,2-Re2Cl6(dppp)] |
2.2458(5) |
staggered |
189 |
[ReCl2(o-P2)2][1,2-Re2Cl6(o-P2)]·4CH2Cl2 |
2.2402(9) |
eclipsed |
281 |
Re2Cl5(µ-dppm)2·2C7H8 |
2.263(1) |
eclipsed |
303 |
cis-[Re2(µ-O2CCH3)2Cl2(µ-dppa)2]PF6 |
2.2757(5) |
staggered |
284 |
cis-[Re2(µ-O2CCH3)2Cl2(µ-Ph2Ppy)2]PF6 |
2.261(1) |
eclipsed |
319 |
trans-[Re2(µ-O2CCH3)2Cl2(µ-dippm)2]- |
2.2705(3) |
eclipsed |
293 |
Cl0.74(ReO4)0.26·CHCl3 |
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|
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trans-[Re2(µ-O2CCH3)2Cl2(µ-dcpm)2]Cl0.8(ReO4)0.2 |
2.271(2) |
eclipsed |
138 |
Re2(µ-O2CCH3)Cl4(µ-dppm)2·2(CH3)2CO |
2.300(1) |
eclipsed |
271 |
Re2(µ-O2CC5H4N)Cl4(µ-dppm)2 |
2.3055(3) |
staggered |
323 |
[Re2Cl4(µ-dppm)2]2(µ-O2CC6H4CO2)·1.5C2H4Cl2 |
2.2939(6) |
eclipsed |
322 |
Re2[(µ-HNC(CH3)O]Cl4(µ-dppm)2·4CH2Cl2·0.833- |
2.3011(3) |
eclipsed |
171 |
C2H5OH |
|
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Re2[µ-HNC(Ph)O]Cl4(µ-dppm)2 |
2.3129(7) |
eclipsed |
173 |
Re2(µ-O2CCH3)Cl4(PPh3)2·H2O |
2.2165(7) |
eclipsed |
318 |
Re2(µ-O2CCH3)Cl4(δ3-L1)g |
2.2454(3) |
eclipsed |
191(b) |
Re2(µ-O2CCH3)Cl4(δ3-L2)g |
2.2403(4) |
eclipsed |
191(b) |
Re2(µ-O2CCH3)Cl4(δ3-L3)·C6H6g |
2.2804(4) |
eclipsed |
191(b) |
Re2(µ-O2CCH3)Cl4(δ3-L4)g |
2.2596(3) |
eclipsed |
191(b) |
Re2(µ-O2CC6H4-2-PPh2)Cl4(δ3-L1)g |
2.2390(3) |
eclipsed |
191(b) |
Re2(µ-O2C-4-quin)Cl4(δ3-L1)g |
2.2536(4) |
eclipsed |
191(b) |
Re2(µ-O2CC6H4-2-PPh2Cl4(δ3-L3)·C2H5OHg |
2.2651(4) |
eclipsed |
191(b) |
Re2(µ-O2C-4-quin)Cl4(δ3-L3)·C2H5OHg |
2.2694(3) |
eclipsed |
191(b) |
[Re2Cl4(δ3-L1)]2(µ-O2CC6H4CO2)·2C2H5OHg |
2.2424(4) |
eclipsed |
191(b) |
[Re2Cl3(µ-dppm)2(mq)]PF6 |
2.2540(5) |
eclipsed |
328 |
aUnless otherwise indicated, where more than one set of data is given for any complex this signifies that more than one crystallographically independent molecule is present in the crystal. In cases where orientational disorder occurs, the Re–Re distance given is that for the dirhenium unit with the highest occupancy or is a weighted average of the distances.
b |
A compound is designated as having a “staggered” geometry if ρav exceeds an arbitrarily chosen value of 10°. |
c |
Distance for the 1,3,6-isomer. |
d |
Distance for the 1,2,7-isomer. |
e |
These are different monoclinic forms of Re2Cl4(µ-dppm)2. |
f |
These are the distances for each of the three dirhenium units in the molecule. |
gThis compound contains a tridentate donor designated as Ln, the identity of which is given in the text (see also ref. 191(b)).
There is no doubt that the 1,3,6,8-Re2X4(PR3)4 compounds possess a μ2/4β2β*2 ground state electronic configuration, a conclusion that was supported by relativistic X_-SW calculations
314Multiple Bonds Between Metal Atoms Chapter 8
on the hypothetical molecule Re2Cl4(PH3)4251 and gas-phase photoelectron spectral studies on volatile Re2Cl4(PMe3)4.244 With this configuration there is no net β bond, and thus no inherent electronic barrier to rotation about the Re–Re bond. An eclipsed rather than a staggered rotational geometry is clearly a consequence of steric factors in the case of this particular isomeric form (8.16). The striking similarity between the spectroscopic and electrochemical properties (vide infra) of the compounds of the type Re2X4(PR3)4 (X = Cl, Br or I) that have been considered up to this point leaves little doubt that they all possess the 1,3,6,8-Re2X4(PR3)4 structure.
Let us now return to the paramagnetic Re2X5(PR3)3 compounds (X = Cl or Br) that were also isolated and characterized in the first report177 of Re24+ and Re25+ compounds. As mentioned already, these compounds are obtained when the tertiary phosphine is PRPh2 (R = Me or Et). In all cases they contain a Re–Re bond order of 3.5 and a μ2/4β2β*1 ground state electronic configuration. It had already been reported long ago22 that with PPh3 only the insoluble, unreduced compounds Re2X6(PPh3)2 are formed (presumably as the 1,7-isomers) consistent with the extent of reduction being dependent on the basicity (and steric bulk) of the phosphine. Subsequently, several other compounds of the type Re2Cl5(PR3)3 were obtained by other less direct means. In the period leading up to the publication of the second edition of this text,10 these procedures involved both the reduction of Re2Cl6(PR3)2 and the oxidation of the analogous Re2Cl4(PR3)4 compounds. An early example was encountered in the reaction between (Bu4N)2Re2Cl8 and PEt3 in methanol-conc HCl which gave a separable mixture of Re2Cl4(PEt3)4 and Re2Cl5(PEt3)3.178 The latter compound was also obtained in high yield upon the treatment of the Re25+ compound [(δ5-C5H5)2Co]Re2Cl6(PEt3)2 (see Section 8.5.2) with PEt3 at room temperature,178 and it is also the product (as are other compounds of this type) from the reaction between [Re2Cl4(PEt3)4]+ and Cl-, as first carried out in an electrochemical cell.228 The latter behavior will be discussed a little later when we deal with the electrochemical properties of these complexes. The compound Re2Cl5(PPrn3)3 has been prepared by the reaction of Re2Cl4(PPrn3)4 with ethanol which results in disproportionation to give a mixture of ReCl(CO)3(PPrn3)2 and Re2Cl5(PPrn3)3.252 Finally, we mention here the chlorine oxidation of Re2Cl4(PMe3)4 in dichloromethane which, depending on the reaction conditions, gives either the 1,3,6-isomer represented in structure 8.18 or the 1,2,7-isomer shown in 8.19.248 The latter product (8.19) requires the presence of an equivalent of PMe3 in the reaction medium. These isomers do not interconvert. This contribution248 is important because it first established the existence of isomers for compounds of this type; X-ray crystal structures were reported248 for both isomers of Re2Cl5(PMe3)3 as well as 1,3,6-Re2Cl5(PEt3)3 (Table 8.4). More recently, the crystal structure of 1,3,6-Re2Cl5(PEt2Ph)3 was reported.253 It should be noted that isomers of the types 1,3,5- and 1,2,5-Re2X5(PR3)3 could easily isomerize to 8.18 and 8.19, respectively, by a simple 90° rotation about the Re–Re bond, and so may not be isolable.
8.18 |
8.19 |
It is noteworthy that the same products Re2Cl6(PR3)2, Re2Cl5(PR3)3 and Re2Cl4(PR3)4 are formed when the dirhenium(IV) complex (Bu4N)Re2Cl9 is used in place of (Bu4N)2Re2Cl8 in reactions with certain phosphines. Thus, with PPh3, PEtPh2 and PEt3 the products are Re2Cl6(PPh3)2, Re2Cl5(PEtPh2)3 and Re2Cl4(PEt3)4, respectively.254 The reduction of [Re2Cl9]- to
Rhenium Compounds 315
Walton
Re2Cl4(PEt3)4 represents formally a four-electron reduction of a metal-metal bonded dinuclear species in which a metal-metal bond is retained and also constitutes a unique example of a redox reaction in which the starting material and product both possess a different type of metalmetal triple bond (μ2/4 and μ2/4β2β*2, respectively).
The redox chemistry of Re2X4(PR3)4 that has been cited previously, together with that of compounds of the types Re2X5(PR3)3 and Re2X6(PR3)2, accords very nicely with the notion that these Re2n+ species are representatives of the μ2/4β2 (n = 6), μ2/4β2β*1 (n = 5), and μ2/4β2β*2 (n = 4) configurations. In the case of Re2X4(PR3)4, such redox chemistry is quite extensive and examples of chemical and electrochemical oxidations are very well documented. Among examples of chemical oxidations are those in which CCl4 oxidizes Re2Cl4(PEt3)4 and Re2Br4(PEt3)4 to produce the Et3PCl+ salts (Et3PCl)2Re2Cl8 and (Et3PCl)2Re2Cl4Br4.177 In the latter reaction, a quantity of Re2Cl4Br2(PEt3)2 is also formed through the reaction of (Et3PCl)2Re2Cl4Br4 with some of the free phosphine that is released during the oxidation.177 Additional examples of chlorocarbon oxidations that produce the [Re2Cl8]2- anion are those of Re2Cl4(PEt3)4 and Re2Cl5(PEtPh2)3.177 Other oxidations include the conversion of Re2Cl4(PEt3)4 to Re2Cl6(PEt3)2 by methanolic-HCl,177 and the aerial oxidation of Re2X4(PR3)4 which produces the cations [Re2X4(PR3)4]+ and/or their neutral analogs Re2X5(PR3)3.25,177,217 The latter oxidations can be accomplished cleanly via electrochemical methods as we shall now discuss.
The oxidation of the dinuclear rhenium(II) compounds Re2X4(PR3)4 has been studied by electrochemistry, the cyclic voltammetric technique having proved especially convenient. The original studies on Re2X4(PR3)4, coupled with related ones on Re2X5(PR3)3 and Re2X6(PR3)2 (see Table 8.2),228 showed that the electrochemical oxidations of Re2X4(PR3)4 to [Re2X4(PR3)4]+ and [Re2X4(PR3)4]2+ (Table 8.5) are followed by the conversion of these cations to Re2X5(PR3)3 and then Re2X6(PR3)2 via coupled chemical steps.228 Electrochemical data for various Re2X5(PR3)3 complexes are also given in Table 8.5; note that the 1,3,6- and 1,2,7-isomers of Re2Cl5(PMe3)3 have quite different sets of values for E1/2(ox) and E1/2(red).248 A noteworthy feature of these systems is that the conversion of Re2X4(PR3)4 to Re2X6(PR3)2 proceeds by both EECC and ECEC coupled electrochemical(E) - chemical(C) reaction series (see Schemes 8.1 and 8.2); the difference between them is the selection of the potential used for the oxidation of Re2X4(PR3)4. This is demonstrated in Fig. 8.16, where curve B shows the appearance of the processes at E1/2(ox) = +0.31 V and E1/2(red) = -0.88 V that signal the formation of Re2Cl5(PPrn3)3 following the bulk oxidation of Re2Cl4(PPrn3)4 to the monocation. Curve C shows that both Re2Cl5(PPrn3)3 and Re2Cl6(PPrn3)2 (the latter characterized by E1/2(red) = -0.11 V) are formed upon oxidation at +1.0 V (i.e. to [Re2Cl4(PPrn3)4]2+). The mechanism of the chemical reactions that follow the electrochemical oxidations of Re2X4(PR3)4 involves the reaction between halide ion and [Re2X4(PR3)4]+ or [Re2X4(PR3)4]2+ to produce Re2X5(PR3)3 and [Re2X5(PR3)3]+, respectively. [Re2X5(PR3)3]+ then reacts further with X- to form the final product, Re2X6(PR3)2.228(b) The halide ion that is available for these reactions, as originally carried out,228 is generated by the disruption of a very small proportion of the dirhenium complex (almost certainly through reaction with adventitious oxygen). These mechanisms were confirmed later in separate experiments243 that involved the addition of halide ion to pure samples of [Re2X4(PR3)4]+ and [Re2X4(PR3)4]2+ (vide infra).
[Re2X4(PR3)4]0 |
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-e |
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[Re2X4(PR3)4]+ |
[Re2X4(PR3)4]0 |
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-e |
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[Re2X4(PR3)4]+ |
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-e |
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+X- |
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[Re2X4(PR3)4]+ |
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[Re2X4(PR3)4]2+ |
[Re2X4(PR3)4]+ |
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[Re2X5(PR3)3]0 |
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+X- |
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[Re2X5(PR3)3]+ |
[Re2X5(PR3)3]0 |
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[Re2X5(PR3)3]+ |
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+X- |
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+X- |
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[Re2X5(PR3)3]+ |
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[Re2X6(PR3)2]0 |
[Re2X5(PR3)3]+ |
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Scheme 8.1. EECC process. |
Scheme 8.2. ECEC process. |
316Multiple Bonds Between Metal Atoms Chapter 8
Fig. 8.16. Cyclic voltammograms in 0.2 M Bu4NPF6-dichloromethane.
(A) Re2Cl4(PPrn3)4; (B) solution A following oxidation at +0.1 V; (C) solution A following oxidation at +1.0 V.
Table 8.5. Voltammetric E1/2 Values for Mixed Halide-Phosphine Complexes of Re24+ and Re25+ in Dichloromethanea
A. Re24+ Compounds
Compound E1/2(ox)(2) E1/2(ox)(1) ref.
1,3,6,8-Re2Cl4(PMe3)4 1,2,7,8-Re2Cl4(PMe3)4 1,3,6,8-Re2Cl4(PEt3)4 1,3,6,8-Re2Cl4(PPn3)4 1,3,6,8-Re2Cl4(PBun3)4 1,2,7,8-Re2Cl4(PEtH)4 1,3,6,8-Re2Cl4(PMe2Ph)4 1,2,7,8-Re2Cl4(PMe2Ph)4 1,2,7,8-Re2Cl4(PMe2Ph)3(PEt2H) 1,3,6,8-Re2Cl4(PEt2Ph)4 1,3,6,8-Re2Cl4(PEtPh2)4 1,3,6,8-Re2Br4(PMe3)4 1,3,6,8-Re2Br4(PEt3)4 1,3,6,8-Re2Br4(PPrn3)4 1,3,6,8-Re2Br4(PBun3)4 1,3,6,8-Re2I4(PMe3)4 1,3,6,8-Re2I4(PEt3)4 1,3,6,8-Re2I4(PPrn3)4 1,3,6,8-Re2I4(PBun3)4 1,3,6,8-Re2I4(PMe2Ph)4 _-Re2Cl4(dmpe)2 _-Re2Br4(depe)2 _-Re2Cl4(dppe)2 _-Re2Br4(dppe)2 _-Re2Cl4(dppee)2 _-Re2Br4(dppee)2 _-Re2Cl4(dppbe)2 `-Re2Cl4(depe)2
`-Re2Br4(depe)2
+0.96b |
-0.23b |
244 |
+1.12b |
-0.16b |
179 |
+0.80 |
-0.42 |
228(b) |
+0.79 |
-0.44 |
228(b) |
+0.82 |
-0.44 |
228(b) |
+1.14b |
+0.03b |
183 |
+0.83 |
-0.30 |
228(b) |
+0.98b |
-0.17b |
180 |
+1.22b |
+0.37b |
262 |
+0.85 |
-0.25 |
228(b) |
+0.84 |
-0.29 |
246 |
+1.01b |
-0.11b |
244 |
+0.83 |
-0.31 |
228(b) |
+0.84 |
-0.38 |
228(b) |
+0.82 |
-0.40 |
228(b) |
+0.98b |
-0.02b |
184 |
+0.77 |
-0.27 |
228(b) |
+0.85 |
-0.22 |
228(b) |
+0.83 |
-0.25 |
228(b) |
+0.94b |
-0.01b |
184 |
+1.10b,c |
+0.21b |
275 |
+1.07b,c |
+0.02b |
275 |
+1.05b |
+0.27b |
276 |
+1.03b |
+0.29b |
276 |
+1.05b |
+0.30b |
275 |
ȵ1.0b |
+0.33b |
275 |
+1.14b,c |
+0.29b |
280 |
+0.88b |
+0.08b |
275 |
+0.89b |
+0.13b |
275 |
Rhenium Compounds 317
Walton
A. Re24+ Compounds
Compound E1/2(ox)(2) E1/2(ox)(1) ref.
`-Re2Cl4(dppe)2 `-Re2Br4(dppe)2 `-Re2I4(dppe)2 `-Re2Cl4(dppee)2 `-Re2Br4(dppee)2 `-Re2Cl4(arphos)2 `-Re2Br4(arphos)2 `-Re2I4(arphos)2 Re2Cl4(µ-dppm)2 Re2Br4(µ-dppm)2 Re2I4(µ-dppm)2 Re2(NCBH3)4(µ-dppm)2g Re2(CH3)4(µ-dppm)2 Re2Cl4(µ-dppa)2 Re2Br4(µ-dppa)2 Re2Cl4(µ-dppE)2 Re2Cl4(µ-dcpm)2 Re2Cl4(µ-dpam)2 Re2Br4(µ-dpam)2 Re2Cl4(µ-dmpm)3 Re2Br4(µ-dmpm)3 Re2Cl4(µ-dppm)(PMe3)2 Re2Cl4(µ-dppa)(PMe3)2 Re2Cl4(µ-dppm)(PEt3)2 Re2Cl4(µ-dppa)(PMe2Ph)2 Re2Cl4(µ-dcpm)(PMe3)2 `-Re2Cl4(dppm)(dppe) `-Re2Cl4(dppm)(arphos) `-Re2Cl4(dppa)(dppe) Re2Cl4(µ-dppm)2(PMe3) Re2Br4(µ-dppm)2(PMe3) Re2Cl4(µ-dppm)2[P(OMe)3] Re2Cl4(µ-dppm)2[P(OEt)3] Re2Cl4(µ-dppm)2[P(OPh)3] Re2Cl4(Ph2Ppy)3
Re2Cl3(Ph2Ppy)2[(C6H5)(C6H4)Ppy] Re2Cl4(Ph2Ppy)2(PEt3) Re2Cl4(Ph2Ppy)2(PBun3) [Re2Cl2(Ph2Ppy)2](PF6)2 Re2Cl4(bdppp)2 Re2Cl2(pyphos)2(pyphosH)
+1.04 |
+0.23 |
282 |
+0.97 |
+0.22 |
282 |
+0.92 |
+0.29 |
282 |
+1.13b |
+0.24b |
275 |
+1.15b |
+0.34b |
275 |
+1.07 |
+0.23 |
282 |
+1.01 |
+0.24 |
282 |
+0.91 |
+0.28 |
282 |
+0.87b,f |
+0.29b,f |
288 |
+0.94b |
+0.34b |
288 |
+0.95b |
+0.34b |
288 |
|
+0.98b |
290 |
+0.59b |
-0.14b |
289 |
+0.94b,f |
+0.40b,f |
288 |
+1.05b |
+0.41b |
284 |
+0.92b |
+0.37b |
287 |
+0.93b |
-0.05b |
285 |
+0.84b |
+0.32b |
288 |
+0.92b |
+0.37b |
288 |
+1.30b,c |
+0.53b |
295 |
+1.33b,c |
+0.58b |
296 |
+1.28b,c |
+0.58b |
244,299 |
+1.26b,c |
+0.65b |
244 |
+1.15b,c |
+0.55b |
244 |
+1.37c,d |
+0.64d |
203 |
+1.40b,c |
+0.49b |
285 |
+0.96b |
+0.32b |
298 |
+0.88b |
+0.31b |
298 |
+1.00b |
+0.35b |
298 |
+1.29b,c |
+0.30b |
300 |
+1.31b,c |
+0.38b |
300 |
+1.39b,c |
+0.31b |
300 |
+1.41b,c |
+0.29b |
300 |
+1.66b,c |
+0.43b |
300 |
+1.20d |
+0.41d |
207 |
+1.06d |
+0.24d |
207 |
+1.15c,d |
+0.27d |
207 |
+1.19c,d |
+0.27d |
207 |
|
+1.38d,h |
207 |
+0.85b |
-0.07b |
197 |
+1.00b |
+0.22b |
302 |
B. Re25+ Compounds
Compound |
E1/2(ox) |
E1/2(red) |
ref. |
1,3,6-Re2Cl5(PMe3)3 |
+0.46b |
-0.75b |
248 |
1,2,7-Re2Cl5(PMe3)3e |
+0.68b |
-0.48b |
248 |
1,3,6-Re2Cl5(PEt3)3 |
+0.34 |
-0.88 |
228(b) |
318Multiple Bonds Between Metal Atoms Chapter 8
B. Re25+ Compounds
Compound |
E1/2(ox) |
E1/2(red) |
ref. |
1,3,6-Re2Cl5(PPrn3)3 |
+0.31 |
-0.88 |
228(b) |
1,3,6-Re2Cl5(PCy2H)3 |
+0.52b |
-0.66b |
181 |
1,3,6-Re2Cl5(PMe2Ph)3 |
+0.46d |
-0.65d |
243 |
1,2,7-Re2Cl5(PMe2Ph)3 |
+0.75b |
-0.40b |
180 |
1,3,6-Re2Cl5(PEtPh2)3 |
+0.44 |
-0.66 |
228(b) |
1,3,6-Re2Cl5(Ph2PCH2CO2Me)3 |
+0.66b |
-0.46b |
263 |
1,3,6-Re2Cl5(Ph2PCH2CO2Et)3 |
+0.61b |
-0.44b |
263 |
1,3,6-Re2Br5(PMePh2)3 |
+0.48 |
-0.55 |
228(b) |
1,3,6-Re2Br5(PEtPh2)3 |
+0.45 |
-0.59 |
228(b) |
(Bu4N)[1,2-Re2Cl6(dppp)] |
+0.39b |
|
189 |
Re2Cl5(µ-dppm)2 |
+0.51d |
-0.36d,i |
203 |
a Unless otherwise stated, data are in volts vs. the saturated sodium chloride calomel electrode (SSCE) with a Pt-bead working electrode and 0.1 M Bu4NPF6(TBAH) as supporting electrolyte.
b Versus Ag/AgCl.
c Ep,a value
d Versus SCE.
eThe reduced complex (Bu4N)[1,2,7-Re2Cl5(PMe3)3] is reported to have E1/2(ox) values of +0.73 V and -0.46 V vs. Ag/AgCl. (ref 261)
f Values are similar to those reported vs. SCE (see ref 203).
g This complex has an irreversible reduction with Ep,c = -1.14V vs. Ag/AgCl. (ref 290). h Reductions observed at E1/2 = -0.82 V and -1.6 V vs. SCE. (ref 207)
i Ep,c value
Standard electrochemical rate constants k have been measured by ac voltammetry for the two sequential one-electron transfers of Re2X4(PMe2Ph)4 (X = Cl or Br) and other triply bonded Re24+ complexes including Re2X4(PMe3)4, as well as for Re2Cl5(PMe2Ph)3 and Re2Cl6(PMe3)2.255 Measurements were carried out in dichloromethane, acetonitrile and N,N-dimethylformamide at platinum electrodes and established the electrochemical reversibility that is in accord with fast electron transfer. In dichloromethane and acetonitrile, k for the first oxidation step of the dirhenium(II) complexes was invariably larger than for the second oxidation; for example, the k values for the +/0 and 2+/+ couples of Re2Cl4(PMe2Ph)4 in CH2Cl2 are 0.65 and 0.28 cm s-1, respectively.255 More recently, the kinetics of the electron self-exchange reaction of the redox couples [Re2X4(PMe2Ph)4]0/+ (X = Cl or Br) have been measured in CH2Cl2 as a function of temperature and concentration by 1H NMR line-broadening experiments.256 The values of the self-exchange rate constants (at 298 K) are 2.3×10-8 M-1 s-1 for X = Cl and 4.2×108 M-1 s-1 for X = Br. In addition, the kinetics of outer-sphere oxidation of Re2Br4(PMe2Ph)4 by cobalt(III) has been studied.257
From the very low value of the potential for the first oxidation of Re2X4(PR3)4 (Table 8.5) it is apparent that mild oxidants should be capable of generating [Re2X4(PR3)4]+. The salt NOPF6 proved to be an excellent oxidant in this regard, and earlier work led to the isolation of [Re2X4(PEt3)4]PF6 (X = Cl or Br) by such a procedure.228(b) Spectroscopic characterizations, using EPR and electronic absorption spectroscopy,228(b) showed that these monocations possess the expected μ2/4β2β*1 ground-state electronic configuration. In a later study, a comparison was made of the low temperature (5 K) electronic absorption spectrum of Re2Cl4(PPrn3)4 and its one-electron oxidized congener [Re2Cl4(PPrn3)4]PF6.251 The trimethylphosphine complexes [Re2X4(PMe3)4]PF6 (X = Cl or Br) have also been prepared by this method,244 while [Re2Cl4(PMe3)4]ReO4 has been obtained by the aerial oxidation of Re2Cl4(PMe3)4.250
Rhenium Compounds 319
Walton
It has also been found that NOPF6 can access the second oxidation of Re2X4(PMe3)4; by this means Re2Cl4(PMe2Ph)4 was oxidized cleanly in two one-electron steps to give [Re2Cl4(PMe2Ph)4]PF6 and [Re2Cl4(PMe2Ph)4](PF6)2.243 In a similar fashion, the reversible oneelectron oxidation of the Re2X5(PR3)3 complexes can be accomplished through the use of NOPF6; for example, Re2Cl5(PMePh2)3 has been oxidized to [Re2Cl5(PMe2Ph)3]PF6.178 Of particular note is the observation that the treatment of [Re2Cl4(PMe2Ph)4]PF6 and [Re2Cl4(PMe2Ph)4](PF6)2 with Cl- forms Re2Cl5(PMe2Ph)3 and Re2Cl6(PMe2Ph)2, respectively,243 thereby confirming the EECC and ECEC mechanisms that were proposed in Schemes 8.1 and 8.2 (vide supra).
The isolation and structural characterization of the complexes Re2Cl4(PMe2Ph)4, [Re2Cl4(PMe2Ph)4]PF6, and, [Re2Cl4(PMe2Ph)4](PF6)2 provided the first opportunity to probe the structural changes that take place in a series of complexes that possess M–M bond orders of 3, 3.5, and 4 and identical sets of monodentate ligands.243 The same basic eclipsed rotational geometry is preserved in all three complexes (D2d virtual symmetry), the structures being as depicted in 8.16. It is also clear that the electrochemical properties of these complexes accord with only minimal structural changes accompanying the electron transfer processes.255 Of most interest is the trend in Re–Re bond lengths which are 2.241(1) Å, 2.218(1) Å and 2.215(2) Å, respectively (see Tables 8.1 and 8.4). Apparently, the Re–Re distances do not respond in a simple and predictable way to β bond order changes because, with the increase in metal core charge (as the dimetal unit is oxidized), there is some decrease in the strength of the μ and/or / bonding contributions to the Re–Re bond resulting from orbital contraction. The X-ray photoelectron spectra (XPS) of representative complexes of the types Re2X6(PR3)3, Re2X5(PR3)3, [Re2X4(PR3)4]PF6 and Re2X4(PR3)4 have been recorded,228(b),258 and although the binding energies of the core Re(4f) electrons are in the expected order (Re26+ > Re25+ > Re24+), interpretations of these chemical shifts are complicated by relaxation effects that occur during the core ionization.
The use of cobaltocene to reduce the complexes of the type Re2Cl6(PR3)2 by one electron to give [(δ5-C5H5)2Co][Re2Cl6(PR3)2] has already been discussed in Section 8.5.2. Likewise, this reagent was shown to reduce Re2Cl5(PMePh2)3 to the Re24+ complex [(δ5-C5H5)2Co][Re2Cl5(P MePh2)3] upon admixing acetone solutions of the reactants.178 This reaction could be expected based on the cyclic voltammetric data reported in Table 8.5 for this complex. Both types of anions react further with an equivalent of the appropriate phosphine ligand with substitution of a halide ligand and the formation of the appropriate neutral mixed halide-phosphine complex,178 as the following reactions show:
[(C5H5)2Co][Re2Cl6(PEt3)2] + PEt3 |
CH2Cl2 |
Re2Cl5(PEt3)3 + [(C5H5)2Co]+ + Cl- |
||
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[(C5H5)2Co][Re2Cl5(PMePh2)3] + PMePh2 |
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CH2Cl2 |
Re2Cl4(PMePh2)4 + [(C5H5)2Co]+ + Cl- |
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The key reactions we have discussed that lead to the interconversion of the various mixed halide-monodentate tertiary phosphine complexes of Re26+, Re25+ and Re24+ are summarized in the redox scheme shown in Fig. 8.17. We now focus our attention on the more recent developments in the field that started in the mid-1990’s, many of which have taken advantage of the transformations that are given in Fig. 8.17. Most important among these contributions are those of Cotton and co-workers who re-visited this chemistry with a further investigation259 of the isomeric 1,3,6- and 1,2,7-Re2Cl5(PMe3)3 compounds (see structures 8.18 and 8.19, respectively) that had first been reported in 1990.248 By minor modifications of the original reaction conditions248 a form of Re2Cl5(PMe3)3 was isolated that contained both 1,3,6- and 1,2,7- isomers in the same unit cell, as well as a new crystalline modification of composition 1,3,6- Re2Cl5(PMe3)3·0.5CH2Cl2. The compound 1,2,7-Re2Cl5(PMe3)3·Bun4NCl was formed by the
320Multiple Bonds Between Metal Atoms Chapter 8
reaction of (Bu4N)2Re2Cl8 with PMe3 in 1-propanol at room temperature.259 All three compounds were structurally characterized (see Table 8.4). Several of the Re2Cl5(PMe3)3 compounds have proven to be useful starting materials. Thus, 1,2,7-Re2Cl5(PMe3)3 can be reduced to its monoanion by cobaltocene, and this in turn reacts with PMe3 to give 1,2,7,8-Re2Cl4(PMe3)4, which was the first example of this type of isomer to be isolated (see structure 8.17 and Table 8.4).179 This isomer has different cyclic voltammetric properties from those of 1,3,6,8-Re2Cl4(PMe3)4 (Table 8.5). When NOBF4 is used to oxidize the compound 1,2,7-Re2Cl5(PMe3)3·Bu4NCl by one-electron , the Re26+ complex 1,7-Re2Cl6(PMe3)2 is formed.179 Both the aforementioned reactions involving 1,2,7-Re2Cl5(PMe3)3 are of types that have been discussed previously and are represented in Fig. 8.17. The reaction between 1,2,7-Re2Cl5(PMe3)3·Bu4NCl and NOBF4 is different in the presence of an additional equivalent of Bu4NCl; in this case, both Re2Cl6(PMe3)2 and the mixed-salt (Bu4N)4[Re2Cl7(PMe3)]2[Re2Cl8] are formed (see Section 8.4.4).185 The reduction of 1,2,7-Re2Cl5(PMe3)3 to form 1,3,6,8-Re2Cl4(PMe3)4, which occurs when the former compound is reacted with PMe3 in 1-propanol, proceeds via the intermediacy of the Re25+ complex [1,3,6,8-Re2Cl4(PMe3)4]Cl which has been isolated and structurally characterized (Table 8.4).260 The isomerization and substitution that occurs when 1,2,7-Re2Cl5(PMe3)3 converts to [1,3,6,8-Re2Cl4(PMe3)4]Cl may in turn proceed260 via the intermediacy of the confacial bioctahedral Re25+ complex (Me3P)2ClRe(µ-Cl)3ReCl(PMe3)4, a compound that has been isolated separately (vide infra).261
Fig. 8.17. Reaction scheme for mixed halide-monodentate tertiary phosphine complexes containing the Re2n+ cores (n = 6, 5 or 4). The chemical reactions are as follows: (a) reaction with PR3 at room temperature; (b) reaction with PR3 under reflux; (c) reaction with NO+PF6- in CH2Cl2 at room temperature; (d) reaction with NO+PF6- in CH3CN at room temperature; (e) reaction with Cp2Co in acetone at room temperature; (f) reaction with one-equivalent of Cl- (g) reaction with one equivalent of PR3; (h) Cl2 oxidation. Note: (1) the reduction of Re2Cl6(PR3)2 to [Re2Cl6(PR3)2]- may occur in some instances directly from the reaction of [Re2Cl8]2- with PR3; (2) the oxidation of Re2Cl4(PR3)4 to [Re2Cl4(PR3)4]+ is possible with other oxidants, including O2.
Another entry into PMe3 complexes that contain the Re25+ and Re24+ cores is through the paramagnetic, non metal-metal bonded, edge-shared bioctahedral complex 1,3,5,7-Re2(µ- Cl)2Cl4(PMe3)4, which is the main product from the reaction of (Bu4N)2Re2Cl8 with PMe3 in benzene at room temperature.179,261 Reduction of this compound with one or two equivalents of KC8 in toluene (or toluene/CH2Cl2) affords the confacial bioctahedron Re2(µ-Cl)3Cl2(PMe3)4 (mentioned above) and 1,2,7,8-Re2Cl4(PMe3)4, respectively. The Re–Re distance in Re2(µ- Cl)3Cl2(PMe3)4 is 2.686(1) Å.261 When the reduction with 2 equiv of KC8 is carried out in benzene in the presence of Bu4NCl the Re24+ complex (Bu4N)[1,2,7-Re2Cl5(PMe3)3] is formed.261 The latter compound is also obtained when 1,2,7-Re2Cl5(PMe3)3·Bu4NCl is reduced with KC8; like its neutral congener 1,2,7-Re2Cl5(PMe3)3248,259 it has a partially staggered rotational