Multiple Bonds Between Metal Atoms / 08-Rhenium Compounds

Rhenium Compounds 341

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an axial position. The ligands that have been used are bis[2-(diphenylphosphino)phenyl]ether, 4,6-bis(diphenylphosphino)dibenzofuran, 2,6-bis(diphenylphosphinomethyl)pyridine, bis[2- (diphenylphosphino)ethyl]amine and N,N-bis[2-(diphenylphosphino)ethyl]trimethylacetami de and these are designated as L1, L2, L3, L4 and L5, respectively, in Tables 8.4 and 8.6 where details of the X-ray crystal structures and electrochemical data for the complexes are given.191 The lability of the acetate group in these complexes has been demonstrated by reactions of certain of these complexes (those that contain ligands L1 and L3) with 4-Ph2PC6H4CO2H, 2- Ph2PC6H4CO2H and quin-4-CO2H, to give products that have structures and properties very similar to those of the µ-acetato derivatives (Tables 8.4 and 8.6).191(b) These same two acetate complexes also react with terephthalic acid to give [Re2Cl4(δ3-L1)]2(µ-O2CC6H4CO2) and [Re2Cl4(δ3-L3)2](µ-O2CC6H4CO2), the structure of the first of these having been established by X-ray crystallography.191 Magnetic susceptibility and cyclic voltammetric measurements show that any electronic coupling between the paramagnetic individual Re25+ units is at most very

weak.191(b)

8.35

A few examples exist of Re24+ and Re25+ complexes that contain bidentate monoanionic ligands, other than carboxylates, in combination with halide and phosphine donor sets. These ligands may bridge the two metal centers or chelate to only one of them. Bis(µ-hydrosulfido) and µ-gem-dithiolato complexes prepared from cis-Re2(µ-O2CR)2Cl2(µ-dppm)2 have already been mentioned.326 The reaction of Re2X4(µ-dppm)2 (X = Cl or Br) with 2-mercaptoquino- line (2-mqH) affords the 1:1 adducts Re2X4(µ-dppm)2(2-mqH) that can undergo a reversible one-electron oxidation with [(δ5-C5H5)2Fe]PF6 to give [Re2X4(µ-dppm)2(2-mqH)]PF6. This oxidation is followed by the slow elimination of HX to give paramagnetic [Re2X3(µ-dppm)2(2- mq)]PF6 in which, in addition to two bridging dppm ligands, there is also a bridging 2-mq ligand bound through its N and S (thiol) atoms.327,328 A crystal structure determination on the chloride derivative shows the Re–Re distance to be 2.2540(5) Å.328 The neutral compounds Re2X3(µ-dppm)2(2-mq) are formed by the electrochemical reduction of the paramagnetic cations and from Re2X4(µ-dppm)2(2-mqH) by treatment with the strong base DBU.328

The reaction of Re2Cl4(µ-dppm)2 with Tl(acac) affords Re2(acac)2Cl2(µ-dppm)2 (8.37) via the intermediacy of Re2(acac)Cl3(µ-dppm)2 (8.36).329 These are the first examples of `-diketonate complexes of Re24+ and both have been characterized by X-ray crystallography (Table 8.4), and found to have cis, trans sets of ReP2 units as shown in 8.36 and 8.37.329

342Multiple Bonds Between Metal Atoms Chapter 8

8.36

8.37

Reactions of Re2X4(µ-LL)2 compounds with carbon monoxide, isocyanides, nitriles and related ligands

By far the most extensive reaction chemistry for the Re2X4(µ-LL)2 compounds has been developed from their reactions with organic ligands such as CO, isocyanides, nitriles and alkynes, some of which involve multi-electron redox changes at the dirhenium unit. Selected aspects of this chemistry are covered in several earlier short overviews of the subject,212,330,331 and these can be consulted for additional insights. One simplifying feature in surveying this chemistry is that, to date, it has involved predominantly the reactions of Re2X4(µ-dppm)2 (X = Cl or Br), although there can be little doubt that related compounds will generally behave similarly and this has been shown to be the situation with the few other systems that have been studied.

We will discuss first the compounds that are formed exclusively with CO, alkyl and aryl isocyanides, nitriles or alkynes, and then turn our attention to compounds that contain two or three of these ligands in combination. Finally we will mention briefly other small molecules such as CS2 and SO2. Structural data for some of the key complexes that have been characterized by X-ray crystallography and which retain Re–Re multiple bonds are summarized in Table 8.7.

A thorough investigation has been made of the reactions between Re2X4(µ-dppm)2 (X = Cl or Br) and carbon monoxide.332-335 The chloride compounds that have been prepared are shown in Fig. 8.26, along with the structures of the 1:1, 1:2 and 1:3 complexes (8.38 - 8.40) as based upon single crystal X-ray structure determinations (see Table 8.7).332-334 The analogous bromide complexes have been prepared in all cases, and the structure of the monocarbonyl has also been established by X-ray crystallography and shown to be like 8.38.335 Two (CO) modes are observed in the infrared spectra of the monocarbonyls and these vary in their relative intensities depending on the solvent used. This information, along with NMR spectral data which clearly indicates that a fluxional process is occurring in solution, suggests that isomers are present, as is also the case for the mono-isocyanide species Re2X4(µ-dppm)2(CNR) (vide infra). A partial X-ray structure determination on a single crystal of the second isomer of Re2Cl4(µ-dppm)2(CO) showed that it has an open structure with no bridging ligands other than dppm.335 Its structure is probably closely akin to one of the structure types encountered with mono-isocyanide adducts of Re2Cl4(µ-LL)2 compounds (vide infra). Although the dicarbonyl complex Re2Cl4(µ- dppm)2(CO)2 has been characterized by X-ray crystallography,332 a disorder problem made it impossible to say if the two CO ligands are cis or trans to each other with respect to the Re–Re axis. However, in light of the derivatization of this dicarbonyl with isocyanides and nitriles, and the structural characterization of these complexes as well as that of the related complex Re2Cl4(µ-dppE)2(CO)2,287 it is clear that the CO ligands are cis as shown in 8.39. The Re–Re bond distance of 2.584(1) Å is far longer than bonds observed in triply bonded complexes with a Re24+ core, and this complex can be viewed332 as possessing a Re–Re double bond.

Table 8.7. Structural data for selected dirhenium complexes that contain Re–Re multiple bonds and are formed from the reactions of Re2X4(µ-LL)2 (X = Cl or Br) with carbon monoxide, isocyanides, and nitrilesa

						Compound		r(Re–Re)Å	Structure Descriptionb	ref.
Re2Cl4(µ-dppm)2(CO)								2.338(1)	A-frame-like (µ-Cl) (8.38 in Fig. 8.26)	333
Re	2	Cl	4	(µ-dppm) (CO)	2			2.584(1)	edge-shared bioctahedron(µ-Cl,CO)c (8.39 in Fig. 8.26)	332
				2
Re2HCl3(µ-dppm)2(CO)2								2.605(1)	edge-shared bioctahedron (µ-H,Cl)c	337
[Re2Cl3(µ-dppm)2(CO)3]PF6								2.582(1)	edge-shared bioctahedron (µ-Cl,CO)c (8.40 in Fig. 8.26)	334
[Re2Cl3(µ-dmpm)3(CO)]PF6								2.3565(7)	open bioctahedron	338
Re	2	Cl	4	(µ-dppm) (CNBu			t)			2.30(1)	A-frame-like (µ-Cl) (similar to 8.38)	341
				2
Re2Cl4(µ-dppm)2(CNXyl)								2.3195(9)	A-frame-like (µ-Cl) (similar to 8.38)	342
Re2Cl4(µ-dcpm)2(CNXyl)·(CH3)2CO								2.2887(3)	open structure (8.42)	342
Re2Cl4(µ-dcpm)2(CNBut)2·CH2Cl2								2.3797(3)	open bioctahedron (8.43)	342
Re2Cl4(µ-dppE)2(CNBut)2·3C2H4Cl2								2.3497(4)	open bioctahedron (8.43)	342
[Re2Cl3(µ-dppE)3(CNBut)3]Cl								2.3451(10)	open bioctahedron (8.44)	342
[Re2Cl3(µ-dppm)2(NCMe)2]Cl								2.272(5)	structure 8.41 with L = MeCN	349
[Re2Cl3(µ-dppm)2(NCEt)2]PF6								2.2661(9)	structure 8.41 with L = EtCN	300
[Re2Cl3(µ-dppm)2(NCPh)2]PF6								2.270(1)	structure 8.41 with L = PhCN	345
[Re2Cl3(µ-dppm)2(NCPh)2]Cl·2CH2Cl2								2.2835(5)	structure 8.41 with L = PhCN	349
[Re2Cl3(µ-dppm)2(1,2-NCC6H4CN)2]PF6								2.265(1)	structure 8.41 with L = 1,2-NCC6H4CN	325
[Re2Cl3(µ-dppm)2(1,4-NCC6H4CN)2]PF6								2.2637(12)	structure 8.41 with L = 1,4-NCC6H4CN	348
Re2Br4(µ-dppm)2(CO)(CNBut)·CH2Cl2·2.5C6H6								2.3805(14)	open bioctahedron (8.49)	335
Re	2	Cl	4	(µ-dppm) (CO)(CNXyl)·CH			OH	2.581(2)	edge-shared bioctahedron (µ-Cl,CO)c (8.48)	333
				2		3
[Re2Br3(µ-dppm)2(CO)(CNXyl)]O3SCF3								2.298(1)	structure 8.41 with L = CO, XylNC	335
[Re2Cl3(µ-dppm)2(CO)(NCMe)]O3SCF3								2.2881(7)	structure 8.41 with L = CO, MeCN	355
[Re2Cl3(µ-dppm)2(CO)2(NCEt)]PF6·CH2Cl2·1/2Et2O								2.586(1)	edge-shared bioctahedron (µ-Cl,CO)c (similar to 8.50 in Fig. 8.28)	356
Re	2	Cl	3	(µ-dppm) (CO)		(CNPri)		2.718(1)	edge-shared bioctahedron (µ-Cl,CO)c	358
				2	2
[Re2Cl3(µ-dppm)2(CO)2(CNBut)]PF6·2CH2Cl2								2.605(1)	edge-shared bioctahedron (µ-Cl,CO)d (8.51 in Fig. 8.28)	358
[Re2Br3(µ-dppm)2(CO)2(CNXyl)]O3SCF3·Me2CHC(O)Me								2.5853(13)	edge-shared bioctahedron (µ-Br,CO)c (similar to 8.50 in Fig. 8.28)	359
[Re2Cl3(µ-dppE)2(CO)2(CNXyl)]O3SCF3·1.53CH2Cl2								2.5767(5)	edge-shared bioctahedron (µ-Cl,CO)c (similar to 8.50 in Fig. 8.28)	287
[Re2Cl3(µ-dppm)2(CO)(CNBut)2](PF6)0.5(OMe)0.5								2.379(1)	open bioctahedron (similar to III and IV in Fig. 8.29)	357,363

Walton		Compounds Rhenium
	343

													Compound																				r(Re–Re)Å	Structure Descriptionb	ref.
	[Re2Cl3(µ-dppm)2(CO)(CNXyl)2](ReO4)0.82Cl0.18																																2.576(1)	edge-shared bioctahedron (µ-Cl,CO)c (isomer VI in Fig. 8.30)	364
	Re2Cl3(µ-dppm)2(CO)(CNXyl)2																																2.7155(9)	edge-shared bioctahedron (µ-Cl,CNXyl)c (isomer V in Fig. 8.30)	362
	[Re2Cl3(µ-dppm)2(CO)(CNXyl)2]O3SCF3·CH2Cl2																																2.3833(8)	open bioctahedron (isomer VII in Fig 8.30)	353
	[Re2Br3(µ-dppm)2(CO)(CNXyl)2]O3SCF3·MeCN																																2.3792(7)	open bioctahedron (isomer VII in Fig 8.30)	359
	[Re2Cl3(µ-dppm)2(CO)(CNXyl)(NCMe)]O3SCF3·MeCN																																2.378(3)	open bioctahedron (similar to III in Fig. 8.29 with MeCN in place of ButNC)	354
	[Re2Cl3(µ-dppm)2(CO)(NCMe)2]ReO4·MeCN																																2.5669(4)	edge-shared bioctahedron (µ-Cl,CO)c	355
	[Re2Cl3(µ-dppm)2(CO)(PMe3)2]Cl·0.5CH2Cl2																																2.6021(6)	edge-shared bioctahedron (µ-Cl,CO)c	377
	[Re2Cl3(µ-dppm)2(CO)2(PMe3)]PF6																																2.593(1)	edge-shared bioctahedron (µ-Cl,CO)c	378
	{Re2Cl3(µ-dppm)2(CO)2[P(OEt)3]}PF6																																2.595(1)e	edge-shared bioctahedron (µ-Cl,CO)c	378
	Re2Cl3[C(CN)3](µ-dppm)2(CNXyl)·H2O																																2.2766(10)	open structure (8.52)	291
	Re2Cl2[C(CN)3]2(µ-dppm)2(CNXyl)·2CHCl3																																2.2856(5)	open structure (8.53)	291
	Re	2	Cl		3	[C(CN)		3	](µ-dppE)						(CO)		·C H			Cl			2										2.5823(6)	edge-shared bioctahedron (µ-Cl,CO)c	291
														2		2		2	4
	Re2Cl3[C(CN)3](µ-dppm)2(CO)(CNXyl)·0.436C2H4Cl2																																2.5672(3)	edge-shared bioctahedron (µ-Cl,CO)c	291
	Re	2	Cl		3	[C(CN)		3	](µ-dppm)						2	(CO)(CNXyl)·C									H	4	Cl		2				2.23776(3)	open bioctahedron (similar to III in Fig. 8.29 with [C(CN) ]- in place of	291
																								2										3
																																		ButNC)
	{[Re2Cl3(µ-dppm)2(CO)2]2[µ-N(CN)2]}Cl·5C2H4Cl2																																2.5839(7)	two edge-shared bioctahedra (µ-Cl,CO)c linked by µ-[N(CN)2]	370
	[Re2Cl3(µ-dppE)2(CO)2]2[µ-Ni(CN)4]·6CH2Cl2																																2.5768(5)	two-edge-shared bioctahedra (µ-Cl,CO)c linked by µ-[Ni(CN)4] (see Fig. 8.31)	370
	Re	2	Cl		3	(µ-dppm) (CO)							[(µ-NC)W(CO)									]											2.5898(4)	edge-shared bioctahedron (µ-Cl,CO)c	291
									2		2										5
	[Re		2	Cl		3	(µ-dppE)			2	(CO)	2		(µ-NCS)]				[Pd			Cl			(SCN)				]·10C		6	H	6	2.5775(4)	two-edge shared bioctahedra (µ-Cl,CO)c linked by µ-[Pd (µ-SCN)(µ-	291,371
																	2		2				2				2							2
																																		NCS)Cl2(SCN)2]
a	Only compounds that contain no more than three of these ligands, either alone or in combination with one another, are listed.
b	All structures contain a pair of trans bridging dppm, dcpm or dppE ligands. Other bridging ligands, when present, are given in parentheses, as are any references to the struc-
	tures if cited elsewhere in the text.
c	Edge-shared bioctahedron with a symmetrical all-cis arrangement of halide ligands.
d	Edge-shared bioctahedron with an unsymmetrical arrangement of halide ligands.

e Two crystallographically independent and essentially identical molecules are present in the unit cell.

	344
8 Chapter		Bonds Multiple
		Atoms Metal Between

Rhenium Compounds 345

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Fig. 8.26. Reaction scheme showing the products of the reactions of

Re2Cl4(µ-dppm)2 with carbon monoxide.

In the presence of TlPF6, a Re–X bond is labilized and the products are [Re2X3(µ-dppm)2- (CO)2]PF6 or [Re2X3(µ-dppm)2(CO)3]PF6, depending upon whether Re2X4(µ-dppm)2(CO) or Re2X4(µ-dppm)2(CO)2, respectively, is used as the reactant (see Fig. 8.26).334 The similarity between the electrochemical and NMR spectral properties of [Re2X3(µ-dppm)2(CO)2]PF6334 and those of the structurally characterized nitrile complexes [Re2X3(µ-dppm)2(NCR)2]PF6 (vide infra) argues for a structure like 8.41 for the bis-carbonyl cations. Support for this comes from their infrared spectra which show the presence of only terminal carbonyl ligands.334

8.41

The aforementioned carbonyl complexes exhibit well-defined electrochemical behavior,332334 with several redox states quite readily accessible. This is clearly demonstrated in the case of [Re2X3(µ-dppm)2(CO)3]PF6 (X = Cl or Br), which can be reduced by cobaltocene in a stepwise fashion to give the lower valent complexes Re2X3(µ-dppm)2(CO)3 and [(δ5-C5H5)2Co][Re2X3(µ- dppm)2(CO)3] (see Fig. 8.26).334 Another aspect of the redox chemistry of the carbonyl complexes is encountered in the case of the halogen oxidation of Re2X4(µ-dppm)2(CO) (X = Cl or Br) to give the cationic species [Re2X5(µ-dppm)2(CO)]+. A crystal structure determination of [Re2Cl5(µ-dppm)2(CO)]PF6 showed that the cation has the edge-shared bioctahedral structure [Re2(µ-Cl)2(µ-dppm)2Cl3(CO)]+ with a formal Re=Re bond (the distance is 2.6607(4) Å).336

An interesting mixed hydride-carbonyl complex is formed upon reacting Re2Cl4(µ-dppm)2 with various carbonyl clusters in the presence of H2 and, also, from its reaction with H2/CO gas mixtures in refluxing toluene.337 The structure of Re2(µ-H)(µ-Cl)Cl2(µ-dppm)2(CO)2 is that of a symmetrical edge-shared bioctahedron with an all-cis arrangment of chloride ligands and a bridging hydride ligand.337

346Multiple Bonds Between Metal Atoms Chapter 8

The carbonyl complexes Re2Cl4(µ-dppE)2(CO)2 and [Re2Cl3(µ-dppE)2(CO)3]O3SCF3 have been prepared from Re2Cl4(µ-dppE)2287 but, surprisingly, Re2Cl4(µ-dcpm)2 does not react with CO285 although it does convert to 1:1 and 1:2 complexes with isocyanides (vide infra). Even though the compound Re2Cl4(µ-dmpm)2 does not exist, the tris-dmpm complex Re2Cl4(µ-dmpm)3 is very stable295 and has been found to react at room temperature with CO in the presence of TlPF6 to form [Re2Cl3(CO)(µ-dmpm)3]PF6,338 which has an open bioctahedral structure with a terminal CO ligand in an equatorial position and a short Re–Re distance (see Table 8.7). The dicarbonyl complexes [Re2X2(CO)2(dmpm)3](H2PO4)2 (X = Cl or Br) are produced upon reacting [Re2(µ-O2CCH3)X2(µ-dmpm)3]PF6 with CO in deoxygenated acetone/HPF6(aq) mixtures.338,339 These compounds have a structure that can be represented as [Re2(µ-X)2(µ-dmpm)(CO)2(dmpm)2]2+, in which a Re–Re single bond is present (2.918(2) Å) and two of the original bridging dmpm ligands have switched to a chelating mode.338,339

In contrast to the simple carbonylation reactions that Re2X4(µ-dppm)2 undergo (Fig. 8.26), the tetramethyl complex Re2(CH3)4(µ-dppm)2 reacts with CO to give the di-µ-methylene complex Re2(µ-CH2)2(CO)4(µ-dppm)2 in which a long Re–Re single bond is present.289 Structure determinations on two different crystals of this complex that contain solvent THF or CH2Cl2 molecules show that the [Re2(µ-dppm)2] units possess chair and boat conformations, respectively.

The reactions of Re2X4(µ-dppm)2 with one equivalent of an isocyanide, RNC (R = Me, But or Xyl), give the monoisocyanide adducts Re2X4(µ-dppm)2(CNR) in high yield.340-342 A partial crystal structure determination of Re2Cl4(µ-dppm)2(CNBut) showed that like the analogous monocarbonyl complex it has an A-frame-like structure (8.38) with a Re–Re distance of c. 2.30 Å.341 Based upon a qualitative treatment of the bonding, this distance can be considered to represent a slightly weakened triple bond. Because of a disorder involving the terminal ButNC and Cl ligands trans to the Cl-bridge, the full structure could not be solved. However, more recently, a similar disorder in Re2Cl4(µ-dppm)2(CNXyl) was satisfactorily modeled and the structure solved (see Table 8.7).342

The cyclic voltammetric properties of these 1:1 complexes show that like other triply bonded dirhenium(II) species they possess two accessible one-electron oxidations.341 Another interesting property is the presence of two ι(C>N) modes in the infrared spectra at frequencies characteristic of a terminally coordinated RNC ligands.341 These findings indicate that the complexes exist as a mixture of isomers, as is the case for Re2X4(µ-dppm)2(CO) (vide supra), but only one of which forms suitable crystals for a crystallographic determination. These isomers interconvert rapidly on the NMR time scale. However, oxidation of these complexes with NOPF6 forms [Re2X4(µ-dppm)2(CNR)]PF6 (X = Cl or Br)341 which show one ι(C>N) mode in their infrared spectra, indicating that only a single isomer is now present. The structure of this other isomer is most likely that shown in 8.42, based upon studies of the 1:1 complexes Re2Cl4(µ-dppE)2(CNXyl) and Re2Cl4(µ-dcpm)2(CNXyl).342 Although both these compounds have solution properties that resemble closely those of Re2Cl4(µ-dppm)2(CNXyl), an X-ray crystal structure determination of Re2Cl4(µ-dcpm)2(CNXyl) revealed that the structure is as shown in 8.42 with a Re–Re triple bond (see Table 8.7).342 The halogen oxidation of Re2X4(µ- dppm)2(CNR) (X = Cl or Br; R = But or Xyl) affords salts of the edge-shared bioctahedral cations [Re2(µ-X)2(µ-dppm)2X3(CNR)]+; the chloride complexes have been reduced by cobaltocene to the neutral paramagnetic Re25+ complexes Re2(µ-X)2(µ-dppm)2X3(CNR).336

Rhenium Compounds 347

Walton

8.42

A variety of complexes with two or three RNC ligands present have also been isolated and some of these structurally characterized.340,342 Which compound is isolated depends upon the reaction conditions and the identity of the phosphine ligand in the Re2X4(µ-LL)2 precursor compound. The first such study, which involved the treatment of Re2Cl4(µ-dppm)2 with two equivalents of ButNC in acetone in the presence of PF6-, gave yellow and green isomers of stoichiometry [Re2Cl3(µ-dppm)2(CNBut)2]PF6.340,343 A comparison of their infrared, 1H NMR and 31P{1H} NMR spectra show that these isomers are structurally very different, with the green isomer very likely having a structure that is similar to 8.41 (with L = ButNC). Subsequently, the salts [Re2Cl3(µ-dppE)2(CNBut)2]X were prepared with X = O3SCF3 or PF6, and shown to have properties very similar to those of the green isomeric form of the dppm complex.342 A novel complex that contains a µ-iminyl ligand, [Re2Cl3(µ-C=NHBut)(µ-dppm)2(CNBut)2]PF6, has been isolated as a by-product in the synthesis of the green isomer and has been structurally characterized.343,344 It has an edge-shared bioctahedral structure with an all-cis arrangement of chloride ligands, a symmetrically bridging µ-C=NHBut ligand and a Re–Re distance of 2.704(1) Å. This blue, paramagnetic compound exhibits a well-defined X-band EPR spectrum. In the absence of salts such as TlO3SCF3 or TlPF6, Re2Cl4(µ-dppm)2 reacts with 2 equiv of ButNC to give Re2Cl4(µ-dppm)2(CNBut)2 via the intermediacy of the 1:1 complex, but this product could not be separated from some [Re2Cl3(µ-dppm)2(CNBut3]+ (vide infra) which is formed.342 However, the bis-isocyanide complexes Re2Cl4(µ-dppE)2(CNBut)2 and Re2Cl4(µ- dcpm)2(CNBut)2 can be isolated and both have the same symmetrical structure shown in 8.43 with axial Re–Cl bonds, and quite short Re–Re distances (see Table 8.7).342 The dppE complex possesses a staggered rotational geometry in the solid state while the dcpm complex is rigorously eclipsed.342 The 1:2 complexes with XylNC have not yet been isolated.

8.43

Complexes that contain three isocyanide ligands i.e. [Re2Cl3(µ-dppm)2(CNR)3]+ (R = But or Xyl), have been isolated as their PF6- salts from the reactions of Re2Cl4(µ-dppm)2 or the mixed isocyanide-nitrile complex [Re2Cl3(µ-dppm)2(CNBut)(NCEt)]PF6 (vide infra) with c. 4 equiv of ButNC and of [Re2Cl3(µ-dppm)2(CNXyl)(NCPh)]PF6 (vide infra) with 2.5 equiv of XylNC.343 While the stoichiometries of these two complexes are identical, their electrochemical redox properties are very different. This suggests that they have different structures. Reduction of the monocation [Re2Cl3(µ-dppm)2(CNXyl)3]PF6 with cobaltocene yields the neutral paramagnetic

348Multiple Bonds Between Metal Atoms Chapter 8

complex Re2Cl3(dppm)2(µ-CNXyl)3 containing (formally at least) the Re23+ core, while oxidation with NOPF6 gives the paramagnetic dication [Re2Cl3(µ-dppm)2)(CNXyl)3](PF6)2. The related green ButNC derivative [Re2Cl3(µ-dppm)2(CNBut)3]PF6 does not possess any reversible redox chemistry.343 In a more recent study, the ButNC complex [Re2Cl3(µ-dppm)2(CNBut)3]+ was isolated as its Cl- salt from the direct reaction of ButNC with Re2Cl4(µ-dppm)2, and the pair of salts [Re2Cl3(µ-dppE)2(CNBut)3]X (X = Cl or PF6) were likewise prepared from Re2Cl4(µ- dppE)2.342 An X-ray crystal structure determination carried out on [Re2Cl3(µ-dppE)2(CNBut)3]Cl has established that it has the open bioctahedral structure shown in 8.44, with a staggered rotational geometry and a short Re–Re distance (see Table 8.7).342 It seems certain that the related [Re2Cl3(µ-dppm)2(CNBut)3]+ species has this same structure. Although all attempts to date have failed to solve the crystal structure of a salt of the [Re2Cl3(µ-dppm)2(CNXyl)3]+ cation, a partially refined structure of its neutral reduced congener Re2Cl3(µ-dppm)2(CNXyl)3 has confirmed that it is the edge-shared bioctahedron (XylNC)ClRe(µ-Cl)(µ-CNXyl)(µ-dppm)2ReCl(CNXyl) with an all-cis arrangement of XylNC ligands and a Re–Re distance of 2.73 Å.342

8.44

A variety of nitriles react with Re2X4(µ-dppm)2, including the polycyano acceptor molecules TCNQ and DM-DCNQI that form neutral complexes of the type [Re2Cl4(µ-dppm)2]2(µ-L), in which the organocyanide bridges link two Re2Cl4(µ-dppm)2 through equatorial positions.312,313 These compounds are discussed in the section dealing with the redox chemistry of Re2X4(µ-LL)2 compounds. Simple organic nitriles RCN react very readily with Re2Cl4(dppm)2 in the presence of KPF6 to yield the stable, bis-nitrile complexes [Re2Cl3(µ-dppm)2(NCR)2]PF6.345 In the original study of this system, the nitriles where R = Me, Et, Ph, or 4-PhC6H4 were used,345 although a wider range of them has subsequently been studied, including a few diand trinitrile ligands (such as 1,2- and 1,4-dicyanobenzene and tris(2-cyanoethyl)phosphine) which bind through only one of their nitrile groups.325,346-348 Several related bromide complexes have been isolated;346 it should be noted that the insoluble complex that is formed upon reacting (Bu4N)2Re2Br8 with dppm in refluxing acetonitrile, and which had originally been formulated as “_-[ReBr2(dppm)]n”,202 is in fact [Re2Br3(µ-dppm)2(NCMe)2]Br.346 The use of 31P{1H} NMR spectroscopy to monitor the formation of [Re2Cl3(µ-dppm)2(NCEt)2]Cl in the reaction between Re2Cl4(µ-dppm)2 and propionitrile shows that these reactions occur in a two step fashion, in which a RCN ligand first coordinates to one of the Re atoms of Re2Cl4(µ-dppm)2 to generate a 1:1 adduct, possibly having a molecular A-frame-like structure (see 8.38), followed by the addition of a second nitrile ligand to the same Re atom with concomitant loss of Cl- to generate [Re2Cl3(µ-dppm)2(NCEt)2]+.345 These conclusions have been supported by a detailed electrochemical study of the formation of these complexes.347 The acetonitrile, propionitrile, benzonitrile, 1,2-dicyanobenzene and 1,4-dicyanobenzene complexes have all been structurally characterized and found to have a structure like that shown in 8.41 (L = RCN) (see also Table 8.7).300,325,345,348,349 The Re–Re bond lengths are very similar to one another, although longer than in the parent Re2Cl4(µ-dppm)2, and the molecules have staggered rotational geometries.

Rhenium Compounds 349

Walton

The bis-nitrile salts react cleanly with NOPF6 to generate the paramagnetic EPR-active dications [Re2Cl3(dppm)2(NCR)2](PF6)2,206 which possess the Re25+ core and a μ2/4β2β*1 groundstate electronic configuration.

A few bis-nitrile complexes have been isolated with phosphine ligands other than dppm, namely, [Re2Cl3(µ-cdpp)2(NCR)2]PF6 (R = Me or Et)286 and [Re2Cl3(µ-dppa)2(NCR)2]PF6 (R = Et or Ph).348 They resemble closely their µ-dppm analogs.

Several of the bis-nitrile complexes react with further nitrile under reflux conditions to afford the green paramagnetic complexes [Re2X3(µ-HN2C2R2)(µ-dppm)2(NCR)]PF6 (X = Cl or Br; R = Me, Et, Pri or Ph),346,350 in which the dimetal unit has served as a template for the reductive coupling of two nitrile ligands. The lability of the nitrile ligand (RCN) in these complexes has been demonstrated by carrying out nitrile exchange reactions, and their structural identity has been confirmed by an X-ray structure analysis of a salt of the edge-shared bioctahedral [Re2Br3(µ-HN2C2Me2)(µ-dppm)2(NCMe)]+ cation, which has shown that the coupled nitrile ligands exhibit a novel µ2-δ2 bonding mode.346,350 The Re–Re distance in this complex is 2.666(1) Å. Distances within the five-membered metallacycle ring, formed from the coupled nitrile ligands, can best be rationalized in terms of contributions from a singly deprotonated diimine ligand (8.45), and a triply deprotonated ene-diamine ligand (8.46). The treatment of these complexes with either [(δ5-C5H5)2Fe]PF6 in acetone or NOPF6 in CH2Cl2 leads to oxidation, and the formation of the red, diamagnetic salts [Re2X3(µ-HN2C2R2)(µ-dppm)2(NC R)](PF6)2.346,350 This chemistry has recently been extended to the analogous reactions between Re2Cl4(µ-dppa)2 and propionitrile, which has led to the isolation of [Re2Cl3(µ-HN2C2Et2)(µ- dppa)2(NCEt)]Cl and the oxidized salt [Re2Cl3(µ-HN2C2Et2)(µ-dppa)2(NCEt)](PF6)2.348

8.45

8.46

The reaction of Re2Cl4(µ-dppm)2 with acetylene at room temperature in dichloromethane or acetone affords both 1:2 and 1:3 complexes as shown in the reaction scheme in Fig. 8.27.351 These complexes have structures that resemble those of the corresponding carbonyl complexes (structures 8.39 and 8.40 in Fig. 8.26) with the important difference that the acetylene complexes contain Re–Re single bonds; the Re–Re bond distances are 2.8094(3) Å and 2.8613(5) Å, respectively. The bis-acetylene complex has also been isolated in the case of Re2Br4(µ-dppm)2. The compound Re2Cl4(µ-dppm)2(µ:δ2,δ2-HCCH)(δ2-HCCH) can be derivatized by isocyanides, while the two terminally bound δ2-acetylene ligands in the tris-acetylene complex are readily displaced by CO and XylNC (see Fig. 8.27). In all cases the products retain the Re–Re single bonds of the parent molecules.351 A quite different and novel reaction course ensues when Re2Cl4(µ-dppm)2 is treated with 1,7-octadiyne.352 In this case, the starting material serves as a reagent for the 2-electron reductive cyclization of the diyne and as a template to stabilize the resulting [C8H7Re2] bridging unit shown in structure 8.47, in which a quadruply bonded Re26+ core is present (the Re–Re distance is 2.2647(3) Å). The µ-C8H7 ligand is formally trianionic.

Very extensive series of mixed carbonyl-isocyanide, carbonyl-nitrile and carbonyl- isocyanide-nitrile complexes have been prepared starting from the preformed adducts Re2Cl4-

350Multiple Bonds Between Metal Atoms Chapter 8

(µ-dppm)2(CO), Re2Cl4(µ-dppm)2(CO)2 and Re2Cl4(µ-dppm)2(CNR). The products of these reactions typically have edge-sharing bioctahedral or open-bioctahedral structures which lead to a dependence of bond order upon structure type. Up to five CO, RNC and RCN ligands in combination with one another have been incorporated into the coordination sphere of the [Re2(µ-LL)2] unit. Once again, the cases where LL is the dppm ligand dominate this chemistry. A complicating factor in some of this chemistry is the existence of structural isomerism, the important features of which are discussed below. Because this chemistry is now so extensive and quite complicated, only a few key aspects of each type of compound will be discussed but the literature citations are complete and can be consulted for further details. The coverage of these specific compounds will be followed by a consideration of complexes that result from combining alkynes with compounds that contain RNC and CO ligands and, finally, other related compounds of interest.

Fig. 8.27. Reaction scheme showing the products of the reactions Re2Cl4(µ-dppm)2 with acetylene.

The first study involved the reactions of the monocarbonyl Re2Cl4(µ-dppm)2(CO) in acetone with one equivalent of an isocyanide to form the neutral complexes of stoichiometry Re2Cl4(µ-dppm)2(CO)(CNR) (R = Pri, But, xylyl or mesityl).333 A comparison of their infrared spectral properties shows that the alkyl isocyanide derivatives have both their /-acceptor ligands terminally bound, but in the aryl isocyanide derivatives the CO ligand bridges the Re–Re bond. Their electrochemical properties and hence their electronic configurations are