Multiple Bonds Between Metal Atoms / 04-Molybdenum Compounds

Molybdenum Compounds 129

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	Compound	E1/2(ox)	E1/2(red)	Other	Solvent	Reference	ref.
				processes		electrode
	_-Mo2Cl4(dppbe)2	+0.45b	−1.23c		CH2Cl2	Ag/AgCl	385
	Mo2(NCO)4(PMe3)4	+0.83b	−1.42		THF	Ag/AgCl	399
		+0.60	−1.57		CH2Cl2	Ag/AgCl	399
	Mo2(NCS)4(PMe3)4	+1.0b	−0.93	E1/2(red) = −1.95	THF	Ag/AgCl	399
		+1.0b	−1.01		CH2Cl2	Ag/AgCl	399
	Mo2(NCS)4(PEt3)4	+0.80	−1.17		CH2Cl2	SCE	451
	Mo2(NCS)4(dppm)2	+0.84b	−0.80	Ep,c = −1.60	CH2Cl2	SCE	451
	Mo2(NCS)4(dppe)2	+0.74b	−0.85	E1/2(red) = −1.58	CH2Cl2	SCE	451

a This process is described as being at a potential near the solvent limit.

b Ep,a value. c Ep,c value.

dSimilar data reported in ref. 439. Values of E1/2(ox) = +0.65 V and E1/2(red) = −1.82 V have been reported with the use of a silver quasi-reference electrode (see ref. 455).

eValues of E1/2(ox) are given in ref. 454 for CH2Cl2 solutions of several Mo2Cl4(PR3)4 complexes. The values quoted (versus Ag/AgCl) are anywhere between 0.04 V and 0.14 V more positive than those cited in this table

depending upon the identity of PR3. f Similar data reported in ref. 452.

g Similar data reported in ref. 451.

The one-electron oxidation and one-electron reduction of the phosphine complexes generate species that possess the electronic configurations μ2/4β1 and and μ2/4β2β*1, respectively, and therefore contain Mo–Mo bond orders of 3.5. While several attempts have been made to isolate salts of the monocations, these efforts have met with limited success. Solutions of the paramagnetic EPR-active [Mo2Cl4(PPrn3)4]+ cation in CH2Cl2 have been generated electrochemically at c. 0 °C,451 while [Mo2Cl4(PBun3)4]PF6 has been formed at −78 °C with the use of [Ag(NCMe)4]PF6 as oxidant.453 These species decompose rapidly at room temperature. An interesting case of electrogenerated chemiluminescence has been encountered in the case of Mo2Cl4(PMe3)4 dissolved in (Bu4N)BF4-THF by pulsing the potential of the Pt electrode between −1.95 and +0.7 V (versus a Ag quasi-reference electrode).458 Emission results from the electron-transfer reaction between the [Mo2Cl4(PMe3)4]− and [Mo2Cl4(PMe3)4]+ species that are generated.

[Mo2Cl4(PMe3)4]− + [Mo2Cl4(PMe3)4]+ Α {Mo2Cl4(PMe3)4}* + Mo2Cl4(PMe3)4

{Mo2Cl4(PMe3)4}* Α Mo2Cl4(PMe3)4 + hι

Electrogenerated chemiluminescence has also been observed upon electrochemical reduction of Mo2Cl4(PMe3)4 in the presence of [S2O8]2− when the potential is pulsed between −0.5 and −2.0 V. The mechanism involves the reaction of [Mo2Cl4(PMe3)4]− with SO4−.458

A different technique has been used to study the arsine complexes Mo2X4(AsEt3)4 (X = Cl or Br), namely, rotating electrode polarography.102 Solutions of these complexes in CH3CN show

oxidations at E1/2 = +0.56 V (X = Cl) and E1/2 = +0.6 V (X = Br) versus SCE. Controlled potential electrolysis at 0 °C has been used to generate solutions of the paramagnetic EPR-active

monocations, which can be re-reduced to their neutral parents.102 The [Mo2X4(AsEt3)2]+ cations have also been characterized by electronic absorption spectroscopy.

In addition to the simple one-electron transfer reactions that these complexes undergo, there are numerous reactions in which the Mo24+ core is oxidized to Mo26+, the resulting complexes containing confacial bioctahedral or edge-sharing bioctahedral structures. The com-

130Multiple Bonds Between Metal Atoms Chapter 4

plexes Mo2Cl4(PR3)4 (R = Et or Prn) are oxidized in refluxing CH2Cl2–CCl4, mixtures to give red (R3PCl)3Mo2Cl9,331 and this same anion is also generated from Mo2Cl4(dppm)2 and _- Mo2Cl4(dppe)2 under similar conditions;368 it can be precipitated as its Et4N+ salt from the latter reaction solutions. The oxidations of Mo2Cl4(PR3)4, where PR3 = PEt3, PBun3or PEtPh2, also proceed photochemically. A maroon colored compound purported to be Mo2Cl6(PEtPh2)3 was prepared459 by broad band UV photolysis of a dichloromethane solution of Mo2Cl4(PEtPh2)4. The reaction of Mo2I4(PMe3)4 with I2 in toluene affords (Me3PH)[Mo2(µ-I)3I4(PMe3)2],460 while the oxidation of Mo2Cl4(PMe3)4 with PhICl2 gives (Me3PH)[Mo2Cl7(PMe3)2], which can be isolated in both syn and gauche isomeric forms.461

Oxidative addition reactions to `-Mo2X4(LL)2 molecules are numerous.390,462-468 They yield edge-sharing bioctahedra in which the LL ligands continue to bridge the metal atoms with the phosphorus atoms trans at each molybdenum atom. The complex Mo2(µ-SPh)(µ-Cl)Cl4(µ- dppm)2 is isolated in low yield (8%) through the reaction of Mo2Cl4(dppm)2 with PhSSPh in CH2Cl2.466 In some cases there is a change in the bonding mode of the dmpe and dppe ligands from bridging to chelating, and dichloromethane may serve as a chlorinating agent. The reactions of RSSR with Mo2Cl4(dto)2 afford Mo2(SR)2Cl4(dto)2 compounds which can also be obtained by reacting K4Mo2Cl8 or (NH4)5Mo2Cl9·H2O with dto and EtSSEt or PhSSPh in refluxing methanol. These later reactions certainly proceed through the intermediacy of Mo2Cl4(dto)2.467,468

4.3.5 Cationic complexes of Mo24+

There are only a few compounds that contain the Mo24+ core entirely surrounded by neutral ligands so that a [Mo2L8]4+ or [Mo2L10]4+ complex results. The first such cation, Mo24+(aq), was prepared in solution201 in 1971, but no solid compound of it has ever been reported and it is not known whether the coordination sphere has 8 or 10 water molecules. The solution was prepared by adding Ba(SO3CF3)2 to K4Mo2(SO4)4 dissolved in 0.01 M CF3SO3H.202,201 The solution of the cation, which has electronic absorption bands at 370 and 504 nm, is stable if not exposed to light or oxygen. Green [Mo2(µ-OH)2(aq)]4+ is formed with evolution of H2 when a solution of Mo24+(aq) in 1 M CF3SO2H is irradiated at 254 nm.206 The kinetics and mechanism of reaction with NCS− and HC2O4− have been investigated.469 From X-ray absorption edge and EXAFS spectra the Mo–Mo distance in the Mo24+(aq) ion has been estimated to be 2.12 Å.470

The [Mo2(CH3CN)n]4+ (n = 8, 9, 10) ions are well established and some of their chemistry has been studied. Structural results are collected in Table 4.11. [Mo2(CH3CN)8](CF3SO3)4 was obtained as a blue crystalline solid.178 It readily loses CH3CN and reacts with acetic acid to form Mo2(O2CCH3)4. [Mo2(CH3CN)10](BF4)4 may be prepared158,179 by reaction of Mo2(O2CCH3)4 and HBF4 in Et2O. This compound is also rather unstable, but gives large dark-blue crystals from acetonitrile. X-ray crystallography reveals a centrosymmetric [Mo2(CH3CN)8(ax-CH3CN)2]4+ ion (Fig. 4.19) with a Mo–Mo distance of 2.187(1) Å. More recently [Mo2(CH3CN)9](BF4)4 has been structurally defined with C4v symmetry and an Mo–Mo distance of 2.180(1) Å.243 These are the only cationic Mo24+ complexes that have been crystallographically defined.

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Fig. 4.19. The [Mo2(NCCH3)10]4+ cation as found in [Mo2(NCCH3)10](BF4)4·2CH3CN.

Some reactions of the [Mo2(CH3CN)8-10]4+ ions have been studied.242 The compound [Mo2(µ- CH3CONH)(CH3CN)6](BF4)3 is obtained by reaction of [Mo2(CH3CN)8](BF4)4 with CH3CONH2 in c. 60% yield or by reaction of [Mo2(CH3CN)8](BF4)4 with H2O in c. 70% yield. Hydrolysis of CH3CN occurs in the latter reaction. The compound [Mo2(µ-CH3CONH)(CH3CN)6](BF4)3 reacts with dppm to give [Mo2(µ-CH3CONH)(µ-dppm)2(CH3CN)2](BF4)3. Reaction of toluidine with [Mo2(CH3CN)8]4+ produces [Mo2(µ-(HNCMeNtol)(CH3CN)6]4+. [Mo2(CH3CN)9](BF4)4 reacts243 with dppe to produce an adduct with a very complex structure in which an Mo–Mo bond (2.180(1) Å) is multiply bridged, and this in turn reacts with traces of water at low temperature to generate another complex product in which the dppe is lost and one CH3CN is hydrolyzed to an acetamido anion, which bridges through its nitrogen atom only. However, by reaction of the dppe intermediate with excess water the cation [Mo2(NHC(CH3)O)2(CH3CN)4]2+ is formed, in which the Mo–Mo distance is 2.144(2) Å.

Table 4.11. Structures of [Mo2(CH3CN)8-10]4+ compounds and their reaction products

	Compound			Crystal	Virtual	r(Mo–Mo)	Twist	ref.
				sym.	sym.		angle
[	Mo2(CH3CN)8(ax-CH3CN)2	]	(BF4)4·2CH3CN	¯	D4h	2.187(1)	zero	179
				1
[Mo2(CH3CN)8(ax-CH3CN)](BF4)4				4	C4v	2.180(1)	50	243
[Mo2(CH3C(O)NH)(CH3CN)6](BF4)3				1	Cs	2.183(1)	NR	242
[Mo2(CH3C(O)NH)2(CH3CN)4](BF4)2				1	C2	2.144(2)	NR	243
[Mo2(CH3C(O)NH)py5(OH)](BF4)2				1	Cs	2.149(1)	NR	242
[Mo2(CH3C(O)NH)(dppm)2(CH3CN)2](BF4)3				1	Cs	2.146(2)	NR	242
(CH3C6H4NH3)[Mo2(HNC(CH3)Ntol)-				1	Cs	2.157(1)	NR	242
(CH3CN)6](BF4)4

The compound [Mo(en)4]Cl4 forms upon heating neat ethylenediamine with K4Mo2Cl8 and was isolated202 as orange crystals upon adding hydrochloric acid to an aqueous solution of the crude product. Such a recrystallization in the presence of p-toluenesulfonic acid produces the p-toluenesulfonate salt.202 [Mo2(en)4]4+ has its β Α β* electronic transition at 20,900 cm−1 and, like [Mo2(aq)]4+, is irreversibly oxidized under a variety of conditions; cyclic voltammetry measurements have shown that this complex exhibits an irreversible oxidation at +0.78 V versus SCE.202 The analogous complex [Mo2(R-pn)4]Cl4, where R-pn = (R)-1,2-diaminopropane, has also been prepared471 by a similar procedure. The CD spectrum of this complex in 0.1 M HCl

132Multiple Bonds Between Metal Atoms Chapter 4

has been interpreted in terms of a structure with bridging R-pn ligands and a staggered rotational geometry (ρ between 45 and 90°).

A complex formulated as [Mo2(EtCO2CH3)4](CF3SO3)4 and proposed to contain ethyl acetate bridges, may be a further example of cationic species.213

4.3.6 Complexes of Mo24+ with macrocyclic, polydentate and chelate ligands

Compounds that have been crystallographically characterized are listed in Table 4.12.

Table 4.12. Structures of Mo24+ compounds with macrocyclic or chelating ligands

	Compound		Crystal	Virtual	r(Mo–Mo)	Twist	ref.
			sym.	sym.		angle
t	t	)4	¯	D2h	2.147(1)	zero	472
Mo2(Bu	(C(O)CHC(O)Bu		1
Mo2(acacen)2			¯	C2h	2.168(1)	0	473
			1
Mo2(But-salophen)2			1	C1	2.203(1)	88	474
Mo2(tmtaa)2			1	D2d	2.175(1)	90	475
Mo2(TPP)2			1	D2d	2.239(1)	18	476
Mo2(o-Me2NCH2C6H4)4			1	C2	2.145(1)	11	477
Mo2(Et2Bpz2)2[Et2B(OH)pz]2			1	D2d	2.156(1)	15	478

The macrocyclic ligand tmtaa2−, shown as 4.28, as Li2tmtaa reacts with Mo2(O2CCH3)4 to give a brown-black product Mo2(tmtaa)2.475,479 The tmtaa ligands are rotated 90° relative to one another which still gives two sets of Mo–N bonds that are essentially eclipsed, but allows the two saddle-shaped ligands to fit snugly together. Cyclic voltammetry of solutions of this complex in (Bu4N)PF6–CH3CN shows four redox processes, two of which correspond to oxidations and two to reductions.479 Oxidation at room temperature with [(δ5-C5H5)2Fe]PF6 affords dark-purple paramagnetic [Mo2(tmtaa)2]PF6,479 whose structure is very similar to that of Mo2(tmtaa)2. The Mo–Mo distance (2.221(1) Å) is 0.046 Å longer than that in Mo2(tmtaa)2, as a result of removing one β electron.

N N

N N

4.28

The treatment of Mo2(tmtaa)2 with the mild oxidant tetracyanoethylene (TCNE) in toluene or acetonitrile gives the biradical compound [Mo2(tmtaa)2]+(TCNE)−, which has been characterized by EPR spectroscopy.480 This complex decomposes to [MoO(tmtaa)]+[C3(CN)5]− in the presence of a trace amount of water, and this compound can in turn be converted to the dimolybdenum radical anion [Mo2(tmtaa)2]− upon reaction with Na/Hg in THF.480 The later species is formed more directly by the reduction of Mo2(tmtaa)2 with Na/Hg.479 When Mo2(O2CCH3)4 reacts with H2tmtaa, only two cisoid molecules of acetic acid are displaced and the tmtaa forms two bonds to each molybdenum atom, thereby bridging them.

Several dimolybdenum(II) porphyrin complexes, Mo2(Por)2, have been prepared in which there is an unsupported Mo–Mo quadruple bond. These have usually been prepared by the vacuum pyrolysis of mononuclear Mo(Por)(PhC>CPh),481 where Por represents the dianionic

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porphyrin ligand, and/or from the reaction of MoCl2(CO)4 with the free porphyrin (H2Por) in oxygen-free toluene in the presence of lutidine.482 These methods have been used to prepare derivatives where Por = octaethylporphyrinato (OEP), mono-meso-substituted OEP-X (where X = formyl, nitro, amine or isocyanate), and meso-tetra-p-tolylporphyrinato (TTP). By utilizing a mixture of H2(OEP) and H2(OEP–CHO) in the second of these procedures, a separable mixture of Mo2(OEP)2, Mo2(OEP)(OEP–CHO) and Mo2(OEP–CHO)2 was obtained.482 Variable temperature 1H NMR studies of the meso-substituted derivatives have provided solution evidence for the presence of Mo–Mo bonds and an activation energy of 10.0 ± 0.5 kcal mol−1 for the barrier to rotation about the Mo–Mo bonds. The resonance Raman spectrum of Mo2(OEP)2 has yielded a Mo–Mo stretching frequency of 341 cm−1, from which an Mo–Mo distance of 2.23 Å has been estimated.483

In one instance a complex has been prepared in which the two porphyrin rings are constrained to be eclipsed by employing a rigid biphenylene bridge to link them. This complex, Mo2DPB, contains the tetraanion 1,8-bis[5-2,8,13,17-tetraethyl-3,7,12,18-tetramethyl)porp hyrin]biphenylene and is prepared by reacting H4DPB with MoCl2(CO)4 followed by chromatography.484 Only in the case of Mo2(TPP)2, which is the initial product from the reaction of Mo(CO)6 with tetraphenylporphyrin (H2TPP), has the structure been determined by X-ray crystallography (Table 4.6).476

There are several molecules in which MoII, which are either bis-chelated or coordinated by a tetradentate ligand, are linked by an unbridged quadruple bond. For example, the reaction of Mo2(O2CCH3)4 with Na[Et2Bpz2] (pz = 2-pyrazolyl) yields several products,478 one of which is Mo2(Et2Bpz2)2(Et2B(OH)pz)2. One ligand of each type is chelated to each Mo atom and the N3OMoMoN3O core is nearly eclipsed. An organometallic example is Mo2(o-Me2NCH2C6H4)4, in which two C6H4CH2NMe2 ligands are chelated to each Mo atom in a cis relationship.477

The structural characterization of the eclipsed `-diketonate complex Mo2(ButCOCHCOBut)4 has also been carried out472 following the synthesis of several complexes of the type Mo2(RCOCHCOR)4. The reduction of the mononuclear molybdenum(IV) complex Mo(acacen)Cl2, where acacen2− = N,N'-ethylenebis(acetylacetoneiminato), with sodium in THF in the presence of diphenylacetylene, affords the dimolybdenum(II) complex Mo2(acacen)2, whose structure is shown in Fig. 4.20.473 The role of the PhC>CPh in the synthesis of this complex may be similar to that in the preparation of various porphyrin complexes of dimolybdenum(II). A comparable complex of a salophen ligand has also been made.474

Fig. 4.20. The structure of the Mo2(acacen)2 molecule.

134Multiple Bonds Between Metal Atoms Chapter 4

4.3.7 Alkoxide compounds of the types Mo2(OR)4L4 and Mo2(OR)4(LL)2

Several such complexes have been prepared and characterized. Entry to this chemistry has involved dimethylamido dimolybdenum(III) starting materials. The first such study, reported in 1984407 showed that the reaction of 1,2-Mo2(Bui)2(NMe2)4 with isopropyl or neopentyl alcohol in hexane results in `-hydrogen atom transfer to form isobutylene, isobutane and Mo2(OR)4(HNMe2)4 (R = Pri or CH2CMe3). Ligand exchange reactions have been used to prepare Mo2(OPri)4L4, where L = py, MeNH2, PriOH or PMe3, and Mo2(OCH2CMe3)4(PMe3)4.406,407 X-ray structure determinations on Mo2(OPri)4L4 (L = py or PriOH) and Mo2(OCH2CMe3)4L4 (L = Me2NH or PMe3) have confirmed406,407 that each of these complexes is the 1,3,6,8 isomer.

The Mo–Mo distances (Table 4.8) are typical of Mo–Mo quadruple bonds, although the mixing of filled oxygen p-orbitals with empty Mo–Mo β* and /* MOs probably tends to make the Mo–Mo bonds slightly longer and weaker than those in similar halide complexes. However, in the cases of Mo2(OPri)4(HOPri)4 and Mo2(OCH2CMe3)4(HNMe3)4, the Mo–Mo bonds are actually shorter than expected because of the formation of strong hydrogen bonds of the type represented in 4.29.

RH

O L

Mo Mo 4

4.29

Similar chemistry with aryloxide ligands has been shown to occur by treating Mo2(NMe2)6 with C6F5OH and 3,5-Me2C6H3OH. The former reaction, when carried out in toluene or a pyridine–benzene mixture and with the use of a large excess of C6F5OH (10-12 equivalents), affords the complex Mo2(OC6F5)4(HNMe2)4.409 Its structure, of the 1,3,6,8 type, is shown in Fig. 4.21. The reaction of Mo2(NMe2)6 with four equiv of 3,5-Me2C6H3OH in hexane gives deep blue Mo2(OC6H3-3,5-Me2)4(HNMe2)4 in 15-30% yield; this yield is increased to 65% if Me2NH is added to the initial reaction mixture.455 A crystal of the novel Mo27+ complex Mo2(µ-NMe2)(µ-OC6H3-3,5-Me2)2(OC6H3-3,5-Me2)4(HNMe2)2 has been isolated from this reaction and structurally characterized (the Mo–Mo distance is 2.414(1) Å).455 The reaction of Mo2(OC6H3-3,5-Me2)4(HNMe2)4 with PMe3 produces Mo2(OC6H3-3,5-Me2)4(PMe3)4; both complexes have electronic absorption spectra characteristic of Mo24+ complexes with the β Α β* transition at 584 and 673 nm, respectively. Interestingly, the redox properties of these two complexes are markedly different from those of the halide complexes of the type Mo2X4L4. Cyclic voltammograms on solutions in (Bu4N)PF6–THF show two one-electron oxidations at

E1/2 = −0.15 V and Ep,a = +0.31 V versus Ag/AgCl for the Me2NH complex and at E1/2 = −0.40 V and E1/2 = +0.24 V versus Ag/AgCl for the PMe3 derivative. While the oxidation of

Mo2(OC6H3-3,5-Me2)4(HNMe2)4 is chemically irreversible, the PMe3 complex can be oxidized electrochemically to its yellow-brown, EPR-active monocation. While this process is reversible, the second oxidation is not.455

The green compound, Mo2(OC6F5)4(PMe3)4, obtained from the reaction of C6F5OH with Mo2(CH3)4(PMe3)4408 is the 1,2,7,8 isomer, although the Mo–Mo distance is about the same as that in 1,3,6,8-Mo2(OC6F5)4(HNMe2)4. Reactions of Mo2(CH3)4(PR3)4 (PR3 = PMe3 or PMe2Ph) with the fluoroalcohols C6F5OH, CF3CH2OH and (CF3)2CHOH all seem to proceed in a similar fashion but the structures of the products (other than Mo2(OC6F5)4(PMe3)4) have not yet been determined.408

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Fig. 4. 21. The structure of the Mo2(OC6F5)4(NHMe2)4 molecule.

The Mo2(OR)4L4 compounds show some interesting chemistry. There are preliminary reports407 of the following reactions:

The reaction of Mo2(OPri)4(HOPri)4 with dmpe in hexane gives421 Mo2(OPri)4(dmpe)2, which can also be obtained from reaction of Mo2(Bui)2(NMe2)4 with Pri(OH) (> 4 equivalents) and dmpe (2 equivalents) in a hydrocarbon solvent. The structure of this compound is of the 1,2,3,4–Mo2X4(LL)2 type as shown in 4.30, but the conformation is also staggered. The Mo–Mo bond distance and staggered geometry are in accord with a triple bond.421 The electronic structures of the model species X4Mo–Mo(PH3)4 (X = OH or Cl) have been investigated by the SCF-X_-SW method.485 It has been concluded the /-donor ligands such as alkoxides inhibit the formation of a polar β-bond between the two metal centers by interacting strongly with the MoIV-based dxy orbital. This would result in a Mo–Mo bond order of three in any ligand conformation; the staggered geometry is preferred for steric reasons. The preferences for the structure (PriO)4MoMo(dmpe)2 over `-Mo2(OPri)4(dmpe)2 apparently reflects the greater steric demands of the isopropoxide ligands as compared to the halide ligands.485

	OR			OR P		P
Mo					Mo
RO				P		P
OR

4.30

136Multiple Bonds Between Metal Atoms Chapter 4

4.4Other Aspects of Mo24+ Chemistry

4.4.1 Cleavage of Mo24+ compounds

The red phosphido compound, Mo2(µ-PBut2)2(PBut2)2, can be prepared by the interaction of LiPBut2 with Mo2(O2CCH3)4 in diethyl ether at −78 °C.486 This compound has a ‘butterfly’ structure and a short Mo–Mo distance (2.209(1) Å) that accords with a multiple bond. The 31P{1H} NMR spectrum of this complex shows two sharp singlets, which is evidence that this structure is retained in solution.486

The interaction between Mo2(O2CCH3)4, Me3SiI, and I2 in THF results in oxygen abstraction from the solvent and the formation of the salt [Mo2(µ-O)(µ-I)(µ-O2CCH3)I2(THF)4]+- [MoOI4(THF)]− and I(CH2)4I.487 The cation contains a metal–metal bonded Mo27+ core.

A further reaction of note is that between Mo2(O2CCH3)4 and the sodium salt of 2-mercap- topyridine in ethanol. This affords a green solid which upon exposure to oxygen is converted into red Mo2O3(C5H4NS)4,390 a complex that contains two terminal Mo=O units and a linear Mo–O–Mo bridge. This reaction is analogous to the reaction between Re2(O2CCH3)4Cl2 and sodium diethyldithiocarbamate which produces Re2O3(S2CNEt2)4. A similar reaction course to this has been found488 to lead to the formation of Mo2O3(SC4H3N2)2(py)2 when Mo2(O2CCH3)4 is reacted with 2-mercaptopyrimidine in methanol and the reaction precipitate is dissolved in pyridine. The dithiocarbamate complex Mo2(S2CNEt2)4 is readily oxidized by air to give Mo2O3(S2CNEt2)4,279 while its oxidation with I2 in THF affords Mo2O3(S2CNEt2)2I2(THF)2.280

The pyridine complexes Mo2X4(py)4 (X = Cl or Br) are oxidized to mer-MoX3(py)3 in the presence of an excess of pyridine under forcing reaction conditions.489 This is an especially noteworthy reaction since the Mo2X4(py)4 compounds are themselves best prepared347 from the dimolybdenum(III) species Cs3Mo2X8H. Another group of cleavage reactions that involve μ-donor ligands include the formation of trans-MoBr2(dppe)2, as one of the products of the reaction between (NH4)4Mo2Br8 and Ph2PCH2CH2PPh2,377 and trans-MoX2(dppee)2 (X = Cl or Br; dppee = cis-Ph2PCH=CHPh2), which are formed in small quantities when K4Mo2Cl8 and (NH4)4Mo2Br8 are reacted with dppee in refluxing n-propanol for several days.380 The compounds trans-MoX2(dppbe)2 (X = Cl or Br; dppbe = 1,2-bis(diphenyl- phosphino)benzene) can be obtained in quite good yield by a similar procedure, together with some [MoOX(dppbe)2]X·nH2O.385

Like other multiply bonded dimetal complexes, those of quadruply bonded Mo24+ are in many instances cleaved by /-acceptor ligands such as CO, NO, and isocyanides.490 Note that there are also examples where /-acceptor ligands give products in which a dimolybdenum unit is retained, such as the conversion of Mo2(O2CCH3)4 to the alkyne complex [Mo2(µ-4- MeC6H4CCH)(µ-O2CCH3)(en)4](O2CCH3)3·2en.194 The reactions of Mo2Cl4(PR3)4 (PR3 = PEt3 or PBut4) with CO in toluene give mononuclear Mo(CO)3(PR3)2Cl2 and trans-Mo(CO)4(PR3)4 as the only identifiable products. In a similar fashion, a variety of phosphine complexes of the type Mo2X4(PR3)4, where X = Cl or Br and PR3 = PEt3, PBun3 or PEtPh2, and Mo2X4(LL)2, where X = Cl or NCS and LL = dppe or dppm, react with NO in dichloromethane to yield the mononuclear complexes Mo(NO)2X2L2 and Mo(NO)2X2(LL).491 These reactions constitute a useful general synthetic method for obtaining dinitrosyls of molybdenum. On the other hand, the cleavage of Mo2(CH3)4(PMe3)4 by NO gives a yellow complex of stoichiometry Mo2O(NO)2 (ONCH3)2(Me3PO)2.492 In a related study, it was found that the only identifiable products from the reactions of nitrosyl chloride with K4Mo2Cl8 and Mo2(O2CCH3)4 were those in which fission of the Mo–Mo bond had occurred. After work-up of the reaction mixtures, K2Mo(NO)Cl5 and Mo(NO)Cl3(Ph3PO)2 (upon the addition of triphenylphosphine oxide) were isolated.493

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A suspension of Mo2(O2CCH3)4 in methanol reacts quickly with phenyl isocyanide494a and other aryl isocyanides494b to yield Mo(CNAr)6. This reduction to Mo0 is in contrast to the related reactions of Mo2(O2CR)4 (R = CH3 or CF3) and K4Mo2Cl8 with alkyl isocyanides,495,496 where the Mo–Mo bond is cleaved but the products that result, the [Mo(CNR)7]2+ ions, where R = Me, CMe3 or C6H11, are derivatives of MoII. This difference in reaction course is in accord with previously documented differences between the stabilities of homoleptic aryl and alkyl isocyanide complexes of molybdenum, viz. Mo(CNAr)6 versus [Mo(CNR)7]2+. When the phosphine-containing complexes Mo2Cl4(dppm)2, Mo2Cl4(dppe)2, and Mo2Cl4(PR3)4 (PR3 = PEt3, PPrn3 or PEtPh2) are used in place of Mo2(O2CCH3)4, seven-coordinate mixed phos- phine-alkyl isocyanide complexes are formed. The [MoCNR)5(dppm)]2+, [Mo(CNR)5(dppe)]2+, [Mo(CNR)5(PR3)2]2+ and [Mo(CNR)6(PR3)]2+ cations have been isolated as their PF6− salts.497 A detailed study of the reactions of Mo2X4(dppm)2 (X = Cl, Br or I) with RNC (R = Pri or But) has shown438 that with one equivalent of RNC in the presence of TlPF6 (in THF) or KPF6 (in acetone), the dimolybdenum(II) complexes [Mo2X3(dppm)2(CNR)]PF6 are formed. When an excess of RNC is used, cleavage of the Mo–Mo bond occurs to give [MoX(CNR)4(dppm)]+, which is in turn converted into [Mo(CNR)5(dppm)]2+ and finally [Mo(CNR)7]2+.438

4.4.2 Redox behavior of Mo24+ compounds

The Mo24+ core has a μ2/4β2 electron configuration. The β electrons are not strongly bound, and the LUMO, β*, is relatively low in energy. The possibilities of oneand two-electron oxidations and reductions under normally accessible chemical conditions therefore suggest themselves. Obviously, the nature of the ligands surrounding the Mo24+ core will strongly affect these possibilities. The electrochemical behavior of Mo2X4L4 and Mo2X4(LL)2 compounds has already been discussed in Section 4.3.4.

There are two main ways to study the redox behavior. One is by electrochemistry (usually the cyclic voltammetry (CV) or differential pulse voltammetry (DPV) methods are used), and the other is by employing chemical oxidants or reductants to produce isolable amounts of the desired products. Commonly, the electrochemistry provides a basis for choosing the most suitable redox reagent, with FcPF6, AgPF6 being the most often used oxidants. Some observed electrochemical oxidation data are present in Table 4.13.

No simple [Mo2X8]3− ion has been isolated. A solution of K4Mo2Cl8 in 6 M HCl shows an oxidation at about 500 mV vs SCE, but the oxidation product, presumably [Mo2Cl8]3−, appears to be very short lived.326

There is only one instance in which chemical reduction has led to an isolable product containing an Mo23+ core.506 This is shown in the following reaction:

1,3,6,8-Mo2(CCSiMe3)4(PMe3)4 K(C10H8)

crypt-222

[K(crypt-222)][Mo2(CCSiMe3)4(PMe3)4]

The necessity of a very strong reductant is in accord with the observation by CV in THF that the reduction potential lies 2.13 V negative from the Fc/Fc+ potential. This and other studies of Mo2(C>CR)4(PMe3)4 compounds394,395,507,508 (and their W analogs) have shown that there is major interaction of the / and/or /* orbitals of the acetylide ligands with the β and/or β* orbitals of the dimetal units.

It has also been reported that pulse radiolysis of a methanol solution of Mo2(O2CCF3)4 gave rise to a new electronic absorption band at 780 nm.509 This band, which decayed rapidly, was assigned to the [Mo2(O2CCF3)4]− ion.

138Multiple Bonds Between Metal Atoms Chapter 4

Table 4.13. Some electrode potentials for Mo24+/Mo25+ processes in paddlewheel compoundsa

Compound		E1/2 (mV)	ref.
Mo2(O2CC3H7)4		450	326
Mo2(O2CC3H7)4	in EtOH	300
Mo2(O2CC3H7)4	in CH3CN	390
[Mo2(O2C(2,4,6-Pri3C6H2)]4		621	498
[Mo2(O2C(2,4,6-Pri3C6H2)]4	in EtOH	488
[Mo2(O2C(2,4,6-Pri3C6H2)]4	in CH3CN	462
Mo2[(O2C(3,5-C6H3(OH)2]4	in C6H5CN	530	90
[Mo2(SO4)4]4−/[Mo2(SO4)4]3−	in 9 M H2SO4	220 (vs SCE)	195
Mo2(DArF)4	Ar = p-MeOC6H4	142	499,500,501
	Ar = p-MeC6H4	231
	Ar = C6H5	316
	Ar = m-MeO	356
	Ar = p-ClC6H4	499
	Ar = m-ClC6H4	581
	Ar = m-CF3	660
	Ar = p-MeC(O)C6H4	676
	Ar = p-CF3C6H4	693
Mo2(DAniF)3(uracilate)		172	502
Mo2(DAniF)3(O2CC>CH)		351	503
Mo2(DAniF)3(O2CCH=CH2)		217	503
Mo2(DAniF)3(O2CCH=CH–CH=CH2)		225	503
Mo2(hpp)4	in Bu4NBF4·3toluene	−1271	504
Mo2[(PhN)2CN(H)Ph]4		−50	505

aIn CH2Cl2 solutions vs Ag/AgCl with Bu4NBF4 supporting electrolyte, where Fc/Fc+ has a value of 440 mV, unless otherwise stated.

The indirect synthesis of a compound510 that could reasonably be assigned a Mo22+ core occurred when the [Mo2Cl8]4− ion was reacted with F2PN(CH3)PF2 to produce Mo2[(F2PN(CH3)PF2]4Cl2, which has the structure shown in Fig. 4.22. The rotational conformation is twisted 21° and the Mo–Mo distance is 2.457(1) Å. Oxidation of Mo24+ compounds to isolable Mo25+ and Mo26+ species has often been observed. All of these isolated oxidation products have been obtained with paddlewheel ligands present. The first observations326 were made electrochemically on Mo2(O2CPrn)4. This was shown to undergo “quasireversible” oxidation in CH3CN, CH2Cl2 and EtOH to [Mo2(O2CPri)4]+ which had a half-life of c. 10−2 s at ambient temperature. EPR spectroscopy at 77 K (gav = 1.941) showed the presence of one unpaired electron delocalized over two molybdenum atoms.

The cyclic voltammogram of Mo2(O2CCH3)4 in methanol is similar to that of the butyrate, with E1/2 = +0.24 V versus Ag/AgCl,18 while measurements on solutions of Mo2(O2CCMe3)4 in

acetonitrile (0.1 M in Bu4NBF4) and THF (0.2 M in Bu4NPF6) have given E1/2 values of +0.38 V versus SCE511 and +0.86 V versus Ag wire,19 respectively (note the difference in referencing

procedures). In the case of DMF solutions of the ferrocenyl species Mo2(O2CCH3)2(FCA)2(py)2 and Mo2(FCA)4(CH3CN)(DMSO), where FCAH = ferrocenemonocarboxylic acid, a reversible oxidation occurred near the potential of the ferrocene–ferrocenium couple but further oxidation led to the destruction of the complexes.16 Cyclic voltammetric measurements on DMF solutions of the 2-acetoxybenzoate complex showed that oxidation of the monocation was followed by a rapid and irreversible decomposition of the complex.18