Multiple Bonds Between Metal Atoms / 04-Molybdenum Compounds

Molybdenum Compounds 89

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ternative synthetic route to the latter compound, the THF complex Mo2(O2CCH3)2Cl2(THF)2 has been reacted with dppm.123 The complex Mo2(O2CCH3)2Cl2(µ-dppm)2 is unstable and decomposes to Mo2Cl4(µ-dppm)2 in both solution and the solid state.123,190 Compounds of the type Mo2(O2CCH3)2X2(µ-LL) (X = Cl or Br) that are much more stable than the dppm complex have been isolated in the case of LL = Ph2PCH2CH2PPh2, cis-Ph2PCH=CHPPh2 and 1,2-C6H4(PPh2)2,190 and, on the basis of their spectroscopic and electrochemical properties, they have all been assigned the type of structure shown in 4.8. While none of these bidentate phosphines react to form the 1:2 complexes of the type Mo2(O2CCH3)2X2(µ-LL)2, the 2-(diphenyl- phosphino)pyridine ligand forms such a compound.126 Red crystalline trans-Mo2(O2CCH3)2Cl2- (µ-Ph2Ppy)2 is produced, along with Mo2(CCH3)4, upon heating Mo2Cl4(Ph2Ppy)2 with acetic acid. However, it is not isolated when Mo2(O2CCH3)4 is treated with Me3SiCl and Ph2Ppy since this reaction gives insoluble green Mo2Cl4(Ph2Ppy)2. The crystal structure of trans- Mo2(O2CCH3)2Cl2(µ-Ph2Ppy)2 shows126 that this compound is centrosymmetric (4.9), although the Mo–Mo–Cl units are bent (c. 163°).

CH3

CH3

C

O

O

O

C

CH3

N

P

O

C

Cl

Cl

O

Mo

Mo

O

P

N

Mo

Mo

P

O

O

X

C

X

P

CH3

4.8

4.9

With the ligand Ph2PN(H)PPh2 (dppa) stable compounds such as that shown in Fig. 4.7 can be obtained168 by the stoichiometric reactions:

Mo2(O2CCH3)4 + 2Me3SiX + 2dppa Α trans-Mo2(O2CCH3)2(dppa)2(ax-X)2

Fig. 4.7. The structure of Mo2(O2CCH3)2(dppa)2Br2.

The pyphos ligand, (2-Ph2P)(6-O)py, has been exploited to allow additional metal ions (Pd2+, Pt2+) to be held in the axial positions of Mo24+ units. For example the type of molecule (which dimerizes) shown in 4.10 has been made151 with R = CH3 or CMe3 and X = Cl, Br or I.

When Mo2(O2CCH3)4 is reacted with a mixture of LiCl (3 equiv) and PMe3 in THF the mono(acetate) complex Mo2(O2CCH3)Cl3(PMe3)3 can be isolated in almost quantitative yield.127

90Multiple Bonds Between Metal Atoms Chapter 4

Its X-ray crystal structure has been determined and shows a geometry (4.11) that minimizes repulsions between the PMe3 ligands. This complex slowly converts to Mo2Cl4(PMe3)4 and Mo2(O2CCH3)4 when dissolved in THF.127

Ph2P

N

O

CH3

R

C

O

O

X

O

O

M

Cl

Mo

Mo

PMe3

Ph2P

Mo

Mo

N

O

O

O

Cl

Me3P

R	PMe3	Cl
X
4.10	4.11

Of the remaining examples of reactions in which two of the carboxylate groups of Mo2(O2CR)4 are displaced, most involve the binding of monoanionic bridging ligands. Exceptions to this include the reactions of Mo2(O2CCH3)4 with sodium acetylacetonate and lithium (4-phenylimino)-2-pentanonide in THF which lead to the formation of Mo2(O2CCH3)2(acac)2116 and Mo2(O2CCH3)2[PhNC(CH3)CHC(CH3)O]2.128 These compounds have a cis disposition of acetates and chelating acac− and [PhNC(CH3)CHC(CH3)O]− ligands. A more complicated system involves the reactions between Mo2(O2CCH3)4 and pyrazolylborate ligands.129 With sodium diethyldipyrazolylborate, the reaction stoichiometry was adjusted to afford either red Mo2(O2CCH3)2[(pz)2BEt2]2 or blue Mo2[(pz)2BEt2]4 (whose structure could not be determined) using 1,2-dimethoxyethane and toluene, respectively, as the reaction solvents. The structure of the mixed ligand complex (recrystallized from carbon disulfide) is similar to those of cis- Mo2(O2CCH3)2(acac)2 and cis-Mo2(O2CCH3)2[PhNC(CH3)CHC(CH3)O]2. A related complex, Mo2(O2CCH3)2[(pz)3BH]2, has been obtained using KHB(pz)3 in place of NaEt2B(pz)2 and it too possesses a cis arrangement of acetate groups (Fig. 4.8).129 This complex was prepared in order to ascertain whether the availability of three nitrogen donor atoms in each ligand (compared to two in Et2B(pz)2−) would force the formation of two axial Mo–N bonds. In fact only one such axial bond is formed (at 2.45 Å) but it is far longer than the normal equatorial Mo–N bond lengths.129 It appears that while the steric and conformational demands of one HB(pz)3 ligand permit the approach of a pyrazolyl nitrogen atom at one of the axial sites, a similar conformation for two of these ligands within the dinuclear complex is not possible.

A complex with pairs of cis acetates and bridging monoanionic ligands is Mo2(O2CCH3)2- (µ-pdc)2(OPPh3), which is prepared by reacting Mo2(O2CCH3)4 with the potassium salt of the dithiocarbamate of pyrrole (Kpdc) and Ph3PO.114 Trans isomers of this general type are encountered in the case of the THF adduct of the bis(xylyl)acetamidinato complex trans-Mo2(O2CCH3)2{[(2,6- xylyl)N]2CCH3}2,130 the o-(dimethylamino)benzyl ligand complex trans-Mo2(O2CCH3)2- [o-(Me2N)C6H4CH2]2,131 the DMF adduct of trans-Mo2(O2CCH3)2(7-azaindolyl)2132 and trans-Mo2(O2CC6H5)2[(Me3SiN)2CPh]2.132 These compounds have all been prepared directly from Mo2(O2CCH3)4. Structural information is contained in Table 4.2. Interest in molybde- num-based catalysts has led to an investigation of the reaction between Mo2(O2CCH3)4 and aluminum isopropoxide. In decalin this reaction was found to yield the orange complex Mo2Al2(O2CCH3)2(OPri7)8 which may be purified by sublimation.191 Its structure consists of an eclipsed Mo2O8 skeleton, with a Mo–Mo distance of 2.079(1) Å, containing two acetate bridges in a trans disposition and two [Al(OPri)4] bridges.133,191 The interest in this molecule lies in its reaction with oxygen and the potential this offers of oxidizing ligand groups.

Molybdenum Compounds 91

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The bis-carboxylate complexes Mo2(O2CCH3)2(mhp)2 (mph is the anion of 2-methyl-6-hy- droxypyridine)192 and Cs2[Mo2(O2CH)2(SO4)2]·2H2O111,112 have been described; full details of their structures have not been established although the standard enthalpy of formation of Mo2(O2CCH3)2(mhp)2 has been determined.192

Fig. 4.8. The structure of Mo2(O2CCH3)2[HB(pz)3]2.

When solutions of the amino acid complexes [Mo2(O2CCH2NH3)4]4+, [Mo2(L-isoleu)4]4+, and [Mo2(D-val)2(L-val)2]4+, which are generated by dissolving K4Mo2Cl8 in acidified aqueous solutions of the appropriate amino acid, are mixed with KNCS, the red crystalline species Mo2(amino acid)2(NCS)4·nH2O are formed.85,171 Each of these complexes has been structurally characterized (Table 4.2), and each possesses a cisoid arrangement of the amino acid ligands and four N-bonded NCS− groups.

While Mo2(O2CCF3)4 reacts with pyridine to form the fairly stable 1:2 adduct,29 reactions of this carboxylate with 2,2'-bipyridyl (bpy) are quite complicated. With 1:2 mole proportions of reagents (Mo2(O2CCF3)4:bpy) four different ‘adducts’ (two 1:1 and two 1:2) have been isolated55,193 depending upon the choice of solvent. It was suggested (mainly on the basis of infrared spectral data) that these complexes possess structures in which the bpy ligands are chelating and the carboxylate ligands are present in one or more of the following modes – bidentate bridging, bidentate chelating, monodentate, and outer-sphere.193 The crystal structure of a 1:2 adduct has shown it to be the ion-pair [Mo2(µ-O2CCF3)2(bpy)2](O2CCF3)2.55 This compound undergoes thermal and photochemical conversion to Mo2(δ1-O2CCF3)4(bpy)2.55 When Mo2(O2CCF3)4 is reacted with bpy in dichloromethane in the presence of (Et3O)BF4 the complex [Mo2(O2CCF3)2(bpy)2](BF4)2·Et2O is produced.193 In the absence of (Et3O)BF4, the 1:1 adduct [Mo2(O2CCF3)3(bpy)]+[O2CCF3]− is the principal product.193 The reaction of Mo2(O2CCH3)4 with neat ethylenediamine (en)134(a) gives [Mo2(O2CCH3)2(en)4](O2CCH3)2·en, which contains two cis bridging acetate ligands, two en bridges, and two monodentate terminally bound en ligands, which are disordered. This complex converts back to Mo2(O2CCH3)4 when heated at 120 °C in the solid state.134(a) When Mo2(O2CCH3)4 in en is reacted with K2Te4, the [Mo4Te16(en)4]2− ion is formed.134(b) This tetranuclear molybdenum cluster actually contains pairs of confacial bioctahedral Mo26+ units with an Mo–Mo bond distance of 2.469(3) Å.134(b)

A few examples are known of dimolybdenum(II) complexes in which there is a single carboxylate bridge present. The compound Cs3[Mo2(O2CH)(SO4)3]·2H2O has been described111,112 but not yet fully characterized. Two complexes that contain one acetate and three other ligand bridges are Mo2(O2CCH3)(ambt)3·2THF (THF is present as lattice solvent and ambt is the anion of 2-amino-4-methylbenzothiazole)135 and Mo2(O2CCH3)[(PhN)2CCH3]3.129 In both cases the anionic ligand is generated by treatment of the protonated form with BunLi in THF/hexane, and then reacted with Mo2(O2CCH3)4. A more complicated molecule is the

92Multiple Bonds Between Metal Atoms Chapter 4

ion-pair complex (C3N2H5)+{Mo2(O2CCH3)[CH3Ga(C3N2H3)O]4}−, which was obtained as its bis-THF solvate upon reacting Mo2(O2CCH3)4 with Na[CH3Ga(C3N2H3)3] (i.e. the tridentate anionic ligand methyl(tris-pyrazolyl)gallate) in THF.136 During the course of this reaction the [CH3Ga(C3N2H3)3]− anion hydrolyzed to give the hexadentate ligand that was identified by a crystal structure determination.136 The ion-pair is present in the gas phase as shown by the mass spectrum of this complex.136 The first example of an alkyne addition to a metal–metal quadruple bond has been encountered in the reaction between Mo2(O2CCH3)4 and 4-MeC6H4C>CH in ethylenediamine.194 Two alkyne containing isomers are formed in an approximate 1:1 ratio, and the X-ray crystal structure of one isomer revealed the structure to be that of the salt [Mo2(µ-4- MeC6H4CCH)(µ-O2CCH3)(en)4](O2CCH3)3·2en, in which the trication contains a perpendicular alkyne bridge and a Mo–Mo distance of 2.489(3) Å. The latter is consistent with a double bond.194

4.1.4 Paddlewheels with other O,O anion bridges

Relatively few are known, mainly those with µ-SO42−, µ-HPO42− and µ-HAsO42−. The known structures as well as structures of some thio analogs are presented in Table 4.3.

Table 4.3. Dimolybdenum compounds with polyoxoanion bridges

Compound

Crystal

Virtual

r(Mo–Mo) Å

Twist angle

ref.

sym.

sym.

K4

[

Mo2(SO4)4

]

·2H2O

¯

C4h

2.110(3)

0

195

1

K3[Mo2(SO4)4]·3.5H2O

1

C4h

2.164(3)

~0

196

Cs2[Mo2(HPO4)4(H2O)2]

1

C4h

2.223(2)

~0

197

(pyH)3

[

Mo2(HPO4)4Cl2/2

]

¯

C4h

2.232(1)

0

197

1

(pyH)2

[

Mo2(HAsO4)4(H2O)2

]

¯

C4h

2.265(1)

0

198

1

Mo2

[

O2P(OPh)2

]

4·2H2O

¯

C4h

2.141(2)

0

199

1

Mo2(OSPEt2)4(THF)

¯

D4h

2.128(2)

0

200

4

Mo2(S2PEt2)4(THF)

¯

C4h

2.123(1)

0

200

1

Mo2(µ-S2PEt2)2(

-S2PEt2)2

1

C2v

2.137(1)

−

200

The compound K4[Mo2(SO4)4] was first reported201 in 1971 and the crystalline dihydrate was characterized crystallographically202 soon after. Its structure is shown in Fig. 4.9. There are several good synthetic routes.195,201-204 The [Mo2(SO4)4]4− ion is red, diamagnetic, and has an absorption band at c. 520 nm which is presumed to correspond to the β Α β* transition. It is easily oxidized to the [Mo2(SO4)4]3− ion,196,205,206 which forms the crystalline compound K3[Mo2(SO4)4]·3.5H2O.

The structure of K3Mo2(SO4)4·3.5H2O resembles that of K4Mo2(SO4)4·2H2O except for the presence of axially bound water molecules (Mo–O distance of 2.550(4) Å) in place of sulfate oxygen.195,196 The Mo–Mo distance is longer in the 3− ion (2.167(1) versus 2.111(1) Å) in accord with the loss of half of the β-bond upon oxidation from μ2/4β2 to μ2/4β1. The magnetic and EPR spectrum195,207 of this complex are in accord with the ground state configuration being

μ2/4β1.

The [Mo2(SO4)4]3− anion has also been obtained in compounds with the formula K4[Mo2(SO4)4]X·4H2O (X = Cl or Br) by the hydrogen peroxide oxidation of a solution of K4Mo2Cl8 in 2 M H2SO4 and 0.3 M HCl, or 0.5 M HBr, to which is added KX.208,209 These molecules are structurally similar to K3[Mo2(SO4)4]·3.5H2O (the Mo–Mo distances are the same) but possess Mo···X axial interactions in place of Mo···OH2. The presence of these continu-

Molybdenum Compounds 93

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ous, linear ···Mo–Mo···X···Mo–Mo···X··· chains confers properties that are advantageous in the study of the electronic structure and spectroscopic properties of the [Mo2(SO4)4]3− anion.209

Fig. 4.9. The structure of the [Mo2(SO4)4]4− ion in K4[Mo2(SO4)4]·2H2O. The linking of these ions to one another is also shown. The water molecules are not coordinated to molybdenum atoms.

Solutions of K3[Mo2(SO4)4]·3.5H2O in 2 M H2SO4 are blue and have spectroscopic properties (e.g., ηmax at 412 nm) that are in accord207 with the preservation of the [Mo2(SO4)4]3− ion or a structurally related, partly aquated sulfate complex. Solutions in other strong acids (hydrochloric or p-toluenesulfonic acid) turn a deep red color as disproportionation to Mo24+ and Mo26+ species occurs.207,210 This disproportionation reaction can be reversed upon the addition of K2SO4, the blue complex K3[Mo2(SO4)4]·3.5H2O being regenerated.207 The Mo26+ species cannot be isolated.

An interesting derivative of [Mo2(SO4)4]4− has been prepared in virtually quantitative yield by the reaction of Mo2(O2CCH3)4 with concentrated H2SO4 in pyridine.211 This molecule is of composition Mo2(SO4)2(py)8, and has been shown by X-ray crystallography to be centrosymmetric with a trans arrangement of bridging sulfate groups and three kinds of pyridine molecule. Four pyridines are bound in equatorial sites, two in axial sites, and two more are present in interstitial positions.211 The Mo–Mo bond length is 2.134(2) Å.

Displacement of the acetate ligands of Mo2(O2CCH3)4, by methylsulfonate and trifluoromethylsulfonate can be accomplished212,213 to produce the analogous ligand-bridged dimolybdenum(II) complexes Mo2(O3SCH3)4 and Mo2(O3SCF3)4. Temperatures of close to 100 °C were required for the reaction between these sulfonic acids and Mo2(O2CCH3)4, the reaction with CH3SO3H having been carried out in diglyme. While purification of Mo2(O3SCF3)4 can be accomplished by sublimation to afford air-sensitive crystals, it has in fact proven difficult to remove the last traces of acetate impurity.213 In an attempt to circumvent this problem an alternative synthetic procedure was investigated, namely, the reaction of Mo2(O2CH)4 with CF3SO3H and (CF3SO2)2O.178 However, this gives the hydrate [Mo2(O3SCF3)2(H2O)4](CF3SO3)2 which cannot be dehydrated, although its reaction with acetonitrile affords [Mo2(NCCH3)8](CF3SO3)4 (see Section 4.3.5). An ethanol solution of Mo2(O3SCF3)4 when treated with formic acid yields the formate complex Mo2(O2CH)4.213 The methylsulfonate complex Mo2(O3SCH3)4 has been converted to the 1:2 adducts Mo2(O3SCH3)4L2 (L = α-butyrolactone or dimethylformamide), to the mixed methylsulfonate-halide complexes (Me4N)2[Mo2(O3SCH3)2Cl4] and (Bu4N)2[Mo2(O3SCH3)2X4] (X = Br or I) upon reaction with the appropriate substituted ammonium halide, and to the octakis(isothiocyanato)dimolybdate(II) anion upon stirring with a dimethoxyethane solution of NH4NCS.212 The reaction of ‘Mo2(O3SCF3)4’ (or more probably [Mo2(O3SCF3)2(H2O)4](CF3SO3)2) with 1,5,9,13-tetrathiacyclohexadecane yields several products,214 in none of which is there a Mo–Mo multiple bond.

94Multiple Bonds Between Metal Atoms Chapter 4

Phosphate-, arsenate-, diarylphosphate-, phosphinate-, and phosphonate-bridged complexes of Mo24+, Mo25+ and Mo26+.

While further oxidation of [Mo(SO4)4]3− to give an isolable species with a triple bond has not been observed, the formation of the triply-bonded dimolybdenum(III) species [Mo2(HPO4)4]2− takes place very easily. Simply by dissolving K4Mo2Cl8·2H2O in aqueous 2 M H3PO4 and allowing the solution to stand in an open beaker for 24 h, with larger cations such as Cs+ or pyridinium also present, purple crystalline materials containing this triply-bonded species are formed.197 The structures of both Cs2[Mo2(HPO4)4(H2O)2], which has axial water molecules, and (pyH)3[Mo2(HPO4)4]Cl, in which there are infinite chains with shared Cl− ions occupying axial positions, have been determined. While the hydrogen atoms of the HPO42− ligands were not observed, it is easy to tell where they are from the outer P–O distances. One on each ligand is about 1.48 Å (P=O) and the other is about 1.54 Å (P–OH). The O=P–OH moieties are so arranged that the overall symmetry of the [Mo2(HPO4)4]2− ion is C4h; however, the inner Mo2O8 portion of the ion has effective D4h symmetry and the bonding can be simply understood as a μ2/4 configuration. The bromide salt (pyH)3[Mo2(HPO4)4]Br has been prepared starting from Mo2(O2CCH3)4, and is isostructural with its chloride analog.198 Both the chloride and bromide complexes show Raman-active ι(Mo–Mo) modes at c. 360 cm−1 and have very similar electronic absorption spectra.198,215

While the above complexes197 were the first dimolybdenum phosphates to be isolated, Bino showed210 soon thereafter that light-purple colored solutions of [Mo2(HPO4)4]2- in 2 M H3PO4 could be reduced by zinc amalgam under nitrogen first to pale-blue/gray Mo25+ and then to deep-red Mo24+ phosphate species. Later, solutions of the dimolybdenum(II) complex [Mo2(HPO4)4]4− were generated by the reactions of K4Mo2Cl8, K4Mo2(SO4)4 or [Mo2(aq)]4+ with H3PO4 under anaerobic conditions.216 The one-electron oxidation of this species was carried out to afford the paramagnetic salt K3Mo2(HPO4)4. While neither of the species [Mo2(HPO4)4]4− or [Mo2(HPO4)4]3− has been structurally characterized by X-ray crystallography, the close structural relationship between them is shown by the reversibility of their electrochemical properties. Cyclic voltammetric measurements on 2 M H3PO4 solutions of [Mo2(HPO4)4]4− (with use of a glassy carbon electrode) show redox processes at −0.67 and −0.25 V versus SCE that have been attributed to the (3−/4−) and (2−/3−) couples, respectively.216 Whereas [Mo2(HPO4)4]4− reacts thermally in 2M H3PO4 to produce [Mo2(HPO4)4]3− and H2 over a period of several days, UV irradiation (η 335 nm) leads to the facile production of [Mo2(HPO4)4]2− and H2, by oneelectron steps via the high energy / Α /* excited state.216,217 The thermal reaction is believed to involve the slow conversion of [Mo2(HPO4)4]4− to [Mo2(HPO4)4]2− which then reacts in an ensuing comproportionation reaction with [Mo2(HPO4)4]4− to give [Mo2(HPO4)4]3−.

In addition to the extensive photochemical studies that have been carried out on these phosphate complexes,216,217 detailed measurements have been made on the electronic absorption spectra of the 4−, 3−, and 2− anions,215,216 and the magnetic properties and EPR spectrum of K3[Mo2(HPO4)4] which possesses the μ2/4β1 configuration, have been examined down to 5 K.216

Several dimolybdenum(III) arsenate analogs of these phosphato complexes have been prepared, namely, Cs2[Mo2(HAsO4)4]·3H2O, (pyH)2[Mo2(HAsO4)4]·2H2O, and (pyH)3[Mo2(HAsO4)4]X (X = Cl or Br). In the case of (pyH)2[Mo2(HAsO4)4]·2H2O its identity was confirmed by X- ray crystallography.198 The structure of the [Mo2(HAsO4)4]2− anion is closely akin to that of its phosphate analog, with a Mo–Mo triple bond distance of 2.265(1) Å. The Mo–Mo stretching frequencies of these arsenate complexes (c. 330 cm−1) are a little lower than those in the Raman spectra of their phosphate analogs (c. 360 cm−1).198,215

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The dimolybdenum(II) diphenylphosphato complex Mo2[O2P(OPh)2]4 is formed upon addition of excess (PhO)2PO2H to Mo2(O3SCF3)4 in methanol.199 While complete displacement of the triflate ligands occurs in this reaction, the use of Mo2(O2CCH3)4 in place of Mo2(O2SCF3)4 gives a product in which only partial replacement of acetate ligands has occurred. The tetrakisdiphenylphosphate complex has also been prepared from (NH4)5Mo2Cl9·H2O.218 The crystal structure of the THF adduct Mo2[O2P(OPh)2]4·2THF has been determined; the two THF molecules are bound weakly in axial positions (Mo–O = 2.656(9) Å).199 This complex readily undergoes a one-electron oxidation as shown by cyclic voltammetry and chemical oxidation with [(δ5-C5H5)2Fe]PF6. The resulting product, {Mo2[O2P(OPh)2]4)}PF6, is paramagnetic with magnetic susceptibility and EPR spectral properties in accord with the presence of one unpaired electron.199

Measurements of the electronic absorption spectra of Mo2[O2P(OPh)2]4 and its one-electron oxidized congener show that the β Α β* transition shifts from c. 20,000 cm−1 to c. 6,500 cm−1.199 In the case of the Mo24+ complex, the chemistry of the 1(ββ*) excited state has been examined.217,218 In the solid state and solution, this complex exhibits weak luminescence upon excitation into the β2 Α ββ* absorption band. Solutions of Mo2[O2P(OPh)2]4 in 1,2-dichloroethane undergo the following photoreaction when excited with visible light (η 530 nm):218

2Mo2[O2P(OPh)2]4 + ClCH2CH2Cl hv ( 530 nm) 2Mo2[O2P(OPh)2]4Cl + CH2CH2

4.2Paddlewheel Compounds with O,N, N,N and Other Bridging Ligands

4.2.1 Compounds with anionic O,N bridging ligands

These compounds fall into two main classes: (1) those with 2-oxopyridine type ligands (4.12), and those with noncyclic amidate ligands (4.13). We include here also several thio analogs. Structural data are collected in Table 4.4.

						R'
			R
						C
X	N		O 4	N				O 4
	Mo		Mo	Mo				Mo
	4.12					4.13

There are two main methods of preparation for the 2-oxopyridine compounds, namely, the reaction of the free ligand or its monoanion with Mo2(O2CCH3)4 or Mo(CO)6.220,222-236,238-240. In general, these compounds do not have axial ligands; the structure of Mo2(mhp)4 is shown in Fig. 4.10 where the ligand arrangement gives D2d symmetry to the central Mo2N4O4 core. The structures of the chp and dmhp molecules are similar. In contrast, the fhp ligand gives a structurally different product, Mo2(fhp)4THF, in which all fhp ligands point in the same direction.223 This completely blocks one axial position but leaves the other one free to accommodate the axial THF molecule. With mhp and chp it is impossible to have all four substituents (Me or Cl) at the same end. Since four F atoms can fit at one end, they do so and this allows one more bond, to axial THF at the other end, to be formed.

96Multiple Bonds Between Metal Atoms Chapter 4

Fig. 4.10. The Mo2(mhp)4 molecule as found in Mo2(mhp)4·CH2Cl2. Note the D2d symmetry of the Mo2N4O4 core.

Table 4.4. Structures of Mo24+ compounds with anionic O,N bridging ligands

			Compound								Crystal	Virtual	r(Mo–Mo)	Twist	ref.
											sym.	sym.	(Å)	Angle (°)
Mo2(mhp)4											1	D2d	2.067(1)	50	219
Mo2(mhp)4·CH2Cl2											1	D2d	2.065(1)	1.3	220
Mo2(mhp)4·CH3OH											1	D2d	2.068(1)	50	219
cis-Mo2(mhp)2Cl2(PEt3)2											1	C2	2.103(1)	50	221
Mo2(chp)4											1	D2d	2.085(1)	3.1	222
Mo2(fhp)4·THF											1	C4v	2.092(1)	50	223
Mo2(dmhp)4·diglyme											1	D2d	2.072(1)	0.3	224
[Mo2(mhp)3(CH3CN)2](BF4)·2CH3CN											1	Cs	2.103(1)	NR	169
Mo2(pyphos)4·CH2Cl2											1	D2d	2.098(2)	50	225
Mo2(pyphos)4·2CH2Cl2											¯	C2h	2.103(1)	zero	226
											1
Mo2(pyphos)4Pd2(TCNE)2											2	D2d	2.097(2)	NR	227
Mo2(pyphos)4Pd2Cl2(CH2Cl2)											2	D2d	2.106(2)	NR	227
Mo2(pyphos)4Pd2Cl4											2	D2d	2.096(3)	NR	225
Mo2(pyphos)4Pd2Br4											2	D2d	2.095(4)	NR	225
Mo2(pyphos)4Pt2Cl4											2	D2d	2.101(2)	NR	225
Mo2(2-O-7-Me-naphthyridine)4											¯	D2d	2.090(4)	1	228
											1
Mo2(2-S-7-Me-naphthyridine)4											2	D2d	2.131(2)	8	228
Mo2	[	t	NC(CH3)O		]						¯	C2h	2.063(1)	0	229
	[	Bu					4				1
Mo2		PhNC(CH3)O		]		4·2THF					¯	C2h	2.086(2)	0	230
	[										1
Mo2		PhNC(CMe3)O					]				¯	C2h	2.070(1)	0	231
								4			1
Mo2[(2,6-xylyl)NC(CH3)O]4·2CH2Cl2											1	C2v	2.083(2)	50	232
Mo2[(2,6-xylyl)NC(CH3)O]4·2CH2Br2											1	C2v	2.086(2)	50	233
Mo2[(2,6-xylyl)NC(H)O]4·2THF											2	D2d	2.113(1)	50	231
Mo2[(2,6-xylyl)NC(CH3)O)]4·2THF											1	C2v	2.097(3)	50	234
											2	C2v	2.093(2)	50	234
Mo2[(2,6-xylyl)NC(CH3)O]4·py·C6H6											1	C2v	2.101(1)	50	234
Mo2[(2,6-xylyl)NC(CH3)O]4·4-pic											2	C2v	2.102(1)	50	234
Mo2(2-mq)4											1	D2d	2.089(1)	50	235
Mo2(dmmp)4·CH2Cl2											1	D2d	2.083(1)	50	236
Mo2[MeNC(PPh2)S]4									]		1	D2d	2.083(1)	50	237
Mo2	[	MeNC(PPh2)S			] [						¯	Ci	2.104(2)	0	237
							2	MeNC(S)PPh2		2	1

Molybdenum Compounds 97

Cotton

The pyphos ligand, 4.14, is a special case because the substituent at the 6-position, Ph2P, is also a potential electron donor. The structure of Mo2(pyphos)4 is shown in Fig. 4.11, where the presence of two Ph2P “claws” at each end can be seen. In a series of papers225-227,241 K. Mashima and A. Nakamura have shown how these “claws” can be used to capture Pd2+, Pd1+ and Pd0 atoms and also Pt2+ ion, and they have explored the chemistry of the various tetranuclear species. The presence of the captured metal atoms has very little effect on the Mo–Mo bond lengths although small changes occur in the ι(Mo–Mo) frequencies in the Raman spectra.

Ph2P N O

4.14

Fig. 4.11. The structure of the Mo2(pyphos)4 molecule showing how two Ph2P “claws” are in place at each end for capturing other metal atoms such as Pd and Pt.

The compounds with noncyclic amidate bridging ligands are generally prepared by reaction of Mo2(O2CCH3)4 with the ligand in anionic form.229-233 Both C2h and D2h arrangements of the Mo2N4O4 core are found.230,232-234 The particular amidato ligand CH3C(O)NH arises in several compounds242,243 as the hydrolysis product of the CH3CN ligand. It is believed that this normally very slow hydrolysis is catalyzed by the metal atoms.

For Mo2(mhp)4, the standard enthalpy of formation has been determined.192 Mass spectral measurements on Mo2(mhp)4, MoW(mhp)4 and Mo2[pyNC(O)CH3]4 have confirmed220,238,244,245 that the dinuclear structure is retained in the vapor phase and an extensive PES study has been carried out on Mo2(mhp)4,244 as well as a gas phase XPS study of Mo2(mhp)4.94 The Raman spectra of the molybdenum-containing mhp complexes gave M–M stretching frequencies of 504, 425 and 384 cm−1 for the Cr–Mo, Mo–Mo and Mo–W bonds.220,246 In the case of the polar molecule Mo2(fhp)4·THF, the ι(Mo–Mo) mode has been assigned to a band at c. 430 cm−1.223

The reaction of Mo2(mhp)4 with a cesium halide and the appropriate hydrogen halide in refluxing methanol produces Cs4Mo2X8 (X = Cl or Br).247 When a Bu4NI/HI(g)/THF mixture is used, Mo2(mhp)4 is converted to (Bu4N)2Mo4I11.247 The synthetic utility of Mo2(mhp)4 is further shown by its reactions with Mo2X4(PR3)4 to form complexes of the type Mo2X2(mhp)2(PR)2.248 An alternative synthetic strategy involves reacting Mo2(mhp)4 with Me3SiCl in the presence of PR3.221

Several dimolybdenum complexes containing the ligand type 4.15, with R' = Ph or Me, have been examined.237 Both N,P and N,S modes of bridging are found. A molecule with the latter is shown in Fig. 4.12.

98Multiple Bonds Between Metal Atoms Chapter 4

S

C

R2P NR'

4.15

Fig. 4.12. The structure of the green isomer of Mo2[Ph2PC(S)NMe]4 showing the occurence of both N,S, and N,P coordination modes.

4.2.2 Compounds with anionic N,N bridging ligands

Anionic bridging ligands with the type of anionic structure shown as 4.16 have emerged as especially important. Table 4.5 lists structurally characterized molecules that contain only one Mo24+ unit bridged by at least one such ligand. Compounds containing two or more Mo24+ units connected by linkers are treated in Section 4.5 and compounds in which the dimolybdenum unit has been oxidized to Mo25+ or Mo26+ are discussed in Section 4.4.2.

X

R R

N N

4.16