Multiple Bonds Between Metal Atoms / 06-X3M _ MX3 Compounds of Molybdenum and Tungsten

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g M. H. Chisholm, J. C. Gallucci, C. B. Hollandsworth, unpublished crystal structure of W2Cp2(NMe2)4.

Table 6.3. M2XaYbZc Compounds (where a + b + c = 6 or 7)

a For Mo2(CH2Ph)2(OPri)4(PMe3), Mo–Mo = 2.235(1) Å, Mo–C (av.) = 2.22(1) Å, Mo–P = 2.581(1) Å, and Mo–O (av.) = 1.88(1) Å.

b For W2I(NMe2)5, W–W = 2.298 (1) Å, W–I = 2.688(1) Å, W–N (av.) = 1.94(1) Å. c For Mo2I(NMe2)5 Mo–Mo = 2.211(1) Å, Mo–I = 2.80(1) Å, Mo–N (av.) = 1.93(1) Å.

d For W2(PPh2)(NMe2)5, W–W = 2.304(1) Å, W–P = 2.432(4) Å, W–N (av.) = 1.95(1).

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f M. H. Chisholm, J. C. Gallucci, and C. B. Hollandsworth, unpublished crystal structure of W2Cp2(NMe2)4.

6.4.1 Mo2X2(CH2SiMe3)4 compounds

Hydrocarbon solutions of Mo2(CH2SiMe3)6 react with 2 equiv of anhydrous HBr to give the orange, hydrocarbon soluble, crystalline compound 1,2-Mo2Br2(CH2SiMe3)4. Based upon spectroscopic data, this dibromide adopts the anti rotamer in solution and in the solidstate.103,115 The bromide ligands in Mo2Br2(CH2SiMe3)4 are substitutionally labile and a wide variety of Mo2(X)(Y)(CH2SiMe3)4 compounds have been prepared by metathetic reactions e.g., X = Y = alkyl, amide, alkoxide, thiolate, phosphide, and X ɒ Y where Y = CH2SiMe3, O2CNMe2, OBut.101,116 Many of these compounds were obtained as oils or waxy solids. Two compounds were crystallographically characterized, 1,2-Mo2(OBut)2(CH2SiMe3)4116 and 1,2- Mo2(NMe2)(PPh2)(CH2SiMe3)4101 and both were found to have the anti-staggered rotamer in the solid-state. Studies of this class of compounds revealed unequivocal information concerning the solution dynamic behavior of M2XnY6-n compounds and the complexities of the seemingly simple metathetic exchange reactions at the (M>M)6+ center. For example, 1,2-Mo2Br2(CH2SiMe3)4 was found to undergo metathesis reactions in hydrocarbon solvents with excess HNMe2 or two equivalents of LiNMe2 to give 1,2-Mo2(NMe2)2(CH2SiMe3)4 and 1,1-Mo2(NMe2)2(CH2SiMe3)4, respectively. Initially, it was thought that 1,1-Mo2(NMe2)2(CH2SiMe3)4 was a kinetic product which in the presence of an excess of HNMe2 isomerized to the 1,2-isomer. However, the 1,1-Mo2(NMe2)2(CH2SiMe3)4 isomer was subsequently shown to undergo aminolysis with NH(CD3)2 without 1,1- to 1,2-isomerization.101 Moreover, it was shown that the reactions

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involving 1,2-Mo2Br2(CH2SiMe3)4 and each of LiNMe2 and HNMe2, proceeded via the common intermediate 1,1-Mo2Br(NMe2)(CH2SiMe3)4.101 Finally, it was shown that the isomerization of 1,1- to 1,2-Mo2(NMe2)2(CH2SiMe3)4 could be catalyzed by the presence of Me2NH2Br and this allowed for speculation concerning the mechanism of metathetic exchange at the (Mo>Mo)6+ center. However, as shown in Scheme 6.1, the ability to isolate kinetically persistent 1,1- and 1,2-isomers (that do not interconvert even at 100 °C) indicates that a relatively high barrier to ligand exchange between the two metal atoms exists.

Scheme 6.1. Reactions of 1,1-Mo2Br(NMe2)R4 where R = CH2SiMe3.

The dynamic behavior of this class of compounds led to the first direct observation of rotation about a triple bond by variable temperature NMR studies. These studies complemented studies of 1,2-M2X2(NMe2)4 compounds, vide infra, and the rotational barriers about M–NR2 bonds are listed in Table 6.4. The restricted rotation about the M–NMe2 bond arises from the preferred alignment of the CNC unit along the Mo>Mo axis to allow Np/ to Modxy /-bonding. This gives rise to the proximal and distal methyl groups with respect to the M>M bond and does not disrupt the MM /-bonding orbitals, which utilize the Mdxz, and Mdyz atomic orbitals. For the series of compounds 1,1-Mo2(NMe2)(X)(CH2SiMe3)4, the rate of proximal to distal exchange follows the order X = NMe2 > OBut > SBut > CH2SiMe3 Ph > Br which correlates well with the relative μ//-donating ability of the X ligands.101 The electronegative bromide ligand leads to the highest rotational barrier. Steric factors can also greatly influence M–NMe2 rotational barriers as was argued for the M2R2(NMe2)4 compounds where R = Si(SiMe3)3 and CH(SiMe3)2.79,117

For some compounds, such as 1,2-Mo2X2(CH2SiMe3)4 where X = Br or OBut, it is not possible to determine whether they exist in solution exclusively in the anti rotameric form or if anti to gauche isomerization is too fast to be frozen out. The cases of 1,1-Mo2(NMe2)2(CH2SiMe3)4 and 1,2-Mo2(NMe2)(PPh2)(CH2SiMe3)4 have been examined in detail.101 In general, the low barriers to rotation about the M>M bond are consistent with the view that a cylindrical triple bond of μ2/4 configuration should have no inherent electronic barrier. For a molecule of the type 1,2-M2X2R4, steric factors may influence this barrier, and for a gauche rotamer with C2 symmetry, the degeneracy of the MM /x and /y MO’s is removed. Herein some electronic barrier may be introduced when the gauche rotamer is thermodynamically preferred, but in all cases that have been studied, these barriers, when measurable, are small.

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6.4.2 1,2-M2R2(NMe2)4 compounds and their derivatives

Particular attention was given to the chemistry of 1,2-Mo2R2(NMe2)4 compounds with respect to developing the organometallic chemistry of dinuclear molybdenum and tungsten compounds. Early attempts at investigating reductive elimination from the dinuclear center focused on 1,2-Mo2R2(NMe2)4 compounds where R contained a `-hydrogen atom, such as in the ethyl and isopropyl ligand. It was found that insertion of CO2 into the MN bond was accompanied by `-CH activation leading to reductive elimination of ethane and ethene. Furthermore, in labeling studies, it was shown that this involved transfer of the `-H atom of one alkyl ligand to the _-carbon of the other ligand:

1,2-Mo2(CH2CD3)2(NMe2)4 + 4CO2 Α Mo2(O2CNMe2)4 + CH2=CD2 + CH2D–CD3

A similar reductive elimination was observed in the reactions of `-H alkyl containing molybdenum compounds with 1,3-diaryltriazines which gave Mo2(ArNNNAr)4, alkane, and alkene.118 Related 1,2-W2R2(NMe2)4 compounds are less prone to reductive elimination though reactions of these compounds with symmetrical anhydrides R'CO2COR' (where R' = Me, But, Ph) pro-

vide a useful and general synthetic route to WW quadruply bonded carboxylate compounds:

W2R2(NMe2)4 + 4R'CO2COR' Α W2(O2CR')4 + 4R'CONMe2 + alkane + alkene

For alkyl compounds lacking `-hydrogen atoms, the reaction with acid anhydrides gave compounds of formula W2R2(O2CR')4 which have the unusual structure in which the axial sites of the paddlewheel W2(O2CR')4 are ligated by alkyl ligands.117 Particularly noteworthy in the structures of W2R2(O2CR')4 compounds are the short WW distances, comparable to those found in species with ditungsten quadruple bonds. One exception, however is seen in the reaction of W2Cp2(NMe2)4 with propionic anhydride which gave incomplete substitution forming W2Cp2(O2CEt)3(NMe2). Electronic structure calculations indicated that W2R2(O2CR')4 compounds most likely have the unusual bonding configuration of /4β2, lacking a formal μ-bond component to the ditungsten multiple bond. This situation is similar to that found for the molecule C2, which also contains an unusually short C–C distance for a diatomic molecule formally lacking a μ-bond. In both cases, however, there is slight, residual μ-bonding as a result of the fact that occupied μ-orbitals are slightly more bonding in character than the populated μ* MO’s are antibonding.119,120

The structure involving axial alkyl ligation, is in marked contrast to the structure seen in W2Me2(O2CNMe2)4 wherein each tungsten center forms five bonds in a pentagonal plane perpendicular to the W>W bond axis.108 However, there would appear to be little difference in energy between these two structural forms as seen from the study of the compound W2(CH2CMe3)2(O2CMe)2(S2CNEt2)2.121 The axially ligated W2R2(O2CR')4 compounds were thermally labile to reductive elimination via a WC homolysis reaction with the stability order R = Me3CCH2 > Me > Ph > PhCH2 which correlates with the accepted stability of organic radicals. The molybdenum analogs were more prone to reductive elimination and only the neopentyl complex Mo2(CH2CMe3)2(O2CMe)4 has been found stable enough for characterization.122

Reactions of 1,2-M2R2(NMe2)4 compounds with alcohols showed a similar trend in that reductive elimination was more favorable for M = Mo. The reactions proceed under kinetic control in which the amides are replaced by alkoxides:113,123

1,2-M2R2(NMe2)4 + R'OH (excess) Α 1,2-M2R2(OR')4 + 4HNMe2

When the alkyl ligand R contains `-hydrogen atoms, e.g., R = Et, Pr, Pri, CH2CHMe2, and for M = Mo, reductive elimination occurs during the course of the reactions to give an alkane and an alkene. When R = Bui and R' = Pr, the compound Mo2(OPri)4(HOPri)4 is obtained in

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contrast to W2(Bui)2(OPri)4. In the presence of a chelating diphosphine ligand, the compound Mo2(OPri)4(dmpe)2 was obtained wherein d6-Mo0 and d2-Mo4+ centers were united by a formal Mo24+ quadruple bond.124

In contrast, W2COT(NMe2)4 reacts with sterically demanding alcohols, ROH (where R = CH2But, Pri, But), to give clean alkoxide for amide exchange products W2COT(OR)4.96 Reactions with less sterically demanding alcohols, R'OH (where R = Me, Et, Pr) give the dimerized products [W2COT(OR')4]2 where two W2COT(µ-OR')(OR')2 units are connected via two symmetrical µ-OR' bridges.125 The ditungsten COT alkoxides do not eliminate COT-H2 even when dissolved in neat alcohol. Dissolving W2COT(OBut)4 in excess PriOH gives W2COT(OPri)4 quantitatively. However, preliminary studies indicate that W2COT(NMe2)4 reacts with ButSH (excess) to make exclusively the COT-eliminated product W2(SBut)2(NMe2)4. This seems to suggest that under conditions of alcoholysis or thiolysis, there is a point at which either amine (HNMe2) or alkyl (COT-H2) can preferentially eliminate.

Preliminary studies also suggest that W2Cp2(NMe2)4 is unreactive towards bulky alcohols such as ButOH.126 This result indicates that the amide might be sterically inaccessible to alcohols. However, reactions with excess CF3CH2OH give a variety of products. Observations on the W2COT(NMe2)4 and W2Cp2(NMe2)4 alcoholysis reactions tend to suggest that alkyl elimination is preferred only in some cases over amine elimination, despite the fact that thermodynamically alkyl exchange should be preferred as M–C bonds are weaker than M–N bonds. It is likely that the formation of an intermediate, having the incoming alcohol hydrogen-bonded to the dinuclear complex plays an important role in deciding the preference for alkyl vs. amine elimination. Efforts are underway to better understand the mechanisms of alcoholysis (and thiolysis) of both W2COT(NMe2)4 and 1,2-Cp2W2(NMe2)4.

6.5M4 Complexes: Clusters or Dimers?

The coupling or oligomerization of MM triple bonds has been a topic of longstanding interest to the Chisholm group. In 1978, they speculated about the reversibility of the reaction below, wherein a metathesis of (MɓM)6+ bonds could occur.127 However, studies of the species present in solution upon both thermal and photochemical excitation provided no evidence for the metathesis product Cp2MoW(CO)4.

Cp2Mo2(CO)4 + Cp2W2(CO)4 Α 2Cp2MoW(CO)4

The mixed metal compound is formed in a thermal or photochemical reaction employing the two Cp2M2(CO)6 compounds (M = Mo and W). The compound Cp2MoW(CO)4 can be detected readily by mass spectrometry and is probably formed by the following reaction sequence:

(i)Cp2M2(CO)6 + hι Α 2CpM(CO)3 (M = Mo, W)

(ii)CpM(CO)3 Α CpM(CO)2 + CO

(iii)CpMo(CO)2 + CpW(CO)2 Α Cp2MoW(CO)4

Subsequently, several M4 clusters were made via coupling of two (M>M)6+ units supported by alkoxide ligands. This work is described in the following sections.

6.5.1 Molybdenum and tungsten twelve-electron clusters M4(OR)12

The reversible coupling of two W2(OPri)6 molecules was discovered in 1986.128 The addition of secondary and bulky primary alcohols (Pri, CH2But, CH2-cyclopentyl, CH2-cyclobutyl, and CH2Pri) to W2(OBut)6 leads to black crystalline clusters W4(OR)12 and/or W4(OR)12(HOR).129,130 The molybdenum analogs Mo4(OR)12 and Mo4(OR)12(HOR) are formed for the less sterically-

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demanding alkoxide ligands but not for R = Pri and CH2But which remain as dinuclear species.129,130

The compound W4(OPri)12 was crystallographically characterized along with W2(OPri)6; the unit cell contained one dinuclear and one tetranuclear species.32 The tetranuclear structure is shown in Fig. 6.4. The central W4 unit is diamond-shaped having alternating short, 2.5 Å and long, WW distances, 2.8 Å, with a significant backbone WW interaction of 2.7 Å. The low temperature 1H NMR spectrum is consistent with expectations based on the C2h symmetry found in the solid state. Upon raising the temperature, two dynamic processes are observed,61 one of which is intramolecular and the other involves the reversible dissociation of the tetranuclear compound to W2(OPri)6:

W4(OPri)12 2W2(OPri)6

The intramolecular process involves the site exchange of the bridging alkoxides without exchange with the terminal OPri ligands. Also one set of terminal ligands exchanges sites but these do not exchange with the other set of terminal ligands. The wing-tip alkoxides may be classified in a pair-wise manner as proximal and distal with respect to the orientation of the methine vector. The terminal alkoxide ligands of the wingtip metals thus interconvert as do the bridging groups, but these transformations do not involve the backbone alkoxides.

Fig. 6.4. Structure of the butterfly W4(OPri)12 cluster.

The explanation proposed for this dynamic process was that the C2h-W4 cluster oscillates about the more symmetric diamond W4 structure wherein the WW distances are equivalent. This leads to the bridging groups becoming equivalent without exchange with the terminal groups. Concomitant with this dynamic process is a correlated motion of the wing-tip alkoxides. The process is shown schematically in Scheme 6.2 and was called the Bloomington Shuffle. The energy of activation of this intramolecular process was estimated to be 13 kcal mol-1. This was determined from the line broadening seen at low temperatures in 1H NMR spectra. At room temperature the dissociative equilibrium is clearly evident by NMR spectroscopy although it is never rapid on the NMR time scale.

The thermodynamic parameters of this equilibrium were found to be ¨Hº = -16 kcal mol-1 and ¨Sº = +60 eu, together with the activation parameters ¨H& = 5 kcal mol-1 and ¨S& = +38 eu for the forward (dissociative), and ¨H& = 10 kcal mol-1 and ¨S& = -40 eu for the back (associative reaction).61 The tetranuclear cluster is favored on enthalpic grounds but disfavored by entropy. The low enthalpic barrier to the association of two M–M bonds is noteworthy and contrasts with organic p/–p/ systems for which the process would be symmetry forbidden according to the Woodward-Hoffman rules.131-133

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As noted earlier, there are interesting analogies in the bonding of early transition metal alkoxide clusters and later transition metal carbonyl clusters. However, perhaps the most amazing characteristic of these 12-electron clusters of Mo and W is the variety of geometries seen for the M4 unit. Clearly, the MM bonding is very sensitive or responsive to the steric and electronic constraints of the attendant ligands. This is even further underscored by a consideration of the linked MɓM bonded dimers to be described next.

6.5.6 Linked M4 units containing localized MM triple bonds

It was previously noted that Mo2(OPri)6 does not show any tendency to form Mo4(OPri)12 akin to its tungsten analog. However, with less sterically-demanding alkoxide ligands, clusters are formed. In an attempt to study the nature of the “dimerization” process, Mo2(OPri)6 and methanol were allowed to react in hydrocarbon solvents. An initial “dimer of dimers” was characterized as [Mo2(OPri)4(µ-OMe)(µ-OPri)]2.142 Its structure, Fig. 6.7, has a rectangular Mo4 unit containing two localized Mo>Mo bonds of 2.22 Å brought together by alkoxide bridges for non-bonding MoMo distances of 3.5 Å.

Fig. 6.7. Structure of the mixed alkoxide cluster [Mo2(OPri)4(µ-OMe)(µ-OPri)]2.

Another rectangular Mo4-containing molecule, [Mo2(OPri)4(µ-OPri)(µ-F)]2, was obtained from the reaction between Mo2(OPri)6 and two equivalents of PF3. Here the “dimer of dimers” was readily cleaved by the addition of PMe3 which gave Mo2(OPri)6 and Mo2F2(OPri)4(PMe3)2.143 Treatment of a hydrocarbon solution of Mo2(OBut)6 with two equivalents of PF3 gave Mo4(µ-F)4(OBut)8 as depicted in 6.4.143,144 Once again, the differing MoMo distances of 2.25 Å and 3.7 Å leave no doubt that the localized triple bonds have been retained. This is further supported by the fact that treatment of Mo4(µ-F)4(OBut)8 with 4 equiv of PMe3 yields Mo2(F)2(OBut)4(PMe3)2. The reactions are:

2[Mo2(OBut)6] + 4PF3 Α Mo4F4(OBut)8 + 4PF2OBut

Mo4F4(OBut)8 + 4PMe3 Α 2[Mo2(F)2(OBut)4(PMe3)2]

From the reaction between 1,2-Mo2Br2(CH2SiMe3)4 and water in the presence of pyridine, the unusual compound Mo4O2(CH2SiMe3)8 was obtained.145 The notable feature of this structure shown in Fig. 6.8 is that each molybdenum atom is three-coordinate and the local ethanelike W2O2C4 core is gauche, whereas in the previously described linked (Mo>Mo)6+ species each Mo atom is four coordinate such that each “L4MoɓMoL4” fragment is square pyramidal.