Multiple Bonds Between Metal Atoms / 04-Molybdenum Compounds

Molybdenum Compounds 149

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In the supramolecular arrays there are two types of ligands:

1.linkers, that connect dimetal units with one or more others, and

2.spectator ligands, which fill all the positions around the dimetal unit that are not occupied by linkers.

The dimolybdenum units that have been used are of the two types shown as (a) and (b) in Fig. 4.29 along with a generic representation of a linker, (c). The four main types of supramolecular structures are shown in Fig. 4.30 although there are a few others that will be mentioned later.

Fig. 4.29. The two types of dimolybdenum fragments, (a) and (b), that can be connected by linkers, (c), to form supramolecular arrays.

Fig. 4.30. The four most common types of supramolecular arrays, with the spectator ligands omitted for clarity.

It is clear that the unit (a) in Fig. 4.29 is suited to form only the pairs shown in Fig. 4.30 when the linkers are of the type shown in (c). For the type of dimolybdenum fragment shown as (b) in Fig. 4.29, it might seem that only squares could be expected, because of the 90° angle subtended at the Mo2 unit by the carboxyl planes of the two adjacent linkers. However, this is too simplistic a view. It is obvious that if the linkers are inherently bent, loops will naturally be favored.

Less obvious is the possibility of forming triangles, since the 60° corner angles of a triangle are far from the 90° angles favored by the type (b) units shown in Fig. 4.29. And yet triangles are sometimes formed, for thermodynamic reasons.

150Multiple Bonds Between Metal Atoms Chapter 4

In order to have a ¨G° of zero for the reaction in which triangles become squares,

4{[Mo2]3L3} = 3{[Mo2]4L4} ¨G° = ¨H° − T¨S°

(where we abbreviate the dimolybdenum core plus its spectator ligands as [Mo2]) ¨H° must equal T¨S°. Now there is considerable strain entailed in forming triangles relative to forming squares which could arise from any or all of three principal distortions:

1.making the angles subtended at metal atoms by the linkers less than 90°;

2.curving and twisting linkers;

3.twisting the angle of internal rotation about the Mo–Mo quadruple bond away from

the preferred eclipsed conformation.

The accumulation of strain energies in the triangle must make a negative contribution to ¨H° in the above equation; that is, squares are enthalpically preferred to triangles.

However, the entropy change as four moles of triangles are converted to three moles of squares is negative so the −T¨S° will be positive and tend to offset the negative ¨H° term. Rough estimates553 of both ¨H° and −T¨S° (at c. 300 K) suggest that each of these terms might have an absolute value of 10-15 kcal mol−1. Since the entropy contribution should be practically independent of the exact identity of the linker, the most promising strategy for obtaining triangles instead of squares is to employ linkers that are flexible – avoiding, however, those that actually prefer (or demand) to be bent since, as already noted, they will give rise to loops for enthalpic reasons alone.

So far, only one successful preparation of a triangle containing [Mo2] components has been reported.554 The two carboxyl groups in the linker are at the 1 and 4 positions of a cyclohexane ring, and this ring is flexible enough to provide, at a low enthalpic cost, the curvature in the sides necessary for the triangle, as shown in Fig. 4.31. This may be contrasted with the result of using the p-xylenediyldicarboxylate which can equally well have the linear conformation 4.38(a), that would make possible the formation of a square, or the bent one 4.38(b) which favors a loop. The formation of a loop is even more entropically favored than a triangle, so that this linker, in conformation 4.38(b), could have been predicted to give rise to a loop in preference to a square or even a triangle and, as shown in Section 4.5.6, this is what it does.

Fig. 4.31. The core of the structure of the molecular triangle [(DAniF)2Mo2(O2CC6H10CO2)]3.

Molybdenum Compounds 151

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4.38

In the following subsections, the entire range of supramolecular assemblies afforded by [Mo2] units (pairs, squares, loops and others) will be discussed. The one and only triangle so far reported was mentioned above. A list of all of the dianions that have been used to link Mo24+ cores into supramolecular arrays is presented in Table 4.16. The numbers assigned there will be used to identify these linkers in all subsequent lists and tables.

Table 4.16. Linkers that have been used to make supramolecular structures of Mo24+ components.

A. Dicarboxylates

OO

CC

OO

A1

FF

O2CCO2

FF

A4

HH

C

O2C CO2

A7

FF

C	CO2
O2C	C
F	F

A10

O2C CO2

C C

HH

O2C C C CO2

A2

B	B
	B B
O2C C B	B C CO2
	B B

BB

A5

HH

		C	CO2
O2C			C
		H	H
		A8
		H2
		C
O2C		C C	C		CO2
		H2
		C
		H2

A11

O2C H

C C C

HCO2

HCO2

C C

O2C H

A3

CO2 Fe

O2C

A6

HH H H

C	C	C
O2C		CO2

HH

A9

CO2

CO2

A12

HCO2

H C C

C C H

CO2 H

A13

A14

A15

152Multiple Bonds Between Metal Atoms Chapter 4

H

H

H

O2C

C

C

C

C

C

C

CO2

H

H

H

A16

A17

A18

H2

CO2

O2C

CO2

O2C

CO2

H2

H

H

H2

CO2

A19

A20

A21

O2C

CO2

O2CCH2

CH2CO2

CH2

O2C

CO2

A22

A23

A24

Me

H

H Me

H

NH2

H

OH

CO2

O2C C

CO2

C

O

O

C

C

O2C

CO2

O2C

C

C

H2

H OH

A25

A26

A27

B. Diamidates

O

NPh

C

O

N

C

C

C

N

O

PhN

O

B1

B2

B3

OMe

OMe

O

O

O

N

N

N

C

C

C

C

C

C

N

O

N

O

N

O

MeO	MeO

B4

B5

B6

Molybdenum Compounds 153

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N

N

O

O

O

O

N

N

N

O

O

N

B7

B8

B9

O

O

O

N

N

N

N

C

C

O N

O

N

N

H

B10

B11

C. Tetrahedral XY42- linkers

SO42-

MoO42-

WO42-

C1

C2

C3

Zn(OMe)42-

Co(OMe)42-

C4

C5

Communication through linkers.

One of the most interesting questions raised by the supramolecular compounds described in this section is the extent to which an electronic change (oxidation or excitation) in one Mo24+ unit will be communicated to the other, or others, in the same higher-order assembly. One convenient way to explore this subject is by electrochemistry, and this has been done on the majority of the supramolecular compounds. The accessible oxidation potentials may be determined by cyclic voltammetry (CV) or by differential pulse voltammetry (DPV), and the difference between the first and second ones, ¨E1/2, (and any succeeding ones) provides a measure of communication.

In the case of a “dimer of dimers” type molecule (Sections 4.5.2 and 4.5.3) ¨E1/2 is related to the stabilities of the neutral, +1, and +2 species by the following equations, in which we continue to use [Mo2] as a shorthand for the dimolybdenum unit together with its spectator ligands and L for the linker. We first define the comproportionation constant, Kc, and then a form of the Nernst equation in which 25.69 is the numerical value of the requisite combination of fundamental constants when ¨E1/2 is in millivolts. For cases where ¨E1/2 is small, it is best evaluated from the pulse voltammogram by employing a method due to Richardson and Taube.555

[{[Mo2]L[Mo2]}+]2

Kc = [{[Mo2]L[Mo2]}][{[Mo2]L[Mo2]}2+]

Kc = exp(∆E1/2/25.69)

The smallest value of the comproportionation constant, Kc, is 4 for purely statistical reasons. If the linkers simply insulate one [Mo2] group from the other in the [Mo2]L[Mo2]+ ion the second oxidation will be as easy as the first (except for the statistical factor) and we will have

¨E1/2 = 25.69 ln 4 = 35.6 mV

On the other hand if the +1 ion is fully delocalized, removal of the second electron will be significantly more difficult than the first and ¨E1/2 values typically exceed 400 mV. Actually, a ¨E1/2 value as low as 36 would occur only when the linker is very long so that the electrostatic

154Multiple Bonds Between Metal Atoms Chapter 4

repulsive effect would be reduced effectively to zero. For the majority of linkers that have been used ¨E1/2 values in the range 100-400 mV have been measured. Such compounds are variously called “moderately coupled,” “partly delocalized” or “class II,” the latter term derived from the Robin-Day classification of charge transfer systems.556

The theoretical problems raised by these intermediately coupled systems are formidable and are much discussed elsewhere.557 The work on supramolecular systems of [Mo2] units, but especially on [Mo2]L[Mo2] molecules and their +1 and +2 ions provides an abundance of new results concerning electronic communication through linkers. Not only are the results new, but they present certain advantages not generally afforded by other classes of compounds, such as those in which mononuclear complexes (e.g., of Ru2+/Ru3+) or organometallic moieties (e.g., ferrocene/ferrocenium) are linked. In the [Mo2]L[Mo2] compounds the nature of the orbitals

(βMo−Mo) from which electrons are removed is unambiguous and their interactions with linker orbitals are well-defined. Moreover, the structural changes in going from Mo24+ to Mo25+, espe-

cially in the Mo–Mo distances, are independently well-established and in each compound they can be determined crystallographically to sufficient accuracy ()0.001 Å) that the distinction between Mo24+, two Mo24.5+ in a delocalized system, and Mo25+, is always clear. The change at each step is about 0.025(1) Å. Moreover, as in other systems, magnetic susceptibilities, EPR and electronic spectra also provide valuable information.

The very schematic representation of a linker in Fig. 4.29(c) indicates only one essential feature, namely that there be two end portions, each consisting of a bent triatomic group with the two outer atoms being donor atoms capable of spanning the two Mo atoms in an Mo2n+ unit. In fact, a very large number of species, mostly dianions, can meet this simple prescription. Table 4.16 is a list of all of those that have actually been used in structurally characterized compounds.

4.5.2 Two linked pairs with carboxylate spectator ligands

The first efforts to link Mo24+ units into larger arrays558 were made by employing the following class of reactions:

	xMo2(O2CCMe3)4 + yHO2CXCO2H
		x	[Mo	(O	CCMe	)	]	(O			2	CXCO			2	) + xMe	CO	H	(when x = 2y)
	2
			2	2	3	3	2									3	2
	and/or x[Mo2(O2CCMe3)2](O2CXCO2) + 2xMe3CO2H																		(when x = y)

At equilibrium the relative amounts of the two stoichiometric products will depend on the ratio x/y. The major products were the 2:1 type. It was implied that the 1:1 type might also be formed, but none have ever been isolated and it is not known if they might be linear chains, triangles, squares, etc.

Several products of the 2:1 type were obtained (as well as some tungsten analogs) each of which had two Mo2(O2CCMe3)3 units linked by a dicarboxylate ion (oxalate, −O2C(1,4-C6F4)CO2−, −O2C(1,1´ Fc)CO2−) or by the linkers 4.39(a) through 4.39(d). Of all these compounds only the one with the linker 4.39(b) was subjected to structure determination by X-ray crystallography, because of the well-known lability of carboxyl groups.39-41

Despite the fact that it has never been possible to carry out a conventional single-crys- tal X-ray structural characterization of any (RCO2)3M2O2CXCO2M2(O2CR)3 compound, such compounds have been extensively studied. From powder diffraction data the crystal packing of the (ButCO2)3Mo2(O2CCO2)-Mo2(O2CBut)3 and (ButCO2)3Mo2(O2CC6H4CO2)Mo(O2CBut)3 molecules was assessed in a semiquantitative way.559-561

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On the basis of these results Chisholm and coworkers have carried out many interesting physical and theoretical studies562-566 of the (ButCO2)3Mo2(O2CXCO2)Mo2(O2CBut)3 compounds and their tungsten analogs. For example, EPR spectra and related physical evidence have led to the conclusion that for oxalato-bridged molecules with both Mo2 and W2, the monocations are delocalized, while for O2CC6F4CO2-bridged species, only the W2 compound is delocalized. These conclusions are, in part, surprising. For the W2 oxalato-bridged compound the comproportionation constant was reported558 to be c. 1012, so that delocalization is expected, and for the O2CC6F4CO2-bridged compound of molybdenum Kc = 13, so that localization is expected. However, for both of the other compounds said to be delocalized, Kc values (104-105) are below the value of c. 106 often cited as the approximate lower limit for delocalization. This, of course, rises the question (which will not be discussed here) of what “delocalization” really means, especially with respect to time scales of various spectroscopies.

4.39

It is particularly worth mentioning that spectroscopic and DFT molecular orbital studies of the oxalato-bridged and O2CC6F4CO2-bridged compounds of both Mo24+ and W24+ have been reported.560,561 For all four compounds the interactions between the M24+ β and β* orbitals and the / orbitals of the bridging ligands are extensive when the molecules are planar. Planarity is electronically favored, although rotational barriers about the C–C bonds are less than 10 kcal mol−1 according to the DFT calculations. The visible spectra are dominated by MLCT transitions. The Mo–Mo stretching modes (by Raman spectroscopy) are at 395-400 cm−1 for the Mo24+ compounds and about 311 cm−1 for those of W24+. In preliminary communications of this work, calculations of more extended compounds (none of which have been made) were also reported briefly564,565 and several overviews of this area have been presented.562,563

It has more recently been shown that 2,5-thiophenedicarboxylate can also serve as a bridge between Mo2(O2CCMe3)3 groups.567

4.5.3 Two linked pairs with nonlabile spectator ligands

The pernicious consequences of the lability of carboxylate ligands with regard to efforts to isolate and study molecules containing two or more dimetal units are overcome by using ligands that are stereoelectronic to carboxylates but less labile.568,569 Amidinate ligands serve this purpose well and experience has shown that one particular ligand, DAniF−, 4.40, is extremely suitable. Thus, by employing (DAniF)3Mo2+ rather than (RCO2)3Mo2+, stable crystalline “dimers of dimers” in which a virtually unlimited range of O2CXCO2 and other types of linkers may be incorporated are readily accessible. Table 4.17 lists all neutral compounds of the type (DAniF)3Mo2(linker)Mo2(DAniF)3 that have been isolated and studied.

156Multiple Bonds Between Metal Atoms Chapter 4

The first two compounds,570a reported in 1998, were obtained by the reactions:

2Mo2(DAniF)3Cl2 + 2NaHBEt3 + (NBun4)2O2C–X–CO2 Α Mo2(DAniF)3O2C–X–CO2Mo2(DAniF)3 + 2NaCl + 2NBun4Cl + 2BEt3 + H2

in which X represents either nothing (i.e., the linker is oxalate) or 1,4-C6F4. The complete structure of the oxalato-bridged molecule is shown in Fig. 4.32. In 2001 a total of twelve compounds of this type were reported,570b in which the linkers were A1 to A12 in Table 4.16. In addition to extending the range of linkers, this report introduced a better method of preparation in which the (DAniF)3Mo2Cl2 compound (previously used together with NaHBEt3) is first dissolved in CH3CN and treated with Zn to produce, in situ, a solution of [(DAniF)3Mo2(CH3CN)2]+, from which the excess Zn is removed by filtration. This avoids the formation of unwanted products that sometimes result from reaction of NaHBEt3 with the linker. All twelve compounds were crystallographically characterized.

		OMe				OMe
			H
			C
			N			N
			4.40
Table 4.17. Compounds with two linked Mo2(DAniF)3+ units
		Distances, Å
Linkera	Mo–Mo bond distancesb		Mo2···Mo2 distance					¨E1/2, mV	ref.
A1	2.090					6.94		212	570
A2	2.095	2.095				9.54		150	570
A3	2.086	2.087				9.19		145	570
A4	2.090					11.30		87	570
A5	2.088					11.61		69	570
A6	2.087					10.95		75	570
A7	2.088	2.089				7.65		108	570
A8	2.090	2.092				9.21		100	570
A9	2.084	2.088				9.01		112	570
A10	2.086	2.087				9.06		121	570
A11	2.082	2.088				10.30		95	570
A12	2.087	2.088				9.78		69	570
A13	2.090	2.093				7.69		172	571
A14	2.092					9.40		130	571
A15	2.087	2.087				10.35		125	571
A16	2.087					11.58		105	571
A17	2.086					13.92		75	571
A18	2.090					16.16		65	571
A19	2.090					11.24		100	572
A20	2.082					15.45		na	573
A22	2.092	2.092				11.10		66	574
A26	2.101	2.086				9.03		na	575
A27	2.088	2.088				9.02		na	575
B1	2.089					11.38		112	576
B2	2.089					11.38		105	576

Molybdenum Compounds 157

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		Distances, Å
Linkera	Mo–Mo bond distancesb		Mo2···Mo2 distance	¨E1/2, mV	ref.
B3	2.07		7.10	191	577
B4	2.094		7.08	190	577
B5	2.093		6.32	540	577
B6	2.095		6.33	523	577
B7	2.084	2.089	7.26	187	502
B8	2.092		7.08	258	502
B9	2.090		7.09	308	502
B10	2.095		7.13	263	502
B11	2.096	2.097	7.32	152	502
C1	2.090	2.094	6.01	228	578
C2	2.108	2.119	6.01	311	578
C3	2.110	2.117	6.08	285	578
C4	2.117	2.111	6.55	212	579,580
C5	2.116	2.114	6.56	207	579,580

a Identification numbers are given in Table 4.16.

b When the Mo2 units are crystallographically independent, both are given. Esd in each case is 0.001 Å or less.

Fig. 4.32. The structure of the [(DAniF)3Mo2]2(O2CCO2) molecule.

In 2003 six more [Mo2]L[Mo2] compounds having dicarboxylate linkers were reported.571 This work was focused on dicarboxylates with conjugated, unsaturated chains of carbon atoms, namely A3, A13, A14, A15, A16, A17 and A18 in Table 4.16. In this and a closely following paper,581 the interactions between the β orbitals of the Mo24+ cores and the / orbitals of the linkers were examined by both spectroscopy and DFT calculations. It was concluded that with saturated linkers (e.g., succinate) or others in which no continuous orbital overlap connects one Mo24+ core to the other, the lowest energy absorption band is localized in each of the independent, non-interacting Mo24+ chromophores. However, with linkers such as A1, A2, A3, A13, A15, A16, A17 and A18, the lowest transitions are best described as Mo24+ β to linker /* MLCT transitions.

The (DAniF)3Mo2(O2C–X–CO2)Mo2(DAniF)3 compounds cover a range of ¨E1/2 values of 213 mV to 65 mV and the distances between the centers of the two Mo24+ unit go from 6.95 Å to 16.15 Å. The magnitude of ¨E1/2 is proportional to the ¨G on introducing a positive charge on the second Mo24+ unit after one is already present on the first one. In the absence of any form of interaction between the two charges other than one that follows Coulomb’s Law, and with

158Multiple Bonds Between Metal Atoms Chapter 4

the further assumption that the effective dielectric constant for the medium that separates the

two charges is the same in all compounds, a plot of ¨E1/2 vs the Mo24+ to Mo24+ distance, d, should be linear. Of course, the effective dielectric constant probably does vary, the Mo25+ to

Mo25+ distance in the product may not always differ by the same amount from the Mo24+ to Mo24+ distance in the neutral molecules, and the end-to-end distances in conformationally nonrigid molecules may be different in solution from what they are in the crystals. Thus, even if the only energy of interaction were Coulombic, perfect adherence to a linear relationship could not be expected. However, major and non-random deviation would vitiate the idea of a purely Coulombic interaction.

In Fig. 4.33 the ¨E1/2 values have been plotted vs d for 19 compounds. Filled circles are for linkers that are either saturated or for other reasons (such as the orthogonality of the / bonds in A14 or the non-planarity of A19) are expected to be poor electronic connectors. These data provide no support for the concept of a linear relationship based on a predominantly Coulombic interaction. Presumably the animadversions already noted, and probably other special features of individual linkers, are too important to ignore.

Fig. 4.33. A plot of ¨E1/2 vs the distance between Mo24+ centers in some compounds with linked (DAniF)3Mo2+ units. The numbers refer to the linkers in Table 4.16.

It is interesting to see that the seven compounds (open circles) with unsaturated moieties connecting the carboxyl groups plus the oxalate bridge (which is planar) form a much better behaved set. The relationship is not linear, but curvature is expected if an electronic connection through the / systems which falls off with 1/dn (n > 1) is superimposed on the Coulombic behavior. Both the Coulombic and the non-Coulombic interactions should go to zero as d Α , and therefore the points should approach a limiting value of ¨E1/2 = 35.6 mV, as explained in Section 4.5.1. This does not seem inconsistent with the limited data available.

Diamidate linkers.

Dicarboxylates are not the only linkers that have produced interesting compounds with linked pairs of Mo2(DAniF)3+ units. A closely related class are diamidate dianions, several of which are shown in Table 4.16. They are of two types, open chain577 and cyclic.502 The compounds made so far with diamidate linkers are listed in Table 4.17. With linkers B1 and B2