Dinuclear (d3-d3) Diolate Complexes of Molybdenum and Tungsten. 3.1 Bridging and Chelating Isomers of M2 (NMe2)2(O～～CHMe～～O)2, Where O～～CHMe～～O Is the Dianion of 2,2′-Ethylidenebis(4,6-di- tert-butylphenol). Kinetic versus Thermodynamic Considerations

Article abstract of DOI:10.1021/ic9708430

Hydrocarbon solutions of Mo₂(NMe₂)₆, and 2,2′-ethylidenebis(4,6-di-tert-butylphenol), HO～～CHMe～～OH (2 equiv), react to give a mixture of two isomers, A and B, of formula Mo₂(NMe₂)₂(O～～CHMe～～O) ₂. Both A and B are shown to contain the bridging μ-O～～CHMe～～O ligands. They are diastereomers differing with respect to the positioning of the CHMe moiety as a result of ring closure with elimination of HNMe₂. Compounds A and B are thermally persistent at 100 °C in the solid state and in toluene, but compound A isomerizes in the presence of pyridine to the thermodynamically favored chelate isomer Mo₂(NMe₂)₂(η ²-O～～CHMe～～O)₂, C. Compound C is related to the previously characterized isomer Mo₂(NMe₂)₂(η²-O～～CH ₂～～O)₂, where the methylene proton that is distal to the Mo-Mo triple bond is replaced by a Me group. Compound B cannot rearrange to this isomer without Mo-O bond dissociation. W₂(NMe₂)₆ and HO～～CHMe～～OH (2 equiv) react to give W₂(NMe₂)₂(η ₂-O～～CHMe～～O)₂, D, which is an analogue of C. The three molybdenum isomers A-C and tungsten complex D have been structurally characterized by single crystal X-ray studies. The results of this work are discussed in terms of the earlier work involving the related 2,2′-methylenebis(6-tert-butyl-4-methylphenoxides) and reveal that the initial ring closure reaction determines the stereochemistry of the subsequent substitution by the biphenoxide at the dinuclear center. Crystal data: for A·PhMe at - 168 °C, a = 16.585(5) A?, b = 19.480(6) A?, c = 10.960(3) A?, α = 98.39(2)°, β = 103.19(1)°, γ = 91.44(2)°, space group P1?; for B·2PhH at - 168 °C, a = 15.167(3) A?, b = 18.210(3) A?, c = 13.964(3) A?, α = 92.81(1)°, β = 100.40(1)°, γ = 71.16(1)°, space group P1?; for C·1.SEt₂O at - 168 °C, a = 16.483(3) A?, b = 16.817(3) A?, c = 14.305(2) A?, α = 106.13-(1)°, β = 108.80(1)°, γ = 71.70(1)°, space group P1?; and for D at - 168 °C, a = 14.638(2) A?, b = 21.188(4) A?, c = 10.273(2) A?, a = 94.80(1)°, β = 95.05(1)°, γ = 100.85(1)°, space group P1?.

Full text of DOI:10.1021/ic9708430

50
Inorg. Chem. 1998, 37, 50-55
Dinuclear (d3-d3) Diolate Complexes of Molybdenum and Tungsten. 3.1 Bridging and
Chelating Isomers of M2(NMe2)2(O∼∼CHMe∼∼O)2, Where O∼∼CHMe∼∼O Is the
Dianion of 2,2′-Ethylidenebis(4,6-di-tert-butylphenol). Kinetic versus Thermodynamic
Considerations
Malcolm H. Chisholm,* Kirsten Folting, William E. Streib, and De-Dong Wu
Department of Chemistry and Molecular Structure Center, Indiana University,
Bloomington, Indiana 47405
ReceiVed July 7, 1997X
Hydrocarbon solutions of Mo2(NMe2)6 and 2,2′-ethylidenebis(4,6-di-tert-butylphenol), HO∼∼CHMe∼∼OH (2
equiv), react to give a mixture of two isomers, A and B, of formula Mo2(NMe2)2(O∼∼CHMe∼∼O)2. Both A
and B are shown to contain the bridging µ-O∼∼CHMe∼∼O ligands. They are diastereomers differing with
respect to the positioning of the CHMe moiety as a result of ring closure with elimination of HNMe2. Compounds
A and B are thermally persistent at 100 °C in the solid state and in toluene, but compound A isomerizes in the
presence of pyridine to the thermodynamically favored chelate isomer Mo2(NMe2)2(η2-O∼∼CHMe∼∼O)2, C.
Compound C is related to the previously characterized isomer Mo2(NMe2)2(η2-O∼∼CH2∼∼O)2, where the
methylene proton that is distal to the Mo-Mo triple bond is replaced by a Me group. Compound B cannot
rearrange to this isomer without Mo-O bond dissociation. W2(NMe2)6 and HO∼∼CHMe∼∼OH (2 equiv) react
to give W2(NMe2)2(η2-O∼∼CHMe∼∼O)2, D, which is an analogue of C. The three molybdenum isomers A-C
and tungsten complex D have been structurally characterized by single crystal X-ray studies. The results of this
work are discussed in terms of the earlier work involving the related 2,2′-methylenebis(6-tert-butyl-4-
methylphenoxides) and reveal that the initial ring closure reaction determines the stereochemistry of the subsequent
substitution by the biphenoxide at the dinuclear center. Crystal data: for A‚PhMe at -168 °C, a ) 16.585(5) Å,
b ) 19.480(6) Å, c ) 10.960(3) Å, R ) 98.39(2)°, â ) 103.19(1)°, γ ) 91.44(2)°, space group P1h; for B‚2PhH
at -168 °C, a ) 15.167(3) Å, b ) 18.210(3) Å, c ) 13.964(3) Å, R ) 92.81(1)°, â ) 100.40(1)°, γ ) 71.16(1)°,
space group P1h; for C‚1.5Et2O at -168 °C, a ) 16.483(3) Å, b ) 16.817(3) Å, c ) 14.305(2) Å, R ) 106.13-
(1)°, â ) 108.80(1)°, γ ) 71.70(1)°, space group P1h; and for D at -168 °C, a ) 14.638(2) Å, b ) 21.188(4) Å,
c ) 10.273(2) Å, R ) 94.80(1)°, â ) 95.05(1)°, γ ) 100.85(1)°, space group P1h.
Introduction
the thermodynamic isomer is not first formed, the kinetic isomer
is persistent even upon heating to ca. 100 °C.5 In reactions
between Mo2(NMe2)6 and methylenebis(6-tert-butyl-4-meth-
ylphenol) the bridged isomer Mo2(NMe2)2(µ-O∼∼CH2∼∼O)2
is kinetically formed, and upon addition of pyridine rearrange-
ment to the thermodynamic chelate isomer Mo2(NMe2)2(η2-
O∼∼CH2∼∼O)2 occurs.1 A study of the isomerization induced
by pyridine (and other Lewis bases) indicated a second order
dependence on the concentration of pyridine (py), leading us
to propose that two molecules of pyridine were bound in the
activated complex which schematically could be represented
by eq 2.1
Within the field of coordination chemistry the chelate effect
has played an influential role in the elucidation of mechanisms
of substitutions and rearrangements at mononuclear centers.2
We have for some time been interested in the development of
the coordination chemistry of d3-d3 dinuclear complexes of
molybdenum and tungsten which show a rich variety of
coordination geometries.3 We have given particular attention
to the so-called ethane-like dimers with M-M triple bonds and
noted the relatively high barrier to bridge formation in such
complexes. Indeed, only one authentic example of the equi-
librium shown in eq 1 has been observed thus far.4
1,2-W2(PR2)2(NMe2)4 h W2(µ-PR2)2(NMe2)4
(1)
Mo2(NMe2)2(µ-O~~CH2~~O)2 + 2Py
Py
As a result of this relatively high barrier it is possible to isolate
1,1- and 1,2-M2X2Y4 compounds under kinetic control; i.e., if
N
O
O
O
O
N
Mo2(NMe2)2(η2-O~~CH2~~O)2 + 2Py
(2)
Mo
Mo
Py
X Abstract published in AdVance ACS Abstracts, December 15, 1997.
(1) Part 2: Chisholm, M. H.; Huang, J.-H.; Huffman, J. C.; Parkin, I. P
Inorg. Chem. 1997, 36, 1642.
(2) (a) Basolo, F.; Pearson, R. G. Mechanisms of Inorganic Reactions. A
Study of Metal Complexes in Solution, 2nd ed.; John Wiley and Sons
Publishers: New York, 1968. (b) Wilkins, R. G. Kinetics and
Mechanisms of Transition Metal Complexes, 2nd ed.; VCH Publish-
ers: Weinheim, Germany, 1991.
(3) Chisholm, M. H. Acc. Chem. Res. 1990, 23, 419.
(4) Buhro, W. E.; Chisholm, M. H.; Folting, K.; Huffman, J. C.; Martin,
J. D.; Streib, W. E. J. Am. Chem. Soc. 1988, 110, 6563.
(5) Chisholm, M. H.; Folting, K.; Huffman, J. C.; Rothwell, I. P.
Organometallics 1982, 1, 251.
S0020-1669(97)00843-4 CCC: $15.00 © 1998 American Chemical Society
Published on Web 01/12/1998

Dinuclear (d3-d3) Diolate Complexes of Mo and W
Inorganic Chemistry, Vol. 37, No. 1, 1998 51
In this work we continue our studies of the substitution of
M2(NMe2)6 compounds by examining reactions of the closely
related 2,2′-ethylidenebis(4,6-di-tert-butylphenol), represented
for simplicity as HO∼∼CHMe∼∼OH and shown diagramati-
cally as follows.
Results and Discussion
Synthesis. Hydrocarbon solutions of Mo2(NMe2)6 and 2
equiv of HO∼∼CHMe∼∼OH react to yield orange solutions
of Mo2(NMe2)2(O∼∼CHMe∼∼O)2. Further replacement of the
NMe2 groups by the biphenol does not occur in the presence of
additional HO∼∼CHMe∼∼OH, presumably because of steric
crowding at the dimetal center. Also as we have shown
previously, as NMe2 groups are replaced by more electronega-
tive and less π-donating ligands, the M-N σ and π bonds are
stabilized and become kinetically less labile toward protonolysis
reactions.6 The kinetically formed solution of Mo2(NMe2)2-
(O∼∼CHMe∼∼O)2 is a mixture of two isomers as is evident
by 1H NMR spectroscopy. The two isomers have been
separated and purified by taking advantage of their different
solubilities in hydrocarbon solvents. Both isomers show, by
1H NMR spectroscopy, four t-Bu singlets in the ratio 1:1:1:1
and two NMe resonances corresponding to proximal and distal
Me groups with respect to the MotMo bond. In one isomer
the NMe resonances are notably upfield with respect to the other.
In both isomers there is a single CH quartet downfield at ca. 7
ppm and a single Me doublet. 1H NMR data are given in the
Experimental Section. The data are consistent with two isomers
each having C2 symmetry.
Figure 1. View of the molecular structure (50% thermal ellipsoid
probability) of A showing the gauche conformation of the central O2-
NMotMoNO2 core.
CHMe∼∼OH this compound was transformed to A and B.
Thus, it was abundantly apparent that the formation of A and
B occurs by separate reaction pathways from a common inter-
mediate, Mo2(NMe2)4(µ-O∼∼CHMe∼∼O), and that the forma-
tion of the second µ-diolate bridge determines the mode of
construction of the isomers.
The reaction between W2(NMe2)6 and HO∼∼CHMe∼∼OH
(2 equiv) under similar conditions gives, quite remarkably, a
single product W2(NMe2)2(η2-O∼∼CHMe∼∼O)2 in greater
than 60% yield. This is remarkable not only in comparison to
the reactions of the related Mo2(NMe2)6 described above but
because in the reaction involving the related HO∼∼CH2∼∼OH
biphenol C-H activation and oxidative addition to the ditung-
sten center occurs. This is a reversible process, as shown in eq
3.1
The addition of pyridine to a mixture of the two isomers A
and B of formula Mo2(NMe2)2(µ-O∼∼CHMe∼∼O)2 in hydro-
carbon solvents gave upon heating a new compound C along
with unreacted B. As shown in the Experimental Section, we
can isolate A and B in pure forms and thus separately establish
that only A is isomerized to C upon heating in the presence of
pyridine. Compound C showed a more simple 1H NMR
spectrum wherein there were two t-Bu singlets of equal intensity,
one CH and one CHMe resonance (a quartet and doublet,
respectively) and proximal and distal NMe signals. By inference
from our earlier work employing the related biphenol
HO∼∼CH2∼∼OH,1 the isomer C could reasonably be formu-
The addition of neutral ligands (HNMe2, pyridine) to W2-
(NMe2)2(η2-O∼∼CHMe∼∼O)2 resulted in no similar C-H
activation.
W2(µ-H)(NMe2)2(η2-O∼∼CH2∼∼O)-
(η3-O∼∼CH∼∼O)L h
W2(NMe2)2(η2-O∼∼CH2∼∼O)2 + L (3)
Solid State and Molecular Structures. The three isomeric
forms of Mo2(NMe2)2(O∼∼CHMe∼∼O)2 have been crystal-
lographically characterized. Compounds A and B contain
gauche O2NMo-MoNO2 cores with virtual (but not crystallo-
graphically imposed) C2 symmetry, as shown in Figures 1 and
2. The bridging µ-O∼∼CHMe∼∼O ligands form nine-
membered rings, and in both isomers the phenyl blades are
roofed or held in a V shape by the bridging tetrahedral CHMe
carbon. In both isomers the CHMe proton is forced in close
proximity to the MtM bond which accounts for its extreme
deshielding, δ 7.00 pm. The Mo to CHMe carbon distances in
A are 3.59 and 3.69 Å and in B are 3.42 and 3.84 Å. The
CHMe protons in A are directed toward phenolic oxygen atoms
and those in B toward the nitrogen atoms of the amide ligands.
Though the conformation of the nine-membered ring is not
rigid (except on the NMR time scale), it should be recognized
that invertomers of A and B lead to two different molecules
lated as being
a
chelating isomer Mo2(NMe2)2(η2-
O∼∼CHMe∼∼O)2 having either the anti NO2Mo-MoO2N core
or one wherein anti h gauche isomerization was fast on the
NMR time scale.
The detailed nature of isomers A and B could not be inferred
from the NMR data, nor was it obvious why only one isomer,
isomer A, could be transformed to isomer C.
1
We did investigate by H NMR spectroscopy the reaction
between Mo2(NMe2)6 and less than 1 equiV of HO∼∼
CHMe∼∼OH in a J. Young NMR tube. Here we observed,
along with the residual Mo2(NMe2)6, only one bridging com-
pound that could be reasonably formulated as Mo2(NMe2)4(µ-
O∼∼CHMe∼∼O). With further addition of HO∼∼
(6) The PES of W2(NMe2)2(OR)4 compounds reveals the changes in IP’s
of the NMe2 lone pairs: Chisholm, M. H.; Tiedtke, D. B.; Gruhn, N.
E.; Lichtenbeger, D. L. Polyhedron, in press.

52 Inorganic Chemistry, Vol. 37, No. 1, 1998
Chisholm et al.
Figure 4. View of the molecular structure (50% thermal ellipsoid
probability) of D and showing different disposition of the phenyl blades.
Table 1. Selected Distances (Å) and Angles (deg) of Compound A
Mo1-Mo2
Mo1-O9
Mo1-O41
Mo2-O23
Mo2-O23
Mo2-O55
Mo2-N6
O9-C10
2.2583(7)
1.925(5)
1.937(3)
1.948(2)
1.948(2)
1.919(3)
1.922(6)
1.361(7)
O23-C22
O41-C42
O55-C54
N3-C4
1.365(2)
1.383(5)
1.381(1)
1.466(4)
1.451(6)
1.444(9)
1.454(5)
N3-C5
Figure 2. View of the molecular structure (50% thermal ellipsoid
probability) of B showing the different CHMe protons direction from
that in A. The disorder in one of the tert-butyl group methyl carbon
atoms is shown.
N6-C7
N6-C8
Mo2-Mo1-O9
Mo2-Mo1-O41
Mo2-Mo1-N3
O9-Mo1-O41
O9-Mo1-N3
O41-Mo1-N3
Mo1-Mo2-O23
Mo1-Mo2-O55
Mo1-Mo2-N6
O23-Mo2-N6
105.4(1)
109.6(1)
103.3(1)
113.9(1)
115.0(2)
109.0(2)
109.1(1)
105.1(1)
103.7(1)
108.7(1)
O23-Mo2-N6
O55-Mo2-N6
Mo1-O9-C10
Mo1-O41-C42
Mo1-N3-C4
Mo1-N3-C5
Mo2-O23-C22
Mo2-O55-C54
Mo2-N6-C7
Mo2-N6-C8
108.7(1)
115.4(1)
153.8(2)
133.2(3)
114.0(3)
134.9(2)
133.7(2)
149.6(4)
134.6(3)
113.8(4)
Table 2. Selected Distances (Å) and Angles (deg) of Compound B
Mo1-Mo2
Mo1-O9
Mo1-O41
Mo1-N3
Mo2-O23
Mo2-O55
Mo2-N6
O9-C10
2.2486(10)
1.916(5)
1.927(5)
1.926(6)
1.916(5)
1.923(5)
1.919(6)
1.376(8)
O23-C22
O41-C42
O55-C54
N3-C4
1.367(9)
1.354(8)
1.376(8)
1.48(1)
1.44(1)
1.46(1)
1.48(1)
N3-C5
N6-C7
N6-C8
Figure 3. View of the molecular structure (50% thermal ellipsoid
probability) of C showing the roof shape of the O∼∼CHMe∼∼O
ligand directed away from the Mo-Mo bond.
Mo2-Mo1-O9
Mo2-Mo1-O41
Mo2-Mo1-N3
O9-Mo1-O41
O9-Mo1-N3
O41-Mo1-N3
O41-Mo1-N3
Mo1-Mo2-Ott
Mo1-Mo2-N6
O23-Mo2-O55
110.3(2)
111.1(2)
98.8(2)
113.9(2)
106.5(2)
115.2(2)
109.9(2)
109.0(2)
99.0(2)
O23-Mo2-N6
O55-Mo2-N6
Mo1-O9-C10
Mo1-O41-C42
Mo1-N3-C4
Mo1-N3-C5
Mo2-O23-C22
Mo2-O55-C54
Mo2-N6-C7
Mo2-N6-C8
115.3(2)
109.2(2)
143.6(4)
166.6(5)
112.5(5)
135.4(5)
166.8(5)
140.7(4)
135.1(5)
115.3(5)
wherein the CHMe group would have to be disposed toward
the MotMo center. Molecules A and B are therefore diaster-
eomers and arise because of the position of the CHMe group
upon ring closure. In A the phenyl groups engulf the NMe2
ligands which presumably accounts for the upfield shift of the
1
NMe signals in the H NMR spectra.
The chelate isomer C is shown in Figure 3 and is closely
related to that seen previously for Mo2(NMe2)2(η2-
O∼∼CH2∼∼O)2. It may be viewed as being related by the
substitution of the distal ∼CHpHd∼ carbon-hydrogen bond by
a distal Me group ∼CHpMed∼. Clearly this conformation is
favored on steric grounds since a proximal CHMe group would
cause internal steric congestion.
The tungsten compound D does, however, differ from its
molybdenum analogue C with respect to the disposition of the
phenyl blades, as is shown in Figure 4. One phenyl blade in a
diolate ligand is nearly parallel to the WtW bond while the
other is nearly perpendicular to it.
113.5(2)
without Mo-O bond dissociation. What was not so im-
mediately obvious to us was why A is transformed to C in
pyridine but B is not. However, if we assume that the role of
pyridine is one of association, as was seen in the earlier study,
then its role is to facilitate terminal to bridge Mo-O bond
interconversions. Again, no bonds are actually ever broken in
a dissociative sense; merely terminal to bridge to terminal site
exchange is promoted by the coordinated pyridine. In this
manner isomer A can be converted to isomer C while B cannot.
If B were to adopt a chelate form related to C, then the CHMe
group would be proximal to the Mo2 center and this would be
sterically disfavored (the Mo to CHMe proton distance in C is
2.37 Å). However, since the η2-O∼∼CHMe∼∼O chelate eight-
membered ring is not rigid, the central CHMe group could be
Listings of selected bond distances and bond angles for the
isomers A-C and D are given in Tables 1 through 4.
Interconversions Involving A-C. Careful considerations
of the structures of A and B reveal that they cannot interconvert

Dinuclear (d3-d3) Diolate Complexes of Mo and W
Inorganic Chemistry, Vol. 37, No. 1, 1998 53
Table 3. Selected Distances (Å) and Angles (deg) of Compound C
Mo1-Mo2
Mo1-O9
Mo1-O23
Mo1-N3
Mo2-O41
Mo2-O55
Mo2-N6
O9-C10
2.2201(7)
1.944(4)
1.965(3)
1.911(5)
1.943(4)
1.953(4)
1.916(4)
1.364(7)
O23-C22
O41-C42
O55-C54
N3-C4
1.374(6)
1.379(6)
1.371(6)
1.462(7)
1.479(8)
1.470(8)
1.467(7)
N3-C5
N6-C7
N6-C8
Mo2-Mo1-O9
Mo2-Mo1-O23
Mo2-Mo1-N3
O9-Mo1-O23
O9-Mo1-N3
O23-Mo1-N3
Mo1-Mo2-O41
Mo1-Mo2-O55
Mo1-Mo2-N6
O41-Mo2-O55
102.1(1)
94.3(1)
106.0(1)
129.1(1)
109.5(2)
111.5(2)
91.2(1)
101.4(1)
106.2(1)
130.0(2)
O41-Mo2-N6
O55-Mo2-N6
Mo1-O9-C10
Mo1-O23-C22
Mo1-N3-C4
Mo1-N3-C5
Mo2-O41-C42
Mo2-O55-C54
Mo2-N6-C7
Mo2-N6-C8
112.5(2)
109.9(2)
132.6(3)
102.7(3)
115.2(4)
133.3(4)
124.4(3)
121.4(3)
135.1(4)
114.3(4)
Figure 5. View of the molecular structure (50% thermal ellipsoid
probability) of W2(H)(NMe2)2(η2-O∼∼CH2∼∼O)(η3-O∼∼CH∼∼O)-
(py) showing the η3-O∼∼CH∼∼O ligand bond to W2.
the first ring then determines that of the second to give the
sterically favored chelate isomer D.
Table 4. Selected Distances (Å) and Angles (deg) of Compound D
It could, however, be that W2(NMe2)6 reacts with HO∼∼
CHMe∼∼OH to give a kinetically labile product W2(H)-
(NMe2)2(η2-O∼∼CHMe∼∼O)(η3-O∼∼CMe∼∼O)(HNMe2)
which then converts to W2(NMe2)2(η2-O∼∼CHMe∼∼O)2.
Possible evidence for this notion comes from the observation
that the crude reaction mixture containing the products from
the reaction between W2(NMe2)6 and 2 equiv of HO∼∼
CHMe∼∼OH obtained from removal of the solvent under a
dynamic vacuum is a brown foamy substance which shows a
W1-W2
W1-O3
W1-O17
W1-N67
W2-O35
W2-O49
W2-N70
O3-C4
2.3363(4)
1.942(1)
1.964(2)
1.927(3)
1.935(2)
1.964(2)
1.917(1)
1.369(1)
O17-C16
O35-C36
O49-C48
N67-C68
N67-C69
N70-C71
N70-C72
1.377(6)
1.370(3)
1.396(6)
1.471(7)
1.443(3)
1.476(7)
1.468(6)
W2-W1-O3
W2-W1-O17
W2-W12-N67
O3-W1-O17
O3-W1-N67
O17-W1-N67
W1-W2-O35
W1-W2-O49
W1-W2-N70
O35-W2-O49
105.6(1)
101.6(1)
101.3(1)
134.2(1)
101.3(1)
108.6(1)
108.9(1)
101.6(1)
102.8(1)
130.2(1)
O35-W2-N70
O49-W2-N70
W1-O3-C4
W1-O17-C16
W1-N67-C68
W1-N67-C69
W2-O35-C36
W-O49-C48
W2-N70-C71
W2-N70-C72
101.2(1)
109.5(1)
136.3(2)
104.9(2)
113.3(1)
134.8(4)
136.3(1)
103.4(2)
113.4(2)
135.2(3)
1
complicated H NMR spectrum. However, recrystallization
from hydrocarbon solvents yields a clean orange crystalline
product W2(NMe2)2(η2-O∼∼CHMe∼∼O)2. In toluene-d8 this
product shows no reactivity toward added pyridine-d5 of the
type shown in eq 3.1 Given the close proximity of the ∼CHMe
hydrogen atom to the W26+ center, one must surely wonder why
C-H activation does not occur in this compound. A possible
clue to this may be found in the structure of the complex W2-
(H)(NMe2)2(η2-O∼∼CH2∼∼O)(η3-O∼∼CH∼∼O)(py) shown
in Figure 5. (The distance between N56 and the hydrogen
related to C35 is 2.67 Å). If the product of CH activation, W2-
(H)(NMe2)2(η2-O∼∼CHMe∼∼O)(η3-O∼∼CMe∼∼O)(py), had
a related structure, the position of the methyl group of the
metallated η3-O∼∼CMe∼∼O ligand would cause unfavorable
internal steric congestion. Thus, an equilibrium of the type
shown in eq 3 might occur but merely lie in favor of W2(NMe2)2-
(η2-O∼∼CHMe∼∼O)2. The stereochemical consequences of
CH activation with respect to bridged isomers of types A and
B are not known. Further speculation is not profitable.
disposed in an anti manner with respect to the Mo2 moiety. But,
in this conformation the phenyl blades of the biphenoxide are
directed inwards (with respect to the Mo2 center) and now the
t-Bu groups at each end of the molecule interact in a most
unfavorable manner.
What about W2(NMe2)2(η2-O∼∼CHMe∼∼O)2? We are
faced with the puzzling situation that the reaction between W2-
(NMe2)6 and 2 equiv of HO∼∼CHMe∼∼OH proceeds directly
to one isomer D, which is an analogue of C described above.
How is it that the ditungsten center avoids the bridged analogues
A and B and why does it not undergo C-H activation of the
type seen previously? Inspection of the structure of D reveals
a short W‚‚‚H-C interaction, as indicated by the W to CHMe
carbon distance of 2.90 Å. It is always difficult to say why
things do not happen and sometimes dangerous to do so because
maybe they do but we do not see them. So we merely speculate
upon the following.
The WtW bond is ca. 0.08 Å longer than the MotMo bond
in related complexes.7 It is perhaps just this extra distance that
disfavors the second NMe2 substitution across the bond and
favors attack at the same metal center. Ring closure would then
occur only to give the chelate isomer, and proton NMR
spectroscopic data indicate that the reaction between W2(NMe2)6
and ca. 1 equiv of HO∼∼CHMe∼∼OH forms, in greater than
90% yield, a chelate isomer formulated as W2(NMe2)4(η2-
O∼∼CHMe∼∼O). The stereochemistry of the formation of
Concluding Remarks
The reactions between Mo2(NM2)6 and HO∼∼CHMe∼∼OH
(2 equiv) reveal that the initial mode of chelation of the diolate,
or ring closure with elimination of HNMe2, occurs across the
Mo-Mo bond. The second substitution and ring closure can
occur in one of two ways so that two isomers A and B are
formed competitively. Isomerization is not possible without
Mo-O bond cleavage, and this does not take place below 100
°C. It is interesting to note that an isomer having a mixed
arrangement of the µ-diolate ligands is not observed.
The addition of pyridine allows the isomerizaton of only A
to the chelate isomer C, which is with respect to A the
thermodynamic isomer. Isomer B cannot rearrange to C without
Mo-O bond cleavage, and a chelate form derived from B is
presumably not favored because of steric factors.
The reaction of W2(NMe2)6 with the HO∼∼CHMe∼∼OH
(7) Cotton, F. A.; Walton, R. A. Multiple Bonds between Metal Atoms,
2nd Ed.; Oxford University Press: Oxford, U.K., 1993.
biphenol (2 equiv) to give only the chelate isomer D is quite

54 Inorganic Chemistry, Vol. 37, No. 1, 1998
Chisholm et al.
Table 5. Summary of Crystal Data
A‚PhMe
B‚2PhH
C‚1.5Et2O
D
chem formula
fw
space group
T (°C)
a (Å)
b (Å)
c (Å)
C71H108Mo2N2O4
1245.53
P1h
C76H112Mo2N2O4
1309.62
P1h
C70H115Mo2N2O5.5
1264.58
P1h
C64H100N2O4W2
1329.19
P1h
-168
-168
-168
-168
16.585(5)
19.480(6)
10.960(3)
98.39(2)
103.19(1)
91.44(2)
3404.30
2
1.215
0.710 69
4.137
0.0579
0.0554
15.167(3)
18.210(3)
13.964(3)
92.81(1)
100.40(1)
71.16(1)
3590.11
2
1.211
0.710 69
3.957
0.0589
0.0556
16.483(3)
16.817(3)
14.305(2)
106.13(1)
108.80(1)
71.70(1)
3494.14
2
1.202
0.710 69
4.055
0.0503
0.0553
14.638(2)
21.188(4)
10.273(2)
94.80(1)
95.05(1)
100.85(1)
3100.75
2
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
F
calcd (g cm-3
)
1.424
wavelength λ (Å)
0.710 69
37.563
0.0280
0.0325
µ (cm-1
R(F)a
)
Rw(F)b
2
a R ) ∑||Fo| - |Fc||/∑|Fo|. b Rw ) {∑w(|Fo| - |Fc|)2/∑wFo }1/2, where w ) 1/[σ2(|Fo|)].
1H NMR data (C6D6): δ 7.87 (d, aromatic protons, 2H, JH-H ) 2.4
Hz), 7.54 (d, aromatic protons, 4H, JH-H ) 2.4 Hz), 7.39 (d, aromatic
protons, 2H, JH-H ) 2.4 Hz), 7.16 (q, CHMe, 2H, JH-H ) 6.9 Hz),
3.87 (s, NMe, 6H), 2.58 (s, NMe,6H), 1.73 (d, CHMe, 6H, JH-H ) 2.4
Hz), 1.55 (s, CMe3, 18H), 1.48 (s, CMe3, 18H), 1.37 (s, CMe3, 18H),
1.26 (s, CMe3, 18H). 13C NMR data (C6D6): δ 165.0, 159.5, 143.9,
142.5, 138.4, 138.3, 136.9, 132.4, 124.6, 122.0, 121.7, 121.0 (phenyl
carbon), 60.2 (NMe), 43.5 (CHMe), 36.5 (CMe3), 35.7 (CMe3), 35.3
(CMe3), 34.9 (NMe), 32.4 (CMe3), 31.3 (CMe3), 31.1 (CMe3), 27.0
(CHMe). Anal. Calcd for C64H100N2O4Mo2‚C7H8: C, 68.47; H, 8.74;
N, 2.43. Found: C, 68.32; H, 8.66; N, 2.33.
remarkable. It is possible that subtle differences in the
substitution reaction pathways of the Mo2(NMe2)6 and W2-
(NMe2)6 may allow for the direct formation of the chelate
complex W2(NMe2)2(η2-O∼∼CHMe∼∼O)2, D, an analogue of
C, without the formation of kinetic bridged (µ-O∼∼CHMe∼∼O)
containing compounds. Alternatively a reversible CHMe met-
allation reaction may lead to ligand isomerization via an (η3-
O∼∼CMe∼∼O) complex (or complexes) which is (are) un-
stable with respect to the W2(NMe2)2(η2-O∼∼CHMe∼∼O)2
complex because of steric factors. It does not seem to be
possible to distinguish between the latter possibilities at this
time.
Mo2(NMe2)2(µ-O∼∼CHMe∼∼O)2, B. The above filtrate was
stripped under a dynamic vacuum, and the residue was washed with
10 mL of hexane in two portions and then dried in vacuum. A mixture
of A and B at a ratio of ca. 1:4 was isolated (A and B total yield:
65%, 386 mg). 110 mg amount of mixture was suspended in 6 mL of
pyridine, and the mixture was gently refluxed for 20 min. After cooling,
the solvent was removed under a dynamic vacuum. The residue was
washed with 8 mL of hexane and then dried under vacuum to give a
yellow powder of pure B (76 mg). Orange crystals of B‚2PhH suitable
for X-ray analysis were obtained by slowly cooling the benzene solution.
1H NMR data (C6D6): δ 7.97 (d, aromatic protons, 2H, JH-H ) 2.1
Hz), 7.56 (d, aromatic protons, 2H, JH-H ) 2.1 Hz), 7.50 (d, aromatic
The present work emphasizes how the use of chelating ligands
at dimetal centers may allow insight into detailed reaction
pathways of substitution chemistry. It is also interesting to note
4+
that in substitution reactions at Mo2 centers with M-M
quadruple bonds chelating isomers are often kinetically formed
and then transformed to the thermodynamic bridged isomers
when the entering ligands are ethylene linked -diphosphines or
-diarsines.7 The mechanisms of their interconversions have been
discussed in terms of an internal flip rearrangement, 8 which is
in effect related to the non-dissociative isomerizations described
herein.
protons, 2H, JH-H ) 2.1 Hz), 7.31 (d, aromatic protons, 2H, JH-H
)
2.1 Hz), 6.99 (q, CHMe, 2H, JH-H ) 7.2 Hz), 4.30 (s, NMe, 6H), 2.98
(s, NMe,6H), 1.68 (d, CHMe, 6H, JH-H ) 7.2 Hz), 1.58 (s, CMe3, 18H),
1.39 (s, CMe3, 18H), 1.32 (s, CMe3, 18H), 1.03 (s, CMe3, 18H). 13C-
{1H} NMR data (C6D6): δ 162.6, 158.1, 143.7, 142.7, 137.8, 137.7,
135.5, 134.7, 124.1, 122.5, 121.5 (phenyl carbon), 58.5 (NMe), 46.6
(CHMe), 35.9 (CMe3), 35.8 (CMe3), 35.3 (CMe3), 32.5 (CMe3), 32.3
(CMe3), 30.9 (CMe3), 30.5 (NMe), 30.2 (CMe3), 24.6 (CHMe). Anal.
Calcd for C64H100N2O4Mo2‚C6H6: C, 68.27; H, 8.67; N, 2.27. Found:
C, 67.95; H, 8.86; N, 2.34.
Mo2(NMe2)2(η2-O∼∼CHMe∼∼O)2, C. Pure A (105 mg, 0.09
mmol) in 8 mL of pyridine was gently refluxed for 20 min, and the
solvents were removed under a dynamic vacuum to yield a brown
powder of pure C (yield: 90%, 94 mg). Amber crystals of C‚1.5Et2O
suitable for X-ray analysis were obtained from slow evaporation of
the ether solution. 1H NMR data (C6D6): δ 7.67 (d, aromatic protons,
4H, JH-H ) 2.4 Hz), 7.46 (d, aromatic protons, 4H, JH-H ) 2.4 Hz),
5.90 (q, CHMe, 2H, JH-H ) 6.3 Hz), 4.37 (s, NMe, 6H), 1.73 (d, CHMe,
6H, JH-H ) 6.3 Hz), 1.72 (s, CMe3, 36H), 1.49 (s, NMe, 6H), 1.34 (s,
CMe3, 36H). 13C{1H} NMR data (C6D6): δ 154.8, 143.4, 139.7, 134.8,
123.0, 122.4 (phenyl carbon), 59.0 (NMe), 39.9 (CHMe), 36.4 (CMe3),
35.0 (CMe3), 32.5 (NMe), 32.2 (CMe3), 30.6 (CMe3), 26.9 (CHMe).
Anal. Calcd for C64H100N2O4Mo2: C, 66.67; H, 8.74; N, 2.43.
Found: C, 67.01; H, 8.84; N, 2.20.
Experimental Section
All reaction manipulations were carried out in a glovebox or Schlenk
line under a nitrogen atmosphere. Solvents were dried by distillation
over sodium dispersion/benzophenone under a nitrogen atmosphere.
1H and 13C{1H} NMR spectra were recorded on a Varian XL-300 NMR
spectrometer and referenced to the residual protio impurities of the
deuterated benzene. M2(NMe2)6 compounds were prepared as described
elsewhere.9 2,2′-Ethylidenebis(4,6-di-tert-butylphenol) was purchased
from Aldrich Chemical Co., Inc.
Mo2(NMe2)2(µ-O∼∼CHMe∼∼O)2, A. A 100 mL Schlenk flask
was charged with Mo2(NMe2)6 (456 mg, 1.0 mmol) and biphenol (880
mg, 2.0 mmol). To the mixture was added 30 mL of toluene. After
the resultant solution was stirred overnight, pure A was deposited as a
yellow precipitate, which was collected by filtration and dried under
vacuum (yield: 31%, 363 mg). Orange crystals of A‚PhMe suitable
for X-ray analysis were obtained by slowly cooling the toluene solution.
(8) (a) Cotton, F. A.; Kitagawa, S. Polyhedron 1988, 7, 463. (b) McVilie,
A.; Peacock, R. D. Polyhedron 1992, 11, 2531. (c) Agaskar, P. A.;
Cotton, F. A. Inorg. Chem. 1986, 25, 15. (d) Cayton, R. H.; Chisholm,
M. H. Inorg. Chem. 1991, 30, 1422.
(9) (a) M ) Mo: Chisholm, M. H.; Haitko, D. A.; Murrillo, C. A. Inorg.
Synth. 1982, 21, 5. (b) M ) W: Chisholm, M. H.; Martin, J. D. Inorg.
Synth. 1992, 29, 137.
W2(NMe2)2(η2-O∼∼CHMe∼∼O)2, D. A 30 mL Schlenk flask was
charged with W2(NMe2)6 (320 mg, 0.5 mmol) and biphenol (441 mg,

Dinuclear (d3-d3) Diolate Complexes of Mo and W
Inorganic Chemistry, Vol. 37, No. 1, 1998 55
1.0 mmol). To the mixture was added 10 mL of toluene. After the
resultant solution was stirred for 6 h, then the volatile components were
removed under a dynamic vacuum to give a foamy solid, which was
dissolved in 5 mL of hexane. Pure D was obtained as a brown
microcrystalline solid (yield: 66%, 440 mg). Orange crystals of D
suitable for X-ray analysis were obtained by slow evaporation of the
hexane solution. 1H NMR data (C6D6): δ 7.63 (d, aromatic protons,
4H, JH-H ) 2.4 Hz), 7.49 (d, aromatic protons, 4H, JH-H ) 2.4 Hz),
5.26 (q, CHMe, 2H, JH-H ) 6.3 Hz), 4.43 (s, NMe, 6H), 1.71 (s, CMe3,
36H), 1.66 (d, CHMe, 6H, JH-H ) 6.3 Hz), 1.54 (s, NMe, 6H), 1.33 (s,
CMe3, 36H). 13C{1H} NMR data (C6D6); δ153.1, 143.7, 139.9, 134.1,
122.8 (phenyl carbon), 60.6 (NMe), 37.1 (CHMe), 36.4 (CMe3), 35.0
(CMe3), 33.0 (NMe), 32.2 (CMe3), 30.6 (CMe3), 26.6 (CHMe). Anal.
Calcd for C64H100N2O4W2: C, 57.83; H, 7.58; N, 2.11. Found: C,
58.12; H, 7.69; N, 1.92.
7.37 (d, aromatic protons, 2H, JH-H ) 2.4 Hz), 4.69 (q, CHMe, 2H,
JH-H ) 7.2 Hz), 1.70 (d, CHMe, 6H),1.53 (s, CMe3, 18H), 1.34 (s,
CMe3, 18H).
Crystallographic Studies. General operating procedures and a
listing of programs have been given previously.10 A summary of crystal
data is given in Table 5.
Full details of the four structural determination are available via the
Internet as indicated in the Supporting Information.
Acknowledgment. We thank the National Science Founda-
tion for financial support.
Supporting Information Available: Tables giving a summary of
data collection, atomic coordinates, and complete listings of bond
distances and angles for A-D (41 pages). Ordering information is
given on any current masthead page. The complete structural reports
are available from the reciprocal data base via Internet at url http://
www.iumsc.indiana.edu. For A‚PhMe request Report No. 97036; for
B‚2PhH, Report No. 97043; for C‚1.5Et2O, Report No. 97048; and for
D, Report No. 96210.
Selected 1H NMR data, as determined by an NMR tube reaction
(C6D6): for Mo2(NMe2)4(µ-O∼∼CHMe∼∼O), δ 7.91 (d, aromatic
protons, 1H, JH-H ) 2.4 Hz), 7.58 (d, aromatic protons, 1H, JH-H
)
2.4 Hz), 7.51 (d, aromatic protons, 1H, JH-H ) 2.4 Hz), 7.46 (d,
aromatic protons, 1H, JH-H ) 2.4 Hz), 7.07 (q, CHMe, 1H, JH-H
)
IC9708430
7.2 Hz), 1.71 (d, CHMe, 3H, JH-H ) 2.4 Hz), 1.70 (s, CMe3, 9H), 1.58
(s, CMe3, 9H), 1.36 (s, CMe3, 9H), 1.29 (s, CMe3, 9H); for W2(NMe2)4-
(η2-O∼∼CHMe∼∼O), 7.50 (d, aromatic protons, 2H, JH-H ) 2.4 Hz),
(10) Chisholm, M. H.; Folting, K.; Huffman, J. C.; Kirkpatrick, C. C. Inorg.
Chem. 1984, 23, 1021.