Synthesis and structural characterization of a 4-coordinate molybdenum(VI) dioxo diaryloxide, MoO2(O-2,6-t-Bu2C6H3)2·Ho-2,6-t-Bu2C6H3

Article abstract of DOI:10.1021/ic991271h

The facile solution synthesis and spectroscopic and structural characterization of 4-coordinate MoO₂(OAr)₂. ArOH (1) where Ar = 2,6-di-tert-butylphenyl are described. MoO₂(O-2,6-t-Bu₂C₆H₃)₂·HO-2,6-t-Bu₂C₆H₃ (1) was synthesized by addition of LiOAr to MoO₂Cl₂ in CH₃CN. Yields are reduced by concurrent oxidation of the phenolic anion to form 3,3',5,5'-tetra-tert-butyl-4,4'-diphenoquinone (4). Relatively strong Mo=O double bonds and p-electron donation from the aryloxide oxygens appear to compensate for the unsaturation of the pseudotetrahedral metal center.

Full text of DOI:10.1021/ic991271h

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Inorg. Chem. 2000, 39, 630-631
Synthesis and Structural Characterization of a 4-Coordinate Molybdenum(VI) Dioxo Diaryloxide,
MoO2(O-2,6-t-Bu2C6H3)2‚HO-2,6-t-Bu2C6H3
Tracy A. Hanna,*,1 Christopher D. Incarvito,2 and Arnold L. Rheingold*,2
Department of Chemistry, Texas Christian University, Fort Worth, Texas 76109, and Department of Chemistry and Biochemistry,
University of Delaware, Newark, Delaware 19716
ReceiVed October 27, 1999
High oxidation state metal alkoxides, aryloxides, and oxoalkox-
ides are interesting because they can be used to produce high-
purity metal oxide materials in chemical vapor deposition and
sol-gel processes. Metal alkoxides have also been cited as models
for heterogeneous catalysts, precursors for supported materials,
and modifiable homogeneous catalysts.3,4 Few molybdenum(VI)
oxoalkoxides have been reported, however, that are suitable for
such uses.
2+
In Mo(VI) structural chemistry, the MoO2 unit has by far
received the most attention. Mo(VI) dioxo compounds are viewed
as models for the active sites of oxo-transfer molybdoenzymes5,6
and for heterogeneous catalysts,4 as well as materials precursors.3
The predominant structural types found for these compounds are
whether the diphenoquinone 4 was a decomposition product of 1
or originating from a side reaction, compound 1 was heated in
octahedral and pseudo-octahedral. Some examples of 6-coordinate
nonoctahedral complexes have been reported, as well as a few
C6D6 to 140 °C. Although black solid was slowly formed, no
5-coordinate complexes.7,8 Four-coordinate MO2(OAr)2 complexes
1
diphenoquinone formation was observed by H NMR.
have not been structurally characterized for M ) Mo, Cr, or W.
A search of the Cambridge Structural Database9 reveals only one
molybdenum dioxo compound of the formula MoO2(OR)2: MoO2-
The source of the 2,6-di-tert-butylphenol in the crystals of 1
appears to be the lithium salt 2 acting as a hydrogen acceptor in
the formation of diphenoquinone. If 1 equiv of 2,6-di-tert-
butylphenol is added to the reaction mixture, the formation of
diphenoquinone 4 is inhibited, though the yield of the desired
product is not substantially improved. The same effect is observed
if the reaction is done in the presence of trace water. Interestingly,
in the synthesis of MoO2(OSiPh3)2 the authors noted that a small
amount of acetonitrile was necessary to avoid the formation of
byproduct (hexamethylsiloxane), possibly due to saturation of the
inner sphere of the Mo dioxo complex.10 In our case a similar
explanation is unlikely to hold, as 2,6-di-tert-butylphenol is clearly
too large to fit into the coordination sphere, and acetonitrile itself
does not have the same inhibition effect.
(OSiPh3)2.10 The tetrahedral structure is known for the MoO4
2-
ion, and MoO2X2 (X ) halide) is tetrahedral in the gas phase.7,8
We have synthesized and structurally characterized a second
example of a 4-coordinate Mo(VI) dioxo species, and a first
example of a structurally characterized MO2(OAr)2 species.
Workers attempting to produce MoO2(OR)2 where R ) bulky
alkyl or aryl have encountered difficulties stabilizing the resulting
compounds in the absence of a donating nitrogen base,11 leading
to the use of cumbersome metal vapor synthetic methods.12 In
contrast to these esoteric techniques, we describe the facile
solution synthesis and spectroscopic and structural characterization
of 4-coordinate MoO2(OAr)2‚ArOH (1) where Ar ) 2,6-di-tert-
butylphenyl.
X-ray quality crystals of 1 were obtained by recrystallization
from pentane. An ORTEP drawing is shown in Figure 1, and
selected bond distances and angles are outlined in Tables 1 and
2.14 The molecular structure of 1 shows a distorted tetrahedral
coordination geometry.
MoO2Cl2 reacts quickly with 2 equiv of LiO-2,6-t-Bu2C6H3
(2) in acetonitrile to form a deep red solution (eq 1). Proton NMR
of the crude reaction mixture reveals three major components:
MoO2(O-2,6-t-Bu2C6H3)2‚ArOH (1),13 HO-2,6-t-Bu2C6H3 (3), and
3,3′,5,5′-tetra-tert-butyl-4,4′-diphenoquinone (4). Repeated crys-
tallization from pentane yields pure orange/yellow crystals of 1
suitable for X-ray diffraction, in 31% yield. In order to determine
The Mo-O bond angles of 1 are similar to those of MoO2-
(OSiPh3)2. All the angles are close to tetrahedral values. The
(13) MoO2Cl2 (117.5 mg, 0.5909 mmol) and 260.7 mg of LiO(2,6-t-Bu2C6H3)
(1.228 mmol) were each dissolved in 5 mL of CH3CN. The MoO2Cl2
solution was added to the LiOAr solution with stirring to cause an
immediate color change to deep red. The mixture was stirred for 1 h,
and then the solvent was removed in vacuo. The product was brought
up in approximately 50 mL of pentane, leaving black undissolved
material, filtered to remove residual solid, and placed in a -35 °C freezer
for 2 days. The resulting solid was recrystallized from pentane to produce
94.4 mg of air-sensitive orange blocky crystals of MoO2(O-2,6-t-
Bu2C6H6)2‚HO-2,6-t-Bu2C6H3 (126.9 mmol, 31.0% yield): mp 124-
125 °C. IR (C6H6): 3636 m, 2961 s, 2874 w, 1466 w, 1426 m, 1402 s,
1395 m, 1366 w, 1264 w, 1231 w, 1206 m, 1186 s, 1119 s, 1109 m, 963
s, 941 s (ModO), 911 s (ModO), 882 w, 795 w, 748 m, 698 w, 685 s.
1H NMR (thf-d8) δ 7.32 (d, 2H, J ) 7.88 Hz), 6.98 (t, 1H, J ) 7.87
Hz), 1.50 (s, 18H). 13C NMR (C6D6) δ 31.8 (tBu CH3), 35.4 (tBu quat),
124.4 (p to O), 126.1 (m to O), 140.0 (aryl quat), 164.4 (C-O) (ArOH
NMR peaks not listed). UV-vis (C6H6): λmax ) 278 (ꢀ 7.9 × 103), 346
(ꢀ 6.1 × 103). Anal. Calcd for C42H64MoO5: C, 67.72; H, 8.66. Found:
C, 67.61; H, 8.82.
(1) Texas Christian University.
(2) University of Delaware.
(3) Hubert-Pfalzgraf, L. G. New J. Chem. 1987, 11, 663-675.
(4) Chisholm, M. H. In Inorganic Chemistry: Toward the 21st Century;
Chisholm, M. H., Ed.; ACS Symposium Series 211; American Chemical
Society: Washington, DC, 1983; pp 243-268.
(5) Berg, J. M.; Holm, R. H. J. Am. Chem. Soc. 1984, 106, 3035-3036.
(6) Berg, J. M.; Holm, R. H. J. Am. Chem. Soc. 1985, 107, 917-925.
(7) Stiefel, E. I. In ComprehensiVe Coordination Chemistry; Wilkinson, G.,
Ed.; Pergamon Press: New York, 1987; Vol. 3, pp 1375-1420.
(8) Pope, M. T. Prog. Inorg. Chem. 1991, 39, 181-257.
(9) Allen, F. H.; Kennard, O. Chem. Des. Autom. News 1993, 8, 31.
(10) Huang, M.; DeKock, C. W. Inorg. Chem. 1993, 32, 2287-2291.
(11) Chisholm, M. H.; Folting, K.; Huffman, J. C.; Kirkpatrick, C. C. Inorg.
Chem. 1984, 23, 1021-1037.
(12) Kim, G.-S.; Huffman, D.; DeKock, C. W. Inorg. Chem. 1989, 28, 1279-
1283.
10.1021/ic991271h CCC: $19.00 © 2000 American Chemical Society
Published on Web 02/04/2000

Communications
Inorganic Chemistry, Vol. 39, No. 4, 2000 631
the neighoring molybdenum atom, possibly incipient bond forma-
tion. Consistent with such an interpretation, the Mo(1)-O(2) and
Mo(1)-O(1) bond lengths (1.861(2) and 1.857(2) Å, respectively)
are significantly shorter than the analogous bond lengths in a
6-coordinate Mo dioxo diaryloxide species (5) (1.938(17) Å).15
In the coordinatively unsaturated MoO2(OSiPh3)2 the Mo-O
internal bond lengths are also relatively short, at 1.815(5) Å.10
The relatively short Mo-alkoxy bonds in our system could
indicate incipient conversion of the alkoxo to an oxo group: one
step in such a process must involve the shortening of the
molybdenum-alkoxy bond. The coordinative unsaturation of this
metal center favors such bond shortening and may decrease the
barrier to interconversion. Indeed, Mo and W oxo alkoxide
clusters such as Mo6O10(O-i-Pr)12 are fluxional on the NMR time
scale, indicating facile interconvertibility of the oxo and alkoxide
groups in coordinatively unsaturated systems.11
Figure 1. ORTEP diagram of MoO2(O-2,6-t-Bu2C6H3)2‚HO-2,6-t-
Bu2C6H3 (1). Thermal ellipsoids are shown at 30% probability; hydrogen
atoms, with the exception of H(1), are omitted for clarity.
Table 1. Selected Bond Lengths (Å)
Mo(1)-O(3)
Mo(1)-O(1)
O(1)-C(1)
1.703(2)
1.857(2)
1.405(4)
1.379(4)
Mo(1)-O(4)
Mo(1)-O(2)
O(2)-C(15)
1.706(2)
1.861(2)
1.392(4)
O(5)-C(29)
Table 2. Selected Bond Angles (deg)
The crowding of the coordination sphere in 1 is evidenced by
the solid-state distance between a terminal oxo group and the
nearest carbon of the adjacent tert-butyl group, ∼3.1 Å (O(4)-
C(27)), as compared to the estimated van der Waals contact for
an O-CH3 group interaction of 3.6 Å.16
The nature of the interaction between the phenol and the MoO2-
(OAr)2 unit is ambiguous. The distance between the phenolic
hydrogen and the closest Mo oxo group (O(4), shown by a dotted
line in Figure 1) is 2.327 (8) Å, and the angle is 115.5°. Although
the bond distance indicates that some hydrogen-bonding interac-
tion may be present, the similarity between the ModO bond
lengths (1.703(2) and 1.706(2) Å) in the solid state argues against
such interaction. H-bonding interaction between the free phenol
and the ModO units is almost certainly absent in the solution
state, as evidenced by the relatively high solution IR stretches
for the ModO bonds.
In conclusion, we have reported the first X-ray structure of a
4-coordinate group VIB metal(VI) dioxo diaryloxide complex.
The synthesis of MoO2(O-2,6-t-Bu2C6H3)2‚HO-2,6-t-Bu2C6H3 (1)
is facile, though yields are reduced by concurrent oxidation of
the phenolic anion to form 3,3′,5,5′-tetra-tert-butyl-4,4′-dipheno-
quinone (4). Relatively strong ModO double bonds and p-electron
donation from the aryloxide oxygens appear to compensate for
the unsaturation of the metal center.
O(3)-Mo(1)-O(4) 106.55(13) O(3)-Mo(1)-O(1) 110.47(11)
O(4)-Mo(1)-O(1) 109.14(12) O(3)-Mo(1)-O(2) 108.81(12)
O(4)-Mo(1)-O(2) 109.14(12) O(1)-Mo(1)-O(2) 111.26(10)
C(1)-O(1)-Mo(1) 170.3(2)
O(1)-C(1)-C(2) 118.4(3)
C(15)-O(2)-Mo(1) 174.2(2)
O(1)-C(1)-C(6) 118.1(3)
angles O(4)-Mo(1)-O(1) and O(3)-Mo(1)-O(2) are 109.14-
(12)° and 108.81(12)°, while O(4)-Mo(1)-O(2) and O(3)-Mo-
(1)-O(1) are 110.50(11)° and 111.26(10)°. This is similar to that
observed in MoO2(OSiPh3)2, which has analogous angles of 109.9-
(3)° and 110.3(3)°.10 The angles between the two bulky ligands
(O(1)-Mo(1)-O(2) ) 111.26°) are comparable as well.
Likewise, the Mo-O bond distances are similar to those
reported for the siloxy complex MoO2(OSiPh3)2. The Mo(1)-
O(3) and Mo(1)-O(4) distances (1.703(2) and 1.706(2) Å, respec-
tively) and the O(3)-Mo(1)-O(4) angle (106.55(13)°) are typical
of those in 6-coordinate Mo dioxo compounds. These values are
also very similar to the corresponding values in MoO2(OSiPh3)2
(1.692(7) Å and 106.4(5)°, respectively).10 The ModO IR (KBr)
stretches for 1 (898 and 884 cm-1) are almost identical to the
stretches observed for MoO2(OSiPh3)2 (898 and 873 cm-1).10
The solution species, however, does not appear to be identical
to that found in the solid state. The solution ModO stretches for
1 (941 and 911 cm-1, C6H6) are much higher than those in KBr
(898 and 884 cm-1). These values are relatively high for
6-coordinate MoO2 complexes, and significantly higher than those
observed for MoO2(OSiPh3)2. They approach those observed in
molybdenum dioxo halide complexes, probably due to the
electron-poor metal center in 1 and increased ModO back-
bonding.
Acknowledgment. This work was supported by the Texas
Christian University Research and Creative Activities Fund. The
University of Delaware acknowledges the National Science
Foundation for their support of the purchase of the CCD-based
diffractometer (Grant CHE-9628768).
Supporting Information Available: Synthetic procedures and spec-
troscopic and analytical data for complex 1, crystal data and structure
refinement, atomic coordinates, bond lengths and angles, anisotropic
displacement coefficients, and H-atom coordinates. An X-ray crystal-
lographic file for 1 (CIF). This material is available free of charge via
the Internet at http://pubs.acs.org.
The most interesting aspect of this structure is that electron
donation from the aryloxy oxygen appears to stabilize the
4-coordinate geometry. The Mo(1)-O(2)-C(15) and Mo(1)-
O(1)-C(1) angles are almost linear (174.2(2)° and 170.3(2)°),
suggesting some interaction between the oxygen lone pair and
IC991271H
(14) Crystal data for C42H64MoO5 (1): monoclinic, P21/c, a ) 11.4344(2)
Å, b ) 19.4277(3) Å, c ) 18.5445(3) Å, â ) 99.1385(6)°, V ) 4067.27-
(12) Å3, Z ) 4, Z′ ) 1, fw ) 744.87 g mol-1, T ) 173(2) K, Dcalc
)
(15) Subramanian, P.; Spence, J. T.; Ortega, R.; Enemark, J. H. Inorg. Chem.
1984, 23, 2564-2572.
(16) Pauling, L. The Nature Of the Chemical Bond; Cornell University
Press: Ithaca, New York, 1960.
1.216 g/cm3, yellow block, GOF ) 1.250, µ(Mo KR) ) 0.71073 Å,
R(F) ) 4.86% for 8275 observed independent reflections (4° e 2θ e
56°), wR2 ) 0.1505. R indices (all data): R ) 0.0650, wR2 ) 0.1675.