Synthesis, structure, and characterization of molybdenum(VI) imido complexes with N-salicylidene-2-aminophenol

Article abstract of DOI:10.1021/ic060370+

Diimido complexes of the type Mo(NAr)₂Cl₂(dme) (dme = 1,2-dimethoxyethane) react with N-salicylidene-2-aminophenol (sapH₂) in methanol in the presence of 2 equiv of triethylamine to form complexes with the general formula Mo(NAr)(1,2-OC₆H₄NH)(sap). The structures of three of these compounds (NAr = 2,6-dimethylphenylimido (1), 2,4,6-trimethylphenylimido (2), 2-tert-butylphenylimido(3)) have been determined by X-ray crystallography. The coordination sphere around the Mo is a distorted octahedron. The oxygen from the 2-aminophenol is trans to the imido nitrogen, whereas the amido nitrogen and the tridentate sap occupy the four equatorial positions. The Mo-N-C imido linkages have angles of 167.5(2)° (1), 163.2(2)° (2), and 162.4(1)° (3). A precursor complex to the imido-amido complex, Mo(NAr)(sap)(OCH₃)₂ (4, NAr = 2,4,6-trimethylphenylimido), has been isolated and characterized. Compound 4 reacts with 2-aminophenol to form 2, with 2-aminothiophenol to form Mo(NAr)(1,2-SC₆H₄NH)(sap) (5), with catechol to form Mo(NAr)(1,2-OC₆H₄O)(sap) (6), with naphthalene-2,3-diol to form Mo(NAr)(naphthalene-2,3-diolate)(sap) (7), with 1,2-benzenedithiol to form Mo(NAr)(1,2-SC₆H₄S)(sap) (8), and with 1,2-phenylenediamine to form Mo(NAr)(1,2-HNC₆H₄NH)(sap) (9). The structures of compounds 5-9 have been determined by X-ray crystallography. With the exception of compound 8, the structures are similar to those of 1, 2, and 3, with the bidentate ligand occupying one axial and one equatorial position. In 8, 1,2-benzendithiolate occupies two equatorial positions, and the nitrogen from sap is located trans to the imido nitrogen. All complexes were characterized by ¹H NMR spectroscopy, cyclic voltammetry, and UV-vis spectroscopy. When a solution of 4 is exposed to moisture-containing air, MoO₂(sap)(CH₃OH) (10) is formed. The structure of 10 was also determined.

Full text of DOI:10.1021/ic060370+

Inorg. Chem. 2006, 45, 5455−5464
Synthesis, Structure, and Characterization of Molybdenum(VI) Imido
Complexes with N-Salicylidene-2-aminophenol
Martin Minelli,* Frances Namuswe, Daniel Jeffrey, Amy L. Morrow, Ilia A. Guzei,† Dale Swenson,‡
Eberhard Bothe,§ and Thomas Weyhermuller§
1
Department of Chemistry, Grinnell College, Grinnell, Iowa 50112
Received March 6, 2006
Diimido complexes of the type Mo(NAr)2Cl2(dme) (dme
aminophenol (sapH2) in methanol in the presence of 2 equiv of triethylamine to form complexes with the general
formula Mo(NAr)(1,2-OC6H4NH)(sap). The structures of three of these compounds (NAr 2,6-dimethylphenylimido
) 1,2-dimethoxyethane) react with N-salicylidene-2-
)
(1), 2,4,6-trimethylphenylimido (2), 2-tert-butylphenylimido(3)) have been determined by X-ray crystallography. The
coordination sphere around the Mo is a distorted octahedron. The oxygen from the 2-aminophenol is trans to the
imido nitrogen, whereas the amido nitrogen and the tridentate sap occupy the four equatorial positions. The Mo
C imido linkages have angles of 167.5(2) (1), 163.2(2) (2), and 162.4(1) (3). A precursor complex to the
imido amido complex, Mo(NAr)(sap)(OCH3)2 (4, NAr 2,4,6-trimethylphenylimido), has been isolated and
−
N−
°
°
°
−
)
characterized. Compound 4 reacts with 2-aminophenol to form 2, with 2-aminothiophenol to form Mo(NAr)(1,2-
SC6H4NH)(sap) (5), with catechol to form Mo(NAr)(1,2-OC6H4O)(sap) (6), with naphthalene-2,3-diol to form
Mo(NAr)(naphthalene-2,3-diolate)(sap) (7), with 1,2-benzenedithiol to form Mo(NAr)(1,2-SC6H4S)(sap) (8), and with
1,2-phenylenediamine to form Mo(NAr)(1,2-HNC6H4NH)(sap) (9). The structures of compounds 5−9 have been
determined by X-ray crystallography. With the exception of compound 8, the structures are similar to those of 1,
2, and 3, with the bidentate ligand occupying one axial and one equatorial position. In 8, 1,2-benzendithiolate
occupies two equatorial positions, and the nitrogen from sap is located trans to the imido nitrogen. All complexes
1
were characterized by H NMR spectroscopy, cyclic voltammetry, and UV
−vis spectroscopy. When a solution of 4
is exposed to moisture-containing air, MoO2(sap)(CH3OH) (10) is formed. The structure of 10 was also determined.
Introduction
it possible to synthesize diimido complexes with a variety
of ligands.5-12 Slight modifications on one of the ligands in
an imido complex can have a significant influence on the
properties of the compounds. We have shown previously that
changing the position of alkyl substituents on the aryl ring
Molybdenum imido complexes have been studied widely
over the past years.1 They are of special interest due to their
involvement in the catalysis of olefin metathesis2 and
the Ziegler-Natta olefin polymerization.3 High-yield syn-
thetic routes for diimido starting materials of the type
Mo(NAr)2Cl2(dme) (dme ) dimethoxyethane)4 have made
(4) Fox, H. H.; Yap, K. B.; Robbins, J.; Cai, S.; Schrock, R. R. Inorg.
Chem. 1992, 31, 2287.
(5) Barrie, P.; Coffey, T. A.; Forster, G. D.; Hogarth, G. J. Chem. Soc.,
Dalton Trans. 1999, 4519.
(6) Jim A. M.; Brandts, J. A. M.; Boersma, J.; Spek , A. L.; van Koten,
G. Eur. J. Inorg. Chem. 1999, 10, 1727.
(7) Galindo, A.; Montilla, F.; Pastor, A.; Carmona, E.; Gutierrez-Puebla,
E.; Monge, A.; Ruiz, C. Inorg. Chem. 1997, 36, 2379.
(8) Hao, H.; Cui, C.; Bai, G.; Roesky, H. W.; Noltemeyer, M.; Schmidt,
H.-G.; Ding, Y. Z. Anorg. Allg. Chem. 2000, 626, 1660.
(9) Ward, B. D.; Dubberley, S. R.; Gade, L. H.; Mountford, P. Inorg.
Chem. 2003, 42, 4961.
(10) Ramnauth, R.; Al-Juaid, S.; Motevalli, M.; Parkin, B. D.; Sullivan,
A. C. Inorg. Chem. 2004, 43, 4072.
(11) Lokare, K. S.; Ciszewski, J. T.; Odom, A. L. Organometallics 2004,
23, 5386.
* E-mail: minelli@grinell.edu.
† Current address: Molecular Structure Laboratory, Chemistry Depart-
ment, University of Wisconsin-Madison, Madison, WI 53558.
‡ Current address: Department of Chemistry, University of Iowa, Iowa
City, IA 52242.
§ Current address: Max-Planck Institut fu¨r Bioanorganische Chemie,
Stiftstrasse 34-36, D-45470 Mu¨lheim, Germany.
(1) (a) Nugent, W. A.; Haymore, B. L. Coord. Chem. ReV. 1980, 31, 123.
(b) Nugent, W. A.; Mayer, J. M. Metal Ligand Multiple Bonds; John
Wiley & Sons: New York, 1988. (c) Wigley, D. E. Prog. Inorg. Chem.
1994, 42, 239. (d) Eikey, R. A.; Abu-Omar, M. M. Coord. Chem.
ReV. 2003, 243, 83.
(2) (a) Schrock, R. R.; Hoveyda, A. H. Angew. Chem., Int. Ed. 2003, 42,
4592. (b) Schrock, R. R. Chem. Commun. 2005, 22, 2773.
(3) Bolton, P. D.; Mountford, P. AdV. Synth. Catal. 2005, 347, 355.
(12) Ciszewski, J. T.; Harrison, J. F.; Odom, A. L. Inorg. Chem. 2004, 43,
3605.
10.1021/ic060370+ CCC: $33.50
Published on Web 06/14/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 14, 2006 5455

Minelli et al.
7.50 (t, 1), 7.15 (t, 1), 6.97 (t, 1), 6.89-6.80 (m, 6), 6.77 (d, 1),
6.66 (t, 1), 6.56 (d, 1), 2.25 (s,3), 2.13 (s, 6). Anal. Calcd. for C28H25-
MoN3O3, 2: C, 61.42; H, 4.60; N, 7.67. Found: C, 61.36; H, 4.69;
1
N, 7.59. H NMR for 3 ((CD3)2SO): δ 14.10 (s, 1), 9.56 (s, 1),
7.89 (d, 1), 7.80 (d, 1), 7.53 (t, 1), 7.21 (t, 1), 7.07 (t, 1), 6.96 (m,
8), 6.87 (d, 1), 6.62 (d, 1), 6.05 (d, 1), 1.27 (s, 9). Anal. Calcd. for
C29H27MoN3O3‚1/2CH2Cl2, 3: C, 58.66; H, 4.67; N, 6.95. Found:
C, 58.82; H, 4.73; N, 6.87.
Figure 1. N-Salicylidene-2-aminophenolate (sap). Aromatic protons are
Preparation of Mo(N-2,4,6-Me3C6H2)(sap)(OCH3)2 (4). The
reaction mixture described above was refluxed for only 2 h instead
of overnight. Better yields were obtained if only 0.36 g (1.7 mmol)
of sapH2 was used. After 2 h, the reaction mixture was cooled in
an ice bath and filtered. The dark brown precipitate, 4, was dried
under vacuum. The yield was 0.30 g (31%). From the red-purple
filtrate, black crystals of 2 precipitated out after several hours at
room temperature. 1H NMR for 4 ((CD3)2SO): δ 9.16 (s, 1), 7.78
(d, 1), 7.68 (d, 1), 7.40 (t, 1), 7.13 (t, 1), 6.90 (t, 2), 6.81 (m, 2),
6.67 (s, 2), 3.16 (s, 3), 3.15 (s, 3), 2.15 (s, 3), 2.12 (s, 6). Anal.
Calcd. for C24H26MoN2O4, 4: C, 57.37; H, 5.22; N, 5.57. Found:
C, 56.54; H, 4.88; N, 5.43.
Reactions of 4 with Bidentate Aromatic Ligands. A 0.2-g (0.4
mmol) portion of 4 was reacted with a slight excess (∼0.6 mmol)
of the aromatic bidentate ligand in 40 mL of methanol. The solution
was refluxed for 1 h and then cooled to room temperature. The
product was filtered off and dried. For crystals, the product was
recrystallized in CH2Cl2/hexane. A precipitate does not form with
1,2-phenylenediamine (see below).
in italic letters.
of the imido ligand can influence the angle of the imido
linkage and the 95Mo NMR chemical shift.13 In this study,
we have used Mo(NAr)2Cl2(dme) (NAr ) 2,6-dimethyl-
phenylimido, 2,4,6-trimethylphenylimido, 2-tert-butyl-
phenylimido) to synthesize imido complexes with the tri-
dentate Schiff base N-salicylidene-2-aminophenol (sapH2)
(Figure 1). These reactions were expected to lead to either
five-coordinate diimido complexes, six-coordinate complexes
with a solvent bound in the sixth coordination site, or dimers
with bridging and terminal imido ligands, as found for the
corresponding oxo-complexes;14-18 however, the final prod-
ucts were quite different from the compounds we expected.
Experimental Section
All reactions were performed under an argon atmosphere using
standard Schlenk techniques unless otherwise noted. The solvents
were dried and distilled prior to use. Mo(NAr)2Cl2(dme) (H2NAr
) 2,6-dimethylaniline, 2,4,6-trimethylaniline, 2-tert-butylaniline)
was synthesized according to Schrock et al.,4 and N-salicylidene-
2-aminophenol, according to Alyea and Malek.19 Triethylamine was
dried over sodium and distilled prior to use. All other compounds
were purchased from Aldrich and were used without further
purification. The elemental analyses were carried out by Atlantic
Microlab, Inc. in Norcross, GA, and by H. Kolbe, Mu¨lheim (Ruhr),
Germany.
Preparation of Mo(NAr)(1,2-OC6H4NH)(sap) (NAr2- ) 2,6-
Dimethylphenylimido, 1; 2,4,6-Trimethylphenylimido, 2; 2-tert-
Butylphenylimido, 3). To a suspension of Mo(NAr)2Cl2(dme) (1.0
g, 2.0 mmol for 1, 1.9 mmol for 2, 1.8 mmol for 3) in 30 mL of
methanol were added 0.43 g (2 mmol) of sapH2 and 0.49 mL (4
mmol) of triethylamine. The mixture was refluxed overnight. The
color of the solution changed from dark yellow-brown to dark red-
purple. The mixture was cooled to room temperature and filtered.
The resulting precipitate was recrystallized in dichloromethane/
hexane to yield dark red crystals. The yield, based on
Mo(NAr)2Cl2(dme), was 0.37 g (35%) for 1, 0.27 g (26%) for 2,
and 0.23 g (21%) for 3. 1H NMR for 1 ((CD3)2SO): δ 14.30 (s, 1),
9.56 (s, 1), 7.92 (d, 1), 7.83 (d, 1), 7.53 (t, 1), 7.18 (t, 1), 7.03 (s,
1), 6.98 (t, 1), 6.92 (d, 1), 6.86 (m, 2), 6.79 (d, 1), 6.67 (t, 1), 6.58
(d, 1), 2.20 (s, 6). Anal. Calcd. for C27H23MoN3O3, 1: C, 60.79;
H, 4.35; N, 7.87. Found: C, 60.74; H, 4.33; N, 7.88. 1H NMR for
2 ((CD3)2SO): δ 14.18 (s, 1), 9.51 (s, 1), 7.89 (d, 1), 7.80 (d, 1),
2-Aminophenol. Yield: 66% of 2.
2-Aminothiophenol. Yield: 70% of Mo(NAr)(1,2-SC6H4NH)-
1
(sap), 5. H NMR for 5 ((CD3)2SO): δ 14.43 (s,1), 9.47 (s, 1),
7.83 (d, 1), 7.77 (d, 1), 7.48 (t, 1), 7.15 (m, 2), 7.07 (d, 1), 6.93 (t,
1), 6.87 (m, 5), 6.82 (t, 1), 6.73 (d, 1), 2.27 (s, 3), 2.25 (s, 6).
Anal. Calcd. for C28H25MoN3O2S, 5: C, 59.67; H, 4.47; N, 4.97.
Found: C, 59.25; H, 4.95; N, 6.95.
Catechol. Yield: 90% Mo(NAr)(1,2-OC6H4O)(sap), 6. 1H NMR
for 6 ((CD3)2SO): δ 9.69 (s, 1), 7.93 (d, 2), 7.64 (t, 1), 7.26 (t, 1),
7.15 (t, 1), 6.86 (m, 3), 6.84 (m, 3), 6.56 (d, 1), 2.28 (s, 3), 2.19 (s,
6). Anal. Calcd. for C28H24MoN2O4, 6: C, 61.32; H, 4.41; N, 5.10.
Found: C, 60.50; H, 4.36; N, 4.98.
Naphthalene-2,3-diol. Yield: 64% of Mo(NAr)(naphthalene-
2,3-diolate)(sap), 7. 1H NMR for 7 ((CD3)2SO): δ 9.73 (s, 1), 7.97
(m, 2), 7.69 (m, 3), 7.28 (m, 9), 6.90 (s, 2), 2.31 (s, 3), 2.28 (s, 6).
Anal. Calcd. for C32H26MoN2O4, 7: C, 64.21; H, 4.38; N, 4.68.
Found: C, 63.70; H, 3.82; N, 4.15.
Benzene-1,2-dithiol. Yield: 60% of Mo(NAr)(1,2-SC6H4S)(sap),
8. 1H NMR for 8 ((CD3)2SO): δ 9.92 (s, 1), 8.10 (t, 2), 7.76(t, 1),
7.36(t, 1), 7.14 (m, 6), 6.98 (s, 2), 6.94 (m, 2), 2.43 (s, 6), 2.34 (s,
3). Anal. Calcd. for C28H24MoN2O2S2, 8: C, 57.92; H, 4.17; N,
4.82. Found: C, 57.48; H, 4.23; N, 4.28.
1,2-Phenylenediamine. The synthesis was carried out as de-
scribed above, but no precipitate formed. When the solvent was
evaporated, an amorphous solid remained. This solid was recrystal-
lized in THF and CH2Cl2. The formation of crystals was aided by
adding a small amount of acetone. Yield: ∼5% of Mo(NAr)(1,2-
(13) Minelli, M.; Hoang, M. L.; Kraus, M.; Kucera, G.; Loertscher, J.;
Reynolds, M.; Timm, N.; Chiang, M. Y.; Powell, D. Inorg. Chem.
2002, 41, 5954-5960.
(14) Syamal, A.; Maurya, M. R. Coord. Chem. ReV. 1989, 95, 183.
(15) Mondal, J. U.; Schultz, F. A.; Brennan, T. D.; Scheidt, W. R. Inorg.
Chem. 1988, 27, 3950.
(16) Rajan, O. A.; Chakravorty, A. Inorg. Chem. 1981, 20, 660.
(17) Craig, J. A.; Harlan, E. W.; Snyder, B. S.; Whitener, M. A.; Holm, R.
H. Inorg. Chem. 1989, 28, 2082.
(18) Cindric, M.; Strukan, M.; Vrdoljak, V.; Kamenar, B. Z. Anorg. Allg.
Chem. 2004, 630, 585.
1
HNC6H4NH)(sap), 9. H NMR for 9 ((CD3)2SO): δ 13.15 (s, 1),
9.69 (s, 1), 9.46 (s, 1), 7.86 (d, 1), 7.84 (d, 1), 7.42 (t, 1), 7.11 (t,
1), 6.8 (m, 6), 6.75 (m, 2), 6.5 (unres, 1), 6.35 (unres, 1), 2.25 (s,
3), 2.14 (s, 6). Anal. Calcd. for C28H26MoN4O2, 9: C, 61.54; H,
4.80; N, 10.25. Found: C, 61.42; H, 4.87; N, 10.25.
Synthesis of MoO2(sap)(CH3OH). When the filtrate from the
synthesis of 4 (above) is exposed to moisture-containing air, in
addition to 2, orange crystals of MoO2(sap)(CH3OH), 10, form and
1
(19) Alyea, E. C.; Malek, A. Can. J. Chem. 1975, 53, 939.
can be isolated. H NMR for 10 ((CD3)2SO): δ 9.26 (s, 1), 7.82
5456 Inorganic Chemistry, Vol. 45, No. 14, 2006

Molybdenum(VI) Imido Complexes
(d, 1), 7.79 (d, 1), 7.54 (t, 1), 7.23 (t, 1), 7.08 (t, 1), 7.00 (m, 3),
6.87 (d, 1), 6.62 (d, 1), 6.05 (d, 1), 3.16, 3.14 (2s, 3). Anal. Calcd.
for C14H12MoNO5, 10: C, 45.42; H, 3.27; N, 3.78. Found: C, 45.65;
H, 3.48; N, 3.81.
Complex 1. A dark purple blade (0.05 × 0.11 × 0.40 mm) of
1 was isolated from the sample and mounted with grease on the
tip of a glass capillary epoxied to a brass pin and placed on the
diffractometer with the long crystal dimension (a axis) ap-
proximately parallel to the diffractometer æ axis. Data for 1 were
collected on an Nonius Kappa CCD diffractometer (Mo KR
radiation, graphite monochromator) at 190 K (cold N2 gas cooling)
using standard CCD techniques, yielding 29 627 data points. Lorentz
and polarization corrections were applied. An empirical absorption
correction was applied using the multiscan technique. Equivalent
data were averaged, yielding 5257 unique data points (R-int )
0.045, 4278 > 4 *sigma(F)). On the basis of preliminary examina-
tion of the data, the space group Pbca was assigned (no significant
exceptions to the hk0, h ) odd; h0l, l ) odd; 0kl, k ) odd systematic
absences were noted). The computer programs from the HKL2000
package were used for data reduction. The preliminary model of
the structure was obtained using XS, a direct methods program.
Least-squares refining of the model vs the data was done with the
XL computer program. Illustrations were made with the XP
program, and tables were made with the XCIF program. All are in
the SHELXTL, v5.1 package. Thermal ellipsoids are drawn at the
35% level unless otherwise noted.
All non-hydrogen, nondisordered atoms were refined with
anisotropic thermal parameters. The tridentate ligand was position-
ally disordered. The ligand’s major site (occ ) 0.726(5)) is
designated with atom labels O1-O14 and was refined with
anisotropic thermal parameters. The ligand’s minor site (occ )
0.274(5)) is approximated by a 180° rotation of the major site about
an axis defined by the Mo1-(C7-N8 bond midpoint) vector. The
atom labels O1′-O14′ are used for the minor site. The conformation
of the minor site was constrained to be the same as that of the
major site. A group isotropic thermal parameter for all non-hydrogen
atoms of the minor site was used in the refinement model. Hydrogen
atom H7 was allowed to refine as an independent isotropic atom.
All remaining H atoms (including those with partial occupancy)
were included in the final refinement using the riding model with
default parameters.
NMR Spectroscopy. The 1H NMR spectra were measured on a
Bruker 400-MHz DRX spectrometer and a Bruker 500-MHz DRX
spectrometer at room temperature. The residual solvent was used
as an internal reference for chemical shift determination.
Infrared Spectroscopy. The IR spectrum of 10 was measured
on a Perkin-Elmer System 2000 infrared spectrometer as KBr pellet.
Electrochemistry. (a) The cyclic voltammograms of 1-3 were
recorded using a PAR 175 Universal Programmer and a PAR 173
Potentiostat. The three-electrode electrochemical cell consisted of
a platinum disk working electrode, a platinum wire auxiliary
electrode, and a silver electrode as a reference electrode. The
potentials are reported as potentials vs silver wire and were corrected
to potentials vs ferrocene as an internal standard. The complexes
were dissolved (5 × 10-4 M) in acetonitrile in the presence of 0.1
M tetrabutylammonium tetrafluoroborate as electrolyte.
(b) Cyclic voltammograms of the complexes were also recorded
at room temperature in solutions of CH3CN containing 0.2 M [(n-
Bu)4N]PF6 (TBAHFP) as supporting electrolyte. Potential control
was achieved using an E.G.&G. potentiostat/galvanostat (model
273A) and M270 software. For the measurements, a conventional
three-electrode arrangement was employed that consisted of a glassy
carbon electrode (2-mm diameter), a Ag/Ag+ (0.01 M AgNO3)
reference electrode, and a Pt wire counter electrode. Small amounts
of ferrocene were added as an internal standard after completion
of each set of experiments, and potentials are referenced versus
the ferrocenium/ferrocene (Fc+/Fc) couple.
Spectroelectrochemistry. (a) A HP 8452 diode array spectrom-
eter with a deuterium lamp and a PAR 173 potentiostat were used,
together with an OTTLE cell that had three electrodes: an OTTLE
as the working electrode, a silver wire as reference electrode, and
a platinum wire as a counter electrode. A 3-mL reservoir held the
solution and the two latter electrodes. The potentiostat was set at
5 mV/s, and the spectra were taken after every 0.02 V at 2-3-min
intervals.20 Solvent and electrolyte were the same as in part a, above.
(b) Controlled potential electrolysis of 8 mL off solution was
performed at -25 °C in a jacketed quartz cuvette with a Ag/Ag+
(0.01 M AgNO3) reference electrode, a Pt mesh working electrode,
and a Pt brush counter electrode. The latter was mounted in its
own compartment, which was separated from the electrolysis solu-
tion by a Vycor frit. The potential for coulometry was chosen ∼0.2
V more negative than the respective reduction potential. Mixing
of the solution during the coulometry was achieved by bubbling
argon through the solution. The 0.5-cm cuvette was mounted in a
Hewlett-Packard HP 8453 spectrophotometer. UV-vis spectra were
recorded in situ during the electrolysis. After the completion of
the electrolysis, samples of the electrolyzed solutions were taken
with gastight syringes and frozen rapidly for EPR analysis.
EPR Spectroscopy. The room-temperature EPR spectra were
measured on a Varian E-6 spectrometer using a quartz flat cell.
The low-temperature EPR spectra were measured on a 9.43-GHz
Bruker E500 spectrometer using 4-mm quartz tubes.
Complexes 2 and 3. Red crystals of 2 and 3 were selected under
paratone oil under ambient conditions and attached to the tip of a
nylon loop. The crystals were mounted in a stream of cold nitrogen
at 100(2) K and centered in the X-ray beam by using a video
camera.
The crystal evaluation and data collection were performed on a
Bruker CCD-1000 diffractometer with Mo KR (λ ) 0.71073 Å)
radiation and the diffractometer-to-crystal distance of 4.9 cm. The
initial cell constants were obtained from three series of ω scans at
different starting angles. Each series consisted of 20 frames collected
at intervals of 0.3° in a 6° range about ω with the exposure time of
10 s per frame. The harvested reflections were successfully indexed
by an automated indexing routine built in the SMART program.
The final cell constants were calculated from a set of 20 610 strong
reflections for 2 and 1002 for 3 from the actual data collection.
The reciprocal space was surveyed to the extent of a full sphere to
a resolution of 0.80 Å. A total of 41 347 data points were harvested
by collecting seven sets of frames with 0.3° scans in ω with an
exposure time 20 s per frame for 2 and 30 s per frame for 3. These
highly redundant datasets were corrected for Lorentz and polariza-
tion effects. The absorption correction was based on fitting a
function to the empirical transmission surface as sampled by
multiple equivalent measurements.21 The systematic absences in
UV-vis Spectroscopy. The UV-vis spectra were measured on
a Perkin-Elmer Lamda 19 spectrophotometer using 0.1-cm quartz
cells. The concentrations were around 1 × 10-4 M in CH3CN.
X-ray Crystallographic Data Collection and Refinement of
the Structures. Crystallographic data of the compounds are listed
in Table 1.
(21) Bruker-AXS. (2000-2003) SADABS V.2.05, SAINT V.6.22,
SHELXTL V.6.10, and SMART 5.622 Software Reference
Manuals; Bruker-AXS: Madison, WI.
(20) DeAngelis, T. P.; Heinemann, W. R. J. Chem. Educ. 1976, 53, 594.
Inorganic Chemistry, Vol. 45, No. 14, 2006 5457

Minelli et al.
Table 1. Crystallographic Data for 1, 2, 3‚0.5CH2Cl2, 5, 6, 7, 8, 9‚acetone, and 10
1
2
3‚0.5 CH2Cl2
5
6
chem formula
fw
C27H23MoN3O3
533.42
C28H25MoN3O3
547.45
C
29.5H28ClMoN3O3
C28H25MoN3O2S
563.51
C28H24MoN2O4
548.43
603.94
space group
a, Å
b, Å
Pbca, No. 61
8.3385 (3)
20.9434( 6)
26.3109 (9)
90
P21/n, No. 14
8.5450 (11)
16.939 (2)
16.750 (2)
90
101.838 (2)
90
2372.9 (5)
4
Pbca, No. 61
12.6310 (14)
17.211 (2)
24.356 (3)
90
P21/n, No. 14
8.2851 (2)
17.1205 (5)
17.5495 (5)
90
99.822 (5)
90
2452.82 (12)
4
P21/n, No. 14
13.9504 (5)
9.9723 (3)
17.1114 (6)
90
90.256 5)
90
2380.47 (14)
4
c, Å
R, deg
â, deg
90
90
90
90
γ, deg
V, Å
4594.8 (3)
8
5249.9 (10)
8
Z
T, K
190 (2)
100 (2)
100 (2)
100 (2)
100 (2)
F calcd, g cm-3
reflns collected/2Θmax
unique reflns/I > 2σ(I)
no. of params/restr
λ, Å/µ(KR), cm-1
R1a/GOFb
wR2c (I > 2σ (I))
residual density, eÅ-3
1.542
1.532
1.515
1.526
1.530
69374/55.0
6257/4278
367/42
0.71073/6.06
0.0604/1.257
0.0887
41347/52.92
4890/4594
327/0
0.71073/5.89
0.0370/1.132
0.0883
41703/52.76
5418/4828
311/0
0.71073/6.33
0.0349/1.028
0.0774
71627/68.60
10215/8651
368/45
0.71073/6.51
0.0383/1.057
0.0836
83529/70.00
10436/9412
368/45
0.71073/5.89
0.0309/1.120
0.0672
0.42/-0.43
0.76/0.62
1.03/-0.71
+1.47/-2.13
+0.72/-0.55
7
8
9‚Acetone
10
chem formula
fw
C32H26MoN2O4
598.49
C28H24MoN2O2S2
580.55
C31H32MoN4O3
604.55
C14H13MoNO5
371.19
space group
a, Å
b, Å
P21/n, No. 14
14.2100 (7)
9.9737 (4)
18.9907 (9)
90
101.339 (5)
90
2638.9 (2)
4
P1h, No. 2
10.2268 (7)
10.5520 (8)
13.0500 (11)
74.123(4)
67.455 (4)
75.855(5)
1235.5 (2)
2
P21/c, No. 14
8.8469 (5)
12.7485 (7)
25.1199 (14)
90
99.995 (5)
90
2790.1 (3)
4
P21/n, No. 14
11.4839 (6)
6.7543 (4)
17.5715 (9)
90
92.878 (5)
90
1361.23 (13)
4
c, Å
R, deg
â, deg
γ, deg
V, Å
Z
T, K
100 (2)
100 (2)
100 (2)
100 (2)
F calcd, g cm-3
reflns collected/2Θmax
unique reflns/I > 2σ(I)
no. of params/restr
λ, Å/µ(KR), cm-1
R1a/GOFb
wR2c (I > 2σ (I))
residual density, eÅ-3
1.506
1.561
1.439
1.811
24122/52.00
5176/3909
324/42
0.71073/5.39
0.0434/1.057
0.0729
16935/55.00
5659/4297
319/8
0.71073/7.29
0.0417/1.033
0.0699
21653/60.00
7440/5638
358/6
0.71073/5.09
0.0544/1.065
0.1391
32182/62.00
4339/3581
201/0
0.71073/9.85
0.0326/1.069
0.0692
+0.56/-0.43
+0.42/-0.49
+0.99/-1.32
+1.53/-0.67
2
2
2
a Observation criterion: I > 2σ(I). R1 ) ∑||Fo| - |Fc||/∑|Fo|, b GOF ) [∑[w(Fo2 - Fc )2]/(n - p)]1/2 c wR2 ) [∑[w(Fo2 - Fc )2]/∑[w(Fo )2]]1/2, where
2
2
2
w ) 1/σ2(Fo ) + (aP)2 + bP, P ) (Fo + 2Fc )/3.
the diffraction data were uniquely consistent for the space group
P21/n for 2 and Pbca for 3; these space groups yielded chemically
reasonable and computationally stable results of refinement.
Successful solutions by the direct methods provided most non-
hydrogen atoms from the E-map. The remaining non-hydrogen
atoms were located in an alternating series of least-squares cycles
and difference Fourier maps. All non-hydrogen atoms were refined
with anisotropic displacement coefficients. All hydrogen atoms were
included in the structure factor calculation at idealized positions
and were allowed to ride on the neighboring atoms with relative
isotropic displacement coefficients. In the structure of 2, the
tridentate ligand is disordered in a 2:1 ratio over two positions, a
fact manifested in somewhat enlarged thermal motion ellipsoids
of some of the atoms within the ligand and in well-resolved
positional disorder of atoms N3 and C22. The latter were refined
with soft restraints. In the structure of 3, there is also one-half
molecule of solvate dichloromethane per Mo complex present in
the lattice. The solvate dichloromethane molecule is disordered over
a crystallographic inversion center.
source and a graphite monochromator (Mo-KR, λ ) 0.71073 Å).
Final cell constants were obtained from least-squares fits of all
measured reflections. Crystal faces of sample 6 were determined,
and the corresponding intensity data were corrected for absorption
using the Gaussian-type routine embedded in XPREP.22 The
Siemens ShelXTL22 software package was used for solution and
artwork of the structure, and ShelXL9723 was used for the
refinement. The structures were readily solved by direct and
Patterson methods and subsequent difference Fourier techniques.
All non-hydrogen atoms were refined anisotropically, except
disordered atoms of the sap-ligand in 7, which were isotropically
refined. Hydrogen atoms attached to carbon atoms were placed at
calculated positions and refined as riding atoms with isotropic
displacement parameters. To determine the degree of protonation,
acidic hydrogen atoms attached to nitrogen were located from the
difference map and were then refined as riding atoms with
corresponding HFIX instructions. The OH proton of the coordinated
methanol molecule in compound 10 was easily located from the
Complexes 5-10. Deep red to black single crystals of 5, 6, 7,
8, 9 and a yellow specimen of 10 were coated with perfluoropoly-
ether and mounted in the nitrogen cold stream of a Nonius Kappa-
CCD diffractometer equipped with a Mo-target rotating-anode X-ray
(22) ShelXTL, V. 5. Siemens Analytical X-ray Instruments, Inc.: Madison,
WI, 1994.
(23) ShelXL97. G. M. Sheldrick; University of Go¨ttingen: Go¨ttingen,
Germany, 1997.
5458 Inorganic Chemistry, Vol. 45, No. 14, 2006

Molybdenum(VI) Imido Complexes
Table 2. Selected Bond Lengths (Å) and Bond Angles (°) for 1, 2, and
3
compd
1
2
3
Mo-N(1)
1.750(3)
2.076(2)
1.975(3)
2.214(3)
2.218(6)
1.939(3)
1.868(7)
2.039(3)
2.091(7)
167.5(2)
166.68(10)
156.7(3)
156.7(3)
74.96(12)
73.9(2)
1.751(2)
2.0744(18)
1.977(2)
2.199(3)
1.7488(17)
2.0540(14)
1.9823(16)
2.2090(17)
Mo-O(1)
Mo-N(2)
Mo-N(3)
Mo-N(3′)
Mo-O(2)
1.978(2)
1.993(2)
1.9828(14)
2.0089(14)
Mo-O(2′)
Mo-O(3)
Mo-O(3′)
Mo-N(1)-C(1)
N(1)-Mo-O(2)
O(3)-Mo-O(2)
O(3′)-Mo-O(2′)
O(3)-Mo-N(3)
O(3′)-Mo-N(3′)
N(1)-Mo-N(3)
N(1)-Mo-N(2)
N(1)-Mo-O(2)
N(1)-Mo-O(3)
163.20(18)
162.71(8)
155.10(11)
162.40(14)
162.73(7)
153.30(6)
71.48(12)
75.71(6)
103.06(12)
90.94(11)
99.63(13)
98.06(15)
101.99(10)
88.67(9)
96.60(9)
95.65(7)
89.90(7)
94.08(7)
103.42(7)
Figure 2. Molecular drawing of Mo(N(2,4,6-Me3)C6H2)(1,2-OC6H4NH)-
(sap), 2.
101.00(10)
Scheme 1. Proposed Reaction Mechanism for the Formation of 4
difference map. An isotropic displacement parameter of 1.3 times
Ueq of the O-atom was used.
All structures suffered from disorder of the sap-ligand, which
was found to be present in both of the two possible bite orientations.
A split atom model was used to account for the disorder. Equal
thermal displacement parameters were refined for the split positions
(EADP Instruction), and geometrical restraints were introduced for
corresponding distances and angles of the SIP-ligand (SAME and
SADI instruction).
have been obtained under similar conditions,6,8,12 we expected
Mo(NAr)2(sap) as the product. When the reaction mixture
is refluxed for only 2 h, a brown complex can be isolated.
Efforts to grow crystals of this complex have been unsuc-
cessful. The complex is fairly stable in coordinating solvents,
Results and Discussion
Syntheses and Structures. The reaction of Mo(NAr)2-
Cl2(dme) with sapH2 in methanol in the presence of 2 equiv
of triethylamine produces Mo(NAr)(1,2-OC6H4NH)(sap).
The yields are around 25%, based on the molybdenum
starting material. Crystals of three complexes with different
alkyl substituents on the imido ligand (NAr ) 2,6-dimethyl-
phenylimido, 1; 2,4,6-trimethylphenylimido, 2; 2-tert-butyl-
phenylimido, 3) were obtained, and the structures were
determined by X-ray crystallography. The crystallographic
data for compounds 1, 2, and 3 are listed in Table 1, selected
bond lengths and bond angles, in Table 2. Figure 2 shows a
molecular drawing of complex 2. In compounds 1-3, the
coordination sphere around the Mo is a distorted octahedron.
The imido linkages have angles of 167.5(2)° in 1, 163.2(2)°
in 2, and 162.4(1)° in 3 and can be considered linear.5 Each
Mo center is also bound to an amido and an imine nitrogen.
The bond lengths increase from Mo-Nimido (1.75 Å) to Mo-
Namido (1.98 Å) and Mo-Nimine (2.20 Å), as expected. The
oxygen from the 2-aminophenol is trans to the imido
nitrogen, whereas the amido nitrogen and the tridentate sap
occupy the four equatorial positions. No lengthening of the
Mo-O bond trans to the imido linkage is observed. Such a
trans influence is seen in Mo(NAr)Cl2(dtc)2 complexes,13 for
example. Since the sap ligand does not have a C2 axis, the
structures reported here have disorders with regard to
inversion of the sap ligand. Mondal et al.15 also found
disorder in crystals of MoO(naphthalene-2,3-diolato)(sap).
The presence of the 2-aminophenol ligand in the complex
was surprising. Since five-coordinate diimido complexes
1
but it decomposes in others. On the basis of the H NMR
spectrum and the elemental analysis, the complex can be
formulated as Mo(N-2,4,6-Me3C6H2)(sap)(OCH3)2, 4. In
methanol, 4 can be reacted with 2-aminophenol to form 2
with a 60% yield.
On the basis of the data above, we suggest the following
pathway for the reaction of Mo(NAr)2Cl2(dme) with sapH2
in the presence of 2 equiv of base. First a diimido complex
with sap is formed (eq 1). The imine carbon of another sapH2
then reacts with one of the imido nitrogens to form a new
Schiff base, HOC6H4CHdN-2,4,6-Me3-C6H2, 2-amino-
phenol, and 4 (eq 2, Scheme 1). Compound 4 then reacts
with 2-aminophenol to form the imido-amido complex, 2
(eq 3).
Mo(NAr)2Cl2(dme) + sapH2 + 2NEt3 f
Mo(NAr)2(sap) + 2NEt3HCl + dme (1)
Mo(NAr)2(sap) + sapH2 + 2CH3OH f
Mo(NAr)(sap)(OCH3)2 (4) +
HOC6H4CHdNC6H2(2,4,6-Me3) + 1,2-HOC6H4NH2 (2)
Mo(NAr)(sap)(OCH3)2 (4) + 1,2-HOC6H4NH2 f
Mo(NAr)(1,2-OC6H4NH)(sap) (2) + 2CH3OH (3)
Although the proposed mechanism requires a 2:1 ratio of
sapH2/Mo(NAr)2Cl2(dme), the synthesis was carried out in
Inorganic Chemistry, Vol. 45, No. 14, 2006 5459

Minelli et al.
Table 3. Selected Bond Lengths (Å) and Bond Angles (°) for 5, 6, 7, 8, 9
compd
5
6
7
compd
8
9
Mo(1)-N(31)
1.7628 (15)
1.9197 (19)
1.889 (4)
2.2244 (19)
2.227 (3)
2.060 (2)
2.100 (3)
1.9779 (13)
1.7585 (10)
1.921 (4)
1.907 (3)
2.211 (2)
2.1961 (18)
2.000 (4)
2.001 (3)
1.761 (3) (N(41))
1.902 (5)
1.874 (6)
2.205 (5)
2.199 (5)
Mo(1)-N(31)
1.734 (2)
1.93 (5)
1.744 (2)
1.989 (5)
1.992 (6)
2.244 (5)
2.204 (5)
2.002 (5)
2.004 (6)
Mo(1)-O(1)
Mo(1)-O(1)
Mo(1)-O(1′)
Mo(1)-O(1′)
1.967 (6)
2.310 (5)
2.256 (5)
1.967 (6)
1.985 (5)
2.4075 (8)
Mo(1)-N(9)
Mo(1)-N(9)
Mo(1)-N(9′)
Mo(1)-N(9′)
Mo(1)-O(16)
2.010 (5)
1.997 (6)
Mo(1)-O(16)
Mo(1)-O(16′)
Mo(1)-O(16′)
Mo(1)-N(28)
Mo(1)-S(28)
Mo(1)-O(28)
1.9514 (9)
1.938 (2) (O(32))
Mo(1)-N(28)
2.077 (2)
Mo(1)-S(21)
2.5001 (5)
Mo(1)-S(21)
2.4333 (8)
Mo(1)-O(21)
2.0859 (9)
174.11 (9)
2.080 (2)
178.0 (3) (1-41-42)
Mo(1)-N(21)
1.970 (2)
167.4 (2)
Mo(1)-N(31)-C(32)
N(31)-Mo(1)-S(21)
N(31)-Mo(1)-O(21)
O(1)-Mo(1)-O(16)
O(1′)-Mo(1)-O(16)
O(1)-Mo(1)-N(31)
O(1′)-Mo(1)-N(31)
N(28)-Mo(1)-N(31)
O(28)-Mo(1)-N(31)
N(28)-Mo(1)-N(9)
O(28)-Mo(1)-N(9)
N(28)-Mo(1)-O(1)
O(28)-Mo(1)-O(1)
N(28)-Mo(1)-S(21)
O(28)-Mo(1)-O(21)
167.58 (12)
167.12 (5)
Mo(1)-N(31)-C(32)
N(31)-Mo(1)-S(21)
N(31)-Mo(1)-N(21)
O(1)-Mo(1)-O(16)
O(1′)-Mo(1)-O(16)
O(1)-Mo(1)-N(31)
O(1′)-Mo(1)-N(31)
S(28)-Mo(1)-N(31)
N(28)-Mo(1)-N(31)
S(28)-Mo(1)-N(9)
N(28)-Mo(1)-N(9)
S(28)-Mo(1)-O(1)
N(28)-Mo(1)-O(1)
S(28)-Mo(1)-S(21)
N(28)-Mo(1)-N(21)
178.1 (2)
101.45 (8)
171.04 (4)
153.45 (14)
11.67 (11)
98.39 (14)
96.74 (14)
174.32 (11)
152.51 (19)
13.0 (2)
99.08 (18) (N(41))
93.7 (2) (N(41))
90.79 (11)
153.3 (3)
11.3 (4)
100.5 (3)
101.9 (6)
156.76 (8)
13.08 (11)
94.01 (8)
99.33 (15)
89.36 (6)
111.7 (6)
9.1 (11)
101.2 (3)
93.3 (5)
101.42 (8)
93.63 (4)
161.34 (6)
108.99 (8)
77.69 (4)
100.21 (18) (32-1-41)
158.67 (13) (32-1-9)
109.47 (16)
165.85 (11)
95.47 (13)
81.0 (3)
161.06 (6)
104.56 (7)
78.31 (4)
97.25 (15)
104.56 (7)
80.21 (3)
78.59 (9)
75.62 (10)
Scheme 2. Reactions of 4 with Bidentate Aromatic Ligands
3. All complexes show some disorder regarding the sap
ligand. Molecular drawings of compounds 6 and 8 are shown
in Figures 3 and 4. With the exception of 8, the structures
of the complexes are similar to those of 1, 2, and 3. The
coordinating atoms of the bidentate ligand occupy one
equatorial and one axial position. In 5, the amido nitrogen
of 2-aminothiophenol is bound in the equatorial position, as
in 1, 2, and 3. We synthesized 7 to be able to compare its
structure with the corresponding oxo complex, MoO(naph-
thalene-2,3-diolato)(sap).15 The coordination spheres differ.
In the oxo complex, sap binds in fac rather than in mer
positions,15 resulting in dihedral angles of 23.5° and 29.5°
a 1:1 ratio because purer products were obtained using this
ratio. Another way to form the imido-amido complexes is
by imine metathesis between Mo(NAr)2(sap) and sapH2.24
The resulting Mo(NAr)(dNC6H4OH)(sap) complexes could
then rearrange into 1-3.
Compound 4 also reacts with 2-aminothiophenol to form
Mo(NAr)(1,2-SC6H4NH)(sap), 5; with catechol to form
Mo(NAr)(1,2-OC6H4O)(sap), 6; with naphthalene-2,3-diol to
form Mo(NAr)(naphthalene-2,3-diolate)(sap), 7; with benzene-
1,2-dithiol to form Mo(NAr)(1,2-SC6H4S)(sap), 8; and with
1,2-phenylenediamine to form Mo(NAr)(1,2-HNC6H4NH)-
(sap), 9 (Scheme 2). The structures of complexes 5-9 have
been determined. The crystallographic data are listed in Table
1, and selected bond lengths and bond angles are in Table
(24) Cantrell, G. K.; Meyer, T. Y. Organometallics 1997, 16, 5381. Cantrell,
G. K.; Meyer, T. Y. J. Chem. Soc. Chem. Commun. 1997, 8035.
Cantrell, G. K.; Meyer, T. Y. J. Am. Chem. Soc. 1998, 120, 8035.
Figure 3. Molecular drawing of Mo(N(2,4,6-Me3)C6H2)(1,2-OC6H4O)-
(sap), 6.
5460 Inorganic Chemistry, Vol. 45, No. 14, 2006

Molybdenum(VI) Imido Complexes
Table 5. 1H NMR Chemical Shifts of the Sap Imine Proton
compd
δ (ppm) in (CD3)2SO
1 (NH, O)
2 (NH, O)
3 (NH, O)
4 (OCH3)2
5 (NH, S)
6 (O, O)
7 (O, O n)
8 (S, S)
9 (NH, NH)
10 (dioxo)
sapH2
9.56
9.51
9.56
9.16
9.47
9.69
9.73
9.92
9.46
9.26
8.70
9.69 ppm that can be attributed to the two amido protons.
The imine proton from sapH2 becomes more deshielded when
the ligand is bound (Table 5). The imine protons of the
complexes are found above 9 ppm, and their chemical shifts
depend on the bidentate ligand. The shielding increases in
the order S,S (benzene-1,2-dithiol) < O,O (naphthalene-2,3-
diol) < O,O (catechol) < O,NH(2-aminophenol) < S,NH(2-
aminothiophenol) < NH,NH (1,2-phenylenediamine). Com-
pound 4, containing two methoxides instead of a bidentate
ligand, has the most shielded imine proton. The imine protons
allow an easy identification of mixtures of these compounds.
The consistent presence of six methyl protons in the
Figure 4. Molecular drawing of Mo(N(2,4,6-Me3)C6H2)(1,2-SC6H4S)-
(sap), 8.
Table 4. Selected Bond Lengths (Å) and Bond Angles (°) for 10
Mo(1)-O(17)
Mo(1)-O(18)
Mo(1)-O(1)
Mo(1)-O(16)
Mo(1)-N(9)
Mo(1)-N(9′)
Mo(1)-O(19)
O(17)-Mo-O(18) 105.24 (8)
O(17)-Mo-O(1) 99.99 (8)
O(18)-Mo(1)-O(1) 102.69 (8)
1.6988 (15) O(17)-Mo(1)-O(16) 97.35 (8)
1.7123 (16) O(18)-Mo(1)- O(16) 95.66 (8)
1.9242 (17) O(17)-Mo(1)-N(9)
1.9948 (17) O(17)-Mo(1)-N(9′)
90.27 (7)
91.2 (6)
1
methanol region of the H NMR spectrum allowed the
2.2800 (19) O(18)-Mo(1)-N(9) 162.55 (8)
formulation of 4 as a dimethoxide complex. On the basis of
COSY spectra, the aromatic protons for 2 and 4 are assigned
in Figure 5. The aromatic protons of the 2-amidophenolate
in 2 and the imido protons in both complexes are more
shielded than the sap protons.
2.27 (3)
2.322 (18)
O(18)-Mo(1)-N(9′) 159.9 (7)
O(1)-Mo(1)-N(9)
O(1)-Mo(1)-N(9′)
81.95 (7)
62.3 (8)
O(17)-Mo(1)-O(19) 173.19 (7)
O(18)-Mo(1)-O(19) 81.39 (7)
In the compounds with the 2,4,6-trimethylimido ligand,
the protons from the methyl group in the p position are more
of the phenolates in the sap ligand. In the imido complexes,
the phenolates are coplanar. The bond lengths of the imine
carbon to the imine nitrogen and to the phenyl carbon of
sap in both complexes are comparable. The structure of 8 is
similar to that of MoO(naphthalene-2,3-diolato)(sap). It is
the only imido complex synthesized here in which the
bidentate aromatic ligand has both binding sites in equatorial
positions and the nitrogen from sap is located trans to the
imido nitrogen. The Mo-imido nitrogen distance in 8 is the
shortest of all complexes studied here (1.734(2) Å).
If a solution containing 4 is exposed to moisture-containing
air, orange crystals of MoO2(sap)(CH3OH), 10, form (eq 4).
The crystallographic data for 10 are listed in Table 1; selected
bond lengths and bond angles, in Table 4. The coordination
sphere of the molybdenum is a distorted octahedron with
the two terminal oxo groups and the coordinated methanol
in mer positions, similar to the structures of the dioxo
complexes with sap derivatives published earlier.17,18 The
ModO stretches for 10 are at 933 and 911 cm-1 in the
infrared spectrum.
Mo(NAr)(sap)(OCH3)2 (4) + CH3OH + 2H2O f
MoO2(sap)(CH3OH) (10) + 2CH3OH + H2NAr (4)
NMR Data. The 1H NMR data for all complexes are listed
in the Experimental Section. The spectra of the complexes
1, 2, 3, and 5 in (CD3)2SO show a singlet for the proton
bound to the amido nitrogen of the bidentate ligand above
14 ppm. Compound 9 has two broad singlets at 13.15 and
Figure 5. Aromatic regions of the proton NMR spectra of 2 and 4; the
sap protons are labeled according to Figure 1; imido ) aromatic protons
from the imido ligand, ap ) protons from the 2-amidophenolate.
Inorganic Chemistry, Vol. 45, No. 14, 2006 5461

Minelli et al.
Table 6. Redox Potentials of Compounds 1-10 in CH3CN, 0.2 M
TBAHFP
compd
E°′ (V vs Fc+/Fc)
Eps(V vs Fc+/Fc)
1 (NH, O)
2 (NH, O)
3 (NH, O)
4 (OCH3)2
5 (NH, S)
6 (O, O)
-1.37
-1.40
-1.37
-1.10
-1.20
-0.85
-0.73
-0.85
-1.55
-1.16 (Ep)
-2.03
-2.06
-2.03
a
-2.17
-1.93
-1.90 (E°′)
-1.86 (E°′)
-1.87
7 (O, O n)
8 (S, S)
9 (NH, NH)
-2.06
-1.99
10 (dioxo)a
a Two smaller, irreversible peaks follow the first reduction.
Table 7. UV-Vis Absorption Spectral Data of Compounds 1-10 in
CH3CN
compd
λ
max (nm)
ꢀ (M-1 cm-1
)
Figure 6. Cyclic voltammogram of 1 in CH3CN, 0.2 M TBAHFP; sweep
rate, 100 mV.
1 (NH, O)
2 (NH, O)
520
520
350 (sh)
510
390 (sh)
350 (sh)
550 (sh)
420 (sh)
350
15 000
15 000
21 000
15 000
24 000
27 000
13 000
32 000
∼50 000
9 000
20 000
27 000
40 000
3 000
12 000
23 000
26 000
42 000
8 500
3 (NH, O)
4 (OCH3)2
5 (NH, S)
6 (O, O)
7 (O, O n)
8 (S, S)
610
360
420 (sh)
360
600 (sh)
410 (sh)
355
475 (sh)
380 (sh)
420
9 (NH, NH)
10 (dioxo)
deshielded than the protons from the methyl groups in the o
position, except in complex 8, in which the protons in the o
positions are more deshielded.
The three methanol protons in 10 appear in two singlets
of approximately equal intensity at 3.14 and 3.16 ppm. This
is probably due to the replacement of CH3OH with (CD3)2SO
(eq 5).16
Figure 7. UV-vis absorption data from the spectroelectrochemistry of 1
in CH3CN, 0.1 M TBATFB.
MoO2(sap)(CH3OH) + (CD3)2SO T
MoO2(sap)(CD3)2SO + CH3OH (5)
with two oxygen atoms. This is unexpected, but can be
attributed to the structural differences between the two
compounds. The benzene-1,2-dithiolate in 8 occupies two
equatorial positions, whereas the bidentate ligands in all other
cases bind in an equatorial and an axial position. It is also
interesting to note that the complexes with the most positive
first reduction potentials have imido linkages with angles
close to 180°, and the complexes with more negative
potentials have angles around 166°.
Using spectroelectrochemistry, we were able to establish
that the reversible reduction is a direct one-electron reduction
from Mo(VI) to Mo(V). Figure 6 shows the cyclic voltam-
mogram of compound 1. The changing visible spectra during
the gradual reduction of complex 1 can be seen in Figure 7.
Plots of Eapplied vs log[O]/[R] show that the reduction is a
one-electron reduction.20 Solutions of the complexes can also
be reduced with sodium amalgam to yield Mo(V) species
Electrochemical and UV-Vis Absorption Data. The
electrochemical data for all compounds are listed in Table
6, and the UV-vis absorption data, in Table 7. The imido
complexes have one reversible and one generally irreversible
reduction between 0 and -2.1 V (Figures 6 and 9). The
reversibility of the second peak depends on the bidentate
ligand, the scan rate, and the temperature. The second
reduction is reversible for compounds 6 and 7 at -25 °C or
at scan rates higher than 200 mV/s at room temperature.
Complex 7 has the most positive reduction potential of the
complexes studied here; complex 9 has the most negative.
Since nitrogen is a good σ donor, it is not surprising that 9
is least likely to accept an electron, whereas 7, with two
electronegative oxygen atoms attached to a naphthalene ring,
can be reduced more easily. Complex 8 with two sulfurs on
the bidentate ligand has the same potential as complex 6
5462 Inorganic Chemistry, Vol. 45, No. 14, 2006

Molybdenum(VI) Imido Complexes
Figure 10. UV-vis absorption spectra of 6 before (green) and after the
reduction with one electron (blue) and after the reduction with two electrons
(red).
Figure 8. Room temperature EPR spectrum of the Mo(V) derivative of
1.
Figure 9. Cyclic voltammogram of 6 in CH3CN, 0.1 M TBAHFP; sweep
rate, 400 mV/s; -25 °C.
with the same visible spectra as those obtained from the
spectroelectrochemistry experiment. A room temperature
EPR spectrum of the chemically reduced 1 is shown in Figure
8. The g value of 1.95 is typical for Mo(V) complexes with
nitrogen- and oxygen-containing ligands.25,26
Figure 11. EPR spectra of 6 after a one-electron reduction and after a
two-electron reduction.
electron reduction, but the potential is significantly more
positive,15 as one would expect with a more electronegative,
multibonded ligand. A second reduction peak is also present
in the cyclic voltammogram of the oxo complex. It becomes
more reversible with increasing sweep rate. This second
reduction peak is found not only in the Mo(VI) monooxo
and -imido complexes but also in Mo(VI) dioxo complexes,
MoO2(sap)(S) (S ) Lewis base solvent),16 and Mo(IV) oxo
complexes, MoO(sap)(S).27 It could represent a reduction
from Mo(V) to Mo(IV)15 or a reduction of the sap ligand.
The fact that this peak is also found in the cyclic voltam-
mogram of Mo(IV) oxo complexes supports the latter
interpretation. The cyclic voltammogram of the Mo(IV))
complex has an irreversible oxidation peak. Even if the scan
is started at a more negative potential than the oxidation
potential from Mo(IV) to Mo(V),27 the reduction peak is still
A cyclic voltammogram of 6 is shown in Figure 9. The
second reduction of 6 is completely reversible at -25 °C
and was determined to be a one-electron reduction. Visible
spectra of 6 before the reduction, after the one-electron
reduction, and after the two-electron reduction are shown in
Figure 10. After the first reduction, the solution shows an
EPR signal with a g value of 1.960 (Figure 11). The EPR
signal decreases to <5% of its original size after the second
reduction. The reduced complex can be reoxidized to the
starting material. On the basis of the ꢀ values, the electronic
transitions are ligand-to-metal charge-transfer bands.
The oxo complex MoO(naphthalene-2,3-diolate)(sap), like
the imido compounds studied here, shows a reversible one-
(25) Scullane, M. I.; Taylor, R. D.; Minelli, M.; Spence, J. T.; Yamanouchi,
K.; Enemark, J. H.; Chasteen, N. D. Inorg. Chem. 1979, 18, 3213.
(26) Xiao, Z.; Bruck, M. A.; Doyle, C.; Enemark, J. H.; Grittini, C.; Gable,
R. V.; Wedd, A. G.; Young, G. C. Inorg. Chem. 1995, 34, 5950.
(27) Boyd, I. W.; Spence, J. T. Inorg. Chem. 1982, 21, 1602.
Inorganic Chemistry, Vol. 45, No. 14, 2006 5463

Minelli et al.
present. This interpretation is consistent with the EPR results
for 6, since the lone electron from the reduction of sip can
couple with the electron from the Mo(V), leading to a
disappearance of the EPR signal.
Compound 10 has two smaller irreversible peaks following
the first reduction peak at -1.16 V and a peak at -1.99 V.
The potentials are comparable to data from the literature.16
ligand. A second, generally irreversible one-electron reduc-
tion is reversible for the catecholate and naphthalene-2,3-
diolate complexes. Complexes with more positive first
reduction potentials tend to have imido linkages with angles
close to 180°.
Acknowledgment. We thank Grinnell College for finan-
cial support of this research. M.M. thanks the University of
Konstanz and especially Prof. P. Kroneck for the opportunity
to spend July 2004 as a visiting professor there and work
on this project. M.M. gives special thanks to Professor K.
Wieghardt at the Max-Planck Institute for Bioinorganic
Chemistry, Mu¨lheim (Ruhr), for the opportunity to spend a
sabbatical semester there in the fall of 2005.
Conclusions
The reaction of Mo(NAr)2Cl2(dme) with sapH2 leads to
the formation of imido-amido complexes, Mo(NAr)(1,2-
OC6H4NH)(sap). The compounds are formed via an inter-
mediate, Mo(NAr)(sap)(OCH3)2, which has been isolated.
This intermediate can be reacted with bidentate aromatic
ligands to form complexes in which, with the exception of
the 1,2-benzenedithiolate complex, the bidentate ligand
occupies an equatorial and an axial position. The Mo(VI) in
the compounds can be reversibly reduced to an EPR-active
Mo(V). The reduction potentials depend on the bidentate
Supporting Information Available: CIF file of crystallographic
data. This material is available free of charge via the Internet at
http://pubs.acs.org.
IC060370+
5464 Inorganic Chemistry, Vol. 45, No. 14, 2006