Imidomolybdenum(IV) porphyrin complexes: Synthesis, characterization, and intermetal imido transfer reactivity

Article abstract of DOI:10.1021/ic051100e

The imido(meso-tetra-p-tolylporphyrinato)molybdenum(IV) complexes, (TTP)Mo=NR, where R = C₆H₅ (1a), p-CH₃C ₆H₄ (1b), 2,4,6-(CH₃)₃C ₆H₂ (1c), and 2,6-(i-Pr)<

Full text of DOI:10.1021/ic051100e

Inorg. Chem. 2005, 44, 7304−7306
Imidomolybdenum(IV) Porphyrin Complexes: Synthesis,
Characterization, and Intermetal Imido Transfer Reactivity
Lisa M. Berreau,† Jinyuan Chen,‡ and L. Keith Woo*,‡
Department of Chemistry, Iowa State UniVersity, Ames, Iowa 50011-3111, and Department of
Chemistry, Utah State UniVersity, Logan, Utah 84322-0300
Received July 1, 2005
The imido(meso-tetra-p-tolylporphyrinato)molybdenum(IV) com-
plexes, (TTP)Mo NR, where R C6H5 (1a), p-CH3C6H4 (1b),
2,4,6-(CH3)3C6H2 (1c), and 2,6-(i-Pr)2C6H4 (1d), can be prepared
by the reaction of (TTP)MoCl2 with 2 equiv of LiNHR in toluene.
Upon treatment of the imido complexes with pyridine derivatives,
Metalloporphyrin complexes have been extremely useful
in investigating a variety of inner-sphere redox processes
involving intermetal halogen,10 oxygen,11 nitrogen,12 sulfur,13
and selenium atom transfer reactions.13 However, extension
of this system to intermetal imido transfer has not material-
ized, even though metalloporphyrin complexes containing
organoimido ligands are becoming more common.14-18
Surprisingly, the only imidomolybdenum porphyrin examples
are Mo(V) complexes, and only one has been structurally
characterized.8,19 We previously reported that treatment of
(TTP)MCl2 (M ) Ti, Zr, Hf; TTP ) meso-tetra-p-tolylpor-
phyrinato) with various lithium amides results in the forma-
tion of M(IV) organoimido complexes.20,21 By extension of
this synthetic method to molybdenum chemistry, we have
been able to prepare new organoimidomolybdenum(IV)
d
)
NC5H4-p-X (X
complexes, (TTP)Mo
between the molybdenum imido complexes, (TTP)Mo
(TTP)Mo NC6H4CH3, and (TTP)Ti( CPh) resulted in
2-PhC
complete imido group transfer and two-electron redox of the metal
centers to give (TTP)Mo( CPh) and (TTP)Ti NC6H5 or
2-PhC
(TTP)Ti NC6H4CH3.
)
CH3, CH(CH3)2, C
NR NC5H4-p-X, were observed. The reaction
NC6H5 or
tN), new six-coordinate
d
‚
d
d
η
t
η
t
d
d
The transfer of an oxygen atom from transition-metal-
oxo complexes to organic or nonmetal substrates is a well-
documented process.1 The related reaction of intermetal
oxygen atom transfer continues to be an area of intense
research and has been extensively reviewed.2 However, fewer
examples involving the isoelectronic imido group transfer
have been reported.3-8 Furthermore, imido group transfer
processes reported to date have largely involved transfer to
nonmetal substrates, e.g., phosphines, alkanes, and alkenes.
Examples of intermetal pairwise exchange reactions involv-
ing oxo, imido, and alkylidene ligands were reported by
Gibson et al. In these latter reactions, no net redox change
occurs.9
(9) Jolly, M.; Mitchell, J. P.; Gibson, V. C. J. Chem. Soc., Dalton Trans.
1992, 1331.
(10) (a) Chapman, R. D.; Fleischer, E. B. J. Am. Chem. Soc. 1982, 104,
1582. (b) Cohen, I. A.; Jung, C.; Governo, T. J. Am. Chem. Soc. 1972,
94, 3003. (c) Reed, C. A.; Kouba, J. K.; Grimes, C. J.; Cheung, S. K.
Inorg. Chem. 1978, 17, 2666.
(11) (a) Woo, L. K.; Goll, J. G.; Berreau, L. M.; Weaving, R. J. Am. Chem.
Soc. 1992, 114, 7411. (b) Woo, L. K.; Hays, J. A.; Goll, J. G. Inorg.
Chem. 1990, 29, 3916.
(12) (a) Woo, L. K.; Goll, J. G. J. Am. Chem. Soc. 1989, 111, 3755. (b)
Woo, L. K.; Czapla, D. J.; Goll, J. Inorg. Chem. 1990, 29, 3915. (c)
Woo, L. K.; Goll, J. G.; Czapla, D. J.; Hays, J. A. J. Am. Chem. Soc.
1991, 113, 8478. (d) Bottomley, L. A.; Neely, F. L. J. Am. Chem.
Soc. 1989, 111, 5955.
(13) (a) Woo, L. K.; Hays, J. A. Inorg. Chem. 1993, 32, 2228. (b) Berreau,
L. M.; Woo, L. K. J. Am. Chem. Soc. 1995, 117, 1314.
(14) (a) Smieja, J. A.; Omberg, K. M.; Breneman, G. L. Inorg. Chem. 1994,
33, 614. (b) Smieja, J. A.; Shirzad, K.; Roy, M.; Kittilstved, K.;
Twamley, B. Inorg. Chim. Acta 2002, 335, 141. (c) Au, S. M.; Fung,
W. H.; Huang, J. S.; Cheung, K. K.; Che, C. M. Inorg. Chem. 1998,
37, 6564.
(15) Groves, J. T.; Takahashi, T. J. Am. Chem. Soc. 1983, 105, 2073.
(16) (a) Che, C. M.; Huang, J. S.; Li, Z. Y.; Poon, C. K. Inorg. Chim.
Acta 1991, 190, 161. (b) Huang, J. S.; Che, C. M.; Poon, C. K. J.
Chem. Soc., Chem. Commun. 1992, 161.
(17) Elliot, R. L.; Nichols, P. J.; West, B. O. Polyhedron 1987, 12, 2191.
(18) Buchler, J. W.; Pfeifer, S. Z. Naturforsch. B: Anorg. Chem., Org.
Chem. 1985, 40B, 1362.
(19) (a) Kim, J. C.; Rees, W. S.; Goedken, V. L. Inorg. Chem. 1995, 34,
2483. (b) Kim, J. C.; Rees, W. S.; Goedken, V. L. Inorg. Chem. 1994,
33, 3191.
* To whom correspondence should be addressed. E-mail: kwoo@
iastate.edu.
† Utah State University.
‡ Iowa State University.
(1) (a) Ostovic, D.; Bruice, R. Acc. Chem. Res. 1992, 25, 314. (b) Holm,
R. H. Chem. ReV. 1987, 87, 1401. (c) Holm, R. H.; Donahue, J. P.
Polyhedron 1993, 12, 571. (d) Jorgensen, K. A. Chem. ReV. 1989,
89, 431.
(2) Woo, L. K. Chem. ReV. 1993, 93, 1125.
(3) Holm, R. H.; Harlan, E. W. J. Am. Chem. Soc. 1990, 112, 186.
(4) Dobbs, D. A.; Bergman, R. G. J. Am. Chem. Soc. 1993, 115, 3836.
(5) Rovis, T.; Evans, D. A. Prog. Inorg. Chem. 2001, 50, 1.
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1079.
(20) (a) Berreau, L. M.; Young, V. G., Jr.; Woo, L. K. Inorg. Chem. 1995,
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7304 Inorganic Chemistry, Vol. 44, No. 21, 2005
10.1021/ic051100e CCC: $30.25
© 2005 American Chemical Society
Published on Web 09/15/2005

COMMUNICATION
1
porphyrin complexes. These compounds undergo a two-
electron intermetal redox process mediated by imido group
transfer. Specifically, treatment of (TTP)ModNR (R ) C6H5,
C6H4-p-CH3) with a low-valent titanium porphyrin complex,
(TTP)Ti(η2-PhCtCPh), resulted in complete intermetal
imido group transfer, a formal two-electron redox process.
We previously showed that (TTP)MoCl2 is a useful
precursor for the preparation of a number of new molybde-
num porphyrin complexes.22 This practical starting material
can be readily converted into new metalloporphyrin imido
complexes. As illustrated in eq 1, anerobic treatment of
The new porphyrin complexes were characterized by H
NMR, UV-vis, mass, and IR spectroscopy. Although the
IR spectrum of 1 contains a band at 753 cm-1 that may
correspond to a ModN stretch,19a coupling with a N-C
stretching mode and/or other vibrations of the organoimido
group complicates the assignment.28 Molybdenum(IV) por-
phyrins have been shown to exhibit both paramagnetic
([TTP]MoCl2)29 and diamagnetic ([TTP]ModO) properties.29
The 1H NMR spectra of the imido complexes exhibit sharp
resonances that are diagnostic of diamagnetic porphyrin
compounds. For example, the 1H NMR spectrum of the five-
coordinate imido complex (TTP)ModNC6H5 exhibits a
singlet for the â-pyrrole hydrogens at 8.73 ppm, multiplets
for the o- and p-tolyl protons at 7.96 (d, 8H) and 7.24 (m,
8H) ppm, and a tolyl methyl signal at 2.36 (s, 12H) ppm.
Particularly characteristic is the ring current effect on the
imido phenyl protons. These hydrogens exhibit upfield
shifts ranging from 0.7 to 1.9 ppm relative to the free
amine: 6.00 (t, 1H, p-H), 5.88 (t, 2H, m-H), and 4.77 (d,
2H, o-H) ppm.30
23
(TTP)MoCl2 with LiNHR resulted in the formation of
molybdenum(IV) organoimido complexes. Relatively pure
(TTP)ModN-Ph (1a) could be obtained by recrystallization
(TTP)ModNR + 2LiCl +H NR
(TTP)MoCl2 + 2LiHNR f
2
1a, R ) C6H5
1b, R ) p-CH3C6H4
1c, R ) 2,4,6-(CH3)3C6H2
1d, R ) 2,6-(i-Pr)2C6H3,
(1)
When excess amine is present, the formation of six-
coordinate complexes is observed. As noted above, this was
a complication during the initial imido synthesis because
neutral amine was produced as a byproduct. The initially
isolated imido complexes would have up to 1 equiv of the
parent amine, bound as a sixth ligand. NMR studies showed
that the position of the â-pyrrole proton resonance of these
adducts depended upon the amount of amine present, with
the signal shifting upfield with an increasing amount of
coordinated amine. A range of 8.4-8.7 ppm has been
observed for the â-pyrrole resonances. Signals for the
coordinated p-toluidine in complex 2 are shifted upfield as
well, appearing at 6.81 (d, 2H, m-H), 6.09 (d, 2H, o-H), and
2.12 (s, 3H, CH3) ppm.30
from a minimal amount of toluene/hexane (ca. 1:3) on
cooling to -20 °C for 1 h. An orange-red solid was isolated
in approximately 60% yield after filtration, washing with
hexanes, and drying in vacuo.24 A similar method was used
to prepare tolyl,25 mesityl,26 and 2,6-diisopropylphenyl27
analogues. In all cases, analytically pure samples were
difficult to obtain because of the inclusion of varying
amounts of amine adduct produced as a side product, 1‚(H2-
NR)x. Only in the case of the mesitylimido complex, 1c, was
an analytically pure material obtained after several recrys-
tallizations.26
(22) Berreau, L. M.; Young, V. G., Jr.; Woo, L. K. Inorg. Chem. 1995,
34, 3485.
(23) Berreau, L. M.; Hays, J. A.; Young, V. G., Jr.; Woo, L. K. Inorg.
Chem. 1994, 33, 105.
The initial amine adducts were labile. Thus, treatment of
the imido complexes 1a‚(H2NR)x and 1b‚(H2NR)x with
pyridine derivatives resulted in the loss of the coordinated
amine and the formation of new six-coordinate adducts,
(TTP)ModNR‚L.31
(24) Analytical data for 1a: 1H NMR (C6D6, 300 MHz, ppm) 8.73 (s, 8H,
â-H), 7.96 (d, 8H, -C6H4CH3), 7.24 (m, 8H, -C6H4CH3), 6.0 (t, 1H,
p-H), 5.88 (t, 2H, m-H), 4.77 (d, 2H, o-H), 2.36 (s, 12H, -C6H4CH3);
UV-vis (toluene) 432 (Soret), 454 (sh), 552, 614 nm; MS (FAB, CH3-
CN, positive ion) m/e 857 (857); IR (KBr, cm-1) bands associated
with the porphyrin ligand, 1475, 1332, 1208, 1182, 1109, 1069, 1005,
844, 800, 724, 523; bands for ModNPh, 753, 686.
(TTP)ModNR‚(H2NR)x + L f (TTP)ModNR‚L + xH2NR
2a, R ) tolyl, L ) 4-methylpyridine
(25) Analytical data for 1b: 1H NMR (C6D6, 300 MHz, ppm) 8.39 (s, 8H,
â-H), 7.88 (d, 8H, -C6H4CH3), 7.27 (m, 8H, -C6H4CH3), 5.81 (d,
2H, dNC6H4CH3), 5.01 (d, 2H, dNC6H4CH3), 2.34 (s, 12H,
-C6H4CH3), 1.37 (s, 3H, dNC6H4CH3); UV-vis (toluene, nm) 302,
430 (Soret), 452 (sh), 550, 570 (sh); MS (FAB, positive ion) m/e 870
(M+, 870).
(2)
2b, R ) phenyl, L ) 4-methylpyridine
3, R ) phenyl, L ) 4-isopropylpyridine
(26) Analytical data for 1c: 1H NMR (C6D6, 300 MHz, ppm) 8.65 (s, 8H,
â-H), 7.95 (d, 8H, o-tolyl), 7.23 (m, 8H, m-tolyl), 5.65 (s, 2H,
m-mesityl), 2.36 (s, 12H, C6H4CH3), 1.42 (s, 3H, p-C6H2Me3), 0.03-
(s, 6H, o-C6H2Me3); UV-vis (toluene) 430 (Soret), 551, 629 nm; MS
(EI, positive ion) m/e 899.2 (M+, 899.3). Anal. Calcd for C57H47-
MoN5‚0.4C7H8: C, 76.83; H, 5.41; N, 7.49. Found: C, 76.18; H, 5.48;
N, 6.88. The toluene solvate (0.4 equiv) was observed in the 1H NMR
spectrum of the sample submitted for elemental analysis.
The 1H NMR spectrum of 2b clearly exhibited a new methyl
signal at 1.63 (s, 3H) ppm corresponding to the new picoline
(28) Nugent, W. A.; Mayer, J. M. Metal-Ligand Multiple Bonds; Wiley:
New York, 1988; Chapter 4, pp 123-125.
(29) Diebold, T.; Chevrier, B.; Weiss, R. Inorg. Chem. 1979, 18, 1193.
(30) Data for aniline: 1H NMR (C6D6, ppm) 7.17 (m, 2H, m-H), 6.70 (m,
2H, o-H), 6.77 (m, 1H, p-H). Data for p-toluidine: 1H NMR (C6D6,
ppm) 6.99 (d, 2H, m-H), 6.33 (d, 2H, o-H), 2.26 (s, 3H, CH3).
(31) Data for (TTP)ModN-tolyl‚4-methylpyridine: 1H NMR (C6D6, 300
MHz, ppm) 8.32 (d, 2H, pic), 7.51 (m, 8H, -C6H4CH3), 7.09 (m,
8H, -C6H4CH3), 6.92 (s, 8H, â-H), 6.2-6.4 (m, 6H, dN-tolyl, pic),
2.24 (s, 12H, -C6H4CH3), 1.86 (s, 3H, dNC6H4CH3), 1.63 (s, 3H,
pic). Data for (TTP)ModNPh‚4-methylpyridine: 1H NMR (C6D6, 300
MHz, ppm) 7.94 (m, 2H, pic), 7.51 (m, 8H, -C6H4CH3), 7.10 (m,
8H, -C6H4CH3), 6.95 (s, 8H, â-H), 6.2-6.6 (m, 6H, dNPh, pic),
2.23 (s, 12H, -C6H4CH3), 1.58 (s, 3H, pic).
(27) Analytical data for 1d: 1H NMR (C6D6, 300 MHz, ppm) 8.79 (s, 8H,
â-H), 8.08 (d, 4H, -C6H4Me), 7.96 (d, 4H, -C6H4Me), 7.26 (d, 4H,
-C6H4Me), 7.21 (d, 4H, -C6H4Me), 6.31 (t, 1H, p-C6H3(i-Pr)2), 6.01
(d, 2H, m-C6H3(i-Pr)2), 2.37 (s, 12H, -C6H4Me), 0.01 (d, 12H,
-CHMe2), -0.15 (m, 2H, -CHMe2). A total of 0.3 equiv of free
(2,6-diisopropylphenyl)amine also existed in the product: 1H NMR
of (2,6-diisopropylphenyl)amine (C6D6) 7.05 (d, 2H, m-H), 6.89 (t,
1H, p-H), 3.12 (s, 2H, -NH), 2.62 (m, 2H, -CHMe2), 1.13 (d, 12H,
-CHMe2). Purification to remove free amine failed because of the
high solubility of the imido product in Et2O.
Inorganic Chemistry, Vol. 44, No. 21, 2005 7305

COMMUNICATION
fragment. Significant shifts are observed for all resonances
associated with the porphyrin ligand for the picoline com-
plexes. For example, the â-pyrrole resonance has been shifted
upfield and is observed as a sharp singlet at 6.92 ppm.
Resonances corresponding to the tolyl groups of the por-
phyrin ligand are also shifted upfield, with the methyl
resonance now appearing at 2.24 ppm. The signals associated
with the tolyl group of the imido ligand are instead shifted
downfield slightly, appearing in the region of 6.2-6.4 ppm.
Similar shifts are observed for the picoline derivative of the
phenylimido complex, 1a.
Other pyridine derivatives also produce new six-coordinate
imido complexes. The addition of 4-isopropylpyridine to 1a‚
(H2NR)x affords the molybdenum imido complex trans-(4-
i-Prpy)(TTP)ModNPh (3), which is isolated in 61% yield
after recrystallization from toluene/hexane (1:3).32
Molybdenum porphyrin complexes bearing multiply bonded
ligands are capable of transferring these terminal ligands to
other species. For example, (TTP)ModX (X ) S, Se) is
reduced by PPh3 to form XdPPh3.22 The resulting Mo(II)
intermediate was trapped with an alkyne. In contrast to this,
(TTP)ModNPh does not transfer its imido ligand to PPh3
after 3 days at ambient temperature. This is also different
from the chemistry of the lighter congener (TPP)CrdNPh
complex, which does transfer its imido ligand to PPh3 to
form the phosphine imine.33
transfer, yielding a known titanium porphyrin organoimido
(TTP)ModNR + (TTP)Ti(PhCtCPh) f
(TTP)Mo(PhCtCPh) + (TTP)TidNR
R ) C6H5 or C6H4-pCH3
(3)
complex, (TTP)TidNR,20 and a known molybdenum(II) η2-
alkyne complex, (TTP)Mo(η2-PhCtCPh).35 Both the six-
coordinated complexes 1‚(H2NR)x and the amine-free com-
plexes, 1, participate in the imido transfer reactions. The
amine-free reactions produce cleaner NMR spectra and are
reported here. In a typical, ambient-temperature reaction, an
NMR tube containing a C6D6 solution of (TTP)ModNPh
was treated with a slight excess of (TTP)Ti(PhCtCPh). The
reaction was monitored over a period of 3 days. After 14 h,
new â-pyrrole resonances corresponding to (TTP)TidNPh
(9.21 ppm)20 and (TTP)Mo(PhCtCPh) (8.99 ppm) began
to appear. Over the course of the reaction, the resonances of
the starting materials diminish, while those of the products
intensify, yielding a final spectrum composed of the reso-
nances of (TTP)Mo(PhCtCPh) and (TTP)TidNPh. The
overall time required for the reaction to go to completion is
approximately 2 weeks. The slow nature of this reaction
could be due to steric problems associated with a possible
µ-imido intermediate. Further studies involving the synthesis
and reactivity of early-transition-metal porphyrinatoimido
complexes are underway.
In an extension of our work with intermetal inner-sphere
reactions, a net two-electron redox process, mediated by
imido group transfer, occurs with these metalloporphyrin
complexes. As shown in eq 3, treatment of (TTP)ModNC6H5
(1a) with a titanium(II) η2-acetylene complex, (TTP)Ti(η2-
PhCtCPh),34 resulted in complete intermetal imido group
Acknowledgment. Financial support for this work was
provided in part by the National Science Foundation and the
Petroleum Research Fund, administered by the American
Chemical Society.
Supporting Information Available: Experimental procedures
for base-free complexes 1a-d. This material is available free of
charge via the Internet at http://pubs.acs.org.
(32) ModNPh‚4-i-Prpy: 1H NMR (C6D6, 300 MHz, ppm) 8.12 (br,
2H, py), 7.67 (d, 8H, -C6H4CH3), 7.54 (s, 8H, â-H), 7.15 (m, 8H,
-C6H4CH3), 6.5 (br, 2H, py), 6.34 (m, 2H, dNPh), 6.09 (t, 1H, d
NPh), 5.74 (m, 2H, dNPh), 2.28 (s, 12H, -C6H5CH3), 0.780 (m,
7H, py).
IC051100E
(34) Woo, L. K.; Hays, J. A.; Jacobson, R. A.; Day, C. L. Organometallics
1991, 10, 2102.
(33) Boujemaa, M.; Murray, K. S.; Nichols, P. J.; Thompson, S.; West, B.
O. Polyhedron 1994, 13, 485.
(35) De Cian, A.; Colin, J.; Schappacher, M.; Ricard, L.; Weiss, R. J. Am.
Chem. Soc. 1981, 103, 1850.
7306 Inorganic Chemistry, Vol. 44, No. 21, 2005