Molybdenum η2-imine complex formation and the reductive coupling of imines

Article abstract of DOI:10.1039/b000355g

Addition of imine PhN=C(Ar)R (Ar = Ph, R = Me; p-MeOC₆H₄, R = H) to (PhN)Mo(TMS-o-pda)(CH₂=CMe₂) 1 [TMS-o-pda = bis(trimethylsilyl)-o- phenylenediamide] affords the corresponding η²-imine complexes (P

Full text of DOI:10.1039/b000355g

2
Molybdenum h -imine complex formation and the reductive coupling of
imines†
Tom M. Cameron,a Carlos G. Ortiz,a Khalil A. Abboud,a James M. Boncella,*a R. Tom Bakerb and
Brian L. Scottb
a Department of Chemistry, University of Florida, Gainesville, FL 32611, USA. E-mail: boncella@chem.ufl.edu
b Chemical Science and Technology Division, Los Alamos National Laboratory, MS J514, Los Alamos, NM 87545,
USA
Received (in Bloomington, IN, USA) 5th January 2000, Accepted 25th February 2000
Published on the Web 17th March 2000
Addition of imine PhNNC(Ar)R (Ar = Ph, R = Me; p-
MeOC6H4, R = H) to (PhN)Mo(TMS-o-pda)(CH2NCMe2) 1
The short imido Mo(1)–N(3) bond length of 1.736(2) Å is
typical of a Mo–N triple bond interaction.9 The Mo(1)–N(4),
Mo(1)–N(2) and Mo(1)–N(1) bond lengths of 1.944(2),
2.009(2) and 1.996(2) Å, respectively, are all consistent with
Mo–N single bonds.10 The C(31)-centered bond angles of the
imine fragment are all < 120° and support a considerable
amount of azametallacyclopropane character in 2b as does the
N(4)–C(31) bond length of 1.414(3) Å.11
Complete conversion of 2a to the corresponding organic
amine (PhNHCH2Ar) was observed by 1H NMR upon exposure
of a solution of 2a to an atmosphere of 15 psi H2 at room
temperature over a 1 week period.12 Unfortunately no catalytic
activity was observed upon treatment of 2a with excess imine
under low pressures (ca. 15 psi) of H2 gas.
[TMS-o-pda = bis(trimethylsilyl)-o-phenylenediamide] af-
2
fords
the
corresponding
h -imine
complexes
(PhN)Mo(TMS-o-pda)[PhNNC(Ar)R] 2a and 2b; analogous
reactions with aldimines RNNC(H)Ar (Ar = p-MeOC6H4,
R = CH2Ph, Et) afford products (PhN)Mo(TMS-o-pda)-
[RNC(H)ArC(H)ArNR] 3a and 3b resulting from reductive
imine coupling, providing the first example of facile and
2
general Mo(IV) h -imine complex formation.
2
Current synthetic methodologies affording h -imine complexes
(azametallacyclopropanes) consist of C–H activation from
methylmetallocene amides,1,2 rearrangements of iminoacyl
complexes,3 reaction between Cp* ZrH2 and ArNC,4 reduction
The reaction of aldimines derived from less hindered primary
amines with 1 affords molybdenum(VI) bisdiamide imido
complexes 3a and 3b from reductive imine coupling
(Scheme 2).‡ Complexes 3a and 3b were isolated in 75% yield
from cold pentane–toluene solutions. Interestingly, only the
rac-coupled form of the metal complexes is isolated as
2
of a low-valent complex with phosphaazaallene,5 and various
2
in situ methods of h -imine complex formation.6 Isolation and
2
characterization of h -imine complexes generated by direct
reaction of imines with metal reductants has for the most part
been unsuccessful, and although two recent reports detail the
2
1
isolation and characterization of ytterbium6 and tantalum7 h -
ascertained from H and 13C NMR spectroscopy. An X-ray
imine complexes via such direct methods, examples with other
early transition metals are, to our knowledge, non-existent. We
report herein structural characterization and direct synthesis of
crystallographic study was performed on a single crystal of
3b.
The molecular structure and selected bond lengths and angles
are shown in Fig. 2.§ The complex adopts a distorted square
pyramidal geometry around molybdenum with the imido ligand
2
molybdenum(IV) h -imine complexes obtained by treatment of
the molybdenum(IV
)
olefin complex (PhN)Mo(TMS-o-
pda)(CH2NCMe2) 18 with appropriate aldimines and ketimines.
Furthermore, aldimine reductive coupling products are isolated
for less sterically demanding aldimines.
Reaction of the aryl amine derived aldimine PhNNC(H)Ar or
2
ketimine PhNNC(Me)Ph with 1 affords the h -imine complexes
2a and 2b, respectively, as green crystals in 70% isolated yield
(Scheme 1).‡ The molecular structure of 2b was determined by
an X-ray crystallographic study, and selected bond distances
and angles for 2b are shown in Fig. 1.§
Complex 2b adopts a five-coordinate square pyramidal
geometry with the imido ligand occupying the apical position.
Fig. 1 Thermal ellipsoid plot of 2b. Selected bond lengths (Å) and angles
(°): Mo(1)–N(1) 1.996(2), Mo(1)–N(2) 2.009(2), Mo(1)–N(3) 1.736(2),
Mo(1)–N(4) 1.944(2), Mo(1)–C(31) 2.200(3), N(3)–C(13) 1.409(3), N(4)–
C(31) 1.414(3), C(31)–C(32) 1.518(4); Mo(1)–N(3)–C(13) 163.0(2), N(2)–
Mo(1)–N(1) 85.28(9), N(4)–Mo(1)–C(31) 39.29(9), N(2)–Mo(1)–N(4)
100.09(9), N(1)–Mo(1)–C(31) 99.20(9), N(4)–C(31)–Mo(1) 60.55(13),
N(4)–C(31)–C(32) 116.9(2), N(4)–C(31)–C(25) 116.6(2), C(32)–C(31)–
C(25) 116.2(2).
2
Scheme 1 The generation of molybdenum(IV) h -imine complexes.
† Electronic supplementary information (ESI) available: spectroscopic data
for 1, 2a, 2b, 3a and 3b. See http://www.rsc.org/suppdata/cc/b0/b000355g/
DOI: 10.1039/b000355g
Chem. Commun., 2000, 573–574
This journal is © The Royal Society of Chemistry 2000
573

of 3a and 3b was similar but required the addition of 2 equivalents of the
appropriate imine.
§ Crystal data: for 2b: C32H40N4Si2Mo, M = 632.80, a = 9.8565(5), b =
18.8443(8), c = 17.9089(8) Å, b = 104.6520(1)°, V = 3218.2(3) Å3,
monoclinic, space group P21/c, Z = 4, T = 203(2) K, final R1 = 0.0431,
wR2 = 0.0831, GOF (on F2) = 1.225.
For 3b: C38H53MoN5O2Si2, M
19.0705(9), c 10.2628(5) Å, b
=
=
763.97, a
97.351(1)°, V
=
9.9600(5), b
=
=
=
1933.3(2) Å3,
monoclinic, space group P21, Z = 2, T = 173(2) K, final R1 = 0.0420, wR2
= 0.0775, GOF (on F2) = 1.021.
Both structures were solved using the direct methods option of SHELXS.
Full-matrix least-squares refinements based on F2 were subsequently
performed using SHELXL 97.13 All non-hydrogen atoms were assigned
anisotropic temperature factors, with corresponding hydrogen atoms
included in calculated positions.
Scheme 2 The reductive coupling of imines.
CCDC 182/1560. See http://www.rsc.org/suppdata/cc/b0/b000355g/ for
crystallographic files in .cif format.
1 P. Berno and S. Gambarotta, Organometallics, 1995, 14, 2159; D. A.
Gately, J. R. Norton and P. A. Goodson, J. Am. Chem. Soc., 1995, 117,
986; L. Buchwald, B. T. Watson, M. W. Wannamaker and J. C. Dewan,
J. Am. Chem. Soc., 1989, 111, 4486; J. A. Tunge, D. A. Gately and J. R.
Norton, J. Am. Chem, Soc., 1999, 121, 4520.
2
2 A Mo(H)(h -imine) complex has been reported and presumably arises
via C–H activation of an amide ligand. Y. Tsai, M. J. Johnson, D. J.
Mindiola, C. C. Cummins, W. T. Klooster and T. F. Koetzle, J. Am.
Chem. Soc., 1999, 121, 10 426.
3 M. J. Scott and S. J. Lippard, Organometallics, 1997, 16, 5857; J. R.
Clark, P. E. Fanwick and I. P. Rothwell, Organometallics, 1996, 15,
3232; L. D. Durfee, J. E. Hill, P. E. Fanwick and I. P. Rothwell,
Organometallics, 1990, 9, 75; L. D. Durfee, P. E. Fanwick, I. P.
Rothwell, K. Folting and J. C. Huffman, J. Am. Chem. Soc., 1987, 109,
4720; K. W. Chiu, R. A. Jones, G. Wilkinson, A. M. R. Galas and M. B.
Hursthouse, J. Chem. Soc., Dalton Trans., 1981, 2088; J. Am. Chem.
Soc., 1980, 102, 7978.
4 P. T. Wolczanski and J. E. Bercaw, J. Am. Chem. Soc., 1979, 101,
6450.
5 J. B. Alexander, D. S. Glueck, G. P. A. Yap and A. L. Rheingold,
Organometallics, 1995, 14, 3603.
Fig. 2 Thermal ellipsoid plot of 3b with 50% probability ellipsoids. Selected
bond lengths (Å) and angles (°): Mo–N(1) 1.754(3), Mo–N(2) 2.008(3),
Mo–N(3) 2.065(3), Mo–N(4) 1.996(3), Mo–N(5) 2.021(3), N(4)–C(19)
1.476(5), N(5)–C(29) 1.468(5), C(19)–C(29) 1.514(5); Mo–N(1)–C(1)
166.6(3), N(2)–Mo–N(3) 80.65(14), N(3)–Mo–N(5) 85.75(12), N(5)–Mo–
N(4) 77.46(12), N(2)–Mo–N(4) 102.59(12).
6 Y. Makioka, Y. Taniguchi, Y. Fujiwara, K. Takaki, Z. Hou and Y.
Wakatsuki, Organometallics, 1996, 15, 5476 and references therein.
7 K. Takai, T. Ishiyama, H. Yasue, T. Nobunaka, M. Itoh, T. Oshiki, K.
Mashima and K. Tani, Organometallics, 1998, 17, 5128.
8 We have recently prepared a variety of Mo(IV)(olefin) complexes
(manuscript in preparation) from the corresponding dichloride species
(C. G. Ortiz, K. A. Abboud and J. M. Boncella, Organometallics, 1999,
18, 4253). Preparation of PhNMo(TMS-o-pda)(CH2NCMe2): To a 278
°C blue ethereal solution of PhNMo(TMS-o-pda)Cl2•THF (1.0 g, 1.7
mmol) was added 2 equivalents of ClMgCH2CHMe2 (2.0 M in Et2O, 1.7
ml) with stirring, which resulted in the emergence of a red-colored
solution. Upon warming to room temperature (ca. 3 h) the reaction
mixture turned green whereupon the solvent was removed in vacuo. The
resulting green residue was extracted twice with 25 ml of pentane. The
solvent was then removed in vacuo affording a green, waxy oil 1 in 65%
yield.
9 P. W. Dyer, V. C. Gibson, J. A. K. Howard, B. Whittle and C. Wilson,
Polyhedron, 1995, 14, 103; P. W. Dyer, V. C. Gibson and W. Clegg,
J. Chem. Soc., Dalton Trans., 1995, 3313; P. W. Dyer, V. C. Gibson,
J. A. K. Howard, B. Whittle and C. Wilson, J. Chem. Soc., Chem.
Commun., 1992, 1666; N. Bryson, M. T. Youinou and J. A. Osborn,
Organometallics, 1991, 10, 3389.
10 M. B. O’Donoghue, W. M. Davis and R. R. Schrock, Inorg. Chem.,
1998, 37, 5149; Z. Duan and J. G. Verkade, Inorg. Chem., 1995, 34,
1576; N. C. Mösch-Zanetti, R. R. Schrock, W. M. Davis, K. Wanninger,
S. W. Siedel and M. B. O’Donoghue, J. Am. Chem. Soc., 1997, 119,
11 037.
occupying the apical position. The Mo–N(1) bond length of
1.754(3) Å is consistent with a molybdenum nitrogen triple
bond interaction.9 The Mo–N(2), N(3), N(4) and N(5) amide
bond lengths of 2.008(3), 2.065(3), 1.996(3) and 2.021(3) Å,
respectively, are within the range expected for Mo–N single
bonds.10 The hydrogen atoms located on the backbone of the
newly formed diamide ligand are related by an H–C(19)–
C(29)–H torsion angle of 90°. Consistent with this torsion angle
there is no observed coupling between these protons in the
solution 1H NMR spectrum at 300 MHz. Complex 3a displays
similar characteristics in its 1H NMR spectrum.
In summary, we have demonstrated that chelate-supported
2
Mo(IV) h -imine complexes can be easily prepared via displace-
ment of olefin from 1 upon reaction with N-aryl imines. In
contrast, imine reductive coupling products were observed for
sterically less demanding imines. We are currently investigating
2
the reactivity of these h -imine complexes with unsaturated
organic molecules.
J. M. B. thanks the National Science Foundation (CHE
9523279) for funding of this work. K. A. A. thanks the NSF and
the University of Florida for funding X-ray equipment pur-
chases, and R. T. B. thanks the Science and Technology Base
programs at Los Alamos.
11 C–N single-bond length 1.45 Å and C–N double-bond length 1.27 Å: M.
Burkr-Laing and M. Laing, Acta. Crystallogr., Sect. B, 1976, 32,
3216.
12 (a) The identity of the amine produced by hydrogenolysis was
confirmed via generation of the amine from the corresponding imine
and LAH and comparison of 1H NMR spectra after appropriate workup.
At this time the nature of the metal containing products of this reaction
are unknown, though we have isolated hydride complexes of closely
related tungsten complexes;12b (b) J. M. Boncella, S.-Y. S. Wang and D.
D. Van der Lende, J. Organomet. Chem., 1999, 591, 8.
Notes and references
‡ All reactions and manipulations were carried out using standard Schlenk
techniques or a dry box under atmospheres of nitrogen and argon. Synthesis
of 2a and 2b: to a green solution of 1 (0.50 g, 1.05 mmol) in pentane at room
temperature was added a pentane solution of the appropriate imine (0.20 g,
1.05 mmol). After stirring for 12 h the pentane solution was concentrated in
vacuo and cooled, affording the appropriate metal complexes. The synthesis
13 SHELXTL/NT Version 5.10, Bruker Analytical X-Ray Instruments,
Inc., Madison, Wisconsin 53719, 1997.
Communication b000355g
574
Chem. Commun., 2000, 573–574