An unusual reduction of an imine bond: The isolation of a stable π-iminium intermediate [3]

Full text of DOI:10.1021/ja0109996

J. Am. Chem. Soc. 2001, 123, 10391-10392
10391
detectable intermediate en route from 4 to 6 is the protonated
complex, mer-[W(CO)3(1H)]+ (5a). The formation of the π-imi-
nium intermediate 5a is accompanied by a purple-to-orange color
change. In this step [(i) in Scheme 1] the W f π*imine charge-
transfer band at 580 nm,9 responsible for the purple color of the
starting material 4, disappears, resulting in the orange-colored
5a. The reaction will proceed no further than 5b if HBF4 is used
in place of HCl. Step (i) is fully reversible on addition of base
(e.g., piperidine).
An Unusual Reduction of an Imine Bond: The
Isolation of a Stable π-Iminium Intermediate
Eric W. Ainscough,* Andrew M. Brodie,*
Anthony K. Burrell, and Steven M. F. Kennedy
Chemistry - Institute of Fundamental Sciences
Massey UniVersity, PriVate Bag 11 222
Palmerston North, New Zealand
X-ray quality crystals of the cation 5b, were obtained by vapor
diffusion of n-pentane into a dichloromethane reaction mixture
of 4 and BF3.Et2O.10 The molecular structure of the cation 5b is
shown in Figure 1.11 The ligand 1 is coordinated unusually in
that the former imino group N-C(1) is bound to the tungsten
atom (W) as a π-iminium donor [N-W {2.221(3) Å}, C(1)-W
{2.219(3) Å}]. The π-iminium coordination mode is consistent
with protonation of the nitrogen lone pair as is the location of a
proton on a difference electron-density map and the observation
of a weak band at 3076 cm-1 in the IR, assignable to ν(N-H).
Also, monitoring the reaction of 4 with 1 equiv of CF3SO3H, by
1H NMR spectroscopy at temperatures from 213 to 293 K, gave
no evidence of metal-protonation which has been observed in
other tungsten carbonyl systems.12 The formation of 5 from 4
may, in part, be ascribed to the release of angle strain at the imine
nitrogen atom for the latter. In the molybdenum analogue of 4,
the C(12)-N-C(1) angle is 115.6(2)° which is 10° lower than
the value for the same angle in the uncoordinated ligand 1.8
However, on protonation in (5b) this increases to 120.7(3)°,
close to the expected value of 120° for an sp2-hybridized N atom.
The N-C(1) bond length at 1.432(4) Å is significantly longer
than found in the molybdenum analogue of 4 at 1.257(3) Å.9 The
bond parameters are similar to those found in other π-iminium
complexes.13 Changes in the spectroscopic parameters indicate a
considerable redistribution of electron density in the complex on
protonation. For example, ν(CO) bands shift by around 100 cm-1
to higher energy, relative to the starting material 4, and the 2J(P,P)
coupling constant decreases to 29 Hz.
ReceiVed April 20, 2001
Ionic hydrogenation of π-systems under mild conditions with
protic reagents, in conjunction with low-valent transition metal
complexes, is now receiving increasing attention as a complement
to the more traditional methods using reducing agents such as
LiAlH4, NaBH4, Na/EtOH, or H2/catalyst systems.1 For example,
protonation of: alkenes leads to σ-alkyl complexes or alkanes;2
the alkyne, PhCtCPh, to both cis- and trans-PhCHdCHPh;3
aldehydes and ketones to alcohols;4 imines to amines;4d,e,5
methyleneamido (CH2dN) to methylimido (CH3-N);6 and CH3Ct
N to CHCH3dNH.7 A recent report by Magee and Norton
discusses the catalytic, asymmetric, ionic hydrogenation of tetra-
alkyl substituted CdN cations.5a
Reported here is an unusual use of hydrochloric acid, as a sole
source of hydrogen atoms, to effect the reduction of the imine
function of the coordinated Schiff base 1 to the amine 2, via a
π-iminium intermediate. The reaction requires a sacrificial
reductantsin this case zerovalent tungsten which is concomitantly
oxidized to W(II). In contrast, the free ligand 1 reacts with acids
to afford the cyclic phosphonium salt 3.8
Unlike the HBF4 product 5b, the HCl product 5a detected in
situ by solution IR,14 reacts with an additional mole of acid,
yielding 6 [step (ii) in the Scheme]. In this case the metal has
undergone a two-electron oxidation (W0 f WII), and the original
imine bond has been reduced to a secondary amine, as supported
by the molecular structure of the seven-coordinate complex 6
shown in Figure 2.15 The angles C(1)-N-C(12) (109.0(2) °) and
C(42)-C(1)-N (111.0(3) °), coupled with a C(1)-N bond
distance of 1.455(4) Å, are supportive of the reduction of the
imine bond.16 Two chlorine atoms are found at bonding distances
from the metal [W-Cl(1), 2.5072(6) Å and W-Cl(2), 2.5050-
(6)] which is consistent with its formal +2 oxidation state.
The facile reduction of 1 to 2 occurs when the complex mer-
[W(CO)3(1)] (4)9 is exposed to HCl gas, the final product being
the complex cis-[WCl2(CO)2(2)] (6) (see Scheme 1). The first
(1) March, J. AdVanced Organic Chemistry: Reactions, Mechanisms and
Structure, 4th ed.; John Wiley & Sons: New York, 1992; pp 771, 910, 918.
(2) (a) O¨ hrstro¨m, L.; Stro¨mberg, S.; Glaser, J.; Zetterberg, K. J. Organomet.
Chem. 1998, 558, 123-130. (b) Bullock, R. M.; Song, J.-S. J. Am. Chem.
Soc. 1994, 116, 8602-8612.
(3) Henderson, R. A.; Lowe, D. J.; Salisbury, P. J. Organomet. Chem. 1995,
489, C22-C25.
(4) (a) Bullock, R. M.; Voges, M. H. J. Am. Chem. Soc. 2000, 122, 12594-
12595. (b) Smith, K.-T.; Norton, J. R.; Tilset, M. Organometallics 1996, 15,
4515-4520. (c) Song, J.-S.; Szalda, D. J.; Bullock, R. M.; Lawrie, C. J. C.;
Rodkin, M. A.; Norton, J. R. Angew. Chem., Int. Ed. Engl. 1992, 31, 1233-
1235. (d) Casey, C. P.; Singer, S. W.; Powell, D. R.; Hayashi, R. K.; Kavana,
M. J. Am. Chem. Soc. 2001, 123, 1090-1100. (e) Minato, M.; Fujiwara, Y.;
Koga, M.; Matsumoto, N.; Kurishima S.; Natori, M.; Sekizuka, N.; Yoshioka,
K.; Ito, T. J. Organomet. Chem. 1998, 569, 139-145.
(5) (a) Magee, M. P.; Norton, J. R. J. Am. Chem. Soc. 2001, 123, 1778-
1779. (b) Kabir, S. E.; Rosenberg, E.; Stetson, J.; Yin, M.; Ciurash, J.;
Mnatsakanova, K.; Hardcastle, K. I.; Noorani, H.; Movsesian, N. Organo-
metallics 1996, 15, 4473-4479.
(6) Shapeley, P. A.; Shusta, J. M.; Hunt, J. L. Organometallics 1996, 15,
1622-1629.
(7) McGilligan, B. S.; Wright, T. C.; Wilkinson, G.; Motevalli, M.;
Hursthouse, M. B. J. Chem. Soc., Dalton Trans. 1988, 1737-1742.
(8) Ainscough, E. W.; Brodie, A. M.; Burrell, A. K.; Fan, X.; Halstead,
M. J. R.; Kennedy, S. M. F.; Waters, J. M. Polyhedron 2000, 19, 2585-
2592.
(9) Ainscough, E. W.; Brodie, A. M.; Buckley P. D.; Burrell, A. K.;
Kennedy, S. M. F.; Waters, J. M. J. Chem. Soc., Dalton Trans. 2000, 2663-
2671.
(10) The same product 5b, as confirmed by 1H NMR, 31P NMR, IR
spectroscopy, and a single-crystal X-ray diffraction experiment, is obtained
when BF3.Et2O is used in place of HBF4, indicating that a residual amount of
HBF4 was present in purified BF3.Et2O.
(11) Crystal system ) triclinic; space group ) P-1; color of crystal )
orange; unit cell dimensions: a ) 10.91880(10) Å, b ) 12.88350(10) Å, c )
17.43280(10) Å, R ) 95.6520(10)°, â ) 105.2640(10)°, γ ) 111.0940(10)°;
temperature ) 203(2) K.; Z ) 2; Final R indices [I > 2σ(I)], R1 ) 0.0279,
wR2 ) 0.0661; GOF ) 1.056.
(12) (a) Siclovan, O. P.; Angelici, R. J. Inorg. Chem. 1998, 37, 432-444.
(b) Vila, J. M.; Shaw, B. L. J. Chem. Soc., Chem. Commun. 1987, 1778-
1779.
(13) Adams, R. D.; Babin, J. E.; Kim, H.-S. Organometallics 1987, 6, 749-
754 and references therein.
(14) See Supporting Information.
(15) Crystal system ) triclinic; space group ) P-1; unit cell dimensions:
a ) 11.0236(2) Å, b ) 12.0855(2) Å, c ) 16.6620(3) Å, R ) 68.9950(10)°,
â ) 85.7360(10)°, γ ) 71.9260(10)°; temperature ) 203(2) K.; Z ) 2; Final
R indices [I > 2σ(I)], R1 ) 0.0206, wR2 ) 0.0531; GOF ) 1.019.
(16) Kennard, O. International Tables for X-ray Crystallography;
Kynoch: Birmingham, 1962; Vol. 3, p 276.
10.1021/ja0109996 CCC: $20.00 © 2001 American Chemical Society
Published on Web 09/27/2001

10392 J. Am. Chem. Soc., Vol. 123, No. 42, 2001
Communications to the Editor
Scheme 1
Figure 1. ORTEP diagram for the cation of 5b. Selected bond lengths
(Å) and angles (deg): W-P(1), 2.4959(8); W-P(2), 2.4703(8); W-N,
2.221(3); W-C(1), 2.219(3); W-C(2), 2.045(3); W-C(3), 2.030(3);
W-C(4), 2.029(4); N-C(1), 1.432(4); N-C(12), 1.451(4); C(1)-C(42),
1.399(5), P(1)-W-P(2), 173.22(3); P(1)-W-N, 74.40(7); P(2)-W-
C(1), 73.32(8); C(4)-W-N, 172.07(12); C(4)-W-C(1), 149.87(13);
C(2)-W-C(3), 176.05(13); C(12)-N-C(1), 120.7(3); N-C(1)-C(42),
120.3(3).
Figure 2. ORTEP diagram for the complex 6. Selected bond lengths
(Å) and angles (deg): W-P(1), 2.4817(6); W-P(2), 2.4785(6); W-N,
2.315(2); W-C(2), 1.961(2); W-C(3), 1.962(3); W-Cl(1), 2.5072(6);
W-Cl(2), 2.5050(6); N-C(1), 1.455(4), P(1)-W-P(2), 123.16(2); P(1)-
W-N, 75.73(7), P(2)-W-N, 85.58(7); C(2)-W-C(3), 70.58(10); Cl-
(1)-W-Cl(2), 80.73(2); C(12)-N-C(1), 109.0(2); N-C(1)-C(42),
111.0(3).
(although we have not detected a metal-hydride species by NMR).
The importance of hydride transfer from the metal in the rate-
determining step has been demonstrated for the catalytic ionic
hydrogenation of CdN bonds using a Ru catalyst but in this case
the catalyst was a preformed metal hydride, that is, [CpRu{(S,S)-
Chiraphos}H].5a Further studies are required to clarify step (ii)
and the relationship of this work to the more general case of ionic
hydrogenation of isoelectronic π-alkene ligands.
In summary, the CdN bond of 4 can be easily reduced using
HCl as the sole source of hydrogen atoms. The requisite electrons
are provided via a concomitant oxidation of the metal from W(0)
to W(II). In the first step of the reaction, the metal-coordinated
nitrogen atom of 5a is protonated, resulting in a slippage from
the σ-bound imine to a π-iminium moiety. The final product 6,
contains the newly formed secondary amine functionality.
The molybdenum analogue of 6 is synthesized in identical
fashion to the tungsten product; however, solution of the X-ray
molecular structure only afforded atom connectivity. The chro-
mium analogue of 6 could not be obtained which is consistent
with the fact that seven-coordinate chromium(II) complexes are
rare.17
The first step in the CdN bond reduction reported here
proceeds with the imine nitrogen receiving a hydrogen atom via
direct protonation [step (i)]. Although the second hydrogen atom
is shown to finally reside on the imino carbon of 5a, as confirmed
by reacting 4 with DCl,18 there is more than one possible pathway
for step (ii): for example, either (a) the displacement of a
coordinated CO from tungsten by chloride accompanied by the
direct protonation of the iminium carbon or (b) the oxidative
addition of 5a by a second molecule of HCl, followed by an
intramolecular hydride migration to the protonated imine bond
Acknowledgment. The authors thank Massey University for financial
support.
(17) Larkworthy, L. F.; Nolan, K. B.; O’Brien, P. In ComprehensiVe
Coordination Chemistry; Wilkinson, G., Gillard, R. D., McCleverty, J. A.,
Eds.; Pergamon: Oxford, 1987; Vol. 3, pp 716-772.
Supporting Information Available: An Experimental Section con-
taining NMR, IR, and mass spectroscopic data, elemental analyses (PDF)
and X-ray crystallographic data (CIF) for 5b and 6. This material is
available free of charge via the Internet at http//pubs.acs.org.
(18) The reaction of DCl with 4 showed the integrals of the methylene
peaks in the 1H NMR spectrum of 6 at 4.70(dd, 3.1 and 11.8 Hz) and 4.18 (t,
11.8 Hz), are both reduced to approximately half, relative to the other 28
aromatic protons, as expected when comparing NH-CH2- with -ND-
CHD-.
JA0109996