Mono- and zerovalent manganese alkyl complexes supported by the α,α′-diiminato pyridine ligand: Alkyl stabilization at the expense of catalytic performance

Article abstract of DOI:10.1021/om0110078

The mono- and zerovalent manganese alkyl complexes supported by the α,α′-diiminato pyridine ligand were analyzed. The reactivity pattern provides both activation of the vanadium metal and a two electron reduction pathway towards rare monovalent vanadium alkyls. The coordination geometry of manganese was identical to that of the Fe and Co supercatalysts with the same distorted square pyramidal arrangement.

Full text of DOI:10.1021/om0110078

786
Organometallics 2002, 21, 786-788
Mon o- a n d Zer ova len t Ma n ga n ese Alk yl Com p lexes
Su p p or ted by th e r,r′-Diim in a to P yr id in e Liga n d : Alk yl
Sta biliza tion a t th e Exp en se of Ca ta lytic P er for m a n ce
Damien Reardon, Ghazar Aharonian, Sandro Gambarotta,* and Glenn P. A. Yap
Department of Chemistry, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada
Received November 21, 2001
Summary: The reaction of {[2,6-(i-Pr)2PhNdC(Me)]2-
(C5H3N)}MnCl2 with MeLi or Me3SiCH2Li afforded
reduction toward rare examples of non-cyclopentadienyl
and non-carbonyl Mn(I) and Mn(0) alkyl derivatives.
gated the behavior of the Mn derivative. Given the high
activity of the V and Fe derivatives, a Mn(II) diimino-
pyridinate complex may also be expected to be another
potent catalyst. However, we anticipated some uncer-
tainties regarding how the high-spin d5 electronic con-
figuration of Mn(II) might affect the stability of the
Mn-C bond and the other critical steps of the catalytic
cycle. It is worth noting that divalent Mn alkyl com-
plexes are stable species, and a substantial number of
homoleptic derivatives have been reported in the lit-
erature.8 Herein we describe our findings.
The reaction of MnCl2(THF)2 with [2,6-(i-Pr)2PhNd
C(Me)]2(C5H3N) in toluene is a straightforward com-
plexation reaction affording {2,6-bis[2,6-(i-Pr)2PhNd
C(Me)]2(C5H3 N)}MnCl2 (1), which was crystallized from
methylene chloride.9 The coordination geometry of
manganese10 is identical to that of the Fe and Co
“supercatalysts”4,5 with the same distorted square py-
In the past few years, efforts to develop alternatives
to group IV metallocene single-site olefin polymerization
catalysts have resulted in major breakthroughs. In par-
ticular, the employment of the sterically hindered [2,6-
(i-Pr)2PhNdC(Me)]2(C5H3N) ligand system, pioneered
by the groups of Gibson,1 Brookhart,2 and Bennett,3 has
enabled the preparation of an unprecedented family of
potent catalysts based on late transition metals. Theo-
retical work has suggested that the tremendous success
of the Fe(II) and Co(II) catalysts4,5 relies chiefly on the
complex electronic configuration capable of maintaining
a low-energy ethylene insertion barrier while providing
a relatively high activation energy for the â-H transfer
and â-H elimination termination steps.6
Our investigation on the trivalent vanadium complex
of this particular ligand7 has outlined a surprising
involvement of the ligand in the catalytic performance
of the alkylating agents. This unusual reactivity pattern
provides both activation of the vanadium metal via
alkylation on the pyridine ring and at the same time a
two-electron reduction pathway toward rare monovalent
vanadium alkyls.
(8) See for example: (a) Andersen, R. A.; Berg, D. J .; Fernholt, L.;
Faegri, K. J r., Green, J . C.; Haaland, A., Lappert, M. F.; Leung, W.
P.; Rypdal, K. Acta Chem. Scand., Ser. A 1988, A42 (8-9), 554. (b)
Cahiez, G.; Alami, M. Tetrahedron Lett. 1986, 27, 569. (c) Kauffmann,
T.; Bisling, M. Tetrahedron Lett. 1984, 25, 293. (d) Leleu, J . Cah. Notes
Doc. 1977, 88, 361. (e) Tamura, M.; Kochi, J . J . Organomet. Chem.
1971, 29, 111.
(9) Complex 1: A solution of 2,6-bis[1-(2,6-diisopropylphenylimino)-
ethyl]pyridine (2.0 g, 4.1 mmol) in toluene (100 mL) was treated with
MnCl2(THF)2 (1.1 g, 4.1 mmol). The mixture was refluxed overnight,
upon which the yellow suspension changed to an orange solution. The
solution was evaporated to dryness, yielding an orange residue, which
was then dissolved in 20 mL of methylene chloride. The orange solution
was filtered and layered with hexane, yielding orange crystals of 1
over a period of 2 weeks (1.8 g, 2.9 mmol, yield 71%). IR (Nujol mull,
cm-1): 3055(m), 2906(s), 1570(m), 1458(s), 1379(s), 1321(m), 1302(w),
1255(m), 1236 (m), 1146(w), 1103(m), 1078 (w), 995(w), 960(w), 868(w),
To expand our understanding of the unique behavior
of this important ligand system, we have now investi-
(1) (a) Britovsek, G. J . P.; Bruce, M.; Gibson, V. C.; Kimberley, B.
S.; Maddox, P. J .; Mastroianni, S.; McTavish, S. J .; Redshaw, C.; Solan,
G. A.; Stro¨mberg, S.; White, A. J . P.; Williams, D. J . J . Am. Chem.
Soc. 1999, 121, 8728. (b) Britovsek, G. J . P.; Dorer, B. A.; Gibson, V.
C.; Kimberley, B. S.; Solan, G. A. (BP Chemicals Limited), WO 99/
12981, 1999 [Chem Abstr. 1999, 130, 252793]. (c) Britovsek, G. J . P.;
Mastroianni, S.; Solan, G. A.; Baugh, S. P. D.; Redshaw, C.; Gibson,
V. C.; White, A. J . P.; Williams, D. J .; Elsegood, M. R. J . Chem. Eur.
J . 2000, 6, 2221. (d) Bruce, M.; Gibson, V. C.; Redshaw, C.; Solan, G.
A.; White, J . P. A.; Williams, D. J . Chem. Commun. 1998, 2523.
(2) (a) Small, B. L.; Brookhart, M. Polymer Prepr. (Am. Chem. Soc.,
Div. Polym. Chem.) 1998, 39, 213. (b) J ohnson, L. K.; Killian, C. M.;
Brookhart, M. J . Am. Chem. Soc. 1995, 117, 6414. (c) J ohnson, L. K.;
Mecking, S.; Brookhart, M. J . Am. Chem. Soc. 1996, 118, 267. (d)
Killian, C. M.; Tempel, D. J ., J ohnson, L. K.; Brookhart, M. J . Am.
Chem. Soc. 1996, 118, 11664. (e) J ohnson, L. K.; Killian, C. M.; Arthur,
S. D.; Feldman, J .; McCord, E. F.; McLain, S. J .; Kreutzer, K. A.;
Bennett, M. A.; Coughlin, E. B.; Ittel, S. D.; Parthasarathy, A.; Tempel,
D. J .; Brookhart, M. S. (DuPont) WO 96/23010, 1996 [Chem. Abstr.
1996, 125, 222773t]. (f) Dias, E. L.; Brookhart, M.; White, P. S. Chem.
Commun. 2001, 423.
(3) (a) Bennett, A. M. A. (DuPont) WO 98/27124, 1998 [Chem Abstr.
1998, 129, 122973x]. (b) Bennett, A. M. A. CHEMTECH 1999, J uly,
24-28.
(4) Small, B. L.; Brookhart, M.; Bennett, A. M. A. J . Am. Chem Soc.
1998, 120, 4049.
(5) Britovsek, G. J . P.; Gibson, V. C.; Kimberley, B. S., Maddox, P.
J .; McTavish, S. J .; Solan, G. A.; White, A. J . P.; Williams, D. Chem.
Commun. 1998, 849.
(6) Margl, P.; Deng, L.; Ziegler, T. Organometallics 1999, 18, 5701.
(7) Reardon, D.; Conan, F.; Gambarotta, S.; Yap, G. P. A.; Wang,
Q. J . Am. Chem. Soc. 1999, 121, 9318.
824(m), 800(w), 769(s), 723(m), 688(w). Anal. Calcd (found) for C33H43
Cl2N3Mn: C 65.24 (63.91), H 7.13 (6.96), N 6.92 (6.64). µeff ) 5.74 µBM
-
.
Complex 2: A suspension of 1 (1.5 g, 2.5 mmol) in freshly distilled
diethyl ether (100 mL) was treated with 2 equiv of MeLi (3.5 mL of
1.4 M solution in diethyl ether, 5.0 mmol). The mixture was stirred
for 2 days, upon which the color changed from orange to maroon. The
solution was filtered and the solvent was reduced to 15 mL to
crystallize for 4 days, upon which burgundy crystals of 2 suitable for
single-crystal X-ray analysis were obtained (0.7 g, 1.3 mmol, yield 52%).
IR (Nujol mull, cm-1): 3058(w), 2923(s), 2854(w), 1586(s), 1462(s),
1378(s), 1319(w), 1255(m), 1236(m), 1145(w), 1104(m), 1091 (m),
997(w), 950 (m), 854(m), 822(w), 798(w), 772(m), 756(m), 724(m),
694(m), 502 (m). Anal. Calcd (found) for C34H46N3Mn: C 74.02 (74.18),
H 8.40 (8.72), N 7.62 (7.41). µeff ) 4.82 µBM. Complex 3: Solid LiCH2-
Si(CH3)3 (0.3 g; 3.4 mmol) was added to an orange suspension of 1
(1.0 g; 1.7 mmol) in toluene (80 mL) at room temperature. A fast
reaction was observed in which the solution changed in color from
orange to dark brown. The solution was stirred for additional 4 h, and
the solvent was removed under vacuum. The resulting brown solid was
dissolved in freshly distilled diethyl ether (80 mL) and centrifuged to
eliminate a small amount of pale-colored solid, and the volume was
further reduced to 50 mL. A dark blue-black solid precipitated upon
standing overnight at room temperature and was separated. The dark
brown mother liquor was placed at 4 °C, in which yellow-brown crystals
of 3 were isolated (0.6 g, 0.6 mmol, 35%). Anal. Calcd (found) for C53H94
-
LiMn N3O4Si: C 68.65 (68.86), H 10.22 (10.34), N 4.53 (4.27). IR (Nujol
mull cm-1): ν 1912(w), 1851(w), 1790(w), 1643(s), 1589(s), 1570(m),
1364(s), 1321(w), 1261(s), 1192(w), 1094(s), 1021(s), 929(w), 861(m),
802(s), 721(m), 691(m), 663(m). µeff ) 3.91 µBM
.
10.1021/om0110078 CCC: $22.00 © 2002 American Chemical Society
Publication on Web 01/29/2002

Communications
Organometallics, Vol. 21, No. 5, 2002 787
F igu r e 1. Thermal ellipsoid plot of 1. Relevant bond
distances and angles: Mn-Cl(1) ) 2.350(3) Å, Mn-Cl(2)
) 2.313(2) Å, Mn-N(1) ) 2.333(5) Å, Mn-N(3) ) 2.318(5)
Å, Mn-N(2) ) 2.201(5) Å, Cl(1)-Mn-Cl(2) ) 119.68(9)°,
Cl(1)-Mn-N(1) ) 103.25(17)°, Cl(1)-Mn-N(2) ) 98.93(17)°,
Cl(1)-Mn-N(3))101.71(16)°,Cl(2)-Mn-N(2))141.36(18)°,
N(1)-Mn-N(3) ) 135.4(2)°, N(1)-Mn-N(2) ) 69.71(19)°.
F igu r e 2. Thermal ellipsoid plot of 2. Relevant bond
distances and angles: Mn-C(34) ) 2.086(7) Å, Mn-N(1)
) 2.175(3) Å, Mn-N(2) ) 2.039 Å, Mn-N(3) ) 2.160(3) Å,
C(34)-Mn-N(1) ) 104.8(2)°, C(34)-Mn-N(2) ) 163.3(3)°,
C(34)-Mn-N(3) ) 110.6(2), N(1)-Mn-N(3) ) 143.09(13)°,
N(1)-Mn-N(2) ) 74.18(12)°, N(2)-Mn-N(3) ) 74.96(12)°.
ramidal arrangement and with the coordination geom-
etry defined by two chlorine atoms and the three
nitrogen donor atoms of the tridentate ligand (Figure
1).
Complex 1 shares with the Fe(II) and Co(II) bis-
(imino)pyridine analogues the same geometry and the
high-spin electronic configuration. Thus, it is at least
surprising that 1 does not exhibit ethylene polymeriza-
tion activity upon activation with methylalumoxane
(Mn:Al 1:1000) or other cocatalysts and under standard
reaction conditions (1 atm of ethylene at room temper-
ature). The high-spin d5 electronic configuration (µeff
)
5.74 µBM) of manganese is obviously responsible for the
lack of activity by either preventing alkylation of the
manganese center by the Al or, more likely, causing
instability of the Mn-alkyl function. In an attempt to
probe this second possibility, complex 1 was treated in
separate experiments with MeLi with different stoichio-
metric ratios. The reaction with 2 equiv of MeLi afforded
the monovalent {2,6-bis[2,6-(i-Pr)2PhNdC(Me)]2(C5H3N)}-
MnCH3 (2) derivative (Figure 2).9 The complex was
obtained in reasonable yield and displays the same d6
high-spin electronic configuration observed for the Fe(II)
complex. The same reaction with 4 equiv of MeLi
afforded instead a quantitative deposition of a very
bright mirror of metallic manganese.
Complex 2 has a distorted square planar geometry10
with the central Mn atom surrounded by three coordi-
nating nitrogen atoms from the bis(imino)pyridine
ligand and one methyl group (Figure 2) and with the
methyl group substantially deviating from the plane
defined by the metal and the three ligand donor atoms.
F igu r e 3. Thermal ellipsoid plot of 3. Relevant bond
distances and angles: Mn-C(34) ) 2.147(6) Å, Mn(N(1) )
2.138(4) Å, Mn-N(2) ) 2.185(4) Å, Mn-N(3) ) 2.160(4)
Å, C(34)-Mn-N(1) ) 109.6(2)°, C(34-Mn-N(2) ) 151.5(2)°,
C(34)-Mn-N(3) ) 109.0(2)°, N(1)-Mn-N(3) ) 141.86(17)°,
N(1)-Mn-N(2) ) 72.94(17)°, N(2)-Mn-N(3) ) 73.08(17)°.
radical initiators for radical polymerization processes.11
Thus, it is tempting to propose a methyl radical elimi-
nation mechanism for reducing the manganese metal
center to the +1 state. However, a very similar reaction
conducted with Me3SiCH2Li afforded instead the zero-
valent [{[2,6-(i-Pr)2PhNdC(Me)]2(C5H3N)}Mn(CH2SiMe3)]-
[Li(OEt2)4] (3).9 This species displays a magnetic mo-
ment consistent with the presence of three unpaired
electrons on a tetracoordinated high-spin d7 zerovalent
manganese [µeff ) 3.91 µB]. The coordination geometry
is remarkably similar to 2 (Figure 3)10 and, besides the
presence of the Me3SiCH2 group replacing the methyl
and of a Li(ether)4 cation in the lattice, is the same
The reduction of the metal center is not particularly
surprising given that Mn(II) complexes are used as
(11) See for example: (a) Sherigara, B. S.; Rai, S. K.; Shivakumar,
K. Indian J . Chem., Sect. A: Inorg., Bio-inorg., Phys., Theor. Anal.
Chem. 2001, 40A(1), 46. (b) Danilovtseva, E. N.; Annenkov, V. V.;
Skushnikova, A. I.; Svyatkina, L. I. J . Appl. Polym. Sci. 2000, 78, 101.
(c) Anisimov, Y. N.; Kolodyazhnyi, A. V.; Grekhova, O. B. Russ. J . Gen.
Chem. 1999, 69, 1784. (d) Rai, S. K.; Shivakumar, K.; Sherigara, B. S.
Eur. J . Polym. 2000, 36, 1339. (e) Sherigara, B. S.; Rai, S. K.;
Shivakumar, K. J . Indian Counc. Chem. 1999, 16, 29. (f) J ocheska,
N.; Petrov, G.; J anevski, A.; Sebenik, A. Eur. Polym. J . 1996, 32, 209.
(g) Cakmak, I. Angew. Makromol. Chem. 1995, 224, 49. (h) Sumi, K.;
Kimura, M.; Nakamura, I. J . Polym. Sci., Part A: Polym. Chem. 1994,
32, 1243.
(10) Crystal data. 1: C33H43Cl2MnN3, Mw ) 607.54, triclinic, P1h, a
) 8.741(2) Å, b ) 9.793(2) Å, c ) 20.843(5) Å, R ) 82.333(4)°, â )
88.424(5)°, γ ) 65.987(4)°, V ) 1614.5(7) Å3, Z ) 2, T ) 203 K, F000
642, R ) 0.0564, wR2 ) 0.1463, GoF ) 1.039. 2: C33H46MnN3, Mw
)
)
927.28, monoclinic, P2(1)/n, a ) 14.159(3) Å, b ) 8.775(2) Å, c )
25.300(5) Å, â ) 94.784(4)°, V ) 3132.4(10) Å3, Z ) 4, T ) 203 K, F000
) 1184, R ) 0.0618, wR2 ) 0.1582, GoF ) 1.043. 3: C53H94N3-
MnLiSiO4, Mw ) 927.28, monoclinic, P2(1)/c, a ) 19.039(3) Å, b )
15.776(3) Å, c ) 18.905(3) Å, â ) 90.810(4)°, V ) 5677.7(16) Å3, Z ) 4,
T ) 203 K, F000 ) 2028, R ) 0.0615, wR2 ) 0.1415, GoF ) 1.024.

788 Organometallics, Vol. 21, No. 5, 2002
Communications
distorted square planar. The Mn atom is coplanar with
the three nitrogen donor atoms and has the alkyl group
significantly deviating from planarity. The Mn-C bond
distance is longer than in 2, as could be expected given
the difference in oxidation state.
not necessarily catalytically inactive.12 Should this point
be confirmed, it will open new interesting perspectives
for further research.
Ack n ow led gm en t. This work was supported by the
Natural Science and Engineering Council of Canada
(NSERC).
The transformation of 1 to 3 is a two-electron process
and can hardly be explained in terms of Mn-C bond
homolytic cleavage given the fact that one alkyl residue
remains attached to the metal. The reaction can be more
easily rationalized in terms of the same mechanism
responsible for the previously reported reduction of the
trivalent vanadium derivative, and that implies alky-
lation/dealkylation of the pyridine ring.7 However, this
obviously does not apply to 2, whose formation is only
a one-electron process. In any event, the fact that
reduction was independently observed with both vana-
dium and manganese strongly suggests that perhaps a
similar reduction may occur also in the case of Fe and
Co derivatives where, however, the low-valent alkyl are
Su p p or tin g In for m a tion Ava ila ble: Complete listing of
structural parameters and crystal data for the complexes. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OM0110078
(12) While this work was in progress,
a reduction of the Co
derivative affording a zerovalent dinitrogen complex was reported: (a)
Gibson, V. C.; Humphries, M.; Tellmann, K.; Wass, D.; White, A. J .
P.; Williams, D. J . Chem. Commun. 2001, 2252. (b) Kooistra, T. M.;
Knijnenburg, Q.; Smits, J . M. M.; Horton, A. D.; Budzelaar, P. H. M.;
Gal, A. W. Angew. Chem. Int. Ed. Engl. 2001, 40, 4719.