Structurally Characterized Neutral Monoalkyl and -aryl Complexes of Manganese(II)

Article abstract of DOI:10.1021/om030645i

The synthesis and X-ray crystal structures of the monomethyl and -phenyl manganese complexes [LMn(μ-Me)]₂ (2) and LMnPh (3) (L = HC(CMeNAr)₂, Ar = 2,6-iPr₂C₆H₃) are reported. These complexes represent

Full text of DOI:10.1021/om030645i

Organometallics 2004, 23, 1177-1179
1177
Str u ctu r a lly Ch a r a cter ized Neu tr a l Mon oa lk yl a n d -a r yl
Com p lexes of Ma n ga n ese(II)†
J ianfang Chai,‡ Hongping Zhu,‡ Hongjun Fan,§ Herbert W. Roesky,*,‡ and
J o¨rg Magull‡
Institut fu¨r Anorganische Chemie der Universita¨t Go¨ttingen, Tammannstrasse 4,
D-37077 Go¨ttingen, Germany, and Labor fu¨r Physikalische und Theoretische Chemie,
Universita¨t Siegen, D-57068 Siegen, Germany
Received October 28, 2003
Summary: The synthesis and X-ray crystal structures of
the monomethyl and -phenyl manganese complexes
[LMn(µ-Me)]2 (2) and LMnPh (3) (L ) HC(CMeNAr)2,
Ar ) 2,6-iPr2C6H3) are reported. These complexes
represent the first structurally characterized neutral
dimeric manganese(II) monoalkyl complexes and mono-
meric manganese(II) monoaryl complexes. Furthermore,
compound 3 shows an interesting coplanar arrangement
of the phenyl group and the chelating ligand.
been used as intermediates without isolation.2 To the
best of our knowledge, only two compounds of this class,
[Mn(nBu)(NPEt3)]4 and [Mn(Me)(NPEt3)]4, have been
structurally characterized containing a heterocubane
structure.8 There is no structural investigation known
of lower aggregated complexes (monomer or dimer).
Keeping this in mind and the recent interesting results9
obtained by taking advantage of â-diketiminato ligands
on the basis of their unique electronic and steric effects
to stabilize unusual complexes such as the monomeric
iron(II) monomethyl complex {HC(CtBuNAr)2}FeMe (Ar
) 2,6-iPr2C6H3),10 we set out to synthesize low ag-
gregated manganese(II) monoalkyl and -aryl complexes
using such ligands. Herein we report the preparation
and molecular structure of the dimeric compound
[LMn(µ-Me)]2 (2) and the monomeric compound LMnPh
(3) (L ) HC(CMeNAr)2, Ar ) 2,6-iPr2C6H3).
There is widespread interest in organomanganese(II)
complexes, especially alkyl and aryl derivatives, since
such complexes have extensive applications in organic
synthesis.1,2 Manganese(II) alkyl and aryl complexes
have been proved to be excellent reagents in C-C
coupling reactions and can be compared to other organo-
transition-metal complexes such as the widely used
organocopper reagents.3 Moreover, they also show good
thermal stability, high chemoselectivity, and excellent
functional group tolerance.1d,3 Some neutral and ionic
The addition of MeLi and PhLi, respectively, to
[LMn(µ-I)]2 11 (1) in toluene at low temperature smoothly
provided the dimeric 2 and the monomeric 3 in moderate
yield.12 However, the successful isolation of pure 2 and
3 was not easy, due to the difficulties in removing small
amounts of unreacted starting material 1. This requires
dialkyl and diaryl derivatives of Mn(II) of composition
MnR2,4 MnR2Bx,1d,4a,5 LiMnR3,3 and Li2MnR4
(R )
4a,6
alkyl, aryl; B ) Lewis base, x ) 1-4) have been
synthesized and characterized. Recently ionic manga-
nese(II) monoalkyl and -aryl complexes Mn(acacen)(R)-
Li(DME) (acacen ) N,N′-ethylenebis(acetylacetiminato)
dianion; R ) Me, Ph, Mes) were reported as bifunctional
carriers of polar organometallics.7 However, neutral
monoalkyl and -aryl derivatives of Mn(II) are rare.
Compounds of composition RMnX (X ) halide) have only
(5) (a) Howard, C. G.; Wilkinson, G.; Thornton-Pett, M.; Hursthouse,
M. B. J . Chem. Soc., Dalton Trans. 1983, 2025. (b) Howard, C. G.;
Girolami, G. S.; Wilkinson, G.; Thornton-Pett, M.; Hursthouse, M. B.
J . Chem. Soc., Dalton Trans. 1983, 2631. (c) Girolami, G. S.; Howard,
C. G.; Wilkinson, G.; Dawes, H. M.; Thornton-Pett, M.; Motevalli, M.;
Hursthouse, M. B. J . Chem. Soc., Dalton Trans. 1985, 921. (d) J acob,
K.; Thiele, K.-H. Z. Anorg. Allg. Chem. 1979, 455, 3. (e) Shakoor, V.
A.; J acob, K.; Thiele, K.-H. Z. Anorg. Allg. Chem. 1983, 489, 115. (f)
Girolami, G. S.; Wilkinson, G.; Galas, A. M. R.; Thornton-Pett, M.;
Hursthouse, M. B. J . Chem. Soc., Dalton Trans. 1985, 1339. (g)
Girolami, G. S.; Wilkinson, G. J . Am. Chem. Soc. 1983, 105, 6752. (h)
Manzer, L. E.; Guggenberger, L. J . J . Organomet. Chem. 1977, 139,
C34. (i) Davies, J . I.; Howard, C. G.; Skapski, A. C.; Wilkinson, G. J .
Chem. Soc., Chem. Commun. 1982, 1077.
* To whom correspondence should be addressed. E-mail: hroesky@
gwdg.de.
† Dedicated to Professor Edgar Niecke on the occasion of his 65th
birthday.
‡ Universita¨t Go¨ttingen.
§ Universita¨t Siegen.
(1) Selected references: (a) Cahiez, G.; Martin, A.; Delacroix, T.
Tetrahedron Lett. 1999, 40, 6407. (b) Boucley, C.; Cahiez, G.; Carini,
S.; Cere`, V.; Comes-Franchini, M.; Knochel, P.; Pollicino, S.; Ricci, A.
J . Organomet. Chem. 2001, 624, 223. (c) Riguet, E.; Alami, M.; Cahiez,
G. J . Organomet. Chem. 2001, 624, 376. (d) Donkervoort, J . G.; Vicario,
J . L.; J astrzebski, J . T. B. H.; Gossage, R. A.; Chiez, G.; van Koten, G.
J . Organomet. Chem. 1998, 558, 61.
(2) Selected references: (a) Cahiez, G.; Masuda, A.; Bernard, D.;
Normant, J . F. Tetrahedron Lett. 1976, 3155. (b) Cahiez, G.; Figadere,
B. Tetrahedron Lett. 1986, 26, 4445. (c) Cahiez, G.; Alami, M.
Tetrahedron 1989, 45, 4163.
(6) Morris, R. J .; Girolami, G. S. Organometallics 1989, 8, 1478.
(7) Gallo, E.; Solari, E.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C.
Organometallics 1995, 14, 2156.
(8) (a) Mai, H.-J .; Neumu¨ller, B.; Dehnicke, K. Z. Naturforsch. 1996,
51b, 433. (b) Riese, U.; Neumu¨ller, B.; Faza, N.; Massa, W.; Dehnicke,
K. Z. Anorg. Allg. Chem. 1997, 623, 351.
(9) Bourget-Merle, L.; Lappert, M. F.; Severn, J . R. Chem. Rev. 2002,
102, 3031.
(10) Andres, H.; Bominaar, E.; Smith, J . M.; Eckert, N. A.; Holland,
P. L.; Mu¨nck, E. J . Am. Chem. Soc. 2002, 124, 3012.
(11) Chai, J .; Zhu, H.; Most, K.; Roesky, H. W.; Vidovic, D.; Schmidt,
H.-G.; Noltemeyer, M. Eur. J . Inorg. Chem. 2003, 4332.
(3) Bartlett, R. A.; Olmstead, M. M.; Power, P. P.; Shoner, S. C.
Organometallics 1988, 7, 1801.
(12) Selected data for 2: yield 52%; mp 190-192 °C; EI-MS m/z (%)
487 (6) [LMnMe]+, 472 (100) [LMn]+. Anal. Calcd for C60H88Mn2N4
(975.22): C, 73.83; H, 9.02; N, 5.74. Found: C, 73.31; H, 8.76; N, 5.64.
Selected data for 3: yield 65%; mp 230-232 °C; EI-MS m/z (%) 549
(4) (a) Andersen, R. A.; Carmona-Guzman, E.; Gibson, J . F.; Wilkin-
son, G. J . Chem. Soc., Dalton Trans. 1976, 2204. (b) Buttrus, N. H.;
Eaborn, C.; Hitchcock, P. B.; Smith, J . D.; Sullivan, A. C. J . Chem.
Soc., Chem. Commun. 1985, 1380. (c) Gambarotta, S.; Floriani, C.;
Chiesi-Villa, A.; Guastini, C. J . Chem. Soc., Chem. Commun. 1983,
1128. (d) Andersen, R. A.; Haaland, A.; Rypdal, K.; Volden, H. V. J .
Chem. Soc., Chem. Commun. 1985, 1807.
(3) [M]+, 471 (100) [M
(549.68): C, 76.41; H, 8.37; N, 5.09. Found: C, 75.72; H, 8.13; N, 4.86.
NMR spectroscopic data for 2 and 3 are not available, due to the
paramagnetism of Mn(II).
-
C6H6]+. Anal. Calcd for C35H46MnN2
10.1021/om030645i CCC: $27.50 © 2004 American Chemical Society
Publication on Web 02/07/2004

1178 Organometallics, Vol. 23, No. 6, 2004
Communications
Sch em e 1
a number of precautions. The RLi reagents should be
in excess, according to the stoichiometric amounts given
in Scheme 1. Sometimes it is necessary to recrystallize
the product from pentane several times. The reaction
of 1 with nBuLi, however, resulted in the formation of
a manganese mirror and LH, which was confirmed by
1
F igu r e 1. ORTEP drawing for complex 2 (35% probability
ellipsoids). Selected bond lengths (Å) and angles (deg):
Mn(1)-N(1) ) 2.125(2), Mn(1)-N(2) ) 2.129(2), Mn(1)-
C(30) ) 2.241(2), Mn(1)-C(30A) ) 2.306(2), Mn(1)-Mn(1A)
) 2.809(1); N(1)-Mn(1)-N(2) ) 89.68(6), N(1)-Mn(1)-
EI-MS and H NMR. The proposed mechanism for this
reaction is obviously the direct intramolecular elimina-
tion of the alkyl group and the chelating ligand. A
similar decomposition pathway was suggested for vari-
ous dialkyl Mn(II) species.13
C(30)
) 117.53(8), N(2)-Mn(1)-C(30) ) 117.18(8),
Complexes 2 and 3 are both yellow crystalline solids,
which are highly sensitive to air and moisture and were
handled in a glovebox under purified nitrogen. Below
the corresponding melting points of 2 and 3 (190-192
°C for 2 and 230-232 °C for 3), no decomposition was
observed. In the EI-MS of 2 the molecular ion peak
M+ was silent; however, half of the molecular mass
[LMnMe]+ was observed (m/z 487, 6%) and the most
intense peak (m/z 472, 100%) was assigned to [LMn]+.
For 3, M+ was observed at m/z 549 (3%), followed by
[M - C6H6]+ (m/z 471) as the most intense peak.
Compound 2 is the first structurally characterized
manganese alkyl complex containing bridging methyl
groups.14 The solid-state structure of 2 (Figure 1) shows
that the manganese centers have a distorted-tetrahedral
geometry. The backbone of the chelating ligand is nearly
planar, and the manganese atom is out of this plane
(0.56 Å). The central core contains an ideal planar four-
membered Mn2(µ-Me)2 ring, which bisects and is per-
pendicular (89.5°) to the two chelating ligands around
them. The distance between two manganese atoms
(2.809(1) Å) indicates that there is some weak interac-
tion rather than a strong Mn-Mn bond.3,4a,5h The
distance is comparable to those of Mn2C2 four-membered
rings reported in the literature such as those in Mn2-
(CH2C6H4NMe2)4 (2.810(3) Å),5h Mn2(CH2SiMe3)4(PMe3)2
N(1)-Mn(1)-C(30A) ) 114.41(7), N(2)-Mn(1)-C(30A) )
114.72(8), C(30)-Mn(1)-C(30A) ) 103.71(7).
F igu r e 2. ORTEP drawing for complex 3 (30% probability
ellipsoids). Selected bond lengths (Å) and angles (deg):
Mn(1)-N(1) ) 2.041(3), Mn(1)-N(1A) ) 2.041(3), Mn(1)-
C(16) ) 2.077(6); N(1)-Mn(1)-N(1A) ) 91.31(16), N(1)-
Mn(1)-C(16) ) 134.27(8), N(1A)-Mn(1)-C(16) ) 134.28(8).
(2.772(1) Å),5i and Mn2(CH2CMe2Ph)4 (2.719 Å).4a The
Mn-C(methyl) bond length (average 2.273(7) Å) is in
the normal range of those (2.22(6)-2.28(1) Å) in the
manganates [Li(TMEDA)]2[MnR4] (R ) Me, Et, Bu,
CH2SiMe3)4a,6 and, as expected, slightly longer than
those (2.060(11)-2.249(4) Å) of terminal Mn-methyl
bonds.
Single crystals suitable for X-ray structural analysis
of 3 were obtained by recrystallization from diethyl
ether. The X-ray structural analysis reveals a mono-
nuclear three-coordinate manganese center with a
terminal phenyl group (Figure 2). Complexes with three-
coordinate manganese are rare.15 The sum of the angles
(13) Tamura, M.; Kochi, J . J . Organomet. Chem. 1971, 29, 111.
1
(14) Crystal data for
/ 2: C30H44MnN2, Mr ) 487.61, monoclinic,
2
space group C2/c, a ) 22.612(5) Å, b ) 14.880(3) Å, c ) 16.481(3) Å, â
) 90.05(3)°, V ) 5545.2(19) Å3, Z ) 8, Fcalcd ) 1.168 Mg m-3, F(000) )
2104, µ(Mo KR) ) 0.495 mm-1. The total number of reflections was
20 857 in the range 3.28 e 2θ e 49.42°, of which 4662 were unique.
Final R indices: R1 ) 0.0327 (I >2σ(I)) and wR2 ) 0.0772 (all data).
The maximum/minimum residual electron density was 0.257/-0.260
e Å-3. Crystal data for 3: C35H46MnN2, Mr ) 549.68, orthorhombic,
space group Pnma, a ) 16.6103(13) Å, b ) 20.943(2) Å, c ) 9.2785(13)
Å, â ) 90°, V ) 3227.8(6) Å3, Z ) 4, Fcalcd ) 1.131 Mg m-3, F(000) )
1180, µ(Mo KR) ) 0.433 mm-1. The total number of reflections was
8044 in the range 3.88 e 2θ e 45.98°, of which 2191 were unique.
Final R indices: R1 ) 0.0514 (I > 2σ(I)) and wR2 ) 0.1296 (all data).
The maximum/minimum residual electron density was 0.199/-0.408
(15) Eaborn, C.; Hitchcock, P. B.; Smith, J . D.; Zhang, S.; Clegg,
W.; Izod, K.; O’Shaughnessy, P. Organometallics 2000, 19, 1190.
e Å-3
.

Communications
Organometallics, Vol. 23, No. 6, 2004 1179
3. The maximum deviation in bond length is less than
0.03 Å, and the bond angle is less than 4°. These results
show that RI-BP86 gives reliable results for compound
3.
One task of the theoretical study is to find a reason
for the coplanarity of the Ph ring and the chelating
ligand. The result distinctly shows that the two rings
in vertical positions are less stable than the coplanar
arrangement by 2.9 kcal/mol. However, if the Ar groups
of the chelating ligand are replaced by H atoms, the two
rings in vertical positions are more stable than the
coplanar arrangement by 57.4 kcal/mol, which indicates
that the coplanar arrangement of the two rings results
from the influence of the Ar groups, not the conjugation
between the two rings. The role of the Ar groups can be
attributed to two aspects: one is steric repulsion, which
is the major factor in the coplanar arrangement. If the
two rings are forced into vertical positions, several short
H(Ph)‚‚‚H(iPr) distances can be found; the other is a
phenyl-phenyl interaction. In 3 the terminal Ph and
Ar groups are nearly in their optimized positions for this
kind of interaction (dihedral angle 83.9°).18 The shortest
C‚‚‚H distance between the phenyl rings is 3.345 Å. The
crystal packing analysis shows that there is no close
interaction with neighboring molecules.
The calculated results do not support the existence
of significant Mn-C and Mn-N d-π conjugation in 3.
This is demonstrated if the terminal Ph is replaced by
a CH3 group, where the Mn-C d-π conjugation is
impossible, while the Mn-C bond length almost does
not change significantly (2.047-2.059 Å). Compound 3
was calculated by the B3LYP method (the ligand is
replaced by C3H5N2) and the molecular orbitals were
localized by Boys methods;19 the most extended d orbital
of Mn was drawn by the MOLDEN 3.8 program20
(Figure 3) and compared with that of Ni in Ni(CO)4. The
d orbital is well localized around Mn in 3, while in
Ni(CO)4 the d orbitals are deformed and overlap with
other atoms. These results indicate that the Mn-C and
Mn-N bonds are of pure σ character, and the rather
short Mn-C and Mn-N distances are attributed to the
low coordination number of the central metal.
F igu r e 3. The most extended d orbital of Mn in compound
3 (a) and of Ni in Ni(CO)4 (b).
at the metal center is 359.87(76)°, which shows that the
manganese center is of planar-trigonal geometry. The
six-membered MnN2C3 ring is essentially planar, which
is in addition coplanar with the terminal phenyl ring
with a mean deviation of ∆ ) 0.02 Å. The Mn-C bond
length in 3 is 2.077(6) Å, which is particularly short
compared to those (2.089(8)-2.1858(4) Å) in the other
terminal manganese aryl derivatives.
The order of the corresponding N-Mn-N angles is 1
(average 94.585(14)°) > 3 (91.31(16)°) > 2 (average
89.68(6)°), while the order of the Mn-N bond lengths
is 3 (average 2.041(3) Å) < 1 (average 2.067(2) Å) < 2
(average 2.127(2) Å). The unexpectedly short Mn-N
bond lengths in 3 are probably due to the low-coordinate
metal center, which is consistent with the theoretical
calculated results.
DFT calculations were carried out to get further
insight into the geometry and electronic structure of
compound 3. RI-BP86 (TZVP for Mn, SV(P) for the other
atoms) in Turbomole 5.516 was used throughout the
calculation, except for the localized orbital in Figure 3,
where B3LYP (6-311g(d) for Mn and 6-31g(d) for other
atoms) in Gaussian 9817 was applied. The reliability of
RI-BP86 was tested by comparing the optimized
geometry with the experimental structure of compound
In summary, monomethyl and -phenyl complexes of
Mn(II) have been successfully synthesized and charac-
terized by using bulky â-diketiminato ligands. Research
on the application of these complexes to C-C coupling
reactions is in progress.
(16) Ahlrichs, R.; Ba¨r, M.; Baron, H.-P.; Bauernschmitt, R.; Bo¨cker,
S.; Deglmann, P.; Ehrig, M.; Eichkorn, K.; Elliott, S.; Furche, F.; Haase,
F.; Ha¨ser, M.; Horn, H.; Ha¨ttig, C.; Huber, C.; Huniar, U.; Kattannek,
M.; Ko¨hn, A.; Ko¨lmel, C.; Kollwitz, M.; May, K.; Ochsenfeld, C.; O¨ hm,
H.; Scha¨fer, A.; Schneider, U.; Sierka, M.; Treutler, O.; Unterreiner,
B.; Arnim, M. v.; Weigend, F.; Weis, P.; Weiss, H. TURBOMOLE 5.5;
University of Karlsruhe, Karlsruhe, Germany, 2002.
Ack n ow led gm en t. We are grateful to the Deutsche
Forschungsgemeinschaft, the Fonds der Chemischen
Industrie, and the Go¨ttinger Akademie der Wissen-
schaften for support of this work.
(17) Frisch, M. J .; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J . R.; Zakrzewski, V. G.; Montgomery, J . A.;
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C.; Clifford, S.; Ochterski, J .; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J . B.; Cioslowski, J .; Ortiz, J . V.; Baboul, A. G.; Stefanov,
B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.;
Martin, R. L.; Fox, D. J .; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.;
J ohnson, B.; Chen, W.; Wong, M. W.; Andres, J . L.; Gonzalez, C.; Head-
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Gaussian, Inc.: Pittsburgh, PA, 1998.
Su p p or tin g In for m a tion Ava ila ble: Text and tables
giving experimental, spectroscopic, and crystallographic details
and details of the calculations. This material is available free
of charge via the Internet at http://pubs.acs.org.
OM030645I
(18) Tsuzuki, S.; Honda, K.; Uchimaru, T.; Mikami, M.; Tanabe, K.
J . Am. Chem. Soc. 2002, 124, 104.
(19) Boys, S. F. Rev. Mod. Phys. 1960, 32, 296.
(20) Schaftenaar, G. MOLDEN 3.8; CAOS/CAMM Center Nijmegen,
Nijmegen, The Netherlands, 2003.