Synthesis and structures of 1,2-bis(imino)acenaphthene (BIAN) lanthanide complexes that involve the transfer of zero, one, or two electrons

Article abstract of DOI:10.1039/b708758f

The syntheses and X-ray crystal structures of the first 1,2-bis(imino)acenaphthene (BIAN) complexes of the lanthanides are described. The Royal Society of Chemistry.

Full text of DOI:10.1039/b708758f

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Synthesis and structures of 1,2-bis(imino)acenaphthene (BIAN)
lanthanide complexes that involve the transfer of zero, one, or two
electrons
Kalyan Vasudevan and Alan H. Cowley*
Received (in Austin, TX, USA) 11th June 2007, Accepted 6th July 2007
First published as an Advance Article on the web 30th July 2007
DOI: 10.1039/b708758f
(i) lengthening of the C–N bonds and (ii) decreases in the C–C and
N–M bond distances. Furthermore, the M2+ to M3+ oxidation
state change would result in shortening of the metal–C5Me5 ring
centroid distance. On the other hand, if metal A ligand back
transfer did not occur, the structure would be that of a bis(diimine)
complex as depicted in structure B.8 The C(1)–C(12) bond
distances in t-Bu-BIAN,3 mes-BIAN,9 and p-MeO-BIAN4 are
The syntheses and X-ray crystal structures of the first 1,2-
bis(imino)acenaphthene (BIAN) complexes of the lanthanides
are described.
Interest in the 1,2-bis(arylimino)acenaphthene (aryl-BIAN) ligand
class stems from the ability of these ligands to function as both
electron and proton sponges. In turn, this desirable combination of
properties is attributable to the presence of both a naphthalene
ring and a 1,4-diaza-1,3-butadiene moiety. The d-block chemistry
of aryl-BIAN ligands is now well established and many transition
complexes have been employed as catalysts.1 In view of the
foregoing, it is surprising that no lanthanide BIAN complexes have
been reported previously. In the present communication, we report
(i) the syntheses and X-ray crystal structures of four lanthanide
BIAN complexes, and (ii) control of zero, one, or two metal A
BIAN electron transfers by the choice of metal, ligand tuning, or
ligand bulk.
˚
1.551(4), 1.528(2), and 1.530(2) A, respectively. Reference to
Table 1 reveals that for complexes 1 and 3 the C(1)–C(12) bond
distances are shorter than those for the free ligands, thus implying
that electron transfer has taken place and that structure A is
adopted in both cases. On the other hand, the C(1)–C(12)
separation in 2 (Fig. 1) is identical to that reported for t-Bu-
BIAN,3 within experimental error, thus indicating the presence of
a single bond at this location and conformity with structure B.
Note also that the nitrogen–carbon bond distances for complexes 1
and 3 are longer than those for 2, which is consistent with the
presence of C–N double bonding in the latter and single bonding
Treatment of (C5Me5)2M?OEt2 (M 5 Sm, Eu) with an
equimolar quantity of the appropriate R-BIAN ligand
(R 5 mesityl,2 tert-butyl,3 p-methoxyphenyl4) in toluene solution
at ambient temperature, followed by work-up of the reaction
mixtures, resulted in 85–90% yields of (C5Me5)2Sm(mes-BIAN) (1)
(C5Me5)2Eu(t-Bu-BIAN) (2) and (C5Me5)2Eu(p-MeO-BIAN) (3),
each of which was characterized by single-crystal X-ray diffrac-
tion.5 In each complex, a metal(g5-C5Me5)2 unit is attached to the
two nitrogen atoms of the BIAN ligand, thus forming a five-
membered MNCCN ring. A summary of pertinent metrical
parameters is presented in Table 1. Since main group fragments
6
7
such as AlR2 and GaI2 are known to transfer an electron from
the metal to a low-lying p* orbital of the BIAN ligand system, it
was reasonable to anticipate that a similar process might occur in
the case of the new lanthanide complexes. As depicted in structure
A, such a one-electron lanthanide A ligand back transfer process
would be evident from metrical parameters of the C2N2M ring by
Fig. 1 View of Eu(II) complex (C5Me5)2Eu(t-Bu-BIAN) (2) showing the
atom numbering scheme and thermal ellipsoids at 50% probability
˚
(hydrogen atoms omitted for clarity). Selected bond distances (A) and
angles (u): Eu(1)–N(1) 2.794(3), Eu(1)–N(2) 2.768(3), C(1)–N(1) 1.295(5),
C(12)–N(2) 1.278(5), C(1)–C(12) 1.548(6), N(1)–Eu(1)–N(2) 59.84(10),
Eu(1)–N(1)–C(1) 120.8(3), N(1)–C(1)–C(12) 117.7(4), C(1)–C(12)–N(2)
119.2(4), C(12)–N(2)–Eu(1) 121.8(3).
Department of Chemistry and Biochemistry, The University of Texas at
Austin, 1 University Station A5300, Austin, Texas, 78712, USA.
E-mail: cowley@mail.utexas.edu; Fax: (+1) 512-471-6822;
Tel: (+1) 512-471-7484
˚
Table 1 Summary of bond distance (A) and angle (u) data for lanthanide BIAN complexes 1–4
Compound
C(1)–C(12)
M–N(1)
M–N(2)
N(1)–C(1)
N(2)–C(12)
C5Me5–M
SMNCCN
(C5Me5)2Sm(mes-BIAN) (1)
(C5Me5)2Eu(t-Bu-BIAN) (2)
(C5Me5)2Eu(p-MeO-BIAN) (3)
(C5Me5)Sm(dpp-BIAN)(thf) (4)
1.444(6)
1.548(6)
1.442(8)
1.414(6)
2.530(3)
2.794(3)
2.454(4)
2.269(3)
2.501(3)
2.768(3)
2.456(4)
2.232(4)
1.352(5)
1.295(5)
1.345(6)
1.387(5)
1.338(5)
1.278(5)
1.334(7)
1.405(5)
2.489(2)
2.670(2)
2.485(2)
2.420(1)
518.1(3)
539.3(3)
536.1(5)
516.7(6)
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in the former. The observation that the metal–nitrogen bond
˚
distances for 2 are y0.3 A longer than those for 1 and 3 is
consistent with the retention of the +2 oxidation state in 2. As
reflected by the sums of bond angles for the MNCCN rings, these
metallacycles are close to planar for 2 and 3. The significant
folding along the N–N vector in the case of 1 is attributable to the
steric demands of the mesityl substituents.
Scheme 1
The proposed structures for 1–3 are also in accord with IR,
1H NMR, and magnetic moment data. Thus, only 2 exhibits an
IR peak at 1636 cm21, which falls in the region reported for the
CLN stretching mode,10 therefore indicating that metal A ligand
electron transfer has not occurred in this case. Moreover, the
detection of a 1H NMR resonance for 3 at d 221.30 supports the
assignment of the +3 oxidation state in this complex since Evans
et al.11 reported a peak with a similar chemical shift (d 219.7) for
the Me5C5 protons of the Eu (+3) complex, [(C5Me5)Eu(OCMe3)-
(m-OCMe3)]2. The magnetic moment values (Evans method) of
1.82, 6.98, and 3.55 BM for 1, 2 and 3, respectively, also support
the oxidation state assignments proposed above.
arrangement is best represented by structure C (Scheme 1). The
angle between the N(1)–Sm–N(2) and N(1)–C(1)–C(12)–N(2)
planes (47.54u) is larger than that in 1 (42.80u) due to the
concerted effects of increased substituent bulk and optimization
of the interaction between Sm and the C(1)–C(12) double bond.
It is likely that the reaction of (C5Me5)2Sm?OEt2 with dpp-
BIAN proceeds via initial displacement of Et2O to form
(C5Me5)2SmII(dpp-BIAN), followed by intramolecular electron
transfer to generate (C5Me5)2SmIII(dpp-BIAN2), which eliminates
[C5Me5]2. In turn, [C5Me5]2 transfers the second electron to the
singly-reduced BIAN ligand.14 Support for this idea stems from
the detection of the oxidized product, [C5Me5]2, in the reaction
mixture by 1H NMR spectroscopy. This type of process has been
elegantly investigated by Evans and Davis15 and dubbed ‘‘sterically
induced reduction.’’
The reaction of (C5Me5)2Sm?OEt2 with one equivalent of the
sterically encumbered ligand, dpp-BIAN12 (dpp 5 2,6-diisopro-
pylphenyl) in THF solution at ambient temperature resulted in loss
of a C5Me5 group and formation of (C5Me5)Sm(dpp-BIAN)(thf)
(4) in 85% yield. Akin to 1 and 3, compound 4 features a five-
membered MNCCN ring. However, a single-crystal X-ray
analysis5 revealed that the ring structure of 4 differs significantly
from those of 1–3 (Fig. 2). Thus, the C(1)–C(12) separation in 4 is
In summary, we have prepared the first lanthanide 1,2-
bis(imino)acenaphthene complexes and demonstrated that the
metal A BIAN charge transfer process can be controlled by (a) the
choice of metal, (b) tuning of the BIAN ligand substituents, or (c)
use of a bulky BIAN ligand.
˚
y0.03 A less than those in 1 and 3. Furthermore, there are short
˚
contacts of 2.692(4) and 2.707(4) A between these carbon atoms
We are grateful to the Robert A. Welch Foundation (F-0003)
for financial support of this work.
˚
and the Sm center, and the N–C bond distances are y0.05 A
longer than those in 1 and 3. The Sm–C5Me5 ring centroid
˚
distance of 2.420(1) A is similar to that of 1 and therefore
Notes and references
indicative of a Sm(III) center in 4. Moreover, the N–C and C–C
bond distances in 4 are very similar to those of the dianionic dpp-
BIAN ligand in magnesium and calcium complexes.13 Overall, the
metrical parameters for 4 imply that two-electron reduction of the
dpp-BIAN ligand has taken place and that the bonding
1 For recent examples, see: (a) P. Preishuber-Pflugl and M. Brookhart,
Macromolecules, 2002, 35, 6074; (b) M. D. Leatherman, S. A. Svedja,
L. K. Johnson and M. Brookhart, J. Am. Chem. Soc., 2003, 125, 3068;
(c) A. E. Cherian, E. M. Lobkovsky and G. W. Coates, Chem.
Commun., 2003, 2566; (d) W. Liu and M. Brookhart, Organometallics,
2004, 23, 6099; (e) D. H. Camacho, E. V. Salo, J. W. Ziller and Z. Guan,
Angew. Chem., Int. Ed., 2004, 43, 1821; (f) B. S. Williams, M. D.
Leatherman, P. S. White and M. Brookhart, J. Am. Chem. Soc., 2005,
127, 5132; (g) A. M. Kluwer, T. S. Koblenz, T. Jonischkeit, K. Woelk
and C. J. Elsevier, J. Am. Chem. Soc., 2005, 127, 15470; (h) D. H.
Camacho and Z. Guan, Macromolecules, 2005, 38, 2544; (i) J. M. Rose,
A. E. Cherian and G. W. Coates, J. Am. Chem. Soc., 2006, 128, 4186.
2 R. van Asselt, C. J. Elsevier, W. J. J. Smeets, A. L. Spek and R. Bendix,
Recl. Trav. Chim. Pays-Bas, 1994, 113, 88.
3 J. A. Moore, K. Vasudevan, N. J. Hill, G. Reeske and A. H. Cowley,
Chem. Commun., 2006, 2913.
4 D. N. Coventry, A. S. Batsanov, A. E. Goeta, J. A. K. Howard and
T. B. Marder, Polyhedron, 2004, 23, 2789.
5 All X-ray data were collected at 153 K on a Nonius-Kappa CCD
diffractometer. Crystal data for 1: C64H74N2Sm, Mr 5 1021.60, triclinic,
¯
˚
space group P1, a 5 12.319(5), b 5 14.294(5), c 5 16.910(5) A, a 5
3
˚
70.589(5), b 5 82.726(5), c 5 69.084(5)u, V 5 2623.2(16) A , Z 5 2, T 5
153(2) K, m 5 1.161 mm21, reflections collected/independent 5 17880/
11973 (Rint 5 0.0373), R1(I . 2s(I)) 5 0.1107. For 2: C47H62EuN2,
Mr 5 806.95, monoclinic, space group P21/n, a 5 10.250(5),
Fig. 2 View of the Sm(III) complex (C5Me5)Sm(dpp-BIAN)(thf) (4)
showing the atom numbering scheme and thermal ellipsoids at 50%
probability (hydrogen atoms omitted for clarity). Selected bond distances
3
˚
˚
b 5 18.436(5), c 5 23.258(5) A, b 5 94.739(5)u, V 5 4380(3) A ,
Z 5 4, T 5 153(2) K, m 5 1.463 mm21, reflections collected/
independent 5 16952/10032 (Rint 5 0.0358), R1 (I . 2s(I)) 5 0.0443
and wR2 (I . 2s(I)) 5 0.1103. For 3: C46H50EuN2O2, Mr 5 814.88,
˚
(A) and angles (u): Sm(1)–N(1) 2.269(3), Sm(1)–N(2) 2.232(4), Sm(1)–C(1)
2.704(4), Sm(1)–C(12) 2.692(4), C(1)–N(1) 1.387(5), C(12)–N(2) 1.405(5),
C(1)–C(12) 1.414(6), N(1)–Sm(1)–N(2) 83.1(1), Sm(1)–N(1)–C(1) 92.4(2),
N(1)–C(1)–C(12) 119.3(3), C(1)–C(12)–N(2) 124.4(4), C(12)–N(2)–Sm(1)
92.6(2).
¯
˚
triclinic, space group P1, a 5 9.5567(19), b 5 10.286(2), c 5 20.283(4) A,
˚
3
a 5 101.15(3), b 5 94.09(3), c 5 104.01(3)u, V 5 1883.4(7) A , Z 5 2,
T
5
153(2) K,
m
5
1.706 mm21
independent 5 12045/8520 (Rint 5 0.0398), R1 (I . 2s(I)) 5 0.0476
,
reflections collected/
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and wR2 (I . 2s(I)) 5 0.0989. For 4: C40H54N2Sm, Mr 5 713.20,
monoclinic, space group P21/c, a 5 18.640(4), b 5 10.563(2),
8 A similar result has been observed with a diazabutadiene ligand. See:
J. A. Moore, A. H. Cowley and J. C. Gordon, Organometallics, 2006,
25, 5207.
9 U. El-Ayaan, F. Murata, S. El-Derby and Y. Fukuda, J. Mol. Struct.,
2004, 692, 209.
10 J. M. Kliegman and R. K. Barnes, Tetrahedron, 1970, 26, 2555.
11 W. J. Evans, J. L. Shreeve and J. W. Ziller, Organometallics, 1994, 13,
731.
12 A. A. Paulovicova, U. El-Ayaan, K. Shibayama, T. Morita and
Y. Fukuda, Eur. J. Inorg. Chem., 2001, 2641.
3
˚
˚
c 5 17.493(4) A, b 5 97.49(3)u, V 5 3414.8(12) A , Z 5 4, T 5 153(2)
K, m 5 1.749 mm21, reflections collected/independent 5 12820/7785
(Rint 5 0.0517), R1 (I . 2s(I)) 5 0.0576 and wR2 5 0.1431 (I . 2s(I)).
Crystals of 1, 2, 3, and 4 were covered with a mineral oil prior to
mounting on the goniometer of a Nonius Kappa CCD diffractometer
equipped with an Oxford Cryostream liquid nitrogen cooling system.
The four data sets were corrected for absorption: CCDC 650007 (1),
650008 (2), 650009 (3) and 650010 (4). For crystallographic data in CIF
format see DOI: 10.1039/b708758f.
13 I. L. Fedushkin, A. A. Skatova, V. A. Chudakova, G. K. Fukin,
S. Dechert and H. Schumann, Eur. J. Inorg. Chem., 2003, 3336.
14 We thank a referee for this suggestion.
6 H. Schumann, M. Hummert, A. Lukoyanov and I. L. Fedushkin,
Organometallics, 2005, 24, 3891.
7 R. J. Baker, C. Jones, M. Kloth and D. P. Mills, New J. Chem., 2004,
28, 207.
15 (a) W. J. Evans and B. L. Davis, Chem. Rev., 2002, 102, 2119; (b)
W. J. Evans, J. Organomet. Chem., 2002, 647, 2.
3466 | Chem. Commun., 2007, 3464–3466
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