Selective oxygen capture in lithium zincate chemistry: The syntheses and solid-state structures of (μ-O)Zn4[N(2-C5H4N)Bz]6 and Bu(t)(μ3-O)Li3(μ6-O)Zn3[N(2-C5H4N)Me]6 (Bz = benzyl)

Article abstract of DOI:10.1039/b005238h

Sequential reaction of ZnMe₂ with the (2-pyridyl)amines HN(2-C₅H₄N)R (R = Bz = benzyl 1, Me 2), Bu(t)Li and oxygen affords species which demonstrate oxo-encapsulation {(μ4-O)Zn₄[N(2-C₅H₄N)Bz]₆ 3}, and both encapsulation and insertion into a C-Li bond {Bu(t)(μ₃-O)Li₃(μ₆-O)Zn₃[N(2-C₅H₄N)Me]₆ 4}; in the solid-state 4 has a new type of facisomeric (μ₆-O)M₃M'₃ octahedral core.

Full text of DOI:10.1039/b005238h

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Selective oxygen capture in lithium zincate chemistry: the syntheses and
solid-state structures of (m-O)Zn4[N(2-C5H4N)Bz]6 and
But(m3-O)Li3(m6-O)Zn3[N(2-C5H4N)Me]6 (Bz = benzyl)
Robert P. Davies, David J. Linton, Ronald Snaith† and Andrew E. H. Wheatley*
Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, UK CB2 1EW.
E-mail: aehw2@cam.ac.uk
Received (in Cambridge) 29th June 2000, Accepted 17th August 2000
Sequential reaction of ZnMe2 with the (2-pyridyl)amines
HN(2-C5H4N)R (R = Bz = benzyl 1, Me 2), ButLi and
oxygen affords species which demonstrate oxo-encapsula-
tion {(m4-O)Zn4[N(2-C5H4N)Bz]6 3}, and both encapsulation
and insertion into a C–Li bond {But(m3-O)Li3(m6-O)Zn3[N(2-
C5H4N)Me]6 4}; in the solid-state 4 has a new type of fac-
isomeric (m6-O)M3MA3 octahedral core.
centre in this species, three describe the pyramid base
[Zn1A…Zn1B 3.130(2) Å] each non-bonding edge of which is
spanned by a [N(2-C5H4N)Bz]2 ligand [Zn–N(2-C5H4N)
2.069(7) Å, Zn–NBz 1.984(7) Å]. The final metal centre (Zn2)
occupies the C3v cluster apex and is stabilised by the pyridyl N-
centres of the remaining three organic residues [Zn2–N(2-
C5H4N) 2.045(7) Å], each of which spans one non-bonding
Zn1…Zn2 [3.089(2) Å] pyramid edge [Zn1–NBz 2.002(7) Å].
At the core of the C3v cluster is a nearly tetrahedral O22 centre
(mean Zn–O 1.903 Å, mean Zn–O–Zn 108.9°). It is noteworthy
that previous examples of oxo-encapsulation by Zn4-tetrahedra
have resulted from the deliberate introduction of water9,10 or
Current interest in the properties of lithium containing hetero-
bimetallic species derives from their ability to effect organic
transformations whose selectivity differs from those afforded
by homometallic organolithium compounds. Hence, whereas
organolithium reagents perform 1,2-addition across a,b-un-
saturated ketones,1 it has been reported lately that in the
presence of group 13 Lewis acids 1,4-addition is preferred,2
with the formation of a lithium aluminate recently being
implicated in this process.3 Also of use in such conjugate
additions,4 lithium organozincates find extensive employment
in synthetic chemistry.5 In spite of this utility, lithium zincate
structural chemistry is not well understood, and of the few
known structures two classes dominate. Ion-separated species
are favoured by the presence of strongly coordinating Lewis
bases6 whereas in their absence ion-associated [Li(m2-C)2]nZn
(n = 1, 2)7 motifs prevail. In the light of advances in the
controlled oxygenation of heterobimetallic compounds8 we
report here on the incorporation of oxygen by mixed Li–Zn
systems and detail the variable reactivity towards oxygen of the
products afforded by sequentially treating (2-pyridyl)amines
with organozinc and organolithium reagents (Scheme 1).
The reaction of a non-donor solution of HN(2-C5H4N)R (R =
Bz = benzyl 1) with ZnMe2 and ButLi was followed by
treatment with air (pre-dried over P2O5) until the evolution of
fumes subsided. Storage of the resultant solution afforded two
crystalline compounds.‡ The characterisation of both species
was made possible by their mechanical separation, with 1H
NMR spectroscopy verifying that one of these products was
ButOLi. The remaining compound was shown by X-ray
crystallography§ to be the trigonal pyramidal complex (m4-
O)Zn4[N(2-C5H4N)Bz]6 3 [Fig. 1(a)] for which formulation
there is a toluene molecule in the lattice. Of the two types of Zn
Scheme 1 Reagents and conditions: i, ZnMe2; ii, ButLi; iii, O2.
Fig. 1 (a) Molecular structure of 3; hydrogen atoms and lattice toluene
molecule omitted and only the ipso-C of Ph shown. (b) Molecular structure
of 4; hydrogen atoms and lattice thf molecule omitted.
† Deceased.
DOI: 10.1039/b005238h
Chem. Commun., 2000, 1819–1820
This journal is © The Royal Society of Chemistry 2000
1819

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carbon dioxide11 whereas 3 is afforded by exposure to
oxygen.
Notes and references
‡ 3: ZnMe2 (0.5 ml, 1 mmol, 2 M in toluene) was added to a stirred 278 °C
solution of 2-(benzylamino)pyridine (0.18 g, 1 mmol) in toluene (5 ml).
After 30 min the mixture was warmed to 240 °C and ButLi (0.59 ml, 1
mmol, 1.7 M in pentane) was added. Treatment of the room-temperature
mixture with (P2O5) dry air (until observable reaction ceased after ca. 1 min)
was followed by reflux and by storage at ambient temperature for 24 h
whereupon needles of ButOLi (by 1H NMR) and blocks of 3 formed.
Mechanical separation allowed the characterisation of 3. Yield 42% (by 1
consumed), mp 140 °C (decomp.). Found: C, 64.96; H, 5.16; N, 11.34. Calc.
for C79H74N12OZn4: C 64.53, H 5.04,N 11.44%. dH(500 MHz, CD3CN),
7.40–6.46 (m, 20H, Ar + PhMe), 4.56 (s, 4H, CH2), 2.33 (s, 1H, PhMe). 4:
ZnMe2 (0.5 ml, 1 mmol, 2 M in toluene) was added to a stirred 278 °C
solution of 2-(methylamino)pyridine (0.11 g, 1 mmol) in hexane (1 ml).
After 30 min ButLi (0.59 ml, 1 mmol, 1.7 M in pentane) was added.
Treatment of the room-temperature mixture with (P2O5) dry air (until
observable reaction ceased after ca. 1 min) and the addition of thf (0.08 ml)
was followed by storage at room temperature for 1 week whereupon blocks
of 4 formed. Yield 32% (by 2 consumed), mp > 300 °C. Found: C, 50.68;
H, 6.48; N, 12.64. Calc. for C44H59Li3N12O3Zn3: C, 51.69; H, 6.45; N,
13.45%. dH(500 MHz, [2H8]thf), 8.00–6.01 (m, 24H, Ar), 3.65 (m, 4H, thf),
2.81 (s, 9H, NMe), 2.53 (br m, 9H, NMe), 1.80 (m, 4H, thf), 1.12 (s, 9H,
But).
Significant structural modification was observed in the
product of the analogous reaction for which R was the less
sterically demanding Me group (2), this process affording 4.‡
X-Ray crystallography§ reveals 4 to be the unique distorted
octahedral oxo-encapuslation complex But(m3-O)Li3(m6-
O)Zn3[N(2-C5H4N)Me]6 [Fig. 1(b)] for which formulation
there is one thf molecule in the lattice. That the Li3 face is m3-
capped by an OBut group points to an oxygen atom having
inserted into a Li–C(But) bond [Li–O2 1.849(14) Å, Li–O2–Li
83.7(7)°].12 The three lithium centres are less strongly bonded
to the encapsulated m6-O centre [Li–O1 2.074(13) Å, Li–O1–Li
73.0(6)°] which in turn interacts with the three Zn centres [Zn–
O1 1.944(4) Å, Zn–O1–Zn 101.4(3)°]. The result—a molecular
fac-isomeric (m6-O)M3MA3 octahedron—has not been seen
before. Instead, existing examples of molecular m6-O hetero-
bimetallic octahedra have either (MMA5)13 or (M2MA4)14
formulations. The only previous report of a lithium containing
heterobimetallic m6-O octahedral complex is that of [RuH-
(SiHPh2)(CO)X2]2·[Li2Ru4OCl8X4] (X = PBut2Me), which
has a trans-isomeric (m6-O)Li2Ru4 core;15 all other examples of
lithium containing m6-O octahedra have been homometallic.16
The coordination spheres of both Li and Zn centres in 4 are
completed by [N(2-C5H4N)Me]2 ligands. Three of these
residues span the 3.007(2) Å Zn…Zn non-bonding distances in
the lower tier of the cluster [Zn–N1 2.038(7) Å, Zn–N2 2.050(7)
Å]. The remaining organic residues circumscribe the non-
bonding Li3 [Li…Li 2.47(2) Å] upper tier but are orientated
such that their N-pyridyl centres are precisely in the Li3 plane
while their NMe-groups span non-bonding heterobimetallic
[Li…Zn 2.75(1) Å] octahedron edges [Zn1–N4 2.065(7) Å].
The sp2-orbital on their deprotonated N-centres almost bisects
the Li–N(Me)–Zn bond angle [N3–C11–N4–M torsional angles
= 42.8° (M = Li1A), 45.4° (M = Zn1)] with the consequent
mismatch in orientation of the NMe-centred lone-pairs with the
Li+ ions in the upper tier probably being responsible for the
respective Li–N(2-C5H4N) and Li–NMe bond lengths of
2.071(14) and 2.160(13) Å. This variability in ligand coordina-
tion contrasts with the uniformly m2- or m3-bridging modes
adopted by ligand heteroatoms in previously reported m6-O
molecular heterobimetallic octahedra.14
Compounds 3 and 4 both incorporate six-membered MO-
MANCN rings by virtue of [N(2-C5H4N)R]2 coordination [M =
MA = Zn (3); both M = MA = Zn/Li and M = Li, MA = Zn (4)].
Further, both species show almost identical Zn3[N(2-
C5H4N)R]3 basal tiers [Fig. 1(a) and (b)], whilst the remaining
three amide ligands exhibit significant variability between 3 and
4. The orientation which they adopt in 3 results in their pyridyl
N-centres bonding only to Zn2 which they render approx-
imately tetrahedral, their deprotonated N-centres bonding only
to Zn1. Conversely, in 4 the upper tier ligands are orientated
such that the pyridyl N-centres lie in the Li3 plane with the
concomitant stabilisation of adjacent Li centres (by NMe
groups) being accompanied by bridging to basal Zn atoms.
Thus, 3 and 4 can be viewed as empirically incorporating a {(m3-
O)Zn3[N(2-C5H4N)R]6}22 ligand which is capable of acting
either as a heptadentate donor to a C3v cation [(ButOLi3)2+ in 4]
or as a tetradentate donor to an R3 cation (Zn2+ in 3) by virtue
of flexibility in the orientations demonstrated by the upper tier
of [N(2-C5H4N)R]2 ligands.
§ Crystal data: for 3: C79H74N12OZn4; M = 1468.98, cubic, space group
Ia3, a = 30.8060(7) Å, U = 29235.2(12) Å3, Z = 16, Dc = 1.335 g cm23
,
Mo-Ka (l = 0.71073 Å), m = 1.350 mm21, T = 180(2) K. 8240 reflections
(4297 unique, q < 25.02°, Rint = 0.0886), data were collected. Refinement
on F2 values of all data gave wR2 = 0.1987, conventional R = 0.0849 on
F values of all reflections with F2 > 2s(F2), 276 parameters. Residual
electron density within ±1.45 e Å23
.
4: C11H14.75Li0.75N3O0.75Zn0.75; M = 255.24, cubic, space group P213, a
= 16.694(10) Å, U = 4882.0(10) Å3, Z = 4, Dc = 1.389 g cm23, Mo-Ka
(l = 0.71073 Å), m = 1.513 mm21, T = 180(2) K. 20356 reflections (2128
unique, q < 22.44°, Rint = 0.0744), data collected. Refinement on F2 values
of all data gave wR2 = 0.2093, conventional R = 0.0744 on F values of all
reflections with F2 > 2s(F2), 205 parameters. Residual electron density
within ±1.02 e Å23
.
CCDC 182/1748. See http://www.rsc.org/suppdata/cc/b0/b005238h/ for
crystallographic files in .cif format.
1 M. T. Reetz, Angew. Chem., Int. Ed. Engl., 1984, 23, 556.
2 K. Maruoka, T. Itoh, M. Sakurai, K. Nonoshita and H. Yamamoto,
J. Am. Chem. Soc., 1988, 110, 3588.
3 W. Clegg, E. Lamb, S. T. Liddle, R. Snaith and A. E. H. Wheatley,
J. Organomet. Chem., 1999, 573, 305.
4 H. B. Mekelburger and C. S. Wilcox, Comprehensive Organic
Synthesis, ed. B. M. Trost and I. Fleming, Pergamon, Oxford, 1991, vol.
2, p. 124.
5 M. Uchiyama, M. Kameda, O. Mishima, N. Yokoyama, M. Koike, Y.
Kondo and T. Sakamoto, J. Am. Chem. Soc., 1998, 120, 4934.
6 M. Westerhausen, M. Wieneke, W. Ponikwar, H. No¨th and W. Schwarz,
Organometallics, 1998, 17, 1438.
7 E. Rijnberg, J. T. B. H. Jastrzebski, J. Boersma, H. Kooijman, G. van
Koten, N. Veldman and A. L. Spek, Organometallics, 1997, 16,
2239.
8 R. E. Mulvey, Chem. Soc. Rev., 1998, 27, 339; A. R. Kennedy, R. E.
Mulvey, C. L. Raston, B. A. Roberts and R. B. Rowlings, Chem.
Commun., 1999, 353; R. P. Davies, D. J. Linton, R. Snaith and A. E. H.
Wheatley, Chem. Commun., 2000, 193.
9 For C2v see, for example: F. A. Cotton, L. M. Daniels, L. R. Falvello,
J. H. Matonic, C. A. Murillo, X. Wang and H. Zhou, Inorg. Chim. Acta,
1997, 266, 91.
10 For C3v see: C.-F. Lee, K.-F. Chin, S.-M. Peng and C.-M. Che, J. Chem.
Soc., Dalton Trans., 1993, 467.
11 A. Belforte, F. Calderazzo, U. Englert and J. Stra¨hle, Inorg. Chem.,
1991, 30, 3778.
12 J. L. Wardell, Comprehensive Organometallic Chemistry, ed. G.
Wilkinson, F. G. A. Stone and E. W. Abel, Pergamon, Oxford, 1982,
vol. 1, pp. 46–47.
13 T. M. Che, V. W. Day, L. C. Francesconi, M. F. Fredrich, W. G.
Klemperer and W. Shum, Inorg. Chem., 1985, 24, 4055.
14 M. Veith, E.-C. Yu and V. Huch, Chem. Eur. J., 1995, 1, 26; P. Miele,
G. Foulon and N. Hovnanian, Inorg. Chim. Acta, 1997, 255, 289; J. L.
Jolas, S. Hoppe and K. H. Whitmire, Inorg. Chem., 1997, 36, 3335;
W. J. Evans, M. A. Ansari and J. W. Ziller, Inorg. Chem., 1999, 38,
1160.
Attempts to elucidate the structural characteristics of the
precursors to 3 and 4 are ongoing, as are studies into the nature
of the competition between oxo-insertion (cf. the ButO fragment
in 4) and oxo-encapsulation [cf. 3 and the (m6-O)Li3Zn3
fragment in 4]. Finally, the integrity of the {(m3-O)Zn3[(2-
C5H4N)NR]6}22 ligand is being investigated (by attempting to
effect substitution reactions, e.g. the conversion of 4 to 3 by
reaction with ZnCl2).
15 R. H. Heyn, J. C. Huffman and K. G. Caulton, New J. Chem., 1993, 17,
797.
16 J. F. K. Mu¨ller, M. Neuburger and B. Spingler, Angew. Chem., Int. Ed.,
1999, 38, 3549.
We thank the UK EPSRC (D. J. L.) for a Studentship and St.
Catharine’s (R. P. D.) and Gonville & Caius (A. E. H. W.)
Colleges for Research Fellowships.
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Chem. Commun., 2000, 1819–1820
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