Synthesis, structural authentication, and structurally defined metalation reactions of lithium and sodium DA-zincate bases (DA = diisopropylamide) with phenylacetylene

Article abstract of DOI:10.1021/om8001813

In a study aimed at developing the diisopropylamido (DA) chemistry of zincates, the new lithium DA-zincate [(TMEDA) · Li(^tBu)(DA) Zn(^tBu)] (4) has been synthesized by an interlocking cocomplexation approach comprising mixing of its three component chemicals, LDA, ^tBu₂Zn, and TMEDA, in a 1:1:1 ratio in hexane solution. Previously made by transamination from the corresponding TMP-zincate, the known sodium congener [(TMEDA)-Na(^tBu)(DA)Zn(^tBu)] (2) was also synthesized by this approach, substituting NaDA for LDA. Closely resembling each other, their molecular structures determined by X-ray crystallography can be categorized as contact ion-pair ates of TMEDA-chelated alkali metal cations linked to trigonal-planar dialkyl-Zn anions via bridging DA ligands. Reaction of 4 and 2 with phenylacetylene affords the bimetallic acetylides [((TMEDA)·Li(C≡CPh)₂Zn(^tBu)} ₂·(TMEDA)] (5) and [((TMEDA)·Na(C≡CPh) ₂Zn(^tBu)}₂] (6), respectively. X-ray crystallographic studies reveal 5 is a pseudodimer (tetranuclear) with two (LiCZnC) rings linked at the Zn atoms by a bridging, nonchelating TMEDA ligand; in contrast 6 adopts a distorted cubane of alternating PhC≡C and metal (2 Na, 2 Zn) corners. For comparison, the synthesis and crystal structures of the neutral zinc complexes [(TMEDA)·Zn(C≡CPh)₂] (7) and [(TMEDA)·Zn(^tBu)(C≡CPh)] (8), formally components of the ate complexes 5 and 6, are also reported. In addition, the ¹H and ¹³C NMR spectra of 2, 4, 5, 6, 7, and 8 recorded from solutions in C₆D₆ are disclosed.

Full text of DOI:10.1021/om8001813

2654
Organometallics 2008, 27, 2654–2663
Synthesis, Structural Authentication, and Structurally Defined
Metalation Reactions of Lithium and Sodium DA-Zincate Bases
(DA ) diisopropylamide) with Phenylacetylene†
‡
,§
§
§
´
´
William Clegg, Joaqu´ın Garc´ıa-Alvarez,* Pablo Garc´ıa-Alvarez, David V. Graham,
Ross W. Harrington,‡ Eva Hevia,§ Alan R. Kennedy,§ Robert E. Mulvey,*,§ and
Luca Russo‡
WestCHEM, Department of Pure and Applied Chemistry, UniVersity of Strathclyde, Glasgow,
Scotland G1 1XL, U.K., and School of Natural Sciences (Chemistry), Newcastle UniVersity,
Newcastle upon Tyne NE1 7RU, U.K.
ReceiVed February 26, 2008
In a study aimed at developing the diisopropylamido (DA) chemistry of zincates, the new lithium
DA-zincate [(TMEDA) · Li(tBu)(DA)Zn(tBu)] (4) has been synthesized by an interlocking cocomplexation
t
approach comprising mixing of its three component chemicals, LDA, Bu2Zn, and TMEDA, in a 1:1:1
ratio in hexane solution. Previously made by transamination from the corresponding TMP-zincate, the
known sodium congener [(TMEDA) · Na(tBu)(DA)Zn(tBu)] (2) was also synthesized by this approach,
substituting NaDA for LDA. Closely resembling each other, their molecular structures determined by
X-ray crystallography can be categorized as contact ion-pair ates of TMEDA-chelated alkali metal cations
linked to trigonal-planar dialkyl-Zn anions via bridging DA ligands. Reaction of 4 and 2 with
phenylacetylene affords the bimetallic acetylides [{(TMEDA) · Li(Ct CPh)2Zn(tBu)}2 · (TMEDA)] (5)
and [{(TMEDA) · Na(Ct CPh)2Zn(tBu)}2 ] (6), respectively. X-ray crystallographic studies reveal 5 is a
pseudodimer (tetranuclear) with two (LiCZnC) rings linked at the Zn atoms by a bridging, nonchelating
TMEDA ligand; in contrast 6 adopts a distorted cubane of alternating PhCt C and metal (2 Na, 2 Zn)
corners. For comparison, the synthesis and crystal structures of the neutral zinc complexes
[(TMEDA) · Zn(Ct CPh)2] (7) and [(TMEDA) · Zn(tBu)(Ct CPh)] (8), formally components of the ate
1
complexes 5 and 6, are also reported. In addition, the H and 13C NMR spectra of 2, 4, 5, 6, 7, and 8
recorded from solutions in C6D6 are disclosed.
regioselectively mono- and dideprotonated,6,7 whereas they are
Introduction
essentially inert to LiTMP and NaTMP. Structural snapshots
Lithium 2,2,6,6-tetramethylpiperidide (LiTMP), one of the
most important utility bases in synthetic chemistry,1 was first
transformed to the TMP-zincate “Li(TMP)Zn(tBu)2” in 1999
in a report by Kondo, Uchiyama, et al.2 Subsequently in 2005
we introduced the first sodium TMP-zincate reagent
[(TMEDA) · Na(tBu)(TMP)Zn(tBu)], 1.3 TMP-zincate bases can
instigate clean deprotonation of a wide range of aromatic
compounds.4 In general TMP-zincates outperform their parent
lithium and sodium amides in their ability to effect highly
regioselective metalations and polymetalations under mild
conditions. In the most spectacular examples to date, N,N-
dimethylaniline is the subject of meta-deprotonation,5 not ortho-
deprotonation as with butyllithium, and the substituent-free,
nonactivated hydrocarbons benzene and naphthalene can be
of key bimetallic complexes taken before and after deprotonation
have established that these are zinc-hydrogen exchange (zin-
cation) reactions, which, since dialkylzinc reagents cannot
replicate due to their generally poor kinetic basicity, are best
thought of as alkali-metal-mediated zincations (AMMZ).8
Surprisingly the zincate metamorphosis of the best known
lithium amide utility base lithium diisopropylamide (LiNiPr2;
LDA) has, in contrast, been largely neglected.
The first study reported seven years ago,9 also by Kondo and
Uchiyama, revealed that DA-zincate “Li(DA)Zn(tBu)2” used in
situ in ether or THF solution without any direct characterization
shows a completely different regioselectivity from that of TMP-
zincate in reactions with bromopyridines (Scheme 1). The power
to control the regioselectivity of a reaction by changing the
amide component provides a strong incentive for developing a
complementary DA-zincate chemistry. Recently we started along
† Dedicated to Professor Ken Wade on the occasion of his 75th birthday.
* Corresponding author. E-mail: r.e.mulvey@strath.ac.uk.
§ WestCHEM, University of Strathclyde.
‡ Newcastle University.
(1) Campbell, M.; Snieckus, V. Encyclopedia of Reagents for Organic
Synthesis; Paquette L. A., Ed.; Wiley: New York, 1995; Vol. 5.
(2) Kondo, Y.; Shilai, M.; Uchiyama, M.; Sakamoto, T. J. Am. Chem.
Soc. 1999, 121, 3539.
(6) Armstrong, D. R.; Clegg, W.; Dale, S. H.; Graham, D. V.; Hevia,
E.; Hogg, L. M.; Honeyman, G. W.; Kennedy, A. R.; Mulvey, R. E. Chem.
Commun. 2007, 598.
(3) Andrikopoulos, P. C.; Armstrong, D. R.; Barley, H. R. L.; Clegg,
W.; Dale, S. H.; Hevia, E.; Honeyman, G. W.; Kennedy, A. R.; Mulvey,
R. E. J. Am. Chem. Soc. 2005, 127, 6184.
(7) Clegg, W.; Dale, S. H.; Hevia, E.; Hogg, L. M.; Honeyman, G. W.;
Mulvey, R. E.; O’Hara, C. T. Angew. Chem., Int. Ed. 2006, 45, 6548.
(8) Mulvey, R. E. Organometallics 2006, 25, 1060.
(9) Imahori, T.; Uchiyama, M.; Sakamoto, T.; Kondo, Y. Chem.
Commun. 2001, 2450.
(4) Mulvey, R. E.; Mongin, F.; Uchiyama, M.; Kondo, Y. Angew. Chem.,
Int. Ed. 2007, 46, 3802.
(5) Armstrong, D. R.; Clegg, W.; Dale, S. H.; Hevia, E.; Hogg, L. M.;
Honeyman, G. W.; Mulvey, R. E. Angew. Chem., Int. Ed. 2006, 45, 3775.
10.1021/om8001813 CCC: $40.75
2008 American Chemical Society
Publication on Web 05/09/2008

Metalation Reactions of Lithium and Sodium DA-Zincate Bases
Organometallics, Vol. 27, No. 11, 2008 2655
Scheme 1. Different Regioselectivity of (TMP)- and (DA)-
Zincates in Reactions with Bromopyridines
Scheme 3. “Interlocking Cocomplexation” Synthesis of 2 (M )
Na) and 4 (M ) Li) from Their Three Component Chemicals
Scheme 2. Transamination Reaction of
1 to Generate
(TMEDA) · Na(tBu)(DA)Zn(tBu)], 2, Which Subsequently Dis-
proportionates to [(TMEDA) · Na(DA)2Zn(tBu)], 3, and
[(TMEDA) · NaZn(tBu)3]
and TMEDA, are nonaggregated in their normal pure forms and
so preorganized to participate in the interlocking cocomplexation
process, the third component, LDA or its sodium congener
(NaNiPr2; NaDA), usually exists in aggregated form. A depro-
tonating agent par excellence,11 LDA exudes a fascinating
structural chemistry in both the solid and solution phases. In
its pure solvent-free form, crystalline LDA exists as a single-
strand helical polymer of near-linear N-Li-N segments,12
while in donor-solvent-free solution it takes the form of various
cyclic oligomers.13 Mixed with TMEDA, LDA crystallizes as
an infinite array of (LiN)2 ring dimers linked by monodentate,
bridging (nonchelating) TMEDA ligands.14 The structure of
hydrocarbon-insoluble NaDA is not yet known, though it is most
probably a polymer. Crystalline [(TMEDA · NaDA)2] adopts a
discrete (NaN)2 ring dimer structure, in which the larger size
of sodium (compared to lithium) permits full N,N-chelation by
the TMEDA ligands.15 Hence, these aggregated forms of LDA
and NaDA must break down to monomeric modifications during
the cocomplexation processes leading to 4 and 2.
Reaction of phenylacetylene with 2 or 4 in a 1:1 stoichiometry
in hexane solution afforded the colorless crystalline products 6
and 5, each surprisingly containing 2 molar equiv of PhCt C-
ligand in its molecular formula (Scheme 4). Repeating the
reaction of 4 with a 4:3 ratio of PhCt CH:TMEDA com-
mensurate with the stoichiometry of 5 increased the (isolated,
crystalline) yield of 5 from 36% to 76%. Because the yield of
5 exceeds 50%, it is unequivocal that lithium zincate 4 is
functioning as a dual alkyl-amido base toward the acetylene.
This ambibasic behavior has precedence in TMP-zincate
chemistry with other types of organic substrate,16 and in general,
the ability of such heteroleptic bases to utilize one (alkyl) or
other (amido) of their component anionic ligands for deproto-
nation purposes is part of the fascination of this area of
this path by reporting in a communication10 that the TMP-
zincate 1 undergoes transamination with diisopropylamine,
DA(H), to generate initially the dialkyl-DA zincate [(TMEDA) ·
Na(tBu)(DA)Zn(tBu)], 2, which subsequently disproportionates
over 24 h to the monoalkyl-diDA zincate [(TMEDA) ·
Na(DA)2Zn(tBu)], 3, and the trialkyl zincate [(TMEDA) ·
NaZn(tBu)3] (Scheme 2). Here, in this paper, we make further
inroads in the development of DA-zincate chemistry by describ-
ing (i) an alternative preparative method for 2 and its full
crystallographic characterization; (ii) the synthesis and crystal-
lographic characterization of the new lithium congener
[(TMEDA) · Li(tBu)(DA)Zn(tBu)], 4; (iii) a comparative study
of the products obtained from the reactions of 2 and 4 toward
phenylacetylene, PhCt CH; and (iv) from the study in iii, the
first crystallographically characterized products of AMMZ
reactions performed by DA-zincate bases in [{(TMEDA) ·
Li(Ct CPh)2Zn(tBu)}2 · (TMEDA)], 5, and [{(TMEDA) · Na-
(Ct CPh)2Zn(tBu)}2], 6. For completeness, we also report the
synthesis and crystallographic characterization of the TMEDA-
chelated homometallic, neutral zinc components of 5 and 6,
namely, homoanionic [(TMEDA) · Zn(Ct CPh)2], 7, and het-
eroanionic [(TMEDA) · Zn(tBu)(Ct CPh)], 8.
(11) (a) Collum, D. V.; McNeil, A. J.; Ramirez, A. Angew. Chem., Int.
Ed. 2007, 46, 3002. (b) Eames, J. Sci. Synth. 2006, 8a, 173. (c) Rathman,
T. L. Speciality Chem. Mag. 1989, 9, 300.
Results and Discussion
(12) Barnett, N. D. R.; Mulvey, R. E.; Clegg, W.; O’Neil, P. A. J. Am.
Chem. Soc. 1991, 113, 8187.
Syntheses. Both 2 and 4 were synthesized by simply mixing
together their component chemicals (alkali metal amide, di-
alkylzinc reagent, and TMEDA) in a 1:1:1 stoichiometry in
hexane solution. Conceptually, one could view these reactions
as “interlocking cocomplexation syntheses” (Scheme 3), wherein
the group 1 amide functions dualistically as an N-Lewis base
toward zinc and an alkali metal-Lewis acid toward one (agostic)
Me arm, the dialkylzinc functions dualistically as a zinc-Lewis
acid toward the N of the amide group and as an electrostatic
(agostic) Me source to an alkali metal cation, and TMEDA
functions solely as an N,N-chelating Lewis base to an alkali
metal. Mechanistically, the picture is decidedly more compli-
cated. Though two of the molecular components, the dialkylzinc
(13) (a) Galiano-Roth, A. S.; Collum, D. B. J. Am. Chem. Soc. 1989,
111, 6722. (b) Galiano-Roth, A. S.; Kim, Y.-J.; Gilchrist, J. H.; Harrison,
A. T.; Fuller, D. J.; Collum, D. B. J. Am. Chem. Soc. 1991, 113, 5053. (c)
Hall, P. L.; Gilchrist, J. H.; Harrison, A. T.; Fuller, D. J.; Collum, D. B.
J. Am. Chem. Soc. 1991, 113, 9575. (d) Gilchrist, J. H.; Collum, D. B.
J. Am. Chem. Soc. 1992, 114, 794.
(14) Bernstein, M. P.; Romesberg, F. E.; Fuller, D. J.; Harrison, A. T.;
Collum, D. B.; Liu, Q.-Y.; Williard, P. G. J. Am. Chem. Soc. 1992, 114,
5100.
(15) Andrews, P. C.; Barnett, N. D. R.; Mulvey, R. E.; Clegg, W.;
O’Neil, P. A.; Barr, D.; Cowton, L.; Dawson, A.; Wakefield, B. J. J.
Organomet. Chem. 1996, 518, 85.
(16) (a) Clegg, W.; Dale, S. H.; Harrington, R. W.; Hevia, E.; Honeyman,
G. W.; Mulvey, R. E. Angew. Chem., Int. Ed. 2006, 45, 2374. (b) Clegg,
W.; Dale, S. H.; Drummond, A. M.; Hevia, E.; Honeyman, G. W.; Mulvey,
R. E. J. Am. Chem. Soc. 2006, 128, 7434. For an example from TMP-
´
aluminate chemistry see: (c) Garcia-Alvarez, J.; Graham, D. V.; Kennedy,
´
(10) Armstrong, D. R.; Clegg, W.; Dale, S. H.; Garcia-Alvarez, J.;
Harrington, R. W.; Hevia, E.; Honeyman, G. W.; Kennedy, A. R.; Mulvey,
R. E.; O’Hara, C. T. Chem. Commun. 2008, 187.
A. R.; Mulvey, R. E.; Weatherstone, S. Chem. Commun. 2006, 3208. (d)
Clegg, W.; Dale, S. H.; Hevia, E.; Hogg, L. M.; Honeyman, G. W.; Mulvey,
R. E.; O’Hara, C. T.; Russo, L. Angew. Chem., Int. Ed. 2008, 47, 731.

2656 Organometallics, Vol. 27, No. 11, 2008
Clegg et al.
Scheme 4. Crystalline Products 5 (top) and 6 (right) from
Reactions of Phenylacetylene with 4 and 2, Respectively
Scheme 5. Formation of
Phenylacetylide with ZnCl2 in the Presence of TMEDA
7
by Metathesis of Lithium
(top) and Synthesis of
8
via Reaction of ZntBu2 and
Phenylacetylene (bottom)
led mainly to the formation of the latter monoacetylide 8.
Analysis of the crude reaction mixture by NMR spectroscopy
revealed that the formation of 8 was essentially quantitative,
though the isolated crystalline yield was 47%, reflecting the good
solubility of 8. On heating the same reactants in toluene solution,
8 was again the major product, though the reaction was
considerably less clean and a trace amount of 7 was detected
in the mixture.
chemistry. If 4 functioned solely as an alkyl base, the putative
intermediate formed would be “[(TMEDA) · Li(Ct CPh)-
(DA)Zn(tBu)]”, whereas exclusive amido activity would gener-
ate “[(TMEDA) · Li(Ct CPh)(tBu)Zn(tBu)]”. Theoretical studies
of TMP-zincate systems17 have suggested that in certain cases
TMP functions as the kinetic base and the alkyl ligand functions
as the thermodynamic base. In the former scenario, “expelled”
TMP(H) can reenter the coordination sphere of the alkali metal
and protonate the alkyl ligand, leading to an ultimate expulsion
of alkane. Most TMP-zincate reactions with organic substrates
that have been experimentally structurally defined retain TMP
ligands in the cocomplexed deprotonated product, but exceptions
are found with nitrile substrates in which both alkyl ligands
remain intact on zinc.16d Irrespective of the kinetics involved
in the production of 5, both DA(H) and butane molecules have
been expelled. Furthermore, as a significant quantity of
di(PhCt C)-containing 5 was still obtained even when a
deficiency of PhCt CH was employed (in the 1:1, base:
PhCt CH stoichiometric reaction), it can be inferred qualita-
tively that the aforementioned putative intermediate
“[(TMEDA) · Li(Ct CPh)(DA)Zn(tBu)]” or “[(TMEDA) ·
Li(Ct CPh)(tBu)Zn(tBu)]” is more reactive than the starting
zincate base 4. The details associated with the synthesis of 6
essentially mimic those of 5. Thus, adding 2 molar equiv of
PhCt CH to the sodium zincate reagent increases the isolated,
crystalline yield of 6 from 37% to 67%. However, the fact that
6 is still the major product when the stoichiometry is incorrect
(1:1, base:PhCt CH ratio) implies any mono PhCt C-containing
intermediate reacts faster with PhCt CH reactant molecules than
does the starting base 1. Again, as the yield of 6 in the
stoichiometrically correct reaction is in excess of 50%, both
DA and t-Bu ligands must have been consumed in the reaction.
Sodium zincate 2 is therefore functioning as a dual base in the
synthesis of 6.
NMR Spectroscopic Studies. All new compounds 4, 5, 6,
7, and 8, as well as 2, were characterized spectroscopically by
1H and 13C NMR studies in C6D6 solution. The 7Li NMR spectra
of 4 and 5 were also recorded. It is noteworthy that the spectra
of 2 and 4 could be recorded from C6D6 solution without any
sign of decomposition. In contrast, the analogous TMP-zincate
1 rapidly metalates C6D6 in an NMR tube.3 This advantage
suggests that DA-zincates should be the zincate reagents of
choice for certain reactions performed in arene media. The
electronic influence of the alkali metal is discernible on
comparing the data from the lithium complex 4 and the sodium
congener 2. All the 1H chemical shifts for 2 and 4 are essentially
equivalent (MeCH, 0.98/0.98; Me′CH, 1.23/1.24; tBu, 1.53/1.54;
CH, 3.40/3.49 ppm for 4/2). This trend is mirrored in the 13C
data (e.g., CH3 of tBu 35.4/35.6; C(Me)3 of tBu 22.4/22.3 ppm
1
for 4/2). The distinct resonances noted within the H and 13C
spectra for each Me arm of the isopropyl groups in both 4 and
2 indicate that the alkali metal-DA-Zn bridges observed in
the crystal structures (vide infra) are maintained in solution.
DA can therefore be regarded as the synergic anchor, which
holds together the bimetallic framework. In contrast, only one
t
type of Bu group is observable for both 4 and 2, consistent
with free rotation about the N-Zn axes and fast exchange of
t
the terminal and bridging Bu groups seen in the crystal.
TMEDA interacts in a similar way with Li in 4 and Na in 2, as
1
evidenced by the chemical shifts of their H resonances (CH3,
1.72/1.68; CH2, 1.58/1.60 ppm for 4/2) from those of free
TMEDA (CH3, 2.11; CH2, 2.33). These substantial chemical
shift differences between 4/2 and the free ligand signify strong
dative interactions to compensate for the coordinative/electronic
unsaturation of the three-coordinate alkali metal centers (four-
coordination is more common). The room-temperature 1H NMR
t
Turning to the neutral zinc complexes 7 and 8, we synthesized
the former by a reproducible metathetical approach reacting the
corresponding lithium acetylide with zinc chloride and TMEDA
in a 2:1:1 stoichiometry in hexane solution (Scheme 5).
Typically the yield was about 70%. Attempts to reprepare 7 by
reacting di-tert-butylzinc with 2 equiv of PhCt CH surprisingly
spectrum of 5 reveals broad resonances for the Bu group and
TMEDA. Two distinct types of TMEDA ligand are present in
the crystal structure of 5: N,N-chelated to Li and N-monodentate
bound to Zn. These observations are consistent with a dynamic
process in solution involving TMEDA. A significant distinction
1
t
with the H spectrum of 6 is the more strongly shielded Bu
resonance (1.46; cf. 1.88 in 6). Some change was to be expected,
as in the crystal structures the zinc coordination sphere of 6
contains three electron-withdrawing PhCt C- ligands, whereas
(17) Uchiyama, M.; Matsumoto, Y.; Nobuto, D.; Furuyama, T.; Yamagu-
chi, K.; Morokuma, K. J. Am. Chem. Soc. 2006, 128, 8748.

Metalation Reactions of Lithium and Sodium DA-Zincate Bases
Organometallics, Vol. 27, No. 11, 2008 2657
One type of TMEDA ligand N,N-chelates to Li, while another
functions in an N-monodentate bridging mode (note the transoid
arrangement of N atoms) between Zn atoms. The distorted
tetrahedral coordination expansion of Zn (compared to its
trigonal planarity in 4), a result of introducing small, linear
PhCt C ligands, is completed by a terminal tBu ligand. We have
described previously the anomalous preferential coordination
(N,N-chelation) of TMEDA to Zn over Li in an “inverse
zincate” structure,20 but the variation here appears to be the
first example where TMEDA binds in a different way to the
distinct metals of an ate complex. In marked contrast, the
dilithium zincate [{(TMEDA · Li}2Zn(Ct CPh)4] exhibits the
more usual chain-terminating TMEDA (N,N-) to Li bonding,
with no TMEDA-Zn bonding.21 A notable feature of 5 is the
dissimilarity in the bonding modes adopted by the two acetylide
ligands. That labeled PhC(4)t C(3) assumes a near-linear
geometry to Zn [C4-C3-Zn, 173.64(15)°] and a near-
perpendicular geometry to Li [C4-C3-Li, 105.37(16)°], in-
dicative of predominately σ- and π-bonding, respectively (vide
infra). Strongly bound to C3 [length, 2.222(4) Å], Li also lies
close to C4 [2.803(4) Å]. In contrast, the corresponding angles
for the other acetylide ligand [C12-C11-Zn, 155.08(17)°;
C12-C11-Li, 127.17(17)°] are significantly more removed
from linear and perpendicular geometries respectively, indicative
of a greater mixing of σ- and π-bonding components. There is
also more deviation in the Li-C(11) [2.260(4) Å] and Li · · · C(12)
[3.148(4) Å] lengths. However, these distinct bonding modes
do not impact on the Ct C bond lengths [C3-C4, 1.218(2) Å;
C11-C12, 1.216(2) Å]. Despite its similar composition to 5,
the sodium product [{(TMEDA) · Na(Ct CPh)2Zn(tBu)}2], 6,
adopts a fundamentally different molecular structure (Figure 4).
A distorted cubane22 with alternating acetylide R-C and metal
(2 Na, 2 Zn) corners, the structure of 6 is completed by chelating
Figure 1. Molecular structure of 2 with hydrogen atoms omitted
for clarity, showing the (agostic) Na · · · CH3 contact as a thin line.
Displacement ellipsoids are shown at the 40% probability level in
Figures 1-6 (25% in Figure 5 to reduce congestion), and labels
are shown for selected atoms.
in 5 there are only two such ligands and a TMEDA N atom.
However, there is very little difference in the corresponding
1H resonances of the PhCt C- ligands in 5 and 6. On the basis
of its molecular structure in the crystal, 6 should display two
distinct PhCt C- ligands, but only one type is observed in
solution. This points to a dynamic process. Reflecting the
similarity of their distorted tetrahedral zinc structures (vide
infra), 7 and 8 display essentially equivalent 1H chemical shifts
for corresponding TMEDA signals (CH3, 2.12/2.08; CH2, 1.79/
1.78 ppm for 7/8), although in 8 the signals are broader,
consistent with existence of a dynamic process.
Solid State Structures. Turning to the results of the X-ray
crystallographic studies, the molecular structures of 2, 4, 5, 6,
7, and 8 are shown in Figures 1–6, respectively. Table 1 lists
selected bond lengths and bond angles for the bimetallic ate
compounds 2, 4, 5, and 6, Table 2 lists the corresponding values
for the homometallic neutral zinc complexes 7 and 8, and Table
3 lists key crystallographic and refinement parameters for all
these compounds.
t
TMEDA and terminal Bu groups on Na and Zn, respectively.
Though ring-stacking can be invoked to describe metal-ligand
cubane structures,23 6 is best interpreted as a structurally
dictating [(PhCt C)2Zn(µ-Ct CPh)2Zn(tBu)2]2- dianion, the two
(PhCt C)3 triangular faces of which are filled by TMEDA-
chelated Na+ cations. For steric reasons the tBu ligands occupy
cisoid positions within the dianion, exo to the cubane (Na2Zn2C4)
core. The origin of the distinction between 5 and 6 is presumably
also largely steric, as Li+ seems too small to support a
(PhCt C)3 triangular face, instead restricted to a (PhCt C)2 edge.
There are two crystallographically distinct PhCt C- anions in
each of the two independent molecules of 6; each molecule has
crystallographic 2-fold rotation symmetry. Within the puckered
(ZnC)2 ring, there is a set of near-linear [Zn2-C31-C32,
158.0(5)°] and near-perpendicular [Zn2B-C31-C32, 121.4(4)°]
Zn-CCPh bonds. Exo this ring, the third Zn-CCPh bond type
also approaches linearity [Zn2-C39-C40, 172.3(5)°]. This
mixture of geometries cannot simply be interpreted as zinc
binding in both a σ- and π-manner, as that situation cannot be
distinguished from the situation where steric constraints involved
in aggregation enforce such a geometry. What the structure of
6 does definitely show is that zinc has the capacity to
The molecular structures of 2 (Figure 1) and 4 (Figure 2)
closely resemble each other and related TMP-zincate structures.4
TMEDA N,N-chelated alkali metals form a strong amido (N)
t
bridge to Zn, which carries a terminal Bu ligand. A distorted
trigonal-planar coordination of Zn is completed by a second
tBu ligand. Its quaternary C atom bonds exclusively to Zn, but
one Me arm forms a long-range contact to the alkali metal in a
long (agostic) interaction [in 2, Na · · · C(4), 2.809(4) Å; in 4,
t
Li · · · C(14), 2.695(4) Å]. Thus this second Bu ligand is
nominally bridging, as the bridge must be weak and easily
t
ruptured in solution (as gauged by the equivalence of the Bu
groups in the NMR spectra, consistent with free rotation about
the Zn-N axes, as mentioned above). Similar gross features
decorate the TMP structures 13 and [(THF) · Li(tBu)(TMP)-
Zn(tBu)] 9.17,18 Substituting DA for TMP also has little effect
on relative dimensions with, for example, Zn-N bond lengths
covering the narrow range 2.034(6)-2.0741(19) Å across the
series of four compounds (mean values: for 2 and 4, 2.057 Å;
for 1 and 9, 2.054 Å).19
(20) Graham, D. V.; Hevia, E.; Kennedy, A. R.; Mulvey, R. E.
Organometallics 2006, 25, 3297.
The centrosymmetric structure of 5 displays a LiCZnC four-
atom ring motif with R-C atom (PhCt C-) bridges (Figure 3).
(21) Edwards, J. A.; Fallaize, A.; Raithby, P. R.; Rennie, M.-A.; Steiner,
A.; Verhorevoort, K. L.; Wright, D. S. J. Chem. Soc., Dalton Trans. 1996,
133.
(18) Clegg, W.; Dale, S. H.; Hevia, E.; Honeyman, G. W.; Mulvey,
R. E. Angew. Chem., Int. Ed. 2006, 45, 2370.
(22) Note that the asymmetric unit contains two independent cubane
molecules. Due to the close similarity between them, only one is discussed
here.
(19) Outside of this work, only one previous Zn-DA bond [length,
1.852(2) Å] has been elucidated crystallographically, though it does not
belong to a zincate but a neutral diiminatozinc amide. See: Chisholm, M. H.;
Gallucci, J.; Phomphrai, K. Inorg. Chem. 2002, 41, 2785.
(23) Clegg, W.; Dale, S. H.; Graham, D. V.; Harrington, R. W.; Hevia,
E.; Hogg, L. M.; Kennedy, A. R.; Mulvey, R. E. Chem. Commun. 2007,
1641.

2658 Organometallics, Vol. 27, No. 11, 2008
Clegg et al.
Table 1. Selected Bond Lengths (Å) and Angles (deg) for Compounds 2, 4, 5, and 6 (M ) Na or Li); 6a and 6b are Two Crystallographically
Independent Molecules
2
4
5
6a
6b
Zn-N(DA/TMEDA)
M-N(DA)
2.050(2)
2.345(2)
2.445(2)
2.460(2)
2.054(3)
2.065(3)
2.809(4)
2.0650(14)
2.033(4)
2.153(4)
2.253(4)
2.0496(19)
2.047(2)
2.695(4)
2.2305(15)
M-N(TMEDA)
2.081(4)
2.103(4)
2.0389(18)
2.466(6)
2.604(5)
2.005(5)
2.481(5)
2.564(6)
1.995(6)
Zn-tBu(terminal)
Zn-tBu(bridging)
M · · · C(tBu)
Zn-C(t C)
2.0495(18)
2.0738(18)
2.039(5)
2.097(5)
2.327(5)
2.625(5)
2.601(6)
2.848(6)
2.935(5)
3.000(6)
3.201(6)
2.025(6)
2.118(5)
2.292(5)
2.628(5)
2.619(5)
2.788(6)
2.991(5)
2.957(5)
3.113(7)
M-C(t C)
M · · · ꢀC
2.222(4)
2.260(4)
2.803(4)
3.148(4)
t
N-Zn- Bu(terminal)
120.47(10)
116.62(10)
122.90(11)
123.19(7)
112.85(7)
123.95(8)
117.13(7)
t
N-Zn- Bu(bridging)
t
tBu-Zn- Bu
N-Zn-C(t C)
96.63(6)
99.43(6)
123.67(7)
109.37(7)
tBu-Zn-C(t C)
123.8(2)
114.1(2)
113.9(2)
107.61(19)
96.36(18)
96.49(18)
120.4(3)
114.8(3)
114.4(2)
107.36(19)
97.90(17)
98.6(2)
(Ct )C-Zn-C(t C)
107.55(7)
N(DA)-M-N(TMEDA)
126.56(8)
148.42(9)
76.12(8)
91.15(9)
110.01(12)
101.45(10)
127.17(8)
138.53(17)
84.98(13)
93.80(13)
106.35(14)
101.63(15)
N(TMEDA)-M-N(TMEDA)
C(tBu)-M-N(DA)
89.52(14)
71.41(18)
74.14(19)
C(tBu)-M-N(TMEDA)
N(TMEDA)-M-C(t C)
121.66(17)
119.39(17)
116.17(16)
116.26(17)
151.50(19)
111.21(17)
127.56(18)
94.80(18)
177.00(18)
104.34(18)
83.46(17)
79.38(16)
72.97(16)
147.86(18)
109.45(19)
132.69(16)
96.30(18)
176.28(18)
102.50(19)
81.13(18)
79.02(17)
74.43(17)
(Ct )C-M-C(t C)
95.81(14)
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
Compounds 7 and 8; 8a and 8b are Two Crystallographically
Independent Molecules
interactions involve short Na-RC contacts [2.601(6)-2.848(6)
Å] and longer Na · · · ꢀC contacts [2.935(5)-3.201(6) Å].
Both 7 and 8 adopt simple mononuclear molecular structures
(Figures 5 and 6, respectively) with distorted tetrahedral zinc
centers. This type of molecular architecture is common in
mononuclear organozinc structures.24,25 Two chemically equiva-
lent molecules are found in the asymmetric unit of 8, but for
brevity only one is discussed here. The TMEDA bite angles
[in 7, N2-Zn-N1, 84.96 (11)°; in 8, Nl-Znl-N2, 82.9(9)°]
are a major factor in the tetrahedral distortion at the four-
coordinate zinc center. These N-Zn-N (dative) bond angles
are therefore the narrowest subtended at the metal, while the
C-Zn-C (anionic ligand) bond angles are the widest [in 7,
7
8a
8b
Zn-N
2.159(3)
2.155(3)
1.942(4)
1.960(4)
2.207(2)
2.227(2)
2.002(3)
2.212(2)
2.216(2)
2.000(3)
Zn-C(t C)
Zn-C(tBu)
N-Zn-N
C-Zn-C
N-Zn-C(t C)
2.031(3)
82.92(9)
126.69(11)
102.53(10)
101.17(10)
2.027(3)
82.81(8)
126.32(12)
103.22(10)
100.65(11)
84.96(11)
134.75(15)
107.12(13)
103.41(13)
105.49(13)
109.77(13)
N-Zn-C(tBu)
116.85(10)
117.30(9)
117.76(10)
116.74(10)
(24) (a) Wooten, A.; Carroll, P. J.; Maestri, A. G.; Walsh, P. J. J. Am.
Chem. Soc. 2006, 128, 4624. (b) O’Brien, P.; Hursthouse, M. B.; Motevalli,
M.; Walsh, J. R.; Jones, A. C. J. Organomet. Chem. 1993, 449, 1. (c)
Lennartson, A.; Hedstrom, A.; Hakansson, M. Acta Crystallogr., Sect. E:
Struct. Rep. Online 2007, 63, m123. (d) Johansson, A.; Wingstrand, E.;
Hakansson, M. J. Organomet. Chem. 2005, 690, 3846. (e) Andrews, P. C.;
Raston, C. L.; Skelton, B. W.; White, A. H. Organometallics 1998, 17,
779. (f) Westerhausen, M.; Wieneke, M.; Rademacher, B. B.; Schwarz, W.
Chem. Ber. 1997, 130, 1499. (g) Westerhausen, M.; Wieneke, M.; Schwarz,
W. J. Organomet. Chem. 1996, 522, 137. (h) Conway, B.; Hevia, E.;
Kennedy, A. R.; Mulvey, R. E. Chem. Commun. 2007, 2864.
(25) Barley, H. R. L.; Clegg, W.; Dale, S. H.; Hevia, E.; Honeyman,
G. W.; Kennedy, A. R.; Mulvey, R. E. Angew. Chem., Int. Ed. 2005, 44,
6018.
compromise from its preferred linear (σ) coordination to the
Ct CPh ligand in order to allow aggregation to take place.
Similarly, although all the Na-Ct C bond angles lie in the range
92.4(3)-96.9(4)°, formally indicative of π-interactions, the steric
effects of aggregation could also play a part in pushing the alkali
metal into these nonlinear geometries. What is certain is that
the Na cation has to compromise more than Zn from a σ-binding
mode to the Ct CPh ligands in building the aggregated structure.
The Na cation’s greater propensity for engaging in such
π-contacts makes this compromise easy. Note that the Na-Ct C

Metalation Reactions of Lithium and Sodium DA-Zincate Bases
Organometallics, Vol. 27, No. 11, 2008 2659
Table 3. Crystallographic Data
2
4
5
6
7
8
formula
fw
C20H48N3NaZn
419.0
C20H48LiN3Zn
402.9
C58H86Li2N6Zn2
1012.0
C52H70N4Na2Zn2
927.8
C22H26N2Zn
383.8
C18H30N2Zn
339.8
cryst syst
space group
a, Å
b, Å
c, Å
monoclinic
C2/c
27.7184(10)
9.8386(3)
19.6286(7)
monoclinic
P21/c
10.0802(2)
14.9984(3)
16.8398(4)
monoclinic
P21/c
12.9978(13)
16.615(2)
15.8875(17)
orthorhombic
Ibca
22.420(3)
22.883(3)
41.069(6)
monoclinic
P21/c
11.6198(6)
9.6815(6)
18.3865(11)
triclinic
P1
j
10.4058(4)
14.1317(15)
14.986(3)
61.934(9)
85.505(7)
89.357(7)
1937.7(4)
4
R, deg
ꢀ, deg
102.215(2)
94.220(1)
118.218(7)
107.610(3)
γ, deg
V, Å3
5231.7(3)
8
1.064
0.96
2539.05(9)
4
1.054
0.97
3023.3(6)
2
1.112
0.83
0.594-0.732
21069(5)
16
1.170
0.96
0.748-0.846
1971.5(2)
4
1.293
1.25
0.687-0.860
Z
Dc, g cm-3
µ, mm-1
transmission
cryst size, mm
Τ, Κ
1.165
1.26
0.619-0.760
0.30 × 0.18 × 0.05 0.40 × 0.30 × 0.30 0.70 × 0.60 × 0.40 0.32 × 0.24 × 0.18 0.30 × 0.20 × 0.12 0.42 × 0.39 × 0.23
150
27.1
123
27.5
150
25.0
150
25.0
123
27.0
150
25.0
θmax
no. of reflns meads
no. of unique reflns
Rint
34 146
5767
0.0810
251
0.0491
0.0943
1.037
10 878
5824
0.0288
279
0.0354
0.0871
1.061
21 467
5266
0.0313
350
0.0265
0.0699
1.108
74 668
9282
0.0537
598
0.0533
0.1654
1.082
22 291
4258
0.1042
230
0.0523
0.1281
1.067
21 170
6743
0.0498
393
0.0379
0.0786
1.019
no. of refined params
R (F, F2 > 2σ)
Rw (F2, all data)
S (F2, all data)
diff map extremes, e Å-3 +0.47, -0.29
+0.36, -0.39
+0.47, -0.25
+0.84, -0.72
+0.58, -0.86
+0.29, -0.35
Cl-Zn-C9, 134.75(15)°; in 8, Cl-Znl-C15, 126.69(11)°]. The
mean bond angles at zinc are essentially equivalent [in 7,
107.58°; in 8, 107.91°]. Comparing Zn-C bond lengths reveals
a modest elongation in the mono PhCt C monomer (mean,
2.016 Å; cf 1.951 Å in 7) due to the presence of the branched
alkyl ligand. The Zn-N bonds show a similar effect (mean,
2.214 Å; cf 2.157 Å in 7). Synthesized by the reaction of lithium
Figure 2. Molecular structure of 4 with hydrogen atoms omitted
for clarity, showing the (agostic) Li · · · CH3 contact as a thin line.
Figure 4. Molecular structure of one of the independent molecules
of 6 with hydrogen atoms omitted for clarity, showing Na · · · ꢀC
(acetylide) contacts as dashed lines.
Figure 3. Molecular structure of 5 with hydrogen atoms omit-
ted for clarity, showing Li · · · ꢀC (acetylide) contacts as dashed
lines.
Figure 5. Molecular structure of 7 with hydrogen atoms omitted
for clarity.

2660 Organometallics, Vol. 27, No. 11, 2008
Clegg et al.
Table 4. Ct C-Metal Bond Angles (deg) in Selected Bimetallic Phenylacetylide Strucutures
phenylacetylide
Ct C-AM
Ct C-M
5
6
7
8
105.37(16)
127.17(17)
96.9(4)
173.64(15)
155.08(17)
169.0(5)
164.2(4)
171.4(3)
169.5(3)
173.6(3)
170.56(17)
171.66(17)
133.87(15)
141.01(15)
133.52(15)
141.02(15)
151.4(3)
175.5a
92.4(3)
[{(TMEDA)Na(Ct CPh)2Mg(DA)}2],27 10
[{LiMg(DA)2(Ct CPh)}2],27 11
91.68(13)
91.67(12)
118.61(16)
122.45(16)
[(THF)3Na(Ct CPh){Sm[N(SiMe3)2]3],28 12
[{(TMEDA)Li}2Zn(Ct CPh)4]21
[(TMEDA)Cd(Ct CPh)2]26
129.7(2)
77.4a
164.5(4)
175.4(4)
163.81
[(TMEDA)2Na(CCPh){Al(iPr)3}]16d
100.69
92.33
84.9a
82a
[{Li(THF)2}{Cp2Zr(C’CPh)(PhC2Ct CPh)}]29
[(por)Zr(Ct CR)3Li(THF)]30
171.9(1)
173a
[{(TMEDA)Li)}2V(Ct CPh)4(TMEDA)]31
[Na(THF){(2,6-iPr2C6H3N(SiMe3)}Al(Ct CPh)3)]2
168a
89a
171a
32
[K(THF){(2,6-iPr2C6H3N(SiMe3)}Al(Ct CPh)3)]2
96.2a
90a
171a
32
[Li(dioxane){(2,6-iPr2C6H3N(SiMe3)}Al(Ct CPh)3)]2(dioxane)32
[(12-crown-4)2Na{Zn(THF)(Ct CPh)3}]33
170a
175a
[Li2{(Ct CPh)3Mg(TMEDA)}2]34
167.8(4)
162.0(4)
176.1(3)
173a
[Pt2(Ct CPh)4(PEt3)2(Pr)2Li2]35
85a
36
[Cp*2Sm(Ct CPh)2K]n
96.6a
81.5(1)
173a
[(TACN)LiAl(Ct CPh)3]37
178.1(2)
a a ) average, AM ) alkali metal, M ) non alkali metal, TACN ) 1,4-diisopropyl-1,4,7-triazacyclononane, POR ) 2,3,7,8,12,13,17,18-octaethyl-
porphyrinato dianion.
phenylacetylide and cadmium bis(hexamethyldisilazide) in
toluene/TMEDA solution, the group 12 homologue to 7,
[(TMEDA) · Cd(Ct CPh)2], is also known:26 essentially isos-
tructural to 7, its more open C-Cd-C bond angle [142.8(2)°]
intimates a modest relaxation of steric strain compared to that
in 7.
The dual (σ, π) bonding ability of the PhCt C- ligand makes
it a useful implement for measuring the relative σ/π bonding
characteristics of different metals in different molecular struc-
tures. A perfect σ-bond exhibiting sp-hybridization at the
acetylido C head would display a Ct C-M (where M ) metal)
bond angle of 180°. The two remaining p orbitals would be
disposed orthogonal to this sp hybrid orbital, and thus bonds of
high π-character would have Ct C-M bond angles closer to
90°. In polar alkali metal acetylides, the latter side-on bonding
is not “π” in the normal organic sense, but is predominantly an
ion-dipole interaction. The interaction could involve both
acetylido C atoms (η2), but as the R-C atom formally carries
the negative charge of the deprotonated acetylene, the contact
with the alkali metal cation will be strongest at the R-C junction.
Table 4 surveys these Ct C-M bond angles for 5-8 and those
of related crystallographically characterized bimetallic alkali
metal-based phenylacetylides.16d,21,26–37 The aforementioned
Figure 6. Molecular structure of one of the independent molecules
of 8 with hydrogen atoms omitted for clarity.
cadmium homologue to 7 is also included. A number of
interesting observations can be made from the comparative data.
Most obviously, in every case where both an alkali metal and
a higher valent metal (zinc, magnesium, aluminum, vanadium,
zirconium, platinum, or samarium) engage a PhCt C- ligand,
the Ct C-metal bond angle is larger for the latter set of metals.
Nearly always the former bond angles approach 90°, whereas
(26) Barr, D.; Edwards, A. J.; Raithby, P. R.; Rennie, M. -A.;
Verhorevoort, K.; Wright, D. S. Chem. Commun. 1994, 1627.
(27) Garcia-Alvarez, J.; Graham, D. V.; Hevia, E.; Kennedy, A. R.;
Mulvey, R. E. Dalton Trans. 2008, DOI: 10.1039/b716215d.
(28) Karl, M.; Seybert, G.; Massa, W.; Harms, K.; Agarwal, S.; Maleika,
R.; Stelter, W.; Greiner, A.; Heitz, W.; Neumuller, B.; Dehnicke, K. Z.
Anorg. Allg. Chem. 1999, 625, 1301.
(32) Schiefer, M.; Hatop, H.; Roesky, H. W.; Schmidt, H.-G.; Nolte-
meyer, M. Organometallics 2002, 21, 1300.
(33) Putzer, M. A.; Neumuller, B.; Dehnicke, K. Z. Anorg. Allg. Chem.
1997, 623, 539.
(34) Schubert, B.; Weiss, E. Chem. Ber. 1984, 117, 366.
(35) Sebald, A.; Wrackmeyer, B.; Theocharis, C. B.; Jones, W. J. Chem.
Soc., Dalton Trans. 1984, 747.
(36) Evans, W. J.; Keyer, R. A.; Ziller, J. W. Organometallics 1993,
12, 2618.
(37) Cui, C.; Schmidt, J. A. R.; Arnold, J. J. Chem. Soc., Dalton Trans.
2002, 2992.
(29) Choukroun, R.; Zhao, J.; Lorber, C.; Cassoux, P.; Donnadieu, B.
Chem. Commun. 2000, 1511.
(30) Kim, H.-J.; Jung, S.; Jeon, Y.-M.; Whang, D.; Kim, K. Chem.
Commun. 1999, 1033.
(31) Kawaguchi, H.; Tatsumi, K. Organometallics 1995, 14, 4294.

Metalation Reactions of Lithium and Sodium DA-Zincate Bases
Organometallics, Vol. 27, No. 11, 2008 2661
Figure 7. ChemDraw representations of the structures of 10, 11, and 12.
the latter are closer to 180°. This reflects the greater propensity
for σ-bonding and stronger carbophilicity of the di- and trivalent
metals compared to that of the alkali metals. Zinc is a
particularly strong acetylidophilic metal as, with the exception
of one case, Ct C-Zn bond angles lie in the range 164.2-175.5°.
The exception of 155.08(17)° occurs in 5 and is necessitated
by the requirement to close a heterobinuclear [Zn(Ct CPh)2Li]
ring within the steric constraints of tetrahedral Zn and Li centers
and is accompanied by an exceptionally large Ct C-Li bond
angle of 127.17(17)°, the second largest for an alkali metal in
the selection, which emphasizes the structural novelty of 5. The
pronounced dependency of the Ct C-metal bond angle on the
structural type is evident from the mixed acetylido-amido
magnesiate entries [{(TMEDA) · Na(Ct CPh)2Mg(DA)}2], 10,
and [{LiMg(DA)2(Ct CPh)}2], 11 (see Figure 7).27 The
Na · · · Mg · · · Mg · · · Na chain arrangement of 10 facilitates an
almost perfect linear and orthogonal coordination of the µ2-
Ct CPh ligand toward Mg and Na, respectively, with bond
angles close to 90° (mean, 91.67°) and 180° (mean, 171.11°)
respectively. On the other hand, the inverse crown motif of 11,
in which the Ct CPh ligands assume a µ2-disposition to Mg
centers within a four-membered (MgC)2 ring, or overall a µ3-
disposition including a long interaction with the Li centers, leads
to Ct C-metal bond angles intermediate between the perfect
180°/90°, being 138.51° (mean) for Mg and 120.53°(mean) for
Li. Here, as the acetylido ligand bridges two Mg centers, it must
donate its electron density essentially equally between them,
and thus a greater mixing of σ- and π-components takes place
within the Mg-C bonds. Steric hindrance would appear to be
largely responsible for the relatively small Ct C-Sm bond angle
[151.4(3)°] in the mixed acetylido-amido samariate
[(THF)3Na(Ct CPh)Sm{N(SiMe3)2}3], 12 (Figure 7), as the
lanthanide is surrounded by three bulky amido ligands in
addition to the acetylide.28 In 12, the Na binds side-on to the
PhCt C- ligand with an RC-Na-ꢀC angle of only 26.30(11)°.
zinc complexes [(TMEDA) · Zn(Ct CPh)2], 7, and [(TMEDA) ·
Zn(tBu)(Ct CPh)], 8, have also been synthesized and found to
have simple distorted tetrahedral mononuclear structures in the
crystal. Finally, as this study has revealed pronounced alkali
metal effects, as well as similarities to and differences from
TMP-zincates, DA-zincate chemistry promises to be a fertile
field for future investigation and exploitation.
Experimental Section
General Procedures. All reactions were performed under a
protective argon atmosphere using standard Schlenk techniques.
Hexane and toluene were dried by heating to reflux over sodium
benzophenone ketyl and distilled under nitrogen prior to use. ZnCl2
was purchased from Aldrich Chemicals as a 1 M solution in diethyl
ether. Butyl sodium38 was prepared according to a literature method.
n-Butyllithium in hexane was purchased from Aldrich Chemicals
and standardized immediately prior to use using salicylaldehyde
phenylhydrazone.39 NMR spectra were recorded on a Bruker DPX
1
400 MHz spectrometer, operating at 400.13 MHz for H, 155.50
MHz for 7Li, and 100.62 MHz for 13C. Satisfactory elemental
analyses of the compounds 2, 4, 5, and 6 could not be obtained
due to their high air- and moisture-sensitive nature.
X-ray Crystallography. Data were collected on Nonius Kap-
paCCD and Bruker SMART 1K diffractometers with Mo KR
radiation (λ ) 0.71073 Å). Further information is given in Table
3. Where applied, semiempirical absorption corrections were based
on repeated and symmetry-equivalent reflections. The structures
were solved by direct methods and refined on all unique F2 values.
2-Fold disorder was resolved and refined satisfactorily for TMEDA
in 4 and 5 and for one phenyl ring in 6; high anisotropic
displacement parameters indicate possible further disorder in 6, but
it could not be resolved. Programs were standard Nonius and Bruker
diffractometer control and integration software, SADABS,40 pro-
grams of the SHELX family,41 and local software.
Synthesis of [(TMEDA) · Na(tBu)(DA)Zn(tBu)] (2). A Schlenk
t
tube was charged with 2 mmol (0.358 g) of Bu2Zn, which was
dissolved in 10 mL of hexane. In a separate Schlenk tube 2 mmol
of BuNa (0.16 g) was suspended in 10 mL of hexane, and 1 molar
equiv of DA(H) (2 mmol, 0.28 mL) was added via syringe. The
resultant creamy white suspension was allowed to stir for 1 h, after
which the hexane solution containing tBu2Zn was added via syringe.
The suspension changed from creamy white to a clear solution.
This was followed by the addition of 1 molar equiv of TMEDA (2
mmol, 0.30 mL). The resultant homogeneous solution was moved
to the freezer to aid crystallization. A crop (0.33 g, 39%) of colorless
crystals formed in solution, which were suitable for X-ray crystal-
lographic analysis. 1H NMR (400.13 MHz, C6D6, 293 K): 0.98 and
Conclusions
An interlocking cocomplexation approach has been used to
synthesize the tricomponent alkali metal zincates
[(TMEDA) · M(tBu)(DA)Zn(tBu)], 2 (M ) Na) and 4 (M ) Li).
X-ray crystallography has established a common structural type
for 2 and 4, a dinuclear contacted ion pair with a strong alkali
metal-N(amido)-Zn bridge. Their reactions with phenylacety-
lene have been explored, leading to the first crystallographically
characterized products of direct zincation reactions performed
by DA-zincate bases, in [{TMEDA · Li(Ct CPh)2Zn(tBu)}2 ·
(TMEDA)], 5, and [{(TMEDA) · Na(Ct CPh)2Zn(tBu)}2], 6.
Lithium acetylide 5 adopts a novel tetranuclear structure with
two (LiCZnC) rings connected at the Zn atoms via a bridging
(nonchelating) TMEDA ligand, whereas sodium acetylide 6
adopts a pseudocubane arrangement of alternating Ct C and
metal (2 Na, 2 Zn) corners. In forming 5 and 6, the DA-zincate
reagents function as flexible, dual alkyl-amido bases. The neutral
3
1.24 (6H each, d, JHH ) 6.1 Hz, CH3-iPr of DA), 1.54 (18H, s,
CH3-tBu), 1.60 (4H, s, CH2-TMEDA), 1.68 (12H, s, CH3-TMEDA),
3
3.49 (2H, septet, JHH ) 6.1 Hz, CH-iPr of DA). 13C{1H} NMR
(38) Schade, C.; Bauer, W.; Schleyer, P. v. R. J. Organomet. Chem.
1985, 295, C25.
(39) Love, B. E.; Jones, E. G. J. Org. Chem. 1999, 64, 3755.
(40) Sheldrick, G. M. SADABS; University of Go¨ttingen: Germany,
2003-2007.

2662 Organometallics, Vol. 27, No. 11, 2008
Clegg et al.
1.95 (24H, s, CH3-TMEDA), 6.97 (4H, t, JHH ) 7.5 Hz, Hpara
(100.63 MHz, C6D6, 293 K): 22.3 (C(CH3)3 of tBu), 26.2 and 27.0
,
3
t
(CH3-iPr of DA), 35.6 (CH3 of Bu), 45.7 (CH3-TMEDA), 49.5
3
C6H5), 7.06 (8H, t, JHH ) 7.5 Hz, Hmeta, C6H5), 7.64 (8H, t,
(CH-iPr of DA), 56.7 (CH2-TMEDA).
3JHH ) 7.5 Hz, Hortho, C6H5). 13C{H} NMR (100.63 MHz, C6D6,
Synthesis of [(TMEDA) · Li(tBu)(DA)Zn(tBu)] (4). A Schlenk
t
t
293 K): 21.3 (C(CH3)3 of Bu), 34.8 (CH3 of Bu), 45.8 (CH3-
TMEDA), 56.9 (CH2-TMEDA), 126.5 (Ct CPh), 127.2 (Cpara
t
tube was charged with 2 mmol (0.358 g) of Bu2Zn, which was
,
dissolved in 10 mL of hexane. In a separate Schlenk tube 2 mmol
of nBuLi (1.25 mL of a 1.6 M solution in hexane) was added to 2
mmol of DA(H) (0.28 mL) in 10 mL of hexane. The resultant clear
solution was allowed to stir for 1 h, after which the hexane solution
containing tBu2Zn was added via syringe followed by the addition
of 1 molar equiv of TMEDA (2 mmol, 0.30 mL). The resultant
clear solution was moved to the freezer to aid crystallization. A
crop (0.43 g, 52%) of colorless crystals formed in solution, which
were suitable for X-ray crystallographic analysis. 1H NMR (400.13
C6H5), 128.6 (Cmeta, C6H5), 132.1 (Cortho, C6H5). Signals for
Ct CPh and Cipso of PhCt C were not observed.
Synthesis of [(TMEDA) · Zn(Ct CPh)2] (7). Phenylacetylene
n
(4 mmol, 0.44 mL) was dissolved in 10 mL of hexane, BuLi (4
mmol, 2.50 mL of a 1.6 M solution in hexane) was added dropwise
to the previous solution, and immediately a white solid precipitated.
The resultant white suspension was allowed to stir for 30 min. This
was followed by the addition of 0.5 molar equiv of ZnCl2 (2 mmol,
2 mL of a 1 M solution in diethyl ether) and 0.5 molar equiv of
TMEDA (2 mmol, 0.3 mL). No changes were observed after this
addition, and the white suspension was stirred at room temperature
overnight. The solvents were then removed, and the residue was
suspended in 15 mL of toluene. The suspension was filtered off to
separate the LiCl formed, and from the filtrate 7 was isolated as a
white solid (0.55 g, 71%) after solvent removal. Anal. Calcd for
C22H26N2Zn (383.8): C, 68.84; H, 6.83; N, 7.30. Found: C, 68.79;
H, 6.85; N, 7.31. 1H NMR (400.13 MHz, C6D6, 293 K): 1.79 (4H,
3
MHz, C6D6, 293 K): 0.98 and 1.23 (6H each, d, JHH ) 6.0 Hz,
CH3-iPr of DA), 1.53 (18H, s, CH3-tBu), 1.58 (4H, s, CH2-
3
TMEDA), 1.72 (12H, s, CH3-TMEDA), 3.40 (2H, septet, JHH
)
6.0 Hz, CH-iPr of DA). 13C{1H} NMR (100.63 MHz, C6D6, 293
K): 22.4 (C(CH3)3 of Bu), 25.0 and 25.6 (CH3-iPr of DA), 35.4
t
(CH3 of tBu), 46.8 (CH3-TMEDA), 48.2 (CH-iPr of DA), 57.1 (CH2-
7
TMEDA). Li NMR (155.50 MHz, C6D6, 293 K, reference LiCl
in D2O at 0.00 ppm): 0.95.
Synthesis of [{(TMEDA) · Li(Ct CPh)2Zn(tBu)}2 · (TMEDA)]
(5). A Schlenk tube was charged with 2 mmol (0.358 g) of tBu2Zn,
which was dissolved in 10 mL of hexane. In a separate Schlenk
3
s, CH2-TMEDA), 2.12 (12H, s, CH3-TMEDA), 7.01 (2H, t, JHH
3
) 7.7 Hz, Hpara, C6H5), 7.11 (4H, t, JHH ) 7.7 Hz, Hmeta, C6H5),
7.69 (4H, d, 3JHH ) 7.7 Hz, Hortho, C6H5). 13C{1H} NMR (100.63
MHz, C6D6, 293 K): 47.5 (CH3-TMEDA), 56.8 (CH2-TMEDA),
108.5 (Ct CPh), 113.7 (Ct CPh), 126.3 (Cpara, C6H5), 128.2 (Cipso,
C6H5), 128.4 (Cmeta, C6H5), 132.0 (Cortho, C6H5).
n
tube 2 mmol of BuLi (1.25 mL of a 1.6 M solution in hexane)
was added to 2 mmol of DA(H) (0.28 mL) in 10 mL of hexane.
The resultant clear solution was allowed to stir for 1 h, after which
t
Synthesis of [(TMEDA) · Zn(tBu)(Ct CPh)] (8). A Schlenk
tube was charged with 2 mmol (0.358 g) of ZntBu2, which was
dissolved in 10 mL of hexane, and 1 molar equiv of pheny-
lacetylene (2 mmol, 0.22 mL) was added via syringe. This was
followed by the addition of 1 molar equiv of TMEDA (2 mmol,
0.30 mL), and the resultant colorless solution was stirred at room
temperature overnight. Standing the solution at -27 °C afforded
colorless crystals of 8 (0.32 g, 47%), which were suitable for
X-ray crystallographic analysis. The crude of the reaction was
the hexane solution containing Bu2Zn was added via syringe
followed by the addition of 1.5 molar equiv of TMEDA (3 mmol,
0.45 mL). Then, 2 molar equiv (4 mmol, 0.44 mL) of phenylacety-
lene was added to the solution, and immediately a yellow solid
precipitated, which dissolved on addition of hot toluene. Standing
the solution on the bench afforded colorless crystals of 5 (0.77 g,
76%), which were suitable for X-ray crystallographic analysis. Note
that adding only 1 molar equiv of the phenylacetylene and TMEDA
also produces 5 but in a smaller yield (0.36 g, 36%). This suggests
that the intermediate reacts quicker with the phenylacetylene than
1
also analyzed by H-13C{1H} NMR, showing only the signals
1
does the starting reagent. H NMR (400.13 MHz, C6D6, 293 K):
of 8. Therefore the yield in the formation of 8 is quantitative.
1.46 (18H, s br, CH3-tBu), 1.80 (12H, s br, CH2-TMEDA), 2.14
(36H, s, CH3-TMEDA), 6.97 (4H, m, Hpara, C6H5), 7.07 (8H, m,
Hmeta, C6H5), 7.54 (8H, m, Hortho, C6H5). 13C{1H} NMR (100.63
MHz, C6D6, 293 K): 34.6 (CH3 of tBu), 46.6 (CH3-TMEDA), 57.2
Note that on adding 2 molar equiv of phenylacetylene at room
t
temperature, the second Bu group is not replaced since the
reaction produces 8 as the unique product. Anal. Calcd for
C18H30N2Zn (339.8): C, 63.62; H, 8.90; N, 8.24. Found: C, 63.80;
H, 8.90; N, 8.23. 1H NMR (400.13 MHz, C6D6, 293 K): 1.44
(CH2-TMEDA), 122.1 (Ct CPh), 126.3 (Cpara, C6H5), 127.9 (Cmeta
,
C6H5), 132.1 and 132.3 (Cortho, C6H5). Signals for Zn-C(CH3)3 of
tBu and Ct CPh and Cipso of PhCt C were not observed. 7Li NMR
(155.50 MHz, C6D6, 293 K, reference LiCl in D2O at 0.00 ppm):
1.24.
(9H, s, CH3-tBu), 1.78 (4H, s brought, CH2-TMEDA), 2.08 (12
3
H, s brought, CH3-TMEDA), 6.98 (1H, t, JHH ) 7.4 Hz, Hpara
,
3
C6H5), 7.08 (2H, t, JHH ) 7.4 Hz, Hmeta, C6H5), 7.61 (2H, d,
3JHH ) 7.4 Hz, Hortho, C6H5). 13C{1H} NMR (100.63 MHz, C6D6,
293 K): 19.4 (C(CH3)3 of tBu), 34.6 (CH3 of tBu), 47.8 (brought,
CH3-TMEDA), 57.1 (CH2-TMEDA), 108.4 (Ct CPh), 118.5
(Ct CPh), 125.8 (Cpara, C6H5), 128.3 (Cmeta, C6H5), 128.8 (Cipso,
C6H5), 132.0 (Cortho, C6H5).
Synthesis of [{(TMEDA) · Na(Ct CPh)2Zn(tBu)}2] (6). A
t
Schlenk tube was charged with 2 mmol (0.358 g) of Bu2Zn,
which was dissolved in 10 mL of hexane. In a separate Schlenk
tube 2 mmol of BuNa (0.16 g) was suspended in 10 mL of
hexane, and 1 molar equiv of DA(H) (2 mmol, 0.28 mL) was
added via syringe. The resultant creamy white suspension was
allowed to stir for 1 h, after which the hexane solution containing
tBu2Zn was added via syringe. The suspension changed from
creamy white to a clear solution. This was followed by the
addition of 1 molar equiv of TMEDA (2 mmol, 0.30 mL). Then,
2 molar equiv (4 mmol, 0.44 mL) of phenylacetylene was added
to the solution, and immediately a yellow solid precipitated,
which dissolved on addition of hot toluene. Standing the solution
on the bench afforded colorless crystals of 6 (0.62 g, 67%). Note
that adding only 1 molar equiv of the phenylacetylene also
produces 6 but in a smaller yield (0.34 g, 37%). This suggests
that the intermediate reacts quicker with the phenylacetylene
than does the starting reagent. 1H NMR (400.13 MHz, C6D6,
293 K): 1.62 (8H, s, CH2-TMEDA), 1.88 (18H, s, CH3-tBu),
Reaction of 8 with Phenylacetylene in Refluxing Toluene. A
Schlenk tube was charged with 2 mmol (0.358 g) of ZntBu2,
which was dissolved in 10 mL of hexane, and 1 molar equiv of
phenylacetylene (2 mmol, 0.22 mL) was added via syringe. This
was followed by the addition of 1 molar equiv of TMEDA (2
mmol, 0.30 mL), and the resultant colorless solution was allowed
to stir at room temperature overnight. At this stage the solution
contains mainly the compound 8. This was followed, after solvent
removal, by the addition of 10 mL of toluene and another
equivalent of phenylacetylene (2 mmol, 0.22 mL), affording a
very pale yellow solution. This solution was refluxed for 1 h,
leading to some gray solid (probably zinc metal). The crude
product of the reaction was analyzed by 1H-13C{1H} NMR,
revealing a very complicated mixture in which the major product

Metalation Reactions of Lithium and Sodium DA-Zincate Bases
Organometallics, Vol. 27, No. 11, 2008 2663
is still the starting material 8 and where compound 7 was
detected in only a trace amount.
Supporting Information Available: Crystallographic details for
compounds 2, 4, 5, 6, 7, and 8 in CIF format and NMR spectra for
these compounds. This material is available free of charge via the
Internet at http://pubs.ac.org.
Acknowledgment. We thank the EPSRC (grant award no.
GR/T27228/01), the Royal Society (University Research
Fellowship to E.H.), and the Spanish MEC-MCyT (Post-
doctoral Research grant no. 2007-0355 to P.G.-A.) for their
generous sponsorship of this research.
OM8001813
(41) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112.