The structural characteristics of organozinc complexes incorporating N,N'-bidentate ligands

Article abstract of DOI:10.1039/b410945g

Dimethylzinc reacts with an excess of N-2-pyridylaniline 6 to give the homoleptic species, Zn[PhN(2-C₅H₄N)]₂ 8. Single crystal X-ray diffraction reveals a solid-state dimer based on an 8-membered (NCNZn)2 core motif. Zn[CyN(2-C₅H₄N)]Me (Cy = c-C ₆H₁₁) 10, prepared by the combination of ZnMe₂ with the corresponding cyclohexyl-substituted pyridylamine, is also dimeric in the solid state but reveals a central (ZnN)₂ metallacycle. Employment of (p-Tol)NH(2-C₅H₄N) (p-To = 4-MeC₆H ₄) 11 yielded the tris(zinc) adduct Zn₃[(p-Tol)N(2C ₅H₄N)]₄Me₂ 12, which incorporates a central chiral molecule of Zn[(p-Tol)N(2-C₅H₄N)] ₂' 12a, that bridges two Zn[(p-Tol)N(2-C₅H ₄N)]Me' 12b units. A similar trimetallic structure is noted when the pyridylaniline substrate 11 is replaced with the bicyclic guanidine 1,3,4,6,7,8-hexahydro-2h-pyrimido[1,2-a]pyrimidine (hppH), affording Zn ₃(hpp)₄Me₂ 13. Spectroscopic studies point to retention of the solid-state structure of 13 in hydrocarbon solution. Reaction of 13 with dimesityl borinic acid, Mes₂BOH (Mes = mesityl), affords Zn₃(hpp)₄(OBMes₂)₂ 14 in which the trimetallic core is retained. This reactivity is in contrast to the closely related reaction of dimeric Zn[Me₂NC{N'Pr}₂]Me 15 with Mes₂BOH, which yielded Zn[Me₂NC{N'Pr}₂] [OBMes₂]'Me₂NC{N'Pr} {NH'Pr} 16 as a result of protonation at the guanidine ligand in addition to the Zn-Me bond.

Full text of DOI:10.1039/b410945g

View Article Online / Journal Homepage / Table of Contents for this issue
F U L L P A P E R
The structural characteristics of organozinc complexes incorporating
N,N-bidentate ligands†
Samuel J. Birch,a Sally R. Boss,a Sarah C. Cole,b Martyn P. Coles,*b Robert Haigh,a
Peter B. Hitchcockb and Andrew E. H. Wheatley*a
a
Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, UK CB2 1EW.
E-mail: aehw2@cam.ac.uk; Fax: +44 (0)1223 336362; Tel: +44 (0)1223 763122
Department of Chemistry, University of Sussex, Falmer, Brighton, UK BN1 9QJ.
b
E-mail: m.p.coles@sussex.ac.uk; Fax: +44 (0)1273 677196; Tel: +44 (0)1273 877339
Received 19th July 2004, Accepted 9th September 2004
First published as an Advance Article on the web 27th September 2004
Dimethylzinc reacts with an excess of N-2-pyridylaniline 6 to give the homoleptic species, Zn[PhN(2-C5H4N)]2
8. Single crystal X-ray diffraction reveals a solid-state dimer based on an 8-membered (NCNZn)2 core motif.
Zn[CyN(2-C5H4N)]Me (Cy = c-C6H11) 10, prepared by the combination of ZnMe2 with the corresponding
cyclohexyl-substituted pyridylamine, is also dimeric in the solid state but reveals a central (ZnN)2 metallacycle.
Employment of (p-Tol)NH(2-C5H4N) (p-Tol = 4-MeC6H4) 11 yielded the tris(zinc) adduct Zn3[(p-Tol)N(2-
C5H4N)]4Me2 12, which incorporates a central chiral molecule of ‘Zn[(p-Tol)N(2-C5H4N)]2’ 12a, that bridges two
‘Zn[(p-Tol)N(2-C5H4N)]Me’ 12b units. A similar trimetallic structure is noted when the pyridylaniline substrate
11 is replaced with the bicyclic guanidine 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine (hppH), affording
Zn3(hpp)4Me2 13. Spectroscopic studies point to retention of the solid-state structure of 13 in hydrocarbon
solution. Reaction of 13 with dimesityl borinic acid, Mes2BOH (Mes = mesityl), affords Zn3(hpp)4(OBMes2)2 14
in which the trimetallic core is retained. This reactivity is in contrast to the closely related reaction of dimeric
Zn[Me2NC{NiPr}2]Me 15 with Mes2BOH, which yielded Zn[Me2NC{NiPr}2][OBMes2]·Me2NC{NiPr}{NHiPr} 16 as
a result of protonation at the guanidine ligand in addition to the Zn–Me bond.
complexes,12 and led to the development of a highly active ROP
system employing the bicyclic guanidine, 1,3,4,6,7,8-hexahydro-
2H-pyrimido[1,2-a]pyrimidine (hppH) as a source of chelating
ligand.
Introduction
The synthetic utility of both diorganozinc reagents1 and alkali
metal organozincates2 is now well established and has been the
subject of recent review.3 Heteroleptic methylzinc complexes
In
a
separate development, it has recently been
incorporating N,O-chelating moieties have lately been isolated,
in the context of the derivatization of PhC(O)N(R)H, as the
metal containing species Zn[PhC(O)N(R)]Me (R = Me, Pri,
Ph). These compounds have been interpreted as precursors
capable of reacting with ButLi to afford the lithium zincates
(thf)2·LiZn(But)2[PhC(O)N(R)] (R = Me, Pri) and {thf·Li[PhC-
(O)N(Ph)]}·{thf·LiZn(But)2[PhC(O)N(Ph)]}.4 Moreover, the
effect of stoichiometry has been found to be important in the
sequential reaction of ZnMe2 with N-2-pyridylaniline and ButLi.
Accordingly, a 1:1 reaction of the first two of these species has
been suggested to yield methyl(amido)zinc complexes5 whereas a
1:2 reaction has been shown to give homoleptic bis(amido)zinc
products instead.6
Amongst the most significant recent advances in applied zinc
chemistry has been the development of well-defined catalysts
for the ring-opening polymerisation (ROP) of cyclic esters,
focusing predominantly on lactide and caprolactone monomers.7
The majority of target catalysts contain an ancillary ligand set,
that stabilises the metal during the polymerisation reaction, and
a labile Zn–O or Zn–N bond at which the polymerisation is
believed to initiate via a coordination–insertion pathway. N,N-
chelating ligands have proven to be amongst the most successful
in fulfilling the role of supporting ligands, with emphasis placed
on the utilization of b-diketimininate ligands8 in conjunction
with zinc centres bearing pendant amide or organooxide units.9
Until recently, only a very limited number of zinc compounds
incorporating the alternative N,N-bidentate moieties amidines
or guanidines had been fully characterized.10,11 However, recent
work has reported a number of mono- and bisguanidinate
established that the formation of new hydridic clusters of
the type [Li8(H)R6]+ (R = N-2-pyridylanilide or guanidinate)
is possible using not only organoaluminium substrates13
but also organozinc species.14 These studies have typically
utilized AlMe3 or ZnMe2 in sequential conjunction with an
N,N-bidentate ligand and ButLi to give a pseudo-cubic Li8
cluster capable of surrounding either an interstitial void
(cf. [Li8(hpp)6]2+{[Li(Me2AlBut2)2]−}2 1)14 or else a hydride
ion (cf. [Li8(H){PhN(2-C5H4N)}6]+[Li(Me2AlBut2)2]− 2 and
[Li8(H)(hpp)6]+[ZnBut3]− 3).13,14 In seeking to understand the
processes active in the formation of cluster compounds such
as 1 and 2, simple dimers of Al[PhN(2-C5H4N)]Me2 4 and
Al(hpp)Me2 5 have been prepared by the straightforward treat-
ment of parent amine15 or guanidine16 with AlMe3. However, the
nature of the organozinc precursor to 3 has not hitherto been
unambiguously established. The importance of stoichiometry
in reactions of the type that yield 3 has already been suggested
by studies into the derivatization of lithium zincates.6 The lately
reviewed field of zincate chemistry3 has also seen the employ-
ment of zinc-bonded N,N-chelating ligands. Sequential reaction
of di(p-Tol)formamidine with LiBun and ZnCl2 in the presence
of an oxygen source has afforded the N-oxygenated formamidi-
nate Zn3(l5-O)Li2[(p-Tol)N(O)C(H)N(p-Tol)]617 and ZnMe2 has
been reacted with N-2-pyridylamines [HN(2-C5H4N)R, R = Ph,
3,5-xy (= 3,5-xylyl), 2,6-xy, Me, Bz (= benzyl)] and LiBun/t under
both anaerobic and (controlled) aerobic conditions to give a
disparate set of heterometallic complexes.5,6 Related work has
been effected using N,N-diphenylbenzamidine18 and has led
to the solvent dependent isolation of heterobimetallic organo-
oxides and a heterobimetallic oxide-encapsulation cluster.19
The studies discussed, combined with recent advances in
the coordination chemistry of N,N-bidentate amidinate20
and guanidinate21 ligands and the development of catalytic
† Electronic supplementary information (ESI) available: NMR spectro-
scopic data for 13 and selected crystallographic data for 8, 10, 12–14, and
16. See http://www.rsc.org/suppdata/dt/b4/b410945g/
3 5 6 8
D a l t o n T r a n s . , 2 0 0 4 , 3 5 6 8 – 3 5 7 4
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

View Article Online
applications of the latter,12 lead us to report here on the role of
stoichiometry in the reaction of N-2-pyridylaniline and -amine
and also guanidine substrates with ZnMe2. The lability of
metal–carbon interactions in the products of zincation is also
tested.
dimeric form that is based on a series of conjoined metallo-
cycles in a folded ladder arrangement (Scheme 2 and Fig. 2).
The transoid arrangements of anilide and methyl ligands about
the central ‘Zn2N2’ metallacycle contrasts with the structure of
the related guanidinate compound, (Zn[Me2NC{NiPr}2]Me)2,12
which was shown to consist of a non-centrosymmetric struc-
ture with syn ligands. The core four-membered (ZnN)2 ring
in (10)2 incorporates both formally deprotonated N-centres,
Results and discussion
The straightforward reaction of N-2-pyridylaniline 6 with
ZnMe2 has been effected employing both a 1:1 and 2:1
ratio of aniline to organozinc substrate. The 1:1 reaction,
intended to yield mono(anilido)zinc compound Zn[PhN(2-
C5H4N)]Me 7, deposits only amorphous material which
spectroscopic analysis suggests to be a crude mixture incor-
porating both mono- and bis(anilido)zinc. However, NMR
spectroscopy suggests the crystalline product of the 2:1 reac-
tion to be the pure bis(anilido)zinc species Zn[PhN(2-C5H4N)]2
8 (Scheme 1). X-ray diffraction substantiates the proposed fully
demethylated formulation of 8, revealing a solid-state dimer
(Fig. 1a and ESI†) based on an 8-membered (NCNZn)2 core
motif (Fig. 1b). Overall the aggregate is based on two metal ions
linking a pair of precisely coplanar anilide groups with essen-



N delocalization is uniform [C7–N1

though as with 8, N
C
1.376(5), C7–N2 1.362(5) Å]. Completion of a distorted tetra-
hedral geometry for each metal ion is achieved by the reten-
tion of metal-centred methyl groups [C12–Zn1 1.955(4) Å]
and the coordination of each metal by a pyridyl N-centre
[N2A–Zn1 2.170(3) Å] yielding (in the same way as for 8)
peripheral ‘NCNZn’ chelate rings [N1A–Zn1–N2A 62.80(12)°].
Surprisingly, strain in the chelating motif is insufficient to
render N1A–Zn1 greater than N1–Zn1 [N1–Zn1 2.129(3),
N1A–Zn1 2.161(3) Å, N1–Zn1–N1A 97.12(12), Zn1–N1–Zn1A
82.88(12)°].




tially uniform delocalization across N
C
N [C1–N1 1.341(6),
C1–N2 1.361(6) Å], with metal–nitrogen distances vary-
ing according to the charge on nitrogen [N1–Zn1 1.943(4),
N2A–Zn1 2.011(4) Å] akin to the related amidoalkane
dimers of Al[PhN(2-C5H4N)]Me2.15 Peripheral to the core of 8,
a second type of pyridylanilide anion acts as a dihapto ligand at
each metal centre, forming a 4-membered chelate ring, the acute
bite angle of which causes severe distortion of the zinc coordina-
tion geometry from ideal tetrahedral [N3–Zn1–N4 65.12(16)°].
As with the bridging anilide ligands within the core, the N–Zn
distances vary within the chelate according to whether they
involve neutral or deprotonated nitrogen centres [N3–Zn1
2.010(4), N4–Zn1 2.127(4) Å].
Scheme 2
Although ligand exchange prevented the structural
elucidation of 7, by analogy to the cyclohexyl derivative
Zn[CyN(2-C5H4N)]Me 10 obtained from the 1:1 reaction of
N-2-pyridyl(cyclohexyl)amine 9 with ZnMe2, it is likely to be
dimeric in the solid state. X-ray diffraction of 10 shows the
Fig. 2 Structure of (10)2. Hydrogen atoms omitted for clarity.
Scheme 1
The predictable manner in which 6 and 9 react with ZnMe2
is not reflected by the product of similar reaction involving
(p-Tol)NH(2-C5H4N) 11 (Scheme 3). Instead, this process
results in formation of a product that features components
of both the 1:1 and the 2:1 reactions between pyridylamine
and diorganozinc reagents and in which the anilide ligands
span metal centres. Tris(zinc) compound Zn3[(p-Tol)N(2-
C5H4N)]4Me2 12 represents the only product isolable from this
reaction and may be considered as consisting of a central, chiral
molecule of the homoleptic complex ‘Zn[(p-Tol)N(2-C5H4N)]2’
12a, which bridges two heteroleptic ‘Zn[(p-Tol)N(2-C5H4N)]Me’
12b units by virtue of the j1,2N-j3N-action of the anilide groups
(Scheme 3).
X-ray diffraction reveals two crystallographically indepen-
dent Zn3 clusters in the unit cell, of which one representative
aggregate will be discussed. Within 12, a trigonal arrangement
of zinc ions (Zn1/5/6 in Fig. 3) is held together by four N-2-
Fig. 1 (a) Structure and (b) Core of (8)2. Hydrogen atoms omitted
for clarity.
D a l t o n T r a n s . , 2 0 0 4 , 3 5 6 8 – 3 5 7 4
3 5 6 9

View Article Online
Fig. 4 Binding modes for the N-2-pyridyl(4-methylanilide) ligands in
12 (R = p-Tol).
The employment of guanidine substrates in place of
pyridylanilines and -amines has yielded diverse products.
For example, it has recently been noted that treatment of
Me2NC{NiPr}{NHiPr} with ZnMe2 gives a cisoid dimer
of formulation Zn[Me2NC{NiPr}2]Me, while analogous
reaction with the bis(trimethylsilyl) substituted guanidine,
(Me3Si)2NC{NiPr}{NHiPr}, has given only the homoleptic
bis(chelate) complex, Zn[(Me3Si)2NC{NiPr}2]2.12 Replacement
of these acyclic guanidines by the bicyclic reagent hppH, leads
to reaction variability of the type already discussed here. Hence,
in contrast to the simple monosubstituted product of reaction
between hppH and Zn[N(SiMe3)2]2,12 treatment of hppH with
ZnMe2 reproducibly gives an analogue of tris(zinc) adduct 12,
Zn3(hpp)4Me2 13, as the only isolable product (Scheme 4 and
Fig. 5). X-ray diffraction reveals that the trigonal arrangement
of zinc ions (Zn1–3) is also generated by the ability of the [hpp]−
groups to adopt the j1,2N-j3N-behaviour noted for anilide
units in 12. Two [hpp]− ligands bond to all three metal centres;
N9/10 straddle Zn1 [N9–Zn1 2.026(4), N10–Zn1 2.020(4) Å]
and Zn2/3 [N9–Zn3 2.175(4), N10–Zn2 2.188(4) Å] while N7
and N12 interact only with Zn2 and Zn3, respectively [N7–Zn2
2.058(5), N12–Zn3 2.043(4) Å]. The remaining ligands each
bond with two metal ions [N3–Zn1 1.971(4), N4–Zn1 1.979(4),
N6–Zn2 2.035(5), N1–Zn3 2.068(4) Å]. As was the case with 12,
the variable ligand behaviour is reflected in the observed delo-
Scheme 3
pyridyl(4-methylanilide) units with the ligands demonstrating
two fundamental types of bridging mode (Fig. 4). Two ligands
each interact with all three metal centres (Fig. 4a) such that their
formally deprotonated N-centres (N3/4) straddle Zn1 [N3–Zn1
2.039(4), N4–Zn1 2.050(4) Å] and Zn5/6 [N3–Zn5 2.120(4),
N4–Zn6 2.210(4) Å]. The pendant pyridyl system (containing
N8/10) of each ligand interacts uniquely with the remaining zinc
ion [N8–Zn5 2.097(4), N10–Zn6 2.150(4) Å]. The remaining two
ligands each bond with only two metal centres, bridging between
Zn1 and either Zn5 or Zn6 (Fig. 4b). Ligand asymmetry allows
this second class of ligand coordination mode to be subdi-
vided such that Zn1 interacts with formally deprotonated N6
[1.999(4) Å] and neutral N5 [2.038(4) Å]. Conversely, Zn5 is
supported by pyridyl N-centre N7 [2.053(4) Å] and Zn6 inter-
acts with charged N9 [2.007(4) Å]. A further issue of ligand



asymmetry can be noted by analysis of the (Zn)N
bonding parameters in 12, with the variable ligand coordination
already described being reflected in these bond lengths. Largely
C
N(Zn)



calisation patterns across N
C
N chelating units. Whereas


essentially symmetrical moieties are noted for the two anions
that bond only to two metal ions the l2-action of N9 and N10
incurs significant extension in their bonds to C20 and C27,
respectively [C20–N7 1.314(7), C20–N9 1.380(7), C27–N10
1.379(7), C27–N12 1.333(7) Å]. Notably, the dissimilarity in
these carbon–nitrogen interactions is less pronounced than
that lately reported in the dimer (Zn[Me2NC{NiPr}2]Me)2,
consistent with the partial activation of Zn–C bonds;12 the
two methyl groups in 13 project endo,endo to the core (mean
C–Zn 2.018 Å).







symmetrical N6 C5 N7 and N5 C7 N9 units are noted

for the two anions that bond only to two metal ions. However,




significant asymmetry is observable in both N4 C26 N8 and
1
3




N3 C32 N10 due to the j -N,j -N action of N3 and N4
[C32–N3 1.402(6), C32–N10 1.355(7), C26–N4 1.397(6), C26–
N8 1.344(7) Å]. The two zinc-bonded methyl groups project
endo,endo to the trigonal metal array (mean C–Zn 1.974 Å).
Scheme 4
1H NMR spectroscopy and COSY suggest that upon
dissolution in a hydrocarbon medium 13 forms multiple species
(identified as #1–4, see Experimental section). Species #2 and
#3 show multiple a-CH2 environments,22 suggesting asymmetry
1
in the bound ligand. H,1H NOESY indicates that [hpp]− and
Fig. 3 Molecular structure of the tris(zinc) adduct 12. Hydrogen
atoms omitted for clarity.
ZnMe groups are retained in close proximity in [2H6]benzene.
3 5 7 0
D a l t o n T r a n s . , 2 0 0 4 , 3 5 6 8 – 3 5 7 4

View Article Online
(Zn[Me2NC{NiPr}2]Me)2 15 and 2 equiv. Mes2BOH. The
isolated product of this transformation can be shown by
X-ray diffraction to be the mixed guanidinate/guanidine
complex Zn[Me2NC{NiPr}2][OBMes2]·Me2NC{NiPr}{NHiPr}
16 (Scheme 5 and Fig. 7).
Fig. 5 Molecular structure of the guanidinate complex Zn3(hpp)4Me2
13. Hydrogen atoms omitted for clarity.
Correlations are observed between ZnMe and a-CH2 signals
in the range d 3.71–3.33 but not to the a-CH2 multiplet at d
3.25 and these data are consistent with retention of the solid-
state structure of 13. More noteworthy is the observation of
NOESY correlations between ZnMe and b-CH2 signals at d
2.46 and 1.65. These are difficult to rationalise in terms of
(hpp)ZnMe monomers or dimers (cf. 10 and its solid-state
aggregate). However, an analysis of non-bonding distances
in 13 suggests that computed inter-hydrogen displacements
(H-atom positions calculated on the basis of a riding model)
are consistent with NOE’s between zinc-bonded methyl groups
and a-hydrogen atoms23 on the j1N-j3N-bridging [hpp]− ligands
(C1–C14 3.600, C2–C3 3.424, H1A–H14 A 2.911 (H1A bonded
to C1 etc.), H2B–H3B 2.949, H2C–H3A 2.982 Å) but only
b-hydrogen atoms on their j1,2N-j3N-bridging counterparts
(C1–C25 3.702, C2–C22 3.593, H1A–H17B 2.313, H1B–H25A
3.025, H1C–H25A 2.386, H2A–H22A 2.558 Å). 13C NMR
spectroscopy shows two resonances (d 162.8, 159.6) for the CN3
component of the guanidinate ligands at ambient temperature.
Likewise, the remaining methylene fragments afford three sets of
four dominant signals (d 49.6–49.1, 45.6–44.3, 25.4–23.6 for c-,
a-, and b-CH2, respectively), consistent with the four environ-
ments for each CH2 fragment demonstrated by the solid-state
structure. Coalescence of these resonances into three singlets
suggests, however, that at 373 K in [2H8]toluene the complex is
in a state of rapid flux.
Fig. 6 Molecular structure of Zn3(hpp)4(OBMes2)2 14. Hydrogen
atoms and mesityl substituents excepting Cipso omitted for clarity.
Scheme 5
The stability of the structural core noted in compound
13 can also be demonstrated chemically by its reaction with
excess Mes2BOH (Mes = mesitylene), used in the prepara-
tion of metal–boroxide species.24 Accordingly, the labile zinc-
bonded methyl groups are exchanged for [OBMes2]−, affording
Zn3(hpp)4[OBMes2]2 14 with concomitant expulsion of methane
(Scheme 4 and Fig. 6). The spectroscopic data for 14 are simi-
lar to that described for the methyl analogue 13, most notably
the room temperature 13C NMR spectrum which clearly shows
two signals for the CN3 carbon atoms and three groups of
resonances at d 49.5–48.5, 45.3–44.0, 25.0–23.3 for c-, a-, and
b-CH2, respectively. These data suggest that the trimetallic core
structure is maintained in solution, as noted for 13. Unfortu-
nately, crystals of 14 have so far proved to be small and weakly
diffracting (see Experimental section). Nevertheless, the essen-
tial structural features are unambiguous and analysis reveals the
cores of 13 and 14, and bonding patterns therein, to be essen-
tially identical. The action of the [hpp]− ions requires that the
endo,endo orientations of C1–Zn2/C2–Zn3 (in 13) are exactly
reproduced by O1–Zn2/O2–Zn3 (in 14).
Fig. 7 Molecular structure of 16. Hydrogen atoms, except H4,
omitted for clarity.
Compound 16 crystallises as the monomeric compound
with a four-coordinate zinc subtended by a symmetrically-
bound anionic guanidinate [Zn1–N1 2.0645(16), Zn1–N2
2.0287(17) Å], a boroxide ligand and a neutral guanidine. An
intramolecular hydrogen bond is observed between the NH of
the neutral guanidine and the ZnO of the boroxide [H4O1
1.98 Å], similar to that previously reported in the compound
This observed reactivity with dimesitylborinic acid
emphasises the robust nature of the [Zn3(hpp)4]2+ core
and contrasts with the analogous reaction between
D a l t o n T r a n s . , 2 0 0 4 , 3 5 6 8 – 3 5 7 4
3 5 7 1

View Article Online
Zn[OAr]Me·Me2NC{NiPr}{NHiPr} (Ar
The C–N distances in the anion are commensurate with
=
2,6-But2C6H3).12
the present work that this is not the case. Asymmetry in the


N chelating units present in 13 is less pronounced than
N
C


that recently seen for (Zn[Me2NC{NiPR}2]Me)2 15,12 and in light
of this the formation of 13 and its reaction with Mes2BOH to
yield 14 are to be expected. More generally, however, whereas
the effect of stoichiometry on reaction appears straightforward,
efforts are ongoing to elucidate competition between (ZnLMe)2
and ZnL2·(ZnLMe)2 structure types. Further spectroscopic
studies can also be expected to shed light on the dynamic
processes suggested for 13 at high temperature.


delocalization across the N
C
N moiety [N1–C19 1.334(3),


N2–C19 1.344(3) Å], while as expected, a more localized
bonding scheme is manifest in the neutral ligand [N3–C28
1.317(3), N4–C28 1.353(3) Å]. The Zn–O distance [1.9251(13) Å]
is significantly longer than those in 14, likely reflecting location
of electron density within the hydrogen bond, thus reducing
the metal–oxygen bond order. A reasonable mechanism for the
formation of 15 involves initial protonation of one of the Zn–C
units with extrusion of methane (not observed), to afford the
mixed boroxide/methyl system ‘Zn2[Me2NC{NiPr}2]2[OBMes2]
Me’ (16a, Scheme 6). A second equivalent of Mes2BOH is able
to protonate one of the guanidinate ligands in preference to the
methyl group, giving 16 and Zn[OBMes2]Me,25 identified in the
crude 1H NMR by a characteristic singlet at d −0.87.
Variations in zinc–nitrogen bond distances in the complexes
reported here can also be interpreted in terms of the suitability
of such systems as polymerisation catalysts.12 In this context, the
observation of extended N–Zn interactions in the novel tris(zinc)
complexes 13 and 14 may represent the presence of metal
centres that are potentially active in this regard. Accordingly,
the use of these systems as polymerisation catalysts and detailed
elucidation of polymer structure will be initiated.
Experimental section
Methods and materials
All reactions and manipulations were carried out under an inert
atmosphere of dry nitrogen, using standard double manifold
and glove-box techniques. Toluene and hexane were distilled
off sodium or sodium–potassium amalgam immediately prior
to use. Aniline and amine substrates N-2-pyridylaniline 6,
N-2-pyridyl(cyclohexyl)amine 9 and N-2-pyridyl(4-tolyl)amine
11 and Mes2BOH26 were prepared using a modified literature
method.27 hppH and ZnMe2 (2 M solution in toluene) were
purchased from Fluka and used without further purification
or prepared according to literature methods.28 Zinc dianilide 86
12
and the guanidinate complex (Zn[Me2NC{NiPr}2]Me)2 were
synthesised via previously reported protocols.
NMR data were collected on either a Bruker Avance
DPX 300 FT NMR spectrometer (300.13 MHz for 1H
and 75.76 MHz for 13C) or a Bruker DRX 500 FT NMR
1
spectrometer (500.05 MHz for H and 125.01 for 13C). Spectra
were obtained at 293 K unless otherwise stated and chemical
shifts are internally referenced to deuterated solvents and calcu-
lated relative to TMS.
Synthesis and characterisation
Scheme 6
Zn[CyN(2-C5H4N)]Me 10. ZnMe2 (0.5 mL, 1.0 mmol,
2.0 M in toluene) was added to a solution of 9 (0.17 g, 1 mmol)
in toluene (1 mL). The reaction mixture was heated to reflux and
then stored for two days at −30 °C whereupon 10 deposited as
colourless blocks. Yield 133 mg, 52%, m.p. 126–128 °C. Found:
C 56.42, H 7.05, N 10.81%. Calcd. for C24H36N4Zn2: C 56.37, H
7.10, N 10.96%. 1H NMR spectroscopy (500 MHz, [2H6]dmso),
d 7.51 (m, 1H, C5H4N), 7.26–7.18 (m, 0.6H, PhMe), 7.14 (m,
1H, C5H4N), 5.96 (m, 1H, C5H4N), 5.91 (m, 1H, C5H4N),
3.02 (m, 1H, NCH), 2.30 (s, 0.2H, PhMe), 1.79–1.05 (m, 10H,
CH2), −0.95 (s, 3H, ZnMe). 13C NMR spectroscopy (125 MHz,
[2H6]dmso), d 166.1 (i-C5H4N), 146.6 (C5H4N), 138.4 (C5H4N),
137.4 (i-PhMe), 129.0, 128.3, 125.4 (PhMe), 104.9 (C5H4N),
103.2 (C5H4N), 52.2 (CHN), 34.3, 25.9, 24.6 (CH2), 21.1
(PhMe), −13.2 (ZnMe).
Conclusions
Dimethylzinc has been used to generate both straightforward
ZnLMe (10) and ZnL2 (8) and also novel tris(zinc) adducts
ZnL2·(ZnLMe)2 (12, 13). The reaction of 13 with excess acid
has led to further functionalization of the metal centres in
ZnL2·(ZnLOR)2 and the isolation and characterization of 14.
While this process has evidenced the labile nature of Zn–C
interactions, it has also demonstrated the stability of the
(Zn3L4)2+ cluster core. This is emphasised by the generation
of the mixed guanidinate/guanidine boroxide compound
ZnL(OR)·LH 16 as a result of the protonation of a Zn–N bond
in the ZnLMe precursor during reaction.
Significant differences are noted in these structural
motifs in spite of the similar protocols described. Whereas
a clear correlation exists between reaction stoichiometry
and the formation of 8 and 10, it is as yet unclear why, for
example, the introduction of a para-tolyl component into
the N-2-pyridylaniline substrate should incur the formation
of trimetallic 12. Similar discrepancies are noted during the
employment of guanidine reagents, with Me2NC{NiPr}{NHiPr}
and hppH giving widely differing products. While the bicyclic
nature of [hpp]− would lead to the expectation, based on recent
data, that a 1:1 reaction between hppH and ZnMe2 should
yield Zn(hpp)Me and that the zinc–carbon bond therein should
be unreactive with respect to proton sources, it is clear from
Zn3[(p-Tol)N(2-C5H4N)]4Me2 12. ZnMe2 (0.5 mL, 1 mmol,
2.0 M in toluene) was added to a solution of 11 (0.17 g, 1 mmol)
in toluene:hexane (0.8 mL:1 mL). The reaction mixture was
heated to reflux and then allowed to cool to room temperature.
Storage overnight afforded yellow crystals of 12. Yield 126 mg,
48% [based on (p-Tol)NH(2-C5H4N)], m.p. 213–215 °C. Found:
C 60.29, H 5.45, N 10.32%. Calcd. for C50H50N8Zn3: C 62.61, H
5.25, N 11.68%. 1H NMR spectroscopy (500 MHz, [2H6]dmso),
d 7.63 (m, 1H, C5H4N), 7.24–7.09 (m, 4H, PhMe), 7.06 (m, 2H,
ArMe), 6.74 (m, 2H, ArMe), 6.16 (m, 1H, C5H4N), 6.06 (m,
1H, C5H4N), 2.27 (s, 2H, PhMe), 2.18 (s, 6H, ArMe), −0.99 (s, 3H,
3 5 7 2
D a l t o n T r a n s . , 2 0 0 4 , 3 5 6 8 – 3 5 7 4

View Article Online
Table 1 Crystallographic data for 8, 10, 12–14 and 16
8
10
12
13
14
16
Formula
Mr
Crystal system
Space group
C22H18N4Zn
403.77
Monoclinic
P21/c
C12H18N2Zn
255.65
Monoclinic
P21/c
C57H58N8Zn3
1051.22
Triclinic
P1
C30H54N12Zn3
778.96
Monoclinic
P21
C64H92B2N12O2Zn3
1279.23
Monoclinic
P21/c
C36H63BN6OZn
672.10
Triclinic
P1
a/Å
b/Å
c/Å
a/°
11.156(2)
8.838(2)
19.132(4)
90
103.11(3)
90
8.011(2)
13.964(3)
10.964(2)
90
101.95(3)
90
13.849(3)
18.301(4)
21.277(4)
102.98(3)
98.82(3)
98.35(3)
5101.7(18)
4
9.839(2)
17.042(3)
10.104(2)
90
98.18(3)
90
13.9605(4)
17.5229(5)
27.138(1)
90
99.867(1)
90
11.4067(2)
12.7166(2)
13.8031(2)
93.498(1)
96.341(1)
101.274(1)
1944.53(5)
2
b/°
c/°
V/Å3
1837.2(7)
4
1199.9(4)
4
1677.0(6)
2
6540.4(4)
4
Z
qcalc
1.460
1.415
1.369
1.543
1.299
1.148
Radiation
Mo–Ka, 0.71073
1.350
Mo–Ka, 0.71073
2.016
Mo–Ka, 0.71073
1.445
Mo–Ka, 0.71073
2.170
Mo–Ka, 0.71073
1.143
Mo–Ka, 0.71073
0.665
l/mm−1
T/K
180
8577
23.83
0.0901
0.0492, 0.1017
244
1.040
180
6403
2117
0.0547
0.0433, 0.1134
137
1.064
180
37276
14094
0.0669
0.0484, 0.1332
1225
1.073
180
19930
5858
0.0530
0.0344, 0.1180
408
1.108
173
29967
8928
0.1784
0.1109, 0.2757
743
1.097
173
20754
6736
0.0309
0.0348, 0.0884
418
1.041
Measured refl.
Unique refl.
Rint
Final R(F), wR(F2)
Parameters
S
Peak, hole/e Å−3
0.337, −0.370
0.748, −0.682
0.520, −0.725
0.689, −0.824
1.965, −0.638
0.943, −0.398
ZnMe). 13C NMR spectroscopy (125 MHz, [2H6]dmso), d 164.5,
147.7, 147.1, 137.4, 131.7, 129.5, 128.3, 126.3, 125.4, 110.3, 107.5
(ArMe + PhMe), 21.1 (PhMe), 20.6 (ArMe), −16.2 (ZnMe).
was added dropwise at room temperature to a stirred solution
of Zn[Me2NC{NiPr}2]Me (0.38 g, 1.50 mmol) which had been
diluted with an additional 20 mL toluene, to afford a colour-
less solution. After stirring for 14 h under ambient conditions
the volatiles were removed under reduced pressure, and the
resultant sticky yellow solid was extracted by filtration from
a small amount of insoluble material using pentane. Storage
at −35 °C yielded 16 as colourless crystals. Yield 310 mg, 30%,
m.p. 125–128 °C. Found: C, 64.36; H, 9.60, N, 12.41%. Calcd.
Zn3(hpp)4Me2 13. A solution of hppH (1.0 g, 7.18 mmol) in
toluene (30 mL) was added at room temperature to a solution of
ZnMe2 (3.6 mL, 7.2 mmol, 2.0 M in toluene) in toluene (20 mL)
to give a colourless solution. After 1 h at room temperature, a
white precipitate formed and the resultant slurry was stirred
for a further 14 h at room temperature. Subsequent heating
to ca. 60 °C and filtration allowed 13 to deposit as colourless
crystals on cooling to room temperature. Yield 1.23 g, 87%
(based on hppH), m.p. 260–261 °C. Found C, 46.27; H, 7.05; N,
21.37%. Calcd. for C30H54N12Zn3: C, 46.26; H, 6.99; N, 21.58%.
1H NMR spectroscopy (500 MHz, [2H6]benzene), d 3.71–3.61
(m, 3H, a-CH2#2, a-CH2#3), 3.53–3.33 (m, 4H, a-CH2#1,
a-CH2#2, a-CH2#3), 3.25 (dt, 1H, a-CH2#4), 3.05 (t, 1H, c-
CH2#4), 3.00 (t, 0.5H, c-CH2#1), 2.91–2.80 (m, 5H, c-CH2#2),
2.72–2.69 (m, 2H, c-CH2#3), 2.46 (m, 1H, b-CH2#4), 1.89 (m,
0.5H, b-CH2#1), 1.81–1.72 (m, 3H, b-CH2#2), 1.68–1.62 (m,
3H, b-CH2#3), 0.29 (s, 3H, ZnMe). 13C NMR spectroscopy
(125 MHz, [2H6]benzene), d 162.8, 159.6 (CN3), 49.6, 49.4, 49.2,
49.1 (c-CH2), 45.6, 45.4, 45.1, 44.3 (a-CH2), 25.4, 25.2, 24.2, 23.6
(b-CH2), −13.3 (ZnMe).
1
for C36H63N6BOZn: C, 64.33; H, 9.44, N, 12.50%. H NMR
spectroscopy (300 MHz, [2H6]benzene), d 9.77 (br, s, 1H, NH),
6.89 (s, 4H, C6H2), 3.31 (br, 4H, iPr), 2.68 (s, 12H, 2,6-Me2) 2.40,
(br, 12H, NMe), 2.27 (s, 6H, 4-Me), 0.87 (br, 24H, iPr). 13C NMR
spectroscopy (75 MHz, [2H6]benzene), d 167.0 (NCN), 138.4
(CH), 144.8 (br, C), 140.7 (C), 135.8 (C), 128.1 (CH), 46.2 (br,
CH, iPr), 39.6 (NMe2), 25.1 (Me, iPr), 23.2 (Me), 21.3 (Me).
X-ray crystallography
Crystallographic data (excluding structure factors) for 8, 10,
12–14 and 16 have been deposited as CCDC reference numbers
244976–244981.
See http://www.rsc.org/suppdata/dt/b4/b410945g/ for crystal-
lographic data in CIF or other electronic format.
Data were collected using
a Nonius Kappa CCD
diffractometer equipped with an Oxford Cryostream low-
temperature device (Table 1). Structures were solved by direct
methods29 and refined against F2 using SHELXL-97.30
Zn3(hpp)4[OBMes2]2 14. A solution of Mes2BOH (0.21 g,
0.795 mmol) in toluene (20 mL) was added dropwise at room
temperature to 13 (0.31 g, 0.398 mmol) in toluene (20 mL) to
give a pale yellow solution. After stirring for 14 h under ambient
conditions the volatiles were removed, and the resultant sticky
yellow solid was extracted from a small amount of insoluble
material using warm hexanes. Storage at room temperature
yielded 14 as colourless crystals. Yield 310 mg, 61%, m.p.
153–155 °C (decomp.). Found: C, 59.81; H, 7.25; N, 13.16%.
Calcd. for C64H92B2N12O2Zn3: C, 60.09; H, 7.25; N, 13.14%. 1H
NMR spectroscopy (300 MHz, [2H6]benzene), d 6.89 (s, 4H,
C6H2), 3.77 (br, m, 2H, CH2), 3.49 (br, m, 3H, CH2), 3.22–2.21
(br, m, 9H, CH2), 2.70 (s, 12H, 2,6-Me2), 2.28 (s, 6H, 4-Me), 1.66
(br, m, 3H, CH2), 1.43 (br, m, 7H, CH2). 13C NMR spectroscopy
(75 MHz, [2H6]benzene), d 163.8, 159.8 (CN3), 146.2 (br, C),
140.4 (C), 135.0 (C), 128.8 (CH), 49.5, 49.1, 48.8, 48.5 (c-CH2),
45.3, 44.7, 44.1, 44.0 (a-CH2), 25.0, 24.8, 23.8 (b-CH2), 23.6
(Me), 23.3 (b-CH2), 21.3 (Me).
Zn3(hpp)4Me2 13. The absolute structural parameter refined to
zero in the least squares.
Zn3(hpp)4[OBMes2]2 14. Crystals were very small and weakly
diffracting and N11 was left isotropic to stop it going non-
positive-definite. Residual electron density near inversion
centres is unresolved and is assumed to be caused by very
disordered solvate.
Zn[Me2NC{NiPr}2][OBMes2]·Me2NC{NiPr}{NHiPr} 16. The
hydrogen atom on N(4) was refined; other hydrogen atoms were
in riding mode.
Acknowledgements
Thanks go to the UK EPSRC (S. R. B., R. H., S. C. C.) for
financial support. We also wish to acknowledge the use of the
EPSRC’s Chemical Database Service at Daresbury.
Zn[Me2NC{NiPr}2][OBMes2]·Me2NC{NiPr}{NHiPr} 16. A
solution of Mes2BOH (0.40 g, 1.50 mmol) in toluene (20 mL)
D a l t o n T r a n s . , 2 0 0 4 , 3 5 6 8 – 3 5 7 4
3 5 7 3

View Article Online
14 S. R. Boss, M. P. Coles, R. Haigh, P. B. Hitchcock, R. Snaith and
References
A. E. H. Wheatley, Angew. Chem., Int. Ed., 2003, 42, 5593.
15 D. R. Armstrong, R. P. Davies, D. J. Linton, P. Schooler, R. Snaith
and A. E. H. Wheatley, J. Chem. Soc., Dalton Trans., 2001, 2838.
16 S. L. Aeilts, M. P. Coles, D. C. Swenson, R. F. Jordan and V. G.
Young, Jr., Organometallics, 1998, 17, 3265.
17 M. L. Cole, D. J. Evans, P. C. Junk and L. M. Louis, New. J. Chem.,
2002, 26, 1015.
18 R. P. Davies, D. J. Linton, P. Schooler, R. Snaith and A. E. H.
Wheatley, Eur. J. Inorg. Chem., 2001, 619 and references cited
therein.
19 A. D. Bond, D. J. Linton, P. Schooler and A. E. H. Wheatley, J.
Chem. Soc., Dalton Trans., 2001, 3173.
20 J. Grundy, M. P. Coles and P. B. Hitchcock, J. Organomet. Chem.,
2002, 662, 178.
21 M. P. Coles and P. B. Hitchcock, J. Chem. Soc., Dalton Trans., 2001,
1169; M. P. Coles and P. B. Hitchcock, Organometallics, 2003, 22,
5201; D. B. Soria, J. Grundy, M. P. Coles and P. B. Hitchcock,
Polyhedron, 2003, 22, 2731.
1 Organozinc Reagents: A Practical Approach, ed. P. Knochel and
P. Jones, OUP, Oxford, 1999.
2 T. Harada and A. Oku, in Organozinc Reagents: A Practical
Approach, ed. P. Knochel and P. Jones, OUP, Oxford, 1999,
pp. 101–117.
3 D. Linton, P. Schooler and A. E. H. Wheatley, Coord. Chem. Rev.,
2001, 223, 53; A. E. H. Wheatley, New J. Chem., 2004, 28, 437.
4 S. R. Boss, R. Haigh, D. J. Linton, P. Schooler, G. P. Shields and
A. E. H. Wheatley, J. Chem. Soc., Dalton Trans., 2003, 1001.
5 R. P. Davies, D. J. Linton, P. Schooler, R. Snaith and A. E. H.
Wheatley, Chem. Eur. J., 2001, 7, 3696.
6 S. R. Boss, R. Haigh, D. J. Linton and A. E. H. Wheatley, J. Chem.
Soc., Dalton Trans., 2002, 3129.
7 B. J. O’Keefe, M. A. Hillmyer and W. B. Tolman, J. Chem. Soc.,
Dalton Trans., 2001, 2215.
8 L. Bourget-Merle, M. F. Lappert and J. R. Severn, Chem. Rev., 2002,
102, 3031.
9 M. Cheng, E. B. Lobkovsky and G. W. Coates, J. Am. Chem. Soc.,
1998, 120, 11018; M. Cheng, A. B. Attygalle, E. B. Lobkovsky
and G. W. Coates, J. Am. Chem. Soc., 1999, 121, 11583; B. M.
Chamberlain,M. Cheng,D. R. Moore,T. M. Ovitt,E. B. Lobkovsky
and G. W. Coates, J. Am. Chem. Soc., 2001, 123, 3229; M. Cheng,
D. R. Moore, J. J. Reczek, B. M. Chemberlain, E. B. Lobkovsky and
G. W. Coates, J. Am. Chem. Soc., 2001, 123, 8738; M. H. Chisholm,
J. C. Huffman and K. Phomphrai, J. Chem. Soc., Dalton Trans.,
2001, 222; M. H. Chisholm, J. Gallucci and K. Phomphrai, Inorg.
Chem., 2002, 41, 2785.
10 J. Barker and M. Kilner, Coord. Chem. Rev., 1994, 133, 219.
11 G. Minghetti, G. Banditelli and F. Bonati, Inorg. Chim. Acta,
1975, 12, 85; J.-K. Buijink, M. Noltemeyer and F. T. Edelmann, Z.
Naturforsch., Teil B, 1991, 46, 1328.
22 We define a as denoting the positions of the methylene units adjacent
to the nitrogen atoms involved in metal bonding, b as the central
methylene group and c as the remaining position within the C4N2
rings of [hpp]−.
23 D. Neuhaus and M. Williams, The Nuclear Overhauser Effect
in Structural and Conformational Analysis, VCH Publishers,
Cambridge, 1989, ch. 10.
24 S. C. Cole, M. P. Coles and P. B. Hitchcock, J. Chem. Soc., Dalton
Trans., 2002, 4168.
25 Zn[OB(mes)2]Me has been independently synthesised by the
reaction of (mes)2BOH with ZnMe2. S. C. Cole, M. P. Coles and
P. B. Hitchcock, Organometallics, in press.
26 H. C. Brown and V. H. Dodson, J. Am. Chem. Soc., 1957, 79, 2302.
27 O. Fischer, Chem. Ber., 1902, 35, 3675.
12 M. P. Coles and P. B. Hitchcock, Eur. J. Inorg. Chem., 2004, 2662.
13 W. Clegg, R. P. Davies, S. T. Liddle, D. J. Linton, P. R. Raithby,
R. Snaith and A. E. H. Wheatley, Angew. Chem., Int. Ed., 1999, 38,
3367.
28 A. L. Galyer and G. Wilkinson, Inorg. Synth., 1979, 19, 253.
29 G. M. Sheldrick, Acta Crystallogr., Sect. A, 1990, 46, 467.
30 G. M. Sheldrick, SHELXL-97 Program for Crystal Structure
Refinement, University of Göttingen, Germany, 1997.
3 5 7 4
D a l t o n T r a n s . , 2 0 0 4 , 3 5 6 8 – 3 5 7 4