Bulky guanidinato and amidinato zinc complexes and their comparative stabilities

Article abstract of DOI:10.1039/c0dt00589d

The preparation of a series of amidinato and guanidinato zinc halide complexes incorporating ligands of varying steric bulk is described, and their thermal stabilities compared. Salt elimination reactions between [M(Giso)] (M = K or Li; Giso = [(ArN)₂CNCy₂]^-, Ar = 2,6-diisopropylphenyl, Cy = cyclohexyl) and ZnX₂ (X = I or Br) have yielded the monomeric complexes [(Giso)ZnI] and [(Giso)Zn(μ-Br) ₂Li(OEt₂)₂]. Both have been crystallographically characterised and the former shown to slowly decompose in solution at ambient temperature to give the carbodiimide, ArNCNAr. In contrast, reactions between alkali metal complexes of a less bulky guanidinate, [M(Priso)] (Priso = [(ArN)₂CNPrⁱ₂]^-) and ZnX₂ have yielded [(IZn)₂(μ-NPrⁱ ₂){μ-N,N′-(NAr)₂CH}] and [(Priso)Zn(μ-Br) ₂Li(OEt₂)₂]. The latter decomposes in solution at ambient temperature, generating ArNCNAr, which was also produced in the preparation of the former. Analogies are drawn between the decomposition of [(Priso)Zn(μ-Br)₂Li(OEt₂)₂] and the carbonic anhydrase catalysed dehydration of bicarbonate. Two bulky amidinato zinc complexes, [{(Piso)Zn(μ-Br)}₂] and [Zn(Piso)₂] (Piso = [(ArN)₂CBu^t]^-) have been prepared, structurally characterised and shown to be markedly more thermally stable than the zinc guanidinate compounds. Attempts to reduce several of the zinc(ii) halide complexes to dimeric zinc(i) compounds were so far unsuccessful, in all cases leading to the deposition of zinc metal.

Full text of DOI:10.1039/c0dt00589d

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PAPER
www.rsc.org/dalton | Dalton Transactions
Bulky guanidinato and amidinato zinc complexes and their comparative
stabilities†
Cameron Jones,*a Leigh Furness,a Sharanappa Nembenna,a Richard P. Rose,a,b Simon Aldridgec and
Andreas Stasch*a
Received 2nd June 2010, Accepted 30th June 2010
DOI: 10.1039/c0dt00589d
The preparation of a series of amidinato and guanidinato zinc halide complexes incorporating ligands
of varying steric bulk is described, and their thermal stabilities compared. Salt elimination reactions
between [M(Giso)] (M = K or Li; Giso = [(ArN)2CNCy2]-, Ar = 2,6-diisopropylphenyl, Cy =
cyclohexyl) and ZnX2 (X = I or Br) have yielded the monomeric complexes [(Giso)ZnI] and
[(Giso)Zn(m-Br)2Li(OEt2)2]. Both have been crystallographically characterised and the former shown to
slowly decompose in solution at ambient temperature to give the carbodiimide, ArN
C
NAr. In
contrast, reactions between alkali metal complexes of a less bulky guanidinate, [M(Priso)] (Priso =
[(ArN)2CNPri2]-) and ZnX2 have yielded [(IZn)2(m-NPri2){m-N,N¢-(NAr)2CH}] and
[(Priso)Zn(m-Br)2Li(OEt2)2]. The latter decomposes in solution at ambient temperature, generating
ArN
C
NAr, which was also produced in the preparation of the former. Analogies are drawn
between the decomposition of [(Priso)Zn(m-Br)2Li(OEt2)2] and the carbonic anhydrase catalysed
dehydration of bicarbonate. Two bulky amidinato zinc complexes, [{(Piso)Zn(m-Br)}2] and [Zn(Piso)2]
(Piso = [(ArN)2CBut]-) have been prepared, structurally characterised and shown to be markedly more
thermally stable than the zinc guanidinate compounds. Attempts to reduce several of the zinc(II) halide
complexes to dimeric zinc(I) compounds were so far unsuccessful, in all cases leading to the deposition
of zinc metal.
redistribution reactions, yielding bis(chelate) zinc complexes.
Introduction
In this respect, it has been shown that the extent of such
redistribution reactions can be attenuated by increasing the steric
bulk of the chelating ligand.9
The
coordination
chemistry
of
anionic
amidinate
([R1NC(R2)NR1]-, R1, R2 = H, alkyl, aryl, silyl etc.) and
guanidinate ([R1NC(NR2 )NR1]-) ligands is extensive and
2
We have developed a series of extremely bulky guanidinate
ligands ([(ArN)2CNR2]- Ar = C6H3Pri2-2,6; R = cyclohexyl (Giso)
or Pri (Priso))10 and have applied these to the stabilisation of met-
allacycles containing s-, p-, d- and f-block metals in low oxidation
states and with low coordination numbers.11,12 Throughout this
work, the stabilising and coordinating properties of the ligands
have been shown to be more akin to those of bulky b-diketiminates
than less bulky guanidinates. Accordingly, we saw the opportunity
to utilise our guanidinate ligands to prepare mono(guanidinato)
zinc complexes which are very resistant towards redistribution
reactions. We were particularly interested in preparing zinc(II)
halide complexes, LZnX (L = bulky guanidinate, X = halide), which
we saw as potential precursors to zinc(I) dimers, LZnZnL. This
seemed a realistic prospect in light of the increasing number
of kinetically stabilised Zn–Zn bonded complexes (including b-
diketiminate coordinated examples13) that have populated the liter-
ature since Carmona’s landmark preparation of [Cp*ZnZnCp*].14
Moreover, given our prior preparation of a related stable guani-
dinato magnesium(I) dimer, [(Priso)MgMg(Priso)],12e and the
well known chemical similarities between Mg and Zn, zinc(I)
dimers were deemed viable synthetic targets. Herein, we report
the synthesis and characterisation of several guanidinato zinc(II)
complexes and compare their stability with that of related bulky
amidinato zinc(II) compounds. In addition, we detail unsuccessful
attempts to reduce these to zinc(I) dimers.
includes complexes incorporating metals from across the periodic
table.1 Such systems have found many applications in areas as
diverse as catalysis,2–4 materials science5 and synthesis.1 Despite
this, the chemistry of zinc amidinate and guanidinate compounds
is relatively poorly developed, though in recent years an increase
in activity in the area has occurred.1b This is likely due to the
realisation that mono(amidinato)- and mono(guanidinato)-zinc
complexes have potential for use as catalysts in, for example,
the ring opening polymerisation (ROP) of cyclic esters, e.g.
D,L-lactide6 and e-caprolactone.7 That said, they are generally
not as effective for this purpose as are related b-diketiminato
zinc complexes.8 The reasons for these differences include the
tendency of the amidinate or guanidinate complexes to undergo
aSchool of Chemistry, PO Box 23, Monash University, VIC, 3800, Australia.
E-mail: cameron.jones@monash.edu, andreas.stasch@monash.edu
bSchool of Chemistry, Main Building, Cardiff University, Cardiff, UK CF10
3AT
cInorganic Chemistry, University of Oxford, South Parks Road, Oxford, UK
OX1 3QR
† Electronic supplementary information (ESI) available: Crystal data,
details of data collections, refinement and ORTEP diagrams for
[{HgI2(NCy2H)}•] and [(Giso)Zn(m-Br)2Li(THF)2]. CCDC reference
numbers 779527–779530 and 779531–779534. For ESI and crys-
tallographic data in CIF or other electronic format see DOI:
10.1039/c0dt00589d
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Scheme 1 i) THF, X = I, [K(Giso)], -KI; ii) Et2O, X = Br, [Li(Giso)]; iii) THF, X = I, [K(Priso)], -KI; iv) Et2O, X = Br, [Li(Priso)]; v) toluene–Et2O or
toluene, X = Br, [K(Piso)], -KBr.
The solution state spectroscopic data for 1 and 2 are consistent
with their solid state molecular structures, which are depicted
Results and discussion
In prior studies we have had most success in stabilising low
oxidation state metal complexes using the very bulky Giso ligand.11
As a result, this was initially chosen for the preparation of potential
zinc(II) halide precursors to the zinc(I) dimer [(Giso)ZnZn(Giso)].
The 1 : 1 reaction of [K(Giso)] with ZnI2 afforded a low isolated
yield of the monomeric complex, 1, after recrystallisation from
diethyl ether (Scheme 1).15 Similarly, a moderate yield of the “ate”
complex, 2, was obtained from the reaction of [Li(Giso)] with
ZnBr2 in diethyl ether. Although both compounds are thermally
stable in the solid state, in C6D6 solutions at ambient temperature
compound 1 was found to very slowly decompose (over weeks) to
yield product mixtures, the only identifiable component of which
in Fig. 1 and 2. To the best of our knowledge there have been
no prior structural characterisations of guanidinato zinc halide
complexes, though several amidinato zinc halide structures have
been reported.9,19 All of these are dimeric and possess four-
coordinate Zn centres. In contrast, the steric bulk of the delocalised
Giso ligand in 1 leads to it being monomeric with a distorted
trigonal planar zinc geometry. As a result, both the Zn–I and
Zn–N distances in the compound and significantly shorter than
˚
˚
the mean separations for such interactions (2.564 A and 2.096 A)
in previously reported complexes.20 The structure of compound
2 reveals it to be an “ate” complex with a distorted tetrahedral
zinc geometry, very similar to that described for the closely related
amidinate complex, [{ButC(NCy)2}Zn(m-Br)2Li(OEt2)2].19a Both
was the carbodiimide, ArN
Attempts to reduce 1 or 2 to [(Giso)ZnZn(Giso)] with KC8
led instead to the deposition of zinc metal and product mixtures
C
NAr.16
˚
˚
the Zn–N (2.012 A mean) and Zn–Br (2.415 A mean) separations
in that compound are comparable to those in 2.
including significant amounts of ArN
C
NAr. We have recently
As the guanidinate ligand, Giso, was found to be apparently un-
suitable for the stabilisation of a dimeric zinc(I) species, attention
turned to the less bulky ligand, Priso. As already mentioned, this
has been utilised in the preparation of the thermally very stable
magnesium(I) dimer, [(Priso)MgMg(Priso)],12e via the potassium
reduction of the magnesium(II) precursor, [(Priso)Mg(OEt2)(m-
I)2Mg(Priso)]. Attempts were made to synthesise a similar zinc(II)
complex by the reactions of [K(Priso)] with ZnI2 in THF, or
[Li(Priso)] with ZnBr2 in diethyl ether. These did not lead to ArN-
,ArN-chelated products (cf. 1 and 2), but instead reproducibly
afforded low to moderate yields of the unexpected products, 3 and
4 (Scheme 1).
shown that hydrocarbon soluble b-diketiminate stabilised magne-
sium(I) dimers can act as milder and more selective reducing agents
than KC8 and other alkali metal reagents in both organic17 and
organometallic synthesis.18 Accordingly, the reaction of 2 with half
an equivalent of [(MesNacnac)MgMg(MesNacnac)] (MesNacnac =
[{(C6H2Me3-2,4,6)NC(Me)}2CH]-)17a was carried out in C6D6,
though this also led to zinc metal deposition and a mixture
of unidentifiable products. The mechanism of formation of the
carbodiimide, and the fate of the NCy2 (Cy = cyclohexyl) fragment
in these reactions, are not known. In contrast, no reduction
occurred in the reaction of 2 with Mg powder in THF. Instead, the
diethyl ether ligands of the zinc complex exchanged with THF to
yield [(Giso)Zn(m-Br)2Li(THF)2], which is isostructural to 2 (see
Supplementary Material).
Compound 3 contains a Zn2N3C heterocycle in which the two
zinc atoms are bridged by a formamidinate ligand and the amide
anion, NPri2-. Clearly, this complex results from the cleavage of
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Fig. 1 Molecular structure of 1 (25% thermal ellipsoid◦s are shown;
˚
hydrogens omitted). Selected bond lengths (A) and angles ( ): I(1)–Zn(1)
2.4204(6), Zn(1)–N(1) 1.970(2), Zn(1)–N(2) 1.981(2), N(1)–C(1) 1.367(3),
C(1)–N(2) 1.359(3), C(1)–N(3) 1.363(3), N(1)–Zn(1)–N(2) 68.50(9),
N(1)–Zn(1)–I(1) 147.08(7), N(2)–Zn(1)–I(1) 144.29(7), N(2)–C(1)–N(1)
109.3(2).
Fig. 2 Molecular structure of 2 (25% thermal ellipsoids are shown;
◦
˚
hydrogens omitted). Selected bond lengths (A) and angles ( ): Br(1)–Zn(1)
2.4016(8), Br(1)–Li(1) 2.540(6), Br(2)–Zn(1) 2.3940(7), Br(2)–Li(1)
2.570(7), Zn(1)–N(2) 2.014(2), Zn(1)–N(1) 2.046(2), N(1)–C(1) 1.341(4),
N(2)–C(1) 1.354(4), N(3)–C(1) 1.387(4), N(2)–Zn(1)–N(1) 66.01(10),
Br(2)–Zn(1)–Br(1) 101.13(3), N(1)–C(1)–N(2) 110.3(3), O(2)–Li(1)–O(1)
108.3(6), Br(1)–Li(1)–Br(2) 92.90(19).
the C–NPri2 bond of the Priso ligand, though the full mechanism
of its formation is unknown at this stage. It is noteworthy, however,
that an NMR spectroscopic analysis of the reaction mixture
that gave 3 showed the presence of significant amounts of the
carbodiimide, ArN
C NAr, i.e. the same product that was
generated from the slow thermal decomposition of 1. Therefore,
it seems likely that the reaction of [K(Priso)] with ZnI2, initially
yields a guanidinato zinc complex [{(Priso)ZnI}n] (n = 1 or 2),
which is less thermally stable than 1, and decomposes via an amide
extrusion process to give 3, amongst other products. A very similar
amide extrusion from an in situ generated guanidinato zinc ary-
loxide, [{Me2NC(NPri)2}ZnOAr¢] (Ar¢ = C6H3But2-2,6), has been
reported to give [(Ar¢OZn)2(m-NMe2){m-N,N¢-(NPri)2CNMe2}],6
which contains a central Zn2N3C heterocycle reminiscent of that
in 3. This reaction was seen as having implications for the
mechanisms of the metal catalysed guanylation of amines21 and
NPri2 bond to generate the carbodiimide, ArN
a zinc amide fragment. Indeed, the solid state structure of this
C
NAr, and
complex (vide infra) reveals this bond to be significantly elongated
˚
(1.511(4) A) in comparison to what would be expected for normal
2
3
24
˚
sp C–sp N single bonds (1.416 A). This, combined with the
rather obtuse ArN
C
NAr angle (139.2(3)◦), leads to the
conclusion that 4 represents an intermediate, or “snapshot”,
in the aforementioned amide extrusion process. Interestingly,
this process can be directly compared to the dehydration of
-
bicarbonate, HCO3 , to yield CO2, which is reversible and is
C
N bond metathesis of carbodiimides.22
extremely efficiently catalysed by Zn2+ in carbonic anhydrase
(CA).25 Many studies have suggested that the intermediate in this
catalysed process contains the bicarbonate anion coordinated to
Compound 4 is unstable in non-coordinating solvents at ambi-
ent temperature and rapidly decomposes to yield product mixtures,
the main and only spectroscopically identifiable component of
2
the zinc centre in an k -O,O¢-fashion, viz. “-Zn{(OH)(O)C O}”
which is the carbodiimide, ArN
C
NAr. Like 2, compound
(cf. the -Zn{(NPri2)(NAr)C NAr} fragment in 4).25,26 Cleavage
of the C–OH bond then occurs to give CO2 and a zinc hydroxide
moiety, “–Zn(OH)”. This is equivalent to the aforementioned
4 is an “ate” complex, but in this case the guanidinate ligand
chelates the Zn centre unsymmetrically through its amino and
amide N-centres, leaving the imino fragment uncoordinated.
This unusual guanidinate coordination mode has only been
seen on one previous occasion in the titanium complexes,
[{Ti[(NMe2)(NPri)C NPri]2(m-E)}2] (E = O or S).23 What is
significant about the guanidinate coordination in 4 is that the
complex appears “pre-organised” to heterolytically cleave its C–
C–NPri2 bond cleavage of 4, which gives ArN
C
NAr (an
isoelectronic analogue of CO2), and an insoluble material which
is presumably a zinc amide species, “–Zn(NPri2)”.
The stability of complexes of the type, [(Priso)ZnX] (X =
halide), seems to be related to the nature of the zinc coordinated
halide. No such species could be isolated for X = I, whereas a
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related metastable “ate” complex, 4, was isolable when X = Br. In
contrast, we have previously prepared [{(Priso)ZnCl}2],27 which
is very stable in solution and shows no signs of decomposition
to give ArN
C NAr. Although the solid state structure of
this compound was not reported, solution NMR spectroscopic
data for it suggest that the Priso ligand chelates the Zn centre
through both NAr units. This preference for “normal” guanidinate
coordination might explain the enhanced stability of the complex
relative to 4 and the likely intermediate in the formation of 3, viz.
[{(Priso)ZnI}n] (n = 1 or 2).
The solution state spectroscopic data for 3 are reminiscent
of it retaining its solid state structure in solution. Due to the
thermal instability of solutions of 4, no NMR spectroscopic
data could be obtained for this compound. Both 3 and 4 were
crystallographically authenticated and their molecular structures
are depicted in Fig. 3 and 4 respectively. That for 3 shows it
to possess two distorted trigonal planar Zn centres with Zn–I
and Zn–N distances close to those in 1. As already mentioned,
the central six-membered Zn2N3C ring of the compound is
close to planar and resembles that in [(Ar¢OZn)2(m-NMe2){m-
N,N¢-(NPri)2CNMe2}],6 and the Be2N3C ring in [(ClBe)2{m-
N(SiMe3)2}{m-N,N¢-(NSiMe3)2CPh}].28 The equivalent backbone
C–N distances in the bridging formamidinate (Fiso) show it to
be delocalised, as in other complexes bearing this ligand, e.g.
[Zn(Fiso)2].29 The structure of the “ate” complex, 4, has been
briefly described above. It is somewhat related to 2, except that the
guanidinate ligand chelates the distorted tetrahedral zinc centre
unsymmetrically through amide and tertiary amine N-centres,
leaving an imine fragment uncoordinated. Although the N(3)–
C(1) distance is very long for a single bond, the bond lengths
within the N(2)–C(1)–N(1) fragment are consistent with partial
delocalisation.
Fig. 4 Molecular structure of 4 (25% thermal ellipsoids are shown;
◦
˚
hydrogens omitted). Selected bond lengths (A) and angles ( ): Br(1)–Zn(1)
2.4285(11), Br(1)–Li(1) 2.512(6), Br(2)–Zn(1) 2.4129(9), Br(2)–Li(1)
2.519(6), Zn(1)–N(1) 1.988(2), Zn(1)–N(3) 2.141(3), N(1)–C(1) 1.351(4),
N(2)–C(1) 1.286(4), N(3)–C(1) 1.511(4), N(1)–Zn(1)–N(3) 66.87(10),
Br(2)–Zn(1)–Br(1) 99.23(4), C(1)–N(1)–Zn(1) 99.51(18), C(1)–N(3)–Zn(1)
88.23(16), Br(1)–Li(1)–Br(2) 94.28(19), N(2)–C(1)–N(1) 139.2(3),
N(2)–C(1)–N(3) 115.4(3), N(1)–C(1)–N(3) 105.4(2).
Considering that the bulky guanidinato zinc halide complexes
were found to be less stable than expected, and not amenable
for reduction to zinc(I) dimers, the possibility of using less bulky
amidinato zinc halide species for this purpose was examined.
The rationale here was that their amidinate ligands should not
be susceptible to N–C cleavage reactions. The 1 : 1.1 reaction
of [K(Piso)] (Piso = [(ArN)2CBut]-) with ZnBr2 in a toluene–
diethyl ether mixture afforded a moderate yield of the dimeric
mono(chelate) complex, 5 (Scheme 1). In contrast, when the same
reaction was carried out in toluene, the homoleptic complex,
[Zn(Piso)2] 6, was generated in good yield, leaving significant
amounts of unreacted ZnBr2 in the product mixture. The homolep-
tic complex results from the latter reaction as the [K(Piso)] in the
mixture preferentially reacts with initially formed 5 (to give 6),
rather than with toluene insoluble ZnBr2 (to give more 5). The
diethyl ether present in the former reaction partially solubilises
the ZnBr2 reactant, leading to the competitive formation of 5
Fig. 3 Molecular structure of 3 (25% thermal ellipsoid◦s are shown;
˚
hydrogens omitted). Selected bond lengths (A) and angles ( ): I(1)–Zn(1)
2.4857(6), Zn(1)–N(1) 1.961(2), Zn(1)–N(3) 1.979(2), N(1)–C(1)
1.320(3), C(1)–N(2) 1.320(3), I(2)–Zn(2) 2.4892(6), Zn(2)–N(2) 1.960(2),
Zn(2)–N(3) 1.978(2), N(1)–Zn(1)–N(3) 118.75(9), N(2)–Zn(2)–N(3)
119.09(9), C(1)–N(1)–Zn(1) 126.77(18), C(1)–N(2)–Zn(2) 126.43(18),
N(1)–C(1)–N(2) 126.5(2), Zn(2)–N(3)–Zn(1) 101.11(10).
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over 6. A number of attempts were made to reduce 5 with either
KC8, Na or [(MesNacnac)MgMg(MesNacnac)], but all led to the
deposition of zinc metal and the formation of PisoH amongst
other unidentifiable products.
The solution state data for 5 are very similar to those for
[{(Piso)Zn(m-Cl)2}],9 whilst those for 6 show it to have four
chemically inequivalent sets of isopropyl methyl groups. This was
thought to reflect steric crowding between the two Piso ligands.
The solid state structures of 5 and 6 are shown in Fig. 5 and 6
respectively. Compound 5 is dimeric, through bridging bromides,
and its crystal structure is isomorphous to the previously reported
complex, [{(Piso)Zn(m-Cl)2}].9 Like that compound, the NCN
backbone of the Piso ligands are effectively delocalised and the
zinc atom has a distorted tetrahedral coordination geometry. This
is also the case for 6, the structure of which closely resembles those
of a number of other bis(chelate) zinc complexes, e.g. [Zn(Fiso)2]29
and [Zn(Aiso)2] (Aiso = (ArN)2CMe-).9 The steric crowding in 6
most probably gives rise to the significant differences between
its Zn–N bond lengths. This crowding and the dihedral angle
between the two ZnN2C least squares planes (59.7◦), seemingly
accounts for the four chemically inequivalent sets of isopropyl
methyl resonances observed in the 1H NMR spectrum of the
compound.
Fig. 6 Molecular structure of 6 (25% thermal ellipsoids are shown;
◦
˚
hydrogens omitted). Selected bond lengths (A) and angles ( ): Zn(1)–N(1)
2.0005(14), Zn(1)–N(3) 2.0143(14), Zn(1)–N(4) 2.1452(14), Zn(1)–N(2)
2.1564(14), N(1)–C(1) 1.354(2), N(2)–C(1) 1.325(2), N(3)–C(30) 1.345(2),
N(4)–C(30) 1.334(2), N(1)–Zn(1)–N(2) 63.65(6), N(3)–Zn(1)–N(4)
63.88(6), N(2)–C(1)–N(1) 110.09(13), N(4)–C(30)–N(3) 110.63(14).
complexes and the carbonic anhydrase catalysed dehydration of
bicarbonate, which yields CO2. In contrast, the thermal stability
of a less bulky amidinato zinc bromide complex was found to
be greater than that of its guanidinato counterparts. Attempts to
reduce several of the zinc(II) halide complexes reported here to
dimeric zinc(I) compounds were unsuccessful, in all cases leading
to the deposition of zinc metal.
Experimental
General considerations
All manipulations were carried out using standard Schlenk and
glove box techniques under an atmosphere of high purity argon
or dinitrogen. THF, hexane and toluene were distilled over molten
potassium, while diethyl ether was distilled over a Na/K alloy.
Melting points were determined in sealed glass capillaries under
argon or dinitrogen and are uncorrected. Mass spectra were
recorded at the EPSRC National Mass Spectrometric Service
at Swansea University. The microanalysis was obtained from
Medac Ltd. IR spectra were recorded using a Nicolet 510 FT-IR
Fig. 5 Molecular structure of 5 (25% thermal ellipsoids are shown;
◦
˚
hydrogens omitted). Selected bond lengths (A) and angles ( ): Br(1)–Zn(1)
2.4352(4), Zn(1)–N(1) 1.996(2), Zn(1)–N(2) 2.003(2), Zn(1)–Br(1)¢
2.4182(4), N(1)–C(1) 1.343(4), C(1)–N(2) 1.345(4), N(1)–Zn(1)–N(2)
66.64(10), Br(1)¢–Zn(1)–Br(1) 97.524(14), N(1)–C(1)–N(2) 109.6(2). Sym-
metry operation: ¢-x+1, -y+2, -z.
Conclusion
1
spectrometer as Nujol mulls between NaCl plates. 1H and 13C{ H}
In summary, a variety of bulky amidinato and guanidinato zinc
halide complexes have been prepared and their thermal stabilities
compared. The guanidinato complexes are generally susceptible
to decomposition via amide extrusion reactions, yielding the
NMR spectra were recorded on either Bruker DXP300 or DPX400
spectrometers and were referenced to the resonances of the solvent
used. [K(Giso)],12g [K(Priso)],10 K[(Piso)],12b GisoH10 and PrisoH10
were prepared using variations of literature procedures. [Li(Giso)]
and [Li(Priso)] were prepared in situ by treating diethyl ether
solutions of either GisoH or PrisoH with one equivalent of a
1.6 M solution of LiBun in hexanes.10 All other reagents were used
as received.
carbodiimide, ArN
C NAr. This is more so for complexes
bearing the less bulky ligand, Priso, than those incorporating the
more sterically demanding guanidinate, Giso. Analogies have been
drawn between the decomposition of the guanidinato zinc halide
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Preparation of [(Giso)ZnI] 1. A solution of [K(Giso)] (1.82 g,
3.13 mmol) in THF (20 cm3) was added over 5 min to a solution of
ZnI2 (1.0 g, 3.13 mmol) in THF (20 cm3) at -78 ◦C. The reaction
mixture was warmed to room temperature to yield a pale yellow
solution. Volatiles were removed in vacuo, the residue washed
with hexane (10 cm3) and extracted into diethyl ether (30 cm3).
Filtration and cooling to -30 ◦C over◦night yielded colourless
crystals of 1 (0.54 g, 24%). Mp: 175–185 C; 1H NMR (400 MHz,
C6D6, 298 K): d = 0.62–0.71 (m, 4 H, CH2), 0.84–1.01 (m, 8H,
C31H49I2N3Zn2: C 43.89%, H 5.82%, N 4.95%, found: C 43.50%,
H 5.99%, N 4.63%.
Preparation of [(Priso)Zn(l-Br)2Li(OEt2)2] 4. A solution of
[Li(Priso)] (1.48 g, 3.15 mmol) in diethyl ether (40 cm3) was added
over 5 min to a slurry of ZnBr2 (0.79 g, 3.51 mmol) in diethyl
ether (10 cm3) at -30 ◦C. The reaction mixture was warmed to
room temperature, whereupon it was concentrated to ca. 20 cm3,
filtered, and cooled to -30 ◦C yielding colourless crystals of 4.
Concentration of the supernatant solution to ca. 8 cm3, and
3
3
CH2), 1.26 (d, JHH = 6.8 Hz, 12 H, CH(CH3)2), 1.34 (d, JHH
=
6.8 Hz, 12 H, CH(CH3)2), 1.67–1.42 (m, 8 H, CH2), 3.60 (m, 2
◦
cooling to -30 C yielded a second crop of 4 (1.21 g, 46%). Mp:
3
H, Cy–CH), 3.62 (sept, JHH = 6.8 Hz, 4 H, CH(CH3)2), 6.89–
◦
decomp. above 65 C; IR n/cm-1 (Nujol): 2174 s, 1611 s 1580 s,
1
7.29 (m, 6 H, Ar–H); 13C{ H} NMR (100.6 MHz, C6D6, 298 K):
1460 s, 1341 s, 1256 m, 1185 m, 1154 m, 1094 m, 1062 s, 1003 m, 975
m, 914 m, 828 m, 798 m, 767 m, 752 s. NMR spectroscopic data
could not be obtained for the compound as it rapidly decomposes
when dissolved in non-coordinating deuterated solvents.
d = 20.2 (CH2), 23.9 (CH(CH3)2), 24.9 (CH2), 25.7 (CH(CH3)2),
27.2 (CH(CH3)2), 34.2 (CH2), 59.3 (Cy–CH), 122.2, 123.7, 140.7,
141.9 (Ar–C), 168.1 (NCN); IR n/cm-1 (Nujol): 1612 m, 1583 m,
1244s,1018s, 893 m, 865 m, 770 m, 722m; MS/EI m/z (%): 733.3
(M+, 4), 608.2 (MH+ - I, 6), 500.5 (GisoH+-Pri, 80); acc. mass MS
(EI) m/z: calc for C37H56IN3Zn: 733.2805, found: 733.2809.
Preparation of [{(Piso)Zn(l-Br)}2] 5. A solution of [K(Piso)]
(1.00 g, 2.18 mmol) in toluene–diethyl ether (1 : 1, 40 cm3) was
added to a slurry of ZnBr2 (0.54 g, 2.40 mmol) in toluene–diethyl
ether (1 : 1, 40 cm3) at -80 ◦C. The reaction mixture was allowed
to warm to room temperature and stirred overnight. The resultant
mixture was filtered, concentrated to ca. 25 cm3 and cooled to
-30 ◦C to yield colourless crystals of 5. Concentration of the
supernatant to ca. 10 cm3 and ◦cooling yielded another crop of
Preparation of [(Giso)Zn(l-Br)2Li(OEt2)2] 2. A solution of
[Li(Giso)] (1.50 g, 2.73 mmol) in diethyl ether (25 cm3) was added
to a slurry of ZnBr2 (0.71 g, 3.15 mmol) in diethyl ether (15 cm3)
at -60 ◦C, over 5 min. The reaction mixture was warmed to room
temperature, filtered, and the filtrate cooled to -30 ◦C to give
colourless crystals of 2. Concentration of the supernatant solution
1
◦
5 (0.75 g, 60%). Mp: 306–308 C; H NMR (300 MHz, C6D6,
to ca. 12 cm3, and cooling to -30 C yielded a second crop of 2
3
◦
298 K): d = 0.93 (s, 18 H, C(CH3)3), 1.23 (d, JHH = 6.8 Hz,
1
(1.30 g, 52%). Mp: 196–202 C (decomp.); H NMR (400 MHz,
C6D6, 298 K): d = 0.70–1.15 (m, 20 H, OCH2CH3, Cy–CH2), 1.25–
1.38 (m, 8 H, Cy–CH2), 1.46 (d, 3JHH = 6.8 Hz, 12 H, CH(CH3)2),
1.57 (d, 3JHH = 6.8 Hz, 12 H, CH(CH3)2), 1.65–1.77 (m, 4 H, Cy–
3
24 H, CH(CH3)2), 1.29 (d, JHH = 6.8 Hz, 24 H, CH(CH3)2),
3
3.64 (sept, JHH = 6.8 Hz, 8 H, CH(CH3)2), 6.94–7.31 (m, 12
H, ArH); 13C{H}NMR (75.5 MHz, 298 K, C6D6): d = 22.6
(CH(CH3)2), 26.8 (CH(CH3)2), 28.9 (CH(CH3)2), 30.3 (C(CH3)3),
42.2 (C(CH3)3), 123.5, 125.3, 142.3, 143.7 (Ar–C), 178.9 (NCN);
IR (Nujol) n/cm-1: 1616 m, 1586 m, 1462 s, 1364 m, 1331 m, 1261
m, 1179 m, 1098 s, 1054 m, 933 m, 801 s, 757 s; MS (EI 70 eV), m/z
(%): 521.2 ((M/2)+ -Pri, 4), 420.6 (PisoH+, 16), 244.1 (ArNCBut+,
100).
CH2), 3.41–3.48 (m, 10 H, OCH2CH3, Cy–CH), 3.58 (sept, 3JHH
=
6.8 Hz, 4 H, CH(CH3)2), 6.99–7.23 (m, 6 H, ArH); 13C{ H} NMR
(100.6 MHz, C6D6, 298 K): d 22.9, 23.0, 25.1, 25.6, 27.0, 28.0
(2 ¥ CH2, 2 ¥ CH(CH3)2, 1 ¥ CH(CH3)2, 1 ¥ OCH2CH3), 35.1
(CH2), 59.4 (Cy–CHN), 68.0 (OCH2CH3), 123.2, 123.4, 143.4,
143.9 (ArC), CN3 not observed; IR n/cm-1 (Nujol): 1612 s 1583 s,
1454 s, 1326 s, 1258 s, 1183 m, 1150 m, 1093 s, 1066 s, 1021 s, 954
m, 898 s, 835 m, 795 s, 770 m, 750s.
1
Preparation of [Zn(Piso)2] 6. A slurry of [K(Piso)] (0.99 g, 2.16
mmol) in toluene (25 cm3) was added to a suspension of ZnBr2
(0.52 g, 2.31 mmol) in toluene (15 cm3) at ambient temperature and
the mixture stirred vigorously overnight. The resultant mixture
was filtered, the filtrate concentrated to ca. 15 cm3 and then cooled
to 4 ◦C, affording colourless crystals of 6·(toluene)1.5 (0.62 g,
Preparation of [(IZn)2(l-NPri2){l-N,N¢-(NAr)2CH}] 3. A so-
lution of [K(Priso)] (1.57 g, 3.13 mmol) in THF (20 cm3) was added
over 5 min to a solution of ZnI2 (1.0 g, 3.13 mmol) in THF (20 cm3)
at -78 ◦C. The reaction mixture was warmed to room temperature,
then volatiles were removed in vacuo. The residue was washed
with hexane (10 cm3) and extracted into diethyl ether (20 cm3).
Filtration, concentration and cooling to -30 ◦C overnight yielded
colourless crystals of 3 (0.67 g, 51% based on ZnI2). Mp = 197–203
◦C; 1H NMR (400 MHz, C6D6, 298 K): d = 1.18 (d, 3JHH = 6.8 Hz,
◦
1
55%). Mp: 280–284 C (decomp.); H NMR (300 MHz, C6D6,
3
298 K): d = 0.42 (d, JHH = 6.8 Hz, 12 H, CH(CH3)2), 0.87 (s,
3
18 H, C(CH3)3), 1.22 (d, JHH = 6.8 Hz, 12 H, CH(CH3)2), 1.38
(d, JHH = 6.8 Hz, 12 H, CH(CH3)2), 1.44 (d, JHH = 6.8 Hz, 12
H, CH(CH3)2), 3.24 (sept, JHH = 6.8 Hz, 4 H, CH(CH3)2), 3.78
3
3
3
3
12 H, NCH(CH3)2), 1.41 (d, JHH = 6.8 Hz, 12 H, CH(CH3)2),
(sept, 3JHH = 6.8 Hz, 4 H, CH(CH3)2), 6.89–7.08 (m, 12 H, ArH);
13C{H} NMR (75.5 MHz, C6D6, 298 K): d = 23.0 (CH(CH3)2), 23.1
(CH(CH3)2), 23.2 (CH(CH3)2), 25.4 (CH(CH3)2), 28.4 (CH(CH3)2,
28.6 (CH(CH3)2), 30.6 (C(CH3)3), 42.3 (C(CH3)3), 123.4, 124.8,
142.9, 144.0 (Ar–C), 178.1 \28;NCN); IR (Nujol) n/cm-1: 1616
m, 1586 m, 1462 s, 1366 m, 1312 m, 1253 m, 1173 s, 1096 m, 1044
m, 965 m, 933 m, 802 s, 760 s; MS (EI 70 eV), m/z (%): 902.7 (M+,
12), 859.6 (M+ - Pri, 10), 845.6 (M+ -C4H9, 46), 244.1 (ArNCBut+,
100); acc. mass, m/z (EI): calc for C58H86N4Zn: 902.6138; found
902.6152.
1.53 (d, 3JHH = 6.8 Hz, 12 H, CH(CH3)2), 3.34 (sept, 3JHH = 6.8 Hz,
3
4 H, CH(CH3)2), 3.77 (sept, JHH = 6.8 Hz, 2 H, NCH(CH3)2),
1
7.03–7.29 (m, 6 H, ArH), 7.54 (s, 1 H, NCH); 13C{ H} NMR
(75.6 MHz, C6D6, 298 K): d = 22.9 (CH(CH3)2), 23.1 (CH(CH3)2),
24.1 (NCH(CH3)2), 28.5 (CH(CH3)2), 49.9 (NCH(CH3)2), 123.8,
125.2, 141.8, 143.7 (Ar–C), 167.1 (NCN); IR n/cm-1 (Nujol): 1593
m, 1254 s, 1174 s, 1130 s, 1054 m, 932 m, 805 m, 761m; MS/EI m/z
(%): 845.1 (M+, 7), 366.2 (FisoH+, 80); acc. mass MS (EI) m/z:
calc for C31H49I2N3Zn2: 845.0593, found: 845.0584; anal. calc. for
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Table 1 Crystal data for compounds 1–6
Compound
1
2
3
4
5
6·(toluene)1.5
Empirical formula
FW
T/K
Crystal system
C37H56IN3Zn C45H76Br2LiN3O2Zn C31H49I2N3Zn2
C39H68Br2LiN3O2Zn
843.09
123(2)
C58H86Br2N4Zn2
1129.87
123(2)
C68.5H98N4Zn
1042.88
123(2)
735.12
150(2)
Monoclinic
P21/n
10.659(2)
18.297(4)
19.481(4)
90
923.22
123(2)
Monoclinic
P21/c
19.350(4)
19.897(4)
12.633(3)
90
848.27
150(2)
Monoclinic
P21/n
12.980(3)
12.870(3)
21.640(4)
90
Triclinic
Triclinic
Triclinic
¯
¯
¯
Space group
P1
P1
P1
˚
a/A
9.3323(19)
14.519(3)
18.076(4)
66.40(3)
85.15(3)
75.77(3)
2175.3(8)
2
10.0698(30
10.7948(4)
14.8844(7)
105.869(2)
102.254(2)
106.999(2)
1411.22(9)
1
12.650(3)
12.723(3)
19.058(4)
100.62(3)
90.59(3)
92.15(3)
3012.3(10)
2
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
105.17(3)
90
103.02(3)
90
93.42(3)
90
3
˚
V/A
3666.8(13)
4
1.332
4738.7(16)
4
1.294
3608.7(13)
4
1.561
Z
Density (calcd)/Mg m-3
m(Mo-Ka)/mm-1
F(000)
1.287
2.434
884
15510
1.329
2.304
592
21361
1.150
0.451
1134
27427
1.539
1528
2.241
1944
3.062
1688
No. of reflections collected
No. of independent
reflections (Rint)
Final R1 (I > 2s(I)) and wR2 R1 = 0.0405
indices (all data)
15002
17945
14940
7856 (0.0268)
7927 (0.0348) 10313 (0.0335)
8431 (0.0316)
6293 (0.0508)
14451 (0.0245)
R1 = 0.0469
R1 = 0.0290
R1 = 0.0416
R1 = 0.0399
R1 = 0.0417
wR2 = 0.0944 wR2 = 0.1071
wR2 = 0.0650
wR2 = 0.1059
wR2 = 0.0819
wR2 = 0.1110
X-Ray crystallography
8 (a) M. Cheng, A. B. Attygalle, E. B. Lobkovsky and G. W. Coates, J. Am.
Chem. Soc., 1999, 121, 11583; (b) M. H. Chisholm, J. C. Huffman and
K. Phomphrai, J. Chem. Soc., Dalton Trans., 2001, 222.
9 N. Nimitsiriwat, V. C. Gibson, E. L. Marshall, P. Takolpuckdee, A. T.
Tomov, A. J. P. White, D. J. Williams, M. R. J. Elsegood and S. H. Dale,
Inorg. Chem., 2007, 46, 9988.
10 G. Jin, C. Jones, P. C. Junk, K.-A. Lippert, R. P. Rose and A. Stasch,
New J. Chem., 2009, 33, 64.
11 C. Jones, Coord. Chem. Rev., 2010, 254, 1273.
12 (a) See for example: C. Jones, C. Schulten, R. P. Rose, A. Stasch, S.
Aldridge, W. D. Woodul, K. S. Murray, B. Moubaraki, M. Brynda, G.
La Macchia and L. Gagliardi, Angew. Chem., Int. Ed., 2009, 48, 7406;
(b) R. P. Rose, C. Jones, C. Schulten, S. Aldridge and A. Stasch, Chem.–
Eur. J., 2008, 14, 8477; (c) C. Jones, D. P. Mills and A. Stasch, Dalton
Trans., 2008, 4799; (d) G. Jin, C. Jones, P. C. Junk, A. Stasch and W. D.
Woodul, New J. Chem., 2008, 32, 835; (e) S. P. Green, C. Jones and A.
Stasch, Science, 2007, 318, 1754; (f) S. P. Green, C. Jones, G. Jin and
A. Stasch, Inorg. Chem., 2007, 46, 8; (g) D. Heitmann, C. Jones, P. C.
Junk, K.-A. Lippert and A. Stasch, Dalton Trans., 2007, 187; (h) C.
Jones, P. C. Junk, J. A. Platts and A. Stasch, J. Am. Chem. Soc., 2006,
128, 2206; (i) S. P. Green, C. Jones, P. C. Junk, K.-A. Lippert and A.
Stasch, Chem. Commun., 2006, 3978.
13 (a) S. Schulz, D. Schuchmann, U. Westphal and M. Bolte,
Organometallics, 2009, 28, 1590; (b) Y. Wang, B. Quillian, P. Wei, H.
Wang, X.-J. Yang, Y. Xie, R. B. King, P. v. R. Schleyer, H. F. Schaefer
III and G. H. Robinson, J. Am. Chem. Soc., 2005, 127, 11944.
14 I. Resa, E. Carmona, E. Gutierrez-Puebla and A. Monge, Science,
2004, 305, 1136.
Crystals of 1–6 suitable for X-ray structural determination were
mounted in silicone oil. Crystallographic measurements were
made using a Nonius Kappa CCD diffractometer. The structures
were solved by direct methods and refined on F2 by full matrix
least squares (SHELX97)30 using all unique data. Hydrogen atoms
have been included in calculated positions (riding model) for all
structures. Details of the modelling of disorder in the crystal
structures of 2, 4 and 6 can be found in their CIF files. Crystal data,
details of data collections and refinement are given in Table 1.
Acknowledgements
We thank the Australian Research Council (fellowships for CJ,
AS and SN). CJ and SA thank the Engineering and Physical
Sciences Research Council (EPSRC), UK (fellowship for RPR).
The EPSRC Mass Spectrometry Service at Swansea University is
also thanked.
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8794 | Dalton Trans., 2010, 39, 8788–8795
This journal is
The Royal Society of Chemistry 2010
©

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Dalton Trans., 2010, 39, 8788–8795 | 8795
©