Synthesis and characterisation of zinc gallyl complexes: First structural elucidations of Zn-Ga bonds

Article abstract of DOI:10.1039/b706402k

Reactions of the anionic gallium(i) heterocycle, [:Ga{[N(Ar)C(H)] ₂}]^- (Ar = C₆H₃Prⁱ ₂-2,6), with two N,N-chelated zinc chloride complexes have yielded the compounds, [{Prⁱ₂

Full text of DOI:10.1039/b706402k

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www.rsc.org/dalton | Dalton Transactions
Synthesis and characterisation of zinc gallyl complexes: First structural
elucidations of Zn–Ga bonds†‡
Cameron Jones,*a Richard P. Rosea,b and Andreas Stascha,b
Received 27th April 2007, Accepted 30th May 2007
First published as an Advance Article on the web 14th June 2007
DOI: 10.1039/b706402k
not dissimilar. Given that the molecular chemistry of zinc(I) has
been initiated with the preparation of remarkable compounds of
the general type, RZnIZnIR,9 comparisons with related Zn–Ga
Reactions of the anionic gallium(I) heterocycle, [:Ga{[N(Ar)-
C(H)]2}]− (Ar = C6H3Pri2-2,6), with two N,N-chelated
zinc chloride complexes have yielded the compounds,
[{Pri2NC[N(Ar)]2}ZnGa{[N(Ar)C(H)]2}] and [(tmeda)Zn-
bonded complexes would be informative. Our preliminary efforts
to form zinc gallyl complexes are reported herein.
{Ga{[N(Ar)C(H)]2}}2] which contain the first crystallograph-
The bulky guanidinate ligand, [Pri2NC{N(Ar)}2]− (Priso−),
ically characterised Zn–Ga bonds.
has been shown to be effective in the stabilisation of
the first structurally characterised Sn–Ga bonded complex
The chemistry of complexes containing metal–metal bonds is
wide ranging and of undoubted fundamental importance. Over
[(Priso)SnGa{[N(Ar)C(H)]2}].4a Accordingly, it was employed
in this study to form the zinc chloride complex, [(Priso)ZnCl]
the years such complexes have found numerous applications in
2,‡ which when reacted with [K(tmeda)][1] in toluene afforded
the zinc gallyl, 3, in almost quantitative yield (Scheme 1). In
order to examine the possibility of preparing a bis(gallyl) zinc
complex, [ZnCl2(tmeda)] was treated with two equivalents of
[K(tmeda)][1] which afforded 4 in good yield. Repeating this
reaction in a 1 : 1 stoichiometry led to no isolable products.
Similarly, the 2 : 1 reaction of [K(tmeda)][1] with [CdCl2(tmeda)]
led to decomposition and deposition of cadmium metal above
0 ◦C, thus attesting to the reducing nature of 1.
synthesis, catalysis and materials chemistry to name but a few.1 We
gained entry to this field with the synthesis of the anionic gallium(I)
heterocycle, [:Ga{[N(Ar)C(H)]2}]− 1 (Ar = C6H3Pri2-2,6),2 which
is a valence isoelectronic analogue of the important N-heterocyclic
carbene (NHC) class of ligand. This has been utilised in a variety of
ligand displacement, oxidative insertion and reductive elimination
reactions that have yielded an array of complexes containing bonds
between gallium and s-, p- or d-block metals. In addition, the
carbenoid character of 1 has proved useful in cycloaddition and
C–H activation reactions.3,4
Until recently, the formation of metal gallyl complexes from the
reactions of 1 with metal halides and their complexes had proved
elusive. Instead of salt elimination occurring, the paramagnetic
gallium(II) dimers, [GaX{[N(Ar)C(H)•]2}]2 (X = halide), were
invariably formed in these reactions.5 These presumably resulted
from an initial oxidative insertion of the Ga(I) center of 1 into
the M–X bond of the metal halide, followed by elimination of the
dimer. We now know that the problems associated with reactions
of 1 with metal halides can be circumvented by pre-coordination
of the metal halide reactant with bulky and/or electron rich
ligands, e.g. NHCs or bulky guanidinates. As a result, neutral
metal gallyl complexes are becoming increasing accessible, which
in the last 12 months have included examples containing the
first structurally characterised Ga–Sn,4a Ga–Nd,6 Ga–Cu7 and
Ga–Ag7 bonds. An extension of this work to the preparation
of zinc gallyl complexes seemed appropriate as Zn–Ga bonded
species were unknown prior to this study, despite the relatively
well developed chemistry of zinc–metal bonded compounds.1,8 It
was also seen of interest as gallium is adjacent to zinc in the
periodic table, and therefore the properties of the two metals are
Scheme 1 Reagents and conditions: (i), (Priso)ZnCl 2, –KCl; (ii), 1/2
(tmeda)ZnCl2, –KCl.
Both 3 and 4 are yellow crystalline solids that are indefinitely
stable under inert atmospheres. The NMR spectra of 3 § are con-
sistent with its proposed structure in that they exhibit resonances
corresponding to five chemically inequivalent sets of isopropyl
1
methyl groups. In contrast, the H NMR spectrum of 4 displays
only two broadened isopropyl methyl doublet resonances, which
suggests rotation of its gallyl groups about the Zn–Ga bonds in
solution. Variable temperature NMR studies (298 K–223 K) led
to further broadening of these signals, but not to resolution of the
spectrum.
Compounds 3 and 4 were crystallographically characterised†¶
and their molecular structures are depicted in Fig. 1 and 2
respectively. It should be noted that the quality of the structure of
aSchool of Chemistry, Monash University, Melbourne, PO Box 23, Victoria,
3800, Australia
bSchool of Chemistry, Main Building, Cardiff University, Cardiff, UK CF10
3AT
† CCDC reference numbers 645310 and 645311. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/b706402k
‡ Electronic supplementary information (ESI) available: Full synthetic
details for 2–4. See DOI: 10.1039/b706402k
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the backbone N–C distances in the guanidinate ligand of 3 are
suggestive of significant delocalisation over its N3C fragment.
Although there have been no previously reported Zn–Ga bonds
for purposes of comparison, the bond lengths characterised in
11
˚
this study are well within the sum of covalent radii (2.50 A ) for
˚
the two elements. Interestingly however, that in 3 it is ca. 0.1 A
shorter than those in 4, probably because of the differences in
the zinc coordination numbers between the two complexes. As the
11
˚
covalent radii of zinc and gallium are the same (1.25 A ), the
Zn–Ga bonds in 3 and 4 are, not surprisingly, of similar lengths to
the four previously structurally characterised ZnI–ZnI interactions
9
˚
˚
(range: 2.305(3) A–2.3994(6) A ).
One feature of the structure of 3 that interested us was the
acute angle between the least squares planes of its two heterocycles
(21.7◦). As the two metal centres of the compound presumably
possess empty p-orbitals orthogonal to the heterocycle planes,
the possibility of singly or doubly reducing 3 to give a complex
with a Zn–Ga bond order of 1.5 or 2 respectively, existed. In this
respect, similar single reductions of digallanes, [R2GaGaR2], have
previously afforded planar radical anions, [R2GaGaR2]•−, with a
Ga–Ga bond order of 1.5.12 Attempts to cleanly reduce 3 with
lithium or KC8 in 1 : 1 or 2 : 1 stoichiometries have so far not
been successful and instead have led to an intractable mixtures of
products.
In conclusion, the first examples of complexes containing Zn–
Ga bonds have been described. We are currently exploring the
reactivity of the metal–metal bonds in the prepared complexes
towards Lewis bases and mild reducing agents and will report on
this in due course.
Fig. 1 Molecular structure of 3 (25% thermal ellipsoids, hydrogen
◦
˚
atoms omitted). Selected bond lengths (A) and angles ( ): Ga(1)–N(4)
1.8633(19), Ga(1)–N(5) 1.867(2), Ga(1)–Zn(1) 2.3230(7), Zn(1)–N(1)
1.986(2), Zn(1)–N(2) 2.0041(19), N(1)–C(1) 1.354(3), N(2)–C(1)
1.356(3), N(3)–C(1) 1.360(3), N(4)–C(32) 1.383(3), N(5)–C(33) 1.392(3),
C(32)–C(33) 1.338(4), N(4)–Ga(1)–N(5) 88.12(9), N(1)–Zn(1)–N(2)
67.40(8), N(1)–C(1)–N(2) 109.5(2).
Acknowledgements
We gratefully acknowledge financial support from the EPSRC
(studentship for R. P. R.), the Australian Research Council (fel-
lowships for C. J. and A. S.), and the EPSRC Mass Spectrometry
Service, Swansea.
Notes and references
§ Selected data for 3: Yield: 96% (yellow crystals). Mp 180–185 ◦C (dec.).
3
1H NMR (400 MHz, C6D6, 298 K): d 0.81 (d, JHH = 6.8 Hz, 12 H,
NCH(CH3)2), 1.13 (d, 3JHH = 6.8 Hz, 12 H, CH(CH3)2), 1.16 (d, 3JHH
=
3
6.8 Hz, 12 H, CH(CH3)2), 1.39 (d, JHH = 6.8 Hz, 12 H, CH(CH3)2),
3
1.51 (d, JHH = 6.8 Hz, 12 H, CH(CH3)2), 3.60 (overlapping m, 6 H,
Fig. 2 Molecular structure of 4 (25% thermal ellipsoids, hydrogen atoms
3
CH(CH3)2), 3.79 (sept, JHH = 6.8 Hz, 4 H, CH(CH3)2), 6.36 (s, 2 H,
˚
GaNCH), 7.03–7.39 (m, 12 H, ArH); 13C{ H} NMR (100.6 MHz, C6D6,
and isopropyl groups omitted). Selected bond lengths (A) and angles
1
(◦): Ga(1)–N(2) 1.905(10), Ga(1)–N(1) 1.903(9), Ga(1)–Zn(1) 2.4491(17),
Ga(2)–N(4) 1.886(10), Ga(2)–N(3) 1.895(8), Ga(2)–Zn(1) 2.4307(17),
Zn(1)–N(6) 2.168(9), Zn(1)–N(5) 2.177(10), N(2)–Ga(1)–N(1) 86.2(4),
N(4)–Ga(2)–N(3) 86.7(4), N(6)–Zn(1)–N(5) 84.7(3), Ga(2)–Zn(1)–Ga(1)
126.00(6).
298 K): d 20.5, 20.7, 22.2, 23.0, 24.2 (CH(CH3)2), 27.0, 27.8 (CH(CH3)2),
=
47.8 (NCH(CH3)2), 121.4 (N C), 121.5, 121.7, 121.9, 123.4, 141.6, 143.9,
144.2, 146.4 (ArC), 168.4 (CN3); IR m/cm−1 (Nujol): 1613 (s), 1584 (m),
1260 (m), 1124 (m), 1107 (m), 1057 (m), 932 (m), 800 (m); MS (EI 70 eV),
m/z (%): 971 (M+, 2), 420 [(ArN)2CNPriH+, 100]; EI Acc. Mass.: calc.
for C57H84N569GaZn: 971.5269, found: 971.5268; 4: Yield: 72% (yellow
◦
1
crystals). Mp 85–87 C. H NMR (400 MHz, C6D6, 298 K): d 1.18 (br.
3
3
4 is poor due to significant disorder of its tmeda ligand and the
weakly diffracting nature of its crystals. Despite this, the molecular
connectivity of the complex is unambiguous and the geometrical
parameters of its ZnGa2N2 fragment are sufficiently accurate to
pass comment. Both compounds are monomeric with distorted
trigonal planar (3) or tetrahedral (4) zinc coordination geometries.
The geometries of the gallium heterocycles in 3 and 4 are similar
to each other and to those in the majority of other complexes
of 1.3,4 As in previously reported zinc guanidinate complexes,10
d, JHH = 6.8 Hz, 24H, CH(CH3)2), 1.33 (br. d, JHH = 6.8 Hz, 24H,
CH(CH3)2), 1.42 (s, NCH2, 4H), 1.47 (s, NCH3, 12H), 3.64 (sept, 3JHH
=
6.8 Hz, 8 H, CH(CH3)2), 6.37 (s, NCH, 4H), 7.13–7.18 (m, ArH, 12H);
13C NMR (75 MHz, C6D6, 298 K): d 23.5, 24.4 (br., CH(CH3)2), 26.7 (br.,
CH(CH3)2), 47.7 (NCH3), 55.8 (NCH2), 121.5 (CN), 121.7 (m-ArC), 123.3
(p-ArC), 144.1 (o-ArC), 146.3 (ipso-ArC); IR m/cm−1 (Nujol): 1587 (s),
1259 (s), 1100 (br), 1020 (br), 800 (s); MS (EI 70 eV), m/z (%): 1071 (M+,
+
2), 446 (Ga{[N(Ar)C(H)]2} , 12), 377 ({N(Ar)C(H)}2H+, 100); EI Acc.
Mass.: calc. for C58H88N669Ga2Zn: 1070.4873, found: 1070.4870.
¶ Crystal data for 3: C57H84GaN5Zn, M = 974.38, triclinic, space group
◦
˚
˚
˚
P-1, a = 11.608(2) A, b = 13.803(3) A, c = 17.997(4) A, a = 88.49(3) ,
2998 | Dalton Trans., 2007, 2997–2999
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◦
◦
3
−3
˚
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b = 81.17(3) , c = 86.69(3) , V = 2844.3(10) A , Z = 2, Dc = 1.138 g cm
,
F(000) = 1044, l(Mo-Ka) = 0.933 mm−1, 150(2) K, 10 462 unique
reflections [R(int) 0.0349], R (on F) 0.0418, wR (on F2) 0.0950 (I > 2rI);
4·(diethyl ether)0.5: C60H93Ga2N6O0.5Zn, M = 1111.21, orthorhombic,
˚
˚
˚
space group Pbca, a = 13.953(3) A, b = 21.070(4) A, c = 46.243(9) A,
V = 13595(5) A , Z = 8, Dc = 1.086 g cm−3, F(000) = 4728, l(Mo-Ka) =
3
˚
1.173 mm−1, 150(2) K, 11 852 unique reflections [R(int) 0.0854], R (on F)
0.1271, wR (on F2) 0.3003 (I > 2rI).
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This journal is
The Royal Society of Chemistry 2007
Dalton Trans., 2007, 2997–2999 | 2999
©