Reactions of a β-diketiminate zinc hydride complex with heterocumulenes

Article abstract of DOI:10.1039/c0cc01329c

The β-diketiminate zinc hydride MesnacnacZnH (1) reacts with CO ₂, C(Ni-Pr)₂ and t-BuNCO at ambient temperature with insertion into the Zn-H bond and subsequent formation of the corresponding formato (2), formamido (3) and formamidinato (4) complexes.

Full text of DOI:10.1039/c0cc01329c

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Reactions of a b-diketiminate zinc hydride complex with
heterocumulenesw
¨
Stephan Schulz,* Tamara Eisenmann, Sarah Schmidt, Dieter Blaser,
Ulrich Westphal and Roland Boese
Received 10th May 2010, Accepted 21st July 2010
DOI: 10.1039/c0cc01329c
The b-diketiminate zinc hydride MesnacnacZnH (1) reacts with
CO2, C(Ni-Pr)2 and t-BuNCO at ambient temperature with
insertion into the Zn–H bond and subsequent formation of the
corresponding formato (2), formamido (3) and formamidinato (4)
complexes.
and ambient temperature with the formation of the corres-
ponding formato complex. In addition, ZnH2 completely
failed to react with CO2.10
We became only recently interested in organozinc hydrides
RZnH and reported on the synthesis of MesnacnacZnH (1)
(Mesnacnac = [HC{C(Me)N(2,4,6-Me3C6H2)}2]) containing
a sterically demanding N,N0-chelating b-diketiminato group.11
1, which was synthesized according to a procedure established
by Harder et al. by reaction of MesnacnacZnCl with
KN(i-Pr)HBH3,12 represents one of the rare examples of
structurally characterized organozinc hydrides.13 Even though
1 forms a hydride-bridged dimer in the solid state, pulsed
gradient spin echo (PGSE) diffusion measurements at 25 1C of
solutions of 1 in different solvents indicated that 1 is
monomeric in solution. Herein we report on reactions of 1
with CO2, C(Ni-Pr)2 and t-BuNCO (Scheme 1).
The effective utilization of abundant, renewable, nontoxic and
inexpensive CO2 as C1-feedstock still is an important target in
respect to the current environmental concerns facing our
planet.1 Unfortunately, the high stability of CO2 has limited
its use in industrial syntheses, so far, but zinc complexes have
been demonstrated in the past to be of potential interest for
this purpose. Zinc alkoxide and carboxylate complexes were
found to serve as living single-site catalysts for the ring-
opening polymerization (ROMP) of lactide2 and the
copolymerization of epoxides and carbon dioxide3 yielding
biodegradable aliphatic polycarbonates. Moreover, bimetallic
calcium/zinc complexes supported by a bridged b-diketiminate
ligand are effective catalysts for terpolymerization reactions of
Reaction of 1 with CO2 under very mild reaction conditions
(ambient temperature, CO2 under normal pressure) for 2
hours resulted in an almost quantitative formation of 2.
1
4
In situ monitoring of the reaction by H NMR spectroscopy
epoxide, cyclic anhydrides and CO2 as well as for epoxide/
CO2 copolymerization.5
showed a continuously decreasing resonance at 4.57 ppm
(Zn–H) and a steadily increasing new singlet at 8.01 ppm
due to the formation of the O2C–H group. IR spectra of pure 2
show two absorption bands due to nas(CO) and nsy(CO). The
difference Dn between both is typically used to classify the
binding mode of the formiate group, with a large difference
of about 200 cmꢀ1 indicating a Z1-coordination, whereas
difference of 100 cmꢀ1 is typical for bridging and chelating
In order to investigate the mechanism of the CO2 activation
in more detail, several reactions of zinc alkoxides RZnOR0
and amides RZnNR0 with CO2 have been investigated
2
experimentally6 and by theoretical calculations.7 These
reactions typically proceeded with insertion into the Zn–N
and Zn–O bond and subsequent formation of zinc carbamate
and carbonate complexes. In addition, pyrazolylborate zinc
hydrides were found to react with CO2 and heterocumulenes
such as CS2 and isothiocyanates RNCS with insertion into the
Zn–H bond and formation of the mononuclear Zn
complexes,8 whereas no reaction was observed with cyclohexyl
carbodiimide.9 Very recently, Driess et al. reported on the
formation of zinc formates via Li-promoted hydrogenation of
CO2 using heterobimetallic hydridozinc alkoxide clusters of
the type [(HZnOt-Bu)4ꢀn(LiOt-Bu)n] (n = 0–3). Li ions were
found to play a distinguished role on the activity of the Zn–H
moiety since [(HZnOt-Bu)4] only showed a very low hydride
transfer (reduction) activity of CO2 at Zn–H sites whereas
[(HZnOt-Bu)3(LiOt-Bu)] reduces CO2 at atmospheric pressure
University of Duisburg-Essen, Universitatsstr. 5-7, S07 S03 C30,
¨
45117 Essen, Germany. E-mail: stephan.schulz@uni-due.de;
Fax: +49 201 1833830; Tel: +49 201 1834635
w Electronic supplementary information (ESI) available: Experimental
procedure, characterization of 2, 3, and 4, IR spectroscopic studies as
well as computational calculations. CCDC 776239 (2) and 776240 (4).
For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c0cc01329c
Scheme 1 Synthesis of 2 and 4.
c
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1.986(2) A), whereas the Zn–O distances within the
mꢀZ1-bridged unit differ by almost 0.04 A (Zn1–O1 2.039(3);
Zn2–O2 1.995(3) A). Comparable structural motifs with
two metal centers bridged by two formato groups with
as-described coordination modii have been previously
observed.14 The C–O distances within the formato groups
differ significantly. The bridging bidentate moiety (CO1–O1
1.233(4), CO2–O2 1.236(4) A) almost shows equal C–O bond
lengths whereas the bridging monodentate unit exhibits a
long (CO2–O3 1.332(5) A) and
a short C–O bond
(CO2–O4A 1.165(7) A) indicating a terminal CQO double
bond. Both the terminal O-atom and the H-atom are
disordered over two sites (SOF 0.5). The C3N2Zn rings in 2
are almost planar with the Zn atoms slightly out of the plane
as was observed in [MesnacnacZn(m-H)]2 and the C–C, C–N
and Zn–N bond lengths within the rings are almost identical
compared to the starting zinc hydride complex.11
In order to evaluate the reactivity of the zinc hydride in
more detail, reactions of 1 with heterocumulenes such as
(i-PrN)2C and t-BuNCO were investigated. These reactions
were also performed at ambient temperature and resulted in an
almost quantitative formation of the corresponding formamido
(3) and formamidinato (4) complexes, respectively.
Fig. 1 Solid state structure of 2; H atoms are omitted for clarity.
formiate groups. 2 shows a difference of 99 cmꢀ1, which points
to a bridging/chelating mode as observed in the solid state
structure (Fig. 1). No further symmetrical or asymmetrical CO
absorption bands could be identified; however, they might be
overlapped by absorption bands of the diketiminato group as
a comparison of 2 with the starting hydride showed (see ESIw).
The molecular structure of 2 in the solid state was investi-
gated by single crystal X-ray diffraction. 2 crystallizes in the
Both reactions were monitored by H NMR spectroscopy.
1
As was observed for the reaction with CO2, the Zn–H
resonance of 1 steadily decreased over a period of 30 min
and new resonances at 7.91 (3) and 7.30 ppm (4) occurred.
Colorless crystals of 4 suitable for a single crystal structure
determination were obtained from a solution in n-hexane
(Fig. 2). 4 crystallizes in the monoclinic space group P21/c
with four molecules in the unit cell.
The formamidinato unit serves as chelating Z2-(NR)2CH
substituent with the Zn–N bond lengths significantly differing
(Zn1–N3 2.024(3); Zn1–N4 2.092(4) A), which most likely
indicates some sterical stress within this complex. The central
Zn atom adopts a distorted tetrahedral coordination sphere.
The endocyclic N1–Zn1–N2 bond angle (97.2(2)1) within the
six-membered C3N2Zn ring is significantly larger than the
N3–Zn1–N4 bond angle (65.7(2)1) within the four-membered
CN2Zn ring and the Zn–N bond lengths within the six-
membered ring are significantly shorter (Zn1–N1 1.968(4);
Zn1–N2 1.970(4) A) than in the four-membered ring
(Zn1–N3 2.024(3); Zn1–N4 2.092(4) A). The N–C bond
ꢀ
triclinic space group P1 with two molecules in the unit cell. 2
forms an unsymmetrically bridged dimer exhibiting two
different coordination modii of the formato unit. The two
Zn atoms are linked by two HCO2 units, with one formato
group serving as bridging bidentate (syn–syn) m-Z2-O2CH
ligand and the second group adopting
a
bridging
m-Z1-OC(O)H coordination mode. The presence of only a
single resonance for the formato moiety in the 1H NMR
spectrum of 2 points to the presence of a monomeric complex
in solution as was observed for the starting hydride complex.
Moreover, a pulsed gradient spin echo (PGSE) diffusion
measurement of a solution of 2 in C6D6 at 25 1C yields a
hydrodynamic radius of 5.48 A, which is very similar to
that observed for monomeric MesnacnacZnH in [D8]THF
(5.26 A), also indicating 2 to be monomeric in solution.
Unfortunately, temperature-dependent 1H NMR spectro-
scopic studies were hindered by the extremely low solubility
of 2 in solvents such as [D8]THF and [D8]Toluene at temperatures
below ꢀ10 1C.
DFT calculations were performed to evaluate the relative
stabilities of monomeric versus dimeric 2. However, the
unsymmetrically bridged form as was observed experimentally
doesn’t represent an energy minimum. Instead, the symmetrically
bridged dimer containing two m-Z2-O2CH groups is energeti-
cally favored by 24.2 kcal molꢀ1 compared to the monomer
(see ESIw).
The Zn–O bond lengths within the bridging bidentate
moiety are almost identical (Zn1–O3 1.971(2); Zn2–O3
Fig. 2 Solid state structure of 4; H atoms are omitted for clarity.
Chem. Commun., 2010, 46, 7226–7228 7227
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lengths of the formamidinato group (C24–N3 1.314(6);
C24–N4 1.311(6) A) indicate perfect delocalization of the
p-electrons. As was observed for 2, the Zn atom in the
C3N2Zn ring in 4 is slightly out of the plane and the C–C,
C–N and Zn–N bond lengths within the rings again are almost
identical compared to those of the starting zinc hydride
complex.
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MesnacnacZnH easily reacts with heterocumulenes under
very mild reaction conditions and its reactivity is more
pronounced compared to the corresponding organo-
magnesium hydride [DippnacnacMgH]2, which was found to
react with cyclohexyl carbodiimide C(NCy)2 with insertion
into the Mg–H bond whereas the reaction with t-BuNCO only
gave a mixture of unidentified products.15 Harder et al.
recently reported on the reactions of an organocalcium
hydride [DippnacnacCaH(thf)]2 with several unsaturated
substrates, but unfortunately no reaction with CO2, C(NR)2
or RNCO were reported.16 In contrast, Holland et al. very
recently reported on reactions of b-diketiminato stabilized
iron hydrides with both CO2 and C(Ni-Pr)2. These reactions
also yielded a formamidinato complex with a chelating
Z2-(NR)2CH unit, whereas the formato complexes showed
two bridging bidentate m-O2CH ligands.17
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heterocumulenes under very mild reaction conditions with
insertion into the Zn–H bond. Further studies on its reactivity
toward other small molecules including other heterocumulenes
as well as C–C and C–X multiple bonded species are currently
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