Convenient synthesis and crystal structure of a monomeric zinc hydride complex with a three-coordinate metal center

Article abstract of DOI:10.1039/b906319f

Convenient syntheses for a β-diketiminate zinc hydride complex, that crystallizes as a monomer with a three-coordinate Zn atom, have been developed.

Full text of DOI:10.1039/b906319f

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Convenient synthesis and crystal structure of a monomeric zinc hydride
complex with a three-coordinate metal centerw
Jan Spielmann, Dirk Piesik, Bernd Wittkamp, Georg Jansen and Sjoerd Harder*
Received (in Cambridge, UK) 31st March 2009, Accepted 8th April 2009
First published as an Advance Article on the web 7th May 2009
DOI: 10.1039/b906319f
Convenient syntheses for a b-diketiminate zinc hydride complex,
that crystallizes as a monomer with a three-coordinate Zn atom,
have been developed.
as a dimer with bridging hydride atoms (Zn–H = 1.766 A).2
The ionic radius of Zn2+ (four-fold coordination: 0.74 A) is
slightly larger than that for Mg2+ (four-fold coordination:
0.71 A).6 However, the complex (DIPP-nacnac)MgH is a
strongly bound dimer with a Mg–H bond distance of 1.96(3) A,7
i.e. considerably longer than the Zn–H bond distance of
1.766 A in [(Mes-nacnac)ZnH]2.2 The magnesium hydride
complex crystallizes even in the presence of THF as a
dimeric solvate [(DIPP-nacnac)MgHꢀTHF]2.7 This difference in
structural behaviour is undoubtedly related to the significantly
more covalent character of Zn complexes. The covalent radius
for Zn (1.22 A) is indeed smaller than that for Mg (1.41 A).8
A comparison of the related complexes (DIPP-nacnac)MgN-
(SiMe3)2 and (DIPP-nacnac)ZnN(SiMe3)2, which crystallize
isomorphously,9 shows that the bonds to Mg are on average
4.5% longer than those to Zn.
Despite the increasing interest in the use of zinc hydride
complexes as reducing agents, models for zinc–enzymes or as
possible precursors in material chemistry,1 only a few well-
defined examples are known. Recently, the first b-diketiminate
zinc hydride,2 [(Mes-nacnac)ZnH]2 (Mes-nacnac
=
CH{(CMe)(2,6-Me2C6H3N)}2) was prepared by reacting the
corresponding zinc fluoride complex with Et3SiH, a method
that was earlier introduced to prepare the scorpionate complex
Tpp-Tol,MeZnH.3
Our recent studies on the mechanism of hydrogen release
in alkaline-earth metal amidoborane complexes,4 directed
our interest to investigations of related zinc amidoborane
complexes. In one of our synthetic approaches we have reacted
the potassium amidoborane, KN(iPr)HBH3, which was
obtained by deprotonation of H2N(iPr)BH3 with KN(SiMe3)2,
The crystal structure of monomeric (DIPP-nacnac)ZnH is
reproduced well by DFT-calculations on the B97-D/TZV2P-
level (Zn–N 1.996 A; Zn–H 1.548 A). At the same level of
theory, however,
a minimum was also found for the
with (DIPP-nacnac)ZnCl (DIPP-nacnac
= CH{(CMe)-
dimer [(DIPP-nacnac)ZnH]2 (Zn–N 2.034 A; Zn–H 1.784 A)
which is even more stable than the monomer by
9.7 kcal molꢁ1 (corrected for ZPE). Similarly, calculations on
[(Mes-nacnac)ZnH]2 reproduce its dimeric crystal structure
(Zn–N 1.994 A; Zn–H 1.768 A). In this case, the slightly
reduced ligand size results in a considerable preference for the
dimer (19.8 kcal molꢁ1).
(2,6-iPr2C6H3N)}2) in THF (Scheme 1). This simple salt
metathesis reaction, however, did not give the expected zinc
amidoborane but instead the zinc hydride (DIPP-nacnac)ZnH
could be isolated in good crystalline yield (71%).z Clean
formation of this metal hydride is likely to proceed through
b-hydride elimination and formation of various oligomeric
‘‘BN’’-species (many signals could be seen in the NMR
spectra).
As the calculation study indicates that, under gas phase
conditions at 0 K, dimeric [(DIPP-nacnac)ZnH]2 is preferred
over the monomeric species, we investigated the complex
dissolved in toluene-d8 with low-temperature 1H NMR
spectroscopy. Gradually cooling the sample to ꢁ80 1C did
The crystal structure of (DIPP-nacnac)ZnHy revealed a
monomeric complex of approximate C2v symmetry (Fig. 1).
The hydride hydrogen atom has been located and was
isotropically refined. The Zn–H bond distance of 1.46(2) A
is one of the shortest reported so far and equals that of 1.46(5) A
in [(Me3PN)ZnH]4, a cubane-like tetramer with bridging N
centers and terminal hydride functionalities.5
As the sum of the valence angles at the three-coordinate Zn
center equals 360.0(7)1, the arrangement of the ligands around
this metal center is perfectly planar. This trigonal coordination
sphere is only distorted by the acute N1–Zn–N2 angle of
97.33(4)1, which is dictated by the chelating DIPP-nacnac ligand.
At first sight, the monomeric nature of (DIPP-nacnac)ZnH
is surprising. The closely related (Mes-nacnac)ZnH crystallized
Fachbereich Chemie, Universitatsstrasse 5, 45117 Essen, Germany.
¨
E-mail: sjoerd.harder@uni-due.de; Fax: +49 201 1832621;
Tel: +49 201 1833684
w Electronic supplementary information (ESI) available: Details for
experimental and calculation work. CCDC 724884. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/b906319f
Scheme 1
ꢂc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 3455–3456 | 3455

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advantages arise from their easy handling and safe storage
without deterioration. Noyori et al. discovered that the
combination of basically inert LiH and catalytic amounts of
ZnCl2 can be used as a reagent in reduction chemistry.12
Additionally, a recent report appeared on the use of the
CaH2–ZnBr2 combination in the reduction of imines.13 This
prompted us to react a solution of (DIPP-nacnac)ZnCl in
THF with commercially available CaH2, the cheapest
available metal hydride. We found quantitative conversion
of the chloride to (DIPP-nacnac)ZnH (Scheme 1) and the raw
product is essentially pure.
The convenient synthesis and the good accessibility of the
hydride functionality in (DIPP-nacnac)ZnH encourages
further exploration of this complex.
Notes and references
Fig. 1 The crystal structure of (DIPP-nacnac)ZnH (ORTEP plot
with 50% probability; overlay with van der Waals radii). Selected
bond distances (A) and angles (1): Zn–N1 1.950(1); Zn–N2 1.952(1);
Zn–H 1.46(2); N1–Zn–N2 97.33(4); N1–Zn–H 131.5(7); N2–Zn–H
131.2(7).
z For (DIPP-nacnac)ZnH: mp 211 1C. Anal. Calc. (Found) for
C29H42N2Zn: C 71.96 (71.89); H 8.75 (8.94)%. 1H NMR (C6D6,
3
3
300 MHz): d 1.11 (d, J(H,H) = 6.9 Hz, 12H, iPr), 1.27 (d, J(H,H)
6.9 Hz, 12H, iPr), 1.68 (s, 6H, Me backbone), 3.18 (sept, J(H,H)
=
=
3
6.9 Hz, 4H, iPr), 4.39 (s, 1H, ZnH), 5.02 (s, 1H, H backbone), 7.12 (m, 6H,
aryl). 13C NMR (C6D6, 75 MHz): 23.2 (iPr-Me), 23.5 (Me backbone),
24.8 (iPr-Me), 28.4 (iPr-CH), 96.0 (CH backbone), 123.9 (aryl), 126.2
(aryl), 141.8 (aryl), 144.6 (aryl), 168.1 (CMe backbone).
not give rise to the appearance of another set of signals
that could be assigned to a dimeric species. However, a
significant shift of the signals for the hydride (Dd = 0.31 ppm)
and the iPr-methyl group directed towards the hydride
(Dd = 0.09 ppm) was observed, i.e. those groups most affected
by possible dimer formation (all other signals hardly shifted).
This indicates a possible monomer–dimer equilibrium which
at higher temperatures shifts to the monomer side on account
of entropy. From pulsed gradient spin echo (PGSE) diffusion
measurements10 on a solution of (DIPP-nacnac)ZnH in
toluene-d8 at 20 1C, a hydrodynamic radius of 5.2(1) A
could be deduced. As measurements on monomeric
(DIPP-nacnac)ZnMe gave an equal hydrodynamic radius,
(DIPP-nacnac)ZnH in toluene at 20 1C is largely monomeric.
(DIPP-nacnac)ZnH is the first monomeric zinc hydride
y Crystal data for (DIPP-nacnac)ZnH: C29H42N2Zn, M = 484.04,
monoclinic, space group P21/n, a = 12.639(3), b = 15.856(3),
c = 14.020(3) A, b = 105.528(10)1, U = 2707.1(10) A3, Z = 4,
Dc = 1.188 g cmꢁ3, F(000) = 1040, m(Mo Ka) = 0.925 mmꢁ1
,
3.8
r
2y
r
61.01, 50 265 reflections measured, 7976 unique
(Rint = 0.045) were used in all calculations, R1 = 0.0278 [I 4 2s(I)]
and wR2 = 0.0777 (all data); max/min. residual electron density:
0.41/ꢁ0.35.
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complexes with a terminal Zn–H functionality show four-fold
coordination at Zn and the only other zinc hydride complex
with a three-coordinate Zn center is a dimer with bridging
hydride atoms: [(2,6-DIPP-phenyl)ZnH]2.11
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The monomeric nature of (DIPP-nacnac)ZnH and its low
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precursor in reduction chemistry, catalysis, material chemistry
or for mimicking zinc–enzyme processes. Therefore we
developed an even more attractive synthetic route for this
potentially useful complex.
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ZnCl and a metal hydride would be preferable over both the
amidoborane, and the zinc fluoride/silane routes.2,3 In such
syntheses highly reactive metal hydrides like NaH or KH are
usually used. In recent years, however, there has been an
increased interest in implementing cheaper, more inert, metal
hydrides like LiH and CaH2. Both hydrides show extremely
high lattice energies and do not react with a large variety of
functional groups (hence the application of CaH2 as a versatile
drying agent for many organic liquids). Other operational
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This journal is The Royal Society of Chemistry 2009
3456 | Chem. Commun., 2009, 3455–3456