Two-coordinate terminal zinc hydride complexes: Synthesis, structure and preliminary reactivity studies

Article abstract of DOI:10.1039/c6cc05445e

The first examples of essentially two-coordinate, monomeric zinc hydride complexes, LZnH (L = -N(Ar)(SiR₃)) (Ar = C₆H₂{C(H)Ph₂}₂R′-2,6,4; R = Me, R′ = Prⁱ (L′); R = Prⁱ, R′ = Me

Full text of DOI:10.1039/c6cc05445e

ChemComm
View Article Online
View Journal
COMMUNICATION
Two-coordinate terminal zinc hydride complexes:
synthesis, structure and preliminary reactivity
studies†
Cite this:DOI: 10.1039/c6cc05445e
Received 30th June 2016,
Accepted 2nd August 2016
a
a
b
a
Michael J. C. Dawkins, Ewart Middleton, Christos E. Kefalidis, Deepak Dange,
a
b
a
Martin M. Juckel, Laurent Maron* and Cameron Jones*
DOI: 10.1039/c6cc05445e
www.rsc.org/chemcomm
The first examples of essentially two-coordinate, monomeric zinc
0
hydride complexes, LZnH (L = –N(Ar)(SiR
3
)) (Ar = C
6
H
2
{C(H)Ph
2
i
}
2
R -
0
i
0
i
0
i
0
†
2
,6,4; R = Me, R = Pr (L ); R = Pr , R = Me (L*); R = Pr , R = Pr (L ))
have been prepared and shown by crystallographic studies to have
near linear N–Zn–H fragments. The results of computational studies
imply that any PhꢀꢀꢀZn interactions in the compounds are weak at best.
Preliminary reactivity studies reveal the compounds to be effective
for the stoichiometric hydrozincation and catalytic hydrosilylation
of carbonyl compounds.
A key area of study in the field of main group/post transition
metal organometallic chemistry is the realisation of catalytic
regimes which achieve synthetic transformations normally the
Fig. 1 Examples of catalytically active terminal zinc hydride complexes
Dip = 2,6-diisopropylphenyl, Mes = mesityl, Py = 2-pyridyl).
(
1
preserve of expensive and toxic heavy transition metal systems.
Within this realm, terminal zinc hydride complexes are emerging not been observed to date, even when extremely bulky ligands at
as both novel synthetic targets, and as efficient catalysts for a zinc are employed. A case in point here are Power’s terphenyl
0
0
i
2
variety of processes. These processes include the hydrosilylation substituted zinc hydrides, e.g. {Ar Zn(m-H)} (Ar = C H (C H Pr -
2
6
3
6
3
or hydrogenation of unsaturated substrates (e.g. CO , ketones, 2,6) -2,6), which exist as three-coordinate hydride bridged dimers
2
2
2
3
aldehydes, imines and nitriles), and the alcoholysis of silanes.
in the solid state. It occurred to us that we might be able to
Several mechanistic pathways have been proposed for these circumvent dimerisation of related LZnH units by incorporation
transformations, which involve, for example, the Zn–H moiety of ‘‘super bulky’’ amide ligands, e.g. –N(Ar)(SiR
3
) (Ar =
0
0
i
0
i
0
participating in substrate insertion/s-bond metathesis chemistry,
C
6
H
2
{C(H)Ph
2
}
2
R -2,6,4; R = Me, R = Pr (L ); R = Pr , R = Me
2e
i
0
i
†
4
or the Lewis acid activation of substrate or silane. All of these (L*); R = Pr , R = Pr (L )) developed in our group. In this
pathways are reliant on the electrophilicity of the zinc centre of respect, we have shown these amides to be more sterically
the catalyst, which suggests that lower coordinate terminal zinc imposing than substituted terphenyl ligands, and have successfully
hydride complexes maybe more catalytically active than currently utilised them for the kinetic stabilisation of an array of unprece-
2
available three- and higher coordinate systems, e.g. 1–4 (Fig. 1).
dented low-coordinate s-, p- and d-block metal–metal bonded
5
6
Clearly, in order to access monomeric, two-coordinate zinc systems, and p-block metal hydride complexes. Most relevant
hydride complexes of the type, LZnH, sterically bulky monodentate to this study are the recently reported zinc(I) dimers and mixed
anionic ligands (L) would be required. However, such systems have valence tri-zinc systems, LZn–ZnL and LZn–Zn–ZnL (L = bulky
amide), which were shown to be devoid of bridging hydride
5b
a
ligands. Here we report the first examples of essentially two-
coordinate, monomeric zinc hydride complexes, and describe pre-
liminary investigations into their use for the stoichiometric and
catalytic transformation of carbonyl compounds.
Monash Centre for Catalysis, School of Chemistry, Monash University, PO Box 23,
VIC, 3800, Australia. E-mail: cameron.jones@monash.edu
b
Universit ´e de Toulouse et CNRS, INSA, UPS, UMR 5215, LPCNO, 135 Avenue de
Rangueil, F-31077 Toulouse, France. E-mail: laurent.maron@irsamc.ups-tlse.fr
Electronic supplementary information (ESI) available: Full details and references
†
Initial attempts to prepare zinc hydride complexes involved
treatment of amido zinc alkyls with silanes or boranes, though
for synthetic, crystallographic and computational studies. CCDC 1488963–1488971.
For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c6cc05445e
7
this route ultimately proved unsuccessful. Subsequently, three
This journal is ©The Royal Society of Chemistry 2016
Chem. Commun.

View Article Online
Communication
ChemComm
Fig. 2 Molecular structure of 5 (25% thermal ellipsoids; hydrogen atoms,
except H(1), omitted).
0
0
Scheme 1 Preparation of compounds 5–7, L CdCdL and L*HgHgL*.
i) NaH(excess), –NaBr; (ii) K[HBEt ], –H . –KI, –BET
1
1
(
3
2
3
.
range observed for reported terminal zinc hydride complexes,
and is close to the sum of the covalent radii for Zn and H
(
1
2
1.53 Å). Although the Zn–H distance for 5 was restrained, the
previously reported amido zinc bromide compounds were reacted N–Zn–H angles for both complexes were freely refined. This
with an excess of NaH, which led to good isolated yields of the revealed them to be close to linear (5 171(2)1, 7 167(2)1;
monomeric, terminal zinc hydride complexes, 5–7, upon work-up cf. 126.2(1)1 for three-coordinate ZnH{N(SiMe
3
)
2
}(IPr), IPr =
), which provides further evidence for the
one structurally authenticated example of a terminal cadmium essentially two-coordinate nature of the compounds (cf. linear
8,9
i
2c
6 3 2 2
(Scheme 1). Given this result, and considering that there is only C{N(C H Pr -2,6)}
3
13
hydride complex, and no reports of terminal mercury hydrides, H–Zn–H ). Moreover, there are no CPhꢀ ꢀ ꢀZn separations in
P
0
the heavier group 12 iodide compounds, L CdI and L*HgI, were either compound that are o2.95 Å (N.B.
treated with excess NaH. However, these reactions led to deposi- C and Zn = 1.95 Å ), though the large steric bulk of the amide
tion of the metal involved and formation of the secondary amines, ligand is the likely cause of the slight deviations from linearity
L H or L*H at ambient temperature. The same products resulted for the N–Zn–H fragments. In contrast, the presumably more
from the reactions of L CdI and L*HgI with K[HBEt
covalent radii for
1
2
0
0
3
] at ꢁ30 1C, Lewis acidic Zn centres in the precursor complexes, L*ZnBr and
0
0
†
but small amounts of the known metal(I) dimers, L CdCdL and L ZnBr, lead to weak CPhꢀ ꢀ ꢀZn interactions (closest ca. 2.85 Å)
5b
5b
L*HgHgL*, were also isolated. These dimers probably result from and narrower angles at the metal centre (ca. 1591). Importantly,
0
intermediate metal hydrides, L CdH and L*HgH, which are the structural characterisation of monomeric 5 and 7 confirms the
unstable to loss of H
2
under the reaction conditions employed absence of any bridging hydride ligands in our previously reported
5b
(cf. higher thermal stability of zinc hydride complexes). It is of zinc(I) dimers, e.g. L*ZnZnL*.
note that the formation of group 12 metal(I) dimers via metal(II)
hydride complexes has precedent in the literature.
The zinc hydride complexes 5–7 are all colourless crystalline details) were carried out on 5 in the gas phase (i.e. 5 ). The
solids that are stable to temperatures in excess of 180 1C in the geometry of the compound optimised to be very similar to that
solid state. The simplicity of their NMR spectra suggest they seen in the crystal structure of 5, while an natural population
In order to probe the electronic structure of 5–7, dispersion
3
corrected DFT calculations (oB97XD, see ESI† for computational
0
s
possess time averaged C symmetry in solution at 20 1C, with analysis (NPA) revealed considerable charge separation over the
the mirror plane including the SiCNZnH moiety. Interestingly, NZnH fragment (N ꢁ1.42, Zn 1.05, H ꢁ0.43). While the HOMO
1
their H NMR spectra all exhibit similar zinc hydride resonance and LUMO of the compound are largely ligand based, the
chemical shifts (d = 1.85–1.87 ppm) that, to the best of our HOMOꢁ11 and HOMOꢁ64 show character corresponding to
knowledge, are at higher field than those for any previously the Zn–H bond, and a N - Zn p-bonding interaction, of the
2
i
reported terminal zinc hydride complex (e.g. 1 d = 4.39 ppm;
2
compound, respectively (Fig. 3). Furthermore, the LUMO+15,
d = 5.60 ppm ). Compounds 5–7 display Zn–H stretching predominantly comprises an empty p-orbital at the Zn-centre.
bands in their infrared spectra that lie at similar frequencies No filled MO could be found that exhibited significant CPhꢀ ꢀ ꢀZn
2
h
ꢁ
1
(
n = 1892–1901 cm ), and which are at markedly higher bonding character, which is consistent with our description of
wavenumbers than found for all other terminal zinc hydrides, the compound as being essentially two-coordinate. It is also
ꢁ
1 2h
ꢁ1 2f
(e.g. 2 n = 1729 cm
,
3 n = 1734 cm
). This is as would be interesting to note that the calculated hydride chemical shift
ꢁ
1
expected for two-coordinate species.
(d = 1.03 ppm) and Zn–H stretching frequency (n = 1980 cm
)
0
X-ray crystal structures of 5 and 7 were obtained, and these for 5 are not dissimilar to the experimental values for 5–7, which
showed the complexes to be essentially isostructural (see Fig. 2 adds weight to the validity of the assignment of the unprecedentedly
1
0
for the molecular structure of 5). The hydride ligands of the high field and high wavenumber values for the latter.
compounds were located from difference maps and refined So as to investigate the reactivity of the two-coordinate zinc
isotropically. The Zn–H distance in 7 (1.50(5) Å) lies within the hydride complexes towards unsaturated substrates, compound 5
Chem. Commun.
This journal is ©The Royal Society of Chemistry 2016

View Article Online
ChemComm
Communication
zinc hydrides under the reaction conditions employed has so far
limited their catalytic potential. We are currently developing more
robust two-coordinate zinc hydride complexes, and will report on
our efforts in this direction in due course.
Notes and references
1
See for example (a) P. P. Power, Nature, 2010, 463, 171; (b) M. Asay,
C. Jones and M. Driess, Chem. Rev., 2011, 111, 354; (c) Catalysis
Without Precious Metals, ed. R. M. Bullock, Wiley, New York, 2010;
(
d) K. Revunova and G. I. Nikonov, Dalton Trans., 2015, 44, 840.
2
Selected recent examples (a) A. Kreider-Mueller, P. J. Quinlivan,
M. Rauch, J. S. Owen and G. Parkin, Chem. Commun., 2016, 52, 2358;
(
b) W. Sattler, S. Ruccolo, M. R. Chaijan, T. N. Allah and G. Parkin,
Organometallics, 2015, 34, 4717; (c) A. J. Roberts, W. Clegg,
A. R. Kennedy, M. R. Probert, S. D. Robertson and E. Hevia, Dalton
Trans., 2015, 44, 8169; (d) P. A. Lummis, M. R. Momeni, M. W. Lui,
R. McDonald, M. J. Ferguson, M. Miskolzie, A. Brown and E. Rivard,
Angew. Chem., Int. Ed., 2014, 53, 9347; (e) C. Boone, I. Korobkov and
G. I. Nikonov, ACS Catal., 2013, 3, 2336; ( f ) P. Jochmann and
D. W. Stephan, Angew. Chem., Int. Ed., 2013, 52, 9831; (g) A. Rit,
T. P. Spaniol, L. Maron and J. Okuda, Angew. Chem., Int. Ed., 2013,
Fig. 3 (a) HOMOꢁ11, (b) HOMOꢁ64 and (c) LUMO+15 of 50.
5
1
2, 4664; (h) W. Sattler and G. Parkin, J. Am. Chem. Soc., 2011,
33, 9708; (i) J. Spielmann, D. Piesik, B. Wittkamp, G. Jansen and
was initially reacted with stoichiometric amounts of benzaldehyde
or trifluoroacetophenone in a J. Young’s NMR tube. These reactions
were complete within seconds under ambient conditions, and
NMR analyses of the resultant mixtures showed near quantita-
S. Harder, Chem. Commun., 2009, 3455.
3
4
Z. Zhu, M. Brynda, R. J. Wright, R. C. Fischer, W. A. Merrill, E. Rivard,
R. Wolf, J. C. Fettinger, M. M. Olmstead and P. P. Power, J. Am. Chem.
Soc., 2007, 129, 10847.
(a) E. W. Y. Wong, D. Dange, L. Fohlmeister, T. J. Hadlington and
C. Jones, Aust. J. Chem., 2013, 66, 1144; (b) J. Hicks, T. J.
Hadlington, C. Schenk, J. Li and C. Jones, Organometallics,
0
0
tive conversions to L Zn(OCH
respectively. It is of note that a single crystal of {L Zn(O =
2
Ph) and L Zn{OCH(Ph)(CF
3
)},
0
2
013, 32, 323; (c) J. Li, A. Stasch, C. Schenk and C. Jones, Dalton
2 2
CHPh)(m-OCH Ph)} was isolated from the former reaction and
Trans., 2011, 40, 10448.
found to be a benzaldehyde coordinated, alkoxide bridged dimer,
as determined by an X-ray crystallographic analysis (see ESI†).
This compound presumably formed due to the presence of a
slight excess of benzaldehyde in the reaction.
5
See for example (a) A. J. Boutland, D. Dange, A. Stasch, L. Maron and
C. Jones, Angew. Chem., Int. Ed., 2016, 55, 9239; (b) J. Hicks,
E. J. Underhill, C. E. Kefalidis, L. Maron and C. Jones, Angew. Chem.,
Int. Ed., 2015, 54, 10000; (c) J. Hicks, C. E. Hoyer, B. Moubaraki,
G. L. Manni, E. Carter, D. M. Murphy, K. S. Murray, L. Gagliardi and
C. Jones, J. Am. Chem. Soc., 2014, 136, 5283; (d) T. J. Hadlington and
C. Jones, Chem. Commun., 2014, 50, 2321; (e) T. J. Hadlington,
M. Hermann, J. Li, G. Frenking and C. Jones, Angew. Chem., Int.
Ed., 2013, 52, 10389; ( f ) J. Li, C. Schenk, C. Goedecke, G. Frenking
and C. Jones, J. Am. Chem. Soc., 2011, 133, 18622.
With a view to test our proposal that 5–7 should be more
catalytically competent than their higher coordinate counterparts,
2
e
comparisons of the activity of 7 (5 mol%) and 1 (3 mol%)
towards the hydrosilylation of benzaldehyde and acetophenone,
using triethoxysilane, were carried out. While moderate yields of
the expected silylether products were obtained (47% and 75%
respectively) using 7 as a catalyst, higher yields were previously
6
7
See for example (a) T. J. Hadlington, B. Schwarze, E. I. Izorodina and
C. Jones, Chem. Commun., 2015, 51, 6854; (b) T. J. Hadlington,
M. Hermann, G. Frenking and C. Jones, J. Am. Chem. Soc., 2014,
136, 3028.
2e
N.B. During the course of this work, the two-coordinate zinc alkyl,
reported for highly active 1 (viz. 498% and 84% respectively).
0
0
00
t
3 6 2 2 2
L ZnEt (L = N(Ar){Si(OBu )}, Ar = C H {C(H)Ph } Me-2,6,4) was
Moreover, catalytic transformations using 1 were more rapid than
those employing 7, presumably because intermediates in the
catalytic cycle involving the latter system showed considerable
decomposition during the catalytic runs (see ESI† for full details).
Similar results were obtained for the hydroboration of the same
substrates using HBpin and 7 (5 mol%).
spectroscopically and crystallographically characterised (see ESI† for
details).
00
8 N.B. Two new amido-zinc bromide complexes, viz. L ZnBr and
BoN)(Me Si)NZnBr(THF) (BoN = B(DipNCH) ), were prepared and
(
3
2
characterised (see ESI†), though their treatment with excess NaH led
to intractable product mixtures.
9 N.B. During one preparation of L*ZnH, several crystals of {L*Zn(m-OH)}
were isolated and structurally characterised (see ESI†). The compound
presumably arose due to traces of adventitious water in the reaction
mixture.
2
In conclusion, the first examples of essentially two-coordinate
zinc hydride complexes have been prepared and shown by crystallo-
graphic studies to have near linear N–Zn–H fragments. The results 10 N.B. The crystal structure of 6 was acquired, but the compound was
shown to co-crystallise with small amounts (ca. 2%) of L*ZnBr (see
of computational studies imply that any PhꢀꢀꢀZn interactions in the
ESI†). Hence, no discussion on the geometry of the NZnH fragment
of 6 is given here.
compounds are weak at best, thus confirming the low-coordinate
nature of these species. Preliminary reactivity studies revealed the 11 As determined from a survey of the Cambridge Crystallographic
Database, June, 2016.
2 B. Cordero, V. Gomez, A. E. Platero-Prats, M. Reves, J. Echeverria,
E. Cremades, F. Barragan and S. Alvarez, Dalton Trans., 2008, 2832.
compounds to be very active towards the hydrozincation of alde-
hydes and ketones. However, when used as catalysts for the hydro-
1
silylation of the same substrate classes, the limited stability of the 13 S. Aldridge and A. J. Downs, Chem. Rev., 2001, 101, 3305.
This journal is ©The Royal Society of Chemistry 2016
Chem. Commun.