Nickel as a Lewis Base in a T-Shaped Nickel(0) Germylene Complex Incorporating a Flexible Bis(NHC) Ligand

Article abstract of DOI:10.1002/anie.201809889

Flexible, chelating bis(NHC) ligand 2, able to accommodate both cis- and trans-coordination modes, was used to synthesize (2)Ni(η²-cod), 3. In reaction with GeCl₂, it produced (2)NiGeCl₂, 4, featuring a T-shaped Ni⁰ and a pyramidal Ge center. Complex 4 could also be prepared from [(2)GeCl]Cl, 5, and Ni(cod)₂, in a reaction that formally involved Ni–Ge transmetalation, followed by coordination of the extruded GeCl₂ moiety to Ni. A computational analysis showed that 4 possesses considerable multiconfigurational character and the Ni→Ge bond is formed through σ-donation from the Ni 4s, 4p, and 3d orbitals to Ge. (NHC)₂Ni(cod) complexes 9 and 10, as well as (NHC)₂GeCl₂ derivative 11, incorporating ligands that cannot accommodate a wide bite angle, failed to produce isolable Ni–Ge complexes. The isolation of (2)Ni(η²-Py), 12, provides further evidence for the reluctance of the (2)Ni⁰ fragment to act as a σ-Lewis acid.

Full text of DOI:10.1002/anie.201809889

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Accepted Article
Title: Nickel as a Lewis Base in a T-Shaped Nickel(0) Germylene
Complex Incorporating a Flexible Bis(NHC) Ligand
Authors: Chris Gendy, Akseli Mansikkamäki, Juuso Valjus, Joshua
Heidebrecht, Paul Chuk-Yan Hui, Guy M. Bernard, Heikki M.
Tuononen, Roderick E. Wasylishen, Vladimir K. Michaelis,
and Roland Roesler
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201809889
Angew. Chem. 10.1002/ange.201809889
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COMMUNICATION
Nickel as a Lewis Base in a T-Shaped Nickel(0) Germylene
Complex Incorporating a Flexible Bis(NHC) Ligand
[
a]
[b]
[b]
[a]
[a]
Chris Gendy, Akseli Mansikkamäki, Juuso Valjus, Joshua Heidebrecht, Paul Chuk-Yan Hui,
[c]
[b]
[c]
[c]
Guy M. Bernard, Heikki M. Tuononen, Roderick E. Wasylishen, Vladimir K. Michaelis, and
Roland Roesler*[a]
Abstract: Flexible, chelating bis(NHC) ligand 2, able to
mostly the related oxidative addition reactions leading to Ni(I)
accommodate both cis- and trans-coordination modes, was used to
and Ni(II) complexes.11 While L
2
Pt(0) moieties have been shown
2
12
synthesize (2)Ni(η -cod), 3. In reaction with GeCl
2
, this produced
to act as lone pair donors in a variety of complexes, there is no
(
2)NiGeCl
Complex 4 could also be prepared from [(2)GeCl]Cl, 5, and Ni(cod)
in a reaction that formally involved Ni-Ge transmetalation, followed
by coordination of the extruded GeCl moiety to Ni. A computational
2
, 4, featuring a T-shaped Ni(0) and a pyramidal Ge center. record of the lighter Ni congeners being able to emulate this
2
,
binding mode.
2
analysis showed that 4 possesses considerable multiconfigurational
character and the Ni→Ge bond is formed through σ-donation from
2
the Ni 4s, 4p, and 3d orbitals to Ge. (NHC) Ni(cod) complexes 9 and
1
0, as well as (NHC) GeCl derivative 11, incorporating ligands that
2
2
cannot accommodate a wide bite angle, failed to produce isolable
2
Ni-Ge complexes. The isolation of (2)Ni(η -Py), 12, provides further
Scheme 1. Synthesis of ligand 2 and its complex 3.
evidence for the reluctance of the (2)Ni(0) fragment to act as a σ-
Lewis acid.
Aiming to investigate the Lewis acid-base chemistry of
NHC) Ni(0) species, we designed the flexible, chelating
2
dicarbene ligand 2 that could stabilize both the linear Ni(0)
starting material and its tri- and tetracoordinated reaction
products expected to feature narrower CNHC-Ni-CNHC angles. It
The Lewis basicity of transition metals plays a key role in the
formation of their complexes.1 This includes the concepts of
(
2
back bonding, as well as Z-type ligands and metal-only Lewis
3
pairs for the most basic metals, with the latter leading to metal-
4
only frustrated Lewis pairs. In agreement with the increase in
was prepared from 1 (Scheme 1), and its reaction with Ni(cod)
2
basicity for the heavier transition elements,1, 5 little has been
reported so far on the chemistry of nickel Lewis bases,3,6 in stark
contrast to platinum.3 Among the most basic compounds of
nickel are the dicoordinate Ni(0) complexes, which are linear
and highly reactive. 7 As opposite to Pd(0) and Pt(0), where
dicoordinate complexes with phosphine and N-heterocyclic
afforded
η -cyclooctadiene
the
pentane-soluble,
complex 3.
thermally
sensitive
2
2
(NHC) Ni(η -cod) and
2
4
(
NHC)
2
Ni(η -cod) complexes have been used extensively as
Ni. 13 The NMR analysis of
precursors in lieu of (NHC)
2
diamagnetic derivative 3 was hindered by broad, poorly resolved
resonances between 25 and -65 ºC. It was postulated that this
was a consequence of conformational fluxionality involving
flipping of the siloxane backbone and the reciprocal
arrangement of the Dipp (2,6-diisopropylphenyl) substituents. A
solid-state 13C NMR spectrum featured four resonances
2
carbene (NHC) ligands are common, L Ni(0) complexes were
isolated exclusively with bulky NHC substituents, even though
the phosphine analogs are commonly postulated active species
in Ni(0)-Ni(II) catalytic cycles.8,9 (NHC)
Ni(0) derivatives proved
2
to be proficient precatalysts in a number of bond-forming
transformations, 10 and their stoichiometric chemistry targeted
2
corresponding to Me Si, and two resonances corresponding to
the coordinated carbene carbons (200.2 and 212.5 ppm), as
expected when they are not related through site symmetry. It
mirrored the solution spectrum acquired at -65 ºC (Figure 1),
[
a]
C. Gendy, P. C.-Y. Hui, Prof. R. Roesler
Department of Chemistry
indicating that
a similar local chemical environment was
preserved on NMR time scale due to reduced conformational
fluxionality.
University of Calgary
2500 University Drive NW, Calgary, AB, T2N 1N4 Canada
E-mail: roesler@ucalgary.ca
[
b]
c]
Dr. A. Mansikkamäki, J. Valjus, Prof. H. M. Tuononen
Department of Chemistry, Nanoscience Centre
University of Jyvӓskylӓ
FI-40014 University of Jvӓskylӓ, Finland
Dr. G. M. Bernard, Prof. R. E. Wasylishen, Prof. V. K. Michaelis
Gunning-Lemieux Chemistry Centre
University of Alberta
X-ray diffractometry revealed that 3 had a Y-geometry at
and η2-1,5-cyclooctadiene
nickel with chelating ligand
2
completing the coordination sphere of the metal (Figure 2). The
C1-Ni1-C19 angle measures 110.40(1)º, and there is substantial
steric crowding around the metal (Figure S10), as well as an
anagostic interaction involving the backbone. The postulated
siloxane backbone ring flipping was modeled using DFT, and a
barrier of 90 kJ·mol-1 was found for swapping the orientations of
the NHC rings, accounting for the observed lack of time-
[
11227 Saskatchewan Drive NW, Edmonton, AB, T6G 2G2 Canada
Supporting information and the ORCID identification number(s) for
the author(s) of this can be found under:
s
averaged C -symmetry at low temperature in solution.
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Angewandte Chemie International Edition
10.1002/anie.201809889
COMMUNICATION
been documented for Ni(I),17 Ni(II),11d,18 and Ni(III),18b,c but so far
not for Ni(0), although it is ubiquitous for the isoelectronic
Cu(I).19
Figure 1. Selected regions from the solution and solid-state 13C NMR spectra
of 3.
Scheme 2. Synthesis of T-shaped Ni(0) complex 4.
Temperature-sensitive 4 was prepared via the reaction of 3
with GeCl (Scheme 2). Its solution NMR characterization was
2
also hindered by conformational equilibria leading to complex
spectra and broad resonances. The solid-state 13C NMR
spectrum featured resonances in the expected ranges, including
a prominent one corresponding to the carbene carbons at 182.2
ppm. An X-ray diffraction experiment revealed a (2)Ni-GeCl
2
structure featuring a T-shaped, tricoordinated nickel with a weak
(
(
<10 kJ·mol-1 by DFT) anagostic interaction trans to germanium
Figure 3). On undergoing the transformation from 3 to 4, the
bite angle of the chelating bis(NHC) ligand widens substantially,
from 110.42(13)º to 167.2(3)º. Both cis- and trans-binding
bis(NHC) complexes are known,14 and interconversions of the
two binding modes are known for non-chelating NHCs.15 The
-1
ring-flipping barrier in 4 was calculated to be only 50 kJ·mol .
Figure 3. Solid-state structure of 4 with 50% probability ellipsoids. Most
hydrogen atoms have been removed for clarity. Selected bond distances [Å]
and angles [º]: Ni1-Ge1 2.2854(11), Ni1-C 1.902(7), 1.884(7), Ge1-Cl 2.340(2),
2
9
.3363(18), Ni1···H 2.47(4), C1-Ni1-C19 167.2(3), C-Ni1-Ge1 92.96(19),
7.26(19), Cl1-Ge1-Cl2 94.69(4), Ge1-Ni1···H 172.8(9).
The singlet DFT solution of 4 showed symmetry breaking,
indicative of multiconfigurational character. Multireference QD-
NEVPT2//CASSCF calculations performed on a simplified model
system 4’ (see ESI) confirmed that the wave function consists of
three major configurations: a closed-shell Ni 3d10 configuration
9
(
21 %) and two open-shell Ni 3d configurations (26 and 21 %)
with a Ni-Ge bonding interaction. The nature of the metal-
metalloid bond can be understood by analyzing the fractionally
occupied natural orbitals of 4’ in terms of the fragments (2’)Ni(0)
and GeCl (Figure 4; see Figure S12 for a more complete
2
diagram). Altogether nine electrons occupy the five Ni3d orbitals
in 4’ (146-150), with the tenth electron distributed over three
orbitals (151-153) composed of the Ge4p orbital of GeCl (orbital
2
34) along with the bonding and antibonding combinations of C2p
and Ni4s-4p orbitals of (2’)Ni(0) (orbitals 118 and 119). Of these,
orbital 151 is clearly metal-metalloid bonding and, for the most
part, responsible of the stability of the complex 4’. Furthermore,
Figure 2. Solid-state structure of 3 with 50% probability ellipsoids. Most
hydrogen atoms have been removed for clarity. Selected bond distances [Å]
and angles [º]: Ni1-C 1.884(3), 1.911(3), Ni1···H 2.51(3), C1-Ni1-C19
110.42(13).
0
0
Very similar (Cy
3
P)
2
Pt -ECl
2
and (Cy
3
P)(IMes)Pt -ECl
2
the 3dz orbital of Ni (orbital 115) shows some delocalization
2
complexes (E = Ge, Sn, Pb; P-Pt-P = 156.90(3) - 163.62(2)º; C-
M-P = 165.11(7) - 166.37(7)º) have been reported.12 In these
analogues the metal-metalloid bond was described as featuring
mainly σ-electron donation from Pt.12b,d,16 The T-geometry has
onto Ge4p (orbital 34), giving rise to a minor bonding component
(orbital 148). Thus, two separate dative Ni→Ge interactions play
a role in stabilizing 4’. In contrast, the GeCl lone pair (orbital 33)
2
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COMMUNICATION
has only a minor influence on the metal-metalloid bond as back-
bonding interactions are virtually absent in 4’ (orbital 145).
described as a classic case of σ-bonding, π-backbonding with a
planar, σ-donating germylene.21 The acute P-Ni-P angles in the
latter compounds (113.81(4) and 118.10(5)º vs. C-Ni-C
167.2(3)º in 4) and the planar coordination environment at
germanium (vs. pyramidal with a Ni-Ge/GeCl angle of 66.2(3)º
2
in 4) strongly suggest a different Ni-Ge binding mode. N-
heterocyclic germylenes (NHGes) have lower Lewis acidity than
GeCl owing to N→Ge-π-donation, and it was of interest to
2
assess the impact of this factor on the Ni-Ge bonding. The
reaction of 3 with a neopentyl substituted NHGe22 resulted in
ligand displacement (Scheme 3) with formation of a homoleptic
4
Ni(NHGe) complex (Figure S6).
Scheme 3. Reaction of 3 with an N-heterocyclic germylene.
Germylene and stannylene complexes
5 and 6 were
prepared as shown in Scheme 4. The broad resonances in the
room temperature 1H NMR spectrum of 5 sharpened upon
cooling in a manner typical for conformational equilibrium in the
slow exchange regime.
A lineshape analysis yielded the
≠
-1
≠
-
activation parameters ΔH = 32.7 kJ·mol and ΔS = -96 J·mol
1·
K , suggesting an associative chloride exchange rather than
-1
2
3
ring flipping dynamics. Complex 5 crystallizes as an ion pair,
(2)GeCl]Cl (Figure S4), while 6 has a disphenoidal (2)SnCl
structure (Figure S6). Reaction of Ni(cod) with 5 led to
transmetalation where Ni(0) replaced GeCl followed by
[
2
2
2 2
Figure 4. Selected natural orbitals of (2’)Ni(0), 4’ = (2’)Ni→GeCl , and GeCl
2
,
with their occupancies.
coordination of the extruded metalloid fragment to Ni(0) to form
complex 4 (Scheme 2). This type of castling reaction relying on
the Lewis-amphoteric properties of germanium is to our
knowledge unprecedented.
The frontier orbitals of 4’ suggest a non-integer oxidation
state of +½ for Ni, assuming that the electrons in orbitals 151-
1
53 are distributed equally between Ni and Ge. However,
considering the relative importance of the 3d10 configuration on
the overall wave function and the lack of any significant
contribution from configurations in which the Ge4p orbital is
doubly occupied, the system is best approximated as a Ni(0)
complex in which the electrons forming the dative metal-
metalloid bond are exclusively from Ni. The preference of 4 for a
T-shaped geometry over the Y-shape usually observed in three-
coordinate Ni(0) complexes19 can be rationalized with the
bonding interaction in orbital 118 of (2’)Ni(0): any bending of the
C–Ni–C moiety would weaken the C-Ni bonds and,
consequently, destabilize also the metal-metalloid bond in 4’
Scheme 4. Synthesis of complexes 5 and 6.
To explore whether the ability of 2 to accommodate a wide
range of bite angles was essential to the chemistry reported
9
1
10
0
24
(
orbital 151). The importance of d s vs. d s electron
above, cis-chelators 7 and 8 were used to prepare nickel
complexes 913d and 10 (Figure S7), as well as germanium
derivative 11 (Scheme 5, Figure S8). Reactions of 9 or 10 with
configurations in the chemistry of nickel and its heavier
congeners has been discussed, 20 and the T-geometry for
tricoordinate d -nickel complexes, albeit in oxidation state I, is
well documented.17
9
GeCl
2 2
, and reaction of 11 with Ni(cod) produced intractable
mixtures of products. While the influence of the N-substituent
(
Dipp vs tBu) on the stability of compound 4 cannot be
The Ni-Ge bond in 4 (2.2854(11) Å) is slightly longer than
that recorded in the related, Y-shaped Ni(0) complexes
25
neglected, this result confirms that the ability of ligand 2 to
stabilize (NHC) Ni-GeCl species is exceptional.
2 2
(
Ph
3 2 2 3 2 3 3 6 2
P) Ni-GeX (X = [(N(SiMe ) ], 2,4,6-(CF ) C H ) (2.206(1)
and 2.1814(7) Å). The Ni-Ge interaction in these derivatives was
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COMMUNICATION
d9s1 electronic structure can play an important role in the
1
0
chemistry of metal complexes usually typecast as 3d .
Experimental Section
Scheme 5. Synthesis of derivatives 9-11.
Experimental details, NMR spectra, computational details, as well as the
relevant crystallography tables are provided in the Supporting Information.
CCDC 1861471-1861479 contain the supplementary crystallographic
data for this paper. These data are provided free of charge by The
Cambridge Crystallographic Data Centre.
The reaction of 3 with pyridine produced an equilibrium
mixture from which pyridine π-adduct 12 was isolated by
crystallization. The η -coordination mode has been observed for
2
L
2
Ni(0) species (L
=
R
3
P, NHC) with quinoline and N-
9
a, 26
a
coordinated pyridine.
coordination in 12 highlights the reluctance of the (NHC)
fragment to act as a σ-Lewis acid.26b
The preference for
π
vs. σ-
Ni(0)
Acknowledgements
2
Financial support for this work was provided by the Universities
of Calgary, Jyväskylä, and Alberta, as well as the NSERC of
Canada in the form of Discovery Grants #262037 to R.R and
#05447 to V.K.M, and a CGS-M scholarship to to J.H.. This
research was undertaken thanks in part to funding from the
Canada First Research Excellence Fund. Computational
resources were provided by CSC-IT Center for Science in
Finland and the Finnish Grid and Cloud Infrastructure (persistent
identifier urn:nbn:fi:research-infras-2016072533).
Keywords: N-heterocyclic carbene • nickel • metal-only Lewis
pairs • germanium • T-geometry
Figure 6. Solid-state structure of 12 with 50% probability ellipsoids. Most
hydrogen atoms have been removed for clarity. Selected bond distances [Å]
and angles [º]: Ni1-C1 1.922(3), Ni1-C19 1.915(3), Ni1···H 2.76(4), 2.82(3),
C1-Ni1-C19 116.28(15).
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[
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complex 4, which displays a T-geometry that is unprecedented
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pyridine adduct 12. Chelating ligands 7 and 8, which cannot
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germylenes due to their lower Lewis acidity. This work proves
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can parallel the remarkable reactivity previously observed only
for the more basic platinum analogues. It also shows that the
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Angewandte Chemie International Edition
10.1002/anie.201809889
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Castling reaction: A metal-only
Lewis pair was obtained via reaction
of either a Ni(0) bis(N-heterocyclic
Chris Gendy, Akseli Mansikkamäki
Juuso Valjus, Joshua Heidebrecht,
Paul Chuk-Yan Hui, Guy M. Bernard,
Heikki M. Tuononen, Roderick E.
Wasylishen, Vladimir K. Michaelis, and
Roland Roesler*
carbene) complex with GeCl
GeCl -bis(N-heterocyclic carbene)
adduct with Ni(cyclooctadiene) . The
2
, or a
2
2
complex Ni-Ge bonding situation is
best described as a one-electron
donation from the T-shaped metal to
the pyramidal metalloid. The near-
linear C-Ni-C arrangement proved
essential to the formation of the Ni-
Ge bond.
Page No. – Page No.
Nickel as a Lewis Base in a T-
Shaped Nickel(0) Germylene
Complex Incorporating a Flexible
Bis(NHC) Ligand
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