Reversible metathesis of ammonia in an acyclic germylene-Ni0complex

Article abstract of DOI:10.1039/d1sc00450f

Carbenes, a class of low-valent group 14 ligand, have shifted the paradigm in our understanding of the effects of supporting ligands in transition-metal reactivity and catalysis. We now seek to move towards utilizing the heavier group 14 elements in effective ligand systems, which can potentially surpass carbon in their ability to operate via 'non-innocent' bond activation processes. Herein we describe our initial results towards the development of scalable acyclic chelating germylene ligands (viz.1a/b), and their utilization in the stabilization of Ni0 complexes (viz.4a/b), which can readily and reversibly undergo metathesis with ammonia with no net change of oxidation state at the GeII and Ni0 centres, through ammonia bonding at the germylene ligand as opposed to the Ni0 centre. The DFT-derived metathesis mechanism, which surprisingly demonstrates the need for three molecules of ammonia to achieve N-H bond activation, supports reversible ammonia binding at GeII, as well as the observed reversibility in the overall reaction.

Full text of DOI:10.1039/d1sc00450f

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Reversible metathesis of ammonia in an acyclic
germylene–Ni0 complex†
Cite this: Chem. Sci., 2021, 12, 5582
a
Philip M. Keil,a Tibor Szilvasib and Terrance J. Hadlington
*
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
´
Carbenes, a class of low-valent group 14 ligand, have shifted the paradigm in our understanding of the
effects of supporting ligands in transition-metal reactivity and catalysis. We now seek to move towards
utilizing the heavier group 14 elements in effective ligand systems, which can potentially surpass carbon
in their ability to operate via ‘non-innocent’ bond activation processes. Herein we describe our initial
results towards the development of scalable acyclic chelating germylene ligands (viz. 1a/b), and their
utilization in the stabilization of Ni0 complexes (viz. 4a/b), which can readily and reversibly undergo
metathesis with ammonia with no net change of oxidation state at the GeII and Ni0 centres, through
ammonia bonding at the germylene ligand as opposed to the Ni0 centre. The DFT-derived metathesis
mechanism, which surprisingly demonstrates the need for three molecules of ammonia to achieve N–H
bond activation, supports reversible ammonia binding at GeII, as well as the observed reversibility in the
overall reaction.
Received 23rd January 2021
Accepted 5th March 2021
DOI: 10.1039/d1sc00450f
rsc.li/chemical-science
attention in the literature, and are typically doubly substituted
by a ligand bearing a phosphine arm; a number of examples of
such systems have been reported by Cabeza and co-workers,8,9
Introduction
Beginning with the inception of transition metal (TM) carbene
complexes by E. O. Fischer,1 research involving the synthesis
and utility of metal carbene complexes has long stood as a pillar
of modern organometallic chemistry.2 This has borne
numerous classes of carbene complex, a vast number of which
have catalytic implications, from highly reactive alkylidene
complexes capable of double-bond metathesis,3 to those
involving N-heterocyclic carbene (NHC) spectator ligands.4
Whilst considerable efforts have also been given to the study of
the heavier tetrylenes in TM complexes, this eld has largely
focused on synthetic access, and less so on reactivity and uti-
lisation in broader synthetic protocols.5 The heavier tetrylenes
have the capacity to act as ‘single centre ambiphiles’, this effect
amplied on descending group 14 due to reduced sp-mixing,6
with a s-donating lone electron pair and a Lewis acidic vacant p-
orbital. Although heavier tetrylenes have been employed as
ligands in catalysis,5c,7 and notably so amidinato silylenes,7
these, like NHCs, are typically spectator ligands due to common
examples being N-heterocyclic in nature. Chelating ligands
employing an acyclic, low-coordinate germylene have seen some
which have been combined with rst-row TM halides so as to
access TMII complexes, oen with tetrylene insertion into TM–X
bonds (X ¼ halide).10 Related (carbonyl-free) rst-row TM0
complexes bearing acyclic, two-coordinate germylene ligands
are rare,11 and should allow for metal–ligand cooperativity
(MLC) due to their amphiphilic nature imparting them the
potential to remain Lewis acidic when bound to a metal centre.
MLC, whereby a metal-bound ligand plays an active role in bond
aDepartment of Chemistry, Technical University Munich, Lichtenbergstraße 4, 85747
Garching, Germany. E-mail: terrance.hadlington@tum.de
bDepartment of Chemical and Biological Engineering, University of Alabama,
Tuscaloosa, AL 35487, USA
† Electronic supplementary information (ESI) available: Experimental procedures
and characterisation data for all new compounds, full details of computational
studies, summarised crystallography data, and complete CIFs. CCDC
2057295–2057309. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/d1sc00450f
Fig. 1 The nucleophilic role of a carbene ligand in reversible ammonia
activation (Piers), and the electrophilic role of a germylene ligand in
reversible ammonia activation (this work).
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activation,12 is a powerful concept in catalysis,13,14 and has
allowed for the facile activation of bonds which are otherwise
challenging to cleave, a prime example being the N–H bonds in
ammonia.15 In a key demonstration of this process, the
phosphino-carbene Ni0 complex A can readily bind ammonia at
Ni in the displacement of its Ph3P ligand, with the nucleophilic
carbene ligand accepting a proton in N–H bond cleavage
(Fig. 1).15c
Given the potentially Lewis acidic nature of heavier tetry-
lenes, we envisage that such ligands can reverse the polarity of
this MLC process, and act as electrophilic binding sites in TM
complexes. This would open a new avenue in the dual-centre
activation of small molecules, and indeed in catalysis. In
order to investigate the capacity of a tetrylene to remain elec-
trophilic in the coordination sphere of a TM, and perhaps more
importantly to be more electrophilic than the TM, we sought to
develop a chelating ligand incorporating an acyclic, low-
coordinate heavier tetrylene. To this end, we have developed
Scheme 1 The synthesis of phosphine-functionalised amine pro-
ligands, their deprotonation, and subsequent synthesis of chloro-
germylene complexes. (i) Ph2PCH2Li$TMEDA, hexane; (ii) DippN(H)Li,
THF; (iii) KH, THF; (iv) GeCl2$dioxane, THF (yields in parentheses). Dipp
¼ 2,6-iPr2-C6H3.
forms, 1H and 31P NMR analyses of crude reaction mixtures
suggest they are formed near quantitatively.17 These crude
products are readily deprotonated with a suspension of KH in
THF, yielding the potassium amides PhiPDippNK and
PhPhDippNK in good isolated yields of ꢀ70 to 75% aer work up,
based on the Ph2PCH2Li$TMEDA starting material. GeII chlo-
ride complexes PhPhDippNGeCl and PhiPDippNGeCl (1a and 1b,
respectively; (Scheme 1)) are generated through combination of
these potassium amides with GeCl2$dioxane in either THF or
toluene, yielding the desired germylene ligands in high yields.
Both 1a and 1b show somewhat complex 1H NMR spectra, when
compared with potassium amides PhiPDippNK and PhPhDippNK,
due to the asymmetric coordination at their GeII centres.
novel
phosphine-functionalised
amine
pro-ligands,
PhPhDippNH and PhiPDippNH (PhPhDippNH ¼ Ph2PCH2Si(Ph)2-
N(H)Dipp; PhiPDippNH ¼ Ph2PCH2Si(iPr)2N(H)Dipp; Dipp ¼
2,6-iPr2-C6H3), which can be used to generate phosphine-
functionalised (amido)(chloro)germylenes which satisfy the
targeted acylic, low-coordinate ligand characteristics when
combined with a low-valent TM centre (Fig. 1). Herein we report
the synthesis of these compounds, specically as their Ni0
complexes, in which the single centre ambiphile ligand centre
(i.e. GeII) maintains its Lewis acidity, and is capable of binding
and activating NH3 and H2O, in the former case reversibly.
Nevertheless, the presence of a single peak in the respective 31
P
{1H} NMR spectra conrms the presence of a single ligand
environment in these compounds. A single crystal X-ray
diffraction analysis of the two germylenes conrms their
monomeric nature (viz. Fig. 2), and shows that the Ph2P moiety
Results and discussion
The phosphine-functionalised germylene ligands feature of the amide ligands binds the GeII centre in both species (1a:
˚
˚
a
novel ancillary ligand scaffold, namely
a
phosphine- dPGe ¼ 2.4547(9) A; 1b: dPGe ¼ 2.458(1) A). Complexes 1a/b bear
functionalised amide. The amine pro-ligands are readily some similarity to complexes reported by Wesemann, incorpo-
accessed through an initial in situ synthesis of phosphine- rating bulky aryl ligands at GeII,18 and Baceiredo, who also
functionalised chlorosilanes, (Ph2PCH2)(R)2SiCl (R
¼
Ph, developed GeII chloride species bearing phosphine-
iPr),16 which can then be reacted with the lithium anilide,
DippN(H)Li, with loss of LiCl. Although the formed pro-ligands,
PhiPDippNH and PhPhDippNH, were not isolated in their pure
Fig. 2 The molecular structures of (a) 1b and (b) 3a, with thermal
ellipsoids at 40% probability. Hydrogen atoms omitted for clarity.
Selected bond distances (A˚) and angles (ꢁ) for 1b: Ge1–Cl1 2.355(1);
Ge1–P1 2.472(1); Ge1–N1 1.925(3); N1–Ge1–Cl1 100.69(1); Cl1–Ge1–
P1 85.98(4); N1–Ge1–P1 85.85(1). For 3a: Ge1–Br1 2.512(1); Ge1–P3 Scheme 2 The reaction of chloro-germylenes 1a/b with NiBr2$DME,
2.455(2); Ge1–N1 1.913(4); N1–Ge1–Br1 103.00(1); Br1–Ge1–P3 leading to reversible complexation and Cl/Br exchange at GeII. Inset:
84.74(4); N1–Ge1–P3 84.98(1).
the molecular structure of dimeric germyl-nickel complex 2.
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functionalised amide ligands.19 In these cases, Ge–P distances
are similar to those in 1a/b.
With (amido)(chloro)germylene ligands 1a/b in hand, we
began our investigations into the complexation chemistry of
these pro-chelating ligands towards nickel dihalides. We found
that both complexes do not readily react with NiCl2, even aer
heating, despite a dark red colouration of reaction solutions.
However, reactions with an excess of NiBr2$DME in toluene with
a small amount of THF led to dark red-brown reaction mixtures,
from which dimeric complex 2 could be isolated as large red-
brown crystals aer ltration (Scheme 2). X-ray structural
analysis of these crystals (Scheme 2, inset) revealed that Ni
inserts into the Ge–P bond of 1a, forming a chelating ligand
motif incorporating P and Ge as we had hoped. Formal oxida-
tive addition of one Ni–Br bond at GeII is also observed, akin to
previous examples reported by Cabeza and co-workers.10 Each
Scheme
3 4 and
Synthesis of halo-germylene Ni0 complexes
subsequent reactions with NH3 and H2O. (i) 1.1NiX2$DME, 2Ph3P, 6Zn,
THF, 2 h (X ¼ Br); ꢀ24 h (X ¼ Cl); X ¼ Cl and/or Br; (ii) 1Ni(cod)2, 2Ph3P,
toluene, 1 h; X ¼ Cl.
NiII centre sits in a square planar geometry, bound by two complexes as the sole products, with the crystalline compounds
bridging Br ligands, Ge, and P. Compound 2 represents
a surprisingly uncommon example of a germyl-nickel complex,
and is unstable in solution in the absence of an excess of NiBr2.
This was clear in an attempt to obtain NMR spectroscopic data
isolated in good yields from diethyl ether extracts of the crude
reaction mixtures. A single crystal X-ray structural analysis of
dark red-brown crystals of 4b reveal a central Ni0, bound by two
Ph3P ligands, and one chelating phosphino-germylene ligand
for 2, where the only observable species is the bromo germylene (Fig. 4(a)). Notably, the chloride moiety in the germylene ligand
PhPhDippGeBr 3a, presumably through elimination of the nickel remains intact. The three-coordinate GeII centre holds
dihalide.20 Reassessing the reaction of chloro germylene 1a with a trigonal planar geometry, and has a relatively short Ge–Ni
NiBr2$DME, we found that addition of a single equivalent leads contact when compared with previously reported examples,
to a ꢀ50 : 50 mixture of 1 and 3, i.e. partial Cl/Br exchange
indicative of some back-bonding from Ni to Ge (vide supra). The
Ni0 centre holds a tetrahedral geometry, with the three Ni–P
(Fig. S24 and S28†). From such a reaction mixture the formation
of one or two single crystals of the germyl-nickel mixed halide
complex 20, the acetonitrile-coordinated monomeric form of 2,
distances being as expected when compared with reported
phosphine-coordinated Ni0 complexes. Complex 4a is essen-
allowed for structural analysis of this ‘intermediate’ (Fig. S80 in tially isostructural to 4b.23 The UV/vis spectra of these species
ESI†). Addition of 6 equiv. of NiBr2$DME allows for full show two clear absorption bands (4a: lmax ¼ 484 nm (3 ¼ 1370
conversion to bromo germylene 3,21 whilst a vast excess (ꢀ10 L cmꢂ1 molꢂ1) and 362 nm (6735 L cmꢂ1 molꢂ1); 4b: 473 nm
equiv.) of NiBr2$DME affects the crystallisation of small (1685 L cmꢂ1 molꢂ1) and 362 nm (9160 L cmꢂ1 molꢂ1)), in-
amounts of germyl-nickel bromide complex 2. We presume that
the presence of an excess of NiBr2$DME is required to maintain
the stability of 2, as in all cases NMR analyses showed only the
free chloro/bromo-germylene ligands. Taken as a whole, this
demonstrates that NiBr2 can act as a facile halide exchange
reagent for the synthesis of bromo germylenes, which are
challenging to access via conventional routes due to the lack of
readily available GeBr2 reagents.22
keeping with related absorptions for Ni-based d–d transi-
tions.24 Likely due to hindered rotation brought about by the
sterics of the chelating ligand system, resonances relating the
ligand are broadened in the 1H NMR spectrum of both Ni0
Although the isolation of useful quantities of nickel complex
2 was not possible, the observation that it exists within the
reaction mixture seemed a promising start. We found that in
situ reduction of this reaction mixture, in the presence of Ph3P,
gave facile access to chelating phosphino-germylene complexes
of Ni0. Room temperature addition of THF to a mixture of 1a/b,
NiBr2$DME, Ph3P, and an excess of Zn powder allowed for the
clean formation of Ni0 complexes 4a/b aer 2 h, albeit as
a mixture of the chloro- and bromo-germylene ligated species.
Employing NiCl2$DME in place of NiBr2$DME led to consider-
ably extended reaction times, but gave clean access to the
chloro-germylene species (Scheme 3). The pure chloro-
germylene analogue can also be accessed through combina-
tion of 1, Ni(COD)2, and PPh3 in the stoichiometric ratio 1 : 1 : 2
(Fig. S31†). Satisfyingly, 31P NMR analysis of crude reaction
Fig. 3 The 31P{1H} NMR spectrum of (a) 4b, (b) in situ addition of NH3
to 5b, and (c) 4b regenerated after sonication and degassing the
sample used for spectrum (b). * ¼ small amounts of 6b; D ¼ small
mixtures in all cases suggest the formation of these novel Ni0 amounts of residual 5b.
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species. The 31P NMR spectra show one broadened peak for the complimentary 1H NMR spectra indicated new singlet 2H
Ph3P ligands, centred at d ¼ 39.8 (4a and 4b) ppm, whilst the resonances at d ¼ 3.13 (5a) and 2.96 (5b) ppm, which were
anking phosphine arm of the chelating ligand presents as tentatively assigned to an NH2 fragment. The IR spectrum of
a well resolved triplet centred at d ¼ 7.1 (4a) and 9.6 (4b) ppm reaction products is in keeping with this, with two weak N–H
(Fig. 3(a)). For the mixed bromide/chloride samples, a second stretching bands observed at n ¼ 3354 and 3461 (5a and
set of peaks relating to the bromide species is observed in the 5b) cmꢂ1. Structural analysis of large orange crystals of the
31P NMR spectrum, overlapping with the rst (Fig. S32 and product of the reaction of 4a with ammonia reveal that the Ge–
S40†).25 Separation of the two compounds proved impossible in Cl moiety in this species has in fact undergone a s-metathesis
our hands, although using the mixture in subsequent chemistry reaction with one N–H bond of ammonia, yielding the bis(a-
discussed herein did not pose any issues.
mido)germylene Ni0 complex 5a (Scheme 3 and Fig. 4(b)).26
Given the ambiphilic nature of tetrylenes, particularly in Presumably, the colourless solid in reaction mixtures is
acyclic derivatives, the design of complexes 4a/b aims to retain ammonium chloride, through the net loss of HCl in this reac-
a degree of Lewis acidity at GeII. This forms the central idea of tion (vide supra). As such, removal of the ammonia atmosphere
non-innocent single centre ambiphile ligands. This character- from reaction mixtures, and sonication of amides 5a/b over the
istic was probed by Density Functional Theoretical (DFT) anal- precipitated NH4Cl with intermittent degassing leads to
ysis of the frontier orbitals in model complex 40, employing regeneration of starting materials 4a/b (Fig. 3), with the loss of
PhMeXylN in place of PhiPDippN/PhPhDipp (PhMeXylN ¼ (Ph2- ammonia (Scheme 3).27 Such a reversible activation of ammonia
PCH2SiMe2)(Xyl)N). We found that the LUMO of 40 is located on is, to the best of our knowledge extremely rare, as is the more
the Ge centre (Fig. 4(a)), and mainly constitutes a vacant p- general redox-innocent metathesis reaction at a tetrylene
orbital which is expected to be Lewis acidic, particularly given centre. Beyond the reaction of 4a/b with ammonia, these species
the overall NPA charge at Ge of +1.13. Still, Natural Bond Orbital also readily undergo a similar metathesis reaction with water in
(NBO) analysis of the Ge–Ni bond indicates strong polarisation the presence of nitrogen bases to facilitate HCl abstraction (i.e.
toward the Ge centre (Table S4†), whilst the HOMO in 40 CyNH2), to yield (amido)(hydroxyl)germylene Ni0 complexes 6a/
(Fig. S85 in ESI†) shows some degree of p-bonding, pointing b (Scheme 3 and Fig. 4(c)). These species can also be accessed by
towards a donor–acceptor description of the Ge–Ni bond. In line the reaction of amide complexes 5a/b with water, with
with the Lewis acidity of the GeII centre in 4a/b, deep red-brown concomitant loss of ammonia. The formation of 6a/b can be
solutions of these compounds readily react with ammonia at 1 clearly observed by 1H NMR and IR spectroscopy, the former
atm pressure to form slightly turbid bright orange reaction containing new 1H resonances pertaining to OH residues (d ¼
mixtures containing ammonia activation products 5a/b. In situ 3.13 (6a) and 2.96 (6b) ppm), and the latter clear OH stretching
31P{1H} NMR spectroscopic analysis indicated the formation of bands (n ¼ 3547 (6a) and 3541 (6b) cmꢂ1). In recent years a urry
a single new product for both systems (Fig. 3(b)), whilst of examples of ammonia activation at low-valent group 14
Fig. 4 Molecular structures of compounds (a) 4b, (b) 5a, and (c) 6b, with thermal ellipsoids at 40% probability. Hydrogen atoms omitted for
clarity, aside from those at N2 and O1 in 5a and 6b, respectively. The LUMO of each compound is inset below the respective structure. Selected
bond distances (A˚) and angles (ꢁ) for 4b: Ge1–Ni1 2.1877(7); N1–Ge1 1.869(2); P1–Ni1 2.201(1); P2–Ni1 2.2079(8); P3–Ni1 2.2055(8); N1–Ge1–Cl1
99.57(7); Ni1–Ge1–N1 133.09(7); Ni1–Ge1–Cl1 126.89(3). For 5a: Ge1–Ni1 2.217(1); N1–Ge1 1.890(2); N2–Ge1 1.819(2); P1–Ni1 2.210(1); P2–Ni1
2.201(1); P3–Ni1 2.1892(9); N1–Ge1–N2 99.07(9); Ni1–Ge1–N1 128.63(6); Ni1–Ge1–N2 132.29(7). For 6b: Ge1–Ni1 2.2077(7); N1–Ge1 1.885(3);
O1–Ge1 1.874(2); P1–Ni1 2.210(1); P2–Ni1 2.202(1); P3–Ni1 2.104(1); N1–Ge1–O1 100.98(1); Ni1–Ge1–O1 128.47(7); Ni1–Ge1–N1 130.49(8).
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centres have been forthcoming, but lead almost exclusively to 0.980; for 60: 1.17/1.07), with predicted Ge–Ni distances (40: 2.161
oxidative addition reactions, thus yielding EIV compounds (E ¼ A; 5 : 2.203 A; 6 : 2.177 A) only slightly contracted relative to
0
˚
˚
˚
C–Sn).28 That oxidative addition is circumvented in reactions of those observed experimentally (e.g. dGeNi for 4b: 2.1877(7) A; 5a:
˚
˚
˚
4a/b with both NH3 and H2O likely stems from the binding of 2.217(1) A; 6a: 2.1956(7) A), possibly due to the increased steric
the Ge lone electron pair to Ni0. Further, it is surprising that the encumbrance in the real complexes. An NBO analysis indicates
Ni0 centre in these complexes remains unchanged, given a large degree of s-character in these bonds, indicative of the
previous examples of MLC in ammonia activation involving greatest contributing factor being Ge / Ni lone-pair donation,
Ph3P-ligated Ni0 complexes.15c,d Overall, the metathesis reac- but, as already described for 40, a high degree of polarisation
tions of ammonia demonstrated here highlight the targeted towards Ge is also apparent, making a donor–acceptor inter-
high Lewis acidity of the developed acyclic germylene ligands, action the best description for the Ge–Ni interaction in these
and are an exciting step towards employing these ligands in complexes.
complexes which can operate via MLC bond activation
processes, where a lower coordinate TM centre is utilised.
Further, the mechanism for the amination of model complex
40 was investigated by means of DFT (Fig. 5), given the reversible
So as to compare the effects of the differing fragments at GeII nature of this interesting process. The most favourable mech-
in the described Ni0 complexes, we performed frontier orbital, anism begins with binding of NH3 at GeII, highlighting the
NBO, Wiberg Bond Index (WBI), Mayer Bond Order (MBO), and Lewis acidity of this centre. This process is near thermoneutral
Natural Population analyses on 40, 50, and 60 (Tables S4–S7†), as (IM1 in Fig. 5, +1.7 kcal molꢂ1), which corroborates reversibility
model complexes of the real systems, again employing PhMeXylN in this key step. Surprisingly, it was found that two further NH3
in place of PhiPDippN/PhPhDipp. We found that 40, 50, and 60 all molecules are subsequently required to drive the metathesis,
show very similar characteristics in general; the main difference which form a H-bond network between the Ge-bound NH3 and
is the interaction of the vacant p-orbital on the GeII centre and Cl ligands. The most challenging step in the overall process is
the lone pair of the Cl/NH2/OH moiety. Bond analysis showed the ion pair formation (TS4 in Fig. 5, +14.5 kcal molꢂ1), which is
the highest bond order for the Ge–NH2 in 50 (MBO: 1.10) which only possible on the potential energy surface (PES) if the H-bond
+
is due to the efficient hybridization of the N lone pair and the network is present, which stabilises the formation of the NH4
vacant p-orbital of Ge that are well-described for low-valent ion. Then, the NH4+ and the additional NH3 molecule facilitate
germanium compounds.29 The LUMO of 50 is the only the removal of Clꢂ. The overall reaction is very close to ther-
example in this series which is not represented by a vacant p- moneutral (50 in Fig. 5, ꢂ0.9 kcal molꢂ1), making this process
orbital at Ge (Fig. 4), most likely due to the described N / Ge reversible simply by changing the reaction conditions, again
donation. Consequently, the Ge–Ni bond order decreases and corroborating the observed experimental results. Finally, we
the Ge–Ni bond length increases (vide infra) due to the smaller also considered (i) the potential involvement of the Ni centre in
back donation from the Ni d-orbital to the Ge vacant orbital. All NH3 activation, and (ii) oxidative addition of an N–H bond at the
systems show some degree of multiple bond character between Ge centre. Both of these mechanisms can be excluded from the
the GeII and Ni0 centres (WBI/MBO for 40 1.20/1.13; for 50: 1.11/ active PES; the Ni–H complex, formed in (i), is relatively
Fig. 5 DFT-derived mechanism for the s-metathesis of ammonia in 40 leading to 50.
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unstable (TS20 in Fig. 5, 19.5 kcal molꢂ1), whilst the Ge oxidative
PhPhDippNK
addition product is not a minimum on the PES (route (ii)).
A yellow suspension of PPh2CH2Li$TMEDA (6.0 g, 18.6 mmol) in
100 mL hexane was cooled to ꢂ78 C. The mixture was stirred
ꢁ
Conclusions
vigorously and Ph2SiCl2 (3.9 mL, 18.6 mmol) was added. The
mixture was allowed to warm to RT overnight. All volatiles were
subsequently removed in vacuo, leaving a yellow oil. DippN(H)Li
(3.4 g, 18.6 mꢁmol) was added to the residue, and the ask
cooled to ꢂ78 C, followed by the addition of 50 mL THF. The
mixture was stirred until dissolution of all solids was observed.
The cold bath was then removed and the reaction allowed to
warm to RT, leading to an orange solution. All volatiles were
removed in vacuo and the oily residue extracted with 50 mL
hexane, and ltered. The solvent was removed in vacuo and KH
(0.9 g, 28.3 mmol) was added. Aer addition of 50 mL THF, gas
started to evolve, and the mixture was vigorously stirred for
a further 16 h. The dark brown suspension was ltered, and all
volatiles were removed in vacuo. To the resulting oil 50 mL
hexane was added, and the mixture treated in an ultrasonic
bath causing the precipitation of copious pale brown powder,
which was ltered and washed multiple times with hexane, and
subsequently dried in vacuo to yield PhPhDippNK as an off-white
powder (8.5 g, 14.3 mmol, 77%). Colourless crystals suitable for
X-ray diffraction analysis were obtained aer two days from
a concentrated THF/TMEDA solution layered with hexane
stored at ꢂ32 ꢁC. 1H NMR (THF-d8, 400 MHz, 298 K): d ¼ 0.91 (d,
In conclusion, we have developed a chelating phosphino-
germylene ligand scaffold, and a facile route to Ni0 complexes
bearing these ligands in a low-cost, one pot synthetic prepara-
tion. These ligands, which are centred around an acyclic (ami-
do)(chloro)germylene, remain Lewis acidic when bound to Ni0.
This has been demonstrated by the facile and reversible acti-
vation of ammonia, as well as the complimentary irreversible
reaction with water, both of which lead to GeII products through
s-metathesis of the Ge–Cl bond, thus circumventing oxidation
at both germanium and nickel. These results form an initial
basis for the single centre ambiphile ligand concept, which we
are currently expanding to further low-valent main group
species, and further rst row transition metals, moving towards
cooperative bond activation involving both the ligand and the
metal centre.
Experimental
General considerations
All experiments and manipulations were carried out under a dry
oxygen free argon atmosphere using standard Schlenk tech-
niques, or in a MBraun inert atmosphere glovebox containing
an atmosphere of high purity argon. THF and diethyl ether were
dried by distillation over a sodium/benzophenone mixture and
3
2
12H, JHH ¼ 6.9 Hz, Dipp-Pri-CH3), 1.97 (d, 2H, JHP ¼ 5.4 Hz,
Ph2P–CH2), 3.94 (hept, 2H, 3JHH ¼ 6.9 Hz, Dipp-Pri-CH), 6.24 (t,
3
3
1H, JHH ¼ 7.4 Hz, Ar–CH), 6.74 (d, 2H, JHH ¼ 7.4 Hz, Ar–CH),
7.00 (m, 6H, Ar–CH), 7.10 (m, 6H, Ar–CH), 7.27 (m, 4H, Ar–CH),
7.48 (m, 4H, Ar–CH). 13C{1H} NMR (THF-d8, 101 MHz, 298 K):
d ¼ 19.7 (d, 1JCP ¼ 29.2 Hz, Ph2P–CH2), 24.9 (Dipp-Pri-CH3), 28.1
(Dipp-Pri-CH), 112.6, 123.00, 127.3, 127.5, 128.3, 128.7, 133.4,
133.6, 135.8, 141.4, 143.9, 144.0, 146.7, 146.7 and 156.2 (Ar–C).
31P{1H} NMR (THF-d8, 162 MHz, 298 K): d ¼ ꢂ19.1 (s, CH2–
PPh2). 29Si{1H} NMR (THF-d8, 99 MHz, 298 K): d ¼ ꢂ47.4 (d, 2JSiP
¼ 14.7 Hz, SiPh2).
˚
stored over activated 4 A mol sieves. C6D6 was dried and stored
over a potassium mirror. All other solvents were dried over
˚
activated 4 A mol sieves and degassed prior to use. NiBr2-
$DME,30 NiCl2$DME,31 DippN(H)Li,32 and PPh2CH2Li$TMEDA33
were synthesized according to known literature procedures. All
other reagents were used as received. Commercial CyNH2, when
used as received, contained enough residual moisture to allow
for the synthesis of 6a. NMR spectra were recorded on a Bruker
1
AV 400 or 500 Spectrometer. The H and 13C{1H} NMR spectra
PhPhDippGeCl, 1a
were referenced to the residual solvent signals as internal
standards. 29Si NMR spectra were externally calibrated with A pale brown solution of PhPhDippNK (7.0 g, 11.7 mmol) in
SiMe4. 31P NMR spectra were externally calibrated with H3PO4. 20 mL THF was added dropwise to a stirring solution of
LIFDI MS spectra were measured at a Waters Micromass LCT GeCl2$dioxane (2.7 g, 11.7 mmol) in 10 mL THF at ꢂ78 ꢁC, and
TOF mass spectrometer equipped with an LIFDI ion source subsequently allowed to warm to RT, resulting in the formation
(LIFDI 700) from Linden CMS GmbH. The samples were dis- of an orange solution. All volatiles were removed in vacuo, the
solved in dry toluene and ltered using a syringe lter under an residue extracted in 20 mL DCM, and ltered. The solvent was
inert atmosphere. The TOF setup was externally calibrated removed in vacuo and the residue washed with hexane to yield
using polystyrene. ESI-MS was performed on an exactive plus 1a as an off-white powder (7.0 g, 10.5 mmol, 90%). Colourless
orbitrap spectrometer from Thermo Fischer Scientic. Infrared crystals suitable for X-ray diffraction analysis were obtained
spectra were measured with the Alpha FT IR from Bruker con- from a concentrated diethyl ether at RT. 1H NMR (C6D6, 400
taining a platinum diamond ATR device. The compounds were MHz, 298 K): d ¼ 0.09 (d, 3H, 3JHH ¼ 6.4 Hz, Dipp-Pri-CH3), 0.68
measured as solids under inert conditions in a glovebox. For the (d, 3H, 3JHH ¼ 6.6 Hz, Dipp-Pri-CH3), 0.86 (d, 3H, 3JHH ¼ 6.5 Hz,
3
ammonia activation experiments water free ammonia 5.0 was Dipp-Pri-CH3), 1.43 (d, 3H, JHH ¼ 6.5 Hz, Dipp-Pri-CH3), 2.54
used.
(m, 3H, Ph2P–CH2/Dipp-Pri-CH), 4.28 (m, 1H, Dipp-Pri-CH), 7.02
Representative procedures for the preparation of (m, 14H, Ar–CH), 7.21 (m, 3H, Ar–CH), 7.46 (m, 2H, Ar–CH), 7.65
3
compounds 1, 3, and 4–6 are given below. Further details for the (m, 2H, Ar–CH), 8.26 (d, 2H, JHH ¼ 7.1 Hz, Ar–CH). 13C{1H}
synthesis and characterisation of all novel compounds are given NMR (C6D6, 101 MHz, 298 K): d ¼ 8.1 (Ph2P–CH2), 21.4, 22.6 and
in the ESI.†
28.0 (Dipp-Pri-CH3), 28.2 and 28.6 (Dipp-Pri-CH), 28.8 (Dipp-Pri-
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CH3), 123.9, 124.6, 125.6, 129.0, 129.1, 129.6, 129.7, 131.1, NMR tube, which was then closed and shaken leading to an
131.5, 132.8, 132.9, 133.3, 133.4, 134.8, 135.4, 137.3, 137.8, immediate colour change from deep red to bright orange, with
140.8, 140.9, 147.8 and 149.7 (Ar–C). 31P{1H} NMR (C6D6, 162 concomitant formation of a colourless solid (NH4Cl). The
MHz, 298 K): d ¼ 4.0 (s, CH2–PPh2). 29Si{1H} NMR (C6D6, 79 solution was ltered, and volatiles removed in vacuo to yield 5a
2
MHz, 298 K): d ¼ ꢂ4.0 (d, JSiP ¼ 13.4 Hz, SiPh2). MS/LIFDI- as an orange powder (15 mg, 0.013 mmol, 83%). Dark orange
HRMS found (calcd) m/z: 665.1466 (665.1490) for [M]+.
crystals suitable for X-ray diffraction analysis were obtained by
storage of a concentrated toluene solution layered with hexane
at ꢂ32 ꢁC. 1H NMR (C6D6, 400 MHz, 298 K): d ¼ 0.50 (d, 6H, 3JHH
¼ 6.6 Hz, Dipp-Pri-CH3), 1.14 (m, 6H, Dipp-Pri-CH3), 2.97 (s, 2H,
Ph2P–CH2), 3.14 (s, 2H, Ge–NH2), 3.79 (hept, 2H, 3JHH ¼ 7.3 Hz,
Dipp-Pri-CH), 6.67 (m, 3H, Ar–CH), 6.81 (m, 6H, Ar–CH), 7.00 (m,
31H, Ar–CH), 7.32 (d, 4H, 3JHH ¼ 6.6 Hz, Ar–CH), 7.48 (s, 9H, Ar–
CH). 13C{1H} NMR (C6D6, 101 MHz, 298 K): d ¼ 20.1 (Ph2P–CH2),
23.6 and 26.1 (Dipp-Pri-CH3), 28.8 (Dipp-Pri-CH), 124.4, 125.2,
127.1, 131.6, 132.3, 134.3, 134.5, 136.1, 136.3, 140.2, 141.1 and
146.3 (Ar–C). 31P{1H} NMR (C6D6, 81 MHz, 298 K): d ¼ 9.3 (t, 2JPP
PhPhDippGeBr, 3a
To a mixture of 1a (200 mg, 0.30 mmol) and NiBr2$DME
(540 mg, 1.8 mmol) was added 5 mL toluene and 1 mL THF,
and the resulting mixture stirred for 1 h. All volatiles were
removed in vacuo and the residue extracted with 5 mL toluene.
The solution was concentrated and layered with hexane
yielding colourless crystals of 3a suitable for X-ray diffraction
1
analysis (127 mg, 0.18 mmol, 54%). H NMR (C6D6, 400 MHz,
3
298 K): d ¼ 0.06 (d, 3H, JHH ¼ 6.6 Hz, Dipp-Pri-CH3), 0.67 (d,
2
3
3
¼ 15.6 Hz, Ph2P–Ni–(PPh3)2), 39.8 (d, JPP ¼ 14.2 Hz, Ph2P–Ni–
3H, JHH ¼ 6.7 Hz, Dipp-Pri-CH3), 0.93 (d, 3H, JHH ¼ 6.6 Hz,
3
(PPh3)2). 29Si{1H} NMR (C6D6, 99 MHz, 298 K): d ¼ ꢂ14.8 (d, 2JSiP
¼ 3.2 Hz, SiPh2); IR, n/cmꢂ1 (ATR): 3354 and 3461 (br, w, Ge–
NH2); MS/LIFDI-HRMS found (calcd) m/z: 966.2156 (966.2253)
for [M ꢂ PPh3]+.
Dipp-Pri-CH3), 1.46 (d, 3H, JHH ¼ 6.6 Hz, Dipp-Pri-CH3), 2.55
(m, 3H, Ph2P–CH2/Dipp-Pri-CH), 4.26 (hept, 1H, 3JHH ¼ 6.9 Hz,
Dipp-Pri-CH), 6.99 (m, 14H, Ar–CH), 7.21 (m, 3H, Ar–CH), 7.46
3
(m, 2H, Ar–CH), 7.60 (m, 2H, Ar–CH), 8.30 (d, 2H, JHH
¼
7.3 Hz, Ar–CH). 13C{1H} NMR (C6D6, 101 MHz, 298 K): d ¼ 8.4
(Ph2P–CH2), 21.4 and 22.6 (Dipp-Pri-CH3), 28.1 and 28.2 (Dipp-
Pri-CH), 28.5 and 29.0 (Dipp-Pri-CH3), 123.1, 124.0, 124.8,
125.7, 128.9, 129.0, 129.1, 129.6, 129.8, 131.0, 131.6, 132.7,
132.8, 133.3, 133.4, 134.5, 135.4, 137.6, 140.5, 140.6, 147.8 and
149.9 (Ar–C). 31P{1H} NMR (C6D6, 162 MHz, 298 K): d ¼ 1.9 (s,
CH2–PPh2). 29Si{1H} NMR (C6D6, 99 MHz, 298 K): d ¼ ꢂ3.9 (d,
2JSiP ¼ 13.9 Hz, SiPh2); MS/LIFDI-HRMS found (calcd) m/z:
709.1012 (709.0984) for [M]+.
[
PhPhDippGe(OH)]Ni(PPh3)2, 6a
‘Wet’ cyclohexylaniline (100 mg, 1.00 mmol) was added to
a stirring solution of 4a (200 mg, 0.16 mmol) in 5 mL toluene
leading to an immediate colour change from deep red to light
red-orange. The mixture was stirred for 30 min. All volatiles
were then removed in vacuo and the residue extracted with
diethyl ether, and ltered. Orange-red crystals of 6a suitable for
X-ray diffraction analysis formed over the course of two hours at
ambient temperature (133 mg, 0.11 mmol, 68%). 1H NMR
(C6D6, 400 MHz, 298 K): d ¼ 0.47 (d, 6H, 3JHH ¼ 6.3 Hz, Dipp-Pri-
CH3), 1.10 (m, 6H, Dipp-Pri-CH3), 2.99 (s, 2H, Ph2P–CH2), 3.77
(hept, 2H, 3JHH ¼ 6.4 Hz, Dipp-Pri-CH), 4.15 (s, 1H, Ge–OH), 6.67
(m, 3H, Ar–CH), 6.83 (m, 6H, Ar–CH), 7.01 (m, 30H, Ar–CH), 7.34
(d, 4H, 3JHH ¼ 7.0 Hz, Ar–CH), 7.54 (s, H, Ar–CH). 13C{1H} NMR
(C6D6, 101 MHz, 298 K): d ¼ 19.4 (Ph2P–CH2), 23.3 and 26.2
(Dipp-Pri-CH3), 28.9 (Dipp-Pri-CH), 124.5, 125.7, 127.2, 128.9,
132.3, 134.5, 134.5, 134.6, 135.7, 135.8, 136.0, 138.8, 139.7,
139.9, 140.0 and 146.6 (Ar–C). 31P{1H} NMR (C6D6, 81 MHz, 298
[
PhPhDippGe(Cl)]Ni(PPh3)2, 4a
To a mixture of 1a (1.60 g, 2.4 mmol), NiCl2$DME (0.53 g, 2.4
mmol), PPh3 (1.26 g, 4.8 mmol), and Zn (0.94 g, 14.4 mmol) was
added 10 mL THF, and the resulting mixture stirred for 24 h at
RT resulting in a deep red reaction mixture. All volatiles were
removed in vacuo and the residue extracted with 20 mL diethyl
ether. Dark red crystals of 4a, which were suitable for X-ray
diffraction analysis, were obtained aer storing the solution
1
at RT overnight (1.55 g, 1.8 mmol, 52%). H NMR (C6D6, 400
2
2
MHz, 298 K): d ¼ 0.49 (d, 6H, 3JHH ¼ 6.6 Hz, Dipp-Pri-CH3), 1.28
(bs, 6H, Dipp-Pri-CH3), 3.03 (s, 2H, Ph2P–CH2), 3.72 (hept, 2H,
3JHH ¼ 6.6 Hz, Dipp-Pri-CH), 6.63 (m, 3H, Ar–CH), 6.94 (m, 36H,
Ar–CH), 7.32 (d, 5H, 3JHH ¼ 6.2 Hz, Ar–CH), 7.52 (s, 9H, Ar–CH).
13C{1H} NMR (C6D6, 101 MHz, 298 K): d ¼ 20.0 (Ph2P–CH2), 23.8
and 26.1 (Dipp-Pri-CH3), 29.2 (Dipp-Pri-CH), 124.0, 125.6, 127.2,
128.8, 128.8, 129.1, 132.3, 134.5, 134.6, 135.3, 136.0, 138.6,
138.9, 142.3 and 145.5 (Ar–C). 31P{1H} NMR (C6D6, 81 MHz, 298
K): d ¼ 7.1 (t, 2JPP ¼ 18.2 Hz, Ph2P–Ni–(PPh3)2), 39.8 (bs, Ph2P–
Ni–(PPh3)2). 29Si{1H} NMR (C6D6, 99 MHz, 298 K): d ¼ ꢂ13.5 (d,
2JSiP ¼ 5.5 Hz, SiPh2). MS/LIFDI-HRMS found (calcd) m/z:
985.1705 (985.1755) for [M ꢂ PPh3]+.
K): d ¼ 9.0 (t, JPP ¼ 15.8 Hz, Ph2P–Ni–(PPh3)2), 40.6 (d, JPP
¼
15.8 Hz, Ph2P–Ni–(PPh3)2). 29Si{1H} NMR (C6D6, 99 MHz, 298 K):
2
d ¼ ꢂ14.4 (d, JSiP ¼ 4.6 Hz, SiPh2); IR, n/cmꢂ1 (ATR): 3547 (m,
Ge–OH); MS/LIFDI-HRMS found (calcd) m/z: 967.2057
(967.2093) for [M ꢂ PPh3]+.
Author contributions
PMK carried out the vast majority of the chemical synthesis and
characterisation. TS carried out all computational work. TJH
designed and planned the project, carried out preliminary
experimental work, and wrote the manuscript.
[
PhPhDippGe(NH2)]Ni(PPh3)2, 5a
Conflicts of interest
Compound 4a (20 mg, 0.016 mmol) was dissolved in 0.4 mL
C6D6 in an NMR tube. An excess of ammonia was added to the There are no conicts to declare.
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S. Bestgen, N. H. Rees and J. M. Goicoechea,
Organometallics, 2018, 37, 4147–4155.
Acknowledgements
´
´
10 (a) J. A. Cabeza, I. Fernandez, J. M. Fernandez-Colinas,
TJH gratefully thanks the Fonds der Chemischen Industrie (FCI)
for generous funding of this research through a Liebig Stipen-
dium, and the Technical University Munich for the generous
endowment of TUM Junior Fellow Funds. TS thanks the
University of Alabama and the Office of Information Technology
for providing high performance computing resources and
support. This computational work was made possible in part by
a grant of high-performance computing resources and technical
support from the Alabama Supercomputer Authority. We also
thank M. Muhr, P. Heiß, and A. Heidecker for the measurement
of LIFDI-MS and IR spectra.
´
´
´
P. Garcıa-Alvarez and C. J. Laglera-Gandara, Chem.–Eur. J.,
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´
´
´
´
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I. Fernandez, P. Garcıa-Alvarez, I. Garcıa-Rubio and
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C. J. Laglera-Gandara, Chem.–Eur. J., 2021, DOI: 10.1002/
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8 (a) L. Alvarez-Rodrıguez, J. Brugos, J. A. Cabeza, P. Garcıa- 16 Procedures modied from those previously reported for
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1
19 J. Berthe, J. Manuel Garcia, E. Ocando, T. Kato, N. Saffon- 27 As ascertained by H and 31P NMR spectroscopic analysis.
´
´
Merceron, A. De Cozar, F. P. Cossıo and A. Baceiredo, J.
Due to the extremely high sensitivity of 5a/b towards
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20 The bromo-germylene, PhiPDippGeBr (3b), can be accessed
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28 For some recent examples, see:(a) T. J. Hadlington,
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21 Yields of 50–55% are obtained aer recrystallisation.
22 As an example, GeBr2$dioxane, used as is GeCl2$dioxane,
can be synthesised from GeBr4, which is a high-cost
starting material: N. Hayakawa, T. Sugahara, Y. Numata,
H. Kawaai, K. Yamatani, S. Nishimura, S. Goda, Y. Suzuki,
T. Tanikawa, H. Nakai, D. Hashizume, T. Sasamori,
N. Tokitoh and T. Matsuo, Dalton Trans., 2018, 47, 814–822.
23 Note that single crystals of pure 4a reproducibly gave poor X-
´
(c) D. C. H. Do, A. V. Protchenko, M. A. Fuentes, J. Hicks,
P. Vasko and S. Aldridge, Chem. Commun., 2020, 56, 4684–
4687.
´
ray diffraction data, not of a publishable standard. However, 29 Z. Benedek and T. Szilvasi, Organometallics, 2017, 36, 1591–
4a co-crystallised with ꢀ19% of 4-Br gave data of
1600.
30 H. Bauer, J. Weismann, D. Saurenz, C. Farber, M. Schar,
¨
¨
a publishable standard. See ESI† for details.
¨
¨
24 C. A. Tolman, W. C. Seidel and D. H. Gerlach, J. Am. Chem.
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5590 | Chem. Sci., 2021, 12, 5582–5590
© 2021 The Author(s). Published by the Royal Society of Chemistry