Bulky bis(aryl)triazenides: just aspiring amidinates? A structural and spectroscopic study

Article abstract of DOI:10.1039/d0dt00285b

The synthesis and molecular structures (by single crystal X-ray diffraction) of s-, p- and d-metal complexes of the sterically demandingN,N'-bis(2,6-diisopropylphenyl)triazenide are reported and the spectroscopic (NMR spectroscopy and infrared spectroscopy) and physical properties of these complexes compared to related formamidinate complexes. Through the use of infrared spectroscopy the s-donor capacity of this ligand is demonstrated to be reduced relative to the structurally isomorphous formamidinate congener, which supports previously advanced theoretical calcluations and DFT results reported herein. These electronic differences are highlighted by the stark contrast in reaction outcomes at rhodium; where [(Dipp₂N₃)Rh(CO)₂] (>1>) is an isolable, stable complex and the formamidinate complex is not. The coordination chemistry of the triazenide ligand for the s-block metal complexes (M = Li, Na, K) has been shown to give structurally isomorphous complexes to the formamidinate analogue. In contrast to the amidinate complexes, these complexes show extreme lability of coordinated, volatile Lewis-bases, which in turn-yields the highly insoluble base-free triazenide complexes. These complexes are also synthesized directly in the absence of donor solvents. This triazenide ligand has proven to be a suitable ligand for stabilising reactive main group hydrides of Group 13 (M = Ga, In) and attempts at the analogous thallium hydride complex by halide-hydride exchange are reported. Finally attempts at the synthesis of low valent main group complexes are reported ([M^IL], M = In, Ga) are also reported, which yield instead disproportionation products ([M^IIIXL₂], M = Ga, In; [{M^IIXL}₂], M = Ga).

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Bulky bis(aryl)triazenides: just aspiring amidinates?
A structural and spectroscopic study†
Cite this: DOI: 10.1039/d0dt00285b
Matthew R. Gyton, ‡a Anthony R. Leverett, a Marcus L. Colea and
a,b
Alasdair I. McKay
*
The synthesis and molecular structures (by single crystal X-ray diffraction) of s-, p- and d-metal com-
plexes of the sterically demanding N,N’-bis(2,6-diisopropylphenyl)triazenide are reported and the spec-
troscopic (NMR spectroscopy and infrared spectroscopy) and physical properties of these complexes
compared to related formamidinate complexes. Through the use of infrared spectroscopy the σ-donor
capacity of this ligand is demonstrated to be reduced relative to the structurally isomorphous formamidi-
nate congener, which supports previously advanced theoretical calcluations and DFT results reported
herein. These electronic differences are highlighted by the stark contrast in reaction outcomes at
rhodium; where [(Dipp2N3)Rh(CO)2] (1) is an isolable, stable complex and the formamidinate complex is
not. The coordination chemistry of the triazenide ligand for the s-block metal complexes (M = Li, Na, K)
has been shown to give structurally isomorphous complexes to the formamidinate analogue. In contrast
to the amidinate complexes, these complexes show extreme lability of coordinated, volatile Lewis-bases,
which in turn-yields the highly insoluble base-free triazenide complexes. These complexes are also syn-
thesized directly in the absence of donor solvents. This triazenide ligand has proven to be a suitable
ligand for stabilising reactive main group hydrides of Group 13 (M = Ga, In) and attempts at the analogous
thallium hydride complex by halide–hydride exchange are reported. Finally attempts at the synthesis of
low valent main group complexes are reported ([MIL], M = In, Ga) are also reported, which yield instead
disproportionation products ([MIIIXL2], M = Ga, In; [{MIIXL}2], M = Ga).
Received 24th January 2020,
Accepted 30th March 2020
DOI: 10.1039/d0dt00285b
rsc.li/dalton
ently large NC(H)N angle and have been utilised to enforce the
close proximity of metal–metal bonded species in transition
Introduction
The use of organoamides as support ligands in coordination metal paddlewheel complexes.12,13 An isoelectronic but fre-
chemistry has expanded dramatically in recent years.1,2 quently overlooked monoanionic N,N′-ligand set is the triaze-
Bidentate N,N′-ligands such as amidinates,3–5 guanidinates6,7 nide (Fig. 1A). Their underutilisation in coordination chem-
and β-diketiminates,8,9 have received particular attention due istry may lie in their inferred inferior donor capacity relative to
to their demonstrable potential to stabilise metal complexes the formamidinate, as indicated by the lower pKa of tria-
featuring unusual oxidation states and/or bonding modes, par- zenes.14 This was further demonstrated by an NPA (natural
ticularly when coupled with the kinetic stability afforded by population analysis) study, which showed significantly
sterically demanding N-substituents. A notable subclass of reduced charge on the terminal N atoms in N,N′-bis(phenyl)
amidinates is the formamidinates, where the C-substituent is triazenide (−0.38) relative to that in N,N′-bis(phenyl)formami-
a hydrogen atom (Fig. 1B). Formamidinate ligands are well dinate (−0.60).15 In spite of the electronic limitations of this
suited to forming bridging interactions,10,11 due to the inher- ligand framework, the ease at which steric bulk can be incor-
porated, enabled by scalable and modular syntheses, as
demonstrated by Niemeyer15–18 and others,19–23 has led to
aSchool of Chemistry, University of New South Wales, Kensington, NSW 2052,
renewed interest in this ligand class.24,25
Australia
bSchool of Chemistry, Monash University, Clayton, VIC 3800, Australia.
In order for the true utility of the triazenide ligand family
to be determined relative to the more common amidinate con-
geners, a comparative structural study is required. There are
E-mail: alasdair.mckay@monash.edu
†Electronic supplementary information (ESI) available: Full details of experi-
mental details, X-ray crystallography, spectroscopic and other analytical data.
CCDC 1971987–1972014, 1976265 and 1976266. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/d0dt00285b
‡Present address: Department of Chemistry University of Warwick, Gibbet Hill
Road, Coventry, CV4 7AL, United Kingdom.
relatively few examples where isostructural metal complexes
−
featuring the N,N′-bis(2,6-diisopropylphenyl)triazenide (N3Dipp2
,
Fig. 1A) and N,N′-bis(2,6-diisopropylphenyl)formamidinate
(Fiso−, Fig. 1B) have been structurally and spectroscopically
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from which a few orange crystals could be separated. X-ray
crystallography revealed these to be a trinuclear rhodium imi-
nocarbamoyl
cluster,
[Li{(Fiso′CO)Rh(μ-CO)}3Cl]
(2).41
Presumably 2 is formed by migratory insertions of a molecule
of CO into the Rh–N bond of the transient [(Fiso)Rh(CO)2], fol-
lowed by trimerisation in the presence of the metathesis bypro-
duct LiCl. The migratory insertion of CO into M–N bonds of
manganese(I), iron(II) and cobalt(I) amidinate complexes has
previously been reported.39,42,43 By contrast, monomeric cis
dicarbonyl complexes were obtained when bulky amidinate
([DippNC(tBu)NDipp]−, Piso−) and guanidinate ([DippNC
(NCy2)NDipp]−, Giso−) ligands were deployed.41
Fig. 1 Monoanionic N,N’-bis(diisopropylphenyl)-substituted ligands
employed in this study: A [N3Dipp2]−, B [Fiso]−.
As monomeric triazenide and formamidinate complexes
featuring terminal CO moieties proved inaccessible by this
route, we envisaged they may be accessed by the carbonylation
of dilute solutions of rhodium diolefin complexes,44 as a
means of frustrating the formation of higher nuclearity carbo-
nyl containing clusters. The diolefin precursor, [(Dipp2N3)Rh
(COD)] (3), was prepared by the metalation of Dipp2N3H with
[{Rh(μ-OEt)(COD)}2] in toluene (Scheme 1). Whilst, the analo-
gous formamidinate complex, [(Fiso)Rh(COD)] (4), was pre-
pared by a metathesis procedure similar to that used for the
preparation of bulky amidinates and guanidinates
(Scheme 1).45 These complexes were obtained in moderate to
good isolated yields as yellow crystalline solids (3: 46%; 4:
characterised. Notable exceptions to this dearth include
studies of groups 226–28 and 11,29,30 as well as zinc22,31 and alu-
minium complexes.32–34 Such a comparative study is ripe to be
made given that complexes of Fiso span the periodic table,
most notably the rare earth metals, making comparative struc-
tural studies viable.11
Herein we report the preparations of rhodium(I) triazenide
and formamidinate complexes, including infrared spectro-
scopic analysis of rhodium(I) carbonyl complexes to determine
potential differences in donating ability of the sterically
similar ligands. Several alkali metal triazenide complexes were
prepared and structurally characterised to enable comparison
of their solid-state structures with the vast library of alkali
metal formamidinate complexes reported. The ability of this
triazenide to kinetically stabilise low oxidation state main
group metals as well as main group metal terminal hydride
moieties despite the hypothesised electronic differences (vide
supra) was also studied.
1
79%). The H and 13C{1H} NMR spectra of 3 and 4 are consist-
ent with their formulations, notably the formamidinate NCH
3
resonance displays rhodium coupling (δ 8.09, JRhH = 2.3 Hz).
Treating a dilute CH2Cl2 solution of 3 with CO(g) resulted in a
rapid colour change from yellow to red (Scheme 1).
Characterisation by IR spectroscopy of an aliquot of the reac-
tion mixture acquired as soon as practicable, displayed two
new absorption bands in the CO-stretching region (2084 and
2028 cm−1). These are mildly blue shifted relative to the bands
Results and discussion
Comparison of steric and donor properties
observed upon carbonylation of 4 (2078 and 2011 cm−1
,
Whilst many methods to assess the donor properties of mono- CH2Cl2 solution) and strongly relative to the amidinate ([(FcC
anionic N,N′-ligands have been reported,35 the IR spectra of {NCy}2)Rh(CO)2] 2052 and 1977 cm−1 (mineral oil mull);39
rhodium dicarbonyl complexes presented as a straightforward ([(Piso)Rh(CO)2] 2063 and 1992 cm−1, CH2Cl2 solution) and
way to assess donor properties of triazenides and related guanidinate ([(Giso)Rh(CO)2] 2055 and 1983 cm−1, CH2Cl2
ligands.36–38 Arnold previously reported a monomeric ferro- solution). These species, prepared by carbonylation of
cene-substituted amidinate complex of rhodium dicarbonyl, rhodium diolefins and assigned as monomeric rhodium cis
prepared by reacting [{Rh(μ-Cl)(CO)2}2] with two equivalents dicarbonyl complexes, are transient as new carbonyl stretches
of
a
lithium amidinate complex.39 The reaction of
[{Rh(μ-Cl)(CO)2}2] with two equivalents of in situ prepared
[(Dipp2N3)Li(THF)2] afforded [(Dipp2N3)Rh(CO)2] (1), as an
orange crystalline solid in high yield. The 1H NMR spectra of 1
display resonances that are sharp and consistent with a high
degree of symmetry in solution. The IR spectrum of 1 (CH2Cl2)
features terminal (2020 cm−1) and bridging (1814 cm−1
)
carbonyl stretches, indicative of a carbonyl bridged oligomeric
structure, which is notably distinct from that of the
dimeric rhodium dicarbonyl benzamidinate complex,
[Rh2{µ-CPh(NPh)2}2(CO)4], where cis terminal dicarbonyl moi-
eties are observed.40
An attempt to prepare the formamidinate analogue by the
Scheme 1 Preparation and carbonylation of the rhodium(I) 1,5-
same route resulted in the formation of an intractable mixture, cyclooctadiene triazenide and formamidinate complexes, 3 and 4.
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Table 1 Selected bond lengths (Å), angles (°) and steric metrics for
complexes 3 and 4
Metric
3
4
Rh–N
N–N {N–C}
Rh-COD alkene centroid
N–N–N {N–C–N}
GRh(L)a {ECA (°)}
%VBur(L)b
2.0905(11)
1.3134(14)
2.0008(13)
106.20(15)
41.31 {160.0}
42.5
2.092(2), 2.104(3)
{1.324(4), 1.305(4)}
1.980(4), 1.991(4)
{114.2(3)}
41.27 {159.9}
39.1
a GRh(L) and ECA values were calculated from the crystal structure data
using the program Solid-G.53 b %VBur values were calculated from the
crystal structure data using SambVca 2.1.54 A sphere of radius 3.5 Å,
included hydrogen atoms and scaled Bondi radii were employed in
these calculations.
Fig. 2 Molecular structure of
1 (50% displacement ellipsoids). All
hydrogen atoms omitted and Dipp groups depicted as wireframes for
clarity. Symmetry operation used to generate # atoms: 1 − x, 1 − y, 1 − z.
Selected bond lengths (Å) and angles (°): Rh(1)–Rh(1)# 2.5113(5), Rh(1)–
C(1) 1.920(3), Rh(1)–N(1) 2.050(2), Rh(1)–N(3)# 2.056(3), Rh(1)–C(2)
2.082(4), Rh(1)–C(2)# 2.166(4), N(1)–N(2) 1.308(3), N(2)–N(3) 1.299(3),
N(1)–N(2)–N(3) 117.0(2).
by using both solid angle (G-parameter and Equivalent Cone
Angle, ECA)53 and %VBur calculations.54 These calculations
(Table 1) suggest that monoanionic N,N′-ligands in 3 and 4 are
indistinguishable based on their steric profiles.
Density functional theory calculations at the BP86/def2-
TZVP level in conjunction with the D3 empirical dispersion
correction were carried out to optimise the full monomeric
molecules [(L)Rh(CO)2] and those of the free anionic ligands.55
These accurately reproduced the molecular geometries (where
available) including ([(Dippnacnac)Rh(CO)2] (Table 2).36,37 The
calculated CO-stretching frequencies of these rhodium cis
dicarbonyl complexes are red shifted relative to the experi-
mental data (Table 2), which can be fitted by linear regression
(Fig. S76†).
are observed upon monitoring the reaction mixtures for
several hours. On the basis of these data, it is evident that the
triazenide is less electron donating than the formamidinate,
as well as the other ligands.46,47
ˉ
Complex 1 crystallises in space group P1 with two unique
half dimer molecules in the asymmetric unit. The molecules
are similar hence only one is depicted and discussed in detail
(Fig. 2). The two rhodium centres are bridged by two triazenide
ligands trans to one another, with the coordination sphere of
each rhodium further furnished by two bridging and two term-
inal carbonyl moieties, affording non-crystallographic D2h sym-
metry, consistent with the simple IR spectrum observed.
Complex 3 crystallises in space group C2/c with a half mole-
cule in the asymmetric unit, whilst 4 crystallises in space
As per Niemeyer’s DFT study,15 significantly reduced charge
on the terminal N atoms is observed for both the free triaze-
nide and in [(Dipp2N3)Rh(CO)2] relative to those of the other
donor sets (Table 2). Examination of the frontier orbitals of
each system reveals the HOMO to be primarily based on the p
orbitals of the terminal nitrogen atoms of the donor set.
Whilst the LUMO has some dyz orbital character which is
π-antibonding to both the π*(C–O) and the p orbital of the
central atom of the donor set. In the specific case of the triaze-
nide, both the HOMO and LUMO display a greater degree of
delocalisation across the entire donor set (Fig. 4). To this end,
the HOMO of [(Dipp2N3)Rh(CO)2] is the most stable, and
unsurprisingly is destabilised with increasing charge on the
coordinated nitrogen atoms (Table 2).56,57 The population of
π*(C–O) orbitals similarly increase as the energy of the HOMO
is raised in the series, consistent with the red-shifting of the
carbonyl stretches. These data are consistent with the pre-
viously advanced argument that triazenides are weaker donor
ligands when compared to analogous monoanionic N,N′-
ligands.
ˉ
group P1 with a full molecule in the asymmetric unit. The
crystal structure of 6 is depicted in Fig. 3 and relevant metrical
data can be found in Table 1. The Rh–N and Rh–C bond
lengths for 3 and 4 are indistinguishable within error,
however, the triazenide in 3 does display a more acute bite
angle relative to the formamidinate in 4. We,21,48–50 and
others51,52 have analysed the overall steric demand of a ligand
Reactivity and structural studies: alkali metal complexes
Having established that the sterically similar ligands possess
different donating abilities, we sought to prepare and structu-
rally characterise alkali metal complexes of the triazenide to
enable comparison with its formamidinate congener, for
which a large library of structural data is available.10 The alkali
Fig. 3 Molecular structure of
hydrogen atoms omitted for clarity. Symmetry operation used to gene-
rate # atoms: 1 − x, y, 3/2 − z.
3 (50% displacement ellipsoids). All
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Table 2 Selected spectroscopic, structural and theoretical parameters associated with monomeric rhodium cis dicarbonyl complexes ligated
monoanionic N,N’-ligands
[(Dipp2N3)Rh(CO)2]
[(Fiso)Rh(CO)2]
[(Piso)Rh(CO)2]
[(Giso)Rh(CO)2]
[(Dippnacnac)Rh(CO)2]
ν(CO)/cm−1
r(CO)/Å
Expt.a
DFTb
Expt.
DFT
Expt.
DFT
Expt.
DFT
Expt.
DFT
Expt.
DFT
Rh
2084
2053
2028
1997
—
1.154
—
1.860
—
2.078
—
60.24
—
104.91
0.06
−0.34
0.00
2078
2049
2011
1991
—
1.156
—
1.853
—
2.103
—
63.13
—
112.06
0.08
−0.55
0.25
2063
2041
1.133(2)
1.156
1.854(2)
1.859
1992
1982
1.133(2)
1.156
1.858(2)
1.859
2.0603(14)
2.079
63.11(6)
62.77
107.61(15)
107.51
0.10
−0.57
0.50
0.351
0.394
−5.127
−2.546
2.581
2055
2034
1.138(2)
1.158
1.847(2)
1.855
1983
1975
1.139(2)
1.158
1.859(2)
1.856
2.0604(14)
2.090
64.13(5)
63.56
107.27(15)
107.96
0.10
−0.61
0.54
0.348
0.406
−4.982
−2.206
2.776
2053
2039
1.136(2)
1.156
1.869(2)
1.866
1988
1982
1.137(2)
1.156
1.871(2)
1.866
2.051(1)
2.065
89.79(4)
89.19
—
—
0.12
−0.54
—
1.154
1.860
2.078
1.156
1.853
2.103
r(RhC)/Å
r(RhN)/Å
2.0457(15)
2.087
2.0423(14)
2.075
2.049(1)
2.065
∠(NRhN)/°
∠(NXN)c/°
NPA Charge
Nd
Xc
Occupancye
Rh–C*
0.352
0.374
−5.208
−3.181
2.027
0.339
0.387
−5.154
−2.683
2.471
0.345
0.397
−5.069
−2.118
2.951
CuO*f
EHOMO/eV
ELUMO/eV
ΔEHOMO–LUMO/eV
a IR data acquired in CH2Cl2 solution, with the exception of [(Dippnacnac)Rh(CO)2] for which the reported IR data was acquired as a KBr disk.36,37
b Unscaled values. c For [(Dipp2N3)Rh(CO)2] X = N, for the other systems X = C. d Mean of the coordinated N atoms, calculated using NBO 7.0.58 e Mean
value. f Sum of the occupancies of the anti-bonding orbitals.
Scheme 2 Preparation alkali metal triazenide complexes 5–7.
these ether complexes readily desolvate under thermal con-
ditions or by application of dynamic vacuum to regenerate the
base-free alkali metal triazenides (see ESI†). This contrasts
with the previously reported complexes of Fiso which tend to
retain their donor solvent ligands.60,61
The 1H and 13C{1H} NMR spectra of these donor co-
ordinated complexes (C6D6, 298 K) exhibit a high degree of
symmetry (C2v), which is indicative of N,N′-chelation of the
alkali metal centre by the triazenide or flexibility fast on the
Fig. 4 Selected orbitals for the optimised molecule: (a) [(Dipp2N3)Rh
(CO)2] (HOMO), (b) [(Fiso)Rh(CO)2] (HOMO), (c) [(Dipp2N3)Rh(CO)2]
NMR timescale.
(LUMO), (d) [(Fiso)Rh(CO)2] (LUMO). Isosurfaces plotted with Chemcraft
with a contour value of 0.05.
Several of these donor ligated alkali metals have been struc-
turally characterised.41 The structures are largely unremark-
able in that they resemble those of their Fiso analogues, in
metal triazenides, [M(N3Dipp2)] (M = Li 5, Na 6 or K 7), were that they are monomeric and bidentate chelation of the alkali
readily obtained as off-white precipitates by treatment of the metals is observed (Fig. 5).10 The M–N and M–O bond lengths
parent triazene with the respective alkali metal base in in these complexes are largely indistinguishable from those in
toluene. Unlike the related N-2,6-terphenyl triazenide com- their Fiso analogues.60
plexes, [M(N3Dmp2)], which show excellent solubility in hydro-
One noteworthy exception is the THF solvate of the potass-
carbon solvents,59 these compounds display negligible solubi- ium triazenide, 7-THF1.5. This compound crystallises from
lity in non-donor solvents such as toluene or n-hexane. They toluene as a disordered linear coordination polymer with an
do however readily dissolve in donor solvents affording the overall empirical formula of [(Dipp2N3)K(THF)1.5]. The major
respective solvate complex (Scheme 2), which in turn display (ca. 50%) component (Fig. 6A) is similar to that of [{K(THF)2K
high solubility in non-donor solvents. It should be noted that (Fiso)2}n],61 with alternating [K(THF)2]+ units that bridge [K
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the donor set, thus enabling the bridging [K(THF)]+ unit to be
coordinated by both of the terminal nitrogen atoms of that
donor set (Fig. 6B). This distinct binding mode leads to a
decreased need for the potassium to coordinate a second
molecule of THF. This adaptability of coordination mode, in
this case to compensate for a reduced number of ether
ligands, is a distinct advantage of the triazenide ligand relative
to its formamidine congeners.
Reactivity and structural studies: main group metal complexes
Fig. 5 Molecular structures (50% displacement ellipsoids) of 6–15c5
(left) and 7–18c6 (right). All hydrogen atoms omitted for clarity.
Previously we were able to stabilise low oxidation state Group
13 metals by deploying a sterically demanding N-2,6-terphenyl
triazenide.62 Following the procedure developed by
Niemeyer,17 a toluene solution of Dipp2N3H was treated with
TlOEt, workup of the resultant red solution, afforded [Tl
(N3Dipp2)] (8) as a deep red crystalline solid (Scheme 3). No
reaction was observed when we attempted to prepare the for-
mamidinate analogue by the same route (Scheme 3) consistent
with the reduced acidity of the formamidinate N–H proton.
Attempts to prepare the indium analogue by the stoichiometric
addition of 7 to either indium(I) chloride or indium(I) iodide
in toluene at −78 °C resulted in the isolation of the dispropor-
tionation products [InCl(N3Dipp2)2] (9) and [InI(N3Dipp2)2]
(10) respectively (Scheme 3). Similarly, the gallium(II) dimer,
[{GaI(N3Dipp2)}2] (11), was isolated upon workup of the reac-
tion of 7 with excess ‘GaI’ in toluene at −78 °C (Scheme 3).63
Repeating the reaction with a stoichiometric amount of ‘GaI’
afforded, [GaI(N3Dipp2)2] (12), upon workup (Scheme 3). These
mirror the disproportionation products obtained when Fiso or
other amidinate ligand were deployed.64,65 Attempts to prepare
the chloride analogue of 11, by reacting 5 with Ga[GaCl4], simi-
larly resulted in disproportionation with [GaCl(N3Dipp2)2] (13)
isolated (Scheme 3). Likewise attempts to prepare indium(II)
triazenide complexes were unsuccessful. By contrast bulky
amidinate and guanidinate ligands are well known to stabilise
low oxidation state main group metals.7,66–68
Fig. 6 Molecular structure of 7-THF1.5 (25% displacement ellipsoids)
illustrating the disordered components: (a) the bis THF form and (b) a
mono THF form. All hydrogen atoms omitted for clarity. Symmetry oper-
ation used to generate # atoms: 1 − x, 3 − y, 1 − z.
(N3Dipp2)2]− units. Single crystals containing exclusively the
[{K(THF)2K(N3Dipp2)2}n] form of the coordination polymer
may be obtained be recrystallisation directly from solutions in
THF, however repeated destructive phase changes (T < 180 K)
and the propensity for this morphology to desolvate at
ambient temperature (vide supra) lead to routine low-quality
structural data. Within the [K(N3Dipp2)2]− units, both triaze-
nide are in the Z-anti isomeric form and coordinate the potass-
ium via η6,κ1-binding modes. The binding of the [K(THF)2]+
units to the adjacent triazenides is distinct from that in [{K
(THF)2K(Fiso)2}n], whereby the central nitrogen atom of the
triazenide donor set also coordinates to [K(THF)2]+ (Fig. 6A).
This mode of binding by the triazenide is reminiscent of that
reported for [Ba(N{SiMe3}2)2(THF)K(N3Dipp2)].26 Two other
lower occupancy disordered forms are observed, which have
the overall formula [{K(THF)K(N3Dipp2)2}n]. These polymers
are distinct from the major component in that the both the Z-
anti and E-anti isomeric forms of the triazenide are observed
in the [K(N3Dipp2)2]− unit. The latter isomeric form binds the
Having failed to stabilise low oxidation state complexes
with the N-Dipp triazenide our attention switched to the stabil-
isation of heavy Group 13 metal hydrides. Our previous studies
Scheme 3 Attempted syntheses of low oxidation Group 13 triazenide
potassium in an η6,κ1-binding mode via the central nitrogen of complexes and isolation of disproportionation products, 8–13.
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Scheme 4 Synthesis of complexes 14–15 and attempted preparation
of the thallium hydride analogue.
have shown that an N-2,6-terphenyl triazenide imparts con-
siderable stability to Group 13 and 14 terminal hydride com-
plexes, with the notable exception of an indium hydride
Scheme 5 Synthesis of Group 14 metal triazenide complexes, 19–21.
complex.62 Jones previously reported that the addition of two intractable precipitate (Scheme 4). An identical outcome was
equivalents of FisoH to LiInH4 resulted in isolation of an observed when the analogous formamidinate complex, [TlCl
indium hydride complex with considerable thermal and (Fiso)2] (18), was treated with Na[BEt3H]. This suggests that
aerobic stability,69 thus the thermal and aerobic stability of despite the considerable thermal stability observed in the
complexes of the sterically similar triazenide were worthy of lighter congeners the triazenide ligand is unable to stabilise
investigation; indeed we have previously shown this to be the the presumed transient thallium hydride species; therefore for
case for [AlH(N3Dipp2)2] (dec. 230–236 °C)32 and [AlH(Fiso)2] main group hydrides the triazenide and formamidinate
(dec. 231–233 °C).33 The addition of two equivalents of ligands are of equal merit.
Dipp2N3H to a cold diethyl ether solution of LiGaH4 or LiInH4
The success of the N3Dipp2 ligand in stabilising heavy
afforded [GaH(N3Dipp2)2] (14) and [InH(N3Dipp2)2] (15) Group 13 hydrides inspired further investigation into the
respectively, as colourless cubes in good yields (Scheme 4). chemistry with main group metals. In this context it is note-
Sharp and strong M–H stretching absorptions were observed worthy that few Group 14 metal triazenide complexes have
in the IR spectra of the complexes 14 and 15 (1912 and been reported.62,73–75 Previously, we prepared [(Dmp2N3)GeCl]
1747 cm−1 respectively). These compare well with the bis for- and [(Dmp2N3)SnCl] by reacting [(Dmp2N3)Li(OEt2)] with an
mamidinate analogues [MH(Fiso)2] (M = Ga, 1911 cm−1; M = equimolar amount of [GeCl2(Dioxane)] or SnCl2 in diethyl
In, 1748 cm−1).33,69 Similarly, the thermal stabilities of 14 (dec. ether.62 Our initial investigation centred on preparing hetero-
190–210 °C) and 15 (dec. 164–172 °C) are indistinguishable leptic complexes ligated by the smaller N-Dipp substituted tria-
from their bis formamidinate analogues (dec. 211–213 °C and zenide, however, under these conditions homolytic complexes,
160–170 °C respectively).33,69 The 1H and 13C{1H} NMR spectra [Ge(N3Dipp2)2] (19) and [Sn(N3Dipp2)2] (20), were isolated in
of 14 and 15 each feature two methyl resonances (δ 14: 1.06, low yield (Scheme 5). The outcomes of these reactions mirrors
1.12; 15: 1.06, 1.14) and a single methine resonance (δ 14: 3.40; that reported by Jones for the sterically similar formamidinate
1
15: 3.39) in C6D6. The H NMR spectrum of 14 also exhibits a ligand, where [Ge(Fiso)2] was the only identifiable product.76
broad hydride resonance at δ 6.59, whilst the hydride resonance Likewise, complex 20 was isolated from the reaction of [Sn(N
was not observed in the 1H NMR spectrum of [GaH(Fiso)2].33
{SiMe3}2)2] with Dipp2N3H in a 1 : 1 stoichiometry (Scheme 5).
The considerable thermal stability of the indium hydride Identical outcomes were recently reported for this triazenide
triazenide complex led us to investigate whether these ligands by Fox and Johnson.75
may be able to stabilise a thallium hydride, which are as yet
As the heterolytic Group 14 triazenide complexes proved
unisolated in monometallic form.70 The synthetic route used inaccessible in our hands from simple divalent precursors, our
to prepare 14 and 15 could not be used to prepare a thallium attention turned to higher coordinate sources of tin(II).
equivalent as there is no available lithium tetrahydridothallate Gratifyingly, the reaction of Dipp2N3H with [({Me3Si}2N)SnBr
counterpart,71 instead we envisaged [TlH(N3Dipp2)2] may be (IEt)], prepared in situ from [Sn(N{SiMe3}2)2] and IEt·HBr,
accessed by a halide–hydride exchange reaction. The precursor afforded the heterolytic tin(II) triazenide complex, [(Dipp2N3)
1
thallium chloride complex, [TlCl(N3Dipp2)2] (16), was isolated SnBr(IEt)] (21). The H and 13C{1H} NMR spectra of 21 feature
as orange crystals from the reaction of anhydrous TlCl3 with single sets of triazenide and NHC resonances consistent with
two equivalents of 5-THF2 (Scheme 4). The bromide analogue, exchange, which is fast on the NMR timescale. Whilst we were
[TlBr(N3Dipp2)2] (17) was similarly prepared by treating an able to isolate a crystal of the heterometallic complex,
in situ prepared solution of TlBr3 (TlBr + Br2)72 with two [(Dipp2N3)SnCl·W(CO)5], suitable for crystallographic charac-
equivalents of 5-THF2, however, difficulty in removing excess terisation, from the reaction of 7 with [(THF)2SnCl2·W(CO)5],
Br2 (cf. preparation of TlBr3) resulted in 17 being isolated in the complicated nature of the reaction mixture prevented
very poor yield. Dropwise addition of a toluene solution of Na further spectroscopic characterisation.41
[BEt3H] (1.0 M in toluene) to a solution of 16 in toluene at
Compound 8 is dimeric in the solid-state (Fig. 7), with each
−78 °C resulted in the immediate formation of a dark grey thallium atom coordinated by four nitrogen atoms, two from
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general, each metal centre is five coordinate with two chelating
triazenide units and one halide atom, giving a heavily dis-
torted trigonal bipyramidal coordination geometry about the
metal centre, with N(3) and N(6) in the apical positions. These
N-atoms are significantly more distant from the metal centre
than the equatorial N(1) and N(4) donors. These geometries
are maintained in 19 and 20 due to the lone pair on the
metals.75 The N–N distances in the triazenide scaffolds of each
complex show close to complete bond delocalisation despite
the noted disparity in the M–N bond lengths (axial vs. equator-
ial). The degree of steric congestion in the bis(triazenide) com-
plexes can be seen in the non-orthogonal placement of the
arene ring planes to the N3 donor planes as would be expected
in the ideal geometry. The M–N bond lengths in 9, 14–16, 19
and 20 are largely indistinguishable from their Fiso
counterparts.33,64,69,76,77 Similarly the In–Cl bond length in 9,
and the metal hydride contacts in 14 and 15 (Table 3) are con-
sistent with those reported for their Fiso analogues (2.3574
(11), 1.65(6) and 1.71(6) Å respectively).33,64,69 Whilst the Tl–Cl
bond length in 16 is somewhat longer than that in 18.
Fig. 7 Molecular structure of
8 (50% displacement ellipsoids). All
hydrogen atoms and the second molecule in the asymmetric unit are
omitted for clarity. Selected bond lengths (Å) and angles (°): Tl(1)–N(1)
2.746(7), Tl(1)–N(3) 2.699(7), Tl(1)–N(4) 2.761(7), Tl(1)–N(6) 2.645(7), Tl
(2)–N(1) 2.727(7), Tl(2)–N(3) 2.734(7), Tl(2)–N(4) 2.64(7), Tl(2)–N(6) 2.757
(7), T1(1)⋯Tl(2) 3.4612(5), N(1)–N(2)–N(3) 110.5(7), N(4)–N(5)–N(6) 110.9
(7).
There are four crystallographically independent molecules
in the asymmetric unit of 21 that have no significant geometric
differences and, therefore, the metric parameters of only one
will be discussed. The compound possesses a distorted square
pyramidal tin centre (Fig. 9). The binding of the triazenide is
each triazenide ligand. This bonding motif is very common for
thallium triazenide complexes reported in the literature.17
Compounds 9–10, 13–20 (with the exception of 19) crystal-
lise in space group P21/c. The representative molecular struc-
ture of 15 is depicted in Fig. 8. Salient metrical parameters for
these compounds are listed in Table 3. All these compounds
are isomorphous with their Fiso analogues.33,64,69,76,77 In
Fig. 9 Molecular structure of 21 (50% displacement ellipsoids). Two
views shown. All hydrogen atoms except for the hydride are omitted for
clarity. Selected bond lengths (Å) and angles (°): Sn(1)–Br(1) 2.7235(5), Sn
(1)–N(1) 2.198(3), Sn(1)–N(3) 2.523(3), Sn(1)–C(25) 2.287(4), N(1)–N(2)
1.325(4), N(2)–N(3) 1.280(4), N(1)–Sn(1)–N(2) 53.14(10), N(1)–Sn(1)–Br(1)
86.61(8), N(3)–Sn(1)–Br(1) 133.41(7), N(1)–Sn(1)–C(25) 105.76(12), N(3)–
Sn(1)–C(25) 82.26(11), C(25)–Sn(1)–Br(1) 88.55(10).
Fig. 8 Molecular structure of 15 (25% displacement ellipsoids and van
der Waals’ spheres; green = hydride, pink = indium, blue = nitrogen,
grey = carbon). View down the In(1)–H(1) vector (left) and normal to In
(1)–H(1) vector (right). All hydrogen atoms except for the hydride are
omitted for clarity.
Table 3 Selected bond lengths (Å) and angles (°) for complexes 9–10, 13–20
Metric
9
10
13
14
15
16
17
18
19
20
M–Xa
M–N(1)
M–N(3)
M–N(4)
M–N(6)
N(1)–M–N(3)
N(4)–M–N(6)
2.3532(5)
2.1771(15)
2.2663(14)
2.1922(15)
2.2562(14)
56.90(5)
2.6933(12)
2.194(9)
2.249(8)
2.195(10)
2.201(8)
57.0(3)
2.1617(9)
1.967(3)
2.112(2)
1.973(3)
2.105(2)
61.8(1)
1.52(3)
1.69(3)
2.4364(7)
2.2802(19)
2.3672(18)
2.2998(19)
2.3523(19)
54.45(6)
2.558(1)
2.264(6)
2.318(5)
2.312(6)
2.307(6)
54.4(2)
2.4251(8)
2.261(2)
2.330(2)
2.235(2)
2.371(2)
58.04(8)
57.95(8)
—
—
1.971(2)
2.147(2)
1.979(2)
2.134(2)
61.22(9)
61.17(9)
2.2061(19)
2.2739(18)
2.1906(19)
2.2758(18)
56.00(6)
1.9917(17)
2.2078(18)
2.0001(18)
2.1902(17)
59.65(7)
59.57(7)
2.236(3)
2.329(3)
2.221(3)
2.325(2)
55.21(10)
55.16(10)
56.91(5)
57.1(3)
61.6(1)
56.26(7)
54.35(6)
54.4(2)
a X = halide or hydride.
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highly asymmetric and the electron density is not completely
delocalised across the triazenide donor set. Similar asym-
metric triazenide binding was observed in the molecular struc-
ture of Te(N3Dipp2)2.75 The Sn–Br bond length is 2.7235(5) Å,
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complexes [(Dippnacnac)SnBr]78 and [(PhC{NtBu}2)SnBr]79
(2.6373(2) and 2.620(2) Å respectively).
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Conclusions
We have prepared a range of transition metal and main group
metal triazenide complexes that complement the substantial
number of formamidinate complexes reported over the past
two decades. Similar stabilities and structural motifs have
been revealed, however several instances of new binding
motifs were uncovered, for example in the linear polymer
[(Dipp2N3)K(THF)1.5], which adapts the coordination behaviour
of the triazenide ligand to variable Lewis-base content in the
coordination polymer chain. The triazenide ligand has been
demonstrated to be a viable “drop-in” alternative ligand for the
stabilisation of main group hydrides, which display compar-
able thermal stability to their formamidinate congeners in-
spite of the reduced σ-donor capacity of this ligand as demon-
strated by IR spectroscopy of carbonyl rhodium complexes of
this ligand.
21 M. R. Gyton, M. Bhadbhade and M. L. Cole, Z. Anorg. Allg.
Chem., 2019, 645, 768–776.
22 N. Nimitsiriwat, V. C. Gibson, E. L. Marshall,
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Conflicts of interest
There are no conflicts to declare.
23 K. N. Buys, M. L. Cole and A. I. McKay, J. Organomet.
Chem., DOI: 10.1016/j.jorganchem.2020.121204.
24 D. Kalden, S. Krieck, H. Gorls and M. Westerhausen,
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Acknowledgements
We are grateful for the financial support from the Australian
Research Council (DP110104759) and the Australian
Government for funding postgraduate scholarships. We also
thank Mr Baptiste Leforestier (University of Warwick) for valu-
able discussions.
26 A. G. M. Barrett, M. R. Crimmin, M. S. Hill,
P. B. Hitchcock, G. Kociok-Köhn and P. A. Procopiou, Inorg.
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