Stitching together SNx units in the coordination sphere of zirconium: Assembly of a tris(imido)sulfite and a hydrazidobis(imido)sulfite

Article abstract of DOI:10.1039/c2cc17594k

Reaction of a zirconium imido and a hydrazinediido complex with bis(trimethylsilyl)sulfur diimide yielded the corresponding formal [2+2] cycloaddition products, the trisimidosulfito complex and the hydrazidobis(imido)sulfito complex containing an unprecedented SN ₂(N-N) unit. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc17594k

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COMMUNICATION
Stitching together SNx units in the coordination sphere of zirconium:
assembly of a tris(imido)sulfite and a hydrazidobis(imido)sulfitew
Thorsten Gehrmann, Matthias Kruck, Hubert Wadepohl and Lutz H. Gade*
Received 5th December 2011, Accepted 6th January 2012
DOI: 10.1039/c2cc17594k
Reaction of a zirconium imido and a hydrazinediido complex
with bis(trimethylsilyl)sulfur diimide yielded the corresponding
formal [2+2] cycloaddition products, the trisimidosulfito
complex and the hydrazidobis(imido)sulfito complex containing
an unprecedented SN2(N–N) unit.
The coupling of the multiply bonded nitrogen ligand in group
Scheme 1 Synthesis of the tris(imido)sulfito-zirconium complex 2
4 metal imides and hydrazinediides with unsaturated organic
substrates has given rise to a multitude of new types of
chemical transformations.1,2 These not only have attracted
interest in their own right but as key steps in catalytic reactions
such as hydroaminations,3 hydrohydrazinations4 or more
complex catalytic domino reactions.5 Whereas the formation
of N–C bonds has been the focus of these studies, the MQN
unit may also undergo coupling reactions with other main group
elements,6,7 with the potential of forming main group imides.
Sulfur nitrogen compounds have been the focus of much
interest for more than half a century,8 initially for their own
sake and later in investigations of their properties as non-metal
conducting materials.9 They have also played an important
role in developing the isoelectronic principle based inter alia on
the synthesis of analogues of the ubiquitous sulfur oxygen
compounds.10 Iminosulfindiamides,10a,11 frequently referred
to as triimidosulfites S(NR)32ꢀ, have been studied both in
terms of their S–N bonding12 and as tridentate capping ligands
for alkali and coinage metals.13 They are typically generated
by addition of a lithiated amine to a sulfur diimide and may be
coordinated to other metals in subsequent transmetallations.
These types of sulfur nitrogen compounds have not been
generated within the coordination sphere of a transition metal,
and the highly reactive nature of the imido and hydrazinediido
ligands in group 4 metal complexes suggests the possibility of
this type of buildup.
by formal [2+2] cycloaddition of the imido complex
bis(trimethylsilyl)sulfur diimide.
1 with
chemistry,1a,14 with one molar equivalent of bis(trimethyl-
silyl)sulfur diimide immediately generated the trisimidosulfito
TBS py
complex [Zr(N2 N )k2-N,N0(TMS–N–S(QN–TMS)NDIPP)]
(2) (Scheme 1). In complex 2 the S(NR)32ꢀ ligand is coordinated
in a k2-N,N fashion with the aryl-N unit derived from the imido
ligand being located next to the coordinating pyridyl ring of the
ancillary tripod such that the ortho CH of the pyridine ring is
directed to the deshielding segment of the anisotropy cone of
the 2,6-diisopropylphenyl substituent. This gives rise to an
unusually high field shifted resonance (8.35 ppm compared to
9–10 ppm in most related complexes) of this proton.
Details of the molecular structure of complex 2 were established
by X-ray diffraction (Fig. 1).z The tris(imido)sulfito ligand adopts
the expected non-planar arrangement [S+(NSN) = 308.81], due
to a stereochemically active lone pair at the sulfur atom. There are
two independent molecules in the asymmetric unit which are
related to one another essentially by inversion of configuration
of S(1). The S–N bond lengths indicate the bonding within this
fragment. The distances between the sulfur atom and the nitrogen
atoms connected to the metal centre are in agreement with bond
lengths characteristic for S–N single bonds (S(1)–N(4): 1.727(2) A
[1.723(2) A for the second independent molecule in the unit cell]
and S(1)–N(5): 1.670(2) [1.682(3)] A), while the significantly
shorter S(1)–N(6) bond is consistent with a double bond
(1.554(2) [1.535(3)] A).8,10
Reaction of the zirconium imido complex [Zr(N2TBSNpy)-
(QNDIPP)(py)] (1), stabilized by a tripodal diamido-pyridyl ligand
previously employed in this field of group 4 metal complex
The corresponding reaction of the previously reported
hydrazinediido complex [Zr(N2TBSNpy)(QNNPh2)(py)] (3)6
with one molar equivalent of bis(trimethylsilyl)sulfur
diimide yielded the corresponding formal [2+2] cycloaddition
Anorganisch-Chemisches Institut, Universitat Heidelberg,
¨
Im Neuenheimer Feld 270, 69120 Heidelberg, Germany.
E-mail: lutz.gade@uni-hd.de
TBS py
product [Zr(N2 N )k2-N,N0(TMS–N–S(QN–TMS)NNPh2)]
w Electronic supplementary information (ESI) available: Character-
isation and crystallographic data for 1, 3 and 4; computational details
and further discussion. CCDC 857217 and 857218. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c2cc17594k
(4) (Scheme 2).
Whereas previous cycloaddition products of hydrazinediido-
zirconium complexes displayed the tendency to undergo
thermal N–N bond cleavage and subsequent transformations,5e,6,7
c
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Chem. Commun., 2012, 48, 2397–2399 2397

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Fig. 2 Molecular structure of complex 4. Selected bond lengths (A) and
angles (1): Zr(1)–N(1) 2.052(2), Zr(1)–N(3) 2.346(2), Zr(1)–N(4) 2.176(2),
Zr(1)–N(5) 2.119(2), S(1)–N(4) 1.725(2), S(1)–N(5) 1.686(2), S(1)–N(6)
1.542(2), Zr(1)–N(4)–S(1) 100.20(8), Zr(1)–N(5)–S(1) 103.80(9), N(4)–S(1)–
N(6) 110.08(11), N(4)–S(1)–N(5) 88.26(9), N(5)–S(1)–N(6) 108.39(10).
Fig. 1 Molecular structure of complex 2. The asymmetric unit contains
two independent molecules. Selected bond lengths (A) and angles (1) [data
for the second independent molecules given in brackets]: Zr(1)–N(4)
2.139(2) [2.117(2)], Zr(1)–N(5) 2.117(2) [2.133(2)], S(1)–N(4) 1.727(2)
[1.723(2)], S(1)–N(5) 1.670(2) [1.682(3)], S(1)–N(6) 1.544(2) [1.535(3)],
Zr(1)–N(4)–S(1) 98.7(1) [98.8(1)], Zr(1)–N(5)–S(1) 101.5(1) [100.8(1)],
N(4)–S(1)–N(6) 109.4(1) [109.39(14)], N(4)–S(1)–N(5) 90.6(1) [90.8(1)],
N(5)–S(1)–N(6) 108.9(1) [108.8(1)].
Fig. 3 Kohn–Sham molecular orbitals (HOMO-4) illustrate the
localised p-bond between N(6) and S(1) with only non-bonding
contributions at N(4) and N(5) in 2 (left) and 4 (right).
Scheme 2 Synthesis of the hydrazidobis(imido)sulfito-zirconium
complex 4 by formal [2+2] cycloaddition of the hydrazinediido
complex 3 with bis(trimethylsilyl)sulfur diimide.
at N(4) and N(5) in both cases, which may be attributed to a
localised S(1)–N(6) double bond (Fig. 3). This interpretation is
further supported by the Wiberg indices for the relevant bonds
[S(1)–N(6) 1.33 (2), 1.31 (4), S(1)–N(4) 0.73 (2), 0.73 (4),
S(1)–N(5) 0.87 (2), 0.88 (4)]. The Wiberg index for
N(4)–N(7) of 1.08 is consistent with an N–N single bond
within the hydrazido fragment. (See ESIw for details.)
The small Wiberg indices for the S–N bonds which include
the nitrogen atoms ligating the metal centre are indicative of
strongly polar bonds. The electron density in these bonds is
polarised towards the electronegative nitrogen atoms, an effect
which is further amplified by the influence of the positively
charged zirconium atom (ligand–metal p-donation). The
resulting electron deficiency at the sulfur atom is stabilised
by conjugation with N(6). Stalke et al. have previously shown
that tris(imido)sulfito ligands provide several resonance forms
stabilising the negative charge by delocalisation.12,13,16 In the
case at hand a localised resonance structure with terminal partial
charges and a single SQN bond accounts for the most significant
contribution (Fig. 4, left).17 The bis(imido)hydrazidosulfite
fragment on its own lacks the competition for electron density,
hence delocalisation is observed and all S–N bonds exhibit
Wiberg indices close to 1.
complex 4 proved to be thermally highly stable and could be
fully characterized.
The hydrazidobis(imido)sulfite ligand in 4 is the first such
fragment to be generated. Based on the spectroscopic data its
coordination mode remained ambiguous. In order to clarify
this aspect and to establish the structural details of the
coordinated S–N fragment in 4 a single crystal X-ray structure
analysis was carried out (Fig. 2).z
The geometry of the non-planar hydrazidobis(imido)sulfite
ligand in 4 is similar to that observed for the tris(imido)sulfite
in 2 [S+(NSN) = 306.71]. The hydrazido unit is directly
bonded to the Zr centre through the Na atom. The S–N units
connected to the metal centre are again consistent with the
presence of S–N single bonds (S(1)–N(4): 1.725(2) A;
S(1)–N(5): 1.686 A) and the non-coordinated S(1)–N(6) unit
having multiple bond character (1.552(2) A).
More details related to the intra-ligand bonding were
obtained by a DFT study carried out for both 2 and 4 at the
B3PW91/6-31G(d) level of theory.15 The analysis of the
Kohn–Sham molecular frontier orbitals established weak
p-bonding between S(1) and N(6) with non-bonding contributions
c
2398 Chem. Commun., 2012, 48, 2397–2399
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Fig. 4 The negative charges may be stabilised by delocalisation within
the p-system. In the metal complex the resonance form on the left
accounts for the most significant contribution resulting in virtually
localised charges on the nitrogen atoms bonded to the metal centre.
4 Review: A. L. Odom, Dalton Trans., 2005, 225.
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¨
Chemie, Bd. 32 Erganzungswerk zur 8. Auflage, Teile 1 – 11,
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The preparation of complexes 2 and 4 has demonstrated that
the pattern of formal [2+2] cycloaddition at the highly polar
MQN bonds of group 4 metal imides and hydrazinediides may
be employed for the assembly of imido-derivatives of 3rd row
main group element imides and hydrazides and thus element-N
coupling in a broader sense.
This work has been supported by the Deutsche Forschungs-
gemeinschaft (SFB 623, TP A7).
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Notes and references
z Crystal data for 2: C39H76N6SSi4Zr, M = 864.70, orthorhombic,
space group P21212, a = 24.884(11), b = 27.141(12), c = 14.484(6) A,
V = 9782(7) A3, Z = 8. T = 100(2) K, 209 306 reflections measured,
24 252 unique [Rint = 0.090] which were used in all calculations. Final
R indices: R(F) [Fo > 4s(Fo)] = 0.0398, wR(F2) [all data] = 0.0874.
Flack absolute structure parameter 0.01(2). Crystal data for 4:
C39H69N7SSi4Zr, M = 871.65, orthorhombic, space group P212121,
a = 10.222(5), b = 19.352(10), c = 23.638(13) A, V = 4676(4) A3,
Z = 4. T = 100(2) K, 110 828 reflections measured, 14 274 unique
[Rint = 0.086] which were used in all calculations. Final R indices: R(F)
[Fo > 4s(Fo)] = 0.0374, wR(F2) [all data] = 0.0788. Flack absolute
structure parameter ꢀ0.05(2).
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c
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Chem. Commun., 2012, 48, 2397–2399 2399