Iron β-diiminate complexes with As2-, As4- And As8-ligands

Article abstract of DOI:10.1039/d0cc05173j

Different substituents at the β-diiminato ligand in low-valent [LFe(tol)] (L = β-diiminato) complexes fundamentally change their reactivity towards yellow arsenic. By using dmp (2,6-dimethylphenyl) as flanking groups, the tetranuclear complexes [(LFe)4As8

Full text of DOI:10.1039/d0cc05173j

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Iron b-diiminate complexes with As2-, As4- and
As8-ligands†
Cite this:DOI: 10.1039/d0cc05173j
Fabian Spitzer, Gabor Balazs, Christian Graßl and Manfred Scheer
*
´
´
Received 29th July 2020,
Accepted 29th September 2020
DOI: 10.1039/d0cc05173j
rsc.li/chemcomm
Different substituents at the b-diiminato ligand in low-valent [LFe(tol)] activations, which yielded different dinuclear products
(L = b-diiminato) complexes fundamentally change their reactivity [(L4Fe)2P4] or [(L3Fe)2(P2)2], if dipp (2,6-diisopropylphenyl) flank-
towards yellow arsenic. By using dmp (2,6-dimethylphenyl) as flanking ing groups are used. However, for dmp (2,6-dimethylphenyl)
groups, the tetranuclear complexes [(LFe)4As8] (L = L1 (1), L2 (2)) are substituted systems, exclusively tetranuclear [(LFe)4P8] (L = L1, L2
isolated. For dipp (2,6-diisopropylphenyl) substituted ligands, dinuc- (E)) with a realgar-like [P8]4ꢁ core were obtained.6 However, for
lear complexes [(LFe)2(cyclo-As4)] (L = L3 (3a), L4 (4a)) are obtained. Not the heavier congener, antimony, this crucial influence of the
only the choice of the ligand impacts the product formation, but also 2,6-position of the aromatic substituents was not confirmed.
the temperature of the crystallization can shift their ratio in the Recently, the isostructural [(LMg)4Sb8] (L = L4, L5 (F))7 was
solid state.
reported, which is supported by dipp as well as mes (2,4,6-
trimethylphenyl; L5) substituted ligands. Therefore, the question
Group 15 element ligand complexes have become an important arises about the reaction products of As4 with b-diiminato iron(I)
class of compounds at the intersection of main-group and precursors in general and about the substituent dependency at
transition-metal chemistry.1 While in the past decades, exten- the b-diiminato ligand towards the formed arsenic moiety in
sive investigations towards the isolation of polyphosphorus particular.
compounds have been carried out, the number of polyarsenic
Herein, we report about the reactivity of yellow arsenic
ligand complexes still remains limited.2 Most of the reported towards four [(LFe)(tol)] (L = L1, L2, L3, L4) complexes. Depen-
compounds are stabilized by cyclopentadienyl or strongly dent from their distinct steric and electronic properties, differ-
donating CO ligands.3 Recently, b-diiminato transition- ent tetranuclear products [(LFe)4As8] (L = L1 (1), L2 (2)) were
metal(I) systems have attracted attention as they enable the obtained, as well as different dinuclear complexes [(LFe)2(cyclo-
activation of yellow arsenic (As4) under remarkably mild As4)] (L = L3 (3a), L4 (4a)) and [(L3Fe)2(As2)2] (3b). These results
conditions.4 Thermodynamically unstable, but kinetically confirm the decisive influence of the ligand’s 2,6-substituents
stable compounds were accessible and gave insight into the (Me vs. iPr). Moreover, they show the importance of the crystal-
subsequent reactivity of As4 towards transition metal com- lization temperature on the solid-state structure composition of
pounds. The first reported b-diiminato polyarsenic complex such highly dynamic systems (vide infra).
was [(L4Cu)2As4] (A), which contains an intact As4 tetrahedron
(Scheme 1).5 Recently, we reported about more electron defi-
cient, binuclear cobalt complexes [(LCo)2As4] (L = L3 (B), L2 (C),
L1 (D)), which in dependency of their used ligands and the
corresponding Coꢀ ꢀ ꢀCo distance in the solid state, stabilize
various rectangular (in B), prismatic (in C) or asterane-like
(in D) As4-ligands.4 The influence of ligand substituents on
the product formation was also recognized in FeI-mediated P4
¨
¨
¨
Institut fur Anorganische Chemie, Universitat Regensburg, Universitatsstrasse 31,
93040 Regensburg, Germany. E-mail: manfred.scheer@chemie.uni-regensburg.de;
Web: http://www.uni-regensburg.de/chemie-pharmazie/anorganische-chemie-scheer/
† Electronic supplementary information (ESI) available. CCDC 2019408–2019412.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/d0cc05173j
Scheme 1 Selected examples of En ligand (E = P, As, Sb) complexes
supported by the b-diketiminato (L) ligand system.
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All reactions were performed under rigorous exclusion of distances are in the range of 2.4379(5)–2.4430(5) Å in 1 and
light and under comparable conditions: A solution of approx. 2.4250(3)–2.4423(3) Å in 2; and therefore, are in line with As–As
200 mg [LFe(tol)] (L = L1, L2, L3, L4) in 10 mL toluene was single bonds (for comparison: As–As single bond in yellow
transferred onto a freshly prepared load of purified yellow arsenic determined by electron diffraction: 2.435(4)10 and
arsenic (excess).8 The reaction mixture was stirred at ambient 2.44(3)11 Å, by DFT calculations:12 2.437 Å). The distance
temperature for 15 – 60 minutes. The solvent was removed in between the bridgehead arsenic atoms are between 2.4929(5)
vacuum and the remaining solid was stored (at least) overnight and 2.4973(3) Å, which corresponds to a moderately elongated
in order to complete the desired decomposition of unreacted As–As single bond. Those findings agree with previously
yellow arsenic into grey arsenic. The remaining solid was reported As–As bond lengths in [{Cp0 0 0Fe(CO)2}2{Cp00 0Fe(CO)}2
dissolved in Et2O or toluene and filtered over Celite in order (m4-Z1:1:2:2-As8)] (G), [{Cp*Cr(CO)3}4(m4-Z1:1:1:1-As8)] (H) and
to remove the formed grey arsenic. After investigation of the [(Cp*2Sm)4(m4-Z1:1:1:1:1:1:1:1-As8)] (I), which are the only exam-
1
reaction solution by H NMR spectroscopy and FD-mass spec- ples for realgar-type [As8]4ꢁ ligand complexes so far.8,13 While
trometry, the solution was concentrated and stored at 8 1C or the coordination mode in I is similar to that found in 1 and 2,
ꢁ30 1C for crystallization.
this differs in G and H. The As–As distances within their [As8]4ꢁ
The reactions of [LFe(tol)] (L = L1, L2) with As4 leads to the ligands are not significantly changed by the different nature of
formation of two tetranuclear and isostructural products the metal centers (Fe or Cr), their coordination modes (Z1 vs.
[(LFe)4As8] (L = L1 (1), L2 (2)) in 3% and 9% for 1 and 2, Z1:1) or the supporting ligand systems (CpR vs. L1, L2). A
respectively. Both compounds are highly soluble in toluene detailed comparison is given in Table S3 in ESI.†
and are red-brown in color. LIFDI mass spectrometry as well as
elemental analysis (for 2) confirms their molecular composi- to the more bulky iPr, the reactions of [LFe(tol)] (L = L3, L4) with
tion. Both complexes are paramagnetic in solution. Compound As4 leads to formation of only dinuclear products [(LFe)2(m-Z4:4
Changing the aromatic substituent in 2,6-position from Me
-
2 has a magnetic moment (meff) of 5.62 mB in C6D6 solution.9 In As4)] (L = L3 (3a), L4 (4a)) (Scheme 2). By crystallization solid
the 1H NMR spectrum of 1 in C6D6 solution, a signal pattern is solutions containing 3a and [(L3Fe)2(As2)2] (3b) (2% crystalline
detected between 311.15 and ꢁ22.82 ppm corresponding to a yield) as well as 4a and [(L4Fe)2(As3)] (4b) (10% crystalline yield)
half of the ligand, which suggests a D2d symmetry on the NMR are obtained (Scheme 2).15 All compounds are readily soluble in
time scale.
Et2O and are red-brown in color. LIFDI mass spectrometry
Single-crystal X-ray structural analysis reveal that the mole- confirms their molecular composition and excludes the for-
cular structures of 2ꢀ2.5 toluene and 1ꢀtoluene are isostructural mation of possible tetranuclear products (like in 1 or 2).
(Fig. 1 for 1). Both compounds contain a realgar-like [As8]4ꢁ
All complexes are paramagnetic in solution and for each a
ligand coordinating to four [LFe] (L = L1 (1), L2 (2)) fragments. signal pattern of a quartered ligand (two perpendicular mirror
The coordination geometry of the Fe metal centres in 1 and 2, planes) is detected in C6D6 solution, which suggests D2h or D2d
respectively, is best described as distorted tetrahedral. All As–As symmetry on NMR time scale. For 4a, the assigned signals in
the 1H NMR spectrum (C6D6) are between 9.41 and ꢁ5.71 ppm.
Since the compounds 3a and 3b co-crystallize in their crystal
Fig. 1 Core structure of 1 in crystals of 1ꢀtoluene (hydrogen and carbon
atoms are omitted for clarity; thermal ellipsoids are drawn at 50% prob-
ability level). A representation of 1 with its complete ligands is shown in the
inset.
Scheme 2 Reactivity of As4 towards different [LFe(tol)] complexes. High-
lighted boxes: Products obtained as ‘‘solid solution’’ (vide infra) of two
components ‘a’ and ‘b’ in the crystal.
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Table 1 Comparison of the structural parameters in [(LFe)2(m-Z4:Z4-As4)]
(L = L3 (3a), L4 (4a)) and [(CpBIGFe)2(m-Z4:Z4-As4)] (J)14
structure, signals of two different sets of L3 ligands are expected
in the 1H NMR spectrum. To our surprise, exclusively one set of
signals is detected between 109.1 and ꢁ15.0 ppm in C6D6
solution. This, on the one side confirms their paramagnetic
character (range of chemical shifts), on the other side suggests
a fast dynamic isomerization in solution. It was expected, that
the hypothetical coalescence and signal splitting of 3a and 3b
could be detected by 1H NMR spectroscopy of a sample of
crystals dissolved at low temperature and by successively
increasing the temperature of the solution. However, the low
solubility of crystals at lower temperature (ꢁ100 1C) prevents
this investigation.
Therefore, we assume that the existence of the two different
isomers 3a and 3b is only present in the solid state and cannot
be observed in solution. Surprisingly, we found that the crystal-
lization temperature influences the ratio of 3a : 3b in the
crystals. In single crystals, grow at 8 1C the ratio of 3a : 3b
is 61.5 : 38.5, while in single crystals grown at ꢁ30 1C the ratio
is 82 : 18.
Due to their similarities, the molecular structure of
[(L4Fe)2(m-Z4:Z4-As4)] (4a) and [(L3Fe)2(m-Z4:Z4-As4)] (3a) are
described comparatively. Both structures are centrosymmetric
and consist of two parallel [LFe] (L3, L4) fragments. They are
bridged by a square-planar cyclo-As4 ring with As–As–As angles
of 89.62(2)/90.38(2)1 in 3a and 88.54(3)/91.46(3)1 in 4a. The
central [Fe2As4] core reveals a slightly distorted octahedral
shape and is depicted in Fig. 2, left side (exemplified for 3a).
The As–As distances are almost equidistant (2.4276(7) and
2.4423(6) Å in 3a and 2.3898(8) and 2.4200(10) Å in 4a). The
values in 4a are comparable to the ones found in the Zintl
phase [(K@18-crown-6)2As4] (2.3871(4) and 2.3898(4) Å) con-
taining a cyclo-[As4]2ꢁ unit.16 The As–As distances in 3a are in
the range of As–As single bonds (2.44(3) Å)10–12 and resemble
best the ones (2.3905(8)–2.4562(7) Å) found in [(CpBIGFe)2
(m-Z4:4-As4)] (J, see Table 1).17 The most remarkable differences
between complexes 3a and 4a are the Feꢀ ꢀ ꢀFe0 distances
(3.787 Å in 3a vs. 3.982 Å in 4a) and the orientation of the As4
ligand related to the [L3,4Fe] planes (see o1/o2 and further
details in Table 1).
Complex
3a
4a
J14
3b
d(Feꢀ ꢀ ꢀFe0) [Å]
d(As–As) [Å]
3.787
2.4276(7)
2.4423(6)
3.982
2.3898(8)
2.4200(10)
3.5414(7)
2.3909(7)
2.4201(7)
2.4331(6)
2.4558(7)
2.4144(7)
—
2.940(5)
2.2414(13)a
3.314(2)b
d(Fe–As) [Å]
2.5516(7)
—
2.5681(7)
2.5707(5)
—
2.6374(5)
2.461(2)
2.4673(17)
2.499(2)
2.5022(19)
0.4(2)
90(2)
25.2(3)
0
2.5130(7)
o1, o2 [1]
3.69(6)
86.69(6)
17.5(1)
0
41.26(5)
50.21(5)
15.19(8)
0
—
Y [1]
F [1]
—
—
a
b
Distance within As2 ligand. Distance inter As2 ligands.
of two [L3Fe] fragments bridged by a pair of As2 ligands (Fig. 2,
right). In comparison to the previously discussed complexes 4a
(3.982 Å) and 3a (3.787 Å), the Feꢀ ꢀ ꢀFe0 distance in complex 3b is
further decreased to a value of 2.940(5) Å.18 Doubtlessly, the
shape of the arsenic ligands in 3b is significantly different. The
distance between both As2 units is 3.314(2) Å, which excludes
any bonding interaction of the arsenic atoms. The As–As
distance within each As2 ligand is 2.2414(13) Å and therefore,
in the range of an AsQAs double bond (e.g. in diarsene
R1AsQAsR2: 2.224(2)–2.2634(3) Å).19 Additionally, it is in accor-
dance with the As–As distances (2.272(2)–2.300(2) Å) of other
As2-ligand containing complexes,20 especially with [{(CpRCo)(m-
Z2:Z2-As2)}2] (R = 1,2,4-tBu3H2 (2.2795(5) Å),19a Me4Et (2.272(1)
and 2.279 Å)19b), which display similar [Fe2(m-Z2:Z2-As2)2] cores.
In summary, the synthesis and characterization of the first
polyarsenic containing b-diiminato iron complexes is reported.
Low-valent iron(I) complexes [LFe(tol)] (L = b-diiminato) posses-
sing different substituted b-diiminato ligands were reacted with
yellow arsenic and several tetranuclear or dinuclear products are
formed. b-diiminato ligands with dmp (2,6-dimethylphenyl) flank-
ing groups yielded tetranuclear complexes of the type [(LFe)4As8]
(L = L1 (1), L2 (2)), stabilizing a realgar-like [As8]4ꢁ unit revealing a
unique bridging coordination mode of the metal moieties. For the
sterically more bulky dipp (2,6-diisopropylphenyl) substituted
ligands, the dinuclear complexes [(LFe)2(cyclo-As4)] (L = L3 (3a),
The second component in the solid solution of 3ab is
[(L3Fe)2(m-Z2:2-As2)2] (3b). It is centrosymmetric and consists
Fig. 2 Central core structures of complexes 3a (left) and 3b (right) in crystals of 3ab (hydrogen and carbon atoms are omitted for clarity; thermal
ellipsoids are drawn at 50% probability level).
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L4 (4a)) and [(L3Fe)2(As2)2] (3b) are formed. There is found a
further dependency of the FeꢀꢀꢀFe distance on the formed cyclo-
As4 or double As2 ligands in the complexes. If the b-diiminato
ligands at the Fe centres allow a closer FeꢀꢀꢀFe distance, a
rectangular shaped As4 unit (double As2 ligands) are formed
whereas, if the distance gets longer a cyclo-As4 unit results. The
shape of the Asn-ligand in the solid state is influenced by the
crystallization temperature, which leads to different ratios of
the arsenic ligand in the solid solution of 3a, b. Such effect has
never seen before and will influence further studies definitively.
This work was supported by the Deutsche Forschungsge-
meinschaft (DFG) within the project Sche 384/40-1.
´
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13 (a) O. J. Scherer, K. Pfeiffer and G. Wolmershauser, Chem. Ber., 1992,
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