Yttrium complexes of arsine, arsenide, and arsinidene ligands

Article abstract of DOI:10.1002/anie.201500173

Deprotonation of the yttrium-arsine complex [Cp′₃Y{As(H)₂Mes}] (1) (Cp′=η⁵-C₅H₄Me, Mes=mesityl) by nBuLi produces the μ-arsenide complex [{Cp′₂Y[μ-As(H)Mes]}₃] (2). Deprotonation

Full text of DOI:10.1002/anie.201500173

Angewandte
Chemie
DOI: 10.1002/anie.201500173
Yttrium–Arsenic Complexes
Yttrium Complexes of Arsine, Arsenide, and Arsinidene Ligands**
Thomas Pugh, Andrew Kerridge, and Richard A. Layfield*
Abstract: Deprotonation of the yttrium–arsine complex
[Cp’3Y{As(H)2Mes}] (1) (Cp’ = h5-C5H4Me, Mes = mesityl)
by nBuLi produces the m-arsenide complex [{Cp’2Y[m-
transfer, and would also furnish new opportunities for using
arsenic ligands to influence the electronic structure and
magnetism of lanthanide(III) complexes. With these possibil-
ities in mind, we now report the first example of a rare-earth
metal arsinidene complex.
Our strategy involved the initial synthesis of a primary
arsine complex of yttrium to establish the metal–arsenic bond,
followed by deprotonation of the {YAsH2R} unit to give
corresponding yttrium–arsenide and yttrium–arsinidene com-
plexes. Thus, adding one stoichiometric equivalent of mesi-
tylarsine to Cp’3Y in toluene led to the formation of
[Cp’3Y{As(H)2Mes}] (1) (Cp’ = h5-C5H4Me, Mes = mesityl),
which was crystallized as colorless blocks in 88% yield
(Scheme 1). To obtain the yttrium arsenide complex, 1 was
À
As(H)Mes]}3] (2). Deprotonation of the As H bonds in 2 by
nBuLi produces [Li(thf)4]2[{Cp’2Y(m3-AsMes)}3Li], [Li-
(thf)4]2[3], in which the dianion 3 contains the first example
of an arsinidene ligand in rare-earth metal chemistry. The
molecular structures of the arsine, arsenide, and arsinidene
complexes are described, and the yttrium–arsenic bonding is
analyzed by density functional theory.
R
are-earth metal compounds containing soft heteroatom
donor ligands have attracted considerable interest in recent
years.[1–6] The combination of Lewis acidic M3+ cations with
heavy p-block donor atoms results in a hard–soft mismatch
that can lead to unusual bonding properties and to distinct
reactivity. Within this context, rare-earth metal complexes of
anionic phosphorus donor ligands such as phosphide (R2PÀ)
have been extensively studied.[7] A key development occurred
in 2008, when a lutetium phosphinidene (RP2À) complex was
structurally characterized and its phosphinidene transfer
reactivity towards aldehydes and ketones demonstrated.[8]
Phosphinidene complexes of other rare-earth metals were
subsequently reported, and their phosphinidene transfer
chemistry and small-molecule activation reactions de-
scribed.[9–14] Despite the increased activity in rare-earth
metal phosphinidene chemistry, the area is considerably
underdeveloped relative to transition metal phosphinidene
chemistry.[15] A terminally bonded phosphinidene ligand
remains a key target in rare-earth metal chemistry, although
a uranium complex of such a ligand was reported recently.[16]
The chemistry of rare-earth metal complexes with arsenic
donor ligands is almost entirely unexplored: arsenide (R2AsÀ)
complexes are rare,[17–22] and arsinidene (RAs2À) ligands are
unknown in rare-earth metal chemistry. The development of
synthetic routes to rare-earth metal arsinidene complexes
could lead to more novel reactivity, such as arsinidene
Scheme 1. Synthesis of the complexes described herein.
dissolved in toluene and one equivalent of nBuLi was added.
The ensuing work-up allowed isolation of the trimetallic
m-arsenide complex [{Cp’2Y[m-As(H)Mes]}3]·toluene (2·tolu-
ene) in 59% yield.Deprotonation of 2 by nBuLi in thf,
followed by crystallization from the same solvent, resulted
in formation the heterobimetallic yttrium–lithium
m-arsinidene complex [Li(thf)4]2[{Cp’2Y(m-AsMes)}3Li]·thf,
[Li(thf)4]2[3]·thf, as orange crystals in 73% yield.
[*] Dr. T. Pugh, Dr. R. A. Layfield
School of Chemistry, The University of Manchester
Oxford Road, Manchester, M13 9PL (UK)
E-mail: Richard.Layfield@manchester.ac.uk
Dr. A. Kerridge
Department of Chemistry, Lancaster University
Lancaster, LA1 4YB (UK)
[**] T.P. and R.A.L. thank the UK EPSRC for financial support through
Responsive Mode grant EP/K008722/01 and Core Capability grant
EP/K039547/1. The National Service for Computational Chemistry
Software (NSCCS) is acknowledged for providing access to the
Slater HPC facility. A.K. thanks the EPSRC for the award of a Career
Acceleration Fellowship (grant EP/J002208/1).
The molecular structure of the yttrium arsine complex
À
1 (Figure 1) features a Y1 As1 bond of length 3.0945(6) ꢀ,
and a Y1-As1-C19 angle of 122.55(7)8. The three Cp’ ligands
5
À
are h -coordinated to yttrium, with Y C bond lengths in the
range 2.672(3)–2.746(3) ꢀ (average 2.706 ꢀ). The IR spec-
À
trum of 1 shows characteristic As H stretches at 2120 and
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201500173.
2154 cmÀ1 (Supporting Information, Figure S9). The 1H NMR
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À
arsenic center carries an exo mesityl substituent. The As H
stretching vibrations were observed in the IR spectrum at
2120 and 2154 cmÀ1 (Supporting Information, Figure S9).
The 1H–13C HSQC spectrum of 2·toluene at 298 K reveals
that two proton environments at d(1H) = 2.51 and 2.60 ppm,
1
with relative integrals of 1:2, do not engage in J coupling to
À
carbon, which identifies them as the As H protons and
indicates that there are two magnetically inequivalent arsenic
1
environments. The H NMR spectrum of 2·toluene features
four resonances in the range d(1H) = 6.78–6.94 ppm and 5.84–
6.51 ppm, which correspond to the six mesityl CH protons and
the 24 Cp’ CH protons, respectively (Supporting Information,
Figures S3–S5). The various CH3 environments occur in the
range d(1H) = 1.72–2.66 ppm.
Figure 1. Molecular structure of 1, with ellipsoids set at 50% proba-
bility.[27] Unlabeled atoms are carbon; hydrogen atoms, except those
bonded to arsenic, are not shown.
Complex 2 is the first rare-earth metal complex of
a primary arsenide ligand; however, several crystallograph-
ically characterized rare-earth metal complexes of secondary
arsenide ligands have been reported.[18–22] A range of synthetic
routes have been employed to access secondary arsenide
complexes, including, for example, deprotonation of Ph2AsH
by the lutetium–lithium methyl complex [Cp2Lu(m-CH3)2Li-
(tmeda)] (tmeda = N,N,N’,N’-tetramethylethylenediamine),
which resulted in the formation of the arsenide-bridged
spectrum of 1 in [D6]benzene (Supporting Information,
Figure S2) features a resonance corresponding to the arsine
protons at d(1H) = 3.12 ppm, the mesityl methyl groups occur
at d(1H) = 2.08 and 2.18 ppm, and the mesityl aromatic
protons occur at d(1H) = 6.67 ppm. The Cp’ methyl group
occurs at d(1H) = 1.95 ppm and the Cp’ CH protons occur at
d(1H) = 5.86 and 5.77 ppm. The 1H NMR spectrum of 1 shows
no evidence for free arsine.
species [Cp2Lu(m-AsPh2)2Li(tmeda)].[17] Activation of As As
À
The only rare-earth metal arsine complex reported to date
contains a macrocyclic amidoarsine ligand coordinated to
bonds by samarium(II) reduction has also been used to access
arsenide complexes such as [Cp*2SmAsPh2], which features
a terminally bonded [Ph2As]À arsenide ligand.[19,20] Lanthani-
de(II) arsenide and arsolyl complexes can be accessed by salt
metathesis reactions of LnI2 with alkali-metal arsenide salts;
for example, Mes2AsK reacts with SmI2 to give trans-
[(Mes2As)2Sm(thf)4].[21,22]
À
yttrium, with Y As bond lengths in the range 2.9545(7)–
2.9968(7) ꢀ.[17] Complex 1 is therefore the first rare-earth
metal complex of a primary arsine ligand.
The molecular structure of the yttrium arsenide 2
(Figure 2) consists of a central Y3As3 chair-like ring, with
each yttrium ligated by two m-arsenide ligands and two h5-Cp’
The structure of the arsinidene-ligated complex dianion 3
(Figure 3) also consists of a central chair-like Y3As3 core, with
three arsinidene ligands bridging the yttrium centers. A
lithium cation caps the core of the structure and bonds to the
three arsenic donors, such that the arsinidene ligands adopt an
À
ligands. The Y As bond lengths in 2 are in the range 2.977(2)–
3.019(2) ꢀ (average 2.998 ꢀ), and therefore they are, on
À
average, approximately 0.10 ꢀ shorter than the Y As bond in
1, which is due to the stronger electrostatic attraction between
À
yttrium and the arsenide ligand. The As-Y-As bond angles are
overall m3-bridging mode. The Y As bond distances in 3 are
2.8574(6)–2.8893(7) ꢀ (average 2.8722 ꢀ), making them
À
in the range 88.66(5)–96.26(5)8. The Y C bond lengths in 2
À
À
are 2.59(1)–2.67(1) ꢀ, and the average Y C distance of
shorter on average than the Y As bonds in 2 by more than
0.12 ꢀ. It is also noteworthy that the Y···Y separations in 2 are
2.63 ꢀ is approximately 0.08 ꢀ shorter than in 1. The Y-As-Y
angles in 2 are 130.56(5), 135.09(6), and 135.66(6)8, and each
Figure 2. Molecular structure of 2, with ellipsoids set at 50% proba-
Figure 3. Molecular structure of 3, with ellipsoids set at 50% proba-
bility.[27] Unlabeled atoms are carbon; hydrogen atoms are not shown.
bility.[27] Unlabeled atoms are carbon; hydrogen atoms are not shown.
2
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5.465–5.548 ꢀ, whereas those in 3 are 5.266–5.314 ꢀ; overall,
therefore, the Y3As3 core of 3 is more compact than that of 2.
replacing the para and Cp’ methyl groups with hydrogen
atoms. Geometry optimizations employing two exchange
correlation functionals were carried out in the gas-phase and
using a continuum dielectric, and the results of the calcu-
lations with the hybrid PBE0 functional, including dielectric
effects, are described.
À
Relative to 2, a broader range of Y C bond lengths, that is,
À
2.59(2)–2.731(6) ꢀ, and a greater average Y C bond length
of 2.67 ꢀ, are found in 3. The distortion of the Y3As3 chair
À
conformation in 3 is reflected in the As-Y-As and Y-As Y
À
bond angles of 91.59(2)–94.87(2)8 and 133.59(2)–136.49(2)8,
respectively. The lithium cation in 3 is ligated by three
arsinidene ligands and resides 0.889(8) ꢀ above the mean
Comparing the calculated and experimental Y As bond
lengths, we find good agreement for 1 (3.113 ꢀ vs. 3.095 ꢀ)
but also that the calculations slightly overestimate the average
distance for 2 (3.061 ꢀ vs. 2.998 ꢀ) and for 3 (2.912 ꢀ vs.
2.872 ꢀ). The discrepancies are most likely due to the
inability of the simulations to fully account for solid-state
intermolecular interactions, and also the effects of the counter
cations on 3. Despite this, the overall trend in the decrease of
À
plane of the arsenic atoms. The Li As bond lengths are
2.539(8), 2.563(7) and 2.615(8) ꢀ, and the As-Li-As angles
are 107.2(3), 108.2(3) and 110.0(3)8. An ortho methyl group
on one mesityl substituent is oriented towards Li1, and the
relatively short Li1···C54 distance of 2.777(8) ꢀ may indicate
an agostic interaction similar to that observed in other lithium
complexes containing CH2R substituents (R = H, alkyl,
silyl).[23]
À
the Y As bond length is reproduced. The atomic charges
were calculated by natural bond orbital (NBO) and quantum
theory of atoms in molecules (QTAIM) analyses (Table 1).
The 1H NMR spectrum of [Li(thf)4]2[3]·thf, recorded
30 min after sample preparation in [D8]thf at 298 K (Support-
ing Information, Figure S6), features two resonances at
d(1H) = 6.71 and 6.80 ppm, both of which integrate to three
protons and correspond to two types of mesityl meta CH
environments. The Cp’ CH protons occur as four resonances
at d(1H) = 6.44, 6.15, 5.07, and 4.85 ppm, each of which
integrates to six protons. Distinct singlets for the ortho, para,
and Cp’ CH3 environments were observed in the region
d(1H) = 1.54–2.62 ppm. The appearance of the 1H NMR
spectrum of [Li(thf)4]2[3]·thf is therefore consistent with the
arsinidene complex possessing a C3 symmetry axis coincident
with Li1 and approximately perpendicular to the Y3 plane.
The 7Li NMR spectrum of [Li(thf)4]2[3]·thf features two
resonances, with a sharp peak at d(7 Li) = À1.64 ppm corre-
sponding to the [Li(thf)4]+ cations and a broader peak at
d(1H) = 4.41 ppm corresponding to the {LiAs3} environment
in [Li(thf)4]2[3]·thf (Supporting Information, Figure S8). An
additional feature of the 1H NMR spectrum of [Li-
(thf)4]2[3]·thf is that, over time, additional resonances which
were observed as minor components after 30 min grow in
intensity (Supporting Information, Figure S7). After a period
of only two hours, the additional resonances account for
a significant component of the NMR spectrum. It was not
possible to identify the decomposition products; however, this
unexpected feature suggests that the arsinidene ligands in
[Li(thf)4]2[3]·thf react with the thf solvent.
À
Table 1: Experimental and calculated Y As bond lengths, atomic charges
(q) for 1–3, and QTAIM-derived topological parameters at the bond
critical points.
1
2
3
[a]
[b]
À
Y As [ꢀ]
3.0945(6)
3.113
+1.09, +0.42
+1.89, +0.86
35.86, 30.36
0.024
2.998[c]
3.061[c]
2.8722[c]
2.912[c]
+1.13, À0.53
+1.82, À0.81
35.89, 32.20
0.038
À
Y As [ꢀ]
qNBO (Y/As)
+1.26, À0.10
+1.85, +0.03
35.89, 31.30
0.030
q
QTAIM (Y/As)
lQTAIM
1
H
d(Y,As)
À0.00577
À0.0338
À0.0592
0.200
0.302
0.438
[a] Experimental. [b] PBE0+COSMO. [c] Average length.
lQTAIM =localization index, 1=electron density (e bohrÀ3), H=energy
density (a.u.), d=delocalization index.
Both types of analysis show an increasing negative charge on
the arsenic donor atom on moving from 1 to 2 to 3. The
QTAIM-derived localization index, l, which provides a mea-
sure of the number of electrons localized on a given atom,
increases by 0.94 from 1 to 2, and again by 0.90 from 2 to 3.
The analogous parameters for yttrium are essentially constant
across the three complexes, which indicates that the observed
À
decrease in Y As bond lengths is due to stronger ionic
interactions. However, the fact that the calculations produce
Dl < 1 implies a small-but-increasing non-ionic contribution
as the negative charge on arsenic increases.
To the best of our knowledge, complex 3 is the first rare-
earth metal complex of an arsinidene ligand, which is
surprising given that arsinidene ligands are well-known in
transition-metal chemistry.[24] Several alkali metal complexes
of arsinidene ligands have also been structurally character-
ized.[25] Complex 3 is related to the rare-earth metal
phosphinidene complexes, particularly the heterobimetallic
À
The Y As bonding was investigated further by a topolog-
ical analysis of the electron density. The parameter 1, which
describes the electron density at the QTAIM-derived bond
critical point (BCP), increases from 1 to 2 to 3, as expected
based on the decreasing Y As bond lengths, and which is
consistent with the change in l. The 1 values indicate an
À
lithium–scandium
complex
[(PNP)Sc(m-dmp)(m-Br)Li]
À
(PNP = N(2-iPr2PC6H3-4-Me)2, 2,6-Mes2C6H3, dme = dime-
thoxyethane).[10] Notably, the arsinidene ligands in 3 adopt
a m-bridging coordination mode, which is an obvious parallel
with rare-earth metal phosphinidene complexes.
The variation in the character of the yttrium–arsenic
bonding in complexes 1, 2, and 3 was investigated using
density functional theory: the calculations were simplified by
enhancement in the non-ionic contribution to the Y As
bonding in the arsinidene complex 3; however, the values are
still markedly less than expected for a typical covalent bond
(1 > 0.2). The values of the energy density (H) at the BCP, and
the values of the delocalization indices (d), which provides
a measure of the number of electrons shared between yttrium
and arsenic, are indicative of considerable ionic bonding
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Received: January 8, 2015
Published online: && &&, &&&&
Keywords: arsenic · arsinidene ligands · lithium ·
.
rare-earth elements · yttrium
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 1 – 5
These are not the final page numbers!

Angewandte
Chemie
Communications
Yttrium–Arsenic Complexes
T. Pugh, A. Kerridge,
R. A. Layfield*
&&&& — &&&&
Yttrium Complexes of Arsine, Arsenide,
and Arsinidene Ligands
As to why: Deprotonation of the yttrium–
arsine complex [Cp’3YAs(H)2Mes] (Cp’=
h5-C5H4Me, Mes=mesityl) yields the m-
arsenide complex [{Cp’2Y[m-
As(H)Mes]}3]. Deprotonation of the
arsenide complex by nBuLi produces
[Li(thf)4]2[{Cp’2Y(m-AsMes)}3Li], which
contains the first example of an arsini-
dene ligand in rare-earth metal chemistry.
The structures and bonding characteristic
of each complex are described.
Angew. Chem. Int. Ed. 2015, 54, 1 – 5
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!