A comparative study of magnetization dynamics in dinuclear dysprosium complexes featuring bridging chloride or trifluoromethanesulfonate ligands

Article abstract of DOI:10.1039/c7cc02457f

We utilized a rigid ligand platform PyCp₂^2- (PyCp₂^2- = [2,6-(CH₂C₅H₃)₂C₅H₃N]^2-) to isolate dinuclear Dy³⁺ complexes [(PyCp₂)Dy-(μ-O₂SOCF₃)]₂ (1) and [(PyCp₂)Dy-(μ-Cl)]₂ (3) as well as the mononuclear complex (PyCp₂)Dy(OSO₂CF₃)(thf) (2). Compounds 1 and 2 are the first examples of organometallic Dy³⁺ complexes featuring triflate binding. The isolation of compounds 1 and 3 allows us to comparatively evaluate the effects of the bridging anions on the magnetization dynamics of the dinuclear systems. Our investigations show that although the exchange coupling interactions differ for 1 and 3, the dynamic magnetic properties are dominated by relaxation via the first excited state Kramers doublet of the individual Dy sites. Compounds 1 and 3 exhibit barriers to magnetization reversal (U_eff = 49 cm^-1) that can be favorably compared to those of the previously reported examples of [Cp₂Dy(μ-Cl)]₂ (U_eff = 26 cm^-1) and [Cp₂Dy(thf)(μ-Cl)]₂ (U_eff = 34 cm^-1).

Full text of DOI:10.1039/c7cc02457f

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A comparative study of magnetization dynamics
in dinuclear dysprosium complexes featuring
bridging chloride or trifluoromethanesulfonate
ligands†
Cite this:DOI: 10.1039/c7cc02457f
Received 30th March 2017,
Accepted 29th June 2017
Corey P. Burns, ‡ Branford O. Wilkins,‡ Courtney M. Dickie,
DOI: 10.1039/c7cc02457f
Trevor P. Latendresse, Larry Vernier, Kuduva R. Vignesh,
rsc.li/chemcomm
Nattamai S. Bhuvanesh and Michael Nippe
*
2ꢀ
2ꢀ
We utilized a rigid ligand platform PyCp2
(PyCp2
= [2,6-
(CH2C5H3)2C5H3N]2ꢀ) to isolate dinuclear Dy3+ complexes [(PyCp2)Dy-
(l-O2SOCF3)]2 (1) and [(PyCp2)Dy-(l-Cl)]2 (3) as well as the mononuclear
complex (PyCp2)Dy(OSO2CF3)(thf) (2). Compounds 1 and 2 are the first
examples of organometallic Dy3+ complexes featuring triflate binding.
The isolation of compounds 1 and 3 allows us to comparatively evaluate
the effects of the bridging anions on the magnetization dynamics of the
dinuclear systems. Our investigations show that although the exchange
coupling interactions differ for 1 and 3, the dynamic magnetic properties
are dominated by relaxation via the first excited state Kramers Scheme 1 Synthesis of compounds 1, 2, and 3.
doublet of the individual Dy sites. Compounds 1 and 3 exhibit barriers
to magnetization reversal (Ueff = 49 cmꢀ1) that can be favorably
ꢀ 9
compared to those of the previously reported examples of [Cp2Dy(l-Cl)]2 closed-shell ligands (such as Clꢀ,7 CpFe(CO)2ꢀ,8 and SSiPh3
)
(Ueff = 26 cmꢀ1) and [Cp2Dy(thf)(l-Cl)]2 (Ueff = 34 cmꢀ1).
as well as radical ligand sets (bpym10 and tppz11). The subtle
interplay between coordination geometry and ligand donor effects
Highly paramagnetic molecules featuring intrinsic magnetic and their resulting effects on the magnetization dynamics are the
anisotropy can display slow magnetic relaxation as well as magnetic subject of current research and systematic experimental comparative
hysteresis. The development of such single-molecule magnets studies are of high importance in this regard.
2ꢀ
(SMMs) could allow the manufacture of high density spin-based
We were interested in utilizing the structurally rigid PyCp2
quantum memory1 or spin qubit devices.2 The large magnetic ligand (PyCp22ꢀ = [2,6-(CH2C5H3)2C5H3N]2ꢀ) for the development of
anisotropy of trivalent lanthanide ions has resulted in lanthanide- novel lanthanide platforms and the investigation of their magnetic
based SMMs with significant energy barriers to magnetization properties. Specifically, we aimed to prepare discrete triflate com-
reversal (Ueff)3 and improved blocking temperatures.4 Organo- plexes of Dy3+ which were previously not structurally or magnetically
metallic lanthanide complexes are a very important class of characterized. Herein we report the structural characterization and
lanthanide-based SMMs and includeꢀf5ascinating examples of magnetic properties of two new dinuclear Dy3+ complexes featuring
single ion magnets such as Er(COT)2 and Cp*2Dy(PhBPh3).6 either bridging ꢀOSO2CF3 ([(PyCp2)Dy-(m-O2SOCF3)]2 (1)) or Clꢀ
In particular cyclopentadienyl anion type ligands are used to ligands ([(PyCp2)Dy-(m-Cl)]2 (3)) as well as the mononuclear complex
stabilize Dy3+ complexes and they continue to attract attention (PyCp2)Dy(OSO2CF3)(thf) (2) (Scheme 1).
in this regard. Examples of magnetically investigated CpR–Dy
Compound 1 can be prepared via the reaction of anhydrous
type complexes include dinuclear Dy3+ complexes with bridging Dy(OSO2CF3)3 with the disodium salt PyCp2Na2 and subsequent
work up (see the ESI†).12 Single crystals of 1 can be obtained by
Department of Chemistry, Texas A&M University, 3255 TAMU, College Station,
TX, 77843, USA. E-mail: nippe@chem.tamu.edu
crystallization from CH2Cl2/hexane solvent mixtures. Dissolution
of 1 in thf and crystallization from thf/hexane solvent mixtures
results in a crystalline material of monomeric 2. A dinuclear
chloride-bridged complex 3 is obtained by the reaction of DyCl3
with PyCp2Na2 in thf and subsequent extraction into CH2Cl2 and
recrystallization. It is important to note that the analogous reaction
† Electronic supplementary information (ESI) available: Experimental details,
X-ray crystallographic data in CIF format, and additional figures and tables.
CCDC 1539709–1539711 and 1553970. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c7cc02457f
‡ These authors contributed equally.
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of two equivalents of NaCp with DyCl3 was previously reported to bridging triflate ligands in 1 result in larger Dyꢁ ꢁ ꢁDy distances
yield the desired dinuclear m2-Cl bridged product only as a mixture of 6.068[1] Å. The very tight crystal packing in crystals of 2
with a polymeric species after sublimation.7 Clearly, the intra- (Fig. S1, ESI†) results in close intermolecular Dyꢁ ꢁ ꢁDy distances
molecular pyridine moiety blocks the formation of a polymeric side of 6.541(1) Å. This close intermolecular proximity likely results
product and allows for a more facile access to the desired dinuclear in intermolecular interactions leading to weak intermolecular
complexes. In contrast to 1, 3 does not dissociate in thf and a magnetic coupling as well as enhanced quantum tunnelling of
crystalline material of 3(thf) was obtained. Thus, the triflate anions the magnetization (vide infra) for 2.
in 1 are more labile leaving groups than the chloride ions in 3.
The static magnetic properties of complexes 1–3 were investigated
The molecular structures of neutral compounds 1, 2, and 3 by means of variable temperature direct current (dc) magnetometry
were investigated by means of single crystal X-ray diffraction using an applied field of 1000 Oe (Fig. 2). The room temperature wMT
studies (Fig. 1, Tables S1 and S2, ESI†). The Dy3+ ions in all values of 27.90, 13.52, and 26.99 emu K molꢀ1 obtained for 1, 2, and
three structures have a ligand bond number of nine, with 3, respectively, are in good agreement with the expected values of
bonds to two Cp and one pyridine moiety and two additional 28.34 emu K molꢀ1 and 14.17 emu K molꢀ1 for di- and mono-
ligands. In complexes 1, 2, and 3 the average Dy–C distances nuclear Dy3+ complexes (6H15/2, S = 5/2, L = 5, and g = 4/3). All
(2.63[1] Å, 2.628[2] Å, and 2.64[1] Å, respectively) and the Dy–N complexes display decreases in their respective wMT values upon
distances (2.554[9] Å, 2.538(2) Å, and 2.581(9) Å, respectively) lowering the temperature with a more significant decrease at low
are fairly comparable for the two triflate complexes 1 and 2 and temperatures for 1 and 3. Such a decrease in wMT values is typically
are only slightly longer in the chloride bridged dimer 3. The observed and ascribed to the depopulation of the Stark sublevels of
Dy–Cl distances in 3 (2.683(3) Å and 2.785(3) Å) are much longer Dy3+ and/or possible magnetic interactions between Dy3+ centres (see
than the Dy–O distances in 1 (2.338[9] Å and 2.346[8] Å) and 2 the computational part).13 Surprisingly, complex 2 features a slight
(Dy–Othf = 2.456(1) Å and Dy–OOTf = 2.335(1) Å).
increase of wMT at very low temperatures (2 K) which may be
For dinuclear complexes 1 and 3, the relative orientation of indicative of intermolecular ferromagnetic coupling but this aspect
the pyridine moieties on each Dy3+ ion to each other could result was not further investigated.
in cis or trans geometries. As has been noted previously for closely
Variable temperature magnetization vs. field measurements
2ꢀ
related lanthanide PyCp2 complexes, the solid structure of 1 were carried out for 1 (Fig. S2, ESI†), 2 (Fig. S4, ESI†), and 3 (Fig. S6,
and 3 solely displays the trans isomer even though cis and trans ESI†). For all three complexes, magnetic saturation is approxi-
isomers can readily interconvert as suggested by variable mately reached at 7 T. The magnetization values are significantly
temperature NMR solution studies.12
smaller than the expected free ion values (10 mB per Dy3+ ion). This
6
The two bridging chloride ligands in 3 allow for an intra- observation is indicative of breaking the degeneracy of the H15/2
molecular Dyꢁ ꢁ ꢁDy distance of 4.252(1) Å whereas the larger ground states by crystal/ligand field effects. Additionally, the
M vs. H/T curves for complexes 1 (Fig. S3, ESI†), 2 (Fig. S5, ESI†),
and 3 (Fig. S7, ESI†) are non-superimposable as a result of the
magnetically highly anisotropic Dy3+ ion(s).
The magnetization dynamics of compounds 1–3 were
probed by alternating current (ac) magnetometry. All three
complexes display signals in the out-of-phase component
(wM00) of the ac magnetic susceptibility in the absence of an
00
externally applied dc field. The maximum of wM of complex 1
(Fig. 3; see Fig. S8 for wM0) remains temperature independent at
temperatures between 010 .8 and 6 K. At a temperature of above 6 K
the maxima of the wM signal become temperature dependent
Fig. 1 Molecular structures of neutral dinuclear [(PyCp2)Dy(m2-O2SOCF3)]2
(top), [(PyCp2)Dy(m2-Cl)]2 (bottom), and mononuclear (PyCp2)Dy(OSO2CF3)(thf)
(center) in crystals of 1, 3, and 2, respectively. Teal = Dy, yellow = S, red =
Fig. 2 Temperature dependence of wMT for 1 (red triangles), 2 (blue
squares), and 3 (black circles) (1000 Oe; 2–300 K). The solid lines are fits
of the data (see text).
oxygen, blue = N, light green = F, green = Cl, and grey = C. Hydrogen atoms
and co-crystallized solvent molecules have been omitted for clarity. The pink
transparent lines indicate gzz directions of the magnetic ground states.
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Fig. 4 Arrhenius plots of magnetization relaxation time data for
1
(red triangles) and 3 (black circles) in the zero dc field. The lines correspond
to fits to the Arrhenius equation yielding Ueff = 70 K (49 cmꢀ1) and 70 K
(49 cmꢀ1), and t0 = 4.8 ꢂ 10ꢀ7 s and 7.2 ꢂ 10ꢀ7 s for 1 and 3, respectively.
t at low temperatures of the Arrhenius plot is indicative of the
presence of other relaxation pathways such as Raman and quantum
tunnelling processes.14 The similarity in the magnetization
dynamics of 1 and 3 is also confirmed by ac measurements
performed for magnetically dilute (Dy : Y = 1 : B12) analogues
Fig. 3 Frequency dependence of the out-of-phase component (wM00) of
the ac susceptibility of 1 (top) and 3 (bottom) at an applied dc field of 0 Oe
in the temperature range of 1.8–15 K (2 Oe switching field).
0
which display almost identical temperature dependences of wM
00
and move to higher frequencies. The latter observation indicates and wM (Fig. S16–S19, ESI†).
the dominant quantum tunnelling of the magnetization (QTM)
These surprising findings were further investigated by
at low temperatures (o6 K) and a transition to a thermally means of ab initio calculations of the CASSCF/RASSI/SINGLE_
activated process at higher temperatures. This transitional ANISO type, using the MOLCAS 8.0 program.15 Both Dy sites of
behaviour of the magnetization dynamics is qualitatively also dinuclear complexes 1 and 3 were calculated separately by
observed for complexes 2 and 3. For the mononuclear complex replacing one Dy3+ ion with a diamagnetic Lu3+ ion and
00
2, the signals in wM are observed at 0 Oe dc field, albeit at displayed very similar properties (Table S3, ESI†). The ground
state Kramers doublets (KDs) of the Dy3+ ions in all three
00
higher frequencies (Fig. S9 and S10, ESI†). Above 9 K, the wM
maxima shift to frequencies beyond the 1 kHz instrument limit complexes are predominantly mJ = ꢃ15/2 in character with
which prevents the extraction of an effective energy barrier to small contributions from mJ = ꢃ11/2 (Fig. 5) with corresponding
magnetization reversal (Ueff) (vide infra). Application of dc fields gz values of 19.3, 18.8, and 19.1 for 1, 2, and 3, respectively. The
00
resulted in significant shifts of the wM maxima to lower first excited state KDs of dinuclear complexes 1 and 3 are mostly
frequencies (as compared to zero field measurements) which mJ = ꢃ11/2 in character whereas for mononuclear 2 a highly
were clearly temperature dependent above 2.5 K. However, for mixed doublet was calculated with contributions from ꢃ5/2, ꢃ3/2,
all fields studied, we obtained ill-defined shoulders in the ac and ꢃ1/2 and others. The energies of the first excited state KDs for
data which are indicative of the presence of multiple, overlapping 1 and 3 are very similar (138 cmꢀ1 and 134 cmꢀ1) and significantly
relaxation pathways (see Fig. S11 and S12, ESI†). The variable higher than those of compound 2 (84 cmꢀ1). Based on the
temperature ac susceptibility of 3 (Fig. 3 and Fig. S13, ESI†) is well calculated probabilities for tunnelling, Raman, and Orbach
00
defined in the absence of dc fields. The maxima of wM remain processes (arrows in Fig. 5), we propose that the magnetic
temperature independent up to a temperature of 4 K (QTM) and relaxation for all three complexes occurs via a thermally assisted
become temperature dependent at higher temperatures.
quantum tunnelling of the magnetization (TA-QTM) process
Cole–Cole plots (wM vs. wM0) corresponding to the above involving the first excited state. The calculated values of 1 and
discussed ac magnetization data for complexes 1 and 3 are 3 are in very good agreement with the identical Ueff values
shown in Fig. S14 and S15 (ESI†), respectively. Quantitative determined experimentally and suggest that the magnetization
relaxation times were extracted by fitting the data for each dynamics in 1 and 3 are dominated by the single ion magnetic
temperature using the generalized Debye model (solid lines in anisotropy. This point is further supported by the small mag-
00
Fig. S14 and S15, ESI†) and used to construct the Arrhenius netic coupling constants ( Jexch = +0.018 cmꢀ1 and ꢀ0.138 cmꢀ1
;
plots (ln t vs. 1/T) as shown in Fig. 4. The barrier height Ueff was Jdipolar = +0.032 cmꢀ1 and 0.0892 cmꢀ1) which were calculated
determined by fitting the linear temperature dependent part of within the Lines model using the POLY-ANISO routine (fits in
the Arrhenius plots (the predominant Orbach relaxation mecha- Fig. 2 and Tables S4, S5, ESI†).16 Interestingly, substitution of
nism). Interestingly, we obtained identical Ueff values for complexes ꢀOSO2CF3 by Clꢀ (in going from 1 to 3) results in a change from
1 and 3 of 70 K (49 cmꢀ1) and pre-exponential factors t0 of 4.8 ꢂ ferro- to antiferro-magnetic coupling ( Jtotal = 0.05 cmꢀ1 and
10ꢀ7 s and 7.2 ꢂ 10ꢀ7 s, respectively. The non-linear dependence of ꢀ0.05 cmꢀ1) but the small magnitude is expected to significantly
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1 and 3 exhibit the same effective barrier to magnetization
reversal of 49 cmꢀ1. The surprising similarity is explained by
computational results that suggest that the magnetic relaxation
dynamics involves TA-QTM via the first excited state of the
single Dy sites. We are currently investigating the reactivity of
complexes 1 and 2 as the increased leaving group ability of
triflate would be expected to allow for facile ligand substitution
chemistry.
MN is grateful to the TAMU Chemistry Department for
generous start-up funds, the LMS for computer time, and
financial support by the Welch Foundation (A-1880).
Notes and references
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Fig. 5 Magnetization blocking barrier for single Dy sites in 1 (top), 2
(center), and 3 (bottom) computed ab initio. The black lines indicate KDs
as a function of computed magnetic moments; green/blue and red arrows
indicate pathways through Orbach/Raman relaxation and QTM/TA-QTM.
The numbers on each arrow are the mean absolute values for the
corresponding matrix element of the transition magnetic moment. The
donut diagrams indicate the nature of the contributing mJ states to KDs
(light grey = combined contributions of several other mJ states).
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2ꢀ
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to the competing formation of polymeric m-Cl bridged systems
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novel organometallic lanthanide complexes. All three complexes
display a slow relaxation of their magnetization and compounds
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