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Dyes and Pigments 110 (2014) 261e269

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Dyes and Pigments

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Luminescence as a tool to study lanthanide-catalyzed formationof carbonecarbon bonds

Zhijin Lin, Matthew J. Allen*

Department of Chemistry, Wayne State University, 5101 Cass Avenue, Detroit, MI 48202, USA

a r t i c l e i n f o

Article history:Received 28 January 2014Received in revised form10 March 2014Accepted 11 March 2014Available online 27 March 2014

Keywords:LanthanidesLuminescenceLuminescence lifetimeCatalysisWater-coordination numberAldol

* Corresponding author. Tel.: þ1 313 577 2070; faxE-mail addresses: zlin@chem.wayne.edu (Z. Lin),

J. Allen).

http://dx.doi.org/10.1016/j.dyepig.2014.03.0200143-7208/� 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

Lanthanide ions have been widely studied in imaging and sensing applications, and they are also strongLewis acids that have the ability to catalyze organic reactions. Here, we review the spectroscopicproperties of lanthanides that enable the calculation of water-coordination numbers. Additionally, theapplication of these calculations to study the mechanism of lanthanide-catalyzed organic reactions isreviewed.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The lanthanides include the 14 elements from cerium to lute-tium that most often exist in the tripositive oxidation state wherethey exhibit interesting luminescence properties. These propertieshave inspired the study of lanthanide ions in anion-sensing [1],cation-sensing [2], biomolecule-sensing [3], medical imaging [4],and light amplification [5]. A number of reviews of the lumines-cence properties of lanthanides have been published including theuse of luminescent lanthanides to sense anions, pH, and oxygen [6];applications of luminescent lanthanides in biomedical imaging [7];lanthanide-based materials in light-emitting diodes [8]; and theuse of lanthanides in ionic liquids including their spectroscopicproperties [9].

In addition to having useful luminescence properties, lantha-nide ions are strong Lewis acids. Unlike other Lewis acidsdsuch asAlCl3, TiCl4, and SnCl4 that readily hydrolyze in waterdlanthanidetrifluoromethanesulfonates or triflates [Ln(OTf)3] are water-tolerant. The use of these salts as water-tolerant precatalysts fororganic reactions has received a great deal of attention over the lastfew decades [10e17], and the use of coordination complexes of

: þ1 313 577 8822.mallen@chem.wayne.edu (M.

these salts as aqueous-phase pre-catalysts and asymmetric pre-catalysts has been reviewed [18,19].

In this article, we summarize a combination of these twoseemingly disparate areas of lanthanide chemistry.We beginwith areview of the principles of lanthanide luminescencewith a focus onthe use of luminescence-decay measurements to study the water-coordination numbers of lanthanide ions. We then describe howthese measurements have been used to study lanthanide-catalyzedcarbonecarbon bond-forming reactions.

2. Lanthanide spectroscopy

Interest in the spectroscopic properties of the lanthanides is duein part to their uniquely narrow absorption and emission peaks,large Stokes shifts, and long luminescence lifetimes. These prop-erties are determined by the electronic structures of the ions.Trivalent lanthanide ions have electronic configurations of [Xe]4fn

(n ¼ 1e14). Because the 4f orbitals are shielded from the environ-ment by the filled 5s2 and 5p6 subshells, the spectroscopic prop-erties of these ions are largely independent from their environmentand ligands. This section details the relationships between elec-tronic structure and absorption and emission.

There are three types of electronic transitions that govern ab-sorption and emission in trivalent lanthanide ions [20]: 4fe5dtransitions, charge-transfer transitions (metal-to-ligand or ligand-to-metal), and 4fe4f transitions. Because there are seven 4f

Fig. 1. Ligand-sensitized lanthanide luminescence: (A) Schematic representation of anantenna absorbing light and then transferring the energy (ET) to an attached lantha-nide complex to enhance the lanthanide emission. (B) Simplified Jablonski diagram ofligand-sensitized lanthanide luminescence where S0 is the ground state of the antenna,S1 is the excited singlet state of the antenna, T1 is the excited triplet state of the an-tenna, and ISC is intersystem crossing.

Z. Lin, M.J. Allen / Dyes and Pigments 110 (2014) 261e269262

orbitals, the trivalent lanthanides have multiple, well-definedtransitions; however, these transitions are usually weak, withmolar absorptivities in aqueous solution of <1 Me1 cm�1. The 4fe5d and charge-transfer transitions are parity allowed and areusually high energy (UV), and the 4fe4f transitions tend to be lowerin energy (often in the visible region). These 4fe4f transitions areoccasionally perturbed by the environment and the occasionalperturbations are useful in studying bond-forming reactions asdescribed later in this article.

The 4fe4f transitions can be divided into electric dipole tran-sitions, magnetic dipole transitions, and electric quadrupolartransitions. Electric dipole transitions between states with thesame parity are forbidden according to Laporte’s parity selectionrule [21]. As a consequence, electric dipole transitions in lanthanideions have low probabilities of occurrence. However, this selectionrule can be circumvented with non-centrosymmetric lanthanidecomplexes that lead to transitions called induced electric dipoletransitions. While electric dipole transitions need special circum-stances to increase their chances of occurring, magnetic dipoletransitions are allowed by Laporte’s parity selection rule [21].Nevertheless, these transitions are weak and not influenced by theenvironment of a lanthanide ion, resulting in intensities frommagnetic dipole transitions being in the same magnitude of in-tensity as induced electric dipole transitions. Finally, like electricdipole transitions, electric quadrupolar transitions are sensitive tothe environment. However, electric quadrupolar transitions areweaker than magnetic dipole transitions and are rarely observeddue to their low intensity [20].

Because direct excitation of lanthanide ions does not alwayslead to sufficient emissions for a given application, ligandsensitization is often used with lanthanide complexes to increaseluminescent emissions. In ligand-sensitized luminescence oflanthanides, a nearby molecule called an antenna is conjugatedto the lanthanide complex as represented in Fig. 1A. Antennamolecules are often aromatic or unsaturated organic moleculesthat absorb light more efficiently than lanthanide ions. Thesemolecules can be conjugated to the lanthanide complex via alinker or by direct coordination to the ion. Excitation of the an-tenna and subsequent energy transfer to a lanthanide ion isdepicted schematically in Fig. 1. In this illustration, the antenna isexcited from its ground state to a singlet excited state and thenreleased to its triplet state through intersystem crossing (ISC).Energy transfer (ET) occurs from the triplet state of antenna tothe excited state of the lanthanide ion. The use of an antenna canincrease the quantum yield of lanthanide ions up to 61% inaqueous solution [22].

Two types of luminescence occur when excited lanthanide ionsreturn to their ground states: fluorescence and phosphorescence.Lanthanide ions can undergo either of these types of luminescenceor both simultaneously. Fluorescence does not involve a spinchange [for example, 4F9/2(excited state) / 4I15/2(ground state) forErIII, where the superscript 4 does not change]. The superscriptportion of the term symbol, the spin multiplicity, is equal to 2S þ 1.If this number does not change, then the spin does not change. Theother type of luminescence, phosphorescence, involves a spinchange [for example, 5D0(excited state) / 7F0(ground state) forEuIII, where the superscript 5 changes to 7]. Compared to organiclumophores and quantum dots, luminescent lanthanides havemany advantages. Lanthanide luminescence occurs over a broadrange of energy that is covered by the ions in aqueous solution fromUV to near-IR [20], a feature caused by themultiple electronic statesof lanthanide ions. The broad range of possible emissions enablelanthanide ions to be used in a diverse range of applications.Furthermore, trivalent lanthanide luminescence is in the form ofsharp emission lines that reduce the loss of spectral information

caused by overlapping peaks. Additionally, for many of the lan-thanides, luminescence lifetimes are long relative to organiclumophores. For example, the luminescence lifetime of EuIII is oftenon the order of milliseconds, while luminescence lifetimes oforganic lumophores are usually on the order of nanoseconds.Another important feature of lanthanide luminescence is thepresence of hypersensitive transitions. Although the luminescenceof lanthanides is minimally perturbed by the environment becauseof the nature of the 4f orbitals, some electric dipole-based emis-sions of the lanthanide ions are sensitive to the environment, andthese transitions are called hypersensitive transitions. For example,in the emission spectra of EuIII-containing complexes, the electricdipole-governed emission (5D0 / 7F2, l ¼ 612 nm) is sensitive toquenching by vibrations in the environment, but the magneticdipole-dominated emission (5D0 / 7F1, l ¼ 591 nm) is relativelyunperturbed by the surroundings. Consequently, the ratio of theemission intensities at these wavelengths (I612/I591) has been usedto study changes in the coordination environment of EuIII. Forexample, changes in the I612/I591 ratio of EuIII were used to studybinding of dipicolinic acid [23]. The I612/I591 ratio increased from0.44 to 3.4 when one equivalent of dipicolinic acid was added toEuIII, and the ratio increased to 7.8 when three equivalents wereadded. The changing ratios are caused by the replacement of theinner-sphere water with dipicolinic acid that changes the sym-metry of the EuIII ion resulting in an induced electric dipole tran-sition that increases the intensity of the emission at 612 nm. Thisratio is a potential tool to study lanthanide-catalyzed organic re-actions that involve coordination changes during the reaction.

The spectroscopic properties of lanthanide ions are importantfor their applications, one of which is the use of luminescencelifetime measurements to study the coordination environment oflanthanide ions in solution. Details of this application are describedin the next section.

Z. Lin, M.J. Allen / Dyes and Pigments 110 (2014) 261e269 263

3. Luminescence lifetime studies of lanthanide coordinationenvironments

3.1. Aqueous solution

The water-coordination number (q) of lanthanide ions is animportant parameter in the study of biomolecules [24], in thedesign of contrast agents for magnetic resonance imaging [25], andin the study of catalysis in bond-forming reactions [26]. Watercoordination has been investigated by methods including X-raydiffraction [27e30], neutron diffraction [31], extended X-ray ab-sorption spectroscopy [32], and lanthanide ion-induced 17O-NMRwater shift measurements [33,34]. These methods are useful, butthey are performed with solids or require high concentrations oflanthanide ions (MemM). In contrast to those methods, lumines-cence lifetime measurements can be used to determine water-coordination numbers in solution with low concentrations oflanthanide ions (�mM).

In the 1960s, Kropp and Windsor reported that the lumines-cence intensities and lifetimes of EuIII and TbIII are enhanced whensolvent molecules (water, methanol, or acetone) are substitutedwith their deuterated counterparts because of changes in theamount of radiationless deactivation of the excited states of thelanthanide ions by vibrations of bonds that contain hydrogen[35,36]. This correlation is illustrated by the different non-radiativepathways of the excited EuIII ion in H2O and D2O as shown in Fig. 2.The excited state of EuIII overlaps better with the energy of thevibrational overtones of OeH oscillators thanwith the energy of thevibrational overtones of OeD oscillators. Therefore, energy transferfrom the excited state of EuIII to H2O is more efficient than to D2O.Based on the different quenching rates of lanthanide luminescencein H2O and D2O, much effort has been invested to quantitativelydetermine the water-coordination numbers of lanthanide ions us-ing luminescence lifetime measurements.

Horrocks and Sudnick used a pulsed dye laser source to excite aseries of EuIII- and TbIII-containing complexes to study theirluminescence-decay rates in H2O ðkH2OÞ and D2O ðkD2OÞ [37]. Thelanthanide complexes that they studied have known coordinationnumbers in the solid state between zero and nine based on X-ray

Fig. 2. Simplified Jablonski diagram outlining some of the possible relaxation path-ways of excited EuIII ions in H2O and D2O: the electric dipole-governed (5D0 / 7F2,l ¼ 612 nm) and magnetic dipole-dominated (5D0 /

7F1, l ¼ 591 nm) emissions (othertransitions including 5D0 / 7F0, 5D0 / 7F3, 5D0 / 7F4, 5D0 / 7F5, and 5D0 / 7F6 arenot shown for simplicity). The vibrational overtones of OeH oscillators effectivelyquench the excited state (5D0) of EuIII ions, reducing luminescence lifetimes. However,quenching in D2O is less efficient because of the smaller overlap between the emissiveEuIII state (5D0) and the vibrational overtones of OeD oscillators.

characterization. The authors directly excited EuIII-containingcomplexes at 461 nm [7F0(ground state) / 5D2(excited state)] andrecorded luminescence lifetimes of the long-lived emissive state(5D0) to calculate the kH2O and kD2O (Fig. 3A). For the TbIII-con-taining complexes, they used 488 nm excitation [7F6(groundstate) / 5D4(excited state)] to study luminescence lifetimes of the5D4 state to calculate kH2O and kD2O (Fig. 3B). After obtaining theluminescence-decay rates of the EuIII- and TbIII-containing com-plexes, the authors calibrated values of kH2OekD2O with the crystal-structure-based water-coordination numbers to empirically deriveequations to calculate q (Table 1, eq’s (1) and (2)). While providing auseful method to study the coordination environment of lantha-nide ions in solution, these initial studies did not consider thecontribution from outer-sphere water molecules and non-waterligands that can depopulate excited states. This missing informa-tion resulted in relatively large uncertainly values of �0.5 watermolecules.

To evaluate the influence of quenching vibrations on the lumi-nescence of lanthanides, Parker and co-workers measured theluminescence lifetimes of EuIII-, TbIII-, and YbIII-containing com-plexes with 20 ligands in H2O and D2O [38]. In their measurements,EuIII- and TbIII-containing complexes were indirectly excited viaantenna molecules at 270 and 274 nm, respectively. Their lumi-nescence lifetimes were recorded at 590 or 619 nm (5D0 / 7F1 or5D0 / 7F2, respectively) for EuIII-containing complexes and at545 nm (5D4 /

7F5) for TbIII-containing complexes (Fig. 3C and D).The YbIII-containing complexes were directly excited at 970 nm(2F7/2 / 2F5/2), and luminescence lifetimes were measured for the2F7/2 / 2F5/2 transition (Fig. 3E). The contribution of outer-spherewater molecules and CeH and NeH oscillators were taken intoconsideration in this study to describe the depopulation of theexcited states of the lanthanide ions. The average contributions ofouter-sphere water to the depopulation of the excited states of EuIII,TbIII, and YbIII were reported to be �0.25, e0.06,and �0.00020 ms�1, respectively. The authors found that NeHoscillators of bound amides quench the excited state of EuIII at adecay rate of 0.075 ms�1 but do not appreciably quench the excitedstates of TbIII or YbIII. They also found that CeH oscillators mini-mally quench the excited states of EuIII and YbIII ions relative to NeH and OeH oscillators. The equations they derived to calculate q forEuIII, TbIII, and YbIII in aqueous solution are shown in Table 1 (eq’s(3)e(5)). The authors suggested that the accuracy of q de-terminations should be high using their equations, but no specificuncertainty values were reported. Additionally, they used com-plexeswith q values less than or equal to 1 to derive their equations.As a result, equations (3) and (4) should be used with caution forestimating q values greater than 1.

Parker and co-workers presented equations that accounted forouter-sphere contributions to quenching, but their equationsoverestimate water-coordination numbers greater than one. Toincrease the accuracy of water-coordination number measure-ments, Supkowski and Horrocks derived an equation for the q ofEuIII that took into consideration complexes with water-coordination numbers greater than one; quenching by outer-sphere water molecules; and quenching by oscillators bound toEuIII including OeH oscillators (nOH), NeH oscillators in amines(nNH), and NeH oscillators in amides (nO¼CNH) [39]. Their updatedequation to calculate q is listed in Table 1 (eq (6)) with an uncer-tainty value of �0.1 water molecules, much lower than the error of�0.5 water molecules with eq (1).

The use of luminescence lifetime measurements to determinethe water-coordination numbers of lanthanide ions in aqueoussolutionwas extended beyond EuIII, TbIII, and YbIII to SmIII, DyIII, andNdIII by Kimura and Kato [40,41]. They used a pulsed laser beam toexcite several SmIII-, DyIII-, and NdIII-containing complexes at

Fig. 3. Simplified Jablonski diagrams of the ions used in the q measurements: Statesnot used for q measurements have been omitted for clarity. (A) EuIII is excited from itsground state (7F0) to an excited state (5D2) that undergoes non-radiative decay to itslong-lived emissive state (5D0). From the 5D0 state, the ion relaxes to a ground state(7F1), emitting at 590 nm. (B) TbIII is excited from its ground state (7F6) to its emissivestate (5D4) and then relaxes to the ground state (7F5), emitting at 545 nm. (C) Anantenna is excited from its ground state (S0) to its singlet excited state (S1). Intersystemcrossing (ISC) to the antenna’s triplet state (T1), intramolecular energy transfer (ET) tothe excited state of EuIII, and non-radiative decay populates the emissive state (5D0) ofEuIII. Relaxation to 7F1 or 7F2 results in emission at 590 or 619 nm, respectively. (D) Anantenna is excited from its ground state (S0) to its singlet excited state (S1). ISC to theantenna’s triplet state (T1), intramolecular ET to the excited state of TbIII, and non-radiative decay populates the emissive state (5D4) of TbIII. Relaxation to 7F5 results inemission at 545 nm. (E) YbIII is excited from its ground state (2F7/2) to its emissive state(2F5/2), and then it relaxes to the ground state (2F7/2) and emits radiation. Emission wasrecorded above 1050 nm to avoid scattering of excitation light. (F) SmIII is excited fromits ground state (6H5/2) to its excited state (4I13/2) and undergoes non-radiative decay topopulate its emissive state (4G5/2). Relaxation to 6H7/2 or 6H5/2 results in emission at559 or 594 nm, respectively. (G) DyIII is excited from its ground state (6H15/2) to anexcited state (4I15/2) and undergoes non-radiative decay to its emissive state (4H5/2).Relaxation to 6H15/2 or 6H13/2 results in luminescence at 478 or 572 nm, respectively.(H) NdIII is excited from its ground state (4I9/2) to its excited state (2G7/2) and undergoesnon-radiative decay to its emissive state (4F3/2). Relaxation to 4I9/2 results in emissionat 890 nm. (I) EuIII is excited from its ground state (7F0) to an excited state (5L6) andundergoes non-radiative decay to populate a long-lived emissive state (5D0). Relaxa-tion to the ground state (7F1) results in emission at 590 nm.

Z. Lin, M.J. Allen / Dyes and Pigments 110 (2014) 261e269264

464 nm (6H5/2 /4I13/2), 454 nm (6H15/2 /

4I15/2), and 593 nm (4I9/2 / 4G7/2), respectively, in H2O and D2O to record luminescencelifetimes for SmIII at 559 or 594 nm (4G5/2 /

6H5/2 or 4G5/2 /6H7/2,

respectively), for DyIII at 478 or 572 nm (4H5/2 / 6H15/2 or 4H5/

2/6H13/2, respectively), and for NdIII at 890 nm (4F3/2 / 4I9/2)

(Fig. 3FeH). After obtaining the decay rates, they empiricallyderived equations to calculate the q values of SmIII, DyIII, and NdIII inaqueous solution (Table 1, eq’s (7)e(9)) [40,41]. The uncertainty of qthey reported is �0.3 water molecules for SmIII and DyIII, but theuncertainty for NdIII was not reported.

All equations to calculate q described so far require measure-ment of luminescence lifetimes in both H2O and D2O. As a result, atleast two measurements need to be performed. To reduce thenumber of measurements, Barthelemy and Choppin measured theluminescence lifetimes of a series of EuIII-containing complexes inthe solid state that were crystallized from H2O or D2O [42]. Theydirectly excited EuIII-containing complexes at 395 nm (7F0 / 5L6)and measured the luminescence lifetimes at 590 nm (5D0 / 7F1)(Fig. 3I). They found the luminescence lifetimes of the complexeswere nearly constant in D2O. To simplify the q calculations, theyassumed that the ligands chelated to EuIII do not contribute to thede-excitation of the excited state, and consequently, they derivedequations for q by measuring luminescence lifetimes of EuIII-con-taining complexes in only H2O (Table 1, eq (10)). This simplifiedequation yields uncertainty values that are similar to the originalHorrocks’s equation (�0.5 water molecules) but requires half of themeasurements by excluding D2O. However, the potential forquenching the excited state of EuIII by coordinated ligands andouter-sphere molecules was not considered in this study and limitsthe usefulness of the equation for solution-based samples.

3.2. Aqueouseorganic mixtures

In addition to strictly aqueous media, luminescence lifetimemeasurements have been used to evaluate the coordination envi-ronment of EuIII-containing complexes in watereorganic mixturesthat are important solvent systems in catalysis and separations.

Luminescence lifetime measurements of EuIII in watereorganicsolvent mixtures have been performed using similar methods as inaqueous solution, using deuterated water mixed with deuteratedorganic solvents to measure kD2O. The luminescence lifetimes ofEuIII in watereorganic mixtures were measured by Tanaka and co-workers [43]. They used the luminescence lifetime measurementsto evaluate the binding ability of water and organic solvents to EuIII

in watereorganic mixtures in a variety of solvent compositions.They found that water outcompeted other solvents in binding toEuIII in the mixtures of water with acetone, acetonitrile, 1,4-dioxane, or methanol. In contrast, organic solvents outcompetewater in binding to EuIII in solutions of water with dimethylsulf-oxide or dimethylformamide [43]. One aspect of quenching thatwas missing from this study was the quenching of luminescencecaused by the organic solvent. This missing piece of informationwas reported by Kimura and co-workers [44]. They systematicallyinvestigated the inner-sphere solvent composition of EuIII ions inwatereorganic solvent mixtures and demonstrated that organicsolvents contribute to the depopulation of the excited state of theEuIII ion. Based on their findings, use of the equations in Table 1 tocalculate q values in watereorganic solvent mixtures is notappropriate.

An equation to measure q in different solvent compositions wasreported by Allen and co-workers [45]. Their equation accounts forthe quantitative de-excitation of excited EuIII ions by inner-sphereorganic molecules and outer-sphere watereorganic solvent mix-tures. They measured the luminescence lifetimes of six well char-acterized EuIII-containing complexes (Fig. 4) in commonly usedwatereorganic solvent mixtures. The equation they derived tocalculate the q values of EuIII-containing complexes is equation (11),where a is the contribution to the depopulation of the excited statesof EuIII-containing complexes that varies as a function of both waterconcentration and organic solvent identity. To derive equations for

Table 1Equations for calculating water-coordination number (q) based on luminescence lifetime measurements.

LnIII Eq to calculate qa Uncertainty (water molecules) Reference

EuIII q ¼ 1:05�kH2OekD2O

�(1) �0.5 [37]

TbIII q ¼ 4:2�kH2OekD2O

�(2) �0.5 [37]

EuIII q ¼ 1:2�kH2OekD2Oe0:25e0:075nO¼CNH

�(3) nrb [38]

TbIII q ¼ 5�kH2OekD2Oe0:06

�(4) nrb [38]

YbIII q ¼ 1� 10e3�kH2OekD2Oe0:2�

(5) nrb [38]

EuIII q ¼ 1:05�kH2OekD2Oe0:31þ 0:45nOH þ 0:99nNH þ 0:075nO¼CNH

�(6) �0.1 [39]

SmIII q ¼ 0:026�kH2OekD2O

�e1:6 (7) �0.3 [40]

DyIII q ¼ 0:024�kH2OekD2O

�e1:3 (8) �0.3 [40]

NdIIIq ¼ 3:58� 10e7

�kH2OekD2O

�e1:97 (9) nrb [41]

EuIII q ¼ 1:05kH2Oe0:70 (10) �0.5 [42]

a the units of kH2O and kD2O are m s�1.b nr ¼ not reported.

Z. Lin, M.J. Allen / Dyes and Pigments 110 (2014) 261e269 265

the a value in different binary solvents, they used complexes 1 and2 that have no inner-sphere water molecules to determine theluminescence quenching by outer-sphere binary solvents (mixturesfrom 1 to 100% water). Then, they subtracted these outer-spherevalues from the quenching rates observed with complexes 3e6 toderive equations for a that take into account quenching from inner-sphere and outer-sphere water and organic solvent. They extrap-olated their findings to provide estimations of q for any concen-tration of water in dimethylformamide, acetonitrile,tetrahydrofuran (THF), acetone, dimethylsulfoxide, ethanol, ormethanol. The equations for calculating a values were adapted intoa web-based application that is free to use [46]. Based on theirfindings, they suggested that eq (11) is more accurate than theequations in Table 1 when using binary solvents.

q ¼ 1:2���kH2O � kD2O

��� a�

(11)

When determining water-coordination numbers of lanthanideions using luminescence lifetime measurements, it is important toconsider that many oscillators (OeH, NeH, and CeH) and solvents

Fig. 4. Structures of EuIII-containing complexes used to derive eq (11) from watereorganic solvent mixtures. The X’s represent coordination sites for water or solventmolecules.

can non-radiatively depopulate the excited states of lanthanideions. The appropriate equation for a specific systemmust be chosento reduce the uncertainty of q in the calculation and to fit theexperimental design. As is discussed in the next section, accuratedetermination of the coordination environment of lanthanide ionsplays an important role in the study of lanthanide-catalyzed car-bonecarbon bond-forming reactions.

4. Study of lanthanide-catalyzed carbonecarbon bond-forming reactions using luminescence measurements

Lanthanide ions are strong Lewis acids that are able to effec-tively catalyze many organic reactions [47e52], and some of thesereactions are performed in aqueous solvent systems. An under-standing the of the reaction mechanisms in these systems wouldenhance the ability to effectively design pre-catalysts, and lumi-nescence is a useful tool to investigate changes in coordination thatare involved in reaction mechanisms.

Luminescence lifetime measurements have been applied tostudy the coordination changes associated with lanthanide-catalyzed carbonecarbon bond formations such as theMukaiyama aldol reaction. The Mukaiyama aldol reaction is apowerful route to form carbonecarbon bonds (Scheme 1) [53], anda study of the Mukaiyama aldol reaction was performed usingluminescence lifetime measurements [54]. Dissanayake and Allenchose Eu(OTf)3 as a pre-catalyst due to its strong Lewis acidity andprominent luminescence properties. They divided the proposedcatalytic cycle into three coordinates (Fig. 5A). The luminescencelifetimes of the EuIII-containing complexes in the 1st, 2nd, and 3rdcoordinates were measured in H2O and D2O solutions to calculatethe water-coordination numbers of each coordinate. Their resultsare shown in Fig. 5B, where the solid circles represent the water-coordination numbers of EuIII in the 1st and 3rd coordinates, andthe hollow circles represent the water-coordination number of EuIII

R1

OSi(CH3)3

H

O

R4

Ln(OTf)310 mol %H2O/THF

R1

R3

O OH

R4R3

R2 R2

Scheme 1. Lanthanide triflate-catalyzed Mukaiyama aldol reaction in watereTHF. Rgroups are aromatic, aliphatic, or H.

Fig. 5. (A) Simplified catalytic cycle of an aqueous Eu(OTf)3-catalyzed Mukaiyamaaldol reaction: x, y, and q represent the number of triflate anions, THF molecules, andwater molecules coordinated to EuIII in the 1st coordinate; x0 , y0, q0 , and z represent thenumber of triflate anions, THF molecules, water molecules, and benzaldehyde mole-cules coordinated to EuIII in the 2nd coordinate; x00 , y00, q00 , and a represent the numberof triflate anions, THF molecules, water molecules and 2-(hydroxyphenylmethyl)cyclohexanone molecules coordinated to EuIII in the 3rd coordinate; (B) water-coordination number of EuIII-containing complexes as a function of solvent composi-tion in the 1st and 3rd (C) and 2nd (B) coordinates of EuIII ions. Dq is the water-coordination number change of EuIII before and after the addition of benzaldehyde.Reprinted with permission from Dissanayake P, Allen MJ. Dynamic measurements ofaqueous lanthanide triflate-catalyzed reactions using luminescence decay. J Am ChemSoc 2009; 131:6342e3. Copyright 2009 American Chemical Society.

Table 2Eu(OTf)3- and Eu(NO3)3-catalyzed Mukaiyama aldol reactions [55].

% H2O in THF (v/v) q withEu(OTf)3

Reaction ratewith Eu(OTf)3(mM s�1)

q withEu(NO3)3

Reaction ratewith Eu(NO3)3(mM s�1)

1 5.8 0.33 3.2 0.0085 8.0 1.0 5.2 0.1110 8.3 1.4 6.8 0.3315 8.6 1.5 7.2 0.4825 8.6 1.45 7.4 1.040 8.5 1.3 7.7 1.3

Z. Lin, M.J. Allen / Dyes and Pigments 110 (2014) 261e269266

in the 2nd coordinate. The water-coordination numbers decreased(Dq) upon addition of benzaldehyde (Fig. 5B), and the authorsinterpreted this observation as the displacement of inner-spherewater by benzaldehyde. Additionally, the largest Dq drop fromthe addition of benzaldehyde occurred when the measurementswere performed in 20% water in THF (v/v), a solvent compositionthat corresponds to the highest reaction yield [11]. The lack ofdifference between the 1st and 3rd coordinates suggests, thatproduct inhibition was not observed with the method. Their studyindicates that the probability of activation of aldehyde by lantha-nide ions is related to reaction yield.

The counteranions of EuIII-based pre-catalysts also influence thereaction rate and yield of the Mukaiyama aldol reaction. Allen andco-workers studied the initial reaction rates and yields of theMukaiyama aldol reaction in watereTHF binary solvents catalyzed

by Eu(OTf)3 or Eu(NO3)3 [55]. They found that the NO3e counter-

anion led to lower initial reaction rates than OTfe until a waterpercentage of 40% (v/v) was reached (Table 2). The qmeasurementswere used to interpret the influence of counter ion identity oninitial reaction rates. The authors suggested that because the NO3

e

salt has a lower q value than the OTfe salt, more counteranions arebound to the EuIII ion, leading to weaker Lewis acidity and, conse-quently, lower reaction rates until there is enough water tooutcompete NO3

e binding.With the knowledge that displacement of inner-spherewater by

aldehyde is important to efficiency in the Mukaiyama aldol reac-tion, Allen and co-workers designed six EuIII-containing complexeswith different side arms as well as open sites for water or aldehydebinding to enhance the enantioselectivity and diastereoselectivityof product formation (Table 3) [56]. The least bulky complexes hadlarger Dq values and higher reaction yields relative to the bulkycomplexes. The authors proposed a mechanism based on lumi-nescence measurements that explains their observations (Scheme2). Based on Dq values and models, benzaldehyde can onlyreplace one bound water molecule on the top position of 7 to form8, which is poised to react with silyl enol ether. Furthermore, thesilyl enol ether can only attack bound benzaldehyde from one di-rection because of steric obstruction from the esters.

In addition to luminescence lifetime measurements, ratiometricmeasurements have been used to study the replacement of inner-sphere water in TbIII-catalyzed reactions. In these studies, a high-ly efficient antenna was used to enhance the luminescence of TbIII

ions. Evidence of coordination of aldehydes to TbIII ions duringcatalysis was reported by Li, Duan, and co-workers [57]. Theymixed4,40,400-nitrilotribenzoic acid (TCA) (10, Fig. 6) and TbIII salts tosynthesize luminescent lanthanideeorganic frameworks (TbIIIeTCA). In TbIIIeTCA, TCA is both an antenna and a luminescent blue-emitter (l ¼ 435 nm). TCA chelates TbIII via its carboxylate groupsand sensitizes TbIII to enhance luminescence at 540 nm. TbIIIeTCAforms suspensions in dichloromethane where it emits at 435 and540 nm after being irradiated at 350 nm. The authors reported thatthe luminescence intensities at 435 and 540 nm both decrease dueto a photo-induced energy transfer (PET) mechanism in whichenergy is transferred from the excited chromophore to thequencher. In the PET mechanism, the chromophore harvests lightto populate its excited state. Instead of transferring the energy toTbIII ions, the excited chromophore transfers energy to thequencher. Consequently, the luminescence of antenna and TbIII

both decrease. Detailed reviews of the PETmechanism can be foundelsewhere [58,59]. In the study by Li, Duan, and co-workers, energywas proposed to be transferred from the excited triarylamine to the

Table 3EuIII-catalyzed Mukaiyama aldol reactions with different ligands [56].

R Yield (%) Dq Syn:anti Enantiomeric ratio (syn)

CH3 85 0.62 26:1 13:1C2H5 82 0.40 26:1 12:1n-C3H7 83 0.45 32:1 13:1n-C4H9 83 0.49 20:1 14:1i-Pr 20 0.19 21:1 9:1t-Bu 18 0.14 18:1 3:1

Fig. 6. Structures of ligands 10 and 11 used to synthesize TbIIIeTCA and TbIIIePT1complexes, respectively.

Z. Lin, M.J. Allen / Dyes and Pigments 110 (2014) 261e269 267

coordinated aldehyde, resulting in lower excitation efficiency ofTbIII. Consequently, TbIII-based (I540) and antenna-based (I435)emissions both decreased upon the addition of aldehyde. Becausethe luminescence of the antenna is directly quenched by benzal-dehyde bound to TbIII, I435 decreased faster than I540; consequently,the ratio I540/I435 was proportional to the amount of addedaldehyde.

TbIIIeTCA has both Lewis acidic (TbIII) and Lewis basic (TCA)sites; therefore, it can be used to catalyze the reaction of aldehydeswith cyanotrimethylsilanes (Scheme 3). Li, Duan, and co-workersscreened four aldehydes and found that 2-nitrobenzaldehydeproduced a reaction yield of 78% [57]. Luminescence measure-ments revealed that when 2-nitrobenzaldehyde was added to asuspension of TbIIIeTCA, the ratio I540/I435 was nearly two timeshigher than the original TbIIIeTCA. This study suggests that alde-hyde coordination leads to high levels of conversion and demon-strates that coordination can be monitored by ratiometricluminescence measurements.

Luminescence emission measurements were also used to studya TbIII-catalyzed cyanosilylation reaction [60]. He and co-workersreported the synthesis of TbIII-containing metaleorganic octa-hedra (TbIIIePT1 or SmIIIePT1) using tris(pyridine-2-ylmethylene)benzene-1,3,5-tricarbohydrazide (PT1) (11, Fig. 6) as a ligand and anantenna. Each lanthanide ion is coordinated by two equivalents of11 through tridentate chelation, and TbIIIePT1 exhibits both TbIII-

Scheme 2. Proposed transition states in the asymmetric Mukaiyama aldol reaction based onof 7 to form 8. Activated benzaldehyde is only attacked by the silyl enol ether from one dpermission from Mei Y, Dissanayake P, Allen MJ. A new class of ligands for aqueous, lant132:12871e3. Copyright 2010 American Chemical Society.

based emission (525 nm) and ligand-based emission (465 nm)when exited at 365 nm. The luminescence of TbIIIePT1 at both 465and 525 nm was suppressed by nitrobenzaldehyde through a PETmechanism from PT1 to aldehyde bound to TbIII. The authors sug-gested that the quenching of luminescence in the TbIIIePT1 systemis caused by the replacement of solvent coordinated to TbIII bynitrobenzaldehyde. They titrated TbIIIePT1 with 4-nitrobenzaldehyde and found that the luminescence intensity ofTbIIIePT1 (525 nm) was inversely proportional to the amount of 4-nitrobenzaldehyde.

The TbIIIePT1-catalyzed cyanosilylation of aldehydes (Table 4)depended on the size of the aldehyde. When bulky 3-formyl-1-phenylene-3,5-di-tert-butylbenzoate was used, the reaction yieldwas lower thanwith less bulky aldehydes. To investigate the reaction,the authors studied the luminescence of TbIIIePT1 (525 nm) uponadding the aldehydes listed in Table 4. Upon addition of 2-nitrobenzaldehyde, 3-nitrobenzaldehyde, and 4-nitrobenzaldehyde,the luminescence of TbIIIePT1 (525 nm) decreased by 35, 30, and 28%,respectively. Addition of 3-formyl-1-phenylene-(3,5-di-tert-butyl-benzoate)onlysuppressed the luminescenceby4%, indicating that thebulky aldehyde did not efficiently coordinate to TbIII resulting in lowyields (Table 4).

In addition to carbonecarbon bond-forming reactions, lumi-nescence lifetimeand intensitymeasurements canalsobe applied to

EuIII luminescence measurements. Benzaldehyde replaces the bound water on the topirection because of the steric hindrance of the ester group as depicted. Redrawn withhanide-catalyzed, enantioselective Mukaiyama aldol reactions. J Am Chem Soc 2010;

Scheme 3. TbIIIeTCA-catalyzed reaction of aldehyde with cyanotrimethylsilane.

Table 4TbIIIePT1-catalyzed reactions of aldehydes with cyanotrimethylsilane [57].

R TbIIIePT1 conversion (%)

95

92

91

13

Z. Lin, M.J. Allen / Dyes and Pigments 110 (2014) 261e269268

study other lanthanide-catalyzed reactions. For example, coordi-nation changes in the lanthanide-catalyzed hydrolysis of RNA havebeen studied [61].Morrowand co-workers used phosphate esters asa model for RNA tomeasure the q values of EuIII-based pre-catalyststhat catalyze the hydrolysis of RNA. The value of q decreased uponaddition of phosphate esters, indicating that replacement of waterby phosphate esters in RNA is crucial to hydrolysis.

5. Conclusions

The unique luminescence properties of lanthanide ions enabletheir coordination environments to be studied. The development ofequations to calculate water-coordination numbers based on lumi-nescence lifetime measurements has been useful in the study of themechanisms of EuIII-catalyzed carbonecarbon bond-forming re-actions, and the techniques summarized in this document potentiallycan be used to enable the study of other lanthanide-catalyzedreactions.

Acknowledgments

The authors acknowledge support from a CAREER Award fromthe National Science Foundation (CHE-0955000) and a SchaapFaculty Scholar Award from Wayne State University.

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