Diazadiene Complexes of the Heavy Alkaline-Earth Metals Strontiumand Barium: Structures and ReactivityVolker Lorenz,† Cristian G. Hrib,† Dirk Grote,‡ Liane Hilfert,† Michael Krasnopolski,§

and Frank T. Edelmann*,†

†Chemisches Institut der Otto-von-Guericke-Universitat Magdeburg, Universitatsplatz 2, D-39106 Magdeburg, Germany‡Lehrstuhl fur Organische Chemie II and §Lehrstuhl fur Anorganische Chemie II, Ruhr-Universitat Bochum, Universitatsstraße 150,D-44801 Bochum, Germany

*S Supporting Information

ABSTRACT: 1,4-Diaza-1,3-diene (=DAD) complexes of theheavy alkaline-earth metals strontium and barium have beensynthesized by direct metalation of N,N′-bis(2,6-diisopropyl-phenyl)-1,4-diaza-1,3-butadiene (1, =DADDipp). The reactionwith Sr metal afforded a mixture of the red enediamide-typederivative (DADDipp)Sr(DME)2 (2, DME = 1,2-dimethoxy-ethane) and black (DADDipp)2Sr(DME) (3), which contains twocoordinated DAD radical anions. With barium, only the radicalanion derivative (DADDipp)2Ba(DME) (4) was formed in 82%yield. For the first time, transfer of a DAD radical anion ligand from an alkaline-earth metal to a rare-earth metal has beenachieved. Reaction of 4 with [{(Ph2SiO)2O}2{Li(THF)2}2]HoCl (5) afforded the novel (DAD)holmium bis(disiloxanediolate)complex [{(Ph2SiO)2O}2{Li(THF)2}2]Ho(DAD

Dipp) (6). All new complexes (2−4 and 6) have been structurally characterizedby X-ray diffraction. In addition, the radical anion complexes 3, 4, and 6 were characterized by their EPR spectra.

■ INTRODUCTION

In 1975, tom Dieck et al. surprised the coordination chemistrycommunity with a paper entitled “2,2′-Bipyridyl - a “bad ligand”for metals in low oxidation states”.1 In this paper the authorsdemonstrated that the π-acceptor capacity of certain 1,4-diaza-1,3-diene (=DAD) ligands such as tBuNCHCHNtBu isabout twice as high as that of the traditionally used 2,2′-bipyridine. This particular contribution by one of the pioneersin the field had undoubtedly a significant impact on thedevelopment of DAD coordination chemistry. Today, 1,4-diaza-1,3-dienes are well established as highly versatile ligands fornearly every element in the periodic table. The list of stableDAD complexes encompasses main-group metals,2 early3 andlate4 transition metals, and the lanthanides and actinides.5 Aunique electronic property of the 1,4-diaza-1,3-dienes is thatthey are redox-noninnocent and can undergo one- and two-electron-reduction steps to afford the corresponding radicalanions and the enediamide dianions, respectively, as shown inScheme 1.6

This electronic flexibility combined with the possibility oftuning the steric properties of the diazadiene moiety byintroducing various substituents at both carbon and/ornitrogen account for the high versatility of DAD ligands incoordination and organometallic chemistry. Scheme 2 illus-trates different coordination modes of DAD ligands which havebeen reported in the literature.2−6 The neutral σ2-chelatingcoordination mode is most common with late transition metals,especially in low coordination states. Both the dianionicenediamide derivatives and the σ2-coordinated radical-anion

complexes are known for various s-, p-, and d-block metals andthe first-row transition metals. In the vast majority of all metalcomplexes the diazadienes act as chelating ligands (Scheme 2).In general, nonchelating coordination modes are also possible,5i

but examples are exceedingly rare.The chemistry of alkaline-earth-metal DAD complexes has

been fairly well developed, especially for magnesium andcalcium.7,8 A large body of research in this area comprisesgroup 2 metal complexes of rigid acenaphthene-based DADligands.8 The majority of these complexes have been preparedby salt metathesis reactions between the metal dihalides andalkali-metal DAD precursors. Yet another suitable syntheticroute is the direct metalation of diazadienes by alkaline-earthmetals. While all coordination modes depicted in Scheme 2have already been realized with alkaline-earth metals, it

Received: June 28, 2013Published: August 13, 2013

Scheme 1. Stepwise Reduction of 1,4-Diaza-1,3-dienes ToGive the Corresponding Radical Anions and EnediamideDianions

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becomes evident that the radical anion and enediamide typecomplexes generally prevail.7 Among the group 2 metals, theDAD coordination chemistry of the heavier elements strontiumand barium remains the least developed.7b,8c A recent paper byMashima et al. reported two synthetic protocols for preparingcomplexes of Ca, Sr, and Ba with the bulky DAD ligand N,N′-bis(diisopropylphenyl)-1,4-diaza-1,3-butadiene (1, =DADDipp).9

Salt metathesis reactions of the metal diiodides with 1 equiv ofthe dipotassium salt of DADDipp gave mono- and dinucleargroup 2 metal complexes containing the DADDipp ligand in itsdianionic enediamide form. The second route involved directmetalation of DADDipp with the respective alkaline-earth-metalpowders in the presence of iodine. This method yielded againthe enediamide type complexes (DADDipp)M(THF)4 for M =Ca, Sr, while barium afforded the iodide-bridged binuclearcomplex [(DADDipp)Ba(μ-I)(THF)2]2 with the DADDipp ligandcoordinated to barium as a radical anion.9

We report here the synthesis and structural characterizationof three new DADDipp complexes of Sr and Ba made by directmetalation in the absence of iodine. Moreover, we discoveredthat the barium complex (DADDipp)2Ba(DME) can besuccessfully employed as a reagent for transferring the DADradical anion from the alkaline-earth-metal center to a rare-earth metal.

■ RESULTS AND DISCUSSIONParallel to the investigations by Mashima et al.9 we also studiedthe formation of new DAD complexes of the heavy alkaline-earth metals Sr and Ba by direct metalation of N,N′-bis(diisopropylphenyl)-1,4-diaza-1,3-butadiene (1, =DADDipp).The main difference was that pure Sr and Ba metals were usedas chips without activation by addition of iodine. The results aresummarized in Scheme 3. When a solution of 1 in DME (=1,2-dimethoxyethane) was stirred with an excess of Sr chips, a deepred color began to develop already after ca. 1 h. In order toensure complete reaction, stirring was continued for 1 week.Two new strontium DAD complexes could be isolated byfractional crystallization directly from the reaction mixture afterremoval of unreacted Sr metal.A minor component (28% yield) in the product mixture was

a red compound which was shown by elemental analysis,spectroscopic data (1H and 13C NMR, MS, IR), and X-raydiffraction to be the enediamide derivative (DADDipp)Sr-(DME)2 (2). Separation of 2 from the second reaction productcould be readily achieved, due to its significantly lowersolubility in DME. Both elemental analysis and NMR datawere consistent with the presence of one DADDipp and two

DME ligands in the molecule, which was already indicative forenediamide dianion coordination of the diazadiene (coordina-tion mode C in Scheme 2). The highest peak in the EI massspectrum at m/z 464 (35% relative intensity) could be assignedto the fragment M+ − 2DME, i.e. [(DADDipp)Sr]+. The 13CNMR spectrum of 2 showed a corresponding NCHresonance at δ 114.5, which is in good agreement with thevalue reported for (DADDipp)Sr(THF)4 (δ 112.7).9 In the 1HNMR spectrum, the isopropyl protons of the Dipp substituentsgave rise to a broad signal at δ 3.98 and a broad singlet at δ1.09. A very broad singlet at δ 5.12 in the 1H NMR spectrum(THF-d8) could be assigned to the NCH protons of thediazadiene backbone, a value typical for dianionic enediamideligation of DAD’s (cf. δ 5.47 (C6D6) reported for (DADDipp)-Sr(THF)4).

9 Particularly notable is the broadening of virtuallyall signals in the 1H NMR spectrum measured at roomtemperature. In particular, the resonance of the olefinic protons(δ 5.12) was barely distinguishable from the baseline. The samephenomenon has already been noted by Mashima et al. for thecomplexes [K(DADDipp)(THF)2Ba(μ-I)(THF)2]2 and(DADDipp)M(THF)4 (M = Ca, Sr) bearing the DAD ligandsin the dianionic enediamide-type coordination mode.9

A single-crystal X-ray diffraction study confirmed thepresence of the monomeric strontium diazadiene complex(DADDipp)Sr(DME)2 (2). Figure 1 depicts the molecularstructure of 2. Crystallographic data for 2−4 and 6 are given inTable 1 in the Supporting Information. The coordination ofchelating DADDipp and two DME molecules leads to adistorted-octahedral coordination geometry around the centralstrontium. The Sr−N distances are Sr−N1 = 2.494(2) Å andSr−N2 = 2.419(3) Å. These can be favorably compared to theSr−N bond lengths reported for (DADDipp)Sr(THF)4 (Sr−N =2.475(4) and 2.458(5) Å).9 The Sr−O distances fall in thenarrow range of 2.537(2)−2.637(2) Å and are unexceptional(cf. Sr−O = 2.448(2)−2.613(4) Å in (DADDipp)Sr(THF)4).

9

The most important structural feature of 2 is the long−short−long sequence in the diazadiene backbone corresponding to a−NCHCHN− bonding situation which is typical fordianionic enediamide coordination of DAD ligands (coordina-tion mode C in Scheme 2). Thus with a value of C1−C2 =1.363(5) Å the central C−C bond is shorter than the two C−Nbonds (N1−C1 = 1.384(5) Å, N2−C2 = 1.407(4) Å). Once

Scheme 2. Different Coordination Modes of 1,4-Diaza-1,3-diene ligands

Scheme 3. Direct Synthesis of DADDipp Complexes of Sr andBa from the Metals

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again there is excellent agreement with the correspondingvalues reported for (DADDipp)M(THF)4 (M = Sr, Ba).9 Thesestructural details in combination with the spectroscopic resultsclearly confirmed the dianionic enediamide coordination of theDADDipp ligand in 2.The major product of the reaction of DADDipp with Sr metal

in DME was isolated with 68% yield in the form of dark red(almost black) crystals by crystallization from the concentratedmother liquor after separation of 2. The fact that no meaningful1H NMR spectrum could be obtained gave an early indicationfor the presence of a radical anion DAD complex. The highestpeak in the EI mass spectrum at m/z 929 (30% relativeintensity) corresponded to the molecular ion of the bis(DAD)complex (DADDipp)2Sr(DME) (3). The peak with 100%relative intensity at m/z 840 could be easily assigned to theunsolvated ion [(DADDipp)2Sr]

+ formed by loss of thecoordinated DME, while a peak at m/z 464 (82%)corresponded to [(DADDipp)Sr]+. Other prominent peakswere assigned to the fragment ions [DADDipp]+ (m/z 378,18%) and [DADDipp − C3H7]

+ (m/z 333, 30%). A single-crystalX-ray diffraction study confirmed the presence of the strontiumradical anion DAD complex (DADDipp)2Sr(DME) (3) (Figure2). The coordination geometry around the central Sr atom canagain be best described as distorted octahedral. In contrast tothe long−short−long bonding sequence for the N−C−C−Nbackbone of the enediamide type coordinated DADDipp ligandin 2, these three bonds are equalized in 3 (N1−C1 = 1.321(4)Å, N2−C2 = 1.330(4) Å, N3−C31 = 1.326(4) Å). This istypical for the radical anionic coordination of the DAD ligand(coordination mode B in Scheme 2). In the Ba complex[(DADDipp)Ba(μ-I)(THF)2]2, which also contains mono-anionic DAD ligands, the central C−C bond (1.401(7) Å) iseven longer than the C−N bonds (1.343(7) and 1.314(7) Å).9

The Sr−N distances fall in the narrow range of Sr−N2.600(3)−2.626(3) Å and are thus significantly longer thanthose in 2 (Sr−N1 = 2.494(2) Å and Sr−N2 = 2.419(3) Å) and(DADDipp)Sr(THF)4 (Sr−N = 2.475(4) and 2.458(5) Å).9 TheSr−O distances are unexceptional with Sr−O1 = 2.578(2) Åand Sr−O2 = 2.588(2) Å (cf. Sr−O 2.448(2)−2.613(4) Å in(DADDipp)Sr(THF)4).

9 The predominant formation of radicalanionic 3 in the reaction of DADDipp with Sr metal in DME ismarkedly different from the same reaction carried out in thepresence of I2 (1 mol %), which led to exclusive formation of

the enediamide derivative (DADDipp)Sr(THF)4, as reported byMashima et al.9

In line with these observations, the analogous reaction of 1with an excess of barium chips in DME yielded exclusively thenew radical anion complex (DADDipp)2Ba(DME) (4). Stirringof the reaction mixture for 1 week at room temperatureproduced a deep red solution, from which (DADDipp)2Ba-(DME) (4) could be isolated in the form of deep red (almostblack) crystals in 92% yield. In this case, the formation of theenediamide derivative (DADDipp)Ba(DME)2 was not observed.Due to the radical anion character of the DADDipp ligands in 4,no meaningful NMR spectra (1H, 13C) could be obtained forthis compound. The highest peak in the EI mass spectrum atm/z 890 (30% relative intensity) corresponded to the fragmentM+ − DME, i.e. [(DADDipp)2Ba]

+. Other prominent peakscould be readily assigned to the fragment ions [(DADDipp)Ba]+

(m/z 514, 25%), [DADDipp]+ (m/z 378, 40%), and [DADDipp −C3H7]

+ (m/z 333, 100%). The molecular structure of 4 (Figure3) confirmed the formation of the barium analogue of 3 and thefirst mononuclear barium DAD complex. With an average of2.769 Å the Ba−N distances in 4 are slightly longer than thosein [K(DADDipp)(THF)2Ba(μ-I)(THF)2]2 (Ba−N = 2.720(4)and 2.706(4) Å). This minor elongation might be traced backto higher steric congestion in 4 as compared to [K(DADDipp)-(THF)2Ba(μ-I)(THF)2]2 in which only one bulky DADDipp

ligand is coordinated to each barium ion. The Ba−O bondlengths to the coordinated DME in 4 are 2.771(2) and2.753(2) Å.The two radical anion complexes 3 and 4 were also

characterized by their EPR spectra. As shown in Figures 4and 5, the EPR spectra of 3 and 4 in toluene display a seven-line pattern due to hyperfine coupling (hfc) to the 14N and 1Hatoms of the ligand. Simulation of the EPR spectrum of 4reveals hfc constants of αN = 5.5 G for two 14N centers and αH= 5.1 G for two 1H atoms. These values are in good agreementwith the hfc constants reported for the barium complexpublished by Mashima et al.9 The EPR spectrum of 3 can besimulated with the same parameters. However, the spectrum

Figure 1. Molecular structure of (DADDipp)Sr(DME)2 (2). Selectedbond lengths (Å) and angles (deg): Sr−N1 = 2.494(2), Sr−N2 =2.419(3), N1−C1 = 1.384(5), N2−C2 = 1.407(4), C1−C2 =1.363(5), Sr−O = 2.537(2)−2.637(2); N1−Sr−N2 = 74.93(9), Sr−N1−C1 = 104.6(2), Sr−N2−C2 = 106.7(2), N1−C1−C2 = 126.1(3),N2−C2−C1 = 125.2(4); (N1SrN2)−(N1C1C2N2) = 14.6.

Figure 2. Molecular structure of (DADDipp)2Sr(DME) (3). Selectedbond lengths (Å) and angles (deg): Sr−N1 = 2.613(2), Sr−N2 =2.600(3), Sr−N3 = 2.626(3), Sr−N4 = 2.613(2), N1−C1 = 1.321(4),N2−C2 = 1.330(4), N3−C31 = 1.326(4), N4−C32 = 1.325(4), C1−C2 = 1.396(5), C31−C32 = 1.403(5), Sr−O1 = 2.578(2), Sr−O2 =2.588(2); N1−Sr−N2 = 67.64(8), N3−Sr−N4 = 66.26(8), Sr−N1−C1 = 110.4(2), Sr−N2−C2 = 110.2(2), Sr−N3−C31 = 111.6(2), Sr−N4−C32 = 112.2(2).

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shows additional lines or splitting which cannot easily beexplained and might be due to impurities.In the course of this investigation we wondered if the new

alkaline-earth-metal DAD complexes could perhaps be used asreagents to transfer the DAD ligand to other metals. As a firstexample in this direction, we studied the reaction between thebarium DADDipp complex 4 and the chloro-functionalizedholmium bis(disiloxanediolate) derivative [{(Ph2SiO)2O}2{Li-(THF)2}2]HoCl (5).

10 Such heterobimetallic lanthanide bis-(disiloxanediolates) have been extensively investigated by ourgroup in recent years.11,12 It was found that the small Sc3+ andY3+ ions form heterobimetallic complexes in which the group 3metal fits into the center of a 12-membered Si4O6Li2 inorganicring system formed by two lithium disiloxanediolate units.Additional chloro functionalities and solvent molecules arearranged in trans positions. In contrast, medium and large Ln3+

ions such as Pr3+, Sm3+, and Ho3+ do not fit into the center of

the 12-membered Si4O6Li2 inorganic ring system but aresignificantly displaced, leading to a series of bis-(disiloxanediolate) complexes which have been termed“inorganic lanthanide metallocenes”.11,12 It was shown thatthis new class of heterobimetallic lanthanide disiloxanediolatesshares structural similarities with the well-known bent metal-locenes containing pentamethylcyclopentadienyl (=C5Me5)ligands. The latter form a large and well-investigated class oforganolanthanides, with many of them displaing high catalyticactivities in various olefin transformations.13 In both cases twobulky ligands are coordinated to the central lanthanide ion in abent geometry, leaving room for functional groups such as thechloro ligand in [{(Ph2SiO)2O}2{Li(THF)2}2]HoCl (5). Thus,we reasoned that replacement of the chloro functionality bymonoanionic DAD ligands should provide access to aninteresting new class of lanthanide bis(disiloxanediolates) ofthe type [{(Ph2SiO)2O}2{Li(THF)2}2]Ln(DAD). These wouldcomplement the well-known lanthanide metallocene derivatives(C5Me5)2Ln(DAD).

5

In fact, treatment of [{(Ph2SiO)2O}2{Li(THF)2}2]HoCl (5)with the barium reagent 4 (molar ratio 2:1) in tolueneaccording to Scheme 4 at reflux temperature led to thedevelopment of a red solution accompanied by formation of afine white precipitate of BaCl2. Filtration and subsequentcrystallization directly from the concentrated filtrate affordedthe desired product [{(Ph2SiO)2O}2{Li(THF)2}2]Ho-(DADDipp) (6) as air-sensitive, orange-red prisms in 57%isolated yield.The isolation of 6 was facilitated by the virtual insolubility of

the barium chloride byproduct in toluene. Because of thestrongly paramagnetic nature of the Ho3+ ion as well as thepresence of a radical anionic DADDipp ligand, no useful NMRdata (1H, 13C, 29Si) could be gathered for 6. In this case, the EImass spectrum was also uninformative, showing only thefragment ion m/z 333 [DADDipp − C3H7]

+ as the highest peak.Fortunately, X-ray-quality single crystals of 6 could be readilygrown by slow cooling of a saturated solution in toluene to 2°C. The X-ray diffraction study (Figure 6) clearly establishedthe successful formation of the target compound[{(Ph2SiO)2O}2{Li(THF)2}2]Ho(DAD

Dipp) (6). For the firsttime, transfer of a DAD radical anion ligand from an alkaline-

Figure 3. Molecular structure of (DADDipp)2Ba(DME) (4). Selectedbond lengths (Å) and angles (deg): Ba−N1 = 2.770(2), Ba−N2 =2.769(2), Ba−N3 = 2.758(2), Ba−N4 = 2.779(2), N1−C1 = 1.322(3),N2−C2 = 1.327(3), N3−C31 = 1.328(3), N4−C32 = 1.326(3), C1−C2 = 1.396(3), C31−C32 = 1.406(3), Ba−O1 = 2.771(2), Ba−O2 =2.753(2); N1−Ba−N2 = 62.56(5), N3−Ba−N4 = 63.03(5), Ba−N1−C1 = 112.7(2), Ba−N2−C2 = 112.4(2), Ba−N3−C31 = 113.4(2),Ba−N4−C32 = 112.9(2).

Figure 4. (a) Experimental EPR spectrum of the strontium complex 3in toluene at room temperature. (b) Simulated EPR spectrum with thesame (hyperfine coupling) hfc constants as for the barium complex, αN= 5.5 G for the two 14N centers and αH = 5.1 G for the 1H atoms (g =2.0027, ν = 9.769633 GHz).

Figure 5. (a) Experimental EPR spectrum of the barium complex 4 intoluene at room temperature. (b) Simulated EPR spectrum with(hyperfine coupling) hfc constants αN = 5.5 G for the two 14N centersand αH = 5.1 G for the 1H atoms (g = 2.0027, ν = 9.770218 GHz).

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earth metal to a rare-earth metal had been achieved with thesynthesis of 6.As illustrated in Figure 6, the molecular structure of 6 adopts

a highly distorted octahedral coordination geometry andfeatures a monoanionic DADDipp ligand. Due to the presenceof two very bulky ligands, i.e. the 12-membered[{(Ph2SiO)2O}2{Li(THF)2}2]

2− ring system and the stericallydemanding DADDipp radical anion, the central Ho3+ ion issterically saturated without addition of a coordinating solvent.The bonding situation within the diazadiene backbonecomprises shortened C−N bond lengths (1.319(5) Å) and anelongated central C−C bond length (1.398(9) Å) inaccordance with the monoanionic (radical anionic) characterof the DADDipp ligand in 6. These values are also in goodagreement with those reported for [(DADDipp)Ba(μ-I)-(THF)2]2 (vide supra)9 or the previously reported scandiumDAD radical anion complex (DADDipp)ScCl2(THF)2 (C−N =1.338(2) and 1.337(2) Å, C−C = 1.399(2) Å).5l The Ho−Ndistances in 6 (2.462(3) Å) are in good agreement with thosereported for other organoholmium complexes comprising Ho−N bonds.14 At 2.243(3) and 2.273(2) Å the Ho−Osilox bondlengths in 6 are slightly elongated as compared to those in thechloro precursor 5 (Ho−Osilox = 2.224(4) and 2.205(4) Å) dueto replacement of the chloro ligand by the bulky DADDipp

radical anion. The radical anionic coordination mode of thediazadiene ligand in 6 was also proven by the EPR spectrum(Figure 7). The spectrum looks quite similar to that of the

barium complex 4 but has slightly different hfc constants and abroader line width. Simulation of the experimental spectrum of6 results in the hfc constants αN = 6.1 G for the two 14N centersand αH = 5.1 G for the 1H atoms.

■ CONCLUSIONSIn summarizing the work reported here, we succeeded in thepreparation of three new diazadiene complexes of strontiumand barium by direct metalation. In the case of strontium, bothmonoanionic and dianionic coordination of the redox-activeligand DADDipp was observed, whereas with barium only theradical anion complex (DADDipp)2Ba(DME) (4) could beisolated in excellent yield (92%). These results nicelycomplement recent findings by Mashima et al.,9 making varioustypes of diazadiene complexes of the heaviest alkaline earthmetals readily available. Moreover, transfer of a DAD radicalanion ligand from an alkaline-earth metal to a rare-earth metalhas been achieved for the first time. Reaction of 4 with[{(Ph2SiO)2O}2{Li(THF)2}2]HoCl (5) afforded the novel(DAD)ho lm ium b i s (d i s i l o x aned io l a t e ) comp l e x[{(Ph2SiO)2O}2{Li(THF)2}2]Ho(DAD

Dipp) (6) as the firstrepresentative of a new class of DAD complexes. All newcomplexes (2−4 and 6) have been structurally characterized byX-ray diffraction. The radical anionic coordination mode of theDAD ligand in 3, 4, and 6 was also proven by EPRspectroscopy.

■ EXPERIMENTAL SECTIONGeneral Procedures. All operations were performed with rigorous

exclusion of air and water in oven-dried or flame-dried glassware under

Scheme 4. Synthesis of the Holmium DADDipp Complex 6 via Ligand Transfer from 4

Figure 6. Molecular structure of [{(Ph2SiO)2O}2{Li(THF)}2]Ho-(DADDipp) (6). Selected bond lengths (Å) and angles (deg): Ho−Osilox = 2.243(3), 2.273(2), Ho−N1 = 2.462(3), Ho−N2#1 =2.462(3), N1−C1 = 1.319(5), C(1)−C(2)#1 = 1.398(9), Li−Osilox= 1.864(7), 1.889(8), 1.881(8); Osilox−Ho−Osilox = 122.12(14),85.28(9), 75.86(9), N1−Ho−N2#1 = 69.21(15). Symmetry trans-formations used to generate equivalent atoms: (#1) −x + 3/2, y, −z +3/2.

Figure 7. (a) Experimental EPR spectrum of the Holmium complex 6in toluene at room temperature. (b) Simulated EPR spectrum with(hyperfine coupling) hfc constants αN = 6.1 G for the two 14N centersand αH = 5.1 G for the 1H atoms (g = 2.0033, ν = 9.769995 GHz).

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an inert atmosphere of dry argon, employing standard Schlenk, high-vacuum, and glovebox techniques (MBraun MBLab; <1 ppm O2, <1ppm H2O). THF, DME, and toluene were dried over sodium/benzophenone and freshly distilled under a nitrogen atmosphere priorto use. All glassware was oven-dried at 120 °C for at least 24 h,assembled while hot, and cooled under high vacuum prior to use. Thestarting materials N,N′-Bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-buta-diene (1; abbreviated DADDipp, Dipp = 2,6-diisopropylphenyl)15 and[{(Ph2SiO)2O}2{Li(THF)2}2]HoCl(THF)2 (5)11 were preparedaccording to published procedures. Strontium and barium metalchips were purchased from Aldrich and used as received. The NMRspectra were recorded in C6D6 or THF-d8 solutions on a Bruker DPX600 (1H, 600.1 MHz; 13C, 150.9 MHz) or a Bruker-AVANCE-DMX400 instrument (5 mm BB; 1H, 400.1 MHz; 13C, 100.6 MHz),1H and 13C shifts are referenced to internal solvent resonances andreported in parts per million relative to TMS. IR (KBr) spectra weremeasured using a Perkin-Elmer FT-IR 2000 spectrometer. Massspectra (EI, 70 eV) were run on a MAT 95 apparatus. X-band EPRspectra were recorded on a Bruker Elexsys E500 EPR spectrometerwith an ER077R magnet (75 mm pole cap distance) and an ER047XG-T microwave bridge. The EPR spectra were measured in toluene.Microanalyses of the compounds were performed using a Leco CHNS923 apparatus. In the case of compounds 2−4 the carbon values werefound to be off by about 1% (lower). This might be traced back to theformation of MCO3 (M = Sr, Ba) during the combustion analysis.Synthesis of (DADDipp)Sr(DME)2 (2) and (DADDipp)2Sr(DME)

(3). A 2.5 g portion (6.6 mmol) of DADDipp (1) was dissolved in 100mL of DME, and 0.61 g (7.0 mmol, in excess) of strontium chips wasadded. The reaction mixture was stirred at room temperature with aglass-coated stirring bar. After 1 h the reaction mixture turned fromyellow to deep red. The reaction mixture was stirred for 1 week, andthe unconsumed strontium together with a gray precipitate wasremoved by filtration. Partial evaporation of 30 mL of DME andcooling of the deep red solution to 5 °C afforded 1.2 g (28%) of(DADDipp)Sr(DME)2 (2) as red crystals. The crystalline compoundwas removed, the volume of the solution was reduced to 40 mL, andthe solution was stored for 1 week at a temperature of 2 °C to form 2.1g (68%) of dark red (almost black) crystals of [(DADDipp)2Sr(DME)](3). The overall conversion of DADDipp to (DADDipp)Sr(DME)2 and(DADDipp)2Sr(DME) was virtually quantitative (ca. 96%).Data for (DADDipp)Sr(DME)2 (2) are as follows. Anal. Calcd for

C34H56N2O4Sr (Mr = 644.4): C, 63.37; H, 8.76; N, 4.35. Found: C,62.47; H, 8.97; N, 4.43. 1H NMR (400 MHz, d8-THF, 193 K): 1.09(s(br), 24H, −CH(CH3)2), 3.24 (s, 12H, DME), 3.80 (s(br), 4H,−CH(CH3)2), 5.12 (s, 2H, CHN), 6.23 (trip, 3JHH = 7.0 Hz, 2H,para-Ar), 6.66 (d, 3JHH = 7.0 Hz, 4H, meta-Ar) ppm. 13C NMR (100MHz, d8-THF, 253 K): 25.3−25.7 (−CH(CH3)2), 28.15, 28.19(−CH(CH3)2), 59.0 (DME), 72.6 (DME), 114.5 (NCH), 120.3(para-Ar), 122.8 (meta-Ar), 140.9 (ortho-Ar), 158.2 (ipso-Ar) ppm.Mass spectrum (EI): m/z 464 (35%) [M+ − 2DME], 378 (55%)[DADDipp], 333 (70%) [DADDipp − C3H9], 203 (35%), 188 (80%),162 (60%), 178 (38%), 146 (100%). IR (KBr disk): ν 3034 m, 2998m, 2954 vs, 2865 s, 1662 w, 1628 w, 1581 s, 1546 w, 1458 s, 1417 vs,1373 vs, 1334 s, 1314 vs, 1265 vs, 1191 m, 1136 m, 1112 vs, 1096 s,1065 vs, 1025 m, 1005 m, 911 w, 863 s, 851 m, 793 m, 760 m, 728 m,685 w, 549 w cm−1. Mp: 143 °C dec.Data for (DADDipp)2Sr(DME) (3) are as follows. Anal. Calcd for

C56H82N4O2Sr (Mr = 930.9): C, 72.25; H, 8.88; N, 6.02. Found: C,71.37; H, 8.75; N, 6.53%. 1H NMR (600 MHz, C6D6, 298 K): becauseof the paramagnetic properties of the radical anionic DAD, it wasimpossible to obtain meaningful 1H NMR spectra. 13C NMR (150MHz, C6D6, 298 K): 23.5, 23.6, 23.7, 24.2, 24.3, 24.4, 28.0, 28.3, 28.5(−CH(CH3)2), 33.0 (br, −CH(CH3)2), 85.5 (br, DME), 81.8 (br,DME), 123.4, 123.8, 124.1, (para-Ar), 125.6 (meta-Ar), 136.6 (ortho-Ar), 148.7 (ipso-Ar), 163.2 (CN) ppm. Mass spectrum (EI): m/z929 (30%) [M+], 840(100%) [M+ − DME], 464 (82%) [M+ − DME,DADDipp], 378 (18%) [DADDipp], 333 (30%) [DADDipp − C3H7]. IR(KBr disk): ν = 3058 m, 2959 s, 2867 m, 1668 w, 1628 m, 1588 w,1539 w, 1457 s, 1433 s, 1382 m, 1361 m, 1314 m, 1261 s, 1178 w,

1159 w, 1108 w, 1076 m, 1036 w, 926 m, 884 w, 868 w, 837 w, 819 w,794 m, 756 s, 679 w, 525 w cm−1. Mp: 160 °C dec.

Synthesis of (DADDipp)2Ba(DME) (4). A 2.5 g portion (6.6 mmol)of DADDipp (1) was dissolved in 100 mL of DME, and 1.0 g (7.3mmol, in excess) of barium chips was added. The reaction mixture wasstirred at room temperature with a glass-coated stirring bar. After 1 hthe reaction mixture changed from yellow to deep red. The reactionmixture was stirred for 1 week, and the unconsumed barium togetherwith a gray precipitate were removed by filtration. Partial evaporationof 40 mL of DME and cooling of the deep red solution to 2 °Cafforded (DADDipp)2Ba(DME) (4) as deep red (almost black) crystals.In this case the formation of the enediamide derivative (DADDipp)-Ba(DME)2 was not observed. Yield: 3.0 g (92%). Anal. Calcd forC56H82BaN4O2 (Mr = 980.6): C, 68.59; H, 8.43; N, 5.71. Found: C,67.72; H, 8.21; N, 6.13. Because of the paramagnetic properties of theradical anionic DAD, it was impossible to obtain meaningful 1H and13C NMR spectra. Mass spectrum (EI): m/z 890 (30%) [M+ − DME],514 (25%) [M+ − DME, DADDipp], 378 (40%) [DADDipp], 333(100%) [DADDipp − C3H7]. IR (KBr disk): ν 3052 w, 3011 w, 2961 vs,2927 sh, 2867 m, 2709 w, 1664 w, 1626 m, 1588 w, 1541 w, 1458 s,1431 s, 1381 m, 1361 m, 1314 s, 1264 vs, 1196 w, 1180 w, 1158 w,1107 w, 1070 m, 1054 w, 1037 w, 1026 w, 982 w, 958 w, 924 m, 884w, 859 w, 835 m, 794 s, 755 s, 525 w cm−1. Mp: 155 °C dec.

Synthesis of [{(Ph2SiO)2O}2{Li(THF)2}2]Ho(DADDipp) (6). A 0.7 g

portion (0.475 mmol) of [{(Ph2SiO)2O}2{Li(THF)2}2]HoCl (5) and0.25 g (0.25 mmol) of (DADDipp)2Ba(DME) (4) were dissolved in 50mL of toluene. The reaction mixture was stirred overnight, refluxed for2 h, and filtered through a P4 glass frit to separate a white precipitate(BaCl2) from the clear red solution. After the volume of the solutionwas reduced under vacuum to 5 mL, the product crystallized in theform of orange-red prisms. Yield: 0.5 g (57%). Anal. Calcd forC82H92HoLi2N2O8Si4 (Mr = 1524.8): C, 64.59; H, 6.08; N, 1.84.Found: C, 64.31; H, 5.89; N, 2.27. Because of the stronglyparamagnetic nature of the Ho3+ ion, it was impossible to obtainmeaningful 1H, 13C, and 29Si NMR data. Mass spectrum (EI): m/z 333(100) [DADDipp − C3H7], 378 (10) [DADDipp], 439 (25), 517 (7),559 (1), 594 (1), 637 (8), 715 (3), 792 (0.5). IR (KBr disk): 3067 m,3048 m, 2999 m, 2962 s, 2869 m, 1959 w, 1892 w, 1825 w, 1774 w,1625 m, 1590 m, 1568 w, 1460 s, 1428 vs, 1384 m, 1362 m, 1316 m,1249 s, 1181 m, 1156 m, 1119 vs, 1083 s, 1034 vs, 1016 vs, 994 vs, 821w, 797 m, 742 s, 701 vs, 683 m, 620 w, 531 vs, 492 s cm−1. Mp: 95 °Cdec.

X-ray Crystallographic Studies. Single crystals of 2−4 wereobtained from saturated solutions in DME at 2−5 °C. In the case of 6,single crystals were grown by cooling a saturated toluene solution to 2°C. The intensity data of 2−4 and 6 were collected on a Stoe IPDS 2Tdiffractometer with Mo Kα radiation. The data were collected with theStoe XAREA16 program using ω scans. The space group wasdetermined with the XRED3216 program. The structure was solvedby direct methods (SHELXS-97) and refined by full-matrix least-squares methods on F2 using SHELXL-97.17 Data collectionparameters are given in Table 1 in the Supporting Information.

■ ASSOCIATED CONTENT

*S Supporting InformationCIF files and tables giving X-ray structural data for 2−4 and 6.This material is available free of charge via the Internet athttp://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*F.T.E.: tel, (+49)-391-6718327; fax, (+49)-391-6712933; e-mail, frank.edelmann@ovgu.de.

NotesThe authors declare no competing financial interest.

Organometallics Article

dx.doi.org/10.1021/om400622d | Organometallics 2013, 32, 4636−46424641

■ ACKNOWLEDGMENTS

Financial support by the Otto-von-Guericke-Universita tMagdeburg is gratefully acknowledged.

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Organometallics Article

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