Unexpected Formation and Structural Characterization of a Dinuclear Sodium Half-Sandwich Complex

inorganics

Article Unexpected Formation and Structural Characterization of a Dinuclear Sodium Half-Sandwich Complex

Nicole Harmgarth 1, Phil Liebing 2 ID , Liane Hilfert 1 ID , Sabine Busse 1 and Frank T. Edelmann 1,*

1 Chemisches Institut der Otto-von-Guericke-Universität Magdeburg, 39106 Magdeburg, Germany; [email protected] (N.H.); [email protected] (L.H.); [email protected] (S.B.) 2 ETH Zürich, Laboratorium für Anorganische Chemie, Vladimir-Prelog-Weg 2, 8093 Zürich, Switzerland; [email protected] * Correspondence: [email protected]; Tel.: +49-391-67-58327; Fax: +49-391-67-42933

Received: 12 April 2018; Accepted: 7 May 2018; Published: 9 May 2018

Abstract: Treatment of N,N0-diisopropylcarbodiimide with sodium cyclopentadienide (NaCp) in a molar ratio of 1:1 in THF solution resulted in formation of the unexpected dinuclear i i sodium half-sandwich complex [NaC5H3{C(NH Pr)(=N Pr)}2-1,2]2 (1) as colorless crystals in low yield. The newly formed ligand, which belongs to the group of 6-aminofulvene-2-aldiminate ligands, coordinates to sodium in an η5-coordination mode via the cyclopentadienyl ring. Dimerization occurs through additional chelating κN,N0-coordination of the amidine substituents. The NMR data of 1 indicated a slow dimer/monomer equilibrium in solution. A serendipitously i i i isolated hydrolysis product,{µ-( PrNH)2C=O}2[NaC5H3{C(NH Pr)(=N Pr)}2-1,2]2 (2), contains the new 6-aminofulvene-2-aldiminate ligand in the N,N0-chelating coordination mode with the cyclopentadiene ring being uncoordinated. In this case, dimerization occurs through the presence of two bridging neutral N,N0-diisopropylurea ligands. Both compounds have been structurally characterized by single-crystal X-ray diffraction.

Keywords: Sodium; amidinate ligand; cyclopentadienyl; aminofulvene-aldiminate; crystal structure

1. Introduction 0 0 − N,N -Chelating anionic ligands—such as the amidinates, [RC(NR )2] , and guanidinates, 0 − [R2NC(NR )2] (Scheme1)—have become indispensable in various fields of organometallic and coordination chemistry [1–9]. These ligands form well-defined complexes with virtually every metallic element in the periodic system. In main group metal chemistry, bulky amidinate and guanidinate ligands have been successfully employed to stabilize low coordination numbers and uncommon oxidation states [10–12]. Numerous transition metal amidinate and guanidinate complexes have been demonstrated to exhibit high catalytic activity in a variety of organic transformations [2,13–16]. Moreover, alkyl-substituted amidinate and guanidinate complexes of various metals have been established as ALD (atomic layer deposition) and MOCVD (metal–organic chemical vapor deposition) precursors for the deposition of thin films of useful materials, such as metals, metal oxides or metal nitrides, among others [17–19].

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. . SchemeScheme 1. Schematic representation of amidinate anions (a) and guanidinate anions (b). Scheme 1. Schematic representation of amidinate anions (a)) andand guanidinateguanidinate anionsanions ((b).). Making the amidinate and guanidinate ligands extremely versatile, the substituents at all three Making the the amidinate amidinate and and guanidinate guanidinate ligands ligands extr extremelyemely versatile, versatile, the substituents the substituents at all atthree all atoms of the N–C–N backbone can be varied. A generally applicable synthetic route to lithium threeatoms atomsof the ofN–C–N the N–C–N backbone backbone can be can varied. be varied. A generally A generally applicable applicable synthetic synthetic route to routelithium to amidinate precursors involves the addition of organolithium reagents to N,N′-disubstituted lithiumamidinate amidinate precursors precursors involves involves the addition the addition of oforganolithium organolithium reagents reagents to to NN,,NN0′-disubstituted-disubstituted carbodiimides, as shown in Scheme 2. The substituents R and R′ can be varied in a wide range and, carbodiimides, as as shown shown in in Scheme Scheme 2.2. The The substituents substituents R Rand and R′ Rcan0 canbe varied be varied in a inwide a wide range range and, consequently, a large library of amidinate ligands has become available [1–9]. Closely related and,consequently, consequently, a large a large library library of ofamidinate amidinate ligands ligands has has become become available available [1–9]. [1–9]. Closely Closely related guanidinate anions can be accessed in a similarly manner by adding lithium diorganoamides, LiNR2, guanidinate anions can be accessed in a similarly manner by adding lithium diorganoamides, LiNR2,, to carbodiimides. 2 to carbodiimides.

Scheme 2. General synthetic route to lithium amidinates. Scheme 2. General synthetic route to lithium amidinates. At the outset of the present study, we wondered if it might be possible to directly attach a At the outset of the present study, we wondered if it might be possible to directly attach a cyclopentadienylAt the outset ring of theto the present amidinate study, N–C–N we wondered backbone if by it mightthe use be of possible sodium tocyclopentadienide, directly attach a cyclopentadienyl ring to the amidinate N–C–N backbone by the use of sodium cyclopentadienide, NaCp,cyclopentadienyl as a nucleophile. ring to the We amidinate now report N–C–N the backbone outcome by of the this use ofreaction sodium and cyclopentadienide, the structural NaCp, as a nucleophile. We now report the outcome of this reaction and the structural characterizationNaCp, as a nucleophile. of the unexpected We now report product. the outcome of this reaction and the structural characterization characterization of the unexpected product. of the unexpected product. 2. Results and Discussion 2. Results and Discussion 2. Results and Discussion The reaction of 1 equiv. of sodium cyclopentadienide, NaCp, with The reaction of 1 equiv. of sodium cyclopentadienide, NaCp, with N,N′-diisopropylcarbodiimideThe reaction of 1 equiv. of was sodium carried cyclopentadienide, out in THF solution NaCp, at with roomN ,temperatureN0-diisopropylcarbodiimide and resulted in N,N′-diisopropylcarbodiimide was carried out in THF solution at room temperature and resulted in formationwas carried of out a inclear, THF light-brown solution at roomsolution. temperature Remova andl of resultedthe solvent, in formation followed of by a clear, extraction light-brown with formation of a clear, light-brown solution. Removal of the solvent, followed by extraction with nsolution.-pentane, Removal provided of thea yellow solvent, solution followed besides by extraction a large withamountn-pentane, of insoluble provided brown a yellow material. solution All n-pentane, provided a yellow solution besides a large amount of insoluble brown material. All attemptsbesides ato large crystallize amount the of latter insoluble from browndiethyl material.ether or THF All failed. attempts Crystallization to crystallize of the the lattern-pentane from attempts to crystallize the latter from diethyl ether or THF failed. Crystallization of the n-pentane filtratediethyl at ether 4 °C orafforded THF failed. colorless, Crystallization needle-like crys of thetals nin-pentane low yield. ﬁltrate An IR at spectrum 4 ◦C afforded of the colorless, product filtrate at 4 °C afforded colorless, needle-like crystals in low yield. An IR spectrum of the product suggestedneedle-like that crystals the expected in low simple yield. addition An IR spectrum reaction (Scheme of the product 3a) had suggestednot taken place, that theas it expected showed suggested that the expected simple addition reaction (Scheme 3a) had not taken place, as it showed threesimple bands addition typical reaction for NH (Scheme functional3a) groups had not at ν taken 3436 place,m (νas NH), as it 3386 showed m (ν threes NH),bands 3223 m typical (νs NH). for A three bands typical for NH functional groups at ν 3436 m (νas NH), 3386 m (νs NH), 3223 m (νs NH). A single-crystalNH functional X-ray groups diffraction at ν 3436 mstudy (νas NH),revealed 3386 the m (νformations NH), 3223 of man ( νunexpecteds NH). A single-crystal dimeric sodium X-ray single-crystal X-ray diffraction study revealed the formation of an unexpected dimeric sodium half-sandwichdiffraction study complex revealed [NaC the formation5H3{C(NHiPr)(=N of an unexpectediPr)}2-1,2]2 ( dimeric1). As sodiumillustrated half-sandwich in Scheme complex3b, the 5 3 i i 2 2 half-sandwich complexi i [NaC H {C(NH Pr)(=N Pr)} -1,2] (1).i As illustratedi − in Scheme 3b, the bis(amidino)-substituted[NaC5H3{C(NH Pr)(=N Pr)} cyclopentadienyl2-1,2]2 (1). As anion illustrated [C5H3{C(NH in SchemePr)(=N3b,Pr)} the2-1,2] bis(amidino)-substituted had formed through i i − bis(amidino)-substituted cyclopentadienyli anioni [C5H3{C(NH− Pr)(=N Pr)}2-1,2] had formed through doublecyclopentadienyl addition of anion N,N [C′-diisopropylcarbodiimide5H3{C(NH Pr)(=N Pr)}2-1,2] to thehad cyclopentadienyl formed through ring. double With addition ca. 9% of double addition of N,N′-diisopropylcarbodiimide to the cyclopentadienyl ring. With ca. 9% (determinedN,N0-diisopropylcarbodiimide after the crystal structure to the cyclopentadienyl had been elucidated) ring. Withthe isolated ca. 9% (determinedyield of 1 was after reproducibly the crystal (determined after the crystal structure had been elucidated) the isolated yield of 1 was reproducibly low,structure although had been synthetically elucidated) us theeful isolated amounts yield (>1 of 1g)was could reproducibly be easily low, prepared. although Notably synthetically and low, although synthetically useful amounts (>1 g) could be easily prepared. Notably and surprisingly,useful amounts the (>1isolated g) could yield beof 1 easily could prepared. not be improved Notably by andcarrying surprisingly, out the reaction the isolated of NaCp yield with of surprisingly, the isolated yield of 1 could not be improved by carrying out the reaction of NaCp with N1 could,N′-diisopropylcarbodiimide not be improved by carrying in a 1:2 out molar the reaction ratio as of suggested NaCp with byN the,N0 -diisopropylcarbodiimidecomposition of the products. in a N,N′-diisopropylcarbodiimide in a 1:2 molar ratio as suggested by the composition of the products. Several1:2 molar experiments ratio as suggested using 2 byequiv. the compositionof iPrN=C=N ofiPr the under products. different Several reaction experiments conditions using afforded 2 equiv. only of Several experiments using 2 equiv. of iPrN=C=NiPr under different reaction conditions afforded only smalliPrN=C=N amountsiPr underof NaCp different as the reactiononly crystalline conditions material. afforded only small amounts of NaCp as the only small amounts of NaCp as the only crystalline material. crystalline material.

Inorganics 2018, 6, 47 3 of 9 Inorganics 2018, 6, x FOR PEER REVIEW 3 of 9 Inorganics 2018, 6, x FOR PEER REVIEW 3 of 9 − − iPr − iPr − iPr iPr N NH ++H X Na H N Na NH a) ++or a) X Na N or Na N N N iPr iPr i + iPr Pr 2 Na+ 2 Na THF + i THF iPr Pr + i 2 iPr N C N iPr iPr Pr 2 iPr N C N iPr − HN NH iPr − HN NH i NH Pr NH iPr N i i i + Pr Pr PrN N N iPr i N Na b) Na+ = Pr N b) Na Na Na iPr = Na N iPr N i N N Pr N iPr i NH N iPr Pr i iPr NH NHPr NH 1 NH NH i 1 i Pr Pr i i Pr Pr Scheme 3. Synthesis of the title compound [NaC5H3{C(NHiiPr)(=NiPr)}2-1,2]2 (1). Scheme 3. Synthesis of the title compound [NaC5H3{C(NH Pr)(=N Pr)}2-1,2]2 (1). Scheme 3. Synthesis of the title compound [NaC5H3{C(NHiPr)(=NiPr)}2-1,2]2 (1). Colorless, needle-like single crystals of 1 suitable for X-ray diffraction were obtained by cooling Colorless,Colorless, needle-like needle-like single single crystals crystals of 1 of suitable1 suitable for X-ray for X-raydiffraction diffraction were obtained were obtained by cooling by a solution in n-pentane to 4 °C. Figure 1 depicts the molecular structures of 1 along with selected acooling solution a solutionin n-pentane in n-pentane to 4 °C. Figure to 4 ◦C. 1 Figuredepicts1 depictsthe molecular the molecular structures structures of 1 along of with1 along selected with bond lengths and angles, while crystallographic details are listed in the Supplementary Materials. bondselected lengths bond and lengths angles, and while angles, crystallographic while crystallographic details are details listed in are the listed Supplementary in the Supplementary Materials. Compound 1 crystallizes in the monoclinic space group P21/n, with one Na atom and one Materials. Compound 1 crystallizes in the monoclinic space group1 P2 /n, with one Na atom and one Compound i1 crystallizesi in− the monoclinic space group P2 /n, 1with one Na atom and one [C5H3{C(NH Pr)(=Ni Pr)}i 2-1,2] ligand− in the asymmetric unit. The dimeric molecular structure is [C[C5HH3{C(NH{C(NHiPr)(=NPr)(=NiPr)}Pr)}2-1,2]-1,2]− ligandligand in inthe the asymmetric asymmetric unit. unit. The The dimeric dimeric molecular molecular structure structure is is completed5 3 by inversion symmetry.2 The ligand in attached to the Na atom in N,N-chelating fashion completedcompleted by by inversion inversion symmetry. symmetry. The The ligand ligand in in attached to the Na atom in N,N-chelating fashion via the two non-protonated N atoms (N1, N3), with the bite angle N–Na–N being 91.03(5)°. The two via the two two non-protonated non-protonated N N atoms atoms (N1, (N1, N3), N3), withwith the the bite bite angle angle N–Na–N N–Na–N being being 91.03(5)°. 91.03(5)◦. The The two Na–N bonds are virtually identical in length (Na–N1 233.1(2) pm and Na–N3 233.8(2) pm). These Na–NNa–N bonds are virtually identical in length (Na–N1(Na–N1 233.1(2)233.1(2) pmpm andand Na–N3Na–N3 233.8(2)233.8(2) pm)pm).. These values are slightly smaller as compared to those in a previously reported sodium amidinate (Na–N values are slightly smaller as compared to those in a previously reported sodium amidinate (Na–N 237.1(3) and 240.7(3) pm; C.N. of Na = 5), which can be attributed to the higher coordination number 237.1(3)237.1(3) and and 240.7(3) 240.7(3) pm; pm; C.N. C.N. of Na of Na= 5), = which 5), which can be can attributed be attributed to the higher to the coordination higher coordination number of sodium in the reference compound [20]. Consequently, it seems like the negative ligand charge is ofnumber sodium of in sodium the reference in the compound reference [20]. compound Conseque [20ntly,]. Consequently, it seems like the it negative seems like ligand the charge negative is located at the amidine moiety at a significant amount. This is in accordance with locatedligand chargeat the is locatedamidine at themoiety amidine at moietya signific at aant signiﬁcant amount. amount. This Thisis in is inaccordance accordance with amidinocyclopentadienide/aminofulvene-aldiminate mesomerism, as outlined in Scheme 4. amidinocyclopentadienide/aminofulvene-aldiminateamidinocyclopentadienide/aminofulvene-aldiminate mesomerism, mesomerism, as as outlined outlined in in Scheme Scheme 44..

. . Scheme 4. Resonance forms of the [C5H3{C(NHiPr)(=NiPr)}2-1,2]− ligand. Scheme 4. Resonance forms of the [C5H3{C(NHiiPr)(=NiPr)}2-1,2]−− ligand. Scheme 4. Resonance forms of the [C5H3{C(NH Pr)(=N Pr)}2-1,2] ligand. Additional η5-coordination of the cyclopentadienyl moiety of the symmetry-related ligand Additional η5-coordination of the cyclopentadienyl moiety of the symmetry-related ligand leadsAdditional to pseudo ηtrigonal-planar5-coordination coordina of the cyclopentadienyltion of the Na atom moiety (regarding of the symmetry-relatedthe η5-Cp moiety ligandas one leads to pseudo trigonal-planar coordination of the Na atom (regarding the η5-Cp moiety as one leadsligating to entity). pseudo The trigonal-planar Na–centroid(C5) coordination separation of is thecomparatively Na atom (regarding short at 244.3(1) the η5 -Cppm, which moiety can as ligating entity). The Na–centroid(C5) separation is comparatively short at 244.3(1) pm, which can onepossibly ligating be entity).explained The by Na–centroid(C5) the low coordination separation number is comparatively of sodium short (cf. at NaCp(DME), 244.3(1) pm, possibly be explained by the low coordination number of sodium (cf. NaCp(DME), whichNaCp(18-crown-6) can possibly and be explainedNa(C4H4Me)(18-crown-6): by the low coordination Na–centroid(C number5-ring) of sodium 254(2)–256(2) (cf. NaCp(DME), pm [21] NaCp(18-crown-6) and Na(C4H4Me)(18-crown-6): Na–centroid(C5-ring) 254(2)–256(2) pm [21] NaCp(18-crown-6)(18-crown-6 = 1,4,6,10,13,16-hexaoxacycooctadecane); and Na(C4H4Me)(18-crown-6): Na–centroid(C Na[C3H3-1,2-(COOMe)5-ring) 254(2)–256(2)2]: Na–centroid(C5) pm [21] (18-crown-6 = 1,4,6,10,13,16-hexaoxacycooctadecane); Na[C3H3-1,2-(COOMe)2]: Na–centroid(C5) (18-crown-6253.04(6) pm =[22]). 1,4,6,10,13,16-hexaoxacycooctadecane); Coordination of further ligands, such Na[C as THFH to-1,2-(COOMe) sodium, is most]: Na–centroid(C5)likely prevented 253.04(6) pm [22]). Coordination of further ligands, such as THF3 3to sodium, is most2 likely prevented 253.04(6)by the steric pm bulk [22]). of Coordination the isopropyl of groups. further Possib ligands,ly for such the as same THF reason, to sodium, the N–H is most hydrogen likely prevented atoms at by the steric bulk of the isopropyl groups. Possibly for the same reason, the N–H hydrogen atoms at byN2 theand steric N4 are bulk not of involved the isopropyl in hydrogen groups. bonding. Possibly for the same reason, the N–H hydrogen atoms at N2 and N4 are not involved in hydrogen bonding. N2 and N4 are not involved in hydrogen bonding.

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FigureFigure 1. Molecular 1. Molecular structure structure of [NaCof [NaCH5H{C(NH3{C(NHiPr)(=NiPr)(=NiPr)}iPr)}2-1,2]-1,2]2 (1)( 1in) inthe the crystal. crystal. Displacement Displacement Figure 1. Molecular structure of [NaC55H33{C(NHiPr)(=NiPr)}22-1,2]2 2(1) in the crystal. Displacement ellipsoids are drawn at the 50% probability level, H atoms attached to C atoms omitted for clarity. ellipsoidsellipsoids are are drawn drawn at at the the 50% 50% probability probability level, level, H H atoms atoms attached attached to to C C atoms atoms omitted omitted for for clarity. clarity. SelectedSelected bond bond lengths lengths (pm) (pm) and anglesand angles (deg.): (deg.): Na–N1 Na–N1 233.1(1), 233.1(1), Na–N3 Na–N3 233.8(2), 233.8(2), Na–C Na–C 263.6(2)–278.5(2), 263.6(2)– Selected278.5(2), bond Na–centroid(C lengths (pm)5-ring) and 244.3(1), angles N1–Na–N3 (deg.): Na–N1 91.03(5), 233.1(1), N1–C6 129.0(2), Na–N3 N2–C6 233.8(2), 136.8(2), Na–C N3–C13 263.6(2)– Na–centroid(C5-ring) 244.3(1), N1–Na–N3 91.03(5), N1–C6 129.0(2), N2–C6 136.8(2), N3–C13 128.8(2), 278.5(2),128.8(2), Na–centroid(C N4–C13 137.6(2),5-ring) N1–C6–N2 244.3(1), N1–Na–N3123.9(1), N3–C13–N4 91.03(5), 124.4(2). N1–C6 Symmetry129.0(2), N2–C6 code: ‘− 136.8(2),x, −y, 2 − z.N3–C13 N4–C13 137.6(2), N1–C6–N2 123.9(1), N3–C13–N4 124.4(2). Symmetry code: ‘−x, −y, 2 − z. 128.8(2), N4–C13 137.6(2), N1–C6–N2 123.9(1), N3–C13–N4 124.4(2). Symmetry code: ‘−x, −y, 2 − z. In the crystal, the dimeric molecule of 1 possesses inversion symmetry with the center of InIninversion thethe crystal,crystal, right the betweenthe dimeric dimeric the moleculetwo molecule Na atoms. of 1 ofpossesses According 1 possesses inversion to the inversion 1H symmetryand 13C symmetry NMR with data the ofwith center 1, the the dimeric of center inversion of 1 13 inversionrightstructure between right found thebetween two in the Na the crystal atoms. two structureNa According atoms. is partly According to the retainedH andto inthe solution. C1H NMR and Both 13 dataC NMRthe of cyclopentadienyl1 ,data the dimericof 1, the structure anddimeric 1 structurefoundisopropyl in thefound crystal resonance in the structure crystal could isstructurebe partlyassigned. retained is However,partly in retained solution. the H inNMR Bothsolution. signals the cyclopentadienyl Both are sign theificantly cyclopentadienyl broadened, and isopropyl and and in both NMR spectra a doubling of all1 resonances is observed. This can be interpreted by the isopropylresonance resonance could be assigned. could be However,assigned. theHowever,H NMR the signals 1H NMR are signals significantly are sign broadened,ificantly broadened, and in both presence of a slow equilibrium between the dimer and a (presumably THF-d8-solvated) monomeric andNMR in spectraboth NMR a doubling spectra ofa alldoubling resonances of all is resonanc observed.es Thisis observed. can be interpreted This can be by interpreted the presence by ofthe a structure in THF-d8 solution. Copies of the spectra can be found in the Supplementary Material. slow equilibrium between the dimer and a (presumably THF-d -solvated) monomeric structure in presence ofWith a slow its two equilibrium amidinate substituents between the in 1,2-positidimer andon ona (presumably the cyclopentadienyl8 THF-d ring,8-solvated) the new monomeric ligand structureTHF-ind8 1solution. belongsin THF- tod Copies 8the solution. group of the of Copies cyclopentadienyl spectra of canthe spectra be found derivatives can in be the calledfound Supplementary 6-aminofulvene-2-aldiminate in the Supplementary Material. Material. (AFA) WithWithligands. its two The amidinate first neutral substituents 6-aminofulvene-2-aldimine in 1,2-positi 1,2-positionon (AFAH)on the cyclopentadienyl cyclopentadienyl compound was first ring, prepared the the new new byligand ligand inin 11 belongsHafner etto al.the the in group group 1963 [23,24].of cyclopentadienyl 6-Aminofulvene-2-aldiminate derivatives derivatives called called ligands 6-aminofulvene-2-aldiminate 6-aminofulvene-2-aldiminate are ambidentate monoanionic (AFA) (AFA) ligands.ligands.ligands TheThe comprising firstfirst neutral neutral both 6-aminofulvene-2-aldimine 6-aminofulvene-2-aldiminecyclopentadienyl and diimine (AFAH) donors,(AFAH) compound as showncompound in was Scheme firstwas prepared 5.first prepared by Hafner by Hafneret al. in et 1963 al. in [23 1963,24]. [23,24]. 6-Aminofulvene-2-aldiminate 6-Aminofulvene-2-aldiminate ligands ligands are ambidentate are ambidentate monoanionic monoanionic ligands ligandscomprising comprising both cyclopentadienyl both cyclopentadienyl and diimine and diimine donors, donors, as shown as inshown Scheme in 5Scheme. 5.

Scheme 5. Resonance forms of 6-aminofulvene-2-aldiminate (AFA) ligands.

As a consequence of this resonance, a 6-aminofulvene-2-aldiminate ligand can coordinate. to a metal atom in a chelating mode through the two imine nitrogen donors as a fulvenealdiminate (A) or SchemeScheme 5. 5. ResonanceResonance forms forms of of 6-aminofulvene-2-aldiminate 6-aminofulvene-2-aldiminate (AFA) (AFA) ligands. in an η5-mode as a cyclopentadienyldiimine. Complexes of this ligand system have been reported for Assome a consequence main group ofmetals, this resonance,as well as afirst 6-aminofulvene-2-aldiminate and second row transition ligandmetals can[25–32]. coordinate Closer to a As a consequence of this resonance, a 6-aminofulvene-2-aldiminate ligand can coordinate to metalinvestigation atom in a chelating of the coordination mode through chemistry the two of 6-aminofulvene-2-aldiminateimine nitrogen donors as a ligandsfulvenealdiminate began in 1998 (A ) or a metalwith atom the indiscovery a chelating of modea straightforward through the synt twohesis imine of nitrogenthe C-phenyl-substituted donors as a fulvenealdiminate precursor in an η5-mode as a cyclopentadienyldiimine. Complexes of this ligand system have been reported for (A) orC5 inH3[1,2-C(Ph)NH] an η5-mode as2H abased cyclopentadienyldiimine. on the reaction of magnesocene Complexes with of this benzonitrile. ligand system The ligand have been some main group metals, as well as first and second row transition metals [25–32]. Closer reportedC5H3[1,2-C(Ph)NH] for some main2H groupwas successfully metals, asemployed well as in ﬁrst a number and second of complexes row transition with Mg [25], metals Al, Ga [25 –32]. investigation of the coordination chemistry of 6-aminofulvene-2-aldiminate ligands began in 1998 Closer[26] investigation and Zr [25]. of In the all coordination these compounds, chemistry the 6-aminofulvene-2-aldiminate of 6-aminofulvene-2-aldiminate anion ligandsacts as begana with the discovery of a straightforward synthesis of the C-phenyl-substituted precursor in 1998 with the discovery of a straightforward synthesis of the C-phenyl-substituted precursor C5H3[1,2-C(Ph)NH]2H based on the reaction of magnesocene with benzonitrile. The ligand C5H3[1,2-C(Ph)NH]2H based on the reaction of magnesocene with benzonitrile. The ligand C5H3[1,2-C(Ph)NH]2H was successfully employed in a number of complexes with Mg [25], Al, Ga C H [1,2-C(Ph)NH] H was successfully employed in a number of complexes with Mg [25], Al, Ga [26] [26]5 3and Zr [25]. 2In all these compounds, the 6-aminofulvene-2-aldiminate anion acts as a

Inorganics 2018, 6, x FOR PEER REVIEW 5 of 9 Inorganics 2018, 6, 47 5 of 9 N,N′-chelating ligand. The same coordination mode was established for some 6-aminofulvene-2-aldiminate complexes of palladium [27,28], copper [29,30] and zinc [27], while the 0 andη5-coordination Zr [25]. In all mode these as compounds, a cyclopentadienyldiimin the 6-aminofulvene-2-aldiminatee has been found anionin some acts AFA as a N complexes,N -chelating of ligand.ruthenium The [27,31]. same Notably, coordination two modemagnesium was established complexes for of somethe AFA 6-aminofulvene-2-aldiminate ligand system bearing 5 complexescyclohexyl ofgroups palladium at the [nitrogen27,28], copper atoms [ 29have,30] be anden zincreported, [27], whilein which the bothη -coordination coordination mode modes as ahave cyclopentadienyldiimine been realized [32]. Deprotonation has been found of N, inN′-dicyclohexyl-6-aminofulvene-2-aldimine some AFA complexes of ruthenium [27,31 (HCy]. Notably,2AFA) twowith magnesiumMeLi followed complexes by ofreaction the AFA with ligand MeMgBr system afforded bearing cyclohexylthe N,N′-coordinated groups at the nitrogencomplex atoms[[1,2-(CyN=CH) have been2C reported,5H3-N,N′]MgMe(THF)] in which both coordinationwith an uncoordinated modes have cyclopentadienyl been realized [32 ring.]. Deprotonation In contrast, 0 oftheN direct,N -dicyclohexyl-6-aminofulvene-2-aldimine reaction of HCy2AFA with MeMgBr resulted (HCy in2AFA) liberation with of MeLi methane followed and byformation reaction of with the 0 0 MeMgBrbinuclear afforded the N,N -coordinatedbromide- complexbridged [[1,2-(CyN=CH) 2C5H3-N,N ]MgMe(THF)]complex with anMg2 uncoordinated(µ-Br)[µ-η5:κN,κ cyclopentadienylN′-1,2-(CyN=CH)2C ring.5H3][1,2-(CyN=CH) In contrast,2 theC5H direct3-κN,κN reaction′] in which of HCyone 2ofAFA thewith Mg MeMgBrcenters is resultedligated inboth liberation to the η of5-cyclopentadienyl methane and formation ring of of one the of binuclear the ligands bromide-bridged and to both N complex donor 5 0 0 Mgatoms2(µ -Br)[of theµ- ηother.:κN, κThus,N -1,2-(CyN=CH) this magnesium2C5 Hderivative3][1,2-(CyN=CH) is the closest2C5H 3structural-κN,κN ] inrelative which of one our of sodium the Mg centerscomplex is 1 ligated, although both the to theligandη5-cyclopentadienyl system in 1 is rather ring unusual, of one of as the it ligandsbears additional and to both secondary N donor amino atoms offunctionalities the other. Thus, attached this magnesium to the imine derivative carbon is atoms. the closest It should structural be relativenoted that of our the sodium 1:1 addition complex of1, althoughN-heterocyclic the ligand carbenes system (NHCs) in 1 is to rather carbodiimides unusual, as has it bears been additional reported. secondary These reactions amino functionalitiesproceed in an attachedanalogous to manner the imine as carbon outlined atoms. in Scheme It should 3a beand noted afford that zwitterionic the 1:1 addition neutral of N amidinate-heterocyclic derivatives. carbenes (NHCs)The resulting to carbodiimides imidazolium-2-amidinates has been reported. These have reactions also been proceed shown in an analogousto be promising manner as ligands outlined in Schemecoordination3a and ch affordemistry zwitterionic [33–35]. neutral amidinate derivatives. The resulting imidazolium-2-amidinates haveWhen also been repeating shown the to be synthesis promising of ligands1, occasionally in coordination small amounts chemistry of [bl33ock-like–35]. crystals of a second productWhen first repeating came out theupon synthesis cooling of of the1, n occasionally-pentane extract. small These amounts were characterized of block-like through crystals their of a secondIR spectrum product as well first as came single-crystal out upon X-ray cooling diffraction. of the n-pentane The IR spectrum extract. Theseshowed were a characteristic characterized ν throughC=O band their at 1674 IR spectrumcm−1, indicating as well the as formation single-crystal of a hydrolysis X-ray diffraction. product. The This IR was spectrum confirmed showed by the a − characteristicX-ray crystal νstructureC=O band determination, at 1674 cm 1, indicatingwhich revealed the formation the presence of a hydrolysis of the dinuclear product. Thisproduct was confirmed{µ-(iPrNH) by2C=O} the2[NaC X-ray5H crystal3{C(NH structureiPr)(=Ni determination,Pr)}2-1,2]2 (2), as which shown revealed in Scheme the presence6. The complex of the dinuclear contains i i i producttwo formula {µ-( PrNH) moieties2C=O} of2[NaC N,N5′-diisopropylurea,H3{C(NH Pr)(=N Pr)}which2-1,2] is2 (2the), as common shown in hydrolysis Scheme6. The product complex of 0 containsN,N′-diisopropylcarbodiimide. two formula moieties ofEvenN,N -diisopropylurea,though the crystals which of is2 thedid common not allow hydrolysis for full product structure of 0 Nrefinement,,N -diisopropylcarbodiimide. the interatomic Evenconnectivity though the crystalswas clearly of 2 did notaccessible. allow for fullJust structure as in refinement, 1, the i i − the[C5H interatomic3{C(NHiPr)(=N connectivityiPr)}2-1,2]− was ligand clearly is attached accessible. to the Just Na as inatom1, the in [Ca 5NH,N3{C(NH′-chelatingPr)(=N mode,Pr)} but2-1,2] the 0 ligandcyclopentadiene is attached moiety to the does Na atom not contribute in a N,N -chelating to metal coordination. mode, but the Consequently, cyclopentadiene in 2 moiety, the ligand does notcan contributebe regarded to exclusively metal coordination. as an aminofulvene-aldiminate Consequently, in 2, the anion, ligand as illustrated can be regarded in the middle exclusively and asthe anright aminofulvene-aldiminate formula in Scheme 4. Coordinative anion, as illustrated saturation in of the sodium middle is andachieved the rightby N, formulaO-bridging in Schemecoordination4. Coordinative of two symmetry-equivalent saturation of sodium (iPrNH) is2 achievedC=O moieties, by N resulting,O-bridging in a distorted coordination tetrahedral of two i symmetry-equivalentenvironment of the metal ( PrNH) atom.2C=O The moieties,bridging coordination resulting in a mode distorted of the tetrahedral N,N′-diisopropylurea environment ligand of the 0 metalleads to atom. a dimeric The bridging structure coordination with a centrosymmetric mode of the NNa,N2O-diisopropylurea2C2N2 eight-membered ligand ring leads as to the a dimeric central structuremotif. with a centrosymmetric Na2O2C2N2 eight-membered ring as the central motif.

Scheme 6. Proposed formation of thei hydrolysis producti Scheme 6. Proposed formation of the hydrolysis product {µ-( PrNH)2C=O}2[NaC5H3{C(NH Pr) {µ-(iiPrNH)2C=O}2[NaC5H3{C(NHiPr)(=NiPr)}2-1,2]2 (2). (=N Pr)}2-1,2]2 (2).

Inorganics 2018, 6, 47 6 of 9

3. Conclusions The reaction of NaCp with N,N0-diisopropylcarbodiimide did not result in formation of the expected sodium amidinate with a cyclopentadienyl ring attached to the central carbon atom i i − of the amidinate backbone. Instead, the newly formed ligand [C5H3{C(NH Pr)(=N Pr)}2-1,2] belongs to the group of ambidentate 6-aminofulvene-2-aldiminate ligands, which can coordinate to metal atoms either as N,N0-fulvenealdiminates or in an η5-coordination mode as a i i − cyclopentadienyldiimine. However, the anion [C5H3{C(NH Pr)(=N Pr)}2-1,2] differs from all previously reported 6-aminofulvene-2-aldiminates in that the two substituents in 1,2-positions comprise additional secondary amino functionalities, providing another potential donor moiety.

4. Experimental Section

4.1. General Procedures All reactions were carried out in oven-dried or ﬂame-dried glassware under an inert atmosphere of dry argon, employing standard Schlenk and glovebox techniques. The solvent THF was distilled from sodium/benzophenone under nitrogen atmosphere prior to use. Sodium cyclopentadienide (NaCp) was prepared according to the published method [36]. N,N0-Diisopropylcarbodiimide was purchased from Sigma-Aldrich (Steinheim, Germany) and used as received. 1H NMR (400 MHz) and 13 C NMR (100.6 MHz) spectra were recorded in THF-d8 or toluene-d8 solution on a Bruker DPX 400 spectrometer (Bruker BioSpin, Rheinstetten, Germany) at 25 ◦C. IR spectra were measured with a Bruker Vertex 70V spectrometer (Bruker Optics, Rheinstetten, Germany) equipped with a diamond ATR unit between 4000 cm−1 and 400 cm−1. Microanalyses (C, H, N) were performed using a LECO CHNS 932 apparatus (LECO Corporation, Saint Joseph, MI, USA).

i i 4.2. Synthesis of [NaC5H3{C(NH Pr)(=N Pr)}2-1,2]2 (1) In a 250 mL Schlenk flask, sodium cyclopentadienide (3.02 g, 34.4 mmol) was dissolved in THF (40 mL) and N,N0-diisopropylcarbodiimide (4.37 g, 34.4 mmol) was added to the light-brown solution at r.t. while stirring. After stirring at r.t. for 48 h, the THF was removed in vacuo and the solid residue extracted with n-pentane (40 mL). A large amount of amorphous brown solid was separated by filtration and the clear yellow filtrate was concentrated in vacuo to ca. 20 mL. Cooling to 4 ◦C for a few days afforded colorless, needle-like crystals which were suitable for X-ray diffraction. Yield: 1.05 g −1 (9%). Elemental analysis calcd. for C38H66N8Na2 (680.98 g·mol ): C, 67.02%; H, 9.77%; N, 16.45%; found C, 66.25%; H, 9.87%; N, 16.11%.

1 ◦ 3 H NMR (400.1 MHz, THF-d8, 23 C): δ 6.04 (s br, 1H, CH–Cp), 5.96 (t, 1H, J = 3 Hz, CH–Cp), 5.82 (s br, 1H, CH–Cp), 5.69–5.70 (s br, 2H, CH–Cp), 5.59 (t, 1H, CH–Cp), 4.36 (s br, 4H, NH), 3.86–4.18 and iPr iPr 3.63–3.70 (s br, 8H, CH ), 0.98–1.11 (m, 48H, CH3 ) ppm. 13 ◦ C NMR (100.6 MHz, THF-d8, 23 C): δ 158.1 (NCN), 113.8 (C–Cp), 108.9, 107.9, 107.4, 106.6, 106.1, iPr iPr 105.6 (CH–Cp), 46.2, 45.7 (CH ), 23.7, 25.5 (CH3 ) ppm.

IR (ATR): ν 3436m (νas NH), 3386m (νs NH), 3223m (νs NH), 3084m (ν CH Cp), 3036m, 2966vs (ν CH3), 2932s (ν CH3), 2869m (ν CH), 2536w, 2224w, 1635vs (ν C=C Cp), 1590vs (ν C=C Cp), 1467s, 1448s, 1382m (δ CH3), 1365m (δ CH3), 1344m, 1310m, 1247m, 1168m, 1127m, 1098m, 1081m, 984w, 963w, 922w, 909w, 868w, 836w, 806w, 720m (δ CH Cp) cm–1.

+ i + i i + MS (EI): m/z (%) 681 (2%) [M ], 318 (100) [{( PrN)2C}2Cp] , 275 (39) [{( PrN)2C}Cp{( PrN)CN}] , 218 i + (87%) [{( PrN)2C}Cp{CN}] . Occasionally, when repeating the synthesis of 1, small amounts of block-like crystals of the hydrolysis product 2 ﬁrst came out upon cooling of the n-pentane extract. These were characterized through their IR spectrum, as well as single-crystal X-ray diffraction. Inorganics 2018, 6, 47 7 of 9

IR (ATR) of 2: ν 3436m (νas NH), 3422m, 3370m (νs NH), 3253m (νs NH), 3230m, 3080w (ν CH Cp), 3057w (ν CH Cp), 2969m (ν CH3), 2932m (ν CH3), 2870m (ν CH), 1674m (ν C=O), 1638m (ν C=C Cp), 1611m (ν C=C Cp), 1591m, 1557m, 1560m, 1489m, 1466m, 1454m, 1446m, 1384m (δ CH3), 1367m (δ CH3), 1309m, 1298m, 1246vs, 1207m, 1180m, 1165s, 1130m, 985m, 868w, 807w, 765w, 747w, 723m (δ CH Cp) cm−1.

4.3. X-ray Crystallography Single crystal X-ray intensity data of 1 and 2 were collected on a STOE IPDS 2T diffractometer (STOE, Darmstadt, Germany) [37] equipped with a 34 cm image plate detector, using graphite-monochromated Mo-Kα radiation, at T = 153(2) K. The structure was solved by direct methods (SHELXS-97) [38] and reﬁned by full matrix least-squares methods on F2 using SHELXL-2016/4 [39]. Crystallographic data for compound 1 have been deposited at the CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies of the data can be obtained free of charge on quoting the depository number 1030712 (Fax: +44-1223-336-033; E-Mail: [email protected], http://www.ccdc.cam.ac.uk).

Supplementary Materials: The following are available online at http://www.mdpi.com/2304-6740/6/2/47/s1, Crystal data and details on structure refinement of 1, 1H and 13C NMR spectra of compound 1. Author Contributions: N.H. performed the experimental work. P.L. carried out the crystal structure determination. L.H. measured the IR and NMR spectra, and S.B. measured the mass spectrum. F.T.E. conceived and supervised the experiments. F.T.E. and P.L. wrote the paper. Acknowledgments: This work was financially supported by the Otto-von-Guericke-Universität Magdeburg. Thanks are also due to Daniel König for carrying out some experiments in the course of this study. Conflicts of Interest: The authors declare no conflict of interest.

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