[3]Ferrocenophanes Containing P3M Bridges (M = Li, Na, K) §

inorganics

Article Metallated [3]Ferrocenophanes Containing P3M Bridges (M = Li, Na, K) §

Stefan Isenberg, Lisa-Marie Frenzel, Clemens Bruhn and Rudolf Pietschnig * ID

Institut für Chemie und CINSaT, Universität Kassel, Heinrich-Plett-Straße 40, 34132 Kassel, Germany; [email protected] (S.I.); [email protected] (L.-M.F.); [email protected] (C.B.) * Correspondence: [email protected]; Tel.: +49-561-804-4615 § Dedicated to Prof. Dr. Dietmar Stalke on the occasion of his 60th birthday.

Received: 19 June 2018; Accepted: 6 July 2018; Published: 11 July 2018

Abstract: Alkali-metal phosphanides can be embedded into a [3]ferrocenophane scaffold giving rise to bicyclic ferrocenophanes [MFe(C5H4PtBu)2P] (M = Li, Na, K). Coordination of the alkali-metal ions takes place via the terminal phosphorus atoms adopting a puckered P3M four-membered ring. All compounds were characterized via single-crystal X-ray diffraction and multinuclear NMR spectroscopy (1H, 31P, 7Li), whereas 13C NMR data could only be recorded for the Li derivative, owing to the limited solubility of its heavier congeners in unreactive solvents.

Keywords: alkali metal; phosphorus; ferrocenophane; phosphide; phosphanide

1. Introduction Alkali-metal phosphanides are versatile reagents for connecting phosphorus centers to various functionalities [1,2]. Apart from this role as important synthetic tools, metal phosphanides show a rich structural diversity [1,3]. While metal-rich phosphanides are well known [4], research on saturated phosphorus-rich phosphanides just started emerging [5,6]. In a similar vein, unsaturated phosphorus-rich phosphanides were employed as building blocks for remarkable supramolecular architectures at the limits of molecular chemistry [7–9]. Based on our own efforts in the chemistry of phosphorus-rich ferrocenophanes, we became interested in exploring the metallation of a P–H functionalized bridging unit in a fully phosphorus-bridged [3]ferrocenophane [10–13]. Herein, we report the results of our investigation targeting the metallation of a stereochemically predeﬁned secondary triphosphane constrained by the [3]ferrocenophane scaffold.

2. Results and Discussion Secondary triphosphane (1, Scheme1) was obtained over the course of our investigations dealing with phosphorus-rich ferrocenophanes. It is stereochemically predeﬁned in a way that only the optically inactive meso forms, cis and trans 1, are obtained, with the terminal bridgehead phosphorus atoms showing opposite absolute conﬁguration, and both t-butyl groups pointing to the same side of the molecule [10]. Reaction of a diastereomeric mixture of cis and trans 1 with n-butyl lithium in hexane in the presence of tetramethyl ethylene diamine (TMEDA) afforded the envisaged lithium phosphanide (2, Scheme1) in the form of a single diastereomer (Scheme1). In the 31P and 7Li NMR spectra, 2 was characterized by a characteristic signal splitting (Figure1). The product showed a 31 P NMR signal at −116.4 ppm (C6D6) for the central phosphorus atom, split into a triplet owing to 1 coupling with the two chemically equivalent terminal phosphorus atoms ( JPP = 271 Hz) resonating at 14.9 ppm. In addition to the already mentioned P–P coupling, the doublet of the latter resonance is split into a quartet of equal intensity due to coupling with 7Li. In line with this, the 7Li resonance

Inorganics 2018, 6, 67; doi:10.3390/inorganics6030067 www.mdpi.com/journal/inorganics Inorganics 2018,, 6,, 67x FOR PEER REVIEW 2 of 9 Inorganics 2018, 6, x FOR PEER REVIEW 2 of 9 showed a triplet with 1JLiP = 50 Hz at 1.4 ppm, indicating symmetric bridging of the two terminal 11 showedphosphorus a triplettriplet atoms withwith via theJLiP lithium== 50 50 Hz Hz atom. at at 1.4 1.4 ppm, indicating symmetric bridging of the two terminal phosphorus atoms viavia thethe lithiumlithium atom.atom.

Scheme 1. Formation of 2 via lithiation of diastereomeric 1. Scheme 1. Formation of 2 via lithiation of diastereomeric 1..

31 7 Figure 1. Expansion from the 31PP (( ((leftleft)) and and ( (mid),), 202 202 MHz) MHz) and 7LiLi (( ((rightright),), 194 194 MHz) MHz) NMR NMR spectra 31 7 ofFigure 2 (Scheme 1. Expansion1 1)) inin benzene-dbenzene-d from the6 solutionsolutionP ((left) at and room (mid temperature. ), 202 MHz) and Li ((right), 194 MHz) NMR spectra of 2 (Scheme 1) in benzene-d6 solution at room temperature. The structural motif of a P3Li four-membered ring was further corroborated using X-ray The structural motif of a P3Li four-membered ring was further corroborated using X-ray diffraction diffractionThe structural on single-crystalline motif of a P23. LiIt four-memberedcrystallized in a ringmonoclinic was fu rtherspace corroboratedgroup (C2/c ) usingwith halfX-ray a on single-crystalline 2. It crystallized in a monoclinic space group (C2/c) with half a molecule of hexane moleculediffraction of on hexane single-crystalline in the asymmetric 2. It crystallized unit. The Pin3Li a unitmonoclinic formed spacea diamond-like group (C2/ puckeredc) with half ring, a in the asymmetric unit. The P3Li unit formed a diamond-like puckered ring, bridging the Cp-rings in bridgingmolecule theof hexane Cp-rings in thein thisasymmetric bicyclic unit.[3]ferrocenophane The P3Li unit (Figureformed a2). diamond-like The lithium puckeredatom showed ring, this bicyclic [3]ferrocenophane (Figure2). The lithium atom showed symmetric contacts to the adjacent symmetricbridging the contacts Cp-rings to the in adjacentthis bicyclic phosphorus [3]ferrocenophane atoms (2.662(3) (Figure Å and 2). 2.548(3) The lithium Å), and atom a distance showed of phosphorus atoms (2.662(3) Å and 2.548(3) Å), and a distance of 3.455(3) Å to the formal phosphanide 3.455(3)symmetric Å to contacts the formal to the phosphanide adjacent phosphorus center. The atoms P–Li distances(2.662(3) Åin and 2 were 2.548(3) close Å), to thoseand a observed distance inof center. The P–Li distances in 2 were close to those observed in anionic 1,3-diphospha-2-silaallylic anionic3.455(3) 1,3-diphospha-2-silaallylicÅ to the formal phosphanide systems center. with The lithium P–Li distances as a counter in 2 wereion [14]. close The to P–Pthose bond observed lengths in systems with lithium as a counter ion [14]. The P–P bond lengths in 2, between the central phosphorus inanionic 2, between 1,3-diphospha-2-silaallylic the central phosphorus systems and the with adjacent lithium terminal as a counter phosphorus ion [14]. atoms, The P–P were bond 2.1710(8) lengths Å and the adjacent terminal phosphorus atoms, were 2.1710(8) Å and 2.1670(8) Å. This triphospha andin 2, 2.1670(8)between theÅ. centralThis triphospha phosphorus metallacycle and the adjacent folded terminal with an phosphorus angle of 135.89(6)° atoms, were (Figure 2.1710(8) 2). The Å metallacycle folded with an angle of 135.89(6)◦ (Figure2). The coordination sphere of lithium was coordinationand 2.1670(8) sphere Å. This of triphospha lithium was metallacycle completed foldedby one withmolecule an angle of TMEDA. of 135.89(6)° The respective(Figure 2). Li–N The completed by one molecule of TMEDA. The respective Li–N distances were placed with 2.113(4) Å and distancescoordination were sphere placed of with lithium 2.113(4) was Å completed and 2.080(4) by Å one in themolecule expected of rangeTMEDA. known The from respective comparable Li–N 2.080(4) Å in the expected range known from comparable compounds [15,16]. Owing to steric effects, compoundsdistances were [15,16]. placed Owing with to2.113(4) steric Åeffects, and 2.080(4) the coordination Å in the expected geometry range around known lithium from differed comparable from the coordination geometry around lithium differed from an ideal tetrahedron with angles of 119.8(1)◦ ancompounds ideal tetrahedron [15,16]. Owingwith angles to steric of 119.8(1)° effects, the to 135coordination.6(2)°, showing geometry larger around values lithium toward differedthe ferrocene from to 135.6(2)◦, showing larger values toward the ferrocene backbone. The Cp rings of the ferrocene unit backbone.an ideal tetrahedron The Cp rings with of angles the ferrocene of 119.8(1)° unit toshowed 135.6(2)°, a nearly showing ecliptic larger conformation values toward (torsion the ferrocene angle of showed a nearly ecliptic conformation (torsion angle of about 2◦), and opened an interplanar angle of aboutbackbone. 2°), andThe openedCp rings an of interplanar the ferrocene angle unit of showed 3.3(1)° towarda nearly the ecliptic lithium conformation TMEDA adduct. (torsion angle of 3.3(1)◦ toward the lithium TMEDA adduct. about 2°), and opened an interplanar angle of 3.3(1)° toward the lithium TMEDA adduct. The structural situation is closely related to the dilithiated bisphosphanide [Fe(Cp-P(Li)tBu)2], where two phosphorus atoms are symmetrically bridged by two lithium atoms forming a P2Li2 four-membered ring in the solid state and in solution [11,15]. Similarly, the spectroscopic data are comparable to the latter bisphosphanide, as well as to other dimeric lithium phosphanides [5,17]. Phosphanide 2 was very sensitive to air and moisture, and was stable only in hydrocarbon solvents. In order to explore the accessibility of the sodium analog of 2, we reacted 1 with sodium hydride in the presence of [15]-crown-5 in tetrahydrofuran (THF) solution. The solution turned light-red over a period of 12 h, indicating the formation of sodium phosphanide (3, Scheme2). The mixture was stirred for an additional 24 h for completion. In the 31P NMR spectra, 3 was characterized by a resonance at −100.3 ppm (THF-d8) for the central phosphorus atom, which splits into a pseudo triplet with a pseudo 1 coupling constant of JPP = 277 Hz. The two chemically equivalent terminal phosphorus atoms showed a single doublet at 21.3 ppm, with a matching coupling constant to the central phosphorus atom.

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showed a triplet with 1JLiP = 50 Hz at 1.4 ppm, indicating symmetric bridging of the two terminal phosphorus atoms via the lithium atom.

Scheme 1. Formation of 2 via lithiation of diastereomeric 1.

Figure 1. Expansion from the 31P ((left) and (mid), 202 MHz) and 7Li ((right), 194 MHz) NMR spectra of 2 (Scheme 1) in benzene-d6 solution at room temperature.

The structural motif of a P3Li four-membered ring was further corroborated using X-ray diffraction on single-crystalline 2. It crystallized in a monoclinic space group (C2/c) with half a molecule of hexane in the asymmetric unit. The P3Li unit formed a diamond-like puckered ring, bridging the Cp-rings in this bicyclic [3]ferrocenophane (Figure 2). The lithium atom showed symmetric contacts to the adjacent phosphorus atoms (2.662(3) Å and 2.548(3) Å), and a distance of 3.455(3) Å to the formal phosphanide center. The P–Li distances in 2 were close to those observed in anionic 1,3-diphospha-2-silaallylic systems with lithium as a counter ion [14]. The P–P bond lengths in 2, between the central phosphorus and the adjacent terminal phosphorus atoms, were 2.1710(8) Å and 2.1670(8) Å. This triphospha metallacycle folded with an angle of 135.89(6)° (Figure 2). The coordination sphere of lithium was completed by one molecule of TMEDA. The respective Li–N distances were placed with 2.113(4) Å and 2.080(4) Å in the expected range known from comparable compounds [15,16]. Owing to steric effects, the coordination geometry around lithium differed from Inorganics 2018, 6, x FOR PEER REVIEW 3 of 9 an ideal tetrahedron with angles of 119.8(1)° to 135.6(2)°, showing larger values toward the ferrocene backbone. The Cp rings of the ferrocene unit showed a nearly ecliptic conformation (torsion angle of InorganicsFigure2018 2. ,ORTEP6, 67 plot of the molecular structure of 2 (left), and expansion of the folded P3Li ring3 of 9 about 2°), and opened an interplanar angle of 3.3(1)° toward the lithium TMEDA adduct. (right). Ellipsoids were drawn at the 30% probability level. Hydrogen atoms were omitted for clarity.

The structural situation is closely related to the dilithiated bisphosphanide [Fe(Cp-P(Li)tBu)2], where two phosphorus atoms are symmetrically bridged by two lithium atoms forming a P2Li2 four- membered ring in the solid state and in solution [11,15]. Similarly, the spectroscopic data are comparable to the latter bisphosphanide, as well as to other dimeric lithium phosphanides [5,17]. Phosphanide 2 was very sensitive to air and moisture, and was stable only in hydrocarbon solvents. In order to explore the accessibility of the sodium analog of 2, we reacted 1 with sodium hydride in the presence of [15]-crown-5 in tetrahydrofuran (THF) solution. The solution turned light-red over a period of 12 h, indicating the formation of sodium phosphanide (3, Scheme 2). The mixture was stirred for an additional 24 h for completion. In the 31P NMR spectra, 3 was characterized by a resonance at −100.3 ppm (THF-d8) for the central phosphorus atom, which splits into a pseudo triplet with a pseudo coupling constant of 1JPP = 277 Hz. The two chemically equivalent terminal phosphorus atoms Figureshowed 2. ORTEPa single plot doublet of the molecular at 21.3 structureppm, with of 2 a(left matching), and expansion coupling of theconstant folded PtoLi the ring central 3 phosphorus(right). atom. Ellipsoids were drawn at the 30% probability level. Hydrogen atoms were omitted for clarity.

Fe Fe

SchemeScheme 2. 2. FormationFormation of of 3 via deprotonation of of1 .1.

Again,Again, single single crystals crystals suitable suitable for for X-ray X-ray diffraction diffraction couldcould bebe obtained,obtained, conﬁrming confirming the the presence presence of the P3Na motif also in this heavier alkali phosphanide (Figure 3). Compound 3 crystallized in a of the P3Na motif also in this heavier alkali phosphanide (Figure3). Compound 3 crystallized in ◦ monoclinica monoclinic space space group group (P21 (P2/c),1 /c),with with a somewhat a somewhat smaller smaller interplanar interplanar angle angle (3.1(1)°) (3.1(1) compared) compared to 2, ◦ andto a2 ,bigger and a torsion bigger torsionangle of angle the ferroce of thene ferrocene backbone backbone (about (about3°). Similar 3 ). Similarto 2, the to P23Na, the unit P3Na formed unit a diamond-likeformed a diamond-like puckered puckeredring which ring was which more was folded more folded(129.63(3)°) (129.63(3) than◦) in than its inlithium its lithium congener. congener. The sodiumThe sodium atom showed atom showed symmetric symmetric contacts contacts to the toadjacent the adjacent phosphorus phosphorus atoms atoms(3.037(2) (3.037(2) Å and Å 3.107(2) and Å),3.107(2) and a distance Å), and of a distance3.868(1) ofÅ to 3.868(1) the formal Å to thephosphanide formal phosphanide center. The center. P–P bond The lengths P–P bond between lengths the latterbetween and the the adjacent latter and phosphorus the adjacent phosphorusatoms were atoms2.184(2) were Å and 2.184(2) 2.174(2) Å and Å. 2.174(2) The coordination Å. The coordination sphere of sodiumsphere was of sodium completed was by completed one molecule by one of molecule [15]-cro ofwn-5. [15]-crown-5. The respective The respective Na–O distances Na–O distances lay in the rangelay inof the2.453(2) range Å of to 2.453(2)2.607(2) Å Å, to with 2.607(2) an average Å, with di anstance average of 2.498(2) distance Å, of which 2.498(2) matched Å, which values matched known in valuesliterature known [18–23]. in literature In contrast [18– to23 ].the In number contrast of to published the number potass of publishedium triphosphanides, potassium triphosphanides, characterized 31 sodiumcharacterized triphosphanides sodium triphosphanides are scarce. Nevertheless, are scarce. Nevertheless, the available the 31P available chemical shiftsP chemical in the shifts literature in the fit literature ﬁt our results quite well [24]. Furthermore, Grützmacher et al. described a P phosphanediide our results quite well [24]. Furthermore, Grützmacher et al. described a P3 phosphanediide3 ion with similarion with bond similar distances bond between distances the between sodium the cation sodium and cation the andphosphanide the phosphanide centers, centers, as well as as well similar as similar described phosphorus–phosphorus distances [25]. Even the Na–P distances in triphosphaallylic described phosphorus–phosphorus distances [25]. Even the Na–P distances in triphosphaallylic sodium were close to those found in 3, despite the quite different bonding situation of the π-conjugated sodium were close to those found in 3, despite the quite different bonding situation of the π- species [26]. conjugated species [26]. To amend this series of closely related alkali-metal triphosphanides, we set out to prepare the To amend this series of closely related alkali-metal triphosphanides, we set out to prepare the corresponding potassium compound, 4 (Scheme3). While the deprotonation of 1 with potassium correspondinghydride was precludedpotassium bycompound, the intrinsically 4 (Scheme low solubility 3). While of the this deprotonation reagent, we were of 1 able with to potassium achieve hydridedeprotonation was precluded using potassium by the intrinsicallyt-butoxide (KO lowtBu). solubility Owing toof thethis limited reagent, solubility we were of KOabletBu to in achieve neat deprotonationhydrocarbons, using the reactionpotassium had t-butoxide to be carried (KO outtBu). in Owing coordinating to the solventslimited solubility such as THF of KO (SchemetBu in3 neat). hydrocarbons,The formation the of 4reactionwas indicated had to by be an carried instantaneous out in colorcoordinating change showing solvents a deep-redsuch as THF solution. (Scheme In the 3). The31 Pformation NMR spectra, of 4 was4 was indicated characterized by an byinstantaneous a resonance color of the change central showing phosphanide a deep-red atom as solution. a triplet In the 31P NMR spectra, 4 was characterized by a resonance of the central phosphanide atom as a triplet

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at −107.3 ppm (THF-d8) with a 1JPP coupling of 268 Hz. The corresponding doublet was located at 16.8 1 atppm. −107.3 ppm (THF-d8) with a JPP coupling1 of 268 Hz. The corresponding doublet was located at 16.8 at −107.3 ppm (THF-d8) with a JPP coupling of 268 Hz. The corresponding doublet was located at ppm.16.8 ppm.

Figure 3. ORTEP plot of the molecular structure of 3 (Scheme 2; (left)) and expansion of the folded Figure 3. ORTEP plot of the molecular structure of 3 (Scheme2;( left)) and expansion of the folded FigureP3Na ring3. ORTEP (right ).plot Ellipsoids of the molecularwere drawn structure at the 30% of 3 probab (Schemeility 2; level. (left)) Hydrogen and expansion atoms of were the omittedfolded P3Na ring (right). Ellipsoids were drawn at the 30% probability level. Hydrogen atoms were omitted Pfor3Na clarity. forring clarity. (right ). Ellipsoids were drawn at the 30% probability level. Hydrogen atoms were omitted for clarity.

SchemeScheme 3. 3. FormationFormation of 4 via directdirect metallation metallation of of1. 1. Scheme 3. Formation of 4 via direct metallation of 1. MicrocrystallineMicrocrystalline and and poorly poorly soluble soluble 44 waswas recrystallized in in the the presence presence of [18]-crown-6of [18]-crown-6 affording affording single-crystallineMicrocrystallinesingle-crystalline samples samplesand poorly suitable suitable soluble for for X-ray X-ray 4 was diffraction. diffraction. recrystallized TheThe heaviest inheaviest the presence alkali alkali metal metal of of[18]-crown-6 ourof our series series showed affording showed a similar P K motif to that in its lighter analogs (Figure4). Compound 4·[18]-crown-6 crystallized in a single-crystallinea similar P3K motif3 samples to that suitable in its lighter for X-ray analogs diffraction. (Figure The 4). Compoundheaviest alkali 4·[18]-crown-6 metal of our crystallized series showed in a ◦ a similartriclinic P3K spacemotif group to that (P −in1), its and lighter showed analogs a less (Figure strained 4). (interplanar Compound angle 4·[18]-crown-6 2.1(5) ) but more crystallized staggered in a triclinic space group (P−◦1), and showed a less strained (interplanar angle 2.1(5)°) but more staggered (torsion angle about 5 ) structure. Similar to 2 and 3, the P3K unit formed a diamond-like puckered ring triclinic(torsion space angle group about (5°)P− 1),structure. and showed Similar a toless 2 andstrained 3, the (interplana P3K unit formedr angle a 2.1(5)°)diamond-like but more puckered staggered ring which was more folded (125.5(1)◦) than in the Li/Na congeners, possibly caused by the introduction of (torsion angle about 5°) structure. Similar to 2 and 3, the P3K unit formed a diamond-like puckered ring whicha sterically was more more folded demanding (125.5(1)°) crown than ether. in the The Li/Na potassium congeners, atom showed possibly slight caused asymmetric by the introduction contacts to of which was more folded (125.5(1)°) than in the Li/Na congeners, possibly caused by the introduction of a stericallythe adjacent more phosphorus demanding atoms crown (3.581(3) ether. ÅThe and pota 3.333(2)ssium Å), atom and ashowed distance slight of 4.227(3) asymmetric Å to the formalcontacts to a sterically more demanding crown ether. The potassium atom showed slight asymmetric contacts to the adjacentphosphanide phosphorus center. The atoms P–P (3.581(3) bond lengths Å and between 3.333(2) the Å), latter and and a distance the adjacent of 4.227(3) phosphorus Å to atomsthe formal thephosphanide adjacentwere 2.187(2) phosphorus center. Å and The 2.192(4) atoms P–P bond Å,(3.581(3) and lengths the Å folding andbetween 3.333(2) angle th ofe Å),latter the and puckered and a distancethe ringadjacent was of 4.227(3) smallestphosphorus Å in thisto atomsthe series. formal were phosphanide2.187(2)The coordinationÅ and center. 2.192(4) The sphere Å,P–P and of bond potassium the lengths folding was between angle completed of th the by elatter puckered one moleculeand the ring adjacent of was [18]-crown-6, smallest phosphorus addingin this atoms upseries. to were a The 2.187(2)coordinationtotal Å coordinationand sphere 2.192(4) of number Å,potassium and ofthe eight wasfolding around completed angle the potassiumof by th one puckerede molecule atom. The ring of respective[18]-crown-6, was smallest K–O adding distancesin this upseries. lay to ina Thetotal coordinationcoordinationthe range sphere number of 2.748(8) of ofpotassium Åeight to 3.039(8) arou wasnd Å, thecompleted showing potassium an by average onatom.e molecule The distance respecti of of [18]-crown-6, 2.88(1)ve K–O Å. distances This adding was inlay lineup in tothe with a rangetotal coordinationof 2.748(8)similar Å distances number to 3.039(8) of in eight the Å, literature aroushowingnd [the18 an,19 potassium average,27,28]. The dist atom. crownance The etherof respecti 2.88(1) showed veÅ. K–O disorderThis distances was at in the line lay carbon inwith the and similarrange ofdistances 2.748(8)oxygen inÅ atoms, theto 3.039(8) literature in which Å, ellipsoids[18,19,27,28].showing stretchedan averageThe crow along distn the etherance ring, showedof indicating 2.88(1) disorder Å. different This atwas positions the in carbon line of thewith and crown similaroxygen in the crystal by rotation about the potassium center. This disorder limited the R value to 0.1104. distancesatoms, in in which the literature ellipsoids [18,19,27,28]. stretched along The thecrow ring,n ether indicating showed different disorder positions at the carbon of the andcrown oxygen in the As already mentioned, phosphanides 2–4 were extremely sensitive to air and moisture, and traces atoms,crystal in by which rotation ellipsoids about the stretched potassium along center. the ring, This indicatingdisorder limited different the positionsR value to of 0.1104. the crown in the of residual water or reactive solvents available for deprotonation, such as THF, led to protonation crystal by rotation about the potassium center. This disorder limited the R value to 0.1104. ofAs2 –already4, resulting mentioned, in the formation phosphanides of 1, which precluded2–4 were theextremely isolation ofsensitive2–4 in an to analytically air and moisture, pure form. and As already mentioned, phosphanides 2–4 were extremely sensitive to air and moisture, and tracesInterestingly, of residual the water protonation or reac of isostructuraltive solvents phosphanides available for2–4 resulteddeprotonation, exclusively such in the as formationTHF, led to tracesprotonationof oftrans residual1 .of We 2 assume– 4water, resulting thator thisreac in selectivitytive the solventsformation most likelyavailable of 1 originated, which for deprotonation,precluded from the preferential the suchisolation attack as THF,of of water2– 4led in toan protonationanalytically ofpure 2–4 form., resulting Interestingly, in the formation the protonation of 1, which of isostructural precluded thephosphanides isolation of 2 –24– 4resulted in an analyticallyexclusively purein the form. formation Interestingly, of trans 1 . theWe assumeprotonation that thisof isostructural selectivity most phosphanides likely originated 2–4 resulted from the exclusivelypreferential in attackthe formation of water of fromtrans 1the. We metal assume side that for this kinetic selectivity reasons, most since likely the originated energy difference from the preferentialbetween cis attack and trans of water 1 was from only thesmall metal in favor side offor the kinetic trans reasons,isomer (0.1since kcal/mol the energy at the difference B3LYP/6- between311+G** cislevel and of transtheory) 1 was[10]. only small in favor of the trans isomer (0.1 kcal/mol at the B3LYP/6- 311+G** level of theory) [10].

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from the metal side for kinetic reasons, since the energy difference between cis and trans 1 was only Inorganics small2018, 6 in, x favor FOR ofPEER the REVIEWtrans isomer (0.1 kcal/mol at the B3LYP/6-311+G** level of theory) [10]. 5 of 9

· Figure 4.Figure ORTEP 4. ORTEP plot of plot 4·[18]-crown-6 of 4 [18]-crown-6 (Scheme (Scheme 33;;( (leftleft)))) and and expansion expansion of the of folded the folded P3K ring P3 (Kright ring). (right). Ellipsoids were drawn at the 30% probability level. Hydrogen atoms were omitted for clarity. Ellipsoids were drawn at the 30% probability level. Hydrogen atoms were omitted for clarity.

The heteronuclear NMR data did not follow a clear trend along the series of alkali-metal cations The heteronuclear NMR data did not follow a clear trend along the series of alkali-metal cations from Li+ to K+ in line with observations reported in the literature [24]. Compared with the parent + + from Li hydrophosphane, to K in line with1, the observations formal phosphanide reported centers in inth deprotonatede literature 2[24].–4 were Compared more shielded with with the parent hydrophosphane,respect to cis 11,, the but deshieldedformal phosphanide with respect to centerstrans 1. Thein deprotonated adjacent phosphorus 2–4 were atoms more were shiftedshielded with respect todownﬁeld cis 1, but upon deshielded interaction with with respect the alkali-metal to trans cations 1. The (δ (adjacent31P): cis 1 :phosphorus−74.3/−9.8 ppm;atomstrans were1: shifted 1 downfield−125.1/ upon− 6.5interaction ppm [10]; with2: −116.4/14.9 the alkali-metal ppm; 3: −cations100.3/21.3 (δ(31 ppm;P): cis4: 1−: 107.3/16.8−74.3/−9.8 ppm). ppm; The transJPP 1: −125.1/ coupling constants of 2–4 were quite similar (2: 271 Hz; 3: 277 Hz; 4: 268 Hz), with values between −6.5 ppm [10]; 2: −116.4/14.9 ppm; 3: −100.3/21.3 ppm; 4: −107.3/16.8 ppm). The 1JPP coupling constants those observed for cis and trans 1 (cis: 295/289 Hz; trans: 107 Hz), indicating reduced P–P interaction of 2–4 werecompared quite tosimilar the cis isomer(2: 2711 Hz;, in which3: 277 the Hz; lone 4: 268 pairs Hz), of all with phosphorus values atomsbetween were those located observed on the for cis and transsame 1 (cis side: 295/289 of the molecule Hz; trans [10].: In 107 turn, Hz), the additionalindicating electron reduced pair atP–P the interaction formal phosphanide compared center to the cis isomer 1in, 2in–4 whichincreased the the lone interaction pairs compared of all phosphorus with trans 1 [ 29atoms,30]. were located on the same side of the molecule [10].With In increasing turn, the size additional of the alkali electron metal, the pair diamond-shaped at the formal ring phosphanide was more folded, center possibly in owing2–4 increased to repulsive interactions between the crown ether and the ferrocene backbone. As a consequence of the interaction compared with trans 1 [29,30]. this steric effect, the torsion angle of the ferrocene unit slightly increased within the series, whereas the Withinterplanar increasing angle size of the of Cp the rings alkali decreased. metal, Similar the diamond-shaped structural effects could ring be observedwas more in triphospha folded, possibly owing to[3]ferrocenophanes repulsive interactions substituted withbetween sterically the demanding crown alkylether groups and [10 the]. The ferrocene M–P2 distances backbone. in 2–4 As a consequencewere all of slightly this steric below effect, the sum the of thetorsion Van-der-Waals angle of radii the [ferrocene31]. However, unit the slightly interaction increased between the within the series, whereasalkali metal the and interplanar P2 was most angle likely electrostaticof the Cp ri inngs nature. decreased. The distances Similar of the structural formal phosphanide effects could be 2 4 observedatom in triphospha to the iron center [3]ferrocenophanes of the ferrocene backbone substituted were almost with identical sterically for – demanding(ca. 3.9 Å), and alkyl matched groups [10]. the value for the parent phosphane, 1, to which they were isostructural [10]. The M–P2 distances in 2–4 were all slightly below the sum of the Van-der-Waals radii [31]. However, the interaction3. Experimental between Details the alkali metal and P2 was most likely electrostatic in nature. The distances of the formalAll phosphanide manipulations atom were carriedto the outiron under center inert of argonthe ferrocene atmosphere backbone using a standard were almost Schlenk identical for 2–4 technique.(ca. 3.9 Å), All and solvents matched were dried the andvalue freshly for distilledthe parent over anphosphane, Na/K-alloy 1 where, to which applicable. they were isostructuralCompound [10]. 1 was prepared according to a previously published procedure [10]. All other reagents were purchased and used without further puriﬁcation. 1H, 13C, and 31P NMR spectra were recorded on a Varian VNMRS-500 MHz spectrometer (Varian Inc., Palo Alto, CA, USA) at room temperature 3. Experimental Details using tetramethylsilane (TMS) as the external reference for 1H and 13C nuclei. 13C–1H correlation was All usedmanipulations for the detailed were chemical carried shift assignmentsout under ininert2. For argon multiplets, atmosphere chemical shifts using were a reportedstandard as Schlenk the center of the respective multiplet. technique. All solvents were dried and freshly distilled over an Na/K-alloy where applicable. Compound 1 was prepared according to a previously published procedure [10]. All other reagents were purchased and used without further purification. 1H, 13C, and 31P NMR spectra were recorded on a Varian VNMRS-500 MHz spectrometer (Varian Inc., Palo Alto, CA, USA) at room temperature using tetramethylsilane (TMS) as the external reference for 1H and 13C nuclei. 13C–1H correlation was used for the detailed chemical shift assignments in 2. For multiplets, chemical shifts were reported as the center of the respective multiplet.

Inorganics 2018, 6, 67 6 of 9

3.1. Synthesis of Compound 2 To a stirred mixture of 0.16 g (0.4 mmol) of 1 and 0.1 mL (0.7 mmol) of TMEDA in 5 mL hexane, 0.16 mL (0.4 mmol; 2.5 M) of nBuLi in hexane was added dropwise at room temperature. After 2 h, the resulting orange precipitate was ﬁltered off and washed with 1 mL of pentane. The product was obtained as an orange solid. Single crystals suitable for X-ray diffraction analysis could be obtained 1 via recrystallization from hexane. Yield: 0.18 g (89%). H NMR (500 MHz, C6D6): δ = 1.52 (m, 18H, tBu), 1.68 (br s, 4H, CH2), 2.01 (br s, 12H, N(CH3)2), 4.04 (m, 2H, Cp), 4.28 (m, 2H, Cp), 4.35 (m, 2H, 13 1 Cp), 5.15 (m, 2H, Cp) ppm; C{ H} NMR (126 MHz, C6D6): δ = 31.1 (m, tBu Cq), 31.3 (m, tBu CH3), 46.5 (s, N(CH3)2), 56.6 (s, CH2), 67.6 (m, Cp), 71.0 (s, Cp), 72.3 (m, Cp), 77.0 (m, Cp), 91.5 (m, Cp Cipso) 31 1 − 1 ppm; P NMR (202 MHz, C6D6): δ = −116.4 (t, 1P, JPP = 271 Hz, P ), 14.9 (dq, 2P, JPP = 271 Hz, 1 7 1 JPLi = 50 Hz, P tBu). Li NMR (194 MHz, C6D6): δ = 1.4 (t, JLiP = 50 Hz).

3.2. Synthesis of Compound 3 Firstly, 0.078 g (0.2 mmol) of 1, 0.005 g (0.2 mmol) of sodium hydride, and 0.022 g (0.2 mmol) of 15-crown-5 were dissolved in THF (2 mL), and stirred for two days at room temperature. A red solid precipitated from the former orange solution, which was separated by ﬁltration. The remaining solid was washed with pentane (2 × 5 mL), and gave 3 as a light-red solid. Recrystallization from THF afforded single crystals suitable for X-ray diffraction analysis. Yield: 0.10 g (78%). 1H NMR (500 MHz, THF-d8): δ = 1.17 (m, 18H, tBu), 3.59 (m, 20H, 15-crown-5), 3.76 (m, 2H, Cp), 4.07 (m, 2H, Cp), 31 4.29 (m, 2H, Cp), 4.66 (m, 2H, Cp) ppm; P NMR (202 MHz, THF-d8): δ = −100.3 (pseudo t, 1P, 1 − 1 JPP = 277 Hz, P ), 21.3 (d, 1P, JPP = 277 Hz, P tBu).

3.3. Synthesis of Compound 4 To a stirred dry mixture of 0.118 g (0.3 mmol) of 1 and 0.034 g (0.3 mmol) of KOtBu, 3 mL of THF was added at room temperature. The solution immediately turned deep red, and stirring was continued for an additional 30 min. All volatiles were removed under reduced pressure, and the residue was kept under vacuum for at least 1 h. Then, 2 mL of THF and 0.080 g (0.3 mmol) of 18-crown-6 were added to the residue, and stirred for 5 min. After removing the solvent in vacuo, the product was obtained as a red solid. Recrystallization from THF afforded crystals suitable for X-ray 1 diffraction analysis. Yield: 0.14 g (65%). H NMR (500 MHz, THF-d8): δ = 1.13 (m, 18H, tBu), 3.14–3.50 (br s, 24H, 18-crown-6), 3.80 (m, 2H, Cp), 4.09 (m, 2H, Cp), 4.13 (m, 2H, Cp), 4.64 (m, 2H, Cp) ppm; 31 1 − 1 P NMR (202 MHz, THF-d8): δ = −107.3 (t, 1P, JPP = 268 Hz, P ), 16.8 (d, 2P, JPP = 268 Hz, P tBu).

3.4. X-ray Crystallography X-ray diffraction measurements were performed on a Stoe IPDS 2 diffractometer (3) (STOE & Cie. GmbH, Darmstadt, Germany) with an image plate detector and monochromated (graded multilayer mirror) Mo Kα radiation (Mo Genix), or on a Stoe StadiVari diffractometer (2 and 4) with a Dectris Pilatus 200 K detector using monochromated (graded multilayer) Cu Kα radiation (Cu Genix) (Xenocs, Sassenage, France). The datasets were recorded with ω-scans, and corrected for Lorentz, polarization, and absorption effects. The structures were solved using direct methods and reﬁned without restraints by full-matrix least-squares techniques against F2 (SHELXT and SHELXL-2014/7) [32]. Details of the structure determinations and reﬁnement for 2–4 are summarized in Table1. Further programs used for analysis and visualization of structural information included WinGX and Mercury [33,34]. CCDC 1847287–1847289 contains the supplementary crystallographic data for this paper (See Supplementary Materials). These data are provided free of charge by the Cambridge Crystallographic Data Centre. Inorganics 2018, 6, 67 7 of 9

Table 1. Crystal data and structure reﬁnement for Compounds 2–4 (Schemes1–3).

2 3 4

Formula C24H42FeLiN2P3,0.5(C6H14) C28H46FeNaO5P3 C30H50FeKO6P3 Formular weight 557.38 634.40 694.56 Temperature (K) 100 100 100 Wavelength (Å) 1.54186 0.71073 1.54186 Crystal system monoclinic monoclinic triclinic Space group C2/c P21/c P−1 Unit-cell dimensions: a (Å) 10.2172(3) 14.2758(6) 10.791(1) b (Å) 17.3386(6) 10.3266(3) 10.7753(10) c (Å) 34.6955(9) 21.4828(9) 16.8369(16) α (◦) 90 90 73.904(7) β (◦) 90.735(2) 99.536(3) 71.663(7) γ (◦) 90 90 69.620(7) Volume (Å3) 6145.9(3) 3123.2(2) 1710.8(3) Z 8 4 2 Calculated density 1.205 1.349 1.348 (mg/m3) µ (mm−1) 5.526 0.685 6.265 Θ-range for data 5.100–68.990 1.446–25.498 2.815–68.998 collected (◦) Data/parameters 5340/318 5814/349 6198/376 Goodness-of-ﬁt on F2 1.021 1.047 1.024 R1 (observed data) 0.0309 0.0531 0.1104 wR2 (all data) 0.0733 0.1559 0.3201 Rint 0.0240 0.0654 0.0772 r.e.d. min/max (e·Å−3) 0.539/0.834 −0.944/0.776 0.451/0.887

4. Conclusions In summary, we demonstrated that secondary triphosphanides of the alkali metals, M = Li, Na, K, could be embedded into a [3]ferrocenophane scaffold, giving rise to bicyclic ferrocenophanes with a two-fold intramolecular P→M donor stabilization as part of puckered P3M rings. The folding of these rings became more pronounced as the alkali-metal cation increased in size. Compounds 3 and 4 were limited in solubility to organic polar solvents, such as THF, while 2 was soluble in nonpolar solvents as well. All alkali-metal cations required coordination of additional donors to complete their coordination spheres. Nevertheless, all compounds were highly sensitive to air and moisture in solution and in the solid state. It is worth noting that the formally hydridic P–H bond (according to a difference in electronegativity) clearly behaved in a protic nature with all reagents under investigation, even though secondary triphosphane lacked any electronegative substituents at phosphorus. Remarkably, the protonation of phosphanides 2–4 led to the stereoselective formation of trans 1, which was otherwise not accessible.

Supplementary Materials: The following are available online at http://www.mdpi.com/2304-6740/6/3/67/s1. Cif and checkcif files of complexes 2–4. Author Contributions: I.S. and L.-M.F. performed the experiments, and analyzed the spectroscopic data; C.B. performed the X-ray interpretation; R.P. provided the materials and wrote the paper. Funding: German Science fund (DFG, PI 353/8-1 and CRC 1319). Acknowledgments: The authors would like to thank the German Science fund (DFG, PI 353/8-1 and CRC 1319) for financial support. Conflicts of Interest: The authors declare no conflict of interest. Inorganics 2018, 6, 67 8 of 9

References

1. Izod, K. Group I complexes of P- and as-donor ligands. Adv. Inorg. Chem. 2000, 50, 33–107. 2. Fritz, G.; Scheer, P. Silylphosphanes: Developments in Phosphorus Chemistry. Chem. Rev. 2000, 100, 3341–3402. [CrossRef][PubMed]

3. Rosenberg, L. Metal complexes of planar PR2 ligands: Examining the carbene analogy. Coord. Chem. Rev. 2012, 256, 606–626. [CrossRef] 4. Driess, M.; Nöth, H. Molecular Clusters of the Main Group Elements; Wiley-VCH: Weinheim, Germany, 2004. 5. Wolf, R.; Schisler, A.; Lönnecke, P.; Jones, C.; Hey-Hawkins, E. Syntheses and Molecular Structures of Novel Alkali Metal Tetraorganylcyclopentaphosphanides and Tetraorganyltetraphosphane-1, 4-diides. Eur. J. Inorg. Chem. 2004, 16, 3277–3286. [CrossRef] − 6. Wolf, R.; Gomez-Ruiz, S.; Reinhold, J.; Boehlmann, W.; Hey-Hawkins, E. The (P4HMes4) anion: Lability, fluxionality, and structural ambiguity (Mes = 2,4,6-Me3C6H2). Inorg. Chem. 2006, 45, 9107–9113. [CrossRef] [PubMed] 7. Dielmann, F.; Heindl, C.; Hastreiter, F.; Peresypkina, E.V.; Virovets, A.V.; Gschwind, R.M.; Scheer, M. A nano-sized Supramolecule beyond the Fullerene Topology. Angew. Chem. Int. Ed. 2014, 53, 13605–13608. [CrossRef][PubMed] 8. Scheer, M.; Schindler, A.; Merkle, R.; Johnson, B.P.; Linseis, M.; Winter, R.; Anson, C.E.; Virovets, A.V. Fullerene C60 as an Endohedral Molecule within an Inorganic Supramolecule. J. Am. Chem. Soc. 2007, 129, 13386–13387. [CrossRef][PubMed] 9. Bai, J.; Virovets, A.V.; Scheer, M. Synthesis of Inorganic Fullerene-Like Molecules. Science 2003, 300, 781–783. [CrossRef][PubMed] 10. Borucki, S.; Kelemen, Z.; Maurer, M.; Bruhn, C.; Nyulászi, L.; Pietschnig, R. Stereochemical alignment in triphospha-[3]ferrocenophanes. Chem. Eur. J. 2017, 23, 10438–10450. [CrossRef][PubMed] 11. Kargin, D.; Kelemen, Z.; Krekić,K.; Maurer, M.; Bruhn, C.; Nyulászi, L.; Pietschnig, R. [3]Ferrocenophanes with bisphosphanotetryl bridge: Inorganic rings on the way to tetrylenes. Dalton Trans. 2016, 45, 2180–2189. [CrossRef][PubMed] 12. Moser, C.; Belaj, F.; Pietschnig, R. Phosphorus-Rich Ferrocenophanes. Phosphorus Sulfur Silicon Relat. Elem. 2015, 190, 837–844. [CrossRef] 13. Moser, C.; Belaj, F.; Pietschnig, R. Diphospha[2]ferrocenophane alias (1,4-dihydrotetraphosphaneoxide): Stereoselective formation via hydrolytic P-P bond formation. Chem. Eur. J. 2009, 15, 12589–12591. [CrossRef] [PubMed] 14. Lange, D.; Klein, E.; Bender, H.; Niecke, E.; Nieger, M.; Pietschnig, R.; Schoeller, W.W.; Ranaivonjatovo, H. 1,3-Diphospha-2-silaallylic Lithium Complexes and Anions: Synthesis, Crystal Structures, Reactivity, and Bonding Properties. Organometallics 1998, 17, 2425–2432. [CrossRef] 15. Hitzel, S.; Färber, C.; Bruhn, C.; Siemeling, U. Phosphido complexes derived from 1,1’-ferrocenediyl-bridged secondary diphosphines. Dalton Trans. 2017, 46, 6333–6348. [CrossRef][PubMed] 16. Less, R.J.; Naseri, V.; Wright, D.S. Formation of an organometallic phosphanediide via main-group dehydrocoupling. Organometallics 2009, 28, 1995–1997. [CrossRef] 17. Reich, H.J.; Dykstra, R.R. Solution Structure of Lithium Benzeneselenolate and Lithium Diphenylphosphide: NMR Identification of Cyclic Dimers and Mixed Dimers. Organometallics 1994, 13, 4578–4585. [CrossRef] 18. Poonia, N.S.; Bajaj, A.V. Coordination chemistry of alkali and alkaline earth cations. Chem. Rev. 1979, 79, 389–445. [CrossRef] 19. Bajaj, A.V.; Poonia, N.S. Comprehensive coordination chemistry of alkali and alkaline earth cations with macrocyclic multidentates: Latest position. Coord. Chem. Rev. 1988, 87, 55–213. [CrossRef] 20. Berry, A.; Green, M.L.H.; Bandy, J.A.; Prout, K. Transition Metal-Hydrogen-Alkali Metal Bonds:

Synthesis and Crystal Structures of [K(18-crown-6)][W(PMe3)3H5], [Na(15-crown-5)][W(PMe3)3H5] and [{W(PMe3)3H5Li}4] and Related Studies. J. Chem. Soc. Dalton Trans. 1991, 2185–2206. [CrossRef] 21. Gjikaj, M.; Adam, A. Complexation of Alkali Triﬂates by Crown Ethers: Synthesis and Crystal

Structure of [Na(12-crown-4)2][SO3CF3], [Na(15-crown-5)][SO3CF3], [Rb(18-crown-6)][SO3CF3], and [Cs(18-crown-6)][SO3CF3]. Z. Anorg. Allg. Chem. 2006, 632, 2475–2480. [CrossRef] 22. Nöth, H.; Warchhold, M. Sodium Hydro(isothiocyanato)borates: Synthesis and Structures. Eur. J. Inorg. Chem. 2004, 2004, 1115–1124. [CrossRef] Inorganics 2018, 6, 67 9 of 9

23. Cole, M.L.; Jones, C.; Junk, P.C. Ether and crown ether adduct complexes of sodium and potassium

cyclopentadienide and methylcyclopentadienide—Molecular structures of [Na(dme)Cp]∞, [K(dme)0.5Cp]∞, Me – [Na(15-crown-5)Cp], [Na(18-crown-6)Cp ] and the “naked Cp ” complex [K(15-crown-5)2][Cp]. J. Chem. Soc. Dalton Trans. 2002, 896–905. [CrossRef] 24. Schmidpeter, A.; Burget, G. Die Reaktion Von Alkaliphosphiden Mit Weissem Phosphor, 11 Bildung Von 1,1,3,3-Tetraphenyl-Triphosphid, 2,3,4,5-Tetraphenyl-Cyclopentaphosphid und Phenyl-Tricycloheptaphosphid. Phosphorus Sulfur Silicon Relat. Elem. 1985, 22, 323–335. [CrossRef]

25. Geier, J.; Ruegger, H.; Worle, M.; Grützmacher, H. Sodium oligophosphanediide ions in the PhPCl2/Na system: Syntheses and structural characterization. Angew. Chem. Int. Ed. 2003, 42, 3951–3954. [CrossRef] [PubMed] 26. Wiberg, N.; Wörner, A.; Lerner, H.W.; Karaghiosoff, K.; Fenske, D.; Baum, G.; Dransfeld, A.;

von Ragué Schleyer, P. The Triphosphide (tBu3Si)2P3Na: Formation, X-ray and Ab initio Structure Analyses, Protonation and Oxidation to Triphosphane (tBu3Si)2P3H and Hexaphosphanes (tBu3Si)4P6. Eur. J. Inorg. Chem. 1998, 1998, 833–841. [CrossRef] 27. Thiele, G.; Vondung, L.; Donsbach, C.; Pulz, S.; Dehnen, S. Organic Cation and Complex Cation-Stabilized (Poly-)Selenides, [Cation]x(Sey)z: Diversity in Structures and Properties. Z. Anorg. Allg. Chem. 2014, 640, 2684–2700. [CrossRef] 28. Kobrsi, I.; Zheng, W.; Knox, J.E.; Heeg, M.J.; Schlegel, H.B.; Winter, C.H. Experimental and theoretical study of the coordination of 1, 2, 4-triazolato, tetrazolato, and pentazolato ligands to the [K(18-crown-6)]+ fragment. Inorg. Chem. 2006, 45, 8700–8710. [CrossRef][PubMed] 29. Hahn, J. Higher order 31P NMR Spectra of Polyphosphorus Compounds. In Phosphorus-31 NMR Spectroscopy in Stereochemical Analysis; Verkade, J.G., Quin, L.D., Eds.; VCH: Weinheim, Germany, 1987; pp. 331–364. 1 30. Albrand, J.P.; Faucher, H.; Gagnaire, D.; Robert, J.B. Calculation of the J(PP) angular dependence in P2H4. Chem. Phys. Lett. 1976, 38, 521–523. [CrossRef] 31. Bondi, A. van der Waals volumes and radii. J. Phys. Chem. 1964, 68, 441–451. [CrossRef] 32. Sheldrick, G.M. A short history of SHELX. Acta Crystallogr. A 2008, 64, 112–122. [CrossRef][PubMed] 33. Farrugia, L.J. WinGX suite for small-molecule single-crystal crystallography. J. Appl. Crystallogr. 1999, 32, 837–838. [CrossRef] 34. Macrae, C.F.; Edgington, P.R.; McCabe, P.; Pidcock, E.; Shields, G.P.; Taylor, R.; Towler, M.; van der Streek, J. Mercury: Visualization and analysis of crystal structures. J. Appl. Crystallogr. 2006, 39, 453–457. [CrossRef]

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