Synthesis and Characterization of a Sulfonyl- and Iminophosphoryl-functionalized Methanide and Methandiide

The synthesis of [H 2 C(PPh 2 =NSiMe 3)(SO 2 Ph)] (1) and its mono-and dimetalation are reported. Due to the strong anion-stabilizing abilities of the iminophosphoryl and the sulfonyl group monometalation to 1-K and dimetalation to 1-Li 2 proceed smoothly with potassium hydride and methyllithium, respectively. Both compounds could be isolated in high yields and were characterized by NMR spectroscopy as well as XRD analysis. The methanide 1-K forms a coordination polymer in the solid state, while in case of the methandiide a tetrameric structure is observed. The latter features an unusual structural motif consisting of two (SO 2 Li) 2 eight-membered rings, which are connected with each other via the methandiide carbon atoms and additional lithium atoms. With increasing metalation a contraction of the P–C–S linkage is observed, which is well in line with the increased charge at the central carbon atom and involved electrostatic interactions. 1. Background In the past 20 years, methandiides with a doubly metalated carbon atom (R 2 CM 2 with M mostly being Li) have attracted intense research interest in organometallic chemistry. This is mainly due to their applicability as ligands for the preparation of carbene-type complexes by simple salt metathesis reactions [1–3]. Thereby, methandiides revealed to be highly efficient ligand systems stabilizing a variety of carbene complexes with main group metals [4–7], early and late transition metals [8–12] as well as lanthanides and actinides [13–16]. The first dilithium compound, which was employed in this chemistry, was the bis(iminophosphorano) system {Li 2 (bipm TMS)} 2 (A, bipm TMS =C(PPh 2 NSiMe 3) 2), which was simultaneously reported by the groups of Cavell and Stephan in 1999 (Figure 1) [17,18]. Unlike all other methandiides reported before [19–21], A was found to be conveniently accessible by double deprotonation and isolable in high yields, thus allowing its application in carbene complex synthesis [22]. The high stability and facile synthesis of A can be explained by the strong anion-stabilizing ability of the P(V) moieties as well as the additional nitrogen donor side-arms, which efficiently coordinate lithium to form stable complexes. Analogously, the corresponding thiophosphoryl system B reported by Le Floch also proved to be a stable and powerful ligand system [23–25]. More recently, our group has focused on non-DPPM derived methandiides, to expand the carbene chemistry of these compounds also to ligands with other substituents. The dilithium compound C with a sulfonyl functionality turned out to be …

In the past 20 years, methandiides with a doubly metalated carbon atom (R 2 CM 2 with M mostly being Li) have attracted intense research interest in organometallic chemistry.This is mainly due to their applicability as ligands for the preparation of carbene-type complexes by simple salt metathesis reactions [1][2][3].Thereby, methandiides revealed to be highly efficient ligand systems stabilizing a variety of carbene complexes with main group metals [4][5][6][7], early and late transition metals [8][9][10][11][12] as well as lanthanides and actinides [13][14][15][16].The first dilithium compound, which was employed in this chemistry, was the bis(iminophosphorano) system {Li 2 (bipm TMS )} 2 (A, bipm TMS =C(PPh 2 NSiMe 3 ) 2 ), which was simultaneously reported by the groups of Cavell and Stephan in 1999 (Figure 1) [17,18].Unlike all other methandiides reported before [19][20][21], A was found to be conveniently accessible by double deprotonation and isolable in high yields, thus allowing its application in carbene complex synthesis [22].The high stability and facile synthesis of A can be explained by the strong anion-stabilizing ability of the P(V) moieties as well as the additional nitrogen donor side-arms, which efficiently coordinate lithium to form stable complexes.Analogously, the corresponding thiophosphoryl system B reported by Le Floch also proved to be a stable and powerful ligand system [23][24][25].More recently, our group has focused on non-DPPM derived methandiides, to expand the carbene chemistry of these compounds also to ligands with other substituents.The dilithium compound C with a sulfonyl functionality turned out to be easily accessible and similarly stable than the bis(phosphonium)-substituted systems [26].The weaker coordination ability of the sulfonyl group also gave way to the formation of transition-metal complexes with open coordination-sides [27][28][29].
group also gave way to the formation of transition-metal complexes with open coordination-sides [27][28][29].Besides their applicability in carbene complex chemistry, methandiides are also interesting reagents because of their structural properties.Analogous to simple organolithium reagents [30][31][32], structure formation is dominated by the presence or absence of additional donor functions within the molecule.Non-functionalized methandiides typically form polymeric structures to complete the coordination sphere of lithium, such as in the case of parent dilithiomethane, H2CLi2 [33], or dilithiated fluorene D (Figure 2) [20].In contrast, donor side-arms lead to the formation of defined cluster structures [19].For instance, the DPPM-based systems A and B and derivatives thereof typically form dimeric structures with a central structural motif consisting of a Li4 and Li2C2 fourmembered ring perpendicular to each other [17,18,[34][35][36][37][38].This motif is also present in the structures of the heavier alkali metal derivatives [39,40].An interesting monomeric structure was isolated by the introduction of the sterically demanding Dipp substitutents (Dipp = 2,6-Di(iso-propyl)phenyl) into the bipm framework together with TMEDA as additional Lewis base [41].Most interestingly, this compound showed in contrast to the van't Hoff rule a planar rather than a tetrahedral geometry at the metalated carbon atom.Such a planar carbon environment was already predicted by Schleyer and coworkers on the basis of theoretical studies [42].The unsymmetrical methandiide C was found to form a complex molecular structure, which however also showed a strong deviation from the ideal tetrahedral geometry of carbon [26].Overall, small changes in the structure of methandiides can easily lead to big differences in their reactivity and the structure formation in the solid state.For example, replacement of the sulfonyl group in C by a sulfoximine moiety lead to an increased reactivity and decreased stability of the methandiide as well as the formation of a different structure in the solid state, which hampered its utilization as ligand in transition-metal chemistry [43].However, due to the efficiency of methandiide C as ligand in carbene complexes, we became interested in the preparation of the iminophosphoryl derivative 1-Li2 (Figure 1).Besides its syntheses we particularly addressed the elucidation of the molecular structures of the mono-and dimetallated compounds in comparison to the thiophosphoryl system.Besides their applicability in carbene complex chemistry, methandiides are also interesting reagents because of their structural properties.Analogous to simple organolithium reagents [30][31][32], structure formation is dominated by the presence or absence of additional donor functions within the molecule.Non-functionalized methandiides typically form polymeric structures to complete the coordination sphere of lithium, such as in the case of parent dilithiomethane, H 2 CLi 2 [33], or dilithiated fluorene D (Figure 2) [20].In contrast, donor side-arms lead to the formation of defined cluster structures [19].For instance, the DPPM-based systems A and B and derivatives thereof typically form dimeric structures with a central structural motif consisting of a Li 4 and Li 2 C 2 four-membered ring perpendicular to each other [17,18,[34][35][36][37][38].This motif is also present in the structures of the heavier alkali metal derivatives [39,40].An interesting monomeric structure was isolated by the introduction of the sterically demanding Dipp substitutents (Dipp = 2,6-Di(iso-propyl)phenyl) into the bipm framework together with TMEDA as additional Lewis base [41].Most interestingly, this compound showed in contrast to the van't Hoff rule a planar rather than a tetrahedral geometry at the metalated carbon atom.Such a planar carbon environment was already predicted by Schleyer and coworkers on the basis of theoretical studies [42].The unsymmetrical methandiide C was found to form a complex molecular structure, which however also showed a strong deviation from the ideal tetrahedral geometry of carbon [26].

Synthesis of the Protonated Precursor 1
group also gave way to the formation of transition-metal complexes with open coordination-sides [27][28][29].Besides their applicability in carbene complex chemistry, methandiides are also interesting reagents because of their structural properties.Analogous to simple organolithium reagents [30][31][32], structure formation is dominated by the presence or absence of additional donor functions within the molecule.Non-functionalized methandiides typically form polymeric structures to complete the coordination sphere of lithium, such as in the case of parent dilithiomethane, H2CLi2 [33], or dilithiated fluorene D (Figure 2) [20].In contrast, donor side-arms lead to the formation of defined cluster structures [19].For instance, the DPPM-based systems A and B and derivatives thereof typically form dimeric structures with a central structural motif consisting of a Li4 and Li2C2 fourmembered ring perpendicular to each other [17,18,[34][35][36][37][38].This motif is also present in the structures of the heavier alkali metal derivatives [39,40].An interesting monomeric structure was isolated by the introduction of the sterically demanding Dipp substitutents (Dipp = 2,6-Di(iso-propyl)phenyl) into the bipm framework together with TMEDA as additional Lewis base [41].Most interestingly, this compound showed in contrast to the van't Hoff rule a planar rather than a tetrahedral geometry at the metalated carbon atom.Such a planar carbon environment was already predicted by Schleyer and coworkers on the basis of theoretical studies [42].The unsymmetrical methandiide C was found to form a complex molecular structure, which however also showed a strong deviation from the ideal tetrahedral geometry of carbon [26].Overall, small changes in the structure of methandiides can easily lead to big differences in their reactivity and the structure formation in the solid state.For example, replacement of the sulfonyl group in C by a sulfoximine moiety lead to an increased reactivity and decreased stability of the methandiide as well as the formation of a different structure in the solid state, which hampered its utilization as ligand in transition-metal chemistry [43].However, due to the efficiency of methandiide C as ligand in carbene complexes, we became interested in the preparation of the iminophosphoryl derivative 1-Li2 (Figure 1).Besides its syntheses we particularly addressed the elucidation of the molecular structures of the mono-and dimetallated compounds in comparison to the thiophosphoryl system.Overall, small changes in the structure of methandiides can easily lead to big differences in their reactivity and the structure formation in the solid state.For example, replacement of the sulfonyl group in C by a sulfoximine moiety lead to an increased reactivity and decreased stability of the methandiide as well as the formation of a different structure in the solid state, which hampered its utilization as ligand in transition-metal chemistry [43].However, due to the efficiency of methandiide C as ligand in carbene complexes, we became interested in the preparation of the iminophosphoryl derivative 1-Li 2 (Figure 1).Besides its syntheses we particularly addressed the elucidation of the molecular structures of the mono-and dimetallated compounds in comparison to the thiophosphoryl system.

Synthesis of the Protonated Precursor 1
In order to examine the influence of the electronic and steric properties of the iminophosphoryl group on the accessibility, reactivity and structure of methandiide 1-Li 2 we chose the protonated precursor 1 with a sterically demanding trimethylsilyl (TMS) group at the imino nitrogen atom, since this group has most often been used in other methandiide systems.To this end, the synthetic strategy outlined in Figure 3 was used.At first, phenyl methyl sulfone was deprotonated with n-butyl lithium and reacted with chlorodiphenylphosphine, followed by a subsequent oxidation of the phosphine moiety with H 2 O 2 [27].Recrystallization from ethanol afforded phosphine oxide 2 as a colourless solid in 87% yield.Next, phosphine oxide 2 was treated with oxalyl bromide in DCM.After filtration, phosphine bromide 3 could be isolated as a colourless to slightly yellow solid in 91% yield [44].
Compound 3 shows a single signal in the 31 P{ 1 H} NMR spectrum at 18.7 ppm in deuterated DMSO and a characteristic doublet at 5.10 ppm with a coupling constant of 2 J PH = 9.95 Hz in the 1 H NMR spectrum.The formation of the iminophosphoryl moiety was achieved by treatment of phosphine bromide 3 with hexamethyldisilazane (HMDS) and subsequent deprotonation with NEt 3 [45].Recrystallization from toluene/hexane yielded the α-iminophosphoryl-substituted sulfone 1 as colourless crystals in 88% yield.The 31 P{ 1 H} NMR spectrum exhibits a single signal at −12.7 ppm and the hydrogen atoms of the methylene bridge resonate as a doublet at 3.94 ppm with a coupling constant of 2 J PH = 9.79 Hz.The crystal structure of iminophosphoryl 1 is depicted in Figure 3 In order to examine the influence of the electronic and steric properties of the iminophosphoryl group on the accessibility, reactivity and structure of methandiide 1-Li2 we chose the protonated precursor 1 with a sterically demanding trimethylsilyl (TMS) group at the imino nitrogen atom, since this group has most often been used in other methandiide systems.To this end, the synthetic strategy outlined in Figure 3 was used.At first, phenyl methyl sulfone was deprotonated with n-butyl lithium and reacted with chlorodiphenylphosphine, followed by a subsequent oxidation of the phosphine moiety with H2O2 [27].Recrystallization from ethanol afforded phosphine oxide 2 as a colourless solid in 87% yield.Next, phosphine oxide 2 was treated with oxalyl bromide in DCM.After filtration, phosphine bromide 3 could be isolated as a colourless to slightly yellow solid in 91% yield [44].
Compound 3 shows a single signal in the 31 P{ 1 H} NMR spectrum at 18.7 ppm in deuterated DMSO and a characteristic doublet at 5.10 ppm with a coupling constant of 2 JPH = 9.95 Hz in the 1 H NMR spectrum.The formation of the iminophosphoryl moiety was achieved by treatment of phosphine bromide 3 with hexamethyldisilazane (HMDS) and subsequent deprotonation with NEt3 [45].Recrystallization from toluene/hexane yielded the α-iminophosphoryl-substituted sulfone 1 as colourless crystals in 88% yield.The 31 P{ 1 H} NMR spectrum exhibits a single signal at −12.7 ppm and the hydrogen atoms of the methylene bridge resonate as a doublet at 3.94 ppm with a coupling constant of 2 JPH = 9.79 Hz.The crystal structure of iminophosphoryl 1 is depicted in Figure 3. 1 crystallizes in the triclinic space group P-1 and shows typical bond lengths and angles compared to those in related compounds.Sole exception is the P1-N1-Si1 angle with an high value of 160.4( 9)° [46][47][48][49].In comparison, the symmetric bis(iminophosphoryl)methane, (Me3SiNPPh2)2CH2, featured an angle of 138.5(1)°.

Preparation of Methanide 1-K and Methandiide 1-Li2
To evaluate the effects of the sulfonyl and the iminophosphoryl group on the deprotonation behavior of compound 1 we next aimed at the synthesis of the corresponding mono-and dimetallated species (Figure 4).Single deprotonation was selectively achieved with a series of different metal bases, as evidenced by a single new signal in the 31 P{ 1 H} NMR spectrum at −1.18 ppm and the signal of the methanide hydrogen at 2.38 ppm with a coupling constant of 2 JPH = 10.8Hz in the 1 H NMR spectrum.Convenient isolation of the methanide could be achieved by employment of KH, which allowed the isolation of 1-K as a colourless solid in 89% yield.1-K is stable over weeks under inert gas atmosphere and exhibits-despite the TMS functionalization-a remarkably low solubility in common organic solvents.Thus all NMR studies had to be performed in deuterated DMSO.Crystals

Preparation of Methanide 1-K and Methandiide 1-Li 2
To evaluate the effects of the sulfonyl and the iminophosphoryl group on the deprotonation behavior of compound 1 we next aimed at the synthesis of the corresponding mono-and dimetallated species (Figure 4).Single deprotonation was selectively achieved with a series of different metal bases, as evidenced by a single new signal in the 31 P{ 1 H} NMR spectrum at −1.18 ppm and the signal of the methanide hydrogen at 2.38 ppm with a coupling constant of 2 J PH = 10.8Hz in the 1 H NMR spectrum.Convenient isolation of the methanide could be achieved by employment of KH, which allowed the isolation of 1-K as a colourless solid in 89% yield.1-K is stable over weeks under inert gas atmosphere and exhibits-despite the TMS functionalization-a remarkably low solubility in common organic solvents.Thus all NMR studies had to be performed in deuterated DMSO.Crystals suitable for X-ray diffraction analysis could be obtained by diffusion of pentane into a saturated solution of compound 1-K in THF.
Inorganics 2016, 4, 40 4 of 12 suitable for X-ray diffraction analysis could be obtained by diffusion of pentane into a saturated solution of compound 1-K in THF.1-K crystallizes as a coordination polymer in the triclinic space group P-1.This observation is well in line with the poor solubility of monoanion 1-K.The asymmetric unit contains a dimeric structural motif with one non-coordinating THF molecule (not shown in Figure 5).The two monomeric subunits of this pseudo C2-symmetric dimer (K1 − K2 = C2 axis) are connected via two potassium atoms.Both show a slightly different coordination environment: While potassium atom K1 is coordinated by the nitrogen atoms of both iminophosphoryl substituents, the oxygen and the phenyl group of the sulfonyl moieties and two protons of a TMS-group, potassium atom K2 is solely coordinated by the oxygens of the sulfonyl moieties and the iminophosphoryl phenyl rings.Completion of the coordination sphere of K2 is finally achieved through coordination of the sulfonyl groups of two adjacent dimers, thus leading to the polymeric structure in the solid state.The potassium atoms show no contacts to the carbon atoms of the methylene groups which leads to a planar geometry with sums of angles of 359(4)° and 360(6)° with slightly widened P-C-S angles of 120.09(18)° and 123.05(18)°, respectively, compared to the neutral compound 1 (from 117.17( 8)).The most important feature of the molecular structure of 1-K is the shortening of the C-P bonds (from 1.8441(15) Å to 1.727(3) Å) and the C-S bond lengths (from 1.7808( 16) Å to 1.638(3) Å) in the P-C-S backbone compared to the protonated precursor 1.This can be explained with electrostatic interactions between the negativ charge at the methylene carbon and the positive charges at the phosphorous and the sulfur.At the same time, an elongation of the P-N bond from 1.5229(13) Å to 1.569(2) Å and 1.562(2) Å, respectively, can be observed which is due to negative hyperconjugation of the lone pair of the methanide carbon into the antibonding σ* orbitals of the P-N bonds.
The preparation of the dianionic species 1-Li2 was achieved by reaction with a slight excess of MeLi in Et2O.Addition of MeLi to a suspension of 1 in Et2O led to a complete solvation followed by the formation of a colourless solid after a few minutes.Removal of the supernatant solution and drying of the obtained solid in vacuo gave dianionic species 1-Li2 in 78% yield.The 31 P{ 1 H} NMR spectrum shows a single signal at 7.63 ppm.The 1 H NMR spectrum confirms the successful double deprotonation through the absence of the protons of the methylene bridge and shows additional coordinating diethyl ether.The 7 Li NMR spectrum shows two signals at 0.19 and 1.67 ppm thus indicating the formation of an unsymmetrical structure with different coordination spheres of the two lithium atoms.This assumption is further supported by the presence of two separate sets of signals for the two phosphorous bound phenyl rings speaking for a diastereotopic behavior.X-ray quality crystals could be obtained by performing the reaction without stirring in a larger volume of solvent leading to the direct crystallization of the product.The crystal structure of methandiide 1-Li2 is shown in Figure 6.1-K crystallizes as a coordination polymer in the triclinic space group P-1.This observation is well in line with the poor solubility of monoanion 1-K.The asymmetric unit contains a dimeric structural motif with one non-coordinating THF molecule (not shown in Figure 5).The two monomeric subunits of this pseudo C 2 -symmetric dimer (K 1 − K 2 = C 2 axis) are connected via two potassium atoms.Both show a slightly different coordination environment: While potassium atom K1 is coordinated by the nitrogen atoms of both iminophosphoryl substituents, the oxygen and the phenyl group of the sulfonyl moieties and two protons of a TMS-group, potassium atom K2 is solely coordinated by the oxygens of the sulfonyl moieties and the iminophosphoryl phenyl rings.Completion of the coordination sphere of K2 is finally achieved through coordination of the sulfonyl groups of two adjacent dimers, thus leading to the polymeric structure in the solid state.The potassium atoms show no contacts to the carbon atoms of the methylene groups which leads to a planar geometry with sums of angles of 359(4) • and 360(6) • with slightly widened P-C-S angles of 120.09(18) • and 123.05(18) • , respectively, compared to the neutral compound 1 (from 117.17( 8)).The most important feature of the molecular structure of 1-K is the shortening of the C-P bonds (from 1.8441(15) Å to 1.727(3) Å) and the C-S bond lengths (from 1.7808( 16) Å to 1.638(3) Å) in the P-C-S backbone compared to the protonated precursor 1.This can be explained with electrostatic interactions between the negativ charge at the methylene carbon and the positive charges at the phosphorous and the sulfur.At the same time, an elongation of the P-N bond from 1.5229(13) Å to 1.569(2) Å and 1.562(2) Å, respectively, can be observed which is due to negative hyperconjugation of the lone pair of the methanide carbon into the antibonding σ* orbitals of the P-N bonds.
The preparation of the dianionic species 1-Li 2 was achieved by reaction with a slight excess of MeLi in Et 2 O. Addition of MeLi to a suspension of 1 in Et 2 O led to a complete solvation followed by the formation of a colourless solid after a few minutes.Removal of the supernatant solution and drying of the obtained solid in vacuo gave dianionic species 1-Li 2 in 78% yield.The 31 P{ 1 H} NMR spectrum shows a single signal at 7.63 ppm.The 1 H NMR spectrum confirms the successful double deprotonation through the absence of the protons of the methylene bridge and shows additional coordinating diethyl ether.The 7 Li NMR spectrum shows two signals at 0.19 and 1.67 ppm thus indicating the formation of an unsymmetrical structure with different coordination spheres of the two lithium atoms.This assumption is further supported by the presence of two separate sets of signals for the two phosphorous bound phenyl rings speaking for a diastereotopic behavior.X-ray quality crystals could be obtained by performing the reaction without stirring in a larger volume of solvent leading to the direct crystallization of the product.The crystal structure of methandiide 1-Li 2 is shown in Figure 6.For bond lengths and angles, see Figure 7.
Methandiide 1-Li2 crystallizes in the monoclinic space group P21/c.The asymmetric unit consists of a pseudo-tetrameric complex, whose S4 symmetry however is broken due to the coordination of only one additional diethyl ether molecule (to Li8).The central structural motif is formed by two almost planar (SO2Li)2 eight-membered rings, which are connected with each other via the methandiide carbon atoms and additional four lithium atoms.The four lithium atoms of the (SO2Li)2 rings are solely coordinated by the sulfonyl moieties and the methanide carbon atoms.These lithium atoms are only three-fold coordinated, except for Li8, which is also coordinated by the ether molecule.The other four lithium atoms (Li1, Li3, Li5 and Li7) are coordinated by the sulfonyl oxygen atoms, the methanide atom and the nitrogen atom of the iminophosphoryl group, thus having a coordination number of four.Overall, this complexation formally leads to two different types of lithium atoms Methandiide 1-Li 2 crystallizes in the monoclinic space group P2 1 /c.The asymmetric unit consists of a pseudo-tetrameric complex, whose S4 symmetry however is broken due to the coordination of only one additional diethyl ether molecule (to Li8).The central structural motif is formed by two almost planar (SO 2 Li) 2 eight-membered rings, which are connected with each other via the methandiide carbon atoms and additional four lithium atoms.The four lithium atoms of the (SO 2 Li) 2 rings are solely coordinated by the sulfonyl moieties and the methanide carbon atoms.These lithium atoms are only three-fold coordinated, except for Li8, which is also coordinated by the ether molecule.The other four lithium atoms (Li1, Li3, Li5 and Li7) are coordinated by the sulfonyl oxygen atoms, the methanide atom and the nitrogen atom of the iminophosphoryl group, thus having a coordination number of four.Overall, this complexation formally leads to two different types of lithium atoms bound to each methandiide carbon atom, which is well in line with the two signals observed in the 7 Li NMR spectrum.A detailed view of a monomeric subunit can be seen in Figure 7.The Li-C bond lengths with an average value of 2.235(6) Å are in the expected range compared to known methandiides featuring sulfonyl or iminophosphoryl groups [3,17,26].The Li-N contacts are (with an average bond length of 2.006 Å) a bit shorter than those found in other iminophosphoryl stabilized geminal dianions, while the Li-O bonds (with an average length of 1.973(6) Å) are well in line with reported data [3,17].The central P-C-S backbone shows an even stronger contraction than in the monoanionic species 1-K, with an average P-C bond length of 1.714(3) Å and an average S-C bond length of 1.608(3) Å speaking for even stronger electrostatic interactions (for a comparison of all bond length, see Table 1).The same strengthening can be observed for the negative hyperconjugation effects, which lead to even further elongated bond lengths for the P-N and especially S-O bonds with average values of 1.586(3) Å and 1.494(2) Å respectively.These observations are consistent with those reported for other mono-and dimetallated systems [14][15][16][17][18][19][20].For example, the bond length changes in 1-Li2 are similar to those found in the corresponding thiophosphoryl compound C (Table 1), despite of the different structures formed in the solid state [26].Another interesting feature of the crystal structure of 1-Li2 concerns the coordination environments of the methandiide carbon atoms which strongly deviate from an ideal tetrahedron.As shown in Figure 7, C(1), S(1), P(1) and Li (1) are almost in one plane, while Li(2) is coordinating almost orthogonally to that plane with an Li-C-Li angle of 72.9(2)°.This has also been observed in the crystal structure of methandiide C. On the basis of theoretical methods [26] this coordination mode can be explained by the two methandiide lone pairs populating two different orbitals, one of sp 2 -and the other of p-symmetry.The Li-C bond lengths with an average value of 2.235(6) Å are in the expected range compared to known methandiides featuring sulfonyl or iminophosphoryl groups [3,17,26].The Li-N contacts are (with an average bond length of 2.006 Å) a bit shorter than those found in other iminophosphoryl stabilized geminal dianions, while the Li-O bonds (with an average length of 1.973(6) Å) are well in line with reported data [3,17].The central P-C-S backbone shows an even stronger contraction than in the monoanionic species 1-K, with an average P-C bond length of 1.714(3) Å and an average S-C bond length of 1.608(3) Å speaking for even stronger electrostatic interactions (for a comparison of all bond length, see Table 1).The same strengthening can be observed for the negative hyperconjugation effects, which lead to even further elongated bond lengths for the P-N and especially S-O bonds with average values of 1.586(3) Å and 1.494(2) Å respectively.These observations are consistent with those reported for other mono-and dimetallated systems [14][15][16][17][18][19][20].For example, the bond length changes in 1-Li 2 are similar to those found in the corresponding thiophosphoryl compound C (Table 1), despite of the different structures formed in the solid state [26].Another interesting feature of the crystal structure of 1-Li 2 concerns the coordination environments of the methandiide carbon atoms which strongly deviate from an ideal tetrahedron.As shown in Figure 7, C(1), S(1), P(1) and Li (1) are almost in one plane, while Li(2) is coordinating almost orthogonally to that plane with an Li-C-Li angle of 72.9(2) • .This has also been observed in the crystal structure of methandiide C. On the basis of theoretical methods [26] this coordination mode can be explained by the two methandiide lone pairs populating two different orbitals, one of sp 2 -and the other of p-symmetry.

General Procedures
All experiments were carried out under a dry, oxygen-free argon atmosphere using standard Schlenk techniques.Solvents were dried over sodium or potassium (or over P 4 O 10 , CH 2 Cl 2 ) and distilled prior to use.H 2 O is distilled water.Organolithium reagents were titrated against diphenylacetic acid prior use. 1 H, 7 Li, 13 C, 31 P NMR spectra were recorded on Avance-500, Avance-400 or Avance-300 spectrometers (Bruker Biospin GmbH, Rheinstetten, Germany) at 22 • C if not stated otherwise.All values of the chemical shift are in ppm regarding the δ-scale.All spin-spin coupling constants (J) are printed in Hertz (Hz).To display multiplicities and signal forms correctly the following abbreviations were used: s = singulet, d = doublet, t = triplet, m = multiplet, br = broad signal.Signal assignment was supported by DEPT and HMQC experiments.Elemental analyses were performed on an Elementar vario MICRO-cube elemental analyzer.All reagents were purchased from Sigma-Aldrich (Munich, Germany), ABCR (Karlsruhe, Germany)) or Acros Organics/Fisher Scientific GmbH (Nidderau, Germany) and used without further purification.Phosphine oxide 2 was, synthesized according to literature procedure [27].

Syntheses
Synthesis of Bromide 3. Compound 3 was prepared in analogues fashion to literature procedure [44].Phosphine oxide 2 (5.00 g, 14.0 mmol) was dissolved in 40 mL DCM.Oxalyl bromide (6.06 g, 28.0 mmol) was slowly added via syringe and the resulting suspension stirred at room temperature over night until no further gas evolution could be observed.The reaction mixture was filtrated and the resulting solid washed three times with DCM (10 mL) giving phosphine bromide 3 as an off white solid (6.39 g, 12.7 mmol, 91%).Synthesis of Compound 1. Compound 1 was prepared in analogues fashion to literature procedure [45].6.73 g (13.4 mmol) of Bromide 3 were suspended in 40 mL MeCN and cooled to −40 • C. 2.89 g (17.9 mmol) HMDS were added under light exclusion and the reaction mixture slowly warmed to room temperature and stirred overnight.The solvent was removed in vacuo and the residue suspended in 30 mL toluene.2.72 g (26.9 mmol) Triethylamine were added and the reaction mixture stirred overnight.The solution was filtered and the solvent removed under reduced pressure.Recrystallization of the crude product in toluene/hexane (1:2) gave way to the product as a white crystalline solid (5.5 g, 11.7 mmol, 88%).

Figure 1 .
Figure 1.Isolated dilithiomethanes used as ligands for carbene complex formation.

Figure 1 .
Figure 1.Isolated dilithiomethanes used as ligands for carbene complex formation.

Figure 1 .
Figure 1.Isolated dilithiomethanes used as ligands for carbene complex formation.

Figure 6 .
Figure 6.Displays of the crystal structure of methandiide 1-Li2.Ellipsoids drawn at the 50% probability level.Non-coordinating diethyl ether molecules and hydrogen atoms omitted for clarity.For bond lengths and angles, seeFigure7.

Figure 6 .
Figure 6.Displays of the crystal structure of methandiide 1-Li 2 .Ellipsoids drawn at the 50% probability level.Non-coordinating diethyl ether molecules and hydrogen atoms omitted for clarity.For bond lengths and angles, seeFigure7.
methandiide carbon atom, which is well in line with the two signals observed in the7 Li NMR spectrum.A detailed view of a monomeric subunit can be seen in Figure7.

Table 1 .
Comparison of structural parameters (average values for S-O bonds, 1