Low-Coordinate Magnesium Sulfide and Selenide Complexes

The reactions of [{(iPrDipNacNac)Mg}2] 1 (iPrDipnacnac = HC(iPrCNDip)2) with Ph3P=O at 100 °C afforded the phosphinate complex [(iPrDipNacNac)Mg(OPPh3)(OPPh2)] 3. Reactions of 1 with Ph3P=E (E = S, Se) proceeded rapidly at room temperature to low-coordinate chalcogenide complexes [{(iPrDipNacNac)Mg}2(μ-S)] 4 and [{(iPrDipNacNac)Mg}2(μ-Se)] 5, respectively. Similarly, reactions of RNHC=S ((MeCNR)2C=S with R = Me, Et, or iPr) with 1 afforded NHC adducts of magnesium sulfide complexes, [{(iPrDipNacNac)Mg(RNHC)}(μ-S){Mg(iPrDipNacNac)}] 6, that could alternatively be obtained by adding the appropriate RNHC to sulfide complex 4. Complex 4 reacted with 1-adamantylazide (AdN3) to give [{(iPrDipNacNac)Mg}2(μ-SN3Ad)] 7 and can form various simple donor adducts in solution, of which [(iPrDipNacNac)Mg(OAd)}2(μ-S)] 8a (OAd = 2-adamantanone) was structurally characterized. The nature of the ionic Mg–E–Mg unit is described by solution and solid-state studies of the complexes and by DFT computational investigations.


■ INTRODUCTION
Well-defined complexes of inorganic fragments in lowcoordination modes are expected to show significantly different properties and a higher reactivity compared with those of solid bulk materials.−4 In the field of well-defined molecular compounds with main group element−chalcogen bonds, 5,6 including heavier carbonyl analogues, 7,8 significant advances have been made in recent years regarding their controlled synthesis, structure, and bonding, but many challenges remain, especially for those of the early main group metals.For magnesium, oxide complexes with an LMgOMgL (L = anionic ligand) unit and low-coordinate oxide ions 9−12 were predominantly obtained by reactions of "highenergy" dimagnesium(I) species 13−15 with nitrous oxide (N 2 O).In addition, some molecules with low-coordinate BeOBe units have been structurally characterized. 16,17Welldefined alkaline earth metal complexes of heavier chalcogenides are very rare, and a majority of complexes with magnesium and sulfur contacts stem from compounds containing sulfur as part of larger anionic ligands (e.g., in magnesium thiolate complexes). 18−24 A rare magnesium disulfide complex (A), see Figure 1, was reported by Ren and Gu from the reaction of a Mg I mimic and S 8 , plus related work for Ca. 25,26Ghosh and Parkin reported a structurally characterized magnesium selenide complex B (Figure 1) that was prepared via an organomagnesium species and H 2 Se, and this work also featured hydroselenide (biselenide) and a related bisulfide species. 27For the related metal zinc, few complexes with bridging low-coordinate chalcogenide ions are known 28−31 plus a unique anionic complex with a terminal Zn−S bond. 32Here we report on the chemistry of lowcoordinate β-diketiminate magnesium sulfide complexes and one related magnesium selenide complex.
■ RESULTS AND DISCUSSION Synthesis.Previously, reactions of dimagnesium(I) complexes with N 2 O afforded low-coordinate oxide complexes, but these reactions can be plagued by the formation of significant quantities of hydroxide byproducts. 9,10,12With the target of a convenient synthesis of LMgEMgL complexes, E = group 16 elements, we studied the reactions of the dimagnesium(I) complex [{( iPrDip NacNac)Mg} 2 ] 1 12 with Ph 3 P�E (E = O, S, Se).The addition of one equivalent of Ph 3 P�O to a yellow benzene-d 6 solution of 1 at ambient temperature showed no reaction as judged by 1 H and 31 P{ 1 H} NMR spectroscopy.
Heating the solution to 60 °C for 3 days suggested only minimal conversion, and heating to 100 °C afforded a slow reaction, a partial conversion of 1, and a color change to orange-red but no formation of the magnesium oxide complex [{( iPrDip NacNac)Mg} 2 (μ-O)] 2. 12 Reacting [{( iPrDip NacNac)-Mg} 2 ] 1 with four equivalents of Ph 3 P�O at 100 °C for 2 days afforded the full conversion of 1 to one main βdiketiminate-containing complex with two different 31 2 for a molecular structure.Biphenyl was detected by 1 H NMR spectroscopy as a byproduct from this reaction.Complex 3 was isolated from n-hexane in 44% isolated yield as a colorless solid that often appears red in earlier crops due to the intense color in solution.For the related reaction of sodium dispersion with Ph 3 PO in THF, the generation of Ph 2 PONa is observed and the formation of NaPh as a byproduct has been suggested due to the onward reactivity with sodium diphenylphosphinite to C−C coupled sodium 5H-benzo[b]phosphindol-5-olate. 33,34The generated NaPh has been shown to then react with the THF solvent to form sodium vinyl alkoxide in this system. 33,34The relatively harsh reaction conditions for the formation of 3 and the color of the solution and the formation of biphenyl as the byproduct suggest that the reaction proceeds via donor adducts of magnesium(I) complexes with elongated Mg−Mg bonds [13][14][15]35 (e.g., Ph 3 PO bisadducts of 1) and could also point to the formation of radicals during the process.
In contrast, reactions of Ph 3 P�E (E = S, Se) with 1, followed by NMR spectroscopy in deuterated benzene, proceeded very rapidly at room temperature, giving high in situ yields of the low-coordinate chalcogenide complexes [{( iPrDip NacNac)Mg} 2 (μ-S)] 4 and [{( iPrDip NacNac)Mg} 2 (μ-Se)] 5 (see Figures 3 and 4, respectively) and Ph 3 P (Scheme 1).The isolation of complexes 4 (54%) and 5 (around 30% yield) was predominantly limited by the need for separation from triphenylphosphine and difficulties in the precipitation of the desired product, especially for 5. NMR spectra for 4 and 5 show, as expected, resonances for highly symmetric complexes in solution.[{( iPrDip NacNac)Mg} 2 (μ-Se)] 5 shows a noticeably upfield 77 Se NMR resonance at −764 ppm.For comparison, Parkin's tris(pyrazolyl)hydroborato-stabilized magnesium hydroselenido complex [(Tp p-Tol )Mg(SeH)] displays a chemical shift at −486 ppm. 27The much more facile activation of Ph 3 P�E (E = O, S, Se) for E = S, Se compared with E = O can be related to the weaker P�E bonds for E = S (ca.301 kJ  mol −1 ) and Se (ca.246 kJ mol −1 ) compared with E = O (ca. 546 kJ mol −1 ).The latter is also larger than that of a typical P− C bond (ca.513 kJ mol −1 ) (cf. the formation of phosphinite 3). 36,37xt, reactions between [{( iPrDip NacNac)Mg} 2 ] 1 and substituted imidazole-2-thiones were studied, which also showed rapid conversions at room temperature to donorsubstituted sulfide complexes of the type [{( iPrDip NacNac)Mg-( R NHC)}(μ-S){Mg( i P r D i p NacNac)}] 6 ( R NHC = (MeCNR) 2 C with R = Me 6a, Et 6b, and iPr 6c) in very high yields, as shown in Scheme 2. Unfortunately, so far these complexes could not be structurally characterized, due to their high solubility and preferential crystallization or precipitation of the uncoordinated sulfide complex 4, especially for the larger R NHC ligand with R = iPr (6c), likely generated in an equilibrium; see Scheme 3. The same compounds (6) were shown by NMR spectroscopy to be obtained nearly quantitatively when sulfide complex 4 was treated with one equivalent of the respective N-heterocyclic carbene ( R NHC); see Scheme 2. NMR spectra of 6a and 6b both show resonances for two different ligand environments, the latter with slightly broader resonances and 13  °C show one β-diketiminate ligand environment only.For 6c, resonances for two ligand sets were observed at low temperature, with coalescence near room temperature.These processes suggest estimated barriers of ΔG ⧧ ≈ 16 kcal mol −1 (6b) and ΔG ⧧ ≈ 14 kcal mol −1 (6c) and depend on the steric demand of the R NHC, with the bulkier one decoordinating and rearranging more easily (Scheme 3), likely via donor-free species 4, which is in line with the repeated observation of crystallization of 4 from such solutions.In a similar manner, the monomeric aluminum(I) complex MeDip NacNacAl: ( MeDip NacNac = HC(MeNDip) 2 ) has been reacted with Ph 3 PS and imidazole-2-thiones to form aluminum sulfide complexes, also forming reduced phosphine or N-heterocyclic carbene units, respectively. 38For comparison and in contrast to the magnesium chemistry reported herein, the reaction of MeDip NacNacAl: with Ph 3 PO proceeded at room temperature, afforded PPh 3 , and transferred one oxygen to the aluminum center without P−C bond cleavage. 39 5, in high in situ yield (67% isolated).Evidently, the nucleophilic sulfide reacted with an electrophilic site at the organic azide to form a bridging {SN 3 Ad} 2− ligand, vide infra.To the best of our knowledge, this is the first example of such a ligand fragment, but it bears a resemblance to magnesium complexes of N−N azide coupled {AdN 3 −N 3 Ad} 2− fragments 40,41 and the {SN 2 O} 2− ligand formed from sulfide addition to N 2 O in a zinc complex. 32At room temperature, complex 7 shows NMR resonances for a more symmetric structure than expected from its molecular structure (i.e., showing one set of ligand resonances suggesting a flexible coordination of the {AdN 3 S} 2− ligand between the ( iPrDip NacNac)Mg + units).At low temperatures, resonances broaden at around −10 °C and Dip-isopropyl group methyl resonances appear to be separated at −45 °C, but the characteristic β-diketiminate backbone methine singlet is not split down to −75 °C, the limit of this experiment (Figure S38), showing the flexible coordination behavior of this system.At high temperatures, complex 7 appears to be quite thermally stable in solution and shows only minimal decomposition after several days at 100 °C in deuterated benzene.
Reactions H NMR resonance at −0.99 ppm which was tentatively assigned to bridging Mg−SH groups.This was later corroborated by a few colorless crystals that were deposited from solution that were structurally characterized as [{( iPrDip NacNac)Mg(μ-SH)} 2 ] 9; see Figure S61.Presumably, the basic nature of the magnesium sulfide can deprotonate the ketone, followed by ligand rearrangements to form [{( iPrDip NacNac)Mg(μ-SH)} 2 ] 9 as part of a product mixture.Further heating led to the formation of some proligand, iPrDip NacNacH.
Molecular Structures.[( iPrDip NacNac)Mg(OPPh 2 )-(OPPh 3 )] 3, crystallized with a full molecule in the asymmetric unit, shows a distorted tetrahedral Mg center sitting significantly above the chelating diketiminate unit (by approximately 0.99 Å) toward the Ph 2 PO ligand which is located approximately perpendicular with respect to the diketiminate, whereas the larger neutral Ph 3 PO donor is approximately in plane with the diketiminate unit.The Mg−O bond distance to the anionic diphenylphosphinite is only slightly shorter than that to the neutral triphenylphosphinoxide (Mg1−O2 1.9000( 16) versus Mg1−O1 1.9243( 14)), and the P−O bond in the P V ligand (P1−O1 1.4972( 13)) is, as expected, shorter than that in the phosphinite (P2−O2 1.5459 (17)).−47 The molecular structure of [{( iPrDip NacNac)Mg} 2 (μ-S)] 4 was determined multiple times, and the best two data sets are included here.In all cases, 4 was obtained with half a molecule of the complex in the asymmetric unit and contains threecoordinate Mg centers.The sulfur positions were found to be disordered on or very close to special positions in the asymmetric unit.As such, the multiple individual bond lengths are not given and a mean value of Mg−S 2.23−2.24Å was obtained.The Mg−S−Mg angles for this model show some variation, with an average around 159°, and are likely flexible and easy to distort.For context, the Mg−S distances in rock salt MgS are ca.2.57 Å and in zinc blende MgS are ca.2.42 Å, showing the expected significant effect of the coordination number on bond distances. 48An Mg−S distance of 2.44 Å can be inferred from single-bond covalent radii 49 and show the significant shortening in 4, but not as low as that determined for diatomic MgS of ca.2.143 Å. 50 The Mg−S bond lengths in 4 are significantly shorter than those observed in Ren's disulfide-bridged NacNac magnesium complex A (2.4518(10) Å and 2.5149(10) Å) and those found for related terminal NacNac magnesium thiolates (e.g., in [( Dip NacNac)Mg(THF)-(SPh)] 25 (Mg1−S1 2.3839(9) Å)).In addition, a few related low-coordinate β-diketiminate metal(II) sulfide complexes of general formula [{(NacNac)M} 2 (μ-S)] exist with M = Fe, 51 Ni, 52 and Sn. 53gSe complex 5 is isomorphous to 4, and again the Se atom is disordered on or very close to special positions.There is, however, one main position in the asymmetric unit which shows two different Mg−Se distances (Se1−Mg1 2.3497(18), Se1−Mg1′ 2.4739(18) Å) among shorter contacts and a relatively acute Mg1−Se1−Mg1 angle of 137.89(6)°, alongside more obtuse angles, and likely again hints that the Se position in this ionic arrangement is easy to distort and preferably bent.Parkin's magnesium selenide complex [{( p-Tol Tp)Mg} 2 Se] B 27 displays comparable Mg−Se bond lengths of 2.404(3) Å and 2.408(3) Å in a four-coordinate Mg environment and a linear Mg−Se−Mg moiety.In solid MgSe, the Mg−Se distances are ca.2.70 Å (rock salt) and ca.2.54 Å (zinc blende), 48 and single bond radii provide 2.55 Å. 49 Complex [{( iPrDip NacNac)Mg} 2 (μ-SN 3 Ad)] 7, crystallized with a full molecule in the asymmetric unit.The central {SN 3 Ad} 2− ligand bridges two Mg centers in a μ-    40,41 The S−N bond (1.8257(12) Å) is noticeably longer than would typically be expected for a S−N single bond (ca.1.74), 49 likely due to electrostatic repulsion in the dianionic unit.To the best of our knowledge, this is the first structurally characterized {SN 3 R} 2− ligand that has some resemblance to the {SN 2 O} 2− coordinated to a zinc center. 32he donor adduct [{( iPrDip NacNac)Mg(AdO)} 2 (μ-S)] 8a with four-coordinate Mg centers shows a linear Mg−S−Mg fragment (cf. the linear arrangement in B) with Mg−S bond lengths of 2.2610(5) Å that are only slightly elongated compared to the mean value found in uncoordinated [{( iPrDip NacNac)Mg} 2 (μ-S)] 4. The hydrosulfide (bisulfide) complex [{( iPrDip NacNac)Mg(μ-SH)} 2 ] 9 (Figure S61) shows the expected dimeric structure with a Mg−S distance of 2.51 Å (mean) and a Mg−S−Mg angle of 89.52 (3)°and can be compared to the shorter Mg−S distance of 2.4424(6) Å in [{( MeDip NacNac)Mg(μ-SnBu)} 2 ]. 24 Computational Studies.A DFT study at the M06L/def2-TZVP level of theory with D3 dispersion addition followed by single-point calculations at the M06-D3/def2-TZVP level of theory (M06-D3/def2-TZVP//M06-L-D3/def2-TZVP) was conducted for the compound series with the full ligand model [{( iPrDip NacNac)Mg} 2 (μ-E)] E = O (2), S (4), Se (5)  and the small ligand model system [{( MeMe NacNac)Mg} 2 (μ-E)] ( MeMe NacNac = HC(MeNMe) 2 ), E = O, S, Se, see Table 1 for selected data).The optimization of both the full ligand sphere and a cut-back model was conducted to gain some insight into the influence of the ligand bulk for these species.The DFT optimized full ligand model species [{( iPrDip NacNac)Mg} 2 (μ-E)] reproduced the overall structures found by X-ray diffraction well, but the sterically smaller models [{( MeMe NacNac)Mg} 2 (μ-E)] reproduce only the linear Mg−O−Mg geometry with coplanar metal−ligand arrangements well but show significantly more bent Mg−E−Mg units for E = S and Se, resulting in sub-90°bond angles.These trends show the importance of the influence of the sterically demanding ligands for the structures of the compounds for E = S and Se and may provide a reason for the significant disorder of the S and Se positions in the molecular structures of 4 and 5.The sub-90°bond angles in [{( MeMe NacNac)Mg} 2 (μ-S)] for E = S and Se appear to be in part due to influences from dispersion forces.54,55 Removing the dispersion addition for [{( MeMe NacNac)Mg} 2 (μ-S)] leads to a more relaxed Mg−E− Mg angle of 106.4°(E = S) but a virtually unchanged angle for E = Se (82.6°), suggesting that these systems are easy to distort (Figure S64).
Inspecting the calculated charges of the Mg and E atoms from natural population analysis (NPA) and the quantum    S2.
C{ 1 H} NMR resonances of 183−184 ppm for the coordinated NHC ligand.The room-temperature 1 H NMR spectrum for 6c shows broader resonances and one ligand backbone CH unit.Complexes 6b and 6c were studied by variable-temperature NMR spectroscopy (Figures S27 and S32), and for 6b, resonances above the coalescence temperature of ca.60
theory of atoms in molecules (QTAIM) analysis in the series shows the most charge separation on Mg with the most electronegative O and less charge separation descending the chalcogen group to S and Se.The negative charge accumulation and slight polarization by the Mg centers at the chalcogen atom (E) can be visualized in the contour plot of the Laplacian of the electron density for [{( iPrDip NacNac)-Mg} 2 (μ-S)] 4 (Figure 7) and in related images for E = O (Figure S65) and Se (Figure S66).High-lying occupied orbitals for [{( iPrDip NacNac)Mg} 2 (μ-E)] show p x and p y orbitals of the chalcogens approximately perpendicular to the Mg−E−Mg bond (for O, HOMO−3 and HOMO−4; for S and Se, HOMO−1 and HOMO−2) and one significantly stabilized p z orbital (for O, HOMO−12; for S, HOMO−12; for Se, HOMO−11) approximately along the Mg−E−Mg bond which is around 1.4 eV (ca.32 kcal/mol, 135 kJ/mol) lower in energy compared to the respective p x and p y orbitals (see Figures S62 and S63).Other occupied orbitals show common β-diketiminate ligand-based features.All of these findings support that these complexes possess highly ionic Mg−E−Mg units that are easy to distort for E = S and Se.■ CONCLUSIONS Reactions of the magnesium(I) complex [{( iPrDip NacNac)-Mg} 2 ] 1 with Ph 3 P�E (E = O, S, Se) required forcing conditions for E = O and afforded the phosphinate complex [( iPrDip NacNac)Mg(OPPh 3 )(OPPh 2 )] 3 highlighting P−C rather than P−O bond cleavage.For E = S and Se, facile reactions afforded the low-coordinate chalcogenide complexes [{( iPrDip NacNac)Mg} 2 (μ-S)] 4 and [{( iPrDip NacNac)Mg} 2 (μ-Se)] 5, respectively, which show low-coordinate Mg−E−Mg units with short Mg−E bonds.Similarly, R NHC�S species

Table 1 .
Selected Metrical Data and Calculated Charges from NPA and QTAIM