Neutral inverse-sandwich rare-earth metal complexes of the benzene tetraanion

Rare-earth metal complexes of the parent benzene tetraanion and neutral inverse-sandwich rare-earth metal arene complexes have remained elusive. Here, we report the first neutral inverse-sandwich rare-earth metal complexes of the parent benzene tetraanion supported by a monoanionic β-diketiminate (BDI) ligand. Reduction of the trivalent rare-earth metal diiodide precursors (BDI)MI2(THF) (BDI = HC(C(Me)N[C6H3-(3-pentyl)2-2,6])2; M = Y, 1-Y; M = Sm, 1-Sm) in benzene or para-xylene by potassium graphite yielded the neutral inverse-sandwich rare-earth metal arene complexes [(BDI)M(THF)n]2(μ-η6,η6-arene) (M = Y, Sm; arene = benzene, 2-M; arene = para-xylene, 3-M). Single crystal X-ray diffraction, spectroscopic and magnetic characterization studies, together with density functional theory (DFT) calculations confirm that these neutral rare-earth metal arene complexes possess an [M3+–(arene)4−–M3+] electronic structure with strong metal–arene δ interactions. The arene exchange reactivity shows that 2-Sm has higher stability than 3-Sm. Furthermore, 2-Sm can behave as a four-electron reductant to reduce unsaturated organic substrates. Particularly, while the reaction of 2-Sm with 1,3,5,7-cyclooctatetraene (COT) yielded (BDI)Sm(η8-COT) (4-Sm), 2-Sm reacted with 1,4-diphenylbutadiyne to afford (BDI)Sm(η4-C4Ph2) (5-Sm), the first rare-earth metallacyclopentatriene complex.


Results and discussion
The pro-ligand (BDI)H and the potassium salt (BDI)K were prepared according to literature procedures. 49,50We choose samarium and yttrium as representative light and heavy rareearth metals, because they have the intermediate ionic radii among their leagues, respectively.In addition, since samarium has a readily accessible +2 oxidation state and benzene is more difficult to reduce than biphenyl, it will be of interest to compare the electronic structures of the target samarium benzene complexes with the samarium biphenyl complexes recently reported by our group. 22The salt metathesis reaction between (BDI)K and YI 3 (THF) 3.5 or SmI 3 (THF) 3.5 gave the trivalent metal precursors (BDI)YI 2 (THF) (1-Y) and (BDI)SmI 2 (THF) (1-Sm) in moderate yields (Scheme 1).The 1 H NMR spectrum of 1-Y in C 6 D 6 shows the characteristic CH peak of the BDI ligand backbone at 5.17 ppm, which shied downeld to 7.86 ppm in 1-Sm probably due to the paramagnetism of the Sm 3+ ion.X-ray crystallography conrmed their monomeric structures (Fig. S14 and S15 †).1-Y and 1-Sm are isostructural and both adopt a distorted trigonal bipyramidal geometry.The oxygen atom of THF and one of the nitrogen atoms occupy the apical positions (O-Y-N: 169.8(2)°;O-Sm-N: 166.5(2)°), and the sum of angles of three equatorial ligands is close to 360°(1-Y: 359.6°; 1-Sm: 359.5°).The average Y-N and Y-I distances in 1-Y are 2.272(5) and 2.935(1) Å, respectively.The average Sm-N (2.311(4) Å) and Sm-I (3.003(1) Å) distances in 1-Sm are slightly longer than those in 1-Y, in line with the ionic radii difference (sixcoordinate Y 3+ 0.90 Å, Sm 3+ 0.96 Å). 60

Synthesis and structural characterization
With 1-M in hand, we then attempted the synthesis of rare-earth metal benzene complexes under reducing conditions.Reduction of 1-M with 2.5 equiv. of potassium graphite (KC 8 ) in benzene at room temperature led to a black suspension within several hours, indicating the full consumption of KC 8 .Aer work-up, the target product 38% and 69% for yttrium and samarium, respectively (Scheme 1).The 1 H NMR spectra of 2-Y and 3-Y are both diamagnetic, consistent with a singlet ground state.The 1 H NMR spectrum of 2-Y in C 6 D 6 features an upeld shied peak for the protons of the bound benzene at 2.37 ppm, which appears as a triplet due to weak 89 Y-1 H coupling (J Y-H = 1.6 Hz), indicating signicant weakening of the aromatic ring current of the bound benzene.The 13 C NMR spectrum of 2-Y in C 6 D 6 also shows an upeld shied triplet for the bound benzene at 65.7 ppm with a J Y-C value of 4.8 Hz.In addition, the 1 H and 13  Moreover, while the 1 H NMR spectra of 2-Sm and 3-Sm are less informative due to the paramagnetism of Sm 3+ ions, the proton signal of the bound ring could be observed as a relatively sharp singlet at 21.73 and 26.70 ppm, respectively, which was further validated by the absence of the signal for 2-Sm synthesized in C 6 D 6 .Notably, immediately aer dissolution in C 6 D 6 , 3-Sm rapidly underwent arene exchange to form [(BDI)Sm(THF)] 2 (mh 6 ,h 6 -C 6 D 6 ) and free p-xylene (Fig. S10 †).The 1 H NMR of 3-Sm could be satisfactorily obtained by measuring the spectrum immediately aer dissolution in toluene-d 8 (Fig. S11 †), owing to the slower arene exchange rate between 3-Sm and toluene.In contrast, 2-Sm showed no evidence of arene exchange in C 6 D 6 even aer pro-long heating at 50 °C, indicating higher stability of 2-Sm than 3-Sm.Similar stability difference between the parent benzene and alkyl substituted benzenes has previously been observed in inverse-sandwich uranium and thorium arene complexes, 25,42 which may be attributed to the stronger d acceptor character of the parent benzene than alkylbenzenes.
The molecular structures of 2-M and 3-M were unambiguously established by single crystal X-ray diffraction and are depicted in Fig. 2. All four complexes are neutral molecules without a counter cation, featuring an inverse-sandwich structure with a m-h 6 ,h 6 coordination mode for the bound arene ligand.A major difference between yttrium and samarium complexes is the latter containing a coordinated THF molecule per samarium, which is probably due to the larger ionic radius of samarium.Moreover, the coordination of THF causes a conformation change: while the two N-Y-N planes are almost orthogonal to each other in 2-Y (84.0(1)°) and 3-Y (85.1(1)°), the two N-Sm-N planes are co-planar in 2-Sm and 3-Sm as enforced by a crystallographic inversion centre.This also results in a difference in the dihedral angle between the N-M-N planes and the bound arene.While the N-Y-N planes are nearly perpendicular to the bound arene plane (2-Y 88.6(1)-89.6(1)°;3-Y 88.5(1)-89.8(1)°),the dihedral angle between the N-Sm-N plane and the bound arene signicantly deviates from orthogonality (2-Sm 57.12(4)°; 3-Sm 60.95(8)°).Similar conformation change from perpendicular to co-planar accompanied with THF coordination has previously been observed in the series of [(NN TBS )Th(THF) n ] 2 (m-h 6 ,h 6 -arene) (n = 0 or 1). 42Despite the difference in the conformation, the arrangement of M-C cent -M (C cent = the ring centroid of the bound arene) is close to linearity (Y-C cent -Y 178.70(2)-179.58(3)°,Sm-C cent -Sm 180.00°), which is the same as [(NN TBS )Th(THF) n ] 2 (m-h 6 ,h 6 -arene) and indicative of similar electronic structures for these inversesandwich f-block metal arene complexes.
The key structural parameters are summarized in Table 1.The average Y-N distances in 2-Y (2.378(5) Å) and 3-Y (2.398(3) Å) are ca.0.1 Å longer than that in 1-Y (2.272(5) Å).For samarium complexes, the average Sm-N distances in 2-Sm (2.510(2) Å) and 3-Sm (2.532(2) Å) are elongated by ca.0.2 Å compared with that of 1-Sm (2.311(4) Å).Such elongation between the metal and the nitrogen donors of the ancillary ligand has previously been observed in inverse-sandwich f-block metal arene complexes supported by ferrocene diamide ligands 20,22,28,42 as well as inverse-sandwich uranium benzene complexes supported by the BDI 0 ligand, 36 which was attributed to the weakening of M-N bonds due to the strong bonding interaction between the metal and the bound arene.In addition, the larger elongation in 2-Sm and 3-Sm may also be caused by the additional THF coordination, since the increase of coordination number leads to a larger ionic radius for Sm 3+ (the ionic radius increases about 0.06 Å when the coordination number increases one). 60The average Y-C cent distances of 2.000(1) (2-Y) and 2.011(1) Å (3-Y) are appreciably shorter than that of [(NN TBS )Y] 2 (m-h 6 ,h 6 20 indicating stronger metal-arene interactions in neutral inverse-sandwich rare-earth metal arene complexes than anionic ones.In addition, the average Sm-C cent distances of 2.095(1) (2-Sm) and 2.109(1) Å (3-Sm) are appreciably shorter than that of [(NN TBS )Sm] 2 (m-h 6 ,h 6 -C 6 H 5 Ph)[K(toluene)] 2 (2.196(7) Å) and [K(18-crown-6)(THF) 2 ] 2 [[(NN TBS )Sm] 2 (m-h 6 ,h 6 -C 6 H 5 Ph)] (2.146(8) Å), 22 consistent with stronger Sm-arene interactions in the neutral compounds as well as the assignment of the +3 oxidation state for samarium.The C-C distances within the bound ring have been correlated with the extent of reduction of the bound arene as well as the distribution of negative charges. 12,13,61The average C-C distances of 2-M (Y 1.452(8) Å; Sm 1.453(4) Å) and 3-M (Y 1.455(7) Å; Sm 1.469(5) Å) are signicantly longer than that of free benzene (1.39 Å), 62 42 in line with quadruply reduced bridging arene ligands in 2-M and 3-M.Moreover, the C-C distances of the bound ring are all within a relatively narrow range for 2-M and 3-M (Table 1, entry 5 & Fig. 3), suggesting the negative charges are evenly distributed among six carbons of the bound ring.This is in accord with previously reported inverse-sandwich f-block metal arene complexes with a [M 3/4+ -(arene) 4− -M 3/4+ ] electronic structure, [20][21][22]42 but different from the benzene 1,4dianion with "two-short, four-long C-C distances" in several rareearth and alkaline-earth metal benzene complexes. 1249-51,61  Notably, the bound rings are not fully planar in 2-M and 3-M, as illustrated in Fig. 3.For 2-Y and 3-Y, the bound rings exhibit a boat conformation with dihedral angles of 9.1°/7.5°(2-Y)and 11.1°/11.6°(3-Y)(dened by the plane of the bridgehead carbon atom and the two neighbouring carbon atoms with the average plane of other four carbon atoms). In contrast for 2-Sm and 3-Sm, the bound rings adopt a chair conformation with dihedral angles of 11.0°/11.0°(2-Sm)and 4.3°/4.3°(3-Sm)(dened by the average plane of the central four carbon atoms with the plane of the carbon atoms above (or below) the central plane and the neighbouring two carbon atoms).The distortion of the bound rings from planarity has been previously observed in the inverse-sandwich rare-earth metal biphenyl complexes [20][21][22] and thorium arene complexes.42 Taking into account the above mentioned structural features, we consider that it is appropriate to assign a [M 3+ -(arene) 4− -M 3+ ] electronic structure for 2-M and 3-M.To the best of our knowledge, 2-Y and 2-Sm represent the rst rare-earth metal complexes containing the parent benzene tetraanion.

Spectroscopic and magnetic characterization
In order to further investigate the electronic structures of these inverse-sandwich rare-earth metal arene complexes, we performed the spectroscopic and magnetic measurements.The UV-vis-NIR absorption spectra of 2-Y, 3-Y, 2-Sm and 3-Sm were recorded in n-pentane (Fig. 4 and S24-S27 †).All four complexes share similar features: (1) sharp intense peaks in the UV region (around 290 nm and 360-370 nm, 3 > 10 4 M −1 cm −1 ) likely originate from the BDI ligand, since they are also present in 1-M (Fig. S22 and S23 †); (2) broad bands covering the entire visible region (3 ∼1000-5000 M −1 cm −1 ) may be attributed to ligand to metal charge transfer, which are similar to the inverse-sandwich samarium biphenyl complexes, 22 but bathochromically shied compared to the inverse-sandwich thorium arene complexes. 42he room temperature solution magnetic moment of 2-Sm was determined by the Evans method 63 to be 0.49 emu mol −1 K and 1.98m B (1.40m B per samarium).This value is comparable to the room temperature solution magnetic moments of [(NN TBS ) Sm] 2 (m-h 6 ,h 6

Density functional theory calculations
We performed DFT calculations to further investigate the electronic structures and bonding interactions of 2-M and 3-M.The DFT calculations were conducted on simplied model complexes with the 3-pentyl groups truncated to methyl groups.The geometry optimization of 2-Y and 3-Y was done with a closed-shell ground state, and the calculated and crystal structures match well (Tables S3 and S4 †).For 2-Sm and 3-Sm, two possible states, 11 A and 13 A, were considered.The optimized structures of 2-Sm and 3-Sm with the 11 A state match well with the crystal structures (Tables S5 and S6 †).For instance, the calculated average Sm-C and Sm-C cent distances of 2-Sm ( 11 A) are 2.53 and 2.07 Å, respectively, close to the experimental values (2.565(2) and 2.095(1) Å).However, the optimized structures of 2-Sm and 3-Sm with the 13 A state show signicant elongation for Sm-C and Sm-C cent distances compared to the experimental values (Tables S5 and S6    lower in energy than the corresponding b orbitals and have some additional 4f characters, which is probably due to energy degeneracy between d bonding orbitals and 4f orbitals.For comparison, the a orbitals in [[(NN TBS )Sm] 2 (m-h 6 ,h 6 -C 6 H 5 Ph)] 2− have negligible 4f characters. 22The difference in 4f participation in the a orbitals of HOMO and HOMO-1 between 2-Sm and [[(NN TBS )Sm] 2 (m-h 6 ,h 6 -C 6 H 5 Ph)] 2− may be attributed to the lower energy level of the p* orbitals of biphenyl than that of benzene, which breaks the energy degeneracy between 4f orbitals and d bonding orbitals.While 3-M mostly resembles 2-M in d bonding interactions, the lower symmetry of p-xylene results in larger energy difference between HOMO and HOMO-1 (Tables S7 and S8 †), which may explain the lower stability of 3-M compared to 2-M.
We also performed population analysis on 2-M and 3-M (Tables S9

Reactivity studies
The unique [M 3+ -(C 6 H 6 ) 4− -M 3+ ] electronic structure and the strong metal-arene d interactions prompted us to explore their potential as multielectron reductants.In addition, the better solubility of neutral compounds in non-polar solvents compared to ion-pair complexes may also be advantageous for reactivity study.We chose 2-Sm as the representative to study its reactivity toward unsaturated organic substrates, since the more readily accessible +2 oxidation state of samarium may play a role in redox reactivity.Treatment of 2-Sm with 2 equiv. of cyclooctatetraene (COT) quantitatively yielded a mononuclear Sm(III) product (BDI)Sm(h 8 -C 8 H 8 ) (4-Sm) with an isolated yield of 95% (Scheme 2).The mononuclear structure of 4-Sm was conrmed by X-ray crystallography (Fig. S20 †).In this reaction, two neutral COT molecules are reduced to (COT) 2− with the concomitant formation of neutral benzene.The reaction of 2-Sm with 2 equiv. of 1,4-diphenylbutadiyne quantitatively afforded the rst rare-earth metallacyclopentatriene complex (BDI) Sm(h 4 -C 4 Ph 2 ) (5-Sm) with an isolated yield of 75%, along with the formation of neutral benzene (Scheme 2).The molecular structure of 5-Sm is shown in Fig. 6.The planar structure of the [h 4 -C 4 Ph 2 ] 2− unit precludes the presence of a 1,4-diphenyl-2butyne-1,4-diyl species. 71The C-C distances for the central four consecutive carbon atoms are 1.320 (5)  similar at 2.449(3), 2.446(3), 2.447(3) and 2.491(3) Å, consistent with a h 4 coordination mode.7][78][79] The room temperature magnetic moments of 4-Sm and 5-Sm were determined by the Evans method to be 0.243 emu mol −1 K (1.40m B ) and 0.304 emu mol −1 K (1.56m B ), respectively, in line with the formation of Sm(III) products.Overall, 2-Sm can serve as a four-electron reductant to reduce unsaturated organic substrates.

Conclusions
In summary, we synthesized and characterized the rare-earth metal complexes of the parent benzene tetraanion and neutral inverse-sandwich rare-earth metal arene complexes for the rst time with a bulky BDI ligand.Structural, spectroscopic and magnetic data agree with a [M 3+ -(C 6 H 6 ) 4− -M 3+ ] electronic structure with strong metal-arene interactions.DFT calculation results support the assignment of the [M 3+ -(C 6 H 6 ) 4− -M 3+ ] electronic structure and unveil the strong d bonding interactions between rare-earth metals and the bound arene.Reactivity studies show that the inverse-sandwich samarium benzene complex can act as a four-electron reductant to reduce unsaturated organic substrates.Our results highlight the advantages of bulky monoanionic ligands in stabilizing neutral inverse-sandwich rare-earth metal arene complexes and the potential of these metal arene complexes as multi-electron reservoirs for synthesis and reactivity.

Fig. 1
Fig. 1 Inverse-sandwich metal complexes of bridging arene anions relevant to this work.
C NMR spectra of 3-Y in C 6 D 6 exhibit a triplet at 2.71 ppm (J Y-H = 1.7 Hz), and two triplets at 73.0 (CMe, J Y-C = 6.8 Hz) and 71.1 (CH, J Y-C = 4.0 Hz) ppm, respectively, similar to 2-Y.Notably, the peak of the two methyl groups of the bound p-xylene also shis upeld to around 0.80 ppm, which is likely due to close proximity to the p face of the aryl group of the BDI ligand (Fig. S18 †).For comparison, the 1 H and 13 C NMR signals of the bound ring in [K(18-crown-6)(THF) 2 ] 2 [[(NN TBS )Y] 2 (m-h 6 ,h 6 -C 6 H 5 Ph)] appear at the range of 3.03-3.83ppm and 54.0-78.8ppm (J Y-C = 6.3 Hz for the peak at 54.0 ppm), respectively. 20The similarity of 1 H and 13 C NMR signals of the bound ring for 2-Y and 3-Y with [K(18crown-6)(THF) 2 ] 2 [[(NN TBS )Y] 2 (m-h 6 ,h 6 -C 6 H 5 Ph)] suggests they may share the same electronic structure as [Y 3+ -(arene) 4− -Y 3+ ].

Fig. 2 X
Fig. 2 X-ray crystal structures of 2-Y (a), 2-Sm (b), 3-Y (c) and 3-Sm (d) depicted as 50% probability ellipsoids.Hydrogens are omitted and the aryl substituents are shown as wireframe for clarity.For 2-Y, only one of the crystallographically independent molecules in an unsymmetric unit is shown.

Fig. 3
Fig. 3 The structural features of the bound rings in 2-M and 3-M.
†).Therefore, based on the DFT results, the ground state of 2-Sm and 3-Sm is the11 A state, in line with the [Sm 3+ -(arene) 4− -Sm 3+ ] electronic structure suggested by structural, spectroscopic and magnetic characterization.The highest occupied molecular orbital (HOMO) and HOMO-1 of 2-Y and 2-Sm (both a and b orbitals) are shown in Fig.5(3-Y and 3-Sm mostly resemble 2-Y and 2-Sm, see Fig. S38 and S39 †).Both HOMO and HOMO-1 feature d bonding interactions between the rare-earth metals and the bound arene.For 2-Y, the HOMO and HOMO-1 are nearly degenerate (−3.55 and −3.59 eV) and composed of slightly over 30% yttrium 4d orbitals and around 58% carbon 2p orbitals of the bound ring.The higher contribution from yttrium-based orbitals in d bonding orbitals in 2-Y than that in [[(NN TBS )Y] 2 (m-h 6 ,h 6 -C 6 H 5 Ph)] 2-(ca.20% of Y 4d orbitals) suggests stronger d bonding interactions and higher covalency in the former, which is in line with the shorter Y-C cent distance in 2-Y than [(NN TBS )Y] 2 (m-h 6 ,h 6 -C 6 H 5 Ph)[K(toluene)] 2 and [K(18-crown-6)(THF) 2 ] 2 [[(NN TBS )Y] 2 (mh 6 ,h 6 -C 6 H 5 Ph)].20In fact, the contribution from yttrium-based orbitals in the d bonding orbitals in 2-Y is on par with the contribution from the thorium-based orbitals in the d bonding orbitals in [(NN TBS )Th] 2 (m-h 6 ,h 6 -C 6 H 6 ) (ca. 30%).42Taking into account that both 2-Y and [(NN TBS )Th] 2 (m-h 6 ,h 6 -C 6 H 6 ) are neutral complexes, and [(NN TBS )Y] 2 (m-h 6 ,h 6 -C 6 H 5 Ph) [K(toluene)] 2 is an ion-pair species, these results support the hypothesis that neutral metal arene complexes will have higher stability over anionic ones due to stronger and more covalent metal-arene d interactions.For 2-Sm, the a orbitals are slightly

Fig. 5
Fig. 5 Kohn-Sham orbitals (isovalue 0.04) with composition analysis for the frontier orbitals of 2-Y (a) and 2-Sm ( 11 A) (b).Only the metal, the atoms directly bound to the metal, and all carbon atoms of the bound arene are shown for clarity.Nitrogen in blue, oxygen in red, metal in cyan, and carbon in grey.
-S13 †).The natural population analysis (NPA) charges for the bound benzene (C + H) are −1.87 and −1.56 for 2-Y and 2-Sm, respectively, which are comparable to those of [[(NN TBS ) M] 2 (m-h 6 ,h 6 -C 6 H 5 Ph)] 2− (M = Y, −1.90; M = Sm, −1.56).20,22The NPA charges for yttrium (1.57) and samarium(1.22)are lower than 3, which reects the covalency in d bonding interactions.For 3-M, the NPA charges for the bound ring of p-xylene and the metal are similar to that of 2-M (TableS13†).The calculated spin density on each samarium is 5.15 for both 2-Sm and 3-Sm, consistent with a 4f 5 electronic conguration.In addition, the average Wiberg bond index for the C-C bonds of the bound ring is around 1.12 for 2-M and 3-M, indicating signicant weakening of the p bonds of the arene compared to neutral arene.The average Wiberg bond indexes for M-C bonds ranging from 0.22-0.27for 2-M and 3-M are even larger than that of M-N bonds (0.19-0.23) (TableS14†), suggesting relatively strong metal-arene d interactions.Overall, the population analysis and calculated bond index support the assignment of the [M 3+ -(C 6 H 6 ) 4− -M 3+ ] electronic structure for these neutral inverse-sandwich rare-earth metal arene complexes.
Scheme 2 Reactivity of 2-Sm as a four-electron reductant.

Table 1
Key structural parameters for 2-M and 3-M a Average values.bFor the boat conformation of 2-Y and 3-Y, dened by the plane of the bridgehead carbon atoms and the neighbouring two carbon atoms and the average plane of other four carbon atoms; for the chair conformation of 2-Sm and 3-Sm, dened by the average plane of the central four carbon atoms and the plane dened by the carbon atoms above and below it together with neighbouring two carbon atoms (see Fig.3for illustration).