A binuclear guanidinate yttrium carbyne complex: unique reactivity toward unsaturated C–N, C–O and C–S bonds

A guanidinato-stabilized binuclear yttrium carbyne complex [(PhCH2)2NC(NC6H3iPr2-2,6)2]2Y2(μ2-Me)(AlMe3)2(μ4-CH) (1) was synthesized via C–H bond activation and its versatile reactivities were investigated. Complex 1 underwent σ-bond metathesis with PhSSPh and nucleophilic addition with PhCN to form the corresponding yttrium thiolate complex 3 and aza–allyl complex 4 respectively. Additionally, the rare yttrium carbide complex 5 was also prepared by treatment of complex 1 with S8. Interestingly, in the reaction with PhNCS, the C 
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Created by potrace 1.16, written by Peter Selinger 2001-2019
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 S double bond was cleaved, followed by C–H bond activation to give the yttrium sulfide complex 7 with a ketenimine dianion ligand. Unexpectedly, the reaction of complex 1 with CO (1 atm) resulted in deoxygenative coupling of CO, to afford mono- or dioxo-yttrium complexes at different temperatures. The mechanism of the possible formation processes of complexes 3 and 9 was elucidated by DFT calculations.

More recently, Cheng et al. discussed a rigid trinuclear scandium carbyne complex [(C 5 Me 5 )Sc(m 2 -Br)] 3 (m 3 -CH) (E) via methyl/halogenido exchange protocol of [(C 5 Me 5 )Sc(Me 2 )] 2 with t BuBr. 8However, a signicant knowledge gap in the area involving binuclear rare-earth-metal alkylidyne complexes was incontrovertible.Additionally, compared to transition-metal alkylidyne complexes, the reactivity exploration of rare-earthmetal alkylidyne complexes remains scarce. 8n the last few decades, investigations on the synthesis and reactivity of rare-earth-metal alkylidene complexes have been documented widely. 9-15Our previous studies on amidinate rareearth-metal carbene derivatives have also shown the unique reactivity of m 3 -CH 2 dianions. 16,17The results indicate that there are signicant ligand effects on the structure and reactivity of these complexes.However, attempts to prepare guanidinate rare-earth-metal methylidene complexes by a similar synthetic process for the synthesis of amidinate methylidene complexes were unsuccessful, and an unexpected binuclear carbyne complex was isolated.Interestingly, this complex shows high and intriguing reactivity under mild conditions, which is distinct from those already reported for other transition-metal carbyne complexes.

Synthesis and structural characterization
The rst binuclear yttrium carbyne complex [(PhCH 2 ) 2 -NC(NC 6 H 3 i Pr 2 -2,6) 2 ] 2 Y 2 (m 2 -Me)(AlMe 3 ) 2 (m 4 -CH) (1) was obtained in 67% yield via a metal-assisted multiple C-H bond activation process when three equivalents of AlMe 3 were added into a stirred toluene solution of the guanidinate yttrium bis(oaminobenzyl) complex 18 at 60 °C for 18 h (Scheme 1).In order to get more insights into the formation process of the yttrium carbyne complex, the same reaction was carried out at room temperature for 6 h which gave a heterobimetallic Y/Al complex (PhCH 2 ) 2 NC(NC 6 H 3 i Pr 2 -2,6) 2 Y(Me)(AlMe 4 ) (2) in 53% yield.It is interesting that complex 2 could turn into complex 1 in high yield in toluene at room temperature for 7 days with the release of CH 4 (ESI Fig. S6 †).This is sufficient to show that complex 2 is an intermediate in the formation of complex 1.
depicted in ESI Fig.
(2.696(6) Å), 23 possibly due to higher coordination numbers of the carbide atoms in latter.The results suggest that the elemental sulfur initiates a bimolecular reaction where the [CH] 3− group was deprotonated by the methyl in the AlMe 3 ligand to form a more stable carbide complex with the release of CH 4 .Additionally, in order to expand the application of complex 1 as a nucleophile in the synthesis of rare-earth derivatives, the reaction of complex 1 with PhNCS was carried out.This reaction yields a yttrium sulde complex [(PhCH 2 ) 2 NC(NC 6 H 3 i Pr 2 -2,6) 2 ] 2 Y 2 (m 3 -h 1 :h 1 :h 2 -CCNPh)(AlMe 3 ) 2 (m 3 -S) (7) with a bridged ketenimine dianion unit (Scheme 3).The single crystal structure of complex 7 reveals that the N1-C1-C2 moiety displays a delocalized electronic framework, in which the C1-N1 methylidene bond with a less sterically demanding C]S bond followed by isomerization.11b The present reaction is also different from previous rare-earth-metal alkylidene complexes in cases where the fragments formed aer the cleavage of the C]S double bond in PhNCS participate in the construction of the free coupled organic molecules. 25arbon monoxide (CO), important C1 feedstock, insertion into metal-carbon bonds and its subsequent transformations were thought to play important roles in organometallic chemistry and industrial catalytic processes (Fischer-Tropsch  synthesis). 26However, the mechanistic details have not been fully understood at the molecular level; in consequence, the reaction of complex 1 with CO (1 atm) was examined at room temperature.Unfortunately, a complicated mixture is detected by using the 1 H NMR spectrum (ESI Fig. S22 †).It is interesting that when the mixture was further heated to 50 °C for 18 h, an unexpected dioxo yttrium complex {[(PhCH  27 To date, the reaction of CO with rare-earth-metal alkylidene complexes generated only rare-earth-metal ketene complexes through nucleophilic addition of an alkylidene group to CO. 11b,15f Although the CO deoxygenative coupling reaction induced by rare-earth-metal phosphinidene and polyhydrido complexes has been well documented, 27,28 the well-dened rare-earth-metal alkyl complex induced deoxygenative coupling reaction of CO was unprecedented.In order to offer more insight into the formation process of complex 8, the same reaction was carried out in toluene at 10 °C for 6 h.To our delight, the yttrium mono- 9) was obtained in 84% isolated yield as colorless crystals (Scheme 4).According to single crystal X-ray diffraction, the C2-C3 bond length (1.065(12) Å) reveals a substantial C^C triple bond character (Fig. 2).And the NMR spectra of 9 show a singlet at d = 1.80 ppm ( 13 C{ 1 H} = 112.4ppm), which is regarded as bridged ethynyl (Fig. S17 and S18 †).It's worth noting that complex 9 could turn into complex 8 almost quantitatively in toluene at 50 °C for 10 h, indicating that complex 9 is an intermediate in the formation of complex 8.[29] DFT calculations The formation of complex 3 from complex 1 was investigated computationally at the DFT level (B3PW91 functional including dispersion).As can be seen from Fig. 3, the dispersion corrections play an important role to account for the reactivity due to the size of the substrate PhSSPh.Indeed, the coordination of the substrate is crucial in this case and implies the rearrangement of the m 2 -Me(Y-Me-Y) to the terminal to allow the coordination of the substrate, which is only favored because of the dispersion forces (stabilization of 27.0 kcal mol −1 ).This is allowed to reach TS5 where the methyl groups attack the sulfur inducing S-S bond breaking with an accessible barrier of 29.0 kcal mol −1 .This barrier is in line with the experimental observation (18 h at 40 °C).Following the intrinsic reaction coordinate, it yields complex 3 whose formation is thermodynamically favorable (−59.4kcal mol −1 ).The reaction cannot occur on the carbyne side as moving the carbyne to the top position is too unstable.
In addition, the formation of complex 9 from complex 1 was also investigated computationally.The computed enthalpy prole (Fig. 4) begins by the coordination of CO to one of the yttrium centers.This coordination is slightly exothermic by 17.2 kcal mol −1 .From this adduct (Int1), the migratory insertion of CO onto the Y-CH bond is observed via TS1.The associated barrier is very low (6.4 kcal mol −1 ) in line with a facile insertion.Following the intrinsic reaction coordinate, it yields the insertion product (Int2) whose formation is exothermic by 10.0 kcal Scheme 4 Reactions of complex 1 with CO.   barrier of 0.2 kcal mol −1 .This reaction yields a vinylidyne complex (Int4), with an extra stabilization of 29.6 kcal mol −1 with respect to Int3.A rotamer of Int4 (Int5) was located on the Potential Energy Surface (PES) with an enthalpy gain of 7.0 kcal mol −1 .In the nal step, the system undergoes C-Al bond formation with concomitant O-Al bond formation via TS4.The associated barrier is 8.6 kcal mol −1 and allows the formation of complex 9.The latter is found to be the most stable species of the whole reaction prole (−83.1 kcal mol −1 ).Th addition of CO on the m 2 -Me(Y-Me-Y) side was also computed and is higher in energy (ESI Fig. S34 †).

Conclusions
In summary, the rst example of a well-dened binuclear carbyne complex 1 has been demonstrated.Complex 1 revealed diverse reactivity towards various substrates, such as nucleophilic addition with benzonitrile and phenyl isothiocyanate to form aza-allyl complex 4 and sulde complex 7, s-bond metathesis towards phenyl disulde to give thiolate complex 3, and insertion of sulde into the Y-C(m 2 -Me) bond along with carbyne deprotonation to the afford carbide complex 5. Unexpectedly, the rare-earth-metal carbyne complex induced deoxygenative coupling reaction of CO was also presented for the rst time.The results have afforded some evidence to suggest that unsaturated substrates are attacked preferentially by the methine rather than the methyl.The observed rich reactivity of this binuclear carbyne complex could provide new insight into the nature of rare-earth-metal carbyne complexes.We are currently probing the reactivity of complex 1 with other molecules.
) 2 ](AlMe 3 )(m 2 -Me) (4) in excellent yield as red blocks (Scheme 2).The X-ray single crystal diffraction study shows the structure of complex 4 in which the trianionic amino-alkenyl [CH(CPhN) 2 ] 3− unit acts as a pentadentate bridge ligand to connect two yttrium atoms in two bonding models (h 5 and h 2 ).In the NMR spectra, the [CH] 3− group appears as a singlet at d = 7.13 ppm ( 13 C: 96.5 ppm), which is shied to a low eld compared to 1 and 3.The average C-N bond length of 1.319(5) Å is longer than the normal C]N double bond distance (1.26 Å), 22 while the C-C distance (av.1.439(6) Å) is between the values observed for a C-C single bond and a C]C double bond, indicating that the three negative charges are delocalized partially in the [CH(CPhN) 2 ] 3− unit.It is interesting that the nucleophilic addition has only taken place on the single [CH] 3− group, and the bridged m 2 -Me(Y-Me-Y) group is inert even if an excess amount of benzonitrile was used under the same conditions, or even heated up to 60 °C for 12 h.The results are clearly different from those of rare-earth-metal methylidene complexes, in which only monoinsertion products [PhC(NC 6 H 3 i Pr 2 -2,6) 2 ] 3 Ln 3 (m 2 -Me) 3 (m 3 -Me)[m-h 1 :h 1 :h 3 -CH 2 C(Ph)N] (Ln = Y, Lu) were yielded.17c Additionally, the present reaction is also different from that of the scandium carbyne complex with PhCN, which afforded the monoaddition product [(C 5 Me 5 )Sc(m 2 -OMe)] 3 [m-h 2 :h 2 :h 3 -CHC(Ph)N] 8 possibly due to the steric hindrance and rigid structure.Amazingly, the reaction of complex 1 with sulfur resulted in carbyne deprotonation to afford the rare carbide complex [single crystal structure of complex 5 reveals that the stabilization of the carbide unit (C 4− ) is achieved by the alkylaluminate ligands.The carbide carbon atom features a unique distorted trigonal bipyramidal geometry, in which three Al atoms lie in the equatorial plane and two Y atoms occupy the axial positions (ESI Fig. S28 †).The bond length of Y-C7(m 5 -C) (av.2.434(3) Å) is shorter than the 2 ]Y(m 3 -O)(AlMe 3 )} 2(8) was isolated in high yield as colorless crystals (Scheme 4).In the NMR spectra of 8, only one singlet at d = −0.16ppm ( 13 C = −2.6 ppm) was ascribed to AlMe 3 (ESI Fig.S18and S19 †).The X-ray single crystal diffraction study shows that two yttrium centers are linked by two oxygen atoms to form a distorted square in the dimeric system (ESI Fig.S31 †), and the Y-O bond lengths (ranging from 2.122(3) to 2.207(2) Å) are comparable to those in

Fig. 3
Fig. 3 Computed (DFT) enthalpy pathway of complex 3 at room temperature with (black) and without (blue) dispersion correction.The Gibbs free energies are given between brackets.

Fig. 4
Fig. 4 Computed (DFT) enthalpy pathway of complex 9 at room temperature with (black) and without (blue) dispersion correction.The Gibbs free energies are given between brackets.
S26. †.Complex 3 is a dimer containing a m 2 -SPh ligand, and the two Y-S bond lengths are 2.799(12) Å and 2.821(13) Å respectively, which are slightly shorter than those in [(C 5 Me 5 ) 2 Y(m-SPh)] 2 , 20 ranging from 2.893(6) to 2.903(6) Å, probably due to the steric hindrance.In the 1 H NMR spectrum, the distinct singlet at d = 2.00 (benzene-d 6 ) ppm is assigned to the [CH] 3− group, which is shied to a high eld compared to 1.The methyl protons in the AlMe 3 unit split into two groups at d = 0.89 and −0.50 ppm at room temperature, while they become one broad peak at d = 0.51 ppm at 60 °C (ESI Fig.-S8 †).19bInterestingly, the 1 H NMR monitoring of this reaction reveals that the chemoselectivity of the reaction of 1 with PhSSPh is independent of the amount of PhSSPh used, and no further reaction was observed even with an excess of PhSSPh.To develop a synthetic methodology of metal N-heterocycle complexes from nitriles, 21 benzonitrile (2 equiv.) was added into the toluene solution of complex 1 at room temperature, obtaining a diinsertion product [(PhCH 2 ) 2 NC(NC 6 H 3 i Pr 2 -2,6) 2 ] 2 Y 2 [m 3 -h 1 :h 2 :h 5 -HC(CPhN