Chromium carbides and cyclopropenylidenes

Carbon tetrabromide can be reduced with CrBr2 in THF to form a dinuclear carbido complex, [CrBr2(thf)2)][CrBr2(thf)3](μ-C), along with formation of [CrBr3(thf)3]. An X-ray diffraction (XRD) study of the pyridine adduct displayed a dinuclear structure bridged by a carbido ligand between 5- and 6-coordinate chromium centers. The carbido complex reacted with two equivalents of aldehydes to form α,β-unsaturated ketones. Treatment of the carbido complex with alkenes resulted in a formal double-cyclopropanation of alkenes by the carbido moiety to afford spiropentanes. Isotope labeling studies using a 13C-enriched carbido complex, [CrBr2(thf)2)][CrBr2(thf)3](μ-13C), identified that the quaternary carbon in the spiropentane framework was delivered by carbide transfer from the carbido complex. Terminal and internal alkynes also reacted with the carbido complex to form cyclopropenylidene complexes. A solid-state structure of the diethylcyclopropenylidene complex, prepared from 3-hexyne, showed a mononuclear cyclopropenylidene chromium(iii) structure.


Results and discussion
Halocarbyne and carbide transfer to aldehydes by the CX 4 -CrX 2 reagent (X ¼ Cl, Br) Falck and Mioskowski have reported a chlorocarbyne transfer reaction to aldehydes by treatment with a mixture of CCl 4 and CrCl 2 in a 1 : 6 ratio. 51 Recently, we have achieved characterization of a trinuclear chromium chlorocarbyne complex, [CrCl(thf) 2 ] 3 (m 3 -CCl)(m-Cl) 3 , obtained from the CCl 4 -CrCl 2 reagent, 52 and the chlorocarbyne complex indeed underwent chlorocarbyne transfer to aldehydes to afford chloroallylic alcohols 1-Cl (Scheme 1a). In addition to formation of 1-Cl, Corey-Fuchs-type homologation products 2, 53 terminal alkynes, were also formed via Cl-abstraction as a formal carbide transfer from the chlorocarbyne complex. A bromocarbyne transfer reaction has also been reported for the bromide analogues CBr 4 and CrBr 2 , 51 which was prepared in situ by reduction of CrBr 3 with LiAlH 4 . We have revisited the classical protocol of preparation of CrBr 2 by treatment of chromium(0) with hydrobromic acid. 54 Although the Cr 2+ ion is still fairly reducing (Cr 3+ /Cr 2+ : À0.424 V vs. SHE), a blue solid of the chromium(II) bromide hydrate [CrBr 2 (H 2 O) 6 ] was readily precipitated out from an aqueous mixture of chromium(0) powder and hydrobromic acid at 0 C. 55 Having a pure solid of anhydrous CrBr 2 in hand, we demonstrated the bromocarbyne transfer to aldehydes by use of the isolated CrBr 2 . To our surprise, in addition to formation of the bromoallylic alcohol 1a-Br and terminal alkyne 2a, an a,bunsaturated ketone 3a was also formed (Scheme 1b). Interestingly, pre-mixing CBr 4 and CrBr 2 in THF prior to treatment with aldehydes resulted in the formation of the a,b-unsaturated ketone 3a as a major product. Unfortunately, formation of unidentiable products by further reactions of 3a with THF 56,57 promoted by some low-valent chromium species lowered the yields of 3a. Combinations of different halogens CCl 4 -CrBr 2 and CBr 4 -CrCl 2 were also attempted, but a mixture of all four products 1a-Cl, 1a-Br, 2a, and 3a was formed in both cases due to halogen-scrambling.

Reduction of CBr 4 with CrBr 2
Given the idea of formation of the a,b-unsaturated ketone 3a, we inquired if the more reactive tetrahalomethane CBr 4 could be further reduced by chromium(II) to form a chromium carbide species rather than a bromocarbyne species. Akin to the reduction of CCl 4 with CrCl 2 recently reported by our group, 52 treatment of CBr 4 with 6 equivalents of CrBr 2 in THF at 0 C produced [CrBr 3 (thf) 3 ] 58 as a brown solid along with a green supernatant. Aer removal of [CrBr 3 (thf) 3 ], a green solid was obtained from the green supernatant and identied as paramagnetic 4-thf in 90% yield (Scheme 2). The green product 4-thf is stable in the solid state at room temperature, but slightly unstable in solution even at low temperature (À78 C) to form [CrBr 3 (thf) 3 ] and some unidentied chromium species. An XRD study of 4-thf revealed a dinuclear structure, namely [CrBr 2 (thf) 2 )][CrBr 2 (thf) 3 ](m-C), where a carbido ligand is bridging between 5-and 6-coordinate chromium centers ( Fig. S35 †). Upon crystallization of 4-thf to obtain a better crystal for the XRD study, complex 4-thf gradually decomposed in solution to form a precipitate of [CrBr 3 (thf) 3 ]. The conclusive structural characterization was performed with a pyridine adduct, 4-py, which was obtained quantitatively by addition of pyridine to 4thf in THF. The molecular structure of 4-py ( Fig. 2a) still maintains a dinuclear chromium moiety bridged by a carbido ligand in a linear fashion (Cr1-C1-Cr2 ¼ 174.5(6) ). The 5coordinate chromium center is best described as distorted square pyramidal (s 5 ¼ 0.29) 59 with the carbido ligand on the apical position along with a short Cr1-C1 bond (1.634(10)Å), while the other chromium center represents a distorted octahedral geometry with a long Cr2-C1 bond (2.035(10)Å). Akin to the reported mononuclear neutral carbides and dinuclear dative bonding carbides, two X-type ligands (halides) and two Ltype ligands (THF, pyridine, PR 3 61 Although the Wiberg bond indices were ca. 2 for the M-C bond in the neutral terminal carbides, an M^C triple bond character has been shown with two pand one s-type bonding orbitals. Molecular orbitals of 4-thf as well as 4-py also depicted two p-bonding interactions of the carbido ligand more delocalized around the 5-coordinate chromium center and a three-center two-electron [Cr-C-Cr] s-interaction (Fig. 2c). Therefore, the canonical structure of the carbido complex 4 could be better described as a dative bonding m-carbide 21-25 than the metallocarbyne character. [27][28][29][30][31] A plausible pathway of formation of the carbido complex 4thf from CBr 4 and CrBr 2 is shown in Scheme 3. Akin to other Cr-C bond formations by reduction of haloalkanes with chromium(II) halides, 62-66 single-electron reductions of C-Br bonds and subsequent radical coupling should proceed to form Cr-C bonds in the CBr 4 -CrBr 2 system. As shown in Schemes 1b and 3, the transient bromocarbyne species could be trapped by aldehydes to afford the bromoallylic alcohols 1-Br. 51 However, as far as we have attempted to isolate the bromocarbyne intermediate, no chromium species other than the carbido complex 4-thf and [CrBr 3 (thf) 3 ] could be obtained by limiting the stoichiometry of CrBr 2 to CBr 4 or controlling the reaction temperature. Although the trinuclear m 3 -chlorocarbyne complex [CrCl(thf) 2 ] 3 (m 3 -CCl) (m-Cl) 3 was obtained from the CCl 4 -CrCl 2 system, 52 a similar trinuclear framework bridged by bromides may be difficult for the bromocarbyne ligand to bridge in a m 3 -fashion due to a larger ionic radius of bromine. Therefore, the transient dinuclear bromocarbyne could be further reduced by CrBr 2 to cleave the last C-Br bond rather than forming a m 3 -carbyne scaffold bridged by bromides, resulting in the formation of the dinuclear carbido complex 4-thf.

Isotope labeling studies
To spectroscopically conrm the carbido ligand delivered from CBr 4 , the isotopologue [CrBr 2 (thf) 2 )][CrBr 2 (thf) 3 ](m-13 C) (4-13 C) was prepared from 13 CBr 4 . An IR spectrum of 4-13 C revealed an absorption of the [Cr-C-Cr] 3-centered vibration at 762 cm À1 , which was red-shied from 788 cm À1 observed in the unlabeled carbide 4-thf (Fig. 2b). Measurement of a 13 C NMR spectrum for 4-13 C was also attempted, but 13 C NMR signals other than solvents could not be located probably due to the paramagnetic feature of the chromium carbide. Akin to the in situ preparation of the CBr 4 -CrBr 2 reagent, the isolated carbido complex 4-thf readily reacted with 2 equivalents of aldehydes (Scheme 4) to form the a,b-unsaturated ketones 3a (22%) and 3b (20%). 55 Accordingly, a 13 C-labeled a,b-unsaturated ketone 3a-13 C was also prepared by use of 4-13 C. The NMR spectrum of 3a-13 C displayed an enriched 13 C NMR signal selectively on the aposition at 130.85 ppm, which was coupled with a-H at 6.10 ppm ( 1 J CH ¼ 157 Hz). The deuterium labeling study was also demonstrated using a deuterated aldehyde, 3-phenylpropanald. The 2 H NMR spectrum of 3a-d displayed deuterium signals on both aand b-positions at 6.15 ppm and 6.88 ppm, respectively, implying that some H-shi event took place from the aldehyde to the a-carbon of the a,b-unsaturated ketone, which was delivered from the carbido ligand. As illustrated in Scheme 5, two plausible pathways to give the a,b-unsaturated ketones 3 could be considered from two canonical structures of 4-thf. Path A shows [2 + 2]-cycloaddition of the rst aldehyde to the Cr^C bond, while Path B represents insertion of the aldehyde into the Cr-C or dative bond. Analyses of the quenched reaction mixture of aldehydes with 4-thf as well as the pre-mixed CBr 4 -CrBr 2 revealed the formation of ketone 5a. The ketone 5a could be formed by hydrolysis of one of the intermediates represented in Path B, implying that the insertion pathway B is the more likely pathway. 67
Spiropentane 70,71 is still a challenging framework to assemble by organic synthetic methods such as reduction of tetrakis(1-haloalkyl)methanes 72,73 due to multiple side-reactions and isomerization. Formal "double-cyclopropanation" by a carbido moiety to alkenes has been reported in the gas phase by use of carbon vapor, 74,75 which was generated by arc discharge, but isomerization of the resulting spiropentanes also proceeded under such harsh conditions. Although the yields of the obtained spiropentanes 6a and 6b are still low, the carbido complex 4-thf underwent "double-cyclopropanation" to alkenes similar to the carbon vapor but without isomerization.

Synthesis of cyclopropenylidene complexes
Reaction of 4-thf with alkynes smoothly proceeded to form a precipitate of [CrBr 2 (thf) 2 ] n . Aer removal of [CrBr 2 (thf) 2 ] n , Scheme 5 Two plausible pathways to afford the a,b-unsaturated ketone 3 by reaction of 4-thf with aldehyde based on the isotope labeling studies.
Formation of a cyclopropenylidene unit by reaction of the ruthenium carbido complex [(Ph 3 P) 2 Cl 2 Ru^C:] with alkyne has been reported by Johnson and co-workers. 84 The ruthenium carbide reacted only with an electron-decient alkyne, dimethylacetylene dicarboxylate (DMAD), to form a cyclopropenylidene complex. In contrast to the ruthenium carbide [(Ph 3 P) 2 Cl 2 Ru^C:], our chromium carbide 4-thf reacted with electron-rich alkynes to form cyclopropenylidene complexes 7a, 7b, and 7c, while no reaction of 4-thf with DMAD in THF was observed along with a gradual decomposition of unreacted 4-thf in solution. Addition of alkenes as well as DMAD to cyclopropenylidene complexes 7a, 7b, and 7c has also been attempted, but identiable products could not be obtained.

Conclusions
This work has shown that the CBr 4 -CrBr 2 reagent possesses bromocarbyne and carbide transfer abilities to aldehydes. In the CBr 4 -CrBr 2 reagent, the rst example of a chromium carbido complex was isolated and structurally characterized. DFT calculations and NBO analyses of the carbido complex proposed a dative bonding m-carbide character. Reactivity studies on carbide transfer of the carbido ligand with aldehydes and alkenes as well as isotope labeling studies have been demonstrated. It is noteworthy that a formal double-cyclopropanation of the carbido complex to alkenes might support the dative bond character of the carbido moiety. In addition, rare examples of cyclopropenylidene complexes have been prepared by treatment of the carbido complex with terminal and internal alkynes. Further investigation of carbide transfer is in progress.

Data availability
All experimental data, NMR spectra, FT-IR spectra, UV-Vis spectra, GC-MS data, crystallographic data, and computational analyses are provided in the ESI. †

Author contributions
K. Irifune prepared compounds and carried out reactions. T. Kurogi performed spectroscopy, crystallography, and computational studies and analyzed the data. T. Kurogi and K. Takai supervised this study and wrote the manuscript. All authors discussed the results and contributed to the peparation of the nal manuscript.

Conflicts of interest
There are no conicts to declare.