Beyond Takai's Olefination Reagent: Persistent Dehalogenation Emerges in a Chromium(III)‐μ3‐Methylidyne Complex

Abstract Reaction of CHI3 with six equivalents of CrCl2 in THF at low temperatures affords [Cr3Cl3(μ2‐Cl)3(μ3‐CH)(thf)6] as the first isolable high‐yield CrIII μ3‐methylidyne complex. Substitution of the terminal chlorido ligands via salt metathesis with alkali‐metal cyclopentadienides generates isostructural half‐sandwich chromium(III)‐μ3‐methylidynes [CpR 3Cr3(μ2‐Cl)3(μ3‐CH)] (CpR=C5H5, C5Me5, C5H4SiMe3). Side and decomposition products of the Cl/CpR exchange reactions were identified and structurally characterized for [Cr4(μ2‐Cl)4(μ2‐I)2(μ4‐O)(thf)4] and [(η5‐C5H4SiMe3)CrCl(μ2‐Cl)2Li(thf)2]. The Cl/CpR exchange drastically changed the ambient‐temperature effective magnetic moment μ eff from 9.30/9.11 μ B (solution/solid) to 3.63/4.32 μ B (CpR=C5Me5). Reactions of [Cr3Cl3(μ2‐Cl)3(μ3‐CH)(thf)6] with aldehydes and ketones produce intricate mixtures of species through oxy/methylidyne exchange, which were partially identified as radical recombination products through GC/MS analysis and 1H NMR spectroscopy.


Results and Discussion
Formation of Methylidyne Complex [Cr 3 Cl 3 (m 2 -Cl) 3 (m 3 -CH)(thf ) 6 ] (1) in the Reaction of Chromium(II) Chloride with Iodoform TheT akai olefination reagent is routinely generated in situ via a3 :1 mixture of CrCl 2 and CHX 3 (Scheme 1). The original report also mentioned the use of a4:1 ratio in case of bromoform which did not significantly affect the yield and E/ Z ratio of the alkenyl halide product. [17] Therefore we pondered about whether use of excessive CrCl 2 would affect, if at all, the formation of Takai reagent B.S urprisingly,t he reaction of CrCl 2 with CHI 3 in a6:1 molar ratio at À35 8 8Cin THF afforded the red methylidyne complex [Cr 3 Cl 3 (m 2 -Cl) 3 -(m 3 -CH)(thf) 6 ]( 1)i nu pt o7 0% yield (Scheme 1, Figure 1) along with the precipitation of three equivalents of CrCl 2 I-(thf) 3 .
Compound 1 is also accessible via B and addition of another two equivalents of CrCl 2 (Scheme 1). Crystallization from the THF solution at À35 8 8Cy ielded compound 1 as am icrocrystalline solid. Repeated crystallization increased the overall yield, but to the expense of co-crystallizing CrCl 2 I(thf) 3. Crystallization from less concentrated solutions gave red plates suitable for X-ray diffraction (XRD) analysis. Thecrystal structure of 1 shows the known tetrahedral M 3 (m 3 -CH) structural motif,w ith three chromium atoms forming an early equilateral triangle ( Figure 1). Three m 2 -bridging chlorido ligands complement the cluster core,r esembling at runcated cube.O ne terminal chlorido and two THF molecules each complete the slightly distorted octahedral coordination of the Cr III atoms. [19] TheCr À (m 3 -CH) distances in 1 of 2.018(3)/2.019(3)/2.022-(3) appear slightly longer than those in Theopoldsc ompound [Cp 3 Cr 3 (m 2 -Cl) 3 (m 3 -CH)] (A;1 .935(10) and 1.949- (14) ), as are the bridging CrÀCl distances (2.3328(7) to 2.4186(7) versus 2.348(4) to 2.360(4) ). [7] Thea verage Cr-Cr distance of 3.167 is also considerably longer than in A (2.82 ), which has been referred to as an unusually short contact (range for Cr À Cr single bonds:2 .65-2.97 ). [7] Correspondingly,b oth the Cr-Cl-Cr and Cr-C-Cr angles are more flat in 1 (81.88(2)-82.96 (2) The 1 HNMR spectroscopic investigation of 1 in [D 8 ]THF did not reveal any signal for the m 3 -CH proton in the range of À500 to 500 ppm, presumably caused by paramagnetic broadening. [20] Also,a ny distinct m 3 -CH vibration band was not detectable by IR-spectroscopy (ESI, Figure S26). The effective magnetic moment of 1 in dissolved and solid form was determined by the Evans method [21] and SQUID magnetic measurements,r espectively.B oth methods consistently point to ferro-or ferrimagnetic coupling between the individual Cr III centers already at ambient temperature.T he derived values of m eff (Evans method:9 .30 m B ;S QUID: 9.11 m B )are significantly larger than those expected for three uncoupled Cr III centers (6.71 m B ). Notably,t he effective moment of solid 1 is nearly temperature independent down to 2K (Figure 2, Figure S30). Af it of the field-dependent molar magnetization M mol (H) at 2Kwith aBrillouin function (the LandØ g-factor was assumed to be 2.0) yields as pin quantum number of S = 4.45(4) which is in line with aS= 9/2  Figure S17). [32] Figure 2. Temperature-dependent molar magnetic susceptibility c mol (T) (black open symbols;l eft ordinate)a nd effective magnetic moment m eff (T)( red filled symbols;right ordinate) as obtained by SQUID magnetic measurements on crystalline powders of 1 and 3 in applied fields H = 3kOe and 10 kOe, respectively.The c mol (T)d ata were corrected for diamagnetic contributions (1: À4.243 10 À4 emu mol À1 ; 3: À3.071 10 À4 emu mol À1 ;calculated from Pascal's constants), [23] and aspin-only g factor of 2.0 was assumed in the calculation of m eff (T). Note, that 1 contains an additionalT HF solvent molecule per formula unit in the crystal packing. ground state ( Figure S32). As imilar large m eff value of 9.61 was found for the related chromium chlorocarbyne complex [Cr 3 Cl 3 (m 2 -Cl) 3 (m 3 -CCl)(thf) 6 ]a ssuming an S = 9/2 ground state. [8] We note,t hat also the tetranuclear Cr III /Cr II complex [Cp R 4 Cr 4 (m 2 -H) 5 (m 3 -H) 2 ]( Cp R = h 5 -tetramethyl-ethyl-cyclopentadienyl) displays at emperature-independent high m eff of 8.1 m B with S = 3.4(2) which is due to intramolecular ferrimagnetic couplings and in line with aS= 7/2 ground state. [22] Compound 1 was found to be infinitely stable in the solid state,while high purity samples showed minor decomposition in THF at À35 8 8Cover several weeks.Thermal decomposition of 1 in THF occurred rapidly above 40 8 8C( as indicated by ag radual color change from red to yellow). Similarly, progressive decomposition of 1 was observed in non-coordinating solvents like toluene as indicated by the formation of aprecipitate as well as decoloration. Utilization of high-purity reactants is crucial for the successful synthesis of 1,aswatercontaining solvents or oxygen-containing impurities of iodoform or CrCl 2 (99.99 %t race metal basis,a nhydrous CrCl 2 ) led to partially inseparable decomposition/side products,a s evidenced for the serendipitous identification of [Cr 4 (m 2 -Cl) 4   Other Cr II and Cr III species present in reaction mixtures,e ven at À50 8 8C, were not assignable by NMR spectroscopy (distinct signals in the range 10 to 50 ppm), but could be removed by crystallization. The 1 HNMR spectrum  3 (m 3 -CH)] (6,r ight), ellipsoids shown at 50 %p robability,l attice solvent and hydrogen atoms (except for the methylidyne hydrogen atom in 6)are omitted for clarity.S elected interatomic distances/angles are listed in the SupportingInformation (Figures S18/S20/S23). [32] Scheme 2. Synthesis of trimetallic half-sandwich methylidyne complexes;representation of the suggested equilibriumofate complex 8 in solution.

Angewandte Chemie
Research Articles of A measured in [D 8 ]THF at ambient temperature shows abroad singlet at d = 30.27 ppm (in CDCl 3 at d = 31.05 ppm), in agreement with the literature. [7] The3 -equivalent reaction of 1 with LiCp* in THF at À50 8 8Cl ed to an instant color change from dark red to dark green. Crystallization from concentrated toluene/n-hexane mixtures gave dark green crystals of [(h 5 -Cp*) 3 Cr 3 (m 2 -Cl) 3

(m 3 -CH)]
(3)f eaturing as tructural motif similar to 1 (Figure 1) and A.C ompound 3 crystallizes in the trigonal space group R3 and displays alocal symmetry of C 3 with Cr-Cr distances of 2.9103(5) ,slightly longer than in A.The Cr À Cl distances of 2.3416(5) to 2.3615(5) as well as the Cr-C-Cr angles involving the central m 3 -CH moiety (92.71 (11)8 8 [7] and substantially below the effective magnetic moment expected in case of three uncoupled Cr III centers (m eff = 6.71 m B ). Ap ossible explanation may be the establishment of antiferromagnetic interactions causing the observed gradual decrease of m eff for solid 3 upon cooling (Figure 2, Figures S31/S33). [7] As imilar temperature-dependent decrease of the effective magnetic moment upon cooling has been observed earlier in the related complex A and considered as an evidence for antiferromagnetic couplings between the chromium ions. [7] Af urther analogy to A is that reaction mixtures of 3 show am ultitude of paramagnetically shifted proton signals,due to partial reduction and decomposition of complex 1.I dentified side products comprise Cp* 2 Cr (d = À6.2 ppm, [D 8 ]THF) [25] and [Cp*CrCl 2 ] 2 (d = À71.5 ppm, CDCl 3 ). [26] Overall, the synthesis of such half-sandwich complexes is extremely sensitive toward change of reaction conditions and choice of precursor.W hile switching the solvent from THF to toluene led to the isolation of trivalent [Cp*CrCl 2 (thf)] (4), probing the direct synthesis of 3 from [Cp*Cr(m 2 -Cl)] 2 /CHI 3 gave only partial halogenido exchange in [(Cp*Cr) 2 (m 2 -Cl)(m 2 -I)] (5)( synthesis details and crystal structures,s ee Supporting Information).
Salt metathesis of 1 with three equivalents of LiCp' in THF at À50 8 8Cg ave ad ark red/violet solution. After several extraction steps,c rystallization from n-hexane yielded dark purple needles of [(h 5 -Cp') 3 Cr 3 (m 2 -Cl) 3 (m 3 -CH)] (6). The crystal structure of 6 is isostructural to A and 3 (Figure 3), with similar Cr À Cl distances of 2.3243(4) to 2.3519(4) . Not unexpectedly,t he Cr-Cr distances of 2.8192(3) to 2.8363(3) match those in A.T he 1 HNMR spectrum of 6 recorded in [D 8 ]THF at ambient temperature shows two broadened signals at d = 35.35 and 30.39 ppm for the aromatic protons of the Cp' ligands,a nd one sharp singlet at d = 0.49 ppm for the SiCH 3 protons (m eff = 2.70 m B ). Again, the 1 HNMR spectrum of the reaction mixture of 6 shows numerous other signals.T op rove similar reaction/decomposition behavior as found for A and 3,chromocene Cp' 2 Cr (7) was synthesized independently from CrCl 2 and LiCp'.C rys-tallization of 7 from n-hexane produced orange crystals suitable for an X-ray diffraction study ( Figure S24). The 1 HNMR spectrum of 7 measured in [D 8 ]THF at ambient temperature displays signals at d = 322.33 ppm, 249.42 ppm, and À3.32 ppm ( Figure S5). Half-sandwich ate complex [(h 5 -Cp')CrCl(m 2 -Cl) 2 Li(thf) 2 ](8)could be crystallized as another side product from reactions in THF.T he crystal structure of deep blue 8 proved the existence of an intramolecular ate complex ( Figure S25). Compound 8 provides further evidence for the equilibrium theory proposed by Rojas et al. (Scheme 2) and explains the virtually non-existent (nonseparable) amount of metathesis salt in the reaction mixtures of 1 and compounds MCp R . [27] Nearly identical solubilities of the side products clearly counteract the isolation of these compounds.I ng eneral, the proneness of 1 to reduction (and the formation of Cp R 2 Cr II )c an be minimized by performing the reactions at low temperatures in less polar solvents.I n THF,t he reactions proceeded with minor impurities only at À50 8 8C, while in n-hexane and n-pentane acceptable results were obtained at À35 8 8C. Toluene is unsuitable as as olvent, since decomposition of 1 was significant within minutes,even at low temperatures.

Reactivity of Methylidyne Complex 1t oward Aldehydes and Ketones
TheT akai and Takai-Utimoto olefination reagents engage in (E)-selective olefinations of aldehydes,with high functional group tolerance. [28,29] Later, reagent extensions involved the formation of (heteroatom-)substituted cyclopropane products. [30] It was of interest how the methylidyne complex 1 would affect such olefination reactions.D irect NMR-scale reactivity studies turned out difficult to interpret because of paramagnetic shifting and broadening.However,filtration of the reaction mixtures over aluminum oxide facilitated the observation of organic products via 1 HNMR spectroscopy. Thec onversions of benzaldehyde and pivaldehyde with 1 in [D 8 ]THF were complete after 1hat ambient temperature. [31] During this period, the mixtures changed color from deep red to turbid green brown, leading to am ultitude of products as detected by GC/MS analysis (see Figures S34 to S43). Most of these compounds are suggested to be formed by radical recombination, involving transient olefinic radical, as aresult from methylidyne/oxy exchange (Scheme 3). 1D and 2D NMR spectroscopies could not resolve the observed overlapping signals of the product mixtures ( Figures S6 to S12). Fore xample,t he benzaldehyde reaction revealed the forma-Scheme 3. Reactionso f1 with aldehydes and ketones. tion of styrene as the only component identifiable by 1 HNMR spectroscopy.S triking was the observation of trace amounts of (2-iodoethenyl)-benzene by GC/MS analysis.A s the synthesis of 1 produces as ubstantial amount of the iodinated side product CrCl 2 I(thf) 3 (approximate solubility of 1mgmL À1 in THF at 17 8 8C), product contamination with iodine seems inevitable.I CP (Inductively Coupled Plasma) analysis of recrystallized samples of compound 1 indicated apersistent iodine content of roughly 3.3 %.
Other causes for the iodine contamination could be the presence of decomposition product 2 or non-reacted Takai reagent [Cr 2 Cl 2 (m 2 -Cl) 2 (m 2 -CHI)(thf) 4 ]( B), which are both easily soluble in THF and hence difficult to separate via crystallization.
Compound 1 did not show any reactivity toward alkynes HC CSiMe 3 ,H C CPh and PhC CPh, neither alkyne metathesis nor insertion/addition-type reactions.T he latter investigations were carried out in [D 8 ]THF and monitored by 1 HNMR spectroscopy over several hours,a lso by heating to the decomposition temperature of 1.F inally,the reactivity of [(h 5 -Cp*) 3 Cr 3 (m 2 -Cl) 3 (m 3 -CH)] (3)t oward benzaldehyde or benzophenone was examined under similar conditions,b ut the 1 HNMR spectra were inconclusive and only indicated decomposition of the methylidyne complex. Further research is needed to elucidate the reactivity of the organometallic compounds.

Conclusion
Thec hromium(III) m 3 -methylidyne complex [Cr 3 Cl 3 (m 2 -Cl) 3 (m 3 -CH)(thf) 6 ]features the ultimate C-X cleavage product in the dehalogenation sequence of haloforms CHX 3 (here: CrCl 2 /CHI 3 mixture). Thed ecent yields of the methylidyne complex enabled as eries of reactivity studies.T he terminal chlorido ligands can be selectively displaced via salt metathesis with alkali-metal cyclopentadienides to afford rare examples of half-sandwich chromium(III) methylidynes,[(h 5 -Cp R ) 3 Cr 3 (m 2 -Cl) 3 (m 3 -CH)].D espite the paramagnetic nature of Cr III ,t hese compounds exhibit only slightly broadened signals in the 1 HNMR spectra, facilitating the observation of in situ derivatizations.T reatment of [Cr 3 Cl 3 (m 2 -Cl) 3 (m 3 -CH)-(thf) 6 ]with ketones and aldehydes led to olefination, entailing the formation of various products probably formed by radical recombination. Them ethylidyne complexes under study do not promote alkyne metathesis reactions or insertions/additions with acetylenes,b ut display exceptional magnetic behavior. Finally,o ur study underlines the importance of complying with correct CrCl 2 /haloform ratios for efficient olefination reactions.