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Non-Chelated Phosphoniomethylidene Complexes of Scandium and Lutetium Weiqing Mao, Li Xiang, Laurent Maron, Xuebing Leng, and Yaofeng Chen J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on November 27, 2017
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Non-Chelated Phosphoniomethylidene Complexes of Scandium and Lutetium Weiqing Mao,† Li Xiang,† Laurent Maron,*,‡ Xuebing Leng,† Yaofeng Chen*,† †
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, P. R. China ‡
LPCNO, CNRS & INSA, Université Paul Sabatier, 135 Avenue de Rangueil, 31077 Toulouse, France
Supporting Information Placeholder ABSTRACT: The first phosphoniomethylidene complexes of scandium and lutetium, [LLn(CHPPh3)X] (L = [MeC(NDIPP)CHC(NDIPP)Me] ; Ln = Sc, X = Me, I, TfO; Ln = Lu, X = CH2SiMe3), have been synthesized and fully characterized. DFT calculations clearly demonstrate the presence of an allylic Ln, C, P π-type interaction in these complexes. Xray diffraction indicates that the scandium iodide complex has the shortest Sc−C bond length to date (2.044(5) Å). These phosphoniomethylidene complexes readily convert into the ylide complexes, and the reactivity is affected by both X 3+ anion and Ln ion. The reaction of lutetium complex with imine shows a rapid insertion of imine into the Lu−C(alkylidene) bond. DFT calculations indicate, that although the bonding situation seems similar to that of the scandium analog, the strong negative charge at the alkylidene carbon is not sufficiently screened by one hydrogen in the lutetium complex, because of a more ionic bonding, and therefore the reactivity of the lutetium complex is much higher.
Mononuclear transition metal alkylidene (or carbene) complexes have attracted intense attention and been exten1 sively studied in the past decades. Such transition metal complexes are not only of fundamental interest for coordination chemistry, but are also known to have important applications in the area of synthetic chemistry. A great number of mononuclear transition metal alkylidene complexes have been synthesized but not involving rare-earth metal (Sc, Y, and lanthanide metal) ions. Due to HOMO/LUMO orbital 0 energies mismatch between the d rare-earth metal ions and the alkylidene groups, the formation of mononuclear rareearth metal alkylidene complexes is thermodynamically 2 unfavorable. Therefore, the alkylidene groups have a strong tendency to bind more than one rare-earth metal ion or one 3,4 rare-earth metal ion and two other metal ions. The only reported mononuclear rare-earth metal alkylidene complexes are those with four- or three-membered metallacycles (Scheme 1), these rings acts as stabilizing and protecting 5-7 agents of the M−C(alkylidene) bonds. Similarly, phos-
phoniomethylidene complexes of transition metal were de8-10 but no rare-earth metal phosveloped forty years ago, phoniomethylidene complex has been reported to date. Herein, we report the synthesis, bonding analysis (DFT) and thermochemistry of the first rare-earth metal phosphoniomethylidene complexes, which also represent the first examples of mononuclear rare-earth metal alkylidene complexes without chelating-assistant. Scheme 1. Previously Reported Mononuclear Rare-earth Metal Alkylidene Complexes.
Scandium dimethyl complex [LSc(Me)2THF] (L = [MeC(NDIPP)CHC(NDIPP)Me] ) was prepared following the 11 1 31 1 method of Piers and coworkers. H and P{ H} NMR spectral monitoring showed that [LSc(Me)2THF] rapidly reacts with the ylide Ph3P=CH2 in C6D6 to give a scandium phosphoniomethylidene complex [LSc(CHPPh3)Me] (1) with concomitant appearance of CH4 (δ = 0.15 ppm) and free THF at room temperature. Monitoring of the reaction at low temperature in toluene-d8 showed that the ligand exchange (THF and ylide) precedes the C− −H bond activation at the ylide CH2 group. A scaled-up reaction in toluene at room temperature provided complex 1 in 95% yield (Scheme 2). This complex readily reacts with Me3SiI or Me3SiOTf to afford two other phosphoniomethylidene complexes, [LSc(CHPPh3)X] (2, X = I; 3, X = TfO), in high yields. Complexes 1− −3 were fully characterized by NMR spectroscopy, elemental analysis and single crystal X-ray diffraction. The molecular structures of 1 and 3 are shown in Figure 1, while that of 2 is given in the Supporting Information (Figure S1). In these complexes, the scandium center adopts a distorted tetrahedral geometry, being coordinated by two nitrogen atoms of L, one carbon atom of [CHPPh3] and one X anion. In 1, the Sc−C(alkylidene) bond length (2.105(2) Å) is 0.19 Å shorter than the Sc−C(methyl) bond length (2.325(2) Å) after the 2 3 difference between sp and sp carbon atom hybridization (0.03 Å) is applied, indicating the Sc−C(alkylidene) multiple
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bond character. The Sc−C(alkylidene) bond lengths in 2 and 3, 2.044(5) and 2.060(3) Å, are shorter than that in 1, which is in line with that methyl is a stronger electronic donor than iodide and TfO. The Sc−C(alkylidene) bond length in 2 is shorter than those in the reported alkylidene complexes [{MeC(NDIPP)CHC(Me)(NCH2CH2NMe2)}Sc{C(SiMe3)PPh2S} ] (2.1134(18) Å) and [LSc{C(SiMe3)PPh2}THF] (2.089(3) Å) (Scheme 1) and is, to the best of our knowledge, the shortest 12 to date. The P−C(alkylidene) bond lengths in 1− −3 (1.673(2), 1.676(5) and 1.673(3) Å) are the same and close to that in Ph3P=CH2 (1.661(8) Å). Such situation was also observed in uranium and thorium complexes, [U(CHPPh3){N(SiMe3)2}3] 10c 10d (1.679(8) Å), [Th(CHPPh3){N(SiMe3)2}3] (1.680(2) Å) , and [(C5Me5)2Th(CHPPh3)X] (X = Br, 1.684(2) Å; X = Cl, 1.686(6) 10e Å), revealing that M− −C multiple bond formation does not greatly affect the P− −C interaction. The Sc–C–P angles are o o o large, 144.40(13) in 1, 152.5(4) in 2, and 141.1(2) in 3, respec1 13 1 tively. In C6D6 solution, H and C{ H} NMR spectra of 1− −3 have been recorded. The CHP and CHP signals (2.50 and 63.1 ppm for 1, 3.48 and 82.3 ppm for 2, 3.57 and 77.2 ppm for 3) are significantly downshifted in comparison with the CH2P 10d and CH2P signals for Ph3P=CH2 (0.81 and -4.18 ppm). It’s also noteworthy that the CHP and CHP signals for 2 and 3 are downshifted in comparison with those for 1, which correlates with the decrease of the Sc−C(alkylidene) bond lengths observed in the solid state of 2 and 3.
same way, the P–C(alkylidene) bonds are computed to be 1.67 Å in line with the experimental ones (1.66 Å). For all three complexes, the HOMO is a three center ScC(alkylidene)-P π interaction in line with a delocalized double bond character (the HOMO of 1 is shown in Figure 2, those of 2 and 3 are given in the Supporting Information). NBO indicates the presence of 3-centers interaction between Sc-C and P and the Wiberg Bond Indexes of Sc–C bonds (0.73, 0.84 and 0.70 respectively for complexes 1, 2 and 3) are in line with the presence of polarized bonds (the associated P–C WBI are 1.27, 1.22, 1.23). For comparison, the Sc–C/P–C WBI were also 0.73/1.21 and 0.75/1.22 for the 4-member and 36,7 and the member rings alkylidene complexes of Scheme 1 Sc–Me WBI in 1 is 0.57. The polarized nature of the bonds is further demonstrated by a Natural Population Analysis (NPA) and the charge carried out by the alkylidene carbon that are 1.59, -1.58 and -1.56 in all three cases whereas the alkylidene hydrogen is +0.26, +0.27 and +0.26. These charges are in line 13 1 with downshift in the NMR ( C and H) as observed experimentally. Computationally, the calculated chemical shifts are 13 1 64.8, 82.2 and 78.9 ( C NMR) and 2.5, 3.5 and 3.6 ppm ( H NMR) for complexes 1, 2 and 3 respectively. These values are in good agreement with the experimental ones.
Scheme 2. Synthesis of Scandium Complexes 1− −3.
Ph3P=CH2 toluene
N N DIPP Me 3SiOTf DIPP Me3SiI DIPP Sc Sc H H I toluene Me toluene r.t. 0.5 h r. t. 1 h PPh3 PPh3 - SiMe4 - SiMe4 2, 88% yield 1, 95% yield N
DIPP
+ - CH 4
N N DIPP DIPP Sc Me THF Me
r. t. 0.5 h
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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N
N DIPP
Sc
TfO
Figure 2. HOMO of complex 1.
N DIPP H
Scheme 3. Thermal Decomposition of Complexes 1 and 3.
PPh3 3, 84% yield
N DIPP
N Sc
Me
toluene o DIPP 75 C, 4 h H -CH 4
N
N Sc
N
DIPP
N Sc
CH2 PPh2
DIPP H H PPh2
PPh3 (B)
(A)
1
4, 55% yield
N DIPP
N Sc
TfO
N N toluene DIPP DIPP 50 oC, 18 h DIPP Sc CH2 H TfO
PPh3 3
N DIPP TfO
PPh2 (A)
N Sc
DIPP H H PPh2
(B) 5, 87% yield
Figure 1. Molecular structure of complexes 1 and 3 (ball and stick representation). DIPP isopropyl groups and hydrogen atoms (except the H atom of the CHP) were omitted for clarity.
In order to get some insights on the bonding properties of complexes 1− −3, DFT calculations (Sc,Lu: RECP, P:6-311G**,631G** other atoms, B3PW91+D3BJ) were carried out. First of all, all optimized geometries are in excellent agreement with the experimental ones. For instance, the Sc–C(alkylidene) distances are computed to be 2.10 Å (1), 2.05 Å (2) and 2.06 Å (3) in perfect agreement with the experimental ones. In the
In contrast to the remarkable stability of other early1 transition metal phosphoniomethylidene complexes, the H NMR spectral monitoring in C6D6 showed 25% of 1, 14% of 2, and 52% of 3 decompose at room temperature in 24 h. At 50 o C, 82% of 1 decomposes in 24 h while 3 completely decomposes in 14 h. The complex 2 is relatively stable, only 43% of 2 decomposes in 24 h. Scaled-up reactions in toluene of 1 at 75 o o C for 4 h and 3 at 50 C for 18 h provided the ylide complexes 4 and 5, and the structures of both complexes were characterized by single crystal X-ray diffraction (Scheme 3 and 3 2 Figure 3). For 1, one sp C−H bond of L and one sp C−H bond of PPh3 substituent were both activated along with CH4
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elimination and a transformation of Sc-CH to Sc-CH2 func2 tionality; for 3, one sp C−H bond of PPh3 substituent was selectively activated. Compared to the CHP and CHP signals for the alkylidene complexes, the CH2P and CH2P signals for the ylide complexes are much upshifted, 0.56 and 12.4 ppm for 4, 0.99 and 17.6 ppm for 5. As expected, the Sc−C(ylide) bond lengths in 4 and 5, 2.410(4) and 2.323(3) Å, are significantly longer than the Sc−C(alkylidene) bond lengths in 1 and 3, 2.105(2) and 2.060(3) Å. The Sc−C(ylide) bond lengths in 4 and 5 are slightly longer than the Sc−C(alkyl) bond lengths in 1 and 4 (2.325(2) and 2.311(4) Å), and the P−C(ylide) bond lengths in 4 and 5 (1.733(4) and 1.736(2) Å) are intermediate between those of typical single and double bonds. Complexes 4 and 5 are best described as two resonance forms, A and B, as shown in Scheme 3.
Lu−C(alkyl) bond remains. Complexes 6 and 7 were also characterized by single crystal X-ray diffraction (Figure 4). In complex 6, the Lu−C(alkylidene) bond length (2.192(11) Å) is 0.15 Å shorter than the Lu−C(alkyl) bond length (2.368(11) Å) 2 3 after the difference between sp and sp carbon atom hybrido ization (0.03 Å) is applied, and the Lu–C–P angle (143.8(7) ) 1 is large. In the H NMR spectrum of 6, the CHP signal appears at δ = 0.70 ppm, which is significantly upshifted in comparison with that of the scandium complex 1 (2.50 ppm). 13 1 But in the C{ H} NMR spectrum, the chemical shift of the CHP for 6 (δ = 64.7 ppm) is very similar to that for 1 (63.1 ppm). The Lu−C(ylide) and P−C(ylide) bond lengths in 7 are 2.461(3) and 1.731(3) Å, respectively, revealing the contribution from two resonance forms A and B, as shown in Scheme 4. 6 rapidly reacts with benzylidenemethanamine to afford a new complex 8 at room temperature (Scheme 4). Complex 8 was isolated in 76% yield and fully characterized, including the single crystal X-ray diffraction analysis (Figure S2 in the Supporting Information). Attempts to prepare yttrium phos1 phoniomethylidene complex did not succeed. H NMR spectral monitoring showed the reaction of [LY(CH2SiMe3)2(THF)] with Ph3P=CH2 gave the phosphoniomethylidene complex, but it quickly decomposes.
Figure 3. Molecular structures of complexes 4 and 5 (ball and stick representation). DIPP isopropyl groups and hydrogen atoms (except the H atoms on the CH2P) were omitted for clarity. Scheme 4. Synthesis of Lutetium Complexes 6 and Its reactivity. N DIPP
Me3 Si
DIPP
N
N Lu
Me 3Si
hexane r. t. 5 min
N Lu
DIPP
+ Ph3 P=CH2
SiMe3
DIPP DIPP H H Me 3 Si PPh2
N Lu
DIPP DIPP N
CH2
PPh 2
N
Me3 Si
7, 82% yield
(A)
DIPP H PPh3
6, 72% yield
toluene r. t. 2 h -SiMe 4 Ph
N Lu
DIPP
DIPP N
H C P Ph2
Ph (B)
N Lu
-SiMe 4 ne lue h to t . 12 . r
N
N DIPP
(A)
Ph
N
N
N Lu
DIPP
P H Ph2 (B)
8, 76% yield
The synthesis of lutetium phosphoniomethylidene complex was also carried out. Reaction of lutetium dimethyl complex [LLuMe2] with Ph3P=CH2 gave a complicated mixture. But when lutetium dialkyl complex [LLu(CH2SiMe3)2] was used, the reaction clearly produced a phosphoniomethylidene complex [LLu(CHPPh3)(CH2SiMe3)] (6) with concomitant appearance of SiMe4 at room temperature. Complex 6 is instable in solution at room temperature, and quickly decomposes into an ylide complex 7 by abstracting one proton from the phenyl ring. With a 5-minute reaction at room temperature and a careful work-up at low temperature, complex 6 can be isolated in 72% yield (Scheme 4). The ylide complex 7 was prepared in 82% yield when the reaction solution of [LLu(CH2SiMe3)2] with Ph3P=CH2 stand at room temperature for 12 h. It’s noteworthy that in 7 one
Figure 4. Molecular structures of complexes 6 and 7 (ball and stick representation). DIPP isopropyl groups and hydrogen atoms (except the H atom on the CHP or the CH2P) were omitted for clarity.
The same computational approach was used to compute complexes 6 and 7. The Lu−C(alkylidene) bond length in 6 is well reproduced computationally (2.21 Å vs 2.19 Å exp.) as well as the Lu−C(ylide) and P−C(ylide) bond lengths in 7 (2.47 and 1.74 Å vs 2.46 and 1.73 Å exp.). Bonding analysis of 6 is very similar to the one found for the scandium complex 1, that is a polarized double bond between Lu and C(alkylidene) (the HOMO is also a 3 centers π bond and the Lu−C/P−C 1 WBI are 0.73/1.28). Interestingly, the H NMR chemical shift of the CHP group is computed to be +0.9 ppm in line with the experimental observation. This upshifted value is in line with a less positive charge on the hydrogen (+0.12) with respect to the +0.23 value in complex 1, whereas the carbon charge is similar in both complexes (-1.5). Therefore, the negative charge at the carbon in complex 6 is not enough screened by one hydrogen so that the system readily abstracts a hydrogen from the phenyl ring. In summary, the scandium dimethyl complex [LSc(Me)2THF] rapidly reacts with Ph3P=CH2 to give a scandium phosphoniomethylidene complex [LSc(CHPPh3)Me] (1). The later complex readily reacts with Me3SiI or Me3SiOTf to afford two other phosphoniomethylidene complexes,
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[LSc(CHPPh3)X] (2, X = I; 3, X = TfO). X-ray diffraction analysis reveals very short Sc−C bond lengths in these complexes, ranging from 2.044(5)~ 2.105(2) Å. DFT calculations indicate some degree of multiple bonding, where the three center π bond (Sc, C and P). Finally, a lutetium phosphoniomethylidene complex [LLu(CHPPh3)(CH2SiMe3)] (6) was synthetized and analyzed by both X-ray diffraction and DFT calculations. The phosphoniomethylidene group in 3 and 6 readily abstracts one hydrogen from a phenyl ring to yield the ylide complexes; for 6 that bears both a phosphoniomethylidene group and an alkyl group, the phosphoniomethylidene shows higher reactivity than the alkyl. The reaction of 6 with imine also reveals the high reactivity of the phosphoniomethylidene functionality. This is a unique new platform to perform new reactivity of the rare-earth metal alkylidene complexes and to determine the influence of the chelating effect. The reactivity of these phosphoniomethylidene complexes is currently under investigation.
ASSOCIATED CONTENT Supporting Information Experimental and computational details and a zip file containing CIFs for 1−8. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author
[email protected] (Laurent Maron);
[email protected] (Yaofeng Chen)
ORCID Laurent Maron: 0000-0003-2653-8557 Yaofeng Chen: 0000-0003-4664-8980
Notes The authors declare no competing financial interests.
ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant Nos. 21325210, 21421091 and 21732007), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB20000000).
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