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Comparison of Two Phosphinidenes Binding to Silicon(IV)dichloride as well as to Silylene Subrata Kundu, Soumen Sinhababu, Mujahuddin M. Siddiqui, Anna V. Luebben, Birger Dittrich, Tao Yang, Gernot Frenking, and Herbert W. Roesky J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b06230 • Publication Date (Web): 16 Jul 2018 Downloaded from http://pubs.acs.org on July 16, 2018
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Journal of the American Chemical Society
Comparison of Two Phosphinidenes Binding to Silicon(IV)dichloride as well as to Silylene Subrata Kundu,1 Soumen Sinhababu,1 Mujahuddin M. Siddiqui,1 Anna V. Luebben,1 Birger Dittrich,*2 Tao Yang,3 Gernot Frenking,*3 Herbert W. Roesky*1 1
Universität Göttingen, Institut für Anorganische Chemie, Tammannstrasse 4, D-37077, Göttingen, Germany. Anorganische und Strukturchemie II, Heinrich Heine-Universität Düsseldorf, Gebäude 26.42.01.21, Universitätsstrasse 1, 40225 Düsseldorf, Germany. 3 Fachbereich Chemie, Philipps-Universität Marburg, Hans-Meerweinstrasse 4, 35032 Marburg, Germany 2
Supporting Information Placeholder ABSTRACT: The cyclic alkyl(amino) carbene (cAAC) anchored silylene with two phosphinidenes was isolated as (cAAC)Si{P(cAAC)}2 (3) at room temperature, which was synthesized from the reduction of (Cl2)Si{P(cAAC)}2 (2) using two equiv. of KC8. Compound 2 resulted from the reaction of two equiv. of (cAAC)PK (1) with one equiv. of SiCl4. Compounds 2 and 3 are the first examples where two phosphinidenes are binding each to a silicon center characterized by single crystal X-ray structural analysis. Furthermore, the structure and bonding of compounds 2 and 3 have been investigated by theoretical methods for comparison.
Recently phosphinidenes of general composition R-P (R= organic substituent) with two lone pairs of electrons at the phosphorus center have created great research interest. They are considered as ligands comparable to carbenes to function as Lewis bases.1 The existence of the highly reactive R-P was first explored by Ecker and Schmidt using a trapping experiment.2 Later on various structurally characterized stable phosphinidene complexes were isolated in the coordination sphere of transition metals.3 Compared to the phosphinidene complexes of transition metals, the main group element phosphinidene complexes are less explored. For example there exists no report on complexes with two terminal phosphinidene ligands of group 14 elements. However, a few examples of bridging phosphinidene complexes of heavier group 14 elements (M = Si (IV), Ge(II), Sn(II), Pb(II)) have been reported.4 Therefore it is a challenging task to prepare terminal phosphinidene complexes of group 14 elements. The enhanced electron donor properties of the phosphinidene were exploited for the stabilization of silicon compounds. So far no complex with two terminal phosphinidene ligands of silicon has been reported. In recent time, N-heterocyclic carbenes (NHCs) and cyclic alkyl(amino) carbenes (cAACs) exhibited enormous application to stabilize various unstable complexes including lower oxidation state main group elements.5 Arduengo et al. reported on the reaction of NHC with cyclic oligomers of alkyl- or arylphosphinidenes to afford the carbene-phosphinidene adducts.6 Immediately after this report, several other groups have isolated a number of carbene stabilized phosphinidenes by applying various synthetic strategies.7 The availability of the two lone pairs on the phosphorus atom of carbene stabilized phosphinidenes was demonstrated by Arduengo et. al. for the first time by the isolation of the bis(borane) adduct [{(IMes)PPh}(BH3)2] (IMes = 1,3-
bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene).8 Recently other groups established that carbene anchored phosphinidenes can be used as precursors for preparing various phosphinidene metal complexes.9 Tamm et al.[9a] introduced a N-heterocyclic carbene (NHC)-stabilized trimethylsilylphosphinidene [IPr:PSiMe3; IPr = :C{N(2,6-iPr2C6H3)CH}2] as a precursor for the preparation of terminal carbene–phosphinidyne transition metal complexes. Hänisch et al.[9c,d] demonstrated the synthesis of group 13 and group 15 complexes using IMesPK as a precursor. Very recently we reported cAAC stabilized lithium phosphinidene [cAACPLi(THF)2]2 which was used for the synthesis of compounds containing P–C, P–Si, P–Ge, and P–P bonds.[9e,f] In this communication we report for the first time, the successful synthesis of two terminal phosphinidene complexes which are attached to a silicon(IV)dichloride moiety (Cl2)Si{P(cAAC)}2 (2) and as well to a silylene species (cAAC)Si{P(cAAC)}2 (3) which have been characterized by single crystal X-ray structural investigation and multinuclear NMR spectroscopy. Scheme 1. Synthesis of compounds 1-3. The arrows indicate the direction of the lone pairs.
The 1:1 reaction of cAACPH and BzK (benzyl potassium) in toluene at -78 °C resulted in the cAACPK salt formation in a quantitative yield as monitored by 31P NMR. The reaction of SiCl4 with two equiv. of cAACPK in toluene at -78 °C resulted in (Cl2)Si{P(cAAC)}2 (2) with two coordinated phosphinidenes as a yellow solid, depicted in Scheme 1. The two equivalent reduction of 2 with KC8 in the presence of one equiv. of cAAC in THF yields the cAAC anchored silylene with two phosphinidenes (cAAC)Si{P(cAAC)}2 (3), which was isolated as orange crystals from hexane solution at -30 °C in 67 % yield (for details see Supporting Information). However, the corresponding reaction
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without using cAAC resulted in an unstable product. Therefore it is crucial to add one equiv. of cAAC to stabilize the silylene. It is worth mentioning that the stabilization of phosphorus(III) coordinated silylene failed from the reduction of {(Mes)2P}2SiCl2 with two equiv. of KC8 which gave complex {(Mes)2P}3SiK-(THF)3}. The latter was supported by only NMR spectroscopy.10 Compounds 2 and 3 were characterized by 1H, 13C {1H}, 31P {1H}, 29Si {1H} NMR spectroscopy and elemental analysis. In the 29 Si {1H} NMR spectrum of 2 and 3 both display triplets at δ= 28.9 ppm (1JSiP = 169 Hz) and δ= +6.9 ppm (1JSiP = 174 Hz), respectively, thus indicating the coupling with the phosphorus atoms. The large difference of the chemical shift between 2 and 3 in the 29Si {1H} NMR spectrum is due to the change of coordination number of silicon from 4 to 3. The 31P{1H} NMR spectrum of 2 exhibits a singlet at δ = +18.7 ppm, while the 31P {1H} NMR of compound 3 exhibits two close singlets at +23.4 ppm and 14.5 ppm, respectively, due to the stereo chemical difference between the two phosphorus atoms. Compounds 2 and 3 were isolated as air- and moisture sensitive materials but they are stable for more than one month in solution or in the solid state at room temperature under an inert atmosphere. Compound 2 melts at 205 °C whereas compound 3 melts at 189 °C. The molecular structure of 2 is shown in Figure 1. Compound 2 crystallizes in the triclinic P-1 space group with a toluene molecule in the asymmetric unit. The central silicon atom is coordinated by two chlorine atoms and two phosphorus atoms of the phosphinidene ligands in a distorted tetrahedral geometry. The Si-Cl bond lengths of complex 2 are 2.1066(4) and 2.1027(4) Å. The bond between the carbene carbon and the phosphorus atom (C1P1/C21-P2) is 1.7501(10)/ 1.7501(10) Å, which is slightly longer than those reported in the literature for cAACP-PcAAC11a and cAACN-PcAAC11b (1.719 Å in both the cases) thus indicating the presence of less double bond character between the phosphorus and the carbene carbon atoms in compound 2. The Si−P bond distances [Si1-P1 2.2238(4), Si1-P2 2.2228(4)] are shorter than the Si−P single bond length {2.2448(7) and 2.2483(7)} in {(Dipp)2P}2SiCl210 and 2.2572(8) Å in LSi(Cl2)−P(cAAC)9e [L= benzamidinate; PhC(NtBu)2].
Figure 1. Molecular structure of 2; hydrogen atoms are omitted for clarity. Anisotropic displacement parameters are depicted at the 50% probability level. Selected experimental and calculated (M06-2X-D3/def2-SVP in parenthesis) bond lengths [Å] and angles [°]: Si1-P1 2.2238(4) (2.247), Si1-P2 2.2228(4) (2.245), Si1-Cl1 2.1066(4) (2.110), Si1-Cl2 2.1027(4) (2.100), C1-P1 1.7501(10) (1.743), C21-P2 1.7513(16) (1.740), N1-Cl1 1.3534(12) (1.356), N2-C21 1.3535(8) (1.357); P1-Si1-P2 137.507(16) (134.0), Cl1-Si1-Cl2 100.810(6) (103.3), P1-Si1-Cl1 112.194(4) (112.7), P1-Si1-Cl2 96.538(15) (97.7), P2-Si1-Cl1 96.586(15) (96.9), P2-Si1-Cl2 108.390(15) (109.3).
The molecular structure of 3 is shown in Figure 2. Compound 3 crystallizes in the triclinic P-1 space group. The asymmetric unit contains two molecules of 3 and one hexane molecule. The structure of 3 exhibits a P-Si-P backbone, in which the central silicon atom adopts a three coordinate geometry with one carbon and two phosphorus atoms. The average P-Si-P bond angle is 118.632°. The average P-Si bond distance (2.245 Å) of 3 is little longer than in compound 2 (2.223 Å). The bond distance between the carbene carbon and the silicon center (C3-Si3) is 1.778(2) Å, which is little shorter compared to CcAAC-silicon bond distance reported in literature [(cAAC)2Si2H2, 1.8173(18) Å; (cAAC)2Si2Me2, 1.8043 (12) Å; (cAAC)2Si2F2,1.822(1) Å]12. The bond angles at silicon show that this atom is essentially planar tricoordinated.
Figure 2. Structure of one molecule of 3; hydrogen atoms are omitted for clarity. Anisotropic displacement parameters are depicted at the 50% probability level. Selected experimental and calculated (M06-2X-D3/def2-SVP in parenthesis) bond lengths [Å] and angles [°]:P1-Si3 2.2508(8) (2.260), P2-Si3 2.2400(8) (2.241), C3-Si3 1.778(2) (1.770), C1-P1 1.7405(2) (1.736), C2-P2 1.739(2) (1.738), C1-N1 1.366(2) (1.365), C2-N2 1.368(2) (1.367); P1-Si3-P2 118.64(3) (120.5), C3-Si3-P1 116.83(7) (118.0), C3-Si3-P2 123.87(8) (120.4). In order to gain further insights into the electronic structure of 2 and 3, we carried out quantum chemical calculations at the M062X-D3/def2-SVP level of theory.13 Figure 2 shows that the calculated bond lengths and angles agree quite well with the experimental values. Table 1 gives the results of NBO calculations of the two compounds. The silicon atoms in 2 and 3 and the SiP2 fragment 3 carry positive charges, whereas the central Cl2SiP2 fragment and the cAAC ligands in 2 are nearly uncharged. Pbonded cAAC ligands in 3 are also nearly neutral, but the Sibonded cAAC fragment carries a significant negative charge of 0.54 e. This could indicate a different bonding situation of the PcAAC and Si-cAAC bond. The Si-bonded cAAC ligand in 3 has a smaller negative charge than the P-bonded cAAC ligands. The PCcAAC and Si-CcAAC bond orders in 2 and 3 suggest a significant degree of multiply bonding. We analyzed the nature of the P-CcAAC and Si-CcAAC bonds in 2 and 3 with the EDA-NOCV method in order to see if the multiple bonds are better described with electron-sharing double bonds or with dative bonds in terms of σ donation and π back donation as shown in Scheme 1. Previous studies of various types of bonds have shown that the absolute values of the orbital term ∆Eorb,
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Journal of the American Chemical Society which gives the energy change that is associated with the bond formation, is a useful indicator for the type of bond. Those fragments, which gives the smallest ∆Eorb values provide the best description of the bonding interactions.13 Tables S1 and S2 of Supporting information gives the numerical results of the EDANOCV calculations where closed-shell fragments or open-shell (triplet) fragments are used as interacting species. The results for compound 2 (Tables S1 and S2) suggest that the use of closedshell fragments for analyzing the P-cAAC bonds gives slightly smaller ∆Eorb values than the triplet fragments. This holds for both cAAC ligands, which have different conformations with regard to the Cl2SiP2 fragment. But the difference between the two values is not very large.
ical investigations. This material is available free of charge via the Internet at http://pubs.acs.org.
Table 1: Calculated NBO partial charges q and Wiberg bond orders P(A-B) of 2 and 3
ACKNOWLEDGMENT
2
3
2
q
3 P(A-B)
Si
0.95
0.73
Si-P1
0.98
0.98
P1
-0.09
-0.08
Si-P2
0.98
0.98
AUTHOR INFORMATION Corresponding Author * Herbert W. Roesky (
[email protected]) * Birger Dittrich (
[email protected]) * Gernot Frenking (
[email protected])
Notes The authors declare no competing financial interests.
H. W. R. is thankful to the DFG for financial support (RO 224/681). GF acknowledges financial support by the Deutsche Forschungsgemeinschaft. Tao Yang is grateful to the Alexander von Humboldt Foundation for a postdoctoral fellowship. Dedicated to Professor Vadapalli Chandrasekhar on the occasion of his 60th birthday.
P2
-0.06
-0.02
Si-C1
1.38
REFERENCES
SiP2
0.80
0.63
P1-C1
1.48
1.53
Cl2SiP2
-0.02
P2-C2
1.50
1.55
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(cAAC)P1
0.03
-0.04
(cAAC)P2
