Reagent for Introducing Base-Stabilized Phosphorus Atoms into

Dec 21, 2017 - Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur 741 246, India. § Anorga...
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A Reagent for Introducing Base-stabilized Phosphorus Atoms into Organic and Inorganic Compounds Subrata Kundu, Soumen Sinhababu, Anna V. Luebben, Totan Mondal, Debasis Koley, Birger Dittrich, and Herbert W. Roesky J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b11977 • Publication Date (Web): 21 Dec 2017 Downloaded from http://pubs.acs.org on December 21, 2017

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Journal of the American Chemical Society

A Reagent for Introducing Base-stabilized Phosphorus Atoms into Organic and Inorganic Compounds Subrata Kundu,1 Soumen Sinhababu,1 Anna V. Luebben,1 Totan Mondal2, Debasis Koley,*2 Birger Dittrich,*3 Herbert W. Roesky*1 1

Universität Göttingen, Institut für Anorganische Chemie, Tammannstrasse 4, D-37077, Göttingen, Germany. Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur 741 246, India. 2

3

Anorganische und Strukturchemie II, Heinrich Heine-Universität Düsseldorf, Gebäude 26.42.01.21, Universitätsstrasse 1, 40225 Düsseldorf, Germany. Supporting Information Placeholder

ABSTRACT: The cyclic alkyl(amino) carbene (cAAC) stabilized monoanionic phosphorus atom in the form of lithium phosphinidene [cAACPLi(THF)2]2 (1) has been isolated as a molecular species and characterized by single crystal X-ray structure analysis. Furthermore the structure and bonding of compound 1 has been investigated by theoretical methods. The utilization of the lithium phosphinidene as a phosphorus transfer reagent for a wide range of organic and inorganic substrates has been investigated. Herein, we report on the preparation of fascinating compounds containing P-C, P-Si, P-Ge and P-P bonds using a single step with a base stabilized phosphorus atom.

The development of a base stabilized phosphorus transfer reagent for the synthesis of mono phospha- organic, inorganic or phosphinidene molecules is of crucial importance in synthetic chemistry. In 1992, Becker et al. reported the synthesis of the first isolable and structurally characterized [PCO]- anion1. Following this landmark discovery, an improved synthesis to access the more stable [NaOCP(dioxane)x] salts have been developed by Grützmacher et al.2 Recently [PCO]- is used as a “P” transfer reagent which generates a phosphorus atom via photochemical or thermal decarbonylation.3 In many cases it has been observed that [PCO]- forms complex heterocycles and cages which limits its function as “P” atom transfer reagent.4 The same group has also reported a few less atom economic reactions, where P7(TMS)3 (TMS = SiMe3) served as a mono phosphorus source.3d Lately Tamm et al. used a N-heterocyclic carbene (NHC) stabilized trimethylsilphosphinidene [IPr:PSiMe3; IPr = :C{N(2,6i Pr2C6H3)CH}2] as a synthon for the preparation of few NHC stabilized phosphinidene-metal complexes, although these studies are limited to transition metals.5 Robinson and coworkers generated a lithium salt of carbene stabilized parent phosphinidene L':PH (L' = :C[{N(2,6-iPr2C6H3)}2CHCLi- (THF)3}] from their carbene stabilized diphosphorus system (IPr2P2) with the reduction of lithium metal.6 Later on Driess et al. have shown that a zwitterionic phosphasilene [LSi:PH; L = CH[(C=CH2)CMe-(NAr)2]; Ar = 2,6-iPr2C6H3)] can transfer the :PH moiety to NHC to form the parent phosphinidene (NHC:PH).7 Furthermore, it is worth mentioning here that Cummins et al. have demonstrated that upon heating, dibenzo-7λ3-phosphanorbornadiene derivatives generate transient phosphinidene intermediates followed by giving a mixture of products.8 Very recently Grützmacher et al. reported the

Lewis acid promoted transfer of phenylphosphinidene from the NHC=PPh adduct to various organic molecules to obtain phosphorus heterocycles.9 It becomes obvious from the literature precedents that no universal phosphorus transfer reagent is known so far, which can transfer a phosphorus atom to organic (aromatic/aliphatic) and inorganic substrates. Therefore the search of new phosphorus transfer reagents remains one of the fascinating chapters of inorganic and organic chemistry. Scheme 1. Synthesis of [cAACPLi(THF)2]2 (1).

Recently NHCs and cAACs received enormous attention for the stabilization of a variety of unstable complexes with lower oxidation state of main group elements.10 Bertrand et al. and few other groups have reported several phosphinidenes which are stabilized by carbenes.11,3d The availability of two lone pairs at the phosphorus atom of carbene stabilized phosphinidene (NHC→PPh) was also demonstrated from the synthesis of the 1:2 adduct of NHC→PPh and BH3 (NHC→PPh·2BH3).12 Till now no structurally characterized alkali-metal phosphinidene is reported. We were interested whether we can stabilize a mono-phosphorus anion by cAAC. We preferred cAAC over NHC for our study due to the stronger σ-donor and π-acceptor properties of cAAC compared to NHC13 and also an anion formation at the phosphorus center was not observed in Robinson’s NHC stabilized parent phosphinidene lithium salt.6 Herein, we report on the synthesis of

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lithium phosphinidene which can be considered as a monoanionic phosphorus atom stabilized by cAAC. We also explored its application as a phosphorus transfer reagent for a wide range of substrates to make various phosphinidene derivatives. A number of them were otherwise not possible to achieve by the conventional route reported in the literature. Previously we have published the synthesis of cAAC stabilized parent phosphinidene (cAACPH).14 The lithiation of cAACPH with MeLi in THF gave the dimer of composition cAACPLi with two THF molecules coordinated to each lithium center (Figure 1). Compound 1 crystallizes in triclinic space group P-1 and the asymmetric unit of 1 consists of two crystallographically unique [cAACPLi(THF)2]2 units. The molecular structure of 1 reveals that each phosphorus atom is bound to one carbene carbon atom and two bridging lithium atoms, which generate a four- membered P2Li2 ring. The average P–Li bond distance found in the P2Li2 core is 2.554(2) Å. The average Li–P–Li bond angle is 82.07°(8) and the average P–Li–P bond angle in the P2Li2 core is 97.93°(7). The four-membered P2Li2 ring is planar. The average P–Ccarbene distance is 1.7042(12) Å, which falls in the range observed for other carbene–phosphinidene systems.11

regarded as the first example of a primary alkyl phosphinidene which is stabilized by cAAC. We were interested in isolating trichloro-silicon and trichloro-germanium phosphinidene, because these species might be useful precursor for the synthesis of low valent Si-P and Ge-P materials. Reactions of SiCl4 and GeCl4 with 0.5 equivalent each of 1 in ether afforded compound cAACPSiCl3 (5) and cAACP-GeCl3 (6) (Scheme 2). Herein a short description of the crystal structure of cAACP-GeCl3 is given because 6 is the first example of a germanium phosphinidene.

Figure 2. Crystal structure of compound 6. Selected experimental values [calculated data at M06-2X/def2-SVP] bond lengths (Å) and angles (deg), C-bound H-atoms are omitted for clarity: P1-C1, 1.7653(12) [1.754]; P1-Ge1, 2.2524(5) [2.279]; C1-N1, 1.3366(14) [1.345]; Ge1-Cl1, 2.1695(6) [2.187]; Ge1-Cl2, 2.1690(6) [2.177]; Ge1-Cl3, 2.1649(6) [2.168]; C1-P1-Ge1, 107.74(4) [108.1].

Figure 1. Crystal structure of compound 1. Selected experimental values [calculated data at M06-2X/def2-SVP] bond lengths (Å) and angles (deg), C-bound H-atoms are omitted for clarity: P1C21, 1.7036(12) [1.706]; P1-Li1, 2.562(2) [2.517]; P1-Li2, 2.563(2) [2.530]; N2-C21, 1.3955(14) [1.390]; Li2-O1, 1.953(2) [1.900]; Li2-O2, 1.993(4) [1.930]; Li1-P1-C21, 122.195(44) [125.1]; Li2-P1-C21, 154.67(6) [151.2]; P1-Li1-P2, 98.063(552) [100.8]; P1-Li2-P2, 97.94(7) [100.8]; Li1-P2-Li2, 82.06(7) [79.2]. The 31P and 7Li NMR spectra of 1 in THF-D8 show singlets at +179.28 and +0.94 ppm respectively. Compound 1 is stable at room temperature in the solid state over one month but decomposes slowly at room temperature in THF/ether solution to give the known cAACP-PcAAC system.15a Phosphinidene chemistry has been attracting a lot of attention in the last few years. Thus, we expected that 1 would be a potential reagent for easy accessing targeted phosphinidene from the reaction of the corresponding halide precursors with 1 and thereby releasing the corresponding lithium halide. The reaction of pentafluoropyridine with 0.5 equivalent of 1 in ether results in a selective substitution at the para position to give compound cAACP-C5F4N (2) in 71% yield (Scheme 2). Pyridine 2phosphinidene (cAACP-C5H4N; 3) and butyl-phosphinidene (cAACP-C4H9; 4) were synthesized from the reaction of 2chloropyridine and butylbromide, respectively with 0.5 equivalent of 1 in ether (Scheme 2). Compounds 2 and 3 represent the first examples of heterocyclic phosphinidene. Compound 4 can be

The molecular structure of 6 is given in Figure 2. The central phosphorus atom is two coordinate to germanium and to the cAAC carbene carbon atom C1. The bond distance between the carbene carbon and the phosphorus atom (C1-P1) is 1.7653(12) Å, which is little longer than literature found CcarbeneP distances11which indicates the presence of less pronounced double bond character between the carbene carbon and the phosphorus atoms. The C1-P1-Ge1 bond angle of 107.74(4)° falls in the typical range for a tetrahedral arrangement, which indicates the presence of two stereo chemically active lone pairs on the phosphorus atom. Scheme 2. Syntheses of the compounds 2-9.

Two fascinating diphosphorus systems with low valent phosphorus atoms, which are stabilized by cAAC or NHC (cAACPPcAAC and NHCP-PNHC), were reported by Robinson et al. and Bertrand et al. respectively.15 In addition to that only a few examples of mixed-valent diphosphorus compounds [(phosphino)phosphinidene] are mentioned due to their difficult synthesis.3b, 16 Herein we report a general procedure for the synthesis of mixed-valent diphosphorus systems. The reaction of 1 with two equivalents of PCl3, CyPCl2 or Ph2PCl gave the three mixed valent (phosphino)phosphinidene compounds cAACP-PCl2 (7),

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Journal of the American Chemical Society cAACP-PCl(cy) (8) and cAACP-PPh2 (9), respectively (Scheme 2). The main attraction of the compounds (7 and 8) is the presence of halide substituents on the phosphorus which can be easily functionalized for the synthesis of more appealing molecules. For a better understanding of the electronic structure and bonding environment, geometry optimization of 1 and 6 was accomplished at the M06-2X/def2-SVP level of theory (for Computational details, see ESI†). Structural optimization of both the singlet and triplet states of 1 and 6 exhibit that the singlet remains the electronic ground state with energy difference (∆ES→T) of 42.0 and 37.3 kcal mol−1, respectively. The energy-minimized geometrical parameters display decent correlation with the X-ray crystallographic data as noticed from alignment and superposition plots of the respective geometries (Figure S1). The formation of 1 from cAACPH and MeLi is highly exergonic with energy value of 178.7 kcal mol-1 suggesting its favorable formation. Interestingly, during the conversion of cAACPH to [cAACPLi(THF)2]2 (1) by reaction with MeLi, the ‘P’ atom changes its formal charge from positive to negative, which is evident from the observed NPA charge values (qP = 0.112 e in cAACPH vs. -0.374 e in 1) (Table S1). NBO population analysis of 1 entails that the P1/P2 are connected with C21/C1 via double bonds with occupancies of 1.971 and 1.949 e respectively. The σ-bond is formed mainly from the sp-hybridized orbital of C21/C1 and sp3 hybridized orbital of P1/P2 atoms, while the π bond is constructed by pure p-orbital of both atoms (Table S2). The C21–P1/C1–P2 σ-bonded electron density is polarized towards C21/C1 end [C21/C1: ~65%], while the π bonded electron density is slightly shifted towards P1/P2 end [P1/P2: ~54%], as pictorially represented by the natural localized molecular orbital plot for C21–P1 bond (Figure 3a). Similar observation gets support from HOMO-2 and HOMO-3 plots (Figure 3b). The N1/N2 atoms donate approximately 65% to form C1–N1/C21–N2 bonds. Additionally, NBO locates two sp hybridized lone pair on P1/P2 atoms with occupancies of 1.784 and 1.716 e respectively (Figure S2). Wiberg bond indices calculated for C21−P1/C1–P2 and C1–N1/C2–N2 bonds are 1.72 and 1.06 respectively, indicating double bond character for the former bonds. The NBO proposed electronic scenario was further investigated by QTAIM calculations.17 The important topological parameters at the (3,-1) bond critical points are tabulated in Table S3. The calculated electron density [ρ(r)] at the BCP of C21−P1/C1–P2 [0.157] and C1–N1/C2–N2 [0.290] bonds together with the respective Laplacian [∇2ρ(r): 0.486 and −0.730] indicate closed-shell interaction in former bond and covalent interaction in later one. Similar to 1, C1 (~65%) atom in 6 has the main contribution towards the formation of C1–P1 σ-bond with occupancies of 1.967 e, while the π bonded electron density is more polarized towards the P1 center (~66%) (Figure 3c). In addition, Ge1 is associated with P1 via the single bond occupancy of 1.908 e. The bond critical parameters clearly suggest the presence of closed-shell type interaction in the C1–P1 [ρ(r)/∇2ρ(r): 0.153/0.318], whereas the covalent interaction is present in the Ge1–P1 bond [ρ(r)/∇2ρ(r): 0.100/-0.096] (Table S3). The calculated Wiberg bond indices for the C1–P1 and Ge1–P1 bonds are 1.43 and 1.03, suggesting a similar trend of double and single bond characters, respectively. Furthermore, we have carried out the EDA calculation to understand the exact bonding nature of the C1–P1 bonds in 1 and 6. EDA serves as a useful tool to obtain a clear picture of the bonding situations.18 In EDA calculations –Dipp substituent at the cAAC moieties was replaced with –Me unit and labelled as 1M and 6M, respectively. The singlet states of two interacting fragments will provide the dative bond, while the interaction between triplet fragments is responsible for electron sharing double bond (Table 1).10d,19 Importantly, the magnitude of ∆Eorb (orbital inter-

actions) between the fragments remains as a characteristics indicator for the most appropriate bonding situation.18

a)

C-P σ-bond Occ (1.971) C (65.6%) – P (34.4%)

C-P π-bond Occ (1.949) C (45.6%) – P (54.4%)

HOMO-2

HOMO-3

C-P σ-bond Occ (1.967) C (65.3%) – P (34.7%)

C-P π-bond Occ (1.905) C (33.2%) – P (66.8%)

b)

c)

Figure 3. a) Shape of some relevant natural bond (NBO) orbitals of 1, b) selected KS-MOs of 1 and c) selected NBOs of 6 at M062X/def2-TZVP//M06-2X/def2-SVP level of theory. Table 1: EDA-NOCV (BP86/TZ2P//M06-2X/def2-SVP) of 1M and 6M.[a] [Fragment definition: 1M : α = [cAACP2Li2(THF)4], β = cAAC and 6M: α' = PGeCl3, β' = cAAC] 1M

6M

α(S)

α(T)

α'(S)

α'(T)

β(S)

β(T)

β'(S)

β'(T) -144.4

∆Eint

-109.2

-135.5

-115.5

∆EPauli

421.8

401.7

346.3

267.9

∆Eel-

-249.1

-224.2

-223.9

-168.3

(46.9%)

(41.7%)

(48.5%)

(40.8%)

-281.9

-313.0

-237.9

-244.0

(53.1%)

(58.3%)

(51.5%)

(59.2%)

stat

[b]

∆Eorb[b] [a]

Energy values are given in kcal mol-1. [b]The values in parentheses give the percentage contribution to the total attractive interactions ∆Eelstat + ∆Eorb. The results collected in Table 1 show that the total orbital interaction term (∆Eorb = -281.9 kcal mol-1) characterizing the singletsinglet fragment combination is lower in magnitude (in absolute values) than triplet-triplet (∆Eorb = -313.0 kcal mol-1) for 1M. It implies that fragments in 1M should be described in terms of dative bonds cAAC P2Li2(THF)4 cAAC. On contrary, in 6M, the orbital terms of both the combinations exhibit comparable values of -237.9 kcal mol-1 for dative and -244.0 kcal mol-1 for electron-sharing bonds. Thus compound 6M can be equally described with a dative bond cAAC PGeCl3 and with a classical double bond of the type cAAC꞊PGeCl3. In conclusion, we report a lithium phosphinidene (1) which is stabilized by cAAC. The lithium phosphinidene can be equally

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considered as a lithium salt of a monoanionic phosphorus atom or the lithium salt of a phosphaalkene. Either way this is the first example of a lithium salt of a phosphorus atom or a phosphaalkene. Quantum chemical calculation suggest that the bonding situation in compound 1 is better represented by closed-shell type interaction cAAC P2Li2(THF)4 cAAC. Moreover, easy access of various novel phosphinidenes has been reported utilizing the lithium phosphinidene as a precursor.

ASSOCIATED CONTENT Supporting Information The cif files of 1 and 6 [CCDC: 1584480 (1), 1584623 (6)] few figures and the details of crystal structure refinements and theoretical investigations. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Herbert W. Roesky ([email protected]) * Birger Dittrich ([email protected]) * Debasis Koley ([email protected])

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT H.W.R. thanks the Deutsche Forschungsgemeinschaft for financial support (RO 224/68-1). T. M. is thankful to the CSIR, India for SRF. D. K. acknowledges CSIR project fund (01(2770)/13/EMR-II) for financial support and Professor Sourav Pal for constant support and encouragement. Dedicated to Professor Dietmar Stalke on the occasion of his 60th birthday.

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