Unprecedented Reaction Mode of Phosphorus in Phosphinidene

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An Unprecedented Reaction Mode of phosphorus in phosphinidene rare-earth complex: a joint experimental-theoretical study Haiwen Tian, Jianquan Hong, Kai Wang, Iker del Rosal, Laurent Maron, Xigeng Zhou, and Lixin Zhang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b11032 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 22, 2017

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Journal of the American Chemical Society

An Unprecedented Reaction Mode of Phosphorus in Phosphinidene Rare-Earth Complex: A Joint Experimental-Theoretical Study Haiwen Tian,a Jianquan Hong,a Kai Wang,a Iker del Rosal,c Laurent Maron,*,c Xigeng Zhou*,a,b and Lixin Zhang*,a a

Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200433, People's Republic of China

b c

State Key Laboratory of Organometallic Chemistry, Shanghai 200032, People's Republic of China

LPCNO, CNRS, and INSA, Université Paul Sabatier, 135 Avenue de Rangueil, Toulouse 31077, France

E-mail: [email protected]; [email protected]; [email protected] Supporting Information Placeholder ABSTRACT: Reactions of trinuclear rare-earth metal complexes bearing functionalized phosphinidene ligand 1 2 2 [L3Ln3(µ2-Me)2(µ3-Me)(μ3-ƞ :ƞ :ƞ -PC6H4-o)] (L = i [PhC(NC6H4 Pr2-2,6)2] , Ln = Y (1a), Lu (1b)) with phenylacetylene, CO2, diisopropyl carbodiimide, isocyanide or PhSSPh lead to the formation of a series of phosphoruscontaining products. The reaction of 1 with CS2 yields two novel P-methyl-phosphindole-2,3-dithiolate dianion complexes, revealing an unusual tandem desulfurization/coupling/cyclization reaction mode of CS2. Possible reaction pathway was determined by DFT calculations. This emphasizes the key role of the reduction power of the formal 2P part of the phosphinidene in the C-S bond cleavage.

Metal promoted oxidative coupling between anionic ligands and reductive cleavage of chemical bonds have proven to crucial in organic synthesis. However, most of the rareearth metals have no redox properties reducing their use in bond formation and cleavage. Since the discovery of the first 1 transition metal phosphinidene complexes in 1975, numerous studies have focused on the development of new phos2 phinidene complexes and their reactivity. Although phosphinidene (M=P) complexes are quite common in transition 3 metal chemistry, analogous rare earth complexes remain scarce due to their lower stability because of the orbital energy mismatch between the hard rare-earth metal (Sc, Y, and 4 lanthanide metal) ions and the soft phosphorus atom. It is therefore a challenge to develop suitable synthetic strategies to access these complexes. To date, only very few examples of the reactivity of the binuclear rare-earth-metal phos4c-4j phinidene complexes have been reported. Chen’s and Kiplinger’s groups showed that these complexes can deliver 4c,4d the phosphinidene unit to ketones whereas Mindiola’s found that its Li-ate complex is active and can deliver the phosphinidene unit not only to ketones but also to phospho4e rus dichlorides and metal chlorides. In 2013, Chen’s group developed a four-coordinate bis-scandium bridged phosphinidene complex that can initiate the homologation of 4g CO. Recently, our group have reported the synthesis and reactivity of a trinuclear rare earth metal phosphinidene 4b complex with an µ3-PPh bridge. All these results indicate that the variation of the bonding modes of the phosphinidene ligands to rare earth metals could lead to specific 4b,4j changes in reactivity.

1

2

2

Recently, we synthesized [L3Ln3(µ2-Me)2(µ3-Me)(μ3-ƞ :ƞ :ƞ PC6H4-o)] (Ln = Y(1a), Lu(1b)) by heating [LLn(μ2-Me)]3(μ34b Me)(μ3-PPh) in toluene. The reactivity of complexes 1 with several unsaturated molecules, such as CO2, CS2, i i PrN=C=N Pr, ArNC, and PhC≡CH, was investigated with a special attention on the effect of the coordination mode and on the presence of the chelating aryl anion. In this communication, a quite unique reactivity of the trinuclear rare-earth metal μ2-bridged phosphinidene complexes is reported together with a mechanistic study at the DFT level on CS2 activation reaction. Treatment of 1a with phenylacetylene gave only the μ3methyl-substituted complex 2a (Scheme 1), which significantly differs from phosphirene or phosphiran complexes obtained in reactions of transition metal analogues with 5 alkynes. Complex 2a is isolated as a yellow crystalline solid 31 in 92% yield. The P NMR spectrum of 2a shows a triplet of peaks at δ = 269.27 ppm, which is similar to the starting material, indicating that the phosphinidene group remained untouched and P atom is still coordinated to two Y atoms. The molecular structure of 2a is further confirmed by X-ray crystal diffraction analysis (See Figure S26, in ESI). Further1 more, monitoring the reaction by H NMR shows that the newly formed product 2a did not react with an excess of phenylacetylene at room temperature.

Scheme 1. Reaction of 1a with Phenylacetylene

When compound 1a reacts with an excess of CO2, the insertion of CO2 molecule occurs into all Y-C and Y-P bonds 6 31 yielding complex 3a (Scheme 2). The P NMR spectrum of 3a shows one doublet at δ -14.68 ppm (J = 16.0 Hz) probably due to the weak coupling between the P atom and the C(Ar) 4g,7 atom. In the same way, one equivalent of diisopropylcari i bodiimide ( PrN=C=N Pr) reacts with 1a to give L2Y2(µ231 Me)[(Me2CHN)2CPC6H4-o)] (4a) in 62% yield. In the P NMR spectrum of 4a, an irregular double peak was observed at δ =

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-1.17 (d, JYP = 22.7 Hz) due to the coupling between P and 89 Y. The structures of complexes 3a and 4a are confirmed by X-ray diffraction (see the Supporting Information, Figures S27 and S28). In contrast to 1a, 1b is unreactive to PhC≡CH i i 31 and PrN=C=N Pr. Indeed, the P NMR monitoring indicates that only a small amount of new products was observed even o heating at 120 C for 10 days. This might be attributed to the increased steric bulk caused by the lanthanide contraction effect, i i which would prevent PhC≡CH and PrN=C=N Pr from approaching the small Lu centers, as seen in other analogues.8

spectroscopies, elemental analysis, and X-ray crystallography. 6a is a trinuclear structure with two μ2-SPh and two P(Me)C6H4-2 dianion ligands, while 7a is a dinuclear struc31 ture containing four μ2-SPh ligands (Scheme 3). P NMR spectrum of 6a displays two singlets at δ -36.60 and -47.98 ppm, which may be attributed to the different coordination environment of two P atoms. Recrystallizing 7a in THF gave LY(SPh)2(THF)2 (8a). However, with complex 1b, no reaction took place under the same conditions.

Complex 1a also reacts with 2-isocyano-1,3dimethylbenzene to form the trinuclear complex 5a by the 1,1-insertion of one ArNC into the Y-P bond (Scheme 2). The bond parameters indicate that the resulting PCN moiety acts as both a bridging and side-on chelating group, in which the negative charge is delocalized over the PCN unit (Figure S29). 4,9 The long Y-P bond length (3.189 Ả) and a singlet, without 31 89 31 splitting due to coupling between P and Y, in the P NMR are in line with only weak interaction exists between the two atoms.

Scheme 4. Reaction of Complexes 1 with CS2 L

L Me Ln P

Ln P Ln L

Me Me Me Ln L

2 CS2

Me S

toluene rt, 48 h

Ln L

S S S Ln

L

Me

Ln = Y (9a), 88% Lu (9b), 87%

Scheme 2. Reactions of 1a with CO2, Carbodiimide and ArNC L L Y

i

Pr

N

N

Me

i Pr i

PrN=C=NiPr

C Me Y

r.t., 84 h, toluene

P

L

OO

CO2 (ex)

1a

O

Y L

O

O O

O O

r.t., 24 h toluene

Me

O

O P

r.t., 1 h, toluene

NC

4a, 62% + Unidentified products

Y

O

Y L

O Me

3a, 91% L Y Me C

N

P Me

Y L

5a, 90% Y L

Me

Scheme 3. Reaction of 1a with PhSSPh L L

Y

Y P

Me

2 L

Me Y

Y Me

L

4 PhSSPh

PhS Me P

toluene, r.t.

L Y

Ph Ph S S

S Ph

Y P

1a 6a, 40%

L

Y L

+ 3/2 L Y S

Me

S Ph Ph 7a, 41% THF LY(SPh)2(THF)2 (8a)

The reaction of complex 1a with two equivalents of PhSSPh yields complexes 6a and 7a. The formation of 6a involves a rare coupling of the phosphinidene dianion with one methyl 31 1 ligand concomitant with ligand redistribution. The P and H NMR monitoring reveal that the chemoselectivity of the reaction of 1a with PhSSPh is independent on the amount of PhSSPh used. When one equiv. of PhSSPh is used, only formation of 6a and 7a as well as some starting material 1a was observed. 6a and 7a do not further react even with an excess of PhSSPh. Complexes 6a and 7a were characterized by NMR

Figure 1. Molecule structures of 9. Isopropyl groups on benzene rings and hydrogen atoms are omitted for clarity. Selected bond lengths (Ả): 9a (Ln = Y): Y1-S1 2.758(2), Y-S2 (av.) 2.669(9), Y-S3(av.) 2.772(2), Y-S4(av.) 2.790(7), C2-S3 1.786(9), C3-P1 1.800(1), C4-C5 1.348(1), C4-S1 1.751(9), C4-P1 1.841(9), C5-C6 1.512(1), C5-S4 1.755(8). 9b (Ln = Lu): Lu1-S1 2.691(2), Lu-S2 (av.) 2.625(2), Lu-S3(av.) 2.730(2), Lu-S4(av.) 2.708(2), C2-S3 1.772(8), C3-P1 1.849(1), C4-C5 1.367(1), C4-S1 1.719(9), C4-P1 1.842(8), C5-C6 1.480(1), C5-S4 1.772(8). The metal-mediated activation of heteroallenes is an area 11,12 of growing interest. Apart from CO2, CS2 is also an important carbon source. CS2 is usually used as a model for CO2 and carbonyl sulfide (COS) because it is more reactive, often displays similar binding modes as the two other het13 eroallenes, and is easily handled as a liquid. Up to now, various binding modes of CS2 to one or more metal centers 14 have been identified. Despite tremendous advances in insertions of CS2 into various metal-ligand bonds and CS2 as a 15 source of thiocarbonyl, examples of fragmenta16 tion/recombination reactions of CS2 are rare. Unlike transition-metal and other rare-earth metal bridged phos4b,4g,4j,17 phinidene complexes, the treatment of complexes 1

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Journal of the American Chemical Society with two equivalent of CS2 in toluene at room temperature afforded the unusual tandem desulfurization/coupling/cyclization products 9a and 9b in excellent yields (Scheme 4). This one-pot sequence of transformations leading to the formations of two C-P bonds and one C-C bond, provides a straightforward method for the construction of 1-methyl-1H-phosphindole-2,3-dithiol rings. Reacting 10 equivalents of CS2 for 3.5 days did not lead to any further 31 reaction. The P NMR spectra of 9a and 9b display a singlet at δ = 3.2 ppm and δ = 5.2 ppm, respectively. The PMe unit shows a sharp singlet at 1.87 ppm for 9a and 1.90 ppm for 9b

Figure

2. 31

Computed

(DFT)

1

in H NMR spectrum in C6D6, respectively. As shown in Figure 1, 9a and 9b are isostructural, and have a trimetallic core with one bridging P-methyl-phosphindole-2,3-dithiolate dianion ligand, which is coordinated to three metal ions in a μ-κ2:κ2-bonding mode through two sulfur atoms (Ln−S bond lengths ranging from 2.758(1) to 2.829(1) Å for yttrium and from 2.690(1) to 2.778(1) Å for lutetium).

Enthalpy

The P NMR monitoring results revealed the presence of two P-containing intermediates when the reaction of 1 with o CS2 was carried out at -25 C. Unfortunately, attempts to isolate these intermediates have not been successful. Therefore, possible reaction mechanisms were computed at the DFT level (B3PW91) in the yttrium case (Figure 2). The incoming CS2 undergoes an outer-sphere insertion onto the YP bond with an accessible enthalpy of activation (20.7 kcal/mol). This is different from the reactivity found for the [4b] phosphine complex where the CS2 is reacting in the inner sphere (this is computed to be higher in energy in this case, see ESI). This yields product B where one of the two former Y-P bonds remains and the second one was replaced by two Y-S interactions. The latter activates a C-S bond that is broken with a barrier of 23.8 kcal/mol to allow the migration of a sulphur atom to the phosphorus (complex C). The formed product is formally a P(IV) one with a formed P=C double bond. This bond easily achieves a [2+2] cycloaddition with the C-C bond of the μ3-phenyl of the former phosphinidene ligand (activation barrier of 11.2 kcal/mol). This cycloaddition leads to a bicyclic intermediate with the six-member ring sharing a C-C bond with a phosphorus containing four-

pathway

at

room

temperature

member ring (complex D). This intermediate is computed to be rather stable with respect to the entrance channel (-32.9 kcal/mol). However, it can react with another incoming CS2 molecule, in the same way as the first CS2 molecule reacted. The outer-sphere P-C coupling occurs with an activation barrier of 14.8 kcal/mol which is similar to the first insertion. This leads again to the formation of a formal P(IV) compound (complex F) with a pendant CS2 molecule, that is in equilibrium with complex D. The formal C=S double bond of the pendant CS2 of F can undergo another [2+2] cycloaddition with the former P=C double bond of the 4 member ring (barrier of 27.2 kcal/mol). This barrier is higher than the previous cycloaddition mainly because a C-S bond breaking is to occur at the same time to allow the addition; this forms an almost linear Y-S-Y moiety. This yields a rather unusual and unstable tricyclic intermediate with two 4 member rings sharing a P-C bond (complex G). The P-C-S-C four-member ring is disrupted (barrier of 31.9 kcal/mol) to allow the thermodynamically favored complex H (-68.0 kcal/mol) , yielding after two easy methyl migrations (barriers of 9.1 and 6.3 kcal/mol) the formation of the highly stable complex 9a.

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In summary, it is demonstrated that trinuclear Lu and Y complexes bearing the μ2-bridged phoshinidene ligand have a versatile reactivity toward unsaturated small molecules such as phenylacetylene, carbodiimide, carbon dioxide, isocyanide, PhSSPh and CS2. Depending on the nature of substrates, a series of derivatives involving ligand substitution, addition or oxidative coupling reactions are prepared. Reaction with CS2 leads to an unprecedented transformation of CS2, forming a cyclic 1-methyl-phosphindole-2,3-dithiolate dianion ligand through an unusual reaction mechanism mainly involving the phosphorus center. In particular, the 2reduction power of the formal P part of the phosphinidene ligand plays a crucial role in the C=S bond cleavage of CS2. These results demonstrate that bonding and reactivity patterns of phosphinidene ligands in trinuclear rare earth complexes can be tuned by an introduction of chelating anionic substituent.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Experimental, crystallographic, and computational details (PDF) Crystallographic data for 2a, 3a, 4a, 5a, 6a, 7a, 8a, 9a and 9b (CIF)

AUTHOR INFORMATION Corresponding Author [email protected]; [email protected]; [email protected]

Notes The authors declare no competing financial interest.

Acknowledgement This work was supported by the National Natural Science Foundation of China (grant nos 21672038, 21372047, 21732007) and 973 program (2015CB856600). The authors acknowledge the HPCs CALcul en Midi-Pyrénées (CALMIPEOS grant 1415).

References (1)

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L L

Me Ln L

L

Me Ln P S

P

2 CS2 S S S Ln

Me

Y 4 PhSSPh

PhS Me P

toluene, r.t.

L Y

Y toluene rt, 48 h L

Me Me

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Y Me

L

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