A new approach to coordination chemistry involving phosphorus-selenium based ligands. Ring opening, deselenation and phosphorus–phosphorus coupling of Woollins’ Reagent

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  A new approach to coordination chemistry involving phosphorus-selenium based ligands. Ring opening, deselenation and phosphorus–phosphorus coupling of Woollins’ Reagent
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  A new approach to coordination chemistry involving phosphorus-seleniumbased ligands. Ring opening, deselenation and phosphorus–phosphorus couplingof Woollins’ Reagent Richard C.S. Wong ⇑ , Mei Lee Ooi Department of Chemistry, University of Malaya, 50603 Kuala Lumpur, Malaysia a r t i c l e i n f o  Article history: Received 19 August 2010Received in revised form 12 November 2010Accepted 18 November 2010Available online 5 December 2010 Keywords: Woollins’ ReagentCongenerRing-openingBond cleavageMechanistic pathwayThermolytic studies a b s t r a c t The facile reaction of [CpCr(CO) 3 ] 2  ( 1 ) with an equivalent of 2,4-bis(phenyl)-1,3-diselenadiphosphetane-2,4-diselenide or Woollins’ Reagent (WR) at ambient temperature gave mainly [CpCr(CO) 2 ] 2 Se ( 3 ) as themain product. A similar reaction with an excess of   1  gave  3  (58%) and  trans -[CpCr(CO) 2 (SePPh)] 2  ( 5 , 25%).However reaction with an equivalent of the triply bonded congener Cp 2 Cr 2 (CO) 4  ( 2 ) at 60   C took 3 h tocomplete and led to the isolation of   trans -[CpCr(CO) 2 (SePPh)] 2  ( 5 , 3%), CpCr(CO) 2 (SeP(H)Ph) ( 4 , 18%) and[CpCr(Se 2 P(O)Ph)] 2  ( 6 , 2%). The ring-opening reaction of WR via an initial homolytic P–Se bond cleavageby CpCr(CO) n   (n  = 2 ( 2A ) or 3 ( 1A )) depicts a new approach to coordination chemistry involving P–Sebased ligands. A mechanistic pathway was proposed according to the evidences obtained from thermol-ysis, NMR and mass spectra studies. All the products of   4 ,  5  and  6  have been structurally characterized bysingle-crystal X-ray diffraction analysis.   2010 Elsevier B.V. All rights reserved. 1. Introduction For the last two decades, mixed pnicogen–chalcogen ligandshave attracted a lot of attention from organometallic chemists.Transition metals especially those at low oxidation states are stabi-lized by the presence of both pnicogen–chalcogen soft donors inthese mixed ligands [1,2]. For example, ligands containingarsenic–sulfur [3–7], phosphorus–sulfur [8–14] and phosphorus– selenium [15–22] have been associated with interesting chemistrydue to their inherent potential liability and strong residual nucle-ophilicity. Mixed ligands especially organo-phosphorus–chalcogenheterocycles such as (  p -methoxyphenyl)-thionophosphine sulfide(LR, Lawesson’s reagent) [23,24] and 2,4-bis(  p -tolylthio)1,3-dithia-2,4-diphosphetane-2,4-disulfide (DR, Davy’s Reagent)[25,26] have proven to be versatile ligands for novel metal-complex formation. Another such ligand is Woollins’ Reagent[27–32], whose reactivity with Group 6 organomolybdenum metalis relatively unexplored. With this background we embarked onthe study of the reactivity of [CpCr(CO) n ] 2  ( n  = 2 or 3) towardsWoollins’ Reagent. 2. Experimental  2.1. General procedures All reactions were carried out using conventional Schlenk tech-niques under an inert atmosphere of argon in a Vacuum Atmo-sphere Dribox equipped with a Model HE 493 Dri-train.  1 H,  13 Cand  31 P NMR spectra were measured on JEOL Lambda and ECA400 MHz spectrometers.  1 H and  13 C chemical shifts were refer-enced to residual C 6 H 6  in C 6 D 6  and  31 P chemical shifts to 85% aque-ous H 3 PO 4  (external standard) for  31 P { 1 H}. IR spectra in Nujolmulls were measured in the range of 4000–400 cm  1 by meansof a Perkin–Elmer 2000 FTIR instrument. Elemental analyses wereperformed by the in-house microanalytical laboratory using a Per-kin–Elmer 2400 Series II CHNS/O System. Mass spectrometric mea-surements, performed by direct injection using electrosprayionization (ESI), were made on an Agilent 6230 LCMS instrument.Electrospray (high resolution) mass spectrometric measurementswere obtained on an Accurate Mass Q-Tof spectrometer. All sol-vents were distilled of sodium/benzophenone under nitrogen priorto use. Silica gel (Merck Kieselgel 60, 35–70 mesh) and Celite (Flu-ka AG) were activated at 140   C overnight before chromatographicuse. 1,3,2,4-Dithiadiphosphetane 2,4-diselenides (Woollins’ Re-agent) was purchased from Sigma–Aldrich. [CpCr(CO) 3 ] 2  was syn-thesized as described by Manning and co-workers [33] fromchromium hexacarbonyl (99% purity from Sigma). 0020-1693/$ - see front matter    2010 Elsevier B.V. All rights reserved.doi:10.1016/j.ica.2010.11.034 ⇑ Corresponding author. Tel.: +60 379674260; fax: +60 379674193. E-mail addresses:  richard@um.edu.my, richardwongcheeseng@gmail.com (R.C.S. Wong).Inorganica Chimica Acta 366 (2011) 350–356 Contents lists available at ScienceDirect Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica   2.2. Reaction of Cp  2 Cr   2 (CO) 6   ( 1 ) with an equivalent of Woollins’ reagent at ambient temperature A deep green suspension of Cp 2 Cr 2 (CO) 6  ( 1 ) (200 mg,0.497 mmol) in toluene (  15 mL) was added WR (264 mg,0.497 mmol) and the mixture stirred at ambient temperature. Adark orange brown suspension was obtained after  ca . 5 min. Thereaction mixture was allowed to continue stirring for 0.5 h. Theresultant dark orange brown solution was filtered through celiteand the filtrate was concentrated to  ca . 3–4 mL before loading ontoa silica gel column (2 cm  9 cm) prepared in  n -hexane. Elutiongave three fractions:(i) A dark purplish orange fraction was eluted with  n -hexane(65 mL) which when concentrated to dryness gave fine darkorange brown crystalline solids of [CpCr(CO) 2 ] 2 Se ( 3 )(70 mg, 0.165 mmol, 33.2% yield) [34,35], identified its  1 HNMR in benzene- d 6  [ d (Cp) 4.36] and TLC against an authen-tic sample [ R f   = 0.64 in 6:3:1  n -hexane/toluene/ether as elu-ent] together with a trace amount of CpCr(CO) 2 (SeP(H)Ph)( 4 ).(ii) A dark orange brown fraction was eluted with toluene(48 mL) which when concentrated to dryness gave a darkbrown oily precipitate (189 mg, 60%). Analyses  via  TLCshowed it consisted of a mixture of [CpCr(CO) 2 ] 2 Se ( 3 ), trans -[CpCr(CO) 2 (SePPh)] 2  ( 5 ) and CpCr(CO) 2 (SeP(H)Ph) ( 4 )in ratio of 8:1:1. Attempts to separate the mixture resultedin failure.(iii) An uncharacterized green fraction was eluted with THF(60 mL) which when concentrated to dryness gave a greenresidue (162 mg).A deep bluish green ring was remained unmoved on top of thecolumn.  2.3. Reaction of excess Cp  2 Cr   2 (CO) 6   ( 1 ) with Woollins’ Reagent at ambient temperature A deep green suspension of Cp 2 Cr 2 (CO) 6  ( 1 ) (101 mg,0.251 mmol) in toluene (  10 mL) was added in 0.125 equivalentof WR (16 mg, 0.031 mmol). The color of the reaction mixturechanged to orange brown immediately. The reaction mixture wasstirred at ambient temperature. After 1 h, no more color changewas observed and the resultant dark orange brown reaction mix-ture was filtered through a sintered-glass funnel. The filtrate wasconcentrated to  ca . 3 mL and loaded onto a silica gel column pre-pared in  n -hexane (1.5 cm  11.5 cm). Three fractions were eluted:(i) A dark orange brown fraction with  n -hexane (10 mL) whichupon evaporation to dryness gave a black crystalline solid  3 (62 mg, 0.146 mmol, 58.2% yield).(ii) A dirty green fraction was eluted with  n -hexane/toluenemixture (4: 1) (18 mL) which upon evaporation to drynessgave green crystalline solids of unreacted Cp 2 Cr 2 (CO) 6  ( 1 )(17 mg, 0.042 mmol, 16.7% recovery).(iii) A greyish brown fraction was eluted with toluene (36 mL)which upon evaporation to dryness gave the dark browncrystalline solids of   trans -[CpCr(CO) 2 (SePPh)] 2  ( 5 ) (45 mg,0.062 mmol, 24.7% yield). Anal. Found:  1 H NMR (benzene- d 6  ):  d  4.01 (s, Cp);  d  7.02–7.72 (m, C 6 H 5 ).  13 CNMR (ben-zene- d 6  ):  d  90.4 (Cp),  d  126.02, 128.89, 129.65, 130.56,138.23 and 138.65 (C 6 H 5 ),  d  249.79, 254.22 (CO). IR:  m (CO)at 1962.47vs, 1951.77vs, 1899.03vs cm  1 ; other peaks at1086.72m, 1063.01m, 1016.07m, 860.22vw, 844.94vw,828.13w, 803.69w, 741.04w, 690.62w, 664.64vw cm  1 (nujol). HR-MS ESI + ( m/z  ): ( 52 Cr,  80 Se): 722.4748[CpCr(CO) 2 (SePPh)] 2 , 691 [Cp 2 Cr 2 (CO) 3 (SePPh) 2 ], 664[Cp 2 Cr 2 (CO) 2 (SePPh) 2 ], 633 [Cp 2 Cr 2 (CO)(SePPh) 2 )], 589[Cp 2 Cr 2 (CO) 2 (SePPh)(SeP)], 533 [Cp 2 Cr 2 (SePPh)(SeP)],502 [Cp 2 Cr 2 (Se 2 PPh)(Se)], 451 [Cp 2 Cr 2 (CO)(SePPh)], 413[CpCr(Se 2 (O)PPh)(O)], 401 [CpCr(Se 2 (O)PPh)], 385[CpCr(Se 2 PPh)],359[CpCr(CO)2(SePPh)],331[CpCr(CO)(SeP-Ph)], 303 [CpCr(SePPh)], 273 [CpCrSe 2 ], 228 [CpCrSeP].  Anal .Calc. for C 26 H 20 Cr 2 O 4 P 2 Se 2 : C, 43.21; H, 2.77; Cr, 14.40; O,8.86; P, 8.59; Se, 22.16. Found: C, 43.37; H, 2.89; Cr, 14.67;O, 8.45; P, 8.62; Se, 22.45%.A deep bluish green ring was remained unmoved on top of thecolumn.  2.4. Reaction of Cp  2 Cr   2 (CO) 4  (  2 ) with Woollins’ reagent at 60   C  A deep green suspension of Cp 2 Cr 2 (CO) 4  ( 2 ) (300 mg,0.867 mmol) in toluene (  15 mL) was added WR (461 mg,0.867 mmol) and stirred at 60   C for 3 h. The resultant dark brown-ish green reaction mixture was concentrated to  ca.  10 mL left andwas filtered through a sintered-glass funnel to remove a deepgreen residue (361 mg). The filtrate was concentrated to  ca . 3 mL left and absorbed onto silica gel (  1 g). The dark brownish greenslurry was evacuated to dryness under  vacuo  and chromato-graphed onto a silica gel column (1.5 cm  9 cm) prepared in  n -hexane. Elution under slight pressure gave four fractions:(i) A greyish brown eluant in  n -hexane–toluene (1:2) (45 mL)when which was concentrated to dryness gave dark browncrystalline solids of   5  (18 mg, 0.022 mmol, 3% yield).(ii) A dark pinkish purple eluant in  n -hexane–toluene (1:1,15 mL) and when concentrated to dryness, gave dark pinkishpurple crystalline solids of CpCr(CO) 2 (SeP(H)Ph) ( 4 ) (55 mg,0.152 mmol, 18% yield). Anal. Found:  1 H NMR (benzene- d 6  ): d  4.26 (s, Cp);  d  5.72 (s, P–H);  d  6.68–7.37 (m, C 6 H 5 );  13 CNMR (benzene- d 6  ):  d  89.53 (Cp),  d  132.71, 132.59, 131.37,129.67, 129.62 and 129.50 (C 6 H 5 ), 249.79, 254.22 (CO).  31 PNMR (benzene- d 6  : proton coupled):  d  45.84, 48.26 (d,  J   = 387.6 Hz). IR:  m (CO) at 1954.93vs, 1942.23vs, 1871.60vs,1853.25sh, and 1847.43sh cm  1 and other peaks at1159.72m, 1110.70m, 1093.53m, 1067.03m, 1055.44m,1014.44m, 921.75m, 912.90m, 846.61m, 823.99m,748.56m, 727.74w, 706.84vw, 690.43m, 685.54m,639.48m, 590.79m, 550.25 m cm  1 (nujol). HR-MS ESI + ( m/z  ): ( 52 Cr,  80 Se): 362.2380 [CpCr(CO) 2 (SeP(H)Ph)], 723[Cp 2 Cr 2 (CO) 4 (SePPh) 2 ], 695 [Cp 2 Cr 2 (CO) 3 (SePPh) 2 ], 666[Cp 2 Cr 2 (CO) 2 (SePPh) 2 ], 633 [Cp 2 Cr 2 (CO)(SePPh) 2 ], 451[Cp 2 Cr 2 (CO)(SePPh)], 413 [CpCr(Se(O)PPh)(O)], 401[CpCr(SePPh)(O)], 362 [CpCr(CO)2(SeP(H)Ph)], 331[CpCr(CO)(SePPh)], 303 [CpCr(SePPh)], 273 [CpCrSe 2 ].  Anal .Calc. for C 13 H 10 CrO 2 PSe: C, 43.09; H, 2.76; Cr, 14.36; O,8.84; P, 8.56; Se, 22.10. Found: C, 43.37; H, 2.58; Cr, 14.39;O, 8.68; P, 8.47; Se, 22.54%.(iii) A dark green eluant in ether (9 mL) which when concen-trated to dryness gave dark brownish green crystalline solidsof Cp 2 Cr 2 (Se 2 P(O)Ph) 2  ( 6 ) (14 mg, 0.018 mmol, 2% yield).Anal. Found:  1 H NMR (toluene- d 8 ):  d  5.04 (s, Cp);  d  6.82–7.08 (m, C 6 H 5 ).  13 C NMR (benzene- d 6  ):  d  126.02 (Cp),  d 129.67, 130.56, 132.46, 138.20 and 138.63 (C 6 H 5 );  31 P NMR (benzene- d 6  ):  d   123.34. I.R:  m  at 1157.67w, 1106.17m,1084.11m, 1058.96m, 1026.21m, 998.64m, 815.55m,741.12w, 728.43sh, 686.71w cm  1 (nujol). HR-MS ESI + ( m/  z  ): ( 52 Cr,  80 Se): 801.5744 [Cp 2 Cr 2 (Se 2 P(O)Ph) 2 ], 413[CpCr(Se 2 (O)PPh)(O)], 401 [CpCr(Se 2 P(O)Ph)], 385[CpCr(Se 2 PPh)], 303 [CpCr(Se 2 P)], 273 [CpCrSe 2 ].  Anal . Calc.for C 20 H 20 Cr 2 O 2 P 2 Se 4 : C, 29.93; H, 2.49; Cr, 12.97; O, 3.99; R.C.S. Wong, M.L. Ooi/Inorganica Chimica Acta 366 (2011) 350–356   351  P, 7.73; Se, 39.90. Found: C, 29.62; H, 2.54; Cr, 12.88; O, 3.96;P, 7.45; Se, 39.46%.(iv) A green eluant in THF (25 mL) which when concentrated todryness gave an uncharacterized deep green crystalline pre-cipitate (361 mg).  2.5. NMR tube reactions The following thermolysis experiments were carried out in tol-uene- d 8  (  0.6 mL) in a 5 mm septum-capped NMR tube at 110   Cand monitored  via  1 H NMR at specified time intervals for 14 h orotherwise stated. The final product composition from each ther-molysis study is tabulated in Table 1.  2.5.1. Thermolysis of CpCr(CO)  2 (SeP(H)Ph) (  4 ) Apurple solutionof  4 (20 mg,0.055 mmol)was thermolyzed for14 h. The resultant dark brown solution consists of   5  (38%) and  6 (11%)togetherwithsomeuncharacterizeddarkbrownprecipitates.  2.5.2. Thermolysis of trans-[CpCr(CO)  2 (SePPh)]  2  ( 5 ) A greyish brown solution of   5  (13 mg, 0.018 mmol) was ther-molyzed for 5 h. The resultant dark green solution consists of   4A (10%) and  6  (70%) together with some uncharacterized dark brownprecipitates.  2.5.3. Thermolysis of Cp  2 Cr   2 (Se  2 P(O)Ph)  2  ( 6  ) A green solution of   6  (22 mg, 0.013 mmol) was thermolyzed for14 h. The resultant dark green solution consists of some uncharac-terized dark brown precipitates.  2.5.4. Co-thermolysis of CpCr(CO)  2 (SeP(H)Ph) (  4 ) and Woollins’ reagent  A purple mixture of CpCr(CO) 2 (SeP(H)Ph) ( 4 ) (20 mg,0.055 mmol) with an equivalent of WR (29 mg, 0.055 mmol) wasthermolyzed for 14 h. The resultant dark brown solution consistsof   4  (19%),  5  (6%) and  6  (31%) together with some uncharacterizeddark brown precipitates.  2.6. Structural studies of   4 ,  5  and  6  Diffraction-quality single crystals were obtained from the solu-tion at ambient temperature. Compound  4  was obtained as darkpurple crystals from  n -hexane/THF at room temperature after 2–3 days. Complexes  5  was obtained as dark brown crystals and  6 as dark brownish green crystals in THF at   28   C after 1 weekand 2 weeks, respectively. The crystals are mounted on quartz fi-bers. Details of crystal parameters, data collection and structurerefinement are tabulated in Table 2. The data were corrected forLorentz and polarization effects with  SMART  suite of programs [36]and for absorption effects with the  SHELXTL   suite of programs [37]. 3. Results and discussion  3.1. Products isolation The facile reaction of [CpCr(CO) 3 ] 2  ( 1 ) with one mole equivalentof WR at ambient temperature completed in 0.5 h had led to theisolation of Cp 2 Cr 2 (CO) 4 Se ( 3 ) as main product together with atrace amount of [CpCr(CO) 2 (SePPh)] 2  ( 5 ), CpCr(CO) 2 {SeP(H)Ph}  Table 1 Time dependent variation of product composition. a Reaction Products (% yield) 4A 4 5 6 Thermolysis of   4 /14 h – – 38 11Thermolysis of   5 /5 h 10 – – 70Co-thermolysis of   4  and Woollins’ reagent/14 h – 19 6 31 a Product yields obtained by integration of Cp resonances in  1 H NMR spectrum of product mixture.  Table 2 Data collection and processing parameters of   trans -[CpCr(CO) 2 (SePPh)] 2  ( 5 ), CpCr(CO) 2 (SeP(H)Ph) ( 4 ) and Cp 2 Cr 2 (Se 2 P(O)Ph) 2  (6 ) . Complexes  trans -[CpCr(CO) 2 (SePPh)] 2  ( 5 ) CpCr(CO) 2 {SeP(H)Ph} ( 4 ) [CpCr(Se 2 P(O)Ph)] 2  ( 6 )Empirical formula C 26 H 20 Cr 2 O 4 P 2 Se 2  C 13 H 10 CrO 2 PSe C 22 H 20 Cr 2 O 2 P 2 Se 4 Formula weight 720.28 360.14 399.08 T   (K) 223(2) 243(2) 223 (2) k  (Å) 0.71073 0.71073 0.71073Crystal system monoclinic monoclinic monoclinicSpace group P2(1)/c  P  2 1 / n P  2 1 / nUnit cell dimensionsa  (Å) 7.9095(8) 6.3855(6) 9.6596(6) b  (Å) 18.688(2) 18.3404(17) 8.0501(5) c   (Å) 9.2154(10) 11.3791(11) 17.2372(11) a  (  ) 90 90 90 b  (  ) 104.246(2) 95.489(2) 103.2300(10) c  (  ) 90 90 90 V   (Å 3 ) 1320.3(2) 1326.5(2) 1304.80(14)  Z   2 4 4 D calc  (Mg m  3 ) 1.812 1.803 2.032Absorption coefficient (mm  1 ) 3.733 3.715 6.553 F  (0 0 0) 708 708 764Crystal size (mm) 0.16  0.10  0.10 0.12  0.06  0.03 0.64  0.18  0.14 h  range for data collection 2.18–25.00 2.11–27.50 2.17–27.50Limiting indices   9 6 h 6 9   7 6 h 6 8   11 6 h 6 12  22 6 k 6 21   22 6 k 6 23   10 6 k 6 9  10 6 l 6 10   14 6 l 6 14   22 6 l 6 20Reflections collected/unique 7378/2329 [R(int) = 0.0452] 9213/3048 [R(int) = 0.0531] 9000/3002 [R(int) = 0.0316]Data/restraints/parameters 2329/0/163 3048/0/163 3002/0/145Goodness-of-fit on  F  2 1.165 1.159 1.026Final  R  indices ( I   > 2 r ( I  ))  R 1  = 0.0561  R 1  = 0.0592  R 1  = 0.0321 wR 2  = 0.1268  wR 2 = 0.1152  wR 2  = 0.0791 R  indices (all data)  R 1  = 0.0720 R1 = 0.0785  R 1  = 0.0395 wR 2  = 0.1317  wR 2  = 0.2105  wR 2  = 0.0820Largest different peak and hole (e Å  3 ) 1.828 and   0.533 0.676 and   0.472 0.658 and   0.372352  R.C.S. Wong, M.L. Ooi/Inorganica Chimica Acta 366 (2011) 350–356   ( 4 ) and an uncharacterized insoluble green residue believed to bedecomposed WR.This observation is comparable to the reaction of   1  with ele-mental selenium [34,35] and mixed ligand P 4 Se 3  [15–17] affording 3  as the main product. It is envisaged that the labile monomericspecies CpCr(CO) 3  ( 1A ) or CpCr(CO) 2  ( 2A ) [38–40] generated fromthe dissociation of   1  reacts with excess WR resulting in its cleavageand subsequent abstraction of Se atom to form  3  as depicted inroute (i) in Scheme 1. However, a similar reaction of   1  with0.125 equivalent of WR gave Cp 2 Cr 2 (CO) 4 Se (58% yield), unreactedCp 2 Cr 2 (CO) 6  ( 1 ) (17% recovery) and  trans -[CpCr(CO) 2 (SePPh)] 2  ( 5 )(25% yield). Here we noticed that the WR was completely con-sumed since  1  was in excess and noticeably the uncharacterizedinsoluble green residue was not formed.The reaction of the triply bonded congener Cp 2 Cr 2 (CO) 4  ( 2 ) withan equivalent of WR required a longer time of 3 h and at an ele-vated temperature of 60   C to yield  4 ( 18%),  5  (3%),  6  (2%) and anuncharacterized insoluble green precipitate (361 mg). It is worthyto note that Cp 2 Cr 2 (CO) 4 Se ( 3 ) was not formed here which agreeswith previous reports involving selenation agents such as Se 8 [34,35] and Ph 2 Se 2  [41].During the thermolysis of   5  which was monitored by  1 H NMR at110   C, we noticed the emergence of a species at  d  4.26 togetherwith the formation of   6  ( d  5.04). We propose that this species ( d 4.26) to be CpCr(CO) 2 (SePPh) ( 4A ) resulting from the loss of ahydrogen since the  1 H chemical shifts for  4  are  d  4.26 (Cp) and  d 5.72 (P–H) (route v). The formation of this phosphinoselenoylideneproduct,  4A , is believed to be the common intermediate specieswhich acts as a precursor to the formation of   5  and  6 . This postu-lation agrees with the NMR tube thermolysis study of   4  at 110   Cfor 14 h which gave  5  (38% yield) and  6  (11% yield) and also theESI mass spectrum of   4  which shows the presence of   trans -[CpCr(CO) 2 (SePPh)] 2  ( m / e  = 722). These observations suggest thatthe thermally unstable  4  is likely to be the primary product whichslowly converts to the secondary product  5  via  4A  (route iv and vi).Under similar condition, cothermolysis of   4  with WR gave unre-acted  4  (19% recovered),  5  (6% yield),  6  (31% yield) and someuncharacterized Cp-containing compounds. As expected, in thepresence of WR more Se atoms will be available for the formationof   6 , hence prolonged thermolysis ultimately resulted in the isola-tion of the eight-membered ring complex [CpCr(Se 2 P(O)Ph)] 2  ( 6 ) asthe final thermolytic product (route vii).  3.2. Mechanistic pathways Similar to its sulfur analogue of LR  [23], WR also underwentP 2 Se 2  ring opening mechanism with a propensity to give severaltypes of fragments as detected by mass spectrometry [42] (Scheme 2). The molecular structure of   4  suggested that its formation islikely to result from the attack of CpCr(CO) n   (n  = 2 ( 2A ) or 3 OCOCCrSePHCrCrCOCOPPSeSeCOOCCrCOOCCrOCCO 2 (ii) (vi)  rearrangement CrOCOCOSeOCOOCOCCrCrSeOCCOCOOC Se (i) PSe 1 . 2 1A3456 17 e -  species - CO CrOCOCCrCOCO 2 Se HH (vii)(iii) -H + OCOCCrSeP 4A (iv) PSe (v) CrOCOC . 2 2A 15 e -  speciesCrSeSePPCr or Scheme 1.  Proposed synthetic pathway for the formation of products  3 ,  4 ,  5  and  6 . R.C.S. Wong, M.L. Ooi/Inorganica Chimica Acta 366 (2011) 350–356   353  ( 1A )) radical at the P 2 Se 2  central ring of the WR to generate thePhPSe   fragment. Further interaction of   1A  or  2A  with a PhPSe  fragment followed by subsequent abstraction of H afforded  4 (route ii and iii). The source of hydrogen is believed to be the coor-dinated Cp, as proposed by Goh and co-workers in a similar reac-tion with LR  [23] which explained the presence of severalunassigned peaks in the Cp region encountered in the productsolutions. Such observation was also present especially in reactionmixtures of   1  and  2  with WR and some chromatographed fractionsas weak intensity Cp resonances between  d  4.07 and 5.55. How-ever, the unstable  4  underwent thermolytic degradation with theloss of hydrogen to give  4A  (route iv). From the thermolysis study,we concluded that the formation of   5  is reversible which proceedswith dimerization of   4A  via P–P bond coupling and decoupling(route v and vi). Similarities in both P–Se (2.1487(14) Å) and Cr–Se (2.6389(9) Å) bond lengths for CpCr(CO) 2 (SeP(H)Ph) ( 4 ) as com-pared to those in  5  (2.1425(17) Å and 2.6309(12) Å, respectively)provides further supportive evidence.Selenation of   5  from the WR ligand with concomitant bondforming and breaking followed by intramolecular bond rearrange-ments with dimerization, decarbonylation and oxygen abstractionhas led to the formation of   6  (route vii). It was not possible for us toconclude experimentally the source of oxygen in  6  as all manipu-lations were performed in the absence of water and oxygen.The ease of fragmentation of the P–Se heterocyclic ligand whichleads to ring opening in WR resembles well with its P–S analogue,LR. Indeed, both ligands behaved showed similar patterns of reac-tivity which yielded some structurally identical products with  1 [23].  3.3. Molecular structures of   4 ,  5  and  6  The molecular structures of   4 , 5 and  6  are shown in Figs. 1–3.Selected bond lengths and angles are listed in Tables 3–5,respectively.The structure of CpCr(CO) 2 (SeP(H)Ph) ( 4 ) possesses a four-legged piano-stool configuration at Cr.  4  contains a Cp moiety atthe apical position while bonded to two CO groups and a  g 2 -arylselenaphosphetane ligand. The complex is isostructural toCpCr(CO) 2 (SP(H)Ar) (Ar = C 6 H 4 OMe) obtained from the reaction of  1  with LR  [23] and [MoCp{ j 2 -OP(OC 6 H 4 OH)R  ⁄ }(CO) 2 ] (R  ⁄ = 2,4,6-C 6 H 2 r  Bu 3 ) from  p -benzoquinone oxidation of [MoCp(CO) 2 {P(O)R  ⁄ }]  to [43].The Cr–P bond is indicated as a single bond with bond distanceof 2.2643(15) ÅA 0 which is similar with some of the reported com-plexes such as CpCr(CO) 2 (SP(H)Ar) (2.2607(8) ÅA 0 ) [23];CpCr(CO) 2 (SPR  2 ) (R = Me, 2.2704(6) ÅA 0 ; Et, 2.2738 (18) ÅA 0 ) [44]. TheCr–Se bond distance of 2.6389(9) Å possessed a single Cr–Se bondwhichis longerthanthose observed in the ( l - g 2 -Se 2 ) complexesof [CpCr(CO) 2 ] 2 Se 2  (2.277 Å) [35] or Cp 4 Cr 4 (CO) 8 (P 2 Se 2 ) (2.566(2) and PSeSeSePSeSePSeM + = 268PSeM + = 188PSeSeSePM + = 454PSeSePM + = 374PSePM + =296 Scheme 2.  Fragmentation of Woollins’ Reagent as detected by mass spectrometry. Fig. 1.  Molecular structure of CpCr(CO) 2 (SeP(H)Ph) ( 4 ). Thermal ellipsoids areshown at the 50% probability level. Fig. 2.  Molecular structure of   trans -[CpCr(CO) 2 (SePPh)] 2  ( 5 ). Thermal ellipsoids areshown at the 50% probability level.354  R.C.S. Wong, M.L. Ooi/Inorganica Chimica Acta 366 (2011) 350–356 
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