Sterically Induced P-C Bond Cleavage: Routes to Substituent-Free

Sep 1, 1995 - PC Bond Cleavage of (silox)3NbPMe3 (silox =Bu3SiO) under Dihydrogen Leads to (silox)3Nb CH2, (silox)3Nb PH or (silox)3NbP(H)Nb(silox)3, ...
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Organometallics 1995, 14, 4247-4256

4247

Sterically Induced P-C Bond Cleavage: Routes to Substituent-Free Phosphorus Complexes of Zirconium Maria C. Fermin, Jianwei Ho, and Douglas W. Stephan* Department of Chemistry and Biochemistry, University of Windsor, Windsor, Ontario, Canada N 9 B 3P4 Received April 7, 1995@ The reaction of Cp2ZrHCl with 1equiv of K[PH(CsH2-2,4,6-t-Bu3)1and excess KH in THF 4. The complex Cp*2yields the dark red product [C~ZZ~H(P(C~H~-~,~,~-~-BU~))K(THF)~I~, Zr(PH(C~H2-2,4,6-t-Bu3))Cl, 5, is derived from the reaction of Cp*~ZrClzwith KPH(C6H22,4,6-t-Bu3). Generation of 5 via the reaction of Cp*,ZrClz with excess KH and phosphine results in P-C bond cleavage as evidenced by the formation of the diamagnetic species (Cp*zZr)a@-Pz),6, and the paramagnetic compound (Cp*2Zr)d,u-P), 7. Compound 7 is the first dimetallaphosphallene to be structurally characterized. In a n alternate synthetic route, reaction of 2 equiv of PHz(CsH2-2,4,6-t-Bu3)with (Cp*2Zr(N2))2(p-N2)yields P-C bond cleavage, as the species Cp*zZr[(PH)zl, 8, is formed. Reaction of 8 with KH leads to the generation of species [Cp*2Zr(P2)1[K(THF),12, 9, which reacts with Cp"zZrCl2 to give 6 quantitatively. In a subsequent reaction, compound 6 undergoes reaction slowly with excess PH2(CsHz-2,4,6-t-Bu3)in the presence of KH to give compound 10. X-ray crystallographic study of 10 revealed the asymmetric unit to contain Cp*2ZrP3K(THF)1.5. This species forms a n infinite polymeric structure in the solid state. The nature of the bonding in species 7,8, and 10 has been examined via EHMO calculations. The chemistry described herein demonstrates that high steric demands may induce P-C bond cleavage, thus offering a feasible metal-mediated route to substituent-free P complexes.

Introduction Interest in the subdiscipline of inorganometallic chemistry has been prompted by academic interest in new reactivity patterns, the notion of metal-mediated syntheses of organic heterocycles, and the possibility of structural and mechanistic insight relating to MOCVD processes. Recently, much interest has focused on early metal inorganometallics. In early metalimides (M=NR),l rich new chemistry has been uncovered principally by the research groups of Bergman and Wolczanski. Related oxide (M-0) and sulfide (M=S)2 systems have also been studied with a view to the incorporation of heteroatoms into organic compounds. In a similar manner, recent studies of Zr-phosphinidenes (M=PR) have revealed a broad range of reactivity including C-H and P-H activation, as well as a variety of insertion or metathesis reactions which offer metalmediated syntheses of both organophosphorus species4 and main group-phosphorus compound^.^ In general, early metal-heteroatom multiple bonds are stabilized

* E-mail: [email protected].

Abstract published in Advance ACS Abstracts, August 1, 1995. (1)(a)Walsh, P. J.; Hollander, F. J.; Bergman, R. G. J . Am. Chem. SOC.1988,110,8729. (b) Walsh, P. J.; Carney, M. J.;Bergman, R. G. J . Am. Chem. SOC.1991,113, 6343. (c)Walsh, P. J.; Hollander, F. J.; Bergman, R. G. J . Organomet. Chem. 1992,428,13. (d) Bennett, J. L.; Wolczanski, P. T. J . Am. Chem. SOC.1994,116, 2179. (e) Schaller, 1994,116, C. P.; Bonanno, J. B.; Wolczanski, P. T. J . Am. Chem. SOC. 4133. (0 Cummins, C. C.; Schaller, C. P.; Van Duyne, G. D.; Wolczanski, P. T.; Chan, A. W. E.; Hoffmann, R. J . Am. Chem. SOC. 1991,113,2985. (g) Banaszak Hall, M. M.; Wolczanski, P. T. J . Am. Chem. SOC.1992,114,3854. (2) (a) Parkin, G.; Bercaw, J. E. J . Am. Chem. SOC. 1989,111, 391. (b) Carney, M. J.; Walsh, P. J.; Hollander, F. J.; Bergman, R. G. J . Am. Chem. SOC.1989,I l l , 8751. (c) Whinnery, L. L.; Hening, L. M.; Bercaw, J. E. J.Am. Chem. SOC.1991,113,7575. (d) Carney, M. J.; Walsh, P. J.; Bergman, R. G. J . Am. Chem. SOC.1990,112,6426. (e) @

0276-7333/95/2314-4247$09.QQIQ

by employing sterically demanding ancillary groups. This is particularly true in the case of phosphinidene derivatives as examplified by the complexes CpzZr(PCsH2-2,4,6-t-Bud(PMed?( M ~ ~ S ~ N C H ~ C H Z ) ~ N T ~ = P R , ~ and (~ilox)3Ta=PPh.~ During the course of the development of synthetic routes to early metal-phosphinidenes, we have observed that the use of sterically demanding substituents may facilitate P-C bond cleavage. Two such examples of this phenomenon, the pseudopyramidal species (CpZr(p3-C5H4))3P, l,gand the planar, mixed-valent compound (Cp2Zr)2(p2-C1)(p3-P)(Cp~ZrClI, 2,1°have been previously reported. In each case these compounds were derived from reactions of low-valent Zr reagents with PH2(CsH2-2,4,6-t-Bu3),clearly the result of P-C bond cleavage. (3)(a)Fagan, P. J.; Nugent, W. A. J . Am. Chem. SOC.1988,110, 2310. (b) Fagan, P. J.;Ward, M. D.; Caspar, J. V.; Calabrese, J. C.; Krusic, P. J. J . Am. Chem. SOC.1988,110,2981. (c) Doxsee, K. M.; Dhen, G. S.; Knobler, C. B. J . Am. Chem. SOC.1989,111, 9129. (d) Buchwald, S.L.; Watson, B. T.; Wannamaker, W. M.; Dewan, J. C. J . Am. Chem. SOC.1989, 111, 4486. (e) McGrane, P. L.; Jensen, M.; Livinghouse, T. J . Am. Chem. SOC.1992,114,5459. (DBuchwald, S. L.; Nielsen, R. B.; Dewan, J. C. J . Am. Chem. SOC.1987,109,1590. (g) (h) Buchwald, S.L.; Nielsen, R. B. J . A m . Chem. SOC.1988,110,3171. Buchwald, S. L.; Fisher, R. A.; Davis, W. M. Organometallics 1989,8, 2082. (i) Fisher, R. A,; Nielsen, R. B.; Davis, W. M.; Buchwald, S.L. J . Am. Chem. SOC.1991,113, 165. (4)Geissler, B.; Wettling, T.; Barth, S.;Binger, P.; Regitz, M. Synthesis 1994,1337. (5) Hou, Z.; Stephan, D. W. J . Am. Chem. SOC.1992,114,10088. (6)Hou, Z.; Breen, T. L.; Stephan, D. W. Organometallics 1993,12, 3158. (7) Cummins, C. C.; Schrock, R. R.; Davis, W. M. Angew. Chem., Int. Ed. Engl. 1993,32,756. (8)Bonanno, J. B.; Wolczanski, P. T.; Lobkovsky,E. B. J . Am. Chem. SOC.1994,116,11159. (9) Ho, J.; Stephan, D. W. Organometallics 1992,11, 1014. (10) Ho, J.; Rousseau, R.; Stephan, D. W. Organometallics 1994,13, 1918.

0 1995 American Chemical Society

Fermin et al.

4248 Organometallics, Vol. 14,No. 9, 1995

bustion analyses were performed by Galbraith Laboratories Inc., Knoxville, TN, or Schwarzkopf Laboratories, Woodside, NY. PHz(CsHz-2,4,6-t-Bu3) was purchased from Quantum Chemical Co. [(Cp*zZr(N2))2(N~)l was prepared by published methods.I4

2

1

. l

Such sterically induced pnictogen-carbon bond cleavages are not without precedent. Evans and co-workers have described the compound (Cp*zSm)z@-Biz),3, which is derived from Bi-C bond cleavage in the reaction of Cp*zSm with BiPhs." Our observations in the zirconium-phosphorus systems suggested to us that control of such P-C bond cleavage reactions would provide a synthetic strategy to substituent-freephosphorus-early metal complexes, a class of compounds which has drawn little attention.12 Furthermore, we reasoned that controlled steric congestion would provide insight regarding the nature of the reaction sequence as P-C cleavage would be shut down. Thus, in this paper we describe in detail a rational, systematic investigation of both the utility and limitations of sterically induced P-C bond cleavage reactions in the synthesis of substituent-free phosphorus-zirconium complexes. In addition, the nature of both the structure and bonding of such Zrbare-P complexes is examined and discussed. A preliminary report of some of the chemistry described herein has been previously comm~nicated.'~ Experimental Section General Data. All preparations were done under a n atmosphere of dry, 02-free N2 employing either Schlenk line techniques or a Vacuum Atmospheres inert atmosphere glovebox. Solvents were reagent grade, distilled from the appropriate drying agents under N2, and degassed by the freeze-thaw method at least three times prior to use. 'H and 13C{'H} NMR spectra were recorded on a Bruker AC-300 operating a t 300 and 75 MHz, respectively. 31Pand 31P{1H}NMR spectra were recorded on a Bruker AC-200 operating at 81 MHz. Trace amounts of protonated solvents were used as references, and chemical shifts are reported relative t o SiMe4 and 85% H3Pod, respectively. FAB mass spectra were recorded employing the Kratos MS-50 a t Georgia Tech. Nitrobenzylglycerol was used as the matrix, and the high-resolution (HR) mass spectral results are reported using the most abundant isotopes. Com(11)Evans, W. J.; Gonzoales, S. L.; Ziller, J. W. J . Am. Chem. SOC. 1991, 113, 9880. (12)(a)Rosenthal, G.; Corbett, J. D. Znorg. Chem. 1988,27,53. (b)

Scherer, 0. J.; Vondung, J.; Wolmershauser, G. Angew. Chem., Znt. Ed. Engl. 1989,28, 1355. (c) Scherer, 0. J.; Schwalb, J.;Swarowsky, H.; Wolmershauser, G.; Kaim, W.; Gross, R. Chem. Ber. 1988, 121, 443. (d) Scherer, 0. J.; Swarowsky, H.; Wolmershauser, G.; Kaim, W.; Kohlmann, S. Angew. Chem., Znt. Ed. Engl. 1987,26, 1153. (e) Hey, E.; Lappert, M. F.; Atwood, J. L.; Bott, S. G. J. Chem. SOC.,Chem. Commun. 1987, 597. (13)Fermin, M. C.; Ho, J.; Stephan. D. W. J . Am. Chem. SOC.1994. 116, 6033.

Synthesis of [C~~Z~H(P(C&-~,~,~-~-BU~))K(THF)~ 4: PH2(CsH2-2,4,6-t-Bu3)(140 mg, 0.5 mmol) in THF (10 mL) was treated with excess KH, generating K[PH(CsH2-2,4,6-tBUS)].A 1equiv amount of CpzZrHCl (130 mg, 0.5 mmol) was added. The mixture was stirred for 0.5 h and stood for 24 h. The solution was filtered, the volume was reduced to about half of the original amount, and pentane was added. Red orange crystals of 4 were deposited in a yield of 10%. 'H NMR (C&, 25 " c ) 6: 7.60 (m, 2H); 5.58 (s, 10H); 3.56 (m, 8H); 1.71 (s, 9H); 1.41 (m, 8H); 1.32 (s, 18H). 31PNMR (C&, 25 "C) 6: 565.5. Anal. Calcd for C3&&1PZr: c, 67.66; H, 8.97. Found: C, 67.60; H, 8.90. Synthesis of Cp*zZr(PH(C&-2,4,6+Bus))Cl, 5: PH2(C,&-2,4,6-t-Bu3) (280 mg, 1.0 mmol) in THF (10 mL) was treated with excess KH, generating K[PH(CsH2-2,4,6-t-Bu3)]. The excess KH was removed by filtration, and Cp*zZrClz (453 mg, 1.0 mmol) was added. The mixture became red-brown, was stirred for 30 min, and stood overnight. The solution was filtered, the volume was reduced, and pentane was diffused slowly into the mixture. Orange-brown crystals of 5 were deposited in 95% yield. An alternative method involves the use of Li[PH(CeH2-2,4,6-t-B~3)1, generated by the reaction of PHz(CsHz-2,4,6-t-Bu3)with BuLi. 'H NMR (CsD.5, 25 " c ) 6: 7.59 (b s, 1H); 7.49 (b s, 1H); 6.17 (d, 1H); 1.79 (s, 30H); 1.56 (s,9H); 1.35 (s, 18H). 31PNMR (THF, 25 "C) 6: 117.0 (IJp-HI = 297 Hz). Anal. Calcd for C3&b&1PZr: c, 67.66; H, 8.97. Found: C, 67.60; H, 8.80. Synthesis of (Cp*2Zr)&-Pz), 6: (i) Compound 5 was generated in THF solution as described above. Excess KH was added to the reaction mixture. The mixture stood for 24 h and was then filtered. The solvent was removed, and the residue was washed with pentane to give compound 6 in 60% yield. (ii) To a solution of 9 (25 mg, 0.05 mmol) in THF (5 mL) was added Cp*pZrC12 (23 mg, 0.05 mmol). The solution was stirred overnight, and then the solvent was removed in vacuo. The residue was washed with a small amount of cold pentane and dried in vacuo. This afforded the brown product 6 in 80% yield. 'H NMR (C6D6, 25 " c ) 6: 2.09 (5). 31PNMR (C&, 25 " c ) 6: 959.5. Anal. Calcd for C4oHsoPzZrz: c , 61.18; H, 7.70. Found: C, 61.02; H, 7.57. FAB-HRMS: mle (calcd) 786.3159; found, 786.3150. Synthesis of (Cp*zZr)&-P),7: From the reaction mixture described for preparation i of 6, the pentane washing was concentrated and stood overnight, affording brown crystals of 7 in 10% yield. EPR (THF): g = 1.989, (up) = 26 G. Synthesis of Cp*zZr((PH)s),8: To a solution of (Cp*zZr(N2))2(N2)(150 mg, 0.185 mmol) in benzene (5 mL) was added PH&sHz-2,4,6-t-Bu3) (206 mg, 0.743 mmol). The mixture was stirred for 30 min, and the solvent was removed in vacuo. The residue was washed with a minimum amount of cold pentane. This afforded 8 as a brown powder in 90%-95% yield. 'H NMR (C&, 25 "C) 6: 1.70 (s, 30H), 4.68 (d of d, 2H). 31PNMR (C6D6, 25 " c ) 6: 134.3, IJP-HI = 310 HZ, IJP-HI = 21.3 Hz. I3cNMR ( C & 3 , 2 5" c ) 6: 117.3, 11.55 ppm. Anal. Calcd for CzoH3zPzZr: C, 56.44; H, 7.58. Found: C, 56.36; H, 7.48. Generation of [Cp*2Zr(Pz)][K(THF),12,9: To a solution of 8 (50 mg, 0.117 mmol) in THF (5 mL) was added KH (10 mg, 0.264 mmol). The mixture was vigorously stirred overnight. Excess KH was removed by filtration, and the solvent was removed in vacuo. The residue was washed with pentane, dried, and isolated as a brown powder in 83% yield (based on NMR). 'H NMR (C6D6,25 "C) 6: 1.66 (SI. 31PNMR (C&, 25 "C) 6: 450.4 (s). I3C NMR (C&, 25 "C) 6: 117.2, 11.68. (14)Manriquez, J. M.; McAlister, D. R.; Rosenberg, E.; Shiller, A. M.; Williamson, K. L.; Chan, S. I.; Bercaw, J. E. J.Am. Chem. SOC. 1978, 100, 3078.

Sterically Induced P-C Bond Cleavage

Organometallics, Vol. 14,No.9,1995 4249 Table 1. Crystallographic Data

4

5

10

7

~

formula fw cryst color cryst size ("3) a (A, b (A) c

(A,

a (deg) P (deg) y (deg) cryst syst space group vol(A3)

Dcalcd (g ~ m - ~ )

z

abs coeff, ,D (cm-') radiation, 1, (A) temp ("C) scan speed (deglmin) scan range (deg) bkgdlscan ratio no. of data collcd 28 range (deg) index range no. of data with FO2> 3u(FO2) no. of variables transmissn factors R (%)a Rw ('%IQ largest Mu goodness of fit

C~~H~~KOZPZ~ 680.11 red-orange 0.42 x 0.20 x 0.28 14.891(7) 14.107(6) 10.192(9) 94.27(6) 73.45(6) 115.53(3) triclinic P1 (No. 2) 1849(2) 1.22 2 4.81 Mo Ka (0.710 69) 24 8.0 (8128)(1-3 scans) 1.0 below Kal, 1.0 above Ka2 0.5 5667 4.5-50.0

Cs8HsoClPZr 674.54 orange-brown 0.35 x 0.35 x 0.30 27.441(2) 12.748(4) 10.026(2)

orthorhombic Pna21 (NO.33) 3783(2) 1.18 4 4.26 ' Mo Ka (0.710 69) 24 8.0 (8/28) (1-3 scans) 1.0 below Kal, 1.0 above Ka2 0.5 3779 4.5-50.0

teJragona1 P421c (NO.114) 4217(5) 1.34 4 6.24 Mo Ka (0.710 69) 24 8.0 (8/28) (1-3 scans) 1.0 below Kal, 1.0 above Ka2 0.5 2193 4.5-50.0

monoclinic P2/c (No. 13) 3033(2) 1.32 4 6.76 Mo Ka (0.710 69) 24 8.0 (8128)(1-3 scans) 1.0 below Kal, 1.0 above Ka2 0.5 5041 4.5-50.0

hhhkl

hkl

hkl

ihkl

1606

1155

857

1189

245 0.558-1.000 9.62 9.72 0.01 2.13

179 0.965-1.000 5.92 5.25 0.06 1.75

95 0.934-1.000 6.56 7.32

116 0.947-1.000 8.23 9.48 0.002 2.28

CJOHGOPZ~Z 754.32 red-black 0.32 x 0.30 x 0.28 14.720(9) 19.464(13)

CZ~H~ZKOI.~P~Z~ 601.86 red 0.30 x 0.32 x 0.22 15.205(4) 13.302(4) 15.481(5) 104.39(2)

Synthesis of Cp*2ZrPsK(THF)1.5,10: Compound 6 (30 mg, 0.038 mmol) was dissolved in THF (5 mL) and treated with a mixture of PHz(CsH2-2,4,6-t-Bu3) (42 mg, 0.152 mmol) and excess KH (4 mg, 0.10 mmol). The mixture was vigorously stirred overnight and stood for 1 week, and the excess KH removed by filtration. The solvent was removed in vacuo, the residue was washed with a small amount of cold pentane, and the product was isolated as a brown solid in 72%-78% yield. This material could be recrystallized from THF/pentane. After the solution stood for several days at -35 "C, red crystals of 10 were deposited in 30%yield. IH NMR (C&, 25 "C) 6: 1.87 (s). 31PNMR (CsD6,25 "C) 6: 490.4 (d), 245.6 (t), J J p - p J = 598 Hz. Anal. Calcd for C26H&F'301.5Zr: C, 51.99; H, 7.05. Found: C, 51.88; H, 6.99. X-ray Data Collection and Reduction. X-ray quality crystals of 4, 5, 7, and 10 were obtained directly from the preparation as described above. The crystals were manipulated and mounted in capillaries in a glovebox, thus maintaining a dry, 02-free environment for each crystal. Diffraction experiments were performed on a Rigaku AFC6 diffractometer equipped with graphite-monochromatized Mo K a radiation. The initial orientation matrix was obtained from 20 machinecentered reflections selected by a n automated peak search routine. These data were used t o determine the crystal systems. Automated Laue system check routines around each axis were consistent with the crystal system. Ultimately, 25 reflections (20" < 28 < 25") were used to obtain the final lattice parameters and the orientation matrices. Crystal data are summarized in Table 1. The observed extinctions were consistent with the space groups. The data sets were collected in three shells (4.5" < 28 < 50.0"), and three standard reflections were recorded every 197 reflections. Fixed scan rates were employed. Up to four repetitive scans of each reflection a t the respective scan rates were averaged t o ensure meaningful statistics. The number of scans of each reflection was determined by the intensity. The intensities of the standards showed no statistically significant change

0.001

2.28

over the duration of the data collections. The data were processed using the TEXSAN crystal solution package operating on a n SGI Challenger mainframe with remote Xterminals. The reflections with Fo2> 3uFO2were used in the refinements. Structure Solution and Refinement. Non-hydrogen atomic scattering factors were taken from the literature t a b u l a t i ~ n s . ' ~ The J ~ Zr and P atom positions were determined using direct methods, employing either the SHELX-86 or Mithril routines. The remaining non-hydrogen atoms were located from successive difference Fourier map calculations. The refinements were carried out by using full-matrix leastsquares techniques on F, minimizing the function w(IFoI 1Fc1)2, where the weight OJ is defined as 4Fo2/2u(Fo2) and F, and Fcare the observed and calculated structure factor amplitudes. In the final cycles of each refinement, all the Zr, P, 0, K, and C1 atoms were assigned anisotropic temperature factors. Carbon atoms were assigned anisotropic thermal parameters, and in some cases cyclopentadienyl and phenyl rings were constrained to be regular pentagons and hexagons, respectively, in order to maintain a reasonable data:variable ratio. Empirical absorption corrections were applied t o the data sets on the basis of either 7p-scan data or a DIFABS calculation and employed the software resident in the TEXSAN package. Hydrogen atom positions were calculated and allowed t o ride on the carbon to which they are bonded, assuming a C-H bond length of 0.95 A. In the case of 4, the hydride on Zr was located via difference map calculations, a t a distance of 1.36 A from Zr. Hydrogen atom temperature factors were fixed a t 1.10 times the isotropic temperature factor of the carbon atom to which they are bonded. All hydrogen atom contributions were calculated but not refined. The final values of R, Rw,and the (15)(a)Cromer, D. T.; Mann, J. B. Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1968,A24,324. (b) Ibid. 1968, A24, 390. (16)Cromer, D. T.; Waber, J. T. International Tables for X-ray Crystallography; Kynoch Press: Birmingham, England, 1974.

Fermin et al.

4250 Organometallics, Vol. 14,No.9,1995 maximum Ala on any of the parameters in the final cycles of the refinements are given in Table 1. The locations of the largest peaks in the final difference Fourier map calculation as well as the magnitude of the residual electron densities in each case were of no chemical significance. Positional parameters, hydrogen atom parameters, thermal parameters, and bond distances and angles have been deposited as supporting information. Molecular Orbital Ca1c~lations.l~Extended Hiickel calculations were performed and visualized employing the Cache Software system operating on a Power Mac 7100 computer. Initial coordinates and geometric parameters were taken from X-ray data. The models for calculations were simplified by use of cyclopentadienyl ligands rather than pentamethylcyclopentadienyl groups.

Results and Discussion

formed the presence of both cyclopentadienyl and supermesityl fragments as well as coordinated THF. Xhray crystallography confirmed the formulation of 4 as the phosphinidene-hydride complex [CpzZrH(PC&2,4,6-t-Bu3)K(THF)zl2,4 (vide infra, Scheme 1). Although the hydride was not detected by lH NMR spectroscopy, recent studies of Zr-hydride anions suggest that the hydride resonance from 4 is probably obscured by the THF signals.18 Attempts to observe the hydride via IR spectroscopy were also unsuccessful; nonetheless, the presence of the hydride was unequivocally confirmed by crystallography. The orange-brown product Cp*zZr(PH(CsH2-2,4,6-tBu3))C1,5, was prepared via reaction of Cp*2ZrC12 with 1 equiv of K[PH(CsH2-2,4,6-t-Bu3)1(eq 3). lH and 31P R'

Synthesis. We have previously reported that elimination of primary phosphine or H2 provides access to the phosphinidene species CpzZr(PCsHz-2,4,6-t-Bu3)(PMe3) (eqs 1 and 2).5,9We reasoned that subsequent NMR data are consistent with the formulation of 5, and this was also confirmed crystallographically (vide infra). Use of excess phosphide in the preparation of 5 did not lead t o further substitution, and thus the diphosphide species Cp*2Zr(PH(CsH2-2,4,6-t-B~3))2 was not readily accessible. In contrast, complexes Cp*2Zr(PH(CsH2-2,4,6-Me3))2 and Cp2Zr(PH(CsH2-2,4,6-tBU3))2 have been prepared.6 Thus it appears that the present combination of pentamethylcyclopentadienyl ligands and supermesityl phosphide presents a I highly sterically demanding environment about the Zr center that precludes formation of the analogous diphosP' P. phide. Generation of 5 via the reaction of Cp*zZrClz with phosphide in the presence of excess KH resulted in further reaction. Solvent removal and washing of the residue with pentane afforded the extraction of a new product, compound 6 in 15%-20% yield. This compound 6 exhibits a singlet 31P{1H}resonance at 959.0 ppm and a lH NMR resonance attributable to pentamethylcyclopentadienyl rings. The absence of resonances attributable to the supermesityl fragment is consistent with P-C bond cleavage. Although the P-C bond cleavage might be induced if an additional spectroscopy is less than definitive, the 31PNMR chemihydride ligand could be incorporated in place of the cal shift does indicate a much more deshielded P stabilizing phosphine (i.e., PMe3). In our initial efforts environment in 6 compared to that seen in 1 (31PNMR to this end, the reaction of CpaZrHCl with 1 equiv of 6: 782.6 ppmLg Numerous attempts to obtain X-ray the KTPH(CsH2-2,4,6-t-Bu3)1and excess KH was perquality crystals of 6 were unsuccessful; however, elformed in THF. The reaction becomes dark red, and emental analysis, FAB-MS, and additional chemical the evolution of gas, presumably H, was observed. data (vide infra) led to the formulation of 6 as (Cp*2Monitoringof the reaction mixture by 31PN M R revealed Zr)zb-Pz) (Scheme 2). Although unconfirmed by X-ray two low-field singlet resonances, a weak signal at 621.0, methods, 6 is thought to be a structural analog of the and a stronger one at 565.5 ppm. These chemical shifts species (Cp*2Sm)&-Biz) 3." are similar to those attributable to the previously In addition to 6, a second product, 7,was isolated from reported species C~~Z~(PC&~-~,~,~-~-BQ)@-C~)L~(DME), the reaction of 5 and KH. The pentane washings of the suggesting that the present products are also phosphininitial residue afford crystals of 7 in 10% yield. This idene-bridged derivatives.6 Species 4, which gives rise paramagnetic product exhibits a doublet EPR resonance to the resonance at 565.5 ppm, was isolated in 10% at g = 1.989, with a P hyperfine coupling constant of yield, while the minor product, resulting in the 31P 26 G consistent with the coupling of a P atom t o a lone resonance at 621.0 ppm, was not isolable and thus unpaired electron on Zr. X-ray crystallographic study remains uncharacterized. lH NMR for species 4 con(vide infra) of 7 revealed the formulation as (Cp*2Zr)z@-PI(Figure 1). (17)Cache Worksystem Software is an integrated modeling, molecular mechanics, and molecular orbital computational software package and is a product of Cache Scientific Inc.

(18)Fermin, M. C.; Stephan, D. W. Unpublished results.

Organometallics, Vol. 14, No. 9, 1995 4251

Sterically Induced P-C Bond Cleavage

Table 2. Positional Parameters for 4,5, 7, and 10 atom

X

Y

2

atom

X

Y

Z

[C~~Z~H(PC~H~-~,~,~-~-BU~)K(THF)Z]~, 4 0.0990(2) -0.1060(4) 0.1476(5) -0.169(2) -0.275(2) 0.229(1) 0.151(1) 0.152(1) 0.229(1) 0.277(1) 0.095( 1) 0.026(1) -0.059(1) -0.043(1) 0.052(1) 0.2684(8) 0.2675(8) 0.361(1) 0.4549(8) 0.4558(8) 0.363(1)

0.3499(2) 0.0761(4) 0.1978(5) 0.150(2) 0.073(2) 0.438(1) 0.473(1) 0.532(1) 0.534(1) 0.476( 1) 0.376(1) 0.412(1) 0.326(2) 0.236(1) 0.267(1) 0.223(1) 0.220(1) 0.260(1) 0.303(1) 0.306(1) 0.266(1)

0.0587(2)

0.173(2) o.io5i2j 0.106(2) 0.198(2) 0.559(2) 0.551(2) 0.623(2) 0.620(2) 0.380(2) 0.493(2) 0.308(2) 0.364(2) -0.138(4) -0.165(3) -0.225(3) -0.219(3) -0.323(5) -0.435(4) -0.450(5) -0.347(3)

0.157(2) 0.049(2) 0.214(2) 0.134(2) 0.357(2) 0.345(2) 0.470(2) 0.298(3) 0.261(2) 0.277(2) 0.148(2) 0.338(2) 0.257(3) 0.263(3) 0.161(3) 0.097(2) 0.139(3) 0.070(6) -0.012(4) 0.006(3)

-0.344(2) -0.264(2) -0.325(2) -0.493(2) -0.464(3) -0.613(3) -0.442(3) -0.463(3) 0.027(2) 0.016(2) 0.099(2) 0.123(3) -0.238(5) -0.351(5) -0.397(3) -0.309(4) 0.144(4) 0.185(7) 0.273(5) 0.261(4)

0.32892(7) 0.2880(3) 0.3544(2) 0.4206(6) 0.3944(7) 0.3677(8) 0.3785(7) 0.4093(7) 0.4600(7) 0.400( 1) 0.3406(9) 0.3717(8) 0.4344(8) 0.2502(8) 0.2386(7) 0.2623(8) 0.291(1) 0.2836(7) 0.2243(9) 0.1994(8) 0.2530(8)

0.4890(1) 0.4096(5) 0.3331(4) 0.497(2) 0.499(2) 0.588(2) 0.638(1) 0.583(1) 0.422(1) 0.432(2) 0.626(2) 0.747(2) 0.611(2) 0.459( 1) 0.533(1) 0.616(2) 0.595(2) 0.501(2) 0.363(2) 0.524(2) 0.712(2)

0.3158(9) 0.2956(8) 0.4020(7) 0.4450(8) 0.4849(8) 0.4904(7), 0.4477(8) 0.4027(7) 0.4423(8) 0.4456(9) 0.3998(8) 0.488( 1) 0.5420(8) 0.560(1) 0.539( 1) 0.575(1) 0.3565(8) 0.3612(9) 0.357(1) 0.3091(8)

0.672(2) 0.460(2) 0.235(1) 0.234(1) 0.197(2) 0.154(1) 0.135(2) 0.174( 1) 0.275(1) 0.385(2) 0.236(2) 0.233(2) 0.120(2) 0.042(2) 0.077(2) 0.202(2) 0.130(2) 0.021(2) 0.166(2) 0.153(2)

0.057(3) -0.031(3) 0.220(2) 0.134(2) 0.179(2) 0.303(3) 0.378(2) 0.331(3) -0.007(2) -0.021(3) -0.081(2) -0.088(3) 0.368(3) 0.267(3)

0.8306(1)

0.4717(1) 0.5000 0.564(1) 0.612(2) 0.568(1) 0.487(2) 0.482(2) 0.590(2) 0.709(2) 0.600(2) 0.423(3)

0.799(2) 0.807(2) 0.733(2) 0.767(2) 0.864(2) 0.889(2) 0.797(2) 0.633(2) 0.715(2) 0.919(2) 0.977(2)

0.404(2) 0.409(2) 0.373(2) 0.311(2) 0.311(2) 0.366(2) 0.464(2) 0.393(2) 0.251(2) 0.243(2) 0.370(2)

-0.017(2) 0.283(1) 0.239(1) 0.190(1) 0.203(1) 0.256(1) 0.344(2) 0.259(2) 0.147(1) 0.168(2) 0.293(1)

0.2807(3) 0.4374(6) 0.3313(7) 0.3876(7) 0.4059(7) 0.576(2) 0.275(2) 0.0778(9) 0.098( 1) 0.133(1) 0.1333(9) 0.0994(9)

0.063(1) 0.049(1) 0.101(1) 0.147( 1) 0.124(1) 0.020(2) 0.105(2) -0.013(2) 0.212(1) 0.159(2) 0.557(3) 0.538(3) 0.688(4) 0.737(4) 0.713(5) 0.649(3)

0.297(1) 0.338(1) 0.427(1) 0.440(1) 0.360(1) 0.200(1) 0.497(2) 0.29512 0.527(2) 0.344(2) 0.641(3) 0.746(3) 0.269(4)

0.050(1) -0.035( 1) -0.029(1) 0.060(1) 0.109(1) 0.075(2) -0.107(1) -0.121(1) 0.098(2) 0.211(1) 0.308(3) 0.278(3) 0.036(3) 0.044(4) 0.114(5) 0.146(3)

1.0000

0.829(2) 0.778(2) 0.701(1) 0.699(2) 0.775(2) 0.909(2) 0.804(2) 0.626(2) 0.615(2) 0.2204(2) 0.5800(4) 0.3578(6) 0.2782(6) 0.3749(6) ‘12

0.652(2) 0.2142(9) 0.1725(8) 0.239(1) 0.3224(8) 0.3069(8) 0.167(1) 0.224(1) 0.0709(8) 0.415(1) 0.379(1)

0.038(1)

0.164(2) 0.086(2) 0.166(2) 0.088(2)

0.181(5)

0.126(5) 0.177(5)

0.508(4)

0.359(4) 0.415(3) 0.409(3) 0.550(3) 0.340(3)

Fermin et al.

4252 Organometallics, Vol. 14, No. 9, 1995 CP*

CP'

CO'

Complexes 6 and 7 are clearly derived from intriguing P-C bond cleavage reactions that occur in this "onepot synthesis". However, because of the poor yields and the generation of both diamagnetic and paramagnetic products, an alternative synthetic route to such substituent-free phosphorus derivatives was sought. The reaction of 2 equiv of PH2(CsH2-2,4,6-t-Bud with (Cp*2Zr(N2))2(~-N2)~~ in benzene at 25 "C proceeds smoothly with the evolution of N2, to give upon subsequent workup a brown product, 8 , in 75%-90% isolated yield. IH NMR showed resonances attributable to the pentamethylcyclopentadienyl ligands and two PH protons. No resonances due to the supermesityl substituents were observed, clearly indicating that P-C bond cleavage had occurred. These spectroscopic data together with the observation of two P-H coupling constants, 310.0 and 21.3 Hz, are consistent with the formulation of 8 as Cp*2Zr[(PH)21(Scheme 2). Similar spectroscopic parameters have been described for the species CpaMo[(PHhl, although this compound was prepared via the reaction of Pq with C ~ ~ M O H ~ . ~ ~ Reaction of 8 with KH proceeds rapidly with the generation of H2 and species 9, which exhibits a 31P{IH} resonance a t 449.5 ppm. The IH NMR spectrum of 9 shows only resonances attributable t o pentamethylcyclopentadienyl and THF protons, consistent with the formulation of 9 as [Cp*2Zr(P2)1[K(THFX12(Scheme 2). Subsequent addition of 1 equiv of Cp*2ZrClz to a solution of 9 yields 6 quantitatively as evidenced by the appearance of the clean 31PNMR resonance a t 959.0 PPm. Compound 9 reacts slowly with excess PH2(C&2,4,6-t-Bu3) in the presence of KH over the period of 1 week. The 31P NMR resonance attributable to 9 is replaced by a doublet a t 490.4 and a triplet a t 245.6 ppm, with a IJp-p/ value of 598 Hz. This magnitude of the coupling constant infers direct P-P bonding. Following workup and recrystallization, red-brown crystals of a new species 10 were isolated in 30% yield. X-ray

22

Figure 1. ORTEP drawing of the asymmetric unit of 4; 30% thermal ellipsoids are shown; hydrogen atoms are omitted for clarity. Scheme 1

THF

\THFk* 4

Scheme 2

R'PH2

CP'

CP'

C20

I

10

crystallographic study of 10 was employed to determine that the asymmetric unit contained CP*~ZTP~K(THF)I.~t-B~3)(PMe3)~~ and is consistent with the anionic nature (vide infra). of the Zr center in 4. The charge of this complex anion Structural Studies. A crystallographic study of 4 is balanced by a potassium atom. Two molecules of THF confirmed its formulation as [CpzZrH(P(CsH2-2,4,6-tare coordinated.to the K with K-0 distances of 2.75(3) Bu3))1K(THF)2. The contents of the asymmetric unit are and 2.71(3) A. The K atom is also loosely associated depicted in Figure 1. Two cyclopentadienyl ligands, a with the P of the phosphinidene ligand of two symmetryphosphinidene, and a hydride complete the pseudotetrelated Zr-complex anions (K-P distances are 3.497(9) rahedral coordination sphere of Zr. The Zr-C bond A). Thus, the Zr anions and K cations form a dimeric distances are typical, while the Zr-P distance is 2.528association in the solid state (Figure 2). The resulting (2) A. This Zr-P distance is slightly longer than the P-K-P and K-P-K angles are 98.0(2) and 82.0(2)', Zr-P distance of 2.505(4)A found in CpzZr(PCsH2-2,4,6respectively. The X-ray structure of 5 is depicted in Figure 3. The (19) Green, J.C.; Green, M. L. H.; Morris, G. E. J . Chem. SOC.,Chem. Commun. 1974, 212. geometry about the Zr is as expected, pseudotetrahedral

Organometallics, Vol. 14,No. 9, 1995 4253

Sterically Induced P-C Bond Cleavage Table 3. Selected Bond Distances

Cdi)

and Angles (deg) for 4,5,7, and 10

[C~~Z~H(PC~HZ-~,~,~-~-BU~)K(THF)Z]Z, 4 Zr(l)-K(l) Zr(l)-C(3) Zr(l)-C(7) K( 1)-P( 1) P(l)-C(ll)

3.993(6) 2.49(2) 2.52(2) 3.497(9) 1.88(1)

K(l)-Zr(l)-P(l) Zr(l)-K(l)-P(l)

Distances 2.528(8) Zr(l)-C(l) 2.46(1) Zr(1)-C(5) 2.55(1) Zr(l)-C(9) 3.67(1) K(1)-0(1)

Zr(l)-P(l) Zr(l)-C(4) Zr(l)-C(8) K(l)-P( 1)

60.0(2) 133.7(2)

Angles Zr(l)-P(l)-K(l) P(l)-K(l)-P(l)

153.8(2) 98.0(2)

2.51(1) 2.47(1) 2.56(1) 2.75(3)

Zr(1)-C(2) Zr( 1)-C(6) Zr(l)-C( 10) K(1)- O(2)

Zr(l)-P(l)--C(ll) K(1)-P( 1)-K( 1)

2.52(2) 2.51(2) 2.53(2) 2.71(3)

117.0(5) 82.0(2)

Cp*zZr(PH(CsH~-2,4,6-t-B~3))Cl, 6 Zr(l)-Cl(l) Zr(l)-C(3) Zr(l)-C(12) P(1)-C(21)

2.484(8) 2.49(2) 2.58(2) 1.88(2)

Distances 2.558(7) Zr(l)-C(l) 2.55(2) Zr(l)-C(5) Zr( 1)-C(14) 2.63(2)

Zr(l)-P(l) Zr( 1)-C(4) Zr(l)-C( 13)

2.58(2) 2.57(2) 2.56(3)

Zr(l)-C(2) Zr( l)-C(11) Zr(1)- C(15)

2.57(2) 2.62(2) 2.60(2)

Angles Cl(l)-Zr(l)-P(l)

97.1(2)

Zr(1)-P( 1)-C(21) (Cp*zZr)z(p-P),7 Distances 2.59(2) Zr( 1)- C(2) 2.54(2) Zr( l)-C(11) 2.55(3) Zr(l)-C(15) Angles

138.8(8)

2.53(2) 2.57(2) 2.57(3)

Zr(l)-C(3) Zr(l)-C(12)

2.52(2) 2.54(3)

166.6(4)

2.62(2) 2.62(2) 2.69( 1) 3.47(1) 2.77(2)

90.2(3) 89.1(3) 118.9(3) 117.9(3) 80.9(4) 34.4(2) 84.1(3) 100.8(5)

140.2(4) 72.3(3) 152.8(3) 136.2(4) 59.8(4) 159.1(5) 119.5(6)

Angles P(l)-Zr(l)-P(3) P(1)-K( 1)- P(2) P(l)-K( 1)-0(1) P(1)-K( 11-P( 3 P(l)-K(l)-0(2) P(2)-K(l)-O(l) P(3)-K( 1)- O( 1) P(3)-K(1)-0(2) Zr(l)-P(l)-K(l) K(l)-P(l)-P(3) Zr(l)-P(2)-P(3) Zr(l)-P(3)-K(l) K(l)-P(3)-K(l) K(1)-P(3 )- P(1) K(1)- O( 1)- K(1)

with two pentamethylcyclopentadienyl ligands, a chloride atom, and a phosphide fragment comprising the coordination sphere. The Zr-P distance of 2.558(7) A is slightly longer than the Zr-P distance of 2.543(3) A seen in the analogous species, CpzZrcl(PCsH2-2,4,6-tBu3).1° While this is attributed t o the relative increase in the steric demands about the Zr center in 5, the electronic effects of the stronger n-donation from the pentamethylcyclopentadienyl ligands cannot be overlooked. A secopd feature that appears to reflect the steric congestion in 6 is the Zr-P-C angle (138.8(8)"). This is some 10" greater than the analogous parameter (Zr-P-C angle 128.4in CpzZrCl(PCsHz-2,4,6-t-B~3) (2)").1° The Zr-C1 distance and P-Zr-C1 angle in 5 of 2.484(8)A and 97.1(2)"compare with the corresponding parameters of 2.494(3) A and 98.61(9)" in CpzZrC1(PCsH2-2,4,6-t-BU3). The results of the structural determination of 7 are depicted in Figure 4. The molecule is simply two bis(pentamethylcyclopentadieny1)zirconiumunits linked by

45.3(3) 150.5(3) 78.8(4) 124.0(3) 85.2(7) 93.2(5) 88.0(3) 94.3(8) 142.7(4) 67.0(3) 75.3(4) 127.3(3) 95.9(3) 171.7(5) 97(1)

P(2)-Zr(l)-P(3) P(l)-K(l)-P(3) P(l)-K(l)-0(2) P(l)-K(l)-P(3) P(2)-K(l)-P(3) P(2)-K(1)-0(2) P(3)-K(1)-0(2) O(l)-K(l)-0(2) Zr( l)-P(l)-P(3) K(1)-P(1)- P(3) K(1) -P(2)- P(3) Zr(l)-P(3)-P(l) K(1)-P(3 1- P(1) K( 1)-P(3 )-P(2 )

44.9(3) 34.9(2) 95.2(6) 151.9(3) 117.5(3) 98.6(7) 94.9(6) 164.8(9) 75.0(3) 122.7(5) 78.0(4) 59.7(3) 78.1(3) 67.7(4)

a single P atom. The Zr-C distances are typical, while the Zr-P distance is 2.545(3) A. This is slightly longer than the Zr-P distances in 4 and CpzZr(PC&-2,4,6t-Bw)(PMe# and is consistent with Zr-P multiple-bond character. The crystallographic symmetry dictates strict 2/m symmetry. Thus, the two Zr-P distances are equivalent, and the geometry at P approaches linearity with a Zr'-P-Zr angle of 166.6(4)". The imposed symmetry dictates that the Cp* centroid-centroid vectors on the two Zr atoms be perpendicular (Figure 4b). This dimetallaphosphaallene20represents the first such species to be structurally characterized, although the related arsina- and stibacumulenes (Cp*Mn(CO)dz@E) (E = As,Sb)have been reported.21 Furthermore, 7 is also a rare example of a mixed-valent Zr(IV)/Zr(III) compound.22 ( 2 0 ) A brief report of the synthesis of [(C~*M~(CO)Z)ZPIX has appeared, although no structural data were reported. Strube, A,; Heuser, J.; Huttner, G.;Lang, H. J. Orgunomet. Chem. 1988,365, c9.

Fermin et al.

4254 Organometallics, Vol. 14, No. 9, 1995

“217

Figure 2. ORTEP drawing of 4 showing the dimeric nature in the solid state; 30% thermal ellipsoids are shown; hydrogen atoms are omitted for clarity.

Figure 4. ORTEP drawing of the asymmetric unit of 7; 30% thermal ellipsoids are shown; hydrogen atoms are omitted for clarity. The two views, a and b, are approximately orthogonal to each other.

Figure 3. ORTEP drawing of 5; 30% thermal ellipsoids are shown; hydrogen atoms are omitted for clarity.

The structure of compound 10 was determined and the contents of the asymmetric unit is depicted in Figure 5. Two pentamethylcyclopentadienyl ligands and three phopshorus atoms constitute the coordination sphere of Zr. The three phosphorus atoms in a plane are bonded to Zr such that the Zr-P distances are 2.550(8), 2.55(11, and 2.853(9)A. The P-P distances average 2.10(1)A. Ignoring the central P atom that exhibits the longer Zr-P distance, the geometry about Z r is pseudotetrahedral. The dissymmetric interaction of the P3 fragment and Zr is in contrast to that typically seen for metal complexes of P3 rings. In a number of such cases, a triangle of P atoms bonds symmetrically to A structurally related species, CpzZrP4the (P(SiMe&)z, 11,was prepared by Lappert et al. via the

reaction of P4 with Cp2Zr(P(SiMe3)2)~.~~ The Zr-P and P-P bond lengths in 10 are somewhat shorter than those observed in 11 (Zr-P distances, 2.632(3), 2.607-

(21) (a)Strube, A.; Huttner, G.; Zsolnai, L. Angew. Chem., Int. Ed. Engl. 1988,27,1529. (b) Strube, A.; Huttner, G.; Zsolnai, L. Z. Anorg. Allg. Chem. 1989,577,263. ( c )Bringewski, F.; Huttner, G.; Imhof, W. J. Organomet. Chem. 1993,448,C3. (22) Ho, J.; Hou, Z.; Drake, R. J.; Stephan, D. W. Organometallics 1993,12, 3145.

(23)Reviews on bare main group metal complexes include: (a) Scherer, 0. J. Angew. Chem., Int. Ed. Engl. 1990,29, 1104. (b) Herrmann, W. A. Angew. Chem., Int. Ed. Engl. 1986,25, 56. (c) DiVaira, M.; Stoppioni, P.; Peruzzini, M. Polyhedron 1987,6 , 351. (24) Hey, E.; Lappert, M. F.; Atwood, J. L.; Bott, S. G. J . Chem. SOC.,Chem. Commun. 1987,597.

Figure 5. ORTEP drawing of the asymmetric unit of 10; 30% thermal ellipsoids are shown; hydrogen atoms are omitted for clarity. SiMe, \

c.P

rSiMe3

11

Sterically Induced P-C Bond Cleavage

Figure 6. ORTEP drawing revealing the extended polymeric nature of 10 in the solid state; 30% thermal ellipsoids are shown; hydrogens, Cp*-methyl carbons, and the carbons of THF molecules are omitted for clarity.

(4);P-P distance, 2.241(4) 81.)24This is consistent with the net charge on anion [Cp2ZrP3]- of 10. Each of the P atoms exhibits an electrostatic interaction with K atoms a t distances ranging from 3.37(1)to 3.61(1)A. Each of the K atoms is situated in the plane of five P atoms. Two THF molecules complete the coordination spheres of K. One of these THF molecules bridges two K atoms, and thus the K-0 distances of 2.63( 1)and 2.77(2) A reflect the terminal and bridging sites. The net result is an infinite lattice in the solid state in which [Cp*2ZrP3]units are bridged by K atoms, yielding the extended array (Figure 6).

Organometallics, Vol. 14, No. 9,1995 4255

Figure 7. Schematic depiction of (a) the frontier orbitals of Cp2Zr. (b) The LUMO, HOMO, and Zr-P bonding orbitals for CpzZr[(PH)z]. (c) The LUMO, HOMO, and Zr-P bonding orbitals for [Cp2ZrP31-. The depicted orbitals were derived from EHMO calculations.

Mechanistic Considerations Figure 8. Truncated orbital energy diagrams of the The reaction of potassium supermesityl phosphide bonding for the Cp2Zr fragment and the species (a) Cp2Zrwith Cp2ZrHCl is presumed to proceed through the [(PH)21and (b) [Cp2ZrP31-. intermediate CP~Z~H(PH(C~H~-~,~,~-~-BU~)) with loss of H2 to generate a transient phosphinidene, in much the same way as that previously suggested for the trapping of the generated phosphinidene by PMe3 to yield Cp2Zr(PCsH2-2,4,6-t-B~3)(PMe3) (eq In the presence of KH, the transient phosphinidene is trapped as 4 (Scheme 1). When larger ancillary ligands, (i.e., pentamethylacyclopentadienyl ligands) are present, the analogous phosphinidene-hydride anion is not stable and P-C bond cleavage results affording 6 and 7. These views are supported by observations made while monitoring the one-pot synthesis of 6 and 7 by 31P{1H} NMR spectroscopy. These experiments confirmed the initial Figure 9. (a) Truncated orbital energy diagram for the formation of 5 in the reaction mixture, as evidenced by Cp2Zr and P fragments and the species (Cp2Zr)2@-P).(b) Schematic depiction of the SOMO and the Zr-P bonding the appearance of the resonance a t 117.0 ppm. Over orbitals. the next l12 h, this resonance gradually diminished and was replaced by a doublet of doublets a t 134.3 ppm, species CP*~Z~H(PH(C~H~-~,~,~-~-BU~)), yielding the attributed to species 8. When the solution was left to is supported transient intermediate Cp*zZr(PH). This stand, this signal was subsequently replaced by resoby the observation of RH in the reaction mixtures via nances attributable to two products, the minor compolH NMR spectroscopy. Subsequent P-H addition and nent 9 and the major product 6, as evidenced by the RH elimination reactions with a second equivalent of resonances at 449.5 and 959.0 ppm. These observations phosphine yields 8. A similar mechanism involving a suggest that 8 and 9 are intermediates en route to the reactive phosphinidene intermediate has been impliIn addition, the isolation of 7 from the P2 product, 6. cated in the formation of Cp*2zr[(P(C&-2,4,6-Me3))21.6ps one-pot mixture, albeit in low yield, is viewed as the Thus, it appears that, in the case of the analogous trapping of a short-lived P1 intermediate. It is notephosphide-hydride intermediates Cp2ZrH(PH(C6H2worthy, however, that the mixed-valent nature of 7 the 2,4,6-t-Bu3)) and Cp*2ZrH(PH(csH2-2,4,6-t-Bu3)), suggests a reaction sequence involving reduction. This lesser steric demands of ancillary ligands in the former aspect of the mechanism remains poorly understood. favor Ha elimination while the greater steric congestion The initial step in the reaction of PH2(C&-2,4,6-tin the latter induces P-C bond cleavage. B u ~ )with (Cp*2Zr(N2))2@-N2)l4is thought to be the Extended Hlickel Molecular Orbital Calculaoxidative addition of the PH bond to Zr (Scheme 2). The The bonding in 7 , 8, and 10 was probed by tions. resulting hydride-phosphide intermediate is reactive. We propose that the steric demands about the metal (25)Ho,J.; Breen, T. L.; Ozarowski, A. J.; Stephan, D. W. Znorg. Chem. 1994,33,865. center induce P-C bond cleavage from the intermediate

Fermin et al.

4256 Organometallics, Vol. 14, No. 9, 1995

EHMO calculations using suitably simplified models. Overlap of the P-P x-bond with the la1 frontier orbital of the CppZr fragment (Figure 7(a)Ip6yields the 2a1 molecular orbital in CppZr[(PH)pl. A a-bonding interaction (bp symmetry) arises from the mixing of P p-orbitals with the bp frontier orbital of the CppZr fragment. The HOMO (2a1) for CppZr[(PH)p]is primarily P p-orbitals with a lesser contribution for the la1 of the CppZr fragment. The LUMO,like most metallocene(IV) derivatives, remains primarily the la1 frontier orbital of the CppZr. Similar bonding occurs in [CppZrPsl- although the bp molecular orbital is more stable than that in CpzZr[(PH)p]- (Figure 8). This is attributed to the greater overlap of the P p-orbitals with the bp metallocene fragment frontier orbital as a result of the greater P-Zr-P angle. This P-Zr-P angle also alters the nature of the HOMO in [CppZrPsl- as more significant overlap of P p-orbitals with the la1 metallocene fragment frontier orbital results. The LUMO is an admixture of the metal-based bl frontier orbital with a contribution from a p-orbital on the central P atom. The primary (T interaction in (CppZr)p@-P)arises from the mixing of the p,-orbital on P with the la1 orbitals of the CppZr fragments (Figure 9). Each of the two remaining (26) Lauher, J., Hoffmann, R. J . Am. Chem. SOC.1976,98,1729.

orthogonal p-orbitals on P are of the correct symmetry for interaction with the bp orbitals of the respective CpzZr fragments. This orthogonal x-interactions are similar to those seen in allenes. The singly occupied orbital in this Zr(IV)/Zr(III) species is of 2a1 symmetry and is the admixture of the two la1 CpzZr fragment orbitals. Summary. The above chemistry demonstrates that a sterically demanding environment may induce P-C bond cleavage and thus provide access to substituentfree P derivatives. The generality of this approach and the utility of the derived products in the construction of main group polyatomic anion complexes is currently being explored.

Acknowledgment. Support from the Petroleum Research Fund administered by the American Chemical Society is gratefully acknowledged. Support from the NSERC of Canada is also acknowledged. Supporting Information Available: Tables of crystallographic parameters, hydrogen atom parameters, and thermal parameters (29 pages). Ordering information is given on any current masthead page.

OM9502502