5962
Journal of the American Chemical Society
/
100:18
range was 4 kbar. The results, shown in Figure 1, may be summarized by the statement that the activation volume for the concerted shift (I) equals -30 f 5 cm3/mol and that for the stepwise reaction (11) is +10 f 2 cm3/mol. These observations clearly confirm that this activation parameter provides a criterion for concertedness in sigmatropic shifts as well as for cycloadditions.
/
August 30, 1978 4
I
-27
(cH,)~-T~cc(cH,), CI'
'P(CH&
T
1
Acknowledgment. The National Science Foundation generously supported this investigation.
C'
[gc.p= 13 H z )
References and Notes Paper 52 in the Series "Kinetics of Reactions in Solutions under Pressure". Summarized in W. J. le Noble,Prog. Pbys. Org. Cbem., 5, 207 (1967);T. Asano and W. J. le Noble, Cbem. Rev., 78,407 (1978). C.Walling and J. Peisach, J. Am. Cbem. SOC.,80, 5819 (1958). W. J. le Noble and B. A. Ojosipe, J. Am. Cbem. SOC.,97,5939 (1975). K. F. Fleischmann and H. Kelm, Tetrahedron Lett., 3773 (1973).W. J. le Noble and R. Mukhtar, J. Am. Cbem. SOC., 96, 6191 (1974):97,5938
(1975). K. R. Brower, J. Am. Cbem. SOC.,83,4370 (1961). C.Walling and M. Naiman, J. Am. Cbem. SOC.,84,2628 (1962). The rearrangement of allylic azides is a closely related case: W. J. le Noble, J. Pbys. Cbem., 67,2451 (1963). K. R. Brower and T. L. Wu, J. Am. Cbem. SOC.,92,5303 (1970). R. C.Neuman and M. J. Amrich, J. Am. Cbem. SOC., 94,2730(1972). C. A. Stewart, J. Am. Cbem. SOC.,94,635 (1972). F. J. Dinan and H. Tieckelman, J. Org. Cbem., 29, 1650 (1964).J. E. Lister 90,4361 (1969).U. Schollkopf and H. Tieckelman, J. Am. Cbem. SOC., and I. Hoppe, Tetrahedron Lett., 4527 (1970);Justus Liebigs Ann. Cbem.,
765, 153 (1971).
W. J. le Noble,* M. Rajaona Daka Department of Chemistry, State University of New York Stony Brook, New York 1 I794 Received March 14. I978
Multiple Metal-Carbon Bonds. 10.1 Thermally Stable Tantalum Alkylidyne Complexes and the Crystal Structure of Ta(q5-C~Me5)(CPh)(PMe3)2C1 Sir: Alkylidene complexes of niobium and tantalum have been prepared by deprotonating (formally2 or actually3) the acarbon atom of an alkyl ligand in a M5+ complex. We now can prepare thermally stable Ta n e ~ p e n t y l i d y n e ~complexes ?~~ similarly by deprotonating a cationic neopentylidene complex. Both neopentylidyne and benzylidyne complexes can be prepared more cleanly by accelerating abstraction of the alkylidene a-hydrogen atom by the alkyl ligand in neutral alkyl/ alkylidene complexes with trimethylphosphine. Addition of 1 mol of PMe3 to red TaCp(CHCMe3)C12' in toluene yields the sparingly soluble yellow adduct, TaCp(CHCMe3)C12(PMe3) (l).sa In T H F in the presence of 3-5 additional mol of PMe3,l reacts with Ph3P=CH2 to give [Ph3PCH3]+CI- (-90% yield). Removing the T H F and extracting the residue with pentane gives moderately soluble, pale yellow, crystalline 2 in 60% crude yield (30% yield of pure, recrystallized 2; eq 1, path A). Calcd for TaC16H32P~Cl:C, 1. PMe,,
I
THF (pathA)
2. Ph,P=CH2
\
I
348
97
(ppm)
35
0
18
(CCMed(PM3)zC1(2).
termediate from which a proton is removed by Ph3P=CH2 to give 2 (cf. the reaction of TaCpz(CHCMe3)Cl with PMe3 to give' [TaCp*(CHCMe3)(PMe3)1+Cland the deprotonation of [TaCpzMez]+BF4- by Me3P=CH2 to give TaCp2(CH2)(CH3)3). Yellow 2 is indefinitely stable at 25 "C in solution. In the solid state it reacts only slowly with oxygen but is more sensitive to water or other protic solvents. It sublimes with little decomposition at 100 "C and 1 p and is a monomer in cyclohexane (mol wt, 527 cryoscopically; theory, 503). The 13C{'HJN M R spectrum of 2 is shown in Figure 1. The most important feature is the 1:2:1 triplet at 348 ppm due to the neopentylidyne a-carbon atom coupled to two equivalent phosphorus nuclei. Some coupling of the neopentyl methyl carbon atoms (C2) to phosphorus can also be seen. In the gated decoupled spectrum the resonances for C5 and C3 are broad (owing to long-range C H coupling), but every other peak is split into the appropriate multiplet. The neopentylidyne Ca resonance is -100 ppm farther downfield than the neopentylidene Ca resonance in TaCp(CHCMe3)C127 and related molecules',2 and is in the same region where the alkylidyne a-carbon resonances in group 6 complexes such as Br(C0)4W=CPh are found.5 The ' H N M R spectrumsb also suggests that the phosphine ligands are equivalent. The reaction of LiCH2CMe3 with TaCp(CHCMe3)Clz at 25 "C in pentane gives TaCp(CHCMe3)(CH2CMe3)CI (3) in 85-90% isolated yield as a pentane-soluble, thermally stable, sublimable, orange oiL9 On adding PMe3 to a pentane solution of 3 at 25 "C 1 mol of neopentane evolves immediately and 2 forms quantitatively (eq 1, path B); no other products can be seen by IH NMR. We believe that PMe3 coordinates to 3 to give a pseudo-five-coordinate complex in which abstraction of the neopentylidene ligand's a-hydrogen atom by the neopentyl ligand is more favorable than it is in pseudotetrahedral 3. It is also possible that abstraction of an a-hydrogen atom from the neopentyL by the neopentylidene ligand is similarly "accelerated", but, since this reaction is degenerate, we cannot tell which a-abstraction process is faster. The reaction of thermally unstable TaCp(CH2Ph)3C110in benzene with 2 mol of PMe3 gives less soluble, red T ~ C P ( C H ~ P ~ ) ~ C I ((by P MIH ~ ~ NMR) ) which dissolves completely on heating to 60 "C for 2 h to give an orange solution which contains 2 mol of toluene (by ' H N M R and GLC) and 4 (eq 2). We do not yet know whether pseudo-six-coordi-
CI-Ta-CH2Ph PhCH,/
+ 2PMe, + 2CHJ'h + C1-Ta-PMes
C ' HJh'
/ \
Me3
c, 'Ph
38.23; H, 6.41; CI, 7.05. Found: C, 38.09; H, 6.26; CI, 6.99. We postulate that [TaCp(CHCMe3)Cl(PMe3)21+CI-is an in0002-786317811500-5962$01.OO/O
51
Figure 1. The 22.63-MHz I3C('H) NMR spectrum of Ta(v5-C5H5)-
4
0 1978 American Chemical Society
(2)
Communications to the Editor
5963
nate TaCp(CHzPh)&l(PMe3) decomposes more readily than pseudo-five-coordinate TaCp(CHzPh)3CI or whether TaCp(CHPh)(CH2Ph)ClL. (x = 0 or 1) is an observable intermediate (cf. Ta(qs-CsMes)(CHPh)(CH2Ph)Cl below). The 13Cspectrum of 4 is similar to that of 2;' the benzylidyne a-carbon resonance occurs at 334 ppm with Jcp = 21 f 1 Hz. Both Ta( q5-CsMe5)(CHCMe3) (CH2CMe3)C1'2a and Ta(qs-C5Mes)(CHPh)(CH2Ph)C1'Zb react instantly with PMe3 to give Ta(q5-CjMes)(CCMe3)(PMe3)2CI (Sa) and Ta(qs-CsMe5)(CPh)(PMe3)2Cl (5b) quantitatively at 25 'C. The I3C N M R spectrum of each is similar to that of the corresponding q5-C5Hs analogue above (for 5a 6ca 354 (Jcp = 19 f 1 Hz); for 5b 6 ~ 345 . ( J c p = 20 f 1 Hz)). 5a and 5b are more soluble in pentane, but otherwise their physical properties are very similar to those of 2 and 4, respectively. They too are monomers (mol wt for 5b, 576 in cyclohexane (average of 565 and 586); theory, 593). Bridging alkylidyne ligands are found in the c ~ m p l e x e s , ' ~ [M(CH2SiMe3)2(CSiMe3)]2 (M = N b or Ta), in which M is Figure 2. Molecular geometry of the Ta(q5-C5Mes)(CPh)(PMe&CI in the same formal oxidation state as Ta in 2,4, or 5, but the molecule (ORTEP diagram; 50% probability contours for all atoms except valence electron count of hypothetical monomeric hydrogen atoms). M(CHZSiMe3)2(CSiMe3) is 10 (and the complex is not crowded) while the valence electron count in 2,4, and 5 is 18 atom. The two "pseudo-trans" angles, CI-Ta-CI and PIand they are relatively crowded molecules. Since the formal Ta-P2, are equivalent with values of 125.54 (22) and 125.41 valence electron count about each metal does not change on (7)', respectively. The four cis angles are CI-Ta-PI = 80.29 dimerization through the alkylidyne ligand, steric hindrance (22), PI-Ta-CI = 76.66 (7), C1-Ta-P2 = 75.76 (7), and may be the primary reason why 2,4, and 5 do not dimerize. P2-Ta-C' = 78.96 (22)'. The tantalum-phosphorus bond We expect the alkylidyne a-carbon atom to be nucleophilic lengths are Ta-PI = 2.567 (2) and Ta-P2 = 2.565 (2) A, while since the alkylidyne ligand is formed in a manner similar to that the tantalum-chlorine distance is 2.548 (2) A. All other disof an alkylidene ligand, and alkylidene ligands are nucleophilic tances and angles in the structure are normal. in TaCp(CHCMe3)C12 and all other complexes with the metal in that formal oxidation state (3+ or 5+ depending on one's Acknowledgments. This work was generously supported by point of view).'-3 the National Science Foundation (Grant NO. CHE76-07410 The molecular geometry of Ta(q5-C~Me5)(CPh)(PMe3)2C1to R. R. Schrock and CHE77-0498 1 to M. R. Churchill). We (5b) has been determined unequivocally by means of a sin- also thank the Francis N. Bitter National Magnet Laboratory gle-crystal X-ray diffraction study. It crystallizes in the cenfor the use of their high field IH and I3C NMR facilities. trosymmetric monoclinic s ace group P21/c with a = 10.5129 (12) A, b = 13.8147 (16) c = 18.1793 (21) A, j3 = 102.945 References and Notes (S)', V=2573.1 (5)W3,andp(calcd) = 1.531 g ~ m - ~ f o r m o l (1) Part 9: R. R. Schrock, L. W. Messerle, C. D. Wood, and L. J. Guggenberger, wt 592.90 and 2 = 4. Intensity data were collected by the 0-20 J. Am. Chem. SOC.,100,3793 (1978). 100,3359 (2) Part 8: R. R . Schrock and J. D. Fellmann, J. Am. Cbem. SOC., scan technique on a Syntex P21 automated four-circle dif11978). -, fractometer and were corrected for absorption via an empirical (3) (a) Part 7: R. R. Schrock and P. R. Sharp, J. Am. Chem. SOC.,100, 2389 method based upon a series of scans ( p 47.3 cm-I). The 97, 6577 (1975). (1978); (b) R. R . Schrock, J. Am. Chem. SOC., (4) One referee vehemently objected to the use of the alkylidyne nomenclature structure was solved via Patterson and difference-Fourier instead of Fischer's5carbyne nomenclature. "Alkylidyne" is not only logical methods; full-matrix least-squares refinement (Ta, CI, P, and alkylidene alkylidyne) but has been used elsewhere in organic (alkyl and organometallic chemistry for at least 10 years. (See, for example, C anisotropic; H isotropic) has led to the final discrepancy alkylidynetricobalt nonacarbonyl cluster chemistry.6) indices RF = 4.1% and R w =~3.5% for those 3 140 reflections (5) (a) E. 0. Fischer and U. Schubert, J. Organornet. Cbem.,100, 59 (1975). with 4' < 20 < 45' (Mo Kcu radiation) and IFo[ > u(lF,,l). (b) E. 0. Fischer, Adv. Organomet. Chem., 14, 1 (1976). (c) E. 0. Fischer. T. L. Lindner, and F. R. Kreissl. J. Organomet. Chem., 112, C27 (1976). All atoms other than the hydrogen atoms of the qs-CsMes li(d) The largest class of alkylidyne complexes consists of examples of the gand were located directly; the molecular geometry is shown type X(C0)5MGCR (X = CI, Br, I; M = Cr, Mo, W: R = CeH5,CH3), most of which are barely stable at room temperat~re.~',~ Other examples, e.g., in Figure 2. (t15-C5H5)(C0)2W~CR,5C are apparently more stable. The central tantalum (5+) atom is surrounded in a "four(6) D. Seyferth, Adv. Organomet. Chem., 14, 97 (1976). legged piano stool" arrangement by the $-CSMeS ligand and (7) S.J. McLain. C. D. Wood, and R. R. Schrock, J. Am. Chem. SOC.,99,3519 (1977). the four monodentate ligands. The tantalum-benzylidyne (8)(a) 'H NMR (T,CD2CI2): 4.01 (d, 1, JHp = 1.6 Hz, =CKMe3). 4.15 (d, 5, linkage (Ta-C]) is 1.849 (8) 8, in length-Le., some 0.40 JHP= 2.9 Hz, t15-C5H5), 8.40 (d, 9, JHP= 8.6 Hz. PMe3), 8.87 (s, 9. shorter than a normal Ta-C(sp3) single bond and -0.18 A =CHCMe3). (b) 'H NMR (T,C6D6): 4.67 (t, 5, JHp 2.5 Hz, Cp), 8.65 (1: 2(br):1, t, 9,JHP 3.5 Hz, PMe3), 8.76 (s, 9, M e 3 ) . shorter than the tantalum-neopentylidene linkage of 2.030 (6) (9) 'H NMR (T,CsDe): 4.08 (s, 1, =C/-CMe3), 4.27 (s, 5, Cp). 7.90 (d, 1, A found in Ta(qS-C~H5)2(CHCMe3)CLi4 Distortion of the CHAHBCM~~, J = 13 Hz). 8.90 (s, 9, =CHCM%), 8.95 (s. 9, CH2CMe3), 9.27 (d, 1. C H A Y C M ~J= ~ , 13 Hz). 13CNMR (gated decoupled, parts per Ta-Ci-C2 angie (171.8 (6)') from 180' presumably results million, C&): 240 (d, =CHCMe3, JCH= 81 HZ), 104 (d, Cp, JCH = 177 from repulsive intermolecular interactions. Nevertheless, it Hz), 79.1 (dd, CHAHBCMe3,J = 100, 110 Hz), 47.3 (s, =CHCMe3), 34.8 (S,CH2CMe3),34.6 (4, =CHCMe3, JCH = 125 Hz), 33.4 (q, CH2C@, JCH seems appropriate to regard the tantalum-benzylidyne linkage = 125 Hz). as essentially a triple bond. The C'-C2 bond length of 1.467 (10) Brown TaCp(CH2Ph)3CIc a n De prepared from Ta(CH2Ph)3C12(R. R. Schrock, (10) A is quite in keeping with its being an unperturbed J. Organomet. Chem., 122, 209 (1976)) and NaCp in THF at -78 OC, followed by warming to room temperature, removal of solvent, and recrysC(sp)-C(sp2) single bond. tallization from benzene-pentane, 'H NMR ( T ,CeD& 4.65 (s,5, Cp), 7.13 Ta-C(cyclopentadieny1) bonding distances are (cyclically) (s, 6, CH2); phenyl region is complex (relative area 15). (11) 13C(1HlNMR spectrum of 4 (parts per million from MelSi in C6D6):334 (t, 2.417 (8), 2.460 (8), 2.518 (Y), 2.455 (9), and 2.397 (9) A; the C', JCP 21 f 1 Hz), 153.2 (Cipso),128.5 and 127.6 (Coand C,,,), 122.7 average value is 2.449 A. The four monodentate ligands define (CA, 98.7 k . C,I. 16.9 It. PMe?. Jrp = 13 f 1 Hzl.
1,
\
~
-
-
%
E
5964
/
Journal of the American Chemical Society
100:18
/
August 30, 1978
hydrogen atom abstraction is easier3band that the final product (in essentially quantitative yield) is a bisneopentylidene complex, la. It is not necessarily true that the immediate precursor is also a bisneopentylidene complex (see later), although this is what we have assumed for the present. The evolution of neopentane from Nb(CH2CMe3)3(CHCMe3)3b is also accelerated dramatically on adding L and the analogous products, lb, can be isolated in -75% yield. All complexes are orange to red, crystalline, sensitive to air and moisture, and moderately soluble in pentane. A molecular weight determination for T~(CHCM~~)~(CH~CM~~)(PM~~)Z in cyclohexane (cryoscopically) showed it to be a monomer (mol wt found, 540 f 40; two runs at 0.006 and 0.01 1 M). The presence of two S . J. McLain,lSaC. D. Wood neopentylidene ligands is suggested by the instantaneous reL. W. Messerle,lSbR. R. S ~ h r o c k * ~ action ~ ~ of 1 with acetone to give 2 mol of di~obutylene.~ Department of Chemistry The IH and 13C N M R spectra of 1 show that (i) it has Massachusetts Institute of Technology a plane of symmetry and (ii) the neopentylidene ligands Cambridge, Massachusetts 02139 are nonequivalent on the N M R time scales (60-270 MHz) at 30 OC. For example, the IH N M R spectrum of TaFrederick J. Hollander, Wiley J. Youngs (CHCMe3)2(CH&Me3)(PMe3)2 at 270 MHz shows three Melvyn Rowen Churchill* CMe3 singlets, a PMe3 triplet ( J H P= 2.4 Hz), a neopentyl H, Department of Chemistry triplet at 7 9.39 ( J H P= 19 Hz), and two neopentylidene H, State University of New York at Buffalo resonances at 7 7.92 and 3.07 ( J H PS 2). On broad-bandBuffalo, New York 14214 decoupling 31Pthe neopentyl H, triplet resonance becomes a Received January 20, 1978 singlet and the broadened neopentylidene H, resonances sharpen. In the gated decoupled 13C spectrum we find two neopentylidene C, doublet resonances at 7 274 and 246 with low C H coupling constants ( J C H= 95 and 85 Hz, respectively, J c p S 5 Hz) characteristic of neopentylidene ligands in Multiple Metal-Carbon Bonds. 11.’ crowded environment^.^ When L = PMe2Ph (and M = Ta) Bisneopentylidene Complexes of Niobium and Tantalirm the gated decoupled 13CN M R spectrum establishes that the Sir: phosphine’s methyl groups are diastereotopic; two P-Me quartets are found (at 6 22.5 and 19.2; J C H = 128 Hz) but only In a recent communication’ we showed that alkylidyne one type of phenyl ring. In the four most likely structures complexes can be prepared from alkyl/alkylidene complexes by “accelerating” (with PMe3) the nondegererate a-hydrogen atom transfer from the alkylidene a-carbon atom to the alkyl a-carbon atom. An interesting question is what happens if a-hydrogen abstraction occurs in a di- or trialkyl-alkylidene complex? Either an alkyl/alkylidyne or a bisalkylidene complex in principle could be formed. Dimeric alkyl-“alkylidyne” A B C7 D complexes are known.Is2 We now want to show that bisneopentylidene complexes (of N b and Ta) can also be the ultimate (A-D) the neopentylidene ligands must not freely rotate about products of accelerated a-hydrogen abstraction reactions in the M=C axis. We favor A, based only on the relatively large di- or trineopentyl-neopentylidene complexes. They can also -CH2-phosphorus coupling constant (1 9 Hz) vs. the relatively be prepared from alkylidyne-chloride complexes by substismall =CH-phosphorus coupling constant (55Hz). tuting the chloride with the alkyl.3a Thermally unstable Ta(CH*CMe3)&1 can be prepared by Ta(CH2CMe3)3(CHCMe3)3b$c is relatively stable thermally adding HCl to Ta(CHlCMe3)3(CHCMe3) at -78 “C in as a solid or in pentane or benzene (for weeks at 25 “C) probpentane.3bWe added PMe3 in an attempt to trap the proposed ably because (i) intermolecular decomposition pathways are decomposition product, M(CH2CMe3)2(CHCMej)Cl;* the slow and (ii) the first observed change is a degenerate intrahigh yield product is, instead, Ta(CHCMe3)2(CI)(PMe3)2 (2a, molecular scrambling of a-hydrogen atoms (AC* * 28 kcal eq 2). Analogous procedures give thermally stable products mol - I ; no neopentane evolves during the scrambling process3b). In the presence of at least 2 mol of L (L = PMe3 or PMe2Ph), M(CH2CMe3),Cl however, neopentane (1 mol) does evolve, comparatively rapidly when L = PMe3 (2 h at 25 “C) but more slowly (2 weeks at 25 “C) when L = PMe2Ph. We therefore propose that a five-coordinate intermediate is formed (eq 1) in which a-
(CH2CMe3)C13is treated with 2 mol of LiCH2CMe3 in pentane at -78 ‘C: 6(CHCMe3) 232 (Jcn = 76 f 2 Hz), T (CKNe3) 6.25, T (CHAH~CMe3) 9.08, T (CHA&CMe3) 9.25 = 14 Hz). (b) Ta(s5-CsMe5)(CHPh)(CH2Ph)CI is formed when Ta(CH2Ph)3C12 is treated with LiC5Me5in THF for 1 day: 8 (CHPh) 222 (Jcn = 85 f 2 Hz), T (CWh) 6.71, T (CHAHBPh) 7.66, T (CH,&Ph) 7.82 (&Ab = 11 Hz). Ta(05-C5Me4Et)(CHPh)(CH2Ph)CI forms similarly and has been analyzed. Calcd for TaC25H3&I: C, 54.91; H, 5.52; CI, 6.48. Found: C. 54.30; H, 5.40; CI, 6.37. (13) (a) W. Mowat and G. Wilkiqson, J. Chem. SOC.,Dalton Trans., 1120 (1973); (b) F. Huq, W. Mowat, A. C. Skapski, and G. Wilkinson, Chem. Commun., 1477 (1971). (14) (a) M. R. Churchill, F. J. Hollander, and R. R. Schrock, J. Am. Chem. SOC., 100,647 (1978); (b) M. R. Churchill and F. J. Hollander. Inorg. Chem., 17, 1957 (1978). (15)(a) National Science Foundation Predoctoral Fellow, 1975-1978; (b) Dow Chemical Central Research Fellow, 1976-1977; (c) Alfred P. Sloan Fellow, 1976-1978.
( Me3CCH2),M=C
/H
‘CMe,
L
“(Me,CCH,),M(CHCMe,)L”
,f,,,L
CH, -M’
L = PMe, or PMepPh
/
(1)
>I’L 3
firM
= Ta b, M = Nb L = PMe, or PMepPh 2a,
(2)
“(MeJCCH,)M(CHCMeJ2L” 4 ,M(CHCMe3)2(CH,CMe,)L2 la, M = Ta
b,M=Nb 0002-7863/78/1500-5964$01 .OO/O
where L = PMe2Ph or M = Nb. Apparently even the larger, less basic PMe2Ph can coordinate to incipient M(CH2CMe3)2(CHCMe3)Cl to give (most likely) thermally 0 1978 American Chemical Society
