Synthesis and Characterization of Re( VII) Alkylidene Alkylidyne

Received September 23, 1991 ... [Re(C-t-Bu)(CH-t-Bu)CI,1, is a versatile precursor to a variety of bisalkoxide complexes of the ..... BuNH3Cl is treat...
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J. Am. Chem. SOC. 1992, 114, 3367-3380

Cp*,TiH to Cp*FvTi, and regenerating H,.

3367

Cp*,TiPr, 99476-28-5; Cp*,TiCH2CMe3, 103351-92-4; Cp*,TiPh, 115564-94-8;Cp*FvTi, 53436-87-6.

Acknowledgment. This investigation was supported by The Netherlands Foundation for Chemical Research (SON) with financial aid from The Netherlands Organization for Scientific Research (NWO). We acknowledge valuable contributions from Mr. A. Kiewiet (MS) and Mr. W. R. Beukema (software). Registry No. Cp*,TiH, 131954-87-5;Cp*,Ti(H)Cl, 115912-71-5; Cp*,TiMe, 99476-26-3;Cp*,Ti(Me-d3),135973-60-3;Cp*,Ti(Me)Cl, 107534-13-4;Cp*2TiEt,99476-27-4;Cp**TiCH=CH2, 13 1954-86-4;

Supplementary Material Available: Tables of rate constants for the thermolysis of Cp*2TiR (R = Et, Pr) in THF, details on the synthesis of Cp*,TiH and (c~*-d,~),TiD, and spectral data and plots of kinetic data for thermal decompositions of Cp*,TiR (R = Me, Pr, CH2CMe3,Ph) catalyzed by Cp*,TiH and the effect of propene on the thermolysis for R = Pr (6 pages). Ordering information is given on any current masthead page.

Synthesis and Characterization of Re( VII) Alkylidene Alkylidyne Complexes of the Type Re( CR’)( CHR’)( OR), and Related Species Robert Toreki, Richard R. Schrock,* and William M. Davis Contribution from the Department of Chemistry 6-331, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139. Received September 23, 1991

Abstract: A convenient one pot synthesis of Re(NAr),(py)Cl, consists of addition of excess trimethylchlorosilane, pyridine, and Reand 2,6-dimethyIaniline (ArNH2) to Rez07or [NH,] [Reo,] in dichloromethane. Re(N-2,6-C6H3-i-Pr2)2(py)C13 (N-t-Bu),CI, can be prepared similarly in high yield. Alkylation of these species with dineopentyl or dineophyl zinc or Grignard reagents affords complexes of the formula Re(NR),(CHR’)(CH,R’) (R = 2,6-C6H3Me2,2,6-C6H3-i-Pr2or tert-butyl; R’ =

CMe3or CMe2Ph). Treatment of Re(NR),(CHR’)(CH,R’) complexes with an appropriate HC1 source yields dimers of the general formula [Re(CR’)(CHR’)(RNH,)Cl,],, which exist as a mixture of two isomers. An X-ray study of [Re(C-tBu)(CH-~-BU)(A~NH~)CI,]~ ( a = 10.05 (1) A, b = 21.65 (3) A, c = 10.99 (1) A, p = 98.28 (9)O, Z = 2, fw = 1031.08,p(ca1cd) = 1.446 g/cm3, space group = P21/n) showed it to contain two bridging halides with mutually cis alkylidene and alkylidyne ligands trans to the bridging halides. Several monomeric derivatives having the general formula Re(C-t-Bu)(CH-t-Bu)L2Clz (L = t-BuNH,, pyridine, ‘/,TMEDA, ‘/,phenylenediamine (pda)) were prepared, and related monoadducts, Re(C-t-Bu)with HCl(g) in dimethoxyethane (CH-t-Bu)(L)Cl,, have been observed in solution. Treatment of Re(C-t-Bu)(CH-t-Bu)(pda)CI, (x > 1). An alternative route to [R~(C-~-BU)(CH-~-B~)CI,]~ consists affords air- and water-stable [R~(C-~-BU)(CH-~-BU)C~,]~ of treatment of Re(O),(CH-t-Bu)(CH,-t-Bu) with HCl(g) in dimethoxyethane. Re(O),(CH-t-Bu)(CH,-t-Bu) is prepared via intermediate Re(NAr)(O)(CH-t-Bu)(CH2-t-Bu). by the acid-catalyzed hydrolysis of Re(NAr)2(CH-t-Bu)(CHz-t-Bu) Re(NAr),(CH-t-Bu)(CH,-f-Bu) and Re(O),(CH-t-Bu)(CH,-t-Bu) conproportionate in solution to give Re(NAr)(O)(CHt-Bu)(CH,-t-Bu). [Re(C-t-Bu)(CH-t-Bu)CI,1, is a versatile precursor to a variety of bisalkoxide complexes of the general (OR = 0-t-Bu, OCMe,(CF3), OCMe(CF,),, 0-2,6-C6H3-i-Pr,,OSi(r-Bu),). Syn and formula Re(C-t-Bu)(CH-t-Bu)(OR), complexes interconvert thermally or photochemically. In syn rotamers anti rotameric forms of the R~(C-~-BU)(CH-~-BU)(OR)~ usually JCH= 120-135 Hz and in anti rotamers JCH= 157-184 Hz. An X-ray study of syn-Re(C-t-Bu)(CH-t-Bu)[OCMe(CF3)2]2(THF)( a = 9.891 (1) A, b = 17.543 (2) A, c = 16.570 (2) A, /3 = 95.90 (2)O, Z = 4, fw = 759.69, p = 1.764 g/cm3, space group = P2,/n) showed it to have a structure approximately halfway between a face-capped tetrahedron (THF trans to the neopentylidyne ligand) and a trigonal bipyramid.

Introduction Rhenium is one of three metals (molybdenum and tungsten being the other two) that are active for the metathesis of olefins in classical metathesis systems.*v2 Although both homogeneous and heterogeneous molybdenum and tungsten catalysts are known, rhenium catalysts of the classical type (e.g., Re207on alumina) are heterogeneous. One of the potential advantages of rhenium catalysts is that they may tolerate functionalities (e.g., the ester in methyl oleate) more than tungsten or molybdenum catalysts., Approximately ten years ago evidence began to accumulate in favor of the highest possible oxidation state for tungsten metathesis catalysts (do if the alkylidene ligand is viewed as a d i a n i ~ n ) . ~ - ~ (1) Ivin, K. J. Olefin Metathesis; Academic: New York, 1983. (2) Dragutan, V.; Balaban, A. T.; Dimonie, M. Olefin Metathesis and Ring-Opening Polymerization of Cyclo-Olefins, 2nd ed.; Wiley: New York, 1985. (3) Mol, J. C. J . Mol. Cutal. 1991, 65, 145. (4) Schrock, R. R. J . Organomet. Chem. 1986, 300, 249.

0002-7863/92/1514-3367$03.00/0

Therefore we felt that it should be possible to prepare wellcharacterized, soluble Re(VI1) alkylidene complexes. At that time organometallic chemistry of Re(VI1) was extremely rare.&I0 We chose to attempt to synthesize complexes of Re(VI1) containing imido ligands in the belief that imido complexes would not be reduced as readily as oxo complexes in alkylation reactions and that unwanted bimolecular reactions might be slowed down or prevented entirely if imido ligands are present instead of oxo ligands. (5) Kress, J. R. M.; Russell, M. J. M.; Wesolek, M. G.; Osborn, J. A. J. Chem. Soc., Chem. Commun. 1980,431. ( 6 ) Kress, J.; Wesolek, M.; Le Ny, J.-P.; Osborn,J. A. J. Chem. Soc., Chem. Commun. 1981, 1039. (7) Kress, J.; Wesolek, M.; Osborn, J. A. J. Chem. Soc., Chem. Commun. 1982, 514. ( 8 ) Mertis, K.; Wilkinson, G. J . Chem. SOC.,Dalton Trans. 1976, 1488. (9) Beattie, I. R.; Jones, P.J. Inorg. Chem. 1979, 18, 2318. (10) Herrmann, W. A. Angew. Chem., Int. Ed. Engl. 1988, 27, 1297.

0 1992 American Chemical Society

3368 J . Am. Chem. SOC.,Vol. 114, No. 9, 1992

Toreki et al.

In 1983, Nugent” reported that Re03(0SiMe3) reacts with (Re(V)) species, and polymers (from cyclic olefins) will be de3 equiv of (t-Bu)NH(SiMe3) in hexane over the course of 2 days scribed in subsequent papers. to yield highly soluble, crystalline, yellow R ~ ( N - ~ - B u ) ~ ( O S ~ M ~ , ) Results in -65% yield. We found that addition of HC1 in ether to this Improved Synthesis of Imido Species. A preparation of Rematerial yielded 1 equiv of tert-butylammonium chloride and (NAr),(py)Cl, (Ar = 2,6-C6H3Me2)that is superior to that inbright orange, highly crystalline Re(N-t-Bu),Cl, in 83% yield.’, volving ArNCO and Re03(OSiMe3)’Sis shown in eq 2. The Re(N-t-Bu),Cl, could be alkylated to give Re(N-t-Bu),R, species, reaction is over in 2 h at 25 OC, and Re(NAr),(py)C13 can be but when R = CH,-t-Bu, Re(N-t-Bu),(CH-t-Bu)(CH,-t-Bu), a isolated in 86% yield after removal of pyridinium hydrochloride yellow oil was formed by a hydrogen abstraction, instead of and recrystallization of the crude product from benzene. The Re(N-t-Bu),(CH,-t-Bu),. An analogous alkylidene complex, procedure is equally successful starting with [NH,] [Reo4], a R~(N-~-BU)~(CHS~M~~)(CH~S~M~,), was formed quantitatively CI upon photolysis of Re(N-t-Bu),(CH,SiMe,),. Unfortunately, 2Arm2 CH2C12 (2) neither alkylidene complex reacted with olefis, even very reactive 0.5 Re20, + 7 Me,SiCl excess py . 3.5 (SiMe&O olefins such as norbornene. Perhaps the most surprising finding - 4 pyHCl c1 was that 3 equiv of 2,4-lutidinium hydrochloride would react with somewhat less expensive rhenium source. It is most convenient Re(N-t-Bu),(CH-t-Bu)(CH,-t-Bu) in dichloromethane as shown as an orto use 3 equiv of aniline in the procedure, even though only two in eq 1 to give [Re(C-t-Bu)(CH-t-Bu)(t-BuNHz)C12]2 ange powder in 70% yield. Four-coordinate species such as Reare necessary, in order for the reaction to proceed relatively rapidly (C-t-Bu)(CH-t-Bu)(O-t-Bu),, Re(C-t-Bu)(CH-t-Bu)(OSiMe3),, and completely. (When only two are used, the product yield is reduced to -50% in approximately the same time period.) The +3lutHCI Re(N-t-Bu),( CH-t-Bu) (CH,-t-Bu) most important step in this type of reaction probably is attack by aniline at the metal followed by proton transfer (either directly -31ut or stepwise employing an external base) to an oxo ligand to yield 0.5 [Re(C-t-Bu)(CH-t-Bu)(t-BuNH2)Cl,]2 (1) the imido analogue and water. The water then reacts with or Re(C-t-Bu)(CH-t-Bu)(CH,-t-Bu), were prepared from [ReMe3SiC1to yield hexamethyldisiloxane and HC1, and the HC1 (C-t-Bu)(CH-t-Bu)(t-BuNHz)C12]2, but these complexes also did is removed from the reaction as the pyridinium salt, thereby driving not react with internal olefins. Addition of a Lewis acid such as what would otherwise likely be an equilibrium (see below) to the AlC13 did yield catalytically active solutions, e.g., Re(C-t-Bu)right. (More complicated variations involving ArNH(SiMe3) (CH-t-Bu)(O-t-Bu), in the presence of aluminum trichloride in cannot be discounted but would be expected to be slower for steric dichloromethane would metathesize 170 equiv of cis-2-pentene reasons than reactions involving ArNH2.) Re(NAr’)2(py)C1315 to equilibrium in 15 min at room temperature.’, (By that time (Ar’ = 2,6-C&,-i-Pr2) can be prepared in a similar fashion in active tungsten catalysts had been prepared by treating tungsten high yield. Related syntheses of tungsten” and molybdenumIs alkyl complexes with Lewis acidse6) However, no catalytically imido complexes have been developed recently. active intermediates were observed, so metathesis by Re(VI1) still The potential generality and superiority of this approach is was not proven. The tedious syntheses of tert-butylimido comillustrated by the synthesis of Re(N-t-Bu),(OSiMe3)’’ (eq 3). plexes prevented any systematic exploration of Re(VI1) at that Upon adding tert-butylamine and trimethylchlorosilane to a stage. CH2Clz Two events led to a reevaluation of the possibility of metathesis 0.5ReZ07 9t-BuNH, + 6Me3SiC1 -6t-BuNH,C1by complexes of the type Re(CR’)(CHR’)(OR),. First, facile -Z.S(SiMe,)20 routes to aryl imido complexes were developed that consisted of Re( N-t-Bu) (OSiMe,) (3) reactions between Re03(OSiMe3)and aryl isocyanates (aryl = 2,6-C6H3X2,x = Me, i-Pr, c1) to give various oxo imido species, suspension of rhenium heptoxide in dichloromethane, a bright, which when treated with pyridinium hydrochloridegave crystalline, lemon-yellow color is generated and flocculent white t-BuNH3C1 green Re(Naryl),(py)CI, complexes in 70-90% overall yield based precipitates immediately. In this case tert-butylamine acts as the on ReO3(OSiMe3).l4J5 Re(NAr)2(py)C13(Ar = 2,6-C6H3Mez) trap for HCl. If the reaction is filtered after 20 min, Re(N-treacted cleanly with 1.5 equiv of dineopentyl zinc at -40 OC to B u ) ~ ( O S ~ Mcan ~ , )be recovered from the filtrate in >90% yield give highly soluble, orange Re(NAr),(CH-t-Bu)(CH,-r-Bu) (compared with a -65% yield after 2 days”). Re(N-t-Bu)$l, quantitatively,a species that is analogous to previously synthesized is not a product of this reaction because the HC1 that is generated Re(N-t-Bu),(CH-t-Bu)(CH,-t-Bu). Second, complexes of the is removed efficiently by tert-butylamine, and Re(N-t-Bu),type M(CH-t-Bu)(NAr’)(OR), (M = Mo or W; Ar’ = 2,6(OSiMe3) therefore is not protonated. However, when the crude C6H3-i-Pr2)had been found to be very active metathesis catalysts reaction mixture containing Re(N-t-Bu),(0SiMe3) and tfor ordinary olefins when OR is strongly electron-withdrawing BuNH3Cl is treated with excess HC1 at 0 “C, the color of the (e.g., OCMe(CF3)2).16 Therefore we became convinced that solution darkens to orange-red and an additional equivalent of Re(CR’)(CHR’)(OR)2 complexes would be active for the mer-BuNH3C1precipitates. R ~ ( N - ~ - B u ) , Cthen ~ , ’ ~can be isolated tathesis of olefins if OR were strongly electron-withdrawing and, from the filtrate as large orange cubes in 85% overall yield (from moreover, that relatively facile routes to such species could be ReZO7). Twenty grams of Re(N-t-Bu),C13 can be prepared in developed using arylimido ligands as “protecting groups” in reabout 3 h, a vast improvement over the previously reported synactions analogous to those that had been outlined previously in is now thesis. Therefore [Re(C-t-Bu)(CH-t-Bu)(t-BuNH2)Clz]2 tert-butylimido chemistry. readily accessible and the preferred route to reported molecules In this paper we describe the synthesis and characterization or Re(C-t-Bu)(CH-tsuch as Re(C-t-Bu)(CH-t-Bu)(O-t-Bu), of complexes of the type Re(CR’)(CHR’)(OR), and related Bu)(CH,-t-Bu),. Excess tert-butylamine reacts with Re(N-tspecies. Reactions of such species with olefins to give metallaBu),Cl, to give Re(N-t-Bu),Cl quantitatively, most likely by cyclobutane complexes, productive metathesis, reduced rhenium double dehydrohalogenation of intermediate Re(N-t-Bu),Cl,( tBuNH,) by tert-butylamine. Alkylation of Re(NAr),(py)Cl, with 1.5 equiv of dineo(11) Nugent, W. A. Inorg. Chem. 1983, 22, 965. pentylzinc in dichloromethane produces Re(NAr),(CH-t-Bu)(12) Edwards, D. S.; Biondi, L. V.; Ziller, J. W.; Churchill, M. R.; Schrock, R. R. Organometallics 1983, 2, 1505. (CH,-~-BU)~~ rapidly and in high yield (eq 4); the alkylideneligand

-

+

,

(13) Edwards, S. Ph.D. Thesis, Massachusetts Institute of Technology, 1983. (14) Horton, A. D.; Schrock, R. R.; Freudenberger, J. H. Organometallics 1987, 6, 893. (15) Horton, A. D.; Schrock, R. R. Polyhedron 1988, 7, 1841. (16) Schrock, R. R.; DePue, R. T.; Feldman, J.; Schaverien, C. J.; Dewan, J. C.; Liu, A. H. J . Am. Chem. SOC.1988, 110, 1423.

(17) Schrock, R. R.; DePue, R. T.; Feldman, J.; Yap, K. 9.;Yang, D. C.; Davis, W. M.; Park, L. Y . ;DiMare, M.; Schofield, M.; Anhaus, J.; Walborsky, E.; Evitt, E.; Kriiger, C.; Betz, P. Organometallics 1990, 9, 2262. (18,) Schrock, R. R.; Murdzek, J. S.; Bazan, G. C.; Robbins, J.; DiMare, M.; 0 Regan, M. J . Am. Chem. SOC.1990, 112, 3875.

Synthesis of Re( VU) Alkylidene Alkylidyne Complexes Table I. Relevant N M R Data for Rhenium Alkvlidene Comolexes‘ com p 1ex ReOdCHCMe,Ph)(CH,CMe,Ph) Re(6j(NAr)(dH-i-Bu) (CH,-;-BU)

J. Am. Chem. SOC.,Vol. 114, No. 9, 1992 3369

6 CHR‘

6 CHR’

12.29 272.4 12.72 278.6 12.26 269.26 Re(NAr),(CHCMe2Ph)(CH2CMe2Ph) 12.00 259.56 Re(N-t-Bu),(CHCMe2Ph)(CH2CMe2Ph) [Re(C-t-Bu)(CH-t-Bu)(ArNH,)C1,], (55%) 14.49 286.3 14.47 [Re(C-t-Bu)(CH-t-Bu)(Ar”H,)CI,I, 14.38 14.54 294.5 13.43 [Re(CCMe,Ph)(CHCMe2Ph)(t-BuNH2)C12l2 296.0 13.48 15.22 R~(C-~-BU)(CH-~-BU)(A~NH~)CI,~ 15.11 15.26 Re(C-t-Bu)(CH-t-Bu)(Ar’NH2)C12d 15.15 15.18 R~(C-~-B~)(CH-~-BU)(~~)CI,~ 15.09 304.4 14.94 R~(C-~-BU)(CH-~-B~)(TMEDAHC~)C~~~ (75%) 303.5 15.06 298.6b 14.53 R~(C-~-BU)(CH-~-BU)(~-BUNH~)~C~~ 295.3 14.04 R~(C-~-BU)(CH-I-BU)(~~)~CI~ 291.7 13.13 R~(C-~-BU)(CH-~-BU)(TMEDA)C~,~ 292.0b 13.52 Re(C-t-Bu)(CH-f-Bu)(pda)Cl2 285.4 13.25 Re(CCMe2Ph)(CHMe2Ph)(pda)C12c 285.8 13.26 [Re(C-t-Bu)(CH-t-Bu)C12]~ 23 1.O 10.15 syn-Re(C-t-B~)(CH-t-Bu)(O-t-Bu)~ (25%) 229.5 11.59 (anti) 230.4b 10.36 syn-Re(CCMe2Ph) (CHCMe,Ph)(0-t-Bu), 228.3b 11.68 (anti) 233.6 syn-Re(C-t-Bu)(CH-t-Bu)[OSi(t-Bu)s]2 10.40 239.6 12.40 (anti) 238.4 10.52 syn-Re(C-t-Bu)(CH-r-Bu) [OCMe,(CF,)], 238.9 11.95 (anti) 238.6b 10.73 syn-Re(CCMe,Ph) (CHCMe,Ph)[OCMe,(CF,)] 238.1b 12.06 (anti) 240.0 10.72 syn-Re(C-f-Bu)(CH-t-Bu)(OAr’), 242.2 12.32 (anti) 248.8 11.08 ~~~-R~(C-~-BU)(CH-~-BU)[OCM~(CF,)~]~ (65%) 251.5 12.48 (anti) 266.8 12.07 syn-Re(C-t-Bu)(CH-t-Bu)(0-f-B~)~(PMe~)~ 265.2 12.50 (anti) 12.84 anti-Re(C-t-Bu)(CH-t-Bu)[OCMe,(CF,)],(PMe,) 283.0 13.00 syn-Re(C-f-Bu)(CH-t-B~)(0Ar‘)~(PMe,) 234.46 11.06 syn-Re(C-r-Bu)(CH-t-Bu)(OAr‘),(t-BuNH,) 239.0b 10.49 ~~~-R~(C-~-BU)(CH-~-BU)(OA~’)~(A~NH,) 10.92 ~yn-Re(C-t-Bu)(CH-f-Bu)(OAr’)~(py) “Spectra recorded in C6D6 unless otherwise noted. bCD2C12. ‘THF-d,.dObservedin situ.

is formed by CY hydrogen abstraction in intermediate Re(NA~),(CH,-~-BU)~. An analogous alkylation employing Zn1.5Zn(CHrr-Bu)2

Re(NAr)(py)C13

-l,)ZnCITpy-CM~~

Re(NAr),(CH-t-Bu)(CH,-t-Bu) (4) (CH2CMe2Ph), gives Re(NAr)2(CHCMe2Ph)(CH2CMe,Ph), a new compound that is similar to the neopentylidene species. (See Table I for NMR data.) The neophyl system has several advantages over the neopentyl system. First, neophyl Grignard reagents can be prepared smoothly, unlike the sometimes fickle neopentyl Grignard reagent. Second, the phenyl ring and the potentially inequivalent methyl groups in the neophyl ligand are two additional NMR probes. Third, neophyl chloride is a small fraction of the cost of neopentyl chloride. If PhMe2CCH2MgC1 is used to alkylate Re(NAr),Cl,, Re(NAr),(CHCMe,Ph)(CH2CMeZPh)is obtained as a dark orange oil contaminated by PhMe2C(CH2)2CMe2Ph.Such a crude product often can be used directly for subsequent chemistry and the PhMe2C(CH2)2CM%Ph impurity removed at a later stage. At this point the neophyl system has not been as extensively explored as the neopentyl system,,but what has been done so far for Re, and more extensively for Mo,I8 suggests that the chemistry of neopentyl and neophyl-based compounds is very similar, as one might expect. Re02(CH-t-Bu)(CH,-t-Bu) has been prepared by photolysis of ReO,(CH,-t-Bu), in pyridine with a medium pressure mercury

JCH

6cR

140 130 139 140 135 130

292.1

127 119

290.1 289.6

127 122 131 121 120 118 122 125 1206 157 124 157 125 155 123 156 122 160 119 161 127 158 110 148

297.7 298.4 286.26 298.4 291.8 295Ab 280.4 293.9 288.5 298.2 287.36 294.8b 294.6 305.9 292.2 300.3 294.0b 297.0b 293.7 301.6 295.8 304.2 292.3 296.8

110 123 128

298.7 293.1b 293.1b 296.2

lamp.lg (In comparison Re(NR)z(CHz-r-Bu)3compounds a p parently decompose thermally to Re(NR),(CH-t-Bu)(CH,-t-Bu) (R = t-Bu or Ar).) ReOz(CH-t-Bu)(CH,-r-Bu) is inactive for the metathesis of olefins. An X-ray study revealed that Reo,(CH-f-Bu)(CH,-t-Bu) has a pseudotetrahedral geometry in which the plane of the alkylidene ligand lies perpendicular to a plane defined by the neopentyl ligand, rhenium, and C, of the alkylidene ligand, viz. H H&

This structure can be explained if the Reo, fragment is viewed as being analogous to a metallocene fragment, with each oxo behaving as a 27r,lu ligand, as has been proposed for the M(NR), fragment in a variety of pseudotetrahedral species.20-21 We propose that the structures of Re(NR),(CHCMe*Ph)(R = t-Bu (CH2CMe2Ph)and Re(NR),(CH-1-Bu)(CH,-t-Bu) or Ar) are analogous to that of ReO,(CH-t-Bu)(CH,-t-Bu) on (19) Cai, S.;Hoffman, D. M.; Wierda, D. A. J . Chem.SOC., Chem.

Commun.1988,1489.

(20) Weinstock, I. A.; Schrock, R. R.; Williams, D. S.;Crowe, W. E. Organometallics 1991, 9, 1. (21) Williams, D. S.; Schofield, M. H.; Anhaus, J, T.; Crowe, W. E.; Schrock, R. R.J . Am. Chem.SOC. 1990,112, 6728.

Toreki et al.

3370 J. Am. Chem. SOC.,Vol. 114, No. 9, 1992 Table 11. Intramolecular Bond Distances (A) and Bond Angles (deg) for the Non-Hydrogen Atoms of [Re(C-f-Bu)(CH-f-Bu)(ArN H,)Cl,], Bond Distances Re-Cl(1) 2.619 (3) 1.76 (1) Re-Cl(2) 2.397 (4) 1.89 (1) 2.21 (1) Re*-Cl(I) 2.673 (4) Bond Angles C( 6)-Re-C( 1) C( 6)-Re-N C( 6)-Re-C1( 2) C(6)-Re-C1( 1) C( 1)-Re-N C( 1)-Re-C1(2) C( 1)-Re-CI( 1) C( I)-Re-CI( 1) N-Re-CI (2)

100.6 ( 5 ) 101.3 ( 5 ) 92.9 (4) 172.3 (4) 96.1 (4) 95.6 (4) 163.9 (4) 87.0 (4) 159.6 (3)

Re-C(6)-C(7) N-Re-Cl( 1) N-Re-CI( 1) C1(2)-Re-C1( 1) C1(2)-Re-CI( 1) Cl(1)-Re-Cl(1) Re-CI( 1 )-Re Re-N-C( 11)

167 (1) 79.0 (3) 78.9 (3) 85.2 ( I ) 85.1 (1) 77.1 (1) 102.9 (1) 120.7 (7)

the basis of NMR data, i.e., the imido ligands are inequivalent and the methylene protons in the neophyl or neopentyl ligands are diastereotopic. Synthesis and X-ray Structure of [Re(C-t-Bu)(CH-t-Bu)(ArNH2)CI2l2. Addition of excess HCl(g) to Re(NAr),(CH-tBu) (C H,-t-Bu) in dimethoxyethane yields [ Re(C-t-Bu) (CH-tBu)(ArNH2)CI,], in 85% isolated yield as pale orange crystals (eq 5). Remarkably, the synthesis of [Re(C-t-Bu)(CH-1-Bu)(ArNH,)Cl,], can be carried out on the benchtop in air with cime

2 R c f Y A r ) ~ ( C 1 l - r - ~ u l ~ ~ I +l ~6~IlC1 r-~ii)

- ArYI12Cl CI

t

(5)

L

Figure 1. Two views of the structure of [Re(C-r-Bu)(CH-r-Bu)(A r NH *)Cl J *.

W(O)(CH-~-BU)(PE~~),C~~,~~ Ta(CH-t-B~),(mesityl)(PMe~),,~~ W(C-t-Bu) (PH Ph) (PMe3)2C12,27and Mo(N Ar')(CH-t-Bu)(OTf'),(dme).I* metal-carbon double and triple bonds are formed in the presence NMR studies show that two very similar isomers of [Re(C'of both oxygen and water. We have no reason to suspect that t-Bu)(CH-t-Bu)(ArNH,)Cl2'J2are present in solution in a 5545 the mechanism of forming [Re(C-t-Bu)(CH-t-Bu)(ArN H2)Cl2I2 ratio (in C6Dn),the major isomer having 6 H, 14.49 (in C6D6), differs substantially from that proposed for the formation of 6 R e , 292.1 (THF-ds), and 6 Re-C, 286.3 (THF-d,; JCH [ R~(C-~-BU)(CH-~-BU)(~-B~NH,)C~~]~.~~ A key feature is that = 130) and the minor isomer having 6 H, 14.47 (in C6D6). (As the alkylidene ligand is the first to lose a proton (to form the a result of the poor solubility of these isomers, quality 13CNMR alkylidyne ligand and an amido ligand), and the alkyl ligand then data for the minor isomer could not be obtained.) The C-H loses a proton to form the alkylidene ligand. coupling constants of the alkylidene ligands in analogous isomers The structure of [R~(C-~-BU)(CH-~-BU)(A~NH,)CI~]~, as of [ R~(C-~-BU)(CH-~-BU)(~-BUNH~)C~~]~~~ are identical (1 20, determined in an X-ray study, is shown in Figure 1. (Bond 120 Hz) and virtually identical in closely related [Redistances and angles can be found in Table 11.) The coordination (CCM~,P~)(CHCMQP~)(A~NH~)CI,]~ (1 19, 127 Hz; Table I). sphere around each metal is a distorted octahedron with relatively The precise nature of the isomers is uncertain. The amine ligands long, approximately equal R K l bonds trans to the neopentylidene could be cisoid, for example, or the alkylidene ligands could be and neopentylidyne ligands. The rhenium-carbon bonds of both anti in an otherwise analogous centrosymmetric dimer. Dithe alkylidene ligand (1.89 (1) A) and the alkylidyne ligand (1.76 mer/monomer or dimer/tetramer equilibria are less likely since (1) A) are similar to those observed in previously characterized the ratio of the two isomers does not appear to vary with conRe(C-t-Bu)(CH-t-Bu)(py),I, (1.873 (9) and 1.742 (9) &.I2 The centration, and, more importantly, what appear to be two isomeric, R e C bond length is also identical to that found in Re(q5pentane soluble monomers can be observed in a 3:1 ratio during CSMeS)(C-t-Bu)Br3(1.755 (6) A).22 Note that the tert-butyl ;he early stages of the reaction. group of the neopentylidene ligand is pointing toward the neoUpon addition of HCl(g) to a solution ot orange Re(NAr),pentylidyne ligand, the syn orientation. The Re-C,-C, angle of (CH-t-Bu)(CH,-t-Bu) in dimethoxyethane, the color darkens and the alkylidene ligand (140 (1)") is among the smallest observed ArNH3C1precipitates. If ArNH3Cl is filtered off and the solution for a high oxidation state syn-alkylidenecomplex. The Re-C,-C, is reduced to dryness quickly in vacuo, the 'H NMR spectrum angle of the alkylidyne ligand (167 (1)") is somewhat smaller than of the residue in C6D6 shows four alkylidene resonances, two of is normally observed in high oxidation state metal alkylidyne which (at 6 14.49 and 14.47) belong to the isomers of [Re(C-tcomplexes, probably for steric reasons.23 (It is bent away from Bu)(CH-~-BU)(A~NH~)CI~]~ mentioned above and two (6 15.22 the syn-neopentylideneligand.) The C( l)-ReC(6) angle between and 15.12) to what we propose are two isomers of Re(C-1the alkylidene and alkylidyne a carbon atoms (100.6 ( 5 ) " ) is Bu)(CH-t-Bu)(ArNH2)CI2. When this crude product is taken comparable to that observed in Re(C-t-Bu)(CH-t-Bu)(py),I, up in pentane, more [ Re(C-t-Bu)(CH-t-Bu)(ArNH2)C12]~ pre(98.1 1 (42)").12 The tendency for mutually cis multiply bonded cipitates out and the proton NMR spectrum of the remaining ligands to repel one another has been observed many times prematerial obtained by removing pentane in vacuo shows that the viously in similar complexes, e.g., W(O)(CH-~-BU)(PM~~)~C~~,*~ aqueous HCI! This result is particularly interesting because

(22) Herrmann, W. A.; Felixberger, J. K.; Anwander, R.; Herdtweck, E.; Kiprof, P.; Riede, J. Organometallics 1990, 9, 1434. (23) Murdzek. J. S.; Schrock, R. R. In Carbyne Complexes; VCH: New York, 1988. (24) Churchill, M. R.; Rheingold, A. L. fnorg. Chem. 1982, 21, 1357.

(25) Churchill, M. R.; Missert, J. R.; Youngs, W. J. fnorg. Chem. 1981, 20, 3988.

(26) Churchill, M. R.; Youngs, W. J. fnorg. Chem. 1979, 18, 1930. (27) Rocklage, S. M.;Schrock, R. R.; Churchill, M. R.; Wasserman, H. J. Organometallics 1982, I , 1332.

Synthesis of Re(VII) Alkylidene Alkylidyne Complexes

J. Am. Chem. SOC.,Vol. 114, No. 9, 1992 3371

set of alkylidene resonances for the monomer have decreased p r o ~ e s s . ) ~ ~An J * amine is an unsatisfactory ligand for general relative to those for the dimer. This behavior is consistent with use since alkoxides will remove amine protons in some circuminitial formation of two isomers of Re(C-t-Bu)(CH-t-Bu)stances to regenerate amido and imido species (see later). Since (ArNH2)C12followed by dimerization in a hydrocarbon solvent. the bases in five-coordinate adducts of the type Re(C-t-Bu)(Dimethoxyethane may coordinate to Re initially and therefore (CH-t-Bu)(L)Cl, are not protonated to give [Re(C-t-Bu)(CHhelp stabilize monomers toward dimerization.) The isomers of t-Bu)Cl,],, we speculated that a bidentate amine adduct of the the monomeric complexes do not appear to be syn and anti roRe(C-t-Bu)(CH-t-Bu)CI, core might stand a better chance of tamers, as JCH, values for the two are identical. (See below for producing [Re(C-t-Bu)(CH-t-Bu)Cl,], upon addition of HCl, Le., in syn and anti rotamers.) discussion of JCHa protonation of one end of the amine ligand might force the other There is abundant evidence that other monomeric complexes end to dissociate and be trapped by a second equivalent of acid. of the type Re(CH-t-Bu)(CHz-t-Bu)(L)C1, form, but unfortuRe(C-t-Bu)(CH-t-Bu)(TMEDA)C12 was synthesized in a nately they cannot be isolated free of the dimeric forms, and manner analogous to that described for Re(C-t-Bu)(CH-ttherefore have been observed only in solution by NMR. For Bu)(py),Cl,. Only a single isomer was observed. Unfortunately, example, when Re(NAr’)2(CH-t-Bu)(CH,-t-Bu) is treated with however, although TMEDA was protonated upon adding excess excess HCl (Ar’ = 2,6-C6H3-i-Pr2),two isomers of monomeric HCl(g) to Re(C-t-Bu)(CH-t-Bu)(TMEDA)Cl,, the proton NMR Re(C-t-Bu)(CH-t-Bu)(Ar‘NH,)Cl, are the only observed orcharacteristics of the resulting complex are virtually identical to ganometallic products initially (6 H, = 15.26 and 15.15). These those described for the Re(C-t-Bu)(CH-t-Bu)(L)Cl, species above, dimerize in pentane (qualitatively much more slowly than those consistent with the formulation Re(C-t-Bu)(CH-t-Bu)(TMEof Re(C-t-Bu)(CH-t-Bu)(ArNH2)C12, as one might expect for DA.HCI)Cl,, i.e., only one end of the TMEDA ligand is prosteric reasons) to give [R~(C-~-BU)(CH-~-B~)(A~’NH~)C~~]~ (6 tonated. Two isomers are observed that have similar values for J C H (122, 127 Hz) in the alkylidene ligands. H, = 14.38 and 14.54). On the other hand some bis adducts can be isolated. For Formation of a stable monoprotonated adduct could be avoided example, addition of excess tert-butylamine to [Re(C-t-Bu)(CHif a diamine is used that has a rigid backbone, since when one ~ - B U ) ( A ~ N H ~ )affords C ~ , ] ~a single isomer of Re(C-t-Bu)(CHend of the diamine is protonated it should not be able to swing t-Bu)(t-BuNH2),C12 in 95% yield as bright yellow fibers (eq 6 ) . away from the metal and therefore should more likely dissociate. It is proposed to be the species shown on the basis of the fact that [Re(C-t-Bu)(CH-t-Bu)(ArNH2)Ci2J2 reacts cleanly with 1,2phenylenediamine (pda) in dimethoxyethane to afford the corL responding monomeric complex, Re(C-t-Bu)(CH-t-Bu) (pda)Cl, xs t-BuKH2 C1, &CH-I-BU (eq 7). When Re(C-t-Bu)(CH-t-Bu)(pda)C12 is treated with 2 0.5 [ R ~ ( C - I - B ~ ) ( C H - ~ - B ~ ) ( A ~ ~ ~ ) C I , I , (6) .&m2 CI’ 1 *C-r-Bu equiv of HCl(g) in dme the diprotonated diamine precipitates out L

-

(z. = r-BuNHz)

H+ CI

the tert-butylamine ligands are equivalent down to -80 OC by NMR (IH and 13C),and the NH2 protons are diastereotopic. The reaction takes several hours to go to completion at 25 OC because of solution and can be filtered off. What remains in solution is the rapidly formed initial product, [Re(C-t-Bu)(CH-t-Bu)(talmost certainly Re(C-t-Bu)(CH-1-Bu)(dme)Cl,, but when the BUNH~)C~ reacts ~ ] ~only , slowly with additional tert-butylamine, dme is removed in vacuo [Re(C-t-Bu)(CH-t-Bu)C12], ( x > 1) is we presume via monomeric Re(C-r-Bu) (CH-t-Bu)(t-BuNH2)Cl2 isolated in >85% yield as a pale orange solid. [Re(C-t-Bu)Once isolated, Re(C-t-Bu)(CH-t-Bu)(t-BuNH,)2C12 (CH-t-Bu)Cl,], is insoluble in noncoordinating solvents but disis stable to solves readily in dimethoxyethane or THF. [Re(C-t-Bu)(CHmoist air for weeks. It does not react with excess gaseous HCl(g) t-Bu)Clz], is stable to air and water for several days in the solid in a solvent such as ether to give [Re(C-t-Bu)(CH-t-Bu)(tstate. Amazingly, a proton NMR spectrum of [Re(C-t-Bu)BuNH2)C12]2. (CH-t-Bu)Cl,], can be obtained in D20, and the complex can be Addition of excess pyridine to a dichloromethane solution of r wovered unchanged! When solutions of [Re(C-f - Bu) (CH- t [Re(C-t- Bu)(CH- t -Bu)(ArNH2)Cl,] yields pale orange Re(CBu)C12], in water, THF, or dme are reduced to dryness in vacuo, t-B~)(CH-t-Bu)(py)~Cl~ quantitatively in less than 20 min. In does not react crystals are obtained first, but they turn to a powder when left contrast, [Re(C-r-Bu)(CH-t-Bu)(t-BuNH,)Cl,], readily with pyridine. (In the presence of SiMe31and pyridine, under vacuum for more than a few minutes at room temperature. We speculate that these crystals are initially at least bis solvates, Re(C-t-Bu)(CH-t-Bu)(py),I, is obtained.I2) These results are consistent with the presence of a small amount of monomeric Le., Re(C-t-Bu)(CH-t-Bu)S,Cl, (S = THF, H 2 0 , I / , dme). Re(C-r-Bu)(CH-t-Bu)(ArNH,)Cl, in equilibrium with [Re(C[Re(C-t-Bu)(CH-t-Bu)Cl,], also is stable to oxygen in T H F as ~-BU)(CH-~-BU)(A~NH~)C~~]~, whereas [Re(C-t-Bu)(CH-tlong as some water ( 2 1 equiv) is present, but in the absence of Bu)(t-BuNH2)C12], does not form monomeric Re(C-t-Bu)(CHwater, it reacts with oxygen slowly (5 h, 1 atm, THF-d,) to yield t-Bu)(t-BuNHz)C12nearly as readily. Only a single isomer of pivaldehyde (50% versus an internal standard) and several unRe(C-t-Bu)(CH-t-Bu)(py),Cl, is observed. Addition of excess identified products. The major product (38%) in the ‘H NMR HCl(g) to R~(C-~-BU)(CH-~-BU)(~~)~C~~ spectrum appears to form at the same time as pivaldehyde. yields 1 equiv of pyAttempts to isolate this presumably organometallic product were ridinium chloride and two isomers of what we propose is Re(Ct-Bu)(CH-t-Bu)(py)CI, in solution, on the basis of NMR charunsuccessful, as it apparently is unstable once the solvent is reacteristics that are similar to those of Re(C-t-Bu)(CH-t-Bu)moved. Although Re(C-t-Bu)(O)Cl,(THF), is an attractive proposal for this transient species, we have no independent evidence (ArNHJCl, (Table I). An insoluble pale pink powder precipitates from CsDs or CD2Clzsolutions of Re(C-t-Bu)(CH-t-Bu)(py)Cl, for this formulation. Similar reactions with oxygen do not occur that we formulate as [Re(C-t-Bu)(CH-t-Bu)(py)Cl,], on the basis to any significant extent in dimethoxyethane, presumably because of its lH NMR spectrum in pyridine-d5. Once [Re(C-t-Bu)dme binds more strongly than T H F to give a pseudooctahedral (CH-t-Bu)(py)CI,], is formed, it does not react with excess species. back again. Since [Re(C-t-Bu)(CH-t-Bu)Cl,], is stable to water at neutral pyridine readily to give R~(C-~-B~)(CH-~-BU)(~~)~C~, Synthesis of Oxo Derivatives and [Re(C-t-Bu)(CH-t-Bu)Cl& pH and loses any coordinated water in vacuo in the solid state, It would be most desirable to synthesize a versatile precursor to we thought that addition of excess HCl to ReO,(CH-t-Bu)any complex of the type Re(CR’)(CHR’)(OR), that either con(CH,-t-Bu) could yield [Re(C-t-Bu)(CH-t-Bu)C12], after retains no base, or a base that is relatively innocuous and labile, moving coordinated water in vacuo from “Re(C-t-Bu)(CH-te.g., 1,Zdimethoxyethane. (Universal precursors of the type Bu) (Hz0),C12” or ,.[Re( C-t-Bu) (CH-t-Bu)(H,O) Cl,] xn (x = 1 M(CH-t-Bu)(NAr’)(OTf),(dme) (M = Mo or W; dme = 1,2or 2). Unfortunately, although ReOz(CH-t-Bu)(CH,-t-Bu) can dimethoxyethane; OTf = triflate) react even with an electronbe prepared in high yield from ReOz(CH,-t-Bu), by photoly~is,~~ withdrawing alkoxide such as OCMe(CF,), and lose dme in the Re02(CH2-t-Bu),itself was originally produced in only 6% yield,

3312 J . Am. Chem. SOC.,Vol. 114, No. 9, 1992

Toreki et ai.

making this an unlikely entry into rhenium alkylidyne chemistry of the mixture of ReNAr)(O)(CH-t-Bu)(CH,-t-Bu) isomers. ’H on a multigram scale. Therefore we explored the possibility of and I3C NMR spectra of the two isomers are very similar (Table hydrolyzing Re(NAr),(CH-t-Bu)(CH,-t-Bu) to give Re02(CHI). Since the structure of Re(NAr)(O)(CH-t-Bu)(CH,-t-Bu) is t-Bu)(CH,-t-Bu). If successful, one step in the synthesis of almost certainly analogous to that of ReO,(CH-t-Bu)(CH,-t-Bu), [Re(C-t-Bu)(CH-t-Bu)C12].(formation of the pda complex) could we propose that in the two isomers the tert-butyl group of the be eliminated. neopentylidene ligand may either point toward the oxo ligand or Re(NAr),(CH-t-Bu)(CH,-t-Bu) reacts with water slowly in toward the imido ligand, viz. C6D6 over a period of 3 h at room temperature to give 1 equiv /r-Bu of 2,6-dimethylaniline and a new organometallic product quanHH2 C H+ titatively (according to IH NMR). The lH NMR spectrum of this product ( 6 H, = 12.29) is similar to that of the starting material; we formulate it as Re(NAr) (O)(CH-t-Bu)(CH,-t-Bu) (eq 8). A second isomer ( 6 H, = 12.72) is typically observed in and that interconversionof the two isomers, e.g., by intramolecular trace amounts (-5%) and is discussed in more detail below. It rotation about the R d bond, is slow on the chemical time scale. should be noted that Re(NAr) (0)(CH-t-Bu) (CH,-t-Bu) contains Addition of 2 equiv of HCl(g) to ReO,(CH-t-Bu)(CH,-t-Bu) simultaneously a metal-carbon double bond, a metal-oxygen in dimethoxyethane affords [Re(C-t-Bu)(CH-t-Bu)C12].in high double bond, and a metal-nitrogen double bond; it may be the yield after removing the dimethoxyethane and water in vacuo (eq only example. Reaction of the crude product (with the aniline 11). Presumably water and/or dme adducts are present initially, present) with HCl(g) forms [R~(C-~-BU)(CH-~-BU)(A~NH~)C~,]~ even in the solid state (see above), but both water and dme are quantitatively. lost in vacuo. This synthetic method bypasses the need to make [Re(C-t-Bu) (CH-t-Bu)(ArNH2)C12] and Re(C-t-Bu)(CH-tpentane Re(NAr),(CH-t-Bu)(CH,-t-Bu) + H20 Bu)(pda)Cl,. Therefore [Re(C-t-Bu)(CH-t-Bu)Cl,]. can be prepared from Re207or [NH4]Re04in four high yield steps (eq Re(NAr)(O)(CH-t-Bu)(CH,-t-Bu) (8) 12). It is the precursor that is required for preparing compounds containing relatively electrophilic alkoxides (see below). Further hydrolysis of Re(NAr)(O)(CH-t-Bu)(CH,-t-Bu) to give ReO2(CH-t-Bu)(CH,-t-Bu) requires several days to go to DME Re(O),(CH-t-Bu)(CH,-t-Bu) + 2HC1 -2~20 completion. When pyridine or aqueous NaOD is added to a C6D6 solution containing Re(NAr)2(CH-t-Bu)(CH2-f-Bu) and water, [R~(C-~-BU)(CH-~-B~)C~,]~ (1 1) no reaction, even to give Re(NAr)(O)(CH-t-Bu)(CH,-t-Bu), is Re207 Re(NAr),(py)C13 observed after several days. However, when aqueous acids such Re(NAr),(CH-t-Bu)(CH,-t-Bu) as acetic acid, HPF6, or HBF4 are added to a C6D6 solution of Reo2(CH-t-Bu)(CH,-t-Bu) Re(NAr),(CH-t-Bu)(CH,-t-Bu), ReO,(CH-r-Bu)(CH,-t-Bu) [R~(C-~-BU)(CH-~-BU)C~,]~ (12) forms within minutes. Unfortunately, ReO,(CH-r-Bu)(CH,-t-Bu) reacts further with acids to give uncharacterized mixtures, so this Synthesis of Akoxide Complexes, Re(CR’)(CHR’)(OR), (R’ method does not appear to be amenable to large scale preparations. = t-Bu or CMe2Ph). [R~(C-~-BU)(CH-~-BU)C~,]~ reacts with However, neutral alumina will catalyze the transformation of 2 equiv of lithium tert-butoxide in tetrahydrofuran to yield preRe(NAr),(CH-t-Bu)(CH2-t-Bu) to ReO,(CH-t-Bu)(CH,-t-Bu) viously reported Re(C-t-Bu)(CH-t-Bu)(0-t-Bu)i2 quantitatively, in high yield (eq 9), although this reaction occasionally does not while addition of 2 equiv of LiOCMe2(CF3) or KOCMe(CF3), proceed smoothly. Several grams of ReO,(CH-t-Bu)(CH,-t-Bu) yields Re(C-t-Bu)(CH-t-Bu)[OCMe,(CF,)], or Re(C-t-Bu)usually can be prepared by this method. (CH-t-Bu)[OCMe(CF3),],, respectively (eq 13). If only 1 equiv THF Re(NAr),(CH-r-Bu)(CH,-t-Bu) + 2 H 2 0 cat. [R~(C-~-BU)(CH-~-BU)C~,]~ + 2MOR 7 ReO,(CH-t-Bu)(CH,-t-Bu) (9) M = K or Li Re(C-t-Bu)(CH-t-Bu)(OR), (13) Hydrolyses of Re(NAr)2(CH-t-Bu)(CH2-f-Bu) and Re(NAr)(O)(CH-t-Bu)(CH,-t-Bu) produce nonvolatile 2,6-diof lithium alkoxide is added to [Re(C-t-Bu)(CH-t-Bu)Cl,]. a 50% methylaniline. It can be removed from the reaction mixture by yield of Re(C-t-Bu)(CH-t-Bu)(OR), is obtained. Re(C-t-Bu)adding zinc dichloride. For example, addition of ZnC12(dioxane) (CH-t-Bu) [OCMe,(CF,)],, like Re(C-t-Bu)(CH-t-Bu)(O-t-Bu),, to a pentane solution containing 2,6-dimethylaniline and Reo2is a low-melting yellow solid that is extremely soluble in pentane. (CH-r-Bu)(CH,-t-Bu) yields a precipitate of ZnC1,(ArNH2),. It can be obtained as yellow crystals from pentane at -40 OC, but Filtration affords relatively pure ReO2(CH-t-Bu)(CH,-t-Bu), these melt to an orange oil at room temperature. All three dewhich then is more easily crystallized or used directly in subsequent rivatives sublime readily (30-40 OC, Torr) but show some reactions. It is important that the zinc dichloride treatment be tendency to decompose when left in the solid state at room temperformed in hydrocarbon solvent; in ether, for example, zinc perature for more than several hours. They are stable indefinitely from dichloride forms an adduct with ReO,(CH-t-Bu)(CH,-t-Bu), in solution (0.1 M in C6D6) or as solids when stored at -40 OC. which it cannot be removed readily. When bisalkoxide complexes are first obtained from [Re(CThe identity of Re(NAr)(O)(CH-r-Bu)(CH,-t-Bu) is further t-Bu)(CH-t-Bu)Cl,],, exclusively one alkylidene complex is obestablished by the fact that it is formed by conproportionation served. When these complexes are heated, a mixture of two and ReO,(CH-t-Bu)(CH,of Re(NAr),(CH-t-Bu)(CH,-t-Bu) alkylidene complexes is obtained, the ratio varying with the steric t-Bu) over a period of 2 days (eq 10). On the basis of these data bulk and electronic nature of the ligands. The new alkylidene alone we cannot tell whether only the oxo and imido ligands are H, resonance is always found downfield of the H, resonance in the initial species. Consistently JCH is relatively low (- 120-125) Re(NAr),(CH-t-Bu)(CH,-t-Bu) + ReO,(CH-t-Bu) in the initial isomer and relatively high (- 155-160) in the second (CH,-t-Bu) 2Re(NAr)(O)(CH-t-Bu)(CH,-t-Bu) (10) isomer (Table I). All data are fully consistent with the isomers (two isomers) being syn and anti rotamers, respectively, the syn rotamer being migrating from one metal to another. (Analogous oxo/imido that in which the substituent on the alkylidene ligand points toward exchange reactions have been observed recently between Mo the alkylidyne ligand (eq 14). (Syn and anti isomers are wellcenters; Mo(NAr’),(O-t-Bu), and MoO,(O-t-Bu), conproporknown in M(CHR’)(NAr’)(OR), c ~ m p l e x e s . ~ Syn ~ ) and anti tionate to give a mixture containing Mo(NAr’)(0)(0-t-B~),.~~) Interestingly, in the conproportionation reaction the isomer of (28) Mitchell, J.; Gibson, V. C., personal communication. Re(NAr)(O)(CH-t-Bu)(CH,-t-Bu) that gives rise to the H, (29) Schrock, R. R.; Crowe, W. E.; Bazan, G. C.; DiMare, M.; O’Regan, M. B.; Schofield, M. H. Organometallics 1991, 10, 1832. resonance at 12.72 ppm predominates, typically comprising -70%

-

,

-

-

--

Synthesis of Re(VZZ) Alkylidene Alkylidyne Complexes

J. Am. Chem. SOC.,Vol. 114, No. 9, 1992 3373

Table 111. Intramolecular Bond Distances (A) . . and Bond Angles (deg) for the Non-Hydrogen Atoms of syn-Re(C-t-Bu)(CH-r-Bu) [OCMe(CF,),],(THF) Bond Distances Re-O(1) 1.954 (7) Re-C(l) 1.75 (1) Re-C(2) 1.85 (1) Re-O(2) 1.954 ( 7 ) Re-O(3) 2.398 (8) C(l)-C(ll) 1.46 (2) C(2)-C(15) 1.47 (2)

O( 1)-Re-O(2) O( 1)-Re-0(3) O( 1)-Re-C( 1) O( 1)-ReC(2)

0(2)-Re-0(3) O(2)-Re-C( 1) O(2)-Re-C( 2)

Bond Angles 128.6 (3) 0(3)-Re-C(1) 73.1 (3) 0(3)-Re-C(2) 104.6 (5) C(1)-Re-C(2) 107.0 (4) Re-O(I)-C(3) 73.8 (3) Re-O(2)-C(7) 100.3 (4) Re-C(1)-C(l1) 110.4 (4) Re-C(2)-C(15)

168.7 (4) 88.7 (4) 102.5 (6) 142.7 (8) 142.9 (8) 175 (1) 151 (1)

,

rotamers can be interconverted, either thermally or photochemically, as discussed in the next section. The final equilibrium values are listed in Table IV. R

R'

Nitrogen or phosphorous base adducts of Re(C-t-Bu)(CH-tBu)(OR), species can be prepared readily by adding excess base to a solution of the alkylidene complex at room temperature. For example, PMe3 yields five-coordinate monoadducts in which the phosphine ligand is firmly bound to the metal on the NMR time scale. The syn rotamer gives rise to a syn adduct and given syn/anti mixture to the same mixture of syn/anti adducts. In both syn and anti rotamers the alkoxide ligands are inequivalent by NMR. One plausible structure is a trigonal bipyramid in which the alkylidyne and alkylidene ligands lie in the equatorial plane (eq 15). This structure is attractive on the basis of the recent

OR

OR

M MoorW

yellow crystals. These adducts become sticky in vacuo as T H F is lost (according to proton NMR studies), but loss of THF soon slows considerably so that the final sticky materials that are obtained contain approximately 0.75 equiv of THF, i.e., Re(Ct-Bu)(CH-t-Bu)(OAr')2(THF), (1 > x > 0.75). All of the Re(C-t-Bu)(CH-t-Bu)(OR), complexes discussed so far react with excess HCl(g) to afford [Re(C-t-Bu)(CH-t-Bu)Cl,]. quantitatively, although isolation and purification is difficult when the alcohol that is formed is relatively nonvolatile. In the first report of Re(C-t-Bu)(CH-t-Bu)(O-t-Bu)z,lz rotamers were not mentioned. However, when Re(C-t-Bu)(CHt- Bu) (0-t-Bu), is prepared from [Re(C-t-Bu) (CH-r-Bu) ( t BuNHz)C12], we find that a mixture of syn and anti rotamers, in fact, is formed. Re(C-t-Bu)(CH-t-Bu)[OCMe,(CF3)], also can be prepared from [Re(C-t-Bu)(CH-t-Bu)(t-BuNH2)C1,],, the product in this case being predominantly the anti rotamer. When [Re(C-t-Bu) (CH-r-Bu)(t-BuNH,)Cl,] is treated with LiOAr', pure syn-Re(C-t-Bu)(CH-t-Bu)(OAr'), is formed. However, when [Re(C-t-Bu)(CH-t-Bu)(t-BuNH2)Cl2], is treated with KOCMe(CF3)2,an unidentifiable mixture of products is formed. In contrast, [Re(C-t-Bu) (CH-t-Bu) (ArNH2)C1,] does not react cleanly with MO-t-Bu, MOCMe,(CF,), or MOCMe(CF,)2 (M = Li or K) to afford Re(C-t-Bu)(CH-t-Bu)(OR), derivatives. (We suspect that bound aniline in [Re(C-t-Bu)(CH-t-Bu)(ArNH2)C12], is more prone to lose a proton to give an amido ligand than bound tert-butylamine, as one might expect on the basis of the relative acidities of the two amines.) When 2 equiv of LiOAr' are added to [Re(C-t-Bu)(CH-tBu)(ArNH,)Cl,] in dichloromethane or tetrahydrofuran, Re(C-t-Bu)(CH-t-Bu)(OAr'),(ArNH,) is obtained in 80% isolated yield as bright yellow crystals (eq 17). [Re(C-t-Bu)(CH-lBu) (2-BuNH,) Cl,] affords Re (C- t-Bu) (CH-t-Bu) (OAr') ,(tBuNH,) in a similar yield, while Re(C-t-Bu)(CH-t-Bu)(OAr'),(py) is obtained from Re(C-t-Bu)(CH-t-Bu)(py),Cl,. The tert-butylamine ligand in Re(C-t-Bu)(CH-t-Bu)(OAr')2(tBuNH,) must be lost readily since treating a pentane solution of Re(C-t-Bu)(CH-t-Bu)(OAr'),(t-BuNHz) with methyl trifluoromethanesulfonate yields a precipitate of white tBuNH2Me+OTf and a solution containing Re(C-t-Bu)(CH-rBu)(OAr'),. [Re(C-f-Bu)(CH-t-Bu)(ArNH2)C12]2 + 2LiOAr'(ether)

,

,

,

CH2C12 -2LiCI

Re(C-t-Bu)(CH-t-Bu)(OAr'),(ArNH,)(1 7)

crystallographic characterization of syn and anti adducts of X-ray Study of Re(C-t -Bu)(CH-t -Bu)[OCMe(CF3)z]z(THF). M(CH-t-Bu)(NAr')(OR), complexes.29 However, the X-ray When reaction mixtures containing syn-Re(C-t-Bu)(CH-tstructure of an analogous T H F adduct (see next section) shows Bu)[OCMe(CF,),], are reduced to dryness in vacuo at room it to be approximately a trigonal bipyramid in which the axial temperature, yellow-orange crystals of syn-Re(C-t-Bu)(CH-tTHF is bound trans to the neopentylidyneligand. Therefore other Bu) [OCMe(CF,),],(THF) are obtained initially. Over a period possible structures for the phosphine adducts cannot be ruled out, of 45 min in vacuo this solid melts to an orange oil that does not and there is no reason why syn and anti structures need be contain T H F and has an NMR spectrum that is consistent with analogous. (In one type of Mo complex it has been shown that it being Re(C-t-Bu)(CH-t-Bu) [OCMe(CF&. In C6D6 solution, syn and anti rotamers have the same basic s t r u c t ~ r e . ~ ~ ) the T H F ligand in syn-Re(C-t-Bu)(CH-t-Bu)[OCMe(CF,),],Phenoxide complexes can be prepared by adding 2 equiv of the (THF) exchanges rapidly on the NMR time scale. appropriate lithium phenoxide to [R~(C-~-BU)(CH-~-BU)C~,]~ in Views of the structure of Re(C-t-Bu)(CH-t-Bu) [OCMedichloromethane (e.g., eq 16). Re(C-t-Bu)(CH-t-Bu)(OAr'), is (CF,),],(THF) are shown in Figures 2 and 3. Since the alkoxide obtained as a dark orange oil. The initial rotamer is again virtually and alkylidene ligands are bent away from the alkylidyne ligand pure syn, but the anti rotamer forms upon heating (slowly) or (100-105°, Table 111) toward the weakly bound T H F ligand (73-89O, Re-O(3) = 2.398 (8) A), the geometry is best described CH2C12 as a face-capped tetrahedron. The structure also could be de[Re(C-t-Bu)(CH-~-Bu)Cl,],+ 2LiOAr'(ether) scribed as a severely distorted trigonal bipyramid in which the Re(C-t -Bu) (CH-t-Bu)(OAr') ( 16) central rhenium atom lies 0.18 A above the plane defined by the three "equatorial" atoms (0(1), 0 ( 2 ) , and C(2)). The alkylidene photolysis (more rapidly). If tetrahydrofuran is the solvent, then and alkylidyne ligands are mutually cis, with the C( 1)-Re-C(2) five-coordinate tetrahydrofuran adducts are isolated as bright Table IV. Activation Parameters and Equilibria Data for Alkylidene Isomerization in Complexes of the General Formula

Re(C-t-Bu)(CH-r-Bu)(OR)," OR AG ' m p b AH 25.3 (2) 19.5 (9) OCMe, OCMe2(CF,) 28.0 (2) 23.4 (9) 25.5 (9) OCMe(CF,), 30.3 (2) "Toluene-d8 solvent. 1,4-Dichlorobenzenewas used as an internal

As * m * c re1 rated % anti (A) -20 (2) 494 72 (1) -15 (2) 25 81 (1) -16 (2) 1 81 (1) standard. *kcal mol-'. CEu. 110 "C.

% anti (hv)

30 (1) 32 (1) 34 (1)

Toteki et al.

3374 J. Am. Chem. SOC.,Vol. 114, No. 9, 1992

8

ldQ(IK

Figure 4. Eyring plot of rate constants for the syn/anti rotamer interconversion in Re(C-r-Bu)(CH-r-Bu)[OCMe,(CF,)] 2.

suggest that each may be donating some A electron density to the metal, although the magnitude of that angle alone is not a good measure of the extent of x bonding in an alkoxide ligand.3' Figure 2. A view of the structure of syn-Re(C-r-Bu)(CH-1-Bu)An interesting feature of this structure is the bonding of the [OCMe(CF,),] ,(TH F). T H F ligand in a position cis to the alkylidene ligand, but not one n that is adjacent to one x-face of the neopentylidene R e = C bond. Therefore, if the T H F were replaced by an olefin, the neopentylidene ligand would not be oriented correctly for a metallacyclobutane ring to form without rearrangement of the ligand sphere or a ninety degree rotation of the alkylidene ligand. Given that alkylidene rotation in this species is extremely slow (ca. s-') at room temperature (see next section) and is even slower in five-coordinate species, it seems highly improbable that alkylidene rotation could precede formation of a metallacycle in this system. Intercomersion of Syn and Anti Rota". As mentioned above, syn and anti rotamers interconvert slowly thermally and more rapidly photochemically (in benzene or toluene upon irradiation with a medium pressure Hg lamp). Several experiments were carried out in order to determine whether alkylidene rotation is in fact an intramolecular process, and how the rate of rotation varies with the nature of the alkoxide. A mixture of syn-Re(C-t-B~)(CH-t-Bu)(O-t-Bu)~ and synRe( CCMe2Ph)(CHCMe2Ph)(0-t-Bu), was photolyzed in C6D6 though Pyrex with a medium pressure mercury lamp for 1 h. Each syn complex turned into a mixture of syn and anti rotamers (30% and 32% anti, respectively), but there was no evidence for any 'crossovern products (e.g., R~(C-~-BU)(CHCM~P~)(O-~-BU)~) Photolysis for an additional 9 h produced no further change. Heating equilibrated mixtures to 60 "C for several hours also yielded no crossover products. These data suggest that alkylidene or alkylidyne ligands do not transfer from one metal to another under the conditions employed for interconversion of rotamers. On the other hand, an NMR spectrum of a mixture of Re(C00) t-Bu)(CH-t-Bu) [OCMe2(CF,) ] and Re(C-t-Bu)(CH-t-Bu) (0Figure 3. Two views of the core geometry of syn-Re(C-r-Bu)(CH-r~ - B Ushowed )~ that Re(C-t-Bu)(CH-t-Bu)[OCMe,(CF,)] (0-t-Bu) Bu)[OCMe(CF,),],(THF). was present within minutes at 25 "C as approximately 90% of angle (102.5 (6)') being typical of mutually cis ligands that are the mixture. Therefore, 0-t-Bu and OCMe2(CF3)ligands exchange rapidly on the chemical time scale at room temperature multiply bonded to the metal, as discussed previously for [Re(C-t-Bu)( CH-t-Bu)(ArN H2)CI2] 2. The neopentylidene R e 4 in complexes of this type. Alkoxide exchange recently also has distance of 1.85 (1) A is slightly shorter than R e 4 distances been found to be rapid in systems of the type M(CH-t-Bu)(Nin [Re(C-t-Bu)(CH-t-Bu)(ArNH2)CI l2 (1.89 (1) A) and Re2,6-C6H3-i-Pr2)(OR)2.28 (C-t-B~)(CH-t-Bu)(py)~1, (1 3 7 3 (9) while the Re-C(2)-CThe rates of thermal interconversion of rotamers in three (15) angle of 151 (1)' is more comparable to that in Re(C-tR~(C-~-BU)(CH-~-BU)(OR)~ compounds (OR = 0-t-Bu, B~)(CH-t-Bu)(py)~1~ (1 50.3 (7)") than in [Re(C-t-Bu)(CH-tOCMq(CF,), and OCMe(CF3)2)were determined by approach Bu)(ArNH2)Cl2I2(140 (1)"). The neopentylidyne is slightly bent to equilibrium kinetics employing conventional NMR techniques. (1 75 (l)"), and the Re-C( 1) distance of 1.75 (1) A is typical of The rate of rotamer equilibration was found to follow first order high oxidation state rhenium alkylidyne complexes. The T H F kinetics over a range of temperatures (-80 to 140 "C) and to ligand is apparently only weakly bound as evidenced by the Rebe independent of concentration in the range 2-1 5 mM. Activation O(3) distance (2.398 (8) A), which is statistically longer than that parameters and equilibrium ratios are shown in Table IV and an in ~~~-R~(CH-~-BU)(NA~')(~~~~F~)~(THF) (2.339 ( 5 ) A),Mand Eyring plot for the five rate constants determined in the by the facile loss of T H F from the complex in vacuo. The ReOCMe,(CF,) case is shown in Figure 4. Full details can be found 0-C angles of the alkoxide ligands (142.7 (8)' and 142.9 (8)' in the Experimental Section. The rotamers of Re(C-t-Bu)(CH-

i),

(30) Schofield, M.H.;Schrock, R. R.; Park, L. Y.OrlQonomefallics1991,

IO, 1844.

(31) Steffey, B. D.;Fanwick, P. E.; Rothwell, I. P. Polyhedron 1990,9,

963.

J . Am. Chem. Soc., Vol. 114, No. 9, 1992 3375

Synthesis of Re( VII) Alkylidene Alkylidyne Complexes

1-Bu)(0-2,6-C6H3-i-Pr2),interconvert at a rate that qualitatively is intermediate between those of Re(C-t-Bu)(CH-t-Bu)[ OCMe,(CF,)], and Re(C-t-Bu)(CH-t-Bu) [OCMe(CF,),] However, Re(C-~-BU)(CH-~-BU)(~-~,~-C~H~-~-P~~)~ decomposes to a significant extent above 100 "C, and so could not be studied thoroughly. Alkylidene ligand rotation is a relatively slow process. For example, Re(C-t-Bu) (CH-t-Bu) (0-t-Bu), requires approximately 45 min at 100 "C to reach equilibrium, while Re(C-tBu)(CH-t-Bu)[OCMe(CF3)2]2 requires 7 h at 144 "C to reach I-RU equilibrium. At 110 "C the calculated relative rates of alkylidene rotation in these two species are approximately 500: 1, respectively. Addition of T H F (up to 10 equiv; free exchange is observed at room temperature) did not change the rate of interconversion of rotamers of Re(C-t-Bu)(CH-t-Bu) [OCMe2(CF,)l2at 1 13 "C, while addition of 1 equiv of PMe, (which forms an adduct in which "Re(V I)" "Re (VI)" PMe, does not exchange on the NMR time scale at room temFigure 5. A schematic representation of the orbitals involved in stabiperature) to Re(C-t-Bu)(CH-t-Bu) [OCMe(CF,),], stopped allizing intermediates in alkylidene ligand rotation (a) M(NAr')(CH-fkylidene ligand rotation entirely in the temperature range where Bu)(OR), (M = Mo, W) and (b) Re(C-r-Bu)(CH-f-Bu)(oR)z. rotation was observed for the pseudotetrahedral species. Therefore, as was found in complexes of the type M(CH-t-Bu)(N-2,6lacyclobutadiene complex in the case of acetylene metathesis), C6H3-i-Pr2)(OR)2,29 alkylidene ligands appear to rotate more presumably on a pseudotetrahedral face that would allow a mereadily in pseudotetrahedral species than in higher coordinate tallacycle to form. There is only one such face in complexes of species. The difference in the barrier to rotation in complexes the type M(CR')(OR),, but if two multiply-bound ligands are of the type M(CH-t-Bu)(N-2,6-C6H3-i-Pr2)(0R), and those represent, then there are two such faces. In acetylene metathesis ported here is dramatic; for M(CH-~-BU)(N-~,~-C~H,-~-P~,)(OR)~ by complexes of the type Re(CR')(NAr)(OR)234attack at one species, values for AG& were usually in the range of 16-1 8 kcal of the two possible faces (C, N, 0) was proposed to lead to a mol-'. relatively stable, inactive type of metallacyclobutadiene inter-

,.

Discussion A potentially important and interesting feature of the chemistry of rhenium(VI1) complexes that contain alkyl ligands that can lose CY protons readily to give alkylidene and alkylidyne ligands is that CY protons are transferred to oxo or imido ligands. Proton "transfer" is perhaps most likely to consist of a stepwise protonation/deprotonation reaction, since in do complexes no CH, bond actually can oxidatively add to the metal to give (e.g.) an alkylidyne/hydride intermediate. From a kinetic perspective it is sensible to p r o p that the rate of protonation of oxo, imido, and carbon-based ligands ( M = t R or M = C H R ) would follow the order 0 > N > C, since two electron pairs are accessible on oxygen and none on carbon. Therefore formation of metal-carbon multiple bonds at the expense of metal-oxygen or metal-nitrogen multiple bonds could be ascribed largely to favorable kinetics, although there does appear to be some thermodynamic preferences for metal-nitrogen or metal-carbon bonds relative to metal-oxygen bonds for metals to the right in the transition series. As we have shown here, metal-carbon multiple bonds can be formed even in the presence of water, and alkylidyne/alkylidene complexes can be stable to water at neutral pH. Presumably other ligands that contain potentially acidic protons will be tolerated by rhenium catalysts of this type and perhaps also relatively reactive functionalities (e.g., the carbonyl group in an aldehyde or ketone). The implications for the development of olefin metathesis catalysts that will tolerate protons and certain functionalities would seem to be significant. Realistically it is unlikely that such functionality-tolerant catalysts will be as active as the more functionality-sensitive electrophilic catalysts. For example, water32*33 and alcohol-tolerant' ruthenium-based catalysts, whose mode of reactivity admittedly has not been elucidated, react readily only with highly strained olefins such as norbornenes. A unifying theme in metathesis by well-defined do alkylidene or alkylidyne complexes is four-coordination, Le., Mo and W complexes of the t p M(CHR')(NAr')(OR)217.18are active olefin metathesis catalysts and M(CR')(OR), (M = Mo or W)23and R~(CR')(NAX-')(OR),~ complexes are active acetylene metathesis catalysts (the latter only in special circumstances). In each case a substrate can attack the metal relatively easily to give a fivecoordinate intermediate metallacyclobutane complex (or metal(32) Novak, B. M.; Grubbs, R. H. J . Am. Chem. Soc. 1%. 110.7542. (33) Novak, €3. M.; Grubbs, R. H. J . Am. Chem. Soc. 1988, 110, 960. (34) Weinstock, I. A.; Schrock, R. R.; Davis, W. M. J . Ani. Chem. SOC. 1991. 112. 135.

mediate, while attack at the other possible face (C, 0, 0) was proposed to lead to metathesis. Complexes of the type M(CHR')(NAr')(OR), are attacked by bases29and, it is proposed, olefin^,^',^^ most readily on the C, N, 0 face. do Re complexes of the type reported here are also members of this class of pseudo-tetrahedral complexes in which either the C, C, 0 or C, 0, 0 face in theory could be attacked and give rise to a metallacyclobutane intermediate. However, the most stable structure of a five-coordinateadduct depends (inter alia) on the nature of the base; the structure of the rhenium PMe, adduct when OR = OCMe(CF,),, according to NMR studies, is different from that found for the rhenium T H F adduct. (A M(CHR')(NAr')(OR)2(quinuclidine) complex also was observed that had a different structure than that of a phosphine adduct, but that structure could not be identified unambigu~usly.~~) It seems more likely that an incoming base or olefin can attack the metal on several faces and that the structurally characterized example reported here is simply the most stable or the most crystalline of the possible T H F adducts. Therefore, knowing the structure of a given base adduct in order to extrapolate to the structure of the weak olefin adduct that forms first in olefin metathesis reactions is perhaps of much less value than we believed initially. Of course, there is also no guarantee that the most stable 'olefin adduct" leads to productive metathesis most quickly. In spite of all the structural knowledge that has been accumulated to date, we are now considerably less certain that we can determine what the fastest metathetical pathway in a high oxidation state alkylidene complex is in a given set of circumstances and whether any observable metallacyclobutane complex actually lies on the fastest reaction pathway. A second common theme in the case of Mo and W complexes of the type M(CHR')(NAr)(OR), and complexes of the type Re(CR')(CHR')(OR), reported here is the presence of syn and anti rotamers. The reason why syn and anti rotamers form is clear; the imido and alkylidyne ligands each use two d orbitals to form two r bonds to the metal, leaving only the orbital that is 6 with respect to the neopentylidyne carbon atom or imido nitrogen atom to form a A bond to the alkylidene carbon atom. Therefore the alkylidene ligand must be bound so that its substituent either points toward or away from the alkylidyne or the imido ligand. The alkylidene ligand can rotate readily about the M==C bond only

-

(35) Feldman, J.; Davis, W. M.; Thomas, J. K.; Schrock, R. R. Orgunomerullics 1990, 9. 2535. (36) Feldman, J.; Schrock, R. R. Prog. Inotg. Chem. 1991, 39, 1.

3376 J . Am. Chem. SOC.,Vol. 114, No. 9, 1992 if the appropriate orbital is available to stabilize the intermediate (Figure 5). In an imido complex (Figure sa) the d orbital in the N-M-C plane can become available to form a M=C ?r bond if the imido nitrogen atom does not donate its electron pair to the metal. However, in an alkylidyne complex the analogous d orbital is used to form the covalent R e C triple bond. A plausible valence bond description of an intermediate or transition state in which the CHR ligand can rotate is shown in Figure 5b. One electron is placed in a metal orbital (not shown) to give formally Re(VI), while the remaining electron is located either on the alkylidyne carbon atom or the alkylidene carbon atom. Neither situation is especially attractive energetically and would account for the relatively high barrier to alkylidene ligand rotation. Such a “diradical” transition state is also attractive in view of the fact that the rate of alkylidene ligand rotation increases dramatically in the presence of light. The reason why C H coupling constants are typically small for syn rotamers and relatively large for anti rotamers can be traced to some small degree of interaction of the electrons in the C-H bond with the metal centers in the syn isomer (now called agostic interactions3’). Low values for alkylidene CH coupling constants (75-100 Hz)were observed routinely in coordinatively unsaturated niobium and tantalum complexes;38in some cases (especially reduced alkylidene complexes) the “distortion” of the alkylidene ligand was severe, with M-C,-C, angles approaching 180° and M-C,-H, angles