Preparation of Tungsten Alkyl Alkylidene Alkylidyne Complexes and Kinetic Studies of Their Formation

Abstract

An equilibrium mixture of alkyl alkylidyne W(CH2SiMe3)3(CSiMe3)(PMe3) (1a) and its bis(alkylidene) tautomer W(CH2SiMe3)2(=CHSiMe3)2(PMe3) (1b) has been found to undergo an alpha-hydrogen abstraction reaction in the presence of PMe3 to form alkyl alkylidene alkylidyne W(CH2SiMe3)(=CHSiMe3)(CSiMe3)(PMe3)2 (2). In the presence of PMe3, the formation of 2 follows first-order kinetics, and the observed rate constant was found to be independent of the concentration of PMe3. The activation parameters for the formation of 2 are Delta H = 28.3(1.7) kcal/mol and Delta S = 3(5) eu. In the presence of PMe2Ph, an equilibrium mixture of W(CH2SiMe3)3(CSiMe3)(PMe2Ph) (3a) and its bis(alkylidene) tautomer W(CH2SiMe3)2(=CHSiMe3)2(PMe2Ph) (3b) was similarly converted to W(CH2SiMe3)(=CHSiMe3)(CSiMe3)(PMe2Ph)2 (4). The observed rate of this reaction was also independent of the concentration of PMe2Ph. These observations suggest a pathway in which the tautomeric mixtures 1a,b and 3a,b undergo rate-determining, alpha-hydrogen abstraction, followed by phosphine coordination, resulting in the formation of the alkyl alkylidene alkylidyne complexes 2 and 4.

Transition metal alkylidene complexes. Pathways in their formation and tautomerization between bis-alkylidenes and alkyl alkylidynes

“…kinetic studies of their formation [40,42] Alkyl alkylidene alkylidyne complexes, a unique class of compounds, contain single, double and triple bonds to one atom in a single molecule. The first such complex, W(CH 2 CMe 3 )(] CHCMe 3 )(^CCMe 3 )(PMe 3 ) 2 (25, Scheme 13) was reported by Clark and Schrock [9].…”

“…In comparison, the conversion in Scheme 15 involves the addition of PMe 3 or PMe 2 Ph to the W complexes [40]. This conversion to give 12 follows first-order kinetics with the following activation parameters: DH 2 s ¼ 13(1) kcal/mol, DS 2 s ¼ À36(4) eu, and DG 2 s 338 K ¼ 25(2) kcal/mol.…”

“…24a forms an adduct (7a) with PMe 3 , which then converts to bisalkylidene 7b. The 7a # 7b mixture under heating then undergoes a-H abstraction reaction [40]. (Me 3 CCH 2 ) 3 W^CCMe 3 (2a), on the other hand, readily undergoes a-H abstraction reaction in the presence of PMe 3 .…”

“…The conversion of the DMPE complexes 9a # 9b is about 25 times faster than that of the PMe 3 complexes 7a # 7b. Since the conversion of the PMe 2 Ph complexes 8a # 8b is about 3.4 times faster than that of PMe 3 complexes 7a # 7b at 348.2(0.1) K[40], the conversion of the DMPE complexes is the fastest.It should be pointed out that the equilibria in Scheme 12 involve phosphine derivatives of (Me 3 SiCH 2 ) 3 W^CSiMe 3 (24a) containing b-Si atoms. Their neopentyl analog (Me 3 CCH 2 ) 3 W^CCMe 3 (2a) reacts with neat PMe 3 at 100 C, giving (Me 3 CCH 2 )W(^CCMe 3 )(] CHCMe 3 )(PMe 3 ) 2…”

“…The second part discusses the tautomerization of silyl alkylidyne (Me 3 CCH 2 ) 2 W(^CCMe 3 )(SiBu t Ph 2 ) (6a) with bis-alkylidene (Me 3 CCH 2 )W(]CHCMe 3 ) 2 (SiBu t Ph 2 ) (6b) [32,38] as well as (Me 3 SiCH 2 ) 3 W(^CSiMe 3 )(PR 3 ) [R 3 ¼ Me 3 , 7a; Me 2 Ph, 8a; Me 2 (CH 2 ) 2 PMe 2 (DMPE-P), 9a] with (Me 3 SiCH 2 ) 2 W(]CHSiMe 3 ) 2 (PR 3 ) (R 3 ¼ Me 3 , 7b; Me 2 Ph, 8b; DMPE-P, 9b) [P refers to a dangling P atom in Me 2 P(CH 2 ) 2 PMe 2 ] [37,39,42]. The conversion of the tungsten phosphine tautomerization mixtures to alkyl alkylidene alkylidyne (Me 3 SiCH 2 )W(] CHSiMe 3 )(^CSiMe 3 )(PR 3 ) 2 [(PR 3 ) 2 ](PMe 3 ) 2 , 10; (PMe 2 Ph) 2 , 11; DMPE, 12], including its pathway [40], will also be presented.…”