Tungsten Alkyl Alkylidyne and Bis-alkylidene Complexes. Preparation and Kinetic and Thermodynamic Studies of Their Unusual Exchanges

Abstract

Preparation and characterization of novel alkyl alkylidyne (Me3SiCH2)3W(⋮CSiMe3)(PMe3) (1a) and bis-alkylidene (Me3SiCH2)2W(CHSiMe3)2(PMe3) (1b) and studies of the exchange between alkylidyne 1a and bis-alkylidene 1b are reported. An adduct between PMe3 and alkyl alkylidyne (Me3SiCH2)3W⋮CSiMe3 (2a), (Me3SiCH2)3W(⋮CSiMe3)(PMe3) (1a), was found to undergo a rare reversible transformation to its bis-alkylidene tautomer (Me3SiCH2)2W(CHSiMe3)2(PMe3) (1b). The X-ray crystal structure of 1b has been determined. The bis-alkylidene tautomer 1b is favored in the 1a ⇌ 1b equilibrium with K eq ranging from 12.3(0.2) at 278(1) K to 9.37(0.12) at 303(1) K, giving the thermodynamic parameters for the equilibrium: ΔH° = −1.8(0.5) kcal/mol and ΔS° = −1.5(1.7) eu. The α-H exchange between 1a and 1b follows first-order reversible kinetics. The activation parameters are ΔH ⧧ = 16.2(1.2) kcal/mol and ΔS ⧧ = −22(4) eu for the forward reaction (1a → 1b) and ΔH ⧧ = 18.0(1.3) kcal/mol and ΔS ⧧ = −21(4) eu for the reverse reaction (1b → 1a). An adduct between (Me3SiCH2)3W⋮CSiMe3 and PMe2Ph, (Me3SiCH2)3W(⋮CSiMe3)(PMe2Ph) (3a), was found to undergo a similar reversible transformation to its bis-alkylidene tautomer (Me3SiCH2)2W(CHSiMe3)2(PMe2Ph) (3b). The 3a ⇌ 3b equilibrium is shifted more to the alkyl alkylidyne adduct 3a [K eq‘ = 4.65(0.11) at 303 K] than the 1a ⇌ 1b equilibrium. The forward 3a → 3b conversion in the PMe2Ph complexes is slower than the 1a → 1b conversion at 303 K, whereas the reverse 3b → 3a conversion is slightly faster than the 1b → 1a conversion.

Organometallic consequences of a redox reaction: Terminal trimethylsilylmethylidene titanium complexes prepared by a one-electron oxidation step

“…Independently, complex 2 can be generated cleanly, via one-electron oxidation of 1 with AgOTf in pentane (76% isolated yield, Scheme 1). 1 H, 13 C, and 19 F NMR spectra are essentially analogous to its solvated counterpart, 2-THF. Despite this similarity, the 1 H NMR spectrum shows a slight deviation in the chemical shift for the alkylidene hydrogen (12.39 ppm), while the 13 C NMR spectrum displays a further downfield chemical shift at 320.1 ppm, congruently with a stronger TiÁ Á ÁH a-hydrogen agostic interaction (when compared to 2-THF) taking place with the four-coordinate metal center (J C-H = 98 Hz) [38,36].…”

“…High-oxidation state transition metal complexes of groups such as 5 [1][2][3][4][5][6][7], 6 [8][9][10][11][12][13]2,14] and 7 [2] share a common ground in stabilizing the terminal trimethylsilylmethylidene ligand (M = CHSiMe 3 ) [2,15,16]. However, the number of terminal trimethylsilylmethylidene metal complexes plummets dramatically when the metal in question represents the lighter congener for each group (e.g., Ti, V, Cr) [2,[15][16][17][18].…”

“…Separation of 3 was achieved by extracting the reaction mixture with hexane, filtering, and crystallizing the filtrate at À35°C. Formation of compound 3 is supported by a combination of 1 H, 13 C, and 19 F NMR spectra as well as comparison of its spectral shifts with the previously reported analogue [ArNC(Me)CHC(Me)@CH t Bu]Ti @NAr(OTf) [38,36]. Our earlier reports revealed that complex (nacnac)Ti@CH t Bu(OTf) had an estimated t 1/2 of 45 min at 57°C and that complete decomposition typically occurred over $2 h [38,36].…”

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

“…Equilibria in alkyl alkylidyne and bis-alkylidene phosphine complexes (Me 3 SiCH 2 ) 3 W(^CSiMe 3 )(PR 3 ) (7a, 8a, 9a) # (Me 3 SiCH 2 ) 2 -W(]CHSiMe 3 ) 2 (PR 3 ) (7b, 8b, 9b) [37,39,42] In the previous section, the tautomerization of silyl neopentyl complexes 6b # 6a was discussed, and the silyl ligand was (Me 3 CCD 2 ) 3 (7a, 8a, 9a), respectively. NMR spectroscopic characterization of the adducts at low temperature suggested that the phosphine ligands coordinate cis to the alkylidyne ligand.…”

“…The first part focuses on preparation of silyl alkylidene complexes (Me 3 ECH 2 ) 2 Ta(]CHEMe 3 )(SiR 3 ) [R 3 ¼ (SiMe 3 ) 3 , E ¼ C, 3a, Si, 3b; R 3 ¼ Bu t Ph 2 , E ¼ C, 4a, Si, 4b] [12,28] and studies of pathways in the formation of the silyl alkylidene (Me 3 SiCH 2 ) 2 Ta(]CHSiMe 3 )-[Si(SiMe 3 ) 3 ] (3b) [28] as well as alkyl alkylidenes (Me 3 ECH 2 ) 3 Ta] CHEMe 3 (E ¼ C, 5a; Si, 5b) [13,41]. 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.…”