Reactivities of a Bis(alkylidene) Complex. Synthesis of a Silyl Bis(alkylidyne) Complex and a Reaction Cycle among Symmetric Bis(alkylidyne), Bis(alkylidene), and Nonsymmetric Bis(alkylidyne) Compounds

Abstract

An unstable bis(alkylidene) complex (RCH 2 ) 4 Ta 2 (dCHR) 2 (Cl) 2 (R ) SiMe 3 , 1), prepared by the addition of 2 equiv of HCl to a symmetrically bridging bis(alkylidyne) complex (RCH 2 ) 4 -Ta 2 (µ-CR) 2 (2a), decomposes through elimination of SiMe 4 to form an unstable alkylidenealkylidyne complex (RCH 2 ) 3 Ta 2 (dCHR)(tCR)(Cl) 2 (3). This conversion of an alkylidene to alkylidyne ligand was found to follow first-order kinetics with ∆H 1 q ) 14.1(0.8) kcal/mol and ∆S 1 q ) -12(3) eu. This is, to our knowledge, the second reported kinetic study of alkylidene f alkylidyne conversion, and the kinetics of this conversion is in contrast to that of (RCH 2 ) 3 TadCHR f (RCH 2 ) 4 Ta 2 (µ-CR) 2 (2a) reported earlier. Substitution of the chlorides by SiR 3 -(R ) SiMe 3 ) leads to a preferential elimination of HSiR 3 and formation of the first silyl bis(alkylidyne) complex (RCH 2 )(R 3 Si)Ta(µ-CR) 2 Ta(CH 2 R) 2 (4). Addition of excess PMe 3 to 1 produces a novel 1,1′-dimetallacyclobutadiene derivative (Me 3 P) 2 (Cl)Ta(µ-CR) 2 Ta(Cl)-(CH 2 R) 2 (5a) with two nonsymmetrically bridging alkylidyne ligands. Both bridging ligands coordinate with CdTa double bonds to the same metal atom (and with C-Ta single bonds to the other metal atom). The two bridging alkylidyne ligands in 5a, which are coordinated to one metal atom in the axial and equatorial position, respectively, are involved in an intramolecular ligand exchange. The kinetic barriers for this exchange were determined to be ∆H 2 q ) 13.6(0.4) kcal/mol and ∆S 2 q ) -5(2) eu in 5a, and ∆H 3 q ) 13.0(0.3) kcal/mol and ∆S 3 q ) -5(1) eu in 5b. Addition of 2 equiv of RCH 2 Li to replace the Cl ligands in 5a converts 5a to 2a, completing a reaction cycle 2a f 1 f 5a f 2a. (Me 3 P) 2 (RCH 2 )Ta(µ-CR) 2 Ta(Cl)-(CH 2 R) 2 (6) was identified as an intermediate in the conversion of 5a to 2a. absences, were uniquely consistent with the space group P21/ n. The structure was solved using the Siemens SHELXTL 93 (Version 5) proprietary software package. The niobium atoms were found by the Patterson method. The structure was completed by successive Fourier syntheses and refined to convergence by successive full-matrix least-squares refinements of F 2 . All non-hydrogen atoms were anisotropically refined. The hydrogen atoms were placed in calculated positions and introduced into the refinement as fixed contributors with isotropic U values of 0.08 Å 2 . Crystal, data collection, and refinement parameters are given in Table 1.

Synthesis and Characterization of Group 4 Amido Silyl Complexes Free of Anionic π-Ligands

“…The Zr atom exhibits a pseudotetrahedral geometry around the metal center. The Si−Zr−N [106.1(2)°] and N−Zr−N [112.6(2)°] angles are very close to those observed in tetrahedral alkyl silyl complexes (Me 3 CCH 2 ) 3 TiSi(SiMe 3 ) 3 [105.4(4) and 113.2(4)°],7b (Me 3 SiCH 2 ) 3 TiSi(SiMe 3 ) 3 [106.4(2) and 113.0(7)°],7b and (Me 3 CCH 2 ) 3 ZrSi(SiMe 3 ) 3 [105.6(8) and 113.0(7)°] and the alkoxide silyl complex (Me 3 CO) 3 MSi(SiMe 3 ) 3 [107.6(3) and 111.4(4)°] 3c 1 ORTEP view of the molecular structure of (Me 2 N) 3 ZrSi(SiMe 3 ) 3 ( 1 ), showing 50% thermal ellipsoids. 2 Selected Bond Distances (Å) and Angles (deg) for 1 Zr−N(1) 2.018(7) C(1)−N(1) 1.434(11) Zr−Si(1) 2.784(4) C(2)−N(1) 1.466(11) Si(1)−Si(2) 2.334(3) C(3)−Si(2) 1.842(10) Si(2)−Si(4) 1.861(10) C(5)−Si(2) 1.914(11)N(1)−Zr−N(1A) 112.6(2) Zr−N(1)−C(1) 137.9(6) N(1)−Zr−Si(1) 106.1(2) Zr−N(1)−C(2) 110.8(5) Zr−Si(1)−Si(2) 112.61(12) Si(1)−Si(2)−C(3) 111.8(3) C(1)−N(1)−C(2) 111.3(7) Si(1)−Si(2)−C(5) 109.0(4) Si(1)−Si(2)−C(4) 114.1(3) Si(2)−Si(1)−Si(2A) 106.16(13) …”

“…Spectroscopic properties [ 1 H, 13 C{ 1 H}, and 29 Si{ 1 H} NMR] of the complexes are consistent with the structure assignments. The 1 H and 13 C{ 1 H} NMR resonances of the Si(SiMe 3 ) 3 ligand in 1 , 2 , and 7 resemble those observed for other group 4 Si(SiMe 3 ) 3 derivatives such as alkyl and alkoxide silyl complexes. 3c, However, the tertiary silicon NMR resonances of the M- Si (SiMe 3 ) 3 ligands in 1 (−124.6 ppm), 2 (−103.5 ppm), and 7 (−121.3 ppm) are significantly upfield shifted from those in the alkyl silyl complexes (Me 3 CCH 2 ) 3 ZrSi(SiMe 3 ) 3 (−85.8 ppm) 7b and (Me 3 SiCH 2 ) 3 ZrSi(SiMe 3 ) 3 (−75.7 ppm) 7b but are close to the corresponding values for (Me 3 CO) 3 MSi(SiMe 3 ) 3 [M = Zr (−115.82 ppm), Hf (−103.47 ppm)] 3c. The upfield shifts of the tertiary silicon NMR resonances in 1 , 2 , and 7 can be attributed to the p(π)−d(π) bonding between the lone pairs on the amido ligands and the metal centers.…”

“…The average Ti−N bond distance of 1.883(5) Å is slightly longer than that in (Me 2 N) 3 TiCl [1.866(4) Å], consistent with the fact that complex 3 is more sterically crowded than (Me 2 N) 3 TiCl. As in the structure of 1 , the Ti−Si bond distance of 2.635(2) Å is slightly longer than those in the more crowded alkyl silyl complexes (Me 3 CCH 2 ) 3 TiSi(SiMe 3 ) 3 [2.618(8) Å] and (Me 3 CCH 2 ) 3 TiSi(SiMe 3 ) 3 [2.603(3) Å] 7b 2 ORTEP view of the molecular structure of (Me 2 N) 3 TiSiPh 2 Bu t ( 3 ), showing 50% thermal ellipsoids. 3 Selected Bond Distances (Å) and Angles (deg) for 3 Ti−N(1) 1.897(5) N(1)−C(1) 1.454(8) Ti−N(2) 1.868(5) N(1)−C(2) 1.454(9) Ti−N(3) 1.884(5) N(2)−C(3) 1.457(9) Ti−Si 2.635(2) N(2)−C(4) 1.459(9) N(3)−C(5) 1.456(8) Si−C(12) 1.893(6) N(3)−C(6) 1.442(9) Si−C(13) 1.920(6) Si−C(19) 1.931(6) Si−Ti−N(1) 108.3(2) Ti−Si−C(12) 105.9(2) Si−Ti−N(2) 104.3(2) Ti−Si−C(13) 111.3(2) Si−Ti−N(3) 101.3(2) Ti−Si−C(19) 118.1(2) N(1)−Ti−N(2) 114.2(2) Ti−N(1)−C(1) 115.1(4) N(1)−Ti−N(3) 113.4(2) Ti−N(1)−C(2) 133.8(5) N(2)−Ti−N(3) 113.9(2) C(1)−N(1)−C(2) 111.0(6) Ti−N(2)−C(3) 113.5(5) Ti−N(3)−C(5) 115.9(4) Ti−N(2)−C(4) 135.0(5) Ti−N(3)−C(6) 131.9(5) C(3)−N(2)−C(4) 111.5(6) C(5)−N(3)−C(6) 112.1(6) …”

“…However, much of early-transition-metal silyl chemistry involves complexes containing cyclopentadienyl (Cp) ligands. Few d° Cp-free silyl complexes of the early transition metals have been reported. 3c,− We recently synthesized a series of Cp-free d 0 alkyl, alkylidene, and alkylidyne silyl complexes of group 4, 5, and 6 metals . It was found that, among these complexes, only those with the bulky ligand Si(SiMe 3 ) 3 are thermally stable.…”

The tris(trimethylsilyl)silyl group in organic, coordination and organometallic chemistry

“…In this case, the bisalkylidyne complex 84 is formed. 184 The reaction of 1 equiv. of CO with the tris(trimethylsilyl)silyl tantalum hydride complex 85 yields the addition product 86, which isomerises within the next 24 h into the cyclic derivative 87.…”