Trimethylsilylated Allyl Complexes of Nickel. The Stabilized Bis(π-allyl)nickel Complex [η³-1,3-(SiMe₃)₂C₃H₃]₂Ni and Its Mono(π-allyl)NiX (X = Br, I) Derivatives

Abstract

Reaction of 2 equiv of K[1,3-(SiMe3)2C3H3] with NiBr2(dme) in THF at -78 degrees C produces the orange pi-allyl complex [1,3-(SiMe3)2C3H3]2Ni (1). Unlike the pyrophoric (C3H5)2Ni, the trimethylsilylated derivative only slowly decomposes in air (from hours to days). Both eclipsed (1a) and staggered (1b) conformations are found in solution; the eclipsed form irreversibly converts to the thermodynamically more stable staggered conformation when heated above 85 degrees C. Single-crystal X-ray structures obtained for both 1a and 1b confirm that the allyl ligands are bound in a trihapto manner to the metals and that trimethylsilyl substituents are in syn, anti arrangements. Density functional theory calculations performed on the bis(allyl)nickel complexes indicate that the substituents exert little effect on the basic metal-ligand geometries. Trimethylphosphine is converted to tetramethyltetraphosphane, (MeP)4, on reaction with 1. In toluene, 3-bromo-1,3-bis(trimethylsilyl)propene reacts with (COD)2Ni to produce the dimeric purple complex {[1,3-(SiMe3)2C3H3]NiBr}2 (2a). Both NMR and X-ray crystallographic data establish that the allyl ligands are staggered and that the trimethylsilyl substituents are in a syn, syn conformation. NMR data indicate that the reaction of one equivalent of 1 with Br2 in benzene produces an analogous complex (2b) with the allyl ligand substituents in a syn, anti configuration. When 1 equiv of 1 is treated with I2 in hexanes, the dark red dimeric complex {[1,3-(SiMe3)2C3H3]NiI}2 (3) is formed. Its X-ray crystal structure demonstrates that both eclipsed (3a) and staggered (3b) allyl conformation are present. The trimethylsilyl groups on the allyl ligands are in syn, anti arrangements in the two forms.

Complexes with Sterically Bulky Allyl Ligands: Insights into Structure and Bonding

“…A staggered MAЈ 2 arrangement with syn,anti trimethylsilyl groups is found for the cobalt and nickel analogues, [17,21,22] although the nickel complex is also found with eclipsed allyl ligands and the iron complex exclusively so. The reason for the difference is unclear.…”

“…[17,[20][21][22] Halide metathetical routes can be used to synthesize all three MAЈ 2 compounds (M = Fe, Co, Ni), although halide exchange can be problematic for nickel (see below), and for iron an additional route starting from a lithium ferrate complex has been described (Scheme 14). [67] Scheme 14.…”

“…Its synthesis from metal halides proved non-routine, in that the reaction of K[AЈ] with NiCl 2 , NiBr 2 , and NiI 2 leads to the formation of the trimethylsilylated hexadiene [(SiMe 3 ) 2 C 3 H 3 ] 2 . In contrast, use of Ni(acac) 2 [44] or the more soluble halide NiBr 2 (dme) [17] leads to the isolation of the yellow-orange product in high yields. Electrochemical investigations of NiAЈ 2 revealed only an irreversible, one-electron oxidation in both thf and dichloromethane.…”

“…The nickel-carbon bond lengths are within 0.04 Å of each other, and the angles between the C 3 planes differ by less than 5°, even though DFT calculations suggest that the potential energy surface for the interplanar angle is nearly flat, and presumably easily changeable by bulky ligand substitutents. [17] The mono(allyl)nickel species AЈNiBr can be formed by the reaction of bis(trimethylsilyl)allyl bromide with Ni(COD) 2 (Scheme 15), or by the direct reaction of NiAЈ 2 with elemental bromine in benzene at 0°C (Scheme 16). The red-purple AЈNiBr is an air-and moisture-sensitive solid.…”

“…For example, unlike the pyrophoric Ni(C 3 H 5 ) 2 , the trimethylsilylated derivative Ni[1,3-(SiMe 3 ) 2 -C 3 H 3 ] 2 only slowly decomposes in air (from hours to days). [17] In addition, the homoleptic thorium complexes Th[(SiMe 3 ) n C 3 H 5-n ] 4 (n = 1, 2) are stable to 90°C, [18] in contrast to Th(C 3 H 5 ) 4 , which decomposes at 0°C. In other cases, the presence of substituent groups aids in crystallization, so that solid-state structures may be obtained; calculations suggest that the X-ray crystal structure of the lithium dimer {Li[1,1Ј,3-(SiMe 3 ) 3 C 3 H 2 ]} 2 is related to that of the unsubstituted complex, which is known to be a dimer from cryoscopy, but which has not been structurally authenticated.…”