The Reaction of Permethylzirconocene and Permethylhafnocene Hydride Complexes with Phosphorous Ylides
3.1 Introduction
The use of phosporous ylides as reagents in organic synthesis is ubiquitous. 1 Only recently, however, have these phosphoranes found application as reagents in the synthesis of organometallic compounds.
Early work in this area focused on the use of phosphorous ylides in chelating or nucleophilic reactions. 2 Little use was made of the phos- phorane as a carbene transfer reagent (Wittig reagent) or as a strong base; properties which were exploited in synthetic organic chemistry.
The basicity of phosphorous ylides was used by Schrock in the preparation of several zirconium and tantalum complexes. The prep- aration of Cp2TaMeCH2 from Cp2TaMe2 and methylenetrimethyl- phosphorane (eq. 1) results not via methylene transfer from the ylide, but rather by proton abstraction from one of the two methyl ligands. 3 Other compounds were prepared using the ylide first as a nucleophile and second as a dehydrohalogenating agent (eq. 2). 4 Phosphorous
ylides have also been successfully employed in the synthesis of several zirconium-ketene complexes by the dehydrohalogenation of zirconium haloacyl compounds. 5
Similar to their use as carbene transfer reagents in organic synthesis, phosphorous ylides have been used to prepare several early transition metal carbene species by the direct transfer of a carbene
fragment from an ylide to an organometallic compound. Schrock has
reported the preparation of tantalum alkylidene complexes by the reaction of phosphine compounds with the appropriate phosphorane (eq. 3). 6 A similar result was later reported by Schwartz in a zirconium system, offering spectroscopic evidence for the first reported example of a zirconium methylene species (eq. 4). 7
Cp2Ta(Me)L + CHR'P~- CP:!Ta(CHR' )Me+ L + P~ (3) L = PMe3 , PMe2Ph
R =Me, Et, Ph R' = H, Me, Ph
(4) To further investigate the possibility of CH2-transfer from phosphorous ylides, their reactions with permethylzirconocene and permethylhafnocene dihydride (Cp~Zr~, Cp~HfH2) has been explored.
These hydride complexes are known to act as powerful hydride trans- fer reagents8 and it was thought that hydride migration to the ylide nucleophilic carbon might be achieved to give a series of alkyl-hydride compounds. The insertion of methylene into a metal-hydride bond to afford a methyl-hydride derivative is known for some of the Group VI and VII transition metals using diazomethane as the car bene source, although the yields are typically low. 9 The insertion of methylene into a metal-hydride bond using a phosphorous ylide had not been previously
reported. This chapter describes the use of methylenetrimethylphos- phorane as a reagent in the synthesis of permethylzirconocene and permethylhafnocene methyl hydride as well as a permethylzirconocene
metallated-ylide complex. The reactivity of this metallated-ylide complex with CO is also discussed.
3. 2 Results and Discussion
3. 2. 1 Synthetic Advantages of Using Methylenetrimethylphosphorane Methylenetrimethylphosphorane (CH2PMe3 ) was chosen for this study for several reasons. First, it is one of the most nuecleophilic of all phosphorous ylides. 1 Coordination of the ylide to the highly electrophilic metal center in Cp*2MH2 (M = Hf, Zr) should proceed quite readily. Second, it is the least serically demanding of all phos- phorous ylides. Because the pentamethylcyclopentadienylligands are themselves quite large, the incoming ylide must be small to success- fully enter the coordination sphere of the Cp*2MH2 complex. Third and last, once the ylide has transferred the methylene fragment to a metal complex, the resulting PMe3 can either be used as a ligand to stabilize the complex if necessary, or simply removed from the reaction mixture by evacuation.
3. 2~ 2 Synthesis and Characterization of Cp*2Hf(H)Me (1)
Bis(pentamethylcyclopentadienyl)hafnium dihydride (Cp*2Hfii2 (!)) reacts with one equivalent of CH2PMe3 at
so•c
after 1. 5 hours toafford PMe3 and Cp*2Hf(H)Me (1) in greater than 95% yield as evidenced by 1H ~R spectroscopy (eq." 5). 10 Although the colorless methyl-
(5)
1 4
hydride complex is extremely soluble in hydrocarbon solvents, it may
be recrystallized from petroleum ether at -78
•c
in 30• yield. The 1H NMR spectrum of 4 consists of a single resonance for the Cp* protons'"'
(01. 90) and a high field singlet at 0 -0. 65 consistent with a methyl group bound to hafnium. 11 The hydride ligand resonance is observed as a broad singlet at 012.97, upfield of the starting complex 1 (015.6).
'"'
A Hf-H stretch is also observed in theIR spectrum at 1600 cm-1•
The formation of Cp*Jif(H)Me from Cp*2Hfil2 and CH2PMe3 is unique in that it is the first example of methylene insertion into a metal-hydride bond using a phosphorous ylide as the carbene source.
Hydrides of the later transition metals are, apparently, not sufficiently hydridic to effect this transformation;12 the protonic nature of later transition metal hydrides probably results in their deprotonation when treated with phosphorous ylides. Complex 4 is also unique in that it is
. '"'
the first example of a monomeric group IV methyl-hydride complex. 13 Although Cp2Zr(H)Me has been reported, it is undoubtedly polymeric and has not been well characterized. 14
The general instability of methyl-hydride complexes has been suggested to account for the rarity of these species. 15,16 Cp*2Hf(H)Me is, however, remarkably stable;
solutions of
1
remain unchanged even after several weeks at 80 • C.3. 2. 3 Mechanism of the Formation of Cp*2Hf(H)Me (1_)
The mechanism of this insertion reaction could follow one of two pathways. In the first (Scheme 3.1), the ylide coordinates to the
highly electrophilic hafnium(IV) center through the nucleophilic carbon giving the 18-electron ylide-dihydride intermediate ~· Loss of PMe3 from 2 affords a dihydrido-methylene intermediate which then
'"'