GENERATION OF THE HIGHLY REACTIVE INTERMEDIATES CP-STAR-2ZR=0 AND CP-STAR-2ZR=S - TRAPPING REACTIONS WITH ALKYNES, NITRILES, AND DATIVE LIGANDS

This paper outlines synthetic procedures that result in the successful generation of the long-sought oxo complex [Cp*2Zr = O] and its sulfur analogue [Cp*2Zr = S]. The generation of [Cp*2Zr = O] has been accomplished at 160-degrees-C by alpha-elimination of benzene from Cp*2Zr(Ph)(OH) (1) and at room temperature by deprotonation of Cp*2Zr(OH)(OSO2CF3) with KN(SiMe3)2. The oxo species reacts with diphenylacetylene and di-p-tolylacetylene to give oxametallacyclobutenes Cp*2Zr(OC(Ph) = C(Ph)) (5a) and (Cp*2Zr(OC-(Tol) = C(Tol)) (5b), which are stable at room temperature. At the elevated temperatures necessary for the alpha-elimination of benzene from 1, however, these complexes rearrange to ortho-metalated oxametallacycles Cp*2Zr(OC(o-C6H4) = C(Ph)(H)) (2a) and Cp*2Zr(OC(o-C6H4CH3) = C(Tol)(H)) (2b), and these are the products isolated in the 160-degrees-C thermolysis of 1. Similarly, generation of [Cp*2Zr = O] at 160-degrees-C in the presence of 1,4-diphenyl-1,3-butadiyne yields the complex metallacycle (eta-5-C5(CH3)5)Zr(OC(Ph) = C(H)- = C (Ph)(eta-5-C5(CH3)4CH2)) (3), in which one Cp* ligand has been integrated into the coordinated enolate moiety. In the presence of excess benzonitrile at both high and low temperature the oxo complex yields the six-membered oxaazametallacycle Cp*2Zr(OC(Ph) = NC(Ph) = N) (4a), formed from insertion of 2 equiv of PhCN into the Zr = O fragment. The connectivities of complexes 2a, 3, and 4a were confirmed by X-ray structure determinations. At room temperature, reaction of [Cp*2Zr = O] with the unsymmetrical alkyne 1-phenyl-1-propyne proceeds regiospecifically, yielding only the metallacycle Cp*2Zr(OC(Me) = C(Ph)) (5d), having the phenyl substituent located alpha to the metal center; this material gives phenylacetone upon hydrolysis. Kinetic, alkyne-exchange, and isotope-labeling studies support a mechanism involving direct elimination of benzene from 1 in the thermal generation of [Cp*2Zr = O]. They also suggest that the rearrangement of the oxametallacyclobutenes 5a,b may proceed by initial reversion of the metallacycle to an oxo-alkyne complex followed by attack of oxygen on the phenyl ring of the alkyne. The isoelectronic sulfido complex [Cp*2Zr = S] is generated by room temperature dehydrohalogenation of Cp*2Zr(SH)(I) (10). In analogy to the earlier generated imido complexes [Cp2Zr = N-R], the sulifido complex can be stabilized and isolated in pure form as the dative ligand adduct [Cp*2(L)Zr = S] (L = pyridine (11a), 4-tert-butylpyridine (11b)). An X-ray structure determination of 11b confirmed the unusual Zr = S bonding mode and represents the first example of this structural type among group 4 metals. Like its oxo analogue, the Zr = S multiply bonded linkage reacts with nitriles and alkynes to form the thiaazametallacycle Cp*2Zr(SC(Ph) = NC(Ph) = N) (12) and the thiametallacyclobutenes Cp*2Zr(SC(R) = C(R)) (13a-c, R = Et, Ph, Tol). The latter structural type was confirmed by an X-ray structure determination of 13b. The thiametallacyclobutenes readily revert to the monomeric terminal sulfido complexes 11 and free alkyne by reaction of the metallacyclobutene with pyridine; with appropriate alkynes and dative ligands, equilibria between the thiametallacyclobutenes and ligand-trapped sulfido complexes can be established and directly observed by NMR spectrometry in solution. Judging from these observations, the sulfido ligand appears to be the most stable linkage in the Zr = X (X = NR, O, S) series.