Studies of the Reactivity of Permethylzirconocene Complexes with Binuclear Transition Metal Carbonyl Compounds

The low valent permethylzirconocene complex {Cp*2ZrN2}2N2 (Cp*≡η5-C5Me5) reacts with binuclear transition metal carbonyl compounds of the general formula Cp2M2(CO)x (Cp≡η5-C5H5) to produce trinuclear complexes. Two major modes of reactivity are observed. Compounds with M-M bonds that are easily reduced such as (CpM(CO)3)2 (M = W, Mo) react with {Cp*2ZrN2}2N2 to produce trinuclear species in which two CpM(CO)3 anions are connected to a Cp*2Zr dication via µ2-(C-η1; O-η1) carbonyl bridges. The structure of the molybdenum complex has been determined by single crystal X-ray diffraction methods. These trinuclear complexes are also produced in the reaction of the Cp*2ZrRR'(R,R' = H,alkyl) with two equivalents of the hydride complexes CpM(CO)3H· The dinuclear complexes Cp*2M(H)(CpMo(CO)3)(M = Zr, L = PMe3; M = Hf, L = CO) have been similarly prepared by treatment of the zirconium or hafnium dihydride with the corresponding molybdenum hydride. The trinuclear complexes react with HX (X = Cl, OH) to produce CpM(CO)3H and Cp*2ZrX2. The other mode of reactivity between {Cp*2ZrN2}2N2 and metal dimers is observed with (CpM(CO)2)2 (M = Fe, Ru). In these cases trinuclear complexes are produced in which two carbonyl ligands are reductively coupled yielding an O2C2 moiety bridging the three metals. This coupling is reversible and the Zr(II) intermediate can be trapped with CO or other metal dimers. The iron analog is also produced in the reaction of Cp*2ZrI2 with Na(CpFe(CO)2). The reaction of {Cp*2ZrN2}2N2 with the bridging methylene complex cis-Cp2Ru2(CO)3(CH2) does not result in carbonyl coupling, yielding instead what appears on the basis of spectroscopic data to be a π-complex between permethylzirconocene and cis-Cp2Ru2(CO)3(CH2) held together only by µ2-(C-η1; C,O-η2) bridging carbonyl ligands. The ruthenium carbonyl coupled product, Cp*2Zr(O2C2)Ru2Cp2(CO)2, reacts under H2 to produce the bridging zirconoxy carbene complex trans-Cp2Ru2(CO)3(C(H)OZr(H)Cp*2). This complex is fluxional on the 1H and 13C NMR timescales. ΔG‡ for this process is 14.0(2) kcal·mol-1 at -4°C. The fluxionality is best explained by a process involving a terminal carbine intermediate. The carbene formation is reversible, and treatment with D2 results in label incorporation in both the hydride and carbene hydrogen positions. Reaction with CO produces Cp*2Zr(CO)2, (CpRu(CO)2)2 and presumably H2. Treatment with BF3 or HBF4 results in C-O scission and produces the bridging methylene complex Cp2Ru2(CO)3(CH2). Labeling studies indicate both methylene hydrogens originate on zirconium. This bridging methylene complex can be synthesized in 60% yield by treatment of (CpRu(CO)2)2 with LiBEt3H in toluene.