Site-Selectivity in the Protonation and Related Reactions of Chalcogenophosphinidene-Bridged Dimolybdenum Cyclopentadienyl Complexes

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Abstract

Reactions of the chalcogenophosphinidene-bridged complexes syn-[Mo2Cp2(μ-κ2P,Z1P4-ZPMes*)(CO)2L] (L = CO, CNtBu; Z = O, S, Se), anti-[Mo2Cp2(μ-κ2P,S1P4-SPMes*)(CO)2L] (L = CO, CNtBu) and [Mo2Cp2{μ-κ2P,Z1P-ZPH}(η6-HMes*)(CO)2] (Z = S, Se, Te) towards sources of H+, Me+, and AuP(pTol)3+ cations were investigated (Mes* = 2,4,6-C6H2tBu3; Cp = η5-C5H5). The latter two electrophiles invariably added to the chalcogen atom to give corresponding derivatives [Mo2Cp2{μ-κ2P,Z1P4-Mes*PZ(CH3)}(CO)3]BX4 [X = F, Z = O, S, Se; P–S 2.144(1) Å when Z = S and X = Ar′ = 3,5-C6H3(CF3)2], [Mo2Cp2{μ-κ2P,Z1P4-Mes*PZ(CH3)}(CNtBu)(CO)2]BF4 (Z = S, Se), and [AuMo2Cp231S2P,S1P4-SPMes*){P(pTol)3}(CO)3]PF6 [P–S 2.113(2), S–Au 2.320(2) Å]. Even when syn and anti isomers of the neutral precursor were used, the corresponding products were invariably characterized by their syn conformation (Z atom close to L ligand). Besides this, methylated derivatives of the chalcogenophosphinidene complexes bearing the formula [Mo2Cp2{μ-κ2P,Z1P-HPZ(Me)}(η6-HMes*)(CO)2](CF3SO3) (Z = S, Se, Te), were found in solution to exist as an equilibrium of corresponding cis and trans isomers differing, in each case, in the relative positioning of the Cp rings with respect to the MoPZ plane. In contrast to the above results, the protonation of all these compounds was quite sensitive to the particular chalcogenophosphinidene ligand and conformation of the complex. Protonation of the HPZ-bridged complexes led to complex mixtures of products that could not be isolated or properly characterized. In contrast, the aryl-bearing substrates reacted selectively to give corresponding complexes [Mo2Cp2{μ-κ2P,Z1P5-ZP(C6H3tBu3)}(CO)3]BX4 (Z = S, X = F; P–S 2.032(2) Å when X = Ar′; Z = Se, X = F) and [Mo2Cp2{μ-κ2P,Z1P5-ZP(C6H3tBu3)}(CNtBu)(CO)2]BAr′4 (Z = S, Se). The reaction of anti-[Mo2Cp2(μ-κ2P,S1P4-SPMes*)(CO)3] with HBF4·OEt2 at 213 K initially gave unstable intermediate anti-[Mo2Cp2{μ-κ2P,S1P4-Mes*PS(H)}(CO)3]BF4, which then rapidly converted at room temperature to the conventional isomer with the S atom close to the metallocene-bound carbonyl ligand (syn isomer). This transformation is in agreement with Density Functional Theory calculations for neutral thiophosphinidene complexes; electrophilic attack at the sulfur atom, which is both an orbital- and charge-favoured event, affords products that are more stable than those resulting from protonation at the metal sites. All these protonation reactions eventually result in an endo addition of H+ to the C6 atom of the Mes* ring with concomitant η4→η5 haptotropic shift of the resulting HMes* group. This conversion involves an easy H migration from S to the C6 atom, computed to take place with a low activation barrier of about 70 kJ/mol.

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