Electronic Structure and Multisite Basicity of the Pyramidal Phosphinidene-Bridged Dimolybdenum Complex [Mo₂(η⁵-C₅H₅)(μ-κ¹:κ¹,η⁵-PC₅H₄)(η⁶-C₆H₃^tBu₃)(CO)₂(PMe₃)]

Abstract

The title phosphinidene complex could be sequentially protonated with HBF4·OEt2 or [H(OEt2)2](BAr'4) to give the phosphido-bridged derivatives [Mo2Cp(μ-κ(1):κ(1),η(5)-HPC5H4)(η(6)-HMes*)(CO)2(PMe3)]X and then the hydrides [Mo2Cp(H)(μ-κ(1):κ(1),η(5)-HPC5H4)(η(6)-HMes*)(CO)2(PMe3)]X2 (X = BF4, BAr'4; Ar' = 3,5-C6H3(CF3)2; Mes* = 2,4,6-C6H2(t)Bu3). Density functional theory (DFT) calculations revealed that the most favored site for initial electrophilic attack is the metallocene Mo atom, but attachment of the electrophile to the phosphinidene P atom gives more stable products. This was in agreement with all other reactions investigated, which invariably involved the attachment of the added electrophile at the P site. Thus, the title compound reacted with S8 at 223 K to give the thiophosphinidene-bridged complex [Mo2Cp{μ-κ(1):κ(1),η(5)-P(S)C5H4}(η(6)-HMes*)(CO)2(PMe3)], a poorly stable molecule which reacted with MeI at room temperature to give the corresponding thiolatophosphido derivative, isolated as [Mo2Cp{μ-κ(1):κ(1),η(5)-P(SMe)C5H4}(η(6)-HMes*)(CO)2(PMe3)](BAr'4) (P-S = 2.128(4) Å) after anion exchange with Na(BAr'4). Reaction of the title compound with MeI proceeded smoothly to give the corresponding methylphosphido derivative, isolated analogously as [Mo2Cp{μ-κ(1):κ(1),η(5)-P(Me)C5H4}(η(6)-HMes*)(CO)2(PMe3)](BAr'4). The related complex [Mo2Cp{μ-κ(1):κ(1),η(5)-P(Me)C5H4}(η(6)-HMes*)(CO)2(PMe2Ph)](BAr'4) (P-C(Me) = 1.841(5) Å) could be prepared analogously from the neutral precursor [Mo2Cp{μ-κ(1):κ(1),η(5)-PC5H4}(η(6)-HMes*)(CO)2(PMe2Ph)]. In contrast, reaction of the title complex with ethylene sulfide involved opening of the C2S ring and formation of new P-C and Mo-S bonds (1.886(7) and 2.493(2) Å, respectively), with displacement of the PMe3 ligand, to give the phosphido-thiolato complex [Mo2Cp{μ-κ(2)(P,S):κ(1)P,η(5)-P(C2H4S)C5H4}(η(6)-HMes*)(CO)2]. All these derivatives of the title complex displayed an unusual trigonal pyramidal-like environment around the bridging P atom, with the added electrophile placed in the Mo2P plane as a result of the directionality of the relevant frontier orbital of the phosphinidene complex, according to DFT calculations.

Phosphinidene-bridged binuclear complexes

“…Initially, this tautomerisation was carried out via acid catalysis, but more recent experiments (2015) proved that it is better accomplished in a two-step procedure first involving full protonation of the parent compound with HPF 6 , and then deprotonation of the resulting cationic phosphido complex using KH [53]. In contrast to the reactivity of its parent compound, addition of simple donors (L) to the cyclopentadienylidene-phosphinidene complex does not occur at the metallocene site, likely due to the reluctance of the C 5 rings to change their hapticity, but at the dicarbonyl fragment, then inducing a pyramidalisation of the phosphinidene ligand which increases substantially the nucleophilicity of the P atom [53]. In fact, the putative pyramidal phosphinidene complexes of formula 52 could be spectroscopically characterised only when the donors used were tertiary phosphines (δ P = 122 ppm for L = PMe 3 ).…”

“…The dimolybdenum complex 52 also reacts with BH 3 ·THF to form the expected adduct 81 (Scheme 44) [53,86], and with some other simple electrophiles such as H + and Me + [53]. Protonation with one equivalent of [H(OEt 2 ) 2 ](BAr′ 4 ) [Ar′ = 3,5-C 6 H 3 (CF 3 ) 2 ], a strong acid with a poorly coordinating anion, led to the stable salt of the phosphanyl-bridged cation 82a, whereas a second equivalent of acid caused protonation at the metallocene site to give the dipositive hydrido complex 83, a strongly acidic species which is deprotonated by addition of weak bases such as THF.…”

“…This could be satisfactorily explained with the aid of DFT calculations on the parent phosphinidene complex, which revealed that the lone electron pair at the P atom (actually the HOMO2 of the molecule) has the required directionality. The latter in turn can be understood in qualitative terms by assuming an sp 2 hybridisation at the P atom to account for the MoP bonds and lone electron pair, while the remaining p orbital is used for bonding to the carbon atom of the C 5 H 4 ring (Figure 2) [53].…”

“…Instead of the corresponding SPR derivative, the cyclic phosphanyl-thiolato complex 84 was obtained in 55% yield (Scheme 45). The latter product likely follows from initial nucleophilic attack of the phosphinidene P atom to one of the C atoms of the C 2 S ring, with concomitant ring opening to afford a zwitterionic intermediate with a negative charge at the S atom, which enables the latter to displace the PMe 3 ligand, thus yielding the final cyclic product [53].…”

“…The dimolybdenum complex 52 reacted with S 8 at 223 K to give the unstable P(S)R-bridged complex 101 (Scheme 55) [53], which reacted with MeI to give, after ion exchange with Na(BAr' 4 ), the more stable phosphanyl complex 102. The structure of the latter was determined through an Xray analysis, which confirmed the unusual trigonal pyramidal-like geometry around the P atom already observed in the alkylated derivatives of this phosphinidene complex (see Section 5.1.1).…”