d/f-Polypnictides Derived by Non-Classical Ln2+ Compounds: Synthesis, Small Molecule Activation and Optical Properties.

Reduction chemistry induced by divalent lanthanides has been primarily focused on samarium so far. In light of the rich physical properties of the lanthanides, this limitation to one element is a drawback. Since molecular divalent compounds of almost all lanthanides have been available for some time, we used one known and two new non-classical reducing agents of the early lanthanides to establish a sophisticated reduction chemistry. As a result, six new d/f-polyphosphides or d/f-polyarsenides, [K(18-crown-6)] [Cp''2 Ln(E5 )FeCp*] (Ln=La, Ce, Nd; E=P, As) were obtained. Their reactivity was studied by activation of P4 , resulting in a selective expansion of the P5 rings. The obtained compounds [K(18-crown-6)] [Cp''2 Ln(P7 )FeCp*] (Ln=La, Nd) are the first examples of an activation of P4 by a f-element-polypnictide complex. Additionally, the first systematic femtosecond (fs)-spectroscopy investigations of d/f-polypnictides are presented to showcase the advantages of having access to a broader series of lanthanide compounds.

Synthesis of Unprecedented 4d/4f-Polypnictogens.

A series of 4d/4f-polyarsenides, -polyarsines and -polystibines was obtained by reduction of the Mo-pnictide precursor complexes [{Cpt Mo(CO)2 }2 (µ,η2:2 -E2 )] (E=As, Sb; Cpt =tBu substituted cyclopentadienyl) with two different divalent samarocenes [Cp*2 Sm] and [(CpMe4nPr )2 Sm]. For the reductive conversion of the Mo-stibide only one product was isolated, featuring a planar tetrastibacyclobutadiene moiety as an unprecedented ligand for organometallic compounds. For the corresponding Mo-arsenide a tetraarsacyclobutadiene and a second species with a side-on coordinated As2 2- anion was isolated. The latter can be considered as reaction intermediate for the formation of the tetraarsacyclobutadiene.

Sterically induced reductive linkage of iron polypnictides with bulky lanthanide complexes by ring-opening of THF.

Reduction of [Cp*Fe(η5-E5)] (E = P, As) with divalent lanthanide reagents usually leads to reduction of [Cp*Fe(η5-E5)] followed by a Ln-E bond formation. In contrast, by using the sterically encumbered reagent [(DippForm)2Sm(thf)2] (DippForm = {(2,6-iPr2C6H3)NC(H)[double bond, length as m-dash]N(2,6-iPr2C6H3)}-), ring-opening of thf and reduction of the polypnictide is observed. This leads to two new 3d/4f polyphosphide or polyarsenide complexes [(DippForm)2Sm(Cp*Fe)E5{(CH2)4O}{(DippForm)2Sm(thf)}], in which [(DippForm)2Sm(thf)2] and [Cp*Fe(η5-E5)] are linked by a ring-opened thf molecule and no Ln-E bond formation is observed.

The approach to 4d/4f-polyphosphides.

The first 4d/4f polyphosphides were obtained by reaction of the divalent metallocenes [Cp*2Ln(thf)2] (Ln = Sm, Yb) with [{CpMo(CO)2}2(µ,η2:2-P2)] or [Cp*Mo(CO)2(η3-P3)]. Treatment of [Cp*2Ln(thf)2] (Ln = Sm, Yb) with [{CpMo(CO)2}2(µ,η2:2-P2)] gave the 16-membered bicyclic compounds [(Cp2*Ln)2P2(CpMo(CO)2)4] (Ln = Sm, Yb) as the major products. From the reaction involving samarocene, the cyclic P4 complex [(Cp*2Sm)2P4(CpMo(CO)2)2] and the cyclic P5 complex [(Cp*2Sm)3P5(CpMo(CO)2)3] were also obtained as minor products. In each reaction, the P2 unit is reduced and a rearrangement occurred. In dedicated cases, a P-P bond formation takes place, which results in a new aggregation of the central phosphorus scaffold. In the reactions of [Cp*2Ln(thf)2] (Ln = Sm, Yb) with [Cp*Mo(CO)2P3] a new P-P bond is formed by reductive dimerization and the 4d/4f hexaphosphides [(Cp*2Ln)2P6(Cp*Mo(CO)2)2] (Ln = Sm, Yb) were obtained.

Intramolecular phosphorus-phosphorus bond formation within a Co2P4 core.

The reduction of [(Cp‴Co)2(µ,η(2:2)-P2)2] (Cp‴ = 1,2,4-tBu3C5H2) with the samarocenes, [(C5Me4R)2Sm(THF)n] (R = Me or n-propyl), gives [(Cp‴Co)2P4Sm(C5Me4R)2]. This is the first example of an intramolecular P-P coupling in a polyphosphide complex upon reduction of the transition metal. The formation of the P-P bond is not a result of the direct reduction of the phosphorus atoms but is induced by a rearrangement of the positive charges between the metal atoms.

P-P bond formation via reductive dimerization of [Cp*Fe(η5-P5)] by divalent samarocenes.

The two new 3d/4f polyphosphide complexes [(Cp*Fe)(2)P(10){Sm(η(5)-C(5)Me(4)R)(2)}(2)] (Cp* = η(5)-C(5)Me(5); R = Me, nPr) were prepared by reductive dimerization of [Cp*Fe(η(5)-P(5))] with samarocenes. They are the first P(10) bridging 3d/4f metal complexes and the first examples of reductive coupling of polyphosphide complexes by divalent lanthanides.

[{(eta(5)-C(5)Me(5))(2)Sm}(4)P(8)]: a molecular polyphosphide of the rare-earth elements.

[{(eta(5)-C(5)Me(5))(2)Sm}(4)P(8)], a molecular polyphosphide of the rare-earth elements having a realgar core structure, was synthesized by a one-electron redox reaction of divalent samarocen and white phosphorus.