Electrophilic Functionalization of a Hexaphosphabenzene Ligand in [(Cp*Mo)2(µ,η6 : 6-P6)].

The electrophilic functionalization of the triple-decker sandwich complex [{Cp*Mo}2(µ,η6:6-P6)] (A) and its mono-oxidized counterpart [{Cp*Mo}2(µ,η6:6-P6)][SbF6] (B) with reactive main-group electrophiles as well as radical scavengers is shown to be a reliable method for the selective functionalization of the hexaphosphabenzene ligand. Depending on the electrophile used, the regioselectivity of the functionalization can be adjusted. Using group 16 electrophiles, the trisubstituted compounds [{Cp*Mo}2{(µ,η3 : 3-P3)(µ,η1 : 1 : 1 : 1-1,3-(SePh)2-2-Br-P3)}][TEF] (1), [{Cp*Mo}2(µ,η3 : 3-P3)(µ,η1 : 1 : 1 : 1-1,2,3-(EPh)3-P3)][SbF6] (E=S (2), Se (3)) as well as the side product [{Cp*Mo}2(µ,η4:4-P4)(µ,η1 : 1-P(SPh)2)][SbF6] (4) are obtained. By switching to phosphenium ions as group 15 electrophiles, the ring-inserted products [{Cp*Mo}2(µ,η3 : 3 : 2 : 2-P7R2)][TEF] (R=Cy (5), iPr (6)) are isolated, showing an unprecedented P7R2 structural motif. Furthermore, the reaction with MeOTf yields the dimeric [{Cp*Mo}4(1,4-Me2-µ4,η1 : 1 : 1 : 1 : 1 : 1-P6)(µ,η3 : 3-P3)2][TEF]2 (7) as the first example of a complex featuring two interconnected cyclo-P6 middle deck ligands. Finally, by combination of the methylation step with Ph2Se2, the mixed group 14/16 complex [{Cp*Mo}2{(µ,η3 : 3-P3)(µ,η1 : 1 : 1 : 11,2-(SePh)2-3-Me-P3)}][OTf] (8) is obtained.

Probing the Isolobal Relation between Cp'''NiP3 and White Phosphorus by Experimental Charge Density Analysis.

An in-depth analysis of the description of bonding within Cp'''Ni-cyclo-P3 (Cp'''=1,2,4-tri-tert-butylcyclopentadienyl, [Ni]P3) employing X-ray diffraction based multipolar modeling, density functional theory (DFT) as well as an "experimental wavefunction" obtained from X-ray restrained wavefunction (XRW) fitting is presented. The results are compared to DFT calculations on white phosphorus - an isolobal analogue to [Ni]P3. A complementary bonding analysis shows insights into the reactivity of [Ni]P3. The isolobal principle is reflected in every aspect of our analysis and the employed methods seamlessly predict the differences in reactivity of [Ni]P3 and P4. Crystallographic modeling, solid-state NMR, and DFT calculations describe the dynamic behavior of the cyclo-P3 unit in the title molecule.

NHC-Stabilized Mixed Group 13/14/15 Element Hydrides.

The syntheses of novel N-heterocyclic carbene (NHC) adducts of group 13, 14 and 15 element hydrides are reported. Salt metathesis reactions between NaPH2 and IDipp ⋅ GeH2 BH2 OTf (1) (IDipp=1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) led to mixtures of the two isomers IDipp ⋅ GeH2 BH2 PH2 (2 a) and IDipp ⋅ BH2 GeH2 PH2 (2 b); by altering the reaction conditions an almost exclusive formation of 2 b was achieved. Attempts to purify mixtures of 2 a and 2 b by re-crystallization from THF afforded a salt [IDipp ⋅ GeH2 BH2 ⋅ IDipp][PHGeH2 BH2 PH2 BH2 GeH2 ] (4) that contains the novel anionic cyclohexyl-like inorganic heterocycle [PHGeH2 BH2 PH2 BH2 GeH2 ]- . In addition, the borane adducts IDipp ⋅ GeH2 BH2 PH2 BH3 (3 a) and IDipp ⋅ BH2 GeH2 PH2 BH3 (3 b) as even longer chain compounds were obtained from reactions of 2 a/2 b with H3 B ⋅ SMe2 and were studied by NMR spectroscopy. Accompanying DFT computations give insight into the mechanism and energetics associated with 2 a/2 b isomerization as well as their decomposition pathways.

Reactivity of [Cp*Fe(η5 -As5 )] towards Carbenes, Silylenes and Germylenes.

The reaction behavior of [Cp*Fe(η5 -As5 )] (I) (Cp*=C5 Me5 ) towards carbenes and their heavier analogs was investigated. The reaction of I with NHCs (NHCs=N-heterocyclic carbenes) results in the first substitution products of polyarsenic ligand complexes by NHCs [Cp*Fe(η4 -As5 NHC)] (1 a: NHC=IMe=1,3,4,5-tetramethylimidazolin-2-ylidene, 1 b: NHC=IMes=1,3-bis(2,4,6-trimethylphenyl)-imidazolin-2-ylidene). In contrast, the reaction of I with Et CAAC (Et CAAC=2,6-diisopropylphenyl)-4,4-diethyl-2,2-dimethyl-pyrrolidin-5-ylidene) leads to a fragmentation and the formation of an unprecedented As6 -sawhorse-type compound [As2 (AsEt CAAC)4 ] (2). The reaction of (LE)2 (L=PhC(Nt Bu)2 ; E=Si, Ge) with I resulted in a rearrangement and an insertion of LE fragments, forming unique silicon- (4: [Cp*Fe(η4 -As4 SiL)], 5 a: [Cp*Fe(η4 -As6 SiL)) and germanium-containing (5 b: [Cp*Fe(η4 -As6 GeL)) cyclic polyarsenic ligand complexes.

Unusual cleavage of phosphaalkynes triple bond in the coordination sphere of transition metals.

The reaction of RCP (R = Me, tBu, iPr) with Co2(CO)8 and Fe2(CO)9 under mild conditions led to unpredictable fragmentations of the CP triple bond and subsequent formation of clusters with a dimeric Co3E (E = P and RC) and an Fe3P(CR) core, respectively.

Ring expansion vs. addition - reactivity of a cyclo-P4 complex towards pnictogenium cations.

A systematic study on the reactivity of the cyclo-P4 complexes [CpRTa(CO)2(η4-P4)] towards pnictogenium cations results in the formation of functionalised interpnictogen cations. Phosphenium ions insert into one of the P-P bonds to give ring-expanded cyclo-P5R2 products. In contrast, an arsenium-functionalised P4AsCy2 ligand displays an interesting borderline case between ring expansion and coordination, while stibenium cations afford addition products. Tuning of the steric and electronic properties of the stibenium ion shows a drastic influence on the reaction outcome.

Nucleophilic Attack at Pentaarsaferrocene [Cp*Fe(η5 -As5 )]-The Way to Larger Polyarsenide Ligands.

By the reaction of [Cp*Fe(η5 -As5 )] (I) (Cp*=C5 Me5 ) with main group nucleophiles, unique functionalized products with η4 -coordinated polyarsenide (Asn ) units (n=5, 6, 20) are obtained. With carbon-based nucleophiles such as MeLi or KBn (Bn=CH2 Ph), the anionic organo-substituted polyarsenide complexes, [Li(2.2.2-cryptand)][Cp*Fe(η4 -As5 Me)] (1 a) and [K(2.2.2-cryptand)][Cp*Fe{η4 -As5 (CH2 Ph)}] (1 b), are accessible. The use of KAsPh2 leads to a selective and controlled extension of the As5 unit and the formation of the monoanionic compound [K(2.2.2-cryptand][Cp*Fe(η4 -As6 Ph2 )] (2). When I is reacted with [M]As(SiMe3 )2 (M=Li ⋅ THF; K), the formation of the largest known anionic polyarsenide unit in [M'(2.2.2-cryptand)]2 [(Cp*Fe)4 {µ5 -η4 :η4 :η3 :η3 :η1 :η1 -As20 }] (3) occurred (M'=Li (3 a), K (3 b)).

Synthesis and Reactivity of Hetero-Pnictogen Diazonium Analogs Stabilized by Transition Metal Units.

The reactivity of the mixed dipnictogen complexes [{CpMo(CO)2 }2 (µ,η2 : 2 -PE)] (E=P, As, Sb) towards different group 14 electrophiles is reported. The resulting library of cationic compounds [{CpMo(CO)2 }2 (µ,η2 : 2 -EPR)]+ (R=Mes (2,4,6-C6 H2 Me3 ), CH3 , CPh3 , SnMe3 ) represents formally inorganic diazonium homologs which are stabilized by transition metal units. Modifying the steric and electronic properties of the electrophile drastically impacts the respective P-R bond lengths and is accompanied by increasing (SnMe3 >CPh3 >CH3 ) dynamic behavior in solution. In contrast to the well-studied organic analogs, the prepared compounds are stable at room temperature. The subsequent reaction of the model substrate [{CpMo(CO)2 }2 (µ,η2 : 2 -P2 Me)][OTf] ([OTf]- =[CF3 SO3 ]- ) with different N-heterocyclic carbenes (NHCs) leads to an addition at the unsubstituted P atom which is also predicted by computational methods. NMR spectroscopy confirms the formation of two isomers sync/gauche-[{CpMo(CO)2 }(µ,η2 : 1 -P(NHC)PMe){CpMo(CO)2 }][OTf]. X-ray crystallographic characterization and additional DFT calculations shed light on the spatial arrangement as well as on the possible formation pathways of the isomers.

Reactivity of Yellow Arsenic towards Cyclic (Alkyl)(Amino) Carbenes (CAACs).

Different cyclic (alkyl)(amino)carbenes (CAACs) were reacted with yellow arsenic. Several products [(CAAC-n)2 (µ,η1:1 -As2 )] (n=1 (1), 4 (2)), [(CAAC-2)3 (µ3 ,η1:1:1 -As4 )] (3) and [(CAAC-3)4 (µ4 ,η1:1:1:1 - As8 )] (6) were isolated due to the differing steric properties of CAAC-1-4. The products contain As2 , As4 or As8 units and represent the first examples of CAACs-substituted products of yellow arsenic. The reactivity of As4 was compared with the reactivities of P4 and the interpnictogen compound AsP3 , which led to a series of phosphorus-containing derivatives such as ([(CAAC-3)3 (µ3 ,η1:1:1 -P4 )] (4) and [(CAAC-3)4 (µ4 ,η1:1:1:1 -P8 )] (7)) and [(CAAC-3)3 (µ3 ,η1:1:1 -AsP3 )] (5). The products were characterized by spectroscopic and crystallographic methods and DFT computations were performed to clarify their formation pathway.

Teaching old tricks to new dogs - rational synthesis of multi-decker complexes featuring cyclo-P5 decks.

A rational synthetic pathway towards a series of cyclo-P5-bearing multi-decker complexes was developed. The combination of [Cp*Fe(µ,η5 : 5-P5)] (A) with low-valent transition metal cations yields a series of triple-decker complexes [Cp*Fe(µ,η5 : 5-P5)MCp'''][WCA] (M = Cr - Ni; WCA = weakly coordinating anion, Cp''' = 1,2,4-tBu3C5H2). Utilizing the arene complexes [M(C6R6)2][WCA]n (M = Fe, Co) instead allowed the synthesis of the unprecedented cyclo-P5-containing quadruple-decker complexes [{Cp*Fe(µ,η5 : 5-P5)}2M][pf]n (M = Fe, Co).

Synthesis and Reactivity of a Cyclooctatetraene-Like Polyphosphorus Ligand Complex [Cyclo-P8 ].

The thermolysis of Cp'''Ta(CO)4 with white phosphorus (P4 ) gives access to [{Cp'''Ta}2 (µ,η2 : 2 : 2 : 2 : 1 : 1 -P8 )] (A), representing the first complex containing a cyclooctatetraene-like (COT) cyclo-P8 ligand. While ring sizes of n >6 have remained elusive for cyclo-Pn structural motifs, the choice of the transition metal, co-ligand and reaction conditions allowed the isolation of A. Reactivity investigations reveal its versatile coordination behaviour as well as its redox properties. Oxidation leads to dimerization to afford [{Cp'''Ta}4 (µ4 ,η2 : 2 : 2 : 2 : 2 : 2 : 2 : 2 : 1 : 1 : 1 : 1 -P16 )][TEF]2 (4, TEF=[Al(OC{CF3 }3 )4 ]- ). Reduction, however, leads to the fission of one P-P bond in A followed by rapid dimerization to form [K@[2.2.2]cryptand]2 [{Cp'''Ta}4 (µ4 ,η2 : 2 : 2 : 2 : 2 : 2 : 2 : 2 : 1 : 1 : 1 : 1 -P16 )] (5), which features an unprecedented chain-type P16 ligand. Lastly, A serves as a P2 synthon, via ring contraction to the triple-decker complex [{Cp'''Ta}2 (µ,η6 : 6 -P6 )] (B).

Structural diversity of mixed polypnictogen complexes: dicationic E2E'2 (E ≠ E' = P, As, Sb, Bi) chains, cycles and cages stabilized by transition metals.

The reactivity of the tetrahedral dipnictogen complexes [{CpMo(CO)2}2(µ,η2:η2-EE')] (E, E' = P, As, Sb, Bi; "Mo2EE'") towards different one-electron oxidation agents is reported. Oxidation with [Thia][TEF] (Thia+ = C12H8S2 +; TEF- = Al{OC(CF3)3}4 -) leads to the selective formation of the radical monocations [Mo2EE']˙+, which immediately dimerize to the unprecedented dicationic E2E'2 ligand complexes [{CpMo(CO)2}4(µ4,η2:η2:η2:η2-E'EEE')]2+ via E-E bond formation. Single crystal X-ray diffraction revealed that, in the case of Mo2PAs and Mo2PSb, P-P bond formation occurs yielding zigzag E2P2 (E = As (1), Sb (2)) chains, whereas Mo2SbBi forms a Sb2Bi2 (5) cage, Mo2AsSb an unprecedented As2Sb2 unit representing an intermediate stage between a chain- and a cage-type structure, and Mo2AsBi a novel planar As2Bi2 (4a) cycle. Therefore, 1-5 bear the first substituent-free, dicationic hetero-E4 ligands, stabilized by transition metal fragments. Furthermore, in the case of Mo2AsSb, the exchange of the counterion causes changes in the molecular structure yielding an unusual, cyclic As2Sb2 ligand. The experimental results are corroborated by DFT calculations.

Substituted aromatic pentaphosphole ligands - a journey across the p-block.

The functionalization of pentaphosphaferrocene [Cp*Fe(η5-P5)] (1) with cationic group 13-17 electrophiles is shown to be a general synthetic strategy towards P-E bond formation of unprecedented diversity. The products of these reactions are dinuclear [{Cp*Fe}2{µ,η5:5-(P5)2EX2}][TEF] (EX2 = BBr2 (2), GaI2 (3), [TEF]- = [Al{OC(CF3)3}4]-) or mononuclear [Cp*Fe(η5-P5E)][X] (E = CH2Ph (4), CHPh2 (5), SiHPh2 (6), AsCy2 (7), SePh (9), TeMes (10), Cl (11), Br (12), I (13)) complexes of hetero-bis-pentaphosphole ((cyclo-P5)2R) or hetero-pentaphosphole ligands (cyclo-P5R), the aromatic all-phosphorus analogs of prototypical cyclopentadienes. Further, modifying the steric and electronic properties of the electrophile has a drastic impact on its reactivity and leads to the formation of [Cp*Fe(µ,η5:2-P5)SbICp'''][TEF] (8) which possesses a triple-decker-like structure. X-ray crystallographic characterization reveals the slightly twisted conformation of the cyclo-P5R ligands in these compounds and multinuclear NMR spectroscopy confirms their integrity in solution. DFT calculations shed light on the bonding situation of these compounds and confirm the aromatic character of the pentaphosphole ligands on a journey across the p-block.

Synthesis of Tetrahedranes Containing the Unique Bridging Hetero-Dipnictogen Ligand EE' (E ≠ E'=P, As, Sb, Bi).

In order to improve and extend the rare class of tetrahedral mixed main group transition metal compounds, a new synthetic route for the complexes [{CpMo(CO)2 }2 (µ,η2 :η2 -PE)] (E=As (1), Sb (2)) is described leading to higher yields and a decrease in reaction steps. Via this route, also the so far unknown heavier analogues containing AsSb (3 a), AsBi (4) and SbBi (5) ligands, respectively, are accessible. Single crystal X-ray diffraction experiments and DFT calculations reveal that they represent very rare examples of compounds comprising covalent bonds between two different heavy pnictogen atoms, which show multiple bond character and are stabilised without any organic substituents. A simple one-pot reaction of [CpMo(CO)2 ]2 with ME(SiMe3 )2 (M=Li, K; E=P, As, Sb, Bi) and the subsequent addition of PCl3 , AsCl3 , SbCl3 or BiCl3 , respectively, give the complexes 1-5. This synthesis is also transferable to the already known homo-dipnictogen complexes [{CpMo(CO)2 }2 (µ,η2 :η2 -E2 )] (E=P, As, Sb, Bi) resulting in higher yields comparable to those in the literature reported procedures and allows the introduction of the bulkier and better soluble Cp' (Cp'=tert butylcyclopentadienyl) ligand.

Redox Chemistry of Heterobimetallic Polypnictogen Triple-Decker Complexes - Rearrangement, Fragmentation and Transfer.

The redox chemistry of the heterobimetallic triple-decker complexes [(Cp*Fe)(Cp'''Co)(µ,η5 :η4 -E5 )] (E=P (1), As (2), Cp*=1,2,3,4,5-pentamethyl-cyclopentadienyl, Cp'''=1,2,4-tri-tertbutyl-cyclopentadienyl) and [(Cp'''Co)(Cp'''Ni)(µ,η3 :η3 -E3 )] (E=P (10), As (11)) was investigated. Compound 1 and 2 could be oxidized to the monocations 3 and 4 and further to the dications 5 and 6, while the initially folded cyclo-E5 ligand planarizes upon oxidation. The reduction leads to an opposite change in the geometry of the middle deck, which is now folded stronger into the direction of the other metal fragment (formation of monoanions 7 and 8). For the arsenic compound 8, a different behavior is found since a fragmentation into an As6 (9) and As3 ligand complex occurs. The Co and Ni triple-decker complexes 10 and 11 can be oxidized initially to the heterometallic monocations 12 and 13, which are not stable in solution and convert selectively into the homometallic nickel complexes 14 and 15 and the cobalt complexes 16 and 17. This behavior was further proven by the oxidation of [(Cp'''Co)(Cp''Ni)(µ,η3 :η2 -P3 )] (19, Cp''=1,3-di-tertbutyl-cyclopentadienyl) comprising two different Cp ligands. The transfer of {CpR M} fragments can be suppressed when a {W(CO)5 } unit is coordinated to the P3 ligand (20) prior to the oxidation and the mixed cobalt and nickel cation 21 can be isolated. The reduction of 10 and 11 yields the heterometallic monoanions 22 and 23, where no transfer of the {CpR M} fragments is observed.

Cationic Functionalisation by Phosphenium Ion Insertion.

The reaction of [Cp'''Ni(η3 -P3 )] (1) with in situ generated phosphenium ions [RR'P]+ yields the unprecedented polyphosphorus cations of the type [Cp'''Ni(η3 -P4 R2 )][X] (R=Ph (2 a), Mes (2 b), Cy (2 c), 2,2'-biphen (2 d), Me (2 e); [X]- =[OTf]- , [SbF6 ]- , [GaCl4 ]- , [BArF ]- , [TEF]- ) and [Cp'''Ni(η3 -P4 RCl)][TEF] (R=Ph (2 f), tBu (2 g)). In the reaction of 1 with [Br2 P]+ , an analogous compound is observed only as an intermediate and the final product is an unexpected dinuclear complex [{Cp'''Ni}2 (µ,η3 :η1 :η1 -P4 Br3 )][TEF] (3 a). A similar product [{Cp'''Ni}2 (µ,η3 :η1 :η1 -P4 (2,2'-biphen)Cl)][GaCl4 ] (3 b) is obtained, when 2 d[GaCl4 ] is kept in solution for prolonged times. Although the central structural motif of 2 a-g consists of a "butterfly-like" folded P4 ring attached to a {Cp'''Ni} fragment, the structures of 3 a and 3 b exhibit a unique asymmetrically substituted and distorted P4 chain stabilised by two {Cp'''Ni} fragments. Additional DFT calculations shed light on the reaction pathway for the formation of 2 a-2 g and the bonding situation in 3 a.

Stabilization of Pentaphospholes as η5 -Coordinating Ligands.

Electrophilic functionalisation of [Cp*Fe(η5 -P5 )] (1) yields the first transition-metal complexes of pentaphospholes (cyclo-P5 R). Silylation of 1 with [(Et3 Si)2 (µ-H)][B(C6 F5 )4 ] leads to the ionic species [Cp*Fe(η5 -P5 SiEt3 )][B(C6 F5 )4 ] (2), whose subsequent reaction with H2 O yields the parent compound [Cp*Fe(η5 -P5 H)][B(C6 F5 )4 ] (3). The synthesis of a carbon-substituted derivative [Cp*Fe(η5 -P5 Me)][X] ([X]- =[FB(C6 F5 )3 ]- (4 a), [B(C6 F5 )4 ]- (4 b)) is achieved by methylation of 1 employing [Me3 O][BF4 ] and B(C6 F5 )3 or a combination of MeOTf and [Li(OEt2 )2 ][B(C6 F5 )4 ]. The structural characterisation of these compounds reveals a slight envelope structure for the cyclo-P5 R ligand. Detailed NMR-spectroscopic studies suggest a highly dynamic behaviour and thus a distinct lability for 2 and 3 in solution. DFT calculations shed light on the electronic structure and bonding situation of this unprecedented class of compounds.