Probing Radical Addition to 1-Phosphabutadienes by Employing Muonium as a "Light Isotope" of Hydrogen.

Understanding free radical addition to multiple bonds is important to elucidating the mechanistic details of addition polymerization reactions, albeit the fleeting radical intermediates are very difficult to detect by conventional methodologies. Muon spin spectroscopy (µSR) is a highly sensitive method that can detect radical species at 106 spins (cf. EPR: 1012 spins, NMR: 1018 spins). Herein, we employ µSR to detect the radical-addition products from three 1-phosphabutadiene monomers, P-analogues of isoprene. We show that muonium (Mu), a "light" H-atom surrogate, adds predominantly at the C4 position of the P1 =C2 -C3 =C4 moiety to give unprecedented 1-phosphaallyl radicals as the major products. Our structural assignments are supported by assignment of muon, phosphorus and proton hyperfine coupling constants using DFT-calculations. A minor radical product is also detected that is tentatively assigned to an PC3 -heterocyclic free radical. On the basis of DFT-predictions, we speculate that its formation may involve initial addition of Mu+ at the C3 position followed by electron capture. These studies provide rare insights into the prospective radical (or cationic) polymerization of 1-phosphabutadienes, which have previously been polymerized using anionic initiation.

Cationic Phosphinidene as a Versatile P1 Building Block: [LC-P]+ Transfer from Phosphonio-Phosphanides [LC-P-PR3]+ and Subsequent LC Replacement Reactions (LC = N-Heterocyclic Carbene).

Cationic imidazoliumyl(phosphonio)-phosphanides [LC-P-PR3]+ (1a-e+, LC = 4,5-dimethyl-1,3-diisopropylimidazolium-2-yl; R = alkyl, aryl) are obtained via the nucleophilic fragmentation of tetracationic tetraphosphetane [(LC-P)4][OTf]4 (2[OTf]4) with tertiary phosphanes. They act as [LC-P]+ transfer reagents in phospha-Wittig-type reactions, when converted with various thiocarbonyls, giving unprecedented cationic phosphaalkenes [LC-P═CR2]+ (5a-f[OTf]) or phosphanides [LC-P-CR(NR2')]+ (6a-d[OTf]). Theoretical calculations suggest that three-membered cyclic thiophosphiranes are crucial intermediates of this reaction. To test this hypothesis, treatment of [LC-P-PPh3]+ with phosphaalkenes, that are isolobal to thioketones, permits the isolation of diphosphirane salts 11a,b[OTf]. Furthermore, preliminary studies suggest that the cationic phosphaalkene [LC-P═CPh2]+ may be employed to access rare examples of η2-P═C π-complexes with Pd0 and Pt0 when treated with [Pd(PPh3)4] and [Pt(PPh3)3] for which analogous complexes of neutral phosphaalkenes are scarce. The versatility of [LC-P]+ as a valuable P1 building block was showcased in substitution reactions of the transferred LC-substituent using nucleophiles. This is demonstrated through the reactions of 5a[OTf] and 6c[OTf] with Grignard reagents and KNPh2, providing a convenient, high-yielding access to MesP═CPh2 (16) and otherwise difficult-to-synthesize 1,3-diphosphetane 17 and P-aminophosphaalkenes.

Ambient Temperature Carbene-Mediated Depolymerization: Stoichiometric and Catalytic Reactions of N-Heterocyclic- and Cyclic(Alkyl)Amino Carbenes with Poly(N-Methylaminoborane) [MeNH-BH2]n.

The reactions of the N-heterocyclic carbenes (NHCs) IDipp and ItBu and the cyclic(alkyl)amino carbene (CAAC) CAACMe with polyaminoborane [MeNH-BH2]n were investigated. Stoichiometric quantities of each carbene were found to cause rapid and complete depolymerization, with the major B-N-containing product identified as the NHC-aminoborane adduct, IDipp-BH2NMeH (1), cyclic borazane [MeNH-BH2]3, or borazine [MeNBH]3 with IDipp, ItBu, and CAACMe, respectively. With substoichiometric quantities of IDipp and ItBu (down to 10 and 2.5 mol %, respectively), complete loss of high molar mass material was also detected, indicating that the depolymerization is catalytic. The main products of the reaction with substoichiometric IDipp were IDipp-BH2NMeH (1) and [MeNH-BH2]3 and with substoichiometric ItBu, [MeNH-BH2]3, and [MeNBH]3 with product ratios dependent on the quantity of NHC used. Under analogous conditions with CAACMe, high molar mass material persisted alongside the formation of [MeNBH]3. Further reactivity studies with cyclic borazane [MeNH-BH2]3 and MeNH2·BH3 provided insights into depolymerization pathways. IDipp showed no reactivity toward [MeNH-BH2]3, whereas with 3 equiv of ItBu and CAACMe, the dehydrogenation product [MeNBH]3, was formed. With MeNH2·BH3, 2 equiv of carbene were used as the first acts to accept dihydrogen; the major products with IDipp, ItBu, and CAACMe were IDipp-BH2NMeH (1), [MeNBH]3, and (CAACMeH)HB═NMeH (2), respectively. The double E-H (E = B, N) bond activation product (CAACMeH)HB═NMe(HCAACMe) (3) was isolated from the reaction between 3 equiv of CAACMe and MeNH2·BH3. A unified mechanism for donor-mediated depolymerization of [MeHN-BH2]n is proposed.

Metathesis of P=C Bonds Catalyzed by N-Heterocyclic Carbenes.

The catalytic metathesis of C=C bonds is a textbook reaction that has no parallel in the widely studied area of multiple bonds involving heavier p-block elements. A high-yielding P=C bond metathesis of phosphaalkenes (ArP=CPh2 , Ar=Mes, o-Tol, Ph) has been discovered that is catalyzed by N-heterocyclic carbenes (NHC=Me2 IMe, Me2 Ii Pr). The products are cyclic oligomers formally derived from ArP=PAr [i. e. cyclo-(ArP)n ; n=3, 4, 5, 6] and Ph2 C=CPh2 . Preliminary mechanistic studies of this remarkable transformation have established NHC=PAr (Ar=Mes, o-Tol, Ph) as key phosphinidene transfer agents. In addition, novel cyclic intermediates, such as, cyclo-(ArP)2 CPh2 and cyclo-(ArP)4 CPh2 have also been observed. This work represents a rare application of non-metal-based catalysts for transformations involving main-group elements.

A thermolytic route to a polysilyne.

We report a safe and convenient method to prepare a new class of network polysilane, or polysilyne ([RSi]n). Simple thermolysis of a readily accessible linear poly(phenylsilane), [PhSiH]n, affords polysilyne [PhSi]n with concomitant evolution of monosilanes. This new polymer shows a hyperbranched structure with unique features not observed in known polysilynes prepared via hazardous Wurtz coupling routes. Despite these differences, our soluble, yellow polysilyne exhibits some important properties associated with the traditional random network structure: it absorbs up to 400 nm in the UV spectrum, yet is stable to photolysis under inert atmosphere. This efficient new synthetic route opens the door to exciting applications for these hyperbranched polymers in materials and device technologies.

Isolable Phosphaalkenes Bearing 2,4,6-Trimethoxyphenyl and 2,6-Bis(trifluoromethyl)phenyl as P-Substituents.

The multistep synthesis and characterization of new P═C analogues of olefins from readily available starting materials is reported. Specifically, the phosphaalkenes TMOP-P═CPh2 (1a: TMOP = 2,4,6-trimethoxyphenyl) and ArF-P═CPh2 [1b: ArF = 2,6-bis(trifluoromethyl)phenyl] have been prepared, isolated, and characterized. In addition, synthetically challenging intermediates, such as the corresponding pyrophoric primary phosphines and bis(trimethylsilyl)phosphines, have been isolated and characterized. The title compounds, TMOP-P═CPh2 (1a) and ArF-P═CPh2 (1b), along with TMOP-PH2 (3a) have been characterized by X-ray crystallography. Importantly, the successful synthesis and isolation of phosphaalkenes 1a and 1b provides a foundation for future investigations of their polymerization, by analogy to the known polymerization of Mes-P═CPh2.

A Smart Phosphine-Diyne Polymer Displays "Turn-On" Emission with a High Selectivity for Gold(I/III) Ions.

A polymeric phosphine sensor is reported that exhibits bright blue fluorescence in the presence of gold(I/III) ions but is nonemissive with other metal ions. Specifically, solutions of a poly(p-arylenediethynylene phosphine) copolymer are 35 or 94 times more emissive when treated with solutions of (tht)AuCl or HAuCl4·3H2O, respectively. Model compound studies confirm phosphine coordination to metals, including gold(I/III) and rhodium(I), and the selective "turn-on" fluorescence was investigated using time-dependent density functional theory calculations.

Diels-Alder reactions of 1-phosphabutadienes: a highly selective route to P[double bond, length as m-dash]C-substituted phosphacyclohexenes.

Kinetically stabilized 1-phosphahaloprenes (2-halo-1-phosphabutadienes) as well as 1-phosphaisoprene undergo a hitherto unknown phospha-Diels-Alder dimerization of the P[double bond, length as m-dash]C-C[double bond, length as m-dash]C units upon heating. The [4+2] cyclodimerization is highly stereo- and regio-selective. The phosphaalkene-substituted phosphacyclohexene product is an unprecedented P(sp2),P(sp3) ligand that is of interest in polymer/materials science and catalysis.

Free Radical Reactivity of a Phosphaalkene Explored Through Studies of Radical Isotopologues.

Muonium (Mu), an H atom analogue, is employed to probe the addition of free radicals to the P=C bond of a phosphaalkene. Specifically, two unprecedented muoniated free radicals, MesP. -CMu(Me)2 (1 a, minor product) and MesPMu-C. Me2 (1 b, major product), were detected by muon spin spectroscopy (µSR) when a solution of MesP=CMe2 (1: Mes=2,4,6-trimethylphenyl) was exposed to a beam of positive muons (µ+ ). The µ+ serves as a source of Mu (that is, Mu=µ+ +e- ). To confirm the identity of the major product 1 b, its spectral features were compared to its isotopologue, MesPH-C. (Me)CH2 Mu (2 a). Conveniently, 2 a is the sole product of the reaction of MesPH(CMe=CH2 ) (2) with Mu. For all observed radicals, muon, proton, and phosphorus hyperfine coupling constants were determined by µSR and compared to DFT-calculated values.

Ammonium and Potassium Salts of a Hexacoordinate Phosphorus(V) Anion Featuring P-O and P-C Bonds.

Ammonium and potassium salts featuring the chiral mer-[P(C6H4CO2)3]-, mer-[1]-, have been isolated. Specifically, treating phosphorane P(C6H4CO2)2(C6H4CO2H), 2, with N-containing bases [Nbase = PhNMe2, PhNH2, pyridine (py), isoquinoline, (-)-brucine, N( n-C8H17)3] afforded ammonium salts [NbaseH]+- mer-[1]-. Each compound was fully characterized spectroscopically, and four were subjected to X-ray crystallographic analysis. Salts were isolated with the racemic mixture of the mer-[1]- anion except in the case of [(-)-brucineH]-Λ- mer-[1], for which the crystal analyzed was enantiomerically pure. The potassium salt, K- mer-[1], was synthesized by treating 2 with KH. The solid-state structure of K- mer-[1] is a coordination polymer consisting of seven- and eight-coordinate K+ ions that are weakly bound by oxygen of either the racemic anion or the methanol solvent. Preliminary NMR and MS data for the formation of the Brønsted acid, H(DMF) n[1], has also been obtained. An estimate of the basicity of anion mer-[1]- was obtained from IR measurements of the N-H stretching frequency for [( n-C8H17)3NH]- mer-[1]. On the basis of these measurements, anion mer-[1]- was ranked similar to the classical weakly coordinating anion, [ClO4]-, in terms of its coordinating ability.

Unconventional ionic ring-deconstruction pathways of a three-membered heterocycle.

Two different ionic deconstructions of a three-membered COP ring have been discovered. The first involves displacement of the C[double bond, length as m-dash]O unit (aldehyde elimination) induced by base whilst the second involves acid-initiated extrusion of the ring-carbon (2-methyl-2-butene). Insight into the latter deconstruction was obtained by quantum chemical calculations.

Phosphorus-Containing Block Copolymers from the Sequential Living Anionic Copolymerization of a Phosphaalkene with Methyl Methacrylate.

Although living polymerization methods are widely applicable to organic monomers, their application to inorganic monomers is rare. For the first time, we show that the living poly(methylenephosphine) (PMPn - ) anion can function as a macroinitiator for olefins. Specifically, the phosphaalkene, MesP=CPh2 (PA), and methyl methacrylate (MMA) can be sequentially copolymerized using the BnLi-TMEDA initiator system in toluene. A series of PMPn -b-PMMAm copolymers with narrow dispersities are accessible (D=1.05-1.10). Analysis of the block copolymers provided evidence for -P-CPh2 -CH2 -CMe(CO2 Me)- switching groups. Importantly, this indicates that the -P-CPh2 - anion directly initiates the anionic polymerization of MMA and stands in stark contrast to the isomerization mechanism followed for the homopolymerization of PA. For the first time, the glass transition of a PMPn homopolymer has been measured (Tg =45.1 °C, n=20). The PMPn -b-PMMAm copolymers do not phase separate and show a single Tg which increases with higher PMMA content.

2-Aminophenolate ligands for phosphorus(v): a lithium salt featuring the chiral [P(OC6H4NR)3]- anion.

Phosphoranes P(OC6H4NR)2(OC6H4NHR) [R = Me (2a), Ph (2b), C6F5 (2c)] were synthesized by treating PCl5 with the respective 2-aminophenol derivative (1a-c, 3.1 equiv.). In one instance, an intermediate species, P(OC6H4NR)2Cl [R = Me (3a)], was isolated and structurally characterized. Deprotonation of the amine moieties (-NH[combining low line]R) in phosphoranes 2a and 2b with a strong alkali-metal base (e.g. n-BuLi) in the presence of a strong-donor solvent (e.g. THF) afforded salts composed of the hexacoordinate P(v)-anions [P(OC6H4NR)3]- (R = Me, [4a]-; Ph, [4b]-). Employing precursor 2a, the salt Li(THF)3fac-[4a]- was isolated. The X-ray crystal of each enantiomer of [4a]- was determined and, to our knowledge, represents the first structurally characterized example of a salt containing a hexacoordinate P(v)N3O3 anion featuring P(v)-N bonds.

Exploring the chemistry of backbone amino(chloro)phosphanyl-substituted imidazole-2-thiones.

Backbone (R'2N)2P-substituted imidazole-2-thiones 2a-c [(R'2N)2P-IMSR,R; IMSR,R = 1,3-dialkylimidazole-2-thione-4-yl, a: R = iPr, R' = Et; b: R = Mes, R' = Et, c: R = Me, R' = iPr] were treated with PCl3 to synthesize R'2N(Cl)P-substituted imidazole-2-thiones 3a-c [R'2N(Cl)P-IMSR,R]. The P-chloro compounds 3a,b were used to explore the chemistry further, i.e. nucleophilic substitution at phosphorus using Ph2CHLi or fluorenyl lithium afforded compounds 4a-c [R'2N(R'')P-IMSR,R; a: R = iPr, R' = Et, R'' = PH2CH; b: R = Mes, R' = Et, R'' = Flu c: R = iPr, R' = Et, R'' = Flu]. Compound 4c was used to access the P-borane adduct 5 [R'2N(R'')(BH3)P-IMSR,R; R = iPr, R' = Et, R'' = Flu] and phosphinoyl-substituted imidazolium hydrogensulfate 6 [[R'2N(R'')(O)P-IMHR,R]HSO4; IMHR,R = 1,3-dialkylimidazolium-4-yl, R = iPr, R' = Et, R'' = Flu]. Compounds 4a,b were transformed into the P-Cl compounds 7a,b [R''(Cl)P-IMSR,R; a: R = iPr, R'' = Ph2CH; b: R = Mes, R'' = Flu] which represent potential starting materials for backbone phosphaalkenyl substituted imidazole-2-thiones. Compound 7a yielded the NHC-stabilized phosphenium salt 8 [[R''(IMe4)P-IMSR,R]Cl; R = iPr, R' = Et, R'' = Ph2CH, IMe4 = 1,3,4,5-tetramethylimidazole-2-ylidene] in reaction with 1,3,4,5-tetramethylimidazole-2-ylidene (IMe4). Attempts to deprotonate 7a,b using nBuLi revealed that nucleophilic substitution of P-Cl to give the P-nBu product occurred preferentially, and dehydrochlorination to the phosphaalkene was not observed.

Polymerization of 1-Phosphaisoprene: Synthesis and Characterization of a Chemically Functional Phosphorus Version of Natural Rubber.

Macromolecules derived from 1,3-dienes, such as polyisoprene (or natural rubber), are of considerable importance in polymer science. Given the parallels between P=C and C=C bonds, the prospect of polymerizing P-containing 1,3-dienes, such as 1-phosphaisoprene, is intriguing due to the unique chemical functionality imparted by the heavier element combined with their structural relationship to natural rubber. Herein, we report the synthesis, characterization and coordination chemistry of the first polymers derived from Mes*P=CR-CH=CH2 (Mes*=2,4,6-t-Bu3 C6 H2 ; R=H, Me). In the case of 1-phosphaisoprene (R=Me), the monomer is isolable and its anionic polymerization affords a polymer that retains P=C bonds in its microstructure. The chemical functionality of these novel materials is demonstrated by forming the macromolecular gold(I) complex where the P=C bond is retained for further chemical elaboration.

[HL2][P(1,2-O2C6Cl4)3] (L = THF, DMF): Brønsted acid initiators for the polymerization of n-butyl vinyl ether and p-methoxystyrene.

The development of solid, weighable Brønsted acids featuring the hexacoordinated phosphorous(v) anion [TRISPHAT]- are reported. H(DMF)2[1] and H(THF)2[1] {[1]- = [P(1,2-O2C6Cl4)3]-} were synthesized and fully characterized by 1H, 31P, 13C and 2D-NOESY NMR spectroscopy, X-ray crystallography, mass spectrometry and elemental analysis. Both, H(DMF)2[1] and H(THF)2[1] are found to be suitable initiators for the polymerization of n-butyl vinyl ether and p-methoxystyrene. Optimal polymerization results for these single-component initiators were generally obtained at -50 °C where high molecular weight polymers were isolated in high yield. 1H NMR spectroscopic analysis and dynamic light scattering of solutions of the isolated polymers provide evidence for a branched structure which presumably arises from chain transfer.

Strong Evidence of a Phosphanoxyl Complex: Formation, Bonding, and Reactivity of Ligated Phosphorus Analogues of Nitroxides.

Facile access to [W(CO)5 (Ph2 P-OTEMP)] is used to initiate a study on the generation, properties, and reactions of transient phosphanoxyl complexes [MLn (R2 PO)], the first example of which could be trapped via heterocoupling with the trityl radical. It is also demonstrated that the phosphorus nitroxyl complex acts as radical initiator in the polymerization of styrene. The quest for P-O versus O-N bond homolysis, as well as the initial steps of the polymerization were studied by DFT methods.

Copper(I) Complexes of Pyridine-Bridged Phosphaalkene-Oxazoline Pincer Ligands.

The synthesis of enantiomerically pure pyridine-bridged phosphaalkene-oxazolines ArP═C(Ph)(2,6-C5H3NOx) (1, Ar = Mes/Mes*, Ox = CNOCH(i-Pr)CH2/CNOCH(CH2Ph)CH2) is reported. This new ligand forms a κ(P), κ(2)(NN) dimeric complex with copper(I) (7) that dissociates into a cationic κ(3)(PNN) monomeric complex upon addition of a neutral ligand {[1a·CuL]OTf (8a-e): L = PPh3 (a), P(OPh)3 (b), 2,6-lutidine (c), 4-DMAP (d), 1-methylimidazole (e)}. The P-Cu bond lengths in 8 are influenced by the π-accepting/σ-donating properties of L, and this can be observed by changes in the δ(31)PP═C NMR shift. The donor-acceptor properties in complexes of type 8 have also been investigated by UV/vis spectroscopy and density functional theory calculations.

Homo- and Heteropolynuclear Complexes Containing Bidentate Bridging 4-Phosphino-N-Heterocyclic Carbene Ligands.

The abnormal reaction of phosphaalkenes with N-heterocyclic carbenes (NHC) offers a convenient method to introduce new functionality at the backbone of an NHC. The 4-phosphino-substituted NHC (1a) derived from 1,3-dimesitylimidazol-2-ylidene (IMes) and MesP═CPh2 is shown to be an effective bifunctional ligand for Au(I) and Pd(II). Several new complexes are reported: 2a: 1a·AuCCl, 3a: 1a·(AuCl)2, 4a: [(1a)2AuC]Cl, 5a: [(1a·AuPCl)2AuC]Cl], and 6a: 1a·(PdC) (AuPCl). The reaction of 4-phosphino-NHC 1b, derived from 1,3-di(cyclohexyl)imidazol-2-ylidene (ICy) and MesP═C(4-C6H4F)2, with (tht)AuCl (2 equiv, tht = tetrahydrothiophene) affords the fascinating tetranuclear 5b [(1b·AuPCl)2AuC][AuCl2]. The molecular structure of 5b features a close Au···Au contact (3.0988(4) Å) between the bis(carbene)gold(I) cation and the dichloroaurate(I) anion. The buried volumes (%Vbur) and Tolman cone angles for representative 4-phosphino-NHCs calculated from structural data are compared to related carbenes and phosphines. The molecular structures are reported for complexes 3a, 4a, 5b, and 6a.

Phosphaalkene-oxazoline copolymers with styrene as chiral ligands for rhodium(I).

The radical-initiated copolymerization of phosphaalkene-oxazoline, MesP[double bond, length as m-dash]C(Ph)CMe2Ox [1, Ox = CNOCH(iPr)CH2] with different loadings of styrene affords poly(methylenephosphine-co-styrene)s [2a (1 : S = 1 : 2): Mw = 7400 g mol(-1), PDI = 1.1; 2b (1 : S = 1 : 5): Mw = 18 000 g mol(-1), PDI = 1.2; 2c (1 : S = 1 : 10): Mw = 16 000 g mol(-1), PDI = 1.3]. Copolymers 2a-2c are demonstrated as viable macromolecular ligands for rhodium(i). By comparison with the crystallographically characterized model P,N-bidentate complex, [Mes(Me)P-CH(Ph)CMe2Ox·Rh(cod)]BF4, the polymer complexes [2·Rh(cod)]BF4 were prepared. The macromolecular metal complexes were characterized by GPC {for [2a·Rh(cod)]BF4: Mw = 14 000 g mol(-1), PDI = 1.2}, UV/Vis spectroscopy, (1)H, (13)C and (31)P NMR spectroscopy. Integration of the (31)P NMR spectra of mixtures of 2 and [Rh(cod)2]BF4 permitted the determination of the mol% of incorporation of monomer 1 in copolymer 2 (2a: 17%; 2b: 5%; 2c: 4%). These results compared favorably with those determined by elemental analysis (2a: 17%; 2b: 6%).