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.

Enabling and Probing Oxidative Addition and Reductive Elimination at a Group 14 Metal Center: Cleavage and Functionalization of E-H Bonds by a Bis(boryl)stannylene.

By employing strongly σ-donating boryl ancillary ligands, the oxidative addition of H2 to a single site Sn(II) system has been achieved for the first time, generating (boryl)2SnH2. Similar chemistry can also be achieved for protic and hydridic E-H bonds (N-H/O-H, Si-H/B-H, respectively). In the case of ammonia (and water, albeit more slowly), E-H oxidative addition can be shown to be followed by reductive elimination to give an N- (or O-)borylated product. Thus, in stoichiometric fashion, redox-based bond cleavage/formation is demonstrated for a single main group metal center at room temperature. From a mechanistic viewpoint, a two-step coordination/proton transfer process for N-H activation is shown to be viable through the isolation of species of the types Sn(boryl)2·NH3 and [Sn(boryl)2(NH2)](-) and their onward conversion to the formal oxidative addition product Sn(boryl)2(H)(NH2).

Expanded-ring N-heterocyclic carbenes for the stabilization of highly electrophilic gold(I) cations.

Strategies for the synthesis of highly electrophilic Au(I) complexes from either hydride- or chloride-containing precursors have been investigated by employing sterically encumbered Dipp-substituted expanded-ring NHCs (Dipp=2,6-iPr2 C6 H3 ). Thus, complexes of the type (NHC)AuH have been synthesised (for NHC=6-Dipp or 7-Dipp) and shown to feature significantly more electron-rich hydrides than those based on ancillary imidazolylidene donors. This finding is consistent with the stronger σ-donor character of these NHCs, and allows for protonation of the hydride ligand. Such chemistry leads to the loss of dihydrogen and to the trapping of the [(NHC)Au](+) fragment within a dinuclear gold cation containing a bridging hydride. Activation of the hydride ligand in (NHC)AuH by B(C6 F5 )3 , by contrast, generates a species (at low temperatures) featuring a [HB(C6 F5 )3 ](-) fragment with spectroscopic signatures similar to the "free" borate anion. Subsequent rearrangement involves BC bond cleavage and aryl transfer to the carbophilic metal centre. Under halide abstraction conditions utilizing Na[BAr(f) 4 ] (Ar(f) =C6 H3 (CF3 )2 -3,5), systems of the type [(NHC)AuCl] (NHC=6-Dipp or 7-Dipp) generate dinuclear complexes [{(NHC)Au}2 (µ-Cl)](+) that are still electrophilic enough at gold to induce aryl abstraction from the [BAr(f) 4 ](-) counterion.

Al-H σ-bond coordination: expanded ring carbene adducts of AlH3 as neutral bi- and tri-functional donor ligands.

Thermally robust expanded ring carbene adducts of AlH3 have been synthesized with a view to probing their ligating abilities via Al-H σ-bond coordination. While κ(2) binding to the 14-electron [Mo(CO)4] fragment is readily demonstrated, interaction with [Mo(CO)3] results in µ:κ(1),κ(1) and µ:κ(2),κ(2) bridging linkages rather than terminal κ(3) binding.

Coordinative trapping of the boron ß-diketiminato system [B(NMesCMe)2CH] via metal-templated synthesis.

A metal templated synthetic approach gives access to [Cp*Fe(CO)(2){B(NMesCMe)(2)CH}][BAr(f)(4)], and represents the first example of coordinative trapping of the elusive [B(NRCMe)(2)CH] fragment.

Salt metathesis for the synthesis of M-Al and M-H-Al bonds.

Salt metathesis has been exploited in the synthesis of M-Al bonds, stabilized by a variety of chelating N-donor substituents at aluminium and including the first examples of such systems featuring ancillary guanidinato frameworks. Importantly, this synthetic approach can be extended to the synthesis of σ-alane complexes through the use of hydride-containing transition metal nucleophiles. Cp'Mn(CO)(2)-[H(Cl)Al{(N(i)Pr)(2)CPh}] synthesized via this route features an alane ligand bound in a more 'side-on' fashion than other alane complexes, although DFT calculations imply that the potential energy surface associated with variation in the Mn-H-Al angle is a very soft one.

Dimethylamine borane dehydrogenation chemistry: syntheses, X-ray and neutron diffraction studies of 18-electron aminoborane and 14-electron aminoboryl complexes.

The reactions of Me(2)NH·BH(3) with cationic Rh(III) and Ir(III) complexes have been shown to generate the 18-electron aminoborane adduct [Ir(IMes)(2)(H)(2){κ(2)-H(2)BNMe(2))](+) and the remarkable 14-electron aminoboryl complex [Rh(IMes)(2)(H)-{B(H)NMe(2))](+). Neutron diffraction studies have been used for the first time to define H-atom locations in metal complexes of this type formed under catalytic conditions.

σ-Alane complexes of chromium, tungsten, and manganese.

Photolytic ligand displacement and salt metathesis routes have been exploited to give access to κ(1) σ-alane complexes featuring Al-H bonds bound to [W(CO)(5)] and [Cp'Mn(CO)(2)] fragments, together with a related κ(2) complex of [Cr(CO)(4)]. Spectroscopic, crystallographic, and quantum chemical studies are consistent with the alane ligands acting predominantly as σ-donors, with the resulting binding energies calculated to be marginally greater than those found for related dihydrogen complexes.

P=C bonds as building blocks for three- and four-membered heterocyclic cations: synthesis, structures and mechanistic studies.

The activation of the P=C bond of phosphaalkenes with electrophiles is investigated as a means to prepare and characterize unusual organophosphorus compounds. Treatment of RP=CHtBu (1a: R=tBu; 1b: R=1-adamantyl) with HOTf (0.5 equiv) affords diphosphiranium salts [RP-CHtBu-PR (CH(2)tBu)]OTf ([2a]OTf and [2b]OTf), each containing a three-membered P(2)C ring. In contrast, the addition of MeOTf (0.5 equiv) to either 1a or 1b affords diphosphetanium salts [RP-CHtBu-P(Me)R-CHtBu]OTf ([3a]OTf and [3b]OTf) containing four-membered P(2)C(2) heterocycles. The phosphenium triflate [tBuP(CH(2)tBu)]OTf ([5a]OTf) and methylenephosphonium triflate [tBu(Me)P=CHtBu]OTf ([7a]OTf) are identified spectroscopically as intermediates in the formation of [2a](+) and [3a](+), respectively. The phosphenium triflate intermediate can be trapped with 2-butyne to afford phosphirenium salt [MeC=CMe-tBuPCH(2)tBu]OTf ([6a]OTf). Treatment of diphosphetanium [3a]OTf with an excess MeOTf affords [Me(2)P-CHtBu-PMetBu-CHtBu](OTf)(2) ([4a](OTf)(2)), a compound containing a diphosphetanium dication. The molecular structures are reported for [2a]OTf, [2b][H(OTf)(2)], [3a]I, [3b]I, [4a](OTf)(2), and [6a]OTf.

Modelling fundamental arene-borane contacts: spontaneous formation of a dibromoborenium cation driven by interaction between a borane Lewis acid and an arene π system.

Reaction of 2,6-dimesityl pyridine (L(py)) with BBr(3) leads to the spontaneous formation of the trigonal dibromoborenium cation [L(py)·BBr(2)](+)via bromide ejection. Systematic structural and computational studies, and the reactivity displayed by a closely related N-heterocyclic carbene (NHC) donor, reveal the role played by arene-borane interactions in this chemistry. [L(py)·BBr(2)](+) features a structurally characterized (albeit weak) electrostatic interaction between the borane Lewis acid and flanking arene π systems.

Phospha-organic chemistry: from molecules to polymers.

Many parallels have been drawn between the chemistry of low-valent phosphorus and carbon. This Perspective is intended to highlight some emerging reactivity of phosphaalkenes, that is phospha-analogues of alkenes, in the areas of asymmetric catalysis and polymer science.

Chemical functionality of poly(methylenephosphine): phosphine-borane adducts and methylphosphonium ionomers.

The chemical functionality of poly(methylenephosphine) n-Bu[MesP-CPh2]nH (2) is examined in reactions with two isoelectronic species, namely BH3 and CH3+. The potential reactivity of polymer 2 is modelled by examining the reactivity of molecular phosphines bearing similar substituents as the polymer. In particular, the phosphine-borane adducts Mes(Me)P(BH3)-CPh2H (4a) and Mes(Me)P(BH3)-CPh2SiMe2H (4b) are prepared from the reaction of BH3.SMe2 with Mes(Me)P-CPh2H (3a) or Mes(Me)P-CPh2SiMe2H (3b), respectively. Treating 3a with MeOTf affords the methylated model compound, [Mes(Me)2P-CPh2H]OTf (5). X-Ray crystal structures are reported for each model compound. The reaction of n-Bu[MesP-CPh2]nH (Mn = 3.89 x 10(4), PDI = 1.34) with BH3.SMe2 affords the phosphine-borane polymer n-Bu[MesP(BH3)-CPh2]nH (6) (Mn = 4.13 x 10(4), PDI = 1.26). In contrast, methylation of phosphine polymer 2 gives n-Bu[MesP-CPh2]x-/-[MesP(Me)-CPh2]yH.(OTf)y (7) where approximately 50% of the phosphine moieties are methylated (from 31P NMR).

Diphosphiranium (P2C) or diphosphetanium (P2C2) cyclic cations: different fates for the electrophile-initiated cyclodimerization of a phosphaalkene.

The synthesis and crystallographic characterization of two new classes of cationic three- and four-membered organophosphorus heterocycles are reported. Treating phosphaalkene tBuP=CHtBu (1) with triflic acid (2:1) afforded an unprecedented asymmetric diphosphiranium triflate [tBuP-CH(tBu)-P(tBu)(CH2tBu)]OTf (2). Surprisingly, the analogous reaction of tBuP=CHtBu with methyl triflate (2:1) afforded a 1,3-diphosphetanium salt [tBu(Me)P-CH(tBu)-P(tBu)-CH(tBu)]OTf (3a). Structural characterization of these novel P2C and P2C2 rings confirmed their identity, and the metrical parameters were consistent with the strain expected for these ring systems. In preliminary mechanistic investigations by NMR spectroscopy, phosphenium species [tBuPCH2tBu]OTf (4) was detected in the stoichiometric reaction of 1 with HOTf while methylenephosphonium [Me(tBu)P=CHtBu]OTf (5) was observed when 1 was treated with MeOTf. The opposite reactivity observed when the P=C bond is treated with MeOTf compared that with HOTf is surprising and, to our knowledge, has not been observed previously in phosphaalkene chemistry. In addition to their fundamental interest, we are interested in these species as ring-closed forms of the propagating species in the cationic polymerization of P=C bonds.