Catalyst Design for Rh-Catalyzed Arene and Alkane C-H Borylation: The NHC Affects the Induction Period, and Indenyl is Superior to Cp.

In order to establish design criteria for Rh C-H borylation catalysts, analogues of the successful catalyst [Rh(Ind)(SIDipp)(COE)] (Ind = η5-indenyl, SIDipp = 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene, and COE = cis-cyclooctene) were synthesized by changing the indenyl and carbene ligands. [RhCp(SIDipp)(COE)] (1) formed alongside the C-C activated, cyclometalated byproduct [RhCp(κ2CAr,Ccarbene-SIDipp')(iPr)] (rac-2; SIDipp' = 1-(6-isopropylphenyl)-3-(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene). Computational modeling of COE dissociation showed that both C-C and C-H activation of the SIDipp aryl group is thermally attainable and reversible under experimental conditions, with the C-C activation products being the more thermodynamically stable species. Oxidative addition of 1 with SiH(OEt)3 gave the Rh silyl hydride [RhCp(H){Si(OEt)3}(SIDipp)] (rac-3). [Rh(Ind)(IDipp)(COE)] (4; IDipp = 1,3-bis(2,6-diisopropylphenyl)-imidazole-2-ylidene), the carbonyl analogue [Rh(Ind)(IDipp)(CO)] (5; νCO = 1940 cm-1, cf. 1944 cm-1 for [Rh(Ind)(SIDipp)(COE)]), and [Rh(Ind)(IMe4)(COE)] (6; IMe4 = 1,3,4,5-tetramethylimidazol-2-ylidene) were also characterized, but attempts to synthesize Rh carbene complexes with fluorenyl or 1,2,3,4-tetrahydrofluorenyl ligands were not successful. For the catalytic C-H borylation of benzene using B2pin2, 1 was inactive at 80 °C, and [Rh(Ind)(SIDipp)(COE)] was superior to all other complexes tested due to the shortest induction period. However, the addition of HBpin to precatalyst 4 eliminated the induction period. Catalytic n-alkane C-H borylation using [Rh(Ind)(NHC)(COE)] gave yields of up to 21% alkylBpin, but [RhCp*(C2H4)2] was the better catalyst.

Rhodium Indenyl NHC and Fluorenyl-Tethered NHC Half-Sandwich Complexes: Synthesis, Structures and Applications in the Catalytic C-H Borylation of Arenes and Alkanes.

Indenyl (Ind) rhodium N-heterocyclic carbene (NHC) complexes [Rh(η5 -Ind)(NHC)(L)] were synthesised for 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene (SIPr) with L=C2 H4 (1), CO (2 a) and cyclooctene (COE; 3), for 1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene (SIMes) with L=CO (2 b) and COE (4), and 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) with L=CO (2 c) and COE (5). Reaction of SIPr with [Rh(Cp*)(C2 H4 )2 ] did not give the desired SIPr complex, thus demonstrating the "indenyl effect" in the synthesis of 1. Oxidative addition of HSi(OEt)3 to 3 proceeded under mild conditions to give the Rh silyl hydride complex [Rh(Ind){Si(OEt)3 }(H)(SIPr)] (6) with loss of COE. Tethered-fluorenyl NHC rhodium complexes [Rh{(η5 -C13 H8 )C2 H4 N(C)C2 Hx NR}(L)] (x=4, R=Dipp, L=C2 H4 : 11; L=COE: 12; L=CO: 13; R=Mes, L=COE: 14; L=CO: 15; x=2, R=Me, L=COE: 16; L=CO: 17) were synthesised in low yields (5-31 %) in comparison to good yields for the monodentate complexes (49-79 %). Compounds 3 and 1, which contain labile alkene ligands, were successful catalysts for the catalytic borylation of benzene with B2 pin2 (Bpin=pinacolboronate, 97 and 93 % PhBpin respectively with 5 mol % catalyst, 24 h, 80 °C), with SIPr giving a more active catalyst than SIMes or IMes. Fluorenyl-tethered NHC complexes were much less active as borylation catalysts, and the carbonyl complexes were inactive. The borylation of toluene, biphenyl, anisole and diphenyl ether proceeded to give meta substitutions as the major product, with smaller amounts of para substitution and almost no ortho product. The borylation of octane and decane with B2 pin2 at 120 and 140 °C, respectively, was monitored by 11 B NMR spectroscopy, which showed high conversions into octyl and decylBpin over 4-7 days, thus demonstrating catalysed sp3 C-H borylation with new piano stool rhodium indenyl complexes. Irradiation of the monodentate complexes with 400 or 420 nm light confirmed the ready dissociation of C2 H4 and COE ligands, whereas CO complexes were inert. Evidence for C-H bond activation in the alkyl groups of the NHC ligands was obtained.

A ruthenium cis-dihydride with 2-phosphinophosphinine ligands catalyses the acceptorless dehydrogenation of benzyl alcohol.

The first ruthenium dihydride complex featuring a phosphinine ligand cis-[Ru(H)2(2-PPh2-3-Me-6-SiMe3-PC5H2)2] was synthesised exclusively as the cis-isomer. When formed in situ from the reaction of cis-[Ru(Cl)2(2-PPh2-3-Me-6-SiMe3-PC5H2)2] with two equivalents of Na[BHEt3], as demonstrated by 31P and 1H NMR spectroscopy, the catalysed acceptorless dehydrogenation of benzyl alcohol was observed leading to benzyl benzoate in up to 70% yield.

An indenide-tethered N-heterocyclic stannylene.

The structure of (µ-1κN:2(η2),κ2 N,N'-(2-{[2,6-bis(propan-2-yl)phen-yl]aza-nid-yl}eth-yl)[2-(1H-inden-1-yl)eth-yl]aza-nido)(1,4,7,10,13,16-hexa-oxa-cyclo-octa-dec-ane-1κ6 O)lithiumtin, [LiSn(C8H16O4)(C25H31N2)], at 100 K has monoclinic (P21/n) symmetry. Analysis of the coordination of the Sn to the indenyl ring shows that the Sn inter-acts in an η2 fashion. A database survey showed that whilst this coordination mode is unusual for Ge and Pb compounds, Sn displays a wider range of coordination modes to cyclo-penta-dienyl ligands and their derivatives.

Functionalised N-Heterocyclic Carbene Ligands in Bimetallic Architectures.

N-Heterocyclic carbenes (NHCs) have become immensely successful ligands in coordination chemistry and homogeneous catalysis due to their strong terminal σ-donor properties. However, by targeting NHC ligands with additional functionalisation, a new area of NHC coordination chemistry has developed that has enabled NHCs to be used to build up bimetallic and multimetallic architectures. This minireview covers the development of functionalised NHC ligands that incorporate additional donor sites in order to coordinate two or more metal atoms. This can be through the N-atom of the NHC ring, through a donor group attached to the N-atom or the carbon backbone, coordination of the π-bond or an annulated π-donor on the backbone, or through direct metalation of the backbone.

On the Basicity of Carboranylphosphines.

Three new carboranylphosphines, [1-(1'-closo-1',7'-C2B10H11)-7-PPh2-closo-1,7-C2B10H10], [1-(1'-7'-PPh2-closo-1',7'-C2B10H10)-7-PPh2-closo-1,7-C2B10H10], and [1-{PPh-(1'-closo-1',2'-C2B10H11)}-closo-1,2-C2B10H11], have been prepared, and from a combination of these and literature compounds, eight new carboranylphosphine selenides were subsequently synthesized. The relative basicities of the carboranylphosphines were established by (i) measurement of the 1JPSe NMR coupling constant of the selenide and (ii) calculation of the proton affinity of the phosphine, in an attempt to establish which of several factors are the most important in controlling the basicity. It is found that the basicity of the carboranylphosphines is significantly influenced by the nature of other substituents on the P atom, the nature of the carborane cage vertex (C or B) to which the P atom is attached, and the charge on the carboranylphosphine. In contrast, the basicity of the carboranylphosphines appears to be relatively insensitive to the nature of other substituents on the carborane cage, the isomeric form of the carborane, and whether the cage is closo or nido (insofar as that does not alter the charge on the cluster). Such information is likely to be of significant importance in optimizing future applications of carboranylphosphines, e.g., as components of frustrated Lewis pairs.

Synergic Deprotonation Generates Alkali-Metal Salts of Tethered Fluorenide-NHC Ligands Co-Complexed to Alkali-Metal Amides.

Synergic combinations of alkali-metal hydrocarbyl/amide reagents were used to synthesise saturated N-heterocyclic carbene (NHC) ligands tethered to a fluorenide anion through deprotonation of a spirocyclic precursor, whereas conventional bases were not successful. The Li2 derivatives displayed a bridging amide between two Li atoms within the fluorenide-NHC pocket, whereas the Na2 and K2 analogues displayed extended solid-state structures with the fluorenide-NHC ligand chelating one alkali metal centre.

Exploiting the Electronic Tuneability of Carboranes as Supports for Frustrated Lewis Pairs.

The first example of a carborane with a catecholborolyl substituent, [1-Bcat-2-Ph-closo-1,2-C2B10H10] (1), has been prepared and characterized and shown to act as the Lewis acid component of an intermolecular frustrated Lewis pair in catalyzing a Michael addition. In combination with B(C6F5)3 the C-carboranylphosphine [1-PPh2-closo-1,2-C2B10H11] (IVa) is found to be comparable with PPh2(C6F5) in its ability to catalyze hydrosilylation, whilst the more strongly basic B-carboranylphosphine [9-PPh2-closo-1,7-C2B10H11] (V) is less effective and the very weakly basic species [µ-2,2'-PPh-{1-(1'-1',2'-closo-C2B10H10)-1,2-closo-C2B10H10}] (IX) is completely ineffective. Base strengths are rank-ordered via measurement of the ¹J 31P-77Se coupling constants of the phosphineselenides [1-SePPh2-closo-1,2-C2B10H11] (2), [9-SePPh2-closo-1,7-C2B10H11] (3), and [SePPh2(C6F5)] (4).

Small bite-angle 2-phosphinophosphinine ligands enable rhodium-catalysed hydroboration of carbonyls.

Two Rh complexes of the 2-phosphinophosphinine ligand 2-PPh2-3-Me-6-SiMe3-C5H2P (1) were prepared: dinuclear trans-[{Rh(CO)(Cl)(µ-1)}2] (2) and chelating [Rh(1)(COD)][B(ArF)4] (3). Despite the widespread use of Rh catalysts for the hydroboration of alkenes, 3 is reported to be the first Rh catalyst for ketone and ketimine hydroboration, with high activity observed at 0.1 mol% loading.

Synthesis and reactivity of fluorenyl-tethered N-heterocyclic stannylenes.

A fluorenyl (Fl) tethered diamine was synthesised by nucleophilic substitution of (bromoethyl)fluorene with a diisopropylphenyl (Dipp) substituted diamine to give FlC2H4N(H)C2H4N(H)Dipp (1a) in good yield (85%). Lithiation of 1a with n-BuLi proceeded with coordination of the Li cation to the aromatic fluorenide ring (2), and with subsequent equivalents of n-BuLi, the secondary amines were then sequentially deprotonated. A fluorenyl-tethered N-heterocyclic stannylene (NHSn) was synthesised from the reaction of 1a with SnN''2 {N'' = N(SiMe3)2} as a neutral dimeric species (5), and this was deprotonated with LiN'' to give the corresponding dianionic fluorenide-tethered NHSn (6). Reactions of [{Rh(cod)(µ-Cl)}2] with the mono-deprotonated ligand 2 led to the formation of a mixed-donor amide-amine Rh(i) compound (7), whereas reactions with the anionic NHSn 6 led to a Rh-fluorenyl complex of low stability with an uncoordinated pendent NHSn arm, which X-ray crystallography showed to be dimeric in the solid state.

Characterizing pressure-induced uranium C-H agostic bonds.

The diuranium(III) compound [UN''2]2(µ-η(6):η(6)-C6H6) (N''=N(SiMe3)2) has been studied using variable, high-pressure single-crystal X-ray crystallography, and density functional theory . In this compound, the low-coordinate metal cations are coupled through π- and δ-symmetric arene overlap and show close metal-CH contacts with the flexible methyl CH groups of the sterically encumbered amido ligands. The metal-metal separation decreases with increasing pressure, but the most significant structural changes are to the close contacts between ligand CH bonds and the U centers. Although the interatomic distances are suggestive of agostic-type interactions between the U and ligand peripheral CH groups, QTAIM (quantum theory of atoms-in-molecules) computational analysis suggests that there is no such interaction at ambient pressure. However, QTAIM and NBO analyses indicate that the interaction becomes agostic at 3.2 GPa.

Uranium(IV) amido-borohydrides as highly active diene polymerisation catalysts.

The synthesis and structural characterisation of the uranium(IV) amido-borohydrides (N'')2U{κ(2)-N(SiMe3)SiMe2CH2BBN-H} and U{κ(2)-N(SiMe3)SiMe2CH2BBN-H}2, and their activity as pre-catalysts for the polymerisation of isoprene are described.

Coordination chemistry of trimethylsilylphosphaalkyne: a phosphaalkyne bearing a reactive substituent.

Trimethylsilylphosphaalkyne binds readily to a variety of transition metals. Binding can take place using either the end-on or side-on mode and to either mononuclear or multinuclear metal complexes. The synthesis, structure and characterisation of eight such complexes, [Cp(2)Zr(PMe(3))(Me(3)SiC≡P)], [(C(6)F(5))(2)FB(C(6)F(4))PCSiMe(3))ZrCp(2)(PMe(3))], [(C(6)F(5))(2)XB(C(6)F(4))(ZrCp(2))(2)P(2)(CSiMe(3))(2)] (X = F/H), [(Me(3)Si-C≡P)(2)Mo(dppe)(2)], [CpMo(CO)(2)PC(SiMe(3))Mo(CO)(2)Cp], [(Ph(3)P)(2)Pt(Me(3)SiC≡P)], [{(dppe)Pd}(2)(Me(3)SiC≡P)] and [Pd(5)(PPh(3))(5)(Me(3)SiC≡P)(3)] are described together with attempts to desilylate some of these complexes.

Spontaneous reduction and C-H borylation of arenes mediated by uranium(III) disproportionation.

Transition-metal-arene complexes such as bis(benzene)chromium Cr(η(6)-C(6)H(6))(2) are historically important to d-orbital bonding theory and have modern importance in organic synthesis, catalysis and organic spintronics. In investigations of f-block chemistry, however, arenes are invariably used as solvents rather than ligands. Here, we show that simple uranium complexes UX(3) (X = aryloxide, amide) spontaneously disproportionate, transferring an electron and X-ligand, allowing the resulting UX(2) to bind and reduce arenes, forming inverse sandwich molecules [X(2)U(µ-η(6):η(6)-arene)UX(2)] and a UX(4) by-product. Calculations and kinetic studies suggest a 'cooperative small-molecule activation' mechanism involving spontaneous arene reduction as an X-ligand is transferred. These mild reaction conditions allow functionalized arenes such as arylsilanes to be incorporated. The bulky UX(3) are also inert to reagents such as boranes that would react with the traditional harsh reaction conditions, allowing the development of a new in situ arene C-H bond functionalization methodology converting C-H to C-B bonds.

Iron(I) in Negishi cross-coupling reactions.

Herein we demonstrate both the importance of Fe(I) in Negishi cross-coupling reactions with arylzinc reagents and the isolation of catalytically competent Fe(I) intermediates. These complexes, [FeX(dpbz)(2)] [X = 4-tolyl (7), Cl (8a), Br (8b); dpbz = 1,2-bis(diphenylphosphino)benzene], were characterized by crystallography and tested for activity in representative reactions. The complexes are low-spin with no significant spin density on the ligands. While complex 8b shows performance consistent with an on-cycle intermediate, it seems that 7 is an off-cycle species.

White phosphorus as a ligand for the coinage metals.

Reaction of equimolar quantities of MX (M = Au, Cu, X = Cl; M = Ag, X = OTf) and GaCl(3) in CH(2)Cl(2) with P(4) leads to phosphorus ligating a cationic coinage metal centre. For Cu and Ag, ion-contacted coordination polymers are formed; for Au, an ion-separated complex is observed that features the [Au(η(2)-P(4))(2)](+) cation, which is the first homoleptic Au-P(4) complex to be characterised in the condensed phase.

Diborane(4) compounds with bidentate diamino groups.

The reaction between B(2)(NMe(2))(4) and 1,2-(NH(2))(2)-4-Bu(t)C(6)H(3) affords the diborane(4) compound 1,2-B(2){1,2-(NH)(2)-4-Bu(t)C(6)H(3)}(2) as the exclusive product whilst the reaction between rac-1,2-(NH(2))(2)C(6)H(10) and B(2)(NMe(2))(4) also affords only the 1,2-isomer, i.e. 1,2-B(2){1,2-(NH)(2)C(6)H(10)}(2), which is shown to be the more stable isomer by computational methods. The previously reported compounds 1,1-B(2){1,2-(NH)(2)C(6)H(4)}(2) and 1,2-B(2){1,2-(NH)(2)C(6)H(4)}(2) both react with four equivalents of Bu(n)Li to give what are presumed to be tetra-anions which react further with MeI, SnClMe(3) or SnClPh(3) to give the tetrasubstituted products 1,1-B(2){1,2-(NMe)(2)C(6)H(4)}(2), 1,1-B(2){1,2-(NSnMe(3))(2)C(6)H(4)}(2) and 1,2-B(2){1,2-(NSnPh(3))(2)C(6)H(4)}(2) respectively. The compound 1,1-B(2){1,8-(NH)(2)C(10)H(6)}(2) has also been prepared from the reaction between B(2)(NMe(2))(4) and 1,8-diaminonaphthalene. Lithiation and subsequent reaction with SnClMe(3), SnCl(2)Me(2) or SnCl(2)Ph(2) affords 1,1-B(2){1,8-(NSnMe(3))(2)C(10)H(6)}(2), 1,1-B(2){1,8-(N(2)-µ-SnMe(2))C(10)H(6)}(2) and 1,1-B(2){1,8-(N(2)-µ-SnPh(2))C(10)H(6)}(2) respectively. All new compounds have been characterised by X-ray crystallography.

Small molecule activation by uranium tris(aryloxides): experimental and computational studies of binding of N2, coupling of CO, and deoxygenation insertion of CO2 under ambient conditions.

Previously unanticipated dinitrogen activation is exhibited by the well-known uranium tris(aryloxide) U(ODtbp)(3), U(OC(6)H(3)-Bu(t)(2)-2,6)(3), and the tri-tert-butyl analogue U(OTtbp)(3), U(OC(6)H(2)-Bu(t)(3)-2,4,6)(3), in the form of bridging, side-on dinitrogen complexes [U(OAr)(3)](2)(µ-η(2):η(2)-N(2)), for which the tri-tert-butyl N(2) complex is the most robust U(2)(N(2)) complex isolated to date. Attempted reduction of the tris(aryloxide) complex under N(2) gave only the potassium salt of the uranium(III) tetra(aryloxide) anion, K[U(OAr)(4)], as a result of ligand redistribution. The solid-state structure is a polymeric chain formed by each potassium cation bridging two arenes of adjacent anions in an η(6) fashion. The same uranium tris(aryloxides) were also found to couple carbon monoxide under ambient conditions to give exclusively the ynediolate [OCCO](2-) dianion in [U(OAr)(3)](2)(µ-η(1):η(1)-C(2)O(2)), in direct analogy with the reductive coupling recently shown to afford [U{N(SiMe(3))(2)}(3)](2)(µ-η(1):η(1)-C(2)O(2)). The related U(III) complexes U{N(SiPhMe(2))(2)}(3) and U{CH(SiMe(3))(2)}(3) however do not show CO coupling chemistry in our hands. Of the aryloxide complexes, only the U(OC(6)H(2)-Bu(t)(3)-2,4,6)(3) reacts with CO(2) to give an insertion product containing bridging oxo and aryl carbonate moieties, U(2)(OTtbp)(4)(µ-O)(µ-η(1):η(1)-O(2)COC(6)H(2)-Bu(t)(3)-2,4,6)(2), which has been structurally characterized. The presence of coordinated N(2) in [U(OTtbp)(3)](2)(N(2)) prevents the occurrence of any reaction with CO(2), underscoring the remarkable stability of the N(2) complex. The di-tert-butyl aryloxide does not insert CO(2), and only U(ODtbp)(4) was isolated. The silylamide also reacts with carbon dioxide to afford U(OSiMe(3))(4) as the only uranium-containing material. GGA and hybrid DFT calculations, in conjunction with topological analysis of the electron density, suggest that the U-N(2) bond is strongly polar, and that the only covalent U→N(2) interaction is π backbonding, leading to a formal (U(IV))(2)(N(2))(2-) description of the electronic structure. The N-N stretching wavenumber is preferred as a metric of N(2) reduction to the N-N bond length, as there is excellent agreement between theory and experiment for the former but poorer agreement for the latter due to X-ray crystallographic underestimation of r(N-N). Possible intermediates on the CO coupling pathway to [U(OAr)(3)](2)(µ-C(2)O(2)) are identified, and potential energy surface scans indicate that the ynediolate fragment is more weakly bound than the ancillary ligands, which may have implications in the development of low-temperature and pressure catalytic CO chemistry.

Coordination chemistry of N-heterocyclic stannylenes: a combined synthetic and Mössbauer spectroscopy study.

The N-heterocyclic stannylenes (NHSns), [(Dipp) N(CH(2))(n)N(Dipp)S n] (Dipp = 2,6- (i)Pr(2)C(6)H(3); n = 2, 1; n = 3, 5) and [((t)Bu) N(CHMe)(2)N((t)Bu)S n] (10) are competent ligands toward a variety of transition metal centers, as seen in the complexes [W(CO)(5)·1] (2), [(OC)(4)Fe(µ-1)(2)Fe(CO)(4)] (3), [(OC)(4)Fe(µ-1)Fe(CO)(4)] (4), [Fe(CO)(4)·5](n) (6, n = 1 or 2), [(OC)(4)Fe(µ-5)Fe(CO)(4)] (7), [Ph(3)PPt(µ-1)(2)PtPPh(3)] (8), [Fe(CO)(4)·10] (11), and [(η(5)-C(5)H(5))(OC)(2)Mn·10] (12). X-ray crystallographic studies show that the NHSns are structurally largely unperturbed binding to the metal, but in contrast to the parent NHCs, NHSns often adopt a bridging position across dinuclear metal units. The balance between terminal and bridging positions for the stannylene is evidently closely balanced as shown by the observation of both monomers and dimers for 6 in the solid state and in solution. (119)Sn and (57)Fe Mössbauer spectroscopy of the complexes shows the tin atoms in such complexes to be consistent with electron deficient Sn(II) centers.

New polycyclic borazine species.

The new polycyclic borazines B(2){1,2-N(2)C(6)H(4)}(2){B(2)(NMe(2))(2)}(2), B(2){1,8-N(2)naph}(2){B(2)(NMe(2))(2)}(2) and B(2)(NPh)(4){B(2)(NMe(2))(2)}(2) have been prepared from diborate(4) anions and two equivalents of B(2)Cl(2)(NMe(2))(2) and have been structurally characterised. Aspects of their structure and bonding are discussed and comparison made with corresponding polycyclic aromatic hydrocarbons.