Michael J. Quayle

OXIDATIVE ADDITION OF BORON-BORON, BORON-CHLORINE AND BORON-BROMINE BONDS TO PLATINUM(0)

Abstract The synthesis and spectroscopic characterisation of the new diborane(4) compounds B2(1,2-O2C6Cl4)2 and B2(1,2-O2C6Br4)2 are reported together with the diborane(4) bis-amine adduct [B2(calix)(NHMe2)2] (calix=Butcalix[4]arene). B–B bond oxidative addition reactions between the platinum(0) compound [Pt(PPh3)2(η-C2H4)] and the diborane(4) compounds B2(1,2-S2C6H4)2, B2(1,2-O2C6Cl4)2 and B2(1,2-O2C6Br4)2 are also described which result in the platinum(II) bis-boryl complexes cis-[Pt(PPh3)2{B(1,2-S2C6H4)}2], cis-[Pt(PPh3)2{B(1,2-O2C6Cl4)}2] and cis-[Pt(PPh3)2{B(1,2-O2C6Br4)}2] respectively, the former two having been characterised by X-ray crystallography. In addition, the platinum complex [Pt(PPh3)2(η-C2H4)] reacts with XB(1,2-O2C6H4) (X=Cl, Br) affording the mono-boryl complexes trans-[PtX(PPh3)2{B(1,2-O2C6H4)}] as a result of oxidative addition of the B–X bonds to the Pt(0) centre; the chloro derivative has been characterised by X-ray crystallography.

Complexation of stable carbenes with alkali metals

Stable diaminocarbenes, including imidazol-2-ylidenes, undergo complexation with lithium, sodium and potassium species; the crystal structure of a complex of 1,3-diisopropyl-3,4,5,6-tetrahydropyrimid-2-ylidene 1 with KN(SiMe3)2 is reported.

Diborane(4) compounds incorporating thio- and seleno-carboranyl groups

The reaction between B2(NMe2)4 and the carborane dithiol C2B10H10(SH)2, followed by addition of HCl affords the [NH2Me2]+ salt of the dianion [B2Cl2(S2C2B10H10)2]2−, which has been characterised by X-ray crystallography. A similar reaction utilising the carborane diselenol C2B10H10(SeH)2 (generated in situ) afforded a compound containing the dianion [B2(Se2C2B10H10)3]2−.

Redox-induced κ2–κ3 isomerisation in rhodium hydrotris(3,5-dimethylpyrazolyl)borate chemistry: the stabilisation of square-pyramidal rhodium(II)

One-electron oxidation of the square-planar, 16-electron complex [Rh(CO)(PPh3)(κ2-Tp′)] with [FeCp2][PF6] gives the square-pyramidal RhII complex [Rh(CO)(PPh3)(κ3-Tp′)][PF6] in which the third pyrazolyl group of Tp′ is N-bound in the axial position by a three-electron two-centre bond (Rh–Naxial 0.13 A longer than Rh–Nbasal).

Chlorobis(triphenylphosphine)nickel(I)

Crystals of the title compound, [NiCl(C(18)H(15)P)(2)], contain one molecule per asymmetric unit with no short intermolecular interactions. This is noteworthy since previous studies have reported that the formally 15-electron species oligomerizes in the solid state. The nickel(I) centre has a distorted trigonal-planar coordination geometry, the origin of which is suggested to be electronic in nature.

2,2'-bipyridyl and 1.10-phenanthroline adducts of the diborane(4) compound B2(1,2-S2C6H4)2

Abstract The syntheses and structures of the 2,2′-bipyridyl and 1,10-phenanthroline adducts of the diborane (4) compound B2 (1,2-S2C6H4)2 are described. In both cases, the bidentate nitrogen donor ligands bridge the B–B bond, one nitrogen being bonded to each boron.

Boron–boron bond oxidative addition to rhodium(I) and iridium(I) centres

The reaction between the diborane(4) compound B2(1,2-O2C6H4)2 and either of the rhodium(I) complexes [RhCl(PPh3)3] or [{Rh(µ-Cl)(PPh3)2}2] afforded the colourless rhodium(III) bis(boryl) species [RhCl(PPh3)2{B(1,2-O2C6H4)}2]. Similar reactions have been carried out with the diborane(4) compounds B2(1,2-O2-4-ButC6H3)2, B2(1,2-O2-3,5-But2C6H2)2, B2(1,2-O2-3-MeC6H3)2, B2(1,2-O2-4-MeC6H3)2, B2(1,2-O2-3-MeOC6H3)2, B2(1,2-S2C6H4)2, B2(1,2-S2-4-MeC6H3)2 and B2[R,R-1,2-O2CH(CO2Me)CH(CO2Me)]2 affording analogous rhodium complexes all of which have been characterised spectroscopically. The complexes derived from the reactions with B2(1,2-O2C6H4)2 and B2(1,2-O2-3-MeC6H3)2 have also been characterised by X-ray crystallography, the structures comprising a five-co-ordinate rhodium centre with a square-based-pyramidal geometry in which the apical site is occupied by a boryl group and the phosphines are mutually trans in basal positions. Reactivity studies have also been carried out for [RhCl(PPh3)2{B(1,2-O2C6H4)}2]. Hydrolysis or alcoholysis with catechol afforded [RhH2Cl(PPh3)3] and either B2(1,2-O2C6H4)2(µ-O) or B2(1,2-O2C6H4)3 and addition of the phosphines PMe3, PEt3 and PMe2Ph afforded the new bis(boryl) compounds cis,mer-[RhCl(PMe3)3{B(1,2-O2C6H4)}2], [RhCl(PEt3)2{B(1,2-O2C6H4)}2] and cis,mer-[RhCl(PMe2Ph)3{B(1,2-O2C6H4)}2], the PEt3 complex having been characterised by X-ray crystallography and shown to be similar to the PPh3 complex. The iridium analogue [IrCl(PEt3)2{B(1,2-O2C6H4)}2] was also prepared from the reaction between [IrCl(PEt3)3] and B2(1,2-O2C6H4)2 and shown by X-ray crystallography to be isomorphous with the rhodium complex. Reactions between [RhCl(PPh3)2{B(1,2-O2C6H4)}2] and the phosphines PPri3, P(C6H11)3, 1,2-bis(diphenylphosphino)ethane (dppe) and 1,2-bis(dicyclohexylphosphino)ethane (dcpe) are also described although these do not result in new rhodium boryl complexes. The reaction between [{RhCl(dppe)}2] and B2(1,2-O2C6H4)2 afforded a compound tentatively assigned as [Rh(dppe)2{B(1,2-O2C6H4)}] with analogous compounds being formed with the diborane(4) compounds B2(1,2-O2-3-MeC6H3)2 and B2(1,2-O2-4-MeC6H3)2. Finally, the reaction between [Rh(PMe3)4]Cl and the diborane(4) compound B2(1,2-O2C6H4)2 is described which affords cis,mer-[RhCl(PMe3)3{B(1,2-O2C6H4)}2]. Analogous reactions with B2(1,2-O2-3,5-But2C6H2)2, B2(1,2-O2-3-MeC6H3)2 and B2[R,R-1,2-O2CH(CO2Me)CH(CO2Me)]2 afforded similar products.

Structure and bonding in redox-active d4, d5 and d6 alkyne complexes: metal–alkyne moieties as electron sinks

X-Ray structural studies of the two redox-related pairs [Cr(CO)2(η-PhCCPh)(η6-C6HMe5)]0/1+ and [Mo(CO)2(η-PhCPh)(Tp′)]0/1+[Tp′= hydrotris(3,5-dimethyl-pyrazolyl)borate] are consistent with the HOMO of the d6, Cr0 alkyne complex being an antibonding M–alkyne π⊥ orbital.

Redox routes to arenechromium complexes of two-, three- and four-electron alkynes; structure and bonding in paramagnetic [Cr(CO)L(η-RCCR)(η-arene)]+

X-Ray structural studies on the redox pair [Cr(CO)2(η-PhCCPh)(η-C6Me5H)]z (z = 0 and 1) show that one-electron oxidation of the neutral complex results in a shortening of the Cr–Calkyne bonds and a lengthening of the Cr–C(O) bonds, consistent with depopulation of a HOMO antibonding with respect to the metal–alkyne interaction. Oxidation leads to an increase in the substitutional lability of the Cr–CO bonds so that [Cr(CO)2(η-RCCR)(η-C6Me6)]+ (R = Ph or C6H4OMe-p) reacts with Lewis bases to give [Cr(CO)L(η-RCCR)(η-C6Me6)]+ {L = CNXyl, P(OMe)3 and P(OCH2)3CEt}, X-ray studies on which show a rotation of the alkyne to align with the remaining Cr–CO bond. ESR spectroscopic studies on [Cr(CO)L(η-RCCR)(η-C6Me6)]+ show delocalisation of the unpaired electron onto the alkyne ligand, consistent with its description as a three-electron donor. The cations [Cr(CO)L(η-RCCR)(η-C6Me6)]+ undergo both one-electron reduction and oxidation, and chemical oxidation of [Cr(CO){P(OCH2)3CEt}(η-p-MeOC6H4CCC6H4OMe-p)(η-C6Me6)]+ with AgPF6 gives the dication [Cr(CO){P(OCH2)3CEt}(η-p-MeOC6H4CCC6H4OMe-p)(η-C6Me6)]2+. Thus the two-electron alkyne of [Cr(CO)2(η-RCCR)(η-C6Me6)] is converted into the four-electron alkyne of [Cr(CO)L(η-RCCR)(η-C6Me6)]2+ by an ECE (E = electrochemical, C = chemical) process in which all of the intermediates have been fully characterised.

Preparation and structure of cis-[Pt(BF2)2(PPh3)2]: the first crystallographically characterised complex containing the BF2 ligand

The platinum compound [Pt(PPh3)2(η-C2H4)] reacts with B2F4 affording the BF2 complex cis-[Pt(BF2)2(PPh3)2] which is characterised by multinuclear NMR studies and X-ray crystallography.