Theoretical Study of the Electronic Structure of Group 6 [M(CO)(5)X](-) Species (X = NH(2), OH, Halide, H, CH(3)) and a Reinvestigation of the Role of pi-Donation in CO Lability

2 + 1 Tricarbonyl Complexes of Technetium(I) with a Combination of N,N-Bidentate Ligands and Ethyl Isocyanoacetate: How Strong Is the Interfering Effect of Chloride Ions on Their Formation?

Technetium(I) 2 + 1 tricarbonyl complexes with a combination of N,N-bidentate ligands (2,2'-bipyridine, bipy; 1,10-phenanthroline, phen) and ethyl isocyanoacetate were prepared and characterized by NMR, IR, UV/visible, and luminescence spectroscopies and by high-performance liquid chromatography (HPLC). The crystal structures of [99Tc(CO)3(bipy)(CNCH2COOEt)](ClO4) (in the form of a solvate with 0.5CH2Cl2) and [99Tc(CO)3(phen)(CNCH2COOEt)](ClO4) (in the form of an adduct with an outer-sphere phen molecule) were determined by single-crystal X-ray diffraction. To evaluate the interfering effect of chloride ions on the formation of the 2 + 1 complexes, the kinetics of the replacement of labile monodentate ligand X in the complexes [MX(CO)3(N∧N)] (M = Re, 99Tc; N∧N = bipy, phen; X = Cl-, ClO4-) by CNCH2COOEt in ethanol were compared. The 99Tc bipy complexes with X = ClO4- (according to the IR data, perchlorate anion in ethanol is displaced from the coordination sphere by the solvent molecule) and X = Cl- are characterized by close ligand replacement rates. In the case of the 99Tc complexes with phen and Re complexes with both phen and bipy, the chloride complexes are appreciably less reactive than the chloride-free complexes. The technetium complexes are considerably more reactive in ligand replacement than their rhenium analogues. In the chloride-containing medium (saline), the complex [99mTc(CO)3(bipy)(CNCH2COOEt)]+ can be prepared under the conditions acceptable for nuclear medical applications, although higher isonitrile concentrations are required as compared to the chloride-free system.

Controlled Synthesis of Well-Defined Polyaminoboranes on Scale Using a Robust and Efficient Catalyst

The air tolerant precatalyst, [Rh(L)(NBD)]Cl ([1]Cl) [L = κ³-(ⁱPr₂PCH₂CH₂)₂NH, NBD = norbornadiene], mediates the selective synthesis of N-methylpolyaminoborane, (H₂BNMeH)_n, by dehydropolymerization of H₃B·NMeH₂. Kinetic, speciation, and DFT studies show an induction period in which the active catalyst, Rh(L)H₃ (3), forms, which sits as an outer-sphere adduct 3·H₃BNMeH₂ as the resting state. At the end of catalysis, dormant Rh(L)H₂Cl (2) is formed. Reaction of 2 with H₃B·NMeH₂ returns 3, alongside the proposed formation of boronium [H₂B(NMeH₂)₂]Cl. Aided by isotopic labeling, Eyring analysis, and DFT calculations, a mechanism is proposed in which the cooperative "PNHP" ligand templates dehydrogenation, releasing H₂B═NMeH (ΔG^‡_calc = 19.6 kcal mol^-1). H₂B═NMeH is proposed to undergo rapid, low barrier, head-to-tail chain propagation for which 3 is the catalyst/initiator. A high molecular weight polymer is formed that is relatively insensitive to catalyst loading (M_n ∼71 000 g mol^-1; Đ, of ∼ 1.6). The molecular weight can be controlled using [H₂B(NMe₂H)₂]Cl as a chain transfer agent, M_n = 37 900-78 100 g mol^-1. This polymerization is suggested to arise from an ensemble of processes (catalyst speciation, dehydrogenation, propagation, chain transfer) that are geared around the concentration of H₃B·NMeH₂. TGA and DSC thermal analysis of polymer produced on scale (10 g, 0.01 mol % [1]Cl) show a processing window that allows for melt extrusion of polyaminoborane strands, as well as hot pressing, drop casting, and electrospray deposition. By variation of conditions in the latter, smooth or porous microstructured films or spherical polyaminoboranes beads (∼100 nm) result.

Sulfur-substituted uranyl stabilized by fluoride ligands: matrix preparation of U(O)(S)F2via oxidation of U(0) by SOF2

A neutral sulfur-substituted uranyl complex [U(O)(S)F₂] in which the SUO²⁺ moiety is stabilized by electron withdrawing fluoride ligands was prepared via oxidation of U(0) by SOF₂ in cryogenic matrixes.

Probing the catalytic potential of chloro nitrosyl rhenium(I) complexes

The reduction of the mononitrosyl Re(II) salt [NMe(4)](2)[ReCl(5)(NO)] (1) with zinc in acetonitrile afforded the Re(i) dichloride complex [ReCl(2)(NO)(CH(3)CN)(3)] (2). Subsequent ligand substitution reactions with PCy(3), PiPr(3) and P(p-tolyl)(3) afforded the bisphosphine Re(i) complexes [ReCl(2)(NO)(PR(3))(2)(CH(3)CN)] (3, R = Cy a, iPr b, p-tolyl c) in good yields. The acetonitrile ligand in 3 is labile, permitting its replacement with H(2) (1 bar) to afford the dihydrogen Re(I) complexes [ReCl(2)(NO)(PR(3))(2)(η(2)-H(2))] (4, R = Cy a, iPr b). The catalytic activity of 2, 3 and 4 in hydrogen-related catalyses including dehydrocoupling of Me(2)NH·BH(3), dehydrogenative silylation of styrenes, and hydrosilylation of ketones and aryl aldehydes were investigated, with the main focus on phosphine and halide effects. In the dehydrocoupling of Me(2)NH·BH(3), the phosphine-free complex 2 exhibits the same activity as the bisphosphine-substituted systems. In the dehydrogenative silylation of styrenes, 3a and 4a bearing PCy(3) ligands exhibit high catalytic activities. Monochloro Re(I) hydrides [Re(Cl)(H)(NO)(PR(3))(2)(CH(3)CN)] (5, R = Cy a, iPr b) were proven to be formed in the initiation pathway. The phosphine-free complex 2 showed in dehydrogenative silylations even higher activity than the bisphosphine derivatives, which further emphasizes the importance of a facile phosphine dissociation in the catalytic process. In the hydrosilylation of ketones and aryl aldehydes, at least one rhenium-bound phosphine is required to ensure high catalytic activity.

Routes to unique palladium A-frame complexes with a bridging fluoro-ligand

Treatment of a toluene solution of [PdMe(2)(Cy(2)PCH(2)PCy(2))](1) with pentafluoropyridine in the presence of traces of water affords the generation of the A-frame complexes [(PdMe)(2){mu-kappa(2)(P,P)Cy(2)PCH(2)PCy(2)}(2)(mu-F)][SiMeF(4)]() and [(PdMe)(2){mu-kappa(2)(P,P)Cy(2)PCH(2)PCy(2)}(2)(mu-F)][OC(5)NF(4)](2b). If the reaction is performed in an NMR tube equipped with a PFA inliner, complex 2b is produced, only. Treatment of 1 with pentafluoropyridine in the presence of an excess water yields the pyridyloxy complex [PdMe(OC(5)NF(4))(Cy(2)PCH(2)PCy(2))](3). Compound [(PdMe)(2){mu-kappa(2)(P,P)Cy(2)PCH(2)PCy(2)}(2)(mu-F)][FHF](2c) bearing a bifluoride anion instead of SiMeF(4)(-) or OC(5)NF(4)(-) can be generated by reaction of 1 with substoichiometric amounts of Et(3)N.3HF. The analogous complex [(PdMe)(2){mu-kappa(2)(P,P)Ph(2)PCH(2)PPh(2)}(2)(mu-F)][FHF] (5c) has been synthesized by addition of Ph(2)PCH(2)PPh(2) to a solution of [PdMe(2)(Me(2)NCH(2)CH(2)NMe(2))](4) in THF and subsequent treatment of the reaction mixture with Et(3)N.3HF. The structure of the A-frame complex 5c has been determined by X-ray crystallography.

Theoretical studies on C–heteroatom bond formation via reductive elimination from group 10 M(PH3)2(CH3)(X) species (X = CH3, NH2, OH, SH) and the determination of metal–X bond strengths using density functional theory

Density functional calculations have been used to investigate C-C, C-N and C-O bond forming reactions via reductive elimination from Group 10 cis-M(PH3)2(CH3)(X) species (X = C-I3, NH2, OH). Both direct reaction from the four-coordinate species and a three-coordinate mechanism involving initial PH3 loss have been considered. For the four-coordinate pathway the ease of reductive elimination to give M(PH3)2 and CH3-X follows the trend M = Pd < Pt < Ni. The reaction of the cis-M(PH3)2(CH3)(NH2) species is promoted by the formation of methylamine adducts. Non-planar transition states are located and the C-heteroatom bond forming processes are characterised by migration of CH3 onto the cis-heteroatom ligand. For a given ligand, X, activation energies follow the trend M = Ni < Pd < Pt. Formation of the three-coordinate M(PH3)(CH3)(X) species is promoted by a labilisation of the cis-PH3 ligand in the four-coordinate reactants when X = NH2 or OH. For the three-coordinate pathway the energy change for reductive elimination to give M(PH3) and CH3-X again follows the trend M = Pd < Pt < Ni and in all cases the initial product is an M(PH3)(XCH3) adduct. The three-coordinate transition states again involve migration of the CH3 ligand onto the cis-X ligand and for X = NH2 or OH activation energies follow the trend Ni > Pd < Pt. For a given metal activation energies in both the four- and three-coordinate pathways increase along the series CH3 < NH2 < OH. These trends in activation energy can be rationalised in terms of the strength of M-CH3/M-X bonding as long as the extent of geometrical distortion required to obtain the transition state geometry is taken into account. Further calculations on cis-Pd(PH3)2(CH3)(SH) suggest that the more common experimental observation of C(sp3)-S compared to C(sp3)-O reductive elimination arises from the greater kinetic accessibility of the former process rather than an intrinsic thermodynamic preference for C-S bond formation. By comparison, the calculations indicate that C(sp3)-N reductive elimination should be feasible from Ni and Pd systems. DF calculations are shown to reproduce the relative homolytic bond strengths determined experimentally for Pt-X bonds. In the cis-M(PH3)2(CH3)(X) systems the M-CH3 homolytic bond strength increases down the group while for M-NH2 and M-OH bonds the trend is M = N approximately equal to Pd < Pt. M-NH2 and M-OH are considerably stronger than M-CH3 bonds and the presence of a heteroatom ligand serves to weaken M-CH3 bonds even further.

Combined computational and experimental study of substituent effects on the thermodynamics of H(2), CO, arene, and alkane addition to iridium

The thermodynamics of small-molecule (H(2), arene, alkane, and CO) addition to pincer-ligated iridium complexes of several different configurations (three-coordinate d(8), four-coordinate d(8), and five-coordinate d(6)) have been investigated by computational and experimental means. The substituent para to the iridium (Y) has been varied in complexes containing the (Y-PCP)Ir unit (Y-PCP = eta(3)-1,3,5-C(6)H(2)[CH(2)PR(2)](2)Y; R = methyl for computations; R = tert-butyl for experiments); substituent effects have been studied for the addition of H(2), C-H, and CO to the complexes (Y-PCP)Ir, (Y-PCP)Ir(CO), and (Y-PCP)Ir(H)(2). Para substituents on arenes undergoing C-H bond addition to (PCP)Ir or to (PCP)Ir(CO) have also been varied computationally and experimentally. In general, increasing electron donation by the substituent Y in the 16-electron complexes, (Y-PCP)Ir(CO) or (Y-PCP)Ir(H)(2), disfavors addition of H-H or C-H bonds, in contradiction to the idea of such additions being oxidative. Addition of CO to the same 16-electron complexes is also disfavored by increased electron donation from Y. By contrast, addition of H-H and C-H bonds or CO to the three-coordinate parent species (Y-PCP)Ir is favored by increased electron donation. In general, the effects of varying Y are markedly similar for H(2), C-H, and CO addition. The trends can be fully rationalized in terms of simple molecular orbital interactions but not in terms of concepts related to oxidation, such as charge-transfer or electronegativity differences.

Influence of filled d pi-manifold and L/L' ligands on the structure, basicity, and bond rotations of the octahedral and d(6) amido complexes TpRu(L)(L')(NHPh) (Tp = hydridotris(pyrazolyl)borate; L = L' = PMe(3) or P(OMe)(3), or L = CO and L' = PPh(3)): solid-state structures of [TpRu(PMe(3))(2)(NH(2)Ph)][OTf], [TpRu[P(OMe)(3)](2)(NH(2)Ph)][OTf], and TpRu[P(OMe)(3)](2)(NHPh)

It has been suggested that the reactivity of pi-donating ligands bound to late-transition-metal complexes is heightened due to high d-electron counts. Herein, the synthesis and characterization of the Ru(II) amine and Ru(II) amido complexes [TpRuL(2)(NH(2)Ph)][OTf] (OTf = trifluoromethanesulfonate) and TpRuL(2)(NHPh) (L = PMe(3) or P(OMe)(3)) are presented, including solid-state X-ray diffraction studies of [TpRu(PMe(3))(2)(NH(2)Ph)][OTf], [TpRu[P(OMe)(3)](2)(NH(2)Ph)][OTf], and TpRu[P(OMe)(3)](2)(NHPh). The pK(a)'s of the Ru(II) amine complexes and the previously reported [TpRu(CO)(PPh(3))(NH(2)Ph)](+) have been estimated to be comparable to that of malononitrile in methylene chloride. In addition, the impact of the filled dpi-manifold (i.e., Ru(II) and d(6) octahedral systems) on barriers to rotation of the Ru-NHPh moieties has been studied. For TpRu(PMe(3))(2)(NHPh) and TpRu[P(OMe)(3)](2)(NHPh), evidence for hindered rotation about the amido nitrogen and phenyl ipso carbon has been observed, and the relative N-C and Ru-N bond rotational barriers for the series of three amido complexes are discussed in terms of the pi-conflict.

Preparation of the octahedral d(6) amido complex TpRu(CO)(PPh(3))(NHPh) (Tp = Hydridotris(pyrazolyl)borate): solid-state structural characterization and reactivity

The reaction of TpRu(CO)(PPh(3))(OTf) (2) with LiNHPh affords the amido complex TpRu(CO)(PPh(3))(NHPh) (3) in 88% isolated yield. The amido complex 3 has been characterized by (1)H NMR, (13)C NMR, (31)P NMR, elemental analysis, cyclic voltammetry, and a solid-state X-ray diffraction study. Variable temperature NMR studies have revealed a rotational barrier around the ruthenium-amido nitrogen bond of 3 of 12 kcal/mol (transformation of the major isomer to the minor isomer). The solid-state structure of 3 discloses a pyramidal amido moiety. Heating benzene solutions of the amido complex 3 and 1,4-cyclohexadiene or 9,10-dihydroanthracene results in no observable reaction. Reaction of complex 2 with excess aniline yields [TpRu(CO)(PPh(3))(NH(2)Ph)][OTf] (4).