Bridged homobimetallic complexes as models of dimetal active centres: the role of metal—metal bonding

An approach to bimetallic catalysts by ligand design

New diphosphines based on benzofurobenzofuran and dibenzodioxocin backbones, forming exclusively bimetallic complexes were designed and synthesized. Depending on the ligand to metal ratio, face-to-face bimetallic complexes or syn-chloride bridged dimeric complexes were formed as main reaction products. The structures of the rhodium complexes of the new ligands 4, 7, 10, 13, 16 were established in solution by NMR, IR, and MS spectroscopy. The molecular structures of the syn-chloride bridged dimeric complexes [Rh(2)(CO)(2)(mu-Cl)(2)(4)] (22), [Rh(2)(CO)(2)(mu-Cl)(2)(10)] (24), and the face-to-face bimetallic complexes [Rh(CO)Cl(4)](2) (17), [Rh(CO)Cl(10)](2) (19), and [Rh(CO)Cl(13)](2) (20) were confirmed by X-ray crystallography. Ligands 4, 7, 10, 13, 16, and SPANphos were tested in rhodium catalyzed methanol carbonylation at 150 degrees C and 22 bar of CO gas, showing high activities under catalytic conditions.

Oxidative Addition of Methyl Iodide to a New Type of Binuclear Platinum(II) Complex: a Kinetic Study

A new organodiplatinum(II) complex cis,cis-[Me2Pt(mu-NN)(mu-dppm)PtMe2] (1), in which NN = phthalazine and dppm = bis(diphenylphosphino)methane, is synthesized by the reaction of cis,cis-[Me2Pt(mu-SMe2)(mu-dppm)PtMe2] with 1 equiv of NN. Complex 1 has a 5d(pi)(Pt) --> pi(imine) metal-to-ligand charge-transfer band in the visible region, which was used to easily follow the kinetics of its reaction with MeI. Meanwhile, the complex contains a robust bridging dppm ligand that holds the binuclear integrity during the reaction. A double MeI oxidative addition was observed, as shown by spectrophotometry and confirmed by a low-temperature 31P NMR study. The classical S(N)2 mechanism was suggested for both steps, and the involved intermediates were suggested. Consistent with the proposed mechanism, the rates of the reactions at different temperatures were slower in benzene than in acetone and large negative deltaS values were found in each step. However, some abnormalities were observed in the related rate constants and deltaS values, which were demonstrated to be due to the associative involvement of the polar acetone molecules in the reactions. The rates are almost 6 times slower in the second step as compared to the first step because of the electronic effects transmitted through the ligands and the steric effects.

Pyrazolyl-bridged iridium dimers. 18.(1) influence of metal-metal bonding on the geometry of diiridium(II) adducts and hydrido-diiridium complexes formed from the diiridium(I) prototype [Ir(mu-pz)(PPh(3))(CO)](2) (pzH = Pyrazole) by dihydrogen addition or protonation

Slow uptake of molecular dihydrogen by the diiridium(I) prototype [Ir(mu-pz)(PPh(3))(CO)](2) (1: pzH = pyrazole) is accompanied by formation of a 1,2-dihydrido-diiridium(II) adduct [IrH(mu-pz)(PPh(3))(CO)](2) (2), for which an X-ray crystal structure determination reveals that (unlike in 1) the PPh(3) ligands are axial, with the hydrides occupying trans coequatorial positions across the Ir-Ir bond (2.672 A). Reaction with CCl(4) effects hydride replacement in 2, affording the monohydride Ir(2)H(Cl)(mu-pz)(2)(PPh(3))(2)(CO)(2) (3) in which Ir-Ir = 2.683 A. At one metal center, H is equatorial and PPh(3) is axial, while at the other, Cl is axial as is found in the symmetrically substituted product [Ir(mu-pz)(PPh(3))(CO)Cl](2) (4) (Ir-Ir = 2.754 A) that is formed by action of CCl(4) on 1. Treatment of 1 with I(2) yields the diiodo analogue 5 of 4, which reacts with LiAlH(4) to afford the isomorph Ir(2)H(I)(mu-pz)(2)(PPh(3))(2)(CO)(2) (6) of 3 (Ir-Ir = 2.684 A). Protonation (using HBF(4)) of 1 results in formation of the binuclear cation Ir(2)H(mu-pz)(2)(PPh(3))(2)(CO)(2)(+) (7: BF(4)(-) salt), which shows definitive evidence (from NMR) for a terminally bound hydride in solution (CH(2)Cl(2) or THF), but 7 crystallizes as an axially symmetric unit in which Ir-Ir = 2.834 A. Reaction of 7 with water or wet methanol leads to isolation of the cationic diiridium(III) products [Ir(2)H(2)(mu-OX)(mu-pz)(2)(PPh(3))(2)(CO)(2)]BF(4) (8, X = H; 9, X = Me).

Key factors determining the course of methyl iodide oxidative addition to diamidonaphthalene-bridged diiridium(I) and dirhodium(I) complexes

The course of methyl iodide oxidative addition to various nucleophilic complexes, [Ir2(mu-1,8-(NH)2naphth)(CO)2(PiPr3)2] (1), [IrRh(mu-1,8-(NH)2naphth)(CO)2(PiPr3)2] (2), and [Rh2(mu-1,8-(NH)2naphth)(CO)2(PR3)2] (R = iPr, 3; Ph, 4; p-tolyl, 5; Me, 6), has been investigated. The CH3I addition to complex 1 readily affords the diiridium(II) complex [Ir2(mu-1,8-(NH)2naphth)I(CH3)(CO)2(PiPr3)2] (7), which undergoes slow rearrangement to give a thermodynamically stable stereoisomer, 8. The reaction of the Ir-Rh complex 2 gives the ionic compound [IrRh(mu-1,8-(NH)2naphth)(CH3)(CO)2(PiPr3)2]I (10). The dirhodium compounds, 3-5, undergo one-center additions to yield acyl complexes of the formula (Rh2(mu-1,8-(NH)2naphth)I(COCH3)(CO)(PR3)2] (R = iPr, 12; Ph, 13; p-tolyl, 14). The structure of 12 has been determined by X-ray diffraction. Further reactions of these Rh(III)-Rh(I) acyl derivatives with CH3I are productive only for the p-tolylphosphine derivative, which affords the bis-acyl complex [Rh2(mu-1,8-(NH)2naphth)(CH3CO)2I2(P(p-tolyl)3)2] (15). The reaction of the PMe3 derivative, 6, allows the isolation of the bis-methyl complex [Rh2(mu-1,8-(NH)2naphth)(mu-I)(CH3)2(CO)2(PMe3)2]I (16a), which emanates from a double one-center addition. Upon reaction with methyl triflate, the starting materials, 1, 2, 3, and 6, give the isostructural cationic methyl complexes 9, 11, 17, and 18, respectively. The behavior of these cationic methyl compounds toward CH3I, CH3OSO2CF3, and tetrabutylamonium iodide is consistent with the role of these species as intermediates in the SN2 addition of CH3I. Compounds 18 and 17 react with an excess of methyl triflate to give [Rh2(mu-1,8-(NH)2naphth)(mu-OSO2CF3)(CH3)2(CO)2(PMe3)2][CF3SO3] (19) and [Rh2(mu-1,8-(NH)2naphth)(OSO2CF3)(COCH3)(CH3)(CO)(PiPr3)2][CF3SO3] (20), respectively. Upon treatment with acetonitrile, complexes 17 and 18 give the isostructural cationic acyl complexes [Rh2(mu-1,8-(NH)2naphth)(COCH3)(NCCH3)(CO)(PR3)2][CF3SO3] (R = iPr, 21; Me, 22). A kinetic study of the reaction leading to 21 shows that formation of these complexes involves a slow insertion step followed by the fast coordination of the acetonitrile. The variety of reactions found in this system can be rationalized in terms of three alternative reaction pathways, which are determined by the effectiveness of the interactions between the two metal centers of the dinuclear complex and by the steric constraints due to the phosphine ligands.

Electronic structure of [(CpCr)[(CO)3M]]mu-Cot (M = Cr, Fe): a theoretical study

The electronic structure of two cyclooctatetraene-bridged dinuclear first-row transition metal complexes of the type [(CpM)[(CO)3M']]mu-Cot (M = Cr; M' = Fe (1), Cr (2)) was investigated by complete active space self-consistent field (CASSCF) calculations. In this context the differences in the binding capabilities of the complex fragments CpM and (CO)3M are discussed on the basis of extended Huckel molecular orbital (MO) calculations. The geometries used for the CASSCF calculations for complex 1 were obtained from the crystal structure. For 2 a model structure was established by geometry optimization using density functional methods. The CASSCF results agree well with the experimental findings and provide insight into the binding situation of the two compounds. Complex 1 can be regarded as being composed of a chromocene-like subunit CpCr(eta5-C5H5) and the fragment (CO)3Fe(eta3-C3H3). A direct metal-metal bond is found, involving one initially singly occupied orbital of each fragment, leading to a doublet ground state for 1 with the remaining unpaired electron localized at the chromium center. For 2 no such direct metal-metal bond can be recognized. A very weak direct metal-metal interaction is induced by electron donation from the Cot2- ligand into a formally unoccupied metal-metal binding orbital combination. In the quartet ground state all three unpaired electrons are localized at the chromium center of the formally doubly positive charged CpCr unit, on which complex fragment [(CO)3Cr(eta5-Cot)]2- acts like a cyclopentadienyl ligand. The coordination sphere of the chromium center of the CpCr unit resembles that of a metallocene metal center and its metal 3d occupation scheme corresponds to that of vanadocene.

Syntheses, Structures, and Reactivity of Polynuclear Pyrazolato Copper(I) Complexes, Including an ab-Initio XRPD Study of [Cu(dmnpz)](3) (Hdmnpz = 3,5-Dimethyl-4-nitropyrazole)

The reaction of [Cu(CH(3)CN)(4)](BF(4)) with 3,5-dimethyl-4-nitropyrazole (Hdmnpz) in the presence of triethylamine yields the new copper(I) complexes [Cu(dmnpz)](3) (1) and (Et(3)NH)(2)[Cu(4)(dmnpz)(6)] (2), depending on the experimental conditions. The reactivity of 1 and 2 toward neutral ligands such as triphenylphosphine, cyclohexyl isocyanide (RNC), and carbon monoxide has been investigated. In particular, both complexes readily react with RNC, giving the dinuclear complexes [Cu(dmnpz)(RNC)](2) (4) and [Cu(dmnpz)(RNC)(2)](2) (5), depending on the copper/RNC ratio, and with PPh(3), affording the dimeric derivative [Cu(dmnpz)(PPh(3))](2) (6). The crystal and molecular structure of 1 has been determined ab initio using X-ray powder diffraction data from conventional laboratory equipment. Crystals of 1 are monoclinic, C2/c, a = 20.057(2) Å, b = 13.816(2) Å, c = 7.883(1) Å, beta = 95.912(4) degrees; R(F) and R(wp) 0.067 and 0.039, respectively, for Rietveld refinement on 3900 data points collected in the range 17 < 2theta < 95 degrees (Cu Kalpha radiation). Crystals of 1 contain planar trimers, with the copper atoms bridged by exo-bidentate ligands and short intermolecular Cu.Cu contacts (3.329(7) Å). For complexes 2 and 4-6, single-crystal X-ray diffraction studies have been performed. Crystals of 2 are monoclinic, P2(1)/c, a = 11.026(1) Å, b = 13.456(2) Å, c = 18.668(5) Å, beta = 92.91(2) degrees, Z = 2. The ionic packing of 2 contains tetranuclear complexes, with the copper atoms connected by six dmnpz ligands. Crystals of 4 are triclinic, P&onemacr;, a = 7.418(1) Å, b = 9.780(1) Å, c = 11.177(3) Å, alpha = 109.61(2); beta = 101.34(3) degrees, gamma = 105.82(1) degrees, Z = 2. Crystals of 5 are triclinic, P&onemacr;, a = 9.506(4) Å, b = 9.957(2) Å, c = 11.658(4) Å, alpha = 86.73(2) degrees, beta = 79.54(2) degrees, gamma = 82.47(4) degrees, Z = 1. Crystals of 6 are triclinic, P&onemacr;, a = 11.379(2) Å, b = 13.592(2) Å, c = 14.409(3) Å, alpha = 81.22(1) degrees, beta = 85.21(1) degrees, gamma = 87.55(1) degrees, Z = 2. 4, 5, and 6 are binuclear complexes bearing one RNC, two RNC, and one triphenylphosphine ligands on each copper atom, respectively. The steric requirement of the dmnpz ligands, in the presence of RNC or PPh(3), forces the inner [Cu(2)(dmnpz)(2)] core of the three complexes into markedly different conformations.

Mononuclear Platinum(II) and Palladium(II) Dithiolate Complexes as Chelate Metalloligands for Preparation of Heterobimetallic d(8)-d(8) Complexes

Palladium-rhodium and platinum-rhodium heterobimetallic bridged dithiolate complexes of general formula [(P-P)M(&mgr;-S-S)Rh(COD)]ClO(4) (COD = 1,5-cyclooctadiene. For M = Pt: P-P = (PPh(3))(2) and S-S = EDT(2)(-) (1,2-ethanedithiolate) (1), PDT(2)(-) (1,3-propanedithiolate) (2), and BDT(2)(-) (1,4-buthanedithiolate) (3); P-P = dppb (1,4-bis(diphenylphosphino)butane) and S-S = EDT(2)(-) (4), PDT(2)(-) (5), and BDT(2)(-) (6); P-P = dppp (1,3-bis(diphenylphosphino)propane) and S-S = EDT(2)(-) (7), PDT(2)(-) (8), and BDT(2)(-) (9). For M = Pd: P-P = dppb and S-S = EDT(2)(-) (10), PDT(2)(-) (11), and BDT(2)(-) (12); P-P = dppp and S-S = EDT(2)(-) (13), PDT(2)(-) (14), and BDT(2)(-) (15)) have been prepared. The crystal structures for complexes 1-3, 6, 9, and 12 have been determined, and hinged structures were found with angles between local coordination planes MS(2)Rh ranging from 111.27 degrees for complex 1 to 151.34 degrees for complex 3. Metal-metal distances show nonbonding interactions between metals. X-ray data for 1: triclinic, P&onemacr;, a = 11.311(6) Å, b = 12.991(6) Å, c = 16.140(6) Å, alpha = 84.86(6) degrees, beta = 75.73(6) degrees, gamma = 86.43(6) degrees, Z = 2, R = 0.0427 (R(w) = 0.1151). X-ray data for 2: triclinic, P&onemacr;, a = 11.084(5) Å, b = 13.094(6) Å, c = 16.338(6) Å, alpha = 86.06(6) degrees, beta = 75.67(6) degrees, gamma = 88.55(6) degrees, Z = 2, R = 0.0470 (R(w) = 0.1264). X-ray data for 3: orthorhombic, Pbca, a = 14.439(6) Å, b = 19.807(6) Å, c = 38.156(6) Å, Z = 8, R = 0.0864 (R(w) = 0.2457). X-ray data for 6: monoclinic, P2(1)/c, a = 20.338(6) Å, b = 14.227(6) Å, c = 14.570(6) Å, beta = 100.94(6) degrees, Z = 4, R = 0.0536 (R(w) = 0.1449). X-ray data for 9: monoclinic, P2(1)/c, a = 20.326(6) Å, b = 14.109(6) Å, c = 14.368(6) Å, beta = 100.90(6) degrees, Z = 4, R = 0.0380 (R(w) = 0.0990). X-ray data for 12: monoclinic, P2(1)/c, a = 20.365(6) Å, b = 14.186(6) Å, c = 14.592(6) Å, beta = 101.04(6) degrees, Z = 4, R = 0.0507 (R(w) = 0.1500).