Amor Rodríguez

Interconversion of Quadruply and Quintuply Bonded Molybdenum Complexes by Reductive Elimination and Oxidative Addition of Dihydrogen

Transition metal complexes that exhibit multiple metal–metal bonding are of fundamental importance in chemistry. Following decades of intense scrutiny of double, triple, and quadruple metal–metal bonds, in 2005 Power and co-workers made an outstanding discovery with the characterization of the first stable molecule with fivefold bonding between two chromium atoms. This report encouraged the search for further examples of quintuply bonded dimetal compounds and was followed by numerous experimental and computational studies. By and large, these studies have focused on chromium compounds. 2–8,11] However, Tsai and coworkers have extended their investigation of the Cr Cr quintuple bond 7,11] to analogous molybdenum complexes and have isolated two closely related compounds supported by monoanionic amidinate ligands, N,N’-disubstituted with 2,6-diisopropylphenyl groups. Recently, another compound with the Mo2 central unit reinforced by three amidinate ligands and a bridging lithium cation has also been reported by the same group of researchers. Identification of the above complexes with fivefold metalmetal bonding has prompted research directed toward understanding their bonding characteristics and the chemical reactivity of their central M M quintuple bond. So far, only a few reports have appeared in the literature concerning mainly the chemical properties of the Cr Cr quintuple bond. Thus, these complexes feature interesting reactivity toward unsaturated molecules like N2O, [19] alkenes, and alkynes, and are able to activate white phosphorous, yellow arsenic, and AsP3. [22] Similar to carbon carbon double and triple bonds, the Cr Cr quintuple bond of an aminopyridinate complex undergoes facile carboalumination, to generate the corresponding Cr Cr quadruple-bond derivative with formally monoanionic bridging CH3 and AlMe2 groups. [23] Disproportionation of Cr to Cr and Cr induced by the addition of [18]crown-6-ether to a Cr Cr quintuply bonded complex has also been demonstrated. During the preparation of this manuscript the reactivity of Mo Mo quintuply bonded compounds with alkynes has been shown. The above results illustrate the potential of quintuply bonded electron-rich M2 centers for bimetallic activation. Herein we describe that the quadruply bonded dimolybdenum dihydride complex [Mo2(H)2{HC(N-2,6-iPr2C6H3)2}2(thf)2] 2 undergoes reductive elimination of H2 from its [(H)Mo Mo(H)] core under UV irradiation (365 nm) to afford the known [Mo2{HC(N-2,6-iPr2C6H3)2}2], 3, [9] with fivefold Mo Mo bonding (Scheme 1B). Because a tetrahydrofuran (THF) solution of the latter reacts readily with H2 to reform 2, our results demonstrate that quadruply and quintuply bonded dimolybdenum complexes may interconvert by means of reductive elimination and oxidative

Zn–Zn‐Bonded Compounds that Contain Monoanionic Oxygen‐Donor Ligands

The last years have witnessed renewed interest in the study of molecular compounds that present a metal–metal bond between atoms of Group 12 metals. For zinc, following the initial report on the structural characterisation of decamethyldizincocene, [Zn2ACHTUNGTRENNUNG(h5-C5Me5)2] (1), a number of complexes of sterically demanding and in many cases chelating ligands have been prepared. Kinetic stabilisation of the Zn Zn bond to prevent disproportionation to Zn and Zn has been achieved by the use of different types of ligands. The initially employed bulky, substituted cyclopentadienyl units were followed by carbon-based m-terphenyl groups, as well as by a variety of chelating N-donor ligands, which have also proved useful to stabilise Mg Mg bonds. Moreover, recent work by Schulz and co-workers has disclosed interesting reactivity of compound 1 that occurs with preservation of the Zn Zn bond and proceeds with elimination of Cp*H (Cp*=C5Me5). [10] Subsequently, a unique Zn2 2+ ion stabilised by coordination of 4-dimethylaminopyridine (dmap), [Zn2ACHTUNGTRENNUNG(dmap)6]2+ , has been isolated and structurally characterised. Since so far Zn Zn bonded complexes with alkoxide or aryloxide ligands (RO ) are not known, we have decided to study the reactivity of 1 towards several alkyl and aryl alcohols. Herein, we present preliminary results on the synthesis and structural characterisation of metal–metal bonded dizinc species featuring Zn O bonds. As reported previously, complex 1 reacts with Me3COH to produce metallic zinc along with the alkoxide [{Zn ACHTUNGTRENNUNG(OtBu)2}x]. This result makes clear that bulkier RO groups are needed to stabilise the dizinc unit and accordingly the reactions of 1 with ArOH (2,6-(2,4,6-Me3C6H2)C6H3OH) and C5Me5OH have been investigated. Initial results were disappointing, as the low-temperature ( 208C) reaction of 1 with ArOH yielded an insoluble white solid that has eluded characterisation so far. Under similar conditions, C5Me5OH led to extensive decomposition, possibly as a result of disproportionation. We then considered providing further stabilisation to the Zn2 unit by carrying out corresponding reactions in the presence of an N-donor ligand. Since 1 is recovered unaltered when crystallised in the presence of an excess of pyridine (despite an evident colour change to yellow that suggests weak [Zn2ACHTUNGTRENNUNG(C5Me5)2]···pyridine interaction), while the more basic dmap provides the metal metal bonded adduct [Zn2 ACHTUNGTRENNUNG(C5Me5)2 ACHTUNGTRENNUNG(dmap)2],[9b] it is clear that sufficiently strong an electron donor is required (pKa values for pyridine and dmap in acetonitrile are 12.53 and 17.95, respectively). With this knowledge, 4-pyrrolidinopyridine (pyr-py, pKa = 18.33) and diaza-1,3-bicycloACHTUNGTRENNUNG[5.4.0]undecane (DBU, pKa = 24.34) have been chosen for this study. As discussed below, these results indicate that pyr-py appears to be more appropriate than DBU for this purpose. Low-temperature H NMR spectroscopic monitoring of the reactions of 1 with ArOH and C5Me5OH (represented in general as ROH), in presence of the aforementioned Ndonor L (pyr-py and DBU), reveals the consumption of both reactants, 1 and ROH, along with concomitant formation of C5Me5H and new zinc–zinc compounds of composition [Zn2 ACHTUNGTRENNUNG(h5-C5Me5)(OR)(L)x], in which a C5Me5 group has been replaced by RO. For instance, treatment of 1 with ArOH in the presence of pyr-py leads to compound 2 as a white solid in 80 % isolated yield (Scheme 1). In its H NMR spectrum in C6D6, the methyl groups of the h [a] M. Carrasco, Dr. R. Peloso, Dr. A. Rodr guez, Dr. E. lvarez, Dr. C. Maya, Prof. E. Carmona Instituto de Investigaciones Qu micas Departamento de Qu mica Inorg nica Universidad de Sevilla Consejo Superior de Investigaciones Cient ficas Avenida Am rico Vespucio 49, 41092 Sevilla (Spain) Fax: (+34) 954460565 E-mail : guzman@us.es Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201001011.

Experimental and Theoretical Studies on Arene‐Bridged Metal–Metal‐Bonded Dimolybdenum Complexes

The bis(hydride) dimolybdenum complex, [Mo2(H)2{HC(N-2,6-iPr2C6H3)2}2(thf)2], 2, which possesses a quadruply bonded Mo2(II) core, undergoes light-induced (365 nm) reductive elimination of H2 and arene coordination in benzene and toluene solutions, with formation of the Mo(I)2 complexes [Mo2{HC(N-2,6-iPr2C6H3)2}2(arene)], 3⋅C6H6 and 3⋅C6H5Me, respectively. The analogous C6H5OMe, p-C6H4Me2, C6H5F, and p-C6H4F2 derivatives have also been prepared by thermal or photochemical methods, which nevertheless employ different Mo2 complex precursors. X-ray crystallography and solution NMR studies demonstrate that the molecule of the arene bridges the molybdenum atoms of the Mo(I)2 core, coordinating to each in an η(2) fashion. In solution, the arene rotates fast on the NMR timescale around the Mo2-arene axis. For the substituted aromatic hydrocarbons, the NMR data are consistent with the existence of a major rotamer in which the metal atoms are coordinated to the more electron-rich C-C bonds.

Quadruply bonded dimolybdenum complexes with highly unusual geometries and vacant coordination sites

New quadruply bonded dimolybdenum complexes of the terphenyl ligand Ar(Xyl(2)) (Ar(Xyl(2)) = C(6)H(3)-2,6-(C(6)H(3)-2,6-Me(2))(2)) have been prepared and structurally characterized. The steric hindrance exerted by the Ar(Xyl(2)) groups causes the Mo atoms to feature unsaturated four-coordinate structures and a formal fourteen-electron count.

Third Kidney Transplantation: A Permanent Medical-Surgical Challenge

The aim of this study was to analyze the 30 third transplantations performed at our center since 1976. They were all from cadaveric donors. Recipient mean age was 40 years (range, 21-57 years). Twenty-one patients (70%) had hepatitis C virus infection and 16 (53%) were hypersensitized (panel-reactive antibodies [PRA] >50%), with a mean time on dialysis since second graft loss of 65 months (range, 1-250 months). The imaging tests showed iliac calcifications in 14 patients (47%). The graft was preferably placed in the iliac fossa (27/30). Twenty-five patients (83%) had prior graft nephrectomy and transplantectomy was performed at the same surgery in 2 cases. Immunosuppressive protocol was quadruple therapy in 23 patients (77%). At a mean follow-up of 43 months, 24 grafts were functioning. Mean serum creatinine was 1.5 mg/dL and Modification of Diet in Renal Disease (MDRD) clearance was 64.5 mL/min. Six grafts were lost: 3 due to acute rejection, 2 due to chronic allograft nephropathy, and 1 due to venous thrombosis. Four patients died: 2 due to infectious complications, 1 due to hepatic encephalopathy, and 1 to an accident with a functioning graft. The acute rejection rate was 30% and 4 patients had an acute humoral rejection episode. The main surgical complication was lymphocele in 7 cases (23%). Estimated survival at 5 years was 76% for grafts and 86% for patients. Graft survival worsened among patients with PRA > 80% and among those who had lost the previous grafts in the first month posttransplantation (P < .05). In conclusion, the outcomes of the third kidney transplantations encourage us to continue with this procedure. However, worse graft survival should be expected among hyperimmunized patients and among those who had lost previous grafts early in their course.

Lithium Di- and Trimethyl Dimolybdenum(II) Complexes with Mo–Mo Quadruple Bonds and Bridging Methyl Groups

New dimolybdenum complexes of composition [Mo2{μ-Me}2Li(S)}(μ-X)(μ-N^N)2] (3a-3c), where S = THF or Et2O and N^N represents a bidentate aminopyridinate or amidinate ligand that bridges the quadruply bonded molybdenum atoms, were prepared from the reaction of the appropriate [Mo2{μ-O2CMe}2(μ-N^N)2] precursors and LiMe. For complex 3a, X = MeCO2, while in 3b and 3c, X = Me. Solution NMR studies in C6D6 solvent support formulation of the complexes as contact ion pairs with weak agostic Mo-CH3···Li interactions, which were also evidenced by X-ray crystallography in the solid-state structures of the molecules of 3a and 3b. Samples of 3c enriched in (13)C (99%) at the metal-bonded methyl sites were also prepared and investigated by NMR spectroscopy employing C6D6 and THF-d8 solvents. Crystallization of 3c from toluene:tetrahydrofuran mixtures provided single crystals of the solvent separated ion pair complex [Li(THF)4] [Mo2(Me)2(μ-Me){μ-HC(NDipp)2}2] (4c), where Dipp stands for 2,6-iPr2C6H3. A computational analysis of the Mo2(μ-Me)2Li core of complexes 3a and 3b has been developed, which is consistent with a small but non-negligible electron-density sharing between the C and Li atoms of the mainly ionic CH3···Li interactions.

Urological complications after simultaneous pancreas-kidney transplantation.

OBJECTIVE We evaluated the incidence of urological complications after simultaneous renal and pancreatic transplantation. PATIENTS AND METHODS We retrospectively reviewed urological complications following 107 simultaneous kidney-pancreas transplantations performed at our institution between March 1995 and June 2008. The 46 women and 61 men were of mean age 37.8 years (range, 25-66). The mean duration of diabetes mellitus was 23.0 years (range, 9-48) and the mean duration of dialysis was 19.9 months (range, 0-70). The exocrine pancreatic secretions were drained to bladder in 58 cases, or enterically in 49 patients. The mean length of follow-up was 51.7 months. RESULTS The most frequent urological complication was urinary tract infection, reported in 63.8% of patients: 42 bladder-drained and 25 enteric-drained (P = .011). Hematuria occurred in 13 patients (12.5%): 12 bladder-drained and 1 enteric-drained (P = .002). Five bladder-drained patients developed bladder calculi. Among 58 bladder-drained patients, reflux pancreatitis occurred in 28 patients and urine leaks related to the pancreatic graft occurred in 7 patients. Conversion of exocrine secretions from bladder to enteric diversion was required in 6 patients. One- and 3-year patient survival rates were 92.7% and 89.1%, respectively. Moreover, 1 and 3-year kidney graft survival rates were 90.6% and 84.4%, and pancreas graft survival rates were 78.1 and 70.3%, respectively. CONCLUSION Simultaneous kidney-pancreas transplantation with bladder drainage is associated with a high frequency of urological complications. Appropriate treatment can resolve most complications. In our opinion, both enteric and bladder drainage seemed to be safe and effective alternatives to manage pancreatic exocrine secretions.

An Unsaturated Four-Coordinate Dimethyl Dimolybdenum Complex with a Molybdenum–Molybdenum Quadruple Bond

We describe the synthesis and the molecular and electronic structures of the complex [Mo2 Me2 {μ-HC(NDipp)2 }2 ] (2; Dipp=2,6-iPr2 C6 H3 ), which contains a dimetallic core with an Mo-Mo quadruple bond and features uncommon four-coordinate geometry and has a fourteen-electron count for each molybdenum atom. The coordination polyhedron approaches a square pyramid, with one of the molybdenum atoms nearly co-planar with the basal square plane, in which the trans coordination position with respect to the Mo-Me bond is vacant. The other three sites are occupied by two trans nitrogen atoms of different amidinate ligands and the methyl group. The second Mo atom occupies the apex of the pyramid and forms an Mo-Mo bond of length 2.080(1) Å, consistent with a quadruple bond. Compound 2 reacts with tetrahydrofuran (THF) and trimethylphosphine to yield the mono-adducts [Mo2 Me(μ-Me){μ-HC(NDipp)2 }2 (L)] (3⋅THF and 3⋅PMe3 , respectively) with one terminal and one bridging methyl group. In contrast, 4-dimethylaminopyridine (dmap) forms the bis-adduct [Mo2 Me2 {μ-HC(NDipp)2 }2 (dmap)2 ] (4), with terminally coordinated methyl groups. Hydrogenolysis of complex 2 leads to the bis(hydride) [Mo2 H2 {μ-HC(NDipp)2 }2 (thf)2 ] (5⋅THF) with elimination of CH4 . Computational, kinetic, and mechanistic studies, which included the use of D2 and of complex 2 labelled with 13 C (99 %) at the Mo-CH3 sites, supported the intermediacy of a methyl-hydride reactive species. A computational DFT analysis of the terminal and bridging coordination of the methyl groups to the Mo≣Mo core is also reported.

Using virtual potential fields for electric wheelchair guidance

TetraNauta is an electric wheelchair guidance system intended for people with heavy motion impairments (such as persons with tetraplegia). It is specially useful when impairments also affect wheelchair steering as it is able to automatically guide wheelchairs between different points in a known environment (a hospital, a school, etc), conditioned with track marks painted on the floor. It also provides a semiautomatic navigation mode, where control is shared between user and navigation system. It is intended for learning wheelchair manipulation and as an aid in places where navigation is difficult or dangerous (i.e. for crossing narrow corridors).