J. CHEM. soc. DALTON TRANS. 1988 1709Preparation and Nuclear Magnetic Resonance Spectroscopy of [Ag,{p-CH,( PPh,),},] [O,SCF,],, a DisiIver(1) Complex with Three Bridging CHJ PPh,),LigandsDagmar Obendorf, Michael Probst, and Paul Peringer"lnstitut fur Anorganische und Analytische Chemie der Universitat lnnsbruck, lnnrain 52a, A - 6020 Innsbruck,AustriaHeinz Falk and Norbert Mullerlnstitut fur Chemie der Johannes Kepler Universitat Linz, A-4040 Linz, Austria[Ag,(y-dppm),] [O,SCF,], [dppm = bis(diphenylphosphino)methane] has been prepared and charac-terized by ,'P n.m.r. spectroscopy. Variable-temperature n.m.r. studies revealed that the Ag-P bondsare kinetically stable on the n.m.r. time-scale below 233 K. At ambient temperature there isintermolecular ligand exchange whereas at intermediate temperatures intramolecular inter-metalexchange of the phosphorus atoms of the dppm ligands occurs.Silver( 1)dppm [dppm = bis(dipheny1phosphino)methanelcomplexes have been reported in the ratios of 3: 3,2: 1, and 2: 2correspondiing to the frameworks (1)-(111) '-, [the formalcharges of the silver(1) compounds are omitted].Silver(1) isknown to co-ordinate up to four unidentate phosphorus-donor ligands. The structures (1)-(111) involve Ag-P ratios of1 : l or 1.2.We report here on the preparation and n.m.r. spectroscopy of[Ag,(~-d~pm),]~ +, framework (IV), in which two metal centresare bridged by three dppm ligands. Examples of this structuraltype are rare: [Pt,(p-dppm),] has been thoroughly character-ized by X-ray methods and multinuclear n.m.r.~pectroscopy.~ Acompound, [Pd,(dppm),], has been formulated on the basis ofelemental analysis and its chemical reaction^-^ Very recentlythe structure of [Au,(p-dmpm),12 +, involving the methylanalogue of dppm [dmpm = bis(dimethylphosphino)methane]has been determined.*Results and DiscussionThe reaction of Ag[O,SCF,] and dppm in the ratio of 2: 3, i.e. aAg-P ratio of 1 : 3, afforded the new homobimetallic complex[Ag2(p-dppfn),][0,SCF3]2 (1) as a colourless, crystalline, air-stable and light-insensitive solid in quantitative yield. Complex(1) is readily soluble in CH2Cl,, acetone, Me,SO and di-methylformamide.The structure of (1) in solution was determined by 31P n.m.r.spectroscopy; the ,'P-{ 'H} n.m.r. spectrum (145.8 MHz)recorded at 213 K is shown in Figure 1.The spectrum arisesfrom the superposition of three different spin systems: ([AI3X),,([A],Y),, and [A],[A*],XY,t attributable to the isotopomers['07Ag2(p-dppm),]2 + (abundance 26.9%), ['"Ag,(p-dppm),12+ (23.2"/,), and [107Ag109Ag(p-dppm)3]2+ (49.9%)respectively. The spectrum is incompletely resolved at this field.Very similar spectra were obtained, however by computersimulation (using the program CYMPLOT) l o using theparameters 'J('07AgP) = 308, 2J(PCP) = 150 Hz. Althoughthe simulated spectra are sensitive to the magnitude of ,J-(PAgP) only values outside a range of 7 k 1 3 0 Hz give simul-ations that are recognizably different to the experimentalresults [Figure l(u)].The chemical shift of (1) is 2.7 p.p.m.The value of 'J(lo7AgP) falls within the range observed forcationic complexes [Ag(PR,),] + (R = alkyl or aryl).'vp PAP P I IAg AgPVPThe ,'P-{ 'H} n.m.r. spectra of (1) in the temperature range298-223 K recorded at 32.4 MHz are shown in Figure 2. Thestructure of (1) is kinetically stable on the n.m.r. time-scale attemperatures below 233 K. At ambient temperature onebroadened signal indicates intermolecular ligand exchange. Atintermediate temperatures there is intramolecular inter-metalexchange of the phosphorus atoms of the p-dppm ligands. Thismechanistic information is obtained from the spectra at 253-293 K (Figure 2): only part of the multiplet collapses withincreasing temperature, whereas the other part remains att Nomenclature from ref.9. Scheme1710cJ. CHEM. SOC. DALTON TRANS. 1988H rFigure 1. (a) Experimental 31P-{ 'H} n.m.r. spectrum (145.8 MHz, 213K) and (b) simulated spectrum of [Ag2(p-dppm),][03SCF,1,constant separation. The presence of the invariant lines ischaracteristic of a mutual exchange process l 2 and they may beassigned to phosphorus transitions involving the silver spin293, and (g) 298 KdFigure 3. 31P-{1H} N.m.r. spectrum (32.4 MHz) of [Ag(dppm),]+ at193 Kstates aa and pp in (1). The collapsing lines are associated withthe silver spin states ap and pa which are permuted by theinterchange of the phosphorus atoms. A possible mechanism(indicated in the Scheme) involves [Ag,(p-dppm),(dppm-PP)12+ as the intermediate.A process which interchanges the phosphorus atoms ofbridging dppm does not appear to have been describedpreviously.Another intramolecular fluxional process involvinJ. CHEM. SOC. DALTON TRANS. 1988 171 1bridging dppm is the phosphine exchange at one of the metals ofa MM'(p-dppm), ring which has been proposed to result in asingle PCH, P resonance.' Fluxionality involving unidentatedppm is more frequently observed intramolecular 'end-over-end' exchange has ample pre~edent.'~ Recently, intramolecularexchange of dppm-PP' and dppm-P at one metal centre hasbeen reported. The structure [Ag,(p-dppm),12 + (IV) is alsopresent in co-ordinating solvents, e.g. dimethylformamide.A complex of stoicheiometry [Ag(dppm),][O,SCF,] (cor-responding to an Ag-P ratio of 1 : 4) does not exist: according to3 1 P n.m.r.studies the disproportionation shown below takesplace in solutions of this composition. [Ag(dppm),] + is alsoproduced as the main product in solutions of compositionAg[O,SCF,]-dppm = 1 : 3 but it could not be obtained ana-lytically pure. Two structures can be proposed for this speciesinvolving either four-co-ordinate silver, [Ag(dppm-PP') (dppm-P),] + or three-co-ordinate silver, [Ag(dppm-P),] +. The ,lP-{'H) n.m.r. spectrum of [Ag(dppm),]+ [Figure 3: 193 K,CH,Cl,, 32.4 MHz; 6 - 10.5, J('07Ag31P) 156 Hz] is thoughtto show the fast-exchange limit of an intramolecular exchangeof the co-ordinated and the free phosphorus atoms of dppm.The slow-exchange limit could not be attained.that prevents the formation of acomplex [Ag(dppm-PP'),] + : dppm chelate rings exhibitP-M-P angles of 67-74".This implies a ring strain whichshould be greater for metals involving a tetrahedral co-ordination geometry (e.g. d" Ag', ideal bonding angle 109O)than for metals involving square-planar or octahedral co-ordination geometry (ideal bonding angle 90O). On the otherhand, there is reduced ring strain associated with the bridgingbonding mode of dppm in (1).The situation found for d' Ag' largely parallels that reportedfor d l o Pt': the compound [Pt(dppm-PP'),] does not exist. Thereduction of [Pt(dppm-PP'),I2+ leads to [Pt,H(dppm-P)(p-dppm),]+ which is formally the protonated form of [Pt2(p-dppm),].' ' In contrast, d l o Hg" forms a complex [Hg(dppm-PP'),]'',' but we were unable to obtain [Hg2(p-dppm),l2+It is probably ring strainExperimentalThe n.m.r.spectra were recorded using Bruker WP-80 and WM-360 spectrometers. Chemical shifts are defined as positive tohigh frequency of the reference, 85% H,PO,; coupling constantsare in Hz. Chemical analyses were performed on a Heraeus EA415 instrument. All reagents were used as commerciallyavailable and experiments were carried out under nitrogen.Preparation o j TrisCp-bis(diphenyIphosphino)methane]-disilver( I) Trifluoromethanesulphonate, (1)-A mixture ofAg[O,SCF,] (0.1 mmol, 25.7 mg) and dppm (0.15 mmol, 57.6mg) in dichloromethane ( 1 cm3) was stirred until a clearsolution was obtained. Evaporation of the solvent revealed[Ag,(p-dpprn),][O,SCF,], as a CH,Cl, solvate.These crystalsreadily lost CH,Cl, on exposure to the atmosphere. Meth-anol ( 1 cm3) was added and the resulting mixture was stirred for15 min. The solvent was removed in vacuo. Recrystallisation ofthe solid residue from methanol afforded colourless crystals ofnon-solvated (l), m.p. 260-265 OC, in almost quantitative yield(Found: C, 55.3; H, 4.2. Calc. for C,,H,,Ag,F,O,P,S,: C, 55.4;H, 4.0%).References1 A. A. M. Aly, D. Neugebauer, 0. Orama, U. Schubert, and H.Schmidbaur, Angew. Chem., Znt. Ed. Engl., 1978, 90, 125; U.Schubert, D. Neugebauer, and A. A. M. Aly, Z. Anorg. 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Commun., 1979, 93 1.Received 3 1st March 1987; Paper 7157