Bruno Crociani

PREPARATION AND REACTIONS OF PALLADIUM(0)-OLEFIN COMPLEXES WITH IMINOPHOSPHINE LIGANDS

Abstract The complexes [Pd(η2-ol){o-(Ph2P)–C6H4–CHNR}] [ol, dimethyl fumarate (dmf), 1,4-naphtoquinone (nq), fumaronitrile (fn); R=C6H4OMe-4, CMe3, Me, bornyl] can be prepared in good yields from the reaction of the allyl derivatives [Pd(η3-C3H5) {o-(Ph2P)–C6H4–CHNR}]BF4 with an excess of NHEt2 in the presence of the activated olefin ol. The complex [Pd(η2-ma){o-(Ph2P)–C6H4-CHNC6H4OMe-4}] (ma, maleic anhydride) is more conveniently obtained via olefin substitution from [Pd(η2-dmf){o-(Ph2P)–C6H4–CHNC6H4OMe-4}]. The α-diimine ligand of [Pd(η2-fn)(py-2-CHNC6H4OMe-4)] is quantitatively displaced by the appropriate iminophosphine to give [Pd(η2-fn){o-(Ph2P)–C6H4–CHNC6H4OMe-4}]. The new zerovalent complexes with P–N ligands are characterized by multinuclear NMR spectroscopy. In solution, olefin rotation or olefin exchange are generally slow. The compound [Pd(η2-fn){o-(Ph2P)–C6H4–CHNC6H4OMe-4}] reacts with a second molecule of iminophosphine yielding [Pd(η2-fn){o-(Ph2P)–C6H4–CHNC6H4OMe-4}2] in which the iminophosphines act essentially as P-monodentate ligands. [Pd(η2-dmf) {o-(Ph2P)–C6H4–CHNC6H4OMe-4}] undergoes fast oxidative addition of allyl chloride to [Pd(η3-C3H5){o-(Ph2P)–C6H4–CHNC6H4OMe-4}]+.

Far-infrared study of palladium(II)–halogen complexes with chelating ligands containing nitrogen and σ carbon as donor atoms

I.r. spectra of palladium(II) halogeno-bridged complexes of the type [Pd(N–C)X]2= azobenzene-2-C,N; NN-dimethylbenzylamine-2-C,N; and 2-methoxy-3-NN-dimethylaminopropyl; X = Cl or (Br) have been recorded in the range (600–120 cm.–1). The bridging-halogen stretching frequencies are discussed on the basis of a planar asymmetric Pd[graphic omitted]Pd unit.Bridge-splitting reactions with mono- and bi-dentate ligands bearing nitrogen or phosphorous as donor atoms are reported and the structures of the resulting products are interpretated on the basis of their far-i.r. spectra. We have also established that the v(Pd–Cl) frequency in the products in the range 300–280 cm.–1 with chloride in the trans-position to the phenyl or alkyl groups.

Some palladium(II) and platinum(II) lead bonded complexes

Abstract Complexes of the type M(PPh 3 ) 2 (PbPh 3 ) 2 [M = Pd, (Ia) and Pt, (Ib)] have been prepared by oxidative addition of hexaphenyldilead to M(PPh 3 ) 4 . The compound Pt(PPh 3 ) 2 (PbPh 3 ) 2 , (Ib), slowly decomposes in dichloromethane to give cis -Pt(PPh 3 ) 2 (PbPh 3 )Ph, (II). which can also be obtained by treating (Ib) with the stoichiometric amount of LiPh. Reaction of Pt(PPh 3 ) 4 with hexamethyldidead gives the complex Pt(PPh 3 ) 2 (PbMe 3 )Me directly. The MPb bonds are easily cleaved by bromine, iodine and hydrogen bromide. The X-ray structure of (II) has been determined using three-dimensional counter data and refined by the least-square method ( R = 0.07). The crystals are monoclinic a = 22.501, b = 10.502, c = 24.120 A, β = 113.43°, space group P 2 1 / c with Z = 4. The complex exhibits a cis configuration, with the coordination around the platinum atom essentially square-planar: the PtPb and PtC(phenyl)bond lengths are 2.698(1) and 2.055(3)A, respectively.

Reactions of 1,4-diazubutadienes with chloro-bridged palladium(II) and platinum(II) allyl derivatives

Abstract The 1,4-diazabutadienes (or α-diimines) RNC(R′)C(R″)NR, DAB, (R = p -C 6 H 4 OMe, R′ = R″ = H, DAB I ; R = p -C 6 H 4 OMe, R′ = H, R″ = Me, DAB II ; R = p -C 6 H 4 OMe, R′ = R″ = Me, DAB III ; R = t-Bu, R′ = R″ = H, DAB IV ) react with the allylic compounds [PdCl(η 3 -2-Y-C 3 H 4 )] 2 (Y = H: all; Y = Me: Meall) and [PtCl(C 3 H 5 )] 4 in the presence of NaClO 4 yielding the cationic complexes [M(η 3 -2-Y-C 3 H 4 )(DAB)]ClO 4 (M = Pd, I; M = Pt, II). The 1 H and 13 C NMR spectra indicate a σ,σ′-N,N′ chelation of DAB. In the complexes with the asymmetric ligand DAB II a fast syn-syn, anti-anti , exchange of the allylic protons occurs at room temperature in CDCl 3 . In acetonitrile a partial dissociation of DAB is observed, with the following order of stability constants; DAB III > DAB I and Pt ⪢ Pd. In the absence of NaClO 4 , equilibria are established involving the starting reactant [PdCl(η 3 -Meall)] 2 , binuclear species {[PdCl(η 3 -Meall)] 2 (DAB)}, III (in which DAB acts as a bidentate bridging ligand) and ionic complexes [Pd(η 3 -Meall)(DAB)] + [PdCl 2 (η 3 -Meall)] − , IV. These equilibria were studied in various solvents by variable temperature 1 H NMR spectra, electronic spectra, molecular weight and conductivity measurements. The complexes III can be isolated as solids and are the predominant species in concentrated solution only with DAB I and DAB IV , both having R′ = R″ = H. With DAB III , the predominant species in CDCl 3 at −40 °C is a complex of type IV. A similar compound, [Pd(η 3 -Meall)(bipy)][PdCl 2 (η 3 -Meall)], is also obtained in the reaction with 2,2′-bipyridine.

Structure of trans-chloro-1,4-bis(ϱ-methoxyphenyl)-1,4-diaza-3methylbutadiene-2-yl-bis(triphenylphosphine)-palladium(II)

Abstract The structure of the compound trans-[PdCl { C(N-ϱ-C 6 H 4 OMe)C (Me)N-ϱ-C 6 H 4 OMe} (PPh 3 ) 2 ] was solved, using a conventional combination of Patterson and Fourier functions, least-squares refinements and electron density difference maps, to a reliability index R of 0.069 for the 2923 observed reflections collected by four-circle diffractometer. The palladium arom is surrounded in a roughly planar fashion by two trans phosphorus atoms, a chlorine atom, and a σ-bonded carbon atom of the diazabutadienyl group. This group assumes a trans configuration, the NCCN fragment being virtually planar and nearly normal to the mean coordination plane. The Pdligand bond lengths are: PdC 1.98(1), PdCl 2.41(1),PDP(1) 2.33(1) and PdP(2) 2.35(1) A.

Reactions of 1,4-diaza-3-methylbutadien-2-ylpalladium(II) derivatives with chloro-bridged rhodium(I) complexes

The reactions of the organometallic 1,4-diazabutadienes, RN=C(R′)C(Me)=NR″ [R = R″ = p-C6H4OMe, R′ = trans-PdCl(PPh3)2 (DAB); R = p-C6H4OMe, R″ = Me, R′ = trans-PdCl(PPh3)2 (DABI; R = R″ = p-C6H4OMe, R′ = Pd(dmtc)-(PPh3), dmtc = dimethyldithiocarbamate (DABII); R = R″ = p-C6H4OMe, R′ = PdCl(diphos), diphos = 1,2-bis(diphenylphosphino)ethane (DABIII)] with [RhCl(COD)]2 (COD = 1,5-cyclooctadiene, Pd/Rh ratio = 12) depend on the nature of the ancillary ligands at the Pd atom in group R′. In the reactions with DAB and DABI transfer of one PPh3 ligand from Pd to Rh occurs yielding [RhCl(COD)(PPh3)] and the new binuclear complexes [Rh(COD) {RN=C(R‴)-C(Me)=NR″}], in which the diazabutadiene moiety acts as a chelating bidentate ligand. Exchange of ligands between the two different metallic centers also occurs in the reaction with DABII. In this case, the migration of the bidentate dmtc anion yields [Rh(COD)Pdmtc] and [Rh(COD) {RN=C(R‴)C(Me)=NR″}]. In contrast, the reaction with DABIII leads to the ionic product [Rh(COD)- (DABIII)][RhCl2(COD)], with no transfer of ligands. The cationic complex [Rh(COD)(DABIII)]+ can be isolated as the perchlorate salt from the same reaction (Pd/Rh ratio = 1/1) in the presence of an excess of NaClO4. In all the binuclear complexes the coordinated 1,5-cyclooctadiene can be readily displaced by carbon monoxide to give the corresponding dicarbonyl derivatives. The reaction of [RhCl(CO)2]2 with DAB and/or DABI yields trinuclear complexes of the type [RhCl(CO)2]2(DAB), in which the diazabutadiene group acts as a bridging bidentate ligand. Some reactions of the organic diazabutadiene RN=C(Me)C(Me)=NR (R = p-C6H4OMe) are also reported for comparison.

Iminophosphine-palladium(0) complexes as catalysts in the alkoxycarbonylation of terminal alkynes

Abstract In the presence of methanesulfonic acid, the palladium(0)-olefin complexes: [Pd(η 2 -ol)(P N)] [ol=dimethyl fumarate or fumaronitrile, P N=1-(Ph 2 P)C 6 H 4 -2-CH NR (R=CMe 3 or C 6 H 4 OMe-4)] catalyse the alkoxycarbonylation of terminal alkynes. Moderately good rates are obtained when the catalysts are promoted with two equivalents of the free P N ligand and a large excess of acid at 120°C. The catalytic data suggest that derivatives of the type [Pd(alkyne)(P N) n ] ( n =2–3) are the active catalytic species.

Structural studies on iminophosphine ligands and their palladium complexes

The crystal and molecular structures of the iminophosphine o-(Ph2P)C6H4CH=NC6H4OMe-4 (1) and its palladium complexes [Pd(η3-C3H5){o-(Ph2P)C6H4CH=NC6H4OMe-p}]BF4 (2) and [Pd(η2-fn){o-(Ph2P)C6H4CH=NC6H4OMe-4}] [fn = fumaronitrile, (3)] have been determined by X-ray analysis. In the free ligand (1), the planar imino group of E configuration is oriented, relative to the PPh2 unit, so that the CH=N hydrogen atom points towards phosphorus, with the nitrogen atom on the opposite side. In (2) and (3) the iminophosphine behaves as a P,N-chelate ligand, this coordination mode being achieved by the imino group rotation of 169.3 ° and 145.3 °, respectively, around its bond with the ortho disubstituted phenyl ring. Complex (2) shows a structural disorder with two different orientations of the allyl ligand. The trigonal planar coordination around the central metal in complex (3) involves the P- and N-donor atoms of (1) and the η2-bound olefin, with a marked lengthening of the olefinic carbon-carbon bond. In both the complexes, the chelate six-membered ring of the iminophosphine with palladium is not coplanar with the N-Pd-P coordination plane, the imino carbon atom and the ortho disubstituted phenyl group lying on the same side out of the N-Pd-P plane, whereas the N-substituent and one of the PPh2 groups are on the opposite side. The 1H-n.m.r. spectra at low temperatures of (2) and (3), and of [Pd(η2-tmetc){o-(Ph2P)C6H4CH=NCMe3}] [tmetc = tetramethyl ethylenetetracarboxylate, (4)] are interpreted on the basis of a non-rigid conformation of the chelate iminophosphine, which undergoes a fast dynamic process whereby the N- and P-substituents move above and below the coordination plane.

Binuclear complexes of 1,4-diaza-3-methylbutadien-2-Yl-palladium(II) derivatives with palladium(II) and platinum(II) chlorides

The organometallic 1,4-diazabutadienes, RN=C(R′)C(Me)=NR [R = p-C6H4OMe, R′ = trans-PdCl(PPh3)2 (DAB); PdCl(LL), LL = 1,2-bis(diphenylphosphino)-ethane (DABI), LL = cis-1,2-bis(diphenylphosphino)ethylene (DABII); Pd(dmtc)-(PPh3), dmtc = dimethyldithiocarbamate (DABIII)] react with ethylene or nitrile derivatives of palladium(II) and platinum(II), [PdCl2(CH2 = CH2)]2, K[PtCl3-(CH2=CH2)], [PdCl2(NCMe)2], [PtCl2(NCPh)2] usually to give binuclear complexes of the type [MCl2{RN = C(R′)C(Me)=NR}] (M = Pd, Pt), in which the 1,4-diazabutadiene group acts as a chelating bidentate ligand. The stability of these compounds in hot 1,2-dichloroethane or acetonitrile markedly depends on the nature of the ancillary ligands on the palladium atom of the group R′. When a chelating diphosphine is present, as in the case of DABI and DABII, the corresponding binuclear complexes are recovered unchanged even after long refluxing times. In the case of DAB and DABIII adducts, a transfer of PPh3 and dmtc ligands from the Pd atom of R′ to the metal atom M of the coordinated MCl2 unit occurs with the formation of trans-MCl2(PPh3)2] and of the new bimetallic complexes, [M(dmtc) {RN=C(R″)C(Me)=NR}] (R″ = cis-PdCl2(PPh3) (DABIV)), respectively. The rates of ligand transfer for the PdCl2 adducts are much higher than for the PtCl2 analogues.

Phenylation of cationic allyl palladium(II) complexes by tetraphenylborate. Synthesis of α-diimine olefin palladium(0) complexes and mechanistic aspects

The cationic allyl complexes [Pd(η3-2-R1C3H4)(N–N′)]+(N–N′=α-diimine ligand; R1= H or Me) react with BPh4– in the presence of activated olefins to give [Pd(η2-olefin)(N–N′)](olefin = fumaronitrile, dimethyl fumarate or maleic anhydride) and PhCh2C(R1)CH2. The palladium(0) derivatives can be isolated in good yield and have been characterized by elemental analysis, molecular weight measurements and standard spectroscopic techniques. The reaction rates increase with increasing π-accepting ability of the α-diimine, with decreasing steric requirements of the imino carbon substituents and with decreasing stability towards palladium–nitrogen bond breaking in the parent cationic compounds. The rates also increase with decreasing relative permittivity and co-ordinating properties of the solvent. Kinetic measurements in aqueous (2% v/v) methanol provide pseudo-first-order rate constants that are independent of both BPh4– and olefin concentrations. This has been interpreted on the basis of extensive ion pairing between the cationic substrate and the BPh4– anion, followed by rate-determining phenyl transfer to the palladium centre and fast reductive elimination of allylbenzenes.