Holger Kopacka

IMIDAZOLINE-2-YLIDENE METAL COMPLEXES WITH PENDANT FERROCENYL SUBSTITUENTS

Abstract Imidazolium and benzimidazolium salts with ferrocenylated alkyl side chains are prepared as progenitors of N-[(ferrocenyl)alkyl](benz)imidazoline-2-ylidene carbenes, which form metal complexes of W(0), Pd(II), and Hg(II). X-ray structures and other spectroscopic properties show that these carbene precursors and complexes are analogous to published non-ferrocenylated complexes with regard to the coordination sphere at the metal, but different in terms of steric protection of the central metal by the bulky ferrocenyl substituents and in terms of distinguishable redox potentials of the ferrocene/ferrocenium couple.

N-HETEROCYCLIC CARBENES WITH N-FERROCENYL-N'-METHYL-SUBSTITUTION : SYNTHESIS, REACTIVITY, STRUCTURE AND ELECTROCHEMISTRY

Abstract Benzimidazolium salts with one N -ferrocenyl substituent and with one N -methyl substituent are prepared by a synthetic sequence involving coupling of the metallocenyl group to a non-cyclic orthophenylene-diamine precursor, ring closure, and oxidation. From these carbene progenitors a range of carbene derivatives are synthesized, including thiourea, azine, and metal complexes with W(0), Pd(II) and Hg(II). Spectroscopic, structural and electrochemical properties in this series of compounds are compared with those of recently published analogous but methylene-spacered benzimidazoline-2-ylidene derivatives, indicating significant electronic communication between the carbene moiety and the N -ferrocenyl substituent.

Functionalized pentamethylferrocenes: Synthesis, structure, and electrochemistry

Abstract The advantageous properties of the Cp * ligand — intensified electron donation, steric bulk, and enhanced solubility in comparison to the ubiquitous Cp ligand — are finding increasing use in organometallic chemistry. A systematic evaluation of synthetic routes to pentamethylferrocene compounds with a wide range of functionalities, including carboxyl, carbonyl, aminomethyl, vinyl, ethynyl, fulvenyl, cyclopentadienylmethyl, and others is reported. Spectroscopic, structural, and electrochemical properties of such functionalized pentamethylferrocenes Fc */2 —R are compared to those of non-methylated ferrocenes Fc—R. The electronic influence of the Cp * ligand in these unsymmetricalferrocenes Fc */2 —R has been studied by cyclic voltammetry measurements, demonstrating a decrease in oxidation potential of −0.276 V in direct comparison to non-methylated ferrocenes Fc—R.

Synthesis and electrochemistry of ferrocenyldiazabutadiene metal carbonyl complexes (Fc-DAB)M(CO)4 [M=Cr,Mo,W]

Abstract The synthesis, characterization (UV–Vis, IR, MS, NMR), and electrochemistry (CV, DPV) of ferrocenyl diazabutadienes and their chromium, molybdenum, and tungsten tetracarbonyl complexes are reported in this study. The properties of these compounds are compared to those of p -methoxyphenyl diazabutadiene analogues, (a) allowing the clear distinction between ferrocene-based and diazabutadiene metal carbonyl localized redox processes in these multimetallic complexes, and (b) showing that the electronic interaction between the peripheral ferrocenyl substituents increases upon complexation to the Group 6 transition metal tetracarbonyl moieties.

Synthesis, platinum(II) complexes and structural aspects of the new tetradentate phosphine cis,trans,cis-1,2,3,4-tetrakis(diphenylphosphino)cyclobutane

Abstract Several novel dimers of the composition [M 2 Cl 4 ( trans -dppen) 2 ] (M=Ni ( 1 ), Pd ( 2 ), Pt ( 3 )) containing trans -1,2-bis(diphenylphosphino)ethene ( trans -dppen) have been prepared and characterized by X-ray diffraction methods, NMR spectroscopy ( 195 Pt{ 1 H}, 31 P{ 1 H}), elemental analyses, and melting points. The intramolecular [2+2] photocycloaddition of the two diphosphine-bridges in 3 produces [Pt 2 Cl 4 (dppcb)] ( 4 ), where dppcb is the new tetradentate phosphine cis,trans,cis -1,2,3,4-tetrakis(diphenylphosphino)cyclobutane. Neither 1 nor the free diphosphine trans -dppen shows this reaction. In the case of 2 the photocycloaddition is slower than in 3 . This difference can be explained by the shorter distance between the two aliphatic double bonds in 3 than in 2 , but also different transition probabilities within ground and excited states of the used metals could be involved. Furthermore, variable-temperature 31 P{ 1 H} NMR spectroscopy of 2 or 3 reveals a negative activation entropy of 2 for the [2+2] photocycloaddition, but a positive of 3 . The removal of chloride from 4 by precipitating AgCl with AgBF 4 , and subsequent treatment with 2,2′-bipyridine (bipy) or 1,10-phenanthroline (phen) leads to [Pt 2 (dppcb)(bipy) 2 ](BF 4 ) 4 ( 5 ) and [Pt 2 (dppcb)(phen) 2 ](BF 4 ) 4 ( 6 ), respectively. In an analogous reaction of 4 with PMe 2 Ph or PMePh 2 , [Pt 2 (dppcb)(PMe 2 Ph) 4 ](BF 4 ) 4 ( 7 ) and [Pt 2 (dppcb)(PMePh 2 ) 4 ](BF 4 ) 4 ( 8 ) are formed. Complexes 1 – 8 show square–planar coordinations, where the compounds 4 – 8 have also been characterized by the above mentioned methods together with fast atom bombardment mass spectrometry ( 7 , 8 ). The crystal structure of 4 reveals two conformations, which arise from an energetic competition between the sterical demands of dppcb and an ideal square–planar environment of Pt(II). The free tetraphosphine dppcb can be obtained easily from 4 by treatment with NaCN. It has been characterized fully by the above methods including 13 C{ 1 H} and 1 H NMR spectroscopy. The X-ray structure analysis shows the pure MMMP-enantiomer in the solid crystal, which is therefore optically active. This chirality is induced by a conformation of dppcb, where all four PPh 2 groups are non-equivalent. Variable-temperature 31 P{ 1 H} NMR spectroscopy of dppcb confirms this explanation, since the single signal at room temperature is split into two doublets at 183 K. The goal of this article is to demonstrate the facile production of a new tetradentate phosphine from a diphosphine precursor via Pt(II) used as a template.

Novel route to carbodiphosphoranes producing a new P,C,P pincer carbene ligand

The reaction of PdCl2, dppm and CS2 in CH(2)Cl(2)/MeOH results in the palladium carbodiphosphorane complex [Pd(Ph(2)PCH(2)Ph(2)PCPPh(2)CH(2)PPh(2)-P,C,P)Cl]Cl.

Semimasked 1,1′-diethynylferrocenes: synthetic concepts, preparations, and reactions

Abstract Due to the inherent instability of 1,1′-diethynylferrocene, the respective coupling chemistry for the access of oligonuclear systems requires stepwise preparative sequences involving consecutive ethyne deprotection, or the conversion of latent ethyne precursor functionalities, respectively. The new derivatives 1-acetyl-1′-ethynylferrocene, 3 , and, preferably, 1-ethynyl-1′-formylferrocene, 9 , turned out to be the most favorable starting compounds. The subsequent synthetic chemistry, as well as the X-ray structures of selected starting and target derivatives are presented. A unique intramolecular coupling product, 12 , represents the first ladder-type tricyclic metallocenophane system exhibiting high ring strain. Supplementary novelties concerning monoacetylenic parent systems are also presented.

Palladium(II), platinum(II), and platinum(IV) complexes containing trans-1,2-bis(diphenylphosphino)ethene or cis,trans,cis-1,2,3,4-tetrakis(diphenylphosphino)cyclobutane: complete X-ray structural characterization of binuclear compounds

Abstract Several novel binuclear PdII, PtII, and PtIV complexes of trans-1,2-bis(diphenylphosphino)ethene (trans-dppen) or cis,trans,cis-1,2,3,4-tetrakis(diphenylphosphino)cyclobutane (dppcb) have been prepared and characterized by X-ray diffraction methods, 195Pt{1H} and 31P{1H} NMR spectroscopy, FAB mass spectrometry, IR spectroscopy, elemental analyses and melting points. The X-ray structure and NMR parameters of [Pt2I4(trans-dppen)2] (1) confirm that homobimetallic complexes of the type [M2L4(trans-dppen)2] (M=Ni, Pd, Pt; L=Cl−, I−, CN−) contain two square planar coordination units joined by two trans-dppen bridges in the solid as well as in the solution state. An analogous structure type is retained in the PtIV compound [Pt2Cl8(trans-dppen)2] (2). In contrast to the corresponding PtII complex of the new tetradentate phosphine dppcb, the X-ray structure of [Pd2Cl4(dppcb)] (3) reveals only one conformation. This can be explained by the longer Pd–P bonds compared with the Pt–P bonds and the weaker square planar stabilization energy in 3. In 3 dppcb acts as a binuclear tetraligate single-bridging ligand combining two square planar coordination centres. The reaction of Pd(CN)2 with dppcb leads to [Pd2(CN)4(dppcb)] (4). The removal of the coordinated chlorides in 3 by AgBF4 followed by subsequent treatment with NaNO2 produces [Pd2(NO2)4(dppcb)] (5). In an analogous reaction with 1,10-phenanthroline (phen) or 2,2′-bipyridine (bipy), [Pd2(dppcb)(phen)2](BF4)4 (6) and [Pd2(dppcb)(bipy)2](BF4)4 (7) are formed. The complexes 4–7 show structure types corresponding to the X-ray structure of 3. The same is true for the treatment of 3 with PMePh2 or PMe2Ph, where [Pd2(dppcb)(PMePh2)4](BF4)2Cl2 (8) and [Pd2(dppcb)(PMe2Ph)4](BF4)4 (9) are obtained. However, the X-ray structure of 8 reveals that the chlorides are coordinated in the solid state, and the crystal structure consists of trans-[Pd2Cl2(dppcb)(PMePh2)2](BF4)2 (10). The flexibility and stereochemical demands of dppcb in the compounds 3–10 and related species are discussed in view of the possible application of PdII complexes containing bidentate tertiary phosphine ligands as catalysts for the alternating copolymerization of ethene and carbon monoxide. In this respect dppcb can be regarded as a combination of two bidentate phosphines, where the cyclobutane ring corresponds to a relatively rigid ligand backbone. This produces strain in the five-membered rings of 3–10 which is released by ‘envelope’-folding in the X-ray structures of 3 and 10.

First true square-planar Hg(II) compound: synthesis and full characterization of trans-[Hg{Ph2PNP(O)Ph2-P,O}2]

Abstract The reaction of Hg(O3SCF3)2 with bis(diphenylphosphino)amine, Ph2PNHPPh2 (dppam), produces the novel, rare face-to-face complex [Hg2(O3SCF3)4(dppam)2] (1). Treatment of 1 with Na2N2O3 leads to trans-[Hg{Ph2PNP(O)Ph2-P,O}2] (2) via regioselective oxidation and simultaneous deprotonation of dppam. 2 is the first true square-planar Hg(II) compound. In 2 the coordination plane and the five-membered rings of the HgNOP2 moieties are completely coplanar indicating strong π-bonding interactions. Both 1 and 2 have been fully characterized by X-ray structure analyses, NMR spectroscopy ( 199 Hg { 1 H }, 31 P { 1 H }, 13 C { 1 H }, 1 H ) , ESI and FAB mass spectrometry, IR spectroscopy, elemental analyses, and melting points. Since in Hg(II) compounds relativistic effects favour linear coordination and Hg(II) donor atom preferences play a significant role, the Hg–P bond length of 2.4042(7) A in 2 is short, whereas the Hg–O bond length of 2.7138(15) A is long. In view of the X-ray structures of 1 and 2, it is shown that the delocalized charge in [Ph2PNP(O)Ph2]− is responsible for the achievement of the square-planar coordination in a Hg(II) compound. A similar π-bonding effect has been observed in several square-planar complexes of Pt(II), Pd(II), and Ni(II) containing cis-1,2-bis(diphenylphosphino)ethene (cis-dppen).

Solution and solid state behaviour of binuclear mercury(II) compounds containing cis,trans,cis-1,2,3,4-tetrakis(diphenylphosphino)cyclobutane: first X-ray structural characterisation of mercury(II) complexes with two different chelating phosphines

Abstract Several novel binuclear Hg II complexes of cis , trans , cis -1,2,3,4-tetrakis(diphenylphosphino)cyclobutane (dppcb) have been prepared and characterised by X-ray diffraction methods, NMR spectroscopy ( 199 Hg{ 1 H}, 31 P{ 1 H}, 1 H), FAB mass spectrometry, IR spectroscopy, elemental analyses and melting points. The tetrahedral coordination of both the Hg II centres in the homobimetallic compounds [Hg 2 L 4 (dppcb)] (L=Cl − ( 1 ), Br − ( 2 ), CN − ( 3 ), NO 3 − ( 4 )), synthesised by the reaction of HgL 2 with dppcb, is indicated by their solution NMR parameters and is confirmed by the X-ray structures of 1 – 3 . Though the Fermi contact term is not always dominant in determining 1 J (Hg,P), the NMR parameters are correlated to the changes in the bond lengths and angles in 1 – 3 . A comparison is given with correlations derived from similar complexes. The reaction of 4 with 2,2′-bipyridine (bipy) or 1,10-phenanthroline (phen) leads to [Hg 2 (dppcb)(bipy) 2 ](NO 3 ) 4 ( 5 ) and [Hg 2 (dppcb)(phen) 2 ](NO 3 ) 4 ( 6 ). Also, for 5 and 6 the NMR data and FAB mass spectra are in agreement with tetrahedral Hg II centres. The treatment of 4 with monophosphines produces trans -[Hg 2 (NO 3 ) 2 (dppcb)L′ 2 ](NO 3 ) 2 (L′=P(CH 2 Ph) 3 ( 7 ), P(CH 2 CH 2 CN) 3 ( 8 ), PPh 3 ( 9 )). In 7 – 9 the typical large 1 J (Hg,P) values are observed for the monophosphines compared with the corresponding parameters for chelating dppcb, which are reduced due to the five-membered ring formation. In the reaction of 4 with the diphosphine Ph 2 PCH 2 PPh 2 (dppm) and the subsequent metathesis with LiAsF 6 , trans -[Hg 2 (NO 3 ) 2 (dppcb)(η 1 -dppm) 2 ](AsF 6 ) 2 ( 10 ) is formed. The X-ray structure of 10 showing coordinated and dangling phosphorus atoms of dppm is the first complete characterisation of a Hg II complex containing two different chelating phosphines. Though in solution the dppm ligands are involved in fast intramolecular end-over-end exchange, the solution structure of 10 corresponds to its solid state structure, which is indicated by unusual 1 J (Hg,P) values. Catalytic amounts of Hg II convert trans -[Hg 2 (NO 3 ) 2 (dppcb)(η 1 -dppm) 2 ](NO 3 ) 2 into trans -[Hg 2 (NO 3 ) 2 (dppcb)(η 1 -P-dppmO) 2 ](NO 3 ) 2 ( 11 ), where dppmO is Ph 2 PCH 2 P(O)Ph 2 . The X-ray structure of 11 is the first complete characterisation of a Hg II compound consisting of chelating phosphine together with phosphinoyl moieties. The solid state structure and the solution NMR parameters of 11 clearly show the presence of a dangling P(O)Ph 2 group. The complexes 1 – 11 illustrate the tendency that polydentate donor ligands often geometrically and entropically restrict the number of accessible structures for Hg II . Especially, the X-ray structures of 1 – 3 , 10 , and 11 indicate the preference of Hg II for tetrahedral and trigonal pyramidal coordinations in compounds containing dppcb.