David A. Bardwell

Donor/acceptor complexes containing ferrocenyl-pyridine ligands attached to a tungsten carbonyl centre: Structural, spectroscopic and electrochemical properties

Abstract A series of ferrocenyl-pyridine ligands (Fc-L) in which a 4-pyridyl side-arm is attached to a ferrocenyl core via a conjugated linker has been attached to tungsten carbonyl cores, giving complexes of the type [(Fc-L)W(CO) 5 ] ( 1–6 ) and [(Fc-L) 2 W(CO) 4 ] ( 7 and 8 ). Three of these complexes were characterised by X-ray crystallography. Complex 2 , [Fc-C(Ph)=CH-C 5 H 4 NW(CO) 5 ]: triclinic, P 1¯; a = 9.329(4), b = 12.115(3), c = 12.745(3) A; α = 66.66(2), β = 71.87(2), γ = 81.40(2)°; Z = 2; 3230 unique data; R 1 = 0.032, wR 2 = 0.090. Complex 3 , [Fc-C(Me)=CH-C 5 H 4 NW(CO) 5 ]: triclinic, P 1¯; a = 11.325(3), b = 12.328(2), c = 16.639(5) A; α = 79.83(2), β = 77.30(2), γ = 82.043(14)°; Z = 4: 5878 unique data; R 1 = 0.035, wR 2 = 0.103. Complex 7 · (CH 2 Cl 2 ) 0.5 , [Fe-CH=CH-C 5 H 4 N 2 W(CO) 4 ·(CH 2 Cl 2 ) 0.5 ]: triclinic, P 1¯; a = 10.290(2), b = 11.751(4), c = 16.328(4) A; α = 88.071(14), β = 83.502(14), γ = 67.50(2)°; Z = 2; 5054 unique data: R 1 = 0.047, wR 2 = 0.125. All of the complexes were fully characterised by 1 H and 13 C NMR, IR, UV—vis and luminescence spectroscopy, FAB mass spectrometry, and electrochemical measurements. The complexes show the expected spectroscopic and electrochemical features for both the ferrocenyl and substituted tungsten carbonyl chromophores. Although the individual molecules possess the necessary structural and electronic features required for second-order non-linear optical behaviour, the fact that the three crystallographically characterised complexes have centrosymmetric space groups precludes bulk solid-state measurements of NLO behaviour.

A STUDY OF CRYSTAL PACKING IN A SERIES OF CLOSELY RELATED SQUARE-PLANAR PALLADIUM(II) AND PLATINUM(II) COMPLEXES

Abstract The crystal structures of the series of four-coordinate complexes [PdL1Cl) · CH2Cl2 (1·CH2Cl2), [PtL1Cl]·CH2Cl2 (2·CH2Cl2), [PdL3Cl][PF6 · CH2Cl2 (3 · CH2Cl2) and [PdL4Cl][PF6 (4) [HL1 = 6-(2-hydroxyphenyl)-2,2′-bipyridine; L3 = 6-(2-dimethylaminophenyl)-2,2′-bipyridine; L4 = 2-(2-dimethylaminophenyl)-1,10-phenanthroline] are compared and contrasted to examine the extent to which aromatic π-stacking interactions contribute to crystal packing. Complexes 1 and 2 have very similar planar molecular structures but stack in different ways; molecules of 1 form a linear stack in which overlap of the aromatic ligands is maximized between adjacent molecules and there is no PdPd interaction, whereas in 2 there are axial PtPt interactions within the linear stack but less overlap between adjacent aromatic ligands. In 3 the ligand L3 is not planar but substantially twisted about the bond between the phenyl and bipyridyl fragments. The phenyl rings of the complex cations form an interleaved stack in the crystal, but the bipyridyl fragments are not involved in any close stacking interactions. Replacing the bipyridyl group by a phenanthrolinyl group in 4 changes the crystal packing; the structure of the individual complex cations is very similar to that of 3, but both fragments of the twisted ligand (the phenyl and phenanthrolinyl parts) are involved in separate stacking interactions with adjacent molecules.

New tricks for an old ligand: cyclometallated and didentate co-ordination of 2,2′ : 6′,2″-terpyridine to ruthenium(II)

Monomethylation of 2,2′ : 6′,2″-terpyridine (terpy) afforded the N-methyl-2,2′ : 6′,2″-terpyridinium cation, [Hmterpy]+. With one of the terminal pyridine ring nitrogen atoms thus protected, it co-ordinates to ruthenium(II) either as a didentate N,N′-donor giving [Ru(terpy)(Hmterpy-N,N′)Cl][PF6]2, or as a cyclometallating terdentate ligand giving [Ru(terpy)(mterpy-N,N′,C)][PF6]2, depending upon the reaction conditions. Both complexes have been fully characterised by spectroscopic and electrochemical methods, and the crystal structures of [Ru(terpy)(mterpy-N,N′,C)][BF4]2·2MeCN and [Ru(terpy)(Hmterpy-N,N′)Cl][PF6]2·2MeCN determined.

Complexes of the terdentate N-donor ligand 6-(2-aminophenyl)-2,2′-bipyridine (L): crystal structures of mononuclear [ZnL2][PF6]2 and tetranuclear [{CuL(MeCN)}4(μ4-PO4)][PF6]5 containing an unusual μ4-bridging phosphate ion

Abstract The new ligand 6-(2-aminophenyl)-2,2′-bipyridine (L) was prepared in high yield by demethylation of 6-(2-N,N-dimethylaminophenyl)-2,2′-bipyridine, and contains two pyridyl donors and one aniline donor. It coordinates as a terdentae N-donor ligand to both first-row and second-row transition-metal ions. The crystal structure of [ZnL2][PF6]2·MeOH reveals that each ligand coordinates meridionally to give a pseudo-octahedral geometry, with the Zn-aniline bonds (2.20 and 2.26 A) being on average slightly longer than the Zn-pyridyl bonds (2.11–2.19 A) and the phenyl rings of the coordinated aniline fragments twisted by ∼40° with respect to the coordinated bipyridyl fragments. Similar complexes [ML2][PF6]2 form with a variety of first-row transition metals, but during recrystallisation of [CuL2][PF6]2 from McCN/diethyl ether small amounts of crystalline [(CuL(MeCN)]4(μ4-PO4)][PF6]5·4MeCN were formed which was also crystallographically characterised. This complex contains four {CuL(MeCN)}2 fragments linked by a central μ4-phosphate ion which donates one oxygen to each copper centre, and is doubly unusual because (i) this is the first discrete coordination complex containing a μ4-phosphate bridge, and (ii) the phosphate ion has arisen from complete metal-catalysed hydrolysis of a [PF6] ion. The electrochemical and spectroscopic properties of [Ru(terpy)L][PF6]2 show that the aniline group is a stronger electron donor and a weaker-field ligand than a pyridyl group.

Co-ordination chemistry of mixed pyridine–phenol ligands; electrochemical, electron paramagnetic resonance and structural studies on monounclear ruthenium(III) and chromium(III) complexes

Syntheses of the new complexes [RuL13]1, [Rul12(acac)]2, [RuL12(bipy)][PF6]3, [CrL13]4, [CrL12(acac)]5 and [CrL22][PF6]6[HL1= 2-(2-hydroxyphenyl)pyridine, HL2= 6-(2-hydroxyphenyl)-2,2′-bipyridine, Hacac = pentane-2,4-dione, bipY = 2,2′-bipyridine] have been carried out. The ruthenium(III) complexes 1–3 all show reversible +3/+4 and +2/+3 waves in their cyclic voltammograms. Together with some previously reported complexes, a full set of electrochemical data is now available for ruthenium complexes with donor sets varying from N6 to N3O3(where N denotes a pyridyl donor and O a phenolate donor); there is a monotonic decrease of 0.75 V in the RuII-RuIII couple per additional phenolate in the co-ordination sphere. Both 1 and 2 have typical rhombic EPR spectra, but the spectrum of 3 is only consistent with formation of a mixture of cis and trans isomers. The chromium(III) complexes show similar variations in the potentials of their metal-centred reductions with donor set although the relationship is not linear. X-Ray analysis showed the both 4 and 6 have pseudo-octahedral crystal structures. Complex 4 crystallises in the chiral space group Fdd2 and therefore spontaneously resolves on crystallisation; 6 has a π-stacking interaction between aromatic residues of adjacent complex units, with an average separation of 3.71 A between the atoms of one aromatic ring and the mean plane of the other.

Coordination chemistry of mixed pyridine-phenol ligands; mononuclear palladium(II) and dinuclear copper(II) complexes of derivatives of bidentate N,O-chelating ligands based on 2-(2-hydroxyphenyl)pyridine

Abstract The new ligands 2-(2-hydroxyphenyl)-4-tbutyl-pyridine (HL1), 2-(2-hydroxyphenyl)-6-methylpyridine (HL2) and 2-(2-hydroxyphenyl)-6-[(2-phenyl)ethyl]pyridine (HL3) were prepared. They are all substituted derivatives of the simple N,O-bidentate chelating ligand 2-(2-hydroxyphenyl)pyridine. HL1 is solubilised derivative bearing a tBu substituent in a position which will not sterically interfere with metal-ion coordination; HL2 and HL3 contain substituents at C6 of the pyridyl ring, adjacent to the N atom, which will therefore sterically hinder metal-ion coordination. The complexes [PdL2] (PdL2] (L = L1, L2 and L3) were prepared and structurally characterised to determine the effects (if any) of the substituents on the structures of the metal complexes. All are four-coordinate complexes with square planar coordination geometry about the Pd(II) ion, but the sterically hindered ligands L2 and L3 have to adopt a different conformation from L1 to allow planar coordination, resulting in “bowl-like” structures. [Cu2(L1)4] was also prepared and found to be a phenolate-bridged dinuclear complex in the solid state, both by X-ray crystallography and EPR measurements. In solution, however, it dissociates to give [Cu(L1)2] monomeric units. Cu(II) complexes of L2 and L3 could not be isolated.

Complexes of thallium(I) and lead(II) with the potentially tetradentate ligand bis[3-(2-pyridyl)-pyrazolyl]dihydroborate

Abstract The complexes [T|;L] and [PbL2] have been prepared and crystallographically characterised, where L− is the potentially tetradentate ligand bis [3-(2-pyridyl)-pyrazolyl]dihydroborate containing two N,N′-bidentate chelating arms linked by a-BH2-fragment. In [T|L] the Tl(I) is coordinated by one tetradentate chelating ligand L−, whose four N donor atoms are approximately coplanar. The Tl(I) ion lies ∼1.4A out of this plane, and the stereochemically active lone pair is assumed to occupy the vacant axial site of the square pyramid. The Tl-N(pyridyl) bonds (2.96–3.17 A) are considerably longer than the TlN(pyrazolyl) bonds (2.61–2.69 A). The molecules lie in a stack along the Tl⋯Tl axis. In [PbL2] there are seven PbN bonds in the range 2.588–2.817 A, which are considered as ‘normal’ PbN interactions, and one much longer interaction (3.055 A) to a pyridyl N atom which, although rather remote, is still oriented towards the metal. The metal ion is therefore ‘7+1’ -coordinate from two tetradentate chelating ligands. Again there is an obvious gap in the coordination sphere which is occupied by a stereochemically active lone pair.

Dinuclear alkoxide-bridged ruthenium(II) complexes with class III mixed-valence states: a structural and spectroelectrochemical study

Reaction of [Ru(bipy)2Cl2](bipy = 2,2′-bipyridine) with NaOMe or KOH in methanol at reflux afforded high yields of the dinuclear complex [{Ru(bipy)2}2(µ-OMe)2][PF6]21. Reaction of 1 with NaOEt in ethanol resulted in exchange of the bridge to give [{Ru(bipy)2}2(µ-OEt)2][PF6]22. The crystal structures of 1 and 2 showed that the complex cations have similar structures in which two Ru(bipy)22+ fragments, which for steric reasons have the same absolute configuration, are linked by two alkoxide bridges. Each dinuclear complex cation is therefore chiral, and there are equal numbers of each enantiomer in the crystals of both 1 and 2. Electrochemical measurements showed that there is a strong electrochemical interaction between the metal centres, with separations of 0.55 and 0.57 V respectively between the two RuII–RuIII couples corresponding to Kc values of 3 × 109 and 6 × 109 for the mixed-valence states of 1 and 2. A spectroelectrochemical study of complex 1 showed that oxidation of the mixed-valence state results in a new transition in the electronic spectrum at ca. 1800 nm with Iµ= 5000 dm3 mol–1 cm–1, which disappears on further oxidation to the RuIII state. The observations that this transition is not solvatochromic, and that the half-width of the peak is much narrower than the value predicted by Hush theory for vectorial intervalence charge-transfer bands, both point to a class III (fully delocalised) mixed-valence state; delocalisation between the dπ orbitals of the two metal centres is facilitated by good overlap with oxygen pπ orbitals in the Ru2O2 core. The magnitude of the electronic interaction Vab is estimated to be 2800 cm–1, similar in magnitude to that of the Creutz–Taube ion.

The coordination chemistry of mixed pyridine-phenol ligands; syntheses and crystal structures of Mn(III) and Ni(II) complexes of 2-(2-hydroxyphenyl)pyridine

Abstract The complexes [Mn(L) 1 ) 3 ] ( 1 ) and [Ni 2 (L 1 2 (dmf) 6 ] [BPh 4 ] 2 ( 2 ) have prepared and crystallographically characterised (L 1 is the N,O-bidentate chelating anion of 2-(2-hydroxyphenyl) pyridine. Complex 1 comprises neutral tris-chelate Mn(III) units, with a mer -N 3 O 3 donor set, which display Jahn-Teller distortion characteristic of the high-spin d 4 configuration. It is typical of other M III (L 1 ) 3 complexes that have recently been characterised, and undergoes a reversible Mn(III)/Mn(IV) couple at +0.03 V versus Fc/Fc + in CH 2 Cl 2 . Complex 2 in contrast is binuclear, with two Ni(II) centres and two ligands L 1 each of which donates its pyridyl residue to one metal and shares its phenolate group between both metals, affording a Ni 2 (μ-O) 2 core. The remaining three sites on each octahedral Ni(II) centre are occupied by O-bound dmf ligands from the recrystallisation solvent. This is the first example of L 1 acting as a bridging ligand via the phenolate oxygen atoms.

Lanthanide complexes of the tetradentate N-donor ligand dihydrobis[3-(2-pyridyl)pyrazolyl]borate and the terdentate N-donor ligand 2,6-bis(1H-pyrazol-3-yl)pyridine: syntheses, crystal structures and solution structures based on luminescence lifetime studies

Lanthanide complexes of two polydentate N-donor ligands containing a mixture of pyridyl and pyrazolyl donors have been prepared. Dihydrobis[3-(2-pyridyl)pyrazolyl]borate (L 1 ) - is a tetradentate ligand with two bidentate chelating pyridyl/pyrazolyl arms linked by an apical BH 2 group; 2,6-bis(1H-pyrazol-3-yl)pyridine (L 2 ) is a terdentate chelating ligand reminiscent of terpyridine. Reaction of L 1 with lanthanide salts gave complexes of the type [M(L 1 ) 2 X] n+ ; the crystal structures of [Eu(L 1 ) 2 (dmf)][ClO 4 ]· 2.5CH 2 Cl 2 , [Tb(L 1 ) 2 (NO 3 )]·2CH 2 Cl 2 and [Tb(L 1 ) 2 (H 2 O)][L 1 ]· H 2 O·0.5CH 2 Cl 2 were determined and all contain two tetradentate ligands L 1 and an ancillary ligand X [dimethylformamide (dmf), nitrate or water] whose nature depends on the reaction/recrystallisation conditions to complete the co-ordination sphere. Luminescence studies of [Tb(L 1 ) 2 (NO 3 )] in water or D 2 O and MeOH or CD 3 OD showed that in methanol the solvation number q is ≈1.8, consistent with displacement of nitrate by the solvent; however in water q ≈ 4.5, indicating additional displacement of some of the N-donor heterocyclic rings of L 1 by co-ordinating water molecules. Reaction of L 2 with lanthanide salts afforded [M(L 2 ) 3 ] 3+ , all isolated as their hexafluorophosphate salts. The crystal structures of three of these (M = Eu, Gd or Ho) showed that they are isostructural and isomorphous, with tricapped trigonal-prismatic nine-co-ordinate geometries similar to that of [M(terpy) 3 ] 3+ (terpy = 2,2′:6′,2″- terpyridine). Luminescence studies of [Tb(L 2 ) 3 ][PF 6 ] 3 gave a solvation number q of 0.6 in methanol, which is small enough to be accounted for by second-sphere solvation effects alone and therefore suggests that the nine-co-ordinate structure is retained in methanol solution. However in water, q is again ≈4.5, due to displacement of some of the donor groups of the L 2 ligands by water.