Brian J. Hathaway

The stereochemistry of the copper(II) ion in the solid-state: some recent perspectives linking the Jahn-Teller effect, vibronic coupling, structure correlation analysis, structural pathways and comparative X-ray crystallography

Abstract It is now over 60 years since the Jahn–Teller theorem was put forward and shown to account for the distorted stereochemistry of copper(II) complexes. Numerous accounts have been written describing the origin of the distortion. However, recent work, involving the emerging field in structural chemistry of comparative X-ray crystallography, has shown that the vibronic coupling mechanism can now be applied to low symmetry systems, suggesting that the original static stereochemistries are all connected and continuously variable. This review of copper(II) stereochemistry involving structural pathways is presented here in an attempt to describe and rationalise these variable stereochemistries. Some recent perspectives and new interpretations linking the Jahn–Teller effect (JTE), vibronic coupling, structure correlation analysis, structural pathways and comparative X-ray crystallography are reported.

Crystal structure and electronic properties of tetrakis[µ3-bis(2-pyridyl)amido]dichlorotrinickel(II)–water–acetone (1/0.23/0.5)

The crystal structure of [Ni3(bipyam–H)4Cl2]·0.23H2O·0.5(CH3)2CO 1[bipyam–H = bis(2-pyridyl)-amide] has been determined using X-ray diffractometer data; C41.5H35.5Cl2N12Ni3O0.75, monoclinic, space group C2/c, with a= 37.525(7), b= 16.237(3), c= 22.550(6)A, β= 110.12(2), Z= 12; 5778 measured and 5519 unique reflections gave a final R value of 0.051. The structure involves two independent Ni3(bipyam–H)4Cl2 units, (1) and (2)[(1) occupies a general position and (2) is on a C2 axis], one non-co-ordinated water molecule [site occupation factor (s.o.f.) 0.23], and one acetone molecule (s.o.f. 0.5). The Ni3N12Cl2 chromophores of (1) and (2) involve a nearly linear Ni3 unit, mean Ni–Ni–Ni angle 178.4(1)°, terminated by the two chloride anions. The four separate bipyam–H ligands act as tridentate ligands, involving co-ordination to three separate nickel(II) ions with Ni–Ni distances 2.443(1)(twice) and 2.431(1)A for units (1) and (2), respectively. If the Ni–Ni separations are ignored the central Ni atom in both units involves a four-co-ordinate rhombic coplanar NiN4 chromophore, generated by the four central amido nitrogens of the bipyam–H ligands. The two terminal Ni atoms involve a square-based pyramidal NiN4Cl chromophore, involving the four pyridine nitrogens of the bipyam–H ligands and an axial Cl atom. A dihedral angle of ca. 54° is involved between the planes of the individual pyridine rings of each bipyam–H ligand and results in a spiral configuration in the Ni3(bipyam–H)4Cl2 unit. The room-temperature magnetic moment µeff of compound 1 is 2.46, consistent with a diamagnetic central NiN4 chromophore and two paramagnetic terminal NiN4Cl chromophores. The electronic spectrum involves a low-energy peak at 6500 cm–1, an intense peak at 17 700 cm–1, and a low-intensity shoulder at 10 800 cm–1, consistent with the two nickel(II) stereochemistries present.

Crystal structure and electronic properties of dibromo- and dichloro-tetrakis[µ3-bis(2-pyridyl)amido]tricopper(II) hydrate

The crystal structures of [Cu3(bipyam–H)4Cl2]·H2O (1) and [Cu3(bipyam–H)4Br2]·H2O (2) where bipyam–H = bis(2-pyridyl)amide, have been determined by X-ray analysis, in the orthorhrombic space group Pnn2: (1), a= 14.092(3), b= 12.895(3), (c)= 11.190(2)A, Z= 2, and R= 0.032 for 2 453 observed and 2 029 unique reflections; (2), a= 14.186(3), b= 13.040(3), c= 11.313(2)A, Z= 2, and R= 0.043 for 1 574 observed and 1 465 unique reflections. The two structures are isomorphous with near isostructural [Cu3(bipyam–H)4X2] units in special positions of two-fold symmetry and a non-co-ordinated water molecule. The Cu3N12X2 chromophores involve nearly linear Cu3 units, Cu–Cu–Cu 178.4°(mean), terminated by the two halide anions. The four separate bipyam–H ligands act as tridentate ligands, involving co-ordination to the three separate copper(II) ions, with Cu–Cu distances of 2.471(1) and 2.468(1)A, for (1) and (2), respectively. If the Cu–Cu separations are ignored, the central Cu atom in both structures involves a four-co-ordinate rhombic coplanar CuN4 chromophore generated by the central amido nitrogens of the four bipyam–H ligands. The two terminal Cu atoms involve a square-based pyramidal CuN4X chromophore, generated by the terminal pyridine nitrogens of the four bipyam–H ligands and an axial halide anion. An average dihedral angle of 48° is involved between the planes of the pyridine rings of the individual bipyam–H ligands, which results in a spiral configuration of the [Cu3(bipyam–H)4X2] units. The spin-only magnetic moment of complex (1) is ca. 1.40 B.M. per Cu atom, consistent with antiferromagnetic coupling between the copper(II) atoms of the trimer. Both complexes are e.s.r. silent, again consistent with strong antiferromagnetic coupling. The electronic spectra of (1) and (2) have a band maximum at 15 500 cm–1, with a high-energy shoulder at 19 230 cm–1, consistent with the two different stereochemistries present.

The crystal structure and electronic properties of catena-bis(2,2′-bipyridyl)-µ-tetrafluoroborato-copper(II) tetrafluoroborate and catena-bis(2,2′-bipyridyl)-µ-perchlorato-copper(II) perchlorate

The crystal structure of [Cu(bipy)2(F2BF2)][BF4](3)(bipy = 2,2′-bipyridyl) has been determined by X-ray analysis [triclinic, space group P, with a= 11.275(4), b= 14.760(5), c= 7.366(3)A, α= 96.44(3), β= 101.63(4), γ= 110.25(4)°, and Z= 2], and that of [Cu(bipy)2(O2ClO2)][ClO4](1) has been redetermined [triclinic, space group P, a= 11.238(4), b= 14.863(5), c= 7.403(3)A, α= 96.28(3), β= 99.49(4), γ= 110.21(4)°, and Z= 2]. The two structures are isomorphous with near isostructural, six-co-ordinate, elongated rhombic trans octahedral CuN4X2 chromophores. The co-ordination of the bipy ligands is approximately planar, with a tetrahedral twist of the CuN4 chromophore [dihedral angle 44.6 and 46.7° for (3) and (1), respectively]. The six-co-ordination is completed by one bridging (and one ionic) tetrafluoroborato and perchlorate anions, in (3) and (1), respectively, at non-equivalent copper–ligand distances consistent with semi-co-ordination. The structure of complex (3) represents the first crystallographic example of a bridging tetrafluoroborato anion and the i.r. spectrum at liquid-nitrogen temperature is consistent with this, showing clear splitting of the ν3 band. The electronic and e.s.r. spectra of the complexes are consistent with their elongated rhombic octahedral stereochemistries and are discussed as a ‘criterion of stereochemistry’ for this geometry in the structural pathway of [Cu(bipy)2X]Y complexes.

Electronic properties and crystal structure of (2,2′-bipyridyl)-catena-µ-(oxalato-O1O2: O1′O2′)-copper(II) dihydrate and aqua(2,2′-bipyridyl)-(oxalato-O1O2)copper(II) dihydrate

The crystal structure of the title compounds [Cu(bipy)(C2O4)]·2H2O (1) and [Cu(bipy)(C2O4)(OH2)]·2H2O (2) have been determined by X-ray analysis. Compound (1) crystallises in the triclinic space group P with a= 9.673(3), b= 6.940(3), c= 9.103(3)A, α= 105.718(3), β= 110.347(3), γ= 97.539(3)°, and Z= 2. The six-co-ordinate CuN2O2O′2 chromophore of (1) involves an elongated rhombic octahedral stereochemistry involving a symmetrically co-ordinated bipy ligand (mean Cu-N 2.007 A) and unsymmetrically co-ordinated bridging oxalate groups (mean Cu-O 1.988 and 2.320 A). Compound (2) crystallises in the triclinic space group P with a= 10.565(3), b= 7.246(3), c= 10.806(3)A, α= 102.467(3), β= 62.119(3), γ= 98.134(3)°, and Z= 2. The CuN2O2O′ chromophore of (2) is basically square pyramidal with a symmetrically co-ordinated bipy ligand (mean Cu-N 1.989 A), and a symmetrically co-ordinated oxalate group (mean Cu-O 1.953 A) in the plane of the square pyramid, and a water molecule at 2.341 A out of the plane. The electronic reflectance spectrum of (1) involves main band at 14 500 cm–1 with a resolved broad band at 9 300 cm–1, while that of (2) involves a single broad band at 15 600 cm–1, a difference that is consistent with the structures and suggests an ‘electronic criterion of stereochemistry’ to distinguish these two structures.

The crystal structure and electronic properties of bis(2,2′-bipyridyl)-copper(II) bis(hexafluorophosphate)

The crystal structure of the title compound, [Cu(bipy)2][PF6]2(1), has been determined by X-ray crystallographic methods, using diffractometer data collection; the structure was solved by the heavy-atom method and by successive Fourier syntheses. Compound (1) crystallises in the tetragonal space group I41/acd, a= 16.228(3), c= 18.954(3)A, Z= 8.319 unique reflections gave a final R= 0.0487. The structure of (1) involves a unique compressed tetrahedral CuN4 chromophore (dihedral angle 44.6°) with four additional non-bonding PF6– anions at 3.3 A. The e.s.r. spectrum is axial (g∥g⊥ > 2.0) and the dark green crystals have an electronic reflectance spectrum with a band maximum at 15 040 cm–1 with a high-frequency shoulder at 16 950 cm–1. This represents the highest electronic transition yet observed in the [Cu(bipy)2X]Y type complexes and establishes an ‘electronic criterion of stereochemistry’ for the compressed tetrahedral CuN4 chromophore.

Crystal structure of bis(2,2′-bipyridyl)monochlorocopper(II) tetrafluoroborate. A relook at the structural pathway of the bis(2,2′-bipyridyl)monochlorocopper(II) cation

The crystal structure of [Cu(bipy)2Cl]BF4(I)(bipy = 2,2′-bipyridyl) has been determined using diffractometer data collection. It was solved by the heavy-atom method and successive Fourier difference syntheses. Compound (I) crystallises in the monoclinic space group P21/n with a= 16.187(4), b= 12.177(3), c= 10.793(3)A, β= 104.49(3)°, Z= 4, and R= 0.058 for 1 451 unique reflections. The CuN4Cl chromophore involves a square-based-pyramidal distorted trigonalbipyramidal stereochemistry. The extent of the square-pyramidal distortion is mirrored in the twin-peaked electronic reflectance spectrum, with absorption maxima at 10 100 and 13 900 cm–1. The structure of complex (I) is compared with those of a series of eight cation distortion isomers of [Cu(bipy)2Cl]Y which are related by a structural pathway. The nine complexes have been subjected to scatter-plot and factor-group analysis and the feasibility of extending the structural pathway to include two [Cu(bipy)2Br]+ and two [Cu(bipy)2I]+ complexes is examined.

A structural profile of the bis(2,2′-bipyridyl)monochlorocopper(II) cation. Crystal structures of bis(2,2′-bipyridyl)monochlorocopper(II) perchlorate and the nitrate trihydrate

The crystal structures of the title compounds [Cu(bipy)2Cl][ClO4], (1), and [Cu(bipy)2Cl][NO3]·3H2O, (2), have been determined by X-ray diffraction methods using three-dimensional diffractometer data; the structures were solved by heavy-atom techniques and successive Fourier synthesis. Compound (1) crystallises in the monoclinic space group P21/c with a= 10.761(5), b= 12.253(5), c= 16.990(5)A, β= 112.18(5)°, Z= 4, and (2) crystallises in the monoclinic space group P21/a with a= 14.305(5), b= 21.213(5), c= 8.234(5)A, β= 114.90(4)°, Z= 4. The [Cu(bipy)2Cl]+ cation in both (1) and (2) involves a square-pyramidal distorted trigonal-bipyramidal CuN4Cl chromophore with the distortion in (1) greater than in (2). From a correlation of the bond-length and bond-angle distortion of the structures of five[Cu(bipy)2Cl]+ cation distortion isomers a structural profile has been constructed between a near regular trigonal-bipyramidal CuN4Cl chromophore of C2 symmetry, and a square-pyramidal chromophore. The electronic reflectance spectra have been established as an ‘electronic criteria of stereochemistry’ for the CuN4Cl chromophore, which involves a single peak at 12 500 cm–1 for the trigonalbipyramidal geometry and twin peaks at 13 240 and 10 470 cm–1 for the square-pyramidal distorted structure.

The range in static stereochemistry of the cation distortion isomers of the [Cu (chelate)2Cl]+ cation, where chelate=bipy, phen or bipyam. The low temperature crystal structure (150 K) of [Cu (bipy)2Cl] [PF6]·H2O and [Cu (phen)2Cl] [BPh4]

Abstract Low temperature crystallography (150 K) has been used to confirm that the two extreme [Cu (chelate)2Cl] [Y] stereochemistries of 3 [Cu (bipy)2Cl] [PF6]·H2O, trigonal bipyramidal and 6 [Cu (phen)2Cl] [BPh4], trigonal bipyramidal distorted square based pyramidal, are genuine and static stereochemistries of the copper (II) ion.

Structural systematics of the [Cu(chelate)3][Y]2 series. An interesting crystallographic structural insight involving vibronic coupling and the Jahn–Teller effect (JTE). The syntheses and low temperature crystal structures of tris(2,2′bipyridyl)copper(II) tetraphenylborate and tris(2,2′bipyridyl)zinc(II) tetraphenylborate

The crystal structures of [Cu(bipy)(3)][BPh(4)](2), 1, and [Zn(bipy)(3)][BPh(4)](2), have been determined at low temperature. 1 and 2 are closely related, but are not isostructural. Both contain a two-dimensional supramolecular construct (SC) involving a sandwich structure. 1 has a six-coordinate CuN(6) chromophore with a regular elongated octahedral stereochemistry and rhombic in-plane bond lengths. The associated tetragonality value, T, of 1 is 0.8868. 2 involves a six-coordinate octahedral chromophore. Differences between 1 and 2 relate to the tendency of copper(II) complexes to undergo a Jahn-Teller (JT) distortion. The zinc(II) cation feels solely the host site strain, whereas the copper(II) cation also involves vibronic JT type coupling. The copper polyhedron geometry is characterized by both phenomena, with the vibronic interaction dominating. Scatter plot analysis involving the tris-chelate copper(II) series suggests that neither pure Q(theta) or Q(epsilon) components or the a(2u) mode operate in isolation over the entire series. All three operate in combination with varying quantifiable contributions, leading to distortion from the regular tetragonal octahedral stereochemistry.