Molybdenum and tungsten mononitrosyl complexes

Coe, Benjamin John (1991). Molybdenum and tungsten mononitrosyl complexes. University of Birmingham. Ph.D.

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Abstract

The primary objective of this thesis is the preparation and characterisation of molybdenum and tungsten mononitrosyl complexes designed to exhibit nonlinear optical (NLO) effects. A part of this work has also been directed at the synthesis of molybdenum mononitrosyl complexes designed to possess liquid crystal phases. Novel materials displaying such "molecular electronic" properties are required for the creation of opto—electronic devices to be used in computing and communications systems of the 21st century. All of the new metallo-organic compounds described have been fully characterised by standard physical techniques including i.r. and \(^1\)H n.m.r. spectroscopy. Cyclic voltammetry and electronic absorption spectroscopy have also been used extensively to investigate molecular electronic properties.
Nine aryl amino- or hydroxy-ligands bearing para-ferrocenyl substituents have been synthesised and used to prepare a total of thirty two heterobimetallic complexes of the form C M(NO) L* (X) (Y) 1 (where M = Mo, X = F, Cl, Br, I, alkoxide, or aryl ferrocenyl ligand CArFc), Y = ArFc; M = W, X = Cl or ArFc, Y = ArFc; L* = tris(3,5-dimethylpyrazolyl)hydroborate) . In such complexes the ferrocenyl group acts as an electron donor and the 16-electron CM(NO)L.*) centre is an electron acceptor. All of these complexes have been tested for powder second harmonic generation (SHG) using modifications of a standard test procedure, and referring the efficiency of the material for SHG to that of urea, taken as unity (≡ = 1>. At least nine complexes have been found to exhibit highly efficient doubling of a 1,907 nm laser wavelength, several showing activities similar to the organic compound DAN which has a Ξ value of about 115.
A single crystal X-ray structure of the complex [Mo(NO)L*(Cl) (HNCeH\(_3\)-3—CH\(_3\)-4—N=NC\(_6\)H\(_4\)-4—Fc )] shows that this molecule, which gives a \( \Xi\) value of 123, crystallises in the noncentrosymmetric space group F21 with only one configuration of the chiral metal centre present. The analogous tungsten complex C [W(NO) L* (Cl ) (HNC\(_6\)H\(_3\)-3-CH\(_3\)-4- N=NC\(_6\)H\(_4\)-4-Fc )] gives a \( \Xi\) value of 53 and has a crystal structure identical to its molybdenum analogue. Results of powder SHG tests show that the Isostructural change of replacing Mo by W always leads to lower SHG efficiencies. This is thought to be because the greater electron density of W renders it a poorer electron acceptor. Powder X-ray diffraction has been used to investigate Isostructural and non-isostructural changes in this class of complexes, and to relate these to observed SHG signals.
The nitroarylferrocenyl ligand precursors have also been tested for SHG at 1,907 nm, but only the stilbene compound 4-Fc-C\(_6\)H\(_4\)CH=CHC\(_6\)H\(_4\)-4-NO\(_2\) shows any frequency doubling. A crystal structure of the related compound 4-Fc-C\(_6\)H\(_4\)N=NC\(_6\)H\(_3\)-2-CH\(_3\)-4-NO\(_2\) shows that this molecule crystallises in the centrosymmetric space group P2\(_1\)∕c in which dipole cancellation precludes the observation of bulk NLO effects.
A number of Stilbenyl and diphenylazo ligands bearing dimethylamino or methoxy substituents have been synthesised and used to prepare forty one (M(NO)L*) complexes. The lack of powder SHG from any of these complexes is thought to be a result of unfavourable structural effects.
Several stilbenyl and biphenyl ligands bearing long alkyl chain substituents have been synthesised and used, along with two mono-aryl ligands, to prepare twenty five donor—acceptor <Mo (NO)L*) complexes. Three molybdenum complexes of the cholesterolate ligand have also been prepared. All of these complexes have been investigated for liquid crystalline behaviour using DSC and polarising microscopy, but no evidence of any mesophases has been found. This is likely to be because the bulky nature of the tripodal L* ligand prevents the fluid ordering found in mesogenic materials.
Four (Mo (NO)L*) complexes containing the chiral resolved ligand (-)-2-(6-methoxy-2-naphthyl)-1-propoxide have been prepared, and a single diastereomer of the complex [Mo(NO)L* (HNC\(_6\)H\(_3\)-3—CH\(_3\)-4—N=NC\(_6\)H\(_4\)-4—Fc ) ( (-)-2-0CH\(_2\)CH (CH\(_3\))C\(_{10}\)H\(_6\)-G-OCH\(_3\)) ] isolated by column chromatography. The complex [Mo(NO)L*(OR)(HNC\(_6\)H\(_3\)-3-CH\(_3\)-4- N=NC\(_6\)H\(_4\)-4—Fc ) ] (R = cholesterolate) has also been prepared but not resolved. These complexes have not been tested for SHG. Several homobimetallic -(Mo(NO)L*) complexes have been synthesised along with six complexes of the form [ Mo CNO) L* CCl ) COAi— 4-NO\(_2\) ] (where Ar = phenyl, Stilbenyl, or diarylazo), and these have been reduced chemically using cobaltocene. This was intended to produce complexes in which the reduced paramagnetic 17-electron Mo centre behaves as an electron donor in NLO-active compounds, but the reduction products have not been adequately characterised. A retinal-derived aniline has been synthesised and used to prepare the first (MoCNO)L*) complex of a polyene ligand. This complex does not appear to show second-order NLO effects, but may be of interest for third-order effects.
Some reactions of the related cationic mononitrosyl dihalide complexes [ Mo (NO) L\(^N\) X\(_2\) ] \(^+\). PF\(_6^-\) (where L\(^N\) = N,N',N"-trimethyl-1,4,7- triazacyclononane, and X = Br or Cl) with potential aryl ligands have also been investigated. However, only one complex, [MoCNO)L\(^N\) (Br) \(_2\) CH\(_2\)NC\(_6\)H\(_3\)-3-CH\(_3\)-4-N=NC\(_6\)H\(_4\)-4—Fc ) ] \(^+\). PF\(_6^-\), has been isolated. This contains a seven coordinate paramagnetic 18-electron Mo centre bearing a neutral amino ligand, but does not show detectable powder SHG. These studies have shown that the cationic (MoCNO)L\(^N\)) dihalides do not display the extensive amide and phenoxide derivative chemistry associated with their L* counterparts.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):