1 . 6 Introductionfor Part 2
From Part I , one may feel that the majority of 'PnNm' ligands investigated are of either PN or P2N2 type ligands. This is indeed the case. Relatively few systems have been studied where the ligand has only three potential donor PIN atoms in the ligand, such as or 'PN2' systems, the ligand 2-(diphenyJphosphino)-N-[2- (diphenyJphosphino)benzylidene]benzeneamine (PNCHP) is one such 'P2N' ligand and is the focus for the research in this thesis.
Q Q
o-p
�CH P- o b
PNCHP
1 . 7 The 'P2N' ligand PNCHP
The 'P2N' Schiff base ligand PNCHP can be visualised as two triphenylphosphane moieties tethered together in the ortho positions by an imine linker. Since the imine bond has no plane of symmetry perpendicular to the C=N axis, the two phosphorus atoms are inequivalent. This feature alone is unusual to diphosphane systems48 and provides an excellent handle for characterising the metal complexes solution structure by 31p NMR. The ligand PNCHP was first synthesised by Duckworth.49 However, the compound was only used as an intermediate in synthesis of the 'P2N' 2-(diphenylphosphino)-N-[(2- diphenylphosphino)benzyl]benzeneamine (PNHCH2P). Consequently, only the coordination chemistry of the PNHCH2P ligand was explored by Duckworth.
Q Q
�P
NH-CH2 P- b o
1 . 8 Previous coordination chemistry of the ligand PNCHP
The first coordination chemistry studies on the PNCHP ligand were carried out by
Halstead.5o In the work, the coordination chemistry of the ligand with nicke
i
(II) wasinvestigated, resulting in the characterisation of the square-planar compounds [Ni(PNCHP-
�P,N,P)X]CI04' where X- is ct", Br- or f and the octahedral compound [Ni(PNCHP)(N03)2]. The perchlorate salts were synthesised in a straight forward manner by reaction of the PNCHP ligand with NiX(CI04) and the nitrate compound formed by reacting one equivalent of PNCHP with Ni(N03h.6H20. However, when two equivalents of the PNCHP ligand were reacted with Ni(CI04h.6H20, in an attempt to obtain [Ni(PNCHP-12p,N,P)2](CI04)2, an analogue of the known compound [Ni(hyzPNN K3p,N,Nh]Cl04 (F.1.7_1),5o two unusual products resulted. The products were [Ni(PNCHP-�P,N,P)Cl]Cl04 and the cyclic phosphonium salt [PN(H)CHP]CI04 (F.I.7- 2).
49 P. A. Duckworth, Polydentate Phosphorus-Nitrogen Hybrid Ligands Containing the 2-Aminophenyl
Group, 1 984, Ph.D. Thesis, University of Sydney.
F.1.7-1 F.1.7-2
Figure 1.17 A bis-'PN2' Ni(II) complex (left) and the cyclic phosphonium salt [PN(H)CHP]CI04 (right)
It was proposed that the coordinated C( in [Ni(PNCHP-�P,N,P)Cl]Cl04 had resulted from the reduction of Cl04- by ethanol, in the presence of nickel and PNCHP, and that the acetic acid generated provided the proton source for formation of [PN(H)CHP]Cl04, as shown by
Equation 1.1.
Ni(CI04)2.6H20 + 2CHjCH20H + 2PNCHP � [Ni(PNCHP)CI]CI04 + 8H20 + CH3COOH + [PN(H)CHPt + CH3COO'
Equation 1.1
The cyclic phosphonium salt [PN(H)CHP]CI04 can be synthesised by direct reaction of PNCHP with HCI04.
Further coordination studies on the PNCHP ligand were carried with the metals copper(I) and silver(I) by Fan.5 1 The copper(I) complexes [Cu(PNCHP)Cl04] and [Cu(PNCHP)L] (L = ligands containing S or N donor atoms) were prepared along with the
silver(I) complex [Ag(PNCHP)CI04] and the dinuclear complex [ { Ag(PNCHP)(SCN)h. Single-crystal X-ray diffraction studies on the [M(PNCHP)CI04] (M = Cu or Ag)
complexes revealed that the PNCHP ligand behaved differently for each complex. For the
5 1 X. Fan, Structural Studies on the Interactions of a PzN Tridentate ligand with Copper(l), Silver(l) and
copper(I) complex, the PNCHP ligand was terdentate with two short Cu-P bonds (2.2032( 1 2) and 2.208( 1 3)
A)
and a Cu-N bond (2. 1 59(5) A). In contrast for the silver(l) complex, the PNCHP ligand was bordering on bidentate with two Ag-P bonds (2.447(3) and 2.438(3) A) and a long and weak Ag-N bond (2.926( 1 0)A).
Both the copper and silver complexes showed a distorted tetrahedral coordination geometry around the metal. Fan also demonstrated the quite different nature of the two inequivalent phosphorus atoms in the ligand. The phosphorus atom closest to the nitrogen atom appeared to have a greater nucleophilicity than the phosphorus atom furthest away. This was concluded from monitoring the reaction of the ligand with sulfur (Ss), where the product S=PNCHP predominated over the product PNCHP=S.In summary, the work of Halstead has shown that the PNCHP ligand has introduced some unpredictable and unusual chemistry for square-planar nickel(II) complexes, whilst in Fan's studies, the PNCHP ligand showed how the effect of having mixed donor atoms within the same ligand can subtly influence a complexes molecular structure and coordination preferences within a tetrahedral coordination environment.
In a previous study, the author set out to establish the coordination preferences of PNCHP in a octahedral coordination environment. The group 6 metal carbonyls were chosen due to the structural information available from C-O stretching frequencies.52 In addition, they offered a entry point into the realms of organometallic chemistry, where such ligand types have shown promise in catalysis.53 The results of these fundamental coordination studies on PNCHP are summarised in the introduction of Chapter 2, suffice to say that six new coordination modes, in addition to those in the complexes above, were fully characterised. The coordination modes of PNCHP, characterised thus far, are given in Figure 1.8.
52 P. S. Braterman, Metal Carbonyl Spectra, Academic Press, 1 975.
53 See for example; G. Francio, R. Scopelliti, C. G. Arena, G . Bruno, D. Drommi and F. Faraone,
Organometallics, 1 998, 17, 338; R. Noyori and S. Hashiguchi, Acc. Chem. Res. 1 997, 30, 97; F. Ungvary, Coord. Chem. Rev. , 1 997, 167, 233; A. Carmona, A. Corma, M. Iglesias, A. San Jose and F. Sanchez, 1. Organomet. Chem. , 1 995, 492, 1 1 .
P N CH P
* =
�
(i) (H)n
* .
....
..... .. � .
....,N=CH P
(iv) (v) · ..
.. ·.
. � p...
..
. ..
... (vii) (viii)P
*
... ... ...
. (iii) (vi) (ix)Figure 1.8 Observed coordination modes of the
PNCHP
ligand(PNCHP = P-N=CH-P)
1 . 9 The Present Study
In this thesis, extension to some of the work above will be presented, along with the coordination chemistry and reactivity of new complexes containing the