N-Only Bridging Thiocyanate, Homo- and Heterobinuclear Complexes A Laboratory Project in Macrocyclic Coordination Chemistry Utilizing a Range of Physical Techniques Josie Hunter, Brian Murphy, and Jane Nelson Open University, 40 University Road Belfast BT7 1SU and Queen's University, Belfast BT9 5AC Much of the current interest i n macrocyclic coordination chemistry stems from the hope that unusual geometric relationships imposed on the metal ions by the macrocyclic donor set may be transformed into unusual bonding situations. In the particular case of binuclear macrocyclic complexes, attention is directed a t the bridgingligand coordinated between metal ions held in the macrocyclic cavity. Where this ligand is constrained within a restricted cavity, there is the hope of observing unusual bridging modes associated perhaps with enhanced ligand reactivity, thus modelling the role of substrate in an enzyme-catalyzed reaction. In such situations there is also interest in the degree of interaction between paramagnetic centers, which in the limit of strong interaction could lead to the binuclear pair acting as one unit in multielectron redox processes. There are many metalloproteins whose function is associated with the occurrence of metal centers in pairs. The atoms in the natural bimetallic site may be identical as, for example, the Cu- - -Cu site in hemocyanin or the Fe- - -Fe site in hemerythrin, or they may be different as in the Cu- - -Zn pair in superoxide dismutase or the Cu - - Fe pair in cytochrome c-oxidase ( I ) . Despite the biological importance of the binuclear site, and research emphasis focusing on the biomimetic approach, few teaching experiments are available that involve the synthesis and characterization of binuclear complexes. The exercises that are described in this paper involve the use of a macrocyclic ligand designed to hold a pair of metal ions in fairly close (4-5 A) proximity; the restrictions imposed by the coordination site are responsible for the unusual mode of coordination of a small hridging substrate, and for the generation of heterobinuclear complexes. These features of the coordination chemistry of the easily prepared (2) macrocyclic ligand L1contribute to a useful open-ended project experiment for the inorganic teaching laboratory a t or above second-year university level. Among other things, the experiment illustrates the way in which use of the large ion Pb2+ directs the Schiff-base condensation reaction toward formation of a "double"macrocycle by [2 21 condensation (i.e., using 2 mol of diamine and 2 mol of dicarbonyl) under mild conditions and in high yield. I t also illustrates the transmetallation process whereby Pb2+is replaced by a smaller ion (i.e., the series of first transition series ions Mn(I1)-Cu(I1)). The main results of the experiment fall under three headings:
+
(1) The product of template synthesis, Pb2L1(NCSk,contains the rare "short" N-only thiocyanate bridge, whose presence is
easily demonstrated by IR spectroscopy. (2) Transmetallation of PbzL1(NCS)~ with Co(I1) or Cu(I1) salts
yields hornobinuclear products. Magnetic and ESR measurements reveal spin equilibrium in the dicobalt and weak antiferromagnetie interaction in the dicopper complex. (3) Transmetallationof Ph2L1(NCS)~ with Mn(II),Fe(II), or Ni(I1) salts yields heterobinuclear complexes.
The experiment will be described in three sections each suitable for one or two days work, and directed a t (I), (2), and (3), respectively. If there are restrictions on time or facilities, the first part, which requires only infrared spectroscopy, can be run alone, as an investigation of the behavior of thiocyanate ligand in different coordination sites. Much of the original work (2,3) on this macrocycle was carried out using perchlorate counterion. Although only in the case of the iron(I1) complex had we any indication of explosive tendency, we have repeated the work using triflate counterion throughout to eliminate any possible hazard. Template Synthesis and Characterlzailon of the Produd
The macrocycle L1 is assembled by (2 + 2) Schiff-basecondensation on a Pb(NCS)ztemplate: Diacetylpyridine, 0.005 mol (0.8 g), lb-diaminopentsne (cadaverine) 0.005 mol (0.6 mL), and freshly ground Pb(NCS)z0.005 mol(1.6 g) arestirred together in 500 mL 1:l MeOHiMeCN solvent mixture at 60-70 'C for 3 h, during which time a yellow color appears and deepens as the turbidity of the suspension diminishes. The gray powder is then filtered from the orange solution, which is concentrated on a rotary evaporator to I00 mL giving PbM (HIO)=as fine crystals. (Yields M = Mn 46% L1(NCS)~(CF3S03)2. M = Fe El%, M = Ni 57%J N.B. For M = Fe, all operations were camed out under nitrogen. Techniques used: (1) IR spectra to demonstrate low-symmetryenvironment of NCSligand. (2) FAB mass spectra to confirm existence of the betemhinudear complex. (3) electronic spectra to provide evidence on coordinationgeometry of the metal ion. The heterobinuclear nature of the products is c o n f i e d by FAB mass spectra, where good agreement is obtained between the observed and calculated isotopic cluster pattern for the most abundant fragment [PbM(NCS)2(CF3S03)IC (Fig. 6). Without the availability of FAB, there is no direct evidence for the heterobinuclear formulation (althouah, - -~~~ - . of course, magnetic susceptibility measurements A d microanalvtical data confirm 1:l Pb:M stoichiometrv for what may bd assumed from its crystalline appearance to be a pure comoound.) ~ i IRe spectra of the heterobinuclear complexes show fairlv strone and sham YC=N imine absorption resolved into two components, 1645and 1630 em-', corresponding respectively to C=N coordinated to Pb2+ and to the transition metal ion M2+.Figure 7 shows the arrangement of ligands in t h e t e t r a t h i o c y a n a t o lead-manganese complex PhMnLt(NCSL - - -- .- (4) ~ . as determined (3) bv X-rav . crvstallo. graphic structure determination. The unusual asymmetric bridging strategy adopted in this complex, where both S ~
~
~
~
Concluslom and Further Dlrecllons Described above is a series of undemanding syntheses leading to stable crystalline products, whose properties can be examined using a wide range of the physical methods tvnicallv used in coordination chemistrv. The exercise thus ..=-combines basic training in coordinatio~chemistrywith motivation toward interpretation of spectroscopic and other physicochemical data. The objectives that may be achieved in the investigation lie in the four main areas set out below. ~
~
~
~
(1) A wide range of binding situationa for the thiocyanate ligand is
Figure 6. F.A.B. mass spectra of the isotopic cluster for the ion: (a) M = Ni, (b) M = Fe. (c)M = Mn. [P~M(NCS)~L'(CF~SO~)]+ 62
Journal of Chemical Education
identified using infrared spectroscopyalone.
(2) Magnetic susceptibilitymeasurements are used to demonstrate
the ?xistenre of spin equilibrium in dicohalt (11)complexes. (3) ESR spectra are used to provide information on geumecry for the dicohslt(1l)and d i c o ~ ~ e r ( com~leaes. llJ Thev also confirm the existence of weak &$fe&oi&aen&ic interaeiion in the dieopper(11) complex by observation of the normally forbidden "half-band" signal. (4) FAB spectra, if available, are able to provide conclusive evidence for the existence of the heterobinuclear PhIM assembly. T h e project is well suited to the development of further open-ended routes for which suggestions could he invited at ahiscussion session. These might include (1) investigation of bridging ligands other than thiwyanate.
(2) transmetallation with ions other than Mn(I1)-Cu(II). (3) use of counterions other than triflate. (4) electrochemical studies on the homo- and heterobinuclear com-
olexes. . (5) mvestigation of enhanced reactivity (e.g., toward hydrolysis)of the bridging thiocynnste lignnd. ~~
Acknowledgment We thank the Open University for the award of Higher Degrees studentships to BPM and to JH. We are also grateful to SERC and Open University Research Committee for purchase of the Faraday magnetic susceptibility balance and the ESR cryostat.
Literature Cited 1. For background, see the offprints "Sfate of the Art", J . Chem. Educ. 1985,62,961>,", 2. Murphy. B. P.: Nolson, J.: Nsiaon, S. M.; Drew. M. 0.B.: Yates. P. C. J C h e m . Soc. Dalton Tmns. 1987,123. 3. Ne1son.J.; Murphy 6.P.: Drew. M. G. B.: Yaks. P. C.; Nelson, S. M. J. Chem. Soc. Dalton Twns. 1988.1001. 4. Cotton. F.A.;Davidson,A.; Ilsley. W. H.;Tmp,H.S.lnargonicChem. 1919.18.279. 5. Van Aibada, C. A,: dcGraaff, R. A. G.: Haarnoot. J. G.; Redljk, J. Inorgonic Chem. 1984.23.1404. 6. Strattan, W.;
Buaeh, D. H.J. Am. Chem. Soc. 1960.82.4834. M. C.B. J. Chem. Soc. Dolton Tram.
7. Ne1son.S. M.: McCann M.:Steuenson, C:Dm.,
~~.~~~ .~~~
(6) reohcement of the lead(11) . . ion in heterobinuclear . ~ attemoted .
complexes by other transition metal ions, a direction in which ( 1 4 ) our own rkeareh is currently propessing.
Volume 68
Number 1 January 1991
63
