O bviously the flexibility o f the angles X - M - X (56-82°) and B - M - B (126-138°) is relatively limited and the X - L n - X angle increases w ith the bite size o f the bidentate ligand.'^’'"^’'^’’^’’^’'^'^’M ost o f the structurally characterised com pounds present a coordination geom etry which is best described as SAP or BCTP. The BCTP or SAP distorted tow ards BCTP are the coordination geometries adopted by the com pounds isolated w ith larger lanthanides and/or w ith bidentate ligands w ith large bite distances, in contrast to the SAP distorted tow ards D O D w hich is preferred by the more sterically congested molecules. [Yb(Tp)2(acac)] appeared not to follow this trend since the SAP coordination geometries observed for two independent molecules in the asym m etric unit have different types o f distortion w hich are probably due to packing effects.
The chem istry o f the lanthanides w ith the more sterically encumbered /m (p y razo ly l)b o rates has been studied here, using a dim ethyl-substituted analogue TpMC’Me because o f its ease o f synthesis, and because it seem ed appropriate to com pare the behaviour o f the complexes o f the larger ligand with the less bulky analogues that w ere the subject o f earlier studies. Reports on these types o f complexes are rare. W e hoped that b y increasing the steric saturation around the metal centre (using x p ^ ‘^’'^°) we could su p p ress ligand redistribution reactions, observed for example by Takats in attem pts to prepare L n T p2X ,'^ w ithout the need for additional ancillary ligands. On the other hand, the steric demands o f this ligand can also be explored and com pared to those o f metallocene analogues. W hile our w ork w as in progress M arques and co-w orkers p rep ared
[Sm C l2(T p“ ' ’“ ')(T H F )] and [Sm (Tp“ ' ' “ ')2]I and T akats’ group prepared [Sm (Tp'’^'’'“ ‘’)2]B Ph4 and [Sm (Tp“ ‘’-“ ')2(PhN N Ph)].
2 . 2 Results and Discussion
2 . 2 . 1 . P r e p a r a t i o n o f L a n t h a n i d e b i s ( ^ r i5-3,5- d i m e t h y l p y r a z o l y l ) b o r a t e T r i f l a t e s . "
The complexes o f formula L n (T p ^ ‘^’'^‘^)2(0 Tf) m ay be prepared by straightforw ard m etathesis o f a slight excess o f the appropriate lanthanide triflate w ith K T p^°’^° in TH F at room tem perature (Eq. 2.2.1):
(Eq. 2.2.1) Ln(0 Tf)3 + 2 K Tp^"'^" — Ln ( Tp ^ ' ' ^ ' )2 0T f + 2 K O T f
T rifiâtes rather than chlorides w ere initially chosen as starting materials, partly because o f the ease o f preparation o f the anhydrous materials but also because the poorly ligating triflate ion m ight be expected to be a better leaving group, leading to fewer problem s w ith purification o f the products.
The reactions proceed sm oothly at room tem perature giving either slightly turbid solutions (due to the form ation o f the sparingly soluble potassium triflate) or a dense precipitate. A fter rem oval o f solvent it was possible to extract the complexes o f the lighter lanthanides into w arm toluene and slow cooling overnight yielded analytically pure complexes. The resulting products for La, Ce, Pr, N d and Sm were obtained as crystalline or m icrocrystalline solids. By contrast, the products o f the reactions for Y and the heavier lanthanides Eu, Gd and Ho failed to dissolve significantly in toluene and it was found necessary to extract the reaction m ixtures w ith dichlorom ethane, from w hich the complexes
obtained by recrystallization from dichlorom ethane/petroleum ether mixtures.
All o f the complexes were o f the expected colours for appropriate trivalent ions. The cerium complex w as found to display a strong luminescence under UV light both in the solid and in solution, w hich w e presum e to result from excitation to the low est excited electronic configuration ( 4 /’5 J ') as has been reported for a wide range o f Ce(III) complexes.^’ The neodym ium com plex displayed a w eaker luminescence than that o f the cerium complex. The rem aining com plexes did not appear to be luminescent.
For the diamagnetic complexes o f Y and La, the 'H and N M R spectra w ere alm ost s u p e rim p o sa b le and stru ctu rally u n in fo rm a tiv e , th e fo rm e r sim p ly sh o w in g three p eak s for the m ethine and two m ethyl groups o f the pyrazolyl rings in the ratio o f 3:3:1, consistent w ith the ligands being equivalent in solution. N M R spectra w ere also recorded for the param agnetic com pounds o f Ce, Pr, N d, Sm and Eu. In each case the p y ra z o ly l resonances w ere shifted from the diam agnetic positions observed for Y and La. In all cases, one o f the two m ethyl signals was significantly broader and m ore shifted than the other (as exemplified by the spectrum o f the sam arium com plex show n in Fig. 2.2.1). This broad resonance was assigned to the m ethyl group in the 3-position on the pyrazolyl ring, which is closer to the paramagnetic m etal centre and is expected to experience a much greater isotropic shift as a result (Fig. 2.2.2). Plots o f 'H chemical shifts versus Vy for the ligand peaks in the paramagnetic species w ere linear and consistent w ith sim ple Curie-W eiss behaviour and therefore w ith m onom eric com plexes w hose structure is invariant w ith temperature.^^ The plot for [N d (T p ’^ ‘^’^ °)2 0Tf] is show n in Fig. 2.2.3. Spectra o f the rem aining lanthanides were too broad to be interpreted.
The sharp difference in solubility betw een com plexes o f identical stoicheiom etry suggested to us that, as a result o f the gradual contraction in the ionic radius across the series, the early com plexes m ight consist o f neutral species w ith a coordinated triflate anion, w hereas those o f the heavier lanthanides m ight exist as an ion pair, the steric demand o f the tris-
/ J
— r — =---
-3 /ppm
Fig. 2.2.1 200MHz 'H NMR spectrum of [Sm(Tp"’' ’% O T f] recorded in CDClj
Sharper and less shifted
Broad and highly shifted resonance expected
/
\
H— B Ln (paramagnetic)
Fig. 2.2.2 The relative effect of a paramagnetic centre on the NMR behaviour of the 3- and 5- substituents in 'j'pM c.M c
MethyH Methine Methyl2 1 0 -1 5 - 0- - 5 - -10- -15- -20- 0.0034 0.0036 0.0038 0.0040 0.0042 0.0044 0.0046 0.0048 irr F ig . 2 .2 .3 P l o t o f 'h c h e m i c a l s h i f t s v e r s u s 1 /T f o r t h e l i g a n d p e a k s o f [ N d ( T p ^ '’^ ') : O T f ] in C D C f t m v(c.F/soi o f free O T f v(c.RsO) o f bonded O T f 1 1 o o J ?oo ■ ( c m - 1 ) F ig . 2 . 2 . 4 S o l i d s t a te I R s p e c t r a o f [ L a ( T p ' ’% O T f ] a n d [ H o ( T p ^ ''^ ')2] O T f i n K B r d is c . 48
dim ethylpyrazolylborate ligands forcing the triflate ion out o f the metal coordination sphere.
Indeed, com parison o f the infrared spectra o f the two types o f complex revealed striking differences. A lthough all the com pounds displayed a prom inent peak in the region o f 2560 cm ’’ due to the B-H stretching vibration, for the complexes o f the smaller ions, Y, Sm, Eu, Gd and Ho, the infrared spectra were essentially superim posable. In particular, th e y displayed a strong band at 1275 cm'% resembling that in ammonium triflate w hich is generally agreed to be ionic. The band could be assigned to a C -F /S -0 com bination band o f the free triflate ion.^^ By contrast, the band is shifted to about 1204 cm'* for the complexes o f the earlier elements La, Ce, Pr and N d (see Fig. 2.2.4) suggesting a significant structural change as a result o f the larger ionic radii o f ions at the beginning o f the series.
In order to establish the nature o f these complexes in solution, infrared spectra were recorded in CDCI3 solution, a solvent transparent betw een 1200 - 1300 cm"’. In contrast to the solid state spectra, the sam arium and europiurp com plexes show ed a peak at 1201 cm’’ suggesting a coordinated triflate ion. In considering these spectra it can be concluded th a t these two complexes appear to exist in solution w ith a coordinated triflate ligand but as separated ion pairs in the solid state and m ay therefore be regarded as being at the cross over point betw een the two structures.
Single crystal X -ray diffraction studies carried out by Dr. G. H. M aunder confirmed the neodym ium complex as being [Nd(Tp^'^’^'^)2 0Tf], the triflate ion lying w ithin the inner coordination sphere o f the metal (Fig. 2.2.5). B y contrast to those complexes prepared previously w ith the unsubstituted Tp ligand, all o f w hich are eight-coordinate (tw o T p ligands plus a bidentate or two unidentate ligands), the neodym ium complex is seven-coordinate. This reflects the increase in steric dem and o f the ligand and p o o r ligating pow er o f the triflate anion. The m olecular structure o f Yb(Tp^^''^^^)2 0Tf, however, consists o f discrete [Y b (T p^‘^’'^‘^)2]^ cations and [CF3SO3]' anions. The metal centre is therefore six-coordinate. (Fig. 2.2.6)
CT21I CI23I rwi Bin CK NO 21 cnii cinn cntii 0 1 2 1
Fig. 2.2.5 The structure of [Nd(Tp'^’=’"'')2 0Tf] Fig. 2.2.6 The structure of [Yb(Tp"'‘^'% ][O Tf]
2.2.2. P r e p a r a tio n o f th e A c e to n itrile A d d u cts
Having prepared seven-coordinate com plexes o f the form ula L n (T p '^ ‘^’^’‘^)2 0T f w hich resem ble the analogous m etallocenes, we set out to explore w hether sim ple adducts could be m ade w ith additional neutral ligands. We chose to look at hard donor ligands such as am ines, pyridines and phosphine oxides w hich are w ell-know n to bind to Ln^^ centres. E xploratory studies w ere carried out using the lanthanum com plex because we thought its large size w ould be best suited to accom m odate an additional ligand. The attem p ted prep aratio n o f such com plexes was carried out sim ply by reaction o f a slight excess o f ligand w ith L a (T p '^ ''^ ')2 0T f in TH F (Eq. 2.2.2):
(Eq. 2.2.2) [L a(T p'^= -% O T £] + L to lu e n e [L a(T p'^'-“ '),(L )O T f]
The resulting solution was cooled slow ly overnight after rem oval o f solvent and extraction o f the com plex into warm toluene. U nfortunately, in each experim ent the starting material w as recovered. Considering the great steric dem and o f ijg^^ds, it is reasonable to suppose that large neutral ligands m ight not be able to bind the m etal centre. In view o f the failure to observe a reaction w ith lanthanum , no experim ents w ith the smaller ions were carried out. Instead the behaviour o f smaller rod-like ligands such as nitriles were
p M.CN KCTTDK McCN)
cm
Fig. 2.2.7 Solid state IR spectra o f [La(Tp^''^")i(MeCN)(OTf )].MeCN (2a) and
[Nd(Tp^^'’^'),(M eCN)2](0 Tf), (2d) recorded in KBr disc
a p
§
1
160 M B 120 0 0 6 0 2 0 0
* Methyl carbon of MeCN
Fig. 2.2.8 Solid state CP-MAS '^C NMR spectra o f [La(Tp"'''"'3 2(MeCN)(0T f )].MeCN (2a)
coordinating solvent, toluene (Eq. 2.2.3):
(Eq. 2.2.3) [L n(T p'^''“ ')2 0Tf] + n R-CN » [Ln(Tp“ '-“ ')2(R-CN)„0 Tf]
In the case o f benzonitrile and 4-cyanopyridine, the starting material w as again recovered unchanged. H ow ever w hen acetonitrile was added to Ln(Tp^^'^^)2 0T f where Ln = La, Ce, Pr, N d, Sm, Y, or Yb, m icrocrystalline products were obtained upon slow cooling o f the solution, w hich had the typical colour expected o f each ion.^ ’H N M R and IR spectra show ed th at the products recovered w hen Ln = Sm, Y, and Yb w ere actually the starting m aterial and virtually free o f acetonitrile. Elem ental m icroanalyses on the La (2a), Ce (2b), Pr (2c) and N d (2d) complexes w ere consistent w ith the proposed formula C35H5oNi4B2p3Ln0 3S and two molecules o f acetonitrile in each complex. ’H N M R spectra o f these com plexes in CDCI3 w ere alm ost superim posable w ith those o f the starting m aterials except for a singlet arising from the m ethyl group o f acetonitrile. The IR spectra o f these com plexes all had a typical B-H stretch around 2560 cm’’ and a C -F /S -0 com bination band suggesting bonded triflate at 1204 cm'*. H ow ever, the spectrum o f com plex 2a show ed two nitrile bands assigned to bonded (2276 cm '’)^"^ and free (2258 cm' *)^^ acetonitrile. The rem aining complexes showed only the bonded nitrile bands.(Fig. 2.2.7) The frequency difference betw een the free and bonded acetonitrile bands is prim arily governed by the relative “softness” o f the Lewis acid m oiety. H owever, the m icrocrystals w ere seen to degrade after removal from the m other liquor, and the reliability o f the IR spectra m ight be disturbed by the gradual loss o f nitrile ligand w hen the crystals are out o f solution.
Since only one acetonitrile peak w as observed in the room tem perature solution 'H N M R spectrum it is reasonable to consider this com plex to exhibit dynam ic behaviour in solution. In order to slow down the dynam ic process by lowering the tem perature, a V T -N M R
CIS C13 C12 CU on Nil N12 Loi, N41 ,N31 N32 1\N 61 NS2 N21 ,N22 C22 C24 C21 C25 C23
Fig. 2.2.9 The molecular structure o f [La(Tp^''^')i(M eCN )(O Tf )].MeCN, 2a.
Fig. 2.2 .10 O R T E P view with 5 0 % probabili ty thenrial ellipsoid s a n d partial labeling sc hem e for the inner co ordination sphere o f 2a.
enough to freeze out the fluxionality. The solid state '^C N M R spectrum o f 2a show ed a single peak assigned to the m ethyl group o f acetonitrile (Fig. 2.2.8). This is surprising since this observation contradicts both the infrared spectra and the results o f our subsequent investigation by X -ray crystallography, which shows the tw o acetonitrile molecules to be in different environm ents. The carbon atoms o f the m ethyl and m ethine groups on all o f the pyrazolyls are also in independent environm ents, indicating a very low sym m etry for this com plex in the solid state.
C r y s ta l S t r u c t u r e o f 2a
In the hopes o f establishing the structure o f the com plexes in the solid state to confirm th a t M eC N w ould bind to the metal centre, a crystallographic study w as carried out. A transparent block-like crystal o f the lanthanum complex suitable for single crystal X -ray diffraction w as grow n by the slow cooling o f a toluene solution. The structure was solved and refined successfully in the triclinic space group P1 as neutral [La(Tp^®’^^)2(C H3CN)OTf] molecules w ith a second molecule o f acetonitrile contained w ithin the lattice. The final atomic coordinates and lists o f selected bond lengths and angles are given in Table A l.l.l - A l .1 .5 (in A ppendix 1). N o significant intermolecular contacts w ere noted. The m olecular structure o f the com plex is show n in Fig. 2.2.9. The central La^^ ion was found to be eight coordinate, w ith an acetonitrile molecule within the metal coordination sphere o f La(Tp'^‘^’^^)2(0 Tf). The two tridentate pyrazolylborate ligands are ben t back in a manner rem iniscent o f a bent metallocene w ith a B-La-B angle o f 142.7°. A lthough the metal coordination polyhedron is som ew hat irregular, polytopal analysis allow s us to assign the geom etry as DOD (this is discussed briefly in appendix 2). The ô and ()) angles are listed in T ab le A2.1.1 (A p p e n d ix 2M he trapezia being defined by the atom s [N (62), 0 (1 ), N (l), N(32)] and [N(42), N (52), N (22), N (12)] respectively. The distortions o f the trapezia im ply a distortion along the pathw ay tow ards BCTP in which the trigonal prism is defined by [N (l), N (12), N (32), N (42), N(52), N (62)] w ith 0 (1 ) and N (22) capping the faces (Fig. 2.2.10).
The average La-Np^ distance is 2.645(7) Â which is similar to that observed by Jones and co-w orkers for the eight-coordinate com plex [Ce(Tp)2(acac)] (2.63(3) Â)^^ and for complex 2e, [La(Tp^^'^^)2(N0 3)], (2.647(3) Â). H owever, all three com plexes show a w ide range o f M -N bond lengths, so a more useful measure o f the metal-to-ligand separation is given b y the mean La-B distance, 3.646(7) Â, similar to that in 2e {vide infra). The tw o py razolylborate groups are m utually staggered and are bent back w ith a B-La-B angle o f 142.7°. By contrast to a metallocene, how ever, the “equatorial plane” defined by the tw o additional ligands lies at 46.4° to the plane containing the B (l)-L a (l)-B (2 ) unit. In addition the arrangem ent o f the pyrazolyl groups around the tw o ligands is effectively C2 sym m etric. Similar pseudo-C2 sym m etry has been noted by Takats in [S m (T p“ ' ' “ ')2(PhN N Ph)],^’ [Sm (Tp“ ' ’“ '=)2(0 2) ] a n d [Sm(Tp'^'-“ ')2(N0 2)].“
A lthough the angles around the boron atom are tetrahedral to w ithin experimental error, both pyrazolylborate groups show considerable distortions from ideal C3 sy m m etry resulting from significant tw ists about one o f the B-N bonds, as measured by the B -N -N - La torsion angle. For both ligands only a single pyrazolyl group show s this tw ist w hile the other two are parallel to the La-B axis to w ithin less than 10°. In each case the tw ist appears to result from the need to accommodate the additional ligands in the coordination sphere while maintaining a constant angle betw een the “extra” ligand and the two arms o f the Tp group. Thus pyrazolyl ring 1 [we defme ring 1 as that w hich contains