Tetrahydropentalenyl-phosphazene constrained geometry complexes of rare-earth metal alkyls †

Reactions of Cp ™ HPPh 2 ( 1 , diphenyl(4,4,6,6-tetramethyl-1,4,5,6-tetrahydropentalen-2-yl)phosphane) with the organic azides AdN 3 and DipN 3 (Ad = 1-adamantyl; Dip = 2,6-di-iso-propylphenyl) led to the formation of two novel CpPN ligands: P -amino-cyclopentadienylidene-phosphorane (Cp ™ PPh 2 NHAd; L Ad H) and P -cyclopentadienyl-iminophosphorane (Cp ™ HPPh 2 NDip; L Dip H). Both were characterized by NMR spectroscopy and X-ray structure analysis. For both compounds only one isomer was observed. Neither possesses any detectable prototropic or elementotropic isomers. Reactions of these ligands with [Lu(CH 2 SiMe 3 ) 3 (thf) 2 ] or with rare-earth metal halides and three equivalents of LiCH 2 SiMe 3 produced the desired bis(alkyl) Cp ™ PN complexes: [{Cp ™ PN}M(CH 2 SiMe 3 ) 2 ] (M = Sc ( 1 Ad , 1 Dip ), Lu ( 2 Ad , 2 Dip ), Y ( 3 Ad , 3 Dip ), Sm ( 4 Ad ), Nd ( 5 Ad ), Pr ( 6 Ad ), Yb ( 7 Ad )). These complexes were characterized by extensive NMR studies for the diamagnetic and the paramagnetic complexes with full signal assignment. An almost mirror inverted order of the paramagnetic shifts has been observed for ytterbium complex 7 Ad compared to 4 Ad , 5 Ad and 6 Ad . For the assignment of the NMR signals [{ η 1 : η 5 -C 5 Me 4 PMe 2 NAd}Yb(CH 2 SiMe 3 ) 2 ] 7 was synthesized, characterized and the 1 H NMR signals were compared to 7 Ad and to other paramagnetic lanthanide complexes with the same ligand. 1 Ad , 2 Ad , 2 Dip , 3 Ad and 3 Dip were characterized by X-ray structure analysis revealing a sterically congested constrained geometry structure.


Introduction
The organometallic chemistry of the rare-earth metals began in the middle of the last century with the synthesis of their tris-(cyclopentadienyl) derivatives by Wilkinson and Birmingham. 1 Since then the cyclopentadienyl (Cp) ligand has remained one of the most ubiquitous ligands of this chemistry.For a long time, rare-earth organometallic chemistry has been dominated by metallocenes, 2 especially when these complexes are used as precursors in various stoichiometric and catalytic processes. 3ore recently, however, considerable attention has been directed towards rare-earth mono-cyclopentadienyl complexes. 4ut the selective synthesis of monomeric rare-earth metal mono-Cp complexes is generally difficult.A great steric bulk of the Cp ligand is advantageous for a successful synthesis.In search of better ligand systems, wide variations of the aromatic cyclopentadienyl framework have been described in the literature. 4,5These variations include for example totally or partly substituted Cp rings and ancillary linked donor atoms like O, N, P or S. The latter are used to form constrained geometry complexes (CGCs). 6n this work we describe the synthesis, NMR studies and molecular structures of two novel cyclopentadienyl-phosphazene (CpPN) ligands with a tetrahydropentalene unit: Cp™PPh 2 NHAd (L Ad H) and Cp™HPPh 2 NDip (L Dip H).This tetrahydropentalene unit is very attractive as it is easily synthesized by condensation of NaCp with Ph 2 PCl and two equivalents of acetone.Compared to other sterically demanding cyclopentadienyls such as C 5 Me 4 R, it is cheap and can easily be synthesized on a large scale.The new ligands bear a sterically very demanding, well crystallizing, electron rich and rigid cyclopentadienyl ring.Therefore we are confident that these and related tetrahydropentalenyl ligands are good alternatives for the commonly used but expensive C 5 Me 4 R building blocks in organometallic chemistry.
Constrained geometry complexes with the cyclopentadienylsilylamido (CpSiN) type ligands, initially developed by Bercaw [7][8][9] and Okuda, 10 became one of the best developed classes of CGCs (Scheme 1; A).In contrast, however, constrained geometry rare-earth metal(III) complexes with different single-atom bridging units in the ligand system have received much less attention and have remained almost unexplored to date.Some examples are CpSiP complexes (B) 11 and CpSiC complexes (C) 12 which have dianionic ligands.][17][18][19] Previously we reported a general and convenient synthetic protocol for a large variety of CpPN type ligands, 20 and their use in the stabilization of highly reactive alkyls of rare-earth and group 4 metals has been claimed. 21Independently, related fluorenyl-and indenyl-phosphazene ligands (FluPN and IndPN) and their rhodium 22 and zirconium 23 complexes were presented by Bourissou and co-workers.The synthesis and characterization of a series of rare-earth metal constrained geometry CpPN complexes [{η 5 ,η 1 -C 5 Me 4 PMe 2 NAd}-M(CH 2 SiMe 3 ) 2 ] (M = Sc, Lu, Y, Sm, Nd, Pr, Ce) and their high catalytic activities in the intramolecular hydroamination/cyclization have been reported by us. 15,17Recently, the organometallic chemistry and catalysis in ethylene polymerization of rare-earth metal CpPN, IndPN and FluPN complexes was studied.CpPN ligands, with less steric bulkiness of the Cp-ring, lead to the coordination of THF, while IndPN adopt a η 3 -bonding fashion and the more bulky FluPN-type ligands display a η 1 -bonding mode. 18Moreover, the reactivity toward various substrates was recently studied and, among others, CpPN amidinate, hydride and terminal imido complexes were synthesized, characterized and their reactivity was probed. 19hese current developments reveal that CpPN type complexes appear to be a promising class of catalysts.Therefore it is of general and fundamental interest to develop novel, sterically most demanding and rigid CpPN type ligands as useful building blocks and to study their stabilizing properties for dialkyls of the smallest and larger rare-earth metal cations (Sc, Lu, Y, Yb, Sm, Nd and Pr).Besides synthetic and XRD structural aspects, the focus of this study will be on the beautiful 1 H and 13 C NMR spectra obtained from paramagnetic organometallic compounds carrying the chelating rigid Cp™PPh 2 NHAd (L Ad H) and Cp™HPPh 2 NDip (L Dip H) ligands with the tetrahydropentalene unit.Assignment of ligand group shifts of paramagnetic organometallic lanthanide complexes is not routinely reported in literature, but it might become a very valuable tool for following catalytic steps.

Synthesis and characterization of the Cp™PN-ligands
The ligands were prepared by a Staudinger reaction (Scheme 2) of the novel phosphane Cp™HPPh 2 (1, diphenyl(4,4,6,6-tetramethyl-1,4,5,6-tetrahydropentalen-2-yl)phosphane) 24 with organic azides (AdN 3 and DipN 3 ; Dip = 2,6-di-iso-propylphenyl).It should be mentioned that compared to our previously published synthesis of 1 we could exchange the highly toxic TlCp by NaCp, which can easily be synthesized on a large scale out of Na and (CpH) 2 . 25The improved ligand synthesis is following the condensation of NaCp with one Ph 2 PCl and two acetone molecules, which are cheap starting materials making the final Cp™PN ligand very attractive as an alternative for commonly used but expensive ligands with a C 5 Me 4 R moiety.
The Staudinger reaction of the highly crowded Cp™HPPh 2 1 with AdN 3 proceeds very slowly.Therefore, under classical Staudinger conditions a reaction time of 10 d is needed in refluxing THF, yielding L Ad H (Cp™PPh 2 NHAd) in only 46% yield.Higher reaction temperatures accelerate the reaction and it was completed after only 2 d in refluxing toluene.The yellow crystalline compound L Ad H was isolated from n-hexane in 75% yield.In contrast, the oxidation with the more electron-poor DipN 3 was completed within 14 h in THF at room temperature.The desired, sterically demanding ligand L Dip H (Cp™HPPh 2 NDip) was obtained in 78% yield after crystallization from cold acetonitrile.
Compared to L Dip H, which is a highly air-sensitive substance with a melting point of 142.4-143.0°C and a high solubility in n-hexane, compound L Ad H is only moderately airsensitive, has a higher melting point (176.5-177.0°C) and is only marginally soluble in n-hexane.Further investigations by means of NMR spectroscopy and X-ray structure analysis show significant dissimilarities in their molecular compositions: compound L Ad H occurs in the form of P-amino-cyclopentadienyl-phosphorane, whereas L Dip H exists in the tautomeric form of a P-cyclopentadienyl-imino-phosphorane.

NMR spectroscopy
The 31 P NMR signal of L Ad H (15.6 ppm) is essentially identical to those of compound C 5 Me 4 PR 2 NHAd (17.6 for R = Me 15 and 12.3 ppm for the main isomer in R = Ph 16 resp.).In contrast, compound L Dip H shows a 31 P NMR resonance at −15.8 ppm.This chemical shift is in good agreement with the iminophosphorane tautomer IndPPh 2 vNR (Ind = indenyl-1; δ P = −8.3 and −16.5 ppm for R = Ph and Dip resp.). 23However, in contrast to C 5 Me 4 PPh 2 NHAd, 16 C 5 Me 4 HPMe 2 NR (R = SiMe 3 , Dip) 20 and IndPPh 2 vNR (R = Ph and Dip), 23 L Ad H and L Dip H show only one sharp resonance in the 31 P NMR spectra, indicating the absence of isomers.Recently, we also observed only one tautomer in C 5 Me 4 HPR 2 NC 6 H 3 R′ 2 (R = Me, R′ = iPr; R = Ph, R′ = Me, iPr) at thermodynamic equilibrium. 18he resonance of the NH-proton in L Ad H at δ H = 2.05 ppm appears as a doublet ( 2 J HP = 5.0 Hz).It does not correlate with any carbon atom in the molecule according to the HMQC correlation spectrum.This confirms the presence of an aminophosphorane.Unlike L Ad H, compound L Dip H shows four resonances in the aliphatic region (besides resonances of isopropyl (Dip) protons) which resemble the pattern of the parent phosphane 1. Especially the allylic CH 2 group at δ H = 3.08 ppm confirms that the ligand is in the imino-phosphorane form in solution (for spectra see ESI †).No tautomerization or isomerization could be observed for L Ad H and L Dip H by multinuclear NMR spectroscopy in C 6 D 6 .

X-Ray structure analysis
The compound L Ad H crystallizes from benzene at room temperature with two disordered solvent molecules per unit cell.Compound L Dip H crystallizes without incorporated solvent molecules.Both compounds crystallize in monoclinic crystal systems (space groups P2 1 /n and P2 1 /c resp.) with 4 units in the unit cell (Fig. 1).Selected bond lengths (Å) and angles (°) for L Ad H and L Dip H are presented in Table 1.In the structure of compound L Dip H one of the iso-propyl groups is disordered and treated with an occupancy factor of 63 : 37.
The cyclopentadienyl rings of both ligands are essentially planar; the largest deviations from the ideal C5 plane are Δ max = 0.004(2) for L Ad H and 0.006(2) Å for L Dip H resp.
As anticipated the P-C Cp bond length in P-ylide L Ad H is quite short, P1-C1 1.704(2) Å, and can be compared with d(P-C Cp ) 1.718(2) Å in Ph 3 PvC 5 H 4 26 while the P-N bond  2) Å) 28 and Ph 3 PvN(tert-Bu) (1.543(2) Å)). 26 The alternating bond order and the bond lengths in the Cp™-moiety are similar to those found in the parent phosphine Cp™HPPh 2 1 24 and lie in ranges typical for cyclopentadiene compounds. 29The P1-C1 1.788(2) Å is significantly larger than that in L Ad H.Moreover, this value is much closer to P-C Ph bond lengths where a C(sp 2 )-P bond is present.other rare-earth metals (Sc, Y, Sm, Nd, Pr and Yb), the complexes were synthesized under essentially the same reaction conditions reported for [{C 5 Me 4 PMe 2 NAd}M(CH 2 SiMe 3 ) 2 ] complexes. 15,17Following this in situ protocol three equivalents of LiCH The complexes with ligand L Dip reveal a different thermostability in comparison with those bearing ligand L Ad .Compounds 1 Dip -3 Dip appear to be less stable than the analogue complexes 1 Ad -3 Ad .Complexes 1 Dip and 2 Dip are still stable at room temperature in solution but decompose fast at elevated temperatures.Yttrium complex 3 Dip is not stable at room temperature both in solution and in the solid state.Thus, for larger metals such as samarium, no stable complex could be isolated with this ligand.Nd) are isolated with a coordinated THF molecule. 18 1H NMR spectra of the diamagnetic 1 Ad -3 Ad and 1 Dip -3 Dip complexes are very similar (for spectra see ESI †); therefore only some main aspects should be discussed here.Because of the η 5 -coordination of the Cp ring, Cp protons appear as one doublet at about 6.2 ppm with a 3 J HP of about 3 Hz.The signals of the methyl and methylene group in the annulated five membered ring are, because of their fixed exo-and endopositions, chemically inequivalent.Consequently, the methyl group resonances appear as two singlets and the resonances of the methylene group appear as two doublets ( 2 J HH about 12 Hz).Silylmethylene protons are, like in [{C 5 Me 4 PMe 2 NAd}-M(CH 2 SiMe 3 ) 2 ], 17 diastereotopic and for that magnetically non-equivalent.They appear as two doublets in all spectra.For yttrium complexes the protons appear as two doublet of doublets due to Y-H-coupling ( 2 J HH = 11.2Hz, 2 J HY = 2.7 Hz). Furthermore, in this case the methylene carbons show a doublet with a 1 J CY coupling of 40.9 Hz in the 13 C NMR spectrum.Both are in the same range as shown in the literature, for example for classic [{η 5 :η 1 -CpSiN}Y(CH 2 SiMe 3 )(thf )] CGC. 30 Lanthanides have a short relaxation time for the unpaired electron so that little line broadening occurs.The mechanism of action within the lanthanides is principally the pseudocontact mechanism, which falls off in a predictable manner with distance (1/R 3 ). 31The direction of shift depends on the anisotropy in the susceptibility, but it also depends on the angle between the principal axis of susceptibility and the vector R to the nucleus.Despite this principal insight, systematic NMR studies on paramagnetic organolanthanide compounds are not a routine characterization method. 32NMR studies are typically restricted to the diamagnetic derivatives (Lu(III), Sc, Y, and La(III)), 2,4 whereas some of the best catalysts are obtained with the paramagnetic metal cations like neodymium or samarium. 3,4Here we report 1   of the resonances at half-height (ν 1/2 ) is shown in the Experimental section.In Fig. 2, the NMR spectrum of f 2 Pr 3+ complex 6 Ad which ranges from +100 to −53 ppm is shown as a representative example (for other spectra see ESI †).

Multinuclear NMR spectroscopy of CpPN complexes
All resonances of the adamantyl and annulated five-ring moiety protons are shifted upfield while the CH 2 SiMe 3 alkyl groups, the phenyl substituents on the phosphorus and the cyclopentadienyl protons are shifted downfield.
Because of the paramagnetic shifting, the signals of the phenyl protons are distributed over a wide range.The o-PhH are furthest downfield shifted due to the relatively small distance to the paramagnetic metal centre.The m-PhH and p-PhH resonances are less shifted as they are located further away from the metal centre.Using the dependence of the paramagnetic shift on the distance to the metal centre, one can specify the methylene and methyl group resonances of the annulated five ring moiety.The methylene and methyl group resonances that are more upfield shifted are the ones closer to the paramagnetic centre (exo-proton/group) leaving the other to be the endo-proton/group.
For the Sm(III) complex the 13 C NMR spectrum was showing similar but only slightly paramagnetically shifted signals compared to those of the diamagnetic homologues.
CpPN complexes of ytterbium(III) have never been described before.The 1 H NMR spectrum of f 13 Yb 3+ complex 7 Ad reaches from −240 to +163 ppm but still shows all the expected defined signals.For unambiguous assignment of all the NMR signals we also synthesized the new complex [{C 5 Me 4  . 17In Fig. 3, the NMR spectrum of 7 Ad is shown as an example (for 7 see ESI †).The analogue praseodymium complex (6 Ad ) displays a downfield shift of the diastereotopic protons Pr-CH 2 -SiMe 3 to 93.03 and 99.51 ppm and an upfield shift of the adamantyl and annulated five-ring moiety protons to the range between −10 and −53 ppm (Fig. 2), whereas for 7 Ad both are shifted to the contrary way: the diastereotopic proton signals are shifted strongly upfield to −225.45 and −239.26ppm and the adamantyl and annulated five-ring moiety signals are shifted strongly downfield to the range between 38 and 163 ppm.This trend can also be assigned for the other proton signals.It is a consequence of the sign variation of spin densities and therefore of the chemical shift within the lanthanide series. 31

Molecular structures of Cp™PN complexes
The molecular structures of 1 Ad -3 Ad , 2 Dip and 3 Dip were established by X-ray structure analyses.Single crystals were obtained by cooling saturated n-hexane (2 Dip and 3 Dip ) or n-pentane (1 Ad ) solution to −30 °C.One pentane molecule is incorporated in the unit cell of structure 1 Ad .Single crystals of 2 Ad were obtained from benzene at room temperature with one solvent molecule per unit cell.Single crystals of 3 Ad were obtained by slowly evaporating a toluene solution, while one toluene molecule is incorporated in the unit cell.Complexes 1 Ad -3 Ad crystallize in the triclinic space group P1 ˉwith the two formal units in the unit cell (Fig. 4).In contrast to 1 Ad the different incorporated solvent molecules in structures 2 Ad and 3 Ad have little effect on the unit cell and for that they are isostructural.Structures 2 Dip and 3 Dip are also isostructural and crystallize in the orthorhombic space group Pbca with 8 formal units in the unit cell.Selected bond lengths (Å) and angles (°) for 1 Ad -3 Ad , 2 Dip and 3 Dip are presented in Table 3.In the structure of the complexes 2 Dip and 3 Dip one of the CH 2 SiMe 3 groups is disordered and treated with an occupancy factor of 56 : 44 and 62 : 38, respectively (Fig. 5).Despite the sterically demanding Cp-moiety 1 Ad -3 Ad , 2 Dip and 3 Dip reveal that the Cp ring coordinates to the metal centre in a typical η 5 mode, while IndPN adopt an η 3 -bonding fashion and the more bulky FluPN-type ligands have a rare η 1 -bonding mode. 18In the solid state, all complexes adopt mononuclear structures, in which the metal atoms reveal a pseudo-tetrahedral coordination by the η 5 -bonded C5-ring and the nitrogen atom of the CpPN-ligand together with two σbonded alkyl groups.The pseudo-tetrahedral environment around the metal centre can be shown by comparison of the C CH2SiMe3 -M-N1, C CH2SiMe3 -M-N1, C CH2SiMe3 -M-C CH2SiMe3 bond angles, which are all very close to 109°.

Conclusions
A new sterically most demanding CpPN chelate ligand system, cheaper than all tetramethyl-based building blocks and therefore more privileged to provide constrained-geometry complexes and catalysts of many more metals, has been developed.By condensation of NaCp with Ph 2 PCl and with two molecules of acetone, followed by carbolithiation and Staudinger reaction of the phosphane Cp™HPPh 2 (1, diphenyl(4,4,6,6-

General procedures
All manipulations were performed under purified argon or nitrogen using standard high vacuum or Schlenk-or Gloveboxtechniques.Solvents were dried and distilled under argon employing standard drying agents.All organic reagents were purified by conventional methods.NMR spectra were recorded   Crystal data were collected with a Stoe-IPDS area-detector diffractometer using graphite-monochromatised Mo-K α -radiation (λ = 71.073pm) at 193 K (L Ad H, 3 Ad , 2 Dip ) or with a Stoe IPDS2 diffractometer at 100 K. Data reduction was carried out using the IPDSI software or X-Area (Stoe). 46The data were empirically corrected for absorption and other effects by using multiscans, 47 except for compound L Ad H, where no improvement in the refinement was achieved through its application.The structures were solved by direct methods (Sir-92, 48 Sir-2004, 49 and SHELXS-97 50 ) and refined by full-matrix least-squares techniques against F o 2 (SHELXL-97). 47C bonded hydrogen atoms were included in idealized positions and refined with fixed isotropic displacement factors.The N-bonded hydrogen atom of L Ad H was located and refined isotropically.The program PLATON 51 was used to check the results of the X-ray analyses.Diamond was used for 30% thermal ellipsoid representations. 52  (1).To 9.43 g Na(C 5 H 5 ) (108 mmol, 1.03 eq.) in 200 mL of n-pentane at 0 °C, 23.16 mL of PPh 2 Cl (105 mmol, 1.00 eq.) was added.The mixture was stirred for 16 h at ambient temperature, and then 10 mL of ethane-1,2diol was added under vigorous stirring.The solution was decanted from the precipitate, the precipitate was washed twice with 20 mL of n-pentane, and the solvent of the transferred solution was evaporated in a vacuum.The following steps were carried out according to the literature, while spectroscopic features match perfectly with the reported ones. 24ynthesis of ligand L Ad H.To a solution of phosphane 1 Cp™HPPh 2 (2.40 g, 6.93 mmol, 1.00 eq.) in 30 mL of toluene, AdN 3 (1.35g, 7.62 mmol, 1.10 eq.) was added and stirred at 120 °C for 17 h.The colour of the reaction mixture progressively turns brown.The reaction proceeding was monitored by 31 P NMR spectroscopy.The solvent was completely removed in a vacuum and the oily residue was dissolved in 10 mL of n-hexane yielding a clear dark brown solution.Upon sonification of the n-hexane solution a yellow powder precipitated.It was filtered off, washed with 3 mL of n-hexane and dried in a vacuum.Yield: 2.33 g (4.70 mmol, 68%).M.p. =

length 1 .
652(2) Å is rather long.This can be better compared with the values found in phosphonium salts [Ph 3 P-NH(iso-Pr)]-Br (P-N = 1.621(3)Å).27The C-C bonds of the Cp ring of L Ad H are conjugated (average bond length d(C-C) = 1.41 Å, maximum C-C bond difference d = 0.07 Å).The parameters resemble the expected values for a cyclopentadienylideneaminophosphorane structure.The structure of L Dip H is unexceptional with a short P-N bond length of 1.556(2) Å, typical for imino-phosphoranes (compare: Ph 3 PvN(2,6-C 6 H 3 ) (1.553(

For
Fig. 1 Molecular structures of L Ad H × 2C 6 H 6 and L Dip H.All hydrogen atoms, except N-H for L Ad H and protons of the C5-ring for L Dip H, have been omitted for clarity.Incorporated benzene molecules and disordered iso-Pr group with lower occupancies have also been omitted for clarity.
tetramethyl-1,4,5,6-tetrahydropentalen-2-yl)phosphane) with organic azides (AdN 3 and DipN 3 ; Ad = 1-adamantyl; Dip = 2,6-di-isopropylphenyl), the two novel chelate ligands Cp™PPh 2 NHAd (L Ad H) and Cp™HPPh 2 NDip (L Dip H) were obtained in high selectivity and yields.Depending on the substituent at the nitrogen atom, they occur either in the P-amino-cyclopentadienylidene-phosphorane (R = Ad) or in the P-cyclopentadienyliminophosphorane (R = Dip) tautomeric form.Neither possesses any NMR detectable prototropic or elementotropic isomers.The rare-earth metal complexes were synthesized following a one-pot protocol, which combines deprotonation and salt elimination methods by addition of 3 equivalents of LiCH 2 SiMe 3 as a base/ligand to a stirred mixture of the corresponding THF or DME solvated rare-earth metal trihalide and the appropriate {Cp™PN}H ligand.The very short synthetic protocol allows the successful isolation of the highly reactive and labile alkyl complexes of early lanthanides, which are usually prone to decompose in solution at ambient temperature.All complexes [{Cp™PN}M(CH 2 SiMe 3 ) 2 ] (M = Sc (1 Ad , 1 Dip ), Lu (2 Ad , 2 Dip ), Y (3 Ad , 3 Dip ), Sm (4 Ad ), Nd (5 Ad ), Pr (6 Ad ), Yb (7 Ad )) were isolated as microcrystalline solids and were completely characterized by microanalysis and partially by X-ray crystal structure determination.As a non-routine characterization method for organolanthanide complexes an extensive NMR study of a series of paramagnetic complexes with assignment of all signals is presented.Paramagnetic complexes Sm (4 Ad ), Nd (5 Ad ), Pr (6 Ad ) reveal an almost mirror inverted signal order compared to previously unknown ytterbium(III) CpPN complexes 7 Ad or its counterpart [{η 1 :η 5 -C 5 Me 4 PMe 2 NAd}Yb-(CH 2 SiMe 3 ) 2 ] 7.

Table 1
Selected bond lengths (Å) and angles (°) for L Ad H and L Dip H H NMR spectra recorded in C 6 D 6 at 27 °C of paramagnetic complexes 4 Ad -7 Ad that show defined, relatively sharp signals with distinctive paramagnetic shifts, indicating a rigid constrained-geometry structure in solution.The assignment of the NMR signals for complexes 4 Ad -6 Ad requires 2D NMR experiments due to the paramagnetic shift.The signals are summarized in Table 2.The width