Variable coordination of a chiral diphosphine containing an amidinium / NHC group within its backbone : μ-P , P ′ , κ 2P , P ′ and κ 3P , C , P ′ coordination modes † ‡

A diphosphine ligand (1·HPF6), which is a potential precursor to a PCNHCP pincer, with a backbone containing two phenylene groups and a central bicyclic 4-aza-2-azoniabicyclo[3.2.1]oct-2-ene unit has been synthesised and coordinated to Pd(II) and Pt(II) to give trans-[M(κ-P,P′-1·H)Cl2]PF6 where M = Pd (2) or Pt (3a). Single-crystal structure determinations of 2 and 3a show the complexes to be isostructural with the diphosphine coordinated in a trans-spanning fashion and the amidinium unit being protonated and non-coordinated. 2 and 3a react with CH3I to give the dimers trans-[Pd2(μ-1·H)2I4](PF6)2, 6, and trans[Pt2(μ-1·H)2I4](PF6)2, 7, as the major products. This bridging mode of coordination of [1·H] + is also seen in trans-[Rh2(μ-1·H)(1,5-COD)2Cl2]PF6, 4, and [Pt2(μ-κ -1·H)(dvdms)]PF6, 5. Upon treatment with KO Bu complexes 2 and 3a undergo deprotonation at the amidinium carbon to give trans-[M(κ-P,C,P′-1)-Cl]PF6 where M = Pd (8), and Pt (9). The related trans-[Rh(κ-P,C,P′-1)(CO)]PF6 (10) is prepared directly from 1·HPF6 and Rh(acac)(CO)2: this and the palladium and platinum complexes 8 and 9 are isolated as isomeric mixtures as a consequence of a conformational isomerism. In situ deprotonation of 1·HPF6 followed by addition of Ag(CF3SO3) gave SAg-[Ag(κ -P,C,P′-1)(CF3SO3)], 11. Some preliminary studies of the reactivity of 2 and 8 in Suzuki-type reactions are reported and the Pt(0) system has been shown to be an active hydrosilylation catalyst.


Introduction
Wide angle diphosphines have been heavily exploited in homogeneous catalysis as demonstrated by the studies of van Leeuwen and others on the hydrocyanation and hydroformylation of alkenes. 1 The success of such systems, e.g. the Xanthos series, resides in the ability of the two phosphine donors to adopt an obtuse bite angle enforced by the rigid nature of the ligand backbone.These relatively fixed bite angles, which typically lie between 110 and 130°, assist in promoting metal-based catalysis but preclude true trans coordination at a square planar metal.Other diphosphines are able to coordinate to give P-M-P angles of ∼180°and these are defined as trans-spanning ligands.1e,f,2 These are generally of two types: those with rigid backbones that strongly favour trans-coordination which can restrict catalysis through 2e redox processes; 2a,3 and those that are flexible and able to promote catalysis through rearrangement to active cis-or monodentate forms.2a,4 Depending on the nature of the intervening ligand architecture, some trans-spanning diphosphines can undergo metallation at an atom in the ligand backbone to give PXP pincer complexes (where X is often carbon). 5Such complexes (which are not restricted to a PCP donor set) are finding increasing employment in homogeneous catalysis as they are usually robust by nature and able to withstand the high temperatures demanded by certain types of transformation notably alkane dehydrogenation. 6They are also proving useful in non-oxidative bond activation through metal-ligand cooperation. 7he range of pincer ligands containing neutral NHC carbon donors includes those with a single central carbene donor 8 and those with two carbenes in the terminal positions 9 while homoleptic tricarbenes tend to be of the facial type. 10We are interested in expanded-ring NHC-based tridentates incorporating a central C NHC donor and have recently reported a dipyridyl-ER-NHC (L py2 ) containing a bicyclic core derived from camphor. 11These earlier investigations established some of the coordination chemistry of the ligand and highlighted its relative flexibility as a N,C NHC ,N′ donor with both planar and non-planar species being isolated.In [Ni(κ 3 -N,C,N′-L py2 )Cl] + where the three donor atoms and the metal are essentially coplanar, twisting in the ligand backbone was observed giving rise to λ and δ conformational isomers.11a However the related Pd(II) and Pt(II) complexes of L py2 proved elusive.Isomerism of a different type was also observed in the [Rh(1,5-COD)(L py2 )] + and [Ir(1,5-COD)-(L py2 )] + .The ligand occupies one axial and two equatorial sites in these trigonal bipyramidal five-coordinate complexes and the source of the isomerism here lies in the relative orientation of the central bicyclic structure and the pyridyl groups leading to endo, endo or exo,exo species.11b As coordination flexibility in multidentate ligands can have a profound effect on catalytic performance, we were interested in exploring the coordination behaviour of NHC based systems bearing donors other than pyridine groups.As part of this broader study we have opted to explore the coordination chemistry of a diphosphino-ER-NHC containing the bicyclic 6/7 membered NHC skeleton, the initial results of which are presented here.

Pro-ligand synthesis
The synthesis of the pro-ligand 1•HPF 6 is highlighted in Scheme 1 and follows an already established procedure for the related dipyridyl analogue.11a The condensation of two mols of 2-(diphenylphosphino)benzaldehyde with 1R,3S-diamino-1,2,2trimethylcyclopentane gives the diimine as an off-white solid.Subsequent reduction with NaBH 4 produces the diamine which is readily condensed with triethylorthoformate in the presence of NH 4 PF 6 to give 1•HPF 6 as a white solid.The salt is relatively air-stable in the solid state but does slowly oxidise when in solution.The two phosphine groups in 1•HPF 6 are different and, although the source of this distinction is remote (H versus CH 3 at the ε position), two signals are observed in the 31 P{ 1 H} NMR spectrum at δ P = −16.6 and −17.5 ppm.The 1 H NMR spectrum shows the NCHN proton at δ H = 7.79 ppm which compares to a value of 8.14 ppm for this hydrogen in the dipyridyl derivative L py2 .11a The only other signals of any diagnostic value in the 1 H spectrum are those for the benzylic hydrogens which occur as three doublets of doublets between 4.84 and 4.42 ppm and a doublet at 4.28 ppm.The pro-ligand is only modestly soluble in alcohol and (although it appears to slowly oxidise in CDCl 3 ) can be crystallised from MeOH in air to give crystals suitable for structural analysis by single-crystal X-ray techniques.The molecular structure of the compound is shown in Fig. 1.The C-N bond lengths to the amidinium carbon in the two independent molecules of the unit cell average 1.315 Å and the C-N-C angles are 123.1°(average).These compare to values of 1.314(3) Å and 123.5(2)°and 1.307(6) Å and 123.0(4)°in the dibenzyl and dipyridyl derivatives respectively.11a (κ 2 -P,P′-1•H) complexes The design of the ligand is such that coordination as a bidentate through both phosphorus atoms can lead to cis-and transisomers although the latter may be encouraged by the lengthy and semi-rigid backbone.Initial interest was to see whether κ 2 -P,P′ coordination could be achieved without activation of the amidinium group to investigate potential cis/trans isomerism in the bidentate systems.When one mol of 1•HPF 6 and K 2 [PdCl 4 ] were heated in EtOH at 70 °C a bright yellow solid precipitated from the solution The isolated complex gives an AB pattern in the 31 P{ 1 H} NMR spectrum with δ P values of 24.7 and 22.3 ppm respectively.The large magnitude (542 Hz) of the 2 J P-P coupling constant suggests that the two inequivalent phosphorus donors are mutually trans.2a,b The compound is confirmed as trans-[Pd(κ 2 -P,P′-1•H)Cl 2 ]PF 6 , 2, through determination of its molecular structure by single-crystal X-ray techniques (Fig. 2).The P-Pd-P bond angles average 173.6°for the two independent molecules in the unit cell and the average Pd-P bond length of 2.344 Å is slightly longer than the 2.322 Å and 2.316 Å observed in related trans-spanning diphosphine complexes of palladium.4a,12 The Pd-Cl bond lengths average 2.293 Å and compare with values of 2.292 and 2.312 Å for the related complexes.4a,12 The amidinium group has not undergone deprotonation during the preparation of 2 but it does appear pre-set for Scheme 1 Synthesis of pro-ligand 1•HPF 6 .− anion is not shown.Thermal ellipsoids are drawn at 50% prob-coordination upon treatment with base; this conclusion derives from the orientation of the NCN carbon which points towards the metal.This conformation is favoured by an intramolecular electrostatic interaction between the positively charged amidinium group and the chloride ligands that manifests itself as a noticeable bending of the two chlorides towards the amidinium group.Although the average Cl-Pd-Cl bond angle of 170.0°is not atypical, as similar or even greater distortions are observed in related trans-Pd(P 2 )Cl 2 complexes, 2a the nature of the distortion in 2 is distinct as the flexing in these other complexes is away from the backbone of the diphosphine as dictated by steric effects.2a,4a The 1 H NMR spectrum of 2 shows the NCN hydrogen at δ H 9.58 ppm, a shift of 1.79 ppm downfield of its position in the uncoordinated ligand.This may be a consequence of the electrostatic ( possibly weak H-bonding) interaction with the Cl ligands or might be a result of its position along the z axis of the palladium.Downfield shifts for hydrogens orientated in relatively fixed positions over a palladium or platinum coordination plane have been noted previously.2i,j All four hydrogens of the methylene groups are inequivalent as evidenced by the presence of four doublets in the 1 H NMR spectrum between 5.02 and 4.61 ppm with 2 J H-H coupling constants of 14.5 and 15.2 Hz.The remainder of the 1 H NMR spectrum is less informative providing little diagnostic information.The amidinium carbon is observed at δ C = 154.5 ppm in the 13 C{ 1 H} NMR spectrum of 2. This is little shifted from its position in the spectrum for the free salt.The remainder of the 13 C{ 1 H} NMR spectrum mimics closely that for 1•HPF 6 except there is no observable coupling between the benzylic carbons and the phosphorus centre.
The complex [Pt(κ 2 -P,P′-1•H)Cl 2 ]PF 6 is isolated as a cream solid from the reaction of 1•HPF 6 with K 2 [PtCl 4 ] in EtOH.Inspection of the 31 P{ 1 H} NMR spectrum of the isolated solid shows it to be a mixture of two complexes: a major species 3a described by an AB pattern in the 31 P{ 1 H} NMR spectrum and a minor species 3b that gives a broad singlet (see below).The two complexes were separated by simple treatment with CHCl 3 which readily dissolved the major compound leaving the minor component as an insoluble solid.
The chloroform soluble complex 3a gives an AB pattern in the 31 P{ 1 H} NMR spectrum at δ P = 20.0 ( 1 J P-Pt = 2523 Hz) and 19.7 ( 1 J P-Pt = 2526 Hz) ppm.The 1 H NMR spectrum is closely similar to that for trans-[Pd(κ 2 -P,P′-1•H)Cl 2 ]PF 6 with a downfield shifted NCHN resonance at 9.90 ppm.The similarity in the spectra for this complex and 2 suggests that they are isostructural which is confirmed upon inspection of the solid state structure (Fig. 3).The Pt-P bond lengths average 2.332(5) Å which are close to the value of 2.304(5) Å seen for Pt(SPANphos)Cl 2 3b and 2.3204(12) Å in a related complex.4c The P-Pt-P angle however is significantly more obtuse at 178.9(2)°in 3a compared to the lower values of 171.9(4)°and 174.43 (5)°seen with the reported complexes.
The broad resonance seen in the 31 P{ 1 H} NMR spectrum of the minor product 3b sharpens to a singlet (δ P = 13.5 ppm, 1 J P-Pt = 2556 Hz) upon heating to 80 °C in d 6 -DMSO.The 1 H NMR spectrum recorded at this temperature showed the amidinium hydrogen at 8.31 ppm which is not significantly shifted from its position in the spectrum of the uncoordinated ligand.The benzylic CH 2 hydrogens are inequivalent and appear as four separate resonances between δ H = 5.80 and 4.77 ppm.One of the methyl groups is seen to resonate at an unusually high position (0.17 ppm) in the 1 H NMR spectrum of 3b which might indicate a residence within the shielding region of one or more of the aromatic rings.Although the two phosphorus centres are inequivalent they clearly have the same chemical shift in the 31 P{ 1 H} NMR spectrum of 3b as only one resonance is seen.The magnitude of the 1 J P-Pt coupling constant would suggest that it is not a cis-isomer as a larger J value would have been expected if the phosphines were trans to chloride. 13Thus the complex has likely trans-P donors but is seemingly not a monomer as the chloroform soluble compound 3a has been shown to be this species.The most likely structure of this complex is therefore the dimer trans-[Pt 2 (μ-1•H) 2 (Cl) 4 ](PF 6 ) 2 , although this could not be confirmed by mass spectrometry as only monomeric species were observed for the compound.

(μ-1•H) complexes
The 1 : 1 reaction of 1•HPF 6 and [Rh(1,5-COD)Cl] 2 in MeOH at near reflux led to the formation of a light yellow solid.Inspection of the 1 H NMR spectrum of the isolated solid revealed the presence of coordinated COD and [1•H] + in a 2 : 1 ratio.Other characteristic features of the 1 H NMR spectrum include the amidinium hydrogen at δ H = 8.18 ppm and the four inequivalent benzylic methylene hydrogens which appear as four doublets between δ H 6.5 and 5.6 ppm.The relative ratio of the [1•H] + and COD resonances coupled with the chemical shift of the amidinium hydrogen would suggest that the isolated complex is [Rh 2 (1,5-COD) 2 (μ-1•H)Cl 2 ]PF 6 , 4. The 31 P{ 1 H} NMR spectrum consists of two doublets at δ P 21.7 and 21.4 with 1 J P-Rh being 146 Hz.Although a coordination shift is evident and both resonances show coupling to rhodium, the lack of any 2 J P-P coupling supports the above assignment where the amidinium diphosphine is coordinating in a bridging mode.The magnitude of the 1 J P-Rh coupling is commensurate with that expected for a triarylphosphine trans to a CvC donor of COD as noted in several complexes of the type [Rh(1,5-COD)(PAr 3 )Cl]. 14he reaction of 1•HPF 6 with Karstedt's complex Pt 2 (dvdms) 3 in THF gave an air-stable cream coloured solid upon work-up.The 31 P{ 1 H} NMR spectrum consisted of two singlets at 20.7 and 18.7 ppm each with Pt-195 satellites ( 1 J P-Pt = 3494 and 3506 Hz, respectively).As for the rhodium complex 4 above, the presence of both a coordination shift and P-Pt coupling confirms coordination of the phosphine donors and the absence of any 2 J P-P coupling indicates a μ bonding mode.Inspection of the 1 H NMR spectrum of the complex reveals the amidinium hydrogen at δ H 8.18 ppm which is exactly as observed in the Rh dimer 4. Integration of selected resonances of the diphosphine and the dvdms reveal the ratio to be 1 : 2 and the most likely formulation of the product is [Pt 2 (dvdms) 2 (μ-1•H)]PF 6 .This accords with similar systems reported by Lappert where comparable values of Δδ and 1 J P-Pt are noted. 15This assignment is further supported upon inspection of the 195 Pt NMR spectrum which shows two doublets of multiplets at δ Pt = −5553 and −5563 ppm for the two platinum centres in the dimer.These chemical shift values again compare with related Pt(PR 3 )(dvdms) 2 complexes. 15,16hen 2 was reacted with excess MeI at 50 °C in CHCl 3 an orange precipitate was formed.Inspection of the 31 P{ 1 H} NMR spectrum of the product showed a singlet at δ P = 21.1 ppm.The 1 H NMR spectrum was similar to that for 2 albeit with notable shifts in δ H for the amidinium hydrogen and the benzylic CH 2 hydrogens.The MS data showed a peak for the complex [Pd(1•H)I 2 ] + at 1063 amu confirming that the chloride ligands had been exchanged for iodide presumably though two successive oxidative addition (MeI)/reductive elimination (MeCl) cycles.The similar spectroscopic features of 2 and 6 suggest that 6 is a monomer containing trans-bound [1•H] + .The orange solid could be recrystallised from MeCN to give crystals suitable for analysis by single-crystal X-ray techniques (Fig. 4).As can be seen from the structure shown in Fig. 4 the complex has rearranged upon recrystallisation to give the [Pd 2 (μ-1•H) 2 I 4 ]-I 0.27 (PF 6 ) 1.73 species containing two bridging [1•H] + ligands.The coordination about each of the Pd centres is the expected square planar arrangement with the P-donors (and by necessity the I − ligands) being mutually trans.The Pd-P bond lengths average 2.33 Å which is slightly shorter than in 2 indicating a small level of strain in the chelate.The P-Pd-P angles are unexpectedly acute at 166°compared to the 174°in 2 while the I-Pd-I angles at 169°are similar to the value of 170°seen for the Cl-Pd-Cl angle in 2. The nature of the bending is such that the palladium atoms point inwards with regard to the cavity defined by the dimetallo macrocycle (Fig. 4).When the crystals were redissolved in CD 3 CN for spectroscopic analysis, the resultant 31 P{ 1 H} and 1 H NMR spectra were complex indicating the presence of a number of species in solution.Substitution of up to four iodides in the dimer by CD 3 CN is one possible source of solution complexity, however it seems likely that the μ-1•H coordination mode of the diphosphine is not so robust that bidentate modes are precluded in solution.
The reaction of 3a or 3b (or a mixture of both) with MeI gave an orange solid 7 that showed a similar pattern to the 3a/b mixture in its 31 P{ 1 H} NMR spectrum.Significantly however the position of the resonances had shifted ∼10 ppm upfield commensurate with the replacement of the two chloride ligands in 3a/b with iodide.This was confirmed by mass spectrometry where a species correlating to [Pd(1•H)I 2 ] + was observed at 1151 amu.The magnitude of the 1 J P-Pt coupling constants were comparable to those in 3a/b as expected for phosphorus donors situated mutually trans.No attempt was made to separate the mixture.
(κ 3 -P,C,P′-1) complexes In the examples discussed above the diphosphine ligand coordinates solely through the phosphorus atoms and the third potential donor (the NHC carbon) is protonated and hence noncoordinating.The solid-state structures of the platinum and palladium complexes of κ 2 -[1•H] + show this carbon to be poised for coordination upon removal of the proton so that tridentate forms should be accessible upon addition of base to 2 and 3a.When 2 was treated with 1.2 equivalents of KO t Bu in THF the colour of the solution changed from yellow to a deep blood-red and the complex [Pd(κ 3 -P,C,P′-1)Cl]PF 6 , 8, was isolated as a red solid  after work-up.Inspection of the 31 P{ 1 H} NMR spectrum of 8 revealed the presence of two sets of AB doublets around 5 ppm suggesting the presence of two isomers in solution.This was confirmed upon observation of the 1 H NMR spectrum where a total of six CH 3 resonances and eight separate doublets for the benzylic methylenes were seen.Some notable features of the 1 H NMR spectrum include an unusual chemical shift for two of the methyl resonances to highfield of TMS and a large δ H separation for the hydrogens of any given benzylic-CH 2 group with one resonating over 2 ppm downfield of its geminal partner.It would appear that the binding of the ligand in the κ 3 -mode has placed one of the methyl groups of each isomer in a region that experiences a local shielding effect presumably from one or more of the aromatic rings.Similarly, one geminal hydrogen of each benzylic group in either isomer resides in a deshielded zone.Isomerisation in heterodonor terdentates containing a central NHC donor has been observed by us previously in square planar and five-coordinate trigonal bipyramidal metal systems. 11Conformational differences in the ligand backbone are the source of the isomerism which is dictated by the desire of the NHC donor to be canted with respect to the coordination plane as shown schematically in Fig. 5; in related square planar metal systems where the NHC does not have pendant donors the NCN link of the carbene donor is known to lie approximately perpendicular to the coordination plane.11a As noted above the 31 P{ 1 H} NMR spectrum of 8 consists of two AB patterns for the two conformers with 2 J P-P coupling appreciably smaller at 436 and 435 Hz than observed in 2.More surprising is the upfield shift in δ P that accompanies the change from κ 2 to κ 3 binding as the reverse (downfield) trend is more often observed upon formation of smaller chelates.These more familiar downfield shifts often reflect the presence of shorter M-P bond lengths and it might be that the transition to the κ 3 mode leads to longer Pd-P bond lengths compared to those seen in 2; unfortunately this could not be confirmed as crystals suitable for single-crystal structure determination were not forthcoming.There was no evidence of 2 J C-P coupling to the NCN carbons in the 13 C{ 1 H} NMR spectrum of 8 where two singlets at 178.2 and 177.2 ppm occur for each isomer respectively.The remainder of the 13 C{ 1 H} spectrum is not of diagnostic value other than to confirm the presence of the two isomers.
The analogous platinum complex [Pt(κ 3 -P,C,P′-1)Cl]PF 6 , 9, was prepared by a similar procedure to that for 8 and isolated as a cream solid.The 1 H NMR spectrum of 9 mimics closely that of 8 as does the 31 P{ 1 H} NMR spectrum although this does have the usual Pt-195 satellites with 1 J P-Pt coupling constants of 2552 and 2570 Hz.These compare with those reported for 3a/b above confirming the retention of the trans-P arrangement in 9.The NCN carbons for each isomer are observed as triplets at 166.2 and 164.9 ppm with 2 J C-P of 9.2 Hz in the 13  2 )](PF 6 ) 2 containing a macrocyclic ligand. 18A larger value of 25 Hz has been reported for a related P,C,P complex containing a central NHC donor.8h As a chelating complex was not obtained from the reaction of 1•HPF 6 with [Rh(1,5-COD)Cl] 2 , access to a κ 3 complex through the addition of base to a preformed [Rh(κ 2 -P,P′-H•1)(L) 2 ] n+/− species was precluded and a different means of synthesis was required.[Rh(acac)(CO) 2 ] was considered an ideal starting material as it contains an internal base (acac − ) for in situ deprotonation after coordination of one or both phosphines.The reaction of 1•HPF 6 with 1 equivalent of [Rh(acac)(CO) 2 ] in THF at reflux gave, after removal of all volatiles, a yellow solid that was recrystallised from MeOH.The 1 H NMR spectrum confirmed the loss of the amidinium proton and the coordinated acac ligand suggesting that the desired reaction had occurred to give [Rh(κ 3 -P,C,P′-1)(CO)]PF 6 , 10.The structure of the complex was confirmed upon closer inspection of the 1 H NMR spectrum which showed the characteristic features for κ 3 -1 coordination, i.e. the presence of two isomers with two methyl resonances upfield of TMS and eight doublets for the benzylic hydrogens with each geminal pair being separated by >2 ppm.The 31 P{ 1 H} NMR spectrum consisted of two almost overlapping ABX multiplets with 2 J P-P having a magnitude of 268 and 274 Hz and 1 J P-Rh = 134 Hz.This value is typical of a trans-phosphine complex 19 being only slightly smaller than the value of 142 Hz for the trans-PPh 3 groups in Rh(PPh 3 ) 3 Cl 20 and is exactly the same as the values quoted for complexes of the type trans-[Rh(L)(PPh 3 ) 2 (CO)]X 21 where L is a saturated five-membered NHC.The infrared spectrum of 10 showed the CO stretch for the coordinated carbonyl ligand at 2008 cm −1 which is directly comparable to the values observed by Lappert and co-workers for trans-[Rh(5-NHC)(PPh 3 ) 2 (CO)]X. 21he examples above are systems in which deprotonation of the amidinium group succeeds coordination of the phosphine donors (this is known for the preformed Pd and Pt complexes 2 and 3a/b and implied for the Rh).The common approach of in situ deprotonation of 1•HPF 6 followed by addition of the resultant free carbene to an appropriate metal precursor had, to this point, not been necessary.However, it was of interest to know if such an approach could be used with 1•HPF 6 as the N,N′-dibenzyl analogue of 1•HPF 6 does furnish complexes via this approach while the N,N′-di(2-pyridyl) derivative does not.11a Part of the problem with benzyl derived NHCs is the facility by which they undergo a 1,2-migration of the benzylic substituent in their unprotected form. 22Fortunately when this approach was attempted in THF with AgOTf the complex [Ag(κ 3 -P,C,P′-1)(OTf )], 11, was isolated as a grey solid.This was encouraging as the common method of preparing Ag-NHC complexes directly from the amidinium salt and Ag 2 O did not work with 1•HPF 6 .The 31 P{ 1 H} NMR spectrum of the isolated solid appears relatively complex because of the presence of P-Ag(107/109) coupling but is essentially two superimposed ABX patterns, one for each of the two isotopomers.The value of the 1 J P-Ag(107) and 1 J P-Ag(109) coupling constants are 378 and 327 Hz respectively, reflecting the inherent difference in the gyromagnetic ratio for the two isotopes.The relatively small value of 65.5 Hz for 2 J P-P is not consistent with the large values of 300-600 Hz expected for trans-orientated P-donors and it would appear that the silver complex does not contain a PCPM plane.This is largely to be expected as d 10 Ag(I) does not tend to form octahedral or square planar complexes and, if 11 is monomeric (although bridging forms of PC NHC P ligands are known 23 only monomeric species were observed in the mass spectrum of 11), then the likely geometry would approximate to tetrahedral as noted for the related [Cu(κ 3 -P,C,P′-1)X] species. 24he absolute magnitude of the 2 J P-P coupling constant compares well with known tetrahedral phosphine containing complexes of silver(I) such as [AgBr(κ 2 -P,P′-dppp)(κ 1 -P-dpppO)] where a value of 58 Hz is observed. 25The 1 J P-Ag coupling constants are somewhat larger than those of 253 and 220 Hz observed in [AgBr(κ 2 -P,P′-dppp)(κ 1 -P-dpppO)]. 25Even though the coordination in the Ag(I) complex does not contain a planar PCPAg unit, there are still two possible isomers as shown in Fig. 6.The related copper(I) systems [Cu(κ 3 -P,C,P′-1)X] show a halidedependant diastereomerism with the R Cu configuration being favoured when X = I − and the S Cu configuration when X = Cl − . 24The 1 H NMR spectrum of 11 shows no evidence of the presence of more than one isomer although some of the signals in the 1 H NMR spectrum are broadened slightly, most notably for one set of benzylic CH 2 resonances (AB pattern) and one of the methyl signals.The absence of a CH 3 resonance upfield of TMS in the 1 H NMR spectrum would suggest that the preferred isomer of 11 has the S configuration at the metal and that this isomer is formed preferentially.Some very slight broadening is also evident for the benzyl methylenes in the 13 C{ 1 H} NMR spectrum of 11 although the spectrum is otherwise well resolved and there is some evidence of a second species in solution as each of the major peaks is associated with a minor peak.The NCN resonance however was not clearly resolved in the spectrum and was seen as a broad unstructured peak at 211 ppm.This is not surprising as this carbon is expected to be coupled to both phosphorus atoms and the silver for each of the isotopomers of the complex.The chemical shift of this carbon is downfield that of the dibenzyl analogue in a complex of the type [Ag(L) 2 ]-OTf 11a but compares to other silver complexes of 6-and 7-membered NHCs. 26here have been very few studies of PCP ligands where the central donor is an NHC carbon.8b,c,e,22 Of these, none contain a ring-expanded NHC and only one consists of two 7-membered chelates.8e The Cu(I) complex of the latter ligand does not show a normal coordination mode but rather the NCN carbon acts as a bridging ligand in a dimeric Cu(I)-Cu(I) complex.8e Such a motif is more common for monovalent silver 27 but is not considered to apply to complex 11 as both phosphines are clearly attached to a single Ag(I) centre as evidenced by the presence of 2 J P-P coupling.It is also noteworthy that the related Cu(I) complex is a tetrahedral monomer. 24

Catalysis
Complexes of staunchly trans-P 2 chelates are not anticipated to catalyse transformations which involve reductive elimination.This is especially so when the catalytic cycle requires a transition from a square planar d 8 complex to a tetrahedral or lower coordinate d 10 system upon reduction, e.g.Pd(II) → Pd(0).Such inactivity is associated with the relative position of the two potential coupling partners: being mutually trans they cannot eliminate.Less rigid P 2 donors may show a preference for transcoordination but their increased flexiblity enables cis-bidentate binding or monodentate coordination (either through bridging or transient loss of one P-donor) and thus facilitate catalysis by cycles that do involve reductive elimination.Both [1•H] + and 1 have been shown to be relatively promiscuous in their mode of metal binding and selected palladium and/or platinum complexes were anticipated to be active for catalysis.
In order to test this assumption some preliminary C-C coupling and hydrosilylation catalytic tests were performed with 2, 8 and 5.The results of the Suzuki coupling of various aryl halides and phenylboronic acid are shown in Table 1.Good conversions were seen for the aryl bromides after 18 h at 80 °C but no activity was observed with the aryl chlorides.The palladium complexes 2 and 8 show little difference in reactivity which may suggest a common catalytic cycle.The fact that κ 3 -P,C,P′-1 has been shown to support d 8 and d 10 metal systems coupled with Fig. 6 Diastereomeric possibilities for κ 3 -1 at pseudo tetrahedral silver(I) in complex 11. the basic conditions of the catalysis tends to suggest the presence of κ 3 -P,C,P′ complexes in the catalytic cycle for both 2 and 8. Platinum complexes have long been known as catalysts for the hydrosilyation of unsaturated hydrocarbons.Zerovalent platinum complexes containing divinyldisiloxane feature prominently in the literature 28 usually with a secondary controlling ligand as Karstedt's complex Pt 2 (dvdms) 3 , although active, is poorly selective. 29Given this history it is not surprising that 5 acts as a catalyst for the hydrosilylation of alkynes with Et 3 SiH (Table 2).Complete conversion is observed for all the alkynes examined after 18 h at 70 °C with a catalyst loading of 0.01 mol%.Regiocontrol is an important consideration in hydrosilylation as alkynes can give a number of products upon addition, e.g. with Et 3 SiH and terminal alkynes as substrates, E and Z 1-triethylsilyl and 2-triethylsilyl products can be formed.Although the thermodynamically more stable 1-substituted E-alkene is usually the major product, the observed selectivity for this isomer is very high for phenylacetylene and oct-1-yne with 5 compared with the best of the reported highly selective Pt(0) catalysts.28g,h,30 This selectivity is reduced for trimethylsilylacetylene where the 2-substituted isomer is produced as 25% of the product: this is not surprising as the alkyne substrate is electronically distinct and tends to form more 2-substituted product in other catalyst systems.28e The internal hex-2-yne gives two products with the 2-isomer predominating (Table 2).Although the selectivity here is modest it is better than the essentially 1 : 1 mixtures observed with complexes of the type Pt(ER-NHC)(dvdms) 2 .28h The reactivity towards alkenes is poor as exemplified by the two entries in the table.Oct-1-ene was chosen as a typical terminal alkene and styrene as a somewhat activated alkene; only the latter showed any conversion under the conditions employed for the reaction.

Crystallography
Data collection was carried out on a Bruker-Nonius Kappa CCD diffractometer using graphite monochromated Mo Kα radiation (λ = 0.71073 Å).The instrument was equipped with an Oxford Cryosystems cooling apparatus.Data collection and cell refinement were carried out using COLLECT 32 and HKL SCALE-PACK. 33Data reduction was applied using HKL DENZO and SCALEPACK. 33The structures were solved and refined with SHELX-97. 34Absorption corrections were performed using SORTAV. 35Generally, all non-hydrogen atoms were generally refined anisotropically, while the hydrogen atoms were inserted in idealised positions with U iso set at 1.2 or 1.5 times the Ueq of the parent atom.For 2, the large asymmetric unit is partially accountable for the weak high angle data and two anion sites show two-fold disorder.Geometric restraints were applied to the PF 6 − during refinement to stop distortion of the anions and restraints were also applied in order to maintain similar atomic displacement parameters for atoms in close proximity.For 3a, geometric restraints were applied to acetonitrile solvent molecules as well as to the atomic displacement parameters to stop distortion.For 6, difference Fourier synthesis showed large spaces occupied by diffuse electron density, interpreted as disordered solvent, and hence SQUEEZE was used. 36The large asymmetric unit combined with the disordered solvent are accountable for the weak high angle data which were omitted from the refinement.Geometric restraints were applied to the PF 6 − during refinement to stop distortion of the anions.Initial examination of the data for 1•HPF 6 and 2 suggested higher crystal symmetry but lower symmetry was necessary for the final ordered complex structures.Both anion sites show two-fold disorder.Geometric restraints were applied to the PF 6 − during refinement to stop distortion of the anions.Restraints were also applied in order to maintain similar atomic displacement parameters for atoms in close proximity.The details of the data collection and structure solution are collected in Table 3.

Conclusions
A diphosphino( pro-NHC) ligand has been prepared containing a central bicyclic core derived from camphor and two closely similar but formally unique RPPh 2 arms.In its pro-ligand form ( protonated NHC) the ligand binds solely through the phosphine donors and, depending on the metal ion, forms trans-chelating monomeric or bridging dimeric complexes.Addition of base to these preformed complexes liberates the NHC for coordination to give κ 3 -P,C,P′ complexes which, for square planar metals, show conformational isomerism.Prior deprotonation of the pro- ligand and subsequent addition of AgOTf gives the pseudo-tetrahedral complex S Ag -[Ag(κ 3 -P,C,P′-1)(OTf )] which is isolated as a single isomer.Selected platinum and palladium complexes have been shown to be effective for the regioselective hydrosilylation of alkynes and for Suzuki coupling with aryl bromides.Studies of this and related ligands are continuing with a view to extending their application in catalysis.

Fig. 1
Fig. 1 Ortep view of the molecular structure of [1•H] + .For clarity the PF 6 − anion is not shown.Thermal ellipsoids are drawn at 50% prob-

Table 1
Summary of the Suzuki coupling for complexes 2 and 8 a