The half-sandwich 18-and 16-electron arene ruthenium iminophosphonamide complexes †

Novel half-sandwich 18e¯ and 16e¯ arene ruthenium iminophosphonamide complexes [( η 6 -C 6 Me 6 )RuCl {(R ’ N) 2 PR 2 }] ( 3a – c ) and [( η 6 -C 6 Me 6 )Ru{(R ’ N) 2 PR 2 }] + (X − ) ( 4a – c ) ( a , R = Ph, R ’ = p -Tol; b , R = Et, R ’ = p -Tol; c , R = Ph, R ’ = Me. X = BF 4 , PF 6 or BAr F4 ) were synthesized. The elongated Ru – Cl bond in the 18e¯ complexes is shown to dissociate even in apolar solvents to form the corresponding 16e¯ cations, which can be readily isolated as salts with non-coordinating anions. The coordinatively unsaturated 16e¯ complexes are stable species due to e ﬃ cient π -electron donation from the nitrogen atoms of the zwitterionic NPN-ligand. The ruthenium iminophosphonamides are moderately active in the ROMP polymerization of norbornene; the 16e¯ complexes 4a,b yield high molecular weight polymers ( M n ∼ 300 × 10 3 ) with a narrow distribution M w / M n ∼ 1.6, while the 18e¯ complexes 3a,b give polymers of lower molecular weight ( M n < 50 × 10 3 ) with a wider polydispersity index M w / M n ∼ 2.5.


Introduction
Among transition metal complexes with κ 2 -N,N-heteroallylic ligands, the iminophosphonamides bearing a coordinated R 2 P(NR′) 2 − anion (NPN) are studied fragmentarily; there have been less than a hundred of molecular structures of NPN complexes established to date, that is in sharp contrast to more than a thousand of transition metal amidinate structures, according to the Cambridge Structural Database (CSD).The IV group metals, 1-6 chromium, 7-10 nickel [11][12][13][14][15] and copper [16][17][18][19][20][21][22] iminophosphonamides are the most studied, which is due to their catalytic application in cyclopropanation, 16,21,23 olefin oligomerization [7][8][9] and polymerization. 2,6,12,15,24,25A few platinum metal group iminophosphonamides have been reported for palladium 24 and ruthenium 26 before 2009, when we started systematic studies of these complexes.We have demonstrated experimentally from the precision X-ray data by determining the deformational electron density for the palladium complex [Pd{( p-i PrC 6 H 4 N) 2 PPh 2 } 2 ] that the iminophosphonamide ligand is zwitterionic N − -P + -N − having single P-N bonds and bearing full negative charges at the nitrogen atoms. 27This result shows a big difference between the electronic properties of iminophosphonamide and amidinate complexes, which previously have been considered as having a similar heteroallylic delocalized π-electronic system.The HOMO orbital of the zwitterionic NPN ligand may have either C 2v or C s symmetry, 28 of which the latter can efficiently donate the π-electron density from the nitrogen atoms to the d xz -orbital of the metal located in the plane of the ligand, similarly to the β-diketiminate complexes (Chart 1A and B).In contrast, the C 2v symmetry of the HOMO orbital in the amidinate ligand allows π-donation only by lateral coordination of the amidinate ligand resulting in a strong folding of the four-membered metallacycle (Chart 1C).Indeed, the electron deficient ruthenium complexes can be stabilized by intramolecular π-coordination of the amidinate ligand, which leads to strong puckering of the Ru-N-C-N metallacycle in the 16e ˉcomplexes [(Cp*)Ru{( t BuN) 2 C(Mes)}] 29 and [(C 6 Me 6 )Ru{( i PrN) 2 CMe}] 30 to 39.9°and 31.5°,respectively.The lateral coordination of the amidinate ligand stabilizes these 16e ˉcomplexes inefficiently since it weakens the M-N σ-bonds; such species are very reactive and can readily coordinate 2e ˉdonors [30][31][32] or other organometallic moieties to form dinuclear µ 2 -amidinate complexes. 33At the same time, the solely reported ruthenium iminophosphonamide, the stable 16e ˉ[( p-cymene)Ru{( i PrN) 2 PPh(NH i Pr)}](BPh 4 ), did not react either with [Et 3 NH]Cl, [PPh 4 ]Cl or with triphenylphosphine and triphenyl phosphite to form 18e ˉadducts; only carbon monoxide or cyanide could coordinate, however the corres-ponding 18e ˉcomplexes were not isolated. 26For this complex the authors proposed a possibility of pseudo π-allyl donation to ruthenium based on a slight puckering of the Ru-N-P-N plane by 13.9°.Since that time the chemistry of ruthenium iminophosphonamides has not progressed, although two structures of 18e ˉ[( p-cymene)RuBr{( p-TolN) 2 PMe( p-TolNH)}] and 17e ˉ[(Cp*)RuCl{( p-TolN) 2 PMe( p-TolNH)}] complexes were deposited in CSD in 2004-2005 but their synthesis and properties were not published.
Recently, an interesting arene ruthenium bis(phosphinimino)methanide complex [(p-cymene)Ru(L)Cl] (L = PhN(PPh 2 ) CH(PPh 2 )NPh) has been shown to exist as a cationic 18e complex with a tridentate κ 3 -C,N,N ligand L. 34 Although this ligand closely relates to iminophosphonamides, the stabilization of the 16e ˉspecies occurs by intramolecular coordination of the methanide group but not by the unpaired electron density from the nitrogen atoms.Indeed the displacement of the methanide group by Cl − or MeCN is highly unfavorable with the ΔG °298 calculated for the latter reaction to be +19 kcal mol −1 .This complex fails to react with CO under ambient conditions, which has also been attributed to the high nucleophilicity of the methanide group and the weak π-basicity of the ruthenium center. 35ere we report the synthesis of a series of new 18e ˉand 16e half-sandwich arene ruthenium complexes with various iminophosphonamide ligands distinguished by the electronic properties of their N-and P-substituents, their physico-chemical and structural data in comparison to other κ 2 -N,N-heteroallylic arene ruthenium complexes, and preliminary catalytic data for the ring-opening metathesis polymerization (ROMP) of norbornene.

Synthesis and characterization of the complexes 3-4
The synthesis of arene ruthenium NPN-complexes (3-4) from the diaminophosphonium salts 1a-c is summarized in Scheme 1.
The diaminophosphonium salts [R 2 P(NHR′) 2 ]Br (1a-c) were prepared in high yields, according to the earlier developed procedure. 36The salts 1a-c can be monodeprotonated with strong bases like NaHMDS or n-BuLi to give the corresponding iminophosphonamines 2a-c, while the more acidic 1a is deprotonated easily with an equimolar amount of Et 2 NH to yield 2a quantitatively.At the same time, NaHMDS is preferred for the synthesis of 2b and 2c; when n-BuLi is employed, the isolation of these iminophosphonamines is very laborious due to their complexation with lithium salts.The new compounds 1b and 2a,b were fully characterized spectroscopically and by elemental analysis, while we were not able to obtain satisfactory elemental analysis for 2c due to its high moisture sensitivity.In 31  4 ) (4c) in nearly quantitative yields as deep-violet solids.All the complexes obtained were fully characterized by NMR spectroscopy and elemental analysis and their molecular structures were confirmed by single crystal X-ray diffraction studies.The selected structural parameters of 3a-c and 4a-c are given in Table 1 and their projections are shown in Fig. 1-6.
The chlorine atom has intramolecular close CH⋯Cl contacts with one ortho-hydrogen (H18A) of the N-tolyl substituents.The H18A⋯Cl distances in 3a (2.762 Å) and 3b (2.813 Å) fall below the sum of the van der Waals radii of 2.95 Å (ref.49  and 50) and the corresponding angles Cl⋯H-C of 140.4°(3a) and 140.8°(3b) are typical of such a type of non-directed interaction.The chlorine atom is almost coplanar to the plane of the tolyl ring involved in the H⋯Cl contact and the corresponding torsion angle Cl-H(18A)-C(18)-C(13) is 7.6°and 8.4°f or 3a and 3b, respectively.A few other intra-and intermolecular close contacts H⋯Cl are observed in 3a,b with the hydrogens of the C 6 Me 6 ligand (the Cl⋯H11C, Cl⋯H11B distances are 2.772, 2.749 Å in 3a and the Cl⋯H11C, Cl⋯H8A are 2.899, 2.871 Å in 3b) and of the P-substituent (the Cl⋯H34A is 2.868 Å in 3a and the Cl⋯H29A is 2.812 Å in 3b).Similar intraand intermolecular close contacts can be seen in 3c between the chlorine atom and the hydrogen atoms of the methyl group at N(2) (Cl⋯H14B is 2.872 Å) and C 6 Me 6 (Cl⋯H10C is 2.840 Å), respectively.
The chelate angle N(1)-Ru-N(2) in 3a-c (68.1-69.4°) is significantly larger than that in the analogous 18e ˉruthenium amidinate complexes (61-62°), [40][41][42][43]51 since the P-N bonds are longer than the corresponding C-N bonds in the amidinates. It s worth noting that the torsion angle Ru-N(1)-N(2)-P is close to 180°showing small puckering of the Ru-N(1)-P-N(2) metallacycle from the planarity (6.3°for 3a, 13.7°for 3b and 4.1°for 3c).The pyramidalization of the nitrogen atoms is rather small in 3a,b, while it is strong for one of the nitrogens in 3c, for which the sum of the angles at the N(2) atom ∑(N) is 344.4°.Recently, a much wider range of puckering angles (up to 23.4°) and relatively strong pyramidalization of one of the nitrogen atoms (348-351°) were observed in the square-planar palladium iminophosphonamide complexes, presumably as a result of steric repulsion between the N-cumyl substituents of the NPN-and co-ligands.27 However this seems to be not the    case of 3c bearing sterically small N-methyl groups; more probably the pyramidalization of N( 2) is due to the high unpaired electron density located fully at the nitrogen atom and being not able to delocalize on any other electron system like aromatic N-tolyl groups in 3a,b.
The chelate angle N(1)-Ru-N(2) in 4a-c (72.1-72.9°) is almost equal to that in 5 (71.8°) 26 and larger by ca.4°than in 3a-c.The pyramidalization of the nitrogen atoms (∑(N) is 355-360°) and the puckering of the plane Ru(1)N(1)P(1)N(2) (5.0-10.3°)are small.This is in sharp contrast to the strong puckering (31.4°) of the chelate metallacycle M-N-C-N in the analogous amidinate 16e ˉarene ruthenium complex [(η 6 -C 6 Me 6 )Ru{(N i Pr) 2 CMe}](PF 6 ), required for additional stabilization of the coordinatively unsaturated species by a π-heteroallyl system. 30he structural peculiarities of the coordinated arene, the flattened RuNPN metallacycle and the short Ru-N bonds in 4a-c are indicative of the strong σ,π-bonding of the iminopho-sphonamide ligand 26 via nitrogen atoms, similarly to β-diketiminates and bis(imidazolin-2-iminates), and in contrast to allylic π-stabilization in metal amidinates.The elongated Ru-Cl bonds in 3a-c and the pyramidalization of the nitrogen atom in the most electron-rich 3c are also in agreement with the zwitterionic structure of the NPN ligand bearing enhanced negative charges at the nitrogen atoms.
In the 31 P NMR spectra of 3a,b the phosphorus resonance (δ 43.9 for 3a and δ 72.4 for 3b) is shifted by ca.47-49 ppm to more positive values compared to the precursors 2a and 2b.Interestingly, in the 1 H NMR spectra of 3a and 3b in CDCl 3 the two chemically inequivalent substituents at the phosphorus atom give only one set of signals for phenyl and ethyl groups, respectively.However, in the 1 H NMR spectra recorded in apolar C 6 D 6 , the P-substituents of 3a and 3b give rise to two sets of the corresponding resonances.Similarly, in the 13 C NMR spectra of 3a and 3b in CDCl 3 the resonances for only one type of phenyl and ethyl groups are observed.It is noteworthy that in 13 C NMR the characteristic doublets for ipso-carbons of phenyls are not found, perhaps due to their strong broadening.Apparently, in polar solvents fast exchange between the two P-substituents takes place.Indeed, heating a solution of 3a and 3b in apolar toluene-d 8 (T = 273-353 K) leads to broadening of the inequivalent phenyl and ethyl resonances, respectively for 3a and 3b, in 1 H NMR, though their coalescence is not achieved (see the ESI †).In a more polar CD 2 Cl 2 solution of 3b at 298 K the signals for only one ethyl substituent are observed, while decreasing the temperature to 193 K gives two separate resonances of methyl groups (Fig. 7).At the coalescence temperature of T c = 238 K in dichloromethane, the estimated exchange rate constant is 1370 s −1 and the free energy of activation ΔG ≠ calculated from the Eyring equation is about 10.4 kcal mol −1 . 52he exchange between the P-substituents R a and R b seems to proceed via a C 2v -symmetric intermediate or a transition state with two equivalent R groups, tentatively the cationic complex [(η 6 -C 6 Me 6 )Ru{R 2 P(N-p-Tol) 2 }] + Cl − (vide infra), formed from 3a,b by dissociation of the chloride anion (Scheme 2).
Although the dissociation of the chloride anion in 18e ˉcomplexes 3a,b is facile in CDCl 3 or CD 2 Cl 2 , the equilibrium concentration of the dissociated 16e ˉform is negligible, as far as their signals in 31 P NMR in CDCl 3 (δ 43.9 for 3a, δ 72.4 for 3b) and C 6 D 6 (δ 43.3 for 3a, δ 71.1 for 3b) remain virtually unchanged.The phosphorus resonance of the 16e ˉcationic complexes 4a (δ 71.9) and 4b (δ 102.3) is strongly shifted by ∼30 ppm to more positive values compared to the neutral complexes 3a,b.In the 1 H and 13 C NMR spectra of 4a,b in CD 2 Cl 2 the P-substituents are chemically equivalent, independent of the temperature (193-298 K) as it is expected for C 2v -symmetric complexes.
In apolar C 6 D 6 complex 3c also gives rise to two inequivalent phenyl groups in the 1 H and 13 C NMR spectra, which is consistent with the non-dissociated 18e ˉchloride complex, though the signals are broadened.Whereas in more polar CDCl 3 complex 3c has violet color and its 31 P resonance (δ 76.9) is strongly shifted to more positive values than in C 6 D 6 (δ 59.8) and becomes close to the signal of the cationic complex 4c (δ 80.8).The 1 H NMR spectra of 3c and 4c in CDCl 3 are nearly identical and in line with the cationic C 2v -symmetric complex.Hence, in sharp contrast to 3a,b, in CDCl 3 solution 3c undergoes facile dissociation to give the cationic complex [(η 6 -C 6 Me 6 )Ru{Ph 2 P(NMe) 2 }] + Cl − , which is prevalent in the equilibrium mixture.Apparently strongly electron-releasing N-methyl substituents enhance the π-donating ability of the NPN-ligand compared to that of the complexes 3a,b bearing weaker N-tolyl donors.
The arene ruthenium complexes having monoanionic β-diketiminate 39 and dianionic bis(imidazolin-2-iminate) 48 ligands have also been reported earlier to undergo facile chloride dissociation, whereas in the amidinate ruthenium complexes the counter-ion dissociation has been observed only for the weakly coordinating triflate ligand but not the chloride. 51ence the capability of the iminophosphonamide ligand to donate electrons and to stabilize the electron-deficient states is much higher than that of the amidinate ligand and comparable to the β-diketiminate and zwitterionic bis(imidazolin-2-iminate) ligands.
In the UV-vis spectra the complexes 4a-c have a broad medium intensity band at λ max 540-550 nm shifted to lower energies compared to the corresponding complexes 3a,b (in CH 2 Cl 2 ) and 3c (in C 6 H 6 ) having a band at λ max 410-450 nm.Similar bands at λ max 520-530 nm have also been observed for 16e ˉpentamethylcyclopentadienyl amidinate, 29 β-diketiminate 47 and bis(imidazolin-2-iminate) 48 ruthenium complexes, which has been attributed to d-d centered transitions. 47he 16e ˉcomplexes 4a-c are remarkably stable in solution to air and moisture in sharp contrast to the 18e ˉcomplexes 3a-c, which hydrolyze to produce the corresponding phosphinoxides R 2 P(O)(NHR′).Apparently the nitrogen atoms in 3a-c are highly basic due to their free electron pairs and thus are prone to the attack by water molecules, whereas in 4a-c these electron pairs participate in π-bonding to the ruthenium atom and are much less basic.Indeed, the susceptibility to hydrolysis is higher for the complex bearing more electron-releasing N-substituents; thus 3c decomposes within minutes in wet CD 2 Cl 2 while 3a,b are stable for hours.Under similar conditions 4a-c do not hydrolyze and do not form the 18e ˉaqua NPN-complexes; their NMR spectra in dried and wet solvents are the same and remain unchanged for days.However, when 10 equivalents of MeCN were added to a solution of 4a in CD 2 Cl 2 the colour immediately changed from violet to red and the phosphorus resonance shifted from δ 72.2 to δ 63.8 indicating the formation of the new complex, presumably the cationic adduct [(η 6 -C 6 Me 6 )Ru(MeCN){Ph 2 P(N-p-Tol) 2 }](PF 6 ).The attempt to isolate it by removing the excess of acetonitrile in vacuo returned the starting complex 4a (violet solution in CD 2 Cl 2 with the phosphorus resonance at δ 71.0, no acetonitrile signal in 1 H NMR). Bubbling carbon monoxide into a solution of 4a in CH 2 Cl 2 leads to the formation of a yellow CO adduct (ν CO = 1984 cm −1 ), however the CO ligand is labile and easily decoordinates upon removing the solvent under reduced pressure to give back 4a.A similar 16e ˉruthenium iminophosphonamide 5 has also been recently reported to coordinate reversibly CO to form the unstable CO adduct (ν CO = 1993 cm −1 ) and to not react with Cl − , PPh 3 , and P(OPh) 3 . 26In sharp contrast, the more electron-deficient arene ruthenium amidinate [(η 6 -C 6 H 6 ) Ru{PhC(N t Bu) 2 }](BAr F 4 ) readily forms the stable carbonyl complex [(η 6 -C 6 H 6 )Ru(CO){PhC(N t Bu) 2 }](BAr F 4 ), in which the CO band is observed at a much higher frequency (ν CO = 2050 cm −1 ). 30On the other hand, the carbonyl band in the arene ruthenium complex with the dianionic dithiolate ligand [(η 6 -C 6 Me 6 )Ru(CO)(SXyl) 2 ] (ν CO = 1965 cm −1 ) 54 is close to that in the CO adduct of the iminophosphonamide 4a and hence indirectly evidences for the zwitterionic nature of the NPN-ligand.Apparently in cationic iminophosphonamides 4a-c the positive charge is predominantly located on the phosphorus atom rather than on the ruthenium atom and thus their electronic properties are more like those of neutral arene ruthenium complexes with dianionic ligands.

ROMP polymerization of norbornene
Half-sandwich arene ruthenium complexes with sterically bulky phosphanes have been shown earlier to be readily acces-  sible precatalysts for ring-opening metathesis polymerization of both strained (norbornene) and low-strain (cyclooctene) cycloolefins and their functionalized derivatives. 55,56Typically, the polymers obtained from norbornene had a numberaverage molecular weight M n of 60-80 × 10 3 and a molecular weight distribution M w /M n of 1.6.We have demonstrated that under the same catalytic conditions the complexes 3a-c and 4a,b activated by trimethylsilyldiazomethane (TMSD) catalyze the polymerization of norbornene, except for almost inactive 4c (Table 2).The 18e ˉcomplexes 3a-c produced polymers of low molecular weight M n = 36-50 × 10 3 , which is close to the expected M n = 23.5 × 10 3 calculated from the catalyst/substrate ratio (1/250), meaning that all ruthenium centers are involved in catalysis.The molecular weight distribution is relatively wide: M w /M n is 2.5-2.7 for 3a,b and is 6.9 for 3c.The polydispersity of the polymer can be significantly improved when 16e ˉcomplexes 4a,b are employed.They yield polymers of a much higher molecular weight M n = 283-341 × 10 3 , which is supposedly due to their limited solubility in chlorobenzene and hence only a minor fraction (about 10%) of these cationic complexes can participate in the catalytic cycle.This assumption would mean that the actual activity of 4a,b is about ten times higher than that of 3a,b.Both types of complexes give a considerable fraction (11-20%) of low-weight oligomers soluble in methanol.
In the absence of TMSD the ruthenium iminophosphonamides studied are inactive.The actual catalytically active species in the ROMP of olefins are carbene complexes, which are typically generated in situ from the precatalysts and TMSD.Recently, the 16e ˉcationic ruthenium amidinate complexes have been reported to react with TMSD to result in migration of the SiMe 3 group to the ruthenium atom and formation of the amidinato-carbene complexes, in which the carbene moiety is inserted into the RuNCN metallacycle. 57These amidinate complexes provided low activity in the ROMP polymerization of norbornene; higher activity was mentioned for polymerization of high-strain norbornadiene catalyzed by the 18e ˉcomplex [(C 6 H 6 )RuBr{MeC(N i Pr) 2 }] although the detailed results were not published. 32Perhaps, a similar carbene inser-tion into the Ru-N bond occurs in the NPN-precatalysts. 58Not surprisingly, the more electron rich iminophosphonamide arene ruthenium complexes 3a,b and 4a,b excel the corresponding ruthenium amidinates in the ROMP polymerization of norbornene.However, our attempts to involve them in the ROMP of low-strain cyclooctene were unsuccessful.
It is noteworthy that strongly electron-releasing N-methyl substituents drastically decrease the activity of the complexes 3c and 4c.The negligible activity of the 16e ˉcomplex 4c could be a result of very efficient stabilization of the unsaturated ruthenium center to make it insusceptible to the reaction with the generated in situ carbene CHSiMe 3 rather than of the solubility issues.The same can be attributed to the reduced activity of 3c, which should partially dissociate in polar chlorobenzene to form stable 16e ˉcationic species (vide supra).
Synthesis of [(η 6 -C 6 Me 6 )Ru{R 2 P(N-p-Tol) 2 }](X) (4a, X = PF 6 ; 4b, X = BF 4 ; 4c, X = BAr F 4 ).General procedure.(a) To a solution of 3a (0.46 g, 0.66 mmol) in CH 2 Cl 2 (30 ml), solid AgPF 6 (0.18 g, 0.72 mmol) was added, the color immediately changed from red to deep violet.The reaction mixture was stirred for 3 h and then filtered through a plug of Celite.The solution was concentrated to 2 mL, further addition of Et 2 O (10 mL) resulted in the precipitation of a product, which was filtered off, washed with Et 2 O (2 × 5 mL) and dried under vacuum.Typical procedure for the ROMP of norbornene A 50 mL round-bottom flask equipped with a magnetic stirring bar and capped with a three-way stopcock was charged with the ruthenium complexes 3-4 (0.03 mmol) and degassed chlorobenzene (20 mL) was added under an argon atmosphere.The solution was stirred for a few minutes at room temperature and then in an oil bath thermostated at 60 °C.Norbornene (1.5 M in chlorobenzene, 5 mL, 7.5 mmol) and eventually trimethylsilyldiazomethane, TMSD (0.1 M in a hexanes-chlorobenzene mixture, 1 mL, 0.1 mmol) were added with a syringe, and the reaction mixture was stirred for 2 h at 60 °C.The conversion was monitored by gas chromatography using norbornane as an internal standard.The resulting gel was diluted with CHCl 3 (20 mL) and slowly poured into MeOH (500 mL) under vigorous stirring.The precipitated polymer was filtered, dried under dynamic vacuum, and characterized by NMR spectroscopy and GPC in THF using a polystyrene calibration.

X-ray crystal structure determination
Single crystals of 3 and 4 were obtained by slow diffusion of Et 2 O to a solution of a complex (3a, 4a-c) in CH 2 Cl 2 , or by diffusion of hexane to a solution of 3b and 3c in benzene.Data collection for all samples was performed on a Bruker SMART APEX II diffractometer (MoKα radiation, λ = 0.71073 Å) equipped with an Apex II CCD detector.Frames were integrated using the Bruker SAINT software package 61 by a narrowframe algorithm.A semiempirical absorption correction was applied with the SADABS 62 program using the intensity data of equivalent reflections.The structures were solved with direct methods and refined by the full-matrix least-squares technique against F 2 hkl in anisotropic approximation with the SHELX 63 software package.The positions of hydrogen atoms were calculated, and all hydrogen atoms were refined in a riding model with 1.5U eq (C m ) and 1.2U eq (C i ), where U eq (C m ) and 1.2U eq (C i ) are respectively the equivalent thermal parameters of the methyl and all other carbon atoms to which the corresponding H atoms are bonded.Detailed crystallographic information is given in Table 3. Crystallographic data have been deposited to the Cambridge Crystallographic Data Centre, CCDC numbers 1475876-1475879, 1494098 and 1494099.

Conclusions
A series of new 18e ˉand 16e ˉhexamethylbenzene ruthenium complexes with the iminophosphonamide ligand have been synthesized and fully characterized.The elongated Ru-Cl bonds and an easy dissociation of the chloride anion in the 18e ˉcomplexes 3a-c as well as short Ru-N bonds in the 16e complexes 4a-c and small puckering of the Ru-N-P-N metallacycle indeed suggest the zwitterionic nature and strong σ,π-donor character of the iminophosphonamide ligand being able to donate 4e ˉor 6e ˉ.Due to charge separation between the phosphorus and the nitrogen atoms, the electronic properties of the NPN-ligand are similar to those of dianionic ligands and therefore it can efficiently stabilize 16e ˉelectron deficient complexes.Among other arene ruthenium complexes with κ 2 -N,N chelating ligands, the properties of 16e īminophosphonamide complexes resemble those of strongly donating monoanionic β-diketiminates and dianionic bis (imidazolin-2-iminates), and render them air stable unlike the analogous air-sensitive 16e ˉamidinate complexes.Thus arene ruthenium complexes 3a,b and 4a,b bearing the strongly donating NPN-ligand perform better than the analogous ruthenium amidinates in the ROMP of strained norbornene.To properly address the unusually low activity of complexes 3c, 4c with the most electron-rich NPN-ligand, which seems to block the activation of the ruthenium center by carbene, a deeper mechanistic investigation is required.The detailed kinetic and thermodynamic studies on the dissociation of the chlorides 3a-c and the coordination of various ligands to their 16e counterparts 4a-c as well as the ROMP mechanistic study and application in the transfer hydrogenation of ketones are in progress and to be reported soon.

Fig. 4
Fig. 4 ORTEP diagram of cation 4a.Ellipsoids are shown at 50% probability, hydrogen atoms and the anion are omitted for clarity.

Fig. 5
Fig. 5 ORTEP diagram of cation 4b.Ellipsoids are shown at 50% probability, hydrogens atoms and the anion are omitted for clarity.

Fig. 6
Fig. 6 ORTEP diagram of the cation 4c.Ellipsoids are shown at 50% probability, hydrogen atoms and the anion are omitted for clarity.

Scheme 2
Scheme 2The exchange between R a and R b substituents via putative dissociation-association of the chloride anion.53