N‐Heterocyclic Carbene Analogues of Nucleophilic Phosphinidene Transition Metal Complexes

Abstract Chloride abstraction from the complexes [(η6‐p‐cymene){(IDipp)P}MCl] (2 a, M=Ru; 2 b, M=Os) and [(η5‐C5Me5){(IDipp)P}IrCl] (3 b, IDipp=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene) with sodium tetrakis[3,5‐bis(trifluoromethyl)phenyl]borate (NaBArF) in the presence of trimethylphosphine (PMe3), 1,3,4,5‐tetramethylimidazolin‐2‐ylidene (MeIMe) or carbon monoxide (CO) afforded the complexes [(η6‐p‐cymene){(IDipp)P}M(PMe3)]BArF] (4 a, M=Ru; 4 b, M=Os), [(η6‐p‐cymene){(IDipp)P}Os(MeIMe)]BArF] (5) and [(η5‐C5Me5){(IDipp)P}IrL][BArF] (6, L=PMe3; 7, L=MeIMe; 8, L=CO). These cationic N‐heterocyclic carbene‐phosphinidene complexes feature very similar structural and spectroscopic properties as prototypic nucleophilic arylphosphinidene complexes such as low‐field 31P NMR resonances and short metal‐phosphorus double bonds. Density functional theory (DFT) calculations reveal that the metal‐phosphorus bond can be described in terms of an interaction between a triplet [(IDipp)P]+ cation and a triplet metal complex fragment ligand with highly covalent σ‐ and π‐contributions. Crystals of the C−H activated complex 9 were isolated from solutions containing the PMe3 complex, and its formation can be rationalized by PMe3 dissociation and formation of a putative 16‐electron intermediate [(η5‐C5Me5)Ir{P(IDipp)}I][BArF], which undergoes C−H activation at one of the Dipp isopropyl groups and addition along the iridium‐phosphorus bond to afford an unusual η3‐benzyl coordination mode.

The carbonyl complex [(h 5 -C 5 Me 5 ){(IDipp)P}Ir(CO)][ BAr F ]( 8) was prepared by leadingastream of carbon monoxide through the reactionm ixture containing 3b and NaBAr F in fluorobenzene solution (Scheme 4). The dark blue color of 3b vanisheda nd turned dark red within af ew minutes. 8 was isolated in analytically pure form as ap urple-reds olid in 51 % yield. In comparison with complexes 6 and 7,t he significantly more deshielded 31 PNMR chemical shift at 596.9 ppm suggests a Z configuration as also observed for the relatedn eutral carbonyl-phosphinidenec omplex [(h 5 -C 5 Me 5 )(Mes*P)Ir(CO)] (d = 804.6 ppm). The IR spectrum of 8 exhibits as trong absorption at 1998 cm À1 ,w hichi sl arger than the value reported for [(h 5 -C 5 Me 5 )(Mes*P)Ir(CO)] (1968 cm À1 ). [5] This differencec ould be ascribed tentatively to as tronger p-accepting ability of the (IDipp)P in comparison with the Mes*P ligand;h owever,t he cationic and neutraln ature of the two complexes must also be considered. The Z configurationo f8 was also confirmed for the solid state by X-ray diffraction analysis, and the molecular structure of the cation in 8 is shown in Figure 4. It displaysa bent (IDipp)P ligand and the typical two-legged piano-stool geometry with Ir-P1-C1 and P-Ir-C4 angles of 113.73(11)8 and 98.54(11)8,w hich is similar to the structuralf eatures of 3b. [15] The IrÀPb ond and IrÀC4 bond lengths of 2.1905(10) and 1.870(4) are both longerc ompared to 2.1783(8) and 1.849(3) in [(h 5 -C 5 Me 5 )(Mes*P)Ir(CO)],w hich suggests slightly higher bond ordersf or the metal-element bonds in the neutral phosphinidene system. The imidazole ring subtends an interplanar angle of 85.86(12)8 with the plane containing the atoms C1-P-Ir-C4-O, revealing an orthogonalorientation.  were also included in this study as representatives of complexes of type I and II.T he computational details and selected contour plots of selected NBOs are given in the Supporting Information (TablesS7-S9). The calculated structuralp arameters are very similar to those found experimentally in the X-ray crystal structures, if available (cf. Ta bles 1a nd 2). In agreement with exclusive observation of E isomersi ns olution and in the solid state in case of the osmium derivatives 4b (L = PMe 3 )a nd 5 (L = Me IMe), the E configurationi sc learly favored thermodynamically by 8.8 and 7.0 kcal mol À1 ,r espectively (Entries 2a nd 3, Ta ble 2). In contrast, the Z configuration is more stable (DH 298K = 6.5 kcal mol À1 )f or the elusivec arbonyl complex( L = CO, Entry 4). For the iridium trimethylphosphine( L= PMe 3 )a nd NHC (L = Me IMe) complexes, however,v ery similare nergies were computed for the E and Z forms,w ith the latter being slightly favoredb y1 .9 and 1.6 kcal mol À1 (Entries 6a nd 7). In principle, this trend is in line with the observation of an E/Z mixturei ns olutionf or the NHC complex 7,w hereas only the E form was observed for the PMe 3 congener 6.S trong stabilization by 13.3 kcal mol À1 of the Z form is again found for the carbonyl species (L = CO, Entry 8), in agreement with the experimental data established for 8.
Comparison of the NBO charges in the neutral and cationic PMe 3 osmium and iridium complexes (entries1/2 and 5/6) reveals av ery similar charged istribution, albeit with as lightly higher polarization of the metal-phosphorus bondsc alculated for the authentic phosphinidenec omplexes of type I and II. Furthermore, the similarity of the Wiberg bond indices( WBI) indicatee qual bond orders, with just am arginal decrease found for the cationic (NHC)Ps ystems. It shouldb en oted, however,that this trend is not reflected by the metal-phosphorus bond lengths, which are virtually identical in the respective osmium and iridium pairs. Substitution of the PMe 3 ligand by Me IMe and CO (Entries3/4 and 7/8) does not revealaclear trend and produces similar values (Table 2). Overall, this bonding situation is best described by the mesomeric structure A (Scheme 1), with the (NHC)P unit acting as ac ationic phosphinidenel igand with highly covalent s-a nd p-contributions. Accordingly, the metal-phosphorus double bond can be described in terms of an interaction between at riplet (NHC)P cation and at riplet metal complex fragment in analogyw ith nucleophilic phosphinidene complexes such as I and II. [3,25] In fact, [(IDipp)P] + has at riplet ground state, which is 8.4 kcal mol À1 (DH 298K )b elow the singlet state, while the neutral phosphinidene Mes*P has as imilar triplet-singlet gap of 7.9 kcal mol À1 (see the Supporting Information). In comparison, the tripletg round state of the parent phosphinidene (HP) has been predicted to be 20-28 kcal mol À1 below the singlet state. [26] In this context,i ti sn oteworthy that, on the other hand, transient or even isolable (phosphino)phosphinidenes have as inglet ground state, which is about 20 kcal mol À1 below the triplet state. [27] The high degree of covalency of the metal-phosphorus double bond is evidenced by almoste qual contributions of the metal and phosphorus atoms to the natural bond orbitals (NBOs) associated with the MÀP p-a nd s-bonds. Furthermore, the expected high 3s character of the phosphorus lone pairi s also confirmed (see the Supporting Information for the composition and the contourp lots of relevant NBOs). As an illustrative example, the NBOs of the iridiumc omplex [(h 5 -C 5 Me 5 ){(IDipp)P}Ir(CO)] + (as in 8)a re shown in Figure 5.

Trapping of a1 6-electron intermediate
The reactivity of phosphinidenec omplexes of type I and II has been explained by involvemento fp utative 16-electron intermediates [(h 6 -Ar)M(PMes*)] (M = Ru, Os) and[ ( h 5 -C 5 Me 5 )M(PMes*)] (M = Rh, Ir), [4] and convincing evidence has   substituent. [28] In our hands, attempts to generate and characterize 16-electron intermediates by chloride abstraction from 2 and 3 provedu nsuccessful. However,t he potential reactivity of these 16-electron intermediates, whichh ave been captured by adding ligands such as PMe 3 , Me IMe and CO as described above,c an be demonstrated by repeated isolation of yellow single crystals from solutions containing the PMe 3 complex 6 (Scheme 5), unfortunately only as an inseparable mixture with the brown rods of 6 (see the Supporting Information). X-ray diffraction analysisr evealed the formation of complex 9 with aC ÀHa ctivated isopropyl group. 9·0.5 C 6 H 6 crystallized in the monoclinic space group P2 1 /c with two independent molecules in the asymmetric unit;a nd cation 1i s presentedi nF igure 6. The structural parameters of both cations are almost identical, and the following discussion is restricted to cation 1. One isopropyl group has undergone CÀH activation and deprotonation at the methine position, affording as ymmetric h 3 -benzyl coordination mode with Ir-C distances of 2.194(5), 2.163(4) and 2.200(5) ;t hese bond lengths are in the same range as the IrÀCd istances to the h 5 -C 5 Me 5 ligand, viz. 2.192(5)-2.300 (4) .T he IrÀPb ondl ength of 2.3360 (12) indicates protonation of the phosphorusa tom, since it is significantly longerc omparedt o2 .1966 (8) in 3b, [15] 2.193(3)/2.205(2) in 7 and 2.1905(10) in 8 (Table 1), but almosti dentical with 2.3460 (6) in [(h 5 -C 5 Me 5 )IrCl 2 {PH(IDipp)}]. [15] However,t he PH hydrogen atom could not be located unequivocally,and therefore the structure of 9 was optimized by DFT calculations (B97-D) with the hydrogen atom located either adjacent or opposite to the h 3 -benzyl moiety.B oth structures are very similar in energy (DH 298K = 0.3 kcal mol À1 ,s uggesting that the hydrogen atom might be disordered over these two positions.T he experimental and calculated parameters are in good agreement,a lthough the calculated bond lengths tend to be consistently longer(Ta ble 3).
Complex 9 shouldh ave formed by dissociation of PMe 3 from complex 6,a nd the resulting 16-electron speciesh as probablyu ndergone CÀHa ctivation alongt he iridium-phosphorusb ond. To the besto fo ur knowledge,t he resulting h 3benzylc oordination mode is rare, [29] although CÀHa ctivation represents ac ommon pathwayo fd ecompositiono fN HC complexesc ontaining carbenes such as IDipp and IMes. [30] Unfortunately,a ttempts to synthesize compound 9 by chloride abstraction from 3b and to characterize it spectroscopically have so far been unsuccessful, and at this stage, the isolation of 9 only allows ag limpse into the potentially interestingr eactivity of cationic 16-  Scheme5.Formation of the h 3 -benzyl complex 9;Dipp = 2,6-diisopropylphenyl. Figure 6. ORTEP diagramofo ne of the two independentc ations in 9·0.5 C 6 H 6 with thermal displacementp arameters drawna tt he 50 %p robability level. The PH hydrogenatom has been placed and refined in aposition adjacent to the h 3 -benzyl moiety.All other hydrogen atoms, the solvent molecule and the BAr F counterion wereo mitted for clarity.Selected bondlengths and anglesa re assembled in Ta ble 3. Table 3. Experimental and calculated structural parameters of 9.
Bond lengths []a nd angles [8]e xp. [a] calc. [b] calc. [c] IrÀP2.3360 (12) The potential reactivity of these species will be furthers tudied, and the reactions with unsaturated substrates such as alkenes, alkynes, carbond ioxide etc. might uncover interesting cycloaddition reactivity along the metal phosphorus double bond. Furthermore, their potential role as homogeneous catalysts for hydroelementation reactions will be investigated.