N-Heterocyclic Carbene-Phosphinidene and Carbene

Article pubs.acs.org/IC

N‑Heterocyclic Carbene-Phosphinidene and CarbenePhosphinidenide Transition Metal Complexes Marius Peters,‡ Adinarayana Doddi,‡ Thomas Bannenberg, Matthias Freytag, Peter G. Jones, and Matthias Tamm* Institut für Anorganische und Analytische Chemie, Technische Universität Braunschweig, Hagenring 30, 38106 Braunschweig, Germany

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S Supporting Information *

ABSTRACT: Half-sandwich complexes of the N-heterocyclic carbene-phosphinidene adduct [(IPr)PH] (1, IPr = 1,3bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) were prepared by its reaction with dimeric complexes of the type [LMCl2]2, which afforded the three-legged piano-stool complexes [LMCl2{HP(IPr)}] (9a/9b: M = Ru/Os, L = η6p-cymene; 10a/10b: M = Rh/Ir, L = η5-C5Me5). Their conversion into the corresponding carbene-phosphinidenide complexes [LMCl{P(IPr)}] (11a/11b: M = Ru/Os; 12a/12b: M = Rh/Ir) with a two-legged piano stool geometry was studied by NMR spectroscopy in the presence of the strong base 1,8diazabicyclo[5.4.0]undec-7-ene (DBU). Alternatively, the complexes 11 and 12 were isolated in high yields from the reactions of the carbene-phosphinidene adduct [(IPr)PTMS] (2) with [LMCl2]2, whereby formation of the metal-phosphorus bonds was accompanied by elimination of trimethylsilyl chloride (Me3SiCl). Theoretical calculations reveal a strong polarization of the phosphorus ligands upon metal complexation, which can be ascribed to the ability of the imidazole moiety to effectively stabilize a positive charge. Dehydrohalogenation of complexes 9/10 to 11/12 affords a significant increase of the metal-phosphorus bond order, with the carbene-phosphinidenide ligand acting as a strong 2σ,2π-electron donor.

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INTRODUCTION Soon after Arduengo and co-workers isolated the first stable carbenes of the imidazolin-2-ylidene type in 1991,1,2 Nheterocyclic carbene (NHC) adducts of the p-block elements became the subject of considerable interest.3−9 Phosphinidene adducts were first synthesized by Arduengo and Cowley by reaction of cyclic oligomers of alkyl- and arylphosphinidenes, e.g., pentaphenylcyclopentaphosphane, (PPh)5, with the respective NHC ligands.10 The representation of the resulting adducts by resonance structures A and B (Chart 1) complies with the notion of inversely polarized phosphaalkenes, which is in line with experimental and theoretical studies;11−15 the potential availability of two lone pairs on the phosphorus atom was demonstrated for the first time by isolation of the bis(borane) adduct [{(IMes)PPh}(BH3)2] (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene).16 A wide range of NHC-phosphinidene adducts was studied by Bertrand, who utilized their 31P NMR chemical shifts as an indicator of the πaccepting properties of carbenes.17 “Carbene-stabilized diphosphorus” [(IPr)P]2 (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene), reported by Robinson,18 represents a related system that was shown to exhibit phosphinidene-like reactivity toward BH3·THF19 or dioxygen.20 Moreover, an analogous [(CAAC)P]2 system (CAAC = cyclic (alkyl)(amino) carbene),21 and more recently, even the isolation of a singlet phosphinidene stable at room temperature were reported by Bertrand.22,23 © 2017 American Chemical Society

Chart 1. NHC-Phosphinidene and NHC-Phosphinidenide Ligands

Surprisingly, the transition metal coordination chemistry of carbene-phosphinidene adducts has attracted only little attention, despite their ease of accessibility. It should be noted, however, that Weber has studied metal complexes of phospha- and arsaalkenes of the type REC(NMe 2 ) 2 extensively, which can be regarded as acyclic carbenephosphinidene and -arsinidene adducts, 11,24 and metal Received: July 14, 2017 Published: August 22, 2017 10785

DOI: 10.1021/acs.inorgchem.7b01798 Inorg. Chem. 2017, 56, 10785−10793

Article

Inorganic Chemistry complexes of diaminocyclopropenylidene-supported arylphosphinidenes are also known.25,26 More recently, Lavoie reported a ruthenium benzylidene complex containing (IMes)PPh,27 and the same ligand was used by Dias for the preparation of bimetallic coinage metal complexes such as [{(IMes)PPh}(MX)2] (M = Cu, Ag, Au; X = Cl, Br).28 Similar complexes with (IPr)PPh were prepared by our group, and cationic digold complexes such as [{(IPr)PPh}{Au(tht)}2][SbF6]2 (tht = tetrahydrothiophene) were employed as single-source catalysts in cycloisomerization and carbene-transfer reactions.29 The same carbene-phosphinidene was used for the preparation of the carbonyl complexes cis-[{(IPr)PPh}Rh(CO)2Cl] and [{(IPr)PPh}M(CO)5] (M = Mo, W), for which the electrondonating properties of this phosphorus ligand were assessed by IR spectroscopy.30 Related tungsten pentacarbonyl complexes were reported by Streubel and co-workers.31 Whereas NHC adducts of aryl- and alkylphosphinidenes were reported as early as 1997,10 carbene stabilization of the parent phosphinidene PH (and also of the parent arsinidene)32 has been achieved only recently. (IPr)PH (1, Chart 1) was initially generated by Driess by silylene-to-carbene PH transfer,33,34 while Robinson had previously reported a lithiated derivative.35 In our hands, 1 was prepared in high yield from the 2,2-difluoroimidazoline IPrF2 (Phenofluor) by reaction with P(SiMe3)3 via the intermediate trimethylsilylphosphinidene species (IPr)PTMS (2, Chart 1).36 Grützmacher reported an alternative protocol for the synthesis of 1 from the corresponding imidazolium chloride using sodium 2-phosphaethynolate, Na(OCP), or P7(TMS)3 as phosphorus-transfer reagents.37 Na(OCP) was also used by von Hänisch and Bertrand for the preparation of (IMes)PH or the more bulky derviative (IAr*)PH with 2,6-bis(diphenylmethyl)-4-methylphenyl (Ar*) substituents.38,39 These (NHC)PH systems were used for the preparation of the carbonyl complexes [{(IPr)PH}M(CO)5] (3: M = Cr, Mo, W),30,40 [{(IMes)PH}W(CO)5] (4),38 and [{(IAr*)PH}Fe(CO)4] (5),39 while the isolation and structural characterization of [{(IMes)PH}(BH3)2] (6)38 and [{(IAr*)PH}(AuCl)2] (7)39 is again in line with the availability of two lone-pairs on the phosphorus atoms (Figure 1). The TMS derivative (IPr)PTMS (2) was also used by us for the preparation of NHC-phosphinidenide transition metal complexes,41 which contain the formally monoanionic ligand [(IPr)P]− (Chart 1). The reaction of 2 with [Rh(COD)Cl]2 (COD = 1,4-cyclooctadiene) furnished the dirhodium complex 8 (Figure 1), which reacted further with [(Me2S)AuCl] to form the trimetallic complex 8·AuCl with a four-coordinate phosphorus atom.36 The Ru/Rh complexes 11a and 12a with a terminal NHC-phosphinidenide ligand were prepared in a similar fashion from 2 and the corresponding dimeric pcymene-ruthenium(II) and pentamethylcyclopentadienylrhodium(III) dichlorides,36 and it was shown that these complexes exhibit evident structural and spectroscopic similarities to terminal arylphosphinidene complexes with isolobal, but formally dianionic [ArP]2− ligands.42−44 In this contribution, we wish to report the preparation of the NHC-phosphinidene complexes 9 (M = Ru, Os) and 10 (M = Rh, Ir) and the attempted conversion into the corresponding NHC-phosphinidenide complexes 11 and 12 by dehydrochlorination. It should be noted that the preparation of the mercury complex [{(IPr)P}2Hg] was recently achieved by the Grützmacher group through the reaction of 1 with HgCl2 in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU),45

Figure 1. Selected examples of NHC-phosphinidene (3−7, 9, 10) and NHC-phosphinidenide (8, 11, 12) complexes; Dipp = 2,6diisopropylphenyl, Mes = 2,4,6-trimethylphenyl, Ar* = 2,6-dibenzhydryl-4-methylphenyl.

which also represents the base of choice for the dehydrochlorination of 9 and 10. In addition, we report herein the preparation and full characterization of the novel osmium and iridium complexes 11b and 12b by use of the TMSphosphinidene derivative (IPr)PTMS (2).

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RESULTS AND DISCUSSION Synthesis and Structures of N-Heterocyclic CarbenePhosphinidene Complexes. The reactions of the parent NHC-phosphinidene 1 with the dimeric metal complexes [(η6p-cymene)MCl2]2 (M = Ru, Os) and [(η5-C5Me5)MCl2]2 (M = Rh, Ir) in toluene afforded the half-sandwich complexes 9 and 10 as red-brown (9a, 10a), yellow (9b), or orange (10b) solids, which precipitated from toluene solution and were isolated in satisfactory yields by filtration (Scheme 1). All complexes are Scheme 1. Preparation of the N-Heterocyclic CarbenePhosphinidene Complexes 9 and 10a

a

(a) Toluene, 0.5 equiv [(η6-p-cymene)MCl2]2 (M = Ru, Os); (b) Toluene, 0.5 equiv [(η5-C5Me5)MCl2]2 (M = Rh, Ir).

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Inorganic Chemistry sensitive toward air and moisture and decompose within one day in solution, e.g., in C6D6, under ambient conditions. The proton-coupled 31P NMR spectra exhibit signals at −56.5 (9a), −76.2 (9b), −57.1 (10a), and −72.8 ppm (10b) with 1JPH coupling constants of 235, 242, 218, and 229 Hz; the rhodium complex 10b shows additional coupling with the 103Rh nucleus, viz., 1JRhP = 81.9 Hz. In comparison with the free ligand (IPr)PH (1) (δ = −133.8 ppm, 1JPH = 165 Hz), a downfield shift and an increase of the 31P−1H coupling constant is observed upon metal coordination. For the carbonyl complexes 3−5 (Figure 1), the same increase of the 1JPH couplings was observed in comparison with the free (NHC)PH species, while ambiguous trends were found for the 31P chemical shifts.30,38−40 The 1H NMR spectra of all four complexes reveal broad signals at room temperature, indicating dynamic behavior in solution by rotation around the phosphorus-carbon bonds. Since coordination of the (IPr)PH ligand renders the complexes C1 symmetric, eight doublets and four quartet-quartets would be expected for the isopropyl hydrogen atoms of the diasterotopic Dipp groups at low temperature, which was indeed confirmed by a variable-temperature study of the osmium complex 9b (see Chapter S3, SI). Rotation around the P−C1 bond is frozen out below a coalescence temperature in the range 235−248 K, which affords a lower limit for the rotational barrier of 12.3 ± 0.2 kcal mol−1. This value falls in the range determined for the barrier in (IMes)EH (E = P, As).32,46 At room temperature, all 1 H NMR spectra show a doublet in the range 4.10−4.54 ppm with 1JPH values identical to those established by 31P NMR spectroscopy. The corresponding signal of the rhodium complex exhibits additional 1H−103Rh coupling of 2JRhH = 2.8 Hz. The molecular structures of the isotypic pairs of complexes 9a/9b and 10a/10b were established by X-ray diffraction analysis; ORTEP presentations of the Os and Ir complexes 9b and 10b are shown in Figures 2 and 3, whereas those of their

Figure 3. ORTEP diagram of complex 10b in 10b·2THF with thermal displacement parameters drawn at the 50% probability levels. Hydrogen atoms (except at phosphorus) and the solvent molecules were omitted for clarity. Selected bond lengths and angles are assembled in Table 1.

resemble those reported for similar half-sandwich phosphine complexes, e.g., for the triphenylphosphine complexes [(η6-pcymene)M(PPh3)Cl2]2 (M = Ru, Os)47,48 and [(η5-C5Me5)M(PPh3)Cl2]2 (M = Rh, Ir).49−52 The metal-phosphorus bonds of 2.3779(4)/2.3869(6) Å and 2.3462(5)/2.3460(6) Å in the 9a/9b and 10a/10b pairs are slightly longer than those in the corresponding PPh3 complexes, i.e., 2.3438(6)/2.355(1) Å for M = Ru/Os and 2.3408(18)/2.3182(14) Å for M = Rh/Ir. The orientation of the (IPr)PH ligand toward the metal complex fragments can be described as approximately eclipsed to Cl1 with comparatively small C1−P−M−Cl1 torsion angles of ca. 20° in 9a/9b and ca. 4° in 10a/10b. The phosphorus atoms display trigonal-pyramidal environments with angle sums of 308−311°, which could be ascribed tentatively to the presence of a stereochemically active lone-pair on the phosphorus atom. Furthermore, a noticeable elongation is observed for the phosphorus-carbene (P−C1) bonds compared to the free NHC-phosphinidene 1 (Table 1). This situation can be rationalized in terms of a stronger polarization of the (IPr)PH ligand upon metal coordination, as illustrated by the resonance structures B in Chart 1 and Table 2 (see below). Synthesis and Structures of N-Heterocyclic CarbenePhosphinidenide Complexes. With the NHC-phosphinidene complexes 9 and 10 in hand, we attempted their dehydrochlorination in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU); this base was also employed for the preparation of mercury complexes of the type [{(NHC)P}2Hg]45 and for the preparation of arylphosphinidene complexes as reported by Lammertsma.53,54 The reaction was first studied with the rhodium complex 10a, since the resulting NHC-phosphinidenide complex 12a had been reported previously.36 Accordingly, equimolar amounts of 10a and DBU were mixed in C6D6, and the reaction was followed by NMR spectroscopy. The conversion can be monitored by decrease of the 31P NMR signal of 10a at −57.1 ppm (1JRhP = 81.9 Hz) and the increase of a low-field signal at 551.8 ppm (1JRhP = 95.4 Hz), which can be assigned to 12a.36 In the 1H NMR spectrum, the reaction proceeds with decrease of the PH signal at 4.10 ppm, and for the hydrogen atoms of the imidazole backbone, the signal at 6.58 ppm for 10a is superseded by the signal at 6.71 ppm for 12a (see Figures S33−34, SI). However,

Figure 2. ORTEP diagram of complex 9b in 9b·C6H6 with thermal displacement parameters drawn at the 50% probability levels. Hydrogen atoms (except at phosphorus) and the solvent molecule were omitted for clarity. Selected bond lengths and angles are assembled in Table 1.

Ru and Rh congeners 9a and 10a are presented in the Supporting Information (Figures S1 and S2). Pertinent structural data are assembled in Table 1. All four complexes display the expected three-legged piano stool geometry around the metal atom with comparatively small P−M−Cl and Cl1− M−Cl2 angles of about 90°. These structural parameters 10787


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Inorganic Chemistry Table 1. Selected Bond Lengths and Angles of 1 and 9−12a

a

compound

P−M

P−C1

M−P−C1

P−M−Cl1

P−M−Cl2

Cl1−M−Cl2

C1−P−M−Cl1

1 9a (M = Ru) 9b (M = Os) 10a (M = Rh) 10b (M = Ir) 11a (M = Ru) 11b (M = Os) 12a (M = Rh) 12b (M = Ir)

− 2.3779(4) 2.3869(6) 2.3462(5) 2.3460(6) 2.2099(6) 2.2230(19) 2.1968(16) 2.1966(8)

1.7510(16) 1.8172(14) 1.817(2) 1.8241(18) 1.829(2) 1.824(2) 1.829(7) 1.822(6) 1.833(3)

− 113.93(5) 113.91(7) 115.33(6) 115.69(7) 112.80(7) 113.0(2) 110.17(17) 111.69(10)

− 90.487(12) 90.393(19) 91.881(16) 92.04(2) 94.24(2) 93.67(7) 96.37(6) 95.00(3)

− 86.075(12) 85.83(2) 90.150(17) 89.50(2) − − − −

− 88.277(12) 86.531(19) 91.848(16) 88.84(2) − − − −

− 20.52(5) 21.34(8) 3.00(6) 5.50(8) −9.15(8) 9.9(3) −16.69(19) −19.99(11)

Distances/Å, angles/°.

Table 2. Natural Bond Orbital (NBO) Charges and the WBIa of Selected Atoms and Bonds

a

compound

charges q(CNHC)

q(P)

q(M)

q(H)

WBI CNHC−P

P−M

P−H

(IPr)PH (1) 9a (M = Ru) 9b (M = Os) 10a (M = Rh) 10b (M = Ir) 11a (M = Ru) 11b (M = Os) 12a (M = Rh) 12b (M = Ir)

0.09 0.21 0.21 0.21 0.22 0.19 0.20 0.17 0.14

−0.04 0.38 0.37 0.34 0.35 0.27 0.23 0.17 0.19

--−0.57 −0.54 −0.31 −0.32 −0.51 −0.47 −0.26 −0.28

−0.02 0.03 0.03 0.02 0.02 ---------

1.37 1.04 1.04 1.02 1.01 1.01 0.99 1.04 1.06

--0.80 0.84 0.76 0.82 1.45 1.49 1.25 1.32

0.96 0.93 0.93 0.93 0.93 ---------

WBI = Wiberg bond indices.

the conversion does not go to completion, and an approximate 1:1 mixture of 10a and 12a is formed. Following the same reaction in THF-d8 indicates ca. 70% conversion. The use of DBU in excess was also unfavorable, and clean conversion between the pairs 9/11 and 10/12 was not accomplished. Stronger bases such as KOtBu and NaN(SiMe3)2 also failed, leading to decomposition and liberation of the free (IPr)PH ligand. Similar observations were made by Severin regarding the attempted deprotonation of the related imidazolin-2-imine complex [(η6-p-cymene)RuCl2{HN(IMes)}].55 In the past, we widely used N-silylated imidazolin-2-imines of the type (NHC)NTMS for the preparation of imidazolin-2iminato complexes by reaction with appropriate metal chlorides and formation of Me3SiCl.56−58 In particular, we synthesized the ruthenium(II) and rhodium(III) NHC-phosphinidenide complexes 11a and 12a by reaction of the P-silylated phosphinidene derivative (IPr)PTMS (2) with the dinuclear complexes [(η6-p-cymene)RuCl2]2 and [(η5-C5Me5)RhCl2]2 in toluene (Scheme 2).36 Herein, the same complexes were prepared in fluorobenzene and isolated in higher yields, i.e., 95% (11a) and 78% (12a). The resulting dark-green complexes showed characteristic low-field 31P NMR signals at 531.5 ppm (11a) and 551.8 ppm (d, 1JRhP = 95.5 Hz, 12a), revealing their relationship with terminal metal phosphinidene complexes,44 which exhibit 31P NMR resonances at even lower field, e.g., at 813 ppm for [(η6-p-cymene)(PCy3)Ru(PMes*)]59 and at 868 ppm (1JRhP = 69 Hz for [(η5-C5Me5)(PPh3)Rh(PMes*)]54 (Mes* = 2,4,6-tri-tert-butylphenyl). The corresponding osmium(II) and iridium(III) complexes 11b and 12b were prepared by similar reactions of 2 with [(η6-

Scheme 2. Preparation of Carbene-Phosphinidenide Complexes 6 and 7a

a (a) Fluorobenzene, 0.5 equiv [(η6-p-cymene)MCl2]2 (M = Ru, Os), (b) Fluorobenzene, 0.5 equiv [(η5-C5Me5)MCl2]2 (M = Rh, Ir).

p-cymene)OsCl2]2 or [(η5-C5Me5)IrCl2]2 in fluorobenzene and were isolated as dark purple and dark blue solids in 82% and 89% yield, respectively. The 31P NMR signals are observed at 354.3 ppm (11b) and 353.7 ppm (12b), which is at higher field than found for their lighter Ru and Rh congeners. Again, these resonances are upfield from those reported for related terminal phosphinidene complex, e.g., 668 ppm for [(η6-p-cymene)(PPh3)Os(PMes*)]53 and 687 ppm for [(η5-C5Me5)(PPh3)Ir(PMes*)].60 Their 1H NMR spectra at room temperature exhibit two doublets and one broad signal for the isopropyl CH3 and CH hydrogen atoms of the Dipp groups, indicating dynamic behavior in solution by rotation around the phosphorus-carbon bond as established previously for 11a and 12a by variable-temperature NMR spectroscopy.36 Single crystals of the 5d metal complexes 11b and 12b were grown from fluorobenzene solutions at −30 °C, and their molecular structures were established by X-ray diffraction 10788


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Inorganic Chemistry

whereas the phosphorus-CNHC bond lengths remain almost unaffected (Table 1). Theoretical Studies. To assess the bonding situation in the complexes 9−12, their structures were optimized applying the density functional theory (DFT) method B97-D, followed by natural bond orbital (NBO) analysis. The computational details are summarized in Chapter S5 and contour plots of selected NBOs are presented in Tables S4−S8. The computed structural parameters are in good agreement with the solid-state structures. The E isomers of 11 and 12 were also calculated, revealing a destabilization (ΔG0) by 5.1 (11a), 6.7 (11b), 12.3 (12a), and 7.4 kcal mol−1 (12b) relative to the experimentally observed Z forms. As expected, metal complexation of (IPr)PH (1) leads to a significant polarization (see Chart 1, vide supra) and weakening of the P−CNHC bond as indicated by a decrease of the Wiberg bond index (WBI) from 1.37 in 1 to 1.01−1.04 in complexes 9 and 10 (Table 2). Simplistically, dehydrochlorination of 9 and 10 creates a negative formal charge at the phosphorus atom, together with a positive formal charge at the metal atom, in the complexes 11 and 12. However, the NBO charges indicate only minor changes (Table 2), suggesting a compensation by phosphorusto-metal charge transfer. Indeed, comparison of the WBI values, which increase from 0.76−0.84 in 9/10 to 1.25−1.49 in 11/12, reveals a significant increase of the metal-phosphorus bond order, whereas the WBI of the P−CNHC bonds remains almost unchanged. This bonding situation is best described by the mesomeric forms B and B′ in Chart 1 and Table 2, with the formally negative carbene-phosphinidenide ligand acting as a 2σ,2π-electron donor. For each complex 11 and 12, an NBO associated with an additional, strongly covalent metalphosphorus π-interaction can be identified, and the expected high 3s character of the phosphorus lone pair is also confirmed; appropriate contour plots are shown in the SI. The theoretical and experimental data thus confirm the close similarity with nucleophilic phosphinidene complexes of the type [LnM PR]44 and the assignment of metal-phosphorus double bonds.36

analysis (Figures 4 and 5). Pertinent structural data are assembled in Table 1; as expected, they resemble those of their

Figure 4. ORTEP diagram of complex 11b with thermal displacement parameters drawn at the 50% probability level. Hydrogen atoms and the solvent molecules were omitted for clarity. Selected bond lengths and angles are assembled in Table 1.

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Figure 5. ORTEP diagram of complex 12b with thermal displacement parameters drawn at the 50% probability level. Hydrogen atoms and the solvent molecules were omitted for clarity. Selected bond lengths and angles are assembled in Table 1.

CONCLUSION The parent N-heterocyclic carbene-phosphinidene adduct (IPr)PH (1) forms half-sandwich complexes with the 4d and 5d metals of group 8 and 9. Treatment of the resulting complexes [LMCl2{HP(IPr)}] (L = η6-p-cymene, M = Ru, Os; L = η5-C5Me5, M = Rh, Ir) with strong bases such as 1,8diazabicyclo[5.4.0]undec-7-ene (DBU) indicate incomplete or unselective dehydrochlorination, and the corresponding carbene-phosphinidenide complexes [LMCl{P(IPr)}] were more conveniently prepared from (IPr)PSiMe3 (2) and the dimeric metal chloride precursors by elimination of Me3SiCl. These complexes exhibit short metal-phosphorus bonds, and DFT calculations reveal that the formally monoanionic (IPr)P ligand acts as a strong 2σ,2π-electron donor with formation of highly covalent metal-phosphorus double bonds. In comparison, the lighter nitrogen-donor imidazolin-2-iminato ligands of the type (NHC)N may act as 2σ,4π-electron donor ligands, in particular, toward early transition metals or metals in a higher oxidation state,56 albeit with formation of significantly more ionic metal-nitrogen bonds.62,63 Accordingly, large and almost linear M−N−CNHC angles are usually observed.62,64−69 In contrast, significantly smaller M−P−CNHC angles are found for the [LMCl{P(IPr)}] complexes reported herein, which can be ascribed to the much higher s character of the phosphorus compared to the nitrogen lone pair. This trend should proceed

lighter 4d metal analogues 11a and 12a (11a/b are isotypic; 12a crystallized as a different solvate than 12b). Both metal atoms display a pseudotrigonal-planar environment (twolegged piano stool geometry) with P−M−Cl angles of 93.67(7)° (11b) and 95.00(3)° (12b). The metal-phosphorus bond lengths are Os−P = 2.2230(19) Å and Ir−P = 2.1966(8) Å, very similar to the values found in [(η6-p-cymene)(PPh3)Os(PMes*)] (2.2195(7) Å)53 and [(η5-C5Me5)(L)Ir(PMes*)] (L = PPh3, 2.2121(5)Å;60 L = CO, 2.1783(8) Å;60 L = NHC, 2.1959(5) Å).61 Similarly to these systems, small M−P−C1 angles in the range 110.17(17)−113.0(2)° are found, in agreement with a high 3s character of the remaining phosphorus lone pair. It should be noted that the carbene moiety in all four complexes 11 and 12 adopts a Z orientation facing the chlorido ligand, whereas the majority of arylphosphinidene complexes display an E configuration. In the solid state, only the iridium carbonyl complex (L = CO) crystallizes in the Z form.60 As expected, the M−P bonds in the complexes 11/12 are shorter than those in their counterparts 9/10, 10789


Article

Inorganic Chemistry

crystals were obtained by diffusion of hexane into a solution of 9b in benzene. 1H NMR (C6D6, 300.1 MHz): δ = 0.96 (d, JHH = 6.9 Hz, 12H, CH(CH3)2), 1.06 (d, JHH = 7.0 Hz, 6H, CH(CH3)2), 1.59 (br. d, JHH = 6.5 Hz, 12H, CH(CH3)2), 1.92 (s, 3H, CH3), 2.91 (sept, JHH = 6.9 Hz, 1H, CH(CH3)2), 3.31 (br, 4H, CH(CH3)2), 4.43 (d, JPH = 242 Hz, 1H, P-H), 4.75 (br., 2H, CHcymene), 4.96 (br., 2H, CHcymene), 6.51 (s, 2H, NCH), 7.13 (m, 4H, CHAr), 7.18 (m, 2H, CHAr) ppm; 13 C{1H} NMR (C6D6, 150.9 MHz): δ = 17.7 (d, JPC = 4.3 Hz, CH3), 23.3 (CH(CDippH3)2), 26.3 (CH(CDippH3)2), 29.1 (CDippH(CH3)2), 30.1 (CH(CH3)2), 30.2 (CH(CH3)2), 74.5 (CCymeneH), 95.8 (CCH(CH3)2), 97.1 (CCH3), 123.6 (NCH), 124.9 (m-CDipp), 131.0 (pCDipp), 133.9 (o-CDipp), 146.7 (NCDipp), 172.6 (d, JPC = 80.8 Hz, CCarbene) ppm; 31P NMR (C6D6, 121.5 MHz): δ = −76.2 (d, JPH = 242 Hz) ppm; MS (EI): m/z (%): 817.2 [M+H]+, 780.3 [M-HCl]+, 646.1 [C27H36ClNOsP]+, 420.2 [(IPr)PH]+ ; elemental analysis calcd (%) for C37H51Cl2N2OsP (815.93): C 54.93, H 6.55, N 3.37; found: C 55.20, H 6.30, N 3.25. Synthesis of [(η5-C5Me5)Rh{HP(IPr)}Cl2] (10a). [Rh(η5-C5Me5)Cl2]2 (147 mg, 0.238 mmol) was suspended in toluene (10 mL) and (IPr)PH (200 mg, 0.476 mmol, 2.0 equiv.) in toluene (5 mL) was added. The reaction mixture was stirred at ambient conditions. After addition of the (IPr)PH the red-brown suspension changed to a dark brown solution. A dark-brown solid precipitated and hexane was added (4 mL). Filtration and drying of the solid in vacuum gives complex 10a as a red-brown powder (274 mg). A second crop was obtained by cooling the mother liquor to −35 °C. The crystalline solid was dried, affording 317 mg of complex 10a (91% yield). Red-brown crystals were obtained by diffusion of hexane into a solution of 10a in THF. 1 H NMR (C6D6, 300.3 MHz): δ = 0.97 (d, JHH = 4.9 Hz, 12H, CH(CH3)2), 1.18 (d, JRhH = 2.3 Hz, 15H, C5(CH3)5), 1.58 (m, 12H, CH(CH3)2), 3.49 (br. s, 4H, CH(CH3)2), 4.10 (dd, JPH = 217 Hz, JRhH = 2.8 Hz, 1H, P-H), 6.58 (s, 2H, NCH), 7.22 (m, 4H, CHAr) ppm; 13 C{1H} NMR (C6D6, 150.9 MHz): δ = 8.9 (d, JRhC = 2.7 Hz, CH3), 23.3 (br. s, CH(CH3)2), 26.5 (br. s, CH(CH3)2), 29.1 (s, CH(CH3)2), 95.4 (d, JRhC = 6.4 Hz, CCH3), 124.3 (NCH), 130.9 (p-CDipp), 134.2 (o-CDipp), 145.9 (br., NCDipp), 172.4 (d, JPC = 90.0 Hz, CCarbene) ppm; 31 P NMR (C6D6, 121.5 MHz): δ = − 57.1 (dd, JPH = 217 Hz, JRhP = 81.9 Hz) ppm; MS (EI): m/z (%): 692.3 [M-HCl]+, 420.2 [(IPr)PH]+, 377.2 [C24H30N2P]+; elemental analysis calcd (%) for C37H52Cl2N2PRh (729.62): C 60.91, H 7.18, N 3.84; found: C 60.82, H 7.09, N 3.89. Synthesis of [(η5-C5Me5)Ir{HP(IPr)}Cl2] (10b). (IPr)PH (200 mg, 0.476 mmol, 2.0 equiv.) in toluene (10 mL) was added to a stirred suspension of [Ir(η5-C5Me5)Cl2]2 (189 mg, 0.238 mmol) in toluene (10 mL). After addition the orange suspension immediately changed the color to a dark-brown solution, which was stirred overnight whereby an orange solid precipitated. Hexane was added (4 mL) and the solid was filtered off and dried in vacuum to afford complex 10b (276 mg). A second crop of 22 mg was obtained by cooling the mother liquor to −35 °C, leading to a total yield of 298 mg (93%). Orange crystals were obtained by diffusion of HMDSO into a solution of 10b in THF. 1H NMR (C6D6, 300.3 MHz): δ = 0.97 (d, JHH = 5.3 Hz, 12H, CH(CH3)2), 1.24 (d, JHH = 1.1 Hz, 15H, C5(CH3)5), 1.60 (br., 12H, CH(CH3)2), 3.55 (br, 4H, CH(CH3)2), 4.22 (d, JPH = 228 Hz, 1H, P-H), 6.57 (s, 2H, NCH), 7.09 (m, 2H, CHAr), 7.22 (m, 2H, CHAr) ppm; 13C{1H} NMR (C6D6, 150.9 MHz): δ = 8.7 (CH3), 23.5 (br. s, CH(CH3)2), 26.4 (br. s, CH(CH3)2), 29.1 (s, CH(CH3)2), 89.0 (CCH3), 124.2 (NCH), 125.2 (m-CDipp), 130.8 (p-CDipp), 134.3 (oCDipp), 147.0 (br. s, NCDipp), 170.4 (d, JPC = 87.9 Hz, CCarbene) ppm; 31 P NMR (C6D6, 121.5 MHz): δ = −72.8 (d, JP‑H = 228 Hz) ppm; MS (EI): m/z (%): 782.3 [M-HCl] + , 420.2 [(IPr)PH] + , 377.2 [C24H30N2P]·+; elemental analysis calcd (%) for C37H52Cl2IrN2P (818.93): C 54.27, H 6.40, N 3.42; found: C 54.20, H 6.34, N 3.68. Synthesis of [(η6-p-Cymene)Ru{P(IPr)}Cl] (11a). Flurobenzene (25 mL) was added to a Schlenk tube containing [(η6-p-cymene)RuCl2]2 (186 mg, 0.304 mmol) and carbene-phosphinidene adduct (IPr)PSiMe3 (300 mg, 0.609 mmol) at room temperature. The resulting dark blue reaction mixture was stirred for 2 d. The solvent was removed under reduced pressure and the residue was washed with

further with ligands containing heavier N-heterocyclic carbenepnictinidene adducts of the type (NHC)E (E = As, Sb, Bi), and with the NHC-arsinidene adducts (IPr)AsR and (IMes)AsR (R = H, SiMe3), the way is indeed paved for the preparation of analogous carbene-arsinidene and carbene-arsinidenide transition metal complexes.32

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EXPERIMENTAL SECTION

All manipulations were carried out using standard Schlenk techniques under a dry argon atmosphere or in dry argon-filled gloveboxes. Solvents were dried by using the MBraun solvent purification system. Fluorobenzene was passed over neutral Al2O3, and then distilled over CaH2. The starting materials, 1,36 2,36 [(η6-p-cymene)RuCl2]2,70 [(η6p-cymene)OsCl2]271 (osmium complexes are known to be carcinogeniccare should be taken), [(η5-C5Me5)RhCl2]2,72 and [(η5C5Me5)IrCl2]273 were prepared according to previously published procedures. 1H, 13C, and 31P NMR spectra were measured on Bruker AV 300 (300 MHz), Bruker DRX 400 (400 MHz), Bruker AV III-400 (400 MHz), Bruker AV III-HD500 (500 MHz), and Bruker AV II-600 (600 MHz) spectrometers. If required, the assignment of signals was supported by 2D experiments (1H-1H COSY, 1H-13C HSQC, 1H-13C HMBC). The chemical shifts are given in parts per million (δ; ppm) relative to residual solvent 1H signals (δ − 7.16 (C6D6) and 3.58 (THF-d8)), or to the 13C resonance of the solvents (δ − 128.06 (C6D6) and 67.21 (THF-d8)). Coupling constants (J) are reported in Hertz (Hz) and splitting patterns are indicated as s (singlet), d (doublet), t (triplet), m (multiplet), sept (septet), and br (broad). All spectra were measured at room temperature unless otherwise stated. The number of protons attached to each carbon atom was determined by 13C-DEPT135 experiments. Mass spectra were recorded on Finnigan MAT 95 (EI) and Finnigan MAT 95XL (SI) systems. Elemental analyses were carried out on a Vario Micro Cube System. Synthesis of [(η6-p-cymene)Ru{HP(IPr)}Cl2] (9a). To a redbrown stirred suspension of [Ru(η6-p-cymene)Cl2]2 (146 mg, 0.238 mmol) in toluene (10 mL) was added a solution of (IPr)PH (200 mg, 0.476 mmol, 2.0 equiv) in toluene (5 mL). The solid disappeared immediately, forming a red solution that was stirred overnight. While stirring, a red-brown solid precipitated from the reaction solution and hexane was added (4 mL); the stirring was continued. The solid was filtered off, washed with a little hexane (3 × 2 mL), and dried in vacuum to afford complex 9a. A second crop was obtained by crystallization from the mother liquor at −35 °C, leading to a total amount of 298 mg (86% yield). Red crystals for X-ray analysis were obtained by diffusion of hexane into a solution of 9a in C6D6. 1H NMR (C6D6, 300.1 MHz): δ = 0.96 (d, JHH = 6.9 Hz, 12H, CH(CH3)2), 1.05 (d, JHH = 6.9 Hz, 6H, CH(CH3)2), 1.59 (br. d, JHH = 5.7 Hz, 12H, CH(CH3)2), 1.88 (s, 3H, CH3), 3.01 (sept, JHH = 6.9 Hz, 1H, CH(CH3)2), 3.45 (br. 4H, CH(CH3)2), 3.77 (d, JPH = 235 Hz, 1H, P-H), 4.54 (br. s, 2H, CHCymene), 6.50 (s, 2H, NCH), 6.98−7.15 (m, 6H, CHAr) ppm; 13C{1H} NMR (C6D6, 150.9 MHz): δ = 17.9 (d, JPC = 6.6 Hz, CH3), 23.2 (br. s, CH(CDippH3)2), 26.4 (br. s, CH(CDippH3)2), 29.0 (br. s, CDippH(CH3)2), 30.0 (s, CH(CH3)2), 30.1 (s, CH(CH3)2), 103.2 (d, JPC = 3.7 Hz, CCH3), 103.7 (s, CCH(CH3)2), 123.7 (NCH), 124.9 (m-CDipp), 131.0 (p-CDipp), 133.9 (o-CDipp), 146.9 (NCDipp), 174.9 (d, JPC = 84.5 Hz, CCarbene) ppm; 31P NMR (C6D6, 121.5 MHz): δ = − 56.5 (d, JPH = 235 Hz) ppm; MS (EI): m/z: 727.2 [M+H]+, 420.2 [(IPr)PH]+, 377.2 [C24H30N2P]·+, 134 [p-cymene]; elemental analysis calcd (%) for C37H51Cl2N2PRu × 0.5 C7H8 (726.77): C 63.27, H 7.42, N 3.56; found: C 63.27, H 7.14, N 3.71. Synthesis of [η6-p-Cymene)Os{HP(IPr)}Cl2] (9b). [Os(η6-pcymene)Cl2]2 (188 mg, 0.238 mmol) was suspended in toluene (10 mL) and a solution of (IPr)PH (200 mg, 0.476 mmol, 2.0 equiv.) in toluene (5 mL) was added; the mixture was stirred at ambient conditions overnight. The yellow suspension changed immediately to a dark-red solution. A yellow solid precipitated and hexane was added (4 mL). The yellow precipitate was filtered off and dried in vacuum to afford complex 9b (290.6 mg). A further crop of 34.1 mg was obtained by cooling the mother liquor leading to 84% yield (326 mg). Yellow 10790


Article

Inorganic Chemistry cold n-hexane (2 mL) and dried under vacuum to obtain complex 11a. Yield: 397 mg (95%). The spectroscopic data (1H and 31P NMR) are similar to those previously reported. Synthesis of [(η6-p-Cymene)Os{P(IPr)}Cl] (11b). To a stirred yellow suspension of [(η6-p-cymene)OsCl2]2 (175 mg, 0.220 mmol) in C6H5F (10 mL) was added (IPr)PSiMe3 (218 mg, 0.440 mmol, 2.0 equiv) in fluorobenzene (10 mL). The resulting dark purple reaction mixture was stirred for 24 h. The solvent was removed under vacuum and the residue was washed with hexane (2 × 1 mL) to obtain 6b as an air- and moisture-sensitive purple solid in 82% yield (283 mg). 1H NMR (C6D6, 300 MHz): δ = 1.07−1.03 (m, 18H, merged protons of cymene and Dipp groups, CH(CH3)2), 1.49 (d, 12H, JHH = 6.6 Hz, CH(CH3)2), 2.09 (s, 3H, CH3), 2.41 (sept, 1H, JHH = 7.2 Hz, CH(CH3)2), 3.46 (sept, 4H, JHH = 6.7 Hz, CH(CH3)2), 3.50 (d, 2H, JHH = 5.4 Hz, CHcymene), 5.35 (d, 2H, JHH = 5.6 Hz, CHcymene), 6.64 (s, 2H, NCH), 7.07−7.14 (m, 6H, CHAr) ppm; 13C{1H} NMR (C6D6, 100.7 MHz): δ = 20.8 (CH3), 23.9 (CH(CH3)2), 24.1 (br, CH(CDippH3)2), 26.4 (CH(CDippH3)2), 29.0 (CH(CH3)2), 33.3 (C Dipp H(CH 3 ) 2 ), 67.7 (C Cymene H), 73.4 (C Cymene H), 85.8 (CCymeneCH), 91.7 (CCH3), 115.8 (d, 3JPC = 20.5 Hz, NCH), 123.9 (m-CDipp), 124.8 (br, p-CDipp), 130.4 (br, o-CDipp), 135.4 (NCDipp), 200.1 (d, JPC = 151.3 Hz, Ccarbene) ppm; 31P NMR (C6D6, 162.1 MHz); δ = 354.1 (s) ppm; HRMS (EI, m/z) for C37H50N2ClPOs; 780.30010 (calcd); 780.30015 (found). MS (EI) (m/z); 780.1 [M]+; 737.2 [M-iPr]+; 644.0 [M-(C10H14)]+; 419.2 [M-ClOs(p-cymene)]+; elemental analysis calcd (%) for C37H50N2PClOs (780.300): C 56.90, H 6.46, and N 3.59; Found: C 56.61, H 6.25, and N 3.64. UV/vis (toluene, λ(nm) ε(M−1 cm−1)): 444.8 (2254.08), 524.5 (2105.50). Synthesis of [(η5-C5Me5)Rh{P(IPr)}Cl] (12a). Fluorobenzene (20 mL) was added to a Schlenk tube containing [(η5-C5Me5)RhCl2]2 (125 mg, 0.202 mmol) and carbene-phosphinidene adduct (IPr)PSiMe3 (200 mg, 0.406 mmol) at room temperature. The resulting dark green-blue reaction mixture was stirred for 3 d at the same temperature and then the solvent was removed under vacuum. The residue was washed with cold n-hexane (3 × 1 mL) and dried to give 12a. Upon cooling the hexane washings to −30 °C another small batch of complex precipitated. Overall yield: 220 mg (78%). The spectroscopic data (1H and 31P NMR) are similar to those of previously reported. Synthesis of [(η5-C5Me5)Ir{P(IPr)}Cl] (12b). To a stirred solution of [(η5-C5Me5)IrCl2]2 (100 mg, 0.125 mmol) in C6H5F (10 mL) was added the carbene-phosphinidene adduct (IPr)PSiMe3 (123 mg, 0.251 mmol, 2.0 equiv) in fluorobenzene (10 mL) at room temperature. The resulting dark blue reaction mixture was stirred for 6 h at room temperature. All volatiles were removed under vacuum and the residue was washed with cold n-hexane (2 × 2 mL). The remaining solid was dried under reduced pressure leading to a dark blue air- and moisturesensitive solid (87 mg, 89%). 1H NMR (C6D6, 200.1 MHz): δ = 1.13 (d, 12H, JHH = 6.9 Hz, CH(CH3)2), 1.56 (d, 12H, JHH = 6.6 Hz, CH(CH3)2), 1.75 (pseudo doublet, 15H, JPH = 2.7 Hz, C5(CH3)5), 3.47 (broad sept, 4H, JHH = 6.8 Hz, CH(CH3)2), 6.73 (s, 2H, NCH), 7.237−11 (m, 6H, CHAr) ppm; 13C {1H} NMR (C6D6, 75.4 MHz): δ = 10.8 (C5(CH3)5), 24.0 (CH(CH3)2), 26.4 (CH(CH3)2), 28.9 (CH(CH3)2), 85.6 (s, C5(CH3)5), 115.8 (d, JPC = 21.3 Hz, NCH), 124.6 (m-CDipp), 130.2 (p-CDipp), 135.6 (o-CDipp), 147.3 (N-C(Dipp)), 165.3 (CCarbene) ppm; 31P NMR (C6D6, 121.5 MHz); δ = 353.7 (s) ppm; HRMS (EI, m/z); 780.3072 (found for M-2H); 780.3078 (calcd); MS (EI): 782.2 [M]+, 739.1 [M-iPr]+, 419.2 [M-ClIrCp*]+; elemental analysis calcd (%) for C37H51N2ClPIr (782.310); C 56.75, H 6.57, and N 3.57; Found: C 57.28, H 6.88, and N 3.40. UV/vis (toluene, λ(nm) ε(M−1 cm−1)): 338 (17107.0), 468 (3141.4), 628 (4210.5).

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Analytical data, X-ray diffraction data, and computational details (PDF) Accession Codes

CCDC 1561571−1561577 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: (+49) 531-391-5309. Fax: +49-531-391-5387. ORCID

Matthias Tamm: 0000-0002-5364-0357 Author Contributions ‡

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. M.P. and A.D. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Deutsche Forschungsgemeinschaft (DFG) through grant TA 189/16-1. M. P. thanks the graduate student program CASUS for a PhD fellowship. The authors wish to thank Dirk Bockfeld for crystallographic assistance.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01798. Structure file (MOL) 10791


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N-Heterocyclic Carbene-Phosphinidene and Carbene

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