Three-Coordinate N-Heterocyclic Carbene Nickel Nitrosyl Complexes

Organometallics 2010, 29, 717–720 DOI: 10.1021/om900706z

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Three-Coordinate N-Heterocyclic Carbene Nickel Nitrosyl Complexes Matthew S. Varonka and Timothy H. Warren* Department of Chemistry, Georgetown University, Box 571227, Washington, D.C. 20057-1127 Received August 10, 2009 Summary: The N-heterocyclic carbene IPr serves as a supporting ligand in a series of low-coordinate nickel nitrosyl complexes which display a range of nitrosyl bonding modes. Attempts to form a cationic two-coordinate nickel nitrosyl led to the three-coordinate {[IPr][IPr0 ]Ni(NO)}þ[BArF4]-, in which one of the IPr ligands exhibits unusual C(5) coordination. Very few three-coordinate, terminal metal nitrosyl complexes have been structurally characterized.1-3 The pogostick complexes CpNi(NO), first reported in 1955,4 and Cp*Ni(NO)5,6 represent relatively rare instances of lowcoordinate transition-metal nitrosyl complexes. More recent three-coordinate examples are neutral nickel β-diketiminate complexes such as [Me3NN]Ni(NO)2 as well as the phosphine-supported cationic nickel nitrosyl species [(dtbpe)Ni(NO)][BArF4] (dtbpe=1,2-bis(di-tert-butylphosphino); BArF4= tetrakis(3,5-bis(trifluoromethyl)phenyl)borate).3 In each case, the ground-state structure possesses a linear nitrosyl ligand, though there is accumulating evidence for several light-induced, metastable [M](η2-NO) species7 such as CpNi(η2-NO)8 and Cp*Ni(η2-NO).6 To extend the synthetic exploration of low-coordinate metal nitrosyls, including the possibility of the cationic, two-coordinate nickel nitrosyl [IPrNi(NO)]þ, we were attracted to the sterically demanding N,N0 -bis(2,6-diisopropylphenyl)imidazole (IPr) supporting ligand.9 Addition of NiI(NO)(THF)210 to the free N-heterocyclic carbene IPr in THF gives the dark blue IPrNi(I)NO (1) in 52% yield (Scheme 1). Unfortunately, crystals of 1 from Et2O were unsuitable for single-crystal X-ray analysis due to severe twinning. Exchange of the iodide anion by addition of 1 equiv of AgOTf to 1 in Et2O gave IPrNi(OTf)NO (2) in

Figure 1. ORTEP diagram of 2. Selected bond distances (A˚) and angles (deg): Ni-C1=1.937(2), Ni-N3=1.630(2), Ni-O2 = 1.946(2), N3-O1 = 1.159(2); Ni-N3-O1 = 163.8(2), C1Ni-N3 = 121.59(8), C1-Ni-O2 = 101.08(7), O2-Ni-N3 = 137.25(8). Scheme 1. Synthesis of 1-3

65% yield (Scheme 1). The X-ray structure of 2 shows a three-coordinate, distorted T-shaped environment at the Ni center (Figure 1).11 For instance, the angle between the κ1-bound triflate and nitrosyl ligands (O2-Ni-N3 = 137.25(8)°) is much larger than the angle between the triflate and the NHC ligands (C1-Ni-O2=101.08(7)°). The nitrosyl ligand is nearly linear (Ni-N3-O1 = 163.8(2)°). Considering the bound nitrosyl as NOþ, 2 is best assigned as a 16-electron Ni(0) complex. Solid-state IR spectra of 1 and 2 show a shift of the vNO stretching frequency from 1766 to 1816 cm-1 (Table 1), respectively, owing to greater nitrosonium character in the electron-poor triflate 2. Addition of the bulky thallium thiolate TlSCPh3 to 1 in THF gives IPrNi(SCPh3)NO (3) in 63% isolated yield (Scheme 1). While both thiolate 3 and triflate 2 are 16-electron Ni(0) complexes, the X-ray structure of 3

*To whom correspondence should be addressed. E-mail: thw@ georgetown.edu. (1) (a) Carty, P.; Walker, A.; Mathew, M.; Palenik, G. J. J. Chem. Soc. D 1969, 1374–1375. (b) Chong, K. S.; Rettig, S. J.; Storr, A.; Trotter, J. Can. J. Chem. 1979, 57, 3119–3125. (2) Puiu, S. C.; Warren, T. H. Organometallics 2003, 22, 3974–3976. (3) Iluc, V. M.; Miller, A. J. M.; Hillhouse, G. L. Chem. Commun. 2005, 5091–5093. (4) Piper, T. S.; Cotton, F. A.; Wilkinson, G. J. Inorg. Nucl. Chem. 1955, 1, 165–174. (5) Schneider, J. J.; Goddard, R.; Kr€ uger, C. Organometallics 1991, 10, 665–670. (6) Fomitchev, D. V.; Furlani, T. R.; Coppens, P. Inorg. Chem. 1998, 37, 1519–1526. (7) (a) Bitterwolf, T. E. Coord. Chem. Rev. 2006, 250, 1196–1207. (b) Coppens, P.; Novozhilova, I.; Kovalevsky, A. Chem. Rev. 2002, 102, 861– 883. (c) Fomitchev, D. V.; Coppens, P. Comments Inorg. Chem. 1999, 21, 131–148. (8) (a) Chen, L. X.; Bowman, M. K.; Wang, Z.; Montano, P. A.; Norris, J. R. J. Phys. Chem. 1994, 98, 9457–9464. (b) Crichton, O.; Rest, A. J. J. Chem. Soc., Dalton Trans. 1977, 986–993. (9) Huang, J.; Nolan, S. P. J. Am. Chem. Soc. 1999, 121, 9889–9890. (10) Haymore, B.; Feltham, R. D. Inorg. Synth. 1973, 14, 81–89.

(11) Crystal data for 2: triclinic, P1, a=9.0451(4) A˚, b=9.9916(5) A˚, c=18.6756(9) A˚, R=80.5300(10)°, β=80.5280(10)°, γ=68.5620(10)°, Z= 2, collected at 173(2) K, μ(Mo KR)=0.752, 16 586 collected reflections, 6029 unique reflections, Rint =0.0342, R1=0.0363 (I > 2σ(I)), wR2= 0.0933 (all data), GOF=1.132.

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Table 1. NO Stretching Frequencies and Ni-N-O Angles compd 1 2 3 5 6 7

NO stretch (cm-1)

Ni-N-O (deg)

1766 1816 1772 1568 1681 1784

n/a 163.8(2) 172.8(2) 129.9(2) 137.5(3), 137.7(3) 164.6(3)

Figure 3. ORTEP diagram of 5. Selected bond distances (A˚) and angles (deg): Ni-C1=1.913(3), Ni-N3=1.779(3), N3-O= 1.163(3), Ni-Cp(centroid) = 1.893(3); Ni-N3-O = 129.9(2), C1-Ni-N3 = 97.3(1), C1-Ni-Cp(centroid) = 134.8(2), Cp(centroid)-Ni-N3=127.7(2).

Figure 2. ORTEP diagram of 3. Selected bond distances (A˚) and angles (deg): Ni-C1=1.944(2), Ni-N3=1.630(2), N3-O= 1.165(2), Ni-S=2.1553(6); Ni-N3-O=172.8(2), C1-Ni-N3 =127.31(9), C1-Ni-S=99.73(6), S-Ni-N3=132.10(7). Scheme 2. Synthesis of 4 and 5

(Figure 2)12 shows modest differences in the coordination sphere as compared to 2. The bulk of the trityl group increases the twist angle between the carbene N1-C1-N2 and S-Ni-N3 planes to 66°, while the analogous twist angle in 2 is 30°. Thiolate 3 possesses a linear nitrosyl ligand (Ni-N3-O=172.8(2)°) with a νNO stretching frequency of 1772 cm-1 (Table 1). Reaction of 2 with 1 equiv of Cp2Ni in THF provides the bright red IPrNi(OTf)Cp (4) as well as the previously characterized nickel nitrosyl CpNi(NO) in a 1:1 ratio (Scheme 2). Instead of Cp2Ni serving as a reducing agent, Cp/NO exchange takes place to convert the 16-electron 2 and 20electron Cp2Ni into the two 18-electron species 4 and CpNi(NO). 1H NMR analysis of the reaction mixture before crystallization of 4 confirms the formation of 4 and CpNi(NO)13 in a 1:1 ratio by their respective Cp resonances at δ 3.91 and 4.96 ppm in benzene-d6. The presence of CpNi(NO) was also confirmed by its νNO stretching frequency at 1811 cm-1. Crystallization of the reaction mixture at -35 °C from (12) Crystal data for 3: triclinic, P1, a=10.6615(5) A˚, b=11.1114(6) A˚, c = 19.6342(10) A˚, R = 74.8880(10)°, β = 82.8800(10)°, γ = 64.5230(10)°, Z = 2, collected at 100(2) K, μ(Mo KR) = 0.568, 21 720 collected reflections, 7937 unique reflections, Rint=0.0388, R1=0.0394 (I > 2σ(I)), wR2=0.0862 (all data), GOF=1.001. (13) Field, C. N.; Green, J. C.; Mayer, M.; Nasluzov, V. A.; R€ osch, N.; Siggel, M. R. F. Inorg. Chem. 1996, 35, 2504–2514.

Et2O afforded crystals of 4 suitable for single-crystal X-ray diffraction. The X-ray structure of 4 (Figure S5, Supporting Information)14 shows a distorted T-shaped coordination environment at Ni with a C1-Ni-O1 angle of 97.68(6)°, similar to that of the chloro analogue IPrNi(Cp)Cl prepared by Nolan, which has a Ccarbene-Ni-Cl angle of 93.86(3)°.15 Inspired by the ready formation of cyclopentadienyl derivatives, we find that the addition of 1 equiv of NaCp in THF to 2 in Et2O readily provides the deep red IPrNi(Cp)NO (5) in 87% yield (Scheme 2). The X-ray structure of 5 (Figure 3)16 shows a stark difference in the nitrosyl coordination from that in 2. The much lower Ni-N3-O bond angle in 5 (129.9(2)°) as compared to that in 2 (163.8(2)°) indicates that 5 possesses a bent nitrosyl. The markedly lowered vNO stretching frequency of 5 at 1568 cm-1 is also in line with this formulation (Table 1).17 Thus, η5 coordination of the Cp ligand in the 18-electron 5 induces a bent conformation of the nitrosyl to avoid becoming a 20-electron species. In an attempt to lower the coordination number via reduction, the addition of 1 equiv of 1% Na/Hg to 1 in ether provides the dinuclear deep purple {IPrNi}2(μ-NO)(μ-I) (6) in 51% yield from ether (Scheme 3). The single-crystal X-ray structure of 6 (Figure 4)18 shows a relatively short Ni-Ni bond at 2.314(1) A˚ in comparison to the structurally related (14) Crystal data for 4: triclinic, P1, a=10.1295(9) A˚, b=12.2420(11) A˚, c = 13.4105(12) A˚, R = 88.6660(10)°, β = 80.4400(10)°, γ = 84.8120(10)°, Z=2, collected at 100(2) K, μ(Mo KR) =0.710, 18 003 collected reflections, 6712 unique reflections, Rint=0.0246, R1=0.0345 (I > 2σ(I)), wR2=0.0866 (all data), GOF=1.037. (15) Kelly, R. A. I.; Scott, N. M.; Dı´ ez-Gonzalez, S.; Stevens, E. D.; Nolan, S. P. Organometallics 1996, 24, 3442–3447. (16) Crystal data for 5: monoclinic, P21/n, a = 9.8436(7) A˚, b = 18.6377(14) A˚, c=16.9527(12) A˚, β=102.9270(10)°, Z=4, collected at 173(2) K, μ(Mo KR)=0.667, 23 638 collected reflections, 5949 unique reflections, Rint = 0.0544, R1 = 0.0450 (I > 2σ(I)), wR2 = 0.1020 (all data), GOF=1.006. (17) Richter-Addo, G. B.; Legzdins, P. Metal Nitrosyls; Oxford University Press: New York, 1992. (18) Crystal data for 6: monoclinic, P21/n, a = 11.903(2) A˚, b = 23.439(4) A˚, c=20.744(4) A˚, β=100.631(2)°, Z=4, collected at 100(2) K, μ(Mo KR) = 1.248, 49 477 collected reflections, 13 455 unique reflections, Rint = 0.0958, R1 = 0.0592 (I > 2σ(I)), wR2 = 0.1385 (all data), GOF=0.956.

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Figure 4. ORTEP diagram of 6. Selected bond distances (A˚) and angles (deg): Ni1-Ni2=2.314(1), Ni1-C1=1.897(4), Ni2C28=1.893(4), Ni1-N5=1.763(4), Ni2-N5=1.758(4), Ni1I =2.538(1), Ni2-I= 2.520(1), N5-O1 =1.222(5); Ni1-N5O1=137.7(3), Ni2-N5-O1=137.5(3), Ni1-I-Ni2=54.44(2), Ni1-N5-Ni2=82.2(2). Scheme 3. Synthesis of 6

719

Figure 5. ORTEP diagram of the {[IPr][IPr0 ]Ni(NO)}þ cation of 7. The BArF4 anion and a molecule of Et2O have been omitted for clarity. Selected bond distances (A˚) and angles (deg): Ni-C1 = 1.970(3), Ni-C30 = 1.931(3), Ni-N3 = 1.650(3), N3-O1 = 1.093(3); C1-Ni-N3 = 120.0(2), C1-Ni-C30 = 121.3(2), C30-Ni-N3=118.0(2). Scheme 4. Synthesis of 7

{IPrNi}2(μ-Cl)2, which has a Ni-Ni bond length of 2.5194(5) A˚.19 As a consequence of the close Ni-Ni distance and the large size of the iodide anion, the Ni1-I-Ni2 bond angle is extremely acute at 54.44(2)°. Crystallographic examples of [M]2(μ-NO) nitrosyls are known in complexes of the first-row metals Cr,20 Mn,21 Fe,22,23 Co,24,25 Ni,26 and Cu.27 The Ni1-N5-Ni2 angle of 82.2(2)° is similar to that of previously synthesized Cp-supported dinuclear {CpCo}2(μNO)225 and {MeCpFe}2(μ-NO)223 complexes, whereas the Ni-Ni distance in 6 is much shorter than the dppm-bridged (dppm = Ph2PCH2PPh2) cationic dimer [Ni2(μ-NO)(CNMe)(dppm)2]PF6 (Ni-Ni=2.471(2) A˚).26 The relatively long N5-O1 bond distance at 1.222(5) A˚, bent Ni-N5-O1 (19) Dible, B. R.; Sigman, M. S.; Arif, A. M. Inorg. Chem. 2005, 44, 3774–3776. (20) Ball, R. G.; Hames, B. W.; Legzdins, P.; Trotter, J. Inorg. Chem. 1980, 19, 3626–3631. (21) (a) Elder, R. C. Inorg. Chem. 1974, 13, 1037–1042. (b) Legzdins, P.; Nurse, C. R.; Rettig, S. J. J. Am. Chem. Soc. 1983, 105, 3727–3728. (22) Karlin, K. D.; Lewis, D. L.; Rabinowitz, H. N.; Lippard, S. J. J. Am. Chem. Soc. 1974, 96, 6519–6521. (23) Kubat-Martin, K. A.; Spencer, B.; Dahl, L. F. Organometallics 1987, 6, 2580–2587. (24) (a) Baumann, F.; Dormann, E.; Ehleiter, Y.; Kaim, W.; Karcher, J.; Kelemen, M.; Krammer, R.; Saurenz, D.; Stalke, D.; Wachter, C.; Wolmershauser, G.; Sitzmann, H. J. Organomet. Chem. 1999, 587, 267– 283. (b) Muller, J.; de Oliveira, G. M.; Pickardt, J. J. Organomet. Chem. 1987, 329, 241–250. (c) Strauss, R. C.; Keller, E.; Brintzinger, H. H. J. Organomet. Chem. 1988, 340, 249–256. (25) Bernal, I.; Korp, J. D.; Reisner, G. M.; Herrmann, W. A. J. Organomet. Chem. 1977, 139, 321–336. (26) Ratliff, K. S.; DeLaet, S. K.; Gao, J.; Fanwick, P. E.; Kubiak, C. P. Inorg. Chem. 1990, 29, 4022–4027 and ref 10 therein. (27) Paul, P. P.; Tyeklar, Z.; Farooq, A.; Karlin, K. D.; Liu, S.; Zubieta, J. J. Am. Chem. Soc. 1990, 112, 2430–2432.

angles at 137.7(3) and 137.5(3)°, and vNO stretching frequency at 1681 cm-1 are all consistent with the bridging nature of the NO ligand (Table 1).17 This formal NiI-NiI dimer gives a diamagnetic 1H NMR spectrum similar to that of the related {IPrNi}2(μ-Cl)2,19 a result of a Ni-Ni bond between the two nickel centers. The iodide anion in 1 and 6 may be removed by addition of 1 equiv of TlBArF428 to ether solutions of these NHC nickel nitrosyls (Scheme 4). In each case we observe rapid precipitation of TlI with formation of a green solution with a new nitrosyl stretch at 1784 cm-1. Curiously, this new νNO frequency is not significantly higher than that in 1, as might be expected for a two-coordinate [IPrNi(NO)]þ species. Crystallization of each reaction mixture reveals the cationic, three-coordinate {[IPr][IPr0 ]Ni(NO)}þ[BArF4]- (7), as characterized by X-ray diffraction (Figure 5).29 The new bisNHC complex 7 exhibits both C(2) and C(5) coordination of the NHC ligands as a consequence of steric crowding by the bulky IPr ligands. This alternate binding mode of the NHC ligand30 has been observed before in other bis-NHC (28) Hughes, R. P.; Lindner, D. C.; Rheingold, A. L.; Yap, G. P. A. Inorg. Chem. 1997, 36, 1726–1727. (29) Crystal data for 7: triclinic, P1, a=12.942(3) A˚, b=18.441(4) A˚, c=19.515(4) A˚, R=99.589(3)°, β=99.181(3)°, γ=92.918(3)°, Z=2, collected at 100(2) K, μ(MoKR) = 0.313, 48 177 collected reflections, 17 665 unique reflections, Rint=0.0519, R1=0.0589 (I > 2σ(I)), wR2= 0.1322 (all data), GOF=1.068. (30) Crabtree, R. Pure Appl. Chem. 2003, 75, 435–443.

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complexes such as Nolan’s [IMes]2PdCl2 (IMes = N,N0 -bis(2,4,6-trimethylphenyl)imidazole).31 The Ni center in 7 is trigonal planar (sum of angles about Ni 359.3(3)) and exhibits a shorter Ni-C bond for the C(5)-bound carbene (Ni-C30= 1.931(3) A˚) as compared to the C(2)-bound carbene (Ni-C1= 1.970(3) A˚). The NO ligand in cationic 7 is linear (Ni-N3O1=164.6(3)°) . Further support for the presence of the C(5)coordinated carbene IPr0 in solution is indicated by a pair of coupled doublets at δ 8.06 and 3.20 ppm (J=1.4 Hz) for its heterocyclic C-H resonances. The upfield signal is presumably a result of the ring current of the N-aryl ring of the “normal” IPr ligand in this crowded trigonal species. Though the formation of 7 from either 1 or 6 is likely complex, attack of a free IPr ligand on the putative [IPrNi(NO)]þ cation could provide 7. Correspondingly, the reaction of 1 with TlBArF4 in the presence of 1 equiv of IPr gives 7 in 34% isolated yield, its isolation hampered by the extreme solubility of 7 in ether (Scheme 4). With NiI(NO)(THF)2 as a convenient entry to Ni nitrosyl chemistry, the N-heterocyclic carbene ligand IPr provides a (31) Lebel, H.; Janes, M. K.; Charette, A. B.; Nolan, S. P. J. Am. Chem. Soc. 2004, 126, 5046–5047.

Varonka and Warren

family of mono- and dinuclear complexes with a range of NO bonding modes. Despite the strong σ-donor ability of NHC ligands,32 the two-coordinate [IPrNi(NO)]þ is too reactive to isolate under our conditions, ravenously attracting another 1 equiv of the IPr ligand to provide the bis-NHC cation 7, in which one of the NHC ligands exhibits unusual C(5) coordination.

Acknowledgment. T.H.W. is grateful to Georgetown University for a research pilot grant and to the donors of the Petroleum Research Fund, administered by the American Chemical Society (Type-AC). M.S.V. thanks the Georgetown University Department of Chemistry for an Espensheid Fellowship. Supporting Information Available: Text and figures giving detailed experimental procedures and spectroscopic and analytical data and CIF files giving crystallographic details for 2-7. This material is available free of charge via the Internet at http://pubs.acs.org. (32) (a) Nolan, S. P. N-Heterocyclic Carbenes in Synthesis; WileyVCH: Weinheim, Germany, 2006. (b) Crabtree, R. J. Organomet. Chem. 2005, 690, 5451–5457.