Synthesis and Thermal Properties of Novel NHC-Stabilized Cobalt

Article pubs.acs.org/Organometallics

Synthesis and Thermal Properties of Novel NHC-Stabilized Cobalt Carbonyl Nitrosyl Complexes Florian Hering, Johannes H. J. Berthel, Katharina Lubitz, Ursula S. D. Paul, Heidi Schneider, Marcel Har̈ terich, and Udo Radius* Institut für Anorganische Chemie, Julius-Maximilians-Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany S Supporting Information *

ABSTRACT: The synthesis of a series of cobalt NHC complexes of the types [Co(NHC)2(CO)(NO)] (NHC = iPr2Im (2), nPr2Im (3), Cy2Im (4), Me2Im (5), iPr2ImMe (6), Me2ImMe (7), MeiPrIm (8), MetBuIm (9); R2Im = 1,3-dialkylimidazolin-2ylidene) and [Co(NHC)(CO)2(NO)] (NHC = iPr2Im (13), nPr2Im (14), Me2Im (15), iPr2ImMe (16), Me2ImMe (17), MeiPrIm (18), MetBuIm (19)) from the reaction of the NHC with [Co(CO)3(NO)] (1) is reported. These complexes have been characterized using elemental analysis, IR spectroscopy, multinuclear NMR spectroscopy, and in many cases by X-ray crystallography. Bulky NHCs tend to form the mono-NHCsubstituted complexes [Co(NHC)(CO)2(NO)], even from the reaction with an stoichiometric excess of the NHC, as demonstrated by the synthesis of [Co(Dipp2Im)(CO)2(NO)] (11), [Co(Mes2Im)(CO)2(NO)] (12), and [Co(MecAAC)(CO)2(NO)] (20). For tBu2Im a preferred coordination via the NHC backbone (“abnormal” coordination at the 4-position) was observed and the complex [Co(tBu2aIm)(CO)2(NO)] (10) was isolated. All of these complexes are volatile, are stable upon sublimation and prolonged storage in the gas phase, and readily decompose at higher temperatures. Furthermore, DTA/TG analyses revealed that the complexes [Co(NHC)2(CO)(NO)] are seemingly more stable toward thermal decomposition in comparison to the complexes [Co(NHC)(CO)2(NO)]. We thus conclude that the cobalt complexes of the type [Co(NHC)(CO)2(NO)] and [Co(NHC)2(CO)(NO)] have potential for application as precursors in the vapor deposition of thin cobalt films.

■

INTRODUCTION Metal deposition has become an important and rapidly growing area in integrated circuit manufacturing. Important methods for thin-film deposition include physical vapor deposition (PVD), chemical vapor deposition (CVD), and atomic layer deposition (ALD).1 PVD is conducted by evaporation or sputtering of the target material under vacuum, using resistive or electron beam heating or bombardment by high-energy particles. CVD and ALD are vapor-phase deposition techniques requiring at least one volatile molecular precursor as a source of the elements in the target thin film. The target material in CVD is formed by thermal decomposition of a volatile molecular precursor upon contact with a heated substrate, whereas in ALD the volatile molecular precursor is adsorbed on the surface of a heated substrate and the resulting monolayer then reacts (often with a volatile coreactant) to give a submonolayer of the target material. Technically, precursor and coreactant pulses are separated by inert-gas or vacuum-purge steps, and the sequence precursor pulse/purge/coreactant pulse/purge is repeated until a film of the desired thickness is obtained. For both chemical methods, precursors are required which have suitable physical and chemical properties, including volatility, thermal stability, and chemical reactivity suitable to make pure films. Cobalt is an important transition metal for giant magnetoresistance applications, spintronics, and microelectronics technology.2 A major motivation for the growth of cobalt films has © 2016 American Chemical Society

been the formation of CoSi2, which is considered as an alternative contact material to TiSi2 because of its wider silicidation window and superior thermal and chemical stability.3 CoSi2 can act as both metallic interconnections and ohmic contacts in silicon-based devices, as Schottky contacts in the field of optoelectronics, and as metal caps for copper interconnects.4 Cobalt metal films have typically been prepared by CVD from several molecular precursors,5a−k such as [Co2(CO)8],5c,d [Co(CO)3NO],4a,5e [Co(CO)3CF3],5f [(η5-C5H5)Co(CO)2],5a,d,f [CoH(CO)3],5g [Co(Allyl)(CO)3],5h [Co(κ2-tBuNCHCHNtBu)2],5i [(η5-C5H5)Co(κ2-iPrNCHCHNiPr)],5j and [(η5-C5H5)Co(η4diolefin)].5k Some of these precursors have also found application in lower temperature growth methods such as ALD.5l−r However, in recent years there has been a growing interest in the development of new organometallic cobalt precursors for both CVD and especially ALD methods. Herein we wish to present synthetic results that open the way to selective and diverse derivatizations of cobalt complexes with NHC ligands, as well as a systematic analysis of the thermal properties of these molecules, as there have been some encouraging reports on the deposition of copper metal films using NHC-stabilized complexes as precursors.6 Received: May 9, 2016 Published: August 16, 2016 2806

DOI: 10.1021/acs.organomet.6b00374 Organometallics 2016, 35, 2806−2821

Article

Organometallics

■

RESULTS AND DISCUSSION Transition-metal complexes bearing N-heterocyclic carbenes (NHCs) have experienced great popularity in organometallic chemistry and homogeneous catalysis over the past decades.7 It is known that the reaction of dicobalt octacarbonyl with NHCs leads to either dinuclear compounds or the heterolytic cleavage of the Co−Co bond8 to afford ionic complexes, which should be of limited use for deposition methods. The first NHC Co complexes were reported by Lappert et al. and were synthesized from the “Wanzlick dimer” and [Co(CO)3(NO)].9 For our study, we set out to more closely investigate the reactivity of NHCs having different electronic and steric properties with [Co(CO)3(NO)] and study the thermal properties of the resulting complexes. The 18-valence-electron (VE) complex [Co(CO)3(NO)] (1) is readily prepared according to a procedure reported by Job and Rovang in a one-pot synthesis starting from dimeric dicobalt octacarbonyl.10 [Co(CO)3(NO)] (1) readily reacts with 2 equiv of various sterically modest NHC ligands at room temperature to give 2-fold NHC substituted complexes [Co(NHC)2(CO)(NO)] (NHC = iPr2Im (2), nPr2Im (3), Cy2Im (4), Me2Im (5), iPr2ImMe (6), Me2ImMe (7), MeiPrIm (8), MetBuIm (9); R2Im = 1,3-dialkylimidazolin2-ylidene) (Scheme 1).

slight excess of the corresponding NHC used. Although 2-fold CO/NHC substitution is quantitative for the complexes 2−8, the isolated yields vary from 60% to 90%, mainly caused by different solubilities of the respective compounds in cold n-pentane. All attempts to completely substitute all carbonyl ligands of 1 using an excess of the NHC to give complexes of the type [Co(NHC)3(NO)] have failed so far, even when the sterically less demanding11 NHC Me2Im was used at elevated temperatures (toluene, 100 °C) or upon irradiation. Single crystals suitable for X-ray diffraction of almost all of the [Co(NHC)2(CO)(NO)] complexes prepared were obtained from saturated solutions in n-pentane at −30 °C. Figure 1 depicts the molecular structures of the compounds containing the “smallest” (Me2Im, 5) and the bulkiest NHC (iPr2ImMe, 6) used. For the solid-state molecular structures of the other complexes see the Figures S78−S83 in the Supporting Information. Important metric values of all of the molecular structures as well as key spectroscopical features are summarized in Table 1. All complexes adopt a tetrahedral geometry around the cobalt atom spanned by two NHCs, a carbonyl and a nitrosyl ligand. The angles between both NHC ligands are within a small range between 95.263(118)° (in 5) and 99.982(88)° (in 6). The angles between the carbonyl and nitrosyl ligands are significantly widened in comparison to the ideal tetrahedral angle (between 115.32(11)° in 6 and 119.27(14)° in 5). This disagrees with expectations from simple arguments: i.e., that the greater steric bulk of the NHC leads to a larger NHC−Co− NHC angle and accordingly to a smaller CO−Co−NO angle. Furthermore, the intersecting angles of the planes Co− C(carbene)−C(carbene) and Co−CO−NO vary from 87.46(12)° in the complex [Co(Me2Im)2(CO)(NO)] (5) to 83.03(7)° in 8, in the latter case most likely caused by the unsymmetrical NHC MeiPrIm. However, none of these intersecting angles differ drastically from that of an ideal tetrahedron (90°). Concerning the Co−C(carbene) bond lengths, increasing steric bulk of the respective NHC enforces only minor elongation of these bonds (see Table 1). Commonly, CO bonds provide good data on the donor capability of NHC ligands;11 the metrics of the carbonyl or nitrosyl framework, however, are generally not good tools for the structural elucidation of these electronic effects, even more so since in some

Scheme 1. Reaction of [Co(CO)3(NO)] (1) with 2 equiv of NHCs: Substitution of Two Carbonyl Ligands To Afford Complexes of the Type [Co(NHC)2(CO)(NO)] (2−9)

For every NHC under investigation a highly chemoselective reaction was observed, in which two CO ligands were replaced by two NHC ligands. In fact, the reaction is so clean that more or less analytically pure product was obtained after removal of a

Figure 1. Molecular structures of [Co(Me2Im)2(CO)(NO)] (5, left) and [Co(iPr2ImMe)2(CO)(NO)] (6, right) in the solid state (ellipsoids set at the 50% probability level). Hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg) in 5: Co−C1 1.962(3), Co−C2 1.962(2), Co−C3 1.726(4), Co−N2 1.671(3), C3−O2 1.168(4), N2−O1 1.190(4); C1−Co−C2 95.26(12), C1−Co−C3 107.55(13), C2−Co−C3 104.36(14), C1−Co−N2 111.81(13), C2−Co−N2 115.67(13), C3−Co−N2 119.27(14), (plane C1−Co−C2)−(plane C3−Co−N2) 87.46(12). Selected bond lengths (Å) and angles (deg) in 6: Co−C1 2.011(2), Co−C2 1.995(2), Co−C3 1.717(2), Co−N2 1.698(2), C3−O2 1.166(3), N2−O1 1.177(3); C1−Co−C2 99.98(9), C1−Co−C3 112.59(11), C2−Co−C3 103.16(10), C1−Co−N2 110.91(10), C2−Co−N2 113.64(10), C3−Co−N2 115.32(11), (plane C1−Co−C2)−(plane C3−Co−N3) 85.31(7). 2807

DOI: 10.1021/acs.organomet.6b00374 Organometallics 2016, 35, 2806−2821

Article

Organometallics

Table 1. Characteristic Spectroscopic and Structural Parameters for Complexes of the Type [Co(NHC)2(CO)(NO)] (2−9)a 2 3 4 5 6 7 8 9

NHC

νCO (cm−1)

νNO (cm−1)

δNCN (ppm)

Co−CNHC (Å)

∠CNHC−Co−CNHC (deg)

iPr2Im nPr2Im Cy2Im Me2Im iPr2ImMe Me2ImMe MeiPrIm MetBuIm

1865 1868 1878 1873 1870 1865 1873 1868

1613 1621 1633 1613 1623 1620 1610 1619

199.5 200.9 200.4 201.9 200.3 199.0 201.0 199.5

1.995 1.978b 1.980 1.962 2.003 1.991b 1.975 c

92.26 97.83b 97.38 95.26 99.98 95.59 94.01 c

a

Left to right: CO stretching frequency; NO stretching frequency; 13C{1H} NMR chemical shift of the NHC carbene carbon atoms; average Co− NHC bond length; angle C−Co−C between the two NHCs. bAverage of two crystallographically independent molecules. cThe X-ray crystal structure is heavily disordered.

Figure 2. 1H NMR spectrum of the reaction mixture of [Co(CO)3(NO)] (1) and 2 equiv of MetBuIm after 2 days at room temperature. The solution reveals resonances for uncoordinated MetBuIm (red), the complex [Co(MetBuIm)2(CO)(NO)] (9, blue) and the mono-NHC complex [Co(MetBuIm)(CO)2(NO)] (19, green).

and its large quadrupole moment induces very broad signals for all atoms coordinated to it, and even measurements with a high-resolution NMR spectrometer (500 MHz with a CryoPlatform) failed to give proper 13C NMR resonances for these atoms. However, for the NHC carbene carbon atoms the crosspeak with the backbone protons in 1H−13C HMBC resonance experiments allowed the identification of these signals in nearly all cases. As noted in Table 1, all NHC carbene carbon atoms give rise to resonances at approximately 200 ppm in the complexes [Co(NHC)2(CO)(NO)]. The IR spectra of these compounds display CO stretching frequencies in the narrow range between 1865 and 1878 cm−1 and NO stretching frequencies between 1610 and 1621 cm−1, respectively. However, no correlation to reported TEP values is observed for those signals.11 Nearly all sterically modest NHC ligands presented in Scheme 1 readily replace two carbonyl ligands of [Co(CO)3(NO)] (1) in diethyl ether at room temperature. For tert-butyl-substituted

cases CO and NO ligands are disordered in the X-ray structures. The 1H NMR spectra of all complexes [Co(NHC)2(CO)(NO)] reveal one set of signals for the NHC ligands with the expected hindered rotation12 of the nitrogen isopropyl and cyclohexyl substituents, respectively, along the C−N axis in 2, 4, 6 and 8. Interestingly, all the NHC resonances for the same groups of hydrogen atoms do not split up on the NMR time scale, which indicates an unstable ligand environment in solution. We assume currently a quick exchange of the CO and the NO ligand on the NMR time scale either by passing through a square planar configuration at cobalt or via a dissociative/ associative mechanism. The detection of the NHC carbene carbon and carbonyl carbon atoms resonances in the 13C{1H} NMR spectra of the complexes was challenging for all the compounds, since these atoms are directly attached to the cobalt atom. The isotope 59 Co (100% natural abundance) with its nuclear spin of I = 7/2 2808

DOI: 10.1021/acs.organomet.6b00374 Organometallics 2016, 35, 2806−2821

Article

Organometallics MetBuIm, however, the formation of the mono-NHC complex [Co(MetBuIm)(CO)2(NO)] (19) was detected for the reaction of 1 with MetBuIm in the 1H NMR spectrum (see Figure 2); hence, the substitution of the second CO ligand occurs at significantly slower rates in comparison to all the other NHCs investigated so far. To obtain full conversion to [Co(MetBuIm)2(CO)(NO)] (9), the reaction has to be performed in toluene at 100 °C. If the steric demand of the NHC is further increased and tBu2Im, the NHC with two tert-butyl substituents at the nitrogen atoms, is used for the reaction, no [Co(tBu2Im)2(CO)(NO)] complex could be identified in solution. Instead, the reaction mixture contains three components (derived from 1 H NMR spectroscopy): namely, the NHC tBu2Im and the complexes [Co(tBu2Im)(CO)2(NO)] and [Co(tBu2aIm)(CO)2(NO)] (10; the superscript “a” indicates coordination of the NHC in its abnormal coordination mode). [Co(tBu2Im)(CO)2(NO)] accounts for a set of two singlets in a 1:9 ratio (6.53 and 1.04 ppm, respectively) in the 1H NMR spectrum, as expected in a “normally” coordinated tBu2Im. Two significantly broadened resonances at 6.73 and 1.30 ppm are attributed to uncoordinated tBu2Im. The third component, [Co(tBu2aIm)(CO)2(NO)] (10), gives rise to a split tBu2Im set consisting of four resonances at 7.43, 6.56, 1.52, and 0.66 ppm in a ratio of 1:1:9:9. The most significant signal here is the low-fieldshifted resonance at 7.43 ppm, which is typically observed for an “abnormally” coordinated mesoionic NHC and attributed to the now-protonated, former NHC carbene carbon atom (NCHN). The often predominant coordination of sterically demanding NHCs in the abnormal mode in comparison to the normal coordination of sterically less demanding NHCs is well documented,13 and the mechanism of NHC to aNHC conversion has been investigated just recently.14 Thus, the tBu2Im ligand shows a comparatively strong tendency to coordinate by the carbon atom at the 4-position due to its high steric pressure at the metal center. To verify the small energetic gap between normal and abnormal coordination of the tBu2Im ligand in this case, we carried out high-level DFT calculations on a triple-ζ basis (def2-TZVPP, BP86, RIDFT). According to these calculations the abnormal complex [Co(tBu2aIm)(CO)2(NO)] (10) is favored in comparison to the normal coordination mode ([Co(tBu2Im)(CO)2(NO)]) by only 1.63 kJ mol−1, which lies in the range of the method’s accuracy. Purple crystals of [Co(tBu2aIm)(CO)2(NO)] (10) were obtained from the mother liquor in diethyl ether at −30 °C (see Figure 3).

Figure 3. Calculated structures of [Co(tBu2Im)(CO)2(NO)] and [Co(tBu2aIm)(CO)2(NO)] (def2-TZVPP, BP86, RIDFT; left) and molecular structure of [Co(tBu2aIm)(CO)2(NO)] (10) in the solid state (ellipsoids set at the 50% probability level; right). Hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Co−C1 2.018(4), Co−C4 1.727(4), Co−N3 1.796(6), C4−O2 1.153(5), N3−O1 1.158(7), C1−C2 1.370(6), N2−C2 1.382(6), C3−N2 1.336(6), C1−N1 1.421(6), C3−N1 1.347(6); C1−Co−C4 108.906(11), C4−Co−C4′ 124.10(18), C1−Co−N3 97.8(2), C4−Co−N3 106.94(11), (plane C1−Co−N3)−(plane C4− Co−C4′) 90.

The cobalt atom in 10 is coordinated with two carbonyl ligands, one nitrosyl ligand, and the tBu2aIm ligand. The solidstate molecular structure of 10 shows a crystallographically imposed mirror plane containing the cobalt atom, the NO ligand, and the imidazole ring and thus reveals (pseudo)Cs symmetry. The bond distance Co−C1 from the cobalt atom to the vinylic imidazole ligand of 2.018(4) Å is only slightly longer in comparison to Co−CNHC bond lengths observed for the other NHC complexes [Co(NHC)2(CO)(NO)] and [Co(NHC)(CO)2(NO)] (see below) within this study (1.962− 2.011 Å; see Tables 1 and 2). The C1−C2 distance lies within the typical range observed for carbon−carbon double bonds as found in the backbone of NHC ligands, and the diazaallylic system is also reflected in the metric data of the ring (C3−N2 1.336(6) Å, C3−N1 1.347(6) Å vs C2−N2 1.382(6) Å, C1− N1 1.421(6) Å). Another example of abnormal NHC coordination at cobalt was presented by Layfield and co-workers just recently. They reported the rearrangement of the arylsubstituted NHC in [Co(Dipp2Im)(N(SiMe3)2)2] from normal to abnormal coordination at elevated temperatures.15

Table 2. Characteristic Spectroscopic and Structural Parameters for the Mono-NHC Complexes [Co(NHC)(CO)2(NO)] (11−19)a 11 12 13 14 15 16 17 18 19

NHC

νCO(A1) (cm−1)

νCO(B1) (cm−1)

νNO (cm−1)

δNCN (ppm)

Co−CNHC (Å)

∠CCO−Co−CCO (deg)

Dipp2Im Mes2Im iPr2Im nPr2Im Me2Im iPr2ImMe Me2ImMe MeiPrIm MetBuIm

2010 2001 2011 2011 2011 2011 2009 2011 2007

1945 1929 1937 1936 1929 1923 1932 1938 1933

1722 1715 1707 1707 1701 1703 1732 1710 1705

197.5 194.3 186.4

1.961 1.970 1.984

108.73 103.93 108.41

1.971 2.002 1.981b

108.65 114.77 102.62b

2.011

107.26

a

13

1

Left to right: CO stretching frequencies (A1 and B1 symmetry); NO stretching frequency; C{ H} NMR chemical shift of the NHC carbene carbon atoms; average Co−NHC bond length; angle between the two carbonyl ligands. bAverage of three crystallographically independent molecules. 2809

DOI: 10.1021/acs.organomet.6b00374 Organometallics 2016, 35, 2806−2821

Article

Organometallics

carbonyl stretching frequencies are detected (irreducible characters: A1 and B1) in the IR spectra. The total symmetric vibrations (A1) are detected as sharp absorptions in a small range between 2007 and 2011 cm−1, whereas the B1-symmetric vibrations emerge between 1929 and 1945 cm−1, usually observed as broader and commonly more intense resonances. However, both CO resonances, as well as the nitrosyl stretching vibrations, are significantly blue shifted in comparison to the analogous 2-fold NHC-substituted complexes 2−9. This shift can be easily explained by the drastically lower π-acceptor capability of the NHC in comparison to the carbonyl (and/or nitrosyl) ligands.16 The more the carbonyl functions are replaced by NHCs, the more electron density is available for π back-bonding to the remaining CO and NO ligands. Consequently, the resonances of [Co(CO)3(NO)] (1) show another substantial blue shift (COA1 2100 cm−1, COE, 2013 cm−1, and NO 1865 cm−1) in comparison to [Co(NHC)(CO)2(NO)] and [Co(NHC)2(CO)(NO)]. The Co−CNHC distances of the complexes [Co(NHC)(CO)2(NO)] differ only marginally from the distances observed for [Co(NHC)2(CO)(NO)] in the solid-state molecular structures, although the electron density at the metal is certainly increased for [Co(NHC)2(CO)(NO)], as IR spectroscopy demonstrates (see Table 2, Figure 4, and Figures S79−S93 in the Supporting Information). The differences in bond lengths of the complexes of the same NHC (Me2Im, 5 and 15; iPr2ImMe, 6 and 16; Me2ImMe, 7 and 17) are a mere 1 pm (see Tables 1 and 2). Cyclic alkylamino carbenes (cAACs) have attracted more and more interest in organometallic chemistry in recent years.11,17 Although these molecules seem to be very similar to the well-explored imidazole-based NHCs on first glance, cAACs themselves18 as well as the resulting transition-metal complexes19 may provide entirely different properties in comparison to the corresponding NHC compounds. Therefore, we investigated the reactivity of [Co(CO)3(NO)] (1) toward 1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene (MecAAC). The mono-cAAC complex [Co(MecAAC)(CO)2(NO)] (20) can be obtained from the reaction of MecAAC with [Co(CO)3(NO)] (1). However, it turned out that the reaction is more sluggish in comparison to all the reactions performed with imidazole-based NHCs, and 20 was obtained in

Interestingly, the Co−C distance of 2.059(2) Å observed for [Co(Dipp2aIm)(N(SiMe3)2)2] is even shorter in comparison to the Co−C bond length in [Co(Dipp2Im)(N(SiMe3)2)2]. Since tBu2Im seems to bring too much steric pressure into [Co(NHC)(CO)2(NO)] complexes and thus rearranges into an abnormal coordination mode, we reacted other bulky NHCs such as Mes2Im and Dipp2Im in the hope of realizing the synthesis of complexes of the type [Co(NHC)(CO)2(NO)]. Accordingly, the transformation of [Co(CO)3(NO)] (1) with Mes2Im and Dipp2Im selectively leads to the mono-NHCsubstituted complexes [Co(Dipp2Im)(CO)2(NO)] (11) and [Co(Mes2Im)(CO)2(NO)] (12), even if an excess of the corresponding NHC is used (see Scheme 2). However, we have Scheme 2. Reaction of [Co(CO)3(NO)] (1) with