Coordination Ability of 10-EtC(NHPr)=HN-7,8-C 2 B 9 H 11 in the Reactions with Nickel(II) Phosphine Complexes

: The complexation reactions of nido -carboranyl amidine 10-PrNHC(Et)=HN-7,8-C 2 B 9 H 11 with different nickel(II) phosphine complexes such as [(PR 2 R’) 2 NiCl 2 ] (R = R’ = Ph, Bu; R = Me, R’ = Ph) were investigated. As a result, a series of novel half-sandwich nickel(II) π , σ -complexes [3-R’R 2 P-3-(8-Pr N =C(Et)NH)- closo -3,1,2-NiC 2 B 9 H 10 ] with the coordination of the carborane and amidine components was prepared. The acidiﬁcation of obtained complexes with HCl led to the breaking of the Ni-N bond with formation of nickel(II) π -complexes [3-Cl-3-R’R 2 P-8-PrNH=C(Et)NH-closo -3,1,2-NiC 2 B 9 H 10 ]. The crystal molecular structure of [3-Ph 3 P-3-(8-Pr N =C(Et)NH)- closo -3,1,2-NiC 2 B 9 H 10 ] was determined by single crystal X-ray diffraction. 3 ) 2 Cl 2 ] (0.47 g, 0.72 mmol) was added by one portion. The pale ‐ yellow color of the reaction mixture was immediately turned to dark red. The reac ‐ tion mixture was stirred at room temperature in air for about 30 min and the solvent was evaporated under reduced pressure. The residue was treated with CH 2 Cl 2 (20 mL) and water (20 mL). The insoluble particles were filtered off and the organic layer was sepa ‐ rated, washed with water (2 × 20 mL) and evaporated under reduced pressure. The column chromatography on silica gel was used for the purification of the substance with hexane:CH 2 Cl 2 (2:1) as an eluent to give maroon solid of 2 (0.28 g, 83% yield). The crystals suitable for X ‐ ray analysis were obtained by slow evaporation from chloroform/hexane (3:1) solution.


Introduction
The dicarbollide dianion [7,8-C 2 B 9 H 11 ] 2− , which is the deprotonated form of 7,8dicarba-nido-undecaborate anion [7,8-C 2 B 9 H 12 ] − (nido-carborane), is known as the inorganic isolobal analogue of the cyclopentadienyl ligand. This makes it the perfect building block in complexation reactions with a wide range of transition metals [1][2][3]. The possibility to substitute hydrogen atoms at the carbon and boron vertices of the carborane cage with various functional groups [4] makes it possible to vary the properties of ligands based on nido-carborane by combining the properties of the nido-carborane nest with the properties of an exo-polyhedral substituent. One of the most interesting and promising tasks in this area is the synthesis of heterobifunctional nido-carborane-based ligands, that can give a firm bound to capture the metal center along with a weak bond, temporarily protecting a metal coordination site. This allows users to obtain labile complexes of transitional metal representing a promising new type of catalysts [5][6][7][8][9][10] and molecular switches [11,12]. There are several examples of such stable metal complexes based on nido-carborane with a side substituent coordinated through oxygen or nitrogen [5][6][7][8][13][14][15]. The utility of such bifunctional ligand systems with the nitrogen donor atom in the side chain has been demonstrated by the complexation of [7-Me 2 NCH 2 -7,8-C 2 B 9 H 11 ]with metals such as nickel [16], iron [17], rhuthenium [17], titanium, zirconium, and hafnium [18,19]. In all cases, the intramolecular coordination of the dimethylamino group of the side substituent with the complexing metal was observed. The possibility of disrupting this coordination in the nickel(II) complexes by displacing the amino group with other soft ligands, such as triethylphosphine or tert-butylisocyanide, has been shown [16].
Earlier, we prepared a series of nido-carborane-based amidines 10-R(CH 2 ) n NHC(Et)=HN-7,8-C 2 B 9 H 11 using the reaction of nucleophilic addition of amines to the 10-propionitrilium derivative of nido-carborane [20]. Therefore, it was of interest to study the possibility of using obtained amidines in the complexation reactions, where nido-carborane itself represents a firm π-acceptor and amidine nitrogen can act as an intramolecular protecting group. In this contribution we report synthesis of a series of new metallacarboranes by the reactions of nido-carborane-based amidine 10-PrNHC(Et)=HN-7,8-C 2 B 9 H 11 with nickel(II) phosphine complexes [(R 2 R'P) 2 NiCl 2 ].

Scheme 1. Synthesis of nickelacarboranes 2-4 and 5-7.
The initial analysis of the proposed structure of complexes 2-4 was carried out using standard methods of NMR and IR spectroscopy and mass spectrometry. The 1 H NMR spectra of complexes 2-4 demonstrated the presence of only one NH signal of the amidine substituent in the region of 5.04-5.53 ppm, as well as the absence of signals of the nido-carborane B-H-B bridge, suggesting that nickel was coordinated both by the pentagonal face of the carborane ligand and by one atom nitrogen of the amidine group. The pattern of the 11 B NMR spectra of complexes 2-4 is characteristic for metallacarboranes and consists of one singlet at~4.0 ppm from the substituted boron atom and a set of four (complex 2) or five (complexes 3 and 4) doublets in the region from −11.2 to −27.4 ppm with total integral ratio of 1:2:3:2:1 (for 2) and 1:2:2:1:2:1 (for 3 and 4). The 1 H NMR spectra of complexes 2-4 also indicated the presence of a single phosphine ligand, while its 13 C NMR spectra demonstrated the characteristic splitting of signals from aromatic and/or aliphatic groups of phosphine ligands. In the 31 P NMR spectra, the signals of phosphine ligands appeared at 29.3 ppm for 2, at −2.6 ppm for 3 and at 11.9 ppm for 4. Such chemical shifts were in good agreement with the data for similar phosphine complexes of other transitional metals [24].
In the IR spectra of complexes 2-4, the NH stretching bands were observed in the region of 3447-3244 cm −1 , whereas the BH stretching bands appeared at~2530 cm −1 . The bands corresponding to the N=C bond were at~1635 cm −1 for 2 and 3, and at 1622 cm −1 for 4. The mass spectra of complexes demonstrated only peak envelopes corresponding to molecular picks of the supposed structures of complexes 2-4.

Structure Parameters
Compound 2 (X-ray) Compound 2 * Compound 2 (calc) * The Ni-C 2 B 3 centroid distances were only slightly different, and differences in orientation of the PPh 3 fragment were not so pronounced (within 15 • of rotation about Ni-P bond). The most significant unequivalence, as expected, was observed for substituents at the N2 and C3 atoms. In spite of that, the system of shortened contacts was quite similar. One can suggest that the observed conformation can be stabilized by intramolecular noncovalent interactions. To confirm that, we carried out quantum chemical calculations of complex 2. In optimized structure, torsion angles, which define the orientation of PPh 3 fragment, differ by ca. 18 • while differences in orientation of the ethyl and propyl groups are less pronounced. Again, the system of shortened contacts, for which bond critical point were localized, was still nearly the same. Those contacts in total added −5.3 kcal/mol to the stabilization of molecular conformation. These results suggest that the variation of substituents at the N2 and C3 atoms would not significantly affect the orientation of the PPh 3 fragment relative to the carborane cage.
An attempt to obtain suitable X-ray diffraction study crystals of 3 and 4 by recrystallization from chloroform unexpectedly led to a change in the color of the solution from dark red to amaranth after~12 h. Thin layer chromatography confirmed the formation of a new product together with the presence of small amounts of original complexes 3 and 4. New complexes 6 and 7 were isolated by column chromatography on silica using dichloromethane as an eluent. An analysis of the NMR spectra of complexes 6 and 7 led to the assumption that the metal atom in the obtained complexes was no longer coordinated by the amidine group ( Figure 2).  In the 1 H NMR spectra of complexes 6 and 7, signals from the second NH proton appeared in low field at 10.37 and 9.90 ppm for 6 and 7, respectively ( Figure 2, items a,b). This signal gave cross-pick in the 1 H-1 H COSY NMR spectrum with the methylene group of the propyl group -CH 2 CH 2 CH 3 (See SI). The 11 B NMR spectra of 6 and 7 confirmed the retention of the metallacarborane skeleton, however their spectral patterns differed from those for complexes 3 and 4 ( Figure 2, items c,d). Since the newly formed complexes were neutral, we assumed that the violation of the coordination of the amidine fragment was caused by the protonation of the second nitrogen atom, and the electroneutrality of the complexes was achieved due to the coordination of the chloride ion by the nickel atom. The driving force behind this process could be the trace amounts of hydrogen chloride normally present in chloroform. To verify this assumption, we resynthesized complexes 3 and 4, dissolved them in acetonitrile and acidified them by small amounts of concentrated hydrochloric acid (Scheme 1). This resulted in an immediate change in color of the complexes from dark red to amaranth (Figure 3). The NMR spectra confirmed the formation of complexes 6 and 7 (See SM). For complex 2, which did not change upon standing in chloroform solution, we carried out a similar acidification procedure with hydrochloric acid. The solution immediately changed its color from dark red to amaranth, but, unlike complexes 3 and 4, the transformation of complex 2 into a similar complex 5 was not complete. According to the NMR spectroscopy data, the reaction mixture contained approximately 90% of complex 5 and 10% of the original complex 2 and the addition of more amounts of hydrochloric acid did not change this ratio. An attempt to purify complex 5 by column chromatography on silica gel with dichloromethane as an eluent resulted in a mixture of complexes 2 and 5 with a new ratio of~5:2, which can be caused by the presence of equilibrium between these complexes and the partial loss of chloride ions on the column (Scheme 2). Scheme 2. Behavior of complex 5 during purification on column chromatography with silica gel. We supposed that the less donor PPh 3 ligand made the Ni-N bond in complex 2 stronger than in complexes 3 and 4 with more donor phosphine ligands PMe 2 Ph and PBu 3 .
Like complexes 6 and 7, the 1 H NMR spectrum of complex 5 contained the signals of two different NH protons: one at 9.09 ppm from the NHPr group and another one at 5.53 ppm from the B-NH-C fragment. However, in contrast to complexes 2-4 in the 1 H NMR spectra of 5-7, the signals of the CH carb groups appeared in low-field at 2.57-2.80 ppm (0.44-1.12 ppm for complexes 2-4). The signals of the ethyl and propyl group of the amidine substituent in 5-7 also underwent a number of changes and in general were shifted to the low field. For example, the signal of the methylene group of the propyl fragment -NHCH 2 CH 2 CH 3 in complex 7 was observed at 3.41 ppm, whereas in complex 4 it appeared at 2.75 ppm. The signal of the methylene group of the ethyl substituent was located at 2.54 ppm for 7 in contrast with 2.22 ppm for 4. At the same time, the signals of the carbon atom of the methylene group of the propyl fragment -NHCH 2 CH 2 CH 3 in 13 C NMR spectra of 5-7 underwent the high field shift from~55 ppm for 2-4 to~46 ppm, whereas the signal of the methylene group of the ethyl fragment demonstrated a slight high field shift from~23-24 ppm for 2-4 to~26-30 ppm for 5-7. In the 11 B NMR spectra of complexes 5-7, the singlet from the substituted boron atom was observed at 1.9 ppm for 5 and at 3.5 ppm for 6 and 7. Other signals appeared as groups of four (complex 5) or five (complexes 6 and 7) doublets in the region from −10.2 to −26.3 ppm with the total integral ratios 1:2:1:4:1 for 5 and 1:2:1:2:2:1 for 6 and 7. The chemical shifts of phosphine ligands in the 31 P NMR spectra of 5-7 were close to those for 2-4. In the IR spectra of 5-7, the NH and BH stretching bands were observed in the region of 3402-3223 cm −1 and~2555 cm −1 , respectively, whereas the bands corresponding to the N=C bond appeared at 1632, 1626 and 1630 cm −1 for 5, 6 and 7, respectively. The mass spectra of complexes 5-7 performed using the MS MALDI technique contained two main sets of signals corresponding to the molecular picks of complexes 5-7 themselves and complexed with the loss of the chloride ligand. For example, the MALDI mass spectrum of complex 6 contained a typical carborane envelope centered at m/z 477.253 corresponding to the molecular ion pick and another one centered at m/z 442.275, that corresponded to the loss of the chloride ligand by complex 6.

Reagents and Methods
The amidine 10-PrNHC(Et)=HN-7,8-C 2 B 9 H 11 (1) was prepared according to procedure from the literature [20]. Dichlorobis(triphenylphosphine)nickel(II), dichlorobis (dimethylphenylphosphine)nickel(II) and dichlorobis(tributylphosphine)nickel(II) were synthesized according to the previously described methods [28]. Tetrahydrofuran was dried using standard procedure [29]. All manipulations were carried out in air. The reaction progress was monitored by thin-layer chromatography (Merck F254 silica gel on aluminum plates) and visualized using 0.5% PdCl 2 in 1% HCl in aq. MeOH (1:10). Acros Organics silica gel (0.060-0. were recorded with a Varian Inova 400 spectrometer. The residual signal of the NMR solvent relative to tetramethylsilane was taken as an internal reference for 1 H and 13 C NMR spectra. 11 B NMR spectra were referenced using BF 3 . Et 2 O as an external standard. 31 P NMR spectra were cited relative to 85% H 3 PO 4 as an external standard. Infrared spectra were recorded on an IR Prestige-21 (SHIMADZU, Kyoto, Japan) instrument. UV/Vis spectra in chloroform were recorded with a SF-2000 spectrophotometer (OKB SPECTR LLC, Saint-Petersburg, Russia) using 1 cm cuvettes. MALDI mass spectra (positive ion mode) were acquired using a Bruker AutoFlex II reflector time-of-flight device equipped with an N 2 laser (337 nm, 2.5 ns pulse). Trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB, ≥98%, Sigma-Aldrich, Louis, MO, USA) was chosen as a matrix, matrix-to-analyte molar ratio in spotted probes being more than 1000/1. High resolution mass spectra (HRMS) were measured on a Bruker micrOTOF II instrument using electrospray ionization (ESI). The measurements were done in a positive ion mode with mass range from m/z 50 to m/z 3000.

Synthesis of [3-Ph 3 P-3-(8-PrN=C(Et)NH)-closo-3,1,2-NiC 2 B 9 H 10 ] (2)
The potassium tert-butoxide (0.34 g, 3.00 mmol) was added to a solution of 1 (0.15 g, 0.60 mmol) in dry tetrahydrofuran (15 mL). The mixture was stirred for~10 min at room temperature and [Ni(PPh 3 ) 2 Cl 2 ] (0.47 g, 0.72 mmol) was added by one portion. The paleyellow color of the reaction mixture was immediately turned to dark red. The reaction mixture was stirred at room temperature in air for about 30 min and the solvent was evaporated under reduced pressure. The residue was treated with CH 2 Cl 2 (20 mL) and water (20 mL). The insoluble particles were filtered off and the organic layer was separated, washed with water (2 × 20 mL) and evaporated under reduced pressure. The column chromatography on silica gel was used for the purification of the substance with hexane:CH 2 Cl 2 (2:1) as an eluent to give maroon solid of 2 (0.28 g, 83% yield). The crystals suitable for X-ray analysis were obtained by slow evaporation from chloroform/hexane (3:1) solution. 1

Synthesis of [3-Ph3P-3-(8-PrN=C(Et)NH)-closo-3,1,2-NiC2B9H10] (2)
The potassium tert-butoxide (0.34 g, 3.00 mmol) was added to a s 0.60 mmol) in dry tetrahydrofuran (15 mL). The mixture was stirred f temperature and [Ni(PPh3)2Cl2] (0.47 g, 0.72 mmol) was added by pale-yellow color of the reaction mixture was immediately turned to d tion mixture was stirred at room temperature in air for about 30 min a evaporated under reduced pressure. The residue was treated with C water (20 mL). The insoluble particles were filtered off and the organ rated, washed with water (2 × 20 mL) and evaporated under redu column chromatography on silica gel was used for the purification of hexane:CH2Cl2 (2:1) as an eluent to give maroon solid of 2 (0.28 g, 83% suitable for X-ray analysis were obtained by slow evaporation from (3:1) solution. 1

Synthesis of [3-Bu 3 P-3-(8-PrN=C(Et)NH)-closo-3,1,2-NiC 2 B 9 H 10 ] (4)
The procedure was analogous to the preparation of 2 using 1 (0.16 g, 0.64 mmol), potassium tert-butoxide (0.36 g, 3.20 mmol) and [Ni(PBu 3 ) 2 Cl 2 ] (0.26 g, 0.77 mmol) in dry tetrahydrofuran (15 mL). The column chromatography on silica gel was used for the purification of the substance with CH 2 Cl 2 as an eluent to give maroon solid of 3 (0.25 g, 80% yield). 1 31  To the N-coordinated complexes 2-4 (0.40 mmol) dissolved in MeCN (10 mL), one drop (~0.1 mL) of concentrated HCl was added at room temperature. The dark red color of solution was immediately changed to amaranth. The solution was stirred for 5 min and evaporated under reduced pressure to give amaranth solid of 5-7. In the case of complexes 6 and 7, the column chromatography on silica gel was used for the purification with CH 2 Cl 2 as an eluent.

Synthesis of [3-Ph3P-3-(8-PrN=C(Et)NH)-closo-3,1,2-NiC2B9H10] (2)
The potassium tert-butoxide (0.34 g, 3.00 mmol) was added to a solution of 1 (0.15 g, 0.60 mmol) in dry tetrahydrofuran (15 mL). The mixture was stirred for ~10 min at room temperature and [Ni(PPh3)2Cl2] (0.47 g, 0.72 mmol) was added by one portion. The pale-yellow color of the reaction mixture was immediately turned to dark red. The reaction mixture was stirred at room temperature in air for about 30 min and the solvent was evaporated under reduced pressure. The residue was treated with CH2Cl2 (20 mL) and water (20 mL). The insoluble particles were filtered off and the organic layer was separated, washed with water (2 × 20 mL) and evaporated under reduced pressure. The column chromatography on silica gel was used for the purification of the substance with hexane:CH2Cl2 (2:1) as an eluent to give maroon solid of 2 (0.28 g, 83% yield). The crystals suitable for X-ray analysis were obtained by slow evaporation from chloroform/hexane (3:1) solution. 1