K[HB(3,5-Me2Pz)3]
2.5.1 Complexes with the K^-coordinated hydrido^m(pyrazolyl)borate ligand
If [ {Ru(r|^-1,4-Me2C6HJCl2)2] is stirred in acetonitrile for only 20 minutes, as opposed
to the 2 hours required in the preparation of [31, and the resulting solution mixture subsequently reacted with Na[HB(Pz)J, a new red crystalline compound can be isolated. The ’H NMR spectrum of this compound exhibits eight resonances (Figure 2.12). Whilst two of the signals (Ô 5.45 and 2.07 ppm) are due to the arene, the remaining six signals (5 7.79, 7.74, 7.72, 6.99, 6.38 and 6.21 ppm) arise from the
R u(II)(arene)poly(pyrazolyl)borates and m ethanes
pyrazolyl groups. This suggests a single complex containing a [HB(Pz)3]’ ligand
coordinated in a mode. Me " A " VN— Me
"■’A ü i
IP H" ‘|*'*iHH'|i"i|iiii|iin|nii|nn|iiii|iiii|iiii|iiii|iin|iiii|iiii|iin|iiii|iiii|iin|nn|iiii|iiii|iiii|nii|iii|i| 7 . 8 7 . 6 7 . 4 7 . 2 7 . 0 6 . 8 6 . 6 6 . 4 6 . 2 6 , 0 5 . 8 5 . 6 p p MFigure 2.12 'H-NMR spectrum of [Ru(r|^-l,4-Me2C6HJ{K:^-HB(Pz)3}Cl] [13] (* signals due to [3’j).
In order to support this formulation, [ {Ru(r|^-1,4-Me2QHJCl2}2] was dissolved in acetonitrile for 20 minutes and then after filtering through celite the solution was evaporated to dryness. After extraction with CHjClj and treatment with two molar equivalents of Na[HB(Pz)3] the reaction mixture was left to stir at room temperature for
2 hours. Filtration of the reaction mixture followed by treatment with diethylether led to precipitation o f an orange powder which was identified as [Ru(r|^-l,4-Me2C6H4){K^- HB(Pz)3}C1] [13]. The major component remaining in the filtrate at this stage was
R u(II)(arene)poly(pyrazolyl)borates an d methanes
identified by 'H-NMR spectroscopy as [Ru(r|^-1,4-Me2C6HJ{K^-HB(Pz)J]C1 [3’], in which the chloride resides outside the first coordination sphere.
The infrared spectra of both [3' ] and [13] are consistent with the presence o f the arene and the hydridorrâ(3,5-dimethylpyrazolyl)borate ligands. It is however surprising to find that the band for the v(BH) stretch in [13] appears at 2449 cm ', only 9 cm ' higher than the corresponding absorption in the free ligand. This contrasts with the large shift observed for the related full mixed sandwich complexes reported in Section 2.2.
The differing coordination modes adopted by the potentially tridentate ligands in [3 ' ] and [13] can be deduced from close inspection of their 'H NMR spectra (Table 2.4). The fact that the spectrum of [13] contains two sets of pyrazolyl signals (relative intensity 1:2) as opposed to the single set exhibited for [3] and [3’] is in accord with the presence o f two inequivalent pyrazolyl environments. The signals for the unique pyrazolyl group appear at lower fields ('H NMR: ô 7.79, 7.72 and 6.38 ppm) compared with the signals for the remaining pyrazolyl groups (6 7.74, 6.99 and 6.21 ppm). In fact the chemical shifts o f the unique pyrazolyl group are comparable with those reported for non coordinated ring in the related complex [Ru {K^-HB(Pz)3} 2(^ % ] " With the exception of
the chemical shifts of and H" in the unique pyrazolyl group, are comparable with those displayed for the free ligand (8 7.70 6.19 ppm H"). It is noteworthy that the signals for the unique pyrazolyl group (8 141.81 C \ 135.74 and 105.48 C ) in the '^C NMR spectrum appear at relatively higher fields, compared to the signals for the remaining pyrazolyl ligands (8 144.70 C \l35.84 and 106.46 C ).
Ru(lI)(arene)poly(pyrazolyl)borates and methanes
Neither [3 '] or [13] require a large anion for isolation, implying the presence of a chloride to balance the charge. This chloride could either be directly bound to ruthenium(II) or reside in the outer coordination sphere, as a component o f a tight ion pair, with a solvent molecule occupying the vacant site. The former situation is more likely for [13]. Not only is this substantiated by the microanalytical data but it also seems reasonable in that if the sixth coordination site on the ruthenium(II) centre is occupied by a solvent molecule this molecule would be labile and there would be a tendency for the [HB(Pz)J' ligand to undergo a rapid hapticity change from to resulting in tris chelation and as a result only one set of observed pyrazolyl resonances. Hence the chloride ligand in [13] must be bonded to the metal ion.
The isolation of this novel bis chelated compound is not restricted to just the para- xylene derivative. Analogous complexes incorporating bulkier arenes, such as mesitylene and hexamethylbenzene can also be prepared (Figure 2.13). As observed for the ^am -xylene derivatives the mesitylene and the hexamethylbenzene complexes, [Ru(Ti'-l,3,5-Me3CA){K'-HB(Pz)3}Cl], [14] and [Ru(r|"-C6Me6){K'-HB(Pz)3}Cl] [15]
also exhibit two sets of pyrazolyl signals in their 'H NMR spectra with 1:2 integral ratio. The microanalytical data on all of these complexes (Table 2.1) are consistent with the proposed formulations.
Ru(II)(arene)poIy(pyrazolyl)borates and m ethanes
R
Ru---
Figure 2.13 Ruthenium(II)(r|^-arene){K^-poly(p3a-azolyl)borate} complexes, (arene =
para-xy\Qne [13], mesitylene [14], and hexamethylbenzene [15]).
When comparing the NMR data for the various arene families of complexes, it is noteworthy that the protons of the pyrazolyl groups in the tris chelated complexes [3] and [3 ' ] generally resonate at lower field than those o f the metallated pyrazolyl groups in [13] (Table 2.4). While a similar pattern is manifest for the mesitylene derivatives the hexamethylbenzene complexes show no such characteristic pattern o f chemical shifts.
A crystallographic study was undertaken for a representative compound, [15], to substantiates the proposed formulations. Tables of atomic coordinates, bond lengths and bond angles are in the experimental section. The molecular structure o f the compound [15] is shown in Figures 2.14.
R u (fl)(aren e)poly(pyra:olyl)borates and m ethanes
G
:io C9 N 32 C 2 4 C 33 C 2 5 C13 C /Figure 2.14 C ry stal structure o f [R.u(r|CC(,Me6)lK "-H B (P z)3}Cl] show ing the atom ic
n u m b erin g schem e.
R u(II)(arene)poly(pyrazolyl)borates an d methanes
Compound [15] exists as half-sandwich complex with, if one assumes the arene occupies three facial sites, a distorted octahedral geometry at the ruthenium centre. The ruthenium atom is 7c-bonded to the arene ligand with an average Ru-C distance of 2.197(7) Â with a separation between the arene plane and the ruthenium atom o f 1.68 A, very similar to that observed for [2] and [10].
The Ru-Cl bond distance of 2.397(2) A is significantly shorter than that reported for [Ru(K^-HB(Pz)3}(PPh3)2Cl]^‘*, 2.453(2) A and for bis chelated derivatives [Ru(r|^-
CA)(K"-triglycine)Cl] 2.441(2) A and [Ru(r|'-C,HJ(K"-/-histidine)Cl] 2.413(2) A'"" but is comparable with those found in [Ru(r|^-C6H6)(PPh2Me)Cl2], 2.410(6) A and 2.407(6)
A.'"'
In addition to being bonded to the arene and the chloride ligand the ruthenium ion is also directly coordinated to two endocycXic nitrogen atoms (N22 and N32) of the pyrazolyl groups, with an average Ru-N distance of 2.082(5) A. It is notable that the bis chelated Ru-N bond lengths falls in the same range as those exhibited for the least sterically congested pyrazolyl groups in [2] and [10]. The bite angle of the chelating ligand, 84.8(2) ° in [15] is not very different from those observed for [2] 84.3 ° or those reported for [Ru(Ti"-C6HJ{K'-B(Pz)J][PFJ,'"'"° 84.2 ° and [Ru(n'-Cp){K'-B(Pz)J]'" 83.8 °. The overall geometry at the boron atom is tetrahedral (average N-B-N angle of 109.0°) with the smallest o f these angles being subtended by the endocycXic nitrogens of the coordinated pyrazolyl groups N(31)-B-N(21) 107.5(5)°.
Ru(Il)(arene)poly(pyrazolyl)borates and methanes