The dimeric mixed-ligand titana­carborane sandwich [1-(Cp)-1-Ti-2-(Me)-3-(SiMe₃)-2,3-C₂B₄H₄]₂

The title dimer, bis­[1-cyclo­penta­dienyl-2-methyl-1-titana-3-tri­methylsilyl-2,3-dicarba-closo-hexaborane(6)], [Ti(C₅H₅)(C₆­H₁₆­B₄Si)]₂, reveals that the centrosymmetric mol­ecule consists of two bent-sandwich titanacarboranes bridged by the B-H-Ti bonds. The average bond distances are Ti-B 2.445 (3), Ti-C(cage) 2.334 (2) and Ti-C(Cp) 2.376 (3) Å, and the corresponding bond angles are Cp-Ti-Cp 163.2 (1) and Cp-Ti-Cb (Cb = C₂B₃ face) 139.9 (1)°; the Ti-H separations are 2.10 (2) and 2.19 (2) Å.

S1. Comment

Group four (titanium group) organometallic compounds have been the subject of numerous structural, spectroscopic and synthetic investigations (Abel et al., 1995; Cardin et al., 1986; Jordan, 1988; Jordan et al., 1990; Corbin et al., 1986). Interest in the design of novel molecular magnetic materials (Miller & Epstein, 1994) has made the study of electron spin-electron spin interactions in dinuclear and polynuclear transition metal complexes a topic of intense research (VanVleck, 1932; Kahn, 1993; Mabbs & Collison, 1992). However, only a relatively small number of symmetrical Ti^III dimers has been completely characterized by EPR, magnetic susceptibility measurements and crystal structure determinations (Samuel et al., 1992; Xin et al., 1994; Grimes, 1995; Hosmane & Maguire, 1990, 1991, 1993). Although our recent paper on this subject described the synthesis and reactivity studies on the dimeric mixed-ligand titanacarborane sandwich, [1-(Cp)-1-Ti-2-(Me)-3-(SiMe₃)-2,3-C₂B₄H₄)]₂, (II) (Cp = η⁵ –C₅H₅), we were unable to grow a suitable single-crystal of this complex and hence its structure was not reported (Hosmane et al., 1997). After allowing a benzene solution of (II) to stand for several months at room temperature in vacuo, suitable crystals of the title compounds were obtained and were subsequently used for X-ray analysis. \sch

The driving force for the formation of (II) is most likely the stability of the Ti^III product, as well as the formation of dihydrofulvalene. When the `carbons adjacent' bis(trimethylsilyl)carborane (Cb) was reacted with TiCl₃, only the monomeric full-sandwich chlorotitanacarborane was formed, irrespective of the stoichiometry used in the reaction. Replacement of an SiMe₃ group with the smaller Me group afforded the [(Cb*)₂Ti]₂^2- dimer exclusively (Hosmane et al., 1997). In the same way, replacement of one of the Cb ligands with the smaller, isolobal and isoelectronic π-donor Cp ligand also resulted in dimer formation, giving the mixed-ligand dimer, (II).

The crystal structure (Figure 1) reveals that (II) is a bent sandwich complex, similar to those found in the Cp system (Corbin et al., 1986; Samuel et al., 1992; Jungst et al., 1977; Peterson & Dahl, 1975; Pattiasina et al., 1987). The (ring centroid)-Ti-(ring centroid) bond angle of 139.9 (1)° is within the range of 130–144° found in the titanocenes (Corbin et al., 1986; Samuel et al., 1992; Jungst et al., 1977; Peterson & Dahl, 1975; Pattiasina et al., 1987), and similar to the analogous angles of 144.6 and 138.8° found by Jordan for the mixed-ligand complexes [(Cp*)Ti(NCMe₂)(C₂B₉H₁₁)] and [(Cp*)Ti(NCMe₂)(MeCN)(C₂B₉H₁₁)], respectively (Kreuder et al., 1995). A comparison of the distances between the Ti and the centroids of the Cp and C₂B₃ carborane rings reveals a surprising insensitivity to either the formal charge on the metals or the nature of the other ligands present. The average Ti—Cp distance of 2.060 (3) Å is close to the average of 2.054 (4) Å found in [Cp₂TiCl]₂ (Jungst et al., 1977) and is also close to the the analogous distances found in a number of substituted titanocenes (Corbin et al., 1986; Peterson & Dahl, 1975; Pattiasina et al., 1987). The average Ti—C₂B₃(centroid) distance of 1.983 (3) Å is close to the values of 2.02 and 1.91 Å found in the Cp*Ti(C₂B₉H₁₁) mixed-ligand sandwich compounds of Jordan (Kreuder et al., 1995), as well as the Ti—C₂B₃(centroid) distance of 1.917 Å reported by Grimes for the cyclooctatetraene(COT) titanacarborane, 1-(η⁸-C₈H₈)-1-Ti-2,3-(Et)₂-2,3-C₂B₄H₄ (Swisher et al., 1984). The latter compound is of interest in that it as well as its cyclopentadienide analogues (Koon & Helmholdt, 1984) are not bent but have a linear ring centroid-Ti-ring centroid arrangement. It is not clear whether the linear structures found in the COT complexes are the results of the size of the [C₈H₈]^2- ligand or the fact that it is a ten-electron donor. In the same way, while both steric and electronic effects are operative in determining the geometry of (II), the relative importance of the two effects is not apparent. However, the influence of steric factors is readily seen in some of the structural features of known titanacarborane bent-sandwich complexes (Hosmane et al., 1997). Nonetheless, substitution of an SiMe₃ group by a sterically less demanding Me, allows additional coordination to form the dimeric structure as found for (II). This is similar to what is found in the titanocenes where the substitution of a Cp ligand in [Cp₂TiCl]₂ by Cp* (Cp* = η⁵-C₅Me₅) resulted in the isolation of the [(Cp*)₂TiCl] monomer (Jungst et al., 1977; Peterson & Dahl, 1975; Pattiasina et al., 1987). Other properties may also be affected. For example, steric crowding by the SiMe₃ groups has been used to account for the lack of reactivity of the coordinated Cl and THF in the zirconacarborane anion, [1-Cl-1-THF-2,2',3,3'-(SiMe₃)₄-1,1'-commo-Zr(2,3-C₂B₄H₄)₂]^- (Thomas et al., 1995). It may well be that the use of different cage-carbon substituents and the locations of the cage-carbon atoms in the bonding face, could be used to control the reactivites of these d⁰ or d¹ bent-sandwich metallacarboranes.

S2. Experimental

The reaction of Cp₂TiCl₂ with the unsolvated `carbons adjacent' dilithium compounds, closo-exo-Li-1-Li-2-(Me)-3-(SiMe₃)-2,3-C₂B₄H₄ (Hosmane et al., 1993), produced the corresponding mixed-ligand sandwich titanacarboranes, [commo-1-Cp-1-Ti(III)-2-(Me)-3-(SiMe₃)-2,3-C₂B₄H₄]₂ (2) in 54% yield as shown in Scheme I (Hosmane, et al., 1997).

S3. Refinement

The hydrogen atoms on carbon atoms were refined using the riding model in SHELXTL with the U_iso equal to 1.5 times of that of the preceding carbon atoms for the methyl groups and 1.3 times for the rings. The C—H distances are equal to 0.97 and 0.96 Å for the CH₂ and CH₃ groups, respectively. The hydrogen atoms attached to boron atoms were located in a difference map, and freely refined.

Computing details

Data collection: CAD-4-PC Software (Enraf-Nonius, 1989); cell refinement: CAD-4-PC Software; data reduction: CAD-4-PC Software; program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXL97; software used to prepare material for publication: SHELXL97.

Figures

Fig. 1. Displacement ellipsoid drawing of the title compound with ellipsoids shown at the 50% probability level [symmetry code: (i) 1 - x, -y, -z].

[1-(Cp)-1-Ti-2-(Me)-3-(SiMe₃)-2,3-C₂B₄H₄)]₂

Crystal data

[Ti(C5H5)(C6H16B4Si)]2	F(000) = 572
Mr = 545.02	Dx = 1.239 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
a = 8.2310 (16) Å	Cell parameters from 25 reflections
b = 10.655 (2) Å	θ = 3–25°
c = 17.042 (3) Å	µ = 0.64 mm^−1
β = 102.26 (3)°	T = 293 K
V = 1460.5 (5) Å^3	Rectangular, black
Z = 2	0.32 × 0.20 × 0.10 mm

Data collection

CAD4 diffractometer	2365 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.018
Graphite monochromator	θmax = 27.0°, θmin = 3.1°
ω–2θ scans	h = −10→10
Absorption correction: ψ scan (North et al., 1968)	k = 0→13
Tmin = 0.862, Tmax = 0.938	l = 0→21
3263 measured reflections	3 standard reflections every 250 reflections
3164 independent reflections	intensity decay: none

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.035	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.089	H atoms treated by a mixture of independent and constrained refinement
S = 1.03	w = 1/[σ^2(Fo^2) + (0.0455P)^2 + 0.2584P] where P = (Fo^2 + 2Fc^2)/3
3164 reflections	(Δ/σ)max < 0.001
170 parameters	Δρmax = 0.31 e Å^−3
0 restraints	Δρmin = −0.26 e Å^−3

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Ti	0.34443 (4)	−0.00578 (3)	−0.09397 (2)	0.03221 (11)
Si	0.22133 (7)	0.33342 (5)	−0.05040 (4)	0.03709 (15)
C1	0.3689 (2)	0.20980 (18)	−0.07164 (11)	0.0314 (4)
C2	0.4083 (3)	0.1824 (2)	−0.15139 (12)	0.0385 (5)
B3	0.5618 (3)	0.0940 (3)	−0.14591 (16)	0.0452 (6)
B4	0.6330 (3)	0.0734 (2)	−0.04870 (15)	0.0376 (5)
B5	0.5011 (3)	0.1479 (2)	−0.00468 (14)	0.0317 (4)
B6	0.5742 (3)	0.2299 (2)	−0.08109 (15)	0.0381 (5)
C11	0.1059 (4)	0.0105 (3)	−0.19996 (18)	0.0691 (9)
H11	0.0670	0.0893	−0.2187	0.090*
C12	0.0574 (3)	−0.0557 (3)	−0.13901 (18)	0.0619 (7)
H12	−0.0203	−0.0297	−0.1100	0.080*
C13	0.1461 (3)	−0.1689 (2)	−0.12874 (17)	0.0573 (7)
H13	0.1390	−0.2307	−0.0911	0.074*
C14	0.2460 (3)	−0.1725 (3)	−0.18433 (17)	0.0639 (8)
H14	0.3165	−0.2379	−0.1913	0.083*
C15	0.2228 (4)	−0.0607 (3)	−0.22843 (16)	0.0715 (8)
H15	0.2760	−0.0382	−0.2693	0.093*
C21	0.3275 (4)	0.2560 (2)	−0.22590 (14)	0.0568 (6)
H21A	0.3676	0.2254	−0.2713	0.085*
H21B	0.3549	0.3433	−0.2179	0.085*
H21C	0.2091	0.2457	−0.2356	0.085*
C101	0.2663 (4)	0.4892 (2)	−0.09116 (19)	0.0678 (8)
H10A	0.3794	0.5123	−0.0689	0.102*
H10B	0.1929	0.5512	−0.0770	0.102*
H10C	0.2498	0.4842	−0.1486	0.102*
C102	0.0023 (3)	0.2885 (3)	−0.09335 (18)	0.0647 (8)
H10D	−0.0709	0.3528	−0.0818	0.097*
H10E	−0.0226	0.2107	−0.0700	0.097*
H10F	−0.0129	0.2787	−0.1504	0.097*
C103	0.2460 (3)	0.3458 (3)	0.06043 (15)	0.0613 (7)
H10G	0.3584	0.3690	0.0841	0.092*
H10H	0.2211	0.2663	0.0816	0.092*
H10I	0.1713	0.4084	0.0727	0.092*
H3	0.618 (3)	0.082 (2)	−0.2002 (14)	0.054 (7)*
H4	0.748 (3)	0.0252 (19)	−0.0170 (12)	0.034 (5)*
H5	0.501 (2)	0.1622 (18)	0.0603 (12)	0.032 (5)*
H6	0.625 (3)	0.326 (2)	−0.0811 (14)	0.053 (7)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ti	0.02982 (18)	0.03192 (19)	0.03155 (18)	−0.00011 (16)	−0.00097 (13)	−0.00250 (16)
Si	0.0344 (3)	0.0339 (3)	0.0416 (3)	0.0059 (2)	0.0050 (2)	0.0028 (2)
C1	0.0294 (9)	0.0322 (10)	0.0316 (10)	−0.0003 (8)	0.0040 (8)	0.0035 (8)
C2	0.0421 (11)	0.0419 (11)	0.0312 (10)	−0.0016 (9)	0.0068 (9)	0.0049 (9)
B3	0.0479 (14)	0.0491 (15)	0.0422 (14)	0.0004 (12)	0.0179 (12)	−0.0029 (12)
B4	0.0312 (12)	0.0360 (12)	0.0448 (13)	0.0006 (10)	0.0066 (10)	0.0017 (10)
B5	0.0267 (9)	0.0314 (11)	0.0353 (11)	−0.0027 (9)	0.0024 (8)	0.0003 (10)
B6	0.0328 (12)	0.0390 (13)	0.0440 (13)	−0.0021 (10)	0.0115 (10)	0.0034 (11)
C11	0.0685 (17)	0.0463 (14)	0.0696 (17)	−0.0031 (14)	−0.0370 (15)	−0.0031 (14)
C12	0.0332 (12)	0.0602 (16)	0.0825 (19)	−0.0039 (11)	−0.0097 (12)	−0.0193 (15)
C13	0.0484 (13)	0.0435 (13)	0.0690 (16)	−0.0120 (11)	−0.0124 (12)	−0.0051 (12)
C14	0.0559 (15)	0.0586 (16)	0.0648 (17)	0.0054 (13)	−0.0152 (13)	−0.0291 (14)
C15	0.084 (2)	0.081 (2)	0.0392 (14)	−0.0175 (18)	−0.0104 (13)	−0.0143 (14)
C21	0.0759 (17)	0.0582 (15)	0.0350 (12)	0.0041 (14)	0.0088 (12)	0.0106 (11)
C101	0.086 (2)	0.0368 (13)	0.081 (2)	0.0036 (13)	0.0187 (16)	0.0093 (13)
C102	0.0362 (12)	0.0651 (17)	0.087 (2)	0.0095 (12)	0.0008 (13)	−0.0033 (15)
C103	0.0673 (17)	0.0689 (18)	0.0491 (14)	0.0163 (14)	0.0157 (12)	−0.0077 (13)

Geometric parameters (Å, º)

Ti—C1	2.330 (2)	C1—B5	1.547 (3)
Ti—C2	2.339 (2)	C1—B6	1.745 (3)
Ti—C15	2.371 (2)	C2—C21	1.520 (3)
Ti—C13	2.371 (2)	C2—B3	1.563 (3)
Ti—C11	2.375 (2)	C2—B6	1.691 (3)
Ti—C14	2.378 (2)	B3—B4	1.650 (4)
Ti—C12	2.384 (2)	B3—B6	1.811 (4)
Ti—B3	2.406 (3)	B4—B5	1.648 (3)
Ti—B5	2.414 (2)	B4—B6	1.790 (3)
Ti—B5^i	2.415 (2)	B4—Ti^i	2.505 (3)
Ti—B4	2.484 (2)	B5—B6	1.776 (3)
Ti—B4^i	2.505 (3)	B5—Ti^i	2.415 (2)
Si—C102	1.859 (2)	C11—C12	1.383 (4)
Si—C103	1.861 (3)	C11—C15	1.392 (4)
Si—C101	1.866 (3)	C12—C13	1.401 (4)
Si—C1	1.878 (2)	C13—C14	1.380 (4)
C1—C2	1.492 (3)	C14—C15	1.400 (4)
C1—Ti—C2	37.26 (7)	C2—C1—B5	109.48 (17)
C1—Ti—C15	113.91 (9)	C2—C1—B6	62.44 (14)
C2—Ti—C15	83.89 (10)	B5—C1—B6	64.98 (14)
C1—Ti—C13	142.58 (8)	C2—C1—Si	125.99 (14)
C2—Ti—C13	136.68 (8)	B5—C1—Si	122.61 (15)
C15—Ti—C13	56.68 (11)	B6—C1—Si	127.62 (14)
C1—Ti—C11	94.64 (8)	C2—C1—Ti	71.70 (11)
C2—Ti—C11	80.92 (9)	B5—C1—Ti	73.94 (11)
C15—Ti—C11	34.10 (11)	B6—C1—Ti	99.08 (12)
C13—Ti—C11	56.62 (9)	Si—C1—Ti	133.30 (10)
C1—Ti—C14	148.00 (9)	C1—C2—C21	121.24 (19)
C2—Ti—C14	116.45 (9)	C1—C2—B3	113.16 (17)
C15—Ti—C14	34.29 (10)	C21—C2—B3	124.44 (19)
C13—Ti—C14	33.80 (9)	C1—C2—B6	66.12 (14)
C11—Ti—C14	56.58 (10)	C21—C2—B6	125.54 (19)
C1—Ti—C12	108.54 (8)	B3—C2—B6	67.50 (15)
C2—Ti—C12	110.61 (9)	C1—C2—Ti	71.04 (11)
C15—Ti—C12	56.47 (11)	C21—C2—Ti	133.95 (16)
C13—Ti—C12	34.28 (9)	B3—C2—Ti	73.10 (13)
C11—Ti—C12	33.79 (10)	B6—C2—Ti	100.36 (12)
C14—Ti—C12	56.41 (10)	C2—B3—B4	104.47 (18)
C1—Ti—B3	65.12 (8)	C2—B3—B6	59.64 (14)
C2—Ti—B3	38.42 (8)	B4—B3—B6	62.09 (15)
C15—Ti—B3	87.01 (11)	C2—B3—Ti	68.48 (12)
C13—Ti—B3	139.29 (10)	B4—B3—Ti	72.84 (12)
C11—Ti—B3	104.31 (11)	B6—B3—Ti	94.52 (13)
C14—Ti—B3	105.64 (10)	B5—B4—B3	105.39 (18)
C12—Ti—B3	138.05 (10)	B5—B4—B6	62.02 (14)
C1—Ti—B5	38.01 (7)	B3—B4—B6	63.37 (15)
C2—Ti—B5	62.91 (8)	B5—B4—Ti	68.07 (11)
C15—Ti—B5	146.73 (10)	B3—B4—Ti	67.75 (13)
C13—Ti—B5	154.40 (9)	B6—B4—Ti	92.46 (12)
C11—Ti—B5	132.40 (9)	B5—B4—Ti^i	67.49 (12)
C14—Ti—B5	167.98 (9)	B3—B4—Ti^i	161.82 (16)
C12—Ti—B5	135.60 (9)	B6—B4—Ti^i	121.79 (14)
B3—Ti—B5	65.96 (9)	Ti—B4—Ti^i	94.17 (9)
C1—Ti—B5^i	119.35 (7)	C1—B5—B4	107.31 (18)
C2—Ti—B5^i	134.18 (7)	C1—B5—B6	62.90 (13)
C15—Ti—B5^i	124.74 (10)	B4—B5—B6	62.91 (14)
C13—Ti—B5^i	86.97 (8)	C1—B5—Ti	68.05 (11)
C11—Ti—B5^i	143.47 (9)	B4—B5—Ti	72.63 (12)
C14—Ti—B5^i	92.23 (9)	B6—B5—Ti	95.18 (13)
C12—Ti—B5^i	115.06 (9)	C1—B5—Ti^i	164.76 (15)
B3—Ti—B5^i	102.15 (9)	B4—B5—Ti^i	73.41 (12)
B5—Ti—B5^i	81.66 (8)	B6—B5—Ti^i	127.50 (14)
C1—Ti—B4	64.57 (7)	Ti—B5—Ti^i	98.34 (8)
C2—Ti—B4	63.48 (8)	C2—B6—C1	51.43 (12)
C15—Ti—B4	124.30 (10)	C2—B6—B5	91.36 (15)
C13—Ti—B4	152.69 (9)	C1—B6—B5	52.12 (12)
C11—Ti—B4	142.44 (10)	C2—B6—B4	93.67 (16)
C14—Ti—B4	128.81 (9)	C1—B6—B4	93.47 (15)
C12—Ti—B4	173.03 (9)	B5—B6—B4	55.06 (13)
B3—Ti—B4	39.41 (9)	C2—B6—B3	52.85 (14)
B5—Ti—B4	39.30 (8)	C1—B6—B3	91.60 (16)
B5^i—Ti—B4	70.70 (8)	B5—B6—B3	94.02 (16)
C1—Ti—B4^i	97.99 (7)	B4—B6—B3	54.54 (14)
C2—Ti—B4^i	132.51 (8)	C12—C11—C15	108.3 (3)
C15—Ti—B4^i	142.78 (10)	C12—C11—Ti	73.45 (14)
C13—Ti—B4^i	86.39 (9)	C15—C11—Ti	72.78 (15)
C11—Ti—B4^i	129.67 (11)	C11—C12—C13	107.9 (3)
C14—Ti—B4^i	111.01 (10)	C11—C12—Ti	72.76 (14)
C12—Ti—B4^i	96.47 (10)	C13—C12—Ti	72.38 (13)
B3—Ti—B4^i	125.20 (9)	C14—C13—C12	108.0 (3)
B5—Ti—B4^i	70.35 (8)	C14—C13—Ti	73.35 (14)
B5^i—Ti—B4^i	39.10 (8)	C12—C13—Ti	73.34 (14)
B4—Ti—B4^i	85.83 (9)	C13—C14—C15	108.2 (3)
C102—Si—C103	107.79 (13)	C13—C14—Ti	72.86 (14)
C102—Si—C101	109.57 (14)	C15—C14—Ti	72.59 (14)
C103—Si—C101	109.10 (13)	C11—C15—C14	107.6 (3)
C102—Si—C1	110.88 (11)	C11—C15—Ti	73.11 (15)
C103—Si—C1	108.03 (11)	C14—C15—Ti	73.12 (14)
C101—Si—C1	111.37 (12)

Symmetry code: (i) −x+1, −y, −z.