Probing mesitylborane and mesitylborate ligation within the coordination sphere of Cp*Ru(PiPr3)+: A combined synthetic, X-ray crystallographic, and computational study

DOI: 10.1021/ic1022328

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Inorganic Chemistry p. 2431 - 2444 (2011)

Update date:2022-07-30

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Authors:

Hesp, Kevin D.

Kannemann, Felix O.

Rankin, Matthew A.

McDonald, Robert

Ferguson, Michael J.

Stradiotto, Mark

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Article abstract of DOI:10.1021/ic1022328

The reaction of Cp^*Ru(PⁱPr₃)Cl (1) with MesBH₂ (Mes = 2,4,6-trimethylphenyl) afforded the mesitylborate complex Cp^*Ru(PⁱPr3)(BH₂MesCl) (2, 66%). Exposure of 2 to the chloride abs

Full text of DOI:10.1021/ic1022328

ARTICLE

pubs.acs.org/IC

Probing Mesitylborane and Mesitylborate Ligation Within the

Coordination Sphere of Cp*Ru(PⁱPr₃)^þ: A Combined Synthetic,

X-ray Crystallographic, and Computational Study

Kevin D. Hesp,^†,§Felix O. Kannemann,^†Matthew A. Rankin,^†,Robert McDonald,^‡

Michael J. Ferguson,^‡and Mark Stradiotto*^,†

^†Department of Chemistry, Dalhousie University, Halifax, Nova Scotia B3H 4J3, Canada

^‡X-ray Crystallography Laboratory, Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada

S Supporting Information

ABSTRACT: The reaction of Cp*Ru(PⁱPr₃)Cl (1) with MesBH₂(Mes = 2,4,6-

trimethylphenyl) aﬀorded the mesitylborate complex Cp*Ru(PⁱPr₃)(BH₂MesCl)

(2, 66%). Exposure of 2 to the chloride abstracting agent LiB(C₆F₅)₄2.5OEt₂

provided [Cp*Ru(PⁱPr₃)(BH₂Mes)]^þB(C₆F₅)₄(3, 54%), which features an

unusual η²-B-H monoborane ligand. The related borate complex Cp*Ru(PⁱPr₃)-

(BH₃Mes) (5, 65%) was prepared from 1 and LiH₃BMes. Attempts to eﬀect the

insertion of unsaturated organic substrates into the B-H bonds of 3 were

unsuccessful, and eﬀorts to dehydrohalogenate 2 using KO^tBu instead aﬀorded the mesitylborate complex Cp*(PⁱPr₃)Ru-

(BH₂MesOH) (6, 48%). Treatment of 1 with benzyl potassium generated an intermediate hydridoruthenium complex (7)

resulting from dehydrogenation of a PⁱPr fragment, which in turn was observed to react with MesBH₂to aﬀord the mesitylborate

complex Cp*(P(ⁱPr)₂(CH₃CCH₂))Ru(BH₃Mes) (8, 47%). Crystallographic characterization data are provided for 2, 3, 5, 6, and 8.

A combined X-ray crystallographic and density functional theory (DFT) investigation of 3 and 5, using Natural Bond Orbital

(NBO) and Atoms in Molecules (AIM) analysis, revealed that 3 and 5 are best described as donor-acceptor complexes between a

Cp*(PⁱPr₃)Ru^þfragment and a bis(η²-B-H) coordinating mesitylborane(borate) ligand. Signiﬁcant σ-donation from the B-H

bonds into the Ru^IIcenter exists as evidenced by the NBO populations, bond orders, and AIM delocalization indices. In the case of 3,

the vacant p orbital on boron is stabilized by RufB π back-donation as well as by resonance with the mesityl group.

’ INTRODUCTION

spectroscopic, crystallographic, and computational methods,^6-9

these are few in comparison to the numerous species featuring

η²-H-H¹⁰or η²-Si-H¹¹ligands.¹²As such, experimental and com-

putational investigations of new families of complexes exhibiting

η²-B-H monoborane ligation are likely to ﬁgure importantly in the

continuing quest to advance our understanding of such metal-

boron species.

The rich and diverse reactivity of coordinatively unsaturated

cationic ruthenium complexes of the type [Cp*RuP_n]^þX^-(Cp* =

η⁵-C₅Me₅; n = 1, 2, or 3; X = outer-sphere anion) is well-demon-

strated,¹³including reactions with organosilanes leading to isolable

η²-Si-H adducts,¹¹as well as to unusual [Cp*(PR₃)(H)₂Rud

SiHR]^þX^-species that are generated via double geminal Si-H

bond activation of RSiH₃.^14-16Glaser and Tilley¹⁴have demon-

strated that such cationic ruthenium-silylene species selectively

catalyze the hydrosilylation of oleﬁns by way of a previously

undocumented mechanism involving direct addition of RudSi-H to

the alkene.¹⁷Notably, data obtained in the course of a compara-

tive synthetic and computational investigation of analogous

neutral and cationic osmium-silylene species conﬁrmed that the

The chemistry of transition metal complexes featuring metal-

boron linkages has developed signiﬁcantly in recent years, resulting

in the characterization of novel metal-borane (MfBR₃), metal-

boryl (M-BR₂), and metal-borylene (MdBR) species by use of

spectroscopic, crystallographic, and quantum chemical techniques.¹

As part of the ongoing eﬀort to uncover fundamentally new metal-

boron bonding motifs and reactivity, and in an eﬀort to gain

mechanistic insights into prominent metal-mediated transforma-

tions including oleﬁn hydroboration,²the dehydrogenative boryla-

tion of hydrocarbons,³and the dehydrogenation of Lewis adducts

including ammonia borane,⁴there is widespread interest in doc-

umenting the bonding and reactivity behavior of B-H containing

substrates within the coordination sphere of transition metals. In

this context, complexes featuring monoborane (RBH₂or R₂BH)

ligands that are bound in an η²-B-H (also denoted σ-B-H) fashion

represent intriguing synthetic targets, as such species ﬁgure directly

in B-H oxidative addition/reductive elimination cycles, and repre-

sent isolable models for σ-C-H species that are implicated in metal-

mediated hydrocarbon C-H bond activation chemistry,⁵as well as

catalytic intermediates in the dehydrogenation of amine-boranes.⁴

While a relatively small number of mostly neutral η ²-B-H mono-

borane complexes have been prepared and characterized by use of

Received: November 7, 2010

Published: February 11, 2011

2011 American Chemical Society

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dx.doi.org/10.1021/ic1022328 Inorg. Chem. 2011, 50, 2431–2444

Inorganic Chemistry

ARTICLE

Scheme 1. Synthesis of 2, 3, and 5 (Mes = 2,4,6-trimethyl-

phenyl; LiB(Ar^F)₄= LiB(C₆F₅)₄2.5OEt₂)

Figure 1. ORTEP diagram for 2, shown with 50% displacement

ellipsoids; selected H-atoms have been omitted for clarity.

The chloroborate complex 2 proved to be a useful synthetic

precursor in the preparation of a novel cationic ruthenium

species (3) featuring MesBH₂as a ligand. When a solution of

2 in hexanes was treated with a solution of LiB(C₆F₅)₄2.5OEt₂

(LiB(Ar^F)₄) in ﬂuorobenzene (Scheme 1), an immediate color

change from orange to orange-yellow was observed. Monitoring

the progress of the reaction by use of NMR methods revealed the

consumption of 2, along with the formation of a single new

phosphorus-containing product 3 (δ ³¹P = 72.0), which was

obtained as an analytically pure solid in 54% yield upon crystal-

presence of a formal cationic charge on the complex is required to

promote alkene insertion into the MdSi-H unit,^14,18thereby

underscoring further the signiﬁcant inﬂuence of charge distribution

on the chemical behavior of main group E-H fragments within the

coordination sphere of reactive transition metal complexes. Intri-

gued by the aforementioned silane reactivity patterns, and encour-

aged by the diagonal relationship between silicon and boron on the

periodic table, we have initiated a program focused on exploring the

chemistry of primary monoboranes with 14-electron Cp*(PR₃)Ru^þ

fragments. We report herein on the synthesis and characterization of

cationic mesitylborane (MesBH₂; mesityl = 2,4,6-trimethylphenyl)

and neutral mesitylborate (MesBH₂X; X = H, Cl, OH) coordination

complexes of Cp*(PⁱPr₃)Ru^þ, including a rare example of a cationic

borane complex featuring bis(η²-B-H) ligation.¹⁹Also presented are

the results of an X-ray crystallographic and quantum chemical

investigation of Cp*(PⁱPr₃)Ru(BH₂Mes)^þand Cp*(PⁱPr₃)Ru-

(BH₃Mes) species, including a comparative analysis of the elec-

tronic structure of these complexes.

lization. Integration of the H NMR spectral data for pure 3

conﬁrmed the presence of signals associated with Cp*, PⁱPr₃, and

Mes groups, as well as a signal in the hydride region (-10.3 ppm,

2H, J_PH= 15.0 Hz) associated with this complex. As well, the

broad doublet observed in the hydride region of the H NMR

spectrum of 3 was observed as a considerably sharper doublet in

the corresponding ¹H{¹¹B} NMR spectrum, thereby suggesting

the presence of some form of B H interaction in solution.

3 3 3

Furthermore, the solid state connectivity in 3 was determined by

use of single-crystal X-ray diﬀraction techniques (Figure 2). A

number of plausible bonding representations can be presented

for 3 (Chart 1), including: the bis(η²-B-H) structure (3-A); the

dihydrido(borylene) formulation (3-B); and alternative struc-

tures featuring a formal positive charge on boron (3-C). How-

ever, on the basis of the crystallographic and computational (vide

infra) characterization data presented herein,²²we view 3-A as

providing the most suitable single representation of the solid

state structure of 3. In this regard, 3 can be viewed as a member

of a very limited class of isolable cationic complexes featuring η²-

B-H monoborane ligation.^7,8

’ RESULTS AND DISCUSSION

Synthetic and Structural Characterization Studies. Treat-

ment of a dark blue hexanes solution of Cp*Ru(PⁱPr₃)Cl (1) with

an equiv of MesBH₂resulted in an immediate color change to

dark orange; ³¹P NMR analysis of the reaction mixture after 0.5 h

revealed the quantitative formation of 2 (Scheme 1), which in

turn was isolated in 66% yield. The assignment of 2 as a C_S-

symmetric chloroborate complex arising from chloride transfer

from ruthenium to boron is supported by NMR spectroscopic

data (e.g., δ ¹H = -11.77, Ru(H)₂B; δ ³¹P = 66.6; δ ¹¹B = 46.5,

Δν_1/2= 462 Hz), as well as single-crystal X-ray diffraction data

(Figure 1). An ORTEP diagram for 2 is provided in Figure 1, while

diffraction data and selected metrical parameters for each of the

crystallographically characterized compounds reported herein are

provided in Tables 1 and 2, respectively. While the overall

connectivity within 2 can be compared with that of Cp*Ru(PMe₃)-

Compound 3 can be described as adopting a piano-stool

structure, in which the Cp*Ru^þfragment is supported by three

neutral two-electron donors: the PⁱPr₃co-ligand as well as two

dative interactions involving the bis(η²-B-H) ligating MesBH₂

group, giving a formal 18 electron conﬁguration. The unusual

bis(η²-B-H) MesBH₂ligand motif featured in 3 has also been

observed in Ru(H)₂(PCy₃)₂(BH₂Mes) (4) reported by

Sabo-Etienne and co-workers.^7eWhile the presence of Ru-H

co-ligands in the neutral complex 4 is of considerable interest in

relation to σ-complex-assisted metathesis processes,^6dthe ab-

sence of such co-ligands in the novel cationic species 3 provides

an opportunity to examine bis(η²-B-H) ligation free of potential

(BH₃Cl),²⁰the Ru-P and Ru B distances in 2 (2.3589(6) Å;

3 3 3

2.162(3) Å) are longer than those found in this comparator com-

Ru-H BH₂Mes interligand interactions. The Ru-H

3 3 3

plex (Ru-P 2.295(1) Å); Ru B 2.122(8) Å),²⁰likely owing to

(1.61(3) Å and 1.60(3) Å) and B-H (1.31(3) Å and 1.28(3) Å)

distances in 3 are indistinguishable from those found in the

precursory complex 2, and are in keeping with the existence of

3 3 3

the greater steric demands of the phosphine and borate ligands

in 2.²¹By comparison, no reaction was observed (¹H and ³¹P

NMR) between 1 and an equiv of either pinacolborane or

catecholborane under similar experimental conditions.

H interactions²³that would appear to be inconsistent with

3 3 3

an alternative description of 3 as the dihydridoborylene species

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Inorganic Chemistry

ARTICLE

Table 1. Crystallographic Data for 2, 3, 5, 6, and 8

empirical formula

formula weight

crystal dimensions

crystal system

space group

a (Å)

₂₈H₄₉BClPRu

C₅₂H₄₉B₂F₂₀PRu

1207.57

C₂₈H₅₀BPRu

529.53

C₂₈H₅₀BOPRu

545.53

C₂₈H₄₈BPRu

527.51

563.97

0.56 ꢀ 0.37 ꢀ 0.13

monoclinic

P2₁/c

0.41 ꢀ 0.26 ꢀ 0.20

orthorhombic

Pbca

0.37 ꢀ 0.20 ꢀ 0.16

triclinic

0.43 ꢀ 0.13 ꢀ 0.06

monoclinic

P2₁/c

0.41 ꢀ 0.25 ꢀ 0.05

triclinic

16.007(2)

10.4284(14)

18.504(3)

18.8734(15)

18.0950(15)

30.283(3)

10.4110(9)

12.7595(10)

21.3876(17)

89.6667(9)

85.9052(10)

87.6112(10)

2831.4(4)

8.3245(7)

17.0809(14)

20.6272(17)

8.5073(3)

17.1327(6)

19.5469(7)

84.4141(4)

86.6989(4)

84.1438(4)

2817.42(17)

b (Å)

c (Å)

R (deg)

β (deg)

106.1982(16)

101.5780(10)

γ (deg)

V (Å³)

2966.1(7)

10341.9(15)

2873.3(4)

_calcd(g cm^-3

μ (mm^-1

)

1.263

1.551

1.242

1.261

1.244

)

0.686

0.443

0.623

0.618

0.626

range of transmission

0.9161-0.6999

55.00

0.9167-0.8394

54.96

0.9080-0.8045

52.80

0.9638-0.7769

55.00

0.9682-0.7830

54.96

2θ limit (deg)

-20 e h e 20

-13 e k e 13

-24 e l e 22

20204

-24 e h e 24

-23 e k e 23

-39 e l e 39

86717

-13 e h e 13

-15 e k e 15

-26 e l e 26

35486

-10 e h e 10

-22 e k e 22

-26 e l e 26

24455

-11 e h e 11

-22 e k e 22

-25 e l e 25

24798

total data collected

independent reﬂections

R_int

6740

11845

35486

6553

12745

0.0266

0.0415

N/A

0.0421

0.0245

observed reﬂections

data/restraints/parameters

goodness-of-ﬁt

5718

9200

29614

5419

10500

6740/0/305

1.063

11845/0/701

1.047

35486/0/612

1.029

6553/0/306

1.032

12745/0/602

1.027

R₁[F_o²g 2σ(F_o²)]

wR₂[F_o²g -3σ(F_o²)]

largest peak, hole (e Å^-3

0.0302

0.0342

0.0352

0.0300

0.0301

0.0761

0.0900

0.1029

0.0771

0.0756

)

0.600, -0.282

0.552, -0.347

0.746, -0.556

0.497, -0.291

0.547, -0.473

Table 2. Selected Interatomic Distances (Å) and Angles (deg) for 2, 3, 5, 6, and 8^a

3^c

3-calc.

5^d

5-calc.

8^d,e

2.201(3)

8^d,e

Ru-B

2.162(3)

2.3589(6)

1.63(3)

1.921(2)

2.3911(6)

1.61(3)

1.60(3)

1.31(3)

1.28(3)

N/A

1.996

2.486

1.78

2.215(2)

2.3524(5)

1.69(2)

2.210(2)

2.283

2.436

1.789

1.328

1.324

1.209

1.599

124.85

113.1

2.151(2)

2.3211(6)

1.58(3)

2.206(3)

Ru-P

2.3576(5)

1.724(17)

1.696(19)

1.281(18)

1.332(19)

1.124(17)

1.593(3)

2.3308(6)

1.69(2)

2.3343(6)

1.64(2)

Ru-H1

Ru-H2

B-H1

1.71(3)

1.77

1.695(17)

1.27(2)

1.58(3)

1.70(2)

1.68(2)

1.29(2)

1.31

1.52(3)

1.35(2)

1.34(2)

B-H2

B-X^b

1.30(3)

1.31

1.252(16)

1.117(18)

1.597(3)

126.93(13)

110.0(9)

1.52(3)

1.31(2)

1.29(2)

1.887(3)

1.586(3)

123.13(16)

105.83(16)

N/A

1.516

170.34

N/A

1.403(3)

1.597(3)

120.22(15)

114.51(19)

1.13(2)

1.14(2)

B-Cipso

Ru-B-Cipso

X-B-Cipso

1.522(3)

172.24(17)

N/A

1.596(3)

122.27(16)

113.6(11)

1.598(3)

124.91(16)

112.8(11)

127.55(13)

110.5(9)

^aData obtained from single-crystal X-ray diﬀraction experiments; related metrical parameters for the computationally derived structures of 3 and 5 are

provided for comparison. ^bX = Cl (2), H3 (5), OH (6), and H3 (8). ^cP-Ru-B-C ortho torsion angles -97.6° (-103.9° calc.) and 110.5° (110.8°

calc.) and the sum of the C-B-H and H-B-H angles = 356.7° (356.8° calc.) (measured in ﬁnal reﬁned structure). ^dIn the two crystallographically

independent molecules of 5 or 8. ^eWithin the major component of the disordered isopropenyl fragments for each of the crystallographically independent

molecules of 8: C21-C22 = 1.304(8) and 1.262(9); C21-C23 = 1.485(7) and 1.481(9).

[Cp*(PⁱPr₃)(H)₂RudBMes]^þB(C₆F₅)₄(cf. 3-B).^24,25The

classiﬁcation of 3 as a bis(η²-B-H) monoborane complex is also

in keeping with the distorted trigonal planar (sp²-hybridized)

nature of boron, as evidence by the sum of the H-B-H and H-

B-C angles (ca. 357°). The coordinated MesBH₂ligand in 3

features a nearly linear Ru-B-C linkage (172.24(17)° in 3;

cf. 177.1(3)° in 4^7e), as well as P-Ru-B-C torsion angles

(97.6° and 110.5° in 3; cf. 90.3-109.2° in 4^7e) that reveal a near

orthogonal alignment of the B-aryl ring relative to the plane

deﬁned by ruthenium, phosphorus, and boron (Figure 2, inset).

The importance of such a geometry in promoting favorable

orbital overlap within the Ru(H)₂B core and extending into the

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Inorganic Chemistry

ARTICLE

Figure 3. ORTEP diagram for one of the two crystallographically

independent molecules of 5, shown with 50% displacement ellipsoids;

selected H-atoms have been omitted for clarity.

Figure 2. ORTEP diagram for 3, shown with 50% displacement

ellipsoids and with selected H-atoms and the B(C₆F₅)₄^-counteranion

omitted for clarity (inset: view down the Ru-B-C vector with

additional C-atoms removed for clarity).

isolated yield by treatment of Cp*Ru(PⁱPr₃)Cl with LiH₃BMes

(Scheme 1), and was characterized by use of spectroscopic and

X-ray crystallographic techniques. The observation of distinct ¹H

NMR signals (300 K) attributable to two equivalent bridging

hydrides (-12.09 ppm) as well as a terminal B-H fragment

(6.77 ppm) is consistent with the crystal structure of 5

(Figure 3). The solid state structural features of 5 mirror those

of the related chloroborate 2, with the exception that slightly

Chart 1. Three Selected Bonding Representations of 3

longer Ru B contacts are observed in each of the crystal-

3 3 3

lographically independent molecules of 5 (2.215(2) Å and

2.210(2) Å, versus 1.921(2) Å in the cationic complex 3). In

contrast to the nearly linear Ru-B-C linkage and trigonal

planar geometry observed for the coordinated bis(η²-B-H)

borane ligand in 3, the Ru-B-C chain in each crystallographi-

cally independent molecule of 5 (126.93(13)° and 127.55(13)°)

is signiﬁcantly bent such that the boron center adopts a distorted

tetrahedral geometry, as expected for a quaternary borate

fragment.²¹No statistically signiﬁcant diﬀerences in the bridging

Ru-H and B-H distances (respectively) were observed within

the structures of 3 and 5.

adjacent aromatic ring is discussed in the context of our

computational investigation of 3 (vide infra).

The Ru B contact (1.921(2) Å) in 3 is signiﬁcantly shorter

3 3 3

than in the chloroborate complex 2 (2.162(3) Å), and is less

than the sum of the covalent radii for ruthenium and boron (ca.

2.1 Å⁶). Indeed, the Ru B distance in 3 is shorter than almost

Reactivity Studies. Given the conceptual relationship be-

tween [Cp*(PR₃)(H)₂RudSiHR]^þX^-species reported by Gla-

ser and Tilley¹⁴and the cationic bis(η²-B-H) species 3, the ability

of 3 to hydroborate unsaturated substrates in a stoichiometric

fashion was examined. However, no reaction was observed upon

exposure of 3 to either diphenylacetylene, styrene, norbornene,

or tert-butylethylene (1 or 10 equiv) at ambient temperature, and

3 3 3

all other crystallographically characterized complexes featuring

ruthenium-boron linkages, including the η²-B-H dialkoxybo-

rane species RuH₂(η²-H₂)(η²-BHR₂)(PCy₃)₂(R₂= pinacolate,

2.173(2) Å; R₂= catecholate, 2.124(2) Å), as well as RuH-

[(μ-H)₂Bpin](η²-HBpin)(PCy₃)₂which contains both second-

ary σ-borane (2.157(5) Å) and dihydroborate (2.188(5) Å)

ligands;^7f,7gruthenaboratranes that feature dative metal-borane

(MfBR₃) interactions (ca. 2.14-2.17 Å);²⁶and ruthenium-

heating of these reaction mixtures at 50 °C afforded ⁱPr₃P BH ₂Mes

as a major product (³¹P NMR) after several hours. By comparison,

the aforementioned cationic ruthenium-silylene complexes reported

by Glaser and Tilley¹⁴react rapidly with R-olefins to afford isolable

Si-H insertion products.

boryl complexes (ca. 2.05-2.17 Å).²⁷While the Ru B contact

3 3 3

in 3 is statistically equal to that observed in 4 (1.938(4) Å^7e), this

distance in 3 is longer than the Ru-B distance found in a

ruthenium-borylene complex (1.780(4) Å).²⁸We are reluctant

To complement the synthesis of 5 and 3, which feature

mesitylborate and mesitylborane ligation (respectively), further

research eﬀorts were directed toward preparing the related

mesitylboryl complex Cp*(PⁱPr₃)Ru(BHMes), which in princi-

ple could rearrange to the mesitylborylene species Cp*(PⁱPr₃)-

(H)RudBMes via R-H elimination.³⁰We initially identiﬁed the

reaction of 2 with KO^tBu as a potential route to Cp*(PⁱPr₃)Ru-

(BHMes); however, instead of eﬀecting the dehydrohalogena-

tion of 2, this reaction gave rise to the mesitylborate complex

Cp*(PⁱPr₃)Ru(BH₂MesOH) 6 (Scheme 2), which in turn was

isolated as an analytically pure solid in 48% yield. While we

cannot unequivocally exclude the formation of 6 as arising from

the reaction of 2 with KOH (formed in situ from KO^tBu and

adventitious water), we feel that such a scenario is unlikely under

the anhydrous reaction conditions employed. Instead, we view 6

to interpret the relatively short Ru B distance observed in the

3 3 3

crystal structure of 3 as providing deﬁnitive evidence of signiﬁ-

cant covalent bonding interactions between ruthenium and

boron, since it is known that the ionic contributions to bonding

can be sizable in transition metal-boron complexes,^1,6and that

the coordination of two geminal η²-E-H bonds (versus a single

η²-E-H interaction) can result in the observation of relatively

short crystallographically determined Ru E distances (E =

3 3 3

B,^7eSi²⁹). In an eﬀort to gain insight into the electronic structure

and bonding within 3, a comparative X-ray crystallographic/

quantum chemical study involving this cationic bis(η²-B-H)

monoborane complex and the related trihydridoborate species

5 was undertaken (vide infra). Compound 5 was prepared in 65%

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Inorganic Chemistry

ARTICLE

Figure 5. ORTEP diagram for one of the two crystallographically

independent molecules of 8 (featuring one of the two disordered

components of the isopropenyl fragment), shown with 50% displace-

ment ellipsoids and with selected H-atoms omitted for clarity.

Figure 4. ORTEP diagram for 6, shown with 50% displacement

ellipsoids; selected H-atoms have been omitted for clarity.

Scheme 2. Reactivity of 2 Including the Formation of 6

Scheme 3. Generation of 7 and Reaction with MesBH₂to

Give 8

as resulting from the net elimination of 2-methylpropene in the

putative reactive intermediate Cp*(PⁱPr₃)Ru(BH₂MesO^tBu).

The structural assignment of 6 is given on the basis of NMR

spectroscopic and X-ray crystallographic data (Figure 4). The

structural features of 6 mirror those observed in the related

chloroborate complex 2, and thus do no warrant further com-

mentary. Further eﬀorts to prepare Cp*(PⁱPr₃)Ru(BHMes) via

treatment of 2 with NaN(SiMe₃)₂, n-BuLi, or K₂CO₃generated

a complex mixture of products from which no pure materials

could be isolated.

The net elimination of R-H in the reaction of an alkylruthe-

nium complex Cp*(PⁱPr₃)RuR with MesBH₂was identiﬁed as

an alternative possible route to Cp*(PⁱPr₃)Ru(BHMes). Given

the utility of Cp*(PⁱPr₃)OsCH₂Ph¹⁸as a synthetic precursor in

Si-H bond activation chemistry, we sought to prepare Cp*-

(PⁱPr₃)RuCH₂Ph via treatment of 1 with benzyl potassium.

Whereas preliminary ³¹P NMR analysis of the reaction mixture

revealed the clean conversion (in the absence of observable

intermediates) to a single phosphorus-containing species

(δ ³¹P = 53.2), full characterization by use of ¹H and ¹³C NMR tech-

niques allowed for the identiﬁcation of this reaction product as the

hydridoruthenium complex 7 (Scheme 3). The formation of 7 can

be viewed as arising from cyclometalation of an isopropyl methyl C-

H fragment in the putative intermediate Cp*(PⁱPr₃)RuCH₂Ph,

followed by reductive elimination of toluene and β-hydride

elimination.³¹Unfortunately, our attempts to isolate 7 in analytically

pure form have thus far been thwarted by the apparent instability

of this compound upon workup.³²Nonetheless, 7 prepared in situ

was found to react quantitatively (³¹P NMR) with MesBH₂to aﬀord

the mesitylborate complex 8, which was obtained as an analytically

pure solid in 47% yield. In contrast to the apparent C₁-symmetry

observed for 7, NMR spectroscopic data for 8 revealed a C_S-

symmetric structure consistent with the presence of an uncoordi-

nated alkenyl fragment in this complex. Furthermore, the assignment

of 8 as a dehydrogenated variant of 5 is consistent with

the observation of unique H NMR (300 K) resonances attribut-

able to two equivalent bridging hydrides (δ H = -12.03) and

a terminal B-H fragment (δ ¹H = 6.86), as well as data obtained

by use of single-crystal X-ray diﬀraction techniques (Figure 5);

the key metrical parameters associated with 8 compare well with

those of 5.

In keeping with the apparent inability of various unsaturated

hydrocarbons to eﬀect the clean dehydrogenation of 3 (vide

supra), no reaction was observed upon exposure of 5 to

norbornene or tert-butylethylene (1 or 10 equiv) at ambient

temperature, and heating of these reaction mixtures at 90 °C

led to the consumption of 5 along with the formation of an

intractable mixture of phosphorus-containing products (³¹P

NMR) including 7 (ca. 10%), PⁱPr₃(ca. 40%), and an as-yet-

unidentiﬁed species (ca. 30%, δ ³¹P = 96) over the course of 48 h.

The thermolysis of 8 under similar conditions was also examined

in anticipation that the pendant alkene might act as a hydrogen

acceptor; once again an intractable mixture of phosphorus-

containing products (³¹P NMR) was generated, including 5

(ca. 10%), 7 (ca. 5%), and an as-yet-unidentiﬁed species (ca.

70%, δ ³¹P = 100).

Computational Approach and Geometry Optimization.

In an effort to complement the X-ray crystallographic analysis of

3 and 5, a comparative quantum chemical investigation of these

complexes was conducted. Electronic structure calculations on

the borane complex 3 and the borate complex 5 were performed

by using the Gaussian03 package³³using the B3LYP³⁴density

functional theory (DFT) method. The main group elements

(C, H, B, P) were described with the 6-31G(d) basis set as

implemented in Gaussian03. To improve the description of the

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B-H hydrogen atoms, the 6-31G(d,p) basis set, which includes

polarization functions on H, was used for these atoms. The

ruthenium center was described with the all-electron, polarized

split-valence basis set of Ahlrichs and May (SVPPalls1),³⁵augmented

with a set of f-type polarization functions (exponent 1.2453897).³⁶

Cartesian basis functions (6d, 10f) are used throughout. For the

electronic structure (NBO, bond order, AIM) analysis, single-point

calculations on the computationally optimized geometries were

performed with the same DFT method and basis sets, but including

scalar relativistic effects by using the second order Douglas-Kroll-

Hess (DKH) Hamiltonian.³⁷

(between complexes 3 and 5) in these distances are in much

closer agreement between calculation and experiment, having

discrepancies equal to or less than 0.01 Å. The only exceptions

are the Ru-H1 and Ru-H2 distances (vide supra). Interest-

ingly, the largest change in interatomic distance occurs for the

Ru B contact, which is signiﬁcantly longer (0.29 Å expt./

3 3 3

0.29 Å calc.) in the mesitylborate complex 5 compared to cationic

borane complex 3. This indicates a much stronger Ru-B

interaction in the borane complex 3, which is further investigated

in the electronic structure analysis of complexes 3 and 5 (vide

infra). A similar but less dramatic increase in interatomic distance

(0.07 Å expt./0.08 Å calc.) on going from 3 to 5 is found for the

B-C_ipsobond. Conversely, the Ru-P distance contracts slightly

(-0.04 Å expt./-0.05 Å calc.) from 3 to 5, indicating that the

weaker Ru-B interaction in 5 is partly compensated by a

strengthening of the Ru-P bond. These changes in Ru-B and

Ru-P distances between 3 and 5 are diﬀerent by an order of

magnitude, thereby conﬁrming that the Ru-P interaction is

Starting from the experimental (X-ray diﬀraction) structures,

full geometry optimizations of complexes 3 and 5 were per-

formed. Frequency calculations on the optimized geometries

characterized these structures as minimum energy points on the

potential energy surface. Selected geometrical parameters of

the structures optimized by use of DFT are reported in Table 2.

In general, the optimized geometries are in reasonable agreement

with the experimental crystal structures. The Ru-B, Ru-P, and

B-H3 (in 5) distances in the optimized structures are about 0.1

Å longer compared to the crystal structures. Such diﬀerences may

be attributable to crystal packing eﬀects absent in the computa-

tional structures, as well as to the approximate DFT method and

ﬁnite basis sets used in the calculation. Also, the DFT-optimized

Ru-H1 and Ru-H2 distances in the borate complex 5 are

elongated by almost 0.1 Å relative to the experimentally deter-

mined structure, while for the borane complex 3 this discrepancy

increases to 0.17 Å. However, the diﬀerence in the experimental

Ru-H1 and Ru-H2 distances between 3 and 5 is not statisti-

cally signiﬁcant because of the large standard deviations of 0.03 Å

(3) and 0.02 Å (5), typical of hydrogen atoms in close proximity

to a metal center. Gratifyingly, the B-H1, B-H2, and B-C_ipso

distances, as well as the reported angles and torsion angles, are

well reproduced in the computationally optimized structures.

Comparative Analysis of the Structural Parameters within

3 and 5. The neutral mesitylborate complex 5 can be viewed as

a derivative of the cationic bis(η²-B-H) monoborane species 3,

where in 5 an additional hydride ligand on the boron center

contributes two electrons to the overall electron count. While

quaternization of the boron center in this manner changes the

hybridization state of boron from sp²to sp³, it does not directly alter

the first coordination sphere of ruthenium in these complexes. It is

therefore interesting to compare the structural parameters around

the Ru and B centers in the complexes 3 and 5. Given the significant

errors that can be associated with the X-ray crystallographic deter-

mination of hydrogen atom positions within close proximity to metal

centers, analysis of the optimized structures of 3 and 5 derived from

DFT calculations was undertaken.²²

much stronger than the malleable Ru B coordination. The

3 3 3

DFT-optimized B-H1 and B-H2 distances as well as the

Ru-H1 and Ru-H2 distances in 3 and 5 are not signiﬁcantly

diﬀerent (within 0.02 Å).

The geometrical consequences of adding a hydride ligand

to the boron center in the cationic bis(η²-B-H) monoborane

complex 3, thereby producing the neutral mesitylborate complex

5, can be summarized as follows:

(1) The geometry of the boron center changes from trigonal

planar (3) to tetrahedral (5).

(2) The Ru-B and B-C_ipsodistances are elongated in 5 with

respect to 3, while the Ru-P bond contracts slightly.

(3) The positions of the bridging hydrogens (H1 and H2) are

unaﬀected.

Molecular Orbital and Bond Order Analysis. To investigate

the electronic structure and bonding in complexes 3 and 5,

molecular orbital (MO) and bond order analysis were per-

formed. The canonical MOs of both systems, including the

frontier orbitals, are highly delocalized and difficult to interpret.

We therefore use localized MOs, in particular the Natural Bond

Orbital (NBO) approach,³⁸which is especially valuable in the

study of donor-acceptor interactions.

In the borane complex 3, the NBO scheme yields three lone

pair orbitals on Ru (electron populations 1.91, 1.85 and 1.69) in

accordance with a Ru^IId⁶description of this system. The P-Ru

bonding orbital (pop. 1.84) has 71% contribution from P and

29% contribution from Ru, thus characterizing this interaction as

predominantly dative, in contrast to a typical covalent (electron-

sharing) bond. Two equivalent B-H bonding orbitals are

obtained (pop. 1.61, 45% B, 55% H) for the boron center in 3.

The electron populations of the B-H bonding orbitals in 3 are

noticeably smaller than in free mesitylborane (pop. 1.99, 46% B,

54% H), indicating a signiﬁcant degree of σ-donation to Ru and

accompanying B-H bond activation. Using second-order per-

turbation theory, the interaction energy of this bis(η²-B-H) σ-

donation into a formally unoccupied lone pair orbital on Ru

(pop. 0.21) is estimated to be 146 kcal/mol. Thus, the NBO

analysis rules out the description of 3 as a borylene complex (3-B

or 3-C) and conﬁrms its nature as the bis(η²-B-H) monoborane

species 3-A.

Signiﬁcant diﬀerences in bond angles occur for the Ru-B-

C_ipsoangle, which changes from almost linear (172.24° expt./

170.34° calc.) in 3 to strongly bent (∼127° expt./124.85° calc.)

in 5. The C_ipso-B-H(1,2) angles decrease from approximately

trigonal planar (∼126° expt./∼121° calc.) in 3 to approximately

tetrahedral (∼115° expt./∼114° calc.) in 5. Along with the

nearly tetrahedral C_ipso-B-H3 angle (110.3° expt./113.1°

calc.) in 5, these structural changes are in keeping with the

expected change in formal hybridization from sp²to sp³on going

from 3 to 5.

Before comparing and contrasting the interatomic distances

within 3 and 5, we note that while the absolute values for

interatomic distances show discrepancies of about 0.1 Å between

the computational and the experimental structures, the diﬀerence

Finally, the formally unoccupied p orbital on B in 3 has a rather

large population of 0.45 electrons, compared to a population of

0.16 in free mesitylborane. This is the result of π back-donation

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from a d lone pair orbital on Ru (pop. 1.69) into the vacant p

orbital on B, with an associated estimated interaction energy of

46 kcal/mol. The orbitals involved in this back-bonding are

depicted in Figure 6. A comparable bonding situation involving

σ-donation and π back-donation has been previously observed

for the neutral mesitylborane-ruthenium complex Ru(H)₂-

(PCy₃)₂(BH₂Mes) (4) reported by Sabo-Etienne and co-

workers.^7e

decreases by about 40% on going from 3 to 5 because of the

absence of Ru-B π back-donation in 5 (bond lengthening of

15%). However, in both 3 and 5 the Ru-B bond order is larger

than the individual Ru-H(1,2) bond orders, demonstrating the

role of the B center in the bis(η²-B-H) σ-donation to Ru.

Similarly, the B-C_ipsobond order decreases on going from 3

to 5 by about 14% because of the loss of resonance stabilization

with the mesityl group in 5, while the Ru-P and the sum of the

Ru-C(Cp*) bond orders increase slightly.

The Ru-H(1,2) and B-H(1,2) bond orders are unchanged

between 3 and 5, as are the populations of the B-H bonding

orbitals and the respective interatomic distances (vide supra).

The sum of the Ru-H and B-H bond orders is close to unity for

both bridging hydrogens, with the B-H(1,2) bond orders being

about twice as large as the Ru-H(1,2) bond orders. The

presence of an additional hydride ligand in the borate complex

5 has only a relatively small eﬀect on the total bond order of

the boron center because of the weakening of the Ru-B and

B-C_ipsobonds.

NBO analysis of the borate complex 5 also yields three

occupied lone pair orbitals on Ru (electron populations 1.93,

1.85, and 1.82) as well as a P-Ru bonding orbital (pop. 1.91,

70% P, 30% Ru). The two σ-donating B-H bonding orbitals

(pops. 1.61 and 1.62, 42% B, 58% H) have lower populations and

are polarized toward the hydride fragments, compared to the

third (non-donating) B-H bonding orbital (pop. 1.97, 49% B,

51% H). With second-order perturbation theory, three dominat-

ing donor(η²-B-H)-acceptor(Ru) interactions are identiﬁed

(electron populations of the diﬀerent accepting Ru orbitals

in parentheses): 66 kcal/mol (0.65), 67 kcal/mol (0.19), and

42 kcal/mol (0.16).

Finally, we compare bond orders in complexes 3 and 5 relative

to the uncoordinated fragments MesBH₂and MesBH₃^-. The

geometries of the fragments were optimized with the same

computational method and basis sets used for the full complexes

(vide supra). As expected for bis(η²-B-H) σ-complexes, the

B-H(1,2) bond orders are signiﬁcantly reduced upon com-

plexation to Ru, by 20-40% depending on the deﬁnition of bond

order. The B-C_ipsobond order increases slightly on going from

free mesitylborane to 3 because of enhanced π-electron deloca-

lization in the presence of Ru-B π back-bonding. The B-H3

In Table 3, Wiberg bond indices³⁹and overlap-weighted

natural atomic orbital (NAO) bond orders⁴⁰are presented for

the Ru and B moieties in complexes 3 and 5. These two

deﬁnitions of bond order show the same characteristic changes

between the cationic borane complex 3 and the neutral borate

complex 5, which also reﬂect the geometrical changes discussed

in the previous section. Most importantly, the Ru-B bond order

and B-C_ipsobond orders are virtually identical in 5 and

MesBH₃

We conclude from the MO and bond order analysis that both

species 3 and 5 are best described as donor-acceptor complexes

between a Cp*(PⁱPr₃)Ru^þfragment and a bis(η²-B-H) coordi-

nating mesitylborane(borate) ligand. Signiﬁcant σ-donation

from the B-H bonds into the Ru^IIcenter exists as reﬂected by

the NBO populations and bond orders. Complexes 3 and 5 do

not show noticeable diﬀerences in the degree of B-H bond

activation. The main change occurs at the B center, where the

formally vacant p orbital of 3 (stabilized by π back-donation from

Ru and resonance with the mesityl group) is instead replaced

by a B-H σ-bond orbital in 5 with concomitant changes in geo-

metry and electronic structure.

Topological Analysis of the Electron Density. Molecular

Graphs. To complement the MO and bond order analysis of

complexes 3 and 5, a comparative analysis of the topology of the

electron density within these complexes was conducted. To the

best of our knowledge, similar computational analyses involving

Figure 6. Orbital interactions in 3, depicting π back-donation from a Ru

valence d orbital (pop. 1.69) into the formally vacant p orbital on B (pop.

0.45). Shown are the (0.2, ( 0.1, and (0.05 orbital isosurfaces.

Table 3. Wiberg Bond Indices and Overlap-Weighted NAO Bond Orders for the Ru and B Centers in Complexes 3 and 5

complex

bond order

Ru-P

Ru-B

Ru-H1

Ru-H2

Ru-C(Cp*)

B-H1

B-H2

B-H3

B-C_ipso

B (total)

MesBH₂

Wiberg

0.98

0.61

0.62

0.96

0.98

0.61

0.62

0.96

1.00

1.04

0.90

3.15

3.46

3.73

3.85

0.74

0.78

0.85

0.46

0.30

1.84

1.92

0.95

0.98

MesBH₃

MesBH₂

NAO

0.83

0.65

0.64

0.80

0.83

0.65

0.80

0.96

1.02

0.87

0.86

2.69

3.51

3.74

3.34

0.72

0.76

0.88

0.58

0.38

0.39

0.38

0.39

1.80

1.85

0.82

MesBH₃

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Figure 7. Molecular graphs displaying the topology of the electron density of 3 (left) and 5 (right). Shown are nuclear attractors (blue spheres), BCPs

(red spheres), RCPs (green spheres), and bond paths (black lines).

4 have not been reported.^7eWhile NBO analysis yields a

chemically intuitive interpretation of bonding in terms of orbital

interactions, the underlying quantities (localized orbitals and

bond orders) are not physical observables and can lead to

ambiguous interpretation. By contrast, the electron density is

an observable quantity which is both routinely measured and

calculated. Topological analysis of the electron density forms

part of Bader’s “Atoms in Molecules” (AIM) theory⁴¹and yields

a rigorous definition of chemical bonding in terms of molecular

graphs. In molecular graphs, atoms that are bonded to one

another are linked by bond paths, and the point of minimum

electron density along the bond path is termed bond critical point

(BCP).⁴²The topological analysis of the electron density of

complexes 3 and 5 was carried out with the AIM2000^41cand

AIMALL^41dprograms.

Molecular graphs for complexes 3 and 5 are shown in Figure 7;

for clarity, we omit bond paths and bond critical points (BCPs)

between atoms on diﬀerent ligands that are not covalently

bonded to one another. In both complexes, the Ru center is

linked to the C atoms of the Cp* ligand by ﬁve BCPs and

associated bond paths, which are intersected by ﬁve ring critical

points (RCPs). This topology is typical for interactions between

metals and an unsaturated ring, where the electron density is

delocalized over the ring perimeter instead of being conﬁned to

individual atomic sites.^41b

Regarding the B-Ru coordination in 3 and 5, the topology of

the electron density shows remarkable diﬀerences. While the

cationic borane complex 3 displays a single BCP and bond path

linking the B and Ru nuclei, no such BCP exists in the neutral

borate species 5. Instead, the borate group is linked to the Ru

center by bond paths emanating from the bridging hydrogens

(H1 and H2). Together with the B-H(1,2) bond paths they

form an almost planar four-membered ring which contains

a RCP (on the line between the B and Ru nuclei). Surprisingly,

no Ru-H(1,2) bond paths are found for the borane complex 3.

We have veriﬁed that the missing Ru-H(1,2) bond paths in 3

are not an artifact of the DFT method or basis set, by performing

a MP2 calculation on a smaller model complex 3s, in which the

Cp*, MesBH₂, and PⁱPr₃ligands of 3 were replaced by Cp,

PhBH₂, and PH₃groups, respectively. The geometry of this

model system was optimized with B3LYP/6-31G(d,p)[main

group elements]/SVPPalls1[Ru]. For the single-point MP2

calculation we employed the 6-311G(d,p) basis set for the main

group atoms and an all-electron, polarized triple-ζ valence basis

set (TZVPPalls2) for the Ru atom (augmented with a set of

f-type polarization functions, exponent 1.2453897).³⁵The topol-

ogies of the MP2 and B3LYP densities of 3s are identical to the

B3LYP density of 3, with a single B-Ru BCP and no Ru-H(1,2)

BCPs.

Inspection of Figure 7 suggests an explanation for the “miss-

ing” Ru-H(1,2) BCPs and bond paths in 3: The ring critical

point on the line between the Ru and B centers in 5 is not at the

center of the four-membered Ru-H1-B-H2 ring, but in close

proximity to the Ru-H(1,2) BCPs. Also, the electron density in

this region is relatively ﬂat, with density values of 0.07 e bohr^-3at

the Ru-B RCP and only 0.08 e bohr^-3at each of the Ru-

H(1,2) BCPs. Together with the bending of the Ru-H(1,2)

bond paths (0.03 Å longer than the Ru-H interatomic dis-

tances), this indicates that the Ru-H(1,2) bond paths are rather

unstable and susceptible to breaking because of small changes in

geometry.⁴²

In 3, RufB π back-donation leads to a large contraction of the

Ru-B interatomic distance by 0.29 Å, concomitant buildup of

electron density, and formation of the Ru-B BCP (0.11 e bohr^-3).

This results in the rupture of the fragile Ru-H(1,2) bond paths.

Similar observations have been made for a related class of σ-

complexes containing a Mn(η²-Si-H) moiety, whereby a Mn-Si

bond path present in [Cp⁰Mn(CO)₂(η²-HSiFPh ₂)] is lacking in

[Cp⁰Mn(CO)₂(η²-HSiHPh₂)] and disappears in the related

[CpMn(CO) ₂(η²-HSiCl₃)] complex upon increase of the

Mn-Si distance by only 0.05 Å.^42,44

The distinct molecular graphs in the Ru-H(1,2)-B region

emphasize the major diﬀerence between complexes 3 and 5,

which is the existence of π back-donation in 3, and the lack

thereof in 5. Since the topology of the electron density is very

sensitive to such diﬀerences, additional AIM properties are

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analyzed in the following section to provide a more complete

picture of molecular structure and bonding in complexes 3 and 5.

Topological Analysis of the Electron Density. AIM Bond

Properties. In Table 4, selected AIM bond properties are

presented for the Ru and B moieties in complexes 3 and 5 as

well as for the fragments Cp*(PⁱPr₃)Ru^þ, MesBH₂and MesBH₃.

Listed are the numerical values at the bond critical point for the

Table 4. AIM Bond Properties (Atomic Units) of Complexes

3 and 5^a

interaction

complex

F_b

r F_b

H_b

ε^e

δ ^f

Ru-P

0.07

0.08

0.14

0.17

0.13

-0.02

0.03 0.78

0.21 0.84

0.23 0.73

Cp^g(PⁱPr₃)Ru^þ0.07

electron density F_b, Laplacian of electron density r F_b, energy

density H_b, bond ellipticity ε, and delocalization index δ.⁴³These

AIM properties have been successfully used to characterize

bonding interactions in transition metal complexes.^41b,42In

particular, the delocalization index provides a bridge between

chemically intuitive bonding models and the rigorous AIM

definition of bonding in terms of the topology of the electron

density, as it quantifies the number of electron pairs shared

between two atoms.⁴²For comparison we also report values for a

typical aromatic C-C bond (from the mesityl group in 3) and a

C-C single bond (from an isopropyl group in 3).

Ru-B

0.11

0.12

-0.05

0.72 0.59

(0.07)^g(0.17)^g(-0.01)^gn.a. 0.19^h

Ru-H1

Ru-H2

Ru-H1

Ru-H2

n.a.

n.a. 0.56^h

n.a. 0.57^h

0.50 0.55

0.51 0.54

n.a.

0.08

0.25

-0.02

B-H1

B-H2

B-Hⁱ

B-H1

B-H2

B-H3

B-H^j

0.15

0.18

0.12

0.17

0.15

-0.29

-0.28

-0.27

-0.04

-0.20

-0.05

-0.15

-0.19

-0.11

-0.18

-0.15

0.27 0.43

0.28 0.54

0.11 0.35

0.10 0.35

0.09 0.50

0.02 0.47

Bond properties for the Ru-P interaction have values that are

typical for dative bonding to a metal center:^41b,42low F_b; small

and positive Laplacian (indicating a local depletion of electron

density and local excess of kinetic energy at the BCP, character-

istic for closed-shell interactions); small and negative H_b; and

signiﬁcant delocalization index δ.⁴³The values in 3, 5, and

Cp*(PⁱPr₃)Ru^þare very similar, with a slightly stronger Ru-P

interaction in 5 as demonstrated by the F_band δ indices.

Bond properties for the Ru-B interaction in 3 are also

indicative of dative bonding. The large bond ellipticity of 0.72

indicates a strong π character,⁴²demonstrating that this inter-

action is dominated by RufB π back-donation. In 5, no Ru-B

BCP exists and δ attains a low value of 0.19, indicating only a

weak interaction between Ru and B.

The Ru-H(1,2) bond indices in 5 are again typical for dative

interactions, in this case bis(η²-B-H) σ-donation to the Ru^IIcenter.

No Ru-H(1,2) BCPs are found for the optimized geometry of 3;

however, the delocalization indices δ(Ru,H) of about 0.56 in 3 (cf.

δ(Ru,H) ≈ 0.55 in 5 and δ(Mn,H) ≈ 0.63 in a series of Mn(η²-

Si-H) complexes⁴⁴) indicate a signiﬁcant sharing of electron pairs

between Ru and the bridging hydrogens (i.e., signiﬁcant σ-donation).

Hence, the AIM delocalization indices unambiguously characterize

both 3 and 5 as bis(η²-B-H) complexes between a Cp*(PⁱPr₃)Ru^þ

fragment and a mesitylborane(borate) ligand, thus resolving the

apparent contradiction between the molecular graphs and the MO

and bond order analysis (vide supra).

MesBH₂

MesBH₃

B-C_ipso

0.20

0.19

0.17

0.15

-0.20

-0.10

-0.13

0.08

-0.22

-0.20

-0.17

-0.14

0.04 0.65

0.18 0.55

0.14 0.51

0.03 0.44

MesBH₂

MesBH₃

CdC(Mes)

C-C(iPr)

0.32

0.24

-0.86

-0.52

-0.32

-0.19

0.22 1.40

0.01 1.01

^aIndividual values discussed in main text are shown in bold. ^bElectron

density at BCP. ^cLaplacian of electron density at BCP. ^dEnergy density

at BCP. Bond ellipticity. Electron delocalization index. ^gNo Ru-B

BCP in 5, values are reported for the RCP on the line between the Ru

and B centers. ^hNo BCPs for Ru-B (5) and Ru-H(1,2) (3). ⁱThe two

B-H bonds in MesBH₂are equivalent by symmetry. ^jAverage value of

three B-H bonds in MesBH₃

evidenced by the larger F_b, H_b, and δ values, in agreement with

the bond order analysis. Both B-C_ipsobond strengths increase

slightly upon complexation to Ru.

An interesting pattern of variation is found for the B-C_ipso

The B-H and B-C_ipsointeractions are characterized as

bond ellipticities: The ellipticity increases from MesBH₃to

shared (covalent) interactions on the basis of their negative

MesBH₂because of the trigonal planar geometry of the borane

group, where the B-H bonds lead to accumulation of electron

density in a plane (coplanar with the Mes group) containing the

B-C_ipsobond. However, the ellipticity also increases from

MesBH₃^-to 5, which we attribute to the weakening of the B-

H1 and B-H2 bonds because of σ-donation to Ru, while the

B-H3 bond is unaﬀected (in fact, slightly strengthened). The

electron density in the B-H3 bonding orbital (perpendicular to

the Mes group) is only partially canceled out by the weakened σ-

donating B-H1 and B-H2 bonds (torsion angles to Mes group

(30°), leading to an accumulation of electron density in a plane

perpendicular to Mes, resulting in a signiﬁcant bond ellipticitiy of

0.14. In free MesBH₃^-, the electron densities of the 3 B-H

bonds (one coplanar, two gauche to Mes) cancel out with respect

to the B-C_ipsobond, leading to a low ellipticity of 0.03. In the

borane complex 3, the B-H(1,2) bonds accumulate electron

values of r F_b, indicating a local concentration of electron

density at the BCP. The only exception is the B-C_ipsobond in

MesBH₃^-, for which the positive Laplacian suggests a strongly

polar interaction. The B-H and B-C_ipsodelocalization indices

are all <1, which shows that the shared electron pair is unequally

distributed between the atoms, leading to polar covalent

bonds.^41bThe B-H(1,2) bonds in 3 and 5 are activated relative

to the free MesBH₂and MesBH₃fragments, respectively, as

shown by the decrease in electron density (ΔF_b= 0.03), energy

density (ΔH_b= 0.04), and delocalization index (Δδ = 0.11 (3),

0.12 (5)) upon coordination to Ru. Conversely, the B-H3 bond

in 5 is slightly strengthened relative to the free mesitylborate. The

borane B-H bonds in 3 and MesBH₂have noticeable ellipticities

of about 0.27, which seems to indicate a π-electron delocalization

from the aromatic mesityl group into the coplanar BH₂unit. The

B-C_ipsobond in 3 is somewhat stronger compared to 5 as

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ARTICLE

density in a plane parallel to Mes, while π-backdonation from the

Ru center leads to accumulation of electron density in a plane

perpendicular to Mes. The two planes of electron density

accumulation are orthogonal to each other, giving a low bond

ellipticity of 0.04 despite the presence of signiﬁcant π-density on

the B center. This situation is similar to the triple bond in

acetylene, where the two orthogonal π bonds give rise to an

overall cylindrically symmetrical bond with ε = 0.

The results of the AIM analysis are consistent with the

conclusions from the MO and bond order analysis: Both 3 and

5 are characterized as donor-acceptor complexes, where a

mesitylborane(borate) ligand coordinates to a Cp*(PⁱPr₃)Ru^þ

fragment in a bis(η²-B-H) fashion. The covalent B-H(1,2)

bonds of the ligand are activated but not broken upon coordina-

tion to the Ru center. Signiﬁcant σ-donation to the metal center

is indicated by the sizable δ(Ru,H) delocalization indices and the

weakening of the B-H(1,2) bonds in both 3 and 5. RufB π

back-donation is manifested in the topology of the electron

density of 3 by the existence of a Ru-B BCP with large bond

ellipticity. No such interaction exists in 5. This distinction

between 3 and 5 is emphasized in the diﬀerent topologies of

the electron density in the Ru-H(1,2)-B region. The AIM

delocalization indices and bond ellipticities are found to be

extremely valuable in the characterization of Ru-P, Ru-B,

and Ru-H dative bonding, as well as for investigating changes

in the covalent B-H and B-C_ipsobonds upon coordination

to Ru.

solvent purification system purchased from mBraun Inc. Dichloro-

methane and diethyl ether were purified over two alumina-packed

columns, while hexanes and pentane were purified over one alumina-

packed column and one column packed with copper-Q5 reactant.

Chloroform-d₁(Aldrich) and fluorobenzene (Alfa Aesar), as well as

benzene-d₆, tetrahydrofuran-d₈, methylcyclohexane-d₁₄, and bromo-

benzene-d₅(Cambridge Isotopes) were degassed by using at least three

repeated freeze-pump-thaw cycles and stored over 4 Å molecular

sieves for 24 h prior to use. All solvents used within the glovebox were

stored over activated 4 Å molecular sieves. Cp*Ru(PⁱPr₃)Cl (1),⁴⁵

mesitylborane (MesBH₂),⁴⁶and LiH₃BMes,^7ewere prepared by using

literature procedures, while LiB(C₆F₅)₄2.5OEt₂(LiB(Ar^F)₄) was

obtained from Boulder Scientific. All purchased and prepared solids

were dried in vacuo prior to use. All other reagents were obtained from

Aldrich (except for PⁱPr₃, Strem) and were used as received. ¹H, ¹³C, ¹¹B,

and ³¹P NMR characterization data were collected at 300 K on a Bruker

AV-500 spectrometer operating at 500.1, 125.8, 160.5, and 202.5 MHz

(respectively) with chemical shifts reported in parts per million down-

field of SiMe₄(for H and ¹³C), 85% H₃PO₄in D₂O (for ³¹P), or

BF₃OEt₂(for ¹¹B). ¹H, ¹³C, and ¹¹B NMR chemical shift assignments

are made on the basis of data obtained from ¹³C-DEPT, ¹H-¹H COSY,

¹H-¹³C HSQC, ¹H-¹³C HMBC, and ¹H-¹¹B HSQC NMR experiments;

NMR signals associated with B(C₆F₅)₄are not reported. Elemental

analyses were performed by Canadian Microanalytical Service Ltd.,

Delta, British Columbia, Canada.

Synthesis of Cp*Ru(PⁱPr₃)(BH₂MesCl) (2). To a glass vial contain-

ing a magnetically stirred deep blue solution of 1 (0.059 g, 0.14 mmol) in

hexanes (3 mL) was added solid MesBH₂(0.018 g, 0.14 mmol) all at once.

The vial was sealed with a PTFE-lined cap and magnetic stirring was initiated.

Over the course of several seconds, the reaction mixture became dark orange.

After 0.5 h, ³¹P NMR data collected on an aliquot of this crude reaction

mixture indicated the quantitative formation of 2. The reaction mixture was

filtered through Celite, concentrated in vacuo to about 2 mL, and stored at

-37 °C. After 24 h, crystals of 2 were isolated by removal of the supernatant

solution by use of a Pasteur pipet; this solution was then concentrated in

vacuo to induce further crystallization. After repeating this procedure, the

isolated crops of crystals were then combined and dried in vacuo, yielding 2

as an analytically pure dark orange crystalline solid (0.052 g, 0.092 mmol,

66%). Compound 2 was found to be thermally sensitive, as well as reactive

toward trace air and moisture; storage under inert atmosphere at or below

-37 °C is recommended. Anal. Calcd. for C₂₈H₄₉BClPRu: C 59.63; H 8.76;

N 0.00. Found: C 59.59; H 8.71; N < 0.3. ¹H NMR (methylcyclohexane-

d₁₄): δ 6.64 (s, 2H, aryl-H), 2.68 (m, 3H, P(CHMe₂)₃), 2.33 (s, 6H, aryl o-

Me), 2.19 (s, 3H, aryl p-Me), 1.37 (s, 15H, C₅Me₅), 1.24 (d of d, ³J_PH= 12.5

’ SUMMARY AND CONCLUSIONS

The synthesis of [Cp*Ru(PⁱPr₃)(BH₂Mes)]^þB(C₆F₅)₄3,

and its characterization by use of spectroscopic, single-crystal

X-ray diﬀraction, and DFT computational methods, establishes

this complex as a member of a very limited class of isolable

cationic species featuring the unusual bis(η²-B-H) monoborane

ligation motif. In contrast to structurally related [Cp*(PR₃)(H)

₂RudSiHR]^þX^-complexes,^14-16eﬀorts to promote the inser-

tion of unsaturated organic substrates into the B-H bonds of 3

were unsuccessful, as were eﬀorts to prepare the neutral mesity-

lboryl complex Cp*(PⁱPr₃)Ru(BHMes). A comparative analysis

of the solid state structure and electronic topology of 3 with

5 reveals important similarities and diﬀerences between these

complexes. Both are best described as donor-acceptor com-

plexes between a Cp*(PⁱPr₃)Ru^þfragment and a bis(η²-B-H)

coordinating mesitylborane(borate) ligand, with the existence

of signiﬁcant σ-donation from the B-H bonds into the Ru^II(d⁶)

center conﬁrmed by the calculated NBO populations, bond

orders, and AIM delocalization indices. Additionally, in the case

of 3 the vacant p orbital on boron is stabilized by π back-donation

Hz, J_HH= 7.5 Hz, 18H, P(CHMe₂)₃), -11.77 (br s, 2H, Ru(H)₂B);

¹³C{¹H} NMR (methylcyclohexane-d₁₄): δ 148.3 (aryl ipso-quaternary),

135.2 (aryl o-quaternary), 133.7 (aryl p-quaternary), 127.3 (aryl-CH), 87.4

(C₅Me₅), 26.5 (P(CHMe₂)₃), 21.0 (aryl o-Me), 20.5 (aryl p-Me), 19.6

(P(CHMe₂)₃), 10.0 (C₅Me₅); ³¹P{¹H} NMR (methylcyclohexane-d₁₄): δ

66.6; ¹¹B{¹H} NMR (methylcyclohexane-d₁₄): δ 46.5, Δν_1/2= 462 Hz.

Storage of a concentrated hexanes solution of 2 at -37 °C provided a

suitable crystal for single-crystal X-ray diffraction studies.

from Ru resulting in a dative Ru B interaction, as well as by

3 3 3

resonance with the mesityl group.

Synthesis of [Cp*Ru(PⁱPr₃)(BH₂Mes)]^þB(C₆F₅)₄^-(3). Compound

2 was prepared in situ by treatment of a solution of 1 (0.042 g, 0.097 mmol) in

hexanes (2 mL) with solid MesBH₂(0.015 g, 0.12 mmol) followed by stirring

at ambient temperature, which caused a color change from blue to dark orange

over the course of several seconds. After 0.25 h of magnetic stirring, a solution of

’ EXPERIMENTAL SECTION

General Considerations. Unless otherwise stated, all manipula-

tions were conducted at ambient temperature in the absence of oxygen

and water under an atmosphere of dinitrogen, either by use of standard

Schlenk methods or within an mBraun glovebox apparatus, utilizing

glassware that was oven-dried (130 °C) and evacuated while hot prior to

use. Celite (Aldrich) was oven-dried for 5 d and then evacuated for 24 h

prior to use. The non-deuterated solvents dichloromethane, diethyl

ether, hexanes, and pentane were deoxygenated and dried by sparging

with dinitrogen gas, followed by passage through a double-column

LiB(C₆F₅)₄2.5OEt₂(0.084 g, 0.097 mmol) in fluorobenzene (2 mL) was

added, causing a color change from dark orange to orange-yellow with the

concomitant formation of a fine precipitate. After an additional 0.75 h of stirring,

the precipitate was removed by filtration of the reaction mixture through Celite.

The solvent and other volatiles were removed in vacuo, generating an oily yellow-

orange solid that was then triturated with pentane (3 ꢀ 2 mL). Subsequent

drying of the residue in vacuo afforded 3 as a yellow solid (0.084 g, 0.067 mmol,

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69%) that was determined to be about 95% pure on the basis of ¹H and ³¹

NMR analysis. Analytically pure 3was obtained as a microcrystalline yellow solid

in 54% yield (based on 1) by successive recrystallizations from concentrated

diethyl ether solutions. Anal. Calcd for C₅₂H₄₉B₂F₂₀PRu: C 51.72; H 4.09; N

0.00. Found: C 51.55; H 4.09; N < 0.3. ¹H NMR (bromobenzene-d₅): δ 6.81

(s, 2H, aryl-H), 2.58 (s, 6H, aryl o-Me), 2.20 (s, 3H, aryl p-Me), 1.78 (m, 3H,

P(CHMe₂)₃), 1.69 (d, J = 1.0 Hz, 15H, C₅Me₅), 0.97 (d of d, ³J_PH= 14.5 Hz,

147.4 (ipso-quaternary), 136.9 (aryl o-quaternary), 135.6 (aryl p-

quaternary), 128.8 (aryl-CH), 92.1 (C₅Me₅), 27.9 (d, J_PC= 18.9 Hz,

P(CHMe₂)₃), 22.2 (aryl o-Me), 21.9 (aryl p-Me), 20.6 (P(CHMe₂)₃)

11.5 (C₅Me₅); ³¹P{¹H} NMR (C₆D₆): δ 77.9; ¹¹B{¹H} NMR (C₆D₆):

δ 59.7 (Δν_1/2= 341.4 Hz). Slow evaporation of a concentrated pentane

solution of 6 at ambient temperature provided a suitable crystal for

single-crystal X-ray diffraction studies.

³J_HH= 14.5 Hz, 18H, P(CHMe₂)₃), -10.3 (br d, J_PH= 15.0 Hz, 2H,

Generation and Characterization of Cp*Ru[K³-P ⁱPr₂-

(MeCdCH₂)]H (7). Compound 1 was prepared in situ by treatment

of a THF-d₈solution (2 mL) of [Cp*RuCl]₄(0.012 g, 0.011 mmol) with

ⁱPr₃P (8.2 μL, 0.044 mmol) followed by magnetic stirring at ambient

temperature. After 0.2 h of magnetic stirring, the blue solution was

quantitatively transferred via pipet to a vial containing solid benzyl

potassium (0.006 g, 0.044 mmol) causing an immediate color change of

Ru(H)₂B); ¹³C{¹H} NMR (bromobenzene-d₅): δ 147.3 (aryl ipso-

quaternary), 146.9 (aryl o-quaternary), 130.2 (aryl p-quaternary), 129.7 (aryl-

CH), 97.6 (C₅Me₅), 27.4 (d, ¹J_PC= 22.3 Hz, P(CHMe₂)₃), 22.1 (aryl o-Me),

22.0 (aryl p-Me), 19.7 (P(CHMe₂)₃), 11.2 (C₅Me₅); ³¹P{¹H} NMR

(bromobenzene-d₅): δ 72.0. We have not been able to observe ¹¹B NMR

resonances for 3, despite prolonged acquisition times using either variable

temperature ¹¹B{¹H} NMR methods or ¹H-¹¹B HSQC NMR techniques, and

by employing baseline correction routines. Furthermore, we have thus far not

been able to obtain satisfactory IR data for 3, possibly owing to the air-sensitivity

of the complex. Storage of a dilute diethyl ether solution of 3 at ambient

temperature provided a suitable crystal for single-crystal X-ray diffraction studies.

Synthesis of Cp*Ru(PⁱPr₃)(BH₃Mes) (5). Compound 1 was

prepared in situ by treatment of a suspension of [Cp*RuCl]₄(0.11 g,

0.099 mmol) in hexanes (5 mL) with ⁱPr₃P (76 μL, 0.40 mmol) followed

by magnetic stirring at ambient temperature. After 0.25 h of magnetic

stirring solid LiH₃BMes (0.083 g, 0.59 mmol) was added all at once

causing a color change from deep blue to dark red with the concomitant

formation of a fine precipitate. After 1 h of stirring, the reaction mixture

was filtered through Celite, concentrated in vacuo to approximately

2 mL, and stored at -35 °C. After 24 h, 5 had crystallized as dark red

crystals and that were isolated by removal of the supernatant solution by

use of a Pasteur pipet; this solution was then concentrated in vacuo to

induce further crystallization. After repeating this procedure, the isolated

crops were then combined and dried in vacuo, yielding 5 as an

analytically pure red crystalline solid (0.14 g, 0.26 mmol, 65%). Anal.

Calcd for C₂₈H₅₀BPRu: C 63.46; H 9.52; N 0.00. Found: C 63.73; H

9.38; N < 0.3. ¹H NMR (C₆D₆): δ 7.00 (s, 2H, aryl-H), 6.77 (br s, 1H, BH),

2.54 (s, 6H, aryl o-Me), 2.41-2.28 (m, 6H, aryl p-Me and P(CHMe₂)₃),

1.43 (d, ⁴J_PH= 1.3 Hz, 15H, C₅Me₅), 1.14 (d of d, ³J_PH= 12.5 Hz, ³J_HH= 7.0

Hz, 18H, P(CHMe₂)₃), -12.09 (br s, 2H, Ru-H₂-B); ¹³C{¹H} NMR

(C₆D₆): δ 135.5 (ipso-quaternary), 132.6 (aryl o-quaternary), 126.7 (aryl-

CH), 84.5 (C₅Me₅),24.2(d,²J_PC=18.6Hz,P(CHMe₂)₃), 21.8 (aryl o-Me),

20.4 (aryl p-Me), 18.8 (P(CHMe₂)₃), 9.9 (C₅Me₅); ³¹P{¹H} NMR

(C₆D₆): δ 62.5; ¹¹B{¹H} NMR (C₆D₆): δ 41.8 (Δν_1/2= 326.2 Hz).

Storage of a concentrated pentane solution of 5 at -35 °C provided a

suitable crystal for single-crystal X-ray diffraction studies.

the mixture to dark red. While H and ³¹P NMR data obtained from the

reaction mixture revealed the clean conversion to 7, attempts to isolate this

compound in analytically pure form have been thwarted by its apparent

instability. ¹H NMR (THF-d₈): δ 2.10-1.99 (m, 1H, P(CHMe_aMe_b)), 1.95

(s, 15H, C₅Me₅), 1.90-1.68 (m, 2H, P(CHMe_aMe_b) and MeCdCH_aH_b),

1.61 (d, ³J_PH= 6.5 Hz, 3H, MeCdCH₂), 1.45 (d of d, ³J_PH= 10.5 Hz, ³J_HH

7.0 Hz, 3H, P(CHMe_aMe_b)), 1.35 (d of d, ³J_PH= 17.0 Hz, ³J_HH= 7.0 Hz, 3H,

P(CHMe_aMe_b), 1.14 (m, 1H, MeCdCH_aH_b), 0.89 (d of d, ³J_PH= 19.5 Hz,

³J_HH= 7.0 Hz, 3H, P(CHMe_cMe_d)), 0.76 (d of d, ³J_PH= 14.5 Hz, ³J_HH= 7.0

Hz, 3H, P(CHMe_cMe_d)), -12.31 (d, ²J_PH= 25.0 Hz, 1H, RuH); ¹³C{¹H}

NMR (THF-d₈):δ91.6 (C₅Me₅), 37.3(d,¹J_PC=21.4Hz, MeCdCH₂), 28.8

(d, ²J_PC= 13.8 Hz, MeCdCH₂), 28.3 (d, ¹J_PC= 23.9 Hz, P(CHMe_aMe_b),

21.6 (d, ²J_PC= 8.8 Hz, P(CHMe_cMe_d), 20.8 (d, ²J_PC= 6.3 Hz, MeCdCH₂),

19.8 (d, J_PC= 7.5 Hz, P(CHMe_aMe_b)), 19.6 (d, J_PC= 20.1 Hz, P-

(CHMe_cMe_d)), 18.7 (P(CHMe_cMe_d)), 18.5 (d, J_PC= 6.3 Hz, P-

(CHMe_aMe_b)), 11.3 (C₅Me₅); ³¹P{¹H} NMR (THF-d₈): δ 53.2.

Synthesis of Cp*Ru[K¹-PⁱPr₂(MeCdCH₂)](BH₃Mes) (8). A

solution of 7 (0.21 mmol) in THF (2 mL) was prepared in situ as

outlined above. The reaction mixture was magnetically stirred for 0.5 h

prior to the addition of solid MesBH₂(0.028 g, 0.21 mmol), which

caused the immediate color change to dark red-yellow. After 0.5 h, ³¹

NMR data collected on an aliquot of this crude reaction mixture

indicated the quantitative formation of 8. The solvent was removed in

vacuo affording a red-orange solid which was extracted into pentane

(3 ꢀ 1 mL) followed by filtration through Celite. The combined

pentane extracts were concentrated to approximately 2 mL and was

stored at -37 °C. After 24 h, crystals of 8 were isolated by removal of the

supernatant solution by use of a Pasteur pipet; this solution was then

concentrated in vacuo to induce further crystallization. After repeating

this procedure, the isolated crops of crystals were then combined and

dried in vacuo, yielding 8 as an analytically pure crystalline orange-red

solid (0.052 g, 0.099 mmol, 47%). Anal. Calcd for C₂₈H₄₈BPRu: C

Synthesis of Cp*Ru(PⁱPr₃)(BH₂MesOH) (6). Compound 2 was

prepared in situ by treatment of a solution of 1 (0.10 g, 0.23 mmol) in

hexanes (5 mL) with solid MesBH₂(0.031 g, 0.23 mmol) followed by

stirring at ambient temperature, during which time the solution turned

from light blue to orange. After 0.25 h of magnetic stirring solid KO^tBu

(0.029 g, 0.26 mmol) was added all at once causing a color change from

deep orange to yellow-red with the concomitant formation of a fine

precipitate. After 3 h of stirring, the reaction mixture was filtered through

Celite, concentrated in vacuo to approximately 2 mL, and stored at

-37 °C. After 24 h, 6 had precipitated as fine yellow powder and was

isolated by removal of the supernatant solution by use of a Pasteur pipet;

this solution was then concentrated in vacuo to induce further pre-

cipitation. After repeating this procedure, the isolated crops were then

combined and dried in vacuo, yielding 6 as an analytically pure yellow

powder (0.059 g, 0.112 mmol, 48%). Anal. Calcd for C₂₈H₅₀BOPRu: C

63.71; H 9.17; N 0.00. Found: C 63.36; H 9.11; N < 0.3. H NMR

(C₆D₆): δ 7.00 (s, 2H, aryl-H), 6.86 (br s, 1H, B-H), 5.59-5.50 (m, 1H,

P(MeCdCH_aH_b)), 5.35-5.29 (m, 1H, P(MeCdCH_aH_b)), 2.53 (s,

6H, aryl o-Me), 2.39 (s, 3H, aryl p-Me), 2.22 (m, 2H, P(CHMe₂)₂), 1.87

(m, 3H, P(MeCdCH₂)), 1.43 (d, J_PH= 1.3 Hz, 15H, C₅Me₅), 1.08 (d of

d, ³J_PH= 14.0 Hz, ³J_HH=7.0 Hz, 6H, P(CHMe_aMe_b)₂), 0.98 (dof d, ³J_PH

13.0 Hz, ³J_HH= 7.0 Hz, 6H, P(CHMe_aMe_b)₂), -12.03 (br s, 2H, Ru-H₂-

B); ¹³C{¹H} NMR (C₆D₆): δ 149.3 (aryl ipso-quaternary), 139.4

(P(MeCdCH₂)), 135.4 (aryl o-quaternary), 132.7 (aryl p-quaternary),

127.3 (aryl-CH), 123.5 (d, J_PC= 7.2 Hz, P(MeCdCH₂)), 85.0

(C₅Me₅), 23.8 (d, J_PC= 10.3 Hz, P(MeCdCH₂)), 22.0 (d, J_PC

21.3 Hz, P(CHMe₂)₂), 21.8 (aryl o-Me), 20.4 (aryl p-Me), 17.9

(P(CHMe_aMe_b)₂), 9.6 (C₅Me₅); ³¹P{¹H} NMR (C₆D₆): δ 66.7; ¹¹B-

{¹H} NMR (C₆D₆): δ 43.0 (Δν_1/2= 296.0 Hz).

61.60; H 9.24; N 0.00. Found: C 61.41; H 9.33; N < 0.3. H NMR

(C₆D₆): δ 6.83 (s, 2H, aryl-H), 3.74 (t, ³J_HH= 3.0 Hz, 1H, -OH), 2.55 (s,

6H, aryl o-Me), 2.40 (m, 3H, P(CHMe₂)₃), 2.28 (s, 3H, aryl p-Me), 1.52

Synthesis of ⁱPr₃P BH₂Mes.⁴⁷To a magnetically stirred solution

of MesBH₂(0.025 g, 0.19 mmol) in CH₂Cl₂(2 mL) was added a

solution of PⁱPr₃(37 μL, 0.19 mmol) in CH₂Cl₂(1 mL) followed by

magnetic stirring; ³¹P NMR analysis of the reaction mixture after 1 h

(s, 15H, C₅Me₅), 1.21 (d of d, J_PH= 12.5 Hz, J_HH= 7.5 Hz, 18H,

P(CHMe₂)₃), -13.52 (br s, 2H, Ru-H₂-B); ¹³C{¹H} NMR (C₆D₆): δ

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ARTICLE

indicated the quantitative formation of ⁱPr₃P BH₂Mes. Removal of the

Present Addresses

^§Department of Chemistry, Yale University, New Haven,

Connecticut, U.S.A.

Department of Chemistry, Massachusetts Institute of Technology,

Cambridge, Massachusetts, U.S.A.

solvent and other volatiles in vacuo afforded a white solid that was dissolved

in pentane (3 mL) followed by filtration through Celite. Removal of the

pentane afforded a microcrystalline white solid (ⁱPr₃P BH₂Mes) that was

determined to be >95% pure on the basis of ¹H and ³¹P NMR data (0.052 g,

0.18 mmol, 95%). ¹H NMR (CDCl₃): δ 6.81 (s, 2H, aryl-H), 2.46 (s, 6H,

aryl o-Me), 2.34 (m, 3H, P(CHMe₂)₃), 2.27 (d, J = 2.5 Hz, 3H, aryl p-Me),

’ ACKNOWLEDGMENT

2.10 (2H, BH₂), 1.27 (d of d, J_PH= 12.5 Hz, J_HH= 7.0 Hz, 18H,

P(CHMe₂)₃); ¹³C{¹H} NMR (CDCl₃): δ 142.0 (d, J_PC= 5.5 Hz, aryl ipso-

quaternary), 133.5 (aryl o-quaternary), 133.4 (aryl p-quaternary), 127.6 (d,

Acknowledgment is made to the Natural Sciences and En-

gineering Research Council (NSERC) of Canada (including a

Discovery Grant for M.S., a Canada Graduate Scholarship for

K.D.H., and a Postgraduate Scholarship for M.A.R.), the Canada

Foundation for Innovation, the Nova Scotia Research and

Innovation Trust Fund, and Dalhousie University for their

generous support of this work. Computational facilities are

provided by ACEnet, the regional high performance computing

consortium for universities in Atlantic Canada. We also thank

Drs. Michael Lumsden and Katherine Robertson of the NMR3

(Dalhousie University) for assistance in the acquisition of

NMR data.

J_PC= 2.9 Hz, aryl-CH), 24.9 (aryl o-Me), 22.4 (d, J_PC= 26.3 Hz,

P(CHMe₂)₃), 20.9 (aryl p-Me), 18.0 (d, J_PC= 1.4 Hz, P(CHMe₂)₃);

³¹P{¹H} NMR (CDCl₃): δ 23.7 (br m); ¹¹B{¹H} NMR (CDCl₃): δ

-32.1, Δν_1/2= 130 Hz.

Crystallographic Characterization of 2, 3, 5, 6, and 8. Crys-

tallographic data were obtained at 193((2) K on either a Bruker

PLATFORM/SMART 1000 CCD diffractometer or a Bruker D8/

APEX II CCD diffractometer using a graphite-monochromated Mo

KR (λ = 0.71073 Å) radiation, employing samples that were mounted in

inert oil and transferred to a cold gas stream on the diffractometer.

Programs for diffractometer operation, data collection, and data reduc-

tion were supplied by Bruker. SADABS was employed as the absorption

correction method for 2, 3, and 6, while Gaussian integration (face-

indexing) was employed for 8. In the case of 5, the crystal used for data

collection was found to display nonmerohedral twinning, and as such

TWINABS was employed as the absorption correction method. Both

components of the twin were indexed with the program CELL_NOW

(Bruker AXS Inc., Madison, WI, 2004). The second twin component can

be related to the first component by 180° rotation about the [-0.052 1

-0.01] axis in real space and about the [0 1 0] axis in reciprocal space.

Integrated intensities for the reflections from the two components were

written into a SHELXL-93 HKLF 5 reflection file with the data

integration program SAINT (version 7.53A), using all reflection data

(exactly overlapped, partially overlapped, and non-overlapped). For each

of 3, 5, and 6, the structure was solved by use of direct methods, while for 2

and 8 a Patterson search/structure expansion was employed. Refinements

were carried out by use of full-matrix least-squares procedures (on F²) with

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’ ASSOCIATED CONTENT

Supporting Information. Single-crystal X-ray diﬀraction

data in CIF format for 2, 3, 5, 6, and 8, and DFT-optimized

molecular structures in XYZ format for 3 and 5. This material is

available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION

Corresponding Author

*E-mail: mark.stradiotto@dal.ca. Fax: 1-902-494-1310. Phone:

1-902-494-7190.

2442

dx.doi.org/10.1021/ic1022328 |Inorg. Chem. 2011, 50, 2431–2444

Inorganic Chemistry

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(H)₂RudSiHR]^þX^-complexes^14,16revealed minimal Ru-H SiHR

3 3 3

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(31) No evidence for chemical exchange involving the alkenyl C-H

and the Ru-H environments in 7 was obtained by use of ¹H-¹H EXSY

NMR techniques (300 K) employing a range of mixing times.

(32) (a) While our investigations were underway, the decomposi-

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dx.doi.org/10.1021/ic1022328 |Inorg. Chem. 2011, 50, 2431–2444

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2444

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